Carbon Nanotubes [CNTS]

CARBON NANOTUBES [CNTS]

Introduction

The discovery of carbon leads to form stable, ordered structures other than graphite and diamond, which have stimulated the researchers worldwide to search for other new forms of carbon. The search was given new impetus when it was shown that C₆₀ could be produced in a simple arc-evaporation apparatus. This C₆₀ is very much useful in the production of carbon nano tubes, so called CNT.

The CNT have remarkable electronic properties and many other unique characteristics. For these reasons they have attracted huge academic and industrial interest.

Definition

Carbon nanotubes (CNT) are molecular-scale tubes of graphitic carbon with outstanding properties. They are among the stiffest and strongest fibres researched till date, with remarkable electronic properties and applications.

Types

Carbon nao-tubes are of two types. viz

(i) (a) Single walled nano-tubes (SWNTs)

 (b) Multi walled nano-tubes (MWNTs)

(ii) Single walled nano-horns (SWNHS).

TYPES OF CARBON NANOTUBES

The nano-tubes possess atleast two layers, often many more, and ranged in outer diameter from about 3 nm to 30 nm. The bonding in carbon nanotubes is sp² hybridization, with each atom joined to three neighbours as in graphite. The tubes can therefore be considered as rolled-up graphene sheets (graphene is an individual graphite layer). There are three distinct ways (Fig. 5.29) in which a graphene sheet can be rolled into a tube, viz.,

(i) Arm chair structure

(ii) Zig-zag structure and

(iii) Chiral structure.

Here the armchair structure and zig-zag structure have a high degree of symmetry. The terms "armchair" and "zig-zag" refer to the arrangement of hexagons around the circumference. The "chiral" term means that it can exist in two mirror-related forms.

The internal arrangements of atoms in the structure of a nanotube is as shown in Fig. 5.30. The structure is specified by a vector, (n,m), which defines how the graphene sheet is rolled up. The carbon nanotube can be produced with various values of 'n' and 'm'.

For example, to produce a nanotube with the indices (7, 6), the sheet has to be rolled up in such a way that the atom labelled (0, 0) has to be superimposed on the one labelled (7, 6). It can be seen from the figure that m = 0 for all zig-zag tubes, while n = m for all armchair tubes.

PROPERTIES OF CARBON NANOTUBES

(i) CNTs have High Electrical Conductivity.

(ii) CNTs have Very High Tensile Strength.

(iii) CNTs are Highly Flexible (can be bent considerably without damage).

(iv) CNTs are Very Elastic (18% elongation to failure).

(v) CNTs have High Thermal Conductivity.

(vi) CNTs have a Low Thermal Expansion Coefficient.

(vii) CNTs are Good Electron Field Emitters.

(viii) CNTs Aspect Ratio.

APPLICATIONS OF CARBON NANOTUBES

(i) The carbon nanotubes are very light in weight, but they are very strong, hence they are used in aerospace.

(ii) They are used in constructing nanoscale electronic devices.

(iii) Carbon nano-tubes (CNTs) are used in battery electrodes, fuel cells, reinforcing fibers etc.

 (iv) CNTs are used in the development of flat panel displays for computer monitors and televisions.

(v) Plastic Composite CNTs are used as a light weight shielding materials for protecting electromagnetic radiation.

(vi) Light weight CNTs are also used in military and communication systems, for protecting computers and electronic devices.

(vii) Semiconducting CNTs are used as switching devices.

(viii) Semiconducting CNTs are also used as chemical sensors to detect various gases.

(ix) Nano-tubes can also serve as catalysts for some chemical reactions.

(x) The unique properties of carbon nanotubes will undoubtedly lead to many more applications in future to produce nano-computers, plastic composites etc.