Direct coagulation casting via increasing ionic strength

The influences of the Idle Time on the microstructures and the mechanical properties of green bodies are described in the present paper. The green bodies were obtained from destabilizing high solids loaded alumina slurries by increasing the ionic strength through an internally catalyzed reaction. We observed that this method induced vertically laminated cracks in coagulated green bodies after casting. The cracks were attributed to the shrinkage of the particle network (syneresis) during coagulation. This process in combination with sedimentation during the reaction effected the vertical cracks, which was not observed when a certain Idle Time before casting was introduced. The Idle Time diminished the final shrinkage still having a castable slurry of reasonably low viscosity. With increasing Idle Time before casting, the failure distance between the vertically directed cracks within the microstructure of the green body became gradually wider. Moreover, the prolongation of the Idle Time distinctively decreased the occurrence of the cracks within the green body and finally disappeared. In this study the microstructure of the green body became dense and homogeneous when the Idle Time was around 150 mins (corresponding to 0.4 mol/l of added ammonium carbonate).