Assembling Nano-Microarchitecture for Electromagnetic Absorbers and Smart Devices

Dielectric Loss Mechanism in Electromagnetic Wave Absorbing Materials

Electromagnetic (EM) wave absorbing materials play an increasingly important role in modern society for their multi-functional in military stealth and incoming 5G smart era. Dielectric loss EM wave absorbers and underlying loss mechanism investigation are of great significance to unveil EM wave attenuation behaviors of materials and guide novel dielectric loss materials design. However, current researches focus more on materials synthesis rather than in-depth mechanism study. Herein, comprehensive views toward dielectric loss mechanisms including interfacial polarization, dipolar polarization, conductive loss, and defect-induced polarization are provided. Particularly, some misunderstandings and ambiguous concepts for each mechanism are highlighted. Besides, in-depth dielectric loss study and novel dielectric loss mechanisms are emphasized. Moreover, new dielectric loss mechanism regulation strategies instead of regular components compositing are summarized to provide inspiring thoughts toward simple and effective EM wave attenuation behavior modulation.

Preparation and Electromagnetic Absorption Properties of Fe73.2Si16.2B6.6Nb3Cu1 Nanocrystalline Powder

In order to decrease and control electromagnetic pollution, absorbing materials with better electromagnetic wave absorption properties should be developed. In this paper, a nanocrystalline alloy ribbon with the composition of Fe73.2Si16.2B6.6Nb3Cu1 was designed and prepared. Nanocrystalline alloy powder was obtained by high-energy ball milling treatment. The effects of ball milling time on the soft magnetic properties, microstructure, morphology, and electromagnetic wave absorption properties of alloy powder were investigated. The results showed that, as time increased, α-(Fe, Si) gradually transformed into the amorphous phase, and the maximum saturation magnetization (Ms) reached 135.25 emu/g. The nanocrystalline alloy powder was flakelike, and the minimum average particle size of the powder reached 6.87 μm. The alloy powder obtained by ball milling for 12 h had the best electromagnetic absorption performance, and the minimum reflection loss RLmin at the frequency of 6.52 GHz reached −46.15 dB (matched thickness was 3.5 mm). As time increased, the best matched frequency moved to the high-frequency direction, and the best matched thickness decreased, while the maximum effective absorption bandwidth ΔfRL<−10 dB was 7.22 GHz (10.78−18 GHz).

CoFe2O4nanoparticles dispersed on carbon rods derived from cotton for high-efficiency microwave absorption

Biomass-derived carbon materials have received a surge of scientific attention to develop lightweight and broadband microwave absorbers. Herein, rodlike porous carbon materials derived from cotton have been fabricated with uniformly dispersed CoFe₂O₄nanoparticles via facile and scalable process. The combination of magnetic particles and carbonaceous material is advantageous to realize the magnetic-dielectric synergistic effect which could effectively promote the dissipation of incident waves, giving rise to an optimal reflection loss value of -48.2 dB over a qualified bandwidth (4.8 GHz) at 2.5 mm. The cotton-derived carbon rods with conductive network not only act as a supporter to carry the CoFe₂O₄nanoparticles, but also provide massive heterointerfaces to facilitate the interfacial polarization. In consideration of the renewable and abundant resource of cotton, the as-prepared CoFe₂O₄/C composites would meet the increasing demand of lightweight and highly efficient microwave absorbers.

High-Performance Ferroelectric Electromagnetic Attenuation Materials with Multiple Polar Units Based on Nanodomain Engineering

The multirelaxation behavior is promising for high-performance dielectric materials based on polarization-controllable high-efficiency electromagnetic attenuation. However, a single polar unit is the main problem that restricts the development of dielectric materials in the field. Herein, by constructing multiple polar units based on nanodomain engineering, enhanced electromagnetic attenuation properties are achieved in La doping BiFeO₃ ferroelectric ceramics. A dual-band attenuation with a maximum reflection loss of -43.4 dB together with a wide effective bandwidth (<-10 dB) of 3.3 GHz in X-band, is acquired in Bi_0.85 La_0.15 FeO₃ which just has a thickness of 1.54 mm. A systematic experimental analysis coupled with potential well modeling suggests that the miniaturization of the ferroelectric domain, from micron to nanoscale, induces an additional interface polarization that is capable of responding to microwave frequency, leading to the formation of dual dielectric relaxation. The way that intrinsic polar unit induces another polar unit through size effect to obtain multiple contributions of electromagnetic loss provides a feasible and universal strategy to design high-performance electromagnetic attenuation materials based on the ferroelectric family.

Dimensional Design and Core-Shell Engineering of Nanomaterials for Electromagnetic Wave Absorption

Electromagnetic (EM) wave absorption materials possess exceptionally high EM energy loss efficiency. With vigorous developments in nanotechnology, such materials have exhibited numerous advanced EM functions, including radiation prevention and antiradar stealth. To achieve improved EM performance and multifunctionality, the elaborate control of microstructures has become an attractive research direction. By designing them as core-shell structures with different dimensions, the combined effects, such as interfacial polarization, conduction networks, magnetic coupling, and magnetic-dielectric synergy, can significantly enhance the EM wave absorption performance. Herein, the advances in low-dimensional core-shell EM wave absorption materials are outlined and a selection of the most remarkable examples is discussed. The derived key information regarding dimensional design, structural engineering, performance, and structure-function relationship are comprehensively summarized. Moreover, the investigation of the cutting-edge mechanisms is given particular attention. Additional applications, such as oxidation resistance and self-cleaning functions, are also introduced. Finally, insight into what may be expected from this rapidly expanding field and future challenges are presented.

Surface modification of helical carbon nanocoil (CNC) with N-doped and Co-anchored carbon layer for efficient microwave absorption

Surface modification and composition control for nanomaterials are effective strategies for designing high-performance microwave absorbing materials (MWAMs). Herein, we have successfully fabricated Co-anchored and N-doped carbon layers on the surfaces of helical carbon nanocoils (CNCs) by wet chemical and pyrolysis methods, denoted as Co@N-Carbon/CNCs. It is found that pure CNCs show a very good microwave absorption performance under a filling ratio of only 6%, which is attributed to the uniformly dispersed conductive network and the cross polarization induced by the unique chiral and spiral morphology. The coating of N-doped carbon layers on CNCs further enriches polarization losses and the uniform anchoring of Co nanoparticles in these layers generates magnetic losses, which enhance the absorption ability and improve the low frequency performance. As compared with the pure CNCs-filling samples, the optimized Co@N-Carbon/CNCs-2.4 enhances the absorption capacity in the lower frequency range under the same thickness, and realizes the decreased thickness from 3.2 to 2.8 mm in the same X band, as well as the decreased thickness from 2.2 to 1.9 mm in the Ku band. Resultantly, a specific effective absorption wave value of 22 GHz g^-1 mm^-1 has been achieved, which enlightens the synthesis of ultrathin and light high-performance MWAMs.

Lightweight TiO2@C/Carbon Fiber Aerogels Prepared from Ti3C2Tx/Cotton for High-Efficiency Microwave Absorption

Carbon fiber aerogel (CFA) derived from cotton wool as a potential microwave absorbing material has received intensive attention owing to the low density, high conductivity, large surface area, and low cost, but its applications are limited by the relatively high complex permittivity. To solve this problem, TiO₂@C (derived from Ti₃C₂T_x) is introduced into CFA to prepare lightweight TiO₂@C/CFA composites based on electromagnetic (EM) parameter optimization and enhanced EM wave attenuation performance. The microwave absorption capacity of TiO₂@C/CFA-2 composite is obviously better than that of CFA. It is confirmed that good impedance matching derived from the combination of TiO₂@C and CFA is the main factor to achieve excellent microwave absorption. Moreover, the improved microwave absorption capabilities are closely related to multiple EM wave absorbing mechanisms including multiple reflections and scattering, dipolar and interfacial polarization, and conductivity loss. TiO₂@C/CFA-2 possesses a maximum reflection loss (RL) of -43.18 dB at a low response frequency of 6.0 GHz. As the matching thickness is less than 2.0 mm, the maximum RL values can still exceed -20 dB, and at the same time, the wide effective absorption bandwidth (EAB) below -10 dB achieves 4.36 GHz at only 1.9 mm thickness. Our work confirms that the lightweight and high-performance TiO₂@C/CFA composites are promising choices and offer a new approach to design and construct carbon-based microwave absorbents derived from biomass.

Morphology optimization strategy of flower-like CoNi2S4/Co9S8@MoS2 core@shell nanocomposites to achieve extraordinary microwave absorption performances

Morphology optimization is an effective strategy to take full advantage of interface polarization for the improvement of electromagnetic wave attenuation capability. Herein, a general route was proposed to produce the flower-like core@shell structured MoS₂-based nanocomposites through a simple hydrothermal process. Through the in-situ hydrothermal reaction between the Mo and S sources on the surface of CoNi nanoparticles, flower-like core@shell structured CoNi₂S₄/Co₉S₈@MoS₂ nanocomposites could be successfully synthesized. By regulating the hydrothermal temperature, the flower-like geometrical morphology of samples could be effectively optimized, and the as-prepared sample (S2) synthesized at 200 °C displayed very excellent flower-like morphology compared to the samples (S1 and S3) obtained at 180 and 220 °C. Owing to the excellent interface polarization effect, the as-prepared S2 presented the evidently superior comprehensive microwave absorption properties in terms of strong aborption capability, wide absorption bandwidth and thin matching thicknesses compared to those of S1 and S3. The as-prepared core@shell structured CoNi₂S₄/Co₉S₈@MoS₂ sample with very excellent flower-like morphology simultaneously displayed the minimal reflection loss of -50.61 dB with the matching thickness of 2.98 mm, and the effective absorption bandwidth of 8.40 GHz with the matching thickness of 2.36 mm. Therefore, we provided a general route for the production of flower-like core@shell structured MoS₂-based nanocomposites, which could make the best of interface polarization to develop high-efficiency microwave absorbers.

Interfacial Assembly of Nanowire Arrays toward Carbonaceous Mesoporous Nanorods and Superstructures

Synthesis of anisotropic carbonaceous nano- and micro-materials with well-ordered mesoporous structures has attracted increasing attention for a broad scope of applications. Although hard-templating method has been widely employed, overcoming the viscous forces to prepare anisotropic mesoporous materials is particularly challenging via the universal soft-templating method, especially from sustainable biomass as a carbon resource. Herein, the synthesis of biomass-derived nanowire-arrays based mesoporous nanorods and teeth-like superstructures is reported, through a simple and straightforward polyelectrolyte assisted soft-templating hydrothermal carbonization (HTC) approach. A surface energy induced interfacial assembly mechanism with the synergetic interactions between micelles, nanowire, nanorods, and polyelectrolyte is proposed. The polyelectrolyte acts not only as a stabilizer to decrease the surface energy of cylindrical micelles, nanowires and nanorods, but also as a structure-directing agent to regulate the oriented attachment and anisotropic assembly of micelles, nanowires, and nanorods. After a calcination treatment, the carbon nanorod and teeth-like superstructure are successfully coupled with Ru to directly produce supported catalysts for the hydrogen evolution reaction, exhibiting much better performance than the isotropic nanospheres based catalyst. This HTC approach will open up new avenues for the synthesis of anisotropic materials with various morphologies and dimensions, expanding the palette of materials selection for many applications.

Heterointerface Engineering in Electromagnetic Absorbers: New Insights and Opportunities

Electromagnetic (EM) absorbers play an increasingly essential role in the electronic information age, even toward the coming "intelligent era". The remarkable merits of heterointerface engineering and its peculiar EM characteristics inject a fresh and infinite vitality for designing high-efficiency and stimuli-responsive EM absorbers. However, there still exist huge challenges in understanding and reinforcing these interface effects from the micro and macro perspectives. Herein, EM response mechanisms of interfacial effects are dissected in depth, and with a focus on advanced characterization as well as theoretical techniques. Then, the representative optimization strategies are systematically discussed with emphasis on component selection and structural design. More importantly, the most cutting-edge smart EM functional devices based on heterointerface engineering are reported. Finally, current challenges and concrete suggestions are proposed, and future perspectives on this promising field are also predicted.

Lightweight and broadband 2D MoS2 nanosheets/3D carbon nanofibers hybrid aerogel for high-efficiency microwave absorption

Three-dimensional (3D) porous molybdenum disulfide nanosheets/carbon nanofibers (MoS₂/CNF) hybrid aerogels were synthesized by using solvothermal method and following carbonization, where two-dimensional (2D) MoS₂ nanosheets were homogenously in-situ grown on the interconnected CNF skeleton derived from bacterial cellulose, forming a hierarchical porous structure. This unique heterogeneous structure of the MoS₂/CNF hybrid aerogels were conducive to electromagnetic loss, including conduction, polarization, multi-scatterings, and reflections, thus resulting in a balanced impedance matching and microwave attenuation capacity. It was found that the resulted MoS₂/CNF hybrid aerogels demonstrate excellent microwave absorbing performance when the only 5.0 wt% fillers were loaded in paraffin. Particularly, MoS₂/CNF-2-900 hybrid aerogel displayed an effective absorption bandwidth of 5.68 GHz and minimum reflection loss (RL_min) value of -36.19 dB at a thickness of 2.0 mm. As the thickness increases to 4.4 mm, the RL_min value of MoS₂/CNF-2-900 hybrid aerogel reaches -48.53 dB. Electromagnetic loss mechanism analysis indicates that such improved microwave attenuation is attributed to proper component, multiple heterogenous interface and hierarchical porous structures. All the results in this work pave the avenue for the development of ultralight microwave absorber with high absorption capacity as well as broad effective absorption bandwidth.

Size-controllable porous flower-like NiCo2O4 fabricated via sodium tartrate assisted hydrothermal synthesis for lightweight electromagnetic absorber

Although the performance of NiCo₂O₄-based absorbers with multiple components has made great progress, the property of pure NiCo₂O₄ is still far from the requirements of high-performance electromagnetic wave absorbers. It is recognized that morphology control is an effective strategy to improve electromagnetic absorbing capacity of absorbers. Herein, this work reported the fabrication of porous flower-like pure NiCo₂O₄ via simple hydrothermal reaction with the assistant of sodium tartarate in where tartaric acid served as a structure-directing agent. It was demonstrated that size distribution and electromagnetic absorbing capacity of the obtained NiCo₂O₄ could be modulated easily by controlling addition of sodium tartrate. It was verified that dipole polarization originated from lattice defect and oxygen vacancy as well as interfacial polarization ascribing to adjacent and interconnected flakes are responsible for the excellent electromagnetic absorbing performance. The obtained porous flower-like NiCo₂O₄ exhibited broad absorption bandwidth at thin thickness as well as proper dissipation ability. This work offers a new strategy to fabricate size-controllable porous flower-like NiCo₂O₄ electromagnetic absorber. It is believed the obtained NiCo₂O₄ will be a promising candidate as a lightweight electromagnetic absorbing material.

Fabrication of clay soil/CuFe2O4 nanocomposite toward improving energy and shielding efficiency of buildings

In this research, the energy and shielding efficiency of brick, fabricated by clay soil, as a practical building material was reinforced using CuFe₂O₄ nanoparticles. Initially, the nanoparticles were fabricated using the sol-gel method and then loaded in the brick matrix as a guest. The architected samples were characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FTIR), diffuse reflection spectroscopy (DRS), field emission scanning electron microscopy (FE-SEM), High-resolution transmission electron microscopy (HRTEM), vibrating-sample magnetometer (VSM), differential scanning calorimetry (DSC) thermograms, and vector network analyzer (VNA) analyses. IR absorption of the tailored samples was monitored under an IR source using an IR thermometer. IR absorption and energy band gap attested that inserting the nanoparticles in brick medium led to the acceleration of a warming brick, desirable for energy efficiency in cold climates. It is worth noting that the brick/CuFe₂O₄ nanocomposite achieved a strong reflection loss (RL) of 58.54 dB and gained an efficient bandwidth as wide as 4.22 GHz (RL > 10 dB) with a thickness of 2.50 mm, meanwhile it shielded more than 58% of the electromagnetic waves at X-band by only a filler loading of 10 wt%. The microwave absorbing and shielding characteristics of the composite are mainly originated from conductive loss, electron hopping, natural and exchange resonance, relaxation loss, secondary fields, as well as eddy current loss. Interestingly, the shielding property of the nanocomposite was significantly generated from its absorbing features, reducing the secondary electromagnetic pollutions produced by the shielding materials applying the impedance mismatching mechanism.

Magnetic Energy Morphing, Capacitive Concept for Ni0.3Zn0.4Ca0.3Fe2O4 Nanoparticles Embedded in Graphene Oxide Matrix, and Studies of Wideband Tunable Microwave Absorption

Nanoparticles of Ni_0.3Zn_0.4Ca_0.3Fe₂O₄ (NZCF) were successfully prepared by the facile wet chemical method coupled with the sonochemical method. These nanoparticles were embedded in a graphene oxide (GO) matrix (NZCFG). Rietveld analyses of X-ray diffraction, transmission electron microscope, scanning electron microscope, and X-ray photoelectron spectroscopy were carried out to extract different relevant information regarding the structure, morphology, and ionic state. A major improvement in saturation magnetization is achieved due to substitution of Ca²⁺ in the ferrite lattice. Interestingly, the observed value of electromagnetic absorption for a sample thickness of 1.5 mm is ∼-67.7 dB at 13.3 GHz, and the corresponding bandwidth is 5.73 GHz. The Cole-Cole plot, the Jonscher power-law fitting, and the Nyquist plot confirm the probability of improved hopping conductance and attractive capacitive behavior in NZCFG. The presence of magnetic energy morphing in combination with a higher attenuation constant, lower skin depth, and various forms of resonance and relaxation makes NZCFG the most suitable for microwave absorption.

Rational construction of porous N-doped Fe2O3 films on porous graphene foams by molecular layer deposition for tunable microwave absorption

Graphene-based materials with porous microstructure have attracted immense attentions due to their wide application in microwave absorption. However, constructing magnetic film with both porous microstructure and uniform pore size by using traditional methods still remains a challenge. To overcome this problem, we reported a facile strategy of molecular layer deposition (MLD) for successfully fabrication of the hybrid-architecture of porous graphene foams and nitrogen-doped porous Fe₂O₃ films. The surfaces of porous graphene foams are uniformly covered by porous Fe₂O₃ films without aggregation and the pore structures are widely distributed. The porous graphene-based composites exhibit remarkably enhanced microwave absorption performance compared to the pristine graphene foams. The minimum reflection loss value is increased by approximately 8 times, reaching -64.36 dB with a thickness of only 2.18 mm. More importantly, the absorption property can be precisely modulated by tuning the MLD cycle numbers and effective absorption bandwidth covers 3.04-18.0 GHz by adjusting the thickness from 1.0 to 5.0 mm. This work provides new insights for exploring novel and high-performance graphene-based microwave absorbents and offers a new idea to rationally design three-dimensional composites with porous magnetic films.

Preparation of self-healing hydrogel toward improving electromagnetic interference shielding and energy efficiency

In this study, a self-healing hydrogel was prepared that is transparent to visible (Vis) light while absorbing ultraviolet (UV), infrared (IR), and microwave. The optothermal features of the hydrogel were explored by monitoring temperature using an IR thermometer under an IR source. The hydrogel was synthesized using sodium tetraborate decahydrate (borax) and polyvinyl alcohol (PVA) as raw materials based on a facile thermal route. More significantly, graphene oxide (GO) and graphite-like carbon nitride (g-C3N4) nanostructures as well as carbon microsphere (CMS) were applied as guests to more dissect their influence on the microwave and optical characteristics. The morphology of the fillers was evaluated using field emission scanning electron microscopy (FE-SEM). Fourier transform infrared (FTIR) attested that the chemical functional groups of the hydrogel have been formed and the result of diffuse reflection spectroscopy (DRS) confirmed that the hydrogel absorbs UV while is transparent in Vis light. The achieved result implied that the hydrogel acts as an essential IR absorber due to its functional groups desirable for energy efficiency and harvesting. Interestingly, the achieved results have testified that the self-healing hydrogels had the proper self-healing efficiency and self-healing time. Eventually, microwave absorbing properties and shielding efficiency of the hydrogel, hydrogel/GO, g-C3N4, or CMS were investigated, demonstrating the salient microwave characteristics, originated from the established ionic conductive networks and dipole polarizations. The efficient bandwidth of the hydrogel was as wide as 3.5 GHz with a thickness of 0.65 mm meanwhile its maximum reflection loss was 75.10 dB at 14.50 GHz with 4.55 mm in thickness. Particularly, the hydrogel illustrated total shielding efficiency (SET) > 10 dB from 1.19 to 18 and > 20 dB from 4.37 to 18 GHz with 10.00 mm in thickness. The results open new windows toward improving the shielding and energy efficiency using practical ways.

Enhanced Microwave Absorbing Ability of Carbon Fibers with Embedded FeCo/CoFe2O4 Nanoparticles

In the face of increasingly severe electromagnetic (EM) wave pollution, the research of EM wave absorbing materials is an effective solution. To reduce the density of traditional absorbing materials, in this work, FeCo/CoFe₂O₄/carbon nanofiber composites were successfully prepared by electrospinning for the EM wave attenuation application. Benefiting from the loss ability of interface polarization, dipole polarization, and magnetic loss, the composites obtained a bandwidth of 5.0 GHz at a 1.95 mm thickness and an absorption peak of -52.3 dB. More importantly, the radar cross section (RCS) reduction of composite coatings calculated by ANSYS Electronics Desktop 2018 (HFSS) can reach 34.5 dBm², and the RCS value is almost less than -10 dBm² when the incident angle is greater than 20°, demonstrating great scattering ability of the material coating to EM waves. This work, combined with the exploration of the mechanism and the simulation analysis of the absorbing coating, will be of significance for the development of absorbing materials.

Effective EMI shielding behaviour of thin graphene/PMMA nanolaminates in the THz range

The use of graphene in a form of discontinuous flakes in polymer composites limits the full exploitation of the unique properties of graphene, thus requiring high filler loadings for achieving- for example- satisfactory electrical and mechanical properties. Herein centimetre-scale CVD graphene/polymer nanolaminates have been produced by using an iterative ‘lift-off/float-on’ process and have been found to outperform, for the same graphene content, state-of-the-art flake-based graphene polymer composites in terms of mechanical reinforcement and electrical properties. Most importantly these thin laminate materials show a high electromagnetic interference (EMI) shielding effectiveness, reaching 60 dB for a small thickness of 33 μm, and an absolute EMI shielding effectiveness close to 3·10⁵ dB cm² g⁻¹ which is amongst the highest values for synthetic, non-metallic materials produced to date.

The properties of graphene/polymer composites are usually limited by the use of discontinuous graphene flakes. Here, the authors report a fabrication method to realise continuous cm-scale graphene/polymer nanolaminates with enhanced electromagnetic interference shielding effectiveness, conductivity and mechanical properties.

3D-Printed Topological MoS2/MoSe2 Heterostructures for Macroscale Superlubricity

Superlubricity is a fascinating phenomenon which attracts people to continuously expand ultralow friction and wear from microscale to macroscale. Despite the impressive advances in this field, it is still limited to specific materials and extreme operating conditions. Introducing a heterostructure with intrinsic lattice mismatch into delicate topologies mimicked from nature provides a promising alternative toward macroscopic superlubricity. Herein, 3D-printed MoS₂/MoSe₂ heterostructures with bioinspired circular-cored square/hexagonal honeycomb topologies were developed. Compared to 3D-printed Al₂O₃, all topological structures with both high hardness and excellent flexural strength achieve more than 30% decrease in the friction coefficient. The circular-cored hexagonal honeycomb composite with 30% area density exhibits a stable ultralow friction coefficient of 0.09 and a low wear rate of 2.5 × 10^-5 mm³·N^-1 m^-1 under 5 N. Even under 10 N, a highly desirable coefficient value of 0.08 can be maintained within 370 s. The extraordinary ultralow friction could be attributed to the small contact area, high lubricant mass loading, efficient collection and storage of both abrasive debris and lubricant, and the self-orientation in the lubricating film. This work provides new insights into developing high-efficiency lubrication devices and aids in the industrial application of macroscopic superlubricity in future life.

MoS2 nanoparticle/activated carbon composite as a dual-band material for absorbing microwaves

In the search for novel high-performance microwave (MW) absorbers, MoS₂ has shown promise as a MW-absorbing material, but its poor impedance matching limits its applications. Herein, a facile hydrothermal method was used to produce a composite consisting of activated carbon (AC) derived from waste biomass and in situ-grown MoS₂ nanoparticles. Its microwave absorption properties were examined in the 2-18 GHz frequency range, and FESEM and HRTEM images confirmed the formation of MoS₂ nanoparticles on the AC. The maximum reflection loss (RL_max) for the MoS₂/AC composite was -31.8 dB (@16.72 GHz) at 20 wt% filler loading. At 50 wt% filler loading, the MoS₂/AC (MAC50) composite exhibited unique dual-band absorption characteristics in the C and K_u bands. An effective absorption bandwidth (RL < -10 dB) of 10.4 GHz (3-5.2 GHz, 9.8-18 GHz) was achieved at various thicknesses that covered the entire K_u band. Therefore, a sole dielectric absorber can easily be tuned to absorb MWs at multiple frequency ranges. The large surface area and conduction losses of AC combined with the superior dielectric loss properties of MoS₂ resulted in improved impedance matching and attenuation ability of the MoS₂/AC composite. Thus, MoS₂/AC is a promising low-cost dielectric absorber for MW absorption applications.

Direct Ink Writing of Highly Conductive MXene Frames for Tunable Electromagnetic Interference Shielding and Electromagnetic Wave-Induced Thermochromism

3D printing of MXene frames with tunable electromagnetic interference shielding efficiency is demonstrated. Highly conductive MXene frames are reinforced by cross-linking with aluminum ions. Electromagnetic wave is visualized by electromagnetic-thermochromic MXene patterns. The highly integrated and miniaturized next-generation electronic products call for high-performance electromagnetic interference (EMI) shielding materials to assure the normal operation of their closely assembled components. However, the most current techniques are not adequate for the fabrication of shielding materials with programmable structure and controllable shielding efficiency. Herein, we demonstrate the direct ink writing of robust and highly conductive Ti₃C₂T_x MXene frames with customizable structures by using MXene/AlOOH inks for tunable EMI shielding and electromagnetic wave-induced thermochromism applications. The as-printed frames are reinforced by immersing in AlCl₃/HCl solution to remove the electrically insulating AlOOH nanoparticles, as well as cross-link the MXene sheets and fuse the filament interfaces with aluminum ions. After freeze-drying, the resultant robust and porous MXene frames exhibit tunable EMI shielding efficiencies in the range of 25-80 dB with the highest electrical conductivity of 5323 S m^-1. Furthermore, an electromagnetic wave-induced thermochromic MXene pattern is assembled by coating and curing with thermochromic polydimethylsiloxane on a printed MXene pattern, and its color can be changed from blue to red under the high-intensity electromagnetic irradiation. This work demonstrates a direct ink printing of customizable EMI frames and patterns for tuning EMI shielding efficiency and visualizing electromagnetic waves.

Architecting functionalized carbon microtube/carrollite nanocomposite demonstrating significant microwave characteristics

Biomass-derived materials have recently received considerable attention as lightweight, low-cost, and green microwave absorbers. On the other hand, sulfide nanostructures due to their narrow band gaps have demonstrated significant microwave characteristics. In this research, carbon microtubes were fabricated using a biowaste and then functionalized by a novel complementary solvothermal and sonochemistry method. The functionalized carbon microtubes (FCMT) were ornamented by CuCo2S4 nanoparticles as a novel spinel sulfide microwave absorber. The prepared structures illustrated narrow energy band gap and deposition of the sulfide structures augmented the polarizability, desirable for dielectric loss and microwave attenuation. Eventually, the architected structures were blended by polyacrylonitrile (PAN) to estimate their microwave absorbing and antibacterial characteristics. The antibacterial properties against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) were scrupulously assessed. Noteworthy, the maximum reflection loss (RL) of the CuCo2S4/PAN with a thickness of 1.75 mm was 61.88 dB at 11.60 GHz, while the architected FCMT/PAN composite gained a broadband efficient bandwidth as wide as 7.91 GHz (RL > 10 dB) and 3.25 GHz (RL > 20 dB) with a thickness of 2.00 mm. More significantly, FCMT/CuCo2S4/PAN demonstrated an efficient bandwidth of 2.04 GHz (RL > 20 dB) with only 1.75 mm in thickness. Interestingly, FCMT/CuCo2S4/PAN and CuCo2S4/PAN composites demonstrated an electromagnetic interference shielding efficiency of more than 90 and 97% at the entire x and ku-band frequencies, respectively.

Functionalized carbonized monarch butterfly wing scales (FCBW) ornamented by β-Co(OH)2 nanoparticles: an investigation on its microwave, magnetic, and optical characteristics

In this research, a bioinspired carbon structure was applied as a novel, unique, green, affordable, light weight, thin, and broadband microwave absorbing material. Briefly, the monarch butterfly wing scales were pyrolyzed and then CBWs were functionalized using oxidative treatments, following that they were ornamented by hexagonal β-Co(OH)₂ nanoparticles to improve their microwave absorbing features based on an innovative complementary method by combining sonochemistry and hydrothermal routes. Noticeably, the polyacrylonitrile (PAN) was used as a practical medium to fabricate the microwave absorbers developing an integrated structure and augmenting the relaxation loss mechanism. Various analyses were applied to identify the prepared samples including x-ray powder diffraction, diffuse reflection spectroscopy, Fourier transform infrared, field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), vibrating sample magnetometer, and vector network analyzer. The net-like morphology of FCBWs were fully coated by the hierarchical hexagonal β-Co(OH)₂ nanoparticles. FCBW illustrated a saturation magnetization of 0.06 emu g^-1 originated from its defects, distortions, dislocations, unique morphology, as well as folding, developing localized magnetic moments. Noticeably, inserting FCBWs narrow the energy bandgap of β-Co(OH)₂ nanoparticles, amplifying their light absorption and polarizability, desirable for the microwave attenuation. As revealed, FCBW/β-Co(OH)₂/PAN nanocomposite gained strong reflection loss (RL) of 68.41 at 9.08 GHz, while FCBW/PAN achieved broadband efficient bandwidth as wide as 7.97 GHz (RL > 10 dB) with a thickness of 2.00 mm. More significantly, β-Co(OH)₂/PAN nanocomposites demonstrated salient efficient bandwidth of 3.62 GHz (RL > 20 dB) with only 2.50 mm in thickness. Noteworthy, the eye-catching microwave absorptions were obtained by only filler loading of 10 Wt%. The remarkable microwave absorbing properties of the samples were generated from their microwave absorbing mechanisms which were scrupulously dissected. More significantly, the negative imaginary parts were obtained, originated from the produced secondary fields.

Developing MXenes from Wireless Communication to Electromagnetic Attenuation

There is an urgent global need for wireless communication utilizing materials that can provide simultaneous flexibility and high conductivity. Avoiding the harmful effects of electromagnetic (EM) radiation from wireless communication is a persistent research hot spot. Two-dimensional (2D) materials are the preferred choice as wireless communication and EM attenuation materials as they are lightweight with high aspect ratios and possess distinguished electronic properties. MXenes, as a novel family of 2D materials, have shown excellent properties in various fields, owing to their excellent electrical conductivity, mechanical stability, high flexibility, and ease of processability. To date, research on the utility of MXenes for wireless communication has been actively pursued. Moreover, MXenes have become the leading materials for EM attenuation. Herein, we systematically review the recent advances in MXene-based materials with different structural designs for wireless communication, electromagnetic interference (EMI) shielding, and EM wave absorption. The relationship governing the structural design and the effectiveness for wireless communication, EMI shielding, and EM wave absorption is clearly revealed. Furthermore, our review mainly focuses on future challenges and guidelines for designing MXene-based materials for industrial application and foundational research.

Chemically Room Temperature Crosslinked Polyvinyl Alcohol (PVA) with Anomalous Microwave Absorption Characteristics

Polyvinyl alcohol (PVA) is a great interest polymer due to its excellent film-forming, emulsifying, microwave dielectrics and adhesive properties. However, PVA is a water-soluble synthetic polymer making it susceptible to environmental factors. In this work, PVA is crosslinked at room temperature using divinyl sulfone (DVS) as a crosslinker, and the obtained crosslinked PVA (XPVA) is water-insoluble. Crosslinking mechanism is proposed, thermal and microwave dielectric properties of X-PVA are studied. The studies revealed that X-PVA has better thermal stability and microwave absorption properties. The obtained minimum reflection loss (RL) of X-PVA is -23 dB (filler-free) with entire X-band (8.2-12.4 GHz) absorption bandwidth (RL ≤ -10 dB), indicating excellent microwave absorption properties. Artificial neural network (ANN) predicted RL of X-PVA also matched well with the experimental data. Electromagnetic power simulation suggests that the microwave power absorption density due to the dielectric loss is intrinsically predominant in X-PVA compared to the pristine PVA. Further, the ratio of electromagnetic energy to heat energy conversion power (absorption) of X-PVA is much higher than pristine PVA, indicating the suitability for self-powered devices. X-PVA also fulfils many commercial requirements such as bulk level facile synthesis, large area fabrications, ultralight, and inexpensive.

Environmentally Friendly and Multifunctional Shaddock Peel-Based Carbon Aerogel for Thermal-Insulation and Microwave Absorption

The eco-friendly shaddock peel-derived carbon aerogels were prepared by a freeze-drying method. Multiple functions such as thermal insulation, compression resistance and microwave absorption can be integrated into one material-carbon aerogel. Novel computer simulation technology strategy was selected to simulate significant radar cross-sectional reduction values under real far field condition. . Eco-friendly electromagnetic wave absorbing materials with excellent thermal infrared stealth property, heat-insulating ability and compression resistance are highly attractive in practical applications. Meeting the aforesaid requirements simultaneously is a formidable challenge. Herein, ultra-light carbon aerogels were fabricated via fresh shaddock peel by facile freeze-drying method and calcination process, forming porous network architecture. With the heating platform temperature of 70 °C, the upper surface temperatures of the as-prepared carbon aerogel present a slow upward trend. The color of the sample surface in thermal infrared images is similar to that of the surroundings. With the maximum compressive stress of 2.435 kPa, the carbon aerogels can provide favorable endurance. The shaddock peel-based carbon aerogels possess the minimum reflection loss value (RL_min) of - 29.50 dB in X band. Meanwhile, the effective absorption bandwidth covers 5.80 GHz at a relatively thin thickness of only 1.7 mm. With the detection theta of 0°, the maximum radar cross-sectional (RCS) reduction values of 16.28 dB m² can be achieved. Theoretical simulations of RCS have aroused extensive interest owing to their ingenious design and time-saving feature. This work paves the way for preparing multi-functional microwave absorbers derived from biomass raw materials under the guidance of RCS simulations.

Highly stretchable and self-foaming polyurethane composite skeleton with thermally tunable microwave absorption properties

Stretchable and lightweight polymer composite material possessing tunable microwave absorption (MA) properties under thermal radiations remain a significant challenge. Here, we proposed a facile strategy to fabricate stretchable, magnetic composite skeletons by incorporating the tadpole-like CNTs@Fe₃O₄nanoparticles into self-foaming polyurethane (PU) matrix and the electromagnetic responsive of CNTs@Fe₃O₄/PU composite foams with different CNTs contents under heating-cooling cycle in a temperature range of 253 -333 K were carefully investigated. Enhanced complex permittivity and shifting peak frequency were observed at elevated temperatures. For instance, the 70-CNTs@Fe₃O₄/PU sample with 15 wt% loading content at 333 K exhibits excellent MA properties including a minimum reflection loss (RL_m) of -66.9 dB and ultrabroad effective frequency bandwidth (RL ≤ -20 dB) of 9.98 GHz at the thickness of 1.58-3.37 mm. Meanwhile, great recoverability in terms of RL-fprofile was achieved in the process of thermal cooling back to 253 K. Such adjustable MA property was attributed to the well-matched impedance and dramatic attenuation ability, benefiting from the temperature-dependant electrical conductivity, abundant interfacial polarization and interior microcellular structures. Besides, the rising temperature increased the sample elongation and electrical conductivity with a slight sacrifice of maximum tensile strength. This stretchable PU skeleton with a unique assembly of CNTs and Fe₃O₄nanoparticles are expected to be promising candidates as smart absorbers for application in the harsh environments.

Growth of NiAl-Layered Double Hydroxide on Graphene toward Excellent Anticorrosive Microwave Absorption Application

High-performance microwave absorbers with special features are desired to meet the requirements of more complex modern service environments, especially corrosive environments. Therefore, high-efficiency microwave absorbers with corrosion resistance should be developed urgently. Herein, a 3D NiAl-layered double hydroxide/graphene (NiAl-LDH/G) composite synthesized by atomic-layer-deposition-assisted in situ growth is presented as an anticorrosive microwave absorber. The content of NiAl-LDH in the composite is optimized to achieve impedance matching. Furthermore, under the cooperative effects of the interface polarization loss, conduction loss, and 3D porous sandwich-like structure, the optimal NiAl-LDH/G shows excellent microwave absorption performance with a minimum reflection loss of -41.5 dB and a maximum effective absorption bandwidth of 4.4 GHz at a loading of only 7 wt% in epoxy. Remarkably, the encapsulation effect of NiAl-LDH can restrain the galvanic corrosion owing to graphene. The epoxy coating with the NiAl-LDH/G microwave absorber on carbon steel exhibits long-term corrosion resistance, owing to the synergetic effect of the superior impermeability of graphene and the chloridion-capture capacity of the NiAl-LDH. The NiAl-LDH/G composite is a promising anticorrosive microwave absorber, and the findings of this study may motivate the development of functional microwave absorbers that meet the demands of anticorrosive performance of coatings.

A procedure and device for determining complex material permittivity using the free-space resonance method

The essential technologies of the complex permittivity of microwave dielectric materials are systematically designed, and the complex permittivity of materials is tested nondestructively by the free-space resonance method. A testing system was built by using a mobile surveying platform, and the complex dielectric constant of the material in the X band was nondestructively tested by using the algorithm of variable physical cavity length and constant physical cavity length. Focusing on the impact of variable physical cavity length on the test results, the cavity calibration technology is proposed to reduce the influence on the complex dielectric constant test of materials. The free-space resonance method was used to test the complex permittivity of polytetrafluoroethylene, glass steel plate (fiber reinforced plastics), and corundum plate. The results show that the test results of complex permittivity obtained by the two algorithms are consistent, and the error of complex permittivity is less than 5%.

Facile synthesis of 3D Ni@C nanocomposites derived from two kinds of petal-like Ni-based MOFs towards lightweight and efficient microwave absorbers

The development of lightweight and high-efficiency microwave absorption materials has attracted wide attention in the field of electromagnetic wave absorption. Herein, two kinds of petal-like Ni-based MOFs were grown on the surface of graphene nanosheets, and then pyrolyzed to obtain new microwave absorbers. The extraordinary microwave absorption performance mainly comes from: the unique petal-like porous carbon framework of MOFs, the 3D conductive network formed by the connection of GNs, the polarization process between the interfaces of multiple heterogeneous components and high impedance matching brought about by magnetic Ni nanoparticles. By adjusting the filling ratio to only 10 wt%, the optimum reflection loss of the prepared composites is up to -53.99 dB, and the effective absorption bandwidth reaches 4.39 GHz when the matching thickness is only 1.4 mm. This work provides not only a facile method for the design and fabrication of two high-efficiency microwave absorbers, but also a reference for the precise control of electromagnetic absorption properties.

Accurately Engineering 2D/2D/0D Heterojunction In Hierarchical Ti3C2Tx MXene Nanoarchitectures for Electromagnetic Wave Absorption and Shielding

The accurate heterojunction engineering in MXene-based composites unprecedentedly boosts their electromagnetic (EM) wave absorption and shielding performance. However, the flocculation of MXene caused by abundant termination groups severely restricts the regulation of heterojunction, which hankers for a revolutionary compositing strategy against unmanageable self-aggregation. Herein, electrically neutral coordination compound with large molecular volume is decorated on Ti₃C₂T_x lamellas to protect them from self-precipitation. A rapid polymerization reaction then controllably assembles them into a hierarchical microsphere composed of superlattice-like 2D/2D polymer/MXene building blocks. In the carbonized Ti₃C₂T_x/C/MoO₂ microspheres, 2D/2D/0D heterojunctions can be precisely tuned to regulate electric/dielectric properties. These heterojunctions simultaneously trigger the intensive interfacial polarization and out-plane electron flowing to exhaust the EM energy as much as possible, confirmed by electron holography. Therefore, our products achieve the first-rate EM wave absorption with an ultrabroad absorption bandwidth of 7.7 GHz at the thickness of 2.5 mm. By altering the heterojunction, the composite acquires excellent EM interference shielding performance with an average shielding effectiveness of 35.9 dB. These accomplishments light a new way to microstructure construction and heterojunction design of MXene-based composites and lay out a profound insight into their EM wave absorption mechanism.

Infinite Approaching Superlubricity by Three-Dimensional Printed Structures

The rapid development of three-dimensional (3D) printing technology opens great opportunities for the design of various multiscale lubrication structures. 3D printing allows high customization of arbitrary complex structures and rapid prototyping of objects, which provides an avenue to achieve effective lubrication. Current experimental observations on superlubricity are limited to atomically smooth clean surfaces, extreme operating conditions, and nano- or microscales. With the in-depth exploration of 3D printed lubrication, construction of multifunctional 3D structures with refined dimensions spanning from micronanoscale to macroscale is increasingly regarded as an important means to approach superlubricity and has aroused great scientific interest. To document recent advances in 3D printing for structural lubrication, a detailed literature review is provided. Emphasis is given on the design and lubrication performance of geometric and bioinspired lubrication structures with characteristic dimensions. The material requirements, merits, drawbacks, and representative applications of various 3D printing techniques are summarized. Potential future research trends aiming at the design strategy and manufacturing process of 3D printed lubrication structures are also highlighted.

Absorption-enhanced EMI shielding using silver decorated three-dimensional porous architected reduced graphene oxide in polybenzoxazine composites

The proliferation of wearable and portable electronic media has increased the demand for highly efficient materials that can be used to create shields against electromagnetic interference.

A Nano-Micro Engineering Nanofiber for Electromagnetic Absorber, Green Shielding and Sensor

The role of electron transport characteristics in electromagnetic (EM) attenuation can be generalized to other EM functional materials. The integrated functions of efficient EM absorption and green shielding open the view of EM multifunctional materials. A novel sensing mechanism based on intrinsic EM attenuation performance and EM resonance coupling effect is revealed. It is extremely unattainable for a material to simultaneously obtain efficient electromagnetic (EM) absorption and green shielding performance, which has not been reported due to the competition between conduction loss and reflection. Herein, by tailoring the internal structure through nano-micro engineering, a NiCo₂O₄ nanofiber with integrated EM absorbing and green shielding as well as strain sensing functions is obtained. With the improvement of charge transport capability of the nanofiber, the performance can be converted from EM absorption to shielding, or even coexist. Particularly, as the conductivity rising, the reflection loss declines from - 52.72 to - 10.5 dB, while the EM interference shielding effectiveness increases to 13.4 dB, suggesting the coexistence of the two EM functions. Furthermore, based on the high EM absorption, a strain sensor is designed through the resonance coupling of the patterned NiCo₂O₄ structure. These strategies for tuning EM performance and constructing devices can be extended to other EM functional materials to promote the development of electromagnetic driven devices.

Integration of bubble phobicity, gas sensing and friction alleviation into a versatile MoS2/SnO2/CNF heterostructure by an impressive, simple and effective method

The engineering of composite surfaces and interfaces of materials at the micro/nano-hierarchical level with multiple functionalities is attracting increasing attention due to their biomimetic technological applications, especially the self-cleaning with gas bubbles, gas sensing and sustainable anti-friction performances. Herein, the ternary MoS₂/SnO₂/CNF (CNF: carbon nanofiber) was designed and assembled by an in situ facile method. Interestingly, its microstructure exhibits a necklace-like morphology. The MoS₂/SnO₂/CNF shows desirable bubble phobicity under water and in a PAO4 environment on various substrates, an acceptable gas-sensing ability to target gas with a detection limit of 5 ppm and fascinating tribological performances for additives in different kinds of base/lubricating oils. These results demonstrate that the necklace-like ternary MoS₂/SnO₂/CNF structure could have numerous applications in one system and may provide a new perspective in composite surface and interface materials engineering.