On the stability of thermoelectric materials: investigating Mg diffusion in Mg2(Si,Sn) at room temperature

Kurzfassung

Due to their ability to convert (waste) heat into electricity, thermoelectric (TE) generators are highly attractive in fields like automotive or aerospace. Magnesium silicide-stannide Mg2(Si,Sn) solid solutions are part of a very interesting family of TE materials due to their low density, as well as cheap, abundant and nontoxic elements. Though Mg2(Si,Sn) TE properties have been optimized [1, 2] and thermoelectric generators have been fabricated successfully [3], previous investigations have also shown limited stability against oxidation in air at high temperature (>430°C) and against Mg loss in inert atmosphere [4, 5]. By integral measurement of the thermoelectric properties we furthermore found that under certain conditions the material also undergoes changes at room temperature (RT). We focus our stability investigations on Mg-poor p-type Mg2Si0.3Sn0.7 and Mg-rich Mg2+dSi0.3Sn0.7 due to their high thermoelectric performance and previous implementation in prototype devices. The samples were synthesized via a melting process and therefore show some compositional variation on a µm-scale. Samples were then stored in different conditions at RT (air, inert or water) to investigate the impact of the storage condition over time and establish a related mechanism. Integral measurement of the TE properties shows that Mg-rich sample deteriorate over time (decrease of charge carrier concentration) while Mg-poor samples remain stable. Inert stored samples are expected to remain stable compared to air stored sample, and yet, it showed slight deterioration over time. SEM/EDX combined with AFM and Kelvin Force microscopy were used to analyze the microstructural changes at the surface of the samples. For samples stored in air they reveal a surface degradation which is clearly selective to Si:Sn content and MgO formation on Sn-rich reasons of the sample surface. Local scanning of the Seebeck coefficient across the depth of the sample shows that the change in properties is a surface effect, indicating a diffusion mechanism towards the surface as driving force for the material change. This work not only establishes the effect of storing conditions on TE properties and microstructure but also proves that it is a magnesium related mechanism comprised of several steps: (i) magnesium evaporation/loss (Mg2+δX → Mg2X + δMg), (ii) magnesium diffusion towards the surface inside the bulk and (iii) magnesium oxidation at the surface (Mg + O2 → MgO). Such correlation between Mg content and TE properties will be further investigated in future work to study the possible synthesis of Mg-poor n-type solid solution to avoid Mg excess in TEG and such degradation over time.