Improved thermoelectric performance of (Zr0.3Hf0.7)NiSn half-Heusler compounds by Ta substitution

The thermoelectric performance of Ta substituted (ZrHf)NiSn-based half-Heusler compounds is studied. Here, Ta is used on the Hf site for controlling the charge carrier concentration in contrast to the widely used Sb substitution on the Sn site. The influence of the Ta content on the thermoelectric and transport properties of (Zr0.3Hf0.7-xTax)NiSn (x = 0, 0.01, 0.05) is investigated by means of Seebeck coefficient, electrical resistivity, thermal conductivity, and Hall coefficient measurements. The results are analyzed in context of the single parabolic band model. Ta substitution increases the charge carrier concentration and suppresses the influence of impurity band, which is present in the pristine (Zr0.3Hf0.7) NiSn. Moreover, Ta substitution decouples and simultaneously increases the density-of-states effective mass (m*) and the charge carrier mobility (mu), leading to a larger weighted mobility mu.(m*)(3/2). The lattice thermal conductivity is slightly suppressed due to increased point defect scattering. As a result, a Figure of Merit of 0.85 is achieved for (Zr0.3Hf0.65Ta0.05)NiSn compound at 870 K, ca. 180% improvement over the unsubstituted sample. (C) 2014 AIP Publishing LLC.