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 were extensively examined as a thermoelectric material. In this talk, I will briefly summarize the physics developed in NaCo2O4, and explain the mechanism of the superior thermoelectric properties of this compound. A key issue is that NaCo2O4 exhibits no structural, electronic, and magnetic transitions from 1 to 1000 K. 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. Since NaCo2O4 is a p-type oxide, an n-type oxide with high thermoelectric performance is indispensable to all-oxide thermoelectric devices. In the latter half of my talk, I will review our strategy how to design an n-type transition-metal oxide, though our attempts are not yet successful.