Barrier Discharge In Air Research Articles

Polytetrafluoroethylene surface modification by filamentary and homogeneous dielectric barrier discharges in air

In this paper, polytetrafluoroethylene (PTFE) films are modified using non-equilibrium plasma generated by homogeneous DBD in air at medium pressure, and the results are compared to those treated by using filamentary DBD in air at atmospheric pressure. The surface properties of PTFE films before and after the treatments are studied using contact angle and surface energy measurement, X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It is found that the plasma treatments modify the PTFE surface in both morphology and composition. The PTFE films modified in both treatments show a remarkable decrease in water contact and a remarkable increase in surface energy. XPS analysis reveals that oxygen-containing polar groups are introduced onto the PTFE surface, and SEM analysis shows that the surfaces of the films are etched after both the treatments. It is found that homogeneous DBD is more effective in PTFE surface modification than filamentary DBD as it can make the contact angle decline to a lower level by introducing more oxygen-containing groups, and the possible reason for this effect is discussed.

Atmospheric‐Air Plasma Surface Modification of Polyethylene Powder

AbstractThe surface modification of polyethylene powder using a plasma reactor based on a dielectric barrier discharge in air at atmospheric pressure and ambient temperature is investigated. The process is inexpensive, and the necessity of any vacuum equipment and technical gases is alleviated. The efficiency of the modification process was successfully demonstrated by ESCA measurements that proved formation of new functional groups at the modified powder surface. The modification effect was also evaluated by means of dynamic capillarity rising measurements. Powder capillarity tests proved significant powder capillarity changes. The reduction of the modification effect was also limited (max. reduction of about 20% during 1 100 d after the modification date).magnified image

Experimental study on repetitive unipolar nanosecond-pulse dielectric barrier discharge in air at atmospheric pressure

Dielectric barrier discharge (DBD) is a typical approach for producing non-thermal plasma at atmospheric pressure. In this experimental study, the DBD is excited by repetitive unipolar nanosecond pulses with a rise time of ∼15 ns and a full-width at half-maximum of ∼30 ns. With a unipolar pulse voltage of 35 kV, the measured discharge current across the DBD circuit has a positive and negative pulse, and the pulse peak value can be hundreds of amperes. The low-speed camera images of the discharge show that both diffuse ‘glow-like’ and filamentary discharges can be observed, and the air gap length, barrier variety and applied repetition rate are the important factors influencing the transition of the two discharge modes. According to a known equivalent circuit and measured data of a typical DBD, electrical parameters including voltage, current and instantaneous power across the air gap and dielectric layer are calculated. The calculated results indicate that there are two consecutive discharges in the air gap, and the secondary discharge fires immediately after the primary discharge extinguishes. The power consumption of the secondary discharge is provided by the dielectric layer and deposited during the primary discharge. With the increase in the repetition rate, the energy deposition in the air gap per second is greatly enhanced and can exceed one joule for a repetition rate up to 100 Hz, and the corresponding charge transfer is also increased. The electron temperature and density are estimated to be approximately 5.1 eV and 1.6 × 1012 cm−3, respectively.

Two-Dimensional Spatially Resolved Cross-Correlational Spectroscopy of the Microdischarge Development in Barrier Discharges in Air

The technique of cross-correlation spectroscopy enables the spatio-temporally, spectrally and phase-resolved measurement of light intensity of statistically occurring microdischarges (MDs) of filamentary barrier discharges in synthetic air at atmospheric pressure. The results visualize the development of a single MD, consisting of the Townsend prephase, the cathode-directed ionization wave, anode glow, and outward-propagating discharges on the dielectric surfaces. These results contribute to a much deeper understanding of the breakdown mechanism.

Numerical simulation of a surface barrier discharge in air

The development of a surface barrier discharge in air at atmospheric pressure under the action of a constant voltage of different polarity is simulated numerically. When the polarity of the high-voltage electrode is negative, the discharge develops as an ionization wave that moves along the dielectric surface. When the polarity is positive, the discharge develops as a streamer that first moves above the dielectric surface and then comes into contact with and continues to develop along it. In the case of a high-voltage electrode of positive polarity, the discharge zone above the dielectric surface is approximately five times thicker than that in the case of negative polarity. The characteristic aspects of numerical simulation of the streamer phase of a surface barrier discharge are discussed. The numerical results on the density of the charge stored at the dielectric surface and on the length of the discharge zone agree with the experimental data.

Comparison of various methods for calculating plasma kinetics in a barrier discharge

Two methods are considered for calculating plasma kinetics of a barrier discharge in air under atmospheric pressure. The effect of fitting parameters on the calculated concentrations is determined using one of the methods. It is shown that both techniques ensure a satisfactory agreement with experiment for concentrations O3, NO3, and HNO3.

Model description of surface dielectric barrier discharges for flow control

This paper presents a study of the development of a surface dielectric barrier discharge in air under conditions similar to those of plasma actuators for flow control. The study is based on results from a 2D fluid model of the discharge in air that provides the space and time evolution of the charged particle densities, electric field and surface charges. The electrohydrodynamic (EHD) force associated with the momentum transfer from charged particles to neutral molecules in the volume above the dielectric layer is also deduced from the model. Results show that the EHD force is important not only during the positive part of the sinusoidal voltage cycle (i.e. when the electrode on top of the dielectric layer plays the role of the anode) but also during the negative part of the cycle (cathode on top of the dielectric layer). During the positive part of the cycle, the EHD force is due to the formation of a positive ion cloud that is periodically interrupted by high current breakdown. The EHD force during the negative part of the cycle is due to the development of a negative ion cloud that continuously grows during the successive high frequency current pulses that form in this regime.

Surface Modification of Non-woven Textiles using a Dielectric Barrier Discharge Operating in Air, Helium and Argon at Medium Pressure

In this paper, polyethylene terephthalate (PET) and polypropylene (PP) non-wovens were modified by a dielectric barrier discharge in air, helium and argon at medium pressure (5.0 kPa). The helium and argon discharges contained a fraction of air smaller than 0.1 %. Surface analysis and characterization were performed using X-ray photoelectron spectroscopy, liquid absorptive capacity measurements and scanning electron microscopy (SEM). The non-wovens, modified in air, helium and argon, showed a significant increase in liquid absorptive capacity due to the incorporation of oxygen-containing groups, such as C—O, O—C=O and C=O. It was shown that an air plasma was more efficient in incorporating oxygen functionalities than an argon plasma, which was more efficient than a helium plasma. SEM pictures of the plasma-treated nonwovens showed that the hydrophilicity of the nonwovens could be increased to a saturation value without causing physical degradation of the surface. The ageing behavior of the plasma-treated textiles after storage in air was also studied. It was shown that during the ageing process, the induced oxygen-containing groups re-orientated into the bulk of the material. This ageing effect was the smallest for the argon-plasma treated non-wovens, followed by the helium-plasma treated non-wovens, while the air-plasma treated non-wovens showed the largest ageing effect.

Study of the ageing behaviour of polymer films treated with a dielectric barrier discharge in air, helium and argon at medium pressure

Plasma treatment of polymers is gaining more and more popularity as a surface modification technique, since it offers numerous advantages over the conventional chemical processes. Plasma surface treatment is an environmentally benign, fast and versatile technology. However, it has one major disadvantage: the induced modification of the surface is not permanent, since the surface tends to recover to the untreated state. This ageing effect is due to the reorientation of induced polar chemical groups into the bulk of the material. In this paper, the ageing of polypropylene (PP) and polyethylene terephthalate (PET) films, treated with a dielectric barrier discharge operating at medium pressure (5.0 kPa) in air, helium and argon, is studied. This study is performed using contact angle measurements and X-ray photoelectron spectroscopy (XPS). Results show that the working gas used during plasma treatment has a significant influence on the ageing behaviour of both PP and PET films. The air-, helium- and argon-plasma treated PP films have a loss in treatment efficiency of 47%, 35% and 25% respectively, while the air-, helium- and argon-plasma treated PET films have a loss in treatment efficiency of 39%, 34% and 29% respectively. These results can be explained by the different cross-linking degrees of the polymer films after plasma treatment. Increasing the cross-linking degree will hinder the movement of the polymer chains and reduce the ageing effect.

Factors influencing the existence of the homogeneous dielectric barrier discharge in air at atmospheric pressure

The characteristics of homogeneous discharge are studied and compared with those of dielectric barrier discharge (DBD) by measuring their electrical discharge parameters and observing their light-emission phenomena. Several factors, including the voltage amplitude, gap distance, electrode configuration and barrier thickness, are experimentally studied to see their effects on the transition of the homogeneous discharge from the stable mode to the unstable mode. The results show that the homogeneous discharge exists only under certain conditions. The important role of the mesh for obtaining the homogeneous discharge is identified through experiments. It is found that the voltage range for maintaining a stable discharge is wider when the barrier thickness is smaller, the gap distance is shorter and the mesh number is greater. It is thought that the corona discharge produced by the wire mesh together with the property of the PET barrier can cause accumulation of charges on the barrier surface, which may provide initial electrons leading to the burst of the homogeneous discharge in the whole air gap when a certain threshold value of the electric field (breakdown value) is exceeded. In view of this assumption, the experimental results for the effects of these factors on the stability of the homogeneous discharge can be qualitatively interpreted.

Progress in Spectroscopic Diagnostics of Barrier Discharges

AbstractSystematic study of barrier discharges (BDs) using cross‐correlation spectroscopy (CCS) has been conducted since 2000 by two research teams at the universities in Moscow and in Greifswald. From the beginning, the investigations were focused on the application of the CCS diagnostics to the BDs in air and in N2/O2 mixtures. Further progress in the research field being considered was aimed on the one hand, at the improvement of the measurement technique and development of the data processing procedures, and on the other hand, at using different working gases and gas mixtures. In this paper, the most interesting and important research results obtained in the course of two last years at both referred above universities using the CCS technique are reported. They correspond to three different subjects of investigation, namely an analysis of axial and radial structure of the microdischrge in air at ambient pressure, an influence of the composition of binary mixtures Ne/O2 upon the mechanism of the BD development, and spatio‐temporal structure of the BD in pure argon. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Treatment of polymer films with a dielectric barrier discharge in air, helium and argon at medium pressure

In this paper, polyester (PET) and polypropylene (PP) films are modified by a dielectric barrier discharge in air, helium and argon at medium pressure (5.0 kPa). The plasma-modified surfaces are characterized by contact angle measurements and X-ray photoelectron spectroscopy (XPS) as a function of energy density. The polymer films, modified in air, helium and argon, show a remarkable increase in hydrophilicity due to the implantation of oxygen-containing groups, such as C–O, O–C O and C O. Atomic oxygen, OH radicals, UV photons and ions, present in the discharge, create radicals at the polymer surfaces, which are able to react with oxygen species, resulting in the formation of oxygen-containing functionalities on the polymer surfaces. It is shown that an air plasma is more efficient in implanting oxygen functionalities than an argon plasma, which is more efficient than a helium plasma. In an air plasma, most of the created radicals at the polymer surface will quickly react with an oxygen particle, resulting in an efficient implantation of oxygen functionalities. However, in an argon and helium plasma, the created radicals can react with an oxygen particle, but can also recombine with each other resulting in the formation of an oxidized cross-linked structure. This cross-linking process will inhibit the implantation of oxygen, resulting in a lower efficiency. In argon plasma, more ions are present to create radicals, therefore, more radicals are able to react with oxygen species. This can explain the higher efficiency of an argon plasma compared to a helium plasma.

Blood Coagulation and Living Tissue Sterilization by Floating-Electrode Dielectric Barrier Discharge in Air

Summary form only given. Thermal plasma discharges have been widely used in the past for treatment of living human and animal tissue. However, an extensive thermal damage and tissue desiccation occur due to extreme temperatures. Some solutions have been offered where the temperature is lowered by short current pulses, addition of noble gases, or significant decrease in the size of treatment electrodes. We propose a method of direct treatment of living tissue that occurs at room temperature and pressure without visible or microscopic tissue damage. The presented floating electrode dielectric barrier discharge (FE-DBD) plasma is proven electrically safe to human subjects and reported results show no gross (visual) or histological (microscopic) damage to skin samples in minutes, complete tissue sterilization from skin flora in seconds, and blood clot formation in seconds of electric plasma treatment. We also observe significant hastening of blood clot formation via electric plasma induced catalysis of natural processes occurring in human blood. Reported results are indicative of selectivity of FE-DBD plasma to inactivation of various foreign micro-organisms on human tissue with little to no influence on the tissue itself. An animal model (hairless mice) will be presented where we show complete sterilization of living animal tissue without any tissue damage even though skin layer of the treated animal is significantly more sensitive than that of a human. Comparison to second animal model (pigs) will also be presented

A computational model of a barrier discharge in air at atmospheric pressure: the role of residual surface charges in microdischarge formation

A two-dimensional (axially symmetric) computational model of the microdischarge formation in a barrier discharge for short (1–2 mm) gaps in air at atmospheric pressure is proposed. A non-homogeneous electric field, caused by a residual non-uniform charging of the dielectric barriers, is considered as an important reason for the filament formation by a Townsend mechanism. The ion–electron emission from the dielectric covering the cathode is treated as the principal secondary process; the secondary emission coefficient in the model is selected to be consistent with the Paschen voltage. For an applied sinusoidal voltage, this follows the microdischarge development on a microsecond scale. It is shown that even a slight inhomogeneity of the initial electric field leads to the formation of a narrow microdischarge channel.The two-dimensional dynamics of the radiation from a microdischarge for the case of the second positive and the first negative systems of nitrogen is simulated and compared with recent experimental data. The effects of the secondary emission coefficient and of a distribution of residual surface charges are investigated. It is shown that the level of inhomogeneity of the residual surface charge distribution does not affect the radius of the microdischarge channel, but affects its two-dimensional structure.The proposed model explains satisfactorily the experimental results for the velocity of the cathode-directed ionizing wave and the emission of the N2 second positive system from a microdischarge.

Blood Coagulation and Living Tissue Sterilization by Floating-Electrode Dielectric Barrier Discharge in Air

Blood Coagulation and Living Tissue Sterilization by Floating-Electrode Dielectric Barrier Discharge in Air

Surface modification of a polyester non-woven with a dielectric barrier discharge in air at medium pressure

In this paper, a polyester non-woven is plasma treated with a dielectric barrier discharge (DBD) in air at medium pressure (5.0 kPa) and at different discharge powers. Results show that an increasing power leads to a better plasma treatment of the sample. The barrier discharge is characterized by a voltage and current waveform, by a Lissajous figure and by Lichtenberg figures. The surface properties of the plasma treated samples are examined using X-ray photoelectron microscopy (XPS) and scanning electron microscopy (SEM). XPS analysis reveals that surface oxidation by the formation of oxygen-containing functional groups enhances the surface wettability. SEM analysis shows that the sample is not etched by the used barrier discharge. This is due to the low surface energy density of the DBD used in this paper. Therefore, a DBD in air at medium pressure provides an efficient modification of the chemical surface properties of textiles without destroying the physical structure.

Asymmetric surface dielectric barrier discharge in air at atmospheric pressure: electrical properties and induced airflow characteristics

The electrical properties of an asymmetric surface dielectric barrier discharge in atmospheric air have been investigated experimentally. The discharge is used for airflow production close to the dielectric surface, and the time-averaged flow velocity spatial profiles have been measured. Velocities of up to 3.5 m s−1 at heights of 1–2 mm are reached when filamentary discharges with current peaks up to 20 mA are produced along the surface. In terms of powers, mechanical powers (output) of a few milliwatts are obtained for electrical powers (input) up to 10 W. Variation laws or behaviour with several discharge parameters (applied voltage waveform, distance between electrodes, dielectric thickness and permittivity) have been experimentally determined.

Axial and radial development of microdischarges of barrier discharges in N2/O2 mixtures at atmospheric pressure

Microdischarges (MDs) of filamentary barrier discharges (BDs) in air and N2/O2 gas mixtures at atmospheric pressure were investigated using the techniques of spatially resolved cross-correlation spectroscopy (CCS) and short exposure time photography (ICCD-camera). The BDs were generated in symmetric discharge cells (configuration of the type ‘glass–glass’) with two semi-spherical electrodes in order to localize the repetitive MDs at a fixed position. In the case of CCS measurements, the MD development was imaged through spatio-temporal distributions of the radiation intensity of the (0–0) transition of the 2nd positive system of molecular nitrogen (λ = 337 nm). Two-dimensional optical scanning of the MD channel (in the axial and radial directions of the MD) was carried out for a BD operated in a gas mixture consisting of 6 vol% O2 and 94 vol% N2. This gas composition had been found to provide the extremely high stability of the discharge that was necessary for the time-consuming scanning procedure. In the middle of the gap, the MD channel diameter was found to be about 0.3 mm and to expand towards both electrodes. With dielectrics, outward propagating discharges were observed. Short exposure time photos of the MDs taken using an ICCD-camera under the same experimental conditions as for the CCS measurements revealed a branched structure of these discharges on the surfaces, not only on the cathode but on the anode as well. The influence of voltage amplitude on the spatio-temporal distribution of individual MDs of a BD in air was investigated using the CCS-instrument operated in a so-called ‘direct start–stop’ mode which enabled statistical analysis of the MD sequences within a time range of about 30 µs.

The transition from a filamentary dielectric barrier discharge to a diffuse barrier discharge in air at atmospheric pressure

A transparent plasma discharge reactor using air was used to investigate the transition from a filamentary dielectric barrier discharge (FDBD) operation regime into the diffuse barrier discharge regime. Recent results of other researchers indicate that the stability of diffuse barrier discharges in nitrogen may be attributed to the lack of a fully formed cathode fall layer when the discharge operates in a regime between the Townsend discharge and a normal glow discharge. We have demonstrated that a diffuse barrier discharge in air exhibits an increased accumulation of electric charge on the electrode's dielectric plates as compared with the FDBD. This may provide a means of stabilizing the discharge in a Townsend-to-glow discharge transition. Unlike operation in nitrogen, a streamer mechanism is involved in the formation of a uniform air plasma, though in a different manner than is associated with the FDBD. Numerous diffuse streamer clusters were observed on pre-charged dielectric plates at the breakdown voltage. Our conclusion is that the macroscopically uniform atmospheric-pressure DBD in air is obtained by the numerous radially expanding streamers that are temporally overlapping.

Spatio-temporal dynamics of discharge domains in a dielectric barrier discharge device

The spatio-temporal correlation of discharge filaments in dielectric barrier discharges in air at atmospheric pressure is studied using an optical method. The stable filament patterns are achieved by using water electrodes, which serve both as coolants and transparent media for observation. It is found that the time interval between the two discharges of a filament in two consecutive half-cycles alternates between the long one and the short one, in general. An equation describing breakdown/termination of discharges is suggested by considering the effect of accumulated charges on the dielectric surface, and the experimental phenomena mentioned above can be explained using this equation. It is found in our experiments that the mapping of the delay time sequence for a filament has metastability with certain fluctuation features. The stable filament pattern can be divided into discharge domains. Each domain consists of several neighbouring filaments. The discharges of the filaments inside a domain are cooperatively correlated, while those of neighbouring domains are most likely competitively correlated.