Formation mechanism of bright and dark concentric-ring pattern in dielectric barrier discharge

In this work,a bright and dark concentric-ring pattern is reported in a dielectric barrier discharge for the first time.The spatiotemporal dynamics of the bright and dark concentric-ring pattern are investigated with an intensified charge-coupled device and photomultiplier tubes.The results indicate that the bright and dark concentric-ring pattern is composed of three concentric-ring sublattices.These are bright concentric-ring structures,dark concentric-ring structures and wider concentric-ring structures,respectively.The bright concentric-ring structures and dark concentricring structures are alternately distributed.The bright concentric-ring structures are located at the centre of the wider concentric-ring structures.The wider concentric-ring structures first form from the outer edge and gradually develop to the centre.The essence of all three concentric-ring structures is the individual discharge filaments.The optical emission spectra of different sublattices are acquired and analysed.It is found that the plasma parameters of the three concentricring sublattices are different.Finally,the formation mechanism of the bright and dark concentricring pattern is discussed.

Effect of dielectric material on the uniformity of nanosecond pulsed dielectric barrier discharge

Dielectric barrier discharge(DBD)is considered as a promising technique to produce large volume uniform plasma at atmospheric pressure,and the dielectric barrier layer between the electrodes plays a key role in the DBD processes and enhancing discharge uniformity.In this work,the uniformity and discharge characteristics of the nanosecond(ns)pulsed DBD with dielectric barrier layers made of alumina,quartz glass,polycarbonate(PC),and polypropylene(PP)are investigated via discharge image observation,voltage-current waveform measurement and optical emission spectral diagnosis.Through analyzing discharge image by gray value standard deviation method,the discharge uniformity is quantitatively calculated.The effects of the space electric field intensity,the electron density(Ne),and the space reactive species on the uniformity are studied with quantifying the gap voltage Ug and the discharge current Ig,analyzing the recorded optical emission spectra,and simulating the temporal distribution of Ne with a one-dimensional fluid model.It is found that as the relative permittivity of the dielectric materials increases,the space electric field intensity is enhanced,which results in a higher Ne and electron temperature(Te).Therefore,an appropriate value of space electric field intensity can promote electron avalanches,resulting in uniform and stable plasma by the merging of electron avalanches.However,an excessive value of space electric field intensity leads to the aggregation of space charges and the distortion of the space electric field,which reduce the discharge uniformity.The surface roughness and the surface charge decay are measured to explain the influences of the surface properties and the second electron emission on the discharge uniformity.The results in this work give a comprehensive understanding of the effect of the dielectric materials on the DBD uniformity,and contribute to the selection of dielectric materials for DBD reactor and the realization of atmospheric pressure uniform,stable,and reactive plasma sources.

On the evolution and formation of discharge morphology in pulsed dielectric barrier discharge

The discharge morphology of pulsed dielectric barrier discharge(PDBD) plays important roles in its applications. Here, we systematically investigated the effects of the voltage amplitude,discharge gap, and O_(2)content on the PDBD morphology, and revealed the possible underlying mechanism of the U-shaped formation. First, the morphological evolution under different conditions was recorded. A unique U-shaped region appears in the middle edge region when the gap is larger than 2 mm, while the entire discharge region remains columnar under a 2 mm gap in He PDBD. The width of the discharge and the U-shaped region increase with the increase in voltage, and decrease with the increase of the gap and O_(2)content. To explain this phenomenon,a two-dimensional symmetric model was developed to simulate the spatiotemporal evolution of different species and calculate the electric thrust. The discharge morphology evolution directly corresponds to the excited-state atomic reduction process. The electric thrust on the charged particles mainly determines the reaction region and strongly influences the U-shaped formation.When the gap is less than 2 mm, the electric thrust is homogeneous throughout the entire region,resulting in a columnar shape. However, when the gap is larger than 2 mm or O_(2)is added, the electric thrust in the edge region becomes greater than that in the middle, leading to the U-shaped formation. Furthermore, in He PDBD, the charged particles generating electric thrust are mainly electrons and helium ions, while in He/O_(2)PDBD those that generate electric thrust at the outer edge of the electrode surface are mainly various oxygen-containing ions.

Experimental study on the effect of H_(2)O and O_(2) on the degradation of SF_(6) by pulsed dielectric barrier discharge

SF_(6) has excellent insulation performance and arc extinguishing ability,and is widely used in the power industry.However,its global warming potential is about 23,500 times that of C0_(2),it can exist stably in the atmosphere,it is not easily degradable and is of great potential harm to the environment.Based on pulsed dielectric barrier discharge plasma technology,the effects of H_(2)O and 0_(2) on the degradation of SF_(6) were studied.Studies have shown that H_(2)O can effectively promote the decomposition of SF_(6) and improve its degradation rate and energy efficiency of degradation.Under the action of a pulse input voltage and input frequency of 15 kV and 15 kHz,respectively,when H_(2)O is added alone the effect of 1% H_(2)O is the best,and the rate and energy efficiency of degradation of SF_(6) reach their maximum values,which are 91.9% and 8.25 g kWh^(-1),respectively.The synergistic effect of H_(2)O and O_(2) on the degradation of SF_(6) was similar to that of H_(2)O.When the concentration of H_(2)O and O_(2) was 1%,the system obtained the best rate and energy efficiency of degradation,namely 89.7% and 8.05 g kWh~(-1),respectively.At the same time,different external gases exhibit different capabilities to regulate decomposition products.The addition of H_(2)O can effectively improve the selectivity of S0_(2).Under the synergistic effect of H_(2)O and O_(2),with increase in O_(2) concentration the degradation products gradually transformed into SO_(2)F_(2).From the perspective of harmless treatment of the degradation products of SF_(6),the addition of O_(2) during the SF_(6) degradation process should be avoided.

Surface charge characteristics in a three-electrode surface dielectric barrier discharge

The surface charge characteristics in a three-electrode surface dielectric barrier discharge(SDBD)are experimentally investigated based on the Pockels effect of an electro-optical crystal. The actuator is based on the most commonly used SDBD structure for airflow control, with an exposed electrode supplied with sinusoidal AC high voltage, a grounded encapsulated electrode and an additional exposed electrode downstream supplied with DC voltage. The ionic wind velocity and thrust can be significantly improved by increasing DC voltage although the plasma discharge characteristics are virtually unaffected. It is found that the negative charges generated by the discharge of the three-electrode structure accumulate on the dielectric surface significantly further downstream in an AC period compared to the actuator with a two-electrode structure. The negative charges in the downstream region increase as the DC voltage increases.In addition, the DC voltage affects the time required for the positive charge filaments to decay.The positive DC voltage expands the ionic acceleration zone downstream to produce a greater EHD force. The amplitude of the DC voltage affects the electric field on the dielectric surface and is therefore a key factor in the formation of the EHD force. Further research on the surface charge characteristics of a three-electrode structure has been conducted using a pulse power to drive the discharge, and the same conclusions are drawn. This work demonstrates a link between surface charge characteristics and EHD performance of a three-electrode SDBD actuator.

Single flow treatment degradation of antibiotics in water using falling-film dielectric barrier discharge

The environmental contamination caused by antibiotics is increasingly conspicuous due to their widespread manufacture and misuse. Plasma has been employed in recent years for the remediation of antibiotic pollution in the environment. In this work, a falling-film dielectric barrier discharge was used to degrade the antibiotic tetracycline(TC) in water. The reactor combined the gas-liquid discharge and active gas bubbling to improve the TC degradation performance. The discharge characteristics, chemical species’ concentration, and degradation rates at different parameters were systematically studied. Under the optimized conditions(working gas was pure oxygen, liquid flow rate was 100 mL/min, gas flow rate was 1 L/min,voltage was 20 kV, single treatment), TC was removed beyond 70% in a single flow treatment with an energy efficiency of 145 mg/(kW·h). The reactor design facilitated gas and liquid flow in the plasma area to produce more ozone in bubbles after a single flow under pure oxygen conditions, affording fast TC degradation. Furthermore, long-term stationary experiment indicated that long-lived active species can sustain the degradation of TC. Compared with other plasma treatment systems, this work offers a fast and efficient degradation method, showing significant potential in practical industrial applications.

Reconfigurable(4, 6^(2)) and(4, 8^(2)) Archimedean plasma photonic crystals in dielectric barrier discharge

Archimedean photonic crystal has become a research area of great interest due to its various unique properties. Here, we experimentally demonstrate the realization of reconfigurable(4, 6^(2))and(4, 8^(2)) Archimedean plasma photonic crystals(APPCs) by use of dielectric barrier discharges in air. Dynamical control on both the macrostructures including the lattice symmetry and the crystal orientation, and the microstructures including the fine structures of scattering elements has been achieved. The formation mechanisms of APPCs are studied by time-resolved measurements together with numerical simulations. Large omnidirectional band gaps of APPCs have been obtained. The tunable topology of APPCs may offer new opportunities for fabricating multi-functional and highly-integrated microwave devices.

Degradation of Aniline Wastewater Using Dielectric Barrier Discharges at Atmospheric Pressure

Aniline is a toxic water pollutant detected in drinking water and surface water, and this chemical is harmful to both human and aquatic life. A dielectric barrier discharge （DBD） reactor was designed in this study to investigate the treatment of aniline in aqueous solution. Discharge characteristics were assessed by measuring voltage and current waveforms, capturing light emission images, and obtaining optical emission spectra. The effects of several parameters were analyzed, including treatment distance, discharge power, DBD treatment time, initial pH of aniline solutions, and addition of sodium carbonate and hydrogen peroxide to the treatment. Aniline degradation increased with increasing discharge power. Under the same conditions, higher degradation was obtained at a treatment distance of 0 mm than at other treatment distances. At a discharge power of 21.5 W, 84.32% of aniline was removed after 10 rain of DBD treatment. Initial pH significantly influenced aniline degradation. Adding a certain dosage of sodium carbonate and hydrogen peroxide to the wastewater can accelerate the degradation rate of aniline. Possible degradation pathways of aniline by DBD plasmas were proposed based on the analytical data of GC/MS and TOC.

Surface Modification of Fluororubber Using Atmospheric Pressure Dielectric Barrier Discharge(DBD)

Fluoride rubber F2311 film, an alternating copolymer of CF2-CFC1 （CTFE） and CH2-CF2 （VF2） components, was treated by atmospheric pressure dielectric barrier discharge （DBD） in air. The surface structure, topography and surface chemistry of the treated F2311 films were characterized by contact angle measurement, atomic force microscopy （AFM）, and X-ray photoelectron spectroscopy （XPS）, respectively. The experimental results showed that a short time air plasma treatment led to morphological, wettability and chemical changes in the F2311 films. The surface hydrophilicity increased greatly after the plasma treatment, the static water contact angle decreased from 98.6° to 32°, and oxygen containing groups （C=O, O-C=O, etc. ） were introduced. Atomic force microscopy revealed that plasma produced by DBD etched F2311 films obviously. The roughness of the samples increased remarkably with the formation of peaks and valleys on the treated surfaces. The increased surface wettability may be correlated with both the introduction of hydrophilic groups due to air plasma oxidation of the surface and the change in surface morphology etched by DBD.

A New Approach to Plasma CVD of TiO_2 Photocatalyst on γ-Al_2O_3 Pellet Filled in Dielectric Barrier Discharges at Atmospheric Pressure

A supported TiO2/γ-Al2O3 photocatalyst has been prepared by γ-Al2O3 pellet-filled dielectric barrier discharges induced plasma CVD at atmospheric pressure and room temperature. The TiO2/γ-Al2O3 photocatalyst exhibits higher photocatalytic activity than Degussa P25, and much higher photocatalytic activity than that prepared by thermal CVD.

Study on the Microsecond Pulse Homogeneous Dielectric Barrier Discharges in Atmospheric Air and Its Influencing Factors

The homogeneous dielectric barrier discharge （DBD） in atmospheric air between two symmetric-columnar copper electrodes with epoxy plates as the dielectric barriers is generated using a us pulse high voltage power supply. The discharge characteristics are studied by measurement of its electrical discharge parameters and observation of its light emission phenom- ena, and the main discharge parameters of the homogenous DBD, such as discharge current and average discharge power, are calculated. Results show that the discharge generated is a homogeneous one with one larger single current pulse of about 2 #s duration appearing in each voltage pulse, and its light emission is radially homogeneous and covers the entire surface of the two elec- trodes. The influences of applied voltage amplitude, air gap distance and barrier thickness on the transition of discharge modes are studied. With the increase of air gap distance, the discharge will transit from homogeneous mode to filamentary mode. The higher the thickness of dielectric barriers, the larger the air gap distance for generating the homogeneous discharge mode. The average discharge power increases non-linearly with increasing applied voltage amplitude, and decreases non-linearly with the increase of air gap distance and barrier thickness. In order to generate stable and homogeneous DBD with high discharge power, thin barriers distance should be used, and higher applied voltage amplitude should be applied to small air gap.

Nonlinear phenomena in dielectric barrier discharges: pattern, striation and chaos

The nonlinear phenomenon is very popular in dielectric barrier discharge （DBD） plasmas. There are at least three kinds of spatial and temporal nonlinear phenomena appearing synchronously or asynchronously in DBDs, i.e. self-organized patterns, striations and chaos. This paper describes the recent research and progress in understanding the nature of these nonlinear phenomena. Patterns are macroscopic structures with certain spatial and/or temporal periodicities generated through self- organization of microscopic parameters. The physics of patterns in DBDs is mainly associated with lateral dynamic behaviors or the lateral non-local effect of charged particles resulting in the lateral development or non-uniformity of discharge. Striations are ionization waves with unique properties determined by transport phenomena, ionization processes and electron kinetics in current-carrying plasmas. The physics of striations in DBDs is mainly associated with the advances in non-local electron kinetics in spatially inhomogeneous plasmas. Chaos is a kind of random and non-periodic phenomenon occunfng in a determined dynamic system, following a series of certain rules while exhibiting random locomotion, and is regarded as an intrinsic and ubiquitous phenomenon in a nonlinear dynamic system. An evolution trajectory including period-doubling bifurcation to chaos was observed in DBDs or DBD-derived plasmas. In a common sense, it is believed that the formation of all the three nonlinear phenomena in a DBD system should be related to the non-local transversal and/or longitudinal dynamics of space charges （i.e. non-local effect） or the localized electric field interaction. Future work is still needed on the underlying physics and should be directed to pursuing the unification of these nonlinear phenomena in DBD.

A Kinetic Study of Ozone and Nitric Oxides in Dielectric Barrier Discharges for O_2/NO_x Mixtures

A simple model is described to simulate kinetic processes in dielectric barrier dis-charges for O2/NOx mixtures. A threshold of ozone production found experimentally is confirmedby the calculations of this modeling, and the underiying chemical reaction mechanisms are dis-cussed. It is also found that the effects of diffusion processes in the period of the lifetime of Oatoms are not important to microdischarge channels with a large radius, i.e. larger than l50 μm.

Particle Densities of the Atmospheric-Pressure Argon Plasmas Generated by the Pulsed Dielectric Barrier Discharges

Atmospheric-pressure argon plasmas have received increasing attention due to their high potential in many industrial and biomedical applications. In this paper, a 1-D fluid model is used for studying the particle density characteristics of the argon plasmas generated by the pulsed dielectric barrier discharges. The temporal evolutions of the axial particle density distributions are illustrated, and the influences of changing the main discharge conditions on the averaged particle densities are researched by independently varying the various discharge conditions. The calculation results show that the electron density and the ion density reach two peaks near the momentary cathodes during the rising and the falling edges of the pulsed voltage. Compared with the charged particle densities, the densities of the resonance state atom Arr and the metastable state atom Arm have more uniform axial distributions, reach higher maximums and decay more slowly. During the platform of the pulsed voltage and the time interval between the pulses, the densities of the excited state atom Ar＊ are far lower than those of the Arr or the Arm. The averaged particle densities of the different considered particles increase with the increases of the amplitude and the frequency of the pulsed voltage. Narrowing the discharge gap and increasing the relative dielectric constant of the dielectric also contribute to the increase of the averaged particle densities. The effects of reducing the discharge gap distance on the neutral particle densities are more significant than the influences on the charged particle densities.

Plasma propagation in single-particle packed dielectric barrier discharges:joint effects of particle shape and discharge gap

In this work,a single Al_(2)O_(3) particle packed dielectric barrier discharge(DBD)reactor with adjustable discharge gap is built,and the influences of the particle shape(ball and column)and the residual gap between the top electrode and particle on the electrical and optical characteristics of plasma are studied.Our research confirms that streamer discharge and surface discharge are the two main discharge patterns in the single-particle packed DBD reactor.The strong electric field distortion at the top of the ball or column caused by the dielectric polarization effect is an important reason for the formation of streamer discharge.The length of streamer discharge is proportional to the size of the residual gap,but the number of discharge times of a single voltage cycle shows an opposite trend.Compared to the column,a smooth spherical surface is more conducive to the formation of large and uniform surface discharges.The surface discharge area and the discharge intensity reach a maximum when the gap is equal to the diameter of the ball.All in all,the results of this study will provide important theoretical support for the establishment of the synergistic characteristics of discharge and catalysis in plasma catalysis.

Formation of honeycomb-Kagome hexagonal superlattice pattern with dark discharges in dielectric barrier discharge

A honeycomb-Kagome hexagonal superlattice pattern with dark discharges is observed in a dielectric barrier discharge system for the first time.The spatiotemporal structure of the honeycomb-Kagome hexagonal superlattice pattern with dark discharges is investigated by an intensified charge-coupled device and the photomultipliers show that it is an interleaving of three different sub-lattices,which are bright-spot,invisible honeycomb lattice,and Kagome lattice with invisible frameworks and dim-spots,respectively.The invisible honeycomb lattices and Kagome lattices are actually composed of dark discharges.By using the optical emission spectra method,it is found that the plasma parameters of the three different sub-lattices are different.The influence of the dark discharges on pattern formation is discussed.The results may have significance for the investigation of the dark discharges and will accelerate the development of self-organized pattern dynamics.

Multiple current peaks and spatial characteristics of atmospheric helium dielectric barrier discharges with repetitive unipolar narrow pulse excitation

Atmospheric dielectric barrier discharges driven by repetitive unipolar narrow pulse excitation are investigated numerically by using one-dimensional fluid models.The one-dimensional simulation focuses on the effects of applied voltage amplitude,pulse repetition frequency,gap width andγcoefficient on the multiple-current-pulse(MCP)discharge.The results indicate that the MCP behavior will lead to the stratification of electron density distribution in axial direction.Traditional MCP manipulating methods,such as reducing the applied voltage amplitude,increasing the applied voltage frequency,adjusting the gap width,cannot regulate MCPs exhibiting in this work.Further analyses reveal that the increasing electric field of the cathode fall region is the basis for the emergence of MCP behavior.

Transition of Streamer, Corona and Glow Discharges in Needle-to-plane Dielectric Barrier Discharge at Atmospheric Pressure Air

To discuss the modes of dielectric barrier discharge(DBD) between needle-to-plane electrodes in air,DBD is generated and observed on a needle-to-plane device at atmospheric pressure air.Fast images of the DBD are taken by using a charge couple device(CCD) cinema with a macro lens,while the electrical and photo-electricity waveforms of the DBD are recorded.The current waveforms show that under an applied voltage of 3 kV,there are numerous short current pulses in both positive and negative half-periods of discharges.However,under 6 kV,there are still the numerous short current pulses in the positive half-periods,but only one wide current pulse in each negative half-period.This difference is also found in the photoelectric signals.The streamer,corona and glow discharges are observed from the images of the discharges at different applied voltages.The structure of glow discharge in the negative period is exactly the same as that of the low pressure glow discharge.However,in the positive period of discharge there is always a streamer.In the negative period of discharge,while the applied voltage increases,the transition from corona to glow discharge is observed.The progress of a transition between streamer and glow discharge at 6 kV during one period is analyzed.The glow discharge appearance is determined by two factors: the discharge current is limited to a certain extent by the dielectric layer; the charges deposited on the dielectric layer during the last half period enhance the intensity of the electric field.At an insufficient applied voltage,the cathode drop leads to no glow discharge,but Trichel pulses.

Experimental and simulated investigation of microdischarge characteristics in a pin-to-pin dielectric barrier discharge(DBD)reactor

Both experimental and simulated studies of microdischarge(MD)are carried out in a dielectric barrier discharge with a pin-to-pin gap of 3.5 mm,ignited by a sinusoidal voltage with a peak voltage of 10 kV and a driving frequency of 5 kHz.Statistical results have shown that the probability of the single current pulse in the positive half-period(HP)reaches 73.6%under these conditions.Experimental results show that great luminous intensity is concentrated on the dielectric surface and the tip of the metal electrode.A 1D plasma fluid model is implemented by coupling the species continuity equations,electron energy density equations,Poisson equation,and Helmholtz equations to analyze the MD dynamics on the microscale.The simulated results are in good qualitative agreement with the experimental results.The simulated results show that the MD dynamics can be divided into three phases:the Townsend phase,the streamer propagation phase,and the discharge decay phase.During the streamer propagation phase,the electric field and electron density increase with the streamer propagation from the anode to the cathode,and their maximal values reach 625.48 Td and 2.31×10^(19)m^(-3),as well as 790.13 Td and 3.58×10^(19)m^(-3)in the positive and negative HP,respectively.Furthermore,a transient glow-like discharge is detected around the anode during the same period of streamer propagation.The formation of transient glow-like discharge is attributed to electrons drifting back to the anode,which is driven by the residual voltage in the air gap.

Effect of Cooling Methods on Methane Conversion via Dielectric-Barrier Discharges

Effects of cooling methods on stability and methane conversion rate using dielectric-barrier discharges （DBD） were systematically investigated in this article. The results showed that the methane conversion rate was as high as 44.43% in a pure methane system at a flow rate of 100 mL·min^-1 and an input power of 234.2 W with air cooling. A dark greenish and soft film-like carbon was deposited on the outer surface of quartz tube when the outer electrode was watercooled, which decreased the methane conversion. With air cooling of inner electrode the selectivity of C2 hydrocarbons was higher than that with other cooling methods, while the C3 hydrocarbons had higher selectivity with flowing water cooling. Cooling the inner electrode could restrain the carbon deposition, but would decrease the methane conversion rate. The stability of both reaction and plasma operation can be improved through cooling the reactor. From thermodynamic analysis, it was found that the effective collisions frequency among the reactant molecules and free electrons （e^-） increased with temperature, which in turn led to a higher methane conversion rate and a change in the distribution of products.