Thermal properties of single crystals of the low-positive thermal expansion material Al2W3O12

Abstract

Investigations of the thermal conductivity and heat capacity of a monocrystalline sample Al2W3O12, along with computational investigations using density functional theory, reveal a complete picture of anisotropic thermoelastic properties of Al2W3O12, a low-positive thermal expansion material. The thermal conductivity at room temperature was essentially isotropic, with a directionally average value of 2.1 ± 0.2 W m−1 K−1, about twice the value reported for polycrystalline samples. The thermal conductivity shows a significant peak at about 20 K, and at 5 K corresponds to a phonon mean free path of about 2000 nm, much shorter than that of a typical high-quality oxide crystal with a more rigid framework. The flexible Al2W3O12 structure also gives rise to low-frequency modes, as evidenced by the high heat capacity in comparison with that of the constituent oxides. The thermal shock resistance figures of merit for moderate and severe thermal shock are both anisotropic, but high, with directionally average values of about 6500 W m−1 and 3300 K, respectively. An additional finding was that the phase transformation from the low-temperature monoclinic phase to the room-temperature orthorhombic phase shows considerable hysteresis and, evidenced by DSC on shards of single crystals, nucleation is a stochastic process.