氩蒸气均匀核化与非均匀核化的分子动力学模拟

本文运用分子动力学模拟方法对氩蒸气的均匀核化和非均匀核化进行了研究.模拟氩蒸气的初始温度为30℃,冷却终温为-220℃.对均匀核化成核现象的团簇分析表明,蒸气凝结分为显热和潜热释放两个过程.提出了一个新的力函数模型,实现了非均匀核化过程的模拟.通过比较冷却速率为0.0002 m/s的均匀核化过程和非均匀核化过程,发现均匀核化的凝结核心是随机的,而非均匀核化的凝结核心以外界引入的核化点为核心.     

汽液界面厚度和分维数

本文应用分子动力学模拟方法进行了汽液界面厚度和分维数的模拟研究。用统计系综方法,以氩原子为对象,对长方形模拟盒中粒子数目为2048个的汽液平衡共存系统进行模拟计算。研究了一定条件下汽液界面厚度的取定对确定界面分维数的影响,根据计算模拟结果,提出了可用15~85法则确定界面厚度和分维数。汽液界面在X方向、Y方向和Z方向具有不同的分维数,充分说明了汽液界面的各性异性性质。     

截断半径变化对汽液界面参数和分维数的影响

本文在分形理论基础上,采用分子动力学模拟方法研究了在平衡状态下的汽液界面参数,得了界面区内的密度、 温度、压力张量、表面张力和分维数的分布。研究表明,利用分形理论可以较好的反映汽液界面的特性。势函数的截断半 径对汽液界面参数和分维数的模拟值存在影响。     

氩蒸汽异质核化的分子动力学模拟研究

建立了一个分子动力学研究蒸汽异质核化的模型,对氩蒸汽异质核化进行MD模拟研究。氩蒸气的初始温度为300 K,冷却终温为80 K,冷却速率为0.0002 m/s。异质核化成核现象表明核化均以固体颗粒为核化中心,核化温度高于均质核化。同时统计了不同氩蒸汽粒子数异质核化的核化团簇的密度、法向压力、切向压力分布,计算团簇的表面...     

过热液氦均匀核化速率的确定

预测不同压力下过热液氦的均匀核化速率是十分重要的,它与液氦的极限过热度密切相关。文中通过回顾动力学理论、分子聚集理论、涨落理论等研究液体均匀核化的方法,对过热液氦的均匀核化速率进行了计算,并且对各种方法进行了比较与分析。结果表明,用能量涨落理论来计算过热液氦的均匀核化速率是一种比较合理的方法。     

用小波和分维数方法处理喘振实验数据

本文针对离心压缩机喘振的实验数据，采用小波分析，发现喘振先兆的频率与后来发生的喘振频率接近，且喘振先兆和喘振均为周期性的脉动．用分维数方法进行处理分析，通过计算压缩机系统不同工况下的关联维数，可以看出关联维数是信号复杂程度的度量．以上工作为实施喘振的主动控制提供了依据．     

液体分子分数布朗运动的研究

本文运用分形理论,通过与标准布朗运动的类比,对实际分子运动的轨迹维数进行了理论分析。运用分子动力学模拟方法,模拟了84K恒温液氩系统中分子的运动过程,统计了不同空间观测尺度及时间观测尺度下分子运动轨迹的分维数。从分子平均自由程理论出发,给出了合理的统计尺度判定标准,并与模拟结果进行了比较,吻合较好。     

分数布朗函数在分子动力学模拟中的应用

本文基于分子动力学模拟的结果,采用分形理论方法提取分维数作为特征数,构成分数布朗运动的位移增量、 速度和速率的概率密度函数来描写微粒的运动。同时,与分子动力学模拟直接统计结果比较,二者吻合较好。     

二元液体中动力学不均匀性与脆性的关系

过冷液体和玻璃有着复杂的结构和动力学性质.目前我们对他的研究是很不够的.当液体的温度远低于它的熔点时,它呈现非指数的驰豫行为.大量的实验和分子动力学模拟的研究表明这种非指数的驰豫行为是由动力学不均匀性引起的.计算机模拟已经提供了一些过冷液体的动力学不均匀性的相关信息.而Agell认为,液体可以粘度随温度的变化分为强,弱液体两种.脆性表示的是随着玻璃态的接近液体性质(如黏度)的改变速度.在这篇文章中我们通过模拟了三种不同二元Lennard-Jones液体,计算了它们的非高斯参数,脆性系数等一些参数,研究动了力学不均匀性和脆性的关系.得出了系统的脆性系数越小他的结构就越稳定,动力学不均匀性就越强的结论.     

气液态分子运动的Hurst指数及相图研究

本文运用分子动力学模拟方法,模拟了不同温度条件下饱和气体和液体氩分子的运动。对气液状态下分子运动在速度空间的演化相图进行了比较。根据分形理论,对实际分子的分数布朗运动特性进行了讨论。应用本文提出的约化规模法,获得了不同温度条件下分子分数布朗运动的Hurst指数和轨迹分维数。饱和液相分子运动的Hurst指数并不随温度单调上升,存在一个极小值。     

可膨胀过热水系统多气核演化过程与临界过热度分析

使用分子动力学方法,研究过热水系统均质沸腾核化过程.采用Langevin 动力学方法控制体积可变系统的温度与压力,更好地模拟了沸腾实际物理过程.得到了液相系统体积连续膨胀、分子间距逐渐增大,最终稳定在汽相的现象学规律.当过热温度较高时,亚稳态液相系统可能在局部形成不同大小的近球形区域：气核,这些气核是不稳定的,处于不断演化之中.通过分析分子所受引力与斥力的共同作用,得到了气核形成与消亡以及多个气核融合的机理.比较了模拟结果与经典沸腾理论的差异,提出了气核生长是比气泡生长更为微观过程的认识.通过研究不同过热 关键词： 过热水系统 分子动力学 气核 临界过热度     

过冷液体钾形核初期微观动力学机理的模拟研究

采用分子动力学方法对液态金属钾凝固过程进行了模拟,根据凝固过程体系平均原子能量、原子成键类型和成团类型,以及均方位移和非Gauss参数等动力学参数的演化特征,对过冷熔体形核初期微观动力学机理进行了研究.结果表明:根据过冷液体钾结晶形核过程热力学、动力学和结构特性的演化规律, 其过冷温度区间可以分为两个明显不同的阶段,潜在结晶核心出现在过冷液体较低温区.过冷熔体钾在形核初期,二十面体团簇结构在α-弛豫阶段逐渐解体,同时具有体心立方(bcc)结构的潜在结晶核心逐步形成,其临界晶核包含约300个原子.     

Ti3Al合金凝固过程晶核形成及演变过程的模拟研究

采用分子动力学方法对Ti₃Al合金的形核机理进行了模拟研究,采用团簇类型指数法(CTIM),对凝固过程不同尺度的原子团簇结构进行了识别和表征,深入研究了临界晶核的形成和长大过程.结果表明,凝固过程体系包含了数万种不同类型的原子团簇结构,但其中22种团簇结构类型对结晶形核过程起关键性作用.在晶核的形成和长大过程,类二十面体(ICO)原子团簇、类BCC原子团簇和缺陷FCC及缺陷HCP原子团簇在3个特征温度点T₁ (1110 K), T₂ (1085 K)和T₃ (1010 K)时达到数量上的饱和,并根据数量和空间分布随温度的变化,得到了它们在形核和长大过程相互竞争的关系.跟踪平行孪生晶粒形成和长大的过程发现,临界晶核是由FCC原子构成的单相结构,并未观察到亚稳BCC相优先形核的过程;平行孪生结构是由FCC单相晶核在沿密排面逐层生长过程中形成的.结果还表明, CTIM相比于其他微观结构表示方法,能更为准确地揭示凝固过程微观结构的转变特征.     

Homogeneous two-dimensional nucleation of guest-free silicon clathrates

The difficulty in synthesizing guest-free semiconductor clathrates complicates the process of determining how these cage-like structures form. This work studies the microscopic mechanism of the nucleation of guest-free Si₁₃₆ clathrate using molecular dynamics simulations with the Stillinger–Weber potential. The homogeneous nucleation of Si₁₃₆, which is realized in a narrow negative pressure range before liquid cavitation, exhibits the characteristic feature of the two-dimensional (2D) mode. The critical nucleus is composed of one to two five-membered rings, and the nucleation barrier is close to 1 k_BT. According to a thermodynamic model based on atomistic nucleation theory, the effective binding energy associated with the formation of 2D critical nuclei is significantly low, which is responsible for the low nucleation barrier of Si₁₃₆ clathrate. In the post-nucleation period, the critical nucleus preferentially grows into a dodecahedron, and the latter continuously grows with sharing face along 〈1 1 0〉.     

Aspects of nucleation and drift processes in a one-dimensional model

We introduce a one-dimensional model involving the nucleation and the drift of many particles. The model originates from interacting kink systems and simulates time evolution in modulated systems. In this model the nucleation rate of a particle depends nonlocally on the density of preexisting particles and the drift of particles is due to a weak and repulsive interaction among them. We first study the statistics of this model in the case that the drift of particles is negligible, and then consider the effects of the drift of particles.     

On the entropic nucleation barrier in a martensitic transformation

The nucleation of martensite in alloys is hindered by a free energy nucleation barrier, hence comprising contributions of the potential energy and the entropy. The leading effect is commonly attributed to the potential energy barrier due to strain fields. In this contribution, we investigate the nature of the entropic barrier by means of molecular dynamics (MD) simulations. We study a transformation process of an undercooled single crystal and examine two nucleation events observed under adiabatic conditions using vibrational mode analysis of the atomic trajectories. Our analysis shows that martensitic nucleations are indicated by transit from a state of uncorrelated into a state of correlated atomic motions. This correlation process is built up locally by a small group of atoms even before the product lattice can be recognized morphologically and it produces vibrational ‘soft’ modes along transformation paths. Phase space analyses unveil that the correlation process is characterized by narrow domains – ‘nucleation channels’ – the atomic trajectories have to pass, connecting the phase space domains of the parent and the product lattice. For a successful nucleation event, the nucleus atoms have to pass this channel collectively, which stochastically represents a rare event. Thermal fluctuations prevent finding the channel at elevated temperature and give rise for entropic stabilization of the parent phase. This ‘entropic nucleation barrier’ is reduced in the undercooled state but still effective, thus preventing the parent phase from collapsing into the product. The entropic barrier may be interpreted as the probability of a group of atoms to simultaneously pass the nucleation channel. Such group then represents a nucleus.     

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.