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Correlation between pseudo-gap electronic structures and thermoelectric properties in Fe_2 VAl intermetallic compounds

Fe_2 VAl金属间化合物赝能隙电子结构与热电性能的相关性

基本信息

批准号：
13450258
负责人：
SODA Kazuo
金额：
$ 0.9万
依托单位：
Nagoya University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (B)
财政年份：
2001
资助国家：
日本
起止时间：
2001 至 2002
项目状态：
已结题

项目摘要

The Heusler-type Fe_2VAl compound has been theoretically predicted to have a pronounced pseudo-gap across the Fermi level and experimentally proved to show the drastic change in the electronic transport properties and the large enhancement of the thermoelectric power on the deviation of the composition from its stoichiometry. The purpose of this research is to experimentally clarify the relation between their electronic structures and thermoelectric properties of the Heusler-type Fe_2VAl-related alloys with use of the normal and inverse photoelectron spectroscopy, and thus to obtain a new guideline for developing thermoelectric materials.In this study, the electronic structures have been investigated for the stoichiometric Fe_2VAl, off-stoichiometric (Fe_<2/3>V_<1/3>)_<100-y>Al_y and Fe_<2-x>V_<1+x>Al, and substituted Fe_2VAl_<1-z>Si_z. It is experimentally shown for the first time that the existence of the pseudo-gap and the relative position of the Fermi level in the pseudo-gap are strongly related to their thermoelectric properties. The change in the electronic structure near the Fermi level and thermoelectric properties with the composition is explained qualitatively by a rigid band model for (Fe_<2/3>V_<1/3>)_<100-y>Al_y and Fe_2VAl_<1-z>Si_z but not for Fe_<2-x>V_<1+x>Al. This arises from the large modification of the electronic structure near the Fermi level due to the transition metal d states in contrast to the sp valence states of Al and Si. These mid-gap d states are responsible for the unusual transport properties at low temperatures. Thus it is important to control the d states near the Fermi level for the further development of these Heusler-type alloys as thermoelectric materials. We suggest that the substitution of the forth element for the Fermi level positioning and the microscopic structural control for the low thermal conductivity are effective for the development.

Heusler型Fe_2VAl化合物在费米能级上具有明显的赝能隙，实验证明其电子输运性质随组分偏离化学计量比而发生剧烈变化，热电功率也随之显著提高。本研究的目的是用正、逆光电子能谱（XPS）研究Heusler型Fe_2VAl系合金的电子结构与热电性能之间的关系，为热电材料的开发提供新的指导。非化学计量比的（Fe_（2/3）V_（1/3））<100-y>Al_y和Fe_<2-x>V_（1+x）Al以及取代的Fe_2 VAl_<1-z>Si_z。首次从实验上证明了赝能隙的存在和费米能级在赝能隙中的相对位置与其热电性能密切相关。用刚性能带模型定性地解释了（Fe_（2/3）V_（1/3））_ Al_y和Fe_2 VAl_ Si_z的费米能级附近的电子结构和热电性能随组分的变化<100-y><1-z>，而对Fe_<2-x>V_（1+x）Al的解释则不然，这是由于过渡金属d价态对费米能级附近的电子结构的大幅度改变而不是Al和Si的sp价态所致。这些中能隙d态是低温下不寻常的输运性质的原因。因此，控制d态在费米能级附近对于进一步发展Heusler型合金作为热电材料是非常重要的。我们认为，用第四种元素代替费米能级定位和用微观结构控制低热导率是发展的有效途径。