Ga Substitution and Oxygen Diffusion Kinetics in Ca₃Co₄O_9+δ-Based Thermoelectric Oxides

Abstract

The influence of Ga doping on the crystal structure and cobalt oxidation state of Ca3Co4O9+δ was determined using synchrotron X-ray diffraction and X-ray absorption near-edge spectroscopy. The results show that Ga is preferentially located on the Co site in the rock-salt-type [Ca2CoO3] layer, up to a maximum occupancy of ∼2.2%, and that doping does not change the cobalt oxidation state of this material. We find that the doping-induced point defect scattering and modification of charge carrier concentration result in lower thermal conductivity and lower electrical resistivity, thereby improving the thermoelectric figure of merit in the Ga-doped Ca3Co4O9+δ material. In addition, the influence of sample density and measuring atmosphere on their thermoelectric performance was found to be significant. It is believed that the interconnected pore structure in the low-density sample facilitates oxygen out-diffusion from the lattice, which leads to anomalous electrical resistivity behavior at elevated temperatures. We show that this behavior can be effectively suppressed by improving the sample density.