Mg2Si-based thermoelectric materials and devices: progress and challenges, in particular interdiffusion phenomena

Kurzfassung

Alternative thermoelectric materials that can substitute the commercially dominant bismuth telluride technology are highly desirable for heat conversion and thermal management applications. Magnesium silicide based solid solutions Mg2X (X = Si, Ge, Sn) are among the most promising thermoelectric (TE) materials due to very good thermoelectric properties, low cost of raw materials and environmental compatibility. We have demonstrated technological maturity with prototypes of p- and n-type Mg2X and p-MgAgSb/n-Mg2X, the latter reaching conversion efficiencies > 6.5% and power densities of ~1 W/cm2, comparable in performance to commercial bismuth telluride modules. However, stable thermoelectric properties are of utmost importance for successful large-scale application. Intrinsic defects like Mg interstitials and Mg vacancies affect the properties of Mg2X significantly, therefore Mg diffusion is a potential concern here. Annealing experiments and in-situ measurements at high temperature show that degradation of Mg2X is a two-step process, where in the first step loosely bound excess Mg sublimates from the surface, reducing the charge carrier concentration, and only in the second step, after the solubility limit of Mg vacancies has been reached, Mg2X decomposes into other phases. Variation of the annealing temperature allows us to develop a kinetic model which can be used to predict material stability at different application temperatures, we also find indications that this process can be decelerated by sealing of the surfaces. Furthermore, we find that diffusion phenomena are relevant even at room temperature, changing the thermoelectric properties on a scale of months to years if stored under laboratory atmosphere. Microstructural investigations by SEM, EDX and AFM indicate that the observed changes are related to Mg diffusion inside the material, in line with conclusions from high temperature experiments. They furthermore show that the diffusion constant is approximately independent of the Si:Sn ratio of the material. Comparison of different microscopic mechanisms of bulk diffusion by first-principles calculations reveals that Mg transport via Mg vacancies is the most relevant mechanism and the increase in vacancy density explains the experimentally observed faster change of Sn-rich Mg2X.