Lattice Thermal Transport in Si-based Nanocomposites for Thermoelectric Applications

Silicon-germanium (SiGe) superlattices (SLs) have been studied for application as efficient thermoelectrics because of their low thermal conductivity, below that of bulk SiGe alloys. However, the cost of growing SLs is prohibitive, so Si-based nanocomposites, made by a ball-milling and sintering, have been proposed as a cost-effective replacement with similar properties. Because the lattice thermal conductivity of SiGe SLs is reduced by scattering from rough boundaries between layers, it is expected that grain boundary properties, for example roughness, orientation, and composition, will also substantially effect thermal transport in nanocomposites, resulting in many ways of adjusting their thermal conductivity by manipulation of grain size, shape, and crystal angle distributions. A model of phonon transport in nanocomposites was developed on the basis of the phonon Boltzmann transport equation. When nanocomposite structures were modeled by using a Voronoi tessellation to mimic the grains and their distribution, agreement with experimentally observed structures was excellent. To accurately treat phonon scattering from a series of atomically rough interfaces between the grains in the nanocomposite, we used a momentum-dependent specularity variable. Our results revealed thermal transport in Si-based nanocomposites is highly anisotropic and suggest further utilization of grain morphology to minimize thermal conductivity.