Relations conductivité/microstructure dans des céramiques composites superconducteur (NASICON)/verre isolant

NASICON (Na3Zr2Si2PO12)/low conductor (Na2Zr4Si16P8O 61 glass) composites ceramics have been synthesized using sol-gel process and low-temperatures sintering. The microstructures have been studied by scanning electron microscopy and the chemical reaction between NASICON grains and the intergranular glassy second phase has been analysed using Raman Microprobe spectroscopy. Conductivity has been measured by impedance spectroscopy for various glass content between 3 and 90 % (by weight) and for various degrees of reactions between NASICON and glass. The percolation threshold is about 5 % and up 10 % the properties are similar to those of the low conducting phase. An conductivity anomaly is observed for 3 % glass content.Des céramiques composites à base de superconducteur ionique NASICON (Na 3Zr2Si2PO12) et de verre peu conducteur (Na2Zr4Si16P8O 61) ont été élaborées suivant une filière « SOL-GEL » par frittage à basse température. Les microstructures obtenues ont été analysées par microscopie électronique et l'inter-réaction entre les deux phases a été étudiée par microanalyse Raman. La conductivité des composites a été analysée par spectrométrie d'impédances complexes pour diverses proportions de verre (entre 3 et 90 % pondéral) et pour différents degrés de réactions (découlant des différentes températures de frittage). Le seuil de percolation se situe vers 5 % et, dès 10 % de verre, les propriétés du composite sont déterminées par le verre. Une anomalie de conductivité est observée pour les composites contenant 3 % de verre

Polychrome enamels, ceramics, glasses and their degradation

International audienceDue to the good chemical stability of chemical bonds forming silicates, glass and pottery are generally well preserved and can be used as dating milestones. After a brief recall of the preparation of (glazed/enamelled) pottery and glass (utensils, stained glass windows) from the technical and historical points of view, the main chemical and physical characteristics of the glassy materials (composition, mechanical and thermal characteristics, porosity, etc.) are presented and discussed in relation to corrosion resistance. The corresponding analytical techniques are addressed. Emphasis is given on the different mechanisms of degradation (surface and bulk corrosion, crazing/peeling, proton/water insertion, lixiviation, oxidation) as well as conservation and restoration practices. Dating/authentication of ancient artefacts by the measurement of Raman signal at the surface of glassy silicates is further presented

Raman Intensity: An Important Tool in the Study οf Nanomaterials and Nanostructures

Analysis of the relative and absolute Raman intensity is very important and powerful tool which allows to understand and characterize the modifications of the crystal/amorphous structure due to: (i) changes of the symmetry, e.g. substitution of the B site ion by the rare earth/lanthanide one and incorporation of mobile species in the case of high temperature protonic conducting perovskites; (ii) changes of the short/long range order, e.g. existence/disappearance of the nanoregions in the case of $PbMg_{1/3}Nb_{2/3}O_{3-x}PbTiO_3$ (PMN-PT) relaxor ferroelectric perovskites; (iii) changes of the nanostructure, e.g. depolymerisation of the Si-O network due to the substitution of the $Si^{4+}$ ions (and associated covalent bonds) by the $M^+$ cations (forming ionic bonds) or by the incorporation of the metal nanoprecipitates in the case of glasses, glazes and enamels

Origin of the carbon rich sliding interface in alkali containing matrix-SiC Nicalon fibre composites

The interfacial Nicalon SiC/Nasicon matrix reaction and its influence on the micro-and macroscopic mechanical properties have been studied by chemical analysis, EDX, SEM, microindentation, fibre tensile and composite flexural strength. Reaction mechanims and relationships with mechanical properties are discussed. An explanation of the origin of the "C" sliding interphase is proposed

The structural and dynamics neutron study of proton conductors: Difficulties and improvement procedures in protonated perovskite

With the expected development of Hydrogen as energy vector, there is a great impetus on the study of thermally stable proton conductors, the core of fuel cells, electrolysers and potential CO2 converters. Prior to a successful industrial application one should first well determine their extremely complex physical and chemical behaviour related to the unique character of the proton. The difficulties in comprehension of the nature of mobile protonic species, their location (especially the differentiation between bulk and surface species) as well as local and long range dynamics are different as a function of the hydration level: i) in hydrates the number of protons not really involved in the conduction is much larger than that of protonic conducting species, ii) in non-hydrated materials, the total amount of conducting protons can be very small, as dopants in semiconductors, and similar to that arising from surface water and physisorbed protonic moieties. The attempts and difficulties to locate and identify the protonic species and their dynamics using the neutron techniques are discussed in the light of representative examples, with emphasis on proton conducting perovskites

De l'analyse micro/nanostructurale et micromécanique à l'imagerie des fibres de renfort d'un composite à matrice métallique

L'amélioration des propriétés thermomécaniques des alliages de titane par incorporation de fibres céramiques rend possible leur utilisation dans certaines pièces tournantes chaudes. Les fibres de renfort étant elles-mêmes des matériaux composites, une évolution de leur état métastable peut conduire à une dégradation des propriétés thermomécaniques. Or, l'état physico-chimique et mécanique de ces fibres est fonction de l'histoire thermique mais aussi des réactions qui se produisent à l'interface fibre-matrice ou au sein même des fibres, sous l'effet des gradients thermiques et chimiques imposés par l'élaboration, puis par l'utilisation du composite. La microspectrométrie Raman est la seule méthode applicable aux matériaux composites qui permette une analyse conjointe des états micro/nanostructural et mécanique (tension, compression) de fibres amorphes ou nanocristallines. Le travail présenté analyse les modifications physico-chimiques et micromécaniques induites par les matrices Ti 6242 sur des fibres SCS-6 (Textron) dans des composites préparés soit par forgeage de feuilles entre lesquelles des fibres sont prises en sandwich, soit par frittage des mêmes fibres revêtues, par CVD, d'une gaine de matrice

Corrosion of ceramic matrix composites

Air stable ceramic matrix composites are promising for thermostructural applications such as aircraft engine parts. Turbine parts are subject to both sulphuric acid and sodium molten salts corrosion due to sulphate traces in engine fuel and to the NaCl air content. The chemical stability is a very important criterion but this point has not received much attention to date. We report here a study of acidic and sodium corrosion of various aluminosilicate matrices : LAS matrices (Li2OAl2O32-6SiO2, nP2O5) in the amorphous, β eucryptite and β spodumene forms, BAS matrix (BaOAl2O32SiO2) in the form of monoclinic and hexagonal celsian, NASICON matrix (Na3Zr2Si2PO12) and mullite matrix. Microstructure damages and ion exchange have been analysed by X-ray diffraction, IR absorption, scanning electron microscopy and Raman microprobe. Drastic corrosion is observed for β spodumene containing composites with the formation of strong hydrogen bond or with the cell expansion due to Li/NA+ exchange. Medium acidic attack occurs for glassy LAS, β eucryptite, BAS and NASICON matrix composites. On the other hand, β eucryptite, NASICON and monoclinic celsian resist to alkaline melts. Mullite matrix composites are never corroded

Raman Intensity: An Important Tool in the Study οf Nanomaterials and Nanostructures

Analysis of the relative and absolute Raman intensity is very important and powerful tool which allows to understand and characterize the modifications of the crystal/amorphous structure due to: (i) changes of the symmetry, e.g. substitution of the B site ion by the rare earth/lanthanide one and incorporation of mobile species in the case of high temperature protonic conducting perovskites; (ii) changes of the short/long range order, e.g. existence/disappearance of the nanoregions in the case of $PbMg_{1/3}Nb_{2/3}O_{3-x}PbTiO_3$ (PMN-PT) relaxor ferroelectric perovskites; (iii) changes of the nanostructure, e.g. depolymerisation of the Si-O network due to the substitution of the $Si^{4+}$ ions (and associated covalent bonds) by the $M^+$ cations (forming ionic bonds) or by the incorporation of the metal nanoprecipitates in the case of glasses, glazes and enamels