Effect of Partial Substitutions of Rare-earth Metals for Na-site on the Thermoelectric Properties of Na_xCo_2O_4 Prepared by the Polymerized Complex Method

摘要

Polycrystalline (Na_(1-y)M_y)_xCo_2O_4 (M=Nd, Sm, Dy and Yb; y = 0.01 approx 0.05) samples were synthesized by a polymerized complex method. The effects of partial substitution of rare-earth metals for a Na-site on the thermoelectric properties of Na_xCo_2O_4 were investigated, and their differences with the sample prepared by a conventional solid-state reaction (SSR) method were evaluated. In case of the Nd, Sm and Dy-substituted samples, each rare-earth metal content in the matrix phase was slightly higher than that of the sample prepared by the SSR method, resulting in the slight increase in the thermoelectric power. Dy-substituted samples with y = 0.03 and 0.05 showed the decrease in the electrical resistivity compared to the non-substituted sample because of the enhanced c-axis orientation. The power factors of the Dy-substituted samples were higher than that of the non-substituted sample over the entire temperature range, and the sample with y = 0.05 showed the maximum power factor value of 1.5 X 10~3 Wm~(-1) K~(-2) at 1073 K. On the other hand, Yb-substituted )samples with y < = 0.03 were almost single Na_xCo_2O_4 phase. The maximum Yb content in the matrix phase corresponded to y = 0.03 from the EDX analysis. The value was remarkably higher than that of the sample prepared by the SSR method. For the Yb-substituted samples with y < = 0.03, the c-axis orientation was enhanced with increasing the nominal Yb composition in conjunction with the rise in the lattice parameter, c. The Yb substitution caused the increase in the electrical resistivity and thermoelectric power compared to the non-substituted sample over the entire temperature range, which is considered to be due to the reduction in the earner density resulted from the Yb substitution for Na-site. The power factor of the sample with y = 0.03 was enhanced over the entire temperature range, and the maximum power factor, 1.5 X 10~(-3) Wm~(-1)K~(-2) was obtained at 1073 K.