Role of nanotube chirality on the mechanical characteristics of pillared graphene

Highlights

• Pillared graphene composed of CNTs and graphene shows unique mechanical properties.

• Pillared graphene under tension and compression possess positive and negative Poisson's ratios, respectively.

• The energy absorption properties of pillared graphene are firstly investigated using molecular dynamics methods.

Abstract

Pillared graphene composed of carbon nanotubes (CNTs) and graphene sheets shows a host of robust properties with diverse applications. Herein, mechanical characteristics of pillared graphene composed of six different chiral CNTs are explored using molecular dynamic (MD) simulations. Pillared graphene shows distinct tensile and compression mechanical characteristics that are greatly dictated by the chirality of CNT pillars. Subjected to planar and CNT-axial directional loads, (6,6) and (8,4) chiral CNTs-dominated pillared graphene are the most mechanically robust structures in terms of tensile strength. Upon compression, four distinct loading stages, including elastic compression, progressive compression, collapse of CNTs and densification, can be identified from their loading curves and re-alignment analysis of CNTs. Besides wrinkling of graphene, compression-induced structural transformations are quantitatively explained by inclination of CNTs in pillared graphene. As a result of de-wrinkling/wrinkling behaviors of graphene sheets, specific pillared graphene structures yield negative Poisson's ratio. Analysis of compression energy-absorption indicators such as crush force efficiency, stroke efficiency and specific energy-absorption reveal that they are good energy-absorbers, with the highest performance of energy-absorption for (7,5) CNT-dominated pillared graphene. The study provides critical understanding of role of CNT chirality on the mechanical properties of pillared graphene for optimal design of high-dimensional hybrid CNT-graphene structures with high mechanical performance.

Introduction

Nanostructured carbon materials contain multi-type allotropic carbon structures such as zero-dimensional (0D) fullerenes, one-dimensional (1D) carbon nanotubes (CNTs), two-dimensional graphene (2D), three-dimensional (3D) polycrystalline diamond, etc.(Rao et al., 2009; Wesolowski and Terzyk, 2011b) 1D CNTs (Iijima, 1991) and 2D graphene (Novoselov et al., 2004) have attracted widespread attention due to their outstanding physical, chemical and mechanical properties (Bernholc et al., 2002; Juarez-Mosqueda et al., 2014; Shenderova et al., 2002; Xie et al., 2011; Zou et al., 2006). 2D monolayer graphene sheet is structurally characterized by a layer of carbon atoms arranged in a hexagonal lattice, and CNTs are physically constructed by curling graphene sheets in a certain direction. As a result of hexagonal lattice of their structures, graphene and CNTs are classified into zigzag, armchair and chiral types of nanostructures (Song et al., 2018).

Because CNTs and graphene are regarded as 1D and 2D structures, respectively, they show intrinsically anisotropic material properties (Sihn et al., 2012). For example, CNTs/graphene possesses both excellent tensile modulus and strength in the tube axial/in-plane direction due to strong networks composed of C–C covalent bonds, but weak shear properties because of weak van der Waals (vdW) bonds between graphene/nanotubes (Cumings and Zettl, 2000; Yu et al., 2000). The extraordinary performances of CNTs and graphene are limited by their dimensionality (Thostenson et al., 2001; Yu et al., 2000). As a result, a number of 3D structures composed of low-dimensional carbon materials such as CNTs and graphene, were synthesized/proposed to achieve unique properties that are superior over those of the parent structures (Matsumoto and Saito, 2002; Sakhavand and Shahsavari, 2017; Xu et al., 2012). For example, an intriguing 3D carbon architecture consisting of CNTs and graphene sheets, named as 3D network nanostructure, was for the first time proposed by Dimitrakakis (Dimitrakakis et al., 2008) et al. for enhanced hydrogen storage. This 3D architecture is structurally characterized by that graphene sheets are uniquely supported by CNTs like pillars, thereby termed as pillared graphene.

Whereafter, a number of investigations (Hu et al., 2010; Loh et al., 2011, 2012; Novaes et al., 2010; Park and Prakash, 2013; Varshney et al., 2010) have been performed to reveal the thermal transport properties of pillared graphene using computer simulations. For example, using nonequilibrium molecular dynamic (MD) simulations, Varshney et al. (2010) studied the thermal transport of pillared graphene in both in-plane and out-of-plane directions of graphene sheets, and revealed that its thermal conductivity is greatly dictated by inter-pillar distance and pillar length. Loh et al. (2011) investigated the phonon transport in pillared graphene using the MD simulations, and showed that thermal conduction in the pillared graphene is primarily contributed by long-wavelength out-of-plane modes. Also, a bidirectional mode propagation mechanism was proposed for reduced heat flux in pillared graphene. Moreover, as a result of its unique structure such as porosity and large ratio of surface-to-volume, it was proposed to show diversity in practical applications such as gas separation membranes (Wesolowski and Terzyk, 2011a), thermal transfer equipment (Varshney et al., 2010), energy storage (Wu et al., 2012), and so on (Niu et al., 2014; Shahsavari and Sakhavand, 2015).

In mechanics, there have been several investigations focused on mechanical characteristics of pillared graphene including elastic and plastic properties, and deformation behaviors (Sihn et al., 2012; Song et al., 2018; Wang et al., 2014, 2017) using a variety of numeric simulations. For example, using finite element method (FEM), it was revealed that the elastic moduli of pillared distance are greatly influenced by pillared length, inter-pillared distance, chirality and volume fraction (Sihn et al., 2012; Song et al., 2018). More recently, using MD simulations, the effects of the geometric structure and size on the deformation behavior and mechanical proprieties of pillared graphene subjected to uniaxial tension and compression were examined (Wang et al., 2014).

To the best of the author's knowledge, however, atomic models of pillared graphene studied so far in the literature are composed of only armchair and zigzag CNTs. As is known, experimentally available CNTs show diversity in chirality. Therefore, there is an interesting question arising as how the chirality of CNTs as supporting pillars affects the mechanical characteristics and energy absorption of pillared graphene remains unknown yet. Towards this end, pillared graphene composed of six different chiral CNTs but with similar tube radius are constructed and their mechanical properties, deformation characteristics such as reversible structural transformations, failure mechanisms, and compression energy absorption, are comprehensively contrasted using MD simulations.