Fabrication and characterization of the Fe3O4@SiO2-rGO nanocomposite: a catalyst for multi-component reactions

A novel nanocomposite is synthesized by covalently modifying reduced graphene oxide (rGO) with Fe₃O₄@SiO₂ nanoparticles. Fe₃O₄ was synthesized using a co-precipitation method, and SiO₂ was then coated onto the Fe₃O₄via a sol-gel method. Graphene oxide was synthesized using the Hummers' method. Furthermore, a hydrothermal method was applied to create the Fe₃O₄@SiO₂-GO composite, and a simple reduction was used to obtain three-dimensional (3D) Fe₃O₄@SiO₂-rGO core-shell spheres. XRD, FTIR, FE-SEM, VSM, BET, TGA, and Raman analyses were used to characterize the prepared nanocomposites. X-Ray diffraction (XRD) and Raman spectra reveal that the nanostructures consist of highly crystallized cubic Fe₃O₄, amorphous SiO₂, and rGO sheets stacked in a disordered fashion. Field emission scanning electron microscopy (FE-SEM) characterization indicates that the form of the Fe₃O₄@SiO₂ core-shell structures is spherical, with an average size of about 25 nm. Magnetic hysteresis loops reveal the super-paramagnetic behavior of the samples at room temperature. All of the results obtained confirm the synthesis of high-quality nanocomposites, which can be a good candidate for use as a catalyst in multi-component reactions.

Recent progress of microwave absorption microspheres by magnetic-dielectric synergy

Designing and developing high-performance microwave absorption (MA) materials for electromagnetic protection and radar detection have received widespread attention. Recently, magnetic-dielectric MA materials have become a research hotspot due to their unique complementary functions and synergy loss mechanism. Herein, we review important research progress of excellent MA systems combining strong magnetic components and dielectric substrates. The functional materials involve magnetic materials, carbon components, semiconductors, polymer and so on. For a comprehensive analysis, current development and challenges are firstly introduced in the background. Modern requirements for microwave energy conversion are elaborated in the following part. To highlight the key points, more attention has been paid to the magnetic-dielectric synergy microsphere: (i) core/yolk-shell structure, (ii) multi-component assembly and (iii) MOF-derived synergy composites. Meanwhile, classical and typical high-performance MA composites with a multi-loss mechanism are also mentioned in this review paper. Finally, the design principles, electromagnetic synergy, future mechanism exploration and device application are presented, which provides guidance for understanding MA materials.

Excellent microwave absorption performances of high length-diameter ratio iron nanowires with low filling ratio

Reducing the filling content of high-density ferromagnetic particles is a key prerequisite for obtaining lightweight absorbers. To this end, large iron nanowires (Fe NWs) with high length-diameters, uniform length of approximately 21 μm and diameters of approximately 60 nm were synthesized through a facile magnetic field-induced in situ reduction method without templates and surfactants. The phase structures, and micromorphology of the high-aspect-ratio Fe NWs were analyzed, and the electromagnetic properties of Fe NWs-paraffin composites were measured with a vector network analyzer at 2-18 GHz. The Fe NWs-paraffin composite with a low filler loading also exhibited satisfactory microwave absorption performance, and the composites filled with 20 wt.% of as-prepared Fe NWs shows a minimum reflection loss (RL_min) of -44.67 dB at 2.72 GHz and effective absorption bandwidth (EAB) with reflection loss below -10 dB reached 8.56 GHz at a layer thickness of 1.42 mm. At a thickness of 3 mm, the RL_min value and EAB (RL ⩽ -10 dB) reached -29.74 dB and 3.28 GHz (3.84-7.12 GHz), respectively. This study suggests that Fe NWs with high-aspect-ratios have promising microwave absorbing applications, and provides a good reference for the preparation of ferromagnetic metal-based lightweight electromagnetic wave-absorbing materials.

Facile Design of Three-Dimensional Nitrogen-Doped Reduced Graphene Oxide/Multi-Walled Carbon Nanotube Composite Foams as Lightweight and Highly Efficient Microwave Absorbers

Graphene foams with three-dimensional (3D) network structure, high porosity, and ultralow density have been regarded as lightweight microwave absorption materials. Herein, nitrogen-doped reduced graphene oxide/multi-walled carbon nanotube composite foams were prepared through a two-step strategy of hydrothermal self-assembly and subsequent high-temperature calcination. Morphology analysis indicated that the 3D networks were composed of overlapped flaky reduced graphene oxide. In addition, the influences of nitrogen doping, calcination temperature, and filler ratios on microwave absorption of composite foams were explored. Results manifested that the microwave absorption of composite foams was remarkably improved with the calcination temperature increased. Dramatically, it was noteworthy that the composite foam obtained under 600 °C calcination (bulk density of ∼10.8 mg/cm³) with an 8 wt % mass filler ratio presented the strongest microwave absorption of -69.6 dB at 12.5 GHz and broadest absorption bandwidth achieved 4.3 GHz (13.2-17.5 GHz) at an extremely low matching thickness equal to 1.5 mm. Moreover, the microwave absorption performance could be conveniently adjusted through modifying the thicknesses, filler ratios, and calcination temperature. The excellent microwave absorption performance of as-prepared composite foams was greatly derived from a well-constructed 3D network structure, significant nitrogen doping, enhanced polarization relaxation, and improved conduction loss. This work proposed a new strategy for fabricating graphene-based composites with a 3D network structure as high-efficiency microwave absorbers.

Controlling the electric permittivity of honeycomb-like core–shell Ni/CuSiO3 composite nanospheres to enhance microwave absorption properties

Controlling impedance matching and enhancing absorption properties are crucial for developing next-generation microwave absorbing materials.

Ultra-High Electromagnetic Absorption Property of One-Dimensional Carbon-Supported Ni/Mo2C and Polyvinylidene Fluoride

A novel one-dimensional carbon-supported Ni/Mo₂C (Ni/Mo₂C-C) nanocomposite with excellent electromagnetic wave absorption properties was successfully synthesized by annealing NiMoO₄@PDA directly, and then the (Ni/Mo₂C-C)/polyvinylidene fluoride (PVDF) composites were fabricated using a simple blending and hot-molding technique. An excellent reflection loss (RL) of −55.91 dB at 9.28 GHz with a low filler loading (15 wt%) and effective bandwidth (RL < −10 dB) of 14.12 GHz in the thickness range of 1.5–5.0 mm was obtained. Dielectric loss is considered to be the dominant mechanism of (Ni/Mo₂C-C)/PVDF, which was confirmed by the Debye relaxation process and attenuation theory.

Stabilization and Mineralization Mechanism of Cd with Cu-Loaded Attapulgite Stabilizer Assisted with Microwave Irradiation

Cadmium (Cd) in soil was stabilized using copper loaded attapulgite (Cu/ATP) in a microwave (MW) system. Excellent Cd stability in soil was achieved with Cu/ATP addition due to higher adsorption energy (1.38 eV) of Cu/ATP for Cd than that of ATP (∼1 eV), confirmed by density functional theory calculations. The strong hybridization of the s, p-orbitals of Cd with the s, p, d-orbitals of Cu on ATP contributed to the strong interactions between Cd and Cu/ATP. The stability performance of Cd in Cu/ATP-treated soil was further enhanced after MW irradiation through a series of phase transformation to more stable Cd-bearing crystalline minerals. The transformation was initiated by MW-induced "hot spots", which created cationic vacancy on Cu/ATP surface and enhanced the solid-state reactions between Cd and Cu/ATP framework. The total bond orders of Cd in the formed CdAl₄O₇ crystalline mineral elevated to 3.38, which was 5-fold higher than that for Cd on Cu/ATP, ensuring the long-term stability of Cd even after 360 curing days. Cd contaminated soil from a former industrial electroplating site was successfully stabilized with the proposed strategy. The research provides an effective stabilization strategy as well as a comprehensive understanding of the mechanism of long-term Cd stabilization.

Metal-organic framework derived cobalt phosphosulfide with ultrahigh microwave absorption properties

Nanostructure composites of ferromagnetic materials embedded in nanoporous carbon (NC) derived from metal-organic frameworks (MOFs) have attracted enormous attention due to their potential application in many fields, such as microwave absorption, energy storage, and conversion. The rational design of nanocomposites holds a determinant factor for overcoming the challenges involving the microwave absorption performance. Herein, CoS₂/NC, CoP/NC, and CoS_2-xP_x/NC with a rhombic dodecahedral structure have been successfully fabricated by using the template cobalt-based MOFs (ZIF-67). A morphology analysis indicates that ferromagnetic nanoparticles are embedded in NC matrix. It is obvious that the rhombic dodecahedron can be maintained after the phosphorization and sulfurization of Co/NC derived from the thermal decomposition of ZIF-67. The microwave absorption performance can obviously be improved by the phosphorization and sulfurization of Co/NC. CoS_2-xP_x/NC exhibits an excellent microwave absorption property and the minimum reflection loss (RL) of CoS_2-xP_x/NC can reach -68 dB at 14.6 GHz with a thickness of 1.5 mm. An RL value less than -10 dB can be achieved in the microwave frequency range of 12.7-17.3 GHz (4.6 GHz) with a thickness of 1.5 mm for CoS_2-xP_x/NC. This article offers a novel way to fabricate cobalt-based materials/carbon composites for an excellent microwave absorber.

Hollow N-Doped Carbon Polyhedron Containing CoNi Alloy Nanoparticles Embedded within Few-Layer N-Doped Graphene as High-Performance Electromagnetic Wave Absorbing Material

Magnetic metal nanostructures have exhibited good electromagnetic wave (EMW) absorption properties. However, the surface of the nanostructures is easily oxidized upon exposure to air, leading to the bad stability of the EMW absorption properties. We use metal-organic framework structure as a template to fabricate hollow N-doped carbon polyhedron containing CoNi alloy nanoparticles embedded within N-doped graphene (CoNi@NG-NCPs). The atomic ratio of Co/Ni can be tuned from 1:0.54 to 1:0.91 in the hollow CoNi@NG-NCPs. Experimental results demonstrate that the EMW absorption properties of the CoNi@NG-NCPs can be improved through the Ni introduction and increased with an increase of the Ni content. Typically, the minimal reflection loss of the optimal CoNi@NG-NCP can reach -24.03 dB and the effective absorption bandwidth (reflection loss below -10 dB) is as large as 4.32 GHz at the thickness of 2.5 mm. Furthermore, our CoNi@NG-NCPs exhibit favorably comparable or superior EMW absorption properties to other magnetic absorbers. In addition, because the CoNi alloy nanoparticles are coated with N-doped graphene layers, their surface oxidation behavior can be efficiently limited. The mechanism of the enhanced EMW absorption property is relevant to the enhanced dielectric loss and better impedance matching characteristic caused by the Ni incorporation.

Fe3O4 nanoparticles decorated on a CuS platelet-based sphere: a popcorn chicken-like heterostructure as an ideal material against electromagnetic pollution

A popcorn chicken-like CuS/Fe₃O₄ heterostructure was fabricated via the solvothermal deposition method to investigate its potential as a high electromagnetic wave absorber.

Flexible, Ultrathin, and High-Efficiency Electromagnetic Shielding Properties of Poly(Vinylidene Fluoride)/Carbon Composite Films

In this study, we fabricated conductive poly(vinylidene fluoride) (PVDF)/carbon composites simply by dispersing multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets into a PVDF solution. The electrical conductivity and the electromagnetic interference (EMI) shielding of the PVDF/carbon composites were increased by increasing the conductive carbon filler amounts. Moreover, we also found that the EMI shielding properties of the PVDF/CNT/graphene composites were higher than those of PVDF/CNT and PVDF/graphene composites. The mean EMI shielding values of PVDF/5 wt %-CNT, PVDF/10 wt %-graphene, and PVDF/CNT/graphene composite films with a thickness of 0.1 mm were 22.41, 18.70, and 27.58 dB, respectively. An analysis of the shielding mechanism showed that the main contribution to the EMI shielding came from the absorption mechanism, and that the EMI shielding could be tuned by controlling the films' thickness. The total shielding of the PVDF/CNT/graphene films increased from 21.90 to 36.46 dB as the thickness was increased from 0.06 mm to 0.25 mm. In particular, the PVDF/carbon composite films, with a thickness of 0.1 mm, achieved the highest specific shielding values of 1 310 dB cm²/g for the PVDF/5 wt %-CNT composite and 1 557 dB cm²/g for the PVDF/CNT/graphene composite, respectively. This was due to the ultrathin thickness. Our study provides the groundwork for an effective way to design flexible, ultrathin conductive polymer composite film for application in miniaturized electronic devices.

A permittivity regulating strategy to achieve high-performance electromagnetic wave absorbers with compatibility of impedance matching and energy conservation

With the variation of permittivity, MoS₂/rGO composites with a broad frequency bandwidth and strong absorbing performance can be obtained due to their optimal compatibility of impedance matching and attenuation loss ability.

Hierarchical porous Ni@boehmite/nickel aluminum oxide flakes with enhanced microwave absorption ability

Hierarchical core–shell composites with porous cores and flaky shells show superior absorption which possess many features, such as lightweight, wide band, small thickness and high efficiency absorption.

Facile design of a ZnO nanorod–Ni core–shell composite with dual peaks to tune its microwave absorption properties

The Ni/ZnO composites show two wide attractive absorption bands, which could broaden the effective absorption bandwidth.

1D Cu@Ni nanorods anchored on 2D reduced graphene oxide with interfacial engineering to enhance microwave absorption properties

In this work, one-dimensional core–shell Cu@Ni nanorods which were anchored on two dimensional reduced graphene oxide (rGO) heterostructures were successfully prepared by a simple co-reduction method.

Lightweight porous Co3O4 and Co/CoO nanofibers with tunable impedance match and configuration-dependent microwave absorption properties

In this study, we fabricated one-dimensional porous Co₃O₄ and Co/CoO nanofibers by calcination of cobalt(ii) oxalate dehydrate precursors in an environment filled with air and N₂, respectively.

A facile one-pot strategy for fabrication of carbon-based microwave absorbers: effects on annealing and paraffin content

The carbon-coated Fe₃O₄ and Fe₃C were synthesized at different temperature. The composites obtained at 700 °C possess higher reflection loss and broader effective bandwidth.

Tunable high-performance microwave absorption for manganese dioxides by one-step Co doping modification

The frequencies of microwave absorption can be affected by the permanent electric dipole moment which could be adjusted by modifying the crystal symmetry of the microwave absorbing materials. Herein, we corroborate this strategy experimentally and computationally to the microwave absorption of manganese dioxides. Nanosized Co-doped cryptomelane (Co-Cryp) was successfully synthesized by a one-step reaction. The introduction of Co(III) induced a change of crystal symmetry from tetragonal to monlclinic, which could lead to an increase of its permanent electric dipole moment. As a result, the frequencies of maximum microwave absorption were regulated in the range of 7.4 to 13.9 GHz with a broadened bandwidths. The results suggested that microwave absorption of manganese dioxides can be tailored with Co doping to expand their potential uses for abatement of various microwave pollutions.

Yolk-Shell Ni@SnO2 Composites with a Designable Interspace To Improve the Electromagnetic Wave Absorption Properties

In this study, yolk-shell Ni@SnO₂ composites with a designable interspace were successfully prepared by the simple acid etching hydrothermal method. The Ni@void@SnO₂ composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results indicate that interspaces exist between the Ni cores and SnO₂ shells. Moreover, the void can be adjusted by controlling the hydrothermal reaction time. The unique yolk-shell Ni@void@SnO₂ composites show outstanding electromagnetic wave absorption properties. A minimum reflection loss (RL_min) of -50.2 dB was obtained at 17.4 GHz with absorber thickness of 1.5 mm. In addition, considering the absorber thickness, minimal reflection loss, and effective bandwidth, a novel method to judge the effective microwave absorption properties is proposed. On the basis of this method, the best microwave absorption properties were obtained with a 1.7 mm thick absorber layer (RL_min= -29.7 dB, bandwidth of 4.8 GHz). The outstanding electromagnetic wave absorption properties stem from the unique yolk-shell structure. These yolk-shell structures can tune the dielectric properties of the Ni@air@SnO₂ composite to achieve good impedance matching. Moreover, the designable interspace can induce interfacial polarization, multiple reflections, and microwave plasma.

Controllable adjustment of the crystal symmetry of K–MnO2 and its influence on the frequency of microwave absorption

Microwave (MW) is becoming an important polluter.

Design of porous C@Fe3O4hybrid nanotubes with excellent microwave absorption

Porous hybridizing nanotubes with a structure of amorphous carbon interspersed with Fe₃O₄were fabricated and demonstrate excellent microwave absorption.

Novel nanoporous carbon derived from metal–organic frameworks with tunable electromagnetic wave absorption capabilities

Core–shell ZIF-8@ZIF-67 crystals, which integrate the properties of single ZIF-8 and ZIF-67, are elaborately designed for the first time by applying a seed-mediated growth technique.

A MWCNT–nanoparticle composite as a highly efficient lightweight electromagnetic wave absorber in the range of 4–18 GHz

The nanocomposite has an excellent EM wave attenuation ability with a wide absorption band (4–18 GHz), strong absorption (−52.4 dB) and low absorber thickness (strong absorption from 4 to 18 GHz at thicknesses between 1.1 and 3.7 mm).

Iron cobalt/polypyrrole nanoplates with tunable broadband electromagnetic wave absorption

The iron cobalt/polypyrrole nanocomposites show an excellent tunable electromagnetic performance due to the synergetic effect between the dielectric and magnetic losses of the nanocomposites.

Synthesis of Cu and Ni chalcogenides and evaluation of their properties for electromagnetic wave absorption

Cu and Ni chalcogenides were synthesized by hydrothermal methods and characterized by XRD and SEM.

Preparation of Honeycomb SnO₂ Foams and Configuration-Dependent Microwave Absorption Features

Ordered honeycomb-like SnO2 foams were successfully synthesized by means of a template method. The honeycomb SnO2 foams were analyzed by X-ray diffraction (XRD), thermogravimetric and differential scanning calorimetry (TG-DSC), laser Raman spectra, scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR). It can be found that the SnO2 foam configurations were determined by the size of polystyrene templates. The electromagnetic properties of ordered SnO2 foams were also investigated by a network analyzer. The results reveal that the microwave absorption properties of SnO2 foams were dependent on their configuration. The microwave absorption capabilities of SnO2 foams were increased by increasing the size of pores in the foam configuration. Furthermore, the electromagnetic wave absorption was also correlated with the pore contents in SnO2 foams. The large and high amounts pores can bring about more interfacial polarization and corresponding relaxation. Thus, the perfect ordered honeycomb-like SnO2 foams obtained in the existence of large amounts of 322 nm polystyrene spheres showed the outstanding electromagnetic wave absorption properties. The minimal reflection loss (RL) is -37.6 dB at 17.1 GHz, and RL less than -10 dB reaches 5.6 GHz (12.4-18.0 GHz) with thin thickness of 2.0 mm. The bandwidth (<-10 dB, 90% microwave dissipation) can be monitored in the frequency regime of 4.0-18.0 GHz with absorber thickness of 2.0-5.0 mm. The results indicate that these ordered honeycomb SnO2 foams show the superiorities of wide-band, high-efficiency absorption, multiple reflection and scatting, high antioxidation, lightweight, and thin thickness.

Facile Synthesis of Novel Heterostructure Based on SnO2 Nanorods Grown on Submicron Ni Walnut with Tunable Electromagnetic Wave Absorption Capabilities

In this work, the magnetic-dielectric core-shell heterostructure composites with the core of Ni submicron spheres and the shell of SnO2 nanorods were prepared by a facile two-step route. The crystal structure and morphology were investigated by X-ray diffraction analysis, transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). FESEM and TEM measurements present that SnO2 nanorods were perpendicularly grown on the surfaces of Ni spheres and the density of the SnO2 nanorods could be tuned by simply varying the addition amount of Sn(2+) in this process. The morphology of Ni/SnO2 composites were also determined by the concentration of hydrochloric acid and a plausible formation mechanism of SnO2 nanorods-coated Ni spheres was proposed based on hydrochloric acid concentration dependent experiments. Ni/SnO2 composites exhibit better thermal stability than pristine Ni spheres based on thermalgravimetric analysis (TGA). The measurement on the electromagnetic (EM) parameters indicates that SnO2 nanorods can improve the impedance matching condition, which is beneficial for the improvement of electromagnetic wave absorption. When the coverage density of SnO2 nanorod is in an optimum state (diameter of 10 nm and length of about 40-50 nm), the optimal reflection loss (RL) of electromagnetic wave is -45.0 dB at 13.9 GHz and the effective bandwidth (RL below -10 dB) could reach to 3.8 GHz (12.3-16.1 GHz) with the absorber thickness of only 1.8 mm. By changing the loading density of SnO2 nanorods, the best microwave absorption state could be tuned at 1-18 GHz band. These results pave an efficient way for designing new types of high-performance electromagnetic wave absorbing materials.

Morphology-Control Synthesis of a Core-Shell Structured NiCu Alloy with Tunable Electromagnetic-Wave Absorption Capabilities

In this work, dendritelike and rodlike NiCu alloys were prepared by a one-pot hydrothermal process at various reaction temperatures (120, 140, and 160 °C). The structure and morphology were analyzed by scanning electron microscopy, energy-dispersive spectrometry, X-ray diffraction, and transmission electron microscopy, which that demonstrate NiCu alloys have core-shell heterostructures with Ni as the shell and Cu as the core. The formation mechanism of the core-shell structures was also discussed. The uniform and perfect dendritelike NiCu alloy obtained at 140 °C shows outstanding electromagnetic-wave absorption properties. The lowest reflection loss (RL) of -31.13 dB was observed at 14.3 GHz, and the effective absorption (below -10 dB, 90% attenuation) bandwidth can be adjusted between 4.4 and 18 GHz with a thin absorber thickness in the range of 1.2-4.0 mm. The outstanding electromagnetic-wave-absorbing properties are ascribed to space-charge polarization arising from the heterogeneous structure of the NiCu alloy, interfacial polarization between the alloy and paraffin, and continuous micronetworks and vibrating microcurrent dissipation originating from the uniform and perfect dendritelike shape of NiCu prepared at 140 °C.

Preparation and electromagnetic wave absorption properties of novel dendrite-like NiCu alloy composite

The dendrite-like NiCu alloys show outstanding microwave absorption properties with the features of thin-thickness, strong absorption and wide-band.

Facile synthesis of a CNT@Fe@SiO2 ternary composite with enhanced microwave absorption performance

The CNTs@Fe@SiO₂ ternary core-structure was prepared by a simple two-step approach consisting a pyrolysis process and then decomposition. At a thin thickness of 1.5 mm, the optimal reflection loss value of the ternary composite is as high as −14.2 dB.