G. Chollon

Thermal stability of a PCS-derived SiC fibre with a low oxygen content (Hi-Nicalon)

The oxygen free Si–C fibre (Hi-Nicalon) consists of β-SiC nanocrystals (≈5nm) and stacked carbon layers of 2–3nm in extension, in the form of carbon network along the fibre. This microstructure gives rise to a high density, tensile strength, stiffness and electrical conductivity. With respect to a Si–C–O fibre (Nicalon NL202), the Si–C fibres have a much greater thermal stability owing to the absence of the unstable SiOxCy phase. Despite its high chemical stability, it is nevertheless subject to a slight structural evolution at high temperatures of both SiC and free carbon phases, beginning at pyrolysis temperatures in the range 1200–1400°C and improving with increasing pyrolysis temperature and annealing time. A moderate superficial decomposition is also observed beyond 1400°C, in the form of a carbon enriched layer whose thickness increases as the pyrolysis temperature and annealing time are raised. The strength reduction at ambient for pyrolysis temperatures below 1600°C could be caused by SiC coarsening or superficial degradation. Si–C fibres have a good oxidation resistance up to 1400°C, due to the formation of a protective silica layer.

Characterization of nearly stoichiometric SiC ceramic fibres

A comparative study of the chemical composition and microstructure of Hi-Nicalon, Hi-Nicalon type S, Tyranno SA, Sylramic and Carborundum fibres has been conducted. This analysis has confirmed results already published but has also evidenced some original features. The Hi-Nicalon type S fibre has a near stoichiometric composition but it still contains some oxygen (≈1 at. %) and free carbon (≈2 at. %). The expected near stoichiometric composition of both the Tyranno SA and the Sylramic fibres is only effective near the edge region, while the core of the fibres contains some amount of free carbon (e.g., up to ≈14 at. % and ≈6 at. % respectively in large diameter fibres) as well as some residual oxygen (≈0.5 at. %). The composition of the Carborundum fibre is very close to stoichiometric SiC except rare and localised free carbon or B4C inclusions. The properties of the different fibres, some of them still beeing at a development stage, are discussed from a chemical and a phase composition point of view, on the basis of what is known about their respective preparation process.

Oxidation behaviour of ceramic fibres from the Si-C-N-O system and related sub-systems

Abstract The oxidation of Si–C–N–O fibres has been investigated. The oxidation rates and the activation energies for the Si–C–O system are similar to those for crystalline SiC. The oxygen and the free carbon concentrations in the ceramics have a limited influence on the oxidation behaviour. As long as the formed silica scale is protective, oxidation kinetics are essentially controlled by the diffusion of oxygen through SiO2. The parabolic rates in the Si–C–N–O and Si–N–O systems are lower and their activation energies higher than those for SiC. Their values strongly depend on the ratios of C and N bonds to Si and continuously vary from those for SiC (Ea=110−140kJ mol−1) to Si3N4 (Ea=330–490 kJ mol−1). The oxidation mechanism might be related to a complex diffusion/reaction regime via the formation of an intermediate silicon-oxynitride (like for Si3N4) or silicon-oxycarbonitride layer. The oxidation behaviour of such complex systems is not significantly influenced by the oxygen nor the free carbon contents. It might be governed by the C/Si and N/Si ratios, limiting the nitrogen concentration gradient of the silicon-oxy(carbo)nitride sub-layer and therefore affecting the diffusion/reaction rates.

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).

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.

Deposition Process of Amorphous Boron Carbide from CH4 ∕ BCl3 ∕ H2 Precursor

Amorphous boron carbide coatings have been prepared by chemical vapor deposition from CH 4 /BCl 3 /H 2 precursor mixture at low temperature (800-1050°C) and reduced pressure (12 kPa). 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 Fourier transform infrared spectrometry and a thermodynamic study of the homogeneous equilibrium, the HBCl 2 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.

Microstructural and mechanical properties of filler-added coal-tar pitch-based C/C composites: the damage and fracture process in correlation with AE waveform parameters

Abstract The mechanical behaviour of undirectional and 2D-laminate coal-tar pitch-based carbon/carbon composites was investigated in tension and bending conditions (3 and 4-point loading tests) at room temperature. One of the major drawbacks of such highly anisotropic structures is their poor resistance to delamination (high sensitivity to local shear components). In order to improve their interlaminar shear strength, carbon blacks were introduced between the unidirectional plies of the 2D-C/C specimens prior to the pitch infiltration. The damage and the failure mechanisms were identified by microscope observations on both UD and 2D-C/C composites. Two families of damage modes were identified, involving limited interfibre cracks and inter-layer delamination cracks homogeneously distributed within the composite. The fracture process is assigned to the propagation of the macrocrack in the failure envelope through a large intra and inter-layer delamination process interrupted by fibres/bundles fracture. Acoustic emission analysis was performed during the tests. An approach using a combination of acoustic emission waveform parameters as indicators of the physical damage of the composite is discussed.

Microstructure and mechanical properties of coal tar pitch-based 2D-C/C composites with a filler addition

In order to improve the flexural and the inter-laminar shear strength of coal tar pitch-based 2D-C/C composites, fillers (carbon blacks and colloidal graphite) have been introduced between the UD layers before the first infiltration of pitch. Matrix parts made of the filler/pitch-based cokes showed fine mosaic microtextures. They were found at the interface between the layers. Whereas the tensile strength is not affected, the flexural strength and the ILSS were significantly increased by the addition of fillers. The original structure of the inter-layer matrix parts and the decrease of the number of flaws were found to be responsible for the improvement of the shear strength of the 0/90° UD layers.

A model SiC-based fibre with a low oxygen content prepared from a polycarbosilane precursor

A model SiC-fibre has been prepared from a polycarbosilane precursor by means of an irradiation oxygen-free curing process. The chemical composition remains unchanged after heat treatments under an inert atmosphere for pyrolysis temperatures of 1600°C. At this temperature, the fibre consists of SiC nanocrystals (mean size 6–10 nm) and free carbon. However, a slow grain growth takes place as the temperature is increased. The fibre retains a high strength at room and high temperatures up to temperatures of 1600 °C when the pyrolysis has been performed under nitrogen. The electrical conductivity was studied as a function of the pyrolysis temperature Tp: For 1100≤Tp≤1200 °C, the conductivity increases by several orders of magnitude due to the reorganization of the free carbon phase at the SiC grain boundaries. Oxidation kinetics of the filaments remain parabolic from 1000–1400 °C.

Correlation between microstructure and electrical properties of SiC-based fibres derived from organosilicon precursors

Abstract The chemical, structural and electrical properties of various SiC-based fibres prepared from the pyrolysis of organosilicon precursors were studied as a function of their maximum (post)processing temperature Tp. The magnitude of the electrical conductivity (σ) and its thermal dependence (the apparent activation energy Ea) are mainly controlled by the carbon excess present in the fibres. The free carbon phase is observed by TEM analysis as turbostratic stacks of aromatic carbon layers. The extent of those carbon domains (in length: La and thickness: N) increases with Tp. The amount of free carbon but above all its microstructure (i.e. the size of the carbon domains and their residual hydrogen content) and its microtexture (isolated domains or interconnected network) govern the electrical properties of the fibres through a percolation effect.