Microwave absorption performance of hierarchical structured composites of greigite and reduced graphene oxide

The greigite (Fe₃S₄)/reduced graphene oxide (RGO) hierarchical structural composites (F–R) with the Fe₃S₄ nanoparticles attached to the RGO layers were successfully prepared via a simple one-pot solvothermal method. The microwave absorption properties were evaluated by calculating the reflection loss (RL) values. The results show that the RGO content and the filler loading of composites in paraffin mixture are very critical to the microwave absorption properties because they can improve the electromagnetic parameters. Sample F–R-3 presents the best microwave absorption capacity, in which an optimum RL value of ??62.3 dB and an effective absorption bandwidth (EAB, RL value?<???10 dB) of 3.04 GHz (14.96–18 GHz) can be obtained when the matching thickness is only 1.29 mm. Meanwhile, the widest EAB reaches 4.08 GHz (13.92–18 GHz) at the matching thickness of only 1.37 mm. Impressively, when the matching thickness is in the range of 1.2–5.5 mm, all RL peaks are below ??20 dB, and the EAB can be 14.98 GHz (3.02–18 GHz), covering the whole C, X and Ku bands. The distinguished absorption property is mainly ascribed to the combined effect of strong loss ability and good impedance matching. Apparently, the F–R composite with strong absorption ability, thin thickness and wide EAB is suitable for the efficient microwave absorber.

Graphical abstract

     

Double layer microwave absorber based on Cu dispersed SiC composites

The present work has been focused on designing an efficient and cost-effective double layer microwave absorber in 8.2–12.4?GHz frequency range. For the same, Cu particles were dispersed in SiC to achieve enhanced microwave absorption by combining the excellent dielectric characteristics of SiC with highly conductive Cu. Cu dispersed SiC composites were prepared by dispersing various weight fractions of Cu particles in the SiC matrix using planetary ball mill. The Cu dispersion in SiC yielded excellent relative complex permittivity values translating into a decrease in the reflection loss (RL) values of dispersed composites as compared to the pristine counterpart. The minimum RL of ?17.18?dB has been observed for 2?wt% Cu dispersed SiC composite at 11.81?GHz with a thickness of 1.3?mm and bandwidth corresponding to ?10?dB is 1.77?GHz. Genetic algorithm approach has been implemented to design double layer microwave absorber to further enhance the microwave absorption of the prepared composites for realizing a cost-effective solution. The optimum double layer results show the RL of ?32.16?dB at 11.05?GHz with 1.67?mm total thickness and bandwidth corresponding to ?10?dB is 2.35?GHz.     

Enhanced microwave absorption performance of graphene/doped Li ferrite nanocomposites

In the present study, Microwave absorbing Li-Sr, Li-Co ferrite nanoparticles and RGO/Li-Sr, RGO/Li-Co ferrite nanocomposites containing Li ferrite and reduced graphene oxide (RGO) were synthesized to further improve the microwave absorption performance of Li ferrite nanoparticles (LiFe₅O₈). The Li-Sr and Li-Co nanoparticles were synthesized by thermal treatment method, the RGO/Li-Sr and RGO/Li-Co nanocomposites were obtained by a polymerization method and were characterized by different techniques. The electromagnetic wave absorption properties of the samples were evaluated by vector network analyzer (VNA) in the frequency range of 2–18 GHz. The magnetic and dielectric loss, impedance matching, and electromagnetic wave absorption of the samples are significantly improved through the addition of RGO. Experimental results revealed that the RGO/Li-Co nanocomposite considerably increased microwave absorption. The minimum reflection loss (RL) of RGO/Li-Co also was found to reach −46.80 dB at the thickness of 3 mm and the effective absorption bandwidth (≤-10 dB) amounted to 6.80 GHz (from 10.52 to 17.32 GHz), which was much higher in comparison with pure Li and Li-Co ferrites nanoparticles. Due to the synergistic effect between magnetic loss and dielectric loss and the good impedance matching, the RGO/Li-Co nanocomposite may be regarded as a new candidate for microwave absorbing materials characterized with a broad effective absorption bandwidth at thin thicknesses.     

Enhanced microwave absorption properties of polymer-derived SiC/SiCN composite ceramics modified by TiC

Porous SiCN(Ti) composite ceramics with good microwave absorbing performance were fabricated by pyrolysis of solid polysilazane modified by tetrabutyl titanate. The introduction of Ti not only acted as active filler to react with free carbon in the matrix to form TiC, but also played the role as catalyst to promote the formation of SiC nanowires. Finally, SiCN(Ti) composite ceramics formed a microstructure containing multi-nanophases and multi-nano heterogeneous interfaces when annealing temperature reached 1500 °C. The complex microstructure annealed at 1500 °C made composite ceramics have good matching impedance, as well as greatly increase the interfacial polarization loss and dipole polarization loss. As a result, the TiC/SiC/SiCN composite ceramics showed the excellent performance of electromagnetic wave absorption in X band. The minimum reflection loss (RL) of samples was ??17.1 dB at the thickness of 1.9 mm, and the maximum effective absorption bandwidth (EAB) of composite ceramics was 3.2 GHz when the thickness of sample was 2.1 mm, which exhibited a promising prospect as a structural and microwave absorbing integration material.

     

Honey-comb carbon nanostructure derived from peach gum to yield high microwave absorption

Lightweight and highly efficient electromagnetic absorptions are crucial for microwave absorption materials due to the fast development of information technology. Bio-derived carbon materials are ideal resistance loss-type microwave absorbers with lightweight. A honey-comb carbon structure has been obtained from peach gum by facile hydrothermal and pyrolysis processes. The skeleton of the as-derived honey-comb carbon structure is composed of carbon nanoparticles with diameters around 40 nm. The honey-comb structures were further carbonized at 850 °C (NPG-850) to have a large specific surface area of 1401.7 m² g^?1 with an average pore diameter of 3.2 nm. A minimum reflection loss (RL) of???59.4 dB was obtained by NPG-850 at a thickness of 2.0 mm with an effective absorption bandwidth (EAB, RL?<????10 dB) of 4.1 GHz (14.7–10.6 GHz). The RL value of the peach gum-derived honey-comb carbon nanostructures is much higher than the reported carbon nanostructures even with thinner thickness and less mass loading, which might due to the multi-reflection effect of the honey-comb structures and the skeleton composition.

     

Fabrication of polypyrrole/nano-exfoliated graphite composites by in situ intercalation polymerization and their microwave absorption properties

Polypyrrole/nano-exfoliated graphite composites were synthesized using an in situ intercalation polymerization of pyrrole into the layers of expanded graphites. The morphologies and nanostructures of obtained composites were characterized by field emission scanning electron microscopy, transmission electron microscopy and X-ray diffraction analysis. Results showed that the in situ intercalation polymerization of pyrrole cation into the layers of expanded graphites could separate graphite into nano-exfoliated graphite sheets. The interactions between polypyrrole and the graphite sheet were also confirmed by Fourier transformed infrared spectroscopy. The fabricated nanocomposites polypyrrole/nano-exfoliated graphite-1.5 showed obvious improvement in microwave absorption compared with those of the polypyrrole or the expanded graphite itself. With a thickness of 2.7 mm, the optimal absorption peak reached −48 dB at 13.4 GHz and the bandwidth corresponding to the reflection loss at −10 dB was 3.4 GHz (from 13.2 to 16.6 GHz). The minimum RL reached −34 dB with a thickness of 2.5 mm for Polypyrrole/nano-exfoliated graphite-1.5. Moreover, it could be observed that the absorption bandwidth with RL below −10 dB was obtained in the frequency range of 5–18 GHz for polypyrrole/nano-exfoliated graphite-1.5 with a thickness of 2–5 mm. This would open a path toward the fabrication of microwave absorption materials of light-weight.     

Structural and microwave properties of Ag-doped strontium hexaferrite

A series of silver-doped strontium hexaferrite with the chemical formula SrAg_ZFe_12-zO₁₉ (0.0?≤?z?≤?1.0) were synthesized by the Co-precipitation method. The crystal structure, morphology, and properties of microwave absorption with Ag concentration were studied. The structural analysis by XRD revealed that the samples are crystallized with an M-type hexagonal structure. The values of lattice parameters, the volume of the unit cell, and X-ray density are increasing with the increase of Ag doping. The least values of Rietveld refinements have confirmed a good correlation between experimental and calculated data. Hexagonal plate-like morphology was observed in SEM images and the grain size decreases with Ag doping. Microwave properties have been measured by a vector network analyzer. Real and imaginary parts of electrical permittivity dependence with the frequencies in X-band (8–12 GHz) have been studied. The Reflection loss (RL) was investigated for all samples in X-band frequencies. Maximum RL of ? 21.95 dB at 10.0 GHz was observed for the composition of silver, z?=?0.4. Improved RL when compared with the pure sample indicating enhanced impedance matching and attenuation constant hence the material can show maximum energy loss for the incident microwaves. The results so obtained are explained based on composition and microwave phenomena. The present studies have confirmed the nature of microwave absorption for Ag-doped strontium hexaferrite.

     

Microwave absorbing behavior of metal dispersed TiO2 nanocomposites

Metal dispersed TiO₂ nanocomposites were prepared by milling process. The microwave absorbing characteristics of the prepared nanocomposites with epoxy were studied in the 8.2–12.4 GHz frequency range for the microwave absorption application. The measured relative complex permittivity of metal dispersed nanocomposite-epoxy indicates higher values in comparison to the pure TiO₂-epoxy nanocomposite. The Reflection loss (RL) values were calculated for thickness from 0.1 to 2.2 mm with an interval of 0.1 mm and the maximum value of RL found for TiO₂-epoxy nanocomposite was −4.96 dB at 10.21 GHz frequency for 2.0 mm thickness. Whereas, RL value is improved to a maximum value of −13.67 dB at 10.13 GHz with Al dispersion (1.8 mm thickness) and −7.24 dB at 10.38 GHz with Ni dispersion (1.3 mm thickness). This study suggests the effectiveness metal particles dispersion for the development of thin microwave absorbers as well as increasing the level of RL.     

Complex Permittivity and Microwave Absorbing Property of Si3N4-SiC Composite Ceramic

Si3N4-SiC composite ceramics were fabricated by chemical vapor infiltration using porous Si3N4 ceramic as preform. The average grain size of SiC was 30 nm. Relationship between SiC content and relative complex permittivity of Si3N4-SiC within the frequency range of 8.2-12.4 GHz (X-band) was investigated. The average real part of relative complex permittivity ε of Si3N4-SiC increased from 3.7 to 14.9 and the relative imaginary part ε increased from 0.017 to 13.4 when the content of SiC increased from 0 to 10 vol.%. The Si3N4-SiC ceramic with 3 vol.% SiC achieved a reflection loss below 10 dB (90% absorption) at 8.6-11.4 GHz, and the minimum value was 27.1 dB at 9.8 GHz when the sample thickness was 2.5 mm. The excellent microwave absorbing abilities of Si3N4-SiC ceramic were attributed to the interfacial polarization at interface between Si3N4 and SiC and at grain boundary between SiC nanocrystals.     

Enhanced microwave absorption properties of flake-shaped FePCB metallic glass/graphene composites

A novel kind of composite absorber, i.e. FePCB/graphene composite, with excellent microwave absorption properties was successfully fabricated by a simple and scalable ball milling method. After being milled, the FePCB particles displayed flaky morphology with large aspect ratio. The complex permittivity and permeability of the flaky FePCB distinctly increased compared with those before milling. Furthermore, with the introduction of graphene, the flaky FePCB/graphene composite exhibited excellent microwave absorption performance with strong absorption and wide absorption band. In particular, for FePCB/graphene composite with an absorber thickness of 2 mm, the reflection loss (RL) reached a minimum of −45.3 dB at 12.6 GHz and the effective absorption bandwidth (RL < −10 dB) covered 5.4 GHz. The enhanced microwave absorption performance of the FePCB/graphene composite was attributed to the high magnetic loss and improved impedance matching which were closely related to the flake-shaped FePCB particles and the introduction of graphene sheets.     

Double-layer microwave absorber based on nanocrystalline Zn0.5Ni0.5Fe2O4/α-Fe microfibers

The microwave absorption properties of the nanocrystalline NiZn ferrite (Zn_0.5Ni_0.5Fe₂O₄) and iron (α-Fe) microfibers with single-layer and double-layer structures were investigated in the frequency range of 2–18 GHz. The double-layer absorbers have much better microwave absorption properties than the single-layer absorbers, and the microwave absorption properties of the double-layer structure are influenced by the coupling interactions between the absorbing layer and matching layer. With the absorbing layer thickness 0.7 mm of α-Fe microfibers–wax composite and the matching layer thickness 1.5 mm of Zn_0.5Ni_0.5Fe₂O₄ microfibers–wax composite, the minimum reflection loss (RL) reaches about −71 dB at 16.2 GHz and the absorption band width is about 9.2 GHz ranging from 8.8 to 18 GHz with the RL value exceeding −10 dB. While, when the absorbing layer is the Zn_0.5Ni_0.5Fe₂O₄ microfibers–wax composite with thickness 1.8 mm and the matching layer is the α-Fe microfibers–wax composite with thickness 0.2 mm, the RL value achieves the minimum about −73 dB at 13.8 GHz and the absorption band width is about 10.2 GHz ranging from 7.8 to 18 GHz with the RL value exceeding −10 dB, which covers the whole X-band (8.2–12.4 GHz) and K_u-band (12.4–18 GHz).     

Low Temperature Solution Synthesis and Microwave Absorption Properties of Multiwalled Carbon Nanotubes/Fe3O4 Composites

Multiwalled carbon nanotubes (MWCNTs)/Fe₃O₄ nanocomposites were synthesized via a simple low temperature solution method. The phase structures and morphologies of the composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the Fe₃O₄ spheres of about 150 nm were linked with MWCNTs. The microwave absorption properties of the MWCNTs/Fe₃O₄ nanocomposites were measured by vector network analysis (VNA). A wide region of microwave absorption was achieved due to dual magnetic and dielectric losses. When the matching thickness is 2 mm, the reflection loss (RL) of the sample exceeding ?10 dB was obtained at the frequency range of 9.9–12.4 GHz, with an optimal RL of ?29.8 dB at 11.04 GHz. A possible mechanism of the improved microwave absorption properties of the composites was discussed.     

Influence of SiO2 fillers on microwave absorption properties of carbonyl iron/carbon black double-layer coatings

Epoxy resin (ER) based double-layer composite coatings were prepared with the thickness of 1.2 mm, employing carbonyl iron (CI) and carbon black (CB) as absorbents in the matching layer and absorption layer respectively. Especially, SiO₂ was introduced into the matching layer as wave-transmission material to improve the matching impendence. The complex permittivity, complex permeability and absorption properties were investigated in 2–18 GHz. With increasing SiO₂ content in the matching layer, the reflection loss (RL) was enhanced in the range 2–18 GHz. When the coating with the optimized SiO₂ and CI weight concentration (SiO₂:CI:ER) of 2:5:1, the optimal RL got to −17.3 dB and the effective absorption band (RL better than −4 dB) reached 5.7 GHz. In comparison, the minimum RL value was only −5.9 dB and the bandwidth (RL better than −4 dB) was just 4.1 GHz for the SiO₂-free composite coating.     

Constructing γ-MnO2 hollow spheres with tunable microwave absorption properties

Hollow γ-MnO₂ sphere was obtained by annealing the precursor at 400 °C with different holding time. The influences of different holding time on the morphology and crystalline structure of final products have been discussed in detail, and the microwave absorption properties of the as-products were also investigated. The results exhibited that finer crystalline feature of the γ-MnO₂ and larger pore size in the hollow γ-MnO₂ sphere could be obtained with the extended holding time. The MnO₂/paraffin composites (50 wt% loading) present extraordinary microwave absorption performance, and the minimum reflection loss (RL) values is −51.3 dB at 4.9 GHz with the thickness of 3.5 mm. The excellent electromagnetic absorption properties can be ascribed to the hollow structure, perfect impedance matching behavior and the multiple interface polarization effect.     

Facile synthesis and novel microwave electromagnetic properties of flower-like Ni structures by a solvothermal method

Flower-like Ni structures composed of leaf-like flakes were synthesized through a facile solvothermal approach independent of surfactants or magnetic force. The evolution of the morphology was closely related to the variation of NaOH and volume ratios of ethylene glycol to water. The microwave absorbing properties of the flower-like Ni wax-composite were evaluated based on the complex permittivity (ε_r = ε′ ? jε″) and permeability (µ_r = µ′ ? jµ″). The Ni wax-composite exhibited excellent microwave absorption performances with a minimum reflection loss of ?46.1 dB at 13.3 GHz, corresponding to a matching thickness of 2.0 mm. In particular, the absorption bandwidth of RL below ?10 dB was 3.6 GHz (11.7–15.3 GHz). The attenuation of microwave could be attributed to the dielectric loss and unique flower-like structure.     

Regulation of Impedance Matching and Dielectric Loss Properties of N-Doped Carbon Hollow Nanospheres Modified With Atomically Dispersed Cobalt Sites for Microwave Energy Attenuation

The rational design of lightweight, broad-band, and high-performance microwave absorbers is urgently required for addressing electromagnetic pollution issue. Metal single atoms (M–SAs) absorbers receive considerable interest in the field of microwave absorption due to the unique electronic structures of M–SAs. However, the simultaneous engineering of the morphology and electronic structure of M–SAs based absorbers remains challenging. Herein, a template-assisted method is utilized to fabricate isolated Co–SAs on N-doped hollow carbon spheres (NHCS@Co–SAs) for high-performance microwave absorption. The combination of atomically dispersed Co sites and hollow supports endows NHCS@Co–SAs with excellent microwave absorption properties. Typically, at an ultralow filler content of 8 wt%, the minimum reflection loss and effective absorption bandwidth of the NHCS@Co–SAs are up to −44.96 dB and 5.25 GHz, respectively, while the absorbing thickness is only 2 mm. Theoretical calculations and experimental results indicate that the impedance matching characteristic and dielectric loss of the NHCSs can be tuned via the introduction of M–SAs, which are responsible for the excellent microwave absorption properties of NHCS@Co–SAs. This work provides an atomic-level insight into the relationship between the electronic states of absorbers and their microwave absorption properties for developing advanced microwave absorbers.     

Surfactant-free synthesis and electromagnetic properties of Co–Ni–B composite particles

The Co–Ni–B composite particles with different mol ratio of Co to Ni were composited of spheres, spheres in-pair, hierarchical assemblies of dentrites, which were surfactant-free synthesized by chemical reduction method in aqueous solution. The complex permeability of the Co–Ni–B composite particles indicated reverse resonant peak at the frequency range of 8–16 GHz, where the complex permittivity showed the positive resonant peak and the μ_r″ of particles showed negative values, caused by the transformation between electric and magnetic energy. The imaginary parts of relative permeability (μ_r″) of Co–Ni–B composite particles indicated one broad resonant peak over the 2–8 GHz range for the high effective anisotropy. A slight decrease in complex permittivity resulted in an excellent impedance matching property. The Co–Ni–B composite alloy particles with mol ratio of 7:3 exhibited reflection loss less than ?20 dB in frequency range of 4.0–14.5 GHz for the absorber thickness of 1.1–3.2 mm, and an optimal RL of ?32.4 dB was obtained at 12.8 GHz with thickness of 1.2 mm. The broadest bandwidth of reflection loss less than ?10 dB from 13.0 to 17.0 GHz, covering almost the whole Ku-band, was obtained for a thickness of 1.1 mm layer.     

Regulating microwave absorption and energy bandgap using cauliflower-like polyaniline coated on La0.8Sr0.2FeO3 nanoparticles

In this research, La_0.8Sr_0.2FeO₃/cauliflower-like polyaniline (PANi) nanocomposite was architected based on a novel complementary method using dodecylbenzenesulfonic acid as a doping agent. The prepared nanocomposite was characterized using Fourier transform infrared, X-ray powder diffraction, field emission scanning electron microscopy, vibrating sample magnetometer diffuse reflection spectroscopy, and vector network analyzer analyses. All of the used analyses attested that the pure structure of materials has been synthesized. Polarizability, energy bandgap, magnetic property, and microwave absorbing features were tailored by loading the various mass fraction of PANi. Exclusive interactions between the nanoparticles with alkaline property and aniline monomers along the experimental route led to the preparation of nanocomposite with unique morphology. Inserting PANi augmented softness and isotropic magnetic property of the prepared nanocomposite, desirable for microwave absorption. Moreover, polyacrylonitrile (PAN) was applied as a novel microwave absorbing matrix. The maximum reflection loss (RL) of La_0.8Sr_0.2FeO₃/PANi_10%/PAN was ??69.24 dB at 12.62 GHz with an efficient bandwidth of 6.47 GHz (RL?<???10 dB) meanwhile the efficient bandwidth was enhanced to 6.97 GHz (RL?<???10 dB) for La_0.8Sr_0.2FeO₃/PANi_30%/PAN nanocomposite.

     

Ferromagnetic and microwave absorption properties of copper oxide/cobalt/carbon fiber multilayer film composites

The copper oxide/cobalt/carbon fiber multilayer film composites were synthesized by thermal oxidation route. In order to investigate the intrinsic reasons for microwave absorption properties of absorbers, the complex permittivity, complex permeability and the microwave absorption properties of composites were studied in the 1-18 GHz range. The strongest reflectivity loss (RL) of microwave absorber was further enhanced to − 42.7 dB (microwave absorption rate > 99.9%) at 10.8 GHz for a layer of 2.0 mm thickness, and the strong absorption (RL < − 10 dB) was obtained between 8.72 and 18 GHz for the thickness of 1.3-2.2 mm. The results indicated that the dielectric loss and magnetic loss led to the excellent microwave absorption property of CuO/Co/CF composites. It is believed to be ideal for making a lightweight, strong absorption and wide-frequency microwave absorbing material.     

Synthesis and characterization of MoS2/Fe@Fe3O4 nanocomposites exhibiting enhanced microwave absorption performance at normal and oblique incidences

Herein, we attempted to prepare MoS₂/Fe@Fe₃O₄ nanocomposites capable of strongly absorbing broadband incident electromagnetic (EM) radiation and probed the effects of their composition on complex permittivity and permeability at 2–18 GHz. Calculations of normal-incidence reflection losses (RLs) based on EM parameters revealed that the Fe@Fe₃O₄ to MoS₂ mass ratio strongly influenced the absorption peak intensity and bandwidth. Specifically, an RL peak of −31.8 dB@15.3 GHz and a bandwidth (RL < − 10 dB) of 4.8 GHz (13.2–18 GHz) were achieved at a thickness of 1.52 mm and a Fe@Fe₃O₄ to MoS₂ mass ratio of 60:40. Further, RL and bandwidth were investigated for oblique incidence, in which case two kinds of EM waves (TE – electric field perpendicular to plane of incidence; TM – electric field in the plane of incidence) were considered. The absorption peaks of TE and TM waves did not exceed −20 dB when the incidence angle increased to 30°, and the bandwidth (RL < − 10 dB) reached 4.2 GHz (TE wave) and 4.0 GHz (TM wave) when this angle was further increased to 40.0° and 50.4°, respectively. Finally, the mechanism of microwave absorption was discussed in detail.