Dynamic convection-driven thermal gradient chemical vapor infiltration
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Dynamic convection-driven thermal gradient chemical vapor infiltration John Y. Ofori and Stratis V. Sotirchosa) Department of Chemical Engineering, University of Rochester, Rochester, New York 14627 (Received 1 December 1995; accepted 1 June 1996)
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.
I. INTRODUCTION
Chemical vapor infiltration (CVI) is used to manufacture ceramic or carbon matrix composites by decomposing a gaseous species to deposit solid material on the interior surface of an initially porous preform. The combination of the reinforcing preform and the deposited matrix produced by CVI provides mechanical properties superior to those of monolithic materials, particularly with respect to fracture toughness.1,2 The retention of the initial preform shape during CVI processing makes possible the fabrication of near-net-shape parts.3 The processing of ceramic composites by chemical vapor infiltration involves complex interactions between chemical reaction and mass transport. These interactions are of primary importance for the determination of deposition uniformity of composites produced by the CVI process, especially in the presence of strong mass transfer limitations. In order to produce by CVI ceramic composites at a commercially acceptable cost, the processing time must be kept at relatively low levels. The main disadvantage of conventional CVI (isothermal isobaric) are the long times required to obtain final products with low density gradients. The main cause for this situation is that diffusion, the principal mechanism of transport of gaseous reactants and products in the pore space of the preform, is a slow process. Several processing schemes have been considered to enhance the density uniformity and reduce the processing time of ceramic composites beyond the levels encountered in the conventional isothermal isobaric CVI process. Most of these processes rely on two approaches, a)
Author to whom correspondence should be addressed. J. Mater. Res., Vol. 11, No. 10, Oct 1996
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