New gel-casting process for alumina ceramics based on gelation of alginate

Sodium alginate—a natural innoxious polymer, which can be dissolved in cold water by mechanical stirring and in which irreversible gelation takes place when enough calcium ions are added—is applied for in situ forming process of ceramics by using chelator to control its gelling behavior. In the present work, the viscosity of sodium alginate solution and factors influencing on it were analyzed. How to control the gelation behavior was discussed. The rheology property of the slurry was examined. An appropriate method to apply the gelation of sodium alginate in order to fabricate alumina ceramic green body was systematically investigated. The viscosity of alumina slurry with 50 vol.% Al2O3 and 1.8 vol.% Ca3(PO4)2 dispersed by 1 wt.% sodium alginate solution is less than 400 MPa s when the shear rate is higher than 50 s−1. The green body has uniform structure and smooth surface, and bending strength is about 8 MPa. After being sintered at 1550 °C for 2 h the linear shrinkage of ceramics composed of 95% alumina is about 15.7% during firing and the relative density of the final product is about 98.7%. Bending strength reaches 320 MPa. SEM images show compact and uniform microstructure.