F.M.A. Margaça

Small angle neutron scattering study of silica gels: influence of pH

Abstract The technique of Small Angle Neutron Scattering (SANS) has been used to investigate the microstructure, at the nanoscale level, of SiO 2 gels at different processing stages. The samples were prepared by the alkoxide route of the sol–gel process with different pH values. They were measured as xerogels after heat treatment at different temperatures and as wet gels at the gel point and after aging at 60°C. SANS measurements confirmed that pH has a strong influence upon the nanostructure of gels. The present work reports the main SANS results obtained in this investigation and their interpretation.

SANS Study of Zirconia-Silica and Titania-Silica Hybrid Materials

New silica-based hybrid materials have been produced by the sol-gel process. Samples with compositions xMO2·(100 − x)SiO2 (with M = Zr, Ti and x ≤ 10 mol%) were prepared with polydimethylsiloxane, silanol terminated, with different molecular weights. In the present work the microstructure of samples prepared with a volume ratio organic/inorganic of 2/3 was investigated by Small Angle Neutron Scattering. The results show that a rather homogeneous hybrid material can be obtained. It is observed that the polymer is well distributed in the inorganic matrix and that the addition of MO2 perturbs its local conformation when it has low molecular weight. The inorganic oxide network in the hybrid was found to develop as in pure inorganic dried gels.

A SANS study of xTiO2SiO2 gels with low titania content

Abstract Silica-titania gels, prepared in acidic conditions, with a low content of TiO 2 , x ≤ 6 mol%, have been studied by small angle neutron scattering as sols, wet gel and xerogels. The wet gel seems to consist of extended linear polymer chains, with a length of about 13 nm, which might be linked together in a ladder-type structure. Xerogels heat-treated at 120°C consist of homogeneous regions of about 12 nm average diameter and pores of average diameter of 3.6 nm located in the free space left by the contact of those regions. For xerogels heat-treated at 850°C, a similar microstructure is obtained for the lower TiO 2 content samples except that the average pore diameter decreases to about 3 nm. For the 6 mol% TiO 2 sample, data are better represented in an Ornstein-Zernick plot which indicates fluctuations with two distinct correlation lengths.

Investigation of Organic-Inorganic Hybrid Materials Prepared by Irradiation

New silica-based hybrid materials have been produced by gamma irradiation of the precursors using a 60Co source. An inorganic mixture of tetraethylorthosilicate with addition of Zirconium propoxide (PrZr) (x = 0, 5 and 10 mol%) was used together with polydimethylsiloxane (PDMS) silanol terminated. Samples with volume ratios organic/inorganic of 4/1, 3/2, 1/1, 2/3 and 1/4 were prepared. The irradiation dose for gel point was observed to depend on the PDMS/ INORG. volume ratio, as expected. That dose increases with decreasing volume ratio. Samples with PDMS/ INORG = 2/3 were irradiated to several irradiation doses above that corresponding to the gel point, which becomes higher for higher fraction of PrZr added. The resulting samples are homogeneous, transparent and flexible being able of swelling in a good solvent of the polymer. The microstructure of the samples was investigated by Small Angle Neutron Scattering. Samples were measured as prepared, and immersed in deuterated cyclohexane. Results indicate the presence of oxide dense clusters interconnected by polymer structures. The addition of PrZr in the preparation stage increases the density of the clusters and decreases their size.

Positronium decay study of zirconia–silica sol–gels

Abstract Positron annihilation lifetime (PAL) and Doppler broadening of annihilation line (DBAL) techniques have been used to investigate the fine-pore structure of the system x ZrO 2 ·(1−x) SiO 2 (x=4,10 mol %) . The samples were prepared by the alkoxide method of sol–gel process. The influences of the ethanol/alkoxide ratio and the heat-treatment on the pore structure have been studied. Pores of two different sizes, R∼3 A and R∼9–11 A were measured in the samples studied. It was established that the heat-treatment had greater influence on the defect structure than the ethanol/alkoxide ratio, for both ZrO2 contents.

Inelastic neutron scattering studies of collective modes in molten CsCl

The authors have measured the inelastic neutron scattering spectra of three samples of molten CsCl with different isotopic chlorine compositions (including the naturally occurring one) using an incident wavelength of 4.1 AA, and also of molten CsCl with natural Cl using 1.08 AA. The experiments were performed on the time-of-flight spectrometers IN6 and IN4 at the Institut Laue-Langevin. By a first-order subtraction they have substantially separated the contributions from mass (acoustic mode) and charge (optic mode) current correlations. A quasi-dispersion curve has been derived from the positions of the peaks in these functions. The acoustic and optic branches are well separated in energy over the measured region (1 AA-1<Q<4 AA-1) with relatively flat dispersion. The modes are strongly overdamped and although the energy of the low-Q optic mode corresponds to the crystal LO there is no evidence of a side peak in Sqq(Q, omega ). Thus the observed optic mode is not propagating, even at the lowest values of Q. Indeed the IN4 measurements show that Sqq(Q, omega ) is very broad (full width 18 meV) at low Q while Smm(Q, omega ) is much narrower. Small maxima in the widths of ST(Q, omega ) at Q approximately=1.7 AA-1 indicate the possible existence of a soft acoustic mode. Classical de Gennes narrowing may be observed at Q approximately=2.0 AA-1, corresponding to a change in diffusion mechanism. The statistical accuracy of the spectra was too low and the differences between scattering lengths too small to enable a second-order separation to be made using results from all three samples.

Solving the problem of SANS instrument optimization

The problem of the optimization of small-angle neutron scattering instruments installed at steady neutron sources is discussed. The optimum solution is shown to be that in which full use is made of either the available luminous area of the source or the available hall space and not necessarily that corresponding to the equal-flight-paths design. Design criteria and their implementation are discussed taking into account space constraints on the instruments layout. It is shown that the performance of currently operating SANS facilities can be substantially improved by following the optimization procedure proposed by the authors.

Design optimization of a small angle neutron scattering spectrometer

Abstract Optimized design parameters of a small angle neutron scattering spectrometer are discussed. It is shown that the detector count rate can be optimized for constant angular resolution and a fixed neutron source area — the maximum luminous area available. This leads to a detector count rate higher than that obtained with a spectrometer optimized for fixed total flight path length and the same angular resolution. The relative gain in intensity depends upon the size of the neutron emitting surface available to the spectrometer. When optimization is hindered by space constraints the gain compared to the conventional design can still be relevant.

Structure of mineral gels

Abstract Small-angle neutron scattering (SANS) measurements have been performed to investigate the nanoscale structure of materials of the systems x TiO 2 · (1 − x )SiO 2 and x ZrO 2 · (1 − x )SiO 2 with x ⩽ 10 mol% at different processing stages. The materials were prepared by sol-gel processing using the alkoxides under strong acidic conditions. Samples were studied as xerogels heat treated at 120°C and 850°C and as wet gels at gel point and after ageing. All samples showed identical microstructure at the gel point of extended linear chains about 10 nm long. The aged gel has a mass fractal structure with fractal dimensions of 1.7–1.9. The 120°C heat-treated xerogels show homogeneous oxide regions with mass fractal structure. For the 850°C heat-treated xerogel the average size of the oxide regions is reduced and it has become more dense compared with the 120°C heat-treated sample.

Oxidation of clofibric acid in aqueous solution using a non-thermal plasma discharge or gamma radiation

In this work, we study degradation of clofibric acid (CFA) in aqueous solution using either ionizing radiation from a60Co source or a non-thermal plasma produced by discharges in the air above the solution. The results obtained with the two technologies are compared in terms of effectiveness of CFA degradation and its by-products. In both cases the CFA degradation follows a quasi-exponential decay in time well modelled by a kinetic scheme which considers the competition between CFA and all reaction intermediates for the reactive species generated in solution as well as the amount of the end product formed. A new degradation law is deduced to explain the results. Although the end-product CO2 was detected and the CFA conversion found to be very high under the studied conditions, HPLC analysis reveals several degradation intermediates still bearing the aromatic ring with the chlorine substituent. The extent of mineralization is rather limited. The energy yield is found to be higher in the gamma radiation experiments.