Design of an overmoded resonant cavity-based reactor for ceramic matrix composites production

The use of high-quality resonant cavities in microwave-assisted reactors is highly desirable. Thanks to the low losses in the cavity walls and to the possibility to reach critical coupling conditions, high energy efficiency can indeed be achieved.This paper describes the design, building, and test of a microwave-assisted Chemical Vapor Infiltration reactor at a pilot scale, based on an overmoded resonant cavity operating at 2.45 GHz. First, a general design method is proposed, in which the dimensions of the cavity are chosen in order to obtain a defined mode density and a high fraction of microwave power dissipated in the sample. Among the modes of this cavity, the most efficient one for the microwave heating of the sample is then determined by means of rigorous numerical analysis.In the graphite reactor cavity built using this approach, the condition of critical coupling is met in a large variety of samples and operating conditions. Moreover, a good agreement between experimental and simulated heating dynamics is obtained, which confirms the validity of the proposed method.As a result, disc-shaped samples with a diameter of the order of 10 cm and a thickness of the order of 1 cm can be brought to an infiltration temperature of about 900 degrees C within 5 min, using a microwave power of 1000 W, with a well controllable and reproducible temperature variation. In the infiltration of Nicalon preforms, more than 60% of the microwave power is dissipated in the sample.