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The Joy of High Tech


Rodford Edmiston

Being the occasionally interesting ramblings of a major-league technophile.

Carbon Nanotubes

            Graphene sheets are single-atom thick layers of carbon atoms laid out in a regular, flat matrix, organized in linked rings similar to benzine rings. (In fact, they are sometimes called benzyne rings.) Stack these sheets one on top of the other and you get graphite. Roll one of these into a tube and you have a Buckytube. This last bit works because the ragged edges of graphene sheets are unstable, with dangling, open bonds, and carbon atoms are good at attaching to other carbon atoms.

            Single-Walled Carbon Nanotubes (SWCNT or SWNT) are, as the name implies, simple, single-layer tubes. Multi-Walled Carbon Nanotubes (MWCNT or MWNT) are two or more SWCNT in a coaxial arrangement. One theory of how these form is that the graphene sheet, instead of forming a simple tube, rolls into a scroll. Bonds then reform to the lowest-energy state, which in this case is concentric tubes. The bond between concentric tubes in is roughly the same as that between sheets of graphite (on the molecular scale, that is). Tests have shown that these concentric tubes can telescope (slide axially with respect to each other, like an old-fashioned captain's telescope) with little resistance and no damage. This property, in itself, could be very useful for some applications.

             Making carbon assume these structures is quite simple, due to the tendencies of carbon atoms to form sheets which then form tubes. Making these tubes stable isn't particularly difficult, since they tend to form hemispherical end closures on their own. However, making long strands free of defects, quickly, efficiently and economically, is something still under development. A lot of people are working on this, for some very good reasons.

            When working on the molecular scale care must be taken in stating physical properties, since these can change radically at macro scales. For instance, molecules are mostly open space, and the cross-sections of Buckytubes even more so. Combine that with the difficulties of actually testing something which makes the finest hairs look like bridge cables and getting consistent and accurate measurements of physical properties becomes quite difficult. Finally, how the tubes are made affects the number and type of defects in the tube walls. Still, good measurements have been made, and the numbers are amazing.

            Tested SWCNT have demonstrated a tensile strength of around 15 million Newtons per square centimeter, and could go as high as 20 million. That compares to 2,152,000 for maraging steel, 1,275,000 for tempered Martensite steel, 775,000 for a good-quality spring steel, 160,000 for modern bridge cable, and 376,000 for Kevlar 49. Strength in compression and shear for SWCNT have been tested to better than 8 million N/cm^2 each. And the density of SWCNT is around 1.3, in comparison to 1.14 for Kevlar and 8 for typical steels.

            The specific properties of Buckytubes depend on the geometry of the rings in the graphene sheet and how they are aligned with respect to the tube axis. If the hexagonal cells are aligned with two of the linear sides parallel to the axis and two perpendicular (which has been termed the "armchair" configuration) the material is a very good conductor, possibly the best known. If the cells are arranged so that a line parallel to the axis runs through two of the corners (where the carbon atoms are, the "zigzag" configuration) and is parallel to two of the sides, the material becomes a semiconductor. For the "chiral" orientation imagine starting with either of the above two descriptions and giving the tube a twist around its axis.

            Armchair Buckytubes are excellent electrical conductors, as mentioned above. Individual SWNT have been observed to carry single electrons ballistically. That is, coherently, with no scattering, in effect acting as a superconductor at room temperature. So far this effect has only been observed for distances of several microns. Current densities as high as 109 Amperes per square centimeter have been observed, and theoretically the value could go as high as 1013 A/cm^2. Even the value already confirmed is well beyond the capability of any other known conductor. Moreover, the high conductivity exists along the length of the tubes, in any other direction even the armchair tubes are semiconductors.

            Diamond was long the champion of heat conductivity, and much experimental work has been done using diamond substrates in electronic circuits, since it is both an excellent electrical insulator and conducts heat away very well. However, Buckytubes have now replaced it as the best known thermal conductor, with at least twice the capacity of diamond. Even more interestingly, the thermal conductivity is directional! (In technical terms, anisotropic.) Because the carbon atoms are tightly packed along the axis, but loosely packed across, a Buckytube large enough to handle would feel cold to the touch like metal on the ends, but warm, like wood, on the sides.

            Buckytubes, because of their strong molecular bonds and low incidence of flaws, are very resistant to chemical change, are very stiff (resistant to mechanical change) and retain most of their unusual properties even at relatively high temperatures. Unlike most very stiff substances, Buckytubes aren't brittle. They can be deformed substantially without permanent (plastic) deformation. In simulations, at least, Buckytubes can be doubled, and snap back without damage when released. Elongation in tension before failure is expected to be 20% to 30%, making them reasonably elastic. Additionally, being pure carbon they are open to application in any process where carbon is currently used.

            With properties like that you could do a lot of impressive things, including build an elevator to orbit. Which is exactly what several people are proposing.

          This document is Copyright 2005 Rodford Edmiston Smith. Anyone wishing to repost it must have permission from the author, who can be reached at: stickmaker@usa.net