Polymer Nanocomposites Based on Nanosized Substituted Ferrites (NiZn)1-xMnxFe2O4 on the Surface of Carbon Nanotubes for Effective Interaction with High-Frequency EM Radiation

To create materials that interact effectively with electromagnetic (EM) radiation, new nanosized substituted ferrites (NiZn)1-xMnxFe2O4 (x = 0, 0.5, and 1) anchored on the surface of multi-walled carbon nanotubes (CNTs) have been synthesized. The concentration of CNTs in the (NiZn)1-xMnxFe2O4/CNT system was from 0.05 to 0.07 vol. fractions. The dielectric and magnetic characteristics of both pristine (NiZn)1-xMnxFe2O4 ferrites and (NiZn)1-xMnxFe2O4/CNT composite systems were studied. The introduction of (NiZn)1-xMnxFe2O4/CNT composites into the amorphous epoxy matrix allows to tailor absorbing properties at the high-frequency by effectively shifting the maximum peak values of the absorption and reflection coefficient to a region of lower frequencies (20-30 GHz). The microwave adsorption properties of (NiZn)1-xMnxFe2O4/0.07CNT-ER (x = 0.5) systems showed that the maximum absorption bandwidth with reflection loss below -10 dB is about 11 GHz.

Effect of Reduced Graphene Oxide on Microwave Absorbing Properties of Al1.5Co4Fe2Cr High-Entropy Alloys

The microwave absorption performance of high-entropy alloys (HEAs) can be improved by reducing the reflection coefficient of electromagnetic waves and broadening the absorption frequency band. The present work prepared flaky irregular-shaped Al1.5Co4Fe2Cr and Al1.5Co4Fe2Cr@rGO alloy powders by mechanical alloying (MA) at different rotational speeds. It was found that the addition of trace amounts of reduced graphene oxide (rGO) had a favorable effect on the impedance matching, reflection loss (RL), and effective absorbing bandwidth (EAB) of the Al1.5Co4Fe2Cr@rGO HEA composite powders. The EAB of the alloy powders prepared at 300 rpm increased from 2.58 GHz to 4.62 GHz with the additive, and the RL increased by 2.56 dB. The results showed that the presence of rGO modified the complex dielectric constant of HEA powders, thereby enhancing their dielectric loss capability. Additionally, the presence of lamellar rGO intensified the interfacial reflections within the absorber, facilitating the dissipation of electromagnetic waves. The effect of the ball milling speed on the defect concentration of the alloy powders also affected its wave absorption performance. The samples prepared at 350 rpm had the best wave absorption performance, with an RL of -16.23 and -17.28 dB for a thickness of 1.6 mm and EAB of 5.77 GHz and 5.43 GHz, respectively.

The Improved Microwave Absorption Performance of the 3D Porous (Ni@NO-C)n/NO-C Composite Absorber

Microwave absorbers that are lightweight and have good stability and high efficiency have attracted much attention for their applications in many contemporary fields. In this work, a 3D porous (Ni@NO-C)n/NO-C composite absorber was prepared using a wet chemistry method with Ni chains and melamine as precursors, in which NO-C (N,O-doped carbon)-encapsulated Ni particles are homogenously dispersed in the 3D porous networks of NO-C in the form of (Ni@NO-C)n chains. The special microstructure of the as-prepared material is proven to be beneficial for the improvement of its microwave absorption performance. The as-synthesized (Ni@NO-C)n/NO-C composite absorber exhibited an effective absorption bandwidth of 4.1 GHz and an extremely large reflection loss of -72.3 dB. The excellent microwave-absorbing performances can be ascribed to the cooperative consequences of dielectric loss and magnetic loss, along with the balance between attenuation capability and impedance matching.

Absorption Band Tunable La-Sr Co-Doped BaCo2-W Type Hexaferrites

La-Sr co-doped Ba1-x(La0.5Sr0.5)xCo2Fe16O27 (x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, respectively) hexaferrites were prepared by the solid-state method. W-type hexaferrite single phase structure with space group P63/mmc was obtained when the doping amount was x < 0.4 and an M-type hexaferrite and a spinel phase with smaller grains gradually replaced the W phase as the primary phases when x ≥ 0.6. The maximum Ms is 76.2 emu/g and the minimum Hc is 60 Oe at x = 0.4, as obtained by VSM analysis. The magnetoelectric properties of the samples were tested at 1-18 GHz with a vector network analyzer and the reflection loss was calculated based on transmission line theory. It was found that the introduction of an appropriate amount of La-Sr provides a large number of porosity defects while increasing the grain size, which can effectively improve the reflection of electromagnetic waves inside the material and dissipate more energy. At the same time, co-doping also makes the resonance frequency of the samples shift to lower frequency, resulting in tunable absorption properties in the C, X and Ku bands. When x = 0.2, the minimum reflection loss is -40.61 dB at 1.5 mm thickness, with the effective absorption bandwidth of 5.76 GHz in the X band; when x = 0.4, the minimum reflection loss is -37.45 dB at 2.5 mm, with the bandwidth of 4.97 GHz in the C band; when x = 0.6, the material has good absorption in both the X and Ku bands with the thickness less than 2 mm. The simple preparation method and good performance make La-Sr co-doped Co2W ferrite a promising microwave absorbing material.

Tunable Electromagnetic and Microwave Absorption Properties of Magnetic FeNi3 Alloys

Magnetic materials have a very broad application prospect in the field of microwave absorption, among which soft magnetic materials become the focus of magnetic materials research because of their high saturation magnetization and low coercivity. FeNi₃ alloy has been widely used in soft magnetic materials because of its excellent ferromagnetism and electrical conductivity. In this work, FeNi₃ alloy was prepared by the liquid reduction method. The effect of the filling ratio of FeNi₃ alloy on the electromagnetic properties of absorbing materials was studied. It is found that the impedance matching ability of FeNi₃ alloy is better when the filling ratio is 70 wt% than that of other samples with different filling ratios (30-60 wt%), showing better microwave absorption characteristics. When the matching thickness is 2.35 mm, the minimum reflection loss (RL) of FeNi₃ alloy with a 70 wt% filling ratio reaches -40.33 dB, and the effective absorption bandwidth is 5.5 GHz. When the matching thickness is between 2 and 3 mm, the effective absorption bandwidth ranges from 7.21 GHz to 17.81 GHz, almost covering the whole X and Ku bands (8-18 GHz). The results show that FeNi₃ alloy has adjustable electromagnetic properties and microwave absorption properties with different filling ratios, which is conducive to selecting excellent microwave absorption materials.

Design and Preparation of Flexible Graphene/Nonwoven Composites with Simultaneous Broadband Absorption and Stable Properties

As the world moves into the 21st century, the complex electromagnetic wave environment is receiving widespread attention due to its impact on human health, suggesting the critical importance of wearable absorbing materials. In this paper, graphene nonwoven (RGO/NW) composites were prepared by diffusely distributing graphene sheets in a polypropylene three-dimensional framework through Hummers' method. Moreover, based on the Jaumann structural material design concept, the RGO/NW composite was designed as a multilayer microwave absorber, with self-recovery capability. It achieves effective absorption (reflection loss of -10 dB) in the 2~18 GHz electromagnetic wave frequency domain, exhibiting a larger bandwidth than that reported in the literature for absorbers of equivalent thickness. In addition, the rationally designed three-layer sample has an electromagnetic wave absorption of over 97% (reflection loss of -15 dB) of the bandwidth over 14 GHz. In addition, due to the physical and chemical stability of graphene and the deformation recovery ability of nonwoven fabric, the absorber also shows good deformation recovery ability and stable absorption performance. This broadband absorption and extreme environmental adaptability make this flexible absorber promising for various applications, especially for personnel wearable devices.

Preparation of Hollow Porous Carbon Nanofibers and Their Performance and Mechanism of Broadband Microwave Absorption

Developing microwave absorbing composites with lightweight and wide absorption bands is an essential direction for electromagnetic wave stealth and shielding application. In this article, PAN/PMMA blend fibers and sheath-core blend fibers with PAN/PMMA as the sheath and PMMA as the core were spun by uniaxial and coaxial electrostatic spinning, respectively. Porous carbon nanofiber (PCNF) and hollow porous carbon nanofiber (HPCNF) were obtained after pre-oxidation and carbonization of the corresponding two precursor fibers. The microwave absorption composite samples with PCNF and HPCNF as absorbents and paraffin as matrix were prepared, respectively. Their electromagnetic parameters were investigated by the reflective-transmission network parameter method. The microwave absorption properties of the corresponding composites were calculated based on a model for a single-layer planewave absorber from electromagnetic parameters. The results showed diversity between the microwave absorbing performance of the composites filled with PCNF and HPCNF. HPCNF performs better than PCNF as an absorbent; that is, the lowest reflection loss of composite filled with HPCNF is -20.26 dB and the effective bandwidth (lower than -10 dB) is to 4.56 GHz, while the lowest reflection loss of a composite filled with PCNF is -13.70 dB, and the effective bandwidth (lower than -10 dB) is 2.68 GHz when the absorbent content is 7%, and the thickness is 3 mm. Much lower reflection loss and a wider absorption band could be expected from HPCNF. The presence of a hollow structure in HPCNF, which may increase the degree of polarization and provide more interfaces for the interference phase extinction of reflected electromagnetic waves, might help to improve the attenuation of electromagnetic waves and broaden the absorption band.

Coupling between the 2D "Ligand" and 2D "Host" and Their Assembled Hierarchical Heterostructures for Electromagnetic Wave Absorption

Constructing the strong interaction between the matrix and the active centers dominates the design of high-performance electromagnetic wave (EMW) absorption materials. However, the interaction-relevant absorption mechanism is still unclear, and the design of ultrahigh reflection loss (R_L < -80 dB) absorbers remains a great challenge. Herein, CoFe-based Prussian blue (PB) nanocubes are coprecipitated on the surface of ultrathin CoAl-LDH nanoplates with the assistance of unsaturated coordination sites. During the subsequent pyrolysis process, CoAl-LDH serves as a "ligand" providing a Co source and reacts with Fe or C in the CoFe-PB "host" to form stable CoFe alloys or CoC_x species. As a result, strong reactions emerged between the CoAl-LDH matrix and the active CoFe-CoC_x@NC centers. Based on the experimental results, the CoAl/CoFe-CoC_x@NC hierarchical heterostructure delivers good dielectric losses (dipolar polarization, interface polarization, and conductive loss), magnetic losses (eddy current loss, natural resonance, and exchange resonance), and impedance matching, resulting in a remarkable EMW absorption performance with a reflection loss (R_L) value of -82.1 dB at a matching thickness of 3.8 mm. Theoretical results (commercial CST) identify that the strong interaction between the 2D CoAl-LDH "ligand" and 2D CoFe-CoC_x "host" promotes a robust heterointerface among the nanoparticles, nanosheets, and nanoplates, which extremely contribute to the dielectric loss. Meanwhile, the coupling effect of nanosheets and nanoplates greatly contributes to the matching performance. This work provides an aggressive strategy for the effect of ligands and hosts on high-performance EMW absorption.

Electromagnetic Wave Absorbing, Thermal-Conductive Flexible Membrane with Shape-Modulated FeCo Nanobelts

Electromagnetic wave (EMW)-absorbing materials, manufactured with composites of magnetic particles, are essential for maintaining a high complex permeability and modulated permittivity for impedance matching. However, commonly available EMW-absorbing materials are unsatisfactory owing to their low complex permeability in the high-frequency band. Herein, we report a thin, flexible EMW-absorbing membrane comprising shape-modulated FeCo nanobelts/boron nitride nanoparticles, which enables enhanced complex permeability in the S, C, and X bands (2-12 GHz). The boron nitride nanoparticles that are introduced to the FeCo nanobelts demonstrate control of the complex permittivity, leading to an effective impedance matching close to 1, consequently resulting in a high reflection loss value of -42.2 dB at 12.0 GHz with only 1.6 mm thickness. In addition, the incorporation of boron nitride nanoparticles improves the thermal conductivity for the heat dissipation of the absorbed electromagnetic wave energy. Overall, the comprehensive study of nanomaterial preparation and shape modulation technologies can lead to the fabrication of an excellent EMW-absorbing flexible composite membrane.

Functionalized-CNT Polymer Composite for Microwave and Electromagnetic Shielding

In this research work, we studied the microwave properties of multi-wall carbon nanotube (MWCNT) surface functionalized with metallic oxides composites. Three different concentrations (5%, 10%, and 20%) of metallic oxides were used, namely cobalt, iron, and cobalt ferrite. The surface-decorated CNTS were impregnated into polyurethane (PU) matrix. The surface-decorated MWCNTs and the MWCNTs-PU composites were characterized using electron microscopy. The dielectric properties of the samples are studied using an open-ended coaxial probe technique in a wide frequency range of (5–50 GHz). The metallic oxide-decorated surface MWCNTs-PU composites demonstrated different microwave-frequency absorption characteristics depending on the concentration of the metallic oxides.

Multi-layered nano-hollow spheres for efficient electromagnetic wave absorption

Ferrite nano-hollow spheres (NHS) are of great significance to improve electromagnetic (EM) wave absorption performance. Herein, the deposition of dielectric SiO₂and ferrimagnetic CoFe₂O₄(CFO) layers on MnFe₂O₄(MnFO) NHS are found as an effective strategy to enhance EM wave attenuation. EM wave absorption properties of as-synthesized bare and bi-layered samples are investigated within a widely-used frequency range of 1-17 GHz. MnFO@CFO bi-layered NHSs exhibit an excellent reflection loss (RL) of -47.0 dB at only 20 wt% filler content with an effective broad bandwidth (BW) of ∼2.2 GHz (frequency region for RL < -10 dB). The attenuation constant is observed to increase from 191.6 Np m^-1to 457.8 Np m^-1for bare MnFO and MnFO@CFO NHSs respectively. Larger interfacial area, additional pairs of dipole, higher magnetic anisotropy, internal reflections and scattering from NHSs are responsible for superior absorption properties of MnFO@CFO NHSs. Moreover, the best impedance matching,∣Z_in/Z₀∣ ∼ 1, promotes the optimum RL in MnFO@CFO at 5.96 GHz. MnFO@SiO₂bi-layered NHSs result in a sufficiently high RL ∼ -30.0 dB with a composite absorber of a thickness of only 3 mm. Analysis from theλ/4 model for best matching thickness (t_m) displays a good agreement between experimental and simulatedt_mvalues. This study demonstrates optimized MnFO@CFO NHS as a highly promising low-cost and lightweight EM wave absorber suitable for practical high-frequency applications.

Tuning the Dielectric and Microwaves Absorption Properties of N-Doped Carbon Nanotubes by Boron Insertion

It is of great significance to regulate the dielectric parameters and microstructure of carbon materials by elemental doping in pursuing microwave absorption (MA) materials of high performance. In this work, the surface electronic structure of N-doped CNTs was tuned by boron doping, in which the MA performance of CNTs was improved under the synergistic action of B and N atoms. The B,N-doped carbon nanotubes (B,N-CNTs) exhibited excellent MA performance, where the value of minimum reflection loss was -40.04 dB, and the efficient absorption bandwidth reached 4.9 GHz (10.5-15.4 GHz). Appropriate conductance loss and multi-polarization loss provide the main contribution to the MA of B,N-CNTs. This study provides a novel method for the design of CNTs related MA materials.

Enhancement of microwave absorption bandwidth of MXene nanocomposites through macroscopic design

MXene, the new family of two-dimensional materials having numerous nanoscale layers, is being considered as a novel microwave absorption material. However, MXene/functionalized MXene-loaded polymer nanocomposites exhibit narrow reflection loss (RL) bandwidth (RL less than or equal to -10 dB). In order to enhance the microwave absorption bandwidth of MXene hybrid-matrix materials, for the first time, macroscopic design approach is carried out for TiO₂-Ti₃C₂T_x MXene and Fe₃O₄@TiO₂-Ti₃C₂T_x MXene hybrids through simulation. The simulated results indicate that use of pyramidal meta structure of MXene can significantly tune the RL bandwidth. For optimized MXene hybrid-matrix materials pyramid pattern, the bandwidth enhances to 3-18 GHz. Experimental RL value well matched with the simulated RL. On the other hand, the optimized Fe₃O₄@TiO₂-Ti₃C₂T_x hybrid exhibits two specific absorption bandwidths (minimum RL value - -47 dB). Compared with other two-dimensional nanocomposites such as graphene or Fe₃O₄-graphene, MXene hybrid-matrix materials show better microwave absorption bandwidth in macroscopic pattern.

Enhanced Microwave Absorption Bandwidth in Graphene-Encapsulated Iron Nanoparticles with Core-Shell Structure

Graphene-encapsulated iron nanoparticles (Fe(G)) hold great promise as microwave absorbers owing to the combined dielectric loss of the graphene shell and the magnetic loss of the ferromagnetic metal core. Transmission electron microscopy (TEM) revealed transition metal nanoparticles encapsulated by graphene layers. The microwave electromagnetic parameters and reflection loss (R) of the Fe(G) were investigated. Graphene provided Fe(G) with a distinctive dielectric behavior via interfacial polarizations taking place at the interface between the iron cores and the graphene shells. The R of Fe(G)/paraffin composites with different Fe(G) contents and coating thickness was simulated according to the transmit-line theory and the measured complex permittivity and permeability. The Fe(G)/paraffin composites showed an excellent microwave absorption with a minimum calculated R of −58 dB at 11 GHz and a 60 wt% Fe(G) loading. The composites showed a wide bandwidth (the bandwidth of less than −10 dB was about 11 GHz). The R of composites with 1–3 mm coating thickness was measured using the Arch method. The absorption position was in line with the calculated results, suggesting that the graphene-coated iron nanoparticles can generate a suitable electromagnetic match and provide an intense microwave absorption. Excellent Fe(G) microwave absorbers can be obtained by selecting optimum layer numbers and Fe(G) loadings in the composites.

Hierarchical Cobalt Selenides as Highly Efficient Microwave Absorbers with Tunable Frequency Response

Microwave absorbing materials have attracted much attention in solving electromagnetic interference and pollution problems. Hierarchical cobalt selenides have been obtained through a facile selenization annealing process. The as-prepared samples exhibit distinct reflection losses (R_L) and frequency responses via tailoring their crystalline configurations, with excellent absorption in Ku, X, or C band. All of the samples show R_L greater than or near -50 dB with effective bandwidths more than 4 GHz, indicating that they may serve as high-efficient and frequency-tunable microwave absorbers. Especially, the sample annealed at 400 °C shows a competitive R_L of -62.04 dB at 9.92 GHz with a thickness of 2.25 mm; meanwhile, its effective absorption bandwidth reaches 5.36 GHz with a thickness as small as 1.56 mm. The cobalt selenides as microwave absorbers exhibit a promising prospect applied in complex electromagnetic environments.

Measurements of Normal Incidence Reflection Loss as a Function of Temperature at the Water-Castor Oil Interface

Reflection loss at the water-castor oil interface as a function of temperature was measured in a direction normal to the interface using a 200-kHz acoustic signal. The acoustic impedance of water increases with temperature, whereas that of castor oil decreases. The measured reflection losses varied from 30 to 65 dB, and a sharp rising peak in reflection loss was observed at the temperature at which the acoustic impedance of water became equal to that of castor oil. This temperature is called the temperature of intromission in this paper. These measurements were compared with the model predictions based on a Rayleigh-reflection model using the measured sound speeds of both fluids. The sound speeds in water and castor oil as functions of temperature are the input parameters of the Rayleigh-reflection model, and were measured directly using an arrival time difference method in the temperature range of 5 to 30 °C. The comparison results imply that temperature is an important factor affecting the reflection at the interface separating the two fluids.

Comparison of Experimental and Modeled EMI Shielding Properties of Periodic Porous xGNP/PLA Composites

Microwave absorbing materials, particularly ones that can achieve high electromagnetic interference (EMI) absorption while minimizing weight and thickness are in high demand for many applications. Herein we present an approach that relies on the introduction of periodically placed air-filled pores into polymer composites in order to reduce material requirements and maximize microwave absorption. In this study, graphene nano platelet (xGNP)/poly-lactic acid (PLA) composites with different aspect ratio fillers were characterized and their complex electromagnetic properties were extracted. Using these materials, we fabricated non-perfect electrical conductor (PEC) backed, porous composites and explored the effect of filler aspect ratio and pore geometry on EMI shielding properties. Furthermore, we developed and experimentally verified a computational model that allows for rigorous, high-throughput optimization of absorbers with periodic porous geometries. Finally, we extend the modeling approach to explore the effect of pore addition on PEC-backed composites. Our composite structures demonstrated decreased fractions of reflected power and increased fractions of absorbed power over the majority of the X Band due to the addition of periodically arranged cylindrical pores. Furthermore, we showed that for xGNP/PLA composite material, reflection loss can be increased by as much as 13 dB through the addition of spherical pores. The ability to adjust shielding properties through the fabrication of polymer composites with periodically arranged pores opens new strategies for the modeling and development of new microwave absorption materials.

Complex Permittivity and Microwave Absorption Properties of OPEFB Fiber-Polycaprolactone Composites Filled with Recycled Hematite (α-Fe2O3) Nanoparticles

Recycled hematite (α-Fe₂O₃) nanoparticles with enhanced complex permittivity properties have been incorporated as a filler in a polycaprolactone (PCL) matrix reinforced with oil palm empty fruit bunch (OPEFB) fiber for microwave absorption applications. The complex permittivity values were improved by reducing the particle sizes to the nano scale via high-energy ball milling for 12 h. A total of 5–20 wt.% recycled α-Fe₂O₃/OPEFB/PCL nanocomposites were examined for their complex permittivity and microwave absorption properties via the open ended coaxial (OEC) technique and the transmission/reflection line measurement using a microstrip connected to a two-port vector network analyzer. The microstructural analysis of the samples included X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectroscopy (FTIR). At 1 GHz, the real (ε′) and imaginary (ε″) parts of complex permittivity of recycled α-Fe₂O₃ particles, respectively, increased from 7.88 to 12.75 and 0.14 to 0.40 when the particle size was reduced from 1.73 μm to 16.2 nm. A minimum reflection loss of −24.2 dB was achieved by the 20 wt.% nanocomposite at 2.4 GHz. Recycled α-Fe₂O₃ nanoparticles are effective fillers for microwave absorbing polymer-based composites in 1–4 GHz range applications.

Absorption and Reflection Contributions to the High Performance of Electromagnetic Waves Shielding Materials Fabricated by Compositing Leather Matrix with Metal Nanoparticles

Leather matrix (LM), a natural dielectric material, features a hierarchically suprafibrillar structure and abundant dipoles, which provides the possibility to dissipate electromagnetic waves (EW) energy via dipole relaxation combined with multiple diffuse reflections. Conventionally, metal-based materials are used as EW shielding materials due to that their high conductivity can reflect EW effectively. Herein, a lightweight and high-performance EW shielding composite with both absorption and reflection ability to EW was developed by coating metal nanoparticles (MNPs) onto LM. The as-prepared metal/LM membrane with only 4.58 wt % of coated MNPs showed excellent EW shielding effectiveness of ∼76.0 dB and specific shielding effectiveness of ∼200.0 dB cm³ g^-1 in the frequency range of 0.01-3.0 GHz, implying that more than 99.98% of EW was shielded. Further investigations indicated that the high shielding performances of the metal/LM membrane were attributed to the cooperative shielding mechanism between LM and the coating of MNPs.

Porous Co-C Core-Shell Nanocomposites Derived from Co-MOF-74 with Enhanced Electromagnetic Wave Absorption Performance

The combination of carbon materials and ferrite materials has recently attracted increased interest in microwave absorption applications. Herein, a novel composite with cobalt cores encapsulated in a porous carbon shell was synthesized via a facile sintering process with a cobaltic metal-organic framework (Co-MOF-74) as the precursor. Because of the magnetic loss caused by the Co cores and dielectric loss caused by the carbon shell with a unique porous structure, together with the interfacial polarization between two components, the ferromagnetic composite exhibited enhanced electromagnetic wave absorption performance compared to traditional ferrite materials. With the thermal decomposition temperature of 800 °C, the optimal reflection loss value achieved -62.12 dB at 11.85 GHz with thin thickness (2.4 mm), and the bandwidth ranged from 4.1 to 18 GHz with more than 90% of the microwave that could be absorbed. The achieved performance illustrates that the as-prepared porous Co-C core-shell composite shows considerable potential as an effective microwave absorber.

Fabrication and electromagnetic performance of talc/NiTiO3 composite

In this study, the electromagnetic (EM) performance of talc/NiTiO3 composite was evaluated. The morphology of talc displayed a lamella structure; there were many nanoparticles of NiTiO3 coated on the talc lamella. Thermal destruction occurred, which increased the surface area from 2.51 m2 g-1 to 79.09 m2 g-1 at the calcined stage at 650°C. The presence of NiTiO3 increased dielectric loss and magnetic loss of talc. The calculation of EM wave absorption of talc/NiTiO3 obtained a maximum reflection loss of -11.94 dB at the thickness of 6.85 mm; the optimum thickness for microwave absorption is 6.3-7.3 mm. This study revealed a new approach for fabricating an EM absorber and broadening applications of both talc and NiTiO3 in EM absorption.

Synthesis of Hierarchical ZnFe2O4@SiO2@RGO Core-Shell Microspheres for Enhanced Electromagnetic Wave Absorption

Hierarchical structured ZnFe₂O₄@SiO₂@RGO core-shell nanocomposites were prepared via a "coating-coating" route, and its structure, composition and electromagnetic properties were characterized. Compared with the binary composites of ZnFe₂O₄@SiO₂, the hierarchical ZnFe₂O₄@SiO₂@RGO ternary composites exhibited enhanced electromagnetic wave (EMW) absorption properties in terms of the effective bandwidth and minimum reflection loss (RL). Furthermore, EMW absorption properties of the prepared samples can be tuned by changing RGO content and thickness of SiO₂ layer to reach the best impedance match. The minimum RL of the sample with a thickness of 2.8 mm can reach -43.9 dB at 13.9 GHz, and its effective bandwidth (RL ≤ -10 dB) was up to 6 GHz. Hence, the obtained products can be a new candidate for lightweight EMW absorbing materials.

Fabrication of NiCo2-Anchored Graphene Nanosheets by Liquid-Phase Exfoliation for Excellent Microwave Absorbers

Graphene nanosheets (GNSs) were prepared by an efficient liquid-phase exfoliation method, and then the NiCo₂/GNS nanohybrids were fabricated using the single-mode microwave-assisted hydrothermal technique. The NiCo₂/GNS composites with different GNS proportions were investigated as microwave absorbers. Morphology investigation suggested that NiCo₂ nanocrystals were uniformly anchored on the GNS without aggregation. The electromagnetic parameters of NiCo₂/GNS nanohybrids could be artificially adjusted by changing the GNS proportion, which led to an exceptional microwave-absorbing performance. A reflection loss (RL) exceeding -20 dB was obtained in the frequency range of 5.3-16.4 GHz for the absorber thicknesses of 1.2-3.2 mm, while an optimal RL of -30 dB was achieved at 11.7 GHz for a thickness of 1.6 mm. The enhanced microwave-absorbing performance indicated that the NiCo₂/10 wt % GNS composite has great potential for use as an excellent microwave absorber.

Electrospinning of Fe/SiC Hybrid Fibers for Highly Efficient Microwave Absorption

Fe/SiC hybrid fibers have been fabricated by electrospinning and subsequent high-temperature (1300 °C) pyrolysis in Ar atmosphere using polycarbosilane (PCS) and Fe₃O₄ precursors. It is found that the introduction of Fe has had a dramatic impact on the morphology, crystallization temperature, and microwave electromagnetic properties of the hybrid fibers. In addition, the Fe particles have acted as catalyst sites to facilitate the growth of SiCO nanowires on the surface of the hybrid fibers. As a result, the permittivity and permeability have been enhanced effectively, and the high reflection loss (RL) has been achieved at a low frequency band with a thin absorber thickness. At an optimal PCS/Fe ratio of 3:0.5, the hybrid fiber/silicone resin composite (35 wt %) with a 2.25 mm absorber thickness exhibits a minimal RL of about -46.3 dB at 6.4 GHz. The wide frequency band (4-9.6 GHz) and thin absorber thickness (1.5-3.5 mm) for effective absorption (<-20 dB) prove that the Fe/SiC hybrid fiber is a promising candidate to work as a highly efficient and lightweight absorber in the C band (4-8 GHz).

Enhanced Microwave Absorption Performance of Coated Carbon Nanotubes by Optimizing the Fe3O4 Nanocoating Structure

It is well accepted that the microwave absorption performance (MAP) of carbon nanotubes (CNTs) can be enhanced via coating magnetic nanoparticles on their surfaces. However, it is still unclear if the magnetic coating structure has a significant influence on the microwave absorption behavior. In this work, nano-Fe₃O₄ compact-coated CNTs (FCCs) and Fe₃O₄ loose-coated CNTs (FLCs) are prepared using a simple solvothermal method. The MAP of the Fe₃O₄-coated CNTs is shown to be adjustable via controlling the Fe₃O₄ nanocoating structure. The results reveal that the overall MAP of coated CNTs strongly depends on the magnetic coating structure. In addition, the FCCs show a much better MAP than the FLCs. It is shown that the microwave absorption difference between the FLCs and FCCs is due to the disparate complementarities between the dielectric loss and the magnetic loss, which are related to the coverage density of Fe₃O₄ nanoparticles on the surfaces of CNTs. For FCCs, the mass ratio of CNTs to Fe³⁺ is then optimized to maximize the effective complementarities between the dielectric loss and the magnetic loss. Finally, a comparison is made with the literature on Fe₃O₄-carbon-based composites. The FCCs at the optimized CNT to Fe³⁺ ratio in the present work show the most effective specific RL_min (28.7 dB·mm^-1) and the widest effective bandwidth (RL < -10 dB) (8.3 GHz). The excellent MAP of the as-prepared FCC sample is demonstrated to result from the consequent dielectric relaxation process and the improved magnetic loss. Consequently, the structure-property relationship revealed is significant for the design and preparation of CNT-based materials with effective microwave absorption.

Recent Advances in Conjugated Polymer-Based Microwave Absorbing Materials

Microwave absorbing materials (MAMs) are paving the way for exciting applications in electromagnetic (EM) pollution precaution and national defense security, as they offer an advanced alternative to conventional reflection principles to fundamentally eliminate the EM waves. Conjugated polymer (CP)-based composites appear as a promising kind of MAM with the desirable features of low density and high performance. In this review, we introduce the theory of microwave absorption and summarize recent advances in the fabrication of CP-based MAMs, including rational design of the microstructure of pure conjugated polymers and tunable chemical integration with magnetic ferrites, magnetic metals, transition metal oxides, and carbon materials. The key point of enhancing microwave absorption in CP-based MAMs is to regulate their EM properties, improve matching of characteristic impedance, and create diversified loss mechanisms. The examples presented in this review will provide new insights into the design and preparation of CP-based composites that can satisfy the high demands of the oncoming generation of MAMs.