2008 Annual Report Conference on Electrical Insulation Dielectric Phenomena

Electrical Discharge Resistant Characteristics of

Epoxy NanocompositesP. Preetha, Sridhar Alapati, S. Singha, B. Venkatesulu and M. Joy Thomas

High Voltage Laboratory, Department of Electrical EngineeringIndian Institute of Science, Bangalore, India 560 012

Abstract- This paper reports the electrical discharge resistantcharacteristics of epoxy nanocomposite systems with SiO2 andA1203 nano-fillers. A comparative study is performed betweenunfilled epoxy systems, nanoparticle filled epoxy systems and abimodal system containing both micrometer and nanometer sizedfillers of the same material. The samples are exposed to surfacedischarges and the levels of surface degradation are analyzedthrough SEM and surface roughness measurements. Significantvariations were observed in the electrical discharge resistantcharacteristics between the different composite systems and it isseen that the introduction of nano-fillers to epoxy is advantageousin improving the electrical discharge resistance of epoxy.

INTRODUCTION

Epoxy based insulating materials are widely used in severalhigh voltage applications and under actual operatingconditions, it is possible that these materials are exposed toelectrical discharges. These discharges in turn can causedegradation of the insulating material over a period of timethereby reducing the life of the equipment. Therefore, there is astrong need for developing electrical insulating materialspossessing good discharge resistant characteristics. Based onthe introductory article by Lewis [1], polymer basednanodielectric systems have been studied for differentinsulating properties. Among the studies reported in the lastcouple of years for the dielectric properties of polymernanocomposites [2], quite a few studies relate to theimprovements in the electrical discharge resistance of thesematerials [3, 4]. Along the same lines, this study also looks atthe electrical discharge resistant properties of two differentepoxy based composite systems - (a) nanocomposite and (b)nano-micro composite. These results have been compared withthose of pure epoxy systems. The nano-micro composite(NMC) was prepared by dispersing both micron and nano sizedfillers in the ratio 4:1. A bimodal mixture of filler materials ofdifferent sizes is considered for this study since it improves thefiller packing density in the epoxy matrix. The dischargeresistance of the examined materials is measured by analyzingthe degradation of the sample surfaces after exposure toelectrical discharges. The extent of degradation is characterizedthrough SEM and surface roughness profile studies.

SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUES

This study utilizes an epoxy system based on bisphenol-Aepoxy resin (CY1300) and hardener (HY956), both supplied byHuntsman. The nano-fillers used in the present study are

Figure 1. SEM image of an epoxy-SiO2 nanocomposite sample

commercially available A1203 and SiO2 particles with AverageParticle Sizes (APS) of 40 nm and 15 nm respectively whereasthe A1203 micrometer sized filler has an APS of 40 ztm. All thefillers are supplied by Sigma Aldrich. The epoxynanocomposite and microcomposite samples for the study areprepared based on the procedure presented in an earlierpublication [5]. The samples have a diameter of 70 mm and athickness of 3 mm and after preparation, the samples are keptin desiccators under vacuum for at least 24 hours to releaseresidual stresses if any before carrying out the surfacedischarge studies. The details of the epoxy composites used forthe present study are shown in table I. The dispersion of fillersin the epoxy matrix was analyzed using a scanning electronmicroscope (Sirion) and the image in figure 1 shows thedispersion of fillers in the epoxy matrix.

TABLE ISPECIMENS USED FOR THE EXPERIMENTAL WORK

Name Filler Filler content (wt%)Epoxy None

Alumina1Epoxy nanocomposite Silica 1

Epoxy nano- Nano-AI203 & micron-microcomposite (NMC) A1203 in the ratio of 1:4

The IEC (b) electrode system (figure 2) has been used for thestudies and the experiments were conducted in a medium of airunder an ac (50 Hz) voltage of 10 kV at three differentdurations of 5 hours, 10 hours and 24 hours. Figure 3 shows aphotograph of the experimental set-up. Before the start of theexperiments, the discharge inception voltages of all thespecimens were found out using an ERA-5 partial dischargedetector. The surface roughness of the samples which wereexposed to discharges is analyzed using SEM (Sirion make)and optical surface profilometer (Veeco make). Figure 4 showsa photograph of the sample exposed to surface discharge.

978-1-4244-2549-5/$25.00 C) 2008 IEEE 718

Rod Electrode (6nri)

Eind cuvattuie radius limm

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tluiiluess 3um)

Plane Electro& (65mmo)

Figure 2. Electrode arrangement used for the experiments

Top Electrode

Discharges

Sample

BottomElectrode

Figure 3. Photograph of the experimental set-up

Region exposed- to surfacedischarges

Figure 4. Image of the sample exposed to surface discharges

RESULTS AND DISCUSSIONS

Discharge Inception Voltage (DIV)The DIV data for all the samples are summarized in table II.

Each data corresponds to an average value of 15 experiments.Even though the DIV is not found to vary much for pureepoxy, nanocomposite and nano-micro composite, still, it isobserved that the discharge area on the sample surfaces is lessin the case of nanocomposites. This variation in discharge areawas also observed for higher durations of exposures too. Thenano-A1203 filled composite aged for 5 hours had a dischargediameter of 2.5 cm whereas the discharge area for samplescontaining micron sized fillers was 3.2 cm and for NMC it was3.5 cm.

TABLE IIDIV FOR DIFFERENT SPECIMENS

Base polymer Filler Wt % DIV (kV)

Epoxy None - 3.025

Epoxy A1203 (nano) 1 3.065

Epoxy A1203(NMMC) 1 3.13

Epoxy SiO2 (nano) 1 2.59

Surface degradation studies using SEMRepresentative SEM images of the surfaces of epoxy

composite samples before and after exposure to surfacedischarges are shown in figure 5. It can be seen from figure 5athat the surface of the unexposed unfilled epoxy sample is verysmooth whereas the surfaces of the other discharge exposedsamples look rough. The roughness on the sample surfacesindicate that the material has eroded due to the electricaldischarges. Two distinct regions can be identified on the roughSEM images - low lying areas where material erosion has takenplace and islands where the material is still intact. The fractionof island regions on the SEM image can give an indication ofthe amount of material degradation, but, in the present study,an analysis using this technique was found to be difficult forthe investigated samples. The discharge exposure durations inthe current study are not long enough to induce significantdegradation of the sample surfaces and therefore the SEMimages of all the different samples looked similar. In such asituation, the extent of material degradation can be only gaugedby measuring the surface roughness of the samples.

Surface roughness measurements using optical profilometerFigure 6 shows the representative 3D surface profiles of the

examined samples. The measure of surface roughness on thedischarge exposed region of the sample is the value of averagesurface roughness (Ra) and the Ra values corresponding todifferent types of samples at different levels of applied voltagesare presented in table III. The values of Ra in table III clearlyshow the dependence of the applied voltage and the influenceof filler particles on the surface roughness of the specimens.For pure epoxy as well as for nanocomposites, the surfaceroughness increases with increasing durations of the appliedvoltage whereas in the case of NMC, a distinct trend is notobserved. In the other important observation from table III, itcan be seen that the values of Ra in the nanocomposites withboth A1203 and SiO2 are lower than that of unfilled epoxy at allexposure durations. This means that nanocomposites haveenhanced resistance to surface degradation under dischargeexposure and this is most likely due to the strong bondingbetween nanoparticles and epoxy at the interface region whichcauses the material to hold on to the nanoparticle and resistdegradation [5]. As compared to neat epoxy andnanocomposites, the values of Ra in the case of NMC do notindicate any trend.

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(a) Unexposed unfilled epoxy specimen

(b) Exposed unfilled epoxy specimen (10 kV, 5 hours)

(c) Epoxy-AI203 (1%) nanocomposite (10 kV, 10 hours)

(c) Epoxy NMC (10 kV, 10 hours)

Figure 5. SEM images of the surface condition of the epoxy compositesamples

(a) Exposed unfilled epoxy specimen (10 kV, 5 hours)

(b) E -A(1%) 301 6 (um

(b) Epoxy-AI203 (1%) nanocomposite (10 kV, 10 hours)

S S W,P- 301 .6 um

(c) Epoxy NMC (10 kV, 10 hours)

Figure 6. 3D surface images of the epoxy composite samples

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(a) Unexposed unfilled epoxy specimen

TABLE IIIAVERAGE SURFACE ROUGHNESS RA (m)

Voltage and Epoxy Epoxy nanocomposite (1%wt.) Epoxy-time duration A1203 SiO2 A1203 NMC

5 hours 483.67 310.66 429.50 635.70

10 hours 620.71 352.38 581.70 1050

10hours24 hour 910.12 718.50 740.2 787.34

CONCLUSION

It is observed that nanofilled epoxy composites with verysmall filler loading have improved resistance to electricaldischarges and the enhancement in the discharge resistance ishigher for longer durations of discharge exposures. The NMCcomposite also shows a better performance against surfacedegradation, but only for long hours of discharge exposure (24hours). Various filler mixture compositions have to be tried outto get optimum mixture of nano and micro fillers which givesan improved discharge resistance along with other electricalcharacteristics of the polymer.

ACKNOWLEDGEMENT

The authors would like to thank the nanoscience centre forthe support in conducting the SEM studies as well as the nano-

tribology laboratory for making available the surfaceprofilometer. The authors would also like to thank Mr. RiazAhmed for the support in setting up the experimental facility.Thanks are also due to Prasad (Materials Engg.) and Rajesh(IPC) for the help in conducting SEM studies.

REFERENCES

[1] T. J. Lewis, "Nanometric Dielectrics", IEEE Trans. Dielectr. Electr.Insul., Vol.1, pp. 812-825, 1994.

[2] T. Tanaka, G. C. Montanari, G. Mulhaupt, "Polymer Nanocomposites asDielectrics and Electrical Insulation-Prospective for ProcessingTechnologies, Material Characterization and Future Applications", IEEETrans. Dielectr. Electr. Insul.,Vol.11, No.5, 2004, pp.763-784.

[3] M. Kozako, N. Fuse, Y. Ohki, T. Okamoto and T. Tanaka, "SurfaceDegradation of Polyamide Nanocomposites Caused by Partial DischargesUsing IEC(b) Electrodes", IEEE Trans. Dielectr. Electr. Insul.,Vol. 1,No.5, 2004, pp.833-839.

[4] T. Imai, F. Sawa, T. Nakano, T. Ozaki, T. Shimizu, S. Kuge, M. Kozakoand T.Tanaka, "Insulation Properties of Nano and Micro Filler MixtureComposites", IEEE Conference on Electrical Insulation and DielectricPhenomena (CEIDP), Nashville, Tennessee, USA October 16-19, 2005,pp. 171-174.

[5] S. Singha and M. Joy Thomas, "Dielectric properties of epoxynanocomposites," IEEE Trans. Dielectr. Electr. Insul., Vol..15, No. 1, pp.12-23, 2008.

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