Compositional modulation is driven by aliovalent doping in n-type TiCoSb based half-Heuslers for tuning thermoelectric transport

Abstract

The ternary intermetallic TiCoSb based half-Heusler (HH) alloys are prominent material exhibiting good p-type thermoelectric (TE) performance. In this work, we studied the implication of V and Nb as n-type aliovalent dopants on Ti crystallographic site of semiconducting TiCoSb based HH alloys for attaining higher TE performance in n-type counterparts. The carrier concentration optimization between semiconducting (Ti1−xVxCoSb) and semi-metallic regime (Ti1−xNbxCoSb) had resulted in maximum TE figure-of-merit (ZT) of 0.22 and 0.52 at 873 K for optimized Ti0.85V0.15CoSb and Ti0.85Nb0.15CoSb HH compositions, respectively. These substitutional alloys were synthesized using arc-melting and consolidated using spark plasma sintering, which resulted in biphasic microstructure with in-situ phase segregation of heterogeneously distributed Ti-rich precipitates at all length scales within the HH matrix, are examined by microstructural analysis using X-ray diffraction and electron microscopy. Interestingly, higher power factor and simultaneous reduction of thermal conductivity is observed in both the doping as a result of optimal carrier concentration and enhanced phonon scattering. However, Nb-doped alloys exhibit higher carrier mobility and more significant lattice thermal conductivity reduction, thus establishing n-type Ti1−xNbxCoSb HH alloys as an equally promising counterpart of p-type TiCoSb for mid-temperature TE power generation.