Molecular and colloidal engineering of ceramics

During the last decade, significant advances have been made in the processing of ceramics by a combination of techniques utilizing molecular precursors and colloids for powder consolidation. Powder consolidation methods have mainly dealt with the formation of unagglomerated powders in the size range of 0·1-1 μm, the preparation of colloidal suspensions that are suitable for the formation of high density compacts by filtration and/or plastic forming techniques, the removal of the processing aids, and the role of consolidation methods on microstructural evolution. In contrast, the molecular and/or sol-gel techniques dealt with processing at a finer dimensional scale of 10-1000 Å with either molecularly homogenous precursors or nanometer-sized particulates that are used in the preparation of gels that display linear viscoelastic behavior. Similar to green compacts of micron-sized powders, these gels are then converted to dense ceramics by heat treatment. This review summarizes the concepts that are common to both of these regimes and points to the synergistic benefits of coupling molecular precursors with colloids in a process path. The emphasis is on the control of the structure of a final product at scale lengths ranging from molecular to micro- and macroscopic dimensions.