High Thermoelectric Figure of Merit Achieved in CuSTe 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 CuSTe (0 ≤ x ≤ 1) alloys by a facile, rapid method of mechanical alloying combined with spark plasma sintering process. The CuSTe 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 CuSTe system changed from the pure monoclinic CuS at x = 0 to monoclinic CuSTe solid solution at 0.02 ≤ x ≤ 0.06 and then transforms to hexagonal CuSTe solid solution at 0.08 ≤ x ≤ 0.1. The phase separation of hexagonal CuTe in the hexagonal CuS matrix occurs at 0.3 ≤ x ≤ 0.7 and finally forms the hexagonal CuTe at x = 1. Owing to the changed band structure and the coexisted CuS and CuTe phases, greatly enhanced power factor was achieved in all CuSTe (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, CuSTe 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 CuS (0.32) and CuTe (0.08), respectively.