Controlled Transformation and Sintering of a Boehmite Sol-Gel by alpha-Alumina Seeding

The transformation, microstructural development, and densification of an α‐alumina‐seeded boehmite sol‐gel was studied. α‐Alumina particles are shown to act as nuclei for the trans‐ formation of θ‐ to α‐alumina and to result in an increase in the transformation kinetics and lowering of the transformation temperature by as much as 170°C. By increasing the seed concentration (i.e., nucleation frequency), a submicrometer aggregate‐free microstructure develops, rather than the vermicular microstructure that usually characterizes the α‐alumina transformation. As a result, the transformed α‐alumina sinters to full density with a submicrometer grain size at 1200°C. It is believed that seeding may represent a unique method for microstructure control in the many ceramic systems that transform by nucleation and growth.