[en] The residual stresses induced in composites when cooling down from the processing temperature were determined using a cylinder model and using a finite element computer program. Various specimen geometries were examined: microcomposites, unidirectional composites and flat substrates coated with one or two layers. Various combinations were investigated involving MoSi₂ as an interphase, SiC is a fiber, a matrix, a substrate or an external coating layer and C as a fiber, a substrate, an interphase or an intermediate coating layer. The influence of factors such as interphase thickness and uncertainty in interphase properties (including Young's modulus and coefficient of thermal expansion) was analyzed. It was shown that trends in distribution of thermal residual stresses (TRS) prevailing in ID composites can be satisfactorily predicted using the analytical cylinder model. The presence of a MoSi₂ interphase induces the highest interfacial stresses but it relieves stresses in the matrix. The presence of a C interphase essentially reduces the interfacial stresses

[en] Texture and residual stresses present in SiC-matrix composites fabricated according to the CVD/CVI process have been approached via XRD experiments (pole figures, sin²φ and ideal orientation methods) performed on model materials (CVD-deposits on plane graphite or sintered-SiC substrates as well as cylindrical microcomposites). Two potential interphase materials have been considered, pyrocarbon and MoSi₂. SiC-coatings deposited on plane graphite substrates, from CH₃SiCl₃-H₂ exhibit a pronounced {111} fiber texture with its axis perpendicular to the substrate surface. MoSi₂ deposited from MoCl₄-SiCl₄-H₂-Ar is not significantly textured and pyrocarbon was assumed orthotropic. MoSi₂ and SiC deposited on plane graphite or sintered-SiC exhibit high levels of in-plane tensile residual stresses (800--1,200 MPa). In graphite/MoSi₂/SiC and SiC/MoSi₂/SiC plane specimens, the residual stresses are tensile in MoSi₂ (800--1,200 MPa) and compressive in SiC (250--400 MPa) whereas in SiC/PyC/SiC, they are compressive in PyC (∼90 MPa) and tensile in SiC (150 MPa). In cylindrical 100 μm microcomposites, the axial and hoop residual stresses are tensile in the MoSi₂ interphase (1,500--1,900 MPa) and SiC matrix (<200 MPa), for SiC/MoSi₂/SiC, whereas they are of much lower intensities in SiC/PyC/SiC. A good agreement is observed between the residual stresses measured by XRD and calculated by a finite element method

[en] The crystal structure of CeMgNi₄ intermetallic compound has been studied by both X-ray and neutron diffraction. Rietveld refinement shows that both 4a and 4c sites are occupied by Ce and Mg. The exchange has been evaluated to be about 15%. The hydrogenation of the sample leads to a decomposition and to the formation of CeH_2.52. Ab initio calculations using pseudo-potential and all-electron DFT methods are performed to explain such an unexpected behaviour. They predict a larger stability of the hydride system in the orthorhombic structure rather than in the cubic one. Anti-bonding Ce-H interactions within the hydride are proposed to assess the observed easy decomposition. Moreover, the metastability introduced by mechano-synthesis (i.e. exchange between Ce and Mg) was also evaluated. (authors)

[en] Hydrogen is now one of the potential vectors of energy for the future. To overcome the problems of hydrogen storage, many works are focused towards the development of new materials. In this paper, new composite materials Mg metal catalysts (Ni or Pd) are synthesized with an original route, the chemical fluid deposition process in supercritical fluids. The two studied materials (Mg-Ni and Mg-Pd) show the potentiality of the CFD route in supercritical fluids to decor surfaces with a structuration from the micrometer scale down to the nanometer one. Regarding hydrogen sorption, the catalytic effect of Ni is higher than the one of palladium. The cyclability is hugely improved with 'SCF materials' in comparison with ball milling ones because the catalysts stay always on the magnesium particle surface. (authors)

[en] The compounds RENi₄Mg have been elaborated by mechanical alloying (MA) and a subsequent heat treatment at 600 C for 1 hour. The lattice parameters are in good agreement with those already reported: 7.165 Angstroms for LaNi₄Mg, 7.030 Angstroms for CeNi₄Mg and 7.038 Angstroms for GdNi₄Mg. The hydrogen sorption behaviours are strongly influenced by the rare earth used: for La compound, hydrogen absorption leads to a structural change and no desorption can be observed, for Ce compound, no reaction towards hydrogen can be noticed and for Gd compound, hydrogen can be absorbed and desorbed reversibly at room temperature. Moreover for GdNi₄Mg, no structural change is reported and only a slight volume expansion can be noticed (e.g. 5%). The MA process has also been used successfully for the elaboration of GdNi_4-xAl_xMg up to x = 1. In these substituted compounds, hydrogen can also been absorbed and desorbed reversibly at room temperature. (authors)

[en] The magnesium-rich composition Gd₁₃Ni₉Mg₇₈ was synthesized from its constituent elements in sealed tantalum tubes in an induction furnace. X-ray diffraction, electron probe microanalysis and dark-field transmission electron microscopy (TEM) images revealed a new compound with a composition ranging from Gd_10–15Ni_8–12Mg_72–78 and low crystallinity. In order to increase the crystallinity, different experimental conditions were investigated for numerous compounds with the initial composition Gd₁₃Ni₉Mg₇₈. In addition, several heat treatments (from 573 to 823 K) and cooling rates (from room temperature quenched down to 2 K h⁻¹) have been tested. The best crystallinity was obtained for the slower cooling rates ranging from 2 to 6 K h⁻¹. From the more crystallized compounds, the structure was partially deduced using TEM and an average cubic structure with lattice parameter a = 4.55 Å could be assumed. A modulation along both a^∗ and b^∗ axis with vectors of modulation q1 = 0.42a^∗ and q2 = 0.42b^∗ was observed. This compound, so-called Gd₁₃Ni₉Mg₇₈, absorbs around 3 wt.% of hydrogen at 603 K, 30 bars and a reasonable degree of reversibility is possible, because after the first hydrogenation, irreversible decomposition into MgH₂, GdH₂ and NiMg₂H₄ has been shown. The pathway of the reaction is described herein. The powder mixture after decomposition shows an interesting kinetics for magnesium without ball milling.