Mechanical Properties Of Special-shape Carbon Nanotubes

Since the appearance of carbon nanotubes (CNTs), scholars of different fields quickly participate in the study of CNTs. The experimenters have succeeded in achieving agglomerated and vertical and horizontal arranged single-walled or multi-walled CNTs macroscopic preparation of varies dimention, and applicating CNTs excellent electrically and thermally conductive, magnetic, optical and mechanical properties to make products with high electrical conductivity, high strength, high hardness, super hydrophobic performance during the last three decades. The number of patents about carbon nano-materials is growing exponentially around the world each year. Obviously, carbon nano-materials have overturned the traditional way of life and the way of thinking. The study of special-shape CNTs has becoming popular based on full understanding of straight CNTs in recent years. Special-shape CNTs not only have excellent properties of straight CNTs, but also can strengthen some properties in a specific aspect because of their special shapes, so that they can be used to design components with special purpose and special structures. This paper focuses on three kinds of typical special-shape CNTs, Y-shape CNTs (YCNT). toroidal CNTs and coil CNTs (CNC), and has systematically researched their structures, thermodynamic stabilities and mechanical properties with use of the molecular dynamics method based on the second generation Brenner potential, and investigated their mechanical properties and failure mechanisms under the load from atomistic simulations.Introducing pentagons, heptagons and octagons into the CNTs can change the curvature and further lead to special-shape CNTs, such as Y-shaped CNTs, toroidal CNTs, and coiled CNTs. In order to study the influence of intrinsic topological defects for their mechanical properties, three branches of Y-shaped CNTs are firstly simultaneously applied axial tension. Molecular dynamics simulation based on the second generation Brenner potential has been employed to investigate failure mechanism of Y-shaped CNTs under tensile loading. It is found that due to the existence of junction heptagonal defects, shear bands induced by the external load may expand outward in the form of spiral and promote the formation of Stone-Wales transformation on where the shear bands are overlaid, which therefore constitutes an effective way to dissipate the energy. With increasing of loading, the deformation develops rapidly followed by the emergence of series of little defect rings, cracks and alkene chain in sequence. Different from CNTs. the failure mechanism of Y-shaped CNTs can be well explained by the shear band theory, and the failure only happens on the main branch. We also study the temperature effect on the mechanical properties of Y-shaped CNTs. It is concluded that the higher the temperature, the smaller the critical strain at which plastic deformation occurs.The mechanical properties and failure mechanisms of coil CNTs are also systematically investigated under an uniaxial tensile load. Based on the positions of heptagon defects relative to tube axis, four types of atomistic models of coil CNTs, which are isomeric to each other, are established. Numerical results of molecular dynamics simulation indicate that the elastic coefficients of the coil CNTs are almost insensitive to the patterns of defect distribution, while the elastic limit strains are highly dependent on the defect distribution. Intrinsic heptagon defects located on the inner side of coil CNTs participate into the formation of dislocation nucleation. One C-C bond of heptagon which is most close to the tube axis firstly occurs the Stone-Wales transformation with around three hexagons. When relatively stable pentagon and heptagon defects are got, the structure ductility is excellent. Its failure strain is the biggest which is high to 185%. In reverse, when unstable octagon defects are got, the structure is relatively easy to fail. Its failure strain can reduce almost half which is only 90%. So it is very hard to make the decision that the inherent defects will reduce mechanical properties.Toroidal CNTs have the outstanding advantages in electromagnetics because of its closed circle structure. We use a piece of straight single-walled CNTs as the growth nuclear to form a series toroidal CNTs, and employ molecular dynamics method to study the relations between geometric parameters and the structural stability. It is concluded that the most stable toroidal structure is the structure that is smooth with high symmetries, and is consist of five repeat units.Inspired by the concept of cutting the fabric and sewing the garments, cutting the graphene sheet and sewing the Y-shaped CNTs with C3 symmetry is developed in the last chapter of this paper. Starting from geometrical parameters of the design goal (for example, chiral and tube diameter of branches, size of center junction), design drawing is inversely deduced on the graphene sheet. Then cut the graphene sheet, sew the Y-shaped CNTs of known size. Besides, in accordance with the above method, we design and build eight kinds of Y-shaped CNTs with chiral of armchair and zigzag, and systematically research chiral and tube diameter influence on the mechanical properties under tensile loading. Results are shown that the Young modulus is irrelevant with the chiral and tube diameter of Y-shaped CNTs in small elastic deformation stage. But in plastic deformation stage, the critical strain of armchair Y-shaped CNTs is obviously higher than zigzag Y-shaped CNTs. The critical strain of armchair Y-shaped CNTs increases as tube diameter decreases, while the critical strain of zigzag Y-shaped CNTs increases as tube diameter increases.Through detailed mechanical properties study and defect evolution analysis for three typical special-shape carbon nanotubes, this paper reveals mechanical properties different from straight carbon nanotubes and exposes defect evolution mechanism at plastic stage for special-shape carbon nanotubes. Our researches provide theory supports for their applications in nano device.