Enhanced Phonon Boundary Scattering at High Temperatures in Hierarchically Disordered Nanostructures

Boundary scattering in hierarchically disordered nanomaterials is an effective way to reduce the thermal conductivity of thermoelectric materials and increase their performance. In this work, we investigate thermal transport in silicon-based nanostructured materials in the presence of nanocrystallinity and nanopores at the range of 300-900K using a Monte Carlo simulation approach. The thermal conductivity in the presence of nanocrystallinity follows the same reduction trend as in the pristine material. We show, however, that the relative reduction is stronger with temperature in the presence of nanocrystallinity, a consequence of the wavevector-dependent (q-dependent) nature of phonon scattering on the domain boundaries. In particular, as the temperature is raised, the proportion of large wavevector phonons increases. Since these phonons are more susceptible to boundary scattering, we show that this q-dependent surface scattering could account for as much as a approximate to 40% reduction in the thermal conductivity of nanocrystalline Si. The introduction of nanopores with randomized positions magnifies this effect, which suggests that hierarchical nanostructuring is actually more effective at high temperatures than previously thought.