Synthesis and thermoelectric properties of Mg2Si-Mg2Sn solid solutions for waste heat recovery
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Mg2Si-Mg2Sn solid solutions are promising thermoelectric materials for vehicle waste-heat recovery owing to their high figure-of-merit (ZT), light weight, low cost, and non-toxicity. The optimized ZT of Mg2Si0.4Sn0.6 peaks at ∼ 750 K because its relatively narrow band gap results in the pronounced bipolar thermoelectric transport at higher temperatures. This bipolar transport can be eliminated by widening the band gap via Ge substitution. Sb-doped Mg2Si0.4Sn0.6−yGey (y=0, 0.1, 0.2) compositions have been synthesized and their thermoelectric properties have been measured. The measurement results suggest that the bipolar conduction was effectively suppressed in the Ge-substituted compositions. In addition, a triple-parabolic-band model has been established to extract of the lattice contribution from the total thermal conductivity. The calculation results indicate that Ge substitutions slightly reduce the lattice thermal conductivity of Mg2Si0.4Sn0.6. Mg vacancies can be introduced in Mg2SixSn1−x due to Mg evaporation and oxidation. To understand the role of Mg vacancies on the electronic structure and transport properties, Mg2Si0.4Sn0.6 with various Mg deficiency and Sb doping levels have been synthesized and their transport properties have been investigated. The results suggest that Mg vacancies create localized hole states inside the band gap instead of moving the Fermi level into the valence band as would be predicted by a rigid band model. This hypothesis is confirmed by density-functional theory calculations, which show that the hole states are trapped at Mg vacancies above the valence band. This localized-states model also provides a good interpretation of the electron-hopping transport behavior in Mg2−δSixSn1−x. The thermal stability issue of Mg2SixSn1−x needs to be addressed be fore the material can be applied in practical thermoelectric devices. It was found that the Mg2Si0.4Sn0.6 pellet could be oxidized at 823 K in inert gas atmosphere. The sample’s carrier concentration decreases significantly due to the Mg vacancies caused by sample oxidation. It has been demonstrated that the atomic-layer deposited Al2O3 coating can effectively protect Mg2Si0.4Sn0.6 from oxidation in inert gas and even in air. This Al2O3 thin-film coating also provides in situ protection to Mg2SixSn1−x during the laser flash measurement and therefore eliminates the measurement errors that occur to the unprotected samples as a result of sample oxidation and graphite exfoliation.