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Ballistic Fracturing of Carbon Nanotubes Sehmus Ozden, Leonardo D. Machado, Chandra Sekhar Tiwary, Pedro Alves da Silva Autreto, Robert Vajtai, Enrique V. Barrera, Douglas Soares Galvao, and Pulickel M Ajayan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07547 • Publication Date (Web): 26 Aug 2016 Downloaded from http://pubs.acs.org on August 30, 2016

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ACS Applied Materials & Interfaces

Ballistic Fracturing of Carbon Nanotubes Sehmus Ozden1‡*, Leonardo D. Machado2,3,‡, ChandraSekhar Tiwary1,‡, Pedro A. S. Autreto 2,4, Robert Vajtai1, Enrique V. Barrera1, Douglas S. Galvao2*, Pulickel M. Ajayan1,* 1

Department of Material Science and NanoEngineering, Rice University, Houston, Texas

77005,USA, 2

Applied Physics Department, State University of Campinas, Campinas-SP, 13083-959, Brazil,

3

Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte,

Natal-RN, 59072-970, Brazil 4

Universidade Federal do ABC, Santo André-SP, 09210-580, Brazil

Abstract Advanced materials with multifunctional capabilities and high resistance to hypervelocity impact are of great interest to the designers of aerospace structures. Carbon nanotubes (CNTs) with their lightweight and high strength properties are alternative to metals and/or metallic alloys conventionally used in aerospace applications. Here we report a detailed study on the ballistic fracturing of CNTs for different velocity ranges. Our results show that the highly energetic impacts cause bond breakage and carbon atom rehybridizations, and sometimes extensive structural reconstructions were also observed. Experimental observations show the formation of nanoribbons, nanodiamonds and covalently interconnected nanostructures, depending on impact conditions. Fully atomistic reactive molecular dynamics simulations were also carried out in order to gain further insights into the mechanism behind the transformation of CNTs. The

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simulations show that the velocity and relative orientation of the multiple colliding nanotubes are critical to determine the impact outcome. KEYWORDS: High Impact, Unzipping, Carbon nanotubes, Junction, MD simulation

A hypervelocity impact (HVI) creates strong shock waves which impart large amounts of energy on both the impacting and the target materials1. Hypervelocity impacts can produce extreme shock pressures on the materials, causing stresses that are much larger than their strength and it is one of the crucial factors to be considered in the design of aerospace structures2,3. Structures used in space vehicles are under the risk of impact with various projectiles at varied velocities in outer space. Such accelerating projectile motion gives rise to ballistic deformation (> 105 /s i.e. high strain deformation). The consequences of these impacts can be destructive on the surface of spacecraft and satellites. To prevent aerospace vehicles from these kinds of damages, spacecraft and satellites designers need to be aware of the response of various components and structural elements under different velocity impact loading conditions. Up to now, different types of materials, such as metals and composites have been used for aerospace applications4-6. One of the major concerns of next generation aerospace vehicles is the need for materials with the unique combination of properties, such as ultralight-weight, flexible, radiation resistant, with low solar absorptivity, high thermal emissivity, electrical conductivity, tear resistance and good mechanical properties7. These requirements are needed for advanced materials and systems that incorporate these functionalities. CNTs are ideal candidates, since nanotubes have the required physical and mechanical properties with the advantage of being light-weight. Carbon nanotubes have been extensively studied in the last two decades for possible use in different types of engineered materials and applications, such as spacecraft,


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satellites, damping and armor design8-17. Before CNTs can be used in aerospace applications, however, their response to high impact ballistic condition must be well understood. Although several theoretical and experimental works have been reported on the hypervelocity impact of polymer composites of CNTs, the ballistic fracturing of pristine CNTs at high impact is still not completely understood18,19. In our previous report, we showed that impacts at 6.9 km/s induced high mechanical strain, causing crack propagation along the tube axis, which resulted in unzipping of nanotubes. The experimental observations were further supported by Molecular Dynamics (MD) simulations, which revealed that the impact outcome depended on the relative orientation of the CNT and the substrate19. Here we report an expanded study on the ballistic fracturing of CNTs under mechanical impacts considering a large range of velocities. We have also studied how nanotubes interacted during collisions. Impact loads were classified into three categories: low-velocity (3.9 km/s), high-velocity (5.2 km/s) and hyper-velocity (6.9 km/s) impacts. Lower velocity values were not investigated because the fraction of undamaged nanotubes was already high at this velocity, and higher velocity values were not tested due to equipment limitations. Three tests were carried out at each category, with a small (~0.1 km/s) velocity variance. Fully atomistic reactive molecular dynamics simulations were used to get further understanding on the response of nanotubes, their deformations, as well as their structural changes under high-energy mechanical impacts. CNTs (multi-walled tubes, diameters from 5-30 nm, lengths 10-30 µm) were synthesized by water-assisted chemical vapor deposition (WACVD) on a Si substrate20. The synthesized CNTs were manually packed as pellets (2mm diameter size) and fed into a two-stage light gas gun (LGG) (Figure S1). The spherically shaped CNT pellets were used as a projectile against an aluminum target, at the three-velocity range mentioned above. The packed nanotubes are



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randomly oriented and hit the target at varied orientations (Figure 1). Because of the high impact, the spherical CNT samples penetrate and/or disperse on the target surface (Figure S4). In order to further understand the mechanism behind experimental results, simulations were carried out using the Reactive Force Field (ReaxFF)21 as implemented in the LAMMPS MD package22. There are ReaxFF parameters designed for carbon nanostructures, which, for instance, were able to obtain good agreement with experiments in the description of the initial stages of CNT formation23. ReaxFF has good accuracy when compared to quantum methods, at a greatly reduced computational cost21, and has compared favorably against other classical force fields at reproducing experimental shock results for hydrocarbons24,25.This enabled us to study large systems (the structures considered here contain up to 32000 atoms) and for long simulation times. Typical simulations were over 500000 time-steps of 0.01 femtoseconds. We have considered single, double and triple-walled tubes, with diameters varying from 1.36 and 2.03 nm and lengths varying from 10 and 20 nm, as well as different chiralities. The targets consisted of 12-6 van der Waals walls: atoms that were less than 10 angstroms (Å) from the target would be attracted/repelled by it if they were farther/closer than 3.55 Å. A potential energy well depth of 0.07 kcal/mol was used. We employed non-periodic boundary conditions. In order to avoid spurious thermal effects before running the MD simulations, we first minimized and then equilibrated the nanotubes at 300K using a standard Nosé-Hoover thermostat. Subsequently, the thermostat was turned off and the structures were shot against the targets. That is, the impact simulations were carried out in the NVE ensemble, in which the number of atoms (N), volume (V) and energy (E) are kept constant. See Supporting Information for additional details. MD simulations allowed us to vary CNT velocities, the number of tube walls and relative tube orientations in relation to the targets. When multiple CNTs were involved, it was important to


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consider not only how they were oriented regarding the target substrate, but also among themselves. The relative tube orientation cases considered here are indicated in Figure 1. For the green, orange and blue configurations, all CNTs are parallel to the target. For the green and orange configurations, one nanotube is atop the other; for the latter, the angle between nanotubes is 90⁰. For the blue configuration, the nanotubes are positioned side-by-side. Finally, for the red configuration, one of the nanotubes is perpendicular to the target. Note that in the side-by-side (blue) configuration both CNTs touch the target simultaneously, while in the vertical (green) configuration only the bottom CNT touches the target – and is then further pressured against it by the top nanotube. In the experiments, the samples were collected from the aluminum target after impacts and characterized by Raman Spectroscopy at laser excitation of 633 nm. CNT Raman spectra typically have two peaks associated to G-band and the D-band, respectively. While the first one is related to sp2 vibrations, the second one comes from the defects on the CNT sidewalls26,27. The defect density in the CNT sidewalls can be determined from the ratio between D-band and G-band (ID:IG), which can be used to obtain information about structural changes26.The ID:IG ratio of the as-grown CNT pellets is 0.6. As shown in figure 2a, the defect density increases as the impact velocity increases. The highest obtained value of ID:IG was 1.08 for the hypervelocity case. The increased ID:IG ratio with increased velocity shows that, as expected, increasing impact velocity results in more CNT structural deformations. Additionally, the structural deformations of CNTs is corroborated by JEOL 2100 field emission gun TEM characterization at operating at 200 kV : large amounts of intact nanotubes can still be found in samples resulting from low-velocity impacts (Figure 2b), while large amounts of graphene nanoribbons (GNRs) produced from fractured tubes, can be found in samples from hypervelocity impacts (Figure 2c). Scanning electron microscope images also reveal similar results (Figure S2).



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While some CNT bundles survive on the target at low and high impact velocity, CNTs were not observed on the target material at hypervelocity impact (Figure S2). Nanotubes before and after velocity impact tests were also characterized using PHI Quantera X-ray photoelectron spectroscopy (XPS) using an AlKα x-ray tube (Figure S3). XPS spectra show that there is no impurity on the surface of the nanotubes after the impact. The peak at 284.6 eV correspondsto C-C covalent bond and the peak at 285.4 eV can be attributed to O-C=O covalent bonds, which is also present in the pristine CNT samples. After the velocity impact, C=C, C-C and O-C=O bonds appear around 283.5 eV, 284.6 eV and 287 eV respectively19, 20. Still regarding the characterization of the CNTs after the impact, it should be noted that the role of some possibly important factors, such as the number of walls and CNT orientation, could not be directly be obtained from the experiments. MD simulations were also carried out to further gain insights on these features. Velocity values used in the simulated impact tests fall in the range between 4.5 and 6.5 km/s, closely mimicking the range used in experiments (3.9-6.9 km/s). Since the ID:IG ratio reveals sizable defect density in the synthesized CNTs, we carried out molecular dynamics simulations to test whether they play a critical role. Our results, presented in figure S7 and movie S3, indicate nanotube fracture patterns are similar for samples without defects, with vacancies, or with Stone-Wales defects. All other simulations were performed using only defect-free CNTs, due to computational limitations. Regarding the number of walls, our simulations showed that, in comparison to the velocity values, this variable played a lesser role in determining the fraction of broken bonds (Figure 2d), but single-walled CNTs exhibited the highest fracture levels. Regarding the relative CNT orientations to the targets, our simulations show that CNT perpendicular configuration greatly increases the fraction of broken


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bonds for lower velocities (Figure 2e). As the velocity value increases, the differences in relation to other configurations decrease. The highly energetic collisions can also lead to the rehybridization of carbon atoms: TEM images (Figures 3a and 3b) show that nanotubes can be transformed into onion-like nanodiamonds and graphitic carbon under hypervelocity impacts. The Diffraction pattern of TEM analysis confirmed their presence in the samples (Figure 3c). We have also carried out MD simulations to determine the conditions that favor the formation of sp3 carbon structures. Regarding CNT orientation, the configuration of nanotubes perpendicular to the target generally precludes the formation of sp3 carbon (Figure 3e). Rather than leading to structural reconstructions, this arrangement results in a high fraction of broken bonds, as shown previously in Figure 2e. Conversely, while the results of Figure 2d showed that SWNT had a greater fraction of broken bonds, the ones of figure 3d reveal that SWNTs have a smaller sp3 carbon fraction.. Increasing the velocity beyond 5.5 km/s actually tended to reduce the sp3 carbon fraction as shown in Figures 3d and 3e. Another interesting finding of the TEM imaging of the high impact collision samples was the observation of welded GNRs and CNTs (Figure 4a and Figure 4b, respectively). The samples used to obtain these images were both from the hypervelocity cases. Once again we employed MD simulations to investigate the conditions that favored the formation of welded structures. Overall, of the four configurations shown in Figure 1, only two yielded junctions for the tested velocities: (i) the one in blue, in which the CNTs fall parallel to the substrate side-by-side; (ii) the one in orange, in which both CNTs are parallel to the substrate, but one is partially on top of the other. Regarding case (i), GNR-GNR junctions were observed when two CNTs unzipped and were close enough to weld after unzipping, but not too close to repelling each other during the



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unzipping process. Figure 4c shows an example in which welding was observed. In this case, two DWNTs with a diameter of 20 Å were separated by a distance of 15 Å, and collided with the target at a velocity of 6.0 km/s. The full collision trajectory is shown in Movie S1, which also highlights a sp3 carbon region formed during the impact. The inter-tube junctions formed during impact exhibit a high level of structural defects. Pentagons, heptagons, octagons, as well as, partially open carbon rings, and out of plane atoms were observed (Figure 4d). Regarding case (ii), both GNR-GNR and CNT-CNT junctions were observed, depending on the collision velocity values. For a velocity of 5.5 km/s, the energy provided during impact was not enough to unzip the nanotubes, but it was enough to create crosslinks between the CNTs at the overlap region. According to previous results19, unzipping occurs when local von Mises stress values are greater than the yield strength of the material for a large fraction of atoms. As this condition is not met for this velocity, unzipping is not observed. However, stress is highest at the overlap region, as the bottom nanotube is pressured both by the target and by the top CNT. The formation of crosslinks indicates that local stress values did surpass the yield strength in this region. For a velocity of 6.0 km/s, tube unzipping occurred, and inter-ribbon bonds were observed at the overlap region (Movie S2). Knowledge of the configurations that yield junctions could be employed to design arrays of covalently bonded CNTs, provided they are properly oriented. We carried out MD simulations to exemplify this concept for the configuration shown in Figure 5. Figure 5a shows a square arrangement of CNTs, while Figure 5b shows the resulting structure after impact against a target at a velocity of 5.2 km/s. We observed inter-tube junctions in the four regions in which the nanotubes overlapped. The evolution of the potential and kinetic energies during impact is presented in Figure 5c. The initial kinetic energy corresponds to the thermal energy of the CNT


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atoms at 300 K. At 1ps the array was shot against the target, leading to a significant increase in the total kinetic energy of the system. The collision occurred at the instant marked by the red dotted line, and during the following picoseconds, the massive kinetic energy was gradually converted into potential energy (Figure S5). This increase was caused by the bonds fractures. Even in the cases in which reconstruction occurred, the resulting atomic arrangements were often less stable than the initial perfect hexagons. Figure 5d highlights one of the four obtained junctions, in which even a nonagon can be observed. More than half of the initially given kinetic energy was converted into potential energy during the course of the collision – and part of it was used to create the bonds that cross-linked the nanotubes. It should be noted that, as long as CNTs are close to parallel to the substrate and their length intersect, inter-nanotube covalent bonding can be formed (Figure S6) . Oriented nanotube structures, such as CNT forests, could aid in achieving such goal. In summary, we performed experiments and MD simulations to investigate the structural and dynamical aspects of CNT high-velocity impact against metallic targets. We investigated how the velocity, number of tube walls and relative orientation of the CNT in relation to the target determine the resulting structures after of high-velocity impact. Our results show that, for a given target, the velocity and orientation of the nanotubes are the key factors that determine whether and how fractures occur during collisions. For certain orientations, fractures and structural rearrangements can occur (for instance carbon hybridization change from sp2 to sp3 ones) leading to the formation of nanodiamonds. TEM microscopy and diffraction patterns were used to confirm the presence of nanodiamonds in samples resulting from hypervelocity (v = 6.9 km/s) impacts. In addition, welded GNRs and CNTs can also result from the highly energetic collisions. MD simulations were used to determine which configurations of CNTs were likely to



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generate junctions at the typical velocities employed in the experiments. We have also used simulations to demonstrate that, with proper control of the velocity and initial CNT arrangements, it is possible to produce covalently bonded nanotube structures with arbitrary shapes. Our results show that high impact collisions can be used to produce nanodiamonds and possibly even complex cross-linked CNT networks in a chemical-free single step process. We hope the present work will stimulate further works along these lines. ASSOCIATED CONTENT Supporting Information. Figure S1: schematic of the experimental set-up for hypervelocity impact. Figure S2: Scanning Electron Microscope (SEM) images of different velocity impact. Figure S3: XPS characterization of nanotubes at different velocity impact. Figure S4: The digital image of CNTs spherical ball and Al target before and after impact test. Figure S5: Energy profile for a single CNT impact. Figure S6: MD simulation results for a non-orthogonal CNT array. Additional simulation details are also presented. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *Email: [email protected] *Email: [email protected] *Email: [email protected] Author Contributions ‡These authors contributed equally.


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ACKNOWLEDGMENTS This work has been supported by U.S. Department of Defense: U.S. Air Force Office of Scientific Research for the Project MURI: “Synthesis and Characterization of 3-D Carbon Nanotube Solid Networks” Award No. FA9550-12-1-0035. LDM, PASA and DSG thank the Center for Computational Engineering and Sciences at Unicamp for financial support through the FAPESP/CEPID Grant # 2013/08293-7. EVB thanks to Johnson Space Center for financial support through the “Advance Transparent Materials for Shielding and Produce Self-Healing Materials for Pressure Wall Use”, Award No. NNX16AC36G. LDM acknowledges financial support from the Brazilian Federal Agency CAPES via its PNPD program. References (1)

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Ballistic Fracturing of Carbon Nanotubes Sehmus Ozden1‡*, Leonardo D. Machado2,3,‡, ChandraSekhar Tiwary1,‡, Pedro A. S. Autreto 2,4, Robert Vajtai1, Enrique V. Barrera1, Douglas S. Galvao2, Pulickel M. Ajayan1,* 1

Department of Material Science and NanoEngineering, Rice University, Houston, Texas

77005,USA,


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2

Applied Physics Department, State University of Campinas, Campinas-SP, 13083-959, Brazil,

3

Departamento de Física Teórica e Experimental, Universidade Federal do Rio Grande do Norte,

Natal-RN, 59072-970, Brazil 4

Universidade Federal do ABC, Santo André-SP, 09210-580, Brazil

‡ These authors contributed equally to this work *Corresponding Author: [email protected], [email protected]



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Figure 1. Schematic view of hyper-velocity experiments and simulations. In the former, a pellet composed of randomly oriented CNTs is shot against an aluminum target. In the latter, a pair of nanotubes at diverse orientations is shot against a solid target.

Figure 2. (a) Raman spectra of CNTs before and after impact at various velocities.(b) TEM images of CNTs after impact at low-velocity and (c) hypervelocity, (d) and (e) results from MD simulations detailing the fraction of broken bonds versus the velocity for various configurations. For (d), we carried out studies with five samples to verify whether multiple simulation runs would yield varying percentage of broken bonds. Overall results were consistent for the different samples. In that figure, dots represent average percentages, and error bars display standard deviations.


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Figure 3. (a) and (b) are TEM images showing nanodiamonds at different magnifications, (c) the diffraction pattern of nanodiamonds, (d-e) results from MD simulations detailing the fraction of sp3 carbon versus velocity for various configurations. For (d), we carried out studies with five samples to verify whether multiple simulation runs would yield the varying percentage of sp3 carbon. Results were consistent for the different samples, with some variation. In that figure, dots represent average percentages, and error bars display standard deviations.



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Figure 4. (a) TEM images of welded GNRs and (b) CNTs (c) Results of MD simulations in which a grain-boundary like region formed at the intersection of two welded GNRs. (d) Zoom-in of the welded region.


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Figure 5. (a-b) are, respectively, snapshots of an array of CNTs before and after impact at 5.2 km/s against a solid target. (c) Evolution of the kinetic and potential energies during the simulation (d) a zoom-in of CNT-CNT junction.


Ballistic Fracturing of Carbon Nanotubes - ACS Applied Materials

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