Vanadium-Doping-Induced Resonant Energy Levels for the Enhancement of Thermoelectric Performance in Hf-Free ZrNiSn Half-Heusler Alloys

Despite Hf-free half-Heusler (HH) alloys being currently explored as an important class of cost-effective thermoelectric materials for power generation, owing to their thermal stability coupled with high cost of Hf, their figure-of-merit (ZT) still remains far below unity. We report a state-of-the-art figure-of-merit (ZT) ∼ 1 at 873 K in Hf-free n-type V-doped Zr 1– x V x NiSn HH alloy, synthesized employing arc-melting followed by spark plasma sintering. The efficacy of V as a dopant on the Zr-site is evidenced by the enhanced thermoelectric properties realized in this alloy, compared to other reported dopants. This enhancement of ZT is due to the synergistic enhancement in electrical conductivity with a simultaneous decrease in the thermal conductivity, which yields ZT ∼ 1 at 873 K at an optimized composition of Zr 0.9 V 0.1 NiSn, which is ∼70% higher than its pristine counterpart and ∼25% higher than the best reported thus far in Hf-free n-type HH alloys. The enhancement of the electrical conductivity is due to the modification of the band structure by suitable tuning of the electronic band gap near the Fermi level, through optimized V-doping in ZrNiSn HH alloys. The reduction in the thermal conductivity has been attributed to the mass fluctuation effects and the substitutional defects caused by V-doping, which results in an abundant scattering of the heat-carrying phonons. The optimized V-doped ZrNiSn HH composition, therefore, strikes a favorable balance between cost and thermoelectric performance, which would go a far way in the realization of a cost-effective (Hf-free) HH based thermoelectric generator for power generation through waste heat recovery.