Effect of Sb Doping on the Thermoelectric Properties of MNiSn (M=Ti, Zr, Hf) Half-Heusler Alloys Fabricated by a Rapid Solidification Process

Among half-Heusler alloys, MNiSn (M = Ti, Zr, Hf) compounds have been extensively investigated due to their unique crystal structure and promising thermoelectric properties. However, forming the MNiSn half-Heusler single phase directly during the solidification process is challenging, necessitating a prolonged annealing step for homogenization. Rapid Solidification Processing (RSP) was employed to synthesize materials with controlled microstructures and high thermoelectric performance within a short processing time. Optimized RSP processes were then used to produce ribbon-shaped samples, aiming to enhance thermoelectric performance through grain size reduction and the synthesis of half-Heusler alloys with minimal amounts of detrimental phases. In the alloy design step, Ti, Zr, and Hf were typically mixed at the M sites to reduce lattice thermal conductivity. To address cost and production efficiency concerns associated with incorporating Hf, alloy compositions with reduced Hf content were designed. A comparative analysis of the thermoelectric properties of Ti0.5Zr0.17Hf0.33NiSn(1-x)Sbx (x = 0.02~0.08) was conducted by varying doping levels of Sb as the dopant element. The addition of Sb led to a gradual increase in electron concentration, resulting in a significant rise in electrical conductivity. However, this increase in electronic thermal conductivity had a detrimental effect on the overall dimensionless Fig. of merit ZT value. The maximum ZT value of 0.92 was achieved in Ti0.5Zr0.17Hf0.33NiSn0.98Sb0.02 at 773K, demonstrating the potential of Hf-reduced half-Heusler alloys synthesized by RSP for thermoelectric applications.