A mathematical model for chemical vapor infiltration with microwave heating and external cooling
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A mathematical model has been used to compute temperature profiles in ceramic preforms that are heated by microwaves. The temperature profiles were then input to a second part of the model describing chemical vapor infiltration of the preform, that is the diffusion of gaseous reactants, heterogeneous reaction, and evolution of the pore structure. Equations were solved numerically for parameters corresponding to the infiltration of SiC preforms by pyrolysis of trichloromethylsilane. While based on some simplifications, the model leads to the conclusion that infiltration proceeds more rapidly, and to a greater extent, with microwave heating/external cooling than in isothermal infiltration. The model suggests that infiltration might be optimized by manipulation of microwave power and external cooling. The computed extent of infiltration is seen to be very sensitive to the initial pore size.
I. INTRODUCTION
Composite materials have long been recognized as offering properties superior to either of their components in many applications. Fiber-matrix composites in which both components are ceramic are likely to find increasing use in many high temperature environments. One partially successful route for the synthesis of ceramic-ceramic composites has been chemical vapor infiltration (CVI). In this method the fibers are preformed into a porous body of the desired shape and the matrix is then produced by chemical vapor deposition into the pores. An obvious difficulty is that the reactions involved (e.g., the thermal decomposition of trichloromethylsilane) require high temperatures and are endothermic. Therefore heat must be provided to the preform. For example, the preform could be placed in a furnace where heat is generated externally to the preform and transported into its interior by conduction and radiation. In this case, the exterior is hotter than the interior and, chemical reactions being faster at higher temperatures, there is a tendency for reaction to occur preferentially near the outside of the preform. This tendency is more severe under circumstances where diffusion of gaseous reactants within the pores of the preform is the rate-controlling step in the infiltration process—for example, if the preform is thick or when high temperatures are exploited in an attempt to shorten processing times. The result is the sealing of the exterior layers of the preform, preventing further infiltration and resulting in a heterogeneous composite with much internal porosity. As described more fully below,
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J. Mater. Res., Vol. 6, No. 4, Apr 1991
it may be possible to overcome this diffusional limitation on CVI by using microwave heating/external cooling to invert the temperature gradient of external heating. The objectives of the mathematical model were to examine this possibility and to provide guidance for future experimental work based on this approach. II. PREVIOUS INVESTIGATIONS
Chemical vapor infiltration has been studied by a number of investigators (e.g., Refs. 1-18). Hann
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