Thermoelectric Properties of Undoped and Doped (Ti0.75Sn0.25)O2

Thermoelectric properties of undoped and doped (Ti0.75Sn0.25)O-2 were investigated for high-temperature thermoelectric conversion application. Nano-composites were formed by annealing above 1000 degrees C. Outside the spinodal dome, ilmenite-type SnTiO3 precipitated from the rutile structure. Thermoelectric properties were measured in the temperature range from room temperature to 1000 degrees C. (Ti0.75Sn0.25)O-2 was doped with both acceptor and donor dopants. Both undoped and doped (Ti0.75Sn0.25)O-2 exhibit n-type electrical behavior independent of the type of the dopant. The electrical conductivity was enhanced three orders of magnitude by donor doping with Nb2O5 or Ta2O5; achieving a maximum of 546 S/m at 850 degrees C. The increase in electrical conductivity was accompanied by reduction of the absolute Seebeck coefficient. Seebeck coefficient reduction of -600 mu V/K was observed between undoped and 4% Ta2O5 doped samples. The solid solution and doping reduced the thermal conductivity to <4 W/mK, far below the parent materials TiO2 and SnO2. Lattice thermal conductivity decreased with increasing temperature, achieving 1.9 W/mK at 900 degrees C for 4% Ta2O5 doping. No further reduction in thermal conductivity was observed in annealed samples containing nano-sized SnTiO3 precipitates. Dimensionless figure of merit (ZT) attained was <0.1.