PhD Program in Industrial Engineering
Curriculum of Chemical Engineering and Materials
Report on 3-years PhD research activities
“Production of Ceramic Matrix Composites using non
conventional energy sources”
PhD candidate: Roberto D’Ambrosio
PhD course cycle: XXXIII
1. Research project: short description
The research activity was focused on the production of SiC-based Ceramic Matrix Composites (CMCs) using Microwave assisted Chemical Vapor Infiltration (MW-CVI) technology, as a sustainable and economic solution to enable the wider utilization of these materials. The experimental tests have been conducted at an innovative pilot plant, designed acting as an overmoded resonator at the frequencies of interest, where the dimensions and materials of the cavity walls were carefully selected in order to achieve a defined mode density and high fraction of microwave power dissipated in the sample. The design as well as the study of the MW-CVI reactor were supported by means of rigorous numerical modelling based on COMSOL Multiphysics software, to determine the temperature profile and heating dynamic of the sample of interest thus controlling the inside-out SiC-deposition reaction. The design of this pilot plant resulted in a robust MW heated reactor, where CMCs can be quickly heated up to the infiltration temperature with reproducible operating conditions, reduced processing times and high chemical reaction efficiencies.
2. Research activities
The three-years PhD research activity work started from the experience gained throughout the European HELM project (FP7/2007-2013 under grant agreement no. 280464), which led to the development of the hybrid MW-CVI pilot plant based on three magnetron sources operating in the ISM frequency band 2.4-2.5 GHz, where all the related experimental activities have been carried out.
First experimental trials on lab-scale preforms (50 mm diameter, 10 mm of thickness), allowed me to notice several critical points both regarding the MW heating and the chemical processing part. Regarding the heating technology, it was soon clear that a detailed knowledge of the electromagnetic behaviour of the overmoded resonant cavity was necessary in order to predict and tune the desired inverse temperature profile inside the preforms of interest as well as the scalability of this technology for bigger samples.
My activities were therefore first dedicated to a detailed understanding of the MW loaded cavity through modelling and experimental MW heating tests, aimed at optimizing the energy efficiency in terms of power dissipated in the sample with respect to the walls and the sample-holder. One of the main achievements was the identification of the conditions in which the desired EM mode, having a relevant fraction of energy distributed in the sample (volume mode), can be efficiently excited with the available magnetrons.
Following, the MW-CVI trials carried out on different pilot scale preforms (100 mm diameter, 10 mm of thickness) allowed to confirm the good control over the desired EM mode, resulting in a high source-applicator energy coupling and stability along different infiltration runs. These results confirmed some of the expected benefits regarding this technology as the inside-out infiltration mechanism with acceptable deposition rates, potentially leading to a significant reduction of the processing times as well as the scalability of the technology. In this framework, great attention was dedicated to the operating conditions granting the best results in terms of chemical efficiency while avoiding thermal degradation of the reinforcement with silica formation, due to plasma establishment using microwaves at low pressures. The latter points have been addressed proposing some optimization of the pilot plant to achieve a finer tuning and control of the operating parameters.
Finally, further steps ahead regarding this technology have been made considering the effect of another fundamental component, which determine the characteristic pseudo-ductile fracture behaviour of non-oxide CMCs, namely the fiber-matrix interphase. In particular, a pyrolytic carbon interphase was deposited on 3rd gen SiC-fibre fabrics which have been subsequently consolidated through a SiC-based slurry infiltration and vacuum bagging process. Despite of the PyC coating presence, the starting spectrum of the sample remained into the operating frequency bandwidth with high coupling levels. The latter confirmed again the stability of the MW cavity designed with respect to the sample properties, varying along with the heating and the infiltration process, confirming that a predictable and robust behaviour of a well-designed overmoded reactor for industrial applications can be achieved.
