Electrical Performance of Epoxy Resin Filled with ... - Science Direct

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ScienceDirect Energy Procedia 110 (2017) 162 – 167

1st International Conference on Energy and Power, ICEP2016, 14-16 December 2016, RMIT University, Melbourne, Australia

Electrical performance of epoxy resin filled with micro particles and nanoparticles Mu Liang*, K L Wong School of Engineering, RMIT University, Melbourne 3000, Australia

Abstract Composite insulators are widely used in the overhead transmission system. Compared with porcelain products, composite has many advantages such as light in weight, hydrophobicity characteristic and superior mechanical property. However, composite products are also suffering from issues such as low tracking resistance and early failures. The use of nano or micro fillers is considered to be one of the methods that can further improve the properties of the composite product. This study is aimed to study the electrical properties of nano and micro filled epoxy resin. SiO2 and Al2O3 nano and micro fillers are used in this research for comparison purposes. The host matrix, which is epoxy resin, is filled with nano particles, micro particles or both nano and micro particles. Micro particles and nano particles are dispersed into epoxy resin using planetary centrifugal mixing technique and degassed in vacuum. In total, seven types of materials are prepared in this study. These materials are (1) neat epoxy, (2) nanocomposite filled with 1 wt% nano SiO2 fillers, (3) micro-composite filled with 20 wt% micro SiO2 fillers, (4) micro-nano composite filled with 1 wt% nano SiO2 and 20 wt% micro SiO2 fillers, (5) nanocomposite filled with 1 wt% nano Al2O3 fillers, (6) micro-composite filled with 20 wt% micro Al2O3 fillers, (7) micro-nano composite filled with 1 wt% nano Al2O3 and 20 wt% micro Al2O3 fillers. In this study, AC electrical breakdown strength test are performed using a sphere-to-sphere setup. The thickness of all the samples is 1 mm ± 0.1 mm in accordance to the IEC standard. Surface partial discharge measurement is also performed to evaluate the surface property. © Published by Elsevier Ltd. This © 2017 2017The TheAuthors. Authors. Published by Elsevier Ltd. is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. Keywords: Nanocomposite; Micro-composite; Electrical Breakdown; Surface Partial Disacharge; Epoxy Resin.

* Corresponding author. Tel.: +61 3 9925 2101; fax: +61 3 9925 2006. E-mail address: [email protected]

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 1st International Conference on Energy and Power. doi:10.1016/j.egypro.2017.03.122

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Mu Liang and K.L. Wong / Energy Procedia 110 (2017) 162 – 167

1. Introduction Nanocomposite is known to provide a better electrical property than micro-composite or conventional composite. Nanocomposite exhibits an enhancement in electrical breakdown strength, a reduction in partial discharge and a longer breakdown time [1-3]. Many inorganic fillers such as SiO2, Al2O3, BN and TiO2 have been studied in the last 10 years. These studies show that conventional polymer filled with nano particles shows a significant improvement in the dielectric properties. A common explanation to this phenomenon is that nano particles have a tremendously large surface area compared to micro particles [4-6]. The nano particles create a large interfacial area between the particles and polymer matrix. This interface is considered to be a major factor in determining the performance of nanocomposite. Recent research showed that the properties of epoxy resin could be largely improved by adding a very small amount of nano BN particles [7]. The results presented in [7] also indicate that nano particles are not only effective at enhancing the neat epoxy but also highly loaded micro-composite. In this study, epoxy resin based nanocomposite, micro-composite and micro-nano-composite are prepared using planetary centrifugal mixer. AC breakdown strength and surface partial discharge are measured to evaluate the performance of the specimens. 2. Specimens Preparation and Experimental Method 2.1. Specimens preparation Nanocomposite specimens used in this study are prepared in the polymer laboratory at RMIT University. Araldite CW177 with Hardener Aradur HY2954 are used to prepare the host polymer and SiO2 nanoparticles are from Evonik Degussa. The nano-material is a hydrophilic fumed silica with a specific surface area of 300 m2/g and the average primary particle size is between 7 nm to 30 nm. Al2O3 nanoparticles are purchased from US Research Nanomaterial, the average particle size is 80 nm. Nano particles are dispersed into epoxy resin via in-situ polymerization method. Planetary centrifugal mixing technique is used to achieve a uniform dispersion. The epoxy resin is degassed at 0.1 Torr in order to remove the air bubbles caused by the mixing and dispersion process. Specimens are then post cured at 100 degree Celsius in a 1 mm thick mould for 14 hours. To fulfill the IEC requirement of electrical breakdown test, specimens are cut into squares of 20 mm × 20 mm × 1mm in dimension. The thickness of the specimens in this study varies from 0.9 mm to 1 mm. The percentage loading of both SiO2 and Al2O3 nanocomposite specimens are 1 wt% (referred as NC_S and NC_A respectively) and the percentage loading of both SiO2 and Al2O3 micro-composite specimens are 20 wt% (referred as MC_S and MC_A respectively). Both of the SiO2 and Al2O3 nano-micro-composite specimens are filled with 20% micro sized fillers and 1% nano sized fillers (referred as NMC_S and NMC_A). Neat epoxy specimens are also prepared for comparison (referred as Neat). All the samples used in this study are listed in Table.1 .

Table 1. Matrix of Samples Types. Samples Nomenclature

Percentage Loading

Fillers Type

Particle Size

Neat

N/A

N/A

N/A

NC_S

1 wt% nano fillers

Silica

7 - 30 nm

NC_A

1 wt% nano fillers

Alumina

80 nm

MC_S

20 wt% micro fillers

Silica

1 µm

MC_A

20 wt% micro fillers

Alumina

1 µm

NMC_S

1 wt% nano fillers and 20 wt% micro fillers

Silica

7 - 30 nm and 1 µm

NMC_A

1 wt% nano fillers and 20 wt% micro fillers

Alumina

80 nm and 1 µm

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Fig. 1. (left) sphere to sphere electrode setup for AC breakdown test; (right) needle to needle electrode setup for surface partial discharge test.

2.2. AC Breakdown Strength Test The samples used in AC breakdown strength test were sheets with a dimension of 20 mm × 20 mm × 1 mm. The breakdown test was carried out based on the IEC60243 standard. The electrodes were stainless steel spheres of 20 mm diameter shows in Fig.1 (left). The electrodes and samples were immersed in oil to prevent surface flashover occurring. The voltage was increased at a rate of 1 kV/s until breakdown occurs. 10 pieces of specimens are tested for each type and median value is taken to evaluate the performance of the samples. 2.3. Surface Partial Discharge Test The surface partial discharge (PD) test is carried out using Omicron MPD600 partial discharge analyzer. The MPD system consists of a measurement unit, a USB controller and the analysis software. The measurement is carried out conforming to IEC60270 standard and the arrangement is shown in Fig.2. The PD threshold is set at 5pC to filter the background noise from the transformer and the test system. The test is carried out in a needle-to-needle electrode system as shown in Fig.1 (right).

Fig. 2. MPD 600 partial discharge measurement set-up.


Fig. 3. Electrical breakdown strength of nano and micro filled epoxy resin

3. Experimental Results 3.1. AC Breakdown Strength Test Results The AC breakdown strength and the Weibull probability plots of breakdown strength for SiO2 and Al2O3 specimens are presented in Fig. 3 and Fig. 4 respectively. Generally, it is expected that a decrease in breakdown strength is observed when micro sized particles are mixed with the epoxy resin [8]. This is due to the presence of defects such as voids and agglomeration produced in the mixing procedure [9]. However, both of the SiO2 and Al2O3 micro-composite specimens in our work shows a slight increase in breakdown strength when compare with the neat epoxy specimens. The enhancement in breakdown strength is 1.7% and 2.3% respectively for SiO2 and Al2O3 specimens. This could be due to better dispersion technique used in the sample preparation and the high vacuum degassing procedure. The thickness of the specimens is also a factor that can affect the results. When the specimens are pressed to thin film (less than 200 µm in thickness [9]), a few tiny voids in the specimen could significantly promote the tree propagation and eventually lead to a total breakdown. The specimens used in our works are 1 mm in thickness. Small number of voids will not affect the performance. Instead, the interfaces between particles and epoxy resin matrix become the dominating factor that improves the dielectric property of the material.

Fig. 4. Weibull probability plot for AC breakdown strength test of Al2O3 (left) and SiO2 (right) specimens.

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Mu Liang and K.L. Wong / Energy Procedia 110 (2017) 162 – 167 Table 2. Weibull distribution scale (α) and shape (β) parameters for ac breakdown values. Samples Nomenclature

Scale parameters (α)

Shape parameters (β)

NE

33.04

45.27

NC_A

33.59

64.16

MC_A

34.32

36.04

NMC_A

35.08

32.44

MC_S

33.71

15.18

NC_S

34.25

18.56

NMC_S

35.03

29.48

The breakdown strength of NC_A sample increased from 32.45kV to 34.05kV compared with neat epoxy and it is higher than the 33.7kV of NC_S sample. Nano-micro-composite specimens show a further improvement in breakdown strength. The breakdown strengths of NMC_S and NMC_A are 34.45kV and 34.6kV respectively. From this results, we can see that the breakdown strength of NMC_S sample is higher than NMC_A. One of the possible reasons is that the size of SiO2 nano particles is smaller than Al2O3 nano particles (e.g. 30nm vs 80nm). Smaller particles can be better dispersed and distributed more evenly between the micro particles and epoxy resin matrix. Table 2. Compares the Weibull distribution scale and shape parameters of the two groups of samples. Compared with Al2O3 filled samples (β varies from 32 to 64), SiO2 filled samples have a relative larger shape parameters (β varies from 15 to 29). The results from scale parameter α are identical to the results from median values. 3.2. Surface Partial Discharge Test Results The surface PD results at voltage of 5kV over a period of 90 seconds are recorded using Omicron analysis software. Dissipation current and PD counts when 5kV voltage is applied to a needle-to-needle arrangement are shown in Fig.5. The results showed that nanocomposite and micro-composite have a better resistance against surface PD. 1% Al2O3 nanocomposite (NC_A) shows the best performance in this group of tests. The dissipation current of NC_A drops significantly from 28nC/s to 0.538nC/s and PD counts from 1613 PDs/s to 5.178 PDs/s when comparing to the neat epoxy sample. The improvement of SiO2 filled specimens is also noteworthy but not as good as Al2O3 filled specimens. The NC_S sample also produces the best results compare to MC_S and NMC_S. The performance of micro-composite is slightly worse than nanocomposite but still much better than neat epoxy. It can be concluded that the order of resistance against surface PD is as follows: nanocomposite >> nano-microcomposite >> micro-composite >> neat epoxy. While unlike Al2O3 filled specimens, the SiO2 filled nano-microcomposite shows a poorer surface PD resistance than micro-composite. This is probably caused by the hydroscopic property of the SiO2 particles. Comparing with micro-composite specimens, the surface PD results of nano-microcomposite are mixed. It can be concluded that Al2O3 specimens (NMC_A) show a decrease in surface PD after adding nano particles into micro-composite (MC_A). But SiO2 specimens (NMC_S) show a reverse results, surface PD increased after nano SiO2 particles are mixed with the micro-composite (MC_S). This observation is probably related to the hydroscopic properties of the SiO2 particles.

Fig. 5. Surface partial discharge over 90 seconds, PD counts (left) and dissipation current (right).


4. Conclusion In this study, neat epoxy and 6 other types of composite samples are prepared. AC breakdown strength and surface partial discharge are investigated. All of the filled specimens show an improvement in the dielectric strength. The breakdown strength of nano-micro-composite increased from 32.45 kV to 34.45kV (Al2O3 specimens) and 34.6kV (SiO2 specimens). Significant reduction in the surface PD counts and dissipation current are observed in the nanocomposite. However, the breakdown strength of the specimens filled with SiO2 and Al2O3 particles shows no significant difference. In the surface PD measurement, Al2O3 specimens showed a better resistance against surface discharge. The experimental results can be concluded as follows. (1) A higher breakdown strength is recorded from nanocomposite samples compare to neat epoxy. (2) Micro-composite showed an improvement in breakdown strength due to the improvement in dispersion using centrifugal mixing method and high vacuum degassing procedure. Better dispersion created larger interface area between the particles and epoxy resin matrix and eventually leads to an increase in breakdown strength. (3) Adding nano particles into micro-composite can further increase the breakdown strength of the material. Similar results are observed in the breakdown strength test of Al2O3 and SiO2 particles. (4) A significant improvement in surface PD resistance is observed on the nanocomposites. Al2O3 nano fillers successfully reduced both of the dissipation current and PD counts by more than 99% at 5kV. The SiO2 samples also reduced the dissipation current and PD counts by 66.4% and 92.9% respectively. Acknowledgements Thanks are due to RMIT polymer laboratory for the sample preparation. Thanks are also due to Omicron for providing the MPD600 partial discharge analyzer and the support from RMIT high voltage laboratory. References [1] Li Z., Okamoto, K., Ohki, Y., Tanaka, T. Effects of nano-filler addition on partial discharge resistance and dielectric breakdown strength of Micro-Al2O3 Epoxy composite. IEEE Transactions on Dielectrics and Electrical Insulation; 2010. 17(3). p. 653-661. [2] Li Z., Okamoto K., Ohki, Y., Tanaka, T. The role of nano and micro particles on partial discharge and breakdown strength in epoxy composites. IEEE Transactions on Dielectrics and Electrical Insulation; 2011. 18(3). p. 675-681. [3] Wang Z, Iizuka T, Kozako M, Ohki Y, Tanaka T. Development of epoxy/BN composites with high thermal conductivity and sufficient dielectric breakdown strength part II-breakdown strength. IEEE Transactions on Dielectrics and Electrical Insulation; 2011. 18(6). p.1973-83. [4] Lewis TJ. Interfaces: nanometric dielectrics. Journal of Physics D: Applied Physics; 2005. Jan 6.38(2).202. [5] Roy, M., J. K. Nelson, R. K. MacCrone, L. S. Schadler, C. W. Reed, and R. Keefe. Polymer nanocomposite dielectrics-the role of the interface. IEEE Transactions on Dielectrics and Electrical Insulation; 2005. 12(4). p. 629-643. [6] Tanaka T. Dielectric nanocomposites with insulating properties. IEEE Transactions on Dielectrics and Electrical Insulation; 2005. 12(5).91428. [7] Tsekmes IA, Morshuis PH, Smit JJ, Kochetov R. Enhancing the thermal and electrical performance of epoxy microcomposites with the addition of nanofillers. IEEE Electrical Insulation Magazine; 2015. 31(3). p. 32-42. [8] Li Z, Okamoto K, Ohki Y, Tanaka T. Role of nano-filler on partial discharge resistance and dielectric breakdown strength of micro-Al2O3/epoxy composites. In2009 IEEE 9th International Conference on the Properties and Applications of Dielectric Materials; 2009. 19. p. 753-756. [9] Fang L, Wu C, Qian R, Xie L, Yang K, Jiang P. Nano–micro structure of functionalized boron nitride and aluminum oxide for epoxy composites with enhanced thermal conductivity and breakdown strength. RSC Advances; 2014. 4(40).21010-7.

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