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 Omega(-1) cm(-1) at 326 K. Furthermore, the samples containing Ta display consistently lower (similar to 10-13%) thermal conductivity of similar to 7 W m(-1) K-1 at 350 K compared to a value of similar to 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 similar to 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 Mossbauer spectroscopy measurements exclude the possibility of mixed Fe occupancies. Although magnetic properties were not strongly modified by the Ta substitution, Nb0.83Hf0.12Ta0.05FeSb shows a magnetic phase transition at similar to 150 K, which is not observed in Nb0.88Hf0.12FeSb. This could be caused by extrinsic defects and microstructure induced by Ta addition. All samples exhibit a similar maximum dimensionless figure of merit value, zT(max), of similar to 0.75 at 800 K, which is comparable to the high efficiency materials previously reported in this system and makes them potential candidates to be utilized as p-type legs in half-Heusler based thermoelectric generators (TEG).