  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.
