Pressure Glow Discharge Plasma Research Articles

Electrical Features of Radio-frequency, Atmospheric-pressure, Bare-metallic-electrode Glow Discharges

Radio-frequency (RF), atmospheric-pressure glow discharge (APGD) plasmas with bare metallic electrodes have promising prospects in the fields of plasma-aided etching, deposition, disinfection and sterilization, etc. In this paper, an induced gas discharge approach is proposed for obtaining the RF, atmospheric-pressure, γ-mode, glow discharges with pure nitrogen or air as the primary plasma-working gas using bare metallic electrodes. The discharge characteristics, including the discharge mode, the breakdown voltage and discharge voltage for sustaining α mode and/or γ mode discharges, of the RF APGD plasmas of helium, argon, nitrogen, air or their mixtures using a planar-type plasma generator are presented in this study. The uniformity (no filaments) of the discharges is confirmed by the images taken by an iCCD with a short exposure time (10 ns). The effects of different gap spacings and electrode materials on the discharge characteristics, the variations of the sheath thickness and the electron number density are also studied in this paper.

Characteristics of atmospheric-pressure, radio-frequency glow discharges operated with argon added ethanol

Rf, atmospheric-pressure glow discharge (APGD) plasmas with bare metal electrodes have promising prospects in the fields of plasma-aided etching, thin film deposition, disinfection and sterilization, etc. In this paper, the discharge characteristics are presented for the rf APGD plasmas generated with pure argon or argon-ethanol mixture as the plasma-forming gas and using water-cooled, bare copper electrodes. The experimental results show that the breakdown voltage can be reduced significantly when a small amount of ethanol is added into argon, probably due to the fact that the Penning ionization process is involved, and a pure α-mode discharge can be produced more easily with the help of ethanol. The uniformity of the rf APGDs of pure argon or argon-ethanol mixtures using bare metallic electrodes is identified with the aid of the intensified charge coupled device images.

The back-diffusion effect of air on the discharge characteristics of atmospheric-pressure radio-frequency glow discharges using bare metal electrodes

Radio-frequency (RF), atmospheric-pressure glow discharge (APGD) plasmas using bare metal electrodes have promising prospects in the fields of plasma-aided etching, deposition, surface treatment, disinfection, sterilization, etc. In this paper, the discharge characteristics, including the breakdown voltage and the discharge voltage for sustaining a stable and uniform α mode discharge of the RF APGD plasmas are presented. The experiments are conducted by placing the home-made planar-type plasma generator in ambient and in a vacuum chamber, respectively, with helium as the primary plasma-forming gas. When the discharge processes occur in ambient, particularly for the lower plasma-working gas flow rates, the experimental measurements show that it is the back-diffusion effect of air in atmosphere, instead of the flow rate of the gas, that results in the obvious decrease in the breakdown voltage with increasing plasma-working gas flow rate. Further studies on the discharge characteristics, e.g. the luminous structures, the concentrations and distributions of chemically active species in plasmas, with different plasma-working gases or gas mixtures need to be conducted in future work.

Simulation of radio-frequency atmospheric pressure glow discharge in γ mode

The existence of two different discharge modes has been verified in an rf (radio-frequency) atmospheric pressure glow discharge (APGD) by Shi [J. Appl. Phys. 97, 023306 (2005)]. In the first mode, referred to as α mode, the discharge current density is relatively low and the bulk plasma electrons acquire the energy due to the sheath expansion. In the second mode, termed γ mode, the discharge current density is relatively high, the secondary electrons emitted by cathode under ion bombardment in the cathode sheath region play an important role in sustaining the discharge. In this paper, a one-dimensional self-consistent fluid model for rf APGDs is used to simulate the discharge mechanisms in the γ mode in helium discharge between two parallel metallic planar electrodes. The results show that as the applied voltage increases, the discharge current becomes greater and the plasma density correspondingly increases, consequentially the discharge transits from the α mode into the γ mode. The high collisionality of the APGD plasma results in significant drop of discharge potential across the sheath region, and the electron Joule heating and the electron collisional energy loss reach their maxima in the region. The validity of the simulation is checked with the available experimental and numerical data.

Modeling of Atmospheric Pressure Glow Discharge Plasmas in Nitrogen-Oxygen Mixtures

A model for atmospheric pressure glow discharge (APGD) plasmas in the nitrogen-oxygen mixtures was constructed. The model is based on electron-neutral, ion-neutral, neutral-neutral gas phase reactions and the one-dimensional fluid equations which consist of particle continuity and Poisson's equations. Optical emission spectroscopy measurements were performed for the APGD plasma at a frequency 100 kHz to evaluate the validity of the model. The emission intensity of the second positive system (SPS) band of nitrogen transition from N2(C3Πu) to N2(B3Πg) and NO-γ system transition from NO(A2Σ+) to ground state were measured as a function of mass fraction of oxygen admixture. The densities of N2(C3Πu) and NO(A2Σ+) calculated by the model show trends similar to the observed emission intensities of SPS and NO-γ system, respectively.

Electrostatic properties of PE and PTFE subjected to atmospheric pressure plasma treatment; correlation of experimental results with atomistic modeling

The use of an atmospheric pressure glow discharge (APGD) plasma was used at Kennedy Space Center (KSC) to increase the hydrophilicity of spaceport materials to enhance their surface charge dissipation and prevent possible electrostatic discharge (ESD) in spaceport operations. Significant decreases in charge decay times were observed after tribocharging the materials using the standard KSC tribocharging test. The polarity and amount of charge transferred was dependant upon the effective work function differences between the respective materials. In this study, PE and PTFE were exposed to a He+O 2 APGD. The pre and post-treatment surface chemistries were analyzed by X-ray photoelectron spectroscopy and contact angle measurements. Semi-empirical and ab initio calculations were performed to correlate the experimental results with some plausible molecular and electronic structure features of the oxidation process. For the PE, significant surface oxidation was observed, as indicated by XPS showing C–O, C O, and O–C O bonding, and a decrease in the surface contact angle from 98.9° to 61.2°. For the PTFE, no C–O bonding appeared and the surface contact angle increased indicating the APGD only succeeded in cleaning the PTFE surface without affecting the surface structure. The calculations using the Parametric Method 3 (PM3) and Density Function Theory (DFT) methods were performed on single and multiple oligomers to simulate a wide variety of oxidation scenarios. Calculated work function results suggest that regardless of oxidation mechanism, e.g. –OH, O or a combination thereof, the experimentally observed levels of surface oxidation are unlikely to lead to a significant change in the electronic structure of PE and that its increased hydrophilic properties are the primary reason for the observed changes in its electrostatic behavior. The calculations for PTFE argue strongly against significant oxidation of that material, as confirmed by the XPS results.

Atmospheric Pressure Glow-Discharge Plasmas with Gas–Liquid Interface

An atmospheric pressure glow-discharge plasma in contact with liquid is generated using a capacitively coupled plasma (CCP) method, by which a boundary region between a plasma (gas-phase) and liquid paraffin (liquid-phase), i.e., gas–liquid interface is considered to be important. A stable atmospheric pressure plasma with liquid is achieved by selecting the appropriate mesh electrode and liquid paraffin. In addition, results of optical emission spectroscopy indicate that carbonic species come from paraffin in the interface region of the plasma. This plasma is accordingly expected to promote the use of an attractive plasma process for creating materials encapsulating various elements in liquids.

Cell treatment and surface functionalization using a miniature atmospheric pressure glow discharge plasma torch

A miniature atmospheric pressure glow discharge plasma torch was used to detach cells from a polystyrene Petri dish. The detached cells were successfully transplanted to a second dish and a proliferation assay showed the transplanted cells continued to grow. Propidium iodide diffused into the cells, suggesting that the cell membrane had been permeabilized, yet the cells remained viable 24 h after treatment. In separate experiments, hydrophobic, bacteriological grade polystyrene Petri dishes were functionalized. The plasma treatment reduced the contact angle from 93° to 35°, and promoted cell adhesion. Two different torch nozzles, 500 µm and 150 µm in internal diameter, were used in the surface functionalization experiments. The width of the tracks functionalized by the torch, as visualized by cell adhesion, was approximately twice the inside diameter of the nozzle. These results indicate that the miniature plasma torch could be used in biological micropatterning, as it does not use chemicals like the present photolithographic techniques. Due to its small size and manouvrability, the torch also has the ability to pattern complex 3D surfaces.

Killing of S. mutans Bacteria Using a Plasma Needle at Atmospheric Pressure

Streptococcus mutans (S. mutans) bacteria were killed using a low-power millimeter-size atmospheric-pressure glow-discharge plasma or plasma needle. The plasma was applied to a culture of S. mutans that was plated onto the surface of an agar nutrient in a Petri dish. S. mutans is the most important microorganism for causing dental caries. A spatially resolved biological diagnostic of the plasma is introduced, where the spatial pattern of bacterial colonies in the sample was imaged after plasma treatment and incubation. For low-power conditions that would be attractive for dentistry, images from this biological diagnostic reveal that S. mutans was killed within a solid circle with a 5-mm diameter, demonstrating that site-specific treatment is possible. For other conditions, which are of interest for understanding plasma transport, images show that bacteria were killed with a ring-shaped spatial pattern. This ring pattern coincides with a similar ring in the spatial distribution of energetic electrons, as revealed by Abel-inverted images of the glow. The presence of the radicals OH and O was verified using optical-emission spectroscopy

Hydrophilic modification of expanded polytetrafluoroethylene (ePTFE) by atmospheric pressure glow discharge (APG) treatment

ePTFE has been used in biomedical applications such as artificial blood vessels. However, the inherent nature of the highly hydrophobic ePTFE surface retards tissue ingrowth and wound healing. APG plasma treatment was applied to modify ePTFE tubes from being hydrophobic to being hydrophilic with AAc or PAA. The plasma-pretreated ePTFE tubes were immersed in AAc or PAA solution, then treated with helium-APG plasma. The spectra of FTIR–ATR adopted the appearance of an absorption peak ascribed to carbonyl group, and XPS showed reduced F / C and increased O / C. The water contact angles after treatment were decreased. An SEM image showed that the fibrils of ePTFE tubes were interconnected with each other to form assembled bundles of fibrils. These results indicated the formation of an insoluble polymerized AAc or PAA layer on the ePTFE surface. Polymerization of AAc or PAA began occurring on the surface of ePTFE and then continued within porous structures. The surfaces of ePTFE were more effectively modified by PAA than AAc. The outer surface was more hydrophilic than the inner surface. When the outer tube of ePTFE was wrapped with PTFE film and processed using the same experimental method, a markedly enhanced surface modification on the inner surface, especially with the use of AAc, was observed. AAc was easier to polymerize on the surface of the ePTFE tube, as a result, the hydrophilicity of the inner surface was increased.

Metal Leadframe Surface Cleaning Prior to Electroplating by Atmospheric Pressure Glow Discharge Plasma

Metal leadframes (Alloy 42) were cleaned using atmospheric pressure glow discharge plasma. Atmospheric pressure glow plasma was generated by a RF source of 13.56MHz with a matching network (300W power) on to the leadframes free from streamers and arc. Argon (Ar) and oxygen (O2) were used as the carrier gas and reactive gas, respectively. The addition of O2 gas to Ar decreased the contact angle of water and increased the surface cleaning rate due to the ncrease of oxygen radicals in the plasma. The chemical characteristics and morphologies of leadframe surface after the plama treatment were analyzed using X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM), respectively. The contact angle of 82° before the plasma treatment was decreased to 5° in the processing condition with oxygen flow rate of 50sccm, treatment speed of 100mm/sec, and input power of 300W. These surface cleaning effect will be very useful in the replacement many steps of wet cleaning before electroplating.

LCD Glass Cleaning by Atmospheric Pressure Glow Discharge Plasma

We report on a novel method for the surface modification of indium tin oxide (ITO) in LCD glass by direct exposure to a dielectric barrier discharge (DBD) at atmospheric pressure and room temperature. To remove the organic contaminants from the surfaces of ITO film in LCD glass, the atmospheric pressure RF glow discharge plasma was used. Argon (Ar) and oxygen (O2) were used as the carrier gas and reactive gas, respectively. The addition of O2 gas to Ar decreased the contact angle of water and increased the surface cleaning rate due to the increase of oxygen radicals in the plasma. The chemical characteristics of ITO surface after the plama treatment were investigated using X-ray photoelectron spectroscopy (XPS), and new carboxyl group bond was produced. The contact angle of 64° before the plasma treatment was decreased to 7° in the processing condition with oxygen flow rate of 50 sccm, treatment speed of 100mm/sec, and input power of 300W. These hydrophilic effect will be very useful in the manufacturing processes of LCD glass.

Effects of Helium Atmospheric Pressure Plasma Treatment on Low-Stress Mechanical Properties of Polypropylene Nonwoven Fabrics

Polypropylene nonwoven fabrics are treated by He atmospheric pressure glow discharge plasma. After plasma treatment, weight loss (%), surface properties (wettability, morphology, and chemical composition changes), tensile strength, low-stress mechanical properties, and air permeability of the fabrics are examined. Scanning electron microscopy analysis shows significant surface morphology changes in plasma-treated polypropylene fiber surfaces, corresponding to reductions in fabric weight. X-ray photoelectron spectroscopy analysis reveals that surface oxidation by the formation of hydrophilic groups enhances the surface wettability of the fabrics. Surface morphology changes with plasma treatment increase fiber-to-fiber friction, playing an important role in enhancing their tensile strength, low-stress mechanical properties, and air permeability.

Stabilization of DC atmospheric pressure glow discharge by a fast gas flow along the anode surface

We have investigated experimentally characteristics of an atmospheric pressure glow discharge (APGD) produced by pin-plate electrodes with a gas flow. An APGD is sustained by DC voltage applied between these electrode with main and auxiliary gas flow. The auxiliary air flow is added to the main flow in a localized region by using a tiny nozzle. It is found for the first time that the glow discharge current is significantly increased when the auxiliary nozzle is set near the anode plate. This finding is very important to clarify the mechanism of a flow-stabilized APGD. An excellent hydrophilic property is observed on a sodium silicate glass surface treated by the APGD plasma only several seconds.

Estimation of point of zero charge for activated carbon treated with atmospheric pressure non-thermal oxygen plasmas

Commercial activated carbon was treated with a non-thermal oxygen plasma under atmospheric pressure using three types of reactors: a dielectric barrier discharge (DBD) parallel-plate reactor; an atmospheric pressure glow discharge plasma jet reactor and a DBD coaxial type reactor. The samples treated with the plasma were characterized by the pH value at the point of zero charge. An estimation of the adsorption properties of samples for copper ions in aqueous solution was also carried out in order to explain the effect of pH value on the adsorption results.

Hydrophobic recovery of VUV/NH3 modified polyolefin surfaces: Comparison with plasma treatments in nitrogen

Film samples of two very pure polyolefins (low density polyethylene, LDPE and biaxially oriented polypropylene, BOPP) were surface-modified by two different methods, namely vacuum ultraviolet (VUV) irradiation with a Kr resonant lamp in low-pressure NH3 gas, and atmospheric pressure glow discharge (APGD) plasma treatment in pure N2 gas. Samples were then stored in air and the time-dependence of surface properties (the surface energy and chemical composition) was monitored using several complementary surface-sensitive techniques: contact angle goniometry (CAG), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). We show that the main mechanism responsible for hydrophobic recovery is the motion of polymer chains and chain segments, which governs an apparent “loss” of functional groups, within the first monolayers of the surface (∼1nm). Finally, comparing BOPP samples modified by both techniques, we show that aging can be reduced by crosslinking near the surface, as illustrated by depth-sensing nano-indentation measurements.

The effect of etching on low-stress mechanical properties of polypropylene fabrics under helium/oxygen atmospheric pressure plasma

Polypropylene nonwoven fabrics were exposed to He/O2 atmospheric pressure glow discharge plasma. Surface chemical analysis and contact angle measurement revealed the surface oxidation by formation of new functional groups after plasma treatment. Weight loss (%) measurement and scanning electron microscopy analysis showed a significant plasma etching effect. It was investigated in low-stress mechanical properties of the fabrics using Kawabata Evaluation System (KES-FB). The surface morphology change by plasma treatment increased surface friction due to an enhancement of fiber-to-fiber friction, resulting in change of other low-stress mechanical properties of fabric.

VUV-induced nitriding of polymer surfaces: Comparison with plasma treatments in nitrogen

Film samples of two very pure polyolefins (low density polyethylene and biaxially oriented polypropylene) have been surface-modified by two different methods, vacuum ultraviolet (VUV) photochemistry in low pressure ammonia, and atmospheric pressure glow discharge plasma treatment in N 2 gas. The results of these two treatments are compared, namely surface compositions (determined by X-ray photoelectron spectroscopy and infrared spectroscopy, ATR-FTIR) and surface energies (determined by contact angle goniometry with several probe liquids). We show that higher concentrations, [N], can be achieved by VUV photochemistry (up to 25%), that N is predominantly bonded as amine or amide groups, and that there exist certain particularities specific to each of the treatment methods investigated.

Novel approach to produce polymerized hydrocarbon coatings using dielectric barrier controlled atmospheric pressure glow discharge plasma

Conventionally, low-pressure (<1 Torr) electrical discharges are used for material processing and thin-film deposition. These schemes suffer mainly due to the high cost of equipment and the complexity of operations. The atmospheric pressure glow discharge plasma is developed using a threaded styled electrode in different configurations, and these reactors are used to produce plasma polymerized coatings, required on plane substrates as self-supporting films to obtain membranes for blocking holes in cavities, and on microballoon targets, which are used as fuel containers for inertial confinement fusion, to avoid DT gas permeation. Helium gas is used as the supporting gas for formation and stabilization of atmospheric pressure glow discharge plasma reactors. Ethylene and acetylene gases are used as monomers to produce plasma polymerized hydrocarbon films. These films are characterized using scanning electron microscopy. Plasma polymerized coatings of thickness 100 nm–10 μm with a smooth surface finish (rms<100 nm) are deposited successfully. The surface finish is further improved using a postdischarge configuration. Preliminary results are very encouraging but further progress is to be made in this area. We are also planning to extend this technique for C:H coating of microballoons, which are used as fuel containers in inertial confinement fusion.

A novel Y-type reactor for selective excitation of atmospheric pressure glow discharge plasma

A novel Y-type atmospheric pressure ac glow discharge plasma reactor has been designed and tested in CO reduction with hydrogen and the reverse water–gas shift reaction. The reactor consists of a Y-type quartz tube with an angle of 120°–180° between the two long arms, two metal rod electrodes serving as high voltage terminals and two pieces of aluminum foil which were wrapped outside of the quartz tubes as a ground electrode. Different combinations of input power applied on this three- electrode system can lead to selective plasmas on one side, two sides, or can also generate a stable arc between the two high voltage terminal electrodes. The ability to selectively activate different species with this type of apparatus can help to minimize side reactions in plasmas to obtain desirable products. The Y-type reactor may provide a novel means to study fundamental problems regarding radical reactions.