Modelling and simulation of materials synthesis: Chemical vapor depositoion and infiltration of pyrolytic carbon

Numerical simulation of materials synthesis based on detailed models for the chemical kinetics and transport processes is expected to support development and optimization of production processes. Exemplarily, chemical vapor deposition and infiltration of pyrolytic carbon for the production of carbon fiber reinforced carbon is studied by recently developed modeling approaches and computational tools. First, the development of a gas phase reaction mechanism of chemical vapor deposition (CVD) of carbon from unsaturated light hydrocarbons (CH4, C2H4, C2H2, and C3H6) is presented. The mechanism consisting of 757 reactions among 230 species is based on existing information on elementary reactions and evaluated by comparison of numerically predicted and experimentally determined product composition for more than 40 stable gas phase compounds in a CVD flow reactor. The reactor was operated at widely varying conditions: 800-1100 °C and 2-15 kPa. Experimentally observed pressure and temperature effects on the species profiles as function of residence time are well predicted. Second, a model and computer code is presented for the numerical simulation of chemical vapor infiltration (CVI) carbon for the production of carbon fiber reinforced carbon. The chemistry model is based on a multi-step reaction scheme for pyrocarbon deposition, derived from the elementary mechanism, and a hydrogen inhibition model of pyrocarbon growth. This chemical model is implemented in transient 2D simulations of chemical vapor infiltration. The coupled models for mass transport (convection and diffusion), chemical vapor deposition and surface growth, gas-phase and surface chemical reactions are numerically solved by a FEM approach. Three sets of experiments were exemplarily simulated with inlet flows of 20 kPa CH4, 20 kPa CH4 with 4 kPa H2, and 20 kPa CH4 with 10 kPa H2, all at a temperature of 1095°C. The continuous infiltration, pyrolysis, and deposition of methane and its consecutively formed CxHy products lead to temporarily and spatially varying species concentrations and porosity inside the carbon felt. The predicted density distribution agrees well with experimental data