Abstract
Ever since the exciting discovery of carbon nanotubes (CNTs), there has been a huge growth in research in material science on finding novel nanostructured materials with advanced material properties. Recently, due to the shrink of feature size in IC technology, nanostructured materials, especially one-dimensional (1-D) nanostructures such as CNTs, nanowires and nanorods, have been considered for use in nanoscale electronic or electromechanical devices as active electronic components or interconnects. Despite of their potential, as claimed, for various engineering applications, the thermal-mechanical properties of nanostructured materials remain not fully determined or clear, not mentioning the effects of the relevant influence factors, such as size, crystal structure and defect. Recent progress in computational methods based on molecular dynamics (MD) methods has allowed the characterizations of the mechanical properties of nanomaterials. The study aims at developing an accurate and effective MD simulation model to explore the thermal-mechanical characteristics of nanostructured materials. The study starts from the evaluation of the fundamental mechanical properties of various single/multi-walled carbon nanotubes (S/MWCNTs), including zig-zag, armchair and hybrid types. The study first focuses on the exploration of the effect of the weak inlayer van der Waals (vdW) atomistic interactions on the mechanical properties of S/MWCNTs. The influence of the axial orientation mismatch between the inner and outer layers of MWCNTs on the associated mechanical properties are also addressed, followed by the investigation of the behaviors of the interlayer shear force/strength of MWCNTs. The effectiveness of the MD simulation is demonstrated through the comparison with the theoretical/experimental data available in literature. Besides, due to the limitation of fabrication technologies nowadays, atomistic defects are often perceived in carbon nanotubes (CNTs) during the manufacturing process. Thus, the second goal of the study is to perform a systematic investigation of the effects of atomistic defects on the nanomechanical properties and fracture behaviors of single-walled CNTs (SWCNTs) using MD simulation. Key parameters and factors under investigation include the number, type (namely the vacancy and Stone-Wales defects), location and distribution of defects. The correlation between local stress distribution and fracture evolution is also discussed. To demonstrate the feasibility of the proposed MD model, the present results are compared with the theoretical/experimental data available in literature. The third goal of the study aims to estimate the elastic properties of three different metal nanowires, namely made of gold (Au), silver (Ag) and cobalt (Co), through MD simulations and nanoindentation testing. The investigation also addresses the effects of the length and cross-sectional area of the nanowires, crystal structure, presumed defect and the variation of grain boundary of the metal crystal on the mechanical properties. Furthermore, tensile test simulation for both the Au (gold) and Ag nanowires is carried out, where the ultimate strength and the necking structure are also evaluated. Verification of the MD simulation model in terms of elastic modulus is made using nanoindentation experiment, and the literature theoretical and experimental data. Finally, the last goal of the study is to establish a multi-material MD simulation model to look into the insight of the effects of self-assembly monolayer (SAM) coating on the interfacial adhesion of an Au-epoxy system and on the bondability of the thermocompression-bonded Au-Au joints. Three different types of functionalized alkanethiol SAMs (SH(CH2)nX, X=CH3, OH, NH2) chemisorbed onto Au substrates, are considered in the investigation. The investigation first explores the elastic properties of these SAMs through uniaxial tensile simulation, followed by exploring the effects of the SAMs on the adhesion behaviors of the Au-epoxy system and the Au-Au system, and those of chain lengths and tail groups of the n-alkanethiolates on the adhesion strength. The study also reports a comparative analysis of the effects of the crystal orientation of Au on the associated interfacial behaviors. The calculated results are partly compared with the published experimental data, and also with each other to identify the optimal SAM candidate in terms of adhesion strength for the Au-epoxy system. The achievements made in this study can not only provide a more thorough and clear understanding of the basic mechanical properties and behaviors of the nanostructured materials and the adhesion behaviors at the Au-Au and Au-epoxy bi-material interfaces, but also give a solid foundation for future research on the nanomechanics and industrial application of the nanostructured materials.