Michel Lahaye

SiC filament/titanium matrix composites regarded as model composites

Two types of large diameter SiC CVD filaments have been investigated on both chemical and mechanical standpoints: a 100μm filament with a tungsten core (from SNPE) and three 140μm filaments with carbon cores and surface coatings (from AVCO). On the basis of microprobe (X-ray, Auger and Raman), X-ray diffraction and SEM analyses, it appears that the former is made of a single homogeneous stoichiometric SiC deposit while the latter are mainly made of two concentric shells (the inner being a SiC+C mixture and the outer almost pure SiC). All the C-core filaments had received a surface coating (either pure pyrocarbon or SiC+C mixture) presumably to protect the brittle SiC deposit against abrasion due to handling in opposition to the W-core filament which seems to have no surface coating at all. The W-core filament, although smaller in diameter, is weaker than the C-core filaments (average UTS of 3 and 4 GPa respectively for a 40 mm gauge length). However, its strength distribution is much narrower (Weibull moduli of 7–8 and 2–5 respectively). Failures of most filaments appear to have a multimodal character.

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.

Large second-harmonic generation of thermally poled sodium borophosphate glasses

Second harmonic generation (SHG) has been obtained in a rich in sodium niobium orophosphate glass by a thermal poling treatment. The thermally poled glass SHG signal has been studied through an original analysis of both transmitted and reflected polarized Maker-fringe patterns. Therefore, the second order nonlinear optical (NLO) efficiency was estimated from the simulation of the Maker-fringe patterns with a stepwise decreasing profile from the anode surface. A reproducible chi(2) susceptibility value as high as 5.0 +/-0.3 pm/V was achieved at the anode side. The nonlinear layer, found to be sodium-depleted up to 5 microm deep inside the anode side, identical to the simulated nonlinear zone thickness, indicates a complex space-charge-migration/ nonlinear glass matrix response process.

Blood haemolysis by ceramics

Ceramics are more and more frequently under consideration for construction of blood-contacting devices, i.e. cardiac valves or cardiac assist devices. This study evaluated the haemolysis eventually initiated in vitro by ceramic powders (Al2O3, ZrO2/Y2O3, AlN, B4C, BN, SiC, Si3 N4, TiB2, TiN, TiC), graphite and diamond. The chemical composition of the powders was studied by X-ray microprobe and various other methods, and BET specific areas were determined. The haemolysis was almost zero for all powders, except AlN which showed slight haemolysis and TiB2 which had high haemolytic power.

Brittle fracture of polycrystalline tungsten

A comparative study of the brittle fracture of three varieties of polycrystalline tungsten has been carried out using mainly impact tests, scanning electron microscopy and Auger spectroscopy. It indicates the occurrence of two modes of brittle fracture, namely cleavage and intergranular fractures, whose proportions depend on temperature and on phosphorus segregation at the grain boundaries. Scanning Auger images indicate unambiguously that phosphorus is only located on the intergranular surfaces. No phosphorus is observed on cleavage planes. Fracture surfaces are the result of the propagation of a crack through the whole sample. As the propagation follows the path which requires the lowest expenditure of energy, it depends both on a geometrical factor and on the respective values of the cleavage (γcl) and intergranular (γl) works of fracture. Our results indicate an increase in γcl and γl with temperature (the effect being more marked in the case of γcl) and a large decrease in γl with phosphorus segregation at the grain boundaries. This impurity produces an intergranular embrittlement of the tungsten.

Interface analysis in Al and Al alloys/Ni/carbon composites

Nature of fibre/matrix interfaces existing in Al/C composites were investigated depending on the presence of a nickel interlayer deposited on carbon fibres and on the composition of the aluminium matrix. Auger and electron microprobe analyses were used. The role of the nickel layer on the chemical evolution of the system after a 96 h heat treatment at 600°C is discussed. The presence of this nickel layer limits the diffusion of carbon into aluminium, and thereby, eliminates the formation of a carbide interphase, Al3C4, which is known to lower the mechanical properties of Al/C composites. The mechanisms differ according to the composition of the matrix. In the case of pure aluminium, an Al-Ni intermetallic is formed after thermal annealing. It does not react with the carbon fibre and so inhibits the growth of Al3C4. In the case of the alloyed matrix (AS7G0.6), the dissolution of the Ni sacrificial layer, after annealing, does not lead to the same Al-Ni intermetallic but a thin nickel layer remain in contact with the carbon fibre avoiding formation and growth of Al3C4 carbide. This difference of behaviour is tentatively ascribed to the presence of silicon that segregates at the fibre/matrix interface.

Enhancement of second harmonic generation signal in thermally poled glass ceramic with NaNbO3 nanocrystals

Glass ceramic composites were prepared by bulk crystallization of NaNbO3 in sodium niobium borate glasses. A homogeneous bulk crystallization of the NaNbO3 phase takes place during heat treatments that produces visible-near infrared transparent materials with ∼30nm NaNbO3 nanocrystallites. Upon thermal poling, a strong Na+ depleted nonlinear optical thin layer is observed at the anode side that should induce a large internal static electric field. In addition, the χ(2) response of the poled glass ceramic composites increases from 0.2 up to 1.9pm∕V with the rate of crystallization. Two mechanisms may be considered: a pure structural χ(2) process connected with the occurrence of a spontaneous ferroelectric polarization or an increase of the χ(3) response of the nanocrystallites that enhances the electric field induced second harmonic generation process.

Towards second-harmonic generation micropatterning of glass surface

Thermal poling of sodium borophosphate niobium glasses, previously coated with a thin silver layer micropatterned by femtosecond laser irradiation, is demonstrated. The field-assisted ion-exchange process for fabricating planar surface in this glass substrate is analyzed. Inside the silver ablated lines obtained by femtosecond laser irradiation, we clearly observe a change in the distribution of the frozen electrostatic field that is modulated by the Ag+/Na+ ion-exchange process during the thermal poling.

Solid-state graphene formation via a nickel carbide intermediate phase

Direct formation of graphene with a controlled number of graphitic layers on dielectric surfaces is highly desired for practical applications but still challenging. Distinguished from the conventional chemical vapor deposition methods, a solid-state rapid thermal processing (RTP) method can achieve high-quality graphene formation on dielectric surfaces without transfer. However, little research is available to elucidate the graphene growth mechanism in the RTP method (heating rate ∼15 °C s−1). Here we show a solid-state transformation mechanism in which a metastable nickel carbide (Ni3C) intermediate phase plays a critical role in transforming amorphous carbon to two dimensional crystalline graphene and contributing to the autonomous Ni evaporation in the RTP process. The formation, migration and decomposition of Ni3C are confirmed to be responsible for graphene formation and Ni evaporation. The Ni3C-assisted graphene formation mechanism expands the understanding of Ni-catalyzed graphene formation and provides insightful guidance for controlled growth of graphene through the solid-state transformation process.

Characterisation of VSe2−xSx solid solutions (0 ⩽ x ⩽ 2)

VSe2−xSx solid solutions are prepared for 0 ⩽ x ⩽ 2. Two types of phases : IT VSe2 and V5S8 are successively obtained when x increases with a gradual change of the lattice parameters in the intermediate range 0.6 ⩽ x ⩽ 1.2. Three types of magnetic behaviors are observed : weak paramagnetism with charge density wave for x 1.2. This magnetization is ascribed to V3+ ions intercalated in the Van Der Waals gap. Electron microprobe chemical analysis is consistent with this interpretation.