  In nanostructures phonon transport behaviour is distinctly different to
transport in bulk materials such that materials with ultra low thermal
conductivities and enhanced thermoelectric performance can be realized. Low
thermal conductivities have been achieved in nanocrystalline materials that
include hierarchical sizes of inclusions and pores. Nanoporous structures
present a promising set of material properties and structures which allow for
ultra-low thermal conductivity, even below the amorphous limit. In this paper
we outline a semiclassical Monte Carlo code for the study of phonon transport
and present an investigation of the thermal conductivity in nanoporous and
nanocrystalline silicon. Different disordered geometry configurations are
incorporated to investigate the effects of pores and grain boundaries on the
phonon flux and the thermal conductivity, including the effects of boundary
roughness, pore position and pore diameter. At constant porosity, thermal
conductivity reduction is maximized by having a large number of smaller
diameter pores as compared to a small number of larger diameter pores.
Furthermore, we show that porosity has a greater impact on thermal conductivity
than the degree of boundary roughness. Our simulator is validated across
multiple simulation and experimental works for both pristine silicon channels
and nanoporous structures.
