Understanding The Impact of Microstructure and Interdiffusion in Mg₂Si-Based Thermoelectric Materials by Integral and Locally Resolved Measurements

Alternative thermoelectric materials that can substitute the commercially dominant bismuth telluride technology are highly desirable for heat conversion and thermal management applications. 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 &gt; 6.5% (T_c = 25 °C; T_h = 300 °C) and power densities of ~1 W/cm², comparable to commercial bismuth telluride modules.<br/>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.<br/>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 highly selective on the Si:Sn ratio of the material, with diffusion being much faster in Sn-rich 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. We can furthermore show that the drift kinetics of the thermoelectric properties is related to the amount of excess Mg added during synthesis and that high-performance samples with improved stability can be synthesized by omitting excess Mg.<br/>The chemical and structural complexity of this material class allows for employing advanced material improvement strategies, in particular selective scattering of charge carriers on (self-assembling) nanostructures (“energy filtering”). This can be achieved by a locally varying electronic band structure, feasible in Mg2Si1-xSnx by changing the band gap via the Sn content x and energetically adjusting the band edges in grains of different compositions by selective doping. To test selective doping strategies, a local charge carrier concentration measurement is highly useful. We will show that transient microprobe measurements of the Seebeck coefficient can be used to that purpose with a local resolution down to ~5 µm. For carrier concentration determination with sub-µm resolution we employ a combin¬ation of the AFM-based Kelvin Probe Force Microscopy for work function measurements, SEM for determination of the composition, first-principles calculations for band offset predictions and modelling based on the Boltzmann transport equation. Our results reveal an inhomogeneous distribution of the dopant Bi among several observed Mg2Si1-xSnx phases and confirm the feasibility to tune the local carrier concentration to control the electronic band structure alignment.