Thermoelectric properties and service stability of Ag-containing Cu2Se

In 1970s, great efforts had been spent to develop Ag-containing Cu2Se-based radioisotope thermoelectric generators (RTG), but unfortunately the activity was completely stopped in 1981 because the Ag-containing Cu2Se-based thermoelectric module demonstrated poor service stability. Since then, a prevailing mindset in thermoelectric society is that Cu2Se, and related Cu-based thermoelectric materials, can not be fabricated into stable modules. In this study, a series of Ag-containing Cu2Se samples are synthesized. Their thermoelectric properties and service stability under current and/or temperature gradient are systematically investigated. It is found that introducing Ag in Cu2Se has significant influence on the thermoelectric properties but little influence on the stability. The critical voltages for Ag-containing Cu2Se samples, which represent the threshold for thermodynamic stability under electric field and/or temperature gradient, are determined by the total content of Cu and Ag rather than just the Cu content. Likewise, electrical conductivity is proposed to be a simple indicator to rapidly predict the stability and thermoelectric performance of Cu2Se-based materials. Based on the measured critical voltages, the rational ranges of leg geometry are recommended to fabricate the stable and efficient Cu1.97Ag0.03Se-based thermoelectric modules.