Plasma Chemistry and Plasma Processing (v.28, #1)

Formation rates of hydrogen peroxide and electrode erosion rates for a range of different electrode materials were determined in a gas–liquid pulsed electrical discharge reactor with a high voltage electrode needle in the liquid and a ground electrode suspended in the gas over the liquid. It was found that the H2O2 formation rates and efficiencies did not depend on the electrode material. Electrode erosion from lowest to highest followed the series: nickel chromium, thoriated tungsten, diamond-coated tungsten, stainless steel, tungsten carbide and tungsten copper alloy. Smooth crater-like morphology was found for nickel chromium, titanium and tungsten carbide and a much finer surface structure with small protrusions for the tungsten, tungsten copper and the copper. Doubling the electrode diameter lead to an increase in the H2O2 formation per eroded length by a factor of three but it also decreased the energy efficiency yield of H2O2 by more than 20%.
Keywords: Gas–liquid electrical discharge; Water; Electrode materials; Hydrogen peroxide generation; Electrode wear

Plasma-induced Degradation of Chlorobenzene in Aqueous Solution by Yongjun Liu; Xuanzhen Jiang (15-24).
Liquid-phase degradation of chlorobenzene (CB), induced by contact glow discharge electrolysis under various reaction conditions, such as, the initial solution pH, current intensity, volume of solution and iron salts was investigated. Experimental results indicated that, in the absence of catalysts, the depletion of CB followed first-order kinetics, where the observed value of the first-order rate constant ‘k’ is directly proportional to the applied current intensity and inversely proportional to the solution volume. Initial solution pH had little effect on the value of k. HPLC and IC analyses showed that the major intermediate products were chlorophenols, phenol, organic acids and chloride ions. During the treatment, a lot of hydrogen peroxide was formed. Role of Fenton’s reaction was examined. A reaction pathway is proposed based on the degradation kinetics and the distribution of intermediate products.
Keywords: Electrical discharge; Non-equilibrium plasma; Chlorobenzene degradation; Water treatment; Reaction mechanism

Catalytic Oxidation of Xylene in Air using TiO2 under Electron Beam Irradiation by Teruyuki Hakoda; Kanae Matsumoto; Akira Mizuno; Takuji Kojima; Koichi Hirota (25-37).
The oxidation of xylene and its irradiation byproducts in air using TiO2 was studied under electron beam (EB) irradiation for the purification of ventilation gases emitted from paint factories. EB irradiation experiments were mainly performed under two different conditions: a TiO2 pellet layer was placed in an irradiation or a non-irradiation space. The results revealed that xylene was decomposed and CO was formed in the gas phase of the irradiation space irrespective of the presence of TiO2 pellets, while CO2 was produced in the gas phase of the irradiation space and on the surface of TiO2 pellets. The total CO2 concentration increased when the pellet layer was in the non-irradiation space. On the other hand, the concentration of CO2 produced on the surface of the TiO2 pellets in the irradiation space was higher than that in a non-irradiation space.
Keywords: Electron beam; Catalyst; TiO2 ; Oxidation; Volatile organic compounds; Xylene

Effects of Temperature on Benzene Oxidation in Dielectric Barrier Discharges by Jing Li; Shu-Pei Bai; Xi-Cheng Shi; Su-Ling Han; Xi-Ming Zhu; Wen-Cong Chen; Yi-Kang Pu (39-48).
Effects of gas temperature on the conversion of benzene and the selectivity of CO and CO2 are analyzed and compared in dielectric barrier discharges. Gas temperature is determined by the method of the optical emission spectroscopy. The result is found that the conversion of benzene increases linearly with gas temperature. The selectivity of CO and CO2 and carbon balance are also investigated.
Keywords: Gas temperature; Plasma; Conversion; Benzene; Selectivity of CO and CO2

In this research, the reforming of simulated natural gas containing a high CO2 content under AC non-thermal gliding arc discharge with partial oxidation was conducted at ambient temperature and atmospheric pressure, with specific regards to the concept of the direct utilization of natural gas. This work aimed at investigating the effects of applied voltage and input frequency, as well as the effect of adding oxygen on the reaction performance and discharge stability in the reforming of the simulated natural gas having a CH4:C2H6:C3H8:CO2 molar ratio of 70:5:5:20. The results showed marked increases in both CH4 conversion and product yield with increasing applied voltage and decreasing input frequency. The selectivities for H2, C2H6, C2H4, C4H10, and CO were observed to be enhanced at a higher applied voltage and at a lower frequency, whereas the selectivity for C2H2 showed an opposite trend. The use of oxygen was found to provide a great enhancement of the plasma reforming of the simulated natural gas. For the combined plasma and partial oxidation in the reforming of CO2-containing natural gas, air was found to be superior to pure oxygen in terms of reactant conversions, product selectivities, and specific energy consumption. The optimum conditions were found to be a hydrocarbons-to-oxygen feed molar ratio of 2/1 using air as an oxygen source, an applied voltage of 17.5 kV, and a frequency of 300 Hz, in providing the highest CH4 conversion and synthesis gas selectivity, as well as extremely low specific energy consumption. The energy consumption was as low as 2.73 × 10−18 W s (17.02 eV) per molecule of converted reactant and 2.49 × 10−18 W s (16.60 eV) per molecule of produced hydrogen.
Keywords: Natural gas reforming; Gliding arc discharge; Plasma; Applied voltage; Input frequency; Partial oxidation

Three-Dimension and Transient D.C. Plasma Flow Modeling by E. Meillot; D. Guenadou; C. Bourgeois (69-84).
The behavior of plasma flow, generated by a D.C. plasma spraying gun, is simulated in a time-dependent 3 D. design. The high-temperature and high-velocity plasma plume is generated by a simple model based on Joule effect. The criterion of validation is the thermal efficiency while the only adjustment parameter is the length of the plasma column inside the anode. The transient plasma flow issuing in air atmosphere is reproduced. The plasma behavior is quite similar to that observed by fast-image video. Moreover, the centerline plasma plume properties are in agreement with experiment measurements, especially close to the torch exit and downstream the laminar-to-turbulent transition.
Keywords: Thermal plasma; Unsteady; Modeling; Heat and acceleration of gas; Air engulfment

When materials processing is conducted in air surroundings by use of an impinging plasma jet, the ambient air will be entrained into the materials processing region, resulting in unfavorable oxidation of the feedstock metal particles injected into the plasma jet and of metallic substrate material. Using a cylindrical solid shield may avoid the air entrainment if the shield length is suitably selected and this approach has the merit that expensive vacuum chamber and its pumping system are not needed. Modeling study is thus conducted to reveal how the length of the cylindrical solid shield affects the ambient air entrainment when materials processing (spraying, remelting, hardening, etc.) is conducted by use of a turbulent or laminar argon plasma jet impinging normally upon a flat substrate in atmospheric air. It is shown that the mass flow rate of the ambient air entrained into the impinging plasma jet cannot be appreciably reduced unless the cylindrical shield is long enough. In order to completely avoid the air entrainment, the gap between the downstream-end section of the cylindrical solid shield and the substrate surface must be carefully selected, and the suitable size of the gap for the turbulent plasma jet is appreciably larger than that for the laminar one. The overheating of the solid shield or the substrate could become a problem for the turbulent case, and thus additional cooling measure may be needed when the entrainment of ambient air into the turbulent impinging plasma jet is to be completely avoided.
Keywords: Impinging plasma jet; Air entrainment; Solid-shield effect; Materials processing; Modeling

The article presents the calculation of thermophysical properties of the mixture water steam–argon which has been used to further enhance the characteristics of plasma torches stabilized by the water wortex. The calculations were performed at the temperatures 400–50,000 K and at 0.1 MPa. First, the composition and thermodynamic properties are determined by classical methods. Further the calculations of viscosity, electrical conductivity and thermal conductivity of the mixture are computed in the 4th approximation of the Chapman–Enskog method. The computation of collision integrals is described with special respect to the interactions of charged particles where the necessary calculations for the Coulomb potential screened at the Debye length were enlarged to cover the 4th approximation. Then the formulae describing the method based on the variational principle of solving the system of Boltzmann integrodifferential equations are shortly introduced and the transport coefficients are presented.
Keywords: Thermophysical properties of thermal plasmas; Ionized gas mixtures; Plasma composition; Thermodynamic properties; Lennard–Jones interaction potential; Screened Coulomb potential; Effective collision cross sections; Collision integrals; Chapman–Enskog method in the 4th approximation; Viscosity; Electrical conductivity; Thermal conductivity

A mixture of acetylene, hydrogen and ammonia (C2H2/H2/NH3) is used to produce carbon nanotubes (CNTs) by a plasma-enhanced catalytic chemical vapor deposition process either without (PE CCVD) or with hot filaments-assistance (PE HF CCVD). A mathematical model based on Chemkin computer package is used for analyzing specific conditions of nanotube synthesis. Simulations are compared with optical emission spectroscopy (OES) measurements. Morphological and structural investigations on the grown carbon nanostructures are also performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was shown that the significant change in the density and the morphology of the CNTs grown in the presence of NH3 could be mainly explained by the gas phase formation of CN and HCN. Both species display a high etching activity, whereas the species C, CH, CH2, CH2(S), C2 and C2H are expected to be the most probable carbon nanotube precursors.
Keywords: Plasma composition; Carbon nanotubes; Plasma enhanced CVD; Chemical kinetics

Plasma Characteristics and Antenna Electrical Characteristics of an Internal Linear Inductively Coupled Plasma Source with a Multi-Polar Magnetic Field by Kyong Nam Kim; Jong Hyeuk Lim; Jung Kyun Park; Geun Young Yeom; Sung Hee Lee; Jae Koo Lee (147-158).
The development of a large-area plasma source with high density plasmas is desired for a variety of plasma processes from microelectronics fabrication to flat panel display device fabrication. In this study, a novel internal-type linear inductive antenna referred to as “double comb-type antenna” was used for a large-area plasma source with the substrate area of 880 mm × 660 mm and the effect of plasma confinement by applying multi-polar magnetic field was investigated. High density plasmas on the order of 3.2 × 1011 cm−3 which is 50% higher than that obtained for the source without the magnetic field could be obtained at the pressure of 15 mTorr Ar and at the inductive power of 5,000 W with good plasma stability. The plasma uniformity <3% could be also obtained within the substrate area. When SiO2 film was etched using the double comb-type antenna, the average etch rate of about 2,100 Å/min could be obtained with the etch uniformity of 5.4% on the substrate area using 15 mTorr SF6, 5,000 W of rf power, and −34 V of dc-bias voltage. The higher plasma density with an excellent uniformity and a lower rf antenna voltage obtained by the application of the magnetic field are related to the electron confinement in a direction normal to the antenna line.
Keywords: Plasma; Large area; Display; Impedance