Integrated Computational Materials Engineering (ICME) to Develop Electrical Contacts for Thermoelectric Devices

Abstract

Energy technology development is guided by international goals for sustainable energy supply to overcome the environmental crisis. Thermoelectric (TE) energy conversion is one among various approaches to feed global energy needs. It is a versatile option for harvesting and recovering waste heat by direct conversion of thermal into electrical energy, having important advantages such as absence of any harmful emission and moving parts. However, there are still challenges in the TE device technology. A crucial one is the contact between the TE material and the metallic bridge to build up a functional module for energy conversion, namely a Thermoelectric Generator (TEG) [1]. With the aim of bringing together the gained experimental knowledge and a robust computational approach, we apply Integrated Computational Materials Engineering (ICME) [2] to accelerate and transform the development of contact solutions as a critical step of the module making. ICME enables integration of material knowledge encoded in databases and materials processing to overcome the current status based on mainly empirical trial. The challenge lies in the intrinsic interdisciplinarity necessary for designing the contacts, a region that develops multiple intermetallic layers as well as new mobile interfaces and has to satisfy mechanical constraints without hindering the conversion performance and functional stability of the TE material. In this work we investigate the phase transformations occurring at the contacting region between TE/metal, in particular for Mg-Si-based TE materials and Cu as metal electrode. We investigate the diffusion and reaction phenomena in the interconnection zone (IZ), the thickness of the IZ, the phases distribution, fraction and precipitates morphology to classify the type of bonding process and identify different joining conditions. We then run thermodynamic and diffusion calculations, using Thermo-Calc and TC-DICTRA module [3], to obtain diffusion profiles, interfaces velocities and elucidate possible diffusion paths by combining phase equilibria and diffusion. We evaluate new possible contact processing conditions and if necessary new contacting schemes, for example, by adding contacting layers. To analyze possible joining process conditions, a vertical section of the Cu-Mg-Si-Sn phase diagram like the one presented in the Figure is extremely useful, temperature vs. Cu global composition may represent all phases to be expected at the IZ at a given contacting temperature. The blue shadowed area indicates the conditions where the Liquid phase is stable and the blue line the Liquid zero phase fraction. Similarly, the red area and line shows where the TE phase is present. The presence/absence of the Liquid phase plays a crucial role on diffusion, segregation phenomena and final microstructure of the IZ. These results are a significant complement to the investigation of the structural integrity of the assembled TE devices by mechanical modelling of the IZ with and without intermetallic layers to understand the stress distribution in the IZ due to thermal and mechanical constraints. And, a critical step towards stablishing an integrated computational modelling approach for TE devices technology. References [1] Ying, P., et al. Nat Commun, 2021, 12, 1121. [2] Xiong, W. and Olson, G. npj Comput Mater, 2016, 2, 15009. [3] Thermo-Calc Software Package DICTRA v2023b. [4] Tumminello S., et al., J. Mater. Chem. A, 2021, 9, 20436-20452.