The case of diffusion reaction in multilayer electrical contacts for mg-si-sn-based thermoelectric devices

Kurzfassung

Thermoelectric (TE) energy conversion 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. Yet, a crucial challenge in TE device technology is the contact between the TE material and the metallic bridge to build up a functional module for energy conversion [1]. Mg-Si-Sn-based compounds are suitable TE materials for high-temperature applications with non-toxic, abundant and inexpensive elements. In particular, the compositions Mg2(Si1−xSnx) with x = 0.6, 0.7 lay within a miscibility gap of the Mg2(Si1−xSnx) C1 solid solution phase, therefore susceptible of undergoing spinodal decomposition sensitive to diffusion, electrical or stress fields. This TE material is optimized for good performance, i.e. optimal Seebeck coefficient and electrical conductivity, in a tradeoff, by adding dopants (for p- and n-type semiconductor legs) and synthesized by ball milling and sintering to achieve good control of the composition. Electrical contacts are needed between the TE legs and a metallic electrode to build up TE devices. Typically, such devices have to work under a temperature gradient. Using Cu as a typical electrode, significant diffusion and reaction at the interconnection zone (IZ) has been observed, accompanied by changes in the Seebeck coefficient that compromise the mechanical strength of the contacts and functionality of the TE material. We approach this technological problem with a CALPHAD-based computational framework, combining thermodynamics, diffusion and mechanical modeling. A thermodynamic description for the quaternary system Cu-Mg-Si-Sn was developed in a previous study [2] where the layer sequence in the IZ after the contacting process was calculated and confronted with experiments. In the quaternary system several phases are found in the IZ, Laves phase C15, 1-Cu3Mg2Si and C1-Mg2Sn. Diffusion simulations with DICTRA [3,4] in C1-Mg2(Si1−xSnx) solid solution are restricted by the lack of atomic mobilities in the C1 phase. Therefore, we initially transform the problem into a single FCC or HCP solid solution phase in contact with a stoichiometric C1 phase to set the design criteria for electrical contacts and extend them to other metallic electrodes. Additionally, diffusion couple experiments were carried out to determine interdiffusion coefficients in C1-Mg2(Si1−xSnx) solid solution which can be used to assess the atomic mobilities in this phase. [1] Ying, P., et al. Nat Commun, 2021, 12, 1121. [2] Tumminello S., et al., J. Mat. Chem. A, 2021, 9, 20436-20452. [3] Andersson J.O., et al., Calphad, 2002, 26, 273-312. [4] Larsson, H., Calphad, 2014, 47, 1-8.