Editorial Feature

Understanding the Properties of Carbon Nanotubes (Buckytubes)

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In 1985, a convergence of events resulted in an unanticipated and unintentional experiment with a new kind of microscope leading to the discovery of a novel molecule made fully from carbon—the same element chemists thought had nothing more to offer.

Buckyballs—60 carbon atoms organized in a soccer ball shape—had been discovered and the chemical realm, not to mention the material and physical realms, would never be the same.

Graphene

To comprehend why buckytubes possess the extraordinary properties they do, one must first explore the building blocks of the tubes. Firstly, graphite must be examined. For a long time, it was believed to be the poor brother of diamond in the family tree; however, graphite appears to be a lot richer in its properties. Like the entrepreneurial son who skipped college, and yet surpassed all expectations, graphite has, notwithstanding its insignificant appearances, struck it rich.

But, for this purpose, the focus will only be on a single layer of graphite, known as graphene, wherein the carbon atoms are organized in hexagons, similar to chicken-wire. Graphene has the merit of being the densest known 2D packing of atoms. No 2D slice through various materials, including diamond, is denser on an atomic scale. This, along with the special nature of the carbon-carbon bonds in this network, endows graphene with the maximum stiffness of any sheet.

Mechanical Properties

This sheet can now be taken and wrapped up into a tube by flawlessly connecting two opposite edges. The resultant tube will have the greatest stiffness, both in bending and in tension. A number of materials are stiff (though not as stiff as this), but can be so brittle that they are often rejected. (Similarly, most materials that are not brittle, like spider silk, which can be stretched about 30% more than its resting length, are not predominantly strong.)

Despite their stiffness, buckytubes are also remarkably tough: they can stretch more than 20% of their resting length, and can be bent over twice, and even tied into a knot and released without any resulting defect. This unparalleled combination of toughness and stiffness makes buckytubes by far the toughest known fibers in tension—around 100 times stronger than high-strength steel at one-sixth the weight. This is an extraordinary factor.

Thermal Conductivity

The praises do not end with the mechanical merits of buckytubes. The tubes are also well-recognized as best conductors of heat, even beating diamond, the previous winner. In contrast to bulk diamond, however, whose thermal conductivity is identical in all directions; buckytubes transfer heat a lot better down the tube axis than sideways from one tube to another. Therefore, a macroscopic crystal of buckytubes, where tubes are crammed together, all arranged alongside one another in the same direction, the sides with the tube ends are cold to touch, similar to metal, while the other sides feel like wood, a good thermal insulator.

Electrical Conductivity

One area of application where buckytubes thoroughly perform is their electrical conductivity. The carbon in the planar graphene sheet is bonded in such a manner as to free up one electron for each carbon atom to meander around freely, instead of staying near its “home” atom. This is similar to metals, where some electrons are not robustly bound to their donor atom, but can effortlessly be pulled any way under the effect of an electric field. The field is obtained, for instance, when a voltage is applied across a metal. Graphene’s comprehensive quantum mechanics results in a semimetal—a situation where the in-plane conductivity is only moderate, akin to that of a poor metal such as lead.

However, when rolled into an ideal tube, something remarkable takes place, again because of quantum mechanics and symmetry. When the graphene sheet is rolled up such that there are carbon-carbon bonds that are perpendicular to the tube axis, the ensuing electronic structure becomes that of a true metal, like gold or copper.

This is the first and only example of a molecule existing as a true metallic conductor. Other methods of rolling up the graphene sheet yield semiconducting tubes with such a small gap that at a few degrees above absolute zero, they exhibit high conductivity, or are akin to silicon in their conductivity.

Any one of these extraordinary material properties would allow buckytubes to be put to use in numerous applications. However, the combination of properties will make this a wonder material that will soon enter innumerable technologies that would make life—and the planet—better, for example, applications like conductive plastics.

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