Effect of Na-site partial substitutions on the thermoelectric properties of layered oxide NaxCo2O4

Polycrystalline samples of (Na_1-yM_y)_x Co₂O₄ (M=K Sr, Y, Nd, Sm and Yb; y=0.01~0.35) were prepared by using a solid state reaction method. The effects of the Na-site partial substitutions on the thermoelectric properties of Na_xCo₂O₄ from room temperature to 1073 K were investigated. For the Sr-substitution sample, the thermoelectric power and the resistivity increased, and the electronic contribution to the thermal conductivity decreased compared to the non-substitution sample. These effects mean the reduction of the carrier density. As a result of that, for the Sr-substitution sample, the figure of merit was slightly improved. On the other hand, for rare-earth-metal-substitution samples, the anomalous substitution effects were observed. That is, in spite of the fact that the electronic contribution to the thermal conductivity decreased, the thermoelectric power decreased and the electrical resistivity increased. For all substitution samples except for Y-substitution, the lattice contribution to the thermal conductivity decreased because of the phonon scattering resulting in the decrease in the thermal conductivity