Prof. WU Xianqian from the Institute of Mechanics of the Chinese Academy of Sciences, Prof. THOMAS Edwin L. from Texas A&M University, and their teams, developed a strategy to fabricate carbon nanotube (CNT) films with specific energy absorption up to 26 MJ/kg. This study was published in Science Advances.

Defects are typically regarded as a detrimental factor that decreases the intrinsic strength of materials, thereby reducing their impact resistance. However, is this principle universal across different material systems and defect scales? Can the internal defects of materials be utilized to enhance the impact resistance?

To answer this question, researchers took multiwalled carbon nanotubes (MWNTs) film as a model material, which is comprised of disordered networks of MWNTs. Using high-energy carbon ion irradiation, they introduced atomic-scale defects into the CNTs, which decreased the strength of individual CNTs. 

However, these atomic-scale defects change the deformation behavior of the MWNT under micro-ballistic impact and enhanced significantly the impact resistance of the MWNT film. The specific energy absorption (SEA) of the MWNT film reached an unprecedented value of 26 MJ/kg, surpassing the existing record of films.‍

To investigate the impact protective performance of MWNT films, researchers performed laser-induced micro-ballistic impact experiments. They found that the high-energy carbon ion irradiation introduced distributed sp² defects into the CNTs, which reduces the strength of individual CNTs, and that the irradiation introduced sp³ bonds on the shells and between adjacent CNTs, which enhanced significantly load-sharing capabilities among the shells and between CNTs. 

During micro-ballistic impact, the sp³ bonds transformed the failure mode of MWNTs from the traditional “Sword-in-Sheath” mechanism to a coordinated fracture mode, fully utilizing the load-bearing capacity of each shell. The introduction of sp² defects created more fracture pathways, dissipating effectively the impact energy. The sp³ bonds between CNTs increased greatly the interfacial strength, enhancing the shear-induced energy dissipation at these interfaces. Moreover, the improved interfacial interactions transformed the macroscopic energy dissipation mode of the CNT network from bending-dominated to a combination of bending and stretching, unlocking additional energy dissipation channels. 

Based on the above synergistic interaction mechanism between CNTs, interface, and network, the SEA of the MWNT films irradiated by the high-energy carbon ions reached 26 MJ/kg. 

This study promotes the understanding of material defects, and provides a theoretical basis for the high-performance design of impact protective materials.