Atomistic Simulations of Mechanical Properties of Circular and Collapsed Carbon Nanotubes With Covalent Cross-Links

Stretching properties of single-walled carbon nanotubes (CNTs) of large diameters are studied in atomistic simulations. The simulations are performed based on the AIREBO empirical interatomic potential for three types of CNTs: Nanotubes with circular cross section, permanently collapsed nanotubes with “dog-bone”-shaped cross sections, and collapsed nanotubes with intra-tube covalent cross-links. In the last case, the cross-links between parallel quasi-planar parts of the nanotube wall are assumed to be formed by interstitial carbon atoms. The calculated equilibrium shape of collapsed nanotubes and the threshold diameter for permanently collapsed CNTs are found to agree with existing literature data. Elastic modulus, maximum stress, and strain at failure are calculated for zigzag CNTs with the equivalent diameter up to 6.27 nm in the temperature range from 5 K to 500 K. The simulations show that these mechanical properties only moderately depend on the diameter of circular CNTs. For collapsed CNTs with and without cross-links, the mechanical properties are practically independent of the CNT diameter for nanotubes with diameters larger than 4.7 nm. The elastic modulus and maximum stress of collapsed nanotubes are found to be smaller than those for the equivalent circular CNTs. The intra-tube cross-linking increases the elastic modulus and strength of collapsed CNTs in up to 50% compared to corresponding collapsed CNTs without cross-links, but reduces the breaking strain. Thermal softening of CNTs with increasing temperature in the range from 100 K to 500 K induces a decrease in the elastic modulus and maximum stress in about 12–33%.