Arc Discharge Method:
The principle of this technique is to vaporize carbon in the presence of catalysts (iron, nickel, cobalt, yttrium, boron, gadolinium, and so forth) under reduced atmosphere of inert gas (argon or helium). After the triggering of the arc between two electrodes, a plasma is formed consisting of the mixture of carbon vapor, the rare gas (helium or argon), and the vapors of catalysts. The va- porization is the consequence of the energy transfer from the arc to the anode made of graphite doped with cata- lysts. The anode erosion rate is more or less important depending on the power of the arc and also on the other experimental conditions. It is noteworthy that a high an- ode erosion does not necessarily lead to a high carbon nanotube production.
It consists of a cylinder of about 30 cm in diameter and about 1m in height, equipped with diametrically op- posed sapphire windows located so that they face the plasma zone in view of observing the arc. The reactor possesses two valves, one for carrying out the primary evacuation (0.1Pa) of the chamber, the other permit- ting it to fill with a rare gas up to the desired working pressure.
In the arc discharge method, a DC bias of 20–30 V is applied between two carbon electrodes in a helium atmosphere. Carbon atoms are ejected from the anode, and accumulate in the form of nanotubes on the cathode. The electrodes are typically 5–20 mm in diameter. As with laser evaporation, the anode includes small quantities of nickel, cobalt or iron, which are also deposited onto the cathode to act as a catalyst. Arc discharges tend to produce narrower and shorter tubes than those obtained from laser ablation (up to $5nm in diameter and around 1mm long). Like laser ablation, arc discharges tend to produce bundles of nanotubes.