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

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

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

BORDEAUX1-BU Sciences-Talence (335222101) / SudocSudocFranceF

Horizontally and on-site grown carbon nanotube membrane for sensitive and selective gas sensing

We report here the fabrication of a sensitive and selective gas sensor using a carbon nanotube (CNT) membrane as the gas sensing transducer. A TiN/Al2O3/TiN stack is used as a support for the CNT growth catalyst, evaporated with a 45° tilt angle on the vertical wall. Thanks to this layout, dense and thin arrays of aligned and horizontal CNTs are grown, both selectively and on-site, in between interdigited electrodes. These features allow better gas limit detection with working conditions close to room temperature and atmospheric pressure. The FET or switch like structure of the sensor allows multiple readouts (conductance, capacitance, resonance) to achieve a better selectivity for gas detection

From pyrocarbon CVD to pyrocarbon CVI

International audienc

Large-scale, in-situ integration of horizontal carbon nanotube arrays into gas sensing devices

The successful experimental demonstration of large-scale and in-situ growth of horizontal, dense and aligned carbon nanotube (CNT) arrays is presented. This new CNT growth process is developed for the direct integration of nanotubes into gas sensing devices. The device electrodes are patterned in a TiN/Al2O3/TiN stack which is used as a support for the CNT growth catalyst. Thereafter, iron is locally deposited on the stack’s vertical sidewall as a catalyst material. With the use of a chemical vapor deposition (CVD) growth process selective with respect to the catalyst underlaying material, dense and thin arrays of horizontally aligned CNTs are directly grown from the Al2O3/Fe surface and bridge the electrode gap. This process enables for the wafer scale integration of dense CNT arrays as basic building blocks in devices such as micro-electromechanical systems (MEMS) for gas sensing applications. These CNT arrays, when contacted in between electrodes, show good electrical characteristics and a sensitive conductance response when exposed to a gas such as NO2. Although further optimization is required, these devices can be expected to show enhanced sensitivity for potential gas identification sensor systems due to their large detection surface

Horizontal growth of dense carbon nanotube membranes for interconnects and sensors

Dense array of horizontal carbon nanotubes (CNTs) connected to electrodes is a basic building block for devices in a wide field of applications such as interconnect lines or micro-electromechanical systems (MEMS) for sensing for instance. For interconnects, maximizing the CNT density means maximizing the number of parallel conduction paths, which is necessary to be competitive with copper. For gas sensing, a membrane made of high density CNTs should promote a high sensitivity by developing a high detection surface. Despite the recognized superior properties of CNTs, their adoption as alternative materials remains limited due to integration difficulties at a wafer scale and low CNT density. Here, two integration schemes fully compatible with the processing of 200 mm wafers using conventional silicon technologies are innovatively proposed to manufacture highly dense horizontal CNT membranes at a wafer scale

Impact of a van der Waals interface on intrinsic and extrinsic defects in an MoSe 2 monolayer

International audienceIn this work, we study growth and migration of atomic defects in MoSe 2 on graphene using multiple advanced transmission electron microscopy techniques to explore defect behavior in vdW heterostructures. A MoSe 2 /graphene vdW heterostructure is prepared by a direct growth of both monolayers, thereby attaining an ideal vdW interface between the monolayers. We investigate the intrinsic defects (inversion domains and grain boundaries) in synthesized MoSe 2 , their evolution amid growth processing steps, and their influence on the formation and movement of extrinsic defects. Electron diffraction identifies a preferential interlayer orientation of 2° between MoSe 2 and graphene, which is caused by the presence of intrinsic IBD defects. Extrinsic defects (point and line defects) are generated by in situ electron irradiation in the MoSe 2 layer. Our results shed light on how to independently modify the MoSe 2 atomic structure in vdW heterostructures for potential utilization in device processing