Modelling of pyrocarbon chemical vapor infiltration

International audienceThe chemical vapor infiltration (CVI) of pyrocarbon is used to produce carbon matrix of C/C composites. This process involves complex physico-chemical phenomena such as the transport of gas mixtures (hydrocarbons and hydrogen) in the reactor and inside the fibrous preform, chemical reactions (pyrolysis and deposition), and the structural evolution of the preform. A global modelling approach has been developed for isobaric CVI. The most difficult point is to find a realistic chemical model for pyrocarbon deposition chemistry, simple enough to be implemented in a 2D or 3D fluid dynamics code. Such a model is proposed in this study, featuring a group of light species leading to smooth laminar pyrocarbon, a group of heavier species (polycyclic hydrocarbons) leading to rough laminar pyrocarbon, and associated homogeneous and heterogeneous reaction kinetics. This model has been developed and validated according to results of pyrocarbon CVD experiments from propane, and isothermal, isobaric CVI in a 1D model porous medium made of compact stacks of 100 μm diameter filaments

CVD and CVI of pyrocarbon from various precursors

International audienceThe control of pyrocarbon (pyC) chemical vapor infiltration (CVI) is a key issue in the processing of high-performance C/C composites with applications in aerospace parts and braking technology. For years, the precise investigation of deposition kinetics and pyC nanometerscale anisotropy has been rehearsed in chemical vapor deposition (CVD) and several variants of CVI with various pore sizes, and using mostly propane, propylene, and methane as source precursors. A literature survey and the analysis of recent experimental data have helped to understand better the role of gas-phase intermediate species in the various nanotextural transitions; a coherent modeling frame, which is suitable for propane, propylene, and methane—the latter having a neatly lower reactivity—has been set up and tested against experimental results from independent teams. The relation between nanotexture and processing conditions is then explained

Quantitative structural and textural assessment of laminar pyrocarbons through Raman spectroscopy, electron diffraction and few other techniques

In pyrocarbon materials, the width of the Raman D band (FWHMD) is very sensitive to low energy structural defects (e.g., disorientations of the graphene layers). The correlation between the two parameters, FWHMD and OA (as derived from selected area electron diffraction: SAED), has allowed to differentiate various pyrocarbons unambiguously. Furthermore, the optical properties of pyrocarbons, i.e., the extinction angle, the optical phase shift and the ordinary and extraordinary reflectance, have been accurately determined at 550 nm by means of the extinction curves method. These results are completed by in-plane and out-of-plane dielectric constant measurements by angular resolved EELS. Moreover, the hybridization degree of the carbon atoms has been assessed by the same technique. About 80% of the carbon atoms of the pyrocarbons have a sp2 hybridization. The lack of pure sp2 carbon atoms, as compared to graphite, might be explained by the presence of sp3-like line defects.Comment: Xavier Bourrat : Present Address = ISTO - CNRS-Universit\'e d'Orl\'ean

Mécanismes chimiques de CVD/CVI de pyrocarbones laminaires issus du propane : cinétique, texture et composition gazeuse

A study of the chemical mechanisms of formation of laminar pyrocarbons by CVD/CVI from propane was conducted on the basis of three experimental approaches, depending on the parameters residence time, pressure and surface/volume ratio. A kinetic study has highlighted two transitions, one with a high residence time that requires a threshold pressure, the other with a very low residence time. These kinetic transitions have been associated with microtexture transitions that can lead to highly organized pyrocarbons, either by increasing residence time and pressure, or by decreasing these two parameters. Thanks to an analysis of the gas phase, in particular by in-situ FTIR spectrometry and MALDI-TOF ex-situ, intermediate chemical species (aliphatic, aromatic, ...) were highlighted, making it possible to propose reaction schemes for the formation of the different categories of pyrocarbon deposited.Une étude des mécanismes chimiques de formation de pyrocarbones laminaires par CVD/CVI à partir du propane, a été conduite sur la base de trois approches expérimentales, en fonction des paramètres temps de séjour, pression et rapport surface/volume. Une étude cinétique a permis de mettre en évidence deux transitions, l'une à fort temps de séjour qui nécessite une pression seuil, l'autre à très faible temps de séjour. Ces transitions cinétiques ont été associées à des transitions de microtexture pouvant conduire à des pyrocarbones très organisés, soit par accroissement du temps de séjour et de la pression, soit par diminution de ces deux paramètres. Grâce à une analyse de la phase gazeuse, notamment par spectrométrie IRTF in-situ et MALDI-TOF ex-situ, des espèces chimiques intermédiaires (aliphatiques, aromatiques, ...) ont été mises en évidence, permettant de proposer des schémas réactionnels pour la formation des différentes catégories de pyrocarbone déposées

Mécanismes chimiques de CVD/CVI de pyrocarbones laminaires issus du propane : cinétique, texture et composition gazeuse

A study of the chemical mechanisms of formation of laminar pyrocarbons by CVD/CVI from propane was conducted on the basis of three experimental approaches, depending on the parameters residence time, pressure and surface/volume ratio. A kinetic study has highlighted two transitions, one with a high residence time that requires a threshold pressure, the other with a very low residence time. These kinetic transitions have been associated with microtexture transitions that can lead to highly organized pyrocarbons, either by increasing residence time and pressure, or by decreasing these two parameters. Thanks to an analysis of the gas phase, in particular by in-situ FTIR spectrometry and MALDI-TOF ex-situ, intermediate chemical species (aliphatic, aromatic, ...) were highlighted, making it possible to propose reaction schemes for the formation of the different categories of pyrocarbon deposited.Une étude des mécanismes chimiques de formation de pyrocarbones laminaires par CVD/CVI à partir du propane, a été conduite sur la base de trois approches expérimentales, en fonction des paramètres temps de séjour, pression et rapport surface/volume. Une étude cinétique a permis de mettre en évidence deux transitions, l'une à fort temps de séjour qui nécessite une pression seuil, l'autre à très faible temps de séjour. Ces transitions cinétiques ont été associées à des transitions de microtexture pouvant conduire à des pyrocarbones très organisés, soit par accroissement du temps de séjour et de la pression, soit par diminution de ces deux paramètres. Grâce à une analyse de la phase gazeuse, notamment par spectrométrie IRTF in-situ et MALDI-TOF ex-situ, des espèces chimiques intermédiaires (aliphatiques, aromatiques, ...) ont été mises en évidence, permettant de proposer des schémas réactionnels pour la formation des différentes catégories de pyrocarbone déposées

Situations de « référence » pour enseigner le numérique au cycle 2

International audienc

Sans famille : Par Hector Malot

Édition originale : Édouard Dentu (Paris), 187