Improving the surface flashover performance of epoxy resin by plasma treatment:a comparison of fluorination and silicon deposition under different modes

This work treats the Al2O3-ER sample surface using dielectric barrier discharge fluorination(DBD-F),DBD silicon deposition(DBD-Si),atmospheric-pressure plasma jet fluorination(APPJ-F)and APPJ silicon deposition(APPJ-Si).By comparing the surface morphology,chemical components and electrical parameters,the diverse mechanisms of different plasma modification methods used to improve flashover performance are revealed.The results show that the flashover voltage of the DBD-F samples is the largest(increased by 21.2％at most),while the APPJ-F method has the worst promotion effect.The flashover voltage of the APPJ-Si samples decreases sharply when treatment time exceeds 180 s,but the promotion effect outperforms the DBD-Si method during a short modified time.For the mechanism explanation,firstly,plasma fluorination improves the surface roughness and introduces shallow traps by etching the surface and grafting fluorine-containing groups,while plasma silicon deposition reduces the surface roughness and introduces a large number of shallow traps by coating SiOx film.Furthermore,the reaction of the DBD method is more violent,while the homogeneity of the APPJ modification is better.These characteristics influence the effects of fluorination and silicon deposition.Finally,increasing the surface roughness and introducing shallow traps accelerates surface charge dissipation and inhibits flashover,but too many shallow traps greatly increase the dissipated rate and facilitate surface flashover instead.     

Deposition of SiC_xH_yO_z thin film on epoxy resin by nanosecond pulsed APPJ for improving the surface insulating performance

Non-thermal plasma surface modification for epoxy resin(EP)to improve the insulation properties has wide application prospects in gas insulated switchgear and gas insulatedtransmission line.In this paper,a pulsed Ar dual dielectrics atmospheric-pressure plasma jet(APPJ)was used for Si C_xH_yO_zthin film deposition on EP samples.The film deposition was optimized by varying the treatment time while other parameters were kept at constants(treatment distance:10 mm,precursor flow rate:0.6 l min~(-1),maximum instantaneous power:3.08 k W and single pulse energy:0.18 m J).It was found that the maximum value of flashover voltages for negative and positive voltage were improved by 18%and 13%when the deposition time was3 min,respectively.The flashover voltage reduced as treatment time increased.Moreover,all the surface conductivity,surface charge dissipation rate and surface trap level distribution reached an optimal value when thin film deposition time was 3 min.Other measurements,such as atomic force microscopy and scanning electron microscope for EP surface morphology,Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy for EP surface compositions,optical emission spectra for APPJ deposition process were carried out to better understand the deposition processes and mechanisms.The results indicated that the original organic groups(C–H,C–C,C=O,C=C)were gradually replaced by the Si containing inorganic groups(Si–O–Si and Si–OH).The reduction of C=O in ester group and C=C in p-substituted benzene of the EP samples might be responsible for shallowing the trap level and then enhancing the flashover voltage.However,when the plasma treatment time was longer than 3 min,the significant increase of the surface roughness might increase the trap level depth and then deteriorate the flashover performance.     

Surface modification of silicone rubber by CF4 radio frequency capacitively coupled plasma for improvement of flashover

The flashover performance of insulating materials plays an important role in the development of high-voltage insulation systems. In this paper, silicone rubber(SIR) is modified by CF₄ radio frequency capacitively coupled plasma (CCP) for the improvement of surface insulation performance. The discharge mode and active particles of CCP are diagnosed by the digital single-lens reflex and the spectrometer. Scanning electron microscopy and x-ray photoelectron spectroscopy are used for the surface physicochemical properties of samples, while the surface charge dissipation, charge accumulation measurement, and flashover test are applied for the surface electrical characteristics. Experimental results show that the fluorocarbon groups can be grafted and the surface roughness increases after plasma treatment. Besides, the surface charge dissipation is decelerated and the positive charge accumulation is inhibited obviously for the treated samples. Furthermore, the surface flashover voltage can be increased by 26.67% after 10 min of treatment. It is considered that strong electron affinity of C–F and increased surface roughness can contribute to deepening surface traps, which not only inhibits the development of secondary electron emission avalanche but also alleviates the surface charge accumulation and finally improves the surface flashover voltage of SIR.     

Non-Thermal Equilibrium Atmospheric Pressure Glow-Like Discharge Plasma Jet

Non-thermal equilibrium atmospheric pressure plasma jet (APPJ) is a cold plasma source that promises various innovative applications,and the uniform APPJ is more favored.Glow discharge is one of the most effective methods to obtain the uniform discharge.Compared with the glow dielectric barrier discharge (DBD) in atmospheric pressure,pure helium APPJ shows partial characteristics of both the glow discharge and the streamer.In this paper,considering the influence of the Penning effect,the electrical and optical properties of He APPJ and Ar/NH3 APPJ were researched.A word "Glow-like APPJ" is used to characterize the uniformity of APPJ,and it was obtained that the basic characteristics of the glow-like APPJ are driven by the kHz AC high voltage.The results can provide a support for generating uniform APPJ,and lay a foundation for its applications.     

The influence of grounded electrode positions on the evolution and characteristics of an atmospheric pressure argon plasma jet

An atmospheric pressure plasma jet (APPJ) in Ar with various grounded electrode arrangements is employed to investigate the effects of electrode arrangement on the characteristics of the APPJ.Electrical and optical methods are used to characterize the plasma properties.The discharge modes of the APPJ with respect to applied voltage are studied for grounded electrodepositions of 10 mm,40 mm and 80 mm,respectively,and the main discharge and plasma parameters are investigated.It is shown that an increase in the distance between the grounded electrode and high-voltage electrode results in a change in the discharge modes and discharge parameters.The discharges transit from having two discharge modes,dielectric barrier discharge (DBD) and jet,to having three,corona,DBD and jet,with increase in the distance from the grounded to the high-voltage electrodes.The maximum length of the APPJ reaches 3.8 cm at an applied voltage of 8 kV.The discharge power and transferred charges and spectral line intensities for species in the APPJ are influenced by the positions of the grounded electrode,while there is no obvious difference in the values of the electron excited temperature (EET) for the three grounded electrode positions.     

Aging Characteristics on Epoxy Resin Surface Under Repetitive Microsecond Pulses in Air at Atmospheric Pressure

Research on aging characteristics of epoxy resin(EP) under repetitive microsecond pulses is important for the design of insulating materials in high power apparatus.It is because that very fast transient overvoltage always occurs in a power system,which causes flashover and is one of the main factors causing aging effects of EP materials.Therefore,it is essential to obtain a better understanding of the aging effect on an EP surface resulting from flashover.In this work,aging effects on an EP surface were investigated by surface flashover discharge under repetitive microsecond pulses in atmospheric pressure.The investigations of parameters such as the surface micro-morphology and chemical composition of the insulation material under different degrees of aging were conducted with the aid of measurement methods such as atomic force microscopy(AFM),scanning electron microscopy(SEM),and X-ray photoelectron spectroscopy(XPS).Results showed that with the accumulation of aging energy on the material surface,the particles formed on the material surface increased both in number and size,leading to the growth of surface roughness and a reduction in the water contact angle; the surface also became more absorbent.Furthermore,in the aging process,the molecular chains of EP on the surface were broken,resulting in oxidation and carbonisation.     

Surface charge accumulation behavior and its influence on surface flashover performance of Al2O3-filled epoxy resin insulators under DC voltages

Surface charge accumulation on insulator surface can have great influence on surface flashover performance. An experimental system is established to investigate surface charge accumulation and decay characteristics ofAl₂O₃-filled epoxy resin insulators in 0.1 MPa SF₆ under DC voltages. Surface potential is recorded by a Kelvin vibrating probe connected to an electrostatic voltmeter. By pre-depositing charges on insulator surface, the influence of surface charges on surface flashover performance is studied. The results reveal that surface charge distribution appearance is the combined effect of electrode injection, back discharge and gas ionization. Surface charge distribution has obvious polarity effect. It is concentrated near the HV electrode under positive voltages and dispersed under negative voltages. The difference in positive and negative surface flashover voltage is attributed to the difference in surface charge distribution under DC voltages of different polarities. Surface charge decay contains two stages, which satisfies the law of double exponential function. At first stage, surface charge decays fast, which corresponds to charges escaping from shallower traps. While it decays slowly at the second stage, which corresponds to charge escaping from deeper traps. Surface charge decay process is dominated by surface conductivity mechanism. The pre-deposited charges on insulation surface have great influence on surface flashover performance. The deposited positive charges can increase positive flashover voltage but decrease negative flashover voltage.     

Discoloration of Congo Red by Rod-Plate Dielectric Barrier Discharge Processes at Atmospheric Pressure

A dielectric barrier discharge (DBD) reactor with a rod-plate electrode configuration was used for the oxidative decomposition of Congo red dye in an aqueous solution.Plasma was generated in the gas space above the water interface under atmospheric pressure.Discharge characteristics were analyzed by voltage-current waveforms.Effects of applied voltage,initial conductivity,and initial concentration were also analyzed.Congo red discoloration increased with increased applied voltage and decreased conductivity.The initial conductivity significantly influenced the Congo red discoloration.Under the same conditions,the highest discoloration rate was obtained at 25 mg/L.The presence of ferrous ions in the solutions had a substantial positive effect on Fenton dye degradation and flocculation.At an applied voltage of 20 kV,about 100％of dye was degraded after 4 min of Fe2+/DBD treatment.Results showed that adding a certain dosage of hydrogen peroxide to the wastewater could enhance the discoloration rate.Possible pathways of Congo red discoloration by DBD plasma were proposed based on GC/MS,FTIR,and UV-vis spectroscopy analyses.     

Trap distribution of polymeric materials and its effect on surface flashover in vacuum

The surface trap parameter can significantly affect the development of surface flashover in vacuum, but the effective mode and mechanism are not very clear yet. The trap parameters of three polymeric materials were tested and calculated by means of isothermal surface potential decay. The flashover experiment was developed under different applied voltages. The results show a positive correlation between the withstand voltage and the deep trap, i.e., the deeper trap energy level is, the higher flashover voltage is. The dynamics process of charge trapping and detrapping was analyzed based on the charge transport model in dielectrics with a single trap level and two discrete trap levels. The time of charge trapping was compared with that of the discharge development. The results show that the charge trapping time is longer than the flashover development time. The way to influence flashover for a trap is not to decrease the secondary electrons in single discharge development, but to change the electric field distribution on the dielectric surface by charge capture.     

On surface elementary processes and polymer surface modifications induced by double pulsed dielectric barrier discharge

In this paper, polyethylene terephthalate and polyethylene films were plasma treated using a dielectric barrier discharge (DBD), which presents a second current pulse with inverse polarity induced at the decreasing applied voltage flank in addition to the main current pulse. The surface modifications of polymer samples were pointed out by two complementary methods: the contact angle method and atomic force microscopy (AFM) technique. The influence of the voltage pulse parameters as width, falling rate and frequency on the secondary discharge formation was studied and correlated with polymer surface modifications. The plasma treatment is improved when the discharge is driven by a voltage pulses with very fast falling flank. After plasma treatment, roughness of the PE films is not considerably changed; only topographical modifications of PE treated samples being observed, while treated PET surfaces present an increased roughness and a new granular structure, with much smaller features compared to the pristine ones.     

Effect of plasma step gradient modification on surface electrical properties of epoxy resin

In this paper, plasma fluorination is combined with plasma silicon deposition to achieve step gradient modification on an epoxy resin surface. The physicochemical characteristics of samples are investigated and the electrical performances measured. The obtained results show that compared with untreated and single treated samples, the samples treated by step gradient modification significantly improve the flashover performance. According to experiment and simulation, the mechanism explanations are summarized as follows. First, it is found that the step gradient conductivity can effectively optimize the electric field distribution of a needle-needle electrode. Then, step gradient modification suppresses the accumulation of surface charge at the triple junction and makes the charge distribution more uniform. Furthermore, it can accelerate the surface dissipation on a high electrical field region and control the dissipation rate on a low electrical field region. All these results can restrain surface discharge and increase the flashover voltage. The step gradient modification method proposed in this paper provides a new idea for improving the surface insulation performance.     

Study on the properties of a ε-Fe_3N-based magnetic lubricant prepared by DBD plasma

The ε-Fe_3N-based magnetic lubricant which is stable and high saturation magnetization has been prepared by a homemade DBD device under the atmospheric pressure. The results show that the NH_3 flow rate, the applied peak-to-peak voltage and the mass ratio of surfactant and carrier lubricant have important effects on the phase structure, the magnetic properties, the size of ferroparticles and the stability of the ε-Fe_3N-based magnetic lubricant. TEM images show the ε-Fe_3N ferroparticles are dispersed in the carrier lubricant homogeneously, and the cluster phenomenon is not observed. The stable ε-Fe_3N-based magnetic lubricant with the saturation magnetization of 50.11 m T and the mean ferroparticle size of 11 nm is prepared successfully. The main particles of the atmospheric-pressure Ar/NH_3/Fe(CO)_5 DBD plasma are NH, N, N~+, Fe, N_2, Ar, H_α, and CO; NH is a decomposition product of NH_3. Fe and N active radicals are two elementary species in the preparation of the ε-Fe_3N-based magnetic lubricant in the atmospheric-pressure DBD plasma. There are two discharge modes for DBD plasma, namely, multi-pulse APGD and filamentary discharge. By increasing the applied peak-to-peak voltage from 4600 to 7800 V, the discharge mode is changed from single-pulse APGD with filamentary discharge to two-pulse APGD with filamentary discharge, and the Lissajous figure also converts from a quadrilateral with one step to two steps on the right-hand side.     

Study on the properties of a ε-Fe3N-based magnetic lubricant prepared by DBD plasma

The ε-Fe₃N-based magnetic lubricant which is stable and high saturation magnetization has been prepared by a homemade DBD device under the atmospheric pressure. The results show that the NH3 flow rate, the applied peak-to-peak voltage and the mass ratio of surfactant and carrier lubricant have important effects on the phase structure, the magnetic properties, the size of ferroparticles and the stability of the ε-Fe3N-based magnetic lubricant. TEM images show the ε-Fe₃N ferroparticles are dispersed in the carrier lubricant homogeneously, and the cluster phenomenon is not observed. The stable ε-Fe3N-based magnetic lubricant with the saturation magnetization of 50.11 mT and the mean ferroparticle size of 11 nm is prepared successfully. The main particles of the atmospheric-pressure Ar/NH3/Fe(CO)5 DBD plasma are NH, N, N⁺, Fe, N₂, Ar, H_α, and CO; NH is a decomposition product of NH₃. Fe and N active radicals are two elementary species in the preparation of the ε-Fe3N-based magnetic lubricant in the atmospheric-pressure DBD plasma. There are two discharge modes for DBD plasma, namely, multi-pulse APGD and filamentary discharge. By increasing the applied peak-to-peak voltage from 4600 to 7800 V, the discharge mode is changed from single-pulse APGD with filamentary discharge to two-pulse APGD with filamentary discharge, and the Lissajous figure also converts from a quadrilateral with one step to two steps on the right-hand side.     

Synthesis of Cu-Doped Mixed-Phase TiO2 with the Assistance of Ionic Liquid by Atmospheric-Pressure Cold Plasma

An atmospheric-pressure dielectric barrier discharge(DBD) gas-liquid cold plasma was employed to synthesize Cu-doped TiO_2 nanoparticles in an aqueous solution with the assistance of[C_2MIM]BF_4 ionic liquid(IL) and using air as the working gas.The influences of the discharge voltage,IL and the amount of copper nitrite were investigated.X-ray diffraction,N_2adsorption-desorption measurements and UV-Vis spectroscopy were adopted to characterize the samples.The results showed that the specific surface area of TiO_2 was promoted with Cu-doping(from 57.6 m~2·g~(-1) to 106.2 m~2·g~(-1) with 3%Cu-doping),and the content of anatase was increased.Besides,the band gap energy of TiO_2 with Cu-doping decreased according to the UV-Vis spectroscopy test.The 3%Cu-IL-TiO_2 samples showed the highest efficiency in degrading methylene blue(MB) dye solutions under simulated sunlight with an apparent rate constant of 0.0223 min~(-1),which was 1.2 times higher than that of non-doped samples.According to the characterization results,the reasons for the high photocatalytic activity were discussed.     

Numerical Study of Pulsed Dielectric Barrier Discharge at Atmospheric Pressure Under the Needle-Plate Electrode Configuration

In this paper,we study the characteristics of atmospheric-pressure pulsed dielectric barrier discharge (DBD) under the needle-plate electrode configuration using a one-dimensional self-consistent fluid model.The results show that,the DBDs driven by positive pulse,negative pulse and bipolar pulse possess different behaviors.Moreover,the two discharges appearing at the rising and the falling phases of per voltage pulse also have different discharge regimes.For the case of the positive pulse,the breakdown field is much lower than that of the negative pulse,and its propagation characteristic is different from the negative pulse DBD.When the DBD is driven by a bipolar pulse voltage,there exists the interaction between the positive and negative pulses,resulting in the decrease of the breakdown field of the negative pulse DBD and causing the change of the discharge behaviors.In addition,the effects of the discharge parameters on the behaviors of pulsed DBD in the needle-plate electrode configuration are also studied.     

Classification and uniformity optimization of mesh-plate DBD and its application in polypropylene modification

The classification of spatial characteristics and discharge modes of dielectric barrier discharge (DBD) are gaining increasing attention in industrial applications, especially in the field of surface treatment of materials. In this work, gray level histogram (GLH) and Fourier energy spectrum based on the digital image processing technology are applied to investigate the spatial structure and discharge mode of mesh-plate DBD. The coefficient of variation (CV) is calculated to describe the uniformity of the discharge. The results show that the discharge mode of mesh-plate DBD changes from periodic discharge to filamentary discharge when the applied voltage increases from 11–15 kV. Moreover, a more regular spatial structure is obtained under lower applied voltages during the discharge process. It is also found that the apertures of mesh electrodes which are below 1mm have smaller values of CV compared to plate electrodes, indicating more uniform discharge. Finally, polypropylene is treated by mesh-plate DBD for surface modification. The hydrophilicity is significantly improved as the water contact angle decreased by 64°, and the dyeing depth is also enhanced.     

In situ and real-time optical investigation of nitrogen plasma treatment of polycarbonate

Polycarbonates are very attractive polymeric materials with extensive applications in optical technology as well as for replacing conventional rigid substrates, such as glass. The use of plasma treatments has been successfully employed for improving their surface properties and increasing adhesion of subsequent functional coatings. The objective of this study is to employ in situ and real-time Fourier transform infrared spectroscopic ellipsometry for a detailed investigation of the effect of nitrogen plasma treatment using a pulsed DC voltage, on the optical properties of polycarbonates. Measurements taken before, during and after plasma treatment showed the formation of a surface overlayer in which macromolecular chain scission takes place. The evolution of the overlayer thickness and its bonding has been investigated in detail. Also, the study of the surface nano-topography of polycarbonates by atomic force microscopy showed that plasma treatment increases the surface roughness.     

Application of persulfate in low-temperature atmospheric-pressure plasma jet for enhanced treatment of onychomycosis

Fungal infection of human nails,or onychomycosis,affects 10%of the world's adult population,but current therapies have various drawbacks.In this work,we employed a self-made low-temperature plasma(LTP)device,namely,an atmospheric-pressure plasma jet(APPJ)device to treat the nails infected with Trichophyton rubrum(T.rubrum)with the aid of persulfate solution.We found that persulfate solution had a promoting effect on plasma treatment of onychomycosis.With addition of sodium persulfate,the APPJ therapy could cure onychomycosis after several times of treatment.As such,this work has demonstrated a novel and effective approach which makes good use of LTP technique in the treatment of onychomycosis.     

Measurement of Electron Density and Ion Collision Frequency with Dual Assisted Grounded Electrode DBD in Atmospheric Pressure Helium Plasma Jet

The properties of a helium atmospheric-pressure plasma jet（APPJ）are diagnosed with a dual assisted grounded electrode dielectric barrier discharge device.In the glow discharge,we captured the current waveforms at the positions of the three grounded rings.From the current waveforms,the time delay between the adjacent positions of the rings is employed to calculate the plasma bullet velocity of the helium APPJ.Moreover,the electron density is deduced from a model combining with the time delay and current intensity,which is about 10¹¹cm^-3.In addition,The ion-neutral particles collision frequency in the radial direction is calculated from the current phase difference between two rings,which is on the order of 10⁷ Hz.The results are helpful for understanding the basic properties of APPJs.     

Degradation of phenol using a combination of granular activated carbon adsorption and bipolar pulse dielectric barrier discharge plasma regeneration

A combined method of granular activated carbon (GAC) adsorption and bipolar pulse dielectric barrier discharge (DBD) plasma regeneration was employed to degrade phenol in water. After being saturated with phenol, the GAC was filled into the DBD reactor driven by bipolar pulse power for regeneration under various operating parameters. The results showed that different peak voltages, air flow rates, and GAC content can affect phenol decomposition and its major degradation intermediates, such as catechol, hydroquinone, and benzoquinone. The higher voltage and air support were conducive to the removal of phenol, and the proper water moisture of the GAC was 20％. The amount of H2O2 on the GAC was quantitatively determined, and its laws of production were similar to phenol elimination. Under the optimized conditions, the elimination of phenol on the GAC was confirmed by Fourier transform infrared spectroscopy,and the total removal of organic carbons achieved 50.4％. Also, a possible degradation mechanism was proposed based on the HPLC analysis. Meanwhile, the regeneration efficiency of the GAC was improved with the discharge treatment time, which attained 88.5％ after 100 min of DBD processing.