Fernanda Margaça

SANS of ZrO2.SiO2 gels

Abstract Small angle neutron scattering (SANS) measurements have been performed to investigate the nanoscale structure of materials of the system xZrO2·SiO2 with x≤10 mol%, at different processing stages. The materials were prepared by sol–gel using the alkoxides method, in strong acidic conditions. Samples studied as xerogels were heat-treated at 120° C and 850° C; as wet gels at the gel point; and after 4 h aging at 60° C. The samples showed extended linear chains ca. 10 nm long at the gel point. The aged gel has a mass fractal structure with fractal dimension of ca. 1.8. The 120°C heat-treated xerogel shows homogeneous oxide regions with 16 nm average diameter and mass fractal nature. For the 850°C heat-treated xerogel the oxide regions average diameter reduced to about 13 nm and presented a sharp and smooth surface, obeying the Porod law.

A study of free-volume hole distributions in by positron annihilation spectroscopy

Positron annihilation spectroscopy has been used to obtain information about the small-pore structure of the system (x = 10, 30 mol%). The pore radius probability distribution functions have been obtained from the ortho-positronium lifetime probability distribution functions, determined from lifetime spectra using the CONTIN (PALS-2) program. The linearity of the variation of S versus W has been used to check the similarity of (or difference between) the defect structures of the samples prepared under different experimental conditions.

Evaluating structural and microstructural changes of PDMS-SiO2 hybrid materials after sterilization by gamma irradiation.

PDMS-SiO2 hybrid materials obtained by sol-gel process have been extensively studied over the past years due to its promising biomedical applications namely as bone substitutes, catheters, and drug delivery devices. Regardless of the intended biomedical application, all these materials should go through a sterilization process before interfacing with a living structure. However, it is unclear whether they undergo structural and microstructural changes when subjected to sterilization by gamma irradiation. This paper addresses this issue by showing that a sol-gel processed biomaterial based on the PDMS-CaO-SiO2 hybrid system suffers only small structural changes when submitted to a radiation dose of 25kGy, the dose usually recommended to achieve a Sterility Assurance Level of 10(-6) when the natural contamination level and microorganism types cannot be calculated. The characterization was assessed by FT-IR, (29)Si-{(1)H} CP-MAS, thermal analysis (DTG), and SEM.

SANS study of the aging of xTiO2-SiO2 gels

Silica-titania gels, prepared in acidic conditions, with contents of TiO2 up to 6 mol%, have been studied by small angle neutron scattering (SANS) as wet gel with increasing aging times. In all samples, small primary particles have been found with a diameter of about 3 nm. At the gel point, cylindrical clusters, ca. 10 nm long, were observed as well. These grow with aging time, becoming branched. The cluster-cluster diffusion limited aggregation model is found to be consistent with the SANS results. The growth process is independent of the TiO2 content. It is mainly influenced by the ratio of the aging time to the gelation time.

Recovery of phenolic compounds from multi-component solution by a synthesized activated carbon using resorcinol and formaldehyde

The adsorption of four phenolic compounds (gallic acid, protocatechuic acid, vanillic acid and syringic acid) is investigated using a synthesized mesoporous carbon on both single and multi-component synthetic solutions. Some correlation of the adsorption capacity of the carbon and the nature of adsorbate could be made, except for gallic acid whose concentration decrease seems to be not exclusively due to adsorption but also to polymerization reaction. In the multi-component mixture, negative effects in the adsorption capacity are observed probably due to competition for the active centers of the adsorbent surface. In desorption studies, ethanol presents better performance than water and acetonitrile. Vanillic acid is the compound with the higher adsorption and interestingly it is then possible to desorb a relatively high amount of it from the adsorbent, which may represent a possibility for a selective recovery of vanillic acid. These results present a potential way to treat the wastewater from the cork industry.

Porosity Assessment of β-Spodumene/Glass Matrix Composites by Small Angle Neutron Scattering

Small Angle Neutron Scattering (SANS) is used to study the microstructure of spodumene/glass matrix composites prepared by sintering of soda-lime glass powder and sol-gel prepared -spodumene (LiAlSi2O6) up to 25wt.%. The glass matrix reinforcement with spodumene results in fracture toughness, hardness and thermal shock resistance increasing. SANS allows distinguishing micro-pores inside -spodumene agglomerates and large sized pores in the glass matrix that result from incomplete densification during the sintering cycle. It is proved that composites with dispersed phase contents below 15 wt.% are homogeneous, whereas for larger contents, such -spodumene particle aggregates are observed inside the glass. Also, for high spodumene contents, e.g. 25wt.%, the crystallisation of anorthoclase in the glass matrix occurs in parallel with the sintering process. Comparison of porosity and total surface area results obtained by SANS proves that the dimensional changes related to such phase transformation result in surplus macro-porosity and cracking, while micro-porosity does not develop.