Microwave plasma synthesis of silicon nanoparticles on a pilot scale

Frederik Kunze, Institute of Energy and Environmental Technology e. V. (IUTA), Duisburg, Germany

Research in the field of highly-specific nanoparticles led to new applications in which the adjustable properties of these particles can be used to optimize a whole range of applications. Especially silicon is of great interest due to the usage as standard semiconductor material. Produced as nanoparticles, it is not only of interest to improve existing applications like solar cells but it is even possible to find new applications. As a result, silicon nanoparticles can be found as anode material in lithium-ion batteries or as sintered bulk material to produce thermoelectric generators.

For many years, these materials were only available in minor quantities from lab-scale synthesis. To bridge this availability gap, a unique cluster of three different types of pilot-plant-scale gas-phase reactors has been installed. Construction and layout of these pilot-plant reactors were based on experiences with reactors in laboratory scale facing the challenges of different upscaling effects like higher mass flows and thermal gradients.

For silicon, the flame reactors widely used in industry are impractical due to the oxidation of silicon during the process. Hot wall reactors are suitable to produce high amounts of silicon nanoparticles, but the cooling rate is not high enough to receive non-agglomerated particles, which could be an issue for some applications. To overcome these challenges, a microwave plasma reactor can be used to produce large quantities of non-oxidized and non-agglomerated silicon nanoparticles.

In this research project, the focus is set on the optimization of a microwave plasma reactor in pilot-plant-scale to obtain a stable process for producing silicon nanoparticles with adjustable properties and high production rates of around 100 g/h. To achieve a long-term stability of the process, a new nozzle was constructed. This nozzle generates a swirl flow, which prevents the deposition of particles on the quartz glass tube and simultaneously stabilizes the particle torch. It was tested in laboratory scale and was upscaled to the pilot-plant-scale. Furthermore, the process parameters were optimized to improve the stability of the process and to get a better understanding of what impact the parameters have on the size and the morphology of the nanoparticles.

With this knowledge about the laboratory and the pilot-plant-scale plasma reactor, a model can be created for upscaling this kind of reactors.

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