Some Researches On Physical And Mechanical Properties Of Nano Materials And Devices By Molecular Dynamics

Materials of nanoscale have attracted great interests owing to their fascinating mechanical, physical and chemical properties. Achieving understanding of the remarkable mechanical properties of the nano-materials is important for the application of nanostructured components in future nano-electromechanical systems (NEMSs). So far, a variety of nano-materials have been studied with both theoretical methods and experimental techniques, among which the computer simulations have played important roles. Many investigations have proved that the molecular dynamics (MD) method is a convenient and effective computer simulation method in studying the mechanical behaviour of nanoscale and nanostructured materials. In this thesis, MD simulations are carried out to investigate the pressure induced phase transition of graphite and the mechanical properties of carbon nanotubes (CNTs) in the radial direction. Furthermore, a new trick of temperature controlling is suggested for reasonable simulation of nano-devices with regular mechanical motion. With the new temperature-controlling scheme, the performance of nano-bearings from BWCNTs is studied to investigate the intertube rotation tribology of CNTs. Lastly, the mechanical behaviour of nano-bicrystal copper is also studied. The results and the main contributions of the thesis are as follows.Firstly, the phase transition of graphite under uniform pressure as well as non-uniform pressure is investigated. A virtual plate is designed to give uniform pressure along the c axis of the graphite. It's found that only ~3GPa of uniaxial pressure may lead to the formation of interlayer sp3 bonds, which is reversible upon further compression. At the pressure of ~400GPa, the graphite layers are buckled to ABC sequence which is an intermediate structure between hexagonal graphite and cubic diamond. The interlayer sp3 bonds form again beyond the pressure of ~800GPa and propagate to a quenchable diamond phase at ~900GPa. Under the non-uniform pressure by ball indenter, the interlayer sp3 bonds, which first form at 36GPa and propagate with the increase of pressure, are reversible until large-scale hybridization occurs and quenchable diamond structure form at a stress level of ~100GPa. The phase transition of graphite explains well with the experimental paradox that graphite under high pressure can crack the diamond anvil.Secondly, a new method of evaluating the true nanohardness upon indentation simulation is suggested by which the deformation mechanism and the mechanical properties of multi-walled CNTs (MWCNTs) in radial direction are studied. The MWCNT is found to be soft in its radial direction with nanohardness rises slowly from about 6GPa to 15GPa, and the soft phase last until all the spaces between adjacent layers of MWCNT are compressed to a critical value of about 1.9?. Beyond the critical stage more and more new bonds form between different layers and a superhard amorphous phase with hardness up to 94GPa is obtained. Though locally compressed to a large radial strain of about 63%, the amorphous phase with a mixture of sp3 and sp2 carbons is completely reversible when unloading, showing super-elasticity. Further indentation afterwards leads to permanent sp3 and even sp rehybridization, the MWCNT is badly damaged and the hardness fluctuates with a maximum of about 124GPa which is comparable to the microhardness of diamond. The simulation results coincide well with the experimental results of cold compression of CNTs reported on PNAS at the same time with our work. The founded unique properties of super-elasticity and super-hardness of CNTs are sure to inspire future applications in special environments of strong impact and/or high pressure.Thirdly, BWCNT oscillators are simulated to demonstrate that traditional temperature controlling scheme of atomistic simulation may result in false prediction of the behaviour of nano-structure with high mechanical velocity. A new algorithm, which gives reasonable results, is suggested to separate the regular mechanical motion of movable components from the random thermal movement of a nano-device and only control the thermodynamic temperature. The temperature-controlling scheme is used to study the mechanical behaviour of BWCNT nanobearings. It's found that rotation tribology is influenced by many factors such as rotation speed, radial size, flexibility of CNTs and temperature. Increase of the rotation speed and the radial sizes of CNTs results in increase of centrifugal force and decrease of intertube distance and thus increase of the intertube friction. The centrifugal force and the thermal motion of atoms will stimulate flexile deformation of CNTs, namely the shape change of the straight axis and the circle cross-section, which will lead to increase of the rotation friction. It's also found that the BWCNTs can act as wear-less nanobearings with ultra-low friction at low rotation speed.Lastly, MD is used to study the influence of grain boundary (GB) and vacancy on the mechanical property of nano copper crystal. The results reveal that GB can promote the growth of vacancy along the interface, while obstructs it from growing into another grain through the interface. In addition, more plastic deformation is found when the initial vacancy runs away from the GB. The simulation also shows that the crystal is weakened by the vacancy in it in the way that the nearer the vacancy to the GB, the lower the strength.