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.     

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.     

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.