碳纳米管组装纳米新材料的物理特性研究
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摘要
1991年Iijima等人在实验上发现了一种称之为碳纳米管的新型纳米材料。这一发现开启了一个纳米研究领域内理论和实验双方面的研究热潮。众多的研究结果表明纳米管具有极其良好的导电导热性,并且作为一种低纬度材料其轴向上的硬度可堪与钻石的硬度相比较。实验和理论上测得的碳纳米管轴向杨氏模量达到了惊人的TPa量级。这些性质昭示了碳纳米管可作为新一代纳米电学和力学器件材料而具有广泛的应用前景。其中,Cumings和Zettl发明了由两个单壁碳纳米管套合而成的双壁碳纳米管(DWCNTs)以制造高速低耗损纳米活塞。这一发明引起了人们很大的兴趣,大量的人力物力被投入到了该项研究中。针对活塞相对平移运动中的滑动摩擦和能量耗散的理论工作取得了诸多可喜的研究成果。同时,实验上也开始了对双壁碳纳米管转动运动的研究。而相较于众多的关于双壁碳纳米管平移运动摩擦力的理论研究成果,在双壁碳纳米管转动摩擦研究这一领域仍留有许多问题有待人们探索。最近几年,金属团簇粒子被成功的填充入碳纳米管中,这使得在碳管中填充生长稳定的金属纳米线成为了可能。这种结合金属纳米线和碳纳米管的新型组装材料预示了在纳米材料这一领域许多新的未知的可能性。填充金属纳米线在改变碳纳米管材料性质的同时自身的物理化学性质也有可能发生变化。而这种材料性质的变化可能由填充碳纳米管来控制。填充金属纳米线在碳管中的稳定结构以及熔点就是本论文工作的另两个主要的研究内容。本文首先介绍了基于Brenner多体经验势和Lennard-Jones (LJ)对势的分子动力学模拟双壁碳纳米管的相对转动运动及其过程中的摩擦研究。然后在引入Finnis-Sinclair半经验势来描述碳纳米管中的Au-Au原子相互作用后,我们研究了利用金团簇填充在碳管中生长金纳米线的相关研究。在此基础上进一步对在碳管中的金纳米线的熔化过程进行了定量研究。具体研究内容和结论如下：
1.我们用分子动力学模拟方法模拟了双壁碳纳米管的内外管相对转动的动力学过程,并测算了在其过程中的摩擦力大小。整个过程中的热损耗是本工作的重点研究问题。计算模型中的内管直径的变化范围为6A到16A；外管直径的变化范围为12A到20A。最小转动频率为0.01rotations/ps,最大转动频率为0.25rotations/ps。模拟结果显示转动中的能量耗散也即系统升温在固定转速下与时间成正比。对于(15,0)@(23,0)Zigzag碳纳米管组成的管组来说,当转动频率为每ps0.05圈时测得的能量耗散率为每转0.59meV/atom左右。这时的相应的平均摩擦力大小为1.75x10-5nN/atom。进一步的研究关于组成纳米转轴的内外管间距离以及内外管间相互接触面积对于转动摩擦的影响,表明相同条件下当内外管间距接近0.34nm时转动摩擦力最小,同时这也是LJ势能的平衡距离。双壁碳纳米管转轴具有高速低损耗的优点,作为纳米转轴而言有着广阔的应用前景。
2.我们用Finnis-Sinclair半经验势模拟研究了Au13 Au55和Au147二十面体团簇在不同直径的单壁碳纳米管中融合的动力学过程。研究主要包括由团簇而成为纳米线的三个主要过程,分别是：单个团簇由外而内的进入纳米管的过程,在纳米管中两个团簇的融合过程,团簇融合之后到形成稳定纳米线结构的过程。研究结果显示只要碳纳米管的半径大于团簇半径0.3nm以上,金团簇就能自由无障碍的进入碳管。团簇在碳管中的方位角由C-Au相互作用决定并有利于团簇在碳管中的融合。融合之后的团簇原子会重新排列以形成圆柱对称的层型结构纳米线。根据纳米管半径的不同,形成的纳米线可能为7度对称或6度对称以及其他对称度的结构。细管中的纳米线呈螺旋状层型纳米线,当管半径比较大时管中纳米线结构会偏向于面心立方(fcc)结构。本文讨论了这些结构的稳定性并详细分析了由多个团簇融合形成的纳米线结构。并由此找到了纳米管半径与其中纳米线结构间的几何关系。利用这种关系可以成功预判纳米管中所形成的纳米线结构。以上这些结果揭示了通过向一定半径的碳纳米管中注入金团簇实现可控生长金纳米线的可能性。
3.本工作最后研究了在纳米管中的金纳米线熔化的物理过程,细致的分析了碳纳米管壁对金纳米线熔点的影响。发现,管内金纳米线的熔点介于无碳管包裹的金纳米线熔点和块体金金属的熔点之间。通过人为改变C-Au相互作用势,进一步证实了碳管壁有助于提升金纳米线的熔点。此外,研究表明碳纳米管直径(1.08nmThe discovery of carbon nanotube (CNTs) by Iijima et al. in 1991 initiated rapid progress in the theoretical and experimental investigation on CNTs. Very excellent electric and thermal conductivities were explored and the nanotubes were shown to be as stiff as diamond in their axial direction with Young modulus of the order of TPa. These properties make the nanotubes promising candidates for nanoelectrical and nanomechanical devices. Since Cumings and Zettl proposed that linear bearings can be made of single-walled CNTs nesting one another, a number ofl theoretical studies has been carried out in this field, particularly in double walled carbon nanotubes (DWCNTs) to address the dynamic friction as well as the energy dissipation during the translational motion of two nanotubes sliding one with respect to the other. Meanwhile, experiments were also carried out to study the rotational motion of nanotubes. Comparing with the translation motion, many questions about the character impacting the rotational motion still remain unknown. Recently, lots of works have addressed the practical process of the filling of CNTs with different metals, and suggest the encapsulation of nanowires in CNTs to be realistic, thereby fostering further studies. Encapsulated nanowires, as well as nanoparticles, may both modify the CNT properties and have their properties determined by the confining CNT. The stable structure and melting temperature of encapsulated nanowires are two interesting and arresting subjects in this respect. In this thesis, molecular dynamics simulations with the many-body Brenner potential and Lennard-Jones (LJ) pair-potential were performed to study the rotational dynamics and interwall friction of DWCNTs firstly. Based on the previous simulation and using a Finnis-Sinclair potential to describe the interaction between Au and Au, we then studied the formation of nanowires by coalescence of small gold clusters inside carbon nanotubes and the melting behavior of encapsulated Au nanowires. Our main results can be summarized as the following:
We investigated the rotational motion and dynamic friction in molecular bearing composed of DWCNTs using molecular dynamics simulations. Thermal effects due to the rotational friction were mainly focused. The diameters of the bearings varied between 6 A and 16 A for inner shafts, and between 12 A and 20 A for outer sleeves. The rotation velocity varied from 0.05 rotations per picosecond to 0.25 rotations per picosecond. The simulations show that the energy dissipation, and hence the temperature of the system increases linearly with rotation time. The value of energy dissipation is around 0.59meV/atom per rotation atω= 0.05 rotations per picosecond for a (15,0)@(23,0) bearing. Correspondingly, the average friction force is around 1.75x10-5nN/atom. The dependence of the energy dissipation on the rotation velocity, interwall distance as well as the contact area of the DWCNT was also discussed. It was observed that the energy dissipation becomes the lowest when the interwall distance of the DWCNT bearing reaches about 0.34nm, the equilibrium distance of the LJ potential between carbon atoms. The low energy dissipation suggests that the DWCNT can be a good candidate as wearless rotational bearing, which supports the previous studies.
The coalescence of Au13, Au55 and Au147 icosahedral clusters encapsulated inside single walled CNTs of different diameters are investigated using molecular dynamics simulation with semi-empirical potentials. Three steps needed for the formation of encapsulated nanowires are followed in detail, namely, the penetration of clusters in CNTs, the coalescence between two clusters inside CNTs and their accumulation to form wires. It is suggested that no significant energy barrier is encountered during the penetration of free clusters into CNTs provided the CNT radius is large enough, that is, about 0.3 nm larger than the cluster radius. The relative orientation of clusters imposed by the CNT favors their spontaneous coalescence. After coalescence of two clusters, the Au atoms are rearranged to form new structures of cylindrical symmetry that may be seven fold, six fold, five fold, helical or fcc depending on the CNT diameter. The thermal stability of these structures is discussed and the structural properties of nanowires formed by accumulation of many clusters in CNTs are analyzed in detail. A geometrical method is presented which allows the prediction of the structure of multi-shell helical wires, when knowing only the CNT radius. These modeling results suggest the possibility of synthesizing metallic nanowires with controlled diameter and structure by embedding clusters into nanotubes with suitable diameters.
The structure and melting behavior of gold nanowires (NWs) in CNTs of different diameters were investigated using molecular dynamics simulations with semi-empirical potentials. The effect of the CNT on the melting of NWs is analysed. The simulations indicated that the predicted melting temperature of the enclosed Au-nanowires is intermediate between that of bulk gold and free standing wires. The role of the CNTs on the melting temperature is evidenced by artificially tuning the strength of the Au-C interaction. The tube diameter has a minor effect on the melting temperature in the range investigated (1.08nm< D<2.09nm). In contrast with isolated NWs, with increasing temperature, the presence of the CNT imposes the melting to initiate in the center of the NW. These findings are in good agreement with previous observations and predictions concerning the stabilizing role of the CNT on the NW structure and, in particular, of their outermost layer structure.
1.我们用分子动力学模拟方法模拟了双壁碳纳米管的内外管相对转动的动力学过程,并测算了在其过程中的摩擦力大小。整个过程中的热损耗是本工作的重点研究问题。计算模型中的内管直径的变化范围为6A到16A；外管直径的变化范围为12A到20A。最小转动频率为0.01rotations/ps,最大转动频率为0.25rotations/ps。模拟结果显示转动中的能量耗散也即系统升温在固定转速下与时间成正比。对于(15,0)@(23,0)Zigzag碳纳米管组成的管组来说,当转动频率为每ps0.05圈时测得的能量耗散率为每转0.59meV/atom左右。这时的相应的平均摩擦力大小为1.75x10-5nN/atom。进一步的研究关于组成纳米转轴的内外管间距离以及内外管间相互接触面积对于转动摩擦的影响,表明相同条件下当内外管间距接近0.34nm时转动摩擦力最小,同时这也是LJ势能的平衡距离。双壁碳纳米管转轴具有高速低损耗的优点,作为纳米转轴而言有着广阔的应用前景。
2.我们用Finnis-Sinclair半经验势模拟研究了Au13 Au55和Au147二十面体团簇在不同直径的单壁碳纳米管中融合的动力学过程。研究主要包括由团簇而成为纳米线的三个主要过程,分别是：单个团簇由外而内的进入纳米管的过程,在纳米管中两个团簇的融合过程,团簇融合之后到形成稳定纳米线结构的过程。研究结果显示只要碳纳米管的半径大于团簇半径0.3nm以上,金团簇就能自由无障碍的进入碳管。团簇在碳管中的方位角由C-Au相互作用决定并有利于团簇在碳管中的融合。融合之后的团簇原子会重新排列以形成圆柱对称的层型结构纳米线。根据纳米管半径的不同,形成的纳米线可能为7度对称或6度对称以及其他对称度的结构。细管中的纳米线呈螺旋状层型纳米线,当管半径比较大时管中纳米线结构会偏向于面心立方(fcc)结构。本文讨论了这些结构的稳定性并详细分析了由多个团簇融合形成的纳米线结构。并由此找到了纳米管半径与其中纳米线结构间的几何关系。利用这种关系可以成功预判纳米管中所形成的纳米线结构。以上这些结果揭示了通过向一定半径的碳纳米管中注入金团簇实现可控生长金纳米线的可能性。
3.本工作最后研究了在纳米管中的金纳米线熔化的物理过程,细致的分析了碳纳米管壁对金纳米线熔点的影响。发现,管内金纳米线的熔点介于无碳管包裹的金纳米线熔点和块体金金属的熔点之间。通过人为改变C-Au相互作用势,进一步证实了碳管壁有助于提升金纳米线的熔点。此外,研究表明碳纳米管直径(1.08nm
We investigated the rotational motion and dynamic friction in molecular bearing composed of DWCNTs using molecular dynamics simulations. Thermal effects due to the rotational friction were mainly focused. The diameters of the bearings varied between 6 A and 16 A for inner shafts, and between 12 A and 20 A for outer sleeves. The rotation velocity varied from 0.05 rotations per picosecond to 0.25 rotations per picosecond. The simulations show that the energy dissipation, and hence the temperature of the system increases linearly with rotation time. The value of energy dissipation is around 0.59meV/atom per rotation atω= 0.05 rotations per picosecond for a (15,0)@(23,0) bearing. Correspondingly, the average friction force is around 1.75x10-5nN/atom. The dependence of the energy dissipation on the rotation velocity, interwall distance as well as the contact area of the DWCNT was also discussed. It was observed that the energy dissipation becomes the lowest when the interwall distance of the DWCNT bearing reaches about 0.34nm, the equilibrium distance of the LJ potential between carbon atoms. The low energy dissipation suggests that the DWCNT can be a good candidate as wearless rotational bearing, which supports the previous studies.
The coalescence of Au13, Au55 and Au147 icosahedral clusters encapsulated inside single walled CNTs of different diameters are investigated using molecular dynamics simulation with semi-empirical potentials. Three steps needed for the formation of encapsulated nanowires are followed in detail, namely, the penetration of clusters in CNTs, the coalescence between two clusters inside CNTs and their accumulation to form wires. It is suggested that no significant energy barrier is encountered during the penetration of free clusters into CNTs provided the CNT radius is large enough, that is, about 0.3 nm larger than the cluster radius. The relative orientation of clusters imposed by the CNT favors their spontaneous coalescence. After coalescence of two clusters, the Au atoms are rearranged to form new structures of cylindrical symmetry that may be seven fold, six fold, five fold, helical or fcc depending on the CNT diameter. The thermal stability of these structures is discussed and the structural properties of nanowires formed by accumulation of many clusters in CNTs are analyzed in detail. A geometrical method is presented which allows the prediction of the structure of multi-shell helical wires, when knowing only the CNT radius. These modeling results suggest the possibility of synthesizing metallic nanowires with controlled diameter and structure by embedding clusters into nanotubes with suitable diameters.
The structure and melting behavior of gold nanowires (NWs) in CNTs of different diameters were investigated using molecular dynamics simulations with semi-empirical potentials. The effect of the CNT on the melting of NWs is analysed. The simulations indicated that the predicted melting temperature of the enclosed Au-nanowires is intermediate between that of bulk gold and free standing wires. The role of the CNTs on the melting temperature is evidenced by artificially tuning the strength of the Au-C interaction. The tube diameter has a minor effect on the melting temperature in the range investigated (1.08nm< D<2.09nm). In contrast with isolated NWs, with increasing temperature, the presence of the CNT imposes the melting to initiate in the center of the NW. These findings are in good agreement with previous observations and predictions concerning the stabilizing role of the CNT on the NW structure and, in particular, of their outermost layer structure.
引文
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