When 10:00 AM - 12:00 PM May 12, 2020
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PHD defense: Novel Composite Strategy to Optimize Thermoelectric Performance

Ruiming Lu (Poudeu Group)

Thermoelectric (TE) technology is a promising strategy to recycle the waste heat. However, the major challenge of commercializing such technology is the poor performance of TE materials. The coupling relations limit the possibility to simultaneously optimize multiple parameters. It is therefore urging to develop novel strategies to improve TE performance.

This thesis is dedicated to exploring novel composite strategy for TE properties regulation. The advantage of making composite materials is the potentially high degree of freedom for properties tailoring. Instead of engineering single-phase material, composite strategy considers more aspects, for example, the choices of constituent components, the microstructures and interfaces to be developed. With different combinations of components and varied microstructures, various interactions are expected. Different from traditional method, novel composite strategy in this research focuses on developing highly interactive interfaces via the in-situ formation of secondary phases, which are chemically close relatives to the matrix materials. For example, full Heusler precipitates in half Heulser matrix and chalcopyrite compounds in Cu2Se matrix. For half Heulser/ full Heusler composites, two phases are co- formed from the same elements through a solid-state reaction at high temperature. In contrast, the chalcopyrite/ Cu2Se composites are constructed by co- forming both phases from the common precursor, CuSe2, through solid-state reactions. Due to the special consideration in selecting constituent components and unique route to construct the composite materials, various interesting microstructures have been developed, leading to very different interactions between precipitates and the matrix, and thus varied effects on TE transport properties. For example, by introducing magnetic full Heusler precipitates in half Heusler matrix, an even stronger modulation in electronic transport has been obtained, which is not observed in heavily doped half Heusler alloys with regular full Heulser precipitates. A two-step synthesis route enables the precise control over the ratio of room-temperature Cu2Se to high-temperature Cu2Se by simply adjusting the amount of CuGaSe2. CuFeSe2 has temperature-dependent solubility in Cu2Se, which leads to the unique dendrite structure and further temperature-dependent doping effect in the composite materials. In contrast, CuAlSe2 tends to agglomerate in Cu2Se and extracts Cu ions from the matrix. Above all, we demonstrated the capability of composite strategy to achieve various modulations in TE properties, which may pave the path to future success of TE technology.