Long term stability of silicide based thermoelectric materials and modules

Slik oppsummerer Antoine de Padoue Shyikira selv avhandlingen:

This dissertation reports on a study conducted to investigate the stability and durability of silicide-based thermoelectric materials (TEM) and modules. Silicide based alloys such as magnesium silicide (Mg-Si-Sn) and manganese silicide (Mn-Si) as n-type and p-type thermoelectric alloys, respectively, have shown promising thermoelectric properties to directly convert heat energy to electricity in the temperature range of 200⁰C - 400⁰C. In addition, the compounds are made of cheap, non-toxic, and abundant raw materials. However, there are also challenges that need to be addressed to insure sustainable material systems for applications such as waste heat recovery. Among the challenges, it is important to find semiconductor materials (n-type and p-type) with matching thermoelectric and thermomechanical properties to insure robust and thermochemical stable modules.

In that regard, this Ph.D. project focused on studying the thermal oxidation of Mn-Si based alloys in air, designing the TEM - electrode contact interfaces, evaluating the performance of the built thermoelectric modules, and finally testing the long-term stability of the modules.

The thermal oxidation studies conducted in the temperature range of 250⁰C - 600⁰C, showed that the composition and synthesis route of the studied materials had major impact on their oxidation potential. Normally, silicon forms a protective layer of silicon-oxide, which is protective (or stops further oxidation) of the remaining material. Comparing Mn-Si alloys powder compacted using different methods, it was found that the denser the bulk material was the more it was robust to oxidation. Moreover, it was also found that the more the alloys were more homogeneous, the level of protection to oxidation increased. Lastly, different dopants (such as Aluminum, Germanium, and Molybdenum) were used to increase the electrical conductivity of Mn-Si alloys, and it was found that with Ge the alloys were more protected against oxidation.

Finally, the built thermoelectric modules stability over time was investigated. The modules were robust over time, however, after around 300 thermal cycles, they showed gradual degradation. The degradation was due to the mechanical failures of the contact interfaces and oxidation of the n-type magnesium silicide. Therefore, further work could investigate protecting the magnesium silicide against oxidation by encapsulation or applying high temperature coating, and further improve the robustness of the contact interfaces against thermal stresses.   

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