Chemical vapor infiltration of SiC with microwave heating

A mathematical model was developed to elucidate the interaction between transport/reaction processes and the evolution of porosity in chemical vapor infiltration with microwave heating (MCVI). The analysis included a set of partial differential equations describing the spatiotemporal variation of gaseous species concentration, composite temperature, porosity, and stress. Maxwell's equations were used to determine the distribution of power dissipated inside the composite. The deposition of silicon carbide was selected as a model chemical system to explore the general features of MCVI. MCVI can provide a favorable temperature distribution in the composite yielding an inside-out deposition pattern, thereby preventing entrapment of accessible porosity. For this temperature profile, tensile stresses develop at the outer regions and compressive stresses are found in the composite core. For a given system there exists a minimum value of the coefficient for heat transfer from the composite surface, h, below which accessible porosity is trapped within the composite. Similarly, there exists a maximum value of the incident microwave energy flux, I₀, above which accessible porosity is trapped within the composite. I₀ and h can be optimized for a given preform to achieve complete densification with minimum processing time. Using the technique of pulsed-power, the processing time can be reduced even further without compromising density uniformity. Power dissipation profiles in the composite depend strongly on preform thickness, microwave frequency, and relative loss factor.