Degree Of Oxygen Dissociation Research Articles

Thermochemical non-equilibrium effects on hypersonic wavecatcher intake at Mach 12

The thermal and chemical non-equilibrium effects on the flow structures and performance of a hypersonic inward-turning intake, for the airbreathing scramjet propulsion, are numerically studied at Mach 12. Three gas models of 1) thermally perfect gas (TPG), 2) chemical non-equilibrium gas (CNEG), and 3) thermochemical non-equilibrium gas (TCNEG), are performed. The influence of wall temperatures is analyzed, with isothermal and adiabatic wall conditions. Wall temperature has a significant effect on the mass capture coefficient, dropping by almost 6% from Tw = 300 K to adiabatic cases. The dissociation of air occurs mainly in the boundary layer near the wall. As the wall temperature decreases, so does the degree of dissociation inside the intake. For adiabatic cases, the results show that the degree of dissociation with TCNEG model is higher than that of the CNEG model. The degree of oxygen dissociation with TCNEG model reaches 54.0% in the symmetry plane, which is 6.3% higher than the value predicted by CNEG model. For the isothermal case at Tw = 2000 K, only a few dissociated oxygen atoms are observed in the isolator with TCNEG and CNEG model, and it is much less than that of adiabatic case. It is suggested that the thermochemical non-equilibrium effect is significant for a Mach 12 intake at adiabatic wall conditions.

Numerical study of high temperature non-equilibrium effects of double-wedge in hypervelocity flow

The high temperature non-equilibrium effects of shock wave interaction and shock wave/boundary layer interaction are important issues for hypervelocity flows. The models of thermochemical non-equilibrium gas (TCNEG), thermal non-equilibrium chemical frozen gas (TNCFG), chemical non-equilibrium gas (CNEG), and thermally perfect gas are used to simulate the double-wedge flows with a total enthalpy of 8 MJ/kg in this study. The unsteady two-temperature Naiver-Stokes equations in the laminar and turbulence flows are solved using the finite volume method. For laminar flow, the shock structures and the heat flux peak for TCNEG model at 170 μs are agreed better with the experiment result compared to reference studies. There are different size vortices in the separation zones, which causes the distributions of the wall heat flux oscillate irregularly. The thermal non-equilibrium effects are the most intense near the attached shock and detached shock, and the degree of oxygen dissociation is the strongest in the subsonic zone near the slip-line. For turbulence flow, the shock structures for the four models are close to Edney's IV interaction. The separation shock position for the TNCFG model is the most upstream, and that for the CNEG model is quite different from the TCNEG model. The intensity of the reflected shocks on the back wedge and its nearby shock interaction largely determine the peak values of the heat flux for the four models.

Increasing the oxygen dissociation degree in the plasma of a pulse-periodic Ar/O2 low-pressure arc

The results of a study on O2 dissociation conditions in the near-anode area of forcibly compressed low-pressure arc discharge are presented. A combined mode of discharge operation was applied to increase the O2 dissociation degree. In this mode, high-current pulses (up to 160 A) of short duration (100 μs) were imposed on a direct current discharge (up to 70 A). Probe measurements showed that a higher atomic oxygen concentration is caused by increasing not only the electron current, but also the voltage drop in the anode part of the discharge in the pulsed phase. A necessary condition for achieving high average values of O2 dissociation degree at high pulse ratio is the increased lifetime of oxygen atoms in the volume. This condition is fulfilled in a gas-discharge system with metal walls used for the deposition of oxide coatings through formation on the walls of amorphous oxide films, which have a low atomic oxygen recombination coefficient. An O2 dissociation degree of ∼0.4 in a low-pressure arc (less than 1 Pa) in Ar/O2 mixture was obtained by optimizing the parameters of the arc operation mode and the condition of the wall surface of the gas-discharge system.

Application of the catalytic probe method for measuring the concentration of oxygen atoms in Ar/O2 plasma of a low-pressure arc

The monitoring of the degree of oxygen dissociation in the discharge plasma is critical for various plasma applications associated with the etching and oxidation of surfaces or the reactive deposition of oxide coatings. The use of existing measurement techniques is limited owing to their complexity, significant error rate, or application conditions. This study deals with the development of a catalytic probe method for measuring the degree of oxygen dissociation in dense arc discharge plasma. A method for measuring and processing the experimental results is presented, which allows the determination of the thermal contribution of the heterogeneous recombination of oxygen atoms at a high total heating power of the catalytic probe by particle streams and plasma radiation. The atomic oxygen concentration was measured in low-pressure arc plasma with a self-heating hollow cathode in an Ar/O2 mixture with changes in the discharge current and oxygen partial pressure over a wide range of 30–70 A and 0.2–0.6 Pa, respectively. It has been demonstrated that the maximum degree of oxygen dissociation (up to 25% of the O2 content) is achieved at the maximum discharge current and is practically independent of the oxygen flow, whereas the highest concentration of atomic oxygen is achieved when the maximum current and O2 flow values are combined. This conclusion is important for technologies based on plasma-chemical processes in high-current discharges.

Volume and surface loss of O(3P) atoms in O2 RF discharge in quartz tube at intermediate pressures (10–100 Torr)

The O(3P) atom loss has been studied in O2 RF plasma in the quartz tube in the intermediate pressure range (10–100 Torr) when the transition from the surface to volume loss is observed. Space- and time-resolved actinometry on Ar and Kr atoms was used to study O(3P) atom loss. The research has shown that such a transition actually takes place. However, it was revealed that the gas temperature plays a significant role in this. Gas temperature was measured spectroscopically using the band emission. It was demonstrated that the gas temperature in the plasma volume was rather high (>1200 K) due to the high values of the specific input RF power which in turn led to a significant O(3P) loss rate decrease in the discharge volume. The atomic oxygen loss is limited by the O(3P) surface recombination as well as by the volume recombination in a thin layer near the wall. It leads to a low integral loss rate and provides a high oxygen dissociation degree. Analysis based on measured [O(3P)]/N, Tgas, O3 spatial profiles and surface loss model including recombination with chemisorbed and physisorbed atoms has revealed the importance of both surface and volume losses. High values of the specific input RF power also lead to increase of the gas temperature near the wall and the temperature of the internal tube surface. As a result, the O atom surface loss rate increases and the volume loss rate near the wall decreases, so overall the contributions of both O(3P) volume and surface recombination are comparable at pressures up to 100 Torr. The O(3P) loss kinetics at intermediate pressures appears to be a rather complex phenomenon and requires at least 1D or even 2D modeling for its correct description. Using global models under similar conditions may lead to dubious results in a detailed study of kinetic mechanisms and processes, but it can be useful for a simple analysis of experimental results.

Investigation of the radially resolved oxygen dissociation degree and local mean electron energy in oxygen plasmas in contact with different surface materials

Energy Resolved Actinometry is applied to simultaneously measure the radially resolved oxygen dissociation degree and local mean electron energy in a low-pressure capacitively coupled radio-frequency oxygen plasma with an argon tracer gas admixture. For this purpose, the excitation dynamics of three excited states, namely, Ar(2p1), O(3p3P), and O(3p5P), were determined from their optical emission at 750.46 nm, 777.4 nm, and 844.6 nm using Phase Resolved Optical Emission Spectroscopy (PROES). Both copper and silicon dioxide surfaces are studied with respect to their influence on the oxygen dissociation degree, local mean electron energy, and the radial distributions of both quantities and the variation of the two quantities with discharge pressure and driving voltage are detailed. The differences in the measured dissociation degree between different materials are related back to atomic oxygen surface recombination probabilities.

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.

Actinometry Measurement of Oxygen Dissociation Degree in a Microwave Discharge Plasma and Effect of Electron Energy Distribution Function

AbstractWe measured the dissociation degree of oxygen molecule in a microwave discharge oxygen plasma by optical emission spectroscopy measurement based on actinometry method. We generated an oxygen plasma in a cylindrical quartz tube (26 mm i.d.) with a discharge pressure 0.5 - 3.0 Torr. The microwave frequency was 2.45 GHz and the output power was set at 600 W. We monitored the oxygen atom density by actinometry using the 844.6 nm line of oxygen atom and the 811.5 nm line of argon. The dissociation degree was estimated at about a few percent. To discuss the validity of the measured dissociation degree, we calculated the population balance equations of the active spices and EEDF (electron energy distribution function) self-consistently. The theoretical calculations were in agreement with the experimental results. We discussed the validity of the dissociation degree of oxygen molecule and some reactions, which play an important role in the microwave discharge oxygen plasma.

アーク風洞プルーム中の原子状酸素数密度分布

Absorption spectroscopy has been applied to the measurement of the arc-heater plumes. Number density distributions of atomic metastable oxygen were obtained from the measured absorption line profile at 777.19 nm in argon-oxygen plumes generated by arcjet type arc-heaters. As a result, it was found that the oxygen is localized off axis at the nozzle exit, and diffuses from outside toward the centerline in the plume. Moreover, numerical simulation was conducted to investigate the diffusion process of the oxygen in the arc-heater. The result indicates that the maximum degree of oxygen dissociation would be at the level of 0.01%.

Diagnostics in helicon plasmas for deposition

and helicon plasmas used for plasma enhanced chemical vapour deposition of films are investigated in the 1 - 10 mTorr pressure and 0 - 800 W rf power ranges. The positive oxygen ions are analysed by energy selective mass spectrometry and Langmuir probes. The oxygen atom concentration is monitored by actinometry and ionization threshold mass spectrometry. In oxygen plasmas it is shown that is the major positive ion, and that the oxygen molecules are far from being completely dissociated, due to a very high oxygen atom recombination frequency on the reactor walls. The dissociation degree increases with the rf power reaching 10% at 500 W. In plasmas, the plasma density and electron temperature decrease as the TEOS fraction increases. In contrast, the degree of oxygen dissociation increases sharply with the addition of a few per cent TEOS, is maximum for about 5% TEOS and decreases as TEOS fraction is further increased. In a 95:5 plasma (5 mTorr, 300 W) the fluxes of oxygen positive ions and atoms impinging onto a floating substrate are estimated to be and respectively. Under these plasma conditions, near-stoichiometric films, with low OH content, are deposited at ambient temperature. The corresponding atom to ion flux ratio is about 250, which suggests the dominant role of oxygen atoms in the deposition kinetics. The comparison of the compositions of layers grown in a 5 mTorr 95:5 plasma at two rf powers confirms the major role of oxygen atoms.

Interaction of titanium with atomic and molecular oxygen

Oxidation of α-titanium has been studied under conditions simulating low Earth orbits i.e. in atomic oxygen at an incident flux of 1016–1017 particles cm–2 s–1. The temperature range was 873–1123 K, in which α-modification of Ti exists. The kinetics of oxidation in atomic oxygen and in low-pressure (0.01–0.1 Pa) diatomic oxygen have been investigated in the initial stages of the process (i.e. before oxide scale formation) in relation to oxygen flux and temperature. The flux of oxygen dissolving in the metal across and metal/gas interface was determined by electrical resistance measurements. Linear rate law was observed at the initial stages of oxidation. For the dependence of reaction rate on diatomic oxygen pressure (or incident flux), the order with respect to pressure was found to be below unity (0.65 at 973 K and 0.82 at 1073 K). A high value of activation energy was obtained, 183 kJ mol–1. A kinetic explanation of these observations has been proposed. Strong acceleration of oxidation in dissociated oxygen was found only at temperatures below 973 K. The reaction rate increase four-fold with a degree of oxygen dissociation of 28.7% at 923 K and two-fold at 973 K. This increase in oxidation rate was confirmed by gravimetry and measurements of the titanium lattice parameter.

Influence of nitrogen on the oxygen dissociation in a DC discharge

The effect of the addition of molecular nitrogen on the oxygen dissociation is investigated in a low pressure gas discharge. The dissociation efficiency is measured as a function of N2 concentration, for a total pressure ranging from 0.4 to 3 Torr in discharge tubes with diameter between 6 and 21 mm. The concentration of O atoms is obtained by the NO+O chemiluminescent reaction. The authors show that for a few percent of nitrogen the oxygen dissociation degree varies by a factor of 1.5-10 depending on the nitrogen concentration, tube diameter and total pressure. The ratio of the dissociation degree for pure oxygen and mixtures of oxygen and nitrogen is calculated, and the authors show that their results imply that, as the tube diameter tends to infinity, the dissociation efficiency tends to one. The authors conclude that the most important reason for the increase in the dissociation of oxygen in the presence of nitrogen is the reduction of wall diffusion losses.

Energy input to the lower thermosphere

The asymmetry in solar heat input to the upper atmosphere at the solstice, and the asymmetry in atomic oxygen production, are largely compensated by a large scale wind system towards the winter polar region. At magnetically disturbed times, atmospheric composition at high winter latitudes changes in such a way as to indicate that polar region heating by magnetic variations, energetic particle inputs, and current systems is more intense than solar heating at low latitudes, thus leading to a reversal of the normal pattern of upper atmospheric circulation. Uncertainties in the intensity of solar radiation responsible for upper atmospheric heating and oxygen dissociation, and uncertainties in the degree of oxygen dissociation in the upper atmosphere, are such that the average rates of eddy mixing may be significantly lower than frequently assumed for the lower thermosphere.

Atomic oxygen transport in the thermosphere

The photodissociation of oxygen in the lower thermosphere is evaluated to obtain its global average value and the hemispheric imbalance. The observed concentrations of atomic oxygen do not reflect this imbalance in production due to the effect of seasonal wind patterns redistributing the atomic oxygen. The wind system necessary to compensate for the imbalance in solar thermal input into the lower thermosphere is found to transport an amount of atomic oxygen sufficient to compensate for the hemispheric imbalance in production. Ionospheric data indicate a winter enhancement in atomic oxygen concentration; to produce this, a higher degree of oxygen dissociation than that normally accepted (i.e. higher than an atomic to molecular oxygen ratio of unity at 120 km) is needed. The concept that the concentrations of atomic oxygen observed over the winter polar region are maintained by transport from lower latitudes requires that eddy diffusion coefficients derived from vertical transport at low latitudes (ignoring horizontal transport) be reduced by about 25 per cent.