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

Des mélanges fleuries présantant une diversité fonctionnelle élevée favorisent-ils les prédateurs des pucerons dans les bandes fleuries?

peer reviewedAmong semi-natural elements in agricultural landscapes, wildflower strips sown at field margins or within fields represent potential habitats for the natural enemies of insect pests. As insects are sensitive to a variety of flower traits, we hypothesised that mixtures with high functional diversity attract and support a higher abundance and species richness of aphid flower visiting predators compared to mixtures with low functional diversity. During a field experiment, repeated over two years (2014 and 2015) in Gembloux (Belgium), aphid predators (i.e., lacewings, ladybeetles and hoverflies) were pan-trapped in five sown flower mixtures (including a control mixture, with three replicates of each mixture) of low to high functional diversity based on seven traits (i.e., flower colour, ultra-violet reflectance and pattern, blooming start and duration, height and flower class, primarily based on corolla morphology). In both years, flower species in the sown mixtures (i.e., sown and spontaneous flowers) were listed, and the realised functional diversity of each plot was calculated. Over the two years, an increase in functional diversity did not result in an increase in the abundance and richness of aphid predators. Moreover, ladybeetles, representing the majority of trapped predators, were more abundant in mixtures with very low or intermediary functional diversity at sowing, especially in 2014. We hypothesise that certain flower species, which were abundant in certain mixtures (and not in those exhibiting the highest functional diversity), attracted predators and were sufficiently represented to support them. Our results present novel information that could be used to the development of flower mixtures that provide effective ecosystem services, such as pest control

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 de pyrolyse des hydrocarbures et dépôt de pyrocarbone par CVD/CVI

A study of the pyrolysis mechanisms of hydrocarbons, leading to the deposition of pyrocarbon by CVD/CVI, was conducted a dual experimental/numerical approach. From the propane precursor, the gas phase was analyzed in situ (FTIR spectroscopy) during infiltration and online (CPG) in CVD or CVI: the role of C2 species in the formation of pyrocarbon called "weakly anisotropic" and that of C3 species in that of pyrocarbon called "columnar laminar" have been identified. Micro- and nano-textural characterizations (MOLP, MET and Raman spectroscopy) of the deposits carried out made it possible to establish distinct growth mechanisms for each of the forms of laminar pyrocarbon. Detailed homogeneous mechanisms describing the pyrolysis of propane and methane have been introduced in various numerical models of 0D and 1D reactors. They were validated by comparison with different experimental analyses of the gas phase. Correlations were established between the composition of the gas phase determined by the models and the deposition of the main microtextures. Effective precursors with an odd number of carbon thus seem to promote the anisotropy of pyrocarbon, while conversely an effective precursor with an even number of carbon leads to the formation of defects and therefore to a fall in anisotropy. On this basis, reaction schemes were pjroposed to describe the formation of the main microtextures deposited from propane and methane.Une étude des mécanismes de pyrolyse des hydrocarbures, conduisant au dépôt de pyrocarbone par CVD/CVI, a été menée suivant une double approche expérimentale/numérique. A partir du précurseur propane, la phase gazeuse a été analysée in-situ (spectroscopie IRTF) en cours d'infiltration et en ligne (CPG) en CVD ou CVI : le rôle des espèces en C2 dans la formation du pyrocarbone dit "faiblement anisotrope" et celui des espèces en C3 dans celle du pyrocarbone dit "laminaire colonnaire" ont été identifiés. Les caractérisations micro -et nano-texturales (MOLP, MET et spectroscopie Raman) des dépôts réalisés ont permis d'établir des mécanismes de croissance distincts pour chacune des formes de pyrocarbone laminaire. Des mécanismes homogènes détaillés décrivant la pyrolyse du propane et du méthane ont été introduits dans diverses modélisations numériques de réacteurs 0D et 1D. Ils ont été validés par confrontation avec différentes analyses expérimentales de la phase gazeuse. Des corrélations ont été établies entre la composition de la phase gazeuses déterminée par les modèles et le dépôt des microtextures principales. Les précurseurs effectifs à nombre impair de carbone semblent ainsi favoriser l'anisotropie du pyrocarbone, tandis qu'à l'inverse un précurseur effectif à nombre pair de carbone conduit à la formation de défauts et donc à une chute d'anisotropie. Sur cette base, des schémas réactionnels ont été pjroposés pour décrire la formation des microtextures principales déposées à partir de propane et de méthane

Mécanismes de pyrolyse des hydrocarbures et dépôt de pyrocarbone par CVD/CVI

Une étude des mécanismes de pyrolyse des hydrocarbures, conduisant au dépôt de pyrocarbone par CVD/CVI, a été menée suivant une double approche expérimentale/numérique. A partir du précurseur propane, la phase gazeuse a été analysée in-situ (spectroscopie IRTF) en cours d'infiltration et en ligne (CPG) en CVD ou CVI : le rôle des espèces en C2 dans la formation du pyrocarbone dit « faiblement anisotrope » et celui des espèces en C3 dans celle du pyrocarbone dit « laminaire colonnaire » ont été identifiés. Les caractérisations micro – et nano – texturales (MOLP, MET et spectroscopie Raman) des dépôts réalisés ont permis d'établir des mécanismes de croissance distincts pour chacune des formes de pyrocarbone laminaire. Des mécanismes homogènes détaillés décrivant la pyrolyse du propane et du méthane ont été introduits dans diverses modélisations numériques de réacteurs 0D et 1D. Ils ont été validés par confrontation avec différentes analyses expérimentales de la phase gazeuse. Des corrélations ont été établies entre la composition de la phase gazeuse déterminée par les modèles et le dépôt des microtextures principales. Les précurseurs effectifs à nombre impair de carbone semblent ainsi favoriser l'anisotropie du pyrocarbone, tandis qu'à l'inverse un précurseur effectif à nombre pair de carbone conduit à la formation de défauts et donc à une chute d'anisotropie. Sur cette base, des schémas réactionnels ont été proposés pour décrire la formation des microtextures principales déposées à partir du propane et du méthane.A study of the hydrocarbon pyrolysis mechanisms preceding pyrocarbon deposition by CVD/CVI was carried out on the basis of an experimental/numerical double approach. Starting from propane, the resulting gas phase was analysed by in-situ FTIR spectroscopy and by on-line GPC in CVD or CVI conditions. These analyses evidenced the role of C2 and C3 species as effective precursors of respectively « weakly anisotropic » and « columnar laminar » pyrocarbons. Specific growth mechanisms for the main types of pyrocarbon were established with the aid of micro – and nano – textural characterizations (PLOM, TEM and Raman spectroscopy). Detailed homogeneous mechanisms describing methane and propane pyrolysis were introduced in various 0D and 1D numerical models of chemical reactors. These mechanisms were validated by confrontation with experimental gas phase analyses. The computed gas phase composition was correlated to the microtexture of the main pyrocarbons deposited from these hydrocarbon sources. Thus, effective precursors with an odd number of C atoms seem to favor the pyrocarbon anisotropy, while effective precursors with an even number lead to defaults and therefore to a decrease of anisotropy. On this basis, reactional routes were proposed to describe the formation of the main pyrocarbons deposited from propane and methane

Mécanismes de pyrolyse des hydrocarbures et dépôt de pyrocarbone par CVD/CVI

A study of the pyrolysis mechanisms of hydrocarbons, leading to the deposition of pyrocarbon by CVD/CVI, was conducted a dual experimental/numerical approach. From the propane precursor, the gas phase was analyzed in situ (FTIR spectroscopy) during infiltration and online (CPG) in CVD or CVI: the role of C2 species in the formation of pyrocarbon called "weakly anisotropic" and that of C3 species in that of pyrocarbon called "columnar laminar" have been identified. Micro- and nano-textural characterizations (MOLP, MET and Raman spectroscopy) of the deposits carried out made it possible to establish distinct growth mechanisms for each of the forms of laminar pyrocarbon. Detailed homogeneous mechanisms describing the pyrolysis of propane and methane have been introduced in various numerical models of 0D and 1D reactors. They were validated by comparison with different experimental analyses of the gas phase. Correlations were established between the composition of the gas phase determined by the models and the deposition of the main microtextures. Effective precursors with an odd number of carbon thus seem to promote the anisotropy of pyrocarbon, while conversely an effective precursor with an even number of carbon leads to the formation of defects and therefore to a fall in anisotropy. On this basis, reaction schemes were pjroposed to describe the formation of the main microtextures deposited from propane and methane.Une étude des mécanismes de pyrolyse des hydrocarbures, conduisant au dépôt de pyrocarbone par CVD/CVI, a été menée suivant une double approche expérimentale/numérique. A partir du précurseur propane, la phase gazeuse a été analysée in-situ (spectroscopie IRTF) en cours d'infiltration et en ligne (CPG) en CVD ou CVI : le rôle des espèces en C2 dans la formation du pyrocarbone dit "faiblement anisotrope" et celui des espèces en C3 dans celle du pyrocarbone dit "laminaire colonnaire" ont été identifiés. Les caractérisations micro -et nano-texturales (MOLP, MET et spectroscopie Raman) des dépôts réalisés ont permis d'établir des mécanismes de croissance distincts pour chacune des formes de pyrocarbone laminaire. Des mécanismes homogènes détaillés décrivant la pyrolyse du propane et du méthane ont été introduits dans diverses modélisations numériques de réacteurs 0D et 1D. Ils ont été validés par confrontation avec différentes analyses expérimentales de la phase gazeuse. Des corrélations ont été établies entre la composition de la phase gazeuses déterminée par les modèles et le dépôt des microtextures principales. Les précurseurs effectifs à nombre impair de carbone semblent ainsi favoriser l'anisotropie du pyrocarbone, tandis qu'à l'inverse un précurseur effectif à nombre pair de carbone conduit à la formation de défauts et donc à une chute d'anisotropie. Sur cette base, des schémas réactionnels ont été pjroposés pour décrire la formation des microtextures principales déposées à partir de propane et de méthane

Binding of Retinoic Acid Receptor Heterodimers to DNA

International audienceThe retinoic acid signaling pathway is controlled essentially through two types of nuclear receptors, RARs and RXRs. Ligand dependent activation or repression of retinoid-regulated genes is dependent on the binding of retinoic acid receptor (RAR)/9-cis-retinoic acid receptor (RXR) heterodimers to retinoic acid response element (RARE). Although unliganded RXR/RAR heterodimers bind constitutively to DNA in vitro, a clear in vivo ligand-dependent occupancy of the RARE present in the RARbeta2 gene promoter has been reported (Dey, A., Minucci, S., and Ozato, K. (1994) Mol. Cell. Biol. 14, 8191-8201). Nucleosomes are viewed as general repressors of the transcriptional machinery, in part by preventing the access of transcription factors to DNA. The ability of hRXRalpha/hRARalpha heterodimers to bind to a nucleosomal template in vitro has therefore been examined. The assembly of a fragment from the RARbeta2 gene promoter, which contains a canonical DR5 RARE, into a nucleosome core prevented hRXRalpha/hRARalpha binding to this DNA, in conditions where a strong interaction is observed with a linear DNA template. However, histone tails removal by limited proteolysis and histone hyperacetylation yielded nucleosomal RAREs able to bind to hRXRalpha/hRARalpha heterodimers. These data establish therefore the role of histones NH2 termini as a major impediment to retinoid receptors access to DNA, and identify histone hyperacetylation as a potential physiological regulator of retinoid-induced transcription

H11-H12 Loop Retinoic Acid Receptor Mutants Exhibit Distinct trans-Activating and trans-Repressing Activities in the Presence of Natural or Synthetic Retinoids

International audienceRetinoids, such as the naturally occurring all-trans-retinoic acid (atRA) and synthetic ligand CD367 modulate ligand-dependent transcription through retinoic acid receptors (RARs). Retinoid binding to RAR is believed to trigger structural transitions in the ligand-binding domain (LBD), leading to helix H1 and helix H12 repositioning and coactivator recruitment and corepressor release. Here, we carried out a detailed mutagenesis analysis of the H11-H12 loop (designated the L box) to study its contribution to hRARalpha activation process. Point mutations that reduced transactivation by atRA also reduced atRA-induced transrepression of AP1 transcription, correlating ligand-induced activation and repression. However, a correlation was not observed with these mutations when tested with another ligand CD367, a synthetic agonist with binding properties identical to those of atRA. Transcription was strongly inhibited in the presence of CD367 for some mutants, thus leading to an inverse agonist activity of this ligand. None of these mutations significantly altered binding affinity for either ligand, indicating that altered transcription was not caused by altered ligand binding by these mutations. Although simple correlations with transcriptional activities were not found, these mutations were also characterized by altered ligand-induced structural transitions, which were distinct for the atRA-hRARalpha or CD367-hRARalpha complexes. These results indicate that amino acids in the L box are involved in specifying trans-repressive and trans-activating properties of the hRARalpha, and support the notion that different agonists induce distinct conformations in the LBD of the receptor

From pyrocarbon CVD to pyrocarbon CVI

International audienc