The effect of element substitution on high-temperature thermoelectric properties of Ca3Co2O6 compounds

Polycrystalline Ca₃Co_1.8M_0.2O₆ (M=Mn, Fe, Co, Ni, Cu) and Ca_2.7Na_0.3Co₂O₆ were synthesized by solid-state reaction to evaluate the effect of substitution on the thermoelectric properties of Ca₃Co₂O₆. Substitution by Mn, Cu and Na appears to increase carrier density, given that electrical resistivity (ρ) and the Seebeck coefficient (S) were simultaneously reduced. Conversely, Fe substitution seems to reduce carrier density, resulting in a simultaneous increase in S and ρ. Cu and Na substitution resulted in a significant decrease in ρ due to enhancement of grain size and grain boundary connectivity, which could have a strong impact on ρ. Not only the intrinsic substitution effect on the electronic state but also this modification of the microstructure plays an important role in improvement of the thermoelectric power factor, particularly in the case of the Na-substituted sample.