UC Riverside

Thermoelectric materials have gained a considerable amount of attention as a practical power source for a wide range of applications including space missions as well as heat scavenging systems in the automobile industry. Though significant research has been performed investigating improved thermoelectric properties as a result of nano-structuring, most of these studies have been focused on efficiency improvements rather than systematic investigations on the effect of microstructure on properties. Three investigations are presented that use pure Silicon and Silicon-Germanium materials to investigate the effects of grain size, porosity, phase boundaries, and grain size distribution on the thermal conductivity in nano-structured materials. These studies provide a deeper fundamental understanding of the effects of microstructure on the thermal conductivity of these materials. It is shown that by decreasing the grain size of pure Silicon to 76nm it is possible to decrease the thermal conductivity by an order of magnitude. The thermal conductivity is further decreased by 74% when the density of the material is decreased to 86%. A study showing the effects of planetary ball milling and Current Activated Pressure Assisted Densification on the homogeneity and microstructure of heterogeneous Silicon-Germanium alloys are shown, as well as preliminary data showing the effects of heterogeneity on the thermal conductivity of these alloys. Finally, the effects of varying the grain size distribution in pure fully dense Silicon materials is shown along with a rule of mixtures based model for bimodal grain size distribution in pure Silicon. The model is shown to accurately estimate the measured thermal conductivity values within 6%.