F. Langlais

Solid-state synthesis and characterization of the ternary phase Ti3SiC2

Ti3SiC2 is the only true ternary compound in the Ti-Si-C system. It seems to exhibit promising thermal and mechanical behaviour. With the exception of its layered crystal structure, most of its properties are unknown, owing to the great difficulty of synthesis. A new procedure of solid-state synthesis with several steps is proposed, which results in Ti3SiC2 with less than 5 at % of TiC. Ti3SiC2 is stable at least up to 1300 °C. Beyond this temperature, it can decompose with formation of non-stoichiometric titanium carbide and gaseous silicon, with kinetics highly dependent on the nature of the surroundings. As an example, graphite can initiate this process by reacting with silicon, while alumina does not favour the decomposition which remains very slow. The oxidation of Ti3SiC2 under flowing oxygen starts at 400 °C with formation of anatase-type TiO2 film, as studied by TGA, XRD, SEM and AES. Between 650 and 850 °C both rutile and anatase are formed, rapidly becoming protecting films and giving rise to slow formation of SiO2 and more TiO2. The oxidation kinetics is slower than for TiC, owing to a protecting effect of silica. By increasing the temperature, both oxidation processes (i.e. direct reaction and diffusion through oxide layers) are activated and an almost total oxidation is achieved between 1050 and 1250 °C resulting in titania (rutile) and silica (cristobalite).

Conversion mechanisms of a polycarbosilane precursor into an SiC-based ceramic material

The pyrolysis of a PCS precursor has been studied up to 1600 °C through the analysis of the gas phase and the characterization of the solid residue by thermogravimetric analysis, extended X-ray absorption fine structure, electron spectrocopy for chemical analysis, transmission electron microscopy, X-ray diffraction, Raman and Auger electron spectroscopy microanalyses, as well as electrical conductivity measurements. The pyrolysis mechanism involves three main steps: (1) an organometallic mineral transition (550 < Tp < 800 °C) leading to an amorphous hydrogenated solid built on tetrahedral SiC, Si02 and silicon oxycarbide entities, (2) a nucleation of SiC (1000 < Tp < 1200 °C) resulting in SiC nuclei (less than 3 nm in size) surrounded with aromatic carbon layers, and (3) a SiC grain-size coarsening (Tp > 1400 °C) consuming the residual amorphous phases and giving rise simultaneously to a probable evolution of SiO and CO. The formation of free carbon results in a sharp insulator-quasimetal transition with a percolation effect.

LPCVD and characterization of boron-containing pyrocarbon materials

Pyrocarbon materials containing various amounts of boron have been prepared by LPCVD from BC13C3H8H2 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.

On kinetic and microstructural transitions in the CVD of pyrocarbon from propane

The microstructure and the deposition rate of pyrocarbon deposited from propane have both been studied as a function of gas phase residence time (tr) and reactor temperature (T) in the vicinity of 1000°C and at a constant pressure of 2 kPa. Transitions between two different types of laminar microstructures, smooth laminar (SL) and rough laminar (RL), have been observed. Simultaneously, transitions between several kinetic regimes have been seen. The kinetic results permit us to interpret changes in carbon microstructure and to develop a qualitative chemical model for the formation of laminar pyrocarbons. This model could explain both kinetic and textural transitions by the occurrence of two parallel routes to carbon formation involving two different families of ultimate carbon precursors.

CVD processing of ceramic-ceramic composite materials

Chemical vapor deposition (CVD), i.e. the deposition of a solid by a chemical reaction involving one or several gaseous chemical species and usually thermally activated, has been used for many years in different kinds of applications (e.g. oxidation or/and wear resistant coatings for cemented carbides, steels or alloys, preforms for drawing graded-index optical fibers, thin films for integrated circuits, coatings for nuclear fuels, etc...). In most cases, the substrates considered here have a rather simple shape and are made of non-porous materials.

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.

Experimental kinetic study of the chemical vapour deposition of SiC-based ceramics from CH3SiCl3H2 gas precursor

Abstract A detailed kinetic study is presented in order to determine the limiting steps in the chemical vapour deposition of SiC from CH3SiCl3 (MTS) H 2 gaseous mixture. On the basis of growth rate measurements, transitions from chemical kinetic control to mass transfer control are found to be induced either by decreasing total flow rate or by increasing temperature and/or total pressure. A pressure-temperature diagram, giving the boundaries between the different domains observed is presented. The chemical kinetics controlled domain includes either 2 or 3 sub-domains, depending on the α = P H 2 P MTS ratio. By taking into account the apparent activation energy, determined in each sub-domain, and the results of the outlet gas phase analysis by mass-spectrometry, three limiting kinetic processes are proposed: CH3SiCl3 homogeneous decomposition, heterogeneous formation of SiC and growth inhibition by HCl.

Correlation Between Homogeneous Propane Pyrolysis and Pyrocarbon Deposition

Pyrocarbon deposition through propane pyrolysis is studied in a 1-D hot-wall CVD furnace. The gas-phase pyrolysis is modeled with a partially reduced kinetic mechanism leading to polycyclic aromatic compounds (PAHs). The C2-C4 and C3 reaction paths are in competition for benzene formation. There is also an independent C3-C5 path leading to naphthalene. The gas-phase concentrations are correlated with experimental data including in situ Fourier transform infrared spectra intensities, pyrocarbon deposition rates, and pyrocarbon nanotextures. Rough laminar pyrocarbon deposition appears to be more related to PAHs than smooth laminar pyrocarbon.

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.