produced by the other two methods.For example,the CVD-produced MWNTs are less crystalline and contain more defects than the arc-discharged MWNTs.The CVD-produced MWNTs are longer and less straight than the arc-discharged MWNTs. The arc discharge method uses two graphite rods;one serving as the cathode and the other serving as the anode,in either helium,argon,or a mixture of helium and argon atmosphere.The graphite rods are placed side by side with a very small gap,typically about 1 mm in size,between them.The pressure inside the reaction chamber is maintained between 100 and 1000 torr. When a stable arc is produced in the gap by passing 50-120 A electric current (at 12-25 V)between the graphite rods,the material eroded from the anode is deposited on the cathode in the form of MWNTs,amorphous carbon,and other carbon particles.To produce SWNTs,the cathode is doped with a small amount of metallic catalyst (Fe,Co,Ni,Y,or Mo).However,the yield of SWNTs is only between 20%and 40%by weight. The laser ablation method uses either pulsed or continuous-wave laser to vaporize a graphite target held at 1200C in a controlled atmosphere of argon or helium inside a tube furnace.The vaporized material is collected on a water- cooled copper collector in the form of carbon nanotubes,amorphous carbon, and other carbon particles.To produce SWNTs,the graphite target is doped with metal catalysts such as nickel and cobalt catalysts.The yield of SWNTs is between 20%and 80%by weight. In the basic CVD method,carbon nanotubes are produced by the decom- position of a carbon-containing gas,such as carbon monoxide and hydrocar- bon gases,or by the pyrolysis of carbon-containing solids,such as polymers,at a high pressure inside a furnace.The temperature inside the furnace is typically between300Cand800°C for making MWNTs,.and60o°Cand1200°Cfor making SWNTs.MWNTs are produced in an inert gas atmosphere,whereas a mixture of hydrogen and an inert gas is used for SWNTs.A high-temperature substrate,such as alumina,coated with catalyst particles,such as Fe,Ni,and Co,is placed in the furnace.The decomposition of the carbon-containing gas flowing over the substrate causes the growth of carbon nanotubes on the substrate.They are collected after cooling the system down to room tempera- ture.Depending on the carbon-containing gas,catalyst,furnace temperature, pressure,flow rate,residence time for thermal decomposition,and so on,the yield can be between 30%and 99%by weight. All three production methods produce carbon nanotubes that are contam- inated with impurities such as amorphous carbon,carbon soot,other carbon particles,and metal catalysts.Several purification processes have been devel- oped to produce cleaner carbon nanotubes.Two of the processes are gas phase oxidation and liquid phase purification.Purification of MWNTs by gas phase oxidation involves heating them in an oxygen or air atmosphere at temperatures >700C.In the case of SWNT,it involves heating in a mixed atmosphere of hydrochloric acid,chlorine,and water vapor at 500C. 2007 by Taylor Francis Group,LLC.produced by the other two methods. For example, the CVD-produced MWNTs are less crystalline and contain more defects than the arc-discharged MWNTs. The CVD-produced MWNTs are longer and less straight than the arc-discharged MWNTs. The arc discharge method uses two graphite rods; one serving as the cathode and the other serving as the anode, in either helium, argon, or a mixture of helium and argon atmosphere. The graphite rods are placed side by side with a very small gap, typically about 1 mm in size, between them. The pressure inside the reaction chamber is maintained between 100 and 1000 torr. When a stable arc is produced in the gap by passing 50–120 A electric current (at 12–25 V) between the graphite rods, the material eroded from the anode is deposited on the cathode in the form of MWNTs, amorphous carbon, and other carbon particles. To produce SWNTs, the cathode is doped with a small amount of metallic catalyst (Fe, Co, Ni, Y, or Mo). However, the yield of SWNTs is only between 20% and 40% by weight. The laser ablation method uses either pulsed or continuous-wave laser to vaporize a graphite target held at 12008C in a controlled atmosphere of argon or helium inside a tube furnace. The vaporized material is collected on a water￾cooled copper collector in the form of carbon nanotubes, amorphous carbon, and other carbon particles. To produce SWNTs, the graphite target is doped with metal catalysts such as nickel and cobalt catalysts. The yield of SWNTs is between 20% and 80% by weight. In the basic CVD method, carbon nanotubes are produced by the decom￾position of a carbon-containing gas, such as carbon monoxide and hydrocar￾bon gases, or by the pyrolysis of carbon-containing solids, such as polymers, at a high pressure inside a furnace. The temperature inside the furnace is typically between 3008C and 8008C for making MWNTs, and 6008C and 12008C for making SWNTs. MWNTs are produced in an inert gas atmosphere, whereas a mixture of hydrogen and an inert gas is used for SWNTs. A high-temperature substrate, such as alumina, coated with catalyst particles, such as Fe, Ni, and Co, is placed in the furnace. The decomposition of the carbon-containing gas flowing over the substrate causes the growth of carbon nanotubes on the substrate. They are collected after cooling the system down to room tempera￾ture. Depending on the carbon-containing gas, catalyst, furnace temperature, pressure, flow rate, residence time for thermal decomposition, and so on, the yield can be between 30% and 99% by weight. All three production methods produce carbon nanotubes that are contam￾inated with impurities such as amorphous carbon, carbon soot, other carbon particles, and metal catalysts. Several purification processes have been devel￾oped to produce cleaner carbon nanotubes. Two of the processes are gas phase oxidation and liquid phase purification. Purification of MWNTs by gas phase oxidation involves heating them in an oxygen or air atmosphere at temperatures >7008C. In the case of SWNT, it involves heating in a mixed atmosphere of hydrochloric acid, chlorine, and water vapor at 5008C.
2007 by Taylor & Francis Group, LLC