Enhanced Phonon Boundary Scattering at High Temperatures in Hierarchically Disordered Nanostructures

TL;DR

This study demonstrates that hierarchical nanostructuring, including nanocrystallinity and nanopores, significantly enhances phonon boundary scattering at high temperatures, leading to greater reductions in thermal conductivity in silicon nanomaterials.

Contribution

It reveals the temperature-dependent nature of phonon boundary scattering in nanostructured silicon, highlighting the increased effectiveness of hierarchical nanostructuring at elevated temperatures.

Findings

Boundary scattering reduces thermal conductivity more at higher temperatures.

Nanocrystallinity causes a stronger reduction in thermal conductivity with temperature.

Introducing nanopores further enhances boundary scattering effects.

Abstract

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 K to 900 K 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 compared to the wavevector independent thermal conductivity value this q dependent surface scattering could account for even a approx. 40 per cent reduction in the thermal conductivity of nanocrystalline Si. Introduction of nanopores with randomized positions enhances this effect, which suggests that hierarchical nanostructuring is actually more effective at high temperatures than previously thought.