To understand why buckytubes have the incredible properties they do, we must first look at the building blocks of the tubes. Start by considering graphite. While long considered the poor brother of diamond in the family tree, graphite turns out to be far richer in its properties. Like the entrepreneurial son who never went to college, yet exceeded all expectations, graphite has, despite its unimpressive appearances, struck it rich. But, for this purpose, we’ll focus on a single layer of graphite, called graphene, in which the carbon atoms are arranged in hexagons, just like chicken-wire. Graphene has the distinction of being the densest known two-dimensional packing of atoms. No 2-D slice through diamond, or any other material, is denser on an atomic basis. This, together with the special nature of the carbon-carbon bonds in this network, gives graphene the highest stiffness of any sheet.
Now take this sheet and wrap it up into a tube by seamlessly connecting two opposite edges. You now have a tube with the greatest stiffness, both in tension and in bending. Many materials are stiff (although not as stiff as this), but so brittle as to be often useless. (Likewise, most materials that are not brittle, such as spider silk, which can be stretched about 30% beyond its resting length, are not particularly strong.) Buckytubes, stiff as they are, are also amazingly tough: they can stretch beyond 20% of their resting length, and can be bent over double, and even tied into a knot and released with no resulting defect! This unprecedented combination of stiffness and toughness makes buckytubes by far the strongest known fibres in tension – about 100 times stronger than high-strength steel at one-sixth the weight! This is remarkable stuff.
The accolades don’t end with buckytubes’ mechanical virtues. The tubes are also the best known conductors of heat, now verified to exceed diamond, the previous winner. Unlike bulk diamond, however, whose thermal conductivity is the same in all directions, buckytubes conduct heat far better down the tube axis than sideways from tube to tube. Thus, a macroscopic crystal of buckytubes, where tubes are packed together, all running alongside one another in the same direction, the sides with the tube ends would feel cold to the touch, like metal, while the other sides would feel like wood, a good thermal insulator.
Where buckytubes really perform, however, is in their electrical conductivity. The carbon in the planar graphene sheet is bonded in such a way as to free up one electron per carbon atom to wander around freely, rather than stay near its “home” atom. This is what happens in metals, where some electrons are not strongly bound to their donor atom, but can easily be pulled this way or that under the influence of an electric field (obtained, for example, when a voltage is applied across a metal). The detailed quantum mechanics of graphene results in a semimetal, a situation where the in-plane conductivity is only moderate, similar to that of a poor metal like lead.
However, when rolled into a perfect tube, something marvellous happens, again due to quantum mechanics and symmetry. When the graphene sheet is rolled up so that there are carbon-carbon bonds that are perpendicular to the tube axis, the resulting electronic structure becomes that of a true metal, like copper or gold. This is the first and only instance of a molecule being a true metallic conductor. Other ways of rolling up the graphene sheet produce semiconducting tubes with such a small gap that at a few degrees above absolute zero they have high conductivity, or are similar to silicon in their conductivity.
Any one of these incredible material properties would give buckytubes uses in many applications. The combination of properties, however, makes this a wonder material that will find its way into countless technologies that will make life – and the planet – better. Applications such as conductive plastics.