Physics of the thermoelectric oxide NaCo/sub 2/O/sub 4/: A guide to new thermoelectric oxides

Since high thermoelectric performance in NaCo/sub 2/O/sub 4/ was found in 1997, various transition-metal oxides have been extensively examined as a thermoelectric material. We will briefly summarize the physics developed in NaCo/sub 2/O/sub 4/, and explain the mechanism of the superior thermoelectric properties of this compound. An important issue is that NaCo/sub 2/O/sub 4/ exhibits no structural, electronic, and magnetic transitions. This implies that a large amount of entropy at high temperatures is not released down to low temperatures, which is an origin for the heavy-fermion-like states and the large thermopower. Since NaCo/sub 2/O/sub 4/ is a P-type oxide, an N-type oxide with high thermoelectric performance is indispensable to all-oxide thermoelectric devices. We will review our attempts to design an N-type transition-metal oxide, though they are not yet successful at present.