INVESTIGATION OF THE SINTERING BEHAVIOUR OF ALUMINA POWDERS CONTAINING LOW AMOUNTS OF NA_2O AND SIO_2

摘要

The initial target of this work was to investigate the effect on sintering behaviour and on the finale microstructure of the main impurities present in alumina powders obtained by Bayer process. Therefore, the amounts of Na_2O and SiO_2 of a commercial powder were modified by doping on purpose. This work shown that the sintering mechanism is controlled by the low amount of intergranular vitreous phase formed from the impurities. Consequently, the ratio Na_2O/SiO_2 strongly influences the properties of this phase and then the shrinkage during sintering, the final density and microstructure. To closer correlate the properties of the intergranular phase with the sintering results, another study is currently performed. A very pure alumina powder (99.998 %) was chosen to be doped on purpose with Na_2O, SiO_2 or mixtures Na_2O + SiO_2. The first point investigated has been the selection of the process for green shaping. Slip casting on specific alumina mould has been developed in order to limit the risk of pollution and to obtain high green densities with small pore what is favourable to the densification during sintering. The first tests of doping show a significant decrease of the densification rate with the increase in SiO_2.