Influence of ionic sizes of rare earths on thermoelectric properties of perovskite-type rare earth cobalt oxides RCoO₃ (R = Pr, Nd, Tb, Dy)

Perovskite-type rare earth cobalt oxide RCoO₃ (R = Pr, Nd, Tb, Dy) ceramics were prepared by reaction sintering of corresponding metal oxide powders, and their thermoelectric performances were evaluated up to 873 K. The electrical conductivity of RCoO₃ increased with increasing temperature while the Seebeck coefficient decreased, which was a typical semiconducting characteristic. The conductivity and the Seebeck coefficient also depend on the ionic radii of the rare earth cations. The former decreased while the latter increased with decreasing ionic radii. Thermal conductivity of RCoO₃ decreased up to certain temperatures and then increased again. The increase above the temperature at which the thermal conductivity reached the minimum value was considered due to the contribution of carrier component on phonon conduction in the present RCoO₃ systems. Thermal conductivity of RCoO₃ decreased with increasing temperature, and at same temperature it increased with decreasing ionic radii of the rare earth elements on the low-temperature side. On the other hand, on the high-temperature side, these dependencies were completely opposite. The figure of merit (Z) of RCoO₃ increased with increasing temperature up to 673 K and then decreased for NdCoO₃ and PrCoO₃, while the Z value of DyCoO₃ still increased. TbCoO₃ and DyCoO₃ showed the highest dimensionless figure of merit, ZT = 0.05 at 873 K in the present RCoO₃ systems.