Reaction and Diffusion Kinetics During the Initial Stages of Isothermal Chemical Vapor Infiltration

Authors

  • Brian W. Sheldon,

    1. Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6063
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    • *

      Member, American Ceramic Society.

    • Division of Engineering, Brown University, Providence, RI 02912.

  • Theodore M. Besmann

    1. Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831–6063
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      Member, American Ceramic Society.


  • R. C. Bradt—contributing editor

  • Presented at the 14th Annual Conference on Composites and Advanced Ceramics, Cocoa Beach, FL, Jan. 14–17, 1990 (Abstract 40-C-90).

  • Supported by the U.S. Department of the Air Force, Air Force Wright Research and Development Center, Wright-Patterson Air Force Base, OH AFWRDC MIPR FY1457–91-N5004, U. S. Department of Energy Interagency Agreement 1692-B081-A1 under Contract No. DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc.

Abstract

Individual bundles of ceramic fibers were infiltrated with SiC to study the reaction and diffusion kinetics during isothermal chemical vapor infiltration (CVI). More uniform infiltration was observed in samples where baffles were placed in the reactor and when HCl was added to the inlet gases. The evolution of the microstructure was modeled using an analytical expression for impinging cylindrical fibers. The transport of reactants was treated using classical descriptions of molecular and Knudsen diffusion in a porous body and also by considering the existence of a percolation threshold. All of these models predict that infiltration should be more uniform than the results that were obtained experimentally. It is possible that this discrepancy occurs because mass transport is more complex than the descriptions that were used. However, a more likely explanation is that the deposition kinetics are significantly more complex than the simple first-order reaction that was used for these models.

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