Synthesis and characterization of Al2−x Sc x (WO4)3 ceramics for low-expansion infrared-transmitting windows

A low thermal-expansion material was synthesized with potential application in thermal-shock-resistant infrared-transmitting windows. The material is derived from a solid solution of Al2(WO4)3, which has positive thermal expansion, and Sc2(WO4)3 with a negative thermal expansion. An optimum composition of Al0.5Sc1.5(WO4)3 was identified by synthesizing solid solutions, Al2−x Sc x (WO4)3, by a solid-state route with compositions ranging from x = 0 to 2.0. A single orthorhombic phase was obtained at all compositions. A composition corresponding to x = 1.5 had a low coefficient of thermal expansion of −0.15 × 10−6/°C in the temperature range 25–700 °C. A low temperature solution combustion process was developed for this optimum composition, resulting in a single-phase powder with a surface area of ~14 m2/g and average particle size (as determined from surface area) of 92 nm. The powder was consolidated by slip-casting, sintering, and hot-isostatic pressing into visibly translucent disks with a peak in-line transmittance of 73 % at 2300 cm−1. Significant infrared absorption in a 1-mm-thick disk of this material begins near 2200 cm−1 and features three absorptions arising from 2-phonon transitions at 2002, 1847, and 1676 cm−1. The infrared and Raman spectra are interpreted in terms of 1-, 2-, and 3-phonon vibrational transitions.