Effects of partial substitution of transition metals for cobalt on the high-temperature thermoelectric properties of Ca3Co4O9+δ -: art. no. 103905

Polycrystalline samples of Ca3CO4-xMxO9+delta (M=Ni, Fe, Mn, Cu; x=0-0.6) have been prepared using sol-gel procedure followed by spark plasma sintering. Their thermoelectric properties have been carefully examined from room temperature to 1000 K. Substitution of Ni and Fe for Co caused the significant increase of electrical resistivity, but the thermopower changed only slightly. The magnitude of the increases in electrical resistivity and thermopower for the Mn-substituted samples was the most pronounced as compared with Ni- and Fe-substituted samples in the same substitution content, which could be explained by the strong localization of charge caused by Mn substitution. Temperature dependence of electrical conductivity showed that the hole hopping conduction mechanism was dominant for all samples. The activation energy increased with increasing substitution content. On the other hand, the Cu substitution resulted in decreases of both electrical resistivity and thermopower, and the activation energy remained unchanged for Cu-substituted sample. These results suggest that Ni, Fe, and Mn substitute the Co in the CoO2 layers, which is the conducting path in the Ca3Co4O9+delta structure, while Cu substitutes the Co in the Ca2CoO3 layers. (c) 2005 American Institute of Physics.