Carbon Nanotubes

Brent Segal

Since their discovery in 1991 by Sumio Iijima, carbon nanotubes have fascinated scientists with their extraordinary properties.il! Carbon nanotubes are often described as a graphene sheet rolled up into the shape of cylinder. To be precise, they are graphene cylinders about 12nm in diameter and capped with end-containing pentagonal rings. One would imagine that a new chemical such as this would be discovered by a chemist, slaving away in front of a series of Bunsen burners and highly reactive chemicals, with a sudden epiphany being revealed in a flurry of smoke or precipitation from a bubbling flask. However, carbon nanotubes were discovered by an electron microscopist while examining deposits on the surface of a cathode; he was performing experiments involving the production of fullerenes, or buckyballs.

This discovery presents one of the key tenets of nanotechnology. Novel tools allow researchers to observe materials and properties at the nanoscale that often have existed for hundreds or thousands of years and to exploit the properties of such materials.

After Iijima's fantastic discovery, various methods were exploited to produce carbon nanotubes in sufficient quantities to be further studied. Some of the methods included arc discharge, laser ablation, and chemical vapor deposition (CVD).[2! The general principle of nanotube growth involves producing reactive carbon atoms at a very high temperature; these atoms then accumulate in regular patterns on the surface of metal particles that stabilize the formation of the fullerenes, resulting in a long chain of assembled carbon atoms.

The arc-discharge methodology produced large quantities of multiwalled nanotubes (MWNTs), typically greater than 5nm in diameter, which have multiple carbon shells in a structure resembling that of a Russian doll. In recent years, single-walled nanotubes (SWNTs) using this method also have been grown and have become available in large quantities. The laser ablation method of carbon nanotube growth produced SWNTs of excellent quality but requires high-powered lasers while producing small quantities of material. The CVD method was pioneered by Nobel Laureate Richard Smalley and colleagues at Rice University, whose experience with fullerenes is nothing short of legendary. This growth technique is aided by a wealth of well-known inorganic chemicals specifically involving the formation of highly efficient catalysts of transition metals to produce primarily single-walled nanotubes.

Figure 13-2 shows a simple carbon nanotube.

Figure 13-2. A simple example of a carbon nanotube. (Courtesy of Dr. Peter Burke, University of California, Irvine.)

Figure 13-2. A simple example of a carbon nanotube. (Courtesy of Dr. Peter Burke, University of California, Irvine.)

Novel Properties

Although carbon nanotubes have a suitably interesting structure, there are a multitude of important properties that impart the potential for novel applications of significant commercial value. Multiwalled and single-walled nanotubes have similar properties, and for illustration, focusing on single-walled nanotubes provides a reasonable primer of the primary features.

Some of these properties include remarkable strength, high elasticity, and large thermal conductivity and current density. Several reports have determined that SWNTs have a strength of between 50 and 100 times that of steel.[3] The elasticity of SWNT is 11.2 terrapascal (TPa), a measure of the ability of a material to return to its original form after being deformed. Imagine a molecule that, on the atomic scale, is as strong as steel but flexible like a rubber band!

Despite these structural properties, SWNTs have a thermal conductivity almost as great as twice that of diamond, which is known to be one of the best conductors of heat. Perhaps nno nf fho mnct imnroccivo nrnnortioc nf QWIMTc in\/nl\/oc thoir olorfriral rnnHi ir1-i\/i1-\/ u/hirh

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