Cd doping and metallic Ag additives in Ca3Co4O9+δ polycrystalline materials are shown to result in improved thermoelectric (TE) transport properties. Carrier concentration and mobility were optimized through the combination of doping and compositional modulation approaches. The formation of filiform Ag nanoinclusions between the interlayers and grain boundaries enhances the anisotropic carrier transport, leading to higher carrier mobility. A spin entropy enhancement due to the change of the net valence of Co induced by Cd substitution on the Ca site was confirmed by X-ray photoelectron spectroscopy. High carrier mobility and enhanced spin entropy results in higher electrical conductivity and Seebeck coefficient, leading to the increase of the power factor. In conjunction, mass fluctuation between Cd and Ca on the same crystal site along with the increase of metallic Ag nanoinclusions effectively lowers thermal conductivity. Consequently, the figure-of-merit, zT, has been improved to 0.31 at 950 K for 10 wt % Ag-modified Ca2.9Cd0.1Co4O9+δ specimen, which is a significant improvement compared to the pristine material. This dual-mode control of electron and phonon transport by including Ag additives and Cd doping offers an approach for tuning the correlated TE parameters.
Keywords: carrier transportation; filiform Ag; nanoinclusions; phonon scattering; thermoelectric.