Research Article Tunable Nanodielectric...

Research ArticleTunable Nanodielectric Composites

Daniel Qi Tan1 Yang Cao12 Xiaomei Fang1 and Patricia C Irwin1

1 GE Global Research Center Niskayuna NY 12309 USA2Department of Electrical and Computer Engineering University of Connecticut Storrs CT 06269 USA

Correspondence should be addressed to Daniel Qi Tan tangecom

Received 12 November 2013 Revised 22 January 2014 Accepted 12 February 2014 Published 25 March 2014

Academic Editor A G Barbosa de Lima

Copyright copy 2014 Daniel Qi Tan et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

This paper presents a progress update with the development of nanodielectric composites with electric field tunability for varioushigh energy high power electrical applications It is demonstrated that nonlinear electricaldielectric properties can be achievedvia the nanostructure and interface engineering A high level summary was given on the progress achieved as well as challengesremaining in nanodielectric engineering towards high energy density capacitors for energy storage and conversion nonlineardielectrics for tunable device and high voltage varistor for surge suppression

1 Introduction

The rapid expansion of renewable energy applicationsdemands higher efficiency and higher density energy stor-age and energy conversion systems [1 2] Various DC-ACAC-AC conversions are needed for solar and wind farmswhile primary and secondary electrochemical devices are inneed for transportation and telecommunication applicationsAdvanced devices and components are critical enablers forthese emerging applications As an example in a typical elec-trical converterinverter state-of-art DC-link capacitors takeup about 30 total volume and weight In addition the wideusage of renewable energy resources such as wind and solarposts challenges in grid stability Advanced passive and activedevices will be needed for future grid for harmonic filtrationstaticdynamic volt-ampere reactive (VAR) compensationtransient suppression and so forth [3 4]

All these advanced apparatus and future electric powerinfrastructure rely on the breakthroughs of material engi-neering and better understanding of the device physics [1]For instance the development of high dielectric constantmaterials and the understanding of the behavior of dielectricmaterials under extremely high electric fields are critical forenergy storage applications [5] In addition better under-standing and control of charge transport cross interfacesare crucial for nonlinear devices The advance in nanotech-nology offers a unique opportunity to engineer materials

with controlled microstructures so that desirable electricalproperties can be greatly enhanced [1 6] On the other handdielectric response of materials can be utilized to facilitatethe formation of nanostructures [7] In this paper progressupdates are given as exemplary cases on the developmentof nanoenabled composite materials for high energy densitycapacitors for energy storage and conversion field tunablenonlinear dielectrics and miniaturized varistors for highvoltagehigh current transient suppression

2 Experimental

In this investigation polyetherimide (PEI) silicone cya-noethyl cellulose polyimide and poly(vinylidene difluoride)were used as the polymer matrices in the investigationNanoparticles of interest include oxides of silicon niobiumaluminum zinc bismuth antimony cobalt titanium andbarium titanate and lead zirconates with the particle size inthe range of 10 to 100 nm Polymer nanocomposites wereprepared by first dissolving a polymer resin in a solvent atroom temperature with a magnetic stirrer and then mixingwith nanoparticles of 2ndash50 vol in a high-energy sonicator(Sonics amp Materials Inc Newtown CT) The films weresolvent cast onto a glass slide and dried at 100∘C for twohours followed by a vacuum dry at 120∘C to 150∘C overnightCeramic composites were formulated with at least 85mol

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2014 Article ID 549275 6 pageshttpdxdoiorg1011552014549275

2 Advances in Materials Science and Engineering

ZnO and other oxide additives A conventional mixed oxideprocess was used for powder processing Green compacts of110158401015840 diameter were pressed using a uniaxial hydraulic press andthen sintered at temperatures of 850ndash1000∘C

DCbreakdown test was conducted followingASTMD149(method A) using a ball-plane electrode configuration Thesample was immersed into insulation oil and DC voltagewas applied at a ramp rate of 500Vs until the samplefailed Dielectric responsesweremeasured using a broadbanddielectric spectrometer from Novacontrol GmbH Scanningelectron microscopy (SEM) imaging was done using a ZeissSupra 55VP (Carl Zeiss AG Oberkochen Germany) Trans-mission electronmicroscopy (TEM) imagingwas taken usinga FEI Tecnai F20 transmission electron microscope

3 Results and Discussions

High energy density capacitor dielectrics field tunable com-posite dielectrics andmetal oxide varistors (MOVs) are takenas examples in this section to demonstrate the achievableelectricaldielectric properties via the nanodielectric andinterface engineering to fulfill the aforementioned develop-ment needs

31 Nanofilled Polymer Film for Capacitors Capacitors rep-resent a big family of non-Faradic energy storage com-ponents used in applications ranging from decoupling incircuit board to series capacitive compensation banks atpower transmission class The state-of-art polymeric filmcapacitors are based primarily on low permittivity polymerssuch as polypropylene or polyester One natural approachfor higher energy density capacitor is the synthesis of higherpermittivity composites with higher dielectric strength byincorporating ceramic fillers into polymer matrix This isbecause the energy density of a film capacitor takes the simpleform of 12120576

119900K1198642 where K is the dielectric constant and 119864 is

the dielectric strength of the polymer filmAlthough the dielectric constant can be increased to

some extent at higher particle loading level increasing thedielectric strength remains a great challenge Instead theaddition of particles exceeding 10wt usually results inthe considerable loss of dielectric strength of the polymerFigure 1 is a graphical comparison of the DC breakdownstrength for various nanofilled PEI films With more con-ductive fillers lower breakdown strength is not surprisingHowever the insulating ceramic fillers of higher breakdownstrength do not improve the electrical field endurance of thenanocomposites

With decreasing particle size interfacial fractionincreases and the particle-polymer interface adhesion alsobecomes critical Poor particle dispersion and interactionwith polymer matrix is shown in Figures 2(a) and 2(b) Itcan be seen that both dry alumina and silica particles areagglomerated and remain to be in poor dispersion evenafter mixing using a high energy ultrasonic vibration Thehigh surface energy and new chemistry on the nanoparticlesurface tend to bond the particles together tightly

In order to minimize the particle surface effect colloidalparticles that were already well dispersed in a solvent were

Si

Al

PZ Oxide-1Oxide-2

Oxide-3Oxide-4

PEI

0

100

200

300

400

500

600

700

30 lt100 200 300 500 720 1400 661

Com

posit

e bre

akdo

wn

stren

gth

(kV

mm

)

Filler breakdown strength (kVmm)

Relationship between fillers and composite properties

Figure 1 The breakdown strength of PEI nanocomposites withvarious fillers of about 5 wt loading PZ stands for lead zirconateAll polymer films were processed using a solvent cast

procured and mixed with a polymer resin in the wet stateFigure 2(c) shows the image of precursor SiO

2particles After

performing in situ polymerization of the polyamic acid apolymer nanocomposite containing 5 vol SiO

2nanoparti-

cles was synthesized Great particle dispersion was achievedas shown in Figure 2(d) In order to achieve high energydensity in a nanodielectric composite a compromise betweenincrease in dielectric constant and decrease of dielectricstrength when doing nanodielectric engineering appears tobe the direction

32 Field Tunable Nonlinear Composites The success of nan-odielectric engineering depends on several factors includ-ing (1) higher dielectric constant polymer matrix (2) highpermittivity ceramic fillers with low (hysteresis) loss (3)proper dispersion and (4) good interfacing between filler andpolymer matrix [8 9] This becomes even more critical whenthe particle polarity is increased resulting in higher dielectricconstant Among the polar ceramicmaterials are ferroelectric(FE) and antiferroelectric (AFE) particles whose impacton dielectric properties and dispersion are still not wellunderstood [9] Figure 3 shows the increase of the effec-tive dielectric constant of nanocomposites from addition ofparticles Typically the dielectric constant contrast betweenhigh-119896 ceramic filler and polymer matrix is so high that theelectric field concentration in polymer phase will reduce thebreakdown strength of the nanocomposites In addition highdielectric constant contrast also leads to low electric fieldpenetration into the particles and thus the reduced overallenergy storage density in the composites

In order to enable the nonlinear tunability of polymerwe leveraged ferroelectric BaTiO

3and antiferroelectric lead

zirconate nanoparticles (nPZ) The electric filed tunablebehavior of the composites was further studied under highelectric fields The nonlinear dependence of the dielectricconstant of the polymer composites was fully exhibited as theelectric field is increased exceeding the coercive field of leadzirconate (nPZ) as shown in Figure 4 The higher the electricfield and loading fraction of particles the higher the dielectricconstant was reached The field tunable behavior of com-posites enabled by the polar particles can be applicable for

Advances in Materials Science and Engineering 3

(a) (b)

(c) (d)

Figure 2 TEM image of 5 vol nanoparticles dispersing in polymer films (a) Dry Al2

O3

(b) dry silica (c) colloidal SiO2

and (d) colloidalSiO2

in a polyimide

0

20

40

60

80

100

120

140

0 20 40 60

Silicone (27)High (19)

Die

lect

ric co

nsta

nt

BaTiO3 fill loading (vol)

k

Figure 3 The dielectric constant of nanocomposites as a function of volume pertentage loading of BaTiO3

nanoparticle in silicone andcyanoethyl cellulose with dielectric constant of 27 and 19 respectively


Dielectric constant versus electric field nano-AFEs

0

30

60

90

0 50 100 150 200D

iele

ctric

cons

tant

PVDF20 NPZ40 NPZ

E (Vm)

Figure 4 Room temperature dielectric constant under an increased electric field Values were obtained from the hysteresis loopmeasurements of a PVDF polymer containing nPZ particles (10Hz)

Figure 5 TEM image of polyimide containing 15 vol 40 nm BaTiO3

nanoparticles through in situ polymerization of 22-bis[4-(34-dicarboxyphenoxy) phenyl]propane dianhydride (BPADA) with 441015840-oxydianiline (ODA) Film was solution cast and dried and completelyimidized at elevated temperatures

various devices such as antenna and phase shifters requiringfrequency tunability A breakdown strength of gt300 kVmmwas also achieved in the polar nanodielectric compositesrendering an energy density of up to 15 Jcm3

TEM imaging shows the great dispersion and particle dis-tribution in the polymer achieved using properly processedparticles and mixing method (Figure 5) The fractured cross-section image of the film reveals only cohesive fracture withinpolymer rather than adhesive failure at interface indicating astrong polymer-particle bonding

33 Nanoenabled Nonlinear Resistor (Varistors) High capac-ity high density power system may be subject to highvoltage transients and surge protection devices are widelyused Compared with other protection devices such as TVSmetal oxide varistor (MOV) as an example of ceramic-basednonlinear dielectric material at low voltage provides a goodcombination of high voltage scalability peak current car-rying capability and fast response speed [10] Nanoenableddielectric composites were also studied in both metal oxidevaristors and polymer based varistors MOV is made of bulkceramic containing predominate grain-boundary barrier

junctions However the underlying oxide processing requireshigh sintering temperatures (1000ndash1300∘C) for densificationAs a result compositional and microstructural heterogeneityand large average grain sizes (typically from a few micronsto tens of microns as shown in Figure 6(a)) as well ashigh porosity caused by exaggerated grain growth duringhigh-temperature sintering limit the performance of existingMOVs Also shown in Figure 6 fine microstructures can beobtained for nanoenabled MOVs which can be sintered atreduced temperatures (lt1000∘C) More varistor junctions ina unit MOV device can be provided so that higher voltagecan be applied or smaller device can be fabricated for anequivalent voltage requirement

Higher breakdown voltage (gt1 kVmm) and excellentelectrical performance were achieved using the nanopowderprecursors new composition and sintering profile Theelectric field dependence of current for the MOV processedunder different sintering conditions is shown in Figure 7 Notonly the resistance of the ceramic composites decreases ina nonlinear way with an increase in the electric field butalso the transition field for the nonlinearity is pushed upby 10 times This will enable miniaturization of MOVs and


(a) (b)

Figure 6 SEM images of microstructures of a commercial metal oxide varistor (a) and nanoenabled MOV sintered at 850∘C (b)

GE nano-MOVCommercial MOV

Current density (Acm2)

Elec

tric

fiel

d (V

mm

)

104

103

102

101

100

100

10minus10

10minus8

10minus6

10minus4

10minus2

120572 sim 60

120572 sim 20

Figure 7 Nanoenabled MOVs with higher breakdown voltage after sintering at lower temperatures The commercial MOVs made of coarsepowder precursors were tested for comparison

high voltage surge suppression Besides when the nano-sizedmetal oxide precursors were mixed with polymers to formcomposites the similar 119868-119881 characteristics can be observedThe field tuning behavior of the resistance can be fulfilled inthe nanodielectric composites

4 Conclusions

It is demonstrated that novel electricaldielectric propertiescan be achieved via the nanodielectric engineering andinterface engineering

(1) Nanodielectric composites can be processed to hostvariety of ceramic particles and great particle disper-sion and bonding with the host polymers can be ful-filled The loading fraction and particle morphologyhowever need to be controlled not to dramaticallylower the dielectric strength

(2) Electrically tunable composites were processed anda nonlinearly dependent dielectric constant wasdemonstrated by mixing polar nanoparticles witheither low-K polyimide polymer or high-K PVDFpolymers The challenges are making the compositetunable at very low electric fields

(3) High energy density (15 Jcm3) can be achieved innanodielectric composites containing nonlinear fer-roelectric particles with proper particle morphologydispersion and particle-polymer adhesion

(4) 10x increase of voltage withstanding capability withlow leakage current and higher nonlinearity can berealized in nanoenabled varistors Refined and uni-form submicron structures can be further explored inpolymer based tunable resistors

Further optimization of the performance of these electricdevicecomponent based such principles is undertaken

Conflict of Interests

The authors declare that they have no conflict of interestsregarding the publication of this paper

Acknowledgment

Part of theworkwas sponsored byUSDepartment ofDefenseunder Contract of FA9451-08-C-0166


References

[1] A S Arico P Bruce B Scrosati J-M Tarascon and W VanSchalkwijk ldquoNanostructured materials for advanced energyconversion and storage devicesrdquo Nature Materials vol 4 no 5pp 366ndash377 2005

[2] A Nourai B P Martin and D R Fitchett ldquoFor new electricitystorage technologies the proof is in AEPrsquos own testsrdquo IEEEPower amp Energy Magzine pp 41ndash46 2005

[3] B P Roberts ldquoGrowing use of large-scale power quality pro-tection systemsrdquo in Proceedings of the Conference on ElectricalSupply Industry Shanghai China 2004

[4] R Baldick ldquoReactive issues reactive power in restructuredmarketsrdquo IEEE Power and Energy Magazine vol 2 no 6 pp14ndash17 2004

[5] Q Tan P Irwin and Y Cao ldquoAdvanced dielectrics for capaci-torsrdquo IEEJ Transactions on Fundamentals andMaterials vol 126no 11 pp 1153ndash1159 2006

[6] Y Cao P C Irwin and K Younsi ldquoThe future of nanodi-electrics in the electrical power industryrdquo IEEE Transactions onDielectrics and Electrical Insulation vol 11 no 5 pp 797ndash8072004

[7] Y Cao S Zhang P Irwin et al GETISCAT report 2010GRC431[8] D Q Tan Q Chen Y Cao P Irwin and S Heidger ldquoPoly-

mer based nanodielectric compositesrdquo Presentation at EMAOrlando Fla USA 2010

[9] Q Tan Y Cao and P Irwin ldquoDC breakdown in polyetherimidecomposites and implication for structural engineeringrdquo inProceedings of the International Conference on Solid Dielectrics(ICSD rsquo07) pp 411ndash414 Winchester UK July 2007

[10] D Q Tan K Younsi Y Zhou P Irwin and Y Cao ldquoNano-enabled metal oxide varistorsrdquo IEEE Transactions on Dielectricsand Electrical Insulation vol 16 no 4 pp 934ndash939 2009

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Advances in

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materials


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ZnO and other oxide additives A conventional mixed oxideprocess was used for powder processing Green compacts of110158401015840 diameter were pressed using a uniaxial hydraulic press andthen sintered at temperatures of 850ndash1000∘C

DCbreakdown test was conducted followingASTMD149(method A) using a ball-plane electrode configuration Thesample was immersed into insulation oil and DC voltagewas applied at a ramp rate of 500Vs until the samplefailed Dielectric responsesweremeasured using a broadbanddielectric spectrometer from Novacontrol GmbH Scanningelectron microscopy (SEM) imaging was done using a ZeissSupra 55VP (Carl Zeiss AG Oberkochen Germany) Trans-mission electronmicroscopy (TEM) imagingwas taken usinga FEI Tecnai F20 transmission electron microscope

3 Results and Discussions

High energy density capacitor dielectrics field tunable com-posite dielectrics andmetal oxide varistors (MOVs) are takenas examples in this section to demonstrate the achievableelectricaldielectric properties via the nanodielectric andinterface engineering to fulfill the aforementioned develop-ment needs

31 Nanofilled Polymer Film for Capacitors Capacitors rep-resent a big family of non-Faradic energy storage com-ponents used in applications ranging from decoupling incircuit board to series capacitive compensation banks atpower transmission class The state-of-art polymeric filmcapacitors are based primarily on low permittivity polymerssuch as polypropylene or polyester One natural approachfor higher energy density capacitor is the synthesis of higherpermittivity composites with higher dielectric strength byincorporating ceramic fillers into polymer matrix This isbecause the energy density of a film capacitor takes the simpleform of 12120576

119900K1198642 where K is the dielectric constant and 119864 is

the dielectric strength of the polymer filmAlthough the dielectric constant can be increased to

some extent at higher particle loading level increasing thedielectric strength remains a great challenge Instead theaddition of particles exceeding 10wt usually results inthe considerable loss of dielectric strength of the polymerFigure 1 is a graphical comparison of the DC breakdownstrength for various nanofilled PEI films With more con-ductive fillers lower breakdown strength is not surprisingHowever the insulating ceramic fillers of higher breakdownstrength do not improve the electrical field endurance of thenanocomposites

With decreasing particle size interfacial fractionincreases and the particle-polymer interface adhesion alsobecomes critical Poor particle dispersion and interactionwith polymer matrix is shown in Figures 2(a) and 2(b) Itcan be seen that both dry alumina and silica particles areagglomerated and remain to be in poor dispersion evenafter mixing using a high energy ultrasonic vibration Thehigh surface energy and new chemistry on the nanoparticlesurface tend to bond the particles together tightly

In order to minimize the particle surface effect colloidalparticles that were already well dispersed in a solvent were

Si

Al

PZ Oxide-1Oxide-2

Oxide-3Oxide-4

PEI

0

100

200

300

400

500

600

700

30 lt100 200 300 500 720 1400 661

Com

posit

e bre

akdo

wn

stren

gth

(kV

mm

)

Filler breakdown strength (kVmm)

Relationship between fillers and composite properties

Figure 1 The breakdown strength of PEI nanocomposites withvarious fillers of about 5 wt loading PZ stands for lead zirconateAll polymer films were processed using a solvent cast

procured and mixed with a polymer resin in the wet stateFigure 2(c) shows the image of precursor SiO

2particles After

performing in situ polymerization of the polyamic acid apolymer nanocomposite containing 5 vol SiO

2nanoparti-

cles was synthesized Great particle dispersion was achievedas shown in Figure 2(d) In order to achieve high energydensity in a nanodielectric composite a compromise betweenincrease in dielectric constant and decrease of dielectricstrength when doing nanodielectric engineering appears tobe the direction

32 Field Tunable Nonlinear Composites The success of nan-odielectric engineering depends on several factors includ-ing (1) higher dielectric constant polymer matrix (2) highpermittivity ceramic fillers with low (hysteresis) loss (3)proper dispersion and (4) good interfacing between filler andpolymer matrix [8 9] This becomes even more critical whenthe particle polarity is increased resulting in higher dielectricconstant Among the polar ceramicmaterials are ferroelectric(FE) and antiferroelectric (AFE) particles whose impacton dielectric properties and dispersion are still not wellunderstood [9] Figure 3 shows the increase of the effec-tive dielectric constant of nanocomposites from addition ofparticles Typically the dielectric constant contrast betweenhigh-119896 ceramic filler and polymer matrix is so high that theelectric field concentration in polymer phase will reduce thebreakdown strength of the nanocomposites In addition highdielectric constant contrast also leads to low electric fieldpenetration into the particles and thus the reduced overallenergy storage density in the composites

In order to enable the nonlinear tunability of polymerwe leveraged ferroelectric BaTiO

3and antiferroelectric lead

zirconate nanoparticles (nPZ) The electric filed tunablebehavior of the composites was further studied under highelectric fields The nonlinear dependence of the dielectricconstant of the polymer composites was fully exhibited as theelectric field is increased exceeding the coercive field of leadzirconate (nPZ) as shown in Figure 4 The higher the electricfield and loading fraction of particles the higher the dielectricconstant was reached The field tunable behavior of com-posites enabled by the polar particles can be applicable for


(a) (b)

(c) (d)


O3



in a polyimide

0

20

40

60

80

100

120

140

0 20 40 60


Die

lect

ric co

nsta

nt


k





0

30

60

90

0 50 100 150 200D

iele

ctric

cons

tant

PVDF20 NPZ40 NPZ

E (Vm)










(a) (b)




Elec

tric

fiel

d (V

mm

)

104

103

102

101

100

100

10minus10

10minus8

10minus6

10minus4

10minus2

120572 sim 60

120572 sim 20



4 Conclusions









Acknowledgment



References


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)

(c) (d)


O3



in a polyimide

0

20

40

60

80

100

120

140

0 20 40 60


Die

lect

ric co

nsta

nt


k





0

30

60

90

0 50 100 150 200D

iele

ctric

cons

tant

PVDF20 NPZ40 NPZ

E (Vm)










(a) (b)




Elec

tric

fiel

d (V

mm

)

104

103

102

101

100

100

10minus10

10minus8

10minus6

10minus4

10minus2

120572 sim 60

120572 sim 20



4 Conclusions









Acknowledgment



References


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





0

30

60

90

0 50 100 150 200D

iele

ctric

cons

tant

PVDF20 NPZ40 NPZ

E (Vm)










(a) (b)




Elec

tric

fiel

d (V

mm

)

104

103

102

101

100

100

10minus10

10minus8

10minus6

10minus4

10minus2

120572 sim 60

120572 sim 20



4 Conclusions









Acknowledgment



References


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




(a) (b)




Elec

tric

fiel

d (V

mm

)

104

103

102

101

100

100

10minus10

10minus8

10minus6

10minus4

10minus2

120572 sim 60

120572 sim 20



4 Conclusions









Acknowledgment



References


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




References


















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials










CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



Research Article Tunable Nanodielectric...

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