Papers by Author: M. Fatima Vaz

Abstract: We addressed the importance of defining a mechanical testing methodology for the compression of human trabecular bone specimens. In fact, currently there are several protocols to test trabecular bone, but a single, standard and validate method has not been accepted yet. In our work, human femoral epiphyses collected from patients with osteoporosis (fragility fractures) and hip osteoarthritis, submitted to total hip replacement surgery, were used. The aims of our work were to develop a mechanical testing methodology for the compression of trabecular bone specimens taking into account the optimization of bone extrinsic and intrinsic variables, in order to establish a patient bone sample database with clinical, structural and mechanical information. Extrinsic variables, such as the effect of specimen preparation, with particular focus on the dimensions of test specimens, and others associated with the compression test, such as the method employed to determine specimen deformation, and hence strain, were evaluated. Also, a new device used to withhold the specimens was developed and tested by the present authors. Although each specimen showed a unique behaviour, even when comparing compression curves between patients from the same disease group, implicating additional complexity and difficulty in the data analysis, the authors managed to assemble the results in two groups related with a possible difference in the deformation mechanisms occurring after yielding.

Abstract: Reticulated cordierite foams produced by a direct foaming method were successfully washcoated with platinum-based zeolite catalysts. For comparison purposes, commercial cordierite monoliths were also washcoated. The effect of the structural properties on the fluid dynamics and catalytic behaviour for the toluene combustion were evaluated. Foam supports revealed highest performances, in terms of conversion into CO2, when compared to conventional honeycomb monoliths. The experimental results suggest that the catalytic behaviour is critically dependent on the fluid dynamics provided by structural characteristics of the supports, such as porosity, density and size of pores. The randomness and tortuosity of foams enhance reactant mixing, as it was evidenced by the higher axial and radial dispersions of the gas flow across the foam structure. This leads to better mass and heat transfers in the reaction system, thus improving the catalytic behaviour.

Abstract: The structure and morphological aspects of highly porous (higher than 90%) cordierite (Mg2Al4Si5O18) foams, prepared by a direct foaming method, have been evaluated by Scanning Electron Microscopy analysis. The resulting ceramic foams consisted of a three-dimensional array of struts forming a well-defined open-cell structure. This type of structure seems very attractive for catalyst support purposes. Attempts have been made in order to control the pore structure since it directly affects the physical properties, namely the mechanical strength. In this respect, the use of a dip coating method to improve the strength of the resulting foams was found to be effective in reducing defects (e.g. pores, flaws) in the struts. Based on image analysis, estimated mean cell sizes were about 550 µm whereas strut thicknesses varied in the range of 60-70 µm. The compressive strength of the developed foams increased by one order of magnitude (0.1 to 1 MPa) by increasing the relative density from 0.06 to 0.18.

Abstract: Portuguese ceramic tiles of different origins from XVIth century to XXth century were studied. In this work we describe microstructural, mechanical and mineralogical characterisation and water absorption studies. Microstructural features (pore size) were determined using Scanning Electron Microscope (SEM) photographs. Mechanical tests (four point bending) were performed and the bending strength was determined from the fracture loads. From water absorption essays the absorption coefficient and the total amount of water retained were obtained and the open porosity was estimated. The maximum water absorbed tends to decrease from XVIth century until XXth century, showing a more pronounced drop for the tiles of the XIXth and XXth centuries. This evolution is also observed in the open porosity values. From image analysis data we also observed that porosity decreases progressively with time and an important drop is observed for XXth century tiles. The bending resistance of tiles from XVIIth to XXth century is almost constant and higher than that of XVIth century samples. This evolution does not correlate with porosity. Advances in ceramic processing, such as higher firing temperatures may lead to the appearance of quartz as almost the only crystalline phase. We consider that an increase in the bending strength is due to a more effective vitrification and a lower porosity.