In situ study of the endotaxial growth of hexagonal CoSi2 nanoplatelets in Si(001)

This investigation aims at studying-by in situ grazing-incidence small-angle x-ray scattering-the process of growth of hexagonal CoSi2 nanoplatelets endotaxially buried in a Si(001) wafer. The early formation of spherical Co nanoparticles with bimodal size distribution in the deposited silica thin film during a pretreatment at 500 °C and their subsequent growth at 700 °C were also characterized. Isothermal annealing at 700 °C promotes a drastic reduction in the number of the smallest Co nanoparticles and a continuous decrease in their volume fraction in the silica thin film. At the same time, Co atoms diffuse across the SiO2/Si(001) interface into the silicon wafer, react with Si, and build up thin hexagonal CoSi2 nanoplatelets, all of them with their main surfaces parallel to Si{111} crystallographic planes. The observed progressive growths in thickness and lateral size of the hexagonal CoSi2 nanoplatelets occur at the expense of the dissolution of the small Co nanoparticles that are formed during the pretreatment at 500 °C and become unstable at the annealing temperature (700 °C). The kinetics of growth of the volume fraction of hexagonal platelets is well described by the classical Avrami equation.Fil: Costa, Daniel Da Silva. Universidade Federal do Paraná; BrasilFil: Huck Iriart, Cristián. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Kellermann, Guinther. Universidade Federal do Paraná; BrasilFil: Giovanetti, Lisandro Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Craievich, Aldo F.. Universidade de Sao Paulo; BrasilFil: Requejo, Felix Gregorio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; Argentin

In situ study of the endotaxial growth of hexagonal CoSi2 nanoplatelets in Si(001)

This investigation aims at studying-by in situ grazing-incidence small-angle x-ray scattering-the process of growth of hexagonal CoSi2 nanoplatelets endotaxially buried in a Si(001) wafer. The early formation of spherical Co nanoparticles with bimodal size distribution in the deposited silica thin film during a pretreatment at 500 °C and their subsequent growth at 700 °C were also characterized. Isothermal annealing at 700 °C promotes a drastic reduction in the number of the smallest Co nanoparticles and a continuous decrease in their volume fraction in the silica thin film. At the same time, Co atoms diffuse across the SiO2/Si(001) interface into the silicon wafer, react with Si, and build up thin hexagonal CoSi2 nanoplatelets, all of them with their main surfaces parallel to Si{111} crystallographic planes. The observed progressive growths in thickness and lateral size of the hexagonal CoSi2 nanoplatelets occur at the expense of the dissolution of the small Co nanoparticles that are formed during the pretreatment at 500 °C and become unstable at the annealing temperature (700 °C). The kinetics of growth of the volume fraction of hexagonal platelets is well described by the classical Avrami equation.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Etude du comportement moléculaire de l'alcane semifluoré F(CF2)8(CH2)18H en couches minces déposées sur substrats liquide et solide (détermination structurale et organisation interfaciale)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Kinetics of sub-liquidus phase separation in silicate and borate glasses. A review

Many silicate and borate glass systems exhibit sub-liquidus immiscibility. The fine-scale microstructure of phase separated systems has been studied mainly by electron microscopy and small angle X-ray scattering. The classical theories of homogeneous nucleation, spinodal decomposition and coarsening, and more recent statistical theories of segregation are outlined. These theories are compared with experimental data for the kinetics of amorphous phase separation. In the early stages of separation, for compositions and temperatures in the region between the binodal and spinodal and in particular close to the binodal, the results are consistent with a homogeneous nucleation process. The classical theory of spinodal decomposition only agrees qualitatively with experimental data for the central region of the miscibility gap. The kinetics of coarsening are in general agreement with theory. Recent predictions from computer simulation studies on model alloys appear to be valid for glass systems. Experimental studies of the influence of amorphous phase separation on crystallization of glasses are also reviewed. In certain circumstances there is clear evidence that amorphous phase separation enhances the rates of crystal nucleation and growth.Beaucoup de silicates et borates vitreux ont une lacune métastable de miscibilité. La microstructure très fine des verres avec séparation de phases a été étudiée principalement par microscopie électronique et diffusion de rayons X aux petits angles. Les théories classiques de nucléation homogène, décomposition spinodale, "coarsening" et les plus récentes théories statistiques de la ségrégation sont exposées. Ces théories sont comparées avec les données expérimentales liées à la cinétique de la séparation de phases. Les résultats associés aux premiers stades, près de la binodale, sont en accord avec un modèle de nucléation homogène. La théorie classique de la décomposition spinodale montre un accord seulement qualitatif avec les résultats expérimentaux dans la région centrale de la lacune de miscibilité. La cinétique de "coarsening" après les stades initiaux, sont en accord général avec la théorie. Des prédictions récentes des études par simulation à l'ordinateur sur des alliages modèles paraissent être valides aussi pour les systèmes vitreux. Les études expérimentales de l'influence de la séparation de phases amorphes sur la cristallisation des verres sont aussi présentées. Ces études montrent des évidences claires permettant de conclure que la séparation de phases favorise la nucléation cristalline.Craievich Aldo F., Zanotto Edgar E, James Peter F. Kinetics of sub-liquidus phase separation in silicate and borate glasses. A review. In: Bulletin de Minéralogie, volume 106, 1-2, 1983. Silicates liquides

Estudo de saxs am sóis, géis e vidros de sílica obtidos pelo processo sol-gel

-Systematic SAXS studies have been performed at the LURE Synchrotran, Orsay, using an intense beam of point like cross-section to obtain information about the sol wet gel dried gel silica glass transformation. The intensity curves have been analysed in term of power law in log-log plots, whose exponent is related to mass and surface fractal dimensions of the structure. It was found that almost all phases present fractal structures and for the case of basic gels, is of hierarchical nature. The aerogels are formed by a dense matrix, with a smooth surface and exhibit a very narrow auto-similarity range that gives a mass fractal dimension

Textural characteristics of silica aerogels from SAXS experiments

Small angle X-ray scattering measurements on silica aerogels give systematically low values for the solid phase density. In order to explain these results, a hierarchical structure is proposed. Textural parameters such as specific surface area, size and density of clusters, and pore size are a function of the number of levels in the structure. Experimental values are consistent with values calculated for a hierarchy of two to four levels, depending on the initial sol concentration. The model can explain the fractal features for lower density aerogels. © 1990

Structural study of fractal silica humid gels

The structures of gels of silica obtained from solutions of TMOS, methanol and water have been studied by small angle X-ray scattering (SAXS) using synchrotron radiation. The SAXS results indicate that all the gels studied exhibit fractal structures. It is inferred that gels obtained under basic conditions are volume fractals built up by surface and volume fractal units for high and low water content, respectively. The gels obtained under acidic conditions are all volume fractals having structural units much smaller than those of basic gels. The influence of aging on the structure of basic gels has also been established

SAXS and BET studies of aging and densification of silica aerogels

Aerogels prepared by hypercritical drying of gels obtained by hydrolysis of TMOS/methanol 50% solution in acid and basic conditions were studied combining SAXS and BET methods. Diffraction results do not seem to reveal a fractal character as the POROD\u27s limit I(q) q4 → Cte was obtained in all cases without well characterised portions in log I(q) vs log q graphs. Aging produces a small increase in the density of the matrix. Densification studies indicate that for moderate temperatures 530°-810°C the poresshow a shrinkage due to a diffusion controlled mechanism with an activation energy ΔE ~ 9 Kcal/mole. For higher temperatures expansion due to bloating was observed. The density of the matrix undergoes only a slight increase