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Materials Design of Oxide Thermoelectrics

For Power Conversion Applications

Kunihito Koumoto

Nagoya University, Graduate School of Engineering, Nagoya 8603, Japan

Email: koumoto@apchem.nagoya-u.ac.jp

Oxide thermoelectrics are very promising as thermoelectric materials with several merits; not involving hazardous elements and excellent high temperature stability etc. The strategic materials design approaches are needed to develop high performance energy conversion materials. My group has already proposed heavily electron-doped SrTiO₃(STO) as a new candidate thermoelectric material which shows ZT=0.37@1,000K, the highest value among n-type oxide materials. Further improvement in ZT relies on how thermal conduction can be suppressed while maintaining the power factor, or how drastic increase in power factor can be achieved. The former approach was attempted by substituting Eu for Sr sites inducing enhanced phonon scattering while power factor was virtually kept unchanged, resulting in a slight increase in ZT to 0.39@1,000K. Another approach was to employ Ruddlesden-Popper phase with layered perovskite structures so that phonon scattering would be enhanced at the internal interfaces. This approach did succeed in reducing thermal conductivity, but power factor decreased more than thermal conductivity because of the distortion of TiO₆ octahedra leading to the reduction of carrier effective mass and hence Seebeck coefficient. The latter approach was to utilize 2DEG in a superlattice structure; undoped STO/STO:Nb superlattices were successfully prepared by PLD method and quantum confinement of electron gas was confirmed to show giant Seebeck coefficient, resulting in an estimated ZT=2.4@300K for one unit cell layer of Nb-doped STO.

References

1. S. Ohta, T. H. Ohta, K. Koumoto et al., Appl. Phys. Lett., 87, 092108 (2005).

2. K. H. Lee, S. W. Kim, H. Ohta and K. Koumoto, J. Appl. Phys., 100, 063717 (2006).

3. H. Ohta, K. Koumoto et al., Nature Mater., 6, 129-134 (2007).

4. K. H. Lee, S. W. Kim, H. Ohta and K. Koumoto, J. Appl. Phys., 101, 083707 (2007).

5. K. H. Lee, H. Ohta, K. Koumoto et al., J. Appl. Phys., 102, 033702 (2007).

6. M. Yamamoto, H. Ohta and K. Koumoto, Appl. Phys. Lett., 90, 072101 (2007).

7. K. Kato, H. Ohta, K. Koumoto, J. Appl. Phys., submitted.

8. Y. F. Wang, K. H. Lee, H. Ohta, K. Koumoto, Appl. Phys. Lett., submitted.