Surface Potential Decay of Negative Corona Charged ... - IEEE Xplore

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Surface Potential Decay of Negative Corona Charged Epoxy/Al2O3 Nanocomposites Degraded by 7.5-MeV Electron Beam Yu Gao , Member, IEEE, Jilong Wang, Fang Liu , and Boxue Du, Senior Member, IEEE

Abstract— Surface potential decay on negative corona charged epoxy/Al2 O3 nanocomposites irradiated with high-energy electron beam has been investigated in this paper. 2-mm-thick laminate nanocomposite samples were irradiated with electron beam at an average energy of 7.5 MeV, and the total accumulated dose was up to 500 kGy. After the irradiation, the nanocomposites were corona charged with dc voltage at −10 kV through a pair of needle to plate electrode. Surface potential of the sample was then measured by means of an electrostatic voltmeter through which trap distribution and carrier mobility were calculated. Differential scanning calorimetry curve for the each sample was measured to understand the change in chemical and physical structures induced by the irradiation. Results obtained indicated that the presence of nanofiller in the nanocomposites played a role in limiting the charge transportation both before and after the irradiation, and the charge transportation behavior was remarkably dependent upon the total irradiation dose. It is found that when irradiated with a certain total dose of electron beam, electron migration within epoxy and its nanocomposites is restricted, which should be attributed to the high-energy irradiation-induced structural change of the material. Index Terms— Carrier mobility, corona charging, epoxy/Al2 O3 nanocomposites, high-energy electron beam, surface potential decay (SPD), trap distribution.

I. I NTRODUCTION

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ITH the rapid development of space technology, a large number of electrical and electronic devices installed in spacecraft or satellites have to face high-energy radiation environment, where radiation rays, such as gamma ray, neutrons, and electrons, are included [1], [2]. Polymer insulating materials have been widely utilized in the devices, and hence, the materials are likely to be exposed to a variety of radiation hazards, by which the chemical and physical structures of the material could be changed, leading to the variation in its electrical properties [3]–[6]. This may shorten the lifetime of the insulation and thereby introduce electrical failure to the Manuscript received January 23, 2017; revised November 28, 2017; accepted January 17, 2018. This work was supported in part by the National Natural Science Foundation of China under Grant 51677127 and Grant 51277131, and in part the National Basic Research Program of China under Grant 2014CB239501 and Grant 2014CB239506. The review of this paper was arranged by Senior Editor D. Rodgers.(Corresponding author: Yu Gao.) The authors are with the Key Laboratory of Smart Grid of Ministry of Education, Tianjin University, Tianjin 300072, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2018.2798931

devices [4], [7]–[9]. Accordingly, from the viewpoint of safety, it is of great importance to investigate the radiation effect on the electrical performance of polymer insulating materials. The polymer nanocomposites are a kind of polymer system, which is added with nanosized inorganic fillers [10], [11]. Since the year 1994 when Lewis put forwards the concept of nanometric dielectrics [12], the polymer nanocomposites have attracted a lot of attention in the field of electrical engineering because of their remarkable electrical and mechanical properties. Previous studies have revealed that by adding proper nanosized particles, excellent electrical property of the nanocomposites, such as low dielectric loss [13], space charge suppression [14], and enhanced resistance to electrical treeing or partial discharge [15], [16], could be achieved. Nelson and Fothergill [17] compared the dielectric properties between microfilled epoxy and nanofilled epoxy with 10-wt% TiO2 as the fillers. It was found that the loss tangent of the nanocomposites was lower than that of the microcomposites at high frequency. Maity et al. [18] investigated the resistance to corona erosion of epoxy nanocomposites by using Al2 O3 and TiO2 as the fillers, and it was reported that the epoxy nanocomposites presented better resistance to corona erosion as compared with the neat epoxy [18]. Ding and Varlow [19] found that the addition of nanosized ZnO particles into epoxy resin improved the resistance to electrical tree growth and increased the time to breakdown. As mentioned earlier, owing to their excellent electrical properties of the nanocomposites, the potential applications, such as insulating material in space environment, have been proposed [20], [21]. However, studies on the electrical performance of the nanocomposites under radiation environment are quite limited. Electron beam irradiation is one of the most common radiation types presented in space environment. It has been reported that the energy of the electron beam irradiation can vary with a very large extent from some keV to tens to hundreds of MeV [22], and a spacecraft has to face the challenge of high-energy electron beam in the range of MeV, which possibly leads to deep dielectric charging in polymer insulating materials and the safety of electrical and electronic instruments installed in the spacecraft is threatened [23]. A number of investigations have been carried out to reveal the charging behavior of the insulating materials in response to the electron beam. Li et al. [24] have investigated the deep dielectric charging in ring structure irradiated by energetic electrons in the range from 0.1 to 5 MeV. It was found that the maximum

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electric field in the three typical insulating materials followed the sequence polytetrafluoroethylene (PTFE) > epoxy resin (FR4) > polyimide (PI) [24]. Yu et al. [25] have used the GEANT4 toolkit and the radiation-induced conductivity model to calculate the maximum possible internal charging potential and electric field in FR4 and PTFE by considering the electron energy in the range from 0.1 to 100 MeV, simulating the situation in geostationary Earth orbit and Jupiter orbits. It was reported that the electric field was dependent upon the shielding layer thickness, dielectric thickness, and ground type [25]. Kang et al. [26] have investigated the effect of electron beam irradiation of 1 MeV on the dynamic mechanical properties of PI films through the dynamic mechanical analysis. It was found that the irradiation led to slight decrease in the β2 subglass relaxations temperature and the decomposition temperature [26]. However, the studies concerning the potential use of polymer-based nanocomposites are quite limited. In particular, the effect of MeV electron beam on the structure changes, and hence, the charge transportation behavior in the nanocomposites is not fully clarified. This paper is carried out mainly in this respect. Epoxy and its nanocomposites have been used as insulating materials in a large variety of applications in the printed circuit board [27], [28], coating [29], radar absorbing materials [30], [31], and solar cell panel, hatches, and antenna of the space shuttle, satellites, and spacecraft [32], which are likely to be exposed to the high-energy irradiation of electron beam. In this paper, effect of 7.5-MeV electron beam irradiation on the charge transportation behavior of epoxybased nanocomposites filled with Al2 O3 has been estimated. Negative dc corona was used to charge the test sample so as to simulate the deposition of lower energy of electrons on polymeric dielectrics. The charge transportation manners are examined by surface potential decay (SPD) measurement, through which the trap distribution characteristics and the charge mobility can be derived. The test results indicate that the charge transportation behavior was dependent upon the total irradiation dose of the electron beam, which was varied as a function of the content of the nanofiller. II. E XPERIMENTAL S ETUP A. Sample Preparation and Electrode Arrangement Neat epoxy, which was commercially available (highly active with low viscous liquid bisphenol A, Yanhai-Resin, Araldite HY-511), was used as the base material. By dispersing nano-Al2O3 (Nanjing Haitai, China) fillers with an average particle size of 20 nm in the neat epoxy at 1, 4, and 7 wt%, the epoxy nanocomposite samples were prepared. The specimen preparation procedures could be systemized as follows. Initially, the Al2 O3 nanoparticles dried in a chamber at 120 °C for 24 h were mixed with epoxy resin by applying magnetic force stir and ultrasonic vibrator to achieve the uniform dispersion of the particles within the polymer matrix. Second, the hardener (LMW polyamide 651# , Tianjin Yanhai Chemical, China) was put into the mentioned mixture and then stirred in a thermostat water bath at 50 °C for 10 min. The mixture was predegassed at 50 °C for 10 min and was cast in a mold. Then, the mixture was put in a vacuum chamber to

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Fig. 1. FTIR spectrum for epoxy nanocomposites with 1-wt% nano-Al2 O3 fillers at various total doses.

be degassed for 1.5 h. The specimen was kept in the vacuum for 24 h and was solidified in air at room temperature. The thickness of the samples was 2 mm, and the dimension was 100 mm × 100 mm. Prior to the SPD test, the samples were irradiated by a high-energy electron beam of 7.5 MeV with a beam density ranging from 0.35 to 0.45 mA/cm2 . The average irradiation dose rate was ∼8.82 kGy/ min, and the total irradiation dose was controlled at 0, 100, and 500 kGy, respectively. As reported by Kang et al. [26], spacecraft and satellites can be exposed to a very large value of irradiation dose up to 1.5–2 × 104 kGy. Actually, for most of the commonly used polymer dielectrics, accumulation of irradiation dose in the range of 102 –103 kGy is rather severe that the use of the material should be limited [33]. In this paper, the total irradiation dose was selected to simulate the situation that the material was not severely degraded that it still had acceptable insulation property. Since the irradiation dose of 500 kGy was achieved by a short period of time, oxygen did not have sufficient time to diffuse into the material bulk, and thus, no remarkable oxidation occurred in the sample, as is shown by Fourier transform infrared spectroscopy (FTIR) spectrum (Bruker Tensor 27, a resolution of 4 cm−1 and a scanning number of 16) in Fig. 1, where the nanocomposites with a nanofiller of 1 wt% are taken as an example. Thus, it could be deduced that the irradiation treatment performed in this paper was comparable to that in vacuum condition. The penetration of electrons within epoxy sample could be roughly estimated by the following equation [34]: L P = (0.543E bm − 0.133)/de

(1)

whereL p is the average penetration depth in cm, E bm is the energy of electron beam in MeV, and de is the density of polymer in g/cm3 . Such an equation holds with 0.8 MeV < E bm < 3 MeV. Assuming E bm = 3 MeV and de = 1.2 g/cm3 for epoxy sample, L p would be 1.2 cm. Accordingly, it is proposed that 7.5-MeV electron beam has the ability to thoroughly penetrate the 0.2-cm-thick epoxy sample. Fig. 2 shows a typical dispersion of the nanofiller within the epoxy, and it can be confirmed that no remarkable agglomeration happened, and thus, the dispersion of the filler is acceptable in this paper. With regard to the electrode configuration, a pair of needle to plate electrode was used for the corona charging with dc

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. GAO et al.: SPD OF NEGATIVE CORONA CHARGED EPOXY/AL2 O3 NANOCOMPOSITES DEGRADED BY 7.5-MeV ELECTRON BEAM

Fig. 2. SEM image of nanocomposites with 4-wt% nano-Al2 O3 fillers at a total dose of 500 kGy.

Fig. 3.

Schematic of the electrode arrangement and the test circuit.

voltage application, which introduced electric charges onto the sample surface. The schematic of the electrode arrangement and the test circuit is shown in Fig. 3. The diameter of the stainless steel needle was 1 mm, and the curvature radius of the tip was about 13 μm. The interval between the needle tip and the sample surface was 2 mm. The needle electrode was connected to the output of a high-voltage dc source, and the plate electrode was grounded. Both the test sample and the electrode system were placed into a temperature controllable test chamber where the relative humidity was kept at 15%. B. Principle of SPD SPD has been considered as a powerful tool for analyzing charge transportation behavior within solid dielectrics [35], [36]. The surface potential of a solid sample is usually established through dc corona charging, after which the potential is allowed to decay under isothermal condition by electric field generated from the implanted electric charges [36]. It has been assumed that the charges are localized by the traps, which are originated from either physical or chemical defects of the material, and the decay of surface potential is closely associated with the charge detrapping behavior [37], [38]. Accordingly, the trap depth as well as the charge mobility could be derived from the SPD measurement. Such a method has been used by a number of research groups to evaluate charge dynamics in polymer, and the concept that various parameters are derived from the same potential decay data has been generally accepted [39]–[41]. Fig. 4 shows a typical scheme of charge transportation behavior within a polymer dielectrics charged with negative

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Fig. 4. Scheme of charge trapping and detrapping process in energy band after negative corona charging. E c is the bottom of the conduction band, E T n , E T 0 , and E T m are the traps of different energy levels, respectively, and E F is the Fermi level.

dc corona. It has been pointed out that the electrons are the main carriers within the material in a negative corona charged polymer. The charged surface in Fig. 4 corresponds to the position defined as x = 0, while the grounded electrode is regarded as the position x = L. Electrons could be transferred from the surface to the bulk and then migrate toward the grounded electrode, leading to the decay of surface potential [42]. It must be stated that although the hopping process exists in the polymer [43], band transport is assumed as the dominate process, and the retrapping event is neglected in the method [44]. Such approximations have been considered to be reasonable when a film or a sheet polymer sample is analyzed [45]–[48]. The trap density has been considered closely related to the decay behavior, which could be expressed as [44], [47] dUs (2) N(E) ∝ t dt where N(E) is the trap density occupied by carriers at trap level E, t is the decay time, and Us is the surface potential. In addition, by defining the demarcation energy E m which indicates the border between emptied and occupied traps [44], it is proposed that E m moves away from the band edge E c with the lapse of time, and thus, the trap depth E = E c − E m can be expressed as a function of the decay time [45], [47], [48] E = kT ln(νt)

(3)

where k is the Boltzmann constant, T is the Kelvin temperature, and v is the attempt to escape frequency. Accordingly, the relationship between |tdUs /dt| and E presents the trap distribution within the corona charged material [47]. Another important parameter related to the charge transportation behavior that could be derived from the SPD measurement is the equivalent carrier mobility μ, which is given by the following expression [49]: −1 μ = L 2 tT−1 Us0

(4)

where Us0 is the initial surface potential at the beginning of the decay, and tT is the transit time at which the implanted charge migrates to the grounded electrode [49]. In this paper, the parameters used for the calculation of trap distribution and carrier mobility are listed in Table I.



TABLE I PARAMETERS USED FOR THE CALCULATION OF TRAP DISTRIBUTION AND CARRIER MOBILITY

C. Experimental Procedure The time period from the completion of irradiation to the test was less than 72 h. All samples were kept in enclosed glass bottles to limit possible postoxidation, which could be confirmed by FTIR spectrum shown in Fig. 1. Since the energy of electron beam was high enough to induce chemical reactions within the samples, the variations in chemical and physical structures caused by the irradiation could be considered to be irreversible. Accordingly, the recovery of samples from the irradiation could be neglected. Before the dc corona charging process, the samples were kept in a drying oven at 50 °C for no less than 8 h so as to remove the possible water absorption, and the epoxy/Al2O3 nanocomposites specimen was cleaned with ethyl alcohol to remove residual charges deposited during sample preparation and/or the electron beam irradiation. A prescan of surface potential was conducted to ensure the removal of charges from the nanocomposites, and it was confirmed that the initial potential was lower than ±10 V. The experiment was performed at the temperature of 20 °C with a relative humidity of 15%. The ambient temperature of a spacecraft can vary from −180 °C to 250 °C [50], and the charge transportation manner obtained at room temperature tends to provide helpful information in understanding the transportation behaviors at both lower and higher temperatures; a dc voltage of −10 kV was applied between the needle and plate electrode for 5 min, and then, surface potential on the epoxy/Al2O3 nanocomposites was established. The potential tended to decrease as the applied voltage was removed, and the surface potential was recorded as a function of time through a Kelvin type electrostatic voltmeter (OP0865). The probe was positioned 3 mm above the sample surface. The total time period for the measurement was 60 min. Based on the SPD measurement, trap depth and carrier mobility were extracted. Moreover, the differential scanning calorimetry (DSC) measurement of the samples was carried out by using the Perkin Elmer Diamond DSC with the temperature between 0 °C and 180 °C at a temperature rise rate of 10 °C/min, from which the glass transition temperature (Tg ) was extracted so as to reveal the influence of nanosized fillers and irradiation on the changes in thermal properties of the nanocomposites. All the tests were performed with at least five specimens at the each condition to check the repeatability of the test results, and the typical results are shown in Section III. III. R ESULTS AND D ISCUSSION The typical SPD behavior of the nanocomposites is shown in Fig. 5. Epoxy doped with 4 wt% of Al2 O3 nanofillers was

Fig. 5. Relationship between the surface potential and the decay time for epoxy nanocomposites with 4-wt% nano-Al2 O3 fillers at various total irradiation doses.

taken as an example. The decay shows a nonlinear manner that follows well with a biexponential curve. It can be observed from Fig. 5 that the potential of nanocomposites irradiated with 500 kGy decays at the slowest rate, whereas the sample irradiated with 100 kGy decays most quickly. The decay of the surface potential on polymer insulating materials has been extensively discussed, and it has been generally accepted that the decay is associated with charge trapping and detrapping processes [51]. Electric charges generated during the corona discharge are driven by the field and migrate to the sample surface. As they arrive at the test specimen, charge transfer is considered to occur, and the charge within the sample bulk becomes electronic in nature [5], [52]. Due to the presence of impurity or molecular chain defect, carrier traps are thought to be present in the bulk, which appears to capture the charges. These charges could be thermally activated so as to detrap and transport to the grounded electrode, leading to the decrease of surface potential. Since no remarkable oxidation occurs in the sample and water vapor absorption is controlled, the internal charge transportation behavior discussed here is thought to be induced mainly by the intrinsic structure of the material and is thereby comparable to that under vacuum condition. It is suggested by Liu and Chen [51] that both deep and shallow traps are likely to contribute to the detrapping process. The initial fast decay of the potential should be ascribed to charge detrapping in shallow traps, while the following slower decay of the potential is due to charge detrapping in deep traps. Fig. 6 shows the relationship between the surface potential and the decay time for epoxy nanocomposites with various nanofiller contents. It is found that with the increase of the nanofiller content from 0 to 7 wt%, the decay of the potential becomes slower, which reveals that the adding of the nanofiller tends to restrict the charge transportation. The typical trap distribution for epoxy/Al2O3 samples irradiated with various doses is shown in Fig. 7. As stated in (2), |tdUs /dt| stands for the trap density at the each energy level. Because the measured data of surface potentials are in good agreement with the fitting curves, as is shown in Figs. 5 and 6, it is reasonable to use the fitting curves to calculate the trap distribution. It can be observed that the measured trap distribution for electron covers the range


Fig. 6. Relationship between the surface potential and the decay time varying as a function of the nano-Al2 O3 filler content at 100 kGy.

Fig. 7. Trap distribution varying as a function of total dose with 4-wt% nano-Al2 O3 fillers.

from 0.64 to 0.85 eV for the samples irradiated with various total doses, whereas the trap center changes obviously. As is shown in Fig. 7, the unirradiated epoxy/Al2O3 sample has a trap center for electron at 0.795 eV, while the samples irradiated with 100 and 500 kGy have the trap centers at 0.790 and 0.815 eV, respectively. It indicates that the electron trap center becomes shallower at lower dose, but the trap center becomes deeper at higher dose. By comparing the results shown in Figs. 5 and 7, it may be proposed that the SPD rate is closely related to the trap center, where a deeper trap center leads to a slower decay of the surface potential. The dependence of the trap center upon the total dose of irradiation for epoxy samples added with various nanofiller contents is shown in Fig. 8. With the increase of the total dose from 0 to 100 kGy, the trap centers for both neat epoxy and epoxy nanocomposites tend to decrease, which indicates that shallow traps are formed within the materials due to the irradiation. However, a further growth of the total dose from 100 to 500 kGy results in the increase of the trap center, which reveals the formation of deep traps at relatively higher irradiation dose. In addition, it can be obtained from Fig. 8 that with the increase of the nanofiller content from 0 to 7 wt%, the trap centers for both unirradiated and irradiated samples appear to be deeper. It suggests that deep traps are formed with the addition of nanofiller, and such a result is in agreement with the decay curve shown in Fig. 6.

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Fig. 8. Relationship between the trap center and the total dose of irradiation.

Fig. 9. Relationship between the potential decay rate and the decay time at different total doses of irradiation.

In order to further explore the charge transportation behavior, the transit time as well as the carrier mobility is calculated through (4). The typical relationship between |dUs /dt| and decay time for epoxy/Al2O3 samples with 4-wt% filler content is shown in Fig. 9. In order to better describe the transit time at which the charge transports from the charged surface to the grounded electrode, lg|dUs /dt| and lg(t) have been used in Fig. 9. It can be observed that a turning point is present in each curve, which corresponds to the transit time. The transit time for samples irradiated with 0, 100, and 500 kGy under negative corona charging is found at t0 = 372 s, t100 = 203 s, and t500 = 480 s, as is shown in Fig. 9. It indicates that the transit time of the material is strongly dependent upon the total irradiation dose. The transit time decreases as the total dose grows from 0 to 100 kGy and then increases with the increase of the total dose to 500 kGy. This reveals that at lower dose of the electron beam irradiation, the transportation of the implanted electrons from the dc corona tends to be accelerated, whereas a further exposure to the electron beam irradiation would cause suppression in electron transportation within the material. According to the transit time determined in Fig. 9, the carrier mobility within the sample can be calculated through (4). Fig. 10 summarizes the influence of electron beam irradiation on the carrier mobility of samples with different nanofiller


Fig. 10.


Effect of the electron beam irradiation on the carrier mobility.

contents. It can be observed that the carrier mobility is generally in the range of 10−12 m2 ·V−1 ·s−1 regardless of the content of nanofiller and the total irradiation dose. With the increase of the total irradiation dose, the carrier mobility increases initially and then appears to decrease. For instance, the electron mobility for neat epoxy before the irradiation is 3.0 × 10−12 m2 ·V−1 ·s−1 . Such a value grows to 3.9 × 10−12 m2 ·V−1 ·s−1 , as the material is exposed to the irradiation with the total dose of 100 kGy. However, as the total irradiation dose reaches 500 kGy, the mobility declines again to 2.5 × 10−12 m2 ·V−1 ·s−1 . Furthermore, it can be observed that with the increase in the nanofiller content, the mobility for electron tends to decrease. This is possibly due to the presence of deep traps introduced by the nanofiller as demonstrated by Tanaka et al. [53], [54] and Shah et al. [55]. It is confirmed again by test data show in Fig. 10 that the charge transportation of epoxy/Al2O3 nanocomposites is facilitated, as the material is exposed to 100-kGy electron beam irradiation but is restricted when the total dose is enhanced to 500 kGy. With the purpose of understanding the change in the charge transportation behavior induced by the electron beam irradiation, measurements are carried out to reveal the variation of physical and chemical structures of the test sample based on DSC. Fig. 11 exhibits the dependence of Tg upon the total irradiation dose for the neat epoxy and the nanocomposites. The method by which Tg is extracted from the DSC curve is shown in the subfigure within Fig. 11. It can be seen from the subfigure that the heat flow grows with the temperature and a step-increase feature can be obtained. Tg is identified by the midpoint of the abrupt transition in heat flow, as is shown in the subfigure in Fig. 11. It is used to estimate the molecular chain mobility within the material. It can be found from Fig. 11 that Tg for both neat epoxy and epoxy nanocomposites increase with the total dose of irradiation from 0 to 500 kGy, which suggests that the movement of molecular chain appears to be difficult. Furthermore, it is also found that Tg decreases as the nanofillers are added into the neat epoxy, revealing the transition temperature from the glass state to the rubber state of epoxy resin could be reduced by the doping of nanofiller. Epoxy as a polymeric insulating material is composed of covalent band, which is capable of being disrupted due to the

Fig. 11. Relationship between Tg and the total irradiation dose for Epoxy/Al2 O3 nanocomposites with a subfigure demonstrating the determination method of Tg .

high-energy irradiation [56]. As the epoxy is irradiated with the 7.5-MeV electron beam, the disruption of the covalent band occurs with the formation of free radicals that may induce oxidation, chain scission, and crosslinking reactions within the material [57], [58]. The very two reaction types, i.e., degradation and crosslinking reactions, are often thought to be the main reactions that have a significant influence on the insulation performance of the material [58]. Actually, both the two reactions occur simultaneously, and the insulation property of the material is dependent upon the one dominant. Epoxy has been generally recognized as a crosslinking type material under high-energy irradiation [7], [8]. With the increase of the total irradiation dose from 0 to 100 kGy, it is considered that the covalent bonds appear to be disrupted. Meanwhile, it is also noticed that Tg increases, which suggests the restriction of molecular chain movement. It is then assumed that in such a range of total irradiation dose, crosslinking reaction takes place insufficiently. The active sites formed during the irradiation are still present, leading to the formation of side chains or end groups, by which the free volume of the material is reduced and the shallow traps are introduced [59]. This leads to the limitation of molecular chain movement and the enhancement of Tg . In addition, the trap depth tends to become shallower, by which the charge transportation is accelerated. Such a special radiation-induced reaction status of the material is thought to be originated from the very high energy of the beam (7.5 MeV); thus, the accumulation of total dose of 100 kGy could be achieved with a very short time, which is not enough for thorough crosslinking reaction within the material. As the total irradiation dose increases up to 500 kGy, it is considered that due to the increase in the number of free radicals and the extension of radiation time period, the collision opportunity of the radicals is enhanced, and the crosslinking reaction tends to occur extensively [60]. The overall reaction result of the material is crosslinking dominant. The formation of the 3-D molecular structure gives rise to the limitation of chain movement, and Tg appears to increase. Furthermore, with the formation of the crosslinking network, the activation energy of the material tends to be enhanced [61], [62], and thus,


the trapped charges become difficult to be thermally activated, and the trap depth tends to be deeper [63], [64]. Accordingly, the charge transportation is limited. It should be mentioned that carrier trap characteristics are closely related to the chemical and physical structures of the polymer [43]. The variation in the microstructures of the polymer tends to alter the trap characteristics [48]. In this paper, the 7.5-MeV electron beam is used to irradiate the epoxy samples, and such an energy level is high enough to induce chemical reactions that would lead to the change in microstructure of the material [65]; thus, the trap distributions are likely to be altered. Although the electron beams are with the same energy at the each case, the different exposure times certainly result in different chemical reactions within the material, as longer exposure time allows the formation of more free radicals, which result in higher opportunity of radical collisions and crosslinking [60]. With regard to the influence of nanofiller content on the trap distribution, it has been proposed that the presence of nanofiller contributes the formation of deep trap because of the interface region between the inorganic particle and the polymer matrix [66], [67].This would be the reason why the trap center tends to be deeper and charge transportation is restricted, as the content of nano-Al2O3 filler increases from 0 to 7 wt% for samples both before and after the irradiation. IV. C ONCLUSION In this paper, the effect of high-energy electron beam irradiation on SPD of epoxy/Al2O3 nanocomposites charged by negative dc corona has been investigated, and the main conclusions can be summarized as follows. 1) With the increase of the total irradiation dose from 0 to 500 kGy, the surface potential tends to decay faster initially but then appears to decay slower. With the enhancement of the nanofiller content from 0 to 7 wt%, the decay of the potential becomes slower. 2) With the growth the total irradiation dose from 0 to 100 kGy, the trap center for both the neat epoxy and the nanocomposites becomes shallower. However, a further increase in the total irradiation dose results in the reduction of the trap center. Regardless of the total irradiation dose, the trap center exhibits a deep trend, as the nanofiller content grows from 0 to 7 wt%. 3) The carrier mobility for the investigated epoxy nanocomposites is in the range of 10−12 m 2 V−1 s−1 , which is not remarkably varied by the 7.5-MeV electron beam irradiation. The mobility presents a rise-an-fall tendency with the total irradiation dose but shows a monotonously decrease tendency with the nanofiller content. 4) Tg for both neat epoxy and epoxy nanocomposites increases with the total dose of irradiation from 0 to 500 kGy. In addition, the adding of the nanofiller leads to the reduction of Tg . In summary, it is revealed in this experimental work that after being irradiated by 7.5-MeV electron beam with a certain accumulated dose, the epoxy nanocomposites appear to form 3-D molecular network that restricts the transportation of

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Yu Gao (M’10) was born in Xinmin, China, in 1981. He received the Ph.D. degree from the School of Electrical Engineering and Automation, Tianjin University, Tianjin, China, in 2009. Since 2012, he has been serving as an Associate Professor in high-voltage and electrical insulation technology. Since 2017, he has been an Academic Visitor with the University of Southampton, Southampton, U.K. His current research interests include the aging phenomena of polymer insulating materials, pulsed power technology, and its application in environment engineering and overvoltage mechanism and protection.

Jilong Wang was born in Fushun, China, in 1992. He received the B.Sc. degree from the Dalian University of Technology, Dalian, China, in 2015. He is currently pursuing the M.S. degree in electrical engineering with Tianjin University, Tianjin, China. His current research interests include surface charge measurement in solid dielectrics and shunt capacitor bank switching overvoltage protection for power system.

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Fang Liu was born in Zaozhuang, China, in 1988. She received the B.Sc. degree in engineering from Tianjin Chengjian University, Tianjin, China, in 2012. She is currently pursuing the M.S. degree in electrical engineering with Tianjin University, Tianjin. Her current research interests include the aging phenomena of epoxy-based nanocomposites and lightening overvoltage protection for power system.

Boxue Du (M’00–SM’04) received the M.E. degree in electrical engineering from Ibaraki University, Ibaraki, Japan, in 1993, and the Ph.D. degree from the Tokyo University of Agriculture and Technology, Fuchu, Japan, in 1996. He was an Associate Professor with the Niigata College of Technology, Niigata, Japan. He is currently a Professor with the Department of Electrical Engineering, School of Electrical Engineering and Automation, Tianjin University, Tianjin, China. His current research interests include dielectric failure mechanisms of polymer insulating materials, electrical insulation technology, and partial discharge measurements. Dr. Du is a member of The Institute of Energy Economics Japan and a Senior Member of the Chinese Society for Electrical Engineering.