Characterization of the thermally contracting tungstates Ta22W4O67, Ta2WO8, and Ta16W18O94

An investigation of the properties of high-purity (> 99 wt %) tantalum tungstates (Ta22W4O67, Ta2WO8, and Ta16W18O94) included determination of density (bulk and theoretical), refined lattice constants, maximum use temperatures, microhardness, heat capacity, thermal expansion (contraction) and diffusivity, calculated thermal conductivity, and electrical resistivity. Usable to approx. = 1700 K in air or inert atmospheres, these tantalum tungstates have theoretical densities of 7.3 to 8.5 g/cm3, are relatively soft (120 to 655 kg/mm2 hardnesses), and are electrical insulators (6 x 103 to 2 x 108 Ω.cm resistivities). The distinguishing properties of the materials are their thermal expansion (average CTE values from +0.6 x 10-6/K to -5.1 x 10-6/K at 293 to 1273 K), thermal expansion hysteresis with minimal observable microcracking, and thermal diffusivity (values from 1.6 x 10-3 to 3.2 x 10-3 cm2/s at 298 K) gradually increasing with temperature