The Development, Properties and Future of Dielectric Nanocomposites

Transcription

1 The Development, Properties and Future of Dielectric Nanocomposites J. Keith Nelson Rensselaer Polytechnic Institute Abstract Nanodielectrics are a 21 st century phenomenon. This paper attempts both to chronicle the rapid development of this new class of dielectric material and to provide some understanding, albeit incomplete, of the way that these materials function. This is done by examining some of the unique properties that arise and looking at diagnostic measures that can bring some insight into the physical and chemical interactions. A particular emphasis is placed on discussing the role of the processing and compounding of these materials since, unless properly controlled, many of the attributes are not manifested. Based on current knowledge, some projections are made on the future directions for this class of material. In particular, areas of application which appear most likely to be served by dielectric nanocomposites are identified and discussed. galleries of nanometric dimensions. However, clay materials usually require very special treatment [9] to provide the exfoliated structure needed. Today, materials are being assembled using inorganic oxides having sizes of 20 nm or less which are particularly vulnerable to agglomeration (with the concomitant erosion of properties) unless steps are taken to prevent it. It has become clear that some of the unique properties attributable to nanodielectrics arise as the result of the large surface area introduced when nanoparticles are included in the structure. Fig. 1 depicts diagrammatically the interaction zones (estimated to be ~10 nm in radial build) thought to surround particles embedded in a polymer. It will be clear from this diagram that, as the size of the particle is reduced, the internal interfacial area becomes very large indeed and Microparticle Interaction Zone 1. History The use of fillers of various kinds to modify the properties or lower the cost of polymeric insulation systems has been common for half a century, but the incorporation of particles of nanometric size is a 21 st century initiative. The possibilities of building nanodielectric structures was anticipated by Lewis [1] in a 1994 paper, and some early work by Henk et al. [2] and a patent by Johnston and Markovitz [3] indicated some anomalous endurance properties. However, it was not really until 2002 when practical trials [4] and systematic studies started to emerge. Since that time there has been a burgeoning of interest in dielectric nanocomposites. This has taken the form of special sessions at conferences and dedicated issues of archival journals [5-8]. The first book [9] devoted to the topic was published in The large volume of emerging literature has to be interpreted with care since it is now known that the properties exhibited by this class of material are highly dependent not only on the formulation but also on the processing; particularly as it relates to the dispersion of particulates in the polymer matrix see Section Introduction and basic principles Nanocomposites are usually defined as filled polymers where the filler entity has at least one dimension less than ~100 nm. This allows for the inclusion of silicates which may have larger dimensions but incorporate Interaction Zone (a) the material becomes dominated by the interface. Indeed, a 10% loading of 15nm particles results in about half the polymer volume being interface. In this sense, although the material may have been fabricated from a particular base polymer and included particulate, the resulting material may have properties which do not necessarily resemble either of its constituents since it will be predominantly interface region. While this may seem somewhat philosophical, it provides the rationale for the recent trend to attempt to tailor the properties through engineered modifications to the interaction zones by physical (e.g. entanglement) or, more usually, chemical means (e.g. through surface functionalization) Surface modification Nanoparticle (b) Fig. 1 - Representation of the interaction zone for (a) a microparticle and (b) an assembly of nanoparticles (not to scale)

2 Since the electrical properties are clearly of paramount importance for an insulation application, the form of particle surface treatment can be pivotal. For example, it has been shown that materials created with polar and non-polar treatments behave differently [10]. The chemical details underlying the choice of coupling agents is a complex issue, but has recently been reviewed by Reed [11] as it relates to the interface, bonding and entanglement. In addition, the chemistry of the interface can have a critical impact on compatibility, morphology, nucleation and free volume effects; all of which impact the electrical properties. An example is shown in Fig. 2 where triethoxyvinylsilane has been used to treat silicon dioxide particles prior to processing into a nanodielectric based on polyethylene. In this case [12], FTIR measurements can be used to show the nature of the coupling both to the silica surface and to the polyethylene main chain. In this way, functionalization can be used to provide a measure of self-assembly and engineer the properties of the resulting material. difficult Transmission Electron Microscopy (TEM) is needed. However, the use of FIB permits the extension of SEM as shown in Fig.3. Assessment of dispersion and distribution is often done subjectively from SEM/TEM images, but this is sufficiently critical that automated and quantified methods are needed such as those advanced by Hui et al. [14]. These methods rely on the digitization of the images using software such as ImageJ. The images are then processed to extract indices from which the dispersion and distribution can be quantified. The theory is provided in Reference [14], and an example of the application of the methodology given in Section 3. CH3 CH Si + CH3 CH3 H H Silica H CH Si Fig. 3 - Comparison of TEM image (left) and FIB/SEM image (right) for a 5% fumed silica polyamide-imide nanocomposite 3. Processing of nanodielectrics PE main chain Si PE + There are several ways of fabricating nanocomposites which may be summarized [15] as: In situ polymerization Melt blending Solvent method Sol-gel method Fig. 2 - The use of a vinylsilane functionalizing agent with a polyolefin-silica nanocomposite [12] 2.2. The importance of dispersion The way in which nanoparticles are introduced is critical. There is a natural tendency for nanoparticles to agglomerate and, in that event, the material reverts to a microcomposite and all the properties which rely on the presence of the interfacial zones are lost. This is particularly noticeable when looking at the electric strength of a material. In general, the introduction of micron-sized filler results in a depression of the electric strength. In contrast, nanodielectrics can exhibit useful improvements in strength, particularly at elevated temperatures. This impact of the processing has recently been highlighted by Calebrese et al. [13] who have advocated the use of the Focused Ion Beam (FIB) technique for use in connection with Scanning Electron Microscopy (SEM) for the evaluation of agglomeration and dispersion. As particulate size gets smaller, the SEM technique becomes marginal, and the more In particular, clay-based nanocomposites are often made by an intercalation and exfoliation process which is an additional complication. As has been emphasized, great care has to be exercised to maintain good dispersion. In this context, the control of moisture is critical, and the use of ultrasonics, an asymmetric centrifuge, and shear enhancement methods have all proved useful in this regard. As an example, Kurimoto et al. [16] have used a novel technique which combined ultrasonic waves and centrifugal force to fabricate alumina/epoxy nanocomposites. The application of ultrasonics was found to be crucial for good dispersion. Similarly, Kochetov [17] combined ultrasonic processing and high shear force stirring to obtain an even dispersion of the corresponding filler in the base material. The use of indices to quantify dispersion as the result of processing changes is illustrated in Table 1 which shows a dispersion index calculated for a nanocomposite processed with and without alumina balls to enhance shear forces during compounding using a dual asymmetric centrifuge with a liquid resin system. The addition of a mixing media (alumina balls) is

3 suggested in this case to help break apart agglomerates during mixing of alumina nanoparticles in a polyamideimide system. The dispersion of the alumina nanocomposites was quantitatively analyzed using the nearest neighbour index and quadrat method [14]. At 5 wt %, the nearest neighbour index value is closer to 1, indicating improved particle dispersion. The skewness value also drops, indicating better particle distribution. Fig. 4 shows SEM images of these materials. Table 1 Quantification of Al 2 3 nanoparticle dispersion in polyamideimide for mixing with and without alumina balls No Alumina Balls (5 %) Alumina Balls (5 %) No Alumina Balls (10%) Alumina Balls (10 %) Skewness st Nearest Neighbour Index Fig. 4 - SEM images of 5 wt % alumina nanocomposites mixed without (left) and with (right) alumina balls. Scale bar is 1 µm. Even using the same mixing methods, high loadings can make dispersion more difficult. For the same alumina/pai system, Table 1 shows that, at 10 wt %, there is only a slight improvement in the nearest neighbour index with the addition of alumina balls during mixing. Similarly, the skewness of the particle distribution is little improved at 10 wt % with alumina ball addition. It should be noted that direct comparison of the quantitative parameters between loadings would require normalization of these parameters, which is not done here. At high loadings, achieving good dispersion can become more difficult. This can manifest itself as a peak or plateau in dielectric properties, and occurs before percolation of the particles would be expected. 4. Properties of polymer nanocomposites It will be clear from the philosophical picture advanced in Section 2 that there is an opportunity to engineer this class of material not only through the selection of the basic constituents and the loading, but also through interface modifications, morphology changes, and particulate aspect ratio. Indeed, one of the exciting features possible is the ability to create some multifunctionality i.e. to enhance a property without the concomitant erosion of other attributes which so often accompany such attempts. The technology has also opened up the possibility of using nanocomposites as a vehicle for new classes of non-linear or smart materials whose properties may be changed by an outside stimulus. Examples of this will be provided later. It is, perhaps, useful to briefly outline those property changes for which there is widespread agreement Electric strength A carefully processed nanocomposite can show useful improvements in electric strength in most systems. However, as indicated in Section 2.2, these advantages are quickly eroded when dispersion is poor or agglomerates exist. Table 2 illustrates the Weibull characteristic DC electric strength value (kvmm -1 ) for a range of silica/xlpe nanocomposites having different surface functionalizations. Results are depicted for temperatures up to 80 ºC. 178% improvement is indicated at 80ºC and a 66 % improvement at room temperature. Table 2 Weibull electric strength characteristic values (shape parameters in parenthesis) for a range of 5 wt% Si 2 /XLPE nanodielectrics. Functionalization: AEAPS = n-(2- aminoethyl) 3-aminopropyl-trimethoxysilane, HMDS = hexamethyldi--silazane, TES = triethoxyvinylsilane [10] Materials Temperature 4.2. Voltage endurance 25 o C 60 o C 70 o C 80 o C XLPE 269 (2.49) 183 (2.65) 129 (3.66) 79 (3.84) XLPE + Untreated Nanosilica 314 (2.07) 260 (2.27) 213 (2.49) 83 (3.09) XLPE + AEAPS Treated Nanosilic 400 (1.69) 266 (2.20) 263 (1.79) 134 (2.11) XLPE + HMDS Treated Nanosilic 336 (1.69) 225 (1.97) 208 (2.14) 128 (2.09) XLPE + TES Treated Nanosilica 446 (1.73) 422 (2.22) 344 (2.17) 220 (2.87) Perhaps the most dramatic improvements in performance are seen in the voltage endurance characteristics under divergent electric fields (7.5 µm radius point-plane sample with a 2 mm gap in Fig. 5) The example provided in Fig. 5 is for a Ti 2 /epoxy material, but many nanodielectrics will exhibit an order of magnitude increase in performance when compared in this way. This is a very substantial improvement and cannot readily be explained by space charge shielding, but is more likely to be due to the scattering effects of the nanophase inclusions which act as a plurality of barriers to inhibit the propagation of electrical treeing discharges.

4 4.3. Erosion characteristics A property that is clearly allied to the voltage endurance is the behaviour of polymers when subjected to degradation and surface erosion under discharge also map into enhanced partial discharge (PD) resistance; particularly at lower magnitudes [42] Internal space charge development Even early work [20] has shown that nanocomposites yield very different magnitude, distribution and behaviour of internal charge when compared with either the base resin from which they are derived or an (a) Fig. 5 - Voltage endurance comparison between 10% loaded microcomposite and nanocomposite (b) Experimental Arrangement Di i i Fig. 6 - Erosion depth data for epoxy-clay composites in relation to the base resin. A & B: Nanocomposites with different preparation methods, C: Micro-nano mixed composite, D: Base Epoxy [18] conditions An example is shown in Fig. 6 taken from work of Tanaka et al. [18] on mass produced epoxyclay nanocomposites with a 5 wt% loading subjected to alternating voltage surface discharges in a rod-plane configuration (see insert in Fig. 6). In this case the erosion depth is used as an indicator of degradation resistance, but other work (Maity et al. [19]) on epoxymetal oxide has shown the same effect by monitoring the surface roughness or the volume of the material eroded in similar circumstances. Although the significant improvement in degradation resistance afforded by this technology is clear from Fig. 6, it also suggests that there may be advantages to be gained through the use of well-chosen micro/nano particulate combinations. The improvements in erosion resistance (c) (d) Fig. 7 - Space charge accumulation and associated internal field for LDPE and LDPE/Mg nanodielectric. (a) and (b) charge density and field for LDPE. (c) and (d) charge density and field for LDPE/Mg. 1 Instantaneous; 2 20 minutes after stress application. (Takada et al. 2008)[ 21]

5 equivalent microcomposite. Unfortunately, however, once again, one cannot generalize except to say that, typically, nanodielectrics exhibit smaller internal space charge density and are characterized by a shorter time constant for charge decay, which is probably due to the nature of the interfacial layer introduced in Section 2. Charge accumulation based on the Maxwell-Wagner effect for dielectric inclusions appears to be mitigated. Clearly, since the field generated by any internal charge will augment that due to an applied field, the space charge in a dielectric has an influence on the perceived applied field required for failure. The internal charge behaviour will also be different for a direct voltage than for a time-variable voltage application which can have a bearing on the relative dielectric integrity exhibited. Clearly the impact is often greatest when heterocharge accumulates close to an electrode which can enhance the surface field and facilitate charge carrier injection. Such behaviour is illustrated in Fig. 7 in which a low density polyethylene 70 µm film is compared with a nanocomposite formed using 1% nanometric magnesium oxide (Mg) with a nominal electric field of 200 kvmm -1. Such an applied field would be substantially higher than the threshold required for charge injection ( 20 kvmm -1 ). The overriding feature of Fig. 7 is the lack of internal charge accumulation for the LDPE/Mg nanocomposite even after 20 minutes with an electric field as high as 200 kvmm -1 [21]. In contrast, the base polymer at the same field has accumulated substantial positive (hetero)charge in front of the cathode which has resulted in a doubling of the field in front of the cathode (compare Fig. 7 (b) and (d)). Such fundamental changes in the internal charge are common, and in some materials [22] have even shown a change of sign of the internal charge which provides evidence for mechanistic differences brought about by even modest loadings of nanoparticulates Mechanical and thermal properties Although beyond the scope of this review, the mechanical and thermal properties of composites can also be enhanced through the introduction of nanoparticulates. This aspect has been reviewed by Irwin [23], and, for example, in polyimide-based nanocomposites, improvements have been obtained in mechanical strength to failure, scratch hardness and thermal conductivity (relative to equivalent micronsized composites), although the latter is only linearly related to the loading. Changes such as these come about for a variety of reasons such as the nucleation of crystallinity, changes in glass-transition [24] and morphology, tethered entanglement [25], etc. In particular, modest increases in polymer operating temperature can be obtained which is critical in many applications Nanodielectrics in an aqueous environment To provide a balanced perspective, the introduction of internal surface area to bring about positive property changes does also introduce a plurality of pathways which can change the gas/liquid permeability of the material. The modeling of these processes has recently been reviewed by Lu et al. [26]. From the insulation perspective, the migration of moisture is clearly of weight gain% UN 12.5VS 5UN 5VS 0.00 XLPE , , , , sqrt(t)/thickness Fig. 8 - Moisture ingress at 100 rh, 20ºC. 12.5UN: 12.5% Untreated. 12.5VS: 12.5% vinylsilance functionalized, etc. paramount importance [27], and the results of experiments to document the water uptake of nanodielectrics are shown in Fig. 8 for a polyethylene/si 2 system [28]. It is clear that, although the XLPE base material is very resistant to moisture ingress, the pathways provided in nanomaterials allow much easier migration of water, particularly at the high loadings. It appears that some mitigation can be provided by functionalization. It is also apparent that the electric strength is affected, but only at humidity W ater tree leng th (um) (a) (b (c) Time (hour) Fig. 9 - Electrochemical tree growth for a polyethylene-si 2 vinylsilane functionalized nanocomposite in comparison with base X-linked polyethylene. (a) XLPE, (b) 5% Si 2 loading, (c) 12.5% Si 2 loading. [28] [Insert: Water tree propagation]

6 levels above about 75% Electrochemical treeing The findings typified by Fig. 8 do raise the important question of the susceptibility of nanocomposites to electrochemical (water) treeing. This phenomenon has been a major issue for the polymer cable industry. Fig. 9 illustrates the results of water treeing studies using Ashcroft-type methodology for the same nanocomposite system shown in Fig. 8. It clearly shows that the propagation of electrochemical trees is substantially restricted despite the propensity of the material to absorb moisture as shown in Fig. 8. This suggests that the scattering mechanism, thought to underlie the improvement in divergent field electrical treeing, dominates even in the presence of moisture Dielectric spectroscopy The ability to make real and imaginary permittivity measurements over a large frequency range and at temperatures which encompass the range from below glass transition to melting/decomposition can provide considerable mechanistic insight and sometimes identify moisture and other contaminants. This is particularly useful for composites because the internal interfaces can contribute to interfacial polarization (the Maxwell- Wagner effect) which usually manifests itself at low frequencies (Bartnikas 1983). 5. Behaviour in mechanistic tests Although property values, such as the electric strength outlined in Section 4.1, are of paramount importance, mechanistic techniques and characterization can also be invaluable from the viewpoint of understanding the Table 3 Candidate tests used to provide insight into the behaviour of nanodielectrics Mechanistic Test Dielectric Spectroscopy Thermally Stimulated Current Dielectric absorption IR absorption Electron paramagnetic resonance Pulsed electroacoustic analysis Differential scanning calorimetry Electroluminescence Thermogravimetric analysis Information Derived Permittivity & loss Interfacial processes Dipolar processes Impurity transport Polymer transitions Trapped charge Mobility determination Transport behaviour Chemical linkages Identification of free radicals and carrier traps Identification/dynamics of internal charge Glass transition Thermal properties Charge carrier energies Degradation Loading Fig Dielectric spectroscopy of an epoxy-si 2 nanocomposite at 298 K and 353 K. (a) dried, (b) water saturated.[28] underlying mechanisms relating to the material specifically as an electrical insulator. In this context, Table 3 outlines some of the techniques that have been used to good effect in the development of nanodielectrics. Although this is by no means a complete list, examples will be used below to show how such mechanistic studies can be used in the informed design of these materials. The example shown in Fig. 10 is taken from a study aimed at examining the effects of water on 3.37% epoxy-si 2 nanocomposites [28]. It is immediately obvious that most of the changes in the structure of the spectrum occur in the frequency range below 10-1 Hz and so conventional power frequency measurements cannot be used for this purpose. Fig. 10(a) shows the real (C ) and imaginary (C ) capacitance for the dry material. C is constant at low frequencies and C ω -1 (where ω is the angular frequency) as would be

7 expected from a classical frequency independent conduction mechanism. In contrast, Fig. 10(b) shows the situation for the same material at high humidity and temperature. Now C is no longer independent of frequency and the slopes of C and C are parallel suggesting that there is a low frequency dispersion [29] or quasi-dc (QDC) behaviour [30]. By consideration of the temperature dependence, an activation energy can be established for this process. Since it is lower than that for the base epoxy (Vantico CY255/HY227), it may be concluded that the unfunctionalized nanocomposite provides pathways for moisture ingress as shown earlier in Fig. 8, for a different material. It has also been found by Zhang and Stevens [31] that this results from an interplay between the interfacial water mobility and the interface bonding which is critically dependent on functionalization [28], and the extent to which the water is bound or free Electroluminescence The detection of light emission from stressed polymers is not an easy technique since electric fields are usually close to breakdown, the magnitude of the emission is Frequency Shift an Epoxy-Ti 2 system as an example, Fig. 11 would indicate that the use of a filler of nanometric dimensions creates a red shift to longer wavelengths of the emitted radiation (when corrected for the photomultiplier response). Since the emission centers in the material are not known, one has to interpret this with care, but if the technique is used in conjunction with an assessment of the propagation of charge fronts into the material in the manner discussed in Section 4.4, it does suggest that the carrier energy is reduced by the introduction of the nanophase material. In turn, this suggests that scattering may have a dominating influence which would be consistent with the enhanced voltage endurance universally observed. The allied technique of photoluminescence, in which the energy to bring about luminescence is supplied, not from the applied field, but from outside photon irradiation, has also been successfully applied to nanocomposites [33]. Like the electroluminescence technique, it permits the nature of electronic states to be investigated by examining the optical absorption and photoluminescence using vacuum ultraviolet photons Dielectric absorption The decay of the current through a dielectric after a direct voltage has been applied can yield valuable insight when the method is applied to composites. Fig. 12 shows a case where polyethylene and its 650 Electroluminescence (a.u.) Wavelength (nm) (b) (a) (c) Fig Frequency resolved electroluminescence spectra for 10 % Ti 2 /epoxy composites [nano (a), and micro (b)] in comparison with the cured base resin (c). very small demanding high sensitivity, and many polymers of interest are not transparent. Since electroluminescence can originate from either charge recombination or from hot electron processes [32], the interpretation is also not always straightforward. Nevertheless, the method does provide valuable insight into the behaviour of nanocomposites when they are compared both with the base resin and also with an equivalent microcomposite of the same loading. Using Fig Dielectric absorption characteristics for X-linked polyethylene and both functionalized and unfunctionalized Si 2 nanocomposites (lower three curves) and an equivalent microcomposite (upper curve). Applied field: 20 kvmm -1 nanocomposites exhibit a power law relationship (approximate linear plot on a log-log scale), but there is a marked difference in the case of a conventional (micro) composite which shows a behaviour typical of interfacial polarization (Raju [34]). It would appear that the build up of internal charge at the internal interfaces is mitigated in the case of nanomaterials which is consistent with the space charge studies highlighted in Section 4.4. It is also possible to use transit time methods to estimate the carrier mobility and such estimates have been made for polyethylene-si 2 and suggest that nanodielectrics exhibit a decreasing

8 mobility with electric field in contrast to the opposite effect seen both for the host polymer and for a micromaterial [10]. However, it is also not certain that the form of the Si 2 is the same for the micro- and nano-particles. nce again, it would appear that nanodielectrics were behaving in a fundamentally different way. In this instance the behaviour would appear to be only dependent on the particle size since a decreasing mobility is found which is independent of the nature of the chemical coupling used. chains are chemically or physically bonded and thus likely to have an ordered structure. This region is the basis for a number of models advanced to try to explain nanodielectric behaviour [9]. The engineering of this region through preferred bonding to establish a measure of self assembly and to affect the properties of the bulk are pivotal in the thrust to develop nanodielectrics through functionalization of the particulates. 6. The emerging picture Some of the examples described here, together with many others that may be found in the literature make it clear that when the particulate dimensions for a filled polymer are reduced to a nanometric scale, then both property and behavioural anomalies appear. Perhaps one of the most remarkable in this regard is the finding by several groups that the inclusion of a high permittivity nanofiller (e.g. Ti 2 having ε r 100) can result in a composite having a relative permittivity which is lower than the base resin from which it was formed. This behaviour cannot be explained by any of the established mixing rules, and is due to the restriction of the mobility of dipolar groups causing a reduction in the polarization associated with the matrix [35]. In this context, the work of Kurimoto et al. [16] is particularly germane. These authors have clearly demonstrated that this reduction in permittivity only comes about when proper dispersion is achieved. This is part of the justification for the emphasis placed on processing in Section 2.2. It has become clear that the fundamental reason relates to the large internal surface area that is created, but it is Fig. 13 Representation of the interaction zone surrounding a nanoparticle inclusion not so obvious that this operates in the same way for all the phenomena studied and for all systems. While it has been postulated that the plurality of interface may result in the scattering of carriers and the interruption of channel propagation to affect voltage endurance, that would not explain, for example, the capacitance effect cited above. The pictorial representation in Figure 13 depicts the region surrounding a particle in which the polymer chains are impacted. At the interface, the Fig. 14 Electrical (left) and mechanical (right) representations of the interfacial zones The exact nature of the interfacial zone is still not clear. However, Lewis [36] has suggested that, from the electrical viewpoint, it may comprise an electrical diffuse layer not unlike the Gouy-Chapman-Stern layer typical of the absorbed layers in liquid dielectrics represented in Fig. 14. Such a situation may imply local conductivity that will not necessarily be reflected in the bulk properties measured externally. Furthermore, the double-layer component would represent a highly polarizable region. The chemical implications of this have been discussed by Reed [11], and some of the techniques in Table 3 can be used to obtain indirect evidence. Furthermore, it is known that the dynamics of charge decay and electroluminescence can be affected which provides some basis for the time constant being altered. The right hand side of Fig. 14 represents an alternative approach in which there is a distribution of mechanical mobility across the interaction zone. The presence of an interfacial zone also has a marked effect on percolation since, for many processes, it will be the overlapping of the zones which will define percolation and not the particles themselves. 7. Applications and future development The rapid development of nanodielectrics based on polymer composites has been largely fuelled by the promise of enhanced properties for conventional insulation applications. Examples of this might include:

9 Enhanced electric strength for high stress duty as found, for example, in cables and capacitors. The universal need for superior electric strength has fuelled commercial interest. Corona resistant insulation for applications such as wire enamels and form-wound machine stators. Multifunctional insulation requirements including elevated temperature and cryogenic applications, the management of insulation thermal conductivity, increasing the energy density of capacitor systems (see Section 7.1), etc. In many of these applications, advantage is taken of an ability to enhance one property without the usual large penalty in some other attribute. However, in most cases, the lack of a clear mechanistic picture has necessitated a trial-and-error approach. This is particularly the case since no two systems are fabricated in the same way see Section Capacitor applications Perhaps the application which has attracted the most interest is that of capacitor dielectrics [9,41] for both embedded and stand-alone applications. High energy density storage has been a goal for half a century, but, although incremental gains have been made, the fundamental problem remains that the energy storage increases as E 2, but only proportionately to the permittivity, ε.r. Attempts to increase ε r which result in a reduction in electric strength are, thus, often counterproductive. Furthermore the inclusion of high ε.r particulates (such as ferroelectric nanometer-sized of conducting nanoparticles (Ag and C, for example), the resulting materials are also often very lossy The way ahead Although most of the particle treatment in the past has been aimed at either trying to achieve exfoliation or to obtain preferential bonding, it is likely that second generation designs may attempt to use the particle surfaces to create materials with smart attributes. A simple example of this is the incorporation of non linearity into the insulating structure to provide for stress relief [37]. Emerging fields such as electroactive polymers are also likely to be facilitated by the application of this technology Furthermore, recent advances in controlled radical polymerization techniques such as nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer polymerization (RAFT) have greatly facilitated controlled synthesis of polymers and have improved the ability to design polymer grafted nanoparticle interfaces [38,39]. These methods comprise a toolbox that allows researchers to design interfaces with several levels of control over chain chemistry, chain length and chain graft density. Block copolymers can be synthesized so than an active polymer can be provided with an outer block to improve compatibility between the filler and the matrix. A schematic of this chemistry approach is shown in Fig. 15. Table 4 Composition and properties of nanocomposites for embedded capacitors. Adapted from [41] Material ε.r. Tan δ Size nm Vol % BaTi 3 /epoxy Pb(Mg 13 Nb 23 ) 3 /PVDF Bimodal BaTi 3 /epoxy /60 75 PMNPT + BaTi 3 /epoxy /50 85 BaTi 3 //PVDF 37 < C Black/epoxy Ag-CB/epoxy Ag Al-Ag/epoxy Ag<20 80 Ag/epoxy Ag-C/epoxy >300, powders) to try to improve storage is often frustrated by the diversion of the stress to the matrix material for loadings far from the percolation limit. This is illustrated in Table 4 drawn from the work of Lu and Wong [41] which indicates that high loadings are needed to realize high values of permittivity in most systems. The Table also shows that, although the highest permittivities may be obtained through the use Fig Schematic showing the general RAFT method used in creating homopolymers and block copolymers grafted from nanoparticles. Advances such as this are likely to become pivotal in attempts to engineer nanodielectrics while maintaining the critical dispersion highlighted in Section 2.2. Silicates (clays) are an important subclass of nanocomposites which usually require special treatment to exfoliate the materials. However, the aspect ratio of the particulate is a variable which has been known for a long time to affect the dielectric performance. In addition to the other phenomena taking place at the interface, the aspect ratio can be used to change the internal electric field at the particle interface. In this context, high aspect ratios have been achieved through electrospinning [40]. It is also possible to fabricate nanocomposites with high aspect ratio nanofillers utilizing imposed dielectrophoretic forces during processing. In this way an anisotropic material can be obtained.

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