Transparent Yttria for IR Windows and Domes - Past and Present

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 yttria’s thermomechanical properties are adequate for a number IR window and dome applications, but only marginal for the most demanding missions. The first fullscale missile domes of transparent yttria manufactured from ceramic powders were developed in the 1980’s under Navy funding. Raytheon perfected and characterized its undoped polycrystalline yttria, while lanthana-doped yttria was similarly developed by GTE Laboratories. The two versions have comparable infrared transmittance, mechanical properties, and thermal expansion, while the undoped material exhibited 2 times higher thermal conductivity. Although conventional yttria’s 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 windtunnel 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 (5 µm) 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. Samples were sintered to a closed pore state at temperatures as low as 1400°C. 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 window/dome materials. The status of on-going Navy-sponsored development of nano-grain yttria is also presented.