THERMOELECTRIC MATERIALS IN PSEUDOGAP SYSTEMS

开发高效热电材料以解决世纪中期可能遇到的能源短缺问题已成为人们关注的焦点。作为提高热电性能效率的最重要因素的塞贝克系数的大小已知由费米能级处的电子状态决定。在弛豫时间近似有效且系统各向同性的情况下，在费米能级处的电子态密度低且斜率陡的系统中将获得高的塞贝克系数。本论文研究了在费米能级上存在赝能隙的体系中开发新的热电材料，研究了Co氧化物、Fe_2 VA_1和Al-Re-Si 1/1立方近似物的无量纲热电优值ZT。它的价值在所有这些系统中几乎达到了统一。在Al-Re-Si系统中，晶体结构已经被精确地确定，并且通过使用由此确定的原子结构计算电子结构。利用得到的电子结构成功地解释了所观测到的塞贝克系数及其温度依赖性。铜氧化物的塞贝克系数也计算通过使用的电子结构确定的角度分辨光电子能谱测量。测得的塞贝克系数和它的温度依赖性再次很好地再现。我们的技术可以应用于任何具有金属导电性的材料。我们认为，在玻尔兹曼输运方程框架下，在费米能级附近的电子结构的详细研究的基础上，本项目已经建立了一种新的技术，以设计一个高效的热电材料。

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.