Thermoelectric Properties of TiNiSn Half Heusler Alloy Obtained by Rapid Solidification and Sintering

Thermoelectric TiNiSn-based half Heusler compound reveals high Seebeck coefficient and electrical conductivity, leading to high power factor. As a consequence of the solidification path, TiNiSn single phase cannot be obtained directly from the liquid phase. Thus, a long annealing step is needed to homogenize the alloy. In this work, we present a new processing route for half Heusler compound formation, combining arc melting of pure elements, rapid solidification of the molten alloy and sintering. Rapid solidification of the molten alloy allows to obtain almost single TiNiSn phase, limiting the formation of the primary TiNi2Sn phase as a consequence of the deep undercooling of the liquid. The rapidly solidified alloy was ground to powder and sintered by open die pressing. As-sintered samples show a density around 95% of the theoretical value. Thermal cycling of the sintered samples shows evolution of the phases, suggesting that after sintering the sample is not in equilibrium yet. After the second thermal cycle thermoelectric properties become reproducible, indicating the attainment of the equilibrium. In conclusion, the proposed processing route allows to obtain dense TiNiSn in bulk form avoiding the time-consuming annealing step, typically used to homogenize this alloy after solidification.