Why gases are use in electric arc discharge method?
Answers
In this review, we focus on the synthesis of multiwalled and single-walled carbon nanotubes (MWNTs and SWNTs, respectively) by an arc discharge method. MWNTs can be obtained in the cathode deposit of the dc arc discharge of pure graphite rods. Ambient gas plays an important role, and pure hydrogen gas is the best gas for obtaining high-crystallinity MWNTs. A thin innermost tube of less than 0.4 nm diameter and a carbon nanowire including a carbon chain can be produced by a hydrogen-ambient dc arc discharge method. The Raman spectra of MWNTs are important because radial breathing modes with frequencies higher than 300 cm –1 and a new Raman band near 1850 cm –1 appear. In the case of SWNT synthesis by an arc discharge method, the incorporation of catalytic metal particles in a graphite anode is necessary, and SWNTs are obtained as soot in an evaporation chamber. By an arc-plasma-jet method wherein two electrodes are placed at a sharp angle, the yield of soot including SWNTs is increased by decreasing the amount of cathode deposit. In particular, the arc evaporation of a graphite rod with a pure Fe catalyst in a gas mixture (hydrogen + inert gas) is effective for obtaining macroscopic SWNT nets. The purification of the SWNTs nets by heat and hydrochloric acid treatments is easy.
Explanation:
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.