Simulation Of Melting And Rapid Solidification Of Silicon

As one of the most important functional materials, silicon is the most abundant element in the Earth's crust after oxygen and has played a key role to decide and impact on development of society. In purification and processing of silicon, melting and solidification are an effective method and an important step. In recent years, with development of the computing simulation, molecular dynamic simulation has become a new experiment means to explore the physical world and it is making possible to research and understand the materials macroscopic properties through the micro-scale and atomic motion view.In this paper, the thermodynamic behavior and structure of silicon during melting and solidification were investigated by using molecular dynamics simulations; moreover the melting mechanism, nucleation in liquid, and liquid-liquid phase transition were studied. Comprehensive comparison of the influence of heating rate, cooling rate and pressure on melting and solidification process was carried out.To ensure the reliability of molecular dynamics simulations, three common potential functions of silicon were investigated by testing the lattice parameter, and thermodynamic properties in the melting process and liquid structure. The Stilling-weber potential was chosen to be used in the following work after comprehensive comparison.For the study of melting process,the equilibrium melting point of silicon was determined by solid-liquid equilibrium method first,and the two factors related to the accuracy, the heating rate and the initial cell size, were also analyzed. It was found that the heating rate will directly affect the overheating melting point of silicon, but the initial cell size has little effect on the superheating melting point because of the periodic boundary conditions. To clearly describe the melting process of silicon, the melting process can be described in three stages: premelting, accelerated melting and relaxation according to the diffusion coefficient, distribution of particle vibration, and the evolution of cluster structures. The true melting process of silicon was revealed through the description of different characteristics of each stage. The strucutre of initial nucleation of silicon was presented, furthermore the nucleation of liquid phase during melting and inherent rules of the structure of voronoi index <2,3,0,0> were analyzed.For the study of solidification process, a new method to calculate the temperature of glass transition point was proposed in this work and an equation was presented, by which the temperature of glass transition point at a given cooling rate can be calculated through the equilibrium melting point and overheating melting point. This method will improve the accuracy of calculating the temperature T_g. The liquid-liquid phase transition in the supercooled liquid silicon was confirmed through the changes of the density, coordination number and structure in this work. The results show that it occurs at 1389 K (at the cooling rate of 1×10¹² K/s). The structural transformation and the structural charateristics of liquid-liquid phase transition were presented. In addition, the influence of cooling rate on the dynamics, structural evolution and liquid-liquid phase transition in solidification process was systematically studied. It is found that the diffusion of particles in the liquid decreases and the coordination number increases with increasing cooling rate. Moreover the liquid-liquid phase transition was restrained when the cooling was beyond a critical value. Finally, the final clusters of glassy solid silicon were obtained by rapid quenching in this work.Additionally, the effect of pressure on the melting and rapid solidification process of silicon was studied. It is found that the liquid-liquid phase transition in the supercooled liquid silicon disappeared when the pressure is higher than 3GPa. Meanwhile, the diffusion of atoms also affected by pressure, thus the process of melting and solidification were influenced as well.