Metastability and microstructure evolution in the synthesis of inorganics from precursors

The microstructure evolution of inorganic materials synthesized by pyrolytic decomposition of precursors often occurs through multi-stage transformation paths which involve different forms and levels of metastability, e.g. nanocrystallinity, formation of alternate crystalline or amorphous phases, and solubility extension. These effects are discussed with examples from recent work on ZrO2 and Al2O3 combined with each other or with one of the following: 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 phenomena share a common conceptual base with similar forms of metastability produced by technologies like rapid solidification and vapor deposition. A thermodynamic foundation, which would be common across technologies, is developed and used to assess the role of kinetics in phase selection and microstructure 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. Kinetic constraints which bias the phase selection away from the energetically most favored structure are usually the result of a requirement for ordering, complex atomic rearrangement, or partitioning during crystallization. The excess chemical energy stored in the system when a metastable phase is selected can lead to undesirable effects during the subsequent transformations, e.g. exacerbated grain coarsening, which hinder microstructural control in technologically important thin film, fiber, and particulate systems. (C) 1998 Acta Metallurgica Inc.