Spectroscopic Diagnosis And Simulation Of The Interaction Between Silicon-based Materials And Oxygen Atoms

Silicon based materials are one of the key components of typical thermal protection materials in hypersonic aircraft.The gas-solid interaction between silicon-based materials and oxygen atoms in dissociated air has important influence on thermal and chemical effects.When the silica protective layer formed on silicon-based material surface by gas-solid interaction,in addition to protecting the internal material from oxidation,the low catalytic properties of the protective layer will also greatly reduce the chemical thermal effect generated by oxygen atom catalysis reactions.However,when the protective layer does not form on the surface,the oxidation of the inner material increases significantly and cause severe stripping damage.The essence of the gas-solid interaction between the silicon-based materials surface and oxygen atoms is the process of heat and mass transfer represented by catalysis and oxidation reactions.The understanding of the reaction and interaction mechanism of surface heat and mass transfer process is the premise and foundation for the effective utilization of protective mechanism and avoid damage effect of the gas-solid interaction.In this paper,based on the key problem of"reaction process of oxygen atom on the surface of silicon-based material",the key technology of“measurement method of oxygen atom concentration and temperature based on laser induced fluorescence diagnosis”was carried out.Based on this technology,the key scientific issues of"high resolution characterization of oxygen atom consumption in gas-solid interaction on silicon-based material surface"and"competitive mechanism of oxygen atom consumption in the process of heat and mass transfer on sili con-based material surface"were studied.Combined with the theory of physical mechanics,a method for studying the interaction between silicon-based material surface and oxygen atom was established based on spectral in-situ diagnosis and surface reaction dynamics simulation:（1）A TALIF test system consisting of a laser inducer,a plasma generator,a fluorescence collector and other devices has been established based on the principle of two-photon laser induced fluorescence（TALIF）diagnosis of oxygen atom.The integration and automatic synchronization control program of the TALIF test system has been developed to achieve accurate in-situ measurements of ground state oxygen atoms in oxygen plasma,and providing a test platform for the study of gas-solid interactions based on particle consumption;（2）The translational temperature and relative concentration of ground state oxygen atoms were obtained by using the Doppler broadening and integral of the TALIF excitation spectrum,respectively.The absolute oxygen atom concentration was quantitatively calibrated by NO₂ chemical titration.According to the discharge glow and emission spectrum intensity,the capacitive/inductive discharge mode boundary composed of gas pressure-discharge power was obtained.The concentration,translational temperature and their spatial distributions of oxygen atoms under different discharge powers,gas pressures and discharge modes were obtained by TALIF diagnosis,which provide references for the discharge environment for the characterization of surface catalytic property based on TALIF diagnosis;（3）Based on TALIF diagnosis and molar fraction gradient theory,the reaction rate and consumption coefficient of oxygen atoms were evaluated.The surface oxygen atom consumption coefficients of single crystal Si（100）andα-quartz at different temperatures were obtained.The surface models of amorphous SiO ₂ andα-quartz（100）were constructed by using the Reactive Force Field（ReaxFF）molecular dynamics（MD）simulation.The number and distribu tion of different kinds of active sites of catalytic reactions at different temperatures were obtained through the flux boundary condition simulation.Based on the finite rate catalytic reaction model,the single reaction event simulation was performed for each basic reaction,and the activation energy,pre-exponential factor and the catalytic coefficient of different SiO₂ surfaces were obtained.The results of oxygen atom consumption coefficient by TALIF test and ReaxFF simulation were consistent with literature data below 1000 K,however,the oxygen atom consumption coefficient increased obviously on Si（100）surface above 1000 K,which was related to the continuous mass transfer process of surface oxidation;（4）Aiming at the phenomenon that the oxygen atom consumption coefficient on Si（100）surface in high temperature range is significantly increased,the ReaxFF MD simulation was used to simulate the atomic oxidation of Si（100）surface at different temperatures.In the initial stage,the oxidation reaction has absolute advantages,all oxygen atoms participate in the surface mass transfer process.After the oxidation reaction has stabilized,the dense SiO₂ layer forms between 300～500 K,the oxidation reaction stops and the catalytic reaction is dominant.The n the oxygen atoms only participate in the surface heat transfer process and the consumption is minimal.The oxidation and catalytic reactions can occur simultaneously when a non-dense silicon dioxide layer is formed between 500～1300 K,and oxygen atoms are involved in both mass and heat transfer processes,so the consumption increases.Oxidation between 1300～1700 K does not form SiO₂ layer,and oxidation plays an absolute role.Almost all oxygen atoms participate in the surface mass transfer process,and oxygen atoms consume the most.（5）The interaction between the SiC material and oxygen atom was studied by the same method.The consumption coefficient of oxygen atoms on SiC surface at1300～2300 K was measured by TALIF method,and ReaxFF simulation was performed on the oxidation mas transfer process ofβ-SiC（100）surface for the phenomenon that the consumption coefficient increased obviously at high temperature.Carbon atoms increase the barrier to the diffusion and oxidation of exotic oxygen atoms.At initial stage of the reaction,all oxygen atoms participate in the surface mass transfer process.Oxidation reaction almost stops when a dense SiO₂ layer forms at lower temperatures,and oxygen atoms only participate in the process of surface catalytic heat transfer,resulting in the minimal consumption of oxygen atoms.When a non-dense SiO₂ layer formed by high temperature oxidation,oxygen atoms are involved in both mass and heat transfer processes,and the oxygen atom consumption increases.It is predicted that when active oxidation occurs at high temperature and low pressure,the oxidation reaction will take an absolute advantage,then the oxygen atoms almost all participate in the surface mass transfer process,and the consumption reaches the maximum.The spectroscopic diagnosis of interaction between silicon-based materials and oxygen atoms can provide an in-situ diagnostic platform and quantitative test results for the study of heat and mass transfer reactions between the gas and surface.The simulation study of surface catalytic properties can be used to obtain surface catalytic coefficients under given conditions and provide input parameters for computational fluid dynamics simulations.The study of the surface oxidation mechanism helps to achieve accurate prediction and effective control of the oxidation reaction degree and oxidation state change.The mechanism of surface catalysis（heat transfer）and oxidation（mass transfer）of oxygen atom on silicon-based materials is revealed,and the competitive relationship between oxygen atom consumption on surface catalysis（heat transfer）and oxidation（mass transfer）reactions is obtained,which could enhance the cognition of coupled reaction between oxygen atom and silicon-based materials,realize the decoupling of coupled heat/mass transfer reactions represented by surface catalysis/oxidation,and provide theoretical guidance for design and preparation of next generation thermal protection materials.This research involves the intersection and fusion of optics,materials science,engineering thermodynamics,plasma physics and quantum mechanics molecular dynamics simulation,which can provide reference for future multidisciplinary research on complex scientific problems.