Instability mechanism in thermoelectric Mg2(Si,Sn) and the role of Mg diffusion at room temperature

Kurzfassung

Mg2(Si,Sn) is a promising thermoelectric material due to its high performance and its non-toxic, abundant, and low-cost constituents. A lot of progress has been made in optimizing its thermoelectric properties and to use it for technological application. Nevertheless, the chemical stability of these materials is a significant challenge to overcome. Temperature dependent measurements of transport properties suggests that the thermal stability of n-type Mg2(Si,Sn) is not only compromised at elevated temperatures but might be limited already at room temperature. In this work, integral thermoelectric properties measurement, locally resolved Seebeck analysis, scanning electron microscopy with energy dispersive X-ray spectroscopy and atomic force microscopy are introduced to assess bulk and surface microstructural changes of n-type samples stored at room temperature in ambient atmosphere for several years. The study reveals changes in carrier concentration and surface degradation for the material over time, driven by Mg diffusion which seems to be highly dependent on the Si:Sn ratio. First-principles calculations conducted by researchers at KERI (South Korea) are employed to develop a microscopic understanding which indicate that Mg diffusion occurs mainly via vacancies at the Mg site. This study elucidates a multi-step degradation mechanism for n-type Mg2(Si,Sn) which consists of: (i) diffusion of Mg from the matrix to the grain boundaries, (ii) diffusion of Mg along the grain boundaries towards the surface and (iii) Mg reaction with moisture, causing deterioration of the material’s thermoelectric properties.