Novel Composite Strategy to Optimize Thermoelectric Performance

Abstract

Thermoelectric (TE) technology is a promising strategy to recycle the waste heat. However, the major challenge of commercializing this technology is the poor performance of TE materials. Previously, there have been reported multiple strategies to optimize thermoelectric transport, such as doping, substitutional alloying and nanostructuring. So far, the best TE performance is mainly achieved by low thermal conductivity, either using materials with intrinsically low thermal conductivities or suppressing thermal transport by strategies like nanostructuring. However, it’s difficult to achieve further improvement in TE performance by simply pushing the lower limit of thermal conductivity. High TE performance must rely on the optimization in electronic transport as well, which is also challenging due to the coupling relations between the transport parameters, for example, electrical conductivity and Seebeck coefficient coupled by carrier concentration. The coupling relations limit the possibility to simultaneously optimize multiple parameters. It is therefore urging to develop novel strategies that can break apart the coupling relations, such as modulation doping and band engineering. 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 materials, the composite strategy includes more aspects to be considered, for example, the choices of constituent components, the microstructures and interfaces to be developed. With different combinations of components and varied microstructures, various interactions will be expected. Nanocomposite strategy was originally applied to decrease thermal conductivity by introducing inert nano particles. The traditional method of constructing composite materials is to mechanically mix the pre-synthesized components, which results in interfaces lack of chemical bonding or interaction. Different from traditional method, novel composite strategy in this research focuses on developing highly interactive interfaces via the in-situ formations of secondary phases, which are chemically close relatives to the matrix materials. For example, full Heusler precipitates in half Heusler matrix and chalcopyrite compounds in Cu2Se matrix. For half Heusler/ 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 a common precursor, CuSe2 through solid-state reactions. Due to the special consideration in selecting constituent components and unique routes to construct the composite materials, various interesting microstructures have been developed, leading to very different interactions between precipitates and the matrix, and thus various 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 was not observed in heavily doped half Heusler alloys with regular full Heusler 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 CuGaSe2 content. CuFeSe2 has a 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 demonstrate the capability of such composite strategy to achieve various modulations in TE properties, which can pave the path to future success of TE technology.