Tensile properties of long aligned double-walled carbon nanotube strands

Abstract

The mechanical properties of well-aligned double-walled carbon nanotube (DWNT) strands with diameters of 3–20 μm and lengths of ∼10 mm were measured using a stress–strain puller. The average tensile strength and Young’s modulus of the tested strands are 1.2 GPa and 16 GPa, respectively. Deformation and fracture processes of these samples are discussed. The tensile strength and Young’s modulus of an individual DWNT bundle were estimated, with values comparable to those of SWNT bundles. The superior mechanical strengths of our as-prepared DWNT strands are expected to give them potential as a high-strength material and a reinforcement in composites.

Introduction

Carbon nanotubes (CNTs) are predicted to possess remarkable mechanical properties, which has been proven by both simulation [1], [2], [3], [4] and experiment [5], [6], [7], [8]. This may make them a potential high-strength lightweight material and reinforcement in composites. Due to the very small dimensions of individual CNTs, it is quite difficult to investigate their mechanical properties. Salvetat et al. [7] measured the elastic moduli of single-walled carbon nanotube (SWNT) ropes by using an atomic force microscope (AFM) and a special substrate, with values of ∼1 TPa for small-diameter ropes. It was also found that the modulus depends strongly on the diameter of the ropes, decreasing by more than one order of magnitude from 3 to 20 nm. Yu et al. [8] performed direct tensile-loading experiment on individual multi-walled carbon nanotubes (MWNTs) with a “nanostressing stage” within a scanning electron microscope (SEM). The results showed that the MWNTs broke in the outmost layer. The tensile strength and Young’s modulus of this layer were 11–63 GPa and 270–950 GPa, respectively. Recent progress in preparing continuous CNTs of macroscopic length makes it possible to exploit their further applications in many cases. Pan et al. [9] prepared ropes of aligned MWNTs with diameters of ⩾10 μm and lengths of ∼2 mm. Tensile tests showed the calculated average Young’s modulus and tensile strength of a single MWNT were 0.45 ± 0.23 TPa and 1.72 ± 0.64 GPa, respectively. Li et al. [10] impregnated aligned 20 mm long SWNT ropes with polyvinyl chloride (PVC) resin to form a composite structure. Tensile strength measurements showed that the average tensile strength of SWNT rope composites was as high as 3.6 ± 0.4 GPa, similar to that of carbon fibers. Zhu et al. [11] developed direct synthesis of ∼20 cm long SWNT strands with tensile strength of ∼1.0 GPa and Young’s modulus ranging from 49 to 77 GPa.

Size of double-walled carbon nanotubes (DWNTs) falls between that of SWNTs and MWNTs with interlayer distance varying from 0.33 to 0.42 nm depending on the geometric properties of the inner and outer layers [12], [13], which is quite different from that of MWNTs (0.34 nm). As a new specific type of CNT, it is expected to have unique physical and mechanical properties and has received increased attention in recent years [14], [15], [16], [17]. Saito’s [18] theoretical calculations have showed that the stability of DWNTs depends only on interlayer spacing, while the potential barrier for relative displacement of the inner and outer layers is found to depend significantly on the chirality difference of the pair. Ru [19] analyzed infinitesimal buckling of a DWNT under axial compression. It was found that inserting an inner tube does not increase the critical axial strain compared to SWNT, despite the fact that the total critical force of DWNT could be increased due to an increase in the cross-sectional area. Li and Chou [20] simulated two different tensile-loading conditions on a DWNT by applying forces on the end of the outer layer and both layers, and demonstrated that the inner layer of a DWNT can be effectively deformed only through direct application of tensile or shear forces. Liew et al. [21] examined the elastic and plastic properties of a (5,5) and (10,10) armchair DWNT under axial tension using molecular dynamics (MD) simulation. The results showed that its Young’s modulus, ultimate stress and maximum strain are around 1.1 TPa, 160 GPa and 28%, respectively. Due to the difficulty in synthesis of highly pure and long DWNTs, experimental measurements on DWNTs’ mechanical properties are not available. Our recent success in synthesizing long well-aligned DWNTs [22] enables us to perform direct tensile tests on macroscopic DWNT strands to investigate their mechanical properties.