TiNiCoSnSb - a new half-Heusler type high-entropy alloy showing simultaneous increase in Seebeck coefficient and electrical conductivity for thermoelectric applications.

A new single phase high entropy alloy, TiNiCoSnSb with half-Heusler (HH) structure is synthesized for the first time by vacuum arc melting (VAM) followed by ball-milling (BM). The BM step is necessary to obtain the single phase. Local electrode atom probe (LEAP) analysis showed that the elements are homogeneously and randomly distributed in the HH phase without any clustering tendency. When the BM was carried out for 1 hour on the VAM alloy, microcrystalline alloy is obtained with traces of Sn as secondary phase. When BM was carried out for 5 h, single HH phase formation is realized in nanocrystalline form. However, when the BM samples were subjected to Spark plasma sintering (SPS), secondary phases were formed by the decomposition of primary phase. Nanostructuring leads to simultaneous increase in S and σ with increasing temperature. The micro (1 h BM-SPS) and nanocrystalline (5 h BM-SPS) alloys exhibited a power factor (Sσ) of 0.57 and 1.02 mWmK, respectively, at 860 K. The microcrystalline sample had a total thermal conductivity similar to bulk TiNiSn sample. The nanocrystalline alloy exhibited a ZT of 0.047 at 860 K. The microcrystalline alloy showed a ZT to 0.144 at 860 K, in comparison to the nanocrystalline alloy.