Binary Nucleation Along Thermodynamically Unfavored Paths

Barbara Wyslouzil是由理论和计算化学计划提供的职业资助，沿着热力学上不受欢迎的路径进行二元成核的实验研究。热力学上有利于成核的路径穿过自由能表面，穿过自由能最低的鞍区。然而，单体撞击率和自由能表面形状的综合作用可以在可混溶的二元体系中驱动通量越过脊线。在部分混溶体系中，通量也可以通过具有较高自由能的鞍座。在这两种情况下，基于鞍形交叉的分析理论都无法预测正确的形核率行为。拟议的工作有两个目标：1)确定真实的化学体系和条件，其中二元成核主要沿着热力学上不受欢迎的路径进行；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一个双脉冲膨胀云室来进行成核速率测量。对小室中粒子形成过程的仔细模拟，加上对临界核组成的了解，应该可以区分成核是否遵循热力学上有利的路径。在许多工程领域中，有意增强或抑制热力学有利相的形成是很重要的。例如，在材料科学中，从挥发性前体通过气相成核来生产亚微米陶瓷颗粒的工艺可以导致材料的烧结温度低于传统生产的陶瓷粉末。在化学加工工业中，成核、生长和再结晶的速度影响着糖、盐等大宗产品和沸石等特种产品的纯度、质量和销售价值。在冶金学中，如果在没有晶相形核的情况下实现大的过冷度，就可能产生不同的和潜在有用的金属形式。对成核的更好的分子水平的了解可能会对大量制造领域的化学加工产生潜在的巨大影响。

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.