Carbon Nanotube Strengthening Through Irradiation-Induced Crosslinking DefectsMark Locascio, M.Sc., Northwestern University, 2009.
Major Professor: Dr. Horacio D. Espinosa.
Pristine multiwalled carbon nanotubes consist of several nested, concentric shells of sp2 carbon. These shells are like rolled-up graphene tubes, and are separated from each other by a 0.34 nm gap. This distance is large enough that van der Waals forces and energy surface corrugation are the only interactions between two adjacent shells. These interactions are not sufficient to transfer load efficiently from one shell to the next.
When the shells are not pristine, however, it is possible to transfer load between two shells up to the continuum theory limit. Defects can be introduced in three ways: removal of atoms, insertion of atoms, or rearrangement of atoms. At least three types of defect have energetically-favorable configurations in which a covalent bond bridges the gap between two adjacent shells. In each case, the formation energy of the defect (computed by both density functional tight-binding methods and molecular mechanics methods) is greater than the non-bonded interactions, indicating a tighter bond, and consequently, more efficient load transfer. This phenomenon confirms the findings of separate experimental tensile tests on multiwalled carbon nanotubes.
Increasing the defect density in the nanotubes increases the load transfer up to the continuum theory limit. When the number of defects present is relatively small, the crosslinking defects undergo sequential shear failure until only the outer shell bears load. At larger numbers of defects, some crosslinks fail, resulting in diminished but significant load transfer that persists up to the failure strain. When the number of defects present is roughly comparable to the number of bonds that would be required to fracture the inner shell, we observe no bridging defect failures and near-optimal load transfer values. Thus, multiwalled nanotubes are shown to have an increased stiffness when crosslinks are present, and crosslinked tubes sustain higher loads at high strain as opposed to pristine tubes. This provides a potential path to customizing the mechanical properties of nanotubes by inducing a specific crosslink density.