Shape forming alumina slurries via Colloidal IsoPressing

The relative density, deformation behavior, and dynamic rheology of bodies consolidated from aqueous alumina slurries, formulated at pH 4 and containing NH4Cl, were studied as a function of the salt concentration (0.1–0.75 M). The smallest salt concentration produced the highest relative density (0.596), which was independent of consolidation pressure. Nearly identical relative densities were obtained for the other salt concentrations, ranging from 0.57 to 0.59 for consolidation pressures of 5–100 MPa, respectively. The consolidated bodies exhibited either a plastic or an elastic behavior when tested in compression. It was observed that the transition from a plastic to an elastic behavior occurred at a critical consolidation pressure. This critical consolidation pressure was directly dependent on the salt concentration. The storage (elastic) modulus for the fluidized, plastic bodies increased with salt concentration. The storage modulus was also observed to be dependent on the time the bodies were allowed to rest after they were subjected to a high shear rate. A pre-consolidated and fluidized slurry formulated to contain 0.5 NH4Cl was extruded into a rubber mold cavity and further consolidated by isopressing at 200 MPa for 2 min. The body not only increased its relative density (0.58–0.62), but it also became elastic to enable its removal from the rubber cavity without shape distortion. No linear shrinkage was detected after the isopressed bodies were dried. Because of the lack of shrinkage, rapid drying was possible. For example, bar specimens (6.4 mm × 12.7 mm × 60 mm) would survive a 34 °C/min heating rate to 300 °C; the specimen was completely dried within 20 min. Because shrinkage does not occur during drying, bodies could be placed directly into a furnace and heated to 1500 °C for 0.5 h to achieve a relative density of >0.99 and an average grain size of 1.6 μm.