摘要 :
Recent molecular beam experiments of high velocity O, N, and O_2 impacting carbon material at high temperature produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with car...
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Recent molecular beam experiments of high velocity O, N, and O_2 impacting carbon material at high temperature produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with carbon has been published using a continuous molecular beam with lower velocity (2000 m/s) and approximately 500 times higher beam flux than previous pulsed-beam experiments. This data is interpreted to construct a new air-carbon ablation model for use in modeling carbon heat shield ablation. The new model comprises 20 reaction mechanisms describing reactions between impinging O, N, and O_2 species with carbon and producing scattered products including desorbed O and N, O_2 and N_2 formed by surface-catalyzed recombination, as well as CO, CO_2, and CN. The new model includes surface-coverage dependent reactions and exhibits non-Arrhenius reaction probability in agreement with experimental observations. All reaction mechanisms and rate coefficients are described in detail and each is supported by experimental evidence or theory. The model predicts pressure effects and is tested for a wide range of temperatures and pressures relevant to hypersonic flight. Model results are shown to agree well with available data and are shown to have significant differences compared to other models from the literature.
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摘要 :
Recent molecular beam experiments of high velocity O, N, and O_2 impacting carbon material at high temperature produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with car...
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Recent molecular beam experiments of high velocity O, N, and O_2 impacting carbon material at high temperature produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with carbon has been published using a continuous molecular beam with lower velocity (2000 m/s) and approximately 500 times higher beam flux than previous pulsed-beam experiments. This data is interpreted to construct a new air-carbon ablation model for use in modeling carbon heat shield ablation. The new model comprises 20 reaction mechanisms describing reactions between impinging O, N, and O_2 species with carbon and producing scattered products including desorbed O and N, O_2 and N_2 formed by surface-catalyzed recombination, as well as CO, CO_2, and CN. The new model includes surface-coverage dependent reactions and exhibits non-Arrhenius reaction probability in agreement with experimental observations. All reaction mechanisms and rate coefficients are described in detail and each is supported by experimental evidence or theory. The model predicts pressure effects and is tested for a wide range of temperatures and pressures relevant to hypersonic flight. Model results are shown to agree well with available data and are shown to have significant differences compared to other models from the literature.
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摘要 :
Direct simulation Monte Carlo (DSMC) is used to perform simulations of molecular beam scattering experiments of a hyperthermal O/O_2 beam striking a vitreous carbon surface. The current DSMC surface reaction model specifies the pr...
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Direct simulation Monte Carlo (DSMC) is used to perform simulations of molecular beam scattering experiments of a hyperthermal O/O_2 beam striking a vitreous carbon surface. The current DSMC surface reaction model specifies the probabilities and characteristic frequencies associated with adsorption, desorp-tion, Langmuir-Hinshelwood, and prompt thermal mechanisms according to reaction rate constants, sticking coefficients, and surface coverage. In addition to the macroscopic information (reaction mechanisms, rate constants, etc.), this DSMC model also includes microscopic information regarding detailed scattering of ablation products from a carbon surface (including sticking coefficients, desorption barriers, angular scattering, etc.). This DSMC solver is used to perform simulations of the molecular beam scattering from vitreous carbon with substrate temperatures ranging from 550-2000 K. Detailed analysis of the experimental time-of-flight (TOF) and angular distributions is used to propose modifications to the carbon surface oxidation model developed by Poovathingal et al., in order to better capture the features of the TOF distribution. We demonstrate that this revised model captures features in the TOF distributions, including the long tail in the CO distribution that indicates a slow CO production process and the fast component in the O distribution that corresponds to impulsively scattered and thermally desorbed O atoms, which were not captured with the previous model. Comparisons between the simulated (DSMC) and experimental TOF distributions, angular distributions, and relative product fluxes show excellent agreement The resulting DSMC model is provided describing the detailed scattering information of the products, and the probabilities and characteristic frequencies of surface reactions based on finite rates.
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摘要 :
Direct simulation Monte Carlo (DSMC) is used to perform simulations of molecular beam scattering experiments of a hyperthermal O/O_2 beam striking a vitreous carbon surface. The current DSMC surface reaction model specifies the pr...
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Direct simulation Monte Carlo (DSMC) is used to perform simulations of molecular beam scattering experiments of a hyperthermal O/O_2 beam striking a vitreous carbon surface. The current DSMC surface reaction model specifies the probabilities and characteristic frequencies associated with adsorption, desorp-tion, Langmuir-Hinshelwood, and prompt thermal mechanisms according to reaction rate constants, sticking coefficients, and surface coverage. In addition to the macroscopic information (reaction mechanisms, rate constants, etc.), this DSMC model also includes microscopic information regarding detailed scattering of ablation products from a carbon surface (including sticking coefficients, desorption barriers, angular scattering, etc.). This DSMC solver is used to perform simulations of the molecular beam scattering from vitreous carbon with substrate temperatures ranging from 550-2000 K. Detailed analysis of the experimental time-of-flight (TOF) and angular distributions is used to propose modifications to the carbon surface oxidation model developed by Poovathingal et al., in order to better capture the features of the TOF distribution. We demonstrate that this revised model captures features in the TOF distributions, including the long tail in the CO distribution that indicates a slow CO production process and the fast component in the O distribution that corresponds to impulsively scattered and thermally desorbed O atoms, which were not captured with the previous model. Comparisons between the simulated (DSMC) and experimental TOF distributions, angular distributions, and relative product fluxes show excellent agreement The resulting DSMC model is provided describing the detailed scattering information of the products, and the probabilities and characteristic frequencies of surface reactions based on finite rates.
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An oxidation model for carbon surfaces is developed where the gas-surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam experiments. In the experiments, a high en...
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An oxidation model for carbon surfaces is developed where the gas-surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam experiments. In the experiments, a high energy beam of oxygen (93% atoms and 7% molecules) was directed at a high-temperature carbon surface. The measurements revealed that CO was the dominant reaction product and that its formation required a high surface coverage of oxygen atoms. As the carbon sample temperature was increased during the experiment, the surface coverage was reduced and the production of CO diminished. Most importantly, the measured time-of-flight distributions of surface reaction products indicated that CO and CO_2 were predominately formed through thermal reaction mechanisms and not impulsive reactive scattering. These observations enabled the formulation of a finite-rate oxidation model including surface-coverage dependence, similar to existing finite-rate models used in computational fluid dynamics (CFD) simulations. However, each reaction mechanism and all rate parameters of the new model are determined individually based on the molecular beam data. The new model is compared to existing models using both zero-dimensional gas-surface simulations and full CFD simulations of hypersonic flow over an ablating surface. The new model predicts similar overall mass loss rates compared to existing models, however, the individual species production rates are completely different. The most notable difference is that the new model (based on molecular beam data) predicts CO as the oxidation reaction product with virtually no CO_2 production, whereas existing models predict the exact opposite trend.
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摘要 :
An oxidation model for carbon surfaces is developed where the gas-surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam experiments. In the experiments, a high en...
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An oxidation model for carbon surfaces is developed where the gas-surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam experiments. In the experiments, a high energy beam of oxygen (93% atoms and 7% molecules) was directed at a high-temperature carbon surface. The measurements revealed that CO was the dominant reaction product and that its formation required a high surface coverage of oxygen atoms. As the carbon sample temperature was increased during the experiment, the surface coverage was reduced and the production of CO diminished. Most importantly, the measured time-of-flight distributions of surface reaction products indicated that CO and CO_2 were predominately formed through thermal reaction mechanisms and not impulsive reactive scattering. These observations enabled the formulation of a finite-rate oxidation model including surface-coverage dependence, similar to existing finite-rate models used in computational fluid dynamics (CFD) simulations. However, each reaction mechanism and all rate parameters of the new model are determined individually based on the molecular beam data. The new model is compared to existing models using both zero-dimensional gas-surface simulations and full CFD simulations of hypersonic flow over an ablating surface. The new model predicts similar overall mass loss rates compared to existing models, however, the individual species production rates are completely different. The most notable difference is that the new model (based on molecular beam data) predicts CO as the oxidation reaction product with virtually no CO_2 production, whereas existing models predict the exact opposite trend.
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Molecular simulations are performed of high temperature dissociated oxygen reacting with an idealized carbon-carbon composite material, where the microstructure is resolved. The Direct Simulation Monte Carlo (DSMC) method is used ...
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Molecular simulations are performed of high temperature dissociated oxygen reacting with an idealized carbon-carbon composite material, where the microstructure is resolved. The Direct Simulation Monte Carlo (DSMC) method is used to simulate the convection and diffusion of reactants towards the microstructure and the transport of surface reaction products away from the microstructure. Simulations are performed with and without gas-phase chemical reactions in order to determine the relative importance of gas-surface reactions compared to gas-phase reactions next to the material surface. The simulations incorporate reaction probabilities for individual gas-surface collisions based on new reactive scattering data obtained in a molecular beam facility. The molecular beam experiments clearly indicate that a majority of surface reaction products were produced through thermal mechanisms. The experiments provide detailed data on the relative magnitude of O, O_2, CO, and CO_2 scattering from a representative material sample, made of vitreous carbon. For a gas-surface temperature of 800K, it is found from the simulations that despite CO being the dominant surface reaction product, a gas-phase exchange reaction forms significant CO_2 within the microstructure region. The amount of CO_2 production within the microstructure region is shown to be dependent on the local Knudsen number, based on the exposed microstructure height. Finally, preliminary simulations are performed for a real Carbon-Carbon (C-C) surface. The surface topology is obtained through X-ray microtomography of an ablated C-C sample, which is triangulated and used directly within a DSMC simulation of the gas-surface interaction.
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摘要 :
Molecular simulations are performed of high temperature dissociated oxygen reacting with an idealized carbon-carbon composite material, where the microstructure is resolved. The Direct Simulation Monte Carlo (DSMC) method is used ...
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Molecular simulations are performed of high temperature dissociated oxygen reacting with an idealized carbon-carbon composite material, where the microstructure is resolved. The Direct Simulation Monte Carlo (DSMC) method is used to simulate the convection and diffusion of reactants towards the microstructure and the transport of surface reaction products away from the microstructure. Simulations are performed with and without gas-phase chemical reactions in order to determine the relative importance of gas-surface reactions compared to gas-phase reactions next to the material surface. The simulations incorporate reaction probabilities for individual gas-surface collisions based on new reactive scattering data obtained in a molecular beam facility. The molecular beam experiments clearly indicate that a majority of surface reaction products were produced through thermal mechanisms. The experiments provide detailed data on the relative magnitude of O, O_2, CO, and CO_2 scattering from a representative material sample, made of vitreous carbon. For a gas-surface temperature of 800K, it is found from the simulations that despite CO being the dominant surface reaction product, a gas-phase exchange reaction forms significant CO_2 within the microstructure region. The amount of CO_2 production within the microstructure region is shown to be dependent on the local Knudsen number, based on the exposed microstructure height. Finally, preliminary simulations are performed for a real Carbon-Carbon (C-C) surface. The surface topology is obtained through X-ray microtomography of an ablated C-C sample, which is triangulated and used directly within a DSMC simulation of the gas-surface interaction.
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Atomic oxygen and molecular nitrogen are the major constituents in the residualatmosphere at low Earth orbital altitudes, and they collide with spacecraft surfaces atrelative velocities of 7.8 km s~(-1).The energy associated with ...
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Atomic oxygen and molecular nitrogen are the major constituents in the residualatmosphere at low Earth orbital altitudes, and they collide with spacecraft surfaces atrelative velocities of 7.8 km s~(-1).The energy associated with these hyperthermalcollisions is in excess of many bond dissociation energies and may help promotematerials degradation by allowing barriers to reaction or to collision-induced dissociation(CID) to be overcome. Spacecraft in low Earth orbit (LEO) are also exposed to highfluxes of vacuum ultraviolet (VUV) radiation, which may degrade materials throughvarious photochemical mechanisms. Fluorinated ethylene-propylene copolymer (FEPTeflon) is commonly used on spacecraft. Many researchers have studied the individualand/or combined effects of atomic oxygen, VUV light, and CID on FEP Teflon, yet thedetailed degradation mechanisms of FEP Teflonin LEO are still a subject of debate.Although not as ubiquitous as a spacecraft material, polymethylmethacrylate (PMMA)has been studied as a model and control polymer because of its well known propensity to"unzip" upon exposure to VUV radiation. A combination of beam-surface scattering,quartz crystal microbalance (QCM), and surface-recession experiments were conductedto study the effects of various combinations of O atoms (in the ground 0(3P) state), Aratoms, and VUV light on FEP Teflon° and PMMA. A laser-breakdown source was usedto create hyperthermal beams containing O and O_2or argon. A deuterium lamp provideda source of VUV light. O atoms with 4 eV of translational energy or less did not reactwith a pristine FEP Teflon surface. Volatile O-containing reaction products wereobserved when the 0-atom energy was higher than 4.5 eV, and the signal increased with0-atom energy. Significant FEP Teflon erosion (-20% of Kaptoe H) was observedwhen it was exposed to the hyperthermal O/O_2 beam with an average O-atom energy of5.4 eV. FEP Teflon and PMMA that were exposed to VUV light alone yielded volatileproducts and mass loss. Similarly, CID by Ar also yielded volatile products and massloss, when the Ar energy was higher than 8 eV. However, the erosion caused by VUVlight and/or CID is not significant compared to that caused by O/O_2. There were noobserved synergistic effects of VUV light and O/O_2 exposure.
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Kapton is ubiquitous in space-structures, yet degrades severely in low Earth orbit (LEO) due to reactions with atomic oxygen (AO). Polyhedral oligomeric silsesquioxane (POSS) is a cage-like structure of silicon and oxygen, surroun...
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Kapton is ubiquitous in space-structures, yet degrades severely in low Earth orbit (LEO) due to reactions with atomic oxygen (AO). Polyhedral oligomeric silsesquioxane (POSS) is a cage-like structure of silicon and oxygen, surrounded by organic groups. Both main-chain (MC) and side-chain (SC) POSS-diamines have been polymerized to form POSS-Kapton-polyimides (POSS-Kapton-PIs) which have comparable resistance to AO. POSS-copolymers form an AO-resistant silica layer upon exposure to AO, which has been evidenced by X-Ray Photoelectron Spectroscopy (XPS). Images of space-flown MC-POSS-Kapton-PIs, physical properties of both MC-POSS-Kaptons and SC-POSS-Kaptons, and transmission electron micrographs of MC-POSS-Kaptons will be presented.
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