Physics of the thermoelectric oxide NaCo2O4: A guide to new thermoelectric oxides

Since high thermoelectric performance in NaCo₂O₄ was found in 1997, various transition-metal oxides have been extensively examined as a thermoelectric material. We will briefly summarize the physics developed in NaCo₂O₄, and explain the mechanism of the superior thermoelectric properties of this compound. An important issue is that NaCo₂O₄ 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 NaCo₂O₄ 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.