Revisiting the Reduction of Thermal Conductivity in Nano- to Micro-Grained Bismuth Telluride: The Importance of Grain-Boundary Thermal Resistance

TL;DR

This paper reevaluates the factors influencing thermal conductivity reduction in nanograined bismuth telluride, emphasizing the significant role of grain-boundary thermal resistance over grain size alone.

Contribution

It introduces first-principles phonon MFP calculations and highlights the importance of grain-boundary resistance in explaining experimental thermal conductivity data.

Findings

Phonon MFPs explain experimental data across various Bi2Te3 nanostructures.

Grain-boundary thermal resistance significantly suppresses high-frequency phonon transport.

Micro-grained Bi2Te3 shows notable thermal conductivity reduction due to boundary effects.

Abstract

Nanograined bulk alloys based on bismuth telluride (Bi2Te3) are the dominant materials for room-temperature thermoelectric applications. In numerous studies, existing bulk phonon mean free path (MFP) spectra predicted by atomistic simulations suggest sub-100 nm grain sizes are necessary to reduce the lattice thermal conductivity by decreasing phonon MFPs. This is in contrast with available experimental data, where a remarkable thermal conductivity reduction is observed even for micro-grained Bi2Te3 samples. In this work, first-principles phonon MFPs along both the in-plane and cross-plane directions are re-computed for bulk Bi2Te3. These phonon MFPs can explain new and existing experimental data on flake-like Bi2Te3 nanostructures with various thicknesses. For polycrystalline Bi2Te3-based materials, a better explanation of the experimental data requires further consideration of the grain-boundary thermal resistance that can largely suppress the transport of high-frequency optical phonons.