The effects of pore structure evolution on the predictions of chemical vapor infiltration models are investigated in this study. A general multicomponent reaction and transport model is used to describe transport and reaction in the pore space, and structure evolution is modeled by representing the void space by a population of cylindrical capillaries (capillary models) or of the solid phase by a population of solid cylinders (fiber models). The capillaries are assumed to be randomly arranged in space without preferred orientation, whereas the fibers are taken to be parallel to a line, parallel to a plane, or without preferred orientation (one-, two-, or three-directional structures, respectively). The obtained results show that the way in which the pore structure evolves during densification plays a dramatic role in determining the overall behavior of the deposition system. In the case of the fiber structures, the results are influenced significantly not only by the directionality but by the direction of diffusion relative to the fiber axes as well.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 142.104.240.194 Downloaded on 2015-07-12 to IP

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The effects of multicomponent mass transport on the predictions of chemical vapor infiltration models are investigated. The dusty-gas model is used as basis for describing multicomponent transport in the pore space of the preforms. Results are presented over a broad range of conditions extending from the bulk to the Knudsen diffusion regime using the full dusty-gas model and a number of variants. These variants are obtained by simplifying the full form of the dusty-gas model and retaining or omitting the viscous flow terms. The presented results demonstrate the importance of using the complete form of the dusty-gas model in most applications and elucidate the implications of some of the simplifying assumptions that are commonly made in describing gaseous mass transport in chemical vapor infiltration.

The operation of the process of chemical vapor infiltration using a combination of pressure pulsing and thermal gradients is theoretically investigated in this study. Past studies had shown that pulsing of the pressure in the gas phase can lead to a dramatic reduction of the density gradients in the densifying structure, in comparison to those seen in isobaric diffusion-driven Pinfiltration, with significant gradients present only in the vicinity of the external surface of the preforms. Using a detailed model for chemical vapor infiltration under unsteady nonisothermal conditions, we show that temperature gradients, created in our study through microwave heating, can, in conjunction with pressure pulsing, eliminate the density gradients in the final product. Moreover, appropriate tuning of the operational parameters can lead to a situation where densification proceeds from the interior of the preform toward the external surface.

A comprehensive study of the forced-flow chemical vapor infiltration process is presented. A rigorous mathematical model accounting for mass transport (by bulk and Knudsen diffusion and viscous flow), chemical reaction, and structure evolution is formulated for the process. We also develop a simplified model for the case in which transport is controlled by viscous flow and use it to derive analytical results of the parametric sensitivity of the process. The effects of pressure, pore structure, flow rate, thermal gradient, reaction reversibility, and periodic flow reversal on the performance of forcedflow chemical vapor infiltration are examined using both the rigorous and the simplified model. The results show that for a given set of operating conditions, there is an optimal flow rate that produces the best deposition uniformity in the preform. However, even for operation with the optimal flow rate, forced-flow chemical vapor infiltration can outperform the isobaric process only for large enough values of pressure and pore size. Another interesting result is that periodic flow reversal can lead to a dramatic improvement of the deposition uniformity, even in the absence of a thermal gradient. * Electrochemical Society Student Member. * * Electrochemical Society Active Memben heating and cooling of different faces of the preform and volume heating by microwaves or radio frequency methods.