Carbon Nanotubes
--> Carbon nanotubes
synthesis methods
There are three main synthesis methods:
[Obviously there are other tecnniques. Reaserch centers are working hard in order to obtain sustainable
(low cost) carbon nanotubes]
Arc-discharge
The carbon nanotubes were produced using an arc discharge
evaporation method similar to that
used for the fullerene synthesis.
The carbon
needles, ranging from 4 to 30 nm in diameter
and up to 1 mm in length, were grown on the
negative end of the carbon electrode used for
the direct current arc-discharge
evaporation of carbon in an argon-filled
vessel.
Laser ablation [Smalley 1996]
With this method can produced high yields
(>70%) of SWNT, laser-vaporization of graphite
rods with small amounts of Ni and Co at 1200 C.
The Nanotube grows until too many catalyst atoms aggregate on the
end of the nanotube. The large particles either detach or become
over-coated with sufficient carbon to poison the catalysis. This allows
the tube to terminate with a fullerene-like tip or with a catalyst
particle.
Chemical Vapour Deposition (CVD)
This method is capable of controlling growth direction on a substrate and
synthesising a large quantity of nanotubes. In this process a mixture of hydrocarbon
gas, acetylene, methane or ethylene and nitrogen was introduced into the reaction
chamber. During the reaction, nanotubes were formed on the substrate by the
decomposition of the hydrocarbon at temperatures 700–900 C and atmospheric pressure.
Discussion
Both
arc-discharge and laser-ablation techniques have the
advantage of high (>70%) yields of SWNT and the drawback that they rely on
evaporation of carbon atoms from solid targets at temperatures >3000 C
makes difficult the purification and application of the samples.
Chemical Vapour Deposition has two main advantages: the nanotubes are obtained at much lower
temperature, although this is at the cost of lower quality, and the catalyst can be deposited
on a substrate, which allows for the formation of novel structures.
