Effects of V Alloying on Thermoelectric Properties of NbFeSb Half-Heusler Materials Codoped with Ti, Zr, and Sn

Abstract

NbFeSb-based alloys are promising <i>p</i>-type half-Heusler materials with excellent thermoelectric performance, thermal stability, and naturally abundant constituent elements. Alloying and doping are powerful techniques for enhancing the thermoelectric properties of half-Heusler materials. This study experimentally investigated the effects of V alloying in NbFeSb codoped with Ti, Zr, and Sn. As the V content increases, the electrical conductivity of Nb<sub>0.8−x</sub>V<sub>x</sub>Ti<sub>0.15</sub>Zr<sub>0.05</sub>FeSb<sub>0.98</sub>Sn<sub>0.02</sub> (<i>x</i> = 0, 0.05, 0.1, 0.2, and 0.3) decreases monotonically because of a simultaneous reduction in carrier concentration and mobility, reducing the power factor up to −39% from 4.13 (x = 0) to 2.52 mW m<sup>−1</sup>K<sup>−2</sup> (<i>x</i> = 0.3) at 298 K. Moreover, the lattice thermal conductivity decreases with increasing <i>x</i> by as much as −40% at the maximum V content of <i>x</i> = 0.3, demonstrating that V addition considerably enhances phonon scattering even in the presence of substitutional dopants Ti, Zr, and Sn. The differences in the mass and size of V and Nb atoms cause a substantial decrease in lattice thermal conductivity. According to our study, the addition of a small V content to Hf-free NbFeSbbased alloys can improve their thermoelectric properties, and a maximum dimensionless figure of merit <i>zT</i> of 0.89 was obtained in Nb<sub>0.75</sub>V<sub>0.05</sub>Ti<sub>0.15</sub>Zr<sub>0.05</sub>Sn<sub>0.98</sub>Sb<sub>0.02</sub> at 973 K.