Significantly improved thermoelectric properties of Nb-doped ZrNiSn half-Heusler compounds

ZrNiSn-based n-type half-Heusler (HH) compounds are listed among the mid-high temperature thermoelectric materials with excellent thermoelectric properties because they have a high power factor (PF). However, the thermal conductivity of HH compounds remains high, especially for the lattice thermal conductivity and bipolar thermal conductivity, which limits their application. In this work, we have enhanced the thermoelectric and mechanical properties of ZrNiSn-based compounds by Nb doping, which is realized by replacing Zr sites with Nb. As a consequence, the PF is enhanced from 3.25 mW mK−2 for pristine ZrNiSn to 4.78 mW mK−2 for Zr0.96Nb0.04NiSn, while the lattice thermal conductivity respectively declines from 4.5 W m-1K−1 to 2.58 W m-1K-1at 1123 K. Combined experimental results and ab-initio calculations revealed the increase in PF is because of that Nb-doping can significantly improve the electrical conductivity through a lot of carrier injection. The reduction of lattice thermal conductivity is primarily due to that Nb doping can generate quantum dots in the matrix causing the phonon scattering for the Nb-rich quantum dots and cause the mass and stress field fluctuations, and Nb-doping can efficiently weaken the bipolar thermal effect further reducing the thermal conductivity. A maximum ZT value of 0.89 within single-doped ZrNiSn compounds is achieved at 1123 K for Zr0.96Nb0.04NiSn, which is 53% higher than the pristine ZrNiSn. Furthermore, Nb doping effectively improves the microhardness, shear modulus, and Young's modulus of the ZrNiSn HH samples. The synthesized Hf-free HH compounds establish Nb as an effective dopant for attaining a high ZT and implying the excellent potential of these materials for industrial applications.