Thermal stability study of Cu_1.97Se superionic thermoelectric materials

With the development of high-temperature thermoelectric materials, especially the superionic copper selenide (Cu2-xSe), thermal stability and cycling thermoelectric performance have become a great concern recently. Here we demonstrate that the excellent repeatability of the thermal cycling data of a simple Cu-Se binary system can be obtained. The thermal stability was systematically studied via measuring the thermal-cycling thermoelectric performances of Cu1.97Se samples with different rates of Se evaporation by adopting a simple and controllable method, i.e., the conventional solid-state sintering method. The samples fabricated using the conventional sintering method exhibited an optimized figure of merit of 0.8 at 800 K with very good thermal repeatability and stability from 400 K to 800 K. The results indicated that the conventional sintering method can also be adopted to fabricate materials with satisfactory thermoelectric performance. It was found that the repeatability of thermal cycling data is directly related to the sintering temperature and indirectly associated with the evaporation of Se. The Cu1.97Se samples sintered at low temperatures experienced less evaporation of Se with good thermal repeatability, while they exhibited relatively low thermoelectric performance due to high carrier concentration. Although the samples sintered at high temperatures show a large variation in the thermal cycling performance, the repeatability could be improved by the further annealing process, which reveals that good thermal stability of thermoelectric materials can be obtained via post-treatment.