Reduced anharmonic phonon scattering cross-section slows the decrease of thermal conductivity with temperature

Without anharmonic phonon scattering, crystals could have infinite or super intrinsic bulk thermal conductivity. Anharmonic phonon scattering rates usually increase with temperature, causing the decrease of thermal conductivity. In this work, we show that the increase of scattering rates is due to the increase of phonon population, while the anharmonic force constant, or phonon scattering cross-section, actually decreases with temperature in general solids. The decrease of scattering cross-section compensates the increase of population, slowing down the power-law increase of scattering rates and decrease of thermal conductivity. In other words, the use of temperature-dependent scattering crosssection increases the thermal conductivity relative to the 0 K ground-state scattering cross-section. We have demonstrated this effect by both first principles (for UO2, Si, Ge, and w-BAs) and classical potentials (for general solids). As an example, after considering the temperature dependency of anharmonic force constants, we have predicted well the thermal conductivity for UO2, which has long puzzled theoretical scientists, from room temperature up to 2000 K (available range of experimental data). The temperature-dependent scattering cross-section results in a more than 15 times increase of thermal conductivity at 2000 K compared to that predicted by using ground-state scattering cross-section. Four phonon scattering becomes prominent at ultra-high temperature (> 1500 K). This work is expected to inspire a broad study of thermal transport in various materials for both fundamental understanding and cutting-edge technologies. (c) 2022 Elsevier Ltd. All rights reserved.

Automated summary

This study reveals that anharmonic phonon scattering rates increase with temperature, while the anharmonic force constant or phonon scattering cross-section actually decreases in general solids. The decrease in scattering cross-section compensates for the increase in phonon population, resulting in a slower increase of scattering rates and decrease of thermal conductivity.