The Kinetics Of Liquid-Solid Interface During Crystallization

Crystallization phenomenon is central to many processes used in chemistry,physics,material and environmental sciences.In liquid metals,random density and thermal fluctuations create transient crystalline clusters even above the equilibrium melting point.When liquids are cooled down to the undercooled region,stable nuclei are available on condition that the thermal fluctuations overcome the free energy barrier for forming a crystalline nucleus.Once a stable nucleus is formed,it will continue to grow.The crystal growth is microscopically regarded as the advance of solid-liquid interface in liquids,and this process is controlled by the kinetics of atom attachment to the interface,the capillarity effect and the diffusion of thermal and mass near interface.We use molecular dynamics methods to simulate the interface energy and crystal growth process of some pure melts and alloys,and analyze the interface kinetics of crystal growth,and reveals the dynamics mechanism of crystal growth with high growth rates.We study the crystallization morphology and crystallization kinetics of liquid droplet.The research of this paper includes:1.We study the crystal growth of spherical Cu,Al,NiAl and CuZr crystals by using MD simulations.The growth rates of the four systems are found to increase with increasing radius of crystals.The interface energies are calculated and the results show that the curvature undercooling induced by the capillary effect has no contribution to the enhanced crystal growth.In this stage,the interface kinetics dominates the crystal growth.We analyze the interface morphology and find that the interface thickness and the Jackson factor increase as the growth proceeds,indicating the interface becomes rougher.Based on these results,under the condition of our study,the increase of the growth rate is interpreted as a result of the increase of solid-liquid interface roughness.An attachment rate of solid-liquid interface,which is defined as the fraction of stable atoms at the crystal interface without leaving.By used to approximate the f-factor,according to the crystal growth model to predict the crystal growth rate changing of Cu,Al,NiAl and CuZr system with the crystal radius,the results qualitatively agree with the values from the direct simulations,thereby reasonably interpreting the mechanism of the accelerated crystal growth.This part of theresearch work can help us to understand the crystal growth interface dynamic effects on crystal growth at the initial stage after nucleation.2.We study the crystallization process of liquid droplet under three conditions.(1)The study found that the free nucleation of water droplets usually occur at internal,with the subsequent crystal growth,the water of cubic and hexagonal structure clinging to grow a cambium structure.(2)We divide the liquid droplet into two parts,and simulate the crystallization under different temperature respectively.The crystallization under this mandatory temperature gradient show that the nucleation is still preferred formed in internal under high temperature,and growth outward rapidly with radial extending.(3)By fix inside of the liquid water,forming a shell limited space,Under the condition,the crystal growth of the water in the shell with circular manner,then release the water in the nuclear,Crystal fill the entire nuclear area with epitaxial growth way.The estimates of crystal growth rate show that limited space is conducive to the crystal growth most.