Metastability and microstructure evolution in the synthesis of inorganics from precursors

The microstructure evolution of inorganic materials synthesized by pyrolyti c decomposition of precursors often occurs through multi-stage transformati on paths which involve different forms and levels of metastability, e.g. na nocrystallinity, formation of alternase crystalline or amorphous phases, an d solubility extension. These effects are discussed with examples from rece nt work on ZrO2 and Al2O3 combined with each other or with one of the follo wing: MgO, Fe2O3, Y2O3, Gd2O3, and PbO+TiO2. Pyrolysis typically occurs at low homologous temperatures, where long range diffusion is constrained, and hence many of the relevant transformations are partitionless. The phenomen a share a common conceptual base with similar forms of metastability produc ed by technologies like rapid solidification and vapor deposition. A thermo dynamic foundation, which would be common across technologies, is developed and used to assess the role of kinetics in phase selection and microstruct ure evolution in the cited oxide systems. Phase hierarchy maps are derived from phase diagrams and used to represent the menu of possible phases, and their relative stability, as a function of temperature and composition. Kin etic constraints which bias the phase selection away from the energetically most favored structure are usually the result of a requirement for orderin g, complex atomic rearrangement, or partitioning during crystallization. Th e excess chemical energy stored in the system when a metastable phase is se lected can lead to undesirable effects during the subsequent transformation s, e.g. exacerbated grain coarsening, which hinder microstructural control in technologically important thin him, fiber, and particulate systems.