Mécanismes de dépôt par voie gazeuse de céramiques base Si-B-C

B-C and Si-B-C ceramic deposits were made by CVD respectively from CH4/BC3/H2 and MTS/BC3/H2 mixtures in a temperature range of 800-1050°C and under reduced pressure (2 to 12 kPa) . The kinetic laws of deposition have been determined in regimes controlled by chemical reactions. The gaseous species were analyzed in situ by IRTF spectroscopy. This study notably established that the reaction intermediate HBCI2 acts as an effective boron precursor during the deposition of B-C and Si-B-C ceramics. Finally, the synthesis conditions were correlated with the chemical composition and the structure of the deposits. B-C ceramics remain amorphous for all conditions and are richer in carbon than B4C. Si-B-C deposits are also amorphous and carbon-rich at low temperatures. The silicon content increases with temperature together with the appearance of SiC nanocrystals. All of these approaches have made it possible to establish the kinetic laws of deposition in a wide range of experimental conditions and to better understand the mechanisms of formation of ceramics in these particularly complex chemical systems.Des dépôts de céramiques B-C et Si-B-C ont été réalisés par CVD respectivement à partir de mélanges CH4/BC3/H2 et MTS/BC3/H2 dans une gamme de températures 800-1050 °C et sous pression réduite (2 à 12 kPa). Les lois cinétiques de dépôt ont été déterminées dans des régimes contrôlés par les réactions chimiques. Les espèces gazeuses ont été analysées in situ par spectroscopie IRTF. Cette étude a notamment établi que l'intermédiaire réactionnel HBCI2 joue le rôle de précurseur effectif de bore lors du dépôt de céramique B-C et Si-B-C. Enfin, les conditions de synthèse ont été corrélées avec la composition chimique et la structure des dépôts. Les céramiques B-C demeurent amorphes pour l'ensemble des conditions et sont plus riches en carbone que B4C. Les dépôts Si-B-C sont également amorphes et riches en carbone à basse température. La teneur en silicium augmente avec la température conjointement à l'apparition de nanocristaux de SiC. L'ensemble de ces approches a permis l'établissement des lois cinétiques de dépôt dans une large gamme de conditions expérimentales et de mieux comprendre les mécanismes de formation de céramiques dans ces systèmes chimiques particulièrement complexes

Mécanismes de dépôt par voie gazeuse de céramiques base Si-B-C

B-C and Si-B-C ceramic deposits were made by CVD respectively from CH4/BC3/H2 and MTS/BC3/H2 mixtures in a temperature range of 800-1050°C and under reduced pressure (2 to 12 kPa) . The kinetic laws of deposition have been determined in regimes controlled by chemical reactions. The gaseous species were analyzed in situ by IRTF spectroscopy. This study notably established that the reaction intermediate HBCI2 acts as an effective boron precursor during the deposition of B-C and Si-B-C ceramics. Finally, the synthesis conditions were correlated with the chemical composition and the structure of the deposits. B-C ceramics remain amorphous for all conditions and are richer in carbon than B4C. Si-B-C deposits are also amorphous and carbon-rich at low temperatures. The silicon content increases with temperature together with the appearance of SiC nanocrystals. All of these approaches have made it possible to establish the kinetic laws of deposition in a wide range of experimental conditions and to better understand the mechanisms of formation of ceramics in these particularly complex chemical systems.Des dépôts de céramiques B-C et Si-B-C ont été réalisés par CVD respectivement à partir de mélanges CH4/BC3/H2 et MTS/BC3/H2 dans une gamme de températures 800-1050 °C et sous pression réduite (2 à 12 kPa). Les lois cinétiques de dépôt ont été déterminées dans des régimes contrôlés par les réactions chimiques. Les espèces gazeuses ont été analysées in situ par spectroscopie IRTF. Cette étude a notamment établi que l'intermédiaire réactionnel HBCI2 joue le rôle de précurseur effectif de bore lors du dépôt de céramique B-C et Si-B-C. Enfin, les conditions de synthèse ont été corrélées avec la composition chimique et la structure des dépôts. Les céramiques B-C demeurent amorphes pour l'ensemble des conditions et sont plus riches en carbone que B4C. Les dépôts Si-B-C sont également amorphes et riches en carbone à basse température. La teneur en silicium augmente avec la température conjointement à l'apparition de nanocristaux de SiC. L'ensemble de ces approches a permis l'établissement des lois cinétiques de dépôt dans une large gamme de conditions expérimentales et de mieux comprendre les mécanismes de formation de céramiques dans ces systèmes chimiques particulièrement complexes

Understanding the CVD process of (Si)–B–C ceramics through FTIR spectroscopy gas phase analysis

International audienceB-C and Si-B-C ceramics are used as self-healing matrices in ceramic matrix composites. They can be processed by CVD respectively from BCl3-CH4-H2 and BCl3-CH3SiCl3-H2 precursors under reduced pressure and relatively low temperature. An investigation of the CVD process is presented for the two systems on the basis of a FTIR in situ analysis of the gas phase. By adding a porous substrate with a high internal surface in the hot zone of the reactor, the consumption of specific species is enhanced, revealing the effective precursors of the solid (e.g., HBCl2 giving rise to boron). In order to better understand the mechanisms of the solid formation, correlations are pointed out between the gas phase analysis, the deposition kinetics and the properties of the coatings. In the B-C system, an amorphous carbon-rich boron carbide (comprising mostly B-C bonds) is obtained according to a heterogeneous reaction between HBCl2 and a carbon effective precursor (e.g. the CH 3 radical), the C content of the coating increasing with the maturation of the hydrocarbon (i.e. with increasing temperature and PCH4). In the Si-B-C system, two chemical processes compete against each other. The first one, occurring at low temperature (800-900°C), is similar to that involved for the B-C system. The second one, gradually prevailing at higher temperature (900-1000°C), is governed by the formation of Si-C bonds according to a heterogeneous reaction between hydrocarbons and chlorosilanes. This process competes with the formation of B-C bonds and gives rise to SiC nanocrystals for the highest temperatures (T > 1000°C), in a kinetic regime controlled by the mass transfers

Deposition Process of Amorphous Boron Carbide from CH[sub 4]∕BCl[sub 3]∕H[sub 2] Precursor

International audienceAmorphous boron carbide coatings have been prepared by CVD from CH4/BCl3/H2 precursor mixture at low temperature (800-1050°C) and reduced pressure (12kPa). A kinetic study has been conducted to determine the kinetic law (including apparent activation energy and reaction orders) related to the deposition within the regime controlled by the chemical reactions. On the basis of an in-situ gas phase analysis by FTIR spectrometry and a thermodynamic study of the homogeneous equilibrium, the HBCl2 species has been identified as an effective precursor of the boron element. The evidence of correlations between the various experimental approaches has supported a discussion on the chemical process involved

Gas Phase Deposition and Characterization of (Si)-B-C Ceramics

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Deposition process of Si–B–C ceramics from CH3SiCl3/BCl3/H2 precursor

International audienceSi-B-C coatings have been prepared by chemical vapour deposition (CVD) from CH3SiCl3/BCl3/H2 precursor mixtures at low temperature (800-1050°C) and reduced pressures (2, 5, 12 kPa). The kinetics (including apparent activation energy and reaction orders) related to the deposition process were determined within the regime controlled by chemical reactions. A wide range of coatings, prepared in various CVD conditions, were characterized in terms of morphology (scanning electron microscopy), structure (transmission electron microscopy, Raman spectroscopy) and elemental composition (Auger electron spectroscopy). On the basis of an in-situ gas phase analysis by Fourier transform infrared spectroscopy and in agreement with a previous study on the B-C system, the HBCl2 species was identified as an effective precursor of the boron element. HxSiCl(4-x), SiCl4 and CH4, derived from MTS, were also shown to be involved in the homogeneous and the heterogeneous reactions generating silicon and carbon in the coating. A correlation between the various experimental approaches has supported a discussion on the chemical steps involved in the deposition process

Mesure continue de l'activité des décarboxylases de la phénylalamine et de la tyrosine.

National audienceUne nouvelle méthode de mesure plus précise de l'activité des carboxylases, basée sur l'évolution des pressions partielles de CO2 dissous est mise au point. Elle est utilisée pour la détermination du pH optimal des décarboxylases de la de la phénylalamine et de la tyrosine

La grande hache polie de La Reversaie, commune de Romagne (Vienne).

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