A.N. Falcão

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.

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.

Structural and optical characterization of topaz implanted with Fe and Co

Natural single crystalline colourless topaz (Al2SiO4(F, OH)2) samples were implanted at room temperature with Fe and Co ions with fluences in the range of 5×1016–2×1017 ions/cm2. Following implantation, the implanted region becomes amorphous in all cases studied and significant changes occur in the absorption spectra. Irradiated samples were subjected to thermal annealings up to 900 °C in steps of 100 °C. At 700 °C almost all F is lost from the implanted region and the implanted ions diffused into the bulk. In contrast, when virgin samples undergo the same thermal treatments, the fluorine loss is not observed. At higher annealing temperatures the implanted ions segregate to the surface. After annealing at 700 °C the samples implanted with the higher concentration of Co do not present colour changes but develop absorption bands centred at 450 and 740 nm. On the contrary, high dose Fe implanted samples submitted to similar annealing protocol show absorption at 280, 350 and 455 nm, becoming yellow. We propose that the absorption bands are related with the crystal field splitting of the transition metal 3d orbitals, probably in a new compound.

Annealing behaviour of natural topaz implanted with W and Cr ions

Abstract Single crystalline samples cleaved from natural topaz were implanted with W and Cr ions with fluences in the range 5×1015–3×1017 ions/cm2. After implantation the implanted region becomes amorphous in all the samples and the absorption spectra reveal significant changes in the near infrared region in the wavelength range 2000–3000 nm. For the lowest doses the transmission has a maximum in this wavelength region. The stoichiometry of the implanted surface changes due to the loss of fluorine after annealing at 600°C. This loss is not observed in virgin samples. Chromium starts to diffuse into the bulk after annealing at 600°C while W profile remains stable up to 800°C. After annealing at 800°C the optical absorption shows a reduction while the damage profile remains without any noticeable change.

Luminescence studies in colour centres produced in natural topaz

Luminescence emission bands originating in colour centres produced in natural topaz have been detected. These bands are studied by steady-state and time-resolved photoluminescence spectroscopy. We found that the energy of the emitting bands are different in colourless samples and in samples submitted to treatment and depend on the treatment itself. Also, there is an emission present in long-lived detection regimes, indicating the possible presence of non-radiative processes. These results are analysed and a possible correlation between structural and colour changes resulting from the irradiation and annealing treatment is addressed.

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.

New LDPE Copolymeric Films with Enhanced Hydrophilic Properties Prepared by Gamma Irradiation

Graft copolymerization induced by gamma radiation has being used as a tool to modify the surface properties of a range of polymeric substrates. This work describes the use of this technique for the grafting of 2-hydroxyethyl methacrylate (HEMA) branches onto low-density polyethylene (LDPE) surface, in order to improve its hydrophilic properties. Sample preparation protocols were selected from previous work in order to obtain films with high grafting yields. The obtained PE-g-HEMA films were characterized by thermal analysis techniques (DSC and TGA), and by Fourier transform infrared spectroscopy (FTIR). The results obtained show that, upon irradiation, there is some loss of cristalinity of the copolymer backbone, but also that the samples keep a good thermal stability. The water uptake of the samples was evaluated. Hydration levels up to near 95 % with a ydration/dehydration average ratio of 1:5 were obtained.

Optimum intensity in small-angle neutron scattering. An experimental comparison between symmetric and asymmetric geometries

Results of an experimental comparison of the performance of symmetric (with the flight path between source and sample, L1, equal to the flight path between sample and detector, L2) and asymmetric (Lt ¢ L2) geometries of a small-angle neutron scattering instrument are reported. The comparison was performed under the assumption that a specific range of scattering vectors is required to be measured with a certain resolution. The measurements were performed at the spectrometer installed at the Ris~ National Laboratory, using as test samples a crystalline powder, a single crystal and a sample that produces a smooth scattering curve at small angles. The results obtained show that, when a specific scattering-vector range has to be measured with a specific resolution, the optimized asymmetric spectrometer performs better than the symmetric instrument. A simple graphical approach for the determination of the best geometry to perform a given measurement is introduced.

Neutron scattering experiments on swollen, uniaxially stretched polymer networks

Abstract Neutron scattering measurements have been performed on uniaxially stretched, swollen polydimethylsiloxane (PDMS) networks produced by electron irradiation of a polydisperse melt. Upon stretching, the correlation length ξ L becomes anisotropic varying continuously from a maximum value observed in the direction parallel to the stress, to a minimum value along the perpendicular direction. The observed angular dependence of ξ L and that of the intensity scattered in the limit of zero scattering vector are very close to those predicted by a classical affine deformation model, suggesting that the connected superstructure of the swollen network is uniaxially deformed in a process that is essentially classical.