Phonon scattering of interfacial strain field between dissimilar lattices

Abstract

Interfaces play a crucial role in controlling the thermal conductivity of a material, as evidenced by the major ongoing research in nanoscale precipitation engineering to improve the performance of thermoelectric materials. To understand their influence on phonon scattering, and thus thermal conductivity, we explored the effect of interfacial strain fields (ISFs) on interfaces with a different misfit between the two dissimilar lattices comprising the interface. We found that phonon scattering depends strongly on the frequency of the incident phonon wave; phonons with frequencies below a critical value pass transparently through the strain zone of the interfaces. Our calculations suggest that the classical acoustic mismatch model (AMM) and the diffusive mismatch model (DMM) represent the two extreme limits of phonon scattering, and that, in reality, scattering lies between these limits. For long-wavelength (low-frequency) phonons, the AMM adequately describes the thermal conductivity of the interfaces. As the frequency of the incident phonons increases, phonon scattering becomes progressively more diffuse to the point where the DMM represents the dominant mechanism for thermal conductivity. Furthermore, we noted that phonon scattering from the interfacial strain field can lower thermal conductivity effectively at high temperatures. We discuss the implications of our findings in explaining the experimental observations of thermal conductivity across precipitates and grain boundaries in thermoelectric materials.