G. Snyder1
Northwestern University1
Thermal transport in complex semiconductors is often analyzed using the phonon gas model including several scattering terms [1]. Experimentally it is hard to uniquely determine which scattering mechanisms are truly dominating. Combining experiment and theory can elucidate the contributing effects that can help engineer thermal conductivity in complex materials. For example, dislocation scattering now appears to be an important mechanism in thermoelectric materials and may even have been overlooked as the primary mechanism for scattering of grain boundaries as evidenced by the temperature dependence of thermal conductivity at low temperature being closer to <i>T2 </i>experimentally rather than the <i>T3</i> traditionally expected for boundary scattering. In addition to scattering, phonon soften can also be important to achieve very low thermal conductivity. Ultra-low thermal conductivity in solids appears to be dominated by diffusions rather than wave like phonons which suggests a better understanding of diffusion heat transport is needed to further engineer low thermal conductivity materials. A reexamination of the traditional phonon gas model with the rigid distinction between harmonic and anharmonic contributions may also be needed as there appears to be a harmonic contribution to thermal expansion which is often assumed to only arise from anharmonic terms that appears to be connected to gas-like phonon pressure. Strategies to find materials with low phonon speed and high scattering rates can be used to engineer good thermoelectrics but also thermal barrier coatings and high power electronics.<br/><br/>[1] Hanus et al, Applied Physics Reviews 8, 031311 (2021); https://doi.org/10.1063/5.0055593