The transformations of boehmite gels during heat treatments at temperatures up to 1150°C have been studied using transmission electron microscopy. Specific attention is directed to the evolution of porosity during the transformations, with isolated small pores seen in λ-Al2O3, aligned small pores in θ-Al2O3, and large cylindrical pores in α-Al2O3. The results are compared with the previous observations of Wilson and Stacey of the transformations which occur on heating boehmite monocrystals. The observed evolution of porosity is related to the results of Kumagai and Messing on the effects of seeding alumina gels with small α-Al2O3 particles, where dense α-Al2O3 structures were obtained at 1200°C. It is suggested that such seeding avoids the formation of wormy crystalline structures with large intragranular pores. Also discussed is the approach of developing alumina sols which will form smectic structures on approaching the leatherhard point during drying of the gels.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Ceramics and Composites
- Condensed Matter Physics
- Materials Chemistry