2, Jet Propulsion Laboratory, Pasadena, California, United States
Thermoelectric materials are the heart of radioisotope thermoelectric generators, which are the main power system for space missions such as Voyager I, Voyager II, and the Mars Curiosity Rover. However, materials currently in use (i.e. SiGe or PbTe based materials) enable only modest thermal to electric conversion efficiencies of 6.5%, warranting the development of material systems with improved thermoelectric performance. One material of interest previously examined at JPL is lanthanum telluride (La3-xTe4), a high-temperature n-type material. La3-xTe4 possesses a defect structure where the La3+ vacancies control the carrier concentration, and this structure possesses an inherently low thermal conductivity. With an optimized vacancy concentration, zT values of 1.2 are achievable at 1275 K. Here we present a study of the thermoelectric properties of neodymium telluride (Nd3-xTe4), another rare earth telluride with a similar structure to La3-xTe4. Density functional theory (DFT) calculations predicted a 100% increase in the Seebeck coefficient due to alteration of the band structure, suggesting an increase in zT compared to La3-xTe4. The high temperature electrical resistivity, Seebeck coefficient, and thermal conductivity were measured and reported for Nd3-xTe4 at various vacancy concentrations.