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S.-J. Wang et al.: Preparation, Microstructure and Properties of Polyethylene/Alumina nanocomposites for HVDC insulation

Preparation, Microstructure and Properties of Polyethylene/Alumina nanocomposites for HVDC insulation Si-Jiao Wang, Jun-Wei Zha, Yun-Hui Wu, Li Ren, Zhi-Min Dang Laboratory of Dielectric Polymer Materials and Devices Department of Polymer Science and Engineering University of Science and Technology Beijing Beijing 100083, P. R. China and Ji Wu College of Materials Science and Engineering University of Science and Technology Beijing Beijing 100083, P. R. China

ABSTRACT In the recent decades, the phenomena of space charge accumulation in the high voltage direct current (HVDC) insulation have been attracted more attention. In this paper, low density polyethylene (LDPE) nanocomposites filled with alumina nanoparticles (nano-Al2O3) were prepared employing melting blend method. Morphologies of nanoparticles and LDPE/Al2O3 nanocomposites were performed by scanning electron microscopy (SEM). Electrical properties of the LDPE nanocomposites were also investigated. Results shown that the nano-Al2O3 particles modified with vinyl silane coupling can effectively enhance the breakdown strength of LDPE nanocomposites. With the nano-Al2O3 particles loading, the volume resistivity of the LDPE nanocomposites was increased, while dielectric permittivity of the nanocomposites was decreased. Space charge of the LDPE nanocomposites was measured by pulsed electro-acoustic (PEA) method. The charge profiles indicated that space charge suppression of the LDPE nanocomposites was better than that of pure LDPE. The excellent insulation properties of the LDPE nanocomposites were attributed to the better interfacial adhesion between the surface-treated nano-Al2O3 particles and the LDPE matrix. Index Terms - Polyethylene, Al2O3, composites, space charge.

1 INTRODUTION DIELECTRIC nanocomposites are a novel class of composite materials that have been received special attention because of their improved properties at very low loading levels compared with conventional filler composites. As we known, when a dielectric is energized, charge may be infected into from the electrode-dielectric interface through process such as Schottky injection or tunneling [1-3]. Charge may also be generated within the dielectric volume under applied of electric field, whereas some of the charge may be conducted away through the dielectric. A large amount of charge is Manuscript received on 7 September 2014, in final form 6 July 2015, accepted 8July 2015.

accumulated, resulted from the traps existing in the dielectric material. The presence of charge accumulation will lead to distortion of electric field distribution within the dielectric materials, resulting electric field enhancement in certain region of the dielectrics. The electric field enhancement could cause material degradation, even premature failure [4-7]. Therefore, the suppression of charge is the important key to dielectric materials. There are many methods applied to suppress space charge, adding organic material such as EVA, MPE, EP, and MAH etc. Ethylene-vinyl acetate copolymers can decreases the current with increasing thickness in the case where positive carriers are injected [8]. Wang X. reported that 1 wt% of MPE added to LDPE can decrease the volume resistivity and space charge accumulation, and increases the DC breakdown strength [9]. Dissado demonstrated that space charge can be injected into

DOI: 10.1109/TDEI.2015.004903

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 22, No. 6; December 2015

epoxy resins at fields as low as 7.14 kV/mm in the sufficient stressing time. The time over which space charge can be retained in the epoxy resin depends upon the depth of the deepest traps occupied and the temperature [10]. Recently, many researchers take concentrate on adding inorganic particle [11, 12]. Nanoparticles normally exist in two forms during dry state. Strong bonds due to sintering will hold the primary particles to exist in aggregated form. The particles will exist in agglomerated form with weak bonding of van der Waals forces. Several methodologies have been used to overcome and separate the weaker attractive forces among the agglomerated nanoparticles [13]. The dispersion of nanoparticles in the polymer matrix and the property of the interface between nanoparticles and polymer are regarded as key factors affecting the electrical insulating properties of polymer nanocomposites [14]. The common solution is to modify the nanoparticles using coupling agents [15-18]. Roy et al [19] reported that the vinylsilane modified nanosilicas offer XLPE a larger increase in the dielectric breakdown strength and double time to failure in voltage endurance strength, as compared to unmodified nanosilica filled materials. Huang et al [20] considered that inorganic particles may reduce the carrier mobility in the composite materials, thereby increasing their insulating properties. Takada et al [17] proposed a trapping model of induced dipole polarization, namely, the deep potential well induced by nanoparticles becomes trapping sites for carriers, which hinder the movement of carriers and avoid the space charge accumulation in the defect positions. So far, a few researches have systematically studied the effects of different nanoalumina surface chemistry on the performances of composite materials, even its excellent dielectric properties. Therefore, this is the main goal of our present work. Compared with the neat LDPE, the dielectric properties, breakdown strength, space charge, and volume resistivity of the LDPE nanocomposites were studied.

2 EXPERIMENTAL 2.1 MATERIALS LDPE (LD200BW) with density of 0.922 g/cm3 and melt flow rate of 2.3 g/10 min was purchased from Sinopec Beijing Yanshan Co. (China). The nanoalumia (nano-Al2O3) particles were supplied by Institute of Process Engineering Chinese Academy of Sciences (China), the diameter of Al2O3 is 50-100 nm, and they were modified by vinyl trimethoxy silane (VMES) prior to use. The different mass contents (0.1, 0.2, 0.5, and 1 %) of nano-Al2O3 particles were mechanically mixed with LDPE granules at the processing temperature of 403 K using the HAAKE PolyLab mixer (HAAKE Rheomix 600, Germany).. The composites were pressed by a hot pressing method at the temperature of 423 K and the pressure of 15 MPa. The prepared films were placed in a vacuum oven at 453 K for 24 h, and then cooled down to room temperature to eliminate thermal history. To banish the remainder charge, the films

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were put between two polished copper plates in a vacuum oven at 353 K for 48 h short-circuiting. 2.2 CHARACTERIZATION The dispersion state of nano-Al2O3 particles in LDPE was observed using a field emission SEM (Hitachi S-4700, Japan) at 20 kV accelerating voltage. The samples for SEM observing were fractured in liquidnitrogen and sputtered with thin gold layer. The Al2O3 nanoparticles was analysized by the fourier transform infrared spectral (FT-IR, Nexus 670). The space charge distribution was tested by pulsed electro-acoustic measurement (PEA, Shanghai Jiao Tong University) under an electrical field of 30 kV/mm for 60 min at 298 K. During the measurement, the samples with the size of 500 μm×7 cm×7 cm were sandwiched between an aluminum electrode with diameter of 12 cm and a semiconductive polymer electrode with diameter of 2 cm. In order to analysis the effect of the additive on crystal form, the LDPE nanocomposites were measured by differential scanning calorimetry (DSC-60, SHIMADZU Company). The weights of the samples were approximately 4 mg. The experiments were performed in the temperature range from 323 K to 433 K under N2 atmosphere with the rate of 10 K/min, and then decreased to 323 K. Direct current (DC) breakdown strength of the samples was measured using a dielectric strength tester (HT-50, Guilin Electrical Equipment Scientific Research Institute, China). The samples with the thickness of 30±5 μm were placed between the copper cylindrical electrodes with the diameter of 6 mm and tested at a voltage rising rate of 500 V/s until the samples were broken down. The whole setup was immersed in silicone oil. Weibull distribution was employed to fit the experimental data and determined the characteristic of DC dielectric breakdown strength according to the IEEE Standard 930-2004 [21]. Dielectric properties of the samples were measured by a high-resolution ALPHA analyzer (Novocontrol, Germany). The samples (500 μm in thick, 10 mm in diameter) were placed between two gold-coated stainless steel electrodes. The relative dielectric permittivity was measured in the frequency range from 102 to 106 Hz at 25 °C. The volume resistivity of nanocomposites was tested using a Keithley electrometer model 6517B with a standard three-electrode system. The samples with the thickness of 500 μm were testing at DC voltage of 50 V.

3

RESULTS AND DISCUSSION

3.1 MICROSTRUCTURE CHARACTERIZATION Figure 1 shows the SEM morphologies of cross section of LDPE nanocomposites. For the nanocomposites filled with Al2O3, more agglomeration can be observed and the Al2O3 particles are in the size range of 300-600 nm. The bonding strength at the interface between the particles and LDPE seems to be loose, the interface is very distinct.

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space charge injected into LDPE material from the anode and cathode. The space charge aggregates near the electrode. The LDPE/Al2O3 composites have inhibited the injection of space charge from the anode. The charge peak of the LDPE/V-Al2O3 nanocomposites in the Figure 3b is smaller than that of the LDPE and LDPE/Al2O3。The charge amount in the material is based on charge density and the thickness of the material. The charge amount Q represented in Figure 3a and 3b is derived from the experimental results according to equation (1). d

Q   |  ( x ) | S  dx …………………………….(1) 0

where Q is total trapped charge amount, ρ(x) is charge density, S is the electrode area, and d is the thickness of the sample. The results are plotted in Figure 3c.

Figure 1. SEM images of the (a) LDPE/Al2O3, (b) LDPE/V-Al2O3 nanocomposites.

However, there is no agglomeration can be seen in the LDPE nanocomposites filled with modified Al2O3 in the Figure 1b, and the average sizes of V-Al2O3 dispersed in LDPE matrix are smaller than that of unmodified Al2O3 ones. These results show the effectiveness of surface modification of Al2O3 nanoparticles in improving their dispersion state in LDPE. It also indicated that the chemical surface modification does improve the interfacial adhesion with LDPE. The possible reason was that the LDPE molecule has the same basic structure repeating unit -CH2- with VMES, it is possible to improve the compatibility of the V-Al2O3 nanoparticles with LDPE, which is more benefited to the dispersion of the nanoparticles. The FT-IR spectrum of unmodified nano-Al2O3 and V-Al2O3 particles is shown in Figure 2. Comparing with the spectrum of unmodified Al2O3 nanoparticles, the C=C absorption peak at 2870 cm-1 existed in that of V-Al2O3, indicating that the VMES is successfully modified on the Al2O3 nanoparticles. The hydroxyl groups on the surface of nano-alumina have been replaced by -O-C2H5. The absorption peak of 3500 cm-1 in the V-Al2O3 is the hydroxyl absorption peak. 3.2 SPACE CHARGE Figure 3 shows the space charge distribution of LDPE nanocomposites polarized at the electric field of 30 kV/mm for 60 min, with the concentration of Al2O3 nanoparticles loading. It can be observed from Figure 3a that the large amount of

Figure 2. FT-IR spectrum of the unmodified Al2O3 and V-Al2O3 nanoparticles

Figure 3c shows the space charge amount in the material and the corresponding concentration of nanocomposites. The space charge in the LDPE/V-Al2O3 is littler than that of the LDPE. But the space charge of LDPE/Al2O3 with 1 wt% nano-particle is larger than another one. The reason is that the amount of unmodified Al2O3 nanoparticles is added into LDPE, agglomeration is liable to occur due to the relatively large surface energy of the nanoparticles, which is incompatible with the LDPE matrix. It has been manifested in the Figure 1. The suppression of space charge of V-Al2O3 is better than that of the unmodified one. The total charge amount in the material is the smallest when the mass fraction of V-Al2O3 nanoparticles is 1%. The nanoparticles induce more deep traps and shallow traps in the LDPE nanocomposites than that of the neat LDPE, trap increase shackles of space charge and reduce the flow of the carrier in the material.


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observed from the Figure 4a, the initial rise of DSC curve of LDPE/Al2O3 samples is lower than that of LDPE, and the nucleation rate in the samples must be large. The increase of the nucleation rate must make the spherules size smaller. Since the dispersed smaller spherulites may induce the deep traps in the bulk of LDPE [20, 23]. From the Figure 4b, the degree of crystallinity of 1 wt% LDPE/V-Al2O3 is less than that of the other concentration filled LDPE/V-Al2O3. When the large amounts of particle are employed, the vinyl of V-Al2O3 insert into the polymer backbone, which may limit the movement of polymer molecular chain and leave little space for additional crystallization, leading to the crystallization decrease. Compared the Figure 4a and 4b, the crystallinity of the modified nanocomposite is higher than that of the corresponding content of unmodified one. It may be due to that the better dispersibility for modified nano-Al2O3 result in the spherulites in the LDPE/V-Al2O3 which is more than that of LDPE/Al2O3. The adding of nano-sized fillers with high dispersibility reduces the injection of space charge into the bulk consequently.

Figure 3. Space charge distribution of nanocomposite at 30 kV for 60 min (a) LDPE/Al2O3, (b) LDPE/V-Al2O3, and (c) the total charge amount of agglomeration in the material.

3.3 DSC DSC test is applicable to the crystallization process of blends. Figure 4 shows the non-isothermal crystallization curve of LDPE nanocomposites with the different concentration at the temperature rate of 10 K/min. Crystallization of polymers include nuclei formation and grain growth. And the growth of crystal grains depends on the crystallization temperature. Known peak starting temperature and the temperature of the crystallization peak is proportional to the difference the crystal growth rate. [22]. It can be

Figure 4. DSC spectrum of the (a) LDPE/Al2O3 and (b) LDPE/V-Al2O3 nanocomposites

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Combining with the results of space charge, the results show that the decrease of space charge in the LDPE nanocomposites especially filled with 1 wt% V-Al2O3 nanoparticles can be attributed to the improvement of the nucleation, decrease of the size of spherulites and the dissipation of charges through boundary regions of smaller spherulites. 3.4 BREAKDOWN STRENGTH

breakdown voltage or time. The shape factors β of LDPE/ V-Al2O3 nanocomposites show much lower values than that of LDPE/Al2O3 nanocomposites, which reflect that the breakdown mechanisms of the LDPE were changed after introducing Al2O3 nanoparticles. The two types of nanocomposites have the similar shape factor, suggesting that the surface modification of the nanoparticle does not change the breakdown mechanisms of the composites.

The dielectric breakdown strength of the LDPE nanocomposites is analyzed within the framework of Weibull statistics. Weibull statistical distribution is the most important method for processing data breakdown strength, which reflects the probability of the material at certain field strength (E) or the probability of failure or breakdown at a certain time (t). Weibull cumulative distribution breakdown field strength can be described as follows.

E P  1  exp[ ( )  ] …………….……….(2)



where P is the cumulative probability of the dielectric breakdown E is the breakdown strength for testing; β is the shape factor related to the dispersion of sample; and α is the characteristic breakdown strength during the cumulative probability of the dielectric breakdown is 63.2%. After twice taking the logarithm, equation (2) can be modified as follows.

log[  ln(1  P )]   log( E )   log( ) …….(3) This point log [-ln (1- P)] and log (E) form a linear relationship in the Cartesian coordinate system. According to IEEE Standard 930-2004, when the number of samples is less than 25, P should be computed by the formula as follows.

Pi 

i  0.44  100% n  0.25

(4)

where n is the breakdown times or voltages in order from smallest to largest and assign them a rank from i = 1 to i = n. The Weibull parameters α and β are shown in the insert table in Figure 5. The α is found to be strongly affected by the filler modification in the nanocomposites and the voltage ramping rate [24]. The introduction of nano-Al2O3 particles reduce α values of the LDPE polymers, which is likely due to the more defects in the nanocomposites that result in charge accumulation and partial discharge. The defects are formed due to the relatively weak interface, voids as well as the larger aggregates. Comparatively, the V-Al2O3 nanoparticles show the relatively higher α value than the unmodified ones, owing to the improved interface, good dispersion level and smaller aggregation [25]. The shape parameter β is a measured range of the failure time or voltages. The larger β is, the smaller is the range of

Figure 5. Breakdown strength of the (a) LDPE/Al2O3 and (b) LDPE/V-Al2O3 nanocomposites.

3.5 DIELECTRICPROPERTIES Figure 6 shows the dielectric properties of LDPE/Al2O3 nanocomposites as function of frequency at room temperature. Figure 6a displays the relative dielectric permittivities (εr) at f=102-106 Hz with the mass fraction of Al2O3. Between 102 and 103 Hz the relative dielectric permittivity of LDPE and LDPE/Al2O3 increases strongly with decreasing frequency. But the LDPE/V-Al2O3 nanocomposites are very stable. Such a behavior is a clear sign of electrode polarization, which origin from the blocking of change carries at sample/electrode interface [26]. The addition of Al2O3 nanopartilces made relative dielectric permittivity of LDPE decreased which indicating that the movements of carries have been inhibited in the functionalized nanocomposites.


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Figure 7. Volume resistivity of the LDPE nanocomposite.

Modified Al2O3 nanoparticles show good dispersion in LDPE matrix when compared with the unmodified Al2O3 nanoparticles. The addictive nanoparticles bring more deep traps and shallow trap, resulting in the effect of suppressing space charge. The suppressing effect of V-Al2O3 nanoparticles is better than that of unmodified one. The incorporation of V-Al2O3 nanoparticles could increase the volume resistivity, and present better breakdown strength when compared with LDPE/Al2O3.

ACKNOWLEDGEMENTS Figure 6. Frequency dependence of relative permittivity of the (a) LDPE/Al2O3 and (b) LDPE/V-Al2O3 nanocomposites.

3.6 VOLUME RESISTIVITY The volume resistivity of nanocomposites is 3.50×1016， 3.57×1016, 3.26×1016, 4.83×1016, 4.26×1016, 4.06×1016, 5.77×1016, 7.56×1016 Ω·cm for respectively, at the DC voltage of 50 V. The volume resistivity of 1 wt% V-Al2O3 nanocomposites is about twice larger than that of LDPE (2.24 ×1016 Ω·cm) as shown in Figure 7. This demonstrates that the Al2O3 nanocomposite have effectively enhanced the ability of low-density polyethylene to resist leakage current, is similar to other references [27, 28]. For the defined equation of volume resistivity (ρ = E/J), where E is the electric field and J is the density of current in the material. The volume resistivity is increase and the density of current is decrease. So the V-Al2O3 particles have inhibited the migration of carrier.

4 CONCLUSIONS In summary, the LDPE nanocomposites filled with the unmodified Al2O3 and V-Al2O3 nanoparticles were prepared by using melt mixing and hot press methods, respectively. The main conclusions are as follows.

This work was financially supported by NSF of China (Grant No. 51207009 and 51377010), the National Basic Research Program of China (973 Program) (Grant No. 2014CB239503), the Hong Kong Scholar Program (XJ 2014048), and the Fundamental Research Funds for the Central Universities (No. FRF-TP-14-016A2), and Beijing Municipal Science & Technology Commission (Grant No. Z131100005913005).

REFERENCES [1] F. Rogti, and M. Ferhat, "Effect of temperature on trap depth formation in multi-layer insulation: Low density polyethylene and fluorinated ethylene propylene", Appl. Phys. Lett., Vol. 104, pp. 031605, 2014. [2] A. A. Sener, and E. Demirhan, "The investigation of using magnesium hydroxide as a flame retardant in the cable insulation material by cross-linked polyethylene", Mater. Design, Vol. 29, pp. 1376–1379, 2008. [3] R. J. Fleming, "Conductivity and space charge in LDPE/BaSrTiO3 nanocomposites", IEEE Trans. Dielectr. Electr. Insul., Vol. 18, pp. 15-23, 2011. [4] H. J. Wintle, "Surface charge decay in insulators with nonconstant mobility and with deep trapping", J. Appl. Phys., Vol. 43, pp. 2927-2931, 1972. [5] B. V. Vanadin, and S. L. Roy, "Impact of concentration gradient of polarizable species on the electric field distribution in polymeric insulating material for HVDC cable", IEEE Trans. Dielectr. Electr. Insul., Vol. 18, pp. 833-839, 2011. [6] S. Ju, M. Chen, H. Zhang, and Z. Zhang, "Dielectric properties of nanosilica/low-density polyethylene composites: The surface chemistry of nanoparticles and deep traps induced by nanoparticles", Polymer Letters, Vol. 8, pp. 682-691, 2014.

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[7] G. Chen, "Space charge and its impact on DC breakdown of polymeric materials", 18th Int’l. Sympos. high Voltage Eng., Vol. OD2-01, pp. 686-691, 2013. [8] Y. Suzuoki, H. Mutot, T. Mizutani, and M. leda, "The role of space charge in the electrical conduction of ethylene-vinyl acetate copolymers", J. Phys. D: Appl. Phys., Vol. 20, pp. 1053-1058, 1987. [9] X. Wang, H. Q. He, and D. M. Tu, "Dielectric properties and crystalline morphology of low density polyethylene blended with metallocene catalyzed polyethylene", IEEE Trans. Dielectr. Electr. Insul., Vol. 15, pp. 319-326, 2008. [10] L. A. Dissado, V. Griseri, W. Peasgood, and E. S. Cooper, "Decay of space charge in a glassy epoxy resin following voltage removal", IEEE Trans. Dielectr. Electr. Insul., Vol. 13, pp. 903-906, 2006. [11] R. J. Fleming, A. Ammala, and P. S. Casey, "Conductivity and space charge in LDPE containing nano- and micro-sized ZnO particles", IEEE Trans. Dielectr. Electr. Insul., Vol. 15, pp. 118-126, 2008. [12] X. Y. Huang, and F. Li, "Effect of nanoparticle surface treatment morphology, electrical and water treeing behavior of LLDPE composites", IEEE Trans. Dielectr. Electr. Insul., Vol. 17, pp. 1697-1704, 2010. [13] C. Y. Chee, N. L. Song, and L. C. Abdullah, "Characterization of mechanical properties: low-density polyethylene nanocomposite using anoalumina particle as filler", Hindawi Publishing Corp. J. Nano., Vol. 15. pp. 1-6, 2012. [14] Y. Yin, J. Chen, J. Yang, and D. Xiao, "Effect of space charge in nanocomposite of LDPE/TiO2", IEEE 7th Int’l. Conf. Properties Applications of Dielectr. Materials Nagoya, Japan, Vol. 3, pp. 913-916, 2003. [15] Y. Geng, M. Y. Liu, J. Li, X. M. Shi, and J. K. Kim, "Effects of surfactant treatment on mechanical and electrical properties of CNT/epoxy nanocomposites". Appl. Sci. Manu., Vol. 39, pp. 1876-1883, 2008. [16] G. Siqueira, and J. Bras and A. Dufresne, "Cellulose whiskers versus microfibrils: Influence of the nature of the nano -particle and its surface functionalization on the thermal and mechanical properties of nanocomposites", Biomacromolecules, Vol. 10, pp. 425-432, 2009. [17] T. Takada, Y. Hayase, and Y. Tanaka, "Space charge trapping in electrical potential wellcaused by permanent and induced dipolesfor LDPE/MgO nanocomposite", IEEE Trans. Dielectr. Electr. Insul., Vol. 15, pp. 152-161, 2008. [18] L. R. Zhou, G. N. Wu, and B. Gao, "Study on charge transport mechanism and space charge characteristics of polyimide films", IEEE Trans. Dielectr. Electr. Insul., Vol. 16, pp. 1143-1149, 2009. [19] M. Roy, J. K. Nelson, R. K. MacCrone, and L. S. Schadler, "Candidate mechanisms controlling the electrical characteristics of silica/XLPE nanodielectrics", J. Mater. Sci., Vol. 42, pp. 3789–3799, 2007. [20] X. Y. Huang, P. Jiang, and Y. Yin "Nanoparticle surface modification induced space charge suppression in linear low density polyethylene", Appl. Phys. Lett., Vol. 95, pp. 242905, 2009. [21] IEEE Std 930-2004, IEEE Guide for the Statistical Analysis of Electrical Insulation Breakdown Data, 2005. [22] X. Wang, H. Q. He, and D. M. Tu, "Space charge distribution and crystalline structure in low-density", IEEE Trans. Dielectr. Electr. Insul., Vol. 21, pp. 35-40, 2006. [23] W. C. Du, W. Zhong, Y. J. Lin, L. Shen, and Q. G. Du. "Space charge distribution and crystalline structure in polyethylene blended with EVOH", Europ. Polym. J., Vol. 40, pp. 1987-1995, 2004. [24] M. Roy, J. K. Nelson, and R. K. M. Crone, "Polymer nanocomposite dielectrics the role of the interface", IEEE Trans. Dielectr. Electr. Insul., Vol. 12, pp. 629-643, 2005. [25] K. Y. Lau, A. S. Vaughan, G. Chen, and I. L. Hosier, "Polyethylene nanodielectrics: the effect of nanosilica and its surface treatment on electrical breakdown strength", 2012 Annual Report Conf. Electr. Insul. Dielectr. Phenom., pp. 21-24, 2012. [26] K. Ishimotol, T. Tanaka, Y. Ohki, Y. Sekiguchi, Y. Murata, and M. Gosyowaki. "Comparison of dielectric properties of low-density polyethylene/MgO composites with different size fillers", IEEE Conf. Electr. Insul. Dielectr. Phenomena,, pp. 208-211, 2008.

[27] S. Okuzumi, Y. Murakami, M. Naga, Y. Sekiguchi, C. C. Reddy and Y. Murata. "DC breakdown strength and conduction current of MgO/LDPE composite influenced by filler size", IEEE Conf. Electr. Insul. Dielectr. Phenom., pp. 722-725, 2008. [28] K. Hinata1, A. Fujita, K. Tohyama, and Y. Murata. "Dielectric Properties of LDPE/MgO Nanocomposite Material under AC High Field", IEEE Conf. Electr. Insul. Dielectr. Phenomena, pp. 313-316, 2006.

S. J. Wang received the Master degree in chemistry from Xi’an University of Science and Technology in 2010. She currently studies in the Department of Polymer Science and Engineering, University of Science & Technology Beijing for Ph.D. degree.

J. W. Zha received the Ph.D. degree in materials science and engineering in 2010 from Beijing University of Chemical Technology. He studied in University of Southampton, as a Joint-Ph.D. student sponsored by China Scholarship Council form 2009 to 2010. He currently works in the Department of Polymer Science and Engineering, University of Science & Technology Beijing. He has published more than 40 journal papers. Y. H. Wu was born in Guangdong Province, China in 1988. He received the B.Sc. degree in 2013 from Chongqing University of Technology, China. He is now a graduate in University of Science and Technology Beijing. His research field is space charge phenomenon in nanocomposites.

L. Ren received the Master degree in 2010. She currently works in the Department of Polymer Science and Engineering, University of Science & Technology Beijing.

Z. M. Dang received his Ph.D. degree in electrical engineering from Xi’an Jiaotong University in 2001. He is currently a Professor in Department of Polymer Science and Engineering, University of Science & Technology Beijing. He is also a member of IEEE on Dielectric and Electrical Insulation. His present research interests are electrical functional materials. He has published more than 180 journal papers with a current citation record of over 3000 times and an H-index of 31. J. Wu currently works in the College of Material Science And Engineering, University of Science & Technology Beijing.