纳米线杨氏模量的尺度效应及其内在机理研究

As a mechanical property, Young’s modulus of nanowires is an important parameter for the application of nanodevices. In many researches, there is a complicate change tentency with size for Young’s modulus of nanowires, and there also have some the physical reasons proposed. The two main reasons are that respectively from surface effect and interior elastic deformation. However, what is the key factor leading to the change of Young’s modulus with size, and what is the main contribution to Young’s modulus, surface modulus or interior modulus, are still not clear. In this study, based on the relation between Young’s modulus and atomic potenial, the inherent relevance among Young’s modulus, bond energy, bond length and lattice direction is put forward. This will make the size effect on Young’s modulus and the physical mechanism behind understood from atomic level. Two research methods, that is thermodynamic theory and molecular dynamic (MD) simulation, will be used in this work. Size-dependent bond energy and bond length will be established, which is in favor of the construction of the size effect of Young’s modulus if the bond role on lattice direction is known. With the help of MD simulation for Young’s modulus of several nanowires, the change rule of Young’s modulus with size is obtained. Moreover, the bond energy and bond length for atoms sited on nanowires is easy to be calculated by MD method, and thus the concerned surface modulus or interior modulus can be determined. Arriving here, the contribtion of surface effect and interior modulus can be quantitatively analyzed, which is helpful for understanding the intrinsic factor which casuses different Young's modulus of nanowires at different size when compared with corresponding bulk value. All this will be in valid the supplement and theoretical guidance for the practical application of nanodevices.

纳米线杨氏模量这一力学性能参数是评价纳米器件实用化的重要指标。研究发现杨氏模量随纳米线尺度的变化趋势多样，其物理本质也众说纷纭，主要集中在表面效应和内部弹性变形两种原因。然而，决定杨氏模量随尺度变化的因素以及在不同尺度下表面效应与内部模量对杨氏模量的决定作用并未十分清楚。本申请拟从杨氏模量与原子势函数的关系出发，提出杨氏模量与键能、键长及晶向之间的本质关联，进而对杨氏模量尺度效应及其物理本质进行原子水平分析。本申请拟采用热力学理论和分子动力学计算相结合的研究方法，构建键能、键长的尺度依赖模型，探索二者与晶向的关系，发展纳米线杨氏模量的热力学模型，加以计算模拟，阐明杨氏模量随尺度变化的规律；通过计算纳米线不同位置上原子的键能、键长，结合理论模型定量表面原子模量和内部原子模量，明确表面效应与内部模量在不同尺度下对杨氏模量的贡献，揭示引起杨氏模量尺度效应的物理本质，为纳米器件的应用提供理论依据。

对于不同的一维纳米线材料，其杨氏模量随尺度变化趋势表现出不同的趋势，并且其内在的物理机理不清楚。在本项目中，我们在原子水平上解析了这种特殊的物理现象。以原子势函数为出发点，针对两个原子之间的弹性形变模型，推导出纳米材料的杨氏模量的特征，从而成功的建立了一维纳米线杨氏模量的尺度依赖模型。用类似的思路，纳米粒子、纳米薄膜的杨氏模量尺度依赖模型也成功建立。理论预测结果与计算、实验结果的一致性验证了我们理论推导的合理性。在本模型中，我们提出了单键模量的概念，基于块体材料原子势函数，解析出块体材料的单键模量。然而，在热力学结合能的帮助下，我们发现对于纳米材料，表面原子和内部原子的单键模量不再与块体材料相同，这是导致纳米材料弹性性能变化的根部原因。在单键模量的基础上，我们发展了纳米材料的表面弹性模量和内部弹性模量，二者均表现出强烈的尺度效应，并且不同的材料二者的变化趋势不同，这就导致了体系杨氏模量不同的变化趋势。因此，在本项目中，我们将表面模量、内部模量的尺度依赖性也进行了定量化的研究，提出为了解析纳米材料的杨氏模量，其表面、内部的贡献是不可忽略的，并且可以将二者的贡献定量化。三年的研究工作基本上完成了项目的研究目的，达到了对纳米材料杨氏模量尺度效应的物理认识。这一工作对于预测纳米结构的力学性能以及认识其内在的物理原因有很好的指导和参考价值，这对于纳米器件稳定性的判断有重要的科学意义。