(15h) Control of Diameter during Cnt Synthesis in the Three Methods

The three primary methods of syntheis of carbon nanotubes, CNTs are a) arch discharge method, b) laser ablation and c)chemical vapor deposition. Carbon nanotubes have diameters less than 1 nm and length of a few microns. Investigators have reported interesting properties of CNT. Young's modulus of more than 1 TPa, thermal conductivity greater than 2000 W/m/K and electrical conductivity in measures qualifying it as a superconductor, application as supercapacitor and so on and so forth. Recently their electronic structure has been studied using resonance Rayleigh scattering. The electric arc discharge process works by utilizing two graphite electrodes in arc welding type process. The welder is turned on and the rod ends are held against each other in an Argon atmosphere to produce or grow carbon nanotube, CNT. Yield of CNT by this process is low and growth of CNT orientitation are random in nature. Sony corporation has patented a technique that offer greater control on the formation of CNT. The direction and region of arc plasma is allowed to be adjusted to gain control on the formation of the CNT. The synthesis apparatus consists of a chamber , first electrode, and second electrodes and an adjusting mechanism. The chamber includes walls which bound the chamber interior. The walls are structured to allow a cooling fluid to flow therethrough. The cooling fluid enters through a cooling fluid inlet port and may exit through a cooling fluid outlet port so as to cool the chamber interior. A inert gas atmosphere can be produced in the chamber using suitable inlet and outlet. The inert gas mixture may comprise of Hydrogen and Argon. The pressure in the chamber is sub-atmospheric and is expected to be 300 torr ? 760 torr. The second method of generating carbon nanotubes is by laser ablation. In 1996 in Science Thess et al had a paper on crystalline ropes of metallic carbon nanotubes that described a laser ablation method for generating carbon nanotubes. In this process, metal catalyst particle such as Nickel-Cobalt alloy is mixed with graphite powder in a planned proportion and the mixture is pressed as to obtain a pellet. A laser beam is irradiated on the pellet. The carbon and the Nickel-Cobalt alloy is evaporated by the laser beam. The carbon vapor is condensed in the presence of metal catalyst. Single Wall Carbon Nanotube, SWNT are formed during the condensation. The early investigators found that the SWNTs were not constant in diameter. One possible reason for this is the variation of the ratio between the carbon vapor and the metal catalyst vapor with time during absorption of laser light. The black graphite powder took more of the light compared with the metal catalyst. This can lead to uneven heating and the temperature of the graphite would increase more compared with the metal catalyst. The metal catalyst can be expected to be left on the surface. This can lead to the variation in the diameter of the carbon nanotubes. Purity is not uniform. A patent by Iijima of NEC corp., Japan, describes a process to produce SWNT of uniform diameter. Here the carbon and metal catalyst are evaporated independently. The thermal CVD apparatus for CNT growth consists of a quartz tube of diameter of 1-2 inches inserted into a tubular furnace capable of temperature control to within ± 1 0C over a 25 cm zone. It is atmospheric pressure CVD with a hot-wall system. The growth is catalyst promoted at temperatures of 500-1000 0 C. Either a cold wall or hot wall system can be effectivel ofr CNT growth. Substrate is smaller than 1 inch and is placed inside the quartz tube. In the thermal CVD process, either carbon monoxide, or some hydrocarbon such as methane, ethane, ethylene, acetylene or other higher hydrocarbons are used without any dilution. The feedstock is metered through a mass flow controller. During a typical growth run, the reactor is purged with Argon or some other inert gas until the reactor reaches the desired growth temperature. Then the gas flow is switched to the feedstock for the specified growth period. At the end the gas flow is switched back to the inert gas while the reactor cools down to 300 0 C or lower before exposing the nanotubes to air. Typical growth rates range from a few nm/min to 2-5 µm/min. CNT growth is largely empirical . There are very little modeling studies undertaken. The effects of reactor length and diameter, flow rate, etc, on the growth characteristics are completely unknown.