Potential and limitations in the application of solution sol-gel method to ceramic processing.

摘要

Because of the inherently irreversible nature of the solution-sol-gel method, the properties of ceramics produced this way are highly dependent on the path followed in their preparation. The greatest potential of the method lies not with the preparation of materials currently available by other means, but rather in the preparation of new materials. These ideas are demonstrated through the preparation of different materials. Alpha aluminum oxide was prepared from alumina gels and the control of microstructure and density by the technique of seeding demonstrated. Microstructure and densification are shown to depend upon the epitaxial or heteroepitaxial nucleation of the stable phase. The one known failure of an isotypic phase, alpha chromium oxide, to seed the formation of alpha alumina at low concentration is shown to result from reaction with the metastable alumina with the loss of the precursor structure type. Spinel/alumina composites showing unusual microstructural development were prepared by doping with magnesium and lithium salts. The utility of nanoporous gel derived silica for the solution of a current materials problem is demonstrated with the preparation of a material having a relative dielectric permittivity of less than two. The dielectric properties of the porous silica nanocomposites are shown to be consistent with Lichtenecker's mixing rule for composites with mixed connectivity patterns between the phases. Finally the dependence of the precise path of relaxation to lower free energy configurations is shown to be dependent upon the method and kinetics of gelation with the preparation of gels of the composition of cordierite. These materials show the presence of a liquidus at temperatures well below the equilibrium melting point of the crystalline composition. The kinetics of crystallization and the path followed in the crystallization process is found to be sensitive functions of the method of gel formation. Seeding for control of phase formation as well as microstructure was also found effective in this system. The common theme throughout these examples is that through control of the path of structural and compositional relaxation to lower free energies, new and different materials can be prepared.