Assessment of low-expansion tungstates for thermal-shock-resistant infrared windows

Abstract

Abstract: Low-thermal-expansion tungstate materials have the potential to be used as thermal-shock-resistant midwave (3-5 µm) infrared windows. Material properties that favor thermal shock resistance are high strength, high thermal conductivity, low elastic modulus, and low thermal expansion. Sapphire, for example, owes its high thermal shock resistance to high strength and high thermal conductivit y. In principle, it is possible to obtain even higher thermal shock resistance if a window materi al with near-zero thermal expansion can be made. This paper assesses recent work on Zr(WO 4 ) 2 and Al 0.5 Sc 1.5 (WO 4 ) 3 . It is concluded that multi-phonon absorption in the midwave spectral region limits the optical capabilities of tungstate materials. These materials have more absorption—and therefore, more emission—than aluminum oxynitride in the 4-5 µm wavelength region. Thermal Shock Resistance For applications including missile domes and industrial process monitoring, an infrared-transparent window material could be heated so rapidly that significant, transient temperature differences develop in the materi al. Different thermal expansion between the warmer and cooler regions can lead to mechanical stress that causes the brittle material to shatter. Resistance to failure from thermal shock for “mild” heati ng rates is favored by the following factors: x High strength permits a material to survive high stress without failure x Low Young’s modulus (ie., low stiffness) creates litt le stress even from high strain x High thermal conductivity does not allow large temperature differences to exist x Low thermal expansion —in the absence of expansion, there is no thermal stress These factors are contained in the Hasselman therma l shock figure of merit, R', for mild heating: R' = S (1- Q) kD E . (1) where the material properties are S = tensile strength, = Poisson’s ratio, = thermal kconductivity, . = thermal expansion coefficient, and E = Young’s modulus. The greater the thermal shock figure of merit, the greater the rate of heat transfer that can be tolerated without mechanical failure. _______________________________