Editorial Feature

The History and Development of Carbon Nanotubes

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In 1985, a convergence of events resulted in a surprising and unplanned experiment with a new type of microscope, leading to the discovery of a new molecule made solely of carbon—the same element which chemists thought had nothing to offer.

Buckyballs—60 carbon atoms assembled in a soccer ball shape—had been discovered and the chemical world, as well as the physical and material worlds, would never be the same.

Fullerenes

As a matter of fact, the discovery involved an infinite class of new molecules—the fullerenes—rather than just one new molecule. Each fullerene—C60, C70, C84, etc,—had the vital characteristic of being a pure carbon cage, with each atom attached to three others as in graphite. In contrast to graphite, every fullerene has precisely 12 pentagonal faces with a varying number of hexagonal faces (for example, buckyball—C60—has 20).

Some fullerenes, such as C60, were spheroidal in shape, and others, such as C70, were oblong as in a rugby ball. In 1990, Dr Richard Smalley identified that, theoretically, a tubular fullerene must be achievable, capped at each end, for instance, by the two hemispheres of C60, linked by a straight segment of tube, with only hexagonal units in its structure. When Millie Dresselhaus heard this idea, she named these imagined objects as “buckytubes.”

Carbon Nanotubes

However, in reality, carbon nanotubes had been found three decades back but had not been completely valued at that time. In the late 1950s, Roger Bacon at Union Carbide, discovered an unusual new carbon fiber while examining carbon under conditions close to its triple point. He noticed straight, hollow tubes of carbon that seemed to be enclosed in graphitic layers of carbon separated by the same spacing as the planar layers of graphite. Back in the 1970s, Morinobu Endo observed these tubes again, created by a gas-phase process. In fact, he even noticed some tubes enclosed in just a single layer of rolled-up graphite.

In 1991, following the discovery and confirmation of the fullerenes, Sumio Iijima of NEC noticed multiwall nanotubes developed in a carbon arc discharge, and after two years, he and Donald Bethune at IBM independently observed buckytubes, that is, single-wall nanotubes. These pure carbon polymers could now be inferred in the perspective of fullerenes, transforming one’s perception of them to molecules, with all that unique designation implies. Nanotubes had been fullerenized.

Multiwall Nanotubes

At present, after 10 years of Iijima’s initial observation, considerable details about nanotubes and tubular fullerenes have been known. It is known that multiwall nanotubes are consistently made with a high frequency of structural defects. (When compared to their larger relations, the 5–20 micron-diameter graphite fibers employed in aerospace and sporting goods applications, multiwall nanotubes are rather reliable structurally, but they often include areas of structural imperfection.).

Any material scientist knows the fact that the material properties of a substance, like strength is certainly degraded by the occurrence of defects. The inherent properties of a material may be the best, but normally the actual properties of the bulk material are only a few percent of what the material would show if it were structurally perfect. For instance, a structural defect like a microcrack in steel wire, will result in catastrophic failure at 1%–2% of the theoretical breaking strength anticipated based on underlying chemical principles.

Buckytubes

On the contrary, buckytubes are fullerenes, and are therefore molecules: perfect, hollow molecules of pure carbon connected together in a hexagonally bonded network to produce the hollow cylinder. The tube is flawless, with either open or capped ends. The single-wall carbon nanotubes have a diameter of 0.7 to 2 nm (typically around 1.0 nm)—100,000 times thinner than a human hair. The lengths of buckytubes are usually hundreds of times their diameters.

Molecular Considerations

The molecular aspect of buckytubes is crucial. Every atom is in the correct position. This characteristic makes them differ greatly from their larger, defective relatives. A molecule is an extremely exclusive thing to a chemist. A molecule is complete and is typically quite satisfied with its identity. When a molecule is given an opportunity to modify, that is, to go through a chemical reaction with other bits of stuff, there is often a comparatively considerable hurdle to overcome.

On the other hand, when a non-molecular “stuff” is confronted with other stuff, it typically changes quickly and introduces the new part to create a larger whole. Metals are of this kind: a portion of metal exposed to more metal (this could occur in the solid, molten, or gaseous phase) will be able to accommodate the addition since it does not have molecular completeness and invariance.

Molecular Invariance

One characteristic of molecular invariance is the predictability and reliability of chemical modification. Using various methods, molecules can be induced to overcome their obstacles to change, for instance, by using heat. However, on the whole, the reaction products are consistent when dealing with molecules. This is not true in the case of modifications undergone by non-molecular things.

No two portions of metal ever appear the same, just as no two snowflakes appear the same: snowflakes are not molecules. The influence of molecular invariance on material properties is similarly insightful. Regardless of the inherent nature of the material, there are no flaws to degrade the properties. The material is received as such.

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