High temperature grain boundary resistance in Yb14(Mg,Mn)Sb11 thermoelectrics

Abstract

Grain boundary electrical resistance has been a limiting factor in the thermoelectric performance of many otherwise promising materials. For example, the recent emergence of Mg₃Sb₂ as a high performing low temperature thermoelectric material is a result of efforts to mitigate grain boundary electrical resistance. Many other materials including half Heuslers, oxides, Mg₂Si and others have also seen improved thermoelectric performance after mitigating grain boundary electrical resistance. However, the focus of grain boundary engineering work has been on low and mid temperature materials. Grain boundary electrical resistance is typically strongest at room temperature, so it has been assumed that high temperature thermoelectrics do not need to be evaluated for detrimental grain boundary electrical resistance. In this study, we demonstrate the importance of grain boundary resistance in all temperature ranges by discovering large grain boundary resistance in the high temperature thermoelectric Yb₁₄MgSb₁₁. While the observed boundary resistance is still largest at room temperature, there is still significant boundary resistance in the measured temperature range. As a result, a 22% increase in thermoelectric figure of merit is acheived at 1000 °C by increasing the grain size. Further, we demonstrate the importance of bulk composition to grain boundary engineering, as substituting Mg for Mn completely removed all measurable grain boundary resistance. These results demonstrate that grain boundary resistance is important in all thermoelectric materials regardless of targeted operating temperature.