【作者】 戴意涛；

【导师】 郭万林；

【作者基本信息】 南京航空航天大学 ， 工程力学， 2009， 博士

【摘要】 碳纳米管和氮化硼纳米管具有独特的几何结构和优异的物理、力学和化学等性质,是构筑纳米器件的重要材料。本文利用从头算量子分子动力学、分子动力学、密度泛函理论以及量子力学和经典力学的混合模型,结合经典力学分析的方法对碳纳米管的轴向高频振动、氮化硼纳米管的电致变形和电磁中性分子的电驱动原理进行了大规模并行计算模拟和理论分析,尝试探索碳和氮化硼纳米管中存在的物理力学耦合行为,以及这种耦合行为在纳尺度器件开发中潜在的应用价值。本文通过对以上问题的研究,取得了如下进展:1)碳纳米管轴向高频振动的研究:人们对碳纳米管的高频振动行为进行了大量的研究,但缺乏量子分子动力学的检验,并且对振动受机电耦合效应的影响也了解甚少。本文以碳纳米管的轴向振动为例,利用从头算量子分子动力学对其振动过程进行了模拟。结果表明,碳纳米管的轴向振动受轴向电场的影响较小,而受电荷注入的影响较大。经典分子动力学方法能较好地描述该振动的主要特征,然而对振动的本征频率的预测与量子分子动力学的结果相比仍有差别。此外,对振动受机电耦合效应的影响,特别是有电荷注入的情况,经典分子动力学不能给出合理的描述。选择合适的参数,弹簧-质量块模型和空心杆模型对轴向振动的基频也可以给出较为准确的预测,然而对于高阶模态,上面两种模型的误差都比较大。此外,利用碳纳米管的轴向高频振动行为,我们提出了一种太赫兹辐射源的工作原理并申请了国家发明专利。2)氮化硼纳米管电致变形的密度泛函研究:寻找具有大应变能密度的智能材料一直是人们努力的方向。我们利用第一原理密度泛函方法对氮化硼纳米管的电致变形行为进行了研究。结果表明,在轴向外加电场的作用下,并且在实验中可以达到的电场强度下,锯齿型氮化硼纳米管的轴向电致变形可达4%,相应的体积功密度比目前已报道的聚合物智能材料的最高值要高100倍以上,比传统压电陶瓷材料要高出3个数量级。氮化硼纳米管的电致变形源于逆压电效应和电致伸缩效应,并且后者引起的变形量可达前者的两倍。考虑到氮化硼纳米管良好的热力学和化学稳定性以及绝缘性,它有望成为一种极富潜力的纳米智能材料。3)电磁中性分子电驱动原理的探索研究:对纳米和分子机器的驱动,特别是对电磁中性分子体系的驱动,是一个极具挑战性的科学和技术难题。它们不含净电荷或磁距、磁畴或电畴,因此不能用均匀的电场或磁场来进行驱动和操作。本文利用半经验量子分子动力学模拟证明:通过控制一端封闭一端开口的单壁碳纳米管上的电荷分布,可以改变它与内部中性分子之间的相互作用,进而实现对中性分子的驱动和操作。当碳纳米管带上均匀分布的正电荷时,可以将其内的中性分子打出;而当它带上均匀分布的负电荷时,则不能将内部的中性分子打出,然而却能将位于其开口端附近的中性分子吸入。这些发现有望为纳米器件和系统的驱动和控制提供一种新的机制。更多还原

【Abstract】 Carbon nanotubes (CNTs) and boron nitride nanotubes (BNNTs) are expected to have great potential applications in building nano devices due to their unique geometry structures and excellent physical, mechanical and chemical properties. Atomistic simulation is a very powerful tool to unveil the complex phenomena in nanoscale and is also helpful for designing new nano devices. In this thesis, exceptional physical mechanics properties and behaviors of CNTs and BNNTs are investigated by using the atomistic methods including the molecular mechanics (MM) and quantum mechanics (QM) as well as the hybrid QM/MM method. The main contents are as follows:1) High frequency longitudinal oscillation of carbon nanotubes. To check the validity of the classical mechanics methods and to study the influence of electromechanical coupling effect on the oscillation behavior, longitudinal oscillation of a (3, 3) CNT is studied by ab initio quantum mechanical molecular dynamics (ab-MD) simulations. It is found by the ab-MD simulations that axial electric field affects very slightly on the longitudinal oscillation behavior of the CNT, but electrical charging can significantly influence the oscillation behavior. Classical MD method can goodishly describe the frequency-domain characteristic of the oscillation, but it can not exactly predict the electromechanical coupling effect, especially when the CNT is electrically charged. Choosing suitable mechanical parameters, both the simple spring-mass model and the hollow rod model can yield good prediction for the fundamental frequency, but can not give accurate descriptions for the higher order modes. Furthermore, based on the high frequency longitudinal modes of the CNTs, a terahertz (THz) source is also proposed.2) Density functional theory studies on the electric-field-induced deformation of BNNTs. Intelligent materials with high work density are essential for nano electromechanical devices. Our density functional theory calculations indicate that the electric-field-induced deformation of zigzag BNNTs can be 4% around field strength of 1.2 V/?. The corresponding volumetric work capacity is nearly ten times higher than those of the best reported polymer intelligent materials, and about 3 orders of magnitude higher than those of traditional piezoceramics. The large electric-field-induced deformation is found to arise from both the converse piezoelectric effect and the electrostrictive effect of BNNTs. Considering the high chemical and thermal stability and electrical insulation, BNNTs they should have great value in potential applications. 3) Quantum mechanical molecular dynamics simulations of nano-gun from CNTs. How to make nano/molecular machines work is a challenging nanotechnology issue, especially to drive magnetoelectrically neutral molecules. Here, we demonstrate by quantum mechanical molecular dynamics simulations on an ideal model that an electrically neutral nanotube or fullerene ball inside a one-end-open carbon nanotube can be driven into movement by properly charging the housing nanotube. It is more interesting that positively charged housing tube can drive the molecule inside it out, like a nano-gun; while negatively charged housing tube can only drive the molecule into oscillation inside it, but can not drive it out. Instead, a negatively charged housing tube can absorb inward a neutral molecule in the vicinity of its open end, like a nano-manipulator. These findings may be helpful for designing new nano devices.更多还原