Diffusion control chemical vapor deposition

Chemical vapor deposition of silanes, along with a subsequent calcination using steam, can be utilized to deposit silica (Si(>2) inside the pore system. By variation oF the temperature, the partial pressure of the silane and the duration of the treatment, location and amount of the deposited material can be controlled [104]. When, for example, tetraethoxy- or tetrame-thoxysilane are used as reacting agents on a mordenite, ZSM-5, or /1-zeolite, then a controlled deactivation of only the external cristallite surface is possible [23, 44]. This is because these are rather bulky molecules which are not able to diffuse into the pore system of the crystallite. Alternatively, an irreversible adsorption of bulky bases may serve to destroy the undesired external acidity. Suitable basic compounds are 4-methylquino-line for ZSM-5 [2] and tributylphosphite for mordenites [71]. 

Chemical vapor deposition is an important technique in the production of all kinds of solid state devices. In the process, the active metal organic vapor is swept into a two-dimensional slit reactor by a carrier gas, and deposition occurs at the hot top and bottom plates. The reaction at the plate surfaces can be written MO —> M - - O. Assuming a laminar operation, develop an expression to compute the rate of loss of MO for diffusion-controlled process. 

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). 

The extent to which gas-phase diffusion can be prevented from controlling the deposition rate is of considerable importance for chemical vapor infiltration (CVI). Low pressures and low temperatures (conditions in the catalytic regime) favor penetration. Both factors slow the deposition rate, however, and very long reaction times would be necessary for this way of doing CVI. Consequently, thermal gradients and forced reactant gas flows are sometimes applied to increase deposition rates. 

Contaminant volatilization from subsurface solid and aqueous phases may lead, on the one hand, to pollution of the atmosphere and, on the other hand, to contamination (by vapor transport) of the vadose zone and groundwater. Potential volatihty of a contaminant is related to its inherent vapor pressure, but actual vaporization rates depend on the environmental conditions and other factors that control behavior of chemicals at the solid-gas-water interface. For surface deposits, the actual rate of loss, or the pro-portionahty constant relating vapor pressure to volatilization rates, depends on external conditions (such as turbulence, surface roughness, and wind speed) that affect movement away from the evaporating surface. Close to the evaporating surface, there is relatively little movement of air and the vaporized substance is transported from the surface through the stagnant air layer only by molecular diffusion. The rate of contaminant volatilization from the subsurface is a function of the equilibrium distribution between the gas, water, and solid phases, as related to vapor pressure solubility and adsorption, as well as of the rate of contaminant movement to the soil surface. 

The mechanism for VLS was proposed by Wagner in 1964. Under deposition conditions, the catalyzer has to form a liquid solution with the desired material. It should also have a low vapor pressure and be chemically inert. In the process, the vapor diffuses into the liquid catalyzer and, as the concentration becomes too high, the growth species precipitate to form the nanowire. The liquid phase is a preferential condensation site, and this causes a higher growth rate of the VLS with respect to the VS. Furthermore, by controlling the dimension and dispersion of the catalyzer, control can be achieved over the diameter of the nanowire. 

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