DTIC ADA588154: Hybrid Carbon Fibers/Carbon Nanotubes Structures for Next Generation Polymeric Composites

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JournalofNanotechnology
Volume2010,ArticleID860178,9pages
doi:10.1155/2010/860178
Research Article
Hybrid Carbon Fibers/Carbon Nanotubes Structures for
Next Generation Polymeric Composites
M.Al-Haik,1C.C.Luhrs,1M.M.RedaTaha,2A.K.Roy,3L.Dai,4J.Phillips,5andS.Doorn5
1DepartmentofMechanicalEngineering,UniversityofNewMexico,Albuquerque,NM87131,USA
2DepartmentofCivilEngineering,UniversityofNewMexico,Albuquerque,NM87131,USA
3AFRL/RXBTThermalSciencesandMaterialsBranch,Wright-PattersonAFB,OH45433,USA
4DepartmentofChemicalEngineering,CaseWesternReserveUniversity,Cleveland,OH44106,USA
5LosAlamosNationalLaboratory,LosAlamos,NM87545,USA
CorrespondenceshouldbeaddressedtoM.Al-Haik,[email protected]
Received2July2009;Revised18December2009;Accepted2January2010
AcademicEditor:QuanWang
Copyright©2010M.Al-Haiketal.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense,
whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
Pitch-basedcarbonfibersarecommonlyusedtoproducepolymericcarbonfiberstructuralcomposites.Severalinvestigationshave
reporteddifferentmethodsfordispersingandsubsequentlyaligningcarbonnanotubes(CNTs)asafillertoreinforcepolymer
matrix.ThesignificantdifficultyindispersingCNTssuggestedthecontrolled-growthofCNTsonsurfaceswheretheyareneeded.
Here we compare between two techniques for depositing the catalyst iron used toward growing CNTs on pitch-based carbon
fibersurfaces.ElectrochemicaldepositionofironusingpulsevoltametryiscomparedtoDCmagnetronironsputtering.Carbon
nanostructuresgrowthwasperformedusingathermalCVDsystem.Characterizationforcomparisonbetweenbothtechniques
wascomparedviaSEM,TEM,andRamanspectroscopyanalysis.Itisshownthatwhilebothtechniquesweresuccessfultogrow
CNTsonthecarbonfibersurfaces,ironsputteringtechniquewascapableofproducingmoreuniformdistributionofironcatalyst
andthusmultiwallcarbonnanotubes(MWCNTs)comparedtoMWCNTsgrownusingtheelectrochemicaldepositionofiron.
1.Introduction selvesthatarenotonlypoorlyadheredtothematrixbutalso
concentrate stresses, compromising the effect of the CNTs
The attractive properties of carbon nanotubes [1] (CNTs) asreinforcement.Sonication[15]andcalendaring[16]have
might be attributed to their unique and minimum defect beenusedtomitigatethisproblem,butthesetechniquesare
nanostructure. Single wall carbon nanotubes (SWCNTs) not effective beyond ∼3% CNTs weight fraction due to the
possess exceptional mechanical [2, 3], thermal, and electric formationofaggregates[17].
properties [4] compared to graphite, Kevlar, SiC, and The extreme difficulty in uniformly dispersing CNTs
alumina fibers. The strength, elastic modulus, and fracture in polymer matrices arises from the large surface area of
properties of CNTs are an order of magnitude higher than CNTs [18]. Dispersion and extrusion techniques have been
most common composites used in civilian and military reported in the literature for producing CNTs composites
applications [5–8]. Moreover, CNTs reinforcement was [19]. The authors utilized high magnetic fields to process
proven to increase the toughness of the polymers and nanocomposites based on SWCNTs [14]. However, in both
compositetoabsorbimpactenergy[9–12]. dispersion and extrusion techniques, producing uniform
Most research to date had focused on using CNTs as and well-dispersed carbon nanotubes composite is difficult
a reinforcement or as a filler in a polymeric matrix by because of the small amount of solid “powder” (carbon)
dispersing and perhaps subsequently aligning single- or comparedwiththelargeamountofliquidpolymer(matrix)
multiwalled CNTs in the matrix [13, 14]. Alignment and in early mixing stages. This often leads to phase separation
dispersion are critical factors that are difficult to control due to the strong van der Waals forces between the CNTs
experimentally using oft-repeated mixing methods. CNTs themselves compared with that between CNTs and the
embedded in a polymeric matrix form aggregates of them- polymermatrix[14].
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2010 2. REPORT TYPE 00-00-2010 to 00-00-2010
4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Hybrid Carbon Fibers/Carbon Nanotubes Structures for Next
5b. GRANT NUMBER
Generation Polymeric Composites
5c. PROGRAM ELEMENT NUMBER
6. AUTHOR(S) 5d. PROJECT NUMBER
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Engineering,10900 Euclid Avenue,Cleveland,OH,44106
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2 JournalofNanotechnology
15kV5mm×25k 2μm 15kV5mm×6k 5μm
(a) (b)
Figure1:(a)SEMimageofcarbonfiberannealedat500◦Ctoremovesizing,(b)SEMimageofcarbonfibercleanedbyacetoneandethanol.
Alternatively, to eliminate the dispersion- related prob- growth of CNTs over carbon fibers surfaces, under similar
lems, CNTs can be controlled-grown in places where they growthenvironments.
are needed. CNTs can be grown over most substrates such
assilicon,silica,andalumina[17,20,21].CNTsgrowthon
2.Experimental
metallicsubstrateswasalsoreported[22].
Recently, several investigations discussed the growth of 2.1.CatalystDeposition
CNTs on the surface of microscale graphitic and/or carbon
structures.ThegrowthofCNTsovercarbonfiberencounters 2.1.1. Electrochemical Deposition. Electrochemical deposi-
two obstacles: (i) the catalyst, if transition metal, can easily tion of catalytic materials is a very complex problem that
diffuse into the carbon substrate [23], and (ii) different entailsmanyparameterssuchassolutionconcentration,DC
phases of carbon can be formed on the graphitic substrate voltage, counter electrode material, time of deposition, and
[24]. Similjanic et al. [25] had reported the growth of potential application scheme (continuous versus stepping
multiwallcarbonnanotubesfromethyleneoncarbonpaper potential, etc.). The long-term objective of this research is
by Ohmically heating the catalyst (Co–Ni). Silicate gel rich to grow “noncontinuous” film or “islands” of CNTs rather
with catalytic metals was used to deposit the catalysts than uniform growth all over the carbon fiber. Continuous
on carbon papers. Li et al. [26] utilized chemical vapor voltage electrochemical deposition will yield a uniform
deposition(CVD)tocontrolthegrowthofcarbonnanotubes growth which is not desirable for the future application
ongraphitefoil.Thecatalystconsistedofstainlesssteel(Fe: intended. This application entails manufacturing a hybrid
Cr: NI 70: 19: 11) deposited by DC magnetron sputtering. composite. Having islands of growth rather than uniform
Similarly, Thostenson et al. [27] utilized magnetron sput- growthwillallowtheepoxymatrixto“flow”andinfiltratein-
tering to deposit stainless steel as catalyst to grow carbon betweenthesurface-grownCNTs.Continuousanduniform
nanotubesoncarbonfiberusingCVDwithacetyleneasthe growth will not allow for the epoxy to “wet” in-between
hydrocarbon.Alternatively,Zhuetal.[28]utilizedincipient these nanostructures. Wetting these CNTs together with
wetnesstodepositironnanoparticlesonthesurfaceofcar- wetting the surface of the pitch carbon fiber will create
bonfibers.TheauthorsutilizedthermalCVDwithmethane the very desirable mechanical interlocking and anchoring
at1000◦CtogrowtheCNTsonthesurfaceofcarbonfibers. mechanisms. Otherwise, if epoxy cannot anchor both the
Other investigators achieved similar growth of CNTs over CNTsandthepitchfiber,thentheoriginalfiberwillnotcarry
carbonfibersatmilderthermalconditions[29]byutilizing anyload;insteadtheCNTs/epoxycomplexwilldothat.Such
Niatcatalystandutilizingcyclopentadieneorbenzeneasthe scenariodefiesthepurposeofhybridcomposites.
hydrocarbon. Beside the sputtering and incipient wetness, Pitch-based carbon fibers were used as substrate mate-
otherinvestigatorsutilizedelectrochemicalcellstodepositNi rials.Thesetypesoffibersarecommonlyusedasreinforce-
catalystoncarbonfibersandusedCH asprecursortogrow mentsintraditionalpolymericfibrouscomposites.Ironwas
4
CNTsonPANandpitch-basedcarbonfibers[30]. chosen as the catalyst material since the gaseous precursor
Despitethenumerousinvestigationscitedearlier,thereis for CNTs synthesis was C H . A solution of 0.05M FeCl .
2 4 3
no single investigation that discusses the effect of utilizing 6H O was prepared with deionized water and pH was
2
different techniques to deposit the catalyst on the nature adjusted to the 3.0 value by suitable KOH addition. To
of the CNTs growth on carbon substrate. The current ensure the air removal, nitrogen gaswas flown through the
studyaimstoofferacomparativeinvestigationoftheeffect cell until no bubbles observed. A bundle of carbon fibers,
of the two leading techniques: DC magnetron sputtering first treated at 500◦C for 90 minutes and then washed with
and electrochemical deposition of the iron catalysts on the acetone and ethyl alcohol to remove the sizing, were used
JournalofNanotechnology 3
6E−04 2.1.2. Metal Sputtering. Iron was deposited on the surface
5E−04 of treated carbon fibers by DC magnetron sputtering of
4E−04 commercial target of high purity iron. The chamber was
nt(A) 23EE−−0044 egpvareas,scsuaunartdeedtwhtaeossmaubbasiatnsrteaatipenrteeedsmsuaptree1rao×tfu15r0e×−w31aT0so−mr6raTbionyrtrian.intTrehodedauwtcoi3nr0kg0i◦AnCgr
re 1E−04
ur duringthesputtering.Thesputteringpowerdensitywaskept
C−1.4 −1.2 −1 −0.8 −0.6 −0.4 −0.2 0 0E+00 at 0.5W/cm2. Each carbon bundle was sputtered on both
−1E−04 facesfor60secondsateachface.
−2E−04
−3E−04 2.2.ChemicalVaporDeposition. AthermalCVDsystemwas
−4E−04 usedforthecarbonnanostructuresgrowth.TheCVDsystem
Potential(V)
isequippedwithquartztubeof1-inchinternaldiameter.The
Figure 2: Cyclic Voltammogramm 0f 0.05M FeCl3. 6H2O/KOH samplesweretransferredinairandlocatedinceramicboats
(pH=3),withscanrate50mV/s. placedinthemiddleofthetube.ArgonwasflownintoaCVD
reactor in order to prevent oxidation of catalyst iron while
raising the temperature. A mixture of 10% H and 90% Ar
2
0.045 wasflowninthereactorat750◦Cfor10minutesat300sccm
toreduceanyoxideformed.C H wasintroducedtothemix
2 2
suchthatthecompositionis10%H,5%C H ,and85%Ar.
0.035 2 2
Thegaseswereintroducedusingseparateflowcontrollersfor
10minutesat300sccm.
0.025
A)
ent( 0.015 2.3.Characterization. ScanningElectronMicroscopy(Hita-
r chi S-4800 High-Resolution Scanning Electron Microscope
ur
C (HRSEM) was employed to examine the deposited catalyst
0.005
before the CNTs growth and also to investigate the purity
and density of the CNTs after growth. High-resolution
−0.005 transmission electron microscope (TEM-JEOL 2010) was
usedtocharacterizethestructureanddiameteroftheCNTs.
−0.015 Raman spectra in the 100–2000cm−1 region were
0 100 200 300 400 500 600
obtainedina180degreeback-scatteringgeometryonacon-
Time(s)
focalRamanmicroscope(KaiserOptical).Sampleexcitation
Figure3:Currentresponsethroughoutpulsevoltammetryexper- was through a 100 X, 0.9 NA microscope objective, using
iment for the 0.05M FeCl . 6H O/KOH (pH = 3.0) system with typically1mWofincidentpowerat514nmfromanargon-
3 2
carbonfibersbrushasworkingelectrode. ionlaser.Ramanscatteredlightwascollectedthroughafiber
opticanddispersed(withapproximately5cm−1 resolution)
ontoaCCDcamerafordetection.Typicalintegrationtimes
were1minute.
asworkingelectrode;seeFigure1(a)and1(b).Thebundles
werearrangedinabrushgeometrybeinggluedononeside
toglassslideformechanicalsupport. 3.ResultsandDiscussion
The counter electrode was a Pt wire and the reference
electrode was saturated calomel electrode (SCE), saturated For the electrochemical deposition the electrolyte-electrode
by KOH, mounted in a Luggin capillary containing the interface was studies to set suitable parameters for the
ironsolutionpreparedforthedeposition.Thevoltammetric deposition experiment. Cyclic voltammetry was utilized
measurements and the potentiostatic experiments were to set a suitable voltage and molarity of the electrolyte
performed by a model 760 series Potentiostat/Galvanostat, solution. Cyclic voltammetry has become a very popular
CH Instruments, Inc. Different deposition conditions were technique for initial electrochemicalstudies of new systems
testedinordertooptimizethedimension,morphology,and and has proven very useful in obtaining information about
densityofFeclusters.Multipotentialstepsdeemedtobethe fairly complicated electrode reactions. Voltametric curves,
mostefficientbyapplyingaseriesofpotentialstepsbetween Figure2,wereobtainedtocollectgeneralinformationabout
0and−0.7Vfordurationof5secondseachstepforatotalof theironelectrodepositionprocessonthepitchcarbonfibers;
5minutes:60steps.Forthesakeofcomparisonwerepeated several peaks were observed at potential between −0.6 and
the same experiment under 0.7V but this time the voltage 0.8V.Therefore,wechoose−0.7Vtobethevoltageforthe
wasappliedcontinuouslyfor5minutes.Finally,toinvestigate nextelectrochemicaldeposition.Upondecidingtheworking
the effect of a higher voltage, the potential stepping and potential, pulse voltammetry was chosen to carry out the
continuous potential deposition experiments were repeated depositionofironoverthecarbonfibers.Pulsevoltammetry
usinga1.1Vpotential. ispreferredchoicetodepositislandsratherthancontinuous
4 JournalofNanotechnology
2kV×8.01kSE 5μm 15kV5mm×80k 500μm
(a) (b)
Figure4:(a)Electrochemicaldepositionofirononcarbonfiber,and(b)HRSEMimageofthedepositedironat−0.7Vusingmultipotential
stepping.
15kV5mm×80k 500μm 15kV5mm×90k 500μm
(a) (b)
15kV5mm×80k 500μm 15kV5mm×100k 500μm
(c) (d)
Figure 5: HRSEM of microstructures seen after electrochemical deposition of iron on carbon fiber, (a) at −0.7V using multipotential
stepping,(b)using−0.7Vcontinuouspotentialdeposition,(c)using−1.1Vusingmultipotentialstepping,and(d)using−1.1Vcontinuous
potentialdeposition.
films of metals [31]. The potential between the working andtheirdifferencesareplottedversuscurrent;seeFigure3.
electrodeandthereferenceelectrodewaschangedasapulse Thescanbeginsatpotentialof−0.7V.Cathodiccurrentsare
fromaninitialpotentialof0toaninterlevelpotentialof−0.7 upward. The time axis corresponds to the half-cycle index
and remains at the interlevel potential for about 2 seconds; m=5seconds.
thenitchangestotheinitialpotential.Thepulseisrepeated, On pitch fibers bundles used as electrode, iron clusters
changing the final potential, and a constant difference is with various shapes and dimensions were observed; see
kept between the initial and the interlevel potential. The Figure4. The deposited iron compounds show great size
values of the current between the working electrode and variation from several microns, Figure4(a), to nanoscale
auxiliary electrode before and after the pulses are sampled particles and rods, Figure4(b). It is also observable at both
JournalofNanotechnology 5
2kV×6kSE 5μm 2kV×90kSE 500nm
(a) (b)
Figure6:(a)IrondepositionusingDCsputteringmethod.HRSEMimageofthesputteredironfilms.
10kV5mm×5k 10μm 10kV5mm×20k 2μm
(a) (b)
Figure7:(a)SEMimageofCNTsgrowthoncarbonfiberusingtheironDCsputteringtechnique,and(b)close-upimageoftheCNTs
grownviaCVDusingironevaporationdeposition.
10kV4.9mm×4k 10μm 10kV5mm×18k 3μm
(a) (b)
Figure8:(a)SEMimageofCNTsgrowthoncarbonfiberusingtheelectrochemicaldepositionofironcatalystand(b)closeupimageof
CNTsgrownusingelectrochemicaldeposition.
6 JournalofNanotechnology
50μm
(a)
10kV5mm×80k 500nm
Figure 9: SEM image of CNTs grown on pitch carbon fiber. The
continuous nature of the deposited catalyst film is not observed;
instead,nanoscaleparticlesorFe/Ccompoundsareobserved.
scales that the iron deposition took the form of islands
rather than continuous films. The high surface coverage
10μm
canbeattributedtotheparametricsetupforthemolarities
of the electrolyte solution, the working potential, and the
(b)
conductivenatureofpitchfibers.
Figure5revealsthatapplyingcontinuouspotentialrather
than potential stepping increased the deposition density
and surface coverage of the iron on the carbon fiber
surface. Furthermore, several nanostructures (<100nm,
Figure5(a)) have disappeared leading to the conclusion of
agglomerationundersuchcontinuouspotentialdeposition,
Figure5(b).Finallyuponapplying−1.1Vsteppingpotential
(Figure5(c)) fewer deposition was observed compared to
the −0.7V, and almost no deposition was observed when
the −1.1V potential was applied continuously, Figure5(d).
5μm
Thus, the −0.7V was more favorable deposition potential
andwelimitedthenextsynthesisandcharacterizationsteps
(c)
for samples with catalytic iron deposited under −0.7V
steppingpotential. Figure 10: TEM images of carbon nanostructures observed for
On the contrary, DC magnetron sputtering technique samples with iron deposition carried out via electrochemical
deposition.
yielded more continuous and uniform deposition of iron
films, Figure6. Nanofeatures could not be observed in the
deposited catalytic iron films, an indication of considerable
thickness. Figure7 shows SEM images of CNTs growth on Figures 10 and 11. Multiwall carbon nanotubes were the
the pitch fiber surface with catalytic iron deposited via DC dominant feature of the growth nanostructures regardless
sputtering while Figure8 shows CNTs growth when iron is ofthecatalystdepositionmethod,Figures10(a)–11(a).One
depositedusingthepulsevoltammetrytechnique. notablefeatureisthepurityofthegeneratedMWCNT;not
The results in Figures 7 and 8 suggest that both manyironparticlesareobservedattheendofMWCNTtips.
techniquesaresuccessfultogrowCNTsonthecarbonfiber However, the electrochemical deposition yielded MWCNT
surfaces. However the iron sputtering shows more dense with smaller external diameter (<15nm) than that for
growth due to the uniform distribution of the iron catalyst the MWCNT grown via the DC sputtered iron (>25nm).
using this technique; evidently a lower cluster/film density The second species of nanostructure comprise of the iron
ofthecatalystcorrespondstoalessdensecarbonnanotubes particles encapsulated within several graphitic shells as
growth. Another interesting feature is that the continuous shown in Figures 10(b) and 11(b). This indicates that iron
film of catalytic iron, shown in Figure7, was shattered into particlesthathavebeenformedduringthereductionstepare
particlesuponthereductionandgrowthstages,Figure9. toolargetogeneratefilamentouscarbon;insteadtheyremain
Moreover, TEM analysis revealed that different types active and get surrounded by layers of graphitic materials.
of carbon nanostructures were synthesized as shown in Thisobservationwascitedbyotherinvestigators,whenusing
JournalofNanotechnology 7
vibration of the hexagonal ring while the D band is
producedbythedefect-inducingvibrationmode,thatis,the
noncrystallinecarbon-specificabsorptionband[33].
Havingtheironcatalystdepositedonthecarbonfiberdid
not show significant difference among the spectra, Figures
12(b) and 12(c). For Figures 12(d) and 12(e), the decrease
in the intensity ratio of D band to G band (I (D)/I (G))
was slightly higher for the carbon fibers with MWCNTs
grownbyDCsputteringofiron.Thisrevealsthatthedegree
50μm of crystalline perfection of the MWCNTs grown using DC
sputtering iron is slightly higher than that of MWCNTs
(a) grown using electrochemical deposition of iron. Also the
width of the D peak for that sample is narrower than
that for the sample where electrochemical deposition was
utilized to deposit the iron. This may be an indication of a
high degree of order in the sample and/or less amorphous
impurities.
4.Conclusions
We presented the results of a comparative study to grow
CNTs on pitch-based carbon fiber surfaces using DC mag-
10μm netronsputteringofironandelectrochemicaldepositionof
the iron catalysts. A bundle of carbon fibers was treated
(b) at 500◦C for 90 minutes and then washed with acetone
and ethyl alcohol to remove the sizing. For electrochemical
deposition, C H was used as the gaseous precursor for
2 4
CNTssynthesisandIronwaschosenasthecatalystmaterial.
Cyclic voltammetry was utilized to set a suitable voltage
and molarity of the electrolyte solution. Pulse voltammetry
was chosen to deposit of iron over the carbon fibers to
enable depositing islands rather than a continuous film.
In metal sputtering method, iron was deposited on the
surface of treated carbon fibers by DC magnetron sput-
tering of commercial target of high purity iron. Carbon
10μm
nanostructures growth was performed using a thermal
CVDsystem.Characterizationforcomparisonbetweenboth
(c)
techniques was done using SEM, TEM, and Raman spectra
Figure 11: TEM images of carbon nanostructures observed for analysis.
samples with iron deposition carried out via DC magnetron
Characterization methods showed that both techniques
sputtering.
were successful to grow CNTs on the carbon fiber surfaces.
However, the iron sputtering shows denser growth due to
the uniform distribution of the iron catalyst. While the
electrochemical deposition resulted in iron deposition in
other catalysts such as Ni and Co [32]. Consulting the the form of islands, DC magnetron sputtering technique
TEM images, it is apparent that the majority of the grown yielded a continuous and uniform deposition of iron films.
MWCNTs have diameters within 10nm. From the close up TEM images proved that both techniques enabled pro-
TEMimages,Figures10(c)–11(c),itseemsthatnotracesof ducing multiwall carbon nanotubes with significant purity
amorphouscarboncanbefoundontheinnerorontheouter regardless of the catalyst deposition method. The results
wallsofthetubes. of Raman spectra analysis revealed that MWCNTs grown
Finally,Ramanspectraanalysisoftheneatcarbonfibers, using DC sputtering of iron have a little higher degree
carbon fibers with catalysts, and carbon fibers with surface of crystalline perfection than those MWCNTs grown using
grownMWCNTsarepresentedinFigure12.Ramanspectra electrochemical deposition of iron. The ease of the DC
analysis was performed in the 100–2500cm−1 spectral sputtering of iron, less process parameters compared to
region.Thespectraforthecarbonfiberspectrashowmainly electrochemical deposition, suggests DC iron sputtering as
twoRamanbandsat1350cm−1(Dband)and1580cm−1(G a leading alternative to produce new hybrid carbon fiber-
band). The G band typically observed in single crystals of carbon nanotubes structures that will enable new types of
graphite and assigned to a doubly degenerate deformation structuralcomposites.
8 JournalofNanotechnology
1000 14000
12000
8000
Intensity(a.u.) 46000000 Intensity(a.u.)14680000000000000
2000
2000
0 0
0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500
Ramanshift(cm−1) Ramanshift(cm−1)
(a) (b)
8000 14000
12000
6000
Intensity(a.u.) 4000 Intensity(a.u.)14680000000000000
2000
2000
0 0
0 500 1000 1500 2000 2500 0 500 1000 1500 2000 2500
Ramanshift(cm−1) Ramanshift(cm−1)
(c) (d)
22000
20000
18000
16000
u.) 14000
a.
( 12000
y
nsit 10000
nte 8000
I 6000
4000
2000
0
0 500 1000 1500 2000 2500
Ramanshift(cm−1)
(e)
Figure12:Ramanspectrumfor(a)carbonfiberwithremovedsizing,(b)carbonfiberwithirondepositionviaelectrochemicaldeposition,
(c)carbonfiberwithirondepositionviaDcmagnetronsputtering,(d)carbonfiberwithsurfacegrownMWCNTs(ironwasdepositedusing
electrochemicalmultipotential,and(e)carbonfiberwithsurfacegrownMWCNTs(ironwasdepositedusingDCmagnetronsputtering).
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