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

Polycrystalline Ca3Co1.8M0.2O6 (M=Mn, Fe, Co, Ni, Cu) and Ca2.7Na0.3Co2O6 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 (image) and the Seebeck coefficient (S) were simultaneously reduced. Conversely, Fe substitution seems to reduce carrier density, resulting in a simultaneous increase in S and image. Cu and Na substitution resulted in a significant decrease in image due to enhancement of grain size and grain boundary connectivity, which could have a strong impact on image. 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.