Plasma Chemistry and Plasma Processing (v.26, #2)

An Iron Catalytic Probe for Determination of the O-atom Density in an Ar/O2 Afterglow by M. Mozetic; A. Vesel; U. Cvelbar; A. Ricard (103-117).
An iron fiber optics catalytic probe has been constructed and applied for the real-time measuring of the O-atom density in an Ar/O2 afterglow. The recombination coefficient for the heterogeneous surface recombination of O atoms on the oxidized iron foil was measured at different temperatures between 400 and 950 K. The coefficient was found to be constant in the entire range of experimental conditions and had a value of 0.41 ± 0.12. The iron fiber optics catalytic probe has an advantage over the classical nickel fiber optics catalytic probe: the probe signal is higher for the iron probe due to a higher recombination coefficient thus causing an easier real-time monitoring of the O-atom density. The O-atom density was measured in an afterglow of microwave plasma created at different discharge powers up to 300 W, at a constant Ar flow rate of 1000 sccm/min and at different oxygen flow rates between 50 and 300 sccm/min. The O-atom density was found to be dependent on oxygen flow. At low oxygen flow rates up to 100 sccm/min, a saturation of the O-atom density was obtained at a certain discharge power, while at high oxygen flow rate the O-atom density kept increasing with the increasing power. The results were explained by gas phase and surface phenomena.
Keywords: Oxygen plasma; Afterglow; Plasma characterization; O-atom density; Dissociation; Thermocouple catalytic probe; Fiber optics catalytic probe (FOCP); 52.70.Nc; 52.50.Dg

Polyacrylonitrile fibers were treated with a nitrogen glow-discharge plasma. The surfaces of untreated and treated fibers were examined with contact angle measurements, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Surface energy calculations of the fibers were carried out from contact angle measurements using the relationships developed by Fowkes. It is found that plasma treatment causes a reduction in water contact angle on the fiber surfaces. The dispersion component of surface energy changes slightly, while the polar component is increased significantly from 14.6 mN/m to 58.7 mN/m and the total surface energy increase is 139%. The increase of surface energy is mainly caused by the introduction of hydrophilic groups on the fiber surfaces after plasma treatment.
Keywords: Polyacrylonitrile; Plasma; Surface energy; Contact angle; polar groups

A Study of Two-Dimensional Microdischarge Pattern Formation in Dielectric Barrier Discharges by Alexandre Chirokov; Alexander Gutsol; Alexander Fridman; Kurt D. Sieber; Jeremy M. Grace; Kelly S. Robinson (127-135).
Although microdischarges in dielectric-barrier discharges (DBDs) have been studied for the past century, their mutual interaction was explained only recently. This interaction is responsible for the formation of microdischarge patterns reminiscent of two-dimensional crystals. Depending on the application, microdischarge patterns may have a significant influence on DBD performance, particularly when spatial uniformity is desired. This paper presents the results of study of regular microdischarge pattern formation in DBD in air at atmospheric pressure. Experimental images of DBD (Lichtenberg figures) were obtained using photostimulable phosphors. A new method for analysis of microdischarge patterns that allow measuring the degree of pattern regularity was developed. Simulated and experimental patterns were compared using the newly developed method and comparison indicates the presence of interaction between microdischarges. Analysis of microdischarge patterns shows that regularity of the patterns increases with the number of excitation cycles used to produce the pattern.
Keywords: Dielectric barrier discharge; Microdischarge patterns formation; Patterns analysis

Pulsed-Spray Radiofrequency Plasma Enhanced Chemical Vapor Deposition of CuInS2 Thin Films by Rene G. Rodriguez; Daniel J. V. Pulsipher; Lisa D. Lau; Endrit Shurdha; Joshua J. Pak; Michael H. Jin; Kublinder K. Banger; Aloysius F. Hepp (137-148).
Thin films of CuInS2 were grown on various substrates at a temperature of 523 K from two metal-organic precursors using radiofrequency plasma enhanced chemical vapor deposition (PECVD). Two precursor molecules, with different solubility properties, were dissolved in appropriate solvents and sprayed into the plasma region in the PECVD chamber. The resulting films were examined for atomic composition, growth rate, crystalline orientation, and uniformity. Films made from each precursor differed in thickness, atomic composition, and crystallinity. The uniformity of the film was fairly good from near the edge to the center of the substrate, and evidence for a chalcopyrite-like structure was found in several samples deposited from one of the precursor molecules.
Keywords: PECVD; CuInS2 ; Liquid-spray; Single-source precursor

Numerical models have been developed using computational fluid dynamics (CFD) analysis program FLUENT V6.02© to investigate the effect of the substrate on the behavior of the plasma flow fields and in-flight particles. Simulations are performed for cases where flat substrates are either present or absent, for the former, the substrate is oriented perpendicularly or inclined to the torch axis. It is shown that although the presence of perpendicular or inclined substrate significantly influences the plasma flow fields at the vicinity of the substrate, the particle behavior remain relatively unaffected. The insignificant effect of the substrate on particle behavior is qualitatively verified by experimental observation using SprayWatch© imaging diagnostics equipment. Images captured by the equipment confirm that the particles travel through the plasma plume with high momentum and show no sudden change in theirtrajectories right before impacting the substrate. Both the numerical and experimental findings show that the freestream model is sufficiently detailed for future work of this nature.
Keywords: Atmospheric plasma spray; CFD modeling; Particle behavior; Inclined substrate

Removal of SO2 from Gas Streams by Oxidation using Plasma-Generated Hydroxyl Radicals by Mindong Bai; Zhitao Zhang; Mindi Bai; Chengwu Yi; Xiyao Bai (177-186).
The key problem for the removal of SO2 by electrical discharge methods is how to obtain the hydroxyl radicals at high concentration and large production rates. With the micro-gap discharge method, O2 and H2O in simulated gas streams (N2/O2/H2O/SO2) are ionized into a large number of OH. radicals to oxidize SO2 into SO3 which reacts with H2O forming H2SO4 droplets at 120 °C in the absence of any catalyst or absorbent. The droplets are captured with an electrostatic precipitator. As a result, conversion of SO2 to primarily H2SO4 is limited by the generation of OH. radicals. By increasing the reduced field and concentrations of O2 and H2O, the amount of OH. radicals increase resulting in more removal of SO2 from gas streams. The removal efficiency of SO2 reaches 100% when the residence time is only 0.74 s. Therefore, a new gas-phase oxidation method for removal of SO2 without NH3 additive is found.
Keywords: Micro-gap discharge; OH. radials; H2SO4 droplet; Removal of SO2

Evaluation of Multiple Corona Reactor Modes and the Application in Odor Removal by Yao Shi; Xin Wang; Wei Li; Tian-en Tan; Jian-jun Ruan (187-196).
Effects of multiple corona reactor modes on pulse characteristics, energy transfer efficiency, and odor (H2S and NH3) removal were investigated experimentally by the wire-plate corona reactor(s). The removal efficiency of H2S was only 91% and the energy consumption was 16.1 Wh m−3 by the single mode with a gas-flow rate of 23 m3 h−1 and an initial concentration of 200 mg m−3. At the same experimental conditions, almost 100% removal efficiency was achieved and the energy consumption was only 12.8 and 14.9 Wh m−3 by the series and parallel modes. In the case of 50 mg m−3 NH3 removal at the same gas-flow rate, the removal efficiencies with the single mode, the series and parallel modes were 64, 92 and 70%, respectively. The energy requirement did not increase at the same residence time under the experimental conditions of the single mode with a gas-flow rate of 11.5 m3 h−1 and the series or parallel mode with a gas-flow rate of 23.0 m3 h−1. The experimental results indicate that the series and parallel modes are effective in saving energy consumption, improving removal ability and efficiency, especially for the series mode.
Keywords: Pulse corona discharge; Wire-plate reactor; In parallel; In series; H2S; NH3

The plasma chemistry of magnetron sputtered Zr and Nb in an Ar/O2 atmosphere has been measured as a function of the O2 partial pressure. The previously reported composition of films deposited onto grounded non-intentionally heated substrates was correlated with the dominant positive and negative ion populations in the plasma. While the oxygen deficient films were grown in the Ar+ dominant mode, the close-to-stoichiometric films were grown in the O+/O dominant mode. The formation of close-to-stoichiometric ZrO2.1 is observed in the compound mode (CM), while the formation of close-to-stoichiometric Nb2O4.7 thin films was reported in addition to the CM also in the transition mode (TM). This may be understood based on the 1.5–1.9 times higher power dissipated in the Nb–Ar–O2 plasma as compared to the Zr–Ar–O2 plasma. We suggest that at larger power O2 dissociation may be more efficient and lead to the presence of sufficient atomic oxygen to fully oxidize the films. This finding may provide a pathway towards a deposition rate enhancement, since compound formation at the substrate is enabled in the TM of the higher power Nb–Ar–O2 plasma and not only in the CM, as in the case of the lower power Zr–Ar–O2 plasma.
Keywords: Zirconia; Niobia; Thin film composition; Plasma chemistry; Magnetron sputtering