Some significant advances in sol-gel processing of dense structural ceramics

Abstract

This review deals with the solution-sol-gel processing of some structural ceramics such as alumina, zirconia, mullite and cordierite and brings out the most significant advances in the preparation of dense ceramics. In the Al2O3 system, seeding of gels with α-Al2O3 and other isostructural seeds led not only to lower crystallization temperature but also enhanced densification with refined microstructure through solid-state epitaxy. This breakthrough of seeding has led to improved abrasive grains with large commercial market. Although highly dense, partially stabilized zirconia ceramics were prepared using monodisperse and spherical or nanophase zirconia sol-gel powders, no commercial applications seem to have been realized thus far. In the Al2O3−SiO2 system, compositionally different sol-gel nanocomposites (diphasic gels) led to enhanced densification of mullite at lower temperatures because of the occurrence of densification and crystallization processes almost simultaneously. Mullite powders derived from the diphasic gel route are a breakthrough and are now commercially marketed by Chichibu Cement Company. Highly dense cordierite was prepared by using three sols, i.e., the compositionally different sol-gel nanocomposites in an analogous manner to that of mullite. Although this process appears to be highly cost-effective, especially for the fabrication of substrates, it has not yet been utilized for commercial applications. The use of sol-gel nanocomposites in the processing of various ceramics is expected to be fully exploited in the future.