Biomedical Engineering Reference
In-Depth Information
A detailed discussion of the growth processes, synthesis
parameters and physical properties of the CNTs grown with different
methods is briefly reported in the successive paragraphs from
Section 1.3.1 to 1.3.4.
9.3.1
Arc Discharge
Iijima [23] first observed nanotubes synthesized by electric arc
discharge. This technique generally involves the use of two high-
purity graphite rods as the anode and cathode. The rods are brought
together under an inert (He, Ar, N
) atmosphere and a voltage
(10-300 V) is applied until a stable arc is achieved. The process
depends on the size of the graphite rods. As the anode is consumed,
a constant gap between the anode and cathode is maintained by
adjusting the position of the anode. Typical rate of the cathode
deposition is about one millimeter per minute, with the current and
voltage in the range of 100 A and 20 V, respectively. The temperatures
reached in the process are in the range 2000-3000°C, more than
sufficient for the carbon atoms to rearrange into the tube structure.
The carbon-based nanomaterial deposit on the cathode to form a
buildup consisting of an outside shell of fused material and a softer
fibrous core containing nanotubes and other carbon particles. The
CNTs produced by arc discharge are very straight, indicative of their
crystallinity. In the as-grown material, there are few defects such as
pentagons or heptagons existing on the sidewalls of the nanotube
due to processing high-temperatures. The by-products of the arc
discharge growth process are multilayered graphitic particles in
various shapes. Purification of CNTs can be achieved by thermal
annealing upon air oxidation to oxidize away the graphitic particles.
This purification process also removes an appreciable amount of
nanotubes.
In order to increase the yield of nanotubes in the carbon material
created, several different metals in concentrations of the order of
1% are incorporated into the target material that is evaporated. The
metals evaporate with the carbon and coalesce into clusters that
form a base from which the nanotubes can grow. MWCNTs can be
obtained by controlling the growth conditions such as the pressure
of inert gas in the discharge chamber and the arcing current. In 1992,
a breakthrough in MWCNTs' growth by arc discharge was first made
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