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International audience; The replacement of single crystals like Er3+:Y3Al5O12 (Er:YAG) by polycrystalline ceramics is one of the ways to increase the output power of lasers sources. Although their low thermal conductivity could limit the overheating phenomenon that appears in the bulk material, their use is still limited due to the difficulties to obtain perfect material transparency quality. To meet with this requirement, lithium fluoride (LiF) was pointed out by some authors as an efficient sintering aid because it promotes densification and grain growth and allows the elimination of impurities. However, its mode of action and evolution inside the microstructure are not well known yet. In this work, sintering of YAG ceramics using LiF as a dopant is carried out on a Spark Plasma Sintering (SPS) apparatus. Attention is focused on the understanding of the LiF behavior during the thermal treatment according to optical and structural characterizations of specimens.

International audience; Nanocrystalline hafnium diboride, carbide and nitride are synthesised by a two-step process, namely a short mechanical activation step (∼1?2 h) of powder mixtures based on partially hydrated hafnium tetrachloride, ph-HfCl4, and on magnesium, followed by a brief annealing step (∼1 h) of the activated powders in flowing argon at 1100?C. HfCl4 is a by-product of the carbochlorination process of extraction of zirconium from ores. Magnesium has a strong reducing capacity and is insoluble in metallic hafnium. The unsought reaction products may then be eliminated during the additional heat treatment or by further leaching. Mechanically activated powder mixtures of ph-HfCl4 + B and of ph-HfCl4 + B + Mg yield either facetted grains or monocrystalline rods of HfB2 according to the composition of the initial mixtures and to the temperature and duration of the annealing treatment. The mean size of the facetted grains ranges between 100 nm and 300 nm. The growth direction of the rods is along the c-axis. Their mean diameters are around 100 nm and their mean lengths are in the range 500 nm?1 um. Hafnium carbide HfC is directly mechanosynthesised from ph-HfCl4 + Mg + C powder mixtures. A subsequent heat treatment in flowing argon allows to better crystallise the grains of HfC (∼100 nm) and to decompose the intermediate products. Annealing of ground ph-HfCl4 + Mg blends in flowing nitrogen for 1 h at 1100?C yields HfN nanoparticles whose size ranges between 10 nm and 30 nm. Characterisations of powders by X-ray diffraction, thermogravimetric analysis, scanning and transmission electron microscopy, Fourier Transform Infrared Spectroscopy were performed to follow the phase evolution and to clarify the reaction mechanisms.

International audience

Texte intégral accessible uniquement aux membres de l'Université de Lorraine; In this work, we have studied the synthesis ofborides, carbides and nitrides of zirconium and hafnium, from powders of chlorides and oxides by mechanosynthesis or bail milling. The interest is to synthesize by a new way, Jess complicated than those usually applied, useful compounds for nuclear industry and to increase the value of secondary products of this industry. The compounds have been synthesized in three stages : bail milling, thermal treatment and washing. The reactional mechanisms have been investigated for ail mixtures at every stage. The products of mechanosynthesis have been characterized mainly by X-ray diffraction, thermal analysis, scanning and transmission electronic microscopy, mass and infra-red spectroscopy. The ball milling conditions (duration and amount of reagents) have been optimized to obtain pure products. After thermal treatrnent under flowing neuter atmosphere, the mixtures have been washed by hydrochloric acid and / or alcohol to extract the secondary products of the reaction. The microstructures of products have been characterized. When the bor ides are synthesized from hafnium tetrachloride, according to the composition of original mixture the morphology of single cristals is either facetted grains or nanorods, the diameter of wich is around 100 nm. We have shown the conditions necessary to obtain the nanorods and we have suggested hypotheses to explain their growth mechanism. In the particular case of hafnium borides, their densification by hot pressing bas been investigated.; Ce travail est consacré à l'élaboration de borures, carbures et nitrures à base de zirconium ou de hafnium à partir de poudres de chlorures et d'oxydes de ces éléments activés par mécanosynthèse ou broyage à haute énergie. L'intérêt tient à la valorisation de sous-produits de l'industrie nucléaire par la mise au point d'une nouvelle voie de synthèse de composés utiles pour celle-ci par un procédé bien plus simple que ceux actuellement employés. La synthèse des composés s'est déroulée en trois étapes: broyage, traitement thermique et lavage. Les mécanismes réactionnels intervenant à chacune de ces trois étapes ont été étudiés pour l'ensemble des mélanges. Les produits de la mécanosynthèse ont été caractérisés principalement par diffraction des rayons X, par analyses thermiques, par microscopies à balayage et à transmission, par spectrométrie de masse et infra-rouge. Les conditions de broyage (temps de broyage, quantité de réactifs), ont été optimisées, dans le but d'obtenir des produits« purs». Après traitement thermique sous atmosphère neutre, les mélanges ont été lavés à l'acide chlorhydrique et/ou à l'alcool, dans le but d'extraire les sous-produits de la réaction. Les microstructures des produits obtenus ont été caractérisées. Lorsque les borures sont synthétisés à partir de chlorures de hafnium, la morphologie des monocristaux se présente sous forme soit de grains facettés, soit de nanobâtonnets d'un diamètre de l'ordre d'une centaine de nanomètres, suivant la composition du mélange initial. Les conditions nécessaires à l'obtention des nanobâtonnets ont été mises en évidence, ce qui a permis de formuler des hypothèses quant à leur mode de croissance. Dans le cas particulier des borures de hafnium, une étude de leur mise en forme par pressage à chaud a été effectuée.