Electrical and thermal properties of superionic KxCu2−xS (x = 0.1, 0.2, 0.25) alloys

The paper presents and discusses the results of experimental studies of the electronic conductivity, Seebeck coefficient and thermal conductivity of KxCu2−xS alloys (x = 0.1, 0.2, 0.25) in the temperature range from 30 to 420°C. According to the results of X-ray phase analysis, the alloys are mixtures of various phases of copper sulfide: the cubic Fm-3m phase Cu1.84S, cubic Fm-3m phase Cu2S, rhombohedral R-3m phase Cu17S9, and metastable tetragonal P43212 phase Cu2S. Differential scanning thermometry determined an endothermic peak extended from 84 to 102°C, caused by phase transitions from the rhombohedral and tetragonal phases to the hexagonal copper sulfide phase. In the temperature range above room temperature, a semiconducting character of conductivity is observed for all samples, which then changes to metallic one (above 102°C, 93°C and 270°C for К0.1Cu1.9S, К0.2Cu1.8S, К0.25Cu1.75S compositions respectively). The activation energies for the electronic conduction are 0.16, 0.22, and 0.39 eV for К0.1Cu1.9S, К0.2Cu1.8S, К0.25Cu1.75S, respectively. In the range from 130 to 270°C, the activation energy for the К0.25Cu1.75S alloy decreases to 0.16 eV. The anomalously high peak of Seebeck coefficient observed for the studied alloys at 360 – 380°C may be caused by the superionic phase transition from the hexagonal to the cubic phase in those fractions of the alloys, which were in the rhombohedral (Cu17S9) and tetragonal (Cu2S) modifications at a room temperature, and then transformed into hexagonal modifications Cu17S9 and Cu2S, respectively, at 84 and 93°C. For the К0.2Cu1.8S composition above 300°C, a strong jump in Seebeck coefficient (up to 4 mV / K) was observed and a decrease in thermal conductivity (from 0.87 to 0.4 W ∙ m−1 ∙ K−1), leading to the very high peak value of the dimensionless thermoelectric figure of merit ZT = 3.5 at 380°С, which is important for possible technical applications.

1. Ministry of Education and Science of the Republic of Kazakhstan - BR05236795