【作者】 彭传校；

【导师】 王丽；

【作者基本信息】 山东大学 ， 材料加工工程， 2009， 硕士

【摘要】 对一维纳米线结构的研究是探讨纳米线其他性能的一个重要前提。随着纳米线在微电机系统、纳米机械、纳米增强材料等领域的应用和推广,对纳米线机械性能的探索具有很重要的意义。而一维纳米结构,如纳米线、纳米管及纳米碳管等因其结构微小、实验较困难,使得分子动力学模拟被广泛应用于纳米线的结构和性能的研究中。本文以分子动力学模拟作为研究手段,探讨了镍纳米线的结构及其力学性能。对于尺寸较大的镍纳米线,能保持宏观状态下的面心立方结构(FCC),但对于不同晶向、不同自由表面的FCC结构,其能量和性质不尽相同。我们模拟了不同尺寸的FCC＜001＞、＜110＞、＜110＞{111}和＜111＞晶向的镍纳米线,其弛豫后的平均原子能量与收缩率随着纳米线尺寸的增加而减小,对于相同尺寸的镍纳米线而言,＜110＞{111}结构的平均原子能量与收缩率最低,这主要与表面原子所占的比例及自由表面的晶向密切相关。不论哪种晶向的镍纳米线,只要自由表面存在非密排面,当尺寸减小到一定程度时,较大的表面应力就会驱使结构转变的发生,形成表面密排的超细纳米线结构。我们通过弛豫退火、拉伸和卷曲组合三种方法探讨了镍超细纳米线的结构。依照超细纳米线表面密排的原则,可以通过卷曲FCC(111)面的三角网格结构构造(m,n)超细镍纳米线,并不是所有构造的(m,n)纳米线都是稳定的,故还得通过能量最小化来获得稳定的纳米线结构。弛豫后,非螺旋的(5,5)和(6,6)纳米线显示了相对较低的结构能;另外对不同横截面形状且径向尺寸较细的FCC结构的纳米线进行弛豫退火,即结构和能量的优化,亦可获得不同的超细纳米线结构,且有很多结构与卷曲组合法获得的相同;在FCC＜001＞晶向的纳米线拉伸模拟中,颈缩部位形成了较长的(5,5)结构,并证实其是从(6,5)结构转变而来的。我们还尝试探讨了多壳镍纳米线结构,其中(12,6)(6,3)结构较稳定,而螺旋多壳结构中(11,6)(4,2)、(13,7)(6,3)和(14,8)(7,4)弛豫前后结构变化较小,相对也比较稳定。稳定纳米线结构的力学性能,也是备受关注。我们探讨了镍不同晶向的FCC结构纳米线的尺寸效应,揭示了纳米线的弹性模量、屈服应力和屈服应变等力学参数随着尺寸的变化规律。对镍纳米线的应变率效应研究,是以FCC＜001＞纳米线为例,当应变率较低时,变形机制主要是滑移变形,且屈服应力和应变基本不变;当施以高应变率拉伸时,其结构将产生非晶转变,且屈服应力也随着应变率的升高而增大。对不同结构的超细镍纳米线进行拉伸模拟,计算了超细纳米线的弹性模量、屈服应力和屈服应变等力学参数,对拉伸变形过程中的原子构型的变化进行了详尽的描述,探讨超细纳米线的变形机理。非螺旋的(5,5)和(6,6)镍纳米线有较高的屈服应力,但塑性较差。而螺旋结构,如(7,4)结构相对就拥有较高的塑性变形能力。纳米线的螺旋特性有助其在拉伸中的塑性变形,其变形是通过减少外壳原子链数m来实现的。更多还原

【Abstract】 The researches on the structures of the one-dimensional nanowires are of great significance in the development of their other properties.Due to the wide applications in areas of nanocomposite strengtheners and components in nanoelectromechanical system(NEMS) devices,knowledge on mechanical properties of nanowires is extremely important.However,the one-dimensional nano-structures, such as metal nanowires,nanotubes and carbon nanotubes,are not available easily in experiment,molecular dynamics(MD) simulation shows its importance in the research of the structures and properties of nanowires.In the paper,MD simulation is performed to explore the structures and mechanical properties of nickel nanowires.The Ni nanowires can keep face-centered-cubic(FCC) structure when their cross-sizes are large enough.However,the energy and properties are diverse for nanowires with different orientations and free surfaces.FCC <001>,<110>, <110>{111} and <111> oriented Ni nanowires have been constructed and relaxed to explore their structural stabilities by calculating their size dependence of the total energy per atom and contraction ratio.The finding indicates that the energy and contraction ratio decrease as size increases,and the <110>{111} nanowires show the lowest level among the simulated nanowires.These are attributed to the ratio of surface atoms and different surface orientations.When the size is small enough,as well as non-closed packing on the free surfaces,large surface stress will induce the structural transforming and closed packing on the surface.Three measures have been taken to explore the structures of ultrathin Ni nanowires named relaxing-annealing circle,stretching and cylindrical folding respectively.According to the closed packing on the surface of ultrathin nanowires,atomic structures of(m,n) Ni nanowires are constructed by cylindrical folding of the 2D triangular FCC(111) lattice sheet.However,not all constructed(m,n) nanowires are stable,thus,the nanowires are relaxed by energy minimization way.The non-helical(5,5) and(6,6) structures show much lower structural energy after relaxation in comparison with the energy and radius of stable nanowires.In addition,some nanowires the same as the cylindrical folding(m,n) nanowires can be obtained by relax-annealing(structure and energy optimization) thin nanowires with FCC structure.The(5,5) structure which turns from(6,5) structure can be formed at the necking part of tensioned FCC nanowires along the <001> crystallographic direction.The Ni nanowires with multi-shell structure(MS) have also been explored.The finding indicates that(12,6)(6,3) structure shows lower energy and the(11,6)(4,2),(13,7)(6,3) and(14,8)(7,4) structures show a little change in atomic configurations during relaxation and can be also regarded as being relative stable.The mechanical properties of the relaxed stable nanowires are the other aspect of our researches.We explore the size effects of the different orientations FCC Ni nanowires and make a summary on the Young’s modulus,yielding stress and yielding strain as a function of the nanowires’ size.The strain rate effects are exemplified by loading FCC <001> nanowires.The nanowires undergo plastic slipping deformation at low strain rates and the yielding stress and strain are on the same level.At high strain rates,the nanowires change from crystalline structure to amorphous phase continuously and the yielding stress increases as the strain rate increases.The mechanical properties of ultrathin nanowires such as Young’s modulus,yielding stress and strain are calculated by tensioned simulation.The deformation mechanisms of ultrathin Ni nanowires are explored by describing the snapshots of atomic configurations at several strains.The non-helical(5,5) and(6,6) nanowires show high yielding stress but low plasticity.The helical structure,such as(7,4) structure,has a good performance in plastic deformation.The helical nanowires are prone to plastic deformation by decreasing the number of helical strands m.更多还原