Electrical and Thermal Transport Property Studies of High-Temperature Thermoelectric Materials.

High-temperature electrical conductivity, Seebeck coefficient, and thermal conductivity measurements were used to investigate the thermoelectric properties of several refractory oxide systems. Particular emphasis was placed on an investigation of lanthanum chromite and yttrium chromite as systems for testing proposed transport models. The substitution of divalent metal ions for La and Y results in the formation of small polarons as charge carriers. Experimentally, results show that the most effective divalent substitutes for La and Y are Sr and Ca, respectively, due to similar ionic size. Both electrical conductivity and Seebeck coefficient exhibit behavior consistent with thermally-activated transport by small polaron hopping. The additional substitution of Mn for Cr or S for Q decreased both electrical conductivity and Seebeck coefficient. The thermal conductivity generally decreased with temperature and dopant concentration. The dimensionless figure of merit for these oxides approached 0.2 at high temperatures.