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

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