原子对分布函数方法对ZrNiSn基half-Heusler热电材料结构缺陷的研究

Thermoelectric materials can convert heat directly into electricity, and vice-versa, by passing currents. Thermoelectricity, which provides a reliable and fully solid-state means of power generation and cooling, could play an important role in a global sustainable energy solution. Among the wide variety of thermoelectric materials, ZrNiSn based half-Heusler compounds have been identified and well-studied as potential high-temperature thermoelectric materials for power generation. However, the knowledge behind the defects and disorder in the half-Heusler compounds is limited due to the lack of powerful and convenient methods. The interrelation between atomic disorder and thermoelectric properties is still not known as it is very difficult to study the structure of partially ordered materials. In this project, atomic pair distribution function technique is proposed to study and predict the disorder of ZrNiSn based half-Heusler thermoelectric materials. The analysis of the total scattering pattern or pair distribution function is an invaluable tool to study the disordered structure. Because pair distribution function analysis makes use of both Bragg and diffuse scattering intensities, it is a powerful method that allows extraction of structural information not only of the long-range material structure (average structure) but also of the local distortions away from the average structure. Therefore, pair distribution function measurement is an ideal technique to study the structures of the nanoinclusions and defects in the materials. By probing the detailed structure and defects information of ZrNiSn based compounds, one could get a better understanding of the interrelation between the material structure and thermoelectric properties. This may help the design of novel, high efficient, cheap and stable thermoelectric materials.

热电材料提供了一种安全可靠、全固态的发电和制冷方式，能够将废热直接转化为电能，在新能源技术领域具有重要的应用前景。ZrNiSn基half-Heusler材料作为一种新型热电材料，其结构无序（如晶体缺陷和第二相纳米颗粒等）对热电性能有非常重要的影响。然而长期以来一直缺少直接、有效的实验方法来研究这些缺陷和第二相纳米颗粒结构。本申请项目将采用对分布函数方法研究ZrNiSn基half-Heusler热电材料中的结构缺陷。由全散射实验获得的对分布函数同时包含布拉格衍射和漫散射的信息，不仅能够提供材料中的长程原子有序结构，还包括了原子短程结构的信息，因此对分布函数方法非常适合用来研究材料的晶体缺陷和纳米颗粒的结构。通过对ZrNiSn基材料的晶体结构和缺陷机理加以细致、翔实的分析，理解其与热电性能的关系，有助于进一步了解热电材料，为寻找和设计具有高性能、低成本的热电材料提供指导。

正确了解材料的晶体结构和结构缺陷对进一步理解和预测材料性能至关重要。本项目针对热电材料及其他能源材料存在的纳米合成、晶体结构和结构缺陷等问题，利用先进晶体学研究方法，如同步辐射X射线衍射（SR-PXRD）和对分布函数（PDF）方法，探索材料的形成过程、结构和缺陷等，揭示材料的制备、结构与性能之间的关系。本项目利用Fullprof，Jana2006等软件对SR-PXRD数据进行Rietveld精修，结合对分布函数（PDF），最大熵法（MEM）等先进分析方法，获得原位合成纳米材料的形成机理和形核与生长动力学过程，以及更加合理、准确地得出材料的晶体结构和结构缺陷。我们揭示了MgAgSb的晶体结构和结构缺陷，合理地解释了MgAgSb材料结构与性能的关系，认为弱化学键和原子缺陷的存在导致了低温相α-MgAgSb的低热导率，而高温相γ-MgAgSb存在很大的无序度，表现为Mg和Ag原子的混合占位以及大量空位的存在，这是导致γ-MgAgSb具有较低热导率的主要原因；提出了水热合成Bi2-xSbxTe3（x = 0-2）纳米材料的形成机制，阐述了纳米材料结构缺陷形成机制；利用粉末XRD解析了新型固态电解质Na3PSe4的晶体结构，发现结构中部分Na占据12d空位，形成Na离子通道，揭示了其离子导电机制；提出了锐钛矿TiO2纳米颗粒的OH缺陷模型，发现TiO2纳米颗粒内部是有序结构，而表面是一层包含OH缺陷的无定型结构。此外，我们还研究了Pt纳米材料、PtRu双金属纳米材料的形核和生长机制，并在一些新型材料的氧还原催化机理方面开展了探索性研究工作。