The role of phase transition by nucleation, condensation, and evaporation for the synthesis of silicon nanoparticles in a microwave plasma reactor - Simulation and experiment

A novel particle model is presented to simulate the synthesis of silicon nanoparticles from monosilane in a laboratory sized microwave plasma reactor. The simulations contribute essentially to the understanding of the particle formation process and the spatial and size distribution of particles observed in the experiment. The model approach features phase transition and explains the observed, tube-shaped particle formation zones by a competing nucleation, condensation, and evaporation process coupled with complex transport phenomena. The simulation results are in excellent agreement with experimental data from Rayleigh scattering and line-of -sight optical absorption with onion-peeling reconstruction (LOSA) measurements of the particle front, as well as with multiline SiO laser-induced fluorescence (LIF) temperature measurements. Particle size distributions determined from transmission electron microscopy (TEM) on thermophoretically sampled particles are in good agreement with the simulation results. Average diameters of 25.8 nm calculated in the simulation compare well to 27.6 nm measured in the experiment. It was found that thermophoresis has a crucial impact on particle trajectories, as it extends the particle residence time within the reactor by about 20% and provides the determining force for particles to escape zones of high temperature in which particles evaporate otherwise. The sectional model features two-way coupled phase transition formulations for the condensing matter, which is formed through the decomposition of monosilane diluted in argon/hydrogen mixtures. The process is investigated by the combination of two simulations with different grid resolutions, which show differences for the high Schmidt number particle phase only. The simulations feature a global monosilane decomposition reaction, while the microwave plasma source is simplified by a local heat source.