Optimizing thermoelectric performance of Cd-doped β-Zn4Sb3 through self-adjusting carrier concentration

Crack-free Zn3.96+xCd0.04Sb3 (x = −0.05, 0.0, 0.05 and 0.1) ingots were successfully synthesized by a melting followed by a precisely controlled slow cooling process. The facile control of Zn content realizes the effective self-adjustment of carrier concentration, as well as the optimization of the thermoelectric figure of merit. The Zn-deficiency and stoichiometric samples are single phase, whereas a slight metal Zn phase can be detected in other two Zn-rich samples existing as forms of numerous evenly distributed nano-clusters with size of 20–50 nm and a spot of micro-scale precipitations embedded in the matrix. In particular, these multi-scale microstructures combined with the subtle variation of interstitial Zn apparently intensify phonon scattering and give rise to a “phonon-glass” feature of Zn-rich samples. However, Zn-deficiency sample benefiting from high Seebeck coefficient, shows a high power factor (>1.0 mW m−1 K−1) in the entire temperature range and a maximum value of 1.26 mW m−1 K−1 at 660 K. As a result, the enhanced effective hole mass by a slight Cd-doping coupled with the extremely low lattice thermal conductivity originated from crystalline complexities lead to a high figure of merit of 1.23 at 660 K for Zn3.91Cd0.04Sb3 sample, which is comparable with the highest value reported by T. Caillat et al. [T. Caillat et al. J Phys Chem Solids 1997; 58: 1119−25]. Furthermore, this study demonstrates a simple and easily-industrialized melting combined with slow cooling technique making the high performance β-Zn4Sb3 a promising candidate for low-grade waste heat recovery.