SPECIAL REPORT
High-tech
ceramics
Howard J. Sanders, C&EN Washington
Eric A. Barringer, assistant professor of ceramics pro cessing at Massachusetts Institute of Technology, puts it this way: "Any large U.S. chemical company that is not now active in the high-tech ceramics field or is not planning to enter the field in the near future must ob viously be daft." His statement may be hyperbole, but it clearly reflects a growing enthusiasm for ceramics as engineering or structural materials. Various observers talk fervently of materials science entering a Ceramics Age. Many sci entists see vastly increased use of ceramics as replace ments for high-performance metals and plastics. Louis
Worldwide sales of high-tech ceramics expected to exceed $12 billion in 1990 Estimated sales, $ billions 25 I
I Hu.s. 20
Γ Β Japan • Others 15 L- Π Total
10 L
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._
-
K ^ 1980
«
^ 1990
Note: Sales do not include those in communist countries. Source: H. Kent Bowen and George B. Kenney of MIT
26
July 9, 1984 C&EN
L·-^^ 1995
E. Toth, program director for ceramics and electronic materials at the National Science Foundation, describes high-tech ceramics as "an exciting, challenging field that is obviously a wide-open frontier/' High-technology ceramics—also known as highperformance ceramics, engineering ceramics, technical ceramics, advanced ceramics, and structural ceram ics—are definitely subjects of growing research and development attention. They have excellent mechanical properties under heavy stress, outstanding electrical and optical properties, and exceptional resistance to high temperatures and corrosive environments. Such mate rials do not include such conventional ceramics as pot tery, dinnerware, cement, building bricks, roof tiles, and window glass. Because high-tech ceramics have high heat resistance, they are used in automotive engines, burner nozzles, and heat exchangers. Their special electrical properties are useful in capacitors, piezoelectric devices, thermistors, solar cells, and integrated-circuit substrates. Their optical properties make them of value as infrared transmission windows, as well as in lasers and high-pressure sodium vapor lamps. Because of their hardness and wear resis tance, they find use in cutting tools and bearings. And because some are biocompatible, they are used as sub stitutes for bones and joints and in artificial heart valves. Much ceramics research during the past 10 or 15 years has focused on what happens to ceramics at the atomic, molecular, or crystal level during actual use. Previously, scientists might have been content merely to determine the point at which a given ceramic fractures when subjected to increasing mechanical stress. Now, they also are interested in determining what basic processes occur when a ceramic breaks.
Gas turbine engines containing parts made of high-tech ceramics operate at much higher temperatures than those with only metal parts. Gary Boyd (left) and Martin Zytkowski of Garrett Corp. check the position of such an engine's silicon nitride turbine shroud. Surrounding the shroud is white aluminum silicate insulation used to minimize heat loss to the engine case In the past, ceramics generally were formulated and fabricated on a hit-or-miss basis. Now, however, the formulating and fabricating of ceramics are based much more on scientific knowledge and theory. One ceramist comments, "In years past, people added a scoop of this or that oxide to a ceramic formulation because somehow it improved the material's properties. Nowadays, we have a far better understanding of exactly how the additive functions. " Another trend is the increasing use of advanced scientific instruments and methods to study the composition and properties of ceramics. In addition, more and more scientists are exploring these materials through the use of mathematical models of ceramic crystals, defect structures, and the mechanical behavior of ceramics—an effort that involves the extensive use of computers. The broad term "ceramic" generally is defined as "an inorganic, nonmetallic material processed or consolidated at high temperatures." Some scientists argue that ceramics are crystalline materials and, therefore, do not include glass. Most ceramists, however, regard glass as a ceramic. Ceramics encompass a wide range of compounds, including silicates, oxides, carbides, nitrides, sulfides, and borides. Although most ceramics contain metal ions,
some do not. Among these are silicon carbide and silicon nitride. For thousands of years, ceramics have been used to make pots, dishes, tiles, and bricks. Artisans in what is now Turkey transformed clay into ceramics some 8500 years ago. For centuries, however, these materials were rejected for engineering use where mechanical properties are critical, because they are too brittle. Indeed, brittleness continues to be the greatest single shortcoming of most ceramics in such applications. For this reason, metals have been the materials of choice. Metals have high strength, great cracking resistance, malleability, resistance to elevated temperatures (within limits), and the ability to produce parts with reproducible properties. Exactly when high-tech ceramics first entered the industrial scene is difficult to pinpoint. One of the earliest such products was silicon carbide, introduced by Carborundum Co. in the 1890s. A major impetus for research on such materials came during World War II, as scientists realized that highperformance ceramics—because of their outstanding electrical properties, heat resistance, and other characteristics—could play an important role in equipment for the war effort. July 9, 1984 C&EN
27
Special Report
Companies and universities are placing increasing emphasis on ceramics High-tech ceramics are getting heightened attention from industry and academia. The U.S. chemical industry, for example, is becoming more active in this field. Items: • In April 1982, Du Pont acquired Solid State Dielectrics of Sun Valley, Calif., a leading producer of dielectric ceramic powders used by other companies to make multilayer ceramic capacitors. • In late 1982, Allied Corp. started an ambitious research program to develop high-performance ceramics, an area in which it had previously done relatively little research. Its current program is focused on such materials as aluminum oxide, barium titanate, yttrium oxide-stabilized zirconium oxide, and, to a lesser extent, carbides and nitrides. George S. Hammond, director of Allied's integrated chemical systems laboratory, points out that his company had considered launching a sizable research program on high-tech ceramics several years earlier. The final go-ahead was triggered, to a large extent, by the company's acquisition in late 1982 of Bendix Corp., which has a lively interest in automotive, aerospace, electronic, and other uses for ceramics. "This acquisition," says Hammond, "provided greatly increased justification for our substantial move into R&D on high-tech ceramics." • Dow Corning in the past two years has boosted its R&D activities on highperformance ceramics. A major impetus has been an approximately $20 million Department of Defense contract to develop high-temperature ceramic fibers and composites.
• In April 1983, W. R. Grace & Co. purchased Diamonite Products of Shreve, Ohio, which specializes in aluminum oxide ceramics used mainly in electronic devices. In February 1984, it acquired Metaramics of Sunnyvale, Calif., which makes ceramic substrates for semiconductors. Jack Rimmer, recently retired head of Grace's industrial chemicals group, says, "The objective of these acquisitions has been to establish Grace as a major participant in the rapidly growing high-tech ceramics industry." • In the past few years, Standard Oil (Ohio) also has become active in the high-tech ceramics field. In 1981, it acquired Carborundum Co. through its acquisition of Kennecott Copper, which had bought Carborundum in 1978. Other U.S. chemical firms increasingly interested in high-tech ceramics include Union Carbide, Dow Chemical, ICI Americas, Cabot, Celanese, Stauffer, American Cyanamid, PPG Industries, and Air Products & Chemicals. U.S. universities, such as MIT, Penn State, Rutgers, Case Western Reserve, and the University of California, Berkeley, likewise are showing greater interest in ceramics research. For example, Rutgers, which has had a department of ceramics since 1902, augmented this department in 1982 by establishing a center for ceramics research, directed by John B. Wachtman Jr. This center is supported partially by a five-year grant from the National Science Foundation as part of a project to promote cooperative research between industry and universities. In the past two years, the Rutgers center has obtained $30,000-a-year
In the postwar years, major stimulus for the creation of new high-tech ceramics was the development of computers and other electronic equipment that could use these advanced materials in capacitors, electronic substrates, thermistors, varistors, piezoelectric devices, and other components. In the past 10 years, one of the greatest impetuses to the development of high-tech ceramics has been interest in developing more efficient automotive engines. Among those under study are gas turbine engines that, because they contain major components made of hightech ceramics, can operate at unusually high temperatures. Some high-tech ceramics can withstand temperatures as high as 1400 °C, whereas even the best superalloys can seldom be used above about 1100 °C. Also, 28
July 9, 1984 C&EN
contributions from each of 18 companies and two government laboratories. Among the corporate sponsors are Alcoa, Allied, Carborundum, Celanese, Corning, Ferro, GTE, Martin Marietta, Lockheed, IBM, Norton, W. R. Grace, Johnson & Johnson, and 3M. Although the amount of ceramics research in U.S. universities is growing, many observers believe that it is still inadequate. Louis E. Toth of the National Science Foundation points out that only about 15 universities in the U.S. have ceramics departments, although a number of others have ceramics professors in other departments, such as materials science. "This number of schools," he declares, "is not sufficient to tackle the large number of major ceramic problems that should be dealt with by universities." He also points out that, if more universities had strong departments of ceramics, more students would be attracted to the field. A major problem today is not so much a shortage of funds for ceramics research but an inadequate supply of people being trained to carry out ceramics research. Rustum Roy, director of the materials research laboratory at Penn State, notes, "If a company wanted to start a large laboratory to do research on ceramics these days, it would have real difficulty staffing it." He believes the current shortage of ceramic scientists could be alleviated if chemistry departments placed greater emphasis on training in inorganic chemistry and if they encouraged some of their chemistry students with leanings toward the materials field to take ceram-
because of this temperature resistance, ceramics may obviate the need for cooling equipment, especially in diesel engines. Apart from their ability to withstand elevated temperatures, high-tech ceramics have many other advantages over metals: • Some are exceptionally hard. The hardest substances known, such as diamond, cubic boron nitride, boron carbide, and silicon carbide, are ceramics. • Many are highly resistant to oxidation and other chemical attack, as well as to erosion. • They are usually lighter than metals, sometimes weighing only about 40% as much. This is important in aircraft, missile, and spacecraft applications, where reduced weight conserves fuel. In a gas turbine engine, a
Penn State leads in papers on ceramics
QE Uofl«nois
ORNL AHradU Rutgers U
100
200
__L _._ 300
400
500
600
700
800
Number of papers* a Denotes number of papers presented at annual meetings of the American Ceramic Society, 1953-83. Source: Rustum Roy, Pennsylvania State University
ics courses. Most ceramists in Japan and the Soviet Union, he says, were trained as inorganic chemists. Recently, Roy carried out a survey to determine, in part, which U.S. organizations have contributed the most to ceramics research by ranking those that have been the greatest sources of papers presented during the past 30 years at the annual meetings of the American Ceramic Society, the pre-eminent U.S. scientific society in the ceramics field.
among the top 10 performers of U.S. ceramics research was Battelle. One country that is vigorously involved in research on high-tech ceramics, as well as in their production, is Japan. A study by H. Kent Bowen and George B. Kenney of MIT indicates that Japan's sales of hightech ceramics in 1980 totaled about $1.9 billion, compared to $1.6 billion for the U.S. By 1990, they estimate, sales will be about $6.5 billion for Japan and $4.5 billion for the U.S.
The universities that originated the most papers were Penn State, MIT, the University of Illinois, Alfred University, and Rutgers. The leading corporate organizations were General Electric and Bell Laboratories. The foremost government laboratories were the National Bureau of Standards and Oak Ridge National Laboratory. The only not-for-profit organization
Japan surpasses the U.S. in its sales of such high-tech ceramic products as ferrites (magnetic compounds), piezoelectric materials, thermistors, and substrates for integrated electronic circuits. However, the U.S. exceeds Japan in sales of hightech ceramics for cutting tools, capacitors, and gas and humidity sensors. The Japanese government has desig-
lightweight ceramic rotor accelerates more rapidly than a heavier metallic rotor because it has less inertia. • Ceramics generally are made from abundant raw materials. Among the most common elements used in ceramics are silicon, which accounts for about 27% of Earth's crust, and aluminum, which constitutes about 8%. • They do not require the increasingly expensive strategic metals needed to make superalloys for hightemperature use. These alloys usually contain such metals as cobalt, chromium, nickel, and tungsten, which the U.S. obtains predominantly from abroad. • High-tech ceramics are potentially less expensive than superalloys, although not nearly so inexpensive as they might appear to be. The silica and aluminum oxide,
nated advanced ceramics as a high-priority field for R&D. In October 1981, Japan's Ministry of International Trade & Industry initiated a 10-year, $120 million R&D program with industry and government laboratories to develop improved high-tech ceramics and find new uses for them. Under this program, $60 million will be allocated to a group of 15 participating companies. Many American scientists say that one of the greatest spurs to U.S. research on high-tech ceramics today is a keen desire to keep up with or surpass recent advances made by Japan. Various ceramists have urged that the U.S. government, like that of Japan, make ceramics a high-priority field for government-sponsored research. Meanwhile, many European countries are expanding their R&D on high-tech ceramics. In West Germany, Rosenthal Glas & Porzellan, Feldmuehle, and other firms are stepping up their research to develop ceramics with greater fracture toughness and wear resistance. Such German automobile companies as Volkswagenwerk and Daimler-Benz are putting special effort into developing ceramics for gas turbine and diesel engines. Last October, the West German government announced plans for a major new R&D effort on advanced materials, including ceramics. Rolls-Royce in Britain and Volvo and Saab in Sweden, as well as other British and Scandinavian companies, also are working to develop high-tech ceramic parts for automobile engines. In addition, companies in France, Italy, and other European countries are doing R&D on advanced ceramics for automobile engines and other uses.
for example, used in making advanced ceramics is not the inexpensive technical-grade material. These raw materials must undergo a succession of purification and costly processing steps before they can be used to produce a high-performance ceramic. In addition, some high-tech ceramics are costly because they are currently made in relatively small quantities. In the years ahead, however, advanced ceramics are expected to be much cheaper than superalloys. • Because of their low coefficient of friction, high compressive strength, and wear resistance, some ceramics can be used in bearings and other mechanical parts without requiring lubrication. Despite their many virtues, most ceramics have had their use greatly restricted in structural applications July 9, 1984 C&EN
29
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Special Report
Uniform 0.3-μ m particles of titanium dioxide powder (above) were made by MIT's solution-precipitation process. Particles made by conventional method ( right) range in size from 0.2 to 1.0 μ m and have large voids because of their most glaring defect— brittleness, leading to severe failure. As one ceramist puts it: "When you drop a metal cup, it may develop a minor dent. When you drop a typical ceramic cup, it usually shatters." Roy W. Rice, head of the ceramics branch of the Naval Research Laboratory, says, "Traditional ceramics fail catastrophically instead of retaining some load-carrying capability after reaching their maximum sustainable load. In contrast, the ductility of metals often allows survival of a component that is temporarily over loaded." Ceramics also can have significant shortcomings be sides brittleness. For example, it is difficult to make ad vanced ceramic parts with uniform physical properties from one batch to the next—or even within a single batch. As a result, ceramic parts often fail to meet speci fications. H. Kent Bowen, director of MIT's materials processing center, says, "The most costly aspect of making high-tech ceramics is the expense of nonreproducibility in manufacturing." Another shortcoming is that the properties of ceramics are more sensitive to imperfections than are the prop erties of metals. Pores at grain boundaries or impurity inclusions, for example, may be acceptable in a metal but disastrous in a ceramic. Also, ceramics are often difficult to make in large, complex shapes. Production of the final shape may re quire expensive grinding. The machining of some ce ramics is difficult, time-consuming, and costly because of their great hardness. Joining ceramic parts to one another and to metals also may be difficult. And they can fail to stay attached at high temperatures. In addition, uniform submicron powders required to produce many high-tech ceramics are expensive to make, and other steps in the production of finished ceramics also may be costly. Still another problem is that during production, high-tech ceramics may develop random, essentially
undetectable microcracks, voids, or other flaws that, by acting as stress concentrators, ultimately may cause a part to fail. In addition, many engineers have gained far more experience and familiarity with metals than with ce ramics. Also, switching to a ceramic component in an engine, for example, usually requires substantial design changes. In recent years, one of the biggest thrusts in ceramics research has been to develop materials with greatly improved mechanical reliability, usually by improving their fracture toughness. One approach has been to re duce the size and concentration of pre-existing flaws in ceramics—for example, through the use of pure, uni form, submicron spherical particles. Such particles can be densely packed in an orderly manner. When sintered, they produce a ceramic with very little void space and a minimum of tiny cracks that may form much larger cracks when the ceramic is later subjected to stress. Random impurities that promote cracking or electrical failure of ceramic materials must be removed. Of course, many ceramics contain deliberate additives to improve their strength and other properties.
Making fine powders The conventional way to make fine powders is to grind a bulk material and pass it through a fine sieve. A problem with this approach is that, although the pow ders may be relatively fine, they may not be uniform in size. Furthermore, the very act of grinding the material may introduce metallic or ceramic impurities. One method for making pure, superfine, uniformsized powders is the sol-gel process. As far back as 1941, Ferenc Ko rosy in Hungary used this method to produce glasses. Rustum Roy and colleagues at Pennsylvania State University pioneered the use of this technique to make finely divided powders and exceptionally homo geneous glasses of silica and the oxides of aluminum, July 9, 1984 C&EN
31
Special Report magnesium, titanium, zirconium, germanium, and other metals. But it was not until researchers at Oak Ridge National Laboratory in the mid-1960s found that the method could be used to make uranium dioxide pellets for nuclear reactors that the process gained its first large-scale industrial use. The sol-gel method involves the conversion of a sol (a fluid colloidal suspension of a solid in a liquid) to a gel (a semirigid colloidal dispersion of a solid in a liquid). In one typical procedure, a metal is reacted with an al cohol to form a metal alkoxide. The metal alkoxide then is dissolved in an appropriate alcohol. Next, water is added to cause the alkoxide to hydrolyze. After the pH of the solution is adjusted, the material polymerizes to form a gel. The gel consists of loosely bonded material and a liquid (water and alcohol). This material, which has roughly the consistency of Jell-O, is heated carefully at 200 to 500 °C to remove the liquid. This converts the gel to finely divided metal oxide powder, with particle sizes in the range of 0.003 to 0.1 μιη. This method can produce extremely homogeneous mixtures of two or more com ponents because the mixing of the ingredients takes place at the atomic level in a liquid rather than in the solid state. An alternate process for making fine, spherical metal oxide powders was developed in 1981 by MIT's Barringer, Bowen, and coworkers. Like the sol-gel process, it starts with the conversion of a metal to a metal alkox ide. However, a gel is not formed because the solutions used are more dilute and their pH is adjusted to prevent gelling. The MIT solution-precipitation process for preparing finely divided titanium dioxide, for example, starts with the preparation of titanium tetraethoxide (Ti[OC2H5]4), which is then dissolved in ethanol. Next, water is added to form amorphous, hydrated titanium dioxide, which is treated ultrasonically to produce a colloidal dispersion. Then, the material is centrifuged, and much of the excess liquid is decanted. Finally, the precipitate is heated to yield a titanium dioxide powder of essentially uniform submicron particles. This method has produced titanium dioxide powder with spherical particles having diameters of about 0.3 to 0.6 μιη that has been used to produce ceramics with more than 99% of the theoretical maximum density and an average grain size of about 0.5 μηι. The process also has been used to produce superpure powders of the ox ides of silicon, zinc, zirconium, and other metals. This method, Barringer emphasizes, can be used not only to produce dry powders but also thick slurries of finely divided oxides. These slurries, containing 30 to 50% by volume of solids, can be cast by a variety of methods. In the slip-casting method, a substantial amount of the fluid is absorbed into the capillaries of the porous mold, with the result that the solids content of the slurry can rise to 60 to 70% by volume. The cast shape is then dried at 100 to 200 °C and sintered at about 1000 °C to form a dense body. Barringer believes his method is superior to the sol-gel procedure because, when the cast material is dried, less shrinkage occurs than with a gel, which may contain 32
July 9, 1984 C&EN
only 30 to 40% by volume of solids. Also, the material is easier to handle than gels and can be formed into r m r e complex shapes. He acknowledges, however, that the sol-gel process has great potential for making thin films, coatings, and fibers. In recent years, various scientists have explored ways to make superfine powders in the vapor phase rather than the liquid phase. They have vaporized materials by using laser beams, plasma arcs, and other techniques. For instance, John S. Haggerty and associates at MIT have used a carbon dioxide laser as a heat source to convert gases to high-purity powders of such materials as elemental silicon, silicon nitride, and silicon carbide. They have formed silicon nitride by reacting silane (SiH4) with ammonia at 1700 °C and produced spherical silicon carbide particles less than 0.1 μπι in diameter by treating silane with ethylene. The particles have extremely small diameters because the reactants are exposed to high temperatures only very briefly, so that the solid nuclei have little time to grow. In addition, the powders are essentially free of agglom erates. Studies are in progress at MIT to determine whether laser beams also can be used to make submicron powders of aluminum oxide, titanium dioxide, boron carbide, and other compounds.
Lisa C. Klein of Rutgers (upper left) examines glass made by sol-gel process. Rutgers' Stephen Dan forth (lower left) uses laser to produce ceramic powder. PPG's Roger A. Steiger (above) looks at Sintrium titanium diboride cone PPG Industries has developed a plasma arc process tor making high-purity titanium diboride powder tor pro ducing ceramics. Hydrogen gas at atmospheric pressure is passed through a high-voltage electric arc and thus ionized at about 2500 ° C The resulting hydrogen plasma is directed into a chamber where it quickly heats the gaseous reactants, titanium tetrachloride and boron trichloride, to form titanium diboride, along with hy drogen chloride. As the products cool rapidly, the titanium diboride crystallizes out in submicron particles. This powder can be converted to hard, electrically conductive ceramics for potential use in electronic components, military armor, automotive engine parts, and electrodes for cells for making aluminum or chlorine and caustic soda. PPG's Sintrium process also has been used in the laboratory to produce silicon carbide, titanium carbide, tungsten carbide, and other powders. The company, which at present is not using its process commercially, is offering it for licensing. Once a ceramic powder is formed, the finely divided material, mixed possibly with other powders, is com pacted and sintered at a high temperature (about 1650 °C for aluminum oxide, 1700 °C for zirconium oxide, or 2050 °C for silicon carbide). During sintering, the ce ramic particles coalesce without actually melting. The most common method of compacting powders uses static pressures. Still largely in the research stage is the use of explosives to compact ceramic powders. \or some years Du Pont has used the explosive technique to produce small, synthetic industrial diamonds (General Electric, on the other hand, uses static pressures for the same purpose).
One of the oldest methods used to increase fracture toughness is to combine a ceramic with another material. Λ classic example is the combination of cement with sand or gravel to form concrete. Such a combination is known as a composite. It con sists of a bulk material (the matrix) that has been com bined with a toughening agent. Starting in the late 1940s, scientific interest centered on increasing the fracture toughness of ceramics by combining them with metals to form so-called cermets, which are basically ceramic particles bonded together by metal films (typically constituting 5 to 15% by volume oi the material). The hope was that the resulting material would have both the high-temperature resistance of ceramics and the high-fracture resistance or flexural strength of metals. Unfortunately, in many early cases, cermets often possessed the brittleness of ceramics and the comparatively low-temperature resistance of metals. However, scientists have developed some highly useful cermets, most notably cobalt-bonded tungsten carbide, which is widely used in drilling and machining tools. In recent years, substantial interest has developed in metal-matrix composites, in which ceramic particles or fibers are bonded together by larger amounts (30 to 70%) of metal. Examples include fibers of silicon carbide and aluminum oxide, both used to reinforce aluminum or titanium. Research has been done on metal fibers to toughen ceramics. Some years ago, scientists at United Aircraft (now United Technologies) attempted to toughen silicon nitride with filaments of such metals as niobium, mo lybdenum, and tungsten. Other researchers investigated the toughening of aluminum oxide or titanium carbide with fibers of nickel or chromium. By and large, these efforts were disappointing. Ex posed to high temperatures during processing or sub sequent use, the metal oxidized and lost strength. Cracking of ceramics can be inhibited by incorporating within them a high percentage (30 to 60% by vol ume) of a ceramic fiber, rather than a metal fiber. Although the ceramic fiber may be made of the same material as the matrix, it is more commonly a different and stronger material. The fibers used may be continu ous filament or cut fibers (staple) that usually measure from about 0.5 to 1.5 cm in length or are even as short as 0.1 cm. Ceramic fibers improve fracture toughness mainly by preventing tiny cracks from growing into larger ones that may cause a stressed ceramic to shatter. To be effective, the fibers should have length-to-di ameter ratios of more than about 50 and should be dis tributed uniformly throughout the matrix. Moreover, no chemical interaction should occur between the matrix and the fibers that would cause the physical properties of the fibers to deteriorate. In the early seventies, researchers at the Atomic En ergy Research Establishment in Harwell, England, re ported that incorporation of fine graphite fibers into silicate-based glass increased its fracture toughness. In 1979, Karl M. Prewo and coworkers at United Tech nologies' research center found that borosilicate glass reinforced with graphite fibers has "significant perforJuly 9, 1984C&EN
33
Special Report mance advantages" over other composites, such as alu minum metal or epoxy resin reinforced with graphite fibers. Such reinforced glass has good flexural strength at elevated temperatures and good resistance to me chanical fatigue. Prewo, John J. Brennan, and associates at United Technologies also studied borosilicate glass reinforced with about 50% by volume of silicon carbide fibers, which are among the strongest fibers known. Unlike borosilicate glass reinforced with graphite fibers, this newer material is less affected by exposure to air at ele vated temperatures. The United Technologies scientists two years ago developed a lithium aluminosilicate glass-ceramic re inforced with about 50% by volume of silicon carbide fibers. The composite, they say, has excellent mechanical properties, such as fracture toughness, at temperatures as high as 1000 °C. In contrast, borosilicate glass rein forced with silicon carbide fibers has high fracture toughness only up to about 600 °C. The new material uses a glass-ceramic (a glass that, by use of special nu cleating agents and heat treatment, has been largely crystallized) rather than conventional noncrystalline glass because of its greater heat resistance. Prewo and Brennan believe the new lightweight composite might be used, for example, as a replacement for superalloys in gas turbine engines. Meantime, William B. Hillig and associates at General Electric Research & Development Center developed composites of elemental silicon reinforced with silicon carbide "fibers" made by exposing carbon fibers to molten silicon. The silicon reacts with the carbon to form, in a matrix of silicon, a linear array of silicon car bide crystals that retains the shape of the original carbon fibers. The composite, which may contain 20 to 80% of ele mental silicon, has been used experimentally as a liner for a combustion chamber operated at 1350 °C. To date, GE has employed this material, called Silcomp, only for internal company use in high-temperature applications, typically in the range of 1100 to 1200 °C. Many scientists foresee further advances in the de velopment of composites made of ceramics reinforced with ceramic fibers stemming from more sophisticated processing and the development of an array of fibers.
Production of ceramic fibers In recent years, methods have been developed to produce a variety of ceramic fibers. For instance, in Japan Seishi Yajima and coworkers at Tohoku University an nounced in 1975 their development of silicon carbide fibers having high tensile strength and good oxidation resistance. They are made by first dechlorinating dimethyldichlorosilane ([CIHbkSiC^) and polymerizing the product to form polydimethylsilane ([—Si(CH3)2 —] n ). This material is then heated to 400 to 500 °C to convert its backbone of silicon atoms to a backbone of alternating silicon and carbon atoms by forming polycarbosilane ([—HS1CH3—CH2—]n)· This polymer, having an average molecular weight of about 1500, is then melt-spun to form polycarbosilane fibers. These fibers are cured in air at 200 °C to become 34
July 9, 1984 C&EN
Extremely tough lithium aluminosilicate glass-ceramic reinforced with silicon carbide fibers, developed by Karl Af. Prewo and coworkers at United Technologies, cannot be penetrated by nail at right Other nail can pierce a composite of graphite reinforced with graphite fibers infusible as a result of a crosslinking reaction. The cured fibers then are heated slowly to about 1250 °C in an at mosphere of nitrogen to drive off all of the hydrogen and some of the carbon atoms (in the form of methane), leaving behind the desired skeleton consisting of al ternating silicon and carbon atoms. The fibers, which range in diameter from 10 to 15 μιτι, retain their physical properties at temperatures above 1000 ° C For more than five years, Nippon Carbon Co. of Japan has been using this process commercially to make its silicon carbide fibers called Nicalon. Since 1982, Dow Corning has been importing and marketing Nicalon fibers in North America. In the future, Dow Corning expects to produce similar fibers domestically. Avco Corp/s specialty materials division also makes a silicon carbide fiber that it has been offering com mercially since 1980 in the form of composites in either aluminum or titanium matrices. The fiber is produced by first forming a carbon filament about 35 μπ\ in di ameter by spinning and then pyrolyzing a modified form of pitch. In a separate step, a mixture of chlorosilanes is reacted with propane at high temperature to form silicon carbide, which is vapor-deposited on the carbon filament. The resulting fiber has a diameter of about 140 Mm. Another ceramic fiber, boron nitride, was developed in the mid-1970s by Isoji Taniguchi and coworkers at Sumitomo Chemical Co. in Japan. They obtained a pat ent for making pure boron nitride fibers by heating a polymer based on triaminotriphenylborazole, but these fibers are still under development. Meanwhile, in the U.K., Imperial Chemical Industries sells commercial quantities of aluminum oxide fibers under the tradename Saffil. The fibers, 1 to 5 μτη in di ameter, are used for insulating furnaces and reinforcing metals such as aluminum.
Du Pont also has marketed experimental quantities of an aluminum oxide fiber, called Fiber FP, for the past four years. Developed by Du Pont research associate Ludwig E. Seufert and coworkers in 1974, the fiber contains more than 99% aluminum oxide and has a di ameter of about 20 μιη. Its properties, the company says, remain unchanged at temperatures as high as 1000 °C. The fiber is used to reinforce such materials as alumi num, magnesium, polyimide resin, epoxy resin, and fused silica. Composites of resin or glass and Fiber FP are being tested for use as radar-transparent materials, and composites of magnesium and Fiber FP are under study for transmission housings of helicopters. At the Tokyo Motor Show in October 1983, Toyota Motor Corp. exhibited its FX-1 sports car containing aluminum connecting rods reinforced with Fiber FP. These connecting rods are about 35% lighter than ones made of steel. Du Pont points out that aluminum rein forced with 35 to 50% by volume of Fiber FP has up to four times the fatigue strength of pure aluminum con necting rods and also greater heat resistance. An aluminum borosilicate fiber that has greater heat resistance than ceramic fibers made of glass or fused silica is marketed by 3M. Developed by 3M ceramist Harold G. Sowman and coworkers, it is known as Nextel 312 and has good strength at high temperatures. Nextel 312 fibers, measuring 8 to 12 μηι in diameter,
Making silicon carbide fiber involves melt-spinning, heating of polycarbosilane CH 3
CI Dimethyldichlorosilane
Si
/
\
CH 3
CI Dechlorination and polymerization
Polydimethylsilane CH,/n Thermal decomposition and polymerization at 400 to 500 °C ,CH 3 H ^ Polycarbosilane ^H
H/n Melt-spinning
Spun fiber Curing in air at 200 °C Infusible fiber
r
Heat treatment in an atmosphere of nitrogen at 1250 °C
Silicon carbide (SiC) fiber Source: Nippon Carbon Co.
can be woven into heat-resistant fabrics that withstand temperatures up to 1200 °C and can be used as furnace curtains, furnace conveyor belts, seals, and reinforce ments in composites. Still under development at 3M is a ceramic fiber called Nextel 440 made of aluminum silicate. It has better reinforcing properties than Nextel 312 and can with stand higher temperatures.
Manufacture of whiskers Of growing interest in the ceramics field are ceramic whiskers. These materials are appreciably thinner than fibers. A silicon carbide whisker usually has a diameter of about 0.5 μιη, whereas a fiber of the same material ordinarily has a diameter of 10 μιη or more. Ceramic whiskers also are shorter. They may range in length from 10 to 80 μτη, whereas the length of some fibers may be measured in centimeters. In addition, whiskers are single-crystal materials; ceramic fibers generally are polycrystalline. Compared to fibers, whiskers produce composites with greater fracture toughness, stiffness, and tensile strength. A whisker can have a tensile strength at room temperature of 3 to 4 million psi, compared to 0.3 to 0.5 million psi for a fiber made of the same material. In ad dition, whiskers generally tend to maintain their phys ical properties at high temperatures better than fibers do. Among the ceramic whiskers of greatest interest are those made of silicon carbide. In the early 1970s, Ivan B. Cutler, a ceramist at the University of Utah, developed a process for growing such whiskers by heating rice hulls, which are excellent sources of silicon and carbon, to nearly 2000 °C. In 1976, Cutler sold his process to Exxon Enterprises, a subsidiary of Exxon Corp., and Hulco Inc., a venturecapital company. The joint effort, called Silag Inc., built a plant at Greer, S.C., to make silicon carbide whiskers commercially. In late 1982, Arco Metals purchased the company, renamed it Arco Metals/Silag Operation, and since early last year has been producing silicon carbide whiskers under the tradename Silar. Used mainly to reinforce aluminum, the whiskers increase aluminum's strength about 40% and its stiffness about 100%, the company says. Silicon carbide whiskers also are used to boost the frac ture toughness of aluminum oxide, zirconium oxide, and other ceramics. Other organizations likewise make ceramic whiskers. Nippon Carbon Co. of Japan manufactures silicon car bide whiskers by pyrolyzing rice hulls. Los Alamos National Laboratory is developing a different process for making silicon carbide whiskers. Versar Inc., of Springfield, Va., produces whiskers of silicon carbide and of aluminum oxide. Boeing makes whiskers of sili con nitride. Currently, ceramic whiskers are used less than ceramic fibers partly because whiskers have not been on the market so long, with the result that companies have had less experience with them. Their use is growing, how ever, as they become increasingly available with the building of new production facilities. July 9, 1984 C&EN
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These silicon carbide whiskers, highly magnified, are used chiefly to strengthen aluminum. They are grown by Arco Metals/Silag Operation by heating rice hulls
Some ceramics can be reinforced not by fibers or whiskers but by ceramic particles of a different material than the matrix. These particles, usually less than 1 μπι in diameter, increase the fracture toughness, thermal shock resistance, and strength of a variety of ceramic matrices. Particulate composites have greater tensile strength than fiber-containing composites but usually have less fracture toughness. Among the many particulate composites available are those consisting of boron nitride particles dispersed in aluminum oxide, graphite particles in boron carbide, tetragonal zirconium oxide particles in aluminum oxide, and silicon carbide particles in aluminum metal.
Partially stabilized zirconium oxide Among the most intriguing particulate composites is partially stabilized zirconium oxide (zirconia), fre quently referred to simply as PSZ. Developed in 1975 by Ronald C. Garvie and coworkers at the Commonwealth Scientific & Industrial Research Organization in Mel bourne, Australia, it consists of a matrix of cubic zir conium oxide containing about 20 to 50% by volume of metastable tetragonal zirconium oxide particles. The partially stabilized material is made by heat-treating cubic stabilized zirconium oxide, containing magnesium oxide, calcium oxide, or yttrium oxide to cause some of the cubic crystals to precipitate as the tetragonal form. When a crack is propagated in PSZ, the stresses at the crack tip allow the adjacent tetragonal particles to transform to monoclinic zirconium oxide. This trans formation tends to inhibit further cracking of the ma terial, partly because the monoclinic form has about 4% greater volume than the tetragonal form and has a dif 36
July 9, 1984 C&EN
ferent shape. This increase in volume and change in shape apply a compressive stress to the crack tip, thus reducing the effective driving force of the crack. Interest in PSZ was stimulated in the mid- and late 1970s when Garvie, as well as Arthur H. Heuer at Case Western Reserve University, reported that this material has unusually high strength and toughness. They found that the tetragonal-to-monoclinic phase transformation in PSZ can increase the fracture toughness of the ceramic by a factor of two or more. In 1976, Nils E. Claussen at Max Planck Institute for Metal Research in Stuttgart, West Germany, reported that aluminum oxide ceramics can be transformationtoughened by incorporating into them about 15% by volume of finely divided tetragonal zirconium oxide. When a crack occurs in the aluminum oxide, the re sulting stresses locally allow the tetragonal zirconium oxide crystals to transform to the monoclinic form, thus impeding further cracking. Aluminum oxide toughened by zirconium oxide has replaced pure aluminum oxide or combinations of aluminum oxide and titanium car bide in cutting tools for metal machining operations. Norton Co. has used this method to make two types of aluminum oxide abrasives containing 25 or 40% zir conium oxide. In a similar way, tetragonal zirconium oxide particles are used to toughen such other ceramics as spinel (MgAl 2 0 4 ) and mullite (3A1 2 0 3 · 2Si0 2 ). Scientists currently are investigating other com pounds that undergo expansive phase transformations and thus can be used as transformation-toughening agents for ceramics. Among these are enstatite (MgSiOs) and dicalcium silicate (Ca2Si04). One aim of this research is to develop materials that are cheaper and possibly more effective than partially stabilized zirconium oxide. Other additives also may enhance the properties of ceramics. The addition of about 1% magnesium oxide to silicon nitride can make the silicon nitride particles denser during sintering at about 1600 °C. Yttrium oxide sometimes is added to the sialons, which are combinations of silicon nitride and aluminum oxide. When the combination is sintered, the yttrium oxide is converted to yttrium silicate and yttrium aluminate. When the material is cooled, these compounds, which are present at the grain boundaries, improve the end product's toughness at high temperatures. Other additives are used to facilitate the sintering and espe cially the hot-pressing of various oxides such as mag nesium oxide; magnesium oxide itself is used to aid the sintering of aluminum oxide.
High-tech ceramics in engines One of the biggest incentives for today's R&D on high-tech ceramics is the potentially huge market for these materials in automotive and other engines. Many observers predict that within 10 or 15 years these ap plications will constitute the largest single market for advanced ceramics. Others, however, are decidedly less sanguine about this potential market. Ceramics with the needed mechanical properties at high temperatures, they argue, will not be available for many years. Indeed, some
skeptics assert that a gas turbine engine containing a high percentage of ceramic parts will never reach the market because of the engine's inadequate reliability. Just about everyone agrees, though, that an automotive or truck engine will never be built completely of ceramics—except possibly as a curiosity. Although parts exposed to very high temperatures in such engines will be made of ceramics, other parts will be made of cast iron, aluminum, or other metals. Conventional metals not only function quite well at relatively low temperatures but also are less expensive. Because auto engines containing a high percentage of ceramic parts can function at higher temperatures than conventional engines, they make more efficient use of fuel and help to conserve energy supplies. In addition, these engines may produce cleaner exhaust emissions and have greater durability than present engines. In conventional piston engines, high-tech ceramics may find increasing use, for example, as piston caps, exhaust manifold liners, valve heads, and turbocharger rotors. The ceramics most likely to be employed are silicon carbide, silicon nitride, partially stabilized zirconium oxide, and lithium aluminosilicate. Turbochargers, which are turbine devices that pump air into piston engines and simultaneously increase the engine's gasoline intake and power output, are likely to be the first widespread use of these ceramics. Toshiba, Kyocera, and Mitsubishi in Japan are expected to begin commercial production of silicon nitride or silicon carbide turbocharger rotors for automobiles in the next year or two. In the future, however, the greatest automotive use of high-tech ceramics is expected to be in gas turbine engines and in adiabatic (uncooled) diesel engines. In gas turbine engines, compressed air is passed continuously through a burner, where fuel is added and burning takes place. The resulting high-temperature gas is expanded through a turbine wheel, which creates the power for the vehicle. The engine can be operated on either petroleum or nonpetroleum liquid fuels, with low exhaust emissions. Even more important, a gas turbine rotor made of a ceramic such as silicon nitride or silicon carbide can be subjected to temperatures as high as 1400 °C, whereas superalloys seldom tolerate more than 1100 °C. Operated at 1400 °C, an engine can use fuel quite efficiently. Ceramics used in gas turbine engines, however, must perform dependably over long periods of time. They must not only withstand high temperatures but high mechanical stresses, such as those created by high speeds of rotation. Some key ceramic parts, such as rotors, have complex shapes that are difficult to fabricate. Ceramic-containing gas turbine engines may find use in nonautomotive applications sooner than in passenger cars, because the operational demands will be less. Nonautomotive uses could include missile engines and auxiliary power units for generating electricity or producing hydraulic power where their light weight is a special advantage. The Department of Energy (DOE) currently supports two large demonstration projects to develop ceramiccontaining gas turbine engines for autos. These pro-
grams, each costing more than $60 million, were begun in October 1979 and are scheduled to continue through November 1985. One of these projects, conducted jointly by the Allison Turbine Operations and the Pontiac Motor divisions of General Motors, is concerned with the development of the AGT-100 engine (AGT meaning "advanced gas turbine"). This is a lightweight, 100-hp, two-shaft engine that has a turbine-inlet temperature of 1290 °C. The engine uses silicon carbide in its high-temperature areas, such as the power turbine and gasifier. Other parts employ such ceramics as aluminum silicate and lithium aluminosilicate. The other major gas turbine project sponsored by DOE is a joint effort of Garrett Corp. and Ford Motor Co. to develop a 100-hp, single-shaft engine known as AGT101. It has a turbine-inlet temperature of about 1370 °C. The AGT-101 uses either silicon carbide or silicon nitride in the turbine and other parts subjected to unusually high temperatures. Because it is a single-shaft engine, very high stress is inflicted on the turbine rotor, which operates at up to 100,000 rpm. Since the rotor is so greatly stressed, it is made of strong and relatively tough silicon nitride. Among the engine's 60 ceramic parts are some made of lithium aluminosilicate. Researchers expect that such gas turbine engines will have a city/highway combined gas mileage of about 43 miles per gal in a 3000-lb car. This would be 30% greater mileage than that of a conventional piston engine. Some observers believe that ceramic-based gas turbine engines for autos will not be on the market in U.S. cars
Edward Fischer of 3M applies intense heat to fabric made of 3M's Nextel 312 aluminum borosilicate fiber. Fabric has good heat-resisting and heat-insulating properties July 9, 1984 C&EN
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Special Report until sometime in the 1990s. But Japanese companies seem much more optimistic. Mitsubishi Heavy Industries expects to be mass-producing ceramic-containing gas turbine engines for autos in three or four years. At the Tokyo Motor Show in October 1983, Nissan introduced its prototype NX-21 sedan with a ceramic-containing gas turbine engine, and Isuzu displayed its new diesel engine using some components made of PSZ.
Diesel engines Much interest in ceramics for engines has centered on the development of adiabatic diesel engines for cars and trucks. Conventional diesels are nonadiabatic (cooled) because the metals they use otherwise could not withstand the engine's high operating temperatures. On the other hand, because ceramics can tolerate higher temperatures, diesels containing them not only are more fuel efficient but do not require the cooling systems that demand repeated maintenance and add to the engine's weight and cost. For some years the Army has supported research by Cummins Engine Co. on ceramic-containing adiabatic diesels for military trucks. In a demonstration last year, one such Cummins engine in a 5-ton truck obtained 9 miles per gal of diesel fuel, compared to the usual 5 to 7 miles per gal for conventional diesel engines. The experimental Cummins engine makes extensive use of PSZ as well as other ceramics such as silicon nitride and silicon carbide. Zirconium oxide coatings are used to insulate the combustion chamber and sustain the high operating temperatures needed for high fuel economy. The engine uses ceramics mostly as coatings for cylinder walls, cylinder heads, piston tops, and valve seats. Ceramics also are used as monolithic parts. DOE also funds research on ceramic-containing adiabatic diesel engines for heavy-duty commercial trucks. The program is concerned partly with the development of high-temperature ceramic components for adiabatic diesels, ceramic coatings for various engine parts, and improved methods for bonding ceramics to metals or to other ceramics. This program, the agency hopes, will increase the efficiency of diesel fuel use in such engines about 50%. Japanese companies also are developing ceramiccontaining adiabatic diesel engines for both trucks and autos. Hitachi has built an experimental automobile powered by such an engine containing parts fabricated from or coated with silicon carbide. Toshiba is using silicon nitride in the adiabatic diesel engines it expects to have on the market in the next four or five years.
Electrical applications Aside from the potential market in engines, the largest single worldwide market today for high-tech ceramics is based on the electrical properties of these materials. According to a survey made by Bowen and George B. Kenney of MIT, about half the world's total 1980 sales of high-tech ceramics went for materials with specific electrical properties. These substances included everything from insulators and piezoelectric materials to semiconductors and ion-conducting materials. One of the principal electrical or electronic uses for 38
July 9, 1984 C&EN
high-tech ceramics is as insulators. Partly because aluminum oxide has high electrical resistance and high density, it is the principal ceramic used in substrates for integrated electronic circuits, wiring, and electronic interconnectors. Other compounds used for these purposes are beryllium oxide and magnesium aluminate. These ceramics are usually superior to plastics in making such substrates because they not only provide good electrical resistance but have good resistance to chemical attack and moisture penetration. In addition, they can withstand the chemical and thermal processes involved in making the substrates. And because they are highly unreactive, they do not contaminate ultrapure silicon chips. L. Eric Cross, Robert E. Newnham, and Amar S. Bhalla at Penn State are investigating new ceramic materials as possible substrates for integrated circuits. Among these compounds are strontium silicate and barium silicate. These ceramics, they point out, have almost the same electrical resistance as aluminum oxide but have a much lower dielectric constant. Also, they can be processed at lower temperatures (800 to 1200 °C, compared to about 1600 °C for aluminum oxide). Aluminum oxide, which has a dielectric constant of about 10, is quite satisfactory for integrated circuits that use silicon chips. However, for integrated circuits employing gallium arsenide chips, substrates with a dielectric constant of about 2 are required to fully utilize the mobility of electrons in gallium arsenide, which is about five times greater than in silicon. In the future, gallium arsenide chips may be packaged in new ceramic substrates to be used in the ultrafast computers currently under development for use in space and radiation environments. Because some high-tech ceramics are excellent dielectrics, they are the mainstay of the capacitor industry. Probably the most widely used such material is barium titanate (BaTiOs), which in 1946 was found to have a dielectric constant about 100 times greater than that of any other material then known. Currently under study are a variety of other dielectric materials such as barium zirconate, which may be used alone or, more commonly, in combination with barium titanate. Another major electrical use for high-tech ceramics is as piezoelectric materials, which generate electricity or electrical polarity when subjected to mechanical stress. The most common single-crystal piezoelectric material is quartz (silicon dioxide), which is used mainly for frequency control in electrical oscillators, such as in watches. At present, the most widely used piezoelectric polycrystalline material is lead zirconate titanate. Its electrical output can be used, for example, to measure pressure. This ceramic also can be used in hydrophones, which permit listening to sound transmitted through water. Cross, Newnham, and coworkers at Penn State have been exploring the use of lead zirconate titanate fibers imbedded in an epoxy resin or polyurethane matrix. Such composites can produce about 100 times more electrical output for a given hydrostatic stress than can lead zirconate titanate alone.
erated noise, absorb electrical current surges, and act as lightning arrestors. Among the most commonly used ceramics for varis tors are additive-containing zinc oxide and, to a lesser extent, silicon carbide. Bell Labs scientists currently are developing varistors consisting primarily of titanium dioxide.
Ion-conducting materials for batteries
At Rutgers, W. Roger Cannon (right) watches student Keith Wilfinger use slip-casting molds to make aluminum oxide toughened with tetragonal zirconium oxide High-tech ceramics also play an important role in thermistors, which are materials whose electrical resis tance varies with temperature. One type is called a negative-temperature-coefficient thermistor because its electrical resistance decreases with increasing temper ature. Such a material is used to measure temperature, control temperature, or to compensate for other circuit components that have a positive coefficient of resis tivity. Among the leading materials of this type are cobalt ferrite (CoFe20 4 ) and nickel ferrite (NiFe 2 0 4 ). David W. Johnson Jr. and coworkers at Bell Labs find that less ex pensive compounds such as manganese ferrite (MnFe20 4 ) and magnesium ferrite (MgFe20 4 ) can be substituted. They find that, if these ceramics are pro cessed carefully (for example, their sintering conditions are controlled precisely), they can be used as negativetemperature-coefficient thermistors with characteristics comparable to those of cobalt- or nickel-containing fer rites. Thermistors with a positive temperature coefficient are used as heating elements and electrical switches. Among the leading ceramics used as positive-tempera ture-coefficient thermistors are barium titanate (BaTiO^), magnesium aluminate (MgAl 2 0 4 ), and zinc titanate (Zn 2 Ti0 4 ). Yet another group of ceramics with useful electrical properties are varistors—materials whose electrical re sistance significantly decreases above a critical applied voltage. Such materials can eliminate electrically gen
A focus of active research these days is high-perfor mance batteries in which the electrolyte is a ceramic rather than a conventional liquid such as sulfuric acid. Solid electrolyte batteries can produce the same amount of electrical energy as a conventional lead-acid battery but with about one third the weight. This weight saving would be important, of course, in battery-operated au tomobiles and other equipment. One such battery is the sodium-sulfur cell, in which the solid electrolyte is the ceramic β-sodium alumina (Na2Al22034), often called merely "/3-alumina." In this cell, which is being explored by such com panies as Ford Motor and GE, the β-alumina is permeable to positively charged sodium ions, which are the con ducting ions, and is impermeable to negatively charged sulfide ions. When the battery is discharged, the sodium metal anode is partially converted to sodium ions, and the re leased electrons are conducted to the external circuit. The sodium ions pass through the solid electrolyte to the elemental sulfur cathode. The sulfur cathode takes up electrons from the external circuit and forms sulfide ions that combine with the sodium ions to produce sodium polysulfide (Na2Sn). Since the battery is operated at about 300 °C, both electrodes are molten. The /3-alumina ceramic permits the movement of so dium ions from one part of the cell to the other, yet provides a physical barrier between the molten elec trodes. The ceramic also acts as an electronic insulator, so the battery does not short itself out. GE researchers report that such batteries are disap pointingly short-lived because /3-alumina tends to de grade fairly rapidly. To help solve this problem, they have developed a family of more durable ceramic elec trolytes called "^"-aluminas," such as Na1.67Mgo.67AI10.33O17·
Scientists at Westinghouse and elsewhere are inves tigating ceramics that can be used as solid electrolytes in fuel cells to replace such liquid electrolytes as phos phoric acid. The solid electrolyte frequently used in fuel cells is zirconium oxide containing a small percentage of magnesium oxide or yttrium oxide. This ceramic is able to conduct oxygen ions at the cell's operating tem perature of about 980 °C. Magnetic properties impart important uses to another group of ceramics, known as ferrites, having the general formulas of MFe20 4 (where M is nickel, manganese, zinc, magnesium, cobalt, and so on for "soft" ferrites) and M r Fei20i9 (where M' is barium, strontium, and so on for "hard" ferrites). The initial commercial devel opment of these materials is generally credited to such companies as N.V. Philips in the Netherlands and TDK Electronics in Japan. July 9, 1984 C&EN
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Worker applies ceramic tiles to body flap to be located in aft portion of Columbia space shuttle. Produced by Lockheed from 99.7% pure silica fibers, the strong, low-thermal-conductivity tiles protect the shuttle from re-entry heat "Soft" ferrite ceramics are used as magnetic recording heads, memory devices, temperature sensors, and components of electrical motors. In such applications, the most frequently used ferrites are those containing manganese-zinc, nickel-zinc, and magnesium-manganese. "Hard" ferrites are used chiefly as magnets. Notwithstanding all these uses of ceramics, it is another use that brought general public awareness of high-tech ceramics to the fore when in April 1981 about 34,000 ceramic tiles were used to protect the space shuttle Columbia from the 1260 °C heat of re-entry into Earth's atmosphere. The upsurge in attention occurred when more than a dozen of these critically important tiles fell off, probably during or shortly after lift-off. The tiles are believed to have come loose because of improper cleaning and priming of part of the spacecraft's metal surface and the faulty application of cement. The shuttle's strong, low-thermal-conductivity ceramic tiles were made of short, 99.7% pure silica fibers supplied by Manville Corp. This material was formed into blocks, sintered at up to 1315 °C, and then cut into tiles by Lockheed Missiles & Space Co. Because of this insulation, the temperature of the shuttle's aluminum skin never exceeded its design limit of 180 °C. Two kinds of silica tiles have been used on both the Columbia and Challenger space shuttles. About 90% of these tiles are made of a material called LI-900, which weighs 9 lb per cu ft. The remaining 10% are made of LI-2200, which weighs 22 lb per cu ft. The heavier material is used where tougher, more durable tiles are needed because they are exposed to higher temperatures. A problem with the denser tiles is their weight. To solve this problem, NASA's Ames Research Center developed a composite consisting of 78% pure silica fibers and 22% aluminum borosilicate fibers (3M's Nextel 312), weighing only 12 lb per cu ft. This material has been installed instead of LI-2200 on the Discovery and Atlantis space shuttles. Besides being less dense, ceramic tiles made of the new composite are more rigid and 40
July 9, 1984 C&EN
stronger than those made solely of silica. One reason is that the boric oxide in the aluminum borosilicate fibers helps to weld these fibers to the silica fibers. Advanced ceramics are finding a vast number of other important uses. Among these are as cutting tools (using diamond, cubic boron nitride, silicon nitride, aluminum oxide, titanium carbide, silicon carbide), fuels for nuclear power plants (uranium dioxide, plutonium dioxide), materials for containing radioactive wastes (borosilicate glass, lithium aluminosilicate), and optical fibers (germanium-doped silica glass, beryllium fluoride glass). Other uses include heat exchangers (silicon carbide, cordierite, aluminum oxide) and ceramics for implants in the human body (aluminum oxide, potassium magnesium silicate, calcium phosphate, isotropic carbon). Roy Rice of the Naval Research Laboratory declares, "Economic forces clearly are pushing the development of new high-tech ceramics. Back in the days when gasoline cost only 30 cents a gal, there wasn't much incentive to develop ceramics for auto engines. Now, however, ceramic engines that are more fuel-efficient are becoming much more attractive. This is particularly true since the cost of high-performance metals is climbing rapidly." Fortunately, techniques are being discovered to make new high-tech ceramics feasible on a commercial scale. For those ceramics already on the market and for those yet to come, these materials not only will be extremely valuable but are sure to generate substantial sales, especially in large-volume uses. It's no wonder that so many organizations have "ceramics fever." D
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