Plasma Chemistry and Plasma Processing (v.32, #6)

Cold atmospheric plasma is considered to be a promising approach for decontamination purposes, e.g. dyeing water decoloration. In order to better understand the complex mechanism of the plasma physics coupled with the plasma chemistry involved in the interaction of the polluted water with the discharge plasma, a novel approach was proposed to study the in situ oxidation process between the plasma and liquid phase in two dielectric barrier discharge (DBD) plasma reactors with different bottom shape (concave vs. plane), by using the planar laser induced fluorescence technique to visualize the process dynamics. Rhodamine B was employed as the tracer dye, which was gradually decomposed by the combined effect of the chemically active radicals (OH, O, H2O2, etc.) as well as the intense UV radiation in the DBD plasma process. The results showed that the DBD plasma filaments induced certain fluctuation on the Rhodamine B liquid layer, which accordingly intensified the mass transfer to a large extent thus accelerated the oxidation process. The comparison of the measured concentration fields in the two DBD plasma reactors illustrated that the DBD reactor #1 with concave bottom showed higher oxidation efficiency than the DBD reactor #2 with plane bottom. Additionally, the experiments demonstrated that the oxidation efficiency in the DBD plasma water treatment was much better than that in the reactor with pure oxidation by ozone gas, which can be further improved by injecting the additional oxygen gas bubbles into the liquid phase in the plasma reactor.
Keywords: Dielectric barrier discharge (DBD); Plasma–liquid interaction; Oxidation process; Planar laser induced fluorescence (PLIF); Visualization

Wet Conversion of Methane and Carbon Dioxide in a DBD Reactor by Torsten Kolb; Thorsten Kroker; Jan H. Voigt; Karl-Heinz Gericke (1139-1155).
The influence of water on the plasma assisted conversion of methane and carbon dioxide in a dielectric barrier discharge (DBD) plug flow reactor was studied. The plasma at atmospheric pressure was ignited by a power supply at a frequency of 13.56 MHz. Product formation was studied at a power range between 35 and 70 W. The concentrations of the three gases were altered and diluted with helium to 3 %. FTIR spectroscopy and mass spectroscopy were applied to analyze the inlet and the product streams. The main product of this process are hydrogen, carbon monoxide and ethane. Ethene, ethine, methanol and formaldehyde are generated beside the main products in this DBD in lower concentrations. The conversion of methane, the ratio of the synthesis gas components (n(H2):n(CO)), and the yield of oxygenated hydrocarbons and hydrogen increases by adding water. The total consumed energy reaches lower values for small amounts of water. Additional water does not influence the generated amount of C2 hydrocarbons and of CO, but decreases the carbon dioxide conversion.
Keywords: Dielectric barrier discharge; Cold plasma; Water; Methane; Carbon dioxide

Methane Conversion to Hydrogen and Carbon Black by DC-Spark Discharge by Mohammad Mahdi Moshrefi; Fariborz Rashidi; Hamid Reza Bozorgzadeh; Seyed Majid Zekordi (1157-1168).
A continuous production of hydrogen and carbon black from methane without CO2 emission in atmospheric pressure has been investigated by non thermal decomposition of methane using a system of direct current (DC)-spark discharge plasma, which has great advantages over other systems, like thermal plasma or catalytic conversion of methane in H2 production. A plasma reactor with specific design of electrodes was employed to examine the reactor performance regarding operating conditions such as feed flow rate, input power and electrodes distance. The experimental results showed that designed reactor increases not only the concentration of the produced hydrogen in continues condition but also guarantees stable plasma. As the methane supply rate increased, the hydrogen concentration decreased but on the other hand the hydrogen volume flow rate increased. In general, under the specified operating condition (power = 21 W and methane flow rate = 150 ml/min), the plasma converter produced a hydrogen concentration of 45 % at hydrogen volume flow rate of 75 ml/min.
Keywords: Hydrogen; Methane; Spark discharge; Rotating electrode

Ethylene Epoxidation in Cylindrical Dielectric Barrier Discharge: Effects of Separate Ethylene/Oxygen Feed by Thitiporn Suttikul; Chakrit Tongurai; Hidetoshi Sekiguchi; Sumaeth Chavadej (1169-1188).
The effects of separate C2H4/O2 feed and C2H4 feed position on the ethylene epoxidation reaction in an AC cylindrical dielectric barrier discharge reactor were investigated. The highest EO selectivity of 34 % and EO yield of 7.5 %, as well as the lowest power consumption of 1.72 × 10−16 Ws/molecule of EO produced, were obtained at a C2H4 feed position of 0.25, an O2/C2H4 feed molar ratio of 1/4, an applied voltage of 13 kV, an input frequency of 550 Hz, and a total feed flow rate of 75 cm3/min. The results demonstrated, for the first time, that the separate feed of C2H4 and O2 could provide better ethylene epoxidation performance in terms of higher EO selectivity and yield, and lower power consumption, as compared to the mixed feed. All undesired reactions including C2H4 cracking, dehydrogenation, oxidation, and coupling reactions are lowered by the ethylene separate feed because of a decrease in opportunity of ethylene molecules to be activated by generated electrons.
Keywords: Epoxidation; Ethylene oxide; Dielectric barrier discharge; Feed position

Effect of Reactor Structure in DBD for Nonthermal Plasma Processing of NO in N2 at Ambient Temperature by Tao Wang; Bao-Min Sun; Hai-Ping Xiao; Ju-ying Zeng; Er-peng Duan; Jing Xin; Chun Li (1189-1201).
In order to improve dielectric barrier discharge reactor structure, an experimental study was carried out into its effect on nitric oxide (NO) removal. Different structures were distinguished by electrode connection, diameter, material and shape of inner electrode, and dielectric material, respectively. The results show that breakdown voltage is lower when a high voltage is applied to the outer electrode in the coaxial reactor; NO removal efficiency decreases with a smaller inner electrode diameter and increases when tungsten is used as the inner electrode material rather than copper or stainless steel. When the inner electrode is screw shaped, it improves the discharge power and NO removal efficiency. A similar higher NO removal efficiency is found when corundum is used as the dielectric material instead of ceramic or quartz. Therefore, these findings should become the basis for modifications to the electrode structure in order to improve NO removal efficiency.
Keywords: Dielectric barrier discharge; Reactor structure; NO removal; Nonthermal plasma

Experimental Study of Liquid Hydrocarbons Pyrolysis to Acetylene in H2/Ar Plasma by Binhang Yan; Pengcheng Xu; Xuan Li; Cliff Yi Guo; Yong Jin; Yi Cheng (1203-1214).
Liquid hydrocarbons including n-hexane, cyclohexane and toluene are pyrolyzed in H2/Ar plasma to investigate the effects of feedstock properties and key operating conditions (e.g., the feedstock specific input power and residence time) on the reaction performance. The experiments verify that the non-aromatic hydrocarbons show better chemical reactivity than partially aromatic substances. Meanwhile, the straight-chain alkanes and cycloalkanes have better yields of ethylene during the pyrolysis. The results also demonstrate that the pyrolysis reactions are almost completed within the first 0.8 ms in Ar/H2 plasma independent of the feed substances (i.e., liquid hydrocarbons), where the increased feedstock specific input power enhances the reactant conversions and correspondingly raises the yields of acetylene. At a feedstock specific input power of 4.7 × 104 kJ/kg, the n-hexane conversion is over 90 % and the yield of acetylene reaches 70 %. In addition, when using n-hexane as the feedstock, very little coke is formed during the course of reaction. Comprehensive comparisons of the current experiments with the data reported in the literature are made to point out the key influencing factors, i.e., the effective mass ratio of C/H (R C/H) in the gaseous phase and the quench temperature. Both two factors would need to be enhanced in order to get a better performance. Finally, the improvements on the specific energy requirement of this process are discussed.
Keywords: Thermal plasma; Liquid hydrocarbons; Pyrolysis; Acetylene; Specific energy requirement (SER)

The plasma enhanced chemical vapour deposition method applying atmospheric dielectric barrier discharge (ADBD) plasma was used for TiOx thin films deposition employing titanium (IV) isopropoxide and oxygen as reactants, and argon as a working gas. ADBD was operated in the filamentary mode. The films were deposited on glass. The films′ chemical composition, surface topography, wettability and aging were analysed, particularly the dependence between precursor and reactant concentration in the discharge atmosphere and its impact on TiOx films properties. Titanium in films near the surface area was oxidized, the dominating species being TiO2 and substoichiometric titanium oxides. The films exhibited contamination with carbon, as a result of atmospheric oxygen and carbon dioxide reactions with radicals in films. No relevant difference of the film surface due to oxygen concentration inside the reactor was determined. The films were hydrophilic immediately after deposition, afterwards their wettability diminished, due to chemical reactions of the film surface and chemical groups involved in the atmosphere.
Keywords: Atmospheric dielectric barrier discharge; Chemical composition; Plasma enhanced chemical vapour deposition; Thin film; Titanium isopropoxide (TTIP); TiOx

Melting Refining Mechanisms in Supersonic Atmospheric Plasma Spraying by Wei Tao Zhao; Jiu Hui Wu; Yu Bai; Zhi Hai Han (1227-1242).
In recent years, Yttria-stabilized zirconia based thermal barrier coatings (TBCs) are deposited by newly-developed high-efficiency supersonic atmospheric plasma spraying (SAPS) technology. The final microstructure of the plasma-sprayed coatings is strongly dependent on the size distribution of spray particles. It has been corroborated through experiments that there is a special phenomenon of particle melting refining in SAPS, as compared with the conventional atmospheric plasma spraying (APS). This phenomenon greatly affects the final particle size and distribution, which has not been explained reasonably up to now. Therefore, it is necessary to investigate the melting refining behavior of in-flight particles to control the particle size and to analyze the coating properties. In this paper, the breakup of particle is presented to characterize the phenomenon of particle melting refining, and the peak of size distribution becomes bigger with increasing the spray distances, which is explained by collision-coalescence. Furthermore, based on the maximum entropy formalism, the particle-size distribution is calculated and the result is in good accordance with the plasma spraying experiment results, which verifies the mechanism analysis presented in this paper. This work could provide more efficient applications of the SAPS technology in high-performance TBCs.
Keywords: Supersonic atmospheric plasma spraying; Melting refining phenomenon; Breakup of particles; Collision-coalescence; Particle-size distribution

The long-distance and dynamic characteristics of low-temperature plasma in a state of flow were studied by observing the glow distance of plasma and detecting the peroxide content formed on the polymer membrane after irradiation. The field emission scanning electron microscopy and accurate weight loss analysis were respectively employed to observe the decreased etching effect of long-distance plasma on uniaxially stretched poly(tetrafluoroethylene) membranes. The effects of plasma excitation time, plasma power, gas flowrate and chamber pressure on the peroxide contents were investigated using polypropylene fiber membranes. The dynamic characteristic of plasma was furthermore discussed by exploring the penetration of plasma flow through membrane pores. The dynamic plasma flow not only exhibited a longer glow distance than static plasma, but also can obtain greater peroxide contents, and even still can penetrate the first membrane and further treat the second membrane layer. The etching and degradation effects of membrane can be significantly reduced in a long-distance and dynamic low-temperature plasma (LDDLTP) flow. The safe distance to minimize the plasma damage to the membrane depends greatly on the plasma conditions. The long-distance acts and complete penetration performances of LDDLTP we have achieved could provide valuable references for membrane modifications in more flexible forms.
Keywords: Low-temperature plasma; Long-distance and dynamic; Glow distance; Peroxide; Polymer membranes

Functionalization of Cotton by In-Situ Reaction of Styrene in Atmospheric Pressure Plasma Zone by Dambarudhar Parida; Manjeet Jassal; Ashwini K. Agarwal (1259-1274).
Cotton fabric was treated using styrene/helium glow plasma at the atmospheric pressure. After the treatment, the substrate was found to turn into a highly hydrophobic material showing water drop disappearance time of 60 min and water contact angle of 133°. The treatment was found to be durable even after vigorous washing. The effect of various parameters, such as discharge voltage and frequency, on fragmentation of styrene inside plasma zone was investigated using optical emission spectroscopy and GC–MS. The types of fragments formed in the plasma zone were correlated with the hydrophobicity of the substrate. The chemical nature of surface of the substrate was analyzed using SEM and Raman spectroscopy to elucidate the possible mechanism of plasma modification.
Keywords: Atmospheric pressure glow plasma; Hydrophobicity; Mechanism; Low frequency plasma

The natural rubber latex (NRL) film taken from medical surgical gloves was surface-modified with a dielectric barrier discharge (DBD) plasma treatment under an air environment. The results showed that surface hydrophilicity of the NRL film increased after the plasma treatment due to the presence of oxygen-containing polar groups on the plasma-treated surface. An increase in plasma treatment time increased the surface roughness of the NRL film, and eventually decreased the mechanical properties. From the obtained results, the optimum plasma treatment time of 20 s was chosen. After immersion in a chitosan solution, the amount of chitosan deposited on the plasma-treated NRL film increased with increasing chitosan concentrations. The chitosan coating smoothed the surface of the plasma-treated NRL film and also improved the mechanical properties. The highest antibacterial activities of the chitosan-coated DBD plasma-treated NRL film against both Staphylococcus aureus and Escherichia coli were achieved when a 2 %(w/v) chitosan solution was used for the coating.
Keywords: Dielectric barrier discharge; Plasma treatment; Natural rubber latex; Chitosan; Antibacterial activities

Effect of Radiofrequency Plasma Assisted Grafting of Polypropylene on the Properties of Muga Silk Yarn by D. Gogoi; J. Chutia; A. J. Choudhury; A. R. Pal; N. N. Dass; D. S. Patil (1293-1306).
Radiofrequency (RF) Ar/propylene glow discharge is utilized for grafting polypropylene onto muga silk yarn at working pressure of 1.2 × 10−1 mbar and in the of RF power range of 20–80 W. The plasma discharge is diagnosed using self-compensated emissive probe to study the variation of ion energy impinging on the substrates with RF power. From chemical compositional analysis, a possible grafting mechanism between propylene and muga yarn is proposed by considering the charge-transfer initiation through the formation of electron-donor–acceptor (EDA) complex. X-ray photoelectron spectroscopy reveals that at RF power values of 60–80 W, the ion sputtering effect becomes dominant over plasma grafting thereby leading to severe destruction in chemical structure of the polypropylene grafted (PP-grafted) muga yarns. The experimental results show that PP-grafted muga yarns exhibit improved mechanical strength and hydrophobic behavior as compared to the virgin yarn. The properties of the PP-grafted muga yarns are observed to be dependent on atomic concentration, surface morphology as well as the results obtained from plasma discharge characteristics.
Keywords: Muga silk yarn; Surface modification; Radiofrequency plasma assisted grafting; Polypropylene; Plasma diagnostics