The Promise of Dielectric Nanocomposites

The Promise of Dielectric Nanocomposites J. Keith Nelson Rensselaer Polytechnic Institute Troy, NY 12180-3590 attendant changes in other attributes, particularly thermal Abstract: Several research groups worldwide have now been conductivity, coefficient of the thermal expansion and thermal able to document some significant improvements that can be endurance [4]. made in the electrical, and other, properties of polymer composites through the incorporation of nanoparticulates. TABLE I Although it is now becoming clear that the mechanisms EXAMPLES OF NANOCOMPOSITE SYSTEMS UNDER INVESTIGATION responsible for these changes are by no means universal, some of the benefits are substantial and rely on the large interface areas which are inherent in the introduction of materials of nanometric Base Polymers Nanomaterials dimensions. Polyolephins Clays Epoxies/phenolics Inorganic oxides By examining a variety of nanomaterials, this contribution Elastomers Carbon nanotubes seeks to review the property changes that can be brought about Ethylene-vinyl copolymers Graphite and examines the possibilities for commercial applications. This Polyethylene terephthalate involves not only the electrical properties, but the implications for the attendant mechanical characteristics and the polymer Polyamides processing necessary for utilization of this emerging breed of Polyimides dielectric material. In this context, the functionalization of the particulate surfaces to provide preferential coupling to the host II. PRINCIPLES polymer will be explored since, by this means, a degree of preferred assembly can be accommodated. Through experimental examples, the use of this technique to tailor the It has become clear that the principal underlying reason for properties of nanodielectrics is illustrated. the changes in properties which are being seen is related to the plurality of interfaces introduced through the use of I. BACKGROUND nanomaterials. Fig. 1 indicates that for quite modest loadings, Prior to a “theoretical” paper by Lewis [1], interest in nanotechnology had been primarily centered on (Volume of interface/ total polymer volume) 1.0 semiconductor, biological and sensor applications. However, the experimental work of Henk et al. [2] suggested that there may be advantages to be gained in the field of bulk electrical 0.8 15 nm insulation. Early in this decade, some of these benefits were documented experimentally in joint US/European work [3], 0.6 although a full understanding of the complex physics and 100 nm chemistry was, as still is, lacking. In the last two years there 0.4 has been a burgeoning interest in this technology by many 1 µm research groups worldwide. The commercial impact is potentially very large since electrical insulation is a huge 0.2 business segment and thus there has also been significant 100 µm patent application activity. This contribution is limited to use 0.0 of nanoparticles (defined loosely as material having one 0.01 0.1 1 dimension less than about 100 nm) incorporated in a polymer Loading, (volume fraction) matrix. However, within this class of composites, there is a great deal of opportunity to tailor the properties of the Fig.1. Surface-to-volume ratios of nanocomposites as a function of resulting material to specific applications. Table 1 provides an nanoparticle loading overview of some of the systems currently under investigation. From the viewpoint of insulating systems most of the activity the surface area associated with the internal interfaces is very has been on clays and inorganic oxides (particularly SiO2, large. In this way, properties associated with the interface may Al2O3, ZnO and TiO2). Although the interest here is primarily become dominant so that the “new” material can then display the electrical properties of this new class of material, it is properties which are not necessarily provided by either of the likely that many of the applications will also take advantage of phases from which it is derived. For this reason the mixing rules for composite structures will no longer apply. Given that Clearly, the detail will depend on the nature of the material, underlying philosophy, it is perhaps clear why the tailoring of but the compounding stage usually requires the application of material properties will often involve modifications of the a considerable amount of shear stress utilizing a twin-screw internal interface. This may occur by physical means, such as extruder, melt mixed and/ or ultrasonic wand. Characterization tethered entanglement of the polymer chains, but can also be is imperative to ensure proper quality control. In addition to brought about chemically through the fuctionalization of the electron microscopy, differential scanning calorimetry is nanoparticle surface. In this way the bonding of the useful to determine the crystallinity (if applicable) and nanoparticles can be engineered to bring about a degree of self transition temperatures. Fig. 2 shows an example of a scanning assembly and, in principle, provide an opportunity to affect the electron micrograph taken of 23 nm SiO2 nanoparticles properties in desirable ways. embedded in a cross-linked polyethylene (XLPE) resin system. III. FORMULATION Particle functionalization can also be carried out prior to compounding to provide preferred coupling sites. This The preparation of nanocomposites has a marked effect on involves the deposition of a thin layer of the desired agent their eventual properties. It is clearly important to insure that (usually a form of silane treatment). A recent contribution by there is proper dispersion of the nanophase material. Clays are Roy et al. [6] provides an example of the use of vinylsilane for an important class of filler, but the layered structure needs to this purpose with good effect. Fig. 3 illustrates the use of IR be intercalated or exfoliated in order to make them effective. spectroscopy to determine the way in which the components An example of this is the formation of layered silicates [5] have assembled. Particle features such as free silanol groups at which are rendered organophilic by means of ion exchange of 3747 cm-1 and the broad peak centering around 3500 cm-1 are inter-gallery sodium ions by protonated primary alkyl amines. gone as the result of compounding, but the peak at 1580- Although exfoliation is not a complication with monodisperse 1680cm-1 representing (-C=C-) double bond which was inorganic oxides, processing is nevertheless pivotal if present in the vinylsilane treated particles (from the vinyl agglomeration is to be prevented for the high surface energies group) is replaced by the peak at 2860-2970 cm-1 representing implicit in the system. The essential steps involved may the single bond of carbon (-CH2-CH2-.). generally be specified as: Dried Vinylsilane particles 4 Dried Vinylsilane in XLPE ● particle drying (after functionalization, if used) monitored by thermogravimetric analysis ● compounding with the base resin (and cross-linking Absorbance, a. u. 3 agent if appropriate) ● casting or molding 2 H-bonded OH ● post curing protocol to remove unwanted 3747 byproducts which will prejudice the electrical properties. 1630 1 0 -1 4000 3500 3000 2500 2000 1500 1000 500 -1 Wave Number, [cm ] Fig. 3. FTIR spectrum of vinylsilane-treated SiO2 before and after incorporation into a XL-polyethylene polymer [6] IV. ELECTRICAL PROPERTIES A. Electric Strength Perhaps the most important property of nanocomposites is 100 the change in electric strength which is found when the filler particles attain nanometric dimensions. This contrasts with the situation for conventional microparticles where substantial reductions in electric strength are typical as a result of the Fig. 2. Scanning electron micrograph of 23 nm SiO2 weak interfaces and defects which are involved. This is nanoparticles dispersed in a cross-linked polyethylene matrix illustrated in Fig. 4 (a) where the DC quazi-uniform field electric strength of a biphenol epoxy resin system is depicted  with both micro- and nano-particulates of TiO2 used to form and additional gains made through changing the interface the composites. The impact of the size of the filler, at the same conditions by functionalization. nominal loading of 10% by weight, is clearly evident. 700 99 Tip Electric Field (kV/mm) Nano Probability (%) 600 (23 nm) 50 500 400 300 Micro (1.5µm) 200 1 100 1 Breakdown Strength (MV/cm) 10 1 10 100 1000 10000 Life (hr) (a (a 1 XLPE 1500 XLPE + untreated nanosilica Probability of Failure Electrode-tip Stress, kV/mm 1400 XLPE + HMDS-treated nanosilica XLPE + aminosilane-treated nanosilica 1300 XLPE + vinylsilane-treated nanosilica 1200 1100 Un tre Tr 1000 ate ea dN t ed 900 an XLPE + TE- treated nanosilica osi Na XLPE + AEAPS-treated silica 800 lic a no XLPE + HMDS-treated nanosilica sil XLPE + untreated nanosilica 700 XL ica XLPE + untreated microsilica 600 PE XLPE only 0.1 500 10 100 1000 400 Breakdown Strength (kV/mm) 100 1000 10000 Time, min (b Fig. 4. Breakdown probability plots for nanocomposites using recessed (b specimens. (a) Epoxy-TiO2, (b) XLPE-SiO2 Fig. 5. Voltage endurance characteristics for nanocomposites using 4 µm tip/plane electrodes. (a) Epoxy-TiO2, (b) XLPE-SiO2 Fig. 4(b) indicates more dramatic improvements in a polyolefin nanocomposite. In this case the base resin, depicted C. Permittivity and Loss by the curve on the left, is seen to be substantially improved Much can be learned about the way these material operate by the incorporation of nanoparticulates. The various curves through studies of thermally stimulated currents and dielectric on the right indicate that additional benefits can be obtained in electric strength by the use of appropriate functionalization 10% w/w TiO2 38 nm particles in epoxy, 393K which is used to affect conditions at the interfaces. In this 1E+7 tan delta vs. frequency instance the most effective agent is the vinylsilane which is 100 1E+6 known to couple well both to the SiO2 surface and to the 10 XLPE chain (see Fig.2). 1E+5 Relative permittivity 1 1E+4 0.1 B. Voltage Endurance 1E+3 0.01 1E-3 1E-1 1E+1 1E+3 1E+5 Analogous results for the voltage endurance under non- 1E+2 uniform field conditions are depicted in Fig. 5 where the nano real 1E+1 improvements are dramatic. These endurance curves are nano imag 1E+0 micron real plotted in terms of the maximum field at the tip of the micron imag point/plane gap and show over 2 orders of magnitude 1E-1 1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 improvement in the voltage endurance in both the Epoxy (a) Frequency/Hz and XLPE (b) systems. Again, it would appear from Fig. 5(b) Fig. 6. Real and Imaginary components of permittivity for nano- and micro- that a substantial benefit is obtained from the nanoparticles Epoxy/TiO2composites [3].  spectroscopy. Although the practical use of these materials is also likely that at least part of the enhanced electric strength usually involves frequencies of 50 Hz and above, it is the very may be due to the redistribution of internal charge. low frequency domain where mechanistic evidence is to be found. For example the change of slope of the real part of the V. THE UNDERLYING PHYSICS AND CHEMISTRY permittivity depicted in Fig. 6 for a thermoset nanocomposite is the result of the mitigation of the Maxwell-Wagner effect In order to properly engineer future insulation systems which is well known for conventional (micron) fillers. using nanocomposites, it is clearly necessary to understand the Of more practical significance is the finding that at power way in which they function. Work conducted in a number of frequencies and higher, the incorporation of a high different laboratories worldwide has indicated that although permittivity nanoparticulate material results in a composite there are common features, not all nanodielectric systems which may exhibit a bulk dielectric constant which is lower behave in the same way. If it is accepted that it is the internal than either the base polymer or the nanoparticle introduced. interface that is dominating the behavior, then that is perhaps This surprising result has been documented in several not too surprising. Although beyond the scope of this review, dielectric systems and contradicts the usual mixing laws. It is evidence for the part played by the internal interface has been thought to result from the tethering of species giving rise to provided by electroluminescence, photoluminescence, dispersion at the interfaces. However, from the practical point thermally stimulated currents, x-ray secondary emission of view it is important since it may be used as a technique for spectroscopy, electron paramagnetic resonance, as well as the lowering the reactive current associated with the material areas cited here. The picture which emerges is one in which which is an important factor in cable dielectrics, for example. the internal interfaces may create or influence: D. Internal Charge Characteristics (1) scattering of electrons in high field regions A common feature of many nanodielectric systems is the behavior of the internal space charge build-up. The recent (2) an interaction zone surrounding the nanoparticles development of systems to measure the internal charge in dielectrics has allowed both the magnitude and dynamics of (3) free volume within the polymer structure the internal charge to be assessed and it appears that there are substantial differences when nanoparticles are incorporated Although, there is evidence that all these factors can play a into a polymer matrix. As an example, Fig. 7 depicts the role, it is clear that the extent to which they will influence the differences seen with a polyethylene/SiO2 nanocomposite in properties does depend on the system concerned, and, more comparison with the base resin (XLPE). For many particularly, on the nature of the interface zone. nanodielectric systems three features are usually dominant: Fig. 8. provides an example which suggests that, in high field regimes where electronic conduction is prevalent, the (a) The magnitude of the internal charge is much less intervention of nanoparticle interfaces creates scattering which for nanocomposites moderates the prevailing electron energies. This may be seen from the electroluminescence spectra where a “red” shift (b) The dynamics of charge decay are much faster for corresponding to an energy of about 1.3 eV is evident when nanocomposites 10% TiO2 nanoparticles are added to an epoxy resin (c) There is often a very different distribution of charge (and therefore of internal field) with nanocomposites. Frequency Shift ◄ Again there are very important practical consequences of this 650 finding. Applications, such as DC cables, that have design 600 (b) Electroluminescence (a.u.) limitations dependent on internal charge can perhaps be 550 reengineered using nanomaterials in the light of this finding. It 500 Calibrated Electric Field (MV/m) Calibrat ed Electric Field (MV/m) Calibrated Signal (C/m-3) Calibrated Signal (C/m-3) 80.0 60.0 Cathode Anode 110.0 100.0 90.0 Cathode Anode 450 (a) 40.0 Charge distribution 80.0 70.0 20.0 0.0 60.0 50.0 400 potentia Charge distribution 40.0 -20.0 30.0 -40.0 -60.0 20.0 10.0 0.0 pot entia 350 (c) -10.0 -80.0 Electric Field -20.0 -100.0 -30.0 -40.0 Electric Field 300 -120.0 -50.0 -140.0 0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 400.0 450.0 500.0 550.0 600.0 -60.0 0.0 50.0 100.0 150.0 200. 0 250.0 300.0 350. 0 400.0 450.0 400 450 500 550 600 650 700 Calibrated potential ( kV) Time in Sec 0.00 Time in hours 0.00 thickness of sample(um) (a) Calibrat ed potent ial ( kV) Time in Sec 0.00 Time in hours 0.00 t hickness of sample(um) (b) Wave le ngth (nm ) Fig. 7. Typical PEA space charge measurements; (a) XLPE only, (b) nano-filled Fig. 8. Frequency resolved electroluminescence spectra for 10 % material [6]. TiO2/epoxy composites [micro (b) and nano (c)] in comparison with the base resin (a).  Since the technique employed involved the use of calibrated volume through particle chemical coupling such as that interference filters and a sensitive photomultiplier tube, there depicted in Fig. 3. is little fine structure available and no information from the In addition to the inherent free volume, which can be absolute value of the output. Nevertheless, the shift in the thought of as a distributed defect structure, there will also be major peak is quite reproducible. The divergent field used effects associated with the interfaces introduced. In the case of (~500 kVmm-1) is similar to that used in the dramatic results a conventional microcomposite, such interfacial defects will of Fig.5 strongly suggesting that scattering plays an important dominate and give rise to poor electric strength performance, role in the improvement of voltage endurance. as depicted, for example, in Fig. 4 (a). In this way, one might Another general feature of this class of materials stems from expect defects which are native to the host polymer (free the modification of the internal space charge such as is shown volume) and those which are the result of the interface regions in Fig. 7. This type of phenomenon has been observed in formed by the particles introduced. Careful investigation of several nanodielectric systems [3, 5-6] and there is increasing the failure statistics of these materials indicates that there are evidence that this is also due to interface behavior. Lewis [7] two regions which can be identified as having two different has recently suggested that there may be a Guoy-Chapman (temperature dependent) breakdown distributions. Such layer associated with these interfaces similar to that which is characteristics are depicted for polyolefin nanocomposites in well known in the electrochemistry of liquids. The dispersion Fig. 9. Assuming a chain scission mechanism, the two of such small particles, even at loadings of a few percent, can segments of the line represent two ranges of defect cause these interaction zones to coalesce creating a pathway distributions, each having its own width and maximum value for local conduction without negatively affecting the bulk that change independently over the temperature range studied conductivity. Such a mechanism would provide an explanation (increase in maximum value but decrease in width of for the changes in internal charge behavior and also would be distribution with increasing temperature). In this way, one expected to reduce the charge time constant seen in pulsed may speculate that, with increase in temperature, both electroacoustic measurements and in light emission behavior distributions get narrower and the upper limits can finally [8]. Theoretical models are beginning to emerge based on such merge giving rise to the characteristics seen in Fig. 9. interaction zones [9] and are likely to form useful tools in the engineering of new materials based on this technology. VI. APPRAISAL The last major feature which is relevant here is the influence of free volume and defects. The free volume theory While filled polymers have been common since the start of of electrical breakdown [10] is compelling on the basis of the the plastics age, nanocomposites are intrinsically different in close association between breakdown strength and free that they appear to be dominated by the characteristics of the volume which is seen as a function of temperature. internal interfaces. These properties appear to arise when the Furthermore, it is known that the free volume of a polymer is infilled material has a similar length scale to that of the affected by the incorporation of nanoparticles [8]. However, polymer chain. the changes in free volume are not always what might be One possible reason for this lies in the relationship to the expected and are likely to be dependent on the radius of gyration. Fig.10 illustrates this, again for a SiO2- functionalization. Indeed, it may be that the advantages seen in polyethylene system. For the microparticles, the bond angles Fig. 4(b) result primarily from the modification of the free will allow hydrogen bonding of the –OH groups which is not 10 100 1000 facilitated in the case of nanoparticles. Early crude attempts to 1 1 engineer nanodielectrics by changing the conditions at the interface do suggest that indeed some degree of tailoring of Region-1 dielectric properties and self assembly may be possible. From the commercial standpoint, the voltage endurance Probability of Failure enhancements documented already should provide the encouragement to examine the properties of scaled-up systems Region-2 using this technology. Dielectrics have long been regarded as “mature” science, but there are here real opportunities to o provide substantial performance enhancements/cost savings. 25 C Furthermore, many insulation applications are not limited by o 0.1 60 C 0.1 electrical properties, but by mechanical and thermal o 70 C o considerations. For example, the nanocomposite giving rise to 80 C the breakdown characteristics shown in Fig. 4 (b) also has a concomitant increase in the melting temperature of about 6%. 10 100 1000 Indeed, an obvious application would be cryogenic dielectrics Breakdown Strength, kV/mm because of the criticality of some of the attendant properties. Fig. 9. Detailed breakdown statistics for an untreated SiO2- XLpolyethylene Similarly, applications, such as DC cables, for which the nanocomposite with a 5% (by weight) oxide loading mitigation of internal space charge is important, are clearly prime candidates for this technology.  nanocomposite where electron trapping is indicated as the H Externa lly dominant mechanism. bonded water H H O H O ACKNOWLEDGMENT d +H O H Si O The author is grateful to the US Electric Power Research H Institute, the UK Science and Engineering Research Council, O Si O Si d- sponsoring Companies of the Philip Sporn Chair at H Rensselaer, and to a large number of collaborators and O Si graduate students. Nan om etric Particle (b) Hy drogen REFERENCES bonded -O H grou ps 1. Lewis, T. J., “Nanometric Dielectrics”, IEEE Trans on Diel. and Elect. Ins., (a) Vol.1, 1994, pp 812-25 Bulk-size partic le 2. Henk P.O., Kortsen T.W. and Kvarts T., “Increasing the electrical discharge endurance of acid anhydride cured DGEBA epoxy resin by dispersion of nanoparticle silica”, High Perf. Polym., Vol. 11, 1999, pp 281- Fig. 10 Schematic illustration of the bond angles associated with (a) 296 micro- and (b) nanoparticles 3. Nelson J.K. and Fothergill J.C., “Internal charge behaviour in nanocomposites”, Nanotechnology, Vol. 15, 2004, pp 586-9 Finally, very recent results [11], again in epoxy resin, have 4. Motori A.,et al., “Improving thermal endurance properties of reiterated the importance of internal charge in nanodielectrics. polypropylene by nanosaturation”, Ann. Rep. Conf. on Elect. Ins. & Diel. Phen., IEEE, 2005, pp 195-8 Fig. 11 shows clearly that, in the case of a microcomposite, heterocharge can build up in front of the cathode. This is in 5. Montanari G.C., “Modification of electrical properties and performance of EVA and PP insulation through nanostructure by organophilic silicates”, IEEE striking contrast to the case for a nanodielectric where, in Trans on Diel. and Elect. Ins., Vol.11, 2004, pp 754-61 6. Roy M., et al. .”Polymer nanocomposite dielectrics – the role of the 15 interface”, IEEE Trans EI. Vol. 12, 2005, pp 629-4 3 ) 10 7. Lewis T.J., “Interfaces are the dominant feature of dielectrics at the Charge (C/m 5 nanometric level”, IEEE Trans EI. Vol. 11, 2004, pp 739-53. 0 8. Nelson J.K. and Hu Y., “Nanocomposite dielectrics - properties and -5 implications”, J. Phys. D (Appl. Phys.),Vol 38, 2005, pp 213-222 -10 Voltage Applied 9. Tanaka T., “Dielectric nanocomposites with insulating properties”, IEEE -15 Trans. DEI. Vol. 12, 2005, pp 914-28 -2.E-04 0.E+00 2.E-04 4.E-04 6.E-04 8.E-04 10. Artbauer J., “Electric strength of polymers”, J. Phys. D., Vol. 29, 1996, pp 446-56 Thickness Voltage (m) Applied 11. Nelson J.K. and Hu Y. “Candidate mechanisms responsible for property 25 changes in dielectric nanocomposites”, IEEE Int. Conf. Prop. & Appl. Of Diel. Mater., Bali, Indonesia, 2006 (to appear) ) 3 15 Charge (C/m 5 -5 -15 Voltage Applied -25 -1.E-04 1.E-04 3.E-04 5.E-04 Thickness (m) Fig. 11. Charge distribution profiles for 10 % TiO2/epoxy composites at an average stress of 20 kVmm-1 both after 1 hr of stressing (indicated) and with the voltage removed. [Top: nano and bottom: micro; cathode on left] equivalent circumstances, a shielding homocharge is seen in front of a cathode in high electric fields situations. Not only would this have implications for both dielectric strength and voltage endurance, but also again points to the scattering action of nanofillers. The free paths in the micromaterial appear to allow impact ionization in contrast to the