Plasma Chemistry and Plasma Processing (v.30, #2)

Effect of C2H4/N2 Ratio in an Atmospheric Pressure Dielectric Barrier Discharge on the Plasma Deposition of Hydrogenated Amorphous Carbon-Nitride Films (a-C:N:H) by Christian Sarra-Bournet; Nicolas Gherardi; Hervé Glénat; Gaétan Laroche; Francoise Massines (213-239).
The goal of this study was to investigate the properties and growth mechanisms of nitrogen-containing carbon-based coatings obtained with an atmospheric pressure dielectric barrier discharge in an N2-C2H4 atmosphere. Radically different chemical compositions were observed depending on C2H4/N2 ratio. With a low C2H4 concentration (<400 ppm) as a function of the residence time in the discharge, two different growth mechanisms were observed consisting of a highly nitrogenated coating (N/C > 0.8) and low hydrogen content. At the short residence time, growth was due to mobile small radicals that procured a smooth yet soluble coating, while at the longer residence time, diffusion-limited aggregation of high sticking N-containing radicals produced a cauliflower-like structure. With a high C2H4 concentration (≥2,000 ppm), a polymer-like coating with relatively lower nitrogen content (N/C ~ 0.2) was observed with a cauliflower morphology for the entire coating. Nanoindentation measurements revealed very different physical properties in the two types of coatings.
Keywords: Dielectric barrier discharge (DBD); Atmospheric pressure; Plasma deposition; Hydrocarbons; Carbon nitride; Optical emission spectroscopy; Surface analysis

The excitation capability of an argon microwave plasma flame expanded at atmospheric pressure has been studied. For this purpose, argon with different proportions of nitrogen was introduced at the end of the expanded flame, where the population densities of the atomic argon levels were still high enough. Optical emission spectra allowed the identification of different excited species in the plasma. When argon containing nitrogen was added at the end of the plasma flame, all argon lines emitted in this region were highly quenched, emission due to species containing nitrogen (NH, CN) was enhanced and a noticeable increase in the emission of N2 (C 3Πu − B 3Πg) was observed. On the contrary, the weak emission of $$ { ext{N}}_{2}^{ + } left( {B^{ 2} sum_{u}^{ + } - X^{ 2} sum_{g}^{ + } } ight) $$ was scarcely affected. According to these results it is possible to conclude that metastable argon atoms from the expanded flame are the main energy carriers when generating N2 reactive species in this plasma zone.
Keywords: High frequency plasma; Microwave plasma; Atmospheric pressure; Optical emission spectra

Dry reforming of CH4 with CO2 to produce syngas was investigated in a plasma reactor without catalysts at atmospheric pressure. The reactants passed through the plasma zone and reacted in milliseconds with high conversions and selectivity due to the localized high temperature. The results showed that both conversions and selectivity were higher when using a DC arc discharge than using a pulsed DC arc. Increasing the input energy density promoted the conversions of reactants. At an input power of 204 W, the conversions of CO2 and CH4 reached 99.3 and 99.6%, respectively, and the selectivity to products was almost 100%, where the molar ratio of CO2/CH4 was 1 with the reactants flow rate of 100 ml/min. Very little coke was formed during the course of reaction. Key parameters such as the pulse frequency, the input power and the total feed flow rate were studied to find the optimum operating condition.
Keywords: Dry reforming; Methane; Carbon dioxide; Pulsed DC arc plasma; Syngas

Carbon Blacks Produced by Thermal Plasma: the Influence of the Reactor Geometry on the Product Morphology by Ramona Pristavita; Norma-Yadira Mendoza-Gonzalez; Jean-Luc Meunier; Dimitrios Berk (267-279).
Carbon black (CB) nanopowders were obtained by plasma decomposition of methane at various flow rates using inductively coupled thermal plasma torch system of 35 kW. Nitrogen was also introduced in some experiments along with the methane. Using a cylindrical shape reactor the obtained powders were composed mainly of spherical particles, non-uniform in terms of particles size with diameters between 30 and 150 nm. The shape and size of this reactor resulted in the presence of recirculation areas enabling the formation of large CB particles and other secondary volatile compounds. Changing the reactor to a conical geometry resulted in the production of CB powders showing a crystalline and flake-like morphology made of sheets having 6–16 graphitic planes. The conical shape avoids the presence of recirculation areas and promotes the formation of a uniform powder morphology throughout the reactor.
Keywords: Carbon black; Carbon nano-flakes; Thermal plasma; Reactor geometry; ICP torch; PEM fuel cell catalyst

CCl4 Decomposition in RF Thermal Plasma in Inert and Oxidative Environments by Tamás Kovács; Tamás Turányi; János Szépvölgyi (281-286).
The decomposition of carbon tetrachloride was investigated in an RF inductively coupled thermal plasma reactor in inert CCl4–Ar and in oxidative CCl4–O2–Ar systems, respectively. The exhaust gases were analyzed by gas chromatography-mass spectrometry. The kinetics of CCl4 decomposition at the experimental conditions was modeled in the temperature range of 300–7,000 K. The simulations predicted 67.0 and 97.9% net conversions of CCl4 for CCl4–Ar and for CCl4–O2–Ar, respectively. These values are close to the experimentally determined values of 60.6 and 92.5%. We concluded that in RF thermal plasma much less CCl4 reconstructed in oxidative environment than in an oxygen-free mixture.
Keywords: Carbon tetrachloride; Decomposition; Thermal plasma

Pulsed plasmas containing organic precursors are becoming increasingly common for multiple applications. To understand the nature of such discharges, in-situ time resolved microwave interferometry measurements of the electron density in a 60 W pulsed inductively coupled 1,3-butadiene discharge have been made. Measurements were also made for continuous wave plasmas at 40, 50, 60 and 70 W power for comparison. The data shows that the time averaged electron densities are independent of pulse width at a particular duty cycle. In addition, time averaged values increase approximately linearly with increasing duty cycle. Such linearity in average density is tied to the ambipolar loss rate. This knowledge is important for understanding the growth kinetics of plasma polymerized films.
Keywords: Pulsed plasmas; Microwave interferometry; Electron density; Plasma polymerization

A study of scavenging effects of iso-propanol, n-propanol, n-butanol and acetone on H radicals, precursors for nonfaradaic products of anodic contact glow discharge electrolysis (CGDE) in aqueous media, has been carried out. A kinetic analysis of the competing reactions of H with different species in the system leads to a primary yield of 9.8 mol (mol electron)−1 each of H and OH radicals in the liquid-phase reaction zone of anodic CGDE. The method is considerably more convenient to apply than those reported earlier based on the use of OH scavengers.
Keywords: Contact glow discharge electrolysis; H scavengers; Nonfaradaic yields; Primary radical yield

Influence of Rubber Formulation on Surface Modifications Produced by RF Plasma by Ana B. Ortíz-Magán; M. Mercedes Pastor-Blas (311-332).
The effectiveness of nitrogen, oxygen and air Radio Frequency (RF) plasma treatments on two styrene-butadiene vulcanized rubbers with a different formulation has been studied. The presence of an antiadherent surface layer containing low-molecular weight ingredients (sulfur-rich vulcanization agents and wax) from SW (Sulfur-Wax) rubber formulation requires an extended plasma treatment capable of removing this surface layer. When the percentage of antiadherent moieties is reduced in ZS (Zinc Stearate) rubber formulation, shorter plasma treatment times are enough to modify rubber surface and increase its polarity by the creation of C–O and C=O polar groups that enhance adhesion towards a polyurethane adhesive. Air and oxygen plasma treatments are more aggressive than nitrogen plasma and therefore they are more effective in removing the antiadherent layer of the outermost rubber surface layer prior to oxidation of the rubber surface.
Keywords: Plasma treatment; Styrene-butadiene rubber; Wax; Zinc stearate; Vulcanization compounding; Adhesion