thermal expansion of carbon nanoﬁber-reinforced multiscale...

Thermal Expansion of Carbon Nanofiber-Reinforced MultiscalePolymer Composites

RONALD L. POVEDA,1 SRINIKET ACHAR,1 and NIKHIL GUPTA1,2

1.—Department of Mechanical and Aerospace Engineering, Polytechnic Institute of New YorkUniversity, Brooklyn, NY 11201, USA. 2.—e-mail: [email protected]

Improved dimensional stability of composites is desired in applications wherethey are exposed to varying temperature conditions. The current study aimsat analyzing the effect of vapor-grown carbon nanofibers (CNFs) on thethermal expansion behavior of epoxy matrix composites and hollow particle-filled composites (syntactic foams). CNFs have a lower coefficient of thermalexpansion (CTE) than epoxy resin, which results in composites with increaseddimensional stability as the CNF content is increased. The experimentalmeasurements show that with 10 wt.% CNF, the composite has about 11.6%lower CTE than the matrix resin. In CNF-reinforced syntactic foams, the CTEof the composite decreases with increasing wall thickness and volume fractionof hollow particle inclusions. With respect to neat epoxy resin, a maximumdecrease of 38.4% is also observed in the CNF/syntactic foams with micro-balloon inclusions that range from 15 vol.% to 50 vol.% in all composite mix-tures. The experimental results for CNF/syntactic foam are in agreement witha modified version of Kerner’s model. A combination of hollow microparticlesand nanofibers has resulted in the ability to tailor the thermal expansion ofthe composite over a wide range.

INTRODUCTION

Thermal stability is a critical parameter inselecting materials for applications that are exposedto temperature variations.1,2 The ability to tailor themechanical and thermal properties of composites isan asset. Some applications, such as substrates forspace mirrors, strive for materials with low coeffi-cient of thermal expansion (CTE), while otherapplications, such as sandwich skins or cores usedin aircraft structures, desire composites with a tai-lored CTE value to match with the adjacent com-ponents.3 Thermal stresses can be minimized atinterfaces by matching the CTE of adjacent mate-rials. Therefore, understanding the trends observedin the CTE of composites with respect to the prop-erties and proportions of the constituent materialsis desired.

In addition to the desired mechanical properties,polymer composites can have better thermal sta-bility when reinforced with phases that are stable athigh temperatures and have low CTE,4 such ascarbon-based nanomaterials.5–7 Carbon nanotubes(CNTs) are extensively used in recent years to

develop composites with high mechanical perfor-mance.8,9 CNT-reinforced nanocomposites have alsoshown better dimensional stability at high temper-atures. By replacing a fraction of the matrix poly-mer with inclusions such as carbon nanofibers(CNFs) or CNTs, a composite with enhanced ther-mal stability can be fabricated.10–12 With increasingfocus on cost-effective materials such as CNFs, theiruse for developing composites with low CTE isexplored in the current study. Hollow ceramic par-ticle-filled polymer matrix composites, called syn-tactic foams, have also shown improved dimensionalstability and a great degree of tailorability.13 In thecurrent work, syntactic foams are reinforced withCNFs to gain the benefit of mechanical propertyenhancement.14,15 along with improvement indimensional stability.

Existing applications of syntactic foams includebuoyancy modules in deep-sea vehicles and under-sea pipe insulation. Syntactic foams are nowreplacing traditional core materials in sandwichcomposites used in aircraft structures,16 for exam-ple, in floor boards and leading edges. These appli-cations demand materials with high mechanical

JOM, Vol. 64, No. 10, 2012

DOI: 10.1007/s11837-012-0402-5� 2012 TMS

1148 (Published online August 30, 2012)

properties coupled with low density. In addition,material with tailored CTE can be extremely usefulin aircraft structures where large variations instresses can occur during their routine service con-ditions. While mechanical properties of syntacticfoams have been extensively studied in existingliterature and although relations are now availablebetween modulus and material parameters,17,18

such information on thermal properties is lacking.Several studies have assessed the CTE value ofCNF-reinforced multiscale composites containingfibers and particles.1,10,12 It has been observed thatthe addition of CNF decreases the CTE of particle-or fiber-reinforced composites. In the presence ofsecond-phase particles along with CNFs, it is diffi-cult to isolate and understand the effect of eachconstituent on the CTE of the composite. The cur-rent work fills a critical gap by studying a variety ofCNF/epoxy composites and CNF/syntactic foams tobuild a comprehensive understanding of the ther-mal expansion behavior of these composites.

Reinforcement of syntactic foams with nanoscalematerials is very attractive because they canstrengthen the matrix in the interparticle regionwithout affecting the packing efficiency orarrangement of hollow particles. Since most appli-cations of syntactic foams benefit from the lowdensity of these composites, high loading of hollowparticles as well as matrix strengthening withnanoscale materials provides new opportunities fortailoring the material properties.

The specimens are tested using a thermome-chanical analyzer (TMA) below the glass transitiontemperature of the matrix resin.19 The experimen-

tal results obtained for CNF/epoxy composites arecompared with Schapery’s bounds for fiber-rein-forced composites. The experimental results forCNF/syntactic foams are compared with predictionsof two theoretical models: a modified Turner’smodel, which takes into account the hollow spheri-cal inclusions inside the composite, and a modifiedKerner’s model, which takes into account the shearmodulus of the matrix material, as well as the hol-low spherical inclusions.

MATERIALS AND METHODS

Constituent Materials

DER 332 epoxy resin with DEH 24 hardenermanufactured by the DOW Chemical Co. (Midland,MI) is used as the matrix. The material composi-tions are schematically illustrated in Fig. 1. Fourcompositions of CNF/epoxy composites and 16 com-positions of CNF/syntactic foams are fabricated forthis study. Glass microballoons (3M, Maplewood,MN) of 220 kg/m3 and 460 kg/m3 densities are usedin 15 vol.%, 30 vol.%, and 50 vol.% in different typesof syntactic foams. CNF/epoxy composites are fab-ricated with 1 wt.%, 2 wt.%, 5 wt.%, and 10 wt.%CNFs, procured from Pyrograf Products, Inc.(Cedarville, OH). Vapor-grown PR-19 XT-PS CNFsare used in the study, where ‘‘XT’’ denotes that theCNFs have been debulked to allow for uniform bulkdensity, and ‘‘PS’’ denotes that the fibers have beenpyrolytically stripped of aromatic hydrocarbons. InCNF/syntactic foams, the weight fraction of CNFs(1%, 2%, 5%, and 10%) are calculated with respect toonly the matrix system. The CNF density is

CNF/epoxy

1 wt.% CNF

2wt.% CNF

5wt.% CNF

10 wt.% CNF

CNF/syntactic foam having glass microballoons

15 vol.% MB 30 vol.% MB 50 vol.% MB

Fig. 1. Illustration of material composition. Sixteen compositions of CNF-reinforced syntactic foams are synthesized with two types of glassmicroballoons (noted as MB in the figure).

Thermal Expansion of Carbon Nanofiber-Reinforced Multiscale Polymer Composites 1149

1950 kg/m3 and the epoxy resin density is 1160 kg/m3, as obtained from the respective manufacturer’sdatasheets. The composition of the fabricated CNF/epoxy composites is given in Table I. The composi-tions of CNF/syntactic foams are listed in Tables IIand III for foams containing glass hollow particles of220 kg/m3 and 460 kg/m3 density, respectively. Thecomposite nomenclature follows the trend where Nrepresents CNFs, followed by microballoon density,microballoon content, and CNF content. The vis-cosity of the mixture during stirring increases athigher CNF volume fractions and makes it difficultto uniformly mix large amounts of microballoonswithout breaking them. Therefore, fewer composi-tions, having low microballoon content, are studiedat high CNF content as listed in Tables II and III.

Specimen Fabrication Method

A mechanical mixer with a high shear impeller isused to obtain uniform dispersion of CNFs in theepoxy resin.14 CNFs are initially mixed in the epoxy

for 30 min at 650 rpm to form a composite slurry.Next, microballoons are added to the slurry andmixed slowly with a wooden dowel for an additional15 min.16 The hardener is added in the end andmixed with the dowel until a slurry of uniformconsistency is obtained. The slurry is poured intoaluminum molds that are placed on top of shakersin an effort to degas the mixture and remove airvoids entrapped in the resin during mixing (suchvoids are called matrix porosity). After 5 min ofshaking the mold, the mixture is left to cure for atleast 24 h. The resulting slab is then postcured inan oven for 2 h at 100�C. The dimensions andweight of CTE test specimens are measured to cal-culate the density of the composites. The theoreticaldensity of composites qc was calculated by the ruleof mixtures

qc ¼ qm/m þ qb/b þ qf/f (1)

where qm, qb, and qf are the densities of the epoxyresin matrix, glass microballoons, and CNFs,respectively. The terms /m, /b, and /f represent thevolume fractions of the epoxy resin matrix, glassmicroballoons, and CNFs, respectively, and are lis-ted in Tables I, II, and III.

Thermomechanical Testing

The CTE of each composite grade was measuredusing a Q400 TMA (TA instruments, New Castle,DE). A schematic of the setup is shown in Fig. 2.Specimens with nominal dimensions of 4 9 4 912 mm3 were used for CTE measurement. Rectan-gular prism-shaped specimens are extracted from

Table I. Composition of CNF/epoxy composites

CNFs(wt.%)

CNFs(vol.%)

Epoxy resin(vol.%)

Hardener(vol.%)

1 0.60 87.20 12.212 1.20 86.67 12.135 3.04 85.06 11.9110 6.20 82.28 11.52

Table II. Composition of CNF/syntactic foams containing 220 kg/m3 density hollow particles

Composite type Microballoons (vol.%) CNFs (vol.%) Epoxy resin (vol.%) Hardener (vol.%)

N220-15-1 15 0.53 74.10 10.37N220-30-1 30 0.47 60.99 8.54N220-50-1 50 0.37 43.53 6.09N220-15-2 15 1.09 73.61 10.31N220-30-2 30 0.85 60.66 8.49N220-15-5 15 2.76 72.14 10.10N220-30-5 30 2.27 59.41 8.32N220-15-10 15 5.46 69.77 9.77

Table III. Composition of CNF/syntactic foams containing 460 kg/m3 density hollow particles

Composite type Microballoons (vol.%) CNFs (vol.%) Epoxy resin (vol.%) Hardener (vol.%)

N460-15-1 15 0.54 74.08 10.37N460-30-1 30 0.47 60.99 8.54N460-50-1 50 0.43 43.48 6.09N460-15-2 15 1.12 73.58 10.30N460-30-2 30 0.97 60.55 8.48N460-15-5 15 2.86 72.05 10.09N460-30-5 30 2.40 59.30 8.30N460-15-10 15 5.68 69.58 9.74

Poveda, Achar, and Gupta1150

the slab using a bandsaw and an IsoMet precisionsaw (Buehler, Lake Bluff, IL) for testing. The spec-imens were heated in an oven for 2 h at 70�C toremove any surface moisture prior to the test. Aglass expansion probe was positioned on top of thespecimen to measure the initial length and theexpansion of the specimen under thermal loading.The temperature was ramped up at the rate of 3�C/min up to a temperature of 90�C, similar to that of aprevious study.13 The CTE value was calculatedfrom the slope of the temperature–thermal straincurve.

RESULTS

Vapor-grown CNFs are observed through atransmission electron microscope in Fig. 3a. Thesenanofibers have a hollow core structure. Previouswork focused on characterizing CNFs has shownthat CNFs comprise a series of truncated cones (orcups) stacked vertically to provide a fibrous struc-ture,20–23 as schematically illustrated in Fig. 3b.Experimental evidence has shown that the struc-ture may be composed of a long narrow grapheneribbon helically folded in the form of a hollow tube.21

A small unit of this kind of structure would alsoresemble stacked cups, as shown in Fig. 3b. Theinteraction between various layers of cup-stacksdetermines the properties of CNFs.22,24 In multi-walled CNTs, the graphene layers run along thelength of the nanotubes and provide higher strengththan that seen in CNFs.20,25

In the current nanocomposite specimens, CNFsare randomly dispersed as observed in the scanningelectron micrograph in Fig. 4a. CNFs are also dis-persed with glass microballoons in CNF/syntacticfoam composites as shown in Fig. 4b. Microscopicobservations show that the nanofibers and glassmicroballoons are wetted with the resin and areuniformly dispersed in the composite microstruc-ture.

The theoretical densities, calculated using Eq. 1and the experimentally measured densities for allcomposites, are listed in Tables IV, V, VI, and VII.In most cases, the calculated and experimentallymeasured densities are within 5% of each other,which illustrates that the matrix void entrapmentor microballoon fracture during composite fabrica-tion are negligible. Since the specimen size is smallfor CTE testing, the quality of the fabricated mate-rial is extremely important for obtaining consistentresults, and the close matching between theoreticaland experimental density values provides confi-dence in the material quality.

Temperature and thermal strain data areobtained from the testing. A representative set ofgraphs is shown in Fig. 5, where data for one spec-imen of each composite is plotted for direct com-parison of trends. At least five specimens of eachmaterial type were tested, and the average CTEwith standard deviations is reported. The figureshows that the CTE is dependent on the weightpercentage of CNF. As the CNF content is increasedfrom 1 wt.% to 10 wt.%, the thermal stability of theCNF/epoxy composites increased, with a maximumof 11.6% CTE reduction at 10 wt.% of CNF.

Cooling gas

Heater

Thermocouple

Specimen stage

Specimen

Expansion probe

Fig. 2. Schematic of TMA experimental setup.

Fig. 3. (a) Transmission electron micrograph of cup-stacked nanofibers showing their hollow structure and (b) schematic representation of cup-stack structure.


The measured values of CTE for CNF/epoxycomposites are presented in Table IV. The CTE ofneat epoxy resin is measured to be 65.4 9 10�6/�C.The CNF/epoxy composites show a monotonouslydecreasing trend from 68 9 10�6 to 57.8 9 10�6/�Cas the CNF content is increased from 1 wt.% to10 wt.%. The difference between the CTE values ofneat resin and composites containing 1 wt.% and2 wt.% CNF is statistically insignificant, but com-posites containing higher CNF content providethe indication of the trend. These measurements

show that the dimensional stability of compositesincreases with CNF content. CNFs have a cup-stacked structure with a hollow core, where indi-vidual cups may expand in diameter and length. Inaddition, the cups may also slide with respect toeach other along the fiber length as the cup diam-eter increase with temperature, as schematicallyrepresented in Fig. 6. This kind of effect provides alow longitudinal thermal expansion because back-sliding between cups may be able to recover some ofthe longitudinal thermal expansion.

A representative set of temperature–thermalstrain plots are compared for various CNF/syntacticfoam compositions in Fig. 7. The CTE of materialsmay be dependent on temperature, and sometimes

Fig. 4. Scanning electron micrographs of (a) 10 wt.% nanofibers dispersed in epoxy resin and (b) 5 wt.% nanofibers dispersed in N460-30 CNF/syntactic foam.

Table IV. Density and CTE values for CNF/epoxyand CNF syntactic foam composites at 1 wt.% CNF

Compositetype

Density (kg/m3) ExperimentalCTE

(31026/�C)Measured Theoretical

CNF/epoxy 1168.8 1164.7 68.0 ± 3.1N220-15-1 1034.9 1023.2 54.1 ± 2.3N220-30-1 918.8 883.5 51.2 ± 2.5N220-50-1 724.9 694.0 41.9 ± 3.4N460-15-1 1059.2 1059.3 54.9 ± 2.9N460-30-1 909.7 955.5 46.0 ± 1.7N460-50-1 835.5 814.0 40.3 ± 4.7

Table V. Density and CTE values for CNF/epoxyand CNF syntactic foam composites at 2 wt.% CNF

Compositetype



CNF/epoxy 1212.2 1169.5 67.0 ± 3.7N220-15-2 1059.7 1027.6 52.6 ± 1.1N220-30-2 827.5 881.7 52.8 ± 4.9N460-15-2 1091.6 1063.8 49.3 ± 2.2N460-30-2 949.4 957.7 42.0 ± 4.1

Table VI. Density and CTE values for CNF/epoxyand CNF syntactic foam composites at 5 wt.% CNF

Compositetype



CNF/epoxy 1198.8 1184.0 60.1 ± 4.6N220-15-5 1077.1 1040.8 49.7 ± 2.2N220-30-5 886.5 896.0 52.1 ± 3.6N460-15-5 1086.5 1077.6 53.1 ± 2.7N460-30-5 937.1 969.0 42.5 ± 5.1

Table VII. Density and CTE values for CNF/epoxyand CNF syntactic foam composites at 10 wt.% CNF

Compositetype



CNF/epoxy 1227.6 1209.0 57.8 ± 3.2N220-15-10 1065.5 1062.2 48.0 ± 1.9N460-15-10 1107.3 1099.9 48.2 ± 3.5


temperature–thermal strain graphs show differentsteps in different temperature ranges. However, inthe current case, no measurable difference in slopeis observed over the entire range of test tempera-ture. According to the graphs, it is evident thatthere are variations in the CTE of the compositesdue to the presence of CNFs as well as glassmicroballoons. The experimentally measured CTEvalues for all composites are presented in Tables IV,V, VI, and VII. The CTE is also observed to decrease

with increasing inclusion density, with a maximumdecrease of 38.4% observed in N460-50-1 CNF, anda minimum decrease of 19.3% observed in N460-15-1 compared to that of CNF/epoxy resin. It isobserved that the CTE of composites decreases withincreasing glass hollow particle volume fraction.Similar data trends have been observed in a paststudy on plain syntactic foams containing only glass

0

1

2

3

4

5

30 50 70 90

Th

erm

al s

trai

n, 1

0-3

Temperature, °C

Neat epoxy1 wt.% CNF2 wt.% CNF5 wt.% CNF10 wt.% CNF

Fig. 5. Typical raw data graphs of CNF/epoxy composites withvarious weight percentages of CNFs.

Longitudinal expansion

Transverse expansion

Cup sliding

Fig. 6. Possible thermal expansion effects in CNFs.

0

1

2

3

4

5(a) (b)

(c) (d)

20 40 60 80 100

Th

erm

al s

trai

n, 1

0-3

Th

erm

al s

trai

n, 1

0-3

Th

erm

al s

trai

n, 1

0-3

Th

erm

al s

trai

n, 1

0-3

Temperature, °C

N220-15N220-30N220-50N460-15N460-30N460-50

220 kg/m3

460 kg/m3

0

1

2

3

4

5N220-15

N220-30

N460-15

N460-30

220 kg/m3

460 kg/m3

0

1

2

3

4

5N220-15

N220-30

N460-15

N460-30

220 kg/m3

460 kg/m3

0

1

2

3

4

5

N220-15

N460-15

220 kg/m3

460 kg/m3

Temperature, °C

Temperature, °C Temperature, °C

20 40 60 80 100

20 40 60 80 10020 40 60 80 100

Fig. 7. CNF/syntactic foams with (a) 1 wt.% CNF, (b) 2 wt.% CNF, (c) 5 wt.% CNF, and (d) 10 wt.% CNF.


particles.13 The CTE of the bulk glass material is anorder of magnitude lower than that of the epoxymatrix resin, which leads to reduction in the CTE ofthe composites.

DISCUSSION

The experimental results on CNF/epoxy compos-ites are compared with the Schapery’s boundscalculated for CTE of unidirectional fiber compos-ites.26,27 The bounded range of CTE can be obtainedas the longitudinal (als) and transverse (ats) CTEpredictions of the model as28

als ¼am/mEm þ af/f Ef

/mEm þ /f Ef(2)

ats ¼ 1þ mfð Þaf/f þ 1þ mmð Þam/m

� al mf/f þ mm/m

� �ð3Þ

where E, m, /, and a refer to Young’s modulus, Pois-son’s ratio, volume fraction, and CTE, respectively.Subscripts m and f refer to matrix and CNF, respec-tively. The values for CNF are obtained from theo-retical studies because direct experimentalmeasurements are not yet available. In this study,the values assigned to CNF are Ef = 300 GPa1,29 andaf = 4 9 10�6/�C.29,30 The CTE of the neat epoxy resinis measured to be am = 65.1 9 10�6/�C in the currentwork. The elastic modulus of the neat epoxy wasmeasured under quasi-static compression and wasfound to be Em = 1511 MPa, which corroborates wellwith the values reported previously on the sameepoxy system.31 CNF/epoxy specimens are also testedunder compression, and the results were used as in-puts for the analytical models. The compressivemodulus values for CNF/epoxy at 1 wt.%, 2 wt.%,5 wt.%, and 10 wt.% are measured as 974 MPa,1095 MPa, 1123 MPa, and 1318 MPa, respectively.The compressive modulus values of CNF/epoxy werelower compared to that of the neat epoxy. However,the compressive modulus increased with increase inwt.% of CNFs. This result corroborates well with thatobserved in a previous study32 for CNF-reinforcedepoxy when CNF wt.% was increased from 0 to 5.These experimentally obtained values are used inSchapery’s model to obtain predictions of CTE values.

Schapery’s upper and lower bounds are plotted inFig. 8, along with experimental results on CNF/epoxy composites. The experimental results arewithin the bounds. Note that the input values of Ef

and af are obtained from theoretical and simulationstudies, which may affect the prediction accuracy.The experimental data in Fig. 8 can be approxi-mated as a linear trend by

a ¼ �1:18wþ 68:54 (4)

where a and w are the CTE of the composite andweight fraction of CNFs, respectively. Since the

upper bound (als) corresponds to the fibers in thedirection of loading and the lower bound (ats) refersto all the fibers being oriented perpendicular to theloading direction, the effectiveness of the reinforce-ment is calculated as a parameter s by comparingthe experimental values with theoretical results as

a ¼ sals þ 1� sð Þats (5)

The best matching is obtained between the theo-retical and experimental results at s = 0.27 by curvefitting. This illustrates that due to the randomnessof CNF in the composite, their reinforcing efficiencyis only 27%. Several other parameters such as cur-viness of nanofibers due to their large aspect ratiomay also affect the results.

The experimental trends for CNF/syntactic foamsare further analyzed using two different analyticalmodels: Turner’s model, which takes into accountthe elastic modulus and the Poisson’s ratio of eachconstituent material,2,33 and the Kerner’s model,which also takes into account the shear modulus ofthe constituents.13,34,35 Both these models aremodified to incorporate the effect of microballoonwall thickness.

The Turner’s model is expressed as

a ¼ am/mKm þ ab/bKb

/mKm þ /bKb(6)

where a is the CTE of the composite; am, /m, and Km

are the CTE, volume fraction, and bulk modulus ofthe composite matrix, respectively; and ab, /b, andKb are the CTE, volume fraction, and bulk modulusof the microballoons, respectively. Using the elasticmodulus E of each constituent material, the bulkmodulus can be found from

K ¼ E

3 1� 2mð Þ (7)

where m is the Poisson’s ratio of the respective con-stituent material. The experimentally measured

0

20

40

60

80

100

0 5 10

CT

E,

×10-6

/°C

wt.% CNF

ExperimentalSchapery LongitudinalSchapery Transverse

Fig. 8. Schapery’s bounds for CNF/epoxy composites are plottedalong with corresponding experimental results.


values of modulus and CTE obtained for the CNF/epoxy composites were assigned to the matrix of theCNF/syntactic foams and the Poisson’s ratio istaken as 0.35, which is in-line with the reportedvalues for epoxy at room temperature.36,37

Given that the glass particles included in thesyntactic foam are hollow, the effective elasticmodulus of the microballoon phase (Emb) is calcu-lated as38,39

Emb ¼Eg 1� 2mg

� �1� g3� �

1� 2mg

� �þ 1þmg

2

� �g3

(8)

Equation 8 takes into account the radius ratio (g) ofthe hollow inclusions in the composite, which is theratio of the inner radius to the outer radius of themicroballoon. The values of g for the same glassmicroballoons were measured in a previous studyand are taken from there.40 Eg is the elastic modu-lus and mg is the Poisson’s ratio of the glass material,which are taken as 60 GPa and 0.21, respectively, asin the previous studies.13,41 Using this approach,the modified Turner’s model is obtained from Eqs. 6through 8 as

The term mm represents the Poisson’s ratio of thematrix material. The Kerner’s model is expressed by

a ¼ am/m þ ab/b þ /b/mðab � amÞ

� Kb � Km

/mKm þ /bKb þ 3KbKm

4Gm

!(10)

where Gm is the shear modulus of the matrixmaterial. The Kerner’s model can also be modifiedin a similar manner to obtain

The experimental CTE and the percentage differ-ence between experimental and model prediction forthe various CNF/syntactic foams are compared inTable VIII. It can be observed in Table VIII that themodified Kerner’s model shows better agreementwith experimental results compared to the modified

Turner’s model. Only 3 of 15 compositions of CNF/syntactic foams show large deviations with Kerner’smodel predictions; most compositions show predic-tions within 10% of the experimental results. Ingeneral, some broken microballoons and localizedmicron-size air voids may be present in the speci-mens, which may cause some difference in theexperimental results compared to the expected val-ues. The CTE values decrease with increasing con-tent of CNFs, content of microballoons of the samewall thickness, and wall thickness at the samemicroballoon content in the composite. Parametricstudies can be conducted to identify parameters thatwill help in fabricating composites with a givenvalue of CTE as per the requirements of an appli-cation.

CONCLUSIONS

In this study, the effect of CNFs on the CTE ofepoxy-based composites is characterized. CNF/epoxy composites and CNF/syntactic foams con-taining an additional ceramic hollow particle phaseare thermomechanically tested. Modified forms ofTurner’s and Kerner’s models are used to obtain

predictions of CTE values for CNF/syntactic foams.The results are summarized as follows:

� The effect of the CNFs on the CTE of CNF/epoxycomposites is significant, where the CTE of theoverall composite is observed to decrease by amaximum of 11.6% with respect to neat epoxy at10 wt.% loading of CNFs.

� The experimental results of CNF/epoxy compos-ites are within Schapery’s bounds. Modification ofpredictive models for randomly dispersed large

aspect ratio nanofibers can help in determiningthe compositions of nanocomposites for obtaininga tailored set of properties.

� The effect of both microballoons and CNFs issignificant on the CTE of CNF/syntactic foams.The CTE of the CNF/syntactic foams is observed

a ¼am/mEm ð1� 2mgÞ þ 1þmg

2

� �g3

h iþ ab/bEgð1� g3Þð1� 2mmÞ

/mEm ð1� 2mgÞ þ 1þmg

2

� �g3

h iþ /bEgð1� g3Þð1� 2mmÞ

(9)

a¼ am/mþ ab/bþ/b/mðab� amÞ�Egð1� g3Þð1�2mmÞ�Em ð1�2mgÞþ ð1þmgÞ

2 g3h i

/mEm ð1�2mgÞþ ð1þmgÞ2 g3

h iþ/gEgð1� g3Þð1�2mmÞþ 3

4Egð1� g3Þð1þ mmÞ

0

@

1

A

(11)


to decrease by a maximum of 38.4% for thecompositions selected.

� The experimental results for CNF/syntactic foamare in general agreement with the trends dis-played by the modified Kerner’s model.

� It is found that CTE of the composites decreaseswith increasing CNF content. Higher microbal-loon wall thickness and volume fraction alsodecrease the CTE of the composite. A combinationof these parameters can be used to tailor the CTEof the composite as per the requirements of theapplications.

ACKNOWLEDGEMENTS

This work is supported by the Office of NavalResearch through Grant N00014-07-1-0419 and theNational Science Foundation GK-12 Fellows Grant0741714. The authors thank 3M, MN for providingmicroballoons and relevant technical information.Gleb Dorogokupets is thanked for assistance inspecimen preparation and testing. Dinesh Pinisettyand Vasanth Chakravarthy Shunmugasamy arethanked for their assistance in theoretical compar-ison and manuscript preparation. Dr. JafarAl-Sharab is acknowledged for TEM imaging.

REFERENCES

1. J.-S. Jang, J. Varischetti, G.W. Lee, and J. Suhr, Compos.Part A 42, 98 (2011).

2. P.K. Rohatgi, N. Gupta, and S. Alaraj, J. Compos. Mater. 40,1163 (2006).

3. S. Jin, JOM 50, 46 (1998).4. N. Suzuki, S. Kiba, and Y. Yamauchi, Mater. Lett. 65, 544

(2011).

5. Z. Han and A. Fina, Prog. Polym. Sci. 36, 914 (2011).6. F. Pervin, Y. Zhou, V.K. Rangari, and S. Jeelani, Mater. Sci.

Eng. A 405, 246 (2005).7. L. Schadler, L. Brinson, and W. Sawyer, JOM 59 (3), 53

(2007).8. Y. Ganesan and J. Lou, JOM 61, 32 (2009).9. S. Bakshi, K. Balani, T. Laha, J. Tercero, and A. Agarwal,

JOM 59 (7), 50 (2007).10. M. Cho, J. Jang, and J. Suhr, J. Nanosci. Nanotechnol. 11,

1098 (2011).11. K. Lozano, JOM 52 (11), 34 (2000).12. K.J. Green, D.R. Dean, U.K. Vaidya, and E. Nyairo, Compos.

Part A 40, 1470 (2009).13. V. Shunmugasamy, D. Pinisetty, and N. Gupta, J. Mater.

Sci. 47, 5596 (2012).14. M. Colloca, N. Gupta, and M. Porfiri, Compos. Part B (2012).

doi:10.1016/j.bbr.2011.03.031.15. M. Dimchev, R. Caeti, and N. Gupta, Mater. Des. 31, 1332

(2010).16. P. Bunn and J.T. Mottram, Composites 24, 565 (1993).17. C.S. Karthikeyan, S. Sankaran, and Kishore, Mater. Lett.

58, 995 (2004).18. P. Rohatgi, N. Gupta, B. Schultz, and D. Luong, JOM 63 (2),

36 (2011).19. T. Lin, N. Gupta, and A. Talalayev, J. Mater. Sci. 44, 1520

(2009).20. H. Shioyama, Carbon 43, 203 (2005).21. J. Vera-Agullo, H. Varela-Rizo, J.A. Conesa, C. Almansa,

C. Merino, and I. Martin-Gullon, Carbon 45, 2751(2007).

22. M. Endo, Y.A. Kim, T. Hayashi, T. Yanagisawa, H. Mura-matsu, M. Ezaka, H. Terrones, M. Terrones, and M.S.Dresselhaus, Carbon 41, 1941 (2003).

23. B. Eksioglu and A. Nadarajah, Carbon 44, 360 (2006).24. M. Endo, Y.A. Kim, T. Hayashi, Y. Fukai, K. Oshida, M.

Terrones, T. Yanagisawa, S. Higaki, and M.S. Dresselhaus,Appl. Phys. Lett. 80, 1450264 (2002).

25. I. Kang, Y.Y. Heung, J.H. Kim, J.W. Lee, R. Gollapudi, S.Subramaniam, S. Narasimhadevara, D. Hurd, G.R. Kirikera,V. Shanov, and M.J. Schultz, Compos. Part B 37, 382 (2006).

26. R.A. Schapery, J. Compos. Mater. 2, 380 (1968).

Table VIII. Experimental values and predictions of CTE using Turner’s and Kerner’s models modified forhollow particle-filled composites

Composite type

Turner’s model Kerner’s model

a (31026/�C) Da* (%) a (31026/�C) Da* (%)

N220-15-1 51.24 5.50 56.97 5.12N220-30-1 38.36 33.37 46.48 10.07N220-50-1 25.24 65.87 33.32 25.63N460-15-1 43.58 25.91 56.22 2.41N460-30-1 29.60 55.47 45.32 1.54N460-50-1 18.22 121.06 32.05 25.66N220-15-2 51.84 1.48 56.14 6.29N220-30-2 39.59 33.46 45.81 15.33N460-15-2 44.61 10.56 55.39 10.95N460-30-2 30.93 35.78 44.66 5.97N220-15-5 46.69 6.36 50.36 1.40N220-30-5 35.84 45.30 41.17 26.49N460-15-5 40.29 31.77 49.69 6.85N460-30-5 28.13 51.13 40.13 5.94N220-15-10 46.30 3.61 48.87 1.84N460-15-10 40.53 18.87 48.07 0.22

*Difference between experimental value and the model prediction


http://dx.doi.org/10.1016/j.bbr.2011.03.031

27. Z.H. Karadeniz and D. Kumlutas, Compos. Struct. 78, 1(2007).

28. C.L. Hsieh and W.H. Tuan, Mater. Sci. Eng. A 460–461, 453(2007).

29. C.Y. Wei and D. Srivastava, Appl. Phys. Lett. 85, 2208(2004).

30. T. Natsuki, Q.-Q. Ni, and S.-H. Wu, Polym. Eng. Sci. 48,1345 (2008).

31. N. Gupta, E. Woldesenbet, and P. Mensah, Compos. Part A35, 103 (2004).

32. M. Sanchez, J. Rams, M. Campo, A. Jimenez-Suarez, and A.Urena, Compos. Part B 42, 638 (2011).

33. Q. Zhang, G. Chen, G. Wu, Z. Xiu, and B. Luan, Mater. Lett.57, 1453 (2003).

34. R.U. Vaidya and K.K. Chawla, Compos. Sci. Technol. 50, 13(1994).

35. E.H. Kerner, Proc. Phys. Soc. Lond. Sect. B 69, 808 (1956).36. M.A. El-Hadek and H.V. Tippur, Optics. Lasers Eng. 40, 353

(2003).37. K. Liang, H. Kunkel, C. Oakley, and W. Huebner, Ultra-

sonics 36, 979 (1998).38. J. Nji and G. Li, Compos. Part A 39, 1404 (2008).39. G. Li, Y. Zhao, and S.-S. Pang, Mater. Sci. Eng. A 271, 43

(1999).40. N. Gupta, R. Ye, and M. Porfiri, Compos. Part B 41, 236

(2010).41. G. Tagliavia, M. Porfiri, and N. Gupta, Int. J. Solids Struct.

47, 2164 (2010).


thermal expansion of carbon nanoﬁber-reinforced multiscale...

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