Northwestern University researchers have found a flaw in the empirical methods commonly used to assess deformations and defects in crystal structures. Now they are looking to quantum mechanics to provide a better perspective on nanotubes and reveal fractures that are otherwise invisible to analysis.
Studies reveal that failure of CNTs under tension along their lengths are likely to occur through defects introduced either during synthesis or because of stress. The focus of attention has been the Stone–Wales defect as it appears to be the lowest energy defect possible in a perfect tube. This defect occurs with a bond rotation that turns four carbon hexagons into two pentagons and two heptagons.
The results of empirical simulations have been compared with quantum mechanical results based on semi-empirical methods and density functional theory (DFT). These techniques they have found an alternative and more feasible failure mechanism for Stone–Wales defects containing carbon nanotubes under axial tension.
One fracture mechanism is based on a ring-opening step that includes failure of the bond between two pentagon units in a Stone–Wales defect. This fracture is not reproduced by the quantum calculations with the bond connecting two pentagons being much stronger than empirical potentials predict. It therefore means the bonds are not the weak link in nanotube fracture.
Quantum mechanics points to another mechanism that involves breaking more bonds within the pentagon rings but still matches certain observed mechanical properties. However this failure mechanism results in a defected tube that is only a little bit weaker than the perfect tube.