Time, temperature, and load: the flaws of carbon nanotubes.

Content Provider	Semantic Scholar
Author	Ruoff, Rodney S.
Copyright Year	2006
Abstract	T he ‘‘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 COC 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 (6, 7) (as a few examples among many interesting studies) have been probed. Analysis based on continuum mechanics (8) discusses the possibility that evolution has optimized composite materials present in biological systems such as bone or abalone such that they are inherently ‘‘flawtolerant.’’ Nanostructures having covalent bonding with (relatively) stiff bonds, in contrast, are not tolerant of, e.g., point defects (a missing atom in the lattice) (9). A single missing atom in a hypothetical CNT of the ‘‘(10,10)’’ type stretching from the surface of the Earth to geostationary orbit (thus containing of order 6 1017 otherwise perfectly bonded atoms) would have a tensile strength 80% that of the hypothetical defect-free tube (9). This reduction in strength and ‘‘end effects,’’ such as have been discussed in a review of the ultimate strength and stiffness of polymers (10), are relevant to the strength of the hypothetical space elevator (11). Even if structures such as space elevators could be defect-free by a remarkable future nanotechnology used to construct them and supposing they were composed largely of CNTs, the question of how long before defects arise can be debated in light of the treatment presented by Dumitrica et al. (3), although this advance in treating the time and temperature dependence of CNT strength (3) is not incorporating potentially reactive chemical environments, radiation, including cosmic rays, cycling of thermal or mechanical loads, or other external perturbations present in the real world (and space!). The synthesis of carbon, boron nitride, and metal dichalcogenide nanotubes (among others), and single-crystal inorganic and metal nanowires (and nanorods, ribbons, plates, platelets, sheets, etc.) enables study of the influence small numbers of atomic-scale defects will have on strength. The ultimate strength has perhaps been measured for a few specimens of microscale whiskers (12) and glass fibers (13). For example, a severalmillimeter-long, 0.34m-diameter -Si3N4 whisker with strength of 59 GPa might have been defect-free (9, 14, 15). Nanostructures can be created with a very broad range of compositions and can be
Starting Page	37
Ending Page	47
Page Count	11
File Format	PDF HTM / HTML
Alternate Webpage(s)	http://utw10193.utweb.utexas.edu/Archive/RuoffsPDFs/141.pdf
Alternate Webpage(s)	http://www.me.umn.edu/~dtraian/pnas-comment.pdf
PubMed reference number	16636280v1
Volume Number	103
Issue Number	18
Journal	Proceedings of the National Academy of Sciences of the United States of America
Language	English
Access Restriction	Open
Subject Keyword	Blood Platelets Bone Tissue Composition Covalent Interaction DNA Packaging Diameter (qualifier value) Elevator Fifty Nine Magnesium Sulfate Mechanics Nanostructured Materials Nanotubes Nanotubes, Carbon Organism Peptide Nucleic Acids Pyschological Bonding Radiation Specimen Vibrissae boron nitride fiberglass physical hard work tensile strength
Content Type	Text
Resource Type	Article