On the Origin of Temperature Induced Performance Degradation of Cu-Contacted Mg₂<i>X</i>-Based (<i>X</i> = Si, Sn) Thermoelectric Materials

Mg₂<i>X</i> (<i>X</i> = 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 Mg₂<i>X</i> 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 further been observed for samples following contacting with a metal electrode, presenting 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 723 K show enhanced material stability, while spatially resolved interface characterization of samples with Cu electrodes indicate comparable degradation for both coated and uncoated samples. 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 the 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 microstructural analysis, we present a methodology to distinguish TE material/electrode-related degradation mechanisms, enabling future advancements in Mg₂(Si, Sn)-based TEGs.