Recombination Of Nitrogen Atoms Research Articles

Thermal Effect of Surface Catalysis in Subsonic Dissociated-Air Jets. Experiment on a High-Frequency Plasmatron and Numerical Modeling

Experiments on heat transfer in subsonic dissociated-air jets are performed on 100 kW VGU-4 induction plasmatron. The heat fluxes to the water-cooled surfaces of copper, silver, tantalum, beryllium, niobium, gold, molybdenum, and quartz are measured at the stagnation point on a cylindrical flat-nosed model, 50 mm in diameter, having rounded edges at 50 and 100 hPa pressures in the low-pressure chamber and the high-frequency (HF) generator power of 30 to 70 kW. At the same pressures and the HF generator powers of 45 and 64 kW the convective heating of a specimen of sintered silicon carbide is studied in the surface temperature range from 1720 to 1910 K. The predominance of the surface catalyticity effect on the heat flux with respect to nitrogen and oxygen atom recombination is demonstrated. Under the experimental conditions the air plasma flow in the discharge channel of the plasmatron, the subsonic jet flow past the cylindrical model, and the heat transfer to the stagnation point on the model are numerically simulated. Basing on the comparison of the experimental and calculated data on the heat fluxes to the surfaces of metals (Tw = 300 K), quartz (Tw = 572–722 K), and silicon carbide (Tw = 1720–1910 K) the quantitative catalyticity gradation of the materials considered with respect to heterogeneous recombination of nitrogen and oxygen atoms is established.

Experimental strategy to determine nitrogen catalytic behavior of high‐temperature woven ceramics

AbstractNew vehicle deceleration strategies have led to the development of deployable thermal protection systems with woven fabric outer layers that can withstand the harsh aerothermal environment of planetary entry. In this work, we present an experimental strategy for testing fabric samples and a method to determine nitrogen catalytic efficiency, γN, from two‐photon laser‐induced fluorescence measurements of recombining nitrogen atoms near the sample surface. Measurements were made on other silicon carbide materials to better understand the influence of the fabric structure and porosity on catalytic recombination. The three test materials studied were a monolithic α‐SiC, a CVD β‐SiC, and a single‐ply, Hi‐Nicalon β‐SiC fabric layup. Extracted catalytic efficiencies for atomic nitrogen recombination for monolithic α‐SiC, CVD β‐SiC, and single‐ply β‐SiC fabric are (6.18 ± 1.55) × 10−4 (at 1460 K), (3.59 ± 0.90) × 10−3 (at 1500 K), and (8.34 ± 2.19) × 10−4 (at 1675 K), respectively. This would categorize these materials as low‐catalytic materials for nitrogen atom recombination in this temperature range.

Changes in the Surface and Volume Properties of a Spacecraft during Absorption and Recombination of Oxygen and Nitrogen Atoms

The results of studying the influence of cosmic space on a spacecraft surface is presented. We showed that the mechanical (volume and surface), optical, and other characteristics of a spacecraft’s materials noticeably change due to long exposure to cosmic vacuum. At extremely low pressures in cosmic vacuum, the external and internal interfaces in the materials can be broken, resulting in either microscopic surface cracks or internal corrosion. We have shown that changes in the properties of surface layers cause changes in the volume mechanical properties of materials (fatigue strength, etc.). Furthermore, noticeable effects of space vacuum on the mechanical properties of a spacecraft’s materials arise as a result of the absence of oxide and other surface protective films. Due to the disappearance of protective gas and oxide films, and also as a result of sublimation of the surface layers to a thickness close to the wavelengths of electromagnetic radiation, the surface roughness is changed and, as a consequence, their optical characteristics (in particular, the absorption of solar radiation and the emissivity factor) are also changed. During orbital motion in sufficiently dense layers of the upper atmosphere of the Earth, as well as under bombardment by charged particles of cosmic radiation, strong plasma-chemical loosening of the surface material of spacecraft and its dispersion into the surrounding space can occur. This process is especially evident for the front (in the direction of travel) parts of spacecraft. The method used to describe the dynamics of the interaction of atomic oxygen colliding with the surface of silicon dioxide is described. The influence of space vacuum on materials is shown. Particular attention is paid to the phenomenon of accommodation of low-orbit satellite systems and the use of information received from satellites.

Damage to porous SiCOH low-k dielectrics by O, N and F atoms at lowered temperatures

Information on the degradation of porous organosilicate glasses (OSGs) by active radicals and atoms is of high importance for their integration as low-k dielectrics in the next generation of ultra large scale integration (ULSI) production. The films’ degradation is caused by depletion of coverage of the pore surface methyl. OSG samples with differing porosities and pore sizes were treated by O, N, and F atoms at lowered temperatures (down to −45 °С) downstream of O2, N2, and SF6 inductively coupled plasma discharges, respectively. It has been shown that lowering the temperature reduces film degradation. In the case of O atoms, this reduction is insignificant, while the effect is much more noticeable for F atoms. In addition, an accumulation of F atoms forms a fluorocarbon layer during F atom treatment. The accumulated fluorine can interact with surface Si atoms, giving rise to film etching. The film degradation under O and F atoms increases with pore size and porosity, due to deeper atom penetration. In the case of N atoms, even a small temperature reduction essentially decreases OSG degradation, while –CH3 modification is less clear in the cases of O and F atoms. Density functional theory and ab initio molecular dynamics simulations of reaction mechanisms show that atom reactions with surface Si–CH3 groups are the initial stages of OSG degradation. Subsequent mechanisms include branched reaction pathways with the formation, modification, and destruction of different surface groups. In the cases of O and F atoms, these reactions lead to the formation of HxCО and CFx bonds. For the direct reaction of N(4S) atoms with –СН3 groups, significant activation energy is required. Under downstream plasma conditions, –СН3 groups are destroyed in reactions with metastable particles, and first of all, with metastable atoms N(2D), N(2P) and molecules , which are also produced inside the pores in the surface recombination of nitrogen atoms.

Luminescence of molecular nitrogen in cryogenic plasmas

Great enhancement of molecular nitrogen luminescence in the afterglow of nitrogen-helium gas mixtures was observed at temperatures ≤ 10 K. The effect is explained by the increased efficiency of the recombination of nitrogen atoms and energy transfer from metastable nitrogen molecules and helium atoms to nitrogen molecules in the cold dense helium vapor.Great enhancement of molecular nitrogen luminescence in the afterglow of nitrogen-helium gas mixtures was observed at temperatures ≤ 10 K. The effect is explained by the increased efficiency of the recombination of nitrogen atoms and energy transfer from metastable nitrogen molecules and helium atoms to nitrogen molecules in the cold dense helium vapor.

Mechanisms of the Recombination of Oxygen and Nitrogen Atoms on Quartz

A heterogeneous recombination of nitrogen and oxygen atoms is considered. The installation and the method to determine the probability of heterogeneous recombination of oxygen and nitrogen atoms by resonance fluorescence spectroscopy under strictly controlled conditions are described. The variation of the recombination probability of nitrogen and oxygen atoms on the quartz surface is studied in the temperature range of 297–1000 K and pressures of 0.01–10 mbar. The regions of pressure and temperature in which recombination occurs predominantly according to the Langmuir–Hinshelwood mechanism or the Eley–Rideal mechanism are distinguished. Data on the recombination activation energy are presented. The recombination of nitrogen and oxygen atoms occurs mainly at various active centers.

Rotationally induced luminescence of nanoclusters immersed in superfluid helium

We studied the influence of rotation speed of a beaker containing superfluid helium (He II) on the intensity of luminescence of collections of nanoclusters immersed in He II. We observed an increase in the α-group emission of nitrogen atoms (2D→4S transition) in nanoclusters which correlated with the increasing of rotational speed of the beaker. Increasing luminescence was also observed by increasing the concentration of molecular nitrogen in the nitrogen-helium gas mixtures used for the formation of the molecular nitrogen nanoclusters. We suggest that this effect is caused by the change of the density of quantum vortices in He II initiated by variation of rotational speed of the beaker. When the density of the vortices is increased, the probability for the nanoclusters to become trapped in the vortex cores is also increased. The collisions in the vortex cores of trapped nanoclusters with nitrogen atoms stabilized mostly on the surfaces of the nanoclusters initiate the recombination of nitrogen atoms resulting in luminescence.

Non-thermal air plasma jets at atmospheric pressure: The flow-dependent propagation in the afterglow

With a bare pin-to-nozzle electrode configuration, an ac-driven non-thermal air plasma jet with a length of several centimeters is generated. Depending on the gas flow rate, the propagation speed of these plasma columns is in the range of 100–350 m/s, which is close to the gas flow velocity. This indicates that the propagation of the plasma jet is determined by the transport of long lifetime species by gas flow instead of the electron-impact excitation and ionization of molecules under a high electric field. In comparison to the N2, O2, and Ar plasma jets, the length of the air plasma jet is much shorter than that of the N2 plasma jet but longer than that of the O2 and Ar plasma jets. A simple kinetic model reveals that the N2 (A3Σ) metastable plays a crucial role in sustaining the length of the N2 and air plasma jets, whose lifetime is strongly dependent on the quenching effect of O2, O, and NO as well as the compensation from the three-body recombination of nitrogen atoms. Based on the flow-dependent mechanism of the plasma jet, a visually uniform air plasma brush with a width of 2.8 cm and a length of 1.6 cm and a visually uniform N2 plasma brush with a width of 4 cm and a length of 4.4 cm have been demonstrated.

Heat Transfer in Subsonic Flows of Dissociated Nitrogen: HF Plasmatron Experiment and Numerical Simulation

Experiments on heat transfer in subsonic jets of dissociated nitrogen have been carried out on a IPG-4 induction plasmatron. The heat fluxes to copper, stainless steel, nickel, graphite, and quartz surfaces at the stagnation point of a water-cooled cylindrical flat-faced model 20 mm in diameter and dynamic pressures have been measured at a pressure of 50 hPa in the test chamber and a power of 35–65 kW of the HF generator. The experiments showed the influence of surface catalytic properties on the heat flux in relation to the nitrogen atom recombination. In the conditions of the experiments, a numerical simulation of nitrogen plasma flows in the discharge channel of plasmatron and the subsonic dissociated nitrogen jet flow around the cylindrical model has been carried out. The experimental and calculated data on heat fluxes to cooled copper, stainless steel, nickel, graphite, and quartz surfaces are compared. The quantitative catalyticity scale of the studied materials in relation to the heterogeneous recombination of nitrogen atoms is established.

Robust reconstruction of the catalytic properties of thermal protection materials from sparse high-enthalpy facility experimental data

Quantifying the catalytic properties of reusable thermal protection system materials is essential for the design of atmospheric entry vehicles. Their properties quantify the recombination of oxygen and nitrogen atoms into molecules, and allow for accurate computation of the heat flux to the spacecraft. Their rebuilding from ground test data, however, is not straightforward and subject to uncertainties. We propose a fully Bayesian approach to reconstruct the catalytic properties of ceramic matrix composites from sparse high-enthalpy facility experimental data with uncertainty estimates. The results are compared to those obtained by means of an alternative reconstruction procedure, where the experimental measurements are also treated as random variables but propagated through a deterministic solver. For the testing conditions presented in this work, the contribution to the measured heat flux of the molecular recombination is negligible. Therefore, the material catalytic property cannot be estimated precisely. Moreover, epistemic uncertainties are rigorously included, such as the unknown reference calorimeter catalytic property.

Femtosecond Laser Excitation in Argon–Nitrogen Mixtures

Argon gas is demonstrated to enhance femtosecond laser electronic excitation tagging at atmospheric pressure and temperature for unseeded velocimetry applications, primarily through two to four orders of magnitude increase of excited species that may radiate through nitrogen’s second positive system at early timescales of interest. The first positive system continues to play an important role in maintaining this emission at longer delays. A detailed kinetic model is implemented to explain this observed behavior in nitrogen and argon mixtures. Dominant processes governing the creation of and include a slower decay of electron temperature through increased ionization processes, reduced nitrogen quenching of electrons, nitrogen atom creation and recombination, the formation and dissociation of , and a number of argon–nitrogen direct and indirect excitation pathways. The production of ions through charge-transfer reactions and affect excited C- and B-state nitrogen population delays at later times (). The pooling reactions play minor roles in the formation of and at timescales useful for measurements in the femtosecond laser electronic excitation tagging argon plasma chemistry. In mixtures where nitrogen is dominant, metastable argon species are less instrumental in direct nitrogen excitation transfer, , than in facilitating further reactions through maintaining a higher electron temperature, whereas this excitation transfer begins to play a larger role as the percentage of argon is further increased. The model yields results that agree with sub-100 torr argon–nitrogen discharge experiments and theoretical results derived from other studies. It is concluded that not one single process can be credited for the enhancement, but a combination of ionization and heating produces the increased emission observed in argon mixtures.

HF-plasmatron experiment and numerical simulation of heat transfer in underexpanded dissociated-nitrogen jets

Experiments on heat transfer in underexpanded supersonic jets of high-enthalpy nitrogen are performed on the VGU-4 induction high-frequency plasmatron at a pressure of 10.4 GPa in a compression chamber. At gas flow rates of 2.4 and 3.6 g/s and HF generator powers of 45 and 64 kW the heat fluxes to the copper, stainless steel, MPG-7 graphite, and quartz surfaces are measured at the stagnation point of a water-cooled cylindrical, flat-ended model, 20 mm in diameter. In the same regimes the stagnation pressures are measured. The effect of the surface catalyticity with respect to nitrogen atom recombination on the heat flux is demonstrated and the qualitative catalyticity scale of the studied materials is established. In the supersonic regimes nonequilibrium nitrogen plasma flow in the discharge channel of the plasmatron and the underexpanded jet flow past the model are numerically simulated for the experimental conditions. The experimental and calculated data on the stagnation pressures and the heat fluxes to cooled surfaces of the metals, graphite, and quartz are compared.

Thermoluminescence Dynamics During Destructions of Porous Structures Formed by Nitrogen Nanoclusters in Bulk Superfluid Helium

We studied the dynamics of thermoluminescence during destruction of porous structures formed by nanoclusters of nitrogen molecules containing high concentrations of stabilized nitrogen atoms. The porous structures were formed in bulk superfluid helium by injection of the products of discharges in nitrogen–helium gas mixtures through the liquid helium surface. Fast recombination of nitrogen atoms during warming-up led to explosive destruction of the porous structures accompanied by bright flashes. Intense emissions from the $$\alpha $$ -group of nitrogen atoms, the $$\beta $$ -group of oxygen atoms and the Vegard–Kaplan bands of N $$_2$$ molecules were observed at the beginning of destruction. At the end of destruction the M- and $$\beta $$ -bands of NO molecules as well as bands of O $$_2$$ molecules were also observed. Observation of the emissions from NO molecules at the end of destruction was explained by processes of accumulation of NO molecules in the system due to the large van der Waals interaction of NO molecules. For the first time, we observed the emission of the O $$_2$$ molecules at the end of destruction of the porous nitrogen structures as a result of the (NO) $$_2$$ dimer formation in solid nitrogen and subsequent processes leading to the appearance of excited O $$_2$$ molecules.

On charged impurity structures in liquid helium

The thermoluminescence spectra of impurity-helium condensates (IHC) submerged in superfluid helium have been observed for the first time. Thermoluminescence of impurity-helium condensates submerged in superfluid helium is explained by neutralization reactions occurring in impurity nanoclusters. Optical spectra of excited products of neutralization reactions between nitrogen cations and thermoactivated electrons were rather different from the spectra observed at higher temperatures, when the luminescence due to nitrogen atom recombination dominates. New results on current detection during the IHC destruction are presented. Two different mechanisms of nanocluster charging are proposed to describe the phenomena observed during preparation and warm-up of IHC samples in bulk superfluid helium, and destruction of IHC samples out of liquid helium.

Adsorption and reactivity of nitrogen atoms on silica surface under plasma exposure

The kinetics of adsorption, desorption and recombination of nitrogen atoms on a silica surface is investigated. Stable nitrogen atoms are grafted to the inner surface of a fused silica discharge tube by a discharge in N2 at 0.53 mbar. After the pre-treatment, the surface is analysed using x-ray photoelectron spectroscopy and an isotopic exchange technique. The latter consists of the exposure of the pre-treated surface with a discharge in the heavy nitrogen isotope 30N2. Nitrogen isotopologues 29N2 and 28N2 produced on the surface are detected using a mass spectrometer and provide information about the coverage and reactivity of adsorbed 14N atoms. It is found that during the pre-treatment, a silicon oxynitride (SiOxNy) layer is formed on the initially clean SiO2 surface. The coverage of N on the surface increases from 5 × 1013 to 5 × 1015 cm−2 for a pre-treatment duration in the range of 10−2 − 104 s. Atoms on the surface demonstrate a distribution of reactivity, which is attributed to a distribution of their binding energies and configurations on the surface. We demonstrate that stable chemisorbed Nads are not the main recombination sites for N atoms on the surface contrary to previous studies. We conclude that recombination takes place mainly on weakly bonding active sites with the binding energy smaller than 1 eV.

Studies on gas breakdown in pulsed radio frequency atmospheric pressure glow discharges

In pulsed RF atmospheric pressure glow discharges, the gas breakdown judged by the rapid drop in the amplitude of the pulsed RF voltage is no longer universally true. The steep increment of the plasma-absorbed RF power is proposed to determine the gas breakdown. The averaged plasma-absorbed RF power over a pulse period is used to evaluate effects of the preceding pulsed RF discharge on the breakdown voltage of the following one, finding that the breakdown voltage decreases with the increment in the averaged plasma-absorbed RF power under constant pulse duty ratio. Effects of the pulse off-time on the breakdown voltage and the breakdown delay time are also studied. The obtained dependence of the breakdown voltage on the pulse off-time is indicative of the transitional plasma diffusion processes in the afterglow. The breakdown voltage varies rapidly as the plasma diffuses fast in the region of moderate pulse off-time. The contribution of nitrogen atom recombination at the alumina surface is demonstrated in the prolonged memory effect on the breakdown delay time vs. the pulse off-time and experimentally validated by introducing a trace amount of nitrogen into argon at short and long pulse off-times.

Simultaneous determination of the recombination probability of nitrogen and oxygen atoms on quartz

The state of the problem of the heterogeneous recombination of nitrogen and oxygen atoms has been considered. A setup for measuring the probability of the heterogeneous recombination of atoms using resonance florescence spectroscopy has been described. The main feature of this method is the possibility of simultaneous registration of the concentration of N and O atoms in the air mixture in strictly controlled conditions. The temperature dependences of the probability of the recombination of nitrogen and oxygen atoms separately and in the combined recombination process on the quartz surface in the range of 300–600 K have been obtained. The proposed methodology also made it possible to start studying the mutual effect of the processes of the heterogeneous recombination of nitrogen and oxygen and to determine the fraction of the cross recombination.

Elaboration of collisional–radiative models for flows related to planetary entries into the Earth and Mars atmospheres

The most relevant way to predict the excited state number density in a nonequilibrium plasma is to elaborate a collisional–radiative (CR) model taking into account most of the collisional and radiative elementary processes. Three examples of such an elaboration are given in this paper in the case of various plasma flows related to planetary atmospheric entries. The case of theoretical determination of nitrogen atom ionization or recombination global rate coefficients under electron impact is addressed first. The global rate coefficient can be implemented in multidimensional computational fluid dynamics calculations. The case of relaxation after a shock front crossing a gas of N2 molecules treated in the framework of the Rankine–Hugoniot assumptions is also studied. The vibrational and electronic specific CR model elaborated in this case allows one to understand how the plasma reaches equilibrium and to estimate the role of the radiative losses. These radiative losses play a significant role at low pressure in the third case studied. This case concerns CO2 plasma jets inductively generated in high enthalpy wind tunnels used as ground test facilities. We focus our attention on the behaviour of CO and C2 electronic excited states, the radiative signature of which can be particularly significant in this type of plasma. These three cases illustrate the elaboration of CR models and their coupling with balance equations.

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.

Navier-Stokes Simulations of Hypersonic Flows withCoupled Graphite Ablation

equilibriumablationwithsurfacemassandenergybalancesfullycoupledwiththenumericalsolverandcanaccount for both surface oxidation and sublimation. The surface temperature is obtained from the steady-state ablation approximation. This numerical procedure can predict aerothermal heating, chemical species concentrations, and carbon material ablation rate over the heat-shield surface of reentry vehicles. Two-dimensional axisymmetric simulations have been performed to numerically reproduce the ablation of a graphite sphere cone that has been tested in the Interaction Heating Facility at the NASA Ames Research Center. The freestream conditions of the selected test case are typical for Earth reentry from a planetary mission. The predicted ablation rate and surface temperatureassumingfrozenchemistryinthe flowshowagoodagreementwiththeavailableexperimentaldata.The agreementisfurtherimprovedfreezingthenitrogenrecombinationreactionatthesurfacetobemoreconsistentwith experimental observation, which has shown nitrogen atom recombination not to occur at the graphite surface.