Effects of Ta Substitution on the Microstructure and Transport Properties of Hf-Doped NbFeSb Half-Heusler Thermoelectric Materials

This investigation demonstrates the effect of partial substitutions of Nb by refractory Ta on the microstructure and thermoelectric properties of Hf-doped NbFeSb materials. All the synthesized samples show a heavily doped semiconducting character with the electrical conductivity higher than 3500 Ω–1 cm–1 at 326 K. Furthermore, the samples containing Ta display consistently lower (∼10–13%) thermal conductivity of ∼7 W m–1 K–1 at 350 K compared to a value of ∼8 W m–1 K–1 at the same temperature for the nonsubstituted sample. The vivid impact of Ta substitutions on reducing the lattice thermal conductivity of NbFeSb based materials is chiefly due to the lattice disorder originating from the mass difference between Ta and Nb atoms, resulting in ∼28% reduction in lattice thermal conductivity of the Nb0.73Hf0.12Ta0.15FeSb sample at 350 K compared to the nonsubstituted sample. The results of our Mosbauer spectroscopy measurements exclude the possibility of mixed Fe occupancies. Although magnetic properties were ...