Abstract
Rheological properties of aqueous suspensions (48 vol%) of zircon-alumina mixtures containing different amounts of polyelectrolyte as dispersant were studied. Slip casting in a plaster mold and pressure filtration at 8 MPa experiments were performed to correlate the rheological properties of the suspensions with the relative density of green casts. Flow curves fitted satisfactorily to the Casson model. The Casson viscosity values remained nearly constant whereas Casson yield stress parameter decreased to a minimum and then increased with increasing the amount of dispersant added. The influence of particle size distribution (PSD) of the powder mixtures on Casson yield stress parameter and on the minimum viscosity was also examined. Green densities increased as yield stress Casson parameter decreased. The mixture of alumina and zircon as fine and coarse powders produced bimodal PSD which maximized the green density of the compacts. Some weakly flocculated suspensions having low apparent viscosity and small yield stress produced casts with relatively high densification. This is attributed to a slight higher viscosity at low shear rates of these suspensions in which settling of particles and /or segregation of components can not occur.
Highlights
Reaction sintering of alumina and zircon is a common route to obtain a mullite-zirconia composite
Rheological properties of aqueous suspensions (48 vol%) of zircon-alumina mixtures were determined to study the effects of the amount of polyacrylate added and of the particle size distribution of the different powder compositions
Rheological properties were correlated with green density of compacts obtained by slip casting and pressure filtration at 8 MPa
Summary
Reaction sintering of alumina and zircon is a common route to obtain a mullite-zirconia composite. Several studies showed that materials composed of mullite and zirconia may be obtained using alumina and zircon powders by the traditional method that consisted in: premix milling, drypressing operations to produce the compacts and sintering. A high solid concentration of fine and colloidal particles are needed for slip casting and pressure filtration processes, but currently these slips are difficult to cast. With large particles (or hard-sphere systems) the colloidal interparticle forces may be ignored and the contribution of hydrodynamic effects to rheology depend on their volume concentration, shape, size, relative orientation and position[3,4,5]. The particle size distribution (PSD) is an important parameter for these concentrated suspensions, acting mainly on the maximum solid volume fraction.
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