Thermoelectric materials in the alloy system Bi2Te3-In2Te3

• Applicant: Professor Dr. Markus Rettenmayr (†)
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials ; Synthesis and Properties of Functional Materials
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 265148924

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.

• DFG Programme: Research Grants