IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 5; October 2009

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Dielectric Spectroscopy of Epoxy Resin with and without Nanometric Alumina Fillers

P. Maity1, P. K. Poovamma3, S. Basu1, V. Parameswaran1 and Nandini Gupta2

Indian Institute of Technology Kanpur, 1Department of Mechanical Engineering,

2Department of Electrical Engineering, Kanpur-208016, India.

3Central Power Research Institute, Bangalore-560080, India.

ABSTRACT

In this work, the complex permittivity of epoxy resin is measured. Earlier, we have shown that the inclusion of nanometric alumina particles (Al2O3), both as-received and pre-processed, improves the ability of a polymeric dielectric (epoxy) to resist degradation when exposed to surface discharges. In this work, we use dielectric spectroscopy to characterize neat epoxy (unfilled) and epoxy nanocomposites prepared with as-received and pre-processed Al2O3 nanoparticles. The dielectric spectroscopy measurements and analyses are carried out in the frequency range of 10-3 Hz to 103 Hz and temperature range of 25 oC to 90 oC. Analyses of the data for neat epoxy indicate the presence of low frequency dispersion below 100 Hz. It is observed that the inclusion of nanoparticles lowers the effective real and imaginary permittivity of the composite material, at low temperatures. At higher temperatures, low permittivities are exhibited only by composites prepared with particles functionalized with silane before use. It is therefore seen that not only the presence of filler particles, but also the nature of the interface affects the dielectric properties.

Index Terms — Dielectric spectroscopy, epoxy nanocomposite, charge mobility, permittivity, functionalized nanoparticles.

1 INTRODUCTION

CONSIDERABLE research has gone into the use of polymer and polymer composites as effective materials for electrical insulation. In recent times, it has been observed that the inclusion of even a small fraction of nanofillers instead of conventional micron sized fillers has a major impact on electrical properties such as breakdown strength, surface degradation resistance due to partial discharges, voltage endurance etc [1-4]. In a previously-published work, we have shown that the resistance of epoxy insulation to surface degradation from electrical discharges increases considerably when even a small fraction of nanosized alumina (Al2O3) is used as filler [5]. The improvement in resistance to surface degradation is further pronounced when the nanoparticles are preprocessed (heated and/or functionalized with silane) before adding to base polymer [6]. In an effort to understand how the incorporation of such fillers changes other essential bulk properties, and the physical processes behind such changes, comprehensive dielectric spectroscopy of neat (unfilled) epoxy as well as epoxy composites with as-received and pre-processed alumina nanofillers, is undertaken in this work.

There are some studies reported in the literature, on the dielectric properties of epoxy composites filled with nanosized ceramic materials. Fothergill et al [7] reported an increase in loss factor at frequencies lower than 0.1 Hz for composites with nanometric TiO2 and ZnO fillers. Singha et al [8] investigated TiO2 and ZnO nanofilled epoxy composites and reported increase in permittivity and tan delta values with increase in filler loading, in the frequency range of 400 Hz to 1 MHz. Zou et al [9, 10] reported the effect of humidity on the dielectric properties of silica filled epoxy composites. The dielectric properties of epoxy composites filled with nano sized and micro sized TiO2 were studied by Nelson et al in the frequency range of 10-4 to 106 Hz [11]. They suggested that interfacial regions surrounding the particles play a dominant role in determining the properties of the nanodielectrics.

In this paper, dielectric spectroscopy studies of epoxy nanocomposites prepared with processed and unprocessed fillers are undertaken at very low frequencies (10-3 Hz to 103 Hz). The dielectric response of the materials at different temperatures is studied. An attempt is made to understand the principal charge transport mechanisms operative in the neat epoxy, and how that is modified by Manuscript received on 9 March 2009, in final form 15 May 2009.

P. Maity et al.: Dielectric Spectroscopy of Epoxy Resin with and without Nanometric Alumina Fillers 1482

Ring guard electrode

Specimen

Measurement Electrode

IDA 200

Ground

Lo

Hi

Φ 75 mm

Φ 50 mm 1 mm air gap

Ceramic Insulator

Φ 75 mm

Figure 1. Experimental setup for dielectric spectroscopy measurement.

the incorporation of fillers. The effect of the interface is also investigated.

2 EXPERIMENTAL

The material considered is epoxy resin (Bisphenol-A) prepared from Araldite LY556® (density 1.17 g/cm3 at 25

oC) and Hardener HY951® (density 0.98 g/cm3 at 25

oC). The as-received nanofillers used are spherical aluminum oxide nanopowder (size 40 to 47 nm, γ phase, density 3.97 g/cm3 at 25 oC and melting point 2040 oC) from Sigma-Aldrich, USA. In order to functionalize the alumina particles, 3-Glycidoxy propyltrimethoxysilane (Dow Corning® Z-6040, purity 98%, density 1.07 g/cm3 at 25 oC) supplied by Dow Corning Chemicals is used.

Apart from neat epoxy (EP), composites with the following fillers are prepared:

1. As-received Al2O3 nanoparticles (AR). 2. Al2O3 nanoparticles heated before use (HN). 3. Al2O3 nanoparticles functionalized with 3-

Glycidoxypropyltrimethoxysilane (GPS) after heating (CHN).

The processing techniques for each of the above specimens are described in detail in [6].

Insulation Diagnostic System IDA 200 is used in order to obtain the real and imaginary permittivity values for neat epoxy and epoxy composites in the frequency range of 10-3 to 103 Hz. All measurements are made with an applied voltage of 140 Vrms. Measurements were repeated at 1 V and 10 V to verify linearity. Figure 1 shows a schematic representation of the experimental set-up. Dielectric specimens of size 100 mm × 100 mm × 2 mm are used. The bottom electrode is a stainless steel plane electrode of 75 mm diameter with standard end-curvature. The top (high voltage/ measurement) electrode is a stainless steel plane electrode of 50 mm diameter. A three electrode system, with a stainless steel guard ring electrode (DIN Standard: 53482) of 75 mm outer diameter and 52 mm inner diameter is used to reduce stray currents. A constant air gap of 1 mm thickness is maintained between the guard and measurement electrodes. The electrode assembly is placed inside a humidity and temperature-controlled oven. All measurements are made at 0% relative humidity. Specimens are dried in a vacuum oven for 24 h at 100 oC and stored in a dry chamber before measurement. Dielectric permittivities and tan delta of all samples are measured at 25, 50, 70 and 90 oC.

3 RESULTS AND DISCUSSION

3.1 DIELECTRIC PROPERTIES OF NEAT EPOXY In this section, we examine the data for neat epoxy

specimens at different temperatures in order to establish the principal polarization mechanisms operative in the bulk matrix.

The frequency variations of real relative permittivity ε ′ and imaginary relative permittivityε ′′ of EP at different

temperatures between 25 oC and 90 oC are shown in Figure 2a and 2b. Typically one would expect a peak in the frequency dependence of the imaginary component ε ′′ together with a increment in the real component ε ′ , signifying a relaxation process. Figure 2b shows no such peak, but rather an almost inverse proportionality with frequency, as one would expect for dc conductivity. However, dc conductivity is characterized by a frequency-independent real permittivity, which is certainly not the case here. Rather, the combined behavior of the real and imaginary permittivities indicates the presence of what is termed the anomalous low frequency dispersion (LFD) by Jonscher [12] or quasi-dc behavior (QDC) by Dissado and Hill [12-15]. Such behavior is characterized as follows:

n−∝′′∝′ ωωεωε )()( , cωω > (1) p−∝′′∝′ ωωεωε )()( , cωω < (2)

where cω represents the point of overlap of the two characteristics, p lies in the range 0.7 to 1 and n is less than 0.3 [14]. This essentially means that the frequency dependence of real and imaginary permittivities persists throughout the frequency range, showing a transition from a small value of the proportionality to a different larger value beyond the characteristic frequency cω . This form of response has been observed in all the nano-composites investigated to date (see [7, 9, 10, 11]).

Figure 3 represents the spectroscopic data for epoxy at 70 oC, as an anomalous low frequency dispersion with

996.0=p and 129.0=n . A similar analysis for the data at other temperatures is performed and the corresponding parameters are shown in Table 1. The frequency independence of the ratio of real and imaginary components is expected to be given by the following relation [9],

)2

cot()()( π

ωεωε p

≈′′′

; 10 << p (3)

Equation 3 predicts a value of 994.0=p , which is in good agreement with the observed slope of 0.996. LFD is observed at all temperatures and the shape parameters are unaffected by temperature.

Dissado and Hill presented a cluster approach to explain LFD in materials. The model is based on the concept of a

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 5; October 2009 1483

(a)

(b) Figure 2. (a) Real (ε ′ ) and (b) imaginary (ε ′′ ) components of relative permittivity of EP specimen at different temperatures.

Figure 3. Dielectric spectroscopy data for EP specimen at 70 oC.

cluster of quasi-mobile charges possessing partial structural regularity. dc conductivity involves charge transport by particle hopping, with each ion having unrestricted access to unoccupied sites on a path connecting the electrodes. When access to sites is restricted, conduction may be envisaged as a combination of intracluster and intercluster motions. At short times, charge displacement over microscopic distances evolves

into identical displacements of charges within the cluster, such that the effective displacement is correlated to the cluster size. At longer times, ion exchange between clusters transports an effective charge over longer distances, and alters internal structures of the intervening clusters. The cross-over occurs at the characteristic time ct ω/1= . When p is large, the range of clusters is wide. For near-unity values of p, the intercluster exchanges carry effective charge over long paths.

Schonhals and Schlosser [16] extended the cluster-based concept in terms of local chain dynamics in amorphous polymers. The molecular movements responsible for the main relaxation of a polymeric solid are highly cooperative at glass transition, and include both intramolecular and intermolecular correlation. Local chain dynamics represent elementary molecular motion; the shape parameter at high frequencies may taken to be mainly connected with small scale motion (in the order of some chain units). The environment of a chain unit varies locally due to the microstructure of the polymer and inhomogeneity of the chains; these effects hinder the diffusional propagation of orientation along the polymer chain, leading to shape parameters less than 0.5. The observed value of the shape parameter n at high frequencies may thus be attributed to local chain dynamics. At lower frequencies, the relaxation process would be dominated by intermolecular correlation. This means that there would exist large scale molecular motions of many chain units of different lengths.

The characteristic frequency ( cω ) for neat epoxy at different temperatures are shown in Table 1, and is seen to increase with temperature. This may be taken to be the frequency (at the given temperature) at which the charge transport process ceases to be determined almost completely by intracluster or short range mobility, and begins to be dominated by intercluster or long range mobility. With increase in temperature, the cross-over from short range to long range mobility of the charge carriers is possible at higher frequencies. A time-temperature superposition assumption is made and master plots are constructed for the low frequency dispersion process, as shown in Figure 4a. The reference temperature is 25 oC. The temperature scale shows how much the plots are shifted to bring them into alignment at different temperatures [10]. Such plots imply that the characteristic times governing the low frequency dispersion (corresponding to characteristic frequency) follows an Arrhenius-type relation of the form given below in equation 4, and shown in Figure 4b.

)/exp( kTE∞=ττ (4)

where E is the activation energy, τ the characteristic time, ∞τ the characteristic time at ∞→T and k the Boltzmann

Table 1. Low frequency dispersion parameters of neat epoxy at different temperatures.

Parameters 25 oC 50 oC 70 oC 90 oC

Slope Characteristic frequency

cω (Hz)

-0.996 8.66

-0.996 10.94

-0.996 33.0

-0.996 49.75

P. Maity et al.: Dielectric Spectroscopy of Epoxy Resin with and without Nanometric Alumina Fillers 1484

(a)

(b) Figure 4. (a) Master plot for neat epoxy specimen. (b) Arrhenius plot for neat epoxy specimen.

constant. Using equation 4, the activation energy is calculated to be 0.26 eV. This value is much lower than that of 2.4 eV found for the base resin of [10], which reduced to about 1 eV with increasing relative humidity (RH).

We thus interpret the dielectric response of neat epoxy specimens principally in terms of low frequency dispersion. Another mechanism that evokes interest among researchers in the context of materials with interfaces is interfacial polarization. Interfacial polarization, or the Maxwell-Wagner-Sillars (MWS) effect, is observed in heterogeneous systems composed of two or more phases. In materials prone to interfacial polarization, space-charge build-up occurs at the macroscopic interfaces as a result of the difference in conductivities and permittivities of the constituents. The accumulation of free charges changes the electric field, in contrast to the other types of polarization (atomic, electronic, dipolar) which are produced by the displacement or orientation of bound charge carriers. Epoxy resin is a thermosetting resin and, and X-ray diffraction (XRD) showed the neat epoxy resin samples to be entirely amorphous in structure. In such a system, interfacial polarization which is more relevant to semicrystalline structures, is unlikely to be

significant [17, 18]. In fact, interfacial polarization would be reflected in a high frequency region where 1−∝′′ ωε and

2−∝′ ωε , with the ratio of the imaginary and real components proportional to frequency. This is not observed in the obtained data. Thus, the possibility of any significant amount of interfacial polarization may be ruled out.

In the following sections, we investigate how the above polarization and loss mechanisms are affected by the presence of various nanofillers in the bulk matrix.

3.2 EFFECT OF INCORPORATION OF NANOFILLERS

In this section, we compare the dielectric properties of neat epoxy and composites prepared with as-received alumina nanoparticles. Figure 5a shows the real and imaginary parts of the electric permittivity at 25 oC for neat epoxy (EP) and as-received Al2O3 nanocomposites (AR) with 1% volume fraction ( pv ) of nanoparticles. The presence of nanoparticles

introduces a significant decrease in the effective low frequency permittivity. In this frequency range, the AR specimens also show lower imaginary permittivity. An analysis of the data in a manner similar to that discussed for epoxy specimen indicates the existence of LFD process. This behavior is seen to persist at higher temperatures. The parameters corresponding to LFD for the AR specimen are given in Table 2. As may be seen in Figure 5b, the difference in behavior between the neat epoxy and as-received nanocomposites decreases with increase in temperature and becomes negligible at and above 70 oC.

The activation energy of the AR specimens is calculated as before using equation 4, and is found to have increased to 0.41 eV from 0.26 eV for EP. In contrast, it was shown in [10] that the addition of silica nano-fillers to the base epoxy resin reduced the activation energy from 2.4 eV to around 1 eV. The characteristic frequency at which cross-over from short range mobility to long range mobility occurs is seen to decrease in comparison to EP specimens. Thus it would appear that in the case of this particular nanocomposite under study, the presence of nanofillers impedes charge carrier transport. Thus, the polarizability of the material is affected by the impediment offered to effective separation of charge centers required for establishment of ‘giant dipoles’. That charge movement in nanocomposites requires greater energy and longer times is reflected in an increase in the activation energy, as well as characteristic time.

It is important to remember that a certain amount of change in polymer property is brought about through the replacement of neat epoxy by the filler material; however in the current work, this is a minuscule 1% of total volume. Much of the nanocomposite behavior is due to interfacial properties, owing to the high interface/volume ratio. There is reason to believe that the incorporation of nanofillers gives rise to an interfacial region with properties different from both the bulk and filler [7, 9, 11, 19 and 20]. The intracluster and intercluster charge mobilities are therefore likely to be predominantly affected in the near vicinity of the nanoparticles i.e. in the interphase region. This difference shows up in the measurements at lower

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 5; October 2009 1485

(a)

(b) Figure 5. Frequency dependence real (ε ′ ) and imaginary (ε ′′ ) components of relative permittivity of neat epoxy (EP) and as-received

Al2O3 nanocomposite (AR) with 1% pv at (a) 25 oC and (b) 70 oC.

(a)

(b) Figure 6. Real (ε ′ ) and imaginary (ε ′′ ) components of relative permittivity of neat epoxy and (i) AR, (ii) HN, and (iii) CHN nanocomposite specimens at temperature (a) 25 oC and (b) 70 oC.

temperatures. With increase in temperature, charge mobilities become comparable in both specimens, either because the contribution of the interfacial region to the effective charge mobility becomes insignificant, or the charge mobilities in the interface regions become similar to that of bulk epoxy at higher temperatures.

Hence, the role of the interface is studied in the next section.

3.3 EFFECT OF PRE-PROCESSING OF NANOPARTICLES

In this section, we investigate the dielectric properties of composites prepared with preprocessed particles. Figures 6a and 6b show real and imaginary components of the relative permittivity at different temperatures, for the differently processed alumina nanocomposites with 1% pv , viz. (i) as-

received Al2O3 (AR), (ii) heated Al2O3 (HN) and (iii) functionalized Al2O3 after heating (CHN). At 25 oC, EP has significantly higher )(ωε′ and )(ωε ′′ than any of the

nanocomposites; this is not true at higher temperatures. At room temperature, all the alumina nanocomposites exhibit almost identical behavior. The trend changes at 70 oC, where CHN specimen shows lower values of )(ωε′ and )(ωε ′′ . A similar trend is observed at 90 oC. Thus, incorporation of both as-received and pre-processed nanofillers reduces real and imaginary permittivities at low temperatures. The effect of as-received nanoparticles is negligible at high temperatures. However, composites prepared with particles functionalized with GPS, are seen to have lowest permittivity at high temperatures, compared to all other specimens (including neat epoxy).

Thus, pre-processing has a significant effect on the behavior of the composite specimens, and this may be

Table 2. Low frequency dispersion parameters of AR specimen at different temperatures.

Parameters 25 oC 50 oC 70 oC 90 oC

Slope Characteristic frequency

cω (Hz)

-0.999 3.63

-0.999 6.69

-0.999 30.89

-0.999 36.59

P. Maity et al.: Dielectric Spectroscopy of Epoxy Resin with and without Nanometric Alumina Fillers 1486

Figure 7. Schematic representation of possible bond formation between epoxy resin and alumina nanoparticles surface for (a) AR, (b) HN and (c) CHN specimens.

Figure 8. Tan delta and complex (storage and loss) moduli for the EP specimen.

attributed to the modified properties at the interface. While there is no direct experimental evidence, an attempt is made to explain the observation in terms of intercluster transport in the interface region. Composites with as-received particles have a significant amount of adsorbed moisture on the particle surfaces, bonded to the dangling OH bonds [6]. This ensures that the as-received nanoparticles are prevented from bonding with the polymer matrix. The situation is different for HN and CHN samples. During preparation of HN samples, heating frees up the hydroxyl groups on the particle surface and makes them available for hydrogen bonding with the polymer. Similarly, functionalizing the nanoparticles modifies the interface by covalent bonding between the silane group in GPS and nanoparticles by formation of Si-O-Al bonds [6]. Figure 7 schematically shows the nature of possible bond formation between epoxy resin and nanoparticles.

As discussed in the previous section, charge transport is limited by the presence of filler particles. This pertains to all the nanocomposites and brings down the permittivity values. At room temperature, the number of charge carriers in the interfacial region is low, and there is no discernible difference across the various nanocomposites. As temperature increases, the contribution of the interfacial region in AR becomes large enough to be noticeable as decreased real and imaginary permittivity at low frequencies. At higher temperatures, the effect of the interface in AR and HN is overridden and there is no significant difference between behavior of EP and AR composites.

At low frequencies intercluster charge transport is significant. That dielectric properties of nanocomposites are affected by modified molecular chain mobility in the vicinity of particles has been reported by Kanapitsas et al [21] for smectite clay nanocomposite. As mentioned earlier in section 3.1, Schonhals and Schlosser [16] stressed the contribution of local chain dynamics to intercluster mobility at glass transition. An attempt is made here to find an explanation for the observed permittivities of CHN specimens at high temperatures in terms of segmental motion.

The storage modulus, loss modulus and tan delta of the various specimens are determined using a Rheometrics model DMTA IIIE Dynamic Mechanical Thermal Analyzer in the flexural mode. The tan delta and complex moduli of specimens with dimensions of 60 mm × 10 mm × 2 mm are

measured from 30 to 220 oC at a heating rate of 4 oC/min and a frequency of 1 Hz in air atmosphere. Figure 8 shows the tan delta and complex moduli of neat epoxy obtained from the DMTA analysis. At around 70 oC, neat epoxy begins to enter the glass transition region as indicated by the sudden drop in the values of the storage modulus in Figure 8. The same is observed for the nanocomposites. The glass transition temperatures, Tg, of neat epoxy and nanocomposites are shown in Table 3. Previous work [10, 11] has shown that the addition of nano-fillers lowers the value of Tg as has been found here.

At low temperatures, polymer chains have limited mobility. When the thermal energy is high enough to cause significant segmental motion of the polymer chains in the matrix, charge

Table 3. Glass transition temperature of the EP, AR, HN and CHN specimens

Specimen Tg (oC)

EP 117 AR 115 HN

CHN 117 113

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 16, No. 5; October 2009 1487

transport is likely to be decided largely by polymer segmental motions. This is likely to occur in the vicinity of the glass transition region. It is important to remember that the polymer segmental motions are equally unrestricted in the EP and AR specimens due to absence of bond-formation. The tight binding of the polymer chains to the particle surface in the silane functionalized samples (CHN), due to covalent bond formation appears to affect conformational motion at the interfaces of the particles. This shows up as reduced charge mobility in the interfacial region, and consequently reduced effective permittivity of the composite near glass transition temperatures. Interestingly, hydrogen bonding at HN interfaces does not seem to be very effective in restraining chain movement, and these composites show behavior similar to AR at temperatures above 50 oC. While the mechanism is not amply clear, it would appear from the above discussion that tight binding of the particle to the base epoxy facilitates charge carrier movement in the interface region, as in CHN samples, while the absence of such bonding in AR or HN specimens is not as conducive to effective charge transport in this region.

The activation energy for the various nanocomposites and epoxy, computed from master plots as discussed in section 3.1, are shown in Table 4. Interestingly, EP and CHN specimen show very similar activation energies while composites with loosely bonded interfaces (AR and HN) have high activation energies.

The nano-composites whose particles bind to the matrix via H-bonding (AR and HN) have a higher activation energy than the base matrix, but in [10] the activation energy reduced with increasing relative humidity, i.e. with an increase of water molecules in the interfacial region. It is not clear why there should be this difference between the two cases. Possibly the charge carrier in the present case is an ionic species from the epoxy resin itself that moves in the free spaces of the epoxy cages. Thus the more fluid (and flexible) interface regions of AR and HN probably impede its progress, giving a higher activation energy. The more tightly bound interface CHN might result in a more rigid structure leaving the carriers the same kind of percolation pathways as in the base resin. In the case of [10], there are very few ions in the base resin and the water possibly frees ionic carriers from the nano-particle surface by shielding the charge from the counter-ion.

3.4 EFFECT OF FILLER LOADING The dielectric spectroscopy experiments are repeated with 2%

pv loading of the respective particles. The corresponding

activation energies are also shown in Table 4, and it may be observed that the functionalized nanocomposites (CHN) continue to show activation energies closest to epoxy. Increase in volume fraction is accompanied by lowering of activation energy. Additionally, characteristic frequencies are seen to consistently increase with filler loading, i.e. smaller times are required for transition to low frequency dispersion. The characteristic frequency at different temperatures for AR and CHN specimens with different loadings is shown in Table 5. This is in keeping with the lowering of temperature and relative humidity with increased concentration of nanofillers observed by Zou et al [10] for transition to QDC. This may be attributable to the overlap of the interfacial regions surrounding the nanoparticles at higher concentrations, providing continuous paths for charge transport.

4 CONCLUSIONS In this work, data on dielectric spectroscopy of neat epoxy as

well as epoxy-alumina nanocomposites prepared with as-received and pre-processed nanoparticles have been presented. The key findings are enumerated below.

1. The frequency dependent dielectric properties exhibit low frequency dispersion. The LFD may be explained in terms of cooperative intracluster and intercluster charge transport. The cluster approach may be extended to local chain dynamics in amorphous polymers at glass transition.

2. Increase in temperature leads to cross-over of short range mobility to long range mobility at higher characteristic frequencies. The cross-over frequency for the EP specimen at 25 oC is 8.66 Hz and which is increased to 49.75 Hz at temperature 90 oC.

3. The incorporation of nanofillers brings down effective real permittivity as well as imaginary permittivity. With increase in temperature, differences between neat and composite specimens are eliminated. The evaluated activation energy of base epoxy resin is 0.26 eV, however, it is 0.41 eV for the 1% as-received nanofilled composite.

4. At higher temperatures (70 oC and above), specimens with functionalized particles (CHN) show lowest permittivity and losses. It is possible that the covalent bonding of CHN particles at the interfaces affects conformational motion, impeding intercluster charge transport in the interfacial region at glass transition.

Table 4. Evaluated activation energy AR, HN and CHN specimens.

Activation energy (eV) Specimen

pv = 1% pv = 2%

AR 0.41 0.38 HN

CHN 0.38 0.27

0.36 0.25

Table 5. Characteristic frequency ( cω ) of AR and CHN specimens at different temperatures and loading.

25 oC 50 oC 70 oC

90 oC Specimen

pv = 1% pv = 2% pv = 1% pv = 2% pv = 1% pv = 2% pv = 1% pv = 2%

AR CHN

3.63 3.42

3.96 6.96

6.69 9.86

8.20 10.17

30.89 19.77

31.53 29.56

36.59 22.62

52.22 39.26

P. Maity et al.: Dielectric Spectroscopy of Epoxy Resin with and without Nanometric Alumina Fillers 1488

It is therefore concluded that the dielectric behaviour of epoxy resin is significantly affected by the incorporation of alumina nanofillers, at low temperatures. At higher temperatures, it is essential to functionalize the particles before use, to see a decrement in real and imaginary parts of the permittivity.

ACKNOWLEDGMENT The work was carried out with funds received from the

Department of Science and Technology, Government of India, under Project no. SR/S3/ME/29/2005-SERCEngg. We are grateful for the valuable suggestions made by the reviewers and the associate editor, and would like to put on record that we have benefitted vastly from the review process.

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Parimal Maity was born in 1979. He received the Master of Engineering degree from Bengal Engineering and Science University, Shibpur, India in 2004. He is currently a Ph.D. scholar at the Indian Institute of Technology Kanpur, India. His research interests are nanodielectrics and multiscale modeling of nanocomposites.

P. K. Poovamma, graduated from Bangalore University in 1978. She is with Central Power Research Institute Bangalore, India, since 1978. Her areas of interest include performance evaluation, condition assessment of insulation systems of in-service high voltage power equipment and establishment of new diagnostic techniques for evaluation of insulation systems in service Power Equipment.

Sumit Basu obtained the Ph.D. degree from the Indian Institute of Science in 1999, and after a post doctoral stint in the Netherlands, joined the Mechanical Engineering Department of the Indian Institute of Technology Kanpur, where he is now an Associate Professor in 2002. His current research interests include studying, primarily through computational methods, structure property correlations at various length scales in thin films, polymers, composites and other heterogeneous materials.

Venkitanarayanan Parameswaran obtained the Ph.D. degree from the University of Rhode Island, USA in the Department of Mechanical Engineering and Applied Mechanics in 1999. He is currently an Associate Professor of Department of Mechanical Engineering at Indian Institute of Technology Kanpur. His current research includes dynamic fracture mechanics, graded composite and nanomaterials.

Nandini Gupta (M’02) obtained the Ph.D. degree from the Indian Institute of Science in 1998. She was a postdoctoral researcher at the University of Windsor, Canada and the Eindhoven University of Technology, the Netherlands. She is currently an Associate Professor in the Indian Institute of Technology, Kanpur, India. She works in the area of dielectrics and insulating materials, partial discharges and treeing, and applications of plasma discharges.