Transparent Yttria for IR Windows and Domes - Past and Present
RAYTHEON INTEGRATED DEFENSE SYSTEMS ANDOVER MA
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Yttria Y2O3 has excellent optical performance through the full mid wavelength infrared MWIR atmospheric transmission band at both ambient and elevated temperatures. Current state-of-the-art yttrias thermomechanical properties are adequate for a number of IR window and dome applications, but only marginal for the most demanding missions. Although conventional yttrias strength and hardness are lower than the more durable but less transmitting MWIR materials sapphire, ALON, spinel, its thermal shock performance is similar. In fact, 7 out of 7 flat yttria windows were successfully wind-tunnel tested under hypersonic conditions simulating representative surface-to-air interceptor missile flights. Recent renewed interest in yttria windows and domes has prompted efforts to enhance mechanical properties by producing materials with micron or nano-size grains. Three vendors were selected to provide nanoscale powders for testing and evaluation, and they were compared to a conventional yttria powder previously used to prepare transparent ceramic yttria. While all of the nanopowders evaluated had impurity levels that were too high to allow processing to full transparency, two were processed to full density and moderate transparency. Ultrasonic attenuation as a technique for measuring particle size distributions in slurries was explored and found to be an invaluable tool when processing colloidal suspensions of nanopowders. In this paper, the optical, thermal, and mechanical properties of conventional transparent yttria are reviewed and compared with other candidate MWIR windowdome materials. The status of on-going Navy-sponsored development of nano-grain yttria is also presented.