Vanadium-free colusites Cu26A2Sn6S32 (A = Nb, Ta) for environmentally friendly thermoelectrics

Abstract

The thermoelectric properties of colusites Cu₂₆A₂Sn₆S₃₂ with A = V, Nb, and Ta have been investigated on high-density sintered compacts over the temperature range of 300–670 K. The ingots melted in evacuated and sealed quartz tubes were found to be composed of two colusite phases with slightly different lattice parameters. The successive hot-pressing converted the two-phase samples to single-phase ones of high density. Scanning electron microscopic analysis revealed the presence of CuS- and Cu₂S-based secondary phases in only the sintered compact with A = Ta. For A = V, Nb, and Ta samples, the heavy hole effective mass of 3m₀–4m₀ (m₀: free electron mass) caused a high Seebeck coefficient, leading to a high thermoelectric power factor of 0.6–0.7 mW K⁻² m⁻¹ at 660 K. The electrical resistivity and lattice thermal conductivity for the A = Ta sample are lower than those for A = V and A = Nb samples, because the secondary phases in the A = Ta sample provide charge carriers and scatter effectively the phonons. As a result, a ZT of 0.8 at 670 K is achieved for the A = Ta system.