TiNiSn half-Heusler crystals grown from metallic flux for thermoelectric applications

Abstract

A thermoelectric converter (TEC), an array of thermocouples composed of p-type and n-type semiconducting elements, directly converting heat to electricity, is subjected to large temperature gradients in practical operation conditions. To prevent mechanical fractures in the thermocouples, the coefficient of thermal expansion (CTE) between the p-type and n-type elements must be matched reasonably well. The current research paves a route for obtaining a TEC based on polycrystalline n-type and single-crystal based p-type half Heusler (HH) TiNiSn thermoelectric elements with practically the same CTEs. For this purpose, HH TiNiSn crystals were grown using the metal flux method, and their crystallographic and thermoelectric properties were characterized. It was experimentally validated that electron donor levels of 8 × 1019-8x1020 cm−3, originated by grain boundaries in poly-crystalline TiNiSn, can be reduced to the ∼7 × 1018cm−3 range, in single-crystals, allowing upon low acceptor doping concentration, to obtain electronically optimal p-type TiNiSn thermoelectric legs.