手性碳纳米管扭转过程中力学性质的研究

发布时间：2018-05-12 10:17

本文选题：手性纳米管 + 分子动力学模拟　；参考：《南京师范大学》2015年硕士论文

【摘要】：碳纳米管自发现以来就被誉为未来的材料,根据其独特的力学特性,可以制作很多有广泛应用前景的纳米器件,譬如在搭建扭转式振荡器时,纳米管可以作为弹簧元件；在搭建纳米发电机时,纳米管可以作为旋转轴承等。在这些应用中,纳米管的扭转性能对纳米器件系统极为重要。本文采用分子动力学模拟的方法探讨了手性碳管扭转的力学行为,并分析了长度、半径、转速、缺陷等对手性碳管扭转过程中力学性质的影响。取得了一些有意义的结果：(1)从整体上看,碳管正(反)向扭转都经历了四个阶段,前面三个阶段,扭矩随切应变的增加而增加,但是扭矩曲线的斜率逐级减小,第四阶段,碳管扭转破裂,扭矩陡降,积累的能量迅速释放。(2)随着扭转角的增加,碳原子离管轴平均距离单调减小,碳管中的碳原子整体上做靠近轴向的运动,管内空间持续下降。(3)手性碳管在扭转过程中,中间部分的碳原子应力增加的最快,而左右两部分的碳原子所受到的应力则较慢,碳管中部原子的局部环境率先出现明显改变,先于其他部位受到较大的应力,随后逐渐将应力向两边传递,从而使整个碳管上的碳原子所受的应力得以增大,导致形变结构以“扭结”的方式向两边展开。(4)正向扭转的轴向力随扭转角单调上升,使碳管趋于伸长,当扭转越过屈服点后,轴向收缩力开始持续下降到零,并最终完成方向的反转而使碳管转为轴向收缩；而反向扭转的轴向力从开始就体现出收缩特性,轴向收缩力大小随扭转进程单调增大。当碳管正(反)向扭转越过屈服点后之后,轴向收缩力以更大的增速随扭转角增大,体现为在碳管扭转弹性屈服点后有更大的轴向弹性模量,且轴向弹性模量随着管长的增加而减小。(5)手性角在约15°之前,正向扭转对应的屈服角随手性角的增加而减小,反向扭转对应的屈服角随手性角增加而增大；当手性角超过15°时,正向扭转的屈服角转而增大,而反向扭转的屈服角却开始减小；即手性碳管扭转时,手性角在15°左右的碳管反向扭转的弹性扭转范围最大,能承受的应变最大,正向扭转的弹性扭转范围最小,能承受的应变也最小。(6)当碳管扭转速率小于等于1°/ps,系统始终处于准平衡状态,即系统在每一步的扭转过程中有足够的弛豫时间达到平衡。当转动速度大于1°/ps,系统开始偏离平衡态,随扭转动速率的增加,扭矩曲线斜率增大,扭转屈服角则先增大到一极值后再减小,而碳管断裂的扭转角单调下降。
[Abstract]:Carbon nanotubes (CNTs) have been regarded as the future materials since their discovery. According to their unique mechanical properties, carbon nanotubes can be used as spring elements in the construction of torsional oscillators. Nanotubes can be used as rotating bearings in building nano-generator. In these applications, the torsional performance of nanotubes is very important for nanodevice systems. In this paper, the mechanical behavior of chiral carbon tube torsion is studied by molecular dynamics simulation, and the effects of length, radius, rotation speed and defects on the mechanical properties of chiral carbon tube torsion are analyzed. Some meaningful results have been obtained: (1) overall, the positive (reverse) torsion of the carbon tube has gone through four stages. In the first three stages, the torque increases with the increase of the shear strain, but the slope of the torque curve decreases step by step, the fourth stage. With the increase of torsion angle, the average distance of the carbon atom from the tube axis decreases monotonously, and the carbon atom in the carbon tube moves near the axis as a whole. In the process of torsion, the stress of carbon atoms in the middle part of the tube increases fastest, while the stress of the carbon atoms in the left and right parts of the tube is slower, and the local environment of the atoms in the middle part of the tube changes obviously. The stress on the carbon atoms in the whole carbon tube is increased by the larger stress on the other parts of the tube, and then the stress is gradually transferred to the two sides of the tube, so that the stress on the carbon atoms in the whole carbon tube can be increased. The axial force of the forward torsion increases monotonously with the torsion angle, which makes the carbon tube tend to elongate. When the torsion crosses the yield point, the axial contraction force begins to decrease to zero. The axial force of the reverse torsion shows the characteristics of shrinkage from the beginning, and the magnitude of the axial contraction force increases monotonously with the torsion process. When the positive (reverse) torsion of the carbon tube passes the yield point, the axial contraction force increases with the increase of the torsion angle, which is reflected in the greater axial elastic modulus of the carbon tube after the torsional elastic yield point. And the axial modulus of elasticity decreases with the increase of the length of the tube.) the chiral angle decreases with the increase of the yield angle of the forward torsion before about 15 掳, and the yield angle of the reverse torsion increases with the increase of the chiral angle, and when the chiral angle exceeds 15 掳, the corresponding yield angle increases with the increase of the chiral angle. The yield angle of positive torsion increases, but the yield angle of reverse torsion begins to decrease, that is, when the chiral carbon tube torsion, the elastic torsion range of the chiral carbon tube with a chiral angle of about 15 掳is the largest and the strain is the largest. When the torsion rate of carbon tube is less than or equal to 1 掳/ s / s, the system is always in a quasi-equilibrium state, that is, the system has sufficient relaxation time to achieve equilibrium in each step of torsion. When the rotational velocity is greater than 1 掳/ ps, the system begins to deviate from the equilibrium state. With the increase of the torsional dynamic rate, the slope of the torque curve increases, the torsional yield angle increases to an extreme value and then decreases, while the torsional angle of the carbon tube fracture decreases monotonously.
【学位授予单位】：南京师范大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：O613.71;TB383.1

【参考文献】