J. Bouix

The chemical behaviour of carbon fibres in magnesium base Mg-Al alloys

The chemical interaction between carbon fibres and Mg-rich Mg-Al alloys was studied at 723–1273 K using optical metallography, scanning electron microscopy and electron probe microanalysis. In a first stage, carbon fibres were heated at 723, 1000 and 1273 K with Mg-Al alloys of different compositions. Two carbide phases were identified at 1000 and 1273 K: an Al4C3 type phase with up to 6 wt.%Mg present in solid solution and a new ternary carbide with the chemical formula Al2MgC2 existing under two hexagonal crystalline varieties. In a second stage, the solid-liquid phase equilibria in the Al-C-Mg ternary system were experimentally established at 1000 K. At that temperature, all the Mg-Al alloys containing more than 19 ± 2 wt.%Al were observed to be in equilibrium with the Al4C3 type phase whereas Mg-Al alloys containing from 0.6 to 19 wt.%Al were found in equilibrium with the ternary carbide Al2MgC2. As for the Mg-Al alloys with an Al content lower than or equal to 0.6 ± 0.2 wt.%, they appeared to be in equilibrium with carbon. These thermodynamic principles being established, the extent, morphology and composition of the reaction zones formed in out of equilibrium conditions at the interface between carbon fibres and Mg-rich Mg-Al alloys were characterized. Attempts were made to determine the influence of different factors such as the fibre nature, the alloy composition, the heating time and the heating temperature on the formation and growth of the ternary carbide Al2MgC2. The observed changes were interpreted in terms of reaction mechanism and kinetics.

Characterization by XPS and SEM of reactive chemical vapour deposited boron carbide on carbon fibre

A process and apparatus used in the large-scale deposition of boron-based films on carbon fibres is described. The deposition occurs when the heated moving fibres react with a BCl3-H2 mixture. Among other advantages, such a process provides for continuous processing at atmospheric pressure. The properties of the as-deposited fibres, ex-Pan based, are discussed in terms of their use in material composites under ambient atmosphere. X-ray photoelectron spectroscopy shows that the filament surface consists of a B4C-BN mixture.

Microtexture and structure of boron nitride fibres by transmission electron microscopy, X-ray diffraction, photoelectron spectroscopy and Raman scattering

Continuous boron nitride fibres have been fabricated by melt spinning and pyrolysis of poly[2,4,6-tris(methylamino)borazine]. The longitudinal mechanical properties depend on mechanical stress and temperature applied during the conversion process. High-performance and low-performance fibres were characterized in order to find relationship between structure and physical properties. In all the cases, photoelectron spectroscopy (XPS) analysis proves that the chemical composition of the fibre is close to stoichiometric BN. The crystallite sizes were measured by means of X-ray diffraction (XRD) and Raman techniques. Cross-sections of separated fibres were investigated by high-resolution electron microscopy (HREM) and transmission electron microscopy (TEM). All the BN fibres have a hexagonal turbostratic structure. With increasing stress and temperature, the tensile strength and the elastic modulus increase. In the high-performance fibres, the 002 layers with an increased distance (about 0.35 nm) showed a mean stacking sequence near to graphite and a preferred orientation of the 002 layers parallel to the fibre axis.

Effect of magnesium on the composition, microstructure and mechanical properties of carbon fibres

This work was undertaken in order to provide more detailed information on the chemical and mechanical behaviour of carbon fibres during the elaboration of graphite-magnesium composite materials. For this purpose, PAN-based T300, pitch-based P55 and P100 carbon fibres were isothermally heat treated, at temperatures ranging from 450 to 700 °C, under a saturated vapour pressure of magnesium. The composition, microstructure and tensile strength of the resulting samples were characterized by chemical and electron probe microanalysis, Raman spectrometry, X-ray diffraction and mechanical test of single filaments. From the results obtained, it has been concluded that highly graphitized fibres such as pitch-based P55 or P100 are not affected by long-time annealing in the presence of magnesium vapour, whereas impure and disorded fibres such as PAN-based T300 undergo some chemical and microstructural modifications decreasing their mechanical properties.

Process for coating carbon fibers with a carbide, and carbon fibers thus coated

A process for continuously coating a strand of continuous carbon filaments by means of a refractory carbide by reactive CVD deposition, which includes the step of causing the strand to travel under mechanical tension with a hermetic enclosure, simultaneously, introducing into the said enclosure a gas mixture consisting of hydrogen and of a volatile compound of an element capable of forming a refractory carbide; and heating the reaction mixture consisting of the strand and the gas mixture so that the formation of the carbide deposit on the filaments takes place by a chemical reaction between the carbon in these filaments and the gas mixture; and the reaction is self-regulated by the diffusion of carbon through the layer of carbide already formed.

AFM study and optical properties of GaAsN/GaAs epilayers grown by MOVPE

Abstract We report a study on surface morphology and optical properties of GaAsN epilayers grown on GaAs (1xa00xa00) vicinal substrates by metalorganic vapor-phase epitaxy (MOVPE), using dimethylhydrazine as the N source. The surface morphology was investigated by atomic force microscopy (AFM) as a function of N composition and growth temperature ( T g ). At T g =530–550°C, we observed a transition from a bunched step/terrace structure towards a surface with indistinct step/terrace structure when increasing the N content. We attribute this evolution to a transition from step-bunching towards 2D-nucleation growth, related to decreased Ga surface-diffusion length. The optical properties were studied using room-temperature (RT) and low-temperature photoluminescence (PL), and photoreflectance (PR). Post-growth thermal annealing were carried out on GaAs-capped layers. A drastic increase of the RT PL intensity and a narrowing of the linewidth were observed under optimum annealing conditions. The influence of annealing temperature and duration were also examined.

Analysis of nitrogen incorporation mechanisms in GaAs1-xNx/GaAs epilayers grown by MOVPE

Abstract The incorporation of nitrogen in Ga(As,N)/GaAs grown by metalorganic vapor phase epitaxy has been investigated as a function of growth conditions. The evolution of the solid composition was studied as a function of growth temperature between 500 and 600°C, and gas-phase composition. For instance, at growth temperatures of 510–560°C, the solid composition varies slowly with T , with an apparent activation energy of 0.6 eV. For T >560°C, an exponential decrease is dominantly observed with an apparent much higher activation energy of 3.7 eV. The effect of the growth temperature is analyzed, taking account for the different surface reaction mechanisms. The observed transition between the two regimes is attributed to an increased desorption of nitrogen at high temperatures. In light of experimental results, we propose a surface kinetics model based on the competitive adsorption of group V precursors. We also deduced an energy barrier for adsorption of group V species.

Dépôt de TiC sur fibres de carbone par CVD réactive: Influence des conditions expérimentales sur les caractéristiques des fibres obtenues

Resume Le revetement uniforme des filaments dune meche de carbone par le carbure de titane est realise par action du tetrachlorure de titane et de lhydrogene sur la fibre elle-meme portee a une temperature superieure a 1200 K. Le traitement est realise en continu et a la pression atmospherique. Les conditions de la CVD (chemical vapour deposition) sont optimisees par determination des proprietes mecaniques a partir de tests de rupture sur monofilaments. Le role de la temperature est predominant: a 1240 K et pour une duree de traitement de 0,8 mn, des fibres C(TiC) dexcellentes qualites sont obtenues ( σ R = 2965MPa). A une temperature superieure, la resistance a la rupture baisse. Pour expliquer ce phenomene, la variation de la taille des cristallites du carbone et du carbure de titane est etudiee en fonction des conditions operatoires.

Deposition and characterization of BN/Si(0 0 1) using tris(dimethylamino)borane

Abstract Boron nitride thin films could be deposited on Si(0xa00xa01) by chemical vapor deposition (CVD) at atmospheric pressure using a single source precursor. IR absorption spectra of films deposited between 750 and 1000°C using B[N(CH3)2]3 (tris(dimethylamino)borane, TDMAB) as the boron and nitrogen source showed a peak absorption at ∼1360xa0cm−1 characteristic of the in-plane vibrational mode seen in h-BN. It was noted that the mode at 800xa0cm−1 is very weak. The observed growth rate varied exponentially with temperature in the range 850–900°C. Ellipsometry measurements were used to investigate the thickness and optical constant of the films. The refractive index, slightly lower than the bulk material, is close to 1.65–1.7 depending on the surface morphology of the films. The surface morphology of thin layers has been observed by atomic force microscopy with an increase of the surface roughness from 0.3 to 3.5xa0nm as the growth temperature increases from 800 to 950°C.

Surface morphology of InGaAs and InP materials grown with trimethylarsenic and arsine on vicinal InP substrates

Abstract In this study, we report on an investigation using atomic force microscope (AFM) to study lattice-matched InGaAs surface layers grown by MOVPE with two different sources of group V elements: trimethylarsenic (TMAs) and arsine. The growth was performed with a home made apparatus with a T shaped reactor. The substrate temperature ranged from 570 to 630°C under pure hydrogen or a mixture of hydrogen and nitrogen as carrier gas. After thermal annealing under PH 3 and growth of InP buffer layer, we could clearly observe the step/terrace like feature of the vicinal surface morphology for 0.2° misoriented substrates. For epilayers grown under nitrogen flow, step bunching was observed with terraces as large as 170xa0nm which is twice larger than the nominal value (80xa0nm). Lattice-matched InGaAs/InP epilayers were grown with arsine and TMAs. For TMAs-grown epilayers, we observed that the roughness depends on V/III ratio and growth temperature. The comparison of the surface morphology of TMAs and arsine-based epilayers assessed by AFM characterisation will be presented and discussed in terms of As-carbon adsorbed species.