Gas Glow Discharge Research Articles

Removal of volatile organic compounds in atmospheric pressure air by means of direct current glow discharges

A nonthermal plasma with an electron density on the order of 10/sup 12/ cm/sup -3/ and a gas temperature of 2000 K was generated in atmospheric pressure air, using a microhollow cathode discharge as plasma cathode. The plasma was sustained in a /spl sim/1 mm/sup 3/ micro reactor, by a voltage of 470 V between the plasma cathode and a planar anode, and at currents ranging from 12 to 22 mA. This direct current glow discharge has been used to study the remediation of methane and benzene, two of the most stable volatile organic compounds (VOCs). The removal fraction for 300-ppm methane in atmospheric pressure air, flowing through the 0.5-mm thick plasma layer, with a residence time of the gas in the plasma of less than 0.5 ms, was measured at 80% with an energy density of 4 kJ/L. For benzene, the remediation rate is as high as 90%, comparable to results obtained with low pressure glow discharges. The energy efficiency for benzene remediation is 0.9 g/kWh, higher than that obtained for benzene remediation in low pressure glow discharges in noble gases. However, the VOC fraction remaining was found to be limited to values of approximately 0.1 and 0.05 for methane and benzene, respectively. In addition to experimental studies, the VOC dissociation mechanism in a VOC/dry air mixture plasma was modeled using a zero-dimensional plasma chemistry code. The modeling results have shown that atomic oxygen impact reactions are the dominant dissociation reactions for VOC destruction in this kind of glow discharge. Diffusion of atomic oxygen to the dielectric walls of the reactor is assumed to cause the observed limitation in the VOC destruction rate and efficiency.

Imaging of atmospheric pressure glow discharges in helium and argon

We present images of atmospheric pressure glow discharges (APGD) in pure noble gases, helium and argon, for a needle-plane (N-P) electrode geometry. The images have been obtained using an intensified charge-coupled device (ICCD) digital camera, at nanosecond exposure times. We present time-dependence of discharge images perpendicular and parallel to the interelectrode gap for both helium and argon.

Two-dimensional optical imaging spectroscopy in a dc driven Mercury-neon-argon glow discharge

Imaging spectroscopy is a powerful tool for visualization of plasma phenomena. A novel type of imaging spectrometer based on an acoustic optical tunable filter (AOTF) was used to select several emission lines for imaging from a mercury: rare gas glow discharge driven by direct current (dc) current. Images taken from this discharge show different locations of light origin for different emission lines. The spatial and wavelength resolutions of the spectrometer are evaluated.

Adsorbtion of polyelectrolyte multilayers on plasma-modified porous polyethylene

Hydrophilic and chemically reactive porous media was prepared by adsorbing functional polymers at the surface of sintered polyethylene membranes. Modification of the membrane was accomplished by first exposing the membrane to an oxygen glow discharge gas plasma to render electrostatic charge at the membrane surfaces. A cationic polyelectrolyte was adsorbed from solution to the anionically charged surface to form an adsorbed monolayer. The adsorption of a second anionic polyelectrolyte allowed further modification of the membrane surface with a polyelectrolyte bilayer complex. In this paper we probe the effect of polymer structure on the conformation and stability of the adsorbed polyelectrolyte monolayers and bilayers on the modified polyethylene surface. Using the wicking rate of deionized, distilled water through the porous membrane to gauge the interfacial energy of the modified surface, we show that the wicking rate of the multilayer membrane can be controlled by varying the chemistry of the adsorbing polyelectrolytes and their molecular weights.

Evaporation–glow discharge hybrid source for plasma immersion ion implantation

The plasma ion sources play a very important role in the plasma immersion ion implantation (PIII) process. In this paper, we report on our newly designed evaporation–glow discharge hybrid ion source for PIII. The high negative substrate bias not only acts as the plasma producer but also provides the implantation voltage. The sulfur vapor gas glow discharge shows that the electrons in the plasma are focused to the orifice of the inlet tube, thereby helping the ionization of the fleeing vapor gases. The sulfur depth profile confirms that this evaporation–glow discharge hybrid source is effective for materials with a low melting point and high vapor pressure.

Adsorption of polyelectrolyte multilayers on plasma-modified porous polyethylene

Hydrophilic and chemically reactive porous media was prepared by adsorbing functional polymers at the surface of sintered polyethylene membranes. Modification of the membrane was accomplished by first exposing the membrane to an oxygen glow discharge gas plasma to render electrostatic charge at the membrane surfaces. Cationic polyelectrolyte polyethylenimine (PEI) was adsorbed from solution to the anionically charged surface to form an adsorbed monolayer. The adsorption of a second anionic polyelectrolyte allowed further modification of the membrane surface with a polyelectrolyte bilayer complex. In this paper we probe the effect of polymer structure on the conformation and stability of the adsorbed polyelectrolyte monolayers and bilayers on the modified polyethylene surface. Using the wicking rate of deionized, distilled water through the porous membrane to gauge the interfacial energy of the modified surface, we show that the wicking rate of the multilayer membrane can be controlled by varying the chemistry of the adsorbing polyelectrolytes and their molecular weights.

Deposition and Wetting Characteristics of Polyelectrolyte Multilayers on Plasma-Modified Porous Polyethylene

Hydrophilic and chemically reactive porous media were prepared by adsorbing functional polymers at the surface of sintered polyethylene membranes. Modification of the membrane was accomplished by first exposing the membrane to an oxygen glow discharge gas plasma to introduce an electrostatic charge at the membrane surfaces. Cationic polyelectrolyte polyethylenimine (PEI) was adsorbed from solution to the anionic-charged surface to form an adsorbed monolayer. The adsorption of a second anionic polyelectrolyte onto the PEI layer allows further modification of the membrane surface to form a polyelectrolyte-bilayer complex. The conformation and stability of the adsorbed monolayers and bilayers comprising the modified surface are probed as a function of the polymer structure, charge density, and solubility. Using X-ray photoelectron spectroscopy analysis, we demonstrate that the presence of the polyelectrolyte multilayers drastically increases the density and specificity of the functional groups at the surface, more than what can be achieved through the plasma modification alone. Also, using the wicking rate of deionized, distilled water through the porous membrane to gauge the interfacial energy of the modified surface, we show that the membrane wicking rate can be controlled by varying the chemistry of the adsorbing polyelectrolytes and, to a lesser extent, by adjusting the polarity or ionic strength of the polyelectrolyte solution.

Microhollow cathode discharges

By reducing the dimensions of hollow cathodes into the hundred micrometer range, stable, direct current, high (atmospheric) pressure glow discharges in rare gases, rare gas–halide mixtures and in air could be generated. The electron energy distribution in these microdischarges is non-Maxwellian, with a pronounced high-energy tail. The high electron energy together with the high gas density, which favors three-body collisions, is the reason for an efficient excimer generation in these microplasmas. Excimer efficiencies from 1% to 9% have been measured for argon, xenon, argon fluoride, and xenon chloride direct current excimer emitters, with a radiant excimer emittance of up to 2 W/cm2 for xenon. Adding small amounts of oxygen to argon has allowed us to generate vacuum ultraviolet line radiation at 130.5 nm with an efficiency approaching 1%. Pulsing xenon discharges with nanosecond electrical pulses has led to an increase in intensity to 15 W/cm2 and to a simultaneous increase in efficiency to more than 20%. Operating the discharges in an abnormal glow mode has allowed us to generate microdischarge arrays without individual ballast. Applications of these plasma arrays are excimer lamps and plasma reactors.

Deterministic Boltzmann solver for electron kinetics in plasma reactors for microelectronics applications

A kinetic module has been developed in the commercial software package CFD-ACE+ and applied to simulations of plasma reactors for microelectronics applications. The kinetic module solves the Boltzmann transport equation (BTE) using two-term spherical harmonics expansion (SHE) of the probability distribution function (PDF). This method reduces the 6D BTE to a Fokker Planck equation in a four-dimensional space (three spatial coordinates+energy) offering a very good compromise between physical accuracy and numerical efficiency. This paper describes the design of the kinetic module and its current status and applications to electron kinetics in gas discharges. The kinetic module is coupled to other modules in CFD-ACE+ for self-consistent kinetic simulations of plasmas. The Fokker–Planck equation is solved for the electron energy probability function (EEPF) providing macroscopic characteristics (electron density, fluxes, rates of electron induced chemical reactions, etc.). Using these quantities, the transport of ions and neutrals in plasmas is simulated using a continuum model. The electromagnetic fields are calculated by solving Maxwell equations in the potential formulation (scalar electric and vector magnetic potentials). Several examples of hybrid kinetic simulations of plasma reactors are described including inductively coupled plasma (ICP), and classical DC glow discharges in electropositive and electronegative gases. The developed Boltzmann solver expands the applicability of computational plasma models to low gas pressures and enhances accuracy and fidelity of plasma simulations.

Emission in the deep vacuum ultraviolet from a plasma formed by incandescently heating hydrogen gas with trace amounts of potassium carbonate

A hydrogen plasma with intense extreme ultraviolet and visible emission was generated from low pressure hydrogen gas (0.1–1 mbar) in contact with a hot tungsten filament only when the filament heated a titanium dissociator coated with K2CO3 above 750°C. The electric field strength from the filament was about 1 V cm−1, two orders of magnitude lower than the starting voltages measured for gas glow discharges. The emission of the Hα and Hβ transitions as well as the Lα and Lβ transitions were recorded and analysed. The plasma seemed to be far from thermal equilibrium, and no conventional mechanism was found to explain the formation of a hydrogen plasma by incandescently heating hydrogen gas in the presence of trace amounts of K2CO3. The temporal behaviour of the plasma was recorded via hydrogen Balmer alpha line emission when all power into the cell was terminated and an excessive afterglow duration (2 s) was observed. The plasma was found to be dependent on the chemistry of atomic hydrogen with potassium since no plasma formed with Na2CO3 replacing K2CO3 and the time constant of the emission following the removal of all of the power to the cell matched that of the cooling of the filament and the resulting shift from atomic to molecular hydrogen. Our results indicate that a novel chemical power source is present and that it forms the energetic hydrogen plasma that is a potential new light source.

Wetting Characteristics of Plasma-Modified Porous Polyethylene.

This paper examines the wetting characteristics of porous polyethylene surfaces modified by exposure to reactive oxygen glow discharge gas plasma, through the direct measurement of the wicking properties of the modified material. It is well-known that oxygen plasma can be used to chemically alter the surface of polyethylene to enhance wetting properties. Chemical and physical modification of a polymer's surface is the consequence of reactions initiated by the collision of high-energy species in the plasma with the polymer surface. The conditions of the plasma treatment, such as electric field strength (power), exposure time, and chamber pressure, govern the frequency and energy of collisions and, thus, determine the nature and degree of the chemical modification of the polyethylene surface. Comparisons of the chemical modification of sintered porous polyethylene surfaces achieved through treatments with reactive oxygen glow discharge gas plasmas generated at various powers, chamber pressures, and times of exposure were made by measuring the wicking rate of distilled and deionized water in the modified materials. A strong correlation was observed between the electric field power used to generate the plasma and the degree of chemical modification of the polyethylene surfaces. In addition, the rate of chemical modification was also found to be a function of the electric field power.

Efficacy of glow discharge gas plasma treatment as a surface modification process for three-dimensional poly (D,L-lactide) scaffolds.

Gas plasma surface modification of three-dimensional poly (D,L-lactide) scaffolds fabricated by a novel vibrating particle fabrication technique was demonstrated to enhance cell adhesion, proliferation, and differentiation over 10 days in culture using human embryonic palatal mesenchyme cells. Characterization of corresponding two-dimensional treated surfaces revealed decreased contact angle measurements of 54.2 +/- 0.6 degrees for treated surfaces compared to 72.3 +/- 0.7 degrees for control surfaces (p < 0.05). SEM of treated surfaces revealed increased surface roughness combined with marked pitting and erosion. This may contribute to increased cell adhesion. WST-1 cell proliferation assay measurements as an index of cell numbers revealed a statistically significant increase in proliferation activity on treated surfaces on days 1 and 4 compared with controls. There was a fivefold increase in WST-1 activity for both control and treated groups over 10 days. Confocal laser micrographs revealed increased cell numbers on treated specimens throughout all layers of the scaffold, indicating that the glow discharge process enhanced cell proliferation throughout the entire scaffold architecture. Scanning electron micrographs demonstrated increased cell adhesion for treated specimens at the polymer surface most evident after days 1 and 4 of culture. Alkaline phosphatase (ALP)-specific activity peaked by day 7 for control and treated surfaces, indicating cellular differentiation. There was a trend for increased protein production on the treated specimens compared with controls at the initial time points although the differences were not statistically significant. These results demonstrated that gas plasma surface modification enhances osteoblast-like cell function in a three-dimensional scaffold model.

Approximations to the electron energy distribution and positive column models for low-pressure discharge light sources

The applicability of `fluid' models based on analytic approximations of the electron energy distribution function (EEDF) and of kinetic models for low-pressure discharge light sources is discussed. Traditionally, `fluid' models of fluorescent lamps assume that the EEDF is Maxwellian up to the energy of the first excited state. It is shown that such an approach is sufficiently accurate in most cases of conventional as well as of `highly loaded' fluorescent lamps. However, this assumption is strongly violated for many rare gas glow discharges for mercury free light sources. As an example, a neon dc discharge is studied. The densities of the four lowest excited states and the electric field have been measured. The experimental results can be fairly well reproduced by a kinetic positive column model. This article was scheduled to appear in issue 14 of J. Phys. D: Appl. Phys. To access this special issue please follow this link: http://stacks.iop.org/0022-3727/35/i=14/

Prebreakdown Phenomena and Formation Processes of Low-Pressure Glow Discharges in Ar, Ar/N2 and Ar/O2 Mixtures

In this research, both the prebreakdown phenomena and the transient processes from the prebreakdown phenomena to the glow discharge in low-pressure Ar gas, Ar/N2 and Ar/O2 mixtures were investigated by electrical and optical methods. The prebreakdown phenomenon in the Ar/N2 mixture was similar to that of Ar gas, and a pulseless component has been observed. The prebreakdown phenomenon in the Ar/O2 mixture differed from those in the Ar gas and Ar/N2 mixture, and formed a pulse discharge. Thus, the prebreakdown phenomenon in the Ar gas has been changed to pulse discharge by mixing a small amount of O2 gas which played an important role in the formation of pulse discharge.

Prebreakdown phenomena and formation process of the glow discharge in low-pressure Ar gas

The prebreakdown phenomena and the formation process of the glow discharge in a low-pressure Ar gas were investigated under a uniform field gap. Prebreakdown phenomena were observed for 0.5 Torr cm⩽pd⩽2 Torr cm (where p is pressure, d the gap distance) in Ar gas under conditions of a slowly increasing voltage. It was observed that the prebreakdown phenomena formed pulse discharges up to the transition to the glow discharge. The amplitudes of the photon and current pulses due to the pulse discharge increased with time, and then decreased as soon as the transition to a steady glow discharge occurred. When the overvoltage or external series resistance was increased, the pulse amplitudes increased with the applied voltage and decreased with the resistance. The characteristics of the prebreakdown phenomena were changed by the shape of the electrodes. The formation mechanism of the glow discharge can be qualitatively explained by that of the streamer in a high-pressure discharge. The transient glow discharge was observed, and its duration increased with an increase in resistance. The instability of the glow discharge was controlled by three factors, namely, Kaufmann’s criterion, the Child–Langmuir law, and the density balance between the production and removal rates of electrons.

Permeation of hydrogen through amorphous ferrum membrane

Hydrogen permeation through amorphous and recrystallized foils of Fe-based alloys was investigated. It was found that surface layers of both types of samples were enriched by metalloids. These layers prevented permeation of hydrogen from molecular and atomic phases. Only the ionization of the gas in glow discharge near the upstream side of foils resulted in big permeation fluxes. Kinetics of flux relaxation to the steady-state values revealed reversible trapping in amorphous alloy. Permeation flux demonstrated Arrhenius linear dependence for recrystallized samples and non-monotonous for amorphous ones. Values of steady-state flux through amorphous foils were several times higher than through ordered samples at temperatures below 200°C. The mechanism of hydrogen transport is proposed taking into account re-emission and diffusion processes. Activation energies for thermal desorption and diffusion are evaluated.

Asymptotic layer-type model of high pressure direct current glow discharge in electronegative gases

Here a self-consistent one-dimensional continuum model is presented for a narrow gap plane-parallel dc glow discharge. The governing equations consist of continuity and momentum equations for positive and negative ions and electrons coupled with Poisson’s equation. A singular perturbation method is developed for the analysis of high pressure dc glow discharge. The kinetic processes of the ionization, electron attachment, and ion–ion recombination are included in the model. Explicit results are obtained for the asymptotic limits: δ=(rD/L)2→0, ω=(rS/L)2→0, where rD is the Debye radius, rS is recombination length, and L is the gap length. The discharge gap divides naturally into four layers with multiple space scales: anode fall region, positive column, transitional region, cathode fall region and diffusion layer adjacent to the cathode surface, its formation is discussed. The effects of the gas pressure, gap spacing and dc voltage on the electrical properties of the layers and its dimension are investigated.

A modified Paschen law for the initiation of a dc glow discharge in inert gases

Breakdown of inert gases in a homogeneous dc electric field is studied experimentally and theoretically at various distances L between the electrodes and radii R of the discharge tubes. It is shown that, for arbitrary geometric dimensions of the discharge chamber and cathode materials, the ratio of the breakdown electric field strength to the gas pressure holds constant at the breakdown curve minimum. A modified Paschen law is obtained, according to which the breakdown voltage is a function of both the product of the gas pressure by the distance L and the ratio L/R.

Measurements of Electric Fields in a Discharge Plasma Using Laser Optogalvanic Detection of the Stark Effect of Argon Atoms

The spatial distribution of the electric field in the vicinity of a cathode in a dc glow discharge in argon gas was studied. The effect of the electric field on the laser-optogalvanic (LOG) spectra of argon atoms was used to determine the electric field. The magnitude of the electric field F was determined by comparing the experimental LOG spectra of argon atoms with a set of corresponding theoretically obtained spectra calculated for different values of F. The effect of the electric field gradient within the spatial width of the laser beam was taken into account.

A Hollow-Cathode Transient Plasma Process for Thin Film Growth

AbstractWe have developed a vapor deposition method that produces a highly ionized transient plasma plume of metallic species in the presence of a low-pressure inert or reactive gas glow discharge. In this process, a transient electrical discharge is formed in a hollow-cathode by a pulse-forming network (PFN) which is triggered by a pulsed CO2 laser. Current pulses with peak currents of 100 kA and pulse widths of about 20 ms have been produced by the PFN. The effect of the PFN power input and the ambient gas pressure on the evaporated material yield is presented. These experiments also showed a higher evaporation rate of carbon in a nitrogen ambient than in an Ar ambient. Carbon films, with rates of deposition exceeding 18A per pulse that are uniform over a large area, have been deposited. The ionic content of the plasma, spatial distribution of ions, and plume expansion dynamics have been investigated by time-of-flight ion probe measurements and optical emission spectroscopy and are presented.