Compositional tuning of ZrNiSn half-Heusler alloys: Thermoelectric characteristics and performance analysis

Nanostructuring has rejuvenated great interest in thermoelectric (TE) based power generation by enabling enhanced performance in nano-grained TE materials over their bulk counterparts. In ZrNiSn based half-Heusler (HH), a promising n-type TE materials, we examine the prospects of nanostructuring by synthesizing nano-crystalline iso-electronically substituted n-type Zr_1-xHf_xNiSn HH alloys to achieve a state-of-the-art TE figure-of-merit ZT ∼ 1.2 at 873 K, which is ∼20% higher than its bulk counterparts and corresponds to high conversion efficiency of ∼9% with output power density ∼7 Wcm⁻², estimated using cumulative temperature dependence model. Enhanced phonon scattering at nano-scale grain boundaries and crystal defects, arising from nanostructuring and mass defect fluctuation in Hf substituted ZrNiSn alloys resulted in significantly reduced thermal conductivity. It was found that the partial substitution at Zr site with its heavier homologue Hf causes variation in the carrier effective mass and carrier concentration, which at lower Hf concentration results in an enhancement in the power factor.