“Archimedes considered his most significant accomplishments were those concerning a cylinder circumscribing a sphere, and he asked for a representation of this together with his result on the ratio of the two, to be inscribed on his tomb.”
And one year after it was told us how to produce carbon spheres in relative abundance (at least, enough to buy a decent quantity from your laboratory chemical supplier), along comes Sumio Iijima telling us how to make cylinders.
Helical microtubules of graphitic carbon
Sumio Iijima, NEC Corporation, Fundamental Research Laboratories, Tsukuba, Japan
THE synthesis of molecular carbon structures in the form of C60 and other fullerenes1 has stimulated intense interest in the structures accessible to graphitic carbon sheets. Here I report the preparation of a new type of finite carbon structure consisting of needle-like tubes. Produced using an arc-discharge evaporation method similar to that used for fullerene synthesis, the needles grow at the negative end of the electrode used for the arc discharge. Electron microscopy reveals that each needle comprises coaxial tubes of graphitic sheets, ranging in number from 2 up to about 50. On each tube the carbon-atom hexagons are arranged in a helical fashion about the needle axis. The helical pitch varies from needle to needle and from tube to tube within a single needle. It appears that this helical structure may aid the growth process. The formation of these needles, ranging from a few to a few tens of nanometres in diameter, suggests that engineering of carbon structures should be possible on scales considerably greater than those relevant to the fullerenes.
Nature 354, 56 - 58 (07 November 1991); doi:10.1038/354056a0
The change from sphere to cylinder brings with it some noticeable difference is properties. Lovers of differential geometry will immediately recognize that a cylinder has no intrinsic curvature. One can roll up a flat map into a cylinder without distorting the picture, which is not the case with a sphere. This is the basis of map projections.
In the case of carbon, we had to mix hexagons with pentagons to make our C60 football. This, combined with the curvature out of the plane, reduces its chemical stability brought on by aromaticity, so a buckyball is considerably more reactive than a graphite sheet.
With a cylinder, we can roll up a sheet consisting of all hexagons – chemical “chicken wire” – as is found in the crystal planes of raphite. This, and losing one axis of curvature, gives us a more stable system. The tubes can also be of greater diameter, and so we get even less distortion of the ideally planar carbon bonds, so we have a much tougher system.
Carbon Nanotubes (CNTs), as these beasties are called, have a wide variety of applications, and I won’t go into them here. The one are in which I did come across them in my work is as reinforcements of plastics, where a small amount can impart quite a lot of strength. The one problem with this type of application is that (as in the picture above) they come horribly tangled, and when one tries to disperse them in the plastic a lot of them remain bound up in clusters, as if one were trying to shred a dish scourer. If one could separate them properly and disperse them randomly in the plastic, a little less would go a much longer way …
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