  Nanoporous materials are of broad interest for various applications, in
particular advanced thermoelectric materials. The introduction of nanoscale
porosity, even at modest levels, has been known to drastically reduce a
materials thermal conductivity, in some cases even below its amorphous limit,
thereby significantly increasing its thermoelectric figure of merit, ZT. The
details of the important attributes that drive these large reductions, however,
are not yet clear. In this work, we employ large-scale equilibrium molecular
dynamics to perform an exhaustive atomistic-scale investigation of the effect
of porosity on thermal transport in nanoporous bulk silicon. Thermal transport
is computed for over 50 different geometries, spanning a large number of
geometrical degrees of freedom, such as cylindrical pores and voids, different
porosities, diameters, neck sizes, pore/void numbers, and surface-to-volume
ratios, placed in ordered fashion, or fully disordered. We thus quantify and
compare the most important parameters that determine the thermal conductivity
reductions in nanoporous materials. Ultimately, we find that, even at the
nanoscale, the effect of merely reducing the line-of-sight of phonons, i.e. the
clear pathways that phonons can utilize during transport, plays the most
crucial role in reducing the thermal conductivity in nanoporous materials,
beyond other metrics such as porosity and surface/boundary scattering.
