Diffuse Dielectric Barrier Discharge Research Articles

Influence of the dielectric thickness on the homogeneity of a diffuse dielectric barrier discharge in air

This work focuses on a diffuse Dielectric Barrier Discharge (DBD) in atmospheric pressure air using an original experimental approach. In air, the role of the capacitance on the discharge regime has not yet been studied even though it affects the discharge current. To address this gap in knowledge, the dielectric capacitance was modified by the means of the dielectric thickness in a plane-to-plane DBD configuration. The discharge regime was analyzed using short exposure time photographs and electrical measurements in order to remove any potential uncertainties regarding the diffuse nature of the DBD in atmospheric pressure air. The influence of the dielectric capacitance on the discharge regime was analyzed thanks to the working domain of the discharge. Additionally, using an original electrical model of the discharge, the effective surface and current density of the discharge was obtained only from electrical measurements. Thus, the crucial role of the maximum current density on the discharge regime was highlighted. The control of the current density by the means of the dielectric capacitance led to the obtention of a diffuse discharge at a frequency up to 5 kHz for the very first time. Considering this high frequency, the possibility of using a DBD in air at atmospheric pressure for coating applications is discussed.

On constriction and striation of a diffuse DBD in large gap filled with moderate pressure helium diluted by the air and easily ionized impurity

The experimental results on studying the plane-to-plane dielectric barrier discharge (DBD) excited in a very large volume (120 × 120 × 120 mm3) are presented. The DBD was powered by sinusoidal voltage with a frequency of 100 kHz. The plasma-forming gas is helium, which is slightly diluted by the air and trace impurity of an easily ionized gas. The total pressure of the working gas was varied within of 20–100 Torr. A moderate gas pressure allowed one the creation of non-typical DBD with a very long plasma column of 12 cm in length. Such a long gap DBD exhibits new features in its behavior compared to the well-known DBD in short gaps of several millimeters in length. Immediately after the discharge ignition, a long and uniform diffuse plasma column occurs that occupies the entire space of the inter-electrode bulk. However, this transversally homogeneous plasma mode is unstable. The ionization-thermal instability develops in the plasma owing to gas heating by the discharge. Therefore, the transversally uniform plasma breaks into several separate narrow current channels that move chaotically relative to each other. After a while, they eventually integrate into a single constricted plasma column located in the center part of the discharge volume. At the very beginning of its formation, the constricted plasma column is longitudinally uniform. But soon, it becomes streated. The strata are almost standing and their shape differs from that in a DC glow discharge at low pressure in a sealed tube. The revealed features in the slow spatial-temporal behavior of the long gap DBD are discussed with the involvement of mathematical analysis describing the small plasma disturbance development in the linear approximation.

Automated Fluid Model Generation and Numerical Analysis of Dielectric Barrier Discharges Using Comsol

MCPlas is introduced as a powerful tool for automated fluid model generation with application to the analysis of dielectric barrier discharges operating in different regimes. MCPlas consists of a number of MATLAB\textsuperscript{\textregistered} scripts and uses the COMSOL Multiphysics\textsuperscript{\textregistered} module LiveLink\textsuperscript{\texttrademark} for MATLAB\textsuperscript{\textregistered} to build up equation-based COMSOL Multiphysics\textsuperscript{\textregistered} models from scratch. The present contribution highlights how MCPlas is used to implement time-dependent models for non-thermal plasmas in spatially one-dimensional and axisymmetric two-dimensional geometries and stresses out the benefit of automation of the modelling procedure. The modelling codes generated by MCPlas are used to study diffuse and filamentary dielectric barrier discharges in argon at sub-atmospheric and atmospheric pressure, respectively. The seamless transition between different levels of model complexity with respect to the considered model geometry is demonstrated. The presented investigation of a single-filament dielectric barrier discharge interacting with a dielectric surface shows that complex phenomena of high technological relevance can be tackled by using plasma models implemented in COMSOL Multiphysics\textsuperscript{\textregistered} via MCPlas.

Modification of powders by atmospheric pressure plasma and embedding into nickel coatings

The aim of this work was to achieve improved dispersibility of hydrophobic dry lubricants in aqueous media through plasma activation. As a result, the use of wet-chemical environmentally questionable substances – for example, certain surfactants – to ensure the dispersibility of such particles can be avoided. These plasma-activated particles can be further embedded in electroless nickel (Ni) coatings to improve wear behaviour. Thus, the hydrophobic polymeric powders polyimide and polyetheretherketone and the ceramic powder hexagonal boron nitride (BN) were activated with atmospheric pressure plasma using a diffuse coplanar dielectric barrier discharge system. The generation of polar chemical bonds on the powder surface resulted in an improved dispersibility of the powders in aqueous media. This led to a higher deposit volume of the particles in electroless nickel dispersion coatings. The modified bonding conditions at the powder surfaces were analysed with infrared and X-ray photoelectron spectroscopy. Optical microscopy and scanning electron microscopy investigations showed the deposit of the activated dispersed phase in the coatings. Furthermore, the wear behaviour of the coating was tested by using the Taber abraser test. The plasma activation led to surface modifications – namely, linkage of polar groups at the powder surfaces – and therefore to improved dispersibility. The deposition of the particles in the electroless nickel coatings was possible with all three materials, and the wear resistance of the coatings was improved.

Influence of air impurities on the transition from a symmetric discharge to an asymmetric discharge in an atmospheric pressure helium diffuse dielectric barrier discharge

The transition from a symmetric and single period (SP1) discharge to an asymmetric and single period (AP1) discharge is a typical nonlinear dynamical phenomenon in dielectric barrier discharges (DBDs) at atmospheric pressure. Considering the presence of air impurities in practical applications which is always unavoidable due to the air-tightness of the DBD reactor, a one-dimensional fluid model with 26 species and 154 reactions is developed to thoroughly investigate the influence of air impurities on the transition from the SP1 discharge to the AP1 one in atmospheric DBDs in a helium mixture with air impurities. In our study case, simulation results show that the discharge experiences the transitions from the SP1 discharge to the AP1 one twice when the air impurity content is increased from 1 to 200 ppm. The first transition (appearing around 60–100 ppm) is due to the additional pre-ionization electrons generated by the higher rate of Penning ionization in the pre-ionization phase, whereas the second transition (occurring around 170–200 ppm) is due to the reduction of the residual electron density which is caused by the decrease in the electron production rates and the change in applied voltage polarity. In addition, as the air impurity content exceeds a certain amount, the rates of Penning ionization are the result of the competition between the increase in the mole fractions of N2 and O2 and the decrease in the molar fractions of He* and He2*.

The Generation and Evolution of a Diffuse Nanosecond Pulsed Dielectric Barrier Discharge in Airflow

Atmospheric-pressure dielectric barrier discharge (DBD) with airflow participation has been widely used in recent years. In this experimental study, a diffuse DBD excited by repetitive nanosecond pulses is achieved in airflow. Upon the introduction of airflow to the gas gap, the discharge state transforms from a filamentary one into a diffuse one at first, followed by the generation of a thin filamentary discharge (TFD) at a high airflow rate. It is observed that the diffuse discharge (DD) and the TFD are ignited from the electrode boundary, which subsequently move toward the downstream region with a velocity below the corresponding airflow rate. Subsequent to hundreds of pulse cycles, a stationary discharge state is formed with the TFD in the upstream region and the DD in the downstream region. The length of the TFD region is associated with the airflow rate, the pulse repetition frequency (PRF), and the applied voltage. The observations presented above are ascribed to the transverse electric field at the electrode boundary and the redistribution of charges under the action of airflow.

Emission and absorption diagnostics of a diffuse dielectric barrier discharge with multiple current peaks in helium at atmospheric pressure

Dielectric barrier discharge (DBD) is a simple means to sustain low-temperature diffuse discharges at atmospheric pressure. As atmospheric pressure plasma processing requires DBDs to work under various operating conditions, even diffuse discharges are often observed to exhibit multiple current peaks. In this work, multiple current peaks helium DBD is obtained by adding small concentration of argon in a nominally pure helium discharge. As the concentration of argon increases, multiple current peaks emerge while the discharge remains diffuse. Absorption spectroscopy is used to record the evolution of the He(23S) metastable atom. In a nominally pure helium discharge He(23S) metastable atom can reach up to 6 × 1011 cm−3. Even if quenching of the He(23S) metastable atom by argon is very effective, He(23S) density is found to still rise above 1010 cm−3 in a multiple current peaks discharge with 1100 ppm of argon. Optical emission spectroscopy of He n = 3 levels is also used to gain insights about the electron temperature (Te) through the means of a collisional-radiative model. For the multiple current peaks discharge with 1100 ppm of argon concentration, Te reaches about 1.0 eV during the primary peak but only about 0.3 eV during the secondary peaks.

Generation of atmospheric pressure diffuse dielectric barrier discharge based on multiple potentials in air

In order to achieve atmospheric pressure diffuse dielectric barrier discharge (DBD) in air, a helical–helical electrode structure with a floating-voltage electrode is proposed in this paper. Results from an electric field distribution simulation indicate that strong electric fields are formed where the helical-contact electrodes’ insulating layers are in contact with each other, as well as near the floating-voltage electrode, which contributes to the production of a large number of seed electrons. The electric field within the air gap is weak (<3 × 106 V m−1), which inhibits the rapid development of electron avalanches and the formation of filament discharge. The experimental result shows that a 3.0 mm width diffuse DBD is generated in air. Moreover, based on the study of the helical–helical electrode with a floating-voltage electrode, a three-dimensional electrode structure is presented, and a three-dimensional diffuse discharge is generated in air by adopting this electrode structure. The plasma studied is stable and demonstrates good diffusion characteristics, and therefore has potential applications in the field of exhaust gas treatment and air purification.

Comparative study on diffuse dielectric barrier discharges excited by unipolar positive versus bipolar pulses in atmospheric air

Diffuse discharges excited by unipolar positive and bipolar pulses can be achieved by a self-designed dielectric barrier discharge (DBD) structure (a metal rod is inserted into a traditional parallel-plate DBD structure) exposed in airflow. For a self-designed DBD excited by unipolar positive pulses, only a primary discharge occurs in a voltage pulse. When the applied voltage is low, a diffuse discharge first appears near the anode. As the voltage further increases, a diffuse discharge appears in a larger area near the anode. Until the applied voltage is high enough, the discharge fills the whole discharge gap. Additionally, there is a priority region around the metal rod for the development of a diffuse discharge. However, for a self-designed DBD excited by bipolar pulses, two separate discharges are observed in a voltage pulse. The primary discharge occurs at the rising front of the voltage pulse, and the secondary discharge (reverse discharge) takes place at the falling front of the voltage pulse. When the applied voltage is low, the diffuse discharge first starts from the priority region around the metal rod placed in the center of the discharge electrode. As the voltage further increases, the diffuse discharge appears in a larger area around the metal rod. The above observations about the different spatial evolutions of diffuse discharge areas excited by unipolar positive and bipolar pulses are mainly ascribed to the difference of a strong local electric field caused by residual charges. This diffuse discharge has potential applications in surface treatment of materials and thin film deposition.

Selection of dielectric material for producing diffuse dielectric barrier discharge plasma at atmospheric pressure

Abstract We report the effect of dielectric layer and its selection criterion to produce diffuse dielectric barrier discharge plasma at atmospheric pressure. The dielectric barriers over electrodes are used to prevent glow-to-arc transition by limiting growth of discharge current. We have analyzed the dielectric properties such as secondary electron emission, dielectric constant, dielectric thickness etc in order to get better power efficiency at breakdown. It is observed that a thin dielectric layer with high dielectric constant is suitable for lowering the breakdown voltage. During one of our experiments, planar-surface discharge plasma was produced with the use of epoxy resin dielectric-covered stripe electrodes. High-voltage high frequency signal was applied between the electrodes to produce the discharge. It is observed that the discharge remain stable & uniform in our parametric range. However, after prolonged discharge operation, degradation of dielectric layer was observed in the form of powder formation over the dielectric insulation. SEM analysis reveals that the powder contains individual as well as cluster of particles of micrometer size dimension. Such micro-particle formation may be useful for some material and biomedical applications, however in other way degraded dielectric would lead to spark formation in plasma. In this context, theoretical and experimental analysis of our results is presented in this paper.

Experimental investigation on the optical emission spectroscopy of dielectric barrier discharge plasma actuators at different atmospheric pressures

The experiment of dielectric barrier discharge (DBD) plasma actuator using an asymmetrical electrodes configuration is conducted to investigate optical emission spectroscopy of the plasma discharge at pressures from 0.02 to 0.1 Mpa in air mixed with various concentrations of Ar (0%, 20%, 50%, and 80%). The measurements were performed with a sinusoidal driven DBD having an amplitude of 12 kV and a frequency of 1 kHz. The diffuse DBD discharge images and the optical emission spectra are recorded successfully, and the effects of pressure and concentration of Ar are investigated, respectively. It is found that the emission intensities of air discharge increase with decreasing the pressure but increasing the concentration of Ar to some extent. Especially, when the pressure is lower than 0.04 Mpa or the concentration of Ar is over 50%, the emission intensities increase obviously rapidly. The experimental results indicate that the low pressure environment and the high concentration of Ar are beneficial to improve the performance of the DBD plasma discharge actuators.

Tuning the water-alumina nanofluids impedance and dielectric relaxation by the diffuse coplanar dielectric barrier discharge

Nanofluids are nowadays representing an exciting tool with significant impact and tremendous potential in several industrial applications. In the present paper, we focus our attention on the tunability of the electrical and dielectric properties of alumina nanoparticle-based nanofluids achieved by preliminary modifying the particle surface charging by a diffuse coplanar dielectric barrier discharge plasma treatment. We produced a parametric study on the tuning of the plasma-treatedalumina-based nanofluid electrical response as a function of the selected discharge feeding gases.

Electron density and temperature in an atmospheric-pressure helium diffuse dielectric barrier discharge from kHz to MHz

Diffuse dielectric barrier discharges are generated over a very wide range of frequencies. According to the targeted frequency, the glow, Townsend-like, hybrid, Ω and RF-α modes are sustained. In this paper, the electrical characterization of the discharge cell together with an electrical model are used to estimate the electron density from current and voltage measurements for excitation frequencies ranging from 50 kHz to 15 MHz. The electron density is found to vary from 1014 to 1017 m−3 over this frequency range. In addition, a collisional-radiative model coupled with optical emission spectroscopy is used to evaluate the electron temperature (assuming Maxwellian electron energy distribution function) in the same conditions. The time and space-averaged electron temperature is found to be about 0.3 eV in both the low-frequency and high-frequency ranges. However, in the medium-frequency range, it reaches almost twice this value as the discharge is in the hybrid mode. The hybrid mode is similar to the atmospheric-pressure glow discharge usually observed in helium DBDs at low frequency with the major difference being that the plasma is continuously sustained and is characterized by a higher power density.

Atmospheric-pressure diffuse dielectric barrier discharges in Ar/O2 gas mixture using 200 kHz/13.56 MHz dual frequency excitation

Atmospheric-pressure diffuse dielectric barrier discharges (DBDs) were obtained in Ar/O2 gas mixture using dual-frequency (DF) excitation at 200 kHz low frequency (LF) and 13.56 MHz radio frequency (RF). The excitation dynamics and the plasma generation mechanism were studied by means of electrical characterization and phase resolved optical emission spectroscopy (PROES). The DF excitation results in a time-varying electric field which is determined by the total LF and RF gas voltage and the spatial ion distribution which only responds to the LF component. By tuning the amplitude ratio of the superimposed LF and RF signals, the effect of each frequency component on the DF discharge mechanism was analysed. The LF excitation results in a transient plasma with the formation of an electrode sheath and therefore a pronounced excitation near the substrate. The RF oscillation allows the electron trapping in the gas gap and helps to improve the plasma uniformity by contributing to the pre-ionization and by controlling the discharge development. The possibility of temporally modifying the electric field and thus the plasma generation mechanism in the DF discharge exhibits potential applications in plasma-assisted surface processing and plasma-assisted gas phase chemical conversion.

Infrared gas phase study on plasma-polymer interactions in high-current diffuse dielectric barrier discharge

A roll-to-roll high-current diffuse dielectric barrier discharge at atmospheric pressure was operated in air and Ar/N2/O2 gas mixtures. The exhaust gas from the discharge was studied using a high-resolution Fourier-transform infrared spectrometer in the range from 3000 to 750 cm−1 to unravel the plasma-polymer interactions. The absorption features of HxNyOz, COx, and HCOOH (formic acid) were identified, and the relative densities were deduced by fitting the absorption bands of the detected molecules. Strong interactions between plasma and polymer (Polyethylene-2,6-naphthalate, or PEN) in precursor-free oxygen-containing gas mixtures were observed as evidenced by a high COx production. The presence of HCOOH in the gas effluent, formed through plasma-chemical synthesis of COx, turns out to be a sensitive indicator for etching. By adding tetraethylorthosilicate precursor in the plasma, dramatic changes in the COx production were measured, and two distinct deposition regimes were identified. At high precursor flows, a good agreement with the precursor combustion and the COx production was observed, whereas at low precursor flows an etching-deposition regime transpires, and the COx production is dominated by polymer etching.

Effect of nitrogen addition to ozone generation characteristics by diffuse and filamentary dielectric barrier discharges at atmospheric pressure

Ozone is widely used for gas treatment, advanced oxidation processes, microorganisms inactivation, etc. In this research, we investigated the effect of nitrogen addition to ozone generation characteristics by atmospheric pressure Townsend discharge (APTD) type and filamentary dielectric barrier discharge (DBD) type ozone generators. The result showed that the ozone generated by the filamentary DBD increases rapidly with the increase of O2 content, and is higher than that by the APTD. On the other hand, it is interesting that the ozone generated by the APTD gradually decreases with the increase of O2 content. In order to clarify why the characteristics of ozone generation by the two kinds of discharge modes showed different dependency to the N2 content, we analyzed the exhaust gas composition using FTIR spectroscopy and calculated the rate coefficients using BOLSIG+ code. As a result, we found that although O2 content decreased with increasing N2 content, additional O atoms produced by excited N2 molecules contribute to ozone generation in case of APTD.

Characteristics of Shallow Traps in the Dielectric Surface and Their Effects on Diffuse Dielectric Barrier Discharge in Air

Diffuse Townsend dielectric barrier discharge (DBD) was realized in 3-mm air gap at atmospheric pressure using special Guangdong Zhuhai Yueke Tsinghua Electronic Ceramics Company, Ltd (GTT) ceramics as the dielectric while discharges with dielectric of other ceramics or quartz were always filamentary. It is supposed that there exist shallow traps ( $E eV) in dielectric surface. In order to experimentally study how the shallow traps influence the generation of diffuse DBD in air and to explore the mechanism of extraordinary extinguishment in Townsend DBD, thermally stimulated current measurements were performed. It was found the trap level of two types of ceramics was 0.34–0.36 eV, lower than 0.61 eV of quartz, which made it more easily to desorb trapped electrons for the ceramics. There was 280.8 nC of trap charge in GTT ceramics, which was much more than that in common ceramics (38.3 nC) and quartz (15.12 nC). Pulse electroacoustic measurement also indicated that GTT ceramics could acquire more seeds electrons with a longer decay time than the other two dielectrics. Scanning electron microscope observations clearly demonstrated denser distribution and better dispersion of grains in GTT ceramics surface, as was supposed to be the reason why it behaved better than other dielectrics. The results prove that there exist shallow traps in dielectric surface, which could offer seed electrons for generation of diffuse DBD. Lower trapping energy and higher density in the material would make it more easily to generate diffuse DBD.

Transitions of an atmospheric-pressure diffuse dielectric barrier discharge in helium for frequencies increasing from kHz to MHz

Recent studies have shown that tuning a dielectric barrier discharge (DBD) in the medium-frequency range (MF: from 0.3 to 3 MHz) allows a low-power and a high-power mode to be sustained. In the present article the effect of the driving frequency on a DBD is studied from the low-frequency range (LF: from 30 to 300 kHz) to the high-frequency range (HF: from 3 to 30 MHz). This is achieved using fast imaging together with electrical and spectroscopic diagnostics. At every frequency, a diffuse discharge is sustained. It is observed that at 25 kHz the discharge is an atmospheric-pressure glow discharge (APGD) while at 15 MHz the discharge behaves as a capacitive discharge in the RF-α mode. The usual LF APGD behavior is observed up to 100 kHz. Above 200 kHz, the positive column remains during the whole cycle so that the hybrid mode is sustained. At 5 MHz, the hybrid mode finally turns into the RF-α mode. In addition to the LF APGD, RF-α and hybrid modes obtained when the applied voltage is significantly higher than the ignition value, two other modes can be reached at low applied voltage. A Townsend-like mode is achieved from 50 to 100 kHz while in the medium-frequency range, the Ω mode is sustained. Moreover, only from 1.0 to 2.7 MHz there is a large hysteresis occurring when the discharge transits back and forth from the Ω to the hybrid mode. It is also found that when the frequency increases from 25 kHz to 15 MHz, the rms current increases over two orders of magnitudes while the rms voltage decreases by about 60%. The gas temperature estimated from N2 rotational spectra is always close to room temperature but the discharge is more energy efficient (in the HF range) as a lower fraction of energy turns into gas heating.

Influence of the excitation frequency on the density of helium metastable atoms in an atmospheric pressure dielectric barrier discharge

Diffuse dielectric barrier discharges in atmospheric-pressure helium can be sustained over a wide range of excitation frequencies (from, but not restricted, 25 kHz to 15 MHz). The aim of the present paper is to identify the specific characteristics of the discharge modes that can be sustained in this frequency range, namely, the atmospheric-pressure Townsend-like discharge (APTD-L) mode, the atmospheric-pressure glow discharge (APGD) mode, the Ω mode, the hybrid mode, and the RF-α mode. This is achieved experimentally, by measuring the density of helium metastable atoms, which are known to play a driving role on the discharge kinetics. This density is measured by means of two absorption spectroscopy methods, one using a spectral lamp and the other one using a diode laser as a light source. The first one provides the time-averaged atom densities in the singlet He(21S) and triplet He(23S) metastable states, while with the second one we access the time-resolved density of He(23S) atoms. Time-averaged measurements indicate that the He(23S) density is relatively low in the APTD-L, the Ω and the RF-α modes (&amp;lt;4×1016 m−3) slightly higher in the APGD mode (2–7×1016 m−3), and still higher (&amp;gt;1×1017 m−3) in the hybrid mode. The hybrid mode is exclusively observed for frequencies from 0.2 to 3 MHz. However, time-resolved density measurement shows that at 1 MHz and below, the hybrid mode is not continuously sustained. Instead, the discharge oscillates between the Ω and the hybrid mode with a switching frequency about the kilohertz. This explains the significantly lower power required to sustain the plasma as compared to above 1 MHz.

Volume Diffuse Dielectric Barrier Discharge Plasma Produced by Nanosecond High Voltage Pulse in Airflow**supported by National Natural Science Foundation of China (No. 51437002)

Volume diffuse dielectric barrier discharge (DBD) plasma is produced in subsonic airflow by nanosecond high-voltage pulse power supply with a plate-to-plate discharge cell at 6 mm air gap length. The discharge images, optical emission spectra (OES), the applied voltage and current waveforms of the discharge at the changed airflow rates are obtained. When airflow rate is increased, the transition of the discharge mode and the variations of discharge intensity, breakdown characteristics and the temperature of the discharge plasma are investigated. The results show that the discharge becomes more diffuse, discharge intensity is decreased accompanied by the increased breakdown voltage and time lag, and the temperature of the discharge plasma reduces when airflow of small velocity is introduced into the discharge gap. These phenomena are because that the airflow changes the spatial distribution of the heat and the space charge in the discharge gap.