  Structure transformation by heat treatment in single-walled carbon nanotubes
(SWCNT) is investigated using molecular dynamics simulation. The critical
temperature for the collapse of pure SWCNT is as high as 4655 K due to strong
covalent carbon-carbon bonding. Above 2000 K, the cross section of SWCNT
changes from circle to ellipse. The self-repairing capability is then
investigated and two efficient processes are observed for the SWCNT to repair
themselves. (1) In the first mechanism, vacancy defects aggregate to form a
bigger hole, and a bottleneck junction is constructed nearby. (2) In the second
mechanism, a local curvature is generated around the isolate vacancy to smooth
the SWCNT. Benefit from the powerful self-repairing capability, defective SWCNT
can seek a stable configuration at high temperatures; thus the critical
temperature for collapse is insensitive to the vacancy defect density.
