Development of Mg3Sb2−Mg3Bi2 Alloys as Thermoelectric Materials

Imasato, Kazuki

doi:https://doi.org/10.21985/n2-n6xm-e202

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Development of Mg3Sb2−Mg3Bi2 Alloys as Thermoelectric Materials

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Following the discovery of n-type Mg3Sb2−Mg3Bi2 alloys, attempts have been undertaken to optimize their thermoelectric performance because understanding of the properties of this system had been limited due to its novelty. In this thesis, the thermoelectric performance of n-type Mg3Sb2−Mg3Bi2 alloys was systematically investigated and significantly improved. Thorough experimental characterization of the effects of defects, composition,microstructure, and electronic band structure were complimented to establish foundational methodologies to optimize this material system. First, by controlling the Mg stoichiometry, the thermodynamic requirement to achieve n-type conduction was determined by the concept called phase boundary mapping. The key to get n-type material is that the material needs to be synthesized in the Mg-excess state to suppress the formation of Mg vacancies. Furthermore, by optimizing the amount of excess Mg, the thermoelectric performance has been improved 20% by minimizing the lattice thermal conductivity. As long as the materials are synthesized in Mg-excess condition, the carrier concentration can be adjusted by extrinsic dopants. While all the reported dopants are anion site substitution, La was discovered as an effective cation site dopant with higher doping efficiency than the anion alternatives. In addition to gaining an understanding of the nature of the atomic defects, the origin of the undesirable low electronic conductivity, which indicated a thermally activated process, was identified as a grain boundary effect. Mitigation of this effect by increasing grain size through various methods led to a multi-fold improvement in the thermoelectric performance. Finally, effects of alloying on Mg3Sb2 with various elements were investigated. The understanding of these microstructure-property relationships and band structure has led to the optimization of n-type Mg3Sb2−Mg3Bi2 alloys for different temperature ranges. With increased bismuth content, the combination of a reduced band effective mass and increased grain size resulted in the discovery that Mg3Sb0.6Bi1.4 has excellent thermoelectric properties, and is the first material in decades to compete against Bi2Te3 for applications near room temperature. These findings will be a significant step towards the realization of terrestrial application of thermoelectric materials and this series of work can be a great example to demonstrate the methodology of optimizing newly discovered thermoelectric materials.

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