Carbonized foams from graphene/phenolic resin composite aerogels for superior electromagnetic wave absorbers

Ultralight graphene/phenolic resin composite aerogels (GPFs) were prepared through the chemical reduction and self-assembly of graphene oxide (GO) in water-soluble phenolic resin, followed by a freeze-drying process; carbonized foams (GPFs(T)) were obtained by the subsequent heat treatment of GPFs at a relatively low temperature (500–700 °C). Although GPFs do not show the qualified reflection loss value of below −10dB, GPFs(T) achieve the greatly enhanced electromagnetic-wave absorbing performance. Specifically, the minimum reflection loss value of GPF₁ (500) reaches −22.7 dB at 14.4 GHz with the absorber thickness of 2.0 mm and the effective absorption bandwidth is up to 5.4 GHz (12.4–17.8 GHz). The evolution of electromagnetic-wave absorbing properties from GPFs to GPFs(T) at different temperatures related with different graphene content is explored. GPFs(T) are expected to exhibit high thermal stability and excellent corrosion resistance property, and especially still maintain ultralight nature (e.g the density of GPF₁ (500) is only 24.3 mg/cm³). Most importantly, little graphene (as low as 7.5 wt% of GO addition for GPF₁(T)) in GPFs(T) guarantees the facile formation of three-dimension (3D) skeleton network and greatly cut downs the carbonization temperature of phenolic resin to achieve the required electromagnetic-wave energy losing ability. The present work provides an effective method to fabricate an ultralight material with exceptional performances including the good electromagnetic-wave absorbing property and the high stability.     

Core-shell MXene/nitrogen-doped C heterostructure for wide-band electromagnetic wave absorption at thin thickness

MXenes have been utilized to fabricate electromagnetic wave (EMW) absorbers owning to large aspect ratio, high electronic conductivity, and favorable hydrophilicity. In this work, the core-shell MXene/nitrogen-doped (N-doped) C heterostructure was firstly prepared via HCl and LiF etching, in-situ polymerization, and carbonization. When mixed with paraffin at a low filler loading of 30 wt%, the MXene/N-doped C hybrid reached a wide effective absorption bandwidth of 5.0 GHz (13.0 GHz–18.0 GHz) at a thin thickness of 1.72 mm. The stronger ability of attenuating EMWs promoted the absorption performance of MXene/N-doped C, overcoming the deficiency in the characteristics of impedance compared with its counterparts. This work provides a new insight in manufacturing MXene-based absorbers to alleviate EMW pollution by delicate structural design and effective multi-component strategy.     

Facile synthesis of a novel flower-like BiFeO3 microspheres/graphene with superior electromagnetic wave absorption performances

In recent years, porous or layered magnetic materials have received increasing attention due to their low density and lightweight. In this work, porous BiFeO₃ microspheres and three-dimensional porous BiFeO₃ microsphere-reduced graphene oxide (RGO) composite (3D porous BiFeO₃/RGO) were prepared by one-step etching processing using pure BiFeO₃ particles as precursors. The precursor undergoes dissolution-recrystallization/reduction process, resulting in large amount of BiFeO₃ fragments and graphene hybrid product, which forms 3D porous BiFeO₃/RGO composite. Electromagnetic (EM) absorption performance measurements exhibit that at low thickness of 1.8?mm, porous BiFeO₃/RGO composite can achieve reflection loss (R_L) value up to ?46.7?dB and absorption bandwidth (defined by R_L <?10?dB) exceeding 4.7?GHz (from 12.0 to 16.7?GHz), testifying outstanding microwave absorbing performance. Compared with pure porous BiFeO₃, improved EM wave absorption ability of as-prepared porous BiFeO₃/RGO composite is attributed to interfacial polarization, multiple reflections, scattering, and appropriate impedance matching.     

Metal-oxide doping enhances electromagnetic wave absorption performance of BaFe12O19 glass-ceramics

Elimination or reduction of electromagnetic wave pollution is receiving increasing attention; development of high-performance wave-absorbing materials has become key to solving this problem. In this study, BaFe₁₂O₁₉ crystals were precipitated in a BaO–Fe₂O₃–B₂O₃–SiO₂ glass system using the melt-quench-second heat treatment method. The microstructure and magnetic properties of the BaFe₁₂O₁₉ glass ceramics and their electromagnetic wave loss characteristics were analyzed. The composition of the crystals in the glass can be modified by introducing other metal oxides, such as ZrO₂ (BFO-Z), TiO₂ (BFO-T), and Al₂O₃ (BFO-A), to enrich the heterogeneous interfaces and increase the dielectric loss of the material. The maximum effective absorption bandwidth of BaFe₁₂O₁₉ glass-ceramics with addition of ZrO₂ (BFO-Z) was increased to 3.20 GHz; the minimum reflection loss was reduced to −45.60 dB. This simple method represents a new direction for fabricating high-performance wave-absorbing materials.     

Porous Fe3O4/C microspheres for efficient broadband electromagnetic wave absorption

Porous Fe₃O₄/C microspheres, which were Fe₃O₄ nanocrystals (～8?nm) embedded in an open nanostructured carbon network, were successfully synthesized via a facile hydrothermal process. The porous Fe₃O₄/C microspheres possessed many distinct attributes that facilitate efficient broadband electromagnetic wave absorption (EMWA). EMWs were attenuated through multiple reflections and absorption in the 3D interconnected porous structure of the microspheres; these processes collectively improved the interaction between the EMWs and the absorber. Additionally, the carbon network and embedded Fe₃O₄ nanoparticles caused significant dielectric losses and magnetic losses, respectively, which also enhanced EMWA. The EMWA characteristics of the microspheres could be precisely tuned via changing the carbon content to achieve optimized impedance matching. Porous Fe₃O₄/C microspheres with a 71.5?wt% carbon content displayed particularly impressive EMWA properties: a maximum reflection loss (RL) value of ??31.75 across broad band frequencies in the range of 7.76–12.88?GHz (RL < ?10?dB) at an absorber thickness of 3.0?mm. These excellent EMWA properties may be attributed to both dielectric loss (carbon) and magnetic loss (Fe₃O₄). Additionally, the 3D interconnected porous structure of the Fe₃O₄/C microspheres is especially favorable for impedance matching.     

Ultralight polymer-derived ceramic aerogels with wide bandwidth and effective electromagnetic absorption properties

In this work, ultralight polymer-derived ceramic aerogels (PDCA) were prepared by a facile method of hydrosilylation crosslink and freeze drying. The electromagnetic absorption properties of ultralight PDCA were investigated for the first time. The PDCA pyrolyzed at 1000 °C shows a uniform three-dimensional (3D) framework with a low bulk density of ∼0.19 g/cm³ and high surface area of 134.48 m²/g. The electromagnetic properties of PDCA were characterized by a vector network analyzer. The minimum reflection and absorption bandwidth of PDCA pyrolyzed at 1000 °C, 1200 °C and 1400 °C are −43.37 dB @ 7.6 GHz, −42.01 dB@12.5 GHz, and −31.69 dB@17.3 GHz and 3.8 GHz, 6.6 GHz and 4.2 GHz, respectively, at the frequency range of 2–18 GHz. The strong electromagnetic absorption and wide bandwidth features of PDCA could be attributed to the multiple reflections of microwaves in the 3D framework, as well as the high dielectric loss and proper conductivity.     

Ultralight,tunable monolithic SiC aerogel for electromagnetic absorption with broad absorption band

SiC aerogel is a novel candidate for electromagnetic absorption (EMA) materials in extreme environments because of its ceramic nature and aerogel morphology. In this study, graphene oxide (GO), which having has been generally serving served as a “brick” in the material community, was used as a building block to prepare SiC aerogel precursors and further develop a novel and designable route for preparing SiC aerogel for EMA. The experimental results suggested that the GO aerogel was well converted into a SiC aerogel with ultra-low density (<4 mg cm^?3). The synthesized SiC aerogel inherited the precursor well in structure while having a wide absorption band (approximately 5.5GHz) and a high absorption frequency-to-thickness correlation different from traditional SiC aerogels. The above idea for designing and assembling SiC aerogels will significantly broaden the practical applications of GO and SiC in EMA, supercapacitors and sensors.     

Design of magnetic triple-shell hollow structural Fe3O4/FeCo/C composite microspheres with broad bandwidth and excellent electromagnetic wave absorption performance

A three-step strategy combining solvothermal and liquid phase reduction method had been developed for preparation of magnetic triple-shell hollow structural Fe₃O₄/FeCo/C (TSH–Fe₃O₄/FeCo/C) composite microspheres. FeCo was used to enhance electromagnetic (EM) wave absorption in different frequency band and broaden effective absorbing bandwidth, while carbon was used to improve impedance matching. Triple-shell hollow structure was designed to enrich the multiple interfaces to favor the interfacial polarization, increase the multiple reflections and scattering, and provide physicochemical protection to Fe₃O₄ core from oxidation. The microstructure and morphology of TSH-Fe₃O₄/FeCo/C composite microspheres were characterized by TEM, XRD and Raman in detail. The results indicated that magnetic Fe₃O₄ was completely covered by FeCo and carbon via layer by layer. As an EM wave absorber, the maximum reflection loss of TSH-Fe₃O₄/FeCo/C composite microspheres was up to -37.4 dB due to better normalized characteristic impedance at a thickness of 2.2 mm and the bandwidth less than -10 dB even reached up to 5.9 GHz. The excellent EM wave absorption performance was attributed to the combination of shell materials (Fe₃O₄, FeCo and carbon) and unique triple-shell hollow structure, which lead to multiple relaxation processes and good impedance matching. Consequently, this work would contribute to the design and preparation of high performance EM wave absorbent with outstanding absorbing property and wider absorption range.     

3D printed SiC nanowire reinforced composites for broadband electromagnetic absorption

SiC nanowire/acrylic resin (SiC_nw/ACR) composites with broadband electromagnetic (EM) absorption capabilities were fabricated by a novel procedure using 3D stereolithography (3D-SL) printing technology. The EM absorption abilities of the composites can be adjusted by tuning the SiC_nw content and the thickness of the printing layer. When the SiC_nw content is 3?wt% and the thickness of the printing layer is 25?μm–50?μm, the SiC_nw/ACR composite has an optimally broad effective absorption bandwidth (EAB) and a high efficiency for EM absorption, whether assessing the C, X or Ku band, because of the high dielectric loss and proper impedance matching between the materials and free space. In the C band (4–8?GHz), the EAB reaches 2.9?GHz, and the reflection loss (RL) reaches ?34.1?dB; in the X band (8–12?GHz), the EAB reaches 4?GHz, which covers the entire X band, and the RL reaches ?34.5?dB; in the Ku band (12–18?GHz), the EAB exceeds 6?GHz, which covers the whole Ku band, and the RL reaches ?34.7?dB. This research is of great importance to the rapid preparation of parts, shells or devices with arbitrarily complex shapes and high efficiency broadband EM absorption abilities.     

Synthesis and characterization of hexagonal BN-based aerogels for absorbing oils

Porous oil absorbents have been extensively studied in recent years. Boron nitride (BN) is an ideal inorganic material to withstand severe absorption conditions due to its excellent inertness and high-temperature resistance. A large challenge of BN aerogels is their high preparation costs due to requiring high temperatures and dangerous atmospheres. A facial synthesis of h-BN-based aerogels by the cast-freezing method is reported here, which has the advantages of producing a light and recyclable material (~0.035 g/ml) at a low cost and with low energy consumption. In this work, carboxymethylcellulose (CMC) is used as the compounder to interact with BN and form an aerogel that has controllable porosity (38–86 μm) and hydrophobicity (~140°). The porous structure, components, and reaction mechanisms are analyzed by SEM, XRD, mercury porosimetry, etc.. The pore distributions can be controlled by the concentrations of CMC and crosslinking agents. The intercalation effect on BN is carried out by CMC, which increases the specific surface area. Finally, the oil absorption performances are measured. The oil absorption capacity is up to 31.55 g/g (~97% ml/ml) because of the high specific surface area of the prepared material; additionally, the capacity of this material shows no obvious decrease after 6 cycles. Therefore, h-BN-based aerogels are expected to be applied in the oil absorption field and have the potential to be applied in the biochemical and nuclear fields.     

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.     

Synthesis of PPy/Ni/RGO and enhancement on its electromagnetic wave absorption performance

The ternary composites of hollow tubular polypyrrole (PPy) and Ni particles based on reduced graphene oxide (RGO) were synthesized by a low-energy method. PPy and Ni particles were uniformly distributed on the surface of RGO. Specifically, multilayer interfaces existed in the hybrid, residual defects and folded structures of RGO and the hollow tubular structures of PPy made a significant contribution to electromagnetic (EM) wave attenuation. The final results showed that the strongest reflection loss (RL) value of PNR-2 sample could achieve ??47.32?dB at 5.76?GHz. The PNR-3 sample's RL value was ??18.21?dB at 15.92?GHz with the thickness of only 1.5?mm, and its corresponding effective absorption bandwidth reached 4.32?GHz (13.68–18?GHz). Hence, with the changes of PPy mass ratio, the impedance matching and attenuation were regulated to realize remarkable EM wave absorption performance including lightweight, thin thickness, wide bandwidth and strong absorption.     

Fabrication of Ni/ZnO/C hollow microspheres decorated graphene composites towards high-efficiency electromagnetic wave absorption in the Ku-band

Developing light-weight, thin thickness and high-efficiency electromagnetic wave (EMW) absorbers is an effective strategy for dealing with the increasingly serious problem of electromagnetic radiation pollution. Herein, nickel/zinc oxide/carbon (Ni/ZnO/C) hollow microspheres decorated graphene composites were facilely prepared through the high-temperature pyrolysis of bimetallic NiZn metal-organic frameworks (MOFs) precursors. Morphological characterization results manifested that the Ni/ZnO/C microspheres with unique hollow structure were almost evenly anchored on the wrinkled surfaces of flake-like graphene. Moreover, the influences of additive amounts of graphene oxide (GO) in the MOFs precursors on the crystal structure, graphitization degree, micromorphology, magnetic properties, electromagnetic parameters and EMW absorption performance were investigated in detail. It was found that the superior EMW absorption performance could be achieved through facilely adjusting the additive amounts of GO in the precursors. As the additive amount of GO was equal to 60 mg, the obtained composite showed the comprehensive excellent EMW absorption performance. Notably, the optimal minimum reflection loss reached ?57.5 dB at 16.5 GHz in the Ku-band under an ultrathin matching thickness of merely 1.34 mm, and the broadest effective absorption bandwidth achieved 5.6 GHz (from 12.4 to 18 GHz) when the thickness was 1.5 mm. Furthermore, the underlying EMW absorption mechanisms of as-prepared composites were revealed. It was believed that our results could be valuable for the structural design and EMW absorption performance modulation for MOFs derived magnetic carbon composites.     

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).     

In-situ synthesis of ternary layered Y3Si2C2 ceramic on silicon carbide fiber for enhanced electromagnetic wave absorption

A novel ternary layered ceramic of Y₃Si₂C₂ was successfully in-situ synthesized on the surface of home-made third-generation KD-SA SiC fiber for the first time by molten salt method aimed at improving the electromagnetic wave (EMW) absorption. After in-situ synthesis of Y₃Si₂C₂ ceramic layer on SiC fiber (SiC_f/Y₃Si₂C₂), significantly improved EMW absorption performance was obtained. The minimum reflection loss (RL_min) of ?16.97 dB was reached in SiC_f/Y₃Si₂C₂ composites at the thickness of only 2.19 mm, and the effective absorption bandwidth (EAB) was up to 5.44 GHz (12.56–18 GHz) at a thin thickness of 2.64 mm. The improvement in EMW absorption of SiC_f/Y₃Si₂C₂ is mainly attributed to enhanced dielectric loss and conduction loss resulting from increased heterogeneous interfaces and multiple reflections and scattering originating from net structure. The SiC_f/Y₃Si₂C₂ could be a promising EMW absorber for application in high-performance EMW absorbing materials.     

Fabrication of hexagonal cerium oxide nanoparticles decorated nitrogen-doped reduced graphene oxide hybrid nanocomposite as high-performance microwave absorbers in the Ku band

With the aim to obtain microwave absorbers simultaneously possessing broad absorption bandwidth, strong absorption intensity and thin matching thickness, nitrogen-doped reduced graphene oxide decorated by cerium oxide particles (NRGO/CeO₂) hybrid nanocomposite was prepared through a hydrothermal and calcination two-step route. Results of micromorphology analysis showed that numerous hexagonal CeO₂ nanoparticles were evenly anchored on the crumpled surfaces of NRGO. Moreover, both nitrogen doping and hybridization with RGO could notably strengthen the microwave absorption capacity of CeO₂. Remarkably, the NRGO/CeO₂ hybrid nanocomposite exhibited the minimum reflection loss of ?57.2 dB at 13.4 GHz (Ku band) under a matching thickness of 1.66 mm and maximum absorption bandwidth of 4.6 GHz (from 13.2 to 17.8 GHz) at an ultrathin thickness of only 1.5 mm. Meanwhile, the hybrid nanocomposites displayed strong absorption intensity (≤-20 dB, 99% absorption) in almost the whole measured thicknesses range. Furthermore, the relationship between absorption intensity and filler loadings was uncovered. The potential microwave absorption mechanisms were further revealed. Therefore, this work opened a novel idea for designing RGO-based hybrid nanocomposites as high-performance microwave absorbers.     

MXene nanohybrids: Excellent electromagnetic properties for absorbing electromagnetic waves

MXenes have been long sought-after for their exceptional electrical conductivity, large family members, mechanical stability, tunable surface groups, and ease of processability. In particular, they have become the leading materials for electromagnetic (EM) wave absorption. Herein, MXenes’ crystal and electronic structures, as well as EM wave absorption related to those properties, are discussed. The structures and EM functions of MXene hybrids are systematically arranged. The crystal and electronic structure, EM characteristics, and EM response of MXene-based hybrids, as well as their relationship are revealed. Moreover, we propose the main challenges, and predict future directions in this rapidly broadening field.     

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

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

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.     

Ultralight and efficient microwave absorption of SiC/SiO2 ceramic aerogels derived from biomass

Rational multicomponent regulation and microstructure design have proven to be effective strategies for achieving high performance electromagnetic wave (EMW) absorbers. Herein, the ultralight hierarchically porous SiC/SiO₂ aerogels (HPSA) were successfully synthesized by an ingenious one-step method to achieve carbonization and carbothermal reduction. The composition of the HPSA and the quantity of SiC/SiO₂ fibers grown by in situ reaction can be controlled by adjusting the amount of silicon source introduced. The results indicate that the composition of HPSA and the quantity of fibers have a significant effect on the EMW absorption properties. When the introduced silicon source concentration was 0.7 mol/L, the HPSA exhibited excellent EMW absorption performance, with a minimum reflection loss (RL_min) of -55.01 dB at 6.00 GHz and a maximum effective absorption bandwidth (EAB_max) of 6.16 GHz. The highly interconnected porous SiC/SiO₂ skeleton structure significantly contributes to the multiple reflection-absorption effect of EMW and provides available pathways for electron conduction losses. The in situ reaction generates SiC/SiO₂ fibers with a large number of stacking faults and heterojunctions, which further promote the dissipation of EMW. In addition, the maximum radar cross section of HPSA under far-field conditions is reduced to 20.21 dB m² compared to the PEC conductive layer, which implies a much lower probability of detection by radar. In brief, this work provides a reference for the use of highly efficient EMW absorbers and electromagnetic stealth materials.