Liquid-liquid Phase Transition Of Silicon Oxide Melt In Nanoconfined Space

Low-dimensional materials have physical and chemical properties that traditional bulk materials do not have,such as size effect,surface effect,etc.Therefore,they have wide application prospects in biotechnology,new energy,electronic devices and other fields.In recent years,low dimensional materials have developed rapidly.However,it is still difficult to produce large size low dimensional materials with few defects with the current technical conditions.In the preparation of traditional materials,melt solidification is often used to prepare materials.However,this method is rarely used in the preparation of low-dimensional materials.Due to the thermodynamic instability of low-dimensional materials,the melt needs a confined space to maintain its low-dimensional shape.We study the structural transformation and liquid-liquid phase transition of silica melt in the confined space,and grasp the influence of the confined space on the structure and phase change of two-dimensional silica melt,which has important guiding significance for the preparation and processing of silica film.In this dissertation,the liquid-liquid phase transition of nanoconfined silica and silica oxide melts were studied by means of molecular dynamics simulation.The effects of slit size,pressure and the interaction between virtual wall and atoms on melt structure,properties and liquid-liquid phase transitions were discussed.The main content of this thesis is as follows:(1)The liquid-liquid phase transition of two-dimensional silica melt confined in the slit between two virtual walls was studied.The effect of the virtual wall on the melt structure and the liquid-liquid phase transition induced by the slit size and pressure were studied.The results show that the slit size and pressure are the main factors affecting the limited silica melt.In the case of strong restriction effect,the atoms in the silica melt would separate.The pressure-confinement phase diagram showed negative slopes for the liquid-liquid phase transition lines,which indicates that at high pressure,the liquid-liquid phase transition is mainly induced by confinement,while under low pressure,the liquid-liquid phase transition is dominantly induced by pressure.(2)Liquid-liquid phase changes of silicon oxides with different silicon-oxygen ratios in confined space were studied.Lateral pressure and slit size are still the main variables in our study.With the decrease of the slit size and the increase of the pressure,a small acromion peak appears between the first peak and the second peak in the pair correlation function,and the abrupt changes in density,P?,potential energy and diffusion coefficient further indicate the existence of liquid-liquid phase transition.Liquid-liquid phase transitions are similar in layering transition in different liquids but their appearances are not synchronousIn this thesis,the structural transformation and liquid-liquid phase transition of silica and silica oxide melts in confined space are systematically explained,which has important guiding significance for the preparation of two-dimensional silica film materials by melt solidification method.