Chemical vapor deposition cost, relative

Sihcon carbide is also a prime candidate material for high temperature fibers (qv). These fibers are produced by three main approaches polymer pyrolysis, chemical vapor deposition (CVD), and sintering. Whereas fiber from the former two approaches are already available as commercial products, the sintered SiC fiber is still under development. Because of its relatively simple process, the sintered a-SiC fiber approach offers the potential of high performance and extreme temperature stabiUty at a relatively low cost. A comparison of the manufacturing methods and properties of various SiC fibers is presented in Table 4 (121,122). 

Since the reports using the catalytic chemical vapor deposition (CCVD) method for the synthesis of N-CNTs published in 1997 by Yudasaka et al. [32] and Sen et al. [33] and by Terrenes et al. in 1999 [34], CCVD has become the most common and reliable technique to synthesize CNTs and N-CNTs due to its simplicity and scalability. Indeed, the CCVD method requires only a furnace, a tubular reactor, a reactive gas mixture, and an appropriate catalyst. In addition, CCVD can be carried out in a continuous mode and at a relatively low temperature compared with the arc discharge and laser ablation ones. The product is extremely pure and, thus, additional purification is not required, which represents a net gain for the cost-effectiveness of the process. A schematic diagram of the conventional fixed-bed CVD synthesis setup is shown in Figure 9.5. 

Aerosol-assisted CVD introduces rapid evaporation of the precursor and short delivery time of vapor precursor to the reaction zone. The small diffusion distance between the reactant and intermediates leads to higher deposition rates at relatively low temperatures. Single precursors are more inclined to be used in AACVD therefore, due to good molecular mixing of precursors, the stoichiometry in the synthesis of multicomponent materials can be well controlled. In addition, AACVD can be preformed in an open atmosphere to produce thin or thick oxide films, hence its cost is low compared to sophisticated vacuum systems. CVD methods have also been modified and developed to deposit solid phase from gaseous precursors on highly porous substrates or inside porous media. The two most used deposition methods are known as electrochemical vapor deposition (EVD) and chemical vapor infiltration (CVI). 

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