Hokkaido University Research Profiles


Stable and Practical Oxide Thermoelectric Conversion Materials

As a result of replacing the sodium ions in the layered cobalt oxide with barium ions of heavier atomic mass, only the thermal conductivity decreased while the electrical properties remained unchanged. We have found that the thermoelectric figure of merit ZT reaches 0.11 at room temperature.

Content of research

Thermoelectric conversion has been attracting attention as a technology to recycle waste heat. Metal chalcogenides are known as thermoelectric materials, but they have thermal and chemical stability and toxicity issues. Layered cobalt oxides are stable at high temperatures and in air, but have the problems that thermal conductivity is high and conversion performance is low. The research group considered the strategy shown in Fig. 1 to reduce the thermal conductivity of layered cobalt oxide AxCoO2. Figure 2 summarizes the thermoelectric properties in the direction parallel to the layers of Ax-substituted AxCoO2 thin films measured at room temperature. The thermal conductivity shows a monotonically decreasing trend with increasing Ax atomic weight. The room temperature thermoelectric figure of merit of Ba1/3CoO2 is 0.11. The figure of merit ZT increases with increasing temperature. By further enhancing the thermoelectric conversion performance, it is expected to realize stable and practical thermoelectric conversion materials.

  • Caption: Crystal structure of layered cobalt oxide (left) and A-ion dependence of thermoelectric conversion performance index ZT (room temperature) (right)

  • Fig. 1 ?(Left) Crystal structure of layered cobalt oxide AxCoO2. (Right) Schematic diagram of heat conduction along the layers of AxCoO2. If babies are placed on a mattress that contains many springs, the mattress springs will not be affected, but if big wrestlers would get onto the mattress, the mattress springs will shrink and stop moving.

  • Fig. 2 ?Thermoelectric properties in the direction parallel to the layer of Ax-substituted AxCoO2 thin films (room temperature). The output factor expressed as [(thermopower)2×(electrical conductivity)] was almost constant independent of Ax, while the thermal conductivity tended to decrease monotonically with the increase in the Ax atomic mass. Ba is the heaviest element that can be selected from alkali metals and alkaline earth metals. Only this lower thermal conductivity was directly reflected in the change in thermoelectric figure of merit, which reached 0.11 for Ba-substituted Ba1/3CoO2, the largest thermoelectric figure of merit for oxides at room temperature.

Potential for social implementation

  • In our laboratory, we are measuring thermoelectric conversion performance index in thin films such as single crystals to bring out the true performance of the materials. If the material can be made into a large bulk by powder or sintering processes, we believe that it can be applied as a thermoelectric conversion material to convert high-temperature waste heat into electricity.

Appealing points to industry and local governments

It is an oxide thermoelectric conversion material that is non-toxic and has strong resistance to heat and oxidation.