Phase transitions in fluctuations and their role in two-step nucleation
Phase transitions in fluctuations and their role in two-step nucleation
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DOI：
10.1063/1.5057429
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发表时间：
2019-02-21
影响因子：
4.4
通讯作者：
Poole, Peter H.
中科院分区：
文献类型：
--
作者：
James, Daniella;Beairsto, Seamus;Poole, Peter H.
我们考虑在稳定或亚稳定的体相中发生的局部涨落的热力学行为。对于一个系统有三个或更多的阶段，我们提出了一个简单的分析，基于经典的成核理论，预测热力学条件下，小波动类似的相具有最低的表面张力与周围的体相，即使这个阶段不具有较低的化学势。我们还确定了随着波动大小的增加，波动可能转换为不同相位的条件，这里称为“波动相变”（FPT）。我们证明了这些现象在模拟的二维晶格模型通过评估的自由能表面，描述了作为其大小和相组成的函数的波动的热力学性质。我们表明，FPT可以发生在一个稳定的或亚稳的体相的波动和过渡是一阶的。我们还发现，FPT是括号内的定义明确的旋节线，这将限制不同阶段的波动的大小。此外，当FPT发生在一个亚稳体相中，我们表明，FPT的叠加上的成核过程中的两步成核（TSN）的结果。我们确定不同的制度的TSN的基础上的成核途径在自由能表面和相关的这些制度的体系统的相图。我们的研究结果澄清了TSN的起源，并阐明了各种各样的现象与TSN，包括奥斯特瓦尔德步骤规则。由AIP Publishing授权出版。
We consider the thermodynamic behavior of local fluctuations occurring in a stable or metastable bulk phase. For a system with three or more phases, we present a simple analysis based on classical nucleation theory that predicts thermodynamic conditions at which small fluctuations resemble the phase having the lowest surface tension with the surrounding bulk phase, even if this phase does not have a lower chemical potential. We also identify the conditions at which a fluctuation may convert to a different phase as its size increases, referred to here as a "fluctuation phase transition" (FPT). We demonstrate these phenomena in simulations of a two dimensional lattice model by evaluating the free energy surface that describes the thermodynamic properties of a fluctuation as a function of its size and phase composition. We show that a FPT can occur in the fluctuations of either a stable or metastable bulk phase and that the transition is first-order. We also find that the FPT is bracketed by well-defined spinodals, which place limits on the size of fluctuations of distinct phases. Furthermore, when the FPT occurs in a metastable bulk phase, we show that the superposition of the FPT on the nucleation process results in two-step nucleation (TSN). We identify distinct regimes of TSN based on the nucleation pathway in the free energy surface and correlate these regimes to the phase diagram of the bulk system. Our results clarify the origin of TSN and elucidate a wide variety of phenomena associated with TSN, including the Ostwald step rule. Published under license by AIP Publishing.