Liquid Liquid Phase Transition Of Metals In Low-Dimensional Confined Space

How to further improve the properties of metals as well as alloys and expand their application range has been the direction of people's efforts.With the development of science and technology,metals and their alloys have new applications in micro nano scale.For example,in the field of catalysis,metal nanoparticles are widely used because of their large specific surface area,good catalytic performance.In the field of two-dimensional materials,metals are ideal substrates,which are widely used to grow graphene,boron nitride and other two-dimensional materials.In order to play the role of metal materials in nano scale,it is necessary to further understand the structure and properties of metal,especially the phase transition behavior.The successful preparation of carbon nanotubes and graphene provides an ideal carrier for the study of the structure and properties of liquid metals in nano confined space.In this paper,molecular dynamics simulation is performed to study the phase transition behavior of liquid metal in the confined space constructed by carbon nano materials,further revealing the close relationship between the phase transition and the layering transformation,which provides a new idea for the preparation of metal materials in the confined space.The main contents are as follows:(1)In the confined space,the separation of lead-rich phase and aluminum-rich phase will take place in Al-Pb homogeneous liquid alloy with the temperature decreasing.The interfacial energy plays an important role in the process of phase separation,accompanied by the chemical heterogeneity change of the system.According to variance,the MSF is defined to describe the chemical heterogeneity,which can effectively characterize the structural change and chemical heterogeneity in the phase separation process.The effects of cooling rate,temperature and alloy composition on the phase transition and chemical heterogeneity were studied.The high cooling rate is unfavorable to the phase separation occurrence.In the liquid temperature range,the higher the system temperature is,the less the phase separation degree is.The phase separation degree is the largest when the atomic composition close to 50%.According to the parameters of chemical heterogeneity,the phase diagram of Al-Pb monotectic alloy is drawn.(2)The liquid aluminum will be stratified in the confined space.The layering transition will take place with the confinement size increasing.This kind of layering transition,accompanied by the sudden change of system potential energy and volume,is a unique phase transition behavior induced by the confinement size.Compared with the atomic layer far away from the confinement wall,the average energy and coordination number of atoms near the confinement wall are higher.The coordination number and diffusion coefficient of the system are closely related to the phase transition.Pressure can not only change the position of system stratification,but also change the layer numbers,leading to stratification transformation.However,temperature has little effect on layering and its transition.The layering transition phase diagram of liquid aluminum in confined space is drawn.In the low pressure region,the phase transition is mainly induced by the confinement size.In the high pressure region,the phase transition is induced by the confinement size and pressure jointly.(3)With the change of density,the phase transition of liquid copper occurs in the confined space.The phase transition is characterized by a negative slope on the pressure-density curve.The reason for phase transition is that the system is forced to adjust the relative orientation of atoms in order to adapt to the density increase.In the initial stage of density increasing,the first peak position of pair distribution function decreases gradually,which suggests that the system adapts to the increase of density by reducing the distance between adjacent atoms.When the atomic distance is reduced to a certain extent,and the density is further increased,the system is forced to adjust the relative orientation between atoms,so that the basic unit of the structure changes from triangle to square like,with the increase of atomic layers number.The density distribution function and structure show that the system is a metastable state in which two phases coexist.With the occurrence of phase transformation,the physical properties of the system,such as viscosity and diffusion coefficient,also change.(4)Due to the template and interface effect in confined space,Cu64Zr36 is prone to crystallize.This kind of crystallization is weak,and the abrupt point on the potential energy curve is not obvious.The confinement size will affect the crystallization temperature:the crystallization temperature shows an oscillatory rule as a function of confinement size.The atomic movement activity and the atomic motion mode are responsible for the oscillation behavior.Interestingly,the proportion of Cu-Zr bond will change with the layering phenomenon taking place in the confined space,and the copper atoms can separate totally from the zirconium atoms.