The Future of Nanodielectrics in the Electrical Power Industrylme.elmat.pub.ro/~florin/lab_MEN/doc/Y.Cao_Nanocmposites.pdf · types of nanocomposites, based on insulating, semicon-ducting

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 11, No. 5; October 2004 797

The Future of Nanodielectrics in the Electrical PowerIndustry

Yang Cao, Patricia C. Irwin and Karim YounsiGE Global Research CenterNiskayuna, NY, 12309 USA

ABSTRACTWhile specialty applications of nanotechnology in the photonics and electronics ar-eas have seen a tremendous growth in the past several years, the use of nanodi-

( )electrics in the electrical industry high power density and high voltage has notshown the same level of activity. In addition to a review of nanodielectrics, we dis-cuss in this paper, our perspective on the current status, development needs andfuture potential to build or engineer nano-structured materials for dielectric appli-cations in the electrical power industry. Short and long-term future research anddevelopment needs are considered from the point of view of industrial applica-tions.

Index Terms — Nanodielectrics, nanocomposites, nano structure, dielectrics,electrical insulation.

1 INTRODUCTIONANODIELECTRICS is the study of dielectric phen-Nomena and materials on the nanometric scale and the

fabrication of structures, devices and systems that havenovel dielectric properties and function because of their

w xnanometric structure 1, 2 . The novel and differentiatingproperties of these structures are developed at a criticalscale of matter, typically under 100 nanometers. The scope

wof nanodielectrics however, can be broad and extensive 2,x3 . Therefore, in this paper, we will use ‘‘simple’’ two-

phase heterogeneous nanodielectric systems as examplesto discuss the long and short-term development needs andapplications of nanodielectrics.

Currently, the synthesis and fabrication of nanometer-sized particles have reached a mature state and they can

w xbe made with controlled composition and size 1 . Becauseof their length scale and high specific surface area,nanoparticles exhibit novel properties as compared withbulk materials. A lot of research has been carried out re-cently incorporating various nanoparticles into existing di-electric systems in a cost effective manner, resulting innanocomposites with improved benefits over conventional

w xfiller systems 4�7 . The improvements can be a combina-tion of electrical, mechanical and thermal enhancementsw x5, 6 , and can be further tailored through organic synthe-sis, material design optimization. These types of ‘‘top-

Manuscript recei®ed on 21 April 2004, in final form 24 June 2004.

down’’research styles will continue to be fruitful with manyimportant engineering applications in the near future.

Meanwhile, a great deal of progress has been achievedin fabricating and assembling nanostructures to high pre-cision and reproducibility, producing interesting optical,

w xphotonic, electronic, and chemical properties 8�13 .While some of the nano structures are formed using di-

w xelectric methods 14, 15 , the dielectric properties of thesenovel structures have not been fully explored. These ‘‘bot-tom-up’’ structures not only provide unique opportunityfor the study of dielectric phenomena at the nanometriclevel, but also have great potential for groundbreaking ap-plication of nanodielectrics in the long term. As an exam-ple, a giant dielectric constant of �1010 was reported fora nano silver assembly in a polymer matrix as a result of

w xquantum confinement of the electronic wave function 16 .It is not difficult to speculate that its potential applica-tions in energy storage and ultra sensitive transducers aretremendous.

2 CURRENT INDUSTRIALAPPLICATIONS

Two-phase heterogeneous nanodielectrics, generallytermed as dielectric nanocomposites, have wide applica-tions in the electrical and electronic industries. Varioustypes of nanocomposites, based on insulating, semicon-ducting or metallic nanoparticles, have been developed tomeet the requirements of specific applications. For in-stance, nanocomposites with anisotropic electrical proper-ties are desirable for improving partial discharge resis-tance of the ground wall insulation of rotating machinery.

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Cao et al.: The Future of Nanodielectrics in the Electrical Power Industry798

2.1 ELECTRICAL APPLICATIONSInsulation integrity is of great importance for all-electri-

cal power applications, including energy conversion, powerw xdelivery, energy storage, and power consumption 17�20 .

Nanodielectrics can enhance the reliability of current sys-tems, and more importantly, can improve their efficiency

by enabling innovative design and the utilization of re-newable energy resources.

With the ever-growing demand for compact and highlyefficient electric machines, the electrical insulation sys-

w xtems are subjected to multiple stresses 21 with greaterw xintensity, and higher repetition rate 18 . While conven-

Ž .Figure 1. a, 2-parameter Weibull statistic plot of the breakdown strength of polyimide PI nanoparticle composites at different loadings ofalumina nanoparticles. The tests were performed with a 500 vrs ramp rate. The samples were 25 �m thick. The inset shows the Weibullcharacteristic breakdown strength vs composition with loadings up to 10wt%. The bars in the inset correspond to the 95% Weibull confidencelimits. From the plots, slightly increase of the breakdown strength of PI with loading is observed; b, Breakdown strength plot for PI silicananoparticle composites tested under the same conditions.

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tionally filled polymeric insulation offers improved electri-cal discharge resistance, thermal expansion coefficient ad-justment, mechanical robustness and heat dissipation, it isoften achieved at the cost of reduced dielectric perfor-

w xmance 17, 19, 20 . The operational field strength is thefundamental design limitation for high voltage systems. Itis possible to push back this design barrier by utilizingnanodielectrics. Several typical nanocomposite systems forapplication in this area, which illustrate the current situa-tion, will now be discussed.

2.1.1 NANOCOMPOSITES WITHINSULATING NANOPARTICLES

An important type of nanocomposite is one with insu-lating nanoparticles such as silica and alumina. As shownin Figure 1, the electrical breakdown strength of a poly-

Ž .imide PI is not affected by these types of nanofillers forloadings up to 10wt%. The study of the conduction mech-

w xanism of these nanocomposites 5 indicates a field en-hanced, thermally assisted ionic-hopping conduction cur-rent

U eEdjs2nd � exp y sinh 1Ž .

kT 2kT

w xas described earlier 22�24 , where U is the activationenergy, d is the hopping distance, and � is the attempt-to-escape frequency, n is the carrier density, k is theBoltzmann constant, T is the temperature and e is theelectronic charge. Figure 2 gives a 3D curve fitting of thesteady-state current density of 10wt% silica filled PI

Ž .nanocomposites to equation 1 . Normally the addition ofconventional micro sized fillers would result in increase of

Figure 2. Field and temperature dependence of the steady-statecurrent density for a PI nanocomposite with 10wt% nano silica. The

Ž .3D curve fitting of the data to equation 1 gives an activation energyw xof 0.68 eV and a hopping distance of 4.3 nm 5 .

Figure 3. Comparison of the conductivity of pure and nano-filledpolyimides. a, conductivity at 100�C; b, conductivity at 150�C. Circlesdenote the bulk conductivity of unfilled polyimides. 2wt% silicananofilled nanocomposites show a significant reduction in conductiv-

Ž .ity at both 100�C and 150�C diamond symbols whereas 10wt%nanofilled composites are less dramatic in conductivity changesŽ .square symbols . More work is needed to understand the complexnature of temperature and filler loading effects on the bulk conduc-tivity of a nanocomposites.

bulk conductivity due to the increased defects and poorfillerrpolymer interactions. Interestingly it is found that2wt% silica nanofiller shows a significant reduction inconductivity at both 100�C and 150�C whereas 10 wt%nanofilled composites are less dramatic in conductivitychanges, indicating a complex nature of temperature andfill loading effects on the bulk conductivity of a nanocom-

Ž .posites Figure 3 .

In addition to the enhancement of partial discharge orŽ .corona resistance Figure 4 , the addition of nanoparticles

shows a marked improvement in ductility and yieldstrength, shown in Figure 5, as well as certain improve-ment of the scratch resistance and thermal conductivityw x6 . The improvement of the mechanical properties is im-portant, because an insulating material might be subjectedto constant vibration or abrasion by the double power fre-quency magnetic force andror high shear stress under

w xrapid thermal loading 25 . As a result, existing high tem-perature thermosetting insulation, which is inherentlybrittle, quite often undergoes voidrcrack formation or de-lamination under these mechanical stresses with subse-quent electrical discharge and catastrophic failure.

Overall, these nanocomposites show combined electri-cal, mechanical and thermal improvements over conven-tional filler systems.

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Figure 4. The enhancement of partial discharge resistance of PIwith a nanofiller at 10 wt% loading. The dotted lines represent the95% confidence limits.

Figure 5. Example of tensile experiments showing an increase inductility of a polymeric system through the addition of nanoparticles.‘‘X’’ denotes a mechanical failure during the tensile stress test. An

unfilled polyimide fails at 2% strain and 6�107 Pa whereas a 5wt%alumina filled PI does not fail until 11% strain and 1.2�108 Pa. TheYoung’s modulus of the two materials is initially similar but thenanofilled sample showed significant increase in elongation prior tofailure.

2.1.2 NANOCOMPOSITES WITH NONLINEARNANOPARTICLES

Even though the short term breakdown strength is re-duced for nanocomposites with nanofillers shown in Fig-

Ž .ure 6 TiO , ZnO, BaTiO , SiC , they form an important2 3class of dielectric materials with special dielectric proper-ties. For instance, SiC and BaTiO composites have been3employed as field grading materials and ZnO is an excel-lent surge arrestor additive because their electrical prop-

Ž .erties conductivity andror permittivity are strongly fielddependent. Recent studies indicate that nanocompositesbased on these materials have unique dielectric propertiesw x26, 27 . LDPErZnO nanocomposites exhibit a lower per-colation threshold than classical percolation theory, whichdescribes a power law dependence of conductivity on filler

w xconcentration p above a percolation threshold of p 26cn

� A py p 2Ž . Ž .c

Figure 6. ac breakdown strengths for PI with various nanofillers at5wt% loading. The tests were performed with 500 vrs ramp rate onsamples of 25 �m thick. The remarkably low breakdown strength ofSiC nanocomposites may be caused by the aggregation of SiCnanoparticles.

Further investigation shows that for polyethylene filledwith micro and nano ZnO fillers, the decrease in resistiv-ity starts at the same critical interparticle distance l ofc

Ž w x.about 40nm Figure 2 of 26 . The critical interparticledistance can be written as

1r24�l s r p y2 3Ž .c cž /3

by assuming a spherical particle shape with a radius of r,and a uniform size distribution and dispersion. Below thecritical interparticle distance l , LDPErZnO nanocom-cposites exhibit a smaller decrease in resistivity with fillerconcentration when compared with conventional compos-

w xites 26 .

The addition of conventional TiO filler to polyethy-2lene has been investigated for DC power transmission ap-

w xplications 28 , in which the space charge accumulation dueto the large thermal gradient across the cable may causecatastrophic failure of the cable insulation system duringpolarity reversal. It was also shown recently that TiO2nanoparticles could mitigate such space charge accumula-tion in an epoxy system, as compared with conventional

w xTiO fillers 27 .2

2.1.3 ANISOTROPIC NANOCOMPOSITES

Anisotropic materials and structures inherently haveunique properties and applications. Polymerrlayered sili-cate nanocomposites represent good examples of suchmaterials with attractive properties, including high me-chanical moduli, heat resistance, barrier behavior, ion-ex-change capability. A comprehensive review of the prepa-ration, characterization, properties and processing ofpolymerrlayered silicate nanocomposites can be found inw x29 . Technologies have been developed so that layeredsilicates can be incorporated into most types of polymersand nanocomposites can be formed through melt-mixing

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Figure 7. Chemical structure of phyllosilicates, a typical nanoclay whose crystal structure consists of layers made up of two tetrahedrallyw xcoordinated silicon atoms fused to an edge-shared octahedral sheet of either aluminum or magnesium hydroxide 29 . The layer thickness is

about 1 nm, and the lateral dimensions of these layers may vary from 30 nm to more than several microns. There are three different types ofw xthermodynamically achievable polymerrlayered silicate nanocomposites, namely, intercalated, intercalated-and-flocculated and exfoliated 29 .

With proper orientation, such layered silicate nanocomposites have remarkable anisotropic properties as shown schematically, i.e., verticalŽ .barrier corona resistance properties and lateral energy dissipation through confined ionic conduction. Such anisotropic electrical and thermal

properties are of great use for rotating machine insulation.

w x30 . As shown in Figure 7, the barrier behavior of theirlayered structure and the adjustable anisotropic ionic con-

w xductivity between the layers 31 for such polymerrlayeredsilicate nanocomposites make them particularly interest-ing for electrical applications.

Mica based insulation systems are used extensively inHV rotating machines for the exceptional partial dis-charge resistance. The discharge resistance lies perpen-dicular to the alumino-silicate plane axis due to the strongin-plane bonding. However, the graphitic, layered micastructure can delaminate under thermal-mechanicalstresses, forming voids to support partial discharge. As aresult, the design field for micarepoxy or micarpolyester

w xsystems is limited to �3 kVrmm 17 . Fully exfoliatedŽ wand orientated for example through shear orientation 32,

x.33 silicate nanocomposites can potentially be engineeredwith good discharge resistance and with improved me-chanical properties. Moreover, supramolecules with simi-lar structure have been fabricated in the laboratory, tar-geting anisotropic ionic conduction, for applications such

w xas fuel cells 34 .

Carbon nanotubes potentially can find their applica-tions in electric power engineering also. Because of theirhigh aspect ratio, randomly distributed carbon nanotubescan induce significant property change at only 1�5% load-

w xing 35 . Compared with conventional carbon black filler,properly distributed and oriented nanotubes within thematrix will lead to many applications, including field grad-ing.

2.2 ELECTRONIC APPLICATIONSThe electronics industry is driving a significant portion

of the nanocomposites research and development. Withthe continuing miniaturization of microelectronic struc-

Žtures, low k dielectrics below 3 as compared with 4.5 for.silicon dioxide are needed to reduce the signal transition

time, power consumption as well as crosstalk through ca-w xpacitive coupling 36�38 . Nanoporous dielectrics com-

posed of an insulating matrix embedded with pores ofnanometer size are candidates. For example, silica aero-gelsrxerogels having closed pores with porosity as high as90% while retaining good dielectric properties have been

w xdeveloped 39 . The relative permittivity can be controlledthrough porosity adjustment and can be as low as 1.3. Thechallenges lie in the compatibility with current systems andin the mechanical robustness to chemical-mechanical pol-

Ž .ishing CMP . Research shows that polymer basednanoporous low-k materials used in a spin-on process

Ž .compete with modified chemical vapor deposition CVDw xfilms 40, 41 . In addition to interconnections in the high-

density integration, advanced packaging requires also loww xpermittivity dielectrics for RF applications 42 . Mean-

while, polymer-ceramic nanocomposites with high dielec-tric constant are needed for ‘‘System-On-Package’’ and

w xembedded capacitors applications 43�45 .

2.3 OTHER APPLICATIONSThere are many other applications of nanodielectrics

such as ionic conductors, nanoceramics for electronics ap-w x Žplications 46 and effective EMIrEMC Electromagnetic

.InterferencerCompatibility shielding. High frequencyEMIrEMC designs require better solutions than conven-tional metallic shielding in electronic enclosures in orderto attenuate unwanted electromagnetic radiation withoutreflection. For wave propagation in inhomogeneous me-dia, RF ‘‘resonant cavity’’ reflections due to the mismatchof the intrinsic impedances at the enclosure interface areproblematic. Dissipative dielectric absorbers with con-

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Figure 8. Stealth-like dielectric absorbing materials with controlledŽ . Ž . Ž .permittivity � , permeability � and conductivity � can offer more

flexible EMIrEMC designs than traditional enclosure confinementby attenuating incident electromagnetic radiation. A conventionalmetallic enclosure can be viewed as a resonant cavity because of re-

Ž Ž . .'flections by mismatching of the intrinsic impedances �r� . Softferrite nanoparticles are useful not only for permeability adjustmentbut also for the enhancement of electromagnetic absorption at highfrequency.

trolled permittivity, permeability and conductivity can po-tentially offer near stealth-like functionality to eliminateundesirable RF reflections, as shown schematically in Fig-ure 8. Such development will have wide applications inthe electronics, telecommunications and automotive in-dustries.

3 DEVELOPMENT NEEDS ANDFUTURE TRENDS

It would be desirable if nanocomposites could be fabri-cated with tailored dielectric properties, i.e., controllablepermittivity and conductivity as a function of temperature,electric field and frequency. As particles shrink to nano-metric sizes the interface becomes increasingly importantand ultimately totally dominant. The understanding of therole interface in nanodielectric is far from being satisfac-tory. Success is limited to a qualitative level for even a

w xsimple nanostructure 1, 47 and, quite often, classical ap-proximations such as the effective medium method are

w xused in practice 45 . Therefore, a good understanding ofthe interaction between nanoparticle and matrix, inter-play between particles, and associated transport phenom-ena is of great importance.

Incorporating particles of nanometric scale has ren-dered interesting physical, chemical modifications of thematrix dielectric system. Again, if we take silica-filled PI

Žnanocomposites nanocomposites with insulating. Ž .nanoparticles as an example Figure 9 , main thermally

stimulated current measurements show clearly the shift ofthe peak current temperature from 185�C for pure poly-imides to about 200�C for polyimides with 2 wt% nanofillerand almost 220�C for polyimides with 5 wt% nanofiller.When the filler loading increases to 10 wt%, the peak

Ž .Figure 9. Thermally stimulated current TSC data for polyimidenanocomposites with various filler loading. Poling was performed witha field of 20 kVrmm at 250�C for 10 minutes. A heating rate of2�Crmin was used during the TSC acquisition. Since dynamic

Ž .thermo-mechanical analysis DTMA shows no major dipolar relax-ation in this temperature range the peaks are likely to correspond tothe release of trapped carriers by the thermally stimulated process.The shift of the TSC peak temperature indicates deep trapping ofcarriers by the incorporated nanoparticles.

temperature reduces to about 210�C. Because a higherpeak current temperature usually corresponds to a higheractivation energy for trapped charges, deeper trapping isintroduced by incorporating nano silica fillers.

In addition to such physical modifications, nanofillerscan induce chemical modifications. A nano TiO filler in2polyethylene induces a dielectric response by a slight oxi-dation of the polymer such that a previously not visibledielectric response is seen clearly on the relaxation maps

w xof polyethylene 48 . Dielectric models need to be devel-oped to better describe such physicalrchemical modifica-tions by nanofillers as well as the dipolar activities at theinterface and the associated transport phenomena.

While these approaches through incorporation ofnanoparticles will make progressive improvements in im-portant engineering fields in a cost effective way, they havelimitations such as wide particle size distribution, particleagglomeration, etc. Nanocomposites based on semicon-ductive nanofillers like TiO have been studied exten-2sively not only because of a chemically active surface, but

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also because of their electronic and optical confinementw x Ž .capability 1 . Semiconductive particles quantum dots

have been assembled into arrays using pattern-formationw xblock co-polymers as templates 49 and more importantly,

their photonicrelectronic properties can be altered withthe particle size, showing clearly quantization effects inwhich the energy levels available for electrons and holesbecome discretized and confined in a nanometer sizerange. This unique property has led to many applicationsin electronics, photonics and potentially dielectrics also.In 1965, Gor’kov and Eliashberg, predicted an enormouslyenhanced polarizability of a sufficiently minute metallic

w xparticle having discrete energy levels 50 . However, suchprediction had to be corrected for the depolarization fieldwithin the particle and the results of high dielectric con-stant were interpreted using an ‘‘interrupted-strand

Figure 10. Formation of an assembly of ultrafine metal particlesconnected to each other to form an interrupted metal strand by theapplication of electric field to an assembly of isolated minute metalparticles. The strong electrostatic force due to the high electric fieldŽ 6 .10 Vrcm in the nanometric air gap between the particles causesthem to deform in the direction of the electric field and finally toattain a percolation configuration with the formation of a metalfilament. The electronic wave functions of these assembly are local-

w xized due to lattice defects at the interfaces between the particles 16 .Ž .a, Transmission electron micrograph TEM of nanowires; b, diame-

Ž .ter of the wire, 15 nm in the magnified view of the wire of a . Fromthe figure it is evident that the wire is formed by the metal nanopar-ticles; c, TEM photo of the wire after the application of voltage pulse;d, filamentary growth of ultra fine particles.

model’’ - a real crystalline compound consisting of a sys-tem of collinear metallic strands, each of which is inter-rupted by a series of perfectly insulating lattice defectsw x51 . According to such a strand model, the dielectric con-stant can be written as

1 2�f q l 4Ž .Ž .s 02

where q is the Fermi-Thomas screening wave vector ofsthe conduction electrons and l is the strand length.0

Many attempts failed because of the poor control of thew xstructure at the nanoscale 52�56 . It is reported only re-

Žcently that, using a polymer nano template with 15 nm. w xpores , researchers 16 were able to fabricate and assem-

ble a silver nanowire as shown in Figure 10. By applying abias, the wire turns into an assembly of ultrafine metalparticles with localized electronic wave function. A dielec-tric constant of 1010 was reported for such a dielectric-

Figure 11. Capacitance and resistance under bias. a, 20 Hz; b, 110 w xKHz, indicating a giant � of 10 16 .

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Ž .Figure 12. Electrostatic self-assembled monolayer ESAM tech-w xnique for fabricating non-centro symmetric structures 58 .

w xconducting particle assembly 16 . Extremely high dielec-tric constant materials would be useful as ultracapacitorsfor energy storage. In addition, because of the surfacesensitivity of such a nano assembly and because of its ul-tra fast response, since it relies on electronic rather than

dipolar polarization, it can be developed into an ultra sen-sitive airborne sensor.

Dielectrics research should also benefit from dramaticprogress in nanotechnology where supramolecular struc-

w xtures have been assembled for novel properties 57 . Forexample, an electrostatic self-assembled monolayerŽ .ESAM technique for fabricating non-centrosymmetric

w xstructures has been reported 58 . Such a technique re-sults in tens of microns thick films with large, stable sec-ond order nonlinear optical responses without electricfield poling. As shown in Figure 12, a multi-layer film isformed by alternately immersing the substrate in aqueoussolutions of a polyanion and a polycation, with one or boththe polyanion and polycation containing polarizable chro-mophores. Similar self-assembly methods have been de-veloped to generate superlattices with oriented giant

w xdipoles for electro-optic applications 59,60 . Nanostruc-tured materials with giant electrostriction as well as or-ganic composite actuator material with high k have also

w xbeen reported 61,62 .

While current research is focused on filled systems, inthe long run, nanostructured dielectric materials and de-vices will have many intentionally designed novel dielec-tric properties and have wide applications. Figure 13 shows

Figure 13. Time scale in the development of nanodielectrics with novel propertiesrapplications for illustration only: a, TEM photo of aluminanano particles dispersion; b, a bright-field TEM detail of the PSroctadecyl-ammonium silicate intercalate and the simulation of the structure: a

w x Ž .styrene 12mer and the octadecyl-ammonium surfactant molecule 63 ; c, SEM photo of carbon nanotube filled Poly methyl methacrylate withw xunique mechanical, electrical and thermal properties 64 ; d, Proposed supramoleculesrassembly with design-in dielectric properties. Some of

w xthe structures taken from 8, 65 .

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schematically our perspective for the time scale in the de-velopment of nanodielectrics. Other than heterogeneousnanodielectric systems as discussed in this paper, homoge-neous nanodielectrics such as ferroelectrics materials withnanosized domains have also many interesting propertiesfor us to explore.

4 CONCLUSIONSASED on recent progress in the nanodielectrics fieldBand the electrical power industry needs, we have

summarized in the present paper our perspective on thedevelopment, applications and time scale in the develop-ment of nanodielectrics in the areas shown schematicallyin Figure 13.

In the short term, incorporation of nanoparticles willmake combined progressive electrical, mechanical andthermal improvements in important engineering fields, re-sulting in many interesting applications such as enamelinsulation, electrical grading, EMI shielding, in a cost ef-fective manner.

With proper materialrprocessing developments,Žnanocomposites based on preformed mineral or syn-

.thetic anisotropic fillers will have unique anisotropicproperties suitable for applications such as electrical dis-charge resistant machine insulation.

In the long run, with the improving of understanding ofdielectric physics at nanoscale, nanostructured dielectricmaterialsrdevices could be engineered with many noveldielectric properties and wide applications in energy stor-age, electro-optic, electro-strictive, and insulating areas.

Fundamental understanding of dielectric properties atthe nanoscale level is of great importance in the develop-ment of functional nanodielectrics for the electric powerindustry. More than ever such development needs will re-quire close cooperation between industry and academia.Major original equipment manufacturers for example arealready investing tremendous research and developmenteffort and resources to position themselves for the comingchallenging decade.

ACKNOWLEDGMENTSThe authors are indebted to the kind help and guidance

from the editors, Prof. L.A. Dissado and Prof. J.C.Fothergill. The authors would like to thank Drs. RandallCarter, George Gao, Clive Reed, Nancy Frost, MichaelTakemori, Michael Minnick, and Mr. Michael Brown forsupport and helpful discussions.

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Editor’s note: Web sites references are no longer used in IEEE journals be-cause they are not permanent and therefore are not archi®al. Howe®er, sincethe nanodielectric area is still emerging at the present time some rele®ant in-formation can only be found on web sites and therefore an exception is madefor this paper.

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