No title

DEKKER,

YORK

This book is printed on acid-free paper. Marcel Dekker, Inc. 270 Madison Avenue, New York, NY 10016 tel: 212-696-9000: 2 12-685”540 ~ a r cDekker ~ l AC Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 44-61-261-8482; 44-61-261-8896 http://www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities. For more i n f o ~ a t i o n write , to Special ~ales/Profe~sional Marketing at the h e a ~ ~ u ~address ers above.

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CA

carbon materials and in many diverse phenomena.The authors review the evidence for stress graphitization revealed by wide rangeof experimental techniques. They reviewthe well-known importantrole of pressure on the stmcturd changes in both graphitizable and nongraphitizable carbon precursors. They also discuss theless obvious roleof various inorganic compounds in accelerating graphitization under pressure. Most of their chapter is devoted, however, to the te~hnologicallyimportant benefits (or potential hazards!)of stresses induced not by external pressure but by fibedmatrix interaction during the preparation of carbo~carboncomposite materials. Examplesof stress-induced preferential orientation of in the matrix carbon (e.g., concentric around cylindrical fibers) are presented for wide range of practical situations. In p ~ i c u l a rthe , authors illustrate theimpacts of stressgraphitizationon the mechanicalandthermal properties of these materials. inagaki and his collaborators in Japan and France review graphite-forming process whose theoretical background-and indeed feasibility-is supported by the concepts discussed inthe previous two chapters.The fact that perfect graphite films canbe prepared in the solid state from thermosetting and thus “nongraphitizing” precursors such polyimides apparently defies conventional wisdom K. Galbraith would say). Nevertheless, whenthin precursor filmis used and the supramolecular planarity is carefully preserved during the critical stage of its carbonization, highly graphitizable carbon films (including those derived from Kaptonusing techniques!)canbeproduced. The authors detail the processes of preparation, carbonization, and graphitization of aromatic polyimide films, including their modification using heteroatom intercalation. They devote special attention to film quality controlling parameters and characterization methods. They demonstrate remarkably strong effectof the precursor molecular structure on carbon graphitizability and thus seem to be optimistic about the technological prospects of these unique carbon and graphite films. Peter A.

R. Radovic

Sylvie ~ o ~ Centre ~ a de~ Recherche y sur Mati&re Diviske, CNRS-Universitk d’Orlkans, Orl6ans, France Y o s ~ i ~ i r o ~ i s ~Faculty i y a ~ a Engineering, Musashi Institute of Technology, Setagaya-ku, Tokyo, Japan ~ i c ~ i o I ~ Graduate a g a ~ i School Engineering, Hokkaido University, Kita-ku, Sapporo, Japan Robert A. eye^^ fornia

University

California, Santa Barbara, Santa Barbara, Cali-

~ b e ~ lDepartment ~ n of Physics, CNRS, Argelliers, France Paul R o ~ x ~ eUnit6 t deChimiedesInterfaces,UniversitkCatholiquede Louvain, Louvain-la-Neuve, Belgium ~ u t o ~ u ~ ~ Department ~ i c of~ Materials i Science,ToyohashiUniversity Technology, Tempaku-cho, Toyohashi, Japan

of

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Preface Contributors to Volume 26 Contents of Other Volumes

V

ix

1. COLLOIDAL AND SUPRAMOLECULARASPECTS OF CARBON Agn& Oberlin, Sylvie ~ o n n a ~and y , Paul G. R o ~ h ~ t I. 11. 111. IV. V, VI. VII. VIII. IX.

Introduction Overview of Carbonaceous Materials Elemental Units Primary Carbonization Change of Volatile Content Change in the Dispersion of ~ o l y ~ o m a tComponents ic Carbon as a Colloidal Material Consequences of Primary Carbonization Conclusion References

2. STRESS GRA~~ITIZATION Michio Inagaki and Robert Meyer I. Pressure 11.under Graphitization 111. GraphitizationinCoexistencewithMineralsunderPressure177 IV. Stress Graphitization CarbonlCarbon in Composites 195

1

2 9 13 86 100 113 131 138 139 149

1S0 152

239 242

V. Conclusions References 3.HIGHQUALITY GRAPHITE FILMSPRODUCEDFROM AROMATIC POLYIMIDES ~ i c h i oInagak~,T s u t o ~Tak~ichi, ~ Yo~hihiro ~ishiya~a, Agn& O b ~ r l i ~

Remarks

Index

245

I. II. Carbonization and Graphitization of Aromatic Polyimide Films III, Control Quality 287 ofFilms Graphite IV. 307 Quality Films Graphite Concluding References

256

3 335

VOLUME l Dislocations and Stacking Faults in Graphite,S. A m e l i n c ~P. , ~elavignette,and M. ~ e e r s ~ h a p Gaseous Mass Transport within Graphite, G. E Hewitt Microscopic Studies of Graphite Oxidation, Thomas Reactions of Carbon with Carbon Dioxide and Steam, Sabri and Me~ster The F o ~ a t i o nof C ~ b o nfrom Cases, ~ o w a r dB. Palmer and Charles Oxygen C ~ e ~ s o ~ tEffects i o n on Graphite Thermoelectric Power,P. L. ~ a Z ~ e ~ Jr.,L. C. Austin, and Eetjen VOLUME 2 Electron Microscopy of Reactivity Changes near Lattice Defects in Graphite, G. R. ~ e n n i g ~ubin~n Porous Structure and Adsorption Properties of Active Carbons, Radiation Damage in Graphite, l% N. Reynol~s Adsorption from Solution by Graphite Surfaces, A. Zettlemoyer andK. S. Electronic Transpo~in Pyrolytic Graphite and Boron Alloys of Pyrolytic Graphite, Klein Activated Diffusionof Gases in Molecular-Sieve Materials,P. L, ~ a l ~ L.e C.~ Austin, and S, P. ~ a n d i ~ O L U M E3 Nonbasal Dislocations in Graphite, Optical Studies of Carbon, Sabri

Thomas and

Roscoe

Action of Oxygen and Carbon Dioxide above l00 Millibars on “Pure” Carbon, M. Lang and Magnier X - b y Studies Carbon, Sabri Ergun Carbon Transport Studies for Helium-Cooled High-Temperat~eNuclear Reac~ i n s e yand , Romberg tors, M. R. Everett, D. V O L U ~ E4 X-Ray Diffraction Studies on Carbon and Graphite, Ruland Va~orization Carbon, Howard B. Palmer and ~ ~ r ~ eShelef c a i Growth Graphite Crystals from Solution, S. B,Au~terman Internal Friction Studies on Graphite, Tsuzuku and M.H.Saito The Formation Some Graphitizing Carbons, D,Brooks and Taylor Catalysis of Carbon Gasification, L. ~ a l k e rJr., , M. SheleJI and R. A. Anderson VOLUME 5 D~position~ Structure, and Properties of Pyrolytic Carbon, J. Bokros The Thermal Conductivity of Graphite, B, Kelly The Study of Defects in Graphite by Trans~issionElectron Microscopy, P. A. Thrower Intercalation Isotherms on Natural and Pyrolytic Graphite, J. Hooley VOLUME 6 Physical Adsorption of Gases and Vapors on Graphitized Carbon Blacks, N. N. ~ v g u and l A. Kiselev Graphitizati~~ of Soft Carbons, Jacques Maire and Jacques Mkring Surface Complexes on Carbons, B. R. Puri Effects Reactor Irradiation on the Dynamic Mechanical Behavior of Graphites and Carbons, R. E. Taylor and D. E. Kline VOLUM~ The Kinetics and ~ e c h a n i s mof Graphitization, D. B. ~ischbach The Kinetics of Graphitization, A. ~ a c a u l ~ Electronic Properties Doped Carbons, Andrk arch and Positive and Negative M~~n~toresistances in Carbons, Delhaes The Chemistry the Pyrolytic Conversion of Organic Compounds to Carbon, Fitzer, Mueller, and W: VOLUME 8 The Electronic Properties of Graphite, L. Spain Surface Properties Carbon Fibers, D. and V: J, Mimeault The Behavior of Fission Products Captured in Graphite by Nuclear Recoil,Seishi Yajima

VOLUME 9 Carbon Fibers from Rayon Precursors, Roger Bacon Control Structure of Carbon for Use in Bioengineering, Bokros, L. ~ G r a n s eand , Scho~n Deposition of Pyrolytic Garbon in Porous Solids, V: K o t l e ~ s ~ VOLUME 10 The Thermal Properties Graphite, B. Kelly and Taylor Lamellar Reactions in Graphitizable Carbons, M. Robert, M,Oberlin, and J. Methods and Mechanisms of Synthetic Diamond Growth, Strons, and H. ent to^ JK

B~ndy,H. M.

VOLUME 11 Structure and Physical Properties of Carbon Fibers, Reynolds Highly Oriented Pyrolytic Graphite, K Deformatio~~ e c ~ a n i ins Carbons, ~s Gwyn Je~ki~s Evaporated Carbon Films, I. S. McLintuck and J. C. Orr VOLUME Interaction of PotassiumandSodiumwithCarbons, Berger, B. M ~ t r o and ~, Hh-old Ortho-~ar~ydrogen Conversion and Hydrogen Deuterium E ~ u i l i b ~ a tover io~ Carbon Surfaces,Y. Ishikawa, L. G. Austin, D. E. and L. JK Thermoelectric andT h e ~ o m a ~ nEffects e ~ c in Graphite, ~~2~~~ and K, S ~ ~ i h ~ r a rafting of ~acromoleculesonto Carbon Blacks, J. B. Donnet, E. ~ d ~ 1 VOLUME 13 The Optical Properties Diamond, Gordon Davies Fracture in Polyc~ystallineGraphite, E. Brockl~h~rst ~ O L U M E14. Lattice Resolutionof Carbons by Electron Microscopy,G. R. Millward andD.A. Je~erson The F ~ m a t i o nof Filamentous Carbon, R. Baker and P. S. Mec~anismsof Carbon Black Formation, Lahaye and G. Pradu VOLUME 15 Pyrocarbon Coating of Nuclear Fuel ~articles, Guilleray, L. M. S, Price Acetylene Black: ~ a n u f a c t ~Properties, e, and Applications, Yvan S ~ ~ ~ o b The Formation. Graphitizable Carbons via Mesophase: Chennical andGnetic Considerations, and P h ~ l L, i~ Jr.

VOLUME 16 The Catalyzed Gasification Reactions of Carbon, D. W: McKee The Electronic Transport Propertiesof Graphite, Carbons, and Related Materials, Ian L. Spain VOLUME 1’7 Electron Spin Resonance and the Mechanism of Carbonization, Lewis and L. S. Singer Physical Properties of Noncrystalline Carbons, l? Delha2s and C a ~ o n a The Effect of Substitutional Boron on Irradiation Damage in Graphite, E. Brocklehurst, B. Kelly, and E. Gilchrist Highly Oriented Pyrolytic Graphite and Its Intercalation Compounds, ~oore VOLUME 18 Impurities in Natural Diamond, D. Bibby A Review of the Interfacial Phenomena in Graphite Fiber Composites, K. R. E. Fornes, D. M e ~ o r y and , R. D. Gilbert A Palladium-Catalyzed Conversion of Amorphous to Graphitic Carbon, , Poppa, and Boudart Holstein, R. D. ~ o o r h e a dH.

L.

VOLUME 19 Substitutional Solid Solubility in Carbon and Graphite, S. Marinkovi~ Kinetics of Pyrolytic Carbon Formation, l? A. Tesner Etch~decoration Electron Microscopy Studies of the Gas-Carbon Reactions, R a l ~ hT Yang Optical Properties of Anisotropic Carbon, R.A. Forrest, H. ~ a r s h C. , Corn~ord, B. Kelly V~LUM 20~ Structural Studies of PAN-Based Carbon Fibers, David Johnson The Electronic Structureof Graphite and Its Basic Origins, ~arie-FranceCharlier and Alphonse Charlier Interactions of Carbons, Cokes, and Graphites with Potassium and Sodium, Harry ~ ~ a r s hNeil , ~ u r d i e , A. S, Edwards, and ~ a n n s - P e t eBoehrn VOLUME 21 Microporous S t ~ c t u rof e Activated Carbons as Revealed by Adsorption ~ e t h o d s , Francisco Rodr~guez-~einoso Angel Linares-Solano Infrared Spectroscopy in Surface Chemistry of Carbons, Jerzy ~ a w a d z ~ i V O L ~ M E22 ~gh-ResolutionTEM Studiesof Carboni~ationand ~raphitization,Agn2s ~ b e r l i n Mechanisms andPhysical Properties of Carbon Catalystsfor Flue Gas Cleaning, t Ha~ald J~ntgen H e l ~ u Kghl

Theory of Gas Adsorptionon Structurally Heterogeneous Solids and Its Application for Characterizing Activated Carbons, VOLUME 23 Characterization of Structure and Microtextureof Carbon Materials by Magnetoresistance Technique, ~oshih~ro E l e c ~ o c h e ~ cCarbonization al of Fluoropolymers, Oxidation Protection of Carbon Materials, Nuclear Grade Activated Carbons and the Radioactive Iodide Problem, VOLUME 24 Early Stagesof Petroleum Pitch Carbonization-Kinetics and Mechanis~s, Thermal Conductivity of Diamond, I: Chemistry in the Production and Utilization of Needle Coke,Isao Interfacial Chemistry and Electrochemistry of Carbon Surfaces, VOLUME 25 Carbyne-A Linear Chainlike Carbon Allotrope, Yu R Small-Angle Scattering Neutrons and X-rays from Carbons and Graphites, Ernst Carbon Materials in Catalysis, R.

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~rgelliers,

CNRS- Univers~te"d ,Orle"ans, Orlkans, Universite" Catholique de ~ o ~ ~Louvain-la-~euve, a i ~ , Belgium

2

I. In~oduction 11. Overview of CarbonaceousMaterials

4

111. ElementalUnits DiffractionTechniques B. TransmissionElectronMicroscopy G, M o l e c u l ~MechanicsCalculations

9 9 9 12

IV, PrimaryCarbonization A, KerogensandCoals B. Asphaltenesand Oil Derivatives (Low OxygenContent) C. Pitches(Devoid of Oxygen) D. ComprehensiveStudies ofKey Materials E. ConstantsandVariantsinCarbonaceousMaterials

13

Change of VolatileContent A. Variation of Heating Rate Oxidation C. Hydroliquef~tionof Coals

64

Pressure

88 91

D. Catalytic Hydroconversion of Heavy Oils and Heavy Residues (Oil Feedstocks)

94

Change in the Dispersion of Polyaromatic Components A.AnisotropicPitches B. CokingCoals

100 100 108

Carbon ColloidalMaterial A.IntroductiontoColloidalSystems B. Critical Aspects of Molecular Association in Carbonaceous Materials C.Colloids,InterfacesandSupramolecularOrganization During Primary Carbonization

113 1

Consequences of PrimaryCarbonization SecondaryCarboni%ation Release) B. ThermalGraphitization

131 13s 135

122 125

IX. Conclusion

138

References

139

Any organic matter, being made of C, H, 0, N, S, provides carbon by releasing heteroatoms (taken here in the sense used in organic chemistry, i.e. atoms other than C and H) and hydrogen. The laboratory process, i.e., carbonization, involves heat treatment under an inert flow whereas the natural process,i.e., coalification or carbonification, is due to the geothermal gradient. As a first approximation, carbonization and coalification are equivalent; both are thermally activated processes 1-51 that are dependent on pressure and time. Carbon precursors are numerous: materials such as pitches (petroleum or coaltar pitches), oils, tars, heavy residues of oils, compounds such anthracene, synthetic organic polymers, cellulose, etc. More or less coalified products, such oil) [3], may also provide carbons. coals [ 5 ] or kerogens (parent rocks Various steps occurring in carbonization are separated by sharp variations in physical or physicochemical properties [3,5].Pri~ary c~bonization begins with softening and ends with total removal of aliphatic compounds causingsolidi~cation,i.e.,obtention of brittlesolidatroomtemperature(semicoke).Upon increasing temperature, heteroatoms are released first in the form of volatile oxygenated compounds(H,O, CO,). Then mostly aliphaticCH groups are lost and hydrocarbons are released (oil window). Liberation volatiles may produce a strongbubbling(violentoutgassing);thesevolatilesarecondensable.Atthe

semicoke stage, H, 0, S or N may still be present in various amounts, 0 or S remaining up to high temperature 2000°C). Secondary Carbonization is a solidstate reorganization producing only noncondensable gases (CH,, H,) issued from the loss of aromatic CH groups (gas window). In principle, pure iscarbon obtained at the endof secondary carbonization. However, this appreciation depends on the sensitivity of the analyses. Secondary carbonization itself is a two-step process marked by a transition inESR data [S-S] and a sharp change in nanotexture [g]. Subsequent to carbonization, t~ee-dimensionalorder may develop (graphitization) in the range 2000-3000”~. At higher temperatures another process involving vapor growth of graphite may interfere. Indeed, the carbon vapor pressure increases from 102 to 105 Pa as the temperature rises from 2850 to 3850°C [lo]. The industrial products called graphites are always less crystallized than natural graphite [ l 1,121 because single crystals of graphite are not produced by geothermal gradient only, They require preliminary strainof carbonaceous rocks by tectonic stresses 13,141. This process called stress graphitization can be reproduced at least partiallyby experiments under pressureandor under stresses 151. Thefinalproperties of either high-temperature carbons or “graphites” are mainly dependent on the precursors. The latter predetermine the characteristicsof primary carbonization and the resulting macroscopic objects (e.g., rnesophase powders, lamellar green cokes, etc.). As the brittle solid stateis reached (end of primary carbonization), the semicoke obtained is a three-dimensional arrangement of newly formed aromatic layer stacks (basic structural units or This ~ a n g e m e nist dependent on the precursor,so that carbons are as numerous as the precursors themselves,An additional number of precursors is provided by the ability to repeatedly carbonize the condensed volatiles. For instance, ofcoking coals produces tars, distillationof tars provides pitches, whereas carbonization of pitches again gives hydrocarbons. To understand the evolutionof the multiple precursors during carbonization, it is first necessary to recognize that all carbonaceous materials contain the same elemental bricks or basicstructural units (BSU).Then the leading thread consists in understanding their multiple three-dimensiona1 arrangements at different scales, based on the occurrence of various microscopic bodies.The latter depend on chemical composition.An external modificationof the chemical composition may even profoundly modify the three-dimensional ~ a n g e m e n of t BSU [91. This chapteris devoted to understanding the mechanism of primary carbonization through a molecular interpretation. It will be developed taking account of the cont~butionof colloid chemistry. To reliably characterize carbonaceous matters and carbons, physicochemical techniques should be combined with diffraction and imaging techniques.A few review papers provide more detail on basic data about carbonization and graphitization, as well as the principles of sophisticated imaging techniques [9,16-211.

l.

U

.d

Van Krevelen diagrams of natural (coals, kerogens)and industrial products. Kerogens, bandsin full line; coals, dashed area; industrial products, dotted area. The dashed arrows indicate the slopes resulting from the loss of CO,, H,O, CH4.

derives from ~ o t t ~ o c o c c uSporopollenin s. is a model precursor of Series I1 andKuckersitebelongs to Series 11. Sporopolleninconstitutesthespores of Lyco~o~iu~ purified by cytoplasm elimination (Fig. 2a). Kuckersiteis originated from after natural oxidation (oxidized KerosenShale). Coals and kerogens of Series I11 are mainly originated from plants (cellulose, lignin), whereas kerogensof Series I and 11derive from algae and spores (lipids, proteins and carbohydrates). The natural evolution runs through three stagesdiagenesis, catagenesis, and metagenesis-during which carbon content increases

Purified spores of ~ y c u ~ u ~ i ~(model ~ of Series kerogens). as obtained, observed by SEM (left) and OM (right). b: the same heated in standard conditions at observed in SEM. (From Ref. 53.)

[25]. Duringdiagenesis,mostlybacteriaareactivelychanging the sediment. During catagenesis (oil window), thermal conversion occurs, equivalent to primary carbonization. Atthe end of catagenesis and during metagenesis noncondensable gasesare produced (gas window), a process equivalent to secondary carbonization. For evaluating oil or gas potentialof a deposit, which is of prime industrial concern, it is important to recognize as a fingerprint the evolution path followed by the material and the degree evolution reached.

erlin et al.

In addition to natural products, industrial materials have to be mentioned that could also havevariousoxygencontentssuchassaccharose-basedproducts, oxidized PAN-based products (fibers), phenolic resins (glassy carbons), etc. The above presentation mentions only carbonaceous materials, the precursors of which have a relatively high oxygen content. Industrial products are available that contain less oxygen or are even devoidof oxygen. The dotted area in Fig.1 represents the zoneof and that are relatively poor in oxygen. For example the first distillation of oil yields an atmospheric residue (AR) such as Arabian light-AR. Distillation of AR under vacuum produces a vacuum residue (VR) such as Arabian light-VR (Aramco) and Safaniya-VR. are defined as the fractionof carbonaceous matter soluble in benzene and insoluble in n-heptane. They are extracted from oils (heavyor light) or from a chloroform soluble fraction of kerogens. These heavy products cover a large range of composition, since some of them are almost devoidof oxygen but contain sulfur (Table 1). The precursors containing only hydrogen and carbon (oxygen free but hydrogen poor), such as pitches, travel along the ordinate. The term designates materials originatingfrom different treatmentsof different precursors. Petroleum pitches are obtainedby treating the residues of refinery distillation (AR and VR). When AR and VR are submitted to vis-breaking, vapor cracking, orcatalytic fluid cracking to produce additional light products, the residues are pitches soluble in toluene (such as Ashland 240). Coal tars and coal-tar pitches are issued from industrial preparationof blast-furnace cokes. They willbe described below since theyareheterogeneous(seeTable 5). It hasto be mentioned here thatnew industrial products often issued from coal tars but containing oxygen are still improperly qualified as pitches. They have to be fully characterized before classification. The same kindof van Krevelen pattern may be used to represent the evolution of any carbonaceous product. Figure 3, which also plots versus (N/Qat, shows the paths followed by PAN-based carbon fiber precursors (D and C produced in a pilot plant, the latter being more markedly stabilized by air) [26]. The curves have an opposite curvature compared to products of Fig. 1: (WC),, decreases first without oxygen loss, revealing only a hydrocarbon loss; CO, and H,O are released at higher temperature. Figure 3 shows that the curves follow different paths for D and C fibers. Correspondingly, finalD and C carbonized fibers have very different mechanical properties despite their tendency to reach similar compositions [20]. In the same manner, even though the ultimate compositions of coalified or carbonized kerogens and coals tend to merge, all of them behave differently in terms of texture, oil potential, and technological uses. At this stage, it is clear that the oxygen content of a material is one of the key factorsdetermining its subsequentbehavior,keepinginmindthatavolatile product obtained by carbonization may be the precursorof a further carbonization.

Van Krevelen diagramsfor fibers D and C carbonized in pilot units. (From Ref.

Diffraction and imaging techniques show the presence of similar elemental units, not only in some precursors but at differentoflevels carbonization or coalification.

Historically, wide-angle x-ray scattering (WAXS) data [27-321 suggested the occurrence of aromatic layer stacks 0.7- 1.5 nm in diameter, two to four layers thick, in carbon blacks, coals, and coal derivatives. More recently, elemental units in various mesophases were estimated from the modesof their histograms to be between 0.58 to 0.84 nm [33]. Small-angle neutron scattering (SANS) of asphaltenes suggested sheet-like aggregation of similar small units less than 1 nm in diameter with a constant thickness of the same order [34-361.

early as 1973 ([3] Chap. 7, [37-401) Boulmier identified (BSU) in kerogens using TEM imaging in high resolution 002 dark field (002DF) 16- 181. Figure 4a shows bright dots homogeneously dispersed in the sample, which are imagesof BSU seen edge-on, using a portion of 002 scattered beams to form the image If two orthogonal partsof the 002 ring (Fig.4c) are used to produce the image (002 orthogonal DF 1 and 2), the bright dots Fig. 4a disappear andare replaced by others also homogeneously distributed as shown in Fig. 4b. The BSU are thus distributed at random, as illustrated by models of Fig.

erli

Imaging elemental units (BSU) in a kerogen. a and b: orthogonal 002DF images. c: sketch of the aperture positions to realize 002DF a (l) and b (2). d: model BSU oriented edge-on and of their visualizationin situations l and2 (double bar). Ref. 39.)

4d. Since BSU produce scattered beams corresponding to hexagonal structure of aromatic molecules. Theyare less (002, 10, andl l), they are necessarily stacks than 1 nm in diameter (with mode at 0.7-0.8 nm in the histograms). They are piled up by two or three. This average size does not vary significantly with the kerogen series (see I, 11, and in Fig. 1) or with the evolution along the van Krevelen path. In poorly evolved precursors, BSU are always distributed at ranat all scales. This situation is common to all dom, so that the materials are isotropic samples located above 0.6-0.7 in van Krevelen diagram (see Fig. In newly formed BSU, the aromatic molecules are neither parallel nor equidistant. The evaluation of interlayer distancesis performed in darkfield( [ 3 ] Chap. 7, [9,16-18,37’,39]). As the incident beamis tilted to explore radially the002 ring (increasing 20 angles corresponding to decreasing intermolecular spacings), the first bright dots to appear are those with the largest spacings (d3).The last ones have the smallest spacing (dl). Figure 5 shows the data obtained for the initial kerogen of Series I (Fig. Sa) and the same carbonized at 600°C (Fig. 5b). The d, spacing is almost ons st ant, near 0.34 nm, the average spacing (d,) decreases from 0.60 to 0.36 nm the sample representative point moves toward the carbon pole in the van Krevelen diagram (Fig. and significant proportionof BSU is found up to 0.6-0.8 nm (d3). The average spacing, measured to be 0.40-0.50 nm by x-ray diffraction on kerogens and coals [41], is equivalent to d,. This feature is also recognized inother materials such saccharose- or PAN-based precursors, well glassy carbons 18,20,42].

~ i s t ~ b u t i oofn the i n t e ~ o l e c u spacing l~ (d) inside BSU, determined by dark field in TEM.a: initial kerogen of Series I. sample heated in standard conditions at 600°C. (From Ref. 39.)

After Boulmier’s work [39,40], abundant data were published between 1973 and 1990that imagedBSU in a large rangeof c~bonaceousmaterials [9],T&ng into accountonly the earliest papers, basic structural units were found in ~ t ~ a c e n e - b a s eproducts d andanthracites 143-451andin thin carbonfilms [46,47], coals [48,49], hard carbons [44], and pitchesThere is no doubt about the maximumsize of these units-which never exceeds nm-since such values are largely above the optimum resolution in TEM 002DF. However, the smallest value is less certain since it is of the same order as the resolution limit.

Molecular mechanics calculations were performed by Vorpagel and Lavin l] to determine what was the more stable association of aromatic molecules in vacuum. To get orientations other than edge-to-edge or edge-to-face, it is necessary toattain at least thesize of coronene. Dimersand trimers of coronene give stable face-toface stacks preferentially with shiftedconfig~rations(Fig. 6a), asdo ovalene and

Geometric arrangements for aromatic hydrocarbo~s.a: coronene trimer. coronene trimer and single molecule of ovalene. c: part of coronene crystal structure. (From Ref. 51.)

In Fig. l , the distinct paths of evolution depend on the precursorelemental conposition and join near the pure carbon pole. Relations between paths and within paths can be clarifed by confro~tationbetween textural and phy$icoche~icaldata.

l. Boulmier ( [ 3 ] Chap. 7, [18,37-40]) has shown that, coalification or carbonization of kerogens progresses, clusters bright dots replace the initial random distribution of BSU (Fig. 7 to be compared with Fig, 4). In orthogonal 002DF l and 2 (Fig. 7a and b) c o ~ p l e m e n t ~images y are obt This clustering could only be explained by parallel preferred orientationo inside volumewithdigitizedcontours(Fig. 7d). The acronym LMQ is given to these volumes, the projection of whic vides the imageof clusters irregular in shape. By using TEM goniometer stage, 002DF images of increasingly tilted samples give data on the shape, the diameter, the thickness, andthe orientation of each LMQ [9,38,39,52-561. The dark areas between the clusters correspond to the average orientation of which is much too twisted or tilted tolet the 002 scattered beam pass through the aperture. The

Imaging of local molecular orientations (LMO) in kerogen. and orthogonal 002DF images. c: sketch of the aperture positions to realize 002DF (1) and b (2). d: model of (double bar) oriented inside LMO and of their visualization insituatio~s1 and 2. (From Ref. 39.)

LMO fill the whole space with a continuous change of orientation from one to the adjacent one. The LMO are roughly isometric, so that their relative area in the image (two dimensional projection), corresponding to the contour of a cluster, approximates their volume fraction. The LMO sizes will be further defined by the l3 1. area of the clusters. The various LMO sizes are discussed in Section The LMO appear suddenly when the representative point of the sample approachesagivenvalue of whichdoesnotexceed0.7.Their size tendsto decrease as oxygen contentof precursors increases. The size is thus both a iandmark of the evolution (naturally or thermally induced) and a criterion to determine which series(I, or 111)an unknown kerogen belongs to and which typeof parent material it comes from. Whether coalified or heat treated, kerogens of the same series show the appearance of LMO in the same range of composition. Villey generalized these observationstoKerosen Shale, Sporopollenin, Kuckersite, and Lignite (Table 1) [52-56]. He correlated infrared and thermal data with the presence and extent of LMO for three seriesof samples: pyrolyzedat 4°C min-1 under 1 atmosphere pressure (standard conditions), under primary vacuum (500 Pa)andundernitrogenpressure(0.5MPa).However,atthatpointthe number of samples available did not cover a sufficient range of chemical composition to establish precisely a scale of LMO sizes. I ~ ~ r Spectroscopy. ~ r e ~ The preparation of carbonaceoussamples for IR analysis is delicate and high performance spectrometers are required due to the high background absorption ([3] Chap. 6, [57,58]). However, adequate procedures and inst~mentsprovide identification and evaluation of the relative concentration of chemical groups. The integrated absorption coefficient K provides a relative evaluation of the concentrationof the groups responsiblefor the absorption band. K is unit of absorbance wave number (kerogen mg)-l cm2. Kerosen Shale, Sporopollenin, kerogens, and coals show the same bands [5,57,58]. OH stretching corresponding to a broad band around 3430 cm-l; CH stretching of CH, and CH, groups at 2920 and 2860 cmR1; C=O stretching of carbonyl (C=O), carboxyl (COOH), ester (COOR), and ketone (>CO) groups near 1710 cm-'; C=O stretching of bridged quinones, C=C stretching of olefins, aromatic rings, and polyaromatic layers at 1600- 1630 cm-l; and Out-of-plane deformation of aromatic CH at 870, 820, and 750 cm-1. Figure 8 presents the residual concentration of aliphatic CH groups (Fig. 8a) and aromatic CH groups (Fig. 8b) for Kerosen Shale (solid line) and Lignite (dashed line). In oxygen-poor Kerosen Shale the release of hydrocarbons extends all along primary carbonization (Fig.The 1). concentration of aliphatic CH groups is maximum a little before 490°C it drops abruptly above that temperature, The concentration is strongly reducedat 500°C. On the contrary, aromatic CH are

C-OR

Plot of elemental c o ~ ~ o s i t i oand n i n ~ r ~ data e d versus HTT Kerosen Shale (solid line), Lignite (dashed line), and ~ ~ o r o ~ o(dotted l l ~ line). ~ n a: K2920 b: q20-70 c: atomic Goncen~~tion ratio d: (Prom Ref.

formed in large a ~ o u n tat s 500°C and then tend decrease, to The sharp decreaseof aliphatic groups is very close to the sharp increase of aromatic CH groups. Sporopollenin providesdata similar to KerosenShale (not given in Fig. 8a and b), oxygen-rich Lignite, theloss of aliphatic C less abundant tostart with, begins a little above 3OO"C, whereas aromatic group concentration gives a plateau ranging between 450°C and about 550°C. Oxygen-rich products tend to preserve a high concentr~tionof oxygenated functional groups at high temperature, as shown in Fig. 8c, which compares the evolution of for Sporopollenin (dotted line) and Lignite (dashed line). Kerosen Shale is not given since (O/C),, is too small. At 600°C Sporo~ollenin

retains 0.032 and Lignite 0.055 in (OK),. Among the oxygenated functional groups able to crosslink aromatic layers are the carboxyl and hydroxyl groups (which form hydrogen bonds), and ether bonds [54,57,58]. It has been[58] shown that two unitsof K,,,, correspond to about wt% of oxygen. Thus, the comparison of elemental analyses, i.e.,(OE'C)~~, and IR data allowed computing an index, IC-oR,reflecting the concentrationof alcohol, phenol, or ether groups. The variation of IC-oRversusHTT(heattreatmenttemperature),presentedinFig.8d, reveals that oxygen cross-linkingis more marked in Lignite, compared to Sporopollenin is retained at higher HTT. Other kerogens are spaced out between these two extremes ([3] Chap. 6, [52-56,57,58]). ~ ~ ~ ~ ~~ Z y Figure s ri ~ . 9 ~presents~ DTA (differential Z thermalanalysis, solid line), TGA (weight loss, dashed line), and DTGA (dotted line) curves of Sporopollenin for standardconditions [52-531. The behavior of thematerial during pyrolysis was followed visually. A first endotherm (1-2-3 on theDTA curve) corresponds to the of labile oxygenated functions (e.g., OH groups). Then the material softens at 436°C (4 an the curve). This point is easy to determine precisely by the disappe~anceof the initially strongly marked decorations of the raw spores (Fig. 2b). Bubbles of hydrocarbon start to be liberated (beginning of

Thermograms of Sporopollenin obtained in standard conditions (N,flow, one atmosphere pressure, 4°C min-1 heating rate). The points 1 to 3 correspond to the endothermic release of labile oxygenated functions, to softening, 5 to the beginning of bubble release, 6 to maximum bubbling, and 7 to solidification. (From Ref. 53.)

aliphatic CH loss) immediately after5. A more violent bubbling occursat 440°C (6). Mostof the weight loss (about 80%) is completed at about 500°C (7). The rate of volatile release shows two maxima situated at 360°C and 440°C. The first corresponds to OH decrease. The second (maximumrate of aliphatic hydrocarbon release) corresponds to the violent bubblin~. In Kerosen Shale, whichis almost oxygen-free, softeningis more pronounced and the same kindof PTA, TGA, and PTGA curves are obtained, exceptfor the endotherm due to the loss of oxygenated functions. The endothermic hydrocarbon release is displaced to higher HTT: features 4 and 5 are very close at 460"C, whereas 6is at 49OoC, whichis also the second maximumof PTGA. The weight loss (90%) does not change appreciably above 490°C. On the contrary, oxygen-rich materials such as Lignite show a badly defined unique endothermat 360°C followedby an exothermic peak at 415°C and a return to the baseline at 500°C. This is consistent with IRdata showing the simultaneous release of aliphatic CH and of C -OR functions (C 0-C and C OH) above 300°C. There is neither meas~ablesoftening nor solidification. The total weight loss is reduced to 55% with a peak at 350°C in DTGA. The major peakof PTGA is shifted to lower HTT, going from Kerosen Shale to Sporopollenin and to Lignite. The LMO was observed by Villey [52-561 a little after the violent bubbli~g (ma~imum rate of aliphatic CH group loss). Since the first detected images are very poor in contrast,it was impossible to assign their precise occurrence.To be able to evaluate the LMO extent, Villey carbonized the samples of 1000°C, well beyond solidification. The exact solidification point was also not determined. It was thus difficult to establish the succession of LMO occurrence, solidification, aliphatic CH loss, and aromatic CH maximum. The spin concentration givenby ESR ([3] Chap.8) gives a maximumof spins at HTT above the maximumof aromatic CH groups. Spins correspond to heavy radicals formed by BSU having lost part of their aromatic CH. Figurel0 shows the spin concen~ationversusHTT for samplespyrolyzedunderstandardconditions: Kerosen Shale (solid line), Sporopollenin (dottedline), and Lignite (dashed line). The curve maximum is progressively displaced to lower HTT from KerosenShale (at or above 6OO0C), to Sporopollenin (520"C), and Lignite (470°C).In conclusion, accordingto Villey [52-561, LMO and solidification occurat a temperature in the range 490-600°Cfor Kerosen Shale with a largeLMO size 200 nm), in 100 mm), in the range 440-520°Cfor Sporopolle~inwith a mediumLMO size the range 350-470°C for Lignite with the smallest LMO size nm). Until recently, the occurrence of LMO was fixed arbitrarily at 475°C and its size was measured at 1OOO"C, following Villey. Better preparation techniques for TEM are now available and their results will be presented in Section IV P. Villey attributed the behaviour of the above samples to an increasing cross-

Kerose~

300

Plot of the spin concentration versus for Kerosen Shale (solid line), Sporopollenin (dotted line), and Lignite (dashed line). (From Ref. 53.)

linking by oxygen, forming ether bonds in the case of Lignite. Natural or experimental oxidation supports this idea as it shifts the representative points of the materials in the van Krevelen diagram and decreases their LMO size, Figure 1l a shows that oxidation brings the Kerosen Shale point (Series 1) near Kuckersite (Series 11) and Kuckersite and Sporopollenin points closer to the Lignite on the coal and Series TIT kerogen paths. Displacement of representative points in the van Krevelen diagram must be analysed in consideration of the effectof simple reactions the composition of the solid and not simply of the compounds formed. The displacements sketched by arrows in Figure 1l b refer to examples of transformations that can be schematized as follows,

ratio

H,O: dehydration H,O(g)

(1)

-CO,: decarboxylation (2) removal of long-chain aliphatic hydrocarbons -C: through oxidation of carbon (4)

removal of hydrogen, for instance by oxidation

CH,

H,O(g)

(5)

+O: oxidation leading to oxygen uptake

(6) given displacement may result from different combinations of these reactions. For instance, the removal long-chain aliphatic molecules may give the same displacement as an adequate Combination of hydrogen loss and oxygen uptake by oxidation. Furtheri n f o ~ a t i o ncould be obtained by simulations based on both mass balance (weight-loss measurements) and displacements inthe van evelen diagram. C o m p ~ s o nwith computed slopes in Fig. 1 1b indicates that o~idationof erosen Shale involves the uptake of (reaction 6) in addition to the loss of hydrocarbons (reaction 3). Oxidation of Kuckersite could be explained by the removal of long-chain hydrocarbons (reaction 3) or an equivalent combination of reactions removing C and H. Oxidation of Sporopollenin involves consumption H compared to C, which is larger than that attributable to removal moieties (reaction 3). uring heat treatment the representative point of each of the oxidized samples Fig. I1a follow the path near which they are arrived (oxidized follows the path of Kuckersite, oxidized Kuckersite and oxidized follow the pathof Lignite). After heat treatment the LMO sizes are those specific to the new path (Fig. l la). The LMO of oxidized Kerosen Shale becomes a little

a: van Krevelen diagrams of kerogens (I, models of kerogem, coals, and oxidized precursors. Solid line: Kerosen Shale in all conditions. Dashedlines: Sporopollenin in different conditions, oxidized Sporopollenin in standard conditions. Dasheddotted line: Lignite in all conditions. arrows indicate the slopes resulting from the release of H,Q, CO,, CH,, and CH, moieties (longaliphatic chains), from the loss of H and C (by oxidation) and from the uptake of Q oxidation).

less than 250 nm. The LMO of oxidized Kuckersite and oxidized Sporopollenin is 5 nm. These results strongly suggest that a key factor determining the evolution upon oxidation and carbonization is not only the oxygen content but the oxygenhydrogen balance. Figure 1l represents also the influence of pyrolysis conditions on the van Krevelen paths. For Kerosen Shale the path (solid line) is unique for standard pressure and vacuum conditions. Correspondingly, the weight loss of Kerosen Shale is about the samefor all conditions (the volatiles are essentially hydrocarbons). There is also practically no influence of expe~mentalconditions on the evolution path of Lignite (alternately dashed and dotted line). In contrast, three distinct paths (dashed lines) occur for Sporopollenin. Vacuum accelerates the release of hydrocarbons, while pyrolysis under pressure deoxygenates the product more markedly. Correspondingly, the weightloss of Sporopollenin (at 500°C)is 75% under atmospheric pressure and 87% under vacuum.At the end of the path, (Fig. l la) the representative points of Sporopollenin pyrolyzed under pressure correspond to an amountof oxygen lower than that of Kerosen Shale. Villey, after Libert [59], attributed these variations to a competition between volatile release, which is diffusion controlled, and chemical reactions[52-561. Volatile release is favored by vacuum, while it is reduced by pressure. Figure 12 is a sketch summarizing Villey’s data. The LMO size is plotted onthe ordinate. On the abscissa all the samples studied are classified by decreasing LMO size (the vertical thick bars represent size histograms, the horizontal thin ones represent the mode).The LMO averagesize is much smaller when Sporopollenin and Kuckersite are pyrolyzed under vacuum as compared with atmospheric pressure (standard). It is larger when the materials are pyrolyzed under pressure. Kerosen Shale appears insensitive to pressure and slightly sensitive to vacuum. Ove~ressureis so effective on Sporopollenin that its LMO size becomes larger than that of Kerosen Shale. Lignite is indifferent to pyrolysis conditions. The authorsof the present chapter have recently recognized interesting tendencies inVilley’s data (seeTable 7) by comparingsamplesheatedatdifferent conditions and classified inthe same order as inFig. 12, i.e.,by decreasing LMO sizes. Other data about kerogensof Series I and I11 heated at s t a n d ~ dconditions (E31 Chap. 7, 1391) has been added. The samples considered are Sporopollenin, pressure; KerosenShale, standard; KerosenShale, vacuum; Kuckersite, standard; kerogens of Series I, standard; Sporopollenin, standard; Sporopolle~in,vacuum; kerogens of Series HI, standard; Lignite, standard. The violent bubbling corresponding to DTGA ~ a x i m u mis produced in a small HTT range (440-490°C). The ma~imum concen~ation of aliphatic groups occursat an almost constant a little before DTGA m a x i ~ u m(436-46OoC), even for Lignite, which has a plateau between 365 and about 550°C.

nm

Ser

Classification of kerogens, coals, and some of their models following decreasing

LMO size (ordinate). The dotted areas represent the three series. The horizontal indicate the modes of the size histograms (vertical

(From Ref. 53.)

rli The maximum concentrationof aromatic CH groups occursin a large HTT range from more than 500°C down to 360-400°C. The maximumconcentration of aromatic CH groups is alsoaboutconstant, between 9 to 15 K units (ordinate of Fig. 8). This is in agreement with the narrow range of BSU diameters upon which they are fixed. The concentration of aliphatic CH groups measured at the maximum of aromatic CH groups is very low or zero. It decreases progressively from less than 30 K units for Kerosen Shale, standard, down to 12 for Sporopollenin, standard. Itis zero for oxidized Sporopollenin, standard; Sporopollenin, vacuum; and kerogens of Series 111 heated at standard conditions, The maximum concentration of aliphatic CH groups itself decreases from 157 down to less than 20 K units. Aliphatic CH loss ends at HTT values decreasing progressively from above 600°C for Sporopollenin pressure down to 400°C for Lignite. The maximum spin concentration occurs at HTT ranging from more than 600°C for Kerosen Shale down to 520°C for Sporopollenin and 470°C for Lignite, all treated at standard conditions. The ~ a ~ i m uspin m concentration also decreases from1020to 3.1019 in the same sequence and always occurs at a constant content of aromatic CH groups (7-8 K units). Despite the great amountof data collected for kerogens, the nonevolved ones (heads the series) and their model substances were studied in detail almost exclusively, so that natural coalification not as well known as its simulation. Data were limited to the occurrence and size of LMO in natural materials. The difficulties in recovering large amountsof purified samples from rocks containing at most of organic matter were partly responsiblefor this situation. This was not the case for coals for which a large amount of sample was available. Low rank coals and kerogensof Series 111represent the products containing the highest amount of oxygen (Fig. l). The characteristic feature of coals is their heterogeneity due to their continental origin (maceral co~position).Figure 13 presents a diagram illustrating the natural evolution of some coals studied here [48,49]. the rank increases, coals essentially lose oxygen CO, as before losing hydrocarbons. During the first stage, (O/C),, decreases at constant (WC),t. Correspondingly, coals are plastic so that their hardness can be measured by Vickers microhardness HV ([S] Chap. 16, [60]) deduced from the print of a pyramidal diamond applied on the sample with a given force. The average HV of plastic coals is about 300 MPa. When the rankis high enough, a brittle solidis obtained that breaks without retaining the diamond print (HV is infinite). The brittleness appears for a carbon contentof about 90 wt% at the semianthracite stage marked

Van Krevelen diagram for some coals. (From Ref. 48 and 49.) The samples marked triangles are those detailed in this paragraph. (From Ref. 48.) The area below the dashed line corresponds to natural semicokes. (From Ref. 49.)

in Fig. 13by dashed line, Oignies with (H/C),, 0.58, (OIC),, 0.027 and wt% 91 is typical natural sernicoke [48,49]. The LMO in natural semicokes are very small 5 nm) and comparable to those of Lignite (see Table 1). In the same manner, SeriesTI1 kerogens have their natural solidification point at 0.40.5 and 0.06 [39]. When carbonized, low-rank coals 0.10) also show very small LMO. When observed without heating, they exhibit what is called in petrography “basic anisotropy.” Itis long range weak statistical orientation first described by BensaYd in TEM (Fig. 14)[48,61,62].As an example,for 100 BSU in sample, l0 are parallel to preferred orientation plane P but distributed anywhere in the image. Among the 90 BSU remaining, all orientations are distributed over variable numberof units (less than10) spread outin the bulk. In Fig.14e and in Fig. 4 and 7c and d, sketches of the aperture positions and corresponding models are given. The 002DF corresponding to position 3 is not restrictive. It is only an illustrationof one of the possible intermediate cases. In Fig. 14a,b and c,d (corresponding to orthogonal 002DF two different particles), bright dots randomly distributed appear for both positionsof the aperture; however, their number is ma~imumonly in Fig. 14a and c defining the preferred orientation plane P (position 1 in Fig. 14e).The stronger the orientation in the sample (Fig.14a), the higher the numberof bright dots in the image of position 1 (compare Fig. 14a and

Imaging of statistical orientation (SO) in coals. Orthogonal 002DF: a and b, marked SO; c and d, faint SO, e: sketch of aperture positions. f model of BSU (double oriented inside SO and of the visualization in situations 1, 2 and 3. P is the preferred orientation plane. (From Ref. 48.)

e-

Top: is orientation o€BSU in real space (twist around z, tilt around x). Bottom: reciprocal space and illustration of the dist~bution tilt and twist angles (pole figure).

c). Since in 002DF there is no complementary image (Fig.14a and b or c and d to compare with Fig. and b), an SO is very different from an LMO. The di~erencesbetween various types of preferred orientation are brought into evidence by the use of a figure of poles [63] (Fig. 15). In a perfect azimuthal orientation of all BSU in a plane,the common 002 reflection lies along the normal to the aromatic layers. If the preferred orientation plane is chosen as ZOX, 002 lies alongY* (case 1). If there is no orientation atall (case 2), the locusof the 002 reciprocal nodes is a sphere centered on the incident beam (powder sphere) around which the002 nodes are uniformly distributed. In the case of an LMO, a small amount of ~sorientationoccurs relative to the preferred orientation plane ZOX (case 3) either by twist around OZ or tilt around OX. The density of 002 nodes decreases inside an elliptical cone Y* delimiting a portion of the powder sphere (figure of poles).The density is maxim~lmalong the trace of the axisY* of the cone.The distribution the densityof nodes (pole figure)is represented by a Gaussian curve, and being defined as the half width of half ma~imumfor tilt and twist angles, respectively.In an SO (case there is no figure poles since are found inall directions (Fig. 14). Only a more orless marked increase of the 002 nodesnumberoccursaround the trace of Y* relative to an overall homogeneous repartitionof nodes on the sphere. In diffraction patterns, located in

the planeX*Y*, 002 is a single reflectionat Y* in case 1. In case 2it is a ring; in case 3, it is an arc centered on the trace of F , the half openingof which gives and incase 4 it is a ring with a weak intensity reinforcement near the trace Y*.of ~hotometricrecording of SAD (selected area diffraction) patterns provides precisely and but requires a minimum area of 1 pm in diameter. As matter of fact, is more often evaluated in 002DF during the exploration of the 002 ring with the objective aperture and is evaluated during the tilting of the sample, which is equivalent to tilting the diffraction pattern. Photometric recording of the imagesalsogives and butrequiresaminimumarea of 100-200 nm in dia~eter.A way to roughly estimate the misorientationis to approximate by eye the overall intensityof the images from the maxi mu^ brightness to the extinction, instead of measuring and Finally, and can be evaluated on a nanoscale (areas of 10-20 nm) by photometric recordingof optical diffraction patterns from the 002 lattice fringes. an example, kerogens of Series I examined in 002DF give and -+30", thecompleteextinctionobserved by eye being SO" [52,53,55,56]. Brooks and Taylor mesophase spheres also examined in 002DF +"25", whereas in 002, lattice fringes they give 15" give and (see Section IV C 2a and The SO of low-rank coals, which renders them weakly optically anisotropic (basic anisotropy), was attributed by BensaYd [48] to stresses causing flow orientations that are persistent due to the low plasticity of these coals. When coals are heat-treatedat 1000°C (HTT arbitrarily used to evidence LMO following Villey) [52,53,55,56], they give a distribution of sizes. Bensaid represented them as histograms [48,62], based upon the classification into ten classes of increasing size, posterior to thatof Villey and described in Section IV B 1 Figure l 9, and Table 3. Tremendous differences between coals lie in the distribution of classes and p ~ i c u l a r l yin the number of the largest LMO, i.e., lamellae of Class 9 and 10 an example, Fig.16 shows the histograms of some typical coals of increasing rank studied by BensaYd (triangles in Fig. 13): Merlebach: (WC),, 0.72, (OK), 0.10, and C wt% 82, where XL is 0% 16a); ontaine: (H/C),, 0.80, 0.08, and C wt% 82, whereZL is 0% (Fig.16b); r (coking coal): (WC),, 0.68, ( O / C ) ~ ~0.04, and C wt% 86, 80% (Fig. 16c); oking coal):(H/C),, 0.75, (OK),, 0.039, and C wt% 89, where L is 86% (Fig. 16d); 0.029,and C wt% 90, where XL is 30% Lens: (H/C),t 0.60, (OIC), (Fig.16e); Oignies(naturalsemicoke),(WC),, 0.58, 0.027,and C wt% 91, where again only Class 1 and 2 LMO are present (XL is 0%) (Fig. 16f).

7

7

7

Histograms of LMO sizes, as defined in Table 3, for coals of increasing rank. (From Ref. 48 and 49.)The proportion represented lamellae (XL 9 10, in is also indicated. (From Ref. 48.)

It is quite clear from Bensaid’s data that a tremendous chemicalco~position gap occurs between natural semicoke and cokes having reached the same L whicharesupposedtobetheirmodel. As a matter of fact, carbonization of Merlebach provides an LMO identical to that found in Oignies. This reveals the tremendous discrepancy between coalification and carbonization recognized later on, in Reference 49, and also studied in detailfor Series TI1 kerogens (Mahakam humic materials coals) (see SectionVA).In comparison, coalification and carbonization superimposefor other seriesof kerogens, heavy oils, or more generally materials with medium or In these cases LMO size is usually constant

and unique for each sample, in contrast to coals, which are mixtures of complex macerals. The gap between low- and high-rank coals is essentially occupied by very peculiar categoryof coals to which Meadow River and Peak Downs belong. These are the coking coals used to prepare the blast furnace cokes. They occupythe zone of vanKrevelen path (see Fig, 13) where the slope changes (end of CO, departure) in range of elemental composition (WC), 0.8-0.6, (OK), 0.046-0.029, and C wt% 86-89. While all coals of lower rank stay uniformly poorly plastic when heat treated before solidification, coking coals soften and develop new types of anisotropic B). Therefore, they bodies offering similarities with oil derivatives (see Section IV will be treated below,so as to provide clearun~erstanding(see Section B). The discrepancy between coalification and carbonization of oxygen-rich materials was attributed to the fact that coalification produces deoxygenation,i.e. decarboxylation in coals, whereas carbonization tends to produce water, in saccharose pyrolysis [44,49]. In the latter, in heat-treated coals, very small LMO occur (Class l). The physical explanation will be given in Section VI B 3.

3. The oldest models of coal consider coal as simple mixtureof major component and oil-like mate~als,which are being extractable by suitable solvents. However, it was rapidly evident that components interact with each other, since simply mixing them again did not restore the initial coal [48]. The mate~alcannot be recomposed after analysis since association between components is not known and even destroyed by the fractionation required for analysis. A so-called micellar modelwasproposed for coalsand coal extractsbyKreulen [64], involving spherical micelles. Coal models were independently deduced from x-ray data without prejudging the importance of associations, since x-rays are only ableto describe elemental units and not their three-dimensional association. Rlaydenet al. [65] proposed disc-like units. Then Hirsch in 1954 [66] (Fig. 17a) and further Neavel in 1982C671 (Fig, 17b) gave more elaborated models. Allof them contain elementalunits roughly s i m i l ~ in size to However these models are not in close agreement with the characteristic statistical orientation of BSU in coals(SO) (see Fig. 14).Van Qevelen ([S] Chap. 25) considers coals network polymersof infinite molecular weight, i.e., insoluble in all solvents, having “sponge structure.” Oil-like products are assumedto be trapped inside. They can be extracted by suitable solvents and are the origin of the hydroc~bonvolatiles. In 1957-1961, van Krevelen proposed the metaplast theory [68,69] without prejudgement about structural model. According to this theory, coals, when they areheattreated,arepostulated to liberate light molecules. The latter form suspensive medium (metaplast) inside whichelemental units move.Simultaneously to its formation, metaplast is destroyed volatiles escaping from the

Hirsch. (From

66.) b:

(From

plastic material. The metaplast formation and its volatilization are constantly repeated, up to its total disappearance (solidification). This model was adopted by group Chap. 7, then subjected to a slight modification. At that time, it was considered that polyaromatic moieties incorporated in an open macromolecule [l91 were associated together, forming stacks already called BSU. This macromolecular concept was also illustratedby the Larsen model for coals [70].However, ESU have not been observed with certainty before a certain degree of evolution for kerogens of Series I and Thus, in nonevolved materials, polyaromatic molecules are not always associated to an appreciable extent. On the basis of this remark andof results concerning kerogens recalled above, we now propose the following representation for carbonaceous materials and their evolution during coalification or carbo~ization. The nonevolved materialsmay be considered as a distribution of polyaromatic moieties carrying side-chains, functionalized according to the chemical composition (Fig. 18). These moieties may be covalently bound togetherby side-chains, thus forming macromolecules,or associated through polar interactions (including hydrogen bonds) involving oxygenated functions. As evolution proceeds, reactionsof dehydration andlor decarboxylation occur

Molecular modelof a SeriesI1 kerogen, fitting elemental and molecular composition (H/C), l. 17; (OK),, 0.053; aliphatic hydrogen: 28%; naphtbenic hydrogen: 21 aromatic hydrogen: 1%; density 1.2. (From Ref. Stars suggest that covalent bonding between such molecules may make a macromolecule.

first. In the case of kerogens of SeriesTI1 and coals, decarboxylation provokes a strongdecrease of Thencarbon-carbonbondsarebroken,liberating aliphatic compounds that create a suspensive medium for the polyaromatic entities (metaplast theory). As a result, softening be may observed, more marked when the precursors have a high hydrogen content (Kerosen Shale, Sporopollenin, kerogens of Series I, etc.), As te~peratureor coalification rises further, an increasing number of light molecules are produced and volatilization increases. Near that stage, BSU are systematically observed as stacks of molecules of 7-10 aromatic rings. evolution proceeds, whatever the carbonaceous product, BSU associate with a preferred orientation, forming LMO. the peculiar case of carbonized model substances and kerogensof Series I and11, this crucialphenome~onoccurs when the rate of release of aliphatic groups is ~ a x i m uIt~is. also marked by a violent LMO f o ~ a t i iso thus ~ clearly bubbli~gand by a maximum in DTGA curves. The related to a maximum freedomof BSU in the suspensive medium.~ o n c o ~ t a n t

ts with the stageof LMO formation, the concentration of aromatic CH that forms the edges of BSU increases. This concentrationis about the samefor any kindof precursor, either coalified or carbonized.The materials still contain aliphatic groups either as residual suspensive medium oras remaining side-chains boundto BSU. After the complete loss of aliphatic CH groups, maximum concentration of aromatic CH occurs, the suspensive medium is totally removed, solidification occurs, and primary carbonization is over. The material consists of BSU closely of but not covalently associated into LMO of defined size, excluding the presence large polyaromatic layers. This is demonstrated byTEM, which shows a constant size of BSU and by the constant aromatic CH concentration at that point. However, the L~~ size itself is variable. It is larger when (O/C),, of the precursor is lower and when bubbling observed during carbonization is more violent. This will be discussed below. Aftersolidification^ a ma~imumof spins is reached (secondary carbonization). The stages of LMO fornation, the removal of aliphatic CH groups, and the ~aximum of aromatic CH and of spins were too close in kerogens and coals to allow a precise sequence determination. This will be tentatively performed below (see Section IV The constant size of BSU deserves additional discussion, distinguishing their thickness and diameter, respectively. The limitation to stacks of 2-3 molecules neither parallel nor equidistant (see Fig. 5) may be due to steric constraints h posed by side-chains. Molecular mechanics calculations11have also shown that shifted dimers and trimers of coronene offer a high stack stability (see Fig. 4). The absence of polyaromatic molecules of smaller diameter than coronene may be related to the lowerstability of their face-to-face configuration, In addition, nonassociated light molecules would have been volatilized (mono- to triaromatics, for instance). The reason for the absence of large diameters maybe due to several factors. Heavier aromatic molecules are probably absent in the precursors; internal molecularm e ~ h a ~ i s ~ slimit the diameter of the polyaromatic moieties.

For materials considered above, the minimum amountof oxygen contained in the precursorsusuallyexceeds 0.025 (see Table l). Asphalts,asphaltenes of oils, kerogens, and coals, as well as some high te~peraturepitches (HT pitches) may reach much lower values of (O/C),t, down to 0.004. They thus represent interesting intermediates with the pitches such as A240, which are devoid of oxygen. Asphaltenes andoil derivatives (oil feedstocks) are defined by their preparative techniques. The samples are described in Section(carbonaceous ~aterials)and

in Tables 1 and 2, which give their elemental composition and textural data. All of the samples are soluble in toluene. Asphaltenes and oil derivatives (see Tables and 2 and Fig. 1) are relatively poor in oxygen but do contain sulfur, the influence of which is unclear. Their study was u n d e ~ by ~ ex-ray ~ and TEM,at first by ~onthioux[7 1-73] thenby Bourrat onnamy [76,78,79], gathering a large number of samples issued from oils or kerogens.

are obtainedby progressive heating. They are measured after a heat treatment at 1000°C followed grinding, as definedfor kerogens. in kerogens and coals, they appear in 002DF as clusters of bright dots. Due to the large range of LMO sizes, definite numerical values cannot be given, but owing to the large number of samples, LMO can be classified in ranges of size, which are given in Table3. Figure 19 gives examplesof LMO sizes increasing fromClass l , to 3, 5 and 8. Lamellae (Class 9 and 10) are either entirely bright (Fig. 19e) or entirely dark in 002 orthogonal DF when they are oriented edge-on. They are always dark when they lie fiat on the supporting film. Because they are broken into very large fragments grinding, their sizes are not d e t ~ r ~ n eHowever, d. a limited number of samples were studied in O M (optical microscopy) [74-791 and classified according to the size of their anisotropic domains (see Section IV C 2 c). This will be developed below (Section IV D 1 b). Table 2 shows that no immediate correlation appears betweenLMO size and elemental co~positionof the precursor.

2. By considering all possible per~utations of (OK),, (S/C), and even (N/C),,of the precursors, Monthioux [71-731 could not find satisfactory agreement between size and elemental composition. However, considering only half of the content added to oxygen improves the results somewhat (correlation factor 0.86). Bourrat determined afterward that only a partof sulfur, stable between 1700 and 200OoC, was a crosslinker 1771. The other was either inactive released at low HTT) or was a modifier, released between 1300°C and l70OoC, which resulted in sample swelling(~uffingeffect). It left almost perfect graphite crystals in the areas where it was lost [74,77,80-831. An interesting detail has to be mentioned. The histograms of LMO sizes are generally reduced to one class for asphaltenes, oil derivatives, and kerogen extracts [78] (Table 2), except for the products very rich in cross linking sulfur such as Boscan [74,75,78] or Cerro Negro (heavy oils from Venezuela) [78].extracts The of Boscan by heptane (IC, or asphaltenes stricto-sensu), hexane (IC6), pentane (IC,) and Boscan progressivelydecrease in crosslinkingsulfurcontent.Theyprovidelamellae

d

fji Td

g U

c-,

ccc

c

of Class

Sizes

Range of LMO sizes (nm)

1 2 3 4 5 6

G5 5-10 10-15 15-25 25-35 35-50 50- 100 200

8 9 l0

Lamellae Refs.

and

and rectified in

and

(crosslinking sulfur lamellae joined to Class8 LMO (crosslinking sulfur 3.1 wt%); then lamellae, Class8 and 7(crosslin~ingsulfur 3.8 wt%); then only Class and 6 ( c r o s s l i n ~ ~ sulfur g 5.0 wt%). Another sampleof Boscan IC, containing 3.4 wt%of crosslinking sulfur provides a histogram with 2,3,5,6, Class and 7, preventing any classi~cation.Asphaltene of Cerro Negro containing 1.6 wt% of crosslinking sulfur also provides various morphologies (lamellae and very unusual ones). Such heterogeneity could due be to sulfur attached to BSU in various ways. Finally, Bourrat and Bonnamy [74-793 understood that labile heteroatoms lost by the precursor well before LMO occurrence (OH groups and some sulfurcontaining functions) did not influence LMO size. Moreover,it became clear amy that crosslinkers were not alone in regulating primary carbonization he hydrogen content should also be accounted for. A factor was empirically determined. By reference to Villey's work solidi~cationand occurrence (which could not be clearly distinguished either in kerogens or in coals) were assumed to occur near 475°C. Elemental analyses were thus performed at this temperature, The concentration of oxygen was taken to be the weight percent 0 present at 475"C, generally stable up to l 600-2000°C. The concentration of hydrogen was takento bethe weight percent H at 475°C.The weight percent cross-linking sulfur was obtained by relating the amount of sulfur determined in the material treated at 1700°C to the mass at 475°C. A quite good relationship was finally obtained between log LMO size measured at 1000°C and FLMOdetermined at 475°C for standard conditions: 4°C

Examples of classes of LMO.

min-lheatingrate(Fig.20)[52-56,76,78,79].InFigure 20 are reported,in crosses, the values obtained for Series I and I1 kerogens, well their model substances (see Tables l and 2), and values for asphaltenes, oil derivatives and kerogen extracts. Black stars represent samples studied in detail in Section IV D (see Reference 109,l 10). On this subjectit is worthwhile to recall previous data [84] correlating elemental analysis to P,, the ability of various carbonaceous matter to graphitize. They were issued from petroleurn, coal tars, polyvinylchloride, saccharose, etc. High degrees of ~raphiti~ability may be obtained with high percentages of sulfur (0.51.5 wt when sulfuris labile between 1500- 1600°C. The sulfur remainingto up 2000°C is harmful. For comparison, more than0.3-0.5 wt% of oxygen persistent above 500°C cannot be tolerated. The only exception is petroleum precursor where P, 0.6, with more than3 wt% of oxygen stable up to 1500°C. The possible presence of inactive stable oxygen, though rare, should be kept in mind. It has been encountered for oxidized carbonaceous matter and coking coals (see Sections V B and VI B).

3. Asphaltenes have been considered suspension (called micellar suspension). In the first colloidal model proposed for oil heavy products [ss], disc-like polyaromatic core is surrounded by shell of lighter aromatic molecules and dispersed inthe suspensivemedium. Another model pointing to spherical micelles of large sizes was proposed by Hoppler [86]. On the basis x-ray diflraction and physicochemicaldata, Yen [87,88~considered asphaltenes polymer substances presenting an organization at different levels (Fig. 21). The macromolecule consists of polyaromatic molecules, postulated to be about 5 nm in size, bound together by sidechains. The polyaromatic moieties in thesame macromolecules or in distinct macromolecules, may assemble in stacks (A in Fig. 21). This association is responsible for the formation of in Fig. 21), which may in turn aggregate into so-called micelles (C, up to 200 nm). The dimensionsof the elemental units in this modeldo not fit with x-ray and TEMdata since theyare too large and too thick. More recently [34-361, it was pointed out that lamellar aggregates are loosely attached by edge-to-edge dipole interactions (SAXS and SANS data). It seems well established from these more recent data that~olyaromaticmoieties form stacksof about 1 nm diameter and 0.4-0.8 nm thickness, similar to BSU. If asphaltenes are extracted from oil by precipitation with heptane, their ~emixingwith the n-heptane soluble fraction (maltene) does not restore oil Chap. 19,[25,481). This points tothe impo~anceof interactions withirreversible character, which are typical of macromolecules and other colloids. macromolecular modelis consistent with that proposed for weakly evolved

0.05 Log LMOsize measured instandard conditions versus measured at LMO occurrence (for stars) and arbitrarily at 475°C (for crosses). Crosses are the data obtained for kerogens, asphaltenes, and oil derivatives. Black stars are the data obtained in Section IV D. (From Refs. 78 and 110.)

Model of asphaltenes according to Yen. (Adapted Ref. A: stacks of polyarornatic molecules. B: particle. C: association of particles also called micelles.

kerogens and coals (see Fig. 18 and Section IV A 3). However, it may be noted that BSU-like stacks have been observed in asphaltenes and not so clearly in weakly evolved kerogensof Series I and 11. This may be explained in two ways, (1) they may be present but their concentration is which do not exclude each other: toolowtobedetectedbythetechniquesused(TEM),whiletheyaremore concentrated in asphaltenes and(2) the association of polyaromatic molecules in stacksmaybeabsent, due tolack of mobility,but it mayoccurwhenthe polyaromatic molecules are brought into a fluid phase. In any case, this emphasizes the importance of molecular associations, the second explanation pointing to the need for molecular mobility to allow association of polyaromatic moieties entities.

PitchesaredefinedanddescribedinSectionII.Inpitches is 0.5-0.8 (see Table 1). Their oxygen content is always below the detection limit; however, oxygen is sometimes reported for industrial materials that are thus improperly called pitches, such as “Brai cited in Table 1. They are entirely soluble in toluene and isotropic at all scales except for the secondary coal-tar pitches described below (see Section IV C 4). Pitches are solid-like at room temperature. When heat-treated, they softenat a softening point temperature (SP) also defined in Europe as the Kraemer Sarnov point After softening, their apparent

viscosity decreases to minimum before increasing to infinity1,89,90]. Hydrocarbon bubbles escape from the softened materials up to maximum rate of volatiles release. As an example DTGA maximum occurs at 450°C for coal-tar pitch having softening point of 55°C. Correspondingly, the chernical composition continuously changes and the molecular weight increases. At the end of hydrocarbon release, solidification occurs.

l. ~ractionation Cnrbonized Pitches In the preceding paragraphs, solvent extraction was considered for coals. It isused for asphaltene preparation (soluble in benzene precipitated by n-heptane). Sporadic data were .obtained on kerogens showing that poorly evolved samples of Series I and I1 could give extractsin CHC1, [91]. The importance of the fractionation process in the study of carbonaceous materials increases from kerogen and coals to intermediate products (oil heavy products, kerogen extracts) and pitches. A notion of scale of solvent should thusbe recalled here. Among various classifications Chap. 19), those of Dryden [92] and Ouchi were usedfor coals. It gives an increasing strength from benzene, toluene, or chloroformto pyridine then to quinoline. These solvents,well anthracene oil (close to quinoline), are almost exclusively used for fractionation of pitches and carbonized pitches. Three classes of compounds are defined on the basis of their solubility: resins, insoluble in quinoline (QI); resins, soluble in quinoline (QS) but not in toluene (TI), characterized by molecular in the range of 400- 1100 amu, with mean number of 725 amu [94]; and resins, soluble in quinoline and in toluene (TS), with. a rangeof molecular from less than 230 to 310-400 amu [33,95]. It should be pointed out that these definitions are operational and cover a range of products, not defined compound. Isotropic pitches, such Ashland 240 (A240) or primary coal-tar pitches (CTP), are y resins. During their carbonization, then resins develop. At the brittle solid point (semicoke), as defined by Brooks and Taylor [96-981, resins reach 100%.

~icroscopi~ (a.) ~ e s o p h ~As eSpheres. Forkerogens,coals,asphaltenes,and oil derivatives, LMO were observed only by TEM above solidification (1000°C) and after grinding. In the case of pitches, anisotropy occurrence and development were usually followed optically on polished sections. In the next paragraph (IV D), observations by TEM after thin-sectioning were added to those in optical rnicroscopy (see Reference 109, 110).

Spherical anisotropic bodies appear in the isotropic pitch matrix it becomes heavier (enriched in resins). These bodies, which may vary in size (typically several pm to tens of pm), arethespheresdiscovered byTaylor C961 and described by Brooks and Taylor [97,98] mesophase, i.e., liquid crystals (here named A). Their optical anisotropy results from their peculiar so-called PAN-AM microtexture. Figure 22a shows meridian sectionof sphere observed between crossed polarizers with retarder plate added. The ordinary index nois in the plane of the aromatic layers and is the largest, i.e., (negative uniaxial). Due to carbon pleochroism, the aromatic layer stacks appear dark when they are parallel to the polarizer plane and magenta when they are pe~endicular.In 1 of Fig. 22a, the ordinary indexngmis parallel to the vibration planeof the polarizer. Due to the PAN-AM texture, blackMaltesecrossappears(extinctioncontoursor isochromatic lines) with two blue and two yellow quadrants. From 2 to 4, the body is rotated clockwiseso that black hyperbolae are formed and then disappear. Beyon 90°Crotation,theextinctioncontoursbecomemagentaandthequadrants reverted [20]. In sections other than meridian, various shapes of extinction contours are observed, moving when rotating the stage. These contours should be

M e ~ dcross-section i ~ through a Brooksand Taylor sphere of mesophase A; the solid curves indicate the orientationof BSUunits, followingthe PAN-AM texture. a: optical micrographs (crossedpolarizers, X plate added). In 1 the ordinary index ngmis parallel to the plane of the polarizer. From l to 4 the rotation of the stage is clockwise. (From Ref. 109.) 002DF TEM micrographs of a thin section of mesophase A. From l to 3 the 002 d counterclockwise. The shaded areas correspond to bright contours in the image. (From Ref. 99.)

predicted by considering Figure38 below. In 002DF, TEM gives images similar to OM (Fig. 22b) [9,20,99,100], with succession of bright crosses (1 and 2), then hyperbolae (3). The succession of the 1, 2, 3, 4 sketches of Fig. 22a, followed by the sharp change to magentaof the extinction contours, or the succession 1,2,3 of Fig. 22b are the only typical features of Brooks and Taylor spheres of mesophase A. In order to ascertain their presence, successive images shown in Fig. 22 should be detected at least once in the sample. If not, other kindsof anisotropic bodiesmay be present. Mesophase A spheres are suddenly produced at the end of~ n i m u m viscosity. As early 1966, Ihnatowicz et al. E891 ascertained that mesophase sphere was liquid dropinside liquid, i.e., an emulsion,so that they proposed the term of demixtion for the sudden occurrenceof these spheres. Mesophase spheres are the first anisotropic bodies to appear in pitches. "hey show local molecular orientation within spherical contour,The term LMO will also be applied to them. After the Taylor [96], Brooks and Taylor [97,98], and Ihnatowicz et al. [W] papers, spheres of mesophase A were considered to be QI, i.e., resins. On that basis, studies in 002DF and by direct imaging of aromatic layers (002 lattice fringes) were carried on by Augui6 on single spheres extracted from their pitch (coal-tar pitch) by solvent equivalent to quinoline: hot anthracene oil [99,100]. Spheres were embedded in resin, then thin-sectioned. The microtexture was verified to be PAN-AM. However,at the nanoscale, lattice plane resolution (Fig. nar text~remade of bent columns withoutlateral coherence. 002DF allowed the evaluation the very local (200 nm) relative misorientation of BSU: t-30", 25". At the nanoscale, in 002LF, (10-20 nm), is smaller 15'). These variations in are due to the influence of the PAN-AM texture adjoined to the sensitivity of mesophase to the microtome knife inducing mechanical twins detectable in 002DF. Later on [loll, thin-sectio~ng of spheres in situ in their pitch was developed. Surprisingly, Fig. 23a shows anarchic nanotexture. Though globally PAN-AM at the OM scale, at the nanoscalesingle spheres of mesophase A were postulated to be patchwork of LMO intermingled with some kind of disordered domains, withdrawn by anthracene oil (Fig. 23b). Reliable data were only obtained owing to image analysis [20,102,103]. The first Fourier transform of Fig. 23ais given in the inset. It contains weak interferencering, reinforced into two arcs and two sectors of small angle scattering. It was filtered by eliminating small angle scattering, inelastic scattering of the supporting film and incident beam, but keeping the whole ring. Figure 23d shows that the columnar arrangement is already present in the sphere even included in its pitch isand associated with noticeable amountof random BSU including 90" misoriented ones. By imaging small-angle scattering only, random distribution of pores, already detected by SAXS [104], appear everywhere in the interstices between the columns and between the BSTJof

Thin section of a single mesophase sphere. a: in situ in its pitch, TEM 002LF image (from Ref. 101) with its Fourier transform in inset. b: extracted by anthracene oil, TEM 002LF image. (From Ref.99.) c and d, image analysisof a (from Ref.102); c: small angle scattering image; d:002 interference image. The white arrow in a, c, and d a point common to the three micrographs.

column (Fig. 23c). single mesophase sphereis thus an intimate association of resins forming a columnar a~angementof associated BSU and disordered lower molecular matter, similar to resins, plausibly made of misoriented having kept some of their sidechains. In addition the pores could be filled inby some kind of resins. Such columnar textures arer e ~ n i s c e n of t discotic liquid crystals [105-1071, which are characterized by columnar ~angements.,either made of regular compact associations (Fig. 24a), orof bent columns Adjacent columns are entirely devoidof lateral coherence. For comparison, Fig, 24c shows a nematic order without columns. Discotic liquid crystals are made of disc-like molecules having sidechains, which associate into anisotropic spherical bodies or volumes with digitized contours. Mesophase A spheres are thus discotic liquid crystals showing PAN-AM texture at the ~ c r o m e t r i cscale. We attribute the columnar n~otextureof mesophase Ato the close relationship between BSU and coronene (see Fig.6 [sl]). In the coronene crystal, molecules are piled up in rigid columns. On the other hand, isolated coronene molecules tend to form shifted stacks. Association of SU may thus tendto follow the same rule. However., in the latter case, associations are necessarily less ordered than in a crystal. Image analysis in Fig. 23 shows that BSU pile up in distorted columns separated by BSU forming wedges. This is in agreement with the perpendicular orientation of the fourth molecule in coronene tetramers, foreseen by m o l e c u l ~ mechanics computation. It has been verified that the lessn u ~ e r o u sand the less SU forming wedges, the less bent the columns [103]. (b.) ~ e s o ~ h aAs ae n ~ o ~r t h e r ~ n i s a t rBodies. o ~ i ~ Taylor defined anisotropic bodies in vitrinite (maceral of coals). Then observed the characteristic PAN-A texture in pitch [97]. From these authors, at mesophase formation the molecular of the TS fraction of the pitchis 395-

Sketches of various types discotic liquid crystals.(From Refs. l06 and 107.) a and arecolumnar,c nematic. The disc-likeelementalunits are devoidoflateral coherence. Each disc could represent a

470 amu, thatof the spheresis 1690 amu. Mesophase spheres appear only above a certain temperature (400°C) at which nucleation, then growth, would occur.The firstanisotropicbodiesfoundinunknownmaterial E971 andsupposedto be mesophase were 0.1 pm in size. However they were not characterized but detected only in SAD patterns. These bodies were thus sufficiently organized to provide patterns distinct from the matrix diffuse scattering. Their growth was not studied. After total conversion to mesophase, it was assumed that the final product was not always a solid semicoke buta very viscous, easily deformable liquid. Sometimes also, intermediate stages between nucleation, growth, and coalescence, though no observed, were inferredby analogy with others. In addition to pitch, Brooks and Taylor examined numerous other, much-less- define^ products [97,98] such as High-temperature coal-tar pitches (secondary pitches), Vitrinites of coals close to igneous rocks, Heat-treated vitrinites, Heat-treated petroleum bitumens, etc. It is worthwhile to note that all these products contain a small amount of oxygen so that they cannot be classified as pitches (devoid of oxygen). Inside each of these partially coalified or carbonized products, anisotropic bodies were formed. Sometimes they wereso small thateven. they could not be recognized spherical (0.1 pm or less). Sometimes they were spherical but too small to allow recognition of the specific figures of PAN-AM mesophases. It is clear that such bodies were not proved to be mesophase A spheres. It willbe demonstrated below that many anisotropic bodies are other kinds of liquid crystals. It turns out that, except in a well-defined pitch, mesophaseA is not a component of carbonaceous matter as frequently as it has been postulated. The fact that bodies were found with sizes varying betyeen 0.1 pm and a few millimeters was interpreted as a growth starting from nuclei [97]. However this wasnotproved,sincetheanisotropicbodyinitially0.1 pm in size wasnot followed in the same materialall along its growth. The same error was made by the authors of the present chapter 1081. The development of mesophase thus deserves further discussion.

~ v o l u t ~ o n ~ e s o p h a s e Spheres. As HTT increases,therearetwo possible ways for spheres A to develop: either by growth after nucleation as assumed by Brooks and Taylor or by coalescence following demixtion. Investigation of anisotropy development in A240 by O M and TEM 109,1101 shows a sudden demixtion spheres (3-6 pm) having the characteristic PAN-AM texture. Hot-stage video sequences reveal a continuous demixtion of these small spheres that coalesce with the larger ones 13 l]. At the beginning of the process, the spherical shape tends to be maintained (Fig. 25a) [89], a small sphere

5 Coalescence of mesophase A spheres observed in OM between crossed polarizers, at increasing stages a and (Brai HT). In the arrow marks some of the points where the extinction contours of two neighboring spheres coincide. (From Ref. 89.)

approaches a large one, it rotates quickly so as to align oneof its extinction contours with one of the extinction contours of the other spherel 11and coalescence occurs(arrowsinFig.2Sb).Thealignment of isochromaticlinesshowsthat coalescence requires alignmentof the aromatic layersof the two spheres with the minimum misfit. If the orientationsof the two spheres are very different, they get into contact and deform without coalescence. When two spheres merge, defects are created so that complicated extinction contours are produced (Fig. 2Sb). These defects are disclinations discovered by Frank in liquid crystals 1121, then studied in detail by White et al. [l 13- 1151. In a three~dimensionalcrystal, a wedge dislocationis produced by the addition or withdrawal of half a lattice plane associated with a dislocation line. Dislocations are mobile and are attracted or repelled by each other. In the e~uilibrium state, they form stable nodes. composite dislocationline is thus formed 1161. In a liquid crystal (two-dimensionally ordered), the notion of dislocation is replaced by that of disclination. The half plane is replaced by half a lattice row and the dislocation line is replaced by a point. composite is cl in at ion line is the locusof

the disclination points. Inthe case of carbonaceous materials, atoms are replaced by aromatic molecules 113-1 151, which are identified BSU in the present paper. Figure represents a complex system where three composite disclinations lines have merged,foming node (nanoscale). Figure 26b represents, at the micrometric scale, the variation of BSU orientation in different domains having coalesced. Two different kinds of nodes, and B, are formed. The core of B corresponds to Fig. 26a. Upon OM examination between crossed polarizers, zones where BSU units are parallel to the analyzer or polarizer are dark They define extinction contours that are sinuous lines and are the tracesof high density of disclinations. themicroscopestage is rotated,theextinctioncontours(or isochromatic lines) move, but the nodes where disclination join lines and B, in Fig. 26b) remain fixed. Nodes are devoid of dark contrast. It already observedfrom Fig. to b that the spherical shape of coalesced spherestends to disappear. Co~e~pondingly, isochromaticdomains(domains homogeneously bright in Fig. 2%) tendform to well-defined areas, limited by the sinuous extinction contours. By adding the X retarder plate the colors are fairly uniform inside each domain; thus the BSU contained inside these areas are in preferred parallel orientation, Each domain approximates thus anLMO. carbonization progresses further, isochromatic domains associate into islands (mo-

a: sketch of a node formation in a liquid crystal (nanoscale). b: formation of isochromatic lines observed in OM (microscale). Node corresponds to the situation described by a. (Adapted from Ref. 113.)

400

450

500

550

Vickers microhardness versus for pitch heated in standard conditions (4°C min-1, full circles) and at min-1 (empty circles). Single mark demixtion spheres (LMO occurrence). Double arrows mark100% anisotropy. (FromRef. 110.)

saic islands), which progressively invade the whole material at the expenseof the residual pitch matrix. When the latter entirely disappears, 100% anisotropy is reached. The product, called bulk mesophase, was supposed to be brittle solid, the semicoke [98]. As a matter of fact, the curves of Vickers microhardness plotted versus HTT and compared to OM data (Fig. 27) show that, inpitch, 100% anisotropy occurs well before solidification 109-1 101. Figure 27 shows A240 carbonizedin standard conditions (heating rate 4°C rnin-l, full circles) and A240 carbonized at 0S"C min-l (open circles). At 4°C rnin-l, LMO occurs at 465°C (110 single mow), 100%anisotropy occursat 5SO"C (230 MPa, double mow), solidification is over at 600°C (asyrnptot). At 0.5" C rnin-l, LMO occurs at 420-430°C (180 MPa, (120 MPa, single arrow), 100% anisotropy occurs at 490°C HTT double arrow), solidification is over at 550°C. At heating rateof 0.S"C min-l, 4 wt% hydrocarbons are still present100% at anisotropy. At that point, isochromatic domainsof very large size (20-100 occupy the whole polished section. Concornitant with 100% anisotropy, macro-

rli

Increasing development of macropores in A240 pitch min”; between crossedpolarizers). (From Ref. 109.) HTT 50OoC, 100% anisotropy;isochromatic domainsand extinction contours beginto elongate aroundthe pores; isometrical isochromatic domainsare in thecenter (double arrow). solidification; the pore wall thickness is m i n i ~ u m .

pores of millimeter size (single arrows in Fig. 28a) occur, around which oriented mosaics develop by apparent elongation of isochromatic domains and extinction contours around the pores. Isometrical isochromatic domains occupy the regions between three or more pores (double arrow in Fig. 28a). Between 100% anisotropy and solidification, macropores inflate, whereas their walls become thinner and isochromatic domains more elongated (Fig. 28b). Well-defined (mosaic-like) isometrical areas have almost disappeared. Everything is frozen when the solid stage is reached. Around each pore the isochromatic domains formskin, inside SU are all oriented almost parallel to the pore surface. Isoc~omatic domains have thus no definite size: they formskin rich indiscli~ations.As this skin is cut obliquely by the plane of the polished section (Fig. 28), anisotropic bands or even fibers seem to be observed around the pore wall trace.the skinis cut in the vicinityof an apex pore (skin almost flat in the polished section), large areas magenta in color are observed with the retarder plate added. They remain magenta when the stage is rotated. The mosaic-like org~izationis obtained only when anisotropy reachesloo%, i.e., well before the brittle solid stage. In the latter, preferred orientation extends over long distances (oriented mosaic: bands and fibers), so that the concept of mosaic and mosaicsize loses significance. The importantfeature to keep in mind for pitches is the impossibility of determining the LMO size, we amount of disclinations present at solidification (see Section In petrography, the size of isochromatic domainsis sometimes evaluatedby the distance between contours 113- 1151 and sometimes by their area in the image 109,117,1181. Mosaics were classified [ l 17,1181 accordingto the domain size into coarse 5 pm, Fig. 29b), medium (2-5 pm, Fig. 29c), fine 2 p,m, Fig. 29d), and oriented (bands or fibers, Fig. 29a). Macropores constitute troublesome feature for industrial artifacts, since they cause swelling and breakage. At first, they were thought to develop after solidification. This is clearly impossible becauseafter solidification the weight loss is too small ( l wt%) to provide such large holes. On the contrary, macropores appear before solidi~cation,when volatiles are still present (4% in A240). Therefore the pressure produced by the outgassing of remaining hydrocarbons provokes inflation of the pores and thinningof the pore walls. At solidification inflation ceases [109,l 101. From LMO occurrence(Fig.23) to solidification(Fig.30)the colum~~ nanotexture is preserved 1031. Figure 30a presents the initial TEM 002 image (thin section)of solidified A240 treated at 550°C min-l)). The first Fourier transform is given in theinset. Figure 30b gives the image obtained after filtering of Fig. 30a. The inset of Fig. 30 compared to that of Fig. 23 is reduced to two arcs without ring and to reduced small-angle scattering, Correspondingly, reduced number of less misoriented BSU appears, associated with reduced number of pores.

mosaic

Optical microscopy of isochromatic domains. a: oriented mosaic. coarse 5 pm). c: medium mosaic (2-5 pm). d: fine mosaic pm). (From Ref. '78.)

A 240 pitch heat treated up to solidification (550"C, min-l). a: TEM 002LF image (thinsection). Inset: first Fourier transform of a. image after filtering of a. (From Ref. 103.)

Data

~ole~~lar ~ra~tionation.Beforeconsideringthevariousdata obtainedonpitches, it is worthwhiletorecallmajorquestionsarisingabout molecular mass and extraction. Most were raised about coals and were critically discussed by van Krevelen ( [ 5 ] Chap. 19, [119-124]). Coals and other carbonaceous materials may be unambiguously characterized by a molecular mass distribution only if the following two conditions are fulfilled: (1) they are constituted by defined molecules (monomers, oligomers or polymers), i.e., entities that do not make covalent bonds (C-C, C-0) with each other and (2) these molecules can be brought into a real solution where they form independentkineticentitiesand do notassociateto form colloidaldispersions. The molecular-massdistributionhasadirectphysicalmeaningonlyifthesetwo conditions are fulfilled. Carbonaceous materials, except and resins, are not completely soluble in any solvent, either because they are madeof macromolecules that are too heavy (reduced entropy driving force), because they are made of network involving covalent bonds, or becausethe constituting molecules form associations that are too strong (hydrogen bonds) or too rigid (steric constrains) to be readily dissolved The molecular mass ~easurementsare usually performed after reductive alkyla-

tionfollowed by hydrogenationwithalkalinemetalsand “solubilization” in benzene [123]. The molecular mass distribution must be interpreted keeping in mind that the chemical treatment may have affected covalent bonds and that the “solution” may be a colloidal dispersion. In any case, even if the material pretreatment is limited to fractionation without alteration of covalent bonds, the organization of a carbonaceous material may not be inferred from a molecular-mass distribution. In addition, the nature of the material (the precursor, its evolution and the treatment applied)itsand properties, respectively, determine and result from the precise organizationo€ the constituents with respectto each other. This organization is the resultof processes that are not, as such, reversible. Forinstance, a mesophase is not restored by mixing the variousisolatedfractionsevenwithoutbreakingcovalentbonds. another example, hydrogen bonds disrupted by pyridine1221 maylead to other types of associations upon reconstruction after pyridine removal. While molecular-mass distribution does not provide precise i n f o ~ a t i o non the organization of a carbonaceous material,it is useful to characterize it as a whole or to characterize someof its ~omponentsby other techniques (e.g., imaging). From initial pitch to singlespheres of mesophase A, the molecul~-mass s~ectrum extends from 200-400 amu (characteristic of resins) up to400 to 4000 amu ina sphere[33,125]. This revealsindirectly the presence of lowmolecularmass entities inside a single sphere. This agrees well with the image analysis data of Fig. 23 [20,21,10~,103~. It also accounts satisfactorilyfor the partial dissolution of mesophase spheres recognized in anthracene oil [126]. For coalesced spheres, a value of up to 6000 amu was obtained [33,125]. Milder fractionation treatments were used by Bhatia et al. [901 to study the carbonizatio~of increasingly heavy products,A filtered coal-tar pitch (CTP) was verified to befree of Q1 by OM control and molecular mass measurement (Table 4). Then CTP and its TSTIand fractions were compared according to apparent viscosity was measured with a rotational viscosimeter as a function of

comparison Between Products of Increasing Molecular Mass (Mnfor (H/Cj,,, Softening Point, ~ i n i ~ uApparent m Viscosity, and Solidification Temperature (H/Cjat TS C’I’P

SP “C

0.60 80 290 toluene-soluble, CTP Ref. 90.

Min. of appar. Solidification a n (arnu) viscosity (Pa (“C) 230 290 pitch. TI

-0 -0 20 toluene-insoluble.

500

HTT. (HQ,, decreases, the softening point increases, the minimum valueof viscosity tends to increase, and the temperature of solidification decreases. The HTT range of the plastic stage thus narrows. Co~espondingly,the average molecular mass increases.The same results were obtainedfor other pitches or mixtures of pitches [SS], which were also compared to coking coals. S u ~ ~ s i n gthe ly sarneresultswerefoundwhen(WC),decreased in pitchandwhen (OK), increased in coals. These data fit remarkably well with those obtained on kerogens. There are thus two ways to narrow the plasticity range: either by increasing the molecular mass through hydrogen loss or by increasing oxygen retention. This is an argument to consider hydrogen as antagonistic to oxygen in the same manner asH had to be introduced in the computation of FLMo(see Sec. B 2). (b.) A sharpminimurnin the viscositycurvesobtainedwithrotationalviscosimeters is almostsystematicallyobservedimmediately before solidification (Fig. 3 1 [95]), This minimum was the origin of pitchbased carbon fiber preparation, is It thus of utmost importance and necessitates a discussion of its reproducibility and meaning. It studied firstby Didchenko et al. 1273, then byCollett and Rand 128,1291, Balduhn and Fitzer [95], and et al. [1303, Figure 31 [95] shows in dotted line the values given by the viscosimeter. The minimum of viscosity occurs at 460-465"C7 immediately before the increase to infinity. Didchenko et al. [127], as well as Collett and Rand 128,1291, attributed this peak to a sol-gel transformation. et al. 301 proposed that a phase inversion was responsiblefor the minimum, the dispersed phase becoming

Qt w i ~ h ~ t Viscosity,

WTT

Formation quinoline insolubles and apparent viscosity versus pitch (SP 70°C). (From Ref. 95.)

coal tar

suddenly the continuous one. Accordingly, the dispersion of mosaic islands issued from residual pitch would suddenly transform into a dispersion of pitch drops in the mosaics. Balduhn and Fitzer1951 correlated the viscosity minimum to mesophase formation. However, many misleading features emerge when comparing these papers. Kfoury et al. [130], as well as Balduhn and Fitzer [9S], found the peak for every pitch sample studied, including A240. On the contrary, Collett and Rand [128, 1291 did not find it for A240. Whittaker and Greenstaff 1311 did not findit. The interesting pointis that the latter authors did not study pitches but atmospheric and vacuum residues of distillation of various crude oils. In addition, their optical ~ ~ r o g r a pobviously hs showed that coalesced mesophase A was associated with something else, which was too small to be resolved OMby (see SectionIV D). As a matterof fact, rotational viscosimeters develop high shear stresses in the viscous medium, so that many authors noticed the viscosimeters had a great influence on anisotropy. Flow orientations occur, producing the so-called bands and fibers and elo~gatingthe disc~inationloops 132-1341. Brooks and TaylorL971 and Brown et al. [l351 suspected also the persistence of a viscous liquid after complete conversion to mosaics, considering that an applied mechanical pressure could C1331 and Whiteet al. 1343 observed clearly orient the mosaics. White and Price that bands occur near bubbles. Unfortunately their observations were notonmade pitches but on highly heterogeneous products (decant oil, asphaltenes of vacuum residues, etc.) that provide a mixtureof bands and ultrafine mosaics qualified as fine-te~turedisotropic mesophase. Another feature arose from the Balduhn and Fitzer paper [9S] in which compa isonwasmadebetweenanisotropyand Q1 contents,withandwithoutshear. Figure 31 shows the viscosity curve in dashed line. Open circles indicate the percentage ofQ1 formed between 400 and 500°C during viscosimetry experiments. The sharp ~ i n i m u m of viscosity at 460-465°C coincides with 60-65% of Q1 att~buted to60-6S% of mesophase (60-65% anisotropy). As a first conclusion, viscosity behaviour was related to mesophase formatio~.However if the percentage of Q1 is measured in the absence of shear (full circles in Fig. 31), it reaches 100%as viscosity jumps to infinity, i.e.,at a temperature very near to that where the viscosity accident occurs 470°C). The second conclusion was that “bulk mesophase is obtained earlier without shear than under shear” [SS]. Nevertheless, the optical micrographs given in the paper show that 100% anisotropy is produced in thetwo cases, but it isin theform of bands under shear and of coarse mosaics without shear. These assertions are close to the concept of the so-called “dormant mesophase” due to Otani 136,1371, “Dormant mesophase” was prepared by partial carbonizationof a petroleu~pitch so as to produce mesophase A spheres resins) dispersed in their residual matrix. The soaked pitch (thermal maturation) was hydrogenated to recover optical isotropy. A further heat treatmen under stress provided suddenly at least 95% of mosaic-like optical anisotropy. Nevertheless, the Q1 content was only30% or less. These experiments and those

considering persistent plasticity in petroleum pitches submitted to shear stresses [95,127-1301 were followed by a very large number of patents postulating immediateoccurrence of 100% anisotropyuponpeculiartreatments of partlycarbonized isotropic pitches. The result was a plastic material able to soften and to acquire flow anisotropy by spinning[20]. Implicitly or explicitly, the above cited papers [95,127- 1301 refer to thede~nitionscommonly adopted after Taylor [97,98] and Ihnatowicz et [891: al. mesophase, QI, and anisotropy percentage are interchangeable notions, ending in a brittle solid when reaching 100%. a result, an apparent contradiction arises that will be cleared up below (see Section D 2).

4. Coal Tars and C o ~ l - T a rPitches Tars are produced in the industrial preparation of blast furnace coke. coking coal is placed in an oven, the wallsof which are heated at 11001200°C. Volatiles are released at the top and condensed outside by water, forming tars. They carry with them impurities that are coal dust and mainly carbon black formed by volatiles cracking at the oven top ( l l 00-1 200°C). These impu~ties will be called free carbon since they are solids and almost pure carbon (part of ing distilled to make pitch, tars are stored, sometimes long enough to favor se~f-inducedincreases in molecular mass. Then and resins develop, respectively, identified by their insolubility or their solubility in quinoline. The resins form spherical anisotropic bodies1381 that contribute, withfree carbon, to the total amountof primary QI. Figure 32 illustrates some spheres surrounded by resins (mow in Fig. 32). It is interesting to remark that these spheres have a microte~ture,alternatively radial, concentric, and radial, to be compared with mesophase spheres, where the orientation of the aromatic layers is always pe~endicularto the interface with the pitch matrix. In soaked tars (thermal xnaturation), resins develop. They are isotropic at OM and TEM scales. However, decantation under stress produces marked flow orientations illustrated in Fig. 33a and b and modeled in Fig. 33c l 391. They recall the oriented mosaics observed in ,4240. Pitches Primary coal-tar pitches are produced by tar direct distillation. They are isotropic y resins, except that Q1 impurities are maintained in the pitch if it is not filtered. During carbonization Q1 decorate the mesophase spheres when they appear [99,100], as illustrated in Fig. 34. This a self-purification of mesophase that will be discussed below. Secondary coal-tar pitches [99,1401 are obtained from primary ones by distillation at high te~perature 1261 or by soaking l 4 l]. They are thus partially carbonized so that they contain and resins sometimes in large amount, up to 25 wt% (Table5). In this case resins are textured and show morphologies common to all samples studied in the above references [99,126,140,141]. In TEM brightfield (Fig. 35), resins appear as liquid-like particles, either drops (Fig. 35a,b,c) or

erli

TENI orthogonal 002DF of anisotropic spherical bodies surrounded byisotropic resins (arrow) in a tar. The orientation of BSU,represented double bars, is radial in the cores and concentric then radial in the shells. (From Ref. 138.)

with irregular contours (Fig. 35d,e,f). In 002DF, clusters of bright dots occur at random in the external parts of the drop (Fig. 35b and c). They are variably extended inside the irregularly shaped liquid-like particles (Fig. 35e and f). They completely simulate various LMO classes observed in kerogens, coals etc. Figure 35e and f correspond to Class 8. During pitch filtration [141,142], liquid-like drops with randomLMO, similar to thoseof Fig. 35d,e,f are found in the cake, wherethere is little stress. Oriented mosaic-like liquid features (similar to Fig. 33) accumulate in the pores the filter which are thus clogged-up. The changeability of resin textures is responsible for numerous l~isconceptionsor confusions in studies of pitches and coals. In secondary coal-tar pitches (CTP), which are pa~iallycarbonized, not only resins develop but spheres of mesophase A resins), which increase the content (secondary Thepreparation secondarypitchesaims to satisfytherequ c ~ s t o ~ e (electrode rs makers). However the same physicochemical conditions edonthefinalproductarefulfilledwith a highvariety of m o ~ ~ o l o g i e s . c o ~ p ~ i two n g pitches that are apparently identical on the basis of such

002DF.

Bows.

of single spheres of mesophase A decorated

requirements9 one could esal~plesof second custorner9sc~emica

QI.

atisfactory and the othernot [99,14-0].Table which all a p p r o ~ i ~ a t e l y f u l fthe il~e~ ments. They have an increasing softenthree contain rnesophase spheres, in increasingamount amples A and B contain resins as liquid-like ut not I. Sample is certainly issued from a particles (larger filtered precursor since it is devoid primary

Composition of Secondary Coal-Tar Pitches Pitch B I

KS

KS ("C) (HIC), 84 85 91.5

resins 0.58 0.54 0.56

Sarnov point. Ref.140.

6.3 13.3 9.7

resins 18.6 22.6 24.8

and e,

Textured resins in secondary coal-tar pitches. a and d: TEM bright fields. b,c orthogonal 002DF. (From Ref. 99.)

primary Q1 (more abundant in The latter consist mainly of free carbon, i.e. A, and I have decreasing industrial qualities, I being weenindustrialqualitiesand characteri~ations h o ~ i n that g custo~er s~eci~cation may b

demonstrate^ the presence c o l u ~ ara~ Terrikre's ~ c r o g r a ~ [102,103], hs onaceous samples and its persistence from Fig, presents the initial lattice fringes image and Fig. 36b presents the filtered image after removal of incident

002LF images (from Ref.43) of ant~acene-basedsamples heat treatedat 500°C (a), '700°C (c) and 900°C (e). b: image analysis of a; d: image analysis of c; insets: Fourier transforms. (From Ref. 102.)

beam and diffuse scattering; the Fourier transform is in the inset. ForHTT 7OO0C, Fig. 36c is the initial image and Fig. 36d is the filtered image (Fourier transform in the inset). Figure 36e is the initial image at in which columns were so marked that it was almost unnecessary to filter the image. In summary, in ant~acene-basedcarbonaceous series, the columnar arrangement occurs without interruption from 500°C up to 1400-1600°C. Correlatively to the reduction in the arc opening, the number of misoriented BSU decreases as HTT increases. elative to Fig. 23 and 30, there is no BSU misoriented up to thus markingthe difference with respect to mesophase A.

S that in many cases the samples, although carefully studied, are not well defined. an example, the category of pitches often includes atmospheric or vacuum residues of oil distillation that contain oxygen. In addition, elemental compositions are not always giv and also optical microscopy characte~~ation is not done oris incomplete. Spherical particles are sometimes qualified mesophase without researching the typical characteristics of mesophase A singlespheresillustrated in Fig.22.Finally, correlation between morphologies (textures at all scales) and physicochemical properties are rarely established.The recent work of Bonnarny [l09,1101 brings more precise and complete information. It makes a bridge between pitches, kerogens, and coals. The precursors studied were a pitch (A240), heavy products of oils (Arabian light atmospheric residue, Safaniya vacuum residue), an asphaltene (14618), and a kerogen (Kuckersite) (see Table 1). They were heated at 0.5"C min-l and at 4°C min-l under standard conditions. Two conditions have to be fulfilled to observe reliably carbonaceous materials not yet solidified. First, they should not be disrupted. They also have to be observed at all scales. Extraction processes produce artifacts already recognized inside mesophase spheresl011 (see Fig. 23a and To avoid them,it isnece to observe LMO in situ in their matrix. To correlate the data obtained by (after grinding) and those obtained by OM on polished sections,it isnecessary to observe the same object using both techniques. Therefore, grinding was discarded and the same areas of thin section and of residual embedded block used as polished section were respectively observed in TEM and All samples described in this paragraph were observed following this method.

l. ~ i ~ ~ o s ~ o ~ i ~ Anisotrop~Occurrence. Pitchesexhibit (see Section IV C 2). Heavy oil products, such Arabian light-AR and Safaniya-VR, also develop spheres of mesophase A (2-5 pm), but these are associated with two other kinds of much smaller spherical bodies (350-1000 nm) and C (about 200 nm) (Fig. 37) [109,110].

TEM 002DF of thinsection of Arabianlight-AR 0.5"C min-l)). Particles A are Brooks and Taylor rnesophase spheres; B and C are other liquid crystals. (From Ref. 110.)

In Fig. 3'7 (TEM 002DF) the largest bodies A are never entirely bright. They always contain dark areas or are entirely dark. A systematic exploration in 002 dark-field by small increments of angle shows that the dark areas deform or invade the whole body, but never disappear. This feature is typical of the PAN-A texture of mesophase A as demonstrated by Fig. 38 [loo]. In this figure the possible sections of a sphere of mesophase A are arranged from top to bottom according to increasing tilt of the north-south axis from meridian to equatorial sections (0" to 90"). Then parallel slices are cut, arranged by their increasing height above the equator (0 to 0.8)from left to right. Above each section its profile is represented, inside which are marked the aromatic layer traces. Because of the scattering conditions, BSU tilted more than10" relative to the incident beam do not scatter a002 beam and thus are not imaged in 002DF [9]. Most of the sections appear entirely or almost entirely dark except l to S, to 10, 13 to etc. Besides limitations due to scattering conditions, there are limitations due to the portionof

"4

rooks and Taylor modelof a mesophaseA sphere by increasing tilting (top to bottom) and parallelslicing (left to right). The profile of aromatic layers in a thin section is given above each planar projection. (From Ref. 99.)

002 beam admitted to pass through the objective aperture; therefore only areas of very reducedsizecould be bright in thesections. As anexample,section 1 provides images sketched in Fig. 22b, whereas other sections such 2asto 5, 8 to 10, 14 and 15, etc. should produce only narrow bright bands, a result, rnesophase A sections are never entirely bright. the contrary [l 09-1 101, or off from bright to gray to d turn from entirely bright to e

total estin~tion,the rni§orie~

a and TEM orthogonal002DF of S ~ a n i ~ a (42S°C, - ~ R 0S"C min-') showing and C liquid crystals (thin section). Two homologous bodies B and C are s i ~ g l e - ~ ~ o w e ~ inbrightor dark sit~ations.The intermediate orientations are d o u b l e - ~ o ~ e (From d. Ref. 78.)

002DF of a thin sectionof Safaniya-VR (450"C, 0.5"C showing and C liquid crystals. C is accumulated in the interstices between B. (From Ref. 110.)

as well as C are oftenmo~odisperseand tightly associated (Fig. 4.0). tend to accumulate without shape distortions in the intersticesof odies not miscible with each other, neither are they miscible with pelled mesophase A so they remain atthe surface of the spheres. The same repulsion is observed between mesophase and I of any other type, namely those inherited from tars lO0,140].

From Arabian light-AR to Safaniya-VR l09,l lo], the amount of bodies increases, whereas that of decreases, or even annibil The pyrolyzed asphaltene 14618 does not contain spheres o kersite even C disappeared. Sp igitized contours typicalof kerogens (Fig.4lb). Their 200 nm). Class 8 C andvolumeswithdigitizedcontoursare they keepa columnar arrangement the nanoscale assh in Fig. 42a and for kerogens in ig.42c [102,103]. because of their variousmicrotext~res,they belongto continuous seriesof different kinds of liquid crystals.

ing

TEM 002DF. a: asphaltene 14618 with C spherical bodies. with digitized contours. (From Ref. 78.)

Kuckersite

a andb: spherical anisotropic bodies C; a: initial 002LF micrograph. (From Ref. 110.)b: image after filtering of a. (From Ref. 103.) c and d: volumes witb digitized contours (kerogen of Series I beat-treated at 600°C) c: initial 002LF ~icrograph.(From Ref. 39.) d: after filtering of c. (From Ref. 103.)

~ n i ~ u t r ~u ~e yv e ~ u ~ ~Enepitch n ~ A240 . the spheres of mesophase A coalesce and form mosaics following continuous process. In Arabian light-AR and Safaniya-VR the presence of B and C perturbs the coalescence of A (Fig. 43).The B and C spherical bodies, repelled by mesophase A, accumulate in the mosaic island (MI) interstices, forming grain b o u n d ~ e s (GB) and triple points (TP) (Fig, 43). The latter (GB and TP) block further mosaic development since they are never resorbed.Et is only at the semicoke stage that anisotropy is complete, Thereis no production of macropores, so that the mosaics arenotoriented.Arabianlight-ARproducescoarsemosaics(seeFig. Db), ~ a f a n i y a - Vproduces ~ mediummosaics (see Fig. 29c).InSafaniya-VR the and C spherical bodies is so large that the mosaics are trapped in continuous anisotropic network of In asphaltene 14618 the spherical bodies C are alone and almost touching each other soon they are formed, so that thereis very mount of isotropic matrix. Coalescence of C spheres and further increase of LMO size is limited. e mosaics are produced. ckersite spherical bodies are replaced by volumes with di~iti%ed contours, the latter being so se to each other that they coalesce in short range of tel~peratures.The finalpresentatthesemicokestagearereduced to Cl 200nm). The L M clcersiteconstitutesthetransitionbetweenA, spherical mesophases of pitches and oil derivatives, on the one hand, and other kerogenswhere from Class8toClass1 5 nm),ontheother hand.Forkerogensha classessmallerthanClass 8, LMO occu~ence (demixtion of anisotropic bodies) andLMO stabilization at the brittle solid stage are so close in HTT that no distinction between them could be made by Villey [ 5 3 ] , Endustrial products containing large amount of oxygen, such as the saccharose-based series (see Table 1 [44]) or phenolic resins, provide L Class 1. Data presented here correspond to 05°C a n6n-l heating rate. As heating rate increases up to 4°C minl, the L sizes are much larger at the brittle solid stage.

The solidificationwas determine^ byVickersmicrohard 109,110]. Figure 44a and present microhardness (HV) v min-1 and 4°C min-1 (standard conditions), respectively.The pro provided the opportunity to define more precisely the temure at which LMO suddenly occurs (single arrows in Fig. The 44). solidification is determined by the asymptotof the Vickers microhardness curve. The main data are sum~arizedin Table 6. From0.5 to 4°C min", the occurrence and solidification increases whereas microhardness at L rence decreases. hat ever the heating rate,LMO occurs in a narrow for all samples:43 5-435°C for 0.5"C min" and 450-465°C for 4°C min-1. The

TEM 002DF of Arabian light-AR heattreated at 5 0.5"C min". Mosaic islands (MI) are formed, separated triple points (TP)or grain boundaries (GB) containing B and C spherical bodies. (From Ref. lo.)

240 A

KUCKE~S~TE

Vickers microhardness plotted versus a: 0.5"C b: 4°C1nin-l. Single arrows: LMO occurrence detected by double arrows: 100% anisotropy occurrence for A240 pitch; for the others this happens at solidification. (From Ref. 110.)

erlli

.

range of corresponding to LMO occurrence narrows from100 MPa at 0.5"C min-l to 50 MPa at 4°Cmin-l. The A240 sample stays almost the same while the others get close to it. Lowering the heating rate favors elimination of volatiles. Correspondingly LMO occurs atlower It may be noted that the increase in hardness at occurrence with a faster volatiles elimination (0.5OC rate min-1) is more marked when the available amount of suspensive medium is more limited (no increase for Arabian-light, 140 to MPa for 14618). Contrary to LMO occurrence, solidification temperatures are spread over large rangeof HTT for the two heating rates: 465-550"C for 0.5"C min-1 and 475"-600°C for 4°C min-1. The temperatures corresponding to 100% anisotropy coincide with solidification except for A240 whereit occurs much before (double mows in Fig. 44). We arenowabletoclarifytheapparentdiscrepanciesandcontradictions encountered in the rheology of pitches (see Section IV C 3 b), During mesophase evolution to solidification, noticeable amount of resins develops. These resins are able to imitate oriented mosaics when under shear [139,141] (see Fig. of shear [99,140,341] (see 33), well as coarse mosaic and other LMO in absence Fig, 35). A measurement of 100% anisotropy could thus correspond to less than 100% QI if convenient percentage resins is present and plasticity is thus maintained. Thisis obviously the casefor pitch A240, whichis still plastic though 100%anisotropic well before solidification. This is also foundfor systems showing the viscosity accident (sharp minimum) described in Fig. 31 [95]. By adding 37% of resins to the 63% ofQ1 measured by Balduhn and Fitzer, a plastic material 100% aniso~opicshould be obtained oriented mosaics. In the same manner, ''dormant me~ophase'~ should contain 70% of resins to explain the results of Otani 11136,1371.The same reasoning helps to understand why the first 100% anisotropicpitchhavingreachedthe carbonfiberswerespunfrom secondary viscosity minimum, i.e., being able to get fibrous orientation during spinning Whittaker and Grindstaff [13l ] could not observe the viscosity ~ n i m u msince , most of their samples were not pitches but oil derivatives for which 100% anisotropy coincides with solidification. Their micrographs showed that B and C spherical bodies were associated with, or even replaced, mesophase A spheres justifying the absence of plastic stage at 100% anisotropy. Figure 45 presents the evolution of (H/Qat versus HTT for the two heating rates. The inflection pointof (HhJat corresponds to theDTCA ma~imumalready observed by Villey (Fig. 9). The LMO occurrence corresponds to this in~ection point. Changing from 0.5"Cmin-" to 4°C min-1, the inflection pointis displaced towards higher HTT from the range 415-435°C to the range 450-465°C. All representative points thenjoin on the same curve beyond450-5OO0C, when C), is about 0.5 or little below. It is worthwhile to remark that this limitis the value (H/Qatof coronene. It the minimum value common to all kerogens when they have reached their maxi mu^ catagenesis (endof oil window) 1441. It roughly corresponds to the solidification.

U

versus

a: heating rate 0.5"C rnin-". b: heating rate

min-l.

(From Ref. 110.)

3. In order to tentatively synthesize thedata discussed in the previous section,it is convenient to add available TR data obtained with asphaltenes, oil derivatives 1451, and pitches[146,147],considering their importancefor kerogens and coals [5,57,58],

Eser and Jenkins11451 studied a large range of petroleum feedstocksas well as their fractions TI, IC, (asphaltenes), and maltenes (toluene- and pentane-soluble fractions) and compared them to ethylene tar and Ashland 240. ~ n f o ~ u n a t e l y , they gave elemental analysis only for the precursors, so that FLMO cannot be measured. In addition someof these precursors contain a large amountof sulfur, an unknown part of which could be a cross-linker, It is thus difficult to try a classification of semicokeanisotropicdomains.owever, itis verifiedamong these products that all those being devoid of oxygen show oriented mosaics as A240 does. All products initially poor in oxygen give coarse, medium, or fine mosaics, the sizes decreasing from maltenes to heavier asphaltenes. of None them contain enough oxygento produce ~ R of Class 8 to Class 1. The ~ T spe (also limited to those of precursors) show an increasing amountof aliphatic with respect to aromaticC going from tol~lene-insolublefractions to asphaltenes and maltenes. Ito[146,147]studiedacoal-tarpitch ((H/Qat 0.56) andtwopetroleum pitchesA240andP4 ((H/C), 1.22). ~nfortunately,carbonizationwasperformed isothermally. However by scaling OM data and(H/C),, values determined by Ito to those of Reference 110, reliable comparison could be obtained at least for A240. At 0.5 hour of heat treatment at 50O0C,(H/C),, is 0.58 for A240 and0.56 for coal-tar pitch. The pitches are isotropic. After hour (H/C), are 0.56 an 2 hours, respectively, mesophase spheres and mosaic islands occur. After are 0.50 and 0.46, the materialis 100% anisotropic with oriented mosaics (apparently equivalent to 490°C and 500°C with 0.5"C min-1 and 4°C min-" for A240 in Figure 44). After 3 hours, (H/C)atare 0.47 and0.44, and the pitches are probably solidified. The aliphatic concentration decreases during heating while the aromatic does not vary appreciably,The aliphatic/aromatic ratio of the nonheated material decreases following the sequence P4, A240, coal-tar pitch. it turns out that the relative concentration of aliphatic versus aromatic is similar for P4 heated at 500°C during 2 hours and unheated A240. For A240 heated at 500°C during 2 hours, the relative concentration of aliphatic to aromatic is similar to unheated coal-tar pitch. For A240 and coal-tar pitch,the demixtion of mesophase spheresis concomitant with the decrease of aliphatic and solidi~cationprobably arises before aromatic decreases. By comparison with kerogens, the same istrend observed. The P4 sample behaves differently as seen by the fact that LMO is present (as soon as 1 houraftertreatment) for 0.64,Itshowsneithertheorientedmosaics (flows) typicalof pitches nor the coarse mosaics typical of precursors wi 1. This suggests the presenceof oxygen in P4.As a matter of fact, t and hydrogen concentrationof P4 more like thatof asphaltene 14618 0.018), which gives the same types of mosaic. A r probable solidification (3 hours), the two bands of aliphatic and aromatic C reached the same intensity.

For the samples examined in this paragraph [lO9,l lo],the occurrence of anisotropy (LMO occurrence) is separated from solidification by tempe~~ture-time interval during which LMO size increases. Figure 46 presents the plots of FLMo versus the temperatures at which these processes take place. Values for FLMo ((0 Sr)/H)atwere measuredat LMO occurrence andat solidification. On the left hand sideof the figure are marked the various anisotropic bodies that are produced at LMO occurrence. These bodies are the Brooks and Taylor mesophase Aspheres for zero, thenA associated to increasing quantities of B and C, thenC alone, and at last the volumes with digitized contours. On the right side handare marked the final sizesof LMO at solidi~cation,i.e., oriented mosaics (bands); then coarse, medium, and fine mosaics; and finally LMO size decreasing fromClass 8 to Class 1, The data obtained the two heating rates, 0.5"C min-l and 4°C min-l, are presented. It appears that the interval between LMO occurrence and solidification decreases F, increases. The dotted lines are extrapolations for LMO sizes lower than that of Kuckersite (Class 8). They show that the interval would disappear only for close to 0.10-0.12, i.e. for LMO of Class 1, c o ~ o to n kerogens Series 111, to coals, and to indust~alprecursors of hard carbons (sacch~ose-basedproducts,phenolicresins,etc.).Howevertheinterval is so small for Class 8 that it was not detected by Villey for kerogen having LMO below that class[52-561. Nevertheless, it is expected to be detectable for smaller classes. It may be noted that increasing the heating rate changes the temper which the two processes occur bytranslation towards higher HT temperature-time interval remains constant. Figure 46 clearly shows that continuous seriesof liquid crystals are produced fromoxygen-freepitches(A240) to kerogens,givingbodiesthatvary from mesophase A spheres to volumes with digitized contours (left-hand side of the figure). Co~espondingly,the process (finalLMO size or solidification) ends with associations, which form continuous series from coarse mosaics to (right-hand side of the figure). The name mesophase was givenby Brooks and Taylor to the spherical bodies floating in pitches. The name local molecular orientation (LMO) was first given to con ti nu it^ from the volumes with digitized contours found in kerogens and coals.

subsequent evolution. at 4°Cmin-l) and Fig.46 [52-56], or specifically at LMO was determinedeither at 475*C, following Villey rrence [78,79,109,110]. In fact LMO occurs for all samples between450 and C, whichis close to 475°C. Morever, it was checked with all data of the group that the elemental composition did not change significantly between 450 and 475°C. In addition, using measured at solidification simply givesgraph of

N

log LMO versus F, that is parallel to that shown in Fig. 20 but shifted to higher FLMO. The option to measure FL,, at solidification was discarded. The final LMO size was determined either after grinding subsequent to HTT at lOOO"C, or after thin-sectioning subsequent to treatment at the solidification poin The choice of 1000°C is justified by an enhanced contrast of LMO in TEM, without change in size, since the latter is fixed at the solidification point. However, grinding was not convenient for the largest LMO sizes, since either bands or mosaics too large for the scale of TEM were broken into irregular lamellae. Thanks to the directc o m p ~ s o nbetween TEM (performedon thin-sections) and OM (same area on residual blocks after thin sectioning), the measurements by of the sizes of mosaic isochromatic domains filled the gap between lamellae and the 8 classes of LMO sizes. The extrapolation of the straight lineof Fig. 20 to F, provides a maximum value of the LMO size in the vicinity of 20 km (value for coarse mosaics). Pitches such as A240 are out of this range. As explained in SectionIV 2 c, the production of macropores indeed makes well-defined isochromatic domains disappear, so that noLMO extent could be measured(see Fig. 28b). Finally Fig.20 was and will be usedas a correlation between logLMO size and FLMO including not only the data obtainedfor kerogens, asphaltenes, and oil derivatives butalso those of coking coals discussed below (see Section VI l3 and Fig. 64) At the heating rate of rnin-l less data are available; however, the same linear relationship is obtained between log size and FLMO. The slope tends LMO todecreaseandtherepresentativepointsaredisplacedtowardslower sizes. In conclusion,it isonly in pitches, which are devoid of oxygen, that mesophase A is observed alone. It is able to coalesce into 100% anisotropic material still containinghydrocarbonsandstillplastic(mixture of and resins)before reaching the solid state (pure resins). The intermediate plastic stage is responsible for macropore development preventing formationof isochromatic domains with well-defined contours. In all other materials, oxygen is present at LMO occurrence. Whenit isin very small amount, other liquid-crystal phases and C) coexist with A. As a consequence, solidification occurs simultaneously 100% with anisotropy (pure resins); the materials finally obtained are nonporous. Increasing oxygen content leads to volumes with digitized contours (LMO of smaller size), and only micropores or nanopores develop during secondary carbonization. Except for pitch,Table 7 summarizesthedata,discussedthroughoutthis chapter,arrangedaccordingtoincreasingHTT for standardconditions(4°C min-1). Characteristic events are indicated in the first row, i.e., softening, apparent viscosity decrease and increase, solidification, and the very beginning of secondary carbonization.The second row corresponds to events as micro- and nanoscale i.e., macromolecule breakage, BSU formation, LMO occurrence, LMO growth,

c-l

and mosaic formation. Subsequent rows are related to physicochemical events (volatilization, DTA, (WC)at,IR data). The bottom of the table gives experimental data obtained at softening, LMO occurrence, and solidification.

In this section we present a global vision of all the data previously presented and discussed (see also Section IV A l d and Table 7). The materials heated at 1000°C may be depicted as follows from the macro-, to the micro-, and finally the nanoscale. Pitches give lamellae of millimeter size after grinding; these originate from coalescence of Brooks and Taylor mesophase spheres at 100%anisotropy. The orientation at long distanceis due to the actionof stress before solidification. These lamellae constitute the pore walls observed by OM without grinding. Asphaltenes andoil derivatives show lamellaethat originate from the isochromatic domains (LMO)of mosaics. These domains vary size in for coarse mosaics 5 pm, issued from A,B, and C mesophase spheres),to medium mosaics(25 pm, issued from B and C spheres), to fine mosaics 2 pm, issued from C spheres). Kerogens are isotropic inO M and give conchoidal fragments upon grinding. The TEM technique allows distinguishing8 classes of LMO sizes decreasing from above 200 mm to less than 5 nm. Coals, being mixtures of macerals, give a large diversity of textures when observed in OM: bands, mosaics of the three classes, optically isotropic material. When ground and examined in TEM, they give histograms of LMO sizes that cover lamellae and the range of 200 nm to less than 5 nm. unified view of materials obtained at different stages of carbonization or coalification may be presented in light of the widespread occurrence of BSU (stack of 2 or 3 polyaromatic moleculesof about coronene size) andof the relationship outlined above (Fig. 46) among LMO, isochromatic domainsof mosaics, and lamellae, which areall associations of oriented but vary insize. Table 7 summarizes the sequence of the main features as a function of HTT. The first step encountered in Table 7 is the macromolecule breakage (also observed in coals [93]) accompanying softening (carbon-carbon bonds are broSU are usually easily observed above a certain degree of evolution, either because they only form at that stage by association of 2-3 polyaromatic molecules or because they are too dilute to be observable. Nevertheless they may be responsible for neutron or x-ray diffraction features reported for asphaltenes [34-361. In poorly evolved materials, the BSU are distributed at random. However in nonheated low-rank coals, they exhibit a weak statistical orientation (SO)

at long distance, which is attributed to the action of mechanical stresses during geological times. When they are formed, BSU have side-chains and are dispersed in suspensive medium made of light molecules, typically aliphatic hydrocarbons (see metaplast theory of coals). The suspensive medium and side-chains confer certain molecular mobility to the system. the releaseof volatiles increase, suddenly demix, co~espondingto the maximum rate of aliphatic CH release (DTGA maximum), This is in agreement with theEact that demixtion occurs when BSU are losing part of their sidechains. At that stage aliphatic CH are still present in large amount, their concentration being higher when FLMO of the precursor is lower. Precursors with high FLMO keep larger concentrationof oxygenated functions when demixtion occurs. The size of the associations varies from few nanomete tens of microns (spheres of mesophase A). The oriente arrangements c~aracteristicof discoticliquidcrystalsp materials. They are assimilated to resins in the case of pitch. In between the columns, iso oriented SU act wedges. They carry aliphatic side-chains becoming resins. arbonization progresses, the compounds that not are removed, molecular mobility decreases and the organization is frozen (S fication). For the same heating rate, the temperature of oriented association demixtion does not vary appreciably according to the precursor (pitch, oil residue, asphalne, kerogen) and is thus not very sensitive to its chemical composition6).(Table emixtion is probably triggered by compromise involving several factors. Certain factors influence the tendency (driving force in the thermodyna~icsense) of oncentrations: of the suspe entities such ic smaller -chains or suspensive medium. Other factors influence the mobility of BSU and thus the rate of the association process: the size of the kinetic entities due to the relative concentration of side-chains and viscositydue to the amount and propertiesof the suspensive medium. In this context, it appears that breaking aliphatic chains plays crucial role:it creates smaller entities that are less solvated and more mobile, and it creates material acting a suspensive medium. When the heating rate increases, demixtion of oriented associations occurs at higher temperature,at lower viscosity, and at a stage givinglower microhardness after cooling at room temperature (Table 6, Fig. 44). Heating at higher rate increases the rate of volatiles formation, which is controlled by chemical reactivity, with respect to the rate of volatiles release, whichis controlled by transport me~hanisms(diffusion)andthuscharacterizedby loweractivationenergy, retention iles controls demixt decreasing solvation, by concentration, and by increa

in pyrolyzed pitches (spheres in oil residues (spheres A, B, C), and in tars (spheres showing specific alternate orientation features, Fig. 32) cover a wide range of properties. In particular, bodies of the same category are able to coalesce of other categories. This with each other, but are not able to coalesce with bodies indicates that they must differ by specific features: remaining aliphatic moieties, traces of heteroatoms, or size and shape of BSU (e.g., coronene, and ovalenecoronene, Fig. 6b). The question then arises to how heteroatoms influence solidi~cation.Oxygen may be involvedin ether bonds betwe thus freezing their relative motion. However noncovalent i play an important role in cross-linking BSU. interactions between coronene moleculesareLondondispersionforces. The U carryingheteroatomsallow polar interactions and are thus not ideally compatible BSU with devoid of heteroatoms.Accordingly,differentsituationsm ur illustratedbysomeexarnples. If comparableamounts of nonmiscibloccur,theywill form twotypes of bodies. As another example, if oxygenatedBSU are much less abundant than the others, they may solubilize in the latter but show a tendency to accumulate at the surface. Surface accumulationof one component may thus control the size of the association and affect strongly the ability to aggregate with other bodies. If oxygenated BSU are too abundant, they rnay form strong networkin which other BSU are distributed. consequence, associations tend to freeze almostsoon as they form. The various processes are not mutually exclusive. However, some may dominate in the caseof kerogens, while others may be more important in materials characterized by low heteroatom concentration (oil derivatives) in which oriented associations retain the properties of a liquid over larger temperature range. It is noteworthy that only certain types of sulfur have the same effect as oxygen on the size of oriented BSU associations and on the solidification temperature. It may be suggested that this is determined by the polarity of the co~esponding functions, that have cross-linking effect similarto that of oxygenated functions, Cross-linking by heteroatoms rnay thus involve covalent bonds (ether or thioether) and noncovalent interactions inherent in the presenceof certain chemical functions. It must be noted that the two typesof interactions considered here do not exclude each other; they may even act sequentially in the courseof carbonization. For instance, interaction of -OH with other polar groups or even electrons bridges are of aromatic rings 1481 rnay act at certain stages, while C formed at later stages. The above discussion demonstrates the importanceof volatile compounds, of heteroatom content, and of the spacial distribution of BSU at different scales to ~ndersta~ding and controlling the evolution and properties of carbonaceous material. Our attention will be focused next on data that address these points in more detail.

The influenceof heating rate during pyrolysis was first recognized by Ihnatowicz et al. 1891 for coals, coal-tar pitches, and mixtures of coal derivatives. After Villey (see Section A 1 a [52-561), systematic studies were u n d e ~ a k eby~ [74,149] on aspbaltenes and on atmospheric and vacuum residuesof oil tion, using heating rates of 4,8, and 25°Cmin-l as well as“flash” heating (introducing the sample into a preheated oven) under an inert gas flow (open system); see also References 109 and 110, Similar studies were later performed by onthioux 150,15 on simulationsof natural evolutionof coals andby Ayache (Purified NE70 (Torbanite), Sporopollenin site (see Table using thee heatingrates:0.013”C min-l, 4°C min-l, and “flash.” addition, thermal treatments were performedat the same heating rates in sealed glass tubes (“closed system”). F~rthermore,Ayache carried out e ~ p e ~ m e n[l521 t s in sealed gold tubes under 30 MPa pressure, in an auto~lave,with heating at 35°C n6n-l up to 500°C and 1 hour residence time, followed by additionalheatingat650°C (“confined system”). To theabove samples were added polyethylene and anthracene 152- 1 p e ~ o ~ experiments ed in““confined system” under100 350°C up to 550°C with 24 hours residence time [l50--151]. as had Villey before andas was also reported in Section A l o u ~ aobserved, t c, that LMO size was largeras the heating rate increased. Ayache and Monthioux provide more details 150- 1521. of ure 47 shows the influence pyrolysis conditions on thermal conversion a coal belongingto the ~ a h a ~ aseries m river delta in Indonesia).The dotted band is that of M ~ a k a mcoals. “Confined system9’pyrolysis provokes a loss of water and predominantly a loss of CO,, followed by a loss of by~rocarbons. In “closed system,”pyrolysis also starts with lossof CO, and H,O, followed by release of H,O and hydroc~bons.In “open system,” pyrolysis results in almost exclusively a loss of H,O and the oxygen concentration remains relatively high. In this case carbonized coal follows the van Krevelen path of saccharose [493. Only pyrolysis in “confined system” reliably simulates the natural behavior coals [150,151]. Ayache [l523 showed that, in pyrolysis of and Torbanite (NE YO), the retention of hydrocarbons increased as the system was more confined and asthe heatingrate increased, Heating rate of 4°C min-1, flash heatingin “closed system,’, then in “confined system,” approximated more and more the natural evolution. Regardlessof the conditions, the residues (even at 650°C) were plastic

0.2

0.4

7 Influence of increasing confinement conditions applied to thermal experiments of natural coalification simulation. Comparison with the natural trend. (From Ref. 150.)

and optically anisotropic. They contained unidentified spherical bodies. After pyrolysis at lOOO”C, all products were lamellar. Sporopollenin and Kuckersite (Series 11) showed similar characteristics but water loss played a more important role in hydrogen loss. After being heat treated at these materials gave lamellae in flash-heated sealed tubes and LMO down to Class 2 (5-10 nm), in “open system” with low heating rate. ForLignite,thermalconversionin “open system” with fast heatingrates resulted only in water loss, and the final samples were out of the coal band. However,therepresentativepoints of thesamplesheatedin“confined systems” between 500 and 650°C are situated in the coal band. If the different pyrolysis conditions are compared in the same range of temperatures, it appearsthatconfinementandhighheatingratesinoppositionwith “closed” then “open systems,” favor retention of hydrocarbons by maintaining hydrogen in place, whilethe oxygenated functions are removed, thus leading to a separation betweendecarboxylation-de~ydrationand hydrocarbon release. This is attributed [55,56,149,151,1521 to the balance between the production of volatiles,

governed by chemical reactions, and the rate of volatiles release transport). The latter is slowed down in confined conditions, and this effect is relatively stronger for hydrocarbons thanfor CO, and H,O. the precursor contains more oxygen (from Torbanite to Lignite), the more sensitive it to the v~iationsin operational conditions since hydrocarbon retention maxi mu^ for Lignite in confined conditions. ydrocarbon retention is an antidote to high oxygen content. It ensures a SU ~ o b i l i t yup to higherconcentrationsandleads to largeroriented BSU asso~iations.

The aim of Joseph [157was to disti~lguishthe influences of oxygenated functions inherent to the material itself from those producedby oxidation. For all samples, oxidation was performed at 200°Cin air up to 164 hours. first series g to decreasing oxygen content was constituted by Lignite, Sporokersite, A ~ A M C Oasphalt (see Table l), and Brooks and Taylor mesophase A spheres extracted by anthracene oil [98,99]. These samples were increasingly oxidized before pyrolysis for c~aracterizationat 1000°C. A second series of samples was prepared by incomplete pyrolysis of the same precursors before oxidation. They were obtained by pyrolysis at standard conditions (4°C min-l) up to well-defined stages characterized by DTA curves and elemental analysis: Sporopollenin heated to points 2,4,5, and 6 (DTCA maximum of Fig. 9) and solidification; Kuckersite pyrolyzed up to points5,6, and solidification. After o~idation,samples were pyrolyzed at 1000°C for characte~zation.Except LMO sizes, no data were available on LMO occurrence and FLMo.

2.

~ ~ e m i c a l , ~ ~ y ~ i c ~ c~ e ~ m ti ~c ~ r al l

Figure 48 presents the evolution of the first series in a van Kievelen diagram, corresponding to samples not yet carbonized, i.e., without LMO. Mesophase A spheres are also presented for comparison. From Lignite to Sporopolleni~, Kuclsersite, and asphalt, longer duration of oxidation is required to reach a stationary (HK),, ratio, whichis always closeto 0.5 (8 hours, l0 hours, 24 hours, and 76 hours, respectively). Once this value is reached, oxidation proceeds only through oxygen uptake. Lignite, whichis the most oxygenated maxim^^ [54] and maximum is quickly oxidized almost entirelydue to hydrogen and hydrocarbon loss (see also Fig.1l b and Reaction 3). From Sporopollenin to mesophase A spheres, the slope of the paths decreases progressively. This indicates that the uptakeof oxygen takes more importance relative to the removal of as pointed out (Fig. 11) in comparison between and Sporopollenin (Section A 1 d).For mesophase

Evolution paths of different precursors during oxidation in air at 200°C for different times. (From Ref. 157,)

spheres (pure uptakeof oxygen), the pathis horizontal (only(OIC), increases) and the oxidation is very slow: 0.1 is reached after 164 hours. All materials tend to the samee~d-productas a result of oxidation: a product called oxycoal by van Krevelen [S], with (H/C),, 0.25 and 0.35, and called oxychar by Joseph 157-1591 with a composition (HQat 0.5. During primary carbonization of increasingly oxidized samples, softening is reduced or disappears while the softening point increases. The DTGA maximum is reduced and occursat lower temperature,as does so~idi~cation, and LMO sizes decrease, except in the case of mesophase spheres whereLMO is already present. After pyrolysisat 1OOO"C, the weightloss decreases from Lignite to asphalt and is replaced by a gain for mesophase A spheres. Infrared data were obtainedon asphalt, Kuckersite, and~poropolleninonly. oxidation time increases, the aliphatic C groups decrease (Fig. 49a) but always remain higher for asphalt. The C=O groups (KlTlo)increase noticeably, the final concentration re~ainingmuch lower for asphalt (Fig, 49b). A comp~isonwas made between(OK), and as in Fig. 8d [52--58].With increasing oxidation, the ether groups and/or OH concentration increases tremendously for asphalt, whereas it decreases for Sporopollenin and Kuckersite. In summary, progressive oxidation gives the same textural and physicochemical data as an increase in oxygen concentration in the starting material.

4

b: K,,,,

B

Infrareddataplottedversustime of oxidation. a: (From Ref. 15'7.)

(aliphatic

groups).

~onsidernow pyrolysis residues (second series of samples defined in Sec. proce~~re). If their representative points happen to coincide withof one the of Fig. 48, their oxidation pathssupe~mposeto those shown in Fig.48 and they end at the sameL M size at 1000°C an example,S~oropolleni~ heat treated at cupies in Fig. 48 a point situated on the oxidation path of sequent oxidation follows the path of oxidation of n the same ~anner, ~uckersite heat treated at its ~ a x i m and u ~ Sporopollenin heat treated little above its ~aximum have r~presentativepoints

situated on the oxidation pathof asphalt in Fig. 48. Upon oxidation, both follow LMO is theoxidationpathofasphalt. If producthavingalreadyreached oxidized, LMO size does not change. These observations confirm the trends observed by Villey and illustrated by Fig. 11 [55,56]. A change in oxygen concentration by oxidation has the same effect the natural variation of oxygen concentration in the precursor. This indicates that the endogeneous oxygenated functions inherent to the precursor and those formed by oxidation have the same cross-linking effect and influence LMO size in the same manner. WhenorientedBSUassociationsarepresentbeforeoxidation(mesophase spheres), oxidation is very slow and the LMO size obtained after subsequent c~bonizationis not affected. This indicates that oxidation at low temperature (200°C) does not disturb the existing oriented associations. Oxidation and crosslinking by the new o~ygenatedfunctions during subsequent carboni~ationare probably restrictedto grain boundaries, was brought to mind by which mentions the occurrence of stable but inactive oxygen in petroleum precursor.

~ ~ o c e d ~ ~ e ydroliquefaction is one of the methods used to obtain liquid products from coals. To achieve hydroliquefaction, high hydrogen pressureis applied at 400-500°C to a load of coal dispersed in solvent mixed with catalyst 160,161]. The socalled solvent is hydrogen donor. In this paragraph the references given [162, 1631 are chosen so to help clarify the respective influences of hydrogen and oxygen. Among the coals studied (see Table 8) are low-rank coal (~ardanne), h i g h - r a ~coal (Escarpelles, semi-anthracite), and an intermediate The end-product of hydroli~uefactionwill be referred to here H tained are at their bestif their potential is ~ a x i m u m(liquid-li~e considering the data previously discussed in this Id betheir LMO sizeaftercarbonization at 10 should increase the oil potential increases.The work of Vogt new concept 162,1631. e increases with hydroli~uefactionef~ciency. Figure 50 shows that L e (oriented mosaic) is obtained by ~ r o c e s ~ i n g th Freyming coal, maxi about450°Cwith a cat out catalyst, but atalmostthesame ture, only medium rnosai ned, corresponding to less suitable 400'6, the results are the same, with or without catalyst, providing us no

Elemental Analyses and Fractionation Data Sample Low rank coals Gardanne Merlebach Freyming Ste Fontaine Coking coals Oaky Creek Meadow River Peak Downs German Creek Norwich Park High rank coals Lens Natural semicokes Oignies Escarpelles Refs.

y resins

H/C

resins

Various Coals Mode histogram LMO sizes

0.80 0.72 0.75 0.80

0.225 0.104 0.1 10 0.080

7 0.4 0.3 S .6

26 25 30 26

cl. cl. 1 cl. 2 cl.

0.78 0.70 0.75 0.7 0.62

0.046 0.040 0.039 0.030 0.029

0' 0.6 0.6 0.9

54 58 69 48

cl. cl. cl. cl. cl.

0.60

0.029

0.8

S7

cl. 10

0.58 0.45

0.027 0.024

0.6 0

0 0

cl. 2 cl. 2

10 S0 10 S0 S0

and

of Class Hydroliquefaction at 250°C gives LMQ of Class 1-2, which is the same that of the initial coal.As the liquefaction abilityof coal increases, i.e., thebetterits HHP, there is anincreasein size of itsHHPrelative to carbonized coal. In the case of inter~ediate-rankcoal such Freyming, the process is optimum (HHP are liquid products); oxygen is almost entirely withdrawn CO, (Reaction 2 and Fig.1l bj and (H/Cj,,is maintained. In the case of low-rank coal such Cardanne, deoxygenation is much lower and part of the hydrogen is removed with oxygen H,O (Fig. 1l b and Reaction5j; the processis less efficient. When the rank of coal is high enough (Escarpellesj,it isinsensitive to liquefaction since natural LMQ is already present,As result, coalsof intermediate rank are most suitable for hydroliquefaction. The previous data show that the ideal hydroliquefaction of coals should be complete deoxygenation, constant value of (HE>,, being maintained. Hydroliquefaction of coals often called hydrogenation, which may be misleading the (HIC), of the coal is maintained constant. In additio~,all the coals able to be hydrolique~edhave constant (WC),, of 0.7-0.8 (see the horizontal partof Fig. 13); they are thus potentially equivalent in oil yield, though they produce HHP of quite different qualities.The key is the conservationof hydrogen

~ r i e n mosaic t ~ (see Fig

coarse

(see Fig

Fig

e

(see Fig

1000"

of

and deoxygenationby decarboxylation (optimum hydroliquefaction),as opposed to water formation (poorest results). This is in quite close agreement with the data of Villey relative to pressure [52-561 and thoseof Monthioux 150,15 and Ayache 1521, relative to changes in heating rate and pressure (see Fig. 11 and 47), as regards the oil potential and hydrocarbon retention. The mechanism commonly adopted for hydroliquefaction coals involves free radicals 160,1631.The solvent is a hydrogen donor which transfers hydrogen to coal so as to saturate the free radicals produced by the ruptureof bonds. This action of hydrogen is evidenced by replacing it with nitrogen 1641, which leads to the f o ~ a t i o n a solid coke instead a liquid product.The solvent recovers its hydrogen through the catalyst action. The authors of the present paper consider

are being saturated by hydrogen as aromatic CH groups and acquire ~obilitythattheydidnothaveincoal(becausetheywere cross-1inlCedby oxygenated functions). The total hydrogen content is constant from the coal to the P produced, but an internal redistribution is made due to the removal of oxygen asCO, giving aromatic CH fixed upon and aliphatic CH providing the suspensive medium.

l. eavyoilsandresiduesarenotsuitable for usewithoutbeingupgraded by decreasing their viscosity so as to obtain lighter products that are often further distilled to produce gasoline or gas oil. The catalytic hydroconversion of heavy oils or heavy derivativesof oils (such as AR or VR) aims to provide thelightest possibleliquidproductin increasin~amounts(technicallycalledrecipe)and decrease or even eliminate a carbonaceous residue that is formed during the process of hydroconversion [l65]. This residue is insoluble in toluene but is plastic, so that its technical name,“ c o ~ c ~is, improper. ’~ Itis harmful sinceit tends to obstruct the pipes. The feedstock studied here 166-1681 is a vacuum residue of a heavy oil see Table l). It was placed under a7.5 Pa hydrogen pressure and ange 400-500°C with a catalyst dispersed in the load, homogeneous dispersion was obtainedby thermal decomposition of a ~olybdenumsalt S, undertheform of smallstacks,oneortwolayersthick mpleswere e x a ~ i n e dwithoutextraction, i.e., intheir isotropic matrix. are liquid, it is necessary to heat treat them up to thebrittle was chosen). Since the“coke” is not liquid but plastic,L, can be observed directly on thin or polished sections without pyrolysis. The aim of an ideal hydroliquefaction process would be to transfer the total amount of the hydrogen fromthe heavy feedstock to a lighter liquid product, the in the ga dehydrogenated residue (66coke77) given in preceding sectiolls have shown that L, the oilpotential. The size of therecipeshouldthus be the largest whereas, simultaneously, that of the residue should be the smallest. The increasing quality of conversion is thus necessarily expressed by a coxnparison between recipes and cokes. The former should tend to larger L, smaller to ones. lat 52 presentshistograms of sizes of therecipes as comparedtothe

Distribution MoS, stacks in the residual “coke” of Safaniya-VR hydroconverted with Nap-Mo catalyst. Inset, SAD pattern. Ref. 166.)

initial product (Fig. 52a).The histograms corresponding to recipesof increasing quality obtained with catalystsof increasing efficiency are given in Fig. 52b to d. In Fig. the mode the LMO size histogram is medium mosaic. In Fig. 52b, converted without catalyst, the modeis oriented mosaic (bands 5 pm in width called fine fibers, FF), similar to e~hibited that by A240 (see Fig. In Fig.52c and d the modeis displaced to larger bands (width 10 pm called then coarse [FG] fibers), the catalyst goesfrom phosphomolybd then molybde~umnaphthenate increases from 1.4 in theiniti decreases from 4.0 to 1.36 wt%. hydro coke decreases drastically from 1 the products (recipes) impro sizes decreasein the cokes observed

LMO size histograms of Safaniya-VR after pyrolysis at 800°C (OM data). a: initial product. recipe obtained after hydroconversion withoutcatalyst. c and d:recipe after hydroconversion with catalysts of improved efficiency. frequency; isotropic; MF: fine mosaic; MG:coarse mosaic; FM, and FG: oriented mosaic (bandsof increasing thickness). (From Ref. 166.)

is not a solid but it is sufficiently evolved to provide x-ray diffraction diagrams characteristic of turbostratic carbon, i.e., with a sharp 002 ring at0.344 nm for the material converted without catalyst. The dfooz becomes wider and displaced to larger average values as conversion improves, i.e., as size of the coke decreases. After conversion in the absence of catalyst, the coke is a mixture of coarse and medium mosaic islands as major components.The minor componentis made of mesophase A spheres (2-10 pm), single or coalesced, associated to the B and C mesophase spheres already present in Safaniya-VR,as in Arabian light, (see Fig.37) dispersed in the residual matrix. With PMA, “coke” the is only made small mesophase spheres (2-15 pm), single or coalesced inside an isotropic matrix. The coke obtained from Nap-Mo conversion is optically isotropic. In order to describe the optically isotropic areasof the cokes more precisely,

TEM 002DF of the “coke” issued from hydroconversionof Safaniya-VR with PMA catalyst. Anisotropic domains of coalesced rnesophase A are associated to single and to isotropic flocks (double arrows). (From drops of isotropic material (single Ref. 166.)

the “cokes” were examined by TEM.In the caseof conversion without catalyst, the mosaic islands are embedded in a residual isotropic matrix. In the case of conversion with PMA catalyst (Fig. the optically isotropic matrixis reduced to anisotropic domains and an isotropic material made of liquid-like drops (single arrow) and a few flocks (double mow). These flocks are similar to the material illustrated in Fig.5 1 at a highermagni~cationand are devoidof residual isotropic medium as inFig. 51. The liquid-likedropsaredrops of recipeaccidentally trapped in the coke. The optimum conversion, using Nap-Mo, providescoke a completely madeof the isotropic flocks described in Fig. 51 and devoid of liquid-like drops. The results obtained by FTIR (Fourier Transform IR spectroscopy) on the cokes are presented in Fig. 54 for Safaniya-VR converted without catalyst, with PMA, and with Nap-Mo. Aromatic groups continuously decrease along this sequence, as well as aliphatic groups, except for the coke issued from

Ham

l

Relative areas of FTIR bands for the “cokes” (TI fractions) issued from hydroconversion of Safaniya-VR without catalyst and with catalysts of increasin~efficiency a: aromatic CH groups. b: aliphatic CH groups. (From Ref. 166.)

In this case,the increase in aliphatic is an effectdue to the light fractions that are physically retainedby the coke (isotropic drops evidencedby TEM in Fig. 53 are found only in this sample). The continuous decrease of both aromatic and groups, maxi mu^ for showsthatthecoke is moreand more strongly dehy~ogenated. he best recipes obtained with the more efficient catalyst (Nap-MO) have sizes larger than the initial load (Safaniya-~R)and a higher hydrogen content. Their and their hydrogen content are much larger than even those the only (WC),, and bands thinner than pitches. As an example, A240 has while in the recipe obtained with Nap-Mo, (WC),, 1.6 and bandsare larger than 10 The FLMO is 0 for A240 and 0.007 for Safaniya-~R. The “cokes” associated with the best recipes have such small sizes that l a the r l y case of the material illusthey are not eas sur ab le in O O ~ ~ F , p ~ i c u in trated in Fig. 5 1. They are almost mpletely dehydrogenated (loss of aliphatic and aroma ti^ groups) so that th SU form free radicals (ESR data) 167,1681. A balance is thus established between the“cokes” and the recipes. gen content of the form decreases, that of the latter increases. In thesame er,thedecrease of LM sizesinthe 66cokes9’ strictlycorrespondstothe limit of the hyd~oconversionis, on the nsive m e ~ uricher ~ , in hydrogen combined in so~ethingalmost solid

andard conditions), is ~ y ~ r o c o n v e ~ e ~ ,

volatiles are not released but a segregation is produced, leading to the recipe, characterized by a high (HIC),, and the “coke,” which is progressively more dehydrogenated. The main feature of hydroconversion, i.e., segregation, may be understood in light of the discussionof Section IVE. Due to thecon~nement(high hydrogen pressure), the escape of volatiles is suppressed. This maintains a large amount of suspensivemediumunderconditions (400-500°C) whichare, as demonstrated with other materials, favorableto BSU association. This leads to a mixture of light compounds (recipe) and of particles constituted of associated U (coke). The role of the catalyst would be to accelerate bond breakage and o~binationand possiblyto make dehydrogenation more selective, favoring the formation of molecules of given characteristics (size, aliphaticIaro~aticratio, thorough dehydrogenation of polyarornatic molecules and is characterized by A striking observationis that the coke though it formed in the presence of a large amountof suspe material is practically devoidof oxygen. This may be explained in different ways. 1, The conditions may be such that U lackuniformity,covering ss, size, shape) comparedto limited) range of characteristics under standard conditions. 2. Oxygen, present in traces in the whe material, might accu~ulatein the coke e to preferential segregation of SU carrying oxygenated functions. e to the low viscosity, the free dicals generated by chain breakage and low concentrations might reco~binequickly enough, so that the U cross-linkingwouldbereciablewithrespecttotherate of of kinetically independe

a~ternativeto crossAccordingly,cross-linking by C-C bondswouldbean linking by heteroato~s. a matter of fact, C-C bond formation between units is a main feature of secondary carbonization. These three speculativee~planationsare not ~ ~ t ~ ~exclusive. a l l y They may be asso~iationswill be ordered over shorter summ~izedas follows. The distances if they are more dissi from one to the other if they are c functions responsible localized i~teractionsbetween each other (polar ‘cal bonds). In contrast to localized interactions, Lon eaker and extend over long ~istances(10 nm), thus easier, The two explanations are consistent wi potential attributed to owever, the third explanation br of FLMo. all the systemsdiscusse~before, ted an increase in the concentration cross-lin~ers(heteroatoms) andlor a depletio~of the suspensive mediu d h r side c ~ ~ i(loss ns of ~obility),In the case of the decrease reflects dehydrogenation of another form of cross-lin~ing

Hydroconversion of oil is a high temperature process (44OoC), which amounts to separating a mixtureof and y resins (the load)into y resins (the recipe), on the one hand, and resins (“c~oite”),on the other. The preparation of anisotropic pitches amounts to obtaining resins from mixture of and y resins. Their great interest lies in their ability to provide high-p~rformancefibers [20]. Available precursors are either petroleum pitches such as A240 or secondary coal-tar pitches.The former are too light and the latter too heavyfor direct use. In first step, petroleum pitch is made heavier by partial carbonization eventually accelerated by vacuum and coal-tar pitches are made lighter by “hydrogenation” 169,l 701. In second step, two processes are employed: either disruption of the course of c~bonizationor fractionation by a series of solvents, so as to recover only resins.

Process (a.) ~ h y ~ i c o c ~ ~ ~ i c a In l l977 and1978Singer [l711 madethefirst mention of a pitch (acenaphthylene pitch) able to be spun because of its suitable viscosity associated with anisotropy 172,1731. After the first Union Carbide patents 174,1751,in1978ChwastiakandLewis 1761 obtained “particular mesophase pitches” from isotropic ethylene-tar pitch andpetroleum pitch, strongly sparged by bubbling nitrogen under pressure during carbonization (instead of being preparedby “simple heat-tr~atment,,). The percentageof toluene-, pyridine- and quinoline-solublefractions obtained at different degreesof carbonization was plotted versus the percentage of anisotropy qualified as mesophase content (Fig. 55). Three different fractions of increasing molecular mass were obtainedassoon carbonizationbegan 20%anisotropy).At100%anisotropy, the petroleum pitch contained a mixture of QI, PI, and TI, with 30% and 30% QI. Rather su~risingly,the conclusion was that mesophase content was not following a single solubility criterion. The same opinion was expressed by Lewis and Lewis 1771. Assimilation between denominationsof a complex phase and of aclass of solubility confusing, since it is knownthatsolventsof increasing strength [92,93] extract different compounds having different chemical compositions and molecular masses(see Section IV C 3a). In the case of Figure 55, at 100% anisotropy, the“mesophase” designates a material containing simultaneously 30%of resins, 40%of resins, and 30% of y resins, This is the reason the term mesophase or liquid crystal employed alone does not specify precisely a material and should be avoided if other textural and chemical characteristics are missing. The term anisotropic pitch has a more specific meaning and shouldbe preferred.

Solubilitiesin toluene, pyridine, and quinoline at different carbonization degrees of “particular mesophase pitch” (gas-sparge) plotted versus anisotropy percentage (cumulative diagram). (From Ref. 176.)

The termmesophase is misleadingeven for materialscarbonizedbyheat treatment without sparging. In A240 pitch, initially isotropic and toluene soluble (100% y resins), only mesophase A spheres are produced. Before solidification, 100%anisotropy is reached with a noticeable content of resins. However, if the precursors contain a very small amount of oxygen, 100% anisotropy coincides with solidi~cationand 100% resins, but mesophaseA isaccompanied by other liquids crystals and C [109,110]. In 1988 Greinke and Singer [94] stirred A240 pitch by nitrogen bubbling in a reactor at 4OO0C, They followed the progress of anisotropy versus time during increasing carbonization, obtaining three samples after8, 11, and 15 hours. They separated the100%anisotropic component called “mesophase”from the residual isotropic fractionby high-temperature centrifugation 3781, providing three products: the “mesophase” fraction, the isotropic fraction, and the whole pitch. The pyridine-insoluble (PI) percentage was measured in all samples and found to be systematically constant at 55% in the “mesophase” fractions. However, PI percentage increased with time in the isotropic component, from 16 to 29 and 33

wt%. The short side-chain protons (measured by NMR) were almost constant, about23-27%,in the threefractions at all times. The molecular was determined after reductive alkylation and “solubilization” the “ m e ~ ~ p h a s e ’ ~ fraction in trichlorobenzene. Artifacts cannot be excluded using such treatments (see SectionIV C 3 a and [33,125]) but interesting results were obtained and these must be discussed here. The PI content could not be identified with “mesophase9’ content, with respect to which it was always lower (see Reference 176). The number-average molecular was constant and equal to about 900 in the socalled mesophasefraction, whereas as.time increased it increased from above 700 to about 900 in the isotropicfraction.Figure 56 showsthemolecular-mass distribution in the three fractions after 15 hours heat treatment 64% of “mesophase”). Thisfigure shows that the three fra~tionshave the same distribution

l200

~olecular distribution of an anisotropicpitch (A24.0, gas-sparge for 15h at 400°C) after fractionation. “mesophase fraction.’’ isotropic fraction. c: pitch. (From Ref. 94.)

and thus contain molecules of similar sizes. The same feature was observed in coals 123,1241. The dominant molecular masses are600 and 900 amu and only the proportions of each size vary between the various fractions. The authors thus arrived at an apparentlycontradictoryconclusionwithrespectto the largely accepted conceptof polyme~~ation during pitch carbonization 179,1SO], Greinke and Singer [94] tried to eliminate this discrepancy by a s s u ~ n gthat reactive molecules able to polymerize were those inthe range 400 to 1100 arnu and that they were progressively transferred from the isotropic phase to mesophase by the stirring of the heated pitch. Some Union Carbide patents on gas sparging are given in References 18 1841. of resins mixed with resins In Section IVD 2, we considered the presence in 100% anisotropicproducts.The data givenhereconfirmandclarifythis assumption since PI fractions necessarily include QS-TI fractions, i.e., resins and, at least in part, resins. A very simple explanation may thusbe offered for the observationsof Greinke and Singer by considering BSU, possiblycarrying short side-chains (protons detected by The BSU are stable units able to associate into various t~ee-dimensionalanisotropic bodies (LMO) more or less labile (either or resins) becausethere is no lateral coherence between liquid crystals (columnar ordering). The columns were assimilatedresins to and the ~ s o r i e n t e dBSU to resins (see Section C 2, [102,103]) reactive moleculesof Greinke and Singer could thus be identified arnu is the molecular massof coronene dimer,900 arnu is that of c According to this new concept, which rules out the concept of poly~erization (formation of covalent bonds), possible redispersion of anisotropic bodies into single BSU may be envisaged. (b.) Gas-spargepitchesnowavailablearealmostentirely TI, i.e., resins. They are thermotropic. The sample described (Pitch G) composition of 93% resins and 7% resins 185- 1871, In optical microscopy between crossed polarizers, the pitch is 100% anisotropic with flow orientations (oriented mosaic-like).The phase shift specificto this pitch was found to be 120150 nm instead of near 234 nm for mesophase spheres [20]. The measure~ents were then performed independently with or without addition of tint plate. The pitch was found to havephase shiftof 160-200 nm and the mesophase shift was nm (unpublished data). These values corroborate the phase shift found er gas-sparge pitches1881.Fig. 5?b, c and d correspond to 002D exploration of the 002 ring. On the one hand, very poor p o~entationof the pitchis evidenced (misorientation 60"). On the other hand, to the major anisotropic constituent are associated anisotropic droplets specific to this process (Fig. 57a) [185,186,188,189]. They are so weakly anisotropic that on doubt could be expressed. Therefore, orthogonal 002 F images were collected thin section of anisotropic pitch containing droplets.The section was deposited upon a holey carbon film to obtain reference blank [20]. The images were scanned along a line AB, from the blank (A), through droplet, through the

Thin section a gas-sparge anisotropic pitch; TEM 002DF images. (From Bonnamy). a: overall view,the specific feature the occurrence of droplets polydispersed in size. b, c, and d: images with 60", and 90" rotation the objective aperture showing the poor preferred orientation the pitch matrix and thedroplets anisotropy; the aromatic layers are sketched by a double bar.

Gray range graduations corresponding to linear record AB TEM 002DF images gas-sparge pitch deposited on holey carbon film. Meas~rement major component and droplets anisotropy. (From Ref. 103.)

anisotropic component, through another droplet, then again through the anisotropic component(B). In Fig.58, the gray range used as a scale is divided into 253 grad~ationsreported on the ordinate. The blank is 226-230. In Fig. 58a, the anisotropic component corresponds to30 and the droplets to50. In Fig. 58b, the anisotropic component corresponds to 100, the droplets to75. Therefore, the anisotropy the droplets is 25 versusfor the anisotropic component. The anisotropy of the dropletis thus clearly demonstrated and found to be one third of the major anisotropic constituent. The preferred orientation planes in the droplet and in the matrix seem to be parallel. Major differences between gas-sparge pitches and isotropic pitches treated by simple heat treatment are, on the one hand, the lower optical phase shift of the former relative to mesophase and, on the other hand, the very poor preferred

orientation. A different phase shift is found for gas-sparge pitches and mesophase, since they are respectively and resins. The lower value foundfor gas-sparge pitches is due to their very poor preferred orientation andor to the different functional groups fixed on the BSU edges. Since the gas-sparge pitches have no common character with mesophases A,B or C, they will be called anisotropic pitches. The improper nameof mesophase or mesogenic pitchesstill given to them in recent papers comes from their anisotrop taking the external appearance of mosaics. (C.) Hot-Stage Car~onization. Hot stage video sequences [ l 891 allow one to understandthedevelopment of thegas-spargeprocess (Fig 59).Duringthe experiment, bubblesof nitrogen are introduced through a syringe, undera pressure of 0.2 MPa,into A240 pitch heated ina hot stage chamber.The temperature was chosen to produce single spheres of mesophase A. In Fig. 59a, the nitrogen bubb (arrow) is just introduced neara sphere of mesophase (double arrow). In Fig. 59b, the sphere is so strongly attractedby the expanding bubble thatit spreads around the bubble, forming an anisotropic deposit (Fig, 59c). A crown around the bubble becomes depletedof spheres, particularlyof the largest ones. The anisotropic shell thickens simultaneously with the bubble expansion (Fig. 59d) accompanyingits turbulent displacement. In Fig.59e, the system blows up and the shell breaks into fragments. Immediately after this failure, a large area of anisotropic pitch (Fig. 59f) is produced. It contains droplets (arrow in Fig.59f) associated with a major constituent as in other gas-sparge pitches. . video sequence provides a model that explains the mech(d.) ~ i s c ~ s s i o n The anism of gas-sparge pitchf o ~ a t i o nThe . mesophase spheresA consist of columnar arrangements of BSU associated with defective BSU at random orientation and distribution. This amounts to a mixture of a heavy component (the columns, i.e., resins, up to 6000 amu) and a lighter one (single BSU, i.e., resins, 600900 arnu), accountingfor the molecular mass distribution. In contact with the gas, BSU tend to adsorb on the bubble surface and high shear is exerted, as described in SectionIV C 2 c, perturbingtheBSUordering.Bubbleexplosiondisperses intimately the BSU of the mesophase A sphere in the residual pitch. Stirring makes the material homogeneous andresins are obtainedi ~ ~ nIt eis,interesting to remark how well the resultsof Mochida et al. [33,125], Chwastiak and Lewis [1’76],andGreinkeand Singer E941 (seeFig. 55 and 56) areexplainedand reproduced. Their hypothesis that molecules of similar sizes are c o ~ m o nto all pitches but in changing proportions may be adaptedby replacing the concept of molecules by that of BSU,whichmayassociate.Columnararrangements resins),typical of mesophase A spheresaredestroyed, so thatorientedBSU association is dispersed, leadingto an increase in resin content and providinga 600-900 amu gas-sparge pitch (coronene dimers and trimers). It also becomes obvious now that different conditions of stirring could leadto different products, from weakly anisotropic droplets of various sizes (15 nm to

pitch.

Part video sequence in bot-stage Ref. 189.)

showing the formation of gas-sparge

1 to heterogeneous pitches containing intact rnesophase spheres floating in isotropic ~ a t r i x188-1891. AT1 kinds gas-sparge pitches are spinnable.

2. ~ r ~ ~ ~ i o ~ ~ t i o ~ ~ i e f e n and ~ o Riggs ~ 190- 1941 fractionated partially carbonized petroleum pitch to obtainonly p resins, which were then heat treated under stress.The S-TI products resins) with a narrow molecularmass ~ p e c t ~ m (800-900 amu) named neomesophase.The products obtained after heat treatment

002DF image of tially carbonized petroleum pitch.

anisotropic pitch obtained by fractionation Ref. 195.)

a par-

under stress are 100% anisotropic (flow orientations) as can be expected for resins. In general, anisotropic pitches obtained by fractionation are very rich in disclinations (Fig.60), like bandsof A240 pitch, and no precise limit can be assigned to isoc~omaticdomains 109,110,1951. are gas-sparge pitches, pitches obtained by fractionation are thermotropic, i.e., isotropic during heating and revers ibly r e t u ~ i n gto anisotropy during cooling. They are thus spinnable and ableto achieve fibrous orientation. Riggs and Diefendorf established a colloidal ofmode fractionated pitch 1941, postulating a spherical micelle (Fig. 61) with a threedimensional gradient in the concentration of various species.

Coking coals (see Section A 2) are medium-rank coals occupying the region following the maximum decarboxylation (see the change of slope in Fig.13).As a matter of fact, the specific featureof coking coals is their ability to give suitable blast furnace cokes1961. Among all softening coals, the best coking coals shou have oxygen below 5 wt%, residual volatile matter at 20-25%, only a medium fluidity(usuallymeasuredbytherate of rotation of theGieslerplastometer [48,196])and,asthemaincondition,aminimumswellingunderheatingto prevent damaging the walls of the coking oven [48,89,196].

Micellar model for an anisotropic pitch obtained by fractionatio~.(From Ref. 194.)

During carbonization and industrial coke making, coking coals develop optically anisotropic bodies [SS]. In addition, opposed to other coals almost entirely made of resins, they contain large amounts of extractable material originating from their major maceral, the vitrinite.

~he~~cal Table 9 presents coals of increasing rank, including typical coking coals (solid line frame) Mac Clure, Peak Rowns, and Dourges, while other good coking coals Oaky Creek, Meadow River, and German Creek are given in Table 8. Between 1966 andlthe present, the available number of colsing coals decreased tremendously since themineswereprogressivelydepleted.Correspondingly9 coals of lower quality were considered and used either alonewith additives.In Table 9 the limits coking coals were thus extended to Camphausen and Water for low-rank coals, and Norwich Park for high-rank coals [197,198]. ever, none of them corresponds to the re~uirementslisted above. InTable 9 aregivensuccessivelytheoxygenpercentage,that of residual volatile matter VM), tbe temperature T,of maxim~~m fluidity (roughly correocc~~rrence)9 the temperatureT, where the plastometer rotation ceases (correspondi~gveryroughlyto solidi~cation),thecontentin resins determined by solvent fractionation [49,197-1991,well the computed values of FLMO. The last four columns give textural data. Tables 8 and 9 show that the

r-

d.

optimum coking properties are those of German Creek and Mac Clure, which contain the maximum amount resins (67-69%). A part resins is detected in TEM imagesintheform of liqu S identical tothosepresentedin detailinFig.35(seeSectionIV C 4e140)andalso identical tothe resinsdescribedin the cake afterp 1421. The resinswerepostulated [49,61,199] to be responsible for the peculiar properties coking coals by increasing the available amount suspensive medium.If Dourges (a good coking coal) is considered [SS],since its Q1 content is much smaller than 70% 45O0C, at it could be inferred that its initial resin contentis much higher than 30% (see Fig. 1 inReference89).It is worthwhiletonotethat,inadditiontocokingcoals, Ihnatowicz et al. [SS] presented datafor numerous materials with oxygen content increasing from (O/C), 0.008 (“Brai HT,” see Table 1) up to 0.09(“Brai BT”). In all cases, it was demonstrated that total insolubility in quinoline coincided with solidification. It was also shown that increasing the heating rate raised the solidification temperature.

~ e ~ t uData ~al Some authors observed the occurrenceof anisotropic bodies during thermal conversion of coking coals [98,135,196]. Theyidenti~edthese bodies as mesophase A without checkingfor the presence of PAN-AM textures. Ihnatowicz et al. C891 observed anisotropic spherical bodies in Dourges (Fig. 62a),at the most 200-300 nm in size. They are entirely bright, entirely gray or entirely dark in 002DE Obviously, following the data givenin Section VI 109,1101, they are not mesophase A but mesophase B or more probably C because theirsize is identical to that of asphaltene14618(Fig.62b).Camphausen,representedinFig. 63, is not mesophase A, B, or C. In this case, spherical bodies are replaced by digitized contours (Fig.63) comparable to kerogens. However, this coal is heterogenous SO that the upper partof Fig. 63 is comparable to Kuckersite (Fig.4lb). The bottom is similar to Class (50-100 nm). Correspondingly LMO size is difficult to estimate. In more recent papersby Fortin 197,1981, the micrographs given shownthat mesophase A with its characteristic PAN-AM texture is never found. Only spherical bodies which may be identified asB C bodies are found in Mac Clure. In Camphausen, only volumes with digitized contours are observed. We have attempted to evaluate F, from the data given by Ihnatowicz et al. [SS] and BensaYd [48,62] and compared themto those obtained more recentlyby Fortin 19’7, 1981for six coals (Merlebach, Black Water, Camphausen, Mac Clure, Peak Downs, and Norwich Park), among which only two are typical coking coals (Table 9). The data of Fortin were exploited by adopting T2 (maximum fluidity measured by Gieseler plastometer) as the temperatureof LMO occurrence, so as was then compared to the average LMO size after to obtain F,,o. The F, heating at 1000°C (last column). In the caseof Dourges [SS], only LMO sizes at LMO occurrence are available from 62a. Fig. However it is possible to extrapolate the final size LMO at 1000°C by considering that spheres B provide medium

002DF images of B anisotropicbodies at LMO occurrence. a: coking coal Dourges (430"C, 0.5"Cmin-') at occurrence (from Ref. 89). b: Safaniya-~R min-1).(From Ref. 78.)

109,l 101. It is thus assumed in Table 9 that mosaics andC fine mosaics 2 values above l pm are plausible. If the physicochemical requirements given for coking coals are considered, Table 9 shows that Black Water and Camphausen cannot be included in the coki coal series because they are too rich in oxygen and volatile nutter. Moreover Black Water and Norwich Park are not fluid enough l971 Black Water and Camphausen have to be discarded because their LMO are too small (200 nm or less, i.e., below the optical microscope resolution) bringing them closer to low-rank coals.

3. ~ ~ s ~ ~ s s ~ o n The comprehensive study of key materials (Section D 109,l 101) now provides a clear understanding of all the data, i n c ~ ~ o r a t i those n g of Ihnatowicz et al. and Fortin [197,198]. I ~ t e ~ r e t a t i oisnfocused on the role of oxygen in the coincidence between 100% anisotropy, 100% resins, and solidification. the oxygen content increases, at first mesophases B, and C are found, then mesophases B and C, then mesophasesC alone, then bodies with digitized contours, at the exclusionof mesophase alone. a matterof fact, mesophase spheres are never produced in coking coals, as could be expected from their nonnegligible oxygen content. Therefore, coking coals join the series of asphaltenes and oil derivatives dueto the resins. The latter acts as a dispersing agent for resins and counteracts cross-linking which is found to dominate in other coals.

002DF image of coking coal Camphausen (500°C, volumes with digitized contours. (From Ref. 89.)

min") showing

The composition reachedby the best coking coals at LMO occurrence (FLMO) would thus approach thatof asphaltenes and oil derivatives, with similar LMO final size after carbonization, whereas the others would be comparable to kerogens. Figure64 gathers allthe data Fig. 20, with the same symbols, and shows that the linear relationship found between log LMO size and FLMO also to applies coals and coking coals (open circles). The best ones range in the vicinity of asphaltenes (Mac Clure, Peak Downs and even Dourges, with FLMOcalculated from elemental analysisof the precursor coal). The other ones (Camphausen and Black Water) approach kerogens with decreasing performances.

This introduction presents the main features of colloidal systems in order to allow discussion of their relevance to carbonaceous materials. ore details can be eferences 200-206.

erlin et

l14

M O size 1000°C 20 pm

lOpm 5 Pm

Pm

500nm

100nm 50nm

10nm 5nm

Heating rate 4OC. FLMO

H 0

0.05

0.10

64 size plotted versus FLMo.Crosses are the data obtained for kerogens, asphaltenes,and oil derivatives in standard conditions and black stars are the data obtained in Section IV D (see Fig. 20).Empty circles are the data obtained for some coals and coking coals.

~e~~ition colloidal system is made of particles having a size in the range of 1 nm to typically 1 pm. Two main features distinguish such particles from small molecules, on the one hand, and from macroscopic bodies, on the other hand: the area of contact with the medium and the incidence of Brownian motion when the medium is a fluid. Comparedtosmallmolecules,thearea of contactwiththemedium, for instance a solvent, is smaller. Consequently, on a mass basis, the enthalpy of interaction with the environment is smaller and the entropy due to translational motion is also smaller. Consequently different states of organization differ only slightlyinterms of free energyperunitmassandnumerousstatesmaybe encountered. Cornpared to macroscopic bodies,a colloidal particle still interacts with an appreciable volume fraction of the surrounding medium. Particles of 1 nm and 1 pm developsurfaceareasthatare of theorder of 1000 and 1 m2gm1 thata surface respectively, depending on shape and specific weight. it is realized If influences its environ~entover a distance which is in the range 1 to 00 nm, it appearsthatthevolumeinfluencedby a colloidalparticle is an appreciable fraction of and may be much larger than the volume of the particle itself. Random displacementof particles, i.e., Brownian motion,is a direct manifestation of translational entropy. Itis responsible for diffusional transport, the diffusion coefficient being inversely proportional to the particle size. molecularmotiontends to opposethe effect of anapplied force, such.as a gravitation interaction with other particles (van der Waals, electrostatic, solvation, interactions) Normal gravity does not influence the distribution of small molecules over a reasonable height and the concentration tends to be the same throughall the space available. particles typically above1 pm, the effect of Brownian motionis negligible compared to the effect of gravity, and particles settle. In the intermediate size range thereis some balance between the two effects, the results depending on particle size and specific weight.

2. ~ ~ a s e s ~ n t e ~ a in ~ e~olloidal s Syste~s (a.) Lyop~ilicand L y o p ~ o ~ ~olloids. ic Colloidsdispersedin a liquidphase, called sols, are traditionally divided into two classes, lyophilic and lyophobic, depending on the affinity for the dispersing liquid. The term “ly~phobic,’~ “hydrophobic9’when the mediumis water, refers tothe poor affinitythe constituting atoms molecules have for the solvent, whichis responsible for their weak solubility, This t e r ~ n o l o g yis of common use but is somewhat ~ s l e a d i n g the , term ‘~hydr~phobic’~ hydrophilic,' referringindeedoften to thewetting properties of surfaces [209]. ~yophiliccolloids are~acromoleculesthat are solvatedby the liquid medium. Thus a lyophilicsol is a solution of macromolecules macroionsandthe dispersion remainsstable, Lyophobic colloids are small solid particles.The latter

consist of atoms and molecules that have low affinityfor the liquid medium and, therefore, do not dissolve. Typical examples are inorganic colloids (metals, oxides, salts, clays) and polymer colloids (latex particles). 1858the concept micelle for a polymolecul~ aggregate Nageli introduced in with an internal crystal s t ~ c t u r e[200]; typical examples were cellulose crystallites [208). The term micelle has also been used for the kinetic units in hydrophobic sols [209]; at present, it is usually restricted to some association colloids described below. (6,) One can choose to treat colloidal system as one-phase or a two-phase system[202]. When the colloidis lyophobic, it is best treated twophase systemsince the particles have a rather limited on effect the propertiesof the dispersion liquid. lyophilic sol is best treated as single-phase system because the colloid does have an appreciable effect on the properties of the dispersion liquid (e.g., vapour pressure, surface tension, viscosity). Many colloidal systems consist of dispersion of one phase in another phase with which it is not miscible. These phasesmay be gaseous, liquid, solid and Table 10 presents an outline of the different combinations with the terminolo~y and some examples. The presence of an interface between two phases is responsible for an additional tern y dA in the variation of the Gibbs free energy of the system in a given transfo~ation.The termy represents the interfacial energy m-2); for liquidgas interface, this is simply the surface tension of the liquid, The area developed the interface is A. This reflects the influenceof broken bonds or mis~atch between molecules at the surface. One consequenceof this additional free-energytern is that the molar Gibbs free energy or chemical potentialis higher the degree of dispersion increases.This explains two phenomena that are important in processing finely dispersed solids:

Overview of Diphasic Colloidal Systems and Common Examples Medium phase

Dispersed Gas Liquid

Gas Aerosol

Liquid Emulsion

Solid

Solid foam Solid emulsion Gel

Fog Aerosol

Solid Smoke, soot, dust Ref. 201.

Sol Gel Colloidal suspension,

Solid dispersion Ceramic, composite materiu~~~

Ostwald ripeningof precipitate, in which small particles dissolve while the large particles become larger, and sharp edges dissolve to increase the radius of curvature; and sintering, key phenomenon in processing ceramics and certain metals, in which solid particles at high temperature (but below the normal melting point) bind together, increase in size, and acquire more rounded shapes, due to different mechanisms such local fusion, evaporation-condensation, andinterdi~sion. A further consequence of the interfacial energytern is that diphasic colloidal systems are not thermodynamically stable. They will evolveto reduce the interfacial area; this explains the coalescence of gas bubbles and liquid droplets and the coagulation of solid particles. These processes may be extremely slow, however, depending on the possibility of close contact between the two surfacesparticles are approaching one another. In practice,colloidal system is said to be stable if the suspended particles keep their identity (gas bubbles, liquid droplets) or remain distinct kinetic units (solid particles) for sufficient time (days, months, years); the colloid stability is thus relative evaluation with only kinetic meaning. These considerations explain that lyophilic and lyophobic colloids are also known reversible and irreversible colloids, respectively. (C.) Many real materials are made of associations of solid particles of colloidal size or dispersions of such particles in matrix. This willbe illustrated here with polymers in view of certain similarities existing with features of carbonaceousmaterials. The exampleschosen illustrate the ~crotextures shown by homopolymers(influence of molecularweightandbranching), by copolymers made of polyolefins (influence of chain rigidity and spatial constraints), and by copolymers made of hydrophobic and hydrophilic segments (influence of chemical composition and polarity). Figure 65 presentsschematicmicrotextures of polyethylene[210],which explain the evolutionof mechanical properties function of density. The basic organization involves network of crystals and tie molecules. This means that segments of the same macromolecule may contribute to different crystals. The crystallinity, crystal size and density decrease, and the number of tie molecules increases from to c. The number of tie molecules increases with molecular weight and with degree of branching; it is also increased by quenching. The incompatibility of chemically different polymers leads to phase separation in most polymer blends and to microphase separation in block andgraft copolymers.Figure illustrates themorphology of annealedstyrene-butadienestyrene block copolymer [2l l Polystyrene segments associateto form parallel cylinders characterized by repeat distance of 31 nm. For films that are thinner than two repeat distances, thecylinder orientation is perpendicular to thesurface. Parallel orientations only exist at specific values of the thickness. If the average thickness of the film is not compatible with the repeat distance, the film develops microscopic variations in thickness. Thick regions are an integer of the repeat

distance and, in the thinner regions, the cylinders orient p ~ ~ e n d i c u l to a r the surface. Figure 66b schematizes the molecular structure of segmented copolymer networkmadeofhydrophobicchains(e.g.,polyacrylate)boundtogether by hydrophilic segments (e.g., polyo~yethylene). The~ght-handside of the figure illustrates that, despite constraints imposed by the structure, hydrophobic and hydrophilic domains may demix, depending on segment size and cross-linking density, The particularity of gels is to show mechanical properties typical of solid (defined shape, elasticity), while containing large amount of liquid. The solid-like prope~iesare due to colloidal particles forming three-dimensional network expanded through the liquid phase, illustrated by Fig. 67 [ZlZ]. The colloidal particles may be lyophobic particles or macromolecules, giving so called particulate gels and macromol~cul~ gels, respectively. In ~arti~ulate gels, the t~ee~dimensional network is made by the loose associationof the solid partic~es(Fig. 67a andb); the difference with respect to coagulation is that very open tridimensional network is formed instead of more less dense flocs, In macromolecular gels (Fig. 67c and d), macromolecules ~onstitute loose network via the local associationof few segments.The bonds may be formed by several mecha~isms:covalent bond (Fig. 6’7d), hydrophobic bonding,ionic bridge, formation of small quasi-crystal by clustering segments belonging to different macromolecular chains (Fig. 67c). The networkmaypossiblybemade of polymer constituted from the same monomer the liquid, in which case the system is called an isogel [212].

and Colloidal

~ ~ p r a m o ~ e cAspects ~lar

11

66 Illustration of segregation inpolymer systems. a: polystyrene (rectangles)polybutadiene (lines)-polystyrene triblock copolymer. (Adaptedfrom Ref. 21 1 b: segmented network made of a hydrophobic polymer (lines) and a hydrophilic polymer (waves); left, molecular structure, right, morphology. White zones in ~orphological schemes are filled up with the same material as that indicated.

Both gel constituents, the liquid phase and the colloidal particles, havea dispersed character but also continuous character, as one may travel through the system while remaining in one constituent or in the other one. The two phases constituting a gel are thus co-continuous. The above discussion shows that a particulate gel is thermodynamically unstable and will unavoidably evolve to be more and more compact, while the liquid phase is expelled; this is called syneresis. In common pratice, the term gel is also used to designate the porous solid obtained after eli~inationof the liquid phase of a gel stricto This is the case of silica gel, broadly used as a desiccant.

The sol-gel transfo~ationshave become important processesin the elaboration of advanced materials. The use of colloidal particles of great purity and constant size allows one to obtain ordered packings (crystalline colloids) which can be densified, for instance, to elaborate glasses at temperatures lower than usual, or to provide high performance ceramics.

3.

~olloi~s

Amphiphilic molecules, characterized by the existence of a hydrophilic end (head; ionic or nonionic) and a hydrophobic end (tail, e.g., hydrocarbo~,fluorocarbon, siloxane chain), tend to self-assemble and to different kinds of structures [201-203,206,207]. onol layers may be formed at the interface between water and a gas or a solid (Fig. 68a). Above a certain concentration, they form micelles, i.e., aggregates organized in such a way that the hydrophobic parts of the molecules are associated together to f o m the heart of the particle, while the hydrophili~parts of the molecule are in contact with water (Fig. 68b). The micelles are typically spherical but may be ovoid or even have cylindrical shape (Fig. 68c). In an organic solvent,invertedmicellesmaybeformedwheretheheart of themicelle is

68 Illustration of various structures formed by amphiphilic molecules: a, monolayer; b, micelle; c, elongated micelle; d, inverted micelle; e, monolayer stabilizing an emulsion; f, bilayer; g, bilayer defining a vesicle; h, hexagonal liquid crystal; i, lamellar liquid crystal;j tridimensional periodic and bicontinuous structures. (Inspiredby Refs. 201 202, 205, and 207.)

constituted by the polar heads of the amphiphilic molecules small and amountof water (Fig. 68d). The heart of micelle may dissolve liquid whichis not miscible with the other liquid phase. There is thus continuity between rnicellar system and emulsion that is stabilized by the presence of an amphiphilic compound at the droplet surface (Fig. 68e). Bilayers may be seen formed by association of the hydrophobic faces of two monolayers or the extension micelles in two dimensions (Fig.68f). They may constitute walls defining vesicles (Fig. 68g).

12 The formation of micelles results essentially from attractive interactions betweenhydrocarbontails,solvation of thehydrophilicheadgroups by water, interactions between solvated head groups (generally repulsive), and geometric and packing constraints. Geometric factors that control the packing of surfactants in association structures can be expressed by the “critical packing parameter,” v/al, where is the volume of the hydrocarbon chain, 1 the maximum effective length that the chains can assume and a the optimal area per molecule. As the critical packingparameterincreases,theparticlesarepreferentiallyspherical micelles 0.33), nonsphericalmicelles,vesicles or bilayers,andfinally l) [206,207]. inverted micelles Liquid crystalline structures may appear end termsof associations made by amphipathicmoleculesinorder to minimizethecontactbetweenwaterand hydrophobic chains. Typical examples are ordered arrays of cylinders (hexagonal, nematic phases) (Fig.68h) and stacksof bilayers (lamellar, smectic phases) (Fig. 6%). Complex three-dimensional periodic structures formed by folding bilayer (Fig. 68j) may be viewed network of interconnected vesicles [205,207].

This section summarizes the key features of molecular association which are of prime importance to understanding the nature and properties of carbonaceous materials.

Liquid crystals are also called crystalline liquids, mesophases, or mesomorphic phases. The designation “liquid crystal” has been very much extended in recent years due to the discovery of novel molecular typesof liquid crystalline substances and of additional phase structures [213,214]. Some structures (mentioned Secin tion VI1A 2 c) found in dispersions of polymers in polymers may be liquid crystals. Compared to isotropic liquids, liquid crystals show higher state of order. Compared to solidcrystals, they have higher intermolecular and intramolecular mobility, i.e., more freedom of molecular rotation, translation, oscillation and intramolecular conformational changes. The borderline between solid crystals and liquid crystals is not clearcut: it varies according to the criterion used and thus depends on the method of investigation [214]. Cubic plastic crystals are made of of which is ordered in the molecules with more or less spherical shape, the position three dimensions but which rotate freely; an example is given by methane. Their viscosity and mechanical constants are much lower than those or din^ solid crystals. In hexagonal plastic crystals, elongated molecules are allowed to rotate around the molecular axis; higher n-alkanes are good examples.

Liquidcrystalscan be classifiedtothreetypes:lyotropic,polymeric,and the~otropic.The most common lyotropic liquid crystals are those formed by water and amphiphilic molecules, described above. A necessary condition for their existence is strong interaction of only one end of the molecule with the solvent. An impo~antvariable controlling the formationof liquid crystals is the relative amount of solvent present in the system, thermotropic liquid crystal is defined by the fact that the transition to isotropic liquid is ruled by temperature. is their high shape anisotropy, which The key featureof the constituting molecules gives rise to the anisotropy various physical properties:rod-like, disc-like and lath-like molecules given calarnitic, discotic, and sanidic liquid crystals, respectively [214]. The most impo~antphase structures of liquid crystals are [214]: l. nematic,themostliquid-likestructure,withone twomolecularaxes oriented parallel to one another in an orientational long-range order; 2. smectic, layer s ~ c t u r e with s different possibilitiesof order inside the layers and different possibilities of mutual ~ a n g e m e n t s the layers; 3, cubicstructureswithmicellar lattice unitsandinterwovennetworks(Fig. 63j); and columnar, columns of parallel disc-like molecules (Fig. 24).

igure 24 shows a sketch of discotic liquid crystals made of columnar st~cturesand nematic structures, respectively.

netic At this stage it w o ~ ~ecalling h the factors that control the transformationof m o l e c u l ~system,whether by chemicalreaction,formation or disruption of molecular associations, or phase transition. Onefactor is the driving force of the transformation, whichis related to thermodynamic considerations. The other one ation of the ~ i b bfree s energy, results from the energy balanceof m o l e c u l ~interactions and from the change of ~ntropy.The e depends on the forces exerted between the different molecules don dispersion forces, polar van der forces, hydrogen bonds, electrostatic forces, covalent bonds) and on their extension (number of bonds, area of contact). This balance should play c ~ c i arole l in the influence of h e t e ~ o a t o ~ s on the covalent or noncovalent cross-linking betweenBSU. It may differentiate olyaro~aticmoieties and aliphatic side-chains with respect to their affi~ityfor suspensive medium, considered solvent. The energy balance explains that “like dissolves like,” i.e., solute will dissolve preferably in solvent having similar properties[21 The area of contact between molecules orp ~ i c l e may s

be affected by steric constraints, illustrated by Fig. 65. Side chains of polyaromatic molecules may thus hinder close interactions between aromatic rings. The influence of entropy appears in many experimental facts. The translational entropy is responsible for the variation of the molar Gibbsfree energy (chemical potential) of compound in an ideal solution according to the logarithm of the concentration. ~ultiplicationof the entropy by temperature in the expression of the free energy explains that heating favors melting, volatilization, and, most often, dissolution. The smaller increase of translational entropy relative to the decrease of enthalpy during the transfo~ationcontributes to the drop of solubility and the rise of melting temperature and molecular association the molar mass increases.Thevibrationalandconfigurationalentropycontributes to lower solubility and an increased melting temperature for rigid aromatic molecule compared to flexible aliphatic chain of similar mass. The former will indeed undergo smaller increase in entropy during the transition from solid phase to liquid phase. Details on the thermodynamicsof solutions can be found in References 215 and 216. In the absence of covalent bond modification, which may be slow step, the rate of process involving molecular association or separation is determined by the rate of mass transfer and, in the case of large molecules, by configurational changes. Both factors are strongly influenced by the viscosityof the medium.The rate of BSU association may thus be affected by the presence of attached sidechains. During insolubilization of large molecules, the probability that all segments adopt at the same time the confo~ationallowing maximum interaction may be very small, thus leading to highly disordered metastable systems65). (Fig. Theefficiency of dissolutionorextractiontreatments maybe influenced by several factors which are related to the organization of the starting material and not to the chemical nature of the molecules expected to dissolve: (1) disordered phase will dissolve more readily thancrystalline phase; (2) certain components may be imbedded in other components that prevent contact with the solvent; and (3) the dissolution of large molecule may be slowed down due to the need for disent~glem~nt from other molecules. It turns out that,for both thermodynamic and kinetic reasons, classification of constituents of complex systems according to solubility criteria must be t&en with caution (Section C 1 and C 3 The same classrnay refer to different compounds, depending on the material. The relative amount determined for class in the material. may be smaller than the amount of these compounds really present For instance, certain compounds may or rnay not be extracted from by coal pyriis swollen dine and be detected resins, depending on whether or not the coal by pyridine. It is thus not surprising that theQ1 content may be much lower than the anisotropy percentage in heated pitches (Section IV C 3 b) or that coal-tar pitchescharacterized by similarmacroscopicpropertieshavedifferentnanotextures and, therefore, different technological performances (Section IV C 4 b).

In this section a new vision of primary carbonizationis given in the lightof landmarkspresentedabove for colloidalsystems(Section VI1 A)andmolecular association (Section VII B), and of further consideration of the interfaces and interfacial phenomena.

of as the first event of strong molecular association is a key The occurr~nc~ featureorientingthewholeprocess of carbonization.Atacertaindegree of evolution, BSU are formed by association of polyaromatic entities saturated by aromatic groups. The observation of molecular association in polymers (Section VI1 A 2 c) indicates that this transformationis compatible not only with the existence of molecules, each made of one polyaromatic moiety carrying side chains, but also with the presence of several polyaromatic moieties in macromolecules and even with the presence of a reticulated network, provided the latter is loose enough (see Sections A 3, IV B 3, and Fig. 18). The BSU formation offersamlogies with the associationof amphiphilic molecules, as the polyaromatic rings and the side chains have quite different interactions with one another and with the suspensive medium (SectionVI1 E3 2). That the polyaromatic moieties are preferred points of association may thus be explained by a balance of interaction energies. Moreover the entropy loss resulting from association is also smaller for polyaromatic moieties than for flexible aliphatic chains.The side-chains are expected to play a inrole limiting the stacks 2to or 3 polyaromatic entities. Their influence may be exerted by the~odynamic factors; moreover they may be responsible for steric hindrance, particularly if they contain double bonds and functions with heteroatoms (responsiblefor BSU heterogeneity in sizes). Thisis evidenced by the fact that,in newly formedBSU, the aromatic moieties are neither parallel nor equidistant (SectionB); they tendto become so as evolution proceeds further. natural evolution and heat treatment may trigger the formation of BSIJ by several modifications which will influence both the driving force and the kinetics of the transformation. Removal of oxygenated functions and breakage of long aliphatic chains modifythe energy and entropy balance of the processof association of polyaromatic moieties.It also releases steric constraints. It facilitates mass transport and conformational changes by reducing the size of mobile entities and creating a suspensive medium responsiblefor lower viscosity. This may explain that BSU-like stacks have been easily observed in asphaltenes and not in weakly evolved kerogens of Series I and 11. At that stage, the representation of the carbonaceous material is consistent with

ormed en are

a viscous liquid, but also with a gel in which the solid-like continuo~sphase is formed by aliphatic chains cross-linked via the (Fig. 67c). evolution SU side-chains are shortened, which should increase the driving force for association of polyaromatic moieties and make them more mobile. the same time the amount of suspensive medium increases, which also favors mobility. One may wonder why polyaromatic association does not proceed progressively, leading to the growthof solid particles. Thisis probably dueto the dist~butionof size (0.7-1 mm), shape (dimers or trimers) and side-chains, all the more so since most ~olyaromaticmoieties have already been involv In most sterns, as evolution proceeds further, is the second major event of strong molecular association (now scale), whichis a key feature o~entingthe courseof carbonization. Note that storing coal tars may, with time, lead to demi~tionof spherical anisotropic bodies ction IV C a). As discussed in Section E, liquid crystal d e ~ ~ t i iso proba n triggered by a combination of several factors:(1) in relation with the driving force of the transformation, one may cite the concentration of and the balance of the intermolecular interactions entropy, keeping in (2) steric that, at that state, side-chains are reduced in size and number; hindrance is reduced due to the smallersize and numberof side-chains; and(3) the mobiledue to the loss of side-c~ains,th

are independent kinetic en und to show a t h e ~ o t r o ~ i c versibly returningto representsonly a poor ~ r e f e ~ e d id crystals. For other c~bonaceous materials, a simple ther~otropic behaviour was not described either because the in~uenceof s~spensivem e d i u ~thatcanredispersethe U onlyveryslowly (because solidi~cationoccurs readily after demi~tion)or re probably because of the continuous ~ ~ a n of g echemical co~positionduring heat tr~atment. ssociatiol~ssolidify dueto the heteroato~s.At a l bonds

carboni~ation.

sed above is the removal of oxyand the breakage of C-C bonds,

~ ~ ~ r a r n ~ l eAspects c~lar whichleadstosegregationbetween BSU ~ i ~ crystals u i ~ andvolatiles. The balance between hydrogen (mainly aliphatic hydrocarbons) and heteroatoms (mainly oxygen), expressed by FLMO, is key controlling thesize of liquid crystals that covers a broad range, from of Class l nm) to coarse mosaic 5 mm). The size ofsmall associations of oriented BSU (highFLMO typical of industrial products such glassy carbons, saccharose-based carbons, etc. andof coals and kerogensof Series 111)is fixed at dernixtion, whichis quickly followed by solidi~cation.When the gap between ~emixtionand solidi~cationis larger (low typical of oil feedstocks and secondary coal-tar pitches) the domain size may increase by coalescence of liquid crystals (Fig. 46). It appears that hydrogen (hydrocarbons) and heteroatoms (oxygen) present at demixtion are balancing each other andthat an increase in onehas the same effect as decrease in the other, as reflected by the influence of F,. Upholding hYd when BSU are able to associate (the~odynamicallyand ally favors the association, revealed as increasing by liquid crystal size: W e n the hydrocarbon contentis increased by using a precursor of higher (Section IV), When volatilizationis slowed down with respectto bond breakageby increasing the pressure, closing the system or increasing heating rate (Section A 1 d, Section TV D, Section A), As a result of internal redistribution of hydrogen in coal hydroliquefaction (Section V C), a result of segregation of hydrocarbon-rich phase in catalytic hydroconversion of heavy oils and heavy residues (Section V D), and Due to the presence of a large concentration of resins in anisotropic pitches (Section VI A) and in coking coals (Section VI B). The main role of aliphatic molecules is to act as suspensive medium all0 easy displace~entof BSU. In the case of pitches, demi~tionof oriented associations (spheresA) takes places froma matrix, which representsa larg ume fraction and disappears a result of volatiles removal and further demixtion. In gas-sparge pitches (Section Al; VI Fig. the applicationof high stress provokes a disruption of columar associations of BSU. As the suspension medium is already limited, the fragments cannot reassociate quickly in an ordered way,leadingto a weaklyanirialwhichmaybein a metastable state, ~ h ~ m i cfunctions al with act in a direction opposed aliphatic to hydrocarbons as regards both thesize of oriented associations md the temperature gap between their demixtion and solidification. It appears (Section V that oxygenated functions brought by oxidation have the same effect as those precursor. As discussed ove (Section IV E), o x y ~ e n a t e ~ nce the size of oriented U associations through several mechanisms:

1. they may be responsible for a limited miscibility of BSU, thus leading to particles with different properties such as spheres A, and 2. they may accumulate at the surface of liquid-crystalline bodies, thus influencing their interfacial energy, limiting their size and decreasing their aptitude to aggregate with other bodies and to coalesce; and 3. they may make a bond (e.g., ether) between neighboring BSU thus limiting mobility and deformability in the system.

The three mechanisms explain a cross-linking effect of oxygenated functions, which may involve either covalent bonds (e.g. ether functions) or polar intermolecular interactions. The same mechanisms explain the influence of oxygenatedfunctionsonthetemperaturegapbetweenliquid crystal demixtionand solidification. The nature of the sulfur containing functions, which have the sane cross-linking effectas oxygenated functions,is not clear. Formationof thio-ether bonds may be considered. With respect to polar van der Waals forces, it may be noted that sulfuris more electropositive than oxygen. a consequence ethylmercaptan (C,H,SH) is less soluble in water than ethanol. On the other hand, SH group is more acidic than OH group (E2151 p. 107). Functions containing -0 bonds are highly polar and sulfate, sulfonate, and sulfoxide are very hydrophilic. The explanations givenhere about the respective roleof ~ y ~ o c a r b o (favorns ing mobility) and oxygenated functions (favoring cross-linking) are consistent with the observation that coals of increasing rank show not only larger average LMO sizes (above Class 1) but also broader distributions of LMO sizes before reaching the cokingcoal range where only lamellae of Class 10 are found [48,49]. Syst~~s It appears from the above discussion that carbonizationis marked by the occurrence of two types of particles: BSU and associations of oriented BSU. SU formation, softening occurs by macromolecule breakage, When U are formed,the carbonaceous material maybe viewed as a gel or a highly vlscous liquid which is a mixture of several compounds. Some may be carrying side-chainsthathaveagoodaffinity for thesurroundingmedium(lyophilic moieties). The main evolution after BSU formation (removal of H,O and CO,, breakage of aliphaticchains)transformsthematerial into asystemwherea (BSU suspensive medium (rich in volatile hydrocarbons) and lyophobic particles id of side-chains) are present. of orie~tedBSU is produced when demixtion of liquid crystals takes place. a coronene dimer is sometimes taken as a model for BSU devoid of ~ide-chains, it is interesting to note that melting and boiling temperatures of coronene are 440 and 525OC, respectively [217]. Demixtionof liquid crystals of BSU (range of 425 to 475°C; Fig. 46) thus occurs in a temperature range where breakage of side-chains allows potential association af almost purely polyaromatic molecules (see Section V11 C but where these polyaromatic molecules

show a tendency to volatilize.The net result, i.e., formation of liquid crystals and solidification, maybe attributed to the heterogeneity BSU size (0.7-1 nm) and of cross-linking or persistence shape (dimers or trimers) and to the occurrence side-chains. After demixtion liquid crystals, materials with high FLMo(kerogen and coals) almost i~mediatelysolidify giving a material in which oriented domains have a colloidal size. ater rials with a lowF, (pitches, oil residues) appear as particular emulsions or sols, i.e., dispersions different types of liquid crystals (Fig. 37).In peculiar cases (primaryQI of coal-tar pitches), solid (carbon blacks) or plastic particles (Fig. 32) are dispersed in an amorphous matrix. In gas-sparge pitches, d i s ~ p ~ of o ncolumnar liquid crystals leads to a weakly anisotropic materid. Low-rmk coals show the presenceof high concentrations BSU that do not form oriented associations (Section IV A 2). A weak anisotropy is due to the existence of a preferred orientationin a populationof randomly distributed BSU, giving a long-range weak statisti~alori~ntation(SO), The absence of oriented association may be attributed to two factors: (1) BSU still carry too many sidechains, which reduces boththe driving force and the kinetics association; and (2) the temperature has not provided asufficientviscosity decreaseto allow easy BSU displacement. The system is a dispersionof BSU in which preferred orientation is produced by flow under tress. Upon carbonization, the balance between hydrocarbons and heteroatoms is such that small LWIO are famed, Hydroconversion of heavy oils and heavy residues (Section V D) illustrates how the interplay between ~ e ~ o d y n a mand i c kinetic factors may be responsible for the complexity of colloidd system behavior. Quite clearly the hydroconversion leads to s e ~ r ~ ~ ~between t i o n a suspensive mediumof low viscosity, which contains BSu carrying side-chains, and a residue. The small LM() size of the residue may be due to the fact that the BSU that “precipitate” lack u n i f o ~ t y , accumulate oxygen present in trace amounts in the material, or are able to crosslink due to high concentrationof free radicals inherentto the catalytic hydroconversion.

4. The deformation of liquid crystals (nematic in this case) by boundaries was found to playaimportantroleinorientingthinsamples(from2 to 100 pm), The molecules are aligned near the substrate and this fixed orientation can be transferred to the bulk E21 It was pointed out [219,220] that the orientation of BSU at interfaces plays a major role in the nanometric texture of carbon bodies. From the data reported in the present chapter,the generally followed by BSU orientation at interfaces is as follows

2. in contact with gas phase (macropores in pitch, nitrogen bubbles in gassparge process), the tendency of BSU to spread along a b u ~ b l eis clear (Section IV C 2 c; Section VI A 1 c, Fig. 59); and 3, in contact with liquid suspensive medium, BSU orient perpendicularly to the interface (mesophase A, Section C 2 a, Fig. 22; outer shellof plastic primary Q1 of coal tars, Fig. 32). These features are particularly illustrated by the cases of PRB (Table and polyethylene high-pressure carbonization in sealed gold tube 153- 1561. In the case of PRB, the wall of the tube was coated by carbon layer. The BSU were found to be parallelto the gold surface inlayer (about 1pm) near the wall. Then they were turned over a thickness of about 10 nm and ran perpendicular over a thickness of again 1 pm at the side of contact with the suspensive medium (Fig. 69). The sane features, but less marked, occurred with polyethylene. Under the sameconditions,anthracenebehavedinthesamemanner[155]:inthebulk deposit inside the tube, the BSU are systematically oriented perpendicularto the interface with the suspensive medium. Coal tars (Section C 4 Fig. 32) show spheres of resins with radial orientation of BSU surrounded successivelyby shell with concentric orientation of BSU, then again shell of radially oriented BSU. These spheres are dispersed in an isotropic matrix of resins. An impo~antfeature is found in gas-sparge pitches (SectionAIV 10;Figs. 5'759). The major component is slightly anisotropic, due to poor preferred orientation. It contains droplets that are still more weakly a~isotropic;however, the preferred orientation in the droplets is parallel to that in the matrix, indicating that preferred orientation may subsist through the interface between two different phases.

SU at interfaces must be considered, keeping in mind the following. 1. an excess free energy(surfaceenergy, inte~acialenergy) is associated with the boundary between two phases. The roundedofshape particles is imposed by the tendency to reduce the area of the frontier. The BSU must adopt an orientation which izes this interfacial energy, In contact with gas phase or orientation maximizesBSU-BSU interaction. In contact with a suspensive medium, the p e ~ e n d i c u orientation l~ favors interactions between the medium and residual side-chains carried by BSU. 2. accu~ulationof certain constituents at interfaces may correspond to two e analyzed by comparison with associationcol10 c ~ i n side-chains g or heteroaton~smay ind r with respect to their environment. On the on may separate into two phases of di~erentproperwith p ~ i c u l a characte~stics r ties; the driving force for their accumulation (adso~tion)is decrease of the interfacial energy and they stabilize the dispersion wayinsimilar to the stabilization ofan emulsion by su~actant(Fig. 68e). theotherhand,theymay separate into two phases of identical or similar ties, in which they are not soluble, thus ~roducingvesicles as surfactants may do (Fig. 68g). of weakly soluble compounds, for instance ontrol the size li~uid explanation of the correlation between FLMoand e presence of oxygen in~ibitsthe spheres. It alsotheability of liquid crystal C spheres, such ad with other bodies, and n the surface. This would explain o the surface is not the same to to thefact that they are parallel i~erential adso~tion also explain theoccu~ence of oblong bodies.

the end of ~rimary carbo~i~ation, a c~bon-richsemicoke is obtained that hasa columnar texture inhel~tedfrom the various mesophases ~onizationis succeeded by the two-step process of sec0 At the brittle solid state (Vickers microh~dness sizes are already fixed. The release of aromatic

erli

1.

into heavy radicals so that ESR shows a maximum of spins in the range of 500°C700°C before decreasing ([3] Chap. 8; [53]) (Fig. 10). Then a maximum broadening of the ESR line occurs between 1400°C and 1600°C [6--83. Mrozowski suggests that the disappearance of some specific defects is responsible for the transition between the char (semi-coke) resonance and the carbon resonance. Such ~liminationof defects by discrete steps has been recognized in TEM 002 lattice fringe images since 1973 by Terri&re[16,43], then Goma [46] and Rouzaud [44,47], for ant~acene-basedcarbons and thin carbon films. (a,) ~ e ~ e Solid~cation e n and 15OO"C-16OO"C. The columnar arrange~ent inherited from the various mesophases (see Fig. 23, 30, 36, 42) persistent up to solidification [ 102,1031improves by a progressive elimination of defective BSU forming wedges between the columns [44]. There are small areas of lateral coherence between adjacent columns, so that the diameter of a distorted layer L, increases, albeit slowly. The STM image of Fig. 70 corresponds to a carbonized pitch-based fiber [221]. At the surface, BSLJ of a few carbon rings are visible. These data demonstrate that, statistically speaking, BSU do not grow in diameter even at 16OO0C,so that there is no polymerization of BSU. (b,) ~ ~ ~ e ~ i aabove t e Z 1600°C. y The removal of the BSU wedges between the columns allows them to merge laterally. ~istortedbut continuous layers are formed and L, rapidly increases, This implies the persistence of in-plane defects which will be suddenly wiped out at 2000°C.

STM image of a carbonized pitch-based carbon fiber. (From Ref. 221.)

( C . ) At 2000°C. Sudden disappearance of the in-plane defects (probably point defects or vacancies) provides complete annealing of the layer distortions [16, 43,44,46,222]. These data fit well withMro~owski'shypothesis [6-81: as long columnar order persists, char resonance is maintained; when columns suddenly merge to form distorted layers, carbon resonance occurs.

~ n ~ u e n cofe The two steps described above occur whatever the final size of mosaics orL " . The columnar ordering (see Fig. 42) is always present upto 1500- 1600°C. Then up to 2000°C, it is replaced by stacks of continuous distorted layers. However, minor changes occur inthe LMO due both to the improved ordering BSU and release. The LMO diameter does not change but the thickness decreases, caused by the better parallelism between BSU and their better stacking order. Stresses thus develop at the LMO boundaries. The only possibilityfor stresses to relax would be breakage of the grain boundaries though already stable andlor decohesions parallel to the layer stacks. Relaxation is easy in compact lamellar carbons where boundaries are far from each other and where cleavages are easy to produce. On the contrary between smallLMO, the density of grain boundariesis higher so that stresses are more easily frozen-in and pores develop. They are small because the shrinkage due to gas release is limited. The pore size decreases from meso- to micro- and nanoporeLMO finalsize decreases. The opening and rapid closing of pores HTT increases was recognized early 1963 in coals by BET and SAXS [223-2261. They were imaged at first by Comte-Trotet using TEM 002 lattice fringes in saccharose-based carbons [227]. Then stereopairs of bright field micrographs E2281 established their morphology similar to crumpled sheets of paper. Starting from flat lamellae, smooth curvatures occur produced by persistent c r o s s - l i ~ n g The . larger the the more numerous the BSU lateral bonds; consequently the lamellae are more crumpled, so that the pores become smaller. The most intense crumpling correspondsClass to 1 5 nm). At 2000°C the newly acquired stiffnessof the layer stacks causes polygonizationof the porous material. The polyhedral pores are now definedby flat faces, each of which is limited by grainboundariesgathering defects.These faces are inherited from the final LMO and define grains having kept the same diameter (Fig. 71a and b illustrate Classes 5 and Above 2000°C the term grain will replace that of final LMO. Below 20OO0Cthe materials are turbostratic and not yet pure since carbon some cross-linkers (oxygen and sulfur) are still present, Moreover, the persistence of distortions implies the occurrence of defects such single atoms, defective or dangling bonds, or vacancies. At about 2000"C, the apparent activation energy jumps from 120-150 kcal mole-1 to 250-280 kcal mole" [229,230]. The measurement of these values resulted from large amountof research work extending between 1960 and 1963. Electronic(diamagneticsusceptibility)andcrystal-

TEM 002LF images of cokes heat-treated above 2000°C. Formation of polyhedral pores, the diameter of their walls inherited from the LMO size. a: class 5. b: class 3. (From Ref. 53.)

ramolecular Aspects

aoo,)

lographic (La, L,, properties were measured functions of HTT and time. All authors recognized two kinetic regimes below and above 2000°C. The same results were obtainedfor pyrocarbons heated above 2000°C by Fischbach [229], who assumed series of first order processes with broad dis~butionof rate constants. The high value of 260 kcal mole-l obtained at high temperature was attributed to self diffusion of defects (vacancies or interstitials). This value approaches thatof single carbon atom migration[83], i.e., it suggests the occurrence of plasticity. This "magic temperature" marks the end of secondary carbonization; it corresponds to clear transition in electronic and structural properties such the plateau of diamagnetic susceptibility, the maximumof the Hall effect, the change of slope of electrical and thermal resistivities, andthe change of sign of magnetoresistance. Itis also the thresholdof graphitization [g, 11 12,30,32,72,74], The hypothesis of carbon plasticity suddenly occurring at 2000°C was recently confirmed by mechanical tensile tests applied not after quenching, usual, but at 2000°C and above [231-2331. The samples studied, initially fragile, turned suddenly to ductile, shown in Fig. 72.

defined above, grains are inherited from It is inside each grain that graphitization occurs. In lamellar carbonsthe grains are almost infinitethat they graphitizealmostcompletely.Graphitizationreallystartsabove2000"C,The decreases at first abruptly up to2200-~3OO0C,then very interlayer spacing slowly to approach the value of graphite (0.337-0.336 nm [11,12]). The turbostratic bands characteristic of the two-dimensional structure modulate progressively into M reflections. The graphitization degree for given product at given HTTis the probabilityP, to get pair of aromatic layers in the graphite order (sequence and spacing). These pairs at random in the bulk. Graphitization is thus only statistically homogeneous transformation [234,235]. Except in the case P, 1, i.e., the caseof natural graphite, crystallitesstricto never appear because each coherent domain always contains turbostraticofpairs layers. For the best graphitizing carbons,P, tends to value of 1, but never reaches it. It stays at maximum of about 0.90-0.93 11,121, that 7 to 10% of turbostratic pairs remain. Deducedfrom the ~odulationof hk bands not sensitiveto stac~ing faults or r h o ~ b o h e ~sequences, al P, is almost nil 0.1) at the magic ture (~000"C-2200*~). The pe~ectionof the aromaticlayers observed thus necessary for achieving three-dimensional ordering. In addition to the valuesfor L, and L, are often given the aromatic layer stack thickness and diameter, Theyare arbitrary values based upon the widthof the reflections. They have no direct physical meaning they are affectedby many artifacts. In practice, L, is simply the reciprocal valueof the width at half maxi-

aoo2

72 Thetensilestress-strainbehaviors (From Ref. 233.)

of carbonfibers at various temperatures.

mum of the 001 reflections. If it is not measured by theWmen formula [29], L, has no meaning at all. The real physical entity to seek is the defective aromatic layer stack, limited in space by grain boundaries.It is given by the shapeof the 000 node, the originof reciprocal space. get it, the increasingL, corresponding to decreasing hk orderis plotted versusHTT. Then it is extrapolated to the originof reciprocal space, i.e.to the 000 node [236]. In practice, apparent values of L, and L, are more easy to obtain and offer a reasonable comparative criterion. Theaboverestrictionsbeingrecognized,relationsbetween P,, L,,L, and various electronic properties are instructive 11,121. Properties involving charge carriers mobility, such as the magnetoresistance Aplp, are dependenton the layer diameter L, (Fig. 73a). Theydo not correlate univocally with P, (Fig. 73b). On the contrary, L, (Fig. 73c) correlates well with P, as could be expected,since a better stacking accompanies reordering. Carbonized coking coals, pitches, and light aromatic compounds (anthracene, ovalene) are graphitizing carbons. The P, value itself is also an approximation since a fully reliable treatment should be the Fouriert r a n s f o ~of the entire reciprocal space. In fact, scattering laws limit reciprocal space exploration to 21X. This is why trials were and are done to make simulationsof the complete pattern and try to fit it with the real one [237-2401.

PI a: relation between~agnetoresistanceand the aromatic layer diameter L,. relation with c: correlation between L, and P,. (From Ref. 11

its

Inporouscarbons,Monthiouxshowed[71-73]thatmaterialsareunable to graphitize completely. He obtained maximum values of P, ranging between 0.75 and zero, associated with minimum ranging from 0.337nm to 0.344nm for increasingly cross-linked materials. Lateron, other papers 174,751cleared up his conclusions by adding numerical values of as a function of FLMO. The key concept connects LMO final size or grainsize, and P, [78,79,157-159,2411. As a consequence, products that do not develop grains but high density of random disclinations, such as A240 or some anisotropic pitches prepared by fractionation, provide limitedor even poor graphitization. On the contrary, materials containing well-defined grains, such as gas-sparge pitches, give highly graphitized fibers. A an example, after heating at 2800°C is 0.337, for A240 pitch, 0.336, for Arabian light-AR, 0.337, for Safaniya-VR, 0.339, for asphaltene 14618, and 0.340, for Kuckersite. From Arabian light to Kuckersite the grain size decreases from 200 pm to 200 nm. As grain size decreases, i.e., as cross-linking increases, the P, maximum decreases and the dOo2 minimum increases. Oil heavy products having LMO Class d 8, some pitch-based carbon fibers, quinones [242], kerogens, etc., are intermediates between graphitizing and nongraphitizing carbons. Products having the smallestLMO size, i.e., the smallest grains (Class1 5 nm) are nongraphitizing. Among them are saccharose, cellulose, or phenolic resinbased carbons (glassy carbons).

aOo2

aOo2

Primary carbonization is marked by two major events: formation of BSU and formation of associations of oriented BSU. These molecular and supramolecular association processes are determined by bond breakage which leads to the removal of volatiles (first H,O and CO, if oxygenated functions are present, then hydrocarbons), progressivelyremovesside-chains of BSU, thus increasingtheirtendency to associate (thermodyna~caspect) and their mobility (kinetic aspect), increases the concentration of BSU (thermodyn~icaspect), and produces a suspensive medium of low viscosity (kinetic aspect). The carbonaceous materials are complex colloidal systems in which typical particles or subsystems may be identified or hypothesized(lyophobicand lyophilic particles, sols, emulsion, vesicles, liquid crystals).The diversity of the org~izationand the practical irreversibility of the an sf or mat ions is not due only to breakage of covalent bonds andto volatilization of constituents. As for many colloidal systems,it is also due to the existence of numerous states differing only slightly in free enthalpy (interplay of enthalpic and entropic effects) and to the reduced mobility (low diffusion coefficient, slow molecular agitation, or Brown-

ianmotionduetothelarge size of particlesinvolved).Interfacesandgrain boundaries may play an important role both throughthe~odynamic(adsorption of weakly soluble constituents, coalescence, aggregation) and kinetic (rate of coalescence aggregation, overall viscosity) aspects. The size of the oriented SU associations and the temperature gap between their demixtion and solidification is controlled by the balance between the concenati ions of hy~rocarbonmoieties and heteroatoms (oxygen, sulfur) at the moment of demixtion. Oxygenated functions broughtby an oxidation prior to carbonization have the same effect as those originating from the precursor. The main of role of aliphatic moietiesis to act as a suspensive medium allowing easy displacement BSU.Chemical functions containing heteroatoms act as cross-linkers either by covalent bonding or polar van der Waals interactions. This picture allows one to rationalize the evolution of different materials during primary carbonization and coalification, which governs not only the secondary carbonization stage but also the graphitization process [9,21]. It provides also a unified visionof numerous industrial processes and materials: essential properties of pitches used for making carbon fibers, influence of heating rate and confinement on carbonization, influence of oxidation, hydroliquefactionof coals, hydroconversion of heavy oils and residues, anisotropic pitches, coke manufacturin~ and coking coals.

The support of Inte~niversityPoles of Attraction Program (Federal Office for Scientific, Technical and Cultural Affairs, Belgium)is gratefully acknowledged. The authors thank Thomas Cacciaguerra for his help in the illustration.

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181. 3, 974, 264. 182. US 976, 729. 183. 4, 327. 184. 4, 209, 500 (1980). 185. K. Carbon 29, 849-855 (1991). 186. Carbon 29, 857-864 (1991). 187. K, International Conferenceon Carbon, bon ’98, (1998), pp. 801 -802. 188, K. Carbon 29, 831-847 (1991). 189. Carbon 32, 11-21 (1994). 190. US 4, 208, 267 (1980). 191. US 4, 33 620 (198 1). 192. 56338. 193, 34910. 194. R. 16th ~iennialon C a r ~ o n 1983, 24-25. 195. Carbon 32, 61 -70 (1994). 196, Le Coke, 1990, p. 564 1989, 197. 99 1). 198. 73, 795-809 (1994). 199. Acad. Sei. 296 Se‘rie 11,757-760 (1983). Col~oidScience, Vol I, ~ y s t e ~(H. s, 200. H, R. 1952, pp. 1-21. 201. to ~ o d e r nColloid Science, 1993, p. 338. 202. R. foundation^ Colloid Science, 1987, p. 673. ~ r i n c i ~ l e s CoZloi~and S u ~ a c e 203. 1997, p. 650. Che~ist~, 204. Fundamentals 1nte~aceand C~lloidScience, Vol I, Funda~entals, 1991, pp. 1.3-4.85. The Colloidal ~ o ~ a i n - ~ h e r e ~ h y , ~ i c s , 205. 1994, p. 515. C ~ e m i s t ~ , ~ iand o l oT ge ~c ~ n ~ l Meet, o~y 206. S, Inte~acesand Colloids-~ri~ciplesand Applications, 1991, p. 433. 207. Inter~olecularand Forces, 1991, 450. 208. F m , Colloid C h e ~ i s tVol ~ , K ~ h e and o ~etho~s ~icine, 1944, pp. 610-667.

H.van Olphen, Clay Colloid C h e ~ i s t2nd ~ , ed., Wiley,New York, p.

Lu, E. Quian, and N. Brown, Polymer

M.A. van Dijk, and E. van den Berg, ~ a c r o ~ o l e c u l e s

A.C. Pierse, Introduction aux Prockdks Sol-Gel, Septima, Paris,

p.

Li~uid C~stals-Physical Properties and Non Linear ~ p t i c a l I. C. Pheno~ena,Wiley, New York, p. D.Demus,in ~ i ~ u Crystals. id ~armstadt: Steinkop~,(H.Stegemeyer, ed.), Springer, New York, pp. K. C. James,Solubility and Rel~tedProperties, Marcel Dekker, New York and Basel, p. J. H. Hildebrand, andE. L. Scott, The Solubili~of Non-Electrolytes, 3rd Ed,, Reinhold Publishing Corporation, New York, p. ~ a n d ~ o o k o f C h e and m i sPhysics, t~ 64th Ed.,CEC Press Inc, Boca Raton, Florida, Pelzl, in ~ i ~C~stals. ~ i d ~armstadt: Steinkop~, (H. Stegemeyer, ed.), Springer, New York, pp. Inagaki, Carbon Y. Hishiyama, Y. Kaburagi, and M.Inagaki, in Che~istryand Physics (P. A. Thrower, ed.), Marcel Dekker, New York, pp. Car~on, et and E. U. Qin, Carbon A. Oberlin, and T. Noda, Tanso chi^. Phys. S. P r e g e ~ ~ and nChiche, chi^. Phys. P. Chiche, S. Durif, d S. Pregemain, chi^. Phys. Durif, chi^, Phys. d J, E, Comte-Trotet, ~ i c r o s c ~pectrosc. . S. de Fonton, A. Oberlin, and

Inagaki,

ate^ Science

Fischbach, in C h e ~ i sand t ~ Physics Carbon, (P. L. Walker, 1 Dekker, New York, pp. t, C h e ~ i sand t ~ Physics of Car~on,Vol7, L. Walker, Jr. ed.), elsker, pp. L, D. Jaffe, and J. E. Jepson, 3rd ~iennial Co~fe~ence on and Y. Sawada, Proceedings 4th J a ~ a n

Intern. ~ A ~ ~P E y ~ ~ ,pp., ras Na~lin,Th8se Ecole des

ines, Paris University,

234. J. Mering and Maire, in d'Etude lis M a s ~ ~ n , 1965, pp. 229-192, 235. Maire, d 'E 236, 237. H. Shi, N. Reimers, 8 2 7 - ~ 3(1993). ~ 238" Y. G. Andrew and T.~ ~ n d s t r ~ m . 28, 534-539 (1995). 239. V. Suresh M.S, Seehra, 34, 1259-1265 (Z996), 240. F. Guillet, Monts, 37260 France, 1996, p. 241. K,Lafdi, S, ~ o ~ n a and ~ y , Oberlin, 30, 533-549 (1992) 242. Edstrom and X. Lewis, "7,85-91, (1969).

I. Introduction

150

II. ~raphitizationunderPressure A. ~ x p e ~ m e nSetup t a ~ andProcedure B. Structure Changes in Carbons with Oriented Texture C. Structure Changes in Carbons with ~ a n ~ o Oriented ml~ Texture D. TextureChanges E.Mechan.ismof ~raphitizatio~ under Pressure F. Changes in. Bulk Properties

152 152

1111. ~ a p ~ i t i z a t i oin.nCoexistence with Minerals under Pressure A. ~xpe~imental Setup B. CoexistencewithCalciumCarbonate C, Coexistencewith Calcium Oxide D. CoexistencewithCalciumHydroxide E. CoexistencewithCalciumFluoride F. Accelerating Effect of Calcium Compounds on ~raphitization

177 178 180 184 186 187

*Retired

154 160 165 171 174

188

G.CoexistencewithVariousMinerals H. Mechanism for Acceleration of Graphitization IV. Stress GraphitizationinCarbon/CarbonComposites A. Stress Graphitization in Carbon-Fiber/Glass-like-Carbon Matrix Composites Carbon/ B. Texture Development of the Matrix in Carbon Composites C. Mechanism of Graphitization in CarbonlCarbon Composites D. Effect of Matrix Texture on the Fracture Behavior and Mechanical Properties of CarbonlCarbon Composites E. Effect of Fiber-Matrix Bond Interface on the Thermal Expansion of CarbonlCarbon Composites F. Effect of Fiber-Matrix Bond on the Mechanical Strength of Thin- all Carbon/Carbon Composite Tubes

193 194

195 196 200 208 213 222 232

Conclusions

239

References

242

Graphite is one of the allotropic forms of carbon and is defined by the chemical bonding of sp2 conjugated orbitals, which results in the formation of hexagonal layers, The word “graphite” means the crystalline structure these hexagonal carbon layers are stackedin parallel arrays with regularity of ABAB.. On the other hand, stackingof layers canalso be irregular, whichis called “turbostratic” [l]. Also, it is possiblethatintermediatestructuresbetweenturbostraticand graphitic,includingtherandomstacking of hexagonalcarbonlayers,canbe controlled by properly selecting the raw materials and heat-treatment conditions. Graphitization is theprocess of improvingthestackingregularitythat is accompanied by the growthof the dimensionsof hexagonal layers and the number stackedparallel layers. The process of graphitization is understood to be thermally activated [2] and has been studied by various authors functions of heat-treatmenttemperatureandresidencetimeusingdifferentrawmaterials. Consequently, various physical properties, such the thermal, ~echanicaland galvanomagnetic propertiesof these materials, canbe altered by orders of magnitude with graphitization, well causing pronounced changes in chemical properties. this structural transformation occurs from a random stacking to t~ee-dimensionallyordered state,the material also changes from being isotropic to beco~inghighly anisotropic, is shown under examination with the optical microscope. Furthennore, the use of such anisotropi~materials matrix in woven carbon fiber composites (typically referred to carbon/carbo~composites) also changes their the~al-mechanicalproperties such strength, strain,

toughness, and thermal expansion It has recently been pointed out [4] that the knowledge of how to obtain particular structure and texture, and therefore certain propertiesof the matrix,is of primary importancefor developing advanced carbon materi~s,including carbon/carbon composites. The regular stacking of hexagonal carbon layers is known to have an interlayer spacing of 0.3354 nm, which is found in well-c~stallizednatural graphite, and large crystallite sizes both along and c-axes. But the turbostratic structure has little larger spacing, approximately 0.344 nm, and usually smaller crystallite sizes. The word “gra~h.itization’~ sometimes been used inbroader sense to mean a heat treatment at high temperatures irrespective of whether t~ee-dimensional graphite structure is developed in the carbon materials ornot, In this review, the tern “graphitization” is only used to mean the development oft~ee- dimension^ graphitic structure.That is, the graphitization process changes the interlayer spacing from about0.344 to 0.3354 nrn and increases the crystallite sizes both. along the and c-axes. The graphitization from turbostratic to graphitic structure is accompanied by volume decrease, can be seen from the decrease in interlayer spacing and the growth crystallite. Because of this volume decrease, it is reasonable to hypothesize the application mechanical stress during the heat treatment of carbon may favor graphitization. Therefore, numerous experiments were undertaken to determine the influence of mechanical stresses on the graphitization process. Mrozowski proposed that the stress generated, due to anisotropic thermal expansion of carbon layers, cancause graphitization to occur.The group of Noda expe~mentallyd e t e ~ i n e that d pressure enhances the graphitization of carbon 181, The study on graphitization under pressure was continued using various carbonaceous precursors that resulted in pronounced effect on the texture of the precursors 19-36], The possible mechanismfor such changes was discussed. The application of pressure in particular temperature range during the early stages of pyrolysis of organic precursors was also found to be important in determining whether graphitization can take place in certain carbon materials [37-41,821. It has been observed that naturally occurring graphitecrystals can have a very highdegree of crystallineperfectionin the presence of otherminerals. The thermal history of these natural graphites, which geologically estimatedfrom the mineralssu~ounding them, has been pointed out to be very mild, temperatures less than 1000°C and pressures about 0.5 GPa [42,43]. These conditions for the form~tionof natural graphite were much lower in temperature than those experimentally determined for the graphitization of carbon materials, ~ o ~ s e ~ u e n t l y , experimental studies were~onductedon the effect of the coexistence ofm i n e ~ ~ s on the graph.itization of carbon in combination with pressure [44-501. significant acceleration of the graphitization was found to take place the in presence of calcium co~pounds. Graphitization can also be accelerated by stresses due to the shrin~age organic precursors that are used to form the m a ~ in x c ~ b o n / c ~ b ocomposites, n

such furfuryl alcohol condensates [51]. Thisis possible if the shrinkageof the organic precursor is inhibited at the interface between the organic matrix and a carbon fiber to which it is bonded [51-54]. In this region, mechanical stress, which was estimated to be few tenths of GPa, is produced and accumulated during the carbonization of matrixprecursor. This causedtheoccurrence of graphitization, which appeared to occur in the vicinity of stress accumulation [55,56]. This graphitization phenomenon has been extensively studied, scientifically and technologically, to develop the proper conditions for altering the thel~al-mechanical properties of this type carbonaceous material since it has unique capabilities for use in advanced technology [57-701, In the present chapter, we review the graphitization of various carbon materials under applied pressures, the acceleration of graphitization with the combination of pressure and coexistence some minerals, and then the graphitization in some types of carbodcarbon composites due to the stress accumulation effect. The structural changes due to stress, i.e., stress graphitization, is mainly described on the basis the experimental results from x-ray powder diffraction, magnetoresistance ~easurements,polarized light microscopy, and high resolution transmission electron microscopy. The changes in thermal-mechanical properties of carbod carbon composite materials that are associated withthe stress graphiti~ationare also discussed in Section IV.

The graphitization of carbons under pressure was first shown using carbon derived from pitch coke at temperature of 1600°C under pressure of GPa [6]. This result was significant because this coke graphitized only 2500°C above under atmospheric pressure. Subsequent studies have shown that the pressure can be reduced to about 0.3 GPa [7] and that even nongraphitizating carbons, such carbon derived from phenolic resins, can be converted to graphite at around 1600°C under pressure [lo]. These experimental results have been reviewed by one of the authors l]. In this section, the experimental results of the graphitizationof various carbon materials under high pressure are summarized after short explanation of the experimental setup and procedure, These results are presented according to the changes in structure and texture in the various carbons, along with some of their bulk properties. Then, possible mechanisms for causing this graphitization phenomena to occur with pressure are discussed.

For most heat treatments under pressure, simple piston-cylinder typee ~ u i p ~ e n t is used. The setup in pressure vessel is illustrated in Fig. 1. The setupbeginswith pressure-transmittingmedium(themineralpyro-

r

FlG. 1 Cell ~ ~ g e i ~for e the n t graphitization of carbon under high pressures [7]. (a) pyroph~llite,(b) steel plate, (c) graphite plate, (d) pyrophyllite plate, (e) graphite plate, (f) sample powder, (8) graphite heater, (h) boron nitride, (i) boron nitride disk, and (j)Mylerasbestos composite paper.

phyllite A120,.4Si02.H20) and a heater g ~ m ~ u f a c t u r efrom d a high-density graphite block that was heat treated to 3000°C). In order to reduce the temperature gradient along the heater and also to obtain better thermal insulation, a disk d of pyrophyllite is placed in the heater. The sleeve and disks of boron nitride, h and i, respectively, act as the protectors to avoid the reaction of carbon heater g and plates c with the pressure transmitter a and d made of pyrophyllite. The steel plates b, placed at the upper and lower parts of the cell, are used for assurance of a good electrical contact between the hard steel of the piston and the graphite plate c. The carbon plate e avoids possible cont~minationof the sample f from boron nitride i. Heating of the sample powder f was done by supplying electrical current to the graphite heater g through the upper and lower pistons. Electrical insulation

between the pistons and cylinder was obtained by placementof a 0 . 2 - m thick Myler-asbestos composite paper The temperature of heat treatment was estimated from a calibration curve between electrical power input and temperature, which was determined under pressure by inserting an alumel-chromel thermocoupleinto the cell packing that su~oundsthe sample coke powder. The calibration curve was d e t e ~ n e up d to 1200°C.Above this temperature, the power-temperature relationship was extrapolated and verified by measuring the melting point of platinum in the cell under 0.5 GPa. The heat treatment temperature was estimated to an accuracy of 50°C. In this cell, a temperature gradient was observed along the heater, but them ~ i m u m temperature difference between the center and the endof the carbon specimen was less than 10% [8,9]. The pressure was calibrated by m~asuringthe volume decrease during the phase transitions of NH,F (0.37 GPa), KN03(0.36 GPa), and AgI (0.30 GPa) at room temperature [8,72]. The pressure exerted on a specimen in this cell is primarily compressive in a uni-axial direction parallel to the motionof the piston[9]. The carbon samples usedfor these graphitization studies under pressure had a wide range of textures from highly ordered, with planar, axial, and point orientations, to random or disordered. The samples used are listed in Table 1, with the a short description of the type of raw material and the preparation ost of the samples were in the powder form with thesize grain being in the range of 40 to 70

In Fig. 2, the changesof 004 diffraction profiles with heat treatment temperature (HTT) under atmospheric pressure and a pressure of 0.5 GPa are compared for the powder of a typical graphitizing carbon,coke PV-7, which has been prepared by the carbonization of polyvinyl chloride at 680°C [7]. Under atmospheric pressure, the 004 diffraction line shifts towards the highangle side and its profile is sharpened in a sym~etricalmanner with increasing TT (Fig. 2a). This effectis characte~sticof various graphitizing carbons under atmospheric and reduced pressures,pre~iously as pointed out by different authors. This behavior suggests that graphitic t~ee-di~ensional stacking occurs random1 in c~stallitesat high temperatures.Conse~uently,the average interlayer spacin alongthec-axis, graduallyaches the spacing of graphite, 0.3345 nrn e profile of the 004 diffraction lines, at 1460 and 1520°C (Fig. 2b), appears to be a composite, consist in^ of two peaks where the i n t ~ r l a yspacings ~r are roug 0.336 and 0.343 nm (54.5 and 28), respectively. Withthe increase in T under pressure, the peak loc

Samples Used for Graphitization Under Pressures Sample Polyvinyl chloride coke

Pitch coke Graphitized fine powder Carbon fiber Fluid coke Gilsonite coke Mesocarbon microbeads

Carbon black Phenol resin char

Carbon beads

Glass-like carbon Sugar coke TS-carbon Coal

Sample code method

Raw materials, preparation

PV-7 PV- l PV- 1 7 PV-20 PC POCO

Polyvinyl chloride, 680°C PV-7,1S00"C PV-7, 1700°C PV-7, 2000°C Pitch coke, graphitized Fine powder (POCO), graphitized

PAN

PAN-based carbon fiber, ca. 1300°C

Fluid Fluid-P Gilso "-C "-P "-M "-PC Black PH-7 PH- 1S PH-20 CB-S-l0 CB-S- l S CB-L-10 CB-L-20 GC- l 0 GC-S Sugar

Fluid coke, as-received Fluid coke, pulverized Gilsonite coke, pulverized Coal tar pitch, 430°C and 2200°C Asphalt, 430°C and 2200°C Coal tar pitch, 430°C and 2200°C Asphalt, as-received and 2200°C Thermal black, as-received Phenol resin, 700°C PH-7, 1500°C PH-7, 2000°C Carbon beads, 1000°C Carbon beads, lS00"C Carbon beads, 1000°C Carbon beads, 2000°C Glassy carbon, ca. 1300°C Glassy carbon sphere, ca. 1300°C Purified sugar, 700°C Phenol nickelocene, 1600°C Anthracite S u b - b i ~ ~ n ocoal us

Anth Coal

Grain size

40-70 40-70

40-70 long, 10 in dia. 100-200 40-70 40-70

0.3 40-70

20 40-70 40-70 40-70 40-70 40-70 40-70

high-angle side grows, but intensityof the peak at the low-angle side decreases. The profile of the 002 diffraction line changed with HTT in exactly the same manner. These changesin diffraction profile mean that the structure of the samples consists of two different crystallites having turbostratic and graphitic characteristics. This suggests that the graphitization process under pressure proceeds the in follo~ingmanner:some crystallites change to thegraphiticform,butother crystallites remain turbostratic, and the relative number of crystallites having graphitic structure increases with higher HTT and longer residence [7,15,1 time

l

l

CuKa

‘ 2 Changes in 004 diffraction profile the polyvinyl chloride coke with heat treatment temperature (a) under atmospheric pressure, and under the pressure 0.5 GPa.

It should be noted that the structural change, i.e., graphitization, observed from diffraction profilesfor the coke PV-7 starts at about 1600°Cunder atmospheric pressure, However at a pressure of 0.5 GPa and at the same temperature, the transfo~ationis almostfinished.Therefore, it appearsthat pressuret greatly accelerates the graphitization processfor this and other similar typesof carbons. This acceleration was also observed at pressures of about 0.3 GPa Under 1 GPa, the graphitizationof the coke was so fast that the gradual change in the diffraction and the appearance of a composite profile were not clearly observed Therefore the studies on stress graphiti~ationof various carbons were performed mostly at a pressure of GPa. The valueof interlayer spacing, was calculated from the diffraction angles of thecomponentpeaksinthe linesaftertheirgraphicalseparation. An example of the graphical separation is shown for both and 004 diffraction profiles in Fig. 3. The relative area of the peak for the component at the higher angle (with the shorter spacing) to the total of area the observed line was used

tress

b) 004 line corrected by Lorentz-Polarization

observed

l

26

25

28

27 53

degree, GuKa

52

55

28

54

degree, CuKa

Graphical separation of composite diffraction line profile into graphitic component C and ~ ~ r b o s t r aone ti~ (a) 002 line and 004 line,

as a measure of the contentof the graphitic component. In Fig. 4, these two pararneters are plotted against HTT for different residence times under 0.5 GPa The process under pressure appears to be divided into two steps, before and after the value reaches about 0.343nm (Fig. 4a). In the first step, the spacing doozdecreases to 0.343 nm with an increaseof HTT. In this step, the profile is broad and seems to consist of a single component. In the second step, however, a new component appears having a value of about 0.336 nm and its relative intensity increases with higher HTT and longer residence time. Examination, with a transmission electron microscope, showed the coexistence of two kindsof flaky particles: one having diffraction spots with hexagonal symmetry, exactly the same as graphite, and the other consisting of diffuse diffraction rings This observation corresponds exactly to the presence of two Component peaks having the spacings of 0.336 and0.343 nm in x-ray diffraction profiles. Therefore,call wethe componentscorrespondingtothepeaks at the high-andlow-anglesidesas graphitic and turbostratic components (G and T components), respectively. In the secondstep of structure change, a big difference is observed, depending on whether the heat treatments were under atmospheric or high pressures. The high pressure process, above 0.3 GPa, may be called heterogeneous graphitization because of the appearanceof composite x-ray diffraction profiles (Fig. 2b). isThis in contrast to the homogeneous process that occurs at atmospheric pressure where a symmetrical single profile shifts gradually theto high-angle side (Fig. 2a). These two processes, heterogeneous and homogeneous, appear to depend on whether a graphitic andtUrbOStrdtiC structure occurscrystallite by crystallite or randomly in

"0. T-component

G-component

0

HTT

HTT

doo2 /7].

10

A;

one crystallite.The activation energy and activation volume for the heterogeneous process under pressure were evaluated 80 to 120 kcal/mole and to -9 cc/ mole, respectively, based on kinetic studies of the formation of the graphiti~ ~ o ~ p o n eunder n t 0.5 GPa [7].In contrast, the activation energyfor the homogeneous graphitization process under atmospheric pressure was determined to be about 260 kca~mol As seen in the heterogeneous process under pressure, the graphitic component appeared in the second step i.e., after the interlayer spacing reached 0.343 nm. This indicates that graphitization, inthe strict m e ~ i n goccurs , in the secondstep of structure change. On the basis of the changes in structural para~etersand properties ['73], there is good correlation with the previous discussion where ~raphiti~ation under atmospheric pressure initiates the so-called graphitizing of soft carbons spacing of about 0.343-0.342 nm. locks of carbon that were heat-treated under high pressure showed dependence of transverse m~gnetoresistan on magnetic field [33] typicalfielddependencesareshowninFig.teredblocks o b t ~ n e fr d V-'7 that were subjected to differen contain different amountsof the gra~hitic co~ponent. The sample heat-treated 1320°C, which contains only a trace of the graphitic ~omponent,shows negative p/p all magnetic fields (Fig. This is characteristic of tu~bos~atic t the one heat-treated 1780°C and cont~ningmore than 60% of the

c! U

0

graphitic component, shows positive values (Fig. 5b), which is characteristic of graphitic samples. For the samples heat treated between 1390 and 1S4OoC,however, nonlinear dependences of Ap/p on the magnetic field are observed. These magnetic field dependences of h p l p were found to be different from those observed on the carbons consisting of a single component, such as pyrolytic carbons and cokes [74]. This behavior of Ap/p wasexperimentallyreproduced by usingamodel sample, that consisted of a 2900'C-treated pyrolytic graphite, sample PG-2900, with crystalline structure (correspondingto G component) and a 19OO"C-treated coke, sample A-1900, with a turbostratic structure (corresponding to T component), which were electrically connected in series [33]. On this model sample, nonlinear dependences of h p / p on magnetic field are observed, as shown inFig, 5c. This supports the coexistence of two structural components, graphitic and turbostratic, in the sample that was heat-treated under pressure. A significant influence on the graphitization of these carbons, with oriented textures,wasobserved if theyreceivedsomeheattreatment at atmospheric pressure prior to being graphitized under pressure 12,271.In Fig, 6, the changes of the 004 diffraction profile with HTT under 0.5 GPa are shown for mesophase spheres(mesocarbonmicrobeads, MM-C)[27]. The sphereswereformed at 430°C in a pitch and the separated from the matrix of isotropic pitch by solvent extraction.Theheattreatmentunderpressurewasdoneontheas-separated spheres (Fig.6a) and also on the same typeof spheres that were pre-heat-treated at 2000°C under atmospheric pressure (Fig. 6b). On HTT under pressure, graphitization of the as-separated mesophase spheres, appears to initiate above 1800°C, although a small amount the turbostratic component does remain even after the 2000°C-treatment under pressure (Fig. 6a). But, almost no graphitization took placefor the spheres that were preheat-treated to 2000°C and then received heat treatment less to than 1900°C under pressure (Fig"6b). Similar retardation of the graphitization process under pressure wasobserved for differentcokesthatreceivedcertainthermaltreatmentsin advance such as polyvinyl chloride coke, PV-7, which was pre-heat-treated to 1500, 1700, and 2000°C under normal pressure 121, Texture of the as-received material also appearsto have an influence on their graphitizability when they receive HTTs under pressure. For example, commercial gilsonite and fluid cokes, which have a concentric orientation of their carbon layers, showed little graphitization eEect even with heat treatment above1500'C under O S GPa, as compared to the sample PV-7 [19,22].

The accelerationof graphitization under a pressure of GPa was observedto be much more pronounced for carbons with random orientationsof basic structural

28OOOC

18OOOC

Cu~a

CuKa

6 Changes in 004 diffraction profile the mesocarbon microbeads (MM-C) with heat treatmenttemperature under pressure 0.5 GPa (a) sample MM-C asseparated at 430°C and (b) sample MM-C pre-heat-treated at under atmospheric pressure.

units, than that of oriented carbons [lo],as illustrated in the previous section, In Fig, 7a, the change in the 002 diffraction profile HTT with under GPa is shown for a carbon prepared from a phenolic resin at 700°C (PH-7). Composite profiles of the 002 line, consisting of a sharp peak with the interlayer spacing of 0.336 nm and a broad peak with 0,343 nm, i.e., graphitic and turbostratic components, are observed around 1300°C (Fig. 7a). In the casesof the carbons with ran do^ testure, the peak for the turbostratic component was so broad that the separation of two component peaks was sometimes not clear,An example of the graphical separationof the 002 profileis shown in Fig.7bfor HTT 1300°C. Two components, corresponding to the turbostratic and graphitic structure, were confirmed to exist under examination with a transmission electron microscope, one with a diffraction pattern of diffuse rings and the other with a pattern of sharp spots with hexagonal symmetry. single sharp peak, indicating 0.336nm interlayer spacing, was obtained after the heat treatments above 1500°C under 0,s GPa. [10,29,32].

observed

27

28

24

28

/degree, CuKa

20

degree,

Change of 002 diffraction profile of the phenol resin carbon with heat treatment tel~perature under 0.5 GPa and an example of graphical separation

(b)

G,

rs

loa.

0

Changes in content of the graphitic component G (a) andits g-value ES arbon with under 0.5 GPa l l]. Residence time?3 min; delocali and G component, localized spin, Residence time, 20 min;delocalized spin and G component, localized Residence time?60 min; delocalized spin and G component, A; localized spin,

was observed on this graphitization processif the same type of beads received a pre-heat-treatment to 2 100'G under at~osphericpressure In changes are shownfor the 002 profile that indicates graphitization on retarded by a little higher te~perature(ca. 100'G), but is comple 1700°C.These resultsfor the carbon beads with a random textureis in those of mesophase spheres with an oriented texture (Fig. 6). For a glass-like cabon carbonized under atmosphe~cpressure

rnin

g*v&%lue

rnin

l

rnin

b)

g.value rnin

I

g-value

11,

CuKa

(b)

CuKa

Changes of diffraction profile of carbon beads (CB-S) with under 0.5 GPa for hr [26]. (a) sample as received (CB-S-lo), and sample pre-heat-treated at 12100°C under atmospheric pressure of nitrogen (CB-S-20).

(GC-S-lo), a broad peak was observed on the samples heat treated toup1600°C under pressureof 0.5 GPa. Above 17OO0C,however, the00.2 profile changedto a sharp peak having a doo2of 0.336 nm [29]. This change was so sudden that a composite profile was difficult to observe, asis shown for carbon beads (Fig. 10).

The change in the texture formed by the arrangementof hexagonal carbon layers during graphitization under high pressure was studied in detail with high resolution electron microscopy by using carbons with random textures, i.e., glass-like carbon and sugarcoke [29,32]. T~oughoutthis review, the word“grain” is used for the starting or original carbon powders and the word “particle” is for those textures that are formed under pressure in the original grains. Some representative transmission electron micrographs, under bright and dark fields, are shown in Fig. 11, along with the position of aperture to assist in following the explanations [32].

Electron micrographs of the glass-like carbon (GC-10) heat-treated under 0.5 GPa (courtesy Mme Oberlin). 1300OC-treated:bright-field and 002 dark-field (b), 1600OC-treated:bright field(c), 002 dark-field withdifferent aperture positions on 002 ring (d and f) and 10 dark-field (e) with a scheme of the particle, 1700'C-treated:bright-field (g) and d~k-fieldwith aperture position shown on ring (h).

At temperature below 1000°C and under pressure of 0.5 GPa, no change was detected in the texture compared to the original glass-like carbon (GC-IO) random distributionof bright dots with about1 nm size were found ina 002 darkfield image. But, after heat treatment above I300"C, partial aggregation of these dots is recognized by comparison of the 002 dark-~eldimage in Fig. 1Ib to the bright-field image of exactly the same area in Fig.1 This suggests the occurrence of oriented 002 planes in these areas. This partial orientation effect is probably due to a stress concentration at the pointof contact among theoriginal carbon grains. Thisconclusion was suggested by the observations that birefringence was optically observedat contact points between angular grains and these areas increased with higher TT under pressure 15,l ~ i m i l ~ lthese y , oriented particl appeared to increase in number and size for glass-like carbons withthe increase in HTT under pressure. After the heat treatment at 1600°C and 0.5 GPa, representative oriented particle is shown in Fig. 1IC-f, with aninsert of the scheme of the particle.The s u ~ o u n ~ i nedges g of this polyhedralparticlearelightenedupunderthe 0 Id imageswithdifferent the centralpart of this o ~ e n t a t i o of ~ s the aperture (Fig. 1l d and f). particle is darkin all the002darkfieldimageseduponlyunderthe 10 dark field image(Fig. Ile). TheT obser~atio~s suggestthe f o ~ a t i o nof hollow particles with thin walls consisting of oriented carbonlayers, The ratio of

theseparticles,containingoriented layers, to thosewiththeoriginalrandom orientation in the carbon increased with increasing For HTTs. example, only a few oriented particles occurred after a treatment at llOO°C, but roughly one half existed at 1300°C. Furthermore, after the treatment at 16OO0C, most of the particles had an oriented texture, either as hollow or flaky particles, and it was difficult to find any particles which had random orientation. Nevertheless, the structure of these oriented particles was still turbostratic, as observed by the x-ray powder-diffraction method. Above 1700"C, an abrupt development of graphitic stacking order in these oriented particles was found to occur using All the particles consisted of large graphite layers, up to a few hundred nanometers, which showedextensive homogeneous Moire patterns.A representative micrograph is shown in Fig. 1l g andh. This suddenincreaseingraphitizedparticleswithtemperatureunder pressure was verified by the data using the x-ray powder-diffraction method, as explained earlier. The same HTT-pressure effect on texture changes was observed on carbon beads, with a spherical shape, and sugar coke, an with irregular perimeter (Table 1). Both of them consist of randomly oriented carbon layers and are known to be nongraphitizing at any degree of HTT under atmospheric pressure. The preferred orientation of carbon layers was measuredby x-ray diffraction on a bar specimen that was cut from sintered blocks [17,25,31,36]. Someof the orientation functions are shown in Fig. 12 for different kinds of samples that were heattreatedat2000°Cunder 0.5 GPa. The normals to thecarbonlayersin crystallites (the c-axis) are preferentially oriented along the direction of compres0"). sion A high degree of orientation alwaysexists for samples obtained from carbons whose original grains have an oriented texture, such as the coke PV-7, Rather sharp orientation functions are also observed under similar sintering conditions even from carbon beads (CB-L-10) and phenol resin char (PH-7), both of which initially have a random orientation of carbon layers. Again, this partial orientation effect of carbon layers under pressure seems to be dueto the stress concentrati~n that exists at the contacts between particles, as seen under TEM in Fig. 11. Carbon beads, which have a random texture and are isotropic, optically become more anisotropic with heat treatment under pressure, which is seen fromthe optical micrographs of cross-sections under polarized light. (Fig. 13) 1261. Heat treatment at 1300°C under 0.5 GPa creates anisotropic areas at the boundbetween adjacent grains thatare in contact. ~ u ~ h e ~ oit rappears e, thereis a small degreeof alignment of these oriented areas in the direction of the compressive load (Fig. 13a)[IS]. At 1500"G, the anisotropic areas appear to grow preferentially in the compressing direction, as seen in Fig. 13b. Above 16OO0C, the isotropic areas completely disappear and the anisotropic areas become very small (Fig. 13c). This texture, observed under an optical microscope, also corresponds

to the sudden change in the crystalline structure, is observed by x-ray diffraction. It appears that these areas are not only preferentially formed, where the accumulated stress is directed parallel to the compressing direction, but also their growth is by the consumptionof the isotropic matrix.This microscopic observation is p~enomenologica~ly consistent with that whichis viewed with transmission electron microscope, described above (Fig. 11). The orientation carbon layers within these grains was estimated by using the analysis of extinction phenomena and pleochroism, and the rotation the samples under polarized light The carbon layers were estimated to align their planes perpendicular to the direction of the concentrated stress. This estimation was supported by the f o l ~ o w i nmeasurement: ~ the 004 diffraction profilesof the coke PV-7 sintered blocks were measured as function of the rotation angle to the compressing direction shown in Fig. 14, The result clearly shows that the graphitic component is formed with its layer planes preferentially oriented perpendicular to the compression direction. Chard et al. pointed out that the formation of anisotropic areas occurred at

"degree Orientation hour

I

in the blocks sintered under 0.5 GPa at 2000°C

Optical micrographs carbon beads(CB-S-10)heat-treated under 0.5 GPa [26]. Under crossed nicols and cross-section parallel to the compression. 1300"C-treated, (b) 15 0 0 ° ~ - ~ e a t eand d , (c) 1600°C-treated.

53

54

56

degree,

004 diffraction profiles of coke sintered at 1540°C under 0.5 GPa as a function of the angle to the compressing direction U

the contact points between the grains of a glass-like carbon, with irregular shapes, under isostatic compression up to 50 MPa at 2650°C. This result reveals that graphitization can only take place at those locations where the stress is concentrated, such contact points between grains. Furthermore, it is essential that this stress be applied at the beginningof the process if graphitization is to take place, This conclusion is supported by the following experimental results under 0.5 GPa if a rod, made of glass-like carbon, was heated while imbedded in powder of glass-like carbon grains,it was graphitized onits surface, at the contact points withthepowders. ut, no graphitization occurred when the rod was similarly heat-treated in a powder of natural graphite.

The sequencesof the graphitization process under high pressure is summarized in Table 2 11 on the basis of experimental results of various carbon materials under 0.5 GPa and at increasingly higher temperatures. Under pressure, the regions where the stress is concentrated, primarily at the contact points between carbon grains, tend to change to an oriented texture above 1100°C. These oriented regions usually consist of hollow particles (submicron in size, much smaller than the starting carbon grains) with walls that are formed by hexagonal carbon layers withrather high degree of preferred orientation, but still with turbostratic stacking, At this stage, the profiles of 002 and diffra~tion lines are broad and gradually shift to the high angle side with increasing heattreatment temperature. This effect may be called the first step of graphitization under pressure or pregraphitization, since this is a preliminary step for the graphitization step that follows, In orderto complete this first step, it necessary to heat

pressure, are inhomogeneous depending on theirsize and processing conditions. Therefore, as these carbons are graphitized in different ranges of HTT,the diffraction profiles appear be to a composite of T andG components. For these carbons, a retardation of the graphitization process was observed witha pre-heat-treatment was performed under atmospheric pressure. This effect is explained as follows. The growth and the partial improvement stacking order of the crystallites occurred during the pre-heat-treatment in these types of carbons. Andso in order to have enough stress concentration at the contact points the grains, higher temperature andor higher pressure treatments are required.

a pressure of 0.5 The heat treatmentof glass-like carbon spheres (CB-S-10) under GPa creates not only significant changes in structure and texture, as explained in previous sections, but also causes the densification of the particles [31,35]. Figure 15 shows SEM ~ c r o g r a p h of s the surfacesof fractured blocks that have been formed under pressure from carbon beads whose sizes are less than 20 pm [126]. Above 1300- 1400"C, the boundaries of the beads are not clearly defined. Also, it is characteristic in these experimentsto form a cactus-like surface on the beads that grows towards the voids amongthe beads. At 1400"C, these cactuses seem to fill the voids and make the beads polyhedra.Clearly, a necking formation is observed between beads above1600°C and the surfaces become rough. Above 18 0 0 " ~flaky , and needle-like small particles are observed on theofsurface beads. In many cases, the characteristic mo~hologyof the starting carbon materials was maintained even after graphitization under pressure [31]. In Fig. 16, the fractured surfacesof the blocks prepared at1800°C under 0.5 GPa are shown for the chopped PAN-based carbon fibers and the fluid coke. In Fig. 17, thechangesinbullsdensity for sintered blocks prepared from diEerent carbon materials are summarized as a function of heat-treatment temTT) under0.5 GPa for 60 minutes. The texture of the starting carbons is either oriented or random and they have received a pre-heat-treatment temperature, T,, above or below 1500°C [35]. The starting carbons, e.g.,CB-S and CB-L, with a random texture and having been pre-heat-treated below 1500"C, give a relatively low bulkdensity, even after the heat treatments under pressure. This is roba ably due to the low bulk density of the starting carbons and their high content of volatile matter. Using the same starting carbons but pre-heat-treated above 00"C, a large increase in density to 1.8-1.9 g/cm3 is observed around 16001700°C. This signi~cantdensificationcorresptheabruptgraphitization of these sta~ingcarbons,aspreviouslymentioever, for theorientedcarbons, pitch coke, and mesophase spheres (P of about1.8-1.9g/crn3 is obtainedaftertheheat treatme~ts because they have a rather high density before the treatments.

~ c a n n i nelectron ~ micrographs of sintered blocks of carbon beads (CB-S-10) under 0.5 GPa 1261. (a) 130O0C-&eated,(b) 1400°C-treated, (c) 1600°C-~~ated, (d) 1800°C-&eated.

tron ~ i c r o ~ r of ~ carbon p ~ s blocks ~ r e p ~ e d1800" -based carbon fibers, and (b) from coke.

176

lnagaki and Meyer CB-S,

CB-L,

PH-15,

PH-20

h I

PC ,

I

MPI- P

GC-S

CB-L

1500

2000

HTT /

O C

FIG. 17 Changes in bulk density of sintered blocks with HTT under 0.5 GPa [35].The samples were prepared from carbons having oriented texture, random texture, and different pre-heat-treatment temperatures T,.

oriented carbons with a T, above 1500"C, including POCO graphitized powders with fine grain less than 5 pm, give the highest density of 2.0-2.2 g/cm3 for heat treatments under pressure. The relative amount of crystallites whose layer planes are perpendicular to the compressing direction [36] is characterized by orientation functions. As discussed earlier, some representative orientation functions are shown in Fig. 12 of blocks that are obtained from different starting carbons and sintered for one hour at 2000°C and 0.5 GPa. It is evident from Fig. 12 that the oriented coke PV-7 has a rather sharp orientation function relative to the compressive direction, in other words, a high degree of preferred orientation of the crystallites. In contrast, carbon beads, with a random texture and pre-heat-treated above 15OO0C,have a very low degree of orientation along the compressing direction. These orientation functions

Stress Graphitization

177 100

100

I

I

I

l

l

1

-

0

0 o\ Y

H

CB-S-10 50

50

'r

'

GC-S

L

0

~

"

"

"

"

'

1500 a)

-x-x-

J

2000

I

l

PAN l

I

l

HTT / " C

FIG. 18 Changes in orientation degree I(90") of sintered blocks with HTT under 0.5 GPa [36].

may be represented by using the relative intensity of the 002 diffraction line at the rotation angle of 90", I(90"). In Fig. 18, the changes of I(90") with HTT under pressure are summarized for different starting carbon materials [36]. The oriented coke PV-7 shows only a small improvement of orientation with HTT, because it has a high degree of orientation due to the alignment of platy grains during the packing of the starting materials in the high-pressure cell. However, the phenol resin char PH-7 and the carbon beads pre-heat-treated at 1000°C (CB-L-lo), give a pronounced decrease in I(90") with an increase in HTT. This is attributed to the development of graphitic structure at the stress-concentrated contact points of the starting grains, as described in Section 11 D. On the other hand, the same starting carbon pre-heat-treated above 1500°C (CB-L-20),resulted in rather high values of I(90") even after the heat treatment at 2000°C under pressure. This result is expected due to the lack of a texture change in these carbons.

111.

GRAPHITIZATION IN COEXISTENCE WITH MINERALS UNDER PRESSURE

In nature, a good quality of graphite crystals has been found in beds of metamorphic rocks, such as limestone. Geologically, the temperature-pressure condi-

l

tions for the formation of these natural graphitecrystals were estimatedto be at a tempera~reof several hundred degrees Celsius and a pressureof several tenths it hasbeenshown of aGPa[42,43]. As explainedintheprevioussection, experimentally that carbons are graphitized very rapidly at temperatures above 1600°C and pressures above0.3 GPa. However, no indication was foundfor the presence of a graphitic structure in carbons that have experienced temperatures below 1300"C, even under high pressuresof GPa. Therefore, them e t ~ o ~ h i c rocks in which the graphitic crystals are embedded, may have an accelerating effect on the graphiti~ationprocess, in addition to the acceleration by pressure itself. Thus, experiments were initiated to d e t e ~ n ife graphitic crystals can be grown under conditions similar to those found in nature, including the coexistence with minerals. Since there have been a few reports about the occurrence of h i g h - ~ u ~ igraphite ty crystals in bedsof limestone (CaCO,), various calcium compounds (natural limestone and calcium carbonate, oxide, hydroxide, and fluoride) were selected theas minerals for these laboratory experiments. In relation to the,mechanism of the acceleration of graphitization by calcium compounds, some other oxides, such as alumina, silica, etc., were also examined. Certain evidence exists that a limited degree of graphite formation can occur in the presence of these compoundsat temperatures below 1000°C, under a pressure of GPa [44-SO]. However, the complete conversion to graphite below 1000°C was not attained during heat-treatment time employed in the present experiments. The results of these experiments are presented in this section to show the degree of graphitization that occursfor different compounds, ranges of temperatures, and under stresses produced by 0.3 GPa.

The experimental setup that was usedis shown in Fig.19 [SO], and is very similar to that described in the last section (Fig. 1) except the carbon sample is sandwiched betweentwo disks of minerals, such as the calcium compounds. In most of the experiments, thecell arrangement in Fig. 19 was used. However, whenthe effect of water vapor was examined,the cell ~ a n g e m e nbt was employed. Here, the sleeve of calcined pyrophyllite is inserted, which prevents the formation of water vapor by the decompositionof as-received pyrophyllite above 900"C,The heat-~eatmenttemperature(HTT)wasestimatedfrom the input of electrical power to the graphite heater by using a calibration curve that t&es into account the heat flow through the upper and lower pistons [44], The pressure was calibrated by detecting the phase transitions of NH,F, KNO,, and AgI in the same pressure cell ~ a n g e m e n ['72]. t The heat treatments were performed at various tem~eraturesbetween 800 and 1500°C under a pressure of 0.3 GPa for residence times between 20 and 240 min. a coke prepared from polyvinyl chloride at The carbon sample used was

x

1 Cell a

Cell b

~ ~ a n g e ~ofepressure nt cells used (courtesyProf. S. Hirano) [50]. steel disk, (b) graphite disk, (c) carbon samplepowder, (d)coexisting mineral disk, (e) graphite heater, (f) pyrophyllite holder, and (g) pyrophyllite sleeve calcined 970°C.

shown PV-7 (see Table l), which was also used for the study of the effect of pressure onthe various stagesof graphiti~ation,described in the previous section. About 180 mg the coke was used for each experimental run and its grain size was limited to between 0.1 and 0.4 m. fferent compoundsof calcium, which were formedinto disks 3.5 mm thick mm in diameter, were selectedfor these experiments. Natural limestone from Gifu Prefecture, Japan, was also used Inorder to avoid possible effectsof impurity atoms that occur in naturally occurring minerals, some reagent grade powders were used for making disks. The powders from various compounds used [44-50] are listed in Table 3, together with the porosity of the disks after they were formed. In the pressure cell, temperature gradient along the compressing direction was expected because, after an experiment, it was observed that elongated calcium carbonate crystals had formed in that direction as consequence of calcium carbonate recrystallization The heat-~eatmenttemperature was controlled to within 20°C by adjusting the electrical power input and th con troll in^ the pressure of the oil press to within For comparison, the heat treatmentsof the same coke highpressurewithoutanycoexistingcalciumcompounds. Also, thecouled samples of coke and calcium compounds were heat treat tures under atmospheric pressure. For the former experiments, the disks of calcium compounds (Fig. 19) were replaced by glass-like carbon plates and pyro-

Minerals Used for the Experiments and Their Preparation Condition, C~stallographic tructure, and Porosity after F o ~ i n ginto a Disk Porosity Co~pounds Limestone CaCO, Ca0-9 CaO- 10 CaO- 15 Ca(Ol-f), C@, A1203 NaCO, MgO MgF,

Preparation condition Akasaka mine Reagent grade Calcined CaCO, at 920°C Calcined CaCO, at 1050°C Calcined CaCO, at 1470°C Reagent grade Reagent grade Reagent grade Calcined silica gel at 1000°C Reagent grade Calcined MgCO, at 600°C Reagent grade

5 20 18 15 13 15 18 l? 13 S0

phyilite disksC4.91. Class-like carbon plates were used to prevent the direct contact of carbon sample with pyrophyllite disks, which played the role of electrical and thermal insulation. For experiments conducted at atmospheric pressure, the sandwiches, consistingof carbon sample and calcium compound disks, were placed in glass-like carbon tube, compressed slightly by using carbon screw, and then heat treated in flowing nitrogen gas. The degree of graphitization was studiedby following the change in the x-ray diBraction profileof the lines. Transmission electron microscopy was also ernployed to identify the formationof any component that had graphitic s t ~ c t u r ~ .

InFig. 20, thechangesindiffractionprofile of the 002 line with compared for thee cases: coexisting with under pressure of without underthesamepressure,andcoexistingwith atmos heric pressure [45]. For the heat treatments at high pressure,cell arrangein Fig. l 9 was used and the disks of were 2 appearance of a sharp peak at the diffraction angle spacing, dO02,of about ,336 nm) is only observed when the carbon is heat treated in the presence of under the pressure. This type of le a p p e ~to s start at 10’70°C andreaches l ~ a ~ i m uintensity m at 1280°C. ausethisprofile is similar to that the graphitic component that is observed for the same type of coke at higher temperatures and under the same pressure, this sharp peak is

k

tr

Q) Q)

considered to indicate the existence of a graphitic component in the samples. The presence of fl&es with a graphitic s t ~ c t u r ein these samples was verified by trans~issionelectron microscopic observations.It should be emphasized that the graphitic component first appears as low as 1070°C in the presence of CaCO,, which is to be compared to above 1500°C without CaCO, at the same pressure 0.3 GPa (see Fig. 2b). Employing the same procedure as described in the previous section 3), the (Fig. observed composite profile of the 002 line was graphically separated into two peaks co~espondingto the graphitic component (G) at the high-angle side and the turbostratic component at the low-angle side. For each peak, the interlayer spacing, dooz,and the crystallite size, Lc(002),along the c-axis were determined from the diffraction angle andthe line width, respectively. These parametersare plotted against HTT asfunction of the residence timeat each HTT and shown in Fig, 21a and b The initial appearance of the G com~onentis indicated by mows for each residence time in Fig. 21. For the turbostratic component, the value of doozdecreases ~raduallyfrom the initial value of about 0,346 nm to about 0.344 nm and the value ofL,(002) increases slightly up to 8 nm with an increase in HTT. the other hand, the dooz value for thegraphiticcomponentseemstobeprallyconstantatabout 0.336 nm but the ~,(002)value increases significantly with the higher HTTs. Fu~hermorethese figures show that thef o ~ a t i o nof the G compo~entdepends strongly onthe residence time, suggestingthat the transition from turbostraticto graphitic components is a rate process. The ratio of the peak area for the graphitic componentto the total area of the observed profile was determined in order to estimate the content of the graphitic component f o ~ e dIf. this is plotted against HTT and as a function of residence time (Fig. 22), it appears that the content of the graphitic component increases with higher HTTs and with longer residence times at each HTT. The same acceleration of the graphitization process for the carbon powder was observed using natural limestone 14.41. From the dependence of the content of graphitic component on HTT,s u m m ~ z e din Fig. 22, the acceleration of ~raphitizationthat is caused by natural limestone seems to be more effective than for the reagent grade CaCO,. The sane experiment, d e s c ~ b above, e~ was conducted except that the CaCO, disks were saturated with water vapor (about 8 wt% at room temperatu~e)and the cell ~ a n g e m e nwas t changedto Fig. l9b The graphitization process, using the coexistence with wet CaCO,, seems to occur at a lower temperature than when the dried CaCO, was used, as shown in Fig. 22. After heat treatment above 1000°C at 0.3 GPa with the coexistence of calcium carbonate, the starting carbon powder c h ~ g e dto a sintered block with a thickness of 2.5-3.1 mm. The dist~butionof the graphitic component was obtained by pic~ingsmall specimens outof the block for analysis by x-raydiffrac-

T -component

HTT

b) io00

A

ti-component

I

I

cl

500

HTT

21 Changes in interlayer spacing doo2an under 0.3 GPa. A and residence time

of 20 min;

and

min 1

t3 0

0

ETT Changes in content of graphitic component for the coke PV-7 with and the coexistence with CaCO, under 0.3 GPa residence time of 20 min; min, with the coexistence of limestone for the residence time 20 and 60 min, and with the coexistence of CaCO, with 8 wt% water for 60

tion.Theamount of graphiticcomponentwaslargeratthecenteralong the thickness of the block as compared to its surfaces that were in contact with the calcium carbonate disks. Also, this component was larger the edge at of the block, which was close to the graphite heater and therefore experienced a higher temperature than at the center of the disk.The distribution calcium in these blocks was measured using the electron microprobe analysis method itand was found to Interestingly, the correspond roughly to that of the graphitic component penetration of calcium carbonate could also be detected by the naked eye.

The graphitization of coke PV-7 was studied under a pressure of 0.3 GPa in coexistence with three calcium oxide disks, which received three different calcination temperatures (Table3), that resulted in di~erentdegrees of c~stallinity Variations of the graphiti~ationprocess on PV-7, shown in Fig. 23, were measur~d changes in the diffraction profiles with HTT, whose duration of each HTT was 60 min. With the coexistence of PV-7 and calcium oxide Ca0-9 that has a relatively small crystallite size of 72 nm, the graphitic component first appears at 1100°C (Fig.23a),and it growssignificantlywithincreasing Closeto100%

graphitic component was obtained after the heat treatment reaches 1500°C. On the other hand, no graphitic component is detected in the specimen that was heat treated, even to 1500°C, with CaO-15 that has a large crystallite size of more than l00 nm. Inthe case of the coexistence of CaO- 10 that has an intermediate size of crystallite, the graphitization behavior seemsto be intermediate between Ca0-9 and CaO- 15. The interlayer spacing doo2of the graphitic component in these heat-treated samples was constant at 0.336 nm but its crystallite size Lc(002)increases with higher HTTs. The 002 profile for the turbostratic component shiftedto the highangle side and was sharpened with the increases in similar to that seen from Fig. 20 for CaCO,. The variations induozand Lc(002),however, weresmall, about 0.343 nm and less than 10 nm, respectively, even after heat treatment at 1500°C. The changes in duo2and Lc(002)for graphitic and turbostratic components were similar to those observed in CaCO, (Fig‘ 21).

The effect of the coexistence of calcium hydroxide disks (Table3) with the coke PV-7 was carried outat 0.3 GPa using thecell ~ a n g e m e nof t Fig. 19a [46]. The

results are s u m m ~ z e das changes in the002 profile and contentof the graphitic component for different HTTs (Fig. 24). After heat treatment at 800°C for 60 minutes, the sharp peakof the graphitic component with a duuzof 0.336 nm, is observed to be superimposed on a broad

20nlin

1201nin

min

ae/degree,CuKa Change in 002 diffraction profile with heat ~ e a t ~ eunder n t 0.3 CPa with the coexistence of Ca(OH)2

band from the turbostratic component, which hasa clooz of about 0.345 nm. Even though the graphitic component may be a small amount (ca. 2%), it is worth noting that the temperatures required for the formation of the graphitic Component in coke is lowered to 800°C for the coexistenceof calcium hydroxide, in comparison with about 1000°Cfor calcium carbonate and calcium oxide. These temperatures for the formation of a graphitic component are much lower as compared to the temperature above 1600°C without the coexistence of any minerals. Also, it can be seen in Fig. 24 that the contentof the graphitic component increases with longer residence times at 800°C. This suggests, again, that the formation reaction of the graphitic component is a rate process. In the samples showing a sharp peak due to the graphitic component, flaky particles were detected and characterized using a trans~issionelectron microscope anda diffraction patternof six-fold symmetryfor single crystalsof graphite was observed. Even when the sample was heat treated as low as 600°C for 60 min, a few graphitic flakes could be found under the electron microscope that had a single-crystaldiffractionpattern,though the graphiticcomponent,probably because of its small amount, was not detected by x-ray diffraction. Any heat treatment above 900°C is difficult to carry out because the calcium hydroxide melts and reacts with the graphite heater.

The heat treatments of coke were carried out in coexistence with calcium shown in Fig. 19, under a fluoride [50] by using cell arrange~ents pressure of 0.3 GPa. After the heat treatment was conducted above 1300°C in cell the arrange~ent the graphitic component was observed and identified using the002 x-ray difaction profile and the transmission electron microscope. The of content graphitic componentincreasedrapidlywithhighe Tsand becameca. 100% above 1500°C. The presence of calciumoxidedetectedinthedisks of calcium fluoride that were in contact with the carbon specimen, heated above 1300" effect is attributed to thereaction ofaF,with watervapor from the phyllitesleeve to producereactiveCaChangesinthecontent of the graphitic component with TT at 0.3 CPa are shown in Fig.25, It also shows, for co~parison,the curve thatis derived by using coke with the coexistence of (see Section C). Inthe cell a ~ a n g e ~ e n t wherenowatervaporwasexpected from the pyrophyllite sleeve, no graphiticco~ponentwas foundeither on the x-ray profile or by electron microscopic observations, even after a heat treat~entat 1500°C. Furthermore,calciumoxidewasnotdetected,but ~ e l t i n gof theCaF2was observed above 1300°C. Therefore, it appears thatthe content of the graphitic component increases with

HTT

5 Changes

content of graphitic cornpon ence of (courtesy Prof. S. [50]. in the cell a with the coexistence of Ca0-9.

in the cell

and

HTT in the presenceof calcium fluoride onlyif water vapor is present, to form in the same way as it did with the coexistence of reactive CaO.

The two graphitization processes, with and without calcium compounds, have certain common factors in some respects, but different in others. Carbons, including thecoke are known to be graphitized under pressures above 0.3 GPa at temperatures above 16OO0C, as explained in the previous section. With the coexistence of calcium compounds, graphitization of the coke was found to be accelerated with calcium hydroxide, starting at a temperature as low as 800°C. Both typesof processes showa dependence on the residence time at each temperature of heat treatment, although the effect of temperature is more pronounced with the coexistence of minerals. During the courseof graphitization, the diffraction profiles, in both bases, area composite of turbostratic and graphitic components. This revealsa heterogeneous transfo~ationof the carbon from the turbostratic to graphitic structures. h the case of the graphitization of pure carbons

under pressure, there is a preferential change the to graphitic structure due to stress concentration points, which occur at and nearthe contact points between adjacent carbon grains. The ~raphitizationof pure carbon under pressure was found to occur when the turbostratic component has a of approximately 0.343 nm, Incases where the coexistence with calcium compounds under pressure occurs, the graphitic co~ponentappears at lower temperatures and even before the value for the doo2of the turbostratic component has reached 0.343 nm. In Fig. 26, the changes in two components, graphitic and turbostratic, are compared using PV-7 with different HTTs at 0.3 GPa, with and without the coexistenceof CaC03. This is shown schematically based on the results of Figs. 4 and 21. The graphitic compon~nt appears at lower temperature with the coexistenceof CaCO, than without it and before the spacing of turbostratic component reaches 0.343 nm. Therefore, it is reasonable to suppose that the occurrence of graphitization is accelerated by the coexistence of CaCO, with carbon. Afterheattreatmentunderpressureandwith the coexistence of calcium compounds, the sintered carbon blocks always contain a certain amount of calcium. Figure 27 shows a representative distribution of calcium in a carbon block, whichhasbeenmeasuredthroughitscrosssectionbyscanningtheelectron of the graphitic componentis an averaged microprobe [48]. Although the content value over a rangeof distance, as shown in Figs. 27a and b, there appears to be a certain direct correspondence between the content of graphitic component and the amount of calcium present in the sintered disk, as mentioned in SectionI11 B.

0.345

RI

CaCQ3

0.340

0.335

1000

1500

2000

Appearance graphitic component by the heat treatment coke (PV-7) under 0.3 GPa with or without CaCO, (schematic).

polyvinyl chloride

of

18

h, mm

L

n

GPa

his suggests that the important role calcium atoms in accelerating the er pr~ssure.This concl~sionis istent for all the examined: carbonate, oxide, e, and fluoride. disks the c a l c i compounds ~~ were found to parbe tially recrystallized. In addition, the appe~anceof the graphitic co o be closely related to the degree recrystallization g. 28, the content of raphitic co~ponentat the center of a sintered carbon block is plotted against the thickness the rec~stallizedlayer in the disks after heat ~ e a t ~ e natt s for three different residence

Thickness of recrystallized

3

mm

Relation between content of grap ic component and thickness residence time of 20 min; layer formed under 0.3 GPa [45].

times [45], It appears, irrespective of heat-treatment temperature and residence time, the amount of the graphitic componentis closely related to the thickness of the recrystallized part in the CaCO, disks. According to Wyllie and Tuttle [42], the melting pointof calcium carbonateis lowered by the addition of water. For instance, the melting point of 1310°C at0.1 CPa is lowered to 1130°C by the additionof 8% water.In these coexistent mineral experiments, it was observed that the coke PV-7 started to be graphitizedat about 900°C when the calcium carbonate had absorbed about 8 wt% water. This is reduction of temperature of more than 100°C over dried calcium carbonate (Fig. 26) [45].Since calcium hydroxide hasmelting point at 840°C under pressure of 0.3 GPa, the heat treatmentsof the coke coexisting with Ca(OH), were performed below this temperature. Interestingly, ~raphitization the carbon was observed at temperatures low 600°C [46]. All of the e~perimentalresults presented above suggest the following two mechanisms for the graphitizationof carbons under pressure: intermediate formation of either calcium metal or calcium carbide [50]. The following chemical reactions represent these two possible mechanisms by using CaO: CaO

C(turb0stratic) 3C(turbostratic)

CO

CaO CO

C(~raphitic) 3C(graphitic)

(1) (2)

Experiments indicate that calcium carbide was present in the carbon Specimens that were heat-treated above1200°C at the interface where the carbon specimen was in contact with the calcium oxide disk. Qn the other hand, calcium carbide was never detected in specimens that were heat-~eatedbelow 12OO"C, and also it is difficult to cause the decomposition of calcium carbide below 1200°C under pressure [50], Therefore, the first mechanism may d o be ~ n a nbelow t 1200°C and the second above 1200°C. The fact that the more reactive calcium oxide reacts with carbon at the lower temperature agrees with the experimental results the from threecalciumoxidediskswithdifferentdegrees of crystallinity[47].When calcium carbonate and calcium hydroxide were usedthe coexisting minerals, a nascent oxide is formed at temperatures around their melting points and it may react with carbon to form a reactive calcium metal. When calcium carbonate was used, the following reaction between carbon dioxide produced from the carbonate andthe carbon sample can acco~ntfor the f o ~ a t i o nof the graphitic component:

The isotope content in the graphitic component formed in coexistence of limestone under a pressure of 0.3 GPa could be estimated because the carbon derived from organic precursors,like has an usually low contentof Cl3+The means resultsaresummarizedinTable 4 [so], wherethenegativevalue for less Cl3 concentration than for the standard gas. But it should be noted that the values represent only therelative abundance, Thus the carbon in limestane contains a much larger amount of Cl3 than in the original sample PW?. The results in Table show that the carbons heat treated in coexistence with

4 Results of Carbon Isotope ~easurements

Content (9%) Original sample Carbon heat-treated at ll00OC in the coexistence of limestone Carbon heat-treated at 1250°C in the coexistence limestone Carbon contained as carbonate group in the limestone

courtesy Prof.

Hirano

-0.66 4-0.00

15

1.20

34

4-20.14

limestone above 1000°C have higher valuesof &Cl3than the starting coke PV-7. This suggests the transformation of carbon atoms from limestone carbonate to carbon during heat treatment. From the 6C13values measured the starting samnt ples, PV-7 and limestone, the values for the graphitic c o ~ p o ~ e (titled [SC13]calc inthe Table) are estimated by assuming that the turbostratic compor?ent has the same&Cl3value asit had in the original PV-7. It appears from the values in Table 4 thatthegraphitic c~mponenthasahigher value,i.e.,ahigher concentration Cl3, after heat t r e a ~ e n with t the coexistence of limestone, than the original PV-7. This result suggests that the CO,, formed by the deco~posi~ion the limestone, reacts with the carbon sample according to Eq. (3) to produce CO. Consequently, CO can take part in the reactions between CaO and the carbon sample [Equations (1) and

The heat ~ e a t m e effect ~ t on coke PV-7 with the coexistence alumina, silica, magnesia, sodium carbonate, and mag~esiumfluoride (Table 3) was det~rmined under a pressure of GPa by using cell ~ ~ g e m e nint Fig. 19 for a residence time of 60 f i n 1501. In Fig. 29, the content of the graphitic component formed is plotted against HTT in the presence of alumina and silica, In both cases, the graphit~zatio~ is

0 NTT

29 Changes content alumina and silica (courtesy

graphitic component with HTT in the coexistence with Hirana)

accelerated above 1200- l.300"C. But at 1500°C, thecontent of graphitic component exceeds 30% for the coexistence with alumina, but only 10%in the case of silica. By the x-ray diffraction method, aluminum carbide was detected in the carbon specimen atthe interface with the alumina disk. However, when silica was used, no silicon carbide was detected, even after the high-temperature treatment was conducted at 0.3 GPa. It is noted that the shape of these curves is similar to those seen in Fig. 25 for calcium compounds that .contain oxygen and fluorine. In the case of magnesia, no peak for the graphitic component was observed on the diffraction profile. But a small number of the particles were found with the transmission electron microscope that had graphitic diffraction pattern after heat ~eatmentsabove 1400°C. The number of graphite particles seemed to increase with higher HTTs. In the case of magnesium fluoride, the graphitic component was very small and could only be detected by using the electron microscope, even after using heat treatment at 1500°C. With the coexistence of sodium carbonate, graphite particles were detected with the electron microscopeafter heat treatment low 500°C for 60 min. The number of particles seemedto increase for higher HTTs and longer residence times, expected, because the process seems to be di~usion-controlled.However, the heat treatments above 800°C were not possible becauseof the melting and decomposition of sodium carbonate.

Combining the experimental results from the coexistence of various calcium comof various minerals (Section I11 C), pounds (Sections111B-I11 F) along with those two possible routesfor graphitization of carbon can be assumed These can be expressed by using an oxide follows; at low temperatures, C(turb0stratic)

n/S,O,-,

CO

C(graphitic)

and at high temperatures,

rnMxOr

(n

my) C(turb0stratic)

M,C,

myC0 (n +my) C(graphitic)

In the case of calcium compounds and alumina, these two routes are possible for heat-treatment conditions under pressure 0.3 and at temperatures up to 1500°C. Even in these cases, the reactivity of the compounds that coexist with carbon is very impo~antfor these two reactions to proceed, has been seen for calciumcarbonate.Theformation reactivecalciumoxide thereaction between the starting calcium fluoride and water vapor under high pressure and te~peraturewas favorable for accelerating the gra~hitizationprocess (Fig. 25). In the casesof silica and magnesia, onlythe first route, Equation(4), seems to

be possible under the present conditions of heat treatment. form silicon carbide, a slightly higher temperature than 1500°C was needed under pressure. the In case of magnesium fluoride, magnesium oxide was not formed by the reaction with water vapor under high pressure and temperature.a consequence, only minute amounts graphite particles were found after the 1500°C treatment. From these experimental results, it is concluded that graphite can be formed under mild conditions (below 1000"~and 0.3 GPa) with the catalytic action of as calcium. If we could realize extremely long residence some metal species, such time under these mild conditions, it appears possible to reproduce graphite crystals in the laboratory that are similar to those formed geologically in nature. Recently, the graphitization of anthracite was reported to occur as low as 600°C in simple shear under pressures of 0.8-1 .0 GPa In this case, there also have been some accelerating graphitization effect of dunite, which was used to transfer the shear stress to the anthracite sample, although the authors did not mention it. ~ e v e ~ h e l e sthis s , result may support the general conclusion of the present section.

In this section, the applicability and importanceof the stress ~raphiti%atio~ phenomena are presente~by numerous examples to illustrate how the thermal and ies of c~bon/carbon co~posites

the number of fibers ina yarn from one thous lso, the ams can be woven into different or even more.

~ ~ i a t i o in n sthe matrix structure and tex d e t e ~ i n i n gthe t ~ e ~ and a l ~echanical conductivity, fracture behavior, strength, and toughness. For example, fractu~e the

I behavior within the composite is controlled by the propagation of cracks within the matrix, at the fiber-matrix interface, and across the fiber, as will be discussed subse~uentl~. By controlling the architecture of the yams and the st~cture/textureof the matrix, can have the highest specific strengths and moduli of any high temperature material, including graphites. This is possible becauseof the low specific density and the high strengths of the carbon fibers, especially above 1500 to ~000'C.Although are known primarily for their high-temperature properties, they also can have unique thermal properties, such as zero coefficient of thermal expansion near room temperature and negative values below room temperature. Yet, the specific thermal conductivities of in this temperature range can exceed that of copper. As a result of these properties, the thermal stress resistance of exceeds that of all other materials above 1700'C, including bulk graphites, In this section, the structural and textural development in the matrix is discussed, especially while itis being subjected to high-temperature heat treatment, Emphasis is placed on the stress g~aphitizationeffect due to the interactions between the carbon fiber and its su~oundingmatrix. In order to show stress graphitization, thecarbon-fiber/glass-like-carboncomposites are selected, where the structurechange to graphite is more clearly demonstrated. Subse~uentsections discuss the influence of the struc~uraland textural changes on the physical properties of tubes in order to show how such changes can enhance the broad applicability of this unique type of carbonaceous material

Composites of carbon fibers with glass-like carbon matrix were made of PANbased carbon fibers that were aligned unidirectionally in furfuryl alcohol condensates and carbonized up to1000°C. Then these composites were heat treated up to 2800°C before ~easurementswere made of density, x-ray data, and magnetoresistance 1 In Fig.30, the x-ray diEractiondata, and profilesof the 004 line of composites with different fiber fractions are compared to those for the component carbons, carbon fiber and glass-like carbon, all samples being heat treated ~800'C. to First, r it should be mentioned that these x-ray profiles for the are very s i ~ i l a to those observed when carbons are heat treated under pressure, as previously described in Section Also for the composites,it is found that the spacing for the component atthe h i ~ h - a n ~side l e of the profileis about 0.336 nm, whichis the same as that observedfor carbon that has been graphitized under high pressures. On the other hand, individual component samples of the PAN-based carbon fiber and the glass-like carbon that were individually processed under exactly the

v01

I

FIG.

004

I

c~bon-fiber/glass-li~e-carbo~

is

[77]. [78].

interaction between these two components in the composites during the high te~peraturetreatment. It shouldalso be pointed out that the asymmetry of the x-ray profile seems be to more pronounced in the composites with lower fiber fractions. This means that composites whose fiber fraction is 30 vol% containa larger amountof graphitized matrix than those with60 vol% fiber fraction. Thisis due to stress grap~itization that is attributed to the graphitizationof the matrixby the interaction betweenthe fibers and the matrix at elevated te~peratures. Optical microscopic observations were carried out, under crossed nicols, on polished cross-sectionsof the (Fig, 3 previously mentioned,it hasbeen

Optical ~icrographsof the cross-section of the composites [Sl].(a) after curing n 30 vol%), (b) after c~bonization(30 vol%), (c) after ~raphitizationup to (d) after graphiti~ationup to 2800°C (30

known that the glasslike carbons by themselves look isotropic under an optical microscope, even after being heat treated at high temperatures. Also, PAN-based carbon fibers, alone, appear isotropic also. But, combiningthe carbon fibers and glass-like carbon matrix ina composite results in large changesof texture in the matrix afterthe composite is heat treatedat high temperatures. For example, after curingthefurfurylalcoholcondensates at 1 noanisotropicregionsare observed (Fig, 31a). But, after the heat treatment at lOOO"C, anisotropic regions develop at the fiber-matrix interfaces (Fig. 3 1b). In these anisotropic regions, from the analysis of pleochroism, the basic structural units of small carbon layers were found to align parallel to the fiber surface andto the fiber axis. These anisotropic regions were greatly enlarged by the high-temperature heat treatment of these composites, such as to (Fig. 31c). Furthermore, because they are isotropic, few fibers are distinctly seen due to the enlargementof the anisotropic regions. The developmentof these anisotropic regions corresponds to that of the graphitic component's peak in the x-ray diffraction profile. After treatment, almost all matrix becomes anisotropic, as shown in Fig. 31d, and isotropic areas due to carbon fiber cross sections are difficult to recognize, though there are some. This graphitization effect is because in a stress fieldis created during heat treatment between each fiber and its surrounding matrix. This field tends to create a texture of concentric orientationof whose c-axis radiates from the center of the fibers. Figure 32 shows the changeof the 002 profile with HTT for composites with a fiber content of ~01%.The line gradually shifts to the high-angle side above and the asymmetry of the profile becomes more evident with increases of HTT. This behavior is also similar to that which is produced under pressure (see Section 11). The magnetoresistance (Ap/p) was measured on these types of composites in order to evaluate the degree of graphitization and orientation of carbon layers along the fiber axis. The dependence of the maximum transverse magnetoresistance (Ap/p),, on the magnetic field are shown in Fig. 33 for the together with individual components of carbon fiber and glass-like carbon. From this figure, the value (Ap/p),,, of the composite changes its sign from negative to positive at a temperature between and suggesting that this is the temperature range where the growthof the graphitic structure startsto occur. It should be mentioned thatthe values of ( A p / p ) ~for ~ the composite after treatment are comparable to the values of nucle~-gradehigh-density isotropic graphite blocks, which received a heat treatment at and therefore have a very high degree of graphitization. In contrast, there is no indication for such a graphitization transformation for either carbon fiber or glass-like carbon, as they retain negative values (Fig. 33) even after being heat treated to above

I

25

26

28/degree,CuKa FIG. 32 Changes of 002 diffraction profile with HTT on the c~bon-fiber/glass-likecarbon composite [Sl].

In order to verify the graphitization effect occurs in the matrix, the magnetoresist~cewas measured on a single fiber that was stripped from the composite after being heat treated at2800°C. It was clearly shown that the fibers stripped from the composites had negative values of magnetoresistance, i.e., were not graphitized even after 2800°C treatment. The same results were obtained from fibers that were stripped from composites of other types of samples.

The distributionof matrix within can be divided into two major categories: “inter-yarn” or between yams, including voids between yams due to weaving geometry, and “intra-yarn” or between fibers in the yarn. Since the stress graphitization effect depends largely onthe degree of interaction between a fiber and its

4

Naqnetic field H

kG

4

Magnetic field H

kG

2

~ c r o g r a p hof a section perpendicular to the fibers in a lilce-cwbon matrix showing bonded and unbonded regions

with glass-

surrounding matrix, which represents the location of the highest percentageof on the intra type of matrix matrix in the composites, we concentrate in this section distribution andits related effects toward changingthe thermal-mechanical properties of CICs. The matrix that surrounds each fiber generally appears to be partially continuous at the interface between the fiber and the matrix. This is evident, as shown in Fig. 34, from an§EM micrograph of PAN-based carbon fibers in a phenolic resinbased matrix. After curing the sample' at l 10°C, it was heat treated at approximately 1000°C. Its cross section was polished using metallographic techniques and then etched with xenon ions accelerated in an electric field [ss]. It is evident from this figure that there are bonded and unbonded regions at the interfaces betweeneach fiber and its surrounding matrix.The unbonded portions are voids that are shown as dark regions in the micrograph. ?tyo types of void shapes are evident, one being elongated along the fiber periphery and the other being more or less round. The former appears to be caused by shrinking of the matrix, probably during pyrolysis, and the latter to be caused by poor wetting of the matrix with the fibers before the pyrolysis step. This example of voids is characteristic of what is typical for most typesof glass-like carbon matrices, such as for a phenolic resin precursor prior to pyrolysis In this micrograph, it is also to be noted that the texture of the matrix appears to be isotropic, even after extensive ion etching. This means that thistype of glasslike carbon matrix does not have a well-aligned textureof but this does not mean there is no orientation of

A method of evaluating thedis~ibutionof an orientation texture of high-temperature heat treatments, is to view the matrix with an SEM,after the samples have been etched with xenon ions [59]. Since the etching effectis greater for low-density carbon and less for the high-density graphite, it is possible to identify if a lamellar texturehas developed atthe ~ber-matrixinterface. Furthermore, the degree of graphitization is inversely proportional to the width of the lamella [60]. This effect is shown by Fig. 35 where bulk samples of carbon derived from pitch are heat treated at the temperatures shown. At 2100°C, the width of However, as the temperature is raised, for the same the lamellae are 1-2 period of heat treatment, these widths are reduced by a factor of six to eight after heat treatment up to 2750"C, In fact, polygonization occurs at this highest temperature. Similar to the bulk samples, matrices in ClCs became oriented with HTT where the a-b planes were parallel to the fiber axis [60], The orientation effect withinthe CIC matrix canalso be seen with cross nicols for interfilament matrix that has been heat treatedat 2100 and 2400°C (Fig. 36) [62]. For the lower temperature case, no lamellar texture is observed. For the latter HTT,however, lamellar texture is very evident, withthea-bplanesbeing oriented parallel to the fiber axis and very uniform along the length of the fiber. The textureof the areas with graphitic structure may not always be u n i f o ~ if the stress field thatis created between the fiber and the matrix is not of an equal value, and therefore, there may benot an equal degree of orientation of at all regions of the matrix. This effectis shown for a transverse section across a yarn (Fig. 37) [67,69], wherethe region with lamellar texture between adjacent fibers coexists witb glass-like carbon, asis clearly seen at the upper right of the figure [52]. There are other less obvious regions, but similar textures, especially at the cusp locations between three adjacent fibers. This nonunifo~ityof texture is ch~acteristicof the stress graphitization effect. It is worth noting that thef o ~ a t i o nof lamellae at the fiber-matrix interfaces inClCs is very sinilar to theanisotropydevelopmentat the cont between carbon grains under high pressures, as described in Section lamellae appear to surround the fiber concentrically and this arrangement suggests that it is composed of numerous a-b layers that are oriented parallel to the fiber axis. To prove this, measurements were made of the dependenceof magnetoresistance on the rotation angle of the magnetic field along two directions, with a and TL rotation [si]. A typicalexample of theresult is showninFig. 3 composite that was heat treated at 2390°C for 3 hours. The results show a constant value in the T-rotation but a pronounced change with rotation angle in the TL rotation, showing a rather strong axial orientation of carbon. layers relative to the fiber axis. E~perimentalresults of the carbon fiber/glass-like carbon composites show that the matrix carbon made from glass-like carbon can be almost completely graphitizedinthevicinity of thefiberwhenthecompositesreceivedaheat

SEM micrographs

the matrix derived

pitch [60].

treat~entat high temperatures. This conclusion has been verified [5 1-53] by bulk density and x-raydiffractionmeasurements on the bulkspecimens,scanning electron microscopy on the fractured surfaces, optical microscopy on the polish cross-sections, and magnetoresistance measurementson the bulk and also onthe nthesecomposites,however,thecarbonfibersarefoundto remain ~on~raphitized, a~though there were indications that at its periphery there was an anisotropic st~cture,possibly due to a very thin sheath effect by the matrix that not readily identified by#lightmicroscopy or SE he ~raphitizationbehavior of the glass-like carbon matrix in

Polarized light micrographs of the section parallel to the fibers in treated at 2100 and 2400°C [62].

heat-

atrix textures between fibers in a

that experienced stress ~ra~hitization

carbon-fiber composites was studied as functions of heat-treatment temperature and residence time [52,54]. The 0 0 profile ~ is graphically separatedinto the peaks two components, graphitic G and turbostratic T, as was done on the carbons that experienced stress graphitization (Fig. 3). In Fig. 39, the interlayer spacing doo2and the ratios of relative intensity for the G component to the total in tens it^ on 004 profile are plotted against The interlayer spacing was measuredfromthepeak for the and is plottedagainstthe l o g ~ i t h mof residence time, log t ach HTTFig. in 40 cirnen, the crystallite ~agnetoresistance at a ~ a g n e t i field c of 1 Tesla. The values o igure 39 shows gr atmospheri under does The crystallite size ~ ~ ( 0also 0increased ~ ) graduallywithanincrease Applyingthesuperimposition method, proposed by ~ischbach to each of the observed values shown in Fig. 40 (closed points), the master curves for doo2and open points). ~ ~ h e n i plots u s of the shift factors, obtained versus reciprocal temperature of heat treatment yields an apparent activation energy about 240 kcalhol, which is very close to that reported for

T rotation

H

Fiber

2tion Magnetic F

0

0

K

Transverse magnetoresistance(Ap/p) on rotation angle the composite heat-treated at 2390OC for 3 hours

at 10

and 77 K for

the graphitization of graphitizing carbons under atmospheric pressure[2]. These results suggest a homogeneous graphitization process has occurred for the G component in the that prior to HTT contained glass-like carbon matrices. A similar graphitization effect was observed for other The composites prepared from PAN-based carbon fibers and phenol resin, which are classified as one of the carbon-fiber/glass-like-carbon matrix composites, showed the same type of structure and texture changes with heat treatment, as mentioned above, The graphitization of glass-like carbonis not limited to composites that contain round fibers. For example, composites have been prepared from flaky natural graphite and bonded witha glass-like carbon matrix. After heat treatments, a high degree of graphitization was verified with measurements of bulk density, electrical resisitivity and averaged interlayer spacing The development of anisotropic areas was also observed on composites prepared from the glass-like carbon using binder of furfuryl alcohol condensates, which gave a glass-like carbon

ki

340

0.

Changes in interlayer spacing and content of graphitic component G in the composite with HTT (courtesy Prof. Yasuda)

matrix after carbonization at 1000°C. The optical micrographs obtained using polarized light for samples that were heat-treated at 1000 and 2800°C are shown inFig. 4.1. The formation of anisotropic areas inthe matrix is easily recognized between the irregularly shaped particles.

With the aid of polarized-light microscopy, the anisotropic regions were clearly seen to start at the interfaces of the carbon fibers and the su~oundingglass-like carbon matrix and to expand gradually into the matrix with increased HTT and residence time. Most glass-like carbons are known to show l shrinkage in volume, about for the furfuryl alcohol condensates, during carbonization up to 1000°C, On the other hand, the carbon fibers employed here are dimensionally stable because they have previously ahad thermal history at least 1200°C. no fiber shrinkage is expected during the carbonization process of thecomposites.Therefore, it is reasonabletoassumethattheanisotropic regions are formed by the accumulation of stress that is caused by the large difference in the volume shrinkage thatexists between the carbon fiber and the matrix glass-like carbon during carbonization. This assumes, of course, thatthere is a strong bonding between the fiber and the matrix during pyrolysis. ~perimentallyit has been shown that the presence stress accu~ulation can exist in a composite natural graphite flakes and glass-like carbon matrix by

g

l000

l0

k

0

Residence time

min

Changes in interlayer spacing and crystallite magnetoresistance ( A P / ~ ) ~ ~ with residence time (courtesy Prof. Kimura) Solid marks are values measured and open marks are shifted values to construct the master curves at 2390°C.

measuring and comparingits thermal expansion values to flakes of natural graphite The results are reproduced in Fig. 42, and shows the differences in the slope, or coefficientof thermal expansion, for natural graphite (NG) ina powder form and in the composites of natural graphite with glass-like carbon matrix derived froma phenol resinby heat treatmentat 700- 1800°C.The larger thermal expansion of the natural graphite (NG) powderis found to be constrained when it was embedded in a glass-like carbon matrix and heated up1000°C. to But, when NG inglass-likecarbonwasheat-treated at 180OoC, its slope seen to be equivalent to the powder, which means that the stress accumulated is released theheattreatmentuptothistemperature.Thestressaccumulatedatthe NG boundary was estimated to reach a value of a few tenths GPa. The stress accumula-

eyer

Polarized light micrographs of the ~lass-like-c~bon/glass-like-c~bon composite 1801. (a) 1000°C treated and 2800°C treated.

tion at the boundary between carbon fiber and matrix was evaluated as a functio the distance from the surfaceof carbon fiberby using a modelof a thick-wall cylinder for the matrix[53]. Experimental verificationof this model was achieved by measuring the optical reflectance of a glass-like carbon under crossed nicols as a function of the distance from the fiber surface in a composite that had been heat treated to 1000°C. The results are shown in Fig. 43. The calculated stress distribution appears to agree with that observed experimentally. Consequently, t maximum stress accumulated at the fiber-matrix interface was calculated to have possibly reached 0.4CPa during carbonization.By assuming the additivityof the stresses from neighboring fibers, the~ a x i m u m stress is estimated to be more than 0.6 GPa. From the shrinkageof the composites,it was also calculated the residual stress at the boundary between fiber and matrix to be 0.7 CPa [53]. Such a largestress accumulation in the region of the interface between fiber and matrix can cause an orientation of carbon layers to become parallel to the fiber surface during carbonization, as clearly seen bySEM (Fig. 37) andby magnetoresistance measurements 38). These experimental results indicate that the stress near the fibers does cause orientation of the matrix. But, the distancefrom the fiber, where orientation and graphitization eEects still exist, is expected to depend on the type of precursor usedfor making the matrix. In the case of Fig. 43, a glass-like carbon was used and the calculated stress decreases as the distance from the fiber increases. Therefore, it is predicted from the model that there should not be any evidenceof a graphitic structureif the distanceis large enough, Thisis in fact found in Fig. 37 where there are regions that are lamellar or graphitic and others that are isotropic or glass-like in appearance coexisting between fibers. explained in previous sections,by t&ng into account the experimentalfact that nongraphitizing carbons, such glass-like as carbons, can be graphitized under

0

l00

200

Ambient temperature

300 OC

Change in interlayer spacing daozfor graphite component in thenatural graphite/ glass-like-carbon composites with ambient temperature [SS].

Fiber

Glass-like carbon matrix

0.4

IO Distance

pm

Changes calculated stress and optical density at the interface between carbon fiber and glass-like-carbon matrix (courtesy Prof. Kimura)

pressures above0.3 CPa, the effectof stress accumulation at atmospheric pressure can be considered to be responsible for the graphitizationof the glass-like carbon matrix in the composites at high heat-treatment temperatures. Thisis one of the major explanations possible for the graphitization process that occurs, provided the accumulated stressis still sufficient at the high temperaturesfor the process to proceed. An alternative explanation for the graphitization of the matrix in composites may be dueto change from random to an orientedtexture of in the glasslike carbon matrix during carbonization up to 100O0C, which is assisted by the stress accumulated in the region of the boundary betweenfiber and matrix 1,821. This concept is consistent with the ho~ogeneousgraphitization process that has been experimentally observed, described in conjunction withFi In reality, the graphitization process may be some combination of the two processes mentioned above, i.e., oriented regions being graphitizedat high temperatures with the assistanceof the accum~latedstress or the stress accumulation that solely influences the orientation of the BSUs during carbonizationat moderate temperatures. Clearly, further experimental evidence is needed to define more precisely details of the stress graphitizatio~process in and other carbonaceous materials. The complex natureof this processis illustrated by Fig. 13where the influence of temperature and pressure are extensively seen to alter the texture of the The isotropic texture of the fiber and matrix are evident in Fig.3 l a where, after curing the matrix, the fibers can be seen to he~rogeneously distribut~ in the matrix, After the sample is carbonized, all the fibers have the same d i a m ~ ~ e r approximately 8 pm. The matrix between them appears to be oriented and varies in width from less than 1 pm to about 3 pm, is shown in the region of close packing of the fibers (Fig. 31b). In some locations the perimeterof few of the fibers seem to be graphitized, although it is difficult to separate where the fibermatrix interfaceis located at this magnification. As HTT the is further increased to 245OoC, thetexture of the extensivelyaltered,Fig.31c,wherelarge oriented regions are shown with dimensions many times larger than the fibers’ diameter of 8 pm. Also, only smaller number of fibers is evident, even though the original fiber content was ~01%. 45 It appears someof the fibers are beginning to be consumedinto the oriented regionsdue to the combination of pressure and temperature. Such an effect would not occur in thisof type at one atmosphere of pressure. As further HTT increases continue, the number of large oriented areas increases and thereis no evidenceof fibers, Fig. 31d. In any event, the crystallite orientation of the fibers, heat-treated in the composite under constraint, is also explained by stress accumulation at the fiberfound to be improved, which matrix interface. The nongraphitized part of carbon fibers seems to be responsible to the lowangle tailin the 002 and 004 diffraction profilesof the composites heat-treatedat

temperatures high 2800°C (Fig. 30 and 32). The change in the 004 profile with the change in fiber fraction (Fig. 30) is also explained by the difference in accumulated stress per bound^ area between the fibers and the amount of matrix in the

One important aspectof understanding the fundamental mechanismsof graphitization is the ability to provide knowledge in the selection and control of the properties of carbonaceous materials by altering the processing conditions, be these materials in the bulk or composite form. In the latter case, the controlof the texture spectrum of the matrix from carbonto graphite is one of the very useful tools used in adjusting the propertiesof for particular en~ineeringappfications, along with the other major pararneters: the type and o~entationsof the fibers. In the following threesections, illustrative examples will be presentedof how various matrix textures influence the mechanical and thermal expansion 1621. properties of The different texturesof the matrix include the voids, orientation and texture of the matrix well the fiber-to-matrix bond strength. This is discussed in the next section with relation to the thermal expansion characteristics of Processingconditions,especiallyheat-treatmenttemperatureandresidence time, control the matrix texture and the characteristics of the fiber-matrix interface region. The proper selection the matrix precursoris the other major parameter in controlling the texture, has already been presented. The fracture modeof the matrix is profoundly affectedby whether the texture of the matrixis glasslike or graphitic. Itis typical that thefracture, in the former case, is caused by single crack that travels through the matrix and the fibers in an unimpededmanner 1701, showninFig. 44. Inthiscomposite,thespaces between the fibers are filled with glass-like carbon that is well bonded to the fibers. Thesingle crack is seen to travel upward from the bottom to center of the picture. It is relatively straight with only slight deviations at the fiber-matrix interfaces. In contrast,for matrix thatis graphitic, multiple cracks are formed, especially if it is subjected to tensile stress. Furthermore, the direction of these cracks is roughly parallel to the oriented carbon layers graphiti~ in regions, seen in Fig. 45 1611. This micrograph is of matrix region, typically in between yarns, thatshowsmultiplecracksthattravelinthesamedirectionasthelamellar boundaries and are generally perpendicular to the directionof the stress. However, anexact p e ~ e n d i c u l ~ i tofy thecrackdirectionandthe stress vector is not necessary cracks have been observed that are almost parallel to the stress vector.

Crack propagationfrom the bottomto top across the fibers in the like carbon-matrix

with glass-

This situationis possible because the tensile strength in the c direction for basal plane is orders of magnitude less than for the a-b direction, and even less strength is expected at the boundaries between lamellae. In this boundary region, the ion etching shows it to be of a lower density and random in texture. Thus, the lamellae are composed of regions of dense graphite that are surrounded by regions of low density and strength, which promotes extensive microcrac~ng.Essentially, the graphitic textures in the matrix control the crack directions, even when the stress is not in a perpendicular orientation relative to the crack’s propagation direction, as illustrated in Fig. 45b. Here, the tensile stress direction is at approximately 45 degrees from the lower left to the upper right of the figure. The crack is progressing from the upper left toward the lower right but in a very tortuous manner becauseof the multidirection of the textureof the matrix, In the middle of the picture the lamellar orientation is almost parallel to the stress field and consequentlyis the directionof the cracks,c o n f i ~ i n the g discussion that was presented earlier in this paragraph. Therefore, the fracturing modes between glass-like carbon and graphite matrices are vastly diEerent. The former is usually a single type of fracture that usually occurs at the fiber-matrix bonds or alternatively travels u~impeded through the fibers and surrounding matrix. In a graphitic matrix there are multiple fractures of varying lengths that occur in the same region, depending primarily on the texture and the orientation of the lamellar structure the matrix. Failure

usually takes place when these cracksjoin together to form a critical crack size. The difference of fracture modes makea big difference to the mechanical properties of the Also the strain characteristics of are very different, being low for glass-like carbon and relatively much higher for the graphite type of matrix. detailed discussion of the influence of bondsis given in SectionsIV E and TV F. The effectof the differenceof texture is illustrated by measuring the toughness of a three dimensional woven composed of PAN-based carbon fibers (T-300) and densified with a pitch-based matrix [62]. Samples were cut from this billet, heat-treated at atmospheric pressure between 2100 and 2800"C, and then tested for toughness. With this variation of heat trea~ments,the matrix texture was observed to go from an ungraphitized glass-like carbon, to a mixture of carbon and graphite, and finally toa highly graphitic matrix. The relative toughness between the samples, a function of HTT, is shown in Fig. 46. The toughness valueis a ~ a ~ i m uatm HTT of 2400"C, and then decreasedat 2500 and 2750°C. These data are considered tobe evidencethat the matrix testure changes havereal effect on the mechanical behavior of This effectis phenomenologically possible as the mode of crack propagation in

HTT Dependenceof relative toughness on carbon fibers and pitch matrix 1621.

the

prepared from PAN-based

47 Schematic illustration like-carbon matrix (low regions, and (c) graphitic matrix.

crack propagation in different matrix glassmatrix of combination of glass-like and graphitic

the matrix can vary for differences in texture, illustrated in Fig. 47. At lower heat-treatment temperatures,the matrix is ungraphitized glass-like carbon (Fig. 4 7 4 and single typeof crack propagates along the matrix, which has modulus E values that are isotropic, indicated by equal-length vectors representing the modulus E. Not much energyis expended in progagating this single type of crack. Consequently, the toughness is low. In Fig. 47b, the matrix combination of regions of glass-like carbon and graphite. Therefore, Ethe values of this matrix tend to become anisotropic, withthe lower value being perpendicular to the fiber axis.Thistype texture facilitates theformation of multiplesmallcracks throughout the matrix that absorb more energy and therefore increase the toughness of this type of matrix texture over the previous one. However, the matrix texture becomes highly graphitic, with longer lamellae, the E values are very anisotropic, which means many large cracks can easily propagate along the matrix and less energy is expended (Fig. 47c). Therefore, the toughness of this highly

graphitic textureis less than the previous one.This study shows how achange of matrix texture does alter impact properties [62]. Another experiment was performed where the variations of the matrix moduli was measured as a function of HTT. The influence of a texture change in the matrix of a singleyam, composed of unidirectional fibers alongits long axis, was found to markedly change its dynamic modulus.A yarn sample 0.25 0.25 34 mm3wasexcisedfromadensified3Dbillet,previouslydescribed.Dynamic modulus E was determined by passing an alternating current along the axis of the sample in the presenceof constant magnetic field perpendicularto the direction of the current. The resulting interaction between these two fields placed a transverse force on the sample. By altering the frequency of the current, the vibrational resonances of the yam samples were determined. The interactions between the fibers and their surrounding matrix are indicatedby changes of the yam's modulus with HTT, shown in Fig. 48 [63]. This change reveals a texture change of the matrix with HTT. Asexpected, the value of E at room temperatureis a maximum when the matrix is still a phenolic resin. Then, E drops asthe phenolic matrixbecomesaglass-likecarbonbetween 1000 and2100°C.Afterwhich evidence of a further textural change occurs as E starts to increase at 2300"C, reaches a maximum at 2400"C, and then decreases2500°C. at The metallographic examination of thematrixtextureshowedthat,indeed, it had been partially

-0

1000

Dependence of dynamic modulus E on HTT

[63].

tranformed from carbon to graphite at 2300°C. But, the effect of HTT in maximizis similar to what happened ing E at 2400°C was not expected. Perhaps this effect to the toughness propertyof C/Cs, discussed earlier. Again, it appears an optimum mixture of glass-likecarbonandgraphitetexturesseems to enhancecertain mechanical properties of CICs, be it in billet form or yarn. Apparentl~,combinations of carbon and graphite textures in C/C matrices can markedly influence the CIC's mechanical properties.This concept is considered particularly relevant for the matrices that are derived from the stress graphitization effect the texture within the yarnis primarily combination of regions of glass-like carbon and graphite, previously shown in Fig. 3'7. The following stress-strain data is provided further support for the concept that the combination of textures can be beneficial in obtaining desired combinations of mechanical properties. Samples of 2D CICs, composed from PAN-based carbon fibers and phenolic resin and heated up to 2480"C, were tested for load deflection in three-point bending test[64]. The resulting curves are compared in Fig. 49. In the case of the as-received sample B, failure occurs in brittle manner at deflection about 1.44 m. But, if another sample,B', of the same type CIC material were given an additional hour of heat treatmentat about 25OO0C,the failure point increased by 45% to deflection value about l .99 mm, clearly more ductile type failure has taken place, In comparison, the as-received sample A, which has been densified in the same mannerthat of B but has been heat treatedfor longer times at 2SOO"C, shows gradual deflection to 2.54 mm, clearly a plastic mode of

Sample B'with additional heat treatment at for 1 h

0

m Load-deflection curves of three

failure. Examinationsby SEM of the matrix surrounding the fibersof these three samples showed that samples B and contained greater amountsof the graphitic component than sample B. The comparison among samples A, B, and B’ showed that the energy required to deform these depended on the degree of the development of graphitic texture that existed in the matrix, which in turn depended on the temperature and duration of heat treatment. Also, experimental observations showedfor that larger am~untsof graphitic matrix, more microcracks occurred in the matrix when the weremechanicallystressed.Therefore, it wasnaturallyexpectedmore microcracks consumed more deformation energy, thereby increasing the toughness valuesof the as is seen by comparing curvesB and B’ in Fig.49. Based on this hypothesis, a quantitative analysis method was developed to evaluate the “relative” amount of graphitic s t ~ c t u r (relative e graphitic index,RGI) that exists around each fiber w~thinsamples t&enfrom the same or different Details of the method for quantitative evaluation of the texture of the matrix will be discussed in Section F.To validate this analysis as a means of predicting the mechanical performanceof different a ~icrographic quantitative evaluation of numerous samples was compared to the composites’ relative degree of plasticity. A total of twenty-five samples, entailing more than 3000 matrix evaluations, were quantitatively evaluated for the “relative” amount of graphite in their matrix textures. Based on thisdata, each of the twenty-five samples was“predicted” to fail either in brittle a or plastic mode. Then, each sample was mechanically tested. The results are summari~edinTable 5 The predi~tionsappear to agree reasonably well with the twenty-one experiment~lydetermined fracturing modes. ~ e v e ~ h e l e sthis s , quantitativemicrost~cturalanalysis approach does provide a nondest~ctivetechnique for predicting the fracture behaviorof that were processed similarly. Furthermore, evaluating process conditions is made possible by pe~iodicallywithdrawing coupons from the processing furnace and evaluating

5 Fracture Behavior Predicted from Texture Examination and Determined by Mechanical Tests of Thin-Wall Tubes Whose Matrix Was Subjected to Stress Graphitization Fracture behavior Plastic (tough) Brittle Not-tested Ref.

Predicted Result by of texture exa~ination mechanical tests

21 4

17 4 4

them to be sure the have been exposed to the specified temperatures and duration times, The impo~anceof attaining the proper heat treatment is demonstrated for matrix composites that were heat treated at different temperatures. Then, the degreeof stress graphitization was microstructurally evaluated and compared to the fracture behavior of these The mechanical test data were derived from a 4-point bend test with a specimen size of 50 10 mm3 and a notch that was 1 mm deep. The carbon fibers were aligned along the sample's l~ngitudinaldirection, The load-displacement curves for the with different HTTs are summarized in Fig. 50 1521. After the initial elastic deformation segment, the composites that were heat treated at 2800°C have plastic-like displacement, as clearly shownby the C-D portion the curve, which is a characteristic type of displace~entbefore the formation. macroscopic cracking in the specimen. All testing was conducted at room temperature" Furthermore, those samples that were heat-treated above 1900°C show varying degrees this plastic effect, where the graphitization of the matrix startsto occur. This behavior is again attributedto the lamellar orientation texture of the matrix, It is susceptible to the formation of microscopic-sized cracks within these ~raphitiz~d regions near fibers, thereby causing stress relaxation to occur and a redistribution of the local mechanical forces. This microcrackingdecreases the samplesmodulus E andincreases its plastic defo~ation. ~ltimately,the microcracks join together to form critical-size macrocracks and

D tr

g

tl

0.8

Displacement different

m

Load-displacement relations on carbon fiberlcarbon composites heat-treated at and tested at room temperature (courtesy Prof. Yasuda)

3.1

HTT Changes in Young's modulus with along and across fiber direction in unidirectional composite (courtesy Prof. Yasuda) [52].

cause failure, as at B and D, with jagged steps of declining strength, similar to that shown in Fig. 49. In Fig. 51, the Young's moduli along and across the fiber axis, which are measured by a dynamic method, are plotted against HTT. The composite shows a strong degree anisotropy in Young's modulus, which is enhanced with the increase in HTT. After ~800'C tr~atment,the Young's modulus along the fiber axis is about 1.7 lo5MN/m2, whichis larger than the3-4 lo4MN/m2 valuefor the pyrolytic carbons along the deposition surface. This high absolute value and anisotropy of Young's modulus again suggests a high degree of orientation of carbon layers, which agrees with the matrix texture observations of composites described above. The previous i n f o ~ a t i oand ~ observations providesa sound basis for believing that the texture of the matrixis one of the most important in deteminingthemechanicalproperties of andthismatrix,inturn, is strongly influenced by the stress-graphiti~ationeffect.

Thebondinterfacebetweenthefiber its surroundingmatrix impo~antfactor that ~ e t e ~ i nthe e s thermal- mechanic^ properties

another The

details of the bond interface are provided by the use of the TEM, as seen in Fig. 52, which shows the transverse sectionof the region thatis composed of portions of six fibers and their matrix, including voids (light regions) The bonds at fiber-matrix interface are indicated by dark regions between the fibers, such as the area indicated by “ 5 nm” in Fig. 52a. The enlarged micrograph of this region is shown in Fig. 52b. Sufficient detail is evident to show that the interface region between two fibers are bonded together by the matrix, whose boundary is indicated by dots in this figure. The insert in Fig. 52b is a further magnification that shows the fiber-matrix interface at this location does wet the surfaces of two fibers and appears to have chemically bonded them together, whereas a mechanical interaction. would have shown an interface between the matrix and fiber. From these TEM observations it appears that, ona nanometer scale, the fibermatrix interface is generally going to be heterogeneous, consisting of bonded regions, voids, and cracks ina matrix that is therefore partially bonded along the perimeter of individual adjacent fibers. Moreinsight into the nature of the influence of thefiber-matrixinterfacewasobtainedfromthethermalexpansion behavior of The following experimental information is provided to illustrate how variationsof bond strengths and matrix texture can alter the dimensional and mechanical characteristics of their wallhraided tubes of 3.8 cm Thermal expansion characteristics of thin-wall tubes, 15.2 cm long, in diameter, and0.8 mm thick, were measured at temperatures between15’7 and +25OoC [65]. The majority of the tubes were composed of three layers of axial yarns at 36 locations withinthe cross section of the tubes that were constrained by additional yams, in a braid architecture. A majority of the fibers in these tubes were either PAN-based or pitch-based carbon fibers. The matrices were generally derived from a phenolic precursor or a combination of a pitch and resin. Each tube received a series of high-temperature treatments between 1030 and 24OO0C, to obtain either a homogeneous glass-like carbon matrix or a combination of carbon and graphite in the matrix, which was derived from the effect of stress graphitization. The matrix textureof each sample was determined by both optical and SEM microscopic techniques, after etching with Xenon ions, as previously described. Thermal expansion measure~entswere made by an interferomet~technique using a laser source. Each tube was placed in a vertical position inside a vacuum-tight enclosure. The change of length of each tube was continuously recorded during a thermal cycle toa precision of 0.2 where the unit is the tube’s changeof length per unit length of tube. When this unit is divided by the variation of temperature, it is equivalent to the coefficient of thermal expansion (CTE) of the being measured. If the typeof fibers, their architecture, and the matrix texture are held constant while the sample is thermally cycled, changes of CTE of such composites is expected tobe indicative of variations of the fiber-matrix (F-M) bonds Such

a change is illustrated in Fig. 53a where the tube is initially cooled from room temperature to 100°C. At -42"C, the CTE -0.8 pd"C. But, from -4.2 to 10O"C, the CTE suddenly decreases to 1.4 p d 0 C which is considered to indicate a change of the F-M bonds. Furthermore,it has tobe mentioned that this latter slope is equal to the CTE value of the fiber in its axial direction. When a second cooling cycle was performed from room temperature to 100°C on the same sample, the resulting CTE is a constant 1.2 p&/"C,as shown in Fig. 53b. Thus, it appears that theF-M bond is stabilized as thereis no change of slope at -42°C [68]. The tube's initial slopeof -0.8 pel"C (Fig. 53a) is attributed toa high degree of F-M bonding betweenthe glass-like carbon matrix, whoseCTE is +4 and the fiber, which has a CTE of 1.4 pa/"C, This means, during the initial portion of the first cycle, the matrix is sufficiently bondedto the fiber, which tends to decrease the C/C's CTE and counteract the fiber's negative expansion effect, Consequently, the F-M interaction results ina value for the tube of pd0G above -42°C. However, below this temperature, very little F-M bonding appears to exist because theC/C's CTE is very nearly equal tothe value of the fiber inits axial direction. This suggests that debonding is the most likelypheno~enonto be occurring. Such a phenomenon is possible because below room temperature the fiber is tending to contract away from the matrix, as its transverse CTE is twoand-one-half times less than that of the matrix. The lack of any change of slope during the second cycle (Fig. 53b) is indicative that no further debonding is occurring. It is not exactly clear why theCTE of the second cycle is 1.2 pd"C compared to 1.4 pd"C at the end of the first cycle. Two kinds of events can possibly be taking place to explain this phenomenon. The first possibility is that there may bea relaxation of internal stresses thatis occurring at the F-M interface or in the matrix between the first and second cycles, thereby providing some additional F-M mechanicalinteractionduringthesecondcycle. The second possibility is thatthe debris, resultingfromtheinitialdebondingevents, is providing a source of increased mechanical friction between some of the fibermatrix interfaces. Subsequent studies, that are not discussed here, showed that both types of events can occur [67]. The inverse situation occurs when the sampleis heated above room temperature as the F-M bond improves because the fiber transversely expands into the matrix, thereby forming strongerF-M bonds. Consequently, the fibers dominate the thermal expansion phenomena in C/Cs as their lengths contract, due to the negative longitudinal expansion of the fiber with increasing temperature. However, along the axis of the fiber, shear force is developing with increasing temperature because the CTE of the fibersis negative, whereas that of the matrixis positive, Therefore, the shear stresses at the F-M interface canbe sufficient, in the extreme, that the F-M bonds can be broken and then the matrix has been observed to dominate the CIC's CTE [68]. However, the usual situation appears to be onlya

250

200 x

150

50

x

0

-1 -200 -150 -100

-50 -0

250 00 150

50 0

-50 -100 -150 -200

0

Changes of microstrain with cooling of thin wall tube heat-treated up to 1100°C (681. (a) the first cooling and the second cooling cycle.

fraction of the bonds are broken in the C/Cs during thermal cycling. Accordingly, the thermal stress loads are redistributed or relaxed so that the expansion/ contraction of the sample occurs incrementally during thermal cycling. This effect is shown in Fig. 54, where C/C’s length was continuously measured, with the precision of 0.2 FE:units, its temperature is increased to 121°C and then cooled back to room temperature. The saw-toothpatternthat is observedinthisfigure is consideredto be indicative of the breaking and making ofF-M bonds probably by frictional effects. This suggests thereis competition between the fiber and the matrixfor control of the CIC’s total length changes, on micron scale, temperature is increased, the negative slope of the pattern is due to the axial contraction of the fiber, whereas the positive slope is caused by the expansion of the matrix, is schematicallyillustratedinFig. 54. The nonuniformnature of thesaw-tooth pattern is attributed to the variability of the shear stresses that are created or destroyed during the sample’s heating and cooling cycle. The amplitude of this pattern is between 1.4 and 2.1 FE, or 7 to 10 times that of the precision of these measurements The total effectof these interactions is to create a hysteresis loop where there may be difference in the length of the sampleat the finishof the thermal cycle. In Fig. 54, the tube became longerby 10 FE units. But, with other typesof samples and different thermal cycling conditions, the tubular CIC samples could be shortened or have no change in length. Therefore, the shape and location of these hysteresis loops are indicative of the degreeof F-M bonding that exists while the sample is being thermally cycled.Since this is a dynamic condition, one approach for comparing the various shapes between different hyteresis loops is by locating the inflection point as the sample’s temperature reaches its maximum or minimum value, for example 121°C in Fig.54. This inflection pointis defined the “end point,,, indicated on the figure and will be referred to subsequently in this section to indicate the relative differences between hysteresis loops of various types of C/C [65]. The location of an end pointis considered to be due to the cumulative effectof the interactions between the fibers and the matrix in the sample that is being themally cycled.Theseinteractionsareprimarilycontrolledbecause of the differential thermal expansion characteristics between the fibers and su~oundits ing matrix.The different coefficientsof thermal expansionfor the fiber and matrix are schematically shownby the upper right graph of vs. temperature in Fig. 55. The glass-like carbon matrix line, MC,is isotropic and have positive slope thatis less than the slope of the fiber in the transverse direction, FT, relative to fiber’s longitudinal axis. Asthe matrix becomes more graphitic, due to heat treatment, its CTE values approach thoseof the fiber, previously described. Assuming there are no residual stresses in the C/C at room temperature, the compressive stresses between the fiber and its adjacent matrix increase above and decrease below room temperature. Conse~uently,the shear strength at the F-M interfaces are altered

(carbon fiber, transverse) (glass-like carbon matrix)

FL (carbon fiber, longitudinal)

Low Temperatur~(-15pC)

High Temperatur~(+l21 C)

Graphite Matrix

.55 Schematicillustration of mutual expansion-s~rinkageof filament and two different matrices (glass-like and graphitic) at two extreme temperatures 157 and 121°C)

and so is resistant to differential slippage between the fibers in the longitudinal direction, F%, and the matrix, MC. Therefore, it is expected that the dimensions of the composites can be changed in ways that are Schematically depicted by the drawings in Fig. 55. They show the relative dimensional changes between a single fiber andits surround in^ matrix as the composite is heated to 121“C or cooledto 157°C. As previously stated, the length of the tubular sample canbe smaller or larger than its initial value after being thermally cycled. Accordingly, the location the end points haseither negative or positive values, on the microstrain vs. temperature graph. The upper left drawing of Fig. 55 depicts a composite, containing a fiber surroundedby a glass-like carbon matrix, whose initial length is indicated by the arrow labeled “As Received Height” (ARH). Experimentally, it has been determined the samples shrink if they are thermally cycled to 121°C, as is schematically depictedby the middle right-hand drawing where the sample height is shown as less than the ARH value. At this condition, the sample’s end point is also lowered, as shown in Fig. 54, because of the F-M interactions, as previously

explained. This suggests there was small compressive force at the F-M interface. But, if the composite received high temperature heat treatment and the matrix was graphitized, the experimental data indicates that even more shrinkage takes place at 121"C. This result seems reasonable because the slopeof MC is now negative, althoughless than FL, Therefore, the compressive force at theF-M interface becomes larger, Consequently, there is less slippage between the fiber and the matrix, andthe maix is put into compression by the shrinking fiber.This causes the compositeto shrink even more andthere is greater gap between the sample's height and the ARH value. If the CIC samples are cooled to 157"C, they grow in height according to the data for carbon matrix, shown by the middle left-hand drawing of Fig. 55, This effect is attributed to the shrinkage of the fiber from the matrix thereby leaving only some slight bonding. result As the fiber is practically free to expand, seen in Fig. 53, and therefore the composite effectively grows slightly, as depicted by the drawing. ut, this growth is accentuated if the matrix becomes graphiticby heat treatment. Then, thermal cycling to 157"C, enhances the growth, shown by the lower left-hand drawing.There are number of possible reasons for this type of growth to occur. For example, the weaker matrix could be expanding in the axial direction of the film becausethe stronger fiberis putting the matrixinto tension the sampleis being cooled.This could take place there is an enhanced degree of bonding at the interface because there is less of thermal expansion difference between the fiber and the graphitic matrix than there was for the carbon matrix. This also means the graphite matrix is less likely to contract it is cooled. Another possible cause may be some growth of the CIC graphite matrix. This growth is possible because the matrix is more susceptible to microcracking due to the tension effect by the fiber's growth [65]. Also, the fiber could be growing because thereis less attachmentto the matrix and, therefore, it has more abilityto separate or straighten out. The thermal expansion data does show that interactions between the fibers and the matrix occurs and the degree of these interactions appears to be function of the textureof matrix that surrounds each fiber. In general, these interactions seem to be reasonably based on the expected physical behavior of the constituentsof the composite, namely the type of fibers and the texture of matrix. But, the exact mechanisms for such interactions have not yet been well defined, For example, do variations of the matrix texture actually change the thermal expansion characteris tics of CICs? To verify textural efTects can occur for thin-wall braided CIC tubes, the end points of the hysteresis loops were measured for four different typesof textures at the two extreme temperatures of 157°C and 121°C [65]. The results are shown in Fig. 56 for matrix textures that range from glass-like carbon (C) to the most graphitic matrix, which containsC plus regions of stress-oriented graphite (GR)

Matrix

.56 Changes in end points thermal cycle curves with matrix C: carbon, PG; pyrolytic graphite; GR: stress-oriented graphite, and I: involute-woven tubes.

and pyrolytic graphite (PG). A sufficient numberof cycles were conducted at each point to be sure the shapes of hysteresis loops were stabilized. The number of cycles that each tube experienced is indicated by the number next to each experimental point. With the exceptionof the involute samples,the scatter at each data point was less than10 PE. Two types of fibers were used, PAN-based UHMU pitch-based P-55, and sixof the eight tubes were braided while the remaining two were of an involute const~ction? indicated by I, and I, on Fig. 56. A quantitative analysis of the texture was carried out to determine the amount of graphite in the matrix by using micrographs, as will be described in Section IV F. This data indicates that shifts in the end points of the hysteresis loops are, indeed, affectedby the textureof the matrix.The curves, onefor fibers and one for P-55 fibers, at 157°C in Fig.56 shows that debonding usually takes place and the samples lengthen with a more graphitic matrix. This is indicated by the

arrow parallelto the ordinate and the relative degree of graphite in the matrices is shown to increase along the abscissa from glass-like carbon C to a graphitic combination of C CR PG. The majority of the data from braided tubes is consistent. Conversely, bonding is increased from room temperatureto 121°C with a more graphitic matrix as the end points are shifted to lowervalues and the length of the were reduced, especiallyfor the tubes composedof pitchbased carbon fiber yarn. The two exceptionsto this data are the tubes prepared by the involute- or layer-type construction. In summary,it appears from this data that altering the texture of the matrix does change theF-M interactions and the thermal-expansion behavior of the thin-wall braided tubes. This means that the more graphitic structure seems to bond better with the fiber at high temperature and debond at low temperature. Furthermore, the type of fiber used also has an important influence. This effect is also expected because the transverse expansionis larger for the pitch-based carbon fibers than for PAN-based fibers. It should also be noted that more than 200 thermal cycles were performed on braided thin-wall samples with a scatter of less than l 0 units at eachdata point. In contrast, the scatter of data pointsis between 70 and 90 units for the involute woven tubesfor only about60 cycles. This differenceof scatter indicates that the geometry of the weave should be considered in terms of its influence on theF-M interactions. From other experimental information and analysis, the braided configuration, with continuous longitudinal yarns along the tube's axial direction, resulted in properties more like that of a tube composedof only unidirectional axial yams. Therefore, it is reasonable to expect these tubes will have less scatter of the data in comparison to involute woven tubes, where the yarns are discontinuous in three dimensions, especially through the thickness of the tube.

Since the thermal expansion characte~sticsof the tubes were shown to be changed by variations of F-M bonding, it would be expected that the mechanical properties of these thin wall tubes would also be affected, especially in compression where this value depends on the F-M interaction. To test this hypothesis, the compressive strength of a number of tubes were measured at approximately I00"C. The compressive strengthsof the as-fabricated tubular composites (control) were determined to be 219 MPa. In contrast, when the tubes were thermally cycled between room- and liquid-nitrogen temperatures prior to being tested, th compressive strengths were only l16 MPa, half that of the control samples. A possible cause for the loss of compressive strength is indicated by SEM micrographs that show the existence of large numbers of gaps at the F-M interface

region after thermal cycling, seen in Fig. 57a. In contrast, the control samples were essentially free from such gaps (Fig. 57b). Therefore,it was concluded the compressivestrength of thetubeswerereducedbylow-temperaturethermal cycling, due to shrinkage of the fiber from its su~oundingmatrix, which broke many of their F-M bonds. This means the chemical bonds, formed during pro5000 cessing, were broken. The ability to readily detect such gaps at magnification with the SEM,gives promise that it might be possible to predict the mechanical properties of by examining their microtextures. Consequently, quantitative method was devised to evaluate the apparent degree of F-M bonding for glass-like carbon matrix in these thin C/Cs and to verify if their mechanical properties could be related to variations of the Fbonding [68]. The approach consists of two parts, follows: Part Quantitative evaluation of texture Classification of the major typesof texture that existat the F-M interfaces, seen with the micrographs, in the rangeof 5000 to 10000 magnification. Determination of the number of fibers that contain bonds that exist in each classification after examining representative number (more than 100) of fibers in two or more representative transverse sections of the C/C being examined. Part (2): Definition and utilization of the index relative bond strengths (RBS) Assignment, on relative scale,of bond strengthfor each major classification or type of bond interface. The RBS values are based on the apparent integrity and texture differences that are observed in the SEM pictures. For example, in Fig. 58, the index bond strength (BS) is arbitrarily assigned value of 1 because no cracks, voids, or pores appear to exist between the matrix and the fiber. In contrast, for debonded interfaces, zero value of BS is used because the gap is more than 80% of the perimeter of the F-M interface. Other values of BS are assigned between these extremes,shown in Fig. 58, depending on the number andcombi~ationsof pores, cracks, and gaps that exist at F-M the interfaces. Based on this typeof classification, the relative bond index RBI is defined by the following equation: RBI

NJRBS),

where Ni is the numberof filaments observed in given classificationi and (R is the relative bond strength for classification i. This same approach was used for determining the relative graphitic index

7 micrographs at the interface between fiber and matrix before and after thermal cycling after thermal cycling and before thermal cycling.

re TYPE OF INTERFACE

FUSE5 ( 1 0 ~ )

INDEX OF BOND ST~ENGTHS (arbitra~scale)

.o

FILA~ENT

NO APPARE~T INTERFACE AT

sooox

PARTIA~LY FUSED ( 6 6 ~ )

0.7

0.66

0

0.3 (113 strength of fused ~ n d )

DENSITY (1

l~PREG~ATE5 VOIDS

Definition of types of the fiber-matrix interface and corresponding indices bond strength

that was discussed in Section I). The ~uantitativeevaluation of the relative amountof graphitic texture that is contained in the matrix is accomplished by the following procedure.

(1): ~lassi~cation of the major typesof texture (RCI),that exist in the matrix around each fiber, including combinations of glass-like carbon, graphite, pores, and voids they tend to influence the degree of plasticity of the matrix. Then,

determine the numberof filaments, Ni, with matrix types(RGI), that surround the fibers in composites being evaluated. Part (2): Definition and utilizationof an arbitraryscale of relative plasticity index (RPI), where glass-like carbon is 0.1 and hi~hlyaligned graphitic texture is .0. Pores, voids, and cracks are considered to modify the plastic nature of the matrix between these extreme textures. Therefore, arbitrary values of RP1 are assigned between0.1 and 1.0, depending on the extent of defects that exist. The RGI is defined by the relationship

where Ni is the number of classifications that are defined.

Using the quantitative indexes RBI and RGI, it is possible to compare the textural difference between samples from various types of CICs. The utility of the quantitative texture methodhas been demonstratedby comparing the RBI values to the measured mechanical propertiesof thin wall tubes similarto those usedfor the measurement of thermal expansion characteristics (Fig. 53) The data and e x a ~ n a t i o nof micrographs indicates that compressive failure of braided, thin-wallCIC tubes appears to originate the at F-M interface for a glasslike carbon matrix. It was of interest to determine if different processing conditions can alter this modeof failure and the compressive strengths of the CICs. If can such differencesbe indicated by changes of texture through the use of the RBI and RC1 analysis method? Therefore,a series of tubular samples were processed under different circumstances, using the same weaving architecture. Five of them contained the PA~-basedcarbon fibers and received three impregnations with phenolic resin followed by heat treatmentof 1100°C. Then, for the fourth and last impregnation, different methods were used. The sixth sample was densified by using a different liquid impregnation process. The legend of these samples is shown in Fig.59, along with the relative compressive strengths and RBI values. Set l (base) was the baseline sample, which received four phenolic resin impregnations and heat treatments.3§et (shear) was densified in the same manner as Setl , but the tube was braided withPAN fibers that had been slightly oxidized to enhance the shear strength at the J-M interface. Set 4 (P75) was also processed in a manner similar to Set l , but the fibers were changed from PAN-based to pitch-based,P7S. The last processing stepfor §et 8 (CVD) was ani~pregnation by the CVD method. Whereas Set 9 (noCVD) had no fourthimpregnationstep, it received a fourthheattreatmentsimilar to that experienced by Set 8. Finally, Set 10 was sample that was impregnated four times with a liquid that was a combination of pitch and phenolic resin, but received heat treatments similar to the other sets. The datain Fig. 59 shows that the compressive values are changed by the pro-

SET

(P-4)

0.5

El31 9 Dependence of compressive strength on relative bond index (RBI) of different

intercessing conditions and these appear to bea function of bonds at the F". faces, as indicatedby changes of the RBI values. Four sets area linear in g~ouping, Set 1 is in the middleof the group. above it is Set 4 that was densified in the same manner but contained pitch-based carbon fibers. The fact that it is in the same group indicates that changing the fiber from PAN-based,MF55,to a weaker pitch-basedfiber, P75, doesnothave a majoreffecton the failuremodein compression. The highest and the lowest points in this grouping are the sample Sets 8 and 9 that did anddidn't receive the CVD densification step, respectively. Since the RBI valuesalsocorrelatedwiththestrengthvalues,thedifference between these two samplesis attributed to the filling of the porosity at the F-M interface for Set 8 by CVD, which is a better method than the phenolic liquid. It seems the most complete filling of pores is Set 10, based on the strength and data. But the methodof densification usedis considered to bea different process from that used for the points in the linear group. This sample received four

impregnations with a differentliquid, designated P-4, that was a combination of phenolic resin andpitch, Set 3 is also out of the group asit has high strength but a low RBI value. This clearly shows the importance of enhancing the interface bonds by roughening the fiber surfaces with an oxidation step, an effect is notthat detectable by microscopic examination even at a magnification of 10,000with the SEM. Here, the process is considered to be dissimilar to that usedfor the linear group as an additional processing step is used to oxidize the PAN-based fibers. Thisinvestigationsuggests canhavetheircompressivestrengthvalues altered by using different processes but not the type of fiber. This behavior is similar to other brittle materials, like ceramics. The effect on the tensile properties of these same typesof tubular samples was also investigated by altering the fabrication methods previously discussed. Set 6 is an additional sample where a third fiber was used,AN-based type MF 40. The sample was densified in a manner similar to the base line, Set Va~ationsof the tensile load RI31 value is shown in Fig. 60. Again, many of the data points exist in a linear grouping. Notably, all of these points have the c o m o n parameter that they contain one type of fiber, MF 55. This is irrespective of whether different processing conditions were used. The two points that are outside of this grouping are fiberrelated, Set 4 contained pitch-based fibers,P75, that are not as strong as PAN-based fiber, MF 55. it was expected that the tensile value of Set 4 would be lower than for the group.Exa~inationof the textural differences of these samples showed that a large percentage of the MF 40fibers in Set 6 were split or fractured along their longitudinal axis. This effect, ftaturally, reduces thetensile strength of the sample belowthe group values, asis evident in Fig. 60. Remarkably,the bond in Set 6 between the M F H fiber ~ and the phenolic resin derived matrix was strong enough to split the fibers as the phenolic resin s h r i ~ i n gduring pyrolysis. The general conclusion from this studyis the tensile strengths of these samples is primarily determinedby the types of fibers used. The secondary in~uenceis the texture ofthe F-M interface, as indicated by the variations of the RBI. It is of interest to note that fillingof pores and cracksis an important factor for improving the tensile properties. However, shear enhancement, Set 3, did not have any effect, All of these data indicate that the fracture behavior in tensionis largely controlledby the type of fibers and thenby the texture at the interface, again, similar to other brittle materials. From these two above studies,it appears the RBI analysis method can be used to monitor processing conditions of by samples at different stages of their fabrication for testing. S u ~ a r i z i n gSections D to F, the behaviorof is dependent onthe inter~ctionbetween adjacent fibers through the influences of the textural characteristics of the s~rrou~ding matrixandthe F-M inte~aces.It has been demons~atedthat the texture of the matrix and the state of Fmine the thermal expansion and mechanical properties of the types of tested.

M

(P751 (MF

V G

0.5 RBI

(RBI)

Furthermore, there can be optimum combinations of disordered glass-like carbon andorientedgraphiticregionsthat enable themaximumthermal-mechanical properties to be obtained for specific engineering applications. Some of the above studies, althoughat the early stages, were presented incite to interest in the utility of making testural changes to the matrix. Oneof the most powerful methods obtaining these optimum matrix textures is through the use of the stress graphitization effect.As yet, this approach has not been exploiteditstofullest potential and therefore further WD efforts are recommended.

Structural changefromdisorderedturbostraticcarbon to three-dimensionally ordered graphite, i.e., graphitization, is associated with a volume decrease. There-

fore, it is reasonable to suppose this processis accelerated by external pressurization. This hypothesis was experimentally verified because graphitization of turbostratic carbon occurs at temperatures above 1500°C under applied pressures above 0.3 GPa, which is more than 1000°C lower than under atmospheric pressure. It was also clearly shown that this structural change to graphite is also associated with and accelerated by internal stresses that are generated by interaction between the material’s constituents, such as at the contact points between grains or at the interface between fiber and matrixcarbo~carbon in composites. In other words, it is a stress-assisted graphitization process that can be called “stress graphitization.” Remarkably, the accelerationof graphitization occursfor nongraphitizing carbons, which normally do not show any significant development of a graphitic structure even at heat treatment temperatures above 3000°C and under atmout, ata pressure of 0.5 GPa, disordered glass-like carbons were completely converted to ordered graphite by heat treatment at 1600°Cfor 1 hour. However, if the same starting carbons were partially graphitized prior to the application of pressure, further graphitization was not pronounced, even at 1 GPa. Stress graphitization,by its nature, produces an ordered structure of graphite in regions where there is sufficient stress, e.g.,at the contact points between grains. Therefore, the processof structural change from carbon to graphite can be heterogeneous under certain stress-distribution and thermal conditions. This heterogeneous natureof stmctural changes was observed by x-ray powder diffraction as composite profiles of diffraction lines and by micrograp~susing the SEM, TEM, and optical light microscopy. The graphitization of carbon under high pressure was found to be accelerated by the coexistence of certain minerals, typicallycalcium compounds. For example, the formationof nascent calcium oxide was considered to be responsible for enhancing graphitization, and the additionof water to calcium compounds, such as CaCO, and CaO, lowered the temperaturefor graphitization. With the coexistence of Ca(OH),, flaky graphite particles were observed after heating to only 800”C,though the complete conversionto graphite was not attained, These results give experimental proof for the geological occurrence of natural graphite under mild conditions, such asa temperature of several hundred degrees Celsius anda pressure of several tenths of a GPa. Thestress ~raphitizationphenomenon is also observedinsometypes of carbo~carboncomposites, even though the composites had never experienced any external pressurization while being processed. Typically it was observed in composites that containeda glass-like carbon matrix that had been heat-treated to above 2200°C under atmospheric pressure. Conversely, the development of the graphitic structure could not be observed in eachof the composite’s component carbons, carbon fibers, and matrix carbon when they were independently heattreated under similar processing conditions. Also, no graphitization happens if the

iti internal stresses in the matrix were not high enough. Thus, it is possible to have gradients of graphite and glass-like carbon coexisting in the matrix between fibers within This graphiteis most pronouncedat the fibers’ surfaces and its extent decreases to carbon, depending on the distance between adjacent fibers. Micrographic e x a ~ n a t i o nof the composites verified that the stress graphitization effect was at a ~ a ~ i m uwhere m the stress accumulation occurredat the F-M interface during the carbonization of the matrix precursors, suchfurfuryl as alcohol condensates and phenolic resin. The temperaturerangeandexactmechanism for generating the graphitic structure by stress accumulationis still debatable. One conceptis that in the lowtemperature range, less than stress accumulation assists in producing preferred orientation of basic structural units of hexagonal carbon layers that subsequently become graphite at high temperatures. The other conceptis that the combination of stress at high temperatures, above l60O0C, accelerates structural changes from disordered turbostratic stacking to three-dimensional graphitic stacking. Furtherinformation is neededtodetermine the relativeimportance between these two mechanisms. Nevertheless, the stress graphitization effect carbonkarbon in composites provides an excellent means of controlling and altering the degreeof heterogeneous texture in the matrixcxbon, consisting of ungraphitized and graphitized regions, and also different bonding states at the fiber-matrix interfaces. These wide ranges of matrixtexturescanresultin a spectrum of alternatives for changing the mechanical, thermal, and electrical properties of the carbonkarbon composites. Therefore, the stress graphitization effect is considered to be oneof the powerful methods for achievingoptimumproperties for theuse of inadvanced engineering applications. But this approach has not yet been fully exploited its to fullest potential.

M. Inagaki would like to express his great thanks to the late Prof. Emeritus T. Noda for his encouragement andalso to Prof. K. Kamiya, Mie University, and Prof.S. Hirano, Nagoya University, for their collaboration. R. Meyer wishes to express his gratitude to those persons who have greatly assisted in the preparation of the informationpresentedherethroughstimulatingdiscussionsandcollaborative efforts, especially to Prof, J. White, University of California, SD; Prof. A. Evans, Harvard University;Dr. X. Bourrat, CNRS,France; Dr. G. Rellick andG. Henderson of the Aerospace Corp.;Dr. R. Pleger and Dr. W. Brauer of DLR, erm many. The authors thank Mme. A. Oberlin, Directeur Emerite of France; Prof. irano, Nagoya University; Prof. E. Yasuda, Tokyo Institute of Technology; Prof. S. ~ m u r aYamanashi , University,for their courtesyof giving permission for publication of their figures.

242

lnagaki and Meyer

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Stress Graphitization

243

31. M. Inagaki, Atsuryoku-Gizyutu 17, 34 (1979). 32. S. de Fonton, A. Oberlin, and M. Inagaki, J. Mat. Sci. 5, 909 (1980). 33. Y. Hishiyama, Y. Kaburagi, A. Ono, M. Inagaki, and K. Kamiya, Carbon 18, 427 (1973). 34. K. Kamiya and M. Inagaki, Carbon 19, 45 (1981). 35. M. Inagaki, Yogyo Kyokai Shi 89, 236 (1981). 36. M. Inagaki, Yogyo Kyokai Shi 90, 516 (1982). 37* H. Marsh, F. Dachille, J. Melvin, and P. L. Walker, Jr., Carbon 2,159 (1971). 38. S . Hirano, F. Dachille, and P. L, Walker, Jr., High Temp.-High Press. 5, 207 (1973). 39. M. Inagaki, M. Ishihara, and S. Naka, High Temp.-High Press. 8,279 (1976). 40. M. Inagaki, K. Kuroda and M. Sakai, High Temp.-High Press. 13, 207 (1981). 41. M. Inagaki, K. Kuroda, M. Sakai, E. Yasuda, and S. Kimura, Carbon 22,335 (1984). 42. P. J. Wyllie and 0. F. Tuttle, Nature 183, 770 (1959); Am. Min. 44, 453 (1959). 43. P. R. Buseck and B. J. Huang, Geochim. Cosmochim. Acta 49,2003 (1985). 44. T. Noda, M. Inagaki, S. Hirano, and K. Amanuma, Bull. Chem. SOC.Jpn. 41, 1245 (1968). 45. T. Noda, M. Inagaki, S. Hirano, and H. Saito, Kogyo Kagaku Zasshi 72,643 (1969). 46. T. Noda, M. Inagaki, S. Hirano, and H. Saito, Bull. Chem. SOC.Jpn. 42, 1738 (1969). 47. S. Hirano, H. Saito, and M. Inagaki, Bull. Chem. SOC.Jpn. 43,2599 (1970). 48. S . Hirano, M. Inagaki, and H. Saito, Bull. Chem. SOC.Jpn. 43,2624 (1970). 49. S . Hirano, M. Inagaki, and H. Saito, Carbon 17, 395 (1979). 50. S. Hirano, Ph.D. Thesis Nagoya Univ., 1970. 51. Y. Hishiyama, M. Inagaki, S. Kimura, and S. Yamada, Carbon 12, 249 (1974). 52. S. Kimura and E. Yasuda, Zairyo Kagaku 20, 36 (1983). 53. S. Kimura, E. Yasuda, H. Tanaka, and S. Yamada, Yogyo Kyokai Shi 83, 122 (1975). 54. H. Tanaka, Y, Kaburagi, and S, Kimura, J. Mat. Sci. 13, 2555 (1978). 55. K. Kamiya and M. Inagaki, Carbon 11, 429 (1973). 56. S. Kimura, Y. Tanabe, N. Takase, and E. Yasuda, Nihon Kogaku Kaishi 1474 (1981). 57. R. A. Meyer and S. R. Gyetvay, Amer: Ceramic Soc. Symposium Series 303, 380 (1986). 58. R. L. Hales and E. M. Woodruff, Proc. 4th Carbon Con$, vol. I, 1959, p. 456. 59. R. D. Reiswig, L. S. Levinson, and T. D. Baker, Carbon 5 , 603 (1967).

60, R. A. Meyer, S. R. Gyetvay, and A. B. Chase, Proc, 17th Carbon Con$, (1985) p. 505. 61. R. A. Meyer, J. E. Zimmer, and M. C. Almond, Aerospace Rpt ATR-74 (7408)-2 (1984). 62. R. A. Meyer, Proc. Carbon Con,, Baden-Baden, Germany, 1986. 63 L. A. Feldman andS. R. Gyetvay, Aerospace Rpt. TOR-0086 (6728-020)1 (1986). 64. M. Buechler, F. Hawkins, and R. A. Meyer, Aerospace Rpt. TOR-0084 (4622-01) (1984). P ~ 3 11 (1994). 65. R. A. Meyer and G. W. Henderson, 39th Inter. S A ~ Symp. Braue, and R. A. Meyer, Proc. 20th Biennial Carbon Con$ 66. R. Pleger, (1991) p. 399. S A ~ Tech. P ~ Series, 20,109 67. L. H. Peebles, Jr., R. A. Meyer, and J. Jortner, (1988). 68. R. A. Meyer, Proc. 19th Biennial Carbon Con$, 1989 p. 332. 69. R. A, Meyer, J. L.White, et al., Aerospace Rpt. TOR-0075 (5626)-2 (1 974). 70. R.A. Meyer, J. S. Evangeledes, et al., Aerospace Rpt. TOR-0081 (672605)-l (1981). 71. M. Inagaki, Tanso 129, 68 (1987). 72. M. Inagaki, S. Hirano, and H. Saito, Yogyo Kyokai Ski 76, 264 (1968). 73. M. Inagaki, A. Oberlin, and T. Noda, Tanso 81, 68 (1975). 74. Y. Hishiyama, Kaburagi, and M. Inagaki, in C h e ~ i s and t ~ Physics Carbon, Vol. 23, (P. Thrower, ed.), 199 1, p. 1. Chard, R. Reiswig, L. Levinson, and T. Bdcer, Carbon 6, 950 75 W. (1968). 76. R.M. Bustin, J-N. Rouzaud, and J. V. Ross, Carbon 33, 679 (1995). Yamada, N o n - C ~ s t . ~ o l i1, d s285 (1969). 77. T. Noda, M. Inagaki, and 78. Y. Hishiyama, Y. Kaburagi, and A. Yoshida, in Sciences and New A~plication Carbon Fibers, Toyohashi Univ. Tech., 1984, p. 21. Nakamura, T. Ishii,and S. Yamada,in Abstracts of S y ~ p o s i ~on m 79. Carbon, Carbon Society Japan, Tokyo, 1964, No. IX-6. 80. M. Inagaki, K. Tamada, and S . Yamada, Tan& 90, 85 (1977). 81. E, Yasuda, Y. Tanabe, H. Machino, and S , Kimura, Tanso 128, 7 (1987). 185, 1316 (1969). 82. A. Korosonov, et al., Dok. Akad., Nauk

~ o k k a i d oUniversity, Eta-ku, Sapporo, Japan

Toyohashi University

t us as hi Institute

Technology, Tempaku-~ho,Toyohashi, Japan

Technology, Setagaya-ku, Tokyo, Japan

CNRS,Argelliers, France

Introduction A.Carbonization PrecursorPolymers B. AromaticPolyimideFilms C. Preparation PolyimideFilms D. Purpose This Chapter

246 246 247 25 1 255

11. Carbonization and Graphitization Aromatic Polyimide Films A.KaptonFilms B. PMDAPPD andPPTFilms C. NovaxFilms D.UpilexFilms E. Larc-TPIFilms F. Comparison Polyimide-De~vedCarbon Films with Other Carbons

256 256 269 27 1 275 278

111. QualityControl of GraphiteFilms A.ControllingFactors B. Control in FilmPreparation

287 287 288

280

C. Controlin Carbo~izationProcess D. ControlinGraphitizationProcess

302 304

Quality of GraphiteFilms MicroscopicQuality B. Qualification by GalvanomagneticProperties C, PropertyModification by Intercalation

307 307 313 315

ConcludingRemarks

327

References

329

Carbon materials are prepared through thermal decomposition (pyrolysis) and carbonization of organic precursors. From most precursors, different kinds carbon materials have been obtained by heat treatment above 1000°C residues of thermal decomposition, though some precursorsdo not give carbon materials because of their decompositionto low molecular weight fragments. Usually these two processes, thermal decomposition and carbonization, cannot be differentiated clearly because they occur successively with temp~ratureand also in different places of the sample, Here, therefore, we call these successive processes by one word “carbonization,” ~ependingon thestate of the materials used in carbonization, the carbonization process is classified into one of three categories, solid-state, gas-phase, or liquidphase carbonization [l ,2]. Though gas-phase carbonization, carbon blacks of digerent sizes and different so-called “textures” (aggregation state of primary particles of carbonblack)and also pyrolyticcarbonswith a widerange of structures have been obtained. Fullerenes are known to be formed in this process. ~iquid-phasecarbonization gives different cokes that are very important raw materials for various carbon and graphite materials.The important role of mesophase, whichis often formed in the carbonaceous liquid during the change to solid carbon [3] is now well understood. ~eedle-likecokes have been developed to manufacture graphite electrodes for ultra-high-power operation of steel refining f u ~ a ~ e s and also mesophase-pitch-based carbon fibers to give high pedormance reinforcing materials for composites [3]. Solid-state carbonization has given many useful carbon materials, for example, various active carbons from different precursors and glass-like carbons from some limited precursors. In solid-state carbonization, the interm~diatesformed by the thermal decomposition of organic precursors not mobile. consequence, the resultant carbon materials keep the shape of the precursors, If we heat the precursor at

high heating rate, carbons with high porosity are formed andif we heat extremely slowlyandmake the sampleshrink,aglass-likecarbonwithverylowgas [S] are permeability is obtained. In these carbons, the basic structural units (BSUs) small hexagonal netsof carbon atoms, as arethe BSUs in other carbon materials prepared through liquid- and gas-phase carbonization. In most carbons formed through solid-state carbonization, however, there is almost no orientation, even locally, of these structural units, as a consequence of the low mobility of intermediates, andthere is therefore no developmentof a graphitic structure even after heat-treatment at high temperatures. These carbons prepared through solid-state carbonization, therefore, have been characterized by their non-graphitizing nature. Recently, however, there have been reports of the preparationof graphite films with high crystallinity from different organic precursors such as POD (poly-pphenylene- 1,3,4-oxadiazole)[6] PPV (poly-phenylene-vinylene)[7],and Kapton [S], which is one of the aromatic polyimide films, through solid-state carbonization followed by heat treatment at high temperatures.

Polyimides have been developed as therrnoresistant polymers and have been used in different fields, especially in the field of electronics Because of their practical and promising applications, there have been commercially available polyimide films with different molecular structures, and, as a consequence different performances in applications. Aromatic polyimides (D in Fig. 1) are formedby a reaction of tetracarboxylic (B) ineitherN-methyl-2dianhydride(A)withequimolararomaticdiamine pyrrolidone (NMP)or N,~'-dimethylacetamide(DMAc), through dehydrationof the intermediate product of polyamic acid (C) by either a thermal or chemical method (irnidization), as shown in Fig. 1. It has been shown in the studies on carbonization and graphitization of these polyimide films that their molecular structure is considered to consist of two parts,imide and bridging parts. From Fig. 1, it is clear that the former comes from the anhydride and the latter from the diamine used. The polyimides, therefore, can be characterized by coupling the names of starting anhydride and diamine, such as PMDMODA, which is the polyimide made from pyromellitic dianhydride (PMDA)and 4,4'-oxydianiline (ODA). In Fig. 2, the imide molecules that have been used in the studies on carbonization and graphitization are summarized by aligning the imide part in a perpendicular direction and the bridging part horizontal. Various commercially available polyimide films have been used in these studies andso the trade names are also cited on the corresponding structures of the main constituent imide molecules; some of them mightbe familiar to readers. However,it has to be e~phasizedthat laboratory-made polyimide films have to be differentiated from commercial films,

aroma ti^ diu~ine

dianhydride

or

1"-8

-"Ow0

polyarnic acid

a r o ~ tpolyi~ide i~ Scheme for the synthesis of aromatic polyimide films.

even though imide molecules comprising both films are the same. will be shown later,the quality of the polyimide films achieved during the preparation of films has a definite effect on the resultant graphite films. For instance, the comm cially available polyimide film "Kapton" ~onstitutedfrom the imide molecules NODA, buthasaquitedifferentbehaviorduringcarbonizationand ation from the film prepared in the laboratory from the A/ODA. Hereafter, therefore, we use the trade names, like

ality

Tg

LARC-W1

261OC

U P m X Tg 285OC

Imide

PIT Tg

Tg=41OOC

R ~

~

N

-

2 Molecular structures of aromatic polyimides used for carbonization and graphitization studies.

commercial films are employed samples and the structure names, like PMDA/ ODA, when the laboratory-made films are used. This wide variety in molecular structure of polyimide is one of the advantages for the study on carbonization and graphitization. As shown later, we are able to obtain high quality graphite film from the polyimide films of PMDAIODA (Kapton), but only glass-like carbon films from BTDAKIABP (Larc-TPI). In precursors for other carbon materials, there are not such wide variety in their molecul*ar structures, which is why we are interested in the carbonization and graphitization of polyimide films. The positions of constituent atoms in these molecules are known for only few polyimides, the planarity of the molecule beingone of the important factorsto get high graphitizability, will be discussed later.The result of structural analysisof [lo].All of the atoms making up Kapton (PMDAIODA)is reproduced in Fig. 3a Kapton molecule areon plane and at the ether oxygen the molecule bends by an angle of about 1209 The glass transition temperature is reported to be

Position of constituent atoms in the molecule Upilex (BPDAIODA)

Kapton (PMDAIODA) (a)and

which is rather high in comparison with the temperature pyrolysis (about 500°C). This high glass transition temperature of Kapton is consistent with the fact that it is relatively rigid organic film. this result, the polyimide molecule which does not contain an ether oxygen, is predicted to be of completely flat and, consequence, its film be to very rigid. Actually, the film synthesized P ~ D A ~ was P Dso brittle that it was brokenby touching it with only the tip of knife, makingit useless plastic film.No glass transition ternpera-

ture was determined. The film prepared in the laboratory and named PPT is constitutedmainlyfromthemolecules PMDAPPD with smallamount of additive to keep the film flexible. Some co-polymers, such PMDAPPD with PMDNODA, are now commercially available. For Upilex (BPDNODA), the results of structure analysis have been reported all atoms being in plane at room temperature, with bend at the ether oxygen. However, the biphenyl bondingitsinimide partis reasonably predicted to twist abovethe glass transition temperatureof 285°C (Fig. 3b). Although there is no report on structure analysis, the polyimide BTDNDABP (Larc-TPI) is reasonably expected to have steric arrangement of constituent atoms in molecule at two carbonyl groups, which are connecting two aromatic rings.

Since aromatic polyimides are insoluble and infusible, polyamic acid, soluble precursor of polyimide prepared from the reaction of dianhydride and diamine,is generally used for processing (Fig. Polyimide films are easily prepared by casting polyamic acid solutions on glass substrates followedby drying and thermal treatment for imidization.

~ y n ~ h e s i s ~ o ~ yAcids a ~ i ~ The reaction equimolar amountsof dianhydride and diamine at room temperature gives polyamic acid. Solvents used are aprotic polar solvents such ~ - m e ~ y l - 2 - p y ~ o l i d o(NMP) n e or dimethylace acetamide (DMAc) 121. The molecular weight of the polyamic acid depends on the synthetic conditions. To obtain high molecular weight polyamic acid, use of equimolar amounts of two monomers is necessary. Useof pure monomers and dry solvent well the removal of moisture from the reaction system are also necessary to gethigh molecular weight polyamic acid. Reaction at or below room temperatureis recommended because partial conversionof polyamic acid to polyimide releases water that can hydrolyze polyamic acid. The order of addition of the two starting reagents is important to get high molecular weight.The addition of powdered dianhydride to solution of diamine at or below room temperature is preferable, because the addition of powder is easier and more quantitative than that of solution. Further, the addition of powder can eliminate the possibilityof hydrolysis of d i a ~ y d ~that d e may occur when dianhydride is dissolved in solvent. m e n powdered dianhydride is added to the solution of diamine, the locally concentrated dianhydride immediately reacts with diamine, soon increasing the viscosityof the solution. The reverse addition, i.e., the addition of diamine into the solution of dianhydride, tends to result in low molecular weight product. This is because of the possible hydrolysis of the dianhydride by the moisture in the solvent.

2. S t ~ ~of iP lo l~~ ~~mAcid ic The viscosity of stored solution is known to decrease with time because of the presence of a small amountof water in the solution 131. Recent studies revealed that molecular weight decreases with timeeven in the absence of water 114,151. Due to the equilibrium shown in Fig. 4, the weight-averaged molecular weight decreases, but the number-average molecular weight does not change. It also known that imidization of the polyamic acid in the solution proceeds during its storage, even though very slowly, to result sometimes in gelation. The imidization forms water and thus accelerates hydrolysisof polyamic acid. To avoid gelation, the polyamic acid solution should be kept at low temperatures.

3. Imidi~ation Polyamic Acid Films Polyimide films are prepared by casting polyamic acid solution on the substrates, followed by drying and imidization. Casting is usually performed directly from the reaction mixture, commonly10 wt% solution polyamic acidin either NMP or DMAc.The cast films are then dried under mild heating at 50°C in vacuo in a or circulating oven. The dried polyamic acid films are then heated in a stepwise manner for imidization, in most of our casesat 100,200, and then 300°C for 1 hour at each temperature. The progress of imidization can be followed through infrared spectroscopy by the appearanceof new absorption peaksat 1780,1720,1380, and 720 cm- 1. The imidization process has been found to be very complicated, as will be explained below. The molecular weight of the polymer changes during thermal

Chemical reactions leading to polyimide through polyisoirnide.

treatment of polyamicacid;uponheatingabove it firstdecreases,but increases again above 25OoC, and reaches a high value above 300°C 161. When polyamic acid films are thermally imidized, the glass transition temperature of the films increases with increasing degree of irnidization. During heating at a given temperature, the degreeof imidization increases accompanied byanincrease in Therefore,if reachesthetreatmenttemperature,the mobility of the polymer chainsis lost and the imidization becomes very slow. In order to complete imidization, further heating at a higher temperatureis required. Thus, thermal treatment at a temperature abovethe of the final product polyimide is necessary for the final stage of imidization. Thermal imidization of polyamic acid films as fixed on a glass substrate is found to resultin the alignmentof polymer chains alongthe film sufiace, which is explained by the constraintof film shrinkage along the filmdue to the releaseof water during imidization; the only shrinkage possible being perpendicular to the film surface. One-step irnidization, at 300°C for example, is also used. It should be noted that the propertiesof the polyimide films are somewhat different depending theon curing condition, due to the difference in ordering of the polymer chains. Chemical imidizationis an alternative methodfor converting polyamic acids to polyimides 171. Commonly acetic anhydride is used as the dehydrating agent and pyridine is used as a basic catalyst. The reaction depends on the type of dehydrating agents, the structure of the polyarnic acids, the reaction temperature, and other factors. For example, the useof acetyl chloride E181 or ~~'-dicyclohexylcarbodiimide l91 led to isoirnide instead of imide, Polyisoirnide is interesting because of the greater solubility, lower and greater degree of melt processability in comparison with the corresponding polyimide. The isoimide structure can be easily converted into the imide structure by treating at high temperatures without evolution of volatiles (Fig. 4).

4. ~ t h e r for ~ o l y i ~ i ~ e As mentioned above, polyamic acid has problems in solution stability and in changing molecular weight during thermal imidization. These problems arise from the presence of carboxylic groups in polyamic acid, andso modification of the carboxylic group into its derivatives has been proposed. Among the derivatives, polyamide alkyl esters were paid attention for their solution stability and ease of preparation. Polyamide alkyl esters were prepared two by different routes (Fig. 5); the polymerizationof the half ester monomer (Method A) [20,21] and the modification of polyamic acid (Method B) [22-241. Polyamide alkyl ester was very stable against moisture: for example, the viscosity of its solution did not change evenin the presenceof water for 40 days [20].The imidization occurs ca. 240°C for ethyl and isopropyl ester, but at a temperature as lowas 180°C for tertbutyl ester [21].

oJ-kJo

Ar

5. ~ r o ~ e r t i e s~ o l y i ~ i ~d ei l ~ s Aromatic polyimides are known because of their outst~dingthermal stability shown from their very high degradation temperatures 121. There is no appreciable weight loss below 500°C. They also have very high glass transition temperatu~e~ melting temperatures, even though melting is observed. These ies are mainly governed by the chemical structure of the polyimides. The highly ordered texture the polyimide films, associated with their chemical structures, is also responsible for properties such tensile modulus, tensile strength, elongation, and thermal expansion coefficient. The texture in the aromatic polyi~idefilms is developed during the thermd imidization when the polyamic acid films are imidized while fixedsubstrate, on mainly from the s ~ i n k a g edue to the evaporation of the solvent used and the reaction product water, The increase of alignment of the molecules in the film along the substrate surface is p ~ i a l l ycounte~edby molecul~ rela~ation effects. Thus, the development of molecular o~entation(texture) is competition between the inducedalign~entof the molecules parallel to the surface of the substrate and the tendency of the molecules to relax and drift towards random configuration VVhen polyamide ester precursors are imidized,thesituationisslightly different from polyamic acids, mainly todue less complexation between the amide

ester and the solvent, and so the decomplexation occurs at a slightly higher temperature (about Since i ~ d i z a associated ~~n with decomplexation occurs at higher temperatures, relaxation effects are more prominent, leading to less orientation of molecules in the resulting polyimide films [26]. ~lignmentof polymer molecules canbe achieved more efficientlyby drawing the films or by spinning them to form fibers using various methods [27]. The tensile properties of the polyimide films depend the films molecular weight (especially on eight-avera~ed molecular weight),its chemical st~cture, and the ordered texture of polymer chains. Rigid polyimide molecules, such as ~ ~ Dproduce ~ films ~ having D a,high modulus butare brittle. Flexible polyimide molecules lead to tough films, but a lower modulus. Thetensile modulus of polyimide films can be increased by aligning polymer chains. In the case of p o l y i ~ d ecold-drawing , of the polyamic acid films followed i~idization by gives films with much higher modulus, due to the uniaxial alignment of the polymer chains [27]. Because polyimide films are used in the electronics industry, the thermal expansion coefficient is an important property. It is known that rigid polyimide gives films have a lower coefficient, while flexible polyimide produces a higher coefficient. The coefficient is also affected by the ordered texture and becomes small when films are imidized as bifixed, especially the caseof rigid polyimide films [28]. I

We have studied carbonization and graphitizationof various aromatic polyimide films fromdiEerent points of view: structureof precursor polyimides, preparation conditions of thinfilms, the conditions of heatingduringcarbonizationand graphitization,andperfection of graphiticstructurecharacterizedbyvarious techniques [22-24,29401. The carbon films as the products of complete pyrolysis and carbonization of polyimides have attracted attention mainly for the reasons: (1) they are carbonized without any change in shape, even e shrinkage by carbonization is not small, being about 40% in linear shrinkage, and (2) someof them give carbon films with high graphitizability that be converted to graphite by a simple heat treatment in an inert atmosphere. In 6, bird cranes are s wn that have been made by a paper folding technique o~gami)from a film of ton, carbonized up to 1300°C and then graphitized at about 3000'C. This photograph shows clearly that the carbonization and graphitization do not disturb the crane shape of the starting polyimide, with a guaranteed homo~eneousshrinkage. In the present chapter, the experimental results related to the prep~ationof high-quali~graphite films are reviewed. Firstly, carbo~izationand graphitization behaviors are s u m m ~ z e d c o n c e ~ ipolyimide ng films with different molecular structures. Secondly, we discuss how the quality of the resultant graphite films can

.

6 Cranes prepared from Kapton film by paper folding technique (Origami). (Courtesy of Dr. H. Hatori of the National Institute for Resources and Environment, Tsukuba, Japan.)

be controlled during each step in their preparation processes, i.e., film preparation, carbonization, and graphitization. And then we review how high quality can be achieved from the viewpoints of microscopy and galvanomagnetic properties and also show that some properties of these high quality graphite films can be modified by the intercalation of different species. In this chapter, various experimental techniques are mentioned for the characterization of the prepared carbon and graphite films but no detailed descriptions of these techniques are given. Readers are asked to refer to the corresponding literature for x-ray diffraction [61], polarized light microscopy [62], high-fidelity scanning electron microscopy (SEM) [63], high-resolution tra n s ~ s s io nelectron microscopy (TEM) [64], scanning tunneling and atomic force microscopy (STM and AFM) [65],and galvanomagnetic property measurements [66].

s A representative aromatic polyimide film is called Kapton, which is commercially available in different thicknesses and has a wide variety of applications [9]. A n u ~ b eof r studies on the c ~ ~ o n i z a t i and o n graphitization of these Kapton films as well as films constituted from the same imide molecules (PMDAIODA) have been carried out since 1975 [8].

High Quality Graphite Films

I

400

257

1

600

I

I

I

800

9

JO

lo00

HTT I "C FIG. 7 Changes in weight and shrinkage along the film surface during the carbonization of Kapton film (25 pm thick) [30].

I.

Carbonization

Focusing on Kapton films with a 25 pm thickness, the changes in film weight and shrinkage with change in carbonization temperature are shown in Fig. 7 [30]. The measurements of weight and size were done at room temperature on samples heattreated at each temperature for 1 hour and then quenched to room temperature. The change in the composition of Kapton decomposition gas with temperature is shown in Fig. 8, which is measured by sampling the gas every 15 minutes while at a constant rate of 4OO"Ch. Gas composition was determined by gas chromatography. The experimental results in Figs. 7 and 8 show that the carbonization of the polyimide Kapton proceeds in 2 steps. The first step occurs in the rather narrow temperature range of 50O-65O0C, showing an abrupt weight decrease, an evolution of a large amount of carbon monoxide, and pronounced shrinkage of the film. In the second step, a small weight loss, evolution of small amounts of methane, hydrogen, and nitrogen, and little shrinkage are observed over a temperature range from 700°C to more than 1000°C. The first decomposition step is seen to be mainly due to a breakage at carbonyl groups in the imide part. The ether oxygen was thought to be released at the end of the first step, based on a comparison of the result using a molecule without ether oxygen (PMDAPPD), as will be discussed later (Section I1 B). The weight loss observed in the film after heat treatment at 700°C coincided roughly with the value calculated by assuming the release of the carbonyl group and ether oxygen. The out-gas of nitrogen in the second step of carbonization is due to the

lnagaki et al.

258 1500 I

u &

00

. 8

.-.1

;Y,

1000

B>

3

9

W

500

0 400

500

600

700

800 900 Temperature / "C

1000

FIG. 8 Spectrum of decomposition gases during carbonization of Kapton film (25 km thick) [40].

decomposition of imide groups in the molecule. The release of nitrogen was found to continue up to temperatures above 2000°C [54], leaving a large number of pores in the film if heated continuously up to 2400°C [34]. No cyane gas was detected during carbonization up to 1100°C. The structural change in the second step of carbonization was found to be reflected in the electrical properties of the film. In Fig. 9, changes in electrical conductivity along the film measured at room temperature using different carbonization temperatures are plotted for different laboratory-made polyimide films [36]. The molecule PMDA/ODA is the constituent of Kapton film. All films show a pronounced increase in conductivity, more than one order of magnitude, between 700 to 800"C, although only small weight loss and shrinkage are observed (refer to Fig. 7). Further carbonization up to 1100°C gives another increase in conductivity, roughly one more order of magnitude. The higher conductivity of PMDA/PPD film compared to other films, including PMDA/ODA, is explained by a higher orientation of the constituent imide molecules with better planarity, as will be discussed later (Section I1 B). The facts that methane and hydrogen evolved and electrical conductivity increased in the second step suggest that the carbonization process mainly occurs in the second step, and the first step may be pyrolysis of polyimide molecules. It is worthwhile mentioning that homogeneous and dense carbon films are obtained

High Quality Graphite Films

259

.:

FIG. 9 Changes in electrical conductivity with HTT on various polyimide films [36]. 0 : P M D M P D , 0: PMDA/ODA(Kapton-type), (7: BTDAIODA, A:6FDA/ODA, A:BTDA/ PPD, BYDNDDM.

even though weight loss is as large as 40% and linear shrinkage as large as 22%. No cracks are observed even under scanning electron microscopy. This is one of the reasons why we are interested in the carbonization and graphitization of the aromatic polyimide films, as explained in Section I D. From observations under the polarized light microscope viewing a crosssection, Kapton films with a thickness of 25 pm were optically isotropic after carbonization up to 550°C [38]. At 65OoC, an anisotropy appeared on both sides around the film surface and became homogeneous over the whole cross section of the film at 800°C, but it was still too weak to be measured, suggesting the presence of a large misorientation of basic structural units (BSUs) in the film. Even after carbonization up to 1000°C, the orientation of BSUs in the film remained poor, despite being improved with the increase in carbonization temperature, the optical path difference determined using a retarder plate being only 35 nm (230 nm for graphite). By using the high-resolution tilted dark field mode of transmission electron microscopy (002DF), two orthogonal orientations were chosen for filtering the

002 Debye-Scherrer ring with the objective aperture. Inthe first, only the BSUs parallel to the film surface appear bright, but in the other only those perpendicula are bright. Since there. is a preferred orientation of BSUs parallel to the film surface, the first imageis bright and the other almost dark after heat treatment at 1000°C. The 1000°C HTT is the lowest where high-resolution images of the carbon layers are clearly obtained. By STM and AFM observations on the surfaceof carbonized films [67,68],a bumpy morphology with diameter ranging from 30 to 100 pm and a height of about 5-30 pm appeared progressively with the increase of HTT above 600"C, though the starting Kapton film hada relatively smooth surface.Two orthogonal periodicities are observed on a smaller scale probably due to the repetitions of Kapton monomers and of the unit cell perpendicular to the polymer chains.

2.

~~up~itizution

In Fig. 10, some representative images of lattice fringes (002LF) are shown for Kapton films heat treatedat different temperatures above 1000°C [38]. Fig. 11 is a schematic illustration of the nanotexture change in their cross sections with changing heat treatment temperature (HTT). Up to 2450°C, it is clear that tiny pores are formed between the carbon layer stacks that are flattened along the film surface, which direction is shown by the line AA', with the increaseof HTT, and sketched in Fig. 11. The nanotexture of these porous films obtained around 2450°C can be schematically represented by Fig. 12, where the orientation of flattened pores and the detailed arrangement of BSUs around pores are explained. At 2500"C, these flattened pores collapse so that largely extended, flat, and perfect layers are suddenly observed (Fig. 10). Above this temperature, a large grain texture is produced, accompanied by a sudden progress of graphitization, which also shownby x-ray diffraction and galvanomagnetic property measurements [39]. The stacking of carbon layers is turbostratic after heat treatment at 2450"C, but above 2550°Cit is improved, being accompaniedby pore collapse, to a graphitic structure, as illustrated in Fig. 11. For the films that are heat treated in the HTT range of 1800-2200°C, AFM observations showed the presence of large flat domains, consisting of distorted hexagons, heptagons, and triangles that suggested the presence of a large number of defects in the carbon layers [67,68]. With an increase in HTT, these domains became numerous, large, and also less defective. Above an HTT of 2400"C, extended terraces witha triangle pattern, whichis characteristic of graphite with an AB layer stacking, are observed. These terraces are separatedby steps with a height difference of 0'33-0.43 nm. From an initial 25 pm, the thickness of the film decreases with an increase in HTT, as shown in Fig. 13 [38], The thickness is precisely measured by optical microscopy from the residual embedded blocks used for thin sectioning. The first abrupt decrease at 550°C corresponds to a massive release of oxygen as CO and

002 lattice fringe images for carbon films derived from Kapton film (25 thick) at different [38]. 2250"C, b) 2450"C, and c) 2550°C.

I 50

t ~ ~ s t r a t i c2550

partially graphitic

g ~ p ~ ~ c

graphitic

turbostratic

Schematic illustration nanotexture in the cross section of carbon films derived from Kapton film at different HTTs [38].

CO, [30], discussed in Fig. 8. However, the release of CH4*H,, and also N, (Fig. 8) in the second step of carbonization, overa wide rangeof temperature from 600 to1300"C,doesnotgiveanythicknesschange,onlyproducing small shrinkage along the film (Fig. 7). Although no appreciable change in film thickness occurs, considerable improvement in structure and texture occurs in the stacking orderof the aromatic layers, i.e., the preferred orientation, andso optical anisotropy pervades the whole cross-section of the film. Above 2000"~,particularly above 2300"C, the second thinning process is observed. This processcorresponds to the large increase in optical anisotropy (the optical path difference raised from35 nrn 1000°C to160 nm at 2450°C) andthe increasing contrast in the 002 lattice fringes of the flattened pores observed in TEM from Fig.loa-c.

Scheme of nanotexture around pore in apto on-de~ived carbon film (381. Three dimensional ~ a n g e m e nof t pores in the film and detailed ~ ~ ~ e m eBSUs n t around pore.

Change in thickness of Kapton film (25

thick) with

This corresponds to sudden breakageof all the defective areas nearthe edges of flattened pores (Fig. 12b) ensuring lateral coalescence andcollapse of the aromatic layer stacks, which yields m ~ i m u mcompactness and complete annealing of the layer distortions.The partial graphitization thus suddenly introduced does not give any further changes in thickness. X-ray structural and galvanomagnetic parmeters also showed abrupt changes in narrow temperature range around 2450°C [39]. In Fig. 14, the values of interlayer spacingdO02determined from both002 and 004 diffraction lines, andcrystallite sizes alongthe c- and a-axis, Lc(O04) and are plotted against HTTof the carbon film prepared from Kapton filmwith25 pm thickness by carbonization up to 900°C [54], The galvanomagnetic properties, maxi mu^ transverse magnetoresistance (Ap/p),,, Hall coefficient R, and residual resistance ration pRT/p77K ratio of electrical resistivity at room temperature to that at 77 K), and electrical resistivityp are also shown for the same films in Fig. 15 [54]. Only the002 diffraction line was observedfor films heat treated up to 1500°C. In the HTT range between 1~00-2200°C,the dO02values measured from the002 and 004 lines are different, suggesting the presence of stacking disorder, and above 2300°C the two values are the same. Above 2200"C, even 006 lines were clearly observed and above 2700°C both 004 and 006 lines showed splitting due to Kaland radiation, which indicates marked improvement in crystallinity in the film, in accordance with the marked increase crystallite in sizes both in and (Fig. 14b and c). shown in Fig.15a, electrical resistivity p decreases rapidlyat first and then onlyslowlywithincreasingHTT.ButintheHTTrangebetween2200and

I

g Tf

lo00

I

Changes in interlayer spacing cloo2 (a), crystallite sizes (b), and La (c) with for the carbon film derived from Kapton (25 pm thick) Diffraction lines used for the measurement are shown in parentheses.

m/

Changes in electrical resistivity p resistivity ratio pRTIp77K(b), Hall coefficient RH (c),andmaximummagnetoresistance ( A p l ~ (d) ) ~with ~ ~ andthose of l ~ a ~ i m umagnetoresistance m (ApIp),,, with magnetic fieldB (e and for the carbonfilm derived from Kapton (25 thick)

2300"C, the resistivity drops discontinuously and then again decreases gradually. From the HTT dependence of pR1'lp77K in Fig. 15b, three HTT ranges can be discerned: 900-1200, 1300-2100, and above 2300°C. In the first range of semiconductor-li~econduction is observed, and in the second, is almost constant and weakly temperature dependent. Above 2300"C, the electrical conduction becomes semiconductive again, which agrees with the development of a graphitic structure.The ratio is higher than 1 for the film heat treatedat 30OO0C,

m

which suggests good crystallinity. The discontinuous change in the HTT rangeof 2100-2300°C is due to the abrupt transformation of the structure from turbostratic to graphitic.The Hallcoefficient determined from the detailed measurements of its magnetic-~elddependence, shows a maximum at the HTT of 2200"C, as shown in Fig. 1%. The R, values for the films heat treated below 2100°C were independent of magnetic-field strength, being negative in the range of 14001600°C and positive in 1700-2100°C. Above 23OO0C, RH drops to a negative value and then gradually increases with the increase in Its dependence on magnetic field was weakat HTTs about 2300"C, but became similarto those for well-crystallized HOPG (refer to Fig. 55) at higher HTT, suggesting the coexistence of two carriers, electrons and holes. was not detected at HTTs below 1500"C, was ~agnetoresistance(Aplp),, negativ~up to 2200"C, and then changed to positive and increased very quickly (Fig. (Fig. 15d).The magnetic field dependence of ( A p / p ) ~as~a functionof

l

l

l

15e andf) showed general trend commonfor graphitizing cokes[66]. Below an TT of 2200°C, the magnetoresistance is negative and increases its absolute value with an increase in magnetic field. At 22OO0C,(Aplp),,, shows trend of saturation at high field, suggesting the coexistence of large mount of turbostratic structure with small amountof graphitic structure. Above 2300°C the magnetoresistance becomes positive and increases markedly with magnetic field. It reaches about 500% in the film heat treated at 3000°C. The anisotropy ratios rr and which are the ratios among (Aplp) values measured in three different directions [66], were low 0.05, showing highly oriented texture in the films heat treated at high temperatures. These changes in galvanomagnetic properties around 2200-2300'~ correspond well to the abrupt changes in structure shown in Fig. 14.

I

In two studiesof structure of Kapton-derived carbon films an abrupt change in structure and, as consequence, a jump in TEM, and galvanomagnetic properties (Figs, 10-15) was observed. However it has to be pointed out thatthe transition temperature from turbostratic to a graphitic structure was slightly different, about 2450°C in the former and about 2250°C in the latter. The reason for this difference is suggested to be the difference in heating rate during heat treatment, 6"Clmin in the former but 2"Clmin in the latter. These abrupt changes in structural parameters, La,and lattice fringes, and in electric, and galvanomagnetic properties-electrical resistivity p, transverse magnetoresistance ( A p l ~ ) Hall ~ ~ ,coefficient and resistivity ratio pRrlp77K---around 2200-2300°C were foundto be very much related to the final departure of nitrogen atoms that were assumed to be substitution~l~ located in hexagonal carbon layers [55,561. In Fig. 16, is shown a model, in carbon layers including spin density contours, for the chemical bonding state of nitrogen, which is proposed from a calculation using semiempirical molecular orbital methods. Extra electron spins accompanied by nitrogen seems to be localized around the C-N bond, in other words, nitrogen acts an acceptor. It also has to be pointed out that above the same HTT range (2200-2400°C) carbon atoms in most carbon materials become mobile, which has been exhibited by a sudden change from fragile to ductile [69,701. These two phenomena, the loss of remainingnitrogenfromhexagonalcarbonlayersandanincreaseinthe mobility of carbonatoms,mayaideachotherandareassumedtostrongly contribute to the structural development, The graphitization behaviorof thin Kapton filmis very similarto those of anthracites(high-rank coals) andnongraphitizingcarbonsunderhighpressure.

A model the chemical bonding state substitutional nitrogen in the hexagonal carbon layer, with excess spin density contours shown

In anthracite whichcontains flattened pores, the pore walls break before 2000°C and yield lamellae, giving better graphitizability than most the graphitizing carbons, such as cokes 1641.The same behavior was observed upon heat treatment of nongraphitizingcarbons,suchasglass-likecarbons,underhighpressures above 0.3 GPa l,72,73].

A molecule of Kapton (or PMDAIODA) film has ether oxygen in the bridging part, where the of the molecule occurs, even though the other partis completely flat, as shown in Fig.On 3a.the other hand, the molecule P M D M P D does not have ether oxygen (Fig. 2). Therefore, there's no kink in the molecule so and high degreeof molecular orientation in the film is expected. However, the synthesized filmof P M D M P D was brittle thatit was broken easilyjust by touching it with the tip of a knife. The carbonization process was studied on the laboratory-madeoffilms PMDAI PPD with irregular shape and compared with thatfor the films of PMDAIODA, both films being prepared in the laboratory by the same procedure 1361. These two films showed the same two-step carbonization as shown for the commercially available Kapton film consisting of PMDAIODA molecules (Fig. In Fig. 17, the spectrum of decomposition gases is shown for the laboratory-made PMDA/

17

PPD

spectrum of decomposition gases evolved during carbonization of a PMDM

PPD film, This was practically the same as that observed for Kapton films (Fig. 8). If the out-gas profile of CO on this P M D ~ P D film is compared with that of Kapton (Fig. 8),it can be pointed out that the former does not show any tailing, a the latter does. Therefore, the ether oxygen in the bridging part is reasonably expected to come outat a little higher temperature than the breakage of carbonyl groups in the imide part. In Fig.9, changes in electrical conductivity along the film at room temperature with carbonization temperature are shown on different polyimide films, including PMDAPPD and PMDAIODA.The P ~ D film~ clearly P reveals ~ higher conductivity than the others, suggesting better orientation of in that film. In the laboratory, an aromatic polyimide film was prepared from PMDA and PPD by replacing 8 mole% of PPD with 4 mole% of 3,3~4,4’-tetraminobiphenyl which was named in the same manner as PPT [74,75]. The PPT films have flexibility asdo other polyimide films, though the main constituent molecules are supposed to be P M D ~ P DThe . ~raphitizationbehavior was studied for this PPT film with a thickness of 45 pm [45,46,49,50]. In Fig. 18, changes in the006 x-ray diffraction profile with HTTis shown. A marked shiftof the profile to the high-angle side, i.e., a decrease in spacing, is found in the HTT range of 210O-220O0C, and a pronounced improvement of crystallinity, which is exhibited by separation of the 006 profile into the doublet due to and peaks, occurs above 2500°C. The interlayer spacing reached the value of graphite, 0.3354 nm,after heat treatment above3000°C [46]. In Fig. 19, the ma~imumtransversemagnetoresistance at 77 K is plotted against HTT [46]. Its development is so marked that its values have to be plotted on a logarithmic scale and it reaches about 1200% for the 3200°C-treate~film, which is not shown in the figure. From electron channeling analysis, the average size of the graphite crystallites was determined to be 10 pm [45]. The changes in x-ray parameters and galvanomagnetic properties with HTT a very similarto those of graphitizing carbons, except that PPT results in the exact while most graphitizing carspacing of 0.3354nm and very high (Ap/p),, bons do not achieve this with heat treatment above 2800°C. or the PPT film, the preferred orientation became strongly marked above 900°C in optical microscope observations and the optical path difference regula increased with increasingHTT. There was no pore development, but a conti~uous i~provementin the stacking order [50]. In Fig. 20, the 002 lattice fringes are shown for thePPTfilmsheattreated at 1500,1800,and2100°C. A sudden annealing, resulting from distortion of the layers and observed by 002LF, appears at about 2100°C and results in flat and per thickness in ge of the TT is compared with ton and Upilex [42] g account the of fact that the startin PPTfilm is thicker (45pm) than the others(25 pm), the thickness PT is quite different from the others. It is particularly though both Kapton and PPT give highly crystallized

FIG. thick)

Change in 006 x-ray diffraction profilewithHTTonthe

PPT film

pm

graphite films after heat treatment above 2800°C. In Kapton, an abrupt drop in thickness occurs in the range of 2100-25OO0C, as shown more preciselyin Fig. 13, which corresponds to the sudden changes in galvanomagnetic properties and also to the loss of nitrogen atoms that remained in the carbonized film. In PPT, however, the thickness of the film decreases gradually in a wide range of HTT between 1100 and rather similar to Upilex A film.

The polyimide film named N o v a consists of a copolymer of P M ~ ~ and O ~ A ~ M ~ A / OasTshown ~ , in Fig. 2. It was studied in detail was the Kapton film and was found to have high gra~hitizability 1,50,59]. In Fig. 22, the interlayer spacing doo2,measured from 002 and 004 lines, is

I

l

2200

m

I

Maximum magnetoresistance (Aplp),,, km thick)

I as a function of HTT for the carbon

films derived from

plotted against HTT [59]. It is characteristic that the HTT dependence of clooz shows clear steps around 3300 and 2000"C, and the values determined from the 002 and 004 lines are the same. This is different from the case of Kapton (Fig.14a). Resistivity p at 77 resistivity ratio pRTIp77K,Hall coefficientR,, and maximum ~agnetoresistance(ApIp),, at 77 are plotted against HTT in Fig. 23 for the Novax-derived carbon films [59]. Above an HTT of 1200°C, p decreases gradually with increasingHTT, with no abrupt change (Fig. 23a).The change of pRTIp77Kwith HTT is very similar to that observed for Kapton-derived carbon films, except that its abrupt decrease occurs at20OO0C, a much lower HTT (Fig. 23b). The changesin R, and ( A ~ I Pwith ) ~ HTT ~ ~ are a little different from those apton-derived cabon film and also graphitizing cokes (Fig. 23c). The is too small to be detected below negative at 1600 and positive in a range of 1800 and 23OO0C, and then negative again above 2400°C. The R, was field independent below22OO0C,but became gradually field dependent above

(a)

002 lattice fringe images of carbon films derived (b) 1800"C, and (c) 2100°C

at different HTTs

Film thickness variations withHTT for different polyimidefilms of the startingfilms of Kapton and Upilex are pm and that 45 Upilex A, Upilex Upilex C, an

Thickness

found to occur suddenly above 2100"C, at slightly lower temperature than for Kapton-derived carbon films.

polyimidefilmUpilex(tradename of Upilex-R)was studied for its carbonization and graphitization behavior becauseof the presence of biphenyl bond in its molecule (Fig. 2) [32,42,57]. For Upilex films with thickness of 25 p,m, various graphitizabilities were found [42]. Since the variationof thickness withHTT was recognized to be one of the useful parametersfor characterizing structural changefor polyimide films, it is illustrated in Fig. 21. For comparison, Kapton and PPT are recalled in the figure. Here, only three samples of Upilex films were used and discussed; graphitizing (sample A) and nongraphitizing (samples B and C). Upilex filmC first shows decrease in thickness below 1000"C, followed by small but significant swelling above 1000°C. The films heat-treated above 1600°C have constant thickness and remain optically isotropic, which is explained by the occurrence of isometric nanopores visible on the 1600°C-treated sample (Fig. 24a). with other carbons, only the stacking order improves with HTT. is tort ion-free layers formthe pore walls between 2000°C (Fig. 24b) and 2800°C (Fig. 24c). The swelling observed is due to the re~angementof BSUs, which f o m at first distorted open set of local orientation (Fig. 25a), and then more tight ~rangementin pore walls (Fig. 25b).The isometric shape of the nanopores (about 5 nm in diameter)is due tothe lack of plasticity preventing stress orientation, which prevents any subsequent graphitization. The film B shows very similar behavior, can be expected from the thickness change shownFig. in 21, On the other hand,the Upilex film follows the same trendKapton, but the decrease in film thickness occurs at much lower HTT (Fig. 21). A marked decrease is observed above1500°Cand reaches ~ n i m u m thickness (about8 pm) by2100°C. A11 films heat treated above 1000°C show preferred orientation parallel to the film surface: the optical path difference was 52 nm after heat treatment at 1000"C, though that of Kapton was 35 nm. This corresponds to statistically long-range orientationof BSUs in the cross section of the film, where flattened pores are developed with an increasing stacking order in their walls as TT increases. The image of 002LF and the selected area diffraction pattern for 2100°C-treated film are shown in Fig. 26. Carbon layers without disto~ionare observed in the film heat treated at temperatures low 2100"C, and marked orientation of layers is revealed by 002 arcs on the di~ractionpattern. It has to be pointed out here that even three-di~ension~ the 12 line is visible in the pattern, indicating the occurrence of graphitic stacking. In Fig.27, the 110 dark-field image and the selected area diffraction pattern are shown for the film heat treatedat 2800°C [42].A spotty di~ractionpattern and

HTT I cient RH tame

Changes in electrical resistivity p resistivity ratio (b), Hall coeffiand maximum magnetoresista~c~ (d) with and m a ~ ~ ~ t ~ ~ e s i with magnetic field B (e and f) for carbon films derived from [B].

8

1

002 lattice fringe images of carbon films derived Upilex C film (25 thick) at different 1600"C, (b) 2000"C, and (c) 2800°C.

with a granular microtexture, very similar to that observed for various glass-like carbons gave broad 002 x-ray diffraction line profile both in reflection and transmission modes.The region between thesetwo has a rough layer microtexture over a thickness of about pm.

For the commercial film Larc-TPI, the developmentthe graphitic structure was found to be strongly restrained expected fromits steric molecular structure (Fig. 2). InFig. 29, thefielddependences ~a~netor LNC-TPIderivedcarbonfilmsareshown function of values of ( ~ p / are ~ ) ~ negative after heat treatment below 2600'~and are positive, but very small, even '~ suggesting the presence of a turbo strati^ structure. Anafter ~ 8 0 0 treatment, isotropy evaluated from magnetoresistance ~ e a s u r e ~ e nwas t s also very low. The

25 110 dark field images and sketch of nanotexture at pore walls in carbon films derived from Upilex C film at (a> 1000°C and (bj 1600°C [42].

6 002 lattice fringe images (a and bj and corresponding selected area diffraction patterns (c and dj for carbon films derived from Upilex A film (25 pm thick) at 2100°C in different places [42].

Selected area diffraction pattern (a) andl10 dark field image (b) for the carbon film derived from Upilex A at 2800°C [42],

0.6

0.6

0.8

1.0

-0.02

-0.05

1.0

29 Dependence of maxim~mmagnetoresistance rived from Larc-TPI thick) on magnetic field strength

of the carbon film de-

If the graphitization degree reached by the heat treatment at temperatures as high as ~ 8 0 0 - 3 0 0 0 ' ~is compared among carbon films derived from polyimides (Kapton, and Novax) by referring to those of graphitizing carbons, three polyimide-derived films give highly crystallized graphite. For example, their spacings are exactly the same as graphite, 0.3354 nrn, as explained before, thou all graphitizing carbons, such as cokes, cannot reach the exact value of 0.3354

FE-SEML image of the cross section of the carbonfilm derived from

at

3000°C

100

20

Weight change (a)and linear shrinkage (b) for thefilms of PODandPPV compared withKapton Kapton, PPV, and A:POD.

temperature-programmed spectrum of decomposition gases

a POD

film

nm even after heat treatment at 3000°C. However, the structural changes with HTT are different among these three films, The carbon films derived from PPT show progressive structural changes with HTT above 2000°C (Fig. 18) and is somewhat similar to those observed for most graphiti~ingcarbons In the Kapton-derived carbon films, onthe other hand, an abrupt change occurs overa narrow rangeof HTT from 2200 to25OO0C, explained before (Figs. 14 and 15). The behavior observed for the Novax-derived carbon films an i ~ t e ~ e ~ i a t e between PPT and Kapton. In Figs. 33 and 34, the structural parameters and and galvano~agnetic and respectively, for the Kapton-derived carbon films properties [(Ap/p),,, were compared with those for graphiti~ingcokes by plotting against HTT Two points have to be mentioned: at HTTs below 2200”C, the development in these parametersis delayed in Kapton-derived carbon films cornpared with cokes, but their development very pronounced in the former above 2300°C. The abrupt change in structure in the range of 2200-2300°C was found to correspond to the

00

Changes in interlayer spacing Kapton-derived film an

2000 2

and crystallite

with

Changes in Hall coefficient (a) and maximummagnetoresistance with HTT [57]. Kapton-derived carbon film and a petroleum coke.

uality ~ r ~ ~Films ~ i t e departure of nitrogen, which remained substitutionally in the hexagonal carbon layers [34,55], previously discussed.

PHlTE FlL From the studies on different polyimide films, the following three fundamental conditions for producing well-crystallized graphite films are obtained [32]: 1. flatness of starting imide molecules, 2. high degree of o~entationof these flat molecules along the film, and 3. less disturbance of these orientations during carbonization and graphitization due to the out-gassing of noncarbon atoms.

Factor 1 concerns the molecular structure of the polyimide used precursor. shown in Fig. 2, the polyimides can have wide variety of molecular structures, but not all yield high quality graphite [36,59,60]. From Kapton, whose molecules are known to be flat, for example, graphite film with high crystallinity can be obtained by selecting the appropriate conditions of film preparation and heat treatment at high temperatures [29,34,38,54]. However, film of Lac-TPI, which has steric conformationof constituent atoms in the molecules gives only film of glass-like carbon even after heat treatment at high temperature 3000°C [43]. Factor 2 can only be controlled by the conditions of film preparation, particularly by those during imidization and selection film thickness [60]. Even starting with moleculesof PMDNODA (the constituent moleculesof Kapton), high degreeof graphitization in the film was not obtained if the correct procedure for film preparation was not employed, will be shown in the following section. Factor 3 is related to the conditions during carbonization and graphitization. It was shown that the heating rate in the carbonization processhas to be selected in relation to the glass transition temperature of the film used because the orientation of the molecules may be modified [48]. During graphitization, the parallel alignment of hexagonal carbon layers is assumedbetodisturbed by the releaseof small amountof remaining nitrogen, which means thatKapton film should be kept at 2200°C before going to higher temperatures [34]. The crystallinity measuredby x-ray parameters, such the interlayer spacing and galvanomagnetic parameters, such maximum transverse magnetoresistance (Ap/p),,, for the graphitized filmsis governed by these three factors. Therefore, simple comparison in crystallinity among the graphite films prepared separately may be misunderstood the effectsof these three factors. For example, this might be true result of comparison among the commercial polyimide

films, which may be prepared under different conditions, and graphite films graphitized in different furnaces.

also among the

1. ~ ~ ~ ~in the t ~ ~i i l n ~ t

The constraint during imidization to form a polyimide film was found to have a strong effect on the crystallinity of the resultant graphite film [35,79,80]. polyamic acid prepared from the anhydride of and diamineof by in a solution of NMP was homogeneously spread out on a glass substrate using a knife and the solvent was vaporized under reduced pressure at 50°C. Fil with a thicknessin the rangeof 20-30 k m were selected to be imidized at 300°C under three different constraint conditions: fixed on the glass substrate (irnidized on a glass, IG), peeledoff from the glass (imidized in free a space, IF), and being stretched by 20% in length at room temperature and then imidized at 300°C (i~idizedunder stretching, IS). The polyimide films prepared under different constraints were carbonized at 900°C and then graphitized at 3000°Cfor l hour 1351. The x-ray and magnetoresistance parameters measured for these three films are summarized in Table l. The films derivedfrom IG and IS, both irnidized under constraint, show a very high degree of graphitization, a relatively small interlayer spacing a large crystallite size along the c-axisL,(002), and high valuesof maximum transverse magnetoresistance ( A ~ / P ) The ~ ~ ~small . value of their anisotropy ratios indicates a high degree of orientation graphite layers along the film surface. In contrast, the film prepared in free space (IF) exhibits a rather low degree of graphitization and a low degree of preferred orientation.The micrographs of these three graphitized films (Fig.35) support the resultsof the structure studies. The above results indicate that preferred orientation of polyimide moleculesin the film, which seemsto be promoted by constraint during imidizatio~,plays an important role in the graphitization of the resultant carbon film.

X-Ray and Magnetoresistance Parameters after Grap~itizationof Three Polyimide Films with Different Degrees of Constraint

IG Irnidized

IF

1s

on glass substrate Imidized in free space Irnidized under stretching 35.

l40 0.3359 26.97 47 0.3362 0.3356 244.0 280

288.4

0.097 0.252 0.049

35 SEM micrographs of cross section of carbon films prepared from polyimide films imidized underdifferent constraints, after heat treatment at 3000°C for 1 hour (a) imidized on a glass substrate (IC), (b) imidized in free space (IF), and (c) imidized under 20% stretching (IS).

2 Structural Parameters of Polyimide Films Prepared at Different Conditions after ~raphitizationat 2800°C for Hour Polyimide film Sample code Themo-ON Thermo-OFF Chem-OFF VDP-ON film Commercial Kapton

Graphitized film

Preparation and i~idizationconditions

A (-1

(nm)

(nm1

(-1

Casting of solution and thermally on substrate Casting solution thermally and without substrate Casting of solution and che~cally without substrate Vacuum deposition and thermally on substrate

0.070

0.3359

>l00

0.3

0.01

0.343

>l00

0.4

>l00

0.4

0

R

0.9 0.346 3

0.091 0.3359 0.3358

A: b ~ e f ~ n g e n cR: e ; R m a n intensity ratio of Source: courtesy of Dr. H.Hatori of the National Institute for Resources and ~ n v i r o n ~ e nTsukuba, t,

The orientation of polyimide molecules in the films prepared by similar procedure was confirmedby measurements of the refractive indicesof the film [80]. As shown in Table2, the films imidized on the glass substrates have larger values of b~efringence than those imidized without a substrate, irrespectiveof whether polymerization has occurred in solution or deposition and whether thermal or chemical imidization has been employed. The films that have high values, i.e., high degree of orientation, give higher c~stallinity, value close to that of graphite, large crystallite thickness, and high ratioof R m a n intensities at 1355 to 1580 cm-1, compared to those with low A. Such pronounced differencein crystallinity is also demonstratedby TEM lattice fringe images shown in Fig. 36. at room temperaThe result in Table1 suggests that imidization under stretching ture exerts favorable effect on graphitization, regardless of the fact that the constr~ntintroduced by stretching during imidizationis usually considered tobe released during heating to 900°C during carbonization. The effectof stretching duringi ~ d i z a t i o n(cold-drawing) was studied in detail for the same polyimide molecules, PMDNODA [41]. As shown in Fig. 3’7, an increase in drawing ratio results pronounced in increase in the electrical conductivity along the films, which were carbonized up to 1000°C for 1 hour. The enhancement of conductivity in the carbonized film is reasonably attributed to the increase in preferred orientation of hexagonal carbon layers parallel to

36 TEM micrographs and electron diffraction patterns (inset) of polyimide films prepared (a) by thermal imidization on a glass substrate (Thermo-ON) and(b) by chemical imidization without a substrate (Chem-OFF) after heat treatment at 2800°C for 1 hour. (Courtesy of Dr. H. Hatori of theNational Institute for Resourcesand Environment, Tsukuba, Japan.)

0

40

60 80 Drawing Ratio

100

Electrical conductivity of cold-drawn polyimide films PMDAIODA carbonized at 1000°C for hour

the film, which results from the orientation of polyimide molecules in the film during cold-drawing. Alignment polymer chains by cold-drawing was much more effective with the rigid polyimide than with the flexible ones For the rigid polyimides such as P M D ~ P and D BPDA/PPD, the effect of cold-drawing on the modulusof the polyimide films is much more remarkable and orientation is considered to be higher than that for the flexible polyimides such as PMDNODA and BPDN ODA, as shown in Fig. 38. The effect of cold-drawing on the orientation of polyimide molecules in the film is also expected to influence orientation in carbonized films, this effect being much more remarkable with rigid polyimides than with flexible ones. Cast films were prepared from three kinds of polyimides, PMDA/PPD, BPDrVPPD, and BPDNODA. Electrical conductivityof the films carbonized at 1000°C for 1 hour was compared with that of the carbonized PMDA/ODA As expected, the effect of cold-drawing on the electrical conductivityof the carbonized films was much more remarkable with the rigid polyimides such PMDAPPD as and BPDN PPD than with the flexible polyimides such PMDNODA as and BPDAIODA,as shown in Fig. 39. It must also be noticed from Fig. 39 that,at a very low draw ratio, BPDA-based

for

PMDNODA, A:PMDAPPD,

polyimides (BPDAJPPD and BPDAJODA) give more conductive carbon films thantheirPMDAcounterparts ( P ~ D and ~ PMDA/ODA). P ~ This can be explained from the glass transition temperature (ZJ and the decomposition ternperature (l$ecomp) of the polyimides. The BPDA-based polyimides, which havea considerably lower 7;: than the PMDA-based polyimides, have to be kept for a longer timein the temperature range between 7;: and l$ecomp during their pyrolysis, which helps them align and orient more than their PMDA counterparts. As ~entionedabove, uniaxial cold-drawing was foundto be effective during the carboni~ationstage at 1000°C [41,47]. However, it was also shown that uniaxial cold-drawingof films of polyimide, BPDAJPPD and PMDAIODA, results in the hindrance of graphitization evaluated by both magnetoresistance and x-ray diff~actionmeasurements [5l]. As shown in Table 3, the maximum transverse magnetoresistance (hplp),, decreased, showing retardation of graphitization, with an increase in the ratio of uniaxial drawing. Anisotropy ratios, and rT, also show that the in-plane orientation is disturbed by cold-drawing. Thus, it was suggested that, with an increase of drawing ratio, the axial orientation of graphite layers increased and the degree of grap~iti~ation decreased.

60 Changes in electrical conductivity of various polyimidefilms with dr C for hour [47]. PMDNODA, A:PMDNPPD

~agnetoresistanceParameters Films Prepared Polyimides Under Different Imidization Conditions, Carbonized at 1000"C, and Then ~ r a ~ h i t i z at e d2800°C Magnetor~sistance Polyirnide film BPDAFDA drawn 2.87% drawn 10.4% drawn 21.7% drawn on glass PMDNODA 0% drawn 25.9% drawn on glass

(hplp),,

Anisotropy ratios

Ata

(%l

rT

0.010 0.004

285 207 0.16 -0.49 333

0.009 0.008 0.377 0.007

0.063 0.016

52 55 39 34 58

59.4 -0.34 69.8

0.035 0.386 0.026

0.013 0.085 0.009

69 4s 64

h"

*Decrease in thic~ness the films after graphitization. b~~gnetoresistance values (Ap/p)Tmi, and were negative and anisotropy ratios could not be calculated. Source: Ref. 5 I.

3. The dependence of c~stallinityin the graphite films on the thickness of the precursor films is also explained on the basis of preferred orientationof polyimide molecules during film preparation. In Table4, the structure parameters are summarized for the graphitized films prepared from two polyimide films, Kapton and Novax, with different thicknesses [44,60]. For these two precursors,prominent effectof precursor thicknessis observed on the crystallinityof the resultant graphite films; both the degree of graphitization and the preferred orientation of the hexagonal layersin the films decrease with an increase in the thickness of the precursor polyimide films. It was experimentally proved that the thicknessof the precursor film governs the orientation of polyimide molecules in the film Films of the polyimide PMDAIODA (Kapton-type) withdifiFerent thicknesses were preparedby thermal imidization of the corresponding polyamic acid both on glass substrate and without substrate. The dependence of birefringence A of the film on its thickness in Fig. 40 reveals that the thicker film has the lower degreeof orientation. This figure shows the effect of constraint on orientation, discussed before; all films imidizedon a glass substrate show higher degreeof orientation than those without substrate, irrespective of film thickness. These strong dependences of orientation of polyimide moleculeson film thickness and on constraint were also proved by an ESR technique using spin probe [SO]. In the case of Upilex films, a wide range of graphitization behaviors from graphitizing to nongraphitizing was observed,as explained before (Section D). Even in a series of Upilex films with a graphitizing nature (Upilex a good linear relation between film thickness after heat treatment at 1000°C and optical path difference is observed [42], shown in Fig. 41. The thinner film has the higher valueof optical path difference, i.e., the higher degree of preferred orientation of the BSUs, giving higher degree of graphitization after the hightemperature treatment.

4 Structural Parameters for the Various Polyimide Films with Different Thickness after Heat Treatment at High Temperature Polyimide Kapton

Novas

Thickness (pm)

HTT ("C)

dooz (nm)

rT

I 0.10

0.0

I pm

40 Changes in birefringence of polyimide films PMDA/OI)A imidized on a glass substrate (PI-ON) and without substrate (PI-OFF) with film thickness. (Courtesy Dr.ofH. Hatori of the National Institute for Resources and Environment, Tsukuba, Japan.)

0

80

Relation between optical difference of path and film thickness after heat treatment at 1000°C for a series of Upilex A films

4.

Use

~ o l y a Acid ~~c

Ester

From the studies of orientation of polyimide molecules on carbonization and graphitization, high-quality graphite film was obtained when imidization of polyamicacidfilmwascarriedoutfixed on glass substrate Thehigh orientation of the molecules afforded by imidization fixed on glass substrate arises from the ordering of the polymer chains along the film surface, which is induced at irnidization by the loss of water molecules and s ~ i n k i n gonly in the direction perpendicular to the film surface. Polyamic acidester alkyl film imidizes by the loss of alcohol groups, larger molecules than water, and shrinks more at I C. It is expected that imidization than polyamic acid film,discussed in Section the useof polyamic acid alkyl ester precursor, insteadof polyamic acid, would give polyimide film with higher orientation of the molecules along the film surface, when it is imidized fixed on glass substrate. Methyl esters of polyamic acid having various esterification ratios were prepared and imidizedto the cast film, followed by carbonization and graphitization As components of polyimide, pyromellitic dianhydride (PMDA) and p-phenylenediamine (PPD) were selected because polyimide molecules prepared from these components are rigid rods, and the effect of orientation of the molecules can be characterized at various preparation steps by measuring the tensile modulus of the polyimide films, electrical conductivityof the carbonized films, and x-ray diffraction and magnetoresistance after graphitization. Polyamic acid prepared from PMDA and PPD was allowed to react withNaH and methyl iodide to produce its methyl ester having various degrees esterificaof tion (Fig.42). Polyimide films obtained by imidizing the cast films fixed on glass substratehadslightlyhighertensilemodulithanthose from polyamicacid, suggesting that higher orientation along the film surface was achieved, The

42 Reaction scheme for the preparation of polyamic acid methyl ester

lnagaki et

polyimide films were then carbonized by heating up to 900°C. The electrical conductivity of the carbonized films from methyl ester was high that from the corresponding polyamicacid. The carbonized films were further heated to 2800°C for graphitization, and their degreesof graphitization and orientationof graphite crystallites function of esterification ratio were studied by x-ray diffraction and magnetoresistance measurements [22]. Both measurements clearly indicate that the graphite films prepared from polyarnic acid methyl ester have high degree of graphitization and high preferred orientation, from measure~entsof mosaic spread by x-ray diffraction (Fig. and anisotropy ratios and d e t e ~ i n e dfrom magnetoresistance measurements (Fig. 43b). It was expected from the above study that polyimide films utilizing polyamic acid derivatives, which lose larger groups from precursors, may afford graphitize films with the higher quality. Therefore, polyamic acid alkyl esters with various alkyl groups were prepared, and the effect thatsize theof the chemical group has on thein-plane orientation of polyimide films and also on the graphitizability was PNIDA and PPD and then allowed to examined. Polyamic acid was prepared from react with NaH and various kinds of alkyl halide to produce various alkyl esters The cast films were imidized fixed on glass substrate togive polyimide films and then carbonized by heating up to 900°C. The carbonized films were further heated to 2800°C for graphitization.

k .g

L!

60

Esterification Ratio

60

Esterification Ratio 9

Esterification ratio dependences of preferred orientation of carbon layers in the polyimide films after graphitization at 2800°C [22]. (a) Mosaic spread and (b) anisotropy ratios and (U),

l

Weight Loss I

l

Weight Loss I

4 Dependences of preferred orientation of carbon layers in the polyimide films PMDMPPD on the size of leaving groups (weight loss) after graphitizationat 2800°C[23]. (a) Mosaic spread and (b) anisotropy ratios rT and X-ray diffraction and magnetoresistance measurements clearly indicate that the graphitized films prepared from polyamic acid alkyl ester have high degree of graphitization. Itis clear that the orientation of the graphitized films increases with the increase of the size of the chemical group lost during imidization from both x-ray diffraction (Fig. and anisotropy ratios and from magnetoresistance measurements (Fig.44b). Thus, in the case of rigid-rod polyimide from PMDA and PPD, polyimide films from alkyl ester of polyamic acid have high graphitizability, and the orientation of the graphitized films increases with increased size of the group eliminated. It is known that orientationof molecules in polyarnic acid enhanced during imidization, in the caseof rigid-rod polyimide,to give highly oriented polyimide films. In the case of serniflexible polyimide, however, it was reported [8l] that relaxation occurs during imidizationto give less oriented polyimide films. It was shown, on the other hand, that even semiflexible polyimide consisting of PMDA and oxydianiline(ODA) gives excellent graphitized films [29,33]. Thus, to examine the versatility of the method to use polyarnic acid alkyl ester for obtaining highly oriented graphitized films, various alkyl esters based on the semiflexible polyimide PMDNODA were examined to see how the size of the alkyl groups affects the carbonization and graphitization [24]. The cast films were imidized fixed on glass substrate to give polyimide films and then carbonized by heating to 900°C.The carbonized films were further heated to 2800°C for graphitization.

Loss I ODA on

loss)

films 2800°C:

X-ray diffraction and magnetor~sistancemeasurements clearly indicate that these graphitized films have a high degree of graphitization. The orientation of the films, however, does not depend on the sizeof the alkyl group lost during imidization for the semiflexible polyimidePMDNODA. This is shown in Fig.45 by plotting the mosaic spread against weight loss during carbonization, which is proportional to the size of the lost group. Thus, it is concluded that the use polyimide precursors that eliminate large groups during imidization is effective in producing polyimide films with a high in-plane orientation. a consequence, graphite films are produced with high crystallinityin the case of rigid-rod polyimide, but not in the case of flexible polyimide.

5. Porous Films A porous carbon film with bulk densityof 0.3-0.4 &m3 and pore volume of 2 cm3/g couldalso be preparedfrom Kapton-type polyimide[82]. The SEM micrographs of the cross sections of polyimide and carbonized films are shown in Fig. 46. The polyimidefilmswerepreparedbycastingtheDMAcsolution of the corresponding polyamic acid on a glass substrate. The gelation of the film was

SEM micrographs cross section of porous polyimide film prepared by phase inversion technique (a) and its carbonized counterpart (b) (courtesy of Dr. H. Hatori the National Institute for Resources and Environment, Tsukuba, Japan).

carried out by immersion in mixture of DMAcwithwateranddriedthen imidizedat200°C. In thepolyimidefilms thus prepared, phaseinversion occurred and large number of pores of rather homogeneous size resulted shown in Fig. 46a. These films were carbonized up to 800°C at heating rate of 3"Clmin. The resultant carbon films kept their porous texture, shown in Fig. 46b, in spite of large shrinkage of the pores, particularly along the normal d~ectionto the film surface.

The heating rate found to be very important to obtaining the maximum crystallinity inthe films, The investigation was done by using polyimide films of Kapton and Novax withthickness of 25 pm, both of them givinga high degree graphitization by conventional heat treatmentto 3000°C As shownin Fig. 3, the Kapton molecule has kink at the ether oxygen, all other atoms being in a plane, and thatof Novax contains two different bridging parts are thatassumed to act in opposite ways above its glass ans sit ion temperature At higher temperature than the average angle atthe kink of the ether oxygen becomes small and the biphenyl bond is twisted due to the steric hindrance between two methyl groups attached to respective aromatic rings (see Fig. The 2). heating rate during carbonization up to 1000°C was changed from 116 to22"C/min.Allthecarbonized films were subjected to heat treatment at 3000°C. The crystallinity was (.l?lp/p),, at liquid-nitrogen evaluated by maximum transverse magnetoresistance temperature. The interlayer spacing could not be used for the characterization of the films obtained because all of the films hadthe values inthe narrow range of 0.3354 to 0.3356 nm. ~~ resiThe experimental results were interpreted by plotting ( . l ? l p / p ) ~against dence time, t, defined by the following equation;

where is the decomposition temperature of these polyimides, which has been definedto be500°C for all polyimides by different e~periments,and R is the heating rate in "Clmin. The p ~ ~ m e t t,e rtherefore, corresponds to the residence time in the temperature rangefrom to and is expressed in minutes.The results are shown in Fig. 47. For Kapton, the value of (Ap/p),,, is higher, and the residence time t is longer, That is, heating slowly as possible is recommended to get better crystallinity. The slowest heatingrate employed inthis study was 1/6"C/min,i.e., l00 hours to heat up to 1000"C, and the ( . l ? l ~ / pvalue ) ~ ~ approached 1000%.The improvement in crystallinityof the resultant graphite filmis assumed tobe due to the improvement in the flatness of the Kapton molecule above because of the decrease in average kink angle at the ether oxygen in the bridging section,

400

600

lo00

Residence TimeI min 4'7 Changes of (hplp),,, with residence time in the temperature range between glass transition temperature 5 and decomposition temperature for two polyimide films

In Novax, on the other hand, a maximum(Aplp),, in is observed around thet value of 120 min. At shorter residence times, i.e., quicker passage through the tempe~dturerange of T the change of (Aplp),,, with t is exactly the same that for Kapton. For residence times longer than 120 min, however, the values become smaller, in other words, the crystallinity of the film deof (Aplp),,, creases. This result onNovas may be explained follows: for residence times t less than120 min, the decrease in the average angle of the kink at the ether oxygen works effectively, as in Kapton, to improve the crystallinity, but the twisting at the biphenyl bond in the componentOTD becomes pronounced for residence times longer than120 min. The reason whythe twisting in the component OTD becomes effective only at long residence times seems to be the difficulty of twisting the aromatic rings with bulky methyl groups, which is necessary to have cooperative movement between the molecules located above and below. A slow heating rate is reasonably assumed to result inslow lossof noncarbon atoms and, consequence, in less disturbance to the molecular orientation in the films. By this mechanism, however, the decrease in crystallinity for long residence times observed on Novax (Fig. 47) cannot be explained. Therefore, the conformational change in polyimide films above their g1ass-transition temperaturesis the

304

lnagaki et al.

main factor in determining the crystallinity of graphite, though slow loss of noncarbon atoms is important to avoid deformation of the resultant film. When the precursor films were thicker than 25 pm, a deformation of the film was observed [33]. In Fig. 48, SEM micrographs of the cross section are shown for the graphitized films prepared from Kapton with different thicknesses. The heating rate during carbonization was about 6"Clmin. For the film with an initial thickness of 25 pm, a well-developed parallel layer texture is observed, since pores are difficult to detect. In the films with greater initial thickness, large pores are formed inside the films. The regions of parallel stacking of layers are thicker and more flat in the film with 50 pm starting thickness than that with 75 pm. These results are due to the departure of decomposition gases during carbonization. For the thicker films a slower heating rate has to be employed. It was also shown by using hollow polyimide fibers that the carbonization conditions were very important for controlling performance as a molecular sieve for gas separation [83-871.

D. Control in Graphitization Process In the graphitization step, the structural changes to graphite are strongly dependent on the molecular structure of the starting polyimides, as shown in Section 11. In addition, this graphitization process also has a certain influence on the quality of the resultant graphite films. Bursting of the graphitized films was observed when the starting Kapton film was as thick as 125 pm [88]. These films were carbonized at 1300°C at a heating rate of 3"C/min and then heated to 2700°C at a rate of 20"C/min. Above 2200"C, surface-layer stripping, swelling, and even bursting were observed on the surface of the films. As a consequence, deformation of the films also occurred. At 2400"C, the films were often broken into pieces. Figure 49 shows the evolution behavior of nitrogen gas as a function of heating temperature. At 2000"C, no outgassing was detected. After about 100 sec at a temperature of 2700"C, however, prominent leakage of nitrogen gas was observed, there was also marked bursting of the film after cooling. The total amount of nitrogen gas liberated was only 0.41 wt%. The deformation due to pore formation in the graphitized films was found even starting with a Kapton film with 25 pm thickness without pre-heat-treatment at 22OO0C, which was recommended above [34,60]. In Table 5, the magnetoresistance parameters, (Ap/p),, and rT, are shown as a function of HTT. Below 2400"C, the development of the graphite structure is not pronounced, but the anisotropy ratio rT from magnetoresistance is so small that the carbon layers are assumed to be well oriented. Above 2700"C, however, the anisotropy ratio increases, i.e., the degree of orientation decreases, with increase in HTT, though graphite structure is developed, as shown by the rapid increase in ( A ~ / P ) ~This ,~. disturbance in orientation of carbon layers was found by SEM observations on the

SEM micrographs of cross section of polyimide Kapton films with different thickness after graphitization [33].

lnagaki et al.

306

3000

r

7 2

2000

5 \

R 1000

0

100

300 200 Time I s

0

FIG. 49 Nitrogen evolution behavior of Kapton film (125 pm thick) at high temperatures (courtesy of Dr. H. Hatori of National Institute for Resources and Environment, Tsukuba, Japan). cross sections of the films to be due to pore formation. Some SEM micrographs are shown in Fig. 50. The film heated to 2400°C shows the formation of pores (Fig. ~OC), though the main part of this film consists of large extended layers along the film surface. The regions around the pores appear to be very similar to exfoliated graphites and the TABLE 5 Magnetoresistance Measured at Liquid-Nitrogen Temperature and a Magnetic Field of 1 T for Carbon Films Prepared from Kapton and Heat-Treated at Different Temperatures for 30 min Without Pre-Heat-Treatment at 2200°C HTT ("C)

(Ap/p>max

2100 2400 2700 3000 Source: Ref. 60.

-0.105 2.53 83.3 275.2

(%)

(Ap/p)Tmin

(%)

-0.008 1 0.064 12.17 9 1.09

(Ap/P)TLmin

-0.008

0.064 16.8 102.1

'T

'TL

0.071 0.025 0.146 0.331

0.077 0.025 0.201 0.371

High Quality Graphite Films

307

orientation of layers along the film surface is disturbed around these regions. Such an exfoliated region increases in the films heat treated at 2700 and 3000"C, even though the layers are locally developed and straightened. This is expected to be due to the evolution of nitrogen gas remaining in small amounts even after carbonization. The existence of nitrogen in carbon films heat treated to 2200°C was clearly detected by XPS and was found to strongly affect the development of structure and properties [55,56]. In order to avoid the formation of pores and to obtain graphite films with high quality from the polyimide film Kapton, it was recommended to keep the films at 2200°C for 1 hour during heatup to 3000°C [34]. The resultant graphite film prepared through 2200°C-holding shows a very high value of (Aplp),,, and a very low anisotropy ratio (rT),as shown on the last line of Table 5.

IV. QUALITY OF GRAPHITE FILMS

A.

Microscopic Quality

In Fig. 5 1, an SEM micrograph of the cross-section is shown for a graphite film prepared from Novax by heating to 3000°C. This figure shows how large an area the graphite layers extend over because they are parallel to the film surface [37]. This SEM observation is consistent with the fact that this graphite film has a magnetoresistance anisotropy ratio as small as 0.006. The preferred orientation of the basal planes of graphite along the film surface was determined by measuring the x-ray 002 line intensity as a function of rotation of the specimen (the orientation function). This is the distribution of the reciprocal lattice points for each graphite basal plane around the normal to the film surface and is characterized by the half-intensity width of this orientation function, the mosaic spread (MS) in units of degrees. Representative values of MS observed for the films derived from polyimides are compared with those of HOPG and other well-crystallized graphites [52,53,89] in Table 6. Sample HOPG, which has been prepared at as high a temperature as 3600°C under a high pressure of about 1 MPa (10 kg/cm2) gives a very low value of MS, about 0.99 In the films derived from polyimides, however, the orientation function is a little broader and gives MS value of 3.5-7'. This is mostly due to the fact that these films have been graphitized without any applied pressure; the high temperature heat treatment of these films is performed by sandwiching them between two high-density graphite blocks, the weight of which exerts a pressure less than 1 g/cm2on the sample film. An attempt to produce an improvement in the orientation of the graphite films was made by applying a large constraint on the precursor polyimide films made by esterification of polyamic acid, but no marked improvement in orientation was attained [22,23]. By compressing a pile of Kapton films of 25 km thickness under 10-15 kg/cm2

uality Graphite Films 6 Preparation Conditions and Mosaic Spread (MS) for Highly Crystallized Graphite Films Used for Electron Channeling and Galvanomagnetic Measurements Sample code

Precursor

PPT 3200

PFT

Kapton 3100

Kapton

Novax3 l00

Novax

HOPG 3600

Pyrolytic carbon Pyrolytic carbon Pyrolytic carbon

PG 3200

ZYA SG

Carbonization conditions

Graphitization conditions

Up to 900°C with 120"Ck and kept for 1 h in N, Up to 900°C with 2OO"Ch and kept for 1 h in N, Up to 900°C with 120"Ck and kept for l h in N,

Up to 3200°C with 3.5-4.9 1SOO"Ck and kept for 7 min in Ar Up to 3100°C with 6.7 lS00"Clh and kept for l 7 rnin in Ar Up to100°C 3 with 1SOO"Ck and kept for 17 rnin in Ar Up to 3600°C under pressure Up to 3200°C for 1 b without pressure

Commercial

MS

6.9

0.9

0.4

Commercial

Source:

during carbonization and then hot-pressing at 100-300 kg/cm2 toa temperature of 2800-30OO0C, highly oriented graphite blocks were prepared and commercialized for use in monochromatorsfor x-ray and neutron beams[89,90]. Some of these graphite blocks are shown in Fig. 52. Most characteristics of the graphite blocksthuspreparedwerereportedtobecomparablewiththose of HOPG, particularly since an MS of around 0.4" could be produced. The size and a-axis orientation of the graphite layers aligned along the film [91]. In Fig. surface is measured by using electron channeling contrast techniques 53, the electron channeling contrast images for graphite films prepared from three polyimide precursorsare compared with those from two pyrolytic graphite specimens [4.9]. The preparation conditionsof these five films witha rather high crystallinity, which are characterized by different techniques such as electron channeling and galvanomagnetic property measurements, as will be discussed below, are summarized in Table 6. The preparation conditionsfor each of the polyimide films were selected to give the highest crystallinity for each polyimide by taking into consideration the controlling factors discussed in the previous section. For example, the heating rate for Novax was 120"Clmin during carbonization, or while heating the Kapton film up to 3 100°C it was kept at 2200°C for 1 hour. For all five films, clear channeling contrast images are observed, which reveals

Graphite blocks prepared from Kapton films (courtesy the Matsus~itaInstitute Tokyo Inc.).

Dr. M. Murakarni of

that these films have a rather high crystallinity. It has to be emphasized that the grains have a constant contrast corresponding to single crystal domains, which makes its basal planes parallel to the film surface, i.e., parallel to the micrographs, and if these grains were too small there was no contrast observed in the micrograph. In these images, the grains with the highest brightness (white regions) have th [l 101 c~stallographicdirection horizontal and black regions have their 1l direction perpendicular. The contrast of these grains changed gradually with the rotation of the specimen in the microscope by white to black and black to white. Detailed analysisof these channeling contrast images with different angles shows a random distribution of the 101 direction in the plane parallel to the film surface [491. The boundaries between the grains with different contrasts are clearly defined more in the films derived from Kapton and PPT (very sigilar to HOPG) than that from Novax, suggesting better crystallinity in Kapton and PTT films than in Novax, This qualitative comparison in crystallinity among films coincides with the ~ ~ a ~ t i t a tevaluation ive of crystallinity by using galvano~a~netic properties discussed in the next section. The average grain sizeD, which corresponds to the average size of the regions with constant contrastin the films, measured from these channeling contrast images. The results from the present five films are listed in Table 7, in order to compare them to the mean free path determined from magnetoresistance in the next section.

Electron channeling contrast images on graphite films prepared from diEerent precursors (a) from PP" at from Kapton at (c) from Novax at (d) pyrolytic graphite heated up to HOPG.

channeling technique. The same phenomenonis observed in theTEM where the mosaic domains containing Moir6 fringes yield values similar to (see Fig, 27 in 11ODF).

~ntercalatio~ of various species into the galleryof graphite layers is an effective way of modifying the properties of graphite. Some functions of graphite intercalation compou~ds(GICs) such as different metals, acids, and metal chlorides have been extensively studied from different viewpoints and some possible applications have been discussed [92,93]. Graphite films preparedfrom polyimides are expected tobe host materialsfor these graphite intercalation compou~ds Only a few papers have been

Spread, G ~ l v a ~Properties, ~ ~ e t Mean ~ ~ Free Path, Highly Crystallized Graphite Filmsa

Grain Size

(A R

Graphite film Kapton, 3100°C Novax, 3100°C

6.7

P E , 3200°C

5.7 8.6

Pyrolytic carbon,

3.32 2.67 3.45 3.60

(77 K, 1 2.54 8.72 12.06

(4.2 1 0.01 1 0.017 0.017 0.020

3200°C HOQG,0,9 3600°C

60 5.50 5.4 112.52 0,005 13.94

"See Table6. bMosaic spread. resistivity ratio dA~~sotro~y ratio in magneto resist an^^. 'Mean free path length along tlrefilm fAverage domainsize in electron c ~ a n n e ~contrast i n ~ image. Ref.

(pm)

71.80 8

57.91 15.75

2.6

1.5

5

Dependence m ~ i ~ transverse u m ~agnet~r~s~stance magnetic field strength B highly crystallized graphite films (see Table at a magnetic field up to 1 77 K and at a magnetic field up to T and 4.2 K.

point-defect concentration both in the films of pristine graphite and the FeCQintercalated films,are calculated from thethemal con~uctivity data shown in Fig. 56. The calculated data are summarized in Table8, together with other transport properties measured, The results in Table 8 show that intercalation greatly decreasesthe structural order; the in-plane coherence length La is reduced by a €actor of 10 and the concentration of point defects measured by the parameter A strongly increases. However, the electronic properties, such as resistivity (p,), residual resistivity ratio (RRR) and room temperature thermoelectric power (S3oo),are greatly improved. Stage 4 MoCIS-GICs were synthesized from highly crystallized graphite films

t-.l

U

M

1

IT

Dependence of Hall coefficient RH magnetic field strength B (a) at a magnetic field to 1 and 77 K and at a magnetic field up to 6.5 and 4.2 K.

10

1 1

10

100

T e m ~ e r ~ tI ~ r e FIG. 56 Temperature dependence of thermal conductivityof a pristine graphite film and its FeCl, .-intercalation compound

Transport Properties of a Pristine Graphite Film and Its Intercalation Compound with FeCly '300

cm) Pristine FeClz-GIC

A

'300

(W/mK)

(nm)

(nm)

(10-24

m2)

-0.11

po: residual resistivity, residual resistivity ratio ~ ~ ~S300: ~ room-te~perature / p ~ , thermoelectric power, tcjoo: roo^-temperature thermal conductivity, La:in-plane coherence length, out-of-plane coherence length, A: parameter propo~ionalto the point defect concentration. Source: Ref. 95.

that were preparedfrom the polyimide films Kapton and PPT by heat treatment at 3200°C Some structural and electronic Parametersthe GICs are compared to the pristine graphite films in Table 9. The intercalation reaction waspedormed at 450°C for 7 days in a glass tube by sealing a mixture of each pristine graphite film and natural graphite powders with to produce molar ratioof ~ o C l ~ to the sum of the pristine graphite film and natural graphite powders of 1/16. From the structural and electronic properties in Table 9, three pristine graphite filmsareknown to havehighcrystallinity,thoughthecrystallinity PPT-l is slightly lower than the others. The intercalation of MoCl, into these graphite films increases electrical conductivity more than one ordermagnitude in KAP-2 and PPT-3, with smaller increases for PPT-1. The change in electrical conductivity with measurement temperature shows that electric conduction in these CICs is metallic, being most pronounced in GIC-KAP-2. Hall coefficients for these GIC films were positive at ail measurement temperatures from300 to 1.3 K, showing that the majority carriers in these GICs were holes, as was expected. In Fig. the dependence of Hall coefficient (RH)and maximum transverse ~agnetoresistance on magnetic field at the temperature of 1.3 are shown for two GIC films, CIC-KAP-2 and GIC-PPT-3. A Shubnikov-de Haas oscillation is clearlyobserved on above3 T, butnoton The amplitude of the oscillation decreases with increasing 1/B with weak modulation, T-l, being the same but the periodof the amplitudeis evaluated to be7.84 loM3 in the twoGIC films. The modulation of the amplitude suggests a Fermi surface of an undulating cylinder with two extrema1 cross sections.

Structural and Electronic Parameters the Pristine Graphite Films and Their Intercalation Compounds with MoCl, Structural parameters Sample

or

MS

Electronic parameters Cr3QOK

04.2K

RRR

RH

('p'p)

7.09

3.53

-7.71

12.95 180.07

KAP-2 GIC-KAP-2

0.6708 .934

PPT-3 GIC-PPT-3

2.315.84 -9.14 4.81 8.77 2.45 1.83 0.6708 1.6345.534.2 1.934 1.98 27.9

0.0308

-5.73 3.45 5.00 1.90 1.45 5.7 12.06 1.63 1.76 18.7 13.0 10.6

0.921I

PPT- 1

0.6708 ~ I ~ - ~ P T - 1.96 1

1.8 2.01 20.1 1.34 4.21 84.6 58.9

2.63

lattice constant along c-axis of graphic (nm), identity period of GIC along c-axis (nm), MS: mosaic spread U: electrical conductivity at 300, 77 and 4.2 K 10-6 S~n-l),MXR:residual resistivity ratio Wall coefficient at l and 77 K 1 0 8 m3C-l), (Aplp): transverse ~agnetoresistan~e at and 77 K. Source: Ref. 97.

I

B

E3 FIG. 57 Dependences of coefficient R, transverse magn~toresistance on magnetic field strength at a temperature of 1.3 K

(hplp),,,

These experimental results on GIC films show that the high crystallinity of the pristine graphite films are reasonably retained after the intercalation of MoCl,. This is also supported from x-ray diffraction patterns on these GIC films, as show for GIC-KAP-2 in Fig. 58. It was experimentally shown that GICs with MoCl, are very stable in air, with only a slight decomposition of these GIC powders observed in boiling water after 15 days [99]. On the same GIC specimens, GIC-PPT and GIC-Kapton, scanning tunneling and atomic force microscopy observations were carried out [98]. Intercalated domains couldbe recognized from the hexagonal pattern due to a single graphite plane, which results from losing the graphiticAB stacking, although a nonintercalated domain with a triangle pattern due to the successive AB stacking of graphite layersis observed. Representative STM images for these two domains are showninFig. 59. Inthesamples ofMoCl,-GICwith the stage 4 structure, intercalated domains were classified into two categories, large and small. For large intercalated domains (Fig. 59a),a superstructure is clearly identified, as shown in Fig. 60, with a certain periodicity. This superstructureis thought to be due to the ordered locationof chloride ions under the graphite layer. A structural model was proposed by assuming the existence of bimolecular units. Most of these large domains have rather straight boundaries with nonintercalated domains, in directions parallel to certain crystallographic axes of graphite and show some corrugations, as shown in Fig. 61. These corrugations extend either along the [l101 or [l201 graphite directions and have a constant periodicity of about 2.5 nm over about a 10 nm range. On the other hand, small intercalated domains have boundaries with an irregular shape and no corrugation, as shown in Fig. 59b. Ternary GICs with FeC13-CHCl, and MoCl,-CHCl, were reported to have

X-ray diffraction pattern of MoCIS--graphite intercalation compound prepared from the Kapton-derived graphite film Kapton-2

Scanning tunneling micrographs of large intercalated domain (a) and small intercalated domain observed for the MoC1,-graphite intercalation compounds (MoCl,-GICs) 1981.

Periodicalarray of bright dots observedon a large intercalated domainin

high electrical conductivity with relatively high stability in air [99]. The GICs were synthesized by keeping the graphite films, which were preparedpolyfrom imide filmof PPT with26 pm thickness and graphitization at 3000°C for 0.5 hour, in a chloroform (CHCl,) solution of either FeC1, or MoC15 at room temperature. It worth noting that the conditionfor the formation of GICs is very mild. In the case of FeC13,two typesof GICs were obtained that had the space sandwiching the intercalates (FeCl, and CHCl,), dl, of 0.94 and 1.18 nm (type I and 11, respectively). The type I was obtained in the dark and type I1 was obtained in the light 1001. TypeI had the same dlvalue the binary GICs with FeCl,, which had been synthesized by the reaction betweena graphite host and FeCl, vapor, but typeTI was assumed to have newstructureconsisting of double layers of FeClz-

~ o ~ ~ aobserved t i ~ atntheboundarybetween i ~ t e r c a ~ adomains t~d in MoC1,-GICs

large intercalated andnon-

octahedra. For these two GICs, electronic properties, such electrical conductivity (U), transverse magnetoresistance (Ap/p), and Hall coefficient (RH)were measured under magnetic field up to 1 T room temperature and 77 K. The results are s u ~ ~ in ~Table z 10. e In ~ the cases of ternary FeC1,-CHC1,-GICs, two ~pecimensof type I and three specimens type I1 structures were measured, The formationof ternary FeC1,-CHC1,-GICs results in an increase in electrical conductivity of about3-5 times at room temperature and about times at 77 K over the pristine graphite films, the enhancement in conductivity being higher in type I than in type I1 compounds. In the case of the ternary MoClsGHC1,-CIC, the increase in conductivity is less pronounced. However, it has to be mentioned that the ternarization,i.e., co-intercalation of CHCl, with either FeCl,

Electronic Parameters of Pristine Graphite Film and Its Ternary Intercalation Compounds with FeC1,-CHCl, and MoC1,-CHCl, a

77

Sample

RT

Pristine1.92 1.65 FeC1,-CHC1,-GIC type 5.10 Specimen-1 Specimen-:! 8.77 5.21 FeC1,-CHC1,-GIC type I1 Specimen-1 13.06.21 16.17.63 Specimen-:! 17.27.94 Specimen-3 MOCl~-CHCl,-GIC 4.72 3.12

106 Sm"]

K

R, [cm3

RT

7777 K

RT

K

-0.076 -0.155 92.67 953.6 0.0043 0.027

0.032 0.030

4.51 6.50

15.35 21.12

0.023 0.016 0.0088 0.015

0.032 0.017 0.023 0.014

1.55 1.59 1.86 2.23

4.59

RT roomtemperature, c:electricalconductivity, R,: Hall coefficientat 1 T, magnetoresistance at 1 Ref. 96.

5.15

5.82 7.33 transverse

LO

22

1021

lo4

10

Contributions of positive holes and negativeelectrons to electrical conductivity a of pristine graphite film and its ternary intercalation compounds withFeC1,-CHCI, (see Table 10) [96].

or MoC15 into the graphite gallery, gives a less pronounced increasein conductivity than for the binary compounds, whichis probably due to back-donation of electric chargefrom graphite layers,The Hall coefficients(R,) of GICs are small and positive, showingthe majority carriers are holes, which is consistent with the small values of magnetoresistance. According to the classical two-carrier model and using a low magnetic field approximation, the concentrations and mobilitiesof two carriers, holes and electrons, nh, b h , and respectively, were calculated from electronic properties shown in Table 10. In Fig. 62, the contributions of electrons and holes for the conductivity U are shown by plotting nh/JLhand nepe against U. In the pristine graphite, the two carriers contribute almost equally to conduction, as has been pointed outin many publications on graphite. In the GICs, however, the contribution of the holes is more than 100 times larger that of the electrons, which is expected because the present GICs are acceptor-type.

From our studies coupled with those of other research groups, the process of carbonization and graphitization of aromatic polyimide films has been well understood (Section 2). The factors that had to be controlled for obtaining highly crystallizedgraphitefilmsweredefinedonthebasis of experimentalresults (Section 3). The results of crystallinity characterization of the resultant graphite films performed by diRerent techniques and also some trial modificationof their properties by intercalation were reviewed (Section 4). Based on the howledge that has been accumulated, some prospects for the science and technology related to polyimide-derived carbon and graphite films are discussed here. From the viewpoint of carbon science and technology, aromatic polyimide films are very interesting precursors mainly because they can give dense and homogeneous graphite films by a simple heat treatment at high temperatures without any compression, except for sandwiching the films between either alumina or graphite plates to avoid deformation. This simple heat treatment is one of the advantages of aromatic polyimide films as carbon precursors. Shrinkage is not small, about 23% along the film and 20% its thickness, in butno cracks and pores are observed even by scanning electron microscopy. By taking this shrinkage into account, carbon films with a rather complicated shape be can prepared by cutting the precursor polyimide film in the desired shape. For example, bridge-shaped specimens for the measurementof galvanomagnetic properties that are about 2.5 mm wide and 12 mm long with five small ears have been prepared. Aromatic polyimide films have already established a wide range of applications, particularlyin the field of microelectronics. To meet various requirements of these applications, many kindsof aromatic polyimide films with various rnolecu-

The authors express their sincere thanks to Prof. H. Konno of the Graduate School of Engineering, Hokkaido University, Prof. Kaburagi and Yoshida of the Faculty of En~ineering,as as hi Institute of Technology, andMr. C, Bourgerette of ~ ~ ~ E S / C N ~ S .

1.

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V. Vignal, H. Konno, S. Flandrois, and M. Inagaki, in press. M. Inagaki and G. Watanabe, Synth. Met. 94, 235 (l 998). Y. Sonedaand M. Inagaki, anorg. a&. Chem. 610, 15’7 (1992). H. Konno, H. Oka, and M. Inagaki, Carbon, in press. M. Inagaki, H. Oka, H. Konno,and Y, Kaburagi, Solid lonics, in press.

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Active carbons, 246 Alumina, 193, 194 Anthracene, 2, 5, 12, 62, 86, 130, 132, 136 Anthracene oil, 41, 43, 44, 54, 88 Asphalt(s), 5, 33, 88-91 Asphaltene(s), 5, 8, 9, 33-40, 56, 64, 69, 71, 76, 77, 80, 82, 83, 86, 111114, 125, 138 Atomic force microscopy, 256, 260, 322 Basic structural unit (BSU), 3, 9-14, 18, 24, 25-27, 30-33, 36, 38, 40, 42, 44, 45, 48, 58, 62, 64, 65, 8185, 88, 94, 98, 99, 103, 106, 123, 125-133, 161, 171, 199, 202, 212, 247, 260, 275 adsorption, 131 association, 45, 78, 83-85, 88, 91, 99, 106, 124, 126-128,138, 139 cross-linking, 85, 99, 123 dehydrogenation, 99 dispersion, 106, 129 displacement, 127, 129, 139 dist~bution,13, 82 edges, 13, 33, 106 formation, 80, 128, 138

[Basic structural unit (BSU)] miscibility, 128 misorient(-ation, -ed), 27, 43, 51, 64, 83, 103, 259 mobility, 83 nonmiscible, 85 orient(-ation, -ed), 13, 42, 51, 58, 8285, 88, 106, 126-131, 138, 139, 199, 202, 203, 212, 241, 247, 259, 260, 270, 275, 295 oxygenated, 85 polymerization, 132 segregation, 99 shape, 85, 129 size, 33, 85, 129 solvation, 83 Bitumen(s), 46 BottTococcus ( ~ ~ ~4-6,u 86 ~ ~ ~ ) , Brownian motion, 115, 138 Calcium compounds, 151, 178- 180, 188-193, 194, 240 Carbon beads, 155, 162, 165, 168, 170, 174- 177 Carbon black, 9, 57, 62, 129, 155, 246 Carbodcarbon composites, 153, 152, 195-241

[Carbon/carbon composites] crack propagation, 2 15 -2 17 CVD (densification) method, 236, 237 fibedmatrix interface, 196, 199, 202, 203, 208, 210-213, 222-232, 233238, 240, 241 filler (yams/fibers), 195 fracture behavior, 195, 213-222, 238 impact properties, 21 liquid impregnation, 236 matrix (binder), 195, 196, 198, 199, 200-208, 209-223, 225, 227-233, 235, 236, 238, 240, 241 relative bond index (RBI), 233, 236238 relative bond strength (RBS), 233, 235 relative graphitic index (RGI), 235, 236 relative plasticity index (RPI), 236 strength, 195, 196, 236-238 thermal conductivity, 195, 196 thermal expansion, 196, 209, 213, 222-232, 236, 238 toughness, 151, 195, 216-218, 220 tubes, 223, 225-239 Carbon fibers, 75, 136, 139, 152, 196, 199, 200, 204, 208, 210, 212-214, 217, 218, 220-223, 225, 227-233, 236-238, 240, 241 mesophase-pitch-based,246 PAN-based, 8, 155, 174, 175, 196, 197, 199, 202, 204, 207, 216, 219, 223, 23 1, 232, 236-238 pitch-based, 55, 132, 138, 223, 231, 232, 236-238 strength, 196 Carbon film(s), 12, 103, 105, 132, 259, 261, 263, 265, 268, 272, 275, 277, 280-287, 328 Carbonification, 2 Carbonization, 2-4, 8 , 9, 13, 30, 3 1, 32, 41, 48, 54, 56, 57, 77, 82, 83, 85, 91, 100, 101,103, 113, 128-130, 139, 152, 154, 208, 210, 212, 241

[Carbonization] gas-phase, 246, 247 hot-stage, 106 Kapton films, 257-260 liquid-phase, 246, 247 primary, 2, 3, 7, 13-85, 89, 125-131, 131-138, 139 secondary, 3, 4, 7, 33, 80, 81 84, 99, 126,131-135,139 solid-state, 246, 247 Catagenesis, 6, 7, 75 Cellulose, 2, 6, 138 Char, 155, 168, 177 Coal(s), 2-4, 6, 8, 9, 12, 16, 24-33, 38, 40-42, 53, 58, 64, 76, 78, 81, 82, 86, 92, 94, 103, 113, 114, 124, 127, 129, 133, 155 anthracite, 4, 155, 195, 268, 269 bituminous, 4 coking, 4, 29, 30, 38, 55, 57, 80, 92, 108-114, 127, 128, 136, 139 hydrogenation of, 92 hydroliquefaction of, 9 1-94, 127, 139 lignite, 4, 5, 16-24, 25, 81, 84, 87-89 semianthracite, 4, 91 subbitu~nous,4, 155 tar(s), 5, 8, 38, 57, 126, 130 vitrinite in, 46, 109 Coalification, 2, 4, 9, 13, 24, 29, 3 1, 32, 82, 87, 139 Coefficient of thermal expansion (CTE), 223, 225, 227, 253, 255 Coke(s), 3, 8 , 30, 93-100, 134, 139, 152, 154, 159, 160, 180, 188, 206, 246, 269, 272, 282, 284 blast furnace, 8, 30, 57, 108 fluid,155, 160, 174, 175 Gilsonite, 155, 160 needle-like, 246, 328 petroleum, 286 pitch, 155, 174 polyvinyl chloride (PVC), 155, 156, 158-160, 168, 169, 171, 173,176179,182,184,186-189,191-193 sugar, 155, 65, 168 Coking, 3

Colloid(a1) association, 20- 122, 13I dispersion(s), 53, 54 hydrophilic, l 15 hydrophobic, 115 lyophilic, 115-1 17, 138 lyophobic, 115-1 17, 138 particle(s), I 117, 118, 120, 126, 128, 131,138 systems,113-122,128-129,138 Columnar (n~o)texture,43, 45, 51, 69, 131 Coronene, 12, 13, 33, 45, 75, 82, 84, 85, 103,106,128 Cracking fluid catalytic, 8 vapor, 8 Crystallite size(S), 1, 182- 184, 186, 206,263, 264,268,285, 288,290

[Diffraction] 268, 270, 273, 277, 293, 297-300, 307, 313, 322 Disclinations, 47, 48, 51, 56, 108, 138 ~islocations,47, 48 Dunite,195

Decant oil, 56 Decarboxylation, 30, 31, 32, 87, 92, 108 Dehydration, 31, 32, 87 Demixtion, 43, 46, 47, 49, 71, 77, 8 I, 83, 84, 126-129, 139 Density (bulk), 174, 176, 196, 199, 203, 204, 207, 214, 300 Deoxygenation, 30, 92, 93 Diagenesis, 7 Diamagnetic susceptibility, 133, 135 Diffraction, 9, 27, 28 optical, 28 profiles (lines), 154-157, 160-162, 165,169,171,173,174,180,181, 184-188, 194, 196-200, 206, 212, 213, 240, 263, 270, 278 selected area (SAD), 28, 46, 95, 275, 279, 280 small-angle neutron scattering (SANS), 9, 38, 82 small-angle x-ray scattering (SAXS), 38, 43, 133 wide-angle x-ray scattering ( ~ A X S ) , 9, 82 x-ray,38,152,168,169,172,182, 187, 194, 196, 204, 240, 256, 260,

Galvanomagnetic properties (measure~ents),256, 260, 263, 267, 268, 270, 271, 273, 277, 284, 286, 287, 311-336, 327, 328 Gas window, 7 Gel(s),116,118-120,125 Geothermal gradient, 2, 3 Glass transition temperature, 249, 250, 253, 254, 287, 293, 302, 303 Glassy (glass-like) carbons(s), 8, 1l , 127, 138, 155, 163, 165, 166, 171, 173,174,179,180,196,197,199, 203, 204, 207-214, 216-219, 221, 223, 225, 229, 230, 232, 233, 235, 236, 239-241,246,247, 249, 269, 278, 280, 287, 328 Grapllite(s), 3, 34, 129, 135, 150-152, 154,171-173,178,187,195,196, 199, 203, 208, 209, 211, 213, 214, 216-220, 223, 230, 231, 235, 239241, 246, 259, 260, 270, 280, 282, 287, 290, 304, 307, 313, 317, 319, 320, 323-325, 327, 328 Graphite intercalation compounds (GIG), 315-317, 319-327 thermal conductivity, 3 16, 17, 3 3 19

Electrical resistivity (or conductivity), 207, 258, 259, 263, 265, 268, 270, 272, 276, 290, 292, 294, 297, 298, 316, 320, 323-326 Electron spin resonance (ESR), 3, 18, 84,98, 132, 162-164,295 Emulsion, 43, 1 16, 138 Fermi surface, 322 Fullerenes, 246 Furfuryl alcohol, 152, 196, 199, 207, 208, 241

Graphitic carbon (structure), 150, 151, 155, 157, 160, 161, 177, 178, 180, 182,188,189,191,194,197,199, 203, 206-208, 210, 220, 239-241, 247, 255, 260, 262, 265-268, 278, 328 Graphitic (G) component, 158, 160163,169,173,174,180,182-184, 186-194, 206, 208, 211, 220 Graphitizability, 38, 197, 249, 269, 271, 275, 298, 299, 328 Graphitization, 3, 135, 139 heterogeneous,157,158,173 homogeneous, 157, 158, 173, 207, 212 Kapton films, 260-269 mechanism,171-174,191,194-195, 208-2 13, 241 stress, 3, 149-241 thermal,135-138 raph hi ti zing carbons (cokes), 135- 137, 138, 154, 207, 266, 269, 270, 272, 275, 277, 282, 284, 328 Hall effect (coefficient), 135, 263, 266, 268, 272, 276, 286, 313, 318, 320, 321, 324, 327, 328 Hard carbons, 12, 78 Heavy oils, 30, 64, 94-99 Highly oriented pyrolytic graphite (HOPG), 266, 273, 307, 311-313, 315-317 Hydroconversion catalytic, 94-99, 127, 129 heavy oils, 127, 129, 139 (heavy) residues, 127, 129, 139 Infrared spectroscopy (data), 15, 16, 17, 76, 77, 84, 89, 90, 97, 98, 252 Interlayer (dW2) spacing, 151, 154, 156, 158, 161, 173, 182, 183, 186, 196, 206-209, 211, 263, 264, 268,270, 271, 274, 282, 285, 287, 288, 290, 295, 302, 328 Isochromatic domains, 48-52, 80, 82, 108

Kapton (see Polyimide films, PMDAJODA), 247-250, 255, 260265, 268-275, 277, 280, 282-287, 290, 295, 300, 302-309,3 11-313, 315-318, 320-322, 328 Kerogen(s), 2, 4, 8-10, 13-24, 33, 36, 38-42, 58, 64, 69, 71, 75-78, 80, 82-85,111,113,114,129,138 series I, 4-6, 11, 15, 19, 22, 28, 31, 32, 38, 40, 41, 70, 125 series 11,4-7,11,15,19,31,32,38, 40,41, 125 series 111, 4, 6, 1 1, 15, 19, 22-27, 29, 32,78, 127 Kerosen Shale, 4-6, 15-24, 33, 81, 88 Kraemer Sarnov (KS)point, 40, 60 Kuckersite, 6, 15, 20-24, 39, 64, 69, 71,78, 81, 84, 86-90, 111, 138 Limestone, 178, 179, 182, 192, 193 Liquid crystal(s), 41, 42, 45, 46-48, 65, 67-69, 78, 80, 100, 101, 103, 122, 123,126-129,131,138 discotic, 45, 83, 123 nematic,123,129 Local molecular orientation 1316, 18, 19, 21-27, 28-30, 32-39, 41-43, 48, 49, 51, 58, 64, 67, 69, 71,73-75,77-84, 86-92,94-96, 98,99,103,109,111-113,127, 129,133,135 size(s), 15, 16, 19, 21, 22, 24, 28-30, 32-34, 36-38, 39, 51, 71,74,7880, 82, 84, 88-91, 93-96, 98, 99, 110,111,113,114,128,129,131, 133,134,138 stabilization, 7 1 London dispersion forces, 85, 99, 123 6, 7 Magnesia,193,194 Magnesium fluoride, 193-195 Magnetoresistance,135-137,152,158160, 196, 199-201, 203, 204, 206, 207, 209,210, 263, 265-268, 270, 272, 276, 278, 282, 286-288, 293-

magnetoresistance]

295, 297-300, 302, 304, 306, 313, 316, 317, 320, 321, 324, 327, 328 anisotropy ratio, 288, 293, 295, 298, 299, 304, 306, 307, 31 3 Maltene(s), 40, 77 Mesocarbon microbeads, 155, 160, 161 Mesophase, 3, 9, 41-52, 54, 56, 57, 60, 62, 64, 65-68, 71, 75, 77, 78, 80, 82, 83, 88, 89, 91, 96, 97, 100103,105-107,111,112,122,130132, 246 Brooks and Taylor, 28, 42, 43, 62, 65, 66, 78, 82, 88 bulk, 49, 56 dormant, 56, 75 PAN-AM (rnicro)texture, 42, 43, 45, 46, 65, 67, 111 spheres,160,163,174 Metagenesis, 6, 7 Metaplast (theory), 30-32, 83 Micellar suspension, 38 Micelle(s), 30, 38, 40, 108, 116, 120-122 Modulus c ~ b o n / c ~ b composites, on 196, 218, 222 polyimide films, 292, 293, 297 Molecul~sieve, 304 Neornesophase,107 ong graphitizing carbons, 138, 152, 168, 210, 240, 247, 268, 269,'275, 277 Nuclear magnetic resonance (NMR), 102 Oil derivatives, 8, 30, 33-40, 41, Oil window, 2, 7 Optical (polarized light) microscopy (OM), 7, 42, 43, 46-50, 52, 56, 57, 60, 64, 77, 78, 80, 82, 103, 106, 110,112,150,152,168,170,172, 198, 204, 205, 208, 210, 223, 240, 256, 259, 261, 270 Oriented mosaic(s), 51, 52, 64, 75, 77, 78, 91, 93, 95, 96, 103

Ostwald ripening, 117 Ovalene, 12, 13, 85, 136 8, 11 Pitch(es), 2, 3, 8, 12, 33, 34, 40-64, 71, 73, 75-78, 80, 82-84, 98, 124, 129, 130, 136, 139, 160, 203, 216, 223, 236, 238 ace~aphthylene, 100 anisotropic,100-108,109,126,127, 138, 139 tar, 2, 5, 8, 40, 43, 46, 54, 55, 57-62, 77, 86, 100, 124, 127, 129 coke,152 gas-sparge,103-108,127,129,138 isotropic, 41, 57, 77, 100, 101, 105, 160 petroleurn, 2, 5, 8, 56, 57, 77, 100, 107,108 secondary, 58, 60, 61 thermotropic,103,108 POCO graphite (graphitized powder), 155,176 Polyarnic acid(s), 247, 248, 253, 254, 288, 297-300 esteri~cation,253, 297-300, 307 imidization, 252-255, 287, 288, 290, 291, 294, 295, 297, 299, 300, 328 stability, 252 synthesis, 25 1 Polyethylene,86,117,118,130 Polyirnide films, 247-255, 259 birefringence, 290, 295, 296 BPDAIODA (Upilex), 250, 251, 270, 271,274,275-281,292-296 BPDAIPDA, 292-294 BTDAIODA, 259 BTDAIDABP (Larc-TPI), 249, 251, 278-280, 282, 287 BTDMPD, 259 259 carbonization, 247-249, 255, 256, 269, 275, 280, 287, 290, 293, 297, 299, 300, 302-304, 307, 3 11, 327 doping, 328

[Polyimide films] graphitization, 247-249, 255, 256, 270, 273, 275, 282, 287, 288, 290, 293, 295, 297-300, 302, 304-307, 327 modification by intercalation, 3 15327 nitrogen evolution, 306, 307 Novax, 271-275, 277, 282, 284, 295, 302, 303, 307, 310-312, 315-318, 328 PMDLLIODA, 247-249, 251, 256, 258, 259, 269, 271, 287, 290, 292296, 299, 300 PMDAIPDA, 255, 292-294,298, 299 P ~ ~ ~ 250, P 251, D 257-259, , 269-27 1 POD, 280, 283 269-271, 274,275, 277, 282, 284, 311-313, 315-318, 320-322, 328 PPV, 280, 283 properties, 254, 255, 315-327 refractive index, 290 Polyvinyl chloride, 38, 154, 156 Porosity (pores) carbon(s), 247, 269 carbon (graphite) films, 262, 270, 275, 277, 279, 300-302, 304, 306, 307, 316, 327 carbonkarbon composites, 237 carbonized pitches, 50, 51 Preferred orientation, 13, 26, 2’7, 32, 48, 51, 103, 105,106, 126, 129, 130, 168, 171, 176, 241, 260, 262, 270, 275, 288, 290, 295, 298, 299, 307, 313, 328 Pregraphitization, 171, 173 Pyro(1ytic) carbons (or graphite), 129, 135, 159-160, 231, 246, 311, 315318 Rarnan (spectroscopy) intensity, 290 Residual resistivity ratio, 316, 319, 320 Residue(s) atmospheric, 5, 8, 56, 64, 86

[Residue(s)] (heavy) oil, 2, 84, 94-99, 129 vacuum, 5, 8, 56, 64, 86, 94 Resins 41, 43, 45, 56-58, 60, 80, 83, 100, 103, 106, 109, 112, 130 41, 42, 45, 53, 57, 58, 60, 61, 75, 80, 83, 100, 101, 103, 106-112, 324, 127, 130 41, 45, 53, 54, 57, 100, 101, 103 phenol(ic), 8, 71, 78, 138, 152, 161, 162, 168, 177, 202, 207, 209, 218, 219, 223, 236, 238, 241 Rock extracts, 8 metamorphic, 178 Saccharose(-basedcarbons), 5, 8, 1l, 30, 38, 71, 78, 86, 127, 133, 138 Scanning electron microscopy 7, 174, 175, 202-204, 210, 213, 220, 231-234, 238, 240, 256, 259,277, 280, 281, 283, 288, 289,300, 304310, 327 Scanning tunneling microscopy (STM), 132, 256, 260, 322-325 Semicoke(s), 2, 3, 25, 28, 29, 41, 46, 49, 71, 77, 92, 99, 131, 132 Shrinkage composites, 208, 230 carbonized (~aphitized)polyimide films, 255, 257-259, 261, 283, 297,302, 327 Silica, 193, 194 Sintering, 117 Sodium carbonate, 193, 194 Softening point, 40, 54, 55, 60, 89, 90 Sol(s), 116, 129, 138 Sol-gel transformation, 55, 120 Solid dispersions, 117, 118 Solidification, 2, 18, 30, 3 1, 33, 41, 4951,54, 55,73-75,77-80, 82-85, 88, 89, 101, 109, 111, 112, 127129, 132,139 Sporopollenin, 5, 15, 17-24, 32, 81, 86-90

Statistical orientation (SO), 25-28, 30, 82, 129 Strain, 151, 197, 216, 219, 226, 228 Strength, 151, 197, 214, 227, 233, 254 Stress, 3, 151, 152, 169, 171, 173, 199, 203, 208-214, 219, 225, 227, 240, 241 accumulation, 208-210, 21 2, 241 co~centration,166, 168, 171, 173, 174,177,189 distrib~tion,210, 240 relaxation, 22 1 tectonic, 3 tress-oriented graphite, 230, 231 Tar(s), 2, 3, 59, 68, 84 distillation, 3, 57 ethylene, 77, l00 Temperature-programmed desorption, 284 Texture, 154, 199, 203, 206, 207, 212, 213-222, 233, 235, 236, 238, 239, 241, 260, 261, 328 lamellar, 203, 210, 221 ordered, 254, 255 oriented,154-160,163,168,171, 173, 174, 176, 212 random(1y oriented), 160- 165, 168, 173, 174, 176, 197, 214 Thermal analysis, 17, 18 Thermal deco~position,246

Thermal expansion, 15 1 Thermoelectric power, 320 Torbanite(s), 4, 5, 86, 87 Transmission electron microscopy (TEM), 9-12, 13-16, 18,25,3335, 40, 41, 43, 44, 46, 51, 57, 58, 61, 62, 64, 65, 67-69, 71-73, 80, 82,97, 103-105,108,110-113, 130,133-135,152,157,161,162, 165, 166, 168, 169, 172, 180, 182, 187, 194, 223, 224, 240, 256, 262, 268, 273,277, 290, 291,315 T~rbost~atic carbon (structure), 96, 133, 150, 151,155,157,158,160,161, 168, 171, 188, 191, 194, 239-241, 260, 262, 265, 267, 268, 278 Turbostratic (T) component, 158, 160, 161,173,174,182,183,186-184, 193, 206 van der Waals forces (interactions), 115, 123,128,139 van Krevelen diagram (path), 4, 6, 8, 11, 19-21, 25, 30, 84, 86, 88 Vesicles,121,122,138 Vickers microhardness 24, 49, 71, 73, 74, 131 Vis-breaking, 8 X-ray photoelectron spectroscopy 307