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

对连续相变的热力学和动力学以及多组分陶瓷材料中稳定纳米级结构的可能性进行理论研究。主要目标是发展理论和计算机模拟技术，以了解基本的热力学和动力学起源，从而形成一类重要的自组装纳米复合材料，这些纳米复合材料可以抵抗连续粗化。为了达到这一目标，将对具有稳定纳米级结构的离子体系进行系统的相稳定性分析和相图计算。关键的结构和热力学因素导致形成热力学稳定的纳米级相将被确定。广泛的计算机模拟将在涉及短程化学和远程库仑相互作用的系统中进行原子有序、组成聚类和微观结构粗化的动力学。本文将发展一种计算机模拟技术来描述通过电偶极子-偶极子相互作用、弹性相互作用和库仑相互作用相互作用的畴的粗化动力学。重点将放在自组装弛豫纳米复合材料的扩散变换上。本文将结合有序-无序转换的现代统计力学理论和使用壳模型原子间势的原子计算机模拟，研究原子有序和组成聚类倾向与阳离子的原子尺寸和电荷价的关系。这项理论研究资助包括分析理论和计算机模拟，以研究陶瓷材料的性质，这些材料经过相的变化，具有新的结构。该研究将确定在相变过程中控制这些结构形成的条件。由于材料是离子型的，这项研究变得复杂了。这会在长距离上产生额外的力，这在理论上是难以处理的。然而，这项工作的结果将对理解和处理这些技术上重要的材料产生重要的影响。＊＊＊

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. ***