2, California Institute of Technology, Pasadena, California, United States
Since the recent discovery of n-type Mg3.2Sb1.5Bi0.49Te0.01 with high performance, n-type Mg3Sb2-based compounds are attracting considerable interest. In this study, we demonstrate how to improve the overall figure-of-merit zT value by tuning the amount of excess Mg. Originally, as Mg interstitials were considered to be responsible for n-type behavior, a substantial amount of nominal Mg (x=0.2) in Mg3+xSb1.5Bi0.49Te0.01 was added to ensure the n-type property.1 However, a thermodynamic investigation has shown that such a large amount of excess Mg is not necessary as long the amount is above a minimum threshold for n-type conduction.2 Here we investigate the effect of minimizing the amount of excess Mg added to synthesize the n-type material. We find a significant reduction in thermal conductivity, leading to an increased zT. We discuss the experimental aspects of controlling the nominal Mg composition.
1 H. Tamaki, H. K. Sato and T. Kanno, Adv. Mater., 2016, 28, 10182–10187.
2 S. Ohno, K. Imasato, S. Anand, H. Tamaki, S. D. Kang, P. Gorai, H. K. Sato, E. S. Toberer, T. Kanno and G. J. Snyder, Joule, 2017, just accepted.