Thermoelectric materials in the alloy system Bi2Te3-In2Te3

块状纳米结构热电材料被认为是目前最有前途的高效热电器件材料。在本项目中，我们的目标是控制体热电材料Bi2Te3-In2Te3中从介观到纳米尺度的几个微观结构特征的微观结构形成，并评估热电性能方面的微观结构-性能关系。计划工作的新颖之处在于迄今为止对微观结构形成的优越控制，因为化学均匀性，晶体取向和晶界密度将独立设计，以实现最佳的热电性能。到目前为止，文献中关于该材料的研究结果主要集中在各向同性多晶样品的微观结构表征，辅以少量的热电性能测量，但没有系统的组合测量数据。本项目的第一步是制备具有均匀In浓度和降低晶界密度的Bi2Te3基取向晶体。为了制备化学均匀的Bi2Te3单晶，提出了一种具有种子晶的区域熔炼技术，以控制化学成分和晶体取向。为了选择合适的样品和种子浓度，对Bi2Te3-In2Te3体系的伪二元相图进行了实验评价。第二步，将首次在Bi2Te3基体中对取向晶体进行热处理实验，并通过高空间分辨率的透射电子显微镜（TEM）对取向关系、组成和纳米结构颗粒周围应力状态进行详细表征。最后，将对制备的纳米结构材料进行热电测量，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、晶体生长偏好取向和微观结构长度尺度的控制，可以分别利用这些特征对塞贝克系数、热电性能各向异性和导热系数的影响。因此，期望Bi2Te3-In2Te3的热电“优值”显著增强。

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.