Long-term corona behaviour and performance enhancing mechanism of SiC/epoxy nanocomposite in SF6 gas environment

Jingrui WANG (王靖瑞), Qingmin LI (李庆民), Yanfeng GONG (公衍峰),Qixin HOU (侯启鑫), Heng LIU (刘衡), Jian WANG (王健) and Hanwen REN (任瀚文)

State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources,North China Electric Power University, Beijing 102206, People’s Republic of China

Abstract Surface coating technology is an effective way to solve the interface insulation problem of DC GIS/GIL basin insulators, but the performance of the coating will change greatly, and the insulation strength will be completely lost, after long-term use in the extreme conditions of corona erosion.In this research, the multi-needle-plate electrode platform was constructed to explore the long-term use performance of SiC-doped nanocomposite exposed to corona discharge in SF6 gas.Samples with a high SiC content have advantages in maintaining physical and chemical properties such as elemental composition, erosion depth, surface roughness and mass loss.The nanocomposite doped with 6 wt.%SiC has prominent surface insulation strength after long term exposure to corona, and the others are close to losing, or have completely lost,their insulating properties.Furthermore, the degradation mechanism of physicochemical properties of composite exposed to corona discharge was investigated with the proposed ReaxFF MD model of energetic particles from SF6 decomposition bombarding the epoxy surface.The reaction process of SF particles and F particles with the cross-linked epoxy resin, and the SiC nanoparticles providing shelter to the surrounding polymer and mitigating their suffering direct bombardment, have been established.The damage propagation depth, mass loss and surface roughness change of nanocomposite material bombarded by SF6 decomposition products is reproduced in this simulation.Finally, the deterioration mechanism of insulation properties for the SiC-doped composite was elucidated with DFT analysis.The band gap of the molecule containing S drops directly from the initial 7.785 eV to 1.875 eV,which causes the deterioration of surface electric properties.

Keywords:corona discharge in SF6,SiC doped nanocomposite,epoxy resin,ReaxFF MD,DFT

1.Introduction

High voltage direct current (HVDC) technology is an important support for renewable energy consumption and the stable operation of new power systems [1-4].The DC gas insulated transmission line (DC GIL) and DC gas insulated switchgear (DC GIS), as alternatives for traditional equipment, are the most important technologies in future power systems.The gas-solid insulation system still faces two severe challenges [5, 6].On the one hand, the charge accumulation on the gas-solid interface under DC voltage leads to serious electric field distortion.On the other hand,metal particles and micro-nano dust produced for multiple reasons will be driven by a DC electric field and adsorbed on the spacer surface.The interaction of these two factors will more easily cause corona discharge and partial discharge,and even insulation failure and severe ablation on the surface [7].

Recent research shows that surface coating technology is one of the most practical methods to alleviate these surface insulation problems[8].Various inorganic nanofillers,such as TiO2, BN, montmorillonite, etc, were applied to modify the coating composites to regulate surface charge [9].Compared with them, silicon carbide (SiC), which has a nonlinear conductivity property, could make the conductivity of epoxy resin composites adapt to the applied electric field [10, 11].Scholars have designed nano or micron SiC/epoxy coatings with nonlinear conductivity for the DC insulator, which can control the electric field distribution pattern along the surface,inhibit dust adsorption and improve the surface insulation strength[12-14].Therefore,the SiC-doped composite coating is regarded as the most promising method to enhance the surface insulation of DC basin insulators.

The basic electrical properties of the SiC coating surface at the early stage of service in SF6gas have been studied.However,there are few studies on the performance change of SiC composite coatings under harsh conditions for long periods.Due to the inevitable existence of corona discharge and partial discharge inside the equipment,SF6would decompose into low fluoride sulfide, fluorine atoms, and so on [7, 15].These particles generated by discharge are not only highly reactive but also in a high temperature state of more than 8000 K, and they would rapidly impact the coating surface with the equivalent energy of 0.65-1.03 eV [16, 17].Under such harsh conditions,a SiC/epoxy coating will be destroyed by chemically active gas by-products, and the microstructure and chemical composition will be greatly changed, and even some unpredictable insulation failures will occur.Although some studies have pointed out that anti-corona materials modified by nano-SiC are used in fusion reactor coils[18],the operating conditions are totally different from the SF6environment.The existing research on the influence of corona discharge on the treated surface,such as a directly fluorinated surface, mainly use corona discharge model experiments to directly characterize the influence of discharge on epoxy surface deterioration and the possible functions of the treatment [7, 19].However, it is still difficult to comprehensively understand the influence mechanism of SF6decomposition products on the surface dissociation of epoxy polymer, so a targeted performance improvement strategy cannot be formed.

Some representative simulation models have been developed to analyze the effects of particle bombardment on another material interface.Rahmani and van Duin [20, 21]studied the degradation mechanism of polymers, such as epoxy and polyimide,under the impact of atomic oxygen in a low earth orbit based on reactive force field molecular dynamic (ReaxFF MD) simulation.Bai et al [22] carried out MD simulation of the phase transformation of fused silica under nanoparticle impact in high power laser systems.Zhao et al [23] elucidated the microscopic mechanism of the interaction between OH radicals in plasma with biofilms using MD simulation methodology.Based on these studies,the model was further developed and applied to discharge fault simulation.Wang et al[24]transformed the dissociation of bisphenol F epoxy resin under partial discharge into the crack process induced by the impact of high-energy particles.Huang et al[25]have applied this method to study polyimide damage caused by DC corona in the air.The ReaxFF MD method provides an effective way to study the decomposition mechanism of SiC-doped composite under the high energetic particle’s bombardment.

The prominent corona resistance characteristics and longterm service stability are the key to supporting the application of SiC-doped nanocoating.In this research, SiC/epoxy nanocomposite was prepared.The platform for a polymer exposed to corona discharge under electrothermal coupling was established, and the surface damage characteristics of nano-SiC/epoxy composite under corona erosion in SF6gas were realized in the experimental study.Furthermore, the change rules of microstructure, elemental composition, surface roughness, mass loss, and surface insulation strength of SiC/epoxy composite exposed to corona for different times were analyzed.Finally, atomic-scale simulation analysis was carried out based on ReaxFF MD to reveal the mechanism of corona erosion-induced changes in the physicochemical properties of the composite surface, and DFT calculations were performed to reveal the microscopic mechanism of longterm corona-induced surface insulation degradation and insulation failure.

2.Sample preparation and key performance test

2.1.Preparation of SiC/epoxy nanocomposite

Diglycidyl ether of Bisphenol A (DGEBA) epoxy resin,methyl tetrahydrophthalic anhydride (MTHPA) and the accelerant, 2,4,6-dimethylaminomethylphenol (DMP-30)were used to prepare the polymer specimens.The nano-SiC with a purity of 99.9%and a particle size of 40 nm is used as the dopant.The preparation of pure epoxy resin polymer and SiC/EP nanocomposite is carried out according to the mature craftsmanship [26], the thickness of the obtained samples is about 1 mm.Nano-SiC doped composites (abbreviated SEC)containing fillers in five different proportions,2 wt.%,4 wt.%,6 wt.%, 8 wt.% and 10 wt.%, were prepared.The SiC/epoxy composite with a mass fraction of 2 wt.% is taken as an example, which is abbreviated as SEC2 in the following description.

2.2.Experimental platform for composite exposed to corona discharge

In order to further study the influence of long-term corona discharge on physicochemical properties and the insulation performance of the composite surface, the experimental platform for polymer exposed to corona discharge under electrothermal coupling was designed and developed in this research,and its main structure diagram is shown in figure 1.The platform is mainly composed of a high voltage power supply, bushing, a seal chamber, an electrode system, an oil heating system, and a gas supply and disposal system.The multi-needle-plate electrode system can simulate the tip discharge fault on the high voltage conductor near the insulator in GIS/GIL,and it can be ensured that each sample is facing three needle electrodes and the distance between the sample and the tip of the needle electrode is 30 mm.For each needle electrode, the radius of curvature of the needle tip is about 100 μm.When the applied voltage is +45 kV, the schematic diagram of electric field distribution between the electrodes is shown in the cloud diagram of figure 1.The electric field strength at the needle tip could be calculated using Mason’s equation [27] as equation (1) and the maximum value of its magnitude could reach 127 kV mm-1, which is much larger than the onset field of positive corona calculated by Nitta’s model [28].

Figure 1.Schematic diagram of experimental platform for polymer exposed to corona discharge under electrothermal coupling.

where U is applied voltage;d is distance from the plate to the needle tip; R is the curvature radius at the tip.

The corona discharge generated by the needle electrode directly sputtered onto the epoxy composite samples’ surface below the electrode.The maximum allowed pressure of the seal test system could reach 0.6 MPa.The temperature inside the chamber can be controlled between 50°C and 120°C by the external oil circulation heating system to simulate the working conditions of the gas insulated equipment, and test experiments were carried out at 90 °C.Due to the toxic components of decomposition products, the sealing property of the chamber should be guaranteed reliably,and the tail gas disposal system can remove harmful substances and recover SF6gas.The experiments were carried out in 0.2 MPa SF6gas for 10 h, 20 h and 30 h respectively.After that, the key physical and chemical properties of the samples were compared and analysed.

2.3.Surface insulation strength and surface potential decay test platform

Since the SiC/epoxy composite is a coating material applied to the DC insulator’s surface, the changes in surface insulation strength and surface potential decay after long-term exposure are the keys to evaluating the feasibility of its application.

The surface flashover test platform shown in figure 2(a)was applied to evaluate the critical surface insulation strength after corona erosion.The flashover test was conducted with finger electrodes in the seal chamber with the SF6gas pressure of 0.1 MPa.The radius of curvature of the hemisphere of the two finger electrodes is 8 mm, and the distance between the two electrodes is 5 mm.Figure 2(b) gives the charge injection and dissipation test system.The sample is placed on the movable platform, and the electrode system consists of a needle, a metal net and a grounded plate.During the experiment, the sample was charged under the corona with the DC voltage of +10 kV for 10 min, and then the power supply was turned off and the sample was moved below the probe to perform measurements of the surface potential change.In each experiment, a data acquisition card was used to record the change of the surface potential within 1800 s.The test experiments were conducted at an ambient temperature from 20°C to 25°C and an air humidity of about 20%.

Figure 2.Schematic diagram of experimental platform for key electric properties.(a)Surface flashover test platform,(b)charge injection and dissipation test system.

3.Variation of key physicochemical and electric properties after long-term service in corona discharge

3.1.Microstructure and elemental composition on eroded area

The surface morphology and element composition of the exposed area of the composite sample obtained by the scanning electron microscope(SEM)at their initial state and after being exposed to corona for 30 h are shown in figure 3.The SEM of the surface and cross section of pure epoxy resin are given in figures 3(a) and (b), and figures 3(f) and (g) show these states of SEC10, which are all smooth and clean.As shown in figure 3(c),after being exposed to corona for a long time, the pure epoxy surface was obviously damaged, and many defects like cracked bubbles appeared.However,taking SEC10 as an example, after the same exposure time, the sample surface shown in figure 3(h) is still relatively smooth with only local imprint.Some important characteristics related to erosion behavior can also be found from the cross sections of the sample.After 30 h exposure, obvious stratification phenomena appeared on the cross sections of both samples.Figures 3(d)and(i)show the specific characteristics,and the surface material thickness of the pure epoxy resin sample and SEC10 is about 24.8 μm and 7.7 μm, respectively.The energy dispersive x-ray spectroscopy (EDX) of these two samples’ surface was further analyzed.Comparing figures 3(e) and (j), it can be found that the characteristic peaks of S and F elements appear on both surfaces after 30 h exposure under corona.This indicates that there is physical and chemical reaction between the exposed surface molecules and the decomposed S- or F-containing groups under the erosion of corona discharge, which stay on the composite surface,penetrate to the deep area and promote the formation of the surface erosion layer.Compared with pure epoxy, the peak of S on the SEC10 surface is lower and the peak of F is similar.

Figure 3.SEM and EDX of epoxy composite initially and after being exposed to corona discharge for 30 h.

To further analyze the different effects of SiC content,the surface layer thickness was defined as the corona erosion depth.The depth of six erosion sites is selected and averaged for each sample, and the average values with different components exposed to corona for 30 h are obtained.The average corona erosion depth of all samples after 30 h exposure is shown in figure 4.The average corona erosion depth of pure epoxy is the largest,which is 23.1 μm.The depth of composite decreases gradually with the increase of SiC mass fraction,and the erosion depth of SEC10 is the smallest at 9.8 μm.The average values of SEC6,SEC8 and SEC10 are between 10 μm and 15 μm, and the values are relatively close.

From the analysis of SEM and EDX,it can be speculated that SiC dopants can effectively resist the behavior of SF6decomposition products eroding more seriously.

Figure 4.The average corona erosion depth of pure epoxy and composites with different components after 30 h exposure.

3.2.Surface roughness and mass loss

Corona erosion will cause changes in the microscopic topography and elemental composition of the sample surface at the microscale level, and it will be directly manifested as the change of surface roughness at the macroscale level.The surface roughness tester was used to measure the surface roughness of these six kinds of composite with different components, and the value was also the average of the six measurement results, as shown in figure 5(a).The doped nanoparticles had no significant effect on the surface roughness of the composites in the initial state.When prepared,the surfaces of these materials are smooth, with a roughness in the range of 0.03-0.05 μm.While the surface roughness of all kinds of samples increases gradually with the increase of exposure time under corona.After exposed to corona for 10 h,the surface roughness of the samples of each component is similar, all around 0.2 μm.It is worth noting that after exposure for 20 h or 30 h,the roughness of the samples doped with SiC nanoparticles is smaller than that of the pure epoxy samples, and the advantages of nano-doping are gradually reflected.Among the sample exposed to corona for 30 h, the roughness value of SEC10 is the smallest.

Figure 5.(a)Surface roughness and (b) mass change of epoxy resin composites exposed to corona discharge for different durations.

The precision electronic balance was used to accurately weigh the mass of these samples, and the obtained sample mass changes are shown in figure 5(b).All mass changes given are the difference between the mass gained after the fixed exposure time and the initial mass.Through comparison and analysis, it can be found that after 10 h exposure, the mass of the six kinds of samples all increased slightly with little difference, all around 0.001 g.After 20 h corona exposure, the mass changes of the composite samples with different components began to show significant differences.The mass of the pure epoxy sample was similar to that of the 10 h exposure one,remaining at around 0.001 g.The mass increase of SEC2 and SEC4 is about 0.0030 g, and the increments of the other samples with higher SiC doping content reach 0.005-0.006 g, which is about twice that of the former.After 30 h corona exposure,the mass of the pure epoxy sample and SEC2 sample decreased compared with that of 30 h exposure.The former was even 0.005 g less than the initial state,and the latter was 0.0005 g less than the 20 h sample.The mass gain of SEC4 at 30 h was the same as that at 20 h exposure,remaining at 0.0020 g.The mass of other SiC-containing samples still maintains an increasing trend after exposure for 30 h.It is not difficult to see that with the increase in exposure time, only the mass of pure EP and SEC2 increased first and then decreased, while the quality of other samples kept increasing.

From the analysis of the above mass changes,we observe that when the doping content of the SiC nanoparticles is low,SF6decomposition products with high reactivity are further converted into gaseous derivatives after reacting with epoxy resin molecules.When the SiC content was higher than 4 wt.%,the nanoparticles effectively suppressed the intensity of the chemical reaction on the surface and reduced the mass loss caused by the decomposition of the epoxy resin into gaseous products.With reference to the aforementioned EDX,it can be inferred that the heavy groups containing an S or F atom formed by SF6decomposition stay on the surface of the corona erosion zone, resulting in mass increase.

3.3.Surface insulation strength and surface potential decay

The DC surface flashover voltage of the composite sample obtained through experiments is shown in figure 6(a),and the surface flashover phenomenon of SEC6 after being exposed for 30 h and photographed by a high-speed camera is shown in figure 6(b).In the initial state, the DC surface flashover voltage of the composite showed a trend of first increasing and then decreasing with increasing SiC doping content.When the doping content is 6 wt.%,the DC flashover voltage is the highest, which is about 16.9% higher than that of the pure sample.The flashover voltage after corona erosion is decreased to different degrees, while the change trend of the flashover voltage of the samples with the doping ratio of SiC is consistent with that of the pure one, which is not affected by exposure time.After being exposed to the corona for 10 h,the flashover voltage of pure epoxy resin decreased by 11.9%compared with the initial state.The flashover voltage decrease percentage of SEC2 and SEC4 is similar to that of pure epoxy,and the drop of SEC6 is slightly smaller, about 9.6%.The flashover voltage of SEC8 and SEC10 has the smallest drop, which is close numerically to the original state.When exposed for 20 h,the surface flashover voltage of pure epoxy resin drops significantly, to about 75.8% lower than that of the original one.At the same time,the flashover voltage of the SiC-doped polymer drops by 50%to 60%,while the flashover voltage of the composite is still higher than that of the pure sample.In particular, the flashover voltage is 6.3 kV and 7.9 kV higher than those of SEC6 and SEC8, respectively.When the corona erosion time reaches 30 h, the insulating ability of pure epoxy resin and SEC2 has completely failed.However,the other composites still retain a certain insulation strength, and the flashover voltage of SEC6 is the highest among them.It can be determined that long-term exposure to corona discharge will lead to great deterioration of the composite surface insulation performance.

Figure 6.DC flashover of epoxy resin after being exposed to corona.(a) Flashover voltage variation.(b) Surface flashover phenomenon.

The pure epoxy sample and SEC6 sample are selected in the initial state and after 30 h exposure for surface potential decay analysis, and the surface potential normalized by the initial value is given in figure 7.For the samples at initial state,the potential decay on the SEC6 surface is much quicker than that for the pure epoxy,since the SiC dopants introduce a large number of shallow traps and reduce the density and energy level of the deep traps.Due to the 30 h exposure, the pure epoxy completely lost its surface insulation and could not obtain the potential decay behavior on its surface.After corona erosion, the potential decay rate of SEC6 is greatly increased.The surface electric conductivities of these four kinds of samples obtained by the three-electrode test system are given in table 1.It can be seen that long-term corona erosion will greatly decrease the electrical conductivity of the composite surface.The conductivity of pure epoxy resin increased from the initial value, 6.56 × 10-15S m-1, to 5.39 × 10-9S m-1after exposure, and this abnormal increase is the main reason for the surface insulation failure.The increase in the conductivity of SEC6 directly leads to the rise in the potential decay rate.

Figure 7.Normalized surface potential decay.

4.Degradation and performance improvement mechanism of SiC doped nanocomposite

ReaxFF MD and DFT are important theoretical methods for the analysis of the microstructure evolution of the composite.The former can show the dynamic behavior of the physical and chemical reaction process and explain the performance change after being exposed to long-term corona, while the latter can provide theoretical explanation support for the electrical properties change.

Since the solid insulation in the gas-insulated apparatus is immersed in SF6gas, the S/F/O reactive force field has always been crucial to studying the dynamic characteristics of its long-term corona behaviour using the ReaxFF MD method.Although S, F and O elements have appeared in the existing force fields, the connection between these three is empty and these force fields cannot be used to study the molecular dynamics behavior of epoxy composite/SF6systems.Based on the C/H/O/S/Si/F force field suitable for the SF6-EP-SiC system which could be obtained from our previous study through the proposed synergetic optimization methodology [29, 30], the model of the highly energetic particles from SF6decomposition bombarding the epoxy surface was constructed to simulate the corona erosion process in SF6gas, and it is given in figure 8(a).In this model,DGEBA and MTHPA are applied as the cross-linker,and the polymerization degree n of the epoxy resin monomer is set to 0, as shown in figures 8(b) and (c).The SiC nanoparticle model shown in figure 8(d) with diameter of 1.5 nm was constructed and optimized.The chemical molecular structures of the material involved are shown in figure 8(e).Besides the pure epoxy model, three SiC/epoxy composite models were also constructed,and their SiC contents were 3 wt.%,6 wt.%,and 9 wt.%,respectively.The pure epoxy model is denoted as Model P,and the other three models are denoted as Model S1,Model S2 and Model S3 in the above order.The crosslinked model was placed at the bottom of the simulation box with a z-axis length of 160 Å, and the z-direction is aperiodic.The most representative particles, SF and F, among the SF6decomposition products [15] are generated at a random position at a fixed height from the composite surface and shot vertically towards the polymer along the z-axis.A particle will inject every 2000 simulation steps from the beginning to the end of the simulation,and simulation steps are carried out with a micro-canonical ensemble (NVE).These energetic particles are all assigned energy similar to the actual situation,0.75 eV [8].An elastic wall was added at the bottom of the box to avoid ‘drift’ of the entire model.All ReaxFF MD calculations were performed with the software AMS.

Figure 8.Molecular structure of (a) bombardment model, (b) DGEBA, (c) MTHPA, (d) SiC nanoparticle with diameter of 1.5 nm and(e) various types of atoms involved in the study.

The meta-GGA calculation method with the 2-zeta basis group, M06-2X/6-31 G**level [31, 32], is employed to investigate and optimize the geometries of the simplified crosslinked epoxy resin and typical reaction products containing an S atom or F atom.The keyword,int=ultrafine,is added to improve the calculation accuracy.The crosslinked polymer molecule contains one DGEBA molecule and one MTHPA molecule, from which all the other reactants are derived.The DFT analysis is conducted with ORCA 4.2 software package [33] and the data processing is performed with Multiwfn [34] and VMD [35].

4.1.Mechanism of change in physicochemical properties

Taking Model P and Model S3 as examples, their ReaxFF MD simulation results were analyzed, and the states of the epoxy composite system after being impacted by different particles were intercepted, as shown in figure 9.It can be found that under the 50 ps continuous bombardment of F or SF,the surface molecules of pure epoxy become loose,while the molecular structure of the surface layer is still relatively tight after doping with SiC nanoparticles.Compared with the pure epoxy resin system,the gas side small molecule products from the SiC doped epoxy composite system are obviously less, indicating that the damage degree of the solid side molecular chain is smaller.At 100 ps, the decomposition degree of the solid side in the reaction system increased significantly compared with that at 50 ps,and more molecular fragments drifted into the gas side.However,the model doped with SiC is better than pure epoxy resin in both the number of small molecules and the degree of looseness.After the bombardment of the same time,the structural changes caused by SF particles are much more obvious than those caused by F particles.There are not only looser structures but also more small molecule products.

It cannot be ignored that the chemical reaction process of incident particles leading to surface polymer chain erosion is basically the same regardless of whether or not SiC nanoparticles are doped.The SF particles are easily combined with the O atoms on the hydroxyl and ester groups in the epoxy resin molecule, thereby destroying the stability of the epoxy resin molecule.One of the most probable reaction pathways for S atoms to stay on the long chain of epoxy resins is shown in figure 10(a),and HF is also generated at the same time.The F particles will take away the H atoms on the C-H structure to generate HF, and at the same time, other F atoms will complement and form C-F bonds.The typical process of F atoms staying on the polymer surface is shown in figure 10(b).In addition, other intermolecular forces between nanoparticles and these highly energetic particles also provide conditions for the residence.It is through the physical and chemical reactions between the SF6decomposition products and the composite surface substances that the surface features are changed,as shown in figure 3.Not only will the density of the surface material change significantly, resulting in a delamination phenomenon,but also S and F elements will appear on the exposed sample surface.These representative physicochemical reaction processes will occur regardless of whether SiC is doped or not.

The improvement mechanism of SiC nanoparticle doping on the corona resistance of epoxy resin is mainly manifested in the following aspects.First, SiC nanoparticles have strong physical and chemical stability, so the nanoparticles exposed to the surface of composites can effectively block the direct bombardment of epoxy resin molecules by incident highenergy particles without reacting with them.Then, nanoparticles can physically or chemically adsorb incident highenergy particles, thereby reducing the chance of reactive particles reacting with the crosslinked epoxy resin system.In addition,through covalent bonds or intermolecular forces,the nanoparticles are closely combined with the surrounding epoxy resin molecules, and the nanoparticles use their own stable structure to enhance the stability of the surrounding epoxy resin molecules,making the epoxy resin molecules less prone to random movement into the unprotected area,thereby reducing the possibility of epoxy resin molecules being attacked by high-energy particles generated by corona discharge.Figure 11 shows the atomic temperature distribution in model P and model S3 suffering the bombardment process at 80 ps.It can be seen that under the continuous bombardment of high-energy particles, the local atomic temperature can reach about 2000 K.A gradient distribution of temperature occurs along the z-axis across the solid side.When the composite is doped with nanoparticles, it will block and adsorb the incident high-energy particles.During this process,the kinetic energy of the incident particles is directly transferred to the nanoparticles themselves, so the temperature of the nanoparticles is higher than that of the surrounding molecule, as shown in figure 11(b).Considering that the thermal conductivity of SiC nanoparticles is tens of times higher than that of epoxy resin matrix,the energy obtained by nanoparticles from blocking and adsorbing incident particles is easier to disperse rapidly inside the particles and to the molecules around the nanoparticles.Localized overheating is thus avoided, thereby mitigating thermal decomposition of solid materials due to temperature buildup.Therefore, the corona erosion suffered by the composite is greatly alleviated.

Table 1.Surface electric conductivity of pure epoxy and SiC/epoxy composite under the electric field of 1 kV mm-1.

To fully grasp the influence of nanoparticles on the corona resistance of crosslinked epoxy, it is necessary to carry out a more in-depth quantitative analysis.These molecules with fewer than six carbon or sulfur atoms formed a compound and gas molecules are defined as the mass loss part, and the curve of the residual mass over time is depicted in figure 12.

Under the bombardment of F particles, the normalized residual mass of model P and model S3 decreases with a slow trend, and the mass of them was about 0.975 at the end.The residual mass of these two models is slightly higher than the original value in the time range of 2-15 ps and 4-38 ps,respectively.The maximum value of the former is only 1.003,while the maximum value of the latter is only close to 1.005.This is closely related to the chemical reaction process in the corresponding time.At the beginning,the F particles are close to the surface,and the main process causing the mass increase is physical adsorption.As the simulation continues, the energy of the system gradually increases, and the reaction between the composite and F particles begins to accelerate,resulting in a decrease in the subsequent residual mass.It is worth noting that the final mass of model P and model S3 is very close, so it can be inferred that the erosion damage caused by F particles is relatively light, and the SiC nanoparticles in Model S3 have not even played a protective role.

The residual mass of model P and model S3 bombarded by SF particles showed a significant trend of first increasing and then decreasing, and the final mass difference between them was obvious.The residual mass of Model P has a maximum value of 1.033, which appears around 51 ps; the maximum value of Model S3 is around 1.024 at 65 ps.Subsequently,the residual mass of both systems turns down.The decline rate of mass for the model P was significantly faster than that for the model S3.

Figure 9.Representative snapshots of(a)model P and(b)model S3 under the impact with F particles, and snapshots of (c) model P and(d) model S3 under the impact with SF particles.From left to right,the simulation times are 50 ps and 100 ps in sequence.

Figure 10.Typical reaction process of (a) SF particle and (b) F particle with cross-linked epoxy resin.

Figure 11.Molecular structure of (a) model P and (b) model S3 under the impact of SF particles at 80 ps.Red balls represent atomic temperature above 2000 K and blue balls represent low temperature below 353.15 K.

At the end of the simulation, the mass of model P is 0.912,and that of model S3 remains at only 0.987,which was even less than that of model S3 under the F particle’s bombardment.This is attributed to the relatively large relative molecular mass of S atoms in SF, and the initial reaction of SF particles on the composite surface is not very violent.Through physical adsorption, chemical adsorption and other processes, it becomes part of the composite, causing the overall quality of the two models to exceed the initial state.On the other hand,the SF particles have strong reactivity and undergo complex and violent chemical reactions with the crosslinked epoxy chain, resulting in a rapid decrease in the mass of model P.The presence of SiC nanoparticles plays an important protective role, so the subsequent mass loss of model S3 is lower.

For experiments and tests, SF6is decomposed under corona discharge, and the epoxy resin sample is bombarded by both of F particles and SF particles, so F and S elements will be found in the EDX of figure 3.Since a large number of SF particles are bound to the composite surface by physical adsorption, they may then be separated from the SiC doped composites surface by other processes.Therefore,the S and F elements on the surface of SEC10 are less than those of pure epoxy resin.It is the effect of SF particles that leads to the situation in figure 6 that the mass of pure epoxy resin first increases and then decreases, and the mass of SiC doped samples increases with the increase of SiC content.

To better clarify the damage degree of epoxy resin systems exposed to highly reactive particles,the average damage propagation depth (DPD) was calculated in this work, which was determined on the basis of a comparison between the normalized mass density profiles of the initial and final states of the systems, along the z-axis, parallel to the particle bombardment direction.The reference value for DPD is set to 0.90 in this research.The F-induced DPD change shown in figure 13 is an average calculated from the last 1000 steps of the simulation.DPD induced by other particles is shown in table 2.For the same composite model,the DPD value caused by SF particles is larger than that caused by F particles,which is inseparable from the higher chemical reactivity of SF particles and S atoms.Under the bombardment of the two kinds of particles, the DPD values of the model all showed a decreasing trend with the increase of the proportion of SiC.The minimum DPD values all appear in the model with SiC mass fraction of 9wt.%, which are 11.4 Å under the bombardment of S particles and 13.8 Å under the bombardment of SF particles.It is the penetration of these particles into the composite that causes the density change, which in turn creates the delamination phenomenon in figure 3.When the SiC content is increased, the DPD of energetic particles can be reduced and the delamination phenomenon can be alleviated.Then, model P and model S3 were processed to remove all gas products and only the molecular chains connected to the matrix were retained.The surface states of Model P and Model S3 were drawn as shown in figure 14.It can be seen that SiC nanoparticle effectively reduces the surface roughness of the composite after high energetic particle erosion and bombardment.The bombardment model constructed in this MD study effectively reflects the role of SiC nanoparticle in resisting corona erosion by comparing with the experimental measurement data given in figure 5(a).

The simulation system data after 100 ps of bombardment by SF particles is processed, and all small molecule products containing C number less than 5 are removed.A python script is used to draw the surface microscopic morphology according to the atomic position distribution of solid substance,as shown in figure 14.The surface roughness values of model P,model S1,model S2 and model S3 were obtained by calculation, which are 5.43 Å, 4.55 Å, 3.75 Å and 3.42 Å,respectively.As the content of SiC nanoparticles increases,the surface roughness after bombardment decreases gradually,which is the same trend as the experimental values after 30 h exposure in figure 5(a).

Figure 12.Normalized residual mass of nanocomposite model.

Figure 13.Normalized density along the z-axis under impact with F.

Table 2.DPD values of different composite models after 100 ps.

4.2.Deterioration mechanism of electric properties

It can be seen from the measured value of surface roughness in figure 5(a) that the surface roughness of the composite increases with increasing corona erosion time.The current research on the influence of roughness on the insulation performance of epoxy resin composites shows that, in the range of micron scale, a proper increase in roughness could improve the DC flashover voltage [36-38].The value of composite surface roughness after corona erosion should meet the applicable range of this law.However, the flashover voltage along the surface decreases rapidly with the increase of corona treatment time in figure 6.This indicates that the roughness is not the key to influence the composite along surface insulation performance after corona erosion.On the other hand,combining the experimental tests in figure 3 with the reaction mechanism analysis in figure 10, it can be speculated that the change of the elemental composition in the corona-exposed and eroded region is an important reason for the surface insulation failure.

Since the HOMO-LUMO gap of the oligomer is closely related to the band gap of an infinitely extended polymer,the band gap can be extrapolated from the HOMO-LUMO gap calculated by a quantum chemical program.The molecule models,DGEBA/MTHPA,F-DGEBA/F-MTHPA,S-DGEBA/S-MTHPA, shown in figure 15, are respectively constructed based on the typical reaction products of F particle and SF particle with crosslinked epoxy resin molecules in figure 10.

The orbital cloud map and energy level orbital submap of the above three single-chain molecules are obtained by calculation, as shown in figure 16.The cloud images on the top and bottom are LUMO and HOMO, respectively.

The LUMO value of F-DGEBA/F-MTHPA is not affected by a small amount of F element substitution, and its value is almost the same as that of DGEBA/MTHPA.There is a slight difference in the HOMO values of the two,and the value of F-DGEBA/F-MTHPA is slightly smaller than that of DGEBA/MTHPA.The energy gaps of DGEBA/MTHPA and F-DGEBA/F-MTHPA are 7.785 eV and 7.844 eV,respectively,and the latter is 0.059 eV larger than the former.The increase of the band gap indicates the effects of F element substitution, elevating the energy required by the electrons and raising the barrier of electron migration between different lattices.Meanwhile,for DGEBA/MTHPA,there are obvious electron and hole traps.The F element makes the electron trap of F-DGEBA/F-MTHPA deeper and the hole trap slightly shallower.It is shown that the F element can change the effect of the potential decay process of the polymer and improve the insulation performance to a certain extent.

For S-DGEBA/S-MTHPA, the S element significantly reduces the LUMO and slightly increases the HOMO, which makes the gap drop to only 1.875 eV, which is no longer an insulating medium.It can be concluded that after the excess SF particle reacts with the crosslinked polymer,the S element resides on the composite surface exposed to the corona,and it is the key factor causing the surface insulation failure and significant increase in conductivity.The longer the exposure time to corona, the more S element is accumulated on the composite surface, which will degrade the surface insulation more seriously.In addition, for the initial state and the samples exposed to corona for different amounts of time, the flashover voltage along the surface has a maximum value when the mass fraction of SiC is 6 wt.%, which is consistent with the nonlinear conductivity characteristics of SiC and its regulation of deep and shallow traps in composites.The SiC nanoparticles maintain the electrical performance along the surface by reducing the accumulation of S elements on the surface exposed to long-term corona.

Figure 14.Surface topography after 100 ps bombardment by SF particles.

Figure 15.Optimized representative molecular models:(a)DGEBA/MTHPA, (b) F-DGEBA/F-MTHPA, (c) S-DGEBA/S-MTHPA.

5.Conclusions

In this study, through experimental research, the long-term service performance of SiC-modified nanocomposite materials under the extreme condition of corona erosion has been established.Moreover, ReaxFF MD and DFT were used to reveal the deterioration mechanism of the composite material by corona erosion and the mechanism of corona erosion resistance of nanocomposite enhanced by SiC nanoparticles from the atomic scale.The detailed conclusions are as follows:

(1) The variation law of key physicochemical properties and insulation strength after long-term service in corona discharge has been established.A multi-needle-plate electrode platform was used to carry out long-term corona erosion experiments on the SiC-doped nanocomposite.With increasing exposure time, various key properties and indicators were deteriorated, and S and F elements were found on the exposed surface.The erosion depth and surface roughness of the sample doped with 10 wt.% SiC remained optimal.Regarding insulating properties, the DC flashover voltages of composites doped with 6 wt.% and 8 wt.% SiC are 6.3 kV and 7.9 kV after 30 h of erosion,respectively, while the others are close to losing, or have completely lost, their insulating properties.

Figure 16.Energy level distribution and molecular orbitals of epoxy resin chain after being exposed to bombardment.

(2) The degradation mechanism of the physicochemical properties of composite exposed to corona discharge was elucidated using ReaxFF MD simulation methodology.Based on the developed reactive force field containing C, H, O, S, Si, and F elements, a model suitable for simulating the corona discharge erosion of solid insulation by high-energy particles bombarding the surface is constructed.Typical chemical reactions of F particles and SF particles with cross-linked epoxy resins leading to the decomposition of composite were found, which explained the change in the elemental composition obtained by EDX.The corona resistance of the nanocomposite was improved by SiC enhancing the interaction with the adjacent epoxy resin long chain and exposing itself to the direct bombardment of highenergy particles.The data on the dynamic damage characteristics of the interface is analyzed by the python script developed.The normalized residual mass and damage propagation depth (DPD) are defined to evaluate the interface features, and a 3D surface topography of the solid part is constructed to compare with the macroscopic experimental results.It can be confirmed that SF particles are more reactive and more destructive, and this is the primary reason for the changes in various physicochemical properties of the nanocomposite surface.

(3) The deterioration mechanism of the insulation properties for the SiC-doped nanocomposite was elucidated with DFT from the atomic-scale perspective.Energy level orbital analysis was performed on typical molecular structures obtained from ReaxFF MD analysis.The band gap of molecules containing F was raised and the insulating properties were enhanced.For the molecule containing S,the band gap drops directly from the initial 7.785 eV to 1.875 eV.Thus,the continuous accumulation of S-containing derivatives on the composite surface after corona erosion leads to the decline and failure of the surface insulation.

(4) Comparing the simulation results to experimental results shows that the corona erosion model based on microscale simulation effectively reflects some of the characteristics in the experiment.The experimental and theoretical analysis methods proposed in this study have important industrial applications and reference value in designing, developing and evaluating the performance of novel corona resistant insulating materials in gas compressed equipment.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Nos.51737005, 51929701, 52177147 and 52127812).