High Thermoelectric Figure of Merit Achieved in Cu₂S_{1- x}Te _x Alloys Synthesized by Mechanical Alloying and Spark Plasma Sintering

Chalcogenides have been considered as promising thermoelectric materials because of their low cost, nontoxicity, and environmental benignity. In this work, we synthesized a series of Cu₂S_{1- x}Te _x (0 ≤ x ≤ 1) alloys by a facile, rapid method of mechanical alloying combined with spark plasma sintering process. The Cu₂S_{1- x}Te _x system provides an excellent vision of the competition between pure phase and phase transformation, entropy-driven solid solution, and enthalpy-driven phase separation. When the Te concentration increases, the Cu₂S_{1- x}Te _x system changed from the pure monoclinic Cu₂S at x = 0 to monoclinic Cu₂S_{1- x}Te _x solid solution at 0.02 ≤ x ≤ 0.06 and then transforms to hexagonal Cu₂S_{1- x}Te _x solid solution at 0.08 ≤ x ≤ 0.1. The phase separation of hexagonal Cu₂Te in the hexagonal Cu₂S matrix occurs at 0.3 ≤ x ≤ 0.7 and finally forms the hexagonal Cu₂Te at x = 1. Owing to the changed band structure and the coexisted Cu₂S and Cu₂Te phases, greatly enhanced power factor was achieved in all Cu₂S_{1- x}Te _x (0 < x < 1) alloys. Meanwhile, the point defect introduced by the substitution of Te/S atoms strengthened the phonon scattering, resulting in a lowered lattice thermal conductivity in most of these solid solutions. As a consequence, Cu₂S_0.94Te_0.06 exhibits a maximum ZT value of 1.18 at 723 K, which is about 3.7 and 14.8 times as compared to the values of pristine Cu₂S (0.32) and Cu₂Te (0.08), respectively.