Constructing BaTiO_(3)/TiO_(2)@polypyrrole composites with hollow multishelled structure for enhanced electromagnetic wave absorbing properties

BaTiO_(3)/TiO_(2)@polypyrrole(PPy)composites with hollow multishelled structure(HoMS)were constructed to enhance the electromagnetic wave absorbing properties of BaTiO_(3)-based absorbing material.BaTiO_(3)/TiO_(2)HoMSs were prepared by hydrothermal crystallization using TiO_(2)Ho MSs as template.Then,FeCl3 was introduced to initiate the oxidative polymerization of pyrrole monomer,forming BaTiO_(3)/TiO_(2)@PPy HoMSs successfully.The electromagnetic wave absorbing properties of BaTiO_(3)/TiO_(2)HoMSs and BaTiO_(3)/TiO_(2)@PPy Ho MSs with different shell number were investigated using a vector network analyzer.The results indicate that BaTiO_(3)/TiO_(2)@PPy HoMSs exhibit improved microwave absorption compared with BaTiO_(3)/TiO_(2)HoMSs.In particular,tripled-shelled BaTiO_(3)/TiO_(2)@PPy HoMS has the most excellent absorbing performance.The best reflection loss can reach up to-21.80 dB at 13.34 GHz with a corresponding absorber thickness of only 1.3 mm,and the qualified absorption bandwidth of tripled-shelled BaTiO_(3)/TiO_(2)@PPy HoMS is up to 4.2 GHz.This work paves a new way for the development of high-performance composite microwave absorbing materials.

Improvement on the wave absorbing property of a lossy frequency selective surface absorber using a magnetic substrate

An equivalent-circuit model is used to analyse the improvement of the wave absorbing performance of the lossy frequency selective surface（FSS） absorber by using a magnetic substrate,showing that it is possible to widen the wave absorbing bandwidth.Three pieces of magnetic substrates are prepared.According to the complex permittivity and permeability,the reflectivity of the corresponding absorber is calculated by the finite difference time-domain（FDTD） method,and the bandwidth of the reflectivity below 10 dB is optimized by genetic algorithm.The calculated results indicate that the wave absorbing performance is significantly improved by increasing the complex permeability of the substrate;the reflectivity bandwidth below 10 dB of the single layer FSS absorber can reach 3.6-18 GHz with a thickness of 5 mm,which is wider than that with a dielectric substrate.The density of the FSS absorber is only 0.92 g/cm 3.Additionally,the absorption band can be further widened by inserting a second lossy FSS.Finally,a double layer lossy FSS absorber with a magnetic substrate is fabricated based on the design result.The experimental result is consistent with the design one.

Analysis of Wave Height in Front of a Wave Absorbing Structure with Rubble Foundation

Wave absorbing structures have been widely applied in many countries. In the present paper, the wave heights in front of a vertical wave absorbing structure with rubble foundation as well as in the wave chamber of the structure are analysed using an approximative calculation method, and the dissipating effect of the structure is verified. On the basis of the results of regular waves, the relative wave heights of irregular waves in front of the wave absorbing structure as well as in the chamber have also been analysed.

Enhanced microwave absorption property of silver decorated biomass ordered porous carbon composite materials with frequency selective surface incorporation

Porous carbon(PC)is a promising electromagnetic(EM)wave absorbing material thanks to its light weight,large specific surface area as well as good dissipating capacity.To further improve its microwave absorbing performance,silver coated porous carbon(Ag@PC)is synthesized by one-step hydro-thermal synthesis process making use of fir as a biomass formwork.Phase compositions,morphological structure,and microwave absorption capability of the Ag@PC has been explored.Research results show that the metallic Ag was successfully reduced and the particles are evenly distributed inward the pores of the carbon formwork,which accelerates graphitization process of the amorphous carbon.The Ag@PC composite without adding polyvinyl pyrrolidone(PVP)exhibits higher dielectric constant and better EM wave dissipating capability.This is because the larger particles of Ag give rise to higher electric conductivity.After combing with frequency selective surface(FSS),the EM wave absorbing performance is further improved and the frequency region below-10 d B is located in8.20-11.75 GHz,and the minimal reflection loss value is-22.5 dB.This work indicates that incorporating metallic Ag particles and FSS provides a valid way to strengthen EM wave absorbing capacity of PC material.

Initiating Binary Metal Oxides Microcubes Electromagnetic Wave Absorber Toward Ultrabroad Absorption Bandwidth Through Interfacial and Defects Modulation

Cobalt nickel bimetallic oxides(NiCo_(2)O_(4))have received numerous attentions in terms of their controllable morphology,high temperature,corrosion resistance and strong electromagnetic wave(EMW)absorption capability.However,broadening the absorption bandwidth is still a huge challenge for NiCo_(2)O_(4)-based absorbers.Herein,the unique NiCo_(2)O_(4)@C core-shell microcubes with hollow structures were fabricated via a facile sacrificial template strategy.The concentration of oxygen vacancies and morphologies of the three-dimensional(3D)cubic hollow core-shell NiCo_(2)O_(4)@C framework were effectively optimized by adjusting the calcination temperature.The specially designed 3D framework structure facilitated the multiple reflections of incident electromagnetic waves and provided rich interfaces between multiple components,generating significant interfacial polarization losses.Dipole polarizations induced by oxygen vacancies could further enhance the attenuation ability for the incident EM waves.The optimized NiCo_(2)O_(4)@C hollow microcubes exhibit superior EMW absorption capability with minimum RL(RLmin)of-84.45 dB at 8.4 GHz for the thickness of 3.0 mm.Moreover,ultrabroad effective absorption bandwidth(EAB)as large as 12.48 GHz(5.52-18 GHz)is obtained.This work is believed to illuminate the path to synthesis of high-performance cobalt nickel bimetallic oxides for EMW absorbers with excellent EMW absorption capability,especially in broadening effective absorption bandwidth.

Multiphase Interfacial Regulation Based on Hierarchical Porous Molybdenum Selenide to Build Anticorrosive and Multiband Tailorable Absorbers

Electromagnetic wave(EMW)absorbing materials have an irreplaceable position in the field of military stealth as well as in the field of electromagnetic pollution control.And in order to cope with the complex electromagnetic environment,the design of multifunctional and multiband high efficiency EMW absorbers remains a tremendous challenge.In this work,we designed a three-dimensional porous structure via the salt melt synthesis strategy to optimize the impedance matching of the absorber.Also,through interfacial engineering,a molybdenum carbide transition layer was introduced between the molybdenum selenide nanoparticles and the three-dimensional porous carbon matrix to improve the absorption behavior of the absorber.The analysis indicates that the number and components of the heterogeneous interfaces have a significant impact on the EMW absorption performance of the absorber due to mechanisms such as interfacial polarization and conduction loss introduced by interfacial engineering.Wherein,the prepared MoSe_(2)/MoC/PNC composites showed excellent EMW absorption performance in C,X,and Ku bands,especially exhibiting a reflection loss of−59.09 dB and an effective absorption bandwidth of 6.96 GHz at 1.9 mm.The coordination between structure and components endows the absorber with strong absorption,broad bandwidth,thin thickness,and multi-frequency absorption characteristics.Remarkably,it can effectively reinforce the marine anticorrosion property of the epoxy resin coating on Q235 steel substrate.This study contributes to a deeper understanding of the relationship between interfacial engineering and the performance of EMW absorbers,and provides a reference for the design of multifunctional,multiband EMW absorption materials.

MoS_(2)-Decorated/Integrated Carbon Fiber:Phase Engineering Well-Regulated Microwave Absorber

Phase engineering is an important strategy to modulate the electronic structure of molybdenum disulfide(MoS_(2)).MoS_(2)-based composites are usually used for the electromagnetic wave(EMW)absorber,but the effect of different phases on the EMW absorbing performance,such as 1T and 2H phase,is still not studied.In this work,micro-1T/2H MoS_(2) is achieved via a facile one-step hydrother-mal route,in which the 1T phase is induced by the intercalation of guest molecules and ions.The EMW absorption mechanism of single MoS_(2) is revealed by presenting a comparative study between 1T/2H MoS_(2) and 2H MoS_(2).As a result,1T/2H MoS_(2) with the matrix loading of 15%exhibits excellent microwave absorption property than 2H MoS_(2).Furthermore,taking the advantage of 1T/2H MoS_(2),a flexible EMW absorbers that ultrathin 1T/2H MoS_(2)grown on the carbon fiber also performs outstanding performance only with the matrix loading of 5%.This work offers necessary reference to improve microwave absorption performance by phase engineering and design a new type of flexible electromagnetic wave absorption material to apply for the portable microwave absorption electronic devices.

Co-Ni Electromagnetic Coupling in Hollow Mo_(2)C/NC Sphere for Enhancing Electromagnetic Wave Absorbing Performance

For enhancing the electromagnetic wave(EW)attenuation and adsorption,rational constructing and homogeneously distributing bimetallic electromagnetic coupling units in hollow structure is an effective way,but hard to achieve.Herein,a CoNi-doped hybrid zeolite imidazole framework was synthesized as precursor,which was further converted into a hollow CoNi-bimetallic doped molyb-denum carbide sphere(H-CoNi@MoC/NC)through a two-step etching and calcination strategy.At the loading amount of 15 wt%,a strong absorption of minimum reflection loss(RL_(min))of-60.05 dB at 7.2 GHz with the thickness of 3.1 mm and a wide effective ad-sorption bandwidth(EAB)of 3.52 GHz at the thickness of 2.5 mm were achieved,which was far beyond the reported MoC-based metallic hybrids.The crucial synergistic Co-Ni electromagnetic coupling effect in the composite was characterized,not only enhanc-ing the dipolar/interfacial polarization,but also promoting the impedance matching,displaying the optimized EW absorbing perfor-mance.

Microstructural evolution and electromagnetic wave absorbing performance of single-source-precursor-synthesized SiCuCN-based ceramic nanocomposites

Copper(Cu)-containing single-source precursors(SSPs)for the preparation of SiCuCN-based ceramic nanocomposites were successfully synthesized for the first time using polysilazane(PSZ),copper(II)acetate monohydrate(CuAc),and 2-aminoethanol via nucleophilic substitution reactions at silicon(Si)centers of PSZ.The synthesis process,polymer-to-ceramic transformation,and high-temperature microstructural evolution of the prepared ceramics were characterized.Dielectric properties and electromagnetic wave(EMW)absorbing performance of the ceramics were investigated as well.The results show that the polymer-to-ceramic transformation finishes at ca.900 ℃,and Cu nanoparticles are homogeneously distributed in a SiCN matrix,forming a SiCN/Cu nanocomposite.After annealing at 1200 ℃,the Cu nanoparticles completely transform into copper silicide(CusSi).Interestingly,the thermal stability of the Cu nanoparticles can be strongly improved by increasing the free carbon content,so that a part of metallic Cu nanoparticles can be detected in the ceramics annealed even at 1300 ℃,forming a SiCN/Cu/Cu_(3)Si/C nanocomposite.Compared with SiCN,the SiCuCN-based nanocomposites exhibit strongly enhanced dielectric properties,which results in outstanding EMW absorbing performance.The minimum reflection coefficient(RC_(min))of the SiCN/Cu/Cu_(3)Si/C nanocomposites annealed at 1300 ℃ achieves-59.85 dB with a sample thickness of 1.55 mm,and the effective absorption bandwidth(EAB)broadens to 5.55 GHz at 1.45 mm.The enhanced EMW absorbing performance can be attributed to an in situ formed unique network,which was constructed with Cu and Cu_(3)Si nanoparticles connected by ring-like carbon ribbons within the SiCN matrix.

在不同吃水与水深比条件下活塞式主动消波装置水动力学性能解析与数值研究

For active wave absorbers in force-control mode,the optimal feedback(control)force provided by the control system depends on the hydrodynamic forces.This work investigates a piston-type wave absorber with different draft-to-water depth ratios,focusing on the frequency-dependent hydrodynamic coefficients,wave absorption efficiency,wave absorber displacement and velocity,and control force.Analytical results were derived based on potential flow theory,confirming that regular incident waves can be fully absorbed by the piston-type active wave absorber at any draft ratio by optimizing the control force.The results for the wave tank with a typical water depth of 3 m were studied in detail.The draft ratio has a strong influence on the hydrodynamic coefficients.At the maximum wave absorption efficiency,the displacement and velocity amplitudes are sensitive to the draft ratio in the low-frequency region,increase with decreasing draft ratio,and are independent of the mass of the wave absorber.The control force required can be extremely large for a draft ratio greater than 1/3.The control force increases significantly as the draft ratio increases.The mass of the wave absorber has a weak influence on the control force.A time-domain numerical method based on the boundary element method was developed to verify the analytical solutions.Perfect agreements between the analytical solutions and the numerical results were obtained.

Biopolymer-Based Aerogels for Electromagnetic Wave Shielding and Absorbing

The rapid development of communication technology and electronic industry has brought unprecedented serious electromagnetic interference(EMI)and electromagnetic wave(EMW)pollution.Although EMI shields and EMW absorbers based on metal or magnetic materials were used to solve these problems,they have long been criticized for their high price,high density and easy corrosion.In order to achieve low density and efficient dissipation of electromagnetic energy,aerogels stand out among manifold materials.However,constructing aerogels with good EMI shielding or EMW absorption performance and acceptable mechanical properties is not an easy task.Burgeoning biopolymers,such as cellulose,lignin,chitin/chitosan and alginate,breathe new life into aerogels for high-efficiency EMW shielding and absorbing.Here,we reviewed the contributions of biopolymers in the fields of aerogels for EMW shielding and absorbing.At the same time,some challenges and outlook were also pointed out,aiming to promote the advance of aerogel-based EMI shields and EMW absorbers as well as the rational utilization of biopolymers.

Research on Electromagnetic Wave Absorption Based on Electrospinning Technology

At present,in order to overcome electromagnetic interference and prevent electromagnetic harm,the research of new and efficient electromagnetic wave absorbing materials has become the research focus in the field of materials science.The one-dimensional structure can promote the impedance matching and attenuation characteristics of the absorbing materials.Electrospinning,as an effective method to prepare nanofibers with high length-diameter ratio,has been widely concerned because it is suitable for struc-tural design of various materials.In this paper,the research progress and absorption properties of nano-fiber materials prepared by electrospinning combined with different processes are introduced.

Carbon nanofibers embedded with Fe-Co alloy nanoparticles via electrospinning as lightweight high-performance electromagnetic wave absorbers

As a lot of electromagnetic pollution and interference issues have emerged,to overcome electromagnetic interference,prevent electromagnetic hazards,and develop new high-performance electromagnetic wave(EMW)absorbers have become a significant task in the field of materials science.In this paper,a three-dimensional(3D)carbon nanofibers network with core-shell structure,embedded with varied molar ratios of iron and cobalt(4:0,3:1,2:2,1:3,0:4),was effectively synthesized(Fe/Co@C-CNFs)via electrospinning.The phase,microstructure,magnetic and EMW absorption properties were studied.It is discovered that Fe/Co@C-CNFs doped with iron:cobalt=1:1 have excellent EMW absorption capacity.When the matching thickness is 1.08 mm,the minimum reflection loss(RL)value is-18.66 dB,while the maximum effective absorption bandwidth(EAB)reaches 4.2 GHz(13.9-18 GHz)at a thickness of 1.22 mm.This is owing to the absorbers'superior impedance matching and multiple reflections as well as the conductivity,dielectric,and magnetic losses of carbon nanofibers embedded with Fe-Co alloy particles.In addition,the radar cross section(RCS)of the absorbers has been calculated by CST Studio Suite,showing that the absorbing coating can effectively reduce the RCS at various angles,especially for Fe/Co@C-CNFs doped with iron:cobalt=1:1.These findings not only provide new insights for the preparation of light-weight and high-performance electromagnetic wave absorbers,but also contribute to energy storage and conversion.

From binary to ternary and back to binary:Transition of electromagnetic wave shielding to absorption among MAB phase Ni_(3)ZnB_(2)and corresponding binary borides Nin+1Bn(n=1,3)

Due to chemical inertness of nickel and boron,the preparation of nickel borides and corresponding layered ternary transition metal borides Ni_(3)ZnB_(2)(MAB phase)has always required high-temperature and/or high-pressure conditions.Yet,an innovative and efficient approach to preparing Ni_(3)ZnB_(2)at only 600℃and without applied pressure is presented in this study.It is discovered that by simply adjusting the temperature,a phase transition from Ni_(3)ZnB_(2)to Ni4B3 with a layered structure could be induced.This transition between the binary-component and the ternary-component brings about significant variation in electromagnetic wave(EMW)shielding/absorption performance of prepared borides.For instance,Ni2B has good EMW shielding performance(42.54 dB in X band)and Ni_(3)ZnB_(2)is of weak EMW shielding(13.43 dB in X band);Ni_(3)ZnB_(2)has poor EMW absorption performance(−5 dB)while Ni4B3 has excellent EMW absorption performance(−45.19 dB)at a thickness of 2.7 mm with effective absorption bandwidth(10.4 GHz).

Fabrication and efficient electromagnetic waves attenuation of three-dimensional porous reduced graphene oxide/boron nitride/silicon carbide hierarchical structures

In this work,hierarchical hybrid composites consisting of porous three-dimensional reduced graphene oxide(3D-rGO)skeleton and lamellar boron nitride(BN)/silicon carbide(SiC)coatings are prepared by chemical vapor infiltration(CVI)process.The graphene framework prepared by 3D printing and frozen self-assembly exhibits a lightweight structure and a perforated conductive network,which extends the transmission path of incident microwaves.The introduced ceramic coatings can effectively tune the impedance matching degree and supply a lossy phase,and the hierarchical structure of the composites enhances the multiple scattering of the incident microwaves.As expected,the 3D-rGO/BN/SiC composites possess an excellent absorbing performance with a minimum reflection loss value of–37.8 dB,and the widest effective absorbing bandwidth(RL<–10 dB)of 5.90 GHz is obtained.The controllable fabrication of composites can provide a guideline for rational design and fabrication of high-performance electromagnetic waves absorbing materials in practical applications.

Experimental Study on Reflection Coefficient of Curved Perforated Plate

A set of experiments is carried out in a towing tank to study the effects of the curvature of perforated plates on the wave reflection coefficient （Cr）. The curvature of a perforated plate can be changed by rotating a reference perforated plate about its origin point according to the parabolic equation y=-x2 A plunger-type wave maker is used to generate regular waves. The reflection coefficients are calculated using Goda and Suzuki’s （1976） method. The results are compared with those of vertical or sloped passive wave absorbers. The comparison shows that a perforated plate with a curved profile is highly efficient in terms of reducing the wave reflection coefficient. A correlation is established to estimate the reflection coefficient of curved perforated plates as a function of both flow and geometry characteristics.

Experimental Study on Performance of Combined Cambered-Type Wave Absorbing Beach

Wave simulation performance and its quality are key factors to reflect the overall capacity and level of an ocean engineering basin. They include wave simulating and absorbing capacity of reflected waves. In order to reduce the influence of reflected waves, various wave absorbing devices are equipped in ocean engineering basins across the world. The experimental investigation into the performance of combined cambered-type wave absorbing beach(CCTWAB) with damping bars equipped in Deepwater Offshore Basin is conducted. The experiment adopts the two-point method. The reflection coefficients are calculated by the method, in which the incident and reflected waves can be separated from the physically simulated composite waves with different periods and wave heights in the time domain. The experimental results indicate that in the range of normal wave heights and periods for model tests, the CCTWAB with damping bars is excellent in eliminating the reflected waves.

Bifunctional Two-Dimensional Nanocomposite with Electromagnetic Wave Absorption and Anti-bacterial Performance

In current electronic information era,the complex application circumstance of 5G devices pursues the exploration of multi-functional electromagnetic wave(EMW)absorbent materials and it has become the crucial focus in industrial development.A two-dimensional(2D)graphite nanosheet decorated by nickel nanocapsules(2D graphite/Ni@C nanocomposite)was fabricated to possess the EMW absorption and the Escherichia coli(E.coli)anti-bacterial performance simultaneously.By adjusting the filling ratio and injecting nitrogen doping,the value of minimum reflection loss is−36.08 dB and the effective absorption bandwidth reaches to 5.12 GHz(from 11.4 to 16.52 GHz)with the mass ratio of 30 wt%and the absorber thickness of 2 mm.This 2D nanocomposite simultaneously gets an excellent anti-bacterial function expressing an E.coli anti-bacterial rate of 92%during 24 h which is significantly correlated to the interaction between the nanostructure of the 2D nanographite and the nickel ion released from Ni@C nanocapsules.This work provides a new approach to develop a promising 2D anti-bacterial EMW absorber.

Morphology controllable urchin-shaped bimetallic nickel-cobalt oxide/carbon composites with enhanced electromagnetic wave absorption performance

The microscopic morphology of electromagnetic wave absorbers influences the multiple reflections of electromagnetic waves and impedance matching,determining the absorption properties.Herein,the urchin-shaped bimetallic nickel-cobalt oxide/carbon(NiCo_(2)O_(4)/C)composites are prepared via a hy-drothermal route,whose absorption properties are investigated by different morphologies regulated by changing calcination temperature.A minimum reflection loss(RL_(min))of-75.26 dB is achieved at a match-ing thickness of 1.5 mm,and the effective absorption bandwidth(EAB)of 8.96 GHz is achieved at 2 mm.Multi-advantages of the synthesized NiCo_(2)O_(4)/C composites contribute to satisfactory absorption proper-ties.First,the interweaving of the needle-like structures increases the opportunities for scattering and multiple reflections of incident electromagnetic waves,and builds up a conductive network to facilitate the enhancement of conductive losses.Second,the carbon component in the NiCo_(2)O_(4)/C composites en-hances the interfacial polarization and reduces the density of the absorber.Besides,generous oxygen va-cancy defects are introduced into the NiCo_(2)O_(4)/C composites,which induces defect polarization and dipole polarization.In summary,the ternary coordination of components,defects and morphology led to out-standing electromagnetic wave absorption,which lightened the path for improving the electromagnetic wave absorption property and enriching the family of NiCo_(2)O_(4) absorbers with excellent performance.

Electrostatic Encapsulation of Cobalt Ions into Crystalline Framework Derived Polymer Aerogel:Ultra-light,Pressure Resistant,Hydrophobic,Photothermal Conversion,Heat Insulation and Infrared Stealth

A variety of electromagnetic wave absorption materials(EMWAs)have been reported,but the integration of powder materials and multifunctional devices should be investigated in-depth to adapt to practical demands.Herein,carbon-coated cobalt composites were prepared by adsorbing magnetic metal cations into an anionic crystalline framework through an electrostatic encapsulate process.Excellent reflection loss(RL_(min))of-40.49 d B and effective absorption bandwidth(EAB)of 5.36 GHz(RL<-10 d B,10.4-15.76 GHz)was achieved with an optimal radar cross section(RCS)reduction of 34.9 dB·m^(2) for the sample tested.For commercial applications,Co@CN-4 was integrated into sodium carboxymethyl cellulose(CMC)aerogel to create an ultra-lightweight composite aerogel that is compressive resistant and heat-holding while also having photothermal conversion capabilities.The hydrophobic modification makes it more widely useful.This study provides a new strategy for EWAMs to integrate versatility and improve their application prospects.