THEORETICAL AND EXPERIMENTAL INVESTIGATIONS ON DIELECTRIC PROPERTIES OF EPOXY AND XLPE NANOCOMPOSITES D.KAVITHA

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1 Synopsis of the PhD Thesis THEORETICAL AND EXPERIMENTAL INVESTIGATIONS ON DIELECTRIC PROPERTIES OF EPOXY AND XLPE NANOCOMPOSITES by D.KAVITHA Department of Electrical and Electronics Engineering Amrita School of Engineering Coimbatore , Tamil Nadu, India. U N I V E R S I T Y April 216 Ettimadai P.O., Coimbatore , Tamil Nadu, INDIA. Phone :+91 (422) 26

2 1. Introduction Polymeric dielectric materials are widely being used in both indoor and outdoor insulation of power apparatus. Epoxy resins are well-known polymer dielectric materials with superior electrical and mechanical properties and are used in the insulation systems of heavy electric equipments such as bushing, transformer etc [L.Hammerton, 1996]. For underground cable insulation, XLPE is the material of choice due to their distinct advantages viz, lighter weight, better electrical and thermal properties, less maintenance, easier terminating and jointing procedure etc [H.Orton et al, 213]. The dielectric strength of an insulating material is defined as the maximum stress which the material can withstand. In these insulations, high electric stress concentration leads to partial discharge (PD) which further leads to formation of electrical treeing. Treeing refers to the degradation of the dielectric material forming tree like breakdown path through the volume of the insulating materials. Moisturized atmosphere (water, salt, pollution, etc.) in combination with electrical stress causes water treeing in the cable insulation. Electrical and water treeing cause breakdown of the insulation material in a relatively short time period. Many efforts have been made in the past few years to improve the dielectric properties of polymeric insulation materials. It is observed that the breakdown voltage of insulations can be extended by dispersing nanomaterials in the base matrix. In addition, incorporation of nanoparticles into a polymer matrix improves the dielectric and mechanical properties and ageing behavior. The term nanodielectric was proposed in 24 to explore nanometric dielectric system associated with nanotechnology. By definition, nanodielectric is a multi component dielectric system possessing nanostructure in which the presence of nanostructure leads to change in one or several of its dielectric and other properties. Nanofillers have high surface area to volume ratio and hence the nanocomposites will have larger interfacial regions, and such regions play an important role in determining the properties of the nanocomposite. The size of the nanofiller, its shape, concentration and dispersion directly influence the properties of the polymer nanocomposites [J.K.Nelson et al, 211]. In this work, it is proposed to identify the best nanofiller that can be used for epoxy and XLPE base materials. Experimental analysis is required to establish the improvement on dielectric properties of nanocomposites, whereas a theoretical study helps in selecting suitable nanofiller and their weight percentage to conduct experimental studies. 1

3 2. Literature survey Theoretical studies of electric field stress on polymer nanocomposites can open up new avenues for the choice of nanofillers with diverse permittivity and its concentration. The nanofiller added into the base polymer alter the electric field stress and consequently electric tree growth. This work is concerned with dielectric properties and treeing phenomena in polymer nanodielectric materials. Hence, it is important to study the electric field distribution in dielectric in detail by theoretical analysis. The major factor influencing tree growth and breakdown is the non-uniformity of electric field distribution, which in turn depends on the applied voltage, frequency, material property etc. The electric field distribution in the volume of the dielectric material, when subjected to external applied voltage, can be determined by using Laplace s equation. A.El-Zein et al in 29 and J. Lee et al in 1992 simulated the electrical tree growth for different electrode system based on electric field concentration using Finite Element Method (FEM). Theoretical analysis of electric field distribution in nanocomposite based on the influence of filler type and its size, shape and composition towards electrical tree growth and water tree behavior are not much available in the literature. Most of the literature on the treeing phenomena is based on experimental studies. In addition, the effect of inherent permittivity and shape of nanofiller on resistance to tree penetration are not discussed in previous studies. So, a theoretical study helps to select the suitable nanofiller to be used in polymer nanocomposites for enhanced dielectric properties. Epoxy nanocomposites have become popular and have attracted attention from researchers for electrical insulation in high voltage applications. S. Singha et al in 211 have discussed the experimental studies on dielectric properties of epoxy nanocomposites which exhibit advantages in both mechanical, dielectric and thermal properties as compared to unfilled and microcomposites of epoxy resin. The relative permittivity of nanocomposites appeared to have a minimum value with a threshold filler concentration. When the filler content is above the threshold value, the permittivity of nanocomposites begins to increase with filler concentration [J.K.Nelson et al, 29 and M.Kozako et al, 25]. In another study, it is seen that the relative permittivity and tan delta values of nanocomposites containing 1wt% nanotio 2 are higher than that for unfilled epoxy [S.Singha et al, 28]. Research on AC breakdown characteristics of epoxy nanocomposite shows that the breakdown voltage (BDV) of nanofilled epoxy is higher than neat epoxy and that there is a certain value of threshold weight percentage of nanofiller above which the breakdown voltage decreases [J.C.Fothergill 2

4 et al, 24 and P. Preetha et al, 211]. In similar results reported, it can be seen that the breakdown voltage is increased in epoxy nanoclay nanocomposite with increase in nanoclay content up to 5 wt% loading, under AC and DC voltages [R. Sarathi et al, 27 and A. Guevara et al, 214]. In another study, it is shown that the partial discharge resistance of nanoclay composites is greater than unfilled epoxy [T.Tanaka, et al, 27]. The size and inherent properties of the nanoparticles added to the base material and their percentage loading have a large impact in the modification of electric field and tree growth [T. Tanaka, et al, 26, S. Raetzke et al, 29 and W. Yang et al, 213]. XLPE cables are widely being used these days for the insulation purposes of high voltage cables owing to their superior electrical and thermal properties. However, inhomogeneity and defects in the cables are inevitable during cable production and laying processes. Besides this, the cables in service are subjected to electrical, thermal, chemical and environmental effects which would lead to the ageing of the cable and ultimately reaching the end of its life [H.Orton, 213]. Hence evaluation of the insulation state of the cable and predicting its residual life are of key importance. One of the main causes of insulation breakdown in medium voltage outdoor XLPE cable insulation is due to water treeing. The initial studies reported on XLPE are on water trees with micro-cavities of dimensions of the order of 1µm or less. It is observed that crosslinking of materials lengthened the time for water tree growth, thereby improving the cable reliability [Le Hui et al, 213]. Researches show that addition of particles on nanometric scale to XLPE improves the electrical, mechanical and thermal properties as well as resistance to water tree growth of the composite [T. Tanaka et al, 211 and Min-Hae Park et al, 213]. As mentioned above, there are many studies on various dielectric properties of polymer nanocomposites with different types of nanoparticles. However, there are no reported literature on a comparative study giving the effect of the filler permittivity, nanoparticle shape and interparticle distance. Some of the aspects discussed below have not been explored in detail and the author feels that there is scope to do the studies on following topics. Theoretical analysis on electric field calculation of nanocomposites for determining the influence of filler size, shape, concentration and its consequence on electrical tree growth and field analysis. Experimental analysis on dielectric properties of polymer nanocomposites and comparison with the theoretical analysis. 3

5 Experimental analysis on partial discharge (PD) characteristics and treeing in polymer nanocomposites under different service conditions. 3. Objective From the literature survey, it is observed that the size, shape and percentage loading of nanoparticles have significant impact on the dielectric behavior of polymer nanocomposite. Hence, the objective of this work is to select suitable nanoparticle and its optimum weight percentage loading to enhance the dielectric properties of Epoxy and XLPE nanodielectrics. The proposed work would meet the following objectives. Development of simulation models based on Finite Element Method (FEM) to predict electrical stress at interface and treeing phenomenon in nanocomposites. Prepare nanocomposites with fillers having various size, shape and various concentrations. Experimental study on material characterization and dielectric properties such as relative permittivity, tan delta, dielectric strength and treeing behavior of epoxy and XLPE nanocomposites as a function of nanofiller type and concentration. Identification of the best nanofiller and their percentage content in polymer to give the optimum performance. 4. Theoretical analysis on electric field distribution of polymer nanocomposite The properties of the nanocomposites are governed by the dispersion of nanoparticles and its interaction with base polymer. Hence it is important to understand the inter particle distances between nanoparticles as a function of particle sizes, shape as well as filler surface area. The inter particle distance D, between nanoparticles for various weight percentages is calculated by equation (1) [Tanaka et al, 27]. Figure 1 represents calculated inter particle distance in epoxy nanocomposites. D= π 6 ρ m 1 1 wt % ρ n wt % 1 1 ρ m ρ n 1/3 1 d (1) where, ρ n Specific gravity of nanoparticle, ρ m Specific gravity of epoxy resin, d diameter of the nanoparticle. The inter particle distance reduces as the particle wt% increases and then it become difficult to control the agglomeration of particles in the base matrix. In all the nanocomposites, at around 15wt%, the inter particle distance becomes 4

6 almost equal to the size of the particle itself. Hence in the present study, up to 15wt% concentration is used for all 4 types of nanoparticles. Figure 1. Calculated inter particle distance in epoxy nanocomposites The effect of filler permittivity and its concentration on dielectric strength and probability of tree growth are studied based on the value of electric field stress at interface region in epoxy and XLPE nanocomposite using COMSOLv 4.2 simulation software. According to the calculated inter particle distance the nanoparticles are placed in base polymer model. Figure 2 shows the electric field stress distribution in epoxy with Al 2 O 3 and TiO 2 nanocomposites and for two different concentration viz. 2.5wt% and 15wt%. Figure 2. Electric field distribution (V/m) in (a) epoxy 2.5wt% TiO 2 nanocomposites, (b) epoxy 2.5wt% Al 2 O 3 nanocomposites (c) epoxy 15wt% TiO 2 nanocomposites, (d) epoxy 15wt% Al 2 O 3 nanocomposites. 5

7 It is observed that, the maximum stress occurs at the interface because of the change in permittivity and there is a clear reduction within the nanoparticles. The volume of stressed region and enhancement in field mainly depends upon the filler permittivity and its concentration. Since TiO 2 has the highest inherent permittivity among the fillers used, maximum field stress at the interface is observed in epoxy- TiO 2 composite as shown in Figure 2. The enhancement in field at the interface will reduce the breakdown strength of the composite at higher wt% loading. 5. Experimental analysis of epoxy and XLPE nanocomposites A Bisphenol-A type epoxy resin (CY 179, density 1.16g/cm 3 ) along with a Triethylene Tetramine (TETA) hardener (HY956, density 1.2g/cm 3 ), supplied by Huntsman are used for the preparation of epoxy nanocomposites. Nanoparticle is weighed according to the weight percentage of sample to be prepared and it is mixed with the calculated amount of epoxy resin. The mixture is ultra sonicated for one hour to uniformly disperse the nanoparticle in the epoxy resin. The calculated amount of hardener is added and it is manually stirred for five minutes. This epoxy nanocomposite solution is poured into a mould of dimension 4mm 4mm 2.5mm. This is allowed to dry for 24 hours in room temperature and is kept in hot air oven for one hour at 6 C and then cured. To prepare XLPE nanoclay nanocomposites polyethylene pellets, peroxide type crosslinking agent supplied by KLJ Polymers & Chemicals Ltd and nanoclay (Nanomer-1.3E 25-3 wt% of Octyldecyl amine salt and d 1 : 2.29 nm, Sigma Aldrich) are mixed in Haake Rheocord 9 at 6 rpm and 19 C temperature for 2 minutes. The nanocomposites are cured in a water bath at 9⁰C for three hours and moulded in hydraulic compression press at 175 kg/cm 2 and 2 C Material characterization of epoxy and XLPE nanocomposites The study of material characterization is equally important as that of electrical properties. Various material characterizations used in this thesis include Scanning Electron Microscope (SEM), Energy Dispersive Spectroscopy (EDS), X-ray Diffraction (XRD), Fourier Transform Infra Red (FTIR) and Differential Scanning Calorimetry (DSC) techniques. Figure 3 (a) and Figure 3 (b) shows the SEM analysis of epoxy with 5wt% and 15wt% loading of Al 2 O 3 nanoparticles. It is observed that the nanoparticles are 6

8 dispersed properly in 5wt% loading than 15wt% of epoxy Al 2 O 3 nanocomposites. At higher wt%, inter particle distance becomes near to particle diameter which results in agglomeration and further increase in electric field at interface. Figure 3 (c) shows the XRD pattern for XLPE nanoclay nanocomposites. The absence of peaks at 2θ of 4.5 indicates that the layered structure of nanoclay is destroyed and layers are exfoliated in the polymer. However, small peak is observed at 2θ of 6.7 for 7.5wt% and 1wt% nanoclay nanocomposites. These peaks indicate the intercalation and slight agglomeration. Figure 3 (d) shows the FTIR spectrum of XLPE with 5wt% nanoclay. Each peak in the FTIR spectrum at a particular wavelength is associated with a specific functional group. The following points are analyzed from various material characterization conducted in this thesis, (i) size and shape of the fillers, (ii) dispersion of the fillers in the polymer matrix (iii) composition of fillers and (iv) chemical structure in nanocomposites and thermal stability of the materials for all prepared samples. Figure 3. SEM analysis of Epoxy (a) 5wt% and (b) 15wt% Al 2 O 3 nanocomposite. (c) XRD pattern for XLPE nanoclay nanocomposites, (d) FTIR of XLPE 5wt% nanoclay 6. Experimental analysis on dielectric properties of epoxy nanocomposites The important dielectric properties of insulators are dielectric strength, relative permittivity, tan delta, etc. These properties are very important as they play an important role in deciding which dielectric would serve the purpose for specific applications. In this section, the effects of inherent properties of nanoparticle and weight percentage loading on dielectric properties of epoxy nanocomposites are studied. 7

9 Relative Permittivity 6.1. Effect of filler permittivity and filler concentration on relative permittivity of epoxy nanocomposites The permittivity and tan delta measurements are performed using PRECISION LCR METER175 in the frequency range of Hz. The measured data presented here are an average value of 6 samples and measurement error is within 2%. Figure 4 shows the relative permittivity of all four nanocomposites at 2 khz. It is clear that the relative permittivity of the nanocomposite increase with increase in filler concentration. Addition of nanoparticles provides additional bound charges at the interface region which influences the relative permittivity [S. Singha et al, 28]. It can be seen that the filler permittivity has a significant effect on the relative permittivity of nanocomposite. As mentioned, four different nanoparticles with a large variation in their permittivity are used in this study. Due to the lower inherent permittivity of nanoclay (~3.1) compared with other nanoparticles, the relative permittivity of the epoxy nanoclay nanocomposite is less compared to other fillers. Among these four nanoparticles TiO 2 (~1) nanoparticle has the highest value of permittivity. Hence, the relative permittivity of epoxy TiO 2 nanocomposite has higher value than other nanocomposites. Also it is observed that the relative permittivity of epoxy CaCO 3 and Al 2 O 3 nanocomposite have nearly the same value at all wt% loading, because the inherent permittivity of Al2O 3 (~9.1) and CaCO 3 (~8.19) are nearly equal Figure 4. Comparisons of relative permittivity LLLLoading epoxy loading nanocomposites with respect to filler permittivity at 2 khz Dielectric loss 5 1 Filler wt% Loading Epoxy TiO 2 NC Epoxy Al 2O 3 NC Epoxy CaCO 3 NC Epoxy nanoclay NC Loss tangent or tan delta depends mainly upon the electric conductivity in the epoxy nanocomposites. The tan delta of any material describes quantitatively, the dissipation of electrical energy due to various physical processes such as electrical conduction, dielectric relaxation or polarization, dielectric resonance and losses from nonlinear processes. It is observed from Figure 5 that tan delta marginally increased with increase in frequency

10 Tan Delta Tan Delta Tan Delta Tan Delta Nevertheless, in all the conditions, the tan delta values are very low (<.8) and well within the range of that for insulators. Compared with other nanofillers, nano-tio 2 has higher inherent permittivity which introduce more additional bound charges into the epoxy TiO 2 nanocomposite at all wt% loading. Due to this reason there is an enhancement in the tan delta value in epoxy TiO 2 nanocomposite than other type of nanocomposites Unfilled 2.5% nano 5% nano 7.5% nano 1% nano 15% nano Unfilled 2.5% nano 5% nano 7.5% nano 1% nano 15% nano x 1 5 Frequency [Hz] x 1 5 Frequency [Hz] (a) Unfilled 2.5% nano 5% nano 7.5% nano 1% nano 15% nano (c) Unfilled 2.5% nano 5% nano 7.5% nano 1% nano 15% nano x 1 5 Frequency [Hz] (b) x 1 5 Frequency [Hz] (d) Figure 5. Variation of Tan Delta with respect to frequency in Epoxy nanocomposite (line connect the experimental data values). (a) Epoxy TiO 2 nanocomposite, (b) Epoxy Al 2 O 3 nanocomposite, (c) Epoxy CaCO 3 nanocomposite and (d) Epoxy Nanoclay nanocomposite Dielectric strength The AC breakdown studies are performed on unfilled epoxy and epoxy nanocomposites using a high voltage test kit as per standard [ASTM-D149-97a, 24]. It is observed as shown in Figure 6 that there is an increase in breakdown strength initially with filler concentration in all the 4 different composites under study. This may be due to the fact that, as the filler concentration increases, the volume fraction of polymer layer reduces and nanoparticles act like a barrier to the flow of conduction path. It can also be seen that, among four types of nanocomposites, epoxy- nanoclay nanocomposite has high value of dielectric strength with all wt% loading. The dielectric strength value of epoxy TiO 2 nanocomposite is found to be lower than that of other epoxy nanocomposites. The theoretical analysis on electric stress in section 4 has shown that the stress at interface is maximum in epoxy TiO 2 nanocomposite compared to all other composite and hence the lower dielectric strength. 9

11 PD Magnitude(pC) PD Magnitude(pC) Figure 6. Variation of dielectric strength of epoxy nanocomposites Resistance to Partial Discharge (PD) PD depends upon various factors like void location, size of the void and material filled in void. Even in identical samples prepared in same atmospheric condition, PD patterns differ. Figure 7 show the average PD magnitude of unfilled epoxy, epoxy TiO 2 and nanoclay nanocomposites with 2.5wt% and 5wt% respectively as a function of applied voltage. The average discharge magnitude is found to be lowest for nanoclay filled nanocomposite. 2 Epoxy 15 Epoxy Epoxy 2.5wt% TiO2 NC Epoxy 5wt% TiO2 NC 1 5 Epoxy 2.5wt% Nanoclay NC Epoxy 5wt% nanoclay NC 1 2 Voltage(kV) (a) 1 Voltage(kV) 2 (b) Figure 7. PD magnitude(pc) in dry (a) epoxy and epoxy 2.5 and 5wt% TiO 2 nanocomposites (b) epoxy and epoxy 2.5 and 5wt% nanoclay nanocomposites as a function of applied voltage Resistance to electrical tree growth Electrical treeing is the breakdown phenomenon due to partial discharge with the presence of micro size voids in the dielectric material. The name electrical is used for electrical treeing to indicate that the processes are due to only the electric field in association with a void. A comparative study is carried out on unfilled epoxy, epoxy 5wt% TiO 2 nanocomposite and 1

12 Length of Electrical Tree(µm) epoxy 5wt% nanoclay nanocomposite to study the effect of shape of nanofiller on resistance to electrical tree growth. The experiments are conducted by applying a constant ac voltage of 3kV, 5Hz between the needle and the plane electrode and the samples are placed inside a test cell containing transformer oil to prevent surface flash over at room temperature. The tree initiation time for epoxy nanocomposite is more than that of unfilled epoxy samples. Length of the electrical tree growth is measured in vertical direction between the tip of the needle electrode and the opposite plane electrode. Addition of nanofillers act as obstacle into the base polymer matrix which further restricts the tree path and increase the tree propagation period. Figure 8 shows the propagation of electrical tree growth after tree initiation. It is observed that the electrical tree length after 2 hours in unfilled epoxy is 26µm. For the same time duration, the length of tree in epoxy nanoclay composites is measured to be only 5 µm. The electrical tree growth is prolonged when suitable nanofiller with optimum weight percentage loading is added into base epoxy materials Unfilled Epoxy 2 Epoxy 5wt% Nanooclay NC 15 Epoxy 5wt% TiO2 NC Time(hours) Figure 8. Propagation of electrical tree growth after tree initiation 7. Experimental analyses on dielectric properties of XLPE nanocomposite This section describes the effect of nanoclay content on dielectric properties of XLPE nanoclay nanocomposites Effect of filler permittivity and filler concentration on relative permittivity and tan delta of XLPE nanocomposite The dielectric properties such as permittivity and tan delta of the XLPE nanoclay nanocomposites are measured using LCR meter as described in section 6.1. Figure 9 (a) shows the variation of relative permittivity of XLPE nanoclay nanocomposites with respect to frequency at different filler concentrations. It is seen that the relative permittivity of 11

13 nanocomposites is increased with increase in filler concentration. The relative permittivity of the composite varies from 2.2 to 3.5 for various weight percentages loading of nanoclay. Figure 9. (a) Variation of Permittivity with respect to frequency in XLPE nanoclay nanocomposites and (b) Variation of Tan delta with respect to frequency in XLPE nanoclay nanocomposites (line connect the experimental data values) Figure 9 (b) shows the value of tan delta as a function of frequency for different loading of nanoclay. It is observed that on addition of nanofillers, tan delta decreases initially and then increases up to 5 wt%. For all wt%, the tan delta values are less than.2 and well within the range of that for insulators Dielectric strength The AC breakdown studies are performed on unfilled XLPE and XLPE nanoclay nanocomposites using a high voltage test kit as mentioned in section 6.3. The dielectric strength test data is presented in the form of Weibull plots. Figure 1 shows the variation of Weibull Scale parameter (α) with percentage loading. It is observed that the breakdown strength increases upto 5wt% loading and after that it reduces and the reduction in breakdown strength is attributed to the non-uniform dispersion of nanomaterials. 7.3 Resistance to water tree growth Here the resistance to water tree growth in unfilled and XLPE nanoclay nanocomposites is studied. Water trees are measured along the direction which is perpendicular to the ground plate. Figure 11 shows the measured value of average water tree length after being stressed at 5kV, 5 khz for 24 hours. It is observed that XLPE nanoclay nanocomposites shows decreased tree length compared with unfilled XLPE. In unfilled XLPE the water tree grows more towards the ground plane under the applied voltage, but in nanoclay nanocomposites, the water trees have a wider fan-shaped structure. Layered structure of nanoclay filler has 12

14 added advantage towards the resistance to water tree growth. The length of tree is lowest in 5wt% loading compared with other wt% loading. 8. Conclusions Theoretical investigation and experimental validation are conducted to analyse the dependence of filler parameters on the dielectric properties of epoxy and XLPE nanocomposites. Simulation studies show that the field enhanced region increases at interface with the increase of filler concentration which leads to degradation in the dielectric strength. It is observed that the electric field stress is influenced by inherent permittivity of the nanoparticle and percentage weight rather than size and shape of the nanoparticle. The dielectric properties of epoxy nanocomposites with four types of fillers viz. TiO 2, Al 2 O 3, CaCO 3 and nanoclay with various weight concentrations are fabricated and analyzed. From experiments it is demonstrated that the dielectric properties viz. relative permittivity, tan delta and breakdown strength of the nanocomposite mainly depends on concentration and inherent permittivity of the filler materials. In all the composites, the relative permittivity increased with the increase concentration of nanoparticles, because of the higher permittivity of nanofillers. The nanocomposite permittivity is found to be highest in epoxy TiO 2 nanocomposite, because TiO 2 has the highest inherent permittivity among the fillers used. There is an increment of about 12% in the relative permittivity at 15% of the TiO 2 filler. Tan delta of the nanocomposites for all the fillers and for all the concentration is found to be well within the range of insulators. Breakdown strength is increased with the addition of nanofillers and it is the highest in nanoclay nanocomposites because nanoclay has the lowest inherent permittivity. Approximately 5% increment in breakdown strength is observed in epoxy-nanoclay composite at 1 wt% loading. Above this concentration, the inter particle 13

15 distance becomes almost equal to the particle diameter which causes a decrease in breakdown strength. The experimental analysis is made to identify a promising nanofiller to enhance the partial discharge and electrical treeing resistance in epoxy nanocomposites. It is found that nanoclay filled epoxy nanocomposites has lower value of PD magnidute than nanotio 2 filled epoxy nanocomposite due to its lower inherent permitivity of nanoclay. Also nanoclay provides a longer path for electrical tree due to the layerd structure. It is observed that the composite with 5wt% of nanoclay fillers has the maximum dielectric strength in XLPE nanoclay nanocomposite. Beyond 5wt%, the reduction in dielectric strength may be attributed to the agglomeration of nanoclay fillers. The growth rate of water tree in XLPE nanoclay nanocomposite is retarded upto 8 % at 5wt% loading of nanoclay. 8.1 Scope for future work In this work, the effect of inherent permittivity, concentartion, size and shape of the filler nanoparticles on the dielectric properties of a base polymer matrix is demostrated. The effect of combination of two or more types of nanofillers on dielectric properties of polymer nanocomposites could be investigated. Also, since the addition of high permittivity nanoparticles gives higher permittivity of the composite at the expense of lower breakdown strength, suitable nanoparticles for energy storage application could also be identified. References 1. ASTM-D149-97a (reapproved 24), Standard Test Method for Dielectric Breakdown Voltage and dielectric strength of solid Electrical Insulating Materials at Commercial Power Frequencies. 2. A.El-Zein, M.Talaat, M.MEl Bahy, "A Numerical Model of Electrical Tree growth in Solid Insulation", IEEE Trans. Dielectr. Electr.Insul, Vol.16, pp , H.Orton, "History of underground power cables", Electrical Insulation Magazine IEEE, Vol. 29, pp , J.C.Fothergill, J.K.Nelson and M.Fu, "Dielectric Properties of Epoxy Nanocomposites containing TiO 2, Al 2 O 3 and ZnO fillers", IEEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), pp , J.K.Nelson, W.Zenger, R.J.Keefe and L.S.S.Feist, "Nano structured Dielectric Composite Materials", U.S. Patent 7,884,149, J.Lee, G.Park, D.Y.Young, S. Kim, S.Hahn and M.Han, "An accurate method for numerical simulation of electrical tree growth process by finite element method", IEEE Intern. Sympos. Electr. Insul. (ISEI), pp. 7-73, L.Hammerton, "Recent Developments in Epoxy Resins", RAPRA Review Reports, Vol. 8, No.7,

16 8. L. Hui, L. S. Schadler and J. K. Nelson, "The Influence of Moisture on the Electrical Properties of Cross linked Polyethylene/Silica Nanocomposites", IEEE Trans. Dielectr. Electr. Insul, Vol. 2, No. 2, pp , Min-Hae Park, Kee-Joe Lim, "Impulse Breakdown Strength of Nano-ZnO/XLPE Nano composite Material on Temperature Rise", Modeling and Numerical Simulation of Material Science, Vol.3, pp , M.Kozako, R.Kido, T.Imai, T.Ozaki, T.Shimizu, T.Tanaka, "Surface roughness change of Epoxy/TiO 2 nanocomposites due to partial discharges," Electrical Insulating Materials Vol.3, pp. 661, P.Preetha and M.J.Thomas, "AC breakdown characteristics of epoxy nanocomposites", IEEE Trans. Dielectr. Electr.Insul, Vol.18, No.5, pp , R.Sarathi, R.K.Sahu and P.Rajeshkumar, "Understanding the electrical, thermal and mechanical properties of epoxy nanocomposites", Materials Science and Engineering:A, Vol , pp , S.Singha and M.J.Thomas, "Permittivity and Tan Delta Characteristics of Epoxy Nanocomposites in the Frequency Range of 1MHz-1GHz", IEEE Trans. Dielectr. Electr. Insul, Vol.15, No.1, pp.2-11, S.Singha and M.J.Thomas, "Dielectric Properties of Epoxy Nanocomposites", IEEE Trans. Dielectr. Electr. Insul.,Vol. 15, No.1, pp , S.Raetzke, Y.Ohki, Takahiro Imai, Toshikatsu Tanaka and Josef Kindersberger, "Tree Initiation Characteristics of Epoxy Resin and Epoxy/Clay Nanocomposite", IEEE Trans. Dielectr. Electr.Insul, Vol. 16, No 5, pp , T.Tanaka et al, "Dielectric Properties of XLPE/SiO 2 Nanocomposites Based on CIGRE WG D1.24 Cooperative Test Results", IEEE Trans. Dielectr. Electr.Insul, Vol. 18, No. 5, pp , T.Tanaka, A.Matsunawa, Y.Okhi, M.Kozako, M.Kohtoh, and S.Okabe, "Treeing Phenomena in Epoxy/Alumina Nanocomposites and Interpretation by a Multicore Model", IEEJ Transactions on Fundamentals of Materials, Vol.126, No.11, pp , T.Tanaka, Y.Tatsuya, Y.Ohki, M.Ochi, M.Harada and T.Imai, "Frequency Accelerated Partial Discharge Resistance of Epoxy/Clay Nanocomposite Prepared by Newly Developed Organic Modification and Solubilization Methods", IEEE Conf. on Solid Dielectrics (ICSD), pp , W. Yang, X. Yang, M. Xu, P. Luo and X. Cao, "The effect of nano SiO 2 additive on electrical tree characteristics in epoxy resin", 213 Annual Report Conference on Electrical Insulation and Dielectric Phenomena, pp ,

17 PUBLICATIONS BASED ON THE RESEARCH WORK A. INTERNATIONAL JOURNAL PUBLICATIONS: 1. D.Kavitha, T.K.Sindhu and T.N.P.Nambiar, "Effect of Nanoparticles on the Dielectric Strength and PD Resistance of Epoxy Nanocomposites", Advances in Intelligent Systems and Computing, Springer, Vol.397, pp , D.Kavitha, T.K.Sindhu and T.N.P.Nambiar, "Modeling of Electric Field Distribution and Electric Tree Propagation in Epoxy Nanocomposites using Finite Element Method", Journal of Electrical Engineering, Vol.15, Edition 3, pp , D.Kavitha, T.K.Sindhu and T.N.P.Nambiar, "Investigation of treeing process in nanofilled epoxy material by finite element method", Journal of Electrical Engineering, Vol.14, Edition 1, pp , D.Kavitha, Neena Alex and Dr.T.N.P.Nambiar, "Classification and study on factors affecting partial discharge in cable insulation", Journal of Electrical Systems, Vol.9, Edition -3, pp , Reethu Mohanan MM, D.Kavitha and T.N.P.Nambiar, "AC Characteristic Study of XLPE Nano-clay Composites", International Journal of Applied Engineering Research, ISSN , Vol.1, No.2, pp , Chettiar Navin Gopalakrishnan, D.Kavitha and N. Kathiravan, "Theoretical Investigation on Electric Field in XLPE Nanocomposite using Finite Element Method", International Journal of Applied Engineering Research, ISSN , Vol.1, No.2, pp , D.Kavitha, T.K.Sindhu and T.N.P.Nambiar, "Impact of Permittivity and Concentration of filler Nanoparticles on Dielectric Properties of Polymer Nanocomposites", Communicated to Materials Science and Engineering: A, Elsevier. B. INTERNATIONAL CONFERENCE PUBLICATIONS: 8. S.Aiswarya Nair, R.Krithika, P.Monika, N.Purnimaa, D.Kavitha, "Influence of Nanofillers on resistance to water tree in XLPE Nano composite", in Proc. ICCPCT, Arjun Jayakrishnan, D Kavitha, A Arthi, Niveditha Nagarajan and MeeraBalachandran, "Simulation of electric field distribution in nanodielectrics based on XLPE", International Conference on Recent Advances in Nano Science and Technology, P.Manswini, Rithika Reddy, D Kavitha and MeeraBalachandran, Mechanical, "Thermal and Dielectric Properties of Cross-linked Polyethylene Nanocomposites", Proceedings of the Third International Conference on Polymer Processing and Characterization (ICPPC), pp 17,