MOF-derived Co@NC particles synthesized via pyrolysis in N₂/H₂ atmosphere for electromagnetic wave absorption

ZIF-67 was employed as precursor to synthesize Co@NC electromagnetic wave absorbing materials through liquid-phase chemical synthesis combined high-temperature pyrolysis at varying temperatures (600 °C, 700 °C, 800 °C, and 900 °C) in this paper. Comprehensive investigation into their microstructure, static magnetic properties, and electromagnetic wave absorption characteristics were conducted. The results demonstrate that the Co@NC particles retain the near-dodecahedral morphology of ZIF-67, with a particle size ranging from 500 to 700 nm and an irregularly protruding surface. X-ray diffraction (XRD) analysis confirms the presence of elemental cobalt and graphitic carbon, revealing enhanced crystallinity with increasing pyrolysis temperature. BET surface area measurements indicate that the highest surface area is achieved at 800 °C, while Raman spectroscopy shows that high-temperature pyrolysis increases the degree of graphitization and concurrently reduces defect density. Magnetic property analysis reveals that the saturation magnetization monotonically increases with higher pyrolysis temperatures, while the remanent magnetization and coercivity reach peak values at 700 °C and 800 °C, respectively, indicating a significant influence of pyrolysis temperature on magnetic domain structure. Studies of electromagnetic parameters reveal that both the real part and imaginary part of the complex dielectric constant decrease with increasing frequency, with dielectric relaxation peaks observed near 14 GHz for samples prepared at 600 °C and 700 °C. High-temperature pyrolysis at 900 °C markedly enhances dielectric loss; however, the polarization relaxation effect diminishes, leading to conduction loss becoming the predominant mechanism. The imaginary part (μ") of the complex permeability exhibits a bimodal feature in samples pyrolyzed at 600–700 °C, while samples pyrolyzed at 800–900 °C present a single resonant peak. The 800 °C pyrolyzed samples demonstrate optimal impedance matching at a thickness of 2 mm, achieving an effective absorption bandwidth (EAB) of 4.88 GHz for the 30 wt% filled sample at a thickness of 1.8 mm and a minimum reflection loss (RLmin) of − 54.13 dB at a thickness of 2.1mm. For the 900 °C samples, the EAB expands to 4.96 GHz at a thickness of 1.6 mm, with an RLmin of − 21.82 dB.