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Fabrication and multifunctional properties of a hybrid laminate with aligned carbon nanotubes grown In Situ
Composites Science and Technology 68 (2008) 2034–2041
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Fabrication and multifunctional properties of a hybrid laminate with aligned carbon nanotubes grown In Situ Enrique J. Garcia, Brian L. Wardle *, A. John Hart, Namiko Yamamoto Massachusetts Institute of Technology, Cambridge, MA 02139, USA
a r t i c l e
i n f o
Article history: Received 31 December 2007 Accepted 27 February 2008 Available online 7 March 2008
Keywords: A. Carbon nanotubes B. Nano composites A. Hybrid composites A. Polymer–matrix composites (PMCs) B. Interfacial Strength B. Electrical Properties
a b s t r a c t A hybrid composite architecture of carbon nanotubes (CNTs), advanced fibers and a matrix is described, from CNT synthesis and characterization through to standard mechanical and electrical laminate tests. Direct growth of aligned CNTs on the surface of advanced fibers in a woven fabric enables enhancement in multifunctional laminate performance, as demonstrated by a 69% increase in interlaminar shear strength and 106 (in-plane) and 108 (through-thickness) increases in laminate-level electrical conductivity. Processes developed include dip-coating of CNT growth catalyst and atmospheric-pressure chemical vapor deposition of dense aligned CNTs. A capillarity-driven mechanism is presented to explain the observed effective and uniform wetting of the aligned CNTs in the interior of the laminate by unmodified thermoset polymer resins. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction New structural concepts, particularly polymer-based nanocomposites, have been pursued in recent years to harness the attractive properties of carbon nanotubes (CNTs) [1–5]. In this work, fabrication of a hybrid advanced composite material system is described that exhibits enhanced multifunctional laminate-level engineering properties. The hybrid system (Fig. 1) is comprised of three parts: advanced fibers (dia. of order microns) organized in tows and woven, a thermoset polymer resin, and dense aligned CNTs (dia. of order nanometers) organized within the polymeric matrix. As illustrated in Fig. 1, CNTs are organized radially around the existing micron-size fibers, and the polymeric matrix binds all the filaments (advanced fibers and CNTs) together. The alignment and dispersion of CNTs within the dense array of woven tows and fibers in the cloth material is achieved by radial in situ growth of CNTs from the surface the woven fibers. The CNTs reinforce the polymer matrix between the advanced fibers, so as to provide enhanced strength and toughness as well as an electrically conductive pathway, as illustrated conceptually in Fig. 2. The resulting laminated structure is described as a hybrid advanced composite laminate, rather than as a nanocomposite [1] (usually nanostructures in a polymer matrix), and is perhaps best described as a radially-
* Corresponding author. E-mail address: [email protected] (B.L. Wardle). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.02.028
aligned CNT fiber reinforced plastic (ACNT-FRPs), or more succinctly ‘fuzzy’ fiber reinforced plastic (FFRP). Significant technical and manufacturing challenges have hindered the development of large-scale CNT-enhanced structures [1,5]. Alignment, dispersion, and adhesion of CNTs in polymer matrices are vital for structural composite applications, and numerous studies and review articles have reported on the difficulties in dispersing CNTs in polymers [1,5–8]. These difficulties are significantly compounded when CNT-modified matrices are introduced into typical aligned-fiber advanced composites, where the CNTcontaining matrix must effectively impregnate a high volume fraction (Vf) of advanced fibers. Due to issues such as agglomeration and poor dispersion, only marginal mechanical property improvements are observed for both nanocomposites [9] and hybrid composites [10] when CNTs are introduced into the bulk matrix. More success has been achieved with nanoscale modification of the interface between composite plies, by growing CNTs on the surface of cloth [11] or placing unaligned CNTs at low Vf on fibers (including at the ply interface) [12,13]. The hybrid architecture investigated here uses a dry form of woven cloth with in situ grown aligned CNTs that are subsequently impregnated with a thermoset resin. Due to the CNT alignment and organization around the fibers, the CNTs offer both interlaminar and intralaminar modification of the composite performance. Our prior studies on wetting of CNTs with commercial unmodified complex thermosets indicate that aligned CNTs readily draw up such polymers through capillary action [14], and that adhesion between the CNTs and the thermosets
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Advanced fibers covered with aligned CNTs
Cloth plies Tows of CNTcovered fibers Fig. 1. Illustration of the hybrid composite developed in this work: (A) Schematic illustration of the architecture composed of a cloth containing fiber tows, covered by CNTs, in a polymer matrix. The two different plies are shown in two different colors; (B) Closer view of the interface cross-section between the two composite plies. The CNTs grown on the surface of each individual fiber interact with the CNTs of the fibers nearby, achieving reinforcement inter-tow (as in the case of the two fibers from the upper ply) and interlaminar (as in the case of CNTs from the upper and lower plies).
Fiber x-section CNT Polymer Matrix
Ply 1 Intraply
Ply interface Ply 2
Intralaminar
Interlaminar
Fig. 2. Illustration of intralaminar and interlaminar reinforcement from the CNTs in the hybrid composite. All dimensions approximately to scale except CNT diameter and volume fraction.
creates an effective composite [15]. It is now well known that tangled CNTs are difficult to disperse and wet with polymers, even with improvements brought about by solvent-modified polymers and functionalized CNTs. Thus, due to the in situ growth of aligned CNTs on the fibers, the uncured polymer is drawn into the CNT-fiber dry form to create a well-consolidated composite.
ences in laminate fabrication (CNT wetting, void fraction, etc.): Buehler EpoThin and West Systems 105 Epoxy Resin, 200 cP and 1000 cP at room temperature, respectively. The resins were unmodified, e.g., no solvent was utilized to reduce the viscosity.
2. Experimental
The first step to manufacture the hybrid composite is the growth of CNTs on the alumina fibers using a modified chemical vapor deposition (CVD) process [18,19]. First, the alumina cloth was soaked in a 50 mM solution of Fe(NO3)3 H2O dissolved in isopropanol for 5 min. and then dried in ambient air. This coats the fiber surfaces with catalyst precursor from which the subsequent heating process creates catalyst nanoparticles on the surface of the fibers. This heating process was conducted using a single-zone atmospheric pressure quartz tube (Lindberg, 22 mm inner diameter) furnace at 750 °C. After the temperature is stabilized, hydrogen (H2) was introduced for 2 min. to reduce the catalyst, and ethylene (C2H4), was introduced to start CNT growth. The C2H4 gas flow rate was reduced from the maximum growth rate (2 lm sec1) to 0.3 lm sec1 for better control of CNT length. The growth rate is helpful in estimating CNT length (10– 100 lm for 0.5–5 min growth time). The CNT lengths are typically much longer than the spacing between the cloth plies (10 lm) and between alumina fibers in the tows (1–5 lm). CNTs grown on the fibers were characterized by SEM for overall morphology (aligned growth, fiber coverage, and coverage over laminate) and were additionally characterized for diameter and distribution by TEM. Aligned CNTs grow radially and densely on
Fabrication of the hybrid architecture is first described, followed by characterization of the laminates and particularly the aligned CNTs that modify the baseline laminated material system. 2.1. Materials Commercially available thick (0.75 mm, area density of 900 g m2) alumina fiber cloth woven in a 0°/90° satin-weave was used as the base composite fiber. Alumina cloth was chosen as it is an inexpensive and readily-available material that can withstand the 750 °C growth temperature (vs. glass fibers), and is compatible with the particular CNT growth chemistry employed. Alumina-polymer composites have been studied for various applications, including enhanced strength at low-temperatures over glass-thermoset systems [16], and currently in integral armor [17] concepts. The alumina fibers are 11 lm in diameter, and each tow consists of 3 K fibers, yielding a dry alumina cloth volume fraction of approximately 65%. Two room-temperature curing thermosets with different viscosities were used to assess any differ-
2.2. Carbon nanotube synthesis and characterization
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Fig. 3. Woven alumina cloth used in laminate fabrication: (A) As-received without CNTs; and (B) With aligned CNTs grown radially from the fiber surfaces, and an individual fiber covered with CNTs. Scale bars on bottom images in A and B are 20 lm; (C) Aligned CNT coverage over multiple fibers in a tow.
the surface of each individual fiber within the tows of the cloth (Fig. 3B, bottom). The surfaces of the alumina fibers before and after CNT growth can be compared in Figs. 3A and B. In contrast to prior work where CNTs are grown on the surface of fibers (e.g., [2]), the CNTs here are extremely long, dense, and aligned (see Fig. 3B and C). SEM inspections reveal that coverage was uniform over the outer surfaces inspected at the given CVD growth conditions, except near the cloth edges where cloth-cutting irregularities resulted in irregular CNT growth. All laminates were trimmed of these irregular edge regions after the composites were cured. Aligned CNTs are noted to grow on all of the fibers, but not always in a uniform radial pattern. Rather, especially for longer CNT lengths, CNTs on an individual fiber tend to separate into 2
or 3 micro-forests as can be see in Fig. 3B bottom and Fig. 3C. CNTs were prepared for TEM by first removing them from the alumina cloth by sonication in 2-propanol, and the removed CNTs were deposited over a copper grid to be inspected under TEM. The TEMs revealed that the CNTs grown on the alumina fibers are multiwalled with a typical outer diameter of 17 ± 2 nm and 8 concentric walls as shown in the example TEMs in Fig. 4. 2.3. Laminate fabrication The procedures for preparing the baseline composites (no CNTs) and hybrid composites (CNTs) are identical except that CNTs are first grown on the cloth in the latter case. In the hybrid composite,
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Fig. 4. TEM images of CNTs grown on alumina fibers after removal and sonication to prepare samples: (A) low-resolution image used to gather diameter distribution data; (B) high-resolution image showing the concentric wall-structure of the CNT.
CNTs are as-grown, i.e., they are not surface treated or otherwise functionalized, before being combined with the unmodified epoxy. The composite fabrication follows a simple hand layup procedure. The alumina cloth was soaked in room-temperature curable epoxy and then the plies are stacked on a vacuum table. The number of cloth layers (plies) varied from 1 to 4. Layers of porous Teflon and bleed paper were used to remove excess epoxy during the curing process. A steel caul plate, non-porous Teflon, and a weave of glass fiber covered the assembly to guarantee uniform distribution of the vacuum. A vacuum bag enclosed the whole assembly and the laminates were cured for 9–12 h under vacuum with 200 kPa of pressure (including the caul plate and the vacuum) at an elevated temperature (60 °C) to promote epoxy flow for better wetting and to accelerate the curing time. The cured composite specimens were trimmed using an abrasive-grit band-saw to form rectangular specimens, and the edges were smoothed by hand with decreasing grit sandpaper (1200, 2400, and 4000 grit). Dimensions of the composites were measured with a caliper (resolution of 2.5 102 mm). The variation of the length and the width were maintained within ± 2%. The sizes of both baseline and hybrid composites were limited to 20 mm 40 mm by the CVD furnace. The average ply thickness for the baseline composites (0.6 mm) was slightly thinner than that of the hybrid composites (0.7 mm), reflecting the increase in volume of the weave due to CNT growth pushing the fibers apart (this is discussed further in the next section). Mass and volume fractions of the CNTs were estimated as follows: Each cloth was weighed using a microbalance (resolution of 1 lg) before and after CNT growth. Assuming that the measured mass ratio between the CNTs and the alumina cloth with catalyst does not change during the epoxy curing process, the mass of the alumina fiber cloth with catalyst and CNTs in the composite can be estimated using the final mass of the trimmed composite. The epoxy mass is estimated by subtracting the mass of the cloth and of the CNTs from the measured total composite mass. The volume fractions were calculated using the epoxy densities and an assumed CNT density (1.4 mg mm3) [20]. The CNT mass fraction are found to be 0.5–2.5%, while the CNT volume fraction was estimated to be between 1% and 3% (purposefully varied via CNT growth time to obtain the data on the effect of CNT volume fraction on electrical properties). The volume fraction of alumina fibers in the hybrid and baseline composite was 60%, and that of epoxy was 40%. Possible catalyst mass reduction from the CVD process and handling were not considered in the CNT mass assessment, leaving this CNT estimate conservative. 2.4. Laminate characterization Optical and scanning electron microscopy (SEM) are employed to characterize the morphology of the baseline and hybrid compos-
ites. Electrical conductivity of the laminates in the in-plane and through-thickness directions, and interlaminar shear strength (ILSS), are characterized using standard ASTM procedures. 2.4.1. Laminate electrical resistivities The DC electrical resistivities of the samples were experimentally obtained according to the ASTM standards D257–99 for the insulating baseline composites and D4496-04 for the moderatelyconductive hybrid composites [21,22]. Volume resistivity was measured in both through-thickness and in-plane directions. Silver paint (SPI Flash Dry Silver Paint) was applied to the relevant surfaces of the composite to create non-guarded electrodes. The 4-probe wiring configuration of the test setup was chosen to minimize voltage loss in the circuit, as shown in Fig. 5A. The voltage applied to the samples was cycled 5 times between 0–20 V. The maximum voltage, 20 V, was within the standards specifications (500 V) but kept low to avoid melting and degrading the epoxy due to Joule heating. The measured current-voltage curves were linear (R2 value >0.9) as shown in Fig. 5A, and the resistance was calculated as the plot slope. The resistivity was calculated based on qv ¼ Rvt A, where qv, A, t, and Rv are the volume resistivity, area, thickness, and volume resistance, for the specimen, respectively. Note that t is the thickness of the laminate in the case of a through-thickness test, but is the length of the laminate for an in-plane resistivity test. 2.4.2. Interlaminar shear strength Experiments were performed to compare the short-beam-shear (SBS) strength of composite laminates made of alumina cloth with and without CNTs grown on the alumina fibers. The experiments measure the effectiveness of the CNTs to reinforce the interface of a laminated composite. A standard 3-point bending test [23] is applied to short beam shear (SBS) specimens to measure the interlaminar shear strength of the laminates. An Instron 8848 MicroTester with a calibrated 2-kN load cell was used to apply and measure static loads ranging from 5 N to 2 kN. The resolution is 1 mN for load and 1 lm for displacement. The specimen dimensions were selected following the standard [23] with length (15 mm) equal to 6 times the thickness (2.5 mm, 4 plies thick) and width (5 mm) equal to twice the thickness. The span for the 3-point bending test is equal to 4 times the thickness. The tow direction is ±45° to the laminate/test axis of the specimen. The laminate is subjected to a 3-point bending test at a constant crosshead speed of 1 mm/min. The load is applied until fracture occurs and the fracture load is used in the calculation of the apparent shear strength of the laminate ply interface. Care was taken to ensure that failure occurs at the mid-plane of the laminate following guidance in the standard. The test was stopped when load dropped by 30% or head travel exceeded the specimen nominal thickness.
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30 2 plies, 0.41% Vf
Volt [V]
Linear line fit (R2=0.99)
20
Ammeter Voltmeter 10
A
V Rwire
0 0
0.05
0.1
Current [A]
Sample
Silver paint electrodes
10 mm
Fig. 5. Laminate characterizations: (A) Electrical resistivity 4-probe test illustration and representative voltage-current data for a 2-layer hybrid composite with 0.4% CNT volume fraction; (B) hybrid short-beam-shear test specimen.
Five specimens were tested for each specimen type (baseline without CNTs and hybrid composite with CNTs). The short beam shear m strength is calculated using rSBS ¼ 0:75P , where rSBS is the SBS bh strength, Pm is the maximum load during the test, and b and h are the width and the thickness of the specimen, respectively. This relation is derived assuming Euler-Bernoulli beam theory, which is only approximately correct for a short beam and for the experimental three point bending loading conditions. The woven configurations of fibers/tows make the interlaminar stress state more complex to analyze, however, the SBS test allows for a quantitative relative comparison of the SBS strength of composites with and without CNTs, and has similarly been used by others for precisely this purpose, e.g., [13]. 3. Results and discussion Results of the laminate fabrication are discussed focusing on the effects of CNTs on laminate characteristics, electrical resistance, and short-beam-shear strength. 3.1. Laminate fabrication and characterization Fabricated laminates are characterized via optical and scanning electron microscopy, and differences between laminates with and without CNTs are noted. Aligned CNTs are grown on the surface of woven fibers, both on the surface of the cloth and on fibers in the interior of the cloth. This is a consequence of the specific CNT growth process utilized, which is a base-growth process wherein the CNTs are extruded from catalyst particles that remain on the surface of the fibers. The carbon source is the ethylene gas that penetrates to the interior of the woven cloth to reach the catalyst particles. As provided previously, the catalyst particles are applied to all the fibers in the cloth by dip-coating in a liquid catalyst bath. CNT volume fraction is kept constant for the shear tests (30 lm long CNTs, 2% composite volume fraction, and 4% volume fraction in the epoxy), and it is varied to determine its effect on electrical
properties. CNT volume fraction is not varied in the usual way by decreasing spacing between aligned filaments (here, CNTs), but by changing the CNT length via extending the CVD growth times to achieve 10 lm, 40 lm, and 100 lm long forests giving approximately 0.6%, 2%, and 3% CNT volume fraction in the composite. Details of the specimen dimensions are shown in Table 1, including the number of plies, average laminate thicknesses, and CNT volume fraction in the overall composite and in epoxy. Note that CNT volume fraction is a function both of the CNT length and the overall laminate consolidation. Average ply thickness increases in the hybrid composites slightly at higher CNT lengths (see trend in Table 1 for 2-ply specimens) because the CNT growth pushes apart the woven alumina fibers at longer CNT growth times/lengths. Longer CNT growth times ( 1 h) have been observed to push apart the woven structure of the alumina cloth, ruining the weave. A key finding in the fabrication process concerns the wetting of the woven plies containing the aligned CNTs by epoxy. Optical and scanning-electron microscopy of laminate cross sections reveals that CNTs are present throughout the laminate interior and that there is no difference in observed void fraction (<2%) between laminates with and without CNTs (see Fig. 6). Nor are there differences noted for the two epoxy resins, having different viscosities. The epoxy, which is applied to the top of each ply during the hand layup process, fully wets the alumina fibers containing aligned CNTs. Cross sections of a laminate without (Fig. 6C) and with (Fig. 6D) CNTs at the same scale indicate excellent consolidation and wetting of the hybrid ply by the epoxy. The spacing between the alumina fibers in the hybrid composite is slightly larger than in the baseline due to the CNT growth pushing the fibers apart as discussed above. The dark rings over the edge of alumina fibers in the hybrid composite (see Fig. 6D) are due to SEM charging that creates dark halos at the interface of the insulating fibers and conductive CNT-epoxy matrix, i.e., the halos obscure the outer diameter of the fiber cross sections. Visualization of individual CNT morphology inside the hybrid laminate has been unsuccessful. Fo-
Table 1 Summary of fabricated baseline and hybrid laminates
SBS tests Electrical tests
# plies (CNT length)
Baseline thickness (mm)
Hybrid thickness (mm)
CNT Vf in composite (%)
CNT Vf in epoxy (%)
4 1 2 2 2 3
2.2 ± 0.1 0.8 1.2 ± 0.05 – – 2.0
2.1 ± 0.1 0.6 1.3 1.4 ± 0.3 1.8 1.8
2 1.2 0.6 2.3 ± 0.02 2.9 2.2
4 1.4 1.5 5.9 ± 0.2 5.6 9.2
(30 lm) (40 lm) (10 lm) (40 lm) (100 lm) (40 lm)
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Fig. 6. Hybrid composite specimen characterization using West Systems epoxy with 2% Vf of CNTs: (A) 3-ply laminate after trimming; (B) SEM image of woven cloth cross section showing fibers in tows reinforced with CNT/epoxy; SEM cross-sectional images of; (C) no-CNT baseline composite and; (D) CNT-reinforced hybrid composite indicating effective wetting (lack of voids) and good fiber distribution after the vacuum curing.
cused-ion-beam (FIB) preparation, cryogenic microtome, and polishing to create TEM samples have failed due to the stiffness and hardness differences between the alumina fibers and CNT-epoxy matrix. The mechanism underlying epoxy wetting of the hybrid laminate interior is next considered. The epoxy is introduced at the top of a ply, and needs to penetrate the ply thickness (0.6 mm) through the dense array of alumina fibers (65% volume fraction) covered with dense forests of CNTs (2 billion CNTs cm2). Given the difficulty of dispersing and wetting CNTs in epoxy, capillaritydriven wetting of the aligned CNT forests is postulated as the underlying mechanism. Previous work with solvents has demonstrated a strong capillary effect of aligned CNT forests [24–27]. Our previous work with complex thermosets, including epoxies
with similar viscosities as those used here, has demonstrated that long (exceeding 0.5 mm) aligned CNT forests are readily and effectively (i.e., no micron-scale voids) wet [14]. Similar wetting behavior has been observed when forming silver-epoxy electrodes for testing of CNT electrical probes [28]. Further, the preferred route for this wetting was observed to be along the axis of the CNT forest, as illustrated in Fig. 7A. It is proposed that the wetting of the interior portions of the laminate are due to capillarity-driven wetting along the aligned CNT axes, with secondary (and less effective, slower) polymer dispersion orthogonal to the aligned CNTs as illustrated in Fig. 7B. This mechanism needs further investigation, perhaps by purposefully growing tangled, rather than aligned, CNTs on the surface of the fibers and performing a relative cross-sectional characterization of tangled vs. aligned CNT hybrid laminates.
Fig. 7. Routes for polymer wetting of the CNTs in the hybrid composite: (A) High-resolution SEM of aligned CNT forest indicating preferred wetting route via capillary action, scalebar = 1 lm; (B) Illustration of CNT wetting in the interior of the composite.
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3.2. Laminate in-plane and through-thickness electrical resistivities
3.3. Interlaminar shear strength A short beam shear test (essentially a 3-point bending test with a short span) was applied (see Fig. 5B) to assess any differences in the baseline and CNT hybrid laminates. The test was performed on 4-ply laminates at a constant crosshead speed, and the fracture load and mode were used to determine the interlaminar shear strength (ILSS) of the laminates. The hybrid laminate showed an improvement of 69% with respect to the unreinforced laminate (see Table 3). Fracture surface inspection of the specimens tested did not reveal any mechanistic understanding of the reinforcement due largely to the small specimen size. Ongoing work is exploring this mechanism via Mode I and II fracture tests that should yield a larger and cleaner fracture surface for inspection. The increase in interlaminar shear strength observed here compares favorably with existing interface reinforcement techniques
Electrical Resistivity [Ohm mm]
Electrical resistance was acquired with a 4-probe current/voltage measurement (see Fig. 5A). Two DC electrical parametric studies were conducted, the first to consider the effect of ply thickness (1, 2, and 3 plies) at constant CNT volume fraction (40 lm of CNT growth), and the second to investigate the effect of increased CNT volume fraction irrespective of ply thickness. Results from the first parametric study are shown in Table 2. The resistance decrease in both the in-plane and through-thickness direction is clear, indicating the effect of the CNTs on this bulk laminate property. The baseline composites, regardless of thickness, are insulating and have an in-plane resistance of 107–108 Ohm mm and a through-thickness resistivity of 109 Ohm mm, regardless of laminate thickness. The in-plane resistivity is expected and observed to be lower (5–400) than the through-thickness value due to the lower resistivity and continuous nature of the alumina fibers (vs. a discontinuous path across ply interfaces) in the laminate plane vs. the more insulating (10 resistivity of alumina) epoxy [29]. Due to the aligned CNTs present in the polymer matrix, electrical resistance of the laminate decreases by several orders of magnitude at low (1–3%) CNT volume fractions, for both in-plane and through-thickness directions, to values on the order of 101– 102 Ohm mm, as shown in Fig. 8. A direct-path network of aligned CNTs was observed with 0.5% CNT volume fraction (clearly above the percolation threshold in pure epoxy). Such an improvement is expected, as plastics realize significantly enhanced conductivity at low loadings of unoriented CNTs [30–32]. Further, the decrease in through-thickness resistivity is two orders of magnitude greater than the in-plane value. This is attributed to the CNT orientation in the through-thickness direction as well as CNT bridging of the insulating epoxy interlaminar region between plies that contributes to the high through-thickness resistivity of the baseline laminate. The nanostructure, specifically orientation of the CNT long axis, in concert with the woven fiber microstructure, clearly plays an important role in determining the effective laminate resistivities.
1.E+10
In-Plane 1.E+08
Through-Thickness
1.E+06 1.E+04 1.E+02 1.E+00 0
0.5
1
1.5
2
2.5
3
CNT Volume Fraction [%] Fig. 8. Electrical resistivity decrease with increasing CNT volume fraction, both in through-thickness and in-plane directions with 1–3 plies of 0–3% Vf of CNTs.
Table 3 Summary of interlaminar strength for the baseline and hybrid laminates Property
Baseline
CNT-reinforced
Change
Interlaminar shear strength (MPa)
20.1 ± 0.9
33.8 ± 1.1
+69%
such as stitching that typically shows a maximum of 30% increase, and only 10% increase at comparable stitch densities to that used here with the CNTs [33]. Work by others using randomly oriented carbon nanofibers (NFs) and CNTs placed between woven plies using different techniques (additional CNT/epoxy layer [34], spraying [35], and unaligned CNTs mixed between tows [36]) showed limited or no reinforcement. Recently, work with electrophoretically-deposited CNTs demonstrated enhanced ILSS to a maximum of 30% increase. Other recent work with functionalized single-wall CNTs sprayed on the surface of glass cloth showed a maximum of 45% increase in SBS (interlaminar shear) strength. Last, other work [11] has demonstrated significant interlaminar toughening in Mode I and II by growing CNTs on the exterior surface of SiC cloth plies. Advances in processing of nanostructures at ply interfaces are yielding improved laminate-level properties, particularly interfacial properties because this interface is more accessible during laminate manufacture than the interior of the ply/laminate. In this work, because aligned CNTs are present in the interior of the laminate as well, significant intralaminar effects from the CNTs are expected (and observed for electrical conductivities as discussed above). 4. Conclusions A hybrid laminate is presented that demonstrates simultaneous enhancement of mechanical and electrical properties. The hybrid laminate is fabricated with a simple hand layup process, and alignment of the in situ grown CNTs on the surface of the advanced fi-
Table 2 Summary of laminate-level resistivities for the baseline and hybrid laminates # plies (CNT length)
1 2 2 2 3
(40 lm) (10 lm) (40 lm) (100 lm) (40 lm)
CNT volume fraction (%)
1.2 0.6 2.3 2.9 2.2
In-plane resistivity (Ohm mm)
Through-thickness resistivity (Ohm mm)
Base
Hybrid
Base
Hybrid
4.4 106 (7.5 ± 0.4) 106 – – 1.3 107
8.4 1.3 102 8.1 ± 1.5 9.7 8.6
– (2.5 ± 0.8) 109 – – –
6.8 101 5.3 102 8.3 101 6.5 101 6.8 101
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bers is used to explain polymer wetting in the interior of the laminate via capillary action. The specific alumina fiber reinforced plastic system considered has some direct applications (such as a front-facing material for integral armor applications), however, it is primarily a model fiber/composite, allowing fabrication, synthesis, and processing investigation as well as relative comparisons of properties with and without CNTs. Insights from the performance of this system will be useful in developing aligned-CNT carbon and glass fiber reinforced plastics (FCFRP and FGFRP). Such composites would be attractive for a host of structural applications. Characterization and modeling of the enhanced properties afforded by the VACNT-FRP architecture should be extended and broadened to include targeted improvements and tailoring of properties such as thermal conductivity [36], current carrying capacity, damping, fire and heat resistance, and wear resistance. Importantly, this architecture needs to be extended to carbon–fiber based advanced composites that form the majority of advanced composites used in aerospace and other high-performance applications. The woven ceramic architecture investigated here has given significant insight into wetting of in situ grown CNTs, laminate consolidation, and relative property improvements, suggesting that carbon–fiber based versions are achievable and can have significantly improved ‘nano-engineered’ laminate properties if aligned CNTs are grown in situ on the fibers. Given the interlaminar strength improvements observed, it is expected that Mode I toughness and other interlaminar properties will improve as well. Significant modeling and experimentation are required to determine optimal characteristics of CNTs for the multifunctional architecture, including length, diameter, volume fraction, and effects of covalent and non-covalent CNT-polymer interactions. Fabrication studies to create scalable and continuous processes for this and similar architectures would enable large-scale hybrid nano-engineered composites for a variety of applications. Acknowledgements This work was supported by Airbus S.A.S., Boeing, Embraer, Lockheed Martin, Saab AB, and Spirit AeroSystems, and Textron Inc. through MIT’s Nano-Engineered Composite aerospace Structures (NECST) Consortium and by MIT’s Karl Chang (1965) Innovation Fund. The authors gratefully thank John Kane and the entire Technology Laboratory for Advanced Materials and Structures (TELAMS) at MIT for valuable discussions and technical support, Sunny Wicks for fabrication assistance, and Dr. Alex Slocum (MIT ME) for valuable input. Enrique Garcia would like to acknowledge support from the La Caixa Foundation, John Hart from the Fannie and John Hertz Foundation, Sunny Wicks from MIT’s Paul E. Gray (1954) Undergraduate Research Opportunity Fund, and Namiko Yamamoto from the Linda and Richard (1958) Hardy Fellowship. References [1] Ajayan PM, Tour JM. Nanotube composites. Nature 2007;447:1066–8. [2] Thostenson ET, Li WZ, Wang DZ, Ren ZF, Chou TW. Carbon nanotube/carbon fiber hybrid multiscale composites. J Appl Phys 2002;91(9):6034–7. [3] Vaia RA, Wagner HD. Framework Nanocompos Mater Today 2004;32:32–7. [4] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [5] Schulte K, Windle AH. Editorial. Compos Sci Technol 2007;67:777. [6] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites. Carbon 2006;44:1624–52. [7] Thostenson ET, Reng Z, Chou TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61:1899–912.
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[8] Thostenson ET, Li C, Chou TW. Nanocomposites in context. Compos Sci Technol 2005;65:491–516. [9] Gojny FH, Wichmann MHG, Kopke U, Fiedler B, Schulte K. Carbon nanotubereinforced epoxy-composites: enhanced stiffness and fracture toughness at low nanotube content. Compos Sci Technol 2004;64:2363–71. [10] Qiu J, Zhang C, Wang B, Liang R. Carbon nanotube integrated multifunctional composites. Nanotechnology 2007;18:275708–19. [11] Veedu VP, Cao A, Li X, Ma K, Soldano C, Kar S, et al. Multifunctional composites using reinforced laminae with carbon-nanotube forests. Nat Mater 2006;5:457–62. [12] Beyakrova E, Thostenson ET, Yu A, Kim H, Gao J, Tang J, et al. Multiscale carbon nanotube–carbon fiber reinforcement for advanced epoxy composites. Langmuir 2007;23:3970–4. [13] Zhu J, Imam A, Crane R, Lozano K, Khabasheku VN, Barrera EV, et al. Processing a glass fiber reinforced vinyl ester composite with nanotube enhancement of interlaminar shear strength. Compos Sci Technol 2007;67:1509–17. [14] Garcia EJ, Hart AJ, Wardle BL, Slocum AH. Fabrication of composite microstructures by capillarity-driven wetting of aligned carbon nanotubes with polymers. Nanotechnology 2007;18:165602. [15] Garcia EJ, Hart AJ, Wardle BL, Slocum AH. Fabrication and nanocompression testing of aligned CNT/polymer nanocomposites. Adv Mater 2007;19:2151–6. [16] Gu YH. Fracture Behavior of Continuous Alumina Fiber Reinforced Epoxy. J Compos Mater 1994;28(13):1227–36. [17] Hogg PJ. Composites in armor. Science 2006;314:1100–1. [18] Hart AJ, Slocum AH. Flow-mediated nucleation and rapid growth of millimeter-scale aligned carbon nanotube structures from a thin-film catalyst. J Phys Chem B 2006;110:8250–7. [19] Garcia EJ, Hart AJ, Wardle BL, Slocum AH. Fabrication and testing of long carbon nanotubes grown on the surface of fibers for hybrid composites. In: 47th AIAA Structures, dynamics, and materials conference proceedings, Newport, RI, May 1–4; 2006 [AIAA-2006-1854]. [20] Krishnan A, Dujardin E, Ebbesen TW, Yianilos PN, Treacy MM. Young’s modulus of single-walled nanotubes. J Phys Rev B 1998;58(20):14013–9. [21] ASTM Standard D257, 1999. Standard Test Methods for DC Resistance or Conductance of Insulating Materials. West Conshohocken, PA: ASTM International; 2005. Available from:
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Fabrication and multifunctional properties of a hybrid laminate with aligned carbon nanotubes grown In Situ Enrique J. Garcia, Brian L. Wardle *, A. John Hart, Namiko Yamamoto Massachusetts Institute of Technology, Cambridge, MA 02139, USA
a r t i c l e
i n f o
Article history: Received 31 December 2007 Accepted 27 February 2008 Available online 7 March 2008
Keywords: A. Carbon nanotubes B. Nano composites A. Hybrid composites A. Polymer–matrix composites (PMCs) B. Interfacial Strength B. Electrical Properties
a b s t r a c t A hybrid composite architecture of carbon nanotubes (CNTs), advanced fibers and a matrix is described, from CNT synthesis and characterization through to standard mechanical and electrical laminate tests. Direct growth of aligned CNTs on the surface of advanced fibers in a woven fabric enables enhancement in multifunctional laminate performance, as demonstrated by a 69% increase in interlaminar shear strength and 106 (in-plane) and 108 (through-thickness) increases in laminate-level electrical conductivity. Processes developed include dip-coating of CNT growth catalyst and atmospheric-pressure chemical vapor deposition of dense aligned CNTs. A capillarity-driven mechanism is presented to explain the observed effective and uniform wetting of the aligned CNTs in the interior of the laminate by unmodified thermoset polymer resins. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction New structural concepts, particularly polymer-based nanocomposites, have been pursued in recent years to harness the attractive properties of carbon nanotubes (CNTs) [1–5]. In this work, fabrication of a hybrid advanced composite material system is described that exhibits enhanced multifunctional laminate-level engineering properties. The hybrid system (Fig. 1) is comprised of three parts: advanced fibers (dia. of order microns) organized in tows and woven, a thermoset polymer resin, and dense aligned CNTs (dia. of order nanometers) organized within the polymeric matrix. As illustrated in Fig. 1, CNTs are organized radially around the existing micron-size fibers, and the polymeric matrix binds all the filaments (advanced fibers and CNTs) together. The alignment and dispersion of CNTs within the dense array of woven tows and fibers in the cloth material is achieved by radial in situ growth of CNTs from the surface the woven fibers. The CNTs reinforce the polymer matrix between the advanced fibers, so as to provide enhanced strength and toughness as well as an electrically conductive pathway, as illustrated conceptually in Fig. 2. The resulting laminated structure is described as a hybrid advanced composite laminate, rather than as a nanocomposite [1] (usually nanostructures in a polymer matrix), and is perhaps best described as a radially-
* Corresponding author. E-mail address: [email protected] (B.L. Wardle). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.02.028
aligned CNT fiber reinforced plastic (ACNT-FRPs), or more succinctly ‘fuzzy’ fiber reinforced plastic (FFRP). Significant technical and manufacturing challenges have hindered the development of large-scale CNT-enhanced structures [1,5]. Alignment, dispersion, and adhesion of CNTs in polymer matrices are vital for structural composite applications, and numerous studies and review articles have reported on the difficulties in dispersing CNTs in polymers [1,5–8]. These difficulties are significantly compounded when CNT-modified matrices are introduced into typical aligned-fiber advanced composites, where the CNTcontaining matrix must effectively impregnate a high volume fraction (Vf) of advanced fibers. Due to issues such as agglomeration and poor dispersion, only marginal mechanical property improvements are observed for both nanocomposites [9] and hybrid composites [10] when CNTs are introduced into the bulk matrix. More success has been achieved with nanoscale modification of the interface between composite plies, by growing CNTs on the surface of cloth [11] or placing unaligned CNTs at low Vf on fibers (including at the ply interface) [12,13]. The hybrid architecture investigated here uses a dry form of woven cloth with in situ grown aligned CNTs that are subsequently impregnated with a thermoset resin. Due to the CNT alignment and organization around the fibers, the CNTs offer both interlaminar and intralaminar modification of the composite performance. Our prior studies on wetting of CNTs with commercial unmodified complex thermosets indicate that aligned CNTs readily draw up such polymers through capillary action [14], and that adhesion between the CNTs and the thermosets
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Advanced fibers covered with aligned CNTs
Cloth plies Tows of CNTcovered fibers Fig. 1. Illustration of the hybrid composite developed in this work: (A) Schematic illustration of the architecture composed of a cloth containing fiber tows, covered by CNTs, in a polymer matrix. The two different plies are shown in two different colors; (B) Closer view of the interface cross-section between the two composite plies. The CNTs grown on the surface of each individual fiber interact with the CNTs of the fibers nearby, achieving reinforcement inter-tow (as in the case of the two fibers from the upper ply) and interlaminar (as in the case of CNTs from the upper and lower plies).
Fiber x-section CNT Polymer Matrix
Ply 1 Intraply
Ply interface Ply 2
Intralaminar
Interlaminar
Fig. 2. Illustration of intralaminar and interlaminar reinforcement from the CNTs in the hybrid composite. All dimensions approximately to scale except CNT diameter and volume fraction.
creates an effective composite [15]. It is now well known that tangled CNTs are difficult to disperse and wet with polymers, even with improvements brought about by solvent-modified polymers and functionalized CNTs. Thus, due to the in situ growth of aligned CNTs on the fibers, the uncured polymer is drawn into the CNT-fiber dry form to create a well-consolidated composite.
ences in laminate fabrication (CNT wetting, void fraction, etc.): Buehler EpoThin and West Systems 105 Epoxy Resin, 200 cP and 1000 cP at room temperature, respectively. The resins were unmodified, e.g., no solvent was utilized to reduce the viscosity.
2. Experimental
The first step to manufacture the hybrid composite is the growth of CNTs on the alumina fibers using a modified chemical vapor deposition (CVD) process [18,19]. First, the alumina cloth was soaked in a 50 mM solution of Fe(NO3)3 H2O dissolved in isopropanol for 5 min. and then dried in ambient air. This coats the fiber surfaces with catalyst precursor from which the subsequent heating process creates catalyst nanoparticles on the surface of the fibers. This heating process was conducted using a single-zone atmospheric pressure quartz tube (Lindberg, 22 mm inner diameter) furnace at 750 °C. After the temperature is stabilized, hydrogen (H2) was introduced for 2 min. to reduce the catalyst, and ethylene (C2H4), was introduced to start CNT growth. The C2H4 gas flow rate was reduced from the maximum growth rate (2 lm sec1) to 0.3 lm sec1 for better control of CNT length. The growth rate is helpful in estimating CNT length (10– 100 lm for 0.5–5 min growth time). The CNT lengths are typically much longer than the spacing between the cloth plies (10 lm) and between alumina fibers in the tows (1–5 lm). CNTs grown on the fibers were characterized by SEM for overall morphology (aligned growth, fiber coverage, and coverage over laminate) and were additionally characterized for diameter and distribution by TEM. Aligned CNTs grow radially and densely on
Fabrication of the hybrid architecture is first described, followed by characterization of the laminates and particularly the aligned CNTs that modify the baseline laminated material system. 2.1. Materials Commercially available thick (0.75 mm, area density of 900 g m2) alumina fiber cloth woven in a 0°/90° satin-weave was used as the base composite fiber. Alumina cloth was chosen as it is an inexpensive and readily-available material that can withstand the 750 °C growth temperature (vs. glass fibers), and is compatible with the particular CNT growth chemistry employed. Alumina-polymer composites have been studied for various applications, including enhanced strength at low-temperatures over glass-thermoset systems [16], and currently in integral armor [17] concepts. The alumina fibers are 11 lm in diameter, and each tow consists of 3 K fibers, yielding a dry alumina cloth volume fraction of approximately 65%. Two room-temperature curing thermosets with different viscosities were used to assess any differ-
2.2. Carbon nanotube synthesis and characterization
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Fig. 3. Woven alumina cloth used in laminate fabrication: (A) As-received without CNTs; and (B) With aligned CNTs grown radially from the fiber surfaces, and an individual fiber covered with CNTs. Scale bars on bottom images in A and B are 20 lm; (C) Aligned CNT coverage over multiple fibers in a tow.
the surface of each individual fiber within the tows of the cloth (Fig. 3B, bottom). The surfaces of the alumina fibers before and after CNT growth can be compared in Figs. 3A and B. In contrast to prior work where CNTs are grown on the surface of fibers (e.g., [2]), the CNTs here are extremely long, dense, and aligned (see Fig. 3B and C). SEM inspections reveal that coverage was uniform over the outer surfaces inspected at the given CVD growth conditions, except near the cloth edges where cloth-cutting irregularities resulted in irregular CNT growth. All laminates were trimmed of these irregular edge regions after the composites were cured. Aligned CNTs are noted to grow on all of the fibers, but not always in a uniform radial pattern. Rather, especially for longer CNT lengths, CNTs on an individual fiber tend to separate into 2
or 3 micro-forests as can be see in Fig. 3B bottom and Fig. 3C. CNTs were prepared for TEM by first removing them from the alumina cloth by sonication in 2-propanol, and the removed CNTs were deposited over a copper grid to be inspected under TEM. The TEMs revealed that the CNTs grown on the alumina fibers are multiwalled with a typical outer diameter of 17 ± 2 nm and 8 concentric walls as shown in the example TEMs in Fig. 4. 2.3. Laminate fabrication The procedures for preparing the baseline composites (no CNTs) and hybrid composites (CNTs) are identical except that CNTs are first grown on the cloth in the latter case. In the hybrid composite,
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Fig. 4. TEM images of CNTs grown on alumina fibers after removal and sonication to prepare samples: (A) low-resolution image used to gather diameter distribution data; (B) high-resolution image showing the concentric wall-structure of the CNT.
CNTs are as-grown, i.e., they are not surface treated or otherwise functionalized, before being combined with the unmodified epoxy. The composite fabrication follows a simple hand layup procedure. The alumina cloth was soaked in room-temperature curable epoxy and then the plies are stacked on a vacuum table. The number of cloth layers (plies) varied from 1 to 4. Layers of porous Teflon and bleed paper were used to remove excess epoxy during the curing process. A steel caul plate, non-porous Teflon, and a weave of glass fiber covered the assembly to guarantee uniform distribution of the vacuum. A vacuum bag enclosed the whole assembly and the laminates were cured for 9–12 h under vacuum with 200 kPa of pressure (including the caul plate and the vacuum) at an elevated temperature (60 °C) to promote epoxy flow for better wetting and to accelerate the curing time. The cured composite specimens were trimmed using an abrasive-grit band-saw to form rectangular specimens, and the edges were smoothed by hand with decreasing grit sandpaper (1200, 2400, and 4000 grit). Dimensions of the composites were measured with a caliper (resolution of 2.5 102 mm). The variation of the length and the width were maintained within ± 2%. The sizes of both baseline and hybrid composites were limited to 20 mm 40 mm by the CVD furnace. The average ply thickness for the baseline composites (0.6 mm) was slightly thinner than that of the hybrid composites (0.7 mm), reflecting the increase in volume of the weave due to CNT growth pushing the fibers apart (this is discussed further in the next section). Mass and volume fractions of the CNTs were estimated as follows: Each cloth was weighed using a microbalance (resolution of 1 lg) before and after CNT growth. Assuming that the measured mass ratio between the CNTs and the alumina cloth with catalyst does not change during the epoxy curing process, the mass of the alumina fiber cloth with catalyst and CNTs in the composite can be estimated using the final mass of the trimmed composite. The epoxy mass is estimated by subtracting the mass of the cloth and of the CNTs from the measured total composite mass. The volume fractions were calculated using the epoxy densities and an assumed CNT density (1.4 mg mm3) [20]. The CNT mass fraction are found to be 0.5–2.5%, while the CNT volume fraction was estimated to be between 1% and 3% (purposefully varied via CNT growth time to obtain the data on the effect of CNT volume fraction on electrical properties). The volume fraction of alumina fibers in the hybrid and baseline composite was 60%, and that of epoxy was 40%. Possible catalyst mass reduction from the CVD process and handling were not considered in the CNT mass assessment, leaving this CNT estimate conservative. 2.4. Laminate characterization Optical and scanning electron microscopy (SEM) are employed to characterize the morphology of the baseline and hybrid compos-
ites. Electrical conductivity of the laminates in the in-plane and through-thickness directions, and interlaminar shear strength (ILSS), are characterized using standard ASTM procedures. 2.4.1. Laminate electrical resistivities The DC electrical resistivities of the samples were experimentally obtained according to the ASTM standards D257–99 for the insulating baseline composites and D4496-04 for the moderatelyconductive hybrid composites [21,22]. Volume resistivity was measured in both through-thickness and in-plane directions. Silver paint (SPI Flash Dry Silver Paint) was applied to the relevant surfaces of the composite to create non-guarded electrodes. The 4-probe wiring configuration of the test setup was chosen to minimize voltage loss in the circuit, as shown in Fig. 5A. The voltage applied to the samples was cycled 5 times between 0–20 V. The maximum voltage, 20 V, was within the standards specifications (500 V) but kept low to avoid melting and degrading the epoxy due to Joule heating. The measured current-voltage curves were linear (R2 value >0.9) as shown in Fig. 5A, and the resistance was calculated as the plot slope. The resistivity was calculated based on qv ¼ Rvt A, where qv, A, t, and Rv are the volume resistivity, area, thickness, and volume resistance, for the specimen, respectively. Note that t is the thickness of the laminate in the case of a through-thickness test, but is the length of the laminate for an in-plane resistivity test. 2.4.2. Interlaminar shear strength Experiments were performed to compare the short-beam-shear (SBS) strength of composite laminates made of alumina cloth with and without CNTs grown on the alumina fibers. The experiments measure the effectiveness of the CNTs to reinforce the interface of a laminated composite. A standard 3-point bending test [23] is applied to short beam shear (SBS) specimens to measure the interlaminar shear strength of the laminates. An Instron 8848 MicroTester with a calibrated 2-kN load cell was used to apply and measure static loads ranging from 5 N to 2 kN. The resolution is 1 mN for load and 1 lm for displacement. The specimen dimensions were selected following the standard [23] with length (15 mm) equal to 6 times the thickness (2.5 mm, 4 plies thick) and width (5 mm) equal to twice the thickness. The span for the 3-point bending test is equal to 4 times the thickness. The tow direction is ±45° to the laminate/test axis of the specimen. The laminate is subjected to a 3-point bending test at a constant crosshead speed of 1 mm/min. The load is applied until fracture occurs and the fracture load is used in the calculation of the apparent shear strength of the laminate ply interface. Care was taken to ensure that failure occurs at the mid-plane of the laminate following guidance in the standard. The test was stopped when load dropped by 30% or head travel exceeded the specimen nominal thickness.
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30 2 plies, 0.41% Vf
Volt [V]
Linear line fit (R2=0.99)
20
Ammeter Voltmeter 10
A
V Rwire
0 0
0.05
0.1
Current [A]
Sample
Silver paint electrodes
10 mm
Fig. 5. Laminate characterizations: (A) Electrical resistivity 4-probe test illustration and representative voltage-current data for a 2-layer hybrid composite with 0.4% CNT volume fraction; (B) hybrid short-beam-shear test specimen.
Five specimens were tested for each specimen type (baseline without CNTs and hybrid composite with CNTs). The short beam shear m strength is calculated using rSBS ¼ 0:75P , where rSBS is the SBS bh strength, Pm is the maximum load during the test, and b and h are the width and the thickness of the specimen, respectively. This relation is derived assuming Euler-Bernoulli beam theory, which is only approximately correct for a short beam and for the experimental three point bending loading conditions. The woven configurations of fibers/tows make the interlaminar stress state more complex to analyze, however, the SBS test allows for a quantitative relative comparison of the SBS strength of composites with and without CNTs, and has similarly been used by others for precisely this purpose, e.g., [13]. 3. Results and discussion Results of the laminate fabrication are discussed focusing on the effects of CNTs on laminate characteristics, electrical resistance, and short-beam-shear strength. 3.1. Laminate fabrication and characterization Fabricated laminates are characterized via optical and scanning electron microscopy, and differences between laminates with and without CNTs are noted. Aligned CNTs are grown on the surface of woven fibers, both on the surface of the cloth and on fibers in the interior of the cloth. This is a consequence of the specific CNT growth process utilized, which is a base-growth process wherein the CNTs are extruded from catalyst particles that remain on the surface of the fibers. The carbon source is the ethylene gas that penetrates to the interior of the woven cloth to reach the catalyst particles. As provided previously, the catalyst particles are applied to all the fibers in the cloth by dip-coating in a liquid catalyst bath. CNT volume fraction is kept constant for the shear tests (30 lm long CNTs, 2% composite volume fraction, and 4% volume fraction in the epoxy), and it is varied to determine its effect on electrical
properties. CNT volume fraction is not varied in the usual way by decreasing spacing between aligned filaments (here, CNTs), but by changing the CNT length via extending the CVD growth times to achieve 10 lm, 40 lm, and 100 lm long forests giving approximately 0.6%, 2%, and 3% CNT volume fraction in the composite. Details of the specimen dimensions are shown in Table 1, including the number of plies, average laminate thicknesses, and CNT volume fraction in the overall composite and in epoxy. Note that CNT volume fraction is a function both of the CNT length and the overall laminate consolidation. Average ply thickness increases in the hybrid composites slightly at higher CNT lengths (see trend in Table 1 for 2-ply specimens) because the CNT growth pushes apart the woven alumina fibers at longer CNT growth times/lengths. Longer CNT growth times ( 1 h) have been observed to push apart the woven structure of the alumina cloth, ruining the weave. A key finding in the fabrication process concerns the wetting of the woven plies containing the aligned CNTs by epoxy. Optical and scanning-electron microscopy of laminate cross sections reveals that CNTs are present throughout the laminate interior and that there is no difference in observed void fraction (<2%) between laminates with and without CNTs (see Fig. 6). Nor are there differences noted for the two epoxy resins, having different viscosities. The epoxy, which is applied to the top of each ply during the hand layup process, fully wets the alumina fibers containing aligned CNTs. Cross sections of a laminate without (Fig. 6C) and with (Fig. 6D) CNTs at the same scale indicate excellent consolidation and wetting of the hybrid ply by the epoxy. The spacing between the alumina fibers in the hybrid composite is slightly larger than in the baseline due to the CNT growth pushing the fibers apart as discussed above. The dark rings over the edge of alumina fibers in the hybrid composite (see Fig. 6D) are due to SEM charging that creates dark halos at the interface of the insulating fibers and conductive CNT-epoxy matrix, i.e., the halos obscure the outer diameter of the fiber cross sections. Visualization of individual CNT morphology inside the hybrid laminate has been unsuccessful. Fo-
Table 1 Summary of fabricated baseline and hybrid laminates
SBS tests Electrical tests
# plies (CNT length)
Baseline thickness (mm)
Hybrid thickness (mm)
CNT Vf in composite (%)
CNT Vf in epoxy (%)
4 1 2 2 2 3
2.2 ± 0.1 0.8 1.2 ± 0.05 – – 2.0
2.1 ± 0.1 0.6 1.3 1.4 ± 0.3 1.8 1.8
2 1.2 0.6 2.3 ± 0.02 2.9 2.2
4 1.4 1.5 5.9 ± 0.2 5.6 9.2
(30 lm) (40 lm) (10 lm) (40 lm) (100 lm) (40 lm)
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Fig. 6. Hybrid composite specimen characterization using West Systems epoxy with 2% Vf of CNTs: (A) 3-ply laminate after trimming; (B) SEM image of woven cloth cross section showing fibers in tows reinforced with CNT/epoxy; SEM cross-sectional images of; (C) no-CNT baseline composite and; (D) CNT-reinforced hybrid composite indicating effective wetting (lack of voids) and good fiber distribution after the vacuum curing.
cused-ion-beam (FIB) preparation, cryogenic microtome, and polishing to create TEM samples have failed due to the stiffness and hardness differences between the alumina fibers and CNT-epoxy matrix. The mechanism underlying epoxy wetting of the hybrid laminate interior is next considered. The epoxy is introduced at the top of a ply, and needs to penetrate the ply thickness (0.6 mm) through the dense array of alumina fibers (65% volume fraction) covered with dense forests of CNTs (2 billion CNTs cm2). Given the difficulty of dispersing and wetting CNTs in epoxy, capillaritydriven wetting of the aligned CNT forests is postulated as the underlying mechanism. Previous work with solvents has demonstrated a strong capillary effect of aligned CNT forests [24–27]. Our previous work with complex thermosets, including epoxies
with similar viscosities as those used here, has demonstrated that long (exceeding 0.5 mm) aligned CNT forests are readily and effectively (i.e., no micron-scale voids) wet [14]. Similar wetting behavior has been observed when forming silver-epoxy electrodes for testing of CNT electrical probes [28]. Further, the preferred route for this wetting was observed to be along the axis of the CNT forest, as illustrated in Fig. 7A. It is proposed that the wetting of the interior portions of the laminate are due to capillarity-driven wetting along the aligned CNT axes, with secondary (and less effective, slower) polymer dispersion orthogonal to the aligned CNTs as illustrated in Fig. 7B. This mechanism needs further investigation, perhaps by purposefully growing tangled, rather than aligned, CNTs on the surface of the fibers and performing a relative cross-sectional characterization of tangled vs. aligned CNT hybrid laminates.
Fig. 7. Routes for polymer wetting of the CNTs in the hybrid composite: (A) High-resolution SEM of aligned CNT forest indicating preferred wetting route via capillary action, scalebar = 1 lm; (B) Illustration of CNT wetting in the interior of the composite.
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3.2. Laminate in-plane and through-thickness electrical resistivities
3.3. Interlaminar shear strength A short beam shear test (essentially a 3-point bending test with a short span) was applied (see Fig. 5B) to assess any differences in the baseline and CNT hybrid laminates. The test was performed on 4-ply laminates at a constant crosshead speed, and the fracture load and mode were used to determine the interlaminar shear strength (ILSS) of the laminates. The hybrid laminate showed an improvement of 69% with respect to the unreinforced laminate (see Table 3). Fracture surface inspection of the specimens tested did not reveal any mechanistic understanding of the reinforcement due largely to the small specimen size. Ongoing work is exploring this mechanism via Mode I and II fracture tests that should yield a larger and cleaner fracture surface for inspection. The increase in interlaminar shear strength observed here compares favorably with existing interface reinforcement techniques
Electrical Resistivity [Ohm mm]
Electrical resistance was acquired with a 4-probe current/voltage measurement (see Fig. 5A). Two DC electrical parametric studies were conducted, the first to consider the effect of ply thickness (1, 2, and 3 plies) at constant CNT volume fraction (40 lm of CNT growth), and the second to investigate the effect of increased CNT volume fraction irrespective of ply thickness. Results from the first parametric study are shown in Table 2. The resistance decrease in both the in-plane and through-thickness direction is clear, indicating the effect of the CNTs on this bulk laminate property. The baseline composites, regardless of thickness, are insulating and have an in-plane resistance of 107–108 Ohm mm and a through-thickness resistivity of 109 Ohm mm, regardless of laminate thickness. The in-plane resistivity is expected and observed to be lower (5–400) than the through-thickness value due to the lower resistivity and continuous nature of the alumina fibers (vs. a discontinuous path across ply interfaces) in the laminate plane vs. the more insulating (10 resistivity of alumina) epoxy [29]. Due to the aligned CNTs present in the polymer matrix, electrical resistance of the laminate decreases by several orders of magnitude at low (1–3%) CNT volume fractions, for both in-plane and through-thickness directions, to values on the order of 101– 102 Ohm mm, as shown in Fig. 8. A direct-path network of aligned CNTs was observed with 0.5% CNT volume fraction (clearly above the percolation threshold in pure epoxy). Such an improvement is expected, as plastics realize significantly enhanced conductivity at low loadings of unoriented CNTs [30–32]. Further, the decrease in through-thickness resistivity is two orders of magnitude greater than the in-plane value. This is attributed to the CNT orientation in the through-thickness direction as well as CNT bridging of the insulating epoxy interlaminar region between plies that contributes to the high through-thickness resistivity of the baseline laminate. The nanostructure, specifically orientation of the CNT long axis, in concert with the woven fiber microstructure, clearly plays an important role in determining the effective laminate resistivities.
1.E+10
In-Plane 1.E+08
Through-Thickness
1.E+06 1.E+04 1.E+02 1.E+00 0
0.5
1
1.5
2
2.5
3
CNT Volume Fraction [%] Fig. 8. Electrical resistivity decrease with increasing CNT volume fraction, both in through-thickness and in-plane directions with 1–3 plies of 0–3% Vf of CNTs.
Table 3 Summary of interlaminar strength for the baseline and hybrid laminates Property
Baseline
CNT-reinforced
Change
Interlaminar shear strength (MPa)
20.1 ± 0.9
33.8 ± 1.1
+69%
such as stitching that typically shows a maximum of 30% increase, and only 10% increase at comparable stitch densities to that used here with the CNTs [33]. Work by others using randomly oriented carbon nanofibers (NFs) and CNTs placed between woven plies using different techniques (additional CNT/epoxy layer [34], spraying [35], and unaligned CNTs mixed between tows [36]) showed limited or no reinforcement. Recently, work with electrophoretically-deposited CNTs demonstrated enhanced ILSS to a maximum of 30% increase. Other recent work with functionalized single-wall CNTs sprayed on the surface of glass cloth showed a maximum of 45% increase in SBS (interlaminar shear) strength. Last, other work [11] has demonstrated significant interlaminar toughening in Mode I and II by growing CNTs on the exterior surface of SiC cloth plies. Advances in processing of nanostructures at ply interfaces are yielding improved laminate-level properties, particularly interfacial properties because this interface is more accessible during laminate manufacture than the interior of the ply/laminate. In this work, because aligned CNTs are present in the interior of the laminate as well, significant intralaminar effects from the CNTs are expected (and observed for electrical conductivities as discussed above). 4. Conclusions A hybrid laminate is presented that demonstrates simultaneous enhancement of mechanical and electrical properties. The hybrid laminate is fabricated with a simple hand layup process, and alignment of the in situ grown CNTs on the surface of the advanced fi-
Table 2 Summary of laminate-level resistivities for the baseline and hybrid laminates # plies (CNT length)
1 2 2 2 3
(40 lm) (10 lm) (40 lm) (100 lm) (40 lm)
CNT volume fraction (%)
1.2 0.6 2.3 2.9 2.2
In-plane resistivity (Ohm mm)
Through-thickness resistivity (Ohm mm)
Base
Hybrid
Base
Hybrid
4.4 106 (7.5 ± 0.4) 106 – – 1.3 107
8.4 1.3 102 8.1 ± 1.5 9.7 8.6
– (2.5 ± 0.8) 109 – – –
6.8 101 5.3 102 8.3 101 6.5 101 6.8 101
E.J. Garcia et al. / Composites Science and Technology 68 (2008) 2034–2041
bers is used to explain polymer wetting in the interior of the laminate via capillary action. The specific alumina fiber reinforced plastic system considered has some direct applications (such as a front-facing material for integral armor applications), however, it is primarily a model fiber/composite, allowing fabrication, synthesis, and processing investigation as well as relative comparisons of properties with and without CNTs. Insights from the performance of this system will be useful in developing aligned-CNT carbon and glass fiber reinforced plastics (FCFRP and FGFRP). Such composites would be attractive for a host of structural applications. Characterization and modeling of the enhanced properties afforded by the VACNT-FRP architecture should be extended and broadened to include targeted improvements and tailoring of properties such as thermal conductivity [36], current carrying capacity, damping, fire and heat resistance, and wear resistance. Importantly, this architecture needs to be extended to carbon–fiber based advanced composites that form the majority of advanced composites used in aerospace and other high-performance applications. The woven ceramic architecture investigated here has given significant insight into wetting of in situ grown CNTs, laminate consolidation, and relative property improvements, suggesting that carbon–fiber based versions are achievable and can have significantly improved ‘nano-engineered’ laminate properties if aligned CNTs are grown in situ on the fibers. Given the interlaminar strength improvements observed, it is expected that Mode I toughness and other interlaminar properties will improve as well. Significant modeling and experimentation are required to determine optimal characteristics of CNTs for the multifunctional architecture, including length, diameter, volume fraction, and effects of covalent and non-covalent CNT-polymer interactions. Fabrication studies to create scalable and continuous processes for this and similar architectures would enable large-scale hybrid nano-engineered composites for a variety of applications. Acknowledgements This work was supported by Airbus S.A.S., Boeing, Embraer, Lockheed Martin, Saab AB, and Spirit AeroSystems, and Textron Inc. through MIT’s Nano-Engineered Composite aerospace Structures (NECST) Consortium and by MIT’s Karl Chang (1965) Innovation Fund. The authors gratefully thank John Kane and the entire Technology Laboratory for Advanced Materials and Structures (TELAMS) at MIT for valuable discussions and technical support, Sunny Wicks for fabrication assistance, and Dr. Alex Slocum (MIT ME) for valuable input. Enrique Garcia would like to acknowledge support from the La Caixa Foundation, John Hart from the Fannie and John Hertz Foundation, Sunny Wicks from MIT’s Paul E. Gray (1954) Undergraduate Research Opportunity Fund, and Namiko Yamamoto from the Linda and Richard (1958) Hardy Fellowship. References [1] Ajayan PM, Tour JM. Nanotube composites. Nature 2007;447:1066–8. [2] Thostenson ET, Li WZ, Wang DZ, Ren ZF, Chou TW. Carbon nanotube/carbon fiber hybrid multiscale composites. J Appl Phys 2002;91(9):6034–7. [3] Vaia RA, Wagner HD. Framework Nanocompos Mater Today 2004;32:32–7. [4] Iijima S. Helical microtubules of graphitic carbon. Nature 1991;354:56–8. [5] Schulte K, Windle AH. Editorial. Compos Sci Technol 2007;67:777. [6] Coleman JN, Khan U, Blau WJ, Gun’ko YK. Small but strong: a review of the mechanical properties of carbon nanotube–polymer composites. Carbon 2006;44:1624–52. [7] Thostenson ET, Reng Z, Chou TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61:1899–912.
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