Developing contacting solutions for Mg2(Si,Sn)-based thermoelectric generators : evaluating Cu and Ni45Cu55 as candidates for contacting electrodes, and establishing the importance of charged point defects in the contacting process

Thermoelectric materials are materials used for several applications such as waste heat recovery. Building a functional thermoelectric device requires well-designed semiconductor/metal interfaces that would guarantee high device efficiency. This metallic layer called “electrode” is added to the thermoelectric material through a “contacting” step, which, though necessary, has received far less attention than thermoelectric material development. This thesis focuses on said contacting step and presents results of joining n- and p-type Mg2(Si,Sn) with several electrodes. Mg2(Si,Sn) materials were selected because they consist of abundant, light-weight, non-toxic and environmental friendly elements. The first evaluated electrodes were Cu and Ni45Cu55, chosen as they have coefficients of thermal expansion close to that of the studied composition Mg2Si0.3Sn0.7. Experiments of contacting with these electrodes showed that Cu resulted in low electrical contact resistance rc (<10 µΩ cm2) with both n- and p-type materials, while Ni45Cu55 resulted in slightly higher values (<30 µΩ cm2). SEM/EDX investigations showed that both electrodes diffused into the Mg2(Si,Sn) material, resulting in the formation of multilayered interfaces. Surprisingly, a change in the thermoelectric properties of the n-type samples was also recorded after contacting with Cu and Ni45Cu55, while no such behavior was observed with the p-type samples. This interaction between electrode material and thermoelectric material was previously observed but has not been yet understood. This phenomenon was therefore investigated using the examples of Ag, Cu and Ni electrodes. In this study, the intrinsic and electrodeinduced charged point defects of the TE materials were considered, and hybrid-DFT calculations were used to obtain their formation energies. A comparison of these defects based on their stabilities and the interplay between them successfully explained the experimentally observed changes in the thermoelectric properties for both n and p-type material types. Based on these findings, such method using hybrid-DFT calculations to predict the behavior of a metallic electrode and a TE material was established as a pre-selection tool for contacting candidates and is expected to facilitate the electrode selection procedure.


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