Chemical and Biomolecular Engineering - Theses
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ItemA new paradigm in near-net-shape advanced ceramic components processing: development of the novel processing technique( 2011)There are two general ways of processing ceramic materials namely dry and colloidal (wet) processing technique. The colloidal processing route has been widely studied and known to produce better ceramic articles. However, the process of removing the solvents through drying by heating them at high temperature is highly energy intensive (especially if the solvent is water). This thesis will discuss an alternative route using high vapour pressure organic solvents to speed up the solvent removal process. This route involves the reduction in pressure (by applying vacuum) to promote drying as oppose to heating. The surface of alumina which is inherently hydrophilic was made hydrophobic through alkylation reaction with alcohols (chem-adsorbed suspensions) or addition of polymers (phys-adsorbed suspensions) such as Hypermer A70 or Solsperse 3000. The purpose of manipulating the surface chemistry was to enable the dispersion of alumina in the organic solvents such as dodecane, cyclohexane or pentane. The viscosity of suspensions was found to decrease with the increase in the length of the stabilising molecules. This was consistent with the calculated reduction in the inter-particles attraction as the extent of the stabilising molecules increases. A high solid loading of alumina (50%vol) in dodecane using Hypermer A70 (3%wt) was achieved while maintaining a low viscosity (0.25 Pa.s at 100s-1). The observed rheological behaviour fitted the Quemada viscosity model quite well when the effective volume of the polymer was accounted for, which indicated the absence of depletion attraction due to the unadsorbed polymer. The optimum concentration of Hypermer A70 was determined using UV-Visible Spectroscopy technique and was found to be 2.8%wt. The vacuum moulding process produced an interesting and intricate formation of macroscopic voids or channels instead of cracks inside the green body upon the removal of pentane. The fraction of the void space was found to decrease linearly with the increase in the initial solids concentration. By extrapolating this relationship, it was shown that the void space should disappear when the initial solid concentration of the suspension was 62.3% vol. Density measurements performed on the denser sections of the green body showed that the density values were very close to the maximum close packing of spheres (on average 60.7% of the theoretical density). This result is also consistent with the data from the air-driven filtration where the volume fraction of the cake (in essence, the green body) was found to be 64.1%, when the suspension was consolidated by the maximum capillary pressure. The close proximity of these values suggests that they are self-consistent and the dense part is consolidated by the capillary pressure at the same time as the formation of channels. The formation of these voids or channels was thought to be a drying-rate dependent process. It was possible to achieve the green and sintered densities of up to 65.7% and 97.1% theoretical density, respectively, when the rate of removal was slowed down.