INVESTIGATION OF THE POTENTIAL OF FORCED-FLOW CHEMICAL-VAPOR INFILTRATION

A comprehensive study of the forced-flow chemical vapor infiltration p rocess 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 al so develop a simplified model for the case in which transport is contr olled by viscous flow and use it to derive analytical results of the p arametric sensitivity of the process. The effects of pressure, structu re, flow rate, thermal gradient, reaction reversibility, and periodic flow reversal on the performance of forced-flow chemical vapor infiltr ation are examined using both the rigorous and the simplified model. T he 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, f orced-flow chemical vapor infiltration can outperform the isobaric pro cess only for large enough values of pressure and pore size. Another i nteresting result is that periodic flow reversal can lead to a dramati c improvement of the deposition uniformity, even in the absence of a t hermal gradient.