Influence of magnesia on colloidal processing of alumina

Abstract

The effects of magnesia (MgO) on electrophoretic and rheological properties of aqueous alumina suspensions and on the characteristics of the slip cast bodies were evaluated. The influence of the dispersants and slip preparation procedure on all processing steps and on the ultimate properties of the sintered bodies was also discussed. The electrophoretic measurements showed a significant decrease in zeta potential when MgO was present in Al 2O 3 aqueous suspension. This destabilising effect of MgO was also confirmed by rheological measurements which showed an increase in viscosity and pseudoplastic behaviour of the slips. In slip casting, the destabilising effect was expressed by a decrease in green body density. Nevertheless, the relative density of the sintered bodies initially increased, reaching a maximum (99.7%) for 0.02 wt% MgO but decreased again with further additions. The results obtained showed that the optimal amount of magnesia resulted from its beneficial effect on densification of alumina, and from its detrimental influence in slip stability and packing behaviour of suspended particles. The slip preparation procedure also affected the ultimate properties of MgO-doped alumina. Namely, the dispersant used, and the order in which the powders were added to the dispersant solution determine the degree of uniformity of the MgO distribution in the mixture, resulting in different packing densities in both green and sintered states. The highest sintered density was obtained when magnesia was the first component added to the solution of the most efficient dispersant used. Considering the observed results, an optimised route for the slip preparation was established.