Metal ions induced dielectric and magnetic loss in Co3O4 for electromagnetic wave absorption

Despite Co₃O₄ has been widely applied in electromagnetic wave (EMW) absorbers, single Co₃O₄ doesn't have excellent EMW absorbing performance. Modification of Co₃O₄ with other metal ions addition is an effective way to improve its impedance matching and EMW attenuation. Herein, CuO/Cu_0.3Co_2.7O₄/Co₃O₄ and NiCo₂O₄/Co₃O₄ composites have been obtained via a facile two-stage strategy, and the influence of Cu²⁺ and Ni²⁺ on the high-frequency and low-frequency EMW absorbing performance of the composites has been investigated as well. The electromagnetic parameters of samples are regulated by adding different metal ions to achieve optimum impedance matching. Dipole polarization and magnetic resonance are the main loss mechanisms. The composite with Cu²⁺ and Ni²⁺ additions exhibits the best EMW absorption with an effective absorption bandwidth (EAB) of 10.8–18.0 GHz for 2.1 mm thickness at high-frequency and 4.5–8.5 GHz for 4.9 mm thickness at low frequencies, respectively. This work offers an effective method for preparing composite materials with multicomponent broadband absorption of oxides.     

Microstructure and enhanced electromagnetic wave absorbing performance of Zn0.6Ni0.3Cu0.1Fe2O4 ferrite glass-ceramic

Here we introduce a controllable route for the efficient synthesis of Zn_0.6Ni_0.3Cu_0.1Fe₂O₄ ferrite glass-ceramic with enhanced electromagnetic wave (EMW) absorbing performance. By adding a certain amount of Zn, Ni, Cu and Fe oxides into the SiO₂–Al₂O₃–B₂O₃–CaO-R₂O glass system, the microstructure of three-dimensional dendritic ferrites combined with amorphous SiO₂-rich phase is constructed through a high-temperature melt and quenching route. The good EMW absorption performance is attributed to the unique combination of amorphous glass and spinel ferrite, which improves the impedance matching of the material and absorbs EMW by the dielectric loss and magnetic loss. Moreover, the dendritic ferrite crystal phase is compounded with the SiO₂-rich amorphous phase to form grain boundaries and crystal-amorphous interfaces, which enhances the interfacial polarization and builds multiple transmission-absorption mechanisms. The results show that the reflection loss peak value of the glass-ceramics containing 60 wt% Zn_0.6Ni_0.3Cu_0.1Fe₂O₄ spinel is ?42.16 dB with the sample thickness of 2 mm, and the effective absorption band range (reflection loss ≤ -10 dB) is 3.76 GHz (13.6–17.36 GHz) at 1.5 mm. This approach presents a scalable and low-cost solution that may be applied to the design of high-efficiency EMW consumption components in the future.     

Enhanced electromagnetic wave absorption performance of SiCN(Fe) fibers by in-situ generated Fe3Si and CNTs

The SiCN(Fe) fibers with excellent one-dimensional microstructure and electromagnetic wave (EMW) absorption performance were synthesized by combining polymer-derived ceramics (PDCs) method and electrospinning. The in-situ generation of Fe₃Si and CNTs by adding ferric acetylacetonate (FA) into the raw materials effectively improved the dielectric properties, magnetic properties and the impedance matching performance of the SiCN(Fe) fibers. The EMW absorption performance of SiCN(Fe) fibers were mainly based on dipole polarization loss, interface polarization loss and eddy current loss. The RL_min value of SiCN(Fe) fibers reached ?47.64 dB at 1.38 mm and the effective absorption band (EAB, RL ≤ ?10 dB) reached 4.28 GHz (13.72–18 GHz, 1.35 mm).     

Ultra-broadband electromagnetic wave absorption of ceramic composite coating by atmospheric plasma spraying

Dy doped ceramic material of Ba₄Fe_2.6Dy_1.4Nb₈O₃₀ with the tetragonal black bronze structure was synthesized by solid-state reaction at 1300°C. The composite powders of Ba₄Fe_2.6Dy_1.4Nb₈O₃₀/FeSiAl with good flowing were prepared by spray drying using the powders slurry. The obtained composite powders were then plasma sprayed to fabricate coating. The microstructure and absorption characteristics of electromagnetic wave (EMW) of the coating materials are investigated. Results show that the composite powders significantly remain the phase stability during atmospheric plasma spraying and adsorbing heredity of EMW for the coating. The coating powders have excellent EMW absorption with the minimum reflection loss of −48.20 dB at the thickness of 3.5 mm. Also, the coating powders display the maximum effective absorption bandwidth of 6.24 GHz at the thickness of 1.5 mm, which covers the X-band. Moreover, the coating was found to have a certain EMW absorption performance at high temperature of 700°C. The Ba₄Fe_2.6Dy_1.4Nb₈O₃₀ ceramic material with low thermal conductivity ensured the electromagnetic absorption capacity of the coating at high temperature. The Ba₄Fe_2.6Dy_1.4Nb₈O₃₀/FeSiAl ceramic composite materials could have the potential applications as the high temperature EMW absorber.     

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.     

Synthesis of nitrogen-doped graphene-based binary composite aerogels for ultralightweight and broadband electromagnetic absorption

The development of ultralightweight and broadband electromagnetic wave (EMW) absorbing materials remains a big challenge. In this work, porous magnesium ferrite microspheres decorated nitrogen-doped reduced graphene oxide (NRGO/MgFe₂O₄) composite aerogels were prepared by a two-step route of solvothermal synthesis and hydrothermal self-assembly. Results of microscopic morphology characterization showed that NRGO/MgFe₂O₄ composite aerogels had a unique hierarchical porous structure. Moreover, the influence of additive amounts of graphene oxide on the electromagnetic parameters and EMW absorption properties of NRGO/MgFe₂O₄ composite aerogels was explored. Remarkably, the attained binary composite aerogel with the content of NRGO of 70.21 wt% exhibited the best EMW absorption performance. The minimum reflection loss reached up to −55.7 dB, and the corresponding effective absorption bandwidth was as large as 5.36 GHz at a thin matching thickness of 1.98 mm. Furthermore, when the matching thickness was slightly increased to 2.29 mm, the widest effective absorption bandwidth was enlarged to 7.1 GHz, covering the entire Ku-band. The magnetodielectric synergy and unique hierarchical porous structure in NRGO/MgFe₂O₄ composite aerogels not only improved the impedance matching, but also greatly enhanced the EMW absorption capacity. It was believed that the results of this work could be helpful for the preparation of graphene-based magnetic composites as broadband and efficient EMW absorbers.     

Hollow porous Ni@SiC nanospheres for enhancing electromagnetic wave absorption

Multi-component and a large number of non-homogeneous interface absorbers have been proven to be the preferred choice of electromagnetic wave (EMW) absorption materials. Herein, hollow porous Ni@SiC nanospheres (HPNS) were successfully constructed by combining a carbothermal reduction preparation strategy and subsequent electroless plating process. The heterogeneous dielectric/magnetic multi-component benefit to excellent EMW absorption properties through forming multiple polarizations, dielectric loss, and magnetic loss effects. Particularly, the HPNS at the optimal ratio with a low filler loading of 20 wt% exhibits the minimum reflection loss (RL_min) value of −62.39 dB at 13.28 GHz and the maximum effective absorption bandwidth (EAB_max) is up to 5.36 GHz (12.64–18 GHz), and the corresponding thin matching thicknesses are 1.96 mm and 1.80 mm, respectively. Furthermore, the maximum radar cross section (RCS) reduction of HPNS over PEC reaches 26.02 dB m² (1.96 mm) under far-field conditions, which means that the prepared HPNS is extraordinarily promising for practical applications. This work guides the preparation of high-performance EMW absorbers as well as radar stealth materials.     

The heterointerface of graphene in-situ growth for enhanced microwave attenuation properties in La-doped SiBCN ceramics

The electromagnetic wave (EMW) absorbing materials are widely applied to attenuate the useless and harmful EMW generated from wireless communication and 5G networks, which could protect the human health and electronic device safety. In this study, La-doped SiBCN ceramics with broadband EMW absorption capability were prepared via generating abundance of heterointerfaces, as graphene were in-situ grown by La₂O₃ catalyzing. The graphene in-situ formed in the ceramics can be attributed to the La atom decreasing the potential energy of the free carbon ring nucleation from −760.9 Ha to −8984.3 Ha. Consequently, the electrical conductivity of the SiBCN ceramics improved from 12.360 S/m to 18.025 S/m, the minimum reflection loss (RL_min) obtained was −26.48 dB at 7.2 GHz and the effective absorption bandwidth (EAB) was 6.32 GHz (11.68–18.00 GHz) at a thickness of 1.7 mm. At 700 °C, the RL_min and EAB values reached −43.18 dB and 4.2 GHz, respectively. The enhanced EMW absorbing capability can be attributed to the rationally tailor the heterointerfaces to improve the polarization loss and conduction loss of the SiBCN ceramics. The interfaces between graphene and amorphous phases generate built-in electric fields and space chare regions to strengthen the interface polarization, while the electrons migrating rapidly in the graphene and other crystals improved the electrical conductivity. The positive effect of heterointerfaces regulation of graphene in-situ growth improved the dielectric loss capacity of the SiBCN ceramics; therefore, this study provides a feasible method to enhance the EMW absorption capability of polymer-derived ceramics.     

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.     

Facile synthesis of Co0.5Zn0.5Fe2O4 nanoparticles decorated reduced graphene oxide hybrid nanocomposites with enhanced electromagnetic wave absorption properties

Herein, reduced graphene oxide/cobalt-zinc ferrite (RGO/Co_0.5Zn_0.5Fe₂O₄) hybrid nanocomposites were fabricated by a facile hydrothermal strategy. Results revealed that the contents of RGO could affect the micromorphology, electromagnetic parameters and electromagnetic wave absorption properties. As the contents of RGO increased in the as-synthesized hybrid nanocomposites, the dispersibility of the particles was improved. Meanwhile, numerously ferromagnetic Co_0.5Zn_0.5Fe₂O₄ particles were evenly anchored on the wrinkled surfaces of flaky RGO. Besides, the obtained hybrid nanocomposites exhibited superior electromagnetic absorption in both X and Ku bands, which was achieved by adjusting the RGO contents and matching thicknesses. Significantly, when the content of RGO was 7.4 wt%, the binary nanocomposites showed the optimal reflection loss of -73.9 dB at a thickness of 2.2 mm and broadest effective absorption bandwidth of 6.0 GHz (12.0–18.0 GHz) at a thin thickness of merely 2.0 mm. The enhanced electromagnetic absorption performance was primarily attributed to the multiple polarization effects, improved conduction loss caused by electron migration, and magnetic loss derived from ferromagnetic Co_0.5Zn_0.5Fe₂O₄ nanoparticles. Our results could provide inspiration for manufacturing graphene-based hybrid nanocomposites as high-efficient electromagnetic wave absorbers.     

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.     

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.     

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 hierarchical PANI@W-type barium hexaferrite composites for highly efficient microwave absorption

Hexagonal barium ferries is a promising and efficient microwave (MW) absorbing material, but the low dielectric loss and poor conductivity have limited their extensive applications. In this work, a simple tactic of coating conductive polymer PANI on hexaferrite BaCo₂Fe₁₆O₂₇ is presented, wherein the dielectric properties are customized, and more significantly, the electromagnetic loss is greatly enhanced. As displayed from structural characterizations, PANI were coated equably on the surface of hexaferrite grains by an in-situ polymerization process. The outcomes exhibit the as-prepared PANI@hexaferrite composite has remarkable electromagnetic wave absorption capacity. When the thickness is 6.0 mm, the minimal RL of ?40.4 dB was achieved at 2.9 GHz. The effective absorption bandwidth (RL < ?20 dB) of 0.65 GHz, 0.53 GHz, 0.65 GHz, 0.52 GHz, 0.46 GHz and 0.39 GHz was achieved separately when the thickness ranges from 4 to 9 mm. The highly efficient MW absorbing performance of PANI@hexaferrite composite were the consequence of multiple loss mechanisms and perfect impedance matching. It is demonstrated that the PANI@hexaferrite composite with excellent MW absorption performance is expected to be potential MW absorbers for extensive applications.     

Sandwich-like preparation of Ti3C2Tx@CoFe@TiO2 nanocomposites for high-performance electromagnetic wave absorption

Electromagnetic wave (EMW) absorbing materials have been widely applied in the fields of military and engineering areas. It is of great significance to develop high-performance EMW absorbing materials. This work assembled the sandwich-like Ti₃C₂T_x based nanocomposites by the microwave-assisted annealing of CoFe-MOF@Ti₃C₂T_x (CFMF@Ti₃C₂T_x) precursors at different temperatures. Results show that, as the heat treatment temperature is 450 °C, the sandwich-like Ti₃C₂T_x@CoFe@TiO₂ nanocomposites present better EMW absorption properties. The minimum reflection loss (RL) value was ?62.9 dB at 17.95 GHz with a thin thickness of 1.2 mm. Moreover, the effective absorption bandwidth (EAB) value was 5.02 GHz (12.74–17.76 GHz) with a thickness of 1.4 mm. The application of microwave-assisted annealing contributed to the formation of CoFe nanoparticles and TiO₂ nanoparticles because of the ultra-fast heating rate. The introduction of the nanoparticles enhanced the multiple polarization, optimized the impedance matching and introduced magnetic loss, leading to the improvement of EMW absorption. When the annealing temperature further increased to 550 °C, the EMW absorbing performance was weakened, which was mainly correlated with the decrement of the interface area due to the increase of the TiO₂ nanoparticle size and CoFe nanoparticle size. Thus, the loss effect of the multiple interface polarization weakens in the EMW absorption. In addition, the high permittivity of Ti₃C₂T_x disappears, which deteriorated the impedance matching and attenuation ability of EMW. Ultimately, sandwich-like Ti₃C₂T_x@CoFe@TiO₂ nanocomposite with satisfactory EMW absorbing properties is established, promising for various EMW absorbing applications.     

Polymer-derived Co2Si(Co)/SiCN composite ceramics with tunable microwave absorption properties

In this paper, Co₂Si(Co)/SiCN composite ceramics were synthesized by simple precursor-derived ceramics method. The phase composition, morphology, and microwave absorption properties of Co₂Si(Co)/SiCN composite ceramics at different pyrolysis temperatures (1000–1400°C) were studied. When pyrolysis temperature was 1300°C, carbon nanowires (CNWs), Co₂Si, Si₂N₂O, SiC and Si₃N₄ were in situ generated and the best electromagnetic wave (EMW) absorption performance was obtained. The minimum reflection loss reached−50.04 dB at 4.81 mm, and the effective absorption bandwidth broadened to 3.48 GHz (14.52–18 GHz) at 1.31 mm. The excellent EMW absorption performance mainly comes from the coexistence of multiple loss mechanisms, including the magnetic loss of Co₂Si, the conduction loss of CNWs, and the heterogeneous interfaces polarization between varieties of nanocrystals and amorphous ceramic matrix. By adjusting the sample thickness from 1 to 5 mm, the effective absorption of S1300 can cover the entire X and Ku bands, from 3.36 to 18 GHz. This study provides a simple way to synthesize high performance ceramic-based microwave absorbing materials.     

Enhanced EMW absorption properties of SiCN/Fe/Ni ceramics modified with Fe/Ni bimetal

The SiCN/Fe/Ni ceramics codoped with iron acetylacetonate (FA) and nickle acetylacetonate (NA) was synthesized by polymer-derived ceramics (PDCs) method in this study. The microstructure, phase composition and electromagnetic wave (EMW) absorption properties of the samples were analyzed. The polarization loss and conduction loss of materials were analyzed by the direct current (DC) multimeter and the contribution rate of polarization loss was more than 94% in the whole frequency band. The results showed that C, SiC, Fe₂Si, Ni₃Si, γ- (Fe, Ni) and CNTs were formed after pyrolysis which provided lots of heterogeneous interface and enhanced the interfacial polarization. Meanwhile, Ni could enter the lattice of Fe and formed a unique electronic configuration, which reinforced the conductivity and stability of Fe. In addition, the in-situ generated Fe₂Si and Ni₃Si provided magnetic loss and conduction loss. The RL_min value of SiCN/Fe/Ni-3 ceramic was ?52.06 dB at 1.54 mm and the effective absorption band (EAB, RL ≤ ?10 dB) reached 4.21 GHz (13.79–18 GHz, 1.43 mm).     

Enhanced microwave absorption features of Ba3Co2Fe24O41 hexaferrite by high lanthanium doping concentration

Controlling material structure and its electromagnetic properties, including complex permittivity and permeability, could enhance the microwave absorption performance of the material in terms of reflection loss and effective absorption bandwidth. In this study, La-substituted barium hexaferrite, Ba_3−xLa_xCo₂Fe₂₄O₄₁ (x = 0, 0.1, 0.3, and 0.5) compounds were successfully prepared using the solid-state reaction method, and their corresponding microstructures, static magnetic properties, and electromagnetic features in 2–18 GHz were investigated. The doping of La content increased saturation magnetization, coercivity, and remnant magnetization. The Ba_2.7La_0.3Co₂Fe₂₄O₄₁ epoxied sample with 3.5 mm thickness possessed an excellent microwave absorption of −47.3 dB at 3.52 GHz, and its corresponding effective absorption bandwidths were 3.75 GHz (2.25–6 GHz) and 0.57 GHz (17.43–18 GHz). It is shown that doping with various La concentrations on Ba₃Co₂Fe₂₄O₄₁ can be used as an effective technique to tune the performance of microwave absorbers based on barium hexaferrite.     

Tunable design of ZnFe2O4@C@BPC hybrid with rich heterogeneous interfaces as a hydrophobic electromagnetic wave absorber

Environmentally friendly microwave absorbers with superior electromagnetic wave absorption, lightweight and hydrophobic ability have received considerable attention in practical applications. However, addressing the above-mentioned characteristics is simultaneously a tremendous challenge. Along these lines, in this work, a lightweight and efficient hybrid material was fabricated by employing simple self-assembly of core-shell ZnFe₂O₄@C nanospheres embedded within longan shell-derived honeycomb-like porous carbon. The results display that the carbon skeleton not only improves the conduction loss, but also promotes the reflection and scattering of EM wave. In addition, the core-shell ZnFe₂O₄@C microspheres are conducive to enhancing the ability of interface polarization and magnetic loss, and further improving the synergistic effect between the dielectric loss and magnetic loss. Furthermore, the unique structure of the ZnFe₂O₄@C@BPC endows it excellent hydrophobicity and avoids water vapor contamination in practical applications. Precisely, at a thickness of 3.4 mm, the minimum reflection loss (RL) is up to ?58.6 dB at 12.9 GHz. Notably, the effective absorption bandwidth (EAB) is as wide as 9.1 GHz (8.9–18.0 GHz), covering almost the entire X and Ku bands. Consequently, this outstanding performance renders the ZnFe₂O₄@C@BPC composite quite attractive for a broad range of applications in lightweight, hydrophobic microwave absorption materials.     

Hollow porous magnetic/magnetic heterostructured CoFe/defective spinel microspheres template-free synthesis and effective modulation of microwave absorption performance

To create a spinel ferrite with excellent performance for electromagnetic (EM) wave absorption in the low frequency range of 4–6 GHz, compositions based on Co_0.75Zn_0.125Fe_0.125Fe₂O₄ (CZF–1) and Co_0.5Zn_0.25Fe_0.25Fe₂O₄ (CZF–2) with multiple elements substituted for A sites were synthesized by using solvothermal method. Hollow porous magnetic/magnetic heterostructure microspheres (HHMs) of CZF–A1 and CZF–A2 with multiple interfaces were prepared by hydrogen–thermal reduction of CZF–1 and CZF–2, and their unique structure and EM absorption properties were investigated in detail. The widest effective absorption bandwidth (EAB) of CZF–A1 and CZF–A2 was 4.1 GHz (13.6–17.7 GHz) and 3.7 GHz (8.0–11.7 GHz) for a corresponding thickness of 1.4 mm and 2.0 mm, respectively. In addition, the minimum reflection loss (R.L_min) of CZF–A1 and CZF–A2 reached –49.1 dB (at f_m = 13.4 GHz) and –45.0 dB (at f_m = 4.2 GHz) at a thicknesses of 1.6 mm and 3.7 mm, respectively. More specifically, in the low frequency region of 4–6 GHz, CZF–A1 and CZF–A2 exhibited excellent EM wave absorption due to the effective regulation of their natural resonance frequency. The EM wave absorption frequency band of CZF–A1 and CZF–A2 samples was able to completely cover the 4–6 GHz frequency region for at coating thickness of CZF–A1 and CZF–A2 was only 3.5 mm and 3.3 mm respectively, and their R.Lmin reached –36.5 dB and –22.6 dB. Moreover, the absorption mechanisms of CZF–A1 and CZF–A2 including magnetic resonance, eddy current loss, interfacial polarization and dipole polarization were also investigated in detail. This research provides new insights and guidance for the development of spinel ferrite-based EM absorbers for high efficiency EM wave absorption in the low frequency (4–6 GHz) region.