Physical properties of NaCo/sub 2/O/sub 4/ and the related oxides: strongly correlated layered oxides as thermoelectric materials

Since we unexpectedly discovered a large thermoelectric power for NaCO/sub 2/O/sub 4/ in 1996, we have been searching a possibility to synthesize a new thermoelectric material using 3d transition metal oxides. In some transition-metal oxides called "strongly correlated systems", the Coulomb repulsion between electrons is strong enough to break down a conventional band picture. Thus, in such a case, the material parameters can exceed the limitation of band calculations, and we hope that NaCo/sub 2/O/sub 4/ is indeed the case. Further investigation has revealed that NaCo/sub 2/O/sub 4/ shows unusually low thermal conductivity, possibly owing to the disorder in the Na layer. This indicates that the layered structure is as important for the thermoelectric performance as the strong correlation.