Thermoelectric properties and high-temperature stability of the Ti1−xVxCoSb1−xSnx half-Heusler alloys

Abstract

Half-Heuslers (HHs) are attracting widespread interest for thermoelectric waste heat recovery. This manuscript extends the known TiCoSb based HH compositions and provides new insight into their high-temperature stability. X-ray powder diffraction revealed an upper solubility limit near x = 0.4 for the Ti_1−xV_xCoSb_1−xSn_x series. Rietveld analysis of neutron powder diffraction data indicated that TiCoSb is stoichiometric, and confirmed that V and Sn are successfully co-substituted. Scanning electron microscopy revealed small grain sizes (<5 μm) with up to 10–15% Ta along the boundaries due to reaction with the sample containment material. Repeated measurement of the electrical resistivity (ρ) and Seebeck coefficient (S) in a He atmosphere demonstrated that TiCoSb degrades rapidly at elevated temperatures. Scanning electron microscopy and X-ray diffraction reveal the formation of amorphous TiO₂ and crystalline CoSb, suggesting that the degradation is driven by exposure to trace amounts of oxygen. The unintentional incorporation of Ta leads to n-type doping and maximum power factors S²/ρ = 0.55 mW m⁻¹ K⁻² at 550 K and S²/ρ = 0.8 mW m⁻¹ K⁻² at 900 K were observed for TiCoSb and the x = 0.3 sample, respectively.