Atomistic simulations on metallic and CNT-based hierarchical nanostructures

Abstract

Properties of nanocrystalline materials are related to their respective microstructures such as precipitates, dislocation forests and grain boundaries. However, when the size of materials studied is below submicroscale, it is essential to take size effect into consideration because physical mechanisms at nanoscale are different from the traditional mechanisms. Therefore, it is critical to develop a fundamental understanding of the mechanisms for deriving mechanical properties and deformation behaviors. Furthermore, recent experiments and theoretical models have shown that the nanotwinned metals with hierarchical twins can be a novel nanostructured design to achieve higher strength and toughness. Molecular dynamics simulation is adopted to study the deformation mechanisms, providing detailed insights into the effect of nanostructures on the mechanical properties and deformation behaviors. On the other hand, another kind of hierarchical structure fabricated from carbon nanotubes is investigated for potential application as a new kind of nanoporous membrane. One kind of hierarchical structure of carbon nanotubes called super square carbon nanotube networks comprises well-aligned nano-sized pores. The motivation of this study is based on the existing mass of research on discovering the graphenes with nanopores that can efficiently separate different nanoparticles such as salt ions from water molecules and toxic gases from atmosphere. The similarity between hierarchical carbon nanotube structure and nanoporous graphene is that both are carbon-based materials and have nano-sized pores. The size of the pores has been verified to be a key factor for selective molecular permeation. Thus, the hierarchical carbon nanotube structure obtained from the appropriate fabrication of carbon nanotubes can act as a new efficient nanoporous membrane for cleaning water and air. This dissertation presents results of molecular dynamics simulation on nanocrystalline materials and hierarchical carbon nanotube structures. Mainly seven parts of the work are presented: (1) Molecular dynamic simulations are adopted to analyze the deformation and failure mechanisms of nanotwinned copper films with a pre-existing crack and a twin plane. Evolution of the microstructure with consideration of the effects of certain factors such as thickness of nanotwinned copper films, the location of the pre-existing crack and the microstructure of the crack tip is examined; (2) A series of molecular dynamics simulations were performed to investigate the deformation and failure mechanisms of nanotwinned copper films with different twin orientations and spacing distributions; (3) A systematic study at atomistic scale on evolution of microstructure of metals by surface grain refinement is performed for idealized surface grain refinement to identify and explicate the deformation mechanisms in comparison with the unrefined nanocrystalline samples. Some new findings, such as earlier formation of deformation twins, are discovered that explain the mechanisms of strengthening by surface grain refinement; (4) Molecular dynamics simulation study is carried out to clarify the formation mechanisms of hierarchical deformation twins in face-centered cubic Ag and Al. Totally different thinning/thickening mechanisms of twin boundaries in nanotwinned Ag and Al are observed; (5) This study focuses on deformation mechanisms of nanotwinned Cu with hierarchical twins by virtue of a series of large-scale molecular dynamics simulations. For the same grain size and the same secondary twin spacing, two softening stages and one strengthening stage are discovered accompanied by the reduction of the primary twin spacing; (6) Super square carbon nanotube networks, acting as a new kind of nanoporous membrane, manifest excellent water desalination performance which demonstrates that nanopores in super square carbon nanotube networks can efficiently filter NaCl salt from water; and (7) Super square carbon nanotube networks are demonstrated to be effective for separation of O2 molecules from SO2/O2 gas. The relationship between selective gas separation ability and some factors such as molecular packing, pore functionalization, and temperature effect are discussed.

Date of Award	2 Oct 2015
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Xiaoqiao HE (Supervisor)