by: Varun Rajan, Brown ’09
In a list of the most important materials in nanotechnology today, carbon nanotubes are ranked near the top.1 Consisting solely of carbon atoms linked in a hexagonal pattern, these cylindrical molecules are far longer than they are wide, similar to rods or ropes. The prefix nano- means one billionth, which refers to a nanotube’s diameter of a few nanometers. In part because of their unique size, shape, and structure, carbon nanotubes (CNTs) are exceedingly versatile. Proposed areas of application for CNTs range from electronics and semiconductors, to molecular-level microscopes and sensors, to hydrogen storage and batteries.2 However, CNTs’ special combination of strength, low density, and ductility has also led to speculation about their role as “superstrong materials”3 in structural applications, such as a “space elevator”4. Before these science fiction claims become engineering feats, basic questions about carbon nanotubes’ mechanical behavior must be answered. In his research over the past twelve years, Dr. Boris Yakobson, a Rice Professor in Materials Science, has tackled several fundamental questions concerning the failure of nanotubes and the behavior of dislocations. The materials science term “dislocation” refers to a line imperfection or defect in the arrangement of atoms in a CNT; dislocations are important because they affect a material’s mechanical properties.5
How do nanotubes fail?
Determining how carbon nanotubes fail, or lose their capacity to support loads, is a complicated yet important matter; it must be fully understood before nanotubes are used in structural applications. In the article “Assessing Carbon Nanotube Strength” Yakobson, along with then-postdoctoral student Traian Dumitrica and graduate student Ming Hua, used computer simulation to model CNTs and investigate their failure.
According to Yakobson, simulations are valuable because “in principle you have full access to the details of the structure.” He added that one of the advantages of simulations is that the researcher has full control over the experimental conditions and variables. With respect to carbon nanotube failure, some of the most pertinent variables include the level and duration of the applied load, as well as the nanotube’s temperature, diameter, and chiral angle – the angle ranging from 0 to 30 degrees that describes how a carbon nanotube is rolled up from a graphite sheet. In addition to affecting the nanotube’s strain (stretch) at failure, these variables also determine the process by which it breaks. Yakobson found that two different mechanisms can cause nanotube failure. At low temperatures, the mechanical failure dominates as the bonds between adjacent carbon atoms literally snap. On the other hand, high temperatures induce the bonds within the nanotube’s carbon hexagons to flip, causing the hexagons to become five- and seven-sided figures. This effect weakens the nanotube structure and initiates a sequence of processes that culminate in complete nanotube failure. Combining the results of numerical simulations and analytical techniques, Yakobson constructed a carbon nanotube strength map: a single figure that illustrates the relationship between the relevant variables, the failure mechanisms, and the failure strain (figure 2). The significance of Yakobson’s research led to its publication as the cover article for the April 18, 2006 issue of Proceedings of the National Academy of Sciences.
How do dislocations behave in carbon nanotubes?
Another area covered by Yakobson’s work was the study of dislocation behaviors. While dislocation dynamics in multiwalled carbon nanotubes might seem to be a subject only a materials scientist could love, this area of research has great bearing on CNT use in mechanical and electronic applications. Multiwalled carbon nanotubes (MWCNTs) can be visualized as many single-walled carbon nanotubes, arranged concentrically like tree-trunk rings and interacting with each other via weak intermolecular forces.6,7 Although somewhat difficult to visualize, dislocations — defects in the atomic structure of a CNT —can be viewed as objects that can move, climb, and collide with one another, leading to the term “dislocation dynamics.”
In his research on this topic, Yakobson collaborated with J.Y. Huang at Sandia National Laboratory and F. Ding, a research scientist in Yakobson’s group. Their experimental procedure involves heating a MWCNT to approximately 2000° C, which causes its dislocations to mobilize. Using a transmission electron microscope, they tracked the motion and interaction of these dislocations over time. Yakobson said that this powerful microscope gives the experimenters “nearly atomic resolution.” A resolving power of this magnitude creates spectacular images that reveal a rather odd phenomenon: one can observe a dislocation climbing a carbon nanotube wall and combining with a dislocation on an adjacent wall to form a larger dislocation loop, which then continues to climb (figure 3). If this process is repeated throughout the MWCNT, its entire structure becomes a mixture of ‘nanocracks’ and kinks. More importantly, adjacent walls are cross-linked together by covalent bonds, whereas formerly they were only weakly connected by van der Waals forces. Cross-linking is important because it “lock[s] the walls together in one entity,” Yakobson said. As a result, there is an increased possibility for transfer between walls, and current can be driven through the cross-linked junction. He also believes that cross-linking is somehow responsible for the mechanical strength in MWCNTs, because the concentric cylinders can no longer easily slide past one another.
Yakobson’s research is simultaneously old and new. It is old because subjects such as dislocation dynamics and material failure are well-understood for many materials. Yet, it is also ingenuous because knowledge in these fields cannot be extended easily to carbon nanotubes.8 Researchers in this field are treading on unexplored ground that will bring the nanotube a step closer toward its applications.
References
1.Arnall, A.H. Future Technologies, Today’s Choices: Nanotechnology, Artificial Intelligence and Robotics; A Technical, Political and Institutional Map of Emerging Technologies, Greenpeace Environmental Trust, London, 2003.
2.Collins, P.; Avouris, P. Scientific American 2000, 62-69.
3.Chae, H.; Kumar, S. Science 2008, 319, 908-909.
4.University of Delaware. Space Tourism To Rocket In This Century, Researchers Predict. http://www.sciencedaily.com/releases/2008/02/080222095432.htm (accessed 02/27/08), part of Science Daily. http://www.sciencedaily.com/ (accessed 02/27/08).
5.Dumitrica, T.; Hua, M.; Yakobson, B. Proc. Nat. Aca. Sci. 2006, 103, 6105-6109.
6.Cumings, J.; Zettl, A. Science 2000, 289, 602-604.
7.Baughman, R.,; Zakhidov, A.; de Heer, W. Science 2002, 297, 787-792.
8.Huang, J.Y.; Ding, F.; Yakobson, B. Physical Review Letters 2008, 100, 035503.