Structure–Activity Relationship in Microstructure Design for Electromagnetic Wave Absorption Applications

Microwave absorbing materials (MAMs) are materials that effectively absorb incident electromagnetic (EM) wave energy, reducing reflection and scattering. They play a crucial role in enhancing electronic reliability, healthcare, and defense security. However, traditional MAMs like ferrites, magnetic metals, and polymers possess certain limitations, including low impedance matching, narrow absorption bandwidth, poor chemical stability, and high filling ratio, which hinder their further development. To address the requirements of lightweight, wideband, and high‐efficiency absorption, precise structural design has emerged as a captivating research focus. Additionally, comprehending the structure–property relationships between these unique microstructures and EM response and loss mechanisms still poses significant challenges. Herein, a comprehensive review of MAMs is presented with varied structural designs encompassing various scales, providing a detailed introduction of the relationship between various potential structural designs of MAMs and their corresponding EM characteristics and loss mechanisms. Moreover, EM theoretical calculation models, characterization, and analysis methods are discussed. Finally, the article proposes the challenges and prospects for the development of structural design EM wave absorbers.


Introduction
The development of information technology has significantly enhanced our daily lives.However, the excessive use of electronic devices can generate a substantial amount of electromagnetic (EM) radiation, posing risk to harm electronic devices and human health.In light of the rapid development of radar detection technology, EM wave stealth materials have emerged as essential elements in military operations and national defense security by minimizing enemy detection and tracking.3] Studies have demonstrated that microwave-absorbing materials (MAMs) can effectively convert EM waves into thermal energy, offering advantages in reducing EM wave reflection and eliminating secondary pollution caused by EM reflection.These materials have proven relatively effective in reducing EM radiation. [4,5]evertheless, traditional absorbing materials like ferrites, magnetic particles, and partially conductive polymers fall short in meeting the requirements of multispectrum, ultra-wideband, functionalization, and long-term service life due to issues such as high density, complicated preparation process, and poor environmental stability. [6]9] The microwave absorption (MA) performance of materials is greatly influenced by their intrinsic EM parameters.The physical and chemical characteristics, such as conductivity, permeability, and dielectric properties, as well as the morphology of materials affect the intrinsic EM parameters of materials, therefore affecting the MA performance.Furthermore, the morphology of a material can induce changes in its physical and chemical properties. [10,11]Currently, there are numerous studies focused on regulating the EM response of MAMs to enhance their absorption performance through adjusting their physical and chemical properties.[14][15] Typically, it is important to note that electromagnetic wave absorption (EWA) is not solely attribute a single loss mechanism, especially for hybrid or composite materials, where the absorption of EM waves involves complex and diverse coupling mechanisms.The dominate EM wave response mechanisms of the same material differ significantly across different dimensions and scales.Nanometer-scale materials, such as nanoparticles, nanowires, and nanotubes, owing to their sizes being much smaller than the wavelength of electromagnetic waves, primarily achieve electromagnetic wave attenuation through surface polarization and localized surface plasmon resonance.As a result, nanoscale heterogeneous interfaces predominantly generate interface polarization.Submicrometer and micrometer-scale materials, whose sizes are comparable to or slightly smaller than the wavelength of electromagnetic waves, have electromagnetic wave attenuation that is primarily achieved through space charge polarization and conductive polarization.Hence, the degree of order in micrometer-scale aggregated structures formed by numerous nanoparticles largely influences the conductive loss.Larger-scale micrometer materials, such as core-shell materials and porous materials, possess sizes that are larger than the wavelength of electromagnetic waves, and they have mainly realized electromagnetic wave attenuation through multiple reflections and diffractions.Millimeter-scale and centimeter-scale absorbing materials, such as metamaterials, whose sizes are much larger than the wavelength of electromagnetic waves, dissipate energy primarily through the generation of resonant units.Moreover, different types of metamaterial absorbers exhibit various electromagnetic loss mechanisms, including dielectric loss, magnetic loss, ohmic loss, etc.
Therefore, the design of multiscale morphological structure plays an important role in effectively improving impedance matching and EM wave attenuation for MAMs.Morphological structure design involves adjusting parameters such as geometric shapes, size, arrangement, etc., of the absorbing material to achieve control and optimization of EM wave absorption (EWA) performance without altering the intrinsic properties of the material.29] For example, Huang et al. were inspired by the linear microstructure of the nepenthes and prepared a carbon fiber MAM with a hierarchical chiral helical structure.By applying spiraldistributed stress to the fibers to create barriers, the stress induced helical electric dipoles by accumulating positive and negative charges at the barriers (Figure 6i1-i3).The number of polarization modes increases with the upgrade of the helical structure, while the chiral barriers transform the polarity vectors of nonchiral electric dipoles into pseudovectors, enhancing the cooperative EM loss capacity.Chiral barriers and helical fibers help locate the EM field, causing strong resonance and attenuation of microwaves.The effective absorption bandwidth (EAB) of the chiral carbon fiber can reach 9.2 GHz. [30]ompared to traditional MAMs, structurally designed absorbing materials offer flexibility in controlling over EM wave absorption, enabling functionality such as multiband absorption, wide-band absorption, and polarization insensitivity.Furthermore, structurally designed absorbing materials allow for ultra-thin and lightweight designs, minimizing the impact on the carrier's performance while enhancing stealth capabilities.33][34] This article aims at reviewing the impact of morphological structure design on the EM response mechanism and enhanced EM loss mechanism of absorbing materials, providing the absorbing mechanism and loss mechanism of materials with different structures, including interface design and regulation, aggregation state design and regulation, multilevel structure design and regulation, and macrolevel metamaterial structure design and regulation.The study summarizes the EM wave loss mechanism of different structures and clarifies the structure/ morphology design and EM absorption relationship using EM theory calculation models and relevant simulation characterization technology.Finally, it proposes challenges and prospects for the future development of functional absorbing materials with morphology structure design.

Design and Regulation of Morphological Structure of Electromagnetic Wave-Absorbing Materials
Excellent performance of MAM is known to result from the perfect synergy between composition and microstructure.Morphological structure design, as an important strategy for adjusting impedance matching and EM wave attenuation, has become the primary means of regulating EM characteristics, response mechanisms, and loss modes.The interaction between MAMs and EM waves exhibits size effects, and different levels of morphological structure design can achieve different EM responses and loss mechanisms.Therefore, achieving efficient EWA relies heavily on morphological structure design based on a certain scale.Morphological structure design can be categorized into three levels: nano, micro, and macro.Nanoscale morphological structure design primarily focuses on designing the nanostructures of MAMs to regulate the resonance effects of incident EM waves.For example, by fabricating 1D or 2D nanostructures such as nanowires, nanotubes, and nanospheres, surface plasmon resonances, localized surface plasmon resonances, and magnetic dipole resonances can be induced, enhancing the scattering and absorption of EM waves.The introduction defects or dopants into nanostructures allow for adjustment of the resonance frequency and intensity, thereby widening the absorption bandwidth.Microscale morphological design primarily refers to constructing unique microstructures on the surface or inside MAMs to regulate the absorption effects of incident EM waves.For instance, adjusting parameters such as porosity and pore size distribution of porous materials like aerogels and foams can improve their conductive and magnetic losses.Designing multilevel MAMs with core-shell structures can enhance interface losses.Additionally, controlling the ordering of aggregate structures composed of multiple particles, such as crystal structure modulation, can improve the dielectric relaxation and magnetic loss of MAMs.Macroscale morphological structure design mainly encompasses the surface or thickness of absorbing materials to regulate the reflection, refraction, scattering, and other effects of incident EM waves.For example, EM metamaterials can flexibly control the frequency, polarization, amplitude, and phase characteristics of scattered EM waves by designing the structure, arrangement, and EM parameters of the units.As a result, metamaterials have found broadly application in multifunctional devices, EM stealth, polarization control, and highperformance antennas, among other fields.
In this section, we primarily focus on introducing the research scope and EWA mechanism of morphology structure design and regulation from the perspectives of different scales, including interface regulation at the microscale (nanolevel), aggregate state regulation at the small scale (submicron-level and micron-level), multilevel structure design at the large scale (micron-level), and metamaterial structure design at the macroscopic scale (millimeter-level and centimeter-level).We discuss the relationship between structure design and absorption performance, as well as the enhanced absorption mechanism.The regulation of morphology structure design can adjust the EM response and MA performance by altering the material's electronic structure, EM parameters, and increasing EM wave attenuation times.

Interface Design and Regulation
[37] Heterogeneous interfaces can be classified into three categories: homogenous interfaces, heterogeneous interfaces, and composite interfaces.Homogeneous interfaces occur within the same material due to factors such as crystal structure, orientation, and phase, such as grain boundaries, twin boundaries, and dislocations.These interfaces primarily affect the mechanical properties of materials, such as strength, toughness, and plasticity.Heterogeneous interfaces refer to interfaces between different materials, such as metal/semiconductor, semiconductor/ semiconductor, metal/ceramic, etc.These interfaces mainly influence the electrical, magnetic, optical, and other properties of the material, such as conductivity, magnetization intensity, refractive index, etc. Composite interfaces are complex interfaces composed of multiple materials, such as multilayer films, nanocomposites, porous materials, etc.These interfaces mainly impact the comprehensive properties of the material, including wear resistance, corrosion resistance, and absorbing performance.This section mainly delves into the variations in EM response mechanisms caused by homogeneous interfaces and heterogeneous interfaces.
The properties of crystal structure and interfaces can induce changes in the band structure, leading to the accumulation and transfer of charges in the vicinity of heterojunctions and affecting the directional transport of charge carriers and conductivity, thereby altering the magnitude of dielectric loss. [38,39]In 2016, Che et al. quantitatively described the abrupt change in electrostatic potential distribution and charge density at the heterojunction interface using off-axis electron holography for the first time (Figure 1a-c).They explained the bipolar phenomenon at the interface of elongated hexagonal double cone La(OH) 3 nanorods and provided a quantitative characterization of the spatial charge distribution and a visualized strategy for electron concentration. [40]The same technique was employed to explain the interface polarization effect in other materials, where charge separation was observed in a polydopamine/α-MnO 2 microspindle.Negative electrons accumulated on the outer oxide side, while positive electrons gathered on the inner polydopamine side, illustrating a typical interface polarization feature (Figure 1d-f ). [41]Furthermore, multiwalled carbon nanotubes (MWCNTs) exhibit directional transport channels for charge carriers, forming a local microcurrent network.There are directional transport channels for charge carriers in MWCNTs, forming a local microcurrent network (Figure 1g,h). [42]ultiple interface polarizations in multielement composite systems are closely related to increasing dielectric loss and enhancing MA performance.Therefore, it is necessary to design the precise interface properties of MAMs and expand heterogeneous interfaces, which provide guidance for the development of MAMs.
The difference in dielectric properties between two materials can affect the distribution of positive and negative charges, leading to an imbalanced charge distribution and the generation of spatial electric dipole moments.This can trigger polarization relaxation, resulting in increased conduction and polarization losses. [43,44]Cao et al. extensively studied the interface polarization effect by using a capacitive structure and an equivalent circuit model (Figure 1i).They have constructed PANI/Fe 3 O 4 / MWCNTs heterojunction interfaces, [45] synthesized graphene, [46] Fe 3 O 4 -MWCNTs, [47] and ZnO@MWCNTs, [48] among others, to control the interface polarization.Using precise cutting of Fe 3 O 4 -N-doped graphene clusters (Fe 3 O 4 -NG), they have uniformly injected small magnetic clusters onto nitrogen-doped graphene, which act as a connective body between NG nanosheets, generating eddy currents and magnetoresonance-induced magnetic loss.The Fe 3 O 4 -NG system demonstrated synergistic effects in dielectric-magnetic synergistic loss due to good conductivity and improved impedance matching (Figure 1k 1 -k 4 ). [49]n addition, lattice strain and stress can affect charge transfer and local distribution. [50,51]When the lattice mismatch is large enough, lattice strain will be generated to stabilize the crystal structure, which will inevitably lead to lattice defects (such as dislocations, vacancies or impurities), causing charge accumulation or loss and providing rich active sites for dipole polarization. [52]he formation of dislocations is energetically favorable to reduce local strain and stabilize the interface structure.For molecules containing polar functional groups such as hydroxyl (-OH), carboxyl (-COOH), and amino (-NH 2 ), these polar groups can increase the polarity of the molecule, thereby enhancing the dielectric constant and dielectric loss.Additionally, polar functional groups can form hydrogen bonds or van der Waals forces with other polar groups, increasing the intermolecular interactions, improving the stiffness and flexibility of the molecular chains, and enhancing the polarization relaxation capability of the molecular chains.By introducing vacancy defects or graphite structures, the conductivity and structural order of the material can be modified, enhancing spatial charge polarization and conduction polarization, and increasing dielectric loss and absorption efficiency.Vacancies can trigger unsaturated active sites, induce free electrons to form dipole moments, and dissipate EM energy through dipole polarization.[58] When the oxygen-containing part of the graphene lattice is lost, the degree of graphitization increases, leading to more vacancies.N atoms occupy the defect area, providing more electrons and holes, promoting electron migration, and increasing conductivity loss (Figure 1j).
In addition, adjusting the configuration of the bonds in the graphene lattice can change the electronic band structure and density of states, thereby increasing dipole relaxation loss.Pyrrolic N and pyridinic N provide electron lone pairs for pi-conjugated systems, producing dipoles and improving dipole relaxation loss, while graphitic N mainly increases conductivity loss.Both are beneficial to improving dielectric loss and EWA performance. [59]eproduced with permission. [40]Copyright 2016, Springer Nature.d) HCM-5 (hollow hierarchical polydopamine@α-MnO 2 ).e) Original TEM image of a single spindle HCM-3.f ) Reconstructed phase image.Reproduced with permission. [41]Copyright 2016, the American Chemical Society.g) Transport in contact MWCNTs.h) Transport in MWCNT network.Reproduced with permission. [42]Copyright 2013, Elsevier.i) Illustrations of interface effects and contact resistance for PANI/Fe 3 O 4 /MWCNTs.Reproduced with permission. [45]Copyright 2012, the American Chemical Society.j) 3D difference charge density of nonperfect graphene.Reproduced with permission. [52]Copyright 2020, John Wiley and Sons.k 1 -k 4 ) Schematic illustration for the microwave absorption of Fe 3 O 4 -NG.Reproduced with permission. [49]Copyright 2017, Elsevier.l) TEM figures and m) corresponding off-axis electron holograms of CMT@CNT/Co composites.Reproduced with permission. [62]Copyright 2019, John Wiley and Sons.n) Electron holography images, and o) reconstructed phase images of Co@NCNT-o composites.Reproduced with permission. [60]Copyright 2020, John Wiley and Sons.
Che et al. prepared a variety of morphological MOF-derived Ndoped carbon materials Co@NC, in which there is a difference in charge density between the cobalt nanoparticles and the carbon material.Positive and negative electrons are located in the positions of Co NPs and C, respectively (Figure 1n,o).Therefore, local interface polarization significantly increases dielectric loss.Additionally, defects and N doping sites can act as polarization centers, leading to dipole polarization, and making additional contributions to EM absorption. [60]part from dielectric behavior, the heterointerface plays a critical role in magnetic response.For instance, the regulation of heterointerface design is closely related to magnetic coupling cooperation.Magnetic coupling interactions refer to exchange interactions between adjacent magnetic nanoparticles' magnetic moments. [61]Che's group constructed a layered tubular C/Co nanoparticle composite material, which introduced a heterojunction interface, avoiding the aggregation of magnetic particles while inducing strong interface polarization.The multi-interface layered structure provided a 3D magnetic coupling network, generating strong magnetic coupling and increasing magnetic loss capacity.The stray field between the magnets could be directly represented by off-axis electron holography and micromagnetic simulations, demonstrating conclusively that magnetic coupling effects made a significant contribution to the attenuation of EM wave energy (Figure 1l,m). [62]eterogeneous interface design introduces various interface effects that contribute to the attenuation of EM waves.These effects include interface polarization resulting from differences in charge distribution density, dipole polarization caused by lattice defects, and magnetic response due to lattice mismatch.By gaining a deep understanding and precise control over the formation mechanism, structural characteristics, and physical properties of heterogeneous interfaces, it becomes possible to optimize and innovate the MA performance.However, interface design needs to be designed on the nanoscale of the material, and most of the research on interface regulation does not clarify the EM response mechanism corresponding to interface effects, making it difficult to achieve accurate interface control.Based on current research, elucidating the structural effects of heterojunction interfaces on electromagnetic response and absorption performance is of significant importance and also poses a challenge for precise interface engineering.

Aggregation State Structure Design and Regulation
Microscale interface design has achieved remarkable results and researchers have expanded the research field of morphology structure design and conducted multiscale structure design for MAMs, to broaden the absorption bandwidth and increase microwave absorption.Manipulation of small-scale aggregate structures at the microscale and nanoscale has garnered widespread attention.Aggregate structures refer to the structural forms of substances in different aggregated states, such as solids, liquids, and gases.When it comes to absorption materials, the emphasis predominantly lies on solid structures.Solid structures are composed of atoms, molecules, or ions, such as crystal structures or amorphous structures. [63,64]Crystal structures are solid structures composed of periodically ordered molecules, atoms, or ions, which give the solid a definite lattice and long-range order.The extent of molecular ordering is leveraged to regulate the dielectric and magnetic losses of materials.Typically, higher molecular order leads to lower dielectric loss and higher magnetic loss, while lower molecular order leads to higher dielectric loss and lower magnetic loss.Thus, by adjusting the degree of molecular ordering, control over the absorption frequency band and intensity can be achieved.For instance, introducing amorphous phases, liquid crystal phases, oriented phases, and other structural forms can increase the material's disorder or dynamics, thereby enhancing dielectric relaxation and magnetic hysteresis losses.
As we know, MAMs materials can be mainly categorized into three types: dielectric absorbers, conductive absorbers, and magnetic loss absorbers.In this section, we will discuss the influence of the extent of microscale aggregation order in different types of absorbers on the EM response mechanisms and loss mechanisms.

Dielectric Absorbing Materials
Dielectric absorbing materials exhibit high dielectric constants and dielectric loss capabilities.[67][68] Dielectric absorbing materials primarily rely on dielectric loss for absorption, which is closely related to the polarization processes in the materials.These polarization processes include electronic cloud displacement polarization, polar molecule dipole reorientation polarization, and wall displacement, and others.Each type of polarization corresponds to different frequency ranges, thus limiting the absorption frequency of dielectric absorbing materials based on their polarization mechanisms.To achieve wideband and efficient absorption, it becomes necessary to utilize the coupling effect of multiple polarization mechanisms or increase the polarization intensity.The design of ordered structures in the material's aggregated state is carried out at the microscale or nanoscale by arranging or combining absorbing materials in an ordered or periodic composite structure.
Dielectric materials mainly consisted of new semiconductor ceramics such as TiO 2 , ZnO, and carbon materials.Rutile and anatase are the two main crystal phases of TiO 2 , and their formation conditions are different.Rutile is typically obtained at room temperature, while anatase is formed at high temperatures, which results in different lengths of bonds and the arrangements of main structural units.The crystal structure of TiO 2 significantly influences the difference in electrical properties, and rutile with a bandgap of 3.0 eV has a higher dielectric loss capacity than anatase with a bandgap of 3.2 eV. [69]Alford et al. regulated the dielectric loss tangent value tanδ ε of TiO 2 by adjusting crystal defects, and the introduction of divalent and trivalent ions with a radius in the range of 0.5-0.95Å into TiO 2 could lower tanδ ε . [70]dditionally, Chen's team found that the ε 0 and ε 00 values of hydrogenated TiO 2 nanocrystals were 3.4 and 70.1 times higher than those of the same material's original nanocrystals, respectively (Figure 2a-c). [71]The average tanδ ε was increased by more than 15 times, indicating superior absorption performance of hydrogenated TiO 2 nanocrystals compared to pure TiO 2 nanocrystals.For the mixture of rutile and anatase phases, the collective motion of the interface dipoles (CMID) at the rutile/anatase and crystal/disordered interfaces enhances the response to the incident EM field, thereby improving the MA performance.Different crystal structures induce interface effects, in which interface polarization occurs when charge is aggregated at the boundary between two regions or materials.This interface effect can be further demonstrated by electron holography. [19]n the regulation of interface design, crystal defects such as doping and vacancies can influence the dielectric properties of materials.Chen et al. demonstrated that 2D electron gas plasmas resonating on the disordered/ordered interface generated during the hydrogenation process can help improve the performance of absorbers.By employing ZnO and TiO 2 as substrates, they significantly increased the complex dielectric constants of the materials (Figure 2d,e). [72]Wurtzite is a crystal structure of ZnO.In thermodynamic equilibrium, positively charged Zn and negatively charged O polar surfaces produce normal dipole moments and spontaneous polarization. [73]Both current dissipation and dipole polarization can increase dielectric loss of ZnO. [74]

Conductive Absorbing Materials
Conductive-absorbing materials are commonly used in the field of EWA, which utilizing the resistivity and surface resistance of conductive materials to absorb EM wave energy.[77] Conductive absorbing materials offer advantages such as simple preparation, low cost, and good Reproduced with permission. [71]Copyright 2013, John Wiley and Sons.d) Real part (ε 0 ) and e) imaginary part (ε 00 ) of the complex permittivity.Reproduced with permission. [72]Copyright 2015, the American Chemical Society.f ) Microwave reflection loss curves of the Fe 3 O 4 /EP and Fe 3 O 4 @TiO 2 /EP composites.Reproduced with permission. [37]Copyright 2012, John Wiley and Sons.g) Real (μ 0 ), h) imaginary (μ 00 ) values of permeability and i) magnetic loss (tanδ μ ), and j) μ 00 (μ 0 ) À2 f À1 [inset of (j): M-H plots at 300 K for all samples] Reproduced with permission. [86]Copyright 2021, Elsevier.
stability compared to magnetic loss-absorbing materials.However, they still face limitations such as high density, large thickness, and impedance mismatch, which hinder their further development.To enhance their absorption performance, the design and control of ordered structures in the materials have become an important approach.For instance, by adjusting the grain size, grain boundary structure, or the formation of conductive channels, the EM response of the material can be regulated, and interface effects can be increased, resulting in efficient microwave absorption.Manipulating ordered structures in conductive materials provides a pathway to optimize their absorption properties.
Metal-organic frameworks (MOFs) are crystalline porous materials composed of metal ions/clusters and organic ligands, which possess advantages such as compositional design, tunable pore structures, high crystallinity, and large surface area, making them widely applicable in the field of electromagnetic wave absorption (EWA). [78]MOFs can easily undergo thermal treatment to transform into nanostructured functional materials, including porous carbon materials, metal oxides, metal sulfides, and their composites.Yang et al. achieved controllable aspect ratios of multilevel rod-shaped NiCo alloy/carbon nanorod@CNTs structures by adjusting the Ni/Co ratio through high-temperature carburization of NiCo-MOF-74 nanorod precursors. [79]The suitable Ni/Co ratio enabled the formation of dual conductive pathways between the NiCo alloy/carbon nanorods and intertwined carbon nanotubes, owing to their high aspect ratio and dense coating.Therefore, the NiCo alloy/carbon nanorods@CNTs composite exhibited additional conduction loss and excellent impedance matching.SiC, as a semiconductor material, has been widely used in MAMs due to its hightemperature oxidation resistance, high fracture toughness, low density, and high melting point.In SiC crystallography, each Si/C atom is surrounded by four C/Si atoms, forming SP 3hybridized tetrahedra with various crystal structures. [80]The stacking sequence of multilayered C-Si bilayers influences its electrical properties.Silicon carbide nanofibers have been widely used as high-temperature EM wave absorbers due to their 1D structure. [81,82]The dielectric performance of these nanofibers can be tuned by the crystal structure.Chen et al. observed that 3C-SiC nanowires exhibited the best conductivity and the highest dielectric loss due to their narrow bandgap. [83]

Magnetic Loss-Absorbing Materials
Magnetic loss-absorbing materials primarily rely on magnetic loss to absorb EM waves.The magnetic loss performance is closely related to the dynamic magnetization process of magnetic domains or magnetic dipoles in the material under the influence of a magnetic field.The dynamic magnetization process includes hysteresis loss, eddy current loss, damping loss, and magnetic aftereffect loss. [84,85]Common magnetic loss materials include ferrites, carbonyl iron powder, and other magnetic materials.Compared to dielectric loss-absorbing materials, magnetic loss-absorbing materials have advantages such as thinner thickness, wider absorption bandwidth, and better low-frequency absorption performance.However, they also have disadvantages such as higher density, heavier weight, and poor impedance matching, which limit their application range.There are two methods to improve the performance of magnetic loss-absorbing materials.One approach involves composite mixing of magnetic materials with dielectric absorbing materials, utilizing the dielectric-magnetic coupling effect to achieve impedance matching and enhanced loss.The other approach entails introducing ordered structures on the surface or inside of the magnetic materials, such as multilayer films/coatings, nanoparticles, and adjust the magnetic domain structure, magnetic anisotropy, or permeability of the magnetic materials to regulate their EM response and scattering characteristics.These ordered structures can increase the surface area and interface effects of the magnetic materials, thereby enhancing their magnetic loss and dielectric loss.
Ferrites possess different crystal structures and can achieve their magnetic resonance frequencies in the microwave band through precise control of their crystal structures.They find broad applications in the EM function field.The spinel ferrite structure formula MFe 2 O 4 (M = Mn, Co, Fe, Ni, Cu, Zn) has tetragonal M 2þ -O and octahedral Fe 3þ -O coordination and generally high magnetic permeability.The composite magnetic permeability of spinel ferrite is mainly related to its chemical composition (Figure 2g-j). [86]Ji et al. grafted various MFe 2 O 4 (M = Zn, Fe, Co, Ni) ferrites onto carbonyl iron plates to form ferrite/iron interfaces.The combination of different ferrites and carbonyl iron can trigger interface polarization behavior, improve impedance matching and EWA capability, and achieve controllable high-frequency absorption. [87]Dai et al. demonstrated that the permeability of Ni 1Àx Zn x Fe 2 O 4 nanocrystals is closely related to the Zn content, and the μ 00 value reaches its maximum when x = 0. [88] The anatase TiO 2 shell was synthesized on the spinel Fe 3 O 4 core by hydrothermal method and calcination process, and the thickness of anatase crystal layer was adjusted by changing the concentration of reactants.The results showed that Fe 3 O 4 @TiO 2 microspheres with thicker wurtzite crystal layer had lower reflection loss and wider EWA bandwidth.This was attributable to the effective complementarity between dielectric and magnetic loss (Figure 2f ). [37]or dielectric absorbers, the degree of microscale aggregation order plays a significant role in their EM response and loss mechanisms.The arrangement of dielectric materials affects their dielectric relaxation behavior and the ability to store and dissipate electrical energy.Generally, Higher crystallinity leads to lower dielectric loss and higher electrical insulation properties.The microscale aggregation order can be controlled through structural design, such as introducing crystal phases, oriented structures, or ordered molecular arrangements.These ordered structures enhance the material's dielectric properties and contribute to efficient EWA.Conductive absorbers, in contrast, rely on the presence of conductive pathways for EWA.The extent of microscale aggregation order in conductive materials affects the formation and connectivity of conductive pathways, thus influencing their electrical conductivity and EWA property.Increasing the level of ordering in conductive materials, such as through aligned or interconnected conductive structures, enhances their electrical conductivity and facilitates efficient EWA through conduction losses.In magnetic loss absorbers, the degree of microscale aggregation order is closely related to their magnetic properties and the ability to dissipate energy through hysteresis losses.Materials with high magnetic ordering, such as well-ordered magnetic domains or aligned magnetic nanoparticles, exhibit enhanced magnetic properties and hysteresis losses, leading to efficient EWA through magnetic dissipation.To summarize, the degree of microscale aggregation order in different types of absorbers, including dielectric, conductive, and magnetic loss absorbers, has a significant influence on their EM response mechanisms and loss mechanisms.By controlling and manipulating the microscale ordering, the absorption properties of MAMs materials can be tailored and optimized for specific EWA requirements.

Multilevel Structure Design and Regulation
The behavior of EM waves in materials involves surface reflection, internal attenuation, and transmission processes.Increasing the number of transmission paths for EM waves within materials can effectively attenuate their propagation. [89,90]owever, in the practical application of EM wave absorbing materials, the narrow absorption bandwidth and large thickness limit their usability.Multilevel structure design is a method that utilizes the characteristics of material microstructures at different scales, such as morphology, composition, and arrangement, to regulate their mechanical, EM, and optical properties at the macroscopic level.Unlike interface design engineering, designing microscale multilevel structures increases the EM wave transmission path through more interfaces to achieve efficient EWA performance over a broader frequency range.Multilevel structure design exhibits several advantages for improving MA performance.1) Broadband absorption: different levels of structures can be optimized for EM waves in different frequency ranges, providing broadband absorption characteristics; 2) Multiple reflection and scattering effects: reflection and scattering between different interfaces in multilevel structures can increase the interaction path between EM waves and the material, enhancing absorption effects.Multiple reflections and scatterings allow EM waves to propagate and be absorbed multiple times within the material, increasing energy dissipation; 3) Tunability and optimized design: by adjusting parameters such as material type, thickness, shape, and arrangement at each level, the best match for absorbing EM waves in different frequency ranges can be achieved.Customization allows meet the specific requirements of different applications, providing sufficient EM wave transmission paths within the material; and 4) Multifunctionality and customization: multilevel structure design enables the multifunctionality of absorbing materials.][93] Overall, multilevel structure design offers a promising approach to enhance the performance and expand the functionality of EM wave absorbing materials.

Core-Shell Structure
Core-shell structure absorbing materials are composed of a core material coated with one or more layers of another material.Due to the unique structure and order combination, core-shell structures are increasingly used in wideband and high-efficiency absorption fields.The heterogeneous interface between the core and the shell can produce an interface polarization effect, enhancing the dielectric loss of the material and improving its MA performance.[96] Moreover, by adjusting the ratio of the core and shell, along with the microstructure and spatial position, the EM properties of the material can be tailored.In addition, the core-shell structure possesses various dimensions such as 0D, 1D, 2D, and 3D (Figure 3), which provides more opportunities for the structural control engineering of materials, and the preparation of high-efficiency absorbing materials.
Significant progress has been made in the development of 0D core-shell composites with excellent MA performance.For instance, Du et al. successfully prepared Fe 3 O 4 @C composite material with a core-shell structure by in situ polymerization and high-temperature carbonization.The introduction of carbon shell improved impedance matching, increased the complex dielectric constant and multiple relaxation, thereby significantly increasing the dielectric loss and improving the absorption characteristics. [97]In another study, Jian et al. designed a FeSiAl@Al 2 O 3 @SiO 2 multilayer core-shell heterogeneous structure, the multilayer structure enriched the polarization gene and propagation path of EM waves.The uneven charge distribution on the heterogeneous interface provided a flexible and effective way to optimize the EWA performance. [98]They reported the fabrication of a Fe 3 O 4 @black TiO 2Àx heterostructure with a coreshell structure.The introduction of the external disordered TiO 2Àx thin layer increases the amount of heterojunction interfaces (Fe 3 O 4 -TiO 2 and TiO 2 -TiO 2Àx ) and defect dipoles, promoting polarization loss.The TiO 2 -TiO 2Àx shell facilitates the penetration of Fe 3 O 4 magnetic lines, endowing Fe 3 O 4 @black TiO 2Àx with superior magnetic-dielectric synergistic dissipation effect.This concept provides a basis for designing broadband microwave absorbers based on core-shell functional structures (Figure 4c,d). [19]espite the great achievements of 0D core-shell structures, there are still some limitations, such as high electro-sorption threshold and insufficient shape anisotropy.In contrast, 1D core-shell materials are composed of 1D dielectric materials (carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon nanocoils (CNCs)).Component modification is carried out on the 1D substrate, allowing 1D core-shell structures to retain the advantages of 0D structures while demonstrating a higher dielectric constant and lower magnetic permeability. [91,99,100]or instance, Ji et al. fabricated core-shell Co@C nanotubes by in situ thermolysis of dopamine precursor in CoOH, in which Co nanoparticles were evenly distributed in the nanotubes without aggregation (Figure 4e).Each Co nanoparticle was tightly covered by carbon layer, forming an obvious core-shell structure (Figure 4f ).Compared with the original Co nanoparticles, Co@C nanotubes had interconnected conductive network channels, and electrons could migrate and jump along the graphite layers, which increased the electrical conductivity loss.Moreover, the exchange resonance and natural resonance magnetic loss induced by the ferromagnetism of Co NPs made Co@C nanotubes exhibit better EM properties (Figure 4i-k). [99]The high aspect ratio of 1D materials allows them to overlap and establish a conductive network even at low filling concentrations, providing ideas for the preparation of lightweight absorbing materials.
2D core-shell absorbing materials are based on 2D layered materials, and component modification is achieved by introducing dielectric or magnetic nanoparticles.The interface interaction of 2D core-shell composite materials can affect the magnitude of dielectric loss.In addition, the special layered structure can promote multiple scattering and reflection of EM waves, enhancing their ability to attenuate EM wave energy.[103][104][105] Peng et al. synthesized Fe 3 O 4 @BaTiO 3 /RGO absorbing materials using a two-step hydrothermal method. [16]n this process, numerous Fe 3 O 4 @BaTiO 3 nanoparticles were uniformly loaded onto the RGO substrate, and the layered graphene oxide (GO) was successfully reduced to RGO under high-temperature and high-pressure hydrothermal conditions (Figure 4a,b).The introduction of BaTiO 3 not only increased the matching degree between the dielectric constant and the magnetic permeability of the composite material but also brought in numerous heterojunctions, increasing interface polarization.The presence of Fe 3 O 4 , as a magnetic absorption agent, generated dipole polarization at the nanometer scale, thereby increasing the dielectric loss. [16]Similarly, Fe 3 O 4 /C 2D core-shell nanosheets prepared by carbon thermal reduction showed stronger MA performance.The combination of carbon nanosheets and Fe 3 O 4 enhanced interface polarization, improved impedance matching, and exhibited strong natural magnetic resonance at high frequency, achieving the synergistic effect of electricmagnetic loss (Figure 4g,h). [106]The adjustable impedance matching, controllable heterointerfaces inducing polarization relaxation loss, and variable low-dimensional modifying components in 2D core-shell composite materials offer promising solutions for enhancing EWA performance.
3D core-shell structures are characterized by high porosity and a large specific surface area, making them highly constructible in terms of shells and cavities.These structures hold great potential for applications in broadband and efficient EWA.Meng's team utilized three-axis and coaxial electrospinning technology to introduce multishell structures (hollow, core-shell, three-shell layers) into graphene aerogel microspheres (GAMs) and prepared ball-in-ball graphene aerogel spheres (BGAS) (Figure 4l, m) and hollow graphene-based aerogel microspheres (HGAS) (Figure 4n).Additionally, biomass porous carbon with a core-shell heterogeneous structure was controllably prepared as Carbon@RGO/Fe 3 O 4 aerogel microspheres. [107,108]By combining controllable construction of multishell structures with reasonable design of multiple components, the double synergistic effect of porous structure and multishell structure was achieved, which increased impedance matching, promoted multiple internal reflections and scattering of EM waves.The interconnection of 3D layered graphene sheets can induce strong dielectric resonance, which provides new strategies for designing the microstructure of GAMs and achieving their broadband and efficient absorption performance.These 3D core-shell MAMs featured low density, high specific surface area, a highly connected conductive network, and excellent reflection loss of EM waves, surpassing the limitations of traditional macroblock  [16] Copyright 2016, Springer Nature.c) Holograms of Fe 3 O 4 @black TiO 2 -x heterostructure.d) The profile of charge density in the region of the red arrow.Reproduced with permission. [19]opyright 2021, Elsevier.e,f ) TEM images of Co@C at F700.Reproduced with permission. [99]Copyright 2020, Elsevier.g) SEM image of the Fe 3 O 4 /C core-shell NSs.h) Frequency dependence of calculated reflection loss curves for paraffin composites containing Fe 3 O 4 /C core-shell NSs with a volume fraction of 21 vol%.Reproduced with permission. [106]Copyright 2018, the American Chemical Society.i-k) Hysteresis loops, real part, and imaginary part of complex permeability of Co@C.Reproduced with permission. [99]Copyright 2020, Elsevier.l,m) SEM image of the cross-section view of the BGAS.n) SEM image of the HGAS.Reproduced with permission. [108]Copyright 2018, the American Chemical Society.absorption agents.They provide valuable guidance for future customized operation and scalable manufacturing of core-shell structures across various scales.
To achieve impedance matching and enhanced loss in core-shell structured absorbing materials, it is necessary to adjust the composition and thickness of the core and shell.0D core-shell structured absorbing materials have advantages such as simple fabrication, uniform morphology, and good dispersion.However, they also have drawbacks, including uneven particle size distribution, unclear interface effects, and unstable absorption performance.In contrast, 1D core-shell structured absorbing materials offer high specific surface area, strong interface effects, good conductivity, and magnetism.Yet, their development is hindered by complex fabrication processes, high costs, and aggregation issues.2D core-shell structured absorbing materials have advantages such as strong interlayer coupling effects, high dielectric constant, and magnetic permeability.Nevertheless, they also encounter difficulties in fabrication, unclear interlayer interfaces, and susceptibility to delamination.3D core-shell structured absorbing materials have low density, large specific surface area, and strong multiple scattering effects.However, they also have drawbacks such as stringent fabrication conditions, difficulty in controlling porosity, and low strength.In summary, to fully utilize the tunability of core-shell materials, it is necessary to leverage their strengths and overcome limitations, aiming to develop lightweight, wideband, and highly efficient MAMs.

Porous Structure
In recent years, porous materials have garnered significant attention due to their excellent EWA performance and tunability, which containing a large number of air or other media within their structure, effectively reducing their density and complex permittivity, thereby improving impedance matching and reducing reflection losses of EM waves.Additionally, the presence of pores and channels inside porous materials leads to multiple phenomena such as reflection, scattering, and diffraction of incident EM waves, increasing the interaction time and distance between the waves and material molecules or particles, thereby enhancing the absorption of EM waves.Moreover, the structural parameters of porous materials, such as pore size, porosity, and morphology of the channels, can be adjusted by modifying the fabrication process, enabling control over their EM parameters and absorption performance.
Porous materials possess excellent composite properties and design flexibility.They can be combined or loaded with other types of materials to enhance or complement their functionality or performance.For instance, porous materials can be combined or loaded with metals or magnetic materials to improve their conductivity or introduce mechanisms such as eddy current losses and natural resonance losses.Porous materials can also be combined or loaded with materials having high dielectric constants or high loss factors to enhance mechanisms such as dielectric losses or interface polarization losses.Furthermore, porous materials can be combined or loaded with other types of porous materials to create hierarchical or gradient structures, allowing for optimization of absorption bandwidth or absorption intensity.
Additionally, porous materials can be designed in various shapes or sizes to accommodate different assembly methods or operating environments, depending on the requirements of specific applications.Some common examples of porous absorbing materials include foam, carbon-based aerogels, and biomimetic porous materials. [109,110]orous Foam: Porous foam is a porous structural material composed of a continuous or discontinuous framework of metal or nonmetal, which can be prepared using methods such as foaming, templating, sol-gel, etc. [111,112] Porous foam exhibits good mechanical strength, electrical conductivity, and thermal conductivity, making it suitable as a matrix or composite component for EWA materials.The performance of porous foam in EWA depends on factors such as the framework material, pore structure, and thickness.Increasing the electrical conductivity of the framework material enhances the reflection of EM waves, while the complex pore structure prolongs the propagation path of EM waves inside the material, leading to increased energy loss.Optimal thickness helps improving impedance matching and reduces reflection losses.For instance, Sheng et al. fabricated hollow SiC foam with a dual interconnected network using simple chemical vapor deposition (CVD) and direct oxidation processes.[113] The 3D conductive network facilitates charge carrier transport, while the hollow structure reduces density and increases the number of reflection interfaces.The multilevel porous structure synergistically enhances mechanical and microwave absorption properties.Porous foam not only achieves ultralow density but also enables broadband absorption through pore design.Cheng et al. combined the excellent EM resonance loss capability of periodic porous structures at low frequencies with the dielectric loss capability of dielectric materials at high frequencies. They propsed a broadband absorption model based on a periodic porous A/B/C structure, where A represents the air phase.[114] Using cellulose nanofibrils (CNFs) and carbon nanotubes (CNTs) as building blocks, Cheng et al. obtained lightweight porous cellulose nanofiber (CNF)/carbon nanotube (CNT) foam with a structure composed of vertically oriented macropores and nanopores using an ice templating method.Thanks to its unique structure, it achieved an effective absorption bandwidth of 29.7 GHz.Additionally, the CNF/CNT foam exhibits ultralow density and high fatigue resistance, which arise from the well-connected porous structure and strong hydrogen bonds between CNF-CNF and CNF-CNT molecular chains.
Carbon-Based Aerogel: Carbon-based aerogel is a 3D porous network material self-assembled from carbon nanomaterials such as carbon nanotubes, carbon nanofibers, graphene, etc.It possesses ultra-low density, high surface area, excellent conductivity, and mechanical properties, making them ideal for MAMs.The EWA performance of carbon-based aerogels primarily depends on factors such as composition, morphology, doping, and composites.The more diverse the composition, the richer the EWA mechanisms.The more porous the morphology, the stronger the interaction between EM waves and the material.Rational doping or composites lead to optimized EM parameters and impedance matching.The MA performance of carbon-based aerogels is mainly influenced by factors such as composition, morphology, doping, and composites.Graphene aerogel (GA) is an example where graphene sheets serve as the main building blocks and are cross-linked to form a 3D network structure with interconnected pores, high surface area, high pore volume, thermal and electrical conductivity, and environmentally friendly characteristics. [115]GA not only addresses the aggregation of graphene sheets and reduces material consumption but also achieves broadband and high-efficiency MA.Zhang et al. controlled the physical structure of GA by adjusting the concentration of GO and studied the influence of internal porous morphology and pore size on MA. [116] With increasing concentration, the structure of the aerogel transitioned from an open network to a semi-closed cell structure with more complete cell walls.However, higher GO concentrations caused significant shrinkage of the gas-filled pores.By adjusting the internal pore structure, self-assembled GA with optimal MA performance was achieved.Cao et al. modified the pore size, pore structure, and magnetic properties of Co-RGO aerogel by adding polyvinylpyrrolidone (PVP) and varying the annealing temperature. [117]he addition of PVP regulated the pore structure and size of Co-MOF, resulting in eddy current loss and magnetic resonance, increased magnetic loss, and improved dielectric performance.Increasing the annealing temperature enlarged the pore size of the aerogel, lengthening the electron migration path within the 3D network structure, and promoting EM wave reflection and scattering.
Biomimetic Porous Structures: Biomimetic materials are new types of materials that mimic the characteristics and properties of biological systems by studying their functions and mechanisms through certain technological means.In recent years, there were many studies of biomimetic structures in MAMs due to the sensitive response of special structures at different scales in living organisms to light, such as chameleons, [118] cephalopods, [119] butterflies, [120] and moths eyes. [121]Among these materials, biomimetic porous structure materials have become an effective means to design and regulate MAMs due to their low density, large specific surface area, optimized impedance matching, and interconnected conductive network.By designing biomimetic structure to provide materials with abundant pores and interfaces, impedance matching and dielectric loss of the materials are improved, scattering and multiple reflections of EM waves are achieved, and attenuation characteristics are enhanced, thereby improving the MA performance.124][125][126] For instance, Rehma et al. synthesized starfish-shaped C/CoNiO 2 by solvothermal and thermal etching, which has high porosity and excellent impedance matching (Figure 5a,b).The C/CoNiO 2 heterojunction structure resulted in interface polarization, electron polarization due to the electron jump between divalent and trivalent Ni ions, and magnetic loss caused by eddy current effects, showing excellent EM attenuation and reflection loss. [124]Li et al. prepared a red mesoporous C/NiCo 2 O 4 material by combining hydrothermal synthesis and carbonization (Figure 5c,d).In addition to the dielectric-magnetic synergistic loss and dipole polarization caused by the heterogeneous structure of C/NiCo 2 O 4 composites, the unique porous red mesoporous structure contains a large number of isotropic nanoneedles, which can achieve multiple reflections and scattering and attenuation of EM waves, demonstrating good microwave attenuation ability. [125] addition, flower-shaped absorbers, serving as bionic porous absorbing materials, possess unique advantages.First, their petal-like shape can increase the contact surface between the material and EM waves, thus improving the material's interface polarization.Second, the special structure of the flower can enhance the scattering of incident EM waves.Third, they play an important role in suppressing eddy currents due to the high resonance peaks generated in the high-frequency range. [127]Zou et al. obtained a porous flower-shaped Ni/C composite material by simple calcination of Zn-doped Ni-MOF under N 2 atmosphere (Figure 5e,f ). [128]The surface microstructure and EM properties of the Ni/C flower-shaped composite material can be regulated by changing the pyrolysis temperature.The large number of porous lamellar layers and large interlayer spacing in the flower-shaped structure increase multiple reflection loss, improve the MA performance of the Ni/C flower-shaped composite material.This MOF-derived 3D flower-shaped composite material provides an excellent strategy for the preparation of new EM-absorbing agents.Furthermore, the EM properties and absorption performance of chemically adjustable heterogeneous 3D flower-shaped Fe 3 O 4 /Fe particles can be regulated by changing the reduction temperature (Figure 5g,h). [122]t the same time, some plants in nature have multilevel structures, such as the micro-nano structures on lotus leaves and pitcher plant surfaces.These structures have special combinations of different scales, and some of the synergies that arise from them can result in unexpected results. [129]Therefore, designing reasonably bionic multilevel structures can also be an effective method for optimizing absorption performance.For example, Zhu et al. prepared TiN fiber material like setaria viridis by electrospinning and hydrothermal synthesis, which consisted of a central fiber and nanoneedles on the outer surface. [130]The central TiN fiber has high continuity, and TiN nanoneedles are uniformly coated on the surface of the TiN fiber.Additionally, a large number of pores are presented in the setaria viridis-like TiN fiber, which is beneficial for impedance matching and dielectric loss.According to the antenna model proposed in similar surface structure materials, TiN nanoneedles can act as "antennas" to receive and attenuate EM wave energy in synergy with the central TiN fiber (Figure 5i). [131,132]Huo et al. synthesized flower-like TiO 2 using electrospinning and hydrothermal methods.The flower-like bionic multilevel morphology generated a large number of heterogeneous interfaces and porous structures, achieving interface polarization, dipole polarization, conductive loss, multiple relaxations, and excellent impedance matching through synergistic effects (Figure 5j). [133]he low density, high surface area, and multilevel porous structure of porous absorbing materials are advantageous for increasing the multiple reflections, scattering, and refraction of EM waves, thereby enhancing the attenuation capability of EM waves.Moreover, the porous absorbing materials can be tailored and optimized for microwave absorption performance through different synthesis methods and precursor adjustments to control their pore structure, morphology, composition, and properties.However, most current methods for fabricating porous absorbing materials require high temperature, high pressure, and complex templating processes, resulting in uneven particle size distribution, high costs, and poor controllability.Additionally, the relationship between the microstructure and EM properties of porous materials is not yet well-established, and there is a lack of effective theoretical models and prediction methods, making precise design and control challenging.The precise design of the micropore structure of porous materials remains a major challenge.

Chiral Spiral Structure
Chiral absorbing materials are isotropic EM materials with a spiral structure.Under the action of an EM field, the induced current is generated in the radial direction of the chiral spiral structure.The generation of induced current and the movement of electrons in the spiral structure leads to the appearance of an electric dipole moment in the chiral spiral structure.The presence of an asymmetrical polarization center interrupts the transmission of induced electron dipoles and significantly increases the polarization relaxation.[136] In comparison to ordinary absorbing materials, chiral absorbing materials offer two advantages: 1) adjusting the chiral parameter is easier than adjusting the dielectric constant and magnetic permeability, thus meeting the requirements of reflectionless over a wider frequency band; and 2) the frequency sensitivity of chiral materials is smaller than that of dielectric constant and magnetic permeability, making it easier to achieve wideband absorption.138] Yang et al. synthesized chiral polyaniline (PANI) with a helical structure using in situ polymerization and self-assembly techniques, by doping with chiral acid as a dopant (Figure 6a). [139]hiral spiral PANI exhibits significantly enhanced EM loss compared with conventional PANI.The improvement in absorption performance is mainly attributed to good impedance matching, dielectric loss, and additional magnetic loss due to the crosspolarization effect generated by the helical structure and chirality (Figure 6b,c).In addition, PANI@HCNT with a double-handed layer structure was prepared by Tian et al. through in situ polymerization. [140]The molecular and nanoscale chirality derived Reproduced with permission. [124]Copyright 2019, Elsevier.c) SEM images of C@NiCo 2 O 4 .d) Three-dimension images of calculated RL values of C@NiCo 2 O 4 .Reproduced with permission. [125]Copyright 2019, Spring Nature.e,f ) Zinc-doped MOF precursor.Reproduced with permission. [128]Copyright 2019, Elsevier.g) SEM images of flower-like Fe 3 O 4 /Fe composites at F-300.h) Schematic illustration of possible loss mechanisms in 3D flower-like Fe 3 O 4 /Fe composites.Reproduced with permission. [122]Copyright 2019, the American Chemical Society.i) Schematic illustration of MA mechanisms of the setaria viridis-like TiN fibers.Reproduced with permission. [130]Copyright 2020, Elsevier.j) Schematic diagram of the MA mechanism of the TiO 2 @SiC/C composite nanofibers.Reproduced with permission. [133]Copyright 2021, Elsevier.and c) conventional PANI measured at 300 K. Reproduced with permission. [139]Copyright 2018, Elsevier.d) Synthetic strategy of MHPFs.e) Schematic illustration of the possible EWA mechanism and advantages for MHPFs-900.Reproduced with permission. [141]Copyright 2021, Elsevier.f ) Schematic diagram illustrating preparation process and formation mechanism of CPANI@HPPy hybrid nanofibers.Reproduced with permission. [143]Copyright 2023, John Wiley and Sons.h) Microwave absorption mechanisms for CSA-RGO AMs and PANI@RGO AMs.Reproduced with permission. [144]Copyright 2023, Elsevier.g) Synthetic mechanism of helical polypyrrole nanofibers.Reproduced with permission. [23]Copyright 2022, Elsevier.i) Schematic diagram of carbon materials with achiral electric dipoles, single-chiral electric dipoles, dual-chiral electric dipoles, respectively.Reproduced with permission. [30]Copyright 2022, John Wiley and Sons.
from chiral and acid-doped PANI and helical carbon nanotubes (HCNTs) can produce a synergistic effect, resulting in multiple relaxation and cross-polarization.Moreover, applying the chiral spiral structure to other structures can significantly enhance the material's absorption performance.For instance, Wu et al. combined the chiral spiral structure with a 3D porous material to prepare 3D magnetic spiral porous carbon fibers (MHPFs) through catalytic self-deposition technology. [141]Under the influence of the external alternating magnetic field, the numerous defects on the carbon fiber and carbon nanotube skeleton act as polarization centers, generating a dipole polarization effect.The heterojunctions between air/MHPFs, carbon nanotubes/cobalt, and Co/carbon matrix generate interface polarization effects.The 3D chiral/helical structure causes cross-polarization effects, and the multilevel porous structure causes multiple reflections and scattering of incident microwaves (Figure 6d,e).Dong et al. utilized a template method to in situ grow ZIF-67 on helical carbon nanotubes (HCNTs), successfully prepared graded spiral C/Co@CNT nanocomposites.C/Co@CNT has a continuous spiral-doped carbon band layer, and its interface effects and double cross-polarization effects make it have excellent microwave energy dissipation ability. [142]Meng's team has extensively researched the adjustment of EM properties through the design of chiral spiral structures and has designed and prepared chiral spiral poly pyrrole nanofibers with efficient absorption performance and corrosion resistance. [23]Through in situ polymerization strategies, a novel super helical nano-microstructure based on chiral poly aniline and spiral poly pyrrole was successfully achieved, where the multiscale-chirality synergistic effect helps to broaden the effective absorption bandwidth. [143]Through in situ doping polymerization of poly aniline, graphene oxide microspheres with the same chiral spiral structure were obtained, and the special chiral spiral structure and EM crosspolarization enhanced the impedance matching and EM wave attenuation ability of microspheres (Figure 6f-h). [144]o achieve enhanced EWA performance without increasing the thickness of absorbers, it is possible to increase the transmission paths of EM waves within the absorbers under low-density conditions. [145]Therefore, by designing and regulating the hierarchical structure of EM wave absorbers, one can achieve adjustment of absorption frequency bands and bandwidths and meet the requirements of MAMs for light, wide-band, and efficient absorption.Consequently, reasonable design of the microstructure morphology of MAMs will greatly promote the in-depth investigation of EM wave response mechanisms such as impedance matching and EM wave co-loss.

Structure Design and Regulation of Metamaterials
The aforementioned multilevel structures, including core-shell structures, porous structures, and chiral helix structures, all involve the precise design of the material's microstructure.However, EM absorbers with these structures usually face the problems of complex processes and a single absorption frequency band.Metamaterial EM absorbers are a type of artificial material that constructs new EM properties that natural materials do not possess by embedding a certain geometric structural unit in traditional substrate materials, usually composed of subwavelength structural units arranged periodically or nonperiodically.The structural unit consists of three layers: a conductive unit layer, a dielectric layer, and a reflective layer.Among them, the conductive unit layer exhibits resonant characteristics, while the dielectric layer, situated between the conductive unit layer and the reflective layer, contributes to increased EM loss due to its thickness.The reflection layer is located at the bottom, mostly made of metal backplates or conductive materials, which reflect EM waves incident on its surface. [146]Compared to traditional absorbing materials, metamaterial EM absorbents not only have extraordinary physical properties such as negative EM parameters [147] and negative refractive index [148] that conventional materials do not possess, but their unit structure size is much smaller than the working wavelength, at the sub-wavelength level.Metamaterials offer versatility in terms of form, enabling complex pattern and structure designs for EWA across a wide range, from microwaves to visible light.From a structural perspective, metamaterial absorbers can be roughly divided into flat array structures, multilayered structures, and 3D structures.
The flat array structure is a periodic arrangement of the same unit structure.By precisely designing and controlling the conductive layer, dielectric layer, and reflection layer of the basic unit, the absorption performance of the metamaterials EM absorber can be significantly improved.For instance, Wen et al. proposed a hybrid metamaterial absorber based on VO 2 film, which incorporates electrically small resonant rings (eSRR) and VO 2 film placed between the inner ring and the substrate.The introduction of VO 2 in the dielectric layer enables dual-band absorption capabilities. [149]Furthermore, the bandwidth can also be adjusted by designing the size and geometry of the conductive layer.For instance, Araújo et al. designed and prepared a conductive layer with a square spiral metal pattern, which was applied to the metamaterials.Due to the redistribution of charge, more EM dipoles are generated, widening the absorption bandwidth and achieving wideband EWA. [150]he above "conductive layer-dielectric layer-reflection layer" structure has certain limitations in achieving ultra-wideband absorption, and can only achieve EWA at specific or relatively narrow frequency bands, with a relatively singular design and material selection.To overcome this limitation and broaden the absorption bandwidth, researchers have attempted to use a multilayered structure, which is a multilayer structure of "multiple conductive layers-multiple dielectric layers".Wu et al., for example, stacked a square conductive ring/F4B substrate and a gear-shaped conductive unit/F4B substrate with different dielectric layers, with a copper plate as the reflection layer. [151]ince different resonant patterns will produce different absorption frequency bands, ultra-wideband absorption can be achieved by using the vertical stacking method.Additionally, the idea of successive stacking of the conductive layer and the dielectric layer can also be used.An example of this approach involves layer-bylayer stacking of a designed conductive layer containing copper with a magnetic dielectric layer, using a metal aluminum plate as the reflection layer.The synergistic effect of the magnetic dielectric layer and the conductive layer can achieve lowfrequency ultra-wideband absorption (Figure 7a). [152]lthough multilayered structures have certain advantages in expanding the absorption bandwidth of EM waves, their high material density limits their practical applications in the field of EWA.To address this issue, the extension of planar metamaterial absorbers to 3D structures presents an ideal solution.Jiang et al. proposed and used 3D printing and screen printing technology to prepare a 3D metamaterial absorber composed of honeycomb and resistive film (Figure 7b), which not only has low density and high strength, but also has a wide EAB, which can meet the requirements of MAMs for lightweight, wideband, and high-efficiency absorption. [153]Additionally, the mechanical metamaterial absorber prepared by 3D printing technology uses an octet-truss structure as the structural unit (Figure 7c), breaking the traditional three-layer structural unit preparation with long cycle and complex preparation process.The absorption rate at 4-18 GHz is over 90%. [154]s a new and efficient EM wave absorber, although the metamaterial absorber demonstrates advantages of lightweight and wideband absorption by structural regulation.However, there are still several challenges that need to be addressed.The key to achieving good impedance matching and internal attenuation for the metamaterial absorber is the synergistic effect between its conductive, dielectric, and reflection layers.However, this effect requires high-precision fabrication, high-cost investment, and is hard to achieve in mass production.To make the metamaterial absorber functional and practical, research should focus on the Figure 7. a) Geometry of the multilayered structure metamaterial.Reproduced with permission. [152]Copyright 2020, Elsevier.b) Unit cell diagram of the 3D MMA and view of unit cell in plane x-z and x-y.Reproduced with permission. [153]Copyright 2020, Elsevier.c) Schematic representation of wave absorption of BMMA and unit cell of BMMA.Reproduced with permission. [154]Copyright 2022, Elsevier.d) Digital and SEM images of real moth-eye surface microstructures.d) Schematic illustration of millimeter-scale MM.Reproduced with permission. [165]Copyright 2019, John Wiley and Sons.e) Unit cell configuration of a compatible metamaterial absorber based on multilayered resistive metamaterial surfaces.Reproduced with permission. [166]opyright 2018, John Wiley and Sons.synergistic effect of component and material structure design, and improve the physical and mechanical properties and multiband absorption properties of the metamaterial absorber.

Structure-Activity Relationship of Structure and Electromagnetic Absorption
In summary, structural regulation has a wide range of applications in regulating the EM response and loss mechanisms of absorbents.However, a significant amount of research has not made clear the structure-property relationship between material structure and EM absorption performance.It is essential to fully understand the relationship between structure and physical and chemical properties and EM response mechanism for the study and development of new types of EM absorbers.In this section, EWA theory calculation models (aggregation induced charge transfer (AICT) model, electron-hopping (EHP) model) and relevant simulation characterization techniques (EM simulation technology, off-axis electron holography technology) utilized to elucidate the factors and structure-property relationship between different structures and EM responses, which provides a theoretical reference for the future use of morphological structure design to regulate the preparation of multifunctional and efficient broadband absorbing materials.
The ability of EM wave reflection and attenuation is a crucial factor in studying the absorption performance of MAMs.To maximize EWA, it is necessary to reduce the reflection and transmission of EM waves and increase the absorption of EM waves as much as possible.According to Maxwell equations, the electric field strength, the displacement vector, magnetic field strength, and magnetic induction intensity of the EM field at the material interface must follow certain boundary conditions, which are closely related to the dielectric constant and magnetic permeability.The effective absorption of EM waves relies on the synergistic impact of dielectric and magnetic losses.As shown in Table 1, there are differences in the EM loss mechanisms of MAMs with different structural designs.The excellent absorption performance of MAMs is mostly achieved through the synergy of dielectric and magnetic loss.
Off-axis electron holography is a technique that uses high-resolution TEM (HRTEM) images to reconstruct electron holograms for phase imaging.This technique is capable of visualizing the charge and electron density in nonuniform areas through colorization.In the context of interface design engineering, heterojunction interface can induce changes in material EM properties due to differences in material properties, including spatial charge distribution and migration, electron band structure and density, magnetic response, lattice defects, and lattice strain.Defects such as dislocations, vacancies, and impurities that result from nonuniform spatial charges and interface lattice mismatch generate spatial charge dipoles, inducing polarization relaxation at the interface and increasing dielectric loss.The heterojunction in composite materials that use the magnetodielectric synergy effect to produce EM wave attenuation not only produces interface polarization, but also generates strong magnetic coupling.In Figure 1a, the phase image of elongated hexagonal bipyramidal La(OH) 3 was obtained by HRTEM image reconstruction of an electron hologram, and no charge polarization was observed outside and inside the rod in the context of average inner potential distribution.However, a large amount of charge accumulation was observed at the nanorod boundary, resulting in dielectric polarization.Two layers of negative charges appeared on the outer and inner surfaces close to the vacuum region, with a layer of positive charges between the two negative charge layers, resulting in bipolarization, thereby increasing dielectric loss. [40]Similarly, the heterojunction can be used to regulate magnetic response.Che et al. prepared CoNi microflowers, which exhibit different stray magnetic field signals depending on morphology and size.With off-axis electron holography analysis, it is determined that the edge magnetic field strength of nanosized particles on the surface of the CoNi microflower becomes highly concentrated when the EM wave is irradiated along the alternating polarization vector direction.Consequently, these dense boundaries serve as multipole couplings with the incident magnetic field, generating strong stray magnetic fields, and magnetoanisotropy (Figure 8f-i). [155]inite element simulation (FEA) technology allows for the visualization of surface current density and volume loss density in materials under the action of an EM field.Surface current  Reproduced with permission. [107]Copyright 2022, Elsevier.f ) Strong stray magnetic lines and g) closed magnetic flux lines of CoNi microflowers.h) The magnetic field of pristine nanoflakes which are parallel to the plane and i) the stray magnetic flux lines of the nanoflakes which are perpendicular to the plane.Reproduced with permission. [155]Copyright 2015, the American Chemical Society.The 3D model of j 1 ) single helical structure, j 2 ) multihelical structure.The electric field simulation results at 13.36 GHz of k 1 ) single helical structure, k 2 ) multihelical structure.The magnetic field simulation results at 13.36 GHz of l 1 ) single helical structure, l 2 ) multihelical structure.Reproduced with permission. [143]Copyright 2023, John Wiley and Sons.m-p) Schematic illustrations of electronic transport mechanism of MWCNTs.Reproduced with permission. [42]Copyright 2013, Elsevier.
density and volume loss density represent impedance-matching and EM wave attenuation ability, respectively.According to EM field theory, the structure can be precisely designed and EM parameters adjusted through high-frequency structural EM simulation in FEA, greatly reducing the trial-and-error cost and preparation time of EM wave absorbers.For EM wave absorbers with a core-shell structure, interface polarization caused by extensive and close contact between the core-shell and shell-shell components and scattering effects have a significant effect on the dielectric properties of materials.Furthermore, the interface between the core and shell and expansive inner surfaces can increase the transmission and attenuation paths of EM waves, causing multiple scattering and reflection, which is beneficial to EWA.Therefore, precise design of interface properties in the core-shell system and expansion of heterojunction interfaces are crucial for obtaining materials with high-efficiency broadband absorption.

Zhi et al. used coaxial electrospinning technology to obtain
Carbon@RGO/Fe 3 O 4 AMs with a core-shell heterojunction structure. [107]The different porous networks formed by the internal core biomass carbon and external shell RGO/Fe 3 O 4 exhibit different reflection, scattering behavior, and EM response upon microwave exposure, achieving sequential loss and multicoupled synergistic attenuation of EM waves.According to the electromagnetic simulation results (Figure 8a-e), at a frequency of 2 GHz the surface current density of the core layer is lower than that of the RGO/Fe 3 O 4 AMs and outer shell, and has a similar change pattern at 10 and 18 GHz.This fully demonstrates that the introduction of the core layer optimizes the overall impedance matching performance of the microspheres, and multiple reflection and scattering in the small pore structure of the core layer will generate new EM energy dissipation.Additionally, the interface effect between RGO and biomass-derived carbon increases additional space charge polarization, synergistically enhancing the loss performance of the core-shell AMs.For chiral helical structures, the cross-polarization and synergistic enhancement effects of helical structures were studied in depth using FEA technology.As shown in Figure 8k 1 ,k 2 , significant polarization enhancement is observed for the response electric field of single-helical and multihelical structures.For single helical structures, the polarization phenomena are strong along the direction of the excitation source and decreased toward the lateral direction (Figure 8k 1 ).From the distribution of induced magnetic fields, it can be seen that cross-polarization occurs in the helical structure, and the polarization direction is affected by the helical parameters (Figure 8l 1 ).Introducing secondary helix arrays enhances the magnetic field signal in both the main helix structure and the nanoscale array (Figure 8l 2 ). [143]Electromagnetic simulation technology plays an important role in the design of metamaterial absorbers.For a better understanding of the absorption mechanism of the designed metamaterial, Araújo et al. utilized electromagnetic simulation technology to quantitatively calculate the electric field distribution and magnetic field distribution of the square spiral metal pattern in the unit structure of the metamaterial at different EM wave frequencies.It was observed that the behavior of surface currents on the inside and on the ground plane of the spiral was different at different frequencies. [150]ccording to the theory of EM waves, it can be inferred that dielectric loss capability is positively correlated with conductivity.In most low-dimensional EM absorbing composite materials, there exist locally anisotropic regions that provide charge carriers for conducting loss, forming a local microcurrent network.In reality, defects such as vacancies, edges, cracks, and impurities exist in low-dimensional materials, which can disrupt the conductive network and hinder charge transport.Therefore, two electronic transport models have been proposed: the electron hopping (EHP) model and the aggregate-induced charge transport (AICT) model. [156,157]In 2013, Wen et al. proposed the AICT and EHP models to describe the migration and hopping of electrons in the microcurrent network of multiwalled carbon nanotubes (MWCNTs).The migration and hopping of electrons are a response to EM waves, and hopping electrons can jump over the interface between MWCNTs, enhancing the microcurrent in the MWCNT network (Figure 8m-p).They also proposed a conductive network equation, demonstrating that the network conductivity (δ network ) was mainly affected by contact conductivity (δ contact ), which is relatively small.Therefore, δ network can be increased by improving the local conductive network or increasing the temperature.Similarly, Li et al. found that the charge migration in Ti 3 C 2 T x and the electron hopping between the core of Ti 3 C 2 T x and the graphene shell in Ti 3 C 2 T x /graphene core-shell hollow microspheres led to a significant amount of field-induced microcurrent formation, significantly increasing conductive loss. [158]3D interconnected conductive networks in hierarchical Co 3 O 4 /N-doped carbon/SCF composites enabled the migration of electrons on a single carbon fiber and the electron hopping between adjacent carbon fibers, which is of great significance for studying the relationship between microscopic interface electron transport and EM response. [159]n recent years, much progress has been made in the fundamental research of EM response, and various physical models and formulas have been proposed, including the EHP model, AICT model, charge transport conductivity network equation, capacitor-like structure, and equivalent circuit model of dipole polarization. [46,49,156,160,161]However, further exploration of the EM response mechanism is necessary, including exploring the synergistic loss mechanisms generated by multiscale and multilevel structures and the relationship between EM, crystal, and electronic structures.

Conclusion and Outlook
The engineering of morphology and structure in EWA materials has shown great potential in improving EM response and MA performance.This article presented a new perspective to illustrate the EM wave loss mechanism of absorption material through structure regulation.By considering various aspects such as micro-scale interface regulation, small-scale aggregate structure regulation, large-scale multilevel structure regulation (core-shell structure, porous structure, chiral helix structure), and macrolevel metamaterial structure regulation, combined with interface effects, conducting networks, dielectric-magnetic synergy, dielectric-dielectric synergy, and magnetic-magnetic synergy, materials' designable structure has made significant progress in the field of absorption.The relationship between structural regulation and enhancement of EM loss mechanisms and MA performance improvement is discussed.EWA theory calculation models and related simulation characterization technologies are discussed emphatically.These composite materials with different scales and hierarchical structures can satisfy the requirements of lightweight, broadband, and high-efficiency absorption.In general, structural design materials provide vast research opportunities and prospects for obtaining reliable EM absorbers, safe electronic information environment, healthy human living environment, and stable defense security.However, opportunities and challenges coexist, and there are still many challenges to be overcome.
The most urgent challenge is to fully understand the EM response mechanisms.In the second chapter, extensive research on EM wave theory has been presented significantly advancing the development of structural design of EWA materials.However, the complicated structure-property relationship based on interfaces, crystal structures, defects, and various special structures, and EM loss mechanism are still unclear, which limits the accurate prediction and regulation of EWA performance.The key issue is to explain the impact of microstructure/morphology on dielectric response (polarization and relaxation processes) and magnetic response under EM field.Our aim is to integrate device physics materials science, and other disciplines involving condensed matter to predict and explore the physical and chemical properties inside EM functional structures using existing models and computational formulas, and to fully understand their microscopic structures through characterization and analysis technologies.
Another challenge lies in the development and application of EM functional devices.With technological advancements and the advent of the 5G era, radar detection technologies at different frequencies have higher requirements for new EM wave absorbers.[164] Inspired by moth eyes, biomimetic hierarchical metamaterials not only have low infrared (IR) emissivity and MA, but also have hydrophobicity (Figure 7d). [165]ased on the research of carbon black (CCB) filled polyethylene (PE) composite films, a multispectrum-integrated metamaterial with broadband and strong absorption capacity in the microwave, visible light, and near-infrared bands was designed (Figure 7e). [166]Introducing Fe 3 O 4 @SnO 2 core-shell nanochain powders with three different contents of 2-ethylhexanoate, deep green, brick red, and bright orange-gray colors were presented, reflecting selective absorption of visible light.The MA performance was enhanced by the dielectric-magnetic synergistic loss of SnO 2 and Fe 3 O 4 , as well as the core-shell structure.The low IR emissivity is attributed to the reflection of the SnO 2 shell layer.This composite material exhibits compatibility with visible light-radar-IR stealth. [167]MAMs must meet the basic requirement of single-spectral absorption in the process of achieving multispectrum compatibility.For example, for radar-IR compatible stealth, the difference in stealth mechanism and response frequency band between the two is very large.To achieve compatibility with stealth conditions, multiscale structural design of MAMs is required.Although multispectrum stealth materials have achieved some achievements, there are still problems such as incomplete research on compatible stealth mechanisms, incomplete preparation processes, and poor material stability and compatibility.
In addition, practicality, multifunctionality (self-cleaning, corrosion resistance, antibacterial, etc.), integration, intelligence (electrical response, thermal response, light response, etc.), and environmental adaptability are the main development trends of EM functional devices.In this regard, simple, continuous, and scalable preparation methods are the focus of attention.Additionally, in response to sustainable development strategies, EM functional devices must also have the characteristics of low cost and environmental friendliness.Currently, lightweight and efficient EM wave absorbers have made significant progress at the laboratory stage, and we hope that they can realize the transformation from laboratory to industrialization.However, considering the demands for materials, devices, and equipment that have substantial impacts in electronic information, aerospace, military, and other fields, there is still a long way to go.We eagerly anticipate the further advancement of EM absorption materials to reach new heights.
Jingyuan Tang is currently an M.S. student at the Key Laboratory of Advanced Technologies of Materials, Southwest Jiaotong University in China.Her scientific interests include the synthesis, mechanism, and applications of the electromagnetic interference shielding materials with external stimulus-responsive and graphene-based fiber materials.

Figure 4 .
Figure 4. a) TEM of images of GO. b) TEM of Fe 3 O 4 /RGO nanocomposites.Reproduced with permission.[16]Copyright 2016, Springer Nature.c) Holograms of Fe 3 O 4 @black TiO 2 -x heterostructure.d) The profile of charge density in the region of the red arrow.Reproduced with permission.[19]Copyright 2021, Elsevier.e,f ) TEM images of Co@C at F700.Reproduced with permission.[99]Copyright 2020, Elsevier.g) SEM image of the Fe 3 O 4 /C core-shell NSs.h) Frequency dependence of calculated reflection loss curves for paraffin composites containing Fe 3 O 4 /C core-shell NSs with a volume fraction of 21 vol%.Reproduced with permission.[106]Copyright 2018, the American Chemical Society.i-k) Hysteresis loops, real part, and imaginary part of complex permeability of Co@C.Reproduced with permission.[99]Copyright 2020, Elsevier.l,m) SEM image of the cross-section view of the BGAS.n) SEM image of the HGAS.Reproduced with permission.[108]Copyright 2018, the American Chemical Society.

Figure 8 .
Figure 8. a-d) At the different frequency, the surface current density of RGO/Fe 3 O 4 AMs and the surface current density of Carbon@RGO/Fe 3 O 4 -3 at outer and inner surfaces separately.e) The electromagnetic simulation of volume loss density at 13.84 GHz: e 1 ) RGO/Fe 3 O 4 , e 2 ) outer, and e 3 ) inner part of Carbon@RGO/Fe 3 O 4 -3 AMs.Reproduced with permission.[107]Copyright 2022, Elsevier.f ) Strong stray magnetic lines and g) closed magnetic flux lines of CoNi microflowers.h) The magnetic field of pristine nanoflakes which are parallel to the plane and i) the stray magnetic flux lines of the nanoflakes which are perpendicular to the plane.Reproduced with permission.[155]Copyright 2015, the American Chemical Society.The 3D model of j 1 ) single helical structure, j 2 ) multihelical structure.The electric field simulation results at 13.36 GHz of k 1 ) single helical structure, k 2 ) multihelical structure.The magnetic field simulation results at 13.36 GHz of l 1 ) single helical structure, l 2 ) multihelical structure.Reproduced with permission.[143]Copyright 2023, John Wiley and Sons.m-p) Schematic illustrations of electronic transport mechanism of MWCNTs.Reproduced with permission.[42]Copyright 2013, Elsevier.

Qing
Qi is currently an assistant professor at the School of Materials Science and Engineering, Southwest Jiaotong University.She received her Ph.D. from the University of Electronic Science and Technology of China and worked as a postdoctoral fellow at the National University of Defense Technology for two years.She is mainly engaged in the research of microwave absorbing materials, electromagnetic shielding materials, supercritical foam materials, and wave transparent material.Fanbin Meng is currently the dean of the Department of Polymer Science and is also an associate professor at the School of Materials Science and Engineering, Southwest Jiaotong University.He received his Ph.D. (2014) from the University of Electronic Science and Technology of China.His research group is mainly engaged in the development of electromagnetic functional materials, intelligent anticorrosive materials, and relative mechanisms.

Table 1 .
EWA characteristics of EM wave absorber composed of various structures.