Ga Substitution and Oxygen Diffusion Kinetics in Ca3Co4O9+δ-Based Thermoelectric Oxides

This is the authors' abstract. We don't add key points for this paper.

The influence of Ga doping on the crystal structure and cobalt oxidation state of Ca 3 Co 4 O 9+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 [Ca 2 CoO 3 ] 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 Ca 3 Co 4 O 9+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.