Fe3O4 Nanoparticle/N-Doped Carbon Hierarchically Hollow Microspheres for Broadband and High-Performance Microwave Absorption at an Ultralow Filler Loading

Radio-Absorbing Magnetic Polymer Composites Based on Spinel Ferrites: A Review

Ferrite-containing polymer composites are of great interest for the development of radar-absorbing and -shielding materials (RAMs and RSMs). The main objective of RAM and RSM development is to achieve a combination of efficient electromagnetic wave (EMW) absorption methods with advantageous technological and mechanical properties as well as acceptable weight and dimensions in the final product. This work deals with composite RAMs and RSMs containing spinel-structured ferrites. These materials are chosen since they can act as efficient RAMs in the form of ceramic plates and as fillers for radar-absorbing polymer composites (RAC) for electromagnetic radiation (EMR). Combining ferrites with conducting fillers can broaden the working frequency range of composite RAMs due to the activation of various absorption mechanisms. Ferrite-containing composites are the most efficient materials that can be used as the working media of RAMs and RSMs due to a combination of excellent dielectric and magnetic properties of ferrites. This work contains a brief review of the main theoretical standpoints on EMR interaction with materials, a comparison between the radar absorption properties of ferrites and ferrite-polymer composites and analysis of some phenomenological aspects of the radar absorption mechanisms in those composites.

State of the art and prospects of Fe3O4/carbon microwave absorbing composites from the dimension and structure perspective

At present, to solve the threat of electromagnetic wave (EMW) radiation pollution to human health, intelligent control and information security, tremendous efforts have been made to manufacture EMW absorbing materials. For ideal microwave absorption materials (MAMs), it is generally necessary not only to pursue strong microwave absorption (MA) over wide effective absorption bandwidth (EAB), but also to take into account the requirements of light weight, thin matching thickness and chemical stability characteristics. It has been found that magnetite (Fe3O4) is the most promising MAM to absorb and dissipate EMW among various absorbers, because of its good mechanical and chemical stability, controllable morphology, high Curie temperature, easy preparation, economy and excellent magnetic properties. However, the application performance of Fe3O4 absorber with single composition is limited by its easy agglomeration, eddy current, high density, and impedance mismatch. In addition, achieving efficient MA metrics with low absorber loading remains a huge challenge. To overcome these limitations, conjugation with dielectric carbon-based materials and special structural designs have been extensively explored as viable solutions to optimize the microwave absorption performance (MAP) of Fe3O4. This paper reviews the recent research progress of Fe3O4/carbon MAMs, and then the influence of dimensions and structures regulations on the MAPs are introduced in detail. Finally, the current existing problems and future development direction of Fe3O4/carbon composites in the field of MA are also presented.

Rational construction of yolk-shell structured Co3Fe7/FeO@carbon composite and optimization of its microwave absorption

The construction of yolk-shell composites with dielectric/magnetic multiple loss mechanisms has become a promising strategy to obtain high-efficiency microwave absorbing materials. An ideal microwave absorber should possess dielectric and magnetic loss abilities, thereby leading to the attenuation and absorption of incident electromagnetic radiation. Herein, the yolk-shell structured CoFe₂O₄@carbon (YS-CoFe₂O₄@C) and Co₃Fe₇/FeO@carbon (YS-Co₃Fe₇/FeO@C) composites were designed and synthesized through a series of processes, which include in-situ coating, heat-treating, etching and subsequent carbonization reduction reaction. The composite materials with specific structure, composition, and electromagnetic parameters could be effectively obtained by controlling the reaction conditions. The combination of alloy with high magnetic loss and carbon with advanced dielectric loss as well as the unique yolk-shell structure endow YS-Co₃Fe₇/FeO@C improved impendence matching and large attenuation constant. The YS-Co₃Fe₇/FeO@C composites show optimized microwave absorption behaviors, the minimum reflection loss is up to -57.6 dB at 12.30 GHz with the of 2.5 mm and the corresponding effective absorption bandwidth is 5.27 GHz (10.10-15.37 GHz). Moreover, the widest effective bandwidth could reach 7.0 GHz (11-18 GHz) with the thickness of 2.3 m. This design provides a novel concept for tuning microwave absorption efficiency of magnetic/dielectric composites to prepare high-performance microwave absorbers.

Customizing Heterointerfaces in Multilevel Hollow Architecture Constructed by Magnetic Spindle Arrays Using the Polymerizing-Etching Strategy for Boosting Microwave Absorption

Heterointerface engineering is evolving as an effective approach to tune electromagnetic functional materials, but the mechanisms of heterointerfaces on microwave absorption (MA) remain unclear. In this work, abundant electromagnetic heterointerfaces are customized in multilevel hollow architecture via a one-step synergistic polymerizing-etching strategy. Fe/Fe₃ O₄ @C spindle-on-tube structures are transformed from FeOOH@polydopamine precursors by a controllable reduction process. The impressive electromagnetic heterostructures are realized on the Fe/Fe₃ O₄ @C hollow spindle arrays and induce strong interfacial polarization. The highly dispersive Fe/Fe₃ O₄ nanoparticles within spindles build multi-dimension magnetic networks, which enhance the interaction with incident microwaves and reinforce magnetic loss capacity. Moreover, the hierarchically hollow structure and electromagnetic synergistic components are conducive to the impedance matching between absorbing materials and air medium. Furthermore, the mechanisms of electromagnetic heterointerfaces on the MA are systematically investigated. Accordingly, the as-prepared hierarchical Fe/Fe₃ O₄ @C microtubes exhibit remarkable MA performance with a maximum refection loss of -55.4 dB and an absorption bandwidth of 4.2 GHz. Therefore, in this study, the authors not only demonstrate a synergistic strategy to design multilevel hollow architecture, but also provide a fundamental guide in heterointerface engineering of highly efficient electromagnetic functional materials.

High-performance electromagnetic wave absorption of NiCoFe/N-doped carbon composites with a Prussian blue analog (PBA) core at 2-18 GHz

Structure design and assembly control are the two key factors in designing new microwave absorbing materials and improving their electromagnetic wave absorption (EMWA) performance; however, balancing the coordination between these factors remains a great challenge. In this manuscript, a coprecipitation method and an in-situ polymerization method were used to construct nitrogen-carbon-doped popcorn-like porous nanocomposites (NiCoFe/NC). The metallic particles were encapsulated in approximately 10 layers of graphite carbon shells, and a NiCoFe/NC core-shell structure was formed. The EMWA properties of the NiCoFe/NC composites were adjusted by varying the divinylbenzene (DVB) to acrylonitrile (AN) content. The optimized NiCoFe/NC composite showed a minimum reflection loss of -57.5 dB and a maximum effective absorption bandwidth (EAB) of 5.44 GHz. The excellent EMWA properties of the NiCoFe/NC composites can be attributed to the synergistic effect among the core-shell structure, popcorn-like structure, magnetic metal, carbon and nitrogen. This effect leads to enhanced impedance matching, interface polarization, dipole polarization, multiple reflection and scattering in the composites. In this paper, an effective strategy for the preparation of high-performance magnetic/dielectric composites is provided by carefully designing a new microstructure.

Regulating Percolation Threshold via Dual Conductive Phases for High-Efficiency Microwave Absorption Performance in C and X Bands

The development of high-efficiency microwave absorbers for C and X bands still remains a challenge, limiting the settlement of corresponding electromagnetic pollution and radar stealth. In this work, a reduced graphene oxide (RGO)/Cu/Fe₃O₄ composite is successfully proposed by a one-step solvothermal method with a GO dispersion content of 5 mL, where Fe₃O₄ exhibits high magnetic loss from natural resonance at the C band, and Cu nanorods and RGO are introduced as dual conductive phases to produce suitable dielectric properties by regulating the percolation threshold. The results show that the existence of Cu nanorods significantly reduces the conductivity and dielectric loss of the composites, optimizing the coordination of attenuation capacity and impedance matching in the C and X bands. Consequently, the obtained RGO/Cu/Fe₃O₄ composite shows outstanding microwave absorption performance with the maximum effective absorption bandwidth (EAB) value of 5.2 GHz at a thin thickness of 3.1 mm, which covers 84% of the C band and 46% of the X band (4.64-9.84 GHz). The performance is superior to the vast majority of previous absorbers in the corresponding bands.

A generalizable strategy for constructing ultralight three-dimensional hierarchical network heterostructure as high-efficient microwave absorber

Using previous models and theories to construct and develop high-efficient microwave absorbers (MAs) should be a strategic and effective ways to optimize the electromagnetic wave attenuation. Herein, the ultralow density and flexible graphene oxide foam (GOF) and reduced graphene oxide foam (RGOF)/MoS₂ nanosheets were designed and fabricated by the method of chemical vapor deposition and hydrothermal reaction. The obtained GOF and RGOF/MoS₂ samples exhibited very excellent microwave absorption properties while their densities were merely 0.0082 and 0.0084 g•cm^-3, respectively. More importantly, benefiting from the excellent synergistic effect between RGOF and MoS₂, the designed RGOF/MoS₂ well inherited the combined advantages of GOF and MoS₂ in terms of strong absorption abilities, broad absorption bandwidth and thin matching thicknesses. The values of minimum reflection loss and effective frequency bandwidth for RGOF/MoS₂ sample could reach up to -62.92 dB with the matching thickness of 2.27 mm and 4.48 GHz with the matching thickness of 2.12 mm, which were very desirable for high-performance MAs. Moreover, the obtained results indicated that the microwave absorption properties of RGOF/MoS₂ sample could be further optimized by regulating the MoS₂ content. Therefore, a new and effective strategy was proposed to develop high efficiency MAs with ultra-lightweight, wide-band, thin thickness and strong absorption capabilities.

Fabrication of Hollow Cube Dual-Semiconductor Ln2O3/MnO/C Nanocomposites with Excellent Microwave Absorption Performance

Metal-organic frameworks (MOFs) have been verified as ideal precursors for preparing highly effective microwave absorbers. However, it is still challenging to fabricate a thin, lightweight, and well-organized nanostructure with strong microwave absorption (MA) capability and wide absorption bandwidth. In this study, hollow cube dual-semiconductor Ln₂O₃/MnO/C (Ln = Nd, Gd, Er) nanocomposites, which are effective microwave absorbers, have been fabricated via one-step high-temperature carbonization of Ln-Mn-MOFs. The effect of band gap on the MA performance of various nanocomposites synthesized at the same carbonization temperature is investigated. Gd₂O₃/MnO/C-800 shows superior MA capacity with maximum reflection loss (RL_max) of -64.4 dB at 12.8 GHz and 1.86 mm-thickness. When the thickness is 1.44 mm, the RL value is obtained as -52.7 dB at 16.8 GHz, and at a low frequency of 4.36 GHz and thickness of 4.59 mm, the RL value reaches -56.4 dB. Further, the effect of temperature on the MA properties of Gd₂O₃/MnO/C is examined. The results reveal that Gd₂O₃/MnO/C-700 has an ultrahigh MA bandwidth of 6.6 GHz, covering the entire Ku bands at 2.09 mm-thickness. Overall, this work demonstrates a facile strategy to construct hollow, homogeneous ternary composites with outstanding MA performance.

Structural Engineering of Hierarchical Aerogels Comprised of Multi-dimensional Gradient Carbon Nanoarchitectures for Highly Efficient Microwave Absorption

Recently, multilevel structural carbon aerogels are deemed as attractive candidates for microwave absorbing materials. Nevertheless, excessive stack and agglomeration for low-dimension carbon nanomaterials inducing impedance mismatch are significant challenges. Herein, the delicate "3D helix-2D sheet-1D fiber-0D dot" hierarchical aerogels have been successfully synthesized, for the first time, by sequential processes of hydrothermal self-assembly and in-situ chemical vapor deposition method. Particularly, the graphene sheets are uniformly intercalated by 3D helical carbon nanocoils, which give a feasible solution to the mentioned problem and endows the as-obtained aerogel with abundant porous structures and better dielectric properties. Moreover, by adjusting the content of 0D core-shell structured particles and the parameters for growth of the 1D carbon nanofibers, tunable electromagnetic properties and excellent impedance matching are achieved, which plays a vital role in the microwave absorption performance. As expected, the optimized aerogels harvest excellent performance, including broad effective bandwidth and strong reflection loss at low filling ratio and thin thickness. This work gives valuable guidance and inspiration for the design of hierarchical materials comprised of dimensional gradient structures, which holds great application potential for electromagnetic wave attenuation.

Rationally designed structure of mesoporous carbon hollow microspheres to acquire excellent microwave absorption performance

In this study, we used a novel and facile hard-template etching method to manufacture mesoporous carbon hollow microspheres (MCHMs). We prove that the dielectric ability and microwave absorption of MCHMs can be adjusted by structural characteristics. When the average particle size of MCHMs is 452 nm, the paraffin composite material mixed with 10 wt% MCHMs can achieve a maximum reflection loss value of -51 dB with a thickness of 4.0 mm at 7.59 GHz. When the average particle size of MCHMs is 425 nm, the effective absorption bandwidth of the paraffin composite material mixed with 10 wt% MCHMs can achieve a broad bandwidth of 7.14 GHz with a thickness of 2.5 mm. Compared with other microwave absorbers, MCHMs possess high microwave absorption capacity and broad microwave absorption bandwidth with as low as a 10 wt% filler ratio. This excellent microwave absorption performance is due to the internal cavity and the mesoporous shell of MCHMs. By rationally designing the structure of MCHMs, excellent microwave absorption performance can be acquired. Meanwhile, this design concept based on a rational design of spherical structure can be extended to other spherical absorbers.

Hierarchical Magnetic Network Constructed by CoFe Nanoparticles Suspended Within "Tubes on Rods" Matrix Toward Enhanced Microwave Absorption

Hierarchical magnetic-dielectric composites are promising functional materials with prospective applications in microwave absorption (MA) field. Herein, a three-dimension hierarchical "nanotubes on microrods," core-shell magnetic metal-carbon composite is rationally constructed for the first time via a fast metal-organic frameworks-based ligand exchange strategy followed by a carbonization treatment with melamine. Abundant magnetic CoFe nanoparticles are embedded within one-dimensional graphitized carbon/carbon nanotubes supported on micro-scale Mo₂N rod (Mo₂N@CoFe@C/CNT), constructing a special multi-dimension hierarchical MA material. Ligand exchange reaction is found to determine the formation of hierarchical magnetic-dielectric composite, which is assembled by dielectric Mo₂N as core and spatially dispersed CoFe nanoparticles within C/CNTs as shell. Mo₂N@CoFe@C/CNT composites exhibit superior MA performance with maximum reflection loss of - 53.5 dB at 2 mm thickness and show a broad effective absorption bandwidth of 5.0 GHz. The Mo₂N@CoFe@C/CNT composites hold the following advantages: (1) hierarchical core-shell structure offers plentiful of heterojunction interfaces and triggers interfacial polarization, (2) unique electronic migration/hop paths in the graphitized C/CNTs and Mo₂N rod facilitate conductive loss, (3) highly dispersed magnetic CoFe nanoparticles within "tubes on rods" matrix build multi-scale magnetic coupling network and reinforce magnetic response capability, confirmed by the off-axis electron holography.