Impurity tracking enables synthesis of TiFe1-xNixSb half-Heusler compounds with high purity

Abstract

Half-Heusler (HH) compounds normally consist of elements with high vapor pressures that may significantly evaporate during high-temperature synthesis. Mechanical alloying (MA) with following processes is a non-melting method that could get rid of the material loss as well as grain coarsening. However, the as-synthesized materials generally suffer from the existence of impurity phases, hampering the establishment of intrinsic structure–property correlation. Unfortunately, the origin of impurity phases remains uncertain. Here, by tracking the impurity phases in TiFe1-xNixSb (x = 0.4, 0.5, 0.6) compounds prepared by high-energy ball milling (BM) and hot pressing, we demonstrated the impurities to be TiO2 and Sb-based alloys. Titanium reacted with oxygen in the voids during the hot pressing process, forming a high density of nanosized TiO2 precipitates embedded in the matrix and leading to an inhomogeneous microstructure with smaller grain sizes and multiple phases. Based on such deduction, a highly pure TiFe0.6Ni0.4Sb sample has been successfully synthesized by reducing the oxygen concentration using spark plasma sintering (SPS) method, and the electronic and thermal transport measurements indicate a more intrinsic and enhanced thermoelectric performance.