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 useful electrical energy through the Seebeck effect, thermoelectric generators (TEG) 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. While Mg2(Si,Sn) TE properties have been optimized and TEGs have been successfully fabricated, previous investigations have also shown limited stability against oxidation in air at high temperature and against Mg loss in inert atmosphere, identifying material stability as one of the major challenges to be overcome for large scale implementation. By integral measurement of the thermoelectric properties we furthermore found that under certain conditions the material also undergoes changes at room temperature (RT). Our stability investigations focus on Mg-poor p-type Mg2Si0.3Sn0.7 and Mg-rich n-type Mg2+δSi0.3Sn0.7 due to their high TE performance and previous implementation in prototype devices. Samples were obtained by a melting route, resulting in the targeted main phase and a small amount (<3 % vol) of Si-rich Mg2Si1-xSnx homogeneously distributed throughout the sample. While those Si-rich islands apparently don’t affect the TE properties, they allow to investigate the relevance of the composition for the material stability on a single sample. Samples were stored in air at RT to investigate the impact of the storage condition over time. By integral measurement of the TE properties, we show that Mg-rich n-type samples degrade over time (decrease of charge carrier concentration) while Mg-poor p-type samples remain stable, due to Mg-rich/poor content variation between both materials. Scanning electron microscopy (SEM) combined with energy dispersive X-ray spectroscopy (EDX) were used to analyse the microstructural changes at the surface of the samples and reveal the formation of magnesium oxide on the main phase of the sample only, while Si-rich regions remain unchanged. This provides evidence that the surface degradation is clearly selective to the Si:Sn ratio. Lastly, carrier concentration profiling by local scanning of the Seebeck coefficient also shows an increasing inhomogeneity of the samples with increasing age on a meso scale. This work not only reports the effect of storing in air at RT on the TE properties and microstructure but also indicates that Mg in Mg-rich samples can be removed from the material by the following multi-step mechanism: (i) Mg sublimation (Mg2+δX → Mg2X + δ Mg), (ii) Mg depletion at the surface from Mg-rich phases and (iii) Mg oxidation at the surface (Mg + O2 → MgO). Such degradation could be stopped by storing the samples in inert atmosphere.