Ultrathin, broadband microwave absorbing materials (MAMs) are crucial for weight-sensitive and space-constrained applications. This study introduces the electromagnetic frequency dispersion coefficient (EFDC), a synergistic dielectric-magnetic parameter that moves beyond conventional complex mechanisms. Our model directly links EFDC to microwave absorption (MA) performance, guiding the design of advanced MAMs. By optimizing EFDC, we achieved an ultra-wide effective absorption bandwidth (EAB) of 7.04 GHz at 1 mm and 9.28 GHz at 1.3 mm. Moreover, the temperature invariance of EFDC ensures consistent MA performance from 298 K to 473 K, despite the differing thermal responses of permittivity and permeability. This principle outlines a systematic design strategy for fabricating ultrathin and broadband MAMs, establishing a robust framework for developing high-attenuation absorbers suitable for complex frequency and thermal environments.

Aerospace vehicles and electronic devices are often operated in elevated temperature environments, which has led to an increased demand for high-temperature microwave absorbing materials (MAMs). The microwave absorption properties of MAMs are influenced not only by the intrinsic material characteristics but also by the micro-, meso-, and macro-structural configurations of the composites. This paper reviews recent advancements in high-temperature MAMs through a multiscale design approach. Due to the temperature sensitivity of permittivity, MAMs tend to exhibit microwave reflection behavior at elevated temperatures. Strategies aimed at regulating conductivity and polarization behavior can enhance impedance matching under these conditions; however, this often results in an unavoidable reduction in loss capability. In contrast to traditional design methodologies, MAMs constructed from subwavelength-scale functional units can achieve remarkable anti-reflection effects through discrete unit distribution, thereby significantly optimizing both dielectric loss capacity and impedance matching. Furthermore, the equivalent electromagnetic parameters of metacomposites are intrinsically linked to their physical properties and structural attributes. This relationship offers a novel strategy for enhancing high-temperature microwave absorption performance. It is anticipated that this review will provide insightful guidance for future explorations into innovative and highly efficient high-temperature MAMs.

The increasing issue of electromagnetic pollution necessitates the development of high-efficiency microwave absorbing materials. Traditional composites present challenges due to temperature sensitivity, complicating impedance matching and loss capabilities across varying temperatures. Rather than concentrating on the micro-scale structures and components typical in traditional design strategies, mesoscopic metacomposites have garnered significant attention due to their capacity to enhance microwave absorption and impedance matching through a discrete distribution of subwavelength-scale functional units in the composites. This review focuses on the applications of mesoscopic metacomposites in improving microwave absorbing performance. The discrete arrangement of subwavelength units improves anti-reflection effects and provides significant intrinsic loss capacity, enabling strong attenuation and effective impedance matching. Additionally, mesoscopic metacomposites facilitate controlled reflection and scattering of electromagnetic waves by carefully designing conductivity, dimensions, and spatial configurations. This presents groundbreaking methods for the further enhancement of microwave absorption efficacy. This review aspires to illuminate the pathway toward the development of thin, lightweight, highly efficient microwave absorbing materials with broadband absorption capabilities.

Electromagnetic wave (EMW) absorption materials with both high-performance and low-cost are demanded to reduce unwanted noise at microwave communication frequencies. Herein, Ni/Ni3ZnC0.7 modified alginate -derived carbon (Ni/Ni3ZnC0.7/C) composites were successfully prepared by constructing a Ni2+-Zn2+-alginate gel, followed by freeze-drying and carbonization processes. The co-introduction of nickel and zinc greatly improved EMW absorption properties. By adjusting the amounts of Ni2+ and Zn2+ ions, Ni/Ni3ZnC0.7/C composites with excellent impedance matching and EMW absorption performance were realized, evidenced by a maximum reflection loss of-40.2 dB (14.8 GHz, 2.0 mm) and an effective absorption bandwidth of 5.4 GHz (12.6-18.0 GHz, 2.0 mm). This work provides a very simple and green strategy for designing lightweight and high-performance EMW absorbers offering considerable potential for practical applications.

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.

Eggshell membrane-derived Co-CoxSy-Ni/N,S-codoped carbon composites (EM-Co-CoxSy-Ni/N,S-C) were synthesized by heating a Co2+-Ni2+-2-methylimidazole/eggshell membrane precursor in a N-2 atmosphere. The dual atom (nitrogen (N) and sulfur (S)) doping of C was realized easily by using eggshell membrane as the joint source of carbon, N and S. The EM-Co-CoxSy-Ni/N,S-C composites showed remarkable electromagnetic wave absorption (EMWA) performance. The minimum reflection loss (RLmin) reached-48.5 dB (10.1 GHz, 3.5 mm) and the Effective absorption bandwidth (EAB) achieved 7.4 GHz (10.3-17.7 GHz, 3 mm). By adjusting the concentration of 2-methylimidazole, impressive RLmin (-41.5 dB, 13.9 GHz) and EAB (6.5 GHz, 11.5-18 GHz) could be achieved at an absorber thickness of 2.5 mm. Eggshell membrane exhibits considerable potential in designing heteroatom doped carbon based EMW absorbents with high-performance and low cost.

Lightweight magnetic polyvinylidene fluoride/ferric oxide/polyaniline nanofiber (PVDF/gamma-Fe2O3/PANI) foams with excellent microwave absorption performance in the K (18-26.5 GHz) and Ka (26.5-40 GHz) bands were successfully fabricated using heat shaping and a sacrificial templating method. By varying the diameter of the PANI nanofibers (50, 100 or 500 nm), the electromagnetic microwave absorption (EMWA) properties of the composite foams could be precisely tuned. PVDF/gamma-Fe2O3/PANI-50 foams with a PANI nanofiber diameter similar to 50 nm delivered the best all-round EMWA performance, demonstrated by minimum reflection loss (RLmin) values of -38.7 dB at 26.33 GHz and -38.4 dB at 28.9 GHz at an absorber thickness of 2.5 mm. Further, the effective absorption bandwidth (EAB) of the composite could be adjusted to cover the entire 18-40 GHz frequency region simply by changing the absorber thickness (from 1.5 to 3.5 mm). This work introduces a simple low cost strategy for designing high performance microwave absorbers. (C) 2021 Elsevier B.V. All rights reserved.

The novel microstructure of magnetic-carbonaceous composites has become a prevalent route to improve electromagnetic wave absorption (EMWA) performance. Flower-like Ni/nitrogen-doped carbon (Ni/NC-X) composites with core-shell synergistic structure, N-doping and Ni-N bonds were rationally constructed and fabricated by hydrothermal methods and thermal decomposition with carbon reduction. The Ni cores were encapsulated within approximately 15 layers of the graphene shell, leading to the generation of a Ni/NC-X core-shell configuration. The EMWA performances of Ni/NC-X composites could be adjusted by modulating the acrylonitrile (AN) content. Benefiting from the synergistic effects of the core-shell configuration, a hierarchically flower-like architecture and the components (Ni and C), the flower-like Ni/NC-0.80 composite showed remarkable EMWA performance with a reflection loss (RL) of - 40.1 dB and a broad effective absorption bandwidth (EAB) of 10.05 GHz. The result of this study establishes a new strategy to prepare magnetic-carbonaceous composites by carefully designing the microstructure.

Many gallate esters have been applied as food additives due to their good biological properties. Herein, nine novel gallate ester derivatives were synthesized by a Friedel-Crafts alkylation reaction and characterized by melting point (m.p.), infrared (IR) spectroscopy, nuclear magnetic resonance (1 H- and 13 C-NMR) spectra, and high-resolution mass spectrometry (HR-ESI-MS). Their antioxidant and antibacterial activities were measured using a series of classical assays. Studies found that the products showed favorable antioxidant and antibacterial activities. Their 1,1-diphenyl-2-picrylhydrazyl free radical (DPPH⋅ ) scavenging effect IC50 values were less than 5.00 μg mL-1 and their reducing power was not less than that of vitamin C (Vc). Furthermore, the antibacterial results showed that the minimum inhibitory concentration (MIC) values of the products were not greater than 8.00 μg mL-1 , and their antibacterial rates were over 95 % at 300 μg mL-1 . The above data add valuable and novel information that gallate ester derivatives can be considered potential food additives to address food safety issues because of their high biological activity and health benefits.

A series of ZnFe2O4@SiO2@PPy nanocomposites with different SiO2 contents were successfully fabricated using a combination of sol-gel and in-situ polymerization processes. Spherical ZnFe2O4 particles (mean diameter similar to 300 nm) were first synthesized, then coated successively with conformal layers of SiO2 and polypyrrole (PPy). The electromagnetic wave (EMW) absorption properties of the resulting ZnFe2O4@(SiO2)(x)@PPy nanocomposites (where x = the volume of TEOS used in the synthesis) were subsequently investigated in the K band (18-26.5 GHz) and K-a band (26.5-40 GHz). Results show that the EMW absorption properties of the nanocomposites can be precisely tuned by controlling the thickness of the SiO2. Compared with ZnFe2O4@PPy, the ZnFe2O4@SiO2)(x)@PPy composites exhibited enhanced reflection losses and broader effective absorption bandwidth (EAB, reflection loss less than -10 dB). The ZnFe2O4@(SiO2)(1.0)@PPy nanocomposite offered the best EMW absorption performance, with a minimum reflection loss (RLmin) of -29.72 dB at 24.96 GHz (EAB of 7.0 GHz, 19.5-26.5 GHz) at 1.5 mm thickness and 36.75 dB at 38.38 GHz (EAB of 9.56 GHz, 30.44-40 GHz) at 1.0 mm thickness. The main microwave absorption mechanisms used by the ZnFe2O4@SiO2@PPy composites were magnetic losses (ZnFe2O4 nanoparticles), dielectric losses (ZnFe2O4, SiO2 and PPy) and interfacial relaxation losses (at ZnFe2O4-SiO2-PPy interfaces). Results guide the development of improved microwave absorbers in the K and K-a bands.

Based on the abundant and low-cost zinc-based acrylate resins, C/ZnO composites were fabricated via one-step carbonization at 700 degrees C in a N-2 atmosphere for 2 h. Zinc-based acrylate resins, which were synthesized by free-radical polymerization of butyl acrylate (BA), acrylic acid (AA) and vinyl acetate (VAc) and dehydration condensation of Zn(OH)(2), provided a common source for carbon and ZnO. These materials demonstrate enhanced electromagnetic wave absorption (EMWA) behavior with tunable microwave absorption bands at 2-18 GHz, which is related to the molar ratio (mol%) of Zn(OH)(2) to acrylate monomers in zinc-based acrylate resins. Remarkably, the 0.11 mol% C/ZnO composite exhibits outstanding absorption properties: the minimum reflection loss (RLmin) at 16.7 wt% loading of 34.66 dB is observed at 3.0 mm and 10.32 GHz, and an RLmin of -24.83 dB is observed at a small thickness of 1.5 mm with an effective absorption bandwidth (EAB) of 3.61 GHz. Moreover, the EAB (RL <= -10 dB) from the C band to Ku band is achieved by simply adjusting the thickness of the absorbers, which are superior to the other hybrids of organic carbon and ZnO. These results provide a new strategy for the preparation of carbon-based composites containing metal oxides and their application in high-performance microwave absorption.

Enormous research effort is currently being directed toward the development of low-cost electromagnetic wave absorbing (EMWA) materials that operate at microwave frequencies. Herein, we report the successful fabrication of novel three-dimensional TiO2-Fe3O4@Polypyrrole (T-F-P) composites with excellent microwave absorption properties between 2 and 40 GHz. By varying the thickness of the T-F-P absorber from 1.5 to 2.5 mm, an effective absorption bandwidth (EAB, reflection loss (RL) ≤ − 10 dB) of 28.12 GHz (11.88–40 GHz) was realized. At a thickness of 2.5 mm, the minimum reflection loss (RLmin) was − 37.49 dB (13.84 GHz) in the frequency range 2–18 GHz; − 41.30 dB (25.30 GHz) in the frequency range 18–26.5 GHz and − 42.65 dB (34.1 GHz) in the frequency range 26.5–40 GHz. Multiple reflections, interfacial polarization, conductive losses and magnetic losses all contribute to the excellent EMWA properties of the T-F-P composites.

The combination of metallic and carbon components offers tremendous advantages in the development of advanced composites for electromagnetic wave absorption (EMWA) applications. Herein, hollow (NiCo)(x)/(MnO)(y)/C microspheres (MSs) possessing hierarchically porous structures were successfully synthesized via a facile microwave-assisted method followed by a carbonization step. The EMWA performance of the (NiCo)(x)/(MnO)(y)/C MSs in the K-band region could be tuned and optimized by adjusting the NiCo/Mn molar ratio, with a NiCo/Mn ratio of 1:1 affording the best EMWA performance. The NiCo/MnO/C MSs exhibited a minimum reflection loss value of -32.3 dB (at 21.4 GHz) and an effective absorption bandwidth (EAB, RL <= -10 dB) of 8.33 GHz (from 18.17-26.5 GHz) at a thickness of only 1.0 mm in the K band. The excellent EMWA performance of the (NiCo)(x)/(MnO)(y)/C MSs composites can be attributed to a strong EMW attenuation capacity and good impedance matching, which arise through the synergistic action of the components (NiCo alloy, MnO and C) and hierarchical pores.

The nickel-N@carbon composite (Ni-N@C) with size of 4.0-5.0 mm was prepared, in which the Ni-N bonds were formed in the carbonization process. The Ni-N bonds between metallic Ni and N provide different surface electron distribution and contribute to the high-frequency electromagnetic wave absorption (EMWA) of Ni-N@C composite. The Ni-N bonds in the Ni-N@C composite have been confirmed by X-ray diffraction, Raman spectrum, X-ray photoelectron spectroscopy. Compared with Ni@C composite and reported Ni-based materials, the Ni-N@C composite shows the stronger reflection loss (RL = -32.31 dB) and wider effective absorption band (EAB, RL < -10 dB) at the high frequency (12.79 -18 GHz), which can be ascribed to Ni-N bonds. Besides, the EMWA characteristics of Ni-N@C composites are flexibly adjusted by tuning the pyrrole monomers to obtain optimized impedance matching, which is beneficial for broadening the EAB. All C, X and Ku bands (3.90-18.0 GHz) can be covered through changing the Ni content and layer thickness for Ni-N@C composites. (C) 2019 Elsevier B.V. All rights reserved.

Flower-like assemblies of nickel/carbon (Ni-x/C) microspheres (MSs) with Ni-O-C bond and "double-layer" carbon shells have been successfully prepared through a solvothermal route and a sintering process combined with carbon reduction. The electromagnetic wave absorption (EMWA) properties of MSs can be tuned by controlling the Ni content. The Ni-2.0/C composite exhibits superior EMWA properties: the reflection loss (RL) can be up to - 47.5 dB (34.6 GHz) and the efficient absorption bandwidth (RL < -10 dB) is 37.04 GHz (2.96-40 GHz) via adjusting the thickness from 1 to 5.5 mm. The outstanding EMWA properties can be benefitted from the synergistic effects of carbon with dielectric loss, Ni particles with magnetic loss and hierarchically porous structures.

Materials incorporating magnetic metal particles into a carbon matrix have drawn tremendous interest as electromagnetic wave (EMW) absorbers. Herein, the tremella-like assemblies of hierarchically porous nickel-cobalt/carbon (NiCo/C) were successfully prepared by a microwave-assisted method, followed by a sintering process. The sintering temperature is a vital factor for formation of carbon matrix and the metal particles. The higher temperature is helpful for the graphitization of carbon matrix and growth of NiCo alloy. The NiCo/C composite obtained at 700 degrees C displayed the best EMW absorption properties: the minimum reflection loss (RLmin) value reaches -41.6 dB at 8.4 GHz and the widest effective absorption bandwidth is up to 34.33 GHz (3.78-38.11 GHz) by adjusting the layer thickness (1.0-5.5 mm) (EAB, RL <= -10 dB). The excellent EMW absorption properties are attributed to the synergistic effect of the multiple components (magnetic NiCo and dielectric C) and hierarchically porous structure.

Fe3O4/C micro-flowers with a three-dimensional (3D) porous morphology (3D-Fe3O4/C MFs, similar to 5-7 mu m) were prepared by a simple hydrothermal method, with a subsequent calcination step. The 3D-Fe3O4/C MFs consisted of radially assembled 2D Fe3O4/C nanoflakes (NFs, thickness similar to 40 nm). When the electromagnetic wave absorption (EMWA) performances of paraffin composites with 10, 30, and 40 wt% 3D-Fe3O4/C MF loading and Fe3O4-based materials were compared, the paraffin composite with 20 wt% loading exhibited a strong reflection loss (RL, -37.6 dB) and a broad effective absorption bandwidth (EAB, RL < -10 dB, 5.2-9.36 GHz). The excellent EMWA performance was due to the synergistic effects of multiple components (carbon and Fe3O4), combined with a hierarchically porous and 3D structure. The lightweight and broad EAB and the strong and tunable EMWA performances of 3D-Fe3O4/C MFs make them attractive for the development of high-performance EMW absorbers. (C) 2019 Elsevier B.V. All rights reserved.

A sustainable method is proposed to prepare carbon-sponge/CoNi composites from sponge to sponge/CoNi followed by sintering. All CoNi nanoparticles (NPs) are uniformly anchored on the carbon-sponge, and the as-prepared products are stable under ultrasound treatment, implying the CoNi NPs are grown in situ on the carbon-sponge with a strong interaction. The electromagnetic microwave absorption (EMWA) performances of the carbon-sponge/CoNi are properly adjusted by controlling the calcination temperature. The carbon-sponge/CoNi obtained at 800 degrees C (C800) displayed excellent EMWA performances; the minimum reflection loss (RLmin) was -39.7 dB, and the effective absorption bandwidth (EAB, RL <= -10 dB) was more than 5.12 GHz (>= 12.88 GHz) with a thin layer thickness of 1.5 mm. The superior EMWA performances of carbon-sponge/CoNi composites benefit from the synergistic effect of magnetic CoNi and the dielectric carbon-sponge. This work can pave a new road for designing lightweight and high-performance carbon composites from sustainable materials.

To obtain outstanding electromagnetic microwave absorption (EMWA) properties, the rambutan-like dielectric–magnetic C@NiCo2O4 material was successfully prepared by a simple hydrothermal method, followed by a carbonization process. Benefiting from the unique rambutan-like structure, the dielectric–magnetic C@NiCo2O4 composites showed excellent microwave attenuation ability: minimum reflection loss (RLmin) value of − 39.0 dB at 17.4 GHz and wide effective absorption bandwidth (EAB, reflection loss exceeding − 10 dB) of 4.16 GHz (> 13.84 GHz) with a matching thickness of only 1.5 mm, which were much better than those of pure C and NiCo2O4. The superior properties might be due to multiple synergistic effects: magnetic loss (NiCo2O4), dielectric loss (C, NiCo2O4), the multi-reflections, scattering and interface relaxation resulting from mesoporous rambutan-like structures, and the dipole polarization to get good electromagnetic matching and high attenuation efficiency.

In this work, polypyrrole-coated ZnFe2O4 (ZnFe2O4@PPy) nanocomposites were successfully synthesized via a simple in-situ polymerization process, then evaluated as electromagnetic wave (EMW) absorbers over the 2-40 GHz frequency range. The ZnFe2O4@PPy nanocomposites exhibited excellent EMW absorption properties, including very low reflection losses ( - 42.31 dB at 30.24 GHz and a thickness of 2.5 mm) and a broad absorption bandwidth of 28.20 GHz (from 9.66 to 37.86 GHz). The EMW absorption properties of the ZnFe2O4@PPy nanocomposites could be adjusted by changing the PPy shell thickness and also the thickness of the absorber (1-2.5 mm). The excellent microwave absorption performance of the ZnFe2O4@PPy nanocomposites is attributable to the synergistic effects of magnetic losses (ZnFe2O4 nanoparticles), dielectric losses (ZnFe2O4 and PPy) and interfacial relaxation losses at ZnFe2O4-PPy interfaces.