UVM Theses and Dissertations
Format:
Print
Author:
Gu, Jingjun
Dept./Program:
Mechanical Engineering
Year:
2013
Degree:
PhD
Abstract:
Carbon nanofibers (CNFs) are high-modulus, high-strength carbon-based nanomaterials relevant to a wide range of practical applications from reinforced fillers for thermal protection systems to catalyst supports and electrode materials for lithium-ion batteries and super-capacitors. Unlike carbon nanotubes (CNTs), CNFs are comprised of oblique graphene layers with a concentric cone-stacked morphology and active open edges. The unique structure of CNFs, however, gives rise to significant variations in failure strength between fibers ofequivalent size and shape that do not exist in CNTs. Yet limited knowledge exists on the relationships between microstructure and mechanical behavior in CNFs. This thesis presents a molecular dynamics simulation study to elucidate the roles of surface structure and backbone microstructure in the mechanical behavior and fracture mechanisms of thermally-treated cone-stacked CNFs.
First, the mechanisms and kinetics of surface bond strengthening during the thermal treatment of cone-stacked CNFs have been studied at the atomic level at temperatures in excess of 2000 K. Two bonding mechanisms, the formation of energetically stable loops from single dangling atoms and the folding of zigzag and armchair graphene bilayer edges, were found to display predominant but distinct kinetics. This analysis suggested a critical transition temperature at 1000 K beyond which bilayer edge folding dominates over the formation of single atom loops and more significantly contributes to surface strengthening effects.
Second, this study shows that the fiber strength can be markedly increased by increasing the cone angle of the backbone microstructure. This phenomenon was found to result from strong covalent bonds in graphene cones taking over loads in fibers with small cone angles, while van-der-Waals interactions dominated the behavior of fibers with large cone angles.
Third, it was found that the reconstruction of surface bonds could not only significantly improve the strength of CNFs, but also provide greater fracture resistance to the propagation of surface cracks during tensile deformation. As a result, enhanced ductility was predicted in thermally-treated cone-stacked CNFs in comparison to conventional graphitic CNFs and CNTs. The underlying mechanisms responsible for these phenomena are also discussed.
This dissertation sheds light on new mechanisms of strengthening and fracture in CNFs at the atomic scale, and suggests new design ideas for making composite materials with superior chemical and physical characteristics by controlling sidewall structure and internal backbone microstructure in individual fibers.
First, the mechanisms and kinetics of surface bond strengthening during the thermal treatment of cone-stacked CNFs have been studied at the atomic level at temperatures in excess of 2000 K. Two bonding mechanisms, the formation of energetically stable loops from single dangling atoms and the folding of zigzag and armchair graphene bilayer edges, were found to display predominant but distinct kinetics. This analysis suggested a critical transition temperature at 1000 K beyond which bilayer edge folding dominates over the formation of single atom loops and more significantly contributes to surface strengthening effects.
Second, this study shows that the fiber strength can be markedly increased by increasing the cone angle of the backbone microstructure. This phenomenon was found to result from strong covalent bonds in graphene cones taking over loads in fibers with small cone angles, while van-der-Waals interactions dominated the behavior of fibers with large cone angles.
Third, it was found that the reconstruction of surface bonds could not only significantly improve the strength of CNFs, but also provide greater fracture resistance to the propagation of surface cracks during tensile deformation. As a result, enhanced ductility was predicted in thermally-treated cone-stacked CNFs in comparison to conventional graphitic CNFs and CNTs. The underlying mechanisms responsible for these phenomena are also discussed.
This dissertation sheds light on new mechanisms of strengthening and fracture in CNFs at the atomic scale, and suggests new design ideas for making composite materials with superior chemical and physical characteristics by controlling sidewall structure and internal backbone microstructure in individual fibers.