Thermoelectric materials in the alloy system Bi2Te3-In2Te3

块体纳米结构热电材料目前被认为是最有前途的高效热电器件元件。在本项目中，我们的目标是控制块体热电材料Bi2Te3-In2Te3从介观到纳米的几个微结构特征的微结构形成，并评估与热电性能相关的微结构-性能关系。计划中的工作的新颖性在于对微观结构形成的优越控制，因为化学均匀度、晶体取向和晶界密度将独立设计，从而实现最佳的热电性能。到目前为止，关于这种材料的文献记载的结果主要涉及各向同性多晶样品的微结构表征，辅之以少量的热电性能测量，但没有系统的综合测量数据。本项目的第一步是制备具有均匀浓度和降低晶界密度的Bi2Te3基取向晶体。为了获得化学均一的Bi2Te3单晶，开发了一种可同时控制化学成分和晶体取向的籽晶裁剪区域熔炼技术。为了选择合适的样品和晶种浓度，对Bi2Te_3-In_2Te_3系统的准二元相图进行了实验评价。在第二步中，将首次对定向晶体进行热处理实验，以在Bi2Te3基质中产生纳米结构的析出物，并将通过高空间分辨率的透射电子显微镜(TEM)进行详细的表征，包括纳米结构粒子周围的取向关系、成分和应力状态的统计数据。最后，对所制备的纳米结构材料进行热电测试，旨在建立纳米结构与热电性能之间的关系。结合化学均匀性、晶体生长择优取向和微结构长度尺度的控制，可以分别利用这些特征对Seebeck系数、热电性能各向异性和热导率的影响。因此，Bi2Te3-In2Te3的热电“优值系数”有望显著提高。

Bulk nanostructured thermoelectric materials are at present considered as the most promising components for high efficiency thermoelectric devices. In the present project, we aim to control microstructure formation with respect to several microstructural features from the meso- to the nanoscale in the bulk thermoelectric material Bi2Te3-In2Te3 and evaluate the microstructure-property relationships with respect to the thermoelectric properties. The novelty of the planned work is in the by far superior control of microstructure formation, as chemical homogeneity, crystal orientation and grain boundary density will be designed independently such that optimum thermoelectric properties are achieved. Results so far documented in the literature on this material mainly concern the micro-structural characterization of exclusively of isotropic polycrystalline samples, complemented by a small number of measurements of thermoelectric properties, but no data of systematic combined measurements. The first step of the present project is to prepare Bi2Te3 based oriented crystals with homogeneous In concentration and reduced grain boundary density. A tailored zone melting technique with seed crystal is developed to control both chemical composition and crystal orientation, aiming to generate chemically homogeneous Bi2Te3 single crystals. For choosing an adequate concentration of sample and seed, the pseudo-binary phase diagram of the Bi2Te3-In2Te3 system is assessed experimentally. In a second step, for the first time heat treatment experiments will be performed on the oriented crystals for generating nanostructured precipitates in the Bi2Te3 matrix, of which detailed characterization including statistics on orientation relationship, composition and stress state around nanostructured particles will be performed via Transmission Electron Microscope (TEM) at high spatial resolution. Finally, thermoelectric measurements will be carried out on the prepared nanostructured materials, aiming to build the relations between the nanostructure and thermoelectric performance. Combining the control of chemical homogeneity, crystal growth preferred orientation and microstructural length scales will allow to exploit the impact of these features on Seebeck coefficient, thermoelectric property anisotropy and thermal conductivity, respectively. Consequently a significant enhancement of the thermoelectric "figure of merit" of Bi2Te3-In2Te3 is expected.