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

We prepared a set of polycrystalline samples of the thermoelectric oxide Na 1.2Co2-xCuxO4 (x = 0, 0.1, 0.2 and 0.3), and investigated the Cu substituti on effects: on their thermoelectric properties. The substituted Cu signific antly enhances the thermopower over the temperature range from 4 to 300 K, whereas it does not appreciably change the resistivity and the thermal cond uctivity. 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 her e are highly incompatible with usual impurity effects that increase the res isitivity leaving the thermopower unchanged. The doped Cu shifts the reflec tivity edge to lower energies, implying that it causes a decrease of the ca rrier concentration and/or an increase of the effective mass.