Ségrégation d'équilibre aux interfaces : systèmes modèles et matériaux

Dans cet article, la ségrégation d'équilibre est présentée comme un phénomène général qui se produit au voisinage des interfaces dans tous les matériaux solides, métaux, semiconducteurs élémentaires, composés. Les techniques permettant de connaître la structure et la composition des interfaces à l'échelle atomique sont rapidement passées en revue. L'équilibre de ségrégation est décrit par une approche globale à partir d'un modèle de solution solide régulière et par une approche locale prenant en compte la structure propre aux interfaces. Le cas particulier des éléments peu solubles dans la matrice est examiné en détail. Des résultats obtenus dans des sysèmes modèles métalliques, semiconducteurs et conducteurs ioniques sont donnés pour illustrer différents aspects de la ségrégation d'équilibre et des approches expérimentales. Les principaux points d'attention à prendre en compte pour passer des systèmes modèles aux matériaux sont évoqués : composition (y compris les impuretés), structure poly-, micro-, ou nano- cristalline, possibilité d'évolution vers l'équilibre dans les conditions de préparation et d'utilisation

Correlation between electrical activity and various structures of Ge grain boundaries

International audienc

Crystal structure of nanocrystalline Pr5Co19 compound and its hydrogen storage properties

International audienc

A methodology suitable for TEM local measurements of carbon concentration in retained austenite

International audienc

Effect of the bainitic transformation temperature on retained austenite fraction and stability in Ti microalloyed TRIP steels

International audienc

Interfacial reactions in relation with adhesion failures in Al/TiN/Ti/SiO

The objective of this work is to determine the origin of a decohesion problem that occurs during the fabrication process of certain integrated circuits. This decohesion takes place at the Ti/dielectric interface with the dielectric being either SiO2 or borophosphosilicate glass (BPSG). The frequency of the decohesion increased when the dielectric is BPSG. In order to understand the reason for the difference in decohesion rates for the two dielectrics, a comparative study was performed before and after annealing. X-ray Diffraction was used to determine the microstructure of the different layers and Transmission Electron Microscopy coupled to Electron Energy Loss Spectroscopy and X-ray Energy Dispersive Spectroscopy was mainly used to characterize the interfaces and nanophases that had formed during annealing. A fundamental difference observed was the Ti/dielectric interface reactivity: in the case of BPSG, an amorphous layer rich in P is formed before the Ti5Si3 silicide. Two hypothesis are put forward in order to explain adhesion failures: a decrease in the rate of Ti5Si3 formation kinetics and/or a decrease of the glass transition temperature induced by P enrichment

Relationship between structure, segregation and electrical activity in grain boundaries

Using the contactless microwave phase-shift technique (μ W-PS) and High Resolution Transmission Electron Microscopy (HRTEM), we show that the twist and mixed parts of a Σ = 51(θ = 16.10∘) grain boundary in germanium (Ge) are electrically active. We also show that we can passivate the electrically active grain boundaries by sulfur segregation which has been studied by energy filtering HRTEM. Atomistic simulations show that the most favorable places for this segregation are the high energy sites of grain boundar

Etude des contraintes résiduelles dans les multicouches Au/Ni

Les matériaux multicouches font aujourd'hui l'objet d'études très complètes de caractérisation physico- chimique pour améliorer leur performance. L'étude des contraintes résiduelles générées lors de leur élaboration constitue un enjeu important dans la compréhension de leurs propriétés mais aussi dans celle des interfaces métal/métal. Cette étude a été entreprise sur une multicouche or/nickel obtenue par dépôt assisté par jet moléculaire. Les contraintes résiduelles ont été déterminées par diWm des rayons X en utilisant une analyse triaxiale des contraintes par la méthode dite des «sin2Ψ».Multilayer materials appears today very studied in order to increase these physimchemical chacte ristics. The determination of residual stresses that occur during the elaboration of multilayers is very important in the knowledge of these properties but also the metal/metal interfaces one. This study was made on Au/Ni multilayers obtained by molecular beam epitaxy. Residual stress wre determined by X-ray diffraction using triaxial analysis of stresses and «sin2Ψ» method

Advanced microscopy techniques resolving complex precipitates in steels

Scanning electron microscopy as well as analytical transmission electron microscopy techniques such as high resolution, electron diffraction, energy dispersive X-ray spectrometry (EDX), parallel electron energy loss spectroscopy (PEELS) and elemental mapping via a Gatan Imaging Filter (GIF) have been used to study complex precipitation in commercial dual phase steels microalloyed with titanium. Titanium nitrides, titanium carbosulfides, titanium carbonitrides and titanium carbides were characterized in this study. Both carbon extraction replicas and thin foils were used as sample preparation techniques. On both the microscopic and nanometric scales, it was found that a large amount of precipitation occurred heterogeneously on already existing inclusions/precipitates. CaS inclusions (1 to 2 μm), already present in liquid steel, acted as nucleation sites for TiN precipitating upon the steel's solidification. In addition, TiC nucleated on existing smaller TiN (around 30 to 50 nm). Despite the complexity of such alloys, the statistical analysis conducted on the non-equilibrium samples were found to be in rather good agreement with the theoretical equilibrium calculations. Heterogeneous precipitation must have played a role in bringing these results closer together