Plasma Chemistry and Plasma Processing (v.21, #3)

Methane Conversion to Higher Hydrocarbons in the Presence of Carbon Dioxide Using Dielectric-Barrier Discharge Plasmas by Chang-Jun Liu; Bingzhang Xue; Baldur Eliasson; Fei He; Yang Li; Gen-Hui Xu (301-310).
Experimental investigation has been conducted to convert methane into higher hydrocarbons in the presence of carbon dioxide within dielectric-barrier discharge (DBD) plasmas. The objectives of cofeed of carbon dioxide are to inhibit carbon deposit and to increase methane conversion. The products from this plasma methane conversion include: (1) syngas (H2+CO), (2) gaseous hydrocarbons containing ethylene, acetylene, and propylene, (3) liquid hydrocarbons, (4) plasma-polymerized film, and (5) oxygenates. The selectivity of products is subject to the DBD plasma-reactive conditions and catalyst applied. The liquid hydrocarbons produced by this way are highly branched, which represents a better fuel production.
Keywords: methane; carbon dioxide; higher hydrocarbon; dielectric barrier discharge

Perfluorocompounds (PFCs) have been extensively used as plasma etching andchemical vapor deposition (CVD) gases for semiconductor manufacturingprocesses. PFCs have significant effects on the global warming and havevery long atmospheric lifetimes. Laboratory-scale experiments were performedto evaluate the effectiveness of CF4 conversion by using dielectric barrierdischarges (DBD). The results of this study revealed that the removalefficiency of CF4 increased with application of higher voltage, gas residence time, oxygen content, and frequency. Combined plasma catalysis(CPC) is an innovative way for abatement of PFC and experimental results indicated that combining plasma with catalysts could effectively remove CF4. Products were analyzed by Fourier transform–infrared spectroscopy (FT–IR) and the major products of the CF4 processing with DBD were CO2, COF2, and CO, when O was included in the discharge process. Preliminary results indicated that as high as 65.9% of CF4 was decomposed with CPC operated at 15 kV, 240 Hz for the gas stream containing 300 ppmv CF4,20% by volume O2, and 40% by volume Ar, with N2 as thecarrier gas.
Keywords: plasma; CF4 conversion; dielectric barrier discharge; PFC; catalyst

The effectiveness of applying packed-bed dielectric barrier discharge(PBDBD) technology for removing acetaldehyde from gas streams wasinvestigated. Operating parameters examined in this study include appliedvoltage, oxygen content, and gas-flow rate. Experimental results indicatethat the destruction efficiency of acetaldehyde predominantly depends onthe applied voltage. Removal of 99% of acetaldehyde has been achieved forgas streams containing 1000 ppmv acetaldehyde, 5% oxygen, with nitrogen asthe carrier gas. The oxygen content in the gas stream plays an importantrole in removing acetaldehyde within PBDBD. A higher CH3CHO removalefficiency is achieved for the gas stream containing less oxygen, since itwill dissipate energy due to its electronegative property. Carbon dioxideis the major end product, which is less hazardous to the environment and tohuman health. However, undesirable products, e.g., NO2 and N2O,CH3OC2H5, CH3COOH, CH3NO2,HCN, CH3NO3, and CH3OH, are detected as well.
Keywords: acetaldehyde; packed-bed reactor; dielectric barrier discharge; plasma

Pulsed Corona Discharge-Induced Reactions of Acetophenone in Water by Yue-Zhong Wen; Xuan-Zhen Jiang (345-354).
The reactions of acetophenone in water by pulsed corona discharges have been investigated to provide fundamental information concerning the reactions of acetophenone in water. Experimental results indicated that photolysis of acetophenone did not involve a hydroxyl radical mechanism and the majority flux of hydroxyl radicals originated from the dissociation of gas-phase oxygen in the plasma channels. The rate constants for photolysis and pyrolysis were determined to be 1.5×10−7 M-s−1, 2.2×10−4 s−1, respectively. The rate constant for the oxidative reactions was measured as 1.2×10−7 M-s−1. Results from this study support the proposal that acetophenone degradation reaction proceed through the oxidative reaction pathway, where molecular oxygen accelerates acetophenone degradation, photolysis, and pyrolysis pathways.
Keywords: advanced oxidation process; pulsed corona discharge; electrical discharge in water; reaction pathway; rate constant; acetophenone

Calculation of Combined Diffusion Coefficients from the Simplified Theory of Transport Properties by V. Rat; J. Aubreton; M. F. Elchinger; P. Fauchais (355-369).
The aim of this study is to check if it is possible to use the combined diffusion coefficients introduced by Murphy at equilibrium in a two-temperature model (electron temperature Te different from that of heavy species Th), such as that defined by Devoto and Bonnefoi for transport properties. On the one hand, the two-temperature (2-T) theory of transport properties was established by Devoto and Bonnefoi by separating electrons and heavy species because of their mass difference. Their simplified theories allow the calculation of transport coefficients (except diffusion) out of thermal equilibrium, but it has to be noted that when Te tends toward Th, the results are those obtained with an equilibrium calculation. On the other hand, Murphy's combined diffusion coefficients describe the diffusive mixing of two nonreactive ionized gases at equilibrium. First, the exact combined diffusion coefficients of Murphy are calculated for an Ar–N2 (50 wt.%) mixture at atmospheric pressure. Expressions of combined diffusion coefficients are then obtained by using the simplified theory of Bonnefoi at thermal equilibrium. The results of the calculation of combined diffusion coefficients from the simplified theory of transport properties, assuming Te=Th, are compared with those of Murphy at equilibrium. It is shown that large discrepancies occur as soon as the ionization degree is over 10%. These results prove that the simplified 2-T theory of transport coefficients cannot be used for the treatment of diffusion, probably because the mass flux of electrons is no longer constrained. Thus, a new theory of transport coefficients has to be developed, taking into account the coupling of electrons and heavy species.
Keywords: combined diffusion coefficients; transport coefficients; thermal equilibrium; thermal disequilibrium

Transient Response of Radio Frequency Inductively Coupled Plasma for Pulse Modulation by K. C. Paul; J. Mostaghimi; T. Ishigaki; T. Sakuta (371-400).
Properties of an argon/hydrogen (89% molar argon) radio frequency inductivelycoupled plasma subject to a periodic power (or coil current) profile isinvestigated both numerically and experimentally. The model is based on atime-dependent, two-dimensional (2-D), axisymmetric model of the radiofrequency (rf) inductively coupled plasma (ICP) under local thermodynamicequilibrium (LTE). The governing time-dependent equations for theconservation of mass, momentum, and energy, along with Maxwell'sequations are solved numerically. The ICP is operated at 1 MHz inductionfrequency with 55 slpm total gas-flow rate. Spectroscopic measurements ofargon line intensities are also made for a similar plasma setup andpredictions are compared with the measured ArI line intensity at 751 nm. Wefound that the plasma responds quickly to a change in the power (or coilcurrent). Two important parameters responsible for plasma heating (Jouleheating term) and for its constriction (Lorentz force) follow the temporalbehavior of the coil current. The response of the theoretically predictedatomic argon line intensity at 751 nm is slower compared to the experimentalobservations. This is thought to be due to the fast response of the electrongas to a change in the electromagnetic fields, which is not represented inan LTE model.
Keywords: duty factor; on-time; off-time; power profile; square-current profile; shimmer current level

The role of particle injection velocity in influencing the nature of alumina coatings obtained by plasma spraying was studied. Previously reported process chart obtained by computational fluid dynamics (CFD) study on the particle states of alumina with respect to particle injection velocity and size was verified experimentally. For this purpose, alumina particles of three different size ranges with a mean size of 25, 40, and 76 μm were subjected to different injection velocities. The coating obtained was analyzed for cross-sectional microstructure and thickness by optical microscopy. In addition, the role of particle injection velocity and size in influencing the coating-deposition efficiency was studied. The experimental results agreed well with the CFD results, which had indicated the dependence of particle trajectory in the plasma plume on the particle injection velocity and size leading to the changes in the extent of melting. While a higher coating thickness and deposition efficiency was obtained with 25-μm particles, with further increase in particle size, a reverse trend was observed. This was attributed to the changes in heat-transfer characteristics of the particles with size, which governed the coating buildup and deposition efficiency.
Keywords: plasma spraying; alumina; computational fluid dynamics (CFD); injection velocity; deposition efficiency

Negative Electron Mobility in Attachment-Dominated Plasmas by N. A. Dyatko; D. Loffhagen; A. P. Napartovich; R. Winkler (421-439).
The temporal evolution of the electron velocity-distribution function(EVDF), the concentration, mean energy, and the drift velocity of theelectrons is studied on a kinetic basis in a weakly ionized Ar/F2mixture plasma under conditions when the electron concentration temporallydecreases as a result of the electron attachment to fluoride molecules. Usingan appropriate relaxation model, the time-dependent electron Boltzmannequation was solved in multiterm and two-term approximations of the velocitydistribution function. The multiterm results confirmed predictions on theoccurrence of negative electron mobilities in such a decaying Ar/F2plasma, which were made in a former study using the conventional two-termapproximation. The investigations particularly showed that this approximationgives almost accurate results for the EVDF and related electron swarm parametersexcept for in the very beginning of the relaxation process. It has been furthershown that for a certain range of the reduced electric field strength, thedrift velocity becomes negative in the process of temporal evolution and remainsnegative even when approaching the hydrodynamic stage of the electronswarm. In addition, the role played by the back heating from the gas byelastic collisions on the EVDF formation is studied and various comparisonswith corresponding Monte Carlo results are performed.
Keywords: negative electron-drift velocity; attachment-dominated plasmas; time-dependent Boltzmann equation; electron velocity-distribution function; multiterm approximation

The technique of pulsed electron-beam high-pressure mass spectrometry wasused to investigate the sequential cationic chemistry in humid air streamsat 4.2×102 Pa and 380 K. The system was then modeled usingthe ACUCHEM program, incorporating thirty-five reactions taken from theformulations given in Part I of the new National Institute of Standards andTechnology (NIST) Chemical Kinetics Database for Humid Air Plasmas. Theresulting temporal ion profiles were found to be in qualitative agreementwith the laboratory data. Analogous pulsed electron-beam measurements werealso carried out with humid air samples containing low levels of toluene,and these results were also reproduced qualitatively by a model incorporatingforty-eight reactions after the inclusion of an unexpected, but crucial,channel involving the reaction of an intermediate air-generated cluster ionwith toluene. The benefits of laboratory validation of predictive databasesin systems for which the literature data are incomplete are emphasized.
Keywords: plasma; electron beam; kinetics; ionic reactions; cluster ions; air; toluene; mass spectrometry; database

Reliable kinetics data are necessary input for models describing the decomposition of gases in electric discharge or electron-beam devices. In this second part of a continuing series, we provide a core database describing the dominant reactions of neutral species in nonthermal low temperature (300–700K) pulsed plasmas containing humid air. Recommended rate constants and extrapolation methods are provided in a manner to facilitate prediction of reactivities as a function of temperature and pressure.
Keywords: plasma; discharge; electron-beam; database; kinetic; thermodynamic; rate constant; humid air; bimolecular