Recombination Of Oxygen Atoms Research Articles

Closed model of oxygen recombination on an Al2O3 surface

On the basis of cluster-approximation quantum-chemical calculations of the interaction of an α-Al2O3 surface with oxygen, the rate coefficients for the elementary steps of the heterogeneous recombination of atomic oxygen are determined in the framework of the Eley-Rideal and Langmuir-Hinshelwood mechanisms. For the diffusion layer near the studied surface, these coefficients are used to calculate the probabilities of heterogeneous catalytic recombination, surface coverage, and heat flux to the surface at temperatures of 200–2000 K and pressures of 1000–7000 Pa. The results are compared to the results a solid-state periodic model, low-temperature plasma etching studies, and empirical models of recombination of atomic oxygen on a SiO2 surface.

Decomposition of toluene in a steady-state atmospheric-pressure glow discharge

Results are presented from experimental studies of decomposition of toluene (C6H5CH3) in a polluted air flow by means of a steady-state atmospheric pressure glow discharge at different water vapor contents in the working gas. The experimental results on the degree of C6H5CH3 removal are compared with the results of computer simulations conducted in the framework of the developed kinetic model of plasma chemical decomposition of toluene in the N2: O2: H2O gas mixture. A substantial influence of the gas flow humidity on toluene decomposition in the atmospheric pressure glow discharge is demonstrated. The main mechanisms of the influence of humidity on C6H5CH3 decomposition are determined. The existence of two stages in the process of toluene removal, which differ in their duration and the intensity of plasma chemical decomposition of C6H5CH3 is established. Based on the results of computer simulations, the composition of the products of plasma chemical reactions at the output of the reactor is analyzed as a function of the specific energy deposition and gas flow humidity. The existence of a catalytic cycle in which hydroxyl radical OH acts a catalyst and which substantially accelerates the recombination of oxygen atoms and suppression of ozone generation when the plasma-forming gas contains water vapor is established.

Characteristics of Inductively Coupled Plasma Test Flow During Catalytic Recombination Processes

The flowfield characterization in an inductively coupled plasma generator is conducted by optical emission spectroscopy and laser absorption spectroscopy as well as direct simulation Monte Carlo computation. A silicon carbide test piece is exposed to a 1-kilowatt oxygen—argon inductively coupled plasma test flow, and the emission intensity profiles of an atomic oxygen spectral line at 844.636 nm and an argon spectral line at 842.4648 nm along the central axis of the quartz tube are first obtained by optical emission spectroscopy with the surface temperature of 300, 1280, and 1430 K. By comparing the measured emission intensity profiles with the computed number density profiles, qualitative agreement is found for the case with the surface temperature of 300 K, while the difference becomes evident for the other two cases. Because the emission intensity obtained by the spectrometer is generally the integrated value along the line of sight, the measured emission intensity profile can be different from the true emission intensity profile along the central axis. Measure of the emission intensity profiles of atomic oxygen and argon in the radial direction by applying the inverse Abel transform to characterize the entire flowfield in the test chamber is attempted. It is found that the emission intensity near the quartz tube is significantly strong around the test piece not depending on the surface temperature of the test piece. Moreover, the electronic excitation temperature is evaluated through a Boltzmann plot, and it is found that it increases toward the test piece. In the direct simulation Monte Carlo computation, the measured emission intensity profiles of atomic oxygen and argon along the central axis for heating cases are qualitatively reproduced when the measured electronic excitation temperature at the exit boundary is assumed. These results show that the emission intensity profiles of atomic oxygen and argon in the entire flowfield around the test piece should be estimated by applying the inverse Abel transform for evaluating the catalytic efficiency of atomic oxygen recombination accurately.

Dynamics of the Oxygen Molecules Scattered from the Graphite (0001) Surface and Comparison with Experimental Data

A quasiclassical trajectory dynamics study of molecular oxygen colliding over a free of defects and clean graphite (0001) surface has been performed with a recently published density functional theory based flexible periodic London–Eyring–Polanyi–Sato potential energy surface (PES). Although the PES was mainly constructed for describing accurately the recombination of atomic oxygen over an O-preadsorbed surface, here we show that this PES is also reliable to study the scattering of O2 over a graphite surface. Thus, several initial conditions have been explored: collision energies (0.2 eV ≤ Ecol ≤ 1.2 eV), incident angles (θv = 0, 45°), surface temperatures (100 K ≤ Tsurf ≤ 900 K), and some rovibrational O2 levels (v = 0, 1, 2 and j = 1, 17, 25). The calculated polar scattering angular distributions are in good agreement with the experimental ones in a wide range of explored conditions. Moreover, the comparison with hyperthermal experimental data, which was unclear in a previous work, has been finally clar...

Characteristics of SiOX thin films deposited by atmospheric pressure chemical vapor deposition using a double-discharge system

SiOX thin films were deposited using a gas mixture of hexamethyldisilazane (HMDS)/O2/He/Ar from a remote-type dielectric barrier discharges (DBD) source, with/without the additional direct-type DBD just above the substrate (double discharge), and the effect of the double discharge on the characteristics of the SiOX thin film was investigated. The increase of HMDS flow rate and the decrease of oxygen flow rate in the gas mixture increased the SiOX-thin-film deposition rate. The improvement of the mechanical properties for SiOX film, in addition to the increase of deposition rate, is believed to be related not only to the higher gas dissociation because of the higher power deposition but also to the lesser recombination of oxygen atoms and dissociated HMDS due to the shorter diffusion length to the substrate.

Characterisation of thermal barrier coatings and ultra high temperature composites deposited in a low pressure plasma reactor

A low pressure plasma process working at 600–800 Pa was used to deposit from aqueous solution ZrO 2–4 mol% Y 2O 3 (Yttria partially stabilized Zirconia–YpSZ) layers and stacks of Ta 2O 5/YpSZ layers for use as thermal barrier coatings (TBC). The observation of the cross section revealed a high porosity. The thermal diffusivity of the layers (1 × 10 −7 m 2 s −1) was measured by a laser flash technique and compared with values obtained on air plasma sprayed material (3 × 10 −7 m 2 s −1). The plasma reactor were also used to deposit ZrB 2–ZrO 2–SiC layers used as Ultra High Temperature Composite (UHTC) from aqueous solutions of zirconyl and Boron nitrates containing suspensions of SiC. Layers up to 100 μm thick were obtained on SiC substrates. XRD was used to study the crystallinity of the layer. The presence of ZrB 2 and SiC phases was confirmed after the deposition. XRD analysis showed that heat treatment at 1073 K under oxidizing conditions led to the loss of ZrB 2 and the appearance of ZrO 2 phases. To understand the behaviour of the layers to interaction with atomic oxygen (combustion for TBC and spacecraft re-entry phase for UHTC), we have measured the atomic oxygen recombination coefficient to determine the number of adsorption sites on the surface of the coatings. This was accomplished by using a low pressure plasma reactor coupled with optical spectroscopic measurements as a diagnostic technique.

Kinetic characteristics of the process of heterogeneous recombination of O(3P) atoms in O2-Ar plasma

The probabilities of the heterogeneous recombination of oxygen atoms have been determined using the method of electron paramagnetic resonance (EPR) in a positive glow gap of O2-Ar (0–90%) mixtures in a wide range of temperatures of the reactor wall. The apparent activation energies of the process of heterogeneous recombination have been calculated.

Effect of Chemical Reaction Rates on Aeroheating Predictions of Reentry Flows

The present work deals with the sensitivity of aero-heating predictions to variations in chemical reaction rates in high enthalpy flows over re-entry capsules. With in this scope, numerical simulations are performed by varying the reaction rate constants over their uncertainty range. FIRE II re-entry capsule at 35 km altitude is chosen as the test case. At these conditions, recombination of oxygen atoms is the dominant chemical reaction in the thermal boundary layer next to the wall. The flow solutions computed by altering the reaction rates in this region are carefully analyzed to understand the physical effects that influence the surface heat transfer rate. It is found that the chemical state of the gas i.e. equilibrium, non-equilibrium or frozen, and the extent of recombination reactions in the boundary layer are the critical factors that determine the sensitivity of the heating rate to variations in the reaction rate constants. The analysis is carried out at different points on the vehicle surface, and the heating rate sensitivity is found to be the highest at the shoulder. This is due to the non-equilibrium chemical state of the gas prevailing in the shoulder expansion region. By comparison, the equilibrium state of the gas on the forebody and the frozen chemistry on the afterbody result in lower sensitivity of the heating rate to variations in the chemical reaction rates.

Recombination and chemical energy accommodation coefficients from chemical dynamics simulations: O/O2 mixtures reacting over a β-cristobalite (001) surface

A microkinetic model is developed to study the reactivity of an O/O(2) gas mixture over a β-cristobalite (001) surface. The thermal rate constants for the relevant elementary processes are either inferred from quasiclassical trajectory calculations or using some statistical approaches, resting on a recently developed interpolated multidimensional potential energy surface based on density functional theory. The kinetic model predicts a large molecular coverage at temperatures lower than 1000 K, in contrary to a large atomic coverage at higher temperatures. The computed atomic oxygen recombination coefficient, mainly involving atomic adsorption and Eley-Rideal recombination, is small and increases with temperature in the 700-1700 K range (0.01 < γ(O) < 0.02) in good agreement with experiments. In the same temperature range, the estimated chemical energy accommodation coefficient, the main contribution to which is the atomic adsorption process is almost constant and differs from unity (0.75 < β(O) < 0.80).

Surface recombination of oxygen atoms in O2 plasma at increased pressure: I. The recombination probability and phenomenological model of surface processes

This work deals with the study of oxygen atom loss on a quartz surface in a glow discharge plasma in pure O2 at increased pressures (5–50 Torr). O atom loss probabilities are obtained from the radial distributions of oxygen dissociation degree measured by the actinometry method. It is shown that the applicability of the actinometry method at high pressures requires the knowledge of the spatial distribution of a reduced electric field for the correct calculation of the electronic excitation rates of oxygen and actinometer atoms. The analysis of the obtained data within the framework of a simple phenomenological model of the surface processes revealed that O atom surface recombination with physisorbed oxygen atoms and molecules (producing O2 and O3, respectively) is the main loss channel for oxygen atoms in O2 plasmas at increased pressures. The oxygen atom loss probability can noticeably grow in comparison with the case of low pressure due to the essential increase in the surface occupation degree by physisorbed atoms and molecules.

Surface recombination of oxygen atoms in O2 plasma at increased pressure: II. Vibrational temperature and surface production of ozone

Ozone production in an oxygen glow discharge in a quartz tube was studied in the pressure range of 10–50 Torr. The O3 density distribution along the tube diameter was measured by UV absorption spectroscopy, and ozone vibrational temperature TV was found comparing the calculated ab initio absorption spectra with the experimental ones. It has been shown that the O3 production mainly occurs on a tube surface whereas ozone is lost in the tube centre where in contrast the electron and oxygen atom densities are maximal. Two models were used to analyse the obtained results. The first one is a kinetic 1D model for the processes occurring near the tube walls with the participation of the main particles: O(3P), O2, O2(1Δg) and O3 molecules in different vibrational states. The agreement of O3 and O(3P) density profiles and TV calculated in the model with observed ones was reached by varying the single model parameter—ozone production probability on the quartz tube surface on the assumption that O3 production occurs mainly in the surface recombination of physisorbed O(3P) and O2. The phenomenological model of the surface processes with the participation of oxygen atoms and molecules including singlet oxygen molecules was also considered to analyse data obtained in the kinetic model. A good agreement between the experimental data and the data of both models—the kinetic 1D model and the phenomenological surface model—was obtained in the full range of the studied conditions that allowed consideration of the ozone surface production mechanism in more detail. The important role of singlet oxygen in ozone surface production was shown. The O3 surface production rate directly depends on the density of physisorbed oxygen atoms and molecules and can be high with increasing pressure and energy inputted into plasma while simultaneously keeping the surface temperature low enough. Using the special discharge cell design, such an approach opens up the possibility to develop compact ozonizers having high ozone yield at the low energy cost of O → O3 conversion.

Analysis of heterogeneous recombination of oxygen atoms on aluminum oxide by methods of quantum mechanics and classical dynamics

To analyze the catalytic properties of heat shield materials of space vehicles, the cluster models of the adsorption of oxygen atoms on aluminum oxide are constructed and the corresponding potential energy surface is calculated on the basis of the density functional theory. Quantum-mechanical calculations showed that it is necessary to take into account the relaxation of the surface monolayers. Using this surface in molecular dynamics, calculations made it possible to obtain the probabilities of the heterogeneous recombination of oxygen atoms on the α-Al2O3 surface, which are in good agreement with experimental data. The calculations performed substantially decrease the amount of experimental investigations necessary reliably to describe the heterogeneous catalysis on promising reusable heat shield coatings for analyzing heat transfer during spacecraft entry into the atmosphere.

Erosion of a-C:H films deposited on W, Mo, and stainless steel under interaction with air glow discharge

An air direct current glow discharge with a hollow cathode was used as source of chemically active oxygen for selective removal of amorphous hydrogenated (a-C:H) films deposited on W, Mo, and stainless steel. The films were removed both directly in the discharge and afterglow region. The film erosion rates depend on the sample position relatively to plasma and decrease in the order: hollow cathode, positive column, afterglow region. It was shown that primary (1–3nm) continuous amorphous and secondary (1–30nm) island-like oxide films were formed on the metal surfaces after removal of the a-C:H films. Polycrystalline island-like oxide films were generated due to recrystallization of the primary films. Material oxidation suppression was caused by reactions of oxygen ion neutralization and atomic oxygen recombination on metals.

Simulation of oxygen atom adsorption on an Al2O3 surface by the density functional method

Several cluster models of oxygen atom adsorption on an Al2O3 surface are constructed on the basis of the density functional method. The performed quantum mechanical computations allow one to reveal a number of important features of the potential energy surface to describe the heterogeneous catalytic processes with the use of molecular dynamics methods. The heterogeneous recombination of oxygen atoms is simulated according to the Eley-Rideal mechanism. It is shown that the potential energy surface should be used with consideration of the internal relaxation of surface monolayers to correctly describe the process under study.

Integrated diffusion–recombination model for describing the logarithmic time dependence of plasma damage in porous low- k materials

This work proposes an extended model that describes the propagation of damage in porous low- k material exposed to a plasma. Recent work has indicated that recombination and diffusion play a more dominant role than VUV light [1–5] in oxygen plasma induced damage. Especially at low depths, the radical concentration is determined by the number of radicals that disappear back into the plasma while the final depth of damage is defined by recombination of oxygen atoms. A logarithmic equation has been proposed to describe the behavior as a function of time. In this work this equation is extended to take diffusion into account, next to recombination. The results are in agreement with experimental data and one-dimensional random walk theory calculations.

Direct Detection of NO Produced by High-Temperature Surface-Catalyzed Atom Recombination

The surface-catalytic recombination of oxygen and nitrogen atoms to form nitric oxide was confirmed by the direct detection of product NO molecules, using single-photon laser-induced fluorescence spectroscopy. Experiments were performed from room temperature to 1200 K in a quartz diffusion-tube sidearm reactor enclosed in a high-temperature tube furnace. Atomic nitrogen was generated using a microwave discharge, and atomic oxygen was produced via the rapid gas-phase titration reaction N + NO → O + N 2 . The use of isotopically labeled titration gases 15 N 16 O and 15 N 18 O allowed for the unambiguous identification of nitric oxide produced by the O + N surface reaction. The absolute number densities of surface-produced NO were determined from separate calibration experiments using 14 N 16 O. Observed variations of the NO number density with temperature and varying O/N atomic ratios at the sidearm entrance are generally consistent with the predictions of a simple reaction-diffusion model of the sidearm reactor that includes surface-catalyzed NO production as a species boundary condition.

Recombination of atomic oxygen on sintered zirconia at high temperature in non-equilibrium air plasma

High temperature ceramic materials are necessary for the design of primary heat shields for future re-usable space vehicles re-entering atmospheric planet at hypersonic velocity. During the re-entry phase on earth, one of the most important phenomena occurring on the heat shield is the recombination of atomic oxygen and this phenomenon is more or less catalyzed by the material of the heat shield. This paper presents some experimental results for the recombination coefficient of atomic oxygen γ based on experiments performed on the MESOX facility using optical emission spectroscopy and actinometry techniques. Experimental results on the recombination coefficient are presented for three types of sintered ZrO 2 in the temperature range 900–2500 K for 200 Pa total air pressure. These three zirconia ceramics differ essentially by the chemical nature of the sintering additives (Y 2O 3, CaO or MgO). A great different behavior of the recombination coefficient versus temperature is observed according to the crystalline structure of zirconia (monoclinic and tetragonal phases) and few influence of the additive is shown.

Effect of energetic ions on plasma damage of porous SiCOH low-k materials

Plasma damage of SiCOH low-k films in an oxygen plasma is studied using a transformer coupled plasma reactor. The concentration of oxygen atoms and O2+ ions is varied by using three different conditions: (1) bottom power only, (2) bottom and top power, and (3) top power only. After plasma exposure, the low-k samples are characterized by various experimental techniques. It is shown that the ion bombardment induced by the bottom power minimizes the plasma damage by increasing the recombination coefficient of oxygen radicals. Contrary to the expectations, the densification of the top surface by ion radiation was limited. The increase in the recombination coefficient is mainly provided by modification of the pore wall surface and creation of chemically active sites stimulating the recombination of oxygen atoms. The results show that a reduction in plasma damage can be achieved without sealing of low-k top surface.

Analysis of the heterogeneous recombination of oxygen atoms on aluminum oxide by methods of quantum mechanics and classical dynamics

On the basis of the density-functional theory, cluster models of the adsorption of oxygen atoms on aluminum oxide are constructed and the corresponding potential-energy surface is calculated. Quantum-mechanical calculations showed that it is necessary to take into account the angular dependence of the potential-energy surface and the relaxation of the surface monolayers. Using this surface in molecular dynamics calculations made it possible to obtain the probabilities of the heterogeneous recombination of oxygen atoms on the α-Al2O3 surface, which are in good agreement with experimental data. The calculations performed substantially decrease the amount of experimental investigations necessary reliably to describe the heterogeneous catalysis on promising reusable heat shield coatings for analyzing heat transfer during spacecraft entry into the atmosphere.

Microstructural characterization of ZrB 2–SiC based UHTC tested in the MESOX plasma facility

Microstructures were investigated for ZrB 2–SiC and ZrB 2–HfB 2–SiC ultra high temperature ceramics that were subjected to a high temperature plasma environment. Both materials were tested in the MESOX facility to determine the recombination coefficient for atomic oxygen up to 1750 °C in subsonic air plasma flow. Surfaces were analyzed before and after testing to gain a deeper insight of the surface catalytic properties of these materials. Microstructural analyses highlighted oxidation induced surface modification. Oxide layers were composed of silica with trace amounts of boron oxide and zirconia if the maximum temperature was lower than about 1550 °C and zirconia for higher temperatures. The differences in the oxide layer composition may account for the different catalytic behavior. In particular, the presence of a borosilicate glass layer on the surface of ZrB 2–SiC materials guarantees atomic oxygen recombination coefficients that are relatively lower than the coefficients measured when only zirconia is present. The oxidation processes of ZrB 2–HfB 2–SiC materials, associated with catalytic tests carried out up to 1550 °C, lead to the formation of hafnia as well as silica, and zirconia. The higher recombination coefficients measured in the case of ZrB 2–HfB 2–SiC materials can be correlated with the presence of hafnia which is probably characterized by higher catalytic activity compared to zirconia. In any case, the investigated materials demonstrate a low catalytic activity over the inspected temperature range with maximum values of recombination coefficients close to 0.1.