Nanostructured materials for thermoelectric applications

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The present context related to the generation and use of cleaner energy favors the development of technologies based on thermoelectric (TE) materials. From those materials, it is possible to fabricate various devices called TE modules that have no moving mechanical parts resulting in reliable performances. The modules can be operated in cooler mode based on Peltier effect or in thermoelectric generator mode where electricity is produced from waste heat. The performance of thermoelectric materials is evaluated from their figure of merit. To obtain high values of the figure of merit, the material should have at the same time high electrical conductivity and low thermal conductivity. Some semiconductors have this particular behavior giving better TE properties. Among the various approaches for reducing thermal conductivity, one of the most promising is to produce nanostructured materials. By having more interfaces between nanocrystals, higher contribution of thermal resistance comes from the lattice component. During the last years, our research group at École Polytechnique has developed a method to produce semiconductor alloys based on bismuth telluride alloys from powder metallurgy. The method consists in alloy synthesis by mechanical alloying followed by hot extrusion. Preliminary characterization of the alloyed powders revealed that particles of sub-micron size are polycrystalline with grain size in the range 5-20 nm. The goal of the proposed research project is to characterize the evolution of nanostructured materials along the production process of bulk thermoelectric materials. The study and analysis of the nanostructures will be done with the help of advanced X ray diffraction and transmission electron microscopy techniques. Within the course of the project, we will also verify the feasibility to produce actual devices and systems built with modules fabricated by using bulk nanostructured TE alloys. The performance of systems will be evaluated on real refrigerators build by one of our industrial partner in the project. They also experiment various system for electricity generation from waste heat at low temperatures on the order of 100°C