LPCVD synthesis of silicon nanodots from silane and for flash mamory devices

The increase of microelectronic device potentialities essentially derives from the reduction of feature size down to the nanometer scale. Multinanocrystals memories are one illustration of this trend. A multi-field study is reported, aiming to better understand phenomena involved in silicon nanocrystals elaboration. A first objective is to better control the density and the size of silicon nanocrystals formed during Low Pressure Chemical Vapor Deposition (LPCVD). Some experimental results will be presented, aiming a reliable estimation of densities and sizes of nanocrystals using several techniques: ellipsometry measurements, SEM and TEM image analysis. These experimental data have been simulated at the reactor scale using the Computational Fluid Dynamics (CFD) code FLUENT. A new kinetic scheme has been developed, which considers the various chemisorption sites existing on SiO2 substrates. This new modelling approach allows to correlate in transient mode the local composition of the gas phase near the substrate with the various sites number and with the density and the size of nanocrystals

Early stages of precipitation and microstructure control in Mg-Rare Earth alloys

International audienceHardening precipitation frequently occurs in Mg-Rare Earth (RE) alloys after heat treatment in the 150癈-200癈 range. Early stages of precipitation have been studied in detail by transmission electron microscopy in two Mg-RE alloys (Mg-Y-Gd and Mg-Y-Nd). Two types of structures may be involved in the precipitation sequence: a DO19 phase and the so-called orthorhombic 邟 phase. The structural relationship between DO19 and 邟 phases has been established in underaged and overaged states from the observations at peak ageing. We show that the earliest precipitates play a key role in the selection of phases developing in overaged states. Depending on the habit plane of the precipitates present in the early states, either the DO19 or the 邟 phase will grow in further ageing. The Mg-Y-Gd and Mg-Y-Nd alloys illustrate the different microstructures resulting from such selection. Due to the selective growth of the 邟 phase, the Mg-Y-Gd alloys are characterized by a fine scale microstructure which provides improved mechanical properties

Plasticite de la silice vitreuse

SIGLET 55628 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

The deviations of the Al6_6Li3_3Cu quasicrystal from icosahedral symmetry : a reminiscence of a cubic crystal

The (Al$_6$Li$_3$Cu) (T$_2$) quasicrystals are known to exhibit large deviations from the icosahedral symmetry. Series of electron diffraction patterns are used to investigate these imperfections in as-cast T, samples. A detailed analysis of the 5-fold and 3-fold symmetry diffraction patterns shows that they are compatible with the m3 point group instead of the m35 icosahedral group. This symmetry reduction is interprétéd as reminiscent of the cubic approximant phase (R-Al$_5$Li$_3$Cu) rather than of higher order approximant phases. This interpretation is supported by previous observations on crystal/quasicrystal phase transformation in the AlLiCu system.Les quasicristaux de phase T$_2$(Al$_6$Li$_3$Cu) montrent d'importantes déviations à la symétrie icosaédrique. Ces imperfections sont mises en évidence par diffraction électronique dans des échantillons de phase T$_2$ brut de coulée. Un examen détaillé des diagrammes de diffraction de symétrie d'ordre 3 et 5 révèle qu'ils sont compatibles avec le groupe ponctuel m3 au lieu du groupe de l'icosaèdre (m35). Cette réduction de symétrie est interprétée comme une réminiscence de la phase cubique approximante (R-Al$_5$Li$_3$Cu) et non l'apparition d'approximant d'ordre plus élevé. Cette interprétation est suggérée par des observations antérieures sur la transformation cristal/quasicristal dans le système AlLiCu

Double shear mechanism and orientation relationships in austenite/ferrite steels

We propose a new approach of structural transformation between lattices by means of simple bounded transformations composed of two "fractional shears". The strength point of this double shear mechanism in lattice transformations is to specify particular orientation relationships. Though the basis of the model concerns the general 3D diffusionless transformation, the case of 2D lattices is presented in detail. When applied to the transformation of fcc (111) plane into the bcc (110) plane, the model is able to predict precise orientation relationships close to those observed in metallurgy (Kurdjumov-Sachs or Nishiyama-Wasserman relationships). This 2D model seems to be particularly relevant for materials such as the duplex steels

Caractérisation et modélisation de la précipitation du carbone de niobium et du cuivre dans les aciers bas carbone

La précipitation de NbC dans les aciers bas carbone permet une amélioration conséquente des propriétés mécaniques. Cependant, aucune caractérisation quantitative détaillée de cette précipitation n'est disponible à ce jour. Cette étude porte sur le suivi des cinétiques de précipitation de NbC dans la ferrite, pour deux alliages Fe-Nb-C modèles. La fraction volumique précipitée ainsi que le rayon moyen sont suivis par diffusion centrale des neutrons, alors que la morphologie et la localisation des précipités sont déterminées par microscopie électronique en transmission. Un modèle spécifique de précipitation est développé, prenant en compte le rôle des dislocations comme sites préférentiels de germination et courts-circuits de diffusion. En complément, l'étude de la précipitation dans un alliage modèle Fe-Cu nous a permis de progresser dans la compréhension des phénomènes de germination et de précipitation anisotherme.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF