Modified chemical-vapor deposition

Engineering of reaction and Chemical vapor deposition Modified chemical vapor deposition... 

Fig. 21. Schematic illustration of the four primary vapor-phase deposition processes used in optical-fiber fabrication outside vapor deposition (OVD), modified chemical vapor deposition (MCVD), plasma vapor deposition (PVD), and vapor axial deposition (VAD) (115).

Chemical Vapor Deposition. Chemical vapor deposition (CVD) of siHcon dioxide from tetraethoxysilane assisted by the presence of oxygen and a plasma is an important technology for the deposition of pure and modified dielectrics for microelectronics (61). An alternative method for the deposition of siHcon dioxide utili2es di-/-butoxydiacetoxysilane (62). 

In the modified chemical vapor deposition (MCVD) technique, the reactants are deposited on the inside of a rotating siUca tube. The hoUow tube is heated from the outside by a moving oxyhydrogen torch. The oxide soot condenses onto the tube walls ahead of the burner, and the soot is then sintered into a glassy layer as the burner passes over it. When deposition is complete, the tube and its contents are collapsed to form a soHd preform rod. 

Inside" processes—such as modified chemical vapor deposition (MCVD) and plasma chemical vapor deposition (PCVD)—deposit doped silica on the interior surface of a fused silica tube. In MCVD, the oxidation of the halide reactants is initiated by a flame that heats the outside of the tube (Figure 4.8). In PCVD, the reaction is initiated by a microwave plasma. More than a hundred different layers with different refractive indexes (a function of glass composition) may be deposited by either process before the tube is collapsed to form a glass rod. 

Modified carbon fibers, 13 383-385 Modified cellulosic membranes, in hemodialysis, 26 825, 826-828t Modified chemical vapor deposition (MCVD), in fiber optic fabrication, 11 136-137, 138, 139 Modified-Claus sulfur recovery process, 23 601, 602... 

Chemical vapor deposition [37,38], and thermal or anodic oxidation of Ti substrates [39,40,41] have been used to prepare polycrystalline thin films of Ti02. For example, thin films of Ti02 prepared by anodic oxidation of Ti, followed by electrodeposition of In20s from 0.5 M 102(504)3 show enhanced optical absorption up to 500 nm [42] with the In203 modified electrode showing enhanced photocurrent and photovoltage partially due to the low electrical resistance (10 Q) and reduced overvoltage of the photoanode. 

T.2.4.3 Modified CVD Processes for Opticai Fiber Production. The principles of chemical vapor deposition can also be applied to the production of optical fibers. The fundamentals of fiber optics were described in Section 6.3.2.7. We concentrate here on the application of CVD to their production. 

This CVD procedure is somewhat different from that used to deposit semiconductor layers. In the latter process, the primary reaction occurs on the substrate surface, following gas-phase decomposition (if necessary), transport, and adsorption. In the fiber optic process, the reaction takes place in the gas phase. As a result, the process is termed modified chemical vapor deposition (MCVD). The need for gas-phase particle synthesis is necessitated by the slow deposition rates of surface reactions. Early attempts to increase deposition rates of surface-controlled reactions resulted in gas-phase silica particles that acted as scattering centers in the deposited layers, leading to attenuation loss. With the MCVD process, the precursor gas flow rates are increased to nearly 10 times those used in traditional CVD processes, in order to produce Ge02-Si02 particles that collect on the tube wall and are vitrified (densified) by the torch flame. 

Up to now three chemical vapor deposition (CVD) techniques have proved suitable for the preparation of high quality optical fibers the outside vapour phase oxidation (OVPO) process8, the modified CVD (MCVD) process9 and the plasma-activated CVD (PCVD) process10. The last mentioned process will be the main subject of this article. To give a better appreciation of the principles the alternative processes will be described briefly. 

Ceramic and semiconductor thin films have been prepared by a number of methods including chemical vapor deposition (CVD), spray-coating, and sol-gel techniques. In the present work, the sol-gel method was chosen to prepare uniform, thin films of titanium oxides on palladium Titanium oxide was chosen because of its versatility as a support material and also because the sol-gel synthesis of titania films has been clearly described by Takahashi and co-workers (22). The procedure utilized herein follows the work of Takahashi, but is modified to take advantage of the hydrogen permeability of the palladium substrate. Our objective was to develop a reliable procedure for the fabrication of thin titania films on palladium, and then to evaluate the performance of the resulting metalloceramic membranes for hydrogen transport and ethylene hydrogenation for comparison to the pure palladium membrane results. 

Cymene or isopropyltoluene is produced via alkylation of toluene with propylene. Cymene is an important intermediate in the production of cresol, and it is also used as an industrial solvent. Again, for both environmental and economic reasons, the use of zeolitic materials for this conversion has been studied. For example, Flockhart et al. have used zeolite Y to effect this reaction (7). They observed that the state of the zeolite, including its degree of ion-exchange and the temperature at which it was calcined, strongly affected the distribution of cymene isomers obtained. In order to enhance the selectivity to para-cymene, the direct precursor to para-cresol, various studies have focused on the use of surface modified zeolites, for example, ZSM-5 materials, including those produced by chemical vapor deposition (CVD) of silicate esters. These species serve to reduce surface acidity and change limit diffusion within the crystal. 

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