107 research outputs found
Elaboration de matrices céramiques par un nouveau procédé hybride : imprégnation de poudres et CVI réactive
Ceramic matrix composites were originally developed for aerospace,military aeronautics or energyapplications thanks to their good properties at high temperature. They are generally made by ChemicalVapor Infiltration (CVI). A new short hybrid process combining fiber preforms lurry impregnation ofceramic powders with an innovative Reactive CVI (RCVI) route is proposed to reduce the productiontime. This route is based on the combination of Reactive Chemical Vapour Deposition (RCVD), whichis often used to deposit coatings on fibres, with the Chemical Vapor Infiltration (CVI).In RCVD, the absence of one element of the deposited carbide in the initial gas phase involves theconsumption/conversion of the solid substrate. In this work, the RCVD growth and the associatedconsumption were studied with different parameters in the Ti-H-Cl-C chemical system. The study hasbeen completed with the chemical products analysis, combining XRD, XPS and FTIR. Then, the partialconversion of sub-micrometer carbon powders into titaniumcarbide and the consolidation of greenbodies by RCVI from H2/TiCl4 gaseous infiltration were studied. The residual porosity and the final TiCcontent were measured in the bulk of the infiltrated powders by image analysis from scanning electronmicroscopy. Depending on temperature, few hundred micrometers-depth infiltrations are obtained.Finally, the results have been transposed to the RCVI into CMC-type preforms. Despite aminimalTiC content of 25% in the overall preform, the results shown a bad homogeneity of the infiltration anda poor cohesion of fibres with RCVI consolidated powder of their environment.Les composites Ă matrice cĂ©ramique ont initialement Ă©tĂ© dĂ©veloppĂ©s pour des applications aĂ©rospatiales, aĂ©ronautiques militaires ou Ă©nergĂ©tiques en raison de leurs bonnes propriĂ©tĂ©s Ă haute tempĂ©rature. Ils sont gĂ©nĂ©ralement fabriquĂ©s par le procĂ©dĂ© CVI (Chemical Vapour Infiltration). Un nouveau procĂ©dĂ© hybride combinant lâimprĂ©gnation de poudre au sein de prĂ©formes, suivie de la CVI RĂ©active(RCVI), est proposĂ© afin de rĂ©duire les temps de production. Cette voie est basĂ© sur lâadaptation du procĂ©dĂ© RCVD Ă lâinfiltration en milieu poreux. En RCVD, lâabsence dâune partie des Ă©lĂ©ments du dĂ©pĂŽt de carbure dans la phase gazeuse implique une consommation/conversion du substrat solide. Dans cette Ă©tude, la croissance et la consommation associĂ©e ont Ă©tĂ© Ă©tudiĂ©es en fonction de divers paramĂštres dans le systĂšme chimique Ti-H-Cl-C. Cette Ă©tude est accompagnĂ©e dâanalyses (DRX, XPS, IRTF) des produits issus de la rĂ©action chimique de formation du TiC. Ensuite, la conversion partielle dâune poudre de carbone submicronique enTiC et la consolidation des zones compactes de poudre par lâinfiltration RCVI utilisant le mĂ©lange gazeux H2/TiCl4 a Ă©tĂ© Ă©tudiĂ©e. La porositĂ© rĂ©siduelle et la teneur en TiC ont Ă©tĂ© mesurĂ©es par analyse dâimage Ă diffĂ©rentes distance de la surface des matĂ©riaux. Selon la tempĂ©rature, plusieurs centaines de micromĂštres infiltrĂ©s ont Ă©tĂ© obtenus. Finalement, les rĂ©sultats ont Ă©tĂ© transposĂ©s Ă lâinfiltration RCVI de prĂ©formes type CMC. MalgrĂ© une teneur minimale de 25% de TiC dans lâensemble de la prĂ©forme, les rĂ©sultats montrent une mauvaise homogĂ©nĂ©itĂ© dâinfiltration et une mauvaise cohĂ©sion des blocs de poudre consolidĂ©s avecles fibres de leurs environnements
Crystal structure of naltrexone chloride solvates with ethanol, propan-2-ol, and 2-methylpropan-2-ol
Non-centrosymmetric Na3Nb4As3O19
A new non-centrosymmetric compound, trisÂodium tetraÂniobium triarsenic nonaÂdecaÂoxide, Na3Nb4As3O19, has been synthesized by a solid-state reaction at 1123â
K. The structure consists of AsO4 tetraÂhedra and NbO6 octaÂhedra sharing corners to form a three-dimensional framework containing two types of tunnels running along the c axis, in which the sodium ions are located. Na+ cations occupying statistically several sites, respectively, are surrounded by seven, six and four O atoms at distances ranging from 2.08â
(1) to 2.88â
(4)â
Ă
. The title structure is compared with those containing the same groups, viz.
M
2XO13 and M
2
X
2O17 (M = transition metal, and X = As or P)
La variĂ©tĂ© ÎČ-NaMoO2(AsO4)
The title compound, sodium dioxidomolybdenum(VI) arsenate(V), ÎČ-NaMoO2AsO4, was prepared by solid-state reaction at 953â
K. In the crystal structure, the AsO4 tetraÂhedra and MoO6 octaÂhedra (both with m symmetry) share corner atoms to form a three-dimensional framework that delimits cavities parallel to [010] where disordered six-coordinated sodium cations (half-occupation) are located. Structural relationships between the different orthoarsenates of the AMoO2AsO4 series (A = Ag, Li, Na, K and Rb) are discussed
K0.8Ag0.2Nb4O9AsO4
The title compound, potassium silver tetraÂniobium nonaÂoxide arsenate, K0.8Ag0.2Nb4O9AsO4, was prepared by a solid-state reaction at 1183â
K. The structure consists of infinite (Nb2AsO14)n chains parallel to the b axis and cross-linked by corner sharing via pairs of edge-sharing octaÂhedra. Each pair links together four infinite chains to form a three-dimensional framework. The K+ and Ag+ ions partially occupy several independent close positions in the interÂconnected cavities delimited by the framework. K0.8Ag0.2Nb4O9AsO4 is likely to exhibit fast alkali-ion mobility and ion-exchange properties. The Wyckoff symbols of special positions are as follows: one Nb 8e, one Nb 8g, As 4c, two K 8f, one Ag 8f, one Ag 4c, one O 8g, one O 4c
AgNa2Mo3O9AsO4
The title compound, silver disodium trimolybdenum(VI) nonaoxide arsenate, AgNa2Mo3O9AsO4, was prepared by a solid-state reaction at 808â
K. The structure consists of an infinite (Mo3AsO13)n ribbon, parallel to the c axis, composed of AsO4 tetraÂhedra and MoO6 octaÂhedra sharing edges and corners. The Na and Ag ions partially occupy several independent close positions, with various occupancies, in the inter-ribbon space delimited by the one-dimensional framework. The composition was refined to Ag1.06(1)Na1.94(1)Mo3O9AsO4
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