Hokkaido University (Hokkaido University) announced on November 2 that it has realized a new “layered cobalt oxide” with a thermoelectric conversion performance index ZT = 0.11, which is the highest class ever at room temperature.

The results were achieved by Zhang Amebashi, a visiting researcher at the Institute of Electronic Chemistry, Hokkaido University (JSPS Foreign Research Fellow), Yusuke Takashima, a graduate student at the Graduate School of Information Science, Hokkaido University, Assistant Professor Johejun (and the Institute of Electronic Chemistry, Hokkaido University), and the Graduate School of the University of Tokyo. This is an international collaborative research team consisting of Assistant Professor Wei, Assistant Professor of the Graduate School of Engineering, Assistant Professor Yuichi Ikuhara, Professor Yuichi Ikuhara, Hokkaido University Institute of Electronic Chemistry and Hokkaido University Graduate School of Information Science, Hiromichi Ota. Details were published in the “Journal of Materials Chemistry A”, a journal of materials science of the Royal Society of Chemistry.

Thermoelectric conversion is attracting attention as a technology for reusing waste heat discharged from factories and automobiles. As a material for thermoelectric conversion, “metal chalcogenide”, which is a compound of metal and chalcogen elements (sulfur, selenium, tellurium), is famous. For example, there is “PbTe” which is a compound of lead and tellurium.

The performance of a thermoelectric material is represented by the conversion performance index “ZT” derived by “(thermoelectricity) squared x (conductivity) x (absolute temperature) ÷ (thermal conductivity)”. The larger this value is, the better it is. For example, in the case of p-type PbTe, ZT at room temperature is about 0.1. However, since PbTe uses lead, it has a problem of toxicity, and there are also problems such as points to be improved in thermal and chemical stability, so it has not been put into practical use on a large scale.

For such metal chalcogenides, metal oxides have been expected to be used as thermoelectric materials in Japan for about a quarter of a century. This is a major reason why many metal oxides are expected to be stable in high temperature environments and in the air.

The “layered cobalt oxide” discovered by Japanese researchers in 1997 was highly expected as a practical thermoelectric material because of its high thermoelectric power and high conductivity. Cobalt oxide (CoO)2) Layers and alkali metal or alkaline earth metal layers are alternately laminated to form an oxide crystal.

However, due to its weakness of high thermal conductivity, the room temperature ZT of the layered cobalt oxide was only about 0.03. As can be seen from the formula for deriving the conversion figure of merit, the lower (smaller) the value of thermal conductivity, the larger the value of ZT. Against this background, the international collaborative research team challenged to raise the ZT value by lowering the thermal conductivity of the layered cobalt oxide.

Layered cobalt oxide is “AxCoO2Is represented by. Its crystal structure consists of an alkaline (A) ion layer and CoO.2It has a layered structure in which layers are alternately laminated. It is known that the thermoelectric conversion performance is higher in the direction along the layer (horizontal direction) than in the direction perpendicular to the layer. AxCoO2The heat conduction along the layer of is mainly CoO2It is said that it occurs when the vibration of the atoms in the layer propagates.

Therefore, CoO2If the alkaline ions above the layer are light, CoO2Since the vibration of the layer propagates without being attenuated, the thermal conductivity becomes high. On the other hand, if the alkaline ion is made heavy, CoO2The study proceeded with the hypothesis that the vibrations of the layers would soon decay.

If the vibration of each atom is a “spring”, heat propagation is similar to the propagation of expansion and contraction of a “spring” (however, unlike a spring, an atom does not vibrate in only one direction). .. Even if there is a mattress with a lot of “springs”, even if you put your baby on the mattress, the “springs” of the mattress will not be affected so much. On the other hand, for example, if a large sumo wrestler rides, the “spring” will shrink and become immobile. Based on these considerations, he predicted that the thermal conductivity could be lowered by making the alkaline ions heavier.

  • ZT

    (Left) Layered cobalt oxide AxCoO2Crystal structure. (Right) AxCoO2Schematic diagram of heat conduction along the layers of. Even if you put your baby on a mattress that contains a lot of “springs”, the “springs” of the mattress will not be affected and heat will be transferred. However, if a large sumo wrestler rides, the “spring” will shrink and become immobile, making it difficult for heat to be transmitted (Source: Hokkaido University Press Release PDF).

The international collaborative research team chose sodium (Na) as the alkaline ion, and first Na0.75CoO2A thin film was made. Then, Na by the ion exchange method0.75The weight is different Ca1/3, Sr1/3, Ba1/3Replaced with AxCoO2A thin film was made. Structural analysis of the prepared thin film was performed, and Na was first produced.0.75CoO2Na ions in the thin film are Ca1/3, Sr1/3, Ba1/3It was confirmed that it was replaced with.

Subsequent measurements of conductivity, thermoelectricity and thermal conductivity at room temperature were performed. A measured at room temperaturexSubstitution AxCoO2When the thermoelectric properties in the direction parallel to the thin film layer are summarized, the conductivity and thermoelectric power appear to be slightly affected by the substitution of alkaline ions.[(熱電能)の2乗×(導電率)]The output factor represented by is AxIt is said that it was almost constant regardless. The current flows through the CoO2Because it is a layer, AxIt can be said that the substitution of ions does not affect the electrical characteristics.

On the other hand, the thermal conductivity is AxIt was confirmed that it tends to decrease monotonically as the atomic weight increases. Barium is the heaviest element to choose from between alkali metals and alkaline earth metals. Only this low thermal conductivity is directly reflected in the change in the thermoelectric conversion performance index, and Ba replaced with barium.1/3CoO2Then, 0.11 was measured, which is the maximum as the thermoelectric conversion performance index of oxides at room temperature.

  • ZT

    AxA replaced with (alkali)xCoO2Thermoelectric properties (room temperature) in the direction parallel to the thin film layer.[(熱電能)2×(導電率)]The output factor represented by is AxThe thermal conductivity is A, while it is almost constant regardless.xIt is said that it showed a tendency to decrease monotonically as the atomic weight increased. Ba is the heaviest element to choose from between alkali metals and alkaline earth metals. Only this low thermal conductivity is directly reflected in the change in the thermoelectric conversion figure of merit, and the barium-substituted Ba1/3CoO2Then, it was found this time that the thermoelectric conversion performance index of oxides at room temperature reaches 0.11 (Source: Hokkaido University Press Release PDF).

Generally, the figure of merit ZT increases as the temperature rises. Currently, the thermoelectric characteristics at high temperatures are also being measured, and it has been confirmed that ZT will increase. In the future, it is expected that stable and practical thermoelectric conversion materials will be realized by further optimizing the composition and enhancing the thermoelectric conversion performance.