Letter pubs.acs.org/NanoLett
High-Strength Carbon Nanotube Film from Improving Alignment and Densification Wei Xu,† Yun Chen,‡ Hang Zhan,† and Jian Nong Wang*,† †
Nano Carbon Research Center, School of Mechanical and Power Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China ‡ School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dong Chuan Road, Shanghai 200240, P. R. China S Supporting Information *
ABSTRACT: A new method is reported for preparing carbon nanotube (CNT) films. This method involves the continuous production of a hollow cylindrical CNT assembly and its condensation on a winding drum. The alignment and densification of CNTs in the film are improved by controlling the winding rate and imposition of mechanical rolling, respectively. The prepared film has a strength of 9.6 GPa, which is well above those for all other man-made films and fibers. KEYWORDS: carbon nanotube film, high strength, alignment, densification, rolling
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By contrast, a CNT sheet is directly assembled and collected at the end of a reactor where the nanotubes are synthesized using a floating catalyst chemical vapor deposition (CVD) method.23−25 The nanotubes are randomly aligned in the sheet, resulting in a high tensile strain but a low strength. A simple mechanical stretching method was used to improve the CNT alignment,26,27 and two-time stretching and pressing to improve both the CNT alignment and density of the sheet.28 Stretching treatment induced discontinuous CNT ends and weak nanotube interactions and thus easy fracturing under tensile force, but pressing treatment tended to eliminate the gaps between CNT layers and thus increase the intertube interactions.28 Although great efforts have been made, all the film materials (including CNT fibers) developed so far exhibited much lower properties (e.g., σb ≤ 2 GPa) than those for individual CNTs.29,30 As a matter of fact, it has appeared to be a big challenge to transfer the microscopic outstanding properties to a macroscopic scale. If this issue is not solved, many projected applications remain elusive. In this study, a new method is proposed for preparing CNT films. We use a floating catalyst approach to produce CNTs and their macroscopic assembly in a form of hollow cylinder continuously. The cylindrical assembly is condensed and deposited on a winding drum to form a CNT film. We use the winding rate and mechanical rolling to improve the CNT alignment and densification. This results in a film with well-aligned and highly packed CNTs and thus a tensile strength of 9.6 GPa, the highest ever reported for
arbon nanotubes (CNTs) have attracted tremendous research since the landmark paper published in the early 1990s1 due to their remarkable mechanical, electrical, and thermal properties. For example, both theoretical calculations2−5 and direct measurements6−9 have suggested superb mechanical properties, such as a tensile strength σb of 13−150 GPa, Young’s modulus (or stiffness) (E) of 200−1500 GPa, and tensile elongation (δ) of ∼15−30%. The fabrication of macroscopic assemblies such as films and fibers is an important step toward real applications. To date, several methods have been proposed for preparing CNT films or, in the case of a larger thickness, sheets and for improving their properties. The solution processes such as vacuum filtration10,11 and solution spraying12 contains two steps, namely the fabrication of nanotubes and combining them into an integrated macroscopic film. Pressing treatment was demonstrated as an effective means to densify the vacuumfiltrated buckypapers.13 This method is limited to short CNTs, and the residual micromolecular surfactant on the tube surface, random orientation, and weak intertube van der Waals interaction always lead to low mechanical and physical properties. Another approach to fabricating a CNT film is based on a nanotube array by domino-pushing, shear pressing and dry drawing.14−16 The dry-drawing from superaligned CNT arrays gives good mechanical, electrical, and thermal properties.17−19 Recently, many efforts such as liquid shrinking and tension were devoted to improve the density and alignment degree.20−22 However, the productivity of the nanotube array is low and the volume production of the nanotube sheet is limited by the corresponding array. © XXXX American Chemical Society
Received: September 23, 2015 Revised: December 28, 2015
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DOI: 10.1021/acs.nanolett.5b03863 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Experimental setup and results. (a) Schematic illustration. Reaction solution is sprayed into a tube reactor and pyrolysized to form a hollow CNT cylinder which is then condensed and deposited on a winding drum. (b) Photograph of a hollow CNT cylinder being blown out from the reactor. (c) Whole film removed from the substrate. (d) Small piece cut from the film.
Figure 2. CNT structure characterization. (a) TEM image of CNTs contained in the film. (b) HR-TEM image showing CNTs with a double-walled structure. (c) Raman spectrum of a CNT film, showing G and D peaks and their intensity ratio. (d) RBM spectrum which was used to determine the diameter of CNTs.
CNT macroscopic assemblies and other fibrous and film materials. Using the experimental setup shown in Figure 1a, a hollow cylinder-like CNT assembly formed on the inner side wall of
the reactor in the low temperature region and was driven out from the reactor by the enclosed N2 (Figure 1b). The floating CNT cylinder deposited as a narrow CNT film on the substrate surface. As the winding drum rotated, it also underwent B
DOI: 10.1021/acs.nanolett.5b03863 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. CNT orientation characterization. The orientations seen from SEM images (samples with deposition of a few layers of CNT cylinders) (a−c) from G peak intensity ratios from polarized Raman spectra along longitudinal and transverse directions, and from ultimate load ratios from tensile testing along longitudinal and transverse directions for unaligned (a,d,g), slightly aligned (b,e,h), and well-aligned samples (c,f,i).
reciprocating lateral motion, resulting in a CNT film with a preset width on the substrate (Figure 1c,d). Avoiding uses of H2 carrier gas and vacuum or sealed system enabled us to carry out the winding process in the open air environment. The cylindrical assembly contained CNTs, mainly doublewalled, and a low content of Fe particles (Figure 2a,b). Raman microscopy showed that the intensity ratio (IG/ID) of G band peak at 1585 cm−1 to D band peak at 1354 cm−1 was 2.74, indicating that the CNTs possessed a high degree of graphitization (Figure 2c). On the basis of the radial breathing mode (RBM) Raman spectrum (Figure 2d), the diameter of the CNTs was estimated to be about 2−4 nm, being consistent with TEM observations. Efforts had been made on tracking the CNT length in TEM and SEM, but no clear results were obtained. However, close inspection of the CNT traces suggested a length larger than 10 μm (Figure S1 in Supporting Information). The CNT alignment in the film is of primary concern. First, we used a horizontal plate substrate for deposition of the CNT cylinder and regularly rotated the substrate to induce the deposition at random orientations (see Experimental Meth-
ods). Then, we used the winding rate to control the microstructure of CNT alignment in the film deposited on the winding drum. A slow winding rate of 4 m min−1 was used for slightly aligned CNTs, and a fast rate of 20 m min−1 for obtaining well-aligned CNTs (See Videos S1 and S2 in Supporting Information for these two winding rates). SEM examination illustrated the increasing degree of CNT alignment from the randomly, to the slightly, and to the well-aligned samples (Figure 3a−c). To quantify the difference of CNT alignment among the three samples, we applied polarized Raman spectroscopy as the intensity of the G band is sensitive to the orientation of the optical electric field with respect to the nanotube axis.31 The intensities of the G-band were recorded when the incident laser beam was respectively placed parallel and perpendicular to the winding direction of the CNT film, and their ratio (IG∥/IG⊥) was adopted to describe the degree of CNT alignment. Results show that the intensity ratios were 1.0, 1.25, and 1.6, respectively, for the randomly, slightly, and well-aligned samples, suggesting an increase of the alignment (Figure 3d− f). Although the random alignment resulted from the regular C
DOI: 10.1021/acs.nanolett.5b03863 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 4. Mechanical property characterization. Thickness measurements and stress vs strain curves from loading along the longitudinal direction. (a,b) Unaligned sample. (c,d) Slightly aligned sample. (e,f) Well-aligned sample.
each sample determined, the stress vs strain curve was constructed for the tensile testing along the longitudinal direction. It was seen that as the alignment degree increased, the tensile strength increased significantly at the direction parallel to the CNT alignment (from 0.277 → 0.566 → 2.8 GPa). Aligning individual CNTs to the sample length ensures a high direct transfer of the axial load of CNTs to the sample. This is closely related to the present winding; the fast removal of the CNT cylinder from the reactor and its fast spinning on the winding drum are all critical to deposition of well-aligned CNTs. Besides, CNT alignment was achieved in situ while the film was being deposited. This is different from previous methods such as stretching the film with randomly oriented CNTs as the film could be expanded at the thickness direction
rotation of the deposition of the CNT cylinder on the plate substrate, the other two should be ascribed to the waviness and misorientation of CNTs self-assembled in the hollow cylinder. That is, at a faster winding rate, more CNTs tended to be straightened and aligned along the gas stream or the winding direction. To study the anisotropy of the film, we performed tensile testing along the longitudinal (∥ to winding) and transverse (⊥ to winding) directions, respectively. The ultimate load ratios (Lm∥/Lm⊥) were 0.94, 1.50, and 4.7, respectively, for the randomly, slightly, and well-aligned samples (Figure 3g−i). This is another piece of evidence for the significant improvement of CNT alignment and thus the load bearing capability at the longitudinal direction with respect to the transverse direction among the three samples. With the thickness for D
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Figure 5. Tensile testing results along the longitudinal direction for the well-aligned sample after rolling. (a) Thin thickness seen from the cross section. (b) Stress vs strain curves showing a high tensile strength. (c) Ribbon sample after fracturing; (d, e) Fracture surface visualized at high magnification.
operated under a no-gapping condition (no gap between the top and bottom rollers, the tightest rolling condition), the film exhibited no damage and remained its original integrity from visualization by naked eyes. Under the examination of SEM, the thickness was found to have been reduced from 550 to 120 nm (Figure 5a), and the film surface became much smoother than before (Figure S2). Corresponding to the thickness reduction as well as a slight increase in film width by ∼20%, the film density was increased from ∼0.53 to 1.85 g cm−3, as estimated by weighing the film with a given width, thickness, and length. A number of tensile tests gave rise to a strength range of 8.0 to 10.8 GPa with an average of 9.6 GPa, a Young’s modulus range of 110−190 GPa with an average of 130 GPa, and a tensile elongation range of 6−10.5% with an average of 8% (Figure 5b). This strength is much higher than that before rolling (2.8 GPa) although the rolling reduced the ultimate load by 17% (3 → 2.5 N, Figure S3), perhaps resulting from the damage of some CNTs induced by the rolling force. It is apparent that the high strength resulted directly from the significant reduction of the sample thickness and enhancement of the packing density by rolling. Previous CNT fibers, especially those produced from CNT aerogels and arrays usually had a low packing density, although liquid shrinking and twisting were often used for densification.32,33 Liquid shrinking cannot make uniform contraction at the radial direction of the fiber. Twisting introduces new voids and deviation of the axial load on CNTs from the external load applied on the fiber,
and some CNTs could be broken to short pieces during stretching.26−28 Note that all the above samples were in the as-prepared state (condensation by alcohol shrinking only). Using the thickness data for each sample, the corresponding strength along the transverse direction was also calculated. It was 0.294, 0.377, and 0.6 GPa, respectively, for the randomly, slightly, and wellaligned samples. That is, although the alignment along the transverse direction became less and less, the strength still increased. This observation indicates that beyond alignment, there might be other factors affecting the measured strength. Comparing the slightly and well-aligned samples, they were actually prepared with about the same amount of reaction solution and thus presumably about the same amount of CNT material. However, these two samples had a large difference in thickness (1.5 vs 0.55 μm, Figure 4c,e) and thus packing density. In the well-aligned sample, fewer CNTs were aligned at the transverse direction, and thus, a lower ultimate load was observed than that for the slightly aligned sample (0.64 vs 1.14 N, Figure 3h,i. However, because the well-aligned sample had a smaller thickness (higher packing density), the calculated strength (load/(width × thickness)) was still higher (0.6 vs 0.377 GPa). Therefore, there is an apparent packing effect on strength. Because the well-aligned sample had the highest strength in the as-prepared state, we applied rolling to enhance the packing density of the CNTs in this sample. Although the rolling was E
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uses, especially in weight-sensitive applications demanding strength, stiffness, and toughness.
which often leads to low net property improvement. Pressing was also applied for densification.13,28 In this case, the film or sheet was small in size and subject to pressing over its entire surface. Such planar pressing could improve the packing at the thickness direction at most. However, in our case of rolling, the pores and gaps between CNTs could have been eliminated, generating an unprecedentedly high packing in the film. First, the load applied on the film was perpendicular to the film and thus could not deviate the good alignment of CNTs. Second, the applied load was linear rather than planar. A linear load during rolling could improve the packing at the thickness direction and the rolling direction as well. Third, the film had a linear contact with the two rollers. The rolling load was applied on the film in a narrow zone along the roller axis. This could have induced a very high rolling pressure, which is essential to achieve a high packing density. In contrast, in the previous densification by pressing, the load was applied on the film surface, and the actual pressing pressure might not be high. It is unknown if CNTs, especially those with large diameters, had been flattened during the repeated rolling and the reduction of the pores within CNTs also contributed to the high densification as collapsed CNTs were already observed before rolling (Figure S4). But, the aligned long CNTs seen at the fracture surface indicate that CNTs hadn’t undergone structure transition even after tensile testing (Figure 5c−e, Figure S5). This observation is consistent with both theoretical calculations34 and experimental study35 that CNTs could bear substantial compression with their structures remaining intact. The present film possesses an exceptional combination of high strength, high ductility, and high modulus. The observed average strength of 9.6 GPa is much higher than those measured for previous CNT fibers and films (up to 2 GPa), the average Young’s modulus of 130 GPa is one of the highest moduli ever measured, and the average elongation of 8% is within previous observations (Figure S6). The high strength and modulus are closely related to the high alignment and close packing induced to the present CNT material. The present film is also superior to conventional Kevlar, poly(p-phenylene benzobisoxazole) (PBO) and carbon fibers (CFs) in terms of strength and ductility.36−39 For example, the current highstrength CFs are polyacrylonitrile (PAN)-based, having strengths up to 7 GPa. But, such CFs have a ductility of only 2%, which is much lower than that for CNT fibers (6−8.5%). The current high-modulus CFs are pitch-based, having moduli up to 965 GPa but strengths only 3 GPa and almost no ductility. The high ductility, combined with high failure strength, means that the work needed to break the CNT films (called toughness) is also high, which will increase the safety factor of CNT composite structures by preventing catastrophic failure. In summary, although previous CNT fibers and films exhibited low tensile strengths (0.5−2 GPa), we reported here strength improvement to 9.6 GPa with an average Young’s modulus of 130 GPa, and tensile elongation of 8%. The strength improvement was demonstrated to be closely related to the CNT alignment achieved by controlling the winding rate during the process of film deposition and the high packing density resulting from the application of rolling. The excellent combination of the mechanical properties puts the present film superior over previous CNT fibers and films as well as other fiberous materials and thus is highly competitive for high-end
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.5b03863. Experimental methods, TEM images, SEM images, load vs strain curve, magnified surface image, comparison of present CNT sample and previous fibers, schematic illustrations of rolling system. (PDF) Video S1: Cylinder running at slow rate. (AVI) Video S2: Cylinder running at fast rate. (AVI) Video S3: The rolling process. (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
J.N.W. conceived and designed the work. W.X., Y.C., and H.Z. did the experimental work. J.N.W. and W.X. wrote the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial supports from National Natural Science Foundation of China (project #: 51271077, U1362104) and Shanghai Nanoscience and Nanotechnology Promotion Center (project #: 12 nm0503300) are greatly acknowledged.
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