Research progress on carbon-based materials for electromagnetic wave absorption and the related mechanisms

YANG Wang, JIANG Bo, CHE Sai, YAN Lu, LI Zheng-xuan, LI Yong-feng*

（State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing), Beijing 102249, China）

Abstract： With the development of electronic information technology, the use of microwaves in military and civilian fields is becoming more and more widespread. The corresponding electromagnetic radiation pollution has become a global concern. Numerous efforts have been made to synthesize thin electromagnetic wave absorbing materials with a low density, wide absorption bandwidth and high absorption. Carbon-based materials have great potential in electromagnetic wave absorption because of their lightweight,high attenuation ability, large specific surface area and excellent physicochemical stability. The attenuation theory of absorption materials and the factors that influence their absorption performance are provided first. Next, we summarize the research status of carbon materials with different morphologies (such as 0D carbon spheres, 1D carbon nanotubes, 2D carbon platelets, and 3D porous carbons) and their composites with various materials such as magnetic substances, ceramics, metal sulfides, MXene and conductive polymers. The synthesis methods, properties and attenuation mechanisms of these absorbers are highlighted, and prospects and challenges are considered.

Key words: Microwave absorbing materials；Carbon materials；Composites；Reflection loss；Microwave absorption mechanism

1 Introduction

Nowadays, electronic information technology and instruments progresses rapidly, bringing convenience but meanwhile inevitable electromagnetic pollution[1–6]. In the civil and military field, electromagnetic wave (EWs) radiation causes severe harm to human health and problems of the operation of sophisticated electronic devices, and threatens national security. Electromagnetic wave absorbing (EWA) materials can make military weapons undetectable to enemy forces, improve the sensitivity of radar detection, protect humans from radiation hazards, and create microwave darkrooms and so on. Therefore, it is urgent to explore high-performance EWA materials to minimize and attenuate EWs by transforming the electromagnetic energy into heat. Specifically, microwave absorbers with thin thickness, low density, wide absorption bandwidth, and high absorption intensity are of intense demand for the practical applications. In addition, other characteristics, such as excellent hydrophobicity, outstanding mechanical properties, thermal insulation, oxidation resistance and corrosion resistance, are also required for absorbers to meet the demands of complicated applications scenarios and extend their service lifetime.

Conventional EWA materials such as silicon carbide, ferrite, and magnetic metal powder have been widely employed. However, they usually suffer from high density and narrow effective absorption bandwidth. Carbon-based absorbers including 0D carbon microspheres, 1D carbon nanotubes (CNTs), 2D carbon platelet, and 3D porous carbons have attracted intensive research attention worldwide owing to their lightweight, high electrical conductivity, large specific surface area, and excellent stability (Fig. 1). But single-component carbon materials usually exhibit limited absorption ability at low fill levels due to the limited loss mechanisms and unmanageable electromagnetic parameters. Incorporation of magnetic materials (magnetic metals, alloys, and oxides) with carbons is an effective approach to improve their EWA properties by enhancing the permeability of complex,introducing additional magnetic loss mechanism, and improving impedance matching due to smaller difference between permittivity and permeability. Moreover, heterogeneous interfaces between magnetic materials and carbons can lead to increased polarization relaxation losses[7,8].

Fig. 1 Schematic diagram of various carbon-based EWA materials.

In addition to magnetic materials, carbons can be composited with various dielectric lossy components such as sulfides, ceramics, conductive materials by regulating their dielectric parameters, heterogeneous interfaces, and conductivity[9–11]. Besides, carbons with multiple hybrid components are reported to demonstrate remarkable performance through various synergistic effects[12–14]. The absorbing properties of carbon materials can be improved by forming composites. However, this may lead to the problems of complex preparation, poor stability and increased costs.

In this review, we first discuss the theory of EWA and the factors that influence the performance of EWA materials. Then, we comprehensively summarize the research progress of pure carbon and carbon-based hybrids or composite materials. Finally, our perspectives and challenges on the development of EWA materials are presented.

2 EWA theory of materials and the factors influencing their performance

When EWs propagate to the surface of absorbing materials, the incident EWs will be divided into three parts, reflected, absorbed and transmitted waves (Fig. 2).

The relative complex permittivity (εr=ε'?jε'') and relative complex permeability (μr=μ'?jμ'') are vital for the EWA performance of absorbents. Specifically, the real parts (ε'andμ') and imaginary parts (ε''andμ'')represent the energy storage capacity and energy dissipation capacity, respectively[15]. On the basis of the transmission line theory, the final performance is reflected by the reflection loss (RL), which is obtained by the following formulas[16]:

Wherecis the velocity of light, and εrandμrare the relative complex permittivity and permeability, respectively,drepresents the absorption thickness,fdonates the frequency of electromagnetic waves,Zinis the input impedance of absorber, andZ0donates the impedance of free space. The smaller the value of RL is, the better the performance will be. RL below?10 dB indicates over 90% absorption, which is the benchmark for practical applications. Usually, the absorption ability is closely related to the polarization loss, conduction loss, magnetic loss, and multiple reflections and scattering.

2.1 EW absorption mechanism

2.1.1 Dielectric loss

Dielectric loss refers to the characteristic electronic interaction between electric field and absorbents that dissipates incident EWs. Usually, the dielectric loss factor (tanδε=ε'/ε'') is used to characterize the dielectric attenuation ability resulted from the conduction loss and polarization loss. The imaginary part of complex permittivity is calculated by the following formula:

whereρis the resistivity. According to this formula,high electrical conductivity has positive effect on theε''. It is worthy to mention that the conduction loss plays a dominant role in dielectric loss for carbon materials. Generally, the polarization loss covers the ion polarization, electron polarization, dipole polarization,and interfacial polarization. However, ion polarization and electron polarization usually occur at high frequency range (103–106GHz) instead of microwave region. Dipole polarization originates from defects,functional groups, heteroatoms, and edges under fastchanging external electric field. As for the relationship between relative complex permittivity and frequency,ε'andε''will decrease with the increasing frequency because the dipole cannot respond to the external electric field quickly, which is called frequency dispersion behavior[17]. In addition, interfacial polarization relaxation always happens in the heterogeneous interface with different conductivities, and the space charge will accumulate at the interfaces and produce interfacial polarization to consume incident microwave energy.

According to the Debye diploe relaxation theory,Cole-Cole semicircles can be used to describe the polarization relaxation process. The relationship ofε'andε''is shown as follows[18]:

whereε∞is the relative permittivity andεsis the static permittivity under high frequency limits. The semicircle curve reveals the presence of polarization process, indicating a Debye relaxation.

2.1.2 Magnetic loss

The characteristic magnetic interaction will occur when one magnetic material is located in electromagnetic field, which is described as magnetic loss[19].The magnetic loss factor (tanδe=ε'/ε'') is usually used to characterize the magnetic attenuation ability. The magnetic effect originates from the relaxation process during magnetization composed of hysteresis loss, domain wall resonance loss, natural resonance, exchange resonance, and eddy current loss[20]. However,the hysteresis loss is negligible in the weak magnetic field, and the domain wall resonance loss usually happens in the MHz frequency range. Thus, the magnetic loss mainly refers to natural resonance, exchange resonance, and eddy current loss in the microwave band[21,22]. The value ofC0is used to distinguish the contribution of eddy current loss:

IfC0keeps constant within the frequency range,the eddy current loss will be the only cause of magnetic loss. The eddy current loss exists in the whole 2–18 GHz range. But the natural and exchange resonances usually happen in lower frequency (2–10 GHz)and higher frequency range (10–18 GHz), respectively.

2.1.3 Multiple reflections and scattering

When EWs propagate into absorbents, multiple reflections will generate inside the materials. As shown in Fig. 2, a part of EWs will be reflected from the latter boundary to the front boundary, and some EW energy can be consumed in the process. This process occurs back and forth multiple times, leading to significant entrapping and dissipation of microwave inside the absorption materials. The multiple reflections are especially important in those porous configurations, multilayered structures, and multi-interface materials[7,23,24], where large surface and solid phase exclusive of void space provide huge number of active sites for the multiple reflections and scattering of EWs. Besides, these porous and hollow structures lead to low density and tunable internal architecture, which in turn improves microwave absorption (MA) performances.

2.1.4 Impedance matching

Ideal MA requires the balance between appropriate impedance characteristic and strong attenuation capability[25]. The former one (Z) means that more EWs spread into the absorbents rather than being reflected in front of surfaces. Generally speaking, the closer the value ofεrandμrare, the more effective complementary effect between the dielectric loss and magnetic loss will be, indicating moderate characteristic impedance. The value ofZcan be calculated from Eq. (1) and following formula:

Fig. 2 Schematic diagram of reflection,absorption and transmission of EWs.

When the value ofZinequals toZ0,Zequals to 1.Then, the EWs all penetrate into the EWA material,and the reflection loss reaches the minimum value.

2.2 Influence factors for EWA materials

2.2.1 Size effect

The dielectric and magnetic properties can be significantly affected by the size of the material due to the quantum size effect, surface effect, and tunneling effect[26]. As size decreases, the surface area and number of atoms on the surfaces with unsaturated coordination increases dramatically, leading to enhanced interface polarization and multiple scattering. Therefore,the relative complex permittivity and dielectric loss are inversely proportional to the particle size[26,27].With increasing sizes, the reduction of surface-tovolume ratio results in the decrease of thermal fluctuation and magnetically disordered surface, contributing to the increase in saturation magnetization strength eventually[28]. Besides, the value of coercivity can be tuned by particle size. Above the critical size, the coercivity decreases as the particle size increases further[27,29]. Large saturation magnetization strength and low coercivity are beneficial to magnetic loss. Thus, nano-scale EWA materials are the mainstream trend of current development.

2.2.2 Filler loading

Microwave absorbents are often mixed with polymers because of the poor processability of powders. Utilizing the molding properties of polymers, the mixture composed of absorbent and polymer matrix is used in the subsequent measurement or practical applications. The degree of mutual contact and continuity of materials are the key factors affecting conductivity. Effective EWA materials often display excellent absorption performance within a certain filling range. If the filler loading is too low, the attenuation ability is very weak. However, with too much high filler loading, the processability of the mixture will be poor. Moreover, too high conductivity will lead to a poor impedance matching. In consequence, most EWs are reflected back rather than absorption. Thus, the filler loading in polymer matrix should be rationally optimized to reach the best value.

2.2.3 Thickness

The thickness (dm) and matching frequency (fm)satisfy the quarter-wavelength cancelation law. The calculation equation is shown as follows[30]:

wheredmis the absorbing thickness,n= 1, 3, 5, …,λis the wavelength of the microwave,fmis the matching frequency, andcis the speed of light. Whendmandfmmeet the above equation, a 180° phase difference exists between the reflected wave from air/absorber interfaces and the reflected wave from absorber/metal backboard interface[31], improving reflection loss. According to the quarter-wavelength cancelation law, the absorption peak will move to the lower frequency with increasing thickness.

3 Carbon materials with different dimensionals for EWA

In recent years, carbon materials emerge as promising candidates for EWA owing to their outstanding properties of high dielectric loss, remarkable chemical stability, tremendous specific surface area and low density. In this part of review, the current research of pure carbon materials with different dimensionals is summarized.

3.1 Zero-dimensional carbon spheres

Carbon spheres are one of the most basic carbon materials. Because of its excellent dielectric loss, 0D carbon spheres are widely studied in EWA field. The hard-template etching strategy is often used to prepare hollow carbon spheres. For example, Zhang el al.designed carbon hollow microspheres with a uniform mesoporous shell via the template-assistant strategy(Fig. 3a)[32]. The pore size and shell thickness could be adjusted by changing the pyrolysis temperature. Interestingly, with a 20% filling content, it exhibited a favorable absorption of ?39.4 dB at 3.6 mm thickness and broadest effective absorption bandwidth of 5.28 GHz at 2.6 mm thickness, which was ascribed to impedance matching, multiple reflection and scattering, and dielectric loss capability (Fig. 3b). Chen et al.prepared mesoporous hollow carbon spheres by the modified st?ber method. It showed a largest bandwidth of 6.2 GHz and minimumRLof ?38.5 dB[33].The hollow carbon spheres displayed a favorable capacity in EWA at a low filling ratio[34,35].

Fig. 3 Schematic illustration on (a) the fabrication and (b) absorption mechanism of hollow carbon microspheres (Reproduced with permission[32]. Copyright 2019, Elsevier), TEM images of (c1) solid carbon nanoparticles, (c2) HPCNs-1, (c3) HPCNs-2 and (c4) HPCNs-3, (d) conduction loss and (e) polarization loss of all HPCNs-m samples and (f) the RL and effective absorption bandwidth of HPCNs-3 (Reproduced with permission[37]. Copyright 2021, Elsevier).

Heteroatom doping can modulate the absorbing properties of carbon spheres. Zhang et al. prepared sulfur-doped hollow carbon microspheres, and the doping ratios could be controlled by changing the amount of thiourea. The absorption curves were greatly regulated by the doping content of S atoms.The S-doped hollow carbon microspheres showed outstanding properties of lightweight, broadband and strong absorption[36]. Besides, the absorption ability of carbon spheres can also be adjusted by morphology construction[37,38]. Tao et al. prepared multi-shell hollow porous carbon particles (HPCNs-m) (m represents the number of shell) as shown in Fig. 3c1–c4 and studied the influence of the shell number of nanoparticles on EWA performance. The result showed that the conduction loss and polarization loss were enhanced with increasing the shell number of nanospheres, promoting the EWA capability (Fig. 3d–e). In addition, the multi-shell hollow structure increased the size of particles, causing more EW reflection on the surface of nanoparticles. Although HPCNs-3 had a poor characteristic impedance, it still displayed aRLof ?18.13 dB and a bandwidth of 5.17 GHz at a thin absorption thickness of 1.6 mm (Fig. 3f)[37].

3.2 One-dimensional CNTs

At present, CNTs are being used as fillers. Wang et al. fabricated multi-walled CNTs (MWCNT)/epoxy composites. They found that the EW absorption ratio was determined by the loading of MWCNTs[39]. Similarly, Nwigboji et al. also reported that loading MWCNTs had an important influence on EW absorption effect. The MWCNT/epoxy composites displayed a significant EWA with 8%–10% mass loading of MWCNTs[40]. In addition to the content of CNTs,CNTs with different configurations have different properties in terms of EWA. Che et al. compared different MWCNTs and revealed that the one with the largest aspect ratio had the best X-band absorption performance due to their alignment trend and good dispersion[41]. Sun et al. used aligned CNT films as lightweight microwave absorbers. By increasing the intersection angles of aligned CNT films, the location of maximum absorption peaks shifted to higher frequency. What’s more, the absorption performance was improved with the more stacked aligned CNT films[42].

Fluorination is often used to modulate the characteristic impedance matching and absorption properties of CNTs. Liu et al. prepared hybrids of fluorinated single-walled CNTs (F-SWCNTs) and pristine single-walled CNTs (p-SWCNTs). Under the action of stress, F-SWCNTs tended to orient because of electrostatic interaction, which was beneficial for the directional propagation of EWs and impedance matching. While the p-SWCNTs guarantee the attenuation ability (Fig. 4a). The degree of impedance matching and attenuation capability can be adjusted by changing their ratio (Fig. 4b–c)[43]. Liu et al. reported a skin-core fluorinated MWCNTs by selective fluorination of outer shell of MWCNTs. The fluorinated layer was conductive to the transmission of microwave,while the existent of inner layer ensured the capability of dielectric loss[44].

Defeated, I dropped my eyes. Then he said with his fingers in the air, decreasing the space between his thumb and forefinger22. Next time we read thin book, I sure to understand every word. His grin was huge. He made me laugh.

3.3 2D carbon platelets

Fig. 4 (a) The absorption mechanisms of hybrids of F-SWCNTs and p-SWCNTs, (b) impedance matching and (c) attenuation constant of hybrids with different ratios (Reproduced with permission[43]. Copyright 2018, Royal Society of Chemistry), (d) the synthesis process, (e) SEM image, (f) 3D RL plot, and (g) absorption mechanism of thin flake graphite (Reproduced with permission[46]. Copyright 2019, Elsevier).

Lamellar carbons can provide high aspect ratio,large mobility of charge carriers, large surface area,and excellent mechanical strength[45]. Duan et al. proposed a shear-assisted supercritical CO2mechanical exfoliation method to prepare thin-layer flake graphite (Fig. 4d–e). It displayed intense absorption withRLvalues below ?30 dB at 4–8 GHz and 10–18 GHz frequency ranges. Besides, itsRLreached a minimum of?49 dB at a thin thickness of 1.49 mm with a 20%filler loading (Fig. 4f). This outstanding performance was attributed to the multiple reflections, scattering and dielectric loss (Fig. 4g). The layer number of peelings and the lateral size of sheets had huge impacts on the performance. The plates with larger lateral sizes provided higher conductivity, while smaller ones filled the gaps between larger sheets, providing polarization loss and improved conduction loss[46].

The chemical exfoliation method can also be used to prepare carbon nanosheets. Song et al. fabricated carbon nanosheets without the sacrificial in electrical properties via direct chemical exfoliation. It is found that the percolation threshold of thickness-decreased carbon nanosheets became lower compared with that of the unexfoliated ones. The exfoliated carbon nanosheets displayed an excellentRLof nearly?60 dB at a 14% mass loading in wax matrix[47].Graphene, as a typical 2D lamellar material, has received extensive attention as EWA materials. Reducing the graphene oxide is a common method to produce graphene platelets. Morphological features, doping and reduction degree of graphene oxide can pose great influences on complex permittivity, characteristic impedance, and attenuation ability. Quan et al. investigated the morphology influence of the graphene oxide precursor. Rippled, folded, and flower-like graphene oxide was used to produce reduced graphene oxide by one-step thermal annealing. The N doping and reduction of graphene oxide were achieved simultaneously during pyrolysis. It was revealed that fewlayered graphene oxide was easier to be reduced, and flower-like reduced graphene oxide showed the highest magnetization. Finally, the flower-like Ndoped graphene displayed the maximumRLof?21.7 dB with the filler loading as low as 10%[48].

CVD and arc discharge can also be used to synthesis graphene nanosheets. The graphene prepared by CVD has the merits of tunable structures, favorable electrical properties and the minimal restacking of nanoplatelets. Li et al. reported nitrogen-rich graphene-like carbon nanosheets by a g-C3N4-directed CVD strategy. The configurations of doped N atoms could be effectively adjusted by changing the annealing temperature, which influenced the conduction loss and final absorption performance. The optimized products exhibited the bestRLvalue of ?50.2 dB at a thickness of 1.8 mm, and the maximum effective absorption bandwidth reached about 5.9 GHz at a 2 mm thickness with a filing content of 5%[49]. Zhou et al. fabricated graphene nanoflakes with nitrogen doping by arc discharge. By tuning the N2dosage in the synthesis process, the nitrogen doping content could be regulated. Nitrogen doping weakened the crystallinity of graphene and introduced abundant defects,leading to the decreased conductivity and improved static magnetization. With increasing the N doping content, the absorption performance decreased, and the impedance degree raised gradually. Ultimately, it got the balance between the EWA capability and characteristic impedance matching. It exhibited more than 99% absorption in the wide frequency range of 5–18 GHz[50].

3.4 3D porous carbons

Porous carbon materials show potentials in EWA application owing to their porous structure, tunable pore sizes and low density.

Recently, various research progresses were made in carbon aerogels and carbon foams with open-hole network structures owing to their outstanding conductivity and mechanical stability. An ultralight and highly compressible graphene foam has been reported by Zhang et al. by consecutive solvothermal, freezedrying and annealing treatments[51]. The performance of graphene foam could be adjusted by tuning the mechanical compression degree, which covers many critical applications including remote sensing, satellite communications, and radar detections[52]. It was found that the 3D cross-linked graphene network had great impacts on EWA. Liu et al. prepared lightweight N-doped graphene foams by self-hydrothermal reaction and freeze-drying. The as-obtained foam was so light that the blossom could hold them without any deformation (Fig. 5a). Meanwhile, it was strong enough to load the same weight as itself, indicating excellent mechanical strength (Fig. 5b). Finally, this fame displayed ?53.9 dB at an only 5% loading(Fig. 5c)[53]. Furthermore, carbon aerogel has also shown much potential in the field of EWA[54,55]. Zhang et al. reported a reduced graphene oxide aerogel with the effective absorption in the whole 2–18 GHz range at a low filling ratio of 2.0% in paraffin wax. Meanwhile, the negative part of imaginary permeability was attributed to the fact that the magnetic energy was radiated out from the composites[55]. Porous carbon aerogels and carbon foams tend to exhibit strong absorption at low filler loading due to the cross-linked conductive network and porous structure. However,the preparation process is relatively complex and expensive.

Fig. 5 (a) Superior strength, (b) light weight and (c) absorption properties of N-doped graphene foams (Reproduced with permission[53]. Copyright 2019, Elsevier), (d) digital image and (e) SEM image of porous carbon derived from wheat fluor dough, (f) EMA performance under the condition of different fermentation time (Reproduced with permission[57]. Copyright 2020, Elsevier), (g) fabrication of hierarchical porous carbon and (h) its corresponding absorption performance (Reproduced with permission[62]. Copyright 2020, Elsevier).

Biomass, with advantages of low cost, wide range of sources, and environmental friendliness, can be used to prepare porous carbon materials[16,56–61].Zhao et al. synthesized a series of hierarchical porous carbon materials from wheat flour dough through simple fermentation and carbonization (Fig. 5d–e).Pore structures were regulated by the fermentation time, leading to adjustable complex permittivity and absorption effect. Under the optimal condition, it realized a strong absorption intensity of ?52.0 dB at a 2.5 mm thickness (Fig. 5f). This method paves the way of large-scale and green production of microwave absorbents[57]. Bai et al. reported a lightweight aerogel using cellulose as raw material through carbonization. As the calcination temperature raised,the carbon crystal size and graphitization degree were improved, leading to a significant increase in conductivity. Combined with impedance matching and attenuation capacity, it demonstrated a minimumRLof?51.24 dB and a wide effective absorption bandwidth of 7.68 GHz at a 20% filler loading. The absorption intensity decreased dramatically after being ground,indicating that the special porous structure was also beneficial for absorption[61].

What’s more, porous carbons synthesized from residual oil take advantage of high value-added utilization, which is in line with sustainable development.Yang et al. fabricated hierarchical porous carbon nanosheets by carbonization of cheap petroleum asphalt (Fig. 5g). With polarization loss, conductions loss, and multiple reflections, it reached ?53.7 dB absorption intensity and a 5.3 GHz absorption bandwidth at a thin thickness of 1.8 mm with a 20% filler content (Fig. 5h). It achieved effective absorption in whole Ku band at thickness of 1.95 mm[62]. Liu et al.utilized a fluid catalytic cracking slurry to synthesize a N-doped porous carbon by CVD. The porous carbon reached ?53 dB with an only 3% filler loading[63]. In conclusion, the synthesis of porous carbons from biomass and industrial residual oil seems to be an economical method.

4 Carbon-based hybrid materials for EWA

4.1 Magnetic carbon

Although pure carbon materials demonstrate some outstanding performance, its mismatched characteristic impedance resulted from high complex permittivity hinders its wide applications. Due to the low coercivity and strong magnetization, magnetic materials can enhance the relative complex permeability,which will lead to strong magnetic loss and improve impedance matching. Thus, carbon materials composited with magnetic metals, alloys, and oxides are developing rapidly recently[64].

A hierarchical porous carbon with tightly embedded Ni nanoparticles (Ni@NPC) was constructed through a facile sacrificial g-C3N4-templating strategy(Fig. 6a). Ni nanoparticles were highly dispersed in porous carbon layers, producing excellent magnetic coupling networks and providing beneficial magnetic loss (Fig. 6b–c). With the synergistic effect of the magnetic loss, dielectric loss, and special porous construction, the optimal sample exhibited aRLof?72.4 dB and 5.5 GHz effective absorption with an ultralow filler loading of 5% (Fig. 6d)[7].

Similarly, magnetic alloy/carbon and magnetic oxide/carbon composites were also reported to possess excellent EWA performance. Xu et al. prepared an ultralight self-supported N-doped reduced graphene oxide aerogel loaded with FeNi. The ultralow density of only 0.013 1 g cm?3led to excellent absorption performance at a filler loading of only 10%[8]. The effect of carbon geometry on dielectric response was investigated by comparing FeNi-cored carbon nanoparticles (FeNi-CNPs) with FeNi-capped CNT (FeNi-CNTs). It revealed that the electrical inductance introduced by the overlapping CNTs would effectively tune dielectric loss ability and impedance matching. Thus, FeNi-CNTs exhibited much better absorption than FeNi-CNPs[65]. Zhao et al. fabricated a lightweight RGO/CNC/CNF/M-NP hierarchical aerogel through hydrothermal self-assembly and subsequent in-situ CVD (Fig. 6e). Due to the porous structure and the synergistic effect resulted from multiple components, moderate electromagnetic parameters and impedance matching could be obtained, resulting in a ?71.5 dB absorption intensity at a 15% filler loading under the optimal condition (Fig. 7)[66]. Wang et al. reported a three-dimensional porous aerogel composed of N-doped reduced graphene oxide and raspberry-like CoFe2O4clusters (CFO/N-rGA). This 3D conductive network demonstrated improved dielectric loss, and the magnetic/dielectric configuration was beneficial to optimize the impedance matching, enhancing the EWA ability. With the filler loading as low as 10%, it reached aRLof ?55.43 dB and a 7.28 GHz effective bandwidth[67].

Fig. 6 (a) Preparation diagram, (b) SEM image, (c) TEM image, and (d) absorption curves of Ni@NPC (Reproduced with permission[7]. Copyright 2021, Elsevier), (e) synthesis procedure of the “3D carbon nanocoil-2D reduced graphene oxide-1D carbon nanofiber-0D metal oxide nanoparticles” hierarchical aerogel (RGO/CNC/CNF/M-NPs) (Reproduced with permission[66]. Copyright 2021, Springer Nature).

4.2 Carbon-based ceramic composites

Ceramic materials such as boron nitride, silicon carbide, silicon nitride displayed unique intrinsic properties including electrical insulation, corrosion resistance and thermal stability. Based on the excellent properties of carbon and ceramic materials, carbon-based ceramic EWA composites were prepared and investigated.

Fig. 7 Microwave loss mechanisms of RGO/CNC/CNF/M-NP aerogel (Reproduced with permission[66]. Copyright 2021, Springer Nature).

As a dielectric material, zinc oxide (ZnO) owns merits of high dielectric loss, lightweight, and lowcost fabrication. It was reported that the synergistic effect of ZnO and carbon would lead to superior EWA performance. Single component ZnO or graphene oxide displayed nearly zeroRLvalue, while the ZnO/RGO composites showed much better performance with a maximum absorption capability of?67.13 dB and a maximum absorption frequency bandwith of 7.44 GHz at a 2.8 mm thickness[68]. Yan et al. has adopted a molecular layer deposition-calcination strategy to precisely modulate the microstructure of ZnO@carbon composites. During the redox process, the voids between ZnO cores and carbon shells were generated. Finally, the as-synthesized ZnO@carbon achieved 1D yolk-shell morphology, in which multiple reflection and scattering sites existed and conductive network was easily formed (Fig. 8a).Compared with the pure ZnO, the EWA performance of ZnO@carbon composites were dramatically improved both in absorption intensity and absorption bandwidth[10].

With high strength, superior environmental tolerance, and adjustable absorption property, SiC exhibits the potential to be used in harsh environments.However, the electrical property of SiC was extremely poor, which could be potentially solved by compositing with conductive carbon materials. Ye et al. reported a SiC/C foam by direct carbonization of melamine foam and subsequent CVD of SiC coating.Excellent thermal stability was revealed for the SiC/C foam by thermogravimetry analysis. The SiC coating prevented the oxidation of SiC/C foam up to 600 ℃with an only 2.55% mass loss, indicating great potential for high-temperature applications[69]. Furthermore,the novel hierarchical SiC nanowire-reinforced SiC/carbon foam was fabaricated. Benefiting from the dipole polarization, interfacial polarization, and multiple reflection mechanisms, the resultant samples achieved aRLof ?31.216 dB and a 4.1 GHz effective absorption bandwidth at a thickness as thin as 1.5 mm[70]. Interestingly, Zhao et al. introduced SiC nanoparticles to address the impedance mismatch defect of carbon nanofiber resulted from its too high permittivity. The carbon fiber matrix was so flexible that it can be bent and twisted, contributing to outstanding flexibility and reliability (Fig. 8b)[71]. Due to the excellent weather resistance of ceramic materials, carbon-based ceramic microwave absorbers have great potential for applications in harsh conditions.

Fig. 8 (a) Outstanding EWA performance of ZnO@carbon composite (Reproduced with permission[10]. Copyright 2020, Elsevier), (b) lightweight and flexible C-SiC nanofiber with strong EWA (Reproduced with permission[71]. Copyright 2021, Elsevier), (c) absorption mechanisms of MoS2/HCS (Reproduced with permission[9]. Copyright 2020, American Chemical Society) and (d) preparation process, TEM image and absorption performance of ZnS@N-doped porous carbon nanoribbons (ZnS@ NPCNRs) (Reproduced with permission[76]. Copyright 2021, Elsevier).

4.3 Carbon-based metal sulfide composites

Metal sulfides are generally considered to be dielectric lossy materials, with the real and imaginary parts of their relative complex permeability equal to approximately 1 and 0, respectively, showing positive contribution as EWA materials. To date, a number of studies have been reported on compositing carbon materials with metal sulfides to realize superior absorption performance.

MoS2is one of the most widely studied metal sulfides due to the defect dipole polarization resulted from atom vacancies, large interface and specific surface area[72]. Ning et al. reported ultrathin MoS2nanosheets encapsulated in hollow carbon spheres(MoS2@HCS) by a template method[9]. The impedance matching degree was improved compared to each single component through the synergistic effect between the MoS2nanosheets and hollow carbon spheres. It demonstrated that carbon spheres promised the electron transmission capability, while the MoS2nanosheets provided abundant active sites for polarization loss and multiple reflections (Fig. 8c).Worm-like MoS2-expanded graphite (MoS2-EG) hybrids were prepared by an in-situ self-assembled hydrothermal process. The honeycomb-like porous structure was beneficial for construction of crosslinked conductive network, moderate impedance matching, reduced density, and multiple reflections and scatterings. Moreover, a huge number of heterogeneous interfaces existed between MoS2nanosheets and expanded graphite, enhancing dipole polarization and interfacial polarization. The MoS2-EG hybrid exhibited aRLof ?52.3 dB at a 1.6 mm thickness with an only 7% filling loading[73].

In addition to MoS2, other metal sulfides have been also investigated. WS2-rGO heterostructure nanosheets were reported to display a 3.5 GHz effective absorption bandwidth at a thin thickness of 1.7 mm[74]. Liu et al. fabricated a CoS2and carbon composite through in-situ vertically developing Co-MOF precursor on carbon cloths, followed by carbonization and sulfuration, which showed great potential in portable EWA electronic devices due to the flexibility of carbon cloth[75]. Interestingly, a one-dimensional ZnS@NPCNR (nanoparticle carbon nanoribbon) structure was developed, and showed a ?56.1 dB absorption intensity inXband (Fig. 8d)[76]. Considering the low cost and high stability, metal sulfides have huge potential in future practical applications.

4.4 Others

Recently, MXene and conductive polymers are also widely explored and possess strong EWA capability. For instance, Cui et al. fabricated 3D porous MXene/CNT microspheres by an ultrasonic spray strategy. Such unique porous networks, multilayer configuration, numerous defects and abundant interfaces improved the dissipation capacity. The as-synthesized absorbent obtained aRLof ?45 dB at 10 GHz. Almost all electromagnetic waves in C, X,and Ku bands could be attenuated by changing the absorption thicknesses[11]. Different 2D materials, such as Ti2C2TxMXene and graphene oxide, were assembled to generate aerogels by electrospun (Fig. 9a–b). The generated heterointerfaces between two components with different conductivities made up the shortcomings of single material, while the masses of groups were beneficial to the relaxation loss. With moderate impedance and attenuation abilities, aRLof ?49.1 dB was achieved at a rather low filling content of 10%(Fig. 9c)[77]. Li et al. synthesized a prism-shaped hollow carbon decorated with polyaniline (HCP@PANI)by a facile carbonization and in-situ polymerization strategy. The hierarchically oriented HCP@PANI achieved aRLof ?64 dB and a 5.0 GHz effective absorption bandwidth. These multiple components can complement each other to realize optimized impedance matching and outstanding EW absorption[78].Furthermore, multi-component carbon-based absorbers have been widely studied. For example,CoFe2O4/CoFe@C[79], CuS/RGO/PANI/Fe3O4[80]and RGO/MXene/Fe3O4[81]all exhibited outstanding EWA performance.

Fig. 9 (a-c) Schematic diagram of the preparation, morphology and absorption property of Ti3C2Tx MXene@graphene oxide hybride aerogel microspheres（Reproduced with permission[77]. Copyright 2020, Elsevier)， (d) excellent properties of 3D hybrid foam (Reproduced with permission[83]. Copyright 2020,American Chemical Society) and (e) EWA, Self-cleaning, and thermal insulation of PCF aerogel(Reproduced with permission[84]. Copyright 2019, Wiley-VCH).

In order to meet the demand of practical use, in addition to excellent absorption performance, superior stability is highly required. Sun et al. reported a Fe3C/N-C with an outstandingRLof ?57.9 dB and meanwhile excellent oxidation resistance and corrosion resistance, which were attributed to the capsulation of Fe3C nanoparticles by a carbon layer[82]. Gu et al. reported a 3D hybrid foam with combined functions of EWA, thermal infrared stealth and thermal insulation (Fig. 9d). The properties of thermal infrared stealth and thermal insulation demonstrated the potential application in protecting equipment from detection in harsh environments[83]. Li et al. reported a polyacrylonitrile/CNT/Fe3O4aerogel (PCF). It possessed strong hydrophobicity and excellent thermal insulation, making it self-cleaning, infrared and microwave stealth (Fig. 9e)[84]. Moreover, magnetic Ti3C2TxMXene/graphene aerogels from directionalfreezing and hydrazine vapor reduction also displayed multifunction including EWA, hydrophobicity,structural robustness, mechanical property, and heat insulation, indicating a wide variety of potential application scenarios[85]. In view of the practical applications and longevity, developing microwave absorbers with versatility and stability is a main trend.

5 Conclusions and prospects

In this review, we first summarized the electromagnetic wave absorption theory and corresponding influence factors. Compared with the traditional EWA materials, carbon materials possess unique advantages such as high dielectric loss, lightweight, chemical stability and superior mechanical properties. We have reviewed the research progress of different morphological carbon materials. Although pure carbon materials show great potentials, the limited attenuation mechanisms and impedance mismatching hinder further improvements dramatically. Compositing carbon materials with other components is a promising solution to tackle above problems. In recent years,new research hotpot emerged in the multifunctionality of EWA to adapt to the demanding application scenarios. While substantial progresses are achieved so far, many opportunities and challenges remain. The following points may be of value for further investigation:

(ⅰ) Most of researches about EWA materials focused on structural construction and performance tests, and current electromagnetic wave attenuation mechanism is still under development. The further investigation of the dissipation mechanisms is of great importance to guide the design and preparation of absorbents in the future.

(ⅱ) Although incorporation of other components in carbon materials can improve the impedance matching and attenuation capacity, it will entail several complications such as complex preparation, high density, poor stability, and increased economic costs.How to solve these problems effectively is a significant challenge.

(ⅲ) At present, the synthesis of EWA is mostly in laboratory scale. The synthetic strategies are usually complicated, and yields cannot meet commercial requirements. Thus, it is essential to explore environmentally friendly, economical, and large-scale preparation methods.

(ⅳ) In addition to performance characterization,the practical applications of the EWA materials should be considered, such as absorbing film and absorbing coating. Apart from the requirements of “thin,light, wide, and strong”, the EWA materials that are resistant to high temperature, oxidation, acids and alkalis, and wear will be the trend of development.

In conclusion, this article provides a review of the state-of-the-art research on carbon-based EWA materials. Although there is still a long way to go before EWA materials can be employed in practical applications, we believe that this review can provide valuable guidance for the future design of first-class EWA materials.

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgements

We gratefully acknowledge the financial supports from the National Natural Science Foundation of China (Nos. 21908245 and 21776308), and Science Foundation of China University of Petroleum, Beijing(No. 2462018YJRC009).