Mechanical Behavior Of Carbon Nanotube-based Covalent Networks And Self-assembled Fibers

Novel carbon nanotube-based assemblies, including covalent networks andself-assembled films and fibers, are expected to deliver the superb mechanicalproperties of carbon nanotubes from nano-scale to macro-scale. The successfulfabrication of these assemblies will contribute to development of high-performancecomposites and multi-functional devices. In this thesis, mechanical behavior ofcovalent carbon nanotube networks and carbon nanotube fibers were studied. Threemain objectives of this research are: tensile modulus and load capacity of theassemblies, macro-structural deformation and failure behavior and its relatedmicro-structural evolution, and effective mechanical properties of assembliescomposites. The main contents are as follows.(1) A generalized molecular structural mechanic method was proposed to studynon-linear deformation and failure behavior of carbon nanotube covalent networks, i.e.super graphene, super square and super carbon nanotube. The structural parameters ofbeams were obtained based on the energy equivalence of strain energies caused by thebeam deformation and steric potential energies caused by bond changing. Thereafter,deformation and failure behavior of the covalent networks were investigated withinquasi-static approximation. The load capacity, Poisson’s ratio and tensile modulus ofthe covalent macro-structures were examined. Effects of type and size of the networkon mechanical properties were evaluated.(2) Two-dimensional and three-dimensional finite element models of carbonnanotube network composites were built. The effective stiffness coefficients andYoung’s modulus of the composites were predicted by using two-scale expansionmethod. The effect of geometric parameters, i.e. tube length and junction angle, onmechanical properties of the composites were evaluated.(3) Cohesive models for interfaces between super graphene/super carbon nanotubeand polymer were established based on Van der Waals forces between the interfaceatoms of carbon nanotubes and polymer. Relation of interfacial stress and interfacialdisplacement were described. The carbon nanotube length of super carbon nanotubeon interfacial strength was investigated.(4) Coarse-grained molecular dynamic models of circular single-walled carbonnanotube and collapsed double-walled carbon nanotube were established. Force filedsof the coarse-grained models were determined by fitting simulation results withfull-atom molecular dynamis simulation results. Then, hierarchical moleculardynamic models of carbon nanotube films and fibers were constructed.Micro-structural evolution and energy change of carbon nanotube films and fibers under various loading conditions, such as twisting, monotonic stretching and cyclicstretching, were studied, which help explaining the approach of gaining strength.Effects of stacking pattern of carbon nanotubes along axial direction, alignment andentanglements on tensile strength, tensile modulus and failure mode were investigatedas well.