Conduction with Large Transport Entropy in Electrically Conducting Oxides and Its Applications to Thermoelectric Materials

We have investigated large transport entropy in metal oxides with particular emphasis on their thermoelectric properties. The summary is as follows: 1.Selective substitution for Na and Co in NaCo_2O_4 by Ni revealed that the substitution for Na decreased the electrical conductivity, whereas that for Co decreased the Seebeck coefficient. These results suggest that Ni at the Co sites disturbs the spin-spin exchange between the Co atoms, which has been considered to be responsible for unusually large thermopower of the oxide. 2.Layered vanadate LiV_2O_4 and LiVO_2 with the layered crystal structure similar to that of NaCo_2O_4 showed large positive thermopower and rather low electrical conductivity. Attempted p-type doping failed to increase the conductivity, probably due to the large electron specific heat of the oxide comparable to those observed for"heavy-fermion" materials, suggesting large transport entropy in these oxides. 3.Large enhancement of thermopower was observed for oxide materials with nanoviod structure. The nanovoid structure was induced by mixing polymer beads of 150 nm in diameter as a void forming agent (VFA) with the starting oxide powder mixture, followed by firing in an inert atmosphere. The electrical conductivity of the resulting sintered samples was almost the same as that of the pristine sample without VFA, yet the thermopower showed intense negative maxima at around 5 00-600 ーC. This enhancement of the thermopower can be attributed to the porous structure in oxides. Moreover, the thermal conductivity of the nanovoid samples showed a marked decrease by 30-35% from room temperature up to 760℃ These results strongly suggest that the nanovoid structure serves as selective scattering centers for phonons.