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(Credit: Chen and Wu.) Figure 7. Key issues related to the development of highly transparent polycrystalline ceramics. at a wavelength of 1064 nanometers (1.6 millimeter thick), although there was no change in the relative density. The above is not consistent with the traditional belief that large grain sizes are beneficial in transparent ceramics (because the subsequent reduction in the number of grain boundaries could produce better transparency). However, the light transmission mechanism for the transparent nanoceramic is different from that of conventional transparent ceramics. According to the newly developed model for light transmission properties of fine-grained Al2O3 ceramics, based on the Rayleigh–Gans–Debye light-scattering theory, transmittance (T) can be calculated as a function of grain size (r), wavelength, and sample thickness. As a result, T∝ exp(–r), indicating that transmittance may increase exponentially with the decreasing grain size in a ceramic. (Along these lines, our group has been measuring the performance and laser-excitation characteristics of the high-quality transparent Sr-FAP ceramics that we fabricated, and we hope to publish the results soon.) Such transparent ceramics, fabricated to meet the requirements for commercialization, will promote the development of technologies for wide use in applications that range from automobile ignition to inertial confinement nuclear fusion. New frontiers open for exploration Despite the many developments related to transparent ceramics discussed above, there remain many other material systems that have intrinsic physical properties and potential applications in diverse fields, yet these materials pose challenges to those who attempt to prepare them to be highly transparent. For example, in regard to highenergy radiation detection and medical imaging applications, only translucent samples of monoclinic-structured lutetium orthosilicate (Lu2SiO5, or LSO), hexagonal-structured gadolinium oxysulfide (Gd2OS, or GOS) lanthanum bromide (LaBr3), and orthorhombicstructured strontium iodide (SrI2) have been thus far made translucent. (Some applications do not require very thick ceramics and, for example, GOS already has found commercial use.) For electrical-property-related applications, researchers also have fabricated translucent samples of PbZrO3– PbTiO3–Pb(Zn1/3Nb2/3)O3 (PZT-PZN), PbZrO3–PbTiO3–Pb(Ni1/3Nb2/3)O3 (PZT-PNN), (Pb,La)(Zr,Ti)O3 (PLZT), (K0.5Na0.5)1–xLixNb1–xBixO3, BaxSr1–xTiO3 (BST), SrxBa1–xNb2O6 (SBN) typically using hot-pressing, HIP, or SPS. Investigators also are giving appreciable attention to SiAlON and tetragonal structured zirconia. They have made samples of these materials with 70-percent and 25-percent transmittance (at a wavelength of 800 nanometers), respectively. Researchers expect significant improvements in the optical quality of these ceramic materials and expect near-future commercial applications. Furthermore, science and technology innovations (Figure 7) will accelerate the demands for new transparent ceramic materials equipped with high-quality and unprecedented physical properties in the near future. New ceramic technology, which generally can be applied to the wide array of material systems, will help open a new frontier in the development of new technologies. About the authors Shi Chen is a postdoctoral researcher at the Kazuo Inamori School of Engineering, New York State College of Ceramics, Alfred University, Alfred, N.Y. Yiquan Wu is an assistant professor of ceramics and materials sciences at Alfred University. Contact: wuy@alfred.edu Acknowledgment We gratefully appreciate the Air Force Office of Scientific Research support and funding for this research (contract FA9550-10-1-0067). References 1A. Ikesue and Y.L. Aung, “Ceramic Laser Materials,” Nature Photonics, 2, 721–27 (2008). 2A. Krell, J. Klimke, and T. Hutzler, “Transparent Compact Ceramics: Inherent Physical Issues,” Opt. Mater., 31, 1144–50 (2009). 3P. Hogan, T. Stefanik, C. Willingham, and R. Gentilman, “Transparent Yttria for IR Windows and Domes—Past and Present”; presented at the 10th DoD Electromagnetic Windows Symposium, Norfolk, Va., May 19, 2004. 4J. Sanghera, S. Bayya, G. Villalobos, W. Kim, J. Frantz, B. Shaw, B. Sadowski, R. Miklos, C. Baker, M. Hunt, I. Aggarwal, F. Kung, D. Reicher, S. Peplinski, A. Ogloza, P. Langston, C. Lamar, P. Varmette, M. Dubinskiy, and L. DeSandre, “Transparent Ceramics for High-Energy Laser Systems,” Opt. Mater., 33, 511–18 (2011). 5J. Sanghera, W. Kim, G. Villalobos, B, Shaw, C. Baker, J. Frantz, B. Sadowski, and I. Aggarwal, “Ceramic Laser Materials,” Materials, 5, 258–77 (2012). 6J.C. Huie, C.B. Dudding, and J. McCloy, “Polycrystalline Yttrium Aluminum Garnet (YAG) for IR Transparent Missile Domes and Windows,” Proc. SPIE, 6545, 65450E (2007). 7M. Allix, S. Alahrache, F. Fayon, M. Suchomel, F. Porcher, T. Cardinal, and G. Matzen, “Highly Transparent BaAl4O7 Polycrystalline Ceramic Obtained by Full Crystallization from Glass,” Adv. Mater., 24, 5570–75 (2012). 8D.C. Harris, “Development of Hot-Pressed and Chemical-Vapor-Deposited Zinc Sulfide and Zinc Selenide in the United States for Optical Windows,” Proc. SPIE, 6545, 654502 (2007). 9R.M. Sullivan, “A Historical View of AlON,” Proc. SPIE, 5786, 2332 (2005). 10R.L. Coble, "Transparent Alumina and Method of Preparation", US Patent 3,026,210, Mar. 20, 1962. n American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 37


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