International Journal of Powder Metallurgy Volume 45 Issue 5

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Focus Issue: Precious Metals

45/5 Newsmaker: Richard Pfingstler Precious Metals: A Valuable PM Player Precious Metal Powder Applications Manufacture of Platinum, Gold, and Palladium Powders Precipitation and Processing of Precious Metal Powders Additive Manufacturing of Precious Metal Dental Restorations

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EDITORIAL REVIEW COMMITTEE P.W. Taubenblat, FAPMI, Chairman I.E. Anderson, FAPMI T. Ando S.G. Caldwell S.C. Deevi D. Dombrowski J.J. Dunkley Z. Fang B.L. Ferguson W. Frazier K. Kulkarni, FAPMI K.S. Kumar T.F. Murphy, FAPMI J.W. Newkirk P.D. Nurthen J.H. Perepezko P.K. Samal D.W. Smith, FAPMI R. Tandon T.A. Tomlin D.T. Whychell, Sr., FAPMI M. Wright, PMT A. Zavaliangos INTERNATIONAL LIAISON COMMITTEE D. Whittaker (UK) Chairman V. Arnhold (Germany) E.C. Barba (Mexico) P. Beiss, FAPMI (Germany) C. Blais (Canada) P. Blanchard (France) G.F. Bocchini (Italy) F. Chagnon (Canada) C-L Chu (Taiwan) O. Coube (Europe) H. Danninger (Austria) U. Engström (Sweden) O. Grinder (Sweden) S. Guo (China) F-L Han (China) K.S. Hwang (Taiwan) Y.D. Kim (Korea) G. L’Espérance, FAPMI (Canada) H. Miura (Japan) C.B. Molins (Spain) R.L. Orban (Romania) T.L. Pecanha (Brazil) F. Petzoldt (Germany) G.B. Schaffer (Australia) L. Sigl (Austria) Y. Takeda (Japan) G.S. Upadhyaya (India) Publisher C. James Trombino, CAE [email protected] Editor-in-Chief Alan Lawley, FAPMI [email protected] Managing Editor James P. Adams [email protected] Contributing Editor Peter K. Johnson [email protected] Advertising Manager Jessica S. Tamasi [email protected] Copy Editor Donni Magid [email protected] Production Assistant Dora Schember [email protected] President of APMI International Nicholas T. Mares [email protected] Executive Director/CEO, APMI International C. James Trombino, CAE [email protected]

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45/5 September/October 2009

Editor’s Note PM Industry News in Review Newsmaker Richard Pfingstler Consultants’ Corner John A. Shields, Jr.

FOCUS: Precious Metals 15 Precious Metals: A Valuable Powder Metallurgy Player P.W. Taubenblat, FAPMI

21 Applications for Precious Metal Powders J.T. Strauss

29 The Manufacture of Platinum, Gold, and Palladium Powders H.D. Glicksman

37 Precious Metal Powder Precipitation and Processing S. Frink and P. Connor

43 Additive Manufacturing of Precious Metal Dental Restorations A.L. Hancox and J.A. McDaniel

DEPARTMENTS 53 Meetings and Conferences 55 APMI Membership Application 56 Advertisers’ Index Cover: Silver powder blended into ink paste is used to create conductive grid on face of solar panels. Photo courtesy Ferro Electronic Material Systems.

The International Journal of Powder Metallurgy (ISSN No. 0888-7462) is a professional publication serving the scientific and technological needs and interests of the powder metallurgist and the metal powder producing and consuming industries. Advertising carried in the Journal is selected so as to meet these needs and interests. Unrelated advertising cannot be accepted. Published bimonthly by APMI International, 105 College Road East, Princeton, N.J. 08540-6692 USA. Telephone (609) 4527700. Periodical postage paid at Princeton, New Jersey, and at additional mailing offices. Copyright © 2009 by APMI International. Subscription rates to non-members; USA, Canada and Mexico: $100.00 individuals, $230.00 institutions; overseas: additional $40.00 postage; single issues $55.00. Printed in USA. Postmaster send address changes to the International Journal of Powder Metallurgy, 105 College Road East, Princeton, New Jersey 08540 USA USPS#267-120 ADVERTISING INFORMATION Jessica Tamasi, APMI International 105 College Road East, Princeton, New Jersey 08540-6692 USA Tel: (609) 452-7700 • Fax: (609) 987-8523 • E-mail: [email protected]

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EDITOR’S NOTE

P

recious metal powders find wide commercial applications because of their unique characteristics and properties, including thermal/electrical conductivity, corrosion resistance, solderability, and stability at high temperatures, coupled with reliability. Applications include conductive adhesives, dental restorations, catalysts, and sensors to monitor temperature and corrosion, and to detect gases. In comparison with ferrous powder metallurgy (PM), particulate precious metal science and technology receives limited exposure and press in industry news coverage. This “Focus Issue,” coordinated by Pierre Taubenblat and Joe Strauss, traces the history of particulate precious metal technology and gives an overview of market dynamics and current and near-term industrial applications. Comprehensive reviews of precious metal powder production methods and the manufacture of dental restorations are also included. The photograph on the front cover illustrates the application of a silver powder-ink paste blend to create a conductive grid on solar panels. In a second visit to the “Consultants’ Corner,” John Shields provides counsel on a range of timely PM issues: the implications of observed differences in transverse rupture strength between 304L and 316L stainless steel in the context of the applicable MPIF standard; the short-term (3 years) and long-term (10 years) outlook and prospects for high-temperature sintering in relation to production/fabricator/end-customer use; and advances in high-temperature-sintering furnace design in terms of productivity and cost. For close to three decades, “Newsmaker” Richard Pfingstler, president of Atlas Pressed Metals, and current president of the Powder Metallurgy Parts Association (PMPA), has been intimately associated with the PM industry. An entrepreneur by nature, he has guided the family business as a respected supplier of PM parts for lawn-and-garden applications, and small motors, avoiding excessive dependence on OEM automotive manufacturers. A strong work ethic, commitment to the professional development of his employees, and customer longevity are trademarks of Dick Pfingstler’s business philosophy.

Alan Lawley Editor-in-Chief

Although I have been a resident of the New World for over 50 years, on occasion I think back to my roots in the Old Country and, in particular, the English Midlands. Fellow Midlander Roger Lawcock recently sent me a vivid description of this area of England, attributed to Scottish writer Samuel Smiles who visited the Wolverhampton area in 1850: “The earth seems to have turned inside out. Its entrails are strewn about; nearly the entire surface of the ground is covered with cinder heaps … coal … is blazing on the surface … by night the country is glowing with fire, and the smoke … hovers over it.” Smiles’ visit occurred toward the end of the Industrial Revolution and predates any thought of the “Green Movement” and “Cap-and-Trade”! While memory has dimmed over the years, I do not recall my boyhood environment being quite as bad as described by Smiles.

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Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

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Powering environmental improvements from powder

w w w. nah. c o m

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PM INDUSTRY NEWS IN REVIEW The following items have appeared in PM Newsbytes since the previous issue of the Journal. To read a fuller treatment of any of these items, go to www.apmiinternational.org, login to the “Members Only” section, and click on “Expanded Stories from PM Newsbytes.”

Copper Powder Trends in Europe The copper powder industry in Europe has been negatively impacted by the global economic downturn and declining automotive production, reports Claus Heitmann, managing director, ECKA Granulate MicroMet GmbH. However, he says that ECKA has positioned itself with new production facilities in England and Germany to take advantage of demand when the global economy returns to a growth cycle. Brazing Paste Partnership Umicore BrazeTec, Umicore AG & Co. KG, Germany, and Höganäs AB, Sweden, announce a partnership agreement for a new filler material for high-temperature brazing stainless steel applications in corrosive environments. The companies will conduct joint marketing and customer projects. Plansee Sales Top a Billion Euros The 2008–09 fiscal year sales of the Plansee Group, Reutte, Austria, gained two percent to 1.1 billion euros (about $1.5 billion). Europe accounted for half of the group’s sales, with Asia and the U.S. sharing equally in the balance. Union Process Expands Union Process, Inc., Akron, Ohio, has added 6,000 sq. ft. of manufacturing space to meet growing

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demand for its toll milling service. As the inventor and developer of attritor technology, the company makes wet and dry size-reduction and dispersing equipment.

N.Y., as the stalking horse bidder for most of its Powertrain operations. The operations include all of Metaldyne’s Sintered Products operations.

PM Parts Maker Closes, Sells Assets According to local media sources, Hazen Powder Parts LLC (HPP), Hazen, Ark., will cease production and is selling its automotive customer list and compacting presses and sintering furnaces to SSI Sintered Specialties, Janesville, Wis. HPP, owned by MNP Corp., Utica, Mich., employs about 70 workers, who have received termination notices.

Winner of Sanderow Technical Paper Award Announced The MPIF Technical Board has announced that “Influence of Chemical Composition and Austenitizing Temperature on Hardenability of PM Steels” by Peter Sokolowski and Bruce Lindsley, Hoeganaes Corporation, is the recipient of the 2009 MPIF Howard I. Sanderow Outstanding Technical Paper Award.

Uncertain Recovery for Powder Maker First-half 2009 sales of Höganäs AB, Sweden, dropped 37 percent to MSEK 2,014 (about $257 million), while powder shipments declined 42 percent. The company notes a gradual improvement in demand and a very satisfactory positive cash flow of MSEK 439 (about $56 million). Metaldyne Offers Sintered Products Operations Metaldyne Corporation, Plymouth, Mich., reports the U.S. Bankruptcy Court for the Southern District of New York has approved Hephaestus Holdings, Inc. (HHI), affiliated with KPS Capital Partners LP, New York,

GKN Sales Dip, Improvement Expected in September Sales of GKN plc, London, UK, for the first six months declined nine percent to £2,174 million (about $3.6 billion). The company’s automotive and powder metallurgy (PM)) businesses returned to profitability in June despite an overall sales plunge of 41 percent. Ultrasonic Fatigue Testing of Hardmetals In September the European Powder Metallurgy Association (EPMA) will launch an eightmonth project on ultrasonic fatigue testing of hardmetals in the gigacycle regime. Leading academic institutions, such as the Technical University Vienna, CEIT San Sebastian, and NPL Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

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PM INDUSTRY NEWS IN REVIEW

London, are cooperating in the study, as are industry companies that will assist in the selection of materials.

partial-pressure sintering and debinding furnaces and three catalytic ovens, reports Claus Joens, Elnik CEO.

Metaldyne Sells Sintered Products Operation Metaldyne, Plymouth, Mich., reported the sale of its assets, including the Sintered Products unit, to MD Investors Corporation. MD submitted the highest and best bid in a 363 U.S. Bankruptcy Law sale.

Copper Powder Firms Fail Makin Metal Powders Ltd., Rochdale, England, a producer of copper and other nonferrous powders, has filed for insolvency. KPMG, which has been appointed administrator for Makin in order to manage the business, is talking with several potential buyers. In a separate development, Ecka Holding Co., Fürth, Germany, has also filed for insolvency.

New MIM Plant Opens Hock Sachsen GmbH has purchased furnace equipment for a new metal injection molding (MIM) plant in Schwarzenberg, Germany, from Elnik Systems, Cedar Grove, N.J. The equipment purchase includes four

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

NanoSteel Wins R&D 100 Award The NanoSteel Company, Inc., Providence, R.I., has won an

R&D 100 Award from R&D magazine for developing its NanoSteel Super Hard Wear Plate for highwear applications. An alternative to composite carbide overlay and monolithic quench-and-temper wear plate, the NanoSteel material has an ultrafine, sub-micron microstructure that provides exceptional resistance to abrasive wear, fine particle erosion, and impact. Powder Company Adds Equipment Advantage Metal Powders, Inc. (AMP), Ridgway, Pa., has acquired a new delivery vehicle to increase its product distribution capacity. They have also taken delivery of a new automated Cleveland vibrating system for screening reground powder, a

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PM INDUSTRY NEWS IN REVIEW

move that will enhance production throughput, according to the company. PIM Feedstock Testing Arburg GmbH + Co KG, Lossburg, Germany, has developed a simple machine-based test method for powder injection molding (PIM) feedstocks using its small Allrounder 170 S machine. The use of this machine, with the Selogica control system, in a quality-control laboratory setting is much more cost effective than a high-pressure capillary viscosimeter, Arburg reports. Starck Reports Sales Decline for 2008 H.C. Starck Group, Goslar, Germany, a producer of refractory metals and products, reports 2008 sales declined 5.4 percent to 856 million euros (about $1.2 billion). Adjusted for the impact of currency fluctuations and the sale of the Group’s Levasil business, says the company, sales were even with 2007 levels. Tungsten Results North American Tungsten Corp., Vancouver, B.C., Canada,

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reports a net loss of Can$0.8 million on sales of Can$15 million for the third quarter of its fiscal year ended June 30, 2009. The loss occurred after a gain of Can$1 million from the sale of a mineral property, compared with a net loss of Can$1.3 million in the third quarter of fiscal 2008. Miba Sales Weaken Miba AG, Laakirchen, Austria, reports first half fiscal 2009–10 sales fell 26 percent to €148.5 million (about $217 million) compared to the previous fiscal year. The Sinter Group (PM parts), which accounts for 37.7 percent of the company’s total sales, posted a 29 percent decrease in sales to €56 million (about $82 million). Crucible Bankruptcy Attracts Buyers Crucible Materials Corp., Syracuse, N.Y., is talking with two potential buyers of its specialty steel mill, services centers, and compaction metals operation, according to American Metal Market. The company filed for Chapter 11 bankruptcy in May.

H.C. Starck Increases Capacity for Tungsten Products H.C. Starck GmbH, Goslar, Germany, has increased production capacity for tungsten catalyst intermediates/precursors based on recurring global shortages of high-quality sodium tungstate and ammonium metatungstate compounds. The company forecasts steady international growth rates for fine chemicals, agrochemicals and special catalysts. Wellman Friction Products Recognized Wellman Products Croup, Solon, Ohio, a division of Hawk Corp., has received Eaton Corporation’s Premier Supplier award for its delivery, quality, product performance, and inventory management. Wellman makes severe-duty and high-performance friction materials for clutch, brake, transmission, and industrial motion-control systems. ijpm

Have you visited APMI Members Only recently? Did you know that you can renew your dues online, update your membership information, find other APMI members, and even read the current and back issues of the International Journal of Powder Metallurgy and PM Newsbytes? It’s important to keep your membership information up-to-date so you don’t miss out on any benefits of membership. Visit www.apmiinternational.org, and click on “APMI Members Only.”

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NEWSMAKER

RICHARD PFINGSTLER

By Peter K. Johnson*

Dick Pfingstler’s story of entrepreneurial success is not unlike others that have come out of Western Pennsylvania, the heartland of PM. The president of Atlas Pressed Metals, DuBois, Pennsylvania, grew up in nearby St. Marys where his father owned a lumber company. “My only exposure to the powder metal (PM) business was delivering materials to local PM parts makers during summer vacations when I was in high school and college,” he says. “I knew about PM because it was the biggest industry in town.” Working in the family business whetted his appetite for finance, which motivated him to major in accounting at Gannon University in Erie. Aiming initially at a career in public accounting, he landed a position with Peat, Marwick & Mitchell (now KPMG) in Cleveland after graduating in 1972. As a professional accountant, he earned the CPA certification and put in long hours conducting corporate audits. He was doing well but the entrepreneurial drive he acquired working in the family lumber business began to stir within him. “I wanted to do something different and run my own shop,” he recalls. “Maurice Hanes, my father-in-law and former coowner of Carbon City Products (CCP), made me think about the PM business.” CCP was a typical Western Pennsylvania PM parts manufacturer founded by Hanes, Dan DeLullo, and Larry Geitner in 1959. Approaching retirement age, the three local entrepreneurs sold the company to private investors in 1977. Hanes stayed involved by signing on as a manufacturer’s representative with CCP’s new owners. Over the next several years Pfingstler had serious discussions with his father-in-law as he sought advice *Contributing editor

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

about starting a new PM business. He wrote a business plan in 1979 and approached several banks about financing. Due to economic conditions, the banks were not interested in financing a start-up business, so his immediate idea was crushed. But a new opportunity presented itself the following year. Learning that Atlas Pressed Metals was up for sale, Pfingstler and his father-in-law purchased the business from Dennis Heindl in 1980. The three-year-old company specialized in self-lubricating bronze bearings, with a primary customer base of bearing distributors. It had 10 employees and sales under $1 million. At 29, Pfingstler’s mettle was tested severely as sales plunged 50 percent within three months of buying the business. “The national economy went down the tubes. It was a rough time,” he says. Running a small manufacturing business was a new experience. He did it all, from sales to production to finance, everything except engineering. Hanes helped his son-in-law with tooling design and developing quotes. On a cold-call sales visit in late 1980, Pfingstler landed an important new customer in Ohio. “The purchasing manager took a liking to me and was unhappy with his current vendor,” he says. “Within six months he transferred 15 to 20 parts to us.” That was a shot in the arm that helped turn the business around. When not making sales calls, Pfingstler learned the PM business hands-on, operating compacting presses and loading sintering furnaces during the daytime and devoting late evenings to finance. Working 12-hour days Monday through Saturday was not uncommon for him. He drafted a five-year ijpm

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NEWSMAKER: RICHARD PFINGSTLER

plan to eventually leave the distributor side of the business and add more structural parts for new markets such as lawn & garden, appliances, and small motors. Early on he adopted a conservative fiscal policy and chose not to become too dependent on OEM automakers. He says he is a hands-on manager but tries to stay out of everyone’s way and let them do their jobs. As one of more than 35 PM shops in Western Pennsylvania, Atlas operates from a 4,182 sq. m (45,000 sq. ft.) plant with 25 compacting presses, with a capacity of up to 181 mt (200 st), 10 sizing presses, and five sintering furnaces. At full capacity the company has about 70 employees, and its direct sales force concentrates on customers east of the Mississippi River. Pfingstler defines Atlas as a family business with a strong German work ethic. “We maintain the Golden Rule in all areas of our business,” he stresses. “A passion for this ethic can only lead to excellence.” Family members include his brothers Tom (engineering) and Joe (sales), sons Jude (sales) and Doug (production), and son-in-law David (quality). Each family member specializes in one job classification. One of the advantages of a family-run company, Pfingstler believes, is that opinions can be freely shared. “We can argue about different views, which keeps everyone on their toes,” he says. In addition, employees have direct access and input to all facets of the business. Aside from internal training programs, Atlas uses courses from Penn State University to educate new employees. In Pfingstler’s opinion small and medium-size family companies will remain a permanent feature of the PM landscape. “There will always be a need for firms where customers feel they are important to the owners,” he says. “For example, we care very seriously about our customers. Some have been with us for more than 20 years.” The growth and success of the North American PM parts industry are important to Pfingstler, as evidenced by his participation in the activities of MPIF. He served on the MPIF Finance Committee for five years and was recently appointed president of the Powder Metallurgy Parts Association (PMPA). “MPIF and PMPA serve to strengthen the PM industry’s identity in the marketplace and are a good central point for information,” he says. “Membership is valuable because you have the opportunity to interact with other people in the

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industry. This provides a better understanding about industry business conditions.” Participating fully in the life of his close-knit local community is a complement to Pfingstler’s professional career. Among his voluntary posts have been chairman of the board of the DuBois Regional Medical Center, president of the YMCA, and president of the Dubois Area High School Educational Foundation. Pfingstler acknowledges his wife Chris for her important role. “It would have been very difficult to put in the long hours to build the business without her support,” he says. “I have been blessed with a loving wife.” ijpm

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CONSULTANTS’ CORNER

JOHN A. SHIELDS, JR.* Q

For several years, a powder metallurgy (PM) part has been produced using 316L stainless steel powder, at 6.7 to 6.8 g/cm3 sintered density. To try to reduce the raw material cost, a test lot was produced using 304L stainless steel powder. Compaction density, lubricant type and quantity, sintering conditions, and sintered density remained unchanged. Surprisingly, the standard MPIF rupture test showed a decrease in strength (~10%) with 304L. In both cases, the total carbon ranged between 0.04 and 0.06 w/o. According to the MPIF standard, no strength difference should exist between the two alloys. What is the explanation for this difference in strength? The first issue to discuss is the use of transverse rupture strength (TRS) to measure the strength of these alloys. The test is not appropriate for ductile materials such as austenitic stainless steel. TRS is calculated using elastic bending formulae, so plastic deformation (or even a large elastic deformation) renders the results inaccurate. MPIF Standard 411 states, “The test … is applicable only to materials of negligible ductility.” It notes that when deflections during the test exceed 0.5 mm (0.02 in.), the test results may be questionable. As deflection increases, the point of maximum load may not be the point of maximum stress. The standard goes further, recommending tensile testing as the appropriate way to measure the strength of ductile materials. Let us assume that the relative ranking of the alloys would not have been different had they been tested in tension. My initial reaction to the question is: “Why the surprise? 316L has 2–3 w/o molybdenum added to the alloy, while 304L has none.” In the spirit of full disclosure, I confess that I spent several years working at the Climax Molybdenum Ann Arbor laboratory, where adding molybdenum to almost anything, including food, was by definition a good thing. The answer does not lie in the molybdenum content alone, though.

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The molybdenum and nickel contents are both ~2 w/o higher in 316L than in 304L, while chromium is ~2 w/o lower. Alloy-hardening rates in ferrite 2 show that nickel is the strongest hardener of the three elements, followed by molybdenum, and finally chromium. It is dangerous to infer effects in austenite from ferritic alloy data, but on a simple lattice-strain basis, one could argue that the differences in hardening rate should be greater in face-centered cubic closepacked austenite than in the body-centered cubic ferrite. In any case, the net effect of the shift in alloy content from 316L to 304L should be to lower the strength. Another point worth mentioning is that the carbon-content range cited in the question is greater than the maximum allowable value for both alloys.3 For the purpose of the comparisons made in the question, this is not so important, but from the standpoint of selling product that meets specification and exhibits the expected corrosion performance, it is critical. Table I reproduces selected property values for 304L and 316L from MPIF Standard 35, obtained on samples sintered in vacuum. I have grouped the alloys at the same sintered density for comparison. The 316L minimum yield strength values are 10% and 20% higher than those of 304L at densities of 6.6 g/cm3 and 6.9 g/cm3, respectively. For 316L, typical yield strength values have a 15% advantage at both sintered density levels, so the experience described in the question is consistent with the numbers in Standard 35, as well as with expectations based on alloy-hardening rates. The specification values are based on work funded by MPIF and the U.S. Navy, and have been published since 1995. 4 The work showed sintering atmosphere to have a significant effect on mechani-

*Principal, PentaMet Associates LLC, 4457 Brooks Road, Cleveland, Ohio 44105-6053, USA; E-mail: [email protected]

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CONSULTANTS’ CORNER

TABLE I. SELECTED MECHANICAL PROPERTIES FROM STANDARD 35 FOR 304L AND 316L STAINLESS STEEL Minimum Required Values σy, MPa σu, MPa Elongation (psi) (psi) %

σy, MPa (psi)

Typical Values σu, MPa Elongation (psi) %

6.6

90 (13,000)

-

15

120 (18,000)

300 (43,000)

23.0

ND*

SS-316L-15

6.6

100 (15,000)

-

12

140 (20,000)

280 (42,000)

18.5

550 (80,000)

SS-304L-18

6.9

120 (18,000)

-

18

180 (26,000)

390 (57,000)

26.0

ND

SS-316L-22

6.9

150 (22,000)

-

15

210 (30,000)

390 (57,000)

21.0

ND

Alloy

Density, g/cm3

SS-304L-13

TRS, MPa (psi)

*Not determined for purposes of the specification

cal properties. As an example,5 for samples sintered to a lower density than cited in the question (6.44–6.45 g/cm3) in dissociated ammonia, 304L had higher yield strength than 316L by ~5%. At higher densities (6.81–6.85 g/cm 3), the margin favoring 304L increased to ~9%. If the sintering atmosphere was hydrogen, the strengths of the two alloys at a density of 6.93 g/cm3 were identical. The case in question did not state what sintering practice was used to produce the parts, only that the practice was identical for both alloys. The results suggest that the parts were sintered in vacuum, but if the test results were compared with data for parts sintered in dissociated ammonia, a disparity would have been observed. This emphasizes the importance of being certain that any test results be analyzed with reference to the appropriate data set. If this is not done, discrepancies can surely arise. Statistical analysis is critical to standards development. This is an area where the wrought stainless industry has an advantage over PM because of the amount of material manufactured and tested on an ongoing basis. The work funded by MPIF and the Navy was an important contribution to the data on PM properties, but the resulting data sets are still smaller than ideal, making the statistical analysis more uncertain because they include information only from those companies willing to commit resources to the development of specifications. It is important for the PM community to be actively involved in the MPIF specification development process. To do so brings benefits to all, including: • Larger data sets that allow more refined statistical analysis of the information • Opportunities to discuss the technical merits

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of specification changes before they are implemented • Opportunities to benchmark manufacturing processes • Specifications that reflect the industry’s capabilities more accurately, providing incentive for raising the norms, and increasing users’ confidence in PM parts sold to specification

Q

Will the need for sintering at higher temperatures increase? If so, for what production/manufacturer/end-customer use? What is the predicted market penetration for this technology 3, 5, and 10 years hence? In 2008, I had the distinct pleasure of making a presentation on sintering refractory metals at the MPIF PM Sintering Seminar held in Cleveland, Ohio. The seminar lasted two full days, with fourteen presenters covering a broad spectrum of topics. It was a fascinating and highly instructive experience, and I highly recommend it to PM companies as a place to learn about the scientific basis for sintering, current sintering practices, atmosphere chemistry, process troubleshooting, furnace design and economics, and emerging sintering technologies. More important, it is an excellent opportunity for one-on-one conversations with academics, raw-material producers, gas suppliers, parts makers, and furnace manufacturers about real-world needs and problems. Nearly every presenter touched on the topic of high-temperature sintering (HTS) in one way or another. My talk dealt with a different HTS regime than normally considered in ferrous PM. Figure 1 shows sintering temperatures for several refractory metals, and includes a reference line for HTS. The

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CONSULTANTS’ CORNER

blue bars show the melting temperatures for iron and the refractory metals niobium, molybdenum, tantalum, rhenium, and tungsten. The red bars show half the absolute melting temperature, approximately the temperature at which diffusion becomes important for metals and a useful reference point when comparing sintering practices. The equivalent Fahrenheit temperature is noted above each bar. Light-blue regions within the blue bars show the temperature range where the metals are normally sintered. Niobium is nearly always produced as a melted product, so none is shown for this metal. Table II condenses the information shown in Figure 1, and employs the concept of “homologous temperature.” This is the temperature of interest (in this case the sintering-temperature range) measured on the absolute temperature scale, divided by the melting point of the metal measured on the absolute temperature scale. Thus, half the absolute melting temperature of a given metal is equivalent to a homologous temperature of 0.5. With the exception of molybdenum, the low end of the refractory metal homologous sintering-temperature range is at the high end of the normal ferrous sintering-temperature range. HTS moves the homologous sintering farther into the range used TABLE II. HOMOLOGOUS SINTERING TEMPERATURES Material

°C

TH

Fe (Conventional) Fe (High-Temperature) Mo Ta Re W

1,000–1,150 1,250 1,700–2,200 2,250–2,800 2,200–3,050 2,200–3,000

0.55–0.64 0.69 0.59–0.69 0.68–0.85 0.64–0.88 0.60–0.82

Figure 1. Sintering temperatures for refractory metals and iron Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

for refractory metals. In this sense, HTS allows ferrous PM parts makers to catch up to the tungsten sintering practices used for a hundred years. The scale of production necessary to support refractory metal sintering is much smaller than that needed to support the sintering of ferrous parts, and refractory metals also have a much higher intrinsic value than ferrous alloys. This means that HTS can be a much larger fraction of the overall finished part cost for traditional PM parts than for refractory metals, so it must produce significant value to be economically attractive. HTS offers some opportunities for improved microstructures and properties, including: • Higher density • Higher alloy-diffusion rates • Enhanced microstructural homogeneity • Improved deoxidation • Improved mechanical properties • Opportunities for new, less-expensive alloy systems • Ability to sinter stainless steels These perceived advantages spurred significant interest in HTS about 15 years ago. Since then, researchers have worked hard to demonstrate them in various alloy systems and applications. However, in my discussions with colleagues in the industry, the consensus is that the penetration of HTS has not been as large as anticipated, amounting to perhaps 10% now. A number of factors have contributed to this: (1.) Traditional approaches have not stood still. Higher compaction pressures, warm compaction, double-press/double-sinter, and other approaches have yielded property improvements that have chipped away at the advantages of HTS (2.) PM parts makers perceive the cost of implementing HTS to be higher than that for traditional sintering. Furthermore, when nearly all customers invoke just-in-time shipment requirements as they do today, buying a single HTS furnace is risky. This means buying two new furnaces, which changes the calculus dramatically. (3.) Competing designs and technologies from high-temperature-furnace manufacturers make economic analysis and direct comparisons difficult. Atmosphere furnaces (usually hydrogen or hydrogen-containing) have advocates for walking-beam, pusher, and ceramic-belt designs. Vacuum-furnace designs that may or may not have low-pressure controlled-atmosphere capability are also available. (4.) High-temperature furnaces require a para-

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CONSULTANTS’ CORNER

digm shift on the part of the PM parts manufacturer. Instead of familiar belt furnaces, process design must deal with new approaches to parts handling, loading, and unloading, which may introduce additional capital costs. Furnace manufacturers have worked hard to make that shift as easy as possible, designing furnaces with ceramic belts, hybrids with loading belts that transition to a walking beam design in the hot zone, and advanced automation of pusher designs. (5.) Higher temperatures can create more demanding maintenance requirements, causing users to think hard before making a commitment to implement HTS technology. Maintenance requirements may also vary from furnace design to design, further complicating economic analysis and comparison. Perhaps most important, there may not yet have been enough of “The Right PM Parts” identified to entice producers to invest heavily in the technology. One thing was clear from the presentations at the MPIF seminar: the economic analysis supporting decisions to move toward HTS is not straightforward. Each producer’s position is unique. The demands of their customer base; the size, volume, and composition of the PM parts they make; the degree to which their existing equipment is depreciated; the way they account for processing costs—all of these factors play into the analysis and decision whether or not to implement HTS technology. Furnace manufacturers take different approaches to maximize equipment productivity. This means that parts manufacturers must define their needs correctly, heed the furnace manufacturers’ recommendations about maximizing productivity, and be willing to go outside their normal process paradigm to take advantage of the technology and make the economic analysis realistic. Not every PM parts maker will find the technology appropriate for its business, but a decision not to go to high temperatures needs to be made on clear and accurate analyses. Tungsten heavy-alloy technology relies on furnaces operating at temperatures of 1,350°C and above. More than one furnace design serves this technology reliably and economically, so I have no doubt that HTS will ultimately penetrate deeper into ferrous PM. Just what that penetration rate will be is hard to say. No one expects to see an explosion of new HTS capacity installed in the near future, given the present economy and its effect on automobile production. However, there may be an opportunity hidden in the bad news. The increas-

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ingly strong push for fuel economy will create weight-savings goals that translate to higher strength and toughness requirements for sintered parts. This may create enough of “The Right PM Parts” to stimulate adoption of HTS.

Q

Highly productive/economical highertemperature sintering furnaces appear to have design/performance limitations. What is being done to rectify this and what will future furnaces look like? This question appears to be a contradiction in terms. If furnaces were highly productive and economical, why would design and performance limitations be an issue? The question itself reflects the disparate and seemingly irreconcilable views present in the community about how to evaluate and implement HTS. It further emphasizes the need for PM parts makers and furnace manufacturers to work closely together to understand both production needs and ways to optimize furnace utilization. Each furnace design dictates particular operational and maintenance requirements for maximum productivity and the lowest overall cost of operation. These requirements will not be the same for different furnace designs, so PM parts makers must carefully weigh the impact of these factors on their operations in order to make an intelligent decision about implementing HTS in their business. Again, I point to tungsten heavy-alloy manufacturing, which uses high-temperature sintering in highly loaded furnaces day-in and day-out with great success. This experience suggests to me that many design and performance issues are more perception than reality.

A

1. Method for Determination of Transverse Rupture Strength of Powder Metallurgy Materials (PM) MPIF Standard 41, Metal Powder Industries Federation, Princeton, NJ, 2008 2. E. Klar and P. Samal, PM Stainless Steels: Processing, Microstructures, & Properties, ASM International, Materials Park, OH, 2007, p. 110. 3. MPIF Standard 35 Structural Alloys Section: Stainless Steel – 300 Series Alloys, Metal Powder Industries Federation, Princeton, NJ, 2007, p. 26. 4. H.I. Sanderow and T. Prucher, “Mechanical Properties of PM Stainless Steel: Effect of Composition, Density, and Sintering Conditions”, Advances in Powder Metallurgy and Particulate Materials, compiled by M. Phillips and J. Porter, Metal Powder Industries Federation, Princeton, NJ, 1995, vol. 3, part 10, pp. 13–28. 5. E. Klar and P. Samal, PM Stainless Steels: Processing, Microstructures, & Properties, ASM International, Materials Park, OH, 2007, p 119. ijpm Readers are invited to send in questions for future issues. Submit your questions to: Consultants’ Corner, APMI International, 105 College Road East, Princeton, NJ 08540-6692; Fax (609) 987-8523; E-mail: [email protected]

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June 27–30 The Westin Diplomat Hollywood (Ft. Lauderdale), Florida

2010 International Conference on Powder Metallurgy & Particulate Materials For complete program and registration information contact: METAL POWDER INDUSTRIES FEDERATION ~ APMI INTERNATIONAL INTERNATIONAL 105 College Road East, Princeton, New Jersey 08540 USA Tel: 609-452-7700 ~ Fax: 609-987-8523 ~ www.mpif.org

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Register by February 28 and Save!

Make plans to attend the only international metal and powder injection molding event of the year!

MIM2010 CONFERENCE (March 30–31) A two-day event featuring presentations and a keynote luncheon

• Focus on demanding applications • Leading process trends • Numerous case studies • Tabletop Exhibition & Networking Reception with representatives from many of the leading companies in the field ...and much more!

Optional One-Day Powder Injection Molding Tutorial Precedes Conference (March 29) Taught by Randall M. German, world-renowned PIM expert An ideal way to acquire a solid grounding in powder injection molding technology in a short period of time • Introduction to the manufacturing process • Definition of what is a viable PIM or MIM component • Materials selection and expectations • Review of the economic advantages of the process

This conference is sponsored by the Metal Injection Molding Association, a trade association of the Metal Powder Industries Federation Visit mimaweb.org or mpif.org for complete program details and registration information

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PRECIOUS METALS

PRECIOUS METALS: A VALUABLE POWDER METALLURGY PLAYER P.W. Taubenblat, FAPMI*

EARLY HISTORY Ancient Egypt If Howard Carter had been a powder metallurgist he would probably have been less than excited by the 118 kg (242 lb.) solid gold inner coffin of Tutankhamun when he discovered the tomb of the boy king in 1922. Instead, he might have focused his attention on two smaller gold daggers that had been wrapped in the mummy’s bandage.1 The daggers’ microgranulation decorative technique from 3,340 years ago is one of the earliest examples of the fabrication of metal powder, the latter defined as finely divided solids, smaller than 1 mm in maximum dimension.2 As shown in Figures 1–4, the granules used in this technique on the daggers, and in other ancient Egyptian objects from the same dynastic period, such as the bracelet from Rameses II’s tomb, appear to be extremely fine. Though there are earlier examples of granulation in the Royal Tombs of Ur, the particles used were significantly larger and

Figure 1. Gold dagger (with iron and nickel blade) and sheath from the Mummy of King Tut (~3,340 years old)

Silver, gold, platinum, and palladium have long been viewed as a niche powder metallurgy (PM) class of materials. Compared with their ferrous cousins that dominate PM applications such as automotive, small machine, and appliance parts, they often receive little attention in PM industry news and surveys. This introductory article to the Focus Issue puts the spotlight on PM precious metals. It traces the history of the technology, reviews market dynamics and applications, and quantifies the market size.

Figure 2. Magnified view of dagger hilt

*President, Promet Associates, 358 North 4th Avenue, Highland Park, New Jersey 08904, USA; E-mail: [email protected]

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PRECIOUS METALS: A VALUABLE POWDER METALLURGY PLAYER

Figure 3. Hinged granulated gold/lapis bracelets from the reign of Ramses II (~3,270 years old)

Figure 4. Magnified view of bracelet

outside the boundaries of PM.3 From a measurement of the gold dagger hilt (11.8 cm), the author was able to estimate the dagger’s finest particles to be <<1 mm and therefore definable as powder. There are two prevailing theories about how these early powders were made in what is known as the New Kingdom Period over three thousand years ago at the peak of the Ancient Egyptian Empire. The techniques were most likely modifications of earlier methods of grinding gold found 5,000 years ago in ancient Egyptian manuscripts.4 One theory is that gold wire was cut into small pieces which were then heated and rolled between two flat surfaces. 5 Another prevailing idea is that powders were produced by grinding larger flakes and chunks of metal in specially

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designed mortars and pestles, often mixing them with mercury to create an amalgam. The mixture was heated and the mercury burned off leaving the finely ground powder.6 As Ancient Egypt declined as a world power, and after 31 BC when Egypt became part of the Roman Empire, powder techniques were used more generally in the mining, refining, and transport of ancient Egyptian gold. At the mine, gold bearing ore was mechanically broken up with large sledge hammers and then mortars and pestles were used to grind the ore into powder. As a crude refining process the powder was washed on an inclining board and the gold dust that was recovered was fused into small nuggets.7 South American Ancient Cultures About 800–900 years ago, and several generations before the arrival of the Spanish conquistadors, Incan metalworkers in Peru and Ecuador (the native populations of the Esmeraldas region), used rudimentary PM techniques to incorporate platinum in jewelry and religious ritual objects. Platinum was found mixed with gold and was separated out probably through manual sorting, particle by particle using an edge (knife blade) on a smooth board.8 Because of the high melting point of platinum (1,768°C), which was far beyond ancient blowpipe and charcoal heating capabilities, an improvised PM technique was used in which ground particles of platinum were mixed with gold powder which was then brought to the melting point of gold (1,064°C). The coated platinum particles were fused together in a sinter-like process, analogous to how cemented carbides are made today,9 with the molten gold as the “glue.” The resulting gold/platinum spongy material was repeatedly forged and heated, slowly forming a homogeneous material.10 18th–19th Century Platinum Pioneers Besides the occasional use of gold powder for paints in the middle ages,11 the significant history of precious metal powders fast-forwards to the 18th century when the French chemist Nicholas Anne de L’Isle succeeded in isolating platinum in the 1750s. L’Isle distributed his platinum as button-size samples among his circle of scientist colleagues in Paris. These efforts remained at the laboratory scale until the Frenchman Pierre

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Fançois Chabeneau working in Spain in 1786 successfully produced malleable platinum by a PM method that was kept secret on the order of the King of Spain for the next 150 years. Most likely the method involved dissolving the mineral in aqua regia, precipitating the platinum with salammoniac (a mineral containing primarily NH4Cl), calcining to a sponge of platinum metal, heating to the highest temperature possible, and then forging the product.12 Chabeneau also demonstrated the importance of knowing people in high places when he became somewhat of a celebrity, with the King personally arranging his lifelong security by giving him a royal palace in Madrid. Through the power of the monarch he was also appointed the director of a laboratory devoted to producing platinum, and given a chair at the school of Natural History with a large annual salary and life pension. Chabeneau’s success allowed Spain to become the global source for the earliest users of platinum, ushering in what became known as “the platinum age in Spain.” It is estimated that over the next three decades, after 1786, Spain produced ~9.43 × 106g ( 330,000 oz.) of platinum.13 Wollaston’s PM Techniques and the Discovery of Palladium Chabeneau’s advancements in the commercial production of platinum were admired and eventually superseded in England by those of William Hyde Wollaston who began his career in medicine and formed a close friendship with the chemist Smithson Tennant. With Tennant’s financial help Wollaston began researching platinum around 1800. By reverse engineering, he discovered how platinum was produced from ~171,420 g (6,000 oz.) of platinum he acquired. Over the next 20 years he personally made over 1.03 × 10 6 g (36,000 oz.) of platinum and, like Chabeneau, generated a vast fortune by selling it for about £36,000. The majority of this production went to a unique niche application: lining the touch holes of pistols and sporting guns to resist the corrosive action of gunpowder fumes. Wollaston’s technique was to decompose platinum salt at low temperature, gently break up aggregates by hand or using wood, wash and remove salts, and then in a familiar process, press and sinter for 20 min so the mass could be subsequently forged.

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

In his initial investigation, Wollaston noticed that aqua regia dissolved platinum and that the residue seemed to contain a new element. After he treated the filtrate with the most poisonous substance available (mercuric cyanide (Hg(CN)2)) a pale yellow precipitate was ignited and produced the metal palladium.14 Because others were working on finding palladium, Wollaston, with a healthy sense of humor and a knack for promotion, decided on a unique strategy: he would anonymously post pamphlets about the discovery around the city and offer it for sale. Russian Rubles The early 19th century was an exciting time for precious metals and the platinum group metals. In 1800 only platinum was known, but by 1845 iridium, osmium, palladium, rhodium, and ruthenium had all been discovered.15 About 20 years after the discovery of palladium, platinum was found in significant quantities in Russia’s Ural Mountains region. To meet the growing need for platinum, Peter Sobolevsky created a process for producing platinum in 1827. His method was to boil the ore in aqua regia of four times its weight, producing soft platinum from calcined chloroplatinate. Platinum sponge was then pressed, heated, pressed again, and hammered into any desired shape.16 Sobolevsky’s process increased in importance nationally and in some cases on a global scale when the Tsar took a personal interest in minting platinum coinage. This was somewhat of a revolutionary idea as it remains the only example of platinum used on a large scale for coinage in the 19th century. From 1828 to 1846 1,373, 691 three-ruble, 14, 847 six-ruble, and 3,474 twelveruble coins were minted.17 Today these coins are rare, selling for about $5,000 in damaged condition to over $75,000 in the as-minted uncirculated state. A representative six-ruble coin is shown in Figure 5. The use of PM for coinage has been explored. In particular, copper–nickel alloys were used with lubricants such as zinc stearate for greater efficiency and improved die wear.18 Using PM methods, silver, gold, or platinum alloys could, in theory, be fabricated in a similar mode for coinage.

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PRECIOUS METALS: A VALUABLE POWDER METALLURGY PLAYER

advantage for the metal in powder form. Both platinum and palladium powders are also used as sintering aids in the PM process itself. These metal powders can effectively lower the temperatures required for sintering by as much as several hundred degrees.19

Figure 5. Russian 1833 six-ruble platinum coin (28.5 mm dia.)

MARKET DYNAMICS AND APPLICATIONS The primary areas of application for precious metal powders have not changed significantly over the last two decades. Twenty-five years ago applications were normally categorized into five main groups: chemical (catalysts), electrical/electronic (contacts, diode heat sinks), medical/dental (implants, amalgams), petrochemical (catalysts), and printing (inks, coatings).2 Today we expand these categories, as shown in Figure 6. The new specialized applications category includes potential drivers of higher growth for the precious metals market. Some of the market drivers and suppressors of growth in each major category are now considered.

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Electric/Electronics This category is probably the largest market for precious metal powders and includes conductive pastes and adhesives, electrical contacts, and multilayer ceramic capacitors (MLCCs). Smaller markets include sensors, thick-film technology, and quartz oscillators. Because silver has the highest electrical conductivity of any metal and is the cheapest noble metal, it has dominated this segment, especially where the need for higher conductivity far outweighs the additional cost. Electrical contacts used by the military are typically designed and built from precious metal powders. Satellite communications is another “mission critical” application where the additional cost of the precious metal is less significant than the property gain.

Chemical Reactions The ability of platinum to catalyze chemical reactions was a characteristic discovered not long after Wollaston began the industrial production of this metal in the late 18th century. The largest application of platinum as a metal is in catalytic converters in automobiles. However, platinum powder is not normally used in these devices. Platinum powder is used as a catalyst by large chemical companies where its high specific surface area (SSA) and ease of dispersion reflect an

Decoration As previously noted, jewelry represents the oldest use for precious metal powders and PM in general. Included in this category are techniques such as granulation that might use precious metal powders directly. New techniques for forming unusual shapes using powders are increasingly being practiced and honed. For example, about 10 years ago Mitsubishi developed “metal clay,” a blend of pure silver powder and an organic material that can be shaped into objects like clay. The organic substance is then burnt off leaving behind a nearly pure silver solid.20 This process is now becoming more widespread as jewelry makers share their know-how through word of mouth.

Figure 6. Precious metal powder applications

Dental Precious metals have been used for centuries to repair teeth. The use of an amalgam or a mercury silver powder combination, usually about 50 w/o Hg and 27 w/o Ag has been successful due to its ease of application, strength, and durability. There is now a backlash about using mercury, and new alternatives are being developed that still use precious metal powders. Material scientists at the National Institute of Standards and

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Technology (NIST) have developed an approach for filing cavities with pure silver powder. Part of the innovation is to remove naturally present silver oxide on the particles by treating the silver powder with a dilute acid solution. The pure silver powder is then worked with regular dental tools until the silver hardens. As new studies confirm the harmful effect of mercury leakage in the mouth, these types of products and procedures, though more expensive, are becoming more widely accepted.21 The demand for precious metal powders, including gold for dental applications, has remained relatively insulated from the economic turbulence that has adversely affected the overall demand for other metal powders.22 Specialized Applications This category is relatively large, as many new niche uses for precious metal powders are constantly being discovered. One area of great interest is silver–zinc battery technology. The military is a large user of silver–zinc batteries for high capacity and high rates of discharge but with a lower priority for battery life, which tends to be limited to 50 cycles for silver–zinc. The U.S. Navy has been using the technology for underwater rescue vehicles, submarines, and torpedoes. 23 EaglePicher has been producing silver–zinc batteries for such specialized applications as the Mars Reconnaissance Orbiter.24 The porosity of powder parts and the unique properties of gold have resulted in the use of gold powders as a carrier metal for radioactive materials.25 Experimental products are being introduced using silver powder in antibacterial coatings. Wind- and solar-energy devices have used precious metal powders and the market for conductive adhesives keeps growing as LCD TV monitor sales continue to rise due to innovations in wafer production. The market for these specialized precious metal powder applications portends healthy growth. However, because of their high price, manufacturers and suppliers are always looking for cheaper alternatives. For example, researchers have created a method for substituting iron powder for platinum in fuel-cell reactions. 26 This and other competitive threats must be taken into account when forecasting the value of U.S. shipments of precious metal powders.

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

Figure 7. U.S. gold and total precious metal powder shipments vs. gold prices

MARKET SIZE As a foundation, sparse U.S. census data were used with some data points on import/export flows of silver powder for 1997 and 2005. Also, the extreme sensitivity of shipment values to precious metal price volatility was factored in. Figure 7 shows the overall market value and the value of gold powder shipments as related to gold pricing. As the price decreased, the lower commodity pricing resulted in a boost in demand and a corresponding greater volume to compensate for lost market share. The U.S. market value for precious metal powders is estimated at ~$160 million in 2008. This number could increase significantly over the next few years as healthy growth is driven predominantly by the specialized applications segment. The rich history of precious metal powders may well be echoed in a modern resurgence of demand for these valuable materials. REFERENCES 1. Personal viewing of one of the two gold daggers (gold blade) found on Tut’s body, The Franklin Institute, Philadelphia, PA, “Tutankhamun and the Golden Age of the Pharaohs” exhibition, 2008. 2. R.M. German, Powder Metallurgy Science, 1984, Metal Powder Industries Federation, Princeton, NJ, p. 3. 3. J. Wolters, “The Ancient Craft of Granulation: A Reassessment of Established Concepts”, Gold Bulletin, 1981, vol. 14, no. 3, pp. 119–129. 4. S. Ya. Plotkin and G.L. Fridman, “History of Powder Metallurgy and its Literature”, Poroshkovaya Mertallurgiya, 1974, vol. 144, no. 12, pp. 94–97. 5. T.G.H. James, The British Museum, “Gold Technology in Ancient Egypt: Mastery of Metal Working Methods”, Gold Bulletin V, 1972, p. 42. 6. P. Ramakrishnan, “History of Powder Metallurgy”, Indian

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Journal of History of Science, 1983, vol. 18, no. 1, p. 110. 7. T.G.H. James, The British Museum, “Gold Technology in Ancient Egypt: Mastery of Metal Working Methods”, Gold Bulletin V, 1972, p. 40. 8. J.C. Chaston, “The Powder Metallurgy of Platinum: An Historical Account of its Origins and Growth”, Platinum Metals Review, 1980, vol. 24, no. 2, pp. 70–79. 9. C. White, “History of Powder Metallurgy”, ASM Handbook, Vol. 7: Powder Metal Technologies and Applications, 1998, ASM International, Materials Park, OH, pp. 3–8. 10. M. Noguez, R. Garcia, G. Salas, T. Robert, and J. Ramirez, “About the Pre-Hispanic Au-Pt “Sintering” Technique for Making Alloys”, International Journal of Powder Metallurgy, 2007, vol. 43, no. 1, pp. 27–33. 11. S. Y. Plotkin and G.L. Fridman, “History of Powder Metallurgy and its Literature”, Poroshkovaya Mertallurgiya, 1974, vol. 144, no. 12, p. 1,028. 12. D. Mcdonald, “William Hyde Wollaston: The Production of Malleable Platinum”, Platinum Metals Review, 1966, vol.10, no. 3, pp. 101–106. 13. J.C. Chaston, “The Powder Metallurgy of Platinum: An Historical Account of its Origins and Growth”, Platinum Metals Review, 1980, vol. 24, no. 2, p. 73. 14. W.P. Griffith, “Bicentenary of Four Platinum Group Metals: Part 1: Rhodium and Palladium—Events Surrounding Their Discoveries”, Platinum Metals Review, 2003, vol.47, no. 4, pp. 178–181. 15. T. Rehren, “The Minting of Platinum Roubles: Part IV: Platinum Roubles as an Archive for the History of Platinum Production”, Platinum Metals Review, 2006, vol. 50, no. 3, p. 120. 16. C. Raub, “The Minting of Platinum Roubles: Part I: History and Current Investigations”, Platinum Metals Review, 2004, vol. 48, no. 2, p. 66. 17. Ibid ., p. 67. 18. “Copper-Nickel P/M Materials, Characteristics and Properties of Copper and Copper Alloy P/M Materials”, Copper Development Association, www.copper.org/ resources/properties/129_6/characteristics_properties.html. 19. C.W. Corti, “Sintering Aids in Powder Metallurgy: The Role of Platinum Metals in the Activated Sintering of Refractory Metals”, Platinum Metals Review, 1986, vol. 30, no. 4, pp. 194. 20. K. Hoshino, M. Morikawa, T. Kohno, K. Ueda and M. Miyakawa, “Precious Metal Article, Method for Manufacturing Same, Moldable Mixture for use in Manufacture of Same and Method for Producing Moldable Mixture”, U.S. Patent No. 5,376,328, December 27, 1994. 21. U.S. Geological Survey Silver Minerals Yearbook—2000, United States Geological Survey, Washington, D.C., p. 70. 22. “Satisfactory overall results in 2008 despite unusual year”, Heraeus Holding GmbH, http://webmedia.her aeus.com/media/holding/datasources_3/040509_bpk_pm /PM_BPK_Satisfactory_overall_results_in_2008_despite_un usual_year_090504.pdf, p. 3. 23. A. Himy, Advanced Battery Technology: New Aerospace, Army, and Navy Battery Applications: The Silver Zinc Battery, Advanced Battery Technology, Boalsburg, PA, June 2002. 24. EaglePicher archived press release 10/2005, http://www.eaglepicher.com/content/view/95/1/. 25. C. Goetzel, Treatise on Powder Metallurgy (Three Volumes), Interscience Publishers, New York, NY, 1950, p. 679. 26. “Revving Up Fuel Cells,” Scientific American UPDATES, June 2009, vol. 300, no. VI, p. 16. ijpm

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PRECIOUS METALS

APPLICATIONS FOR PRECIOUS METAL POWDERS Joseph Tunick Strauss*

SOLDER, BRAZE PASTES, AND INKS These PM technologies utilize the addition of an organic liquid to the powder to form a paste in order to facilitate controlled application of the powder. For example, solder or braze pastes are a mixture of atomized alloy powder and a liquid media to form a homogeneous mixture that can be applied with precision to a joint using automated dispensing equipment. In this case the liquid media is removed in a subsequent processing step (heating) and the metal powder is melted and flows into the joint to form a structural bond. Jewelry solder pastes are commonly available in most karat-gold alloys as well as silver-based alloys and palladium and platinum alloys. Structural brazes, for example, 82Au18Ni, are used as a structural bond for stainless steels and superalloys. Silver-based brazes are also used in structural application for bonding ferrous and copper alloys. Electronic applications include gold–tin and gold–germanium solder pastes. Many of the new lead-free solders, although primarily tin based, contain silver and represent a tonnage application. The alloy powders used in these applications are produced by atomization in which the molten alloy is disintegrated into fine droplets that subsequently solidify into powder particles. Precipitated powders can also be used when a pure metal is required, or when fine powder is needed. One such application is for electronic conductive paths. Conductive inks using precipitated gold, silver, palladium, and platinum are commonly used. Some conductive inks rely on particle-toparticle contact within the medium for the conductive path. In other cases the medium is removed and the powder sintered to form the conductive path. Recently, there have been sources of atomized silver powder fine enough to be used in conductive inks; however, most of the ink applications utilize precipitated powders. In some cases the powders are milled to allow them to “leaf” or overlay each other for enhanced electrical conductivity.

The use and growth of powder metallurgy (PM) as a manufacturing process reflects a reduction in part cost and enhanced properties compared with competing technologies. PM is also a captive technology in the processing of some materials. These attributes have resulted in the growth and penetration of PM into numerous materials processing and manufacturing sectors. A number of commercial PM technologies are directly applicable to the precious metals industries. These include parts made by pressing and sintering and by metal injection molding (MIM), solder and braze pastes, thermal spray coatings, and oxide dispersion strengthened (ODS) alloys. In addition, precious metal powders embrace biological/dental applications, and the use of powder conversion in refining is unique to the precious metals industries. In this overview, current and near-term industrial applications for precious metal powders and their processing are reviewed.

DENTAL APPLICATIONS The most common precious metal used in dental applications is an amalgam based on silver–mercury for filling cavities in which silver, silver–copper, and silver–copper–tin powders are mixed with mercury. *Engineer/President, HJE Company, Inc., 820 Quaker Road, Queensbury, New York 12804, USA; E-mail: [email protected]

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APPLICATIONS FOR PRECIOUS METAL POWDERS

The liquid mercury allows a limited time for filling and handling before eventually diffusing into the silver and silver alloy powders and solidifying. Although there are competing materials for filling cavities, such as epoxy–ceramic composites, the mercury–silver alloy amalgam is the most frequently used. The silver and silver–copper powders are made by atomization. The silver–copper–tin powders can be made by atomization, machining, or filing. Machining or filing produces acicular particles, which provide the “crunch” or “feel” that some dentists prefer. Caps and implants use precious metal powders in several fabrication methods. For example, as a bonding layer (coping) for the porcelain outer cap and the implant or tooth surface. In one approach, a flexible sheet made from gold, gold alloy, or palladium alloy powder and a polymer is applied to a refractory die which mimics the tooth or implant stud. The polymer is burned out and the powder sinters to form a thin layer for subsequent enameling.1,2 Rapid manufacturing (RM) can also be used to make the coping without having to make the die.3,4 RM of metal parts relies on the use of metal powder as the build material. The dentist makes an impression of the tooth, whose surface contour is digitized. This digital information is sent to the RM facility and used to direct the RM machine to make the coping directly from the powder without the need for a refractory die, with an attendant reduction in labor. This is a developing technology and the subject of much competition among RM companies. Another method of making copings using precious metal powder is by electrophoretic deposition (EPD). This method is analogous to plating in that it relies on an electric field to drive the charged species towards an electrode. However, rather than have ions in solution, the solution is a colloid of charged particles. These particles can be as large as 30 µm dia.5 Thus, the build-up rate is relatively rapid and the thickness can be substantial (~mm). In addition, mixtures or composites can be co-deposited as well as layered structures. Densification occurs during a subsequent sintering operation. EPD is usually associated with ceramic materials but metallic materials are also possible and achievable.6 Coping production using EPD is currently available from one source.1

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THERMAL SPRAY AND COLD SPRAY Thermal spraying of powders onto a substrate has been available for decades. Flame metallizing (and more recently plasma spraying and highvelocity oxyfuel (HVOF)) have been used for building up worn surfaces or to apply protective coatings. One parameter that affects the economics of this technology is the material utilization efficiency, i.e., the amount of material that adheres to the surface with respect to the amount sprayed. This efficiency is typically <50% in most cases, which would limit its application for more expensive powders. However, there are currently two applications that are in use: (i) abradable sealing surfaces for high-temperature turbo machinery7 utilizing a composite coating comprised of chromia, silver, and a glass; the chromia provides wear resistance while the silver and the glass act as high-temperature lubricants; the silver is made by gas atomization to provide a spherical powder that flows readily in the thermal spray apparatus; (ii) pure silver coatings for conductivity and corrosion resistance; the powder is applied by cold spray (kinetic metallization) in which the powder is propelled at a high velocity to impact a surface at ambient temperature; the velocity provides the kinetic energy for deformation and some sintering (heating). NET-SHAPE PARTS Although items of jewelry have been made successfully by PM methods, the use of PM in jewelry manufacturing is limited. Radially and axially symmetric wedding bands, coins, and coin blanks are candidate items for press-and-sinter PM. Coin blanks are typically made by stamping sheet. Ring blanks can be made from stamping sheet or slicing tubing. In each case the production of scrap affects the economics of the process. The yield for conventional coin blanking via the cast/roll/ stamp process is approximately 35% and the yield for ring blanks produced via the cast/roll/ tube/slice process is approximately 50%.8 In contrast, press-and-sinter PM processing yields approximately 90% product from each melt. The majority of the savings is due to the efficiency of the pressing operation since only the material needed for each blank is used, rather than putting added value into material that will ultimately be scrap. Figure 1 is an example of a gold coin made from water-atomized powder via pressing

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Figure 2. Two as-sintered medallions and a dental implant test piece (polished) manufactured by MIM using 18-karat gold powder. U.S. dime included for scale Figure 1. Coin made by Tanishq (division of Titan, India) by pressing and sintering of atomized gold powder. Both sides of coin (~18 mm dia.) are shown

and sintering. The savings are amplified for materials that require significant work to melt and process, such as platinum alloys. The growing market for platinum wedding bands should stimulate investigation into powder processing. Ring blanks have been manufactured by die compaction and sintering. In this case the primary detriment to residual porosity is its negative impact on surface finish. For this reason these pressed-and-sintered parts are then subjected to chipless deformation processes, such as re-pressing in a closed die or open platen, and ring rolling, which densifies the part and also burnishes the surface. The quality of the final product is at least equal to that of rings made by conventional processes. A patented method to manufacture weddingring blanks via pressing and sintering has been used in production.9 Hafner proved that quality wedding bands could be made via MIM but the economics were not favorable unless the cost of the powder could be reduced.10 This perspective was that of a powder or MIM feedstock provider, rather than that of a parts manufacturer. MIM has been employed successfully in other jewelry-part applications, specifically parts made by investment casting. MIM significantly reduces the unit cost by eliminating many of the steps and much of the labor associated with investment casting. Clasps and other generic jewelry hardware (known as findings) are prime candidates for MIM, as they represent the low-margin, high-volume sector of the jewelry industry. At this time there are only two MIM applications for precious metal findings (sterling silver clasps) in the U.S. Interestingly, in Europe most of the jewelry parts

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

being considered for fabrication by MIM have been relatively low production numbers for highend jewelry.11 Figure 2 shows several 18-karat gold parts manufactured by MIM. Despite the economic advantages that PM allows in the manufacture of jewelry, its use is not widespread. The primary reason for this is the lack of availability of precious metal powder and the infrastructure and equipment for the relatively small jewelry-scale manufacturing volumes. Conventional gas-atomization and water-atomization technologies are adequate for processing most precious metal alloys; however, most atomization facilities have equipment of far greater capacity than needed for the precious metals market. The high cost of the material and volatile market prices deter traditional powder suppliers from supplying this potential market. RM has also been applied to the manufacturing of jewelry. In this case the ability of RM to make geometries too complex to cast or form otherwise is a primary driver. Figure 3 shows CAD representations of a portion of a bracelet and the part made by RM in 18-karat gold. The individual links

Figure 3. CAD representation of a geometrically complex bracelet section made via RM.12 Maximum dimension of link ~13 mm

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Figure 4. Charms and trinkets made from silver PMC. Chevron Trading Post and Bead Company, Asheville, North Carolina. Maximum dimension ~25 mm

are complex but could be made individually by conventional methods. RM allows the pieces to be linked together as manufactured, an attribute impossible to achieve by other methods. Another approach using powders in the manufacture of jewelry utilizes precious metal clays (PMC). The product is popular with artisans and craftsmen in that it requires a minimum of equipment. The clay is a mixture of precipitated precious metal powder, an organic binder, and water. The clay is crafted by hand, dried, and then sintered in an air kiln. The finished parts generally do not exceed 80% of the pore-free density with a matte surface finish. Secondary finishing, such as burnishing, results in some luster on the surface. PMCs are only available as pure silver, gold, or platinum since any non-precious metal additions, such as copper, preclude the ability to use an air kiln for sintering. Figure 4 shows representative trinkets and charms made from silver PMC. BULK MATERIALS There is only limited use of PM in the manufacture of non-jewelry parts from precious metals. Rather than manufacture discrete net-shaped parts, PM can be used to produce precious metals in bulk form such as rod and strip. In this case PM is used to make materials with compositions and properties not possible by traditional cast or wrought processing. For example, silver–nickel and silver–graphite alloys for electrical contacts cannot be produced by casting. Silver powder and nickel powder are blended and then extruded to produce a fully dense billet, which can be further reduced to rod. Silver–graphite alloys can be pressed directly into contact parts. Silver–cadmium, silver–tin oxide and platinum–yttrium oxide dispersion-strengthened alloys are also made by the PM route. The procedure is to atomize the

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alloy with the alloying additions (cadmium, tin, yttrium) in solution. The powder is heat treated in air, which diffuses into the alloy where the reactive element (cadmium, tin, yttrium) reacts with oxygen to form a stable oxide. The powder is then consolidated by pressing or extrusion to obtain full density. The powder step allows oxidation to occur rapidly and completely since diffusion of oxygen through a large cast ingot would be very time and energy consuming. POROUS MATERIALS Pressing-and-sintering methods are a common way to make filters or bodies with controlled permeabilities. Powders with specific particle-size distributions are compacted and sintered under controlled processing parameters to produce the required permeability or filtering capacity. There is only a limited use of precious metals in precision filters for highly corrosive applications.13 There are other PM technologies that are capable of producing filters and catalyst from precious metals with specific permeabilities or a high internal specific surface area (SSA). One method involves the use of a binder/carrier with a foaming or gassing agent which produces pores during the molding operation. The part is then debound and sintered under conditions that preserve the porous internal structure. Another process uses relatively fine powder pressed with an extractable phase. The extractable phase is removed and the remaining material is sintered. It is the particle size, particle-size distribution, and volume percent of this extractable phase that controls the permeability or filtering capacity of the bulk material. This enables the filter properties or the specific surface area of the material to be relatively independent of the particle size of the matrix. This permits the use of a wide variety of powders for this application. Most precious metal catalysts for oil and polymer applications use meshes woven from wire. The production of wire involves the addition of significant added value from the base bulk metal. However, PM processing using precipitated powders, such as platinum sponge (not usually considered an “engineered material”) has potential in this application. Catalysts could be made directly from the precipitated material rather than having to process the wire and woven mesh. The PM part would have a greater specific surface area,

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strength, and three-dimensional (3D) capacity than that of woven cloth. Considerable savings can be realized by directly processing powder precipitated from the prior refining process; work in this area is ongoing. REFINING This is an atypical application of powders in that the powders are merely a precursor to dissolution. Typically, alloys to be refined are made into shot or grain to increase the surface area and thereby enhance dissolution. The specific surface area is relatively low, and since the kinetics of the dissolution reaction are surface-area limited, it would benefit the process to maximize the specific surface area. Atomization, specifically water atomization, is an inexpensive method to greatly increase the surface area of the precursor. Figure 5 is a plot of relative specific surface area vs. particle diameter. The plot assumes spherical particles with a monosize distribution and unit density for relative magnitude. The SSA for water-atomized powder will be much higher when a normal distribution and an irregular morphology are accounted for. “Popcorn” is a precious metal term for irregularly shaped coarse shot. Water atomization is capable of increasing the specific surface area by approximately two orders of magnitude in relation to that of a small grain. Gold-alloy powders can be completely dissolved in less than 8 h14 while non-diluted platinum powder scrap requires one tenth the time to be taken into solution compared with shot.15 The plot also shows the increased SSA of gasatomized powder relative to that of water-atomized powder. However, the unit cost for gas atomization is significantly greater than for water atomization. In addition, the cost of a water-atomization system is significantly lower and is a more robust process. The cost of water atomization is generally ~$0.55/kg ( $0.25/lb.)16 while the cost of gas atomization can exceed $2.2/kg ($1.00/lb.).17 Another advantage of producing powder for the refining precursor is the ability to use more benign dissolution chemistries. In theory, it may be possible to use hydrochloric acid only for refining gold scrap;18 if the particles are fine enough, they will dissolve prior to passivation by the silver chloride coating. In practice, peroxide is added to the hydrochloric chemistry. 19 Substantial cost

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

Figure 5. Relative SSA vs. particle diameter

savings can be realized by reducing the quantity of the acids and by environmental management. SUMMARY The industrial uses of precious metal powders involve many of the same processes utilized in conventional PM technologies and applications. With respect to the fabrication of parts, the limited use of precious metal powders is due to the undeveloped infrastructure needed to support it, specifically their availability. Precious metal powders have uses and involve technologies not found in conventional PM due to their unique physical and chemical properties. REFERENCES 1. “Sintercast Gold: Pure gold material for sintering in paste foil, yellow colour”, Nobil Metal, http://www.nobilmetal.it/ english/pages/products/dati_lega/st_sintercast_gold_eng. pdf. 2. Precious Chemicals, Ltd., http://www.captek.com. 3. J.T. Strauss, “Rapid Manufacturing (RM) and Precious Metals”, Proc. 23rd Santa Fe Symposium, 2009, edited by E. Bell, Met-Chem Research, Albuquerque, NM, 2009, pp. 395–416. 4. A.L. Hancox and J.A. McDaniel, “Additive Manufacturing of Precious Metal Dental Restorations”, Int. J. Powder Metall., 2009, vol. 45, no. 5, in press. 5. O.O. Van der Biest and L.J. Vandeperre, “Electrophoretic Deposition of Materials”, Annu. Rev. Mater. Sci., 1999, vol. 29, pp. 327–352. 6. W. Hasz, 2006, GE Global Research, Niskayuna, NY, private communication. 7. C. DellaCorte and B.J. Edmonds, “Self-Lubricating Composite Containing Chromium Oxide”, U.S. Patent No. 5,866,518, February 2, 1999. 8. J.T. Strauss, “P/M (Powder Metallurgy) and Potential Applications in Jewelry Manufacturing”, Proc. 12th Santa Fe Symposium, 1998, edited by E. Bell, Met-Chem Research, Albuquerque, NM, 1998, pp 389–433.

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9. P. Raw, “Mass Production of Gold and Platinum Wedding Rings Using Powder Metallurgy”, Proc. 14th Santa Fe Symposium, 2000, edited by E. Bell, Met-Chem Research, Albuquerque, NM, 2000, pp. 251–270. 10. K. Wiesner, “Metal Injection Moulding (MIM) Technology with 18ct Gold—A Feasibility Study”, Proc. 17th Santa Fe Symposium, 2003, edited by E. Bell, Met-Chem Research, Albuquerque, NM, 2003, pp. 443–462. 11. W. Niedermann, 2005, Hilderbrand & CIE, SA, Geneva, Switzerland, private communication. 12. T. Norlen, “Jewelry Manufacturing Using Selective Laser Technique”, Jewelry Technology Forum, Vicenza Fair, Italy, May 2006, pp. 24–35. 13. N. Sopchak, 2004, Mott Metallurgical, Fairfield, CT, private communication. 14. R. Rubin, 2006, Republic Metals Corporation, Miami, FL, private communication. 15. G. Normandeau, 2003, Imperial Smelting and Refining, Markham, Ontario, private communication. 16. J. Peterson, 1999, Windfall Specialty Powders, St. Marys, PA, private communication. 17. T. Tingskog, 2006, Industrial Consultant, Aliso Viejo, CA, private communication. 18. T. Santala, 1995, Touchstone Metals, Providence, RI, private communication. 19. J. Grundy, 2005, Industrial Consultant, San Diego, CA, private communication. ijpm

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MPIF STANDARD 35

Materials Standards for

PM STRUCTURAL PARTS Newly Revised and Expanded Standard 35, Materials Standards for PM Structural Parts, 2009 Edition, the most recent version of the standard, has adopted the practice of significant figures usage. Developed by the powder metallurgy (PM) commercial parts manufacturing industry, each section of the standard is clearly distinguished by easy-to-read data tables and explanatory information for each material listed. Detailed explanatory notes and definitions, along with data cited in both Inch–Pound and SI units, help to make the standard more user friendly. This standard provides the design and materials engineer with the latest engineering property data and information available in order to specify materials for structural parts made by the PM process. Make sure that your quality assurance/laboratory staff and your sales and marketing personnel/representatives have the latest edition of this standard. Order enough for your own company use and for free distribution to your existing and potential customers. Keep a supply handy for future trade shows, plant visits, etc. Please note that publication of the 2009 Edition of this standard renders the 2007 Edition (and prior editions) obsolete. Previous editions should no longer be distributed but destroyed.

The 2009 Edition contains: • Significant Figures Usage Applicable data and temperature display throughout the standard have been matched to no greater precision than is required by the applicable test method standard. • NEW Materials & Mechanical Property Data Hybrid Low-Alloy Steel FLN4-4405(HTS), as-sintered & heat treated This is the first, high temperature sintered material in this material system. Sinter-Hardened Steels FLC2-5208 FL-5305 • Other NEW Data 300 Series Stainless Steels Fatigue strength values for the SS-304H & SS-316H materials Soft Magnetic Alloys Unnotched Charpy impact energy values for the FF-0000, FY-4500, FY-8000 and FS-0300 materials • NEW Engineering Information Hardenability (Jominy) Data Sinter-Hardened Steels: FLC2-5208 & FL-5305 Corrosion Resistance Data SS-304H & SS-316H

• NEW & REVISED verbiage throughout the standard • Updated Index Alphabetical Listing & Guide to Material Systems & Designation Codes Used in the family of MPIF Standard 35 publications This standard is a must-have document for every engineering professional. MPIF PUBLICATION, 84 pages, 2009 ISBN: 978-0-9819496-0-4 Item # N1027 (softcover format) List $50 APMI Member $45 MPIF Member $40 Item # N1027cd (CD-ROM format) List $50 APMI Member $45 MPIF Member $40 Item #1027e (electronic version, pdf format) List $50 APMI Member $45 MPIF Member $40 For Quantity Discounts, Please Contact the MPIF Publications Department

To Order: FAX: 609-987-8523 Phone: 609-945-0009 E-mail: [email protected] or visit www.mpif.org Metal Powder Industries Federation 105 College Road East, Princeton, NJ 08540-6692

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PRECIOUS METALS

THE MANUFACTURE OF PLATINUM, GOLD, AND PALLADIUM POWDERS Howard D. Glicksman*

PRECIOUS METAL POWDERS Precious metals, including platinum, gold, and palladium, are the preferred electronic materials for high-performance applications. They can provide excellent conductivity, corrosion resistance, solderability, and stability in air during the firing of metal–ceramic films.1 The cost of using these expensive materials is more than outweighed by their ease of firing, subsequent stability in air, and ease of attachment by soldering and welding.2 Precious metal thick-film pastes containing platinum, gold, and palladium are used in a variety of sensors, including systems for measuring oxygen, humidity, dew point, wind speed, flow rate, pressure, and temperature. These sensors make use of the catalytic characteristics of the metals as well as their moisture resistance, heat resistance, and conductivity.3 The conductor pastes used in thick-film applications generally contain four components consisting of a precious metal powder, a glass powder as a bonding agent for joining the metal to the substrate, an organic vehicle containing an organic resin which imparts the needed rheology to the paste for screen printing, and a solvent to control the viscosity and solids content. The particle sizes of the metal powders range from <0.1 to 5 µm. Uniform size distributions and spherical morphology are prerequisites for conductive pastes. In the past, precious metal powders such as platinum and palladium have been prepared via the thermal decomposition of metal salts. Metal salts include nitrates, halides, ammoniates, carbonates, and oxides that decompose and form volatile products. The resulting platinum or palladium metal powders are aggregated with appreciable amounts of impurities. A similar process involves the solid-state reaction of heat treating the metal oxides or metal salts in hydrogen. This also produces aggregated particles that contain appreciable amounts of metal oxide as an impurity.4 The majority of platinum, gold, and palladium powders are prepared by chemical precipitation from an aqueous salt solution. 1,5 Homogeneous nucleation is the prerequisite for small-sized, smooth, and dispersed particles. This requires that the nucleation stage be separated from the growth stage.6 Any heterogeneous nucleation has to be

Platinum, gold, and palladium precious metal powders are used in the electronics industry for the manufacture of fired conductors, multilayer ceramic capacitors, and conductive adhesives. They can also be used in sensors for measuring temperature and corrosion, and for the detection of gases. Platinum, gold, and palladium are used because they provide excellent conductivity, reliability, corrosion resistance, solderability, and stability at high temperatures. This article describes methods used to produce powders of platinum, gold, and palladium. The majority of these precious metal powders are made using aqueous chemical processes with suitable reducing agents and protective surfactants to control the size of the particles and to keep the particles from agglomerating. Other processes described include thermal decomposition of metal salts, mechanical forming of powders and flakes, and spray pyrolysis to form spherically shaped particles.

*Research Fellow, DuPont Electronic Technologies, 14 T.W. Alexander Drive, Research Triangle Park, North Carolina 27709, USA; E-mail: [email protected]

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suppressed to produce monosized particles. In addition to controlling the nucleation mechanism, it is important to prevent aggregation or coagulation of the fine particles after reduction and precipitation. Van der Waals forces of attraction are responsible for coagulation, whereas electrostatic repulsion forces arising from the electric double layers surrounding the particles can be used to act against coagulation. The adsorption of a surfactant onto the surface of the particles will change the surface potential and help to prevent aggregation. Another choice is to precipitate the precious metal powder directly from an organic phase. This process, with its uniform and enhanced steric stabilization with no stabilization by electrostatic repulsion, produces uniform, small-sized, dense particles.1,2 Chemical precipitation begins with the preparation of a metal-soluble salt solution by dissolving the metal in nitric acid, hydrochloric acid (with chlorine), or aqua regia (a mixture of hydrochloric acid and nitric acid). When aqua regia is used, the nitrosyl compound must be removed by continuous boiling with the addition of more hydrochloric acid. Reduction of the platinum-group metal salts in an aqueous solution by reducing agents such as formaldehyde, sodium formate, and hydrazine result in a precipitate of finely divided metal powder. Most often, the reduction is carried out in an alkaline solution, although acidic reductions are known.7 It has been shown that small-sized (<100 nm) high-purity gold or palladium powders can be made by reducing the metallic salt with hydrazine in an alcohol solvent with alkaline hydroxide.8 Continuous comminution of precious metal powders, including platinum and palladium, in the presence of a proper lubricant and solvent, produces two-dimensional, plate-like precious metal flakes. Gold is generally too soft to be made into and used as flake, though gold flake can be directly precipitated (see section on gold powders). Various types of milling equipment can be used, including ball mills, vibratory mills, and attritor mills. The size, shape, and other physical characteristics of the milled particles are controlled by the morphology of the starting metal powder, the lubricant chosen, the solvent, the milling media, and the milling conditions, such as temperature, rpm, powder-to-solvent ratio, and overall time.5 Non-agglomerated particles can be made by a process involving the use of polyol as the reducing

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agent. This process produces agglomerated particles. To obtain an easily dispersed, monosized powder, the growth step must be controlled by adding a polymeric protective agent that is absorbed onto the surface of the particle. Polyvinylpyrrolidone is an example of a suitable stabilizing protective agent.9 Fine particles (<100 nm) of platinum, gold, and palladium can be made as stable colloidal dispersions in a suitable solvent. These are made by reducing these metals rapidly in a solution that contains a stabilizer. Suitable reducing agents are citrate, formaldehyde, hydrazine, hydrogen, hydrogen peroxide, carbon monoxide, and alcohols.9 To reproducibly control the size of platinum, gold, and palladium nanoparticle dispersions, a weaker reductant is used. Extremely small-sized particles were made in an aqueous system using potassium bitartrate. Stabilizers are used, including N-vinyl-2 pyrrolidone, polyethylene glycol, or 3, 3-thiodipropionic acid. The size of the nanoparticles decreases when the ratio of the concentration of the metal to the stabilizer is decreased.10 Many metal powders have been made by spray pyrolysis because this technique allows for enhanced control over size distribution and stability with regard to oxidation. 11 This process involves the following steps: making a metal salt solution, forming an aerosol with control over the size distribution of the droplets, and then flowing this aerosol through a furnace where the salt-containing droplets lose water and decompose, and the resulting powder particles densify. Powders produced this way are spherical, dense, low-surface-area powders. PLATINUM POWDERS Platinum has a high bulk density, does not corrode, and is chemically resistant. It is also used for applications which require reliability and durability in severe conditions.12 Platinum is used to provide temperature measurements, corrosion-resistant electrical contacts, electrodes, and sensors for detection of oxygen and toxic gases.13 Oxygen sensors are used in automobiles to monitor the exhaust gases to ensure that the autocatalysts are working and to set the correct fuel-to-air ratio. Other types of platinum sensors can be used as flow-rate sensors since they have a more-rapid response than the conventional wire-type sensors.3 Platinum powders are used in thick-film pastes

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as spherical or flake-shaped particles with a size range from 0.1 to 20 µm. Scanning electron micrographs (SEM) of representative platinum powders are shown in Figure 1. These pastes, containing the platinum powders, are sprayed, brush-painted, or screen-printed onto inert surfaces that are then fired in temperatures from 500°C to 1,500°C to form strongly bonded conductors that can be soldered.13 In addition, small amounts of platinum powders can be added to silver thick-film pastes to improve the bondability and leach resistance of the formed conductor film. Platinum powders are also added to gold powder thick-film compositions to impart improved solder-leach resistance. These platinum gold conductors are the standard for high-reliability, solderable-conductor applications where cost is not a significant issue.14 An aqueous salt solution of chloroplatinic acid (H2PtCl6) is the most common starting material for the manufacture of platinum powder. It can be prepared by dissolving platinum sponge in aqua regia. After the nitric acid is driven off by continuous boiling with the addition of hydrochloric acid, the solution is then precipitated by a suitable reducing agent at the correct pH level. Chloroplatinic acid can also be produced through oxidation by chlorine in hydrochloric acid. This process has the advantage of not having any residual nitrates. Reducing agents that are used to precipitate platinum powder include sodium formate, formic acid, sodium borohydride, sodium hydrosulfite, hypophosphorous acid, and hydrazines. The pH of the solution may be adjusted by the addition of reagents such as sodium, potassium or ammonium hydroxide, or similar alkaline carbonates.

Platinum metal powder having an average particle size from 0.5 to 2 µm can be produced from a platinum chloride solution. The platinum in solution is complexed with ammonia and then reduced with a suitable reducing agent such as hydrazine.15 A similar process can be used to produce alloys of platinum with gold and palladium.16 Sodium borohydride can be used in the preparation of platinum alloys including ruthenium–platinum, platinum–palladium, platinum– rhodium, and platinum–iridium.7 Fine platinum particles are active and commercially important as catalysts. One method of making platinum powder with a size of ~30 nm is the reduction of chloroplatinic acid with sodium borohydride. The reaction is: H2PtCl6 + NaBH4 + 3H2O → Pt(s) + H3BO3 + 5HCl + NaCl + 2H2 (g)

(1)

An organic protective agent such as poly vinylpyrrolidone is generally added to prevent coagulation of these small platinum particles.17 Pure, spherical, highly crystalline platinum particles with a narrow size distribution can be made using an aerosol decomposition process. A chloroplatinic acid solution (H 2PtCl 6) or other suitable platinum solution, is made into an aerosol of droplets which are then passed through a furnace and made into very dense platinum particles. Temperatures of 1,100°C or higher are needed to produce pure platinum powder. This process can also be used to make alloys of platinum with palladium.18 GOLD POWDERS Gold powder has been used by civilizations for

Figure 1. Platinum powders. SEM

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a long time. Gold powder is mentioned in many ancient Chinese books in reference to preparing medicines and to make paint for decorating silk or paper.19 Initial uses involved first making pure gold into thin foil or wire and then continuing to pound it to make it into a usable powder. A variation of this process was to take very small pieces of gold foil and mix them with powders such as calcium carbonate or magnesium silicate. This mixture was then ground in a mortar and pestle to give a usable gold powder. In the second century, a chemical method was described in which the gold was mixed with mercury to form an amalgam and then pounded in a mortar and pestle.19 Thick-film gold conductor compositions have been developed commercially for microcircuit conductors that have high adhesion strength and fine line resolution. These thick films are also used for chip and wire-bonded electrical terminal contact metallizations, and for etchable pastes for thermal-printer head and microwave applications. The reliability of gold conductors makes them highly desirable for a variety of applications in electronics and communication systems that are used in military, medical, aerospace, and instrumentation applications. Gold’s reliability is due to its superior resistance to oxidation and tarnish. Using gold may cost more initially, but much more would be sacrificed in terms of lost operating time and repair expenses if the electrical properties of the non-gold substitute would deteriorate due to the formation of oxide or sulfide films.14,20,21 In addition, pure gold conductors have high conductivity, excellent wire bondability, excellent migration resistance, and are generally compatible with other components of thick-film materials. This makes gold particularly suitable for resistor terminations, wire and die-attach pads, and highly reliable electrical interconnections.14 Pure-gold conductors exhibit poor solder leach resistance. The gold conductors can be made to be solderable through the addition of platinum. This imparts solderability at the expense of conductivity. Platinum–gold conductors are the industry standard when cost is not an issue and high reliability and solderability is required.14,22 There are many chemical methods for producing gold powders and each one may include variations involving pH, dilution, and temperature. Many processes for making gold powder use an

32

aqueous solution of chloroauric acid (HAuCl4) as the starting salt solution. This gold chloride solution is made by dissolving gold grain or shot in aqua regia (3 to1 mixture by volume of hydrochloric acid and nitric acid) followed by heating to decompose and remove the nitric acid. Gold chloride solution can also be produced using chlorine as the oxidizing agent in hydrochloric acid. Reduction and precipitation of gold powder from chloroauric acid solution may be accomplished through the use of active metals such as zinc, magnesium, and iron, by inorganic reducing agents such as ferrous sulfate, sodium sulfite, sulfur dioxide, and hydrogen peroxide, or organic reducing agents such as formic acid, formaldehyde, or others.15,16,23,24 The characteristics of the gold powder produced, such as surface area, particle size, particle-size distribution, and particle shape, are dependent on the conditions under which the powder was made. Physical characteristics influence the chemical processability and determine the appearance, usefulness, and efficiency of the gold powder in a particular application. The morphology and shape of the gold particles is determined by the conditions of the precipitation process. Gold powder with nodular or irregularly shaped particles is produced when no particle morphology modifiers are used during the reduction of the gold salt solution. Heating the solution increases the rate of reduction and produces smaller-sized gold particles. Solutions that are basic can also increase the rate of reduction.25 The presence of free hydrochloric acid can retard or even prevent reduction from occurring. The shape of gold particles can be affected by the chemical reductant used. The use of colloidal materials during precipitation provides control over the formation of aggregates by inhibiting particle-to-particle adhesion and growth. Control of the nucleation stage followed by particle growth has made possible the precipitation and formation of gold powders with closely controlled particlesize ranges.26 Reproducibility and morphology are the keys to the applications for gold powder and are the driving force for choosing one process over another. Examples of gold powders are shown in Figure 2. For some thick-film applications, it is important that the gold powder be highly deagglomerated and easily dispersed. More-easily dispersed gold

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Figure 2. Gold powders. SEM

powder can be made using potassium sulfite as the reducing agent with the addition of a protective colloid such as gum arabic or polyvinyl alcohol.27,28,29 Fatty acids, fatty acid alcohols, and fatty acid amines can also be used as coatings for the gold particles to control the particle size and act as lubricating agents when using the gold powders.30 Gold particles coated with PVP can be made in ethylene glycol. The reduction of gold in the ethylene glycol process requires high temperatures (>120°C) to make monodisperse particles. The higher the temperature, the smaller the size of the precipitated particles.9 The shape characteristics of gold powder are almost entirely spherical when the reductants are sulfur dioxide, sulfites, and related sulfur-containing compounds. Potassium sulfite or sodium sulfite can be directly added to aqueous gold chloride solution to produce spherical gold particles with controlled size and density:31 2HAuCl4 + 3Na2SO3 + 3H2O → 2Au(s) + 3H2SO4 + 6NaCl + 2HCl

(2)

This reaction is dependent on pH, the ratio of gold chloride to sulfite, the mixing, and the temperature. If high temperatures are used, a mixture of spheres and flat flakes can be made.24 Two-dimensional gold flakes are precipitated when oxalic acid is used to reduce gold chloride solution at elevated temperatures. If another reducing agent such as hydroquinone is used, the process produces spherical particles with a size ranging from 0.5 to 2.0 µm.32 Gold powder containing a mixture of gold flakes and gold spheres can be made by reducing an aqueous gold chloride solution with a mixture of hydroquinone and oxalic acid in the presence of a protective colloid such as gum arabic, methylcellulose, sodium algi-

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

nate, or gelatin.29 Gold powder can also be made mechanically. Sheets of gold are reduced in thickness by rolling and beating to produce a foil between 0.01 and 0.02 mm thick that is then beaten to produce leaves 0.2–0.5 µm thick. These leaves are broken down further during a dispersion process to enable them to be used as fine gold powder. Larger gold powders and gold flakes can be made through standard ball milling of gold powder.26 Spray pyrolysis (aerosol) has been used to make high-purity, phase-pure, spherical gold particles. The spray pyrolysis process involves the atomization of an aqueous gold solution to form droplets which are then directed though a heated furnace. In the furnace, the droplets first lose water by evaporation following which the gold salt particles decompose to give spherical, fully dense gold particles. The gold particles are then collected using a filter and are used without any subsequent processing. This process makes micron and submicron particles with a distribution of sizes that is dependent on the size distribution of the droplets that flow through the furnace.33,34 An example of gold powder produced by spray pyrolysis is shown in Figure 3. Dispersions of fine gold particles are important for scientific and practical applications. Colloidal dispersions of gold have many potential applications, including catalysis, pigments, biology, sensors, heat-reflecting coatings, and decorative arts.

Figure 3. Gold powder produced by spray pyrolysis. SEM

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Most colloidal dispersions of gold are produced using the reduction of gold compounds in solution. The reduction can be induced by radiation or through the use of reducing agents such as borohydrides, hydroxylamine, hydrazine, citric acid, alcohols, polyols, and many others. In most cases, a protective species is needed to stabilize the dispersion of the gold particles. Protective materials include natural polymers such as gelatin and gum arabic, and synthetic polymers such as polycarbonate, polysulfonates, polyvinyl alcohol, polyvinylpyrrolidone, and polyacrylates. A rapid and reproducible method for making colloidal dispersions of gold involves the use of iso-ascorbic acid: 2HAuCl4 + 3C6H8O6 = 2Au + 3C6H6O6 + 8HCl (3) The reaction proceeds rapidly at room temperature and is stable without the addition of any dispersion-stabilizing additives.35 PALLADIUM POWDERS Palladium is used in the electrical and electronics industries for automotive, telecommunications, and high-technology consumer applications.36 Thick-film palladium-containing conductors are used in electrical contacts, multilayer ceramic capacitors, thermocouples, resistor networks, and various hybrid applications. Palladium’s corrosion resistance, high melting point, contact resistance, and reasonable electrical conductivity account for its use as an electrical contact material where long life and reliability are essential. Palladium can be preferred for lowcurrent, long-life relays operating at low contact forces.13,36 Palladium pastes are used in multilayer ceramic capacitors made at high temperatures because of its high melting point (1,773°C). Silver–palladium is used in capacitors prepared at lower fired temperatures. The higher the firing temperature, the more palladium is used. The properties of the powders intended for the internal electrodes of multilayer ceramic capacitors are extremely important as compatibility is required between the metal powder and the organic medium of the thick-film paste, and between the paste itself and the surrounding dielectric material.37 The presence of palladium serves to inhibit the solder leaching of silver from the conductor circuit line and it reduces the migration of the silver into the dielectrics or resistors.14 Palladium powders

34

are also used in chip resistors, resistor networks, and hybrid conductors for automotive, telecommunications, and even high-technology consumer applications.38 Soluble palladium salts such as palladium chloride, palladium nitrate, and palladium bromide are used as starting materials for the production of palladium powders. Chloropalladous acid (H2PdCl4) is the most frequently used palladium salt. Subtle variations in the reaction parameters can have a large effect on the physical characteristics of the metal powder formed. Reducing agents that are effective for gold or platinum precipitations are also applicable for the manufacture of palladium powder. Hydrazine, formaldehyde, hypophosphorous acid, hydroquinone, sodium borohydride, formic acid, and sodium formate can be used. The equation for the palladium precipitation by sodium formate reduction of a palladium chloride solution is:4 H2PdCl4 + HCOOH → Pd + CO2 + 4HCl

(4)

Important variables in the reduction process for making palladium powder include the type of palladium salt used as the source of palladium, the type of complexing agent (if any), the pH, the choice of the reducing agent, the concentration of the reactants, the reaction temperature, the mode and degree of agitation of the solution, the solution viscosity, and the presence of additives or surfactants. Subtle changes to the precipitation parameters can produce different sizes ranging from <0.1 µm to >5 µm and different morphologies including nodular, dendritic, irregular, and spherical palladium particle shapes.37 Examples of palladium powders are shown in Figure 4. Metal powders such as zinc and copper can be

Figure 4. Palladium powder. SEM

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used to produce palladium powder from acidic solution; the reaction is: H2PdCl4 + Zn(s) → Pd(s) + ZnCl2 + 2HCl

(5)

Powders prepared by this technique may require special treatment to reduce the amount of impurities present and the amount of residual palladium oxide. Palladium powders made this way are large, irregularly shaped and tend to be agglomerated. Spherical, dense palladium powder can be produced using spray pyrolysis. A precursor palladium salt solution (such as palladium chloride or palladium nitrate) is atomized by an aerosol generator, the droplets are evaporated and reacted in a furnace, and the resulting spherical palladium powder is collected. The parameters controlling the particle properties include the choice of palladium salt, reactor temperature, solution concentration, aerosol droplet size, reaction atmosphere, and the residence time in the reactor. By controlling these parameters, spherical, dense palladium particles can be made with average sizes ranging from 0.1 µm to ~5 µm.39 Fine palladium powder can be made using a polyol process. Conditions are difficult to control and therefore the nucleation step predominates over growth, making particles <100 nm. Lowering the temperature helps to make larger particles, though an additional reducing agent, such as hydrazine, is needed. A palladium amine complex dissolved in ethylene glycol can be reduced by the addition of hydrazine to make approximately 150 nm particles at room temperature without stirring.9 SUMMARY The majority of platinum, gold, and palladium powders used in industry are prepared by chemical precipitation from an aqueous salt solution. To produce small, monosized, easily dispersed particles, the nucleation stage must be separated from the growth stage. The choice of reducing system plays a major role in controlling the nucleation state and the growth of the particles. To maintain and use these easily dispersed particles, a surfactant is adsorbed onto the particle surface to prevent aggregation or coagulation. Research into new and improved methods for producing platinum, gold, and palladium powders will continue to be important. Cost and yield will drive the use of these expensive metals and one

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

has to learn to tailor the size, shape, and density of the platinum, gold, and palladium powders to make the most efficient use of them. REFERENCES 1. G. Demopoulos and G. Pouskouleli, “Hydrochemical Preparation of Fine Precious Metal Powders”, J. of Metals, 1988, vol. 40, no. 6, pp. 46–50. 2. R.G. Loasby and P.J. Holmes, “Development of Thick Film Technology”, Handbook of Thick Film Technology, edited by P.J. Holmes and R.G. Loasby, Emerald Group Publishing Limited, Bingley, UK, 1976, p. 5. 3. Precious Metals Science and Technology, edited by L.S. Benner, T. Suzuki, K. Meguro, and S. Tanaka, 1991, International Precious Metals Institute and Ilse V. Nielsen Historical Publications, Austin, TX, pp. 319–333, and pp. 584–585. 4. O.D. Neikov, S.S. Naboychenko, I.V. Mourachova, V.G. Gopienko, I.V. Frishberg and D.V. Lotsko, “Production of Noble Metal Powders“, Handbook of Non-Ferrous Metal Powders, 2009, Elsevier, Oxford, U.K., pp. 423–435. 5. H.D. Glicksman, “Production of Precious Metal Powders: Silver, Gold, Palladium, and Platinum”, ASM Handbook, Powder Metal Technologies and Applications, 1998, ASM International, Materials Park, OH, vol. 7, pp. 182–187. 6. R.C. Flagan, “Generation of Particles by Reaction”, Powder Technology Handbook, Second Edition, edited by K. Goth, H. Masuda and K. Higashitani, 1997, Marcel Dekker, New York, NY, pp. 443–458. 7. Platinum Suppl. Vol. A1, Gmelin Handbook of Inorganic Chemistry, 1986, Springer-Verlag, Berlin, Germany, p. 126 8. T. Hosokura, “Method for Producing Metal Powder”, U.S. Patent No. 6,156,094, December 5, 2000. 9. Fine Particles Synthesis, Characterization, and Mechanisms of Growth, 2000, edited by T. Sugimoto, Marcel Dekker, Inc., New York, NY, pp. 430–496. 10. Y. Tan, X. Dai, Y. Li and D. Zhu, “Preparation of Gold, Platinum and Silver Nanoparticles by the Reduction of their Salts with a Weak Reductant – Potassium Bitartrate”, J. Mater. Chem, 2003, vol. 13, pp. 1,069–1,075. 11. T.T. Kodas and M.J. Hampden-Smith, Aerosol Processing of Materials, 1999, Wiley-VCH, New York, NY, pp. 440–460. 12. C.F. Vermaak, The Platinum-Group Metals: A Global Perspective, 1995, Mintek, Randburg, South Africa, pp. 165–180. 13. Chemistry of the Platinum Group Metals, edited by F.R. Hartley, 1991, Elsevier, Germany, pp. 9–31. 14. W. Borland, “Thick Film Hybrids”, Electronic Materials Handbook, Vol. 1, 1989, ASM International, Materials Park, OH, pp. 332–353. 15. O. Short, “Process of Preparing Noble Metal Powders”, U.S. Patent No.3, 620,713, November 16, 1971. 16. O. Short, “Process of Preparing Noble Metal Alloy Powders”, U.S. Patent No.3, 620,714, November 16, 1971. 17. P. Van Rheenen, M. McKelvy, R. Marzke and W. Gaunsinger, “Platinum Microcrystals”, Inorganic Syntheses, 1986, v ol. 24, pp. 238–242.

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18. T.T. Kodas, M.J. Hampden-Smith, J. Caruso, D.J. Skamser, Q.H. Powell and C.D. Chandler, “Methods for Producing Platinum Powders”, U. S. Patent No. 6,165,247, December 26, 2000. 19. Z. Huaizhi and N. Yuantao, “Techniques Used for the Preparation and Application of Gold Powder in Ancient China”, Gold Bulletin, 2000, vol. 33, no. 3, p. 108. 20. J. Savage, “Conductor Materials”, Handbook of Thick Film Technology, edited by P.J. Holmes and R.G. Loasby, 1976, Emerald Group Publishing Limited, Bingley, UK, p. 106. 21. R.R. Davies, “New Development in the Use of Gold in Electronics and Communications”, Precious Metals, edited by R.O. McGachie and A.G. Bradley, 1981, Pergamon, Toronto, Canada, pp. 193–199. 22. M.L. Topfer, Thick-Film Microelectronics: Fabrication, Design, and Applications, 1971, Van Nostrand Reinhold, New York, p. 46. 23. B. P. Block, S. A. Bartkiewicz, T. Moeller, J. D. Chrisp, P. Gentile and L. O. Morgan, “Gold Powder and Potassium Tetrabromoaurate(III)”, Inorg. Synth., 1953, vol. 4,p. 15. 24. D.J. Arrowsmith and K.J. Lodge, “Gold Powder Production for Thick Film Electronic Applications”, Trans. Inst. Metal Finishing, 1987, vol. 65, pp. 120–126. 25. T.K. Rose and W.A.C. Newman, “Chemistry of the Compounds of Gold”, Metallurgy of Gold, Seventh Edition, Met-Chem Research, Inc., Boulder, Co., 1986, pp. 68–80. 26. N. Collier, “Advances in Gold Powder Technology”, Gold Bulletin, 1977, vol. 10, no. 3, pp. 62–66. 27. D.J. Langlois, “Process for Gold Precipitation”, U.S. Patent No. 3,869,280, March 4, 1975. 28. O.A. Short, “Gold Metallizing Compositions”, U.S. Patent No. 3,717,481, February 20, 1973. 29. O.A. Short and R.V. Weaver, “Process for Making Gold Powder”, U.S. Patent No. 3,725,035, April 3, 1973. 30. V.R. Driga, “Gold Powder”, U. S. Patent No. 3,843,379, October 22, 1974. 31. O.A. Short, “Process for Manufacturing Gold Powder”, U.S. Patent No. 3,771,996, November 13, 1973. 32. O.A. Short and R.V. Weaver, “Gold Powder”, U.S. Patent No. 3,811,906, May 21, 1974. 33. D. Majumdar, T.T. Kodas and H.D. Glicksman, “Gold Particle Generation by Spray Pyrolysis”, Adv. Mater., 1996, vol. 8, no.12, pp. 1,020–1,022. 34. H.D. Glicksman, “Method for Making Gold Powders by Aerosol Decomposition”, U.S. Patent No. 5,616,165, April 1, 1997. 35. D. Andreescu, T.K. Sau and D.V. Goia, “Stabilizer-Free Nanosized Gold Sols”, J. of Colloid and Interface Science, 2006, vol. 298, pp. 742–751. 36. P.D. Gurney and R.J. Seymour, “The Platinum Group Metals in Electronics”, Chemistry of the Platinum Group Metals, edited by F.R. Hartley, Elsevier, Germany, 1991, pp. 594–616. 37. E.M. Wise, Palladium, Recovery, Properties, and Uses, Academic Press, New York, NY, 1968. 38. G. Ferrier, A. Berzins and N. Davey, “The Production of Palladium Powders for Electronic Applications”, Platinum Metals Rev., 1985, vol. 29, no. 4, pp. 175–179. 39. T.C. Pluym, S.W. Lyons, Q.H. Powell, A.S, Gurav, T.T. Kodas, L. Wang and H.D. Glicksman, “Palladium Metal and Palladium Oxide Particle Production by Spray Pyrolysis”, Mat. Res. Bull., 1993, vol. 28, pp. 369–376. ijpm

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PRECIOUS METALS

PRECIOUS METAL POWDER PRECIPITATION AND PROCESSING Sean Frink* and Phil Connor**

METAL POWDER PRODUCTION Metal powders can be produced by either “top-down” or “bottom-up” processes. “Top down” processes are based on mechanical pulverization and grinding, and they can be used to produce fine powders from bulk materials. However, malleable materials are difficult to handle with these methods, and they cannot produce the degree of fineness of “bottom-up” techniques. As a result, “bottom-up” processing techniques such as atomization, thermal decomposition, evaporation–condensation, and precipitation are preferred when producing fine metal powders.1 Atomization includes, but is not limited to, gas aspiration, spinningdisk processes, pressure atomization, and quench atomization. Atomization techniques rely on precise control of critical parameters such as gas/fluid type, nozzle-exit velocity, gas-spray velocity, jet angle, and temperature to produce the required particle sizes, particle-size distributions, and morphologies.2 Also, as particle size is reduced for use in electronics applications, it becomes increasingly difficult to produce the required particle-size distribution, to control over-sized particles, and to provide homogeneity with atomization. Classification techniques can be used to segregate the resultant powder into finer, tighter particle-size distributions, but this reduces yield and increases processing costs. Alternative atomization processes such as thermal decomposition and evaporation–condensation reactions have similar limitations.

Precious metal powders are widely used in industry because they are relatively inert, provide catalytic activity, and provide useful thermal/electrical conductivities. Several techniques can be used to produce precious metal powders, but of all the methods available, aqueous chemical precipitation is one of the most versatile because it can produce a wide range of particle sizes, distributions, morphologies, and surface chemistries. Silver powders produced with aqueous chemical precipitation can be further post-processed to generate silver flakes that provide unique electrical, thermal, and rheological properties when formulated into conductive pastes, paints, and adhesives. Innovative reaction chemistries can also lead to directly precipitated flakes and other novel materials.

Figure 1. Representative powder types produced by precipitation: (a) agglomerated fine-particle-size, general-purpose silver powder with a medium-range surface area and low density; (b) non-agglomerated high-purity, fine spherical silver powder; and (c) mid-range particle-size silver flake produced by postprecipitation mechanical milling. SEM *R&D Manager, Metal Products, **Global Applied Technology Manager, Metal Products, Ferro Electronic Material Systems, 3900 South Clinton Avenue, South Plainfield, New Jersey, 07080, USA; E-mail: [email protected]

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In contrast, precipitation is a more versatile method that can be rapidly scaled economically from laboratory to full-scale production. Figure 1 shows scanning electron micrographs (SEM) of representative powders that can be produced by various precipitation and post-precipitation processing techniques. PRECIOUS METAL POWDERS Of all the precious metals, silver has the highest electrical conductivity and, with the exception of diamond, exhibits the highest thermal conductivity. This pairing of preferred properties has led to its use in many applications requiring these characteristics, notably as conductive traces for display panels, as conductive adhesives, and as contacts for solar cells. In fact, of the more than 28,000 mt of silver produced annually, roughly 6,200 mt, or ~23%, are used in electrical/electronics applications.3 Japanese suppliers dominate much of the metal powder market for electronics. Table I lists the leading metal powder suppliers and their estimated powder sales in 2005. Of the top eight suppliers, only DuPont and Ferro are not based in Japan.4 Frequently, demanding electrical/electronic applications require additional physical and chemical powder characteristics to ensure compatibility with vehicle systems that are used to carry and apply materials during processing, and substrates that provide support and various functional properties during manufacturing. Most applications use spherical particles or flakes. Polymer thick films, conductive adhesives, and copper paste for passive components take advantage of the performance-enhancing characteristics of flakes. Most metal particles used in conductive adhesives are in flake form, while copper pastes use a blend of flake and powder with the flake accounting for an

estimated 20% of the metal content. Table II cites relevant market segments and applications, as well as metal types and tonnage.4 A variety of manufacturing techniques have been developed to meet these requirements. While this contribution to the Focus Issue details silver precipitation, the methods discussed apply to various precipitated metal powders, including gold, palladium, and platinum. These materials can be produced in micron or ultrafine submicron, narrowly dispersed precious metal powders and flakes via various chemical-precipitation and mechanical-processing techniques. In addition, multiplereaction chemistries and processing techniques have been developed. These techniques enable end users to capitalize on the ability of powders produced using chemical precipitation to meet the requirements of a wide range of applications with TABLE II. METAL POWDER MARKET BY APPLICATION (2005) Segment

Application

Powder

Usage (mt)

EMI Shielding

Conductive Paint

Ag/Cu

120

Electroless Primer

AgO

20

Conductive Elastomer

Ag/Al

112

Form-in-Place Gaskets

Ag/Cu

36

Polymer Thick Silver Through-Hole Films Membrane Switches Conductive Adhesives

Fired ThickFilm Paste

TABLE I. LEADING METAL POWDER SUPPLIERS Supplier JFE Mineral Shoei Chemical Mitsui Mining and Smelting Sumitomo Metal Mining Dowa Kogyou DuPont Ferro Fukuda Tanaka Kikinzoku

38

Metal Powder(s) Ni Ni, Ag, Ru, Ag/Pd Cu, Ag Ni, Ru, Ag, Ag/Pd Ag, Cu Ru, Ag/Pd Ag, Cu, Ag/Cu, Ag/Pd Ag, Cu Ru, Ag/Pd

Ag

299

Ag

96

Polymeric Die-Attach Adhesives for IC

Ag

111

Isotropic Conductive Adhesives for LED

Ag

7

Isotropic Conductive Adhesives for Other Applications

Ag

15

Hybrid Circuits

Ag/Pd Ag/Pt Cu

8 56 16

Low-Temperature Co-Fired Ceramics

Ag Ag/Pt Cu

14 3 4

Ni Cu

1,300 332

Ru Ag/Pd

8 206

Ag Ag

369 128

Multilayer Ceramic Capacitors Resistor Displays Photovoltaics Total

3,260

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respect to electrical, thermal, and rheological properties when formulated into conductive pastes, paints, and adhesives. PRECIPITATION TECHNIQUES Precipitation of silver from silver salts in a liquid medium by a reducing agent can be represented by the sum of two half-cell reactions: Ag+ + e- → Ag and R → R+ + e-, yielding the general redox equation: Ag+ + R → Ag + R+

(1)

where R represents an applicable reducing agent. This general reaction proceeds from left to right under standard state conditions provided that the sum of the two half-cell potentials (ΔE) is positive. Because the standard reduction potential for Ag+ is 0.799 V, the reducing agent must be chosen to ensure that its reduction potential is <0.799 V. In fact, it should be ~0.3 to 0.4 V <0.799 V for the reaction to proceed at an acceptable rate. 5 In practice, metal powder precipitations are rarely performed under standard state conditions, and numerous reaction factors other than ΔE come into play that can affect the rate and thermodynamics of precipitation. These include, but are

not limited to, surfactant and solvent effects, temperature, and pH. Many process parameters can affect the resultant particle-size distribution. For example, precipitation must be performed with efficient mechanical mixing and controlled addition of reagents to ensure homogeneity within the solution at the beginning of the process. This early stage of the process produces highly reactive transient intermediates often called “embryos.” 5 Efficient mixing also ensures that the formed metal particles are dispersed uniformly in the liquid medium. The embryos can then form larger particles of uniform size by precipitation of additional metal atoms onto the embryonic surfaces. Process conditions are then controlled to generate the required individual particle sizes. Particles produced by this method tend to be faceted monocrystalline powders, as illustrated in Figure 2. Both powders exhibit high dispersability, sintered density, and conductivity. In addition, crystals can be produced through agglomeration and a build-up of embryos to form spherical polycrystalline powders, as shown in Figure 3. Once precipitation is completed, additional processes must be controlled to prevent further

Figure 2. (a) Early-stage precipitated high-purity crystalline silver powder with medium surface area, and (b) high-purity crystalline fine silver powder. SEM

Figure 3. Agglomerated spherical polycrystalline powders: (a) high-purity coarse silver powder with high density, uniform particle size and low surface area, (b) high-purity silver powder with low surface area, and (c) high-purity ultrafine silver powder. SEM

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modifications that can occur. For example, Ostwald ripening occurs because larger particles have greater thermodynamic stability than smaller particles, causing the larger particles to grow at the expense of the smaller ones. Another pathway for particle growth involves the uncontrolled aggregation of smaller particles into larger particles resulting in nonspherical polycrystalline powders. These secondary particle-growth mechanisms can be controlled through the use of various process conditions. FLAKING Multiple milling methods can be used to flake powders. Mill type, milling media, process conditions, and additives all contribute to the physical and functional characteristics of the flake. Figure 4 illustrates the different flake morphologies that can be produced by manipulating critical process parameters during milling. In the process, the material is progressively flattened until individual flakes eventually start to break down. Another factor that significantly affects the properties of the flake is the input powder. Figure 5 demonstrates the wide range of flake morpholo-

gies and characteristics that can be produced. CHARACTERIZATION OF PRECIOUS METAL POWDER AND FLAKE Once a powder or flake is synthesized, a range of analytical methods is required to quantify critical characteristics such as particle size and particle shape. Material characteristics can often be correlated to final performance characteristics in customer-specific applications. Common methods for measuring particle size include sieving, Fisher sub-sieve size analysis, sedimentation, and dynamic light scattering. Particle shape and surface structure can be probed through techniques such as apparent density, tap density, BET surface area, and SEM imaging. No single measurement can be used to universally quantify the size or shape of a non-spherical powder. Many of these measurements are indirect evaluations of shape that can be influenced by several factors. Other methods such as SEM are limited to statistically small samples of the powder population. These characterization methodologies are reviewed in the context of precious metal powders. One of the simplest, most direct, and widely

Figure 4. Progressive flattening of powders to form flakes. SEM

Figure 5. Examples of milled powders to produce flake: (a) fine high-purity flake, (b) mid-size flake, and (c) coarse flake. SEM

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used methods of determining particle size is sieving. This is done by shaking a sample of powder through multiple pans, each with a wire cloth bottom of progressively smaller openings than the preceding one. The amount of material retained in each pan can then be measured and the particle-size distribution established. The principle drawback of this method is that sieves with openings less than ~10 µm are difficult to make and use. Particle size can also be estimated by measuring the pressure drop of a gas flowing through a bed of the compacted powder. Pressure drop is governed in part by the specific surface area of the powder and this can then be converted to particle size using simple algorithms or it can be read directly from the apparatus in the case of the Fisher subsieve sizer. This method is suitable only for comparisons of similar materials and, therefore, is used only for control purposes. It is not suitable for fine powders because their pore sizes are small enough to invalidate the underlying assumptions of the equations used to analyze the results.6 Sedimentation measures the rate of settling of a free-falling particle in a liquid medium. The results are directly proportional to particle mass if the particles are spherical but, in practice, they can be used for particles whose maximum-tominimum-diameter ratio does not exceed 4.7 For small particles (≤2 µm), the terminal velocity of the particle approaches the velocity of the convection currents due to thermal gradients or liquid displacement, in which case centrifugal settling becomes necessary. As particle size decreases further, other methods are needed to determine particle size. Light scattering instruments measure the size-dependent light-scattering characteristics of particles. This is a fast method for determining the particlesize distribution, in part because it is capable of rapidly sampling a statistically significant number of particles. Commercial instruments are available that use this theory, and both wet and dry samples, with sizes ranging from hundreds of microns to tens of nanometers, can be evaluated. When using light-scattering methods, care must be taken to ensure proper sample dispersion and that appropriate algorithms are chosen to analyze the scattering data. The density of a bulk material is a physical constant calculated by dividing its mass by its

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

volume. In the study of powders, material packing is a critical factor that is related to particle-size distribution, shape, surface texture, and chemistry. Tap density is used as an indirect measure of these characteristics because it is more consistent than a simple bulk-density or packed-density measurement. To quantify tap density, a set mass of material is sampled and systematically compacted by repeatedly dropping the powder container from a small fixed height until the powder volume no longer changes. Volume is measured for a given mass of material to give the tap density, which can correlate with paste solids loading and, in some cases, ease of handling. The surface characteristics of a powder can further be probed by studying its ability to absorb a gas onto its surface, using the well-known BET surface-area-measurement technique. 8 Gas absorption depends on several factors, including the gas used, sample preparation, and the presence of internal vs. external pours. Surface-area measurements can be indicative of powder–solvent interactions and, when combined with lightscattering particle-size distribution data, can be used to determine the degree of agglomeration in the powder. Visual or electron imaging is the only direct measurement of particle size and shape; all other methods measure the variation in some property that correlates to particle size. Care must be taken when using this method, however, because the number of particles evaluated in an image can be relatively small and may not be representative of the entire sample population. Therefore, it is important to remember that microscopic imaging techniques are only qualitative methods for evaluating particle shape and size and should always be used in conjunction with other methods. Characterizing both size and surface properties is critical because changes in either entity can greatly affect the rheological properties of formulations containing the powder. This is especially important for inks, pastes, and paints, where rheology has a critical effect on product quality. OUTLOOK In addition to standard flake manufacturing, novel morphologies can be produced by modifying critical process parameters. Examples are shown in Figure 6. All of the techniques cited enable the produc-

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Figure 6. Novel particle morphologies: (a) silver fibers, (b) high surface area, low density flake, and (c) gold platelets. SEM

tion of multi-ton quantities of precious metal powders for demanding electronic materials applications such as electrically and thermally conductive adhesives, passive component terminations, polymer thick film, membrane touch switches, and radio frequency identification antennas. These powders can also be used in spray shielding paints, display pastes, and formin-place gaskets. Hybrid integrated circuits and low-temperature co-fired ceramics (LTCC) also depend on these materials. REFERENCES 1. C.R. Veale, Fine Powders Preparation, Properties, and Uses, 1972, John Wiley & Sons, New York, NY.

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2. A. Lawley, Atomization: The Production of Metal Powders, 1992, Metal Powder Industries Federation, Princeton, NJ. 3. P. Klapwijk , “The Silver Institute World Silver Survey 2009”, GFMS Limited, May 13, 2009, The Silver Institute, New York, NY; www.gfms.co.uk. 4. “Market Assessment and Forecast: Metal Powders in Electronics, Report # 3376”, October 2006, Prismark Partners LLC, Cold Springs, NY. 5. D.V.Goia and E. Matijevic, “Preparation of Monodispersed Metal Particles”, New. J. Chem., 1998, pp. 1,203–1,215 6. T. Allen, Particle Size Measurement, Fourth Edition, 1990, Chapman & Hall, New York, NY. 7. R.R. Irani and C.F. Callis, Particle Size: Measurement, Interpretation, and Application, 1963, John Wiley & Sons, New York, NY. 8. S. Brunauer, P.H. Emmett and E. Teller, “Adsorption of Gases in Multimolecular Layers”, J. Amer. Chem. Soc., 1938, vol. 60, no. 2, pp. 309–319. ijpm

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PRECIOUS METALS

ADDITIVE MANUFACTURING OF PRECIOUS METAL DENTAL RESTORATIONS Anita L. Hancox* and Jeffrey A. McDaniel**

INTRODUCTION Dental restorations can be made from various materials and techniques. One of the most frequently prescribed bonded restoration is a PFM. These have been available since the 1960s, and metal has been used in the mouth for more than 4,000 years.1 PFMs are produced by fusing porcelain to an underlying metal structure called a coping. They combine the strength of a metal coping and the aesthetic appearance of porcelain to create a natural looking tooth known as a crown or bridge. A completed PFM crown on a die is shown in Figure 1. A precious metal coping manufactured with 3D printing is also pictured. Crowns and bridges are types of dental restorations most often used when decay or damage to a tooth is too severe for it to be filled. In these cases, a dental professional will remove a portion of the tooth above the gum line, leaving the tooth’s root system intact. The crown, or cap, is fashioned to fully cover the remaining tooth. A bridge is a series of adjacent copings and framework used to replace one or more missing teeth. Crowns and bridges are also used in conjunction with dental implants. Today, the materials used in permanent crowns and bridges fall into two categories: metal-base and all-ceramic. Metal-based crowns are stronger and create a more-realisticlooking restoration than a ceramic crown or bridge. All-ceramic crowns can be produced at a reduced cost to the laboratory and dentist, but the aesthetic properties and overall performance of the crowns lag behind those of precious metal restorations. More than 60 million metal crowns and bridges are produced each year in the U.S. For more than half a century the majority of crowns and bridges pro- Figure 1. PFM crown on a die and 3D-printed precious metal coping duced in the U.S. and Canada have

The additive manufacturing technology known as threedimensional printing (3DP) has been developed for the production of dental copings in porcelain-fused-to-metal crowns (PFMs). The coping is the metal foundation on which porcelain is fused to produce permanent, fully functional dental restorations. Traditionally, metal copings are manufactured utilizing inefficient and labor-intensive metalcasting using the lost-wax process. 3DP addresses the disadvantages associated with traditional, indirect methods by providing an automated solution for the direct digital production of precious and semiprecious metal copings. A review of the additive manufacturing process, including a description of subsequent thermal processing, is presented. In addition, details of the precious metal alloy systems utilized, and features of the finished dental restorations, are documented.

*Materials Engineer, **General Manager, imagen, LLC, 127 Industry Boulevard, Irwin, Pennsylvania 15642, USA; E-mail: [email protected]

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been PFMs. Manufacturing techniques have remained time and labor intensive and dependant on melting and casting, or requiring the removal of metal by machining/milling. Each year, millions of PFMs are created by either metalcasting into molds or by manual layering processes. A small percentage of PFMs are manufactured by machining non-precious alloys. Crowns and bridges fabricated by direct laser sintering of cobalt–chromium powder alloys reflect a growing market.2 Today, most metal copings are produced via a multi-step fabrication procedure involving the lostwax process by dental laboratories. In the U.S. alone, there are more than 12,000 laboratories servicing more than 170,000 dentists. Imagen’s 3D powder metal printing process is a direct-tometal fabrication method available to the dental industry. 3DP is an additive manufacturing technology that produces 3D parts directly from CAD files.3 In the present study, 3DP has been used for the direct fabrication of metal copings. The 3Dprinting process uses an ink-jet printhead to introduce binder onto a smooth bed of metal powder. The binder connects the powder particles together to create a partially dense preform. Typically, subsequent thermal processing includes curing, sintering, and optional infiltration steps. Dental laboratories have hundreds of alloys to choose from when creating a PFM crown or bridge. The American Dental Association has created three categories to describe these alloys: high noble (HN) with a noble metal content of ≥60 w/o and a gold content of ≥40 w/o; noble (N) with a noble metal content of ≥25 w/o; and predominantly base (PB) with a noble metal content of <25 w/o. The decision to use a particular alloy depends on the clinical type of restoration required, the location of the restoration in the mouth, cost, and the type of porcelain used. Previous work has led to the development of an HN 3D-printed commercial alloy (imagenBright™). This alloy was approved as a medical device by the FDA in 2006. It is a gold–platinum–palladium alloy containing 87 w/o gold. The goal of this work was to develop a silverfree HN alloy suitable for the additive manufacturing of single-unit crowns and multi-unit bridges. It is now commercially available and is referred to in this paper as a gold–palladium alloy. EXPERIMENTAL APPROACH Powder Characteristics and Composition The gold–palladium alloy composition chosen

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for this study was based upon a typical HN casting alloy currently used in the dental industry.4 In this application of 3D printing, the print and infiltrant alloy compositions were chosen so that, when combined, the final composition would be similar to the target cast-alloy composition. The compositions of the print and infiltrant alloys are given in Table I and the powder particle-size distribution of each powder component is given in Table II. A scanning electron micrograph (SEM) of the print alloy powder is shown in Figure 2. The gold–palladium alloy chosen for this study is one of five types of noble metal alloys used for PFMs. It is an HN composition that is used in order to provide good corrosion resistance. The alloy was designed to be silver-free in response to “greening,” an industry concern related to the green color formed when silver-containing metals are fired with certain porcelain materials.5 A small amount of ruthenium is added as a grain refiner to improve both mechanical properties and tarnish resistance.5 Additionally, a typical metal coping is processed to create surface oxides before the first layer of ceramic is applied. These oxides contribute to the metal-to-ceramic bond strength;6 to this end, indium was added to the alloy studied as an oxide former. A metal-toceramic bond is important, as it reduces the risk of chipping or delamination under normal to excessive biting forces. CAD File Preparation The additive manufacturing process begins with a CAD file. In this application, a tooth die is

Figure 2. Gold–palladium print alloy powder. SEM

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TABLE I. ALLOY COMPOSITION (w/o) Element

Print Alloy

Infiltrant Alloy

Gold Palladium Ruthenium Indium

39.8 49.25 0.2 10.75

97 — — 3

TABLE II. PARTICLE-SIZE DISTRIBUTION OF POWDERS* D10 D50 D90 Mean Standard Deviation

Print Alloy (μm)

Infiltrant Alloy (μm)

8.4 20.1 33.5 20.5 10.3

4.5 14.9 42.4 20.3 13.7

*Powders obtained from HJE Company, Inc. scanned and dental design software used to create a virtual coping. An approach gaining in popularity is the use of intraoral scanners that scan the actual geometry of the patient’s mouth to reduce error associated with taking an impression in the dentist’s office and manufacturing a stone tooth die preparation in the dental laboratory. The virtual coping is designed to have a specific thickness as well as a virtual die spacer to achieve the correct fit when the crown is inserted into the mouth. Figure 3 shows an example of a tooth die stone, a scan of the die, and a virtual coping CAD file. Printing The printer accommodates 25 to 30 virtual coping CAD files loaded at one time. The files are then digitally sliced into 50 µm layers. Most copings range from 140 to 180 layers. The “print” powder is loaded into the build box of the

machine and spread to form a level layer. Spreading is achieved by using a roller to move powder from a feed area to a build area. The roller counter-rotates while it travels over the powder bed and this combined motion creates a wavefront of powder that is deposited as a layer over the build area. The printhead deposits binder onto the powder bed according to the 2D cross section of the current layer. A feed piston moves up to supply a 50 µm layer of powder, and a build piston moves down to accept the 50 µm layer. The roller spreads the powder from the feed side to the build side, creating a fresh layer of powder. The system advances the CAD file to the next layer, and the printhead deposits binder according to the 2D cross section. This process is repeated until the copings are complete. During the printing process, the copings are supported in free space by the unbound or “loose” powder. Figure 4(a) shows a build box loaded with precious metal print alloy. The feed and build compartments that make up the build box are each 70 × 50 × 35 mm. Figure 4(b) shows a cross section of various copings during printing. Figure 4(b) is a screen capture of the cross section being printed in 4(b). It shows how the individual copings are arranged in the build; these are used as a map to track copings during subsequent processing. The dimensional accuracy of a coping is affected by the printing process parameters, including the print-powder composition and powder-particle size. The most critical dimension of a coping is called the “margin line.” Although not necessarily true in all cases, the margin line is the gum line. Technically, the margin line is the point at which the coping meets the preparation. Figure 5(a) shows a technician marking the margin with a red

Figure 3. (a) model of arch and anterior tooth die requiring restoration, (b) 3D scan of tooth die obtained by dental scanner, (c) virtual coping CAD file created with dental design software

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Figure 4. (a) build and feed boxes loaded with precious metal print alloy, (b) one layer of binder deposited onto print bed according to the cross section of copings being manufactured in one print run, (c) screen capture of cross section being printed in (b)

pencil. This is a technique frequently used to reveal the margin for inspection of the fit. Figure 5(b) shows an infiltrated coping on a die. The coping must follow the margin line intimately to prevent damage and tooth decay. If there are significant gaps (>50 µm), food will work its way between the coping and tooth. Therefore, printing parameters were chosen to

maximize resolution. Resolution is determined by many factors, including the drop size of the binder, the particle size of the powder, the layer thickness, and the powder–binder wettability. Understanding the interaction among these parameters can lead to improved resolution and dimensional accuracy. For example, a thin wall (~0.3 mm) of a gold–platinum–palladium alloy was printed in the vertical

Figure 5. (a) marking the margin with a red pencil; (b) infiltrated coping meeting the margin line on a die

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Figure 6(a). 3DP gold–platinum–palladium alloy wall at low magnification. SEM

Figure 6(b). 3DP gold–platinum–palladium alloy at high magnification. SEM

position. Figures 6(a) and 6(b) are micrographs of a cured section of this wall at low and high magnifications, respectively. The powder agglomerate in the center of Figure 6(b) represents a single drop of binder. When the binder drop contacts the powder bed, it pulls powder particles toward it, forming an agglomerate and leaving holes on either side. Figure 6(a) shows that this agglomerate formation is repeated across the wall. The agglomerates result in the surface finish shown in Figure 7. This sample was printed with a solvent-based-binder system, a binder drop size of ~100 pL, and a powder classified to -45/+5 µm size fraction. Figure 8 is a micrograph of a cured gold–palladium wall section (~0.3 mm). This sample was printed with a solvent- based-binder system, a binder drop size of ~50 pL, and a -45/+5 µm powder. It shows no obvious agglomerate formation, indicating improved powder–binder wettability. The resultant coping shown in Figure 9 maintains a relatively smooth surface finish and improved dimensional accuracy. The latter combination of process parameters was used throughout this study.

Curing and Depowdering The purpose of curing is to provide green strength to the copings by hardening the binder. After printing, the entire build volume, including the printed copings and supporting loose powder, is removed from the printer and placed into a curing oven. After curing, the copings are depowdered and removed from the loose powder. Using an ultrasonic brush, the loose powder is removed and collected for subsequent print runs. Because the loose powder is not altered during the process and can be reused, there is virtually no loss of precious metal print material. Figure 10 is a micrograph of a cured gold–palladium coping. The binder forms necks between the powder particles and connects the particles together. Although a small amount of binder is present, the copings have sufficient green strength for handling. In addition, this small amount of binder led to short debinding times (<10 min) that were incorporated into the sintering cycle.

Figure 7. Early gold–platinum–palladium alloy coping with rough surface finish

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

Figure 8. 3DP alloy wall. SEM

Sintering and Infiltration Both sintering and infiltration utilize standard

Figure 9. Gold–palladium coping with improved surface finish

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processed in order to evaluate the proposed gold–palladium alloy system.

Figure 10. Cured gold–palladium coping showing powder particles bonded together by binder at the neck regions. SEM

dental furnaces common to the industry. After depowdering, the green copings are placed into a quartz dish filled with a ceramic support material. They are arranged in the support material so that the margin of the coping is facing up to create a “cup.” In order to minimize distortion, sintering cycles were developed to minimize shrinkage. Sintered copings are ~60% of the pore-free density. Micrographs of a partially sintered gold–palladium coping are shown in Figure 11. The sintering necks, which have replaced the binder, create a rigid skeleton. This precious metal print alloy skeleton is then ready to be infiltrated. At this point, a second precious metal alloy is introduced as an infiltrant. A specified amount of infiltrant powder is weighed and transferred into the “cup” of the sintered coping. The infiltrant melts and, through capillary action, “wicks” into the remaining void space to create a fully dense coping. The copings are sintered at 1,150°C for 15 min in air and infiltrated at 1,075°C for 6 min in vacuum. A graphite “getter” is placed in the furnace to prevent oxidation. Infiltrated copings are then porcelain coated to create the finished crown.

Copings Figure 12 shows a coping in each stage of the process: cured, sintered, infiltrated, and porcelain- coated. The final composition of the infiltrated copings was determined using inductively coupled plasma (ICP) and is given in Table III. Three infiltrated copings were sectioned for metallographic characterization. Using image analysis, the level of porosity was measured in five areas of each of the three copings in accordance with ASTM specification E562. The level of porosity ranged from 0.5 v/o to 3 v/o with an average porosity level ~2 v/o. This is comparable with porosity levels typical in cast alloys.8 Figure 13 is a micrograph of an as-polished surface, exhibiting 1 v/o porosity. The thickness of the infiltrated coping designs ranged from 0.35 mm to 0.45 mm. During infiltration, the infiltrant alloy not only fills the voids, but it also forms a layer on the entire surface of the coping. The average layer thickness

Testing Mechanical properties were evaluated in terms of the response to tensile loading. Tensile specimens were prepared and tested according to ISO 22674.7 RESULTS AND DISCUSSION The goal of this work was to develop a silverfree HN alloy suitable for the additive manufacturing of single-unit crowns and multi-unit bridges. Both copings and standard test specimens were

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Figure 11. Partially sintered gold–palladium coping showing solid-state neck formation at particle-to-particle contacts. SEM

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There is no evidence of the infiltrant surrounding the print-alloy grains, consistent with an efficient solubility between the two materials.

Figure 12. Cured, sintered, infiltrated, and porcelain-coated gold–palladium copings

is 0.030 mm, and this surface layer is advantageous for two reasons: color and biocompatibility. First, because the infiltrant alloy is a gold color, the layer also becomes a gold color. The coping takes on a golden hue or “straw” color when the gold-colored infiltrant coats the surface of the gray-colored print alloy. The color is important in creating the final aesthetics of a PFM crown. A golden hue creates a warmer, more aesthetic result as the porcelain allows some light to penetrate to the metal’s surface. It also avoids the blue or gray line seen subgingivally (below the gum line) that can occur when white-colored nonprecious metals, such as nickel-base and cobaltbase alloys are used. 1 In Figure 13, the layer appears as a pale yellow band on both the top and bottom of the cross section. Biocompatibility is the second advantage of the gold layer. The infiltrant is 97 w/o Au and this high gold content at the surface of the coping promotes healthy gum tissue by taking advantage of the tarnish-resistant and corrosion-resistant properties of pure gold.1 Figure 14 is an optical micrograph of an etched cross section of gold–palladium coping. It shows a single-phase microstructure, which is comparable with the microstructure of a cast gold alloy of similar composition, Figure 15. During infiltration, the infiltrant alloy reacts with the print-alloy skeleton to create a homogeneous microstructure.

Figure 13. As-polished gold–palladium surface containing 1 v/o porosity. Optical micrograph

Figure 14. Etched cross section of gold–palladium coping. Optical micrograph

TABLE III. COMPOSITION OF 3DP ALLOY Element (w/o) Au Pd In Ru

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72.5 20.0 7.5 <1

Figure 15. Comparable cast dental gold alloy. Optical micrograph. Courtesy The Argen Corporations, San Diego, CA

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Standard Test Specimens In order for a PFM alloy to be commercially viable, it must pass the criteria specified in ISO 22674;7 this standard classifies PFM alloys into six types according to their intended use, minimum yield strength, and minimum percentage elongation. The specification for a Type 4 alloy states that at least four of six specimens must meet a yield strength of 360 MPa. If the four or more specimens that meet the requirement for yield strength also meet the requirement for elongation at fracture (minimum of 2%), the metallic material meets the Type 4 requirements. 7 The tensile properties are listed in Table IV, confirming that the alloy meets both the yield strength and elongation criteria. The yield strength of all six specimens is ≥370 MPa. Five of the six specimens exceed the minimum ductility requirement of 2% elongation. Thus, this material may be used for the applications included in the standard; these include wide-span bridges, removable parTABLE IV. TENSILE PROPERTIES OF GOLD–PALLADIUM ALLOY Sample ID

UTS MPa (psi)

0.2% YS MPa (psi)

Elongation %

A B C D E F Average

387 (56,100) 399 (57,800) 479 (69,500) 457 (66,300) 440 (63,800) 439 (63,600) 433 (62,900)

370 (53,700) 399 (57,900) 408 (59,200) 398 (57,700) 386 (56,000) 374 (54,200) 389 (56,500)

0.56 4.10 3.61 3.42 3.02 3.33 3.00

Figure 16. Quantitative “screwdriver test” shows residual procelain on the metal surface of a gold–palladium coping, indicating a strong metal-to-ceramic bond

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tial dentures, and dental appliances with thin sections. A critical requirement of a PFM crown is the strength of the bond between the dental porcelain and the metal coping. There are a number of explanations for bond strength, including mechanical retention and compression bonding. However, the primary mechanism of the metal-toceramic bond is the chemical bond between the oxides in dental porcelain and the oxides on the metal surface.5 Therefore, oxide formers such as indium, iron, and tin are added to dental alloys to maximize bond strength. Although this explanation for the metal-to-ceramic bond is accepted by the dental community, it is not completely understood. One theory suggests that metal surface oxides dissolve or are dissolved by the first layer of porcelain, which is called the opaque layer. This oxide dissolution results in the formation of shared electrons and, thus, bonding between the metal and ceramic.5 While the dental industry practices the use of regulatory and ISO standards, informal but informative bench tests are commonly used to imitate restorations in the oral environment. For example, a metal-to-ceramic bond can be evaluated by a qualitative test known as the “screwdriver test.” A slice is made through the porcelain of a crown so as not to damage the metal coping. A screwdriver is then placed in the groove and turned to remove the porcelain from the crown. The surface of the exposed metal is then examined for residual porcelain. The absence of residual porcelain signals the potential for delamination failures. If there is evidence of porcelain residue attached to the metal coping, it is unlikely that any delamination problems will occur in the mouth. The screwdriver test was performed on the gold–palladium alloy. Figure 16 shows residual porcelain on the metal surface, indicating a strong metal-ceramic bond. IMPACT OF PM ON PRECIOUS METAL DENTAL APPLICATIONS Advantages of 3DP over Traditional Methods The advantages of direct metal 3DP over traditional dental-laboratory methods include improved clinical value and performance of dental restorations, mass customization, and reductions in product and labor costs. As demand for dental restorations is likely to increase over the next sev-

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eral years, in spite of the global economic downturn, these advantages will become even more apparent. In the U.S., an aging population with access to quality healthcare will provide older individuals the opportunity to receive many more restorative dental products than in previous generations. In addition, the segment of the population turning to cosmetic dentistry should continue to grow. To meet this demand, dental laboratories are turning to CAD/CAM processes to improve manufacturing capabilities, reduce delivery times, and cut costs. Clinical Benefit Today, most restorations are manufactured from impressions taken by a dentist during a patient’s first or (all too often) second visit. The impression material is a semi-liquid molding material that is placed in a patient’s mouth and allowed to harden for a period of several minutes. This “goo” becomes the mold from which the die is formed. This time-intensive, and somewhatuncomfortable, procedure is often inaccurate, leading to dental restorations that do not fit properly. Clinically, a poorly fitted crown or bridge may degrade a patient’s dental health through the premature failure of the crown or bridge, or increase the risk of tooth and gum decay. As digital impression-taking technologies such as intraoral dental scanners become widely accepted, more and more restorations will be manufactured directly from digital data. In contrast to conventional impression-taking techniques, digital impression taking is hygenic, fast, and accurate. Perhaps most important, the patient is not subjected to a mouth full of impression material. Laboratory Benefit The concepts of lean manufacturing and mass customization have converged within the dental laboratory, creating a new concept of customized, “mass-artistry.” The PFM can be viewed as the canvas upon which a skilled artisan (dental-laboratory technician) creates a lifelike dental restoration. As each person’s tooth is as unique as a fingerprint, dental technicians go to great lengths to recreate the color, tone, and texture of the restoration. A well-crafted restoration will go unnoticed to even the trained observer. Utilizing PM in conjunction with additive manufacturing

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enables the dental-laboratory technician to spend more time on the “art” instead of on the preparation of the “canvas,” increasing productivity without sacrificing clinical value. Today, dental professionals manufacture crowns and bridges primarily via investment casting and high-speed milling, processes that result in significant waste and inefficiency. The investment casting process, while aided by new CAM rapid-prototyping systems, still requires significant quantities of metal casting ingot. The amount of metal in a finished crown is less than half the weight required to cast it. In contrast, 3DP uses significantly less metal. Further, the 3DP alloy is never subjected to repeated metallurgical stresses. Dental casting alloys usually contain one or more elements of low melting point, such as indium or zinc. These elements are added to lower the melting point of the alloy and to act as an oxygen scavenger during melting.6 Both are important in achieving successful castings; however, each time the alloy melts a percentage of those elements is lost as both elements form oxides. The result is a change in final alloy composition from one casting to another. This inconsistency in composition is avoided in the additive manufacturing process where each coping is manufactured from virgin powder. Perhaps the most obvious and significant advantage of PM as it is used in precious metal dental applications is cost. Dental materials such as gold, platinum, and, now, high-quality ceramics continue to increase in price. For example, the price of gold has doubled since 2005 to a high of >$35/g ($1,000/oz.) in early 2009. Some dental ceramics cost the equivalent of >$40/g ($1,143/oz.) The weight of the average HN coping is ~1.0 g, so before a skilled dental laboratory technician can place the first coat of shading porcelain on the PFM, the cost in that single unit can be as much as twice its weight in gold when casting and preparation are factored in. For the gold–palladium alloy composition in the current study, the per-unit cost of a single PFM decreased nearly 20% compared with the gold–platinum– palladium compositional counterpart (imagenBright™) and other traditional HN alloys. More than 99% of the unused PM is immediately recycled for use, greatly reducing the waste associated with traditional manufacturing processes.

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ADDITIVE MANUFACTURING OF PRECIOUS METAL DENTAL RESTORATIONS

SUMMARY The additive manufacturing technology known as 3D printing is a viable alternative to the lostwax process for producing metal copings for PFM restorations. Two HN 3DP alloys (imagenBright™ and imagenNatural™) were made available commercially in 2006 and 2008, respectively. The latter alloy system is suitable for both single-unit crowns and multi-unit bridges. Currently, 3DP is the only CAD/CAM solution available for the production of precious metal copings. Its advantages over the traditional technique include improved accuracy and compositional consistency, reductions in material waste and labor costs, and increased flexibility in the design of more complicated restorations, such as multi-unit bridges. Other industries using precious PM materials, such as in medical and electronic applications, as well as jewelry, can also benefit from this process. Finally, additive manufacturing provides a medium through which novel alloy systems may be developed to reduce material costs while maintaining the required mechanical, aesthetic, and biocompatible requirements of dental restorations. ACKNOWLEDGEMENTS The authors acknowledge the assistance of the

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following companies in carrying out this study: Albensi Dental Laboratory, Irwin, Pennsylvania, The Argen Corporation, San Diego, California, HJE Company, Inc., Queensbury, New York, and RJ LeeGroup, Inc., Monroeville, Pennsylvania. REFERENCES 1. K.J. Anusavice, Phillips’ Science of Dental Materials, Eleventh Edition, 2003, Elsevier Science, St. Louis, MO. 2. “Dental e-Manufacturing Solutions”, Electro Optical Systems, http://www.eos.info/en/applications/dental. html 3. E. Sachs, P. Williams, D. Brancazio, M. Cima and K. Kremmin, “Three Dimensional Printing: Rapid Tooling and Prototypes Directly From a CAD Model” Proc. Manufacturing International ’90, 1990, vol. 4, pp. 131–136. 4. Argedent 70SF Alloy Specification Sheet, The Argen Corporation, http://www.argen.com. 5. W.P. Naylor, Introduction to Metal-Ceramic Technology, Second Edition, 2009, Quintessence Publishing Co, Inc., Hanover Park, IL. 6. R.G. Craig and J.M. Powers, Restorative Dental Materials, Eleventh Edition, 2002, Mosby, An Affiliate of Elsevier, St. Louis, MO. 7. International Standard ISO 22674:2006(E), Dentistry— Metallic Materials for Fixed and Removable Restorations and Appliances, International Organization for Standardization, Geneva, Switzerland. 8. D. Ott, “Chaos in Casting: An Approach to Shrinkage Porosity”, Gold Bulletin, 1997, vol. 30(1), pp. 13–19. ijpm

Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

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MEETINGS AND CONFERENCES

2009

2010

EURO PM2009 INTERNATIONAL POWDER METALLURGY CONGRESS & EXHIBITION October 12–14 Copenhagen, Denmark www.epma.com/pm2009

INTERNATIONAL CONFERENCE & EXHIBITION ON POWDER METALLURGY PROCESSING OF PARTICULATE MATERIALS AND PRODUCTS & 36TH ANNUAL TECHNICAL MEETING January 28–30 Rajasthan, India www.pmai.in

HIGHMATTECH 2009 INTERNATIONAL CONFERENCE October 19–23 Kiev, Ukraine www.hmt.kiev.ua CERAMITEC 2009 11TH INTERNATIONAL TRADE FAIR FOR MACHINERY, EQUIPMENT, PLAN, PROCESSES AND RAW MATERIALS FOR CERAMICS AND POWDER METALLURGY October 20–23 Munich, Germany www.ceramitec.de 96TH ASM ANNUAL MEETING AT MS&T 2009 October 25–29 Pittsburgh, PA www.matscitech.org PM COMPACTING/TOOLING SEMINAR October 27–28 Cleveland, OH MPIF* SEVENTH INTERNATIONAL LATINAMERICAN CONFERENCE ON POWDER TECHNOLOGY November 8–10 ~o Paolo, Brazil Atibaia, Sa www.metallum.com.br/ptech 2009 2009 CHINA (SHANGHAI) POWDER METALLURGY & ADVANCED CERMAICS EXHIBITION & CONGRESS November 9–10 Shangahi, China www.china-pmexpo.com/en Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

MIM2010: INTERNATIONAL CONFERENCE ON INJECTION MOLDING OF METAL, CERAMICS AND CARBIDES MARCH 29–31 Long Beach, CA MPIF* POWDERMET2010: MPIF/APMI INTERNATIONAL CONFERENCE ON POWDER METALLURGY & PARTICULATE MATERIALS June 27–30 Hollywood (Ft. Lauderdale), FL MPIF* BASIC PM SHORT COURSE July TBA MPIF* PRICM 7 7TH PACIFIC RIM INTERNATIONAL CONFERENCE ON ADVANCED MATERIALS AND PROCESSING August 1–5 Cairns, Australia www.materialsaustralia.com. au/scripts/cgiip.exe/WServ ice=MA/ccms.r?PageID=190 70

ILASS 2010 23RD ANNUAL CONFERENCE ON LIQUID ATOMIZATION AND SPRAY SYSTEMS September 6–8 Brno, Czech Republic www.ilasseurope2010.org 7TH INTERNATIONAL SYMPOSIUM ON ALLOY 718 & DERIVATIVES September 10–13 Pittsburgh, PA www.tms.org PM SINTERING SEMINAR September TBA MPIF* TITANIUM 2010 October 3–5 Orlando, FL www.titanium.org PM2010 WORLD CONGRESS October 10–14 Florence, Italy

2011 POWDERMET2011: MPIF/APMI INTERNATIONAL CONFERENCE ON POWDER METALLURGY & PARTICULATE MATERIALS June 19–22 Chicago, IL MPIF*

*Metal Powder Industries Federation 105 College Road East, Princeton, New Jersey 08540-6692 USA (609) 452-7700 Fax (609) 987-8523 Visit www.mpif.org for updates and registration. Dates and locations may change

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Advances in Powder owder Metallurgy & Particulate articulate Materials—2009 Proceedings of the 2009 International Conference on Powder Metallurgy & Particulate Materials

ISBN: 978-0-9819496-1-1

Now available on a fully searchable CD-ROM in a format that preserves the original color of all figures—contains 110 technical papers encompassing over 1,200 pages ALSO AVAILABLE in a complete set of two printed softcover volumes (limited quantities)

CONTENTS: Part 1—Design & Modeling of PM Materials, Components & Processes Part 2—Particulate Production Part 3—Compaction & Forming Processes Part 4—Powder Injection Molding (Metals & Ceramics) Part 5—Pre-Sintering & Sintering Part 6—Secondary Operations Part 7—Materials Part 8—Refractory Metals, Carbides & Ceramics Part 9—Advanced Particulate Materials & Processes Part 10—Material Properties Part 11—Test & Evaluation Part 12—Applications Part 13—Management Issues (For a complete listing of all paper titles, visit the Publication section of our Web site www.mpif.org)

2009 Advances in PM on CD-ROM 2009 Advances in PM Complete Softcover Set Previous Proceedings Still Available at Great Discounts: 2008 Advances in PM on CD-ROM 2008 Advances in PM Complete Softcover Set 2007 Advances in PM on CD-ROM 2006 Advances in PM on CD-ROM 2005 Advances in PM on CD-ROM 2004 Advances in PM on CD-ROM 2003 Advances in PM on CD-ROM 2002 Advances in PM on CD-ROM 2001 Advances in PM on CD-ROM 2000 Advances in PM on CD-ROM Special Offer: 2000–2008 Advances in PM on 9 CD-ROMs Price Total from above _________ Shipping* _________ Handling 5.00 Total Order _________

List $ 925 1,000

APMI $ 850 925 List 1,100 1,250 900 850 800 750 750 1,000 675 600 2,700

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Please invoice (P.O. must be attached) Charge to my credit card Visa Mastercard American Express Card number_______________________

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Name on card______________________ Expir. date________ Security code_____ Signature__________________________ Mail To: Metal Powder Industries Federation 105 College Road East Princeton, NJ 08540-6692 USA Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

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ADVERTISERS’ INDEX

ADVERTISER

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ACE IRON & METAL CO. INC. ________(269) 342-0185 ______________________________________________________5 ACUPOWDER INTERNATIONAL, LLC ___(908) 851-4597 ________www.acupowder.com ___________________________36 CENTORR VACUUM INDUSTRIES______(603) 595-9220 ________www.centorr.com _______________________________8 ELNIK SYSTEMS ____________________(973) 239-6066 _________www.elnik.com __________________________________26 GLOBAL TITANIUM _________________(313) 366-5305 ________www.globaltitanium.com ________________________42 HOEGANAES CORPORATION _________(856) 786-2574 ________www.hoeganaes.com ___________INSIDE FRONT COVER NORTH AMERICAN HÖGANÄS INC.______(814) 479-2003 _________www.nah.com ____________________________________3 PIM INTERNATIONAL _______________+44 (0)1743 369660 ____www.pim-international.com______________________28 QMP ____________________________(734) 953-0082 ________www.qmp-powders.com ________________BACK COVER SCM METAL PRODUCTS, INC.________(919) 544-7996 ________www.scmmetals.com ____________INSIDE BACK COVER UNION PROCESS __________________(330) 929-3034 ________www.unionprocess.com _________________________20

ADVERTISER’S REQUEST FOR INFORMATION FAX FORM Need more information on products or services seen in this issue?

Complete the form below and fax to the advertiser(s) of your choice. Fax numbers are listed in the advertisers’ index above.

To:___________________________________ Fax #: ______________________________________ Company: _________________________________________________________________________ Please send me more information on:_____________________________________________________ _________________________________________________________________________________ as advertised in the __________ issue of the International Journal of Powder Metallurgy. Please send information to: Name: Title: ________________________________________________________________________ Company: _________________________________________________________________________ Address:___________________________________________________________________________ City:____________________________ State:_______________ Postal Code: ___________________ Country:___________________________________________________________________________ Phone:___________________ Fax:___________________ E-Mail: ___________________________

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Volume 45, Issue 5, 2009 International Journal of Powder Metallurgy

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