Molecular Dynamics Simulations of Helium Behaviour in Titanium Crystals

Molecular dynamics simulations are performed to investigate the behaviour of helium atoms in titanium at a temperature of 30OK. The nucleation and growth of helium bubble has been simulated up to 50 helium atoms. The approach to simulate the bubble growth is to add helium atoms one by one to the bubble and let the system evolve. The titanium cohesion is based on the tight binding scheme derived from the embedded atom method, and the helium-titanium interaction is characterized by fitted potential in the form of a Lennard-Jones function. The pressure in small helium bubbles is approximately calculated. The simulation results show that the pressure will decrease with the increasing bubble size, while increase with the increasing helium atoms. An analytic function about the quantitative relationship of the pressure with the bubble size and number of helium atoms is also fitted.     

Melting Behaviour of Shell-symmetric Aluminum Nanoparticles: Molecular Dynamics Simulation

用基于镶嵌原子方法势能的分子动力学模拟研究了含有561个原子的铝纳米粒子. 利用总势能和比热来计算铝纳米粒子的熔点：二十面体、十面体、切去顶端的八面体铝纳米粒子的熔点分别是540±10、500±10和520±10 K. 均方位移、键参数和回转半径的变化趋势与势能和比热的变化一致. 通过拟合均方位移得到了Kohlraush-William-Watts弛豫法则中的弛豫时间和伸缩参数,计算表明在高温区域弛豫时间和温度之间遵循标准阿伦尼乌斯关系.     

单轴应力场下钨中氦扩散行为的分子动力学模拟研究

本文采用分子动力学方法研究了沿<100>及<111>晶向的单轴应变对钨中单个氦原子扩散的影响。结果表明,应变会使得金属钨材料发生相变,且引起相变的临界应变随温度升高而减小。相变起始的应变在达到抗拉强度的应变附近。计算结果表明,拉应变使得单个氦原子在钨中的扩散系数发生骤降,在不同应变下扩散系数变化平缓。沿<100>晶向氦扩散系数随应变的增大而线性减小,而<111>晶向则出现了震荡变化趋势。研究结果表明,沿<100>晶向应变达到+0.15%时阿纽列斯方程不再适用,而沿<111>晶向应变大+5%阿纽列斯方程仍然适用;沿<111>晶向随应变增加氦扩散激活能减小,说明应变使得单个氦原子在钨中迁移性增强。     

Strain Rate Sensitivities of Face-Centred-Cubic Metals Using Molecular Dynamics Simulation

We use dislocation theory and molecular dynamics （MD） simulations to investigate the effect of atom properties on the macroscopic strain rate sensitivity of f cc metals. A method to analyse such effect is proposed. The stress dependence of dislocation velocity is identified as the key of such study and is obtained via 2-D MD simulations on the motion of an individual dislocation in an fcc metal. Combining the simulation results with Orowan＇s relationship, it is concluded that strain rate sensitivities of fcc metals are mainly dependent on their atomic mass rather than the interatomic potential. The order of strain rate sensitivities of five fcc metals obtained by analysing is consistent with the experimental results available.     

Annealing Behaviour of Helium Bubbles in Titanium Films by Thermal Desorption Spectroscopy and Positron Beam Analysis

Helium-containing Ti Glms are prepared using magnetron sputtering in the helium-argon atmosphere. Isochronal annealing at different temperatures for an hour is employed to reveal the behaviour of helium bubble growth. Ion beam analysis is used to measure the retained helium content. Helium can release largely when annealing above 970K. A thermal helium desorption spectroscopy system is constructed for assessment of the evolution of helium bubbles in the annealed samples by linear heating （OAK/s） from room temperature to 1500K. Also, Doppler broadening measurements of positron annihilation radiation spectrum are performed by using changeable energy positron beam. Bubble coarsening evolves gradually below 680K, migration and coalescence of small bubbles dominates in the range of 68-970 K, and the Ostwald ripening mechanism enlarges the bubbles with a massive release above 970K.     

Molecular Dynamics Simulation of Strontium Titanate

The molecular dynamics method is used to simulate the thermophysical properties of SrTiO3 thermoelectric material in the temperature range 300-2200 K. The Morse-type potential functions added to the Busing-Ida type potential for interatomic interaction are used in the simulation. The interatomic potential parameters are determined by fitting to the experimental data of lattice parameters with temperature and the data reported in literature. The linear thermal expansion coefficient, heat capacity and lattice contributions to the thermal conductivity are analyzed. The results agree with the data reported in the literature.     

半晶态聚合物的分子动力学模拟

采用粗粒化聚乙烯醇模型,应用分子动力学方法模拟熔融态聚合物经过缓慢冷却、局部结晶形成半晶态聚合物的过程.静态结构因子的演变显示出在结晶初期小角散射强度的增大先于布拉格峰的出现,这与小角/大角X射线散射实验现象相一致.模拟得到的半晶态聚合物呈现为折叠链构成的晶区与非晶区交杂在一起的结构形态,与缨状微束结构模型相一致.研究发现在不同的冷却阶段具有不同的有序结构形成机制.从结晶温度到玻璃化温度的凝固过程中,存在分子链的伸展和伸直分子链之间平行排列两种形式的结构转变;而在玻璃化温度之后,材料的活性只允许调整伸直分子链之间的相对排列位置.     

不同热浴下Co团簇熔化行为的分子动力学模拟

运用分子动力学方法,采用Berendsen热浴和Nose-Hoover热浴分别研究了Co_n(n=13,55,147)团簇的熔化特性,模型采用Gupta相互作用势.模拟结果表明:两种热浴对钴团簇熔点及预熔化区间给出了基本一致的描述.所研究团簇体系在给定温度下长时间内各Co团簇中单个原子的速率(速度)分布与麦克斯韦速率(速度)分布曲线符合很好.     

不同热浴下Co团簇熔化行为的分子动力学模拟

运用分子动力学方法,采用Berendsen热浴和Nose-Hoover热浴分别研究了Con (n=13,55,147)团簇的熔化特性,模型采用Gupta相互作用势.模拟结果表明：两种热浴对钴团簇熔点及预熔化区间给出了基本一致的描述.所研究团簇体系在给定温度下长时间内各Co团簇中单个原子的速率(速度)分布与麦克斯韦速率(速度)分布曲线符合很好.     

Dielectric Behaviour of Liquid Crystals

The influence of the molecule structure on the dielectric anisotropy of nematic liquid crystals was experimentally investigated and discussed from the point of view of the Maier-Meier theory. Further, the reorientation time of the dipoles in different smectic phases was measured and correlated to the structure of the phases. Some technical aspects are discussed in connection with the dielectric relaxation phenomenon.     

Molecular Dynamics Simulation of Bubble Nucleation in Explosive Boiling

Molecular dynamics （MD） simulation is carried out for the bubble nucleation of liquid nitrogen in explosive boiling. The heat is transferred into the simulation system by rescaling the velocity of the molecules. The results indicate that the initial equilibrium temperature of liquid and molecular cluster size affect the energy conversion in the process of bubble nucleation. The potential energy of the system violently varies at the beginning of the bubble nucleation, and then varies around a fixed value. At the end of bubble nucleation, the potential energy of the system slowly increases. In the bubble nucleation of explosive boiling, the lower the initial equilibrium temperature, the larger the size of the molecular cluster, and the more the heat transferred into the system of the simulation cell, causing the increase potential energy in a larger range.     

Molecular Dynamics Simulation of Thermal Conductivity in Si--Ge Nanocomposites

Thermal conductivity of nanocomposites is calculated by molecular dynamics （MD） simulation. The effect of size on thermal conductivity of nanowire composites and the temperature profiles are studied. The results indicate that the thermal conductivity of nanowire composites could be much lower than alloy value; the thermal conductivity is slightly dependent on temperature except at very low temperature.     

Molecular Dynamics Simulation of Water Confined in Carbon Nanotubes

Molecular dynamics simulations are performed for water confined in carbon nanotubes with various diameters （11.0-13.8 A ）. The simulations under an isobaric pressure （one atmosphere） by lowering temperatures from 300 K to 190K are carried out. Water molecules within variously sized tubes tend to transform from disorder to order with different configurations （four-water-molecule ring, six-water-molecule ring and seven-water-molecule ring） at phase transition temperatures, which may be lowered by the increasing tube radius. It is also found that the configurations of water in （10, 10） tube are not unique （seven-molecule ring and seven-molecule ring plus water chain）.     

Gold Nanobelt Reorientation by Molecular Dynamics Simulation

The embedded atom method is used to study the structure stability of gold nanobelt. The Au nanobelts have a rectangular cross-section with （100） orientation along the x^-,γ- and z-axes. Free surfaces are used along the x- and y-directions, and periodic boundary condition is used along z-direction. The simulation is performed at different temperatures and cross-section sizes. Our results show that the structure stability of the Au nanobelts depends on the nanobelt size, initial orientation, boundary conditions and temperature. A critical temperature exists for Au nanobelts to transform from initial （100） nanobelt to final （110） nanobelt. The mechanism of the reorientation is the slip and spread of dislocation through the nanobelt under compressive stress caused by tensile surface-stress components.     

Copper Nanobelt Reorientation by Molecular Dynamics Simulation

Atomic simulations using embedded atom method （EAM） are performed for Cu （100） nanobelts to study the structural and mechanical behaviour. Cu （100） nanobelts are along[001] taken as the z-axis and have a rectangular cross section in the x - y plane, with [100] and [010] taken as x and y axes. The periodic boundary is used along the z-axis to simulate an infinitely long nanobelt, and other surfaces are free. The simulations are carried out under the mechanical loading with an elongation strain rate of 8.0×10^8 s^-1 along the z-axis. The results show that the nanobelt undergoes a transition from the initial structure with a （100） axis and {100} lateral surfaces to a new structure with the （112） as the z-axis and the lateral surfaces are {111} and {110} respectively, instead of the original {100} surfaces. The mechanism of the structural transition is ascribed to the dislocation propagation through the nanobelt under the external stresses.     

Stabilities and Dynamics of Protein Folding Nuclei by Molecular Dynamics Simulation

To understand how the stabilities of key nuclei fragments affect protein folding dynamics, we simulate by molecular dynamics(MD) simulation in aqueous solution four fragments cut out of a protein G, including oneα-helix(seq B: KVFKQYAN), two β-turns(seq A: LNGKTLKG and seq C: YDDATKTF), and one β-strand(seq D:DGEWTYDD). The Markov State Model clustering method combined with the coarse-grained conformation letters method are employed to analyze the data sampled from 2-μs equilibrium MD simulation trajectories. We find that seq A and seq B have more stable structures than their native structures which become metastable when cut out of the protein structure. As expected, seq D alone is flexible and does not have a stable structure. Throughout our simulations, the native structure of seq C is stable but cannot be reached if starting from a structure other than the native one, implying a funnel-shape free energy landscape of seq C in aqueous solution. All the above results suggest that different nuclei have different formation dynamics during protein folding, which may have a major contribution to the hierarchy of protein folding dynamics.     

Molecular Dynamics Simulation of Thermal Decomposition of Hydrogen

Physics of Atomic Nuclei - The molecular dynamics method was used for simulation of thermal decomposition of hydrogen molecules. As a test substances, we used liquid hydrogen. Interaction between...     

Cu-Ni合金结构的分子动力学模拟

采用EAM作用势对Cu Ni合金的结构特性进行了MD模拟研究 .通过FZ结构因子可发现 ,Cu含量的变化对结构因子的波动影响很小 ,键取向序参数和键对也表现出相似的变化规律 ,这表明液态Cu Ni合金对成份变化不敏感 ,体系中的化学序较弱 .将Cu70 Ni3 0 合金熔体的FZ结构因子与Waseda的实验结果进行对比 ,发现二者吻合得较好 ,表明EAM势可以很好地描绘Cu Ni合金的结构特性 .在快速冷却过程中 ,除了Cu2 0 Ni80 合金外 ,其他合金成份的双体分布函数的第二峰都发生了劈裂 ,标志着体系最终形成了非晶结构 ,而Cu2 0 Ni80 合金的双体分布函数却表现出晶体峰的特征 .通过对键取向序参数、键型指数以及铜镍原子的有效扩散系数的分析表明 ,在快速冷却过程中 ,Cu2 0 Ni80 合金最终形成了hcp晶体结构     

氯化铵水溶液的分子动力学模拟

采用分子动力学模拟方法,使用Material Studio软件中的COMPASSⅡ力场,模拟了温度为298 K时质量分数分别为1%、5%、10%、20%及28%的氯化铵溶液中离子的动力学性质.发现随着浓度增加,水分子周围不再有明显的第二配位圈,NH_4~+和Cl~-配位数都减少,离子水化数、水化半径也随之减小.水分子中的H比NH_4~+中的H对N_((NH4))的影响作用要强的多.在氯化铵水溶液,NH_4~+中的N及H都是由周围水分子中的氧原子来靠近,而Cl~-则由周围水分子中的氢原子去靠近. NH_4~+及Cl~-的加入确实破坏了溶液中的氢键,但是NH_4~+与水分子间也生成了新的氢键,并且生成的数量大于被破坏的数量,但是氢键力却变弱了,这是之前研究当中没有人提过的现象.随着浓度增大,H_2O、NH_4~+及Cl~-的扩散系数逐渐降低,而降低幅度排序为NH_4~+ Cl~- H_2O.     

超高压水的分子动力学模拟

 采用平衡分子动力学（EMD）方法,模拟研究了温度范围为243~348 K、压强范围为0.1~400 MPa条件下水的热力学性质、结构和动力学性质,模拟结果与实验值吻合较好。模拟结果表明,随着压强的增大,水分子间的氢键作用增强,扩散系数减小;随着温度的升高,水分子间的氢键作用减弱,有序程度下降,扩散系数增大。但在过冷水中,扩散系数随压强的增大有增加的趋势。