摘要
在纳米尺度,许多物质和材料都具有非常奇特的物理、化学、力学性能和规律,而根据这些特性制成的各类纳米器件和结构拥有非常广阔的应用前景。本文使用以量子力学为基础的密度泛函理论、Hartree-Fork从头算理论以及半经验量子力学算法,结合经验的分子动力学模拟和量子分子动力学模拟,并配合连续介质力学建模方法,针对关键纳米材料结构和系统中的碳纳米管力电耦合效应和失效行为、单壁碳纳米管电致伸缩变形、基于碳纳米管振荡器中的能量转换和耗散、铜纳米线在碳纳米管内的结构相变与破坏、多壁碳纳米管各层手性存在的特殊规律和生长方式、以及生物纳米复合材料结构中缺陷的自修复等一系列极为重要的低维结构的物理力学问题进行模拟计算和创新原理设计。在分子物理力学理论研究指导下,利用大规模并行计算模拟和力学理论建模,尝试探索碳纳米管系统中特异的物理力学耦合行为以及纳尺度材料的结构性能和生长机理,并理解生物材料自优化机理,主要有以下一些新的发现:
1.纳米机电系统的发展要求智能材料具有大的应变和高功率密度以获得优良的能量转换能力,碳纳米管因其优越的物理力学性能以及对外场(光、电、磁等)的高敏感性很有可能成为满足上述要求的纳智能材料。我们通过基于Roothaan-Hall方程的半经验量子力学计算和量子分子动力学模拟,揭示出机械载荷和电场共同作用下碳纳米管的特殊物理力学耦合行为、电致破坏机理和力电耦合作用对纳米管电学性能的重要影响。然后使用更为精确的Hartree-Fork从头算法和密度泛函理论,发现单壁碳纳米管在沿其轴向的外加电场作用下,碳纳米管的轴向具有超过10%的巨电致伸缩变形,所得到的单位体积功率密度要比已报道的铁电、电致和磁致伸缩材料以及电致驱动聚合物高出近3个量级以上,而单位质量功率密度比其它已知材料高出4个量级以上。
2.多壁碳纳米管因其极低的层间相互作用,在纳米轴承和高频纳米振荡器方面有着潜在的应用前景。对于碳纳米管振荡器中存在的能量耗散问题,我们通过纳秒量级的分子动力学模拟,发现双层碳纳米管的层间相互作用和摩擦力是与其手性和结构以及环境温度密切相关的,对于手性匹配的双层碳纳米管振荡器系统,它在振荡中的能量耗散率要明显地大于手性不匹配系统的能量耗散率。手性匹配的双层碳纳米管系统在低温条件下的振荡时间只能持续几个纳秒,而手性不匹配的双层碳纳米管系统在低温条件下振荡时间却能达到几十个纳秒。但在较高温度条件下,系统的能量耗散率显著提高,温度对层间摩擦力的影响会远高于碳纳米管手性造成的差异。
At nanoscale, many kinds of materials and structures have superior mechanical, electronic and chemical characteristics and the advanced functional nano-elements become more and more important in potential applications. Atomistic simulations have been played important roles in scaling down to nanoscale and can help in the elucidation of their properties and in the development of new methods for their fabrications and applications. In the thesis, exceptional properties and behaviors and the coupled effects of low-dimensional materials and structures have been investigated by using the atomistic methods including molecular mechanics and quantum mechanics as well as the continuum theory.
Using the semi-emprical quantum mechanics as well as quantum-molecular dynamics techniques based on the Roothaan–Hall equations and the Newton motion laws, we have investigated the coupled mechanical and electronic behaviours of single-walled open carbon nanotubes (CNTs) under applied electric field and tensile loading. Different failure mechanisms and mechanical properties are found for CNTs subjected to electric fields and that subjected to tensile load. Electronic polarization and mechanical deformation induced by an electric field and tension load can significantly change the electronic properties of a CNT. The coupling of mechanical and electrical behaviours is an important characteristic of CNTs. Mechanisms for converting electrical energy into mechanical energy are essential for the design of diverse nanoscale devices such as sensors, actuators, artificial muscles, robotics, optical fiber switches and so on. Materials having special piezoelectric, electrostrictive and electrochemical properties play important roles in conversion between electrical and mechanical energy. Our researches have found that single-walled carbon nanotubes have exceptionally high axial electrostrictive deformation using ab initio and density functional quantum mechanics simulations.
Both armchair and zigzag open-ended tubes and a capped tube are modeled and in all of them external electric fields induced axial strains can be greater than 10% for field strength within 1V/?. The corresponding volumetric and gravimetric work capacities are predicted to be three and six orders higher than that of the best known ferroelectric, electrostrictive, magnetostrictive materials and elastomers respectively.
Multiwalled carbon nanotubes (MWNTs) have broad prospects as components in nanomechanical devices due to many exceptional electrical and mechanical
引文
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