ZnFe2O4@SiO2@Polypyrrole nanocomposites with efficient electromagnetic wave absorption properties in the K and Ka band regions

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 ~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 re?ection 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 re?ection loss (RL_min) 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.     

Design of hierarchical core-shell ZnFe2O4@MnO2@RGO composite with heterogeneous interfaces for enhanced microwave absorption

In order to overcome the problems caused by electromagnetic pollution, the design and development of high-performance microwave absorbers is urgently required. In this work, a hierarchical ZnFe₂O₄@MnO₂@RGO composite was successfully fabricated via a facile and rapid hydrothermal method. Its unique core-shell structure and synergistic effect between multiple components are beneficial for electromagnetic wave absorption. The morphology, elemental composition, microstructure and microwave absorption characteristics were systematically studied. With a filler loading of 20 wt%, the composite presents a minimum reflection loss (R_Lmin) of ?46.7 dB and an effective absorption bandwidth (EAB) as wide as 5.2 GHz at a thickness of 2.5 mm. The superior absorption ability profits from a special microstructure, good impedance matching, multiple attenuation features, interfacial polarization, and the synergistic effect of dielectric and magnetic loss. Consequently, this work lays a foundation for the design of high-performance electromagnetic wave absorbers with multicomponent heterogeneous structures.     

ZnFe2O4@Polypyrrole nanocomposites as an efficient broadband electromagnetic wave absorber at 2–40 GHz

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.     

Facile synthesis of Mn3O4 hollow polyhedron wrapped by multiwalled carbon nanotubes as a high-efficiency microwave absorber

Investigating lightweight and high-efficiency electromagnetic wave (EM) absorbers is evolving as a desirable approach to solve the electromagnetic pollution. In this study, Mn₃O₄ hollow polyhedron wrapped by multiwalled carbon nanotubes (Mn₃O₄/MWCNTs) was successfully prepared by one-step hydrothermal treatment. Interestingly, the Mn₃O₄ polyhedron as a unique hollow structure can serve as a microwave receiver and the incident EM waves hardly escape in the intricate networks, which could be repetitiously attenuated and consumed. The Mn₃O₄/MWCNTs composite with a filler loading of 20 wt% exhibits most outstanding EM absorption performance over the whole frequency of 2–18 GHz. The optimal reflection loss (R_L) achieves −53.8 dB at 11 GHz, and the effective absorption bandwidth (R_L exceeding −10 dB) reaches 4.1 GHz (9.1–13.2 GHz) with a thickness of 2.5 mm. The effective absorption bandwidth (R_L < −10 dB) up to 13.7 GHz (85% absorption over 2–18 GHz) was achieved by adjusting the thickness from 1.5 to 4 mm. The remarkable EM absorption performances benefit from the synergistic effects of suitable impedance matching, dielectric loss, interfacial polarizations and relaxation polarizations. These results indicate that Mn₃O₄/MWCNTs composite with lightweight and high-efficiency microwave absorption properties could serve as a prospective microwave absorber in practical applications.     

Enhanced dielectric and microwave absorption properties of Cr/Al2O3 coatings deposited by low‐power plasma spraying

Low‐power plasma‐sprayed Cr/Al₂O₃ coatings have been developed for their potential application as broad bandwidth, thin thickness, lightweight, and strong microwave‐absorbing materials. The dielectric and microwave absorption properties of the as‐sprayed coatings were studied in the X‐band (from 8.2 to 12.4 GHz). High complex permittivity of the coatings was obtained because of a large number of internal boundaries and the conductive networks. Meanwhile, a significant enhancement of microwave absorption properties of the coating was achieved due to the enhanced interfacial polarization and conductance loss. The reflection loss (RL) <?10 dB of the Al₂O₃–15Cr coating was obtained from 9.8 to 11.4 GHz by choosing an appropriate coating thickness, and an optimal minimum reflection loss (RL_min) of ?45.35 dB was achieved at 10.3 GHz with a thin thickness of 1.32 mm.     

Construction of core-shell BaFe12O19@MnO2 composite for effectively enhancing microwave absorption performance

BaFe₁₂O₁₉, a traditional ferrite, has always been extensively investigated as a microwave absorption material because of the application value. Herein, the core-shell BaFe₁₂O₁₉@MnO₂ composite was designed and constructed successfully by a facile hydrothermal method. By introducing nanostructured MnO₂, a typical dielectric loss medium, the electromagnetic wave absorption performance is effectively enhanced. The core-shell structure contributes to high interface polarization, thereby promoting the attenuation of electromagnetic waves. In addition, the optimal temperature of the hydrothermal reaction was explored through the characterization of the morphology and the analysis of microwave absorption performance. The result exhibits that the maximum reflection loss of the prepared BaFe₁₂O₁₉@MnO₂-170 reaches -54.39 dB and the effective absorbing bandwidth reaches 4.64 GHz. The simple preparation method and attractive performance make BaFe₁₂O₁₉@MnO₂ a promising candidate as microwave absorbers.     

Tunable magnetic coupling and dipole polarization of core-shell Magnéli Ti4O7 ceramic/magnetic metal possessing broadband microwave absorption properties

There are few reports on the application of Magnéli Ti₄O₇ in the field of electromagnetic wave absorption. Herein, we designed and prepared a Ti₄O₇/magnetic metal composite via an electrostatic assembly with in-situ reduction reaction. This system utilized distinct magnetic coupling deriving from the subtle designed structures and magnetic-dielectric synergy. The core-shell magnetic metallic nanoparticles/oxygen deficient Ti₄O₇ are useful microwave absorbers in terms of wide broadband, strong absorption and ultra-low filler amount. The optimal reflection loss of Ti₄O₇@CoNi composite was −43.6 dB at 2.5 mm, meanwhile the effective absorbing band could reach over 5.12 GHz at only a 2.7 mm thickness. The results confirm that the dependence of the electromagnetic characteristics of the absorber on the filler ratio, frequency, and absorber thickness. Therefore, this work may be beneficial in constructing core-shell structured magnetic metal/Magnéli Ti₄O₇ composites to tune electromagnetic parameters and strengthen electromagnetic absorption.     

Apium-derived biochar loaded with MnFe2O4@C for excellent low frequency electromagnetic wave absorption

Given the rapid development of electrommunication and radar detection technologies, low frequency electromagnetic wave materials have received more and more attention. Herein, the Apium-derived biochar loaded with MnFe₂O₄@C has been successfully prepared by using co-solvothermal and calcination method. The cladding carbon layer on MnFe₂O₄ NPs is migrated from biochar via thermal diffusion, and the biochar is covered with the ferrite NPs as well. Thus, the combination of dielectric and magnetic loss endows the composite with excellent low frequency electromagnetic absorption ability i.e. the optimal microwave absorbing intensity is −48.92  dB at 0.78 GHz with an extended effective absorbing bandwidth of 0.38–1.78 GHz for only 2.5 mm thickness, being ascribed to nature resonance, multiple interfacial and surface polarization, strong electromagnetic attenuation ability and good impedance matching property in detail. This bio-based ferrite composites have great potential in preparation of MAMs due to the advantages of extraordinary performance, lightweight property, environmental protection and easy degradation.     

Comparative study of microwave absorption properties of Ni–Zn ferrites obtained from different synthesis technologies

Spinel ferrites are widely used for electromagnetic wave (EMW) absorption applications. In this study, spinel Ni–Zn ferrites with excellent microwave absorption properties were synthesized. Their EMW absorption characteristics and interaction mechanisms were studied to lay the foundation for the study of the role of Ni–Zn ferrite as a magnetic substrate for composites. Herein, Ni_0·5Zn_0·5Fe₂O₄ was prepared by the hydrothermal method (H-NZFO) and the sol–gel auto-combustion method (S-NZFO); both samples exhibited distinct microwave absorption properties. The S-NZFO absorber (thickness = 3.72 mm) demonstrated the best dual-zone microwave absorption with two strong reflection loss peaks at 5.1 and 10.5 GHz. The corresponding effective absorption bandwidth (EAB) reached 9.0 GHz, which covered part of the S-band and all of the C- and X-bands. These results were attributed to the high saturation magnetization, outstanding complex permeability, and multiple magnetic loss channels of S-NZFO. The H-NZFO sample exhibited excellent absorption capability and matching thickness. At a thickness as low as 1.71 mm, the minimum reflection loss (RL_min) of the H-NZFO absorber reached -60.2 dB at 13.1 GHz. The maximum bandwidth corresponding to RL below -10 dB was 4.6 GHz. These results can be attributed to small particle size, high complex permittivity, and multiple dielectric loss channels of H-NZFO. The observed wide effective absorption bandwidth of S-NZFO and strong microwave absorption capability of H-NZFO suggest the potential of both materials as substrates for efficient microwave absorbers in military as well as civilian absorption applications.     

Polymer-derived Co2Si@SiC/C/SiOC/SiO2/Co3O4 nanoparticles: Microstructural evolution and enhanced EM absorbing properties

In this work, porous core-shell structured Co₂Si@SiC/C/SiOC/SiO₂/Co₃O₄ nanoparticles were fabricated by a polymer-derived ceramic approach. The in situ formation of mesopores on the shell, microstructural, and phase evolution of resulting nanoparticles were investigated in detail. The obtained nanoparticles-paraffin composites possess a very low minimum reflection coefficient (RC_min) −60.9 dB, broad effective absorption bandwidth 3.50 GHz in the X-band and 15.5 GHz in the whole frequency range (from 2.5 to 18 GHz). The results indicate outstanding electromagnetic wave (EMW) absorbing performance among all the reported cobalt-based nanomaterials, due to the reasons as follows: (a) The unique core-shell structure as well as complex phase composition of SiC/C/SiOC/SiO₂/Co₃O₄ in the shell, result in a large number of heterogeneous interfaces in the nanoparticles; (b) Nanoparticles have both dielectric and magnetic loss; (c) Mesopores in the shell prolong the propagation path of EMW, thereby increasing the absorption/reflection ratio of EMWs. Thanks to the material structure design, the resulting core-shell structured cobalt-containing ceramic nanoparticles have great potential for thin and high-performance EMW absorbing materials applied in harsh environment.     

Fabrication of microwave absorbing CoFe2O4 coatings with robust superhydrophobicity on natural wood surfaces

A superhydrophobic wood surface with microwave absorption property was prepared based on the formation of CoFe₂O₄ nanoparticles and subsequent hydrophobization using fluorinated alkylsilane (FAS). Meanwhile, sticky epoxy resin was worked as a caking agent by adhering abundant of CoFe₂O₄ nanoparticles to wood surface. The as-prepared superhydrophobic coatings on wood maintain stable superhydrophobicity after suffering a significant abrasion. Moreover, the complex permeability and permittivity of the coated wood composites were measured in the frequency range of 2–18 GHz by vector network analysis. The microwave absorption properties were elucidated by the traditional coaxial line method. The results show that the as-prepared wood composites have excellent microwave absorption properties at the frequency of 16 GHz, and the minimum reflection loss can reach −12.3 dB. The approach presented may provide further routes for designing outdoor wood wave absorbers with a specified absorption frequency.     

Microwave absorption properties of Ni0.6Zn0.4Fe2O4 composites with nonmagnetic nanoferrite percentages

Microwave absorption materials need to be thin and lightweight and possess strong wave absorption ability and a wide absorption frequency band. To satisfy these conditions, we changed the microstructure of the composite by mixing ferrite material with different particle sizes. Specifically, we mixed nanosized nonmagnetic ZnFe₂O₄ powder into Ni_0.6Zn_0.4Fe₂O₄ powder, investigated the microwave absorption properties depending on the packing fraction. The crystal structure of the synthesized ferrite powders was analyzed through XRD, and the particle size was analyzed using a PSA and SEM. The density of the powders, which is required to measure the packing fraction, was determined via the gas disposition method, and the magnetic properties of the composites were analyzed using a VSM. The reflection loss represents the electromagnetic wave absorption characteristics, and it was calculated by substituting the measured permittivity and permeability values, into the equation based on the transmission line theory. The Ni_0.6Zn_0.4Fe₂O₄/ZnFe₂O₄ composite showed 99.9% absorption with a high packing fraction, and the absorption peak shifted to high frequencies. These characteristics suggest that the absorption ability and frequency range of the electromagnetic-wave-shielding composite can be easily controlled. Because of the high-absorption characteristic, absorption frequency control, and cost effectiveness, this composite can be applied to products such as thin electromagnetic-wave-shielding sheets.     

Tunable microwave absorption performance of carbon fiber-reinforced reaction bonded silicon nitride composites

The hybrid network of Si₃N₄ whiskers and conducting carbon fiber has great potential for microwave absoprtion applications. The high electrical conductivity of the carbon fiber helps to transform the microwave transparent Si₃N₄ into microwave absorbing materials. Herein, the microwave absorption performance of 5–20 vol % of carbon fiber reinforced reaction bonded Si₃N₄ (C_f-RBSN) composites have been discussed in detail. The C_f reinforcement tuned the X-band dielectric properties of the RBSN composites. The 5 vol % C_f-RBSN composite exhibit a minimum reflection loss (RL_min) of ?36.16 dB (99.998% microwave absorption) at 11.89 GHz and a high specific reflection loss of 920 dB. g^?1 for 5.9 mm thickness, while 20 vol % C_f-RBSN composites resulted in RL_min of ?22.86 dB at 11.56 GHz with a low thickness of 1.5 mm. Thus, the superior microwave absorption performance of the prepared lightweight composites results from the multiple interfacial polarization, dipole polarization, and conduction loss due to the 3D network of interconnected Si₃N₄ whiskers and C_f.     

Nano-SiC-decorated Y2Si2O7 ceramic for enhancing electromagnetic waves absorption performance

To improve the electromagnetic (EM) wave absorption performance of rare earth silicate in harsh environments, this work synthesized dense SiC–Y₂Si₂O₇ composite ceramics with excellent EM wave absorption properties by using the polymer permeation pyrolysis (PIP) process, which introduced carbon and SiC into a porous Y₂Si₂O₇ matrix to form novel composite ceramics. SiC–Y₂Si₂O₇ composite ceramics with different numbers of PIP cycles were tested and analysed. The results show that the as-prepared composites exhibit different microstructures, porosities, dielectric properties and EM wave absorption properties. On the whole, the SiC–Y₂Si₂O₇ composite ceramics (with a SiC/C content of 29.88 wt%) show superior microwave absorption properties. The minimum reflection loss (RL_min) reaches ?16.1 dB when the thickness is 3.9 mm at 9.8 GHz. Moreover, the effective absorption bandwidth (EAB) included a broad frequency from 8.2 GHz to 12.4 GHz as the absorbent thickness varied from 3.15 mm to 4.6 mm. In addition, the EM wave absorption mechanism was analysed profoundly, which ascribed to the multiple mediums of nanocrystalline, amorphous phases and turbostratic carbon distributed in the Y₂Si₂O₇ matrix. Therefore, SiC–Y₂Si₂O₇ composite ceramics with high-efficiency EM wave absorption performance promise to be a novel wave absorbing material for applications in harsh environments.     

Fabrication of novel silicon carbide-based nanomaterials with unique hydrophobicity and microwave absorption property

Novel SiC-based nanomaterials, namely the nitrogen and aluminum co-doped SiC@SiO₂ core-shell nanowires and nitrogen-doped SiO₂/Al₂O₃ nanoparticles, have been fabricated through a facile thermal treatment process based on the chemical vapor deposition and vapor-liquid reaction. These nanomaterials show remarkable hydrophobicity with a water contact angle (CA) over 140°, which are aroused by the surface zigzag morphology of the nanostructures and the hydrocarbyl groups generated during the preparation process. Moreover the nanocomposites also exhibit relatively prominent microwave absorption (MA) properties in the frequency range of 2.0-18.0 GHz. The minimum reflection loss (RL) value as low as −23.68 dB can be observed at 14.16 GHz when the absorber thickness is 2.6 mm with a loading rate of 16.7 wt%. And the nanocomposites-based absorbent can achieve an effective absorption bandwidth (RL < −10 dB) of 4.48 GHz with the absorbent thickness of 2.5 mm. This enhanced microwave attenuation performance can be attributed to multiple polarizations and perfect impedance matching conditions, as well as multiple internal reflections. These marvelous properties make these N and Al co-doped SiC@SiO₂ core-shell nanowires and N-doped SiO₂/Al₂O₃ nanoparticles display extensive application potential as MA materials in harsh environment.     

Fabrication and microwave absorbing property of WO3@WC with a core-shell porous structure

Special microstructure can significantly improve the microwave absorption property of rare materials. In this paper, porous WC powders were successfully synthesized by spray granulation method. Then, WO₃@WC materials with core-shell porous structure can be prepared after 410 °C heating treatment at different time to form the outer WO₃ oxidation layer. In addition, the microstructure, morphology, phase analysis and electromagnetic property were fully studied by investigating the WC-based materials in different structures. For WO₃@WC core-shell porous materials, when the coating thickness was 2.1 mm, the maximum reflection loss can reach ?19.4 dB at 12.6 GHz, which shows quite good microwave absorbing effects. The core-shell porous structure enhances the original microwave absorption performance due to the multiple reflection reflections and polarizations.     

Design of controlled-morphology NiCo2O4 with tunable and excellent microwave absorption performance

In recent years, the high-performance microwave absorbers with strong loss, broad frequency bandwidth, thin thickness and light weight have been intensively investigated to address the problem of electromagnetic pollution and improve stealth technology. Considering the fact that microwave absorption performance is quite sensitive to morphology, studying NiCo₂O₄ with different morphologies is a valuable step towards developing a high-performance microwave absorber. The different morphologies are prepared by adjusting the addition of the structure-directing agent NH₄F. When the amount of added NH₄F is 1 mmol, a flower-like NiCo₂O₄ morphology (NC–F1) is obtained with a large specific surface area of 158.97 m²/g and pore volume of 0.3525 cm³g^-1, which easily generates conductive loss, polarization loss, and multiple scattering, thereby enhancing its microwave absorption performance. The maximum reflection loss reaches −50.3 dB at 3 mm, and the effective bandwidth is 4 GHz with the matching thickness of 2 mm when the fill ratio is only 30 wt% in the epoxy resin. As the thicknesses range from 1.5 mm to 5 mm, the effective bandwidth is 14.2 GHz (3.8 GHz–18 GHz) and covers the entire C, X, and Ku bands. Therefore, the defined-morphology NiCo₂O₄ is expected to be a novel wide-band and strong-loss microwave absorber.     

Rhombic Fe2O3 lumps doping hollow ZnFe2O4 spheres through oxidative decomposition process implanted into graphene conductive network with superior electromagnetic wave absorption properties

Conductor-dielectric-magnetic multicomponent coordination composites with rhombic Fe₂O₃ lumps doping hollow ZnFe₂O₄ spheres through oxidative decomposition process implanted into graphene conductive network (hollow ZnFe₂O₄ spheres/rhombic Fe₂O₃ lumps/rGO composites) were successfully constructed by a facile method. The countless hollow ZnFe₂O₄ spheres were compactly attached to the curled-paper rGO and larger sized-rhombic Fe₂O₃ lumps were relatively dispersed. Among, the hollow structure of ZnFe₂O₄ spheres could attenuate the electromagnetic wave by multiple reflections and scatterings. Intriguingly, hollow ZnFe₂O₄ spheres reacted with GO to form intermediate rhombic Fe₂O₃ lump products, which ameliorated the hetero-interfaces structure and helped to improve impedance matching by weakening the strong magnetic ZnFe₂O₄ (M_s = 91.2 emu/g) and high conductive rGO after the introduction of weakly magnetic Fe₂O₃ semiconductor. Moreover, all three components could induce dielectric polarization losses, such as multilayer or dipole polarization. Therefore, the maximum absorption of ternary composites was up to ?64.3 dB at 7 GHz and 3.4 mm, simultaneously, and a bandwidth exceeding ?10 dB was 4.2 GHz at 1.7 mm. Meanwhile, with a thin thickness range of 1.5–5 mm, the absorption bandwidth below ?10 dB was from 2 to 18 GHz which occupied for 91.5% of whole study frequency range. These results provided a new approach and reference for the design and property regulation of electromagnetic materials at electronic communications, aerospace and military radar flied.     

Coupling CoFe2O4 and SnS2 nanoparticles with reduced graphene oxide as a high-performance electromagnetic wave absorber

The reduced graphene oxide (RGO)/CoFe₂O₄/SnS₂ composites have been successfully synthesized by two-step hydrothermal processes. TEM results show that CoFe₂O₄ and SnS₂ nanoparticles with both diameters about 5–10 nm are well dispersed on the surface of graphene. Compared with RGO/CoFe₂O₄ composites, the as-prepared RGO/CoFe₂O₄/SnS₂ composites exhibit excellent electromagnetic (EM) wave absorption properties in terms of both the maximum reflection loss and the absorption bandwidth. The maximum reflection loss of RGO/CoFe₂O₄/SnS₂ composites is −54.4 dB at 16.5 GHz with thickness of only 1.6 mm and the absorption bandwidth with the reflection loss below −10 dB is up to 12.0 GHz (from 6.0 to 18.0 GHz) with a thickness in the range of 1.5–4.0 mm. And especially, they cover the whole X band (8.0–12.0 GHz), which could be used for military radar and direct broadcast satellite (DBS).     

Morphology-controlled preparation and tunable electromagnetic wave absorption performance of manganese dioxide nanostructures

The development of electromagnetic wave (EMW) absorption materials with lightweight, wide absorption bandwidth, thin thickness, and strong EMW absorption performance has become a hotspot. Herein, the morphology-controlled preparation of α-manganese dioxide (α-MnO₂) was successfully obtained via a facile hydrothermal method, and the EMW absorption performance of α-MnO₂ was investigated in detail. The results indicated the as-obtained Mn-1.0-120 possessed the best EMW absorption performance with minimum reflection loss of −53.43 dB at about 5.2 GHz with a thickness of 4.1 mm originated from the synergistic effects of multiply scattering, dielectric loss, and magnetic loss. This contribution demonstrates that the MnO₂ has promising candidates with a tunable EMW absorption performance for applications in the electromagnetic field in the future.