Plasma Chemistry and Plasma Processing (v.37, #4)

Contact glow discharge electrolysis (CGDE)/plasma electrolysis (PE) which is associated with the formation of a light emitting plasma around an electrode in a high conductivity electrolyte solution at moderate voltages up to ~1 kV, has in recent years attracted considerable interest as a tool for generating a large quantity of heat and a high yield of solvent-split radicals. These potentialities of CGDE/PE have, in fact, been exploited by a large number of investigators for applications ranging over areas as varied as synthetic chemistry, waste water treatment, degradation of polymers, electrosurgical tools, surface engineering, nanoparticle fabrication, machining and micro-machining, hydrogen production with very encouraging results. The article reviews comprehensively these results.
Keywords: Contact glow discharge electrolysis; Electrolytic plasma technology; Superabsorbent composites; Waste water treatment; Plasma electrolytic oxidation; Nanoparticle fabrication; Micromachining

Numerical Modelling of Wood Gasification in Thermal Plasma Reactor by Ivan Hirka; Oldřich Živný; Milan Hrabovský (947-965).
Biomass gasification for synthesis gas production represents a promising source of energy based on plasma treatment of renewable fuel resources. Gasification/pyrolysis of crushed wood as a model substance of biomass has been experimentally carried out in the plasma-chemical reactor equipped with gas–water stabilized torch which offer advantage of low plasma mass-flow, high enthalpy and temperature making it possible to attain an optimal conversion ratio with respect to synthesis gas production in comparison with other types of plasma torches. To investigate this process of gasification in detail with possible impact on performance, a numerical model has been created using ANSYS FLUENT program package. The aim of the work presented is to create a parametric study of biomass gasification based on various diameters of wooden particles. Results for molar fractions of CO for three different particles diameters obtained by the modeling (0.55, 0.52 and 0.48) at the exit are relatively good approximation to the corresponding experimental value (0.60). The numerical results reveal that the efficiency of gasification and syngas production slightly decreases with increasing diameter of the particles. Computed temperature inhomogeneities in the volume of the reactor are strongest for the largest particle diameter and decrease with decreasing size of the particles.
Keywords: Plasma modelling; CFD; Thermal plasma reactor; Biomass; Gasification; Syngas

Preparation of Silicon Nanopowder by Recycling Silicon Wafer Waste in Radio-Frequency Thermal Plasma Process by SeungJun Lee; Tae-Hee Kim; Dong-Wook Kim; Dong-Wha Park (967-978).
Silicon nanopowders were prepared from silicon waste by using radio-frequency thermal plasma. Silicon waste, generated from the manufacturing process of silicon wafers, was pulverized to form micrometer-sized silicon starting powder. In order to obtain as much silicon nanopowder as possible from thermal plasma processing, the enhancement of vaporization and the quenching rate of the silicon starting powder were considered as major factors. A counter-flow injection apparatus (CFIA) was introduced for improved vaporization and homogeneous nanoparticles. It was designed to inject argon as a quenching gas in the direction opposite the thermal plasma flame flow. The controlled location of the CFIA injection nozzle and the flow rate of the quenching gas affect the residence time of the injected staring powder by recirculating flow and the vapor density by gas mixing. The variation of the flow pattern inside the reactor and the characteristics of the products were investigated to determine the optimal processing environment to prepare uniform and small silicon nanopowder particles. The environment was defined by two parameters: the flow rate of the counter quenching gas and the distance between the torch and CFIA nozzles. The flow rate of the quenching gas was controlled from 30 to 70 L/min. The distance between the torch and CFIA nozzles was adjusted from 150 to 350 mm. When the quenching gas flow rate of 70 L/min and the distance of 350 mm were applied, the uniform and smallest silicon nanopowders were obtained.
Keywords: Silicon waste; Silicon nanopowder; Radio-frequency thermal plasma; Counter-flow injection apparatus

The paper presents experimental results pertaining to the reduction of oxide mixtures namely (Fe2O3 + CuO) and (Fe2O3 + Co3O4), by low-temperature hydrogen plasma in a microwave hydrogen plasma set-up, at microwave power 750 W and hydrogen flow rate 2.5 × 10−6 m3 s−1. The objective was to examine the effect of addition of CuO or Co3O4, on the reduction of Fe2O3. In the case of the Fe2O3 and CuO mixture, oxides were reduced to form Fe and Cu metals. Enhancement of reduction of iron oxide was marginal. However, in the case of the Fe2O3 and Co3O4 mixture, FeCo alloy was formed within compositions of Fe70Co30, to Fe30Co70. Since the temperature was below 841 K, no FeO formed during reduction and the sequence of Fe2O3 reduction was found to be Fe2O3 → Fe3O4 → Fe. Reduction of Co3O4 preceded that of Fe2O3. In the beginning, the reduction of oxides led to the formation of Fe–Co alloy that was rich in Co. Later Fe continued to enter into the alloy phase through diffusion and homogenization. The lattice strain of the alloy as a function of its composition was measured. In the oxide mixture in which excessive amount of Co3O4 was present, all the Co formed after reduction could not form the alloy and part of it appeared as FCC Co metal. The crystallite size of the alloy was in the range of 22–30 nm. The crystal size of the Fe–Co alloy reduced with an increase in Co concentration.
Keywords: Reduction of oxide mixtures; Reduction of cupric oxide; Reduction of iron oxide; Reduction of cobalt oxide; Low-temperature hydrogen plasma; FeCo alloy

Preparation of Boron Carbide from BF3 and BCl3 in Hydrogen Plasma of Arc RF Discharge by P. G. Sennikov; R. A. Kornev; A. I. Shishkin (997-1008).
A comparative study was carried out of the process of plasma chemical deposition of boron carbide from hydrogen plasma containing the mixtures of BF3 + CH4 and BCl3 + CH4 sustained by RF arc (13.56 MHz) discharge. It was shown that in the case of synthesis of B4C from a mixture of BF3 + CH4, carbon and complex coordination compound [X3B]H+ (R3B·FH) are formed as the by-products of condensed products. In the case of synthesis of B4C from the BCl3 + CH4 mixture, the only condensed product is carbon. Mechanisms for the formation of boron carbide on the surface of heated electrodes are proposed. The main feature of these mechanisms is the preliminary deposition of a graphite layer from CH4 and then the precipitation of boron with the participation of the radicals BF2, BF and BCl. B4C samples were obtained and the impurity composition, morphology and structure of bulk boron carbide samples obtained using both of its halides were studied. It was found that in both cases a carbon phase is present in boron carbide samples. The main impurities entering the B4C, in the case of using a mixture of BF3 + CH4, is silicon, and in the case of a mixture of BCl3 + CH4, is tungsten.
Keywords: RF arc discharge; Boron carbide; Boron fluoride; Boron chloride

Coating quality is affected by arc and plume instabilities during plasma spraying. In closed chamber plasma spraying, gradual drift is one of the intermediate instabilities, which is mainly due to the electrode erosion. This work focuses on the source of the gradual drift of the plasma jet and the influence on coating quality. The ambient state inside the chamber was controlled by a ventilation system and a vacuum system. The variation in the plasma jet was observed by a particle flux image device based on a CCD camera. The optical spectrum of the plasma plume was measured and analyzed through an optical spectrometer. The results indicated that the addition of hydrogen to plasma gas induced the change in the plasma jet length and width with changing rates depending on the chamber state and the ventilation power. With poor ventilation, the intensity of Hα emission was found to become gradually stronger while Hβ and Hγ were found to become weaker. On closing the chamber and retaining enough ventilation power, it was observed that the ambient gas slowly turned red. Simultaneously, the coating weight and thickness were slightly decreased meanwhile the porosity ratio was obviously increased. The red ambient gas has been proved to be able to acidify the city water with pH value decreased from 7 to 1–3. Without hydrogen, the plasma jet was found to be stable without reddening and variation, but the plasma enthalpy was unfortunately low.
Keywords: Plasma spray; Reddening phenomenon; Plasma stability; Emission intensity; Coating quality

Experimental Comparison of Methane Pyrolysis in Thermal Plasma by Tianyang Li; Christophe Rehmet; Yan Cheng; Yong Jin; Yi Cheng (1033-1049).
Methane pyrolysis via thermal plasma was investigated experimentally on a 2 kW DC arc plasma setup in argon atmosphere. Two widely applied methane pyrolysis profiles, i.e., pre-mixing methane and argon before fed into plasma torch, and injecting methane into pure argon plasma jet at torch outlet, were compared. Performances of methane pyrolysis regarding to methane conversion, acetylene selectivity, acetylene specific energy requirement (SER), and plasma stability were concluded. Results showed that pre-mixing methane and argon before fed into plasma torch would be efficient in converting methane and acetylene production, with higher conversion of methane and lower SER to acetylene at a given specific energy. Also, methane in arc zone would cause periodic fluctuations of plasma voltage and power, which could be reduced by controlling methane fraction in feed. On the other hand, when methane was injected into argon plasma jet at torch outlet, the energy efficiency in converting methane and producing acetylene would be lower. And the plasma would barely participate in the reaction other than providing heat, but the erosion of electrode was much slower and slighter. It was also validated that the SER of acetylene was limited by the thermal loss of the setup due to size-effect of reactor.
Keywords: Methane; Acetylene; Thermal plasma; Pyrolysis; Arc stability

Prediction of Dielectric Properties of Air Plasma for Circuit Breaker Application Based on a Chemically Non-equilibrium Model by Yi Wu; Hantian Zhang; Bing Luo; Fei Yang; Hao Sun; Tianwei Li; Li Tang (1051-1068).
Dielectric properties of air plasma in a model circuit break were investigated. A chemically non-equilibrium (non-CE) model was used to simulate the arc dynamic behavior inside the nozzle during current zero period. Distribution of the critical breakdown electric field Ecr of hot air was derived from the electron energy distribution function by solving the Boltzmann transport equation, using calculated temperature, pressure and species composition. Then, the electric field at applied recovery voltage (Ea) was calculated. The probability of dielectric breakdown inside the nozzle can be predicted by comparing Ea and Ecr. The results show that neglect of departure from chemical equilibrium may lead to the overestimation of the dielectric recovery strength of circuit breaker arc during the first several hundred microseconds after current zero.
Keywords: Chemically non-equilibrium model; Dielectric properties; Critical breakdown electric field; Boltzmann equation analysis

Synthesis of Carbon–Metal Multi-Strand Nanocomposites by Discharges in Heptane Between Two Metallic Electrodes by A. Hamdan; H. Kabbara; M.-A. Courty; M. S. Cha; J.-M. Martinez; T. Belmonte (1069-1090).
We studied composite wires assembled from electric field-driven nanoparticles in a dielectric liquid (heptane) to elucidate the exact processes and controlling factors involved in the synthesis of the multi-phase nanocomposites. Filamentary wires are synthesized by a two-step process: (1) abundant nanoparticle production, mostly of carbonaceous types, from heptane decomposition by spark discharge and of metal nanoparticles by electrode erosion and (2) assembly of hydrogenated amorphous carbonaceous nano-clusters with incorporated metal nanoparticles forming wires by dielectrophoretic transport while maintaining a high electric field between electrodes kept sufficiently separated to avoid breakdown. Four types of nanocomposites products are identified to form at different steps in distinctive zones of the setup. The black carbonaceous agglomerates with metal spherules made by electrode erosion represent the pyrolytic residues of heptane decomposition by spark discharge during step 1. The filamentary wires grown in the interelectrode gap during step 2 get assembled by dielectrophoretic transport and chaining forces. Their great stability is shown to express the concurrent effect of polymerization favoured by the abundance of metal catalysts. The nature, abundance, and transformation of solid particles from the source materials versus discharge conditions control the morphological and compositional diversity of the wires. The production of mineral and metal nano-particles traces the efficiency of dielectrophoresis to separate compound particle mixtures by size and to co-synthesize nanostructured microcrystals and nanocomposites. The link between impurities and the variability from nano- to micro-scales of the synthesized products provides an innovative contribution to the knowledge of nanocomposite synthesis triggered by electric field.
Keywords: Discharge in liquids; Nanoparticle synthesis; Metal/carbon nanocomposite; Dielectrophoresis

Investigation of Cold Atmospheric Plasma-Activated Water for the Dental Unit Waterline System Contamination and Safety Evaluation in Vitro by J. Pan; Y. L. Li; C. M. Liu; Y. Tian; S. Yu; K. L. Wang; J. Zhang; J. Fang (1091-1103).
The disinfection of the inner surface of a medical device has long been a challenge for the central sterile supply departments. Dental unit waterline system (DUWLs) foster the attachment of microorganisms and development of biofilm, which lead to continuous contamination of the outlet water from dental units; this contamination may be responsible for a potential risk of infection due to the exposure of patients and medical staff. The present study investigated the disinfection effects of cold atmospheric plasma-activated water (CAPAW) on DUWLs using a model of 5-day-old Enterococcus faecalis biofilm. The results showed that the colony-forming unit was reduced from 107 to 0 after 5 min of treatment. The physicochemical properties of CAPAW were evaluated, including the pH value, oxidation reduction potential, and NO radical. The results showed that the inactivation mechanisms were mainly triggered by the reactive oxygen/nitrogen species. Additionally, CAPAW had a metal corrosion rate same as that of deionized water. We conclude that CAPAW can be applied as an appropriate alternative disinfectant against biofilm contamination of DUWLs.
Keywords: Cold atmospheric plasma-activated water; Dental unit waterline system; Biofilm; Safety evaluation

Enhancement of Germination and Seedling Growth of Wheat Seed Using Dielectric Barrier Discharge Plasma with Various Gas Sources by Yiran Meng; Guangzhou Qu; Tiecheng Wang; Qiuhong Sun; Dongli Liang; Shibin Hu (1105-1119).
The influences of non-thermal discharge plasma treatment on wheat seed germination and seedling growth were investigated using a dielectric barrier discharge (DBD) plasma system at atmospheric pressure and room temperature. DBD plasma with various gas sources (oxygen, air, argon, and nitrogen) was employed in this study. Germination characteristics, seedling growth parameters, surface changes of the seed coat, permeability, and soluble protein of the seedlings were measured after the DBD plasma treatments. The experimental results showed that moderate-intensity DBD plasma had active impacts on wheat seed germination and seedling growth. Germination potential significantly increased by 24.0, 28.0, and 35.5% after 4 min of the air plasma, nitrogen plasma, and argon plasma treatments, respectively, compared with the control; and the shoot and root length also increased; however, no enhancement was observed after the oxygen plasma treatment. Scanning electron microscope analysis showed that etching effects on the seed coat occurred after the air plasma, nitrogen plasma, and argon plasma treatments, which affected the hygroscopicity and permeability of the wheat seed. In addition, moderate-intensity DBD plasma could also activate several physiological reactions in wheat seed, resulting in the increase of soluble protein production in wheat seedlings.
Keywords: DBD plasma; Wheat seed; Seed germination; Seedling growth

A novel dielectric-barrier-discharge (DBD) loop reactor was designed for the efficient degradation of cyanide anion (CN) in water. The circulation of cyanide water as a falling film through plasma gas discharge zone enhanced gas–liquid mass and energy transfer and induced formation of H2O2 which was associated with the efficient destruction of CN. It was observed that among different discharge gases, the CN degradation rate decreased in the order of Ar > air > H2/air mixture. Depending on discharge voltage, the treatment time for complete removal of 100 ppm CN in this DBD loop reactor is in the range 120–300 min. The dose of Cu2+ catalyst in combination with in situ production of H2O2 enhanced the destruction of CN apparently in this DBD loop reactor. The treatment time for complete degradation of 100 ppm CN decreased from 180 min with Ar DBD discharge alone to 40 min with 40 mg/L dose of Cu2+ ion in water, making it an efficient means to degrade cyanide water.
Keywords: Cyanide; Plasma; Discharge; Loop reactor; Catalysis

Synthesis of CdS Quantum Dots Using Direct Plasma Injection in Liquid Phase by M. Shariat; M. Karimipour; M. Molaei (1133-1147).
In this work, we introduce a new method using non-equilibrium atmospheric pressure plasma jet (N-APPJ) for the synthesis of semiconductor quantum dots in liquid media with low power consumption at room temperature. In this method, the solution containing CdSO4, Na2S2O3 and thioglycolic acid (TGA) was treated by N-APPJ. Firstly, the key role of plasma applied voltage on the power consumption and the produced plasma reactive species was considered. We then continue to demonstrate that the optical properties such as absorption and photoluminescence of cadmium sulfide quantum dots (QDs) can carefully be controlled with the plasma parameters. Here, the effects of two major plasma parameters such as the plasma applied voltage and treatment time on the size of CdS QDs were investigated. The size of nanoparticles increases with the increase of the voltage and treatment time. It was also displayed that the change of the concentration of Na2S2O3 and TGA in the solution had no significant effect on the size of QDs in plasma method. Finally, X-ray diffraction, X-ray fluorescence spectrometry and transmission electron microscopy were employed to determine the composition, size and crystallinity of prepared CdS QDs.
Keywords: Plasma–liquid interaction; Cadmium sulfide nanoparticles; Non-equilibrium atmospheric pressure plasma jet

An Array of Micro-hollow Surface Dielectric Barrier Discharges for Large-Area Atmospheric-Pressure Surface Treatments by T. Homola; R. Krumpolec; M. Zemánek; J. Kelar; P. Synek; T. Hoder; M. Černák (1149-1163).
A robust, commercial micro-hollow plasma source was used to generate atmospheric-pressure plasma, of surface area 18 × 18 mm, in ambient air, nitrogen and argon. An electrode system consisting of 105 micro-hollow surface dielectric barrier discharges was powered by sinusoidal high-voltage at a frequency of 26.7 kHz. The influence of the plasmas on the polycarbonate surface was investigated by means of surface energy measurements and X-ray photoelectron spectroscopy. It emerged that short plasma exposures led to significant increases in surface energy. It is suggested that this may arise out of incorporation of polar groups on the polycarbonate surface. A thermal camera was used to monitor the plasma source surface temperatures for the gases at flow rates ranging from 0 to 5 L/min. It was found that the temperature of the micro-hollow ceramic when operated upon in ambient air decreased significantly from 147 °C at 0 L/min to 49 °C at 5 L/min. In order to investigate further the thermal properties of the plasma, optical emission spectroscopy was employed to monitor the vibrational and rotational temperatures of the plasma generated in ambient air. CCD camera spectroscopic measurements estimated plasma thickness and temperature distribution at high spatial resolution.
Keywords: Micro-hollow plasma; Surface dielectric barrier discharge; Ambient air plasma; Plasma treatment; Polycarbonate surface

Characteristics of Dielectric Barrier Discharge Ozone Synthesis for Different Pulse Modes by Dingkun Yuan; Can Ding; Yong He; Zhihua Wang; Sunel Kumar; Yanqun Zhu; Kefa Cen (1165-1173).
This paper features the pulse polarity effect on ozone generation efficiency by adjusting the applied voltage and the flow rate in a coaxial dielectric barrier discharge reactor. Results show that utilization of unipolar pulse has better performance when compared with the bipolar mode, but on the other hand, utilization of the positive pulse has slightly higher efficiency than that of negative mode. Meanwhile, changing the gas flow rate shows a minor effect on ozone generation. Utilization of bipolar pulse would decrease the breakdown voltage and ozone generation efficiency when compared with unipolar pulse while it would lead to higher ozone concentrations at fixed applied voltage. The maximum ozone yield reaches 186.9 g/kWh at 6 kV positive pulse with ozone concentration of 11.9 g/Nm3.
Keywords: Ozone generation; Dielectric barrier discharge; Pulse polarity; Ozone production efficiency

Low Temperature Catalytic Hydrolysis of Carbon Disulfide on Activated Carbon Fibers Modified by Non-thermal Plasma by Kunlin Li; Ping Ning; Kai Li; Chi Wang; Xin Sun; Lihong Tang; Sijian Liu (1175-1191).
In order to improve the carbon disulfide (CS2) catalytic hydrolysis efficiency of activated carbon fibers (ACFs), ACFs surface was modified by non-thermal plasma (NTP). In particular, the effects of modification conditions on the catalyst properties were studied, including the reactor structure, modification atmosphere, modification time, output voltage and discharge gap. The catalytic activity study showed that ACFs with NTP modification enhanced CS2 catalytic hydrolysis. The optimal reactor structure, modification atmosphere, modification time, output voltage and discharge gap was a coaxial cylinder, an N2 atmosphere, 5 min, 7 kV and 7.5 mm, respectively. The effect of the NTP modification on the micro-structural properties of the ACFs was characterized using scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) and X-ray photoelectron spectroscopy (XPS) methods. The results showed that NTP modification improved the dispersion of functional groups and increased the number of oxygen-containing and nitrogen-containing functional groups, thus the catalytic activity could be enhanced. The present results indicated that NTP modification was an effective way to manipulate ACFs surface properties for the CS2 catalytic hydrolysis reaction.
Keywords: Non-thermal plasma; Activated carbon fibers; Carbon disulfide; Catalytic hydrolysis; Surface modification

Experimental Study on Purification of Diesel Particulate Matter by Non-thermal Plasma Technology by Linbo Gu; Yixi Cai; Yunxi Shi; Jing Wang; Xiaoyu Pu; Hui Xu; Yingxin Cui (1193-1209).
In order to investigate the effects of non-thermal plasma (NTP) on diesel particulate matter (PM), an engine test bench was built up. An engine exhaust particle sizer (EEPS) was introduced to analyze the emission concentration and size distribution of PM and a thermo-gravimetric analyzer was used to analyze the effects of NTP on the composition of the particulate matter in the exhaust gas. The results show that the size distribution interval of the particle mass concentration falls behind that of the quantity concentration under various loads. When the diesel engine operating speed is 2400 rpm and the load is 25%, after NTP, the proportions of the nucleation mode particles and the accumulative mode particles exhibit a small fluctuation while the proportion of ultrafine particles decreases by 10% due to their large quantity concentration. Under the dual effect of DPF and NTP, the particle quantity concentration decreases by 98%. In order to investigate the effect of NTP on the composition of the PM, a thermo-gravimetric analysis of the particles obtained before and after NTP was carried out. The results show that the proportion of volatile matter falls by 16.05% and solid carbon accounts for an increase of 7.29%. NTP has the ability to improve reduction activity of particles and make particles easier to be oxidized at a lower temperature.
Keywords: Diesel engine; Non-thermal plasma; Particulate matter; Particle size distribution

In this paper, a single-gap pseudospark discharge chamber was built and tested with several electrode gap distances and pressures to explore the characteristics of the pseudospark under nanosecond pulsed voltages. Experimental results show that with the development of the discharge, anode plasma firstly forms near the anode, and then moves towards the cathode. The ionization initial voltage decreases with the increase of the electrode gap distance and pressure in our pseudospark discharge configuration. By analysing the emission spectra, it is concluded that the major emission spectra are composed of the second positive bands, and the electron energy of the plasma is in the level of several tens of electron volts. The mechanism of how the pressure and gap distance influence the characteristics has been discussed with the simulation of the collision ionization process using the Monte Carlo Collision method. Based on the simulation and experimental results, it is inferred that the breakdown voltages is decided by the ionization process of the electron with the neutral gas in the main gap, and a possible reason is put forward to explain the reason that U is a function of p 2 d.
Keywords: Single-gap pseudospark; Nanosecond pulse voltages; Breakdown characteristics

Evaluation of the Effect of Plasma Treatment Frequency on the Activation of Polymer Particles by Hisham M. Abourayana; Vladimir Milosavljević; Peter Dobbyn; Denis P. Dowling (1223-1235).
This study investigates the influence of treatment frequency (1–150 kHz) on the atmospheric plasma activation of both silicone and polyethylene terephthalate (PET) particles. These polymer particles with diameters in the range 3–5 mm, were treated using either helium or helium/oxygen gas mixtures, in a barrel atmospheric plasma system. The level of polymer particles activation was monitored using water contact angle measurements. The effect of plasma treatment frequency on barrel heating was monitored using an infrared thermographic camera, the maximum barrel temperature after 15 min treatment was found to be 98 °C at a frequency of 130 kHz. Optical emission spectroscopy was used as a diagnostic tool to monitor changes in atomic and molecular species spectral intensity with experimental conditions, as well as a change in electron energy distribution function. Electrical characterisation studies demonstrated an increase in plasma power with increasing frequency, in the range investigated. X-ray photoelectron spectroscopy analysis indicate an increase of oxygen content on polymer surfaces after plasma treatment. For silicone particles, the minimum polymer water contact angle was obtained by using a frequency of 130 kHz. After 15 min treatment time, the water contact angle decreased from 141° to 11°. While for PET particles the optimum treatment frequency was found to be 70 kHz, resulting in a water contact angle decreased from 94° to 32°. This lower frequency was used due to the partial melting of the PET (Tg of 80 °C), when treated at the higher frequency.
Keywords: Atmospheric plasma; Barrel reactor; Surface activation; Frequency

One-Step Synthesis of Silicon Oxynitride Films Using a Steady-State and High-Flux Helicon-Wave Excited Nitrogen Plasma by Tianyuan Huang; Chenggang Jin; Jun Yu; Yan Yang; Lanjian Zhuge; Xuemei Wu; Zhendong Sha (1237-1247).
A steady-state and high-flux helicon-wave excited N2 plasma was used to oxynitride Si substrates for the synthesis of silicon oxynitride (SiON) films. X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) have been extensively used to characterize surface quality of the SiON films, and it is found that a large amount of nitrogen (N) can be incorporated into the films. The result of XPS depth profiles shows that the N concentration is high near the surface and the oxide/Si interface. In the UPS spectra, absence of the reappearance of surface states suggests a resistance to clustering of the oxynitride layer. The N2 flux and Ar mixture quantity can facilitate tuning of the dissociation characteristics in N2 discharge. By modulating the N2 fractions, the N+ density reaches maximum at a N2/(N2 + Ar) flow-rate ratio of 0.5, resulting in incorporation of more N atoms into the SiON films. Considering the easy control of N2 plasma, our work opens up a new avenue for achieving high-yield SiON films at low temperature.
Keywords: Nitrogen helicon plasma; Silicon oxynitride (SiON) films; Mixture compositions; Ion energy distributions; XPS

The thermal unimolecular decomposition of SiH4 + ion and its related reverse reactions, SiH3 + + H and SiH2 + + H2, have been investigated by ab initio molecular orbital and quantum statistical variational RRKM theory calculations. The potential energy surface has been calculated at different levels of theory; the results at the highest level, CCSD(T)/CBS//CCSD(T)/6-311++G(3df,2p), show that the decomposition of SiH4 + can mainly occur via a barrierless channel giving SiH2 + + H2 lying 11.8 kcal/mol above the reactant, or via a transition state forming SiH3 +···H complex to be followed by a barrierless decomposition yielding SiH3 + + H lying 23.5 kcal/mol above the reactant. Barrierless processes were calculated using the CASPT2 and CASSCF methods with the 6-311++G(3df,2p) basis set and fitted with Morse potentials. The rate constants were predicted by solving master equations based on the RRKM theory at the E,J-resolved level; the results show that the channel SiH4 + → SiH2 + + H2 is predominate under PEVCD conditions. For H- and H2-capturing by SiH3 + and SiH2 + ions, respectively, the rate constants were found to be weakly dependent on temperature at the high-pressure limit and decrease rapidly with pressure. The calculated heats of formation of the SiH x + (x = 2–4) ions are in close agreement with available thermochemical data.
Keywords: PECVD; Silicon thin film; SiH4 + ; Mechanism; Kinetics

This paper represents emission spectrometric analysis of a hollow-cathode glow discharge plasma with helium–oxygen mixed gas for surface treatment of a cobalt-based alloy, together with surface analysis of the resulting oxide layer. A Co–28Cr–6Mo alloy was employed as a specimen. The objective of this work is to obtain plasma information for the operating conditions to be optimized for producing a stoichiometric oxide layer on the alloy surface. Helium atomic lines, atomic and ionic lines of oxygen atom, and band heads of oxygen molecule ion were observed in the emission spectra. These intensities drastically changed depending on a mixing ratio of helium–oxygen mixed gas; particularly, the emission intensity of the molecular bands was largely enhanced in the mixed gas plasma compared to the pure oxygen plasma. This band spectrum is assigned to an electronic transition from the 4 Σ g to 4 Π u states of oxygen molecule ion, whose excitation energies are 18–19 eV from the ground state of oxygen molecule. It is thus suggested that a Penning-type ionization process with metastables of helium atom (1s2s 1 S 0, 20.6 eV and 3 S 1, 19.8 eV) is an excitation mechanism how the number density of the corresponding excited state can be elevated in the helium–oxygen mixed gas plasma. This effect, which more populated the excited oxygen species in the plasma, also exerted influence on the resultant oxide layer, such as the chemical composition and the layer thickness. Surface analysis by X-ray photoelectron spectroscopy indicated that an oxide layer consisting of iron and chromium oxides was formed by this plasma treatment, and that chromium atom was enriched in it. The thickness of the oxide layer varied with a mixing ratio of the plasma gas.
Keywords: Atomic emission spectrometry; Hollow-cathode glow discharge; Helium–oxygen mixed gas plasma; Penning-type ionization; Surface modification; Co–Cr–Mo alloy

Experimental and Theoretical Optimization of Radio Frequency Hollow Cathode Discharge by Xin-Xian Jiang; Wei-Ping Li; Shao-Wei Xu; Feng He; Qiang Chen (1281-1290).
In this paper, the electron density in radio frequency (RF) hollow cathode discharge (HCD) is investigated in experiments and in simulation based on two-dimensional (2D) fluid model. The role of hole diameter on plasma density in RF HCD have been explored at various gas pressures. It is found that the optimal hole size for maximum plasma density decreases along with the increase of gas pressure, which is confirmed by simulated results. The simulations reveal that the high plasma density is owing to the hollow cathode effect in the hollow cathode. It is obtained that the optimal hole diameter in RF HCD is approximately sum of twice thickness of plasma sheath and triple the electron-neutral mean free path.
Keywords: Plasma; Hollow cathode discharge; Radio frequency

Erratum to: Experimental and Theoretical Optimization of Radio Frequency Hollow Cathode Discharge by Xin-Xian Jiang; Wei-Ping Li; Shao-Wei Xu; Feng He; Qiang Chen (1291-1291).