Thermoelectric properties of NaCo2-xCuxO4 improved by the substitution of Cu for Co

We prepared a set of polycrystalline samples of the thermoelectric oxide Na1.2Co2-xCuxO4 (x = 0, 0.1, 0.2 and 0.3), and investigated the Cu substitution effects: on their thermoelectric properties. The substituted Cu significantly enhances the thermopower over the temperature range from 4 to 300 K, whereas it does not appreciably change the resistivity and the thermal conductivity. As a result, the thermoelectric figure of merit increases by five times at 100 K from x = 0 to 0.3. The Cu substitution effects observed here are highly incompatible with usual impurity effects that increase the resisitivity leaving the thermopower unchanged. The doped Cu shifts the reflectivity edge to lower energies, implying that it causes a decrease of the carrier concentration and/or an increase of the effective mass.