High temperature thermoelectric properties of co-doped Ca3−xAgxCo3.95Fe0.05O9+δ (0≤x≤0.3)

We have fabricated a series of co-doped misfit-layered cobaltites Ca3−xAgxCo3.95Fe0.05O9+δ with x=0.0, 0.1, 0.2 and 0.3 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. The level-off thermopower behavior of misfit-layered cobaltites in the temperature range of 200 and 300 K is often observed. Instead, the thermopower of Ca3−xAgxCo3.95Fe0.05O9+δ increases with increasing temperature up to 700 K, which could be associated with the strong temperature dependence of quasiparticle resonance. Partial substitution of Ag+ for Ca2+ in the Ca2CoO3 sublattice results in simultaneous increase of conductivity and thermopower. Considering that both the sublattices of CoO2 and Ca2CoO3 could make contribution to the transport properties, it is therefore likely that the conduction behavior of the Ca2CoO3 sublattice is activated type in the framework of two-carrier system. Ca2.9Ag0.2Co3.95Fe0.05O9+δ exhibits the highest power factor and highest figure of merit (zT) among this series of samples.