Effects of Zn/Bi Double Doping on the Charge Transport and Thermoelectric Properties of Tetrahedrites Cu12−xZnxSb4−yBiyS13

Abstract

Synthetic tetrahedrite (Cu12Sb4S13) is considered a promising p-type thermoelectric material owing to its intrinsically low thermal conductivity, relative nontoxicity, and presence of light elements Cu and S. Since Cu12Sb4S13 has a high carrier (hole) concentration, its power factor can be optimized by decreasing the carrier concentration via doping. In this study, Zn doping is performed to optimize the power factor by reducing the carrier concentration, and Bi doping is performed to reduce the lattice thermal conductivity through additional phonon scattering by atomic mass fluctuation and lattice distortion. Cu12−xZnxSb4−ySbyS13 tetrahedrites (0.1 ≤ x ≤ 0.4 and 0.1 ≤ y ≤ 0.4) were prepared using mechanical alloying and hot pressing, and the effects of double doping of Zn and Bi on the charge transport and thermoelectric properties were examined. X-ray diffraction analysis revealed the successful formation of the tetrahedrite phase, but a small amount of skinnerite phase (Cu3SbS3) was produced with an increase in the Bi content. The lattice constant increased with an increase in Zn and Bi doping contents, as the ionic radii of Zn and Bi were larger than those of Cu and Sb, respectively. With an increase in the Zn content and a decrease in the Bi content, the carrier concentration decreased. Consequently, the electrical conductivity decreased, whereas the Seebeck coefficient increased. The decline in the electrical conductivity with Zn doping dominated the increase in the Seebeck coefficient, which in turn reduced the power factor. However, the power factor did not change significantly with a change in the Bi content. The lattice thermal conductivity was close to the theoretical minimum value in tetrahedrite upon substituting with Zn and Bi, and thus, the thermal conductivities of all the specimens were less than 0.94 Wm−1 K−1 at temperatures ranging from 323 to 723 K. Consequently, the dimensionless figure of merit, ZT = 0.77, was obtained for Cu11.9Zn0.1Sb3.9Bi0.1S13 at 723 K, which was attributed to a relatively high power factor (0.83 mWm−1 K−2) and low thermal conductivity (0.76 Wm−1 K−1).