Researches On The Mechanical Behaviors Of Nanowire And Nanowire Composite

These multi-functional materials of nanowires and nanotube(CNT) based composites have been studied intensely for nearly a decade mainly due to their unique mechanism, the potential application in nano-electronic devices and their mechanical, magnetic and optical properties. Especially in the aspect of mechanical properties, nanowires have shown the strong yield stress and high Young's modulus. They will be the important choices of functional devices in the future. Therefore, the detailed investigations on the mechanical properties of nanowires, nanotube and their composites provide significant contribution to the design of nanometer-sized devices. Last century's final decade witnessed a revolution in terms of new and powerful experimental techniques. These new techniques revolutionized the understanding of matter at its atomic level, not only due to the fact that they permit to image atoms, but also because some of them allow atomic manipulation. In this thesis, we have applied the computer simulation method to investigate the stretching and compressive behavior of nanowires and their composites. These researches have made us understand the evolution mechanism of nanowires and we also hope to find nano materials with excellent properties in this way.We have employed the molecular dynamics(MD) based methods of simulated annealing, energy minimization and first principle based density function theory(DFT) to investigate the optimized structures of carbon, Ni-Al alloy and copper nanowires. We have found that the nanowires encapsulated into CNT show extraordinarily helical and multi-shell structures. And we have also measured the mechanical properties such as the stress-strain curves, strain-strain energy curves and Young's modulus etc. The simulation results show that the ultra-thin carbon nanowire has excellent super-plasticity and its maximum strain is about 245% without failure. The initially helical structure is stretched into a single atomic chain. Not all the carbon nanowires have super-plasticity, when the diameters of carbon nanowires increase, the superplasticy will disappear. In the process of stretching the composite of carbon nanowires encapsulated in CNT, when CNT begins to break, the atoms consisting of CNW adhere with the CNT and further stretching leads the composite to a nanobridge. In the process of stretching of Ni-Al nanowires, the out layer atoms will move along the preferential orientation to fill a point-defect, leading to the phenomenon of necking and the addition of atomic layers. Young's modulus is related with the content of Ni and the distribution of Ni-Al atoms. The compressive behavior of copper nanowires encapsulated in CNT is also investigated. The embedment of copper nanowires really enhances the stability and critical strain of CNT, but it is related with the aspect ratio (length/diameter). When the aspect ratio is over a certain value, the stability and critical stress of the composite being viewed as a long column will be lower than the hollow CNT. Young's modulus of the composite decreases as the length increase, and it is also related with the diameters. Young's modulus decreases when the diameter increases, and when the diameter is over a certain value, Young's modulus will reach a certain value.