Phase structure and thermoelectric properties of Cu<sub>1.8–<i>x</i></sub> Sb<i><sub>x</sub></i> S thermoelectric material
Authors: Zhao Ying-Hao , Zhang Rui , Zhang Bo-Ping , Yin Yang , Wang Ming-Jun , Liang Dou-Dou ,   School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Published: June 2021

Cu<sub>1.8</sub>S-based materials have become potential thermoelectric materials due to their rich raw material reserves, low toxicity, and excellent electrical and thermal properties. In this study, a series of Cu<sub>1.8–<i>x</i></sub> Sb<i><sub>x</sub></i> S (<i>x</i> = 0, 0.005, 0.02, 0.03, 0.04) bulk materials is synthesized by using mechanical alloying combined with spark plasma sintering process. This preparation method can shorten the preparation cycle of materials, and effectively improve the research and development efficiency of thermoelectric (TE) materials due to its simple process. The effects of different Sb doping amounts on the structure, micromorphology, and thermoelectric transport properties of Cu<sub>1.8–<i>x</i></sub> Sb<i><sub>x</sub></i> S phase are investigated. The results show that when 0 ≤ <i>x</i> < 0.02, the bulk samples are single-phase Cu<sub>1.8</sub>S. With the further increase of Sb doping to 0.02 ≤ <i>x</i> ≤ 0.04, the second phase CuSbS<sub>2</sub> is formed when Sb content exceeds the solid solubility limit of <i>x</i> = 0.02 in Cu<sub>1.8</sub>S, all Cu<sub>1.8–<i>x</i></sub> Sb<i><sub>x</sub></i> S bulk samples exhibit p-type conductivity characteristics. Benefitting from the synergistic phonon scattering effect by multiscale defects, such as point defects (<inline-formula><tex-math id="M1">\begin{document}${\rm{Sb}}_{{\rm{Cu}}}^{ \bullet\bullet }$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201852_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201852_M1.png"/></alternatives></inline-formula>, <inline-formula><tex-math id="M2">\begin{document}$ {\rm{V}}_{\rm{S}}^{ \bullet \bullet } $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201852_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="12-20201852_M2.png"/></alternatives></inline-formula>), nanopores, secondary phases (CuSbS<sub>2</sub>), and dislocations, the thermal conductivity <i>κ</i> declines significantly from 1.76 W·m<sup>–1</sup>·K<sup>–1</sup> (<i>x</i> = 0) to 0.99 W·m<sup>–1</sup>·K<sup>–1</sup> at 723 K for the Cu<sub>1.76</sub>Sb<sub>0.04</sub>S sample. Finally, the peak dimensionless TE figure of merit (<i>ZT</i> ) value of 0.37 is achieved at 723 K for Cu<sub>1.77</sub>Sb<sub>0.03</sub>S resulting from a low thermal conductivity of 1.11 W·m<sup>–1</sup>·K<sup>–1</sup> combining an appropriate power factor of 563 μW·m<sup>–1</sup>·K<sup>–2</sup>, which is 12% higher than that (0.33) of pristine Cu<sub>1.8</sub>S. Although the Sb doped Cu<sub>1.8</sub>S-based samples have lower thermal conductivity <i>κ</i>, the reduced power factor cannot be offset by reducing the thermal conductivity <i>κ</i>, so the TE figure of merit (<i>ZT</i> ) value is not significantly improved. Therefore, there is still much room for improving the performance of Sb doped Cu<sub>1.8</sub>S-based thermoelectric material, and its thermoelectric performance can be further optimized through nano-second phase recombination, energy band engineering, and introducing multi-scale defects, etc. Our results suggest that the introduction of Sb into thermoelectric materials is an effective and convenient strategy to improve <i>ZT</i> value by reducing thermal conductivity <i>κ</i>.