Page 387 - Fiber Fracture

ATOMIC TRANSFORMATIONS 369

Fig. IO. Sample configurations showing the time evolution of a carbon addimer on (10,lO) tube under 3%
strain: (a) (7-5-5-7) defect forms after 4 ps; (b) bond rotation leads to defect with one rotated hexagon,
346 ps; (c) two hexagons, 421 ps; and (d) three hexagons at 2.35 ns. The bonds that need to be rotated in
order to incorporate more hexagons are the ones emanating from the vertex of the pentagons, pointing away
from the defect.

This behavior is to be contrasted with that of the strained (17,O) zigzag tube. Here, there
are no oscillations in the variation of energy, indicating that hexagons are all added to
the structure in a similar manner. Under 5% strain, the defect with three hexagons has
the lowest energy. However, the energy differences between adjacent configurations are
quite small, so that energetically the structures are nearly degenerate. Structures with
a larger number of hexagons may therefore readily be formed at finite temperatures.
Under the larger 10% strain, the formation energy now decreases continuously so that
the wrapping of the defect about the circumference of the nanotube is favored. The
above results have been obtained with the many-body Tersoff-Brenner potential, but
qualitatively similar results have also been obtained with a more accurate tight binding
model (Orlikowski et al., 1999).