Understanding Carbon Nanotubes

As we all know, the element carbon has many structurally different forms (allotropes). Among the different allotropes, for example graphite and diamond; carbon nanotubes (CNT) are cylindrical molecules of carbon that exhibit certain characteristic properties. In simpler terms, carbon nanotubes can be considered as a graphite sheet, in the shape of a cylinder. They belong to the structural family 'fullerenes' or 'buckminsterfullerenes', that exist in the form of hollow tube, sphere or ellipsoid. Let's try to understand in brief about the history, physical properties and use of carbon nanotubes.

History of Carbon Nanotubes

The actual discovery of carbon nanotubes was in the late 1950s, when Roger Bacon (Union Carbide) studied carbon under different conditions. He found a strange carbon structure, in form of hollow tubes that resemble graphitic layers. In 1985, buckyballs (C₆₀) in which carbon atoms are arranged in the shape of a soccer ball, was discovered. However, the credit for discovery of carbon nanotubes goes to Sumio Iijima of NEC Corporation (Japan), when he observed the formation of multi-walled carbon nanotubes, while conducting an experiment on electrical arc discharge technique, in 1991.

Physical Properties of Carbon Nanotubes

The term carbon nanotubes is given, as their diameter measures in terms of a few nanometers. Though, the length of carbon nanotubes can be several millimeters, the diameter is about 10,000 times less than a human hair. In fact, carbon nanotube has the largest length-to-diameter ratio that can be as large as 1:28,000,000. Following are the physical properties of carbon nanotubes.

• There are two types of carbon nanotubes viz, single-walled carbon nanotube (SWNT) and double-walled nanotube (DWNT). As the name suggests, the former one comprises a single graphene cylinder; whereas, in MWNT, there are more than one graphene cylinders.
• The unique characteristics of carbon nanotubes is because of the sp² bonds (each carbon atom bonded to the neighboring three atoms) that binds the molecules, the one-dimensional structure, shape and size.
• Some unique properties of carbon nanotubes are high tensile strength, high thermal conductivity and high resilience.
• Carbon nanotubes undergo non-reversible deformation or plastic deformation when given extremely high tensile strain. Due to their hollow structure, they change their shape (buckling) under compressive stress.

Carbon nanotubes can act as metals (conductivity higher than copper) or semiconductors, depending upon the arrangement of the graphene layers. Thus, the electrical property of carbon nanotubes differs.

Uses of Carbon Nanotubes

Carbon nanotubes play a major role in nanotechnology, especially in scanning electron microscopes, nano-electronic devices, nano-mechanical devices and flat-display devices. They are found to be a potential material for use in 'space elevator' (earth-to-space cable) and other aerospace applications. In recent times, suggestions are being made to use carbon nanotubes in making potentially strong and bullet-bounced armors. Due to high thermal conductivity, researches for making use of carbon nanotubes in building computers and televisions have already been emphasized. However, in spite of the peculiar mechanical and electrical properties of carbon nanotubes, the commercial application is rather rare, which is mainly due to the expensive cost of production.

Whether carbon nanotubes are toxic or not is still controversial, some scientists claim that nanotubes are as hazardous as asbestos. According to them, inhalation of carbon nanotubes can cause potential health complications like breathing problems, and even cancer. Current researches are ongoing to determine the overall effects of carbon nanotubes.