SONG Zhiming ¹, LIU Bin ¹, YI Peng ², HAN Xuhui ¹, LIU Xiaofang ²

(1. AVIC Manufacturing Technology Institute, Beijing 100024, China; 2. School of Materials Science and Engineering,

Beihang University, Beijing 100191, China)

Extended abstract:[Background and purposes] In recent years, there has been growing attention in academia and industry on the development of high-performance electromagnetic wave (EMW) absorbing materials. However, creating lightweight broadband absorbers remains a challenge in terms of practical applications. EMW absorbing materials primarily rely on the magnetic loss of magnetic materials and/or the dielectric loss of dielectric materials to convert EMW energy into thermal energy for dissipation. Among various magnetic materials, Fe₃O₄ plays an irreplaceable role in EMW absorption due to its high saturation magnetization, low cost and compatible dielectric loss in the gigahertz frequency range. Nevertheless, the high density, large matching thickness and narrow absorption bandwidth of Fe₃O₄ pose significant challenges for practical applications. In contrast, one-dimensional (1D) structures not only retain the characteristic properties of lightweight, chemical stability and high dielectric loss, but also exhibit anisotropic structures and large aspect ratios. Additionally, researchers have found that the minimum reflection loss (RL) of hollow carbon materials with mesopores is nearly four times that of non-porous hollow carbon materials and nine times that of dense carbon materials. According to Maxwell's EMW theory, composites consisting of Fe₃O₄ and one-dimensional (1D) mesoporous carbon materials can leverage their respective advantages by optimizing the composition and structure of the composites to balance r and r, thereby enhancing EMW absorption performance. Additionally, numerous studies have demonstrated that composites composed of multi-component heterostructures significantly enhance the EAB. This enhancement is primarily ascribed to the numerous interface polarization losses generated by the additional heterostructure interfaces, which also improve the overall impedance matching of the composites. In this study, we leverage the advantages of magnetic/carbon composites, one-dimensional (1D) mesoporous carbon and multi-component heterostructures to prepare a composite of 1D mesoporous carbon-coated manganese oxide (Mn₃O₄ and MnO, denoted as Mn_xO_y) embedded with Fe₃O₄ nanoparticles (Mn_xO_y/C@Fe₃O₄). This composite was synthesized and its formation mechanism and microstructure were analyzed in detail. At the same time, the influence of this Mn_xO_y/C@Fe₃O₄ structure on EMW properties and absorbing performance was further discussed.[Methods] Firstly, MnO₂ nanowires were synthesized by using a simple hydrothermal method. Then, the MnO₂ nanowires served as templates for the synthesis of MnO₂/PDA@Fe³⁺ composites through the in-situ polymerization of dopamine and Fe³⁺ adsorption. Finally, 1D mesoporous carbon-coated manganese oxide composite embedded with Fe₃O₄ nanoparticles (Mn_xO_y/C@Fe₃O₄) composites were obtained after heat treatment at 550 ℃ in N₂. The crystal structure of the samples was analyzed using X-ray diffractometer with Cu Ka irradiation. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM) were used to observe microstructure and morphology of the samples. Nitrogen sorption measurements were obtained at 77 K on a Quantachrome surface area and pore size analyzer to measure the specific surface area and pore size distribution. XPS analysis was performed on X-ray photoelectron spectrometer with monochromatic Al Ka radiation. Magnetization curves of the samples were recorded with a Quantum Design physical property measurement system (PPMS-9) at room temperature. The electromagnetic parameters of the Mn_xO_y/C@Fe₃O₄ composites were measured using an Agilent N5230C network analyzer in the frequency range of 2−18 GHz. For electromagentic testing, the Mn_xO_y/C@Fe₃O₄ composites and paraffin wax were mixed at 50 ℃ according to the mass ratio of 15 wt.%, 20 wt.% and 25 wt.%, and pressed in a special mold to make coaxial rings (inner diameter=3.04 mm, outer diameter=7 mm), which were denoted as S-1, S-2 and S-3, respectively.[Results] SEM images illustrate the preparation process of 1D mesoporous carbon-coated manganese oxide embedded with Fe₃O₄ nanoparticles composites (Mn_xO_y/C@Fe₃O₄). Most of the manganese oxide (Mn_xO_y) was reduced to granular after heat treatment, while the outer carbon layer remains its 1D morphology and the carbon layer is interspersed with Fe₃O₄ nanoparticles. The diffraction peaks of MnO₂ nanowires align well with the body-centered tetragonal α-MnO₂. For the Mn_xO_y/C@Fe₃O₄ composites, the signals of α-MnO₂ disappears, followed by the emergence of Mn₃O₄ and three prominent diffraction peaks for the cubic MnO. In addition, four weak diffraction peaks correspond to the magnetite Fe₃O₄, consistent with the HRTEM results. The corresponding nitrogen adsorption-desorption isotherm and pore size distribution curve are presented to further analyze the mesoporous structure of composite. The surface composition and element valence states of the Mn_xO_y/C@Fe₃O₄ composite were investigated by using XPS. The polarization relaxation processes were analyzed according to the Debye theory which describes the relationship between ε′ and ε″. Besides the polarization loss, the contribution of the conduction loss plays an important role for the overall dielectric loss. The magnetization curve of Mn_xO_y/C@Fe₃O₄ exhibits typical ferromagnetic behavior. The permittivity parameter (C0), defined as C0 =µ''(μ′)⁻²f⁻¹ determine the contribution of eddy current effect to magnetic loss. The tanδε values are all larger than those of tanδμ for the three samples, indicating that the loss capacity of Mn_xO_y/C@Fe₃O₄ composites is mainly derived from the dielectric loss. Although tanδμ is smaller, it plays an important role in improving the impedance matching of Mn_xO_y/C@Fe₃O₄ composites. When the filler loading is 15 wt.%, the RL of sample S-1 is about −10.0 dB at the thickness of 1.5 mm with narrow EAB. As the filler loading increased to 20 wt.%, the RL of sample S-2 reached −62.0 dB at a thickness of 2.2 mm and the EAB was 6.4 GHz at a small thickness of 1.7 mm. When the filler loading is further increased to 25 wt.%, the microwave absorption performance of sample S3 decreased significantly with a little region of RL<−10.0 dB at the thickness of 5.0 mm. The values of |Zin/Z0| of the three samples at thicknesses of 1.5−5.0 mm were calculated. Due to good impedance matching of S-2, the incident EMW can enter the material and then can be dissipated through dipole polarization loss, interface polarization loss, conduction loss, eddy current loss and natural ferromagnetic resonance loss.[Conclusions] 1D Mn_xO_y/C@Fe₃O₄ was synthesized via a process involving the coating of polydopamine, adsorption of Fe (Ⅲ) salts and heat treatment, using MnO₂ nanowires as templates. The multi-component heterostructure of the Mn_xO_y/C@Fe₃O₄ composite (Mn₃O₄, MnO, Fe₃O₄, and C) enhances the interfacial interactions between the different phases, providing increased interface polarization loss under the action of an alternating electromagnetic field. The numerous defects and terminal groups in the mesoporous carbon provide abundant dipole polarization centers. Additionally, the presence of mesopores reduces the weight of the material while increasing the multiple scattering losses of the electromagnetic waves within the material. The 1D carbon structure in the matrix forms a conductive network between adjacent fibers, facilitating electron migration and transition, thereby enhancing conductive loss. The incorporation of magnetic Fe₃O₄ nanoparticles introduces eddy current loss and natural ferromagnetic resonance loss, thus increasing magnetic loss. Moreover, the synergistic effect between dielectric and magnetic losses improves the impedance matching of the material, leading to excellent EMW absorption performance.

Key words: electromagnetic wave absorbing materials; impedance matching; ultralight; broadband