Revisiting the Thermal Conductivity of Nanoporous Materials

Although the thermal conductivity of nanoporous materials has been investigated in the past, previous models have overestimated the small pore limit. Various authors had proposed a cylindrical boundary geometry to mimic the pore’s environment. This permits to solve the phonon Boltzmann equation analytically [1] or numerically [2], but for fixed porosity it leads to a saturation of the thermal conductivity at small pore diameters. We show that such saturation is a spurious effect of the cylindrical boundary approximation. By implementing a Monte Carlo calculation with correct boundary conditions, we obtain considerably different thermal conductivities than predicted by the cylindrical boundary geometry. The approach is illustrated in the case of Si and SiGe nanoporous materials.