【作者】 韩立波；

【导师】 傅容珊； 罗胜年；

【作者基本信息】 中国科学技术大学 ， 固体地球物理， 2010， 博士

【摘要】 分子动力学模拟是人类认识物质世界的一种崭新的实验方式,人类可从原子、分子尺度上去研究和理解材料宏观的性质。近年来,随着分子动力学势函数的发展和计算机计算能力的大幅度提高,分子动力学模拟逐渐成熟,分子动力学模拟技术广泛应用到地球科学领域特别是应用于地球物理学研究中。分子动力学数值模拟实验有助于我们研究、理解在地球深内部环境中物质的状态和性质,特别是在研究地幔底部、外核直至内核的物质结构、组成、各向异性、状态方程等一系列重要的基础问题。物质熔化是地球内部一类重要的基本物理过程。为了分析和理解这一类过程,本文将采用分子动力学数值模拟的方式对物质熔化相关的过程进行研究。本文综述了纳米尺度下材料的热力学行为,对如何采用分子动力学这一原子模拟技术分析材料热力学性能的数值方法进行了深入系统的阐述。我们采用分子动力学方法模拟了单晶铜的缺陷熔化过程,铜在静高压和冲击高压下的熔化过程,以及铜的冲击断裂过程,探讨了铜在冲击断裂中的一些力学行为。同时,还利用分子动力学模拟了纳米铜球粒子在撞击刚壁过程中的弹塑性行为。从固体材料熔化及力学行为原子尺度模拟这一角度系统阐述了分子动力学数值模拟技术,讨论了分子动力学模拟中一些关键问题,如温度控制、压力控制、原子势函数等。缺陷结构普遍存在于真实晶体中,特别是在地球内部。我们采用分子动力学方法模拟了有缺陷结构的单晶铜熔化行为。本文重点研究了层错缺陷和晶界缺陷。无论插入型层错还是抽出型层错,对熔化温度的影响基本可以忽略。层错缺陷的存在使得可以同时观察到均匀成核和非均匀成核,但只是稍微增加了成核速率和非均匀成核的概率。对于晶界缺陷本文研究了两种有代表性的对称(110)倾斜晶界,一种是低能量的∑=11/(113)／50.48°,另外一种是高能量的∑=27/(552)／148.41°。结果表明,在晶界区域内连续局部熔化会出现在不连续的整体熔化前,而连续的固态无序化会出现在局部熔化前。对前一种晶界缺陷,晶界区域的预熔化可以忽略不计,局部熔化发生在热动力学熔点附近。整个系统存在约13%的过热。对于后一种晶界缺陷,局部熔化存在着相当可观的预熔化,而整个系统体熔化过热程度已经可以忽略。本文研究了金属铜在静高压下和冲击高压下的熔化行为。对于静高压条件下金属铜的熔化,本文分别利用两相法和过热-过冷回滞曲线法获得了铜在高压下的平衡熔化曲线,两种方法结果非常接近。最大过热和过冷程度独立于压力,近似保持常数。对于冲击熔化行为,我们研究了沿三个主要晶向(100),(110)和(111)冲击加载铜的熔化。沿(100)和沿(110)、(111)晶向冲击的熔化行为截然不同。对于<100>晶向,固体承受了最大约20%的过热,熔化时有明显的温度降。对于其它两个晶向,其熔化过程为准连续过程,并且存在着大约7%的预熔化。本文借助分子动力学模拟研究了极高应变率下固体铜和液体铜由平面冲击导致的断裂。从自由表面速度历史推断断裂强度σsp和应变率是合理的一阶近似。对弱冲击强度来说断裂强度各向异性较为显著,并随着冲击强度增大逐渐递减。固体中空洞在缺陷位置成核。固体经受弱冲击时,断裂发生时没有伴随着拉伸熔化出现,而对于更强冲击或者当断裂温度Tsp足够高时,部分熔化可能出现在断裂之前或者伴随着断裂出现。断裂温度Tsp在研究的应变率范围内对断裂起着决定性作用。无论是固体铜还是液体铜,断裂强度σsp都随着断裂温度Tsp的增加而降低,并且对液体铜来说其σsp-Tsp成反幂指数关系。本文研究了纳米铜球粒子撞击刚壁的弹塑性行为。铜球粒子在撞击过程中表现出了了高度弹性行为。对较小的粒子(半径小于1 nm),热涨落导致速度恢复系数波动很大,从而使得粒子表现出超弹性行为。对于较大的粒子(半径在2-15 nm范围内),铜球粒子仍然表现出了高度弹性,减小的速度恢复系数是由于碰撞过程中出现了塑性,这将导致不可逆的生热现象发生。更多还原

【Abstract】 Molecular Dynamics(MD) is a brand new method to explore real world. Ma-croscopic properties of materials can be studied and understood at the atomic and molecular scale. Recently along the development of molecular dynamics potential functions and advance of computation speed, MD becomes more and more mature. MD simulations have been applied in many earth science fields, e.g. geophysics. MD simulations are very helpful to study and understand the state and properties of materials in the Earth interior, especially to study the structures, compositions, anisotropies and equations of state(EOS) of the materials in the lower mantle and outer core, even in the inner core. Melting is one of the most important processes for materials in the Earth interior. To understand this process, we conduct MD simulations to study the melting behaviors.Thermodynamic behaviors of nano-scale materials are explained in this the-sis.Numerical method of simulating thermodynamics behaviors of materials by MD is elucidated in detail. Melting behaviors of single crystal copper with defective structures, melting behaviors of copper under hydrostatic and shock wave loading to high pressures and spallation of copper under shock wave loading are simula-ted with MD.The mechanical behaviors of copper during spallation are analyzed. Moreover, elastic and plastic behaviors of copper nanoparticles impact on a rigid wall are also simulated.The methodology of simulating the thermodynamic behaviors of solid ma-terials by molecular dynamics method is elucidated indetail.Some key techniques ,such as the control of temperature and stress, potential functions are emphasized.Defective structures are omnipresent in real solids, especially in the interior of the Earth. MD simulations are performed to investigate melting behaviors of cop-per with defective structures. This thesis focuses on stacking faults(SF) and grain boundary(GB).Both intrinsic and extrinsic SF induce negligible reduction in the temperature at melting. The existence of SF makes it is impossile to observe homo-geneous and heterogeneous nucleations simultaneously, but only slightly increases the nucleation rate and probability of nucleation at heterogeneous nucleations sites. Two representative types of symmetric (110) tilt GBs are explored, one isΣ=11/(113)/50.48°with low GB energy and the other isΣ= 27/(552)/148.41°with high GB energy. The results show within the GB region, continuous local melting precedes discontinuous bulk melting, while continuous solid state disor- dering may precede local melting. ForΣ= 11/(113)/50.48°with low GB energy, premelting of the GB region is negligible and local melting occurs near the ther-modynamic melting temperature. The GB region as a whole is superheated by about 13%before its bulk melting. The the case ofΣ= 27/(552)/148.41°with high GB energy, considerable premelting is observed for local melting, while the bulk melting occurs with negligible superheating.Melting behaviors of copper under hydrostatic and shock wave loading to high pressures are studied. In the case of hydrostaic melting of copper, equilibrium mel-ting curve are obtained by both two-phase method and superheating-supercooling hysteresis method. They show very similar results. The amount of superheating or supercooling is independent of pressure and keep a constant nearly. For shock-induced melting of copper, we investigate melting under shock wave loading along three main crystallographic directions:(100). (110) and (111). Melting behaviors under shock wave loading along (100) and (110), (111) are very different. Fr the former, solid undergoes about 20%superheating before it melts with a pronoun-ced temperature drop. For the latter, melting is quasi-continuous and premelting about 7%is observed.MD simulations are carried out to study spallation in solid and liquid copper incuded by planar shock loading at exteme strain rates. It is a reasonable firs-order approximation deducing spall strengthσsp and strain ratesεfrom free surface velocity history. The anisotroy inσsp is pronounced for weak shocks and decreases for stronger shocks. Voids are nucleated at a defective site in a solid. For weak solid shocks, spallation occurs without tensile melting, for stronger shocks or if the temperature right before spallation Tsp is sufficiently high, spallation may be accompanied or preceded by partial melting. Tsp appears to have a dominant effect on spallation for the narrow range ofεstudied here.σsp decreases with increasing Tsp for both solid and liquid copper andσsp-Tsp follows an inverse power law for liquids.Elastic and plastic behaviors of copper nanoparticles impacting on a rigid wall are studied. The copper nanoparticles can be highly elastic during impact. Thermal fluctuations induce large fluctuations in the velocity restitution coefficient for small particles, and thus cause superelastic. Copper nanoparticles are still highly elastic for larger particles. The reduced restitution coefficient is due to plasticity during impact, which gives rise to irreversible heating.更多还原