Mechanical properties of fused carbon nanotube networks

Abstract

The extraordinary mechanical, electrical, and thermal properties of carbon nanotubes make them ideal building blocks for multifunctional materials. However, disorganized network assembly and non-ideal constituent carbon nanotubes make macroscopic production-level networks significantly less ideal than individual carbon nanotubes. By creating covalently bonded molecular junctions in place of weaker van der Waals interactions, carbon electrical graphitization offers a way to improve the mechanical properties of the network with controlled current pulses. This study investigates the possibility of improving the mechanical performance of macro-scale carbon nanotube structures through electrical graphitization. Tensile testing of 160 µm diameter, 7 mm long carbon nanotube yarns shows that increasing current results in delayed onset of widespread slippage of internal filaments and more brittle fracture. These changes correlate with increased graphitic transformation, demonstrating experimentally that fusion can enhance macro-scale carbon nanotube networks. The results are consistent with a dual mechanism in which Joule heating defects dominate at lower currents and graphitic transformation dominates at higher currents. The evolution of mechanical properties as a function of total energy delivered further demonstrates the dual effects. At a high current of 1 A and moderate delivered energy, a 35% increase in linear strength and a 22% decrease in fracture strength were measured, suggesting a high degree of graphitization with a relatively low density of defects. Even higher values of delivered energy decrease the fracture strength by nearly 50% while barely changing the linear strength, indicating that excessive defect formation overwhelms graphitization. By varying other parameters, such as average power and frequency, the possibility of decoupling the dual effects was demonstrated. These findings enhanced the linear strength and stiffness from 0.16 N/tex and 8.3 N/tex up to 0.23 N/tex and 10.2 N/tex, respectively, with the potential for further improvements. Out-of-plane tearing was also used to study macroscopic carbon nanotube structures. Significant changes in tearing toughness upon fusion are demonstrated and attributed to the fact that tearing characterizes the strength and interactions of carbon nanotube filaments in a small area, perpendicular to the axial direction. Torsional testing is also used to characterize the networks' load-deformation behavior and capacity for elastic energy storage.