Advanced Ceramics

ALBERT R.C.WESTWOOD and STEPHEN R.WINZER

During the past five years, ceramics has undergone a revolution almost as dramatic as the more familiar one in electronics. Novel approaches to preparing and processing ceramic solids have been developed, ingenious ways of circumventing the age-old problem of brittleness have been introduced, and new markets have begun to open up in such areas as sensors, orthopedics, photonics, and heat engines.

Today’s advanced ceramics represent developments well beyond the imagination of even the few farsighted scientists of 25 years ago who first perceived the remarkable potential of ceramic solids and established “ductile” engineering ceramics as a suitable objective for materials researchers to pursue. It was with this goal in mind that an interdisciplinary Conference on the Mechanical Properties of Engineering Ceramics was held at North Carolina State University in March 1960. Earl Parker and co-workers13 had just confirmed that ionic crystals could exhibit considerable ductility, as Joffe and others4,5 had first demonstrated in the 1920s. They had decided that ionic solids were intrinsically ductile but readily embrittled by notches, surface films, and internal barriers to dislocation motion. Accordingly, the 1960s saw, for example, the alloying of oxides and of carbides6 to increase the ductility of ceramic solids by influencing their interatomic bonding and, hence, ease of cross slip. But it proved impossible to overcome notch brittleness, that is, to arrest a crack once initiated by some preexisting flaw or by a dislocation pileup at some obstacle such as a grain boundary7 (see Figure 1).

The late 1950s also saw initiation of the Basic Sciences and Electronics Divisions of the American Ceramic Society. By 1960, papers were being published on phase equilibria and sintering behavior in multicomponent oxide



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Advancing Materials Research Advanced Ceramics ALBERT R.C.WESTWOOD and STEPHEN R.WINZER During the past five years, ceramics has undergone a revolution almost as dramatic as the more familiar one in electronics. Novel approaches to preparing and processing ceramic solids have been developed, ingenious ways of circumventing the age-old problem of brittleness have been introduced, and new markets have begun to open up in such areas as sensors, orthopedics, photonics, and heat engines. Today’s advanced ceramics represent developments well beyond the imagination of even the few farsighted scientists of 25 years ago who first perceived the remarkable potential of ceramic solids and established “ductile” engineering ceramics as a suitable objective for materials researchers to pursue. It was with this goal in mind that an interdisciplinary Conference on the Mechanical Properties of Engineering Ceramics was held at North Carolina State University in March 1960. Earl Parker and co-workers1–3 had just confirmed that ionic crystals could exhibit considerable ductility, as Joffe and others4,5 had first demonstrated in the 1920s. They had decided that ionic solids were intrinsically ductile but readily embrittled by notches, surface films, and internal barriers to dislocation motion. Accordingly, the 1960s saw, for example, the alloying of oxides and of carbides6 to increase the ductility of ceramic solids by influencing their interatomic bonding and, hence, ease of cross slip. But it proved impossible to overcome notch brittleness, that is, to arrest a crack once initiated by some preexisting flaw or by a dislocation pileup at some obstacle such as a grain boundary7 (see Figure 1). The late 1950s also saw initiation of the Basic Sciences and Electronics Divisions of the American Ceramic Society. By 1960, papers were being published on phase equilibria and sintering behavior in multicomponent oxide

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Advancing Materials Research FIGURE 1 Cracks formed at the boundary of an MgO bicrystal, caused by piling up of edge dislocations. From Westwood.7 systems, SiC as a candidate material for the leading edge of wings on hypersonic aircraft, the dielectric behavior of zirconates and simple niobates, and the use of garnets and ferrites for computer memories.8 However, most papers were still concerned with refractories for furnace walls and with enamels and whitewares—the mainstays of the ceramics industry at that time. Parker9 succinctly summed up the state of affairs 25 years ago when he said in his closing remarks to the North Carolina State meeting, “The most important thing we have learned in this conference is how little we really know about complex ceramic materials…. Each speaker, talking for something like half an hour, or even less, was running out of things to say by the time he was finished.” The years 1960–1980 have served as the long period of slowly increasing investment in research and development and technical advances that often precedes the S-curve development of a new industry. We are now entering the initial growth phase of the advanced ceramics industry, in which scientific understanding and developments are exploited in diverse areas, completely new applications are appearing, and companies are beginning to compete for market share. This chapter briefly reviews some of the ways in which the

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Advancing Materials Research FIGURE 2 Interactions of ceramics science with other technical fields. field of ceramics has influenced, or been influenced by, other technical fields, especially chemistry, physics, metallurgy, medicine, and mechanical engineering. Thus, the focus of this chapter might be termed “ceramics at the interfaces” (see Figure 2). CERAMICS AND CHEMISTRY: CERAMIC SYNTHESIS Traditionally, ceramics have been used in chemistry as catalysts, catalyst substrates, and corrosion-resistant reaction vessels. Today, ceramics are beginning to be used as chemical-specific sensors for the detection of oxygen, hydrogen, carbon monoxide, and more complex organic species such as propane or isobutane.10 They also are being used as durable containers for active chemical and nuclear wastes, the new lead-iron phosphate glasses being a thousand times more resistant to leaching than standard borosilicate glasses (see Figure 3).11 Chemistry, on the other hand, has profoundly influenced the emergence of advanced ceramics. Through the development and application of novel chemical routes for the synthesis of micron-sized inorganic powders, or through rediscovery of “ancient” routes, it has become feasible to produce

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Advancing Materials Research FIGURE 3 Improved resistance of lead-iron phosphate glass to aqueous corrosion (90°C for 30 days) over conventional borosilicate glass. From Sales and Boatner.11 Reprinted, with permission, from B.C.Sales and L.A.Boatner, Science 226, 45 (1984). © 1984 by the American Association for the Advancement of Science. raw materials of sufficient purity and size consistency for the preparation of advanced ceramics. Conventional chemical approaches for making fine particulate solids involve colloidal suspension followed by removal of the solvent. But if the suspension is simply dried by heating or evaporation, coarse crystals or agglomerates usually result. An alternative route is through sol-gel chemistry, first used in 1864 by Thomas Graham to make silica gel. It involves three steps: (1) producing a concentrated solution of a metallic salt in a dilute acid (the sol); (2) adjusting the pH, adding a gelling agent, and evaporating the

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Advancing Materials Research liquid to produce a gel; and (3) calcining the gel under carefully controlled atmospheric conditions to produce fine particles of the requisite ceramic. This approach is especially useful for oxide-based ceramics such as Al2O3, ZrO2, and TiO2. An alternative sol-gel process for producing colloidal dispersions is the hydrolysis of metal alkoxides, the products of reactions between alcohols and metal oxides. The advantages of this approach are that alkoxides can be purified by distillation and that the subsequently precipitated hydroxides tend to be pure, uniform, spherical, submicron-sized particles that, upon sintering, retain their uniformity and fine grain size (see Figure 4).12 Sol-gel processes have been used to produce a variety of glass and ceramic fibers, including high-purity SiO2 fibers for optical waveguides. Starting with Si(OEt)4 as a precursor, fibers with optical attenuations of 6 dB/km have been made.13 Research is now under way to produce non-oxide fibers for infrared waveguides. Kamiya and co-workers14,15 have also spun TiO2-SiO2 and ZrO2-SiO2 fibers exhibiting extremely low coefficients of expansion and high thermal stability. Barium titanate films 200 angstroms thick have been prepared by dip coating, though whether their dielectric properties are suitable for electronic applications was not reported.16 Vanadium pentoxide gels have been prepared for antistatic coatings17 and tungstate gels for optical display materials.18 Other developments include the production of UO2 spheres for nuclear applications.19 These are made by dropping spray-formed sol spheres through a heated column of inert liquid. Gelation occurs during the fall, after which the spheres (30–1,200 µm in diameter) are collected and fired to near-the-oretical density. This process has also been applied to the encapsulation of radioactive waste. The principal problems with the sol-gel route are economic (a sol-gel silica glass can cost $20/lb), long processing times, and product porosity, although porosity can be advantageous in the production of porous structures, such as catalyst supports. A comprehensive review of sol-gel processing has been published by Mackenzie.20 Another route particularly appropriate for the production of such ceramic fibers as SiC and Si3N4 involves the thermal degradation of polymers. Again this is not a new discovery; polysilazanes were pyrolized to form Si3N4 as long ago as 1881.21 More recent work by Baker, Grisdale, and Winslow22 in the 1950s led to the production of silicon-carbon bodies, but the field really came of age in the mid-1970s with the pioneering work of Yajima,23 which permitted Nippon Carbon to bring SiC fibers to the marketplace. Their process involves heating polydimethylsilane in an autoclave to yield a polycarbosilane with a molecular weight of approximately 2,000. This is melt-spun into polymer fibers of 10–20 µm in diameter, and the fibers are then partially oxidized by heating in air at 200°C to give them sufficient

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Advancing Materials Research FIGURE 4 (a) TiO2 powder, about 0.35 µm in diameter, produced by the hydrolysis of metal alkoxides. (b) TiO2 ceramic of >99 percent theoretical density made from powder above. Average grain size is 1.2 µm. From Barringer and Bowen.12 Reprinted with permission. strength to retain their shape at the pyrolizing temperature of 1,200–1,400°C. The resulting SiC fibers are similar in dimensions to the original polymer fiber, and can be woven into mattes. Various ceramic materials have now been produced by the polymer degradation route, including BN, AlN, Si3N4, Si-Ti-C, and SiC-B4C.24,25 Other routes for producing ceramic particles involve vapor-phase reactions. For example, researchers at the Massachusetts Institute of Technology and

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Advancing Materials Research Rutgers University have produced fine particles of SiC and Si3N4 by reacting SiH4, NH3, and C2H4 in a CO2 laser beam. The powders produced are typically less than 0.1 µm in diameter.26 In other work, water vapor has been reacted with aerosol droplets of alkoxides to produce either pure or mixed oxide powders of the order of 1 µm in diameter.27 Although the approaches just described are technically promising, their cost-effectiveness remains a problem. Thus it is sometimes worthwhile for the industrial scientist to revisit a long-established approach and see if improved mechanistic understanding can reveal some new opportunity for improving its efficiency. For example, it is known that silica and carbon can interact through the gaseous phase SiO, which adsorbs on the carbon and reacts to produce SiC and CO. If the carbon particles are small, this reaction can be controlled so that the size and shape of the product SiC particle is strongly determined by that of its carbon precursor. In this way, SiC particles 1 µm in diameter have been produced at 1,600°–500°C below traditional SiC production temperatures.28 Estimates indicate that this approach, scaled up, could produce submicron-sized SiC for sale at $5 to $7 per pound. CERAMICS AND METALLURGY: METAL-MATRIX COMPOSITES The classic challenge to the metallurgist has been to increase the strength and stiffness of metallic alloys without significantly reducing their ductility. Considerable success has been achieved with respect to strength, and ferrous alloys exhibiting yield strengths greater than 1,800 MPa and fracture toughness of 80 MPa m1/2 are now available. But economical ways of increasing alloy stiffness have been more difficult to come by. Over the past 20 years, the development of metals reinforced with graphite or ceramic fibers has been vigorously pursued, yet the resulting materials remain expensive, and the production of complex shapes from them is difficult. Joining fiber-reinforced metal components also presents problems. However, new approaches to improving specific stiffness are emerging, driven by the need for lightweight space structures. One such approach has resulted in the new aluminum-lithium alloys now coming to market. These alloys exhibit a specific modulus more than 20 percent greater than that of aluminum. Another approach, using rapid solidification technology, has produced aluminum-base materials of complex nonequilibrium chemistry with strengths greater than 600 MPa, ductilities greater than 9 percent at 20°C, and substantial strength retention to about 350°C. One might conjecture, however, that the intrinsic chemical instability of such materials could lead to unpredictable behavior during complex operating conditions involving elevated temperatures, cyclic stressing, and active environments. A third approach, now under development at Martin Marietta Laboratories, is based on the intrinsic stability of ceramic particles and their known ability

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Advancing Materials Research to increase stiffness when present in sufficiently small size (about 1 µm), high concentration, and homogeneous distribution. Invented by an interdisciplinary team, the new materials, termed XD alloys, exhibit specific moduli up to 40 percent greater than the base metal.29 The process by which these alloys are produced is proprietary, but the prospects for such materials appear very encouraging because, once formed, they can be processed (for example, cast, rolled, extruded, or welded) by conventional means. To date, the XD process for producing ceramic-stiffened metallic materials has been demonstrated for Al, Cu, Fe, Mg, Ni, Pb, and Ti alloys, and also for Ti and Ni aluminides. Their outstanding properties apparently result not only from the uniform dispersion of fine ceramic particles but also from the “cleanliness” of the ceramic particle-metal interface, a result not readily achieved in conventional powder metallurgy processing. Figure 5 compares the modulus of XD-Al alloys with SiC-reinforced Al, and Figure 6 shows the improved high-temperature performance of XD-7075 Al over the conventional 7075 alloy. Other demonstrated advantages of XD-aluminum alloys FIGURE 5 Relation between Young’s modulus and concentration of ceramic phase for XD-Al and SiC-Al alloys. Insert shows typical microstructure of XD-Al material. From Christodoulou, Brupbacher, and Nagle.29

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Advancing Materials Research FIGURE 6 Improvement of high-temperature properties of XD-7075 alloy over conventional alloy. From Christodoulou, Brupbacher, and Nagle.29 include superior weldments (because of the stability of the ceramic phase), more than a 10-fold increase in resistance to wear, greater resistance to fatigue due to crack branching, and production and processing costs much less than those of SiC-Al, with an eventual selling price of less than $10 per pound. Although XD alloys are not inexpensive, their improved performance may lead to their early use in space structures, transatmospheric vehicles, and piston rods. Other interesting developments are occurring in the field of in situ precipitated composites. The possibilities for such materials include metal-matrix structures containing 108 high-strength reinforcing ceramic whiskers per cubic inch or, conversely, ceramic matrices containing high densities of metal fibers,30 for example, tungsten fibers in ZrO2.31 Applications include novel magnetic and electronic devices, such as the electron emitter, photovoltaics and capacitors, anisotropic heat conductors, superconducting materials, catalysts, and advanced superalloy-based structural materials.

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Advancing Materials Research CERAMICS AND MECHANICAL ENGINEERING: TOUGH STRUCTURAL CERAMICS The major problem to be overcome before ceramics can be seriously considered for structural applications is that of notch brittleness. Great progress has been made in this direction over the past few years, and future toughness KIC values in excess of 20 MPa m1/2 are now beginning to be achieved through various toughening approaches. These approaches include reducing the size and concentration of preexisting crack initiators (such as microcracks and pores) by decreasing grain sizes to the micron range and increasing densities to 99 percent or so, and introducing various synthetic crack-retarding entities, some of which are illustrated in Figure 7. Of these approaches, the incorporation of crack-closing particles is receiving the most scientific attention. Such transformation-toughened ceramics contain dispersed small particles of a metastable phase that transform crystallographically when the strain field of a crack passes through or near them. In this way some of the energy of the crack is absorbed. If the transforming particles FIGURE 7 Crack-retarding entities used to produce toughness in ceramics: (a) phase-transforming particles, e.g., ZrO2 (tetragonal)→ZrO2 (monoclinic); (b) fibers with weak fiber-matrix interfaces; and (c) other cracks.

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Advancing Materials Research FIGURE 8 Partially stabilized ZrO2 (a) unstressed, showing coherent tetragonal precipitate particles; and (b) stressed by indenter, revealing transformed monoclinic particles near indentation (arrows) and untransformed tetragonal particles elsewhere. From Porter and Heuer.32 Reprinted with permission. also increase in volume, they can apply a compressive stress to the crack tip, reducing its effective driving force. Further crack-retarding interactions occur at the particle-matrix interface, and within the particle itself. The best known example of this behavior occurs in partially stabilized ZrO2 (PSZ). In this case a two-phase ZrO2 is produced by partially stabilizing the tetragonal ZrO2 phase through additions of up to 10 percent Y2O3, MgO, or other oxide. A typical structure is shown in Figure 8.32 Note the relatively high volume, crystallographic orientation, and small size (0.5–2 µm) of the tetragonal phase. When a crack cuts through this material, the tetragonal (T) phase transforms locally into a monoclinic (M) structure, and toughening occurs by the mechanisms described above. Polycrystalline PSZ exhibits strengths of 500–1,500 MPa and fracture toughnesses of about 12–16 MPa m1/2 at room temperature. PSZ can also provide strengths of up to 700 MPa at 1,500°C.33 Noncubic ZrO2 particles can be incorporated into other ceramic substances, such as polycrystalline alumina, and recent studies34 have investigated the influence of stabilizer element (Y2O3) concentration on the stress required to initiate the T→M transformation in ZrO2 particles in the vicinity of a crack tip, and so on its fracture toughness. The value of KIC at room temperature for Al2O3–20% [ZrO2(Y2O3)] increases from 4 MPa m1/2 to greater than 10 MPa m1/2 as the Y2O3 content is decreased from 3 to 1 mole percent (see Figure 9). Fracture toughness also increases with number of ZrO2 particles that transform per unit volume of the crack-tip stress region. Thus, for a given Y2O3 content, KIC increases with volume fraction of T-ZrO2 particles. However, for each concentration of Y2O3, KIC reaches some maximum, and

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Advancing Materials Research FIGURE 9 Fracture toughness as a function of composition for Al2O3-ZrO2(Y2O3) composites. From Becher.34 Reprinted, with permission, from P.F.Becher, Acta Metall. (in press), © 1986 by Pergamon Press, Ltd. the stress required to transform a ZrO2 particle increases with Y2O3 content. Thus the maximum in KIC occurs at higher ZrO2 concentrations as the Y2O3 content increases, as is evident from Figure 9. The value of KIC also increases as testing temperature is decreased, for example, from 7 MPa m1/2 at room temperature to greater than 14 MPa m1/2 at—195°C (78 K) for Al2O3– [18.5 mole percent ZrO2–1.5 mole percent Y2O3]. Another method of toughening ceramics is to incorporate fibers or whiskers, and encouraging progress has been made with this approach, too. For example, the fracture toughness of a lithium aluminosilicate glass containing approximately 50 percent SiC fibers ranged from about 17 MPa m1/2 at room temperature to 25 MPa m1/2 at 1,000°C, at which temperature the matrix began to soften appreciably.35 Whiskers of SiC can also produce useful and relatively temperature-insensitive increases in the toughness of polycrystalline Al2O3. Moreover, conventional ball milling and sintering (to 95 percent theoretical density) procedures can be used in fabrication. For example, Al2O3 composites containing 20 volume percent SiC whiskers (7 µm diameter× 30 µm long) typically exhibit KIC values of 7–9 MPa m1/2 at room temper-

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Advancing Materials Research ature (see Figure 10), and values as high as 12 MPa m1/2 are now being reported.36 In contrast to PSZ ceramics, these values are retained to about 1,000°C (see Figure 11), as are strengths in the range of 800 MPa. Composites of Al2O3 reinforced with SiC whiskers compete well with the glass ceramic-matrix materials containing fine SiC fibers (see Figure 10) and have the advantages of being less sensitive to orientation than fiber-reinforced solids and easier to manufacture. Such materials are now used to make tough cutting tools and wear-resistant spray nozzles. But potentially the most explosive future application for tough ceramics is, of course, auto engine components. All-ceramic engines of the conventional piston-gasoline type are not considered likely, because of design limitations and because substantial fuel savings are not foreseen.37 It seems more likely that the next 5–10 years will see the gradual introduction of various engine parts, such as those shown schematically in Figure 12. Glow plugs and turbocharger rotors already have been introduced, with different ceramic materials having been used for different parts. The increased use of ceramic coatings is also foreseen.38 In the longer term (5–15 years), ceramic-dependent adiabatic diesel and FIGURE 10 Fracture toughness of Al2O3 and lithium aluminosilicate glass reinforced, respectively, with SiC whiskers and fibers. Courtesy of Paul F.Becher, Oak Ridge National Laboratories, Oak Ridge, Tennessee.

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Advancing Materials Research FIGURE 11 Temperature dependence of fracture toughness of Al2O3-SiC (whisker) components.36 FIGURE 12 Potential uses of advanced ceramic components in auto engines.

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Advancing Materials Research gas turbines will be marketed. The objective will be to eliminate radiators, water, oil, various pumps, and so forth to produce engines that are lighter, more efficient, and less polluting because of higher operating temperatures. Ceramics now being developed for such applications include SiC, Si3N4 (for turborotors, valves, piston caps, etc.), PSZ (for combustion chamber components), and aluminum silicates (for regenerator cores). However, the auto engines of the mid-1990s will certainly use more complex ceramics than these, probably ceramic alloys with toughness and durability optimized by thermomechanical treatments, and with surfaces processed (glazed) to minimize the potential consequences of small flaws introduced by abrasion or erosion. CERAMICS AND MEDICINE: PROSTHETICS Ceramics are extremely resistant to corrosion by body fluids and can be formed with surface characteristics that closely simulate those of natural bone. Thus their use in surgical applications is increasing.39 Materials now being used or evaluated include alumina, calcium and aluminum triphosphates, hydroxy apatites, and various glasses. Interesting developments include Kyocera Corporation’s use of single-crystal sapphire to produce products ranging from hip prostheses to dental implants and Corning Glass Works’ introduction of a castable glass ceramic that is machinable and contains a pure mica that provides a natural translucency to false teeth. This material is stronger than a tooth’s own enamel. Porous aluminas are being developed that permit infiltration of connective tissues to depths of 100 µm. Bioglass, introduced in 1972 by Hench, Splinter, and Allen40 and based on the system Na2O-CaO-CaF2-P2O5-SiO2, forms a chemical bond with bone. It can be used by itself if strength is not an issue, or as an interface between body tissues and a stronger steel implant. Cerevital, a product of the German firm E.Leitz Wetzlar, is a glass ceramic with a composition similar to that of Bioglass, and also has been used to coat implants of metal femoral head devices. The bond formed is stronger than the bone itself. Trisodium phosphate, various calcium phosphates, and polylactic acid/ carbon compounds are finding increasing use as resorbable bioceramics. These materials provide a porous ceramic that forms a temporary scaffold or space filler in human tissue. Initially its small pore size gives strength, but gradually the ceramic dissolves, and the space is filled with bone or tissue.41 The chemistry of these compounds can be adjusted to match the kinetics of the regeneration process. Rapid advances expected in this field include the integration of miniaturized implantable sensors and power sources to produce “active” prosthetic devices.

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Advancing Materials Research CERAMICS AND PHYSICS: ELECTRONIC AND PHOTONICS COMPONENTS Electronic applications now constitute the major market for advanced ceramics used as substrates, capacitors, piezoelectrics, and resistors, for example. Emerging rapidly, however, is photonics, and the growth of this field has been greatly accelerated by the introduction of low-loss optical fibers for broadband data transmission. High-silica fibers containing less than 10 parts per billion of hydroxyl and doped with phosphorus, germanium, and boron oxides can now provide losses of less than 1 dB/km in the 1.5-µm range. Materials with much lower losses (less than 10–2 dB/km), and operating in the 2–4-µm range, will be needed for the next generation of transoceanic cables, and it seems likely that they will be made from fluoride-based materials. Potentially, such materials could reduce losses to 10–3– 10–4 dB/km, in principle permitting transmission across the Atlantic, or the Pacific, without repeater stations.42 Today’s most advanced communication systems are hybrids of digital electronic and optical devices, and this situation is likely to persist for the next 10 years or so. Subsequently, however, the emergence of vastly more efficient and totally photonic systems will be based on integrated optical devices somewhat analogous to today’s integrated circuits.43 Ceramics such as LiNbO3 and LiTaO3, in single-crystal form, are of current interest as materials for optical modulators, switches, splitters, and other waveguide devices, functioning usually through the change in refractive index induced by an electric field. Newer materials such as SrBaNb2O6 and NaBaNb2O6 exhibit electro-optic coefficients 2 to 5 times greater than those of LiNbO3, and so will permit more compact devices, providing that problems in growing single crystals of these materials can be overcome. Another area of interest to ceramists is the multilayer technology being developed for packaging VLSI electronic devices. Figure 13 shows a recent example of the state of the art, namely, the multilayer multichip module (MCM) developed by IBM Corp.44 This consists of about 23 layers of alumina, co-fired with molybdenum metallization. Interconnects between layers are achieved by punching holes and filling them with molybdenum frit before firing. Such modules can include more than 300,000 vias and 500 cm of integral wiring for power and data transmission to and through the chip. Such a module typically carries nine chips, each equivalent to 700 circuits. Today, modules containing more than 40 layers are being introduced, and the trend is toward the use of lower-melting-point lead borosilicate glasses, Pb2Fe2Nb2O6 layers as capacitors, RuO2 layers for resistors, and copper or gold interconnects. Such MCMs can be fired at temperatures below 1,000°C. Just as the composites approach can be used to extend the strength and toughness of ceramics, so it can be used to develop superior or completely

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Advancing Materials Research FIGURE 13 Schematic of multilayer multichip module (MCM) developed by IBM Corp., approximately 50 mm square and containing 23 layers of alumina. From Seraphim and Feinberg.44 © 1981 by International Business Machines Corporation; reprinted with permission. new electronic or magnetic properties for ceramic materials. The key concept here is connectivity45—that is, the manner in which the component phases are self-connected in three dimensions and whether they are electrically connected in parallel or series. Ten connectivity patterns exist for a two-phase solid, and by use of relationships derived from the study of such patterns, materials with specific properties can be designed. For example, the BaTiO3 barrier-layer capacitor has at the grain boundaries an NaNbO3 phase that is connected in three dimensions. The BaTiO3 grains are not interconnected, so this is termed a 3–0 composite. Normally, the polarization of BaTiO3 capacitors saturates at high voltages, with the dielectric constant decreasing by as much as a factor of 2.45 But separating the grains of the ferroelectric BaTiO3 with the thin layer of antiferroelectric NaNbO3 compensates the saturation effect to provide a flat voltage response.46 Using the same approach, Skinner, Newnham, and Cross47 have shown how piezoelectric composites with figures of merit 10 to 100 times those of one-phase PZT ceramics could be made by embedding rods of PZT in a polymer. THE FUTURE This chapter has reviewed the progress made in ceramics over the past 25 years. What of the next 25 years? Predictions are always difficult and usually

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Advancing Materials Research err on the conservative side because they underestimate scientific ingenuity, capitalist entrepreneurism, and the breakthroughs in understanding or processing capability that open up completely unexpected paths of development. Given such caveats, it is expected that multicomponent, self-reinforced ceramic alloys, heat treated to optimize properties, protected by compressive layers applied by ion bombardment or laser glazing, and joined by lasers, electron beams, or novel cements, will become respected members of the engineers’ portfolio of useful structural materials by the year 2000.37 By 2010,48 photonics will have become a dominant technology based on integrated ceramic devices. Coated-fiber sensors will translate electrical, magnetic, and pressure variations into optical signals for real-time processing. Massively parallel “thinking” computers, based on photonics, will be extensively used. Ultra-large-scale integrated electronic chips will be based on doped ceramic materials. Various types of optoelectronic, acousto-optic, and other types of sensors, modulators, and switches based on complex ceramic compositions will be widely used in automated and robotic systems at home and in industry. Bioceramic prostheses will be in common use, and nuclear energy will be the power source of choice, with advanced ceramics used in fuels, structures, and disposal operations. ACKNOWLEDGMENTS It is a pleasure to acknowledge useful discussions with numerous colleagues during the preparation of this paper. Especially valuable were those with members of the Metals and Ceramics Division at the Oak Ridge National Laboratory, including J.Stiegler, V.J.Tennery, P.F.Becher, C.J.McHarge, and G.S.Painter; and with associates at Martin Marietta Laboratories, notably K.W.Bridger, L.Christodoulou, and J.Skalny (now with W.R. Grace and Co., Columbia, Maryland). NOTES 1.   A.E.Gorum, E.R.Parker, and J.A.Pask, J. Am. Ceram. Soc. 41, 161 (1958). 2.   T.L.Johnston, R.J.Stokes, and C.H.Li, Philos. Mag. 4, 1316 (1959). 3.   E.R.Parker, in Mechanical Properties of Engineering Ceramics, edited by W.W.Kriegel and H.Palmour (Interscience, New York, 1961), p. 65. 4.   A.Joffe, N.W.Kirpitschewa, and M.A.Lewitsky, Z. Phys. 22, 286 (1924). 5.   W.Ewald and M.Polany, Z. Phys. 28, 29 (1924). 6.   R.G.Lye, G.E.Hollox, and J.D.Venables, in Anisotropy in Single Crystal Refractory Materials, edited by F.W.Valdiek and S.A.Mersol (Plenum, New York, 1986), p. 445. 7.   A.R.C.Westwood, in Mechanical Properties of Engineering Ceramics, edited byW. W.Kriegel and H.Palmour (Interscience, New York, 1961), p. 89. 8.   E.Albers-Schoenberg, Ceram. Bull. 30, 136 (1960). 9.   E.R.Parker, in Mechanical Properties of Engineering Ceramics, edited by W.W.Kriegel and H.Palmour (Interscience, New York, 1961), p. 595. 10.   N.Ichinose, Bull. Am. Ceram. Soc. 12, 1581 (1985). 11.   B.C.Sales and L.A.Boatner, Science 226, 45 (1984).

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