Nanoscale View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
PAPER
Cite this: Nanoscale, 2014, 6, 1748
View Journal | View Issue
Integrated random-aligned carbon nanotube layers: deformation mechanism under compression† Zhiping Zeng,a Xuchun Gui,*a Qiming Gan,a Zhiqiang Lin,a Yuan Zhu,a Wenhui Zhang,a Rong Xiang,a Anyuan Caob and Zikang Tangac Carbon nanotubes have the potential to construct highly compressible and elastic macroscopic structures such as films, aerogels and sponges. The structure-related deformation mechanism determines the mechanical behavior of those structures and niche applications. Here, we show a novel strategy to integrate aligned and random nanotube layers and reveal their deformation mechanism under uniaxial compression with a large range of strain and cyclic testing. Integrated nanotube layers deform sequentially with different mechanisms due to the distinct morphology of each layer. While the aligned layer forms buckles under compression, nanotubes in the random layer tend to be parallel and form
Received 2nd September 2013 Accepted 24th November 2013
bundles, resulting in the integration of quite different properties (strength and stiffness) and correspondingly distinct plateau regions in the stress–strain curves. Our results indicate a great promise
DOI: 10.1039/c3nr04667b
of constructing hierarchical carbon nanotube structures with tailored energy absorption properties, for
www.rsc.org/nanoscale
applications such as cushioning and buffering layers in microelectromechanical systems.
Introduction Demand is increasing for porous nanomaterials that can absorb and dissipate energy for civil and military applications.1 In recent years, a variety of nanoporous materials have been developed for use as light-weight energy dissipation materials, such as ultralight metallic microlattices and nanosilica,2,3 molecularly intercalated nanoakes,4,5 periodic bicontinuous composites,6–9 and carbon nanomaterials.10–14 Selection of appropriate materials and design of the geometrical structure are two effective approaches to construct energy dissipation materials with high mass- or volume-specic energy absorption.6,15 Carbon nanotubes (CNTs) are widely used as the basic units to assemble macroscopic materials with low density, high porosity, and excellent compression capability, with potential applications as energy absorption and protective materials.16–18
a
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China. E-mail: [email protected]
b
Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China
c Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
† Electronic supplementary information (ESI) available: Low-magnication SEM images showing the deformation process of the double-layered structure, SEM images of sponge–array double-layered structure under the compressive strain of 32% and 52%, and compressive stress–strain curves of a CNT array and sponges separately for 50 cycles. See DOI: 10.1039/c3nr04667b
1748 | Nanoscale, 2014, 6, 1748–1755
Previous investigations have indicated that vertically aligned CNT arrays exhibit an anisotropic structure, and unique response under compression along their length.19 Under external impact, the CNT arrays dissipate energy by structural buckling and friction between buckled CNTs.20 The mechanical response of CNT arrays under impact or cyclic compression is directly related to the deformation of microscopic structure.21 Interestingly, in situ observations have shown that CNT arrays deform by collectively forming a buckled structure.11,20–23 Recently, porous isotropic CNT sponges with high structural exibility and robustness have been reported by our group.16 These sponges can tolerate large compressive strains repeatedly without collapse, and exhibit high energy dissipation ability.24,25 A typical geometrical structure design for protective materials is to adopt layered structures combining hard and so materials for tailored strength and toughness.6 This approach has proven to be effective in the design of energy dissipation systems, for example, multilayer structures consisting of alternating layers of aligned CNT arrays and metal foil,26 CNT arrays and vermiculite;17 CNT arrays and polymers have been developed to form highly efficient vibration and energy dampers.15,27 Although a number of CNT-based energy absorption materials have been reported, they usually contain other inorganic or polymeric components that are brittle or unstable at high temperature. Recently, we reported a well-dened all-CNT tandem composite consisting of an aligned (array) and a random (sponge) layer connected in series, with a wider range characterized by relatively low cushioning coefficients compared to individual arrays or sponges.28
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Paper
However, the deformation and energy dissipation mechanism of the all-CNT tandem composite under compression is still unclear. And the energy absorption and dissipation of the layered structure under cyclic compression require further verication. Here, we focus on the design and fabrication of multilayered structures of rigid aligned CNT arrays and so CNT sponges. The resulting all-CNT structure shows high energy dissipation and complete recovery without damage to the composite structure aer large strain compression cycles, similar to that of individual CNT arrays.11,15,18–22 The deformation mechanism of the layered CNT structure also has been studied by in situ SEM observations, which allows us to explore the relationship between microstructure evolution and mechanical properties in detail. Design of multilayered structures consisting of CNTs with tailored morphologies and properties could benet mechanical recovery, energy dissipation and cushioning applications.
Results and discussion Vertically aligned CNT arrays and random CNT sponges were synthesized by chemical vapor deposition (CVD) as described before.28 To fabricate double-layer and multi-layered structures constructed of CNT arrays and sponges, a catalyst precursor and carbon source of ferrocene–xylene solution (producing array) was rapidly switched to ferrocene–dichlorobenzene (producing sponge) to grow different layers in a controlled manner. Because the synthesis process was continuous, the composites were formed as a seamless block with good adhesion between different layers. Fig. 1a shows a photograph of a sponge–array
Nanoscale
double-layered composite. The CNTs in the anisotropic array and isotropic sponge have certain interpenetration at the interface to give reasonable interfacial combination with a tensile stress of about 0.12 MPa (as illustrated in Fig. 1b). Aer the tension test, we observed that some CNTs from the sponge are still adhered to the array surface at the fractured section.28 The structure of the sponge–array layered composites is stable during compression. To evaluate the deformation behavior of these layered structures under large compressive strain, we performed mechanical tests with in situ SEM observations. Fig. 1c shows a series of low-magnication SEM images of a double-layered composite under uniaxial cyclic compression. Under load, the sponge deformed rst, and then the array started to deform aer the sponge was fully compressed, indicating distinct mechanisms in these two structures. The strain of the composite (3c), the sponge (3s) and the array (3a) in the composite at different stages was respectively calculated from its thickness change relative to its original height (Fig. 1c). At relatively small strains, it was mainly the sponge to deform under compression while the array was stable. For example, a composite's strain of 23% was realized entirely by a sponge's strain of 56%, while the array's strain was negligible. As 3c increased to >50%, the sponge was fully densied and then the array started to deform at increasing strain. When 3c reached up to 56%, 3a was about 25%, while the sponge had been squeezed into a thin lm with much reduced thickness (difficult to measure under SEM). Upon load release, the array recovered from its deformation rst and then the sponge also recovered almost to its original composite height. The residual strain of the composite was about 0.5% generated from the sponge layer.
Fig. 1 CNT sponge–array double-layered structure and its deformation under axial compression. (a) Photograph of sponge–array doublelayered structure. (b) Illustration of the composite structure consisting of isotropic sponge on an anisotropic array. The layers interpenetrate to join tightly together. (c) Low-magnification SEM images showing the deformation process of the double-layered structure as the compressive strain gradually increased to 56% and then decreased to 0.5%. The strain of the composite, sponge and array is labeled 3c, 3s and 3a, respectively.
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 1748–1755 | 1749
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Nanoscale
Further SEM images showing the composite under different strain are presented in Fig. S1 in the ESI,† with corresponding high-magnication SEM images in Fig. S2 and S3.† To reveal the morphology-dominated deformation mechanism related to the array and sponge layer, the double-layered composite was observed by high-magnication SEM during cyclic compression, as shown in Fig. 2. For the top sponge layer, as the compressive strain increased, the isotropic intertwined CNT network started to deform such that CNTs were gradually aligned along the direction perpendicular to the compressive force and formed bundled structures. At large strain, the amount of parallel bundles and bundle diameters increased continuously (Fig. 2a). Upon unloading, the bundle structure recovered to uniform networks (debundling of CNTs and recovery of pores). Owing to the random distribution of CNTs, their contact points (nodes) in the sponge network can be reversibly attached and detached to enable large degree compression and elastic recovery. In contrast, the bottom array layer showed considerably higher modulus and stiffness than those of the sponge, due to its distinct structure and morphology compared to the sponge. The CNT sponge consisted of a random nanotube network with thinner diameter (25–40 nm), while a CNT array consisted of vertically aligned nanotubes with thick walls and large diameter (50–80 nm).29 This led to higher rigidity of array and correspondingly higher compressive stress, as compared with the sponge. Under uniaxial compression, the aligned CNTs were forced to form buckles along the axis direction, while the random CNTs in the sponge layer were squeezed together with closer contact, resulting in different microstructure evolution processes as seen in SEM. This buckling phenomenon has been observed previously, which explains higher stiffness and elastic
Paper
recovery of CNT arrays compared to random sponges.22,29 The number of buckles produced in an array was proportional to the compressive strain, and the wavelength was about 10 mm at a strain of 25%, consistent with that found in a CNT array column.18 During unloading, the CNT array recovered to its original height without any residual CNT curvature or delamination due to complete straightening of the buckles (Fig. 2c). During loading, the interface of the composite was clearly visible although there might be certain interpenetration between CNTs in different layers as the top sponge gradually moved toward the bottom array (Fig. 2b). We also explored the relationship between the mechanical properties and the morphology change in a double-layered composite, in a compression cycle accompanied by in situ SEM observations (Fig. 3). Here, two same-size blocks were prepared from the same double-layered composite (Fig. 1a) in order to record its mechanical properties and deformation mechanism. Obviously, the stress–strain (s–3) curve showed double stress plateaus, in which the rst stress plateau at about 0.01–0.1 MPa (3 is about 2–42%) was from the relatively so sponge and the second stress plateau (smoothly varies from 1.1 to 3.0 MPa, corresponding strain of 54% to 73%) was produced by the stiff array deformation. At low stress, the deformation of the composite was mainly dominated by that of the sponge. When the stress was 0.11 MPa, the strain of the composite was 42.5%. At this point, the sponge was almost fully squeezed, whereas the array remained straight (Fig. 3c). As the strain of the composite continued to increase to 63.5%, the sponge was almost squeezed to a lm. But the part of array only had a strain of about 27% and formed a few buckles. In this region, the loading increased rapidly and the second large stress plateau was observed. The second stress plateau region is steeper as
High-magnification SEM observations reveal the deformation behavior of different parts of the double-layered composite. (a) The isotropic sponge formed bundle structures under compression. (b) The interfacial area of the composite. The dashed line indicates the interface between sponge and array. (c) The anisotropic array formed a buckled structure under compression, and then recovered to its original shape as the compression was withdrawn. Scale bars: 10 mm.
Fig. 2
1750 | Nanoscale, 2014, 6, 1748–1755
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Paper
Nanoscale
In situ SEM images of sponge–array double-layered structure under loading–unloading compression cycles. (a) Compressive stress– strain (s–3) curves of the sponge–array composite, which showed double stress plateaus. (b) to (h) in the s–3 curves indicate different compressive stages, and the corresponding morphologies are shown in (b) to (h). (b–e) SEM images of the sample in loading stages with a strain of 0%, 42.5%, 54.0% and 63.5%, respectively. (f–h) SEM images of the sample in the unloading stage with a strain of 54.6%, 49.5% and 12.0%, respectively. The white circles in (b)–(h) indicate enlarged areas.
Fig. 3
compared to the horizontal plateau region of individual arrays.19,30 This is probably due to the density gradient with regard to height in the CNT array. Experimental and theoretical results showed that a linear variation in the yield-like property along the sample height resulted in the slope of the stress plateau, and a constant property along the sample height resulted in a at stress plateau.30–32 In our experiment, the CNT was synthesized by the oating catalyst CVD method. The density in different parts of the CNT is not a constant. Upon unloading, the array rst recovered to straight morphology and then the sponge recovered its shape. Combining CNT layers with different deformation mechanisms resulted in unique behavior with the features from both an individual sponge and array. The double-layered composite can sustain cyclic compression at large strain (3 ¼ 70%). The s–3 curves of 50 cycles followed a similar route with the appearance of two distinct plateau regions during loading and unloading processes, indicating structural stability under cyclic stress conditions (Fig. 4a). The compressive stress at the set maximum strain (3 ¼ 0.7) decreased with increasing cycles, indicating degradation of mechanical strength. Our previous work showed that the maximum stress and mechanical strength of a CNT sponge at a
This journal is © The Royal Society of Chemistry 2014
given strain were stable.16 Therefore, the degradation in mechanical strength of the composite was related to the CNT array layer. Repeated cyclic compression also produced residual deformation (about 34% aer 50 cycles) in the composite, as seen from the gradual shi (to the right) of the loading and unloading curves with increasing cycle number. This feature was similar to sponges under cyclic compression.16 The deformation of composites was also rate-independent at low compressive rate. We separately tested sponge and array layers under the same compressive stress (2.25 MPa) in the rst cycle. Aer 50 cycles, it can be seen that the array strength has decreased from 2.25 to 1.6 MPa but without residual strain, while the sponge showed signicant plastic deformation (Fig. 4b). More cyclic compression curves of the array and sponge are presented in Fig. S4.† The stress degradation in an individual array was consistent with the array–sponge composite for 55 cycles, and the residual strain of the composite was mainly produced from the sponge layer (Fig. 4c and d). The s–3 curves of the composites also showed signicant hysteresis loops during compression experiments. In the rst cycle, the work done in the loading stage was about 282.8 kJ m3 and the energy released in unloading was about 101.0 kJ m3, providing a high energy dissipation of 181.8 kJ m3 and an
Nanoscale, 2014, 6, 1748–1755 | 1751
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Nanoscale
Paper
Compression properties and energy absorption of the sponge–array double-layered composite. (a) Compressive stress–strain (s–3) curves of the sponge–array composite at a set maximum strain of 3 ¼ 70% for 50 cycles. (b) First and 50th cycle s–3 curves for a composite (middle curve) and its sponge (right) and array layer (left) tested separately. (c) Compressive stress recorded for the composite and array under cyclic compression at the same maximum compression stress in the first cycle, showing the same stress. Inset, deformation developed by cyclic compression of the composite, sponge and array. The deformation strain is calculated as the percentage of the deformed height at the end of each cycle relative to the original height. (d) Absorbed and released energy of the composite under cyclic compression. Inset, dependence of the energy loss coefficient of the composite on the compression cycle number. Fig. 4
energy loss coefficient (DE/E) of 0.64, which is comparable to those of molecularly intercalated nanoakes (0.55)4 and ultralight metallic microlattices (0.77)2,4 (Fig. 4d). But the energy loss coefficient of the composites is lower than that of sponge (0.68) and the literature reports of vertically aligned CNT bundles11,33 (about 0.94), which is because the sponge and CNT bundles are more so and have lower density. Aer 10 cycles, the energy loss coefficient (DE/E 0.42) was nearly constant and higher than those of typical nickel foams and a CNT/vermiculite layered composite.17 The energy absorption, dissipation and loss coefcient of the composite, sponges and arrays in different cycles are presented in Table S1 in the ESI.† It can also be improved by increasing the ratio of sponge. This large energy dissipation resulted from the microstructure deformation and friction between adjacent CNTs, which was useful for constructing much light-weight buffer layer for large energy absorption. During loading and unloading, the random CNTs' contact points from sponge could be attached and detached, and the aligned CNTs from array could bend (even, curl) and straighten. In these processes, most elastic energy could be dissipated. The above deformation mechanism of sponges and arrays was also found in more complex hierarchical systems, for example, a triple-layered structure consisting of two sponge layers sandwiched by an aligned array (Fig. 5). The two sponge layers had different densities and mechanical strengths tuned
1752 | Nanoscale, 2014, 6, 1748–1755
by the synthesis parameters. Fig. 5a shows that at a lower strain (17%), the top low density sponge (sponge 1, about 14.3 mg cm3) layer deformed rst. When the strain increased to 35%, the top sponge was fully compressed and the bottom sponge (sponge 2, about 33.5 mg cm3) layer with a higher density started to deform. This sandwich structure demonstrated a sequential deformation mechanism in which the two sponge layers were compressed rst while the array (about 320.0 mg cm3) in the middle was the last to deform. The strain of the composite was determined by the strain of each layer. It can be calculated by: 3composite ¼ x13sponge 1 + x23sponge 2 + x33array, where x1, x2, and x3 are the thickness ratios of sponge 1, sponge 2 and array in the composite, like the double-layered composite.28 The deformation pattern (from low density sponge to array) did not change when the composite was turned over to let the lowest density in the bottom of the pile. The s–3 curve of the triple-layered composite exhibited three stress plateaus corresponding to the different deformation stages of each layer in the composite (Fig. 5b). At lower stresses (1 MPa), both sponge layers had been densied, and then the array started to deform by forming buckles similar to that in the double-layered composite. The buckle formation normally started at the top of
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Paper
Nanoscale
Microscopic deformation and compression properties of a sponge–array–sponge triple-layered composite. (a) In situ SEM observations of the deformation of the triple-layered composite. (b) Compressive stress–strain (s–3) curves of the triple-layered composite, showing three compressive stress plateaus.
Fig. 5
arrays due to the lower density and more drawbacks in this region resulted from the uctuated growing environment of this layer. Here we obtained a sandwich material with relatively so surface (sponge) layers and a hard core (array), all consisting of only CNTs. This result indicated a possibility of combining multiple CNT layers with tailored microstructures to enable controlled deformation of each layer in predened sequence.
Conclusion We systematically investigated the deformation mechanism of all-CNT energy absorption materials composed of random sponges and aligned arrays integrated into double- and triplelayered structures, and correlated the microstructure evolution with stress–strain curves. We found that random CNTs in the sponge layer mainly deformed by forming parallel bundles along the direction perpendicular to the compressive stress, while aligned CNT arrays formed buckles under compression. For both double- and triple-layered structures, the sponge layers deformed rst producing low stress plateaus and large strain, while the array layer deformed last producing a stress plateau region with a much larger stress and modulus. Our results could be useful for further understanding the deformation mechanism of compressible CNT structures, and designing hierarchical composites with tailored microstructures and energy absorption properties.
Experimental Synthesis of CNT sponge, array, double-layered and multilayered composites CNT sponge, array and their composites were synthesized by CVD processes using the same system containing a tube furnace, as reported in our previous work.28 For growing CNT sponges, ferrocene was dissolved in dichlorobenzene to prepare a carbon source solution with a concentration of 60 mg mL1, and the supply rate was set as 0.1–0.4 mL min1. For growing CNT arrays, ferrocene dissolved in xylene at a concentration of 20 mg mL1 was used as carbon source solution and the supply rate was 0.4 mL min1. To fabricate double-layered composites, CNT arrays were rst grown on quartz for a growth period of 60 minutes. Aer that, the ferrocene–xylene carbon source
This journal is © The Royal Society of Chemistry 2014
solution was immediately switched to ferrocene–dichlorobenzene solution to grow a sponge layer on top of the array within a controlled growth period. In order to fabricate triple-layered composites, a double-layered structure was peeled off from the substrate by a scalpel and turned over to let the array on the top. Then a third CVD process was used to grow a sponge layer on the other side of the array to obtain a sponge–array–sponge composite. During the CVD process, we adopted a reaction temperature of 850 C, and a mixture of carrier gas of argon and hydrogen at ow rates of 2000 and 300 sccm, respectively.
Characterization of the CNT composite The microstructure and morphology of sponge, array and their composites were characterized by scanning electron microscopy (S-4800). By a miniature compression station, in situ SEM observations of the layered composite during cyclic compression were recorded. To measure the densities of individual sponges and arrays, each layer CNT in a triple-layered structure was separated by a scalpel. The density of each layer was obtained by dividing the mass by its volume.
Mechanical properties of the CNT composite Mechanical properties were determined using Instron 5943 equipped with a 1 kN load cell and tow at-surface compression stages. CNT composites were cut into small blocks by a scalpel with sizes of about 7 7 3 (thickness) mm3. The top compression stage was chosen to move down to contact the sample surface and the compression rates were set in the range of 1 mm min1.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant no. 51102286), the Guangdong Natural Science Foundation (Grant no. S2012040007893), the Beijing Science and Technology Program (Grant no. Z121100001312005), and the Fundamental Research Funds for the Central Universities (no. 12lgpy16).
Nanoscale, 2014, 6, 1748–1755 | 1753
View Article Online
Nanoscale
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Notes and references 1 J. H. Lee, J. P. Singer and E. L. Thomas, Micro-/ Nanostructured Mechanical Metamaterials, Adv. Mater., 2012, 24, 4782–4810. 2 T. A. Schaedler, A. J. Jacobsen, A. Torrents, A. E. Sorensen, J. Lian, J. R. Greer, L. Valdevit and W. B. Carter, Ultralight Metallic Microlattices, Science, 2011, 334, 962–965. 3 S. O. Kucheyev, M. Stadermann, S. J. Shin, J. H. Satcher, S. A. Gammon, S. A. Letts, T. van Buuren and A. V. Hamza, Super-Compressibility of Ultralow-Density Nanoporous Silica, Adv. Mater., 2012, 24, 776–780. 4 C. C. Yu, Z. G. Chen, H. Li, J. Turner, X. C. Zeng, Z. H. Jin, J. Y. Jiang, B. Youssef and L. Tan, Molecularly Intercalated Nanoakes: A Supramolecular Composite for Strong Energy Absorption, Adv. Mater., 2010, 22, 4457–4461. 5 L. Y. Sun, R. F. Gibson, F. Gordaninejad and J. Suhr, Energy absorption capability of nanocomposites: A review, Compos. Sci. Technol., 2009, 69, 2392–2409. 6 J. H. Lee, L. F. Wang, M. C. Boyce and E. L. Thomas, Periodic Bicontinuous Composites for High Specic Energy Absorption, Nano Lett., 2012, 12, 4392–4396. 7 A. Misra and P. Kumar, Periodic architecture for high performance shock absorbing composites, Sci. Rep., 2013, 3, 2056. 8 J. Sargianis, H. I. Kim and J. Suhr, Natural Cork Agglomerate Employed as an Environmentally Friendly Solution for Quiet Sandwich Composites, Sci. Rep., 2012, 2, 403. 9 J. H. Lee, L. F. Wang, S. Kooi, M. C. Boyce and E. L. Thomas, Enhanced Energy Dissipation in Periodic Epoxy Nanoframes, Nano Lett., 2010, 10, 2592–2597. 10 M. De Volder, S. H. Tawck, R. H. Baughman and A. J. Hart, Carbon Nanotubes: Present and Future Commercial Applications, Science, 2013, 339, 535–539. 11 S. Pathak, E. J. Lim, P. Pour Shahid Saeed Abadi, S. Graham, B. A. Cola and J. R. Greer, Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An in-Situ Study, ACS Nano, 2012, 6, 2189–2197. 12 B. J. Carey, P. K. Patra, G. M. Hahm and P. M. Ajayan, FoamLike Behavior in Compliant, Continuously Reinforced Nanocomposites, Adv. Mater., 2013, 23, 3002–3007. 13 K. H. Kim, Y. Oh and M. F. Islam, Graphene coating makes carbon nanotube aerogels superelastic and resistant to fatigue, Nat. Nanotechnol., 2012, 7, 562–566. 14 H. Hu, Z. B. Zhao, W. B. Wan, Y. Gogotsi and J. S. Qiu, Ultralight and Highly Compressible Graphene Aerogels, Adv. Mater., 2013, 25, 2219–2223. 15 A. Misra, J. R. Raney, L. De Nardo, A. E. Craig and C. Daraio, Synthesis and Characterization of Carbon Nanotube– Polymer Multilayer Structures, ACS Nano, 2011, 5, 7713– 7721. 16 X. C. Gui, J. Q. Wei, K. L. Wang, A. Y. Cao, H. W. Zhu, Y. Jia, Q. K. Shu and D. H. Wu, Carbon Nanotube Sponges, Adv. Mater., 2010, 22, 617–621.
1754 | Nanoscale, 2014, 6, 1748–1755
Paper
17 Q. Zhang, M. Q. Zhao, Y. Liu, A. Y. Cao, W. Z. Qian, Y. F. Lu and F. Wei, Energy-Absorbing Hybrid Composites Based on Alternate Carbon-Nanotube and Inorganic Layers, Adv. Mater., 2009, 21, 2876–2881. 18 O. Yaglioglu, A. Y. Cao, A. J. Hart, R. Martens and A. H. Slocum, Wide Range Control of Microstructure and Mechanical Properties of Carbon Nanotube Forests: A Comparison Between Fixed and Floating Catalyst CVD Techniques, Adv. Funct. Mater., 2012, 22, 5028–5037. 19 A. Y. Cao, P. L. Dickrell, W. G. Sawyer, M. N. Ghasemi-Nejhad and P. M. Ajayan, Super-compressible foamlike carbon nanotube lms, Science, 2005, 310, 1307–1310. 20 S. B. Hutchens, L. J. Hall and J. R. Greer, In situ Mechanical Testing Reveals Periodic Buckle Nucleation and Propagation in Carbon Nanotube Bundles, Adv. Funct. Mater., 2010, 20, 2338–2346. 21 M. R. Maschmann, G. J. Ehlert, S. J. Park, D. Mollenhauer, B. Maruyama, A. J. Hart and J. W. Baur, Visualizing Strain Evolution and Coordinated Buckling within CNT Arrays by In Situ Digital Image Correlation, Adv. Funct. Mater., 2012, 22, 4686–4695. 22 P. Pour Shahid Saeed Abadi, S. B. Hutchens, J. R. Greer, B. A. Cola and S. Graham, Effects of morphology on the micro-compression response of carbon nanotube forests, Nanoscale, 2012, 4, 3373–3380. 23 M. R. Maschmann, Q. H. Zhang, R. Wheeler, F. Du, L. M. Dai and J. Baur, In situ SEM Observation of Column-like and Foam-like CNT Array Nanoindentation, ACS Appl. Mater. Interfaces, 2011, 3, 648–653. 24 M. Xu, D. N. Futaba, T. Yamada, M. Yumura and K. Hata, Carbon Nanotubes with Temperature-Invariant Viscoelasticity from-196 degrees to 1000 degrees C, Science, 2010, 330, 1364–1368. 25 X. W. Tang, X. C. Gui, Y. L. Liu and B. Wang, Rheological Model Predicting Compressive Responses of Carbon Nanotube Networks, RSC Adv., 2013, 3, 14473–14479. 26 J. R. Raney, F. Fraternali, A. Amendola and C. Daraio, Modeling and in situ identication of material parameters for layered structures based on carbon nanotube arrays, Compos. Struct., 2011, 93, 3013–3018. 27 Y. H. Lin, Y. C. Lai, C. L. Lu and W. K. Hsu, Excellent cushioning by polymer-concreted arrays of aligned carbon nanotubes, J. Mater. Chem., 2011, 21, 12485–12488. 28 Z. P. Zeng, X. C. Gui, Z. Q. Lin, L. H. Zhang, Y. Jia, A. Y. Cao, Y. Zhu, R. Xiang, T. Z. Wu and Z. K. Tang, Carbon Nanotube Sponge–Array Tandem Composites with Extended Energy Absorption Range, Adv. Mater., 2013, 25, 1185–1191. 29 C. H. Cao, A. Reiner, C. H. Chung, S. H. Chang, I. Kao, R. V. Kukta and C. S. Korach, Buckling initiation and displacement dependence in compression of vertically aligned carbon nanotube arrays, Carbon, 2011, 49, 3190– 3199. 30 S. Pathak, N. Mohan, E. Decolvenaere, A. Needleman, M. Bedewy, A. J. Hart and J. R. Greer, Local relative density modulates failure and strength in vertically aligned carbon nanotubes, ACS Nano, 2013, 7, 8593–8604.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
Soening–Hardening Solids, Acta Mech. Sin., 2012, 28, 1115–1124. 33 S. Pathak, J. R. Raney and C. Daraio, Effect of morphology on the strain recovery of vertically aligned carbon nanotube arrays: An in situ study, Carbon, 2013, 63, 303–316.
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
31 S. B. Hutchens, A. Needleman and J. R. Greer, Analysis of uniaxial compression of vertically aligned carbon nanotubes, J. Mech. Phys. Solids, 2011, 59, 2227–2237. 32 A. Needleman, S. B. Hutchens, N. Mohan and J. R. Greer, Deformation of Plastically Compressible Hardening–
Nanoscale
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 1748–1755 | 1755
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
PAPER
Cite this: Nanoscale, 2014, 6, 1748
View Journal | View Issue
Integrated random-aligned carbon nanotube layers: deformation mechanism under compression† Zhiping Zeng,a Xuchun Gui,*a Qiming Gan,a Zhiqiang Lin,a Yuan Zhu,a Wenhui Zhang,a Rong Xiang,a Anyuan Caob and Zikang Tangac Carbon nanotubes have the potential to construct highly compressible and elastic macroscopic structures such as films, aerogels and sponges. The structure-related deformation mechanism determines the mechanical behavior of those structures and niche applications. Here, we show a novel strategy to integrate aligned and random nanotube layers and reveal their deformation mechanism under uniaxial compression with a large range of strain and cyclic testing. Integrated nanotube layers deform sequentially with different mechanisms due to the distinct morphology of each layer. While the aligned layer forms buckles under compression, nanotubes in the random layer tend to be parallel and form
Received 2nd September 2013 Accepted 24th November 2013
bundles, resulting in the integration of quite different properties (strength and stiffness) and correspondingly distinct plateau regions in the stress–strain curves. Our results indicate a great promise
DOI: 10.1039/c3nr04667b
of constructing hierarchical carbon nanotube structures with tailored energy absorption properties, for
www.rsc.org/nanoscale
applications such as cushioning and buffering layers in microelectromechanical systems.
Introduction Demand is increasing for porous nanomaterials that can absorb and dissipate energy for civil and military applications.1 In recent years, a variety of nanoporous materials have been developed for use as light-weight energy dissipation materials, such as ultralight metallic microlattices and nanosilica,2,3 molecularly intercalated nanoakes,4,5 periodic bicontinuous composites,6–9 and carbon nanomaterials.10–14 Selection of appropriate materials and design of the geometrical structure are two effective approaches to construct energy dissipation materials with high mass- or volume-specic energy absorption.6,15 Carbon nanotubes (CNTs) are widely used as the basic units to assemble macroscopic materials with low density, high porosity, and excellent compression capability, with potential applications as energy absorption and protective materials.16–18
a
State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China. E-mail: [email protected]
b
Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China
c Department of Physics, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
† Electronic supplementary information (ESI) available: Low-magnication SEM images showing the deformation process of the double-layered structure, SEM images of sponge–array double-layered structure under the compressive strain of 32% and 52%, and compressive stress–strain curves of a CNT array and sponges separately for 50 cycles. See DOI: 10.1039/c3nr04667b
1748 | Nanoscale, 2014, 6, 1748–1755
Previous investigations have indicated that vertically aligned CNT arrays exhibit an anisotropic structure, and unique response under compression along their length.19 Under external impact, the CNT arrays dissipate energy by structural buckling and friction between buckled CNTs.20 The mechanical response of CNT arrays under impact or cyclic compression is directly related to the deformation of microscopic structure.21 Interestingly, in situ observations have shown that CNT arrays deform by collectively forming a buckled structure.11,20–23 Recently, porous isotropic CNT sponges with high structural exibility and robustness have been reported by our group.16 These sponges can tolerate large compressive strains repeatedly without collapse, and exhibit high energy dissipation ability.24,25 A typical geometrical structure design for protective materials is to adopt layered structures combining hard and so materials for tailored strength and toughness.6 This approach has proven to be effective in the design of energy dissipation systems, for example, multilayer structures consisting of alternating layers of aligned CNT arrays and metal foil,26 CNT arrays and vermiculite;17 CNT arrays and polymers have been developed to form highly efficient vibration and energy dampers.15,27 Although a number of CNT-based energy absorption materials have been reported, they usually contain other inorganic or polymeric components that are brittle or unstable at high temperature. Recently, we reported a well-dened all-CNT tandem composite consisting of an aligned (array) and a random (sponge) layer connected in series, with a wider range characterized by relatively low cushioning coefficients compared to individual arrays or sponges.28
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Paper
However, the deformation and energy dissipation mechanism of the all-CNT tandem composite under compression is still unclear. And the energy absorption and dissipation of the layered structure under cyclic compression require further verication. Here, we focus on the design and fabrication of multilayered structures of rigid aligned CNT arrays and so CNT sponges. The resulting all-CNT structure shows high energy dissipation and complete recovery without damage to the composite structure aer large strain compression cycles, similar to that of individual CNT arrays.11,15,18–22 The deformation mechanism of the layered CNT structure also has been studied by in situ SEM observations, which allows us to explore the relationship between microstructure evolution and mechanical properties in detail. Design of multilayered structures consisting of CNTs with tailored morphologies and properties could benet mechanical recovery, energy dissipation and cushioning applications.
Results and discussion Vertically aligned CNT arrays and random CNT sponges were synthesized by chemical vapor deposition (CVD) as described before.28 To fabricate double-layer and multi-layered structures constructed of CNT arrays and sponges, a catalyst precursor and carbon source of ferrocene–xylene solution (producing array) was rapidly switched to ferrocene–dichlorobenzene (producing sponge) to grow different layers in a controlled manner. Because the synthesis process was continuous, the composites were formed as a seamless block with good adhesion between different layers. Fig. 1a shows a photograph of a sponge–array
Nanoscale
double-layered composite. The CNTs in the anisotropic array and isotropic sponge have certain interpenetration at the interface to give reasonable interfacial combination with a tensile stress of about 0.12 MPa (as illustrated in Fig. 1b). Aer the tension test, we observed that some CNTs from the sponge are still adhered to the array surface at the fractured section.28 The structure of the sponge–array layered composites is stable during compression. To evaluate the deformation behavior of these layered structures under large compressive strain, we performed mechanical tests with in situ SEM observations. Fig. 1c shows a series of low-magnication SEM images of a double-layered composite under uniaxial cyclic compression. Under load, the sponge deformed rst, and then the array started to deform aer the sponge was fully compressed, indicating distinct mechanisms in these two structures. The strain of the composite (3c), the sponge (3s) and the array (3a) in the composite at different stages was respectively calculated from its thickness change relative to its original height (Fig. 1c). At relatively small strains, it was mainly the sponge to deform under compression while the array was stable. For example, a composite's strain of 23% was realized entirely by a sponge's strain of 56%, while the array's strain was negligible. As 3c increased to >50%, the sponge was fully densied and then the array started to deform at increasing strain. When 3c reached up to 56%, 3a was about 25%, while the sponge had been squeezed into a thin lm with much reduced thickness (difficult to measure under SEM). Upon load release, the array recovered from its deformation rst and then the sponge also recovered almost to its original composite height. The residual strain of the composite was about 0.5% generated from the sponge layer.
Fig. 1 CNT sponge–array double-layered structure and its deformation under axial compression. (a) Photograph of sponge–array doublelayered structure. (b) Illustration of the composite structure consisting of isotropic sponge on an anisotropic array. The layers interpenetrate to join tightly together. (c) Low-magnification SEM images showing the deformation process of the double-layered structure as the compressive strain gradually increased to 56% and then decreased to 0.5%. The strain of the composite, sponge and array is labeled 3c, 3s and 3a, respectively.
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 1748–1755 | 1749
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Nanoscale
Further SEM images showing the composite under different strain are presented in Fig. S1 in the ESI,† with corresponding high-magnication SEM images in Fig. S2 and S3.† To reveal the morphology-dominated deformation mechanism related to the array and sponge layer, the double-layered composite was observed by high-magnication SEM during cyclic compression, as shown in Fig. 2. For the top sponge layer, as the compressive strain increased, the isotropic intertwined CNT network started to deform such that CNTs were gradually aligned along the direction perpendicular to the compressive force and formed bundled structures. At large strain, the amount of parallel bundles and bundle diameters increased continuously (Fig. 2a). Upon unloading, the bundle structure recovered to uniform networks (debundling of CNTs and recovery of pores). Owing to the random distribution of CNTs, their contact points (nodes) in the sponge network can be reversibly attached and detached to enable large degree compression and elastic recovery. In contrast, the bottom array layer showed considerably higher modulus and stiffness than those of the sponge, due to its distinct structure and morphology compared to the sponge. The CNT sponge consisted of a random nanotube network with thinner diameter (25–40 nm), while a CNT array consisted of vertically aligned nanotubes with thick walls and large diameter (50–80 nm).29 This led to higher rigidity of array and correspondingly higher compressive stress, as compared with the sponge. Under uniaxial compression, the aligned CNTs were forced to form buckles along the axis direction, while the random CNTs in the sponge layer were squeezed together with closer contact, resulting in different microstructure evolution processes as seen in SEM. This buckling phenomenon has been observed previously, which explains higher stiffness and elastic
Paper
recovery of CNT arrays compared to random sponges.22,29 The number of buckles produced in an array was proportional to the compressive strain, and the wavelength was about 10 mm at a strain of 25%, consistent with that found in a CNT array column.18 During unloading, the CNT array recovered to its original height without any residual CNT curvature or delamination due to complete straightening of the buckles (Fig. 2c). During loading, the interface of the composite was clearly visible although there might be certain interpenetration between CNTs in different layers as the top sponge gradually moved toward the bottom array (Fig. 2b). We also explored the relationship between the mechanical properties and the morphology change in a double-layered composite, in a compression cycle accompanied by in situ SEM observations (Fig. 3). Here, two same-size blocks were prepared from the same double-layered composite (Fig. 1a) in order to record its mechanical properties and deformation mechanism. Obviously, the stress–strain (s–3) curve showed double stress plateaus, in which the rst stress plateau at about 0.01–0.1 MPa (3 is about 2–42%) was from the relatively so sponge and the second stress plateau (smoothly varies from 1.1 to 3.0 MPa, corresponding strain of 54% to 73%) was produced by the stiff array deformation. At low stress, the deformation of the composite was mainly dominated by that of the sponge. When the stress was 0.11 MPa, the strain of the composite was 42.5%. At this point, the sponge was almost fully squeezed, whereas the array remained straight (Fig. 3c). As the strain of the composite continued to increase to 63.5%, the sponge was almost squeezed to a lm. But the part of array only had a strain of about 27% and formed a few buckles. In this region, the loading increased rapidly and the second large stress plateau was observed. The second stress plateau region is steeper as
High-magnification SEM observations reveal the deformation behavior of different parts of the double-layered composite. (a) The isotropic sponge formed bundle structures under compression. (b) The interfacial area of the composite. The dashed line indicates the interface between sponge and array. (c) The anisotropic array formed a buckled structure under compression, and then recovered to its original shape as the compression was withdrawn. Scale bars: 10 mm.
Fig. 2
1750 | Nanoscale, 2014, 6, 1748–1755
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Paper
Nanoscale
In situ SEM images of sponge–array double-layered structure under loading–unloading compression cycles. (a) Compressive stress– strain (s–3) curves of the sponge–array composite, which showed double stress plateaus. (b) to (h) in the s–3 curves indicate different compressive stages, and the corresponding morphologies are shown in (b) to (h). (b–e) SEM images of the sample in loading stages with a strain of 0%, 42.5%, 54.0% and 63.5%, respectively. (f–h) SEM images of the sample in the unloading stage with a strain of 54.6%, 49.5% and 12.0%, respectively. The white circles in (b)–(h) indicate enlarged areas.
Fig. 3
compared to the horizontal plateau region of individual arrays.19,30 This is probably due to the density gradient with regard to height in the CNT array. Experimental and theoretical results showed that a linear variation in the yield-like property along the sample height resulted in the slope of the stress plateau, and a constant property along the sample height resulted in a at stress plateau.30–32 In our experiment, the CNT was synthesized by the oating catalyst CVD method. The density in different parts of the CNT is not a constant. Upon unloading, the array rst recovered to straight morphology and then the sponge recovered its shape. Combining CNT layers with different deformation mechanisms resulted in unique behavior with the features from both an individual sponge and array. The double-layered composite can sustain cyclic compression at large strain (3 ¼ 70%). The s–3 curves of 50 cycles followed a similar route with the appearance of two distinct plateau regions during loading and unloading processes, indicating structural stability under cyclic stress conditions (Fig. 4a). The compressive stress at the set maximum strain (3 ¼ 0.7) decreased with increasing cycles, indicating degradation of mechanical strength. Our previous work showed that the maximum stress and mechanical strength of a CNT sponge at a
This journal is © The Royal Society of Chemistry 2014
given strain were stable.16 Therefore, the degradation in mechanical strength of the composite was related to the CNT array layer. Repeated cyclic compression also produced residual deformation (about 34% aer 50 cycles) in the composite, as seen from the gradual shi (to the right) of the loading and unloading curves with increasing cycle number. This feature was similar to sponges under cyclic compression.16 The deformation of composites was also rate-independent at low compressive rate. We separately tested sponge and array layers under the same compressive stress (2.25 MPa) in the rst cycle. Aer 50 cycles, it can be seen that the array strength has decreased from 2.25 to 1.6 MPa but without residual strain, while the sponge showed signicant plastic deformation (Fig. 4b). More cyclic compression curves of the array and sponge are presented in Fig. S4.† The stress degradation in an individual array was consistent with the array–sponge composite for 55 cycles, and the residual strain of the composite was mainly produced from the sponge layer (Fig. 4c and d). The s–3 curves of the composites also showed signicant hysteresis loops during compression experiments. In the rst cycle, the work done in the loading stage was about 282.8 kJ m3 and the energy released in unloading was about 101.0 kJ m3, providing a high energy dissipation of 181.8 kJ m3 and an
Nanoscale, 2014, 6, 1748–1755 | 1751
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Nanoscale
Paper
Compression properties and energy absorption of the sponge–array double-layered composite. (a) Compressive stress–strain (s–3) curves of the sponge–array composite at a set maximum strain of 3 ¼ 70% for 50 cycles. (b) First and 50th cycle s–3 curves for a composite (middle curve) and its sponge (right) and array layer (left) tested separately. (c) Compressive stress recorded for the composite and array under cyclic compression at the same maximum compression stress in the first cycle, showing the same stress. Inset, deformation developed by cyclic compression of the composite, sponge and array. The deformation strain is calculated as the percentage of the deformed height at the end of each cycle relative to the original height. (d) Absorbed and released energy of the composite under cyclic compression. Inset, dependence of the energy loss coefficient of the composite on the compression cycle number. Fig. 4
energy loss coefficient (DE/E) of 0.64, which is comparable to those of molecularly intercalated nanoakes (0.55)4 and ultralight metallic microlattices (0.77)2,4 (Fig. 4d). But the energy loss coefficient of the composites is lower than that of sponge (0.68) and the literature reports of vertically aligned CNT bundles11,33 (about 0.94), which is because the sponge and CNT bundles are more so and have lower density. Aer 10 cycles, the energy loss coefficient (DE/E 0.42) was nearly constant and higher than those of typical nickel foams and a CNT/vermiculite layered composite.17 The energy absorption, dissipation and loss coefcient of the composite, sponges and arrays in different cycles are presented in Table S1 in the ESI.† It can also be improved by increasing the ratio of sponge. This large energy dissipation resulted from the microstructure deformation and friction between adjacent CNTs, which was useful for constructing much light-weight buffer layer for large energy absorption. During loading and unloading, the random CNTs' contact points from sponge could be attached and detached, and the aligned CNTs from array could bend (even, curl) and straighten. In these processes, most elastic energy could be dissipated. The above deformation mechanism of sponges and arrays was also found in more complex hierarchical systems, for example, a triple-layered structure consisting of two sponge layers sandwiched by an aligned array (Fig. 5). The two sponge layers had different densities and mechanical strengths tuned
1752 | Nanoscale, 2014, 6, 1748–1755
by the synthesis parameters. Fig. 5a shows that at a lower strain (17%), the top low density sponge (sponge 1, about 14.3 mg cm3) layer deformed rst. When the strain increased to 35%, the top sponge was fully compressed and the bottom sponge (sponge 2, about 33.5 mg cm3) layer with a higher density started to deform. This sandwich structure demonstrated a sequential deformation mechanism in which the two sponge layers were compressed rst while the array (about 320.0 mg cm3) in the middle was the last to deform. The strain of the composite was determined by the strain of each layer. It can be calculated by: 3composite ¼ x13sponge 1 + x23sponge 2 + x33array, where x1, x2, and x3 are the thickness ratios of sponge 1, sponge 2 and array in the composite, like the double-layered composite.28 The deformation pattern (from low density sponge to array) did not change when the composite was turned over to let the lowest density in the bottom of the pile. The s–3 curve of the triple-layered composite exhibited three stress plateaus corresponding to the different deformation stages of each layer in the composite (Fig. 5b). At lower stresses (1 MPa), both sponge layers had been densied, and then the array started to deform by forming buckles similar to that in the double-layered composite. The buckle formation normally started at the top of
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Paper
Nanoscale
Microscopic deformation and compression properties of a sponge–array–sponge triple-layered composite. (a) In situ SEM observations of the deformation of the triple-layered composite. (b) Compressive stress–strain (s–3) curves of the triple-layered composite, showing three compressive stress plateaus.
Fig. 5
arrays due to the lower density and more drawbacks in this region resulted from the uctuated growing environment of this layer. Here we obtained a sandwich material with relatively so surface (sponge) layers and a hard core (array), all consisting of only CNTs. This result indicated a possibility of combining multiple CNT layers with tailored microstructures to enable controlled deformation of each layer in predened sequence.
Conclusion We systematically investigated the deformation mechanism of all-CNT energy absorption materials composed of random sponges and aligned arrays integrated into double- and triplelayered structures, and correlated the microstructure evolution with stress–strain curves. We found that random CNTs in the sponge layer mainly deformed by forming parallel bundles along the direction perpendicular to the compressive stress, while aligned CNT arrays formed buckles under compression. For both double- and triple-layered structures, the sponge layers deformed rst producing low stress plateaus and large strain, while the array layer deformed last producing a stress plateau region with a much larger stress and modulus. Our results could be useful for further understanding the deformation mechanism of compressible CNT structures, and designing hierarchical composites with tailored microstructures and energy absorption properties.
Experimental Synthesis of CNT sponge, array, double-layered and multilayered composites CNT sponge, array and their composites were synthesized by CVD processes using the same system containing a tube furnace, as reported in our previous work.28 For growing CNT sponges, ferrocene was dissolved in dichlorobenzene to prepare a carbon source solution with a concentration of 60 mg mL1, and the supply rate was set as 0.1–0.4 mL min1. For growing CNT arrays, ferrocene dissolved in xylene at a concentration of 20 mg mL1 was used as carbon source solution and the supply rate was 0.4 mL min1. To fabricate double-layered composites, CNT arrays were rst grown on quartz for a growth period of 60 minutes. Aer that, the ferrocene–xylene carbon source
This journal is © The Royal Society of Chemistry 2014
solution was immediately switched to ferrocene–dichlorobenzene solution to grow a sponge layer on top of the array within a controlled growth period. In order to fabricate triple-layered composites, a double-layered structure was peeled off from the substrate by a scalpel and turned over to let the array on the top. Then a third CVD process was used to grow a sponge layer on the other side of the array to obtain a sponge–array–sponge composite. During the CVD process, we adopted a reaction temperature of 850 C, and a mixture of carrier gas of argon and hydrogen at ow rates of 2000 and 300 sccm, respectively.
Characterization of the CNT composite The microstructure and morphology of sponge, array and their composites were characterized by scanning electron microscopy (S-4800). By a miniature compression station, in situ SEM observations of the layered composite during cyclic compression were recorded. To measure the densities of individual sponges and arrays, each layer CNT in a triple-layered structure was separated by a scalpel. The density of each layer was obtained by dividing the mass by its volume.
Mechanical properties of the CNT composite Mechanical properties were determined using Instron 5943 equipped with a 1 kN load cell and tow at-surface compression stages. CNT composites were cut into small blocks by a scalpel with sizes of about 7 7 3 (thickness) mm3. The top compression stage was chosen to move down to contact the sample surface and the compression rates were set in the range of 1 mm min1.
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant no. 51102286), the Guangdong Natural Science Foundation (Grant no. S2012040007893), the Beijing Science and Technology Program (Grant no. Z121100001312005), and the Fundamental Research Funds for the Central Universities (no. 12lgpy16).
Nanoscale, 2014, 6, 1748–1755 | 1753
View Article Online
Nanoscale
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
Notes and references 1 J. H. Lee, J. P. Singer and E. L. Thomas, Micro-/ Nanostructured Mechanical Metamaterials, Adv. Mater., 2012, 24, 4782–4810. 2 T. A. Schaedler, A. J. Jacobsen, A. Torrents, A. E. Sorensen, J. Lian, J. R. Greer, L. Valdevit and W. B. Carter, Ultralight Metallic Microlattices, Science, 2011, 334, 962–965. 3 S. O. Kucheyev, M. Stadermann, S. J. Shin, J. H. Satcher, S. A. Gammon, S. A. Letts, T. van Buuren and A. V. Hamza, Super-Compressibility of Ultralow-Density Nanoporous Silica, Adv. Mater., 2012, 24, 776–780. 4 C. C. Yu, Z. G. Chen, H. Li, J. Turner, X. C. Zeng, Z. H. Jin, J. Y. Jiang, B. Youssef and L. Tan, Molecularly Intercalated Nanoakes: A Supramolecular Composite for Strong Energy Absorption, Adv. Mater., 2010, 22, 4457–4461. 5 L. Y. Sun, R. F. Gibson, F. Gordaninejad and J. Suhr, Energy absorption capability of nanocomposites: A review, Compos. Sci. Technol., 2009, 69, 2392–2409. 6 J. H. Lee, L. F. Wang, M. C. Boyce and E. L. Thomas, Periodic Bicontinuous Composites for High Specic Energy Absorption, Nano Lett., 2012, 12, 4392–4396. 7 A. Misra and P. Kumar, Periodic architecture for high performance shock absorbing composites, Sci. Rep., 2013, 3, 2056. 8 J. Sargianis, H. I. Kim and J. Suhr, Natural Cork Agglomerate Employed as an Environmentally Friendly Solution for Quiet Sandwich Composites, Sci. Rep., 2012, 2, 403. 9 J. H. Lee, L. F. Wang, S. Kooi, M. C. Boyce and E. L. Thomas, Enhanced Energy Dissipation in Periodic Epoxy Nanoframes, Nano Lett., 2010, 10, 2592–2597. 10 M. De Volder, S. H. Tawck, R. H. Baughman and A. J. Hart, Carbon Nanotubes: Present and Future Commercial Applications, Science, 2013, 339, 535–539. 11 S. Pathak, E. J. Lim, P. Pour Shahid Saeed Abadi, S. Graham, B. A. Cola and J. R. Greer, Higher Recovery and Better Energy Dissipation at Faster Strain Rates in Carbon Nanotube Bundles: An in-Situ Study, ACS Nano, 2012, 6, 2189–2197. 12 B. J. Carey, P. K. Patra, G. M. Hahm and P. M. Ajayan, FoamLike Behavior in Compliant, Continuously Reinforced Nanocomposites, Adv. Mater., 2013, 23, 3002–3007. 13 K. H. Kim, Y. Oh and M. F. Islam, Graphene coating makes carbon nanotube aerogels superelastic and resistant to fatigue, Nat. Nanotechnol., 2012, 7, 562–566. 14 H. Hu, Z. B. Zhao, W. B. Wan, Y. Gogotsi and J. S. Qiu, Ultralight and Highly Compressible Graphene Aerogels, Adv. Mater., 2013, 25, 2219–2223. 15 A. Misra, J. R. Raney, L. De Nardo, A. E. Craig and C. Daraio, Synthesis and Characterization of Carbon Nanotube– Polymer Multilayer Structures, ACS Nano, 2011, 5, 7713– 7721. 16 X. C. Gui, J. Q. Wei, K. L. Wang, A. Y. Cao, H. W. Zhu, Y. Jia, Q. K. Shu and D. H. Wu, Carbon Nanotube Sponges, Adv. Mater., 2010, 22, 617–621.
1754 | Nanoscale, 2014, 6, 1748–1755
Paper
17 Q. Zhang, M. Q. Zhao, Y. Liu, A. Y. Cao, W. Z. Qian, Y. F. Lu and F. Wei, Energy-Absorbing Hybrid Composites Based on Alternate Carbon-Nanotube and Inorganic Layers, Adv. Mater., 2009, 21, 2876–2881. 18 O. Yaglioglu, A. Y. Cao, A. J. Hart, R. Martens and A. H. Slocum, Wide Range Control of Microstructure and Mechanical Properties of Carbon Nanotube Forests: A Comparison Between Fixed and Floating Catalyst CVD Techniques, Adv. Funct. Mater., 2012, 22, 5028–5037. 19 A. Y. Cao, P. L. Dickrell, W. G. Sawyer, M. N. Ghasemi-Nejhad and P. M. Ajayan, Super-compressible foamlike carbon nanotube lms, Science, 2005, 310, 1307–1310. 20 S. B. Hutchens, L. J. Hall and J. R. Greer, In situ Mechanical Testing Reveals Periodic Buckle Nucleation and Propagation in Carbon Nanotube Bundles, Adv. Funct. Mater., 2010, 20, 2338–2346. 21 M. R. Maschmann, G. J. Ehlert, S. J. Park, D. Mollenhauer, B. Maruyama, A. J. Hart and J. W. Baur, Visualizing Strain Evolution and Coordinated Buckling within CNT Arrays by In Situ Digital Image Correlation, Adv. Funct. Mater., 2012, 22, 4686–4695. 22 P. Pour Shahid Saeed Abadi, S. B. Hutchens, J. R. Greer, B. A. Cola and S. Graham, Effects of morphology on the micro-compression response of carbon nanotube forests, Nanoscale, 2012, 4, 3373–3380. 23 M. R. Maschmann, Q. H. Zhang, R. Wheeler, F. Du, L. M. Dai and J. Baur, In situ SEM Observation of Column-like and Foam-like CNT Array Nanoindentation, ACS Appl. Mater. Interfaces, 2011, 3, 648–653. 24 M. Xu, D. N. Futaba, T. Yamada, M. Yumura and K. Hata, Carbon Nanotubes with Temperature-Invariant Viscoelasticity from-196 degrees to 1000 degrees C, Science, 2010, 330, 1364–1368. 25 X. W. Tang, X. C. Gui, Y. L. Liu and B. Wang, Rheological Model Predicting Compressive Responses of Carbon Nanotube Networks, RSC Adv., 2013, 3, 14473–14479. 26 J. R. Raney, F. Fraternali, A. Amendola and C. Daraio, Modeling and in situ identication of material parameters for layered structures based on carbon nanotube arrays, Compos. Struct., 2011, 93, 3013–3018. 27 Y. H. Lin, Y. C. Lai, C. L. Lu and W. K. Hsu, Excellent cushioning by polymer-concreted arrays of aligned carbon nanotubes, J. Mater. Chem., 2011, 21, 12485–12488. 28 Z. P. Zeng, X. C. Gui, Z. Q. Lin, L. H. Zhang, Y. Jia, A. Y. Cao, Y. Zhu, R. Xiang, T. Z. Wu and Z. K. Tang, Carbon Nanotube Sponge–Array Tandem Composites with Extended Energy Absorption Range, Adv. Mater., 2013, 25, 1185–1191. 29 C. H. Cao, A. Reiner, C. H. Chung, S. H. Chang, I. Kao, R. V. Kukta and C. S. Korach, Buckling initiation and displacement dependence in compression of vertically aligned carbon nanotube arrays, Carbon, 2011, 49, 3190– 3199. 30 S. Pathak, N. Mohan, E. Decolvenaere, A. Needleman, M. Bedewy, A. J. Hart and J. R. Greer, Local relative density modulates failure and strength in vertically aligned carbon nanotubes, ACS Nano, 2013, 7, 8593–8604.
This journal is © The Royal Society of Chemistry 2014
View Article Online
Paper
Soening–Hardening Solids, Acta Mech. Sin., 2012, 28, 1115–1124. 33 S. Pathak, J. R. Raney and C. Daraio, Effect of morphology on the strain recovery of vertically aligned carbon nanotube arrays: An in situ study, Carbon, 2013, 63, 303–316.
Published on 27 November 2013. Downloaded by University of Michigan Library on 30/10/2014 09:54:43.
31 S. B. Hutchens, A. Needleman and J. R. Greer, Analysis of uniaxial compression of vertically aligned carbon nanotubes, J. Mech. Phys. Solids, 2011, 59, 2227–2237. 32 A. Needleman, S. B. Hutchens, N. Mohan and J. R. Greer, Deformation of Plastically Compressible Hardening–
Nanoscale
This journal is © The Royal Society of Chemistry 2014
Nanoscale, 2014, 6, 1748–1755 | 1755
