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Surface flashover that occurs on the surface of epoxy resin (EP) is one of the main causes of insulation failure in the power system. The newly emerging polar groups on the surface are highly desirable for enhancing the surface flashover performance of EP. Here, we present a facile ultraviolet (UV) irradiation method to enable the controlled introduction of C–OH and C = O groups on the EP surface. The resultant UV-irradiated EPs (UV-EPs) are highly uniform with little variation in surface roughness while exhibiting tunable polar content with the irradiation time. We also present the first systematic investigation into the effect of UV irradiation time on the surface flashover voltage of UV-EPs, with a maximum increase of 23.07% compared to the pristine EP. Mechanistic studies suggest that the enhancement of the surface flashover voltage is mainly dominated by the density of the deep trap, which is strongly dependent on the UV-induced C–OH and C = O groups on the surface. Besides that, the UV-EPs also display long-term stability due to the stable presence of polar groups, which further demonstrates the feasibility of this method for the development of EP with excellent properties. [ABSTRACT FROM AUTHOR]

Surface flashover is an inevitable insulation issue for basin-type insulators in gas-insulated switchgears/lines, which significantly challenges the reliability of the electrical power systems. Previous studies have indicated that polymer/semiconductor-filler composite coatings effectively improve the insulation properties; however, the influence mechanism of the coating materials on flashover has not been demonstrated from a molecular perspective. In this work, epoxy/silicon-carbide (EP/SiC) composites were coated onto an EP substrate. The energy-level structure, surface trap, surface charging, and DC flashover voltage in SF6/N2 were calculated and characterized, and the process by which the tailored molecular energy level influences surface charge transport and flashover characteristics was elucidated. The incorporating of SiC particles reduced the width of the bandgap and introduced shallow traps, which improved carrier mobility and surface conductivity. Quantitative analysis of charge transport indicated that the improved carrier mobility and reduced surface trap level accelerated the surface charge dissipation. This reduced the tangential electrical field distortion and surface charge density and further impeded gas ionization. When the SiC concentration was 15 wt%, the flashover performance improved by 20.88%. This study describes the mechanism by which the EP/SiC coating regulates the surface charge distribution to improve the surface flashover performance by establishing a relationship among the microscopic molecular energy-level structures, mesoscopic charge transport, and macroscopic discharge phenomena. [ABSTRACT FROM AUTHOR]

Charge accumulation on the surface of insulation dielectric used in high voltage DC equipment is one of the important causes of surface flashover. Surface charge accumulation is closely related to surface trap characteristics. In this paper, the surface trap energy level distribution of polyimide (PI) was calculated and analyzed by the corona charging method and surface potential decay method. Besides, electron and hole trap characteristics were studied by changing the polarity and amplitude of the charging voltage. Furthermore, the influence of surface charge polarity and quantity on the flashover voltage was researched. Experimental results indicate that the energy level of traps corresponding to the maximum trapped charge density ranges around 0.94 eV. In addition, the energy level of the hole trap is slightly higher than that of the electron trap. The maximum charge density captured by the electron trap and hole trap is about 1.17 × 1021 and 1.03 × 1021 eV−1 m−3, respectively. The polarity of the surface charge can influence the conductivity characteristics of PI, the surface heterocharge reduces the volume conductivity by about 80% while the surface homocharge improves the volume conductivity of PI by about 6.4 times. Besides, both negative and positive surface charges can improve surface conductivity. The polarity of surface charge influences the flashover characteristics of PI, the flashover voltage increases by about 37.8% with the increase of homocharge density and decreases by about 23.8% with the increase of heterocharge density, which is related to the change of electric field near the cathode caused by surface charge accumulation. [ABSTRACT FROM AUTHOR]

The proliferation of urbanization and the integration of new energy sources have stimulated the development of gas-insulated transmission lines and switchgear (GIL/GIS). In particular, the compact DC GIS in offshore converter platforms will significantly reduce footprints for DC switchyards, exhibit exceptional climatic resistance, and facilitate the cost-effective connection of remote offshore wind farms and submarine links. Nevertheless, insulators used in GIS/GIL always suffer from surface charge accumulation under DC stress, which could distort and enhance the local electric field and thus trigger a flashover at the gas–solid interface if it exceeds certain magnitude levels. This susceptibility becomes a major concern affecting the reliability of DC gas-insulated systems. Beyond these engineering-related challenges lie fundamental physics problems involving mechanisms of charge accumulation and charge-induced flashover which still require exploration. To this end, this paper presents an overview of recent advancements on this topic whilst highlighting relevant issues to be addressed. Specifically, the surface charge accumulation phenomena under DC fields are reviewed, and the charging mechanisms are summarized from macroscopic to microscopic perspectives. Further, the correlation between surface charge and surface flashover is discussed. Moreover, recent developments in tailoring methods for surface charging are also presented. Finally, perspectives are given on current research progress and future needs. [ABSTRACT FROM AUTHOR]

To date, numerical simulation techniques for surface flashover are still under development. In this work, a DC surface flashover numerical simulation model is constructed based on a gas–solid coupling surface flashover theoretical model with a multilayered structure at the gas–solid interface. Considering the effects of solid, gas, and gas–solid interaction on surface flashover, bipolar charge transport in the solid surface layer, collision ionization in the gas phase layer, secondary electron emission, and gas adsorption in the gas surface layer are combined to calculate the surface flashover voltage. By initializing model parameters, surface charge transport dependent dc surface flashover voltages of epoxy composites in compressed nitrogen are calculated. The results indicate that the surface flashover voltage increases with surface deep trap level, deep trap density, shallow trap density, and carrier mobility; however, surface flashover voltage decreases with surface shallow trap level and surface charge density. To further investigate the effects of surface trap on surface flashover, a "U-shaped" curve is constructed to describe the relationship between surface flashover voltage and surface trap level by the simulation method which shows good agreement with experimental results. The simulation indicates surface flashover voltage of epoxy composites is influenced by surface deep and shallow traps in the solid surface layer—shallow traps mainly influence surface charge dissipation, while deep traps mainly influence electron emission on the solid surface. The value of Ptr/Pde is crucial for the dominating surface trap in surface flashover. [ABSTRACT FROM AUTHOR]

Surface flashover is likely to be triggered when a high electric field is applied due to its voltage being much lower than the breakdown voltage of either gas or solid, which restricts the development of advanced power electrical and electronic devices. To improve dc surface flashover performances in a vacuum and further explore the effects of deep traps on surface flashover, conductive multiwall carbon nanotubes (MWCNTs) and nonconductive nano-titania (TiO2) particles were incorporated into the epoxy (EP) matrix to tailor the surface deep trap level. Surface trap parameters, surface conductivity, surface charge density, and surface flashover voltage were calculated and measured. It indicates that surface flashover voltage improves by 23.07% and 14.45% compared to pristine EP when solely introducing MWCNTs or nano-TiO2 particles into the EP matrix, while it increases by 36.04% for C0.1T2 as simultaneously introducing both particles. When adding small amounts of particles, the surface charge transport is suppressed due to the introduction of bonded layers in the interfacial region, and the surface deep trap level is improved. As the surface deep trap level increases, charge transport and electron emission processes in the solid surface layer are hindered, which reduces the surface positive charge density and the distortion of the electric field and further results in the reduction of surface conductivity and the improvement of dc surface flashover voltage in a vacuum. The explanation for the effects of surface deep traps on surface flashover is essential for further improvements of surface flashover. [ABSTRACT FROM AUTHOR]

Spacecraft operating environment is rather complex, including high vacuum and energetic electron radiation. Vacuum-insulation interface is usually the weakest link of the insulation system since surface flashover usually occurs at the interface. There are several theories for surface flashover, above which the secondary electron emission avalanche (SEEA) and the electron triggered polarization relaxation (ETPR) are the mainstream. Since researchers have usually assumed that there has been a preexisting positive charge on the insulation surface, when the insulation surface is negatively charged by electron beam irradiation, these theories cannot be directly applied. In this article, the particle tracking method is used to simulate the initial and secondary electron trajectories in the initial phase of surface flashover, and to explain the influence of negative surface charge on surface flashover. The results show that: on the one hand, the negative deposited charge weakens the electric field at the cathode triple junction (CTJ), inhibits the initial electron emission, and hinders the initiation of flashover; on the other hand, the negative deposited charge repels the secondary electrons away from the insulation surface and hinders the multiplication of the secondary electrons and the development of flashover. [ABSTRACT FROM AUTHOR]

In this paper, surface trap distribution and the affected surface charge transport properties in epoxy/Al2O3 nanocomposites were studied to provide theoretical and experimental basis for improving surface flashover performance in vacuum. Four different contents of nano-Al2O3 particles were doped into epoxy and different trap distribution of samples was calculated from the surface potential decay method. Surface charge injection was identified by a direct current integrated charge [DCIC-Q(t)] method. Secondary electron emission (SEE) coefficient were obtained combing testing and fitting method. Results showed improved charge injection resistance, reduced charge mobility and SEE coefficient at a slight concentration of 0.5 wt% and 1 wt% by introducing deep traps. Continuing to increase nanoparticles in matrix was proved to reduce trap depth and have adverse effects. The surface flashover voltages reached the highest at 1 wt% nano-doping, which was in accordance with the percolate threshold loading content for trap depth, charge injection field and SEE coefficient. Surface charge injection caused the local field enhancement at the counter electrode and increased the possibility of surface flashover. Deep traps over 1 eV contributed to carrier capture, reduced charge injection and had a positive effect on increasing surface flashover voltage. [ABSTRACT FROM AUTHOR]

Surface flashover properties should meet stringent requirements for the use in electrical equipment under high voltage DC environments. With the aim of enhancing surface flashover properties and understanding the mechanisms involved at ambient conditions, epoxy nanocomposites with different weight proportions of nano-TiO2 were prepared. A silane coupling agent was selected to functionalize the TiO2, the effect of which on nanoparticles dispersion and distribution was perused through scanning electron microscopy and infrared spectroscopy. DC surface flashover measurements in five normally used ambient conditions (N2, air, CO2, SF6 and vacuum) were conducted and results were analyzed relating to loading content, gas parameters, surface trap parameters as well as electrical conductivity of nanocomposites. Outcomes indicate that with low content of nanoparticles, the density of deep traps was increased, showing a positive effect on inhibiting surface flashover. When the nanoparticle content is higher than 1 wt%, the overlapped transitional region reduced the energy level of traps, resulting in a poor display of surface flashover voltages. It was found that the nanoparticle content had a greater influence on surface flashover in vacuum. Electron negativity, ionization coefficient and mean free path of gases were taken into account to explain why surface flashover in SF6 appeared to be the highest, whereas in N2, it was the lowest. At last, effects of gas-solid interactions on surface flashover combining surface-trap characteristics, secondary electron-emission contributions and gas molecule impact-ionization characteristics were analyzed. [ABSTRACT FROM AUTHOR]

A satellite operates in a harsh environment including high vacuum and radiations. The vacuum-dielectric interface is the most vulnerable area to flashover discharge. Moreover, the presence of electron radiation will further decrease the flashover voltage. However, the influence of electron radiation on surface flashover remains unclear. We have performed a study on vacuum DC surface flashover during electron beam irradiation as a function of electron kinetic energy and incident direction on polyimide representative of spacecraft dielectrics. The electron radiation was accompanied by electron deposition and bombardment effects on the polyimide surface. The surface flashover voltage was experimentally found to significantly depend on the kinetic energy and incident angle of the electron beam. By calculating the trajectories of incident electron beam, we further found that the surface flashover voltages are closely connected to the irradiated area. During low energy electron beam irradiation, kinetic electrons are repelled away from the dielectric surface by the applied electric field, and deposited electrons will be dominant in the flashover process, promoting the surface flashover voltage. However, when the electron beam energy is high, kinetic electrons can overcome the applied electric field and strike the dielectric surface to generate secondary electrons, which will replace the field-emission electrons to serve as seed electrons, and initiate the flashover at a much lower applied voltage. These observations and analyses are expected to benefit the research into mitigating spacecraft discharge, and promote the knowledge of surface flashover. [ABSTRACT FROM PUBLISHER]

Since surface flashover of insulators in vacuum restricts the operation of the power equipment, improvement of surface flashover performance is of considerable importance in various fields. Previous studies show that surface flashover performance is closely related to the trap parameters of materials. In this paper, the crystallization behavior of low density polyethylene (LDPE) is modified by phenolphthalein. It results in the change of the trap parameters, leading to the enhancements in surface flashover performance. Results of differential scanning calorimeter (DSC) and scanning electron microscope (SEM) show that the crystallization behavior of LDPE is greatly changed by phenolphthalein. The crystallinity increases initially and then decreases with the increase of phenolphthalein concentration, and its maximum value occurs at 0.4 wt%. The spherulite size of LDPE composites decreases obviously with the increasing phenolphthalein concentration. The results of thermally stimulated current (TSC) show that both the shallow trap (? peak) and the deep trap (? peak) become deeper with the increase of phenolphthalein concentration. Furthermore, the impulse and dc surface flashover voltages present the optimum values at 1 wt% phenolphthalein concentration, which can be enhanced by 30.8% and 48.4%, respectively. It is concluded that LDPE with the smaller spherulite size indicates the deeper trap and the better surface flashover performance. This paper reveals the relationship between surface flashover performance and crystallization behavior of LDPE, and proposes a new method to enhance the surface flashover performance of semi-crystalline polymers in vacuum. [ABSTRACT FROM AUTHOR]

Charge transport behaviors of dielectric under electron radiation have an important influence on surface flashover performance. In this paper, charging process and model of dielectric during electron radiation are investigated based on the synergistic effect of electron movement on dielectric surface and charge transport properties in dielectric surface layer. For one thing, electrons accumulated in dielectric surface layer will form a reverse electric field acting against kinetic electrons flowing to dielectric surface, which affects the energy and density of electrons reaching the surface. For another, the changes of electron energy and density will further affect the dynamic processes of electron deposition, transport and accumulation in the surface layer. A dielectric charging model is proposed, and charge transport characteristics in dielectric surface and surface layer are obtained, including the temporal behavior of the incident kinetic electrons flowing to dielectric surface, the temporal behavior of secondary electrons and surface conduction charge, as well as the spatial and temporal behavior of the interior potential and electric field. Based on surface potential decay curves of polyimide under different electron energy levels (3-11 keV), the surface potential decay process and model are investigated. The surface trap distribution of polyimide shows different properties under different energy levels. The shallow trap level increases slightly with the increase of electron energy, ranging from 0.81 eV-0.85 eV, while the deep trap level remains unchanged about 0.94 eV. Besides, the amount of trap charge density gradually increases with the increase of electron energy. [ABSTRACT FROM AUTHOR]

The vacuum surface flashover voltage of dielectric material irradiated by electron-beam is very low, which is the key scientific issue for developing high-voltage and high-power spacecraft technology. This paper proposes a novel research way of considering the combined impact of incident kinetic electrons flowing to dielectric surface and deposited electrons in dielectric surface layer on dc surface flashover performance in vacuum. The experiments on dc surface flashover of polyimide in vacuum after electron irradiation and during electron irradiation were investigated. The experimental results indicated flashover voltage showed a distinctly different variation tendency under the two cases. The dc surface flashover voltage of polyimide after electron irradiation became higher than that without electron irradiation, because of the effect of deposited electrons in dielectric surface layer. During electron irradiation, the dc surface flashover voltage exhibited a dramatic drop from 26.9 kV (energy: 5 keV) to 10.5 kV (energy: 30 keV). Furthermore, the surface flashover characteristics exerts different changes for incident electrons of different energies levels. It has been experimentally indicated that the energy and incident trajectory of incident kinetic electrons flowing to polyimide surface are altered by the combined action of the reverse electric field created by deposited electrons and the applied electric field. As a result, the surface flashover characteristics were changed. This work provides a new insight into the vacuum surface flashover of dielectric material under electron irradiation, and can be used as the research basis of surface flashover mitigation methods in space environment. [ABSTRACT FROM PUBLISHER]

In order to understand aging mechanism of solid insulating materialscaused by surface flashover, this paper presents the electrical degradation characteristics of cross linked polystyrene (XLPS) by repetitive impulse surface flashovers in vacuum. The surface of samples underwent flashover repeatedly under different impulse voltage situations. Each flashover voltage and current was recorded, and the flashover energy was calculated to present flashover properties under each condition. Experimental results indicate that there are different degradation phenomena of XLPS after undergoing those impulse flashovers. The flashovers with small current have positive effect on surface flashover property of insulating material. However, the large currents when flashovers occurred play a destruction role in surface properties of XLPS. It is said that finite numbers of repetitive impulse flashovers under small flashover currents removed the contaminants, water and gas on the sample surface. In addition, it represents a "conditioning" effect and stable situation of flashover properties under these situations due to removal of emission sites, micro protuberance of electrodes and sample surface, especially at the cathode triple junction. However, repetitive flashovers under high flashover current (≫1 kA) led to surface degradation. This phenomena attributes to the high flashover energy changed the sample surface state, destroyed the sample surface and formed apparent carbonized channels and regions on sample surface. So flashover becomes unstable in this situation. Through summarizing and analyzing the experimental results, the flashover energy was proposed as a main factor to characterize the phenomenon of degradation by repetitive impulse flashovers in vacuum. [ABSTRACT FROM AUTHOR]

In this paper, a new organic insulation system is proposed to improve the surface-flashover performance in vacuum by inserting permittivity-gradient multilayer organic composite material between the electrode and the dielectric. Simulation technique was used to calculate and analyze the electric-field distribution of this new insulation system. The result shows that the electric-field strength at the vacuum–insulator–electrode triple junction decreases significantly by optimizing the permittivity and conductivity of the inserted layer. On the basis of this analysis, a new improved system of polyethylene composite material–polyethylene–polyethylene composite material was prepared and studied. The permittivity and conductivity of the composite material are controlled by the additive content in the carbon black semiconductive master batch. Moreover, it is indicated in our result that by properly adjusting the permittivity and conductivity through additive content, the surface-flashover performance in vacuum can be greatly improved. The highest first flashover voltage reaches 91 kV and increases by 75% compared with that of the polyethylene standard samples. It is also confirmed that by decreasing the electric-field strength at the vacuum–insulator–cathode triple junction, the vacuum surface-flashover performance can be improved significantly. [ABSTRACT FROM AUTHOR]

• Propose a surface flashover model involving gas molecules adsorption in compressed gas. • Establish a relation between surface flashover and gas molecules adsorption by experimental method and molecular simulation. • Clarify the role of gas adsorption in surface flashover. • Offer a new perspective to study surface flashover. In surface science, when a high voltage is applied, a penetrating discharge occurs along the gas-solid interface, which is called surface flashover. Although the electron stimulated gas desorption is a crucial process for surface flashover in vacuum, the underlying gas-solid interaction physics process for surface flashover is still ambiguous in compressed gas. In this study, the effect of gas adsorption on surface flashover is taken into consideration, surface flashover voltage, gas breakdown voltage, gas adsorption quantity are tested at various gas pressures and temperatures in N 2 , and the surface gas density is calculated. It is found that the difference between surface flashover voltage and gas breakdown voltage (V f / V b) has a positive correlation with the relative gas adsorption ability (V a × T / p or N sur / N av) in a wide gas pressure, temperature and density ranges. The results indicate surface flashover occurs in the gas adsorption layer, which density is higher than the average gas density in the gas phase. The gas adsorption improves the surface flashover voltage by impeding electrons collision ionization process in compressed gas. The findings in this research provide a new perspective for studying surface flashover, and offer a route to improve surface flashover performances by tailoring surface adsorption characteristics. [ABSTRACT FROM AUTHOR]

The influencing mechanism of electron beam irradiation on surface flashover of epoxy resin/Al 2 O 3 microcomposite was investigated. Epoxy resin/Al 2 O 3 microcomposite samples with a diameter of 50 mm and a thickness of 1 mm were prepared. The samples were irradiated by electron beam with energies of 10 and 20 keV and a beam current of 5 μA for 5 min. Surface potential decay, surface conduction, and surface flashover properties of untreated and irradiated samples were measured. Both the decay rate of surface potential and surface conductivity decrease with an increase in the energy of electron beam. Meanwhile, surface flashover voltage increase. It was found that both the untreated and irradiated samples have two trap centers, which are labeled as shallow and deep traps. The increase in the energy and density of deep surface traps enhance the ability to capture primary emitted electrons. In addition, the decrease in surface conductivity blocks electron emission at the cathode triple junction. Therefore, electron avalanche at the interface between gas and an insulating material would be suppressed, eventually improving surface flashover voltage of epoxy resin microcomposites. [ABSTRACT FROM AUTHOR]

In the present paper, a new three-layered 45o insulation structure is proposed and investigated to further improve the surface flashover performance of the 45o vacuuminsulator configuration. This structure was prepared by inserting organic composite between the electrode and insulator. With a numerical simulation method, the electric field distributions in this structure were analyzed. The result shows that by adjusting the permittivity of the inserted layer between the electrode and insulator, the electric field at the cathode and anode triple junction can be suppressed significantly in order to prevent the appearance of the surface flashover in vacuum. On the basis of these theoretical analyses, we conducted a series of vacuum flashover experiments about the new three-layered insulation structure of polyethylene composite-polyethylene- polyethylene composite. In our experimental result, it is indicated that when the permittivity of the inserted layer change in certain range, the surface flashover performance in vacuum of the 45o vacuum-insulator configuration can be improved, e.g. the highest first impulse flashover voltage reaches to 81 kV and increases by 32% compared with the polyethylene standard samples. [ABSTRACT FROM PUBLISHER]

A new insulation structure of Mo/Al2O3 cermet-Al2O32 ceramic-Mo/Al2O3 cermet was designed to improve the surface flashover performance in vacuum through co-firing, where the volume addition of metal Mo was managed from 2 vol% to 50 vol% and there could exist one or two layers with different Mo addition in Mo/Al2O3 cermet. The influences of Mo addition of Mo/Al2O3 cermet on the surface flashover voltage in vacuum of samples were experimentally investigated, indicating that the surface flashover voltage of three and five-layer structures were improved by 77 % and 117 %, respectively. Through electrostatic field simulation, it was found that the improvement is caused by the weakening of the electric field at vacuum-insulator-cathode triple junction. On the basis of an in-depth investigation, it was also concluded that Mo particles of Mo/Al2O3 cermet layer can decline the surface charging and secondary electron emission near the triple junction. Then with a further increase of Mo addition, the decreasing trend of flashover voltage after the growing has been reasonably explained by investigating the configuration of Mo particles in Mo/Al2O3 cermet. [ABSTRACT FROM AUTHOR]

Nanodielectrics, which are concentrated in polymer matrix incorporating nanofillers, have received considerable attention due to their potential benefits as dielectrics. In this paper, short-term breakdown and long-term failure properties of nanodielectrics have been reviewed. The characteristics of polymer matrix, types of nanoparticle and its content, and waveforms of the applied voltage are fully evaluated. In order to effectively comment on the published experimental data, a ratio k has been proposed to compare the electric properties of the nanodielectrics with the matrix and assess the effect for nanoparticles doping. There is evidence that the short-term breakdown properties of nanodielectrics show a strong dependence on the applied voltage waveforms. The polarity and the cohesive energy density (CED) of polymer matrix have a dramatic influence on the properties of nanodielectrics. Nanoparticle doped composites show a positive effect on the long-term failure properties, such as ageing resistance and partial discharge (PD) properties of nanocomposites are superior than microcomposites and the matrix. The larger the dielectric constant and CED of the matrix become, the more significant improvements in long-term performance appear. Based on the reported experimental results, we also present our understandings and propose some suggestions for further work. [ABSTRACT FROM PUBLISHER]

[Display omitted] • Obtain DC surface discharge performances after long-term UV irradiation. • Illustrate the influence of UV irradiation on surface discharge in dry/humid air. • Surface discharge in dry air is determined by degradation reactions caused by UV. • Surface discharge in humid air is governed by oxidization reactions caused by UV. • Clarify the UV-induced DC discharge mechanisms from microscopic to macroscopic. In the modern DC electrical transmission system, high-energy photons interact with silicone rubber (SIR) chains during long-term ultraviolet (UV) irradiation, it degrades the surface properties and triggers surface discharge (flashover) along the SIR-air interface, threatening the secure operation of the advanced electrical equipment. To further comprehend how UV-aging influences the flashover performance, in this work, the chemical bonds, surface charge transport parameters, and flashover performance of UV-aged SIR samples are characterized, and the influence mechanisms of UV-aging on surface discharge in dry and humid air are comprehensively clarified through the investigation from "molecular-mesoscopic-macroscopic" scales. The results indicate both degradation and oxidization reactions occur during UV-aging. The degraded molecular chains dominate the discharge in dry air, while oxidization reactions determine the discharge in humid environment: The degradation reactions reduce the surface trap depth and increase surface conductivity, leading to large surface leakage current and surface charge dissipation rate, causing flashover voltage in dry air increases and decreases as UV-aging time increasing. The oxidization of the chains introduces –OH and –COOH groups into chains, enhancing permittivity and adsorbing H 2 O molecules, significantly increasing surface leakage current, causing surface discharge voltage in humid air to decrease as UV-aging time increases. [ABSTRACT FROM AUTHOR]

[Display omitted] • Obtain flashover characteristics in vacuum during and after EBI (Electron beam irradiation). • Discover EBI crosslinks EP molecular chains and promote V f (Surface flashover voltage in vacuum) long-term after EBI. • Explain V f increases with deposited electrons near molecules short-term after EBI. • Illustrate the effects of radiation electron trajectories on reduced V f during EBI. • Clarify the comprehensive influence mechanisms of EBI on flashover performances. In surface science, surface flashovers are discharge phenomena that occur at the gas/vacuum-solid interface. Spacecraft in the universe suffers from high-energy electron irradiation while it works under a high DC operating voltage, and thus, surface flashover threatens the safe operation of spacecraft and limits human's deeper exploration of the universe. To unravel the influence mechanisms of electron beam irradiation (EBI), surface flashover voltages (V f) of epoxy composites during EBI, short-term after EBI, and long-term after EBI are investigated, and the behaviors of molecular structure, deposited electrons near chains, and radiation electrons under EBI and their effects on V f are separately clarified. For long-term after EBI, EBI induces crosslinking of molecular chains and introduces deep traps to the solid surface, which impedes electron emission and surface charging, and thus V f increases. For short-term after EBI, the deposited electrons suppress charge injection to the solid, and V f further increases. During EBI, the high-energy radiation electrons stimulate gas desorption and directly participate in plasma propagation, leading to a sharp decrease in V f. This work comprehensively explains the influence mechanisms of EBI electron modification, deposition, and migration on the V f , which offers a theory to guide the prevention of flashover for spacecraft. [ABSTRACT FROM AUTHOR]

[Display omitted] • An ultraviolet irradiation in vacuum technique enhancing flashover voltage of SIR. • Illustrate the influence of UVIV on the chain structure of SIR. • Establish a relation between crosslinking of SIR and surface trap parameters. • Explain the effects of surface traps on surface charging and flashover. • Clarify the UVIV improved flashover mechanisms from microscopic to macroscopic. In surface science, surface flashover, which poses a severe danger to the safe operation of modern silicone rubber (SIR)-based electrical equipment, is a continuous discharge event that happens when an intense electric field is produced along the gas-solid interface. To improve the flashover performance, the ultraviolet irradiation in vacuum (UVIV) technique is proposed to modify the SIR surface and the DC flashover threshold increases approximately 14.8 % after 6 h UVIV treatment. Then, the influence mechanisms of UVIV are investigated from microscopic molecule modification to macroscopic flashover performance: The UVIV generates high-energy photons to break C H and Si C bonds in SIR chains, and atoms, group, side chains, and free radicals in chains recombine through the thermally dynamic, causing SIR chains to crosslink. The crosslink chain structure enhances the deep trap energy around the Si O Si bond and introduces large amounts of deep traps. The increment in surface deep trap energy and density captures electrons. It establishes a high electrostatic near the cathode barrier that impedes subsequent electron injection, causing surface charge density decreases far away from the cathode. The reduced surface charges restrict the collision ionization in gas, finally increasing the DC surface flashover voltage through UVIV treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • The artificial damp simulation experiments were conducted in different acidic and alkaline environments. • The physicochemical and electrical properties of XLPE and SIR specimens are evaluated. • Strong alkali has the strongest corrosive effect on specimens, with the most severe degradation of electrical properties. • The Material Studio is used to simulated the effect for electrical properties of breakage molecular chain. The insulation performance of the cable accessories is critical to the safe operation of the power grid. The environment in which the distribution cables are laid affects the insulating properties of the cable accessories. In this paper, the artificial damp simulation experiments were conducted in different acidic and alkaline environments. The physicochemical and electrical properties of specimens were evaluated. The patterns and laws of volume resistivity, breakdown strength, and surface flashover behavior before and after etching were investigated. The results indicated that the resistivity of specimens declined gradually with the extent of etching time. After 28 days, the resistivity of XLPE etched in strong acid (pH = 1) and strong alkali (pH = 13) solutions dropped rapidly, and the SIR decreased by 34.2% (pH = 1) and 26.5% (pH = 13), respectively. The breakdown strength of the two specimens exhibited an overall trend of decline. The XLPE was reduced by 19.08% (strong acid), and 20.61% (strong alkali), respectively, and the SIR was reduced by 15.59% and 17.50% after 28 days compared to the untreated specimens. The flashover voltage of SIR is more severely reduced than that of XLPE. The patterns and principles of the influence of acidity and alkalinity and ion concentration on the electrical properties of the specimens were discussed based on the experimental results. The work has an important guideline for the moisture-proof material selection and damp fault analysis of the cable accessories. [ABSTRACT FROM AUTHOR]

[Display omitted] • Propose coating an assisted functional polythiourea on epoxy to significantly improve the flashover voltage in gases. • Establish the positive correlation between the flashover voltage and gas adsorption quantity. • Comprehensively clarify the effect of active function site characteristics on the gas adsorption quantity. • Reveal the underlying mechanism of the gas adsorption quantity on the flashover voltage. Surface flashover as a well-known discharge phenomenon on the gas–solid interface can lead to the failure of power equipment. Although intensive studies have been devoted to improving the flashover voltage in gases, the perspective on exploiting gas characteristics has so far been ignored. We here directly utilize a polythiourea-assisted coating on epoxy to achieve beneficial gas adsorption and enhance the flashover voltage. The results show that the flashover voltage of the polythiourea-assisted epoxy is significantly improved in both N 2 and SF 6 compared with the blank epoxy. Combining the experimental and computational results, it is proved that the striking improvement in the flashover performance is attributed to the increased gas adsorption quantity resulting from the unique thiourea units in polythiourea. Although both esters in epoxy and thioureas in polythiourea act as active function sites for guest gases, polar thioureas not only lead to pronounced charge displacements but also own the larger density, which further enhances the adsorption energy and promotes the gas adsorption. The resulting increased gas adsorption quantity prevents the collision ionization effectively, thus improving the flashover voltage. Our research innovatively proposes a functional polythiourea-assisted coating that can enhance the flashover voltage by improving the gas adsorption ability. [ABSTRACT FROM AUTHOR]

Surface flashover is a serious threat to the stable operation of advanced electrical applications. Ozone modification has recently been developed for improving the surface flashover performance. However, the effect of ozone on surface properties is still not fully elucidated. In this work, various concentrations of ozone are used to modify epoxy composites and their DC flashover voltages in SF 6 at 0.1 MPa are evaluated. The results show that the flashover voltage of sample modified by 130 mg/L ozone for 4 h is improved by 22.6%, from 31.29 kV to 38.35 kV. It is proved to be highly related to the surface chemistry and trap properties. The X-ray photoelectron spectroscopy (XPS) spectra shows that some C–H and C C bonds on the surface are oxidized to C–O and C O bonds. The results of simulation and surface trap demonstrate that the oxidized C–O and C O bonds change the frontier orbitals and density of states, thus leading to a lower deep trap density. The lower trap density alleviates the generation of electron avalanche and eventually improves the flashover voltage. This work reveals the mechanism of ozone modification improving surface flashover voltage and provides a promising route to enhance the surface flashover performance. ● Revealing the mechanism of ozone modification on improving surface flashover performance of epoxy composites; ● Exploring the correlation between the surface chemistry and deep trap characteristics by experimental and simulation methods; ● Clarifying the influence of surface deep trap density on surface flashover voltage. [ABSTRACT FROM AUTHOR]

Surface property plays a crucial role in the surface electrical breakdown or surface flashover. Although the main electronic processes have been proposed, it is still not clear how thick the surface layer is involved in the interaction with the transporting carriers. In the present study, the effective surface layer thickness (L f) of an isotactic polypropylene (iPP) is directly probed by precisely manipulating the iPP layer thickness (d iPP) deposited on glass substrates. It is found that the direct-current surface flashover voltage varies significantly with d iPP from 17.5 kV of the bare glass substrate to the saturated value of 21.0 kV when d iPP exceeds 200 nm. The flashover voltage shows an exponential dependence on d iPP , from which L f is estimated about 45 nm. Morphologies of iPP thin films are investigated by atomic force microscopy, which exhibits film thickness dependent growth behaviour from a discontinuous layer with scattered iPP agglomerates to continuous layer with seaweed-like structure enclosed by amorphous boundaries. It is concluded that the main electronic interaction between transporting carriers and iPP is confined within the 200 nm thick surface layer, signifying the importance of its morphological and electrical properties in the further understanding and improvement of the surface flashover. Unlabelled Image • Directly probing the effective surface layer thickness of a polymer dielectric • Determine the surface layer thickness and its morphological evolution • Clarify the influence of surface layer property on the surface flashover • Reveal the thickness dependence of the surface flashover voltage • Offer a new approach for studying the solid dielectric-gas interface [ABSTRACT FROM AUTHOR]