Influences of Bi and Cu substitutions on the power factor of Ca3Co2O6

Polycrystalline samples of Ca3Co2O6-based solid solutions, (Ca1−xBix)3Co2O6, Ca3(Co1−xCux)2O6 and (Ca1−xBix)3(Co1−xCux)2O6, were prepared by solid state reaction at 1243 K in air. The single phases were obtained at x≤0.02 for (Ca1−xBix)3Co2O6, Ca3(Co1−xCux)2O6 and (Ca1−xBix)3(Co1−xCux)2O6, and the lattice parameters increased with increasing x. The electrical resistivity (ρ) of the solid solutions exhibited semiconducting behavior in 300-1170 K, and the values of electrical resistivity decreased with increasing x (3.8×10−2 Ωcm for (Ca0.98Bi0.02)3Co2O6, 5.7×10−2 Ωcm for Ca3(Co0.98Cu0.02)2O6 and 4.8×10−2 Ωcm for (Ca0.98Bi0.02)3(Co0.98Cu0.02)2O6 at 1170 K). Seebeck coefficient (S) of the solid solutions were smaller than that of Ca3Co2O6 below 800 K, however, the values were almost the same as that of Ca3Co2O6 (around 140 μV/K) above 1000 K. The power factor (S2/ρ) tended to increase with increasing temperature and x, and the maximum values were 5.0×10−5 W/(m K2) for (Ca0.98Bi0.02)3Co2O6, 3.7×10−5 W/(m K2) for Ca3(Co0.98Cu0.02)2O6 and 4.0×10−5 W/(m K2) for (Ca0.98Bi0.02)3(Co0.98Cu0.02)2O6 at 1170 K.