Liquid-Liquid Phase Transition And Solidification Of Confined Melts

Nanomaterials with perfect structures and excellent performance can be fabricated through the solidification of melt if the confined space is served as a template. Notably, the confined melt displaying different structures and thermodynamic properties from the bulk, has hereditary effects on the solidified structure of materials. For this reason, a better understanding of the low-dimensional confined melts’ structural characteristics and solidification behavior, as well as the interfacial effect of confined space, would not only be an important complement of the phase transition theory, but also have vital guiding significance on the advanced technology development of 2D materials preparation and processing.By means of molecular dynamics simulations, this thesis studies the liquid-liquid phase transition (LLPT) in the confined liquid silicon carbide and discusses the induced effects of confined space, pressure, and wall-liquid forces on LLPTs. The structural evolution of 2D liquid cobalt during rapid cooling is also studied to reveal the correlation between the icosahedral order and the crystalline medium-range order. The main content is as follows:(1) We report theoretical evidence of an LLPT in liquid silicon carbide under nanoslit confinement and plot its pressure-confinement phase diagram. The LLPT is characterized by layering transitions induced by confinement and pressure, accompanying the rapid change in density. During the layering transition, the proportional distribution of different coordinated structures exhibits remarkable change. The tricoordinated structures lead to the microphase separation between silicon (with the dominant tricoordinated, tetracoordinated, and pentacoordinated structures) and carbon (with the dominant tricoordinated structure) in the layer close to the walls. Besides, a strong layer separation between silicon atoms and carbon atoms is induced by strong wall-liquid forces, which theoretically provides the potential for a novel 2D silicon carbide material. Importantly, the pressure-confinement phase diagram with negative slopes for LLPT lines indicates that, under high pressure, the LLPT is mainly confinement-induced, but under low pressure, it becomes dominantly pressure-induced.(2) We show the direct observation of the synergy effect and pinning effect of the icosahedral order on the freezing behavior of 2D liquid cobalt during rapid cooling. The icosahedron would contribute to nucleation through the synergy with other short-range ordered structures and participates in crystal growth via assimilation, but when crystals grow, the pinning effect should be overcome, consuming energy. A semi-ordered morphology with maze-like nano-patterns emerges due to the cooperation between the synergy effect and the pinning effect. These findings shed light on the correlation between the icosahedral order and the crystalline medium-range order.The results presented in this thesis show the phase characteristics and the clusters evolution in confined melts, providing the theoretical evidence for the LLPT of confined silicon carbide and having important guiding significance on the structural design and fabrication of novel 2D silicon carbide materials and metallic films.