Binary Nucleation Along Thermodynamically Unfavored Paths

芭芭拉Wyslouzil是由职业生涯补助金的支持，从理论和 计算化学程序进行实验研究的二进制 沿着结晶不利的路径成核沿着。 的 成核的热力学有利路径穿过自由能 表面通过鞍区的最低自由能。 但是，在这方面， 单体冲击速率和聚合物的形状的组合效应 自由能面可以驱动通量越过可混溶双星的脊 系统 通量也可以通过较高的驱动力驱动通过鞍座。 部分混溶体系的自由能 在这两种情况下， 基于鞍形交叉的分析理论将无法预测 正确的成核速率行为。 拟议的工作有两个目标：1） 识别二元成核真实的化学系统和条件 主要沿着一条不受欢迎的道路前进; 2） 设计和进行实验，清楚地证明二元成核 可以以这种方式发生。 该项目的实验部分将使用 双脉冲膨胀云室， 测量. 仔细模拟颗粒形成过程， 室，连同知识的组成的关键 核，应该能够区分成核是否具有 遵循的是一条受人青睐的道路。故意增强或抑制一个人的形成， 结晶有利相在许多领域都很重要， 工程. 例如，在材料科学中， 亚微米陶瓷颗粒通过气相成核从挥发物 前体可以导致材料具有较低的烧结温度， 常规生产的陶瓷粉末。 在化学处理中 形核、生长和再结晶的速率影响着 大宗产品的纯度、质量和销售价值，如糖 和盐以及特殊产品如沸石。 在冶金学中， 如果大的话，可以生产不同的和潜在有用的金属形式， 在没有晶相成核的情况下实现过冷。 一 更好的分子水平的理解的成核可以有一个 对大量化学品的加工产生潜在的巨大影响， 制造业领域。

Barbara Wyslouzil is supported by a CAREER grant from the Theoretical and Computational Chemistry Program to perform experimental studies of binary nucleation along thermodynamically unfavored paths. The thermodynamically favored path for nucleation crosses the free energy surface through the saddle region with the lowest free energy. However, the combined effect of the monomer impingement rates and the shape of the free energy surface can drive the flux over a ridge in a miscible binary system. The flux can also be driven through the saddle with the higher free energy in a partially miscible system. In both cases, the analytical theories based on a saddle crossing will not predict the correct nucleation rate behavior. The proposed work has two goals: 1) to identify real chemical systems and conditions where binary nucleation proceeds primarily along a thermodynamically unfavored path; and 2) to design and conduct experiments that clearly demonstrate binary nucleation can occur in this manner. The experimental part of this project will use a two pulse expansion cloud chamber to conduct nucleation rate measurements. Careful modeling of particle formation processes in the chamber, together with knowledge of the composition of the critical nucleus, should make it possible to distinguish whether nucleation has followed the thermodynamically favored path. The deliberate enhancement or suppression of the formation of a thermodynamically favored phase is important in many fields of engineering. In material science for example, processes which produce submicron ceramic particles by vapor phase nucleation from a volatile precursor can lead to materials with lower sintering temperatures over conventionally produced ceramic powders. In the chemical processing industry the rates of nucleation, growth and recrystallization affect the purity, quality, and sales value of bulk products such as common as sugar and salt as well as specialty products such as zeolites. In metallurgy, different and potentially useful forms of metals may be produced if large undercoolings are achieved without nucleation of a crystal phase. A better molecular level understanding of the nucleation could have a potentially large impact on chemical processing in a large number of manufacturing areas.