Plasma Chemical Kinetics Research Articles

Film deposition on the inner surface of tubes using atmospheric-pressure Ar–CH4, Ar–C2H2 and Ar–C2H2–H2 plasmas: interpretation of film properties from plasma-chemical kinetics

A hard hydrocarbon film is deposited on the inner surface of glass tubes using a filamentary discharge at atmospheric pressure in Ar–C2H2–H2 and Ar–CH4 mixtures. Under similar conditions, a soft film is deposited with a high deposition rate in an Ar–C2H2 mixture. These differences in film hardness and deposition rate are interpreted on the basis of carbon and hydrogen elemental composition in the plasma.The deposition rate is varied along the axis of the tubes in the Ar–C2H2–H2 plasma. This can be controlled by controlling the substrate (tube) temperature. Chemical erosion of the deposited film by hydrogen atoms is the probable reason for this effect.The plasma conditions (gas temperature, electron distribution function and electron density) are characterized by applying optical emission spectroscopy (OES), microphotography and numerical simulation for all three gas mixtures. The density of hydrogen atoms in the inter-electrode region of the tube is determined by applying OES in all gas mixtures. The rates of precursor molecule excitation and follow-up plasma-chemical reactions are calculated on the basis of the determined plasma parameters. Correlations between plasma conditions and film properties are discussed.

Optimization and performance of atmospheric Fused Hollow Cathodes

Experimental results on the atmospheric hollow cathode plasma generation and performance, using a special configuration with tunable wall separations, are presented. The influence of the gas and type of the power used for generation on the optimum size of the cathode slit is investigated. The experimental results are in agreement with the hollow cathode model. The plasma source/plasma reactor design is of utmost importance for control of plasma-chemical kinetics. An example of the atmospheric hollow cathode plasma application for the NOx conversion is given. The energy consumption and plasma characteristics are discussed.

Conversion of Nitrogen and Carbon Oxides by the Atmospheric Hollow Cathode Discharges

We have successfully tested the fused hollow cathode with aerodynamic stabilization as a 100% oxidation catalyst in conversion of NO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> in air mixtures. Plasma chemical kinetics and the processing window width are controlled by plasma characteristics. Results show that, besides the plasma source design itself, the material of the electrodes plays a crucial role. It was found that, by using graphite electrodes, a 100% removal of NO from the air mixtures is possible without necessity of adding hydrocarbons. This paper presents results of oxygen screening, discusses the mechanisms of the process, and reports also on the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> reduction in some discharge regimes.

Numerical simulation of an oxygen-fed wire-to-cylinder negative corona discharge in the glow regime

Negative glow corona discharge in flowing oxygen has been numerically simulated for a wire-to-cylinder electrode geometry. The corona discharge is modelled using a fluid approximation. The radial and axial distributions of charged and neutral species are obtained by solving the corresponding continuity equations, which include the relevant plasma-chemical kinetics. Continuity equations are coupled with Poisson's equation and the energy conservation equation, since the reaction rate constants may depend on the electric field and temperature. The experimental values of the current–voltage characteristic are used as input data into the numerical calculations. The role played by different reactions and chemical species is analysed, and the effect of electrical and geometrical parameters on ozone generation is investigated. The reliability of the numerical model is verified by the reasonable agreement between the numerical predictions of ozone concentration and the experimental measurements.

An integral approach to considering the evolution of a microwave streamer

We present a model of a microwave streamer based on analytical relations that allows its evolution to be described both during its elongation along the external electric field and after its stop. The equations for the electric field amplitude at the streamer center, the equations that describe the dynamics of the longitudinal and transverse sizes, and the equations of plasma chemical kinetics are self-consistently solved in terms of this model. Comparison with numerical 2D simulations of the electrostatic stage in air is made. We derive simple analytical relations that allow the power released in a plasma channel to be estimated on the fly. Almost the entire energy contribution is shown to be made after the termination of the streamer elongation.

Theoretical investigation of ozone production in negative corona discharge

The purpose of this work is to highlight the chemical kinetics in plasma of pure oxygen. A comprehensive model of ozone generation in wire-to-cylinder negative corona discharge is presented. The model combines the physical processes in the discharge with the chemistry of ozone formation. It is based on an extensive reaction scheme including the major electronic and ionic processes. The importance of excited atomic and molecular states is demonstrated. The obtained results show clearly that the surface material affect quite well the ozone decomposition.

Chemical kinetics of an argon/methane plasma in a hydrogen reforming reactor

We have studied the characteristics of a plasma reactor for the promotion of chemical reactions aimed to hydrogen reforming from gaseous hydrocarbons. A plasma torch-like electrode configuration was powered by a limited current, high voltage DC source. Using as process gas an argon/methane mixture at atmospheric pressure, we have produced an intermittent spark discharge which has been investigated at different operating parameters. Experimental information on the discharge have been used to perform a numerical simulation of the plasma chemical kinetics. The evolution of the gas-phase composition during and after the interaction in the discharge region was calculated under different operating conditions. The results have been used to predict the hydrogen production yield in the experiments and to study the role played by the different discharge parameters in order to determine the optimal interaction between plasma and gas mixture.

Singlet oxygen production and quenching mechanisms in travelling microwave discharges

Experimental and theoretical studies of singlet oxygen excitation in travelling microwave (TMW) discharges are presented. Singlet oxygen O2(a1Δg) concentrations and atomic oxygen mole fraction have been measured for different pressures, input powers and distances from the MW resonator. It was shown that a steady-state TMW discharge with a coaxial cavity resonator could provide a maximal O2(a1Δg) yield of 22% for 2 Torr of pure oxygen and 27–30% for He : O2 = 1 : 1 mixture. The two-dimensional (r, z) model developed for calculations of plasma-chemical kinetics, heat and mass transfer was used for simulation of processes in the TMW discharge under study. Effects of gas pressure, gas flow rate and input power are studied and compared with experimental measurements of O2(a1Δg) concentrations and atomic oxygen mole fractions.

Plasma catalytic hybrid processes: gas discharge initiation and plasma activation of catalytic processes

Catalytic reactions of a gas mixture can efficiently be induced by pre-treatment using a gas discharge plasma or by combined treatment in a plasma catalytic hybrid reactor. The effects of plasma treatment can be excitation of molecules, formation of short lived radicals, formation of long lived intermediate species, emission of UV-radiation, or simply gas heating. By dielectric barrier discharge (DBD) pre-treatment of Diesel engine exhaust selective catalytic reduction (SCR) of nitric oxides on a V 2O 5-WO 3/TiO 2-catalyst could be induced at temperatures as low as 100 °C. Due to the low specific plasma input energy density ε sp of about 10 J/l gas heating does not play a role. Plasma catalytic methane steam reforming using a Ni-catalyst was performed in a dielectric packed bed reactor at temperatures down to 200 °C. Since for reforming ε sp values exceed 1 kJ/l both non-thermal plasma effects and gas heating contribute to the plasma catalytic hybrid process. A review focusing on the plasma-chemical kinetics is given.

Effect of repetition rate of a pulsed microwave diamond forming plasma on the density of C2

The relative density of C2 in a pulsed microwave discharge, used for diamond deposition, was measured by optical emission spectroscopy. It was found that the average density of C2 increases with increasing plasma repetition rate (200 Hz–8 kHz) and approaches a limit at high frequencies. These results are explained with a simple plasma chemical kinetics simulation of the pulsed discharge.

Modeling of formaldehyde destruction under pulsed discharge plasma

The chemical reactions that induce the decomposition of formaldehyde have been investigated using the Monte‐Carlo simulation method. The simulation results, of plasma chemical reaction kinetics in pulsed discharge, showed that the pulse width of applied voltage should be in the range of 100–500 ns to maintain a higher density of radicals. The higher electron density was due to a higher radical density. Furthermore, increasing the content of oxygen and H2O in gas would obtain a higher radical density. Under the simulation conditions, the radical oxygen played a vital role, and the reaction time for 50% of the formaldehyde degradation (100 ppm) was approximately 50 μs.

Removal of NF/sub 3/ from semiconductor-process flue gases by tandem packed-bed plasma and adsorbent hybrid systems

A tandem hybrid gas cleanup system, consisting of a BaTiO/sub 3/ packed-bed plasma reactor and a CaCO/sub 3/ adsorbent filter, was used to study the removal of NF/sub 3/ from semiconductor-process flue gases. Plasma-chemical kinetics of N/sub 2/-NF/sub 3/-O/sub 2/-H/sub 2/ gas mixtures suggested byproducts observed in the experiments. The laboratory-scale system showed NF/sub 3/ removal at atmospheric pressure. Typically, 100% NF/sub 3/ abatement was achieved with an inlet concentration of 5000 ppm and a gas residence time in the reactor less than 10 s.

Parametric optimization of discharge pumped XeCl lasers

The data gained on discharge pumped XeCl laser systems over the last 15 years were analyzed to identify common inherent regularities. It is obvious that there are three major classes of phenomena involved, namely: the electrical processes in the pumping circuit, the plasma-chemical kinetics, and the discharge contraction. It has been demonstrated that under certain conditions they can be considered separately. Such an approach allows one to estimate a priori the optimal pumping conditions and the electrical circuit parameters to obtain the required laser energy. The results presented could be used in the engineering calculations of high-performance excimer laser systems to significantly reduce the cost of the design and the optimization stages.

Phenomenology of a dual-mode microwave/RF discharge used for the deposition of silicon oxide thin layers

A remote plasma enhanced chemical vapour deposition (RPECVD) reactor has been developed to deposit silicon oxide films. It consists of a microwave discharge (created by surface waves) along a quartz tube and a capacitively coupled radiofrequency (RF) discharge on a planar electrode (substrate holder) perpendicular to the tube and facing the gas flow. Plasma diagnostics have been performed in Ar, , Ar - and Ar - He - discharges, at two different positions of the reactor. The densities of electrons (a few ), argon metastables (a few ) and oxygen atoms ( - ) have been determined as a function of different plasma parameters, using microwave interferometry and optical emission spectroscopy (self-absorption and actinometry) respectively. In the case of , the gas flow has little effect on the local equilibrium along the discharge tube due to fast quenching on the walls and quenching in the plasma bulk by slow electrons and oxygen molecules. In contrast, the O atom density profile is governed by the gas flow velocity due to a slow recombination probability on the walls. However it clearly appears that the O atom recombination probability is much larger in the microwave discharge region (due to ion bombardment and plasma heating of the walls) than in the afterglow region. We have also shown that the fraction of O atoms with respect to molecules is enhanced by using helium and/or argon dilution due to the production of O atoms by the quenching reactions of or metastables with . Comparing the densities of electrons, argon metastables and oxygen atoms and their respective rate constants for the reaction with silane , we have deduced that the plasma chemical kinetics leading to silicon oxide deposition can be summarized in a simple scheme: electrons dissociate into O atoms and produce which subsequently react with to enhance the O atom density. In the flowing afterglow, is almost entirely decomposed by O atoms, the direct electron impact dissociation being negligible except when applying an RF discharge in the substrate region.

Kinetic processes in non-equilibrium carbon monoxide discharges. II. Self-consistent electron energy distribution functions

Abstract Self-consistent electron energy distribution functions (EDFs) in carbon monoxide discharges have been calculated by solving the Boltzmann equation coupled to the vibrational master equation and to the plasma chemical kinetics operating in the discharge. The results show a strong coupling between EDF and the vibrational distribution and the chemical composition of the plasma. The consequent temporal evolution of EDFs is analyzed on the basis of the relevant relaxation times characteristic of different processes occurring under plasma conditions.

ChemInform Abstract: ELEMENTARY CHEMICAL PROCESSES AND KINETICS IN A NON‐EQUILIBRIUM AND QUASI‐EQUILIBRIUM PLASMA

AbstractVortrag (136 Literaturhinweise), gehalten auf dem Internat.

Elementary chemical processes and kinetics in a non-equilibrium and quasi-equilibrium plasma

Abstract