Chemical Processing of Ceramics (Materials Engineering, 8)

Chemical Processing of

Ceramics

MATERIALS ENGINEERING

1. Modern Ceramic Engineering: Properties, Processing, and Use in Design. Second Edition, Revised and Expanded, David W. Richerson 2. lntroductianto Engineering Materials: Behavior, Properties, and Selection, G. T. Murray 3. Rapidly SolidifiedAlloys: Processes StructuresApplications, edited by H0 ward H. Liebermann 4. Fiber and Whisker Reinforced Ceramics forStructuralApplicationS , David Belitskus 5. Thermal Analysis of Materials, Robert F. Speyer 6 . Friction and Wear of Ceramics, edited by Said Jahanmir 7. Mechanical Properties of Metallic Composites, edited by Shojiro Ochiai 8. Chemical Processing of Ceramics, edited by Burtrand 1. Lee and Edward J. A. Pope 9. Handbook of Advanced Materials Testing, edited by Nicholas P. Cheremisinoff andPaul N. Cheremisinoff 6

Additional Volumes in Preparation

Chemical Processing of

Ceramics edited 14y

Burtrand I. Lee Clemson University Clemson. South Carolina

Edward J. A. Pope MATECH Westlake Village, Calihrnia and University of Utah Salt LakeCity#Utah

Marcel Dekker, Inc.

New York*Basel*Hong

Kong

Library of Congress Cataloging-in-Publication Data

Chemical processingof ceramics I edited by Burtrand I. Lee, Edward J . A. Pope. cm.p. Includes bibliographical references and index. ISBN 0-8247-9244-0 (alk. paper) 1. Ceramicmaterials. I. Lee,BurtrandInsung. II. Pope,Edward J . A. TP810.5.C48 1994 666-dc20 94-25396 CIP The publisher offers discounts on this book when ordered in bulk quantities. For more information, write to Special SalesProfessional Marketing at the address below. This book is printed on acid-free paper.

Copyright 0 1994 by Marcel Dekker, Inc. All Rights Reserved. Neither this book nor any part may be reproducedor transmitted in any formor by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher. Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 Current printing (last digit): 10987654321 PRINTED IN T I E UNITED STATES OF AMERICA

Foreword

The value of a book is not only in its contents but in the creative idea it further encourages. These are a few ideas about the past and the future that occur to me as stimulated by chapters in this book. The volume is a fine resource of references to the use of chemistry in materials and should, indeed, encourage thinking about such things. The early use of chemistry for particular compounds was usually not with materials chemistry in mind but rather was chemistry for chemistry’s sake. The very early nature of much chemistry, now rightly readdressed for materials, appears to surprise many who believe (or wish it to be believed) that their field is new. Many chemical methods are naturally revived with new materials goals in prospect. Mechanochemical studies and aerogels are in vogue at the moment, the former more than 100 years old and the latter 60 years old. Only very ocoasionally will someone, with enough time and energy to review the early literature, produce a historical perspective as Hlavacek did for thermite processing also known as fast propagating reactions. So we have come to realize that although the “spin” on chemistry for materials is new, the processes often have a long and steady historical progression. Also not new are the phenomena seen in nanostructures-all sol-gel (e.g., Chapter 13 “all aerogels are nanocrystalline”) and life chemistry is nanostructural, and chemists long have been able to prepare molecular clusters and nanocrystalline powers by vaporization or, better, by decomposition of precursors (typically hydroxides, carbonates, nitrates, acetates, citrates, and so on). Even the recent production of biphasic or polyphasic nanostructures from polymers had been preceded. by the decomposition of mixed crystals [e.g., CaMg(Co3)2, dolomite, to CaO and MgO]. Carbon and S i c fibers (NicalonB) are nanostructural. In this book, nanocrystalline cobalt from 1966 is mentioned (Chapter 18). Naturally, the nanostructural works of chemists could not be directly examined before the advent of electron microscopes, but more indirect iii

iv

FOREWORD

methods (sometimes forgotten today) had given us a good knowledge of what was going on. Although chemical synthesis has a long history, many new, simple syntheses are being discovered and many more are mentioned in this book, e.g., the use of alkali iodides to make the molybdenum bronzes (Chapter 3) and creative uses of electrochemistry (Chapter 15). Many new structure types, some related to known types, are yet to be found by suitable new methods as discussed by Rao in Chapter 3. For example, many regular intergrowth structures are little lower in free energy of formation than the separate-layer types standing alone and so can be quite difficult to attain. Nevertheless they may have unique and useful properties. For every rule there are exceptions, and this is peculiarly true in chemistry; it seems to be taken by many as gospel that precursorsto complex ceramics, chemically mixed at the molecular level, must always be preferable to other kinds of mixtures but “it ain’t necessarily so.” However, of all the possible methods of chemical mixing, the default recourse to the Pechini method seems curious as it is hardly the simplest or most desirable method in most cases. To elaborate: if unwanted intermediates are produced at lower temperatures on the way to the desired compounds, then the purpose of molecular mixtures may be defeated. This is true in some ferroelectrics and special routes involving deliberately unstable or transient intermediates can be preferable, as mentioned in Chapter 17. It was noted early that for the P-alumindmagnetoplumbitefamily, chemical mixing, which, indeed, produced products at lower temperatures (usually desirable), also led to syntactic intergrowth problems; this has recently been seen again in the high-temperature superconductors (HTSC). (There are no new ceramic phenomena in the HTSC.) In this case also, the production of special intermediates (e.g., Pb-doped-2122 en route to Pb-doped-2223) can be beneficial. Early alkoxide work was certainly done without modem materials in mind; rather the early sol-gel chemistry was with absorbents and catalyst supports in view. Much synthetic work has been done on alkoxides, but a peculiar theoretical schizophrenia exists about them and their relation to oxides. Studies show that bond lengths and angles are very similar in alkoxide clusters to those found in oxides and polyoxyanions. This is mentioned several times in the book (e.g., Chapters 1, 2, and 21). If it looks like a duck, etc., it probably is a duck, which is to say that the bonding in alkoxides is very similar to that in oxides. Alkoxides are not covalent and oxides ionic; they are similar. Modem X-ray methods to determine actual charges on “ions” in oxides show them to be very small. While modeling Madelung potential methods, etc., often work, they are as ballet or opera is to love, merely an artistic (and often useful) imaginative representation. This will become more apparent as workers are attracted to this field and as old shibboleths fade. Even with the extensive work on alkoxides described in this book, there are

FOREWORD

V

surprising new (old!) developments worth mentioning especially the use of polyhydric alcohols to attain unusual coordination states in silicon (viz. 5-fold coordinate with glycols and 6-coordinate with catechols). New synthetic possibilities will further enhance the power of alkoxide chemistry. The area of heterometallic alkoxides (as discussed in Chapter 2) is very important and there are many cheap ways of synthesizing them, including the reactions of chlorides and similar chemicals with already formed alkoxides. People who claim that alkoxides are too expensive to be used should realize that in any real commercial process the alcohols would be recycled. Difficulties with alkoxides-sol-gel, other than the silicon cases mentioned several times in this book, suggest to me that in the future syneresis, an area that surprisingly is little studied (mentioned in Chapter 17), should pay dividends. Those who may believesthat large-scale chemical methods are unlikely to be used for ceramics should consider the following: the earliest synthesis of Si3N4 was by the chemical reaction of Sic14 with ammonia and not the, perhaps, more familiar reaction of Si plus N2 (elemental Si was not readily available in the early days). Today the largest volume of Si3N4 powder (>300 tons per year) is made by that earliest reaction route and much of it in the future seems destined for automobiles. Carbothermal nitridation of Si02 to produce Si3N4 was only later done with fertilizers in mind. As discussed in Chapter 5, an unexpected related manifestation of this is the production of S i c by pyrolysis of rice hulls. It was for the longest time believed that p-Si3N4 was the high-temperature form, or could only be produced at higher temperatures than the a-form; chemical methods have revealed otherwise, as when pSi3N4 forms from prepolymers at 1150°C (Chapter 15). This exemplifies how chemical synthesis, often designed simply with the goal of making something, serendipitously reveals results in conflict with traditional wisdom that has been acquired via a different viewpoint. In chemical processing particular methods are rarely an answer to many problems but they may address niche uses of materials (the centrifugal pipe lining use of “thermite processing” is such a use). Although much early chemistry exists in the literature, and is not difficult to find, there is also the fact that much experimentation occurred that was never published. I know of all kinds of preliminary tests that were done and not followed up in the light of their perceived limited value at the time. It is inevitable that this ground will be gone over again and while the original workers cannot take credit for unpublished work, the modem “discoverers” should recognize that the oldsters knew much more than they let on, but inevitably they could not recognize the pertinence of their knowledge to the materials problems of today. Peter E. D.Morgan

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Preface

Despite many recent advances in material science and engineering, the performance of ceramic components in severe conditions is still far below the ideal limits predicted by theory. Modem ceramics have been primarily the products of applied physics and parallel the developments of physical metallurgy. The emphasis on the relation between behavior and microstructure has been fruitful for ceramic scientists for several decades, It has been recently realized, however, that major advances in ceramics during the next several decades will require an emphasis on molecular-level control. Organic chemistry, once abhorred by ceramic engineers trained to define ceramics as “inorganic-nonmetallicmaterials,” has become a valuable source of new ceramics. It has recently become known that as the structural scale in ceramics is reduced from macro to micro and to nano crystalline regimes, the basic properties are drastically altered. A brittle ceramic material has been shown to be partially ductile, for example. The impetus and the ultimate goal in chemical processing of ceramic materials are to control physical and chemical variability by the assemblage of uniquely homogeneous structures, nanosized second phases, controlled surface compositional gradients, and unique combinations of dissimilar materials to achieve desired properties. Significant improvements in environmental stability and performance should result from such nanoscale molecular design of materials. The unique properties of ceramics with comparable mechanical performance may be realized by the molecular level or nanoscale fabrication of the chemical building blocks of the materials. A number of books are available that deal with the chemical processing aspect of ceramic materials but most of them are conference proceedings. This book is written by many authors who are actively involved in the field of chemical processing of ceramic materials. The authors are from the intemational materials community-Japan, Germany, Korea, France, India, and the United States, where they practice chemical principles in the fabrication of superior ceramic materials. vii

viii

PREFACE

This book presents current development and concepts in chemical techniques for production of state-of-the-art ceramic materials in a truly interdisciplinary fashion. The twenty-three chapters are divided into six parts reflecting topical groups. It first discusses the starting materials-how to prepare and modify them in the molecular precursor stage. Powders are the most heavily used form of starting ceramic materials. Synthesis, characterization, and behavior of ceramic powders are presented in Parts I1 and 111. In the fourth part, forming and shaping of ceramic components via sol-gel technique are discussed. Fabrication of nonoxide ceramics is covered in Part V. In the last part of this book, several specific examples of classes of ceramic materials fabricated by chemical processing including thin films, membranes, and superconductors are presented. These classes of examples are chosen on the basis of the current demand and active research. The topics of basic principles of sub-gel technique, sintering, and postsintering processing are not reviewed in this volume since there are other excellent books dealing solely with these topics. Although this book is edited, it is organized to reflect the sequence of ceramic processing and the coherent theme of chemical processing for advance ceramic materials. Hence this book is suitable as a supplementary textbook for advanced undergraduate and graduate courses in ceramic science and materials chemistry as well as an excellent reference book for practicing chemists and materials scientists and engineers. A s shown by some of the data presented in this book, the results from chemical processing are not yet up to the real applications of ceramic materials. It is evident that, through further research, the full potential of chemical processing for ceramic materials that perform up to the theoretical limit can be realized. It is the authors’ and the editors’ desire to bring the ceramist and chemist closer together to produce superior ceramic materials. Burtrand I. Lee Edward J. A. Pope

Contents

Foreword Peter E. D.Morgan Preface Contributors

iii vii xi

1. Precursor Chemistry 1. Molecular Design of Transition Metal Alkoxide Precursors

Jacques Livage 2. Metal Alkoxides for Electrooptical Ceramics Liliane G. Hubert-Pfalzgraf

3 23

II. Powder Synthesis and Characterization

3. Chemical Synthesis of Metal Oxide Powders C. N. R. Rao 4.

Multicomponent Ceramic Powders T. Mah, E. E. Hemes, and K. S. Mazdiyasni 5. Chemical Synthesis of Nonoxides Christian Russel and Michael Seibold 6. Techniques for Characterization of Advanced Ceramic Powders S. G. Malghan, P. S. Wang, and V. A. Hackley

61 75 105 129

111. Powder Processing 7. Colloid Interface Science for Ceramic Powder Processing Hyun M. Jang 8. Ceramic Nonaqueous Particles Media in

Burtrand I. Lee 9. Synthesis and Dispersion of Barium Titanate and Related Ceramic Powders Ki Hyun Yoon and Kyung Hwa Jo

157 197 215

ix

CONTENTS

X

10. Rheology and Mixing of Ceramic Mixtures Used

in Plastic Molding Beebhas C. Mutsuddy

239

W . Sol-Gel Processing 11. Processing of Monolithic Ceramics via Sol-Gel

J. Phalippou 12. Bulk Optical Materials from Sol-Gel Edward J. A. Pope 13. Aerogel Manufacture, Structure, Properties, and Applications Jochen Fricke and Joachim Gross 14. Fractal Growth Model of Gelation Edward J. A. Pope

265 287 311 337

V. CeramicsviaPolymerChemistry 15. Nonoxide Ceramics via Polymer Chemistry Kenneth E. Gonsalves and Tongsan D. Xiao 16. Polymer Pyrolysis Masaki Narisawa and Kiyohito Okamura

359 375

VI. Processing of Specialty Ceramics 17. Processing of Lead-Based Dielectric Materials Hung C. Ling and Man F. Yan 18. Synthesis of Magnetic Particles Masataka Ozaki 19. Chemistry and Processing of High-Temperature Superconductors Shin-Pei Matsuda 20. Preparation and Properties of Tantalum Oxide Thin Films by Sol-Gel T. Ohishi 21. Crystalline and Amorphous Thin Films of Ferroelectric Oxides Ren Xu 22. Ceramic Membrane Processing C. Guizard, A. Julbe, A. Larbot, and L. Cot

Index

397 421

445

465

481 501

533

Contributors

L. Cot Centre National de la Recherche Scientifique, Montpellier, France Jochen Fricke Physikalisches Institut der Universiut, Am Hubland, Wurzburg, Germany Kenneth E. Gonsalves University of Connecticut, Storrs, Connecticut Joachim Gross Physikalisches Institut der Universitat, Am Hubland, Wurzburg, Germany

C. Guizard Centre National de la Recherche Scientifique, Montpellier, France V. A. Hackley National Institute of Standards and Technology, Gaithersburg, Maryland

E. E. Hermes Wright Paterson Air Force Base, Ohio Liliane G. Hubert-PfalzgrafUniversitk de Nice-Sophia Antipolis, Nice, France Hyun M. Jang Pohang University of Science and Technology, Pohang, Republic of Korea Kyung Hwa Jo* Yonsei University, Seoul, Korea

A. Julbe Centre National de la Recherche Scientifique, Montpellier, France A. Larbot Centre National de la Recherche Scientifique, Montpellier, France Burtrand 1. Lee Clemson University, Clemson, South Carolina Hung C. Ling AT&T Bell Laboratories, Princeton, New Jersey Jacques Livage Universite Pierre et Marie Curie, Paris, France *Currenr affiliation: Daewoo Corporation, Ltd.,

Seoul, Korea

xi

xii

CONTRIBUTORS

l: Mah UES, Inc., Dayton, Ohio S. G. Malghan National Institute of Standards and Technology, Gaithersburg, Maryland Shin-Pei Matsuda Hitachi, Ltd., Ibaraki, Japan

K. S. Mazdiyasni General Atomics, San Diego, California P. E. D. Morgan Rockwell Science Center, Thousand Oaks, California Beebhas C. Mutsuddy Michigan Technological University, Houghton, Michigan Masaki Narisawa University of Osaka Prefecture, Osaka, Japan

T. Ohishi Hitachi, Ltd., Ibaraki, Japan Kiyohito OkamuraUniversity of Osaka Prefecture, Osaka, Japan Masataka Ozaki Yokohama City University, Yokohama, Japan

J. Phalippou Universitb de Montpellier 11, Montpellier, France Edward J. A. Pope MATECH, Westlake Village, California, and University of Utah, Salt Lake City, Utah C.

N. R. Rao 1ndian.Instituteof Science, Bangalore, India

Christian Russel Universitat Jena, Jena, Germany Michael Seibold OSRAM GmbH, Augsburg, Germany

P. S. Wang National Institute of Standards andTechnology, Gaithersburg, Maryland Tongsan D. Xiao University of Connecticut, Storrs, Connecticut Ren Xu University of Utah, Salt Lake City, Utah Man F. Yan AT&T Bell Laboratories, Murray Hill, New Jersey Ki Hyun YoonYonsei University, Seoul, Korea

I PRECURSOR CHEMISTRY

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1 Molecular Design of Transition Metal Alkoxide Precursors Jacques Livage Universitk Pierre et Mane Curie Paris, France

I. INTRODUCTION Transition metal alkoxides are much more reactive toward hydrolysis and condensation than silicon alkoxides. This arises mainly from the larger size and lower ,electronegativity of transition metal elements. Coordination expansion becomes a key parameter that controls the molecular structure and chemical reactivity of these alkoxides. Hydrolysis and condensation rates of silicon alkoxides must be increased by acid or base catalysis, whereas they must be carefully controlled for the other metal alkoxides. The chemical modification of transition metal alkoxides leads to the development of a new molecular engineering. The chemical design of these new precursors allows the sol-gel synthesis of shaped materials in the form of fine powders, fibers, or films.

II. HYDROLYSIS AND CONDENSATION OF METAL ALKOXIDES Sol-gel chemistry is based on the hydrolysis and condensation of metal alkoxides M(OR),. These reactions can be described as follows [l]: M-OR M-OH

+ H20 ”-+ M OH + ROH hydrolysis + R 0 -M +M- 0 -M + ROH condensation

Condensed species are progressively formed, giving rise to oligomers, oxopolymers, colloids, gels, or precipitates. 3

4

LNAGE

Actually these reactions correspond to the nucleophilic substitution of alkoxy ligands by hydroxylated species XOH, as follows: M(OR), + AXOH _ j [M(OR),.AOX),I + xROH where X is hydrogen (hydrolysis), a metal atom (condensation), or even an organic or inorganic ligand (complexation). They can be described by a S$ mechanism [2]:

H” \ O-M-O’-R+

H

\

06+M”-OR/ X

/

X

Step 1

H” / XO-M--OXO-M+ROH \ R Step 3

Step 2

1. The first step corresponds to the nucleophilic addition ofnegatively charged H06 groups onto positively charged metal atoms M&. It leads to an increase in the coordination number of the metal atom in the transition state. 2. The second step is a transfer, within the transition state, of the positively charged proton toward a negatively charged OR group. 3. The positively charged protonated alkoxide ligand ROH is then removed.

The chemical reactivity of metal alkoxides toward hydrolysis and condensation mainly depends on the positive charge of the metal atom 6nn and its ability to increase its coordination number N [2]. Asa general rule, the electronegativity of metal atoms decreases and their size increases when going down the periodic table, from the top right to the bottom left (Table 1). The chemical reactivity of the corresponding alkoxides toward hydrolysis and condensation then increases.

Table 1 Electronegativity x, PartialCharge 6, Ionic Radius r, and Maximum Coordination NumberN of Some Metal Alkoxides

Alkoxide

X

6

Si(OiPr)4 Ti(OiPr)4 Zr(0iPr)rl Ce(OiPr)4 PO(OEt)3 VO(OEt)3

1.74 1.32 1.29 1.17 2.11 1S 6

0.32 0.60 0.64 0.75 0.13 0.46

?-(A) 4 7 8 4 6

0.40 0.64 0.87 1.02 0.34 0.59

N 6

MOLECULAR DESIGN

OF TRANSITION METAL ALKOXIDE

5

Silicon alkoxides are not very reactive. Gelation occurs within several days after water has been added [3]. Hydrolysis and condensation rates must be increased viaacid or base catalysis. The hydrolysis rate of Ti(OEt)4 ( h = lO-3/M/s) is about five orders of magnitude greater than that of Si(OEt)4 ( h = 5 x lO-g/M/s). Cerium alkoxides are very sensitive to moisture. They must be handled with care in a dry atmosphere; otherwise, precipitation occurs as soon as water is present [4]. Alkoxides of highly electronegative elements, such as PO(OEt)3, cannot be hydrolyzed under ambient conditions [5], whereasthe corresponding vanadium derivatives VO(OEt)3 readily give gels upon hydrolysis [6].

111. MOLECULARSTRUCTURE OF TRANSITION METAL ALKOXIDES Silicon alkoxides have beenwidelyused for thesol-gelsynthesis of silicabased glasses andceramics. Silicon remains fourfold coordinated (N = 4) in the precursor as well as in the oxide. All silicon alkoxides Si(OR)4 are therefore monomericandtetrahedral. Their reactivity decreases whenthe size of the alkoxy group increases; thisis mainly caused by steric hindrance factors, which play a major role during the formation of hypervalent silicon intermediates [l]. The situation is completely different with other metal alkoxides M(OR),. The coordination number N of the metal atom in the oxide M o a is usually larger than the oxidation state z. As a consequence, coordination expansion appears to be a general tendency for most metal alkoxides. Positively charged transition metal atomsMz+ tend to increase their coordination number by using theirvacant d orbitals to accept electrons from nucleophilic ligands (nucleophilic addition). This currently occurs via oligomerization (OR bridges), solvation or the formation of oxoalkoxides (p-oxo bridges) [2].

A.

Oligomerization

Sharing alkoxy groups is the easiest way for metal alkoxides to increase the coordination of the metal atom without changing their stoichiometry. In pure alkoxides, coordination expansion currently occurs viathe formation ofOR bridges. Therefore oligomeric as well as monomeric molecular precursors can be found. Oligomerization depends on physical parameters (concentration and temperature) and chemical factors(solventand oxidation state of themetal atom or steric hindrance of alkoxide groups) [7]. Bulky secondary or tertiary alkoxy groups tend to prevent oligomerization, and the steric hindrance of alkoxy ligands appears to be a major parameter. Oligomeric species [Ti(0Et)4ln(n = 2 or 3) have been evidenced for titanium ethoxide whereas Ti(OiPr)4 and Ti(OtAm)4 remain monomeric (Fig. 1). X-ray absorption experiments at the Ti-K edge have been performed on the

6

LNAGE

'OEt

Figure 1 Molecular structure of titanium alkoxides. (a) = TiOiPr4; (b) [Ti(oEt)4]3.

neat alkoxides in the liquid state [8]. Prepeaks can be seen in X-ray Absorption Near Edge Structure (=S) spectra just before the absorptionedge (Fig. 2a). They correspond to 1s + 3d electronic transitions and are very sensitive to the local symmetry around titanium. A sharp prepeak is observed in Ti(OtAm)4, suggestingthatTi is fourfold coordinated, whereastheweakerprepeakin Ti(OEt)4 should correspondto a fivefold coordination. Moreover, two Ti-0 distances (1.82 and 2.05 A) and Ti to Ti correlations (3.12 A) can be observed in Ti(OEt)4, evidencing the oligomeric structure of this alkoxide. Such correlations are not seen in the Extended X-ray Absorption Fine Structure (EXAFS) spectrum of Ti(OtAm)4, which exhibits a single Ti-0 distance (1.81 A; Fig. 2b). The chemical reactivity of metal alkoxides strongly depends on their molecular structure. Oligomeric titanium alkoxides, in which Ti has a higher coordination number, are less reactive, allowing better control of hydrolysis and condensation reactions. Monodispersed Ti02 powers are usually obtained from Ti(OEt)4 rather than Ti(OiPr)4 [9,10]. Oligomerization becomes more and more important as the N-Z difference increases, that is, as the oxidation state Mz+ decreases. Divalent metals give insoluble polymeric alkoxides [M(OR)2],, (#+ = Fe, CO, Ni, Cu, . . .). This was a real drawback for the sol-gel synthesis of high-Tc superconducting ceramics, such as YBa2Cu307-~Bulky ligands, such as 2-(2-ethoxy-ethoxy)ethoxide had then to be used to prevent oligomerization and obtain soluble copper oxide molecular precursors [l l].

B. Solvation Metal alkoxides are not miscible with water, so that sol-gel reactions must be performed in the presence of a common solvent, such as an alcohol. Coordination expansion can then also occur via solvation. Solvate formation is often

MOLECULAR DESIGN OF TRQNSITION METAL ALKOXIDE

7

Ti-0

.2 '

.¶

.6

.3

I

.

1wnm

.. :.

.

J

4

:i :g J

E

R

r 45.02 .18

4.96

5

Ti-0

.9 -

1.2

.6

-

.3

-

!

.!/ i'i/ I J

4.96

Ti(OEtI4

E I

4.98

I

5

I

5.02

Figure 2 X-ray absorption of titaniumalkoxides at the Ti-Kedge.(a) = X m S ; (b) = EXAFS (Fourier transform).

observed when alkoxides are dissolved in their parent alcohol. The stability of such solvates increases with the size and the electropositive character of the metal, that is, when going down the periodic table [7]. Molecular complexity increases with the size of the metal atom. Ti(IV), for instance, gives monomeric (Ti(OiPr)4 species, whereas Zr(IV) and Ce(1V) give dimeric species, [Zr(OniPr)4,iPrOnH]2and[Ce(OiPr)4(iPrOH]2. Such dimers can be crystallized from the solution. Their structure, studied byX-ray diffraction, shows that alcohol molecules are directly bonded to the metal atom to increase its coordination (Fig. 3) [12,13]. Because coordination expansion could occur either via alkoxide bridging or solvation, the molecular complexity of metal alkoxides can be tailored by an appropriate choice ofsolvent.[Zr(OnPr)41n oligomers are formed (n I4) in nonpolar solvents, such as cyclohexane, allowing slow hydrolysis rates and the formation of clear gels. Less condensed solvates are formed in propanol (n = 2); hydrolysis becomes much faster and leads to precipitation [14].

8

LNAGE Pro H

PrO

OPr

H OPr

Figure 3 Molecularstructure of [Ce(OiPr)4, PirOHIz.

C.

Formation of Oxoalkoxides

Alkoxide groups can also bridge two different metal atoms, leading to the formation of heteroalkoxides, which are often used a precursors for the sol-gel synthesis of multicomponent ceramics [15]. Condensation can even go one step further, leading to p-oxo bridges via ether elimination. M-OR+RO-M"-0-"M+ROR This typically occurs when alkoxide solutions are heated under reflux to provide better mixing of the components. Condensation is favored by the smaller size of oxo ligands and their ability to exhibit highercoordination numbers, M"oxo ligands, for instance,havebeenbeenobservedin Na2FeaO(OCH3)18; 6CH30H [16]. Oxo bridges favor the coordination expansion of metal atoms, and oxoalkoxides are more stable thanthe corresponding alkoxides and of course less reactive toward hydrolysis and condensation [17]. Large and electropositive metals are known to give oxo-alkoxides, such as Pb40 (OEt)6. This Pb(I1) precursor undergoes complete dissolution in ethanol when Nb(OEt)5 is added, giving rise tothecrystallization of [Pb604(OEt)4][Nb(OEt)5]4.Such heterometallic alkoxides have the correct Pb/Nb stoichiometry for the sol-gel synthesis of PbMgln NbmO3 (PMN) ceramics [18]. Heterometallic alkoxides are often more soluble than their parent alkoxides, a property that can be advantageously used for the sol-gel chemistry of nonsoluble alkoxides. They also provide molecular precursors withthecorrect M I M stoichiometry inwhichsome M-0-M' bonds are already formed [15]. Alkoxide bridges are usually hydrolyzed during the sol-gel synthesis, but oxo bridges are strong enough not to bebroken.CrystallineBaTiOscanbe obtained at rather low temperatures when a mixture of Ti(OiPr)4 and Ba(0iPr)z is heated under reflux before hydrolysis [19]. It was shown recently that heterometallic tetrameric species [BaTiO(OiPr)4,iPrOH]4arethen obtained in which Ti-O-Ba bonds are already formed in the solution [20].

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

9

W. CHEMICALMODIFICATIONOF TRANSITION METAL ALKOXIDES Most metal alkoxides are very reactive toward hydrolysis and condensation. They must be stabilized to avoid precipitation. The chemical control of these reactions is currently performed by adding complexing reagents that react with metal alkoxides at a molecular level, giving rise to new molecular precursors of different structure, reactivity, and functionality. Chemical modification is usually performed with hydroxylated nucleophilic ligands, such as carboxylic acids or P-diketones. In most cases complexation by XOH species can be described as a nucleophilic substitution, as follows: M(OR),

A.

+ xXOH +[M(OR),-AOX),I + xROH

CarboxylicAcids

Acetic acid (AcOH=CH3-COOH) is often used as an acid catalyst for the solgel synthesis of Si02. Adding a small amount of AcOH significantly decreases the gel time of silicon alkoxide systems. In similar conditions, this effect appears to be morepronouncedthan with inorganic acids, such as HCl [21]. However, the reverse effect is observed for Ti(OR)4. Adding acetic acid prevents precipitation and increases the gel time [22]. An exothermic reaction takes place when acetic acid is added to titanium alkoxide in a 1:l ratio. A clear solution is obtained. X-ray absorption experiments show that the coordination of titanium increases up to six(Fig. 4a), whereas two Ti-0 distances (1.80A and 2.06A) and Ti-Ti correlations (3.1 1 A) are observed on the ,Fourier transform of the EXAFS spectrum (Fig. 4b). Infrared spectroscopy can be used to study how acetate groups are bonded to tita-

(a)

Cb)

Figure 4 X-ray absorption at the Ti-K edge of pi(OiPr)3(OAc)]. (a) = XANES; (b) = EXAFS (Fourier transform).

WAGE

10

nium. The frequency shift AV between the two bands corresponding to the symmetric v, = 1450 cm" and antisymmetric v, = 1580 cm" stretching vibrations of the acetate group COO- is typical of bidentate bridging acetate groups (AV = 130 cm"). Acetate groups actually behave as complexing nucleophilic ligands and react with titanium alkoxides as follows: Ti(OiPr)4+ AcOH +[Ti(OiPr),(OAc)] + iPrOH During this stoichiometric reaction, the coordination number of Ti increases from four to six, and oligomeric species [Ti(OiPr)3(OAc)In ( n = 2 or 3 are formed (Fig. 5) Esterification occurs when more than 1 mol AcOH is added. Acetic acid in excess reacts with alcohol molecules released during complexation, providing the in situ generation ofwaterandgiving rise to more condensedspecies. Small oligomeric species are obtained in the presence of small amounts of water. Single crystals of hexameric Ti604(OiPr)~?(OAc)4,for instance, have been obtained upon aging an equimolar mixture of AcOH and Ti(OiPr)4 in a closed vessel (Fig. 6). Water, provided via esterification reactions arising from acetic acid in excess, leads to the slow hydrolysis of alkoxide groups, which are replaced by oxo bridges [23]. Most organic groups can be removed and clear transparent polymeric titanium dioxide gels are obtained in the presence of an excess of water. Upon hydroysis, alkoxy groups are removed first, whereas bidentate acetate ligands remain bonded to titanium. They prevent further condensation and increase the gelation time. The functionality of the new molecular precursor de-

Figure 5 Chemical modification of titanium alkoxides. (a) = ri(OiPr)3 (0&)]2; = [Ti(OiPr)3(acac)].

(b)

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

I1

0" OW

Figure 6 Molecularstructure bridges (hatched circles).

of [Ti604(0iPT)12(OAc)4]:Ti(solidcircles);

oxo

creases, leading to the growth of anisotropic particles. TizO(0Ac)rj oxoacetates, made of chainlike polymeric species, are obtained when an excess of acetic acid is added [24].

B.

P-Diketones

Strongly complexing P-diketones are currently employed to stabilize highly reactive metal alkoxides, such as W(OEt)6 [25]. Aluminum sec-butoxide, modified by ethylacetoacetate (etac), appears to be quite attractive as a precursor for the sol-gel synthesis of multicomponent ceramics, such as cordierite. Al(OsBu)z(etac) is more soluble and less reactive thanthe corresponding alkoxide [26]. Complexation is also observed with acetylacetone (acacH=CH3-CO-CHzCO-CH3). Its enolic form contains hydroxyl groups and reacts with metal alkoxides as a chelating ligond. Oligomers are not readily formed, and for a stoichiometric acacni = 1 ratio, the nucleophilic substitution leads to monomers in which Ti is only fivefold coordinated (Fig. 5b). Ti(OiPr)4+ acacH +Ti(OiP&(acac)

+ iPrOH

12

LWAGE

Oligomeric compounds are formed only upon hydrolysis [27]. Titanium coordination becomes six as soon as water is added, whereas Ti-Ti correlations can be seen on the Fourier transform of the EXAFS spectrum. Strongly chelating acac ligands are not removed upon hydrolysis (except at low pH). Condensation is then prevented, and only small oligomers are formed [28].

V. MOLECULARENGINEERING OF METALALKOXIDE PRECURSORS Metal alkoxides react with nucleophilic complexing ligands. Two chemical parameters can then be used to design new molecular precursors. Metal alkoxides react with water molecules (hydrolysis) and nucleophilic species (complexation). The first reaction is followed by condensation and leads to the formation of larger oligomeric or polymeric species, whereas complexing ligands prevent condensation and favor the formation of smaller species. It therefore becomes possible to design sol-gel-derived materials by controlling the following two chemical parameters: the nature and amount of complexing additive XOH (x = X / M ) and the hydrolysis ratio (h = H20/M). A large variety of oligomeric species can then be obtained upon hydrolysis and condensation. Molecular clusters, chain polymers, or colloidal particles can be synthesized, depending on the relative amount of hydrolysis and complexation. A s a general rule, more condensed species are obtained as x decreases and h increases.

A. HYDROLYSIS RATIO The hydrolysis and condensation of metal alkoxides M(OR)z leads to oligomers in which metal atoms tend to acquire their maximum coordination number. The formation of these species can be controlled by the hydrolysis ratio h = H20/M. In the presence of a large excess of water (h >> z). all alkoxide groups are removedand colloidal species are formed.Theylead to hydrous oxides MOzI2.xH20 similar to those synthesized from aqueous solutions. The adsorption and dissociation of water molecules at the oxide/water interface leads to the formation of charged particles. All alkoxide groups are not removed when h I z and chain oxopolymers are formed. They can be advantageously employed for drawing fibers or making coatings. The dielectric constant of the organic solvent is rather low, the surface charge of polymeric particles is very small, and vander Waals interactions prevail. For very low hydrolyis ratios (h -c 1) condensation is mainly governed by the formation of p o x 0 and alkoxo bridges. Solute molecular oxoalkoxides are then formed and can often be isolated as single crystals from the solution.

I3

MOLECULAR DESIGN OF T M S I T I O N METAL ALKOXIDE

Their structure appears tobe closely related to that of the corresponding polyanions formed in aqueous solutions. The first hydrolysis product ( h = 0.6) of titanium ethoxide, for instance Ti704(OEt)20, contains [Ti70241 units isostructural with MO7@& [29]. Larger species, such as Ti1008(0Et)24 ( h = 0.8) and Ti16016(OEt)32 (h = l), are formed when more water is added [30,31]. They have been obtained upon the controlled hydrolysis of Ti(OEt)4 and isolated as single crystals from the solution (Fig. 7). The molecular structure of these species in the solution has been studied by x-ray diffraction and 1 7 0 nuclear magnetic resonance using enriched water as a reagent [32]. Niobium ethoxide leads to Nb8010(OEt)20, which exhibits the same structure as the paratungstate [33]. In the presence of an organic base, NMwOH, a decamer, is obtained [Nblo028(NMe4)6*6H20], whichis isostructural with the decananadate polyanion (V10028)6- [34].

B. Complexation The chemical reactivity of metal alkoxides toward hydrolysis and condensation can be modified by complexation. Less electronegative alkoxide ligands are hydrolyzedpreferentially,whereasstronglybonded complexing groups are more difficult to removed. They prevent condensation, and gelation or precipitation rates decrease upon complexation. Complexing ligands behave as termination reagents. They cause the effective functionality toward condensation to be reduced, resulting in less condensed and more anisotropic polymeric species. Precipitation can then be avoided, and gelation is promoted.

b Figure 7

Structure of somemolecularclustersobtained via thecontrolled hydrolysis of Ti(OEt)4. (a) = [Ti704(0Et)20]; (b) = [Ti1008(OEt)24].

14

LNAGE

The reaction of titanium alkoxides with acetylacetone leads to several molecular compounds.[TiO(acac)z]z is formed in the presence ofan excess of acetylacetone or upon hydrolysis of Ti(acac)2(OR)2. Single crystals have been isolated;andx-raydiffraction experiments show dimers with sixfold coordinated Ti atoms linked through oxygen atoms. Strongly complexing acac ligands cannot be hydrolyzed easily, and condensation does not go any further (Fig. 8) [35]. Lager molecular species can be obtained with smaller ainounts of acetylacetone. Single crystals of Tii8022(0Bu)26(acac)2were obtained recently [36]. They are made of 18 [Ti061 octahedra sharing edges or comers and correspond to x = 0.1. h = 1.2. Complexing acac ligands remain outside the Ti18022 core of the molecule (Fig. 9). These examples show that condensation can be tailored via complexation and hydrolysis. Gels are obtained in the presence of an excess of water and for small values of x (x I0.3, h 2 10). Clear sols are obtained when Ti(OnBu)4 is hydrolyzed in the presence of acetylacetone. The mean hydrodynamic diameter of colloidal particles was measured by quasi-elastic light scattering. It appears to increase from 2 to 40 nm as the hydrolysis ratio increases from h = 1 to h = 4 for x = 0.3). It decreases from 40 to 4 nm when the amount of acetylacetone increases from x = 0.3 to x = l (for h = 4) [28]. Similar results have been found with zirconium alkoxides. Solvated dimeric species are formed when zirconium propoxide is dissolved in its parent alco-

Figure 8 Molecular structure of [TiO(acac)2]2 according to Ref. 35: Ti (solid); p OXO bridges (hatched); other atoms (C and 0) (open).

-

MOLECULAR DESIGN OF TRANSITION METAL

Figure 9

ALKOXIDE

15

Molecular structure of [Ti18022(0Bu)26(acac)2](h = 1.2 and x = 0.1).

hol. Tetameric Zr40(0nPr)1o(acac)4 species are obtained with acetylacetone (x = acacnr = 1) in the presence of a very small amount of water ( h = 0.2). They have been isolated as single crystals and their structure determined by x-ray diffraction (Fig. 10) [37]. Larger molecular clusters, such as [Zr1006(OH)4(0nPr)l8L6], with kallylacetoacetate, can be formed by decreasing x (x = 0.6) and increasing h (h = l) (Fig. l l ) [38]. Monodispersed zirconia powders have been prepared by Rinn and Schmidt [39] via the acid hydrolysis of Zr(OiPr)4 in ethanol in the presence of hydroxypropylcellulose as a stabilizer to avoid aggregation and acetylacetone as a complexing agent. Particles from about 200 to 10 nmin diameter were obtained depending on the relative amount of each reagent. Dimericsolvated solute species [Cez(OiPr)s(iPrOH)z] are formed when cerium isopropoxide is dissolved in its parent alcohol. These solutions are very sensitive to moisture and can be stabilized with acetylacetone. Molecular clusters are formed when a small amount of water is added. Red crystals have been

LNAGE

16

0

Zr

e

b 0 (oxo bridge)

Figure 10 Molecularstructure of [Zr4PO(OPr)lo(acac)4] (X = 1 and h = 0.2).

grown from a solution in which x = acac/Ce = 2. They correspond to hexameric species [Ce604(0H)4(acac)l2] in which the oxohydroxo core (Fig. 12) is isostructuralwiththe inorganic analog [Ce604(OH)4(S04)6], whichwas observed in aqueous solutions of cerium sulfate [ 131. The hydrolysis of P-diketonate-modified cerium(IV) isopropoxide leads to the formation of colloidalsolutions or gels. The complexationratio (x = acac/Ce) appears to be the key parameter to tailor the size of cerium oxide particles. Precipitation is observed when x e 0.1, whereas sols are obtained when 0.1 < x e 1. The mean hydrodynamicdiameter of these particles decreases from 450 to 15 A when x goes from 0.1 to 1. Only solute molecular clusters are formed when the complexing ratio becomes larger than 1 (1 Ix I2).

VI.ORGANICALLYMODIFIEDTRANSITIONMETAL ALKOXIDES Organically modified silicates can be conveniently synthesized from precursors, such as R4Si(OR), which contain nonhydrolyzable Si-C bonds so that organic moieties are not removed during the hydrolysis-condensation process. Organic groups R can behave as either network modifiers or network formers

MOLECULAR DESIGN

0

Zr

Figure 11

e

OF TRANSITION METAL

ALKOXIDE

17

0 (oxo & hydroxo bridges)

Molecular structure of [ZrloOs(OPr)ls(acac)6] (x = 1 and h = 0.8).

when polymerizable organic ligands are used. They allow the formation of an organic network in addition to the inorganic. The organic groups impart new properties to the organic network, such as flexibility, hydrophobicity, or refractive index [40]. The chemistry of hybrid organic-inorganic gels is mainly developed around silicon-containing materials. Titanium and zirconium alkoxides are sometimes used as cross-linking reagents for the condensation of difunctional silicon precursors, such as [(CH3)2Si(OC2H5)2]. However, these alkoxides do not really behave as cross-linking agents. They lead to the formation of Poly Dimethyl Siloxane (PDMS)-Ti02 nanocomposites in which Ti02 nanoparticles are embedded in a PDMS matrix [41]. This certainty arises from the higher reactivity of these alkoxides and their well-known catalytic activity toward the condensation of siloxanes. Therefore, PDMS chains are formed on one side and Ti02 or ZrO2 nanoparticles on the other side. Si-O-Ti bonds have not yet been clearly evidenced. The catalytic role of titanium and zirconium alkoxides to-

18

LNAGE

Figure 12 Molecularstructure of [Ce604(OH)4(acac)l2].

ward the condensation of siloxanes is quite interesting for film formation. The presence of long chains in the film makes the material more flexible, allowing the deposition of crack-free transparent coatings several micrometers thick. Hybrid organic-inorganic compounds containing transition metal species were recently synthesized using organically modified polyoxometallates linked, together through W-O-Si-C bonds [42]. The organically modified precursor is obtained via thereaction of GSiW11039 witha trichlorosilane RSiCl3, R=vinyl (CH=CH2), allyl (CH2CH=CH2), or 3-methacryloxypropyl [-CH2(CH2)2OOCC(CH3)=CH2]. Each [SiW1104o(SiR)2]4- carries two binfuctional R groups, available for organic polymerization reactions performed in the presence of radical initiators. Organically modified transition metal oxide gels are difficult to synthesize. Transition metals have a lower electronegativity than silicon. The M-C bond becomes more polar and would be broken upon hydrolysis. Complexing hydroxylated organic ligands must then be used to form nonhydrolyzable M-0-C

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

19

bonds between the metal atom and the organic species. Complexation appears to be the only way to synthesize hybrid gels involving transition metal oxides. However, very few results have been reported so far in literature. According to the results reported previously, carboxylic acids R-COOH could be conveniently employed as complexing species. Prehydrolyzed titanium butoxide (H2013 = 1) oligomers have been complexed with unsaturated organic acids, such as cinnamic acid(C6H5-CH=CH-COOH).Copolymerizationwasthenperformedwith styrene in the presence of benzoyl peroxide. Transparent brown polymers were thus obtained. They are very stable against water and cannot be dissolved in most organic solvents [43]. Acrylic or methacrylic acids can also be used as polymerizable complexing ligands [a]. They were recently reported for the sol-gel synthesis of zirconium oxides via the copolymerization of zirconium oxide sols and organic monomers [45]. Carboxylates are rather weak complexing ligands, however, and most of them are removed upon hydrolyis when an excess of water is added. Chelating P-diketones are stronger complexing ligands. Organically modified Ti02 gels, which exhibit photochromic properties, have been synthesized from an allyl acetylacetone-modified Ti(OnBu)4 alkoxide. A double polymerization process was initiated via the partial hydrolysis of alkoxy groups and the radical polymerization of allyl functions. The polymerization of the allyl function is slow, however, and organic polymerization is not very effective [46]. New approaches have been explored using functionalized chelates with both a strong chelating end and a highly reactive methacrylate group, such as aceto acetoxy ethyl methacrylate and methacryl amido salicylic acid (Fig. 13).

lor

HC

\

HC

I

0

Figure 13 Chelating ligands for the synthesis of Z r O 2 hybrid gels. (a) = Aceto acetoxy ethyl methacrylate (AAEM); (b) = methacyl amido salicylic acid (MASA).

20

LNAGE

Zirconium-oxopoly-polyAAEMcopolymers have been synthesized via the chemical modification of Zr(OnPr)4 precursors by M M . Both polymerization reactions were run simultaneously, leading to hybrid organic-inorganic polymers. The zirconium oxo core is made of oxoalkoxo AAEiM-modified species in which Zr is likely in a sevenfold coordination, as in monoclinic zirconia. Zirconium oxo species are chemically bonded to polymeric methacrylate chains via the P-diketo complexing groups [47]. The complexation ratio ( A A E W r ) seems to control the structure and texture of these hybrid gels, giving rise to more or less open structures. Organic and inorganic polymerization processes are presumably related, and such a hybrid material could be described as an interpenetrating polymer network (Fig. 14). Both organic and inorganic polymerizations of zirconium propoxide modified by methacryl amido salicylate were performed using similar procedures. This MASA ligand exhibits a stronger complexing power and a higher steric hindrance. Therefore, for equivalent complexation ratios (MASA substitutes two alkoxo groups), simultaneous polymerizations of methacryl amido salicylate-modified zirconium precursors lead to smaller zirconium oxopoly methacryl amido salicylate copolymers [47].

Figure 14 A hybrid organic-transitionmetaloxidenetwork.

MOLECULAR DESIGN OF TRANSITION METAL ALKOXIDE

21

Other routes could be used, such as the following: Copolymerizationbetweenhybridinorganic-organicsystems,zirconiumoxopoly-MM, andathirdorganicpolymerizablecomponent,suchas styrene. The complexation of zirconiumalkoxideswithligandscarryinganazobis group from which radical polymerization could be initiated Moreover,hybridorganic-inorganiccopolymerscouldbesynthesizedfrom many other metal oxide gels (rare earth, aluminum and others).

REFERENCES 1. Brinker, C. J., and Scherer, G.W., Sol-Gel Science, Academic Press, New York, 1989. 2. Livage, J., Henry, M., and Sanchez, C. Prog. Solid Sate Chem., 18, 259 (1988). 3. Klein, L., Annu. Rev. Muter. Sci., 15, 227 (1985). 4. Toledano, P., Ribot, F., and Sanchez, C., C.R. Acud. Sci. Fr., 311, 13 15 (1990). 5. Livage, J., Barboux, P., Vandenbore, M. T., Schmutz, C., and Taulelle,F., J. NonCryst. Solids 147-148, 18 (1992). 6. Nabavi,M.,Sanchez,C.,andLivage, J., Eur. J. SolidStateInorg. Chem, 28, 1173 (1991). Metal Alkoxides, Academic 7. Bradley,D. C., Mehrotra, R. C.,andGaur,D.P. Press,London,1978. 8. Babonneau, F., Doeuff, S., Leaustic, A., Sanchez, C., Cartier, C., and Verdaguer, M., Inorg. Chem., 27, 3166 (1988). 9. Barringer, E. A., and Bowen, H. K., J. Am. Cerum. Soc., 65, C-l99 (1982). 10. Barringer, E. A., Bowen, H. K., Langmuir, 1, 414 (1985). 11. Scozzafava, M. R., Rhine, W. E., and Cina, M. J., Better ceramics through chemistry, IV, Muter. Res. Soc. Symp. Proc., 180, 697 (1990). 12. Vaartstra, B. A., Huffman, J. C., Gradeff, P.S., Hubert-Pfalzgraf, L., Daran,J. C., Parraud, S., Yunlu, K., and Caulton, K. G.,Inorg. Chem., 19, 3126 (1990). 13. Toledano, P., Ribot, F., and Sanchez, C., Acta Crystullogs, C46, 1419 (1990). 14. Kundu D., and Ganguli, D., J. Muter. Sci. Lett., 5, 293 (1986). 15. Caulton, K. G., and Hubert-Pfalzgraff, L., Chem. Rev., 90, 969 (1990). 16. Hegetschweiler, K., Schmalle, H. W., Streit, H. M., Gramlich, V., Hund, H. U., and Emi, I., Inorg. Chem., 31, 1299 (1992). 17. Livage, J. and Sanchez, C., J. Non-Cryst. Solids 145, 11 (1992). 18. Hubert-Pfalzgraf,L.,Papiemik, R., Massiani,M.C.,andSepte,B.,Betterceramics through chemistry, IV, Muter. Res. Soc. Symp. Proc., 180, 393 (1990). 19. Mazdiyasni,K. S., Dolloff, R. T., and Smith, J. S., J. Am Cerum. Soc., 52, 523 (1969). 20. Yanovsky, A. I., Yanoskaya, M. I., Limar, V. K., Kessler, V. G., Turova, N. Y., and Struchkov, Y. T., J. Chem Soc. Chem. Cornmuc., 1605 (191). 21. Pope, E. J., and Mackenzie, J. D., J. Non-Cryst. Solids, 87, 185 (1986). 22. Doeuff, S., Henry, M., Sanchez, C., and Livage, J., J. Non-Cryst. Solids, 89, 206 (1987).

22

WAGE

23. Doeuff, S., Dromzee,Y.,Taulelle, F., andSanchez,C., Znorg. Chem., 28, 4439 (1989). Muter. Res. Bull., 25, 1519(1990). 24. Doeuff, S., Henry,M.,andSanchez,C., 25. Unuma, H., Tokoka, T., Suzuki, Y., Furusaki, T., Kodiara, K., and Hatsushida, T., J. Mater. Sci. Lett., 5, 1248 (1986). 26. Babonneau,F.,Coury,L.,andLivage,J. Non-Cryst. Solids, 121, 153(1990). Chem. Mater., I , 240(1989). 27. Uaustic, A.,Babonneau,F.,andLivage,J., 28. Uaustic, A., Babonneau, F., and Livage, J., Chem. Mater., I , 248 (1989). 29. Watenpaugh, K.,andCaughlan,C.N., Chem. Commun., 2,76 (1967). F. S., 30. Day, V. W., Eberslcher,Klemperer,W.G.,Park,C.W.,andRosenberg, Chemical Processing ofAdvunced Materials (L. L. Hench, and J. K. West, eds.), Wiley, New York, 1992, p. 257. 31. MossetA.,andGaly,J., C.R. Acad. Sci. Fr., 307, 1747(1988). 32. Day, V. W., Eberspacher, T. A., Klemperer, W. G., Park, C. W., and Rosenberg, F. S., J. Am. Chem. Soc., 113, 8190 (1991). 33. Bradley, D. C., Hurthouse, M. B., and Rodesiler, P. F., J. Chem. Soc. Chem. Commun., 1112 (1968). 34. Graeber, E. J. and Momson, B., Acta Crystallogr., B33, 2136 (1977). 35. Smith,G.D.,Caughlan,C. N., andCampbell, J. A., Znorg. Chem., 11, 2989 (1972). 36. Toledano,P., In, M.,andSanchez,C., C.R. Acad. Sci. Fr., 313, 1247 (1991). C.R. Acad. Sci. Fr., 31 1, 1161 (1990). 37. Toledano, P., In, M., and Sanchez, C., 38. Livage,J.Sanchez,C.,andToledano,P., Metaloxideclustersand colloids, Mater. Res. Soc. Symp. Proc., San Francisco 272, 3 (1992). 39. Rinn,G.,andSchmidt,H., CeramicTransactions (G.L.Messing,E.R.Fuller and H. Hausner, eds.), Am. Ceram. Soc., I, 23 (1988). 40. Schmidt,H., J. Non-Cryst.Solids, 73, 681(1985). 41. Dir6, S., Babonneau, F., Sanchez, C., andLivage,J., J. Mater.Chem., 2, 239 (1992). 42. Judeinstein,P., Chem.Mater., 4, 4 (1992). 43. Suvorov,A.L.,andSpaski, S. S., Proc. Acad. Sci. USSR, 127, 615(1959).

44. Schubert, U., Arpac, E., Glaubitt, W., Helmerich, A., and Chau, C.,Chem. Mater.,

4, 291 (1992). 45. Nass, R., and Schmidt, H., Sol-Gel Optics, SPIE, 1328, 258(1990). J. Non-Cryst. Solids, 100, 46. Sanchez, C., Livage, J., Henry, M., and Babonneau, F., 650 (1988). 47. Sanchez,C.,andIn,M., J. Non-Cryst. Solids 147-148, 1(1992).

2 Metal Alkoxides for Electrooptical Ceramics M a n e G. Hubert-Pfalzgtaf Universid de Nice-Sophia Antipolis Nice, France

1.

INTRODUCTION

Ceramics are one of the most recent classes of electrooptical materials. Table 1 summarizes the various compositions associated with transparent electrooptical ceramics [l]. These materials are mostly ferroelectric lead-containing titanates or niobates.Amongthe various types of precursors ofmetal oxides (e.g., carboxylates and nitrates), alkoxides have especially attractive features. These include high purity, the lability of the coordination sphere, which allows their tailoring according to specific applications, their easy transformation into oxides with formation of volatile by-products [2,3], and their ability to form homogeneous solutions under a large variety of conditions, and, for multicomponent systems, via heterometallic alkoxides [4]. As a result of their versatility, they can meet the requirements for sol-gel as well as metal organic chemical vapor-phase deposition (MOCVD) applications [5]. This chapter deals with the synthesis and general properties of homo- and heterometallic &oxides that are or might be used in chemical routes to electrooptical ceramic materials. Some considerations concerning structure, reactivity, and tailoring of their properties are also given. Emphasis is given to the most recent results.

II. HOMOMETALLICALKOXIDES For all elements involved in electrooptical ceramics, their homometallic alkoxides, of the general formula M(OR)n, or oxoalkoxides, MO(OR),, are either 23

on

24

HUBERT-PFALZGRAF

Table 1

Composition of ElectroopticalCeramics

Composition La)(Zr, (Pb, Ti103 PSN Ti103Nb)@ Pb(Sc, (Pb, La)(Hf, (Pb, Sr)(Zr, Ti)O3 (Pb, Ba, Sr)(Zr, Ti)@ (Pb, La, Li)(Zr, Ti)@ Ba, (Pb, Sn)(In, Zr, Ti)@ (Pb, M)(Zr, Ti)@; M = Bi, Sr (Pb, Bi)(Zr, Ti)03

PLZT M(Ta, Nb)03; M = Li, K PLHT PSZT (Pb, La)(Mg, Nb, Zr, Ti)@ PLMNZT PBSZT (Ba, BLTN Nb)@ La)(Ti, (Pb, PLLZT La)Nbz06 PBLN PSIZT (Pb, La)(Zn, Nb, Zr, Ti)@ PBLNZT PMZT (Sr. Ba)Nb206 SBN PBZT

commercially available or can be readily prepared in high yield (strictly anhydrous conditions are required for theirhandling. Table 2 lists their general properties and main synthetic routes.

A.

'

Synthesis

A variety of synthetic pathways, depending on the starting material, are generally available for most elements. Their value can be quite different in terms of selectivity of the reaction and yield and purity of the resulting metal alkoxide. The most electropositive metals (alkaline, alkaline-earth metals, yttrium, and lanthanides) can be oxidized directly by alcohols. The rate of the reaction is largely dependent on the surface properties of the metal and on the nature of the alcohol, especially its acidity and the bulkiness of the OR group. Thus the reaction proceeds easily with alkaline metals (Li, Na, and K), the primary alcohols being more reactive than the secondary or the tertiary. The same variation is observed for the alkaline-earth metals, the reaction being the least favored for magnesium [6,7]. In some cases, it might be necessary to catalyze the reaction by mercury or by gaseous ammonia. Ammonia was observed to be efficient toward barium when classic catalysts (Hg and 12) failed; it also avoids contamination [49]. Activation of yttrium and lanthanides by trace amounts of mercury(I1) salts (chloride or acetate) is generally required, although sonochemistry can be a way to activate the metals in the absence of mercury derivatives [50]. Nonotable reaction is observed between yttrium and lanthanides and methanol, ethanol, or t-butanol, but the reaction proceeds well with isopropanol and functional alcohols, such as 2-methoxyethanol [51,52]. Synthesis of metal alkoxides directly from the metal can lead to degradation reactions either of the solvent (tetrahydrofuran, THF, for instance [ 181) or of the reactant 1131; they appear especially easy with barium. Alcoholysis reactions applied to alkoxides (alcohol interchange reactions) or to metal silylamides generally make it possible to overcome these limitations.

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

25

These reactions are generally achieved in the presence of an excess of alcohol, and thus the alkoxides might be obtained-in solution as solvates

[m.

(113:

M+ROHexcess+

M(OR)n(ROH)x+;H2

(1)

The lability of the alcohol molecules and thus the stability of the solvates is quite variable. Such formulas as Ba(OR)2 or M(OR)3 (M = In, Y, Ln) have been traditionally used to describe alkoxides of these electropositive elements. Many of these alkoxides have now been more precisely defined by x-ray diffraction studies, which have revealed quite different formulas. In fact, the removal of the alcohol molecules (under vacuum) often leads to oxoalkoxides. Pentanuclear oxo or hydroxo units seem to be a common feature for tri- and divalent metals and are illustrated by MsO(OiPr)13 (M= In, Sc; Ln = Y, Yb, . , .) and Bas(OH)(OR)s adducts [R = Ph, 3,5-tBuzCtjH3, CH(CF3)2], and even P-diketonates [53]. It is thus important to emphasize that the formulation, the properties, and the reactivity of metal alkoxides based on large elements may be different according to their “history”-used in situ or isolated [50]. Substitution reactions are another general approach to metal alkoxides, according to Eq. (2):

MG,

+ nROH +M(OR),(ROH), + nGH

(2)

G = NR2, R, H, Cl, . . . . These substitution reactions are especially attractive for obtaining a pure product when the by-product is volatile and thus the anionic ligand G, an amide group NR2, an alkyl group R, or a hydride H. Alcoholysis reactions of trimethylsilylamino derivatives M[N(SiMe3)2In are quite a versatile route to a variety of metal alkoxides, although they require the prior synthesis of the metal silylamide derivatives (commercially available only for Li and Na). This procedure is particularly attractive for metals whose alkoxides are poorly soluble, such as lead, zinc, or iron isopropoxides, since contamination with halide or alkali residues can be overcome. It also allows a large choice of R groups, and lanthanide tertiobutoxide derivatives, for instance, become accessible [39]. The use of appropriate (bulky or functional) alkoxide or aryloxide groups has also opened a way to soluble and/or volatile lead(II) and zinc derivatives [23,29]. Reactions according to Eq. (3) are generally extremely fast and may favor the formation of oxo derivatives, as observed for bismuth [U. N(SiMe3)2],

+ nROH+

M(OR),

+ nHN(SiMe3)2

(3)

Alcoholysis of metal alkyl derivatives is based on the elimination of a volatile by-product as well:

MR, + nR‘OH +M(OR’),

+ nRH

(4)

E

P Q

a g

m

B B

z"z"

B

z"

E B

P

z"

Bm

%

e ....

m

8 2 .-aM m 0

c)

2m 0 c)

x

8 z.. 3 2

.-9ax

c a 8

27

28

Q

d

i

d

e

s

0 .P - 0 -

M

"'-

B

v)

s

29

30

HUBERT-PFALZGRAF

It is of practical use for metals whose alkyl derivatives are commercially easily available, namely, lithium and magnesium. Metal hydrides can be used as starting material mainly for alkali and alkaline-earth metals. AlthoughZn(0R)z alkoxides derived from classic OR groups are insoluble soluble tetranuclear zinc alkyl or hydridoalkoxides [ZnX(OR’)]4 (X= R or H) can be obtained starting from zinc alkyls or hydride [30,31]. Finally, other general routes involve halides, mostly chlorides, as starting materials, according to Eq. (5):

M = alkaline metals. Substitution by alcohols in the presence of a base is the most general route to early transition metal alkoxides, especially for groups IV (Ti, Zr, and Hf) and V (Nb and Ta), and they are commercially available, at least for some R groups. Less labile halides (lead, bismuth, or lanthanide chlorides) require the use of alkali metal alkoxides to achieve substitution. Even so, the reaction may proceed with a poor yield and/or offer final products containing halide or alkali metals, the latter being present either as an impurity or as part of a definite product (heterometallic species). When halides are used as a starting material for metal alkoxides, contamination by chlorides can also result from a limited purification: variable amounts of Cl or Na are often found in commercial titanium isopropoxide Ti(OiPr)4. Substitution reactions applied to acetates generally result in a poor selectivity and thus a low purity of the metal alkoxides; their interest is thus limited despite the low cost of the‘acetates. Reactions between hydrated acetates and refluxing 2-methoxyethanol have been used as a way to obtain acetatoalkoxides in sol-gel processing, but most products are poorly characterized. Electrosynthesis by anodic dissolution ofthe metal in absolute alcohol appears for many elements to be a promising, inexpensive way to obtain large amounts of metal alkoxides [54]. The process goes smoothly and has good current yields for metals, such as scandium and lanthanides, and early transition metals (Ti, Zr, and Nb) using tetrabutylammonium bromide as an electrolyte. Oxo or hydroxo compounds are obtained for more electropositive metals (Mg and alkaline-earth metals), however, probably as a result of alcohol dehydration reactions. Finally, alcohol exchange reactions are a means of obtaining more appropriate precursor, starting from commercial or easily accessible metal alkoxides (see Sec. KC).

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

B.

31

PhysicalProperties and StructuralAspects

Most of the metal alkoxides of interest for electrooptical ceramics are solids (less often liquids) that can be purified by recrystallization, sublimation, or distillation. They are all moisture sensitive, and handling under an inert atmosphere and with anhydrous solvents is thus required. Their unequivocal characterization and formulation are best achieved by x-ray diffraction studies (on monocrystals). Studies on solutions (molecular weight data, nuclear magnetic resonance, NMR, with ‘H, 13C, or metal nuclei) are a means either to establish whether the solid-state structure is retained or, in the absence of x-ray data, to establish the molecular structure and eventually stereolability [48]. Mass spectrometry provides information on the stability of the oligomers or the heterometallic species in the vapor phase. The information gainedby infrared spectroscopy is limited; the technique is mostly useful for the identification of solvates M(OR)n(ROH)x (vOH absorption 3400-3100 cm-1 or of chemically modified (heteroleptic) alkoxides (probe for the vC0 stretching of P-diketonate or carboxylate ligands, for instance). The solubility and volatility of homoleptic alkoxides are mainly determined by the degree of oligomerization m,which depends on the nature of the metal (size and thus coordination number) and on the steric demand of theOR group. Methoxides are thus the least favorable. With the exception of magnesium, lead, and zinc and dn transition metals, isopropoxides are reasonably soluble for all metals of interest for electrooptical ceramics. The insolubility of the first can be overcome by addition of another metal alkoxide (see Sec. 111) or by using a more bulky ligand, such as aryloxide (2.6-di-tert-butylphenoxide)or triphenylsiloxide PhsSiO, or functional alcohols (for instance, OHC2H& with X = OR’ for alcoxyalcohols or X = NR’2 for alkanolamines), which provide intramolecular donor sites [52].Although the aryloxide or triphenylsiloxide group; generally lead to soluble species (often monomeric with a low coordination number for the metal), they might be of limited interest as a result of a low ceramic yield and high organic content of the resulting oxides. Fluorinated OR groups lead to an increase in volatility, reduce the tendency to hydrolysis, and favor the formation of adducts [19]. The bulky tert-heptyloxide groups (R = CMeEtiPr) generally represent a good means of achieving volatility [5]. The metals involved in the composition of electrooptical ceramics are large, oxophilic, and electropositive. High coordination numbers are thus generally required. Recent years have seen, as a result of x-ray studies, the reformulation of many alkoxides based on di-or trivalent large elements as oxo- or hydroxoalkoxides. It is wellknownthat M(OR)n alkoxides are often oligomersdimers, trimers, or tetramers-as a result of the OR group acting as a doublebridging (p2) or triple-bridging ligand (p3), which allows the metal to attain its usual coordination number. Oxo 02- or hydroxo OH- ligands are even more

32

HUBERT-PFALZGRAF

versatile and can behave as p2 or p3 ligands but can also accommodate four, five, or even six metals. As a result, oxo or hydroxo ligands appear to be a means of achieving the high coordination numbers required by the large di- or trivalent element in the absence of neutral ligands. Barium seems to be especially prone to formation of oxo or hydroxo derivatives, and this observation appears in the earlier literature as a tendency to give carbon-deficient or unstable metal alkoxides. The size effect of the mental is illustrated by comparison of Sr and Ba phenoxides: the strontium derivative is a tetranuclear nonoxo unit in which the metal is pseudooctahedral [16]; the barium derivative is a pentanuclear oxo or hydroxo aggregate, which allows Ba, which is larger than Sr, to be sevencoordinate for the basal metal atoms [ 181. Polydentate alcohols, such as triethanolamine, can encapsulate the metal and drastically improve the stability toward hydrolysis [14]. Figure 1 summarizes the basic [M(OR)& oligomers, as well as the oxo or hydroxo aggregates that are observed. Dimeric units (Fig. 1A) are illustrated by [M(OR)5]2 (M= Nb, Ta) or [M(OiPr)4(iPrOHh (M = Zr, Hf); [Pb(OtBu)2]3 and La3(OtBu)gL2 are typical examples of openand closed trinuclearunits (Fig. 1B and C), respectively; tetramers display a [Ti(OMe)4]4 type of structure (Fig. 1D); (also found for divalent or trivalent metals but with additional neutral ligands as for [Nd(OiPr)3(iPrOH)]4or Sr4(0Ph)8(PhOH)z(THF)6) or a cubanelike, (Fig. 1E) as for [K(OtBu)]4 or [ZnR(OR')]4 [S]. The structurally characterized penta- and hexanuclear aggregates are all oxo or hydroxo clusters.M4(p&o)o6 cores (Fig. 1F) havingan adamantanelike structure are found for lead and barium alkoxides. The M5(pS-o)Oy (Fig. 1G) core seems to display a special stability for large di- or trivalent metals. Hexanuclear aggregates derive from the pentanuclear by the addition of a sixth

Figure 1

Basicaggregates for M(OR),lm alkoxides or oxoalkoxides.

33

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

X = OR,R

E

PR G

M5°14

OR

OR

I

I

H Figure 1

6R

Continued

metal (Fig. 1 H ) or by condensation of adamantane units. Oxo or hydroxo aggregates are mostly highly soluble despite their nuclearity-the OR groups ensure a lipophilic layer-and they are also sometimes volatile. NMR and mass spectrometry data have shown that these aggregates are generally retained in solution as well as in the gas phase. They result from C-0 bond cleavage reactions (with formation of alkenes) andor oxidation reactions (especially for

34

HUBERT-PFMZGRAF OR'

bridging-chelating

bridging P2 - q'

P2 - ? l 2

triply- bridging P3

M

triply-bridging P3 -y'

terminal

-q2

Figure 2

Coordination modes of 2-methoxyethanol.

alkaline-earth metals) and are present before hydrolysis; in some cases pb(II)], further condensation is observed by recrystallization, especially when dialkyl ethers or siloxanes are easily formed [24,25]. 2-Methoxyethanol displays numerous coordination modes (Fig. 2) but is basically a bidentate and assembling ligand; [M(OC2H40Me)n]malkoxides are thus large oligomers (m= 9, M = Ca; m = 10, M = Y), sometimes infinite polymers [M = Bi, Pb(II)]. Despite the large values of the nuclearity, however, they are soluble (with the exception of lead) because of cyclic (M = Y) or compact structures (M = Ca) or dissociation of the polymer in solution, as observed for Bi [32]. Aryloxides are, with the exception of barium, either monomers or dimers [30,38]. Alkoxide ligands OR and to a lesser extent trialkylsiloxo R3SiO and aryloxo OAr are n donors [3]. l 7 donation is especially favored for do elements (Ti, Zr, I%,. Ta, and lanthanides). Asthe OR ligand undergoes oxygen p to metal d n'bonding, the M-0 distances can become very short for terminal OR groups (4 covalent radii); the M-0-C angles open up to nearly linear M-0-C units. The n bonding as well as the thermal stability of the M-0-C bond decreases with the metal oxidation state and by moving to the late transition metals and is illustrated by more acute M-0-C angles.

C. Chemical Modifications In addition to using bulky OR groups, reduction of the molecular complexity can also be achieved by a polar solvent. Since formation of complexes is quite

35

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

limited for metal alkoxides because of their preferred autoassociation, giving oligomers [48], one of the few appropriate ligand or polar solvents is an alcohol [Q. (6)]. This is illustrated by the fact that many metal alkoxides are obtained as solvates (the M-OHR interaction is often stabilized byH bonding with an adjacent OR group): the group IV isopropoxides (M(OiPr)4(ihOH)]z (M = Zr, Hf) are typical examples [42]. It should also be noted that since the structure may be different according to the solvent [niobium alkoxides (R = Me, Et) are dimers in toluene but are monomeric Nb(OR)s(ROH) adducts in the parent alcohol] [48], the reactivity can be modified, especially toward other metal derivatives (Fig. 3). 1 -[M(0R),lm m

+ xROH

1 2 [M(OR),(ROH),,],

m’ m

(6)

The electronegative alkoxo or aryloxo groups make the metal atoms highly prone to nucleophilic attack. The alkoxides easily react with the protons of a large variety of organic hydroxy compounds, such as alcohols, silanols, glycols, carboxylic acids, P-diketones (Fig. 3). These exchange reactions convert homoleptic M(OR), into heteroleptic alkoxides M(OR),-XZ, and thus reduce the functionality of the precursor (2= OR). They also illustrate the use of alkoxides as versatile starting materials and as precursors whose coordination sphere can be easily modulated for specific applications. Such modifications of the coordination sphere of the initial alkoxide are often achieved by the use of the so-called additives, which are in fact chemical modifiers [3,55a]. 1. Alcoholysis Alcoholysis reactions are generally performed to “tailor” the volatility or the rheology, to decrease the hydrolysis rates, or, finally, to increase the solubility of the metal alkoxides. As a synthetic route, alcoholysis reactions with classic alcohols are usually achieved n refluxing cyclohexane, the reactions being promoted by azeotropic distillation of the alcohol [2]. In contrast with silicon alkoxides, however, alcohol interchange reactions proceed to some extent (depending onthe OR group and on the stoichiometry) at room temperature for most elements [S]. This has been shown, for instance, by metal N M R on niobium and titanium alkoxides. “Uncontrolled” alcohol interchange reactions may thus occur when a metal alkoxide is used in an alcohol different from the parent alcohol. Such reactions lead to a modification (decrease) of the apparent functionality of the precursors, which in turn can affect the properties (morphology, porosity, and so on) of the resulting oxide. Functional alcohols, such as alcoxyalcohols-mainly 2-methoxyethanol-or alkanolamines (diethanolamine and triethanolamine), are actuallythemost commonly used to slow hydrolysis rates and thus to “stabilize” metal alkoxides. Although such exchange reactions occur at room temperature, Some heat-

36

U

0

v

x e p:

0

::

W

C

p:

h

0 E v

0

X

V

0

1 % X

::

ea

p:

n

0 v

n

x m ea X U

/-\

E

U

HUBERT-PFALZGRAF

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

37

ing may be necessary for completion of the reaction, especially if stoichiometric amounts of reactants are used. Thus the reaction between zirconium isopropoxide [Zr(OiPr)4(iPrOH)]2 and four equivalents of 2-methoxyethanol leads to Zr(OCZH40Me)3(0iPr) a room temperature. The presence offunctional alkoxo groups around the metals generally results in an increase in its coordination number, and thus hydrolysis becomes more difficult [57]. Alcoholysis reactions using 2-methoxyethanol generally also offer more soluble products [52]. A s already mentioned,this observation is no longer valid for lead(I1) since the methoxyethoxide derivative; are polymeric and insoluble [23]. The reactivity of a variety of glycols (CH&(OH)2 toward titanium alkoxides, zirconium isopropoxide, and niobium and tantalum ethoxides has been studied by Bradley et al.[2]. Complete substitution generally resultsinpoorly soluble products; such derivatives asM(OR),~X[O(CH~)~O]~ generally remain soluble and sometimes volatile. Further reactivity, for instance with acetylacetone, has established that the Ti-OR bond is more labile than the Ti-glycolate bond. Trialkylsilanols R3SiOH (R = Me, Et) are also a way of improving solubility and controlling hydrolysis [58]; however, they have rarely beenused, probably as a result of their limited stability and thus commercial availability. 2. Reactions with Carboxylic Acid and P-Diketones Substitutionreactionswith carboxylic acids, such as acetic acid, or with Pdiketones, mainly acetylacetone,are other means of controlling hydrolysis rates by decreasing the functionality of the precursor. Carboxylic acids RCO2H are often added to the sol-gel system as an acid catalyst [59,60]. In fact, they are not merely a source of H+, but a chemical modifier, and carboxylate groups substitute for the OR groups. Although P-diketonate and acetate ligands both generally lead to an increase in the metal coordination number, their behavior is quite different. Carboxylates act as assembling and oxo donor ligands, and thus have a tendency to increase the nuclearity of the aggregates; diketones are chelating ligands and thus decrease the oligomerization. This behavior is well illustrated by the modification of titanium alkoxides.Reactionbetween [Ti(OEt)& and acetylacetone gives monomeric and dimeric species (according tox-ray absorption near-edge structurestudies[61], and condensation into hexanuclear oxo clusters is observed when acetic acid is used (x-ray structures of Ti6(~2-o)2(C13-o)2(oR)8(oAC)8 (R = Et. iPr, nBu) [46]. 3. Hydrolysis Hydrolysis reactions of alkoxides proceed by nucleophilic attack of the metal M s . (7)], and polymerization occurs via the reactive hydroxoalkoxides [Eqs. (7b) and (7c)l. Hydrolysis-polycondensationreactions are governed by numerous factors (Table 3). The metals involved in the composition of electrooptical ceramics are electropositive and oxophilic, and thus the hydrolysis of their ho-

HUBERT-PFALZGRAF

38

Table 3 HydrolysisParameters of Homo-andHeteroleptic Alkoxides Electronegativity of the metal and polarity of the M-0-Cbond Nature of alkoxo groupR Modifies the molecular complexity Rate increases with chain lengthening Sensitive to hydrolysis TertiaryR > secondaryRprimaryR OR > OSiR3 pH (acid or basic catalysis) Solvent and dilution Temperature Degree of hydrolysis h (h = [HzO]/M(OR)n) h < n: fibers, chains, coatings h c 1: molecular clusters h > n: gels, tridimensional polymers Modified precursors M(OR)n-xZx(Z= OH, OAc, B-dik,...) Rate decreases with Functionality of the precursor (number of the OR groups) Increase in metal coordination number Hydrolytic susceptibility OR > OAc > B-dik

moleptic alkoxides is often extremely facile and much easier than for silicon. Introduction of other ligands 2 slows hydrolysis rates and promotes anisotropic growth. M(OR),

+ H20

-

H\

0 - hf(OR),.1 H- --OR

2M(OH)(OR),-1+ (OR),-$4M(OH)(OR),-I + M(OR),*

/

> M(OH)(OR),-l(7a)

0 -M(OR),-1 + H20 (OR)n-lM- 0 “M(OR),-1 + ROH

(7b) (7C)

The early stages of the hydrolysis-condensationof [M(OR),Im can be characterized byx-ray diffraction for oxophilic metals (Table 4). These include centrosymmetric polynuclear oxoalkoxides of titanium, zirconium, andniobium. Their structure is often related to that of polyoxoanions: Ti704(OEt)20 is isostructural with Mo70246, and Nbg010(OEt)20 has a cagelike Structure that compares well with that of the paratungstate H~W120421G.These oxoalkoxides are soluble as a result of their closo structure, and 1 7 0 NMR has been developed as a technique to follow the hydrolysis of titanium(1V) alkoxides with

39

40

HUBERT-PFALZGRAF

H217O. Since enrichment in 1 7 0 occurs selectively at their oxide (as opposed to alkoxide) oxygen sites, the different types of oxo ligands (h,n = 2-4) can be determined, the most encapsulated oxo ligands being the most shielded [63]. Similar types of molecular oxo clusters are observed for alkoxides modified by acetylacetone [65a,66]. Although the early stages of the hydrolysis-polycondensation are extremely rapid, removal ofall alkoxide ligands at room temperature generally requires quite a large excess of water [57]. Hydrolysis studies have also been performed on several lead(1I) alkoxides: Pb(OR)2 (R= CMefit, CHMeCH2NMe2) in THF [23]. The hydrate 3PbOeH20 was obtained at low temperature as the kinetic product; its conversion to massicot and then to litharge is catalyzed by an increase in the pH (which may be caused by alkali metal alkoxide residues). Since 3PbOeH20 is less stable than massicot and litharge, it could act as a more reactive component in ceramic oxide preparations.

D. Thermal Decomposition Thermolysis reactions have mostly been studied for group IV metal alkoxides, although metal oxide depositshavebeen obtained for many metals [5]. Oxoalkoxides are often formed asintermediates,but mechanistic studiesremain scarce [68]. The decomposition can be enhanced by hydrolysis as a result of either residual water on the surface or dehydration reactions of tertiary alcohols.

111.

HETEROMETALLIC ALKOXIDES

Another aspect of the lability of the metal alkoxide bond is the easy formation of heterometallic or mixed-metal species [4]. Such compounds have also been called “double alkoxides” in comparison with double salts; however, their description as heterometallic is more appropriate in view of their covalent character and the existence of species having three different metals. The formation of heterometallics is a means of overcoming the poor solubility of polymeric metal alkoxides (Zn and Pb), of achieving homogeneity at a molecular level for multicomponent systems, and of providing a better homogeneity of the final material. If they display a convenient volatility and stability, they can also be used as “single-source” precursors (in which the different elements are incorporated in a single molecule) for MOCVD applications and thus improve the transport of poorly volatile elements and reduce possible interdiffusion phenomena [5]. Table 5 collects the heterometallic alkoxides, associating elements present in the formulation of electrooptical ceramics, although the stoichiometry may be quite different from those required for materials.

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

A.

41

Formation of HeterometallicAlkoxides

Heterometallic alkoxides are generally obtained either by reactionbetween Lewis acids and bases or by condensation with elimination of volatile or insoluble by-products. 1. Lewis Acid-Base Reactions Formation of heterometallic alkoxides by simple mixing, often at room temperature and in nonpolar solvents, is one of the most general routes and corresponds to the "complexation" step in the sol-gel process. This general reaction [Eq. (8)] appliestonearlyallalkoxides,withthe exception of thesilicon Si(OR)4:

yM'(0R')nI

+ M(0R)n +MM'y(OR)n(OR)nty

(8)

Determination of the stoichiometry of the reaction is best achieved if it is conducted in a solvent in which one of the reactants is poorly soluble. Since the heterometallic species is more soluble than the parent alkoxide, the progressive dissolution of the most insoluble species allows control of the stoichiometry between the two different metals. Metal N M R (7Li and 93Nb) may also be used as tool to follow the reaction, as shown for LiNb(OR)6 [91]. The following reactions, which all proceed at room temperature, are examples of this approach: L[Mg(OiPr)2], m M(OR)~ + "(OR)

+ 2Nb(OiPr)5 +MgNb2(OiPr)l2 250c > "'(OR)6

(9) (10)

M = Nb, Ta; M' = Li, Na, K, . . . Alkali metal alkoxides are often involved in the preparation of metal alkoxides [Eq.51; heterometallic alkoxides can thus be a side product if the stoichiometry of the reaction is poorly controlled ( o n ) . For electropositive metals, such as barium, the alkoxide might be generated in situ: 2Ba + [Zr(OiPr)4(iPrOH)]2 dBa2Zr2(pOR)2(OR)18+ H2

(11)

Systematic studies of the influence of the R group and the solvents on the stoichiometry of such reactions as Eq. (8) remain limited, although the Turova group has studied ternary-phase diagrams [M(OR)n, M'(OR)n, and solvent] and proposed compositions for many systems. The Ba-Ti system is one of the best characterized and has shown dependence on the R group (oxo compound with R = zW,nonoxo compound for R = Et) as well as on the stoichiometry of the reactants:

5 E .e

4 .. h E

2

P

0

C

B

e,

.^

z m C

Y

.e

8

%

S X -

52%

F? E

43

44

HUBERT-PFALZGRAF

[BaTi2(OEt)g(EtOH)4][H(OEt)2] +

+

Addition reactions similar to h.(8) can also be observed between anhydrous acetates and alkoxides. Such reactions can even occur at room temperature, but they may require heating (depending on the lability of the metal acetate bond) and/or be less selective and give redistribution products Fomometallic acetatoalkoxides, M(OR)n-dOAc)x] as well. Mg(0Ac)z + 2Nb(OiPr)s 250c > MgNb2(OAc)2(OiPr)lo

(14)

Although metal b-diketonates can be solubilized in the presence of a metal alkoxide and thus form heterometallic species, at least-as intermediates, most reactions are more in favor of the formation of redistribution products [M(OR)n-x(P-dik)x]as the final result [92]. 2. Elimination of Volatiles The formation of heterometallic alkoxides is sometimes driven by that of a volatile by-product, mainly a dialkyl ether. Typical examples are obtaining the oxoalkoxides Ba4Ti404(0iPr)16(iProH)~ and Pb6Nb404(0Et)24:

When acetates are involved, esters can be the volatile by-product and thus favor the formation of the mixed-metal species; this approach can overcome the noncommercial availability of some alkoxides [89]: 3Pb(OAc).y3H20 + Zn(OAc)z.2HzO

> P~Zn~(OAc)4(0Cz&OMe)4 + (17)

As a general observation, it is important to note that reaction between alkoxides and other metal derivatives, such as acetates, can be strongly dependent on the solvent and/or the addition order of the reactants [84]. Thus, no reaction is observed between anhydrous Pb(OAc)2 andNb(0Et)s in THF at room temperature, but the acetate dissolves readilyin toluene at room temperature in the presence of the niobium alkoxide with formation of a Pb-Nb species [go]. Another general feature is that in heterometallic acetatoalkoxides obtained by this route, the metalintroduced as an alkoxideappearspredominant. More condensed species are obtained if the reaction is achieved at higher temperatures [93]: Pb(0Ac)z + [Zr(OiPr)4(iPrOH)]z 250c> PbZr3(~O)(~OAc)z(~OipT)5(OiPr)~ + '

(18 )

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

45

In some cases, the stoichiometry between the two metals M and M' in the isolated species is different from that of the reaction medium, suggesting that other metal species are present [Eq. (12)].

3. Elimination of an Insoluble By-product Such reactions are generally based on the elimination of an alkali salt according to Q. (19): MCl,

+ M"M',(OR),

+ nM'C1

"+MIM'y(OR)y]n

(19)

M"= Li, Na, K, TI(I)3 Although they allow control

of the stoichiometry between the M and M', metals, they have not been much developed so far for species-associating metals involved in electrooptical ceramics. They are mostly limited totheuse of the complex alkoxide anionZrz(OiPr)s-,andLa-Zr species were recently built up by this route [87]. In contrast with the preceding synthetic routes based on mixing simple compounds, Q. (19) implies the prior synthesis of a heterometallic alkoxide based on an alkali metal and further removal of the salt.

B. PropertiesandStructuralAspects Mixed-metal alkoxides are generally covalent [4]. As a result, they are soluble in organic nonpolar solvents (often less in alcohols), generally more than the parent alkoxides. Some can besublimed without disproportionation, the volatility following the same variation as for the homometallic alkoxides (OtBu > OzR > OEt > OMe).Mass spectrometry, for instance, established thatheterometallic LiNb fragments were retained in the gas phase for LiNb(OEt)6. Their structure is mainly determined by the overall nuclearity of the aggregate (MxMy and derives from the basic structural moiety in which the different metals display the usual coordination numbers expected for the bulk of the OR [4]. Numerous mixed-metal homoleptic alkoxides for which the stoichiometry betweenthetwo metals is 1: 1 have a structurethat derives from that of [Ti(OMe)4]4 (Fig. 1D). &iNb(OEt)6], is an infinite polymer; this explains its poorsolubility,but it is also favorable for obtaining fibers [70]. Trinuclear mixed-metal species generally display a triangular Mkf2(p3-OR)2(p-OR)3core (type Fig. IC), the coordination sphere eventually being completed by neutral ligand L as for SrTi2(0iPr)s(iPrOH)3. Association of such triangular units via bridging OR groups or via incommon apex gives such compounds as [BaZr2(p-OR)4(OR)4]2(Fig. 4J) or a 1:4 stoichiometry, such as BaZr4(OR)i8' (type Fig. 4K), respectively [81]. Thestructure of the Pb-Nb oxoalkoxide, Pb6Nb404(OEt)24, derives from that of the lead, with the oxo ligands acting as donors toward Nb(OEt)5 moities (Fig. 4L), which in turn increase the lead coordination number by three double-bridged OR groups. Similarly,

46

HUBERT-PFALZGRAF

Ba4Ti404(0iPr)ia(iP1OH)~ (Fig. 4M) can be considered a Ba4(~-0)4tetrahedron with oxo groups bearing Ti(OR), moeities. MgNb2(p-OAc)2(p-OiPr)4(0iPr)6 corresponds to an open polyhedron of the type shown in Fig. IB, the acetate ligands connecting the central Mg atom to the neighboring Nb ones. The presence of chelating or assembling ligands may modify the basic arrangement between the different metals, however.

C. Reactivity Data on the reactivity of mixed metal alkoxides are limited. Their reactivity remains dominated by the lability of the metal alkoxo bond, but a problem that needs to be addressed is that of the maintaining or not of the heterometallic unit, of the stoichiometry between the different metals, and finally of the ho-

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

47

mogeneity, at a molecular level when they are reacting. Dissociation into the parent alkoxides upon dissolution of the heterometallic species can be a spontaneous process, even in a nonpolar solvent,andoccurs, for instance, for Pb604(OEt)e[Nb(oEt)s]4, asshown by207Pb NMR [79].Suchdissociation phenomena can be promoted by a polar solvent (alcohol or THF), depending on the nature of the metals, and thus the choice of the solvent becomes even more important than for homometallic alkoxides [4]. Common modifiers usedinthesol-gel process are functionalalcohols, acetic acid, and acetylacetone. Alcoholysis reactions are the most documented; KiNb(OEt)6Imfor instance, was converted to LiNb(OC2H40Me)rj in the presence of 2-methoxyethanol [91]. Assembling ligands, such as 2-methoxyethanol or acetic acid, seem more favorable than P-diketones for the stabilization of mixed-metal alkoxides. However, the poorly controlled addition of an additive may sometimes modify the mixed-metal species by “extracting,” at least partially, one metal as an insoluble product or as a complex. Information on the hydrolysis of mixed-metal alkoxides is even scarcer than for the homometallic, Partial hydrolysis of [LiNb(OEt)6lo0leads to the dimeric heterometallic alkoxide [LiNbO(OEt)4(EtOH)]2, in whichboth the stoichiometry between the two metals and their coordination numbers are retained [93]. On the other hand, the partial hydrolysis of solutions of methoxyethoxides of Ba and Ti (1:l molar ratio) offers BwTi13018(OC2H40Me)24 (30% yield). Its structure corresponds to a tetrahedron ofBaO3unitssurimposedon a TiOs(Ti03)n core; this Ti13042 core is related to that of the aluminum salt [Al1304(0H)24(H20)]7+ [83].The important modification of the stoichiometry between the two metals illustrates the complexity of the sol-gel chemistry of multimetallic systems and shows that important structural rearrangements can occur.

IV. APPLICATION TO CERAMICS A.

Sol-Gel Process

PLZT and PNM are, with L i m o s and LiTaOs, the systems related to electrooptical ceramics that have been the most studied.Chemical low-temperature routes are particularly attractive for lead-containing materials, since lead oxide is quite volatile in comparison with other metal oxides; thus control of the stoichiometry of lead-based materials is tedious. To overcome the difficulty inhandling alkoxides and their availability, commonly accessible salts, such as carboxylates (acetates or the more soluble 2ethylhexanoates), carbonates, nitrates, or hydroxides, have often been used in conjunction with metal alkoxides (mostly n-propoxides or n-butoxides for Ti and Zr and ethoxides for Nb or Ta), the solvent being an alcohol. The poor re-

48

HUBERT-PFUGRAF

activity of barium acetate is overcome by adding acetic acid [46a] or by using carbonate or, more often, barium hydroxide Ba(OH)~8Hz0[94]. Commercial ionic precursors are often available as hydrates, and this must betaken into account when speculation about the intermediates that may form is proposed. Carboxylate groups (acetates or 2-ethylhexanoates) are generally removed only at relatively high temperatures (400°C); they appear to favor porous materials and grain boundaries [95]. Additives for hydrolysis are either acids (0.1-0.2 M m03 or acetic acid) or ammonia solutions. Although leadoxide has also been used [96], acetate hydrates are the precursors most often used for the introduction of lead or lanthanum in the multimetallic systems required for PLZT materials,and they are often associated with the useof 2-methoxyethanol [97-991. This solvent appears as a way to achieve deshydratation by refluxing (boiling point 124"C), as well as condensation (elimination of organic esters as has been shown by infrared (vCOZ = 1730 cm-1) [loo]. Studies on the influence of the precursor remain limited. X-ray absorption fine structure analysis of the sol and gel precursors of PZT (lead acetate in methanol, n-propoxides of Zr and Ti, and acetic acid) have shown that an uniform composition exists in both states on a microscopic level with Ti-O-Z linkages [100a]. The choice of the titanium precursor Ti(OR)4, R = nF'r or iPr, was reported to have an effect on the microstructure development, particularly for PbTiOs films [97]. Clusters and grains were observed in the n-propoxide and were attributed to a less continuously cross-linked networks compared with films derived from the isopropoxide. Using the modified alkoxide Ti(OiPr)z(acac)z instead of Ti(OiPr)4 allowed an increase in the thickness of crack-free PbTiOs films up to 1 m under base-catalyzed conditions [loll. Using the same titanium precursor associated with lead acetate and pentane-lJ-diol instead 2of methoxyethanol permitted the production of films up to approximately 5pm in thickness by repeated coatings before fuing [ 1021. All alkoxide routes appear to be limited toobtaining PNM [88a] and lithium or potassium niobate or tantalate [102a]. For the latter, the formulation of the mixed-metal alkoxide MM'(OR)6, M = Nb, Ta, is in agreement with that of the material. Epitaxial thin films of LiNbO3 on sapphire could be obtained [103]. The useof a mixed-metal MgNbz(0R)lz species (R = Et [105], CzH40Me [104]) has been shown to promote the formation of the perovskite phase for PNM.

B. MOCVD Obtaining electrooptical thin films by MOCVD techniques has been less investigated, although volatile oxide precursors-alkoxides 'or p-diketonatesexist for nearly all elements. For PbTiOs films titanium isopropoxide has been

METAL ALKOXIDES FOR ELECTROOPTICAL CERAMICS

49

associated with an alkyl derivative, PbEU using argon as the carrier gas. No additional oxygenwasnecessary. The film deposited at a quartz substrate heated at 500°C was shown to be conducting and dense, with good surface morphology and strong [l001 texture direction [106]. Pb(0tBu)z was used by Trundle and Brierley as the volatile lead source with Ti(OiPr)4, giving 5 pm thick PbTiOs deposited at 450"C, annealed .in air at 800-900°C [107]. Although these authors could not deposit PbSc0.5Tm.503 (PST) directly from the metal precursors, the PST perovskite phase was obtained in atwo-stage process: deposition of cubic ScTaO4 by MOCVD using Ta(0Et)s and a scandium-fluorinated P-diketonate Sc(fod)3 (fodH = heptafluorodimethyloctanedione), followed by diffusion of PbO from a surface layer [5]. The heterometallic alkoxide LiNb(0R)a has the correct stoichiometry for depositing LiNbO3. Although LiNb(0R)a is volatile, control of the deposition parameters is sometimes hampered by disproportionation reactions. The problem has been overcome by using a lithium P-diketonate Lithd and Nb(0Me)s [5]. LiNbO3 has been deposited on a variety of substrates (-45O"C), but epitaxial layers require annealing in oxygen at higher temperatures.

V.

OUTLOOK

Metal alkoxides are readily available for all elements involved in electrooptical ceramics. The selection of appropriate OR groups-bulky or functional and thus polydentate-allows adjustment of their physical properties: solubility or volatility. An almost unlimited number of mixed-metal compositions is accessible under mild conditions, often at room temperature, by mixing metal alkoxides and carboxylates or P-diketonates. The molecular composition of these solutions can be quite complex and comprise mixed-metal species with different stoichiometries. To date, 2-methoxyethanol has been used predominantly for the sol-gel processing of thin layers and thus for the chemical modification of the metalalkoxides. However, methoxyethanol is teratogenic and can cause neurological and hematological damage, even at the ppm level. Alternative solgel systems, based for instance on other difunctional and polyfunctional alcohols and on hydroxyacids, for example, should be developed to promote the sol-gel process as a practical method. More systematic studies regarding the relationship between precursors, influence of additives, and properties of the final material are also required.

ACKNOWLEDGMENTS The author is grateful to the CNRS for financial support, and to the contributions and enthusiasm of coworkers, whose names are listed in the references.

50

HUBERT-PFALZGRAF

ABBREVIATIONS acacH acetylacetone (2,5-pentanedione) acetate OAc ArR R-substituted phenyl group:2,6-RzCaH3 ArR$ R-substituted phenyl group:2,6-Rz-4-MeCsH3 ArF phenylgroupsubstitutedbyCF3 groups: 2,4,6-(CF3)3C,jH2 Bz benzyl CH2C6Hs P-dikHP-diketone = (O)CRCH2C(O)R thdH 2,2,6,6-tetramethylheptane-3,5-dione

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Sakashita, Y . , Ono, T., Segawa, H., Tominaga, K., and Okada, M., Preparation and electrical propertiesof MOCVD-deposited PZT thin films,J. Appl. Phys., 69, 8352 (1991).

88a. Munozaguado, M. F., Gregorkiewitz, M., and Larbot,A., Sol-gel synthesisof the binary oxide (Zr, Ti)Oz from the alkoxides and acetic acid in alcoholic medium, Muter. Res. Bull., 27,87(1992), Laaziz, I., Larbot, A., Julbe, A., Guizard, C., and Cot, L., Hydrolysis of mixed titanium and zirconium alkoxides by an esterification reaction, J. Solid Sture Chem., to be published; Laaziz, I., Larbot, A., Foulon, 1. D., and Cot, L., zr2Ti404(oiPr)8(oiPr)2( OAC),~, submitted for publication. 89. Francis, L. F., Payne, D. A., and Wilson, S. R., Crystal structure of a new lead zinc acetate alkoxide, PbzZnz(OCzH40Me)4(0Ac)4, Chem Murer., 2,645-647 90. 91.

(1990).

Veith,M.,Hans, J., Stahl, L., May,P.,Huch,V.,andSebald, A., Bimetallic alkoxygermanate(II)-stannate (II) andplumbate(II), Z. Nuturjiorsch. B, 46b, 403-424 (1991).

Eichorst, Howard, K. E., and Payne, D. A.,NMR investigations of lithium alkoxide solutions, J. Non-Cryst. Sol., 121, 773 (1988). 92. Sirio, C., Poncelet, O., Hubert-F'falzgraf, L. G., Daran, l. C., and Vaissermann, J., Reactions between copper P-diketonates and metal alkoxides as a route sol-to uble and volatile copper(I1) oxide precursors: Synthesis and molecular structure Polyof Cu4(p-3,nl-OC2H40iPr)4(acac)4 and (acac)Cu(p-OSiMe~)zAl(OSiMe~)~, hedron, 11, 177 (1992). 93. Turova N. Y., Turevskaya, E. P., Kozlova, N. I., Rogova, T. V., Kessler, V. G., and Kucheiko, S. I., Transition metal alkoxides-precursors of oxides materials, J. Non-cryst. Sol., in the press (1992). 94. Thule, P. P., Raghavan,S., and Risbud, S. H., Comparison of Ba(OH)2, BaO and

of bariumtitanatebythealkoxide Baasstartingmaterialsforthesynthesis method, J. Am. Cerum Soc., 70, C108 (1987). 95. Hsueh, C. C., and McCartney, M. L., Microstructural development and electrical properties of sol-gel prepared lead zirconate-titanate thin films,J. Muter, Res., 6, 96.

2208 (1991).

Yanovskaya, M.I., Turevskaya, E. P., Turova, N. Y., Dambekalne, M. Y . , Kolganova, N. V., Ivanov, S. A., Segalla, A. G., Belov, V. V., Novoselova, A. V., andVenevtsev, Y. N., Transparentceramic(Pb, La) (Zr, Ti)03 preparedby alkoxy technology, Inorg. Muter., 23, 584 (1983).

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97.Budd,K.D.,Dey, S. K.,andPayne,D.A.,Sol-gelprocessing of PbTiO3, PbZrO3, PZT and PLZT thin films, Br. Ceram. Proc., 36, 107 (1985). 98.Ramamurthi, S. D.,andPayne,D.A.,Structuralinvestigations of prehydrolized precursors used in the sol-gel processingof lead titanate,J. Am. Ceram. Soc., 73, 2547 (1990). of finePbTiO3powdersby 99.Cheng,M. J., ZhaoZ.,andQuangD.,Preparation hydrolysis of alkoxide, Chem. Mater., 3, 1006 (1991). 100. Seth, V. K., Schulze, W. A., and Condrate, R. A., Sr. Vibrational spectral characterization of a lead lanthanum zirconate titanate during various stages of solgel processing, Specrro. Lett., 24, 1299 (1991). 100a.AhlfAanger, R., Bertagnolli,H.,Ertel, T., Kolb, U., PeterD.,Nass,R.,and Schmidt, H., First evidence of the preformation of an inorganic network in solgel processing of lead zirconate titanate, obtained by EXAFS spectroscopy,Ber. Bun. Gesel. Phys. Chem., 95, 1286 (1991). 101. Milne, S. J., and Pyke, S. H., Modified sol-gel process for the production of lead titanate films, J. Am. Ceram. Soc., 74, 1407 (1991). 102. Philipps, J. F., and Milne, S. J., Diol-based sol-gel system for the production of thin films of PbTiO3, J. Muter. Chem., 1, 893 (1991). 102a.Amini,M.M.,and Sooks, M.D.,Preparation of single-phaseKNbo3using bimetallic alkoxides, Better Ceramics Through Chemistry W , 180, 675. (1990). 103. Nashimoto, K., and Cima, M. J., Epitaxial LiNbo3 thin films prepared by a solgel process, Mater. Lett., IO, 348 (1991). 104. Francis, L. F., Oh, Y. J., and Payne, D. A., Sol-gel processing and properties of leadmagnesiumniobatepowdersandthinlayers, J. Muter. Scien., 25, 5007 (1990). 105. Chaput, F., Boilot, J. P., Lejeune, M., Papiernik, R., and Hubert-Pfalzgraf, L. G., Low-temperature route to lead magnesium niobate, J. Am. Ceram. Soc., 73, 1355 (1989). 106. Kwak, B. S., Boyd, E. P., and Erbil, E., Metalorganic chemical vapor deposition of PbTiOs thin films, Appl. Phys. Lett., 53, 1702 (1988). 107. Trundle,C.,andBrierley,C. J., Precursors for thinfilmsoxidesbyPhotoMOCVD, Appl. Surf: Sci. 36, 102 (1989).

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POWDER SYNTHESIS AND CHARACTERIZATION

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3 Chemical Synthesis of Metal Oxide Powders C. N. R. Rao Indian Instituteof Science Bangalore, India

1.

INTRODUCTION

Synthesis of oxide materials provides an excellent case study of the contribution of solid-state chemists to materials synthesis [1,2]. Although tailoring oxides of the desired structure and properties remains the main goal of solid-state chemistry and materials science, it is not always possible to do so. One can evolve a rational approach to the synthesis of solids [3], but there is always the element of surprise encountered not so uncommonly. A well-known example of an oxide discovered serendipitiously is NaMo,06 containing condensed MO, octahedral metal clusters [4]. This was discovered in an effort to prepare the lithium analog of NaZn2M0308. Another such serendipitous discovery is that of the phosphorus-tungsten bronze Rb$8W320112, formed by the reaction of phosphorus present in the silica of the ampule during the preparation of the RbW bronze [5]. Novel solids of the type cu$o6s8, called Chevrel phases, were also discovered accidentally. There are many examples of rational synthesis. A good example is Sialon, in which Al and oxygen were partly substituted for Si and nitrogen in silicon nitride, Si3N,. The fast Na+ ion conductor Nasicon was synthesized based on understanding the coordination preferences of cations and the nature of oxide networks formed bythem. The zero-expansion ceramic C%.5Ti2P3012,possessing the Nasicon framework, was later synthesized based on the idea that the property of zero expansion would be exhibited by two- or three-coordination polyhedra linked in to leave substantial empty space in the network [3].

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Traditionally, oxides have been prepared by the so-called ceramic method, which involves mixing and grinding various powders and heating them at high temperatures, with intermediate grinding when necessary. The trend nowadays is to avoid such a brute force method to obtain better control of structure, stoichiometry, and phasic purity. Some of the chemical methods of synthesis of oxides that are especially noteworthy are (1) the precursor method, (2) coprecipitation and soft chemistry routes, (3) the sol-gel method, (4) intercalation and ion exchange, and (5) topochemical methods. We examine examples of synthesis by some of these methods and briefly describe the synthesis of certain novel oxide materials, including superconducting cuprates, it should be noted that a variety ofconditions, often bordering on the extreme, such as very high temperatures or pressures, very low oxygen fugacities, and rapid quenching, have been employed in oxide synthesis. The low-temperature chemical routes, however, are of greater interest.

II. PRECURSOR METHOD In the ceramic method, diffusion distances between the reacting cations are rather large. The diffusion distances are markedly reduced by incorporating the cations in the same solid precursor. Synthesis of complex oxides by the decomposition of compound precursors has been known for some time. For example, thermal decomposition of L~CO(CN)~-SH,O and LaFe(CN),.H,O in air readily yields LaCo0, and LaFeO,, respectively. BaTiO, can be prepared by the thermal decomposition of Ba[TiO(C204),], and LiCrO, can be prepared from L I [ C ~ ( C ~ O ~ ) ~ ( H , O Unfortunately, )~]. compound precursors are not always easy to find. In such instances, precursor solid solutions can be used effectively. Carbonates of such metals as Ca, Mg, Mn, Fe, CO, Zn, and Cd are all isostructural, possessing the calcite structure. We can therefore prepare a large number of carbonate solid solutions containing two or more cations in different proportions [6,7]. The rhombohedral unit cell parameter uR of the carbonate solid solutions varies systematically with the weighted mean cation radius. Carbonate solid solutions are ideal precursors for the synthesis of monoxide solid solutions of rock salt structure. The carbonates can be decomposed in vacuum or in flowing nitrogen to yield single-phase solid solutions of monoxides of the type Mnl-pxO (M = Mg, Ca, CO, or Cd) of rock salt structure. Oxide solid solutions of M = Mg, Ca, and CO would require 770-970 K for their formation; those containing cadmium are formed at even lower temperatures. The facile formation of the oxides of rock salt structure by the decomposition of calcite carbonates is a result of the close (possibly topochemical) relationship between the structures of calcite and rock salt. The monoxide solid

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solutions can be used as precursors for preparing spinels and other complex oxides. Besides monoxide solid solutions, a number of ternary and quaternary oxides possessing novel structures can be prepared by decomposing carbonate precursors containing the different cations in the required proportions, in air or oxygen. Thus, one can prepare Ca,Fe,O, and CaFe204 by heating the corresponding carbonate solid solutions in air at 1070 and 1270 K,respectively, for about 1 h.Ca,F%O, is a defect perovskite with ordered oxide ion vacancies and has the well-known brownmillerite structure (Fig. 1) with the Fe3+ ions in alternate octahedral (0)and tetrahedral (T) sites. Two new oxides of similar compositions, Ca$0,05 and CaCo204, have been prepared by decomposing the appropriate carbonate precursors in oxygen atmosphere around 940 K. Unlike Ca,Fe,O,, in C%Mn,O,, anion vacancy ordering in the perovskite structure gives a square-pyramidal (SP) coordination around the transition metal ion (Fig.1). One can also synthesize complex oxides of the type Ca2FeCo05, Ca,Fe,,Mno~405, Ca3Fe2Mn08, and so on, belonging to the A#,O,,, family by the carbonate precursor route. In the Ca-Fe-0 system, there are several other oxides, such as CaFe407, CaFel2Olg, and CaFe204(FeO), (n = 1, 2, 3), that can, in principle, be synthesized starting from the appropriate carbonate solid solutions and decomposing them in a proper atmosphere. A good example of a multistep solid-state synthesis achieved starting from carbonate solid solution precursors is provided by the Ca2Fez-$4nXO, series of oxides (Fig. 2). The structure of both the end members, Ca2F%0, and Ca2Mn,05, are derived from that of the perovskite (Fig. 1). Solid solutions be-

-

-C - L a -

b

(0)

A

-O ( b)

Figure 1 Structure of (a) C%F905 (brownmillerite) and (b) C%Mn,O,. Oxygen vacancies are shown by open circles.

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I

JI

BM

Figure 2 Ca3F%MnOs (I) is reduced topochemically to Ca2Fe4BMm05 transforms to the brownmillerite (BM) structure on annealing in vacuum.

(m; (II)

tween the two oxides are expected to show oxygen vacancy ordered superstructures with Fe3+ in octahedral and tetrahedral coordinations and Mn3+ in square-pyramidal coordination, but they cannot be prepared by the ceramic method. These solid solutions have indeed been prepared starting from the carbonate solid solutions, Ca2F~-Mnx(C03),. The carbonates decompose in air around 1200-1350 K to give perovskitelike phases Ca2Fe--$fnxOcy (y < 1). The compositions of the perovskitelike oxides prepared from the carbonate precursors with x = 2/3 and 1 are Ca3Fe+4n8 and Ca3Fel,Mnl,08.,. X-ray and electron difraction patterns show that they are members of the A,,Bn03,,-1 homologous series of anion vacancy ordered superstructures with n = 3 (A3B308+J. Careful reduction of Ca3Fe2Mn08 andCa3Fe1.5Mn1.508.1in dilute hydrogen at 600 K yields Ca3Fe2Mn07.5 and Ca2FeMn05 (CagFe1.5Mn1.507.5),respectively (Fig. 2). During this step, only Mn4+ in the parent oxides is topochemically reduced to Mn3+, and Fe3+ remains unreduced. The mostprobable superstructure of Ca3F%Mn07.5 involves SP, 0, andT polyhedra along the b direction. On heating in vacuum at 1140 K,however, it transforms to the more stable brownmillerite structure with only 0 and T coordinations (Fig. 2). A variety of complex metal oxides of perovskite and related structures can be prepared by employing hydroxide, nitrate, and cyanide solid solutions precursors as well [ 8 ] . For example, hydroxide solid solutions of the general formula Ln1-Jblx(OH)3, where Ln = La or Nd and M = Al, Cr, Fe, CO or Ni) and Lal-x-,"~"y(OH)3 (where M' = Ni and M" = CO or Cu), crystallizing in the

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65

rare earth trihydroxide structure, can be decomposed at relatively low temperatures (-870 K) to yield LaNiO,, NdNiO,, LaNil-xCox03, LaNil-xCuxO,, and others. Anhydrousalkaline-earth metal nitrates A(N03)2 (A = Ca, Sr, Ba)and Pb(NO,), are isostructural. One can therefore readily prepare nitrate solid solutions of the formula Al+.Pbx(N03)2,which are ideal precursors for the preparation of such oxides as BaPbO,, B%PbO,, and Sr2Pb0,. Oxides of the type LaFe0.5C00.503and 5Nd&003, which cannot be madereadilyby the ceramic method, have been prepared by the decomposition of cyanide solid solutions: LaFeo,Coo.5(CN)6.5.H20 and L% 5N+.5C~(CN)6.5-H20, respectively.

Ill. LA2C0205 AND La2Ni205 BYTOPOCHEMICAL REDUCTION Both LaNiO, and LaCoO, crystallize in the rhombohedral perovskite structure, and the anion-deficient nonstoichiometry of these oxides is interesting. Occurrence of the homologous series La,Ni,O,,, on the basis of a thermogravimetric study of the decomposition of LaNiO, was proposed some time ago. It was not known, however, whether a similar series exists for cobalt. Controlled reduction of LaNiO,andLaCoO, in hydrogen shows the formation of La2Ni2O5 and L%Co205, representing the n = 2 members of the homologous series LaB,03,1 (B = CO or Ni). La2Ni205.can be prepared by the reduction of LaNiO, at 600 K in pure or dilute hydrogen [9] and La2C0205 by the reduction of LaCo0, in dilute hydrogen at 670 K. Both oxides can be oxidized back to the parent perovskites at lowtemperatures. Neither La2Ni2O5 nor La2C02O5 can be prepared by the solid-state reaction of La203 and the transitionmetaloxide.X-rayand electron diffraction data reveal that La2C0205 adopts the brownmillerite structure. The x-ray diffraction pattern of La2Ni205 is different from that of La2C0205 but could be indexed on a tetragonal cell related to cubic perovskite. The formation of La2C0205 and L%Ni205 by the reduction of LaCoO, and LaNiO,, respectively, is caused by the topochemical nature of the reduction process. The reduction of the high-temperature superconductor YB%Cu30, to YBa2Cu306 is also a topochemical reaction. Similarly, many of the reactions involving insertion of atomic species into host oxides are topochemical.

IV. MO,-xW,O,BYTOPOCHEMICALDEHYDRATION Many of the modem developments in solid-state chemistry owe much to the investigations carried out in MOO, and WO, the crystallographic shear planes being a major discovery. WO, crystallizes in a Reo3-like structure, but MOO,

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possesses a layered structure. MOO, can be stabilized in the WO, structure by partly substituting tungsten for molybdenum. These solid solutions can be prepared by the ceramic method (by heating MOO, and WO, in sealed tubes around 870 K) or by the thermal decomposition of mixed ammonium metallates. These methods do not always yieldmonophasicproducts,however, owing to the difference in volatilities of MOO, and WO,. We therefore sought to prepare Mo1,Wx03 by the topochemical dehydration of Mol,Wx03.H20. Topochemical dehydration, it is to be noted, is a gentle process. Since Mo03-H20 and WO,.H,O are isostructural, the solid solutions between them are prepared readily by adding a solution of MOO, and WO, in ammonia to hot 6 M HNO,. The hydrates, Mol,Wx03.H20, crystallize in the same structure as Mo03.H20 and W03.H20, with a monoclinic unit cell. The hydrate solid solutions undergo dehydration undermild conditions (around 500 K), yielding M o ~ - ~ W ~which O ~ , crystallize in the Reo3-related structure of WO,. Thermal dehydration of W03.H20 and M o , - ~ W ~ O ~ Hgives ~ O rise to oxides of Reo, structure. MOO, obtained by the dehydration of Mo03.H20, however, has a layered structure; this reaction is reported to be topotactic. The nature of the dehydration reaction was studied by an in situ electron diffraction study in which the decomposition occurs as a result of beam heating [IO]. Electron diffraction patterns clearly show how W 0 3 - H 2 0transforms to WO, topotactically with the required orientational relationships. The mixed hydrates, M O ~ - ~ W ~ O , . H also ~ O , undergo topotactic dehydration with similar orientational relations. What is interesting is that when the dehydration of Mo03.H20 is carried out under mild conditions (e.g., electron beam heating), MOO, in the Reo, structure is formed instead of the expected layered structure. The Reo, structure of MOO, is metastable and is produced only by topotactic dehydration under mild conditions. We believe that the preparation of Reo3 like MOO, by mild chemical processing is significant.

V. BRONZES OF MO,-flxO, REACTION

BYASOLID-STATE

Oxides of the type AxW03 (A = alkali metal) are known as tungsten oxide bronzes. These are readily prepared by the insertion of the alkali metal into WO,. The corresponding molybdenum bronzes are more difficult to prepare. High pressures and electrochemical methods are generally employed to synthesize some of them. A simple solid-state reaction between an alkali iodide and MOO, or Mol,WxO3 (under dry conditions) has been found to yield such molybdenum oxide bronzes [l l]. The following reaction represents a simple means of making these bronzes:

CHEMICAL SYNTHESIS OF METAL OXIDE POWDERS

VI.

67

INTERCALATION

Intercalation chemistry has become an important aspect of solid-state chemistry, and a large variety of guest molecules are accommodated in the cavities, cages, channels, or interlayer spaces of host solids. Many oxides act as hosts, and interesting properties emerge from this property. Many of the intercalation reactions are topochemical in nature. One of the interesting intercalation reactions involving oxides is that involving chemical or electrochemical intercalation or disintercalation of alkali metal ions in oxides of the type LiMO,, where M = V or Co. When Li, for example, is removed from LiVO,, the resulting VO, remains in the metastable structure of the parent LiVO,. Other such metastable structures of oxides have been prepared by the preferential removal of intercalated species [l].

VII.

ION EXCHANGE

Ion exchange is employed effectively to synthesize new oxides [l]. Thus Na in p-alumina is readily exchanged by other cations. Such exchange reactions are well known in silicates, especially zeolites. Examples of simple ion-exchange reactions are as follows: U 1 0 2 + AgN03 p "AgA102 + KNO3 a - LiFeOz + CuCl +CuFeOz + LiCl NaCrO;! + LiN03 +LiCrOz + NaCl It has been possible to exchange Li in LiNbO, by protons to obtain HNbO,. Such alkali metal-proton exchange reactions are common in layered oxides (e.g., H2Ti307 and HLaNbO,).

VIII.

ALKALI FLUX METHOD

One of the early examples of the use of strong alkaline media for the synthesis of oxides is that of Pb2Ru2-.Pbx07y, which has the pyrochlore structure. The method stabilizes higher oxidation states of metals. Alkali carbonate fluxes have been traditionally used to prepare many oxides (e.g., LaNiO,). Recently, superconducting La,CuO, was prepared by the reaction of L%O, and CuO in a molten mixture of NaOH and KOH around 520 K [12]. Superconducting Ba,_.K$iO, has been prepared by the use of molten KOH [13].

IX.

INTERGROWTH STRUCTURES

Several systems form chemically well-defined recurrent ordered intergrowth structure with large periodicities, rather than random solid solutions with vari-

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able composition. However, the ordered intergrowth structures themselves frequently show the presence of wrong sequences. The presence of wrong sequences or lamellae is best revealed by a technique more suited to the study of local structure.High-resolution electron microscopy (HREM) enables direct examination of the extent to which a particular ordered arrangement repeats itself: the presence of different sequences of intergrowths, often of unit cell dimensions. Selected area electron diffraction, which forms an essential part of HREM, provides useful information (not generally provided by x-ray diffraction) regarding the presence of supercells caused by intergrowth or defect ordering. Many systems forming ordered intergrowth structures have come to be known in recent years [14]. These systems generally exhibit homology. If the M O 3perovskite structure is cut parallel to the (1 10) planes, slabs of the composition An-lBn03, are obtained; if these slabs are stacked, an extra sheet of A is introduced, giving rise to the family of oxides of the general formula A#n03n+2. Typical members of this family are Ca2Nb2O7 (n = 4) NaCa4Nb,017 ( n = 5), and Na,Ca,Nb,O, (n = 6). High-resolution electron microscopy and x-ray studies show that an ordered intergrowth structure with n = 4.5 with the composition NaCa8Nb903*corresponds to alternate stacking of n = 4 and n = 5 lamellae. In Fig. 3 we show the lattice image of the ordered intergrowth in NaCa8Nb903,. Between n = 4 and 4.5, a large number of or-

Figure 3 High-resolution electron microscopic image of the n = 4.5 memberinthe series.

A,,B,,O,,,

CHEMICAL SYNTHESIS OF METAL OXIDE POWDERS

69

dered solids are found, with the b parameter of the unit cell ranging anywhere from 58.6 A in the n = 4.5 compound to a few thousand angstroms in longer period structures. These solids seem to belong to the class of infinitely adaptive structures. There is a family of oxides of the general formula Bi@A,1B,03,3, discovered by Aurivillius, in which the perovskite slabs, (An-1Bn03n+1)2-,n octahedra thick, are interleaved by (Bi202)2+layers (Fig. 4). Typical members of this family are Bi2W06 (n = l), Bi3Til,Wo.,0, (n = 2), Bi4Ti3CrO12(n = 3), and Bi,Ti3Cro15 (n = 4), andtheyhavebeeninvestigatedindetailby HREM. These oxides form intergrowth structure of the general formula Bi4Arn+&3,,, +n03(rn+fl)+6; involving alternate stacking of two Aurivillius oxides with different n values; the method of preparation' simply involves heating the compo-

0 0

eo 0

eo 0 0

Bi7M5021

Bi9M7027

"

Figure 4 The f i i t three members of a homologous series structures of Bi4Am+,&lm+f103~m+fl~4 formed by the Aurivillius family of oxides, where A is also takento be Bi. Bi cations are shown as filled circles and oxide ions as open circles. BO, groups are shown in polyhedral form.

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nent oxides or carbonates of metals. Ordered intergrowth structures with (m,n) values of (1, 2), (2, 3), and (3, 4) have been fully characterized by x-ray diffraction and HREM. What is most amazing is that such intergrowth structures with long-range order are indeed formed, although either member (mor n) can exist as a stable entity. These materials seem to be truly representative of recurrent ordered intergrowth. The periodicity found in the recurrent intergrowth solids formed by the Aurivillius family of oxides is indeed impressive. The relative ease withwhich WO, forms tetragonal, hexagonal, or perovskite-type bronzes by interaction with alkali and other metals is well known. The new family ofintergrowthtungsten bronzes (ITB) involving the intergrowth of nWO, slabs and one to three strips of the hexagonal tungsten bronze (HTB) is of relevance to our discussion here. In these intergrowth tungsten bronzes of the general formula MxW03, x is generally 0.1 or less, and depending on whether the HTB strip is one or two tunnels wide, ITB are classfied as belonging to (0, n) or (1, n) series (Fig. 5). HTB strips of two-tunnel width seem to be most stable in ITB, and many ordered sequences of the (0, n) and the (1, n) series have been identified. Recently, ITB phases of Bi have been characterized, and in this system the HTB strips are always one tunnel wide, Displacement of adjacent tunnel rows caused by the tilting of WO, octahedra often results in doubling of the long-period axis of the ITB. Evidence for the ordering of the intercalating Bi atoms in the tunnels has been found in terms of satellites around the superlattice spots in the electron diffraction patterns. Among the other systems exhibiting ordered intergrowth, special mention must be made of hexagonal barium femtes MpYq (M = BaFeI2Ol9 and Y = Ba2Me,2022, where Me is Zn, Ni, Mg, and so on). A large number of intergrowth structures of this family have been identified.

X.

SUPERCONDUCTING CUPRATES

The discovery of a superconducting cuprate with a T, above 77 K created a sensation in early 1987. Wu et al., who announced this discovery first, made measurements on a mixture of oxides containing Y, Ba, and Cu. In this laboratory, we independently worked on the Y-Ba-Cu-0 system on the basis of solid-state chemistry [15]. We knew that Y,Cu04 could not be made and that substituting Y with Ba in this cuprate was not the way to proceed (unlike in La2-€!axCu04). We therefore tried to make Y3Ba3Cu5OI4by analogy with the known La3Ba3Cu6OI4and varied the Y/Ba ratio as in Y3,rBa3+xC~6014. When x = 1, we obtained YBa2Cu3 (T, 90 K). We knew the structure had to be that of a defect perovskite from the beginning, because of the route we followed for the synthesis. We briefly examine some preparative aspects of the various types of cuprate superconductors [16]. The cuprates are ordinarily made by the traditional ce2:

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71

I I ,6)

Figure 5 The (1, n) intergrowth tungsten bronzes. Hexagonal tunnels of HTB strips separate the WO, slabs shown in polyhedral form. (After Kihlborg, 1979.)

ramic method (mix, grind, and heat), which involves thorough mixing of the various oxides and/or carbonates (or any other salt) in the desired proportion and heating the mixture (preferably in pellet form) at a high temperature. The mixture is ground again after some time and reheated until the desired product is formed, as indicated by x-ray diffraction. This method may not always yield the product with the desired structure, purity, or oxygen stoichiometry. Variants of this method are often employed. For example, decomposing a mixture of nitrates has been found to yield a better product in the 123 compounds of the type YBa2Cu307 by some workers; some others prefer to use BaO, in place of BaC03 for the synthesis. The sol-gel method has been conveniently employed for the synthesis of 123 compounds, such as YBa2Cu30, and other cuprates. The sol-gel method

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provides a homogeneous dispersion of the various component metals when a solution containing the metal ions is transformed into a gel by adding an organic solvent, such as glycol or an alcohol, often in the presence of other chemicals, such as organic amines. The gel is then decomposed at relatively low temperatures to obtain the desired oxide, generally in fine particulate form. Materials prepared by such low-temperature methods may need to be annealed or heated under suitable conditions to obtain the desired oxygen stoichiometry as well as the characteristic high T,. A total of 124 cuprates of the type YBa2Cu,08, lead cuprates, and bismuth cuprates have all been made by this method; the first two are particularly difficult to make by the ceramic method. Coprecipitation of all the cations in the form of a sparingly soluble salt, such as carbonate, ina proper medium, followed by thermal decomposition, has been employed by many workers to prepare cuprates. One of the problems with the bismuth cuprates is the difficulty in obtaining phasic purity (minimizing the intergrowth of the different layered phases). The glass or the melt route has been employed to obtain better samples. The method involves preparing a glass by quenching the melt; the glass is then crystallized by heating it above the crystallization temperature. Thallium cuprates are best prepared in sealed tubes (gold or silver). Heating T1203 with a matrix of the other oxides (already heated to 1100-1200 K) in a sealed tube is preferred by some workers. It is important that thalliumcuprates are not prepared in open furnaces since T1203, which readily sublimes, is highly toxic. To obtain superconducting compositions corresponding to a particular copper content (number of Cu02 sheets) by the ceramic method, one often must start with various arbitrary compositions, especially with the T1 cuprates. The real composition of a bismuth or a thallium cuprate superconductor is not likely to be anywhere near the starting composition. The actual composition can be determined by analytical electron microscopy and other methods. Heating oxidic materials under high oxygen pressures or in flowing oxygen often becomesnecessary to attain the desired oxygen stoichiometry. Thus h 2 C u 0 4 and La&a,-,Sr,Cu2O6 heated under high oxygen pressure become superconducting,with T, of 40 and 60 K, respectively.With the 123 compounds, one of the problems is that it loses oxygen easily. It therefore becomes necessary to heat the material in an oxygen atmosphere at an appropriate temperature below the orthorhombic-tetragonal transition temperature. Oxygen s t 6 ichiometry is not a problem with the bismuth cuprates, however. The 124 superconductorswere first preparedunder high oxygenpressures. It was later found that heating the oxide or nitrate mixture in the presence of N%02 in flowing oxygen is sufficient to obtain 124 compounds.Superconducting Pb cuprates, on the other hand, can only be prepared in presence of very little oxygen(N2witha small percentage of 02).For the electron superconductor Nd2-xCexCu04, it is necessary to heat the material in an oxygen-deficient at-

CHEMICAL SYNTHESIS OF METAL OXIDE POWDERS

73

mosphere; otherwise, the electron given by Ce merely gives an oxygen-excess material. It may be best to prepare Nd2_,Ce,Cu04 by a suitable method (say, decomposition of mixed oxalates or nitrates) and then reduce it with hydrogen. Many thallium cuprates, as prepared by the sealed tube method, have excess oxygen; they become superconducting only on heating in vacuum or hydrogen. Several other novel strategies have been employed for the synthesis of superconducting cuprates. For example, a Eu-Ba-Cu alloy precursor has been oxidized to obtain EuBa$u,O, [17], and hyponitrite precursor has been employed to prepare YBa,Cu,O, [18]. More interesting in the synthetic strategies are those in which structural and bonding considerations are involved in the synthesis. One such example is the synthesis of modulation-free superconducting bismuth cuprates [19]. Superconducting bismuth cuprates, such as Bi2CaSr2Cu208, exhibit superlattice modulation. Since such modulationhad something to do with the oxygen content in the Bi-0 layers and lattice mismatch, Bi3+ was substituted partly by Pb2+ to eliminate the modulation without losing the superconductivity.

XI.

CONCLUDINGREMARKS

The area of oxide synthesis has become trulyextensive, with newertypes of materials being prepared everyday by employing a variety of novel methods [20]. We have touched on only some of the important methods, and there are many more. For example, we have not discussed the commonly employed coprecipitation method (involving the simultaneous precipitation of the cations as carbonates and oxalates, for example) followed by decomposition or the sol-gel method. This method does not always give precursor solid solutions, but coprecipitated materials on decomposition readily give the desired oxides. Another novel method of preparing oxide powders is by the combustion method, involving the spontaneous ignition of a mixture of metal nitrates in presence of a fuel, such as urea or glycine [20]. This method can be employed to prepare powders of most oxides, including dielectrics and superconductors. The use of templatemolecules to synthesizeporous solids involving silicates andphosphates is noteworthy. The ingenuity with which properties of oxides are modified drastically by appropriate substitutions or by the modification of the structure forms an important part of synthetic strategies. Other methods of interest are those involving fine particles, vapor transport, and electrochemical methods

1201.

ACKNOWLEDGMENT The author thanks the Department of Science and Technology and the Indo-European Economic Community collaborative program for support.

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REFERENCES 1. Rao, C. N. R., and Gopalakrishnan, J., New Directions in Solid State Chemistry, Cambridge University Press, London, 1989. 2. Rao, C. N. R., and Gopalakrishnan, J., Acc. Chem. Res., 20, 228 (1987). 3. Roy, R., Solid State lonics, 32133, 3, (1989). 4. Torardi, C. C., and McCarley, R. E.,J. Am. Chem. Soc., 101, 3963, (1979). 5. Giroult, J. P., Goreand, M., Labbe, P. H., and Raveau B., Acta Crystallogr., 1336, 2570, (1980). . 6. Vidyasagar, K., Gopalakrishnan, J., and Rao, C.N.R., Inorg. Chem., 23, 1206, (1984). 7. Rao, C. N. R., Gopalakrishnan, J., Vidyasagar, K., Ganguli, A. K., and Ramanan, A.,J. Mater. Res., 1, 280 (1986). 8. Vidyasagar, K., Gopalakrishnan, J., and Rao, C. N. R., J . Solid State Chem., 58, 29 (1985). 9. Vidyasagar, K., Reller, A., Gopalakrishnan, J., and Rao, C. N. R., J. Chem. Soc. Chem. Commun., 7 (1985). 10. Ganapathi, L., Ramanan, A., Gopalakrishnan, J., and Rao, C. N. R., J. Chem. Soc. Chem. Commun., 62 (1986). 11. Ganguli, A. K., Gopalakrishnan, J., and Rao, C. N. R., J. Solid State Chem., 74, 228 (1988). 12. Ham, W. K., Holland, G. F., and Stachy, A. M., J . Am. Chem. Soc., 110, 5214 (1988). 13. Schneemeyer, L. F., Thomas, J. K., and Siegrist, T., Nature, 335, 421 (1988). 14. Rao, C.N.R., and Thomas, J. M., Acc. Chem. Res., 18, 113 (1985). 15. Rao, C. N. R., Ganguly, P., Raychaudhuri, A. K., Mohan Ram, R. A., and Sreedhar, K.,Nature, 326, 856 (1987). 16. Rao, C. N.R., Nagarajan, R., and Vijayaraghavan, R., Supercond. Sci. Technol., 6, 1(1992). 17. Matsuzaki, K., Inone, A., Kimura, H., Aoki, K., and Masumoto, K.,Jpn. J . App. Phys., 26, L1310 (1987). 18. Horowitz, H. S., Mclain, S. J., Sleight, A. W., Druliner, J. D., Gai, P. L.,Van Kavelaar, M. J., Wagner, J. L., Biggs, B. D., and Poon, S. J., Science, 243, 66 (1989). 19. Manivannan, V., Gopalakrishnan, J., and Rao, C.N.R., Phys. Rev., B.43, 8686 (1991). 20. Rao, C.N.R., Mater. Sci. Eng., in print (1992).

4 Multicomponent Ceramic Powders T. Mah UES,Inc., Dayton, Ohio

E. E. Hermes Wright Paterson Air Force Base, Ohio

K. S. Mazdiyasni General Atomics, San Diego, California

1.

INTRODUCTION

The chemical processing of ceramics, especially ceramic powder syntheses, has drawn a considerable amount of attention over the past two decades. The reason for this is the demand for reliable and advanced ceramic components for high-performance applications. This increased interest in the chemical synthesis of ceramic powders is illustrated by the vast number of publications and ongoing investigations [ 1-91. This chapter focuses only on the syntheses of oxide ceramic powders. The precursors of these powders are metal-organic compounds, mainly metal alkoxides. The different varieties of ceramic powders synthesized by mixed metal alkoxide precursors are the focal points of this chapter. Numerous references on the fundamental principles of sol-gel processing are available in the literature and within other chapters of this book [10-12]. This chapter is divided into three major sections: (1) metal alkoxide precursor synthesis methods used by the present authors, (2) silicate powder syntheses, and (3) nonsilicate powder syntheses.

II. PRECURSOR SYNTHESIS In this section, only the following synthesis methods are described ammonia, metal/alcohol, metal halide/alcohol, ester exchange, and alcohol exchange. De75

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tailed synthesis techniques for metal alkoxides can be found in Metal Alkoxides by Bradley et al. [ 101.

A.

Ammonia Method

The simplest andmost economic method for the large-scale productionof alkoxides involves the addition of anhydrous metal halides to a mixture of anhydrous alcohol in a diluent (benzene, n-hexane, or toluene) in the presence of anhydrous ammonia [13,14]:

whereM is many of the transition metals, such as titanium, zirconium, hafnium, tantalum, and niobium, and R is the organic group. The transition metal tetrakis-isopropoxide is readily purified by fractional distillation or recrystallization. The removal of NH4Cl by filtration is always cumbersome and time consuming: the ammonia method may be carried out in the presence of amides or nitriles. For this particular method, the metal alkoxide separates out as the upper layer, and the ammonium chloride remains in the solution of the amide or nitile at the lower layer; thus, the filtration step is eliminated [13,15].

B.

Metal/Alcohol Reaction Method

The alkoxides of some metals of interest, such as aluminum, yttrium, and the rare earths, can be made by reaction with alcohol using a reactioncatalyst (e.g., HgC12 and HgI2) [16-181: M + ROH

HgC12 > M(OR), exothermic

+ EH2 2

where n is the valence of metal M, and R is the organic group. Most alkali or alkaline-earth metal alkoxides can be prepared by this reaction without using reaction catalysts.

C.

Metal Halide/AlcoholReaction Method

A typical example of this method is the synthesis of various alkoxides of sili-

con [19]: S i c 4 + 4ROH

exothermic

'Si(OR)4 + 4HC1

where R is the ethyl, n-propyl, isopropyl, and so on, group.

MULTICOMPONENT CERAMIC POWDERS

D.

77

EsterExchangeReaction

A valuable method for converting one alkoxide to another is an ester exchange reaction [20]. This method is particularly suited for the preparation of the tertiary butoxide from the isopropoxideand r-butyl acetate. The reaction is as follows:

M(OR)4 + 4R’OOCCH3 _ _ j M(OR’)4 + 4ROOCCH3 where M is zirconium, titanium, hafnium, and thorium, for example, R is the isopropyl group, and R’ is the tert-butyl group. Since there is a large difference between the boiling points of the esters, the fractionation is simple and quite rapid. Another advantage appears to be the lower rate of oxidation of the esters compared with that of alcohol.

E.AlcoholExchangeReaction Substitution of other branched R groups with the lower straight-chain alcohols has been carried out in an alcohol interchange reaction [lo]:

M(OR)4 + 4R‘OH-

M(OR‘)4 + 4ROH

The distillation of azeotrope drives the reaction to completion. Compounds prepared by this method [21] include the secondary pentoxides, secondary hexoxides, secondary heptoxides, tertiary butoxides, tertiary heptoxides of transition metals, such as yttrium, and also rare earth elements. Increasing the molecular weight and branching of the attached organic group result in the rise in the melting point and stability toward hydrolysis and thermal decomposition. These characteristics are very important in controlling the hydrolysis of multicomponent mixed alkoxides.

111.

ALKOXY-DERIVEDCERAMICPOWDERS

The oxide ceramic powders produced through mixed-metal alkoxides have many advantages over powders prepared conventionally; some of the advantages are lower temperature processes, higher purity, more homogeneous distribution of constituents, finer particle size, and easier compositional alteration. Most metal alkoxides exposed to moisture and/or heat cause decomposition of the alkoxide and thus provide forming methods for fine ceramic powders (notable examples for decomposition are thermal decomposition and hydrolytic decomposition). Mazdiyasni[22,23] reported the direct pyrolysis of the metal alkoxides, which form very fine ceramic powders. The overall thermal decomposition of a liquid zirconium tertiary butoxide to ZrO2 is as follows: Zr(OR)4

*

S

ZrO2 + 2ROH + olefin

MAH ET AL.

78

The thermal decomposition method is very rapid, and the products formed are volatile olefins, alcohols, and fine ceramic powders. The hydrolytic decomposition of metal alkoxides, with subsequent dehydration, has been used to form many types of fine ceramic powders. The decomposition of the metal-organic is initiated via the hydrolytic reaction of water, with subsequent thermal dehydration of the resulting precipitates. The general decomposition is described as a two-step process:

M(OR), M(OH),

-

+ nH2O +M(OH), + nR(0H) MO,/;! + "nH 2 0 2

hydrolysis

dehydration

The hydrolysis reaction usually occurs at room temperature and dehydration occurs below 600°C. resulting in the formation of very fine ceramic particles, that is, 2-5 nm [22]. This method has been successfully used to make high-purity submicrometer-sized oxides from several metal alkoxides [23,24]. Focus on multicomponent oxide powder synthesis through the two-step hydrolysis and dehydration of metal alkoxides constitutes the remainder of this chapter. When mixtures of alkoxides are hydrolyzed, the different hydrolysis and polycondensation rates of each individual alkoxide can cause complications. These complications may lead to local inhomogeneities. If one species is hydrolyzed faster than the other, which is almost always the case, then differential precipitation occurs during hydrolysis, leaving the other species substantially unreacted. Examples of these are found in the following sections of this chapter. The classes of ceramic powder are divided into two categories (silicates and nonsilicates) and are discussed separately in the following sections.

A.

Silicate Powders

Within the category of silicate powders there are two different varieties: (1) glass and/or glass-ceramics and (2) crystalline. The first variety includes various compositions of glasses and glass-ceramics (e.g., lithium aluminosilicate and magnesium aluminosilicate), and mullite and zircon are typical examples of silicates that belong to the latter variety. 1. Multicomponent Silicate Glass and Glass-Ceramics Hydrolyses of multicomponent metal alkoxides were reported by Dislich [25,26] for silicate glass and by the present authors [27,28] for glass-ceramics. Dislich [25] synthesized the eight-component glass through a complexation of mixed alkoxides, followed by hydrolysis andcondensation. The processing flowchart for the synthesis of magnesium aluminosilicate (MAS) (with Li, Zr, and Nb) glass-ceramic oxide powder, hydroxides, and their mixtures for ce-

MULTICOMPONENT CERAMIC POWDERS

79

ramic matrix composite matrix material is shown in Fig. 1 [27]. Synthesis of the individual alkoxides was accomplished using one of the appropriate techniques previously described. The individual alkoxides combined into proportions to yield the desired oxide composition. Solid alkoxides, such as Mg(OC2H5)2 and AI(OC3H7)3,are mutually soluble in liquid alkoxide, such as Si(OCzH5)4. As the metal alkoxides are combined together and refluxed, the possibility exists that some of the alkoxides may form complex mutimetal-organic ligands distributed evenly throughout the solution. Within these mixedmetal alkoxides, the formation of double alkoxide between Mg(OCzH5)z and AI(OC3H7)3 is a strong possibility. In fact, the existence of the double alkoxide, MgA12(0R)g, is well known [10,29], indicating that the distribution of the multiple-metal hydroxides, in the solution after hydrolysis, may not be completely uniform at the molecular scale as a result of the preferential hydrolysis of some metal alkoxides and/or metal-alkoxide complexes. Although it has been documented [30] that the MgA12(OR)g double alkoxide does not break down to its constituents during hydrolysis, the resulting multicomponent metal hydroxides should have, at minimum, a uniform compositional distribution.

2. Multicomponent Crystalline Silicates There are a vast number of publications about mullite (3A1203*2Si02), ranging from synthesis and processing to its applications andproperties [31,32]. The interest and importance of mullite as a candidate material for various applications has been manifest by the number of symposiums on the topic. Hy-

Figure 1

Processingflowchart for thesynthesis of magnesiumaluminosilicate.

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MAH ET AL..

drolytic decomposition of the mixed-metal alkoxide route to synthesize stoichiometric mullite powder was carried out by one of the present authors approximatelytwo decades ago [33].Aluminum tris-isopropoxide and silicon tetrakis-isopropoxide were synthesized. The mixed alkoxides were refluxed in excess isopropyl alcohol for 16 h before hydrolysis to ensure thorough mixing. The hydroxyaluminosilicate was prepared by slowly adding the alkoxide solution to ammoniated triply distilled deionized water according to the reaction 6Al(OC3H7)3 + 2Si(OC3H7)4 + xH20 H20

+

NH3

> 2A13Si(OH)l3 .xHz0 + 26C3H7OH

The resulting hydroxyaluminosilicate was repeatedly washedwith propyl alcohol and dried in vacuum at 60°C for 16 h: vacuum 2A13Si(OH)13 F 3&03 60°C

*

dry iso-

2Si02 + 13H20

At this stage of preparation, the mixed oxide was amorphous to x-ray diffraction. Transmission electron microscopy ("EM) photomicrographs of the as-prepared powders, calcined at 600°C statically for 1 and 24 h and dynamically for 24 h, are shown in Fig. 2. In Fig. 2a, needlelike crystallites of the very fine asprepared particulates are evident. The crystallite growth (agglomeration of small particles followed by rapid growth) occurs during the calcination process, leading to very large but well-defined acicular or prismatic particulates [(Fig. 2b-d)l. The as-prepared powders are extremely active, with a surface area of -550 m2/g; however, the surface area is reduced to 280 m2/g when the powders are calcined at 600°C for 1 h. High-temperature x-ray diffraction (m) studies showed that the crystallization of stoichiometric mullite took place between 1185 and 1200°C. However, the previously mentioned hydrolytic reaction is very sensitive to experimental conditions, such as the presence of acid or base catalysts, reaction temperature, and molar ratio of alkoxides to H2O. Three critical experimental parameters (pH, temperature, and reaction time) that affected the rate of hydrolysis were systematically studied by Paulick et al. (34). The volume ratio of 100 ml mixed alkoxide [Al(OC3H7)3 and tetraethylorthosilicate (TEOS)] solution to 500 m1of an H20/CH30H (3:l ratio) solution was used for all the experiments. It was found that in an acid environment (pH 2) the hydrolytic decomposition was completed within 7 h at room temperature. Stoichiometric mullite was the only product obtained, as evidenced by both elemental analysis and x-ray diffraction. Under alkaline conditions (pH lo), the degree of hydrolytic decomposition was both time and temperature dependent. The TEOS was only partially hydrolyzed even after heating at 70°C for 20 h. The oxide powder obtained was composed of 65% A1203 and 35% mullite. The degree of hydrolysis is only moderately affected by temperature, and it is not affected by the reaction time.

MULTICOMPONENT CERAMIC POWDERS

81

The difference between acid-versus base-catalyzed hydrolysis of mixed TEOS and Al(OC3H7)3 can be accounted for by the different reaction mechanisms of TEOS hydrolysis [l l]. Nucleophilic hydrolysis and condensation in alkaline solutions tend to produce highly cross-linked species, which may not be completely hydrolyzed. In contrast, in acid solution, the electrophilic reaction mechanism favors the production of weakly cross-linked species that tend to be completely hydrolyzed [35]. The specific surface area of the as-calcined mullite powder obtained under acid hydrolysis conditions is 197 m2/g. Clusters of particles develop during concentration of the hydroxide slurry. These particles polymerize on further drying, causing bound -OHgroups in the clusters, thereby resulting in hard agglomerates instead of fine powder. One can make a compromise through the partial hydrolysis of TEOS [36-391 and then add AI(OC3H7)3 to accomplish complete hydrolysis and yet achieve fine powder sizes.

B.

Nonsilicate Powders

A few varieties of nonsilicate oxide ceramic powderssynthesizedthrough

alkoxide processing are presented here. These oxide powders were developed for the use of electronic, optical, and high-temperature structural applications. For each material, we start with a brief description of the synthesis, which is followed by powder characteristics (e.g., particle sizes, morphologies, and size distributions) and densification behavior andsome properties of dense material. 1. Spinel The precursor alkoxide syntheses for spinel (MgA1204) were carried out through the metal/alcohol reaction methodusing HgC12 as acatalyst. This methodwas described in the previous section. The two metal alkoxides, Al(OC3H7)3 and Mg(OC2H5)2, were mixed together and gently heated in isopropyl alcohol until solid Mg(OC2H5)2 was completely dissolved and reacted, producing a clear solution, and then allowed to cool to room temperature [34]. During the refluxing process, magnesium and aluminum double alkoxide was formed, The formation and structural characterization of the MgA12(0R)8 double alkoxide can be found in references [10,29,30]. The double alkoxide was readily hydrolyzed by water, without acid or base catalyst. No differential hydrolytic decomposition of mixed alkoxide was found in this system to hinder the formation of ultrafine homogenous spinel powder. The XRD analysis (Fig. 3) of the ultrafine powder, obtained after calcination at 550"C, shows amorphous characteristics. The powder was calcined at 1000°C for 1 h, and XRD analysis showed broad peaks of spinel, which is a characteristic diffraction pattern for fine particles. Distinct, sharp XRD peaks of spinel can be observed after the powder is hot pressed at 1550°C. Elemental analysis of the powder calcined at 550°C gave a 1:l MgO/A1203 ratio. Emission spectrographic analysis of the as-calcined powder indicated that no measurable impurities were pre-

Figure 2 Transmissionelectronphotomicrographs of mullitepowders (a) as-prepared, (b) calcined at 6OOOC for 1 h, (c) calcined for 24 h with tumbling, and (d) calcined for 24 h without tumbling.

MULTICOMPONENT CERAMIC POWDERS

83

84

MAH ET AL.

MULTICOMPONENT CERAMIC POWDERS

85

sent. The surface area of the as-calcined powder was 260 m2/g, which corresponds to an average particle size of 6.5 nm. The transmission electron photomicrograph of the as-calcined powder (Fig. 4) showed the particle size to be of the order of 10 nm, which is in agreement with the calculated value. The average particle size of the powder, heat treated at 1000°C for 1 h, increased to about 30 nm, and also the particles were faceted. 2. Electronic Ceramics A few multicomponent electronic ceramics, fabricated through metal alkoxide processing, are described in this section. PZT and PLZT. The development of polycrystalline lead lanthanum zirconate-titanate (PLZT) electronic ceramic monoliths, which fully transmit incident light, requires methods for controlling stoichiometry, impurity content, porosity, grain size, and so on. Alkoxy-derived PLZT powders were prepared by hydrolytic decomposition of mixed-metal alkoxides [40]. The Zrand Ti alkoxides were synthesized by the ammonia method, and the lanthanum trisisopropoxide was synthesized by themetal/alcoholreaction method, which were described in the previous section. The Pb alkoxide was prepared by the reaction of anhydrous lead acetate, Pb(C2H302)2, with sodium isoamyloxide, NaOCsHl I [40]: Pb(C2H302)2 + 2NaOR

-

Pb(OR)2 + 2NaC2H302

where R is the isoamyl group. The solvated lead alkoxide solution was filtered to a clear, colorless solution. PLZT powder is typically prepared by dissolving the mixed-metal alkoxides in a mutual solvent, such as isoamyl alcohol, followed by hydrolytic decomposition to yield a nominal zirconate-titanate molar ratio of 65:35 containing 10atom%La. The preferred general formula of Pb1-~Lax(Zr,Ti~)1-~/403 is referred to in the literature[41,42]. The resulting hydroxide was washed repeatedly with high-purity water and then with isopropanol. The washed hydroxide was dried undervacuum at 60°C toyield white amorphous powder. The as-preparedPLZTpowderswere calcined at 500°C for 30 minutes to 1 h and then ground in a B4C mortar to break the large agglomerates. TEM photomicrographs of the powder calcined at 500°C for 15 min~tesand 1 h is shown in Fig. 5. The cubic symmetry of the material is quite evident; the electron diffraction patterns indicate a cubic structure, which is in agreement with the X R D analysis. Because of the rapid outward diffusion of PbO in this system, calcination at a higher temperature and for a longer time at 500°C invariably resulted in massive agglomeration or partial sintering of the particle. Thermograms (TG) for the as-prepared PLZT powders are shown in Fig. 6. The TG of the powders, unwashed and washed withisopropyl alcohol, was run inan ambient atmosphere from room temperature to 1000°C. Both types of

86

MAHETAL..

Figure 4 Transmission electron micrographsof dispersed spinel powder calcined at (a) 550°C and (b) 1000°C inair for 1 h.

Figure 5 Transmissionelectronmicrographs 500°C for (a) 15 minutes and (b) 1h.

of PLZT particulatescalcinedat

88

MAHETAL.

powder showed an initial 5% weight loss from 70 to 11O"C, caused mostly by the loss of alcohol. The additional weight loss observed within this temperature range is attributed to the loss of surface-absorbed water. The isopropanol washing was effective in removing this water, as evidenced by a difference of -15% in the weight loss in this region. Continued weight loss occurs as the temperature is increased and the remaining carbonaceous material is removed. The differential thermal analysis (DTA) curve for the as-prepared PLZT powder is also included in Fig. 6. Dehydration of the powder results in the endotherm observed at 110°C. The exothermic peak at -340°C is attributed to the nucleation and growth of the very fine as-prepared particles to large crystallites. No further peaks were observed while heating to 1200°C. When the sample was cooled and rerun, no additional peaks were noted, indicating retention of the crystalline phase as formed. The microstructure, typical of a PLZT body prepared by cold pressing and sinteringunderthestated experimental conditions, is showninFig.7a;the compact was thermally etched at 800°C for 30 minutes. The fine-grained microstructure is quite uniform, with internal and grain boundary porosity virtually nonexistent. Bodies fabricated similarly but sintered at higher temperatures

30

r

I

252 5.

S

,i

-

200 . 2 v) v)

'15 'c1 5 c I

-

_"

DTA

!2

G

I

/

K L IO O -

0/ 0

"_"". ~/clltormc-w*w€o

L

TGA

O

I

W

1

l

1

I

; 0TEMPERATURE 2 b o 2('c) ;0&&4bosso TG

Figure 6

Bzz

R

S

and

DTA of Alkoxy-derived PLZT

TG and DTA of akoxy-derived PLZT powder.

I

I

MULTICOMPONENT CERAMIC POWDERS

89

(W Figure 7 Scanning electron microscopy (SEM):typical microstructure of PLZT prepared by cold pressing and sintering at (a) 1120OC for 8 h and (b) 1220°C for 4 h.

90

MAHETAL.

(<1200"C), exhibited a microstructure with a much larger grain size (Fig. 7b) and no significant improvement in optical quality. Titanates. Various types of titanate powders, both doped and undoped, were synthesized through the simultaneous hydrolytic decomposition of appropriate metal alkoxides. One of the important concerns for the development of ferroelectric materials, such as BaTiO3, are the impurities. Although the presence of such impurities may not be a priori adverse, it is important to understand their influence on the final properties or to eliminate them completely, if necessary. The metal alkoxide processing is appropriate tosatisfy these purposes by controlled doping of certain elements to ultrapure material. Stoichiometric highpurity submicrometer BaTi03 powders were prepared through a mixed-metal alkoxide process[43]. Titanium tetrakis-tertiary amyloxide wasprepared through the alcohol exchange method:

-

T i c 4 + 4C3H70H + 4NH3 Ti(OC3H7)4 + 4CsH110H

C6H6 3 Ti(OC3H7)4 + 4 N h C 1 5°C

reflux 24 h

Ti(OCsH11)4+ 4C3H7OH

The barium bis-isopropoxide waspreparedby the metal/alcohol reaction method. Appropriate amounts of these alkoxides were dissolved in a mutual solvent, such as isopropyl alcohol or benzene, for a barium and titanium molar ratio of 1:1. The solution was refluxed for 2 h with vigorous stirring before the hydrolysis reaction. Drops of deionized triply distilled water were slowly added to the solution, which was continuously stirred. The reaction was carried out in a COzfree atmosphere. The hydrated oxide was dried in vacuum or in a dry helium atmosphere at 50°C for 12 h. At this stage the oxide was a finely divided, stoichiometric titanate with 50-150 A (maximum agglomerate size
I

Figure 8 Transmission electron micrographs of BaTi03 powder (a) as prepared and (b) calcined at 700°C.

92

MAH

W

W

P A R T I C L E diameter, ('A)

Figure 9

Particle size distribution of high-purityBaTiO3.

Figure 10 SEM microstructure of deeplyetchedBaTi03.

ETAL.

MULTICOMPONENT CERAMIC POWDERS

93

type of material. The synthesis of BaTiO3 powders doped with various rare earth oxides (e.g., La203, Nd203, and Sc2O3) has been reported [44,45]. The advantage of rare earth element doping to BaTiOs is that the grain size can be reduced considerably. A TEiM replica photomicrograph (Fig. 11) of h 2 0 3 doped BaTiOs shows the absence of pores withinthe grains andthe grain boundaries. The microstructure shows a uniform fine grain size (1-1.5 pm). Furthermore, no segregation of a second phase or impurities are visible along the grain boundaries. However, the Sc203-doped BaTiOs specimens were of relativelylowdensity (5.4 g/cm3)and contained a second-phase dendritic structure with very small bubbles characteristic of a glassy phase. 3. Yttria-Stabilized Zirconia and Hafnia

XRD analysis over the temperature range 250-120O0C, upon heating the submicrometer ZrO2 powders synthesized through hydrolytic decomposition of Zr

Figure 11 Electronmicrograph of LazO3-doped BaTi03, cold pressedandsintered at 1300°C for 4 h in air.

94

MAHETAL.

alkoxide, has been reported [46,47]. The following sequence of phases was observed as the temperature increased to 1200°C: amorphous, to metastable cubic, to metastable tetragonal, to tetragonal + monoclinic, to monoclinic + tetragonal, and finally to tetragonal. Calcination studies were undertaken to observe the growth of primary crystallites into large particles and the influence upon growth of tumbling in different atmospheres, such as air, oxygen, hydrogen, and vacuum [46-48]. Figures 12 and 13 are typical electron micrographs' showing, the effect of calcination at 500°C in air for 24 h without tumbling and calcination at 750°C in air for 24 h with tumbling, respectively. Tumbling is seen to be an effective method of controlling crystallite growth; the atmospheric conditions have a negligible effect during the presintering stage of processing these powders. Similar experiments were carried out for Hf02[49], and DTA analysis showed the nucleation and growth of small crystallites at 420"C, leading to conversion to monoclinic HfO;! at 480°C. Y203-stabilized (6-7 mol%) ZIQ and HfO2 powder production and sinteringstudieshavebeencarriedout[50,51]. Zirconium, hafnium,and yttrium alkoxides were synthesized using one of the appropriate methods described previously. Hydrolytic decomposition followed by similar procedures have already been described. The typical particle size distribution of of Y2Os-stabilized (7 mol%) Hf02 after 24 h calcination at 750°C is shown in Fig. 14. The crystallite size was measured directly from the electron photomicrographs. The powder had a mean particle size of -30 nm; 90% of the particles were less than 35 nm in diameter.

Figure 12 Transmissionelectronmicrograph of ZrO2 powder calcined at 500°C in air for 24 h without tumbling.

MULTICOMPONENT CERAMIC POWDERS

'0.1 A

m

95

,

Figure 13 Transmission electron micrograph of ZrO2powder calcined at 750°C in air for 24 h with tumbling.

Rhodes and Haag [52] studied the particle size distribution of Y2Og-stabilized (6 mol%) ZrO2,and typical cumulative particle size distribution curves for two different batches of powders are shown in Fig. 15, which is a common

96

MAHETAL.. 9991 99.8

-

99 98

80

-

60 50 40 3070

v)

0 W

t-

4 lv)

z

4

K

W

-I -1

4

I I I I I

LOT 4 5 - 6 5 -

F

LOT44-6.5-550

20-

I

l-

-I

-

90-

N

I

-

95-

W

I

IO

-

5-

I U)

;r

2I-

0.20.10.050.01a005 0.5

Figure 15 Particle size distribution of 6.5 mol% Y203-ZrOz "zyttrite."

characteristic of most ceramic powders prepared by this technique. The effect of calcination temperature on the specific surface area (measuredby the Brunauer-Emmett-Teller (BET) method) of the Y203-stabilized Hf02 is shown in Fig. 16. The as-prepared powders are extremely active, with surface areas of 210 m2/g. The surface area decreased rapidly up to 40O0C, mainly because

MULTICOMPONENT CERAMIC POWDERS

97

of the nucleation and growth of thefine particles to large crystallites. From 400 to 750"C, the surface area decreased only slightly to 120 m2/g. Above 800"C, the surface area decreased rapidly with increasing temperature to 10 m2/g at 1200°C. This curve suggests that a calcination temperature of 750°C is desirable to retain a relatively highly surface-active powder for the best sintering results. Figures 17 and 18 show the microstructures of a ZrO2 with 6 mol% Y2O3 (sintered at 1500°C for 3 h) and Hf@ with 7 mol% Y203 (sintered at 1650°C for 4 h). The electron photomicrographs show negligible internal porosity and insignificant porosity at the grain boundaries. These materials sintered to the excess of 95% of theoretical densities in a matter of few minutes at several hundred degrees lower thannecessary for the conventional ceramic powder routes. Both materials are fine grained, and the grain sizes ranged from 2 to 5 pm. Such grain sizes and microstructures ordinarily cannot be obtained for oxides as refractory as ZrO2 or HfO2, which are typically sintered at temperatures ranging from 1800 to 2200°C and firing times of 24-48 h or longer. Such a conventional sintering process makes achivement of low porosity concomitant while maintaining a fine, uniform grain size extremely difficult.

98

MAHETAL.

i

l

Figure 17

Electron micrograph (replica ' E M ) of cold-pressed and sintered 6 mol%

Y203-zr02.

4. Y3A1501-2 (Yttrium Aluminum Garnet, YAG)

Numerous reports [53-62] regarding the synthesis of YAG have appeared in the literature, and in most cases the final product was either a mixture of various yttrium aluminate phases or stoichiometric YAG of undesirably large particle size. The phase inhomogeneity in the final product, through alkoxide processing, was caused by the formation of a Y-A1 double alkoxide, which is a very stable compound and, hence, extremely resistant to hydrolysis [63]. For the Y and A1 isopropoxides, the two react to form a stable Y-A1 double isopropoxide according to the reaction Y(OiPr)3 + 3Al(OiPr)3

-

Y[Al(OiPr)&

The formation of this double alkoxide further complicates the precursor chemistryinvolved in the synthesis. Since Y and A1 isopropoxides are initially mixed in a ratio corresponding to the stoichiometry of YAG ( 3 5 = Y/Al), the reaction between the two alkoxides results in a solution containing a mixture of Y-Al double isopropoxide (1:3) and excess Y-isopropoxide. During hydrolysis, Y-isopropoxide hydrolyzes much faster than the double alkoxide, causing precipitation of Y-(OH)x to occur, which eventually leads to inhomogeneity in the final product.

MULTICOMPONENT CERAMIC POWDERS

99

I

Figure 18 Electronmicrograph(replica "EM) of cold-pressed andsintered 7 mol% Y203-Hf02.

27Al nuclear magnetic resonance ( N M R ) studies revealed that the coordination of aluminum in the mixture of yttrium and aluminum isopropoxides was predominantly tetrahedral, suggesting the formation of a Y-AI double alkoxide [63]. To avoid the double alkoxide formation, one of the isopropyl groups was replaced by ethyl acetoacetate (EAA) to reduce the reactivity of yttrium isopropoxide with aluminum isopropoxide. Equimolar quantities of yttrium isopropoxide and ethyl acetoacetate were mixed and refluxed under isopropanol for 6 h. This solution was then added to a stoichiometrically measured amount of aluminum isopropoxide. *TA1 NMR spectra of Al-isopropoxide mixed with EAA-modifiedY-isopropoxideshowed no evidence of the formation ofthe double alkoxide. Infrared spectroscopy provided convincing evidence for the substitution of the isopropyl group. Hydrolysis of the solution mixture was carried out by the dropwise addition of the alkoxide into a water (excess)-isopropanol bath at room temperature. The transparent sol was then aged for 4 h to allow condensation to occur. Fourier transforminfraredspectroscopy showed that during hydrolysis, isopropyl groups were the first to be removed.

MAHETAL..

l00

The gel obtained was then treated to evaporate the solvent, dried at 125°C for 8 h, and calcined at 950°C for 1 h. X-ray diffraction analysis of the powder obtained showed peaks corresponding to single-phase crystalline YAG. Complementing the x-ray results, DTA analysis showed only one exothermic peak occurring at 920"C, corresponding to the formation of YAG. The resulting powder is very fine (submicrometer) and, more importantly, is a single phase.

IV. SUMMARYAND CONCLUSIONS Powder synthesis using the alkoxy precursor technique exhibits processing flexibility not available in traditional high-temperature solid-state reaction of the mechanicallymixed,andmilled oxides and/or the inorganic salt constituents. With proper process control, impurities can be reduced to very low levels, yielding the desired electrical, optical, thermal, and mechanical properties. In addition, selected elements may be purposelyadded in controlled amounts to influence these properties. The alkoxide decomposition process for the preparation of single-phase and homogeneous mixed oxides of high purity and fine grain size has been demonstrated and extended to include a variety of mixed-phase systems. Extremely high purity, in excess of 99.95%, can be obtained for a wide variety of oxides. Particle size ranges below 50 nm are easily achieved while maintaining this exceptionally high level of purity for noncrystalline glasses and crystalline oxide ceramics. Because of the high reactivity of the alkoxy-derived powders, low processing temperatures can be used to densify oxide monolith or as matrices for fibrous reinforced ceramic composites with very fine grain size. In addition to processing advantages, dopants can be added more easily in the liquid phase, resulting in a composition tailored to the user's need. The control over purity and homogeneity afforded by alkoxy-derived powderprocessing allows control of the characteristics of the grain boundaries, which are likely to be central to the issues of electrical, optical, and structural properties of ceramics at service temperatures. Finally, contrary to popular belief, chemically derived powder is an economically feasible and viable method for the production of large-volume raw material for manufacturing reliable high-performance, advanced high-technology ceramic components.

REFERENCES 1. Vincenzini, P. (ed.), Ceramic Powders. Proceedings of the 5th International Meet-

ing on Modem Ceramics Technologies, Elsevier, Amsterdam, 1983. 2. Messing, G.,Mazdiyasni, K. S., McCauley, J. W., andHaber,

R. A. (eds.), Ceramic Powder Science, Advances in Ceramics, Vol. 21, American Ceramics Society, Westerville, OH, 1987.

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IO1

3. Brinker, C. J., Clark, D. E., and Ulrich D.R. 4.

5. 6. 7. 8.

9. 10. 11. 12.

(eds.), Better Ceramics Through Chemistry I , M R S Symposium Proceedings, Vol. 32, Pittsburgh, PA, 1984. Brinker, C. J., Clark, D. E., and Ulrich, D. R. (eds.), Better Ceramics Through Chemistry II, M R S Symposium Proceedings, Vol. 73, Pittsburgh, PA, 1986. Brinker, C. J., Clark, D. E., and Ulrich, D. R. (eds.), Better Ceramics Through Chemistry III, M R S Symposium Proceedings, Vol. 121, Pittsburgh, PA, 1988. Zelinski, B. J. J., Brinker, C. J., Clark, D. E., and Ulrich, D. R. (eds.), Better CeramicsThroughChemistry N,MRSSymposiumProceedings,Vol. 180, Pittsburgh, PA, 1990. Hench, L. L., and Ulrich, D. R., (eds.), Ultrastructure Processings of Ceramics, Glasses, and Composites, Wiley & Sons, New York, 1984. Hench, L. L., and Ulrich, D. R. (eds.), Science of Ceramic Chemical Processing, Wiley & Sons, New York, 1986. Mackenzie, J. D., and Ulrich, D. R. (eds.),Ultrastructure Processingof Advanced Ceramics, Wiley-Interscience, New York, 1988. Bradley,D.C.,Mehrotra,R.C.,andGaur,D.P., MetalAlkoxides, Academic Press, (London), 1978. Ller, R. K.,The Chemistry of Silica, Wiley & Sons, New York, 1979. Brinker, C. J., and Scherrer, G. W., Sol-Gel Science, Academic Press, San Diego,

1990. 13. Bradley, D. C., Mehrotra, R. C., and Wardlaw, J., J . Chem. Soc., (London), 1963 (1953). J. S.. J. Am. Ceram. Soc., 50, 532 14. Mazdiyasni, K. S., Lynch, C. T., and Smith, (1967). 15. Herman, D. F., U.S. Patents 2,654,770 and 2,655,532 (1953). 16. Mazdiyasni, K. S., Lynch, C. T., and Smith, J. S., Inorg. Chem., 5, 342 (1966). 17. Brown, L. M., and Mazdiyasni, K. S., Inorg. Chem., 9, 2783 (1970). 18. Adkins, H.,and Cox, J., J. Am. Chem. Soc., 60, 1151(1938). 19. Bradley, D. C., Mehrotra, R. C., and Wardlaw, J., J. Chem. Soc. (London), 5020 (1952). 20. Mehrotra, R. C., J. Am. Chem. Soc., 76, 2266 (1954). T., Mazdiyasni, K. S., andSmith, J. S., in Decomposition of 21. Lynch, C.

Organometallic Compounds to Refractory Ceramics, Metals and Metal Alloys(K. S. Mazdiyasni, ed.), University of Dayton Press, Dayton, OH, 1967. 22. Mazdiyasni, K. S., Ceram. Int., 8, 42 (1982). 23. Mazdiyasni, K.S., Lynch, C. T., and Smith, J. S., J. Am. Ceram. Soc.,48(7), 372 (1965). 24. Yoldas, B. E., J. Mat. Cer. Sci., 12, 1203 (1977). 25. Dislich, H., Angew. Chem. Int., Ed. Engl., 10, 363 (1971). 26. Dislich, H., in Transformation of Organometallics into Common and Exotic Materials (R. Laine, ed.),NATOAS1 Series E, No. 141, Nijhof, Dordrecht, 1988, pp. 236-249. 27. Hemes, E. E.,and Mazdiyasni, K. S., in NASA Conference Publication 2406 (J. D. Buckley, d . ) , NASA, Langley, VA, 1985, pp. 217-228.

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28. Mazdiyasni, K. S., Paulick, L. A., and Hermes, E. E., in NASA Conference Publication 2406 (J. D. Buckley, ed.), NASA, Langley, VA, 1985, pp. 191-195. 29. Mehrotra,R. C., J . Non-Crystal. Solids, 100, 1(1988). 30. Jones, K., Davies,T.J.,Emblem, H.G., andParkes, P.,in MaterialsResearch Soc. Symp. Proceedings, Vol. 73 (C. J. Brinker, D. E. Clark, and D. R. Ulrich, eds.), Materials Research Society, Pittsburgh, PA, 1986, pp. 11 1-1 16. 31. Somiya, S., Davis,R. F., andPask,J.A.(eds.), CeramicTransactions, Vol. 6, Mullite and Mullite Matrix Composites, American Ceramics Society, Westerville,

OH, 1990. 32. Symposium F MulliteProcessing,Structure,andProperties, 43rd PacificCoast Regional Meeting, Am. Cerum. Soc. Bull. 69(9), 1526 (1990). 33. Mazdiyasni, K. S., and Brown, L.M., J . Am. Cerum. Soc., 55(1l), 548(1972). 34. Paulick, L.A., Yu, Y. F., andMah, T., inAdvancesinCeramics,Vol. 21 (G. 35. 36. 37. 38. 39. 40. 41. 42. 43.

Messing, K. S. Mezdiyasni, J. W. McCauley, and R. A. Haber, eds.), American Ceramics Society, Westerville, OH, 1987, pp. 121-129. Keefer, K.D., inMaterialsResearch Soc. Symp.Proc.,Vol. 32, MaterialsResearch Society, Pittsburgh, PA, 1984, p. 15. Thomas, I. M., U.S. Patent 3,791,808(1974). Yoldas, B. E., J . Muter. Sci., 12, 1203 (1977). Yoldas, B. E., J . Muter. Sci., 14, 1843 (1979). Brinker, C. J., and Mukerjee, S. P., J . Muter. Sci., 16, 1980 (1981). Brown, L. M., and Mazdiyasni, K. S., J . Am. Cerum. Soc., 55(1l), 541(1972). Haertling, G. H., and Land, C. E., J. Am. Cerum. Soc., %(l), 1 (1971). Haertling, G.H., J . Am. Cerum. Soc., 54(6), 303 (1971). Mazdiyasni, K. S., Dolloff, R. F., and Smith, J. S., J . Am. Cerum. Soc., 52(10),

523 (1969). 44. Mazdiyasni, K. S, and Brown, L. M., J. Am. Cerum. Soc., 54(1l), 539 (1971). 45. Mazdiyasni, K. S., and Brown, L.M., Am. Cerum. Soc., 55(12), 633 (1972). 46. Mazdiyasni, K. S., Lynch, C. T., and Smith, J. S., J. Am. Cerum. Soc., 49(5), 286 (1966). 47. Phillippi, C. M., and Mazdiyasni, K. S., J . Am. Cerum. Soc., 54(5), 254 (1971). 48. Mazdiyasni, K. S., in M R S Symposium Proceedings, Vol. 32 (C. J. Brinker, D. E. Clark,and D. R. Ulrich,eds.),MaterialsResearchSociety,Pittsburgh,PA, 1984, pp. 175-186. 49. Mazdiyasni, K. S., and Brown, L. M., J. Am. Cerum. Soc., 53(1), 43 (1970). 50. Mazdiyasni,K. S., Lynch,C.T.,andSmith,J. S., J . Am. Cerum. Soc., 50(10),

532 (1967). 51. 52. 53. 54. 55. 56.

Brown, L. M., and Mazdiyasni, K. S., J. Am. Cerum. Soc., 53(1l), 590 (1970). Rhodes, W. H., and Haag, R.M., Tech. Report AFML-TR-70-209(1970). Messier, D.R.,and Gazza, G. E., Am. Cerum. Soc. Bull., 51(9), 692 (1972). De With, G., and van Dijk, H.J.A., Muter. Res. Bull., 19, 1669 (1984). Takamori, T., and David, L.D. Am. Cerum. Soc. Bull., 65(9), 1282 (1986). Hardy, A.B., Gowda,G.,McMahon, T. J.,Riman,R.E., Rhine, W. E., and

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Bowen, H.K., in UlwaswuctureProcessing of AdvancedCeramics (J.D. MacKenzieandD. R. Ulrich,eds.),WileyandSons, NewYork, 1984, pp.

407428. 57. De With, G., Philips J. Res., 42, 119 (1987). 58. Haneda, H., Watanabe, A., Matsuda, S., Sakai, T., Shirasaki, S., and Yamamura, H.,in Proc. 4th International Symposium on Science and Technology of Sinter-

ing,Tokyo,Japan (S. Somiya, M. Shimada, M. Yoshimura,and R. Watanabe. eds.), 1987, pp. 383-386. 59. Inoue, M., Otsu, H., Kominami, H., and Inui, T.,J. Am. Ceram. Soc., 74(6), 1452

(1991). A., J. Mater. Sci. Lett., 10, 101 60. Yamaguchi, O., Takeoka,K.,andHayashida, (1990). 61. Yamaguchi, O., Takeoka, K., Hirota, Takano, H., and Hayashida, A., J . Mater. Sci., 27(5), 1261 (1992). 62. Apte, P., Burke, H., and Pickup, H., J . Mater. Res., 7(3), 706 (1992). 63. Sambasivan, S., Keller, K., Sim, S., and Mah, T., J. Mater. Sci., in press (1993).

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5 Chemical Synthesis of Nonoxides Christian Russel Universitit Jena Jena, Germany

Michael Seibold OSRAM GmbH Augsburg, Germany

1.

INTRODUCTION

In the last two decades a great deal of interest has been focused on nonoxide ceramics (see, e.g., Refs. 1-5) that is mainly because of their potential use as high-temperature, high-strength materials. The main efforts have been conducted in the field of silicon carbide and nitride ceramics. In addition, aluminum nitride has attracted attention because of its high thermal conductivity, whichwillbecomeincreasinglyvitalin future substrate applications. Other nonoxides, such as boron carbide and various transition metal carbides, nitrides, and borides, are prospective materials for cutting tools and grinding media. All these nonoxides exhibit strong covalent bonding, high strength, and high hardness. They are extremely difficult to sinter because of low self-diffusion coefficients. Therefore, most of the nonoxide powders must be densified either by hot pressing or by using sintering additives. The fabrication of dense ceramics by hot pressing or hot isostatic pressing methods need extensive equipment. The high-temperature properties, however, are preserved. Sintering additives lead inmany cases to the presence ofglassy phases within the microstructure and hence harmed mechanical properties, such as reduced hightemperature strength. Until recently, the industrial production of nonoxide powders was predominantly based on two different high-temperature processes: carbothermal re105

106

RUSSEL AND SEIBOLD

duction [6,7] and direct synthesis from the elements. In the past few years, routes via the gas phase, which are also high-temperature or even plasma reactions, have increasingly been investigated. Powders synthesized at high temperatures show low sinterability caused by large crystallite sizes. Generally they cannot be pressurelessly densified without sintering additives. Moreover, routes via tractable polymer precursors have been studied. They can be used for the production of ceramic fibers, coatings, and monoliths and also for the formation of powders with tailored morphology and hence high sinteractivity. In contrast to oxides, no common reaction path for the synthesis of nonoxide ceramics via tractable precursors is yet known. Therefore, in the past few years many different synthesis reaction schemes for both nitrides and carbides have been discussed in the literature. Although silicon carbide and nitride have been under intense investigation (e.g., Ref. l), many other reactions leading to other nonoxides, such as aluminum nitride, boron nitride, boron carbide, and some transition metal carbides and nitrides, have been reported (e.g., Refs. 1-5). Polymeric compounds even allow the pressureless densification of nonoxide powders without the use of sintering additives in some cases.

II. HIGH-TEMPERATUREROUTES TO NONOXIDES Three main processing schemes for the formation of nonoxide ceramics are known, carbothermal reaction, direct synthesis, and gas-phase synthesis, which usually require reaction temperatures well above 1200°C. Carbothermal reaction as well as the direct synthesis from the elements can be varied over a wide range byusing different carbon sources, such as graphite or other carbon sources.

A.

Carbothermal Reduction

Conventional routes are high-temperature processes that usually require temperatures ranging from 1200 to 1600°C. Oxides and carbon are used as starting compounds, and the reactions are carried out in a nitrogen or inert gas atmosphere for the formation of nitrides (e.g., Refs.6 and 7) and carbides [8-IO], respectively.

It has been mentioned that this reaction can be carried out not only using elemental carbon but also withany carbon precursor, such as phenolic resins. They provide a more intensified mixing of the starting sources. Another varia-

CHEMICAL SYNTHESIS OF NONOXIDES

I07

tion of this method is the combined use of sol-gel technique together with complexation agents, such as furfuryl alcohol [IO], which act as the carbon source during calcination. This method has been used for the formation of TiN and favors much lower formation temperatures than usually required.

B.

Direct Synthesis

Another common route for the production of both nitride and carbide powders is direct synthesis from the elements. For the formation of nitrides, the elements are heated in a nitrogen atmosphere. Me + $N2

MeNx

All nitrides suitable for potential application in the field of materials science show a highly negative Gibbs free enthalpy of reaction according to Eq. (3). Kinetic hindrances, must be considered, however. This gains importance if the metal of focus exhibits a low melting point; in this case the nitridation temperatures are usually restricted below this value. Formation of aluminum nitride, for example, using this method is difficult to achieve because at temperatures belowthe melting point the nitridation rate is far too low for a commercial process. In Ref. 12, however, an interesting pathway is shown: a thin aluminum nitride layer was formed by nitridating at temperatures below the melting point at the surfaces of very finely grained aluminum powder; the temperature was subsequently raised above the melting point, and the nitridation process was completed. Notethat nitridation is a diffusion-controlled process, and hence the temperature cannot be raised until the thickness of the nitride layer around every aluminum particle prevents joining of the aluminum particles. Nitridation of other elements, such as boron, silicon, or transition metals, can easily be achieved at temperatures below the melting point. From a kinetic point of view, the particle size should be as small as possible. The surface of each particle is covered by a thin oxide layer, however, leading to oxygen contamination. In contrast to carbothermal reactions, direct nitridation does not entail an oxygen removal mechanism. Using coarse particles, however, requires remarkably higher temperatures or soaking times. For silicon the reaction rates strongly depend on the amount of impurities [13]. To obtain carbides, the elements are heated together with elemental carbon or another carbon source in an inert gas atmosphere. Me + XC ”+ MeC, Phenolic resins or many other C-containing compounds can be used for this purpose [9,10],

108

RUSSEL AND SEIBOLD

C.Gas-PhaseReactions Gas-phase reactions for the formation of nonoxide powders have been known for a long time. In the last few years many of these routes were reinvestigated and other precursors have been developed. This is mainly because of their potential to yieldvery fine grained nonoxides, withmean grain sizes in the nanometer range. Routes via the gas phase have also been intensively studied for the direct formation of coatings that is, CVD (chemical vapor deposition). All reaction paths basically considered as a CVD process can be tailored for the formation of powders; the processing parameters must be optimized. Metal halides with covalent bonding, which thus can be easily evaporated, and ammonia [14,15]are used. MeX,

4n + -NI43 3

MeN,D

+ nNH4X

(5)

The productionofsiliconnitride [16,17]andaluminumnitride [18-211 has been extensively studied; plasma reactions [21-231 were also reported. Instead of ammonia nitrogen may be used [24,25].Hydrocarbons and metal halides for the formation of carbides are worth mentioning [26,27].

111.

POLYMERIC ROUTES TO MAIN GROUP ELEMENT CARBIDES

A.

Silicon Carbide

A s mentioned, the sinterability of pure silicon carbide strongly depends on the properties of the powder used. Powders made by high-temperature routes are predominantly composed of different hexagonal Sic polytypes (e.g., 2H, 4H, and 6H). Incontrast, S i c produced by polymeric routes almost exclusively consists of p-Sic. In principle, Sic can be formed by the pyrolysis of either polysilanes or polycarbosilanes. The preparation of polysilanes is described in detail in Chap. 2 and was first reported by Kipling in 1924 [28]. The concept most commonly used now was developed by Yajima et al. [29],who used dimethylchlorosilane as a starting substance.

R

I I

Cl-Si-Cl

+Me

R Alkali metals and Me (liquid Na/K alloys are frequently cited [29-321) act as reducing agents of the chlorosilanes. It has been found that the polysilane

CHEMICAL SYNTHESIS OF NONOXIDES

109

transforms to a polycarbosilane [see Eq. (7) and Refs. 2,4, and 29-32] during thermal treatment.

In the past few years, similar reactions using numerous dichlorosilane derivatives were investigated to yield preceramic polymers. Some routes use several different types of chlorosilanes as starting materials (e.g., Refs. 33-35) Analogous to the preparation of polysilanes, polycarbosilanes can also be synthesized using a similar route. Alkyl chloroalkyl chlorosilanes are used as starting materials for this purpose [2]. R

I I

Cl"si"CH2Cl+

Na, K

(8)

R The reduction of dichlorosilanes or chloroalkyl chlorosilanes stimulates the formation of linear chains, whereas the same reaction using trichlorosilanes or similar compounds may support cross-linked products. Cross-linking can also be obtained by the use of vinyl silanes [33-351 or Si-H-modified compounds. A silylation reaction occurs at higher temperatures, also leading to branched products. Routes via polysilanes or polycarbosilanes favor products with comparably low molecular weight in many cases (Fig. 1). During calcination, high quantities of silicon compounds are evaporated and subsequently low ceramic yields are obtained. A notable weight increase in the ceramic residue is sometimes found if the pyrolysis is carried out under either reflux or pressure (see, e.g., Ref. 2). High ceramic yields are derived only with highly cross-linked polymeric products, whereas high molecular weights are not a sufficient prerequisite. A high amount of cross-linking can be achieved by using trichlorosilanes, chloroalkyl dichlorosilanes, and Si-H- and vinyl-modified compounds. For example, the reaction of ClCH2Si(CH3)2Cl with metallic potassium results in only traces of S i c [2]; the reaction of bothClCH2Si(CH3)2Cl and CH3SiCl3 yields ceramic residues of 30.8 wt% [34]. Coreduction of (CH3)3SiCl, (CH3)2SiCH=CH2, and (CH3)SiHClz with potassium allows a ceramic yield of 57 wt% [34]. In some cases, the use of dichlorosilanes containing aromatic substituents has led to an increase in ceramic product: for example, for the coreduction of(CH3)2SiC12 and (C&)2SiC12 with sodium, a ceramic yield of 54% was observed [2].

RUSSEL AND SEIBOLD

- " a

I

6

4

2

0

-2

chemical shift. ppm

Figure 1 29Si nuclear magnetic resonance spectra of different polycarbosilane precursors (molecular weight distributions Mnranging from 320 to 1750) with respect to treatment temperatures.

The pyrolysisof polysilanes and polycarbosilanes is usually carried out using inert gas (e.g., argon) as pyrolysis atmosphere. A general problem asse ciated with the pyrolytic formation of carbides is the desired stoichiometry of the calcined products: in contrast to nitrides, excess carbon cannot be evape rated during calcining; it may therefore contaminate the powders obtained as an elemental impurity and thus influences the physical, especially mechanical and electrical, properties of the sintered ceramic bodies. The volatiles evaporated during pyrolytic treatment of carbosilanes to form a network structure are H2 and CH4, and they depend onthe structure of the polycarbosilane used (Fig.2).

CHEMICAL SYNTHESIS OF NONOXIDES

112

RUSSEL AND SEIBOLD

Silicon-containing volatile compounds are predominantly SiH4 and H3SiCH4. High cross-linking, however, suppresses the evaporation of Si compounds.

B.

Boron Carbide

Some routes for the preparation of boron carbide ceramics were investigated recently. The highest ceramic yield (80 wt%) [5] was obtained using the thermal decomposition of boronylpyridine. Other routes, such as the pyrolysis of triphenyl boron or 1,2-C2BloH12,resulted in ceramic yields of only 2 and 4 wt%, respectively [4]. It should be said that many boron nitride precursors also yield boron carbide if calcined in an inert gas atmosphere.

W. POLYMERICROUTES TO MAIN GROUP ELEMENT NITRIDES A.Silicon

Nitride

Silicon nitride has found many applications in which structural features play an important role. Conventionally derived silicon nitride is fairly difficult to sinter, and pressureless sintering requires large amounts of sintering aids (e.g., 8% A1203 and 2% Y203). The fiist “chemical route”, which has been applied for the formation of silicon nitride at comparably low temperature, is diimid precipitation, already reported in 1830 [36]. During the reaction of silicon tetrachloride with liquid ammonia, the intermediate, Si(NH2)4 was presumed. A precipitation of different polymeric products [e.g., the diimid, Si(NH)2] subsequently occurs, which finally yields silicon nitride. This reaction involves a multiple-step mechanism. Simplified chemical reactions are shown in Eqs. (9) to (11) (for a more detailed description, see, e.g., Refs. 37-40). Sic14 Si(NH2)4 + 8NH3

+4 m C l

(9)

Equation (9) shows that high quantities of ammonium chloride are formed during the reaction that must be evaporated during calcining. In addition, a highly exothermic reaction is much easier to control if a solvent, such as n-hexane [38], is used. The required formation temperature of only 500°C is fairly low. Until recently, diimid precipitation has been the focused of numerous papers,

CHEMICAL SYNTHESIS OF NONOXIDES

I13

and in principle the formation of Si3N4, with much better sintering properties than conventionally produced powders, is possible. One should take into consideration that the precipitate is completely insoluble, however, and therefore, the formation of powders with controlled morphology is difficult and must be determined by the first reaction step. The route does not enable the production of fibers or coatings. Routes via polysilazanes, in contrast, lead to soluble or fusible intermediates that are prerequisites for the formation of coatings and fibers. Although a route enabling the formation of polysilazanes was already reported by Stock in 1921 [41], numerous routes, all leading to polysilazane intermediates, have been discussed in the literature only recently [4249]. Dichlorosilanes and ammonia or alkyl amines are used as starting compounds. R

I I

Cl-Si-Cl

+ 3NH3

(12)

_j

R Inanalogy to the formation of polysilanes, products with low molecular weights are obtained, usuallya mixture of cyclomers (with n = 3-6) and oligomers (with n < 10). Pyrolysis of these products thus leads to the evaporation of silicon-containing compounds and, in many cases, to almost negligible ceramic yields. Cross-linking andhigher molecular weights in principle can be obtained by using trichlorosilanes and vinyl- or Si-H-modified starting materials. The chemical reactions that occur are summarized in Eqs. (13) and (14). These reactions, in principle, might be used to tailor the polysilazane structures, similar to the chemistry of polysiloxanes

I

I

Cl-Si-Cl

+ 3nC1-Si-C1I

c1

R

I

-m

I -Si-CH=CH2+H-SiI R

F

R

R

M12R,

-NH3RCl

I I

].

N-Si N-Si-N Si-N

-NH

l I

"Si-CH2-CH2-Si-

R

(13)

I I

114

RUSSEL AND SEIBOLD

Further cross-linking reactions, leading to products of high molecular weight, have been reported by Seyferth et al. [1,50]. Thus the formation of two-dimensional linked polymers occurs if low-molecular-weight oligomers containing Si-H and N-H bonds have reacted with potassium hydride (residual potassium hydride can be removed by adding CHJ). High ceramic yields of about 85 wt% furthermore prove the necessity for cross-linked structures. Equation (15) shows the reaction mechanism proposed by Seyferth et al. [1,50]. The initial oligomer was synthesized byadding ammonia to chlorosilane and converted without further cross-linking into a negligible quantity of ceramic products during calcining.

Another method for the formation of cross-linked products is the catalytic dehydrogenation of Si-H and N-H bonds using Ru3(C0)12 as a catalyst [43].

R

I

nx H-Si-H

I

H

+ ax[RSiHNH],

_j

[RSiHNH],

(16)

Catalyst

I

I1

with R = phenyl, ethyl, or hexyl. The reaction mechanism was assumed as follows: first linear intermediates (I) and subsequently bridged, branched, or ring polymers (11) are formed (for a detailed discussion, see Ref. 43). Because of the high degree of cross-linking, high ceramic yields are obtained. Similar to polysilanes in terms of synthesis and calcination behavior, a large varietyof polysilazanes have already beenreported. The condensation of CH3NH2 with (CH3)2SiC12 or H2SiC12 resulted in ceramic yields of about 40% [47,51]. When HSiC13 and [Si(CH3)3]2NHwere used, the ceramic yield was in the range of 70-90% [44,49]. Using hydrazine N2H4 as a nitrogen donor and triethylamine instead of [Si(CH3)3]2NH ammonia, the ceramic yields were 72% with HCH3SiC12 as chlorosilane compound (Fig. 3) [47]. With the method of

115

CHEMICAL SYNTHESIS OF NONOXIDES

too W &-

cc

2 9)

W

80

60

40 20 0

200

400

600

800

l000

temperature, 'C

Figure 3 Thermogravirnetric analysis of a polysilylhydrazine precusor, pyrolyzed in nitrogen atmosphere.

Seyferth using potassium hydride to stimulate higher degrees of cross-linking, ceramic yields of 85% could be achieved [1,51]. The pyrolysis of polysilazanes results in fairly different chemical compositions of the residue; the calcination gas is varied. Although this effect strongly depends on the type of polysilazane, some more or less general conclusions can be drawn. Calcining in an inert gas atmosphere, such as argon, results in the formation of a highly carbon-contaminatedsilicon nitride or even in SiC/Si3N4 composites or solid solutions, depending on the calcination temperature. In contrast, if the calcination is carried out in ammonia or nitrogen, the product is pure silicon nitride. The formation of oxynitrides can be achieved by calcining in nitrogen atmosphere with substantial oxygen content or by oxidizing the polysilazane before or after calcining in nitrogen (Fig. 4). The main weight loss during calcination usually occurs in the temperature range between 200 and 6OO0C, strongly depending on the type of polysilazane used. For example, polyphenylsilazane exhibits its mainweight loss in a temperature range of 350-600"C; poly-(N-methy1)silazane loses weight in a two-stepprocess at temperatures of about 250 and 380°C (Fig. 5).

B. Aluminum Nitride Aluminum nitride powder synthesized by high-temperature routes can be sintered to a density of more than 97% of the theoretical density by adding calcium or yttrium compounds as sintering aids [52-541. Therefore, new processing routes to aluminum nitride predominantlyaim at powderswith lower quantities of cationic impurities. In addition, the formation of aluminum nitride coatings [S-581 or fibers [59] has become subject of extensive research.

116

RUSSEL AND SEIBOLD 8OO0C " " . = . W

/

.-5

1550°c

v)

aJ

U

.C

1700OC

40

35

30

25

20

l5

BR4GG angle, degrees

Figure 4 X-raydiffraction ( X R D ) patterns of a polysilazane-polysiloxane"hybrid" polymer with respect to different pyrolysis temperatures.

The first low-temperature route for the formation of aluminum nitride was discovered in the 1950s by Wiberg with Amberger and May [60,61]. Three-dimensionallypolycondensed iminoalanes [e.g., Al(NR)l.s] are formed using aluminum hydride and ammonia or primary organic amines as starting materials. It should be added that aluminum hydride is extremely difficult to handle

I17

CHEMICAL SYNTHESIS OF NONOXIDES 0

Q 0

00

5: 0

cr

a 0

0

200

400

600

1000

800

temperature, 'C

Figure 5 TGA profile of a phenyl-substituted polysilazane (x) compared with hexyl groups containing polymers.

because of its sensitivity to air and moisture. Nevertheless, these routes have been reinvestigated during the last few years r62-651. Fairly similar reactions can be carried out using aluminum alkyls as initial compounds [66-69]. AIR'3

+ 3NH2R

-3RH

Al(NHR)3

-1SNH2R

'A(m)1.5

(17)

with R' and R = H or alkyl. Tractable AlN precursors cannot be produced by adding aluminum chloride to ammonia or primary amines because, in contrast to silicon chloride, a substitution does not occur and different adducts (AlCl3 xNH3 with 1 5 x < 4) are formed, which can be evaporated without decomposition. If, alkali amides are used instead of ammonia or amines, however, the substitution takes place and highly polycondensed polyiminoalanes are formed [68].

-

3KNH2

+ AlCl3

-3KCl

AI(NH2)3

-1SNH3

'Al(m)1.5

(18)

Polyiminoalanes can also be synthesized using a high-pressure route, first reported by Cucinella et al. [70]. Another, fairly different route was recently reported by Schleich [71]. condensation of the starting materials, hexamethylsilazane and aluminum chloride, resulted in the formation of the dimeric and soluble compound ClAlNHSi(CHs)3. By heating to temperatures above 20O0C,ClSi(CH3)3 is evaporated and the polymeric compound (CIAINNH)n [72] is formed, which can be transformed to aluminum nitride by calcining in

RUSSEL AND SEIBOLD

118

an inert gas atmosphere. This route involves the disadvantage that silicon contamination cannot be completely avoided. Calcining polyiminoalanes usually results in the formation of aluminum nitride at comparably low temperatures. If the calcination is carried out in ammonia, carbon contamination is easier to avoid, calcining in argon usually results in the formation of black carbonaceous residues in the range of 10-20 wt% carbon. In general, crystallization is promoted by using ammonia and is prevented if argon is used.

C.

Boron Nitride

Most routes to boron nitride referred to in literature make use of borazole derivatives [4,73-801. A large variety of different ligands, R and R’, have already been mentioned.

R

I

N

R-B’

‘B-R

The easiest way to obtain boron nitride from these precursors involves only heat treatment: for example, borazoles undergo condensation reactions without previous evaporation. The ligand R should either be -NH2 or NHR, and a variety of different ligands R’ are possible. The ceramic yields are found in the range of 26% (R’ = H, R = NHCsHs [4]) and 55% (R’ = H, R = NH2 [4]). Although the starting materials themselves are not polymeric, comparably high ceramic yields are obtained. With R’ = SiR3 and R = NH2, spinnable compounds have been obtained, which might open up possibilities for the formation of boron nitride fibers. In some cases, the formation of white boron nitride with relatively a low oxygen and carbon content could be achieved [75]. Recently, the preparation ofwhiteboron nitride from polyborazylene wasreported [79,80]. Infrared spectra (Fig. 6) of pyrolyzed residues were found to be close to those reported for CVD samples of hexagonal boron nitride. For the B-chloroborazoles (R = Cl), however, the use of condensating agents is required. Either [(CH3)3Si]2NR [73,77,78] or(CH3)Si-C = CSi(CH3)3 [73] have already found use. The borazole rings are connected via BNR-B and B - M - B bonds, respectively, and trichlorosilane [73,76] is formed. In some cases, a solid of gellike consistency is obtained. Calcining of these polymers leads to relatively high degree of carbon contamination, even in an ammonia atmosphere. Calcining under inert gas often results in the formation of BN/B4C composites with desirable properties.

CHEMICAL SYNTHESIS OF NONOXIDES

I19

wavenumbers, cm”

Figure 6 Infraredspectrum of a themolyzed borazeneoligomerheated in vacuum to 275OC.

In analogy to the route of Wiberg for the formation of polyiminoalanes and subsequently of aluminum nitride [61], NaBH4, BCb, and ammonia may be used as starting materials. The intermediate formation of diborane, B2H6, was assumed [81] in this case.

The polymeric compound (BNH2),, is formed as an intermediate and may be thermally decomposed to boron nitride. The synthesis is difficult to carry out because all compounds and intermediates are extremely sensitive to air and moisture. Polymeric adducts of decaboranes, B10H12, are used in many other studies. Diphosphines or diamines (L”) [81-SS] could act as condensating agents that favor the formation of B-L”L-B groups. Most of these polymers are stable toward oxidation and hydrolysis and soluble in polar organic solvents, such as dimethylformamide ( D m or hexamethylphosphorus triamide (HMPT). During pyrolysis in either ammonia or argon, white boron nitride in 70% ceramic yieldand boron carbonitride with ceramic yields ofup to 90% are formed, respectively.

120

V.

RUSSEL AND SEIBOLD

POLYMERICROUTES TO TRANSITIONELEMENT CARBIDES AND NITRIDES

Transition element carbides and nitrides are applied as cutting tools because of their extreme hardness and wear resistance. In some cases nitrides and carbides (e.g.,of titanium) form solid solutions over the entire compositional range; other transition metal nitrides and carbides exhibit fairly different structures and are not completely soluble. Carbon contents within the range of few percentage points usually do not influence the mechanical properties of transition metal nitrides, and vice versa. Hence, completely carbon-free nitrides or nitrogen-free carbides are not required, especially for the titanium compounds. Until now, polymeric routes to transition metal nitrides or carbides have not been as numerously reported in the literature as those of the main group elements. They have been developed in most cases to produce thin coatings on various substrates or fibers. Seyferth and h4ignari [86] reported a route to titanium nitride and titanium carbide via tetrakis-dialkylamine titanium compounds. A polymeric or oligomeric product can be obtained by substituting the secondary amine by a primary amine [87,88]. The initially formed tetrakis-alkylamine polycondenses and leads a double-bridged polymer or oligomer. Calcining in argon or ammonia results in the formation of titanium carbide or titanium nitride, respectively.

I

R

I

R

I

R

I

R

Analogously to silicon nitride, titanium nitride precursors can also be prepared by the reaction of titanium tetrachloride with fluid ammonia [89,90]. This leads to the precipitation of highly polymeric products, which can be transferred to metal nitrides or carbonitrides by calcining in ammonia or an inert gas atmosphere. Zirconium and niobiumpolymers were prepared by fairly similar routes, the ammonolysis of the tetrakis-dialkylamines [88] or the chlorides [89,90] in liquid ammonia. After pyrolysis, nitrides or carbonitrides were obtained in high ceramic yields. The formation of titanium nitrides or carbonitrides via the calcination of titanium acetilides, such as Ti4(C=C)(NH)7 or NaTi4(W)(NH)7NH2 at temperatures I 8OOOC was reported by Maya [91].

CHEMICAL SYNTHESIS OF NONOXIDES

121

It should be mentioned that sol-gel routes have also been described in the literature, which finally yield titanium nitride [l l]. Additives, such as furfuryl alcohols, are used in this case, which decompose during the calcination in ammonia and form amorphous carbon. Subsequently carbothermic reactions occur, and finally titanium nitride is formed.

VI.

ELECTROCHEMICAL ROUTES TO CARBIDES AND NITRIDES

A large variety of precursors have been prepared lately with an electrochemical route [92-991. The metal is anodically dissolved in an organic electrolyte, which can be directly converted into a ceramic coating [55-581 or powder without further purification. The method was first reported by Seibold and Russel [92] for the formation of aluminum nitride, who used an electrolyte consisting of propylamine, acetonitrile, and tetrabutylammoniumbromide as a supporting electrolyte to achieve sufficient electrical conductivity. The electrodes were formed by sheets of various metals. The assumed chemical reactions at the electrodes are as follows: Me xNH2R + xexNH2R + Me

-

MeX++ exNHR- + H2

+Me(=),

x2

+ ;H2

The metal is oxidized at the anode, the propylamine is reduced at the cathode, and the corresponding anion and gaseous hydrogen are formed. As an intermediate, monomeric Me(NHR), is assumed. These intermediates condense to oligomeric or polymeric compounds.

The electrolytes were dried and subsequently calcined. The main weight loss occurs at temperatures in the range of 150-350°C (e.g.,Ref.92).A further slight weight loss occurred up to 550"C, and then the weight remained nearly constant. It should be noted that the thermogravimetric analysis TGA profiles were fairly similar and only slightly influenced by the type of metal. The ceramic yields, however, strongly depended on the type of metal used and were in the range of 20-50 wt% [92,96,98,99].

I22

RUSSEL AND SEIBOLD

Figure 7 XRD patterns of aluminum nitride precursors calcined in ammonia at differenttemperatures: (a) driedprecursor; (b) 600°C; (c) 800OC; (d) 1100°C; and (e) 1400°C.

Allprecursors are amorphous up to calcination temperatures ofaround 600°C. At higher temperatures, in most cases powders with extremely small crystallite sizes of around 2 0 4 0 nm are formed (Fig. 7). A further increase in calcination temperature promotes crystal growth.Withaluminum nitride, a white powder with a low oxygen and carbon content is obtained [97]. Other main group element precursors exhibit fairly different behaviors: Mg and Ca precursors yield metal cyanamide [99]. Calcination of the transition element precursors (Fig. 8) results in the formation of nitrides, carbonitrides, or carbides. For the titanium-containing precursors, TiN/TiC solid solutions can be obtained [96]; the quantity of carbon strongly depends on the calcination atmosphere applied (argon, 31 wt%; ammonia, 5.1 wt%). In contrast, chromium-containing precursors are pyrolyzed in ammonia to CrN; calcining in argon leads to the formation of Cr3C2 [98]. Other products formed are ZrN, TaC, NbC, MoC, and WC. Y-containing precursors lead to the formation of YNTy0.45Co.55 mixtures (Table 1) [99]. Because of the small

CHEMICAL SYNTHESIS OF NONOXIDES

70 *

60

50

40

30

123

20

28 (CUL)P

Figure 8 XRD patterns of various ceramicproductsobtainedaftercalcining:(a) CaCN2; (b) CrN (c) Cr3C2; (d) YN (open circles); Yo.45Co.55 (closed circles); and (e) TaC.

crystallite sizes, the powders obtained exhibit veryhighsinterability. A s already shown for aluminum nitride powders, pressureless sintering without the addition of sintering aids at 1750°C results in densities > 98% of theoretical density (t.d.) [92]. The oxygen contents of the pyrolyzed powders were in the range of 1.5-2.0 wt% for aluminum nitride, calcium cyanamide, and the transition metal compounds investigated. The electrochemical synthesis of polymeric precursors is characterized by a broad versatility. Most of the metals can be handled easily and electrolytically dissolved and transferred into polymericprecursors; thus this processing scheme seems to be a general approach and an alternative to already known methods. In addition, all precursor solutions can be mixed with one another, and therefore a wide range of new composites or solid solutions is possible in the near future.

124

RUSSEL AND SEIBOLD

Table 1 NonoxidesDerivedbyElectrochemicallySynthesizedPrecursors Metal dissolved Ceramic yield

(%)

AI Ti

42 20

zr

41 40

Cr

Ta Ca Mg Y

31 34 21 39

Calcination gas Product NH3 NH3 N2 NH3 NH3 N2

AIN TiN Ti(C, N) ZrN CrN

Ar

C4C2 TaC CaCN2 MgCN2

NH3 NH3 NH3 NH3

cm2

m,y0.45c0.55

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SYNTHESIS CHEMICAL.

OF NONOXIDES

127

71. Schleich, D. M., U.S. Patent 4,767,607 (1988). J . Mater.Sci.Lett., 9, 222-224 72. Riedel, R., Petzow,G.,andKlingebielU., (1990). 73. Maya, L., and Angelini, P., J. Am. Ceram. Soc., 73, 297 (1990). R., Ceram. Eng. Sci.Proc., 1171 74. Bender,B. A., Rice,R. W., andSpann,J. (1985). 75. Tanigushi, I., Harada, K., and Maeda, T., Japan Kokai 76 53OOO (1986). 76. Narula, C. K., Paine, R. T., and Schaeffer, R., in (C. I. Brinker, D. E. Clark, and D.R.Ulrich,eds.), M R S Pr. Vol. 73, BetterCeramicsThroughChemistry I1 Pittsburgh, PA, 1986, pp. 383-388. Chem. Mater., 3, 77. Paciorek,K. J.L., Nakahara,J. M., andHoferkamp,L.A., 83-87(1991). 78. Rye, R. R., Tallant, D. R., Borek, T. T., Lindquist, D. A., and Paine, R. T., Chem. Mater., 3, 286-293 (1991). 79. Fazen, P. J., Remsen, E. E., and Sneddon, L. G., Polymer Preprints, 32, 544-545 (1991). 80. Paine, R. T., Janik, J.F., Borek, T. T., Lindquist, D. A., Duesler, E. N., Smith, D. M., Kodas, T. T., and Datye, A. K., Polymer Preprints, 32, 546-547 (1991). W., in M R S Proc.Vol. 121, BetterCeramics 81. Seyferth, D., andSmithRees, 82. 83. 84.

85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95.

Through Chemistry Ill (C. J. Brinker, D. E. Clark, and D. R. Ulrich, eds.),Pittsburgh, PA, 1986 pp. 449-454. Seyferth, D., and Rees, W. S., Chem. Mater., 3, 1106-1 116 (1991). Schroeder, H., Reiner, J. R., and Knowles, T. A., Inorg. Chem., 2, 393 (1963). Niedenzu, K., and Buschbeck, K.-C., Gmelin Handbook of Inorganic Chemistry, 8th ed., Vol. 54, Boron Compounds: B-H Compounds, Springer, Berlin, 1979, pp. 151ff. Maya, L., in Better Ceramics Through Chemistry III (C. J. Brinker, D. E. Clark, and D. R. Ulrich, eds.) M R S hoc. Vol. 121, Pittsburgh, PA, 1988, pp. 455-460. Seyferth, D., and Mignani, G., J. Mater. Sci. Lett. 7, 487-488 (1988). Lappert, M. F., Power, P. P., Sanger, A. R., and Srivastava, R. C.,Metal and Metalloid Amides, Ellis Horwood, Chinchester, 1980. Brown, G. M., and Maya, L., J . Am. Ceram. Soc., 71, 78-82 (1988). Maya, L., Inorg. Chem., 25,4213-4217 (1986). Maya, L., Inorg. Chem., 26, 1459-1462 (1987). Maya, L.,in M R S Symp. Proc. Vol. 73, Better Ceramics Through Chemistry I1 (C. J. Brinker, D. F. Clark, and D. R. Ulrich, eds. Pittsburgh, PA, 1986, p. 401. Seibold, M., andRussel, C., inMRSSymp. Proc. Vol. 121, BetterCeramics Through Chemistry I l l (C. J. Brinker, D. F. Clark, and D. R. Ulrich, eds.), Piasburgh, PA, 1988, pp. 477-482. Seibold, M., Viemeusel, U., and Russel, C., in Ceramic Powder Processing Seience (H.Hausner,G.C.Messing,and S. Hirano,eds.),DeutscheKeramische Gesellschaft, Koln, 1989, pp. 173-179. Seibold, M., and Russel, C., J . Am. Ceram. Soc., 72, 1503-1505, (1989). Russel,C.,Hofmann,T.,Kulig,M.,andSeibold., Silikattechnik, 40, 425-429, (1989).

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6 Techniques for Characterization of Advanced Ceramic Powders S. G. Malghan, P. S. Wang, and V. A. Hackley National Institute of Standards and Technology Gaithersburg, Maryland

1. IMPORTANCE OF POWDERCHARACTERIZATION The practical performance of a ceramic component, the microstructure of the ceramic, and powder behavior during processing are strongly dependent on the physical and chemical characteristics of starting powders [l]. In addition, in the manufacture of advanced ceramic components, detailed information on powder characteristics is required to achieve reproducibility and cost competitiveness. The choice of starting powder characteristics depends on the intended microstructure and application of the final ceramics. For instance, although total oxygen may be one of the most important measurements in silicon nitride powders because oxygen participates in the formation of a grain boundary silicate phase, the total Y2O3 content may be the most desired parameter in zirconia powder processing because it controls phase composition and transformation. The requirements of powder characterization are complicated by the fact that a large number of properties must be defined to understand its characteristics completely. A review of Table 1 shows that this task is not simple because the powders are complex materials consisting of numerous features. Therefore, ceramists are faced with the question of what characteristics of powders to measure for a given process. Commonly, an answer to this question lies in the development of parametric relationships between the powder properties and their influence on the performance of the ceramic. The next question is how to measure the powder characteristics. A program has been in progress 129

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Table 1 Properties of AdvancedCeramic Powders That Constitute Their Characterization Physical properties Specific surface area Primary particle size and size distribution Agglomerate size and size distribution Porosity, total quantity and pore size distribution Density Phase composition Crystalline phases, quantity and identification Amorphous material, quantity Chemical composition Major element concentration Minor impurities (10 ppm to =l%) Trace impurities (10 ppm) Inorganic elements Organic elements Composition of impurities Surface composition Major elements Minor elements Trace elements Inorganic species Organic species Crystallinity

since 1985 under the auspices of the International Energy Agency to develop procedures for the analysis of ceramic powders [2].An overall goal in this task is to define procedures that provide repeatable and reproducible data. The development of standard reference materials (SRM) stands at the forefront of this task since the SRM are necessary for the improvement of measurement quality [3]. In this chapter, we address the available methods for the characterization of ceramic powders, and selected methods have been briefly described. Primarily, for these methods, a description of operating principles, type of data to be obtained, and measurement limitations are presented. In addition, nuclear magnetic resonance (NMR) and x-ray photoelectron spectroscopy ( X P S ) techniques have been described in more detail because their treatment, as applied to powders, is not available elsewhere. Thermal techniques, such as differential thermal analysis, are not covered because of their specialized use on powders. For interested readers, a number of references have been cited for additional information.

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POWDERS

I31

II. PHYSICAL PROPERTIES The major physical properties of ceramic powders constitute size distribution of primary particles and agglomerates, specific surface area, density, porosity, and morphology (e.g., shape, texture, and angularity).

A.SizeDistribution In the measurement of particle or agglomerate size distribution, the major distinction is that agglomerates are made of primary particles. Hence, often there is a need to determine the true size of the underlying primary particles. In such cases, the size distribution of particles may be determined by a number of techniques, as shown in Table 2, which lists the commonly used methods [3]. With

Table 2 Commonly Used Methods of Particle Size Analysis, Nominal Size Ranges, and Measurement Parameters Nominal particle Measurement Method Coarse particles > 10 pm, sieving Dry Wet Fine particles < 10 pm Field scanning Optical microscopy Electron microscopy Gravity sedimentation Pipette Photoextinction X-ray absorption Radiation scattering, Laser diffraction, scattering Stream scanning Resisitivity Optical Ultrasonic attenuation Column hydrodynamic chromatography Sedimentation fieldflow fractionation Laser Doppler velocimetry Centrifugal sedimentation Photoextinction Mass accumulation X-ray absorption

>l0

Geometric

>2 0.5-1000

Image

0.01-10 1-100 0.5-100 0.1-130

Stokes

0.03-900

Geometric

0.05-500

Dynamic/Stokes

1-500 1 0 0 1 m 0.1-1 .o 0.01-1.0 0.01-3.0 0.05-100 0.05-25 0.1-5

Dynamic/Stokes

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such a large number of choices, the selection of a system for a given application depends on a number of criteria, including the size range of the application, throughput of the instrument, accuracy, precision, reproducibility, resolution, versatility, cost, and manufacturer support [4]. The three methods described in the following text are microscopy, gravity sedimentation, and light diffraction. Their selection is based purely on their technical diversity.

1. Microscopy Three commonly used direct viewing techniques are optical, scanning electron, and transmission electron microscopy. Optical microscopy is used in the size range 1-150 pm for the determination of morphology, agglomeration, and size distribution using automated counting devices. Despite significant improvements in optics, a major disadvantage is its depth of focus, which is about 0.5 pm at x1000 [5]. Scanning electron microscopy (SEM) is a versatile technique in which a beam of electrons at 5-50 keV scans the specimen surface. The resultingx-rays, backscattered electrons, andsecondary electrons are detected and analyzed by a number of techniques. Magnifications of up to ~100,000can be achieved at resolutions finer than 20 nm. Depth of focus and magnification are inversely proportional. Automated image analysis systems are available to analyze particle size distribution.However, the major issues remain sample preparationand dispersion of powder, that is, particles tend to agglomerate when deposited on SEM stubs and thereby do not provide complete and true data for primary particles. Intransmission electron microscopy (TEM), 5-0.001 pm particles deposited on thin membranes are examined at 10-100 times better resolution than that in SEM. TEM is more often used as an analytical tool for surface characterization and texture rather than for routine particle size distribution. 2. Gravity Sedimentation The underlying principle behind the gravity sedimentation method is Stokes’ law, which describes the relationship between the settling velocity of particles in a fluid medium of known density and viscosity [6]. A number of instruments are available in which the settling velocity of particles is measured by x-ray absorption, light absorption, and density changes. One such instrument is the Sedigraph* by the Micromeritics Corporation in which the particle size distribution is determined by x-ray absorption [6]. This instrument offers an excel-

*Certain trade names and company products are mentioned in the text or identified in illustrations to specify adequately the experimental procedure and equipment used. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the products are necessarily the best available for the purpose.

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lent measurement capability in the 0.2-50 pm range of a relatively concentrated suspension containing up to 6% by weight powder in an appropriate dispersion fluid. The data are presented as a continuous plot of percentage weight Of particles smaller than the stated size or other convenient formats. heparation of an appropriate dispersion is a key step in obtaining repeatable data [7]. The dispersion preparation for a given powder requires two Sets of data:(1) the amount and intensity of energy application to break agglomerates into primary particles; and (2) the pH at which the isoelectric point, the point at which the particles carry a net zero charge, occurs in a given aqueous solvent containing a surfactant for stabilization of particles. For each powder, it is necessary to develop an appropriate data for these parameters. 3. Light Scattering Instruments based on light scattering employ a laser beam to radiate particles in a stream, and the resulting scattered light is analyzed using the Fraunhofer and Mie theories to obtain size distribution data [g]. When the size of particles is very small with respect to the wavelength of light or when the refractive index of the particles is very close to that of the dispersion medium, such as that of a liquid to form a suspension, the relatively simple equations of Raleigh and Gans can be employed. For colloidal suspensions, however, it is necessary to resort to the Mie theory, which includes the restrictive conditions that the particles be spherical and intrinsically isotropic. With the advent of desktop computers, the application of the Mie theory has become a reality for polydisperse systems. Mie formulated equations to delineate light scattering by particles by considering electrical fields within and outside each particle. The equations for scattering intensity for each scattering angle involved refractive index difference between particle and suspending medium, the wavelength of incident light, and the spherical diameter of the particle. For particle diameter in the range of %o-10 times the wavelength of incident light, scattering from different portions of the particle is out of phase, which results in interference and reduced intensity. The net effect is an angular distribution of scattered light in the forward direction. This information is utilized in Mie theory computations. A number of instruments are available by which 0.03-900 pm particles can be measured in a matter of several minutes from dispersed suspensions. One major advantage of these systems is their ability to handle particulate systems containing different densities and refractive indices. A requirement of these instruments is the refractive index data, which may not be readily available for many powders. If the light-scattering particulate systems are not monodisperse, the particle size distribution obtained from these systems represents an average value, related but not identical to the weight-average particle size distribution. These averages are expected to differ depending on particle shape, particle size distribution, and degree of anisotropy.

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B.

Specific SurfaceArea

Particles consist of both internal and external surface area. The external surface area represents that caused by exterior topography, whereas the internal surface area measures that caused by microcracks, capillaries, and closed voids inside .the particles. Since the chosen surface area technique should relate to the ultimate use of the data, not all techniques are useful for fine powders. The commonly used approaches are permeametry and gas adsorption according to the Brunauer, Emmet, and Teller (BET) equation [g]. Because of simplicity of operation and speed of operation, permeametry methods have received much attention. The permeametry apparatus consists of .a chamber for placing the material to be measured and a device to force fluid to flow through the powder bed. The pressure drop and rate of flow across the powder bed are measured and related to an average particle size and surface area. Especially for porous powders, permeametry data include some internal surface area, thus decreasing their value. In the BET method, the volume or weight of adsorbed gas as a function of partial pressure is measured. Most commonly, NZgas is adsorbed by the powder at liquid N2 temperature from a gas stream of NZand He. The NZadsorbed and later desorbed is measured by thermal conductivity in one type of equipment. The BET equation describes the equilibrium between vapor and adsorbate in which multilayer adsorption occurs. The BET equation is typically written in the form PIP0

._

1 '

+

"

V(1- PIP,)

V,C

(C - 1)P& vmc

where PIP0 is relative pressure (the ratio of the equilibrium pressure to the saturation pressure at a specific temperature), V is the amount of gas adsorbed, and Vm represents V for monolayer coverage. The constant C is related to the energies of adsorption and gas liquefaction. Sample preparation is the key to obtaining reproducible data. Sufficient outgassing, minimal surface contamination, and the absence of microporosity are important aspects of the proper analysis of surface area. BET measurements are carried out in either singlepoint or multipoint mode, in which multipoint data are normally higher and more representative than those of the single point [2].

C.

Density

Theoretical, true, and tap density are three types of densities associated with ceramic powders. Theoretical density is determined by atomic composition and lattice parameters; tap density is determined by a prescribed procedure, which

CHARACTERIZATION OF ADVANCED CERAMIC POWDERS

I35

consists of measuring the volume of a known weight of powder after a certain number of mechanical taps. Therefore, tap density provides a relative measure of fill density or degree of compactibility of the powder. In the determination of true density, the powder volume is determined by the volume of gas displaced by a known weight of powder. Helium pycnometry is the commonly used method for this application. Helium is the most frequently used gas because of its inertness and small size, which enables it to penetrate even the smallest pores. Therefore, contribution tothe volume by pores can beaccounted for the measurement of true density.

D. Porosity Porosity in a powder can come from both closed and open pores. Two primary methods for the determination ofporosityare gas adsorption andmercury porosimetry. It is assumed that gas adsorption is favored in small capillaries because ofthe overlapping surface potentials, which result in capillary condensation.Pores from 1.5 to 100 nm in diameter are determined using the Kelvin equation, which relates capillary radius r to the ratio of the vapor pressure P and the equilibrium vapor pressure of the same liquid over a plane surface Po, as follows:

where y is the surface tension of the liquid, V is the molar volume of the liquid, 8 is the contact angle between the liquid and the wall, R is the gas constant, and T is the absolute temperature [lo]. Adsorption in the microporosity range ( r c 1.5 nm) often exhibits a Langmuir typeof isotherm, normally a characteristic of monolayer adsorption. The second principal method for porosity measurement is mercury porosimetry, in which a nonwetting liquid, such as mercury, is forced into capillaries of radius r. The force Foutcaused by interfacial tension y is calculated as F , ~ = 2 m y COS e

(3)

where 8 is the contact angle for mercury. The applied pressure P is related to the force driving the mercury Fin into a capillary and is given by -Fin

= P m2

Equating the two forces results in the Washburn equation [l l]:

(4)

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136

Mercury porosity measurements are carried out in the pressure range 14 Pa (ambient) to 415 Pa, which corresponds to pore radii from 2 nm to 200 pm.

E.

Morphology

The morphological analysis of particles constitutes the measurement of size, shape, and texture, which describes the surface profile of a particle image. The primary methods of morphology analysis are microscopy techniques, depending on the size of particles. In recent years, a large number of software packages for processing image data fromautomated microscopes have enabled faster accumulation and interpretation of data. However, the major drawbacks of microscopic techniques are representativeness of the sample examined and two-dimensional examination of the particle surface. The data from the shortest dimension may not be adequately represented.

111.

BULKCHEMICAL COMPOSITION

A large number of techniques are available, depending on the specific need to

analyze major chemical components, minor components, nonmetallic impurities, and metallic impurities (see Table 3). Further, the applicability of a technique depends on the concentration of impurities in the powder, a problem of considerable magnitude for ultrapure powders. A generalproblemwith the techniques'that require dissolution of the powders is their resistance to chemical attack and lack of complete dissolution. The two major methods of placing the powders in solution are acid dissolution and flux decomposition followed by dissolution [2,3]. The nonavailability of standard reference powders and procedures is a serious drawback to all the analysis methods. It is not possible to cover all the available techniques; therefore, only selected techniques with applicability to bulk and minor impurities are described here.

A.

InductivelyandDirectlyCoupledPlasma

These techniques, used for qualitative and quantitative measurements, utilize excitation of atoms in a plasma to obtain their characteristic emissions that are analyzed by photodetection. The powder is dissolved, and the resulting solution is injected into the plasma. The fast speed of analysis and applicability to a wide range of elements are strong points, but these methods are subject to errors resulting from powder dissolution. Currently, a number of industrial laboratories are addressing this issue.

B.

Atomic AbsorptionSpectroscopy (AAS)

A A S is a versatile method in which an analyte is atomized in a flame, thus emitting spectral lines. The characteristic spectral lines that correspond to the

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Table 3 Methods for BulkChemicalandImpurityAnalysis of Metallic and Nonmetallic Impurities in Ceramic Powders Bulk chemical analysis X-ray fluorescence spectroscopy Atomic absorption spectroscopy Inductively coupled plasma emission spectroscopy Direct-current plasma emission spectroscopy Arc emission spectroscopy Gravimetry Combustion Kjeldahl Impurities Neutron activation analysis Mass spectrometry Electrochemical Coulometry Selective-ion potentiometry Potentiometric titration Argentometric Ion chromatography Nuclear magnetic resonance Electron paramagnetic resonance energy required for an electronic transition from the ground state to an excited state are emitted. The absorption of radiation from the light source depends on the population of the ground state, which is directly proportional to solution concentration. The absorption is measured by the difference in transmitted signal in the presence and absence of the test element [ 121. However, this technique is also subject to errors resulting from powder dissolution.

C.

X-rayFluorescenceSpectroscopy

The powder in the form of fine particulates or dissolved in solution is excited in an x-ray source, and the characteristic fluorescence intensity is analyzed for qualitative and quantitative data. Standards are required for the conversion of x-rayintensities to absolute concentrations. The preparation of appropriate standards that include background similar to that in the solution tobe analyzed is critical to obtaining accurate data.

D.

Nuclear Magnetic Resonance and Electron Paramagnetic Resonance (EPR)

In principle, NMR utilizes the atomic nuclear transition, induced by radio frequency irradiation, between two quantized nuclear energy levels in a magnetic

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AL.

field. This transition caused by resonance in a magnetic field occurs in all atomic nuclei, except in atoms with even numbers of both mass and atomic number. The nuclear spin numbers are zero for atoms withboth mass and atomic numbers that are even, and consequently, nuclear energy splitting does not occur in a magnetic field and these atoms are not active in NMR. All other isotopes in the periodic table are NMR active. ‘H, W , 15N,19F,27Al, W i , 31P, 47Ti, 63Cu, 69Ga, 89Y, and 91Zr are some examples. For an atom with a nuclear spin number I , there are 2 I + 1 quantized nuclear energy levels: -I, -(I - l), . .. (I - l), I. These levels are practically equal in energy outside a magnetic field. If a magnetic field is applied to this atom, however, the nuclear energy split into 2 I + 1 levels separated by (yh/27c)H, where H is the magnetic field strength, h is the Planck constant, and y is the nuclear magnetogyric ratio. The nuclear energy E is expressed by

E=--

rh m,H 2x

where MI = I, (I - l), .. . , -(I - l), -I. If a sample is irradiated by a radiofrequency energy equal to the energy separation, resonance occurs, the nuclei at the ground state are excited to a higher energy state, and an absorption signal is observed. The NMR is therefore an absorption spectroscopy, not an emission spectroscopy (e.g., X P S ) . The magnetogyric ratio is a constant for a specific isotope of interest. However, the magnetic field H is an effective field to the nuclei including the applied external field, the field induced by the electrons around the nuclei, and even the field produced by other parts of the molecule. For Si atoms, for example, 95.3 atom% are 28Si, which have a nuclear spin I = 0 and are not NMR active ( 2 I + 1 = 1, only one energy level; therefore, no resonance can occur). There are 4.7 atom% 29Si in natural abundance, however, and 29Si has a nuclear spin I = H. In an external magnetic field of 9.4 T, for example, the 29Si nuclear energy is split into two levels separated by approximately 79.5 MHz, with mr = H and -H and with H lower in energy. In addition, the actual magnetic field experienced by the 29Si nuclei in Sic, Si3N4, and Si02 is different and not exactly 9.4 T because the electron density and distribution around the Si nuclei in these compounds are different. This difference makes it possible for us to observe the absorption signals at different field strengths and is the “chemical shift.” Chemical shift is a comparative value and is most often referenced to tetramethylsilane [TMS, Si(CH3)4]. This compound can be used as a reference to three frequencies: ‘H, W , and 29Si. The frequency difference between the sample and that of TMS is often divided by the frequency of the spectrometer to give a value in parts per million (ppm), which is spectrometric frequency independent. Other factors, such as spin-spin interaction with neighboring atoms, affect Si nuclear energy level splitting. For nuclei with I 2 1, the nuclear quadrupole

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effect also alters the energy level splitting. A more in-depth coverage of N M R theory is found elsewhere [13,14]. N M R spectroscopists are also interested in nuclear spin relaxation times. The relaxation time measures the time required for an excited nucleus to return to the ground state. Two types of relaxation times are involved: spin-lattice relaxation time T i , the time constant for thermal equilibrium between the nuclei and crystal lattice, and spin-spin relaxation time T2, the time constant for thermal equilibrium between nuclei themselves. Information on molecular dynamics can be obtained from these relaxation times. Generally, Ti = T2 for lowviscosity liquids and Ti >> T2 for solids. A combination of information in molecular dynamics (from relaxation times), molecular structure (from spinspin interaction), molecular identification (from resonance frequency and chemical shift), and spin density (from signal intensity) make the NMR an extremely versatile tool. The application of NMR spectroscopy to ceramic powders is a relatively new area, but it has potential. N M R application in solids is difficult because of large dipole-dipole interactions that result in line broadening and thus poor resolution. However, by the application of modem instrument technology and discoveiy of pulsed Fourier transform N M R , scientists have developed various line-narrowing techniques to improve resolution [15]. One of the most often utilized techniques is magic angle spinning (MAS). Molecules in solids are not mobile and tumbling as they are in the liquid or gaseous state. This induces large dipolar interactions between two spin centers. If we physically spin the sample, at several kilohertz, the anisotropic part of the nuclear dipole is eliminated. The dipole-dipole interaction is a function of (3 cos2 8 - l), where 8 is the angle between the static magnetic field and internuclear vector. Setting this term to zero, the “magic” angle is 54”44’ [16]. N M R has been applied most successfully for high-temperature superconductors, YBa2Cu307-6 [17-211. The studies involve mainly 89Y,63Cu, and 65Cu NMR and nuclear quadrupole resonance of this compound below, above, and at the critical temperature. Nuclear relaxation, Knight shift, and crystal structure are often examined at these temperatures. 29Si and *7Al N M R have been applied to silicates, aluminosilicates, and surface-adsorbed molecules on ceramic materials [22]. N M R can also be a powerful technique to determine the surface area [23], porosity, pore size, and molecular diffusion into the crystallattice [24]. The technique is basedon the different relaxation times (or linewidths) of the mobile molecules and adsorbed molecules, the latter losing certain degrees of freedom. The crystal structure of B a T i e has been characterized by 137Ba, 47Ti, and 49Ti NMR at various temperatures up to the Curie point [25]. Structural changes in the alkaline-earth silicate glasses induced by phosphorus were also studied by 31P and 29Si NMR [26]. 29Si MAS NMR is a very useful technique to study the crystal-phase composition and transition in silicon nitride and carbide [27,28]. The a phase of

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these compounds has more than one Si crystal site; while p phase has only one. This yields different line patterns for different phases. MAS NMR has been used to study high-temperature reactions, sintering, the formation of surface oxide and suboxide, and reaction products with the sintering aid [29-341. The theory of EPR (also known as electron spin resonance) is very similar to that of the NMR except the frequency falls into gigahertz region because the mass of electrons is smaller. Electrons are negatively charged, so that the splitting of energy is inverted in its order compared with the nuclei [35]. Since only molecules with unpaired electrons are EPR active, the application of EPR to ceramics is limited.However,EPR is a unique technique for characterizing powders with dangling bonds (as in carbon atoms), impurities, and defects.

E. Combustion This technique is limited to such elements as carbon, nitrogen, and oxygen that can be liberated in gaseous form during decomposition. The concentration of gas is determined by a property of the gas. Commonly a flux is used to aid the combustion [3,36].

IV.

PHASE COMPOSITION

A.

X-rayPowderDiffraction(XRPD)

This is the most widely used method for the determination of the phase composition of powders. The x-ray diffractometer contains a source of monochromatic x-rays that irradiate the sample and are diffracted from atomic planes and detected. The angle of diffraction of x-rays by the crystalline planes is characteristic of the crystal structure, and the intensity of scattered radiation is characteristic of the atomic composition. In recent years, automated data processing has enabled higher accuracy andspeed. A number of problems are encountered in the quantitative determination of phases in fine powders. Some of these are overlap of phase peaks (e.g., in silicon nitride), orientation of grains, and presence of coarse particles. The last produces distortion of the diffraction data. A number of standard reference materials for XRPD have been developed for use in improving the quality of data [37].

V. A.

SURFACECHEMICAL COMPOSITION BY SPECTROSCOPY X-ray Photoelectron Spectroscopy and X-ray-Induced Auger Electron Spectroscopy (XAES)

X P S (also known as electron spectroscopy for chemical analysis) and XAES provide not only the surface elemental composition but also reveal the oxida-

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tion states of the elements. X P S , especially, has emerged as one of the most important techniques for studying the chemistry of the ceramic-powdersurface as a result of oxidation at high temperatures. In X P S , when soft x-rays, such as MgKa (1253.6 eV), impinge on a powder surface, photoelectrons with binding energy BE are ejected from the surface with kinetic energy (KE). The relationship among the irradiation energy hv (1253.6 eV in the case of MgKa), KE, and BE is as follows: KE=hv-BE-eS

(7)

where is the work function of the spectrometer. Since is a constant for a specific spectrometer, hv is known and KE can be measured; therefore, BE can be calculated. Often, residual carbon in the form of hydrocarbon (-CH) is found on the powder surface as a contaminant, and the C 1s photoelectron peak at 284.5 eV binding energy is used as a reference to calculate the binding energies of other signals. From the binding energy of the emitted electrons, the surface elemental composition and chemical state can be derived. Detailed information on surface analysis techniques, comparison of XPS and Auger electron spectroscopy (AES), and photoelectron binding energies can be found in the respective references [3840]. For quantitative surface composition, the p