Influence of the amount of Na2O and SiO2 on the sintering behavior and on the microstructural evolution of a Bayer alumina powder

The target of this work is to investigate the effects of the relative amounts of the main impurities usually encountered in an alumina powder obtained from the Bayer process. On that purpose, the amounts of Na2O and SiO2 present in a commercial powder, have been modified by doping. The sintering behavior and the microstructure of the sintered ceramics have been investigated. The ratio Na2O/SiO2 strongly influences the shrinkage rates during sintering, the final density and the microstructure. From the commercial powder used (P172SB, Aluminium Pechiney Company, Alcan Group, France), it is shown that an increase in Na2O content significantly slows down the densification and also the grain growth. The effect of an increase in SiO2 additions is less obvious even if the dilatometric curves also show decreases in the sintering rates, particularly in the temperature range 1200-1400 °C. Contrary to Na2O, the addition of SiO2 leads to abnormal grain growth and provokes an increase in the aspect ratio of the grains.