neng guo et al- nanoparticle, size, shape, and interfacial effects on leakage current density,...

8/3/2019 Neng Guo et al- Nanoparticle, Size, Shape, and Interfacial Effects on Leakage Current Density, Permittivity, and Br

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pubs.acs.org/cmPublished on Web 01/05/2010r 2010 American Chemical Society

Chem. Mater. 2010, 22, 15671578 1567DOI:10.1021/cm902852h

Nanoparticle, Size, Shape, and Interfacial Effects on Leakage CurrentDensity, Permittivity, and Breakdown Strength of Metal

Oxide - Polyolefin Nanocomposites: Experiment and Theory

Neng Guo, Sara A. DiBenedetto, Pratyush Tewari, Michael T. Lanagan,* ,Mark A. Ratner,* , and Tobin J. Marks* ,

Department of Chemistry and the Materials Research Center, Northwestern University, Evanston,Illinois 60208-3113 and Center for Dielectric Studies, Materials Research Institute, The Pennsylvania State

University, University Park, Pennsylvania 16802-4800

Received September 11, 2009. Revised Manuscript Received December 2, 2009

A series of 0 - 3 metal oxide - polyolefin nanocomposites are synthesized via in situ olefin polymeriza-tion, using the following single-site metallocene catalysts: C 2-symmetric dichloro[ rac -ethylenebisindenyl]-zirconium(IV), Me 2Si(

tBuN)( 5-C5Me 4)TiCl 2,and( 5-C5Me 5)TiCl 3 immobilized on methylaluminoxane(MAO)-treated BaTiO 3, ZrO 2, 3-mol%-yttria-stabilized zirconia, 8-mol %-yttria-stabilized zirconia,

sphere-shapedTiO 2 nanoparticles, and rod-shapedTiO 2 nanoparticles.The resulting composite materialsare structurally characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM),transmission electron microscopy (TEM), 13C nuclear magnetic resonance (NMR) spectroscopy, anddifferential scanning calorimetry (DSC). TEM analysis shows that the nanoparticles are well-dispersed inthe polymer matrix, with each individual nanoparticle surrounded by polymer. Electrical measurementsreveal that most of these nanocomposites have leakage current densities of 10

- 6- 10- 8 A/cm 2; relative

permittivities increase as the nanoparticle volume fraction increases, with measured values as high as 6.1.At the same volume fraction, rod-shaped TiO 2 nanoparticle - isotactic polypropylene nanocompositesexhibit significantly greater permittivities than the corresponding sphere-shaped TiO 2 nanoparticle -isotactic polypropylene nanocomposites. Effective medium theories fail to give a quantitative descriptionof the capacitance behavior, but do aid substantially in interpreting the trends qualitatively. The energystorage densities of these nanocomposites are estimated to be as high as 9.4 J/cm 3.

Introduction

Future pulsed-power and power electronic capacitors willrequire dielectric materials with ultimate energy storage den-sitiesof>30 J/cm 3, operating voltages of>10 kV,and milli-second - microsecond charge/discharge times with reliableoperation near the dielectric breakdown limit. Importantly,at2 and 0.2 J/cm 3, respectively, the operating characteristicsof current-generation pulsed power and power electroniccapacitors, which utilize either ceramic or polymer dielectricmaterials, remain significantly short of this goal. 1 An order-of-magnitude improvement in energy density will require thedevelopment of dramatically different types of materials,which substantially increase intrinsic dielectric energy den-sities while reliably operating as close as possible to the die-lectric breakdown limit. For simple linear response dielectric

materials, the maximum energy density is defined in eq 1,

U e 12

r0 E 2 1

where r is the relative dielectric permittivity, E the dielec-tric breakdown strength, and 0 the vacuum permittivity(8.8542 10

- 12 F/m). Generally, metal oxides have largepermittivities; however, they are limited by low breakdownfields. While organic materials (e.g., polymers) can providehigh breakdown strengths, their generally modest permit-tivities have limited their application. 1

Recently, inorganic - polymer nanocomposite materialshave attractedgreat interest, becauseof their potential forhigh energy densities. 2 By integrating the complementary

*Authorsto whomcorrespondenceshouldbe addressed. E-mail addresses:[email protected] (M.T.L.), [email protected] (M.A.R.), [email protected] (T.J.M.).

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properties of their constituents, such materials can simul-taneously derive high permittivity from the inorganic in-clusions and high breakdown strength, mechanicalflexibility, facile processability, light weight, and tunabilityof the properties (polymer molecular weight, comonomerincorporation, viscoelastic properties, etc.) from the poly-mer host matrix. 3 In addition, convincing theoretical argu-ments have been made suggesting that large inclusion-

matrix interfacial areas should afford greater polarizationlevels, dielectric response, and breakdown strength. 4

Inorganic - polymer nanocomposites are typically pre-pared via mechanical blending, 5 solution mixing, 6 in situradical polymerization, 7 and in situ nanoparticle syn-thesis. 8 However, host - guest incompatibilities intro-duced in these synthetic approaches frequently result in

nanoparticle aggregation and phase separation over largelength scales, 9 which is detrimental to the electrical prop-erties of the composite. 10 Covalent grafting of the poly-mer chains to inorganic nanoparticle surfaces has alsoproven promising, leading to more effective dispersionand enhanced electrical/mechanical properties; 11 how-ever, such processes may not be optimally cost-effective,nor may they be easily scaled up. Furthermore, the

development of accurate theoretical models for the di-electric properties of the nanocomposite must be accom-panied by a reliable experimental means to achievenanoparticle deagglomeration.

In the huge industrial-scale heterogeneous or slurryolefin polymerization processes practiced today, SiO 2 isgenerally used as the catalyst support. 12 Very large localhydrostatic pressures arising from the propagating poly-olefin chains are known to effect extensive SiO 2 particlefracture and lead to SiO 2- polyolefin composites.

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2O

3layer to moderate the large anticipated

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Article Chem. Mater., Vol. 22, No. 4, 2010 1569

polyolefin - ferroelectric permittivity contrast. If too

large, such contrasts are associated with diminishedbreakdown strength and suppressed permittivity. 18,19

In a brief preliminary communication, we reportedevidence that high-energy-density BaTiO 3- and TiO 2-isotactic polypropylene nanocomposites could be pre-pared via in situ propylene polymerization mediated byanchoring/alkylating/activating C 2-symmetric dichloro-[rac -ethylenebisindenyl]zirconium(IV) (EBIZrCl 2) on theMAO-treated oxide nanoparticles (see Scheme 1). 20 Theresulting nanocomposites were determined to have rela-tively uniform nanoparticle dispersions and to supportremarkably high projected energy storage densities ; ashigh as 9.4 J/cm 3, as determined from permittivity anddielectric breakdown measurements. In this contribution,we significantly extend the inorganic inclusion scope toinclude a broad variety of nanoparticle types, to investi-gate the effects of nanoparticle identity and shape on theelectrical/dielectric properties of the resulting nanocom-posites, and to compare the experimental results withtheoretical predictions. We also extend the scope of metallocene polymerization catalysts (see Chart 1) andolefinic monomers, with the goal of achieving nanocom-posites that have comparable or potentially greater pro-cessability and thermal stability. Here, we present a fulldiscussion of the synthesis, microstructural and electricalcharacterization, and theoretical modeling of these nano-composites. It will be seen that nanoparticle coating withMAO and subsequent in situ polymerization are crucialto achieving effective nanoparticle dispersion, and, simul-taneously, high nanocomposite breakdown strengths (ashigh as 6.0MV/cm) andhigh permittivities (ashigh as 6.1)

can be realized to achieve energy storage densities as high

as 9.4 J/cm3

.

Experimental Section

I. Materials and Methods. All manipulations of air-sensitivematerials were performed with rigorous exclusion of O 2 andmoisture in flamed Schlenk-type glassware on a dual-manifoldSchlenk line or interfaced to a high-vacuum line (10 - 5 Torr), orin a dinitrogen-filled MBraun glovebox with a high-capacityrecirculator (


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solvent 1,1,2,2-tetrachloroethane-d 2 was purchased fromCambridge Isotope Laboratories ( g 99 at.% D) and was usedas-received. Methylaluminoxane (MAO; Sigma - Aldrich) waspurified by removing all the volatiles in vacuo from a 1.0 Msolution in toluene. The reagents dichloro[ rac -ethylenebisin-denyl]zirconium(IV) (EBIZrCl 2), and trichloro(pentamethyl-cyclopentadienyl)titanium(IV) (Cp*TiCl 3) were purchasedfrom Sigma - Aldrich and used as-received. Me 2Si( tBuN)( 5-C5Me 4)TiCl 2 (CGCTiCl 2) was prepared according to publishedprocedures. 23 n -Si wafers (root-mean-square (rms) roughnessof 0.5 nm) were obtained from Montco Silicon Tech (SpringCity, PA), and aluminum substrates were purchased fromMcMaster - Carr (Chicago, IL); both were cleaned accordingto standard procedures. 24

II. Physical and Analytical Measurements. NMR spectrawere recorded on a Varian Innova 400 spectrometer (FT 400MHz, 1H; 100 MHz, 13 C). Chemical shifts ( ) for 13 C spectrawere referenced using internal solvent resonances and arereported relative to tetramethylsilane. 13 C NMR assays of polymer microstructure were conducted in 1,1,2,2-tetrachlor-oethane-d 2 containing 0.05 M Cr(acac) 3 at 130 C. Resonanceswere assigned according to the literature for isotactic polypro-pylene, poly(ethylene- co-1-octene), and syndiotactic polystyr-ene, respectively (see more below). Elemental analyses wereperformed by Midwest Microlabs, LLC (Indianapolis, IN).Inductively coupled plasma - optical emission spectroscopy(ICP-OES) analyses wereperformed by Galbraith Laboratories,Inc. (Knoxville, TN). PowderX-ray diffraction (XRD) patternswere recorded on a Rigaku DMAX-A diffractometer with Ni-filtered Cu K R radiation ( = 1.54184 A ). Pristine ceramicnanoparticles and composite microstructures were examinedwith a FEI Quanta sFEG environmental scanning electronmicroscopy (SEM) system with an accelerating voltage of 30kV. Transmission electron microscopy (TEM) was performedon a Hitachi Model H-8100 TEM system with an acceleratingvoltage of 200 kV. Samples for TEM imaging were prepared bydipping a TEMgrid into a suspension of nanocomposite powderin acetone. Polymer composite thermal transitions were mea-sured on a temperature-modulated differential scanning calori-meter (TA Instruments, Model 2920). Typically, ca. 10 mg of samples were examined, and a ramp rate of 10 C/min was usedto measure the melting point. To erase thermal history effects,all samples were subjected to two melt - freeze cycles. The datafrom the second melt - freeze cycle are presented here.

III. Electrical Measurements. Metal - insulator - metal(MIM) or metal - insulator - semiconductor (MIS) devices fornanocomposite electrical measurements were fabricated by firstdoctor-blading nanocomposite films onto aluminum (MIM) orn -Si (MIS)substrates,followed by vacuum-depositingtop goldelectrodes through shadowmasks. Specifically, a clean substrate

was placed on a hot plate heated to just below the polymer-nanocomposite melting point. A small amount of the polymernanocomposite powder was placed in the center of the substrateand left until the powder began to melt. Once in this phase, thepolymer nanocomposite is spread over the center of the sub-strate using a razor blade. The sample was removed from theheat, cooled, and then pressed in a benchtop press to ensureuniform film thicknesses and smooth surfaces. Gold electrodes500 A thick were vacuum-deposited directly on the films

through shadow masks that defined a series of different areas(0.030, 0.0225, 0.01, 0.005, and 0.0004 cm 2) at 3 10- 6 Torr (at0.2 - 0.5 A /s). Electrical properties of the films were character-ized by two probe current - voltage( I - V ) measurements using aKeithley Model 6430 Sub-Femtoamp Remote Source Meter,operated by a local LABVIEW program. Triaxial and lowtriboelectric noise coaxial cables were incorporated with theKeithley remote sourcemeter andSignatone(Gilroy, CA) probetip holders to minimize the noise level. All electrical measure-ments were performed under ambient conditions. For MISdevices, the leakagecurrent densities (represented by the symbolJ , givenin unitsof A/cm 2) were measured with positive/negativepolarity applied to the gold electrode to ensure that the n -Sisubstrate was operated in accumulation. A delay time of 1 s wasincorporated into the source - delay - measure cycle to settle thesource before recording currents. Capacitance measurements of the MIM and MIS structures were performed with a two-probedigital capacitance meter (Model 3000, GLK Instruments, SanDiego, CA) at ( 5 and 24 kHz. Several methods have beendeveloped to measure the dielectric breakdown strength of polymer and nanocomposite films. 1a,25 In this study, variousmethods were examined (e.g., pull-down electrodes 25 ), and thetwo-probe method was used to collect the present data becausethe top gold electrodes had already been deposited for leakagecurrent and capacitance measurements. The dielectric break-down strength of the each type of composite film was measuredin a Galden heat-transfer fluid bath at room temperature with ahigh-voltage amplifier (Model TREK 30/20A, TREK, Inc.,Medina, NY) with a ramp rate of 1000 V/s. 26 The thicknessesof the dielectric films were measured with a Tencor P-10 stepprofilometer, and these thicknesses were used to calculate thedielectric constants and breakdown strengths of the film sam-ples (see Table 2, presented later in this work).

IV. Representative Immobilization of a Metallocene Catalyston Metal Oxide Nanoparticles. In the glovebox, 2.0 g of BaTiO 3nanoparticles, 200 mg of MAO, and 50 mL of dry toluene wereloaded into a predried 100-mL Schlenk reaction flask, whichwas then attached to the high vacuum line. Upon stirring, themixture became a fine slurry. The slurry was next subjected toalternating sonication and vigorous stirring for 2 days withconstant removal of evolving CH 4. Next, the nanoparticles werecollectedby filtration andwashed with fresh toluene (50 mL 4)to remove any residual MAO. Then, 200 mg of metallocenecatalyst EBIZrCl 2 and 50 mL of toluene were loaded in the flaskcontaining the MAO-coated nanoparticles. The color of thenanoparticles immediately became purple. The slurry mixturewas again subjected to alternating sonication and vigorous

Table 1. XRD Linewidth Analysis Results for the Oxide - PolypropyleneNanocomposites

powder 2 (deg)

full width athalf maximum,

fwhm (deg)crystallite

size, L (nm) a

BaTiO 3 31.412 0.254 35.6BaTiO 3- polypropylene 31.649 0.271 32.8TiO 2 25.360 0.317 27.1TiO 2- polypropylene 25.358 0.361 23.5

a Crystallite size ( L) is calculated using the Scherrer equation: L =0.9 /[B(cos B) where is the X-ray wavelength, B the full width at half maximum (fwhm) of the diffraction peak, and B the Bragg angle.

(23) Stevens, J. C.; Timmers, F. J.; Wilson, D. R.; Schmidt, G. F.;Nickias, P. N.; Rosen, R. K.; Knight, G. W.; Lai, S. Eur. PatentApplication EP416815A2, 1991 .

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(25) Claude, J.; Lu, Y.; Wang, Q. Appl. Phys. Lett. 2007 , 91, 212904/1 212904/3.

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stirring overnight. The nanoparticles were then collected byfiltration and washed with fresh toluene until the toluene

remained colorless. The nanoparticles were dried on the high-vacuum line overnight and stored in a sealed container in theglovebox at - 40 C in darkness.

V. Representative Synthesis of an Isotactic PolypropyleneNanocomposite via In Situ Propylene Polymerization. In theglovebox, a 250-mL round-bottom three-neck Morton flask,which had been dried at 160 C overnight and equipped with alarge magnetic stirring bar, was charged with 50 mL of drytoluene, 200 mg of functionalized nanoparticles, and 50 mg of MAO.The assembled flask was removed from the glovebox andthe contents were subjected to sonication for 30 min withvigorous stirring. The flask was then attached to a high vacuumline (10 - 5 Torr), the catalyst slurry was freeze - pump - thawdegassed, equilibrated at the desired reaction temperature using

an external bath, and saturated with 1.0 atm (pressure controlusing a mercury bubbler) of rigorously purified propylene whilebeing vigorously stirred. After a measured time interval, thepolymerization was quenched by the addition of 5 mL of methanol, and the reaction mixture was then poured into 800mL of methanol. The composite was allowed to fully precipitateovernight andwas then collected by filtration, washedwith freshmethanol, and dried on the high vacuum line overnight toconstant weight.

VI. Representative Synthesis of a Poly (ethylene- co -1-octene )Nanocomposite via In Situ Ethylene 1-Octene Copolymeriza-tion. In theglovebox,a 250-mLround-bottom three-neck Mortonflask, which had been dried at 160 C overnight and equip-ped with a large magnetic stirring bar, was charged with 50 mL

of dry toluene, 200 mg of functionalized nanoparticles, and50 mg of MAO. The assembled flask was removed from the glo-

vebox and the contents were subjected to sonication for 30 minwith vigorous stirring. The flask was then attached to a highvacuum line (10 - 5 Torr), the catalyst slurry was freeze - pump -thaw degassed, equilibrated at the desired reaction temperatureusing an external bath, and saturated with 1.0 atm (pressurecontrol using a mercury bubbler) of rigorously purified ethylenewhile being vigorously stirred. Next, 5 mL of freshly vacuum-transferred 1-octene was quickly injected into the rapidly stirredflask using a gas-tight syringe equipped with a flattened sprayingneedle. After a measured time interval, the polymerization wasquenched by the addition of 5 mL of methanol, and the reactionmixture was then poured into 800 mL of methanol. The com-posite was allowed to fully precipitate overnight and was thencollected by filtration, washedwith fresh methanol,and dried on

the high vacuum line overnight to constant weight. Film fabri-cation of the composite powders into thin films for MISelectrical testing was unsuccessful due to the high incorporationlevel of 1-octene.

VII. Representative Synthesis of a Syndiotactic PolystyreneNanocomposite via In Situ Styrene Polymerization. In the glove-box, a 250-mL round-bottom three-neck Morton flask, whichhad been dried at 160 C overnight and equipped with a largemagnetic stirring bar, was charged with 50 mL of dry toluene,200 mg of functionalized nanoparticles, and50 mg of MAO. Theassembled flask was removed from the glovebox and the con-tents were subjected to sonication for 30 min with vigorousstirring. The flask was then attached to a high vacuum line (10

- 5

Torr) and equilibrated at the desired reaction temperature using

Table 2. Electrical Characterization Results for Metal Oxide - Polypropylene Nanocomposites a

entry compositenanoparticle

content b (vol%)melting temperature,

T mc ( C) permittivity d

breakdownstrength e (MV/cm)

energy density,U f (J/cm 3)

1 BaTiO 3-iso PP 0.5 136.8 2.7 ( 0.1 3.1 1.2 ( 0.1

2 BaTiO 3-iso PP 0.9 142.8 3.1 ( 1.2 >4.8 >4.0 ( 0.6

3 BaTiO 3-iso PP 2.6 142.1 2.7 ( 0.2 3.9 1.8 ( 0.2

4 BaTiO 3-iso PP 5.2 145.6 2.9 ( 1.0 2.7 1.0 ( 0.3

5 BaTiO 3-iso PP 6.7 144.8 5.1 ( 1.7 4.1 3.7 ( 1.2

6 BaTiO 3-iso PP 13.6 144.8 6.1 ( 0.9 >5.9 >9.4 ( 1.3

7 sTiO 2-iso PP g 0.1 135.2 2.2 ( 0.1 >2.8 >0.8 ( 0.1

8 sTiO 2-iso PP g 1.6 142.4 2.8 ( 0.2 4.1 2.1 ( 0.2

9 sTiO 2- iso PP g 3.1 142.6 2.8 ( 0.1 2.8 1.0 ( 0.110 sTiO 2-

iso PP g 6.2 144.8 3.0 ( 0.2 4.7 2.8 ( 0.2

11 rTiO 2-iso PP h 1.4 139.7 3.4 ( 0.3 1.0 0.40 ( 0.35

12 rTiO 2-iso PP h 3.0 142.4 4.1 ( 0.7 0.9 0.22 ( 0.09

13 rTiO 2-iso PP h 5.1 143.7 4.9 ( 0.4 0.9 0.23 ( 0.08

14 ZrO 2-iso PP 1.6 142.9 1.7 ( 0.3 1.5 0.18

15 ZrO 2-iso PP 3.9 145.2 2.0 ( 0.4 1.9 0.32

16 ZrO 2-iso PP 7.5 144.9 4.8 ( 1.1 1.0 0.20

17 ZrO 2-iso PP 9.4 144.4 6.9 ( 2.6 2.0 1.02 ( 0.73

18 TZ3Y - iso PP 1.1 142.9 1.1 ( 0.1 N/A N/A19 TZ3Y - iso PP 3.1 143.5 1.8 ( 0.2 N/A N/A

20 TZ3Y - iso PP 4.3 143.8 2.0 ( 0.2 N/A N/A21 TZ3Y - iso PP 6.7 144.9 2.7 ( 0.2 N/A N/A

22 TZ8Y - iso PP 0.9 142.9 1.4 ( 0.1 3.8 1.07 ( 0.0423 TZ8Y - iso PP 2.9 143.2 1.8 ( 0.1 2.8 0.5924 TZ8Y - iso PP 3.8 143.2 2.0 ( 0.2 2.0 0.4125 TZ8Y - iso PP 6.6 146.2 2.4 ( 0.4 2.2 0.61

a Polymerizations performed in 50 mL of toluene under 1.0 atm of propylene at 20 C. b From elemental analysis. c From differential scanningcalorimetry (DSC). d Derived from capacitance measurements. e Calculated by dividing the breakdownvoltage by thefilm thickness, which is measuredusinga Tencor p10 profilometer. f Energy density ( U ) is calculated from thefollowing relation: U = 0.5 0rE b

2 , where 0 is thepermittivity of a vacuum,r the relative permittivity, and E b thebreakdownstrength. g Thesuperscriptedprefix sdenotes sphere-shapedTiO 2 nanoparticles. h The superscriptedprefix r denotes rod-shaped TiO 2 nanoparticles.


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an external bath. With 1.0 atm of rigorously purified argonbubbling (pressure control using a mercury bubbler), 5 mL of freshly vacuum-transferred styrene was quickly injected into therapidly stirred flask using a gas-tight syringe equipped with aflattened spraying needle. After a measured time interval, thepolymerization was quenched by the addition of 5 mL of metha-nol, and the reaction mixture was then poured into 800 mL of methanol. The composite was allowed to fully precipitate over-night and was then collected by filtration, washed with freshmethanol, and dried on the high vacuum line overnight to con-stant weight. Unfortunately, film fabrication of the compositepowders into thin films for MIS electrical testing was unsuccess-ful, because of the high melting point of the polystyrene.

Results

The goal of this study was to synthesize well-dispersedpolyolefin-based nanocomposite dielectric materials viain situ supported metallocene polymerization catalysis,to investigate the effects of matrix polymer and nanopar-ticle identities, loading, and shape on the electrical/di-electric properties of the resulting nanocomposites, and

to compare the experimental findings with theoreticalcalculations. In the first part of the Results section, wedemonstrate that diverse polyolefin nanocomposites canbe synthesized from a variety of olefinic monomers andpolymerization catalysts via in situ polymerization meth-odologies. Next, we discuss microstructural characteriza-tion of the nanocomposites, demonstrating that nano-particle agglomeration can be satisfactorily disruptedwith the present in situ synthetic approach. In the remain-ing parts of this section, we analyze, in detail, the effects of nanoparticle composition, volume fraction, and shape onthe nanocomposite leakage current density, relative per-mittivity, breakdown strength, and ultimate energy sto-

rage density.I. Synthesis of Metal Oxide - Polyolefin Nanocompo-

sites. As one of the most commonly used polymers inlarge-scale power capacitors, isotactic polypropylene of-fers greater stiffness, lower shrinkage, and less deteriora-tion of dielectric properties at higher temperatures thanother types of polypropylene. 17 Therefore, the C 2-sym-metric metallocene catalyst dichloro[ rac -ethylenebisin-denyl]zirconium(IV) (EBIZrCl 2), with demonstrated se-lectivity for highly isospecific olefin polymerization, 27

was chosen for immobilization on the surfaces of MAO-treated metal oxide nanoparticles, to create metaloxide - isotactic polypropylene nanocomposites (recallScheme 1).

The nanocomposites were characterized using a fullcomplement of spectroscopic and analytical method-ologies. XRD line width analyses using the Scherrerequation 28 indicate that the microstructures and coher-ence lengths of the individual nanoparticles remain

largely unchanged throughout the olefin polymeriza-tion/deagglomeration process (see Table 1). Solution-phase 13 C nuclear magnetic resonance (NMR) spectros-copy (shown in Figure 1) indicates that the presentpolypropylenes are highly isotactic, as evidenced by themeasured isotacticity index, [ mmmm ] = 83%. 29 Thenumber-average molecular weightsof the polypropylenesare estimated to be >18 000, based on the relative peakintensities of the repeat unit methyl peak versus thepolymer-chain end methyl peak in the 13 C NMR spectra.Differential scanning calorimetry (DSC) confirms theabsence of extensive amorphous regions in the compo-sites, because only isotactic polypropylene melting fea-tures (142 - 147 C) are detected. 30 Powder XRD data forthe nanocomposites also reveal the presence of mono-clinic R -phase crystalline isotactic polypropylene (2 =14.2 , 17.0 , 18.6 , and 21.8 ).31 It is observed that themelting temperatures of the nanocomposites generally(and not surprisingly) increase as the nanoparticle load-ing increases (see Table 2, presented later in this work),which is reasonably due to attractive interactionsbetweenthe nanoparticles and the crystalline regions of the iso-tactic polypropylene. 32

Linear low-density polyethylene (LLDPE) is anotherpolymer that is widely used in power capacitors. 33 Com-pared to isotactic polypropylene, the chain branching inthe LLDPE affords betterprocessability. 34 Therefore, thesterically open constrained geometry catalyst Me 2Si-(tBuN)( 5-C 5Me 4)TiCl 2 (CGCTiCl 2)35 was utilized tosynthesize BaTiO 3- LLDPE nanocomposites via in situethylene 1-octene copolymerization. 35 Figure 2 pre-sents a representative solution-phase 13 C NMR spectrumof the nanocomposite, with the 1-octene incorporation

Figure 1. 13 C NMR spectrum of an isotactic polypropylene BaTiO 3nanocomposite (100 MHz, C 2D 2Cl4 solution, 130 C).

(27) (a) Lee, I. M.; Gauthier, W. J.; Ball, J. M.; Iyengar, B.; Collins, S.Organometallics 1992 , 11 , 2115 2122. (b) Wild, F. R. W. P.; Zsolnai,L.; Huttner, G.; Brintzinger, H. H. J. Organomet. Chem. 1982 , 232 ,233 247.

(28) (a) Jenkins, R.; Snyder, R. L. In Introduction to X-ray PowderDiffractometry ; Winefordner, J. D., Ed.; Wiley: New York, 1996; pp89 - 91. (b) Scherrer, P. G

ott. Nachr. 1918 , 2, 98 100.

(29) (a) Busico, V.; Cipullo, R.; Monaco, G.; Vacatello, M. Macro-molecules 1997 , 30, 6251 6263. (b) Busico, V.; Cipullo, R.; Corradini,P.; Landriani, L.; Vacatello, M.; Segre, A. L. Macromolecules 1995 , 28,1887 1892. (c)Zambelli, A.;Dorman, D. E.;Brewster, A. I. R.;Bovey,F. A. Macromolecules 1973 , 6, 925 926.

(30) Miller, S. A.; Bercaw, J. E. Organometallics 2006 , 25, 3576 3592.(31) Auriemma, F.; De Rosa, C. Macromolecules 2002 , 35, 9057 9068.(32) (a) Friedrich, K., Fakirov, S., Zhang, Z., Eds. In Polymer Compo-

sites: From Nano- to Macro-Scale ; Springer: New York, 2005; pp169 - 185. (b) Jordan, J.; Jacob, K. I.; Tannenbaum, R.; Sharaf, M. A.;Jasiuk, I. Mater. Sci. Eng.: A 2005 , 393 , 1 11. (c) Zhang, Q.; Yu, Z.;Xie, X.;Mai, Y. Polymer 2004 , 45, 5985 5994. (d) Xie,X.; Liu,Q.; Li,R.;Zhou, X.;Zhang, Q.;Yu, Z.;Mai, Y. Polymer 2004 , 45, 6665 6673.

(33) (a) Ke, Q.; Huang, X.; Wei, P.; Wang, G.; Jiang, P. J. Appl. Polym.Sci. 2007 , 104 , 1920 1927. (b) Hosier, I. L.; Vaughan, A. S.; Swingler,S. G. J. Polym. Sci., Part B: Polym. Phys. 2000 , 38, 2309 2322.

(34) Kulshrestha, A. K.; Talapatra, S. In Handbook of Polyolefins ;Vasile, C., Ed.; Marcel Dekker: New York, 2000; pp 1 - 70.

(35) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998 , 98, 2587 2598.


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level calculated to be 25.0 mol%. 36 DSC measurementsalso confirm the formation of LLDPE, with a typicalmelting temperature of 125.3 C. 34 Syndiotactic polystyr-ene hasgreaterheat resistance 37 than isotactic polypropy-lene, which can only operate at


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IV. Permittivity, Breakdown Strength Measurements,and Leakage Current Density. In the preliminary commu-nication of this work, we reported that anchored metal-locene-catalyzed in situ polymerization is a promisingmethod to achieve well-dispersed ferroelectric nanopar-ticles in a polymer matrix. We speculate that, in a well-dispersed nanoparticle composite, interfaces between theceramic nanoparticles and polymer phases create effec-tive electron scatterers and trapping centers, thus redu-cing the breakdown probability (see more in theDiscussion section). 40 Recent dielectric characterizationstudies have shown that polymer structural relaxations

and dipolar trap states are strongly influenced by thepolymer/oxide interface. 41 Moreover, well-dispersedceramic nanoparticles should block degradation treegrowth and thus increase the long-term breakdownstrength. 42 Here, we employ different metal oxide parti-cles of varying permittivity for the enhancement of com-posite permittivity properties. In addition, all of thenanoparticles used in this study were coated with MAO

Figure 4. Electron microscopic characterization of (a) as-received pristine ZrO 2 nanoparticles (SEM) and (b) a 7.5-vol%-ZrO 2-iso PP nanocomposite

(TEM).

Figure 5. Electron microscopic characterization of (a) as-received pristine 3-mol%-yttria-stabilized zirconia (TZ3Y) nanoparticles (SEM) and (b) a 6.7-vol %-TZ3Y - iso PP nanocomposite (TEM).

Figure 6. Electron microscopic characterization of (a) as-received pristine 8-mol%-yttria-stabilized zirconia (TZ8Y) nanoparticles (SEM) and (b) a 6.6-vol %-TZ8Y - iso PP nanocomposite (TEM).

(40) Fujita, F.;Ruike, M.;Baba, M. IEEEInt. Symp. Electr. Insul. 1996 ,2, 738 741.

(41) Tewari, P.; Rajagopalan, R.; Furman, E.; Lanagan, M. T. J.Colloid Interface Sci. 2009 , 332 , 65 73.

(42) (a) Smith, R. C.; Liang, C.; Landry, M.; Nelson, J. K.; Schadler, L.S. IEEETrans.Dielectr.Electr.Insul. 2008 , 15,187 196. (b)Imai, T.;Sawa, F.; Nakano, T.; Ozaki, T.; Shimizu, T.; Kozako, M.; Tanaka, T.IEEE Trans. Dielectr. Electr. Insul. 2006 , 13, 319 326. (c) Uehara,H.; Kudo, K. IEEE Trans. Dielectr. Electr. Insul. 2005 , 12, 1266 1271.


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as part of the polymerization process, and, as a result of this treatment, the nanoparticles have a thin Al 2O3 (r 10) layer created by ambient exposure of the MAO-coated nanoparticles. This is anticipated to benefit the

breakdown stability and permittivity enhancement bycreating a graded permittivity decrease and reduced fieldcontrast between the nanoparticles and the polymer. 17 Asone example, the high-permittivity BaTiO 3-

iso PP com-posites have a materials permittivity ratio of 1200:2.3,andICP-OES analysis indicates that the estimated Al 2O3coating thickness on BaTiO 3 is 1 nm, creating a gradi-ent of permittivities (1200:10:2.3). In the present model-ing study (see discussion below), we neglect the 1-nm-thick Al 2O3 layer: however, we are currently investigatingthe effects of varying the Al 2O3 layer thickness, both inconjunction with electrical measurements and with effec-tive medium modeling. 43

By varying the nanoparticle identity, we can investigatea range of permittivities to assess possible correlationswith composite dielectric breakdown strength and effec-tive nanocomposite permittivity. For example, a largepermittivity metal oxide (such as BaTiO 3) should affordgreater increases in effective composite permittivity, aswell as decreased breakdown strength, as can be seen byrearranging U =0.5 0rE b

2 for E b .17b,44 Because ZrO 2 has

a lower permittivity ( 18- 34, depending on the yttriacontent) 45 than TiO 2

46 andBaTiO 3,47 it is anticipated that

the resulting polymer nanocomposites will exhibit largerintrinsic breakdown strengths and potentially enhancedenergy storage densities. In the present study, it is ob-served that, at low nanoparticle loadings, the yttria-stabilized ZrO 2- and ZrO 2- polypropylene nanocompo-site films exhibit somewhat lower permittivities than neatpolypropylene (see data given in Table 2), despite the fact

that the permittivities do increase as the nanoparticle load-ings increase. This result is most likely due to the significantfilm porosity visible during sample fabrication, 2a,48 and, forthis reason, Figure 8 shows only the ZrO 2 nanocompositeeffective permittivities for comparison to the other types of composites. Note thatbreakdownfieldstrengths are knownto be dependent on a variety of experimental conditions, 49

and an exhaustive statistical analysis 50 is required to makeprecise engineering-level comparisons between differentnanoparticle - polymer composite films.

Interestingly, the present data show that the largesteffective permittivities at low volume fractions are mea-sured for polymer composites that have rod-shaped TiO

2nanoparticle inclusions, and not for composites that havethe larger-permittivity BaTiO 3 nanoparticles. This maybe explained by the greater degree of electric field exclu-sion that arises from the rod-shaped inclusions (see dis-cussion below of the permittivity, in terms of effectivemedium models). 51 In addition, the rod-shaped TiO 2composite films have significantly lower breakdownfields than the corresponding sphere-shaped TiO 2-, Ba-TiO 3-, and ZrO 2-based polypropylene nanocompositefilms, which might be expected, because enhanced loca-lized electric field around the TiO 2 nanorods facilitatesincreased charge injection. Moreover, the high aspect

ratios of the TiO 2 nanorods are more likely to approach

Figure 7. Representativeplot of capacitance( C ) versuselectrodearea ( A)for a 2.6-vol%-BaTiO 3- iso PP nanocomposite.

Figure 8. Variation in polypropylene nanocomposite film dielectric per-mittivity, relative to nanoparticle volume fractionand composition.Notethat the effective permittivity of pristine isotactic polypropylene is 2.3.

(43) Li, Z.; Fredin, L.; Lanagan, M. T.; Ratner, M. A.; Marks, T. J.,manuscript in preparation.

(44) Li, J. Y.; Zhang, L.; Ducharme, S. Appl. Phys. Lett. 2007 , 90,132901/1 132901/3.

(45) (a) Joo, J. H.; Choi, G. M. Solid State Ionics 2006 , 177 , 1030 1057.(b) Lanagan, M. T.; Yamamoto, J. K.; Bhalla, A.; Sankar, S. G. Mater.Lett. 1989 , 7 , 437 440.

(46) Maliakal, A.; Katz, H.; Cotts, P. M.; Subramoney, S.; Mirau, P. J.Am. Chem. Soc. 2005 , 127 , 14655 14662.

(47) Ohno, T.; Szuki, D.; Suzuki, H.; Ida, T. KONA 2004 , 22, 195 201.

(48) (a) Kloster, G.; Scherban, T.; Xu, G.; Blaine, J.; Sun, B.; Zhou, Y.Proc. IEEE Int. Interconnect Technol. Conf., 5th 2002 , 257 259. (b)Windless, H.; Raj,P. M.; Balaraman,D.; Bhattacharya, S. K.; Tummala,R. R. Proc. 51st Electr. Comput. Technol. Conf. 2001 , 1201 1206.

(49) (a)Gefle,O. S.;Lebedev, S.M.; Pokholkov, Y.P.; Gockenbach,E.;Borsi, H. J. Phys. D: Appl. Phys. 2004 , 37 , 2318 2322. (b) Helgee,B.; Bjellheim, P. IEEE Trans. Electr. Insul. 1991 , 26, 1147 1152.

(50) (a) Abernethy, R. The New Weibull Handbook ; Abernethy: NorthPalm Beach, FL, 1993; pp 2 - 3.(b) Wolters, D. R. Verwey, J. F. InInstabilities in Silicon Devices ; Elsevier: Amsterdam, The Netherlands,1986; Chapter 6, pp 345 - 355.

(51) (a) Calame, J. P. J. Appl. Phys. 2008 , 104 , 114108-1 114108-11. (b)Calame, J. P. J. Appl. Phys. 2006 , 99, 084101 - 1 084101 - 11. (c)Gershon, D.; Calame, J. P.; Birnboim, A. J. Appl. Phys. 2001 , 89,8110 8116.


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percolation at lower volume fractions, thus creating en-hanced conduction pathways. Nevertheless, the range of measured breakdown voltages (fields) of the presentnanocomposites is impressive (3 - 10 kV (1 - 6 MV/cm),which is indicative that that metal oxide nanoparticleinclusion can significantly enhance polymer dielectricbreakdown strengths (values of 0.4 MV/cm have beenreported for films of iso PP, 52a and 6 MV/cm 52b for iso PPfilms impregnated with benzyltoluene). 53 The corre-sponding projected energy storage densities of the presentcomposites vary from 0.3 (ZrO 2-

iso PP andsTiO 2-

iso PP) to 9.4 J/cm 3 (BaTiO 3-iso PP), which rivals

or exceeds those reported for conventional ceramic,54

polymer, 55 and composite 2a,16,56 dielectrics. Energy den-sity is related to the dielectric breakdown strength, whichis a statistically based process. The energy densities forlarge-scale capacitors that are fabricated from thesenanocompositeswill ultimately be dependent on reducingdefects.

The leakage current densities of the nanocompositefilms prepared in this investigation ( J leakage , depicted inFigure 9) are generally within the range of 10

- 8- 10- 6 A/

cm 2 at 100 V. As the nanoparticle loading increases, mostof the present nanocomposites exhibit depressed J leakagevalues (Figure 9a) initially, presumably reflecting modifiedcharge transport and interruption of conduction path-ways within the composite structure by the nanoparticle

inculsions. 57 However, at the highest nanoparticle loadings( 0.07 volume fraction), which are far below the percola-tion threshold ( 32% 44 ), the polyolefin nanocompositesexhibit significantly enhanced leakage current densities,possibly because of film-fabrication-induced anisotropicagglomeration. 57,58 This current density pattern with in-creasing volume fraction (an initial decrease, followed byan increase in J leakage ) is most clear in the BaTiO 3-

iso PPcomposites (green curve, Figure 9a). No obvious correla-tions between oxide permittivity and J leakage are observed.For example, ZrO 2 has the smallest permittivity, yet theZrO 2 polymer - nanoparticle composites (including the

yttria-stabilized nanoparticles) have the largest leakagecurrent densities over the entire volume fraction range(see Figure 9), which is most likely attributable to space-charge differences 59 that occur in the different typesof composites. Nonetheless, the leakage current densitiesremain relatively small (


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nanoparticle agglomeration and does not overcome themicrometer-scale or greater phase separation, which canlead to local dielectric breakdown and degrade nanocom-posite electrical properties. 10 In contrast, the presentsupported-metallocene in situ polymerization approachminimizes these limitations by achieving homogeneousnanoscale dispersion of the metal oxide phase: eachindividual nanoparticle is surrounded by polymer chains

that propagate in situ from the high-productivity surface-immobilized metallocenium catalyst centers, 13 therebyaffording significantly enhanced composite dielectricproperties (energy densities as high as 9.4 J/cm 3).

II. Function of the MAO Cocatalyst. In nanocompo-sites that have very large contrasts in relative permittivitiesbetween host and guest materials, large disparities in theelectric fields within the constituent phases occurs, 44,60 thuspreventing the simultaneousrealizationof maximum energydensities for both constituents. 1a For the present nanocom-posites, however, the largest achieved energy density (ashighas 9.4 J/cm 3) isobserved forBaTiO 3-

isoPP composites,which have the largest materials permittivity ratio of 522:1. We reasonably speculate that the thin Al 2O3 coat-ing on the nanoparticles created by ambient exposure of theMAO co-catalyst coating acts as a dielectric buffer layerbetween the high-permittivity BaTiO 3 nanoparticlesand thelow-permittivitypolypropylene(PP)matrix. In thisway, theBaTiO 3 polarizability can be fully utilizedforcharge storagein this type of core - shell composite. 43

III. Effect of Nanoparticle Shape on NanocompositePermittivity. Experiment versus Theory. The most com-mon effective medium models are derived for the simplecaseof a spherical dielectric inclusion embeddedin a sphereof host material. However, most materials do not occurnaturally as spheres; therefore, effective medium modelsforother inclusion shapes havebeen developed. 61,62 Simpleanalytical solutions for the effective permittivity ( eff ) canonly be achieved for ellipsoids, whereas all other shapesrequire numerical solutions. 63 The polarization of thecomposite is a function of the inclusion geometry andorientation with respect to the applied field. The depolar-ization factors ( N x , N y, N z) describe the extent to which theinclusion polarization is reduced, according to its shapeand orientation along each semiaxis of the ellipsoid.The depolarization factors are calculated from integrals,e.g., eq 3,

N x ax a yaz2 Z

01

s ax 2 ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffis ax 2s a y2s az2p ds

3

where ax , a y,and a z are the semiaxes of the ellipsoid.64 For

spheres, all three depolarization factors are equal ( 1/3,1/3,

1/3), and forellipsoids (infinite needles) the depolarizationfactors are 0, 1/2,

1/2, respectively (1, 0, 0, respectively, fordiscs). For the case of spherical inclusions, the effectivepermittivities are estimated using the Maxwell - Garnett

(MG) effective medium theory (eq 4),2a,65

and for the caseof randomly aligned ellipsoidal inclusions, the effectivepermittivities are estimated using the Polder - Van Santen(PVS) formalism (eq 5), where a is the relative permittiv-ity of the TiO 2 inclusions, b the relative permittivity of isotactic polypropylene, f a the volume fraction of TiO 2,and the N j represents the depolarization factors.

64

eff ba 2b 2 f a a - ba 2b - f aa - b

4

eff b f a3

a - b

X j x, y, z

eff eff N j a - eff

5

Figure 10 shows that a substantial enhancement in eff ispredicted for ellipsoidal inclusions,compared to sphericalinclusions. The enhancement is clearly greatest for flatdisklike and for needlelike shapes, because of their largerdipole moments (versus spherical shapes). 64

- 67 Theseobservations motivated the present experimental studyof TiO 2- isotactic polypropylene nanocomposites withdifferent nanoparticle inclusion shapes. Although theeffective medium models are only qualitatively accurate 64

for the sorts of nanocomposites discussed here (where thedielectric contrast between polymer and filler is large, andthe inclusion sizes are small), they nevertheless can act asguides to understanding the capacitance trends.

The experimental results comparing TiO 2 nanocompo-sites with spherical and ellipsoidal inclusions are plotted,along with the modeling predictions, in Figure 11. Itcan be seen that the composites that contain ellipsoidal

Figure 10. Normalized calculated effective permittivity (( eff - b)/(a - b)) for polypropylene composite dielectrics with spherical inclu-sions (eq 4, denoted by the solid line) and ellipsoidal inclusions (eq 5,denoted by the dashed line).

(60) (a) Takada, T.; Hayase, Y.; Tanaka, Y. IEEE Trans. Dielectr.Electr. Insul. 2008 , 15, 152 160. (b) Murata, Y.; Murakami, Y.; Nemoto, M.; Sekiguchi, Y.; Inoue, Y.; Kanaoka, M.; Hozumi, N.; Nagao, M. IEEE Conf. Electr. Insul. Dielectr. Phenomena 2005 ,10, 158 161. (c) Okoniewski, A.; Perlman, M. M. J. Polym. Sci. PartB: Polym. Phys. 2003 , 32, 2413 2420.

(61) Ma, D.; Siegel, R. W.; Hong, J.; Schadler, L. S.; M artensson, E.;

Onneby, C. J. Mater. Res. 2004 , 19, 857 863.(62) Brosseau, C.; Beroual, A.; Boudida, A. J. Appl. Phys. 2000 , 88,

7278 7288.(63) Green, N. G.; Jones, T. B. J. Phys. D: Appl. Phys. 2007 , 40, 78 85.

(64) Sihvola, A. Electromagnetic Mixing Formulas and Applications ;Institution of Electrical Engineers: London, 1999; pp 61 - 84.

(65) (a)Adohi,B. J.-P.;Bouanga, C. V.;Fatyeyeva, K.;Tabellout,M. J.Phys. D: Appl. Phys. 2009 , 42, 015302. (b) Yang, R.; Qu, J.; Marinis,T.; Wong, C. P. IEEE Trans. Compon. Packag. Technol. 2000 , 23,680 683.

(66) Jones, S. B.; Friedman, S. P. Water Resour. Res. 2000 , 36, 2821 2833.

(67) Jylha, L.; Sihvola, A. J. Phys. D: Appl. Phys. 2007 , 40, 4966 4973.


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inclusions have larger effective permittivities at lowervolume loadings than composites that contain sphericalinclusions. Remarkably, the effective permittivities for sphe-rical inclusions rise very slowly over the range of volume

fractions, exactly as the Maxwell - Garnett formalism predi-cts(seeFigure 10). In markedcontrast, theeffective permitti-vity of the composites that have inclusions with ellipsoidalshapes increases considerably as the inclusion volume frac-tion increases, which, again, is consistent with the trendpredicted for ellipsoidal inclusions using eq 5. 19,51

These results demonstrate that the presentmethodof insitu polymerization for nanocomposite synthesis is highlyeffective in disrupting nanoparticle agglomeration and,therefore, achieving both high storage capacities as wellas relatively accurate modeling of the permittivities with-in a straightforward effective medium description. Futuresynthetic and modeling efforts will focus on the permit-tivitygradient established by the interfacial Al 2O3 layer tomanipulate and understand the interaction between theoxide nanoparticles and the polymer matrix. 43,59

Conclusions

We have synthesized and microstructurally/electricallycharacterized a series of well-dispersed metal oxide -polyolefin nanocomposites via a scalable, in situ supported

metallocene-catalyzed olefin polymerization process.This versatile, scalable approach offers effective controlover composite composition. By straightforwardlyvarying the nanoparticle identity, shape, and the metal-locene catalyst, a wide array of nanocomposites withdesirable dielectric and mechanical properties can becatalytically synthesized in situ. Generally, the leakagecurrent densities of these nanocomposites ( J leakage

10- 6

- 10- 8

A/cm2

at 100 V) initially decrease as thenanoparticle volume fraction increases to 0.07, atwhich point J leakage begins to increase. The relativepermittivities of the nanocomposites increase as thenanoparticle volume fraction increases. At the sameinclusion loading, polypropylene nanocomposites withrod-shaped TiO 2 nanoparticles exhibit significantlygreater relative permittivities than those with sphere-shaped TiO 2 nanoparticles, in agreement with our the-oretical predictions. The energy storage densities of theBaTiO 3- polypropylene nanocomposites are projectedto be as high as 9.4 J/cm 3 . To achieve higher energystorage densities, both the experimental and theoreticalfindings presented here suggest that rational selection of the identity and shape of the metal oxide nanoparticlesand engineering of the nanoparticle - polymer interfaceare essential.

Acknowledgment. This research was supported by ONRMURI Program (N00014-05-1-0766), by DOE (86ER13511),and made use of Central Facilities supported by the MRSECprogramof theNationalScience Foundation (DMR-0520513)at the Materials Research Center of Northwestern University.The SEM and TEM analyses were performed in the EPICfacility of the NUANCE Center at Northwestern University.The NUANCE Center is supported by NSF-NSEC, NSF-MRSEC, Keck Foundation, the State of Illinois, and North-western University. We thank Mr. M. Russell for assistancewith the SEM measurements and Dr. A. Facchetti for helpfuldiscussions.

Supporting Information Available: TEM images of the 6.7-vol %-BaTiO 3 - iso PP and 6.2-vol %- sTiO 2- iso PP nanocompo-sites. This material is available free of charge via the Internet athttp://pubs.acs.org.

Figure 11. Comparison of experimental effective permittivities for poly-propylene nanocomposites having sphere- and ellipsoid-shaped TiO 2nanoparticles versus the predictions of eqs 4 and 5.

neng guo et al- nanoparticle, size, shape, and interfacial effects on leakage current density,...

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