Department of Chemistry, University of Kansas, Lawrence, Kansas 66045, USA.
J Chem Phys. 2016 Dec 7;145(21):211914. doi: 10.1063/1.4962424.
In this work, we examine the effect of surface structure on the heterogeneous nucleation of Pb crystals from the melt at a Cu substrate using molecular-dynamics (MD) simulation. In a previous work [Palafox-Hernandez et al., Acta Mater. 59, 3137 (2011)] studying the Cu/Pb solid-liquid interface with MD simulation, we observed that the structure of the Cu(111) and Cu(100) interfaces was significantly different at 625 K, just above the Pb melting temperature (618 K for the model). The Cu(100) interface exhibited significant surface alloying in the crystal plane in contact with the melt. In contrast, no surface alloying was seen at the Cu(111) interface; however, a prefreezing layer of crystalline Pb, 2-3 atomic planes thick and slightly compressed relative to bulk Pb crystal, was observed to form at the interface. We observe that at the Cu(111) interface the prefreezing layer is no longer present at 750 K, but surface alloying in the Cu(100) interface persists. In a series of undercooling MD simulations, heterogeneous nucleation of fcc Pb is observed at the Cu(111) interface within the simulation time (5 ns) at 592 K-a 26 K undercooling. Nucleation and growth at Cu(111) proceeded layerwise with a nearly planar critical nucleus. Quantitative analysis yielded heterogeneous nucleation barriers that are more than two orders of magnitude smaller than the predicted homogeneous nucleation barriers from classical nucleation theory. Nucleation was considerably more difficult on the Cu(100) surface-alloyed substrate. An undercooling of approximately 170 K was necessary to observe nucleation at this interface within the simulation time. From qualitative observation, the critical nucleus showed a contact angle with the Cu(100) surface of over 90°, indicating poor wetting of the Cu(100) surface by the nucleating phase, which according to classical heterogeneous nucleation theory provides an explanation of the large undercooling necessary to nucleate on the Cu(100) surface, relative to Cu(111), whose surface is more similar to the nucleating phase due to the presence of the prefreezing layer.
在这项工作中，我们使用分子动力学（MD）模拟研究了在 Cu 衬底上从熔体异质形核 Pb 晶体的表面结构的影响。在之前的研究中[Palafox-Hernandez 等人， Acta Mater. 59, 3137（2011）]，我们通过 MD 模拟研究了 Cu/Pb 固-液界面，观察到在 625 K 时，高于 Pb 熔点（该模型为 618 K）时，Cu(111)和 Cu(100)界面的结构有显著差异。Cu(100)界面在与熔体接触的晶面中表现出明显的表面合金化。相比之下，在 Cu(111)界面上没有观察到表面合金化；然而，在界面处观察到形成了 2-3 个原子层厚的、相对于块状 Pb 晶体略微压缩的预冻结层的结晶 Pb。我们观察到，在 Cu(111)界面上，在 750 K 时不再存在预冻结层，但在 Cu(100)界面上的表面合金化仍然存在。在一系列过冷 MD 模拟中，在 592 K 下，即在 5 ns 的模拟时间内，在 Cu(111)界面上观察到 fcc Pb 的异质形核，过冷度为 26 K。Cu(111)上的形核和生长以近乎平面的临界核进行。定量分析得到的异质形核势垒比经典形核理论预测的均匀形核势垒大两个数量级以上。在表面合金化的 Cu(100)衬底上，形核困难得多。在这个界面上观察到形核需要大约 170 K 的过冷度。从定性观察，临界核与 Cu(100)表面的接触角超过 90°，表明形核相在 Cu(100)表面的润湿性差，这根据经典的异质形核理论，解释了在 Cu(100)表面形核所需的较大过冷度，相对于 Cu(111)，由于存在预冻结层，Cu(111)表面更类似于形核相。
