Plasma Chemistry and Plasma Processing (v.35, #4)

Three-Phase AC Arc Plasma Systems: A Review by Laurent Fulcheri; Fréderic Fabry; Sabri Takali; Vandad Rohani (565-585).
Most arc plasma systems are based on DC plasma technologies. However AC plasma systems can offer significant advantages versus DC plasma systems particularly in terms of efficiency, cost and reliability. They are also likely to overcome some of the limits of classical DC systems for some specific large scale high power applications. This paper presents a literature review of three-phase AC plasma systems which have been developed by the most active research groups on multi-phase AC plasma systems in the United States, Norway, Germany, Russia, France, and Japan for about 50 years.
Keywords: Three-phase AC plasma; Electric arc

Thermal Ablation of Stabilized Zirconia/Metal Coated Polyimide Matrix Composites Via Plasma Spray Process by Wenzhi Huang; Haifeng Cheng; Chaoyang Zhang; Yongjiang Zhou; Xueqiang Cao (587-603).
Thermal protection coating systems consisting of different bond coats and stabilized zirconia were fabricated on the silica glass fiber reinforced polyimide matrix composites via air plasma spray process. Influence of the different bond coats on thermal ablation resistance of the coating system was investigated. The weight loss of the sample decreased from (7.01 ± 0.42) to (1.93 ± 0.16) %, while the corresponding mass ablation rate of the sample was reduced from 0.48 ± 0.02 to 0.22 ± 0.02 mg s−1, which was attributed to the protection of the coating system with Zn interlayer and partially stabilized zirconia as top layer. After thermal ablation test, failure modes of these coatings were the formation of the cracks or the delamination in the metallic interlayers. Residual stress, thermal stress and further oxidation of the substrate were responsible for the final failure of the coatings.
Keywords: Plasma spray; Thermal ablation resistance; Residual stress; Failure

Assuming a seven species model of nitrogen plasma, thermodynamic and transport properties are calculated under thermal equilibrium and non-equilibrium conditions. Species densities, mass densities, specific heat, enthalpy, viscosity, thermal conductivity, collision frequency, and electrical conductivity are studied as a function of temperature, pressure, and different degrees of thermal and chemical non-equilibrium. Effect of both electrons and ions are considered in the shielding for charged–charged interactions under shield coulomb potential and obtained results are found to be in excellent agreement with experimentally obtained results, especially at higher temperatures. Accounting up to third ionized states, results are presented with temperatures ranging from 300 to 50,000 K, the ratio of electron temperature (Te) to the heavy particle temperature (Th) ranging from 1 to 20 and the pressure ranging from 0.1 to 7 atmospheres. Under atmospheric pressure, results obtained for thermodynamic equilibrium (Te = Th) are compared with number of published results under similar conditions. Computed properties under different degrees of chemical non-equilibrium are also discussed.
Keywords: Nitrogen plasmas; Thermodynamic and transport properties; Thermal non-equilibrium; Chemical non-equilibrium

Physical Parameters and Chemical Composition of a Nitrogen DC Discharge with Water Cathode by Sergey A. Smirnov; Dmitriy A. Shutov; Elena S. Bobkova; Vladimir V. Rybkin (639-657).
This paper reports the results of the experimental study and chemical composition modeling for a DC nitrogen discharge burning above water cathode in the pressure range of 0.1–1 bar and at discharge current of 40 mA. The gas temperature, vibrational temperatures, reduced electric field strength, cathode voltage drop, and emission intensities of some nitrogen bands were obtained from experiment. The modeling chemical composition of plasma was carried out on the basis of these data. At modeling, the combined solution of Boltzmann equation for electrons, equations of vibrational kinetics for ground states of N2, O2, H2O and NO molecules, equations of chemical kinetics and plasma conductivity equation were used. The calculations agree with the measured bands intensities for the second positive system of N2 and vibrational temperatures of $${ ext{N}}_{2} ( { ext{C}}^{3}Pi _{ ext{u}} )$$ N 2 ( C 3 Π u ) . In the frame of the model proposed, the data of other studies were explained. The second kind collision of electrons with N2 vibration excited states was shown to affect strongly the electron gas parameters. The electron average energy and electron density are given. The difference between the properties of the discharges in N2 and air at the same conditions are discussed as well.
Keywords: DC nitrogen discharge; Water cathode; Modeling; Plasma composition; Electron parameters

Effect of Low-Temperature Plasma on the Structure of Seeds, Growth and Metabolism of Endogenous Phytohormones in Pea (Pisum sativum L.) by Tibor Stolárik; Mária Henselová; Michal Martinka; Ondřej Novák; Anna Zahoranová; Mirko Černák (659-676).
The objective of this study was to determine the influence of low-temperature plasma (LTP) on seed surface modification, water uptake by seeds, seed germination and vigor of seedlings, as well as changes in the content of endogenous hormones in pea, (Pisum sativum L. var. Prophet). The study’s authors used diffuse coplanar surface barrier discharge as the source of LTP in various duration times of treatment (from 60 to 600 s). The SEM analysis showed that LTP induced significant changes on the seeds’ surface, which was related to water permeability into the seeds. LTP increased the germination percentage of pea seeds as well as the growth parameters (root and shoot length, dry weight), and the vigor of seedlings and the effects of LTP also depended on exposure time. The LTP-pretreatment produced changes in endogenous hormones (auxins and cytokinins and their catabolites and conjugates), which correlated with increased growth of the pea seedlings. The results suggested an interaction among the modification of seed structure demonstrated by LTP in the induction of faster germination and hormonal activities related to plant signaling and development during the early growth of pea seedlings.
Keywords: Endogenous hormones; Germination; Growth parameters; Low-temperature plasma; Pea seed structure

Influence of Precursor Functionality on In Situ Reaction Dynamics in Atmospheric Pressure Plasma by Prasanta Kumar Panda; Manjeet Jassal; Ashwini K. Agrawal (677-695).
In situ reaction of various long chain hydrocarbon precursors with cellulosic substrate inside plasma zone were investigated. Each of the selected precursor compounds had C12 alkyl chain as the hydrophobic moiety to impart hydrophobicity to the substrate and a functional group to aid reaction with the active sites of the substrate. The selected compounds were dodecyl acrylate (DA, with acrylate group), lauryl alcohol (LA, with hydroxyl group), dodecanoic acid (DODAC, with carboxylic acid) and dodecane (DD, without any functional group). Treatment was carried out in-situ with these compounds using helium gas as a carrier. After the treatment, durable hydrophobicity was obtained in all cases with water absorbency time of substrate increasing to >1 h in the case of DA and LA, up to 20 min in the case of DD and only up to 9 min in the case of DODAC. The functional groups were found to have profound effect on the extent of functionality obtained and the plasma parameters required to obtain effective reaction with the substrate. Both plasma zone and the functionalized surfaces were characterized to understand the chemistry involved with the different precursors.
Keywords: DBD plasma; Hydrophobic; Cellulose; Hydrocarbon compound; Viscose

Surface Plasmas Versus Volume Plasma: Energy Deposition and Ozone Generation in Air and Oxygen by Muhammad Arif Malik; David Hughes; Richard Heller; Karl H. Schoenbach (697-704).
The energy deposition and the ozone generation in a shielded sliding discharge were compared with those of a simple surface discharge and a pulsed corona discharge in atmospheric pressure air and oxygen. All discharges were generated by applying 160 ns, high voltage pulses. Under the same conditions, the highest energy deposition per pulse was obtained with the shielded sliding discharge. The energy deposition was lower by a factor of four for the surface discharge plasma and by an order of magnitude for the pulsed corona discharge plasma. Replacing air with the more electronegative oxygen caused a decrease in the deposited energy due to electron attachment. The threshold voltage for plasma formation in oxygen in a shielded sliding discharge was approximately 5 kV, three times less than that of the surface discharge (≥15 kV) and four times less than that of the pulsed corona discharge (≥20 kV). A new finding of this study is that, whereas the decrease in energy in the pulsed corona discharge was ≥50 %, and that of the simple surface discharge ≥40 %, it was negligible in the shielded sliding discharge. Since the ozone generation scales with energy density, the results show that plasma reactors based on the nanosecond sliding discharge principle have major advantages in compactness, ignition voltage, and in the use of oxygen, rather than air, compared to surface discharges and corona discharges.
Keywords: Nonthermal plasma; Electronegative gas; Ozone; Shielded sliding discharge; Pulsed corona discharge; Sliding discharge

Ozone production using a hollow-needle-to-mesh negative corona discharge, with the needle electrode inserted in a dielectric tube, has been investigated. The air to the discharge could be supplied either through the needle electrode or reversely through the mesh electrode, which could additionally be covered with globules of TiO2 photocatalyst. The effects of these parameters on the electrical characteristics of the discharge, on ozone production and on the transition of the discharge from the glow to the streamer regime, have all been investigated. We found that reverse air supply substantially affects the electrical characteristics of the discharge when the needle electrode is inserted in the dielectric tube. Placement of TiO2 globules on the mesh decreases discharge voltage for a simple needle as well as for all positions of the needle inside the dielectric tube. The best performance from the standpoint of discharge ozone generation showed discharge with a supply of air through the needle electrode inserted in the dielectric tube with TiO2 globules on the mesh electrode.
Keywords: Corona discharge; Ozone; Dielectric tube; Photocatalyst; Airflow

An Integrated Approach to Understanding the Mass Transfer and Reaction Processes in a Pulsed High-Voltage Discharge Reactor by Mengdan Zeng; Kang Zhao; Yang Lu; Yongjian Ouyang; Deqi Liu; Mei Wang; Yimei Ma (721-738).
The effect of pulsed high-voltage discharge on mass transfer and reaction processes in a needle-plate reactor was studied. Bubble size distributions and characteristic parameters were determined by photography, implying that inlet air bubbles could be broken into smaller ones effectively by discharge. The mass transfer parameters including specific interfacial area (a), volumetric mass transfer coefficient (k L a), and liquid-side mass transfer coefficient (k L) were obtained by Danckwerts-plot, reflecting the mass transfer processes. In addition, 4-chlorophenol (4-CP) decomposition was determined under the same experimental conditions. The results showed that the conclusions of 4-CP removal were coincided with the statistical results of bubble images, illustrating that higher peak voltage, pulsed frequency and appropriate air flow rate are beneficial to mass transfer and reaction. However, there was a slight difference between 4-CP removal and mass transfer parameters at higher air flow rate. Therefore, it is more reasonable to optimize the operating conditions of a pulsed high-voltage discharge reactor via combining image acquisition and mass transfer parameters with the results of pollutant degradation.
Keywords: Pulsed high-voltage discharge; Plasma; Bubble size distribution; Gas–liquid mass transfer process; 4-chlorophenol decomposition

This paper reports on polymerization of hexamethyldisiloxane (HMDSO) using an atmospheric pressure dielectric barrier discharge plasma jet. The aim of the study is to contribute to the knowledge of thin film deposition using a low cost technique of atmospheric pressure plasma. The monomer HMDSO was used as a precursor for polymerization. The discharge was powered using a laboratory made resonant power supply working with sinusoidal voltage signal at a frequency of 8 kHz. The coatings were characterized using Fourier transform infrared spectroscopy, atomic force microscopy, growth rates and surface free energy measurements. The hydrophobic nature of the films was found to be decreased with increasing the plasma power. Fourier transform infrared spectroscopy gave an indication of the dominated inorganic content of the surface at higher discharge. An average growth rate of 220 nm min−1 was achieved at a monomer flow rate of 5 sccm and discharge power of 12.5 W. The films obtained using plasma jet were found to be stable in aqueous media and well adhered with substrate.
Keywords: Atmospheric pressure plasma jet; Glass substrate; Polymerization; Surface properties

Plasma Functionalization of Carbon Nanofibers with Vapors of Ammonia/Water by Ernesto Hernández-Hernández; María G. Neira-Velázquez; Hilda Guerrero-Alvarado; José F. Hernández-Gámez; Pablo González-Morones; Carlos A. Ávila-Orta; Yibran A. Perera-Mercado; José J. Borjas-Ramos; Marissa Pérez-Álvarez; Anna D. Iliná; Pascual Bartolo-Pérez (757-768).
The plasma treatment and characterization of carbon nanofibers (CNFs) using vapors of ammonia/water (NH3/H2O) is reported. CNFs were treated at an input power of 20 and 40 W for 60 min using an inductive RF plasma of 13.56 MHz. During the plasma treatment, the ions were measured in situ using an optical emission spectrometer. Changes in the chemical composition, morphology and percent of functional groups grafted onto the CNFs were studied by infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and thermogravimetrical analysis. As a result of the treatment, a higher oxygen and nitrogen concentration on the surface of CNFs has been obtained, resulting in a good dispersion of the treated CNFs in water and acetone. The emission spectra indicated the formation of ions of CO, which are associated with the purification of the CNFs as indicated by an increase of the sp2/sp3 ratio from 4.3 to 7.5. The fibrillar structure of the treated CNFs was not damaged by the NH3/H2O plasma treatment.
Keywords: Carbon nanofibers; Plasma treatment; Functionalization

Preparation and Characterization of Sulfurized Tungsten Doped Non-hydrogenated Diamond-Like Carbon Films by Zhimin Liu; Wen Yue; Song Wang; Zhiqiang Fu; Chengbiao Wang; Jiajun Liu (769-783).
A tungsten doped non-hydrogenated diamond-like carbon (W-DLC) film was prepared by ion beam assisted deposition, and then W-DLC film was sulfurized by low temperature ion sulfurization. The structural analyses were performed on the scanning electron microscope, optical profilometer, Raman spectroscope and transmission electron microscope equipped with energy dispersive spectroscope. The results showed that the low temperature ion sulfurization treatment increase ID/IG ratio from 3.5 to 4.0 and 3.4 to 3.8 for sulfurized DLC and W-DLC films, respectively. The microstructures of sulfurized DLC film changed from amorphous to nanocrystalline/amorphous structure after low temperature ion sulfuration. The sulfurized W-DLC film was composed of nanocrystallites β-WC1−X, WS2 and FeS, which uniformly dispersed in the amorphous DLC matrix.
Keywords: Diamond-like carbon films; Microstructure; Low temperature ion sulfuration; Transmission electron microscope

Novel Zinc Oxide Nanostructures Fabrication by Oxygen Plasma Surface Modification and Improvement of Ractopamine Detection by Zhihong Zhang; Xiaodong Dong; Junfeng Tian; Shunli Liu; Yu Shi; Fufeng Yan; Shaoming Fang (785-798).
A feasible method for different ZnO nanostructures fabrication via the O2 plasma surface modification was reported in the present work. After the as-prepared ZnO nanoparticles were dispersed in the aqueous solution of Na2CO3, the resultant nanomatter was treated by O2 plasma for different times at high plasma input power of 200 W. It shows different nanostructures of ZnO were observed, such as nanowire, nanosheet, nanoneedles, and nanoparticles. Also, the chemical and crystal performances of the resultant ZnO nanostructures were depended on the duration of plasma. On these bases, the formation mechanism of new nanostructured ZnO-related materials was discussed. In comparison with the pristine ZnO, the plasma modified ZnO nanostructures (p-ZnO) exhibited a relative high electrochemical performance and sensitivity toward the detection of ractopamine (RAC) with a detection limit of 1.18 ng mL−1 within the range of 5–500 ng mL−1. It hints that the p-ZnO nanostructure could be used as a new alternative electrochemical biosensor for the detection of the food additives.
Keywords: ZnO nanostructures; Oxygen plasma; Electrochemical impedance spectroscopy; Ractopamine detection

A model of the direct photoelectric conversion of concentrated solar radiation in a plasma ignited in a heat pipe filled with a mixture of sodium vapor and krypton is developed. The model considers the non-homogeneous distribution of the alkali atom density in the heat-pipe volume and the thermionic effect of a cathode. The model treats a hot plasma core in a local thermal equilibrium (LTE) state and takes into account non-equilibrium layers near the converter walls. The model is employed to calculate an open-circuit voltage, a plasma resistance, a short-circuit current, an energy flux of positive ions directed toward the cathode, and a conversion efficiency of the solar radiation. Two different approaches were used to estimate a value of the electron temperature in the ionization non-equilibrium layer near the cathode. We assumed within the framework of an isothermal approximation that the electron temperature in the ionization layer near the cathode is equal to the temperature of the LTE plasma. This isothermal model predicted a rather low value (approximately 3 %) for the conversion efficiency. We found within the framework of a two-temperature model that the reduction of the electron temperature by 20 % compared with the LTE plasma temperature took place at the outer boundary of the ionization layer near the cathode. This non-isothermal model predicts a rather high value (approximately 33 %) for the conversion efficiency for a 300× solar radiation concentration ratio.
Keywords: Low temperature plasma physics; Photovoltaic effects; Energy conversion