a b s t r a c tDue to their superior material properties, carbon nanotubes (CNTs) have many potential applications such as actuators, composites and electronic components. These CNT-based devices may experience high temperature during manufacture and operation. This leads to thermal expansion and residual stress in devices, and affects the device reliability. Therefore, the coefficient of thermal expansion (CTE) of CNTs is an important property for CNT-based devices. Although the CTE in radial direction of CNTs have been investigated using X-ray diffraction, there is no experimental determination for the axial CTE of CNTs and only numerical and theoretical predictions exit. Here, we investigated the CTE of aligned multi-walled CNT (MWCNT) reinforced epoxy composites in the MWCNT alignment direction in the temperature range of 30e60 C. We demonstrated that the CTE of the composites in the MWCNT alignment direction became negative by addition of more than 10.4 vol.% MWCNTs, and the axial CTE of MWCNTs was deduced to be À1.2 Â 10 À5 K À1 using the rule of mixtures. In addition, the thermal contraction phenomenon was observed in thin sheets of MWCNTs alone. The negative CTE may be explained by out-ofplane atomic vibrations predicted by theoretical and numerical studies.
In this study, the nominal tensile strength, Young's modulus and Weibull scale and shape parameter of the nominal tensile strength distribution of the MWCNTs synthesized by a thermal chemical vapor deposition (CVD) method were investigated by conducting uniaxial tensile tests. 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 and remnant catalysts even though crystalline graphene layers were aligned with the MWCNT axis. The nanotube tested in this study fractured at the structural defects such as discontinuous flaws and kinks and bends, suggesting that the tensile strength of the CVD-grown MWCNTs used in this study was dominated by the above-mentioned structural defects. The tensile-loading experiments demonstrated that the nominal tensile strength, Young's modulus and Weibull scale and shape parameter of the as-grown MWCNTs were 5.2 ± 2.1 GPa, 210 ± 150 GPa, 5.9 GPa and 2.7, respectively. The MWCNTs used in this study showed larger Weibull scale parameter values compared with both the CVD-grown and arc-discharge-grown MWCNTs evaluated an earlier study. This suggested that there was an optimal nanotube structure for increasing nominal tensile strength; not too weak but also not too strong inter-tube coupling to permit an adequate load transfer between the nanotube walls and thus a consequent clean break fracture. We also investigated the effects of the thermal annealing on the mechanical properties of the MWCNTs. The structural changes observed after annealing led to no significant impact on the nominal tensile strength of the MWCNTs, which was mainly due to incomplete removal of the structural defects by thermal annealing.
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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.
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