Carbon nanotube fibers (CNTFs) are regarded as a promising candidate material for use in high frequency AC transmission line, electromagnetic shielding cable, antenna, lightweight high-strength composites due to their advantages in low-density, high conductivity and high-strength. Recent advances are made in the fabrication of CNTFs, especially those spun by a wet-spinning technology. However, CNTFs fabricated by a conventional wet spinning method generally have irregular cross-section and non-uniform radial structure, resulting in a rapid double diffusion between chlorosulfonic acid (CSA) in the spinning solution and acetone coagulation bath and a significant radial stiffness gradient in the fiber, thus easily leading to core-shell and wrinkled structures. In this work, we selected N-Methylpyrrolidone (NMP) as a coagulation bath to decrease the diffusion rate between CSA and coagulation bath. Single-wall carbon nanotube fibers (SWCNTFs) with a circular cross section could be continuously spun in NMP. We further introduced ethanol into NMP coagulation bath to control the double diffusion rate and viscosity to improve the draw ratio of SWCNTFs. As a result, SWCNTFs with circular cross-section, high orientation, and desirable electrical and mechanical properties were fabricated.

Single-wall carbon nanotubes (SWCNTFs) with a high crystallinity and a high purity were synthesized by a floating catalyst chemical vapor deposition method. The SWCNTs were put into a beaker, and transferred to a glove box in Ar atmosphere. CSA was then added to the glass bottle, and agitated for 30 min to obtain a liquid crystal (LC) phase. The LC spinning solution was transferred into a stainless steel syringe in Ar atmosphere and extruded through a spinneret into coagulation bath by an injection pump to form macroscopic fibers. An appropriate draw ratio was applied to the SWCNTFs through a drawing device, and fibers were then wound on a PTFE roll and dried under 30 ℃ for 12 h. In this work, all the fibers were spun at a maximum draw ratio that they could endure.

The SWCNTFs spun from NMP coagulation bath have a higher circularity of 0.80, while their alignment and packing density is relatively low due to the smaller draw ratio. We therefore introduces ethanol into the NMP coagulation bath to tune the double diffusion rate to increase the applied draw ratio. The results show that the maximum draw ratio applied to SWCNTFs increases with the increase of ethanol content and reaches to the maximum value of 169% at the ethanol content of 75%. The obtained SWCNTFs have a high circularity of 0.87 and a high orientation Herman factor of 0.987. We analyzes the mechanical behavior of the nascent fiber in the coagulation bath, clarifying how the coagulation bath affects the formation of wet-spun fibers. The results show that optimizing the coagulation bath viscosity effectively reduces a frictional resistance during fiber formation, while an optimized double diffusion rate promotes a denser packing within the surface solidification layer. These synergistic effects collectively enable higher fiber draw ratios and facilitate the development of a highly oriented microstructure with a circular cross-section.

This study investigated the radial structural heterogeneity and surface wrinkling in conventional wet-spun CNTFs via developing a novel NMP-based coagulation bath. The NMP coagulation bath significantly reduced the double-diffusion rate of CSA, enabling the fabrication of SWCNTFs without a core-shell structure, while improving radial structural uniformity. Ethanol was then introduced into the NMP coagulation bath to improve the draw ratio and orientation of the fibers. The optimized SWCNTFs showed a high circular cross-section of 0.87, which was twice more than that of previously reported SWCNTFs (i.e., 0.37). The fibers simultaneously had high orientation Herman factor of 0.987, high mechanical properties with tensile strength of (1047±44) MPa and Young's modulus of (24.0±3.26) GPa, as well as high electrical characteristics with conductivity of (5.74±0.28)×10⁶ S/m and ampacity of (3.99±0.07)×10⁸ A/m². The developed SWCNTFs with a homogeneous structure and a regular morphology could have a promising potential for applications in high-frequency transmission wires and fiber reinforced composite materials.