Numerical Simulation On Production Of Granular Polysilicon By Fluidized-bed Silane Pyrolysis

As an energy-efficient,high-performance and environment-friendly method,the fluidized-bed silane pyrolysis for production of granular polysilicon has received extensive attention,while the domestic research on this process is still in its infancy.The experimental conditions of silane pyrolysis are very harsh,especially for industrial-grade fluidized bed device.Just taking the experimental means to carry out its research needs a huge economic cost,which restricts research and development of this technology.Therefore it is with much significance to establish a multi-scale coupled model for describing the chemical vapor deposition process of fluidized-bed silane pyrolysis.In this thesis,various types of silane pyrolysis kinetic models in the literature were firstly analyzed and compared in detail.As a result,considering contribution of silylene?SiH₂?to Si deposition,two silane homogeneous kinetic models were used to obtain the SiH₂ concentration from silane gaseous decomposition,and three heterogeneous kinetic models were implemented to evaluate the rate of silane surface deposition.Then six possible silane pyrolysis kinetic models were proposed.Then coupled with population balance equations,a Eulerian-Eulerian two-fluid model based on kinetic theory of granular flow was applied to simulate the evolution of silicon particle growth by chemical vapor deposition of silane pyrolysis in a three-dimensional slugging fluidized bed reactor in the silane-hydrogen system using Fluent.This CFD-PBM coupled model could take the particle diameter distribution and their changes into account.Verification analysis shows that the established coupled model is effective.The mechanism of the silane homogeneous reaction was introduced into the Fluent by Chemkin.The applicability of six silane pyrolysis kinetic models were analyzed in detail.The results show that the simulated growth rate is in good agreement with Hsu's experimental data under the combination kinetic models of Ho's homogeneous and Furusawa's heterogeneous ones,in which a complete silane decomposition is achieved.In addition,the result shows that Si deposition mainly occurs in the dense zones and the periphery of the bubbles,and the contribution of SiH₂ with rather low concentration to Si growth rate is about 10%of the one of silane.Finally,a three-dimensional model of the industrial-grade fluidized-bed silane pyrolysis was established.Considering agglomeration and wall deposition,the mechanism of particle growth in the fluidized bed was further enriched.And effects of inlet composition,inlet velocity and reaction temperature on Si growth rate were analyzed in detail.The optimum operating conditions are proposed as follows:the silane inlet concentration is 10%²0%,the inlet velocity is 3.5⁶u_mf,the reaction temperature is 923⁹73K.In addition,high hydrogen concentration near the wall by distributing the silane inlet concentration is supposed to be effective to reduce the wall deposition.