Thermoelectric properties of NaxCoO2-δ (x≈0.5) system; focusing on partially substituting effects

Polycrystalline samples of a 3d-transition-metal oxide Na_x CoO_2-δ (x≈0.5) system have been synthesized using a standard solid-state reaction method, and the thermoelectric properties have been measured. Although it has been, in general, believed that oxides are not suitable for thermoelectric materials, Na _xCoO_2-δ system shows a good thermoelectric performance, i.e. a high electrical conductivity, a large thermoelectric power and a low thermal conductivity in a wide temperature range from room temperature to 1073 K. The unexpected large thermoelectric power is likely to come from a strong electron-electron correlation. In this study, we have tried to partially substitute alkali-metal, alkaline-earth metal, transition-metal, and rare-earth element for Na or Co site in order to investigate the transport properties of Na_x CoO_2-δ. Due to the alkaline-earth-metal-doping, the thermoelectric power was enhanced while the conductivity was reduced. This indicates that the effective carriers are electron holes. On the other hand, the Ag-doping improves both the conductivity and the thermoelectric power, suggesting that the transport properties of Na_xCoO_2-δ are not classical metallic behaviour