Probing Thermal Stability of Cu-Contacted Mg2X-Based Solid Solutions (X=Si, Sn) at Elevated Temperatures: Understanding Degradation Mechanisms

Kurzfassung

Mg₂X (X =Si, Sn)-based solid solutions are promising candidates for mid-to-high temperature waste heat recovery, but developing long-term stable thermoelectric generators (TEG) remains challenging. While Mg2X demonstrates excellent thermoelectric performance, it is susceptible to Mg loss, causing changes in carrier concentration through the formation of intrinsic defects. More severe degradation has furthermore been observed for samples following contacting with a metal electrode, posing an additional challenge for TEG technology advancement. We propose Atomic Layer Deposition (ALD) to first enhance the material stability by suppression of Mg sublimation and second to differentiate between the different degradation mechanisms occurring at high operating temperatures. Our primary results from integral electrical conductivity and Seebeck coefficient measurements on coated samples without a Cu electrode at 450°C show enhanced stability, while spatially resolved mappings indicate comparable degradation rates for both coated and uncoated samples with Cu electrodes. This confirms Mg-Cu interdiffusion as the primary degradation mechanism for contacted samples with a higher relevance than Mg sublimation. Finally, Wavelength-Dispersive Spectroscopy is used to reveal that the observed reduction in charge carrier concentration during contacting is primarily due to Mg diffusing into the Cu electrode, not by the diffusion of Cu into the TE material as speculated previously, making this study the first to experimentally demonstrate the relevance of Mg loss into the electrode. By combining ALD coating to inhibit Mg loss with micro¬structural analysis, we present a novel methodology to distinguish degradation mechanisms, enabling future advancements in Mg2 (Si,Sn)-based TEGs