29-05-2013, 12:06 PM
Arc Discharge Method
Arc Discharge.pptx (Size: 1.83 MB / Downloads: 23)
Carbon Nano Tubes
Sumio Iijima (born May 2, 1939) is a Japanese physicist, often cited as the discoverer of carbon nanotubes.
In 1991 he observed Multiwalled carbon Nanotube and in 1993 he discovered Single wall carbon Nanotube.
Carbon nanotubes (CNTs) also called buckytubes are allotropes of carbon with a cylindrical nanostructure.
Carbon nanotube (CNT) is a new form of carbon, configurationally equivalent to two dimensional graphene sheet rolled into a tube.
The ends of a nanotube might be capped with a hemisphere of the buckyball structure. It is just a few nanometers in diameter and several microns long. They are light, flexible, thermally stable, and are chemically inert. They have the ability to be either metallic or semi-conducting depending on the "twist" of the tube.
Types of SWNTs
Nanotubes form different types, which can be described by the chiral vector (n, m), where n and m are integers of the vector equation R = na1 + ma2.
Inert atmosphere/Gas
Inert gas is meant for cooling / condensation of the sample
The chamber must be connected both to a vacuum line with a diffusion pump and to helium supply.
The anode is a long rod of 6mm diameter & the cathode is a short rod of 9mm diameter
Cooling of electrode
Efficient cooling of the electrodes & the chamber are essential to produce good quality nanotubes and also to avoid excessive sintering.
Without proper cooling-sintering occurs-with a hard deposit of mass
With proper cooling-sintering does not occurs- forms a uniform deposit
i.e., Homogeneous deposit with aligned bundles of nanotubes.
Growth mechanisms in Carbon Nano Tubes
For the synthesis of multiwalled nanotubes in the arc, three types of mechanism have been put forward, which could be labelled ‘‘gas’’, ‘‘solid’’ and ‘‘liquid’’.
The gas phase models assume that nanotube nucleation and growth occur as a result of direct condensation from the vapour, or plasma, phase.
In solid phase models, the nanotubes and nanoparticles do not grow in the arc plasma, but rather form on the cathode as a result of a solid state transformation.
tip-growth model & base-growth model
A hydrocarbon vapor when comes in contact with the “hot” metal nanoparticles, first decomposes into carbon and hydrogen species; hydrogen flies away and carbon gets dissolved into the metal. After reaching the carbon-solubility limit in the metal at that temperature, as-dissolved carbon precipitates out and crystallizes in the form of a cylindrical network having no dangling bonds and hence energetically stable. Hydrocarbon decomposition (being an exothermic process) releases some heat to the metal’s exposed zone, while carbon crystallization (being an endothermic process) absorbs some heat from the metal’s precipitation zone. This precise thermal gradient inside the metal particle keeps the process on.
Now there are two general cases. (Fig. 3a) When the catalyst-substrate interaction is weak (metal has an acute contact angle with the substrate), hydrocarbon decomposes on the top surface of the metal, carbon diffuses down through the metal, and CNT precipitates out across the metal bottom, pushing the whole metal particle off the substrate (Fig. 3a(i)). As long as the metal’s top is open for fresh hydrocarbon decomposition, the concentration gradient exists in the metal allowing carbon diffusion, and CNT continues to grow longer and longer (Fig. 3a(ii)). Once the metal is fully covered with excess carbon, its catalytic activity ceases and the CNT growth is stopped (Fig. 3a(iii)). This is known as “tip-growth model”.