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 μr and εr, 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 MnxOy) embedded with Fe3O4 nanoparticles (MnxOy/C@Fe3O4). This composite was synthesized and its formation mechanism and microstructure were analyzed in detail. At the same time, the influence of this MnxOy/C@Fe3O4 structure on EMW properties and absorbing performance was further discussed.
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 (MnxOy/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 MnxOy/C@Fe3O4 composites were measured using an Agilent N5230C network analyzer in the frequency range of 2−18 GHz. For electromagentic testing, the MnxOy/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.
SEM images illustrate the preparation process of 1D mesoporous carbon-coated manganese oxide embedded with Fe3O4 nanoparticles composites (MnxOy/C@Fe3O4). Most of the manganese oxide (MnxOy) 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 MnxOy/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 MnxOy/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 MnxOy/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 MnxOy/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 MnxOy/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.
1D MnxOy/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 MnxOy/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.
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