Plasma Chemistry and Plasma Processing (v.31, #5)

Nitrogen Functionalization of Carbon Black in a Thermo-Convective Plasma Reactor by Nicholas Larouche; Emmanuel St-Aubin; Matthieu Balmayer; Barry Stansfield (635-647).
We have developed a simple plasma reactor for functionalizing powders. The reactor uses thermo-convective cells to mix the powder during the treatment and to ensure high homogeneity. The reactor has been used to treat batches of 50–150 mg of carbon black in a gas mixture of He/N2 at 40 Torr. By using repetitive treatments on fractions of carbon black recovered from the cap of the treatment vessel (due to its fluidization during the treatment), we were able to fix a concentration of nitrogen functionalities of 13% at. N/C (as measured by XPS). We report the working conditions of the reactor and the results obtained.
Keywords: Powder; Plasma treatment; Surface functionalization; Nitrogen; Carbon black; Hollow cathode; Convection

Oxidation of Propylene with Oxygen and Air in a Barrier Discharge in the Presence of Octane by Sergey Kudryashov; Andrey Ochered’ko; Andrey Ryabov; Galina Shchyogoleva (649-661).
Oxidation of propylene with oxygen, air and a mixture of nitrogen–oxygen in a barrier discharge is investigated. The selectivity towards formation of propylene oxide in pure oxygen is shown to be as high as 45 wt% and the propylene conversion ratio is found to be 12.9 wt%. In the oxidation with air, the propylene oxide selectivity is 23 wt%, while the conversion is 7.5 wt%. The values of propylene conversion and selectivity towards formation of propylene oxide in a barrier discharge are consistent with those obtained by the thermocatalytic methods for production of propylene oxide.
Keywords: Barrier discharge; Oxidation; Propylene; Propylene oxide; Octane

Experimental Study of Hydrocarbons Synthesis from Syngas by a Tip–Tip Electrical Discharge at Very High Pressure by Vandad Rohani; Samuel Iwarere; Frederic Fabry; Delphine Mourard; Eduardo Izquierdo; Deresh Ramjugernath; Laurent Fulcheri (663-679).
This paper is dedicated to the study of hydrocarbons synthesis from a syngas using a plasma process based on the generation of a electrical discharge at very high pressure. We report experimental results performed using a He/H2/CO mixture with the ratio 40%/48%/12% at a pressure of 2.2 MPa, through three modes of treatment: one continuous mode and two intermittent modes including a relaxation time between two successive discharges. The study shows that significant quantities of C1, and saturated and unsaturated C2 molecules are synthesized at these experimental conditions. It also indicates the better efficiency of the intermittent treatment modes for the C2 molecules synthesis. Estimates of the chemical synthesis balance occurring in the reactor during an instantaneous treatment are also presented in the manuscript. In addition, the study indicates a preliminary scan of other synthesized molecules which have been identified by varying the operating pressure in the continuous treatment mode.
Keywords: Hydrocarbons synthesis; Syngas; Electrical discharge; Very high pressure; Fischer–Tropsch

The miniaturized atmospheric pressure glow discharge (APGD) generated between a solid electrode and a flowing small size liquid cathode (dimension 2 mm) was investigated here using optical emission spectroscopy. The discharge was studied in an open air atmosphere, and the spectral characteristics of the plasma source was examined. Analysed APGD was operated at a discharge voltage of 1,100–1,700 V, a discharge current of 20 mA and gaps between a solid anode and a liquid cathode in the range from 0.5 to 3.5 mm. The emission intensities of the main species were measured as a function of various experimental conditions, including the solution flow rate, the gap between the electrodes, and the concentration of hydrochloric acid. The excitation temperature, the vibrational temperatures calculated from N2, OH, and NO bands, and the rotational temperatures determined from band of OH, N2 and NO, were found to be dependent on these experimental parameters. The electron number density was determined from the Stark broadening of Hβ line. Additionally, the ionization temperature and degree were calculated using the Saha–Boltzmann equation, with the ion to atom ratio for magnesium (MgII/MgI). The results demonstrated that T exc(H), T vib(N2), T vib(OH), T vib(NO) and T rot(OH) were well comparable (~3,800–4,200 K) for selected plasma generation conditions (gap ≥2.5 mm, HCl concentration ≥0.1 mol L−1), while the rotational temperatures determined from band of N2 (~1,700–2,100 K) and band of NO (~3,000 K) were considerably lower. The electron number density was evaluated to be (3.4–6.8) × 1020 m−3 and the ionization temperature varied, throughout in the 4,900–5,200 K range.
Keywords: Atmospheric pressure glow discharge; Optical emission spectrometry; Plasma diagnostics; Liquid cathode; Miniaturized plasma source

Inactivation of Microcystis aeruginosa with Contact Glow Discharge Electrolysis by Xing-Long Jin; Qing Xia; Xiao-Yan Wang; Jun-Jie Yue; Dong-Bin Wei (697-705).
This work investigated the inactivation of Microcystis aeruginosa (MA) with contact glow discharge electrolysis (CGDE). The influences of applied voltage, current and treatment time on the inactivation rate were critically examined. Based on the above results, the optimal conditions were chosen to sufficiently utilize chemically active species and enhance the inactivation of MA. Under the optimal conditions (voltage: 530 V; current: 30 mA; treatment time: 20 min), the inactivation rate of algae was more than 90% within 5 days incubation after inoculating. At the same time, the concentrations of Chlorophyll-a and dehydrogenase decreased, which demonstrated that 20 min CGDE treatment could effectively inhibit the growth of MA and caused deterioration of cell integrity. The present work would provide strong evidence to support the utilization of CGDE on the inactivation of MA in aqueous solution.
Keywords: Inactivation; Microcystis aeruginosa ; Contact glow discharge electrolysis; Chlorophyll-a; Dehydrogenase

Qualitative By-Product Identification of Plasma-Assisted TCE Abatement by Mass Spectrometry and Fourier-Transform Infrared Spectroscopy by Arne M. Vandenbroucke; Minh Tuan Nguyen Dinh; Jean-Marc Giraudon; Rino Morent; Nathalie De Geyter; Jean-Francois Lamonier; Christophe Leys (707-718).
In this study, the formation of by-products resulting from the decomposition of trichloroethylene with a negative DC glow discharge has been investigated. By combining the qualitative data from mass spectrometry and Fourier-transform infrared spectroscopy, the formation of phosgene, dichloroacetylchloride and trichloroacetaldehyde have been confirmed as incomplete oxidation products. Hydrogen chloride, chlorine, carbon monoxide and carbon dioxide were also detected. Also, formation of ozone was found in the outlet stream. Based on this information, it is possible to chose an appropriate catalyst to combine with non thermal plasma. In this way emission of harmful, incomplete oxidation products can be further reduced at a lower energy cost.
Keywords: Non-thermal plasma; VOC; Negative DC glow discharge; By-products; FT-IR; MS

Nickel Nanoparticles Formation from Solution Plasma Using Edge-Shielded Electrode by G. Saito; S. Hosokai; M. Tsubota; T. Akiyama (719-728).
Because of the easy massproduction, synthesis of metallic nanoparticles from a solution plasma is an attractive method. However, a solution plasma produces a highly inhomogeneous electric field via transition to full-plasma, and the products are partially oxidized and agglomerated, with a wide size-distribution. Here, we show a simple method of suppressing oxidation of products. An electrode tip was shield by a glass tube and a voltage of up to 180 V was applied with the electrolyte of 0.1 M NaOH solution. Significantly, the edge-shield was quite effective for maintaining partially glow discharge. The results were (1) surface temperature of the electrode less than 100°C, (2) main phase of metallic nickel evaluated by XRD, and (3) nanoparticles of an average size of 220 nm. These results showed the potential for an application to the production of nanoparticles.
Keywords: Nickel; Glow discharge; Pure metallic

The homogeneity of a helium dielectric barrier discharge, working at atmospheric pressure and containing oxygen as contaminant, is assessed by mapping the spatial distribution of oxygen metastable atoms in relation to the uniformity of surface properties. Tunable diode laser absorption spectroscopy is used to monitor the time evolution of the absorption coefficient corresponding to the oxygen metastable atoms on the 35S2 level, as a function of the laser absorbing path, whereas bi-dimensional Abel transform is used to obtain local information on the space distribution of the metastable atoms in the discharge. The radial distribution of the surface properties is investigated using atomic force microscopy, contact angle measurement and X-ray photoelectron spectroscopy. The results show that the oxygen metastables density has complex space–time behavior, and the spatial distribution of the reactive species yields specific radial profile of the surface properties of a polymer film depending on the treatment time.
Keywords: Dielectric barrier discharge; Absorption spectroscopy; Oxygen metastables; Polymer; Surface analysis; Radial profile

Plasma polymerization of aniline is carried out in a radiofrequency plasma reactor and the effect of polymerization time is examined in the structural, optical and optoelectronic properties of deposited films. Conjugated structures of polyaniline like films are obtained with unique and broad optical absorption band in the ultraviolet and entire visible region. The width of the absorption band increases and hence the optical band gap decreases with polymerization time. The optical constants are extracted by Swanepoel method and the optical dispersion parameters are determined by employing the Wemple-DiDomenico single oscillator model. The films exhibit similar thermal stability in air and argon atmosphere in the region of interest for optoelectronic applications. The photoluminescence study suggests, like the chemically synthesized polyaniline, the benzenoid units to be responsible for fluorescence. The fluorescence peaks due to defect states confirm the formation of spectroscopic units in the plasma polyaniline films.
Keywords: Conjugated polymer; Plasma polymerization; Extended conjugation; Optical dispersion; Photoluminescence

Spectroscopic Characterization of a Steam Arc Cutting Torch by V. Sember; A. Mašláni; P. Křenek; M. Heinrich; R. Nimmervoll; H. Pauser; M. Hrabovský (755-770).
Optical emission spectroscopy has been used to investigate the characteristics of a plasma jet produced by a steam arc cutting torch operated in air at atmospheric pressure. A procedure has been developed for simultaneous determination of temperature and pressure in the plasma jet as well as an effective nonequilibrium factor. It is based on comparison of a few experimental and simulated spectral quantities. The experimental data were obtained from the spectrum of Hβ and OII lines centred at 480 nm. The existence of the shock wave structure characteristic of an underexpanded jet can clearly be deduced from the measured properties. In the first expansion region, the centreline pressure drops from about 1.4 atm at the nozzle exit to about 0.7 atm a few tenths of millimeter downstream. On the contrary, the centreline temperature remains almost unchanged within this region and reaches the value of about 23,000 K.
Keywords: Plasma cutting; Optical emission spectroscopy; Steam arc; Supersonic plasma jet

Influence of Gas Type on the Thermal Efficiency of Microwave Plasmas for the Sintering of Metal Powders by Aidan Breen; Vladimir Milosavljević; Denis P. Dowling (771-785).
Microwave plasmas have enormous potential as a rapid and energy efficient sintering technology. This paper evaluates the influence of both plasma atmosphere and metal powder type on the sintering temperatures achieved and the properties of the sintered powder metal compacts. The sintering is carried out using a 2.45 GHz microwave-plasma process called rapid discharge sintering (RDS). The sintering of three types of metal powder are evaluated in this study: nickel (Ni), copper (Cu) and 316L stainless steel (SS). An in-depth study of the effects of the plasma processing parameters on the sintered powder compacts are investigated. These parameters are correlated with the mechanical performance of the sintered compacts to help understand the effect of the plasma heating process. The substrate materials are sintered in four different gas discharges, namely hydrogen, nitrogen oxygen and argon. Thermocouple, pyrometer and emission spectroscopy measurements were taken to determine the substrate and the discharge temperatures. The morphology and structure were examined using scanning electron microscopy and X-ray diffraction. The density and hardness of the sintered compacts were correlated with the plasma processing conditions. As expected higher densities were obtained with powders with lower sintering temperatures i.e. nickel and copper when compared with stainless steel. Under the power input and pressure conditions used, the highest substrate temperature attained was 1,100°C for Cu powder sintered in a nitrogen atmosphere. In contrast under the same processing conditions but in an argon plasma, the temperature achieved with SS was only 500°C. The effect of the plasma gas type on the sintered powder compact chemistry was also monitored, both hydrogen and nitrogen yielded a reducing effect for the metal in contrast with the oxidising effect observed in an oxygen plasma.
Keywords: Microwave plasma; Sintering; Thermal measurements; Emission spectroscopy

This paper reports the investigation of the excimer UV emission efficiency in Ne/Xe/HCl mixture in terms of the homogenous model of a dielectric barrier discharge. The computer model developed is based on the Ne/Xe/HCl gas mixture chemistry, the circuit and the Boltzmann equations. The effects of operating voltage, HCl concentration in the mixture as well as gas pressure on the discharge efficiency and the 172, and 308 nm photon generation, under typical experimental operating conditions, have been investigated and discussed. Calculations suggest that the overall conversion efficiency from electrical energy to VUV emission in the lamp is greater than 27%, and it will be very affected at high voltage amplitude and high gas pressure with a significant dependence on the HCl concentration in the gas mixture.
Keywords: Modeling; DBDs; Excimer lamp; Ne/Xe/HCl mixtures; Kinetic scheme; Efficiency