Swelling of Hot‐Pressed A1 2 O 3

Fully dense aluminas, prepared by hot‐pressing, were found to swell during annealing at 1600°C in air, but not during annealing in a reducing atmosphere (po 2 = 10 ‐7 Pa). The reaction followed the relation p ‐ po = ‐K log t, where p o and p are the initial and final densities, respectively, t is the time, and AT is a constant. The rate of swelling was enhanced by MgO solute. The reduction in density resulted from the nucleation and growth of grain‐boundary pores. Pore formation was attributed to the reaction of carbon and sulfur impurities at the boundaries with oxygen, which had diffused down the grain boundaries from the ambient, to form CO/CO 2 and SO 2 gas at high pressures. Preliminary results indicate that this reaction can be avoided by preannealing powders in flowing oxygen prior to hot‐pressing. The consequences of internal gas‐forming reactions to other processes such as high‐temperature creep and sintering are also discussed.