Microstructure and thermoelectric properties of Cu2Te-Sb2Te3 pseudo-binary system

Abstract

Systematic investigation of microstructure in pseudo-binary alloys of (Cu2Te)x-(Sb2Te3)100-x (x = 11.91, 32.60, 50.0, 62.02, 73.95, 85.50, 95.65) and their thermoelectric properties had been studied. Alloys were prepared by solid-state synthesis method. Powder X-ray diffraction confirmed the presence of Cu2Te and Sb2Te3 phases which were supported by results of electron probe microanalysis. Scanning electron micrographs of (Cu2Te)62.02-(Sb2Te3)37.98 showed eutectics which are predominantly rod-like with a small amount of degenerate eutectics at the boundaries. The microstructure of hypereutectic alloys comprises of Sb2Te3 major phase along with Cu2-δTe minor phase whereas hypoeutectic alloys consist of eutectic and Cu2Te proeutectic constituent. Transport properties were measured in the temperature between 350 and 600 K. Temperature dependent electrical conductivity (σ) for all samples showed metal-like behavior. Positive Seebeck coefficients (S) indicate that majority carriers were holes. A systematic decrease in S and increase in σ with Cu content was due to an increase of carrier concentration caused by the easy formation of Cu vacancies in Cu2-δTe. A different trend of temperature-dependent thermal conductivity for hypoeutectic, hypereutectic and eutectic alloys could be due to the influence of composition and/or microstructure variation. The most striking results of the present investigation is the significant improvement of high temperature figure of merit of Sb2Te3 with small addition of Cu2Te (∼11%) with a value of zT of 0.6 at 600 K coupled with an increase mechanical strength as reflected by the increase in hardness.