Computational modeling of silicon nanoparticle synthesis in a laser-driven aerosol reactor.

摘要

A general two dimensional (2D) model has been developed for silicon nanoparticle synthesis by silane thermal decomposition driven by laser heating in a tubular reactor. This fully coupled model includes fluid dynamics, laser heating, gas phase and surface phase chemical reactions, and aerosol dynamics, which includes particle transport and evolution by convection, diffusion, thermophoresis, nucleation, surface growth and coagulation processes. A moment method, based upon a lognormal particle size distribution, and a sectional method are used to model the aerosol dynamics. The simulation results obtained by the two methods are compared. The sectional method is capable of capturing the bimodal behavior that occurs locally during the process, while the moment method is computationally more efficient. The effect of operating parameters, such as precursor concentration, gas phase composition, inlet gas velocity and laser power input, on the characteristics of the particles produced are investigated.;Based on the above general 2D model, another 2D model that closely simulates the silicon nanoparticle synthesis by silane thermal decomposition in the six-way cross laser-driven aerosol reactor in our lab was developed. This model incorporates fluid dynamics, laser heating, gas phase and surface phase chemical reactions, and aerosol dynamics, with particle transport and evolution by convection, diffusion, thermophoresis, nucleation, surface growth, coagulation and coalescence processes. Because of the complexity of the problem at hand, the simulation was carried out via several sub-models. First, the chemically reacting flow inside the reactor was simulated in three dimensions in full geometric detail, but with no aerosol dynamics and with highly simplified chemistry. Second, the reaction zone was simulated using an axisymmetric two dimensional CFD model, whose boundary conditions were obtained from the first step. Last, a two dimensional aerosol dynamics model was used to study the silicon nanoparticle formation using more complete silane decomposition chemistry, together with the temperature and velocities extracted from the reaction zone CFD simulation. A bivariate moment model was used to describe the evolution of particle size and morphology. The model predicted that spherical particles are produced at the center of the reaction zone, while non-spherical particle aggregates are formed at the outlet of the reaction zone. Precursor concentration, peak temperature and residence time are shown to be major parameters affecting reactor yield and the characteristics of the product particles.;Gas phase kinetics are a key component of our model of silicon nanoparticle synthesis. However, description of cluster growth to a critical nucleus size that can be treated as a solid particle can involve thousands of distinct elementary reactions. Exploration of such large reaction mechanisms is facilitated by the use of automated reaction mechanism generation, in which a computer constructs a detailed reaction mechanism according to a pre-specified set of rules. In order to utilize the silane decomposition reaction mechanisms generated in this way, one must translate the mechanism from the compact description used in automated reaction mechanism generation codes to more conventional descriptions used in reacting flow simulations. Thus, this work set up a framework for translating their mechanisms produced by automated mechanism generation software, into a form that is readily usable in our simulations. First, the string code representation of each species is translated into a bond electron matrix (BEM). Then a group additivity scheme is used to process the BEM to find out the types and quantities of the groups that make up the molecule. A straightforward walking ring finding algorithm is used to find the smallest set of independent rings with smallest sizes. At last, the thermochemical properties are estimated based on the contribution from each group at various temperatures. Those thermodynamics data in turn were fit to a standard polynomial form that can serve as input for CHEMKIN or other similar packages for calculation of thermodynamic, kinetic and transport properties. We have carried out preliminary studies using this newly translated kinetics model for modeling silicon nanoparticle synthesis by silane decomposition in a plug flow reactor with constant temperature and pressure. (Abstract shortened by UMI.)