复杂Zintl相化合物热电材料的设计与载流子浓度调节

Thermoelectric materials are a special class of compounds which can conveniently realize the energy conversion between heat and power. Based on this knowledge, thermoelectric materials are immediately found to be so attracting and prospecting in solving the current world-wide problems such as energy shortage, exhaust pollution and global warming. Moreover, thermoelectric generators made of solid-state materials are free of moving part, which makes them silent and scalable, especially suitable for fabricating small, reliable, portable devices. Despite so attracting potential applications, the practical utilization of thermoelectrics is greatly limited due to its commonly poor energy conversion efficiency. Currently commercially available thermoelectric materials are found decades ago. Extensive disorders are introduced into their crystal structures through alloying substantially with the isoelectronic species, thus the phonons can be scattered while the pathway for the electron transport are almost intact. To achieve high efficiency, recently, there have been much interest focused on materials such as filled skutterudites and clathrates. In this compounds, the loosely bonded atoms filling the cages in the crystal structure are known as "rattlers", which are believed to be the cause of extremely low, glass like thermal conductivity. However, glasses actually make very poor thermoelectrics due to the lack of "electron-crystal" property. This non-crystalline structure will result in notable electron scattering, which cause low mobility and poor electrical conductivity. These ideas bring so-called "Phonon Glass-Electron Crystal(PGEC)" and "Rattler" concepts for the strategy in improving the thermoelectric efficiency of various materialse..In this project, we plan to design new thermoelectric materials based of complex Zintl phases. Zintl phases are a special class of polar intermetallics composed of atoms with very different electro-negativity. Thus, the electrons can be viewed as completely transferred from the more electro-positive ones to the more electro-negative ones and all constituent elements can achieve a 8-electron close-shell configuration. Typically, Zintl phases are semiconductors or insulators. Complex Zintl phases seem to fit the PGEC requirement very well considering the following three aspects: the semi-conducting character will ensure necessarily high effective mass and Seebeck coefficient; Periodic structure with strong covalent bonds in the polyanionic framework brings the "electron-crystal" property which ensures the high mobility and electrical conductivity; The very complex crystal structure of Zintl phases and the electron precise nature may also cause interstitials, defects and disorders in the crystal, all of which can result in very low lattice thermal conductivity and are critical in achieving high figure of merit.

热电材料对于解决目前全球面临的能源短缺，尾气污染，温室效应等问题均有着极其重要的意义，寻求高转化效率的新一类热电材料已成为目前国际交叉前沿领域的研究热点。对于复合物热电材料的研究，目前较受关注的有以下三类：Skutterudites， Clathrates以及Zintl相化合物。其中，复杂Zintl相化合物由于其具有较低的晶格热导率，有希望构建高转化效率的热电材料，因此在近期研究中倍受关注。本项目我们立足于Zintl相化合物方面的长期工作基础，系统地研究此类化合物的设计合成，晶体结构，电子能带,载流子浓度的调节及其对热电性质的影响。通过对上述科学问题研究并总结规律，以促进相应交叉前沿领域的发展。该项目的成功实施，不仅具有重要的学术价值，还可能带来较大的经济效益和社会效益。

复杂Zintl相化合物的合成，结构与性能研究一直是材料学研究领域的一个重要方向，无论是与之相关的基础研究，理论研究，还是应用研究都对固体化学和材料学发展具有重要意义。 基于复杂Zintl相化合物的热电材料，在高温热电应用方面已经展现出了极大的潜力，近年来已成为国际功能材料研究领域的一个热点。在该项目中,我们立足Zintl相化合物方面的长期工作基础，深入地开展此类化合物的合成，晶体与电子能带结构，热电性能优化等方面工作。该项目经过为期四年的实施，已顺利达成预期目标，并在以下几方面取得了重要成果：1）在本项目中，我们开展了大量的探索性基础研究工作，系统开展了基于V族元素的Zintl化合物的实验合成与晶体结构研究工作，发现了大量新颖的Zintl相化合物，甚至包括一些首次发现的结构类型，这些基础性研究工作，为后期的材料理论和性能研究奠定了良好的基础；2）在材料应用基础研究方面，我们发现了几个热电性能优异的新热电体系，例如Ca1-xRExAg1-ySb (RE = 稀土金属元素)，Ca9Zn4+xSb9，A14MgSb11（A＝二价碱土或稀土元素）等，并对相应体系开展了深入细致的性能与优化研究；3）针对高温Zintl相热电合金合成与纯化方面的难点，我们通过不断的努力，发展了一种快速制备方法，并自主开发了一套高温微感应炉合成设备，用于解决了高熔点，易挥发金属在合金化的过程中计量比偏离以及纯化困难的问题，并应用该设备合成了大量常规固相合成无法获得以及纯化的Zintl相化合物。受本项目资助，所获得的主要成果如下：1）以第一通讯人发表高水准SCI论文16篇；2）申请热电材料方面的国家发明专利2项目，已授权1项；3）培养博士生4名，硕士生2名。