J.-C. Legrand

Methane conversion by an air microwave plasma

Activation of methane is carried out by means of an air microwave plasma (2.45 GHz). The experiments cover the absorbed microwave power range 350–650 W (20–50 W cm3 with 17–62%, of methane in the gas mixture, with pressures of 10–66 mbar and flow rates of 140‐700 ml min1. Methane, dioxygen, and dinitrogen consumptions as well as C2 hydrocarbons, carbon monoxide, and dihydrogen yields are analyzed hr gas chromatography. The distance of methane addition from the end of the discharge plays an important role in the composition and the concentration of the products obtained. This distance mainly determines the energy concentrated in the active species of the plasma when they react with methane. A kinetic mechanism jar the activation and decay of inethane and for the formation of C2 hydrocarbons and carbon monoxide is discussed based on the experimental results and kinetic data in the literature.

Mechanisms of methane decomposition in nitrogen afterglow plasma

Abstract Chemical reactions initiated by the dissociation of methane in the nitrogen flowing afterglow have been studied by the computer modelling. The input experimental data were obtained from the microwave CH4/N2 plasma. The modelling of methane decomposition was based on a macroscopic kinetic approach. The 24 neutral and excited species were introduced: electrons, hydrocarbons, radicals, neutral and excited gases, and nitrogen containing species. Between these species 61 chemical reactions were studied. It was found that in the flowing afterglow conditions, where the energy of excited particles is reduced, the limited amount of reactions is really important. With the aid of this simplified model of 18 chemical reactions the yield of stable products and the detailed kinetics of their creation in the dependence on activity of individual species were studied. Special attention was devoted to the study of reaction kinetics in dependence on the afterglow time in accordance with experimental data.

Kinetics of reactions in CH4N2 afterglow plasma

Abstract In this paper chemical reactions initiated by the decomposition of methane in the nitrogen flowing afterglow are studied by computer modelling with the input data from microwave plasma. The model consists of 61 reactions between 24 kinds of species. The simulation technique is based on a macroscopic kinetic approach which provides the final concentrations of stable products—C 2 H 6 , C 2 H 4 , C 2 H 2 , HCN, H 2 together with CH 4 and N 2 . It was found that the stiffness of the set of equations describing the decomposition process can be monitored by the C and H mass balance.

Rotational Temperature in Helium, Argon, and Oxygen Microwave-Induced Plasmas: Comparison with Translational and Solid Surface Temperatures

Translational temperature (Tg), rotational temperature of OH (Tr), and surface temperature (Ts are determined in helium, argon, and oxygen plasmas. Measurements are carried out at different pressures and different flow rates as a function of microwave power input by using various methods (line broadening, pressure rise, spectroscopy, thermocouple, pyrometry). The three temperatures depend on the nature of the gas (oxygen < helium > argon) and increase with power input and pressure. Results show that Tg is of the same order of magnitude as Ts. But in all experimental conditions, Tr is always higher than Tg. Therefore, Tr is inadequate for estimating Tg. When no direct determination of Tg is available, Ts seems to give a better estimation of this important parameter.

Use of a dihydrogen plasma afterglow for the reduction of zeolite-supported gold-based metallic catalysts

Abstract The afterglow of a microwave plasma (2450 MHz) of dihydrogen is used for the preparation of gold-based metallic catalysts. It contains hydrogen atoms at a sufficiently low temperature for the formation of nanoparticles, whereas the reduction of gold at high temperature often leads to large particles. The study concerns the search for optimal conditions for the preparation of monometallic (Au, Pd) or bimetallic (Au/Pd) catalysts. The position of the sample in the afterglow and the exposure time are varied. According to transmission electron microscopy, the particles are less than 5 nm in size.

Measurement of carbon atom density in the flowing afterglow of a CH4-N2 microwave plasma

Abstract Optical emission spectroscopy in a flowing afterglow of a CH4(0.01–0.075%)-N2 microwave plasma is used to measure absolute carbon atom concentrations. From the intensities of the nitrogen first positive and of the CN violet bands, and by means of a kinetic mechanism, the C atom concentration is found to be in the range 1–13 × 1013cm−3 at 180 W and at pressures between 12.5 and 50 mbar. An increase of C concentration versus flow time is observed, indicating that the major part of the C atom density is produced along the afterglow.

Study of Spectral Emission Behaviour in the Glow and Microwave Discharges of Oxygen

Because of the very important role of oxygen plasmas in various applications, both direct current and microwave discharges have been analysed from the point of view of emission spectra properties. In both cases silica discharge tubes with practically the same diameters were used. The following transitions of the oxygen discharges were studied: the atomic lines at 777.4 nm ( 5 P - 5 S) and 747.7 nm ( 3 D - 3 P 0 ), and the head at 759.4 nm of the atmospheric system (b 1 Σ + g , v = 0 - X 3 Σ - g , V = 0).

Concentrations of Active Species in the Flowing Afterglow of a Nitrogen Microwave Plasma

Measurements of nitrogen atom density, by means of NO chemical titration, along with an evaluation of the densities of some excited species N2(B, v=11), N2(B, v=2), N2(C, v=0), and N2+(B,v=0), by means of a spectroscopic study of some bands of dinitrogen, are achieved along the flowing afterglow of a dinitrogen microwave plasma (2450 MHz) for several pressures. The concentrations obtained are in the following range: [N]∼10+15, [N2(B, 2)]∼10+9–10+10, [N2(B, 11)]∼10+8–10+9, [N2(C, 0)]∼10+6–10+7, [N2+(B,0)]∼10+6-10+8(cm-3). From a kinetic study of the formation and decay of excited and charged species, an estimation of N2(A, v), N2(X, v, and N2+(X) densities can be derived: [N2(A, v)]∼10+12, [N2(X, v≥6)]∼10+15–10+16, [N2(X, v≥12)]∼10+14–10+15, [N2+(X)]∼10+10(cm-3).

Surface oxidation by microwave-induced plasma of candidate composite materials for space shuttle protection

Oxidation resistance of composite materials (SiC/SiC, C/SiC and C/C) which can be used to protect shuttles is studied in oxygen microwave-induced plasmas (MIP). These plasmas contain the same energetic species (electrons, ions, radicals, excited atoms or molecules) as those produced by the shock wave resulting from re-entry into the atmosphere. The plasma is sustained in a silica tube located in a resonant cavity and microwave energy is supplied by a generator operating at 2450 MHz with variable power from 15 to 1000W. Experiments are conducted at pressures from 100 to 1000 Pa with temperature ranging from 1100 to 1300 °C. The atomic oxygen flow rate is about 6.1019 at sec−1 cm−2. The wafers are exposed to the plasma for 10 to 25 h for periods of 15 or 30min. Kinetic behaviour of the material is studied by gravimetry and surface characteristic modifications are analysed by BET Krypton isotherms at 77 K and electron spectroscopy for chemical analysis (ESCA). Gravimetric results, measurements of specific surface area by krypton adsorption and ESCA analysis show that the samples of SiC/SiC and C/SiC are quite resistant to the oxygen plasma even after 25 h exposure. The mass loss is small and the specific surface area (BET) increases but is always lower than 1 m2 g−1. ESCA analysis shows that the surface evolves by change of the superficial silicon carbide into silica. The C/C samples behave quite differently: without any protection they disappear in less than 5 min. With an antioxidant protective layer, this material can be oxidation resistant. The BET and ESCA measurements show that the attack leads to a sintering of the silica which gathers on the fibres, reducing the protection of the matrix.

Reduction of reaction mechanisms in plasma chemistry

Summary form only given. When studying the chemically active plasmas by the methods of computational physics, the macroscopic kinetic approach proved to be very convenient. The models are based on the systems of chemical reactions between active species in the discharge, from which the continuity equations for concentrations of individual species are derived. However, in many cases the kinetic scheme of the model is very complicated and the set of input chemical reactions must be reduced first in order to be able to solve the model. Various techniques for model simplification can be found in the literature - e.g. approaches based on the sensitivities of individual reactions, on the corresponding reaction rates, on time-scale separation, etc. The technique used in our contribution is based on the monitoring of the speed of every reaction during the kinetic calculations, resulting in weight factors of reactions describing the relative importance of individual reactions in kinetic scheme1. The main goal is to preserve only the reactions influencing profoundly the resulting concentrations of main products or products important for given application.