Research on the Effect of the Graphite Defect Type and Doping Structure on Microwave Absorption Properties in Nitrogen-Doped Carbon Fiber

Heterogeneous atom doping, linked to atomic-scale symmetry disruption in the local microstructure, effectively creates microwave absorbers with broad effective bandwidth and high absorption capacity. Crystal defects resulting from breaking the crystal structure’s symmetry are one of the ways to achieve modulation of the microwave absorption frequency. This study synthesized nitrogen-doped carbon fiber (NCF) using a straightforward and controllable method involving electrostatic spinning and graphitization of carbon fibers. Since the defects induced by atomic-scale nitrogen substitution are nucleation sites, the appearance of a large number of dispersed nanocrystalline graphites is accompanied by a high density of heterogeneous interfaces, which enhances interfacial polarization. Introducing nitrogen atoms alters defect types in graphite crystals, thereby controlling the microwave absorption frequency. Nitrogen atoms enter graphite crystals in three nitrogen-doped configurations: pyrrole nitrogen, pyridine nitrogen, and graphite nitrogen. Introducing nitrogen atoms changes the type of defects in graphite crystals, controlling the frequency of the microwave absorption effects. The NCF absorber, graphitized at 700 °C and with a thickness of 2.35 mm, demonstrates strong microwave absorption, covering the entire Ku-band with an effective bandwidth of up to 6.50 GHz. The optimized N-doped carbon fiber with a thickness of 1.13 mm achieves a minimum RL value of −62.11 dB and an effective bandwidth of 3.74 GHz (14.26–18.00 GHz). These findings suggest that N-doped carbon fibers offer benefits such as being lightweight, highly efficient, easy to prepare, cost-effective, and having adjustable absorption frequencies.