J. Vicens

Optimized conditions for an enhanced coupling rate between Er ions and Si nanoclusters for an improved 1.54-μm emission

The present article deals with the optimized processing conditions leading to the highest density of Si nanoclusters which play the role of sensitizing centers for the nearby Er ions within a silica matrix. The layers were obtained by reactive magnetron sputtering under a plasma of Ar mixed to different rates of hydrogen, and were subsequently annealed at various temperatures. The increase of the dilution degree of the Ar plasma with hydrogen was found to multiply the nucleation sites whose density foreshadows that of the Si nanoclusters formed upon annealing. Both hydrogen content and annealing temperature govern the growth of the clusters. The maximum density of efficient sensitizing centers was obtained for hydrogen rate in the plasma of 50% and annealing at 900 °C. This has directly led to the enhancement of the coupling rate between the Si nanoclusters and the Er ions, as reflected by the ten times increase of the proportion of optically active ions, compared to that for standard conditions. In paral...

The creep mechanism of ceramic matrix composites at low temperature and stress, by a material science approach

Abstract This paper deals with the creep mechanism for ceramic matrix composites reinforced by long ceramic fibers in a ceramic or glass-ceramic matrix, tested at low stresses (

Sol-gel synthesis of SiBOC glasses

Abstract Silicon oxycarbide glass containing boron was prepared by pyrolysis of a sol-gel precursor in inert atmosphere. A structural characterization of the gel as well as of the SiBOC glass at various stages of the pyrolysis treatment has been performed. The precursor gels consists of SiOSi bonds with boron-rich regions in which BOB  and/or BOH bonds are present. During the pyrolysis boron atoms are incorporated into the silicate network leading to the formation of a silicon(boron)oxycarbide glass. A further increase of the pyrolysis temperature results in the crystallization of SiC. The glass structure can thus be described as a borosilicate matrix with a fine dispersion β-SiC nanocrstals.

Cryomilling of Al/AlN powders

Abstract Elemental powders of 80 vol% 5000 Al-alloy (3 wt.% Mg) and 20 vol% AlN were milled in different mechanical conditions and at three different temperatures (in liquid nitrogen (cryomilling), low temperature (about −50°C) and room temperature). The effects of these parameters on the milling efficiency of Al and AlN and on the mechanical alloying were investigated. The cryomilling of both Al and AlN resulted in a rapid decrease in size of the coherently diffracting domains (

Flame temperature effect on the structure of SiC nanoparticles grown by laser

Small SiC nanoparticles (10 nm diameter) have been grown in a flow reactor by CO2 laser pyrolysis from a C2H2 and SiH4 mixture. The laser radiation is strongly absorbed by SiH4 vibration. The energy is transferred to the reactive medium and leads to the dissociation of molecules and the subsequent growth of the nanoparticles. The reaction happens with a flame. The purpose of the experiments reported in this paper is to limit the size of the growing particles to the nanometric scale for which specific properties are expected to appear. Therefore the effects of experimental parameters on the structure and chemical composition of nanoparticles have been investigated. For a given reactive mixture and gas velocity, the flame temperature is governed by the laser power. In this study, the temperature was varied from 875°C to 1100°C. The chemical analysis of the products indicate that their composition is a function of the temperature. For the same C/Si atomic ratio in the gaseous phase, the C/Si ratio in the powder increases from 0.7 at 875°C up to 1.02 at 1100°C, indicating a growth mechanism limited by C2H2 dissociation. As expected, X-ray diffraction has shown an improved crystallisation with increasing temperature. Transmission electron microscopy observations have revealed the formation of 10 nm grains for all values of laser power (or flame temperature). These grains appear amorphous at low temperature, whereas they contain an increasing number of nanocrystals (2 nm diameter) when the temperature increases. These results pave the way to a better control of the structure and chemical composition of laser synthesised SiC nanoparticles in the 10 nm range.

AlN dispersed reinforced aluminum composite

Abstract In this paper the first results are presented on nitriding of aluminium powder and the fabrication of Al-AlN composites after milling and hot-pressing. Nitriding appears to follow a complex process. High energetic milling of these powders is an important factor in obtaining homogeneous materials with AlN nanometric grains. TEM and EDX nanoanalyses have shown that Al grains are surrounded by AlN nanocrystals, with some A12O3 needles and AlON crystals. Physical properties — thermal expansion, thermal conductivity, electrical conductivity, hardness, Youngs modulus, fracture strength — of these composites change with the AlN content, and the values for 0 vol.% AlN (process powders) always correspond to higher or lower values than for pure Al (unprocessed powders), reflecting the fact that processing introduces impurities. A comparison of composites fabricated from composite powders and from a mixture of Al-AlN commercially available powders is interesting. Generally these new composites exhibit better properties than those for Al-SiC or Al-Al2O3 composites with an apparently similar reinforcement content.

Understanding the creep behavior of a 2.5D Cf-SiC composite. II. Experimental specifications and macroscopic mechanical creep responses

Abstract Macroscopic results for a 2.5D C f –SiC composite creep tested in tension are presented. After the development and the optimization of a new accurate high temperature tensile device, tests were conducted in argon, under a reduced pressure, for stresses ranging from 110 to 220 MPa and temperatures between 1273 and 1673 K. The macroscopic mechanical creep responses of the composite were analyzed and interpreted. Since ceramic matrix composites (CMCs) contain constituents of a different nature, with an influence of a structural aspect, it is not possible to apply the hypotheses of homogeneity and isotropy as described in Dorn’s theory. Consequently, the physical meaning of the mechanical parameters, obtained by such a classical treatment, is limited. It is then necessary to discuss the global creep responses using an approach based on damage mechanics, which is more consistent with the specific features of the CMCs. This new approach adopted here reveals less classical parameters to be more accurate indicators of the creep behavior and the strain mechanisms of the 2.5D C f –SiC composite.

Understanding the creep behavior of a 2.5D Cf–SiC composite-I. Morphology and microstructure of the as-received material

Abstract A carbon fiber-reinforced silicon carbide matrix composite (2.5D C f –SiC composite) is characterized from both a morphological and a microstructural point of view, as a prerequisite to an investigation of its creep behavior. Using automatic image analysis, various morphological parameters have been characterized: the texture of the woven fibrous preform, the fiber size distribution and the volume fraction of macropores. An original study of the pre-existing microcracks has also been conducted to enable a quantitative estimate of the damage in the as-received composite to be made. A microstructural investigation down to the nanometric scale, via transmission and high resolution electron microscopies, has revealed the intimate structure of each constituent. Thus, the classical structural elements of the carbon fibers (i.e. basic structural units and areas of local molecular orientation) have been identified and measured. The texture of the pyrocarbon interphase has been clearly established especially at the fiber/pyrocarbon and pyrocarbon/matrix interfaces. Finally, the matrix presents the common features of chemically vapor deposited matrices.

Understanding of the creep behavior of SiCf-SiBC composites

Abstract This paper deals with some creep results on a new generation of a ceramic matrix composite with a self-healing matrix. This investigation is based on the creep curves, the observation of damages and their quantification by image analysis, and the application of the damage mechanics approach. The results obtained herein illustrate the concept of the damage-creep mechanism.

SiCf–SiBC composites: microstructural investigations of the as-received material and creep tested composites under an oxidative environment

SiCf–SiBC composites fabricated by Snecma Propulsion Solide (St Médard en Jalles, France) were investigated by SEM and HRTEM in the as‐received state and after creep tests performed in air, in a temperature range 1423–1573 K, under 170 and 200 MPa. These composites are reinforced by Hi‐Nicalon fibres (Nippon Carbon). A pyrocarbon interphase was first deposited on the fibres. The matrix was then deposited on the fibrous preform by several chemical vapour infiltrations (CVI). As a result the matrix is multilayered and based on the Si–B–C ternary system. This matrix is self‐sealing: this is due to the presence of boron inducing the formation of a sealant glass if the material is heated in an oxidative environment. This glass will protect fibres and fibre/matrix interphases against oxidation. Hi‐Nicalon fibres as well as the different matrix layers were studied by HRTEM and EDX. Some investigations were carried out on the creep‐tested specimens in order to characterize modifications observed in the different constituents of the composites, particularly at the interfaces between the matrix layers and at the fibre/matrix interface. It was shown that several matrix layers crystallized during the creep tests. Moreover, a thin silica layer was observed at the pyrocarbon/matrix interfaces. Differences between the behaviour of the same type of material creep tested under neutral atmosphere are discussed.