Lattice and radiative thermal conductivity variations through high p, T polymorphic structure transitions and melting points

Abstract

A steady-state radial heat flux method is used to determine the apparent, lattice and radiative, thermal conductivity and its p, T-dependence up to 6 GPa and over a wide temperature range from 300 to 1600 K. The method employs a differential thermocouple to resolve small changes in temperature gradient due to a line source placed in a sample space subjected to well-defined uniform test temperatures. Measurements are made using an on-line computer. The method is shown to be eminently suitable for determining: (1) the p, T-dependence of the phonon conductivity of cubic single crystals and polycrystalline samples; (2) minima in the apparent thermal conductivity marking the onset of radiative contributions; (3) isolation of phonon and radiative components at high T; (4) conductivity variations caused by progressive polymorphic structure transformations; and (5) conductivity variations through high-pressure melting points into the liquid phase. Results for cubic structures such as MgO and NaCl give good agreement with existing standard values at low temperatures. The conductivity of MgO goes with the inverse of the temperature which is expected from 3-phonon processes. The conductivity of NaCl is of the form λ αT −1.32 with the deviation most likely due to thermal expansion effects. At higher temperatures, a radiative contribution was observed in NaCl and CaCO 3. Calculated values of the extinction coefficient of NaCl increase slightly with pressure.