Understanding Carbon Nanotubes

As we all know, the element carbon has many structurally different forms (allotropes). Among its different allotropes, carbon nanotubes (CNT) are cylindrical molecules, that exhibit certain characteristic properties. In simpler terms, they 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 tubes, spheres, or ellipsoids.

The actual discovery of carbon nanotubes happened in the late 1950s, when Roger Bacon (Union Carbide), studied carbon under different conditions. He found a strange structure, in the form of hollow tubes, that resembled graphitic layers. In 1985, buckyballs (C₆₀), in which the atoms are arranged in the shape of a soccer ball, was discovered.

However, the credit for discovery of CNT 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.

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

• There are two types, viz, single-walled carbon nanotube (SWNT) and multi-walled nanotube (MWNT). As the name suggests, the former one comprises a single graphene cylinder; whereas, in MWNT, there are more than one graphene cylinders.

• Their unique characteristics are because of the sp² bonds (each carbon atom bonded to the neighboring three atoms), that bind the molecules, the one-dimensional structures, shapes, and sizes.

• Some of its unique properties are high tensile strength, high thermal conductivity, and high resilience.
• They 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.

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

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 also found to be a potential material for use in the 'space elevator' (earth-to-space cable), and other aerospace applications. In recent times, suggestions are being made to use them in making potentially strong and bullet-bounced armors.

Due to high their thermal conductivity, researches for making use of them in building computers and televisions have already been emphasized. However, in spite of their peculiar mechanical and electrical properties, 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 they are as hazardous as asbestos. According to them, inhalation of these can cause potential health complications like breathing problems, and even cancer. Current researches are ongoing to determine their overall effects.