Insulating Properties of Low Density Polyethylene/Alumina Nanocomposites

Transcription

1 American Journal of Engineering and Alied Sciences Original Research Paer Insulating Proerties of Low Density Polyethylene/Alumina Nanocomosites 1 Si-Jiao Wang, 1,2 Jun-Wei Zha, 1 Yun-Hui Wu, 1 Hong-DA Yan and 1 Zhi-Min Dang 1 Deartment of Polymer Science and Engineering, University of Science and Technology Beijing, Beijing , P. R. China 2 State Key Laboratory of Power Transmission Equiment and System Security and New Technology, Chongqing University, Chongqing, , P. R. China Article history Received: Revised: Acceted: Corresonding Author: Jun-Wei Zha Deartment of Polymer Science and Engineering University of Science and Technology Beijing Beijing , P. R. China zhajw@ustb.edu.cn Abstract: High-Voltage Direct Current (HVDC) cable has become an imortant factor in the develoment of HVDC transmission. However, there are still difficulties such as sace charge distribution in the develoment of HVDC cables. In this study, Low Density Polyethylene (LDPE) nanocomosites filled with nano alumina (nano-al 2 O 3 ) articles with or without modification were reared by emloying melting blend method. The results show that the modified nano-al 2 O 3 articles with vinyl silane couling can be homogeneously disersed in LDPE matrix. Sace charge of the LDPE nanocomosites was tested by Pulsed Electro-Acoustic (PEA) method. The PEA sectrum indicated that decay charge effect of the LDPE/Al 2 O 3 nanocomosites is better than that of neat LDPE. The sace charge accumulated in the nanocomosites can be effectively decayed. The J-V curves show the nano-al 2 O 3 decrease the carrier mobility. The excellent insulation roerties of the LDPE nanocomosites were attributed to the better interfacial adhesion between the surface-treated nano-al 2 O 3 articles and the matrix. Keywords: Polyethylene, Al 2 O 3, Comosites, Sace Charge, Decay, J-V Curves Introduction Low Density Polyethylene (LDPE) is widely used as bulk insulation in extruded cables, but under High- Voltage Direct Current (HVDC) the sace charge accumulated in the LDPE can greatly distort the electrical field and then influence the aging roerty. The reason is robably that the hysical and chemical defects existed in LDPE could lead to the increasing local electric field so that the insulating roerties would decrease. To solve the roblem, the inorganic nanoarticles and olarity substance have been added into PE. The effect of nano-sized article additives on the electrical roerties of dielectrics is currently of considerable interest (Olthuis and Bergveld, 1992; Liu and Chen, 2013; Wu et al., 2014; Kim et al., 2013; Fleming et al., 2011; Ju et al., 2014; Takada et al., 2008; Hosier et al., 2010). In this study, LDPE nanocmosites filled with the nano-al 2 O 3 articles with and without surface modification were reared. Sace charge characteristics in LDPE nanocomosites were measured using the Pulsed Electro Acoustic technique (PEA). The charge decay in both tyes of the samles was also tested. Tras and carrier mobility of LDPE nanocomosites were also discussed. Exerimental Materials LDPE (LD200BW) with density of g/cm 3 and melt flow rate of 2.3 g/10 min was urchased from Sinoec Beijing Yanshan Co., China. The nano- Al 2 O 3 articles were sulied by Institute of Process Engineering Chinese Academy of Sciences (China) and they were modified by vinyltrimethoxysilane (VMES, V-Al 2 O 3 ) rior to use. The reaction route is resented in Fig Si-Jiao Wang, Jun-Wei Zha, Yun-Hui Wu, Hong-DA Yan and Zhi-Min Dang. This oen access article is distributed under a Creative Commons Attribution (CC-BY) 3.0 license.

2 nanocomosite. It could be indicated that a large amount of charges inject into the LDPE. However, the nano- Al 2 O 3 can inhibit the charges to inject into the interior of nanocomosite and revent most of charges gather on the surface of the samles (Nelson and Hu, 2005). The current density-voltage (J-V) curves obtained from the equilibrium value of olarization current at different voltages were shown in Fig. 3. The J-V curves are consistent with the well-known Mott and Gurney equation (Meunier et al., 2001). At low alied voltages the J-V characteristics may follow Ohm s law: Fig. 1. Schematic of the reactions between the original Al 2 O 3 and the silane. The different contents (0.1, 0.2, 0.5, 1 wt%) of nano- Al 2 O 3 articles were mechanically mixed with LDPE granules using the HAAKE PolyLab mixer (HAAKE Rheomix 600, Germany) with the rocessing temerature of 130 C. The neat LDPE and Al 2 O 3 /LDPE nanocomosite films were reared by a hot-ress method at the temerature of 150 C and the ressure of 15 MPa. The reared films were laced in a vacuum oven at 80 C for 24 h and then cooled down to room temerature to eliminate thermal history. To banish the remainder charge, the films were ut between two olished coer lates in a vacuum oven at 80 C for 48 h short-circuiting. Characterization Carrier mobility of the samles was tested with a Keithley electrometer model 6517 B. The samles with the diameter of 54 mm and the thickness of 0.4 mm were reared for testing. The olarization current was obtained after short-circuiting for 5 min. The olarization voltage was set at 1 kv. The sace charge distribution was tested by PEA (Shanghai Jiao Tong University) carried out under an electrical field of 30 kv/mm for 60 min at 25 C. During the measurement, the samles with the size of 400 µm 7 cm 7 cm were sandwiched between an aluminum electrode and a semi-conductive olymer electrode (the diameter is 12 cm and 2 cm, resectively). Results and Discussion Figure 2 shows olarization currents measured for 5 min. The current decrease exonentially with time increasing and subsequently reaches an equilibrium value. The olarization currents curves of LDPE reach the equilibrium value at 100 s, while the Al 2 O 3 /LDPE nanocomosites filled with 1 wt% nano-al 2 O 3 only at 50 s. The equilibrium value of olarization current of LDPE is much larger than that of 1 wt% Al 2 O 3 /LDPE V J = q0µ (1) d Where: 0 = The density of thermally generated free carriers inside the samle µ = The carrier mobility d = The thickness of the samle. The sloe of the Equation 1 was given as follow Equation 2: k1 = q oµ (2) Due to the increased sace charge, quadratic relationshi of J-V curves fitting is obtained at high alied voltages as shown by Equation 3: 2 9 V J = εε r 0µ (3) 3 8 d where, ε r is the relative ermittivity of the material and ε 0 = F/m, is the ermittivity of vacuum. Then the carrier mobility µ are calculated from the fitting constants k, which have given as follows Equation 4: 9 k = 3 r 0 8d εε µ (4) The carrier mobility of Al 2 O 3 /LDPE nanocomosites calculated by the above equations is cm 2 / (V s) and which of the V-Al 2 O 3 /LDPE nanocomosites is cm 2 / (V s), which is lower than the ublished values of LDPE (Meunier et al., 2001). The addition of Al 2 O 3 decreases the carrier mobility of comosites, esecially the Al 2 O 3 modified by silane couling agent, about an order of magnitude. Figure 3 shows the Al 2 O 3 /LDPE nanocomosites follow Ohm s law at low alied voltages (0-200 V) in the J-V curves. It can be seen that the area of the Ohm s law of the V-Al 2 O 3 /LDPE nanocomosites is larger than that of Al 2 O 3 /LDPE nanocomosites without modification, indicating that the V- 406

3 Al 2 O 3 /LDPE nanocomosites have better insulation. The J-V curves of Al 2 O 3 /LDPE nanocomosites and V-Al 2 O 3 /LDPE nanocomosites are similar to schottky curves. It demonstrates that the main mechanism of charge injected into from electrode to dielectric interface is Schottky injection. Fig. 2. Polarization currents curves of Al 2 O 3 /LDPE and V-Al 2 O 3 /LDPE nanocomosites at the alied of 1 kv Fig. 3. Current density-voltage (J-V) curves from the equilibrium value of olarization currents at V of Al 2 O 3 /LDPE and V- Al 2 O 3 /LDPE nanocomosites (a) (b) 407

4 (c) Fig. 4. Sace charge decay after removal of 30 kv stressing in 120 min on LDPE samle (a) neat LDPE (b) Al 2 O 3 /LDPE nanocomosites (c) V-Al 2 O 3 /LDPE nanocomosites The sace charge decay for both tyes of LDPE samles are shown in Fig. 4. Both ositive and negative charges were introduced into the neat LDPE near the cathode and anode, resectively, was observed in Fig. 4a. Many researchers believe that electrons were injected into the samles from the two electrodes. Fig. 4b shows the negative charges in the samles, migrated in the bulk, reducing the height of the ositive eak. After 10 min, the negative eak totally disaears at the anode and negative charges are now accumulated in the bulk of samles. The electrons in Al 2 O 3 /LDPE comosites have a higher mobility than the neat LDPE, they may disaear very quickly. The reason is that the charges move from the anode to the cathode by negative traed charges. It is likely that a otential energy barrier exists for charges moving from samle to the anode. As a result, the charges gather near the electrode and some of them get retraed near the cathode. As can be seen from Fig. 4c, the injected charge in the cathode is fairly smaller than the Al 2 O 3 /LDPE comosites. It may due to that the silane couling can enhance the comatibility between LDPE and nano-articles and decrease the defects in the interface. It makes the nano-al 2 O 3 become effective traing site to fix carriers and decrease the carrier mobility. A charges injected from electrodes generate in the dee tra induced by the nano-al 2 O 3 after electric voltage alied. The charges are difficult to move and accumulate near the electrodes to roduce an additional electric otential reversed to the alied voltage. It inhibits the dissociation of small molecule and revents further injection of the charges. Therefore, the sace charge inside the samle is reduced. Conclusion In summary, the LDPE nanocomosites filled with the unmodified Al 2 O 3 and V-Al 2 O 3 nanoarticles were reared by using melt mixing and hot ress methods, resectively. The main conclusions are as follows. Modified nano-al 2 O 3 articles show good disersion in LDPE matrix comared with the unmodified nano- Al 2 O 3 articles. The addictive nanoarticles bring more dee tras and shallow tra, resulting in the effect of suressing sace charge. The suressing effect of V-Al 2 O 3 /LDPE nanoarticles is better than that of unmodified one. The additives of V-Al 2 O 3 could decrease the carrier mobility. The V-Al 2 O 3 /LDPE nanocomosites show excellent decay charge effect. Acknowledgment This work was financially suorted by NSFC (Grant No and ), the National Basic Research Program of China (973 Program) (Grant No.2014CB239503), Beijing Municial Science and Technology Commission (Grant No. Z ), Fundamental Research Funds for the Central Universities (No. FRF-TP A2) and Visiting Scholarshi of State Key Laboratory of Power Transmission Equiment and System Security and New Technology (Chongqing University) (2007DA ). Author s Contributions Si-Jiao Wang articiated in all exeriments and contributed to the writing of the manuscrit. Jun-Wei Zha 408

5 designed the research lan, organized the study, coordinated the data-analysis and contributed to the revising of the manuscrit. Yun-Hui Wu and Hong-Da Yan articiated in the exeriments. Zhi-Min Dang coordinated the data-analysis and contributed to the revising of the manuscrit. Ethics The authors declare no cometing financial interest. References Fleming, R.J., A. Ammala, P.S. Casey and S.B. Lang, Conductivity and sace charge in LDPE/BaSrTiO 3 nanocomosites. IEEE Trans. Dielectr. Electr. Insulat., 18: DOI: /TDEI Hosier, I.L., A.S. Vaughan and S.G. Swingler, An investigation of the otential of ethylene vinyl acetate/olyethylene blends for use in recyclable high voltage cable insulation systems. J. Mater. Sci., 45: DOI: /s Ju, S., M. Chen, H. Zhang and Z. Zhang, Dielectric roerties of nanosilica/low-density olyethylene comosites: The surface chemistry of nanoarticles and dee tras induced by nanoarticles. Exress Polymer Lett., 8: DOI: /exressolymlett Kim, Y.J., S.T. Ha, G.J. Lee, J.H. Nam and S.H. Nam, Investigation of sace charge distribution of low-density olyethylene/go-gnf (grahene oxide from grahite Nanofiber) nanocomosite for HVDC alication. J. Nanosci. Nanotechnol., 13: DOI: /jnn Liu, N. and G. Chen, Changes in charge traing/detraing in olymeric materials and its relation with aging. Proceedings of the IEEE Conference on Electrical Insulation and Dielectric Phenomena, Oct , IEEE Xlore Press, Shenzhen, : DOI: /CEIDP Meunier, M., N. Quirke and A. Aslanides, Molecular modeling of electron tras in olymer insulators: chemical defects and imurities. J. Chem. Phys., 115: DOI: / Nelson, J.K. and Y. Hu, Nanocomosite dielectrics-roerties and imlications. J. Phys. D: Alied Phys., 38: DOI: / /38/2/005 Olthuis, W. and P. Bergveld, On the charge storage and decay mechanism in silicon dioxide electrets. IEEE Trans. Electr. Insulat., 27: DOI: / Takada, T., Y.J. Hayase and Y. Tanaka, Sace charge traing in electrical otential well caused by ermanent and induced dioles for LDPE/mgo nanocomosite. IEEE Trans. Dielectr. Electr. Insulat., 15: DOI: /T-DEI Wu, K., Z. Lv, Q.D. Xia, Y. Hong and L.A. Dissado, Sace charge formation and conductivity characteristics of PE and oil imregnated aer under a temerature gradient. Proceedings of International Symosium on Electrical Insulating Materials, Jun , IEEE Xlore Press, Jaan, : DOI: /ISEIM