Plasma Chemistry and Plasma Processing (v.32, #2)

Basic phenomena of the reduction of carbon dioxide to reusable organic materials including methane and methanol were investigated by using a radio frequency impulse discharge in a low gas pressure range without catalysis. The discharge took place under different discharge parameters such as voltage, gas flow rate, gas-mixing ratio, and gas residence time, where the carbon dioxide was mixed with hydrogen at total gas pressure of 1–10 Torr. Organic materials such as methane and methanol were observed. Carbon monoxide was a major product from carbon dioxide. Methane was the dominant organic species produced by the discharge. The concentration of methane increased with discharge voltage, and its volume fraction attained 10–20% of the products containing carbon that came from carbon dioxide. This fraction was also dependent on the mixing ratio of carbon dioxide and hydrogen. We also observed the formation of methanol, though its fraction was low, a few %, compared with methane.
Keywords: Reforming of carbon dioxide; Methane; Methanol; Impulse discharge; Low pressure

Interactive Phenomena of a Rotating Arc and a Premixed CH4 Flame by Nakyung Hwang; Jongguen Lee; Dae Hoon Lee; Young-Hoon Song (187-200).
The interactive phenomena between a rotating arc and a premixed methane/air flame have been investigated. The effects of the rotating arc on the flame were observed: both lean and rich flammability limit was extended with the rotating arc on. More interestingly, the opposite interactions, i.e., the effects of the flame on the rotating arc, were also observed: the arc length, angular speed, and electrical characteristics are affected by the flame. An analysis of the optical emission spectra showed that the rotating arc generates chemically active species such as excited N2 molecules and O and H atoms. An analysis of the gas products indicated that the concentrations of the major gas products such as CO2, CO, and H2 were not significantly affected by the rotating arc, although the flammability limit are significantly extended. Unlike these major gas products, minor products like NOx emissions increased by an order of magnitude in the presence of a rotating arc under certain air/fuel conditions.
Keywords: Plasma; Combustion; Premixed flame; Interaction; Rotating arc

Here, we report a new and rapid way to decompose Zn(OH)2 for the fabrication of ZnO nanotube using dielectric barrier discharge (DBD) plasma initiated at ambient condition. X-ray diffraction, field emission scanning electron microscope and high-resolution transmission electron microscopy were employed to characterize the fabricated ZnO nanotube. The results show that hexagonal hollow tubes in a wurtzite phase are obtained. Compared to the ZnO powder prepared by the thermal calcination, the DBD plasma made ZnO nanotube shows an enhanced performance for H2S removal at low temperature.
Keywords: ZnO; Nanotube; Plasma; H2S; Removal

Synthesis of Nanosized Silicon Carbide Through Non-Transferred Arc Thermal Plasma by Prabhakar Rai; Yun-Su Kim; Sang-Ki Kang; Yeon-Tae Yu (211-218).
A thermal plasma system was used for the preparation of nanosized SiC powder. First SiC was synthesized by solid-state reaction using waste silicon and activated carbon powders and then plasma processing was carried out to form nanosized SiC. Phase and structural analysis was carried out by X-ray diffraction which confirmed the formation of SiC in both cases. Plasma treatment did not show any kind of change in structure and phase of SiC; except little free silicon. Morphological investigation showed the formation of 20–30 nm spherical SiC particles after plasma treatment which was initially 1–5 μm. It was found that DC current played an important role in the reduction of particle size. It was proposed that nanosized SiC was formed due to the dissociation of grains from their grain boundary due to strong plasma gas stream.
Keywords: SiC; Nanoparticles; Thermal plasma processing; Powder technology; Electron microscopy

Synthesis and Characterization of Nanostructured a-C:H (Hydrogenated Amorphous Carbon) Thin Films by Gaseous Thermionic Vacuum Arc (G-TVA) Deposition Technique by Rodica Vladoiu; Victor Ciupina; Mirela Contulov; Virginia Dinca; Aurelia Mandes; Vilma Bursikova (219-229).
The aim of this contribution is to present the properties of the nanostructured hydrogenated carbon thin films and to study their growth carried out in a special deposition technique based on Thermionic Vacuum Arc method. The Gaseous Thermionic Vacuum Arc (G-TVA) technology is an original deposition method performed in a special configuration, consisting of a heated thermionic cathode which provides an electron beam on the anode. The surface free energy was evaluated by contact angle and their optical properties were studied by Filmetrics F20 spectrometry system. Structure of the film has been investigated by Raman spectroscopy as well as the mechanical properties like hardness, wear resistance, film-substrate adhesion. The films showed two distinct Raman characteristic peaks located at 1,350 cm−1 (D-line) and 1,550 cm−1 (G-line), broad for Si and very sharp for glass substrates. The G-TVA enables to prepare soft (hardness ~6 GPa) or hard (~24 GPa) films.
Keywords: Hydrogenated carbon; G-TVA; Raman spectroscopy; Mechanical properties

Dissociative Excitation of C2H2 in the Electron Cyclotron Resonance Plasma of Ar: Production of CH(A2Δ) Radicals and Formation of Hydrogenated Amorphous Carbon Films by Haruhiko Ito; Katsuaki Koshimura; Saori Onitsuka; Kohtaro Okada; Tsuneo Suzuki; Hiroki Akasaka; Hidetoshi Saitoh (231-248).
The dissociative excitation reaction of C2H2 with the electron-cyclotron resonance plasma of Ar was investigated based on the electrostatic-probe measurements and on the optical emission spectroscopy of the CH(A2Δ–X2Π) transition. The density, n e, and the temperature, T e, of free electrons were controlled by adding H2O molecules externally into the reaction region, and the dependence of the CH(A2Δ–X2Π) emission intensity on the addition of H2O was observed to compare with the evaluated dependencies based on n e and T e. The mechanism of production of CH(A2Δ) was found, predominantly, to be the electron impact with the contribution of 10–20% of the electron-impact dissociation of C2H radicals; the contribution of the ion–electron recombination was negligible. Hydrogenated amorphous carbon films were fabricated using the same reaction system. The atomic compositions, Raman spectra, and the hardness of films were discussed in terms of the variations of n e and T e upon the addition of H2O molecules.
Keywords: ECR plasma; Optical emission spectrum; C2H2 ; CH(A–X) transition; Hydrogenated amorphous carbon

Moisture Removal from Natural Jute Fibre by Plasma Drying Process by M. M. Morshed; M. M. Alam; S. M. Daniels (249-258).
Low temperature plasma process is an effective alternative method compared to the conventional vacuum drying method for removing moisture. Plasma drying removes the moisture from fibres faster and to a lower level than conventional methods. It also improves the surface properties of the fibres. The jute fibre was treated with inert gas argon plasma without damaging the fibre. The OES was used to monitor the moisture desorbed from the fibre during processing. The XRD results revealed a change in the macromolecular structure as well as the crystallinity of the treated fibre. The FTIR and TGA provided the evidence of moisture removal from the fibres. It was found that the plasma treated fibres contain less than 1.8% (wt.) moisture which is a promising result when compared with conventional drying processes.The average tensile strength of the plasma treated fibres increased by 12.5% compared with those treated with the conventional vacuum dry process.
Keywords: Natural jute fibre; Moisture; Argon plasma; Optical emission spectroscopy (OES)

Surface Modification of Banana Fibers by DBD Plasma Treatment by Fernando Ribeiro Oliveira; Laura Erkens; Raul Fangueiro; António Pedro Souto (259-273).
Banana fibers, an environmentally friendly raw material freely available, were physically modified by atmospheric dielectric barrier discharge (DBD) plasma treatment of different dosages. The influence of the plasma treatment applied on the banana fibers was performed considering the mechanical properties, wettability, chemical composition and surface morphology. These properties were evaluated by tensile tests, static and dynamic contact angle, Fourier transform infrared spectroscopy, energy dispersive spectroscopy, X-ray diffractometry, conductivity and pH of aqueous extract, differential scanning calorimetry and scanning electron microscopy images. We compare untreated and treated fibers with three different DBD plasma dosages. The results of this study showed considerable modifications in banana fibers when these are submitted to plasma treatment.
Keywords: Dielectric barrier discharge (DBD) plasma; Surface modification; Wettability; Banana fibers

Plasma-Electrospinning Hybrid Process and Plasma Pretreatment to Improve Adhesive Properties of Nanofibers on Fabric Surface by Narendiran Vitchuli; Quan Shi; Joshua Nowak; Rupesh Nawalakhe; Michael Sieber; Mohamed Bourham; Marian McCord; Xiangwu Zhang (275-291).
Electrospun nanofiber mats are inherently weak, and hence they are often deposited on mechanically-strong substrates such as porous woven fabrics that can provide good structural support without altering the nanofiber characteristics. One major challenge of this approach is to ensure good adhesion of nanofiber mats onto the substrates and to achieve satisfactory durability of nanofiber mats against flexion and abrasion during practical use. In this work, Nylon 6 nanofibers were deposited on plasma-pretreated woven fabric substrates through a new plasma-electrospinning hybrid process with the objective of improving adhesion between nanofibers and fabric substrates. The as-prepared Nylon 6 nanofiber-deposited woven fabrics were evaluated for adhesion strength and durability of nanofiber mats by carrying out peel strength and flex resistance tests. The test results showed significant improvement in the adhesion of nanofiber mats on woven fabric substrates. The nanofiber-deposited woven fabrics also exhibited good resistance to damage under repetitive flexion. X-Ray photoelectron spectroscopy and water contact angle analyses were conducted to study the plasma effect on the nanofibers and substrate fabric, and the results suggested that both the plasma pretreatment and plasma-electrospinning hybrid process introduced radicals, increased oxygen contents, and led to the formation of active chemical sites on the nanofiber and substrate surfaces. These active sites helped in creating crosslinking bonds between substrate fabric and electrospun nanofibers, which in turn increased the adhesion properties. The work demonstrates that the plasma-electrospinning hybrid process of nanofiber mats is a promising method to prepare durable functional materials.
Keywords: Adhesion; Plasma; Electrospinning; Surface; Nanofibers

Many studies suggest strong hydrophilicity of plasma treated polyester surfaces. However, no studies have been reported on the influence of plasma on the antibacterial activity of polyethylene terephthalate. First samples were padded with triclosan as antibacterial agent with different concentrations. Second samples were treated by oxygen plasma with different operating frequency and treating time, respectively. Afterwards, plasma treated samples were padded with triclosan in same conditions. The results revealed that the antibacterial activity slighlty increased after treating with triclosan. SEM images and FTIR spectra showed that horizontal channels were brought about on the fiber surface and then better surface roughness and wettability were obtained by plasma. Fibers were fully coated with triclosan after plasma and the antibacterial activity increased with increasing operating frequency and reaction time. Finally, the samples treated with triclosan after plasma gave acceptable results and showed the best antibacterial activity for Staphylococcus aureus and Escherichia coli.
Keywords: Polyethylene terephthalate; Plasma; Surface modification; Antibacterial property

Introduction of Primary Amino Groups on Poly(ethylene terephthalate) Surfaces by Ammonia and a Mix of Nitrogen and Hydrogen Plasma by Jessie Casimiro; Bénédicte Lepoittevin; Caroline Boisse-Laporte; Marie-Geneviève Barthés-Labrousse; Pascale Jegou; François Brisset; Philippe Roger (305-323).
With the aim of introducing primary amino groups on the surface of poly(ethylene terephthalate) (PET), two methods were compared—the use of ammonia or a combination of nitrogen and hydrogen low-pressure microwave plasma. Several plasma parameters were optimized on the reactor to increase the –NH2 surface density, which was estimated by colorimetric titration and X-ray photoelectron spectroscopy (XPS). These techniques show that whatever the plasma treatment, almost 2 –NH2/nm² are incorporated on PET films. Emission spectroscopy highlighted a correlation between the density of primary amino groups and the ratio between an NH peak intensity and an Ar peak intensity (INH/IAr). Variation in surface hydrophilicity with aging in air after plasma treatment was monitored with contact angle measurements and showed a hydrophobic recovery. This was confirmed by XPS, which suggests also that surfaces treated by NH3 plasma are more stable than surfaces treated by N2/H2.
Keywords: Aging; ESCA/XPS; Microwave plasmas; Optical emission spectroscopy (OES); Primary amino groups

An optimal gas supply method for the micro discharge plasma generated along a quartz glass electrode, which was useful for the maskless fabrication of electrode grooves for surface electrodes on solar cells, was examined. We here constructed an electrode system with gas inlet and outlet holes. The gas supply directly to the plasma region contributed to reduce byproducts on the surface being etched, and then it was confirmed that the uniform etching was achieved in the case where the micro-discharge plasma locally produced at the etching area.
Keywords: Maskless etching; Micro-discharge plasma; Atmospheric pressure; Optimal gas supply

Etching Characteristics and Mechanisms of TiO2 Thin Films in HBr/Ar and Cl2/Ar Inductively-Coupled Plasmas by Hanbyeol Jang; Alexander Efremov; Daehee Kim; Sungchil Kang; Sun Jin Yun; Kwang-Ho Kwon (333-342).
The TiO2 etching characteristics and mechanisms in HBr/Ar and Cl2/Ar inductively-coupled plasmas were investigated under fixed gas-mixing ratio and bias power conditions. It was found that in both systems, an increase in gas pressure from 4 to 10 mTorr results in a non-monotonic TiO2 etching rate, while a variation of input power in the range 500–800 W causes a faster-than-linear acceleration of the etching process. Plasma diagnostics performed by Langmuir probes and zero-dimensional plasma modeling provided data on plasma parameters, steady-state densities, and fluxes of the active species on the etched surface. The model-based analysis of the etching mechanism showed that for the given set of processing parameters, the TiO2 etch kinetics correspond to the transitional regime of ion-assisted chemical reaction in which a chemical-etch pathway dominates.
Keywords: TiO2 ; HBr/Ar plasma; Cl2/Ar plasma; Etching mechanism

Active Species Generated by a Pulsed Arc Electrohydraulic Discharge Plasma Channel in Contaminated Water Treatments by O. L. Li; N. Takeuchi; Z. He; Y. Guo; K. Yasuoka; J. S. Chang; N. Saito (343-358).
Pulsed arc electrohydraulic discharge (PAED) direct plasma technique was applied for various types of contaminated water treatment. The experimental system consists of a spark-gap switch type pulse power supply (0.5 kJ/pulse) and a 3 L stainless steel reactor with eccentrically configured rod-to-rod electrodes. The current and voltage waveforms are fundamentally different for different conductivity water. Double pulse current discharges were observed for pond water with relatively higher conductivity (637 mS/m) while a single pulse current discharge was observed for lake water with relatively lower conductivity (78 mS/m). From the optical emission spectrum and UV dosimeters, UV-A, UV-B, excited molecules and radicals including N2*, O, H, OH, O3 etc. were observed during the discharge period. Both optical emission and UV intensities in pond water are slightly lower than lake water. The decay time of the UV-A, N2*, OH, H and O radicals were around 0.6 ms, where the discharge period ended around 0.4 ms. The results indicate that the radicals existed longer than the discharge period. The pH, dissolved oxygen and conductivity were changed during the course of PAED treatment. The ions and radicals such as H·, O·, H+, OH· etc. generated by PAED may cause alterations density decay time and the active species in water were present for a longer period. The reduction of total organic carbon (TOC) in pond water reached 80% after 5 min of PAED treatment. Based on local thermal plasma equilibrium (LTE) model, LTE thermal plasma chemical composition model for 1 mol water vapour was used to compare to present experiments. PAED discharge in contaminated pond water generated peak concentration of OH = 28% and O2H = 0.012% mol in gas-phase and migrated to water-phase via gas–liquid interfaces generated by arc and micro-bubbles to form more stable O3, H2O2, H2 and O2. The model suggested that the reduction of TOC occurred in gas/plasma phases or liquid-phase side of gas liquid interfaces.
Keywords: Pulsed arc electrohydraulic discharge; Active species; Plasma density; Pond water treatment

In this work, light emissions and radicals formed by plasma of contact glow discharge electrolysis were investigated. The plasma was generated by glow discharges at the tip of a Pt anode in contact with a sulfuric acid solution. Emissions of H atoms and OH radicals were observed when the applied voltage was above 430 V. When the applied voltage increased to 450 V, emissions of O atoms were additionally detected. The emission intensities of these radicals and atoms increased with the increasing applied voltage. When the applied voltage exceeded 460 V, thermal radiation from the Pt anode was apparent in the visible and near infrared region. Electron temperature of the plasma increased with the applied voltage from 1.0 × 104 to 1.5 × 104 K by comparison of the intensities of Hα and Hβ lines. The mean electron density was estimated to be 7.4 × 1017 cm−3 by the method of Stark broadening.
Keywords: Emission spectrum; Glow discharge; Water

In the field of plasma modeling for electrical discharge machining (EDM) process, radiative losses are many times include in the energy equation using net emission coefficients (NEC). This is a sink term that accounts for the radiative emissions from the plasma to the surrounding environment. The purpose of this study is to calculate NECs for EDM arc plasmas. Two common plasma mediums, hydrocarbon oil and deionized water, are considered in this work assuming the EDM plasma is in local thermodynamic equilibrium, homogenous and isothermal. NECs are calculated for pressures of 1 and 10 bar for working temperatures commonly seen in EDM plasmas, 5,500–10,000 K. Continuum contributions such as molecular photodissociation, molecular photoionization, electron-molecule bremsstrahlung, electron-atom bremsstrahlung, ionic radiative recombination, and electron–ion bremsstrahlung are included in the calculation of NEC. In addition, the line radiations are calculated by overlapping lines considerations. Results show that radiative heat loss of EDM plasma in the deionized water dielectric medium is less than hydrocarbon oil one. Presented results for the NEC can serve as input data for various numerical models of electrical discharge machining process.
Keywords: Electrical discharge machining; Plasma modeling; Radiative heat loss; Net emission coefficient

Double Langmuir probe measurements are performed during the deposition of tin and chromium coatings in order to examine the degree of ionisation and the deviation from thermal equilibrium of these metal plasmas.
Keywords: Metal plasmas; Langmuir probes; Thermal nonequilibrium; Metal vapour deposition

Plasma: The Fourth State of Matter by K. T. A. L. Burm (401-407).
The plasma state is frequently referred to as the fourth state of matter in the sequence: solid, liquid, gas, and plasma. The statement implies that plasma is another phase. Each state is achieved by adding heat to the previous state. The first three states are the three common phases achieved via phase transitions. The statement that plasmas are the fourth state of matter is examined considering phase transitions. It is shown that the transition from gas to plasma is not a phase transition similar to the other phase transitions at which transitions the differential of the Gibbs free energy equals zero. Therefore, strictly speaking, plasmas are better not called the fourth state of matter.
Keywords: Plasma fundamentals; Plasma thermodynamics; Phase transitions