Theoretical Investigation of Diffusional Phase Transformations and the Possibility of Stable Nanoscale Structures in Ionic Ceramics

9311898 Chen A theoretical investigation will be carried out on the thermodynamics and kinetics of continuous phase transformations and on the possibility of stable nanoscale structures in multicomponent ceramic materials. The main objective is to develop theories and computer simulation techniques for understanding the fundamental thermodynamic and kinetic origins leading to the formation of an important class of self-assembled nanocomposites which resist continuous coarsening. To reach this objective, systematic phase stability analysis and phase diagram calculations for ionic systems with stable nanoscale structures will be performed. The key structural and thermodynamic factors leading to the formation of thermodynamically stable nanoscale phases will be identified. Extensive computer modeling will be conducted of the kinetics of atomic ordering, compositional clustering and microstructural coarsening in systems involving both short-range chemical and long- range Coulomb interactions. A computer simulation technique will be developed which can describe the coarsening dynamics of domains interacting with each other through electric dipole-dipole interactions, elastic interactions and Coulomb interactions. The focus will be on the diffusional transformations in the self- assembled relaxor nanocomposites. The relation of atomic ordering and compositional clustering tendencies to the atomic sizes and charge valences of cations will be investigated through a combination of modern statistical mechanics theory of order- disorder transformations and atomistic computer simulations using shell-model interatomic potentials. %%% This theoretical research grant involves both analytical theory and computer simulation to study the properties of ceramic materials which undergo changes of phase into materials having novel structures. The research will identify the conditions which control the formation of these structures during the phase change. The research is com plicated by the fact that the materials are ionic. This causes additional forces over long distances which are difficult to handle theoretically. However, the results of this work will have important ramifications for understanding and processing these technologically important materials. ***