Rare earth sulfides as high-temperature thermoelectric materials

Abstract

The rare earth sulfides appear promising for use as high-temperature power conversion thermoelectric materials because of low lattice thermal conductivity and good chemical stability above 1000°C. We are making an intensive study of cerium sulfide, and to a lesser extent lanthanum sulfide and thorium sulfide, in the stoichiometric range in which these compounds are semiconducting. Some measurements have been made on barium sulfide-cerium sulfide solid solutions. Electrical, optical, thermoelectric, and magnetic measurements have been made on crack-free cast samples of centimeter dimensions prepared by vacuum melting in molybdenum crucibles. Doping of the semiconductor by molybdenum is not believed to be serious, in view of the low Mo concentrations (≈ 100 ppm or less) indicated by chemical analysis and of the high carrier concentrations measured. Measurements of the thermoelectric parameters have been made at elevated temperatures using small area contact techniques as well as more conventional methods. Values of zT = 0·3 have been measured in cerium sulfide, without doping. Carrier concentration and Hall mobility have been measured by a simple and sensitive a.c. method. Mobilities measured to date in cerum sulfide are low ( ∼ 1 cm 2 V-sec ) and carrier concentrations are high (up to 10 22/cm 3) depending on the stoichiometry. Conduction mechanisms consistent with the data at hand are discussed.