An atomistic simulation study of the mechanisms and kinetics of surface bond strengthening in thermally-treated cone-stacked carbon nanofibers

Bonding mechanisms and rates between the active edges of a cone-stacked CNF are examined by molecular dynamics simulations at temperatures up to 2273 K. Thermally treated nanofibers subjected to tensile deformation show a substantial increase in the elastic strain limit, albeit no change in elastic modulus, due to the resistance of surface bonds to crack propagation. Two bonding mechanisms; i.e., the formation of energetically stable loops from single dangling atoms and the folding of zigzag and armchair graphene bilayer edges, are shown to display predominant, yet distinct kinetics. This study reveals a critical transition temperature at 1000 K beyond which bilayer edge folding dominates over the formation of single atom loops in strengthening the surface of CNFs. This study also underscores the critical roles played by surface bond types, numbers, and distributions on the large failure strength dispersion observed experimentally in CNFs.