Structural Evolution And Solidification Behavior Of Confined Melt

In the last decades, low-dimensional materials have gained rapid development. Especially, the experimental preparation of graphene in2004has led to an explosion of interest in low-dimensional materials. Low-dimensional materials have broad application prospects in electronic device, energy storage, biotechnology, composite materials and other fields, because low-dimensional materials have many specific physical and chemical properties different from the bulk ones, such as size effect, quantum effect, surface effect and so on. In the traditional industrial production, the solidification of melt is usually used to prepare materials, however the structures and phase transition of low-dimensional melts is still limited till now and few people use the method of solidification to prepare the low-dimensional materials. Due to thermodynamic stability, low-dimensional melts should be confined to maintain the shape. The shape, size, and attractive force determine the structure and property of the inner melt. To study the structure evolution rules and phase transition behavior characteristics is of great significance in completing the phase transformation theory, improving the traditional material processing technology, and developing the advanced preparation technology.In this dissertation, molecular dynamics (MD) simulations are performed to study the structural evolution and solidification behavior of the low-dimensional melt in confined space to reveal the structure evolution rules and phase transition mechanism and propose a new method to prepare low-dimensional materials by the liquid-solid relations during solidification. The main content is as follows:(1) We have studied the structure evolution of liquid silicon confined to2D spaces with the decreasing temperature. Looking closely at the presented data, the cooling process can be divided into four stages:high-temperature liquid; liquid-liquid phase transition (LLPT); liquid-solid phase transition (LSPT); and optimization stage. The LLPT in liquid carbon can be treated as the preparation stage of the LSPT. In the stage of LLPT, the liquid carbon changes from a low-density low-coordinated liquid to a high-density highly coordinated liquid, and the chain structures firstly self-assemble into some ring structures and then aggregate into some unstable islands. The threefold coordinated structures play an important role in the formation of atomic rings. The inheritance of the threefold coordinated structures provides essential condition to form rings and islands. In the stage of LSPT, the islands can stable exist as a nucleus and begin to grow outwards by incorporating the discrete chains and rings. Eventually, the carbon islands will encounter each other and combine to form a complete polycrystalline film. These results provide theoretical foundation for using solidification to produce graphene and other functional devices.(2) In addition, the structure evolution of confined liquid silicon in2D space has been investigated and the effect of slit size, temperature, inner pressure and attractive force on the structure of inner melt is also discussed. It is found that the liquid silicon undergoes an observable layering transition from a low-density low-coordinated liquid to a high-density highly coordinated liquid with the increase of the slit size, accompanied by the sudden change of potential energy, coordination number and atomic volume. Moreover, an apparent phase separation has been observed that atoms located at different positions usually have different coordinated number. In the stage of2-3layer transition, each original layer will first split into a4-fold coordinated sub-layer and a5-fold coordinated sub-layer. The confined silicon also exhibits a layering transition with the increase of the pressure, accompanied by the sudden decline in potential energy and volume. The above changes in the isotherms all indicate the weakening of layering transition with the increase of temperature. Increasing temperature will weaken the layering transition, because the higher temperature will induce much stronger thermal motion that can break the local bonding, make the liquid silicon more disorderly and impede the generation of ordering layers. The strong interaction between the wall and silicon will not only raise the melting point of confined silicon, but also tightly adsorb, the atoms on the slit wall.(3) The solidification behavior of2D confined silicon is further investigated and the effect of cooling rate and slit size on the solidification behavior is also discussed. In the solidification process, the crystal nucleuses with hexagonal structure grow outwards by incorporating the disordered structures and changing them into hexagonal structures, and finally combine to form a complete graphene-like polycrystalline film. The increase of cooling rate will reduce the increase of disordered amorphous structures. The change of slit size will not only influence the melting point of confined silicon but also affect the freezing structure. In addition, the finding of structure correlation between the liquid and solid silicon provides a now possibility for the preparation of low-dimension silicon materials using solidification.(4) The interaction between the heterogeneous nucleus and melts is closely related with the purification, refining, inoculation and modification, which ultimately determines the microstructure and macroscopic property of solidification structure. In this dissertation, by studying the heterogeneous nucleation of copper melts in semi-confined spaces constructed by bending carbon film, it is found that the existence of heterogeneous surface will not only influence the thermodynamic property of melts but also affect the structure evolution. The liquid and solid structures have structural relations with the heterogeneity surface. The simulation result clearly shows that copper atoms become layered at the liquid-solid interface in a half-cylinder shell pattern resembling the substrate. This kind of shape control decays with increasing distance from the wall and the outmost layers transform into twin crystal with two fcc wedges. It is found that the final structures have striking correlations with the curvature radius, central angle and arc length of the substrate. On the substrate, the solid copper have different structures, such as single crystal, twin crystal, cylindrical shell,"C"-shaped layer, tangent plane mode and so on. The final structures depend on the competition between the C-Cu van der Waals interaction and the Cu lattice energy among the crystal.This dissertation systematically studies structural evolution and solidification behavior of the confined melt. These findings provide physical and dynamic insights into the structures and phase transition mechanisms of confined melts, and provide theoretical guidance for the further preparation of low-dimensional materials.