Mechanical nanotubes

Whereas multi-wall carbon nanotubes require no catalyst for their growth, either by the laser vaporization or carbon arc methods, catalyst species are necessary for the growth of the single-wall nanotubes [156], while two different catalyst species seem to be needed to efficiently synthesize arrays of single wall carbon nanotubes by either the laser vaporization or arc methods. The detailed mechanisms responsible for the growth of carbon nanotubes are not yet well understood. Variations in the most probable diameter and the width of the diameter distribution is sensitively controlled by the composition of the catalyst, the growth temperature and other growth conditions. 

The other striking feature of nanotubes is their extreme stiffness and mechanical strength. Such tubes can be bent to small radii and eventually buckled into extreme shapes which in any other material would be irreversible, but here are still in the elastic domain. This phenomenon has been both imaged by electron microscopy and simulated by molecular dynamics by lijima et al. (1996). Brittle and ductile behaviour of nanotubes in tension is examined by simulation (because of the impossibility of testing directly) by Nardelli et al. (1998). Hopes of exploiting the remarkable strength of nanotubes may be defeated by the difficulty of joining them to each other and to any other material. 

Key Words—Carbon nanotubes, vapor-grown carbon fibers, high-resolution transmission electron microscope, graphite structure, nanotube growth mechanism, toroidal network. 

Fig. 10. Growth mechanism proposed for the helical nanotubes (a) and helicity (b), and the model that gives the bridge and laminated tip structure (c).

In Fig. 13 is shown the 002 lattice images of an as-formed very thin VGCF. The innermost core diameter (ca. 20 nm as indicated by arrows) has two layers it is rather straight and appears to be the primary nanotube. The outer carbon layers, with diameters ca. 3-4 nm, are quite uniformly stacked parallel to the central core with 0.35 nm spacing. From the difference in structure as well as the special features in the mechanical strength (as in Fig. 7) it might appear possible that the two intrinsically different types of material... 

Other mechanism for doping the tubules. Doping of the nanotubes by insertion of an intercalate species between the layers of the tubules seems unfavorable because the interlayer spacing is too small to accommodate an intercalate layer without fracturing the shells within the nanotube. 

In the polygonized nanotubes observed by Liu and Cowley]12,13], the edges of the polygon must have more sp character than the flat faees in between. These are defeet lines in the sp network. Nanotubes mechanically deformed appear to be rippled, indicat-... 

In this paper we elaborate models of perfect tubule connections leading to curved nanotubes, tori or coils using the heptagon-pentagon construction of Dunlap[ 12,13]. In order to understand the mechanisms of formation of perfectly graphitized multilayered nanotubes, models of concentric tubules at distances close to the characteristic graphite distance and with various types of knee were built. (Hereafter, for the sake of clarity, tubules will be reserved to the individual concentric layers in a multilayered nanotube.)... 

A POSSIBLE MECHANISM FOR THE GROWTH OF NANOTUBES ON A CATALYST PARTICLE... 

Key Words —Nanotubes, mechanical properties, thermal properties, fiber-reinforced composites, stiffness constant, natural resonance. 

The yield strengths of defect-free SWNTs may be higher than that measured for Bacon s scroll structures, and measurements on defect-free carbon nanotubes may allow the prediction of the yield strength of a single, defect-free graphene sheet. Also, the yield strengths of MWNTs are subject to the same limitations discussed above with respect to tube slippage. All the discussion here relates to ideal nanotubes real carbon nanotubes may contain faults of various types that will influence their properties and require experimental measurements of their mechanical constants. 

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