Associate Professor, Department of Applied Physics, Waseda University
We discovered a large thermopower and anomalous properties in the layered cobalt oxide NaCo2O4 in the study of reference materials concerning high-temperature superconductors (HTSC). This study has accelerated the research field of oxide thermoelectric: the energy conversion technology between heat and electricity using oxide ceramics.
The discovery of high-temperature superconductivity in copper oxides, in 1986, has made two great impacts on basic science. One is to deny the superstition that only a mega-science can discover new physics. The other is to remove the wall between physics and chemistry; physicists have begun to synthesize many samples, and chemists have begun to measure physical properties very precisely. I am one such physicist, who makes samples, measures properties precisely, and thinks deeply about the meaning of the measured data.
NaCo2O4 is an old material, which was synthesized and identified in the 1970s by French chemists. We succeeded in making single crystals and measured the resistivity and thermopower (voltage proportional to the applied temperature difference). We found that the measured values were as good as those for the state-of-the-art thermoelectric material Bi2Te3 and reported a possibility that NaCo2O4 was applicable to thermoelectric devices.
This made a great impact to the community because the common thought at that time was that oxides were too poor to be used for thermoelectric devices. We tried to uncover the secret of this oxide, and finally arrived at the concept that the magnetism and electron-electron interaction were crucial. This idea had not been considered in thermoelectric materials thus far.
Chemists are eager to make many new materials and identify their structures. Once they have done so, they are often satisfied and leave other properties untouched. The high thermoelectric performance in NaCo2O4 is a prime example, and I believe that this indicates the raison d’être of physicists developing new materials. We physicists can find interesting and useful functions which have been sleeping quietly in old materials that chemists synthesized a long time ago. Post-HTSC physicists, including myself, will search for new functions in existing materials rather than in new materials.