Large mass field fluctuation and lattice anharmonicity effects promote thermoelectric and mechanical performances in NbFeSb half-Heusler alloys via Ti/Zr/Hf stepwise doping

Abstract

Thermoelectric materials require not only high performance to maximize the energy-conversion efficiency but also good mechanical properties to guarantee machinability and reliable operation. In this work, multi-element doped NbFeSb-based alloys were designed from the stepwise doping of Ti, Zr, and Hf atoms into Nb sites and then by increasing the Ti/Zr/Hf contents, which led to increases in the configurational entropy. Ti/Zr/Hf doping at Nb sites increased the hole-carrier concentration n and improved the electrical-transport properties, whereby the Nb_0.82Ti_0.06Zr_0.06Hf_0.06FeSb sample obtained the highest PF value of 40.3 μW cm⁻¹ K⁻² at 973 K. Moreover, Ti/Zr/Hf doping decreased the lattice thermal conductivity by the strengthened point defect scattering and anharmonicity effect, resulting from the largely increased mass field fluctuation Γ_m and Grüneisen parameter γ, respectively. The lowest κ_lat of 3.6 W m⁻¹ K⁻¹, which was two times lower than that of the pristine NbFeSb, was realized in the Nb_0.82Ti_0.06Zr_0.06Hf_0.06FeSb sample at 973 K. As a result, the Nb_0.82Ti_0.06Zr_0.06Hf_0.06FeSb sample achieved the highest zT of 0.74 at 973 K and an average zT of 0.45 between 333 K and 973 K. Moreover, the highest compressive strength and microhardness could be achieved as 1061 MPa and 1124 H_V, respectively, much higher than the reported state-of-the-art TE materials.