Production Of Nitrogen Atoms Research Articles

Numerical study of nitrogen oxides chemistry during plasma assisted combustion in a sequential combustor

Plasma Assisted Combustion (PAC) is a promising technology to enhance the combustion of lean mixtures prone to instabilities and flame blow-off. Although many PAC experiments demonstrated combustion enhancement, several studies report an increase in NOx emissions. The aim of this study is to determine the kinetic pathways leading to NOx formation in the second stage of a sequential combustor assisted by Nanosecond Repetitively Pulsed Discharges (NRPDs). For this purpose, Large Eddy Simulation (LES) associated with an accurate description of the combustion/NOx chemistry and a phenomenological model of the plasma kinetics is used. Detailed kinetics 0-Dimensional reactors complement the study. First, the LES setup is validated by comparison with experiments. Then, the NOx chemistry is analyzed. For the conditions of operation studied, it is shown that the production of atomic nitrogen in the plasma by direct electron impact on nitrogen molecules increases the formation of NO. Then, the NO molecules are transported through the turbulent flame without being strongly affected. This study illustrates the need to limit the diatomic nitrogen dissociation process in order to mitigate harmful emissions. More generally, the very good agreement with experimental measurements demonstrates the capability of LES combined with accurate models to predict the NRPD effects on both turbulent combustion and NOx emissions.

High temperature non-equilibrium flow characteristics of impinging shock/flat-plate turbulent boundary layer interaction at Mach 8.42

There are fewer reports on the impinging shock/boundary layer interaction in the high Mach number and high-temperature flow than that in the supersonic flow. High-temperature flow characteristics of the impinging shock/flat-plate turbulent boundary layer interaction (IS/FTBLI) at Mach 8.42 are numerically investigated by solving two-dimensional Reynolds averaged Navier–Stokes equations coupling with the thermal–chemical non-equilibrium model. An impinging shock is formed by the wedge with a 10° deflection angle. The inviscid flow parameters ahead of the cowl of a Mach 12 inlet are selected as the free-stream condition of this study. The primary emphasis of this study lies in understanding the thermal–chemical non-equilibrium effects in the IS/FTBLI. Moreover, the chemical non-equilibrium effects similar to previous reports from others are utilized for the comparative analysis. Our findings reveal that the vibrational or thermal non-equilibrium effects exhibit maximum prominence subsequent to the intersection of the impinging shock with separation shock, as well as in the convergence area of compression waves during the flow reattachment. On the other hand, the chemical non-equilibrium effects predominantly result from oxygen dissociation and atomic nitrogen production within the boundary layer; the chemical reactions are most intense within the separation zone. By comparing with a thermally perfect gas, a reduction in the flow separation is observed in the chemical non-equilibrium effects, but the flow separation is enhanced in the thermal–chemical non-equilibrium effects. The insights gained from our research are expected to contribute to the development of flow control technology in hypersonic IS/FTBLI scenarios and aid in configuring wave structures in the inner compression section of high Mach number scramjet inlets.

Time evolution of atomic nitrogen density in pure-nitrogen-pulsed barrier discharge at sub-atmospheric pressure

Ground state atomic nitrogen N(2p34S) was analyzed using two-photon absorption laser-induced fluorescence (TALIF) in sub-atmospheric pressure nitrogen pulsed barrier discharge under needle-to-hemisphere electrode configuration. By reducing the pressure from 90 to 30 kPa, the discharge form transitioned from multiple filaments to a single column, improving the reacting region uniformity. The TALIF measurement revealed that the amount of atomic nitrogen near the needle anode increased over tens of microseconds after the discharge, and this N-production during afterglow was enhanced by reducing the pressure. Reducing the pressure from 90 to 30 kPa extended the half-life period of atomic nitrogen near the anode by 350 μs, while maintaining the peak amount of atomic nitrogen. The lifetime extension with the same amount of atomic nitrogen helped improving the chemical activity near the anode. The origin of the N-production during afterglow was not identified as a single factor, but its time constant indicated the contribution of N(2P) quenched by the ground state atomic nitrogen, along with the quenching of N(2D), which was previously considered as a major source of afterglow production of the ground state atomic nitrogen. Under 30 kPa, higher discharge energy resulted in faster and larger amount of atomic nitrogen production during afterglow, which indicates the involvement of highly excited particles including metastable atomic nitrogen. In contrast, the decay rate of atomic nitrogen did not depend on the discharge energy. This suggests that the increasing discharge energy broadens the N-productive region while maintaining the local N density.

A novel plasma nitriding process utilising HIPIMS discharge for enhanced tribological and barrier properties of medical grade alloy surfaces

The demand for improvement of lifetime and biocompatibility of medical implants is ever growing. The materials used in this challenging application must display enhanced tribological properties, biocompatibility and reduced metal ion release in long-term clinical performance. Surface modification techniques such as nitriding, can significantly improve the in-service behaviour of the medical grade alloys used for implants. This communication reports on a novel approach for nitriding of CoCrMo alloy using High Power Impulse Magnetron Sputtering, (HIPIMS) discharge for the first time. The new nitriding process has been successfully carried out in an industrial size Hauzer 1000-4 system enabled with HIPIMS technology at the National HIPIMS Technology Centre at Sheffield Hallam University, UK. Due to the significantly enhanced production of molecular (N2+) and atomic Nitrogen (N+) ion species in the HIPIMS discharge, which are primarily responsible for the nitrided layer formation, a fourfold increase in the process productivity as compared to the state of the art nitriding process was achieved. The surface layers produced exhibit excellent mechanical and tribological properties such as high hardness, high fracture toughness and wear resistance. The protection properties of the nitrided layer against corrosion in the aggressive environments of simulated body fluid (Hank’s solution) are remarkably augmented. Furthermore, the data showed that the amount of metal ions released from the HIPIMS nitrided samples was reduced by a factor of 2, 4 and 10 for Co, Cr and Mo ions respectively thus demonstrating the reliable barrier properties of the nitrided layer.

Drastically Increase in Atomic Nitrogen Production Depending on the Dielectric Constant of Beads Filled in the Discharge Space.

Nitrogen activation, especially dissociation (production of atomic nitrogen), is a key step for efficient nitrogen fixation, such as nitrogen reduction to produce ammonia. Nitrogen reduction reactions using water as a direct hydrogen source have been studied by many researchers as a green ammonia process. We studied the reaction mechanism and found that the nitrogen reduction could be significantly improved via efficient production of atomic nitrogen through electric discharge. In the present study, we focused on packed-bed dielectric barrier discharge (PbDBD) using dielectric beads as the packing material. The experimental results showed that more atomic nitrogen was produced in the nitrogen activation by the discharge in which the discharge space was filled with the dielectric beads than in the nitrogen activation by the discharge without using the dielectric beads. Then, it was clarified that the amount of atomic nitrogen increased as the dielectric constant of the beads to be filled increased, and the amount of atomic nitrogen produced increased up to 13.48 times. Based on the results, we attempted ammonia synthesis using water as a direct hydrogen source with the efficiently generated atomic nitrogen. When the atomic nitrogen gas generated by the PbDBD was sprayed onto the surface of the water phase and subsequently reacted as a plasma/liquid interfacial reaction, the nitrogen fixation rate increased by 7.26-fold compared to that when using the discharge without dielectric beads, and the ammonia production selectivity increased to 83.7%.

Controlled production of atomic oxygen and nitrogen in a pulsed radio-frequency atmospheric-pressure plasma

Radio-frequency driven atmospheric pressure plasmas are efficient sources for the production of reactive species at ambient pressure and close to room temperature. Pulsing the radio-frequency power input provides additional control over species production and gas temperature. Here, we demonstrate the controlled production of highly reactive atomic oxygen and nitrogen in a pulsed radio-frequency ( MHz) atmospheric-pressure plasma, operated with a small % air-like admixture (/ at ) through variations in the duty cycle. Absolute densities of atomic oxygen and nitrogen are determined through vacuum-ultraviolet absorption spectroscopy using the DESIRS beamline at the SOLEIL synchrotron coupled with a high resolution Fourier-transform spectrometer. The neutral-gas temperature is measured using nitrogen molecular optical emission spectroscopy. For a fixed applied-voltage amplitude (234 V), varying the pulse duty cycle from 10% to 100% at a fixed 10 kHz pulse frequency enables us to regulate the densities of atomic oxygen and nitrogen over the ranges of – and – , respectively. The corresponding 11 K increase in the neutral-gas temperature with increased duty cycle, up to a maximum of K, is relatively small. This additional degree of control, achieved through regulation of the pulse duty cycle and time-averaged power, could be of particular interest for prospective biomedical applications.

Dissociation of nitrogen in a pulse-periodic dielectric barrier discharge at atmospheric pressure

Nitrogen molecule dissociation in a pulse-periodic atmospheric-pressure dielectric barrier discharge is numerically analyzed. It is shown that the quenching rate of predissociation states at atmospheric pressure is relatively low and the production of nitrogen atoms in this case can be adequately described using the cross section for electron-impact dissociation of N2 molecules taken from the paper by P.C. Cosby [J. Chem. Phys. 98, 9544 (1993)].

Detection of atomic oxygen and nitrogen created in a radio-frequency-driven micro-scale atmospheric pressure plasma jet using mass spectrometry

In this paper we show mass spectrometry results for a radio-frequency-driven micro-atmospheric pressure plasma jet (μ-APPJ) discharge obtained using a mass analyzer with triple differential pumping allowing us to sample directly in ambient atmospheric pressure environment (Hiden HPR-60). The flow of the buffer gas (mixture of helium and 1% oxygen) was 2 slm and 3 slm and the excitation frequency was 13.56 MHz. We monitored production of atomic oxygen and nitrogen in the plasma for different flows and powers given by the RF power supply. These measurements were made for energies of electrons emitted from the ionization filament below the threshold for dissociation of O2 and N2. In addition to oxygen and nitrogen atoms, yields for O2, N2, NO and O3 are recorded for different powers and gas flows. It is shown that the μ-APPJ is symmetrical and operates in α-mode. The power transmitted to the discharge was below 5 W in all measurements.

Simulation of N-atom production in dielectric-barrier discharge in nitrogen at atmospheric pressure

A plasma-chemical model of atomic nitrogen production in a Townsend dielectric-barrier discharge in nitrogen at atmospheric pressure is presented. On the basis of the comparison with measured densities, a significant discrepancy between the calculated and the measured production rate of nitrogen atoms is observed and discussed.

Production mechanism of atomic nitrogen in atmospheric pressure pulsed corona discharge measured using two-photon absorption laser-induced fluorescence

To study the production mechanism of atomic nitrogen, the temporal profile and spatial distribution of atomic nitrogen are measured in atmospheric pressure pulsed positive corona discharge using two-photon absorption laser-induced fluorescence. The absolute atomic nitrogen density in the streamer filaments is estimated from decay rate of atomic nitrogen in N2 discharge. The results indicate that the absolute atomic nitrogen density is approximately constant against discharge energy. When the discharge voltage is 21.5 kV, production yield of atomic nitrogen produced by an N2 discharge pulse is estimated to be 2.9 − 9.8 × 1013 atoms and the energy efficiency of atomic nitrogen production is estimated to be about 1.8 − 6.1 × 1016 atoms/J. The energy efficiency of atomic nitrogen production in N2 discharge is constant against the discharge energy, while that in N2/O2 discharge increases with discharge energy. In the N2/O2 discharge, two-step process of N2 dissociation plays significant role for atomic nitrogen production.

Space-resolved characterization of high frequency atmospheric-pressure plasma in nitrogen, applying optical emission spectroscopy and numerical simulation

Averaged plasma parameters such as electron distribution function and electron density are determined by characterization of high frequency (2.4 GHz) nitrogen plasma using both experimental methods, namely optical emission spectroscopy (OES) and microphotography, and numerical simulation. Both direct and step-wise electron-impact excitation of nitrogen emissions are considered. The determination of space-resolved electron distribution function, electron density, rate constant for electron-impact dissociation of nitrogen molecule and the production of nitrogen atoms, applying the same methods, is discussed. Spatial distribution of intensities of neutral nitrogen molecule and nitrogen molecular ion from the microplasma is imaged by a CCD camera. The CCD images are calibrated using the corresponding emissions measured by absolutely calibrated OES, and are then subjected to inverse Abel transformation to determine space-resolved intensities and other parameters. The space-resolved parameters are compared, respectively, with the averaged parameters, and an agreement between them is established.

Numerical simulation of atomic nitrogen formation in plasma of glow discharge in nitrogen-argon mixture

We consider the problem of determining the content of atomic nitrogen as an active component responsible for the efficiency of metal surface modification in plasma of stationary low-pressure glow discharge in nitrogen-argon mixture (widely used in this technology). The influence of the gas mixture composition on the rate constant of molecular nitrogen dissociation, which determines the atomic nitrogen production, has been calculated, The parameters of plasma have been experimentally determined using the method of double probes. The electron energy distribution function is found by numerically integrating the Boltzmann equation in a two-term approximation for the molecular nitrogen-argon mixture.

Quintet electronic states of N2

We use large scale ab initio calculations to investigate the valence and valence-Rydberg quintet states of N(2), their transition moments and their spin-orbit couplings to the close lying triplet electronic states. In addition to the A' (5)Sigma(g)(+) and the C" (5)Pi(ui) states already known, we identify two weakly bound states (2 (5)Sigma(g)(+) and 2 (5)Pi(u)) at approximately 95,300 and 106,200 cm(-1) above N(2)(X (1)Sigma(g)(+), v=0). The other quintets are viewed to be repulsive in nature. Our potentials and couplings are used later to derive a set of accurate spectroscopic data for these quintets, their spin-orbit constants, and to elucidate the quintet-triplet dynamics and the role of these newly identified quintets for the production of cold atomic nitrogen.

SPECTROSCOPIC CHARACTERIZATION OF AN ATMOSPHERIC PRESSURE DBD AFTERGLOW UNDER UNIPOLAR AND BIPOLAR PULSED EXCITATION

This experimental work deals with the study via spectroscopic diagnostics of the afterglow issued from an atmospheric pressure Dielectric Barrier Discharge (DBD) reactor driven by two distinct High Voltage (HV) generators. The first High Voltage generator delivers unipolar (positive) square pulses of variable rise/fall time, frequency, amplitude and pulse duration. The second one delivers bipolar HV pulses of variable frequency and amplitude. After a primary electrical analysis of the behavior of the two systems, attention is drawn in the production of active species in the case of N2 and Air DBD's in the afterglow. The aforementioned species' densities are estimated in these experimental conditions via optical emission spectroscopy for the N2 afterglow, whereas the well known absorption method is used in the case of the Air afterglow.It is shown that the unipolar mode of excitation is more efficient in the production of ozone whereas the bipolar one excels in the production of atomic Nitrogen and N2(A) metastables. These species can be used in a wide variety of surface and gas treatment applications in atmospheric pressure conditions.

Characteristics of the electron temperature in the downstream regions of N 2/Ar ECR plasmas

Characteristics of the electron temperature in the downstream regions of N 2/Ar ECR plasmas are discussed. The remarkable local heating of electrons was observed under certain experimental conditions of the magnetic field and the mixture ratio. The resulting electron temperature which was increased locally can influence the production of atomic nitrogen, which plays an important role for the nitridation process, especially at low pressure operation. As a result of the analysis of the experimental data, it is considered that the extraordinary wave (X-wave) contributed to the local heating due to the upper hybrid resonance (UHR).

Effect of plasma temperature and plasma pulsation frequency on atomic nitrogen production

An electromagnetic surface wave launched by a waveguide (type surfaguide [M. Moisan, Z. Zakrzewski, J. Phys., D., Appl. Phys. 24 (1991) 1025]) powered by a microwave generator (2.45 GHz) generates a plasma which produces atomic nitrogen by dissociation of molecular nitrogen. This plasma takes place in a quartz tube where an argon/nitrogen mixture is flowing. The dissociation rate depends directly on the power density injected into the discharge. The microwave power is therefore pulsated in order to be able to work with high instantaneous power while preserving a low mean power. The frequency of pulsation has a great influence on the effectiveness of the dissociation process: an optimal frequency has been be determined which depends on various parameters of the discharge such as power and tube diameter. The optimal conditions of pulsation seems to be due to a thermal effect and more precisely can be correlated with the radial profile of the gas temperature in the discharge. The dissociation fraction of the molecular nitrogen in atomic nitrogen is determined by following the decrease of the molecular nitrogen signal when the plasma is ignited. The latter is measured by mass spectrometry and by optical emission in the post-discharge. In order to understand the thermal effect inducing an optimum pulse frequency, the discharge is also characterized by optical spectroscopy: the change in concentration of species like argon atoms, neutral and ionized molecular nitrogen has been followed as a function of the pulse frequency and the gas discharge temperature measured by means of the rotational bands of N2+.

Characteristics of plasma parameters in N2/Ar ECR plasma for nitridation

Characteristics of plasma parameters in N2/Ar ECR plasma are investigated experimentally and numerically. Especially we focused on the characteristics of some plasma parameters from the viewpoint of high-efficiency production of atomic nitrogen N. As a result, it was found experimentally that the electron temperature increased locally in the downstream region within limited conditions of pressure, magnetic field and mixture ratio. In addition, one-dimensional fluid simulation was carried out successfully and indicated that N2+ can have a unique axial distribution corresponding to the experimental conditions.

Influence of Nitrogen Species on InN Grown by PAMBE

AbstractActive nitrogen species produced by an Oxford Applied Research HD-25 plasma source have been monitored by optical emission spectroscopy and quadrapole mass spectroscopy. Both techniques confirmed that at higher RF powers and lower flow rates the efficiency of atomic nitrogen production increased; emission spectroscopy confirmed that this was at the expense of active molecular nitrogen (N2*). InN films grown on (0001) sapphire/GaN with higher relative molecular content were found to have lower carrier concentrations than the corresponding films grown with higher atomic content. However, electrical properties of films grown on (111) YSZ showed insensitivity to the active nitrogen content. Etching experiments revealed that films grown on sapphire/GaN were nitrogen-polar, while films grown on YSZ were In-polar, suggesting that film polarity can greatly influence the effect active species have on growth. Lattice relaxation, as measured by reflection high-energy electron diffraction, revealed that the N-polar films grown under high relative molecular flux relaxed fully after ∼60 nm of growth, while the corresponding In-polar film relaxed fully within the first several nm of growth.

Absolute Calibration of TALIF of Atomic Nitrogen by NO Titration?Experimental and Theoretical Analysis

The method of absolute calibration of the TALIF signal of atomic nitrogen by NO titration in the afterglow of a flow tube reactor has been analyzed using TALIF and emission spectroscopy. An increase in production of atomic nitrogen at titration beginning and an asymmetric parabolic curve for emission of NO‐β‐bands have been observed. Of three possible explanations of these effects the model of wall recombination was favored: it led to fitted wall recombination coefficients referring to wall loss factors, which where in the same dimension as literature values and was able to simulate a hysteresis effect, which has been observed experimentally. According to the observations the titration end point cannot be used to determine the atomic nitrogen density in the afterglow before any NO has been added into the system as the titer produces part of this density itself. But for absolute calibration purposes referring to the wall recombination model the linear decrease of the TALIF signal with increasing titer concentration can still be used as straight line of calibration. Only a correction of the calibration factor by a factor of 0.59 for 5 mbar titrations has to be taken into account as the titer shows wall interaction not only in a catalytic way, but is partly consumed.

Spatial and temporal characteristics of atomic nitrogen in a pulsed microwave discharge

The temporal and spatial characteristics of atomic nitrogen in a pulsed microwave discharge with 2.46 GHz field frequency with 20 Hz microwave pulse repetition rate, between 50 and 500 µs pulse duration and pressures from 2.5 to 20 mbar have been analysed experimentally by TALIF and explained by modelling. In the temporal profiles, in the middle of the plasma an unexpected increase in atomic nitrogen density within 20 µs after switching off microwave power supply has been observed. This could be explained by a kinetic plasma model taking into account the excitation of ground state nitrogen atoms N(4S) to the electronically excited states N(2D) and N(2P) by electron impact. As a second effect, the spatial and temporal density profiles of atomic nitrogen in the pulsed plasma showed a dominant diffusion behaviour in the phase of decreasing atomic nitrogen density between two microwave pulses. Fitting a model of diffusion led to diffusion coefficients of the order of 100 cm2 s−1, which correspond to translational temperatures of about 500 K. In this model, additional production of atomic nitrogen during the late afterglow proved to be important in weakening the density decrease.