Long-term thermal stability

Thermal stability of thermoelectric (TE) materials for long-term range is a requisite issue for the large scale fabrications and applications. We investigate the long-term stability of n-type as well as the p-type half-Heusler materials based TE.

The effect of thermal cycling upon the thermoelectric performance of several half-Heusler materials is investigated and correlated with the impact on the structural properties.

Microstructural investigations of the p-type Ti0.5Hf0.5CoSb0.85Sn0.15 by SEM and EDX (as shown in Fig. 1a) prove that the as-cast sample undergo an intrinsic phase separation into two half-Heusler phases: a Ti-rich and Hf-rich compositions.

Characteristic microstructure is already obtained in the as cast samples and is not effected by the consequent annealing steps (Fig. 1b). The microstructure is stable even after 500 heating and cooling cycles (Fig. 1c). The as cast sample exhibits a higher thermal conductivity and decrease slightly after annealing (Fig. 1d), but the values stabilize and lie within the measurement error after annealing and further heat treatment leading to as lower values. The intrinsic phase separation, which is responsible for the outstanding thermoelectric properties, is stable upon the repeated heating and cooling. The zT as well is stable upon thermal cycling (Fig. 1ed) and reaches the same values of  1.2 with progressing heat treatment.

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