Plasma Chemistry and Plasma Processing (v.26, #3)

The Observation of Plasma Structures in Electrolyte Solution by A. I. Maximov; L. A. Kuzmicheva; A. Y. Nikiforov; J. V. Titova (205-209).
The passage of the electrical current (the voltage 1–2 kV) through dielectric tubes filled with electrolyte solutions causes the formation of luminous plasma zones moving along the tube. The visual observations and camera control (24 frames/s) show the possibility of the simultaneous existence up to three luminous zones. They may be as cylinder with the length up to four diameters of the tube, disc or ring. The estimations result in the lifetime of these luminous objects ≤ 0.2 s. The discharge current oscillography demonstrates the sequences of the attenuated pulses modulated with the frequency of 50 Hz. This type of plasma-solution system demonstrates high efficiency modification of the polymer materials.

Modeling results are presented to compare the characteristics of laminar and turbulent argon thermal plasma jets issuing into ambient air. The combined-diffusion-coefficient method and the turbulence-enhanced combined-diffusion-coefficient method are employed to treat the diffusion of ambient air into the laminar and turbulent argon plasma jects, respectively. It is shown that since only the molecular diffusion mechanism is involved in the laminar plasma jet, the mass flow rate of ambient air entrained into the laminar plasma jet is comparatively small and less dependent on the jet inlet velocity. On the other hand, since turbulent transport mechanism is dominant in the turbulent plasma jet, the entrainment rate of ambient air into the turbulent plasma jet is about one order of magnitude larger and almost directly proportional to the jet inlet velocity. As a result, the characteristics of laminar plasma jets are quite different from those of turbulent plasma jets. The length of the high-temperature region of the laminar plasma jet is much longer and increases notably with increasing jet inlet velocity or inlet temperature, while the length of the high-temperature region of the turbulent plasma jet is short and less influenced by the jet inlet velocity or inlet temperature. The predicted results are reasonably consistent with available experimental observation by using a DC arc plasma torch at arc currents 80–250 A and argon flow rates (1.8–7.0)×10−4 kg/s.
Keywords: Thermal plasma; laminar and turbulent jets; comparative study; modeling

Humidity Effect on Toluene Decomposition in a Wire-plate Dielectric Barrier Discharge Reactor by Guo Yu-fang; Ye Dai-qi; Tian Ya-feng; Chen Ke-fu (237-249).
Laboratory-scale experiments were performed to evaluate the humidity effect on toluene decomposition by using a wire-plate dielectric barrier discharge (DBD) reactor at room temperature and atmospheric pressure. The toluene decomposition efficiency as well as the carbon dioxide selectivity with/without water in a gas stream of N2 with 5% O2 was investigated. Under the optimal humidity of 0.2% the characteristics of toluene decomposition in various background gas, including air, N2 with 500 ppm O2, and N2 with 5% O2 were observed. In addition, the influence of a catalyst on the decomposition was studied at selected humidities. It was found that the optimum toluene removal efficiency was achieved by the gas stream containing 0.2% H2O, since the presence of water enhanced the CO2 selectivity. In addition, the toluene removal efficiency increased significantly in a dry gas stream but decreased with an increase in the humidity when the Co3O4/Al2O3/nickel foam catalyst was introduced into the discharge area.
Keywords: Humidity; Dielectric barrier discharge; Catalyst; Plasma; Toluene

Modification of Hexatriacontane by O2–N2 Microwave Post-Discharges by V. Hody; T. Belmonte; C. D. Pintassilgo; F. Poncin-Epaillard; T. Czerwiec; G. Henrion; Y. Segui; J. Loureiro (251-266).
Etch rates of hexatriacontane (HTC) as high as ~10 mg s−1 m−2 in late O2 post-discharge are obtained at 333 K where no significant UV nor VUV irradiation occurs. Introducing N2 in the gas mixture helps control the ratio of O/O2 densities, which is shown to play a key role in the functionalization or etching of the HTC. The oxygen atoms are required for any further modification of the HTC because they initiate the formation of the radical chains by abstraction of one hydrogen. O(3P) atoms do not contribute directly to break the alkane chain close to room temperature but they can functionalise it. O2 is the important reactive species for the etching because of the role played by the peroxide groups on the scission of the hydrocarbon chains.
Keywords: Cleaning; Post-discharge; Hexatriacontane; 34.50-Dy Interactions of atoms; molecules; and their ions with surfaces photon and electron emission neutralization of ions

Preparation and Application of Ca0.8 Sr0.2 TiO3 for Plasma Activation of CO2 by Ruixing Li; Qing Tang; Shu Yin; Tsugio Sato (267-276).
Despite a large interest in plasma-assisted catalytic technology (PACT), very little has been reported about the catalytic effects of different dielectric barriers on a dielectric barrier discharge (DBD) reaction. In the present study, Ca0.8Sr0.2TiO3, that possesses a high permittivity, was prepared by liquid phase sintering and used as a dielectric barrier in a DBD reactor to break CO2. The mechanical and dielectric properties of Ca0.8Sr0.2TiO3 were greatly enhanced by adding 0.5 wt.% Li2Si2O5. A DBD plasma was successfully generated by using this Ca0.8Sr0.2TiO3 as a dielectric barrier and 18.8% CO2 conversion was achieved with the residence time of 0.17 s at the frequency of 8 kHz, which was much higher than with those using an alumina or a silica glass barrier. It was found that the plasma power increased with the increasing of the permittivity, and finally very dense and strong microdischarges were initiated to decompose CO2.
Keywords: Carbon dioxide; Decomposition; Plasma; Dielectric barrier discharge; CaTiO3 ; SrTiO3

Plasma produced by a (1064 nm) Nd:YAG laser focused onto a graphite target at different nitrogen pressures in the range of 1–90 mTorr, was studied spectroscopically. In the spectral range of 350–600 nm, emission lines of CI neutral carbon (501.12, and 505.21 nm), NI neutral nitrogen (493.5 nm), CII (426.72, 463.7, 515.11 nm), and CIII ions (465.02 and 569.59 nm), and NII ions (501.06, and 500.73 nm), were dominating. Bands of C2 Swan (d3Πg → a3Πu, Δ ν=2, 1, 0, −1), and CN Violet (B2Σ +→ X2Σ+, Δ ν=1, 0, −1) systems, and ionic emissions from the First Negative system N2+ (band head at 391.44 nm), were faintly observed under our specific experimental conditions. From the band intensities, vibrational temperature for CN and C2 was calculated to be 1.25 and 0.31 eV at 90 mTorr, respectively. The electron density and temperature, measured by Stark broadening, assuming a local thermodynamic equilibrium (LTE), were found to be 2.1× 1017 cm−3 and 0.33 eV at 1mTorr, respectively. The validity of the LTE is discussed according to the results discussed. Pressure dependence shows a decrease in the vibrational temperature when nitrogen pressure increases, while the electron density and temperature increase.
Keywords: Optical emission spectroscopy; Laser ablation; Plasma properties; Fullerene related materials; 61.48.+c; 52.38.Mf; 52.25-b; 52.70. Kz; 78.30.–j; 81.05.Tp

Modelling of Carbon Tetrachloride Decomposition in Oxidative RF Thermal Plasma by Tamás Kovács; Tamás Turányi; Katalin Főglein; János Szépvölgyi (293-318).
Decomposition of carbon tetrachloride in a RF thermal plasma reactor was investigated in oxygen–argon atmosphere. The net conversion of CCl4 and the main products of decomposition were determined by GC–MS (Gas Chromatographic Mass Spectroscopy) analysis of the exhaust gas. Temperature and flow profiles had been determined in computer simulations and were used for concentration calculations. Concentration profiles of the species along the axis of the reactor were calculated using a newly developed chemical kinetic mechanism, containing 34 species and 134 irreversible reaction steps. Simulations showed that all carbon tetrachloride decomposed within a few microseconds. However, CCl4 was partly recombined from its decomposition products. Calculations predicted 97.9% net conversion of carbon tetrachloride, which was close to the experimentally determined value of 92.5%. This means that in RF thermal plasma reactor much less CCl4 was reconstructed in oxidative environment than using an oxygen-free mixture, where the net conversion had been determined to be 61%. The kinetic mechanism could be reduced to 55 irreversible reaction steps of 26 species, while the simulated concentrations of the important species were within 0.1% identical compared to that of the complete mechanism.
Keywords: Carbon tetrachloride; Oxygen; Thermal plasma; Decomposition; Modelling

A low pressure radio frequency discharge was used to deposit films by mixtures of oxygen and titanium (IV) isopropoxide (TTIP) at powers of 200 W on films of polyethylene-terephthalat and samples of quartz glass. In the non-thermal plasma, films of rather pure TiO2 could be deposited as revealed by X-ray photo-electron spectroscopy. Besides the film growth rate and the chemical composition, the spectral behaviour of the spectral transmittance of visually transparent films was determined in the range from 200 to 500 nm. Furthermore, the absorptance of films has been derived at characteristic spectral positions of the transmission spectra of the films. Accordingly, cut-off wavelength was found to increase with deposition time from 5 to 10 min as well as with the concentration of TTIP in a range below 1.7%. At 310 nm, the spectral absorption coefficient (extinction coefficient × concentration) was 12 μm−1. While keeping other parameters constant, this coefficient decreased by 4 μm−1 due to an increase of the concentration of TTIP from 1.7% to 8%. Simultaneously, the surface roughness increased as revealed by profilometry. Thus, since the chemical structure of films was found to change only marginally, a decrease of the film density is likely to cause the observed dependence of the absorption coefficient with increasing precursor concentration.
Keywords: Titanium dioxide; Plasma enhanced chemical vapour deposition (PECVD); Polymeric substrate; UV absorption; XPS