Large Improvement of the Mechanical Strength of Carbon Nanotube Films by Joule Heating Dominated Post Treatments.

Carbon nanotube (CNT) films prepared via floating catalyst chemical vapor deposition generally suffer from residual iron impurities, structural defects, and weak inter-tube interfaces, which severely limit their mechanical performance. Here, we propose a post-treatment approach, which is dominated by Joule heating, to substantially improve the mechanical properties of CNT films. Acid washing after Joule heating effectively removes iron catalyst and amorphous carbon, increasing the specific strength from 0.64 N/tex to 2.96 N/tex. Pre-stretching induces alignment of the CNTs along the stretching direction, further raising the specific strength to 5.57 N/tex. Subsequent Joule heating not only raises graphitization degree and repairs lattice defects but also transforms the weak van der Waals contacts between tubes into continuous carbon networks, leading to network densification and locking of the aligned structure. The final specific strength reaches 7.04 N/tex and true tensile strength 8.05 GPa, surpassing previous representative carbon materials. The purification mechanism of Joule heating depends on the initial iron content of the film: for high-iron films, iron melts, migrates and forms Fe/Fe3C@C core-shell particles, which can be converted into hollow carbon shells via acid etching; for low-iron films, iron is removed via atomic diffusion and evaporation. This work provides a fast, controllable and synergistic technical route for the preparation of high-performance CNT macrostructures.