Phase diagram of ternary Cu-Ga-Te system and thermoelectric properties of chalcopyrite CuGaTe2 materials

Abstract

Among all types of thermoelectric materials, chalcopyrite CuGaTe2 has been viewed as a promising candidate for use of thermoelectric generator due to its high figure-of-merit (zT) at the high temperatures. Herein the 923 K isothermal section of ternary Cu-Ga-Te system is determined, using various thermally-equilibrated Cu-Ga-Te alloys, and ternary CuGaTe2 phase is stabilized within the compositional region of 48.0–53.0 at%Te and 25.0 at%-30.0 at%Cu. Moreover, the solubility of Cu in binary Ga2Te3 and Ga3Te4 compounds at 923 K is negligible, while that in GaTe phase reaches 7.9 at%Cu. The as-determined isothermal section, depicting the phase stability regime of CuGaTe2, provides options for precisely locating the compositions of CuGaTe2-based materials that lead to promising and reproducible thermoelectric properties. A zT peak of 0.6 has been achieved in the Bridgman-grown Cu25Ga26Te49 alloy at 750 K, which is nearly eight times higher than the neighboring Cu28Ga25Te47 alloy, presumably due to the fact that the Cu25Ga26Te49 alloy, which exhibits high phase purity of CuGaTe2, has lower lattice thermal conductivity (κL∼1.3 (W/mK) and higher power factor (PF∼11.2 (μW/mK2)), comparing with that of Cu28Ga25Te47 alloy (κL∼1.8 (W/mK) and PF∼1.9 (μW/mK2)), which locates in a three-phased CuGaTe2+Cu2Te + Cu9Ga4 region, with only a slight deviation in the starting composition.