Substrate concentration vapor phase reactions

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

Chemical vapor deposition (CVD) is an atomistic surface modification process where a thin solid coating is deposited on an underlying heated substrate via a chemical reaction from the vapor or gas phase. The occurrence of this chemical reaction is an essential characteristic of the CVD method. The chemical reaction is generally activated thermally by resistance heat, RF, plasma and laser. Furthermore, the effects of the process variables such as temperature, pressure, flow rates, and input concentrations on these reactions must be understood. With proper selection of process parameters, the coating structure/properties such as hardness, toughness, elastic modulus, adhesion, thermal shock resistance and corrosion, wear and oxidation resistance can be controlled or tailored for a variety of applications. The optimum experimental parameters and the level to which... 

Inside processes such as modified chemical vapor deposition (MCVD) had a different origin. Chemical vapor deposition (CVD) had long been used In the electronics industry for fabrication of silicon devices and was adapted to produce silica layers inside substrate tubes [18j. In CVD, the concentration of reactants is very low to inhibit gas phase reaction in favor of a heterogeneous wall reaction which produced a vitreous, particle-free deposit on the substrate. This is fine for the 1000 A films required for semiconductor processing, but fails to produce thick deposits required for fiber. CVD was therefore reversed, the reactant concentration was increased and large volumes of particles were produced inside the silica substrate tube. They deposited on the tube wall and were sintered to glass. 

Finally, it is interesting to follow the total C/Si ratio of the last column of Table I. The overall result of vapor treatment creates a surface with a 27% reduced overall carbon surface concentration. It is also felt the trimethyl silyl blanket created by the in-situ gas phase HMDS reaction has a much more stable carbon surface population, as well as, producing a reduced carbon surface layer concentration. This effect combined with the dehydration of the substrate (1 ), which is well known to occur, creates a surface condition of the wafer conducive to low day to day variation in wafer surface condition. 

Chain initiation occurs when two monomer radicals are coupled to form a dimer biradical and proceeds further." This is an endothermic reaction requiring a heat of formation of 16 kcal/mol. Because of energetic concerns, chain initiation is unlikely to happen in the gas phase at low pressure. When the monomers are adsorbed onto the surface of the substrate, it is believed that, the high local concentration of monomers promotes the formation of biradicals assisted by van der waals forces. Models developed for vapor deposition polymerization of parylene-N indicate that initiation is a third order reaction with an activation energy of 24.8 kcal/mol. 

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