Development of Mg2(Si, Sn)-based thermoelectric generators: investigating contacting solutions, fabrication process, evaluating measurement reliability and optimization paths via modelling

Thermoelectric (TE) technology has the ability to convert heat into electricity and is therefore attractive in the context of the search for new green energy sources. The aim of this thesis is to successfully build, characterize and model eco-friendly TE generators (TEG) made of p- and n-type Mg2(Si,Sn) TE materials.

The TE materials are already well-researched but a compatible electrode, electrically interconnecting the TE elements, is missing on the road towards a TEG. Moderately satisfying electrodes were reported in literature, but lacked chemical or mechanical stability. In this work, we find Al to be a promising electrode for Mg2(Si,Sn): we report no cracking nor delamination, a stable interface through annealing and contact resistivities below 10 μΩcm² and stable TE materials directly after joining. This is an essential milestone towards the TEG.

We then successfully build and characterize a full Mg2(Si,Sn)-based TEG. The first efficiency measurement of such a module is reported and we measure a high power density of 0.9 W/cm². We combine the TEG characterization with an analysis based on constant property model (CPM) to identify loss mechanisms. An increase of the inner resistance of the TEG, attributed to cracking due to high thermal stress, is observed. It is predicted that both efficiency and power output could be realistically increased by 30% solely by preventing this cracking.

Finally, we test the hypothesis that the TEG mechanical failure is due to the ceramic plate by using an open module design with bare Cu bridges. Additional voltage probes are soldered to monitor the resistance of each leg during the measurement. This innovative approach consistently shows that the resistance of the n-type legs systematically increases even at low temperatures. It is found that increasing the cross-section of the legs solves the issue when the hot side temperature remains below 300 °C; suggestions for even further improvement are given.

Overall, we identify the first truly promising electrode for Mg 2(Si,Sn) and use it to successfully build one of the first eco-friendly Mg2(Si,Sn)-based TEGs. We report the first efficiency measurement for such a module and implement a different design to improve the performance. Strategies for further improvement are identified and tested with innovative measurement and modelling techniques.



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