UC Riverside

Thermoelectric materials are unique in their ability to directly convert thermal energy into electrical. Various applications include: energy scavenging, spot cooling and heating, and interstellar power generation. Current research is geared towards improving thermoelectric materials for greater energy conversion efficiency. Mg2Si has been a good candidate for study due to its low toxicity, low density, and availability of constituent elements. Recent efforts to improve efficiency of bulk thermoelectric materials have included, but are not limited to, changing the electronic band structure through doping or altering the microstructure. While the former has been well studied and verified, the latter remains relatively untouched experimentally. There is evidence that thermoelectric materials with low dimensionality show improvement in certain properties. Thin films and nanowires are known to have lower thermal conductivities than their bulk counterparts. The goal of this work is to provide evidence of microstructural effects on the thermoelectric properties of bulk Mg2Si.

The three relevant properties for thermoelectric materials are thermal conductivity, electrical conductivity, and the Seebeck coefficient. Because commercial measurement systems are costly and have limited measurement ranges and accuracies, the development of custom equipment is sometimes necessary. An electrical AC current source is developed and used to measure the electrical conductivity and, through the 3ω method, thermal conductivity of bulk samples. The measured values are comparable, with a 3% agreement, to a separate well-established system.

The bulk samples are consolidated from powder using the Current Activated Pressure Assisted Densification (CAPAD) process. It is known that different processing temperatures alter the amount of grain growth during the densification process. Samples processed at higher temperatures show increasing thermal and electrical conductivities while the Seebeck coefficient decreases. These trends lead to a maximum figure of merit value with the sample processed at an intermediate temperature.