DTIC ADA584375: Optimization of Nickel Nanocomposite for Large Strain Sensing Applications

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Optimization of nickel nanocomposite for large strain sensing W911NF-08-1-0350
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611102
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Oliver K. Johnson, George C. Kaschner, Thomas A. Mason, David T.
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Brigham Young University
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14. ABSTRACT
A novel large strain sensor has been developed using a silicone/nickel nanostrand/nickel coated carbon
fiber nanocomposite system. The effect of conductive filler volume fraction on the piezoresistive response
of the nanocomposite sensor has been studied in order to determine the optimal composition for use in
large strain/motion sensing applications. Electromechanical testing of various compositions revealed that
optimum performance was achieved using 11 vol% nickel nanostrands with 2 vol% nickel coated carbon
15. SUBJECT TERMS
Nanocomposite, Strain Sensor, Piezoresistive, Nanostrand
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Report Title
Optimization of nickel nanocomposite for large strain sensing applications
ABSTRACT
A novel large strain sensor has been developed using a silicone/nickel nanostrand/nickel coated carbon
fiber nanocomposite system. The effect of conductive filler volume fraction on the piezoresistive response
of the nanocomposite sensor has been studied in order to determine the optimal composition for use in
large strain/motion sensing applications. Electromechanical testing of various compositions revealed that
optimum performance was achieved using 11 vol% nickel nanostrands with 2 vol% nickel coated carbon
fiber in the silicone matrix. Initial results indicate that this nanocomposite is capable of sensing strains
of over 40% elongation.
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Optimization of nickel nanocomposite for large s ...
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SensorsandActuatorsA166 (2011) 40–47
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Sensors and Actuators A: Physical
journal homepage: www.elsevier.com/locate/sna
Optimization of nickel nanocomposite for large strain sensing applications
OliverK.Johnsona,∗,GeorgeC.Kaschnerb,ThomasA.Masonb,DavidT.Fullwooda,GeorgeHansenc
aDepartmentofMechanicalEngineering,BrighamYoungUniversity,435CrabtreeBuilding,Provo,UT84602,USA
bLosAlamosNationalLaboratory,30BikiniAtollRd,LosAlamos,NM87545-0001,USA
cConductiveComposites,LLC,855S600W,HeberCity,UT84032-2256,USA
a r t i c l e i n f o a b s t r a c t
Articlehistory: Anovellargestrainsensorhasbeendevelopedusingasilicone/nickelnanostrand/nickelcoatedcarbon
Received7July2010 fibernanocompositesystem.Theeffectofconductivefillervolumefractiononthepiezoresistiveresponse
Receivedinrevisedform21October2010 ofthenanocompositesensorhasbeenstudiedinordertodeterminetheoptimalcompositionforusein
Accepted25December2010
largestrain/motionsensingapplications.Electromechanicaltestingofvariouscompositionsrevealedthat
Available online 31 December 2010
optimumperformancewasachievedusing11vol%nickelnanostrandswith2vol%nickelcoatedcarbon
fiberinthesiliconematrix.Initialresultsindicatethatthisnanocompositeiscapableofsensingstrains
Keywords:
ofover40%elongation.
Nanocomposite
© 2011 Elsevier B.V. All rights reserved.
Strain
Sensor
Piezoresistive
Nanostrand
1. Introduction repeatability,focuswasplacedonminimizationofthemeasure-
mentuncertaintyduetomeasurementerrorbetweensensors(εerr).
The sensing of mechanical strain is integral to many engi- Silicone/nickelnanostrand(Si/NiNs)compositeshavebeenshown
neering applications such as ballistic testing, biomechanics (e.g. tobeeffectiveatmeasuringstrainsinexcessof40%elongation[6].
advanced prosthetics), haptic interfaces, structural health moni- Theirexponentialpiezoresistiveresponseprovideshighmeasure-
toring,etc.However,atpresent,mostconventionalstraingauges mentresolution.Atthesametimetheirelastomericmatrixandlow
can only measure strain reliably up to a few percent [1]. Sev- Young’smodulus(∼6.5MPa)allowthemtodeformtolargestrains
eralmethods,suchasinterferometricstrain/displacementgauges without significantly stiffening the substrate to which they are
(ISDG)[2],digitalimagecorrelation(DIC)[1],anddifferentialdig- attached.However,duetothepercolativenatureofchargetrans-
italimagetracking(DDIT)[3]havebeendevelopedinordertobe portinCPCs,Si/NiNsnanocompositeshaveanidiosyncraticcritical
abletomeasurelargerstrains;however,suchmethodsareexpen- strain,εc,belowwhichtheirpiezoresistiveresponseisnearlyzero.
sive,complicatedandcumbersome.Thereisanobviousneedfor In order to improve the material response of this nanocompos-
asimple,inexpensivestraingaugetechnologythatcouldbeeasily ite system such that it becomes viable for use as a large strain
integratedintothemanyengineeringdesignsinwhichlargestrains sensor,astudywasconductedtoexaminetheeffectoffillervol-
mustbesensed. umepercentageonthepiezoresistiveresponseofthematerial.As
Conductivepolymercomposites(CPCs)haveshownpromiseas partofthisstudytheeffectoftheadditionofasecondconductive
inexpensivelargestrainsensors[4–7];however,theirusehasbeen fillerparticleofadistinctlengthscale,namelynickelcoatedcarbon
limitedduetotheirprohibitivelackofreproducibility[8,9].Acan- fibers(NCCFs),wasinvestigated.Theoptimizedcompositionwas
didatelargestrainsensorwillhavethefollowingcharacteristics: showntobecapableofsensingstraininexcessof40%elongation
(1)anobviousabilitytosensestrainsinexcessofthosecapable whileshowingsignificantrepeatabilitybetweengauges.Thisini-
usingconventionalmethods;(2)highmeasurementfidelity;and tialoptimizationofanovelSi/NiNs/NCCFnanocompositematerial
(3)goodrepeatabilitybetweensensors.Theobjectiveofthisstudy demonstratesthegreatpotentialofthismaterialforuseasaninex-
wastodesignasensorcapableofmeasuringstrainsofεmax≥40% pensivestraingaugetechnologythatcanmeasurestrainsgreater
elongation. In order to ensure high measurement fidelity and than8timesthoseofconventionalgauges.
2. Theory
∗
Correspondingauthor.Tel.:[phone].
Inpreviouswork,tensiletestingofvariouselastomericmate-
E-mailaddresses:[email protected],
[email protected](O.K.Johnson). rials filled with NiNs demonstrated the extreme piezoresistive
0924-4247/$–seefrontmatter© 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.sna.2010.12.022
Author's personal copy
O.K.Johnsonetal./SensorsandActuatorsA166 (2011) 40–47 41
Considerforamomenttheproblemofelectricalconductivityfor
aCPCinthecontextofacontinuumbondpercolationmodel.Inthis
casethenodesrepresenttheconductivephaseandthebondsrep-
resent conductive paths through the non-conductive phase (due
to, e.g. tunneling, electron hopping, etc.). When a bond is open
thereisnoconductionbetweenadjacentnodes,whenitisclosed
thereisconductionbetweennodes.InthecaseofSi/NiNscompos-
itesthisisthoughttobeduetoquantummechanicaltunneling,in
whichcaseabondisopenonlyiftheconductivephaseparticlesare
within∼1nmofeachother[11].Wenotethatinthisformulation
eachconductivephaseparticleisreducedtoapointontheper-
colationgraph,andthebondsconnectconductivephaseparticles
onlyalongtheshortestpathbetweenparticles(i.e.notcentroidto
centroid).Ifarepresentativeelementor‘unitcell’ofagivengraph
isexamined(Fig.2)thethresholdprobabilityforconductioncan
bededuced.Foraconductivepathtoexistthroughanodeatleast
twoofthebondsconnectedtothatnodemustbeclosed.Itwould
thenbeexpectedthatthethresholdprobabilitybeapproximately
equaltothenumberofrequiredclosedbondsdividedbythetotal
numberofavailablebondlocations.Forthehoneycomblatticein
Fig. 2 one would expect a critical probability of pc≈2/3, for the
square lattice pc≈1/2, and for the triangular lattice pc≈1/3 and
indeedthesevaluesmatchwellwiththosereportedintheliterature
Fig.1. TypicalpiezoresistiveresponseofSi/NiNscomposite.
[12].Fig.2alsoshowsarepresentative‘unitcell’foracontinuum
modelwherethecentralnodeisconnectedto6adjacentnodes,
responseofelastomer/NiNscomposites[6].However,thepotential for which we would again expect pc≈1/3. This is to say that by
applicabilityofthesepreviouslytestedcompositesystemstostrain increasingthenumberofbondsattachedtoeachnodethepercola-
sensingapplicationswaslimitedbytheirpercolationlikeresponse. tionthresholdcaneffectivelybelowered.Thisfactcanbeexploited
As these composites were pulled in tension, an effective critical tomanipulatethecriticalstrainatwhichSi/NiNsnanocomposites
strain,εc,wasobserved.Onceεcwasexceededthesystemwould begin to respond piezoresistively. By some means, as this mate-
begintorespond,similartothecriticalvolumefractioninperco- rialisstrainedthespatialdistributionofNiNswithrespecttoone
lationtypesystems(Fig.1)[10].Intheelastomer/NiNscomposites another (which directly corresponds to the distribution of bond
previously studied, εc typically occurred between 10% and 20% lengthsinthecontinuumpercolationgraph)isalteredsuchthatthe
elongation. materialpassesfromasub-criticalpercolationregimethroughthe
Intraditionalpercolationtheory,asystemisrepresentedbyan percolationthresholdandintoahighlyconductivesuper-critical
undirectedgraphofnodesandconnectinglines.Thelinesinthis regime. By increasing the initial (unstrained) number of poten-
latticearecalled‘bonds’andthenodesorintersectionsofthelines tialpathsbetweenconductivephasenanoparticlesthethreshold
arecalled‘sites’.Foraparticularsimulation,sites(inthecaseofsite (εc)canbeloweredandtheamountofstrainrequiredtocrossthe
percolation)orbonds(inthecaseofbondpercolation)areeither threshold should be less. Increasing the proximity of conductive
closedoropenwithagivenprobability,p.Aspincreases,clusters phase particles for a static system (i.e. one that is not subjected
of closed sites or bonds will develop and one can define a criti- tomechanicalloading)istypicallyaccomplishedbyincreasingthe
calorthresholdprobability,pc,whichisequivalenttothenumber volume fraction of the conductive phase. However, in nanocom-
fractionofclosedsitesorbondsrequiredtocreateaclusterthat posites this strategy can require prohibitively large quantities of
spansthelattice.Thispercolationthresholdisintimatelyrelatedto expensivenanoparticles.Analternativemethodtomodifytheini-
thestructureofthegraph.Thegraphstructureoftraditionalper- tialbondlengthdistributionistointroduceasecondconductive
colationtheoryisthatofaregularlattice.However,theintrinsic phaseofadistinctlengthscale—effectivelysuperimposingthetwo
structureofsomephysicalsystemsdoesnotlenditselfwelltoan respectivepercolationgraphs.Theeffectofthisistocreate‘super-
ordered lattice representation. For cases of this type continuum nodes’or‘super-sites’inthecompositegraphthatconnectsmall
percolation theory is used. In continuum percolation theory the clustersandcreatelargerclustersatlowervolumefractions(Fig.3).
locationsofsitesarenotlimitedtodiscretepointsonthelattice; The creation of these ‘super-nodes’ significantly reduces the
rather, they are allowed to exist at any point in the continuous thresholdprobabilityascanbeseenfromFig.4,wherea‘super-
domain.Thisallowsforthemodelingofrandomnetworks,suchas node’ connecting 50 clusters is schematically represented. The
theoneunderconsideration. expectedthresholdforthiscasewouldbepc≈1/25.
Fig.2. Severaltypicallatticesforpercolationsimulations.
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42 O.K.Johnsonetal./SensorsandActuatorsA166 (2011) 40–47
Fig.3. Schematicofthecreationof‘super-nodes’bytheinclusionofasecondphaseofdistinctlengthscale.(a)A2DrepresentationofthedistributionofNiNs(thinblack
bifurcatedstructures)andNCCF(grayrectangularstructure)inthesiliconematrix.(b)Continuumpercolationrepresentationof(a),whereblacksites(blackcircles)represent
discreteNiNs,graysites(graycircles)representNCCFs,andbonds(blacklines)representconductivejunctions(i.e.locationswherethedistancebetweenconductivephase
particlesis∼1[11]).
Whiletheintroductionofthislargersecondphasedoesincrease ensuretheusefulnessandapplicabilityoftheresultingnanocom-
thevolumefractionofconductivephasematerial(withfewerand posite material as a large displacement strain sensor, the metric
lessexpensiveparticles),themostimportanteffectisthatofmanip- of εerr is also used to identify the ideal composition. In previous
ulatingthebondlengthdistributionsuchthattheaveragedistance electromechanical testing of Si/NiNs composites there was little
betweenconductivephasedomainsisdecreased.Asthematerial tonoresponsemanifestedwithNiNsvolumefractionsbelow7%.
ismechanicallyloadedthisiswhatdirectlycausestheconsequent Conversely, mechanical instability was observed at volume frac-
decreaseinεc.Thereis,however,acompetingincreaseincompos- tionsabove15%.Thesepreviousresultsguidedourselectionofthe
itestiffnessbecauseofthelowercomplianceinthelargerphase, followingcompositionsfortesting:
whichisalimitingfactorforεmax.Therefore,thisstudyiscouched 7%NiNs + 4%NCCF
asanoptimizationprobleminwhichsimultaneousminimization
ofεcandmaximizationofεmaxissought.Theabovetheoryaswell
aspriorworksuggeststhatbyfindingtheidealvolumefractions 9%NiNs + 1%NCCF
of the nano-phase and the larger macroscopic phase conductive
fillersthespatialdistributionofconductivephaseparticlescanbe
9%NiNs + 2%NCCF
indirectly optimized so as to achieve this objective. In order to
9%NiNs + 3%NCCF
11%NiNs + 1%NCCF
11%NiNs + 2%NCCF
where filler content was measured in volume percentage. The
following compositions were also tested: 7% NiNs+1% NCCF, 7%
NiNs+2%NCCF,and7%NiNs+3%NCCF.However,therewaslim-
itedpiezoresistiveresponseandthereforethedataisnotincluded
inthisstudy.Threetrialswereperformedforeachofthecompo-
sitionsthatweretested;however,forboththe9%NiNs+1%NCCF
and9%NiNs+2%NCCFcompositionsthedataforoneofthetrials
wastoonoisyanddifferedsignificantlyfromtheothertwotrials
andwasconsequentlythrownout.
3. Experimentalprocedures
3.1. Themold
Small‘dog-bone’tensilesampleswerepreparedusinganalu-
minummold.Toaccommodatebothsmallandlargebatcheseach
Fig.4. Schematic‘unit-cell’fora‘super-node’. slot in the mold was made for a set of three dog-bone samples
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O.K.Johnsonetal./SensorsandActuatorsA166 (2011) 40–47 43
with 20wt% nickel (∼80nm thick) and cut to lengths of about
2mm.
The silicone base was mixed with the NiNs in a planetary
centrifugal mixer in order to achieve uniform dispersion of the
nanoparticles.TheSi/NiNssolutionwasthenscreenedthrougha
40gaugemeshtoeliminateanylargeparticlesthatmayhaveinfil-
tratedthesolutionandtoprovidesomedegreeofuniformparticle
size. To this concentrated solution were added the NCCF, addi-
tionalsiliconebase,andthecross-linkingcatalyst.Withincreasing
fillervolumepercentageitbecamenecessarytoaddsmallamounts
of solvent as well, in order to decrease the viscosity sufficiently
suchthatpropermixingcouldoccurandtodecreaseshearstresses
that could destroy the unique structure of the NiNs particles.
This mixture was then mixed again in a planetary centrifugal
mixer.
After mixing, the solution was then placed in the mold using
a metal spatula. This method of filling the mold introduced air
Fig.5. Dog-bonemoldschematic(unitsincm). bubblesintothesamples.Therefore,inordertopreventporosity
inthecuredsamples,theyweredegassedundervacuumatroom
(Fig.5).Channelswerecutintothemoldalongitslengthsothatcop- temperature.Thesampleswerethencuredat100◦Covernight.
perwirescouldbecastintothedog-bonesamplesforimprovedand
consistentelectricalcontact(Figs.5and6).Themoldwastreated 3.3. Testing
with an aerosol polytetrafluoroethylene (PTFE) mold release
in order to facilitate post-cure removal of the nanocomposite MechanicaltestingwasperformedonthesamplesusinganMTS
samples. 880MaterialTestSystem.Themetallicclamshellgripsthatwere
usedwereinsulatedfromtherestofthetestsystemusingcustom
3.2. Nanocompositepreparation madePhenolicspacers.Inordertopreventslippage,anabrasive
coveringwasappliedtothegrips.Eachsamplewasconnectedin
The matrix material for the Si/NiNs/NCCF nanocomposite in serieswitha120(cid:3)±0.01%precisionresistor.Thisvoltagedivider
this study was Dow Corning’s® two part silicone elastomer, Syl- circuitwasshieldedandexcitedbya10VDCsource.Thesamples
gard184.Theprincipalconductivefillerthatwasusedwasnickel were pulled in tension at a strain rate of 2.5mm/min until frac-
nanostrands.NiNsarehighaspectrationanoparticleswithaunique ture.Strainwascalculatedfromthedisplacementofthetestsystem
bifurcatedstructure[6,7,13,14].TheyaremadebyConductiveCom- cross-headandthegaugelengthofthesamples.
posites,LLCthroughaproprietarylowtemperature,atmospheric Aseachsamplewaspulled,thevoltagedropacrossitdecreased.
pressurechemicalvapordeposition(LTAPCVD)process[13].Forthe Theresistivityofthesamplewascalculatedfromthechangeinvolt-
largersecondphase,carbonfiberswereused,whichwerecoated ageusingthefollowingformula,derivedfromthevoltagedivider
Fig.6. Materialschematic.
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44 O.K.Johnsonetal./SensorsandActuatorsA166 (2011) 40–47
Fig.7. PiezoresistiveresponseofcandidateSi/NiNs/NCCFcompositions.
ruleandthedefinitionofvolumeresistivity: Exceptingthe7%NiNs+4%NCCFcomposition,allofthetested
(cid:2) (cid:3) (cid:2) (cid:3)
R V tw compositionsachievedgreaterthan40%elongation.
(cid:4)= 1 0 × (1)
V −V l
s 0
5. Discussion
where R is the resistance of the 120(cid:3) series resistor, V the
1 0
measured voltage signal, Vs the excitation voltage, t the sample 5.1. Performancecomparison
thickness, w the sample width, and l the distance between the
coppercontacts. Thenegativegaugefactorthatwasobservedshowsadiametri-
callyoppositeresponsetothatoftypicalconductivefilledpolymer
4. Results composites,butwhichischaracteristicofthisnanocompositesys-
tem [6,7,15,16]. For clarity, gauge factor is defined by(cid:5)R/R=Gε,
The response of each of the tested compositions is shown in whereRiselectricalresistanceandGisthegaugefactor.Ingeneral,
Fig.7.Inallcasesthevolumeresistivitydecreasedsignificantlyas theincreaseinresistivitywithstrainofconventionalconductive
thematerialwasstrainedintension.Averagedoverthethreetri- filled polymer composites is thought to be a result of the net
alsforeachcomposition,the9%NiNs+1%NCCFand9%NiNs+2% increaseindistancebetweenconductivefillerparticles[15,17–19].
NCCFcompositionsdroppedaboutthree-and-a-halfordersofmag- However,reportsofanotherelastomer/nickelfilamentsystemhave
nitudeinresistivity;the11%NiNs+1%NCCF,aboutthreeordersof beenmadethatshowanegativegaugefactor,whichisattributed
magnitude;the9%NiNs+3%NCCFand11%NiNs+2%NCCF,about to increased filament alignment upon tensile loading [20]. The
two-and-a-half orders of magnitude; and the 7% NiNs+4% NCCF decrease in resistivity of the Si/NiNs/NCCF system is believed to
almostoneorderofmagnitude. be largely attributable to the unique bifurcated structure of the
Fig.8. Comparisonofgaugefactor,G,asafunctionofstrainforallofthetestedcompositions.Only0–15%elongationisshownforclarityandbecauseallcompositions
approachedthesamegaugefactorvaluewithincreasingstrain.
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O.K.Johnsonetal./SensorsandActuatorsA166 (2011) 40–47 45
Fig.9. Comparisonofεcandεmaxforeachcomposition.Thevaluesreportedinthis
figurearethearithmeticmeansforalltrialsthatwereconsidered.
Fig.10. Schematicrepresentationofhowεerrwascalculatedfromtherangeofthe
NiNs particles [7]. In fact, previous work has shown that these volumeresistivityforagivenstrain.
nanocompositeshaveanegativegaugefactorinbothtensionand
compression[13].
AsisapparentfromFig.7,whenNCCFcontentisincreasedthere tivitywasequaltotheminimumresistivity.εerr isdefinedasthe
differencebetweenthesetwoquantities.Thisprocessisillustrated
isadramaticcorrespondingincreaseintheoverallconductivityof
inFig.10.
thecomposite.Thecompositeconductivityisincreasedaboutan
This represents, in essence, the predicted uncertainty in the
orderofmagnitudeforeachadditionalvolumepercentofNCCF.
strain level indicated by the gauge—based on the uncertainty in
IncreasingNCCFcontentalsotendstodecreasethemagnitude
themeasuredresistivitysignal.Themeasurementerrorprofileof
ofthegaugefactor,ascanbeseenfromFig.8wherethepeakgauge
eachcandidatecompositionisshowninFig.11.
factors generally become less and less negative with increasing
NCCFcontent. We note that unlike the previously discussed parameters (εc,
Thecriticalstrain,εc,wascalculatedasthelaststrainlevelfor εmax, G), εerr is not an intrinsic property of the nanocomposite
whichthevolumeresistivitywaswithin5%ofthemaximumvol-
umeresistivity.Aspredicted,εcdiddecreasewithincreasingNCCF
contentforboththe9%NiNsand11%NiNscompositions(Fig.9).
Althoughtherewasnocomparativesamplereportedtotestthis
trendforthe7%NiNs+4%NCCFsample,wenotethat–asstated
previously–lowerNCCFcontentsamplesweretestedat7%NiNs
loadings but there was no response. This is to say that εc was
neverreachedatNCCFcontentlowerthan4%forthesesamples.
Thiscorroboratestheresultsoftheothersamplesbecauseεceffec-
tivelydecreasedfrominfinitytoafinitevalueasNCCFcontentwas
increased.
Fig. 9 also indicates a general dependence of the maximum
elongationonbothNiNsandNCCFcontent.Increasingthevolume
percentageofeithersignificantlydecreasesthemaximumelonga-
tionofthenanocompositesystembeforefracture.Thisisthought
to be due to the fact that increased filler volume fraction leads
toalargernumberofinhomogeneitiesinthecompositematerial.
Theseinhomogeneitiesactasstressrisersandleadtovoidforma-
tion[21].Thus,increasingeitherNiNsorNCCFcontentleadstoa
statisticalincreaseinvoidformationprobabilityandanaccompa-
nyingdecreaseinthestrainrequiredtocausefailure.Additionally,
theextremedisparityinelasticmodulusbetweentheNiNs/NCCFs
andthesiliconematrixmayleadtoseparationatthefiller/matrix
interfaces[22],alsocausingvoidformation.
5.2. Measurementerror
In order to quantify repeatability between gauges, the maxi-
mumandminimumresistivityofalltrialswascalculatedforeach
composition as a function of strain. The smallest possible strain
thatameasuredresistivitycouldrepresentwasgivenbythestrain
atwhichthemeanresistivitywasequaltothemaximumresistiv-
ity.Similarly,thelargestpossiblestrainthatameasuredresistivity
couldrepresentwasgivenbythestrainatwhichthemeanresis- Fig.11. Measurementerrorprofilesforeachcandidatecomposition.