Direct Electron Impact Dissociation Research Articles

Spatially resolved TALIF investigation of atomic oxygen in the effluent of a CO2 microwave discharge

A diagnostic setup for one-dimensionally spatially resolved two-photon absorption laser-induced fluorescence (TALIF) detection of ground state oxygen atoms ( 2p43P2,1,0 ) is developed. The goal of this study is to investigate the evolution of temperatures and absolute number densities of oxygen atoms along the effluent of a low-pressure CO2 microwave discharge in order to gain insights into some of the mechanisms governing the post-discharge regime. The plasma source is operated at conditions of 600 W– 1200 W of absorbed power with flow rates of 74 sccm and 370 sccm pure CO2 at pressures between 1.2 mbar and 5 mbar with specific energy inputs up to 111.9 eV/molecule. These operating conditions exhibit high CO2 conversions (up to 90%) at low energy efficiencies (2%–7.4%), due to direct electron impact dissociation driving the conversion process resulting in splitting of CO2 into CO and metastable oxygen atoms. The TALIF measurements yield spatially resolved translational temperatures between 1000 K– 1600 K for most operating conditions and axial positions along the effluent. Reference measurements with xenon 6p′[3/2]2 are used for absolute number density calibration. The resulting axially resolved number density profiles of ground state atomic oxygen increase along the effluent, even at considerable distances of several centimeters from the active discharge, before they reach a maximum between 5×1020 m−3 and 2.2×1021 m−3 depending on the condition, and decrease after that. This behavior indicates the potential significance of quenching of metastable oxygen atoms within the post-discharge regime of the investigated CO2 discharges. The measured spatially resolved number density evolutions are qualitatively consistent with quenching via wall collisions being the dominant deactivation mechanism, underlining the importance of particle-wall interactions.

KINETIC SIMULATON OF CO2 CONVERSION IN LOW-PRESSURE ELECTRODELESS PLASMA

Kinetic model of processes in low-pressure inductively coupled plasma describing the carbon dioxide conversion is presented together with the modeling results. The model takes into account only direct electron impact dissociation of carbon dioxide and is valid at the lowest gas pressures and at a reduced electric field > 150 Td. The influence of the gas mixture composition and the plasma density on the electron distribution function has been studied. In the low power limit when e-e collisions don't play a significant role the EEDF is strongly non-Maxwellian, but with the plasma density increase, EEDF is approaching Maxwellian distribution. Nitrogen and argon were studied as additions to CO2. The influence of the calculated distribution function on the energy efficiency of carbon dioxide conversion has been studied. It was concluded that the electron temperature is the key parameter for the energy efficiency, which increases by a factor of 6 with temperature change from 3 to 10 eV and at Te = 10 eV reaches values of more than 6 %. Comparison of the calculation results with experimental data shows satisfactory agreement.

Synergistic CO2 plasma catalysis: CO production pathways and effects of vibrationally excited species

Non-thermal plasmas provide a unique method of converting CO2 into CO and other fuels. However, how to improve the conversion rate and energy efficiency simultaneously still needs to be investigated. This paper proposes a novel plasma synergistic catalysis model developed by coupling the CO2 plasma and the reactions on the surface of Ni/γ-Al2O3 catalysts. Our simulation results show that the conversion rate and energy efficiency are 32.3 % and 4.6 % under the plasma without catalysts, respectively. The direct electron impact dissociation consumes over 70 % energy cost of CO production and therefore is the key reason for the low energy efficiency. The plasma synergistic catalysis model suggests that Ni/γ-Al2O3 catalysts can efficiently dissociate the vibrationally excited CO2v (especially high-level CO2v) molecules. The conversion rate and energy efficiency are increased to 39.2 % and 5.6 %, respectively. These results are in good agreement with the reported experimental and computational results, and indicate that improving the vibrational level of CO2v is a good way to improve the plasma catalytic efficiency. It is also shown that the effects of the CO2v vibrational level can be enhanced by increasing the pulsed power, repetition frequency, reducing the rising time, and controlling other process parameters.

Extended plateaux in the vibrational and electron distribution functions of O2/O reacting plasmas in discharge and post-discharge conditions

An accurate state to state model describing the plasma kinetics in reacting oxygen plasma has been built up giving particular attention to vibrational–translational O–O2 processes, as well as O–O2(v) dissociation and recombination, direct electron impact dissociation, and resonant electron–vibration (e–V) processes for O2. Moderate and low pressure situations are investigated for both discharge and post-discharge conditions. In both cases, an important role of three body recombination of O atoms and of multi-quantum e–V processes is observed, the latter being particularly important at low pressure. The interplay of the different processes forms an extended plateaux on the vibrational distribution function (vdf) of O2 in both discharge and post-discharge conditions. A comparison between our calculated vdf and the corresponding experimental and theoretical results of Annusova et al. shows a good qualitative agreement. Declining plateaux are also observed in the electron energy distribution function. In this case, superelastic electronic collisions involving high lying electronic states of the O atom form important maxima over the declining plateau.

Energy pooling mechanism for catalyst-free methane activation in nanosecond pulsed non-thermal plasmas

Methane activation at low temperature remains a major challenge for its utilization in the field of reduction of greenhouse gases and generation of value-add products, while non-thermal plasmas provide a promising catalyst-free methane activation approach. In this work, an innovative energy pooling mechanism for catalyst-free methane activation at low temperature enabled by the nanosecond pulsed non-thermal plasma in argon and methane gas mixture is investigated by both optical diagnostic and kinetics modelling. Importantly, the evolution of the absolute density of the hydrogen atom at the ground state, as a real-time and in-situ indicator of the methane dissociation degree, is measured using the two-photon absorption laser induced fluorescence (TALIF) method. A simple zero-dimensional reaction kinetics model is built-up to perform a quantitative interpretation on the density evolution of active species. Especially, a good agreement is achieved between the measured hydrogen atom density and that from the kinetics model, validating the reaction pathways of active species. It is demonstrated that, except for the direct electron impact dissociation during the pulse-on period, both the charge transfer between argon ion and methane and the quenching of argon metastable species by methane contribute to a further increase of the hydrogen atom density during the pulse-off period, i.e. the argon ions and metastable species can function as an energy pooling for the formation of hydrogen atom and hydrocarbon radicals. The innovative mechanism proposed in this work may contribute to the catalyst-free and cost-effective strategies for methane utilization at low temperature.

Kinetic studies and mechanism development of plasma assisted pentane combustion

The present study explores the chemical kinetics of low temperature oxidation and pyrolysis of pentane/O 2 /He mixtures in a nanosecond repetitively-pulsed DBD discharge. Time-dependent TDLAS measurements and steady state GC sampling are conducted to quantify species evolution in a plasma discharge. An improved kinetic model of plasma assisted combustion and pyrolysis of pentane is developed with updated electron impact dissociation reactions and low temperature reactions involving excited species. This kinetic model is then validated against the experimental data. The results show that a nanosecond plasma discharge causes significant low temperature fuel pyrolysis and oxidization as well as fast gas heating. The original kinetic model fails to predict many intermediate species such as CH 4 , C 2 H 2 , and CH 2 O due to missing corresponding reaction pathways and inaccuracies in electron impact cross section areas. The new model shows a dramatic improvement in modeling both pentane pyrolysis and oxidation, with good prediction of the time histories of pentane, CH 2 O, C 2 H 2 and H 2 O, and slight under-prediction of CH 4 . There is still a large discrepancy in OH prediction and measurement in fuel oxidation. The results show that direct electron impact dissociation pathways play a critical role in plasma assisted fuel pyrolysis and oxidation and that plasma generated radicals and excited species such as O and O( 1 D) enhance low temperature fuel oxidation.

Simulation of spatio-temporal variation of OH radical density in atmospheric-pressure streamer discharge

The spatio-temporal variation of OH radical density in an atmospheric-pressure plasma discharge is investigated via two-dimensional numerical simulation. The behaviours of OH are characterized in four regions in the inter-electrode gap, namely the ‘Hot anode’, ‘Secondary streamer’, ‘Primary streamer’, and ‘Near-cathode’ regions. In these regions, temporal variations of averaged reduced electric field, electron density, and gas temperature differ and they affect the temporal variation of OH density. In all regions, a relatively large amount of OH is produced by the dissociation reactions of H2O with electronically excited nitrogen molecules and oxygen atom rather than the direct electron-impact dissociation reactions. In the Hot anode and Secondary streamer regions, the instantaneous density of OH just after the discharge is high. However, the subsequent OH decay rates differ in these regions owing to the difference in gas temperatures. The reaction mechanism is discussed by dividing the effect of the diffusion-convection term and chemical reaction term in the continuous equation for OH radical density.

An investigation of CO2 splitting using nanosecond pulsed corona discharge: effect of argon addition on CO2 conversion and energy efficiency

The plasma chemical splitting of carbon dioxide (CO2) to produce carbon monoxide (CO) in a pulsed corona discharge was investigated from both an experimental and a numerical standpoint. High voltage nanosecond pulses were applied to a stream of pure CO2 and its mixture with argon, and the gaseous products were identified using Fourier transform infrared spectroscopy. Due to the shape of pulses, the process of CO2 splitting was found to proceed in two phases. The first phase is dominated by ionization, which generates a high electron density. Then, during the second phase, direct electron impact dissociation of CO2 contributes to a large portion of CO production. Conversion and energy efficiency were calculated for the tested conditions. The conversions achieved are comparable to those obtained using other high pressure non-thermal discharges, such as dielectric barrier discharge. However, the energy efficiencies were considerably higher, which are favorable to industrial applications that require atmospheric conditions and elevated gas flow rates.

Synergistic CO2 conversion by hybridization of dielectric barrier discharge and solid oxide electrolyser cell

Hybrid reactor of Dielectric Barrier Discharge (DBD) and Solid Oxide Electrolyser Cell (SOEC) was fabricated and CO2 decomposition characteristics were investigated. For the case of the DBD reactor alone, the CO2 conversion saturates with residence time. In contrast, the saturation is not observed for the case of the hybrid system where CO2 conversion increases monotonically with increasing residence time. This is because the reverse reaction to regenerate the CO2 in plasma is suppressed by the oxygen removal by SOEC in hybrid system and consequently CO2 decomposition reaction proceeds irreversibly. We have examined the experimental results by analytical modeling and reaction mechanism for the CO2 decomposition in the hybrid reactor of DBD and SOEC is deduced. The time‐dependent CO2 conversion for the only plasma reactor and hybrid reactor can be reproduced by a reversible and irreversible first‐order reaction model, respectively. The analytical model can also reproduce the dependence of CO2 conversion on the plasma input power very well. Interestingly, not only the reverse reaction rate but also the forward dissociation reaction rate is reduced by the SOEC in the case of hybrid reactor. The analytical modeling suggests that CO2 decomposition in the conventional DBD plasma reactor proceeds not merely by direct electron impact dissociation but also by reaction with reactive oxygen.

Low temperature oxidation and pyrolysis of n-heptane in nanosecond-pulsed plasma discharges

The present study seeks to explore the parametric effects of oxygen concentration, argon dilution and plasma discharge frequency on pyrolytic and oxidative reaction pathways of n-heptane assisted by non-equilibrium plasma discharge. Low temperature reaction pathways of n-heptane/O2/Ar mixtures with a nanosecond repetitively pulsed plasma discharge are experimentally investigated in both in situ time-dependent TDLAS and steady state gas sampling diagnostics. Fuel consumption is found to be more effective in higher argon than higher oxygen concentrations, indicating higher electron number densities with argon dilution is more effective than direct electron impact dissociation of oxygen. Steady state sampling results suggest a linear trend of n-heptane dissociation and product species formation with increasing plasma frequency, with different major product species for oxidation and pyrolysis. In the time-dependent measurements, the comparison between experiments and numerical modeling show that formation of a major intermediate species, formaldehyde, is significantly under-predicted while fuel and water production are over-predicted. This discrepancy suggests missing reactions in the current model, possibly involving excited alkyl peroxide radicals.

In situ species diagnostics and kinetic study of plasma activated ethylene dissociation and oxidation in a low temperature flow reactor

In situ measurements by mid-IR laser absorption spectroscopy of C2H4/Ar dissociation and C2H4/O2/Ar oxidation activated by a nanosecond repetitively pulsed plasma have been conducted in a low temperature flow reactor (below 500K) at a pressure of 60Torr for both a continuously pulsed plasma discharge mode and a burst mode with 150 pulses. The measurements of the in situ diagnostics are validated and complemented by gas chromatography in the continuous discharge mode. A recently developed kinetic mechanism (HP-Mech-Plasma) for plasma activated C2H4 oxidation is assembled. The experiments of plasma activated dissociation show that the formation of acetylene by direct electron impact dissociation and dissociation by excited and ionized argon collision reactions is the major fuel consumption pathway. Plasma activated C2H4 oxidation experiments show that there exist three fuel consumption pathways, (1) a plasma activated low temperature fuel oxidation pathway via O2 addition reactions; (2) a direct fragmentation pathway via collisional dissociation by electrons, ions, and electronically excited molecules; and (3) a direct oxidation pathway by plasma generated radicals. It is found that the plasma activated low temperature oxidation pathway is dominant and leads to a large amount of formaldehyde formation with less acetylene and negligible large hydrocarbon molecules as compared to the dissociation experiment. The results also indicate that the latter two fuel consumption pathways are strongly dependent on O2 and Ar concentrations due to their effect on the production of atomic oxygen and excited Ar. Although the current model improves the overall prediction over USC-Mech II for plasma activated dissociation and oxidation, both models fail to predict quantitatively the H2O and CH4 formation. The present data provide good targets for future model development in plasma-assisted combustion.

Kinetic effects of non-equilibrium plasma-assisted methane oxidation on diffusion flame extinction limits

The kinetic effects of low temperature non-equilibrium plasma assisted CH 4 oxidation on the extinction of partially premixed methane flames was studied at 60 Torr by blending 2% CH 4 by volume into the oxidizer stream of a counterflow system. The experiments showed that non-equilibrium plasma can dramatically accelerate the CH 4 oxidation at low temperature. The rapid CH 4 oxidation via plasma assisted combustion resulted in fast chemical heat release and extended the extinction limits significantly. Furthermore, experimental results showed that partial fuel mixing in the oxidizer stream led to a dramatic decrease of O concentration due to its rapid consumption by CH 4 oxidation at low temperature. The products of plasma assisted CH 4 oxidation were measured using the Two-photon Absorption Laser-Induced Fluorescence (TALIF) method (for atomic oxygen, O), Fourier Transform Infrared (FTIR) spectroscopy, and Gas Chromatography (GC). The product concentrations were used to validate the plasma assisted combustion kinetic model. The comparisons showed the kinetic model over-predicted the CO, H 2O and H 2 concentrations and under-predicted CO 2 concentration. A path flux analysis showed that O generated by the plasma was the critical species for extinction enhancement. In addition, the results showed that O was produced mainly by direct electron impact dissociation reactions and the collisional dissociation reactions of electronically excited molecules with O 2. Moreover, these reactions involving electron impact and excited species collisional dissociation of CH 4 contributed approximately a mole fraction of 0.1 of total radical production. The present experiments produced quantitative species and extinction data of low temperature plasma assisted combustion to constrain the uncertainties in plasma/flame kinetic models.

Neutral and ion chemistry in low pressure dc plasmas of H2/N2 mixtures: routes for the efficient production of NH3 and NH4+

The chemistry in low pressure (0.8-8 Pa) plasmas of H(2) + 10% N(2) mixtures has been experimentally investigated in a hollow cathode dc reactor using electrical probes for the estimation of electron temperatures and densities, and mass spectrometry to determine the concentration of ions and stable neutral species. The analysis of the measurements by means of a kinetic model has allowed the identification of the main physicochemical mechanisms responsible for the observed distributions of neutrals and ions and for their evolution with discharge pressure. The chemistry of neutral species is dominated by the formation of appreciable amounts of NH(3) at the metallic walls of the reactor through the successive hydrogenation of atomic nitrogen and nitrogen containing radicals. Both Eley-Rideal and Langmuir-Hinshelwood mechanisms are needed in the chain of hydrogenation steps in order to account satisfactorily for the observed ammonia concentrations, which, in the steady state, are found to reach values ~30-70% of those of N(2). The ionic composition of the plasma, which is entirely due to gas-phase processes, is the result of a competition between direct electron impact dissociation, more relevant for high electron temperatures (lower pressures), and ion-molecule chemistry that prevails for the lower electron temperatures (higher pressures). At the lowest pressure, products from the protonation of the precursor molecules (H(3)(+), N(2)H(+) and NH(4)(+)) and others from direct ionization (H(2)(+) and NH(3)(+)) are found in comparable amounts. At the higher pressures, the ionic distribution is largely dominated by ammonium. It is found that collisions of H(3)(+), NH(3)(+) and N(2)H(+) with the minor neutral component NH(3) are to a great extent responsible for the final prevalence of NH(4)(+).

Plasma parameters and chemical kinetics of an HCl DC glow discharge

The investigations of plasma parameters and active particles kinetics in an HCl DC glow discharge system were carried out. The investigation combines plasma diagnostics by electric probes and plasma modeling based on the self-consistent solution of Boltzmann kinetic equation and the balance equation of chemical kinetic for neutral and charged particles. It was shown that the direct electron impact dissociation of the HCl molecules represents the main mechanism for the formation of the Cl and H atoms while the total balances for all kinds of neutral particles are noticeably influenced by the volume atom–molecular reactions. The analysis of the vibrational kinetics shows low population of the vibrational energy levels for HCl molecules, but the role of the HCl V >0 in the negative ions formation process cannot be neglected. The assumption of the Eley-Rideal recombination kinetics for both chlorine and hydrogen atoms provides a good agreement between the calculated and experimental values of reduced electric field ( E/ N) and the total flux of positive ions.

Plasma parameters and volume kinetics in Cl2/O2 mixtures

We investigated plasma characteristics, plasma mass content and kinetic dependencies of both neutral and charged particle formation and decay in Cl2/O2 gas mixture. For these purposes we used a combination of experimental methods (optical emission spectroscopy, Langmuir probe) and a plasma modeling on the base of self-consistent solution of Boltzmann kinetic equation together with balance kinetic equations for neutral and charged particles in a quasi-stationary approximation. It was found that the change of O2/(Cl2+O2) mixing ratio from 0 to 0.2 leads to an increase of electron average energy and electron energy distribution function (EEDF) deformation. The main mechanisms of Cl and O atom formation are the direct electron impact dissociation of corresponding molecules while the contribution of all possible “secondary” processes is not significant in the case of a relatively low O2 addition.

Inductively coupled Cl2/Ar plasma: Experimental investigation and modeling

Electrophysical and kinetic characteristics of Cl2/Ar plasma were investigated to understand the influence of the addition of Ar on the volume densities and fluxes of active particles, both neutral and charged. Our analysis combined both experimental methods and plasma modeling. It was found that addition of Ar to Cl2 leads to deformation of the electron energy distribution function and an increase of the electron mean energy due to the “transparency” effect. Direct electron impact dissociation of Cl2 molecules represents the main source of chlorine atoms in the plasma volume. The contributions of stepwise dissociation and ionization involving Ar metastable atoms were found to be negligible. Addition of Ar to Cl2 causes the decrease of both electron and ion densities due to a decrease in the total ionization rate and the acceleration of heterogeneous decay of charged particles.

Emission, thermocouple, and electrical measurements in SF6/Ar/O2 SiC etching discharges

In SiC etching plasma devices, we have recorded plasma emission from Ar, F, and O atoms in SF6/Ar/O2 rf discharges as a function of pressure, input power, and mixture fraction. At fixed power, the emission intensities rise nearly linearly with increasing pressure between 100 and 300 mTorr; with pressure increases to 600 mTorr, the emission intensity rolls off due to the increase in collisional de-excitation. At fixed pressure, Ar and O atom emission shows a similar functional dependence on input power with a roll off at the higher powers due to decreasing reduced electric field strength (E/n, where n denotes the number density). In contrast, the F atom emission increase with increasing power is nearly linear. This reflects the fact that F atoms are produced by dissociative attachment of SF6 (for lower E/n conditions) in addition to direct electron impact dissociation. Electrical measurements, with a variable interelectrode gap discharge, indicate that the electric field to pressure (E/p) ratio does drop with increasing input power. Thermocouple measurements show that the ground electrode temperature increases with increasing power. The dissociative attachment of SF6 increases with increasing temperature as well. The SiC etch rate increases nearly linearly with input power up until the polymer buildup becomes the rate limiting process. At fixed pressure, the Ar emission from the 750 nm line decreases with increasing additions (up to 10%) of O2. This is due to resonantly enhanced quenching of the 4p (13.5 eV) manifold by oxygen atoms. In marked contrast, the F atom emission intensity increases suggesting F atom production by neutral species chemistry. Over the pressure (100–600 mTorr) and power (20–60 W) range studied, the Ar 750 nm emission line serves as a good actinometer for the 704 nm F line provided that there is not a high degree (or changing degree) of O2 dissociation. Resonant deactivation of the 750 nm line through collisional interaction with O atoms, can make the 750 nm line problematic. Under such conditions our previous work indicates that the Ar 641 nm line can provide an alternative actinometer. The excited state of the 641 nm transition lies above the O atom ionization limit making it immune from resonant quenching.

On mechanisms of argon addition influence on etching rate in chlorine plasma

Parameters of Cl 2/Ar plasma were investigated aimed to understand the mechanism of Ar addition influence on etching rate acceleration. Analysis was carried out on the base of combination of experimental methods and plasma modelling. It was found that the addition of Ar to chlorine under a constant total pressure condition cause changes in plasma electro-physical properties (EEDF, mean electron energy) due to the ‘transparency’ effect. Direct electron impact dissociation of Cl 2 molecules was found as the main source of chlorine atoms while the contributions of dissociative attachment and stepwise dissociation involving Ar metastable atoms are negligible. It was supposed that the main reason of etching rate increasing in Cl 2/Ar mixture plasma is connected with simultaneous action of Ar on volume chemistry and the heterogeneous stage of etching process.

Volume and heterogeneous chemistry of active species in chlorine plasma

Chlorine plasma parameters and the mechanisms of both volume and heterogeneous reactions were investigated using a combination of experimental methods and plasma modeling. It was found that increasing gas pressure within the range 20–200 Pa leads to sufficient deformation of the electron energy distribution function and to corresponding changes in kinetic and transport coefficients. Direct electron impact dissociation and ionization were found to be the main mechanisms for radical and ion generation, while contributions from dissociative attachment and dissociative ionization were negligible. Heterogeneous recombination of chlorine atoms is the dominant decay channel, which is described by a first-order kinetic mechanism.

Kinetics of the reactions involving CF2 and CF in a pure tetrafluoromethane plasma: I. Production of CF2 and CF via electron-impact dissociation

The kinetics of the production and loss of CF2 and CF radicals in a glow discharge in pure CF4 is investigated by the laser-induced fluorescence method. The effective rate constants for electron-impact dissociation of CF4 molecules along the pathways toward CF2 and CF radicals are determined within a wide range of the reduced electric field (80–250 Td). It is shown that, along with the direct electron-impact dissociation of CF4, the radicals are also produced via the dissociation of the CxFy polymer fluorocarbon particles that form in the plasma. A detailed analysis of the kinetics of the radical production and loss in a modulated discharge made it possible to evaluate the contribution of the electron-impact dissociation of CF4 to the production of radicals and, consequently, to determine the dissociation rate constants $$k_{CF_2 } $$ and k CF. A comparison of the obtained $$k_{CF_2 } $$ and k CF values with the results of calculations by the Monte Carlo method and the literature data on the cross sections for electron-impact dissociation of CF4 molecules enabled the normalization of these cross sections in the threshold region and the construction of the model cross sections for the electron-impact dissociation of CF4 into neutral products. The calculated cross sections allow a satisfactory description of the experimental results throughout the entire range of E/N under study. A significant scatter (up to 100%) in the experimental data on $$k_{CF_2 } $$ and k CF at low values of E/N is related to the considerable contribution of the CxFy polymer molecules (and, probably, CxF y + ions and fluorocarbon grains) to the production of CF2 and CF radicals both in the plasma volume and on the surface of a fluorocarbon film covering the discharge tube wall.