Electronic transport of co-doped misfit-layered cobaltites

We have fabricated two series of co-doped misfit-layered cobaltites Ca3Co4O9+delta, Ca3-xYbxCo4-yAgyO9+delta with (x = 0.05, y = 0.05), (x = 0.05, y = 0.10), and (x = 0.05, y = 0.10), and Ca3-xEuxCo4-yAgyO9+delta with (x = 0.05, y = 0.05), (x = 0.05, y = 0.10), and (x = 0.05, y = 0.10) using conventional solid state reaction. The electrical resistivity and thermopower were measured between 300 and 700 K. For all the samples, the temperature dependence of electrical resistivity exhibits broad maximum, indicating disappearance of quasiparticle resonance. Unlike the level-off thermopower behavior between 200 and 300 K, which is often observed for the misfit-layered cobaltites, the thermopower of all the samples increases with increasing temperature up to 700 K, which could be associated with the strong temperature dependence of quasiparticle resonance. Considering that both the sublattices of CoO2 and Ca2CoO3 could make contribution to the electronic transport, it is plausible to explain the variation of the size and temperature dependence of electrical resistivity and thermopower in the framework of two-carrier system. Among the samples, Ca2.95Eu0.05Co3.95Ag0.05O9+delta exhibits the highest power factor of 3.36 mu W cm(-1)K(-2) at 700 K. This value represents an improvement of about 110 % compared to the undoped Ca3Co4O9+delta. Ca2.95Eu0.05Co3.95Ag0.05O9+delta had the highest dimensionless figure of merit of 0.037 at 300 K, representing an improvement of about 84 % compared to the undoped Ca3Co4O9+delta.