A mathematical model that describes the combined diffusion and reaction of trichloromethylsilane in a porous preform under typical chemical vapor infiltration (CVI) conditions is presented. The model utilizes a single pore to demonstrate the importance of the pore geometry, totalgaseous pressure, temperature, and imposed temperature gradient on the degree of achieved densification. By focusing attention on a single pore, it is possible to account for changes in the pore shape that occur during deposition of silicon carbide on the pore boundaries. Process conditions for operation in the reaction-controlled regime in order to attain complete densification are quantitatively identified even for the longer pores and the relatively higher temperatures used.Composite materials are becoming increasingly important in the aerospace, automobile, nuclear, solar energy, and manufacturing industries. Carbon fibers imbedded in a carbon matrix exhibit significant advantages, including low density to strength ratio, and excellent behavior towards friction and ablation at even high temperatures. Moreover, the use of multidirectional reinforcement leads to a quasi-isotropic behavior. However, carbon-carbon composites exhibit a few drawbacks, most important of which is their poor resistance towards oxidation at relatively low temperatures (500~176 On the other hand, monolithic ceramics (silicon carbide, silicon nitride, alumina) combine good strength and resistance to oxidation. Their main disadvantage is their sensitivity to crack propagation and poor resistance to mechanical and thermal shocks. In order to combine the advantages of both classes of materials and overcome their weaknesses, it has been suggested to even partially replace the carbon matrix, by a refractory material. More specifically, silicon carbide (SIC) and titanium carbide (TIC) are compatible with carbon and have improved mechanical properties and good oxidation resistance at high temperatures due to a protective oxide formed on their surfaces (1-3).Typical fabrication stages of fiber-reinforced ceramics include, among others, extrusion, hot-pressing, and sintering. However, during these processes the fibers may be chemically or mechanically damaged and the final product is often not of high purity but of high porosity. A most promising alternative is to subject a fibrous preform of the desired shape to the comparatively low-stress and lowtemperature process of chemical vapor infiltration (CVI) of appropriate chemical precursors and subsequent deposition of the desired product on the fibers until complete densification is achieved. Extreme care must be taken so that the deposition process is uniform and complete filling of the voids is achieved, rather than simple overcoating. To this end, the combined diffusion and reaction process must be limited by the chemical reaction allowing for indepth diffusion. Due to this requirement, the overall process is extremely slow and 300-600h of operation have been required for the full densification of laboratory-scale sampl...
The process of chemical vapor infiltration for den sification of porous ceramic preforms consists of reactant diffusion and decomposition followed by solid product deposition in the porous body. A mathematical model for isothermal and nonisothermal CVI is presented where the transient porous structure is approximated by a Bethe lattice. Concepts of percolation theory are used to account for utilized, unutilized, and filled pore space. Diffusion coefficients are evaluated using the effective medium theory. The effect of pore connectivity, porosity distribution, operating pressure, inlet temperature, hot-face temperature, and length of the preform on densification is studied. Optimum conditions for low residual porosities and uniform densification are suggested.Ceramic composite materials consis%ing of continuous fibers or whiskers imbedded in a brittle ceramic matrix exhibit improved mechanical properties and increased resistance to oxidation in corrosive environments (1-3). Their improved mechanical properties include (i) high strength to density ratio, since mechanical forces are mainly transmitted through the lighter fiber reinforcement and (ii) in-) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 193.0.65.67 Downloaded on 2015-05-28 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 193.0.65.67 Downloaded on 2015-05-28 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 193.0.65.67 Downloaded on 2015-05-28 to IP
A mathematical model for forced-flow chemical vapor infiltration (CVI) for densification of porous ceramic composites is presented. The process consists of mass transport of reactants by both convection and diffusion into a porous preform, decomposition of reactants, and subsequent deposition of solid product on pore surfaces. The preform is represented by a Bethe lattice, and percolation theory is used to account for utilized, unutilized, and blocked pore space. The effective medium approach used earlier for modeling conventional CVI is extended here to include forced flow of reactants into the preform. The effect of reactant flow rates, hot face temperature, and pore size distribution on densification is studied and compared with the conventional process. Optimum conditions for uniformity and low residual porosity in the final product are suggested.Composite materials are increasingly being regarded as the materials of the future. Their high strength-to-weight ratio and excellent friction and ablation resistance make them ideal choices for replacement of metals in several applications in the aerospace, automobile, nuclear, and other manufacturing industries. An important category of such materials is ceramic matrix composites. These composite materials, which consist of fibers or whiskers imbedded in a brittle ceramic matrix, combine the excellent mechanical properties of the fibers with increased chemical inertness of the ceramic matrix in corrosive environments (1-3). Sintering, hot-pressing, and chemical vapor infiltration (CVI) are among the more commonly used techniques for manufacturing ceramic composites. However, it is difficult to sinter a composite material containing desirable volume fractions of fibers or whiskers (>20%) (4). On the other hand, hot pressing is applicable only to relatively simple shapes and may cause damage to the fiber structure. As a ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-06-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-06-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-06-13 to IP ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 138.251.14.35 Downloaded on 2015-06-13 to IP
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