Thermal conductivity of simple and tubular nanowire composites in the longitudinal direction

This work establishes a generic model to study phonon transport and the thermal conductivity of periodic two-dimensional nanocomposites in the longitudinal direction (along the wire axis direction). More specifically, the generic model is applied to study the thermal conductivity of silicon-germanium composites with simple silicon nanowire and tubular silicon nanowire inclusions in a germanium matrix, and cylindrical nanoporous silicon materials. The results show that the effective thermal conductivity changes not only with the volumetric fraction of the constituents but also with the radius of the nanowires and cylindrical pores due to the nature of the ballistic phonon transport. The smaller the wire/pore diameter, the smaller is the thermal conductivity of the periodic two-dimensional nanocomposites for a given volumetric fraction. Composites with tubular nanowire inclusions have a lower effective thermal conductivity than simple nanowire composites due to the introduction of additional surface scattering through the pores associated with tubular nanowires. Results of this study can be used to direct the development of both high-efficiency thermoelectric materials and thermal interface material containing high-thermal-conductivity particle or wire inclusions.