High-Pressure Synthesis of Thermoelectric Materials

Citation

Staneff, Geoffrey D. (2005) High-Pressure Synthesis of Thermoelectric Materials. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HWWN-NS80. https://resolver.caltech.edu/CaltechETD:etd-05262005-004951

Abstract

New thermoelectric materials were synthesized using high-pressure techniques. The synthetic limits of cerium filling in cobalt triantimonide were explored and a model for successful synthesis conditions was generated. The high-pressure technique expanded the practical filling limit from x=0.07 to x=0.5 in the CexCo4Sb12 system. Explorations of cerium filled ruthenium-rhodium triantimonide were also undertaken, using our previous work to guide the synthesis attempt. This material was predicted to have outstanding thermoelectric properties, but our investigations did not confirm this result. The filled skutterudite, Ce(Ru0.67Rh0.33)4Sb12, exhibited the low thermal conductivity anticipated, approximately half the total thermal conductivity of a binary CoSb3 skutterudite over a wide range in temperatures. The electrical properties were insufficient to produce a high efficiency thermoelectric, but these results did suggest incorporated rare earth filling atoms do not fully ionize when incorporated into the skutterudite structure. Modification of the carrier type should therefore be investigated by solid solution on the transition metal or pnictide sites and not the filling ion.

The high-pressure synthesis techniques developed for skutterudite synthesis were then employed to sinter nano-scale silicon-germanium compounds. A sintering figure of merit was created to justify the use of high-pressure synthesis, which later proved to be a useful tool for planning subsequent experiments. Sintered silicon, germanium, and silicon-germanium composites were obtained, with the nano-scale grain structure and chemical heterogeneity of the starting powder aggregate intact.

Future work in high-pressure synthesis should be undertaken to quantify the sintering figure of merit presented in this work. The ability to produce a dense multiphase material with a controlled nano-structure should provide a great boon to thermoelectrics research. The high-pressure synthesis technique provides an method to mix two thermoelectric materials without homogenizing them. Both the two-phased solid and the network of nano-scale grains provide powerful tools for minimizing the thermal conductivity of a thermoelectric device component.

Thesis Files