Effects of structural defects on strength and fracture properties of multi-walled carbon nanotubes

Abstract

In this study, we report the relationship between the mechanical properties and structural defects of multi-walled carbon nanotubes (MWCNTs) synthesized by a chemical vapor deposition (CVD) method. The tensile strength, Young's modulus and Weibull modulus of the individual MWCNTs were determined by conducting uniaxial tensile tests using a manipulator tool operated inside a scanning electron microscope. In addition, the structural defects which induced the failure of the MWCNTs were observed by a transmission electron microscope (TEM). TEM observations revealed that the MWCNTs exhibited several types of structural defects: discontinuous flaws such as holes; kinks and bends; impurities i.e., catalysts even though highly crystalline layers were almost perfectly aligned with the MWCNT axis. The tensile-loading experiments demonstrated that the average tensile strength, Young's modulus and Weibull modulus of the 23 MWCNTs were 5.2 ± 2.1 GPa, 210 ± 140 GPa and 2.7, respectively. The MWCNTs underwent failure leaving either a clean break or a very short sword and sheath failure, suggesting that a significant interwall load transfer might be facilitated by the irregular wall structures as mentioned above. These mechanical characteristics and fracture modes were reasonably consistent with those of previously reported CVD-grown MWCNTs. In order to identify the structural defects controlling the fracture of the MWCNTs, structural analyses were conducted by comparing TEM images captured before and after their breaking. The TEM images of the individual MWCNTs revealed that the defects described above induced the nanotube failure. These suggest that the tensile strength of the CVD-grown MWCNTs used in this study is dominated by the structural defects in particular discontinuous flaws and kinks and bends. The results reported here indicate that improvement and optimization of synthesis methods are needed to prepare stronger MWCNTs with less structural defects reported here.