Time, temperature, and load: The flaws of carbon nanotubes

The “mechanics of nanostructures” is of intrinsic and practical interest. An acorn turning into an oak tree can lead one to consider the (often unknown) mechanical forces exerted by, and acting on, nanostructures present in the tree. A mantra of nanotechnology [which may ultimately outpace (1) “natural” evolution] is having “a place for every atom and every atom in its place” (www.foresight.org/nano/whatismm.html). What level of perfection might be achieved considering the known laws of physics and the constraints of chemistry? In principle, there is no limitation to achieving essentially perfect covalent bonding in material structures. With increasing atom number, a size is eventually reached where the defect-free structure is not the most stable (consider the role of entropy) (2), but it may be kinetically stable if there are high barriers to the nucleation of defects. In a recent issue of PNAS, Dumitrica et al. (3) consider carbon nanotubes (CNTs) and, building on prior theoretical work by themselves and others, present the pathways to failure caused by tensile load as a function of time and temperature. Because CNTs can have different chiralities, the issue of the orientation of the C—C bonds in the different CNTs is treated and shown to critically influence the ultimate strength, the type of defects that nucleate and how they grow or propagate, and the modeled time to failure (3).

The possibility of having structures entirely free of defects would seem more likely for small structures than large structures, and living organisms routinely achieve such perfection. The remarkable mechanics of biological motors (4, 5) and viral DNA packaging and ejection …

*E-mail: r-ruoff{at}northwestern.edu