金奈米線的機械性質模擬

Abstract

本研究利用分子動力學模擬(Molecular Dynamics, MD)，探討奈米金屬線受到拉伸負荷時，材料變形及差排機制。本文所模擬的材料為金，使用的勢能函數為原子嵌入法(Embedded Atom Method, EAM)，以此計算結構在變形的過程中應力應變關係及其楊氏模數、降伏應變、降伏應力、差排成核等機械性質。模擬的過程中，藉由中心對稱參數(Centro-Symmetry Parameter, CSP)來探討及觀察其金屬滑移平面發展過程，及其滑移面上應力與能量的變化情形。 此外討論尺寸效應、應變率效應及方向性對奈米金屬線應力應變曲線的影響。尺寸效應的部份，因為奈米結構的表面存在表面應力，而表面應力對材料特性有很大的影響，影響程度取決於表面積佔整體結構的比例大小。拉伸應變率主要對奈米線的降伏應變有較大的影響，且同時對材料的延展性也有不同程度的供獻。奈米尺度下的方向特性對材料的楊氏係數有決定性的影響，因金屬本身即為非等向性材料。 另一方面，本研究除了分析完美結構奈米線外，對含缺陷奈米線有進一步討論與比較。分別模擬了含空孔及含溝槽奈米線對材料機械性質的影響，在本研究發現在缺陷率很小(約1 %)的情況材料楊氏係數幾乎不會造成影響，但降伏應力會有大幅度減少的現象。而奈米線拉伸到臨界應變量的狀態下，使其平衡至穩定並觀察穩定前後的能量變化，探討降伏應力及能量變化的關係。在不同尺寸下單位體積能量變化幾乎保持定值，而在方向性、缺陷率及缺陷型式的部份能量變化都與降伏應力有相同的趨勢。

In this study, molecular dynamic simulations is used to explore the mechanical properties of nanowire and to investigate the material deformation and dislocation mechanism while the nanowire under tensile load. We chose gold as the simulation material of this research, and embedded atom potential is adopted to describe the atomic interactions. During the nanowire deformation, the young’s modulus, yielding strain, yielding stress and the dislocation nucleation properties are of concern. In the simulation process, centro-symmetry parameter of atoms were calculated to quantify and visualize the dislocation plane expansion. On the other hand, the local stress as well as energy variation were also discussed. The size effect, strain rate effect, and anisotropic behavior were discussed in this study. In part of size effect, the surface stress exist on nano-scale structure so that the ratio of surface area of total structure dominant the material properties. The strain rate effect will affect the yield strain and yield stress, and the ductility increase with increasing strain rate. The orientation of nanowire plays an important rule of Young’s modulus owing to the view point of nano-scale, face-center-cubic structure is an anisotropic material. In addition to the perfect nanowire structure, the nanowire with defect were also discussed in this research. The mechanical properties of the notch nanowire and the vacancies nanowire were simulated, respectively. The defect rate under 1% models will not change the young’s modulus, but the yielding stress decreasing significantly. Under the critical strain of tensile, let it equilibrium to stable state, and find out the potential energy variation before and after equilibrium, and discussing the relation between yielding stress and energy variation. The energy variation per unit volume under different nanowire size are almost consistent and irrelevant to the yield stress, however, the orientation, defect rate, defect type has the same tendency to yield stress.