Upadhyayula V Varadaraju

A half-Heusler (HH) type high entropy alloy (HEA) Ti2NiCoSnSb has been synthesized by a fast powder metallurgy route for the first time. Mechanical alloying (MA) by wet milling produced a powder with a minor fraction of the HH phase. The dry milling route resulted in the desired single-phase HH material. Consolidation of the nanocrystalline mechanically alloyed (MA) powder by spark plasma sintering (SPS) resulted in a majority HH phase. Interestingly, the nanocrystalline alloy exhibited simultaneous enhancement in the Seebeck coefficient and electrical conductivity, with a maximum ZT of 0.13 at 973 K observed for the dry milled alloy. The band structure obtained by density functional theory (DFT) was in good agreement with the ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectroscopy results. The DFT calculations and microstructural analysis suggest that phase separation strongly influenced the thermoelectric properties. The band structure calculations provided a good rationale for the phase evolution and thermoelectric properties.

Powder metallurgy route has been employed to synthesize nanocrystalline Ti2NiCoSn1−xSb1+x (x = 0.3, 0.5, 0.7, 1) alloys for thermoelectric applications. Atom probe analysis confirmed the homogeneous distribution of elements in the half-Heusler phase at a scale of few nanometers. A combination of nanostructuring, lattice distortion and interfacial scattering brings about a reduction in lower thermal conductivity which brings forth an improvement in ZT. Ti2NiCoSb2 exhibited the highest ZT of 0.26 due to the increments effected by higher power factor and lower thermal conductivity.