  This paper studies thermal transport in nanoporous silicon with a significant
specific surface area. First, the equilibrium molecular dynamics approach was
used to obtain the dependence of thermal conductivity on a specific surface
area. Then, a modified phonon transport kinetic theory-based approach was
developed to analyze thermal conductivity. Two models were used to evaluate the
phonon mean free path in the porous materials. The first model approximates the
dependence of the mean free path only with the specific surface area, and the
second one investigates the dependence of the mean free path variation with the
porosity in the peculiar case of a highly porous matrix. Both models
approximate molecular dynamics data well for the smaller porosity values, while
the first model fails for large porosities. The second model matches well with
molecular dynamics simulations for all considered ranges of the porosities.
This work illustrates that the phonon mean free path dependence with the
porosity/volume fraction of composite materials is essential for describing
thermal transport in systems with significant surface-to-volume fractions.
