THERMOELECTRIC MATERIALS IN PSEUDOGAP SYSTEMS

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Attention has been focused on the development of highly efficient thermoelectric materials to resolve energy shortage problem possibly encountered in the midst of the 21 century. The magnitude of the Seebeck coefficient, which serves as the most important factor to raise the efficiency of the thermoelectric performance, is known to be decided by electronic states at the Fermi level. A high Seebeck coefficient will be achieved in a system where the electron density of states at the Fermi level is low and the slope is steep, provided that the relaxation time approximation is valid and that the system is isotropic. In the present work, we carried out the research to develop new thermoelectric materials in systems where the pseudogap exists at the Fermi level and, hence, satisfy the conditions above.The dimensionless figure of merit ZT was studied in substances, which include Co-oxides, Fe2VA1 and Al-Re-Si 1/1-cubic approximants. Its value has reached almost unity in all these systems. In the Al-Re-Si system, the crystal structure has been precisely determined and electronic structure was calculated by using the atomic structure thus determined. The observed Seebeck coefficient and its temperature dependence were successfully interpreted by using the electronic structure thus obtained. The Seebeck coefficient in Cu-oxides was also calculated by using the electronic structure determined from the angle-resolved photoelectron spectroscopy measurements. The measured Seebeck coefficient and its temperature dependence were again well reproduced. Our technique can be applied to any materials characterized by a metallic conduction. We believe that a new technique to design a highly efficient thermoelectric material has been established in this project on the basis of the detailed study on the electronic structure near the Fermi level within the framework of the Boltzmann transport equation.