Molecular Dynamics Simulation For The Structural Evolution In Liquid Tin

Liquid tin is used as a solvent liquid metal or reaction medium which has a good applied prospect due to its low melting point, stable range characteristics and used for recycling nuclear waste, which avoids for organic solvent vulnerable to radiation damage, reduces the waste production, improves the utilization rate of fuel and reduces the danger of radiation. Meanwhile as a fusion reactor of plasma facing materials (PFM) or the first candidate materials of wall materials, liquid tin has a high recycling, low tritium-retention surface and allowing higher than 2000 ? operating temperatures with the advantage of having five or more orders of magnitude lower vapour pressure than lithium. However as a heavy impurity tin contamination to reactor core plasma, tin situates in the periodic table of the IV main group and chemical bonds of tin shows part of the covalent and attribute part of metal which is different from the same group of other metal containing only a single chemical bonds. Those features make it become the first selection of object of study. In recent years, researchers focus on liquid metal structure evolution and phase transformation. Macroscopic property depends on its microscopic structure of material, through the study of liquid tin macroscopic properties can reflect the characteristics of micro-structural evolution. The recent studies found that these only study liquid tin structure changing with temperature and pressure were studied, the influence of the diffusion coefficient varying with temperature, the influence of viscosity changing with temperature, the influence of failed to reveal the characteristics of the structure of the tin liquid will also not directly associated structure and macroscopic property.In order to study the structure evolution characteristics in liquid tin, molecular dynamics simulation method combining the MEAM potential under normal pressure has been used to investigate the thermodynamics characteristics of metal tin (alpha-Sn, beta-Sn) and the structure evolution characteristics of liquid tin and the corresponding macroscopic properties. This paper focuses on analyzing temperature and pressure on the influence of structure, self-diffusion and viscosity, the relationship between temperature, pressure and the short-range order, short-range ordered structures, self-diffusion coefficient and the shear viscosity. The result shows that alpha-Sn and beta-Sn thermodynamics result is consistent with the experimental results, which reveals the relationship between them and the temperature and reflects structure and thermal dynamic properties of alpha-Sn and beta-Sn. The structure of liquid tin are simulated and analyzed as the temperature ranged from 773K to 1873K,using the radial distribution function (RDF)and common neighbor analysis(CNA).The result shows that the short-range ordered degree in liquid tin decrease with the increase of temperature and are obviously divided into two segments at around 1200K. The structural change of liquid tin is confirmed by simulating self-diffusion of liquid tin from 600 to 2000K and viscosity of liquid tin at a temperature range of 773K to 1873K.The radial distribution function near the melting point was simulated as the pressure ranged from 3.0 to 19.4GPa and analyzed by CNA. The result shows that with the increase of the pressure, the characteristic bonded pairs existing in FCC or HCP of short-range ordered structure are gradually converted into the characteristic bonded pairs in BCC near the melting point of liquid tin, the short-range ordered structure has been gradually changed from close packing structure to non-close packing structure in liquid tin. we also simulate the temperature and pressure on the influence of the self-diffusion and shear viscosity, the result show that the self-diffusion coefficient with temperature increases, the shear viscosity decreases with the increase of temperature; the self-diffusion coefficient decreases with the increases of the pressure, the diffusion activation energy increases with the increases of the pressure, the self-diffusion of the liquid tin spreads slower and the shear viscosity and the activation energy for viscous flow increases with the increases of the pressure, the viscous flow of liquid tin becomes worse.