Plasma Chemistry and Plasma Processing (v.31, #1)

The non-equilibrium atmospheric-pressure parallel-plate helium dielectric barrier discharge (DBD) driven by a realistic 20 kHz distorted-sinusoidal voltage waveform has been investigated by means of simulations and experiments. A self-consistent one-dimensional fluid modeling code considering the non-local electron energy balance was applied to simulate the helium DBD. The effect of selecting plasma chemistry was investigated by comparing simulations with experiments. The results show that the simulations, which include more excited helium, metastable helium and electron–ion-related reaction channels, can faithfully reproduce the measured discharged temporal current quantitatively. Based on the simulated discharge properties, we have found that there is complicated mode transition of discharges from the long Townsend-like to the “dark current”-like, then to the short primary Townsend-like and the short secondary Townsend-like for the helium DBD that is driven by a realistic distorted-sinusoidal voltage power source. Discharge properties in different periods of discharge are discussed in detail in the paper.
Keywords: Townsend-like discharge; Atmospheric pressure plasmas; Helium; Fluid modelling; Dielectric barrier discharge

The Influence of Corona Electrodes Thickness on the Efficiency of Plasmachemical Oxidation of Acetone by Mikhail N. Lyulyukin; Alexey S. Besov; Alexander V. Vorontsov (23-39).
Current–voltage characteristics (CVC) and acetone vapors oxidation in atmospheric pressure corona discharge (CD) of negative and positive polarity were studied in double wires-to-plate geometry. Negative CD was more stable than positive one towards breakdown and allowed to reach higher current. CVC for negative CD can be well described by model for cylindrical geometry with correction in one coefficient only. The ignition voltage of negative CD obeys Peek’s law. The rate of acetone vapors oxidation increased with the increase of corona wire radius in the range of discharge power 15–60 W. The highest oxidation rate was obtained at power 60 W, negative polarity and wire radius 400 μm but the highest energetic efficiency (g/kWh) was obtained at maximum wire radius (400 μm) and minimum power level (15 W). The oxidation rate was directly proportional to the volume of discharge area which grows together with the increase of wire radius.
Keywords: Air purification; Negative corona discharge; Wire-plate geometry; Efficiency; VOC

Understanding OH Yields in Electron Beam Irradiation of Humid N2 by Karen L. Schmitt; Theodore S. Dibble (41-50).
Recent experimental work by Hakoda et al. attempted to determine the G-value of OH radical in electron-beam irradiation of humid N2 using the conversion of CO to CO2 to titrate OH. Kinetic modeling is used to simulate the experiment of Hakoda et al. in an attempt to determine the validity of their assumptions and the reason the observed G-value was higher than expected. The model predicts CO2 production that is roughly 25% lower than observed and OH production about 10% higher than concluded from experimental data. This apparent contradiction is resolved by understanding the detailed chemistry of the OH + CO reaction and the fate of the thermalized HOCO intermediate. Analysis of the reactions producing OH indicate that the sources of OH are significantly different in humid air than in humid N2, and the G-value for OH is much higher in humid air than humid N2.
Keywords: Electron beam; Hydroxyl radical; ·OH; Kinetics; HOCO

Mechanism of Calcium Ion Precipitation from Hard Water Using Pulsed Spark Discharges by Yong Yang; Hyoungsup Kim; Andrey Starikovskiy; Young Cho; Alexander Fridman (51-66).
Direct pulsed spark discharge treatment was found to be able to induce the precipitation of calcium ions in supersaturated hard water (Yang et al. in Water Res 44:3659, 2010). The present study investigated possible pathways for the plasma-induced precipitation. Both UV and reactive species were found not the major factors that stimulate the precipitation in present setup. A transient hot-wire method was used to investigate the effect of plasma-induced local micro-heating. Approximately 15% drop in the calcium-ion concentration was observed, indicating that the local micro-heating effect could be the major contributing factor. Additionally, a nanosecond pulsed corona discharge was used to investigate the non-thermal effect of plasma, and a maximum 7% drop in the calcium hardness was observed. Calcite with rhombohedron morphology was observed in both cases, similar to the structure observed in the spark discharge treatment case, indicating that calcium-ion precipitation process could be attributed to both the thermal and non-thermal effects produced by the plasma.
Keywords: Physical water treatment; Pulsed spark discharge; Hard water; Calcium carbonate precipitation

Direct Oxidation of Methane to Methanol Over Cu-Based Catalyst in an AC Dielectric Barrier Discharge by Liang Huang; Xing-wang Zhang; Lin Chen; Le-cheng Lei (67-77).
In this paper, the conversion of methane to methanol on CuO/Al2O3 and Mo–CuO/Al2O3 catalysts in a plasma reactor was tested. A comparison between catalytic and plasma-catalytic systems had been made in tested temperature range of 50–300°C. Experimental results showed that plasma-catalytic system demonstrated a much better methane conversion than catalytic system in tested temperature range and Mo–CuO/Al2O3 revealed a higher catalytic activity than CuO/Al2O3 for methanol synthesis. Furthermore, an Arrhenius plot was made in order to deduce the mechanism of plasma activation, which revealed that the presence of plasma decreased the activation energy for both catalysts. In the case of Mo-CuO/Al2O3 catalyst, the enhanced activity for methanol synthesis was assumed due to the oxygen vacancies on Mo–CuO/Al2O3 catalyst, which can utilize plasma-induced species to improve the catalytic efficiency.
Keywords: Methane; Plasma-catalytic; Dielectric barrier discharge; Activation energy

Efficiency of CO2 Dissociation in a Radio-Frequency Discharge by Laura F. Spencer; Alec D. Gallimore (79-89).
One possible solution to mitigating the effects of high atmospheric concentrations of carbon dioxide (CO2) is the use of a plasma source to break apart the molecule into carbon monoxide (CO) and oxygen. This work experimentally investigates the efficiency of dissociation of CO2 in a 1-kW radio-frequency (rf) plasma source operating at 13.56-MHz in a low-pressure discharge. Mass spectrometry diagnostics are used to determine the species present in the discharge, and these measurements are used to calculate the energy efficiency and conversion efficiency of CO2 dissociation in the rf plasma source. Experimental results have found that the conversion efficiency of CO2 to CO can reach values near 90%, however energy efficiency reaches a maximum of 3%. A theoretical energy cost analysis is also given as a method to evaluate the effectiveness of any plasma system designed for CO2 emissions reduction.
Keywords: Carbon dioxide; Efficiency; rf plasma

New Process Simulation Procedure for High-Rate Plasma Jet Machining by Johannes Meister; Thomas Arnold (91-107).
Surface figuring using chemically reactive plasma jet machining (PJM) is a promising non-conventional technology for deterministic ultra-precision machining of optical components. Based on chemical reactions between plasma generated radicals and the surface atoms this technology is capable to fabricate complex shaped free form surfaces. Since the material removal rate during PJM depends strongly on the surface temperature which itself is influenced by the jet heat flux to the surface, the arising nonlinear effects on the etch result have to be regarded. Conventionally applied dwell time calculation algorithms do not consider those effects leading to significant machining errors in some cases. In order to improve the machining procedure with respect to deterministic material removal yielding predictable results a process simulation model has been developed. This model considers spatio-temporal variations of surface temperature and temperature dependent material removal and is able to predict the final workpiece topography after machining.
Keywords: Plasma jet machining; Optical manufacturing; Numerical simulation; Plasma etching; Temperature dependency

An optical emission spectroscopy (OES) method has been used to measure the concentration above the melt of some radioelements, 137Cs, 60Co and 106Ru during the vitrification of radioactive wastes by thermal plasma. This method allows the study of non homogeneous optically thin plasmas exhibiting a symmetry plane without sophisticated tomographic systems. Local plasma temperatures above the melt have been evaluated using measured relative intensities of spectral lines of the plasma-forming gas. Radioelement concentrations in the plasma are deduced from the intensity ratio of the radioelement–gas spectral lines. A computer model is used to simulate the volatility of 137Cs, 60Co and 106Ru, during the treatment of radioactive wastes by thermal plasma. This model is based on the calculation of system composition using the free enthalpy minimization method, coupled with the equation of mass transfer at the reactional interface. In this study, the OES measurements and the computer model allowed the determination of this radioelement volatility depending on the parameters like plasma current, and the nature of atmosphere furnace. The results of the model are compared to those obtained by OES measurements.
Keywords: Radioactive wastes; Volatility; Thermal plasma; OES diagnostic; Modeling and simulation

Direct Measurement of the Gas Entrainment Into a Turbulent Thermal Plasma Jet by Hai-Xing Wang; Fu-Zhi Wei; Xian Meng; Xi Chen; Dong-Sheng Han; Wen Xia Pan (127-138).
An experimental study is conducted to investigate the entrainment characteristics of a turbulent thermal plasma jet issuing from a DC arc plasma torch operating at atmospheric pressure. The mass flow rate of the ambient gas entrained into the turbulent plasma jet is directly measured by use of the so-called “porous-wall chamber” technique. It is shown that a large amount of ambient gas is entrained into the turbulent plasma jet. With the increase of the gas mass flow rate at the plasma jet inlet or the plasma torch exit, the mass flow rate of entrained ambient gas almost linearly increases but its ratio to the jet-inlet mass flow rate decreases. The mass flow rate of the entrained gas increases with the increase of the arc current or jet length. It is also found that using different ways to inject the plasma-forming gas into the plasma torch affects the entrainment rate of the turbulent plasma jet. The entrainment rate expression established previously by Ricou and Spalding (J. Fluid Mech. 11: 21, 1961) for the turbulent isothermal jets has been used to correlate the experimental data of the entrainment rates of the turbulent thermal plasma jet, and the entrainment coefficient in the entrainment rate expression is found to be in range from 0.40 to 0.47 for the turbulent thermal plasma jet under study.
Keywords: Turbulent plasma jet; Gas entrainment; Entrainment rate measurement; Experimental study

The effect of varying process parameters on atmospheric plasma characteristics and properties of nanometre thick siloxane coatings is investigated in a reel-to-reel deposition process. Varying plasma operation modes were observed with increasing applied power for helium and helium/oxygen plasmas. The electrical and optical behaviour of the dielectric barrier discharge were determined from current/voltage, emission spectroscopy and time resolved light emission measurements. As applied power increased, multiple discharge events occurred, producing a uniform multi-peak pseudoglow discharge, resulting in an increase in the discharge gas temperature. The effects of different operating modes on coating oxidation and growth rates were examined by injecting hexamethyldisiloxane liquid precursor into the chamber under varying operating conditions. A quenching effect on the plasma was observed, causing a decrease in plasma input power and emission intensity. Siloxane coatings deposited in helium plasmas had a higher organic component and higher growth rates than those deposited in helium/oxygen plasmas.
Keywords: Atmospheric pressure plasma; Plasma diagnostics; Aerosol precursor; Thin films

Mass spectrometry has been used to assess plasma composition during a low-energy plasma-enhanced chemical vapor deposition (LEPECVD) process using argon-silane-hydrogen (Ar-SiH4-H2) gas mixtures with input flows of 50 sccm Ar, 2–20 sccm SiH4 and 0–50 sccm H2 at total pressures of 1–4 Pa. Energy-integrated ion densities, residual gas analysis and threshold ionization mass spectrometry have been used to characterize the transition from amorphous (a-Si) to nano-crystalline silicon (nc-Si) deposition at constant LEPECVD operating parameters. While relative ion densities have a marked decrease with H2 input, the densities of SiHx (x < 4) radicals show evolution trends depending on the SiH4 and H2 input. For conditions leading to nc-Si growth a turning point is reached above which SiH is the main radical. Observed SiHx density trends with H2 input are explained based on kinetic reaction rates calculated from previously obtained Langmuir probe data.
Keywords: Mass spectrometry; Silane; nc-Si; Hydrogen dilution; LEPECVD

Anti-Fog Layer Deposition onto Polymer Materials: A Multi-Step Approach by L. Maechler; C. Sarra-Bournet; P. Chevallier; N. Gherardi; G. Laroche (175-187).
A coating with anti-fog properties was developed for a polycarbonate (PC) substrate using a multi-step process. A silicon-containing multilayer was first deposited by atmospheric pressure Townsend discharge (APTD) using hexamethyldisiloxane (HMDSO) precursor to ensure cohesion between the coating and the PC and to protect the polymer against dissolution in solvents during subsequent spin-coating processes. The multilayer was also developed in a nitrogen environment in order to provide amino groups on the surface, which allowed for the subsequent covalent bonding of the anti-fog layer. This anti-fog layer was made of a spin-coated (poly(ethylene-maleic anhydride) (PEMA) layer, used as a linking arm, followed by a spin-coated poly(vinyl alcohol) (PVA) layer, for its anti-fog properties. Surface analyses (XPS, FTIR, and AFM) confirmed that the silicon-containing multilayer was successfully deposited with a good control obtained for each step. The anti-fog coating, also tested with promising results, displayed a negligible loss of optical transmission across the visible spectrum.
Keywords: Plasma deposition; Atmospheric pressure townsend discharge (APTD); Hexamethyldisiloxane (HMDSO); Anti-fog properties; Poly(ethylene-maleic anhydride); Poly(vinyl alcohol)

Interaction Mechanisms Between Ar–O2 Post-Discharge and Stearic Acid I: Behaviour of Thin Films by E. A. Bernardelli; T. Belmonte; D. Duday; G. Frache; F. Poncin-Epaillard; C. Noël; P. Choquet; H.-N. Migeon; A. Maliska (189-203).
Interactions between a late Ar–O2 post-discharge and thin films (~1 − 2 μm) of stearic acid (SA), a C17 aliphatic chain with as end-group a carboxylic acid function, are studied. Thin films grown by evaporation are made of separated droplets and are efficiently etched by the post-discharge to get a clean surface after treatment. The dewetting of the droplets during the first minutes of the treatment leads first to a more homogeneous film in thickness which is etched until isolated islands appear and progressively shrink as a function of time. The etching of the SA occurs via C–C bonds scission when peroxide or hydroperoxide compounds are synthesized from radicals initiated by reaction with atomic oxygen from the post-discharge. A strong functionalization by OH groups grafted on the aliphatic skeleton of the SA is also observed. C = O formation is observed for longer treatment times possibly due to oxidation of OH grafted on the aliphatic chain or to a modification of the branching ratio of the C–C bond scission mechanism due a temperature rise. The acid functional group is likely not modified by the treatment.
Keywords: Stearic acid; Plasma cleaning; Post-discharge

Interaction Mechanisms Between Ar–O2 Post-Discharge and Stearic Acid II: Behaviour of Thick Films by E. A. Bernardelli; T. Belmonte; D. Duday; G. Frache; F. Poncin-Epaillard; C. Noël; P. Choquet; H.-N. Migeon; A. Maliska (205-215).
We study the interaction of thick films (~4 mm) of stearic acid (SA), a C18 alkane skeleton with an acid function, with late Ar–O2 post-discharge. Contrary to what is observed with thin films of SA (~2–3 μm) which are efficiently etched (part I), only functionalization is observed over the first 2 h of treatment with a plasma source operated in the continuous mode, whatever the temperature. The heat released by surface reactions affects non-linearly the temperature of the substrate. Pulsing the source at a frequency ranging from 0.1 to 1 kHz slows down the functionalization process but does not allow any etching of the material. On the contrary, the SA can be etched as thick films by pulsing the oxygen flow rate at a frequency below 50 mHz. By pulsing the reactive gas, the time averaged value of the [O]/[O2] ratio is decreased, limiting the functionalization processes due to oxygen atoms, and the mean temperature is lowered, decreasing the diffusion length of O2 (and/or possibly O2*) species in the SA which are responsible for the scission of C–C bonds of radicals.
Keywords: Stearic acid; Plasma cleaning; Post-discharge

Film Chemistry Control and Growth Kinetics of Pulsed Plasma-Polymerized Aniline by Dominique Debarnot; Tiphaine Mérian; Fabienne Poncin-Epaillard (217-231).
The polymerization of aniline under continuous and pulsed RF-plasma conditions is studied using the same plasma reactor. The effects of input power, on and off-times, frequency and duty cycle variations on the growth kinetics and on the chemical structure of the obtained layers are examined. The chemical structure of the films is characterized using Fourier Transform Infra-Red, X-ray photoelectron and UltraViolet–Visible spectroscopies. The thickness of the films is determined by profilometry. The results show a strong dependence of film chemistry and deposition rates on the discharge power and on-time. The film deposited by pulsed plasma grows mainly during the plasma-on period. Furthermore, this work shows that the retention of aromatic rings can be evaluated by Fourier transform infrared analysis whereas oxidation degree of plasma polyaniline can be determined by X-ray photoelectron and UV–Vis spectroscopies.
Keywords: Pulsed discharge; Plasma polymerization; Aniline; Kinetics; Ring retention

Surface Characterization and Antimicrobial Activity of Chitosan-Deposited DBD Plasma-Modified Woven PET Surface by Pannee Sophonvachiraporn; Ratana Rujiravanit; Thammanoon Sreethawong; Seiichi Tokura; Sumaeth Chavadej (233-249).
In this work, a woven PET with an antimicrobial activity was prepared by depositing chitosan on its surface. Firstly, the hydrophilic property of the PET surface was achieved by a plasma treatment using dielectric barrier discharge (DBD). The hydrophilic property of the PET surface was characterized by wickability and contact angle measurements. The XPS analysis revealed an increment of oxygen-containing polar groups, such as C–O and O–C=O, on the PET surface after the plasma treatment, resulting in an enhanced hydrophilic property. The plasma-treated PET specimen was further deposited with chitosan by immersing in a chitosan acetate aqueous solution. The effects of temperature, chitosan concentration, and number of rinses on the amount of deposited chitosan on the PET surface were investigated. The disappearance of the above-mentioned polar groups from the PET surface was clearly observed after the chitosan deposition, indicating the involvement of these functional groups in interacting with the chitosan. The chitosan-deposited plasma-treated woven PET possessed an exceptionally high antimicrobial activity against both E. coli (gram-negative bacteria) and S. aureus (gram-positive bacteria).
Keywords: Dielectric barrier discharge; Hydrophilic property; Woven PET; Chitosan deposition; Antimicrobial activity