Densification Simulation And Experimental Study Of SiC Cladding With Large Aspect Ratio By Chemical Vapor Deposition

Because of its excellent physical and chemical properties,SiCf/SiC composites have great application prospects in the field of nuclear energy,especially in the application of nuclear fuel cladding tube.However,the deposition rate and uniformity of SiC coating deposited on the cladding pipe with large aspect ratio can not be guaranteed.The method of completely passing the experiment not only has high cost,but also has a large workload.The numerical simulation technology has the advantages of shortening the process scheme design cycle and reducing the experimental cost.In this study,taking trichloromethylsilane(MTS)as the reaction source gas,the chemical reaction kinetics software Chemkin Pro and computational fluid dynamics software ANSYS Fluent were used to simulate the process of preparing SiC coating on the substrate by chemical vapor deposition(CVD),and then the correctness and reliability of the simulation model were verified by CVD SiC experiment.The main research contents and results of this paper are as follows:Taking MTS-H2 system as the reaction system,the equilibrium of MTS-H2 system was analyzed by using equilibrium model in Chemkin pro software,and 1000℃ was determined to be the best equilibrium temperature;The deposition rate of SiC decreases with the increase of pressure;The addition of carrier gas H2 is beneficial to promote SiC deposition.The content of SiCl2 and SiCl4 is the largest in the gas phase byproducts.The mole fraction of SiCl2 increases with the increase of temperature,and the mole fraction of SiCl4 decreases with the increase of temperature;The mole fraction of SiCl4 increases with the increase of pressure,and the mole fraction of SiCl2 decreases with the increase of pressure;The mole fraction of SiCl4 and SiCl2 increases with the increase of the mole fraction of carrier gas H2.The SiC deposition process in a 1.5m chemical vapor deposition reaction chamber is simulated by using the planar shear flow model and the cylindrical shear flow model,respectively.Among them,the cylindrical shear flow model is closer to the experimental condition of CVD SiC.The CVD SiC model is established by using ANSYS Fluent to simulate and analyze the flow field characteristics in the reaction chamber of the chemical vapor deposition furnace,and explore the effects of reaction temperature,furnace pressure,carrier gas H2 flow and matrix quantity on the CVD SiC deposition rate.The analysis results of flow field characteristics are consistent with the SiC deposition rate:the reaction temperature 1200℃ is the boundary temperature of CVD SiC deposition rate.When it is lower than 1200℃,CVD SiC is mainly controlled by surface chemical kinetics,and when it is higher than 1200℃,the reaction is mainly controlled by mass transfer;The SiC deposition rate decreases gradually with the increase of furnace pressure,and the SiC deposition uniformity becomes worse.The SiC deposition rate increases gradually with the increase of carrier gas H2 flow.Under the condition of 1000-1200ml/min,the SiC coating has good deposition uniformity;The deposition rate of SiC decreases with the increase of the number of substrates,and the deposition uniformity also deteriorates.The process conditions of CVD SiC were optimized by numerical simulation with single factor test method.The reasonable process parameters of CVD SiC were obtained as follows:reaction temperature of about 1000℃,furnace pressure of 400Pa,Ar flow of 1200ml/min and carrier gas H2 flow of 1000ml/min.In order to verify the accuracy of the finite volume model,SiC coatings were prepared on the surface of SiCf/SiC Composites by CVD.Macroscopically,the deposition rate of CVD SiC coating was calculated by weighing and measuring with electronic balance,and the deposition uniformity was calculated by standard deviation formula Δ=2.17238×10-7,with good deposition uniformity;At the microscopic level,the CVD SiC coating was characterized by XRD and analyzed by morphology,and the experimental results were obtained β-SiC,the SiC coating deposition is relatively flat.When the deposition position is far away from the air inlet,the SiC deposition thickness decreases gradually,indicating that the deposition rate decreases gradually along the air flow direction.The simulation results verify the reliability of the model through experimental comparison.