Elaboration and TEM structural study of interfaces in composites produced by precipitation

Model ceramic matrix composites have been manufactured in a wide range of materials using the precipitation of a metal (Cu, Ni, Cr) in a ceramic matrix (nitride AIN or oxides MgO, Al2O3) providing, in each case low energy configurations at the heterophase interfaces. In connection to microelectronic applications, copper metallic particles precipitate in AIN after implantation by copper ions and anneal of the ceramic matrix. Faceted particles are imaged by HRTEM and are associated to a low energy structural and chemical configuration. Internal reduction experiments have been carried out on (Mg,Ni)O, (Mg,Cu)O and (Al,Cr)2O3 mixed oxides; the morphology, chemical composition and orientation relationship of the different precipitates are obtained through TEM observations and discussed in terms of interfacial energy and precipitate growth mechanism and kinetics. Conventional and high resolution TEM in conjonction to structural models have allowed a comprehensive description of the interface

Tuning the thermoelectric properties of A-site deficient SrTiO3 ceramics by vacancies and carrier concentration

Ceramics based on Sr0.8La0.067Ti0.8Nb0.2O3-δ have been prepared by the mixed oxide route. The La1/3NbO3 component generates ∼13.4% A-site vacancies; this was fixed for all samples. Powders were sintered under air and reducing conditions at 1450 to 1700 K; products were of high density (>90% theoretical). Processing under reducing conditions led to the formation of a Ti1-xNbxO2-y second phase, core-shell structures and oxygen deficiency. X-ray diffraction (XRD) confirmed a simple cubic structure with space group Pm3[combining macron]m. Transmission electron microscopy revealed a high density of dislocations while analytical scanning transmission electron microscopy at atomic resolution demonstrated a uniform distribution of La, Nb and vacancies in the lattice. X-ray photoemission spectroscopy and thermogravimetry showed the oxygen deficiency (δ value) to be ∼0.08 in reduced samples with enhanced carrier concentrations ∼2 × 1021 cm-3. Both carrier concentration and carrier mobility increased with sintering time, giving a maximum figure of merit (ZT) of 0.25. Selective additional doping by La or Nb, with no additional A site vacancies, led to the creation of additional carriers and reduced electrical resistivity. Together these led to enhanced ZT values of 0.345 at 1000 K. The contributions from oxygen vacancies and charge carriers have been investigated independently

INTERFACES BETWEEN SiC AND METALS

Une étude des réactions chimiques se produisant aux interfaces entre métal et carbure de silicium a été effectuée pour divers métaux (Fe, Ni, Cr) et leurs alliages . Les chemins de réaction, les cinétiques de réaction, la morphologie macroscopique et la microstructure de la couche de réaction ont été détermines. Un modèle a été développé pour expliquer l'influence de chaque composant métallique sur la composition et la structure de la couche de réaction.Chemical reactions occuring at metal - silicon carbide interfaces have been investigated for different metals (Fe, Ni, Cr) and their alloys. Reaction paths, reaction kinetics, macroscopic morphology and microstructure of the product scale have been determined. A model has been developed to explain the influence of each metal component on the composition and structure of the product scale

High temperature creep performance and microstructure of SiC-C-SiC composites

The high temperature creep behaviour of a 2D SiC/C/SiC composite is studied at temperatures ranging from 1200 to 1673K, under stresses from 50 to 500MPa. Due to the microstructural evolution of the material, steady state creep is never reached ; creep toughening is observed. Microcrack formation and propagation in the interface and in the matrix were identified as contributing to the deformation