Power factors of Ca₃Co₂O₆ and Ca ₃Co₂O₆-based solid solutions

A polycrystalline sample of Ca₃Co₂O₆ was prepared by solid state reaction of CaCO₃ and Co₃O ₄ at 1243 K in air. The electrical resistivity (ρ) of Ca ₃Co₂O₆ exhibited semiconducting behavior and ranged from 1.6×10² to 1.3×10^-1 Ω cm with temperature from 400 to 1170 K. The Seebeck coefficient (S) showed positive value, and decreased from 660 to 140 μV/K with increasing the temperature from 350 to 1170 K. The power factor (S²/ρ) tended to increase with increasing temperature, and the maximum value was 1.6×10^-5 W/(m K²) at 1150 K. Samples with the starting compositions of (Ca_1-xA_x)₃Co ₂O₆ (A=Y, La, and Bi, x=0.03) and Ca₃(Co _1-yM_y)₂O₆ (M=Cr and Cu, y=0.03) were also heated at 1243 K in air to prepare Ca₃Co₂O ₆-based solid solutions. The changes of lattice parameters and widths of X-ray diffraction peaks for the obtained samples indicated substitutions of Y, La, Bi for Ca, Cr and Cu for Co, although the samples included trace amount of impurities. Substitutions of Bi and Cu increased the power factors of the solid solutions, and the maximum values were 3.4×10^-5 W/(m K²) (A=Bi at 1130 K) and 2.3×10^-5 W/(m K²) (M=Cu at 1150 K). The Seebeck coefficients of the solid solutions with Y and La were negative below 400 K.