In a study published in Science, Prof. ZHANG Jin and Assoc. Prof. JIAN Muqiang from Peking University and the Beijing Graphene Institute, Prof. WU Xianqian from the Institute of Mechanics of the Chinese Academy of Sciences (CAS), Assoc. Prof. GAO Enlai from Wuhan University, Prof. ZHANG Yongyi from the Suzhou Institute of Nano-Tech and Nano-Bionics of CAS, and their collaborators, reveal a strategy to fabricate carbon nanotube fibers with dynamic strength up to 14 GPa.  

Especially when developing fibers for high strain rate scenarios such as battlefield protection and space debris capture, it is particularly important to achieve ultra-high dynamic strength and high energy absorption capability. 

Carbon nanotubes are considered one of the ideal building blocks for the next generation of high-performance fibers since they are lightweight, strong, high modulus, and highly conductive both electrically and thermally. With these excellent properties, they promise to meet the needs of high strain rate applications. 

The researchers therefore proposed an innovative multiscale structural optimization strategy. First, the carbon nanotube fibers produced by floating catalyst chemical vapor deposition were purified and functionalized. Then the fibers were subjected to progressive stretching in a chlorosulfonic acid (CSA) solution containing poly(p-phenylene-2,6-benzobisoxazole) (PBO), followed by mechanical densification. This strategy led to improvements in interfacial interactions, nanotube alignment, and densification within the fibers, achieving a breakthrough in both quasi-static and dynamic strength.  

The cross-scale ordered assembly of carbon nanotubes endows the fibers with excellent mechanical properties. The quasi-static strength of the carbon nanotube fibers reached 8.2 GPa, and the traditional ballistic performance evaluation index, Cunniff velocity, exceeded 1100 m/s. In addition, the fibers exhibit good electrical conductivity.   

To disclose the impact protection performance of the carbon nanotube fibers, a mini-split Hopkinson tension bar was employed to study the mechanical behavior of the fibers under high strain rate loading. The results indicated that as the tensile rate increased, the fibers experienced a transition from ductile to brittle failure behavior, producing significant strain-rate-strengthening effects in the fibers. When the strain rate was approximately 1400 s^–1, the dynamic strength of the fibers reached 14 GPa, surpassing all other high-performance fibers.   

To investigate the dynamic response of the fibers, a laser-induced high-velocity transverse-impact test was undertaken under simulated ballistic impact loading. The results showed that the specific energy dissipation power of the fibers reached (8.7 ± 1.0) × 10¹³ m kg^–1 s^–1, far exceeding that of traditional ballistic fibers such as Kevlar.  

These findings indicate that carbon nanotube fibers have great potential for use in impact protective engineering.  

The synergistic enhancement of interfacial interactions, nanotube alignment, and densification of carbon nanotube fibers is essential for achieving their excellent mechanical properties. In-situ Raman testing and molecular dynamics simulations indicate that strong interactions between PBO and carbon nanotubes enhance intertube interactions and stress transfer. Coarse-grained simulation results suggest that during progressive stretching, the addition of PBO reduces fiber porosity, increases density, and decreases stress concentration.

Under high-speed loading conditions, a higher proportion of carbon nanotubes in the fibers breaks. In addition, the fiber fracture mode transitions from intertube sliding to more synchronous carbon nanotube fractures, thus endowing the fibers with superior dynamic mechanical properties.  

Carbon nanotube fibers, which are characterized by extremely high dynamic strength, have good potential for application in aerospace and impact protection. This study provides a feasible route for harnessing the intrinsic strength of individual carbon nanotubes at the macroscale in order to fabricate impact-resistant fiber materials.