碳纳米管阵列储氢的分子动力学模拟

采用分子动力学方法,模拟了常温和不同压强下,氢在不同管径和管间距的单壁碳纳米管阵列(SWCNTA—S ingle-walled Carbon Nanotube Arrays)中的物理吸附过程.重点研究了压强、管径和管间距对SWCNTA(管内和管间隙)物理吸附储氢的影响.发现氢分子主要储存在SWCNTA的管壁附近,适当地增大管径和管间距可有效增加SWCNTA的物理吸附储氢量,使其在常温下具有较高的储氢能力,并给出了相应的理论解释.计算结果表明,在常温和中等压强下,SWCNTA的物理吸附总储氢量(重量百分比)可达4.2%,从而为同等条件下SWCNTA具有较高储氢能力的实验结果提供了直接的理论支持.     

多壁碳纳米管与基体之间界面热输运分子动力学模拟

碳纳米管与基体之间的热耦合对纳米结构界面热输运起主导作用。采用非平衡态分子动力学模拟了垂直生长多壁碳纳米管与Si基体之间的热耦合,得到界面热导的温度效应和作用力效应并利用声子输运理论进行了分析。低温下多壁碳纳米管与Si之间声子的态密度匹配较好,界面热输运能力主要取决于界面两侧低频声子的耦合振动,高温下近界面区域内热输运能力受高频声子的非弹性散射影响而逐渐增强。随着范德瓦尔斯力增强,界面热导线性增大。     

水溶液中石墨烯剥离的分子动力学模拟

通过分子动力学模拟方法研究柔性石墨烯在水溶液中剥离的热力学机制。为了表征石墨烯与水溶剂之间的相互作用,研究了石墨烯周围水溶剂的结构性质;通过施加外力的方法剥离石墨模型中最外层的石墨烯,计算剥离过程中体系自由能的变化,并对剥离过程中微观结构变化以及石墨烯周围水溶剂结构性质进行分析。结果表明:石墨烯与水之间的相互作用较小;剥离石墨烯需要克服一定的能垒;石墨烯之间的相互作用决定了自由能能垒的大小,溶剂诱导的贡献对剥离起到促进作用;受限区域内的水溶剂分子对剥离起到重要的作用。     

水-超临界二氧化碳界面的分子动力学模拟

运用分子动力学模拟方法研究20 MPa, 318.15 K条件下水超临界二氧化碳界面的微观结构及其自扩散性质,模拟采用TIP3P (transferable intermolecular potentials 3P)水分子模型和EPM2 (elementary physical model 2)二氧化碳分子势能模型.研究结果表明:水相和二氧化碳相形成明显的界面,界面层的分子有特定的取向;自扩散表现出明显的各向异性特点,并且在界面层二氧化碳与水的扩散趋于一致.     

纳米尺度水薄膜液-气相变分子动力学模拟

采用分子动力学模拟,研究了纳米尺度水薄膜厚度和系统温度对液-气相变蒸发率的影响.模拟了水薄膜厚度为2nm时,温度(375~425K)对相变蒸发率的影响;同时模拟了温度400K条件下,水薄膜厚度(2、3、4nm)对相变蒸发率的影响.结果表明:相同水薄膜厚度下蒸发率随着温度的升高而增大,相同温度下蒸发率随水薄膜厚度的增大而增大,蒸发率随时间呈指数形式递减.相关结果可为微热管及其他依靠内部液体相变传热的散热器件设计提供参考.     

Molecular dynamics simulations of non-Fourier heat conduction

Unsteady heat conduction is known to deviate significantly from Fourier＇s law when the system time and length scales are within certain temporal and spatial windows of relaxation. Classical molecular dynamics simulations were used to investigate unsteady heat conduction in argon thin films with a sudden temperature increase or heat flux at one surface to study the non-Fourier heat conduction effects in argon thin films. The studies were conducted with both pure argon films and films with vacancy defects. The temperature profiles in the argon films showed the existence of mechanical waves when the thin film was suddenly heated and the wave nature of the heat propagation. The flux phase relaxation time, Zq, and the temperature phase relaxation time,τt, were calculated from the temporal variations of the energy flux and temperature distribution in the film. Comparisons of the MD temperature profiles with temperature profiles predicted by Fourier＇s law show that Fourier＇s law is not able to predict the temperature variations with time. Different film thicknesses were also studied to illustrate the variation of the time needed for the films to reach steady-state temperature profiles after a sudden temperature rise at one surface and to illustrate the finite speed of the energy waves.     

磁场对水内能作用的分子动力学模拟

将水系统视为正则系统，采用分子动力学模型方法计算了磁场条件下水的内能，比热和径向分布函数，结果表明，磁场可以影响径向分布函数g(r)的分布，使水的结构发生变化，从而导致水的内能，比热发生改变，水的内能，比热，径向分布函数伴峰的高度与磁场强度的关系均呈多极值特征，且在常温下磁感应强度为0．25T时，磁场作用最明显。     

磁场对水内能作用的分子动力学模拟

将水系统视为正则系综 ,采用分子动力学模拟方法计算了磁场条件下水的内能、比热和径向分布函数。结果表明 ,磁场可以影响径向分布函数 g(r)的分布 ,使水的结构发生变化 ,从而导致水的内能、比热发生改变。水的内能、比热、径向分布函数伴峰的高度与磁场强度的关系均呈多极值特征 ,且在常温下磁感应强度为 0 .2 5T时 ,磁场作用最明显。     

烷烃油滴在超临界二氧化碳中溶解的分子动力学模拟

采用分子动力学模拟方法研究5种正构烷烃油滴在超临界CO2(sc CO2)中的溶解过程及其微观作用机制。结果表明:烷烃在sc CO2中的溶解能力随其链长的增加而逐渐降低。链长小于18时,烷烃易溶于sc CO2;链长大于18时,sc CO2对烷烃的溶解能力较弱。色散作用在sc CO2溶解烷烃的过程中起主要作用。色散作用随烷烃链长的增加而减弱,其强弱导致了不同链长的烷烃在sc CO2中的溶解能力不同。短链烷烃分子与CO2接触的概率较大,同时烷烃分子在sc CO2中的伸展程度随其链长的增加而减弱,导致烷烃与CO2间色散作用强弱不同。     

高岭石层间尿素-水体系的分子动力学模拟

用分子动力学（MD）方法对尿素-高岭石和尿素-水-高岭石两个体系进行模拟,计算结果表明,在300K时,高岭石平衡层间距随着尿素分子的增多不断增大,增幅逐渐减小;尿素在水溶液中的摩尔分率在0.58左右时高岭石平衡层间距有最大值。尿素在高岭石片层间成双层分布,分别通过O=C键和N—H键与高岭石表面原子形成氢键。水在层间主体相成三层分布,有部分水分子吸附在高岭石表面上。     

Water conduction through the hydrophobic channel of a carbon nanotube.

Confinement of matter on the nanometre scale can induce phase transitions not seen in bulk systems. In the case of water, so-called drying transitions occur on this scale as a result of strong hydrogen-bonding between water molecules, which can cause the liquid to recede from nonpolar surfaces to form a vapour layer separating the bulk phase from the surface. Here we report molecular dynamics simulations showing spontaneous and continuous filling of a nonpolar carbon nanotube with a one-dimensionally ordered chain of water molecules. Although the molecules forming the chain are in chemical and thermal equilibrium with the surrounding bath, we observe pulse-like transmission of water through the nanotube. These transmission bursts result from the tight hydrogen-bonding network inside the tube, which ensures that density fluctuations in the surrounding bath lead to concerted and rapid motion along the tube axis. We also find that a minute reduction in the attraction between the tube wall and water dramatically affects pore hydration, leading to sharp, two-state transitions between empty and filled states on a nanosecond timescale. These observations suggest that carbon nanotubes, with their rigid nonpolar structures, might be exploited as unique molecular channels for water and protons, with the channel occupancy and conductivity tunable by changes in the local channel polarity and solvent conditions.     

不同结构单壁碳纳米管热传导的分子动力学模拟

采用基于线性响应理论的平衡态分子动力学方法,计算了扶手椅型和锯齿型单壁碳纳米管(SWNTs)的导热系数,并给出其随管长、直径和温度的变化.在此基础上,对2种结构SWNTs的热传导性能进行了比较.模拟结果表明:SWNTs的导热系数在管长较短时随着管长的增加有所浮动但并不收敛,随温度的升高,导热系数将降低;2种结构SWNTs的导热系数均随着直径的减小而升高,且在相同直径时,不同结构碳纳米管的导热系数相差不大,这与输运方程进行理论求解的结果较为一致.     

用分子动力学方法对Cystatin蛋白的研究

通过动力学模拟研究野生型和I66Q突变型cC的动力学特性,为设计和筛选治疗Cystatin C引起的脑淀粉样血管病药物提供重要的理论依据.通过分子动力学模拟的方法,选择可引起脑淀粉样血管病的Cystatin蛋白为模型,探讨了在形成致病状纤维过程中起到关键作用的蛋白质结构位点.研究分子动力学模拟显示,突变型cC较野生型更易形成淀粉样纤维,突变型蛋白的L1部位较野生型具有更高的柔性,更易通过结构域交换形成二聚体进而引发淀粉样纤维沉淀.该研究结果对于设计治疗相关疾病如疯牛病等的药物也有一定的借鉴意义.     

Molecular dynamics simulation of the deposition process of hydrogenated diamond-like carbon （DLC） films

The deposition process of hydrogenated diamond-like carbon （DLC） film greatly affects its frictional properties. In this study, CH3 radicals are selected as source species to deposit hydrogenated DLC films for molecular dynamics simulation. The growth and structural properties of hydrogenated DLC films are investigated and elucidated in detail. By comparison and statistical analysis, the authors find that the ratio of carbon to hydrogen in the films generally shows a monotonously increasing trend with the increase of impact energy. Carbon atoms are more reactive during deposition and more liable to bond with substrate atoms than hydrogen atoms. In addition, there exists a peak value of the number of hydrogen atoms deposited in hydrogenated DLC films. The trends of the variation are opposite on the two sides of this peak point, and it becomes stable when impact energy is greater than 80 eV. The average relative density also indicates a rising trend along with the increment of impact energy, while it does not reach the saturation value until impact energy comes to 50 eV. The hydrogen content in source species is a key factor to determine the hydrogen content in hydrogenated DLC films. When the hydrogen content in source species is high, the hydrogen content in hydrogenated DLC films is accordingly high.     

Molecular dynamics simulation of the ice nucleation and growth process leading to water freezing

Upon cooling, water freezes to ice. This familiar phase transition occurs widely in nature, yet unlike the freezing of simple liquids, it has never been successfully simulated on a computer. The difficulty lies with the fact that hydrogen bonding between individual water molecules yields a disordered three-dimensional hydrogen-bond network whose rugged and complex global potential energy surface permits a large number of possible network configurations. As a result, it is very challenging to reproduce the freezing of 'real' water into a solid with a unique crystalline structure. For systems with a limited number of possible disordered hydrogen-bond network structures, such as confined water, it is relatively easy to locate a pathway from a liquid state to a crystalline structure. For pure and spatially unconfined water, however, molecular dynamics simulations of freezing are severely hampered by the large number of possible network configurations that exist. Here we present a molecular dynamics trajectory that captures the molecular processes involved in the freezing of pure water. We find that ice nucleation occurs once a sufficient number of relatively long-lived hydrogen bonds develop spontaneously at the same location to form a fairly compact initial nucleus. The initial nucleus then slowly changes shape and size until it reaches a stage that allows rapid expansion, resulting in crystallization of the entire system.     

Molecular dynamics simulation of wetting behavior at CO2/water/solid interfaces

We used molecular dynamics simulation to demonstrate the microscopic wetting behavior of two solid model surfaces for the first time. Hydrophilic and hydrophobic features were modeled in a dense CO₂ fluid environment under various densities. The water droplet loses contact with the surface under the influence of higher density CO₂ fluids on the hydrophobic surface. For the hydro-philic surface, no separation between the water droplet and the surface was observed. However, the contact angle of the water droplet on the hydrophilic surface was found to increase with the fluid density. The effect of dense CO₂ fluid on the surface wettability can be interpreted in terms of enhanced interactions from the surrounding CO₂ molecules.     

石墨烯纳米压痕实验的分子动力学模拟

我们基于分子动力学(Molecular Dynamics), 建立了石墨烯纳米压痕实验的数值模型, 以模拟压痕实验过程, 得到典型实验过程的力-位移曲线, 并进而讨论压头下压速度, 压头半径以及边界条件等因素对实验结果的影响. 论文测得石墨烯弹性模量为1 TPa, 强度为240 GPa. 加载过程中, 压头加载到临界压入深度hc时, 石墨烯试件在压头处撕裂破坏. 给定最大压入深度, 对石墨烯进行加载—卸载—再加载试验, 发现当最大压入深度h_max小于h_c时, 石墨烯发生的是完全弹性变形; 当最大压入深度h_max大于h_c时, 卸载和再加载过程中石墨烯能基本恢复原貌, 但仍有少数C—C键较长而无法恢复, 成为石墨烯再加载时的破坏起点, 石墨烯的破坏力和位移都显著下降. 另外, 还发现大于0.05 nm/ps的压头速度和压头半径对石墨烯临界压入深度和破坏力都有显著影响.     

纳米粒子介电泳的分子动力学模拟

为研究微流体环境下纳米粒子的介电泳现象并分析其介电泳特性,采用非平衡态分子动力学方法对纳米胶体粒子及其周围溶剂粒子进行建模.介电泳模拟之前,通过对系统能量和温度的趋衡过程进行模拟,使纳米胶体所处的微流体系统达到稳定状态,并得出系统能量以及温度变化过程的趋衡图.对纳米胶体模型施加非均匀电场,使胶体电偶极化.变化非均匀电场强度,研究胶体模型失效的一般规律.发现随着非均匀电场强度的增加,小离子有不断脱离大离子表面的趋势,胶体模型失效的临界电场强度参数为Eo=15s/(eó).此外,对不同极性的纳米胶体的介电泳现象进行模拟,发现在正介电泳情况下,胶体的电偶极距不断增大,且电偶极距大的胶体有较大的介电泳速度和位移.     

纳米尺度下对黏滑现象的分子动力学模拟

采用分子动力学模拟的方法研究了硅针尖在金刚石基底滑动的黏滑现象,讨论了纳米尺度下温度、滑动速度、载荷等因素对黏滑摩擦的影响.模拟结果表明,在纳米尺度下,原子排列规则的两固体间的滑动摩擦力呈现出周期性的锯齿型变化.摩擦力曲线的波动周期取决于滑动过程中基体沿滑动方向的晶格常数,同时受接触面原子排列结构变化的影响.在较低温度范围内,滑动摩擦力随温度的升高近似呈线性减小,对滑动摩擦力的波动周期和振幅影响不大.在一定速度范围内,滑动摩擦力主要受黏着作用的影响,滑动摩擦力的大小随速度的增大近似呈线性增大.在一定载荷范围内,滑动摩擦力随载荷的增大近似呈线性增大,振动周期变大.     

超晶格薄膜热传导的分子动力学模拟

采用非平衡态分子动力学方法模拟了超晶格薄膜的热传导性能，并对其主要影响因素作了分析．模拟结果显示，周期长度固定的超晶格薄膜，界面热阻在总热阻中的比例和导热系数同周期数无关；当超晶格薄膜的膜厚不变时，导热系数将随着周期长度的增大而增大，但由于超晶格薄膜晶格常数的不匹配，使其内部发生明显的几何形变，这种变化关系也愈加复杂，同时周期长度的增加，平均界面热阻也随着增大，揭示了界面热阻不仅取决于界面层的物理条件，而且也与构成的介质内部形变有着重要关系．