  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.
