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A typical CVD setup is shown in Figure 1.8. A long quartz tube (C) is inserted into two ceramic tubes (D). Electrical heating wire is wrapped over two ceramic tubes (D). These two ceramic tubes are arranged to maintain two different temperatures (A1 and A2). In the quartz tube (C) two quartz boats (B1 and B2) are kept. In boat B1 a known amount of precursor which is to be pyrolyzed is kept. In boat B2 catalyst powder is spread over the boat. Two gas cylinders (K and H) are connected to the long quartz tube (C) via two controllers (E) and flow meter (F). The outlet of tube C is connected to two bubblers (G). Temperature of A1 is maintained at a value which can boil the precursor kept in quartz boat B1, which is normally kept at around 400 °C. Temperature of furnace A2 is maintained at a suitable temperature where the precursor gas can be decomposed in the presence of the catalyst to get the CNF. Before setting the temperatures of two furnaces, inert gas like argon gas (H) is purged through the quartz tube (C) to remove the presence of oxygen. The temperature of furnace A2 is set to a required temperature (which could be in the range of 600–1000 °C). When the required temperature has attained the temperature of the furnace, A1 is set to boiling temperature of the precursor. During this process the flow of carrier gas is maintained which helps to carry the vapor of precursor to the furnace A2 for completing the process of pyrolysis. Undecomposed gas and carrier gas are allowed to escape to atmosphere through the bubblers (G). At the end of pyrolysis CNF is collected from the furnace (A2).

It is possible to control the length of carbon nanofibers by controlling the rate of gas flow in the quartz tube (C). In addition, direction of fiber growth can also be controlled by adjusting the direction of gas flow. If adjusted properly, the variable parameters of the process can yield CNF with submicrometer diameters and lengths of a few to 100 μm.

Figure 1.8 A sketch of a typical CVD unit. A1 and A2 are two furnaces operating at two different temperatures. B1 and B2 are two quartz boats used for keeping precursor and catalyst respectively. CNF is grown on the boat B2 as well as over the entire area covering the furnace A2, C is the quartz tube in which growth of CNF takes place, D is ceramic tube over which heating wire is wrapped to create required temperature, E and F are gas valves and flow meter to regulate flow rate of gas. G is a gas bubbler to prevent backflow of gas into the quartz tube and allow the escape of unreacted gas into the atmosphere, H is the gas cylinder containing the carrier gas and K is precursor gas like methane or acetylene.