Modeling Of Chemical Vapor Deposition Mechanism With Methane

Chemical vapor deposition is a process involving the dissociation and/or chemical reactions of gaseous reactants in an activated environment and the formation of a stable solid product.This process is a non-equilibrium system with heterogeneous reactions.Precursors in chemical vapor deposition are flexible and by adjusting the process parameters,the deposition rates,the deposition structure and the deposition position could be controlled.Moreover,refractory materials could be deposited at relatively low temperatures on complex shaped components uniformly with good reproducibility and adhesion.Now chemical vapor deposition is the main and the most industrially promising process for the preparation of carbon/carbon composites.The research of its complex mechanism would be beneficial to shorten the preparation time,to lower the manufacture costs and to control the material properties accurately.In this thesis,computer simulations have been done to analyze the mechanism of chemical vapor deposition with methane.The main researches are as following:A multi-step reaction model for the initial pyrolysis of methane has been set up and applied in studying the thermodynamic and kinetic properties at M06-2X/def2-TZVP level based on density functional theory.Results show that most of the reactions in the model are endothermal and more elementary reactions are going to happen spontaneously with the increasing temperature.The equilibrium constants are relative large above 1200 K,which indicates well conversion ratios of the reactants and a better choice for experimental conditions.Chemical kinetic analysis shows that the activation energies of the direct decomposition reactions are higher,followed by those of methyl radicals attacking reactions.Reactions with hydrogen radicals have lower activation energies.The activation energy of methane dehydrogenation at 1200 K is 272.4 k J·mol^-1,which is quite close to the apparent activation energy of methane pyrolysis in experiments(272 k J·mol^-1).The highest activation energy for reactions with hydrogen atom at 1200 K is 185.7 k J·mol^-1,which is expressed in ethynyl radical attacked by hydrogen atom to form diatomic carbon and hydrogen molecule.Based on the thermodynamic and kinetic analysis of initial methane pyrolysis and the data given by various researchers,a gas-phase reaction mechanism for methane pyrolysis including 909 elementary reactions and 241 species has been developed and verified with the main gas mole fractions during the pyrolysis process.The main reaction for methane consumption is methane attacked by methyl radical to produce ethyl radical and hydrogen molecule,which also motivates the formation of C₂ species and benzene.There are three stages in the gas reactions of methane pyrolysis:an incubation stage with the sensitivity coefficients(showing how the gas concentrations depend on the reaction parameters of a elementary reaction)being small at the short residence times,a main stage with the sensitivity coefficients varying dramatically with the increasing of the residence time and a stable stage with almost constant sensitivity coefficients and constant mole fractions of different gas components.The main reaction path of methane pyrolysis at the short residence time is limited by the few reactants in the gas mixture and only a few of reactions in the mechanism are working.As the residence time increases,more heavy species appear in the main reaction path.Benzene is formed from benzyl radical,C₃ species and C₄ species.Acetylene is the important intermediate in the main and stable stages of methane pyrolysis.A heterogeneous reaction mechanism for pyrocarbon deposition is developed and leads to a complete mechanism combining with the gas-phase reaction mechanism for the simulation of pyrocarbon deposition rates and gas mole fractions during the deposition process.This simulation shows well agreement with the experiments and is better than the simulations with previous mechanisms.The simulations reveal that at 1323 K,C₁ species forms low textured pyrocarbon at a low deposition rate.At 1348 K–1398 K,the pyrocarbon precursors are C₁ species,C₂ species and C_?6 species.As the temperature increases,the ratio of C_?6 species deposition rate increases and the texture of pyrocarbon is higher.Along with the pyrocarbon deposition,more hydrogen molecules produce,which are especially from C₁species deposition.Up to 1 s,the process of methane pyrolysis is still out of the stable reacting stage,as the pyrocarbon deposition proceeds continuously.A group of equations for studying the binary diffusion in a porous network is derived.The equations can estimate gas fluxes,effective transport conductances and gas concentrations in the porous network considering both the diffusion characters in the whole porous medium and the diffusion in a single pore.The interactions between gas molecules and wall molecules are based on soft ball collisions.The equations are easy to be solved with any kind of porous network.In the network with a continuous pore size distribution,the porosity of the network shows a more obvious influence on the flux than the mean length of pores.A species and pressure-dependent optimal temperature is predicted to obtain a maximum gas flux.Methane inhibits the hydrogen diffusion,while hydrogen benefits the methane diffusion only at low methane pressures.In the network with a bimodal pore size distribution,the selectivity of methane and hydrogen undergoes a sharp transition at the percolation threshold,when the porosity ratio of two pore sizes varies.When the smaller pore is impermeable to the larger species methane,it cannot go through the pore network with the porosity ratio smaller than the percolation threshold.When both of the pore sizes are larger than both of the molecule sizes,methane inhibits the diffusion of hydrogen,leading to a clear nonlinear effective conductivities and concentration distributions in the porous medium.