A numerical investigation of the irradiation of CNT bundles for enhancement of mechanical properties by improved inter-tube coupling

The ultra-high stiffness and strength of carbon nanotubes (CNTs), of the order of 1 TPa and 100 GPa respectively, has stimulated intense interest in CNT-based composites, including the development of super strong fibres from CNT bundles. However, the mechanical properties of such fibres are generally far lower than that of individual CNTs, due to the weak van der Waals shear interactions between neighbouring shells and tubes, which severely limits load transfer. This deficiency affects not just shear and bending properties, but also the tensile strength and toughness when such fibres are used in composite materials, since load is generally introduced by the matrix to the outer tubes in the fibre, and must be transferred through inter-tube shear if the inner tubes are to share the load. Additionally CNTs generally do not run the full length of the fibre so inter-tube shear load transfer is essential if the fibre is to behave as a coherent entity. Without it, sword-in-sheath type fibre failure occurs in which only a few of the CNTs are actually fractured, with the rest pulled out with minimal resistance. In this thesis, carbon ion irradiation of carbon nanotube (CNT) bundles to enhance mechanical performance is investigated using classical molecular dynamics. Strategies to achieve inter-tube cross-linking for improved shear response without a drastic reduction in tensile strength due to induced defects are considered. Irradiation energies of 50–300 eV/ion, fluences of 4 x 1013 cm-2 to 2 x 1014 cm-2, and dosages of 2– 60 MGy on 7-tube bundles are studied. Within 100–200 eV/ion, the level of crosslinking is directly proportional to dosage and therefore controllable. Lower energy irradiation produces smaller-sized defects, so 100 eV/ion is the preferred energy. More than 10 different types of cross-link and a variety of defects are created. The defect level becomes excessive if either the energy or the fluence is set too high. Extension to larger bundles however is significantly more challenging. In 19-tube bundles, 500 eV/ion is required to form cross-links with the centre CNT, and at this energy careful control of fluence is required to avoid excessive damage. Thus ion irradiation for improving mechanical properties is best suited to small bundles. However, a scenario whereby small bundles are irradiated prior to twisting into bundles is suggested as a possible future method for producing macro-scale cross-linked CNT fibres. 4 The mechanical properties of irradiated CNT bundles are also investigated using molecular dynamics. Bundles irradiated with carbon ions with energy 50–300 eV/ion, and fluence between 4 x 1013 cm-2 and 2 x 1014 cm-2, are mechanically tested. With careful control of irradiation parameters, it is observed that shear and toughness parameters increase by an order of magnitude, while tensile properties reduce by only 30–40%; in real CNT fibres with discontinuous CNT filaments the reduction would be much less. The nano-scale interface response resembles that of micro-scale composites, in which interstitial C atoms play a key role. This makes C ion irradiation an attractive option over irradiation by electrons or other types of ions, since the extra C atoms can provide the required interstitial atoms. Within a certain cross-link density range, the interface shear modulus, shear stress at bonding onset and frictional sliding stress after debonding are all linearly related to cross-link density making controlled design of fibre shear properties feasible. A possible post-treatment with very low energy irradiation is proposed for healing holes and partially restoring tensile strength.