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(a) (b) (c) Figure 5. (a) ZnS ceramic disk, (b) its SEM image, and (c) IR transmittance spectrum. the formation of both of these transparent polycrystalline materials. More recently, Sanghara, et. al, fabricated transparent ytterbium-ion-doped CaF2 ceramics using cold isostatic pressing and hot-pressing techniques.5 The CaF2 ceramics, designed for high-power laser applications, have a much higher transmittance in the visible region than Kodak’s CaF2. The researchers explored fabrication by hot-pressing fluoride single crystals into deformed polycrystalline transparent ceramics with uniformly distributed submicrometer grain sizes. After hot-pressing CaF2 single crystals in a molybdenum alloy mold at 1,300°C with 250-megapascal pressure, the single crystals transformed into a polycrystalline CaF2 ceramic composed of irregularly shaped grains. Lithium fluoride (LiF) and CaF2–SrF2–YbF3, highly transparent ceramics, also can be prepared via this method, and they exhibit a little higher efficiency than their single-crystal counterparts for laser performance. Sulfides: In the middle of the 20th century, scientists identified zinc sulfide (ZnS, Figure 5) as a good candidate for windows that could transmit farther into the IR region than MgF2 and CaF2. The discovery occurred in a period when the military became interested in research on IR-transparent ceramics for airborne reconnaissance, target designation, high-power lasers, and other applications. 8 Kodak, again, led the way, and prepared a cubic-phase ZnS ceramics, via hot-pressing in a metal alloy mold. The material was transparent in the IR region and translucent (40 percent at 600 nanometers) in the visible region. One difficulty with fabricating of ZnS transparent ceramics is that the highest temperature that can be used for sintering is limited to approximately half of the melting temperature, because the Transmittance (%) material undergoes a cubic to hexagonal phase transformation at about 1,000°C. Years later, researchers at other companies, including Raytheon, obtained similar IR optical-quality ZnS ceramics with 2–8 micrometer grain size through chemical vapor deposition. After post- HIP treatment, they converted the translucent, orange-yellow ZnS into a clear and colorless multispectral, highly transparent ceramic. A major issue that results from this treatment is that the grains grow to hundreds of micrometers in size. This results in a 30-percent decrease in mechanical strength and a 35-percent decrease in hardness. Nevertheless, transparent ZnS ceramics made by this method are sold as commercial IR window and optical products. Mixed anionic system: In the late 1950s, scientists discovered cubic-phase aluminum oxynitride (Al23O27N5, or AlON) and its spinel-type phase. This discovery catalyzed significant interest in new transparent armor materials and mechanically stable thermal electromagnetic windows and domes.9 Investigators prepared the first closeto transparent AlON ceramics using pressureless reactive sintering. Beginning with a powder mixture, in appropriate ratios, of AlN and Al2O3, they fired the material at 2,025°C for one hour under flowing nitrogen. They also could form transparent AlON from reactive hotpressing with powder mixtures of Al2O3 and AlN at 1,900°C under 20 megapascals for one hour. Because both of these methods are successful, transparent AlON with improved optical quality is prepared using processes that include AlON powder synthesis, consolidation, and green body firing to full density. AlON powders can be synthesized several ways, including simple reactions of Al2O3 and Wavelength (nm) (Credit: Chen and Wu.) AlN, carbothermal reductions of Al2O3, plasma-arc synthesis, and self-propagating high-temperature synthesis. The sintering technology relies on the addition of a sintering aid through vapor-phase transport and pressureless sintering in a nitrogen atmosphere. This results in a consolidated ceramic with an inline transmission of at least 50 percent in the 0.3–5.0-micrometer range. Post-HIP treatment could produce highly transparent AlON ceramics in the visible region. In the beginning of this century, during an effort to refine the phase diagram of Al2O3 and AlN, researchers found that transient liquid-phase reactive sintering is very effective in obtaining high-quality, visibly transparent AlON ceramics. This requires a two-step sintering process that is comprised of, first, sintering at a temperature to introduce a liquid, and, second, cooling to a lower temperature and allowing the liquid to resorb, producing the single-phase transparent AlON. Among commercial manufacturers of transparent AlON is the Surmet Corporation (see cover story, “Transparent polycrystalline cubic spinels protect and defend”), which uses the material for IR-transparent, hightemperature, ballistic, and blast-resistant windows, all of which are four-times harder than fused silica glass and 85 percent as hard as sapphire. Non-cubic-phase transparent ceramics Oxides: GE was the first to study transparent alumina in the 1950s, for use as a high-temperature lamp envelope because of the material’s unique high strength and great chemical resistance. With the successful fabrication of the material, GE began manufacturing and selling alumina Lucalox bulbs in 1961. Almost entirely sintered, the densified American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 35


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