Oxidation Behavior and Integration into High Power Density Thermoelectric Generators of Commercial Half-Heusler...

Half-Heusler (HH) alloys are among the best high-temperature thermoelectric (TE) materials able to operate above 700 °C. Here, we report on a detailed experimental study of the oxidation behavior of commercial n- and p-type HH alloys, Hf_0.6Zr_0.4NiSn_0.98Sb_0.02 and Hf_0.2Zr_0.75Ti_0.05CoSb_0.8Sn_0.2, respectively, and of their integration into high-power-density TE generators (TEGs). Oxidation behaviors were investigated from room temperature to 800 °C in air on consolidated small pieces and large pellets using thermogravimetric/differential scanning calorimetry (TG/DSC). Combined with scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDXS) and powder X-ray diffraction (PXRD), these experiments enabled the identification of the oxidation products and the determination of the oxidation pathway followed by each alloy. n-type HH initially slowly oxidizes upon heating to 350 °C before rapidly oxidizing above this temperature, giving rise to a significant mass loss in the form of gaseous Sb₄O₆. The inward diffusion of oxygen and outward diffusion of Sb led to the formation of an inner core separated from an outer layer by an empty space. In spite of the higher amount of Sb in the p-type alloy, its oxidation behavior differs, with no significant mass loss observed. In contrast, Sb reacts with Co to form oxides near the surface. A two-couple TEG was fabricated with leg height L = 2.0 mm. A maximum output power density p_max of 7.5 W cm^-2 was achieved under a temperature difference ΔT of 575 K. Of particular relevance, reducing L does not necessitate the insertion of mechanical buffers due to the excellent mechanical strength of these alloys. Using shortened legs allows for an increase in p_max at lower ΔT, thereby reaching a temperature range over which these alloys could remain stable against oxidation if further passivation optimization could be realized.