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(a) (b) (c) (d) Figure 7. Example transparent polycrystalline ceramics components: (a) γ-AlON panel for armor laminate; (b) magnesium-spinel lens for sensor pod systems; (c) γ-AlON reconnaissance window for aircraft; (d) γ-AlON hyper-hemispherical dome for IR countermeasure systems. hardness that makes γ-AlON good for ballistic protection also provides field durability for optical components. • Laser igniter windows At present, high-current electrical pulses ignite the propellant of small- and medium-caliber cannons, but the technology is fraught with problems, such as premature ignition from stray electromagnetic fields and hazardous compositions. Laser ignition may solve some of these issues, and initial tests show that γ-AlON maintains optical and mechanical stability during the high pressures and temperatures experienced with multiple firings. Beyond defense: Nonmilitary applications Just as defense applications exploit the optical and mechanical properties of γ-AlON and magnesium-spinel, these materials also could solve many nondefense and industrial-materialsrelated problems. For example, there are energy-related applications for oil and gas drilling, phosphors, LED technology, solid-state lasers, and lamp envelopes. In the medical arena, they can be used for prostheses, scintillator hosts, and medical equipment sensors in the mid-IR transmission range. Acknowledgement Sreeram Balasubramanian contributed to this article. (Credit: Surmet.) About the authors Mohan Ramisetty, Uday Kashalikar, Lee Goldman, and Nagendra Nag are in Surmet's advanced materials R&D group. Suri Sastri is founder, CEO, and chairman. Contact: Mohan Ramisetty, mramisetty@surmet.com. References 1K.A. Wickersheim and R.A. Lefever, “Optical Properties of the Synthetic Spinel,” J. Opt. Soc. Am., 50, 831–32 (1960). 2G.R. Rigby, G.H.B. Lovell, and A.T. Green, “Some Properties of the Spinels Associated with Chrome Ores,” Br. Ceram. Soc. Trans., 45, 137–48 (1946). 3G. Yamaguchi and H. Yanagida, “Study on the Reductive Spinel—A New Spinel Formula AlN–Al2O3 Instead of the Previous One Al3O4,” Bull. Chem. Soc. Jpn., 32, 1264–65 (1959). 4J.A. Salem, “Transparent Armor Ceramics as Spacecraft Windows,” J. Am. Ceram. Soc., 96 1 281–89 (2013). 5D.C. Harris, “Durable 3–5 mm Transmitting Infrared Window Materials,” Infrared Phys. Technol., 39, 185–201 (1998). 6P.J. Patel, G.A. Gilde, P.G. Dehmer, and J.W. McCauley, “Transparent Armor,” The AMPTIAC Newsletter, 4 3, Fall (2000). 7“Army Materials Research: Transforming Land Combat though New Technologies,” AMPTIAC Quarterly, 8 Nov 4 (2004). 8E. Strassburger, “Ballistic Testing of Transparent Armor Ceramics,” J. Eur. Ceram. Soc., 29, 267–73 (2009). 9K.R. Bray, R.L.C. Wu, S. Fries-Carr, and J. Weimer, “Aluminum Oxynitride Dielectrics for Multilayer Capacitors with Higher Energy Density and Wide Temperature Properties,” Thin Solid Films, 518, 366–71 (2009). 10F. Zhang, L. An, X. Liu, G. Zhou, X. Yuan, and S. Wang, “Upconversion Luminescence in γ-AlON:Yb3+,Tm3+ Ceramic Phosphors,” J. Am. Ceram. Soc., 92 8 1888–90 (2009). 11C.-F. Chen, F.P. Doty, R.J.T. Houk, R.O. Loutfy, H.M. Volz, and Pin Yang, “Characterizations of a Hot-Pressed Polycrystalline Spinel:Ce Scintillator,” J. Am. Ceram. Soc., 93 8 2399–402 (2010). n Manufacturing transparent ceramics and components at Surmet Surmet was founded in 1982 and entered the advanced ceramics business in 2002 when it licensed and subsequently bought the γ-AlON technology from Raytheon Co. Here are a few of the company’s accomplishments since its founding: • Powder synthesis. Powder synthesis requires high-temperature furnaces capable of reaching close to 2,000ºC, with atmosphere control and uniform temperature control. Careful blending, crushing, and milling processes all play a role in powder preparation. Over the past 10 years, Surmet has developed processes for manufacturing γ-AlON powders in tonnage quantities. • Fabrication. Parts larger than about 4 inches by 4 inches are susceptible to fracturing during forming and densification because of considerable shrinkage during sintering. Through careful control of processing protocols, Surmet fabricates parts with areas of several square feet. • Components. Highlight achievements for Surmet components for military applications include – Qualified and FAA certified transparent γ-AlON-based armor windows installed in production commercial armored aircraft and helicopter systems; – γ-AlON windows measuring approximately 14 inches by 25 inches for reconnaissance pods; and – γ-AlON GRIN lenses with the required gradients (in development with DARPA support). Further work will increase magnitude and size of gradients using materials and processes compatible with large-volume manufacturing. n (Credit: Surmet.) Lee Goldman in front of AlON heat treating furnaces. American Ceramic Society Bulletin, Vol. 92, No. 2 | www.ceramics.org 25


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