A 50:50 vol% MgO-Y 2 O 3 nanocomposite with~150 nm grain size was prepared in an attempt to make 3-5 lm infraredtransmitting windows with increased durability and thermal shock resistance. Flexure strength of the composite at 21°C is 679 MPa for 0.88 cm 2 under load. Hardness is consistent with that of the constituents with similar grain size. For 3-mm-thick material at 4.85 lm, the total scatter loss is 1.5%, forward scatter is 0.2%, and absorptance is 1.8%. Optical scatter below 2 lm is 100%. Variable intensity OH absorption (~6% absorptance) is observed near 3 lm. The refractive index is 0.4% below the volume-fraction-weighted average of those of the constituents. Thermal expansion is equal to the volumefraction-weighted average of expansion of the constituents. Specific heat capacity is equal to the mass-fraction-weighted average of heat capacities of the constituents. Thermal conductivity lies between those of the constituents up to 1200 K. Elastic constants lie between those of the constituents. The Hasselman mild thermal shock resistance parameter for the composite is twice as great as that of common 3-5 lm window materials, but half as great as that of c-plane sapphire.

Infrared absorption of high-quality, commercial, polycrystalline MgAl 2 O 4 spinel is ∼40% greater in the range of 3.8 to 5.0 μm than the value predicted by the computer code OPTIMATR®, which has been used for window and dome design for more than 20 years. As a result, spinel and a-plane sapphire windows designed to support the same external pressure with the same probability of survival have approximately the same infrared absorptance in the range 3.8 to 5.0 μm. c-Plane sapphire has greater absorptance than spinel in the range 3.8 to 5.0 μm. Spinel has two weak absorption bands near 1.8 and 3.0 μm. At 1.064 μm, the laser calorimetric absorption coefficient of spinel is 10 to 50 times greater than that of sapphire. New measurements of specific heat capacity, thermal expansion, thermal conductivity, elastic constants, and refractive index (including dn∕dT ) of spinel are reported. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0 Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI. Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms Optical Engineering 087113-2 August 2013/Vol. 52(8) Harris et al.: Optical and thermal properties of spinel with revised (increased) absorption. . . Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 05/26/2015 Terms of Use: http://spiedl.org/terms