多壁碳纳米管外壁高温蒸发的分子动力学模拟

采用经典分子动力学（MD）方法,使用EDIP（environment-dependent interatomic potential)势描述C纳米管内C原子之间相互作用,对多壁C纳米管由于Stone-Wales缺陷引起外层管高温剥落蒸发现象进行了计算模拟.研究结果表明,高温下多壁C纳米管外层管Stone-Wales缺陷处C原子剧烈振动导致C—C键断裂形成悬键,并逐渐向四周扩散导致外层管剥落蒸发.利用Lindemann指数作为判据,得出多壁C纳米管外层管出现剥落蒸发的温度为2290 K左右,与Huang Jianyu等实验中观测到多壁C纳米管外层管剥落蒸发现象产生的温度2000 ℃基本一致.关键词：多壁C纳米管分子动力学Stone-Wales缺陷剥落蒸发     

碳纳米管振荡的分子动力学模拟

运用分子动力学模拟方法,模拟了三种碳纳米管振荡器内管的振荡运动.结果显示：振荡器的内管越短,振荡的频率越大,且受到的轴向回复力的波动也越大.内管在沿着管轴振荡的同时,还绕着管轴旋转,转动的动能有明显涨落并与内管管长密切相关.该研究对于开发碳纳米管的相关应用技术有指导意义.关键词：分子动力学模拟多壁碳纳米管振荡     

纳米液滴蒸发过程的分子动力学模拟

本文从分子水平对纳米氮液滴蒸发的微观过程进行了分子动力学研究,其中液氮分子间采用球形截断的LJ势能函数.从微观模拟信息中获取了相应的宏观性质,示出了液滴在蒸发过程中的分子位形的变、密度分布、双体分布函数和压力分布,探究了纳米液滴在蒸发过程中界面现象的物理机理.     

碳纳米管对接成异质结器件的计算模拟

基于Stone-Wales缺陷演变理论与分子动力学、Monte Carlo计算方法, 进行了碳纳米管(CNTs)对接成异质结器件的计算模拟.首先, 提出了一种模拟CNTs端帽位置变化的新算法, 并计算模拟了单根CNT的端帽从开口到闭合的过程. Stone-Wales缺陷演变被设计模拟这些端帽变化的跃变过程, 以模拟C–C键的生成与断裂, 而分子动力学则作为跃变后构型弛豫的渐变模拟. 同时, 研究了不同管型CNTs的端帽打开并对接形成异质结的过程.研究结果显示, 对接初期在对接处先产生大量的缺陷, 以促进反应的发生. 这些缺陷趋向于演变成稳定的六元环结构, 或者五元环/七元环的结构, 使异质结趋于稳定.关键词：碳纳米管Monte CarloStone-Wales缺陷分子动力学     

多壁碳纳米管弯曲导致的扭转

文章介绍了多壁碳纳米管弯曲的分子动力学模拟,从原子尺度上解释了由于屈曲失稳而导致的起皱现象,并发现多壁碳纳米管弯曲时会呈现出非线性的力学响应。同时,在模拟过程中,观察到了弯曲诱发扭转的现象,并揭示出扭转变形的内在起因是曲率诱导的晶格错配。     

多壁碳纳米管弯曲导致的扭转

文章介绍了多壁碳纳米管弯曲的分子动力学模拟,从原子尺度上解释了由于屈曲失稳而导致的起皱现象,并发现多壁碳纳米管弯曲时会呈现出非线性的力学响应。同时,在模拟过程中,观察到了弯曲诱发扭转的现象,并揭示出扭转变形的内在起因是曲率诱导的晶格错配。     

一种新型的三壁碳纳米管螺旋振荡器:分子动力学模拟

本文提出了一种新型的三壁碳纳米管螺旋振荡器,通过对内管施加轴向激励和中管施加旋转激励的方式,来同时获得内管和中管的螺旋信号输出.采用分子动力学方法研究了该振荡器在拉转耦合下的振荡行为.在模拟过程中,固定的外管充当振荡器定子的作用,内管和中管在分别施加一定的初始激励后保持自由振荡.模拟结果表明,在内管拉出距离一定的情况下,内管的自激发旋转频率随着中管初始旋转激励频率的增加而增加,且最终趋于一个稍低于旋转激励的稳定值.当施加的初始旋转频率在400 GHz以内时,内管达到稳定的旋转频率ω_I与旋转激励频率ω_M0的关系为ω_I=46e⁽0.0045ω_M0).尽管提高初始旋转激励频率可以提高内管的旋转频率,但随着中管初始旋转频率的增加内管的轴向性能下降,不稳定振荡加剧.同时中管轴向振荡的稳定性与施加在其上面的初始旋转激励的频率有关,过高的初始旋转频率不仅会加大非轴向摆动距离,导致轴向振荡性能下降,而且旋转损耗比也将随着初始旋转频率的增加而增加.因此,合理的控制初始旋转频率的幅值是设计低损耗三...     

多壁碳纳米管与金属表面间接触行为的分子动力学模拟

碳纳米管与金属表面间的界面接触行为对碳基电子器件的性能研究具有重要意义.本文利用分子动力学模拟方法研究了端部开口和闭口的多壁碳纳米管与金属表面间的界面接触构型和粘着强度.模拟结果表明,多壁碳纳米管在金属表面上的位置和形状变化是因为减少的范德瓦耳斯能转变成碳纳米管的内能;多壁碳纳米管与金属表面的结合能为负值,表明两者存在粘着作用;多壁碳纳米管水平地接触金属表面时的粘着强度受初始间距、接触长度和金属种类影响,即范德瓦耳斯能作用的初始间距阈值约为1 nm且最终平衡状态时的两者间距约为0.3 nm;随着两者接触长度增加,粘着强度增大,铂与碳纳米管的粘着强度比钨和铝的大;端部开口和闭口的碳纳米管与金属表面垂直接触时的粘着强度都比水平接触时的小,两端开口的碳纳米管在金属表面上发生径向压缩变形,最终形成带状结构;而两端封闭的碳纳米管在金属表面上发生轴向压缩变形;在碳纳米管场效应晶体管中,两端开口的多壁碳纳米管与单壁碳纳米管一样变形成带状结构,并且各个管壁之间以及最外层管壁与金属电极之间的间距相当,该原子尺度的间距(约0.34 nm)保证电子从金属隧穿到最外层管壁,并在内层管壁之间径向迁移.     

固壁加热的分子动力学模拟研究

本文应用分子动力学模拟方法,讨论了NVE系综中不同初始晶格类型对模拟系统平衡温度和截断半径对模拟系统平衡总能的影响.建立了一种新的固壁加热模型,模拟研究了由铂组成固壁对流体氩快速加热过程.在本文的模拟条件下,由于固壁与液体之间的快速加热作用,固壁与液体之间产生的蒸汽推动液体向上移动,可以明显地观察到Leidenfrost现象的产生.     

高盐溶液蒸发过程中的分子动力学模拟与研究

随着我国工业的快速发展,如何减量化处理工业排放的高盐废水已经成为了亟待解决的环境问题.在处理高盐废水的各种工艺中,热蒸发技术因具备脱盐效果好,灵活性好等特点得到了广泛应用.本文从微观角度出发,采用分子动力学模拟方法研究了LiCl、KCl、CaCl₂三种溶液在400 K和500 K两种温度加热条件下的蒸发过程,分析了Li⁺、K⁺、Ca²⁺,Cl^-四种盐离子对蒸发速率、水分子取向、氢键、溶液结构等性质的影响.研究结果表明温度的提升会对蒸发速率产生极大影响,温度的提高不会改变配位水化层的位置但会明显减少离子的配位水分子数,有利于提高活跃水分子的占比和蒸发速率.     

Control of one-dimensional transport in multi-wall carbon nanotubes by wall destruction

We have investigated the damage in multi-wall carbon nanotubes (MWNTs) destroyed by electrical breakdown and focused ion beam bombardment (FIBB). The transport properties of a MWNT destroyed by electrical breakdown have been compared with those of a MWNT destroyed by FIBB. Also the Tomonaga–Luttinger transport (TLT) model has been applied to each type of destroyed MWNT. The MWNT destroyed by FIBB showed TLT behavior because of the weak destruction of the remaining walls. However, in the case of MWNTs destroyed by electrical breakdown, three-dimensional variable-range hopping (VRH) was observed in the low temperature transport. This suggests that the local damage has been caused by strong breakdown. There exists a clear difference between the effects of electrical breakdown and FIBB. Wall destruction by FIBB could be applied to control the one-dimensional transport of MWNTs.     

Molecular dynamics simulation of silver bromide nanostructures in single-walled carbon nanotubes

Nanostructures formed upon filling single-walled carbon nanotubes of different diameters (ranging from 11.5 to 17.6 ?) with silver bromide have been investigated using the molecular dynamics method. The results of molecular dynamics computer simulation have demonstrated that, in such tubes, AgBr nanotubes in the form of rolled-up two-dimensional crystalline networks (including structures both with a trigonal coordination and with a tetragonal coordination of ions) can be produced as well as fragments of the NaCltype structure, which is typical of bulk AgBr crystals. In the initial stage of their filling, the carbon nanotubes in the silver bromide melt are deformed, on average, to a greater extent than those in a similar system with AgI. After taking out from the melt, the degree of deformation of the nanotubes decreases and, in the majority of cases, AgBr nanotubular structures based on a hexagonal network are formed inside them.     

Molecular dynamics simulation of an argon cluster filled inside carbon nanotubes

The effects of the diameters of single-walled carbon nanotubes （SWCNTs） （7.83A to 27.40A） and temperature （20 K-45 K） on the equilibrium structure of an argon cluster are systematically studied by molecular dynamics simulation with consideration of the SWCNTs to be fixed. Since the diameters of SWCNTs with different chiralities increase when temperature is fixed at 20 K, the equilibrium structures of the argon cluster transform from monoatomic chains to helical and then to multishell coaxial cylinders. Chirality has almost no noticeable influence on these cylindrosymmetric structures. The effects of temperature and a non-equilibrium sudden heating process on the structures of argon clusters in SWCNTs are also studied by molecular dynamics simulation.     

Dislocation dynamics in multiwalled carbon nanotubes at high temperatures

Dislocation dynamics dictate the mechanical behavior of materials. Dislocations in periodic crystalline materials have been well documented. On the contrary, dislocations in cylindrical carbon nanotubes, particularly in multiwalled carbon nanotubes (MWCNTs), remain almost unexplored. Here we report that a room temperature 1/2<0001> sessile dislocation in a MWCNT becomes highly mobile, as characterized by its glide, climb, and the glide-climb interactions, at temperatures of about 2000 degrees C. The dislocation glide leads to the cross-linking of different shells; dislocation climb creates nanocracks; and the interaction of two 1/2<0001> dislocations creates kinks. We found that dislocation loops act as channels for mass transport. These dislocation dynamics are drastically different from that in conventional periodic crystalline materials due to the cylindrical, highly anisotropic structures of MWCNTs.     

Molecular dynamics simulation of SnF2 nanostructures in the internal channels of single-walled carbon nanotubes

A molecular dynamics simulation of solid tin(II) fluoride nanostructures formed in internal channels of single-walled carbon nanotubes (SWCNTs) has been performed using two types of model potentials—without and with inclusion of the polarization of ions. For the potential taking into account the polarization of ions, an ordered SnF₂@SWCNT structure is reproduced: in SWCNT(10, 10), it has the form of the SnF₂ internal nanotube. At the same time, the SnF₂@SWCNT(11,11) structure is substantially disordered (glass-like). It has been found that heating of the SnF₂@SWCNT model system produces a superionic state characterized by a high mobility of fluorine ions without migration of tin ions. The model potentials disregard the covalent character of Sn-F bonds and the specific interactions of a lone electron pair of the Sn²⁺ ion. This makes it impossible to completely reproduce the properties of SnF₂ at normal pressures. However, some characteristics of the SnF₂ high-pressure modification can be reproduced if the polarization of ions is taken into account.     

Molecular dynamics simulation of premelting of metals at high pressure

By the molecular dynamics method, we study the process of premelting of iron and aluminum at high pressures under conditions of contact with amorphous argon. We calculate the thickness of the disorder layer of the metal forming at the component interface at various temperatures. We discuss the possibility that premelting affects interpretation of the measurement results of melting curves in diamond anvils.     

Field-induced evaporation of carbon nanotubes

We report here an experimental observation of field emission from arrays of multiwall carbon nanotubes. Current densities in the range 10–30 mA/cm² with excellent long-term stability were recorded. A detailed study of the destruction of nanotubes at extreme operation conditions is performed. We established that field evaporation of nanotubes accompanies field emission from a cold cathode at electric fields higher than 2 V/?. Electron microscopy of the evaporation products reveals irregularly shaped carbon nanoparticles with a hollow core. The diameter of the particles is ∼20 nm. A mechanism of the process is proposed and discussed. Received: 6 October 2000 / Accepted: 28 April 2001 / Published online: 27 June 2001     

Molecular dynamics simulation on surface modification of carbon black with polyvinyl alcohol

A novel and reasonable model of carbon black (CB) was built to investigate the surface encapsulation modification of CB particles with polyvinyl alcohol (PVA) by molecular dynamic simulation. The modification process of PVA on CB surface was intuitively exhibited, which indicated that the encapsulation films were formed by stacking PVA molecules layer by layer on the surface of CB. The dispersion stability of unmodified and modified CB particles was compared both by simulation and by scanning electron microscope graphs. The simulation results indicated that surface modification had great effect on reducing the interaction energy of CB/water interface so as to alleviate agglomeration phenomenon significantly. Moreover, the influences of PVA amount on the morphology and the compatibility of PVA/CB interface were also investigated. The amount of PVA would neither influence the arrangement of single PVA molecule nor change the orientation of the single PVA layer but only decide the thickness of the modification film. Besides, the energy analysis results indicated that a suitable thickness of PVA coating could improve the dispersion stability of CB particles.     

Synthesis of multi-wall carbon nanotubes by simple pyrolysis

A new pyrolysis technique has been developed for the synthesis of multi-walled carbon nanotubes (MWCNTs). In this simple method diethyl ether and nickelocene is pyrolysized in a reaction quartz tube without using carrier gas. The samples are prepared at pyrolysis temperatures of 650 and 950 ^°C and the effect of temperature on the tube morphology investigated. Purification has been done following the standard oxidation and acid bath treatment. The as-synthesized and purified nanotubes have been characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), transmission electron microscope (TEM), thermogravimetric analysis (TGA) and micro-Raman spectroscopy. The technique has great advantages such as low cost and easy operation for the production of CNTs.