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Multiscale Carbon Nanotube-Carbon Fiber Reinforcement for Advanced Epoxy Composites E. Bekyarova,† E. T. Thostenson,‡ A. Yu,§ H. Kim,| J. Gao,† J. Tang,§ H. T. Hahn,| T.-W. Chou,‡ M. E. Itkis,§ and R. C. Haddon*,§ Carbon Solutions Inc., RiVerside, California 92507, Department of Mechanical Engineering and Center for Composite Materials, UniVersity of Delaware, Newark, Delaware 19716, Mechanical and Aerospace Engineering Department, UniVersity of California, Los Angeles, California 90095, and Center for Nanoscale Science and Engineering, Departments of Chemistry and Chemical & EnVironmental Engineering, UniVersity of California, RiVerside, California 92521 ReceiVed September 20, 2006. In Final Form: December 29, 2006 We report an approach to the development of advanced structural composites based on engineered multiscale carbon nanotube-carbon fiber reinforcement. Electrophoresis was utilized for the selective deposition of multi- and singlewalled carbon nanotubes (CNTs) on woven carbon fabric. The CNT-coated carbon fabric panels were subsequently infiltrated with epoxy resin using vacuum-assisted resin transfer molding (VARTM) to fabricate multiscale hybrid composites in which the nanotubes were completely integrated into the fiber bundles and reinforced the matrix-rich regions. The carbon nanotube/carbon fabric/epoxy composites showed ∼30% enhancement of the interlaminar shear strength as compared to that of carbon fiber/epoxy composites without carbon nanotubes and demonstrate significantly improved out-of-plane electrical conductivity.
1. Introduction The use of fiber-reinforced high-performance composites has expanded substantially in advanced aerospace systems because the fiber architecture can be designed to meet the final performance requirements. Furthermore, 2-D and 3-D textile materials can be formed into the final net-shape of the desired product.1,2 Extensive research and development in carbon fiber-reinforced composites led to remarkable improvements in the performance of the system, which exhibits excellent in-plane properties. However, these composites typically show poor out-of-plane performance that is dominated by the polymer matrix. Another critical drawback in textile structural composites is the presence of matrix-rich regions formed in the gaps between the interlaced fiber bundles. These regions, where cracks easily initiate and propagate, are difficult to reinforce with traditional microscale fiber reinforcement. Various nanoscale materials have been explored for selective reinforcement of matrix-rich regions with limited success. It has been suggested that carbon nanotubes (CNTs) are the ideal candidate for selective reinforcement of matrix-rich interlaminar regions3,4 because of their nanoscale diameter and outstanding mechanical, electrical, and thermal properties. This approach benefits from the microscale reinforcement provided by traditional fibers and from the complementary reinforcement on the nanoscale offered by carbon nanotubes. It has been demonstrated that the combination of traditional carbon fibers with carbon nanotubes led to enhanced fiber/polymer interfacial load transfer.3 Recently, it was reported that the use of epoxy * Corresponding author. E-mail:
[email protected]. † Carbon Solutions Inc. ‡ University of Delaware. § University of California, Riverside. | University of California, Los Angeles. (1) Chou, T. W.; Ko, F. Textile Structural Composites; Elsevier: Amsterdam, 1988; Russian Edition, Moscow. (2) Chou, T. W. Microstructural Design of Fiber Composites; Cambridge University Press: Cambridge, U.K., 1992. (3) Thostenson, E. T.; Li, W. Z.; Wang, D. Z.; Ren, Z. F.; Chou, T. W. J. Appl. Phys. 2002, 91, 6034-6037.
composites reinforced with multiwalled carbon nanotube-coated 2-D woven fabric of SiC led to dramatically enhanced out-ofplane mechanical and electrical properties.5 It is important to note that previous studies have employed chemical vapor deposition (CVD) to deposit CNTs on the fiber surface. Although the CVD process is an efficient technique for the growth of CNTs on a variety of surfaces, the use of high temperatures and predeposited catalysts, taken together with the difficulties in processing large panels, imposes serious limitations on the practical application of this technique for the fabrication of carbon nanotube-reinforced structural composites. Furthermore, high-temperature processing with CVD removes any sizing that may be applied to the fiber during manufacturing, and the CVD reaction may also degrade the fiber strength. In this study, we demonstrate that electrophoresis is an efficient technique for the deposition of CNTs on the surface of carbon fibers (CFs) and that the nanoengineered CNT/CF structures may be utilized for the reinforcement of structural composites. With this process, deposition can be accomplished with or without the removal of fiber sizing. Electrophoresis is a simple and versatile technique that can be readily automated and utilized for industrial applications. The fabricated CNT/CF preforms were successfully infiltrated with epoxy resin by vacuum-assisted resin transfer molding (VARTM). The manufactured multiscale hybrid composites reinforced with carbon nanotube-coated carbon fabric showed ∼30% enhancement of the interlaminar shear strength, and importantly, they have preserved in-plane mechanical properties. In addition, the composites showed significantly enhanced outof-plane electrical conductivity. 2. Experimental Section Two types of carbon nanotubessmultiwalled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs)swere used for the electrophoretic deposition. The MWNTs were produced by (4) Thostenson, E. T. Carbon Nanotube-Reinforced Composites: Processing, Characterization and Modeling. Ph.D. Thesis, University of Delaware, 2003. (5) Veedu, V. P.; Cao, A. Y.; Li, X. S.; Ma, K. G.; Soldano, C.; Kar, S.; Ajayan, P. M.; Ghasemi-Nejhad, M. N. Nat. Mater. 2006, 5, 457-462.
10.1021/la062743p CCC: $37.00 © 2007 American Chemical Society Published on Web 02/28/2007
Nanotube-Carbon Fiber Reinforcement for Composites
Langmuir, Vol. 23, No. 7, 2007 3971 The in-plane and the out-of-plane electrical conductivities were obtained from four-point measurements using an Agilent 4263B LCR meter, and the electrical measurements were conducted on specimens with dimensions of 14 mm × 25 mm × 1.2 mm (width × length × thickness). The surface of the samples was polished to expose the fibers and to ensure good contact with the silver paint electrodes. Measurements were performed on four specimens from each set of composites.
3. Results and Discussion
Figure 1. Deposition of carbon nanotubes on a carbon fiber surface by electrophoresis. a CVD process that uses xylene as a carbon source and ferrocene as a catalyst.6 The as-grown MWNTs with an average length of ∼10-20 µm were refluxed in nitric acid to cut the nanotubes into smaller fragments with an average length of ∼2-6 µm and also to introduce carboxylic acid groups, which facilitate the dispersion of MWNTs in aqueous media. The SWNTs are purified electric arcproduced SWNTs (P3-SWNT, Carbon Solutions Inc., www.carbonsolution.com), which are functionalized with carboxylic acid groups.7,8 The relative carbonaceous purity of the SWNTs is 90% as estimated by the solution-phase near-IR (NIR) spectroscopic technique using reference sample R2.9 The nanotube materials were dispersed in water by ultrasonication (bath sonicator, sonic power 270 W) to obtain dispersions of 0.05 mg/mL. The carbon fiber used in this study was Magnamite IM7 (Hexcel Corporation), a high-performance, intermediate modulus carbon fiber utilized extensively in aerospace structures. For the electrophoretic deposition, carbon fabric IM7 (10 cm × 15 cm) was fixed in a stainless steel frame, and two stainless steel plates were positioned on both sides of the carbon fabric as counter electrodes. The carbon fabric was immersed in the nanotube dispersion, and a positive potential of 10 V/cm was applied to the carbon fabric (Figure 1). The carbon nanotube-carbon fiber preforms were infiltrated with epoxy using vacuum-assisted resin transfer molding (VARTM, Figure 2). The epoxy resin (EPON 862, Hexion Specialty Chemicals, Inc.) was mixed with the curing agent (Epi-Kure W, Hexion Specialty Chemicals, Inc.) at 100/26.4 parts by weight. In the VARTM process, illustrated in Figure 2a, the carbon nanotube/carbon fiber preform is sealed in a vacuum bag, and the epoxy resin is then infused into the carbon fibers under vacuum. After the infiltration of epoxy, the composites were cured for 6 h at 130 °C. The scanning electron microscopy (SEM) images of the asdeposited carbon nanotubes on the carbon fiber were taken with a Philips XL-30 microscope at 20 kV, and the images of the multiscale hybrid composites were taken with a JEOL JSM-7400F microscope at 3 kV. The samples were coated with Pt/Au by sputtering. The interlaminar shear strength was measured using the shortbeam method (ASTM D 2344) by testing small specimens in threepoint flexure at a span-to-thickness ratio of 4 to promote shear failure between the plies. A minimum of six specimens were tested from each set of composites. The tensile tests (ASTM D-3039) were performed on four specimens from each set of composites. (6) Sen, R.; Govindaraj, A.; Rao, C. N. R. Chem. Phys. Lett. 1997, 267, 276280. (7) Hu, H.; Yu, A.; Kim, E.; Zhao, B.; Itkis, M. E.; Bekyarova, E.; Haddon, R. C. J. Phys. Chem. B 2005, 109, 11520-11524. (8) Yu, A.; Bekyarova, E.; Itkis, M. E.; Fakhrutdinov, D.; Webster, R.; Haddon, R. C. J. Am. Chem. Soc. 2006, 128, 9902-9908. (9) Itkis, M. E.; Perea, D.; Niyogi, S.; Rickard, S.; Hamon, M.; Hu, H.; Zhao, B.; Haddon, R. C. Nano Lett. 2003, 3, 309-314.
3.1. Electrophoretic Deposition of Carbon Nanotubes on a Carbon Fiber Surface. The developed method for the deposition of carbon nanotubes (CNTs) on the surface of carbon fibers by the use of electrophoresis is based on the ability of carbon nanotubes to respond to an electric field. This behavior of CNTs has been widely explored for the separation of metallic and semiconducting nanotubes,10 purification,11 and the assembly of functional structures and fibers.12-14 Uniform carbon nanotube films have been successfully deposited on conducting metallic surfaces by applying an electric field.15 The electrophoretic deposition process allows the formation of uniform films on objects with complex shapes and rough surfaces. We found that carbon nanotubes were deposited uniformly on the surface of a carbon fiber from aqueous nanotube dispersions upon applying a dc potential between the carbon fiber and the counter electrodes. Because of their negative charge, carbon nanotubes migrate toward the positive carbon fiber electrode and are subsequently deposited on the fiber surface (Figure 1). The negative charge is attributed to the carboxylic acid groups introduced into the nanotubes during the nitric acid treatment,7,8 and the adsorption of hydroxyl anions from the dispersion may also contribute to the net negative charge.15 We studied the effect of the pretreatment of the carbon fabric on the electrophoretic deposition. In the first experiments, before the electrophoretic deposition of carbon nanotubes, the carbon fabric was annealed at 700 °C in vacuum to remove the polymer sizing applied to the fabric, followed by oxidative treatment that consists of refluxing in nitric acid. The oxidative treatment aims at introducing functional groups, removing the weak outer layers of the fiber, and texturing the fiber surface. Thus, the pretreatment is anticipated to impart hydrophilicity to the carbon fiber and increase the interfacial area, which is expected to facilitate the deposition of carbon nanotubes. In addition, the oxidative treatment is known to increase the fiber surface roughness; strong oxidative conditions can result in nonuniform etching of the carbon fiber and loss of fiber tensile strength. In these experiments, we examined the effect of refluxing the fibers in concentrated 16 M nitric acid for 30 min and in 7 M HNO3 for 1 h. SEM observations showed that these treatments did not damage the surface of the carbon fiber and the difference in the oxidative treatments did not affect the electrophoretic deposition of carbon nanotubes. In another experiment, we deposited MWNTs on the surface of an as-received carbon fabric. The results indicate that the pretreatment of the carbon fabric has no effect on the deposition of carbon nanotubes on the carbon fiber surface by electrophoresis; (10) Krupke, R.; Hennrich, F.; v. Lohneysen, H.; Kappes, M. M. Science 2003, 301, 344-347. (11) Xu, X. Y.; Ray, R.; Gu, Y. L.; Ploehn, H. J.; Gearheart, L.; Raker, K.; Scrivens, W. A. J. Am. Chem. Soc. 2004, 126, 12736-12737. (12) Gommans, H. H.; Alldredge, J. W.; Tashiro, H.; Park, J.; Magnuson, J.; Rinzler, A. G. J. Appl. Phys. 2000, 88, 2509-2512. (13) Tang, J.; Zhou, O. AdV. Mater. 2003, 15, 1352-1355. (14) Zhang, J.; Tang, J.; Yang, G.; Qiu, Q.; Qin, L. C.; Zhou, O. AdV. Mater. 2004, 16, 1219-1222. (15) Gao, B.; Yue, G. Z.; Qui, Q.; Cheng, Y.; Shimoda, H.; Fleming, L.; Zhou, O. AdV. Mater. 2001, 23, 1770-1773.
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Figure 2. (a) Schematic and (b) photograph illustrating the infiltration of the carbon nanotube/carbon fiber performs with vacuum-assisted resin transfer molding (VARTM).
Figure 4. SEM micrograph of an epoxy composite fabricated with SWNTs/carbon fabric reinforcement showing carbon nanotubes in the narrow matrix-rich regions between the fibers within a fiber bundle in the multiscale hybrid composite. (Arrows point to some of the many nanotubes.)
Figure 3. SEM images of carbon fibers (CFs) with (a) SWNTs and (b, c) MWNTs deposited by electrophoresis. The CFs shown in panels a and b were annealed in vacuum and treated with nitric acid, whereas the CFs in panel c were used as-received.
in all cases, we observed a homogeneous deposition of both SWNTs and MWNTs on the surface of the carbon fabric (Figure 3). The electrophoretic deposition of the nanotubes was completed after 2 h. To facilitate the deposition of carbon nanotubes, sodium hydroxide (
