مطالعه شبیه سازی MD تطبیقی ​​بر روی خواص مکانیکی نانولوله های کربنی تک دیواره زیگزاگ در حضور نقص سنگ پرتاب کننده ولز

Using molecular dynamics simulation, we investigate the influence of Stone-Thrower-Wales defects in the mechanical behavior of a zigzag (5, 0) single-walled carbon nanotube considering two different interatomic potential functions, the Tersoff–Brenner bond order potential and the Tight-Binding potential. The nanotube is subjected to axial stretch and the potential energy is computed for gradually increasing values of strain. From the energy–strain curve the mechanical characteristics like Young’s modulus, tensile strength and ductility are computed using both the potentials, firstly with a perfect lattice and then by introducing an increasing number of Stone-Thrower-Wales defects. Significant reduction in the values of the mechanical properties is observed with changes in the plastic deformation pattern. Experimental data compares reasonably well with our calculated values of the mechanical constants. Such investigations will help designing carbon nanotube based composites.

Using carbon nanotubes (CNT) as reinforcing agents to design and fabricate strong composites with desirable mechanical properties needs thorough understanding of the mechanical behavior of such nanotubes. Topological defects such as Stone-Thrower-Wales (STW) [1] defects that are either inherently present in the CNTs or introduced into them in the manufacturing process; degrade their mechanical properties to a large extent. Such defect is produced by 90° rotation of a C–C bond and thus forming two pentagons and two heptagons (Fig. 1a and b). Although many theoretical and experimental studies have been carried out to explore the mechanical behavior of carbon nanotubes, a wide variation in their results have been reported so far showing discordances of the theoretical values with the experimental ones. Theoretical studies [2], [3], [4], [5], [6] and [7] show a wide range of Young’s modulus from 0.1 to 5.5 TPa while tensile strength varies from 5 to 150 GPa, depending on the different methods of calculation, different CNT chiralities or lengths and different potentials employed to define the C–C bond in the plane of the graphene sheets. Theoretically overestimated values of nanotube properties can be attributed to the presence of various defects in the CNT structures. The role of vacancy defects or holes on the mechanical properties of CNTs was studied in many theoretical investigations such as of Mielke et al. [8], Lee et al. [9], Xiao and Hou [10] and Wang et al. [11]. The contribution of different authors in this field is tabulated in Table 1. As the exact knowledge of the stiffness or strength of the nanotubes is important for their use as the reinforcements in the next generation composites, and as the defects in general improve the adhesion of the CNTs to a polymer matrix [30], the defects are deliberately introduced into CNTs to achieve certain functionality. Moreover, defects may serve as a mediator to form a strong covalent bond for single-walled (SWCNT) or multi-walled (MWCNT) carbon nanotube bundles and thus improve their mechanical performances. Defects are easy to create, either chemically or by other processes they can be introduced in the CNT structure. Adding suitable number of STW defects, a CNT can be fully exploited for various applications. In this paper the dependence of the mechanical properties on the STW defects of a zigzag single-walled carbon nanotube (SWCNT) is studied with Tersoff–Brenner (TB1) potential and Tight-Binding potential (TB2) and the results are compared.

Comparing the results of our calculations with two different interatomic potentials used in the case of a perfect nanotube and a nanotube with 1 and 2 STW defects, respectively, it can be concluded that the inclusion of STW defects in a SWCNT structure, generally degrade their mechanical properties. Due to the cut-off function in the Brenner’s bond order potential it may fail to reveal the exact failure mechanism of the CNTs. Tight-binding potential on the other hand gives results which are closer to the experimental values regarding failure stress or ductility. For Tight-Binding potential we have obtained 9% maximum strain with 1 defect which matches with the available experimental value. So these calculations reveal that STW defects in one hand can serve as a main factor for the fracture of CNTs and on the other side, the introduction of odd or even number of defects in the tube structure may facilitate some specific applications.