Combustion Synthesis of Thermoelectric Materials for Deep Space Exploration

The goal of the project was to develop advanced methods for fabrication of thermoelectric materials such as nanostructured silicon and silicon-germanium based materials. One focus of the research was on using mechanically activated self-propagating high-temperature synthesis (MASHS). The research objectives included the determination of the optimal conditions for MASHS of silicon and silicon-germanium materials from silica, germanium oxide, and magnesium silicide, the densification of silicon-based materials, and the determination of thermophysical and thermoelectric properties of the densified materials. Further, the oxidation kinetics of magnesium particles was investigated for the applications to chemical heat integrated power systems for deep space missions, which potentially provide very high energy densities and long lifetimes compared to the best batteries. It was determined that high-energy ball milling for several minutes effectively improved the reactivity of Mg2Si/SiO2 mixtures through the formation of composites, consisted of submicron and micron-sized particles. The use of Mg2Si instead of Mg decreased the temperature during SHS and, after leaching, produced micron-scale agglomerates of Si nanoparticles, in contrast with sponge-like Si particles obtained in the case of using Mg. The addition of 15−25 wt% sodium chloride further decreased the combustion temperature, leading to the formation of nanocrystalline silicon powder with crystallite size of about 30 nm. Densification of nanostructured silicon powders produced by ball milling was conducted by a liquid phase sintering approach using tin as the liquid phase. After sintering, most of the tin remains in the densified silicon. By annealing the sample, some tin is expelled from the sample producing pores. The addition of tin results in a higher charge carrier mobility and a lower thermal conductivity than that of nanostructured silicon. The oxidation of spherical and non-spherical (flakes) Mg particles in oxygen flow was investigated using isothermal and non-isothermal thermogravimetric analysis. The Avrami-Erofeev model of simultaneous nucleation and growth provides the best fit with the experimental thermogravimetric curves. The Mampel-Delmon analysis has shown that for 30 −60 μm particles, the nucleation is relatively slow, leading to the sigmoidal thermogravimetric curve, while for 300 μm particles and flakes, the entire process can be considered as a growth of a non-protective oxide layer. The apparent activation energy of Mg oxidation is likely in the range of 200 – 230 kJ/mol, independently of the particle size and shape.