A review on composite strategy of MOF derivatives for improving electromagnetic wave absorption

To address the electromagnetic wave (EMW) pollution issues caused by the development of electronics and wireless communication technology, it is urgent to develop efficient EMW-absorbing materials. With controllable composition, diverse structure, high porosity, and large specific surface area, metal-organic framework (MOF) derivatives have sparked the infinite passion and creativity of researchers in the electromagnetic field. Against the challenges of poor inherent impedance matching and insufficient attenuation capability of pure MOF derivative, designing and developing MOF derivative-based composites by compounding MOF with other materials, such as graphene, CNTs, MXene, and so on, has been an effective strategy for constructing high-efficiency EMW absorbing materials. This review systematically expounds the research progress of MOF derivative-based composite strategies, and discusses the challenges and opportunities faced by MOF derivatives in the field of EMW absorption. This work can provide some good ideas for researchers to design and prepare high-efficiency MOF-based EMW absorbing materials in applications of next-generation electronics and aerospace.

Facile synthesis of Fe3O4 nanoparticles/reduced graphene oxide sandwich composites for highly efficient microwave absorption

Component regulation and microstructure design are two effective strategies to adjust electromagnetic parameters and improve the microwave absorption performance of materials. In this study, a facile synthesis strategy consisting of ultrasonic dispersion, blast drying, and roasting is proposed to build a sandwich-like graphene-based absorbent, in which Fe₃O₄ nanoparticles with adjustable content are sandwiched uniformly between reduced graphene oxide nanosheets. The sandwich structure can form multiple interfaces, prevent the aggregation of nanoparticles, facilitate interface polarization, and endow the material with multiple electromagnetic loss mechanisms, which is very beneficial for impedance matching and microwave attenuation. Notably, the effective absorption bandwidth achieves 5.7 GHz, and the minimum reflection loss value is -49.9 dB. In addition, the synthesis process is simple and suitable for large-scale production and possible industrial applications. Thus, this facile route to fabricate sandwich-like graphene-based absorbents provides new ideas and approaches for designing new graphene-based nanocomposites.

One Pot Self-Assembling Fe@PANI Core-Shell Nanowires for Radar Absorption Application

The one-pot process, which combines the polymerization of polyaniline (i.e., PANI) with subsequent reduction of iron nanowire (i.e., Fe NW) under a magnetic field, was developed to produce Fe@PANI core-shell nanowires. The synthesized nanowires with various PANI additions (0-30 wt.%) were characterized and used as microwave absorbers. Epoxy composites with 10 wt.% absorbers were prepared and examined using the coaxial method to reveal their microwave absorbing performance. Experimental results showed that the Fe NWs with PANI additions (0-30 wt.%) had average diameters ranging from 124.72 to 309.73 nm. As PANI addition increases, the α-Fe phase content and the grain size decrease, while the specific surface area increases. The nanowire-added composites exhibited superior microwave absorption performance with wide effective absorption bandwidths. Among them, Fe@PANI-90/10 exhibits the best overall microwave absorption performance. With a thickness of 2.3 mm, effective absorption bandwidth was the widest and reached 3.73 GHz, ranging from 9.73 to 13.46 GHz. Whereas with a thickness of 5.4 mm, Fe@PANI-90/10 reached the best reflection loss of -31.87 dB at 4.53 GHz.

Facile Synthesis of Metal Oxide Decorated Carbonized Bamboo Fibers with Wideband Microwave Absorption

Aiming at the disadvantages of high cost, complex processes, low yield, and narrow bandwidth of carbon-based microwave absorbing materials, this paper provides a novel and efficient method for synthesizing metal oxide/carbonized bamboo fibers using renewable natural bamboo fibers as a carbon source. The results suggested that the metal oxides such as NiO and Fe₃O₄ were uniformly dispersed on the carbonized bamboo fibers and proved that the dielectric component NiO and magnetic component Fe₃O₄ can significantly improve the microwave absorption performance of the carbonized bamboo fibers. As expected, the NiO/carbonized bamboo fibers showed excellent microwave absorption performance due to the appropriate complex permittivity, high impedance matching, and attenuation coefficient. A wide effective bandwidth of 6.4 GHz with 2.2 mm thickness is achieved, covering the entire Ku-band. Remarkably, the reflection loss (RL) values less than -10 dB covered the whole X-band at a thickness of 3.0 mm. This work reveals the potential of carbonized bamboo fibers-based composite as an economic and broadband microwave absorbent and offers a new strategy for designing promising microwave absorption materials.

Novel Hybrid Nanomaterials Based on Poly-N-Phenylanthranilic Acid and Magnetic Nanoparticles with Enhanced Saturation Magnetization

A one-step preparation method for cobalt- and iron-containing nanomaterials based on poly-N-phenylanthranilic acid (P-N-PAA) and magnetic nanoparticles (MNP) was developed for the first time. To synthesize the MNP/P-N-PAA nanocomposites, the precursor is obtained by dissolving a Co (II) salt in a magnetic fluid based on Fe3O4/P-N-PAA with a core-shell structure. During IR heating of the precursor in an inert atmosphere at T = 700−800 °C, cobalt interacts with Fe3O4 reduction products, which results in the formation of a mixture of spherical Co-Fe, γ-Fe, β-Co and Fe3C nanoparticles of various sizes in the ranges of 20 < d < 50 nm and 120 < d < 400 nm. The phase composition of the MNP/P-N-PAA nanocomposites depends significantly on the cobalt concentration. The reduction of metals occurs due to the hydrogen released during the dehydrogenation of phenylenamine units of the polymer chain. The introduction of 10−30 wt% cobalt in the composition of nanocomposites leads to a significant increase in the saturation magnetization of MNP/P-N-PAA (MS = 81.58−149.67 emu/g) compared to neat Fe3O4/P-N-PAA (MS = 18.41−27.58 emu/g). The squareness constant of the hysteresis loop is κS = MR/MS = 0.040−0.209. The electrical conductivity of the MNP/P-N-PAA nanomaterials does not depend much on frequency and reaches 1.2 × 10−1 S/cm. In the argon flow at 1000 °C, the residue is 77−88%.

Rational design of two-dimensional flaky Fe/void/C composites for enhanced microwave absorption properties

Owing to their unique electromagnetic properties and structure anisotropy, two-dimensional (2D) magnetic metal flakes are attracting special attention for applications as microwave absorption materials. However, the conductive network formed by the connected metal flakes may lead to impedance mismatching and reduced performance. In this study, a facile and rational strategy was developed to fabricate yolk-shell-structured 2D flaky Fe/void/C composites by using α-Fe₂O₃ hexagonal flakes as the template, followed by the coating of polydopamine (PDA) on the composite surface and calcination under H₂/Ar. The volume shrinkage from Fe₂O₃ to Fe and PDA to carbon led to the formation of several irregular holes in Fe flakes and void space between the Fe cores and carbon cages. The thickness of carbon cages of the composites can be tailored by the simple modulation of the synthetic parameters. As a result of the synergistic effects of multiple chemical components, the shape anisotropy of iron flakes, and unique yolk-shell structures, the optimized sample exhibited excellent microwave absorption properties. With a matching thickness of only 1.6 mm, the strongest reflection loss (RL) was up to -27.80 dB at 14.72 GHz, and the effective absorption bandwidth (EAB, RL < -10 dB) reached 6.40 GHz (11.60-18.00 GHz), which can cover the whole Ku-band. This study provides a novel approach to adjust and balance the permeability and permittivity of 2D magnetic metal flakes, which may promote the practical applications of flaky magnetic metal materials in microwave absorption.

Improved methanogenesis in anaerobic wastewater treatment by magnetite@polyaniline (Fe3O4@PANI) composites

The magnetite@polyaniline (Fe₃O₄@PANI) composites with different Fe₃O₄ loadings were prepared, and their effect on methane production in anaerobic systems was investigated. The Fe₃O₄@PANI composite with a 40% loading of Fe₃O₄ showed a better performance on accelerating methane production rate than other composites. The methane production rate was increased by 26.98% at the Fe₃O₄@PANI dosage of 0.6 g L^-1. The results of the contact angle and CLSM revealed that Fe₃O₄@PANI had a good bio-affinity and contact directly with bacteria and archaea. Then the mechanisms related to the enhancement of methane production by the composites were explored by the species annotation and enzyme activity. It showed that Fe₃O₄@PANI promoted the enrichment of DIET-related functional bacteria and archaea and improved the enzyme activity related to the acetoclastic methanogenic pathway.

New Inverse Emulsion-Polymerized Iron/Polyaniline Composites for Permanent, Highly Magnetic Iron Compounds via Calcination

The hydrophilic initiator potassium persulfate (KPS) was converted into a hydrophobic molecule by complexing with cetyltrimethylammonium bromide (CTAB) at both ends of the molecule (CTAPSu). Inverse emulsion polymerization thus proceeded inside micelles dispersed in the affluent toluene with CTAPSu as the initiator. Polyaniline (PANI) formed inside the micelles and entangled with Fe₃O₄ nanoparticles already esterified with oleic acid (OA). Iron composites consisted of OA-esterified Fe₃O₄ nanoparticles covered with PANI after de-emulsification. After calcination at 950 °C in an argon atmosphere, the resultant iron compound was a mixture of α-Fe (ferrite) and Fe₃C (cementite), as determined by X-ray diffraction. Eventually, the calcined iron compounds (mixtures) demonstrated superparamagnetic properties with a high saturation magnetization (Ms) of 197 emu/g, which decayed to 160 emu/g after exposure to the atmosphere for four months.

Ag2CO3 containing magnetic nanocomposite as a powerful and recoverable catalyst for Knoevenagel condensation

In this paper, the synthesis, characterization and catalytic application of a novel magnetic silica-supported Ag2CO3 (MS/Ag2CO3) with core-shell structure are developed. The MS/Ag2CO3 nanocomposite was prepared through chemical modification of magnetic MS nanoparticles with AgNO3 under alkaline conditions. The structure, chemical composition and magnetic properties of MS/Ag2CO3 were investigated by using VSM, PXRD, FT-IR, EDX and SEM techniques. The MS/Ag2CO3 nanocomposite was used as an effective catalyst for the Knoevenagel condensation under solvent-free conditions at 60 °C in an ultrasonic bath. The recovery and leaching tests were performed to study the nature of the MS/Ag2CO3 catalyst under applied conditions.

Superparamagnetic, High Magnetic α-Fe & α″-Fe16N2 Mixture Prepared from Inverse Suspension-Polymerized Fe3O4@polyaniline Composite

Oleic acid (OA)-modified Fe₃O₄ nanoparticles were successfully covered with polyanilines (PANIs) via inverse suspension polymerization in accordance with SEM and TEM micrographs. The obtained nanoparticles were able to develop into a ferrite (α-Fe) and α″-Fe₁₆N₂ mixture with a superparamagnetic property and high saturated magnetization (SM) of 245 emu g⁻¹ at 950 °C calcination under the protection of carbonization materials (calcined PANI) and other iron-compounds (α″-Fe₁₆N₂). The SM of the calcined iron-composites slightly decreases to 232 emu g⁻¹ after staying in the open air for 3 months. The calcined mixture composite can be ground into homogeneous powders without the segregation of the iron and carbon phases in the mortar without significantly losing magnetic activities.

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.

Multi-scale magnetic coupling of Fe@SiO2@C-Ni yolk@triple-shell microspheres for broadband microwave absorption

Magnetic core@shell and yolk@shell microspheres have received extensive attention; however, the realization of microwave performance enhancement remains a critical challenge for their actual applications. Herein, inspired by multi-scale magnetic coupling interactions, a synchronous in situ reduction process was developed to successfully fabricate Fe@SiO₂@C-Ni (FSCN) yolk@triple-shell microspheres based on the Fe₃O₄@SiO₂ substrate. Owing to the unique multi-scale magnetic coupling interactions in their delicate structure, (i) in each microsphere, abundant Ni NPs with optimized size could affect the density distribution and orientation of the magnetic stray field radiating from the Fe core, and (ii) via the coupling interactions between adjacent composite microspheres, the saturation magnetization was significantly enhanced to support strong magnetic loss capability. Moreover, the special yolk@multi-shell structure offered an optimized impedance balance, facilitating the propoagation of the incoming microwaves into the absorber. Both multiple interfacial polarization and synergistic effects from magnetic units (Fe and Ni) and dielectric shell (SiO₂ and carbon) contributed to electromagnetic wave attenuation. The FSCN composite material exhibited excellent absorption performance with an intense reflection loss (-45.5 dB) and bandwidth absorption (8.2 GHz and 9.8-18 GHz) at the film thickness of only 2 mm. Our new findings provide important design implications for functional spheres and high-performance lightweight microwave absorbers.

Review of electromagnetic interference shielding materials fabricated by iron ingredients

Iron (Fe) and its counterparts, such as Fe2O3, Fe3O4, carbonyl iron and FeO, have attracted the attention of researchers during the past few years due to their bio-compatibility, bio-degradability and diverse applications in the field of medicines, electronics and energy; including water treatment, catalysis and electromagnetic wave interference shielding etc. In this review paper, we aimed to explore iron based materials for the prevention of electromagnetic interference (EMI) by means of both reflection and absorption processes, including the standard methods of synthesis of Fe-based materials along with the determination of EMI performance. It is customary that a proper combination of two dielectric-losses, i.e. electrical and magnetic losses, give excellent microwave absorption properties. Therefore, we focused on the different strategies of preparation of these iron based composites with dielectric carbon materials, polymers etc. Additionally, we explained their positive and negative aspects.

Interface Modulation of Core-Shell Structured BaTiO₃@polyaniline for Novel Dielectric Materials from Its Nanocomposite with Polyarylene Ether Nitrile

The core-shell structured polyaniline-functionalized-BaTiO₃ (BT@PANI) nanoparticles with controllable shell layer thicknesses are developed via in-situ aniline polymerization technology and characterized in detail. The results prove that the PANI shell layer with the adjustable and controllable thicknesses of 3⁻10 nm are completely stabilized on the surface of the BaTiO₃ core. In addition, the BT@PANI nanoparticles are regarded as the hybrid nanofillers to prepare PEN/BT@PANI nanocomposite films with a PEN matrix. The research results indicate that the surface functionalized nanoparticles facilitate the compatibility and dispersibility of them in the PEN matrix, which improves the properties of the PEN/BT@PANI nanocomposites. Specifically, the PEN/BT@PANI nanocomposites exhibit thermal stability, excellent permittivity-frequency, and dielectric properties-temperature stability. Most importantly, the energy density of nanocomposites is maintained at over 70% at 180 °C compared with that at 25 °C. All these results reveal that a new way to prepare the high-performance PEN-based nanocomposites is established to fabricate an energy storage component in a high temperature environment.

Synthesis and Characterization of BaTiO₃/Polypyrrole Composites with Exceptional Dielectric Behaviour

Higher concentrations of ceramic fillers induce brittleness in the ceramic/polymer hybrids which restrict their applications to limited fields especially when such hybrids are prepared for their use as dielectrics. We have synthesized and characterized different BaTiO₃-polypyrrole (PPy) composites by changing the concentration of BaTiO₃ from 1% by weight of PPy taken to 5 wt % to explore its effect on the dielectric parameters of the final product and found that the BaTiO₃-polypyrrole composite with weight ratio of 0.05:1 exhibited highest dielectric constant, lowest dielectric loss and thermally most stable. All the composites were prepared using in-situ polymerization of pyrrole in an aqueous dispersion of low content of BaTiO₃ in the presence of small amount of Hydrochloric acid. These composites were characterized for their microstructure and crystallinity by X-ray diffractometer (XRD), Fourier transform infrared (FT-IR) spectroscopy and scanning electron microscopy (SEM) while thermal stability by thermo gravimetric (TGA) analysis. An impedance analyser (LCR meter) was utilized to investigate the dielectric parameters. FT-IR data confirmed the presence of the two phases and their interaction, inferred from the shifting of normal PPy peaks. The data obtained from XRD confirmed the presence of crystallites of 2.8 to 5 nm with dominant crystallinity of the filler, TGA analysis (25 to 600 °C) confirmed the higher thermal stability induced on successive addition of the filler into the prepared composites as compared to that of pure PPy in a wide temperature range which is unusual for such a low % age addition of the filler. The SEM analysis together with XRD results reveal that the successive introduction of BaTiO₃ particles produced crystallites of 2 to 5 nm size which bonded together and changed the hemispherical shaped larger grains of the matrix to regular shaped smaller grains. The dielectric constant of the composites was enhanced with filler contents from 178 to 522 at 1 MHz for 1 wt % and 5 wt % BaTiO₃ respectively. It was concluded that the introduction of BaTiO₃ into the polymer matrix with this new procedure has greatly affected the polymerization process, thermal stability, morphology and dielectric properties of the host matrix and has resulted in a novel series of the composites which may have broad applications.

Low-Cost Carbothermal Reduction Preparation of Monodisperse Fe3O4/C Core-Shell Nanosheets for Improved Microwave Absorption

This paper demonstrates a facile and low-cost carbothermal reduction preparation of monodisperse Fe₃O₄/C core-shell nanosheets (NSs) for greatly improved microwave absorption. In this protocol, the redox reaction between sheet-like hematite (α-Fe₂O₃) precursors and acetone under inert atmosphere and elevated temperature generates Fe₃O₄/C core-shell NSs with the morphology inheriting from α-Fe₂O₃. Thus, Fe₃O₄/C core-shell NSs of different sizes ( a) and Fe₃O₄/C core-shell nanopolyhedrons are obtained by using different precursors. Benefited from the high crystallinity of the Fe₃O₄ core and the thin carbon layer, the resultant NSs exhibit high specific saturation magnetization larger than 82.51 emu·g^-1. Simultaneously, the coercivity enhances with the increase of a, suggesting a strong shape anisotropy effect. Furthermore, because of the anisotropy structure and the complementary behavior between Fe₃O₄ and C, the as-obtained Fe₃O₄/C core-shell NSs exhibit strong natural magnetic resonance at a high frequency, enhanced interfacial polarization, and improved impedance matching, ensuring the enhancement of the microwave absorption. The 250 nm NSs-paraffin composites exhibit reflection loss (RL) lower than -20 dB (corresponding to 99% absorption) in a large frequency ( f) range of 2.08-16.40 GHz with a minimum RL of -43.95 dB at f = 3.92 GHz when the thickness is tuned from 7.0 to 1.4 mm, indicating that the Fe₃O₄/C core-shell NSs are a good candidate to manufacture high-performance microwave absorbers. Moreover, the as-developed carbothermal reduction method could be applied for the fabrication of other composites based on ferrites and carbon.

Selective removal of toxic Cr(VI) from aqueous solution by adsorption combined with reduction at a magnetic nanocomposite surface

The adsorption of toxic hexavalent chromium (Cr(VI)) and its reduction to trivalent chromium (Cr(III)) are important processes for the treatment of industrial wastewater. Conducting polymers can adsorb and reduce Cr(VI) to less toxic Cr(III) but have low adsorption capacities due to agglomeration of particles and are difficult to separate from treated water. In this study, magnetic polypyrrole (PPy)-polyaniline (PANI)/iron oxide (Fe₃O₄) nanocomposite was synthesized for the selective removal of Cr(VI) in aqueous solution. PPy-PANI/Fe₃O₄ nanocomposite was characterized using various techniques including ATR-FTIR, FE-SEM, HR-TEM, EDX, TGA, XRD, VSM and XPS analyses. PPy-PANI/Fe₃O₄ nanocomposite (0.05g) removed 99% of Cr(VI) from aqueous solution (100mg/L, pH 2). Speciation studies confirmed Cr(VI) adsorption and reduction to Cr(III) by the PPy-PANI/Fe₃O₄ nanocomposite in solutions with initial pH of 2 and 3 and that no Cr(VI) reduction occurred at pH values of 4 and above. The Langmuir maximum adsorption capacity for Cr(VI) removal by PPy-PANI/Fe₃O₄ nanocomposite at pH 2 was 303mg/g at 25°C. PPy-PANI/Fe₃O₄ nanocomposite was highly selective for Cr(VI) removal and could be used for three consecutive treatment cycles without loss of adsorption capacity. Moreover, the magnetic nanocomposite could be separated from the reaction fluid using an external magnet. PPy-PANI/Fe₃O₄ nanocomposite is therefore a promising magnetic adsorbent for the treatment of industrial wastewater.

Recent Advances in Conjugated Polymer-Based Microwave Absorbing Materials

Microwave absorbing materials (MAMs) are paving the way for exciting applications in electromagnetic (EM) pollution precaution and national defense security, as they offer an advanced alternative to conventional reflection principles to fundamentally eliminate the EM waves. Conjugated polymer (CP)-based composites appear as a promising kind of MAM with the desirable features of low density and high performance. In this review, we introduce the theory of microwave absorption and summarize recent advances in the fabrication of CP-based MAMs, including rational design of the microstructure of pure conjugated polymers and tunable chemical integration with magnetic ferrites, magnetic metals, transition metal oxides, and carbon materials. The key point of enhancing microwave absorption in CP-based MAMs is to regulate their EM properties, improve matching of characteristic impedance, and create diversified loss mechanisms. The examples presented in this review will provide new insights into the design and preparation of CP-based composites that can satisfy the high demands of the oncoming generation of MAMs.

Pulse electromagnetic fields enhance extracellular electron transfer in magnetic bioelectrochemical systems

BACKGROUND

Microbial extracellular electron transfer (EET) is essential in driving the microbial interspecies interaction and redox reactions in bioelectrochemical systems (BESs). Magnetite (Fe₃O₄) and magnetic fields (MFs) were recently reported to promote microbial EET, but the mechanisms of MFs stimulation of EET and current generation in BESs are not known. This study investigates the behavior of current generation and EET in a state-of-the-art pulse electromagnetic field (PEMF)-assisted magnetic BES (PEMF-MBES), which was equipped with magnetic carbon particle (Fe₃O₄@N-mC)-coated electrodes. Illumina Miseq sequencing of 16S rRNA gene amplicons was also conducted to reveal the changes of microbial communities and interactions on the anode in response to magnetic field.

RESULTS

PEMF had significant influences on current generation. When reactors were operated in microbial fuel cell (MFC) mode with pulse electromagnetic field (PEMF-MMFCs), power densities increased by 25.3-36.0% compared with no PEMF control MFCs (PEMF-OFF-MMFCs). More interestingly, when PEMF was removed, the power density dropped by 25.7%, while when PEMF was reintroduced, the value was restored to the previous level. Illumina sequencing of 16S rRNA gene amplicon and principal component analysis (PCA) based on operational taxonomic units (OTUs) indicate that PEMFs led to the shifts in microbial community and changes in species evenness that decreased biofilm microbial diversity. Geobacter spp. were found dominant in all anode biofilms, but the relative abundance in PEMF-MMFCs (86.1-90.0%) was higher than in PEMF-OFF-MMFCs (82.5-82.7%), indicating that the magnetic field enriched Geobacter on the anode. The current generation of Geobacter-inoculated microbial electrolysis cells (MECs) presented the same change regularity, the accordingly increase or decrease corresponding with switch of PEMF, which confirmed the reversible stimulation of PEMFs on microbial electron transfer.

CONCLUSION

The pulse electromagnetic field (PEMF) showed significant influence on state-of-the-art pulse magnetic bioelectrochemical systems (PEMF-MBES) in terms of current generation and microbial ecology. EET was instantaneously and reversibly enhanced in MBESs inoculated with either mixed-culture or Geobacter. PEMF notably decreased bacterial and archaeal diversities of the anode biofilms in MMFCs via changing species evenness rather than species richness, and facilitated specific enrichment of exoelectrogenic bacteria (Geobacter) on the anode surface. This study demonstrates a new magnetic approach for understanding and facilitating microbial electrochemical activities.

Decomposition of MOFs for the preparation of nanoporous Co3O4 fibres and sheets with excellent microwave absorption and photocatalytic properties

Unique nanoporous Co₃O₄ fibres and sheets were successfully fabricated via a facile hydrothermal route (150 °C) and subsequent annealing process at 500 °C in air.

Synthesis, Characterization, and Microwave Absorption Properties of Reduced Graphene Oxide/Strontium Ferrite/Polyaniline Nanocomposites

Strontium ferrite nanoparticles were prepared by a coprecipitation method, and reduced graphene oxide/strontium ferrite/polyaniline (R-GO/SF/PANI) ternary nanocomposites were prepared by in situ polymerization method. The morphology, structure, and magnetic properties of the ternary nanocomposites were investigated by X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), TEM, Raman, and VSM. The microwave-absorbing properties of the composites were measured by a vector network analyzer. The XRD patterns show the single phase of strontium hexaferrite without other intermediate phases. TEM photographs reveal that strontium ferrite nanoparticles are uniformly dispersed on the surfaces of R-GO sheets. The R-GO/SF/PANI nanocomposite exhibited the best absorption property with the optimum matching thickness of 1.5 mm in the frequency of 2-18 GHz. The value of the maximum RL was -45.00 dB at 16.08 GHz with the 5.48-GHz bandwidth. The excellent absorption properties of R-GO/SF/PANI nanocomposites indicated their great potential as microwave-absorbing materials.

Facile Hydrothermal Synthesis of Fe3O4/C Core-Shell Nanorings for Efficient Low-Frequency Microwave Absorption

Using elliptical iron glycolate nanosheets as precursors, elliptical Fe3O4/C core-shell nanorings (NRs) [25 ± 10 nm in wall thickness, 150 ± 40 nm in length, and 1.6 ± 0.3 in long/short axis ratio] are synthesized via a one-pot hydrothermal route. The surface-poly(vinylpyrrolidone) (PVP)-protected-glucose reduction/carbonization/Ostwald ripening mechanism is responsible for Fe3O4/C NR formation. Increasing the glucose/precursor molar ratio can enhance carbon contents, causing a linear decrease in saturation magnetization (Ms) and coercivity (Hc). The Fe3O4/C NRs reveal enhanced low-frequency microwave absorption because of improvements to their permittivity and impedance matching. A maximum RL value of -55.68 dB at 3.44 GHz is achieved by Fe3O4/C NRs with 11.95 wt % C content at a volume fraction of 17 vol %. Reflection loss (RL) values (≤-20 dB) are observed at 2.11-10.99 and 16.5-17.26 GHz. Our research provides insights into the microwave absorption mechanism of elliptical Fe3O4/C core-shell NRs. Findings indicate that ring-like and core-shell nanostructures are promising structures for devising new and effective microwave absorbers.

Excellent Electromagnetic Absorption Capability of Ni/Carbon Based Conductive and Magnetic Foams Synthesized via a Green One Pot Route

Electromagnetic microwave absorption materials have attracted a great deal of attention. Foams for the low density and tunable porosity are considered as ideal microwave absorbents, while with the requirement of improving their inherent electromagnetic properties. In this manuscript, an innovative, easy, and green method was presented to synthesize an electromagnetic functionalized Ni/carbon foam, in which the formation of Ni nanoparticles and carbon occurred simultaneously from an affordable alginate/Ni(2+) foam precursor. The resultant Ni/carbon foam had a low density (0.1 g/cm(-3)) and high Ni nanoparticles loading (42 wt %). These Ni nanoparticles with a diameter of about 50-100 nm were highly crystallized and evenly embedded in porous graphite carbon without aggregation. Also, the resultant foam had a high surface area (451 m(2) g(-1)) and porosity and showed a moderate conductivity (6 S/m) and significant magnetism. Due to these special characteristics, the Ni/carbon foam exhibited greatly enhanced microwave absorption ability. Only with 10 wt % of functional fillers being used in the test template, the Ni/carbon foam based composite could reach an effective absorption bandwidth (below -10 dB) of 4.5 GHz and the minimum reflection value of -45 dB at 13.3 GHz with a thickness of 2 mm, while the traditional carbon foam and nano-Ni powder both showed very weak microwave absorption (the minimum reflection value < -10 dB). This foam was demonstrated to be a lightweight, high performance, and low filler loading microwave absorbing material. Furthermore, the detailed absorption mechanism of the foam was investigated. The result showed that the derived strong dielectric loss, including conductivity loss, interface polarization loss, weak magnetic loss, and naoporosity, contributes a great electromagnetic absorption.

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 γ-Fe2O3@SiO2@polypyrrole core/shell/shell nanospheres with flexible controllability of electromagnetic properties

A possible formation mechanism for γ-Fe₂O₃@SiO₂@PPy core/shell/shell nanospheres with flexible controllability of electromagnetic properties is proposed.

Enhanced Electromagnetic Wave Absorption Properties of Poly(3,4-ethylenedioxythiophene) Nanofiber-Decorated Graphene Sheets by Non-covalent Interactions

Graphene sheets (GNs) have high conductivity, but they exhibit weak electromagnetic (EM) wave absorption performance. Here, poly (3,4-ethylenedioxythiophene) (PEDOT) nanofibers were decorated on the surface of GNs in which the residual defects and groups act as the active sites and therefore are beneficial for the deposition of PEDOT nanofibers. The SEM images display that PEDOT nanofibers are successfully decorated on the surface of GNs through in situ polymerization. The diameter of the PEDOT nanofibers were ranged from 15 to 50 nm with hundreds of nanometers in length. The EM wave absorption properties of graphene, PEDOT, and GNs-PEDOT were also investigated. Compared to pure graphene and PEDOT, the EM wave absorption properties of GNs-PEDOT improved significantly. The maximum value of R L was up to -48.1 dB at 10.5 GHz with a thickness of only 2 mm. Meanwhile, the absorption bandwidth of R L values below -10 dB was 9.4 GHz (5.8-12.3, 12.9-15.8 GHz) in the thickness of 1.5-3 mm. The enhancement is attributed to the modification of PEDOT and the unique structure of nanofibers. On one hand, the deposition of PEDOT nanofibers on the surface of GNs decreases the conductivity of graphene, and makes impedance match better. On the other hand, the unique structure of PEDOT nanofibers results in relatively large specific surfaces areas, providing more active sites for reflection and scattering of EM waves. Therefore, our findings demonstrate that the deposition of conducting polymers on GNs by non-covalent bond is an efficient way to fabricate strong EM wave absorbers.

Constructing Uniform Core-Shell PPy@PANI Composites with Tunable Shell Thickness toward Enhancement in Microwave Absorption

Highly uniform core-shell composites, polypyrrole@polyaniline (PPy@PANI), have been successfully constructed by directing the polymerization of aniline on the surface of PPy microspheres. The thickness of PANI shells, from 30 to 120 nm, can be well controlled by modulating the weight ratio of aniline and PPy microspheres. PPy microspheres with abundant carbonyl groups have very strong affinity to the conjugated chains of PANI, which is responsible for the spontaneous formation of uniform core-shell microstructures. However, the strong affinity between PPy microspheres and PANI shells does not promote the diffusion or reassembly of two kinds of conjugated chains. Coating PPy microspheres with PANI shells increases the complex permittivity and creates the mechanism of interfacial polarization, where the latter plays an important role in increasing the dielectric loss of PPy@PANI composites. With a proper thickness of PANI shells, the moderate dielectric loss will produce well matched characteristic impedance, so that the microwave absorption properties of these composites can be greatly enhanced. Although PPy@PANI composites herein consume the incident electromagnetic wave by absolute dielectric loss, their performances are still superior or comparable to most PANI-based composites ever reported, indicating that they can be taken as a new kind of promising lightweight microwave absorbers. More importantly, microwave absorption of PPy@PANI composites can be simply modulated not only by the thickness of the absorbers, but also the shell thickness to satisfy the applications in different frequency bands.

One-dimensional carbon nanotube@barium titanate@polyaniline multiheterostructures for microwave absorbing application

Multiple-phase nanocomposites filled with carbon nanotubes (CNTs) have been developed for their significant potential in microwave attenuation. The introduction of other phases onto the CNTs to achieve CNT-based heterostructures has been proposed to obtain absorbing materials with enhanced microwave absorption properties and broadband frequency due to their different loss mechanisms. The existence of polyaniline (PANI) as a coating with controllable electrical conductivity can lead to well-matched impedance. In this work, a one-dimensional CNT@BaTiO3@PANI heterostructure composite was fabricated. The fabrication processes involved coating of an acid-modified CNT with BaTiO3 (CNT@BaTiO3) through a sol-gel technique followed by combustion and the formation of CNT@BaTiO3@PANI nanohybrids by in situ polymerization of an aniline monomer in the presence of CNT@BaTiO3, using ammonium persulfate as an oxidant and HCl as a dopant. The as-synthesized CNT@BaTiO3@PANI composites with heterostructures were confirmed by various morphological and structural characterization techniques, as well as conductivity and microwave absorption properties. The measured electromagnetic parameters showed that the CNT@BaTiO3@PANI composites exhibited excellent microwave absorption properties. The minimum reflection loss of the CNT@BaTiO3@PANI composites with 20 wt % loadings in paraffin wax reached -28.9 dB (approximately 99.87% absorption) at 10.7 GHz with a thickness of 3 mm, and a frequency bandwidth less than -20 dB was achieved from 10 to 15 GHz. This work demonstrated that the CNT@BaTiO3@PANI heterostructure composite can be potentially useful in electromagnetic stealth materials, sensors, and electronic devices.

Fabrication of electromagnetic Fe3O4@polyaniline nanofibers with high aspect ratio

High aspect ratio Fe₃O₄@polyaniline nanofibers prepared show better magnetization saturation and conductivity value compared to Fe₃O₄@polyaniline microspheres.

Preparation of polyaniline (PANI)-coated Fe3O4 microsphere chains and PANI chain-like hollow spheres without using surfactants

Polyaniline (PANI)-coated Fe₃O₄ microspheres chains and PANI chain-like hollow spheres are successfully prepared without using surfactants.

Shell thickness-dependent microwave absorption of core-shell Fe3O4@C composites

Core-shell composites, Fe3O4@C, with 500 nm Fe3O4 microspheres as cores have been successfully prepared through in situ polymerization of phenolic resin on the Fe3O4 surface and subsequent high-temperature carbonization. The thickness of carbon shell, from 20 to 70 nm, can be well controlled by modulating the weight ratio of resorcinol and Fe3O4 microspheres. Carbothermic reduction has not been triggered at present conditions, thus the crystalline phase and magnetic property of Fe3O4 micropsheres can be well preserved during the carbonization process. Although carbon shells display amorphous nature, Raman spectra reveal that the presence of Fe3O4 micropsheres can promote their graphitization degree to a certain extent. Coating Fe3O4 microspheres with carbon shells will not only increase the complex permittivity but also improve characteristic impedance, leading to multiple relaxation processes in these composites, thus the microwave absorption properties of these composites are greatly enhanced. Very interestingly, a critical thickness of carbon shells leads to an unusual dielectric behavior of the core-shell structure, which endows these composites with strong reflection loss, especially in the high frequency range. By considering good chemical homogeneity and microwave absorption, we believe the as-fabricated Fe3O4@C composites can be promising candidates as highly effective microwave absorbers.

Polarization enhancement of microwave absorption by increasing aspect ratio of ellipsoidal nanorattles with Fe3O4 cores and hierarchical CuSiO3 shells

The shape anisotropy of the nanostructured nanorattles is one of the key factors that affect their microwave absorption performance. In the present study, the microwave absorption performance of ellipsoidal Fe3O4@CuSiO3 nanorattles with different aspect ratios was investigated. Results demonstrated that the ellipsoidal nanorattles with the aspect ratio of 3-4 exhibited about 20% enhancement of microwave absorption intensity compared with spherical Fe3O4@CuSiO3. Generally, as the aspect ratio increased from 2.0 to 3.5, the microwave absorption peak was enhanced monotonously from -20 dB to -30 dB. It was found that the ellipsoidal nanorattles with larger aspect ratio exhibited higher coercivity and double resonance peaks of the real part of complex permittivity, resulting in the improvement of microwave absorption performance. Our research gives insights into the understanding of the anisotropic effect of nanorattles on microwave absorption performance.