Nitrogen-Doped Helical Carbon Nanofibers for Enhanced Microwave Absorption

Helical carbon nanofibers possess significant potential for microwave absorption (MA) owing to their unique 3D architecture. However, their practical performance is often constrained by poor impedance matching and limited attenuation mechanisms. Heteroatom doping represents an effective strategy to modulate electronic structure and defect chemistry, thereby enhancing dielectric loss behavior. In this work, nitrogen-doped helical carbon nanofibers (NHCNFs) were fabricated via chemical vapor deposition followed by flotation-assisted N-doping, enabling precise regulation of nitrogen content and defect structures. The effects of N-doping concentration on microstructure, electronic properties, electromagnetic parameters, and MA performance were systematically investigated. Moderate nitrogen incorporation was found to significantly increase defect density and introduce abundant pyridinic-, pyrrolic-, and graphitic-N species, which synergistically enhance dipole polarization loss while optimizing conductive loss. Combined with the intrinsic spring-like helical geometry that promotes multiple scattering, polarization conversion, extended electromagnetic propagation paths, magnetic loss and cross-polarization loss, the NHCNFs exhibit well-balanced impedance matching and strong attenuation capability. As a result, NHCNFs-30 achieves a minimum reflection loss of −40.8 dB at 14.5 GHz with a thin matching thickness of 2.5 mm and an effective absorption bandwidth of 6.12 GHz. Furthermore, CST simulations demonstrate pronounced radar cross-section reduction for NHCNFs-based coatings over a wide angular range. This work provides a scalable and lightweight design strategy for high-performance carbon-based microwave absorbers through the synergistic integration of heteroatom doping and geometric structural engineering, offering promising prospects for electromagnetic protection and stealth applications.