Controllable Preparation of Carbon-Based Micro- and Nanomaterials with Different Helical Morphologies for Electromagnetic Wave Absorption

Abstract

Chiral helical structures offer unique electromagnetic loss mechanisms─including cross-polarization and multiple reflections─that are unavailable in curled or straight morphologies. Currently, the precise and controllable synthesis of carbon-based helical materials remains challenging, and the underlying mechanism by which sulfur-based catalysts regulate their growth is still unclear. Moreover, direct comparative experiments on the absorption performance of chiral helical versus curled structures are lacking. In this study, by controlling the sulfur-to-nickel ratio, three types of carbon materials with nanoscale line diameters and distinctly different morphologies were successfully synthesized: carbon microcoils (CMCs), curled carbon nanofibers (CCNFs), and straight carbon nanofibers (CNFs). The growth mechanisms of these three materials were systematically investigated, and their microwave absorption properties were compared within the 2–18 GHz frequency range. Mechanistic analysis reveals that incorporating an appropriate amount of sulfur activates the catalytic activity of large-scale nickel-based microparticles, while the surface poisoning effect of elemental sulfur balances the carbon deposition rate, enabling precise control over the nanoscale morphology. Regarding microwave absorption performance, CMCs demonstrated the best absorption capability, achieving a minimum reflection loss of −43.26 dB (at 2 mm thickness and a filling ratio of 15%) and an effective absorption bandwidth (RL < −10 dB) of 5.84 GHz, significantly outperforming CCNFs (−20.20 dB, 5.12 GHz) and CNFs (−19.19 dB, 3.86 GHz). These results directly confirm the prominent advantages of the chiral helical structure over curled and straight morphologies in microwave absorption, positioning CMCs as a lightweight and high-efficiency absorbing material with broad application prospects in electromagnetic interference shielding, military radar stealth, and civilian radiation protection.