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

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 recomputed 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.