Papers by Author: Maria Elisabete V. Costa

Abstract: The adsorption of the drug 5-Flourouracil (5-FU) on Hydroxyapatite (HAP) particles having different surface conditions is studied at 37°C. Commercial HAP particles, either as received or soaked in citrate solutions are used as drug adsorbents and compared to a third category of HAP particles which consists of granules obtained by spray drying suspensions of HAP nanoparticles previously precipitated in citrate solutions. The adsorbed amount of 5-FU on the HAP particles surface is conditioned by the existing pre-adsorbed citrate ions which contribute to modify the particle surface charge and thus decrease the drug adsorption. However the obtained results also demonstrate that the particle surface charge is not the unique factor to govern the 5-FU adsorption on the HAP particles surface.

Abstract: Porous chitosan/brushite composite scaffolds were prepared by a freeze-drying technique, starting from brushite suspensions in chitosan solutions. The obtained scaffolds showed a regular macroporous and interconnected structure with brushite particles uniformly distributed in the chitosan matrix. The variation of the brushite concentration affected the microstructure of the final freeze-dried scaffold, in particular, its porosity and its average pore size. The yield strengths of the composite scaffolds could also be improved by the increase of the brushite content.

Abstract: To grow single crystals within the solid solutions between PbZrO3 and PbTiO3, PbZrxTi1- xO3 or PZT, is very difficult when compositions are around the morphotropic phase boundary (MPB). In this work, the PZT single crystals were grown by the high temperature solution method, using fluxes with a constant composition, different thermal profiles and different flux/PZT ratios. This approach was successful used to obtain PZT single crystals with compositions near the MPB, of 2 × 2 × 2 mm3 size and without second phases. The specific experimental conditions to obtain these crystals are analysed and discussed.

Abstract: A quantitative model for the calculation of spontaneous polarization of BLSF ceramics as a function of the degree of texture is presented using a general formalism based on the texture analysis via an orientation distribution function. The March-Dollase equation was selected to fit the measured texture distribution because its fitting parameters can be related to experimentally measurable stereological values obtained from SEM images. The results are applied to the SrBi2Ta2O9 (SBT) system, which is a well-known member of the BLSF family. The textured SBT ceramics were fabricated by templated grain growth (TGG) and the microstructure and degree of texture were evaluated by SEM analysis, which allowed correlating the texture development to the sintering parameters. Enhanced ferroelectric properties were measured perpendicularly to the uniaxial pressing direction of the ceramics, revealing the influence of the grains orientation, anisotropy and volume fraction of textured material on the ferroelectric properties. Finally, the predicted values for the spontaneous polarization as a function of the degree of texture were compared with those measured from the hysteresis loops.