Improved Thermoelectric Performance for β-Zn4Sb3 by Na/Zn Co-doping

In this study, β-Zn4Sb3 was prepared using the Zn-NaCl mixed-flux method based on the stoichiometric ratio of Zn/Sb/NaCl = (4−x):3:(7+x) (x = 0, 0.1, 0.2, 0.3). X-ray diffraction results showed that all samples were hexagonal rhombic crystal structures with the space group R3¯c\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$R\overline{3}c$$\end{document}. A second phase of ZnSb was detected in the samples with x = 0.2 and x = 0.3. In addition, the crystal-cell volume and lattice parameter of all samples increased for the larger ion radius of Na+ than Zn2+. The transmission electron microscopy image and corresponding area electron diffraction pattern demonstrated that the prepared samples had a complete crystal structure. All samples had hole conduction and exhibited p-type conduction behavior. The carrier concentration of the material increased with increased Na content and decreased Zn, ranging from 8.89 × 1019 cm−3 to 17.82 × 1019 cm−3. Moreover, the thermal conductivity (κ) of x = 0–0.2 samples decreased with increased Na content; however, the sample with x = 0.3 had a higher κ owing to the appearance of a ZnSb phase. Thermogravimetric differential thermal analysis showed that no weight loss occurred below the melting point. Eventually, the sample with the nominal composition of β-Zn3.8Sb3NaCl7.2 achieved a ZT value of 1.43 at 690 K.