【作者】 张海燕；

【导师】 孙得彦；

【作者基本信息】 华东师范大学 ， 凝聚态物理， 2016， 博士

【摘要】 界面是当前凝聚态物理与材料科学的重要研究方向。固液界面作为界面的一种,其结构与性质决定了晶体的生长、结晶成核以及表面浸润等众多现象或过程。由于实验很难直接获得固液界面的结构与动力学性质,多数关于固液界面的理论研究主要采用计算机模拟的方法。目前,已有大量关于界面结构和动力学性质的研究,但有关移动界面伴随的缺陷俘获和动态界面结构的研究几乎是空白。本文采用分子动力学模拟方法,对简单金属固化过程中的缺陷俘获与动态固液界面结构进行了研究,主要研究结果概况如下:一、采用分子动力学模拟方法,计算了简单金属Ni和Al固液界面的迁移率,并建立了全温区固化的生长模型。我们发现,两种金属尽管非常不同,但生长速度总是在某个特征温度(T*)附近达到最大值;高于这个温度,两种金属类似,随着过冷温度(定义为与熔点的差)的增加生长速度单调增加;低于这个温度,两种金属截然不同,Ni的生长速度几乎不变,而Al的生长速度随过冷温度的增加快速减小到零。通过理论分析,我们发现,T*的存在可能与生长机制的改变有关。我们结合高温Broughton-Gilmer-Jackson(简称BGJ)模型和基于扩散的Wilson-Frenkel(简称W-F)模型建立了一个拓展的生长模型。该模型不仅从一个侧面佐证了生长机制的变化,同时给出了在整个生长温区的生长模型。二、结合多种结构分析方法,研究了金属Ni和Al固化过程中的动态固液界面结构。我们发现,从晶体层到液体层过渡的界面区域,动态固液界面的结构与平衡态的明显不同。随着生长速度或过冷温度的变化,界面处原子浓度会做明显的相应变化。特别是,在高过冷温度,界面处的原子浓度甚至可以远小于液体的浓度。界面附近的二维结构的分析表明,对于(100)生长方向,在高于T*的温区,界面处存在类气体层;在低于T*的温区,有明显的类玻璃态层出现。这个二维结构的转变,也许是发生生长机制转变的诱因或结果。三、采用分子动力学方法,我们系统研究了 Ni和Al两种金属固化过程中伴随的缺陷俘获。结果表明,缺陷浓度总体随着界面温度的降低而单调增加。缺陷浓度对生长速度有着很强的依赖性,但并非生长速度的单值函数;界面生长速度达到最大值之前,缺陷浓度随着界面生长速度的增加而增加,但是在界面生长速度达到最大值后却相反。通过对生长过程的微观分析,我们讨论了缺陷密度与生长速度和扩散系数之间的可能关系。扩散与生长过程都可以看成输运过程,而产生的效果恰恰相反。扩散过程导致原子输运到界面,而生长过程消除了界面附近的液体原子,二者之间的竞争最终导致了空位的产生。四、系统研究了 Ni与Al固液界面的动力学、动态结构、缺陷俘获在(100)与(110)两个生长方向上的差异。结果表明两种金属的界面结构存在着明显的各向异性,尤其是在深过冷温度下,界面处的二维径向分布函数明显不同;对于相同的过冷温度,(100)方向的生长速度总是大于(110)方向的;(100)方向的缺陷浓度大于(110)方向。更多还原

【Abstract】 Interface is important research direction of condensed matter physics and material science.As a typical interface,the structure and properties of solid-liquid interfaces strongly affect the crystal growth,surface infiltration,and many other phenomenon or process.Because the experiment is difficult to directly obtain the information regarding to interface structure and dynamics properties,most of theoretical studies are carried out by computer simulations.Currently the structure and properties of solid-liquid interface at equilibrium are well investigated.However the structure associated with the moving interface remains to explore.In this thesis,by using the molecular dynamics simulation,we have studied the dynamic structure of moving interface and associated defect traping.The main results include:1st.By using molecular dynamics simulations,we have studied the solidification of simple metals(Ni and Al).We found that,for both metals at certain temperature(T*)the growth velocity reaches a maximum.Above this temperature,the two metals behavior similarly,the growth rate increases monotonously with the increase of undercooling temperature(defined as the difference between the melting temperature and the system temperature).Below this temperature,they are quite different,the growth velocity of Ni is almost constant,but the growth velocity of Al decreases to zero with the increase of undercooling temperature.Through theoretical analysis,we speculate that the existence of T*may be associated with the change of growth mechanism.Combining Broughton-Gilmer-Jackson(BGJ)model and Wilson-Frenkel(W-F)model,we develop a growth model,which can describe well the growth velocity via temperature in wide range of under cooling.2nd.We characterize the dynamic structure of solid-liquid interface during solidification of Ni and Al.We found,as the increase of under cooling,the atomic density near interface presents a minimium,which is even lower than that of liquid.For(100)orientation,two-dimension(2D)structure analysis of the solid-liquid interface indicates that,above T*,a gas-like layer exists at the interface.Below T*,this layer has the character of glass.The change in 2D structure of interface may be the refection of the change in growth mechanism.3rd.Using molecular dynamics simulations,we have systematically studied the defect trapping accompaning with solidification process of Ni and Al.The results show that,only vacancy is observed,and vacancy concentration always increases with the under cooling.Vacancy concentration has a strong dependence on growth velocity,but not a single value function of it.Before the interface growth velocity reaches the maximum,defect concentration increases with interface growth velocity increases,but after the interface growth velocity reached the maximum it is on the contrary.We have also discusse the prossable connection between defect concentration and growth velocity and diffusion coefficient.Diffusion process leads to transport atom to interface,but growth process eliminates liquid atoms near the interface,the competition between them eventually leads to the generation of vacancy.4th.We have also investigated the anisotropy of the dynamics structure of solid-liquid interface and defect trapping between(100)and(110)orientation.Results show an evident anisotropy.Especially,under the deep undercooling,2D radial distribution function is significant different.For the same undercooling temperature,the growth velocity of the(100)direction is always greater than that of(110)direction,and the defect concentration of the(100)direction is greater that of the(110)direction.更多还原