Elsevier

Diamond and Related Materials

Volume 74, April 2017, Pages 154-163
Diamond and Related Materials

An investigation on tensile properties of coiled carbon nanotubes using molecular dynamics simulation

https://doi.org/10.1016/j.diamond.2017.02.023Get rights and content

Highlights

  • Relaxation of CCNTs with and without Stone-Wales defects are presented and compared.

  • By decreasing the rising angle of CCNT, its yield strength and elastic slope decreases while its failure strain increases.

  • A (4,4) CCNT exhibits the largest toughness at temperature of 100 K and the rising angle of 12.5 degrees.

  • The spring constant of (4,4) CCNT was measured as 8 Nm 1 per pitch.

Abstract

A coiled carbon nanotube (CCNT) can be formed from the distortion of parent toroidal carbon nanotube with a uniform pitch length and a uniform spring rise angle. In this research molecular dynamics simulation was carried out to assess the tensile properties of three CCNT having indexes of (3,3), (4,4), and (5,5). The results indicated that Stone-Wales defects are necessary for thermodynamic stability of the CCNTs. The stress-strain curves showed that the yield stress, yield strain, and failure strain are decreased with increase in temperature. The force-displacement curves revealed that the spring constant of these materials is highly depended on the tube diameter and rising angle, while it is not noticeably depended on temperature. However, the toughness of the (4,4) CCNT at different temperatures and rising angles indicated that the CCNT has the highest toughness at lower temperatures and rising angles. Also, it was shown that a (4,4) CCNT presents its largest toughness at temperature of 100 K and the rising angle of 12.5°.

Introduction

The research on the carbon nanotubes in recent years has led to widespread use of these materials in various fields such as nano-springs and biosensors [1]. Since a coiled carbon nanotube (CCNT) has a helical morphology and the extraordinary tensile properties of a straight carbon nanotube (CNT), it may present a greater potential for such applications [2], [3].

So far, the computer simulations have been carried out on CCNTs composed of only hexagonal carbon rings [4], [5], as well as, on the CCNTs containing Stone-Wales defects [6], [7], [8], [9]. It should, however, be noted that no experiments have been performed yet to identify accurately and quantitatively the existence of the Stone-Wales defects and researchers have not been able to set standards to detect them systematically [10]. Thus, it is necessary to study the mechanical behavior of the CCNTs for better understanding of their applications. Chen et al. [11] determined the spring constant and maximum strain of a double wall CCNT with tubular diameter of 126 nm. They clamped the CCNT between the two cantilevers of atomic force microscope and stretched up to 42% strain. Their results showed that CCNTs behavior is similar to an elastic spring with constant of 0.12 Nm 1 and no plastic deformation was observed in their experiments. In another research [12], an equipped electron scanning microscope was used to study the mechanical and electrical properties of several CCNTs. The results showed that CCNT can be stretched up to strains of > 200%. Liu et al. [8] performed atomic quantum simulations on several thin CCNTs with tubular diameter and pitch length lower than 1 nm under the axial load. They reported that CCNTs show superelastic behavior and are able to withstand the strain as high as 60% in tension and 20–35% in compression. Wang et al. [7] studied the mechanical properties of CCNT using the uniaxial tensile and compression loads. The results of their simulation showed that the yield stress and yield strain of CCNT under compression are 9.2 GPa and 16%, respectively, while the yield stress and yield strain in tension are 14 GPa and 30%, respectively. Also, in their research the spring constant was calculated as 10.1 Nm 1. In another research [6], the mechanical behavior of coiled carbon nanotubes of different diameters was investigated at different temperatures using a tensile load. The results of this research have confirmed that by increasing the temperature and decreasing the diameter of CCNTs, the tension force is decreased. Ghaderi and Haji Esmaeili [9] measured the force and fracture strain of several straight and helical nanotubes with different diameters using molecular dynamics finite element method under the tensile load. Their results exhibited that by increasing the diameter of helical nanotubes, the fracture force is increased, while the fracture strain is constant. Ju et al. [3] did the molecular dynamics simulation and tensile test on (5,5), and (10,10) single-walled carbon nanocoils, as well as, on (5,5)@(10,10) double-walled carbon nanocoils. They demonstrated that the nanocoils exhibit superelastic characteristics and a high strength similar to that of carbon nanotube of the same chirality. In a recent study [4], carbon nanosprings (CNS) without topological defects were analyzed to evaluate their spring stiffness, a three-turns CNS showed a spring constant of 0.36 N/m and maximum elongation of 38% in elastic deformation. It should, however, be noted that despite the works conducted so far, the thermodynamic equilibrium of coiled carbon nanotubes without Stone-Wales defects has rarely been investigated in the literature so further study is needed for comparison between either models for thermodynamic stability. In addition, many questions have remained yet without any answer regarding the mechanical behavior of the CCNTs. Among them, the effect of increasing the rising angle and the combined effects of temperature and rising angle on the force-displacement, stress-strain curve, and toughness of this material have not been evaluated yet. Hence, in this work, by considering the CCNTs with or without Stone-Wales, their thermodynamic stability is assessed using molecular dynamics simulation. Further, several types of CCNTs having indexes of (3,3), (4,4), and (5,5) are selected to determine their ultimate strength, elastic slope, spring constant, toughness, and fracture mechanisms.

Section snippets

Methodology

In this research, two structures of CCNTs were constructed. In the first structure, the CCNT is free from Stone-Wales defects and the carbon rings are only hexagonal. This structure is the same as that proposed in Refs. [4], [13], [14]. In the second structure, topological defects i.e. pentagonal and heptagonal carbon rings, as the Stone-Wales defects, were added to the CCNT structure. The latter structure was based on the Chuang et al. method [15]. In this model, the CCNT is derived from

Comparing the modeled structures for CCNT

The two structures constructed in the previous section, were relaxed in NPT ensemble of zero bar pressure and temperature of 300 K. The potential energy diagram versus time step and snapshots of these two structures are presented in Fig. 3, Fig. 4, respectively. As it is seen after 10,000 fs, the CCNT constructed by hexagonal carbon rings, is converted to a straight nanotube with several fractures between the carbon atomic bonds. On the other hand, the nanotube containing the Stone-Wales defects

Conclusions

The uniaxial tensile test on coiled carbon nanotubes was simulated by molecular dynamics simulation and the effect of different parameters such as temperature, tube diameter, and rising angle on the tensile behavior of these materials was investigated. The following conclusions can be derived from the results;

  • 1.

    Relaxation of CCNTs without Stone-Wales defects shows that these CCNTs convert to the straight CNT after a short time indicating that the existence of pentagonal and heptagonal carbon

Acknowledgements

The authors would like to thank the research boards at Sharif University of Technology, Tehran, Iran for the provision of the research facilities used in this work.

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