P. Maity et al.: Degradation of Polymer Dielectrics with Nanometric Metal-oxide Fillers due to Surface Discharges

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ABSTRACT

Manuscript received September 29, 2006, in final form 8 July 2007.

Recent research has indicated that dielectric properties of polymer insulating materials might be improved by the inclusion of nanosized particles dispersed in the polymer matrix. Insulating materials in power apparatus are often exposed to surface discharges in the course of normal operation. Surface degradation due to continued exposure to such discharges may cause deterioration of the surface, and could ultimately lead to catastrophic failure. The current work investigates the effect of inclusion of nanometric particles on the ability of a polymeric dielectric to resist degradation when exposed to surface discharges. The dielectric material used was epoxy resin, while nanosized alumina (Al2O3) and titania (TiO2) were used as fillers. Surface discharge tests were carried out on the specimens. The degraded surfaces were studied using a Scanning Electron Microscope (SEM). Surface roughness measurements were made using a Laser Surface Profilometer (LSP). It was observed that the degradation was greater for unfilled epoxy specimens than that for filled ones. Atomic Force Microscopy (AFM) and Energy Dispersive X-ray analysis (EDX) were used to identify surface changes in the dielectric material due to degradation. It has been conclusively shown that addition of even very small volume fractions of nanoparticles increases the resistance of the material to degradation due to surface discharges. A possible mechanism for surface degradation in nanocomposites has been proposed. Index Terms — Polymer nanocomposite, nanodielectric, surface degradation, electrical insulation, epoxy resin, roughness profile.

1 INTRODUCTION D E S P I T E co n s id e r ab le r e sea rch in t he a rea o f nanotechnology, very little effort has been so far directed at exploring the possibilities of using polymer composites with nanometric fillers as improved dielectric materials. And yet, there is reason to believe, as has been underlined by Lewis [1] and Frechette et al [2] that nanodielectrics could find effective use in the electrical engineering industry, for example, as bulk insulating material in high-voltage

apparatus or as thin films in the electronics industry. Fillers are routinely used in polymeric dielectric materials for various reasons: improving discharge resistance, thermal or electrical conductivity, controlling costs. However conventional microsized fillers generally impact the electric strength of polymeric materials negatively. Recent research has shown that composite dielectrics with nanosized fillers may exhibit rather different and often more attractive electrical characteristics [3, 4]. It would be useful to undertake a systematic and extensive study of how the various electrical characteristics of interest are affected by the use of nanosized particles.

Degradation of Polymer Dielectrics with Nanometric Metal-oxide Fillers due to Surface Discharges

Parimal Maity, Sumit Basu, Venkitanarayanan Parameswaran Department of Mechanical Engineering Indian Institute of Technology Kanpur

Kanpur-208016, India

and Nandini Gupta Department of Electrical Engineering, Indian Institute of Technology Kanpur

Kanpur-208016, India

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 1; February 2008 53

During normal operation and also under fault conditions, insulating materials within electrical apparatus may be exposed to varying degrees of surface discharges. This could be due to partial discharges from neighboring sites in the dielectric assembly. These discharges impinge on the dielectric material, causing erosion and material degradation, and could ultimately lead to dielectric failure. Dielectric materials with improved resistance to surface discharges would help in ensuring better performance and greater reliability of the apparatus in service. In this paper, the effect of using metal-oxide nanofillers in a dielectric polymer with respect to its resistance to surface degradation when exposed to surface discharge, has been studied.

In a similar study, Kozako et al [5] have reported that polyamides with layered nanosilicate are more resistant to partial discharges than polyamides without silica fillers. In the present paper, polymer nanocomposites with metal-oxide fillers were exposed to surface degradation tests. The extents of surface degradation for neat epoxy resin, composites with nanosized aluminum oxide (Al2O3) and titanium oxide (TiO2) fillers, were studied. Tests on epoxy with microsized Al2O3 fillers were also performed for comparison. An attempt has been made to understand the surface degradation mechanism, and how it leads to improved properties in the case of nanofilled epoxy.

2 EXPERIMENTAL

2.1 MATERIALS

In the formulation of the dielectric material, epoxy resin was selected as the polymer matrix. The epoxy resin (Bisphenol-A) was prepared from Araldite LY556® (density 1.17 g/cm3 at 250C) and Hardener HY951® (density 0.98 g/cm3 at 25 0C), supplied by Vantico Performance Polymers Pvt. Ltd., India. The nanofiller materials used in the dielectric composites were respectively spherical aluminum oxide nanopowder (size 40 to 47 nm, γ phase, density 3.97 g/cm3 at 25 0C and melting point 2040 0C) and titanium (IV) oxide nanopowder (size 25 to 70 nm, 99.9% purity, density 4.26 g/cm3 at 25 0C and melting point greater than 350 0C). Additionally, microsized aluminum oxide (size 10 µm, 99.8% purity, density 3.97 g/cm3 at 25 0C and melting point 2040 0C) particles were used as microfillers. Nanofillers and microfillers were supplied by Sigma-Aldrich, USA.

2.2 SAMPLE PREPARATION

The required amount of aluminum oxide nanoparticles (0.5% by volume of the composite) was added to 90 ml of acetone and mechanically stirred for 10 minutes in a beaker. The mixture was then sonicated with a 12 mm diameter acous-

Figure 1. TEM micrograph showing uniform dispersion of Al2O3 nanoparticles in the base matrix at 0.5% volume fraction.

-tic probe operating at 20 kHz for 30 minutes. During sonication, the mixture was kept in an ice bath, and sonicated in an on-off mode (5 s on, 9 s off) to prevent excessive evaporation of acetone due to heat build up. After sonication, epoxy resin in the required amount was added to the acetone- particle mixture and mixed in a conical flask. This mixture was then heated to 100 0C for one hour to remove the acetone. Degasification and further removal of acetone was done by desiccating the mixture for 2 h under vacuum. The weights of the ingredients were monitored throughout the process to ensure that acetone was completely expunged from the system. After adding the required amount of hardener, the mixture was poured into acrylic moulds (110 mm × 250 mm × 3 mm) and cured at room temperature for 24 h. All samples were post cured for 4 h at 100 0C, and left in the oven for at least 24 h for cooling. The nanoparticle-filled specimens were observed under a Transmission Electron Microscope (TEM) in order to examine the dispersion of the metal-oxide particles in the base resin. It can be seen from Figure 1 that reasonably uniform dispersion of the particles was achieved. The same procedure was also followed for dispersing nanosized TiO2. In case of Al2O3 microfillers, they were directly dispersed in the epoxy resin by 30 minutes of sonication, degassed in vacuum for 2 h and cured in acrylic molds.

2.3 SURFACE DEGRADATION TEST

Epoxy-Al2O3 and epoxy-TiO2 specimens, both with metal-oxide components of 0.5% by volume were used. The specimens were of size 100 mm × 100 mm × 3 mm. Surface discharge tests were carried out on the specimens using IEC (b) electrode system. This system consists of a rod-plane electrode assembly as shown in Figure 2. The electrodes were made of stainless steel and the diameter of the rod electrode was 6 mm with 1 mm end curvature. Tests were carried out on neat epoxy and nanofilled epoxy composite specimens simultaneously, in order to ensure identical ambient

P. Maity et al.: Degradation of Polymer Dielectrics with Nanometric Metal-oxide Fillers due to Surface Discharges 54

conditions and discharge magnitudes. The entire system was enclosed in an acrylic cell with silica gel inside, to achieve controlled humidity conditions. A constant voltage of 12 kV at 50 Hz power frequency was applied for different durations, up to 18 hours. Specimens were degraded due to the discharges along the rim of the high voltage rod electrode.

After the tests, the specimens were cleaned by subjecting them to very low intensity sonication in a bath of de-ionized water. The degraded area was observed using scanning electron microscopy (SEM). Roughness measurements of the degraded surface were made with Laser Surface Profilometer ( Mahr

Figure 2. IEC (b) rod-plane electrode assembly.

(a)

(b)

Figure 3. High resolution camera photograph of degraded surfaces (after 6 h) of (a) neat epoxy resin specimen (b) Al2O3 nanofilled composite specimen.

Perthometer PKG 120). Further, Atomic Force Microscopy (AFM) and Energy Dispersive X-Ray analysis (EDX) were done for selected cases to understand the mechanism of surface degradation.

3 RESULTS AND DISCUSSION

Figure 3 shows images of the surfaces of neat epoxy and nanofilled Al2O3 composites, after 6 hours of degradation, taken with a 782 × 582 pixels camera. The innermost dark circular region corresponds to the undegraded region directly under the rod electrode, where discharges are absent. The discharge takes place in the small annular air gap around the periphery of the rod electrode, and it is this region which seems to be the most degraded. The discharge propagates in air to some distance along the surface of the solid specimen. This causes a larger circular region around the rod electrode to suffer degradation; the extent of degradation diminishes with distance from the edge of the electrode, as the farther surfaces are exposed to lower discharge levels. The surfaces of all degraded specimens were scanned along a radial line (indicated in Figure 3) with the Laser Surface Profilometer (LSP) in order to obtain surface roughness measures.

The texture of a real surface is comprised of surface waviness (longer wavelength deviations) and roughness (fine irregularities). A measured surface profile (hereafter termed depth profile) includes both waviness and roughness. A roughness profile is the modified profile, obtained by filtering the depth profile to attenuate the longer wavelengths associated with waviness. Surface degradation mostly affects surface roughness, while waviness is due to the original surface finish; roughness profiles, rather than depth profiles were therefore used to study the effect of surface degradation. Figures 4a and 4b show the surface roughness profiles of neat epoxy and nano-sized alumina-filled epoxy composite, after 6 hours of degradation. The profiles indicate the indentation and protrusions on the mean surface (degraded). The spikes indicate the presence of indentations or craters on the surface,

Figure 4. Roughness profile of (a) neat epoxy (b) nanofilled TiO2 composite after 6 h of degradation.

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Table 1. Surface roughness parameters of neat epoxy, nanofilled Al2O3, nanofilled TiO2, and microfilled Al2O3 composites before degradation (BD) and after 6 h of degradation (AD) over 5 mm distance from the edge of the electrode.

Neat epoxy resin Nanosized Al2O3

filled epoxy resin Nanosized TiO2 filled epoxy resin

Microsized Al2O3 filled epoxy resin

Surface Roughness parameters

BD AD BD AD BD AD BD AD

Roughness Average Ra (µm)

0.05 3.04 0.08 1.54 0.07 1.15 0.04 2.13

Average Maximum Height Rz (µm)

3.13 115.36 3.21 24.44 3.03 11.15 2.3 54.79

Root Mean Square (RMS) Roughness Rq (µm)

0.14 8.38 0.17 2.09 0.16 1.45 0.15 6.97

Total Roughness Rt (µm) 6.71 268.17 5.76 54.51 6.28 17.71 5.95 228.75

while their magnitudes indicate approximate crater depths. It can be seen in Figure 4a that the peaks indicating the presence of craters extend to larger distances from the periphery of the rod electrode in case of pure epoxy, compared to nanofilled epoxy composites. It should be noted that farthest from the electrode, the profiles are similar for neat epoxy and the composite; this part of the profile primarily indicates the roughness of the undegraded sample.

Surface roughness parameters of the composites and neat

epoxy are shown in Table 1. The roughness parameters before (BD) and after degradation (AD) are placed side by side for comparison. The Roughness Average (Ra) is the area between the roughness profile and its mean line i.e. the integral of the absolute value of the roughness profile height h(x) over the evaluation length. The Average Maximum Height (Rz) is a parameter that totals the average of the heights of the highest peaks to the average of the depths of the deepest valleys, over the evaluation length. The Root-Mean-Square (RMS) Roughness of the surface (Rq) is the square root of the integral of the square of roughness profile height h(x) over the evaluation length. The Total Roughness (Rt) is the sum of the height of the highest peak and depth of the deepest valley in the roughness profile within the evaluation length. Values of all the above parameters were seen to increase after degradation.

After degradation, the Average Roughness (Ra) is smaller for the nanofilled Al2O3 composite (1.54 µm) than for neat epoxy (3.04 µm). Nanofilled TiO2 specimens show an even lower value (1.15 µm). Microfilled specimens have lower average surface roughness than epoxy (2.13 µm), though higher than either of the nanocomposites. The Average Maximum Height of the profile (Rz) is also considerably higher for neat epoxy, and so are the other parameters, viz. Rq (Root Mean Square Roughness) and Rt

(Total Roughness). It is clearly evident therefore, that after exposure to surface discharges of equal magnitude and equal durations, the nano-filled specimens suffered lower degradation than both neat epoxy and the micro-filled specimens.

Figure 5 shows SEM images of the degraded surfaces of an epoxy specimen. Figure 5a shows an image taken at the edge of the rod electrode (approximately the same point where the LSP scan begins), at a magnification of 1000×. Figures 5b and 5c show images taken respectively 2 and 6 mm away from the edge, in a radial direction (as in the LSP scans). The darker regions in the SEM pictures indicate low-lying areas, i.e. regions which have suffered degradation. Lighter regions indicate crests, where either degradation has occurred in lower proportions or not at all. In Figure 5c, which is taken at a point far from the electrode edge, it is evident from the typical size of the degraded channels formed on the surface (indicated by the darker regions), that only a small amount of degradation has taken place, and most of the original surface is left intact. As one moves closer to the edge of electrode, extent of degradation increases, with larger chunks of material missing.

The lighter regions, indicating islands around which degradation has taken place, shrink in area near the electrode. Moreover, darker regions begin to form on the tops of the islands themselves, indicating that material has also been eroded from the tops of the islands. This is certainly true in Figure 5a, where the difference in height between islands and craters (regions of material removal) appear to have reduced, as indicated by the contrast.

Figure 6 shows SEM images of neat epoxy specimens, and specimens with fillers (microsized Al2O3, nanosized Al2O3, and nanosized TiO2 respectively), 2 mm away from the edge of the electrode after 18 hours of degradation. Compared to Figure 6a for neat epoxy, the channels in Figure 6c for nanofilled alumina specimen are much thinner, and rather discontinuous. Most of the surface appears to have remained intact. The islands are smaller but much more numerous. It would appear that erosion takes place in bigger chunks for neat epoxy, while comparatively smaller chunks of material are eroded out from the nanocomposite surface. Figure 6d for TiO2 filled nano--composite shows even smaller and less continuous channels.

P. Maity et al.: Degradation of Polymer Dielectrics with Nanometric Metal-oxide Fillers due to Surface Discharges 56

Figure 5. SEM images of neat epoxy resin in the backscattered mode at (a) 0.0 mm, (b) 2.0 mm and (c) 6.0 mm away from edge of the rod electrode after 18 h of degradation.

(a) (b)

(c) (d) Figure 6. SEM images of degraded surfaces in the back-scattered mode of (a) neat epoxy, (b) microfilled Al2O3, (c) nanofilled Al2O3, (d) nanofilled TiO2 epoxy composites, at 2 mm away from the edge of the rod electrode after 18 h of degradation.

The nanofilled samples, thus, suffer lower surface degradation, with TiO2 specimen having lower degradation than Al2O3

specimen. Figure 6b for microparticle filled specimen shows that the channels are thicker than in Figure 6c. They are however thinner than in Figure 6a; the islands are also bigger and more numerous. Microparticles thus appear to help prevent degradation, but not as effectively as nanoparticles. These observations from the SEM images in Figure 6 corroborate the roughness measures reported in Table 1.

In order to trace the degradation of the specimen surface over time, the degraded surfaces were scanned with the LSP every three hours, for a period of eighteen hours. Figure 7 shows the successive profiles of degraded neat epoxy specimen. The surface was scanned each time along a fixed radial line drawn outwards from near the edge of the rod electrode, up to a distance of 7.5 mm. Figure 7a shows the surface profile of undegraded neat epoxy, i.e. before being

~5μm

~15μm

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 1; February 2008 57

Figure 7. Depth profiles of the (a) undegraded and degraded neat epoxy after (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, (f) 15 h and (g) 18 h of degradation.

Figure 8. Depth profiles of the (a) undegraded and degraded nanofilled TiO2 composite after (b) 3 h, (c) 6 h, (d) 9 h, (e) 12 h, (f) 15 h and (g) 18 h of degradation.

P. Maity et al.: Degradation of Polymer Dielectrics with Nanometric Metal-oxide Fillers due to Surface Discharges 58

exposed to surface discharges. The subsequent plots in Figure 7b through 7g show the degraded surfaces after every additional three hours degradation period, with respect to the undegraded surface.

Degradation begins from the edge of the electrode,

indicated in the figure. Thus, in all the profiles, an initial undegraded region is seen, corresponding to an area under the rod electrode. After 3 h of discharge, as seen in Figure 7b, the degradation is localized to a small region close to the electrode, and there is no substantial degradation beyond 1.5 mm from the edge. Over time, however, the degradation extends to regions far from the electrode. The entire scanned surface gets progressively degraded over time; after 18 h large amounts of degradation appear to have occurred between 1-5 mm from the edge of the electrode. This exercise was repeated for nanofilled Al2O3, microfilled Al2O3 and nanofilled TiO2 composite specimens. Figure 8 shows the corresponding profiles for nanofilled TiO2 sample. They clearly show that degradation over time is lower in this case. While the region of maximum degradation remains the same as for epoxy, amount of degradation is much less. Also, for the nanocomposite, there is very little degradation at 7.5 mm, the farthest end of the scan profile shown in Figure 8g, even after 18 h. However, Figure 7g for neat epoxy shows significant amount of degradation at 7.5 mm after 18 h.

These profiles were further utilized to compute the total

volume of material eroded from the surface after every three hours, for the four different materials. The volume of material eroded was calculated as cAR2π , where A is the difference in area under the surface profile curves obtained before and after degradation (i.e. the area between the dotted curve and the solid curve in each figure). Rc is the centroid of this area from the centre of the rod electrode. The assumption here is that the amount of degradation is dependent solely on the radial distance from the edge of the rod electrode; the material degrades uniformly in all radial directions. Figure 9 shows a plot of the volumes of material removed from each material as a function of the exposure time. The neat epoxy sample shows the maximum amount of degradation right from the outset, and continues with a high rate of degradation. Specimens with equal volume fractions of nanosized Al2O3 and TiO2 fillers show equal degradation over the first six hours, though lower than neat epoxy. Over time, the rate of degradation for the nanofilled TiO2 specimen remains almost constant while that for the nanofilled Al2O3 specimen increases slightly. Thus, there is a significant difference in the volumes of material removed from the nanofilled Al2O3 and TiO2 specimens after eighteen hours. The volumes for both nanofilled specimens are significantly lower than that of epoxy, irrespective of time. Strangely, the microfilled Al2O3 specimen shows the least degradation in the initial hours. After 12 h, the rate of degradation picks up substantially, and the values for the volume of eroded material quickly overtake those for both nanofilled specimens.

At the end of eighteen hours, the largest amount of material is eroded from the neat epoxy specimen, followed by the microfilled Al2O3 specimen. The nanofilled specimens suffer lower amounts of material erosion, with TiO2 being more resistant. One point to be noted, for all materials, is that after a substantial amount of material is eroded from the surface, the rate of degradation picks up, possibly due to increased surface roughness. However, this escalation of the rate of degradation is hardly significant for nanofilled samples, especially for the TiO2 sample. Basically, nanocomposites continue to degrade at almost similar rates even after a considerable time.

The above results clearly show that neat epoxy degrades most and fastest, while incorporating nanoparticles in the bulk matrix significantly improves resistance to surface erosion.

3.1 PROPOSED MECHANISM OF SURFACE DEGRADATION IN NANO-COMPOSITES

In this section, a possible mechanism of surface

degradation in particle-reinforced epoxy is proposed, which could explain the improved resistance of nanocomposites to surface degradation. It may be noted that the above proposed mechanism is philosophically similar to the multi-core model proposed by Tanaka et al [6] to explain various phenomena in nanocomposite dielectrics.

Figure 10a shows schematically the proposed mechanism

of surface degradation in nanocomposites. A small region with an approximate area 500 µm × 500 µm located in the highly degraded region near the rod electrode may be considered; a region of this size is expected to contain approximately 0.6 millions of nanoparticles with mean diameter of 50 nm. Stages-I and II show two stages in the degradation process; the scale is exaggerated in order to visualize the particles.

Neat epoxy, in the absence of reinforcements in the bulk

matrix, gets eroded fast. Large chunks of material are eroded from the entire surface. However, for the nanocomposites, it is reasonable to assume that metal-oxide particles act as reinforcements in the bulk matrix. Thus, when exposed to discharges, the particles do not get dislodged easily. Additionally, possible phase changes in the particle-epoxy interface region might make the volume of material in this region resistant to degradation. This is a reasonable conjecture; researchers have reported the existence of comparatively large interface regions around nanomaterials embedded in composites, which exhibit modified material properties [7]. Nelson et al [8] also attributed improved properties of nanofilled epoxy to possible changes at the interfaces.

Thus, initially, the epoxy present between nanoparticles

begin to get eroded, and very fine degradation channels are

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formed, as shown in Stage-I of Figure 10a. With further exposure to discharge, the channels get deeper and wider; however growth of the channel in any direction is arrested when it encounters a nanoparticle. So channels are contained in the inter-particle regions. Further, as the channel grows deeper, nanoparticles emerge from the volume of the material, and degradation is arrested. This is shown in Stage-II. The width as well as depth of the channels, are thus d e t e r m i n e d b y t h e i n t e r - p a r t i c l e d i s t a n c e s ,

Figure 9. Volume of eroded material in neat epoxy, microsized Al2O3, nanosized TiO2, and Al2O3 filled composite over time. which would be typically of the order of 0.3 µm in a nanocomposite with 0.5% (by volume) particle loading for a regular arrangement of particles. More precisely, the channel sizes are determined by the combined sizes of the particles and the interface region surrounding each particle. Degradation thus proceeds by erosion of epoxy between particles, and over time, many more metal-oxide particles are exposed. These particles continue to sit on the surfaces, albeit at different depths from the base-line of the original undegraded surface, while epoxy layers get depleted. This could explain the narrowness of the channels formed in the SEM images of nanocomposite surfaces shown in Figures 6c and 6d. The islands in these images represent undegraded epoxy bonded to nanoparticles. Some particles will indeed be thrown off with increased exposure to discharges, but emergence of a new layer of particles from beneath the first layer, will continue to help in arresting degradation. Nanocomposites thus will continue to degrade at an approximately constant rate; this was observed in Figure 9.

Figure 10b shows schematically a similar degradation

process in microcomposites. Epoxy in between the particles is eroded, and inter-particulate channels formed. However, an area of 500 µm × 500 µm contains approximately only 16 particles with mean diameter of 10 µm. The inter-particle distances are of the order of 50 µm. This means that while the particles and the interface regions around the particles resist degradation as in nanocomposites, the degradation channels are typically much wider and deeper. This is corroborated by the SEM

images for a microcomposite surface shown in Figure 6b. The channels get deeper with time; because of the larger inter-particle distances, it takes considerably larger time for a second layer of microparticles to emerge and arrest the growth of the channels in the vertical direction. Also, when a few microparticles get thrown off due to discharges, they lay bare large volumes of un-reinforced epoxy material which then erode easily. This could possibly explain the sudden increase in the rate of material erosion in micro-filled specimen after 12 h, as seen in Figure 9.

It is important to remember that the main difference in

the use of microparticles and nanoparticles lies in the magnitudes of the inter-particle distances, and the sizes of the interface regions. Smaller the particle, smaller the inter-particle distance, scaling as a factor of the radius. As mentioned earlier, this contributes in limiting the size of the degraded regions. Additionally, the size of the interface increases as the size of the particles decrease, scaling as a factor of (radius of the nanoparticle)-1. Nanocomposites thus have overwhelmingly larger interface regions which resist erosion; this would explain the large difference in the volume of material eroded in nanofilled alumina and microfilled alumina specimens at the end of eighteen hours, as shown in Figure 9.

The mechanism proposed above essentially means that the number of nanoparticles on degraded surfaces should increase with time. In an attempt to find corroboration for the mechanism proposed above, further investigation of the degraded surfaces were undertaken. Energy Dispersive X-Ray analysis (EDX) was performed in order to understand how the element constituents on the surface change with degradation. While the main constituents in neat epoxy understandably were carbon (C) and oxygen (O), the Al2O3 filled samples showed the presence of aluminium (Al), and the TiO2 filled specimens showed the presence of titanium (Ti). The atomic percentages of Al and Ti on the nanocomposite surfaces were seen to vary with distance from the rod electrode. Far from the electrode edge, the percentages were seen to match very closely with values for undegraded surfaces (0.42% for Al and 0.43 % for Ti). Figure 11 shows plots of the variation of atomic percentages of Al and Ti, with distance from the edge of the electrode. The percentages were seen to rise as one approached the electrode, i.e. towards more degraded regions. If, as proposed, with degradation, more and more nanoparticles are exposed, it would explain the increase in metal percentage. Regions closer to the electrode are exposed to higher discharge, and show a greater increase in the metal content than the far-off regions. The increase in Ti was seen to be more than that for Al.

Figures 12b through 12d show the Atomic Force Microscopy (AFM) images of a small surface (900 nm × 900

P. Maity et al.: Degradation of Polymer Dielectrics with Nanometric Metal-oxide Fillers due to Surface Discharges 60

Figure 10. Degradation mechanism of (a) nanofilled Al2O3 composite, (b) microfilled Al2O3 composite specimen.

nm) of an alumina filled nanocomposite, 2 mm away from the edge of the rod electrode, after 6, 12 and 18 h of degradation respectively. Figure 12a shows the three-dimensional topographic image of a similarly degraded epoxy surface after 6 h of degradation. Particles are visible on the surfaces of the nanocomposite, while none are visible on the neat epoxy surface. The two-dimensional topographs and phase images were matched closely to ensure that the spherical protrusions seen on the nanocomposite surfaces actually correspond to a material phase different from the bulk epoxy, and are therefore not mere polymeric protrusions. A comparative study of Figures 12b through 12d, show clearly the difference in the number of particles sitting on the surface, after suffering degradation for different time-intervals. The number of particles in Figure 12b, after 6 h of degradation is considerably low, and close to the approximate number of 18 particles expected on a 900 nm × 900 nm surface of a specimen with 0.5% volume fraction of nano-particles. However, after 18 h, this number is seen to increase substantially in Figure 12d. Thus it is seen that with time, an increased number of metal-oxide particles are exposed on the degraded surfaces.

The EDX analysis and AFM images both provide evidence to the fact that nanoparticles are resistant to erosion. Taken in conjunction with results reported above, they lend credibility to the proposed mechanism of surface degradation in nanocomposites.

It is seen that the TiO2 filled nanocomposite shows higher

resistance to degradation, and there is evidence that more TiO2 particles are exposed in the more-degraded regions near the electrode (indicated by a higher rise in Ti content compared to Al near the electrode in Figure 11). While this could be because of the different characteristics of the material of the particle, it is more likely that the presence of smaller sized TiO2 particles (23-70 nm as opposed to 40-50 nm) is

responsible. Lower size would mean correspondingly greater interface region, and so lower volumes of epoxy would be eroded. It is however not fully clear at this point of time, whether size or material or both are responsible for better surface properties of the TiO2 nanocomposite.

4 CONCLUSIONS

Comparative studies of surface degradation of neat epoxy, nanocomposite and microcomposites were undertaken. It was clearly seen that neat epoxy specimens are most vulnerable to degradation by surface discharges. Microparticles help in reinforcing the bulk epoxy material, to a certain extent. However, the resistance to surface degradation is considerably improved if even a small volume fraction (0.5%) of nanoparticles is used instead of microparticles. It is possible that use of TiO2 fillers instead of Al2O3 may be more advanta-

Figure 11. Atomic percentage of Al and Ti on the surfaces of

nanocomposites after 18 hours of degradation.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 15, No. 1; February 2008 61

Figure 12. AFM images of a 900 nm × 900 nm surface (2 mm from the electrode edge) of (a) neat epoxy after 6 h of degradation and Al2O3 filled nanocomposite after (b) 6 h, (c) 12 h, (d) 18 h of degradation. -geous, but it could be that size of the nanoparticles is the more dominant factor, rather than the material.

A mechanism has been proposed by which degradation is resisted by nanofillers, and this has been corroborated by experimental evidence. Particles as well as epoxy bonded to metal-oxide particles resist erosion. Nanoparticles provide greater interface region for the same metal-oxide content, and thus a correspondingly larger volume of epoxy belong to the interfacial zone and is resistant to erosion. Degradation takes place in small isolated regions that form channels around existing nanoparticles. With time, more and more nanoparticles appear on the surface, helping to arrest or retard degradation of the surface. It remains to be seen if increasing the nanofiller content has any appreciable effect.

ACKNOWLEDGEMENT

The work was carried out with funds received from the Department of Science and Technology, Government of India, under Project nos. SR/S3/ME/29/2005-SERCEngg. and DST/INT/US-NSF/RPO-159/04.

REFERENCES [1] T. J. Lewis, “Nanometric dielectrics”, IEEE Trans. Dielectr. Electr. Insul.,

Vol. 1, pp. 812-825, 1994. [2] M. F. Frechette, M. L. Trudeau, H. D. Alamdari and S. Boily,

“Introductory remark on dielectrics”, IEEE Trans. Dielectr. Electr. Insul., Vol. 11, pp. 808-818, 2004.

[3] T. Tanaka, G.C. Montanari and R. Mulhaupt, “Polymer nanocomposites as dielectrics and electrical insulation-perspectives for processing technologies, material characterization and future applications”, IEEE Trans. Dielectr. Electr. Insul., Vol. 11, pp. 763-784, 2004.

[4] M. Roy, J. K. Nelson, R. K. MacCrone, L. S. Schadler, C. W. Reed, R. Keefe and W. Zenger, “Polymer Nanocoposites dielectrics—the role of the interface”, IEEE Trans. Dielectr. Electr. Insul., Vol. 12, pp. 629-641, 2005.

[5] M. Kozako, N. Fuse, Y. Ohki, T. Okamoto, T. Tanaka, “Surface degradation of polyamide nanocomposites caused by partial discharge using IEC (b) electrodes”, IEEE Trans. Dielectr. Electr. Insul., Vol. 11, pp. 833-839, 2004.

[6] T. Tanaka, M. Kozako, N. Fuse and Y. Ohki, “Proposal of a multi-core model for polymer nano-composite dielectrics”, IEEE Trans. Dielectr. Electr. Insul., Vol. 12, pp. 669-681, 2005.

[7] J.G. Williams, M.E. Donnellan, M.R. James and W.L. Morris,“Properties of interphase in organic matrix composite”, Materials Sci. Eng., Vol. A126, pp. 305-312, 1990.

[8] J. K. Nelson and Y. Hu, “Nanocomposite dielectrics-properties and implications” J. Phys. D: Appl. Phys., Vol. 38, pp. 213-222, 2005.

P. Maity et al.: Degradation of Polymer Dielectrics with Nanometric Metal-oxide Fillers due to Surface Discharges 62

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

Sumit Basu obtained the Ph.D degree from the Indian Institute of Science in 1999. He was a post doctoral researcher in the Delft University of Technology and The University of Groningen in the Netherlands. He joined the Mechanical Engineering Department of the Indian Institute of Technology Kanpur as an Assistant 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 1999. He is currently an Assistant 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 post-doctoral researcher at the University of Windsor, Canada and the Eindhoven University of Technology, the Netherlands. She is currently an Assistant Professor in the Department of Electrical Engineering, Indian Institute of Technology, Kanpur, India. She works in the

area of dielectrics, polymer insulating materials, partial discharges and treeing, and applications of plasma discharges.

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