Controlled synthesis and electromagnetic wave absorption properties of core-shell Fe3O4@SiO2 nanospheres decorated graphene

Fe₃O₄/reduced graphene oxide (RGO) nanocomposite was synthesized by a simple hydrothermal method and then SiO₂ coated onto Fe₃O₄ by a modified Stӧber method. The transmission electron microscopy and field emission scanning electron microscopy characterization indicate that masses of Fe₃O₄@SiO₂ core-shell structure nanospheres attached to the RGO sheets, and that the thicknesses of SiO₂ shells are about 20–40 nm. The X-ray diffractograms and Raman spectra illustrate that the synthesized samples consist of highly crystallized cubic Fe₃O₄, amorphous SiO₂ and disorderedly stacked RGO sheets. The magnetic hysteresis loops reveal the ferromagnetic behavior of the samples at room temperature. In addition, the Fe₃O₄@SiO₂/RGO paraffin composite exhibit excellent electromagnetic wave absorption properties at room temperature in the frequency range of 2–18 GHz, which are attributed to the effective complementarities between the dielectric loss and magnetic loss. For Fe₃O₄@SiO₂/RGO-1 and Fe₃O₄@SiO₂/RGO-2 nanocomposite, the minimum reflection loss can reach −26.4 dB and −16.3 dB with the thickness of 1.5 mm, respectively. The effective absorption bandwidth of the samples can reach more than 10.0 GHz with the thickness in the range of 1.5–3.0 mm. It is demonstrated that such nanocomposite could be used as a promising candidate in electromagnetic wave absorption area.     

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).     

Magnetic reduced graphene oxide nanocomposite as an effective electromagnetic wave absorber and its absorbing mechanism

A magnetic reduced graphene oxide (MRGO) composite consisting of graphene oxide and Fe₃O₄ particles in the range of 5–20 nm has been prepared by the one-pot hydrothermal process. RGO nanosheets provide flexible substrates for nanoparticle decoration, while Fe₃O₄ nanoparticles can also effectively prevent nanosheets to restack each other. Compared with previously literature, the synthesized RGO-Fe₃O₄ composite exhibits excellent electromagnetic wave absorption. The minimum reflection loss (R_L) value of −49.05 dB has been observed at 14.16 GHz with a thickness of 2.08 mm. The absorption bandwidth (R_L<−10 dB) corresponding to the minimum R_L is 4.60 GHz. The electromagnetic wave absorption properties of the RGO-Fe₃O₄ composite have been interpreted through the quarter-wavelength matching model.     

Fabrication and microwave absorption properties of magnetite nanoparticle–carbon nanotube–hollow carbon fiber composites

Absorbents with “tree-like” structures, which were composed of hollow porous carbon fibers (HPCFs) acting as “trunk” structures, carbon nanotubes (CNTs) as “branch” structures and magnetite (Fe₃O₄) nanoparticles playing the role of “fruit” structures were prepared by chemical vapor deposition technique and chemical reaction. Microwave reflection loss, permittivity and permeability of Fe₃O₄–CNTs–HPCFs composites were investigated in the frequency range of 2–18 GHz. It was proven that prepared absorbents possessed the excellent electromagnetic wave absorbing performances. The bandwidth with a reflection loss less than −15 dB covers a wide frequency range from 10.2 to 18 GHz with the thickness of 1.5–3.0 mm, and the minimum reflection loss is −50.9 dB at 14.03 GHz with a 2.5 mm thick sample layer. Microwave absorbing mechanism of the Fe₃O₄–CNTs–HPCFs composites is concluded as dielectric polarization and the synergetic interactions exist between Fe₃O₄ and CNTs–HPCFs.     

Phenolic resin-based composite sheets filled with mixtures of reduced graphene oxide, γ-Fe2O3 and carbon fibers for excellent electromagnetic interference shielding in the X-band

Composite sheets consisting of phenolic resin filled with a mixture of reduced graphene oxide (RGO), γ-Fe₂O₃ and carbon fibers have been produced by compression molding. Its electrical conductivity lies in the range 0.48–171.21 S/cm. Transmission and scanning electron microscopy observations confirm the presence of nano particles of γ-Fe₂O₃ (～9.8 nm) and carbon fiber (～1 mm) which gives flexural strength to composite sheets. Thermogravimetric analysis show that the thermal stability of the sheets depend upon the amount of RGO and phenol resin in the composite. Complex parameters, i.e., permittivity (ε* = ε′ ? iε″) and permeability (μ* = μ′ ? iμ″) of RGO/γ-Fe₂O₃/carbon fiber have been calculated from experimental scattering parameters (S₁₁ and S₂₁) using theoretical calculations given in Nicholson?Ross and Weir algorithms. The microwave absorption properties of the sheets have been studied in the 8.2–12.4 GHz (X-Band) frequency range. The maximum shielding effectiveness observed is 45.26 dB, which strongly depends on dielectric loss and volume fraction of γ-Fe₂O₃ in RGO matrix.     

The effect of GO loading on electromagnetic wave absorption properties of Fe3O4/reduced graphene oxide hybrids

Ideal electromagnetic absorbing materials with lightweight and high efficiency have broad application outlook in military and civil fields. In this work, a 3D nanostructure material by hybridizing Fe₃O₄ nanocrystals and reduced graphene oxide (Fe₃O₄/rGO) were synthesized through an environmental-friendly one-pot solvothermal method. The effect of GO loading on electromagnetic (EM) wave absorption characteristic of Fe₃O₄/rGO was investigated. The introduction of rGO sheets not only prevented Fe₃O₄ from agglomerating, also improved the absorption performance of Fe₃O₄/rGO hybrids. With an appropriate addition, Fe₃O₄/rGO obtained a minimum reflection loss (RL) of −22.7 dB and the absorption bandwidth was 3.13 GHz (90% absorption).     

Dielectric and electromagnetic wave absorption properties of reduced graphene oxide/barium aluminosilicate glass–ceramic composites

BaAl₂Si₂O₈ (BAS) glass–ceramic powders were prepared by sol–gel method. Graphene oxide (GO)/BAS mixture powders were prepared by a simple mixing process of GO and BAS. Dense and uniform reduced graphene oxide (RGO)/BAS composites were fabricated by the hot-pressing of GO/BAS, which was accompanied by the in-situ thermal reduction of GO. Microstructure, phase composition, dielectric and electromagnetic wave (EM) absorption properties of RGO/BAS were investigated. The results reveal that RGO can promote the hexacelsian-to-celsian phase transformation of BAS. In the frequency range from 8 GHz to 12 GHz, the complex permittivity of RGO/BAS increases with increasing RGO content. The composite with 1.5 wt% of RGO shows good EM absorbing ability. When the sample thickness is 2.1 mm, the minimum reflection coefficient (RC) reaches −33 dB, and the effective absorption bandwidth is more than 3.1 GHz.     

Enhanced radar and infrared compatible stealth properties in hierarchical SnO2@ZnO nanostructures

Hierarchical SnO₂@ZnO nanostructures are successfully synthesized in a large scale by using a simple hydrothermal method. The SnO₂ nanowires epitaxially grow on the non-polarized plane of ZnO nanorods with a six-fold symmetry. The radar wave absorbing and infrared emissivity properties of hierarchical SnO₂@ZnO nanostructures are studied. Such hybrid hierarchical SnO₂@ZnO nanostructures show enhanced radar and infrared compatible stealth properties than ZnO or SnO₂. The minimum reflection loss (RL) is −23.51 dB at 9.2 GHz with a bandwidth (RL<−10 dB) of 3.5 GHz and the average infrared emissivity in middle-infrared band and far-infrared band are around 0.65 and 0.89, respectively.     

Preparation of Fe3O4 particles from copper/iron ore cinder and their microwave absorption properties

In this study, magnetic Fe₃O₄ particles were prepared from copper/iron ore cider by precipitation oxidization method. The yield of Fe was 82.6 at%. XRD, TEM, SEM, EDS and microwave network analyzer were used to characterize the particles. The results showed that Fe₃O₄ particles were well crystallized and possessed an octahedral morphology, and the crystal size was about 200 nm; the sample with 70 wt% Fe₃O₄ exhibited the optimal absorbing ability, the minimum reflection loss was ?42.7 dB at 14.08 GHz and the bandwidth less than ?10 dB was about 4.2 GHz when the sample thickness was 1.9 mm. It was clearly demonstrated that the Fe₃O₄ particles prepared from copper/iron ore cider could be used as an effective microwave absorbing material.     

Hollow polyaniline/Fe3O4 microsphere composites: Preparation,characterization, and applications in microwave absorption

Hollow polyaniline/Fe₃O₄ microsphere composites with electromagnetic properties were successfully prepared by decorating the surface of hollow polyaniline/sulfonated polystyrene microspheres with various amounts of Fe₃O₄ magnetic nanoparticles using sulfonated polystyrene (SPS) as hard templates and then removing the templates with tetrahydrofuran (THF). The synthesized hollow microsphere composites were characterized by FT-IR, UV/Vis spectrophotometry, SEM, XRD, elemental analysis, TGA, and measurement of their magnetic parameters. Experimental results indicated that the microspheres were well-defined in size (1.50–1.80 μm) and shape, and that they were superparamagnetic with maximum saturation magnetization values of 3.88 emu/g with a 12.37 wt% content of Fe₃O₄ magnetic nanoparticles. Measurements of the electromagnetic parameters of the samples showed that the maximum bandwidth was 8.0 GHz over ?10 dB of reflection loss in the 2–18 GHz range when the Fe₃O₄ content in the hollow polyaniline/Fe₃O₄ microsphere composites was 7.33 wt%.     

Structural,magnetic, and textural properties of iron oxide-reduced graphene oxide hybrids and their use for the electrochemical detection of chromium

Superparamagnetic Fe₃O₄ nanoparticles were anchored on reduced graphene oxide (RGO) nanosheets by co-precipitation of iron salts in the presence of different amounts of graphene oxide (GO). A pH dependent zeta potential and good aqueous dispersions were observed for the three hybrids of Fe₃O₄ and RGO. The structure, morphology and microstructure of the hybrids were examined by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared spectroscopy, Raman and X-ray photoelectron spectroscopy. TEM images reveal lattice fringes (d₃₁₁ = 0.26 nm) of Fe₃O₄ nanoparticles with clear stacked layers of RGO nanosheets. The textural properties including the pore size distribution and loading of Fe₃O₄ nanoparticles to form Fe₃O₄–RGO hybrids have been controlled by changing the concentration of GO. An observed maximum (～10 nm) in pore size distribution for the sample with 0.25 mg ml^?1 of GO is different from that prepared using 1.0 mg ml^?1 GO. The superparamagnetic behavior is also lost in the latter and it exhibits a ferrimagnetic nature. The electrochemical behavior of the hybrids towards chromium ion was assessed and a novel electrode system using cyclic voltammetry for the preparation of an electrochemical sensor platform is proposed. The textural properties seem to influence the electrochemical and magnetic behavior of the hybrids.     

Cation distribution and magnetic analysis of wideband microwave absorptive CoxNi1−xFe2O4 ferrites

Co_xNi_1−xFe₂O₄ ferrites (x=0, 0.2, 0.4, 0.4, 0.6, 0.8 and 1) were prepared by a sol-gel auto-combustion method. The samples were structurally characterized by X-ray diffractometry (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray analysis (EDX), and Fourier transform infrared spectroscopy (FTIR). The XRD patterns confirmed single phase formation of spinel structure. Cation distribution estimated from XRD data suggested the mixed spinel structure of ferrite. The EDX analysis was in good agreement with the nominal composition. The results of FTIR analysis indicated that the functional groups of Co-Ni spinel ferrite were formed during the combustion process. According to FE-SEM micrographs, by addition of cobalt ion the average particle size of substituted nickel ferrite was gradually became smaller from 450 nm to 280 nm. Magnetic measurement using vibrating sample magnetometer (VSM) showed an increase in saturation magnetization and coercivity by Co²⁺ substitution in nickel ferrite. For Co_0.8Ni_0.2Fe₂O₄ sample, M_s and H_c reaches as high as 93 emu/g and 420 Oe, respectively. The reflection loss properties of the nanocomposites were investigated in the frequency range of 8–12 GHz, using vector network analyzer (VNA). Cobalt substitution could enhance reflection loss of NiFe₂O₄ ferrite. The maximum reflection loss value of the Co²⁺ substituted Ni ferrite was ~ −26 dB (i.e. over 99% absorption) at 9.7 GHz with bandwidth of 4 GHz (RL<– 10 dB) through the entire frequency range of X-band.     

Synthesis of cross-linked graphene aerogel/Fe2O3 nanocomposite with enhanced supercapacitive performance

In this work, cross-linked graphene aerogel (CL-GA) and its composite with Fe₂O₃ nanoparticles (NPs) were synthesized through a one-step hydrothermal procedure by using p-phenylenediamine (PPD). Structural characterizations revealed that in the preparation of the composite PPD acts as a cross-liker and provides high surface area by decreasing restacking of graphene sheets and functions as nitrogen source simultaneously. The electrochemical characteristics of the nanocomposite were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS) and Fast Fourier transform continues cyclic voltammetry (FFTCCV). The results show that cross-linked graphene aerogel/Fe₂O₃ (CL-GA/Fe₂O₃) nanocomposite displays enhanced supercapacitive performance, where it has capacitance of 445 at 1 A g⁻¹, high energy density of 63 W h Kg⁻¹, and 89% capacitance retention after 5000 cycles in 3 M KOH. Presence of PPD considerably improved supercapacitive performance of nanocomposite as a result it could be promising material in synthesis of efficient graphene/metal oxide-based electrode material for high performance supercapacitors.     

Control of dispersion frequency of BaTiO3-based ceramics applicable to thin absorber for millimeter electromagnetic wave

Elements such as B, Li and Na were doped to barium titanate, BaTiO₃ in order to control dielectric dispersion. Addition of 3 mol% Li₂O lowered the dispersion at frequency of 0.53 MHz, while addition of 3 mol% B₂O₃ or Na₂O did not affect dispersion frequency. BaTiO₃ doped with 0.3 mol% Li₂O showed dielectric dispersion at around 2.5 GHz. An electromagnetic (EM) wave absorber using the doped BaTiO₃ plate was tried to produce for millimeter frequency range. A matching layer of 0.5 mm thick ceramic plate with relative permittivity 21 was attached to it to suppress reflection of incident EM wave due to the discontinuity at the boundary between the BaTiO₃ and air. The obtained EM wave absorber had reflectivity of −45 dB at 31 GHz and −25 dB at 95 GHz, respectively.     

Effect of La-Ni substitution on structural,magnetic and microwave absorption properties of barium ferrite

In this paper, La-Ni substituted barium ferrite nanoparticles were prepared by a co-precipitation method. The morphology, structure, magnetic and microwave absorption properties of samples were accomplished by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), vibrating sample magnetometer (VSM) and vector network analysis. From the results evaluation, it can be seen that the magnetoplumbite structure for all of the samples have been formed and the average crystallite sizes of Ba_1−xLa_xFe_12−xNi_xO₁₉ nanoparticles within in the range of 50.9–65.5 nm. Ba_0.9La_0.1Fe_11.9Ni_0.1O₁₉ exhibits a remarkable reflection loss of −13.5 dB at 13.05 GHz with a matching thickness of 1.5 mm. The reflection loss results indicate that Ba_0.9La_0.1Fe_11.9Ni_0.1O₁₉ nanoparticles may be used as a potential for thin microwave absorbers.     

Reduced graphene oxide anchored with δ-MnO2 nanoscrolls as anode materials for enhanced Li-ion storage

We developed a one-pot in situ synthesis procedure to form nanocomposite of reduced graphene oxide (RGO) sheets anchored with 1D δ-MnO₂ nanoscrolls for Li-ion batteries. The as-prepared products were characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). The electrochemical performance of the δ-MnO₂ nanoscrolls/RGO composite was measured by galvanostatic charge/discharge cycling and electrochemical impedance spectroscopy. The results show that the δ-MnO₂ nanoscrolls/RGO composite displays superior Li-ion battery performance with large reversible capacity and high rate capability. The first discharge and charge capacities are 1520 and 810 mAh g⁻¹, respectively. After 50 cycles, the reversible discharge capacity is still maintained at 528 mAh g⁻¹ at the current density of 100 mAh g⁻¹. The excellent electrochemical performance is attributed to the unique nanostructure of the δ-MnO₂ nanoscrolls/RGO composite, the high capacity of MnO₂ and superior electrical conductivity of RGO.     

A high capacity NiFe2O4/RGO nanocomposites as superior anode materials for sodium-ion batteries

NiFe₂O₄/reduced graphene oxide (NFO/RGO) nanocomposites were prepared by a facile one-step hydrothermal method and used as anode for sodium ion batteries (SIBs). The crystal structures, morphologies and electrochemical properties of as-prepared samples were evaluated by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/discharge tests. The results show that NFO/RGO-20 (20 wt%) delivers the highest reversible capacity of ~450 mA h g⁻¹ at 50 mA g⁻¹ after 50 cycles with good cycling stability. The excellent sodium storage performance of NFO/RGO should be attributed to the synergistic effect between NFO and RGO to form conductive network structure, which offers the increased specific surface area, the facilitated electron transfer ability and the buffering of volume expansion.     

Flexible polymer composites with enhanced wave absorption properties based on beta-manganese dioxide nanorods and PVDF

Beta-manganese dioxide (β-MnO₂) nanorods have been fabricated on a large scale by a simple hydrothermal process in a wild condition. Several characterizations such as XRD, SEM, TEM and FESEM have been employed. The wave absorption properties of β-MnO₂/PVDF nanocomposites have been investigated. The results indicated that the β-MnO₂/PVDF nanocomposites exhibit enhanced wave absorption properties. The minimum reflection loss of the β-MnO₂/PVDF nanocomposite reaches − 30.1 dB (> 99.9% attenuation) at 8.16 GHz with a filler loading of 40 wt.%, and the frequency bandwidth less than –10 dB is from 7.12 to 9.20 GHz. The main microwave absorbing mechanism has been also discussed.     

Magnetic and microwave properties of BaFe12O19 substituted with magnetic,non-magnetic and dielectric ions

Barium hexaferrite particles were synthesized with conventional solid state reaction route. 1% boron (B₂O₃) was added to the initial mixture of oxides to inhibit crystal growth at lower temperatures. Magnetic (Mn²⁺, Co²⁺, Ni²⁺and Cu²⁺), non-magnetic (Zn²⁺) and dielectric (Ti⁴⁺) ions were replaced by one Fe³⁺ ion of barium hexaferrite to shift the ferromagnetic resonance frequency to low frequencies and to increase the magnetic and dielectric losses. The structural and morphological characterization of samples was done by X-ray powder diffractometer and scanning electron microscopy. Magnetic and microwave properties were determined by vibrating sample magnetometer and vector network analyzer, respectively. The maximum saturation magnetization and the highest reflection losses of −34 dB at 10 GHz, with absoption bandwidth of 1.6 GHz at −20 dB, were observed in Cu²⁺–Ti⁴⁺ and Zn²⁺–Ti⁴⁺ substituted samples. The mechanism of microwave energy dissipation is due to the impedance matching at matching thickness. It was also observed that as the sample thickness increases, the resonance frequency decreases exponentially.     

Synthesis of hierarchical core-shell NiFe2O4@MnO2 composite microspheres decorated graphene nanosheet for enhanced microwave absorption performance

A ternary functional composite NiFe₂O₄@MnO₂@graphene was synthesized successfully via a facile method. The phase constitution, microstructures, morphologies and chemical compositions of the samples were investigated by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS). It was observed that the NiFe₂O₄ nanoparticles were coated by hierarchically MnO₂ shells and distributed on the surface of graphene. Investigations of EM wave absorption indicated that NiFe₂O₄@MnO₂@ graphene composite has the strongest reflection loss of −47.4 dB at 7.4 GHz at the matching thickness of 3 mm, compared to NiFe₂O₄ and NiFe₂O₄@MnO₂, and its maximum absorption bandwidth (<−10 dB) is 4.3 GHz (from 5.1 to 9.4 GHz). The enhanced microwave absorption performance can be attributed to the hierarchical structure of MnO₂, void space between MnO₂ and graphene, and better impedance matching of ternary composite. The above results indicate that the novel hierarchical NiFe₂O₄@MnO₂@graphene composite, with intense absorption and wide absorption bandwidth, would be a promising absorber with less EM wave interference.