Thermal diffusivity and thermal conductivity of single-crystal MgO and Al₂O₃and related compounds as a function of temperature

Thermal diffusivity (D) was measured up to ~1,800 K of refractory materials using laser-flash analysis, which lacks radiative transfer gains and contact losses. The focus is on single-crystal MgO and Al₂O₃. These data are needed to benchmark theoretical models and thereby improve understanding of deep mantle processes. Measurements of AlN, Mg(OH)₂, and isostructural BeO show that the power law (D = AT^-B) where T is temperature holds for simple structures. Results for more structurally complicated corundum Al₂O₃with and without impurity atoms are best fit by CT^d+ ET^fwhere d ~ -1 and f ~ -4, whereas for isostructural Fe₂O₃, f is near +3 and multiphase ilmenite Fe_1.12Ti_0.88O₃is fit by the above power law. The positive temperature response for hematite is attributed to diffusive radiative transfer arising from electronic-vibronic coupling. We find good agreement of k and D data on single-crystal and non-porous ceramic Al₂O₃. For the corundum structure, D is nearly independent of T at high T. Although D at 298 K depends strongly on chemical composition, at high temperature, these differences are reduced. Thermal conductivity provided for MgO and Al₂O₃, using LFA data and literature values of density and heat capacity, differs from contact measurements which include systematic errors. The effect of pressure is discussed, along with implications for the deepest mantle.