Diffusive Phonons in Nongray Nanostructures

TL;DR

This paper develops a hybrid modeling approach combining Fourier's law and Boltzmann transport equations to accurately predict phonon-mediated heat transfer in nanostructured thermoelectric materials, validated against experiments.

Contribution

It introduces a new boundary condition for diffusive phonons and a hybrid modeling method that improves accuracy in complex nanostructures.

Findings

Derived the short MFP limit of the suppression function, challenging previous assumptions.

Extended Fourier's law with Robin boundary conditions for diffuse surfaces.

Achieved excellent agreement between the hybrid model and experimental data.

Abstract

Nanostructured semiconducting materials are promising candidates for thermoelectrics due to their potential to suppress phonon transport while preserving electrical properties. Modeling phonon-boundary scattering in complex geometries is crucial for predicting materials with high conversion efficiency. However, the simultaneous presence of ballistic and diffusive phonons challenges the development of models that are both accurate and computationally tractable. Using the recently developed first-principles Boltzmann transport equation (BTE) approach, we investigate diffusive phonons in nanomaterials with wide mean-free-path (MFP) distributions. First, we derive the short MFP limit of the suppression function, showing that it does not necessarily recover the value predicted by standard diffusive transport, challenging previous assumptions. Second, we identify a Robin type boundary condition describing diffuse surfaces within Fourier's law, extending the validity of diffusive heat transport in terms of Knudsen numbers. Finally, we use this result to develop a hybrid Fourier/BTE approach to model realistic materials, obtaining excellent agreement with experiments. These results provide insight on thermal transport in materials that are within experimental reach and open opportunities for large-scale screening of nanostructured thermoelectric materials.