Decomposition chemical vapor

Tetravalent silicon is the only structural feature in all silicon sources in nature, e.g. the silicates and silica even elemental silicon exhibits tetravalency. Tetravalent silicon is considered to be an ana-logon to its group 14 homologue carbon and in fact there are a lot of similarities in the chemistry of both elements. Furthermore, silicon is tetravalent in all industrially used compounds, e.g. silanes, polymers, ceramics, and fumed silica. Also the reactions of subvalent and / or low coordinated silicon compounds normally lead back to tetravalent silicon species. It is therefore not surprising that more than 90% of the relevant literature deals with tetravalent silicon. The following examples illustrate why "ordinary" tetravalent silicon is still an attractive field for research activities Simple and small tetravalent silicon compounds - sometimes very difficult to synthesize - are used by theoreticians and preparative chemists as model compounds for a deeper insight into structural features and the study of the reactivity influenced by different substituents on the silicon center. As an example for industrial applications, the chemical vapor decomposition (CVD) of appropriate silicon precursors to produce thin ceramic coatings on various substrates may be mentioned. 

For our experiments, a carbon fiber cloth (Figure 3 a) was prepared by carbonization of viscose under neutral atmosphere for 15 minutes, successively at 400, 700 and 1000°C. The carbon cloth was coated with pyrolytic carbon, using chemical vapor decomposition of propylene (2.5 ml/mn) diluted in nitrogen (100 ml/mn) during 10 minutes at 900°C. The resulting composite carbon material exhibits a very low irreversible capacity and 1.5 times the reversible capacity of graphite9 11. 

In the case of carbon nanotubes (CNTs), numerous syntheses methods have been developed during the last years, for example, the discharge between two graphite electrodes, laser ablation, hydrocarbon decomposition, and catalytic chemical vapor decomposition (CCVD) however, the most applied methods are arc discharge, laser ablation, and CVD [177-179]. 

Energy is provided to this endothermic reaction by the conduction, convection, and radiation from the furnace walls. This type of decomposition can also be caused on a hot substrate to produce a ceramic film. This process is called chemical vapor decomposition (CVD). 

Early reports of the formation of carbon nanotube (CNT) arrays in templates are given in [51]. Aligned CNT with a diameter of about few nm with sufficient graphitic character were made by chemical vapor decomposition (CVD) using propylene gas. The CNTs were made with Fe and Co catalysts or without catalysts. The... 

Arsenic sulfide has been prepared by the chemical vapor decomposition of a mixture of arsine (ASH3) and hydrogen sulfide [135a]. 

Homyak, G.L. Grigorian, L. Dillon, A.C. Parilla, P.A. Jones, K.M. Heben, M.J. A Temperature Window for Chemical Vapor Decomposition Growth of Single-Wall Carbon Nanotubes. J.Phys. Chem B 2002, 106, 2821-2825. 

CNTs can be first synthesized by arc discharge, laser ablation, and chemical vapor decomposition. CNTs are dispersed in a polymer matrix by melt blending under a high temperature and high shear force, which is also compatible with current industrial practices (Moniruzzaman and Winey, 2006). Melt blending is simple and useful for thermoplastic polymers, such as, polyethylene, polypropylene, polycarbonate, poly(methyl methacrylate)... 

CNTs can be made by means of arc-discharge, laser ablation, high pressure carbon monoxide decomposition (HiPCO), and chemical vapor decomposition (CVD) processes. The HiPCO process delivers high quality SWCNTs but the upscaling is difficult. Arc-discharge, laser ablation and CVD systems are currently used to produce SWCNTs as well as MWCNTs in laboratory scales. However, until now only CVD processes have been upscaled successfully to produce MWCNTs in commercially relevant amounts. For this purpose the synthesis is usually performed in fluidized beds [10-12]. Currently (2011), industrial grades of MWCNTs can be purchased for less than 130 USD/kg. 

S. Sivaram, Chemical Vapor Decomposition, Van Nostrand Reinhold, part 1, New York, (1995) p. 14. 

Nitrogen-doped CNT synthesis can be carried out in two different ways, either directly during synthesis of CNTs in situ method) or by post-treatment of the as-synthesized CNTs ex situ method). For both approaches, various synthetic techniques such as arc discharge, laser ablation, chemical vapor decomposition (CVD), and organometallic functionalization have been employed. Among them the CVD method, or low-temperature synthesis, is the most commonly employed due to its versatility and the facility for scale-up process. 

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