摘要
硅和锗是电子工业领域的重要材料。硅锗纳米管展示出优异的化学、力学性能以及高热导率,可应用到纳米钻孔机,纳米镊子,电子显微镜等方面。硅锗纳米管的物理和化学性质除了受尺寸影响之外,还受组成和原子排列的影响。此外,硅锗纳米管呈现出特殊的结构特征。这些特性为硅锗纳米管的制备和设计提供了空间。因此,研究硅锗纳米管的结构、热融和力学性能具有非常重要的理论和实际意义。
由于纳米管体系小,结构复杂,单靠实验手段很难获得纳米管的结构和化学、力学性能的相关信息。因此,人们必须借助各种不同的计算模拟方法来研究纳米管。采用计算模拟方法不但能研究硅锗纳米管的结构、热演化现象和力学性能,还能解释实验现象并可以进而指导其制备。研究纳米管的计算模拟方法主要有两种,即量子化学计算和分子模拟方法。量化计算方法较准确,但计算量巨大,适用范围受限。因此,在研究稍大体系纳米管的性质时,最常用的是基于经验势能的分子模拟方法,主要包括分子动力学(molecular dynamics, MD)和蒙特卡洛(Monte Carlo, MC)方法。
本论文采用量子化学方法研究了硅锗纳米管的两种特异构型,同时采用基于Tersoff势能函数的MD方法研究了硅锗纳米管的热融和力学性能。其主要结论如下:
1.采用第一性原理(Ab-initio Method)方法研究了间隔式和层状式排列的扶椅型及锯齿型硅锗纳米管的结构和能量稳定性。发现扶椅型硅锗纳米管呈现“褶皱”状结构,而锯齿型硅锗纳米管呈现“齿轮”状构型。扶手椅型硅锗纳米管比锯齿型硅锗纳米管更稳定。间隔排列(Type1)的硅锗纳米管比层状排列(Type2)的硅锗纳米管更稳定。
2.系统研究了两种原子排列方式的扶椅型和锯齿型SiGe(n=4-10)纳米管的熔化和热演化过程。硅锗纳米管在热熔化过程中,首先由初始管状结构演变成为压缩的纳米线形状,再在更高的温度下转变为缩聚的结构。通过研究发现大尺寸硅锗纳米管比小尺寸纳米管具有更高的“似熔化”温度,这表明大尺寸管比小尺寸管更稳定。间隔式排列的纳米管比层状式排列的纳米管具有更高的热稳定性。锗取代硅纳米管的“似熔化”温度随着锗含量的升高而降低。原子排列方式、构型和能量稳定性是影响SiGe纳米管热熔性能的主要因素。
3.系统研究了两种原子排列方式的扶椅型和锯齿型硅锗纳米管(n=4-10,13)的力学性能。通过研究发现在拉伸过程中,硅锗纳米管由原始管状结构转变为延伸管状结构,最后在临界应变下出现屈服结构。硅锗纳米管的杨氏模量与原子排列方式、结构、尺寸有紧密联系。间隔式排列的硅锗纳米管具有较大的杨氏模量。温度和拉伸应变速率对硅锗纳米管的力学性能有较大影响。在较高温度和较慢拉伸应变速率下,硅锗纳米管的临界屈服应变值有所下降。硅锗纳米管的临界屈服应变与结构、尺寸有较大影响。间隔式排列的扶椅型硅锗纳米管的临界屈服应变最大。通过TST(过渡态理论),我们预测硅锗纳米管在300K、拉伸速率5%/h的实验条件下的临界屈服应变为3.38%。
Silicon and Germanium are important materials in electronic industry. SiGe nanotubes show excellent thermal and mechanical properties suggesting that they could be used as nanodrillers, nanotweezers, and microscopy tips. The major interest in SiGe is that their properties depend not only on size but also on composition and atomic arrangement. In addition, SiGe nanotubes may display unique structures and properties.
However, structural and related properties of SiGe nanotubes cannot be understood purely from experimental data for their small scales and complex structures. Therefore, it is necessary to use molecular simulation and ab-initio simulation methods to understand structure and properties of these materials.
In this work, we adopt ab-initio method to study structural detail of SiGe nanotubes and also use Tersoff-based molecular dynamics method to investigate thermal and mechanical properties of SiGe nanotubes. Some important conclusions were addressed in a systematic here as follows:
1. Structures and energetic stability of SiGe nanotubes were studied by using Ab-initio Method. It is found that zigzag SiGe nanotubes exhibit Gearlike configuration and armchair nanotubes show puckering configuration. The simulation results indicate that large-diameter nanotubes are more stable than small-diameter ones. Moreover, the type 1 (alternating atom arrangement type) nanotubes are more energetically favorable than the type 2 (layered atom arrangement type) nanotubes.
2. Thermal evolution study of SiGe nanotubes were investigated by using MD simulation. It is found that during the melting process, the initial nanotubes transform into the compact nanowire first, and then chang into agglomerate structures at higher temperature. It was also found that large-diameter nanotubes exhibit higher melting-like temperature than small-diameter nanotubes, which means that the large-diameter nanotubes are more stable. The melting-like temperatures of Ge-substituted silicon nanotubes decrease with increase of the Ge concentration. In addition, the atomic arrangement type, structural character and energetic stability are the primary governing factors in thermal behaviors of SiGe nanotubes.
3. Effects of structural, temperature and strain rate on mechanical properties of SiGe nanotubes were investigated by adopting MD simulation. It is found that three structural transformations were taken place during the extending tests, from initial structure, tensile structure, to critical structure deformation. Simulations indicate that Young's modulus is closely dependent on its diameter, chirality and arrangement-structure. Type1 (alternating atom arrangement-type) armchair SiGe nanotube exhibits the largest Young's modulus. The higher temperature and lower strain rate result in the lower critical strain and tensile strength. It is also found that the critical strains of SiGe nanotubes are significantly dependent on the tube diameter and chirality. Furthermore, based on transition state theory (TST) model, we predict that the critical strain of SiGe (6,6) typel nanotube at 300 K, stretched with a strain rate of 5%/h, is about 3.38%.
引文
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