Xavier Bourrat

LPCVD and characterization of boron-containing pyrocarbon materials

Pyrocarbon materials containing various amounts of boron have been prepared by LPCVD from BC13ue5f8C3H8ue5f8H2 precursor mixtures. By increasing the BCl3(C3H8 + BCl3) ratio up to 85%, the incorporation of boron can reach 33 at.%. A small amount of boron (e.g. 8 at.%) highly enhances the structural anisotropy of pyrocarbon, as evidenced by optical microscopy, X-ray diffraction and transmission electron microscopy (selected area diffraction and lattice fringes techniques). X-ray photoelectron spectroscopy has shown that a large fraction of the boron atoms are included by substitution in the carbon layers; the remaining boron atoms belong to a boron-rich amorphous part of the material. As the boron content increases beyond 8 at.%, the structural anisotropy of the boron-rich pyrocarbon decreases, due to the limited growth and stacking of the carbon layers. Also, amorphous boron-rich regions are more and more abundant as the total amount of boron increases. The oxidation resistance of the C(B) materials is better than that of pure pyrocarbon. This is mainly due to the improvement of the structural organization for the low boron content materials and to the coating of the whole material with a stable boron oxide for materials with a higher boron content.

Pyrocarbon anisotropy as measured by electron diffraction and polarized light

This work deals with the measurement of pyrocarbon anisotropy on very thin fiber coatings used to control the interfacial behavior in carbon/carbon composites. Differentiation of the various pyrocarbons was performed through computerized image analysis of the electron diffraction patterns by measuring the azimuth opening of the carbon 002 diffraction arcs. This orientation angle decreases when the texture switches from rough to smooth laminar. The relationship with the polarized light measurement technique at a lower resolution is discussed.

Plasma processing: a step towards the production of new grades of carbon black

Abstract More than 99% of the carbon black is presently produced by incomplete combustion processes; the by far dominant one is the ‘furnace process’ developed 60 years ago. This paper presents a new route in which the cracking operation is carried out in the absence of oxygen thanks to an external electric energy supply given by a plasma arc.

Micro/minicomposites: a useful approach to the design and development of non-oxide CMCs

Abstract Micro (one single filament) and mini (one single fiber tow) non-oxide composites (C/C; C/SiC and SiC/SiC) with simple (PyC or BN) or complex interphases [C (B) or (PyC-SiC) n multilayers] are fabricated in a short time by CVD/CVI. The fiber/matrix interfacial zone is characterized by AES and TEM. Tensile tests are used to assess the mechanical properties and the Weibull statistical parameters of both the fiber and matrix, as well as the fiber–matrix interfacial parameters (τ i ; l d ; G ic ). The tensile stress–strain behavior has been modelled. The tensile curves exhibit the same features as those previously reported for real nD-composites. Lifetime at high temperatures in air is characterized through static/cyclic fatigue tests and modelled. It is improved by replacing conventional pyrocarbon by highly engineered interphases. The micro/mini composite approach is used in the optimization of processing conditions and to derive parameters necessary for the modelling of the thermomechanical and chemical behavior of composites with more complex fiber architectures.

Electrically conductive grades of carbon black : structure and properties

Abstract Carbon structure of nine commercial conductive blacks, comprising benzene- and acetylenebased blacks was analyzed. This series was selected for its use in dry cells (Leclanche type). TEM (transmission electron microscopy) and X-ray diffraction were employed. First, it has been confirmed that the classical “crystallite” as defined by La and Lc is not the basic building unit of the black structure. This unit is seen to be the carbon layer itself as defined by its size and state by means of TEM phase contrast mode. First, the sizes of the distorted layers were seen to range at three different scales [i.e., less than 1 nm, about 5 nm or over (few tens and over)]. Then taking into account the distortions as a second parameter, five different types were discriminated, say five different turbostratic stacking orders. Type I is defined by a layer size of 0.8 nm and a coherent stack of three layers. In type II, layer size is larger: 1 nm in average and perfect enough to stack by four. Type III has the largest layer size compatible with a concentric texture of the particle: about 5-nm large. The number of layers stacked together is ten in average. Type IV has distorted layers over 5 nm in diameter and stacked by 5 6 , whereas type V has the largest layer sizes (few tens) often straight and stacked by 7 8 . Those largest layer sizes exist exclusively with the “hollow-shell” morphology. Following this model, all the productions were actually characterized by one or few populations of aggregates with a typical turbostratic stacking order and related to properties.

Regenerative laminar pyrocarbon

This work reports on a dense laminar pyrocarbon infiltrated at low temperature, not previously described. By using pulse-CVI and/or toluene for example, a pyrocarbon can be developed with a density as high as that of rough laminar pyrocarbon (RL) but with a regenerative texture. It will be referred to as regenerative laminar (ReL). This pyrocarbon is characterized by a high density (d=2.11), a high bireflectance and the maximum value for the extinction angle: Ae=22°, the latter being related to its high anisotropy. Contrary to rough laminar, regenerative laminar exhibits a smooth extinction of the Maltese-cross in polarized light. This is related, by means of TEM, to the regeneration of thin cones all along the growth. Rough laminar is not regenerative: it contains only primary cones. The regeneration of cones is found to be due to the lateral extent of the layers; on the contrary, rough laminar is grown from small layers which cannot propagate lattice defects (no regeneration). The experimental evidence is based on the infiltration of preforms by using toluene as precursor and pressure-pulsed chemical vapor infiltation (P-CVI). A reference carbon/carbon, RL, is also obtained with the same preform (3D PAN-based needled preform) by using the classical isobaric–isothermal chemical vapor infiltration with a methane–propane mix (I-CVI).

TEM structure of (PyC/SiC)n multilayered interphases in SiC/SiC composites

Two generations of multilayered interphases, composed of carbon and silicon carbide, have been developed to act as a mechanical fuse in SiC/SiC composites with improved oxidation resistance. Pyrocarbon is an ideal interfacial material, from the mechanical point of view, whereas SiC has a good oxidation resistance. In the multilayered interphase, the carbon mechanical fuse is split into thin sublayers, each being protected against oxidation by the neighbouring SiC-based glass former layers. A first generation of multilayers as synthesised by means of isobaric-CVI with sublayers with micrometric thickness. Then, in order to push forward the concept, pressure pulsed-CVI was involved to deposit nanometric scale sublayers. In this work, transmission electron microscopy was developed to characterise the two generations of materials. The microstructure of the layers and the influence of the fibrous preforms on the structure of the layers were studied. Examinations were then performed on the loaded samples and damaging mode characterised at nanometric scale.

Chemical, microstructural and thermal analyses of a naphthalene-derived mesophase pitch

Abstract A detailed characterisation of a synthetic naphthalene-derived mesophase pitch, in its as-received state and during pyrolysis, has been performed. The study has been conducted by means of various techniques and with a particular attention to Raman microspectroscopy. The Raman spectra show features in common with the naphthalene precursor, i.e., a broad and complex band at 1150–1500 cm−1 and a multicomponent G band at 1600 cm−1. These features correspond to the vibration modes of the molecules of the pitch and more especially to the non-aromatic C–C bonds involved in alkyl groups, aryl–aryl bonds or naphthenic rings. The pyrolysis of the pitch into coke takes place within a narrow temperature range (480–500xa0°C) through the elimination of hydrogen and light alkanes resulting from the breaking of homolytic C–H bonds and naphthenic cycles, respectively. This process initiates a swelling of the pitch. The analysis of the Raman features shows that the structure of the pitch is only slightly affected within this temperature range. Conversely, significant structural changes of the material (as shown by the vanishing of the multicomponent bands at 1600 and 1150–1500 cm−1) are evidenced beyond 750xa0°C, simultaneously with a hydrogen release and an increase of the true density. This phenomenon corresponds to the extension of the graphene layers of the coke and the formation of a distorted carbon network.

Low temperature pyrocarbons: a review

Este artigo e uma sintese das pesquisas recentes na area dos pirocarbonos. Pirocarbono e uma forma de carbono preparada por deposicao quimica em substratos quentes (acima de 900 oC) mediante pirolise de hidrocarbonetos. Aplicacoes se encontram nas areas de materiais compositos termostruturais, de reatores nucleares ou de biomateriais. Muito recentemente, um avanco importante foi obtido na compreensao dos processos de crescimento. Uma classificacao dos pirocarbonos de baixa temperatura e apresentada, que se fundamenta na medicao da quantia de defeitos e do grau de anisotropia, fazendo uso da espectroscopia Raman. Ela traz uma relacao bem estabelecida entre mecanismos de crescimento, estrutura e propriedades dos pirocarbonos.

Structure and oxidation of C / C composites: role of the interface

The oxidation of N ex-PAN carbon fibre preform as well as the oxidation of the related carbon/carbon (C/C) composites are compared with the oxidation of their counterparts based on M ex-PAN carbon fibres. The oxidation test is conducted on polished specimens by thermogravimetry. In order to localise and quantify the oxidation, SEM observation was performed on the specimens before and after the test. Whatever the matrix, oxidation of the composite is controlled by the oxidation of the preform itself. The N preform oxidizes more slowly than the M reference. The oxidation features of M-based materials are related to catalytic effects because of the presence of pitting at the interface. Oxidation of N-based composites is localised on the external surface and at the fibre/matrix interface. In the matrix, oxidation is controlled by the anisotropy of the pyrocarbon: smooth laminar pyrocarbon (C/C1) oxidising more slowly at the surface than rough laminar pyrocarbon (C/C2), but deeper in the bulk at the fibre/matrix interface.