Michel A. Aegerter

Fabricação de sílica vítrea pelo método sol-gel

- nThe synthesis of glasses using the sol-gel method appeared recently of great interest either from the technological or from the scientific point of view. We describe the process used to obtain pure vitreous silica (gel formation — hypercritical drying — densification) a well as some physical characterisitics of the various products fabricated.

Dielectric measurements of silica aerogels

The dielectric response of aerogels of silica is measured down to 1.7 K and compared to a theoretical calculation using the Looyengas relation applied to two different model structures. Helium atoms start to condense on the pore walls at T — 13K and fill them at the liquefaction temperature. A drop of the dielectric constant accompanied by a large loss peak is observed near 35K. The origin of the absorption lies in a thermally activated relaxation process and is probably due to a large number of structural defects similar to those proposed for bulk silica.

Synthesis, characterization and applications of lead and barium titanate materials prepared by the sol-gel method

Sol-gel processing represent a promising method for the fabrication of dielectric materials. In this study we review the preparation and characterization of lead and barium titanate derived materials in form of gels, powders and films. The relative merits and problems of the different processing techniques are investigated. The physical properties of these materials obtained either in the amorphous or crystalline form and possible applications of technological interest are finally presented.

Preparação de sílica amorfa monolitica e deposição de filmes de TiO2 pelo método sol-gel

O metodo sol-gel foi usado para obter silica vitrea pura e depositar filmes finos de TiO2 sobre substratos de vidros pela tecnica dip-coating;. Descrevemos os processos e algumas caracteristicas fisicas dos produtos obtidos.

Sintese de catalisadores tipo V2O5/TiO2 e MoO3/TiO2 pelo processo sol-gel com secagem supercrítica

- nTwo series of supported V2O5/TiO2 and MoO3/TiO2 catalysts were synthesized by sol-gel process with supercritical extraction by carbon dioxide. The textural and structural properties of these aerogels were characterized by surface area measurement, X-ray diffraction and SEM analysis. This preparation method allowed to obtain mixed oxides with high surface area (~= 130 m2/g) and good dispersion of V2O5 and MoO3 on TiO2. The average particles size was 20 nm.

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

- nSystematic 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.

Caracterização elétrica e estrutural de cerâmicas supercondutoras BSCCO preparadas com controle granulométrico dos pós precursores

- nResults are shown of grain control of powder used in the fabrication of high temperature superconductors (HTSC) pellets. The powder was obtained using co-precopilation of oxalates of the elements composing the Bi:Sr:Ca:Cu:O system, with nominal composition (2:2:2:3). After various calcinations the grains of sizes 40-120 µm were separated using sieves of appropriate mesh. The electric and structural characterization of pellets was made using electrical resistivity technique (DC FOUR PROBE), transport current density, scanning electron microscopy (SEM) and, X-ray diffraction (XRD). The chemical composition was studied with energy dispersive X-ray microanalysis (EDX). The pellets made with grains having average diameter about 40 µm show critical temperature To~=110 K, compatible with the phase (2:2:2:3) and current density about 50 A/cm2 at 5K.

Preparação de géis secos monolíticos de SiO2 e SiO2-TiO2 através de secagem hipercrítica

- nHomogeneous SiO2 and SiO2-TiO2 gels are made by hidrolysis and polymerization of metal alkoxides in alcoholic solution. The drying of these gels is performed in an autoclave at super critical conditions with a nitrogen atmosphere. The influence of the pressure on the drying process and some features of the resulting gels are discussed.

Transparent and Antiglare Conducting Coatings Deposited by Wet Chemical Processes

In2O3:Sn (ITO) sols made of crystalline nanoparticles fully redispersable in an ethanol solution containing hydrolyzed organosilanes have been developed to obtain conducting transparent and antiglare coatings on plastic and glass substrates at low temperature by spin, dip and spray coating processes. The coatings are cured by UV irradiation and/or by a low temperature heat treatment (T < 130°C) in air or reducing atmosphere. The electrical, optical, textural and mechanical properties of the coatings are reported.

Raman spectroscopy of 2Na(2)0.1Ca0.3SiO(2) glass and glass-ceramics

2Na(2)0.1Ca0.3SiO(2) glass and glass-ceramics are investigated by conventional and microprobe Raman spectroscopy. The glass spectrum can be deconvolved into 6 gaussian bands located at 860, 920, 975, 1030 cm(-1) attributed to stretching of Si-nonbridging oxygen (NBO) bond and at 625, 660 cm(-1) attributed to Si-O-Si bond vibrations. Partially crystallized glass is formed as spherulites imbedded in the amorphous phase. Their Raman activities show that during crystallization the number of SiO4 tetrahedra having 2 NBO increases and that the Si-O-Si bonds soften. Highly crystallized samples still contain a small amount of amorphous phase.