Dayse Iara Dos Santos

A SAXS study of silica aerogels

Abstract Aerogels produced by hypercritical drying of gels from hydrolysis of TMOS in various pH conditions and subjected to a densification process were studied by SAXS using the LURE synchrotron facility. The evaluation of scattering data combined with BET measurements leads to a model of aerogels consisting of a light density matrix in which meso- and macro-pores are embedded. No fractal features were observed for the gels, the Porods limit having an exponent n = 4. This could mean that either these aerogels are not fractals or that the SAXS method suffers from an inherent ambiguity for fractal dimension D = 3.

A SAXS study of kinetics of aggregation of silica sols

Abstract SAXS in situ experiments on the evolution of TMOS solutions during hydrolysis and polycondensation lead to power laws with scaling exponents α ≈ 2. It is suggested that this could be the result of the polydispersity of the samples and that only an apparent fractal dimension can be obtained in this way. Kinetic studies tend to indicate that agglomeration in the sol is the result of a diffusion-controlled process.

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 PORODs 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.

Structural study of fractal silica humid gels

Abstract 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.

Optical properties of dye molecules adsorbed on fractal structures

Abstract We present preliminary measurements of the optical properties of Rhodamine B molecules adsorbed or trapped in porous matrices such as Vycor 7930 glass, silica xerogels and aerogels. The electronic transfer between donor and acceptor molecules adsorbed on such fractal structures is also discussed; a fast sub-picosecond resolution experiment is proposed to study this process.

SAXS study of isothermal sintering of silica aerogel

Abstract Isothermal sintering of SiO 2 aerogels was followed by small angle X-ray scattering using synchrotron radiation. An improved treatment of SAXS data which takes into account the presence of fluctuations explains the discrepancy previously observed with He-pycnometric methods. The matrix density of the gel is shown to increase in several steps, first by a diffusion-controlled mechanism and then by viscous flow.

Structural studies of fractal silica aerogels

The structural properties of silica aerogels of various compositions and prepared in acidic, neutral and basic conditions are studied by SAXS, BET, density and TEM techniques. All structures can be described as mass fractals (D 2 matrix having average size of - 13.5 A (basic), - 8.5 A (neutral) and

Superconducting films made by spin-coating with acetate solutions

Metallic silver substrates were spin-coated with several layers of mixed acetate solutions containing bismuth, lead, strontium, calcium, and copper. The viscosities of the cation solutions were modified by the addition of polyvinyl alcohol. The films were heat treated at various temperatures in air, O{sub 2}, and 1% O{sub 2} (balance N{sub 2}) atmospheres. Bismuth cuprate films with transport critical current densities {approx}500 A/cm{sup 2} were obtained in this work. New conditions of coating and sintering have been tried to produce superconducting films.

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.

Vidros porosos de alto teor de sílica para armazenamento de rejeitos nucleares. - Parte I : Preparação e caracterização da separação de fase espinodal

- We have studied the phase separation behavior by spiondal decomposition of two sodium borosilicate glasses of the following composition (weight%) 8Na2O-32B2O3-60SiO2 (glassA) and 8Na2O-27 B2O3-65SiO2 (glass B). From photographs of glass samples obtained with a scanning electron microscope, we have determined the growth of the mean size r of the microstructures of the east phase as a function of the time of the thermal treatment (0-100 h) and for different temperatures (580, 600, 620 and 640°C). The results are in agreement with the theory of Lifshitz-Slyozov-Ardell.