Structural transformations of bioactive glass 45S5next term with thermal treatments

International audienceWe report on the structural transformations of Bioglass® during thermal treatments. Just after the glassy transition, at 550 °C, a glassy phase separation occurs at 580 °C, with the appearance of one silicate- and one phosphate-rich phase. It is followed by the crystallization of the major phase Na2CaSi2O6, from 610 to 700 °C and of the secondary phase, silico-rhenanite, at 800 °C. The latter evolves from the phosphate-rich glassy phase, which is still present after the first crystallization. In order to control the processing of glass-ceramic products from Bioglass®, crystallization kinetics were studied via differential scanning calorimetry measurements in the range of 620–700 °C and temperature–time–transformation curves were established

Informations de volume en microscopie électronique à balayage (application à l'étude de la microstructure des matériaux et de son évolution sous traction)

Ce travail porte sur le développement des techniques de caractérisation en microscopie électronique à balayage à pression contrôlée. Il s'agit d'une part de mettre au point la tomographie électronique, en utilisant le mode d'imagerie STEM-in-SEM, à l'échelle mésoscopique (non ou mal couverte par les techniques existantes de tomographie). Ce développement devra permettre une caractérisation tridimensionnelle d'un volume plus important que proposé par la tomographie électronique en TEM. D'autre part, les réaliser des essais de traction in situ afin d'examiner les matériaux à différentes étapes de vie : l'état initial de la structure, le comportement dynamique sous sollicitation et son état après l'endommagement. Grâce à la présence de gaz au sein de la chambre du microscope, il est possible d'étudier le comportement mécanique des matériaux non conducteurs. En outre, des informations de volume peuvent être obtenues en appliquant la méthodologie développée pour la tomographie électronique.The present work describes a development of two characterization techniques using controlled pressure scanning electron microscopy (CPSEM): electron tomography and in situ tensile tests. We developed electron tomography in CPSEM using STEM-in-SEM imaging mode. The aim of this development is to provide a three-dimensional characterization technique at the mesoscopic scale which is covered neither by X-ray tomography (insufficient spatial resolution) nor by electron tomography in a transmission electron microscope (too small reconstructed volume). Moreover the CPSEM enables the observation of uncoated non-conductive samples thanks to the presence of gas molecules in the microscope chamber. In situ tensile tests can be then performed in order to follow the evolution of material microstructure under mechanical load. Furthermore, investigations of deformation and failure behaviour of materials in transmission imaging mode are also possible and can provide information in sample volume.VILLEURBANNE-DOC'INSA LYON (692662301) / SudocSudocFranceF

Principes de base de la préparation d’échantillons en science des matériaux

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Microstructural characterization of metal matrix composites by analytical electron microscopy

Metal Matrix Composites have several advantages over the matrix alloys, such as higher stiffness and strength, but they suffer from lower fracture toughness. This is, of course, dependent on the chemical composition of the matrix and of the reinforcement and also on the microstructure and on the processing techniques. The reactions between the different phases in the materials, in particular the matrix / reinforcement interfaces condition strength and toughness. The purpose of the present paper is to outline the potential of different analytical electron microscopy techniques (direct imaging, EDS, EELS, convergent beam diffraction) for the characterization of magnesium segregation in the case of oxide and non-oxide reinforcements. This allows to identify MgAl2O4 spinel at the interface between alumina and an aluminium base matrix, and to understand the different chemical reactions which are involved in the processing.Certaines propriétés mécaniques des matériaux Composites à Matrices Métalliques (CMM) sont améliorées par rapport à celles des alliages de base conventionnels, notamment la rigidité, la résistance, la dureté et la stabilité en température. Ces propriétés sont en relation directe avec la composition chimique de la matrice et des renforts, avec leur microstructure mais aussi avec le mode d'élaboration du composite. L'état des zones réactionnelles entre les différentes phases dans ces matériaux, en particulier des interfaces matrice / renforts, conditionne la rigidité et la résistance à la rupture. Le but de cette communication est de souligner les possibilités de différentes techniques d'analyse en microscopie électronique (imagerie, EDS, EELS, diffraction en faisceau convergent) pour la caractérisation de ségrégation de magnésium dans le cas de renforts oxydés et non-oxydés. Ceci permet d'identifier le spinelle MgAl2O4 à l'interface alumine / matrice à base d'aluminium, et de comprendre les différentes réactions chimiques mises en jeu lors de l'élaboration du matériau

Morphological changes of graphite spheroids during heat-treatment of ductile cast-irons

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Clay dispersion and aspect ratios in polymer-clay nanocomposites

International audienceIt has clearly been shown in the literature that the properties achieved by polymer clay nanocomposites are often related to their structures and to the states of dispersion of the silicate platelets in the polymer matrices. Unfortunately, up to date most techniques used in a standard procedure do not allow a correct interpretation of polymer-clay nanocomposite structure and dispersion. In a recent work, we proposed an image analysis procedure (I.A.P.) based on TEM/OM observations to characterize the clay dispersion in polymer clay nanocomposites. The I.A.P. allows a very fine description of the nanocomposites microstructure. Nevertheless this analysis method shows some limits like the representativity of the sample analyzed volume. The purpose of this work is to discuss about the accuracy of the parameters extracted from the I.A.P. We propose SAXS experimental developments for evaluating the thickness distribution of the clay tactoids. The good agreement between the results of the two techniques confirms the validity of the I.A.P. methodology. Moreover, other experiments were performed in order to understand the abnormally low platelet lengths and aspect ratios determined from TEM micrographs. Wet-STEM observations revealed that clay platelets were not broken during the extrusion process. And, low platelet lengths and aspect ratios were shown to originate from the preparation of the ultramicrotomed sections and from TEM projection effects induced by the clay platelet wavy shape

A history of scanning electron microscopy developments: Towards "wet-STEM" imaging

International audienceA recently developed imaging mode called "wet-STEM" and new developments in environmental scanning electron microscopy (ESEM) allows the observation of nano-objects suspended in a liquid phase, with a few manometers resolution and a good signal to noise ratio. The idea behind this technique is simply to perform STEM-in-SEM, that is SEM in transmission mode, in an environmental SEM. The purpose of the present contribution is to highlight the main advances that contributed to development of the wet-STEM technique. Although simple in principle, the wet-STEM imaging mode would have been limited before high brightness electron sources became available, and needed some progresses and improvements in ESEM. This new technique extends the scope of SEM as a high-resolution microscope, relatively cheap and widely available imaging tool, for a wider variety of sample

Reaction-induced phase separation mechanisms in modified thermosets

International audienceThe reaction-induced phase separation in amorphous thermoplastic-modified epoxy systems was observed in situ using methods of different observation windows: small angle X-ray scattering (SAXS), light transmission (LT) and light scattering (LS). The transmission electron microscopy (TEM) technique was concurrently used to get direct representations of morphologies at different levels of the phase separation process. The selected systems were bisphenol-A diglycidylether cured with either 4,4′-diaminodiphenylsulfone or 4,4′-methylenebis[3-chloro,2,6-diethylaniline] in the presence of polyetherimide. The phase separation mechanisms involved were found to be greatly dependent on the initial modifier concentration and on the ratio of the phase separation rate with respect to the polymerization rate. Experimental results showed that, for modifier concentrations close to the critical fraction, the system was directly thrown into the unstable region, even at a low polymerization rate, and phase separation proceeded by spinodal demixing. On the other hand, for off-critical compositions the homogeneous solution demixed slowly via the nucleation and growth mechanism. In spite of the evolution of the phase diagram with reaction extent, the system remains in the metastable state whatever the cure temperature. The cure temperature has a strong effect on the extent of phase separation, since sooner or later vitrification of the thermoplastic-rich phase occurs and stops the evolution of morphologies. A post-cure allows the phase separation process to go further and sub-particles can be generated depending on the precure and post-cure temperatures

Grafting characterization of natural rubber latex particles: Wet-STEM imaging contributions

International audienceNatural rubber latexes investigated in this study have been chemically modified by seeded emulsion polymerization. Depending on the water affinity of the monomer involved (MMA or DMAEMA), the expected result was the grafting of the corresponding polymer inside or on the surface of the latex particles. The present article focuses on the grafting characterization of these modified natural rubber latexes. In this purpose, non-imaging classical experimental methods such as dynamic light scattering and nuclear magnetic resonance have been completed by microscopy techniques, including transmission imaging in scanning electron microscopy (SEM) and a recent imaging mode called wet-STEM. It consists in transmission imaging in an Environmental SEM operating in the wet-mode, allowing transmission observations of particles suspended in a liquid layer with good resolution and contrast. In the present study, we have adopted a comparative characterization approach between a nongrafted natural rubber latex and two grafted ones. Such an approach indeed contributes to highlight the particles morphology resulting from chemical modification using either MMA or DMAEMA. Transmission images in SEM of thin foils are relatively well interpreted and are completed with wet-STEM images of latexes in their native state, bringing important contributions for grafting characterization