EVALUATION OF ISOTHERMAL CHEMICAL-VAPOR INFILTRATION WITH LANGMUIR-HINSHELWOOD TYPE KINETICS

A model has been developed for the mathematical description of isother mal chemical vapor infiltration (ICVI) processes. Three types of adsor ption of the reactive species are incorporated into the kinetic equati ons: weak associative adsorption, strong associative adsorption, and d issociative adsorption. The kinetic models are based on Langmuir-Hinsh elwood equations. Weak associative adsorption of the reactive species gives rise to an exponentially shaped final deposition profile, wherea s strong associative and dissociative adsorption result in a sigmoidal ly shaped deposition profile in the pore. This profile originates from a shift in deposition mechanism from a layered growth (weak associati ve adsorption) to a moving front growth (strong associative and dissoc iative adsorption). For weakly adsorbing reactive species, the residua l porosity of a preform can be decreased by lowering the process press ure. With strong associative or dissociative adsorption, the reverse e ffect can be found at the investigated process conditions. This effect is caused by the change in reaction-rate dependence of the concentrat ion inside the pore. Application of the concept of a generalized Thiel e modulus shows that for all investigated kinetic models the residual porosity of densified preforms is smaller than 1% when the Thiele modu lus is kept below 0.02. For weak associative adsorption, this region c an be achieved by lowering the pressure. With strong associative and d issociative adsorption of the active species, it is realized by an inc rease in concentration and pressure.