Porous Alumina Ceramics Prepared by the Hydrolysis-Assisted Solidification Method

Abstract

We used the hydrolysis-assisted solidification (HAS) process to form the alumina green bodies from the suspensions. A total of 2.5 wt% of AlN powder, with respect to the solids content, was added to an aqueous alumina suspension containing 53.5 vol% of solids. During the hydrolysis of the AlN powder, the aluminum hydroxide reaction product precipitated on the surface of the alumina particles. The effect of the precipitated aluminum hydroxide on the density and the bending strength, after sintering the alumina samples prepared by the HAS process at temperatures up to 1300°C, was investigated. In comparison with conventionally formed samples, a similar strength was measured at a lower sintered density for the partially sintered alumina prepared by the HAS process. These improved properties could be ascribed to the formation of aluminum hydroxide in the form of boehmite lamellas on the surface of the particles during the HAS consolidation process. Boehmite lamellas topotactically transform with temperature into various transition aluminas; however, the change in the crystal structure did not affect the morphology, which remained the same up to 1100°C. These lamellas hindered the densification of the samples up to 1100°C, but at higher temperatures, the lamellas transformed into nanosized α-alumina particles, which enhanced the neck growth, resulting in a very high measured flexural strength at sintered densities up to 70%.