Enhancement in thermoelectric properties of ZrNiSn-based alloys by Ta doping and Hf substitution

Though half-Heusler alloys are listed among the mid-high temperature thermoelectric materials with excellent thermoelectric and mechanical properties, their thermal conductivities remain high and limit their practical applications. In this work, the thermal conductivities and electrical conductivities of ZrNiSn-based alloys were synergistically optimized by Ta doping and Hf substitution. The samples were fabricated using levitation melting combined with spark plasma sintering. When compared with the pristine ZrNiSn, the electrical conductivity in Zr _0.68Hf _0.3Ta _0.02NiSn increases from 7.3 × 10 ⁴ to 12.9 × 10 ⁴ S·m ⁻¹, and the lattice thermal conductivity declines from 4.1 to 2.7 W·m ⁻¹·K ⁻¹ at 923 K. As a result, an enhanced ZT of 0.94 for Zr _0.68Hf _0.3Ta _0.02 NiSn is achieved, which is a nearly 56% enhancement over pristine ZrNiSn. Furthermore, the microhardness, Young`s modulus, and shear modulus in Zr _{1 -x-y}Hf _xTa _yNiSn ( x = 0, 0.2, 0.3, 0.4; y = 0, 0.02) alloys are substantially improved upon increasing the Ta and/or Hf content, far exceeding those in other conventional thermoelectric materials.