Properties of Carbon Nanotubes (CNTs)

PROPERTIES OF CNTS

(a) Electrical Properties

i. Carbon nanotubes are metallic or semiconducting depending on the diameter and chirality (ie., how the tubes are rolled).

ii. The energy gap of semiconducting chiral carbon nanotubes is inversely proportional to the diameter of the tube as shown in fig 5.18.

The energy bandgap decreases with increase of diameter of the CNTs.

iii. The energy gap also varies along the tube axis and reaches a minimum value at the tube ends. This is due to the presence of localized defects at the ends due to the extra energy states.

iv. In SWNTs, conduction occurs through discrete electronic states that are coherent between the electrical contacts (hundreds of nanometers).

This means that nanotubes can be treated as quantum wires atleast at very low temperatures.

(b) Mechanical Properties

i. The strength of the carbon-carbon bond is very high therefore any structure based on aligned carbon-carbon bonds will ultimately have high strength.

ii. Young's modulus of CNT is about 1.8 TPa (10¹² Pa about 10 times larger than that of steel).

Nanotubes have therefore high ultimate tensile-strength

iii. One of the important properties of nanotubes is their ability to withstand extreme strain.

iv. The carbon nano tubes can recover from severe structural distortions. This is due to the ability of carbon atoms to rehybridize.

(c) Physical Properties

i. Nanotubes have a high strength-to-weight ratio (density of 1.8 g/cm³ for MWNTs and 0.8 g/cm³ for SWNTs). This is indeed useful for lightweight applications. This value is about 100 times that of steel and over twice that of conventional carbon fibres.

ii. The surface area of nanotubes is of the order of 10 - 20 m2/g which is higher than that of graphite.

(d) Chemical properties

Nanotubes are highly resistant to any chemical reaction. It is difficult to oxidize them and the onset of oxidation in nanotubes is 100°C higher than that of carbon fibres.

As a result, temperature is not a limitation in practical application of nanotubes.

(e) Thermal properties

Nanotubes have a high thermal conductivity and the value increases with decrease in diameter.