absorbing materials can be divided into conversion type and consumption type according to The action mechanism,and the conversion type is resistance type absorbing materials;the consumptive type is dielectric type absorbing material and magnetic medium type absorbing material^[46]。 Resistive absorbing materials mainly include carbon black,carbon fiber,silicon carbide and Graphene(GN),etc.Their absorbing forms are based on conductive loss and dielectric loss.Through the interaction with the external electric field,the incident wave is converted into heat energy to achieve the purpose of absorbing electromagnetic waves.However,according to the free electron theory,too high conductivity is not conducive to impedance matching,resulting in electromagnetic wave reflection on the surface of the material,thus reducing the absorbing performance of the material^[47][48]。 dielectric absorbing materials mainly include barium iron titanate ceramics,which absorb electromagnetic waves based on dipole polarization and interface polarization,and produce Dielectric relaxation loss electromagnetic waves in an external electric field^[49]。 magnetic dielectric absorbing materials include ferrite,carbonyl iron and metal powder,and their absorbing forms are based on Magnetic loss to consume electromagnetic waves^[50]。

in order to improve the microwave absorbing properties of carbon nanomaterials,two kinds of carbon nanomaterials with different dimensions can be compounded.For example,one-dimensional carbon nanotubes and two-dimensional graphene are compounded to form a three-dimensional structure,so that the defect degree and the specific surface area of the composite material are improved,the dipole polarization is promoted,the dielectric property of the material is enhanced,and the loss of electromagnetic waves in the material is increased.Zhang et al.Grew nitrogen-doped carbon nanotube arrays on ultrathin reduced graphene oxide sheets by freeze-drying and high-temperature carbonization processes using CoNi alloy as a catalyst and melamine as a carbon source,and the experimental process is shown in Fig.3(a)^[80]。 As shown in Fig.3(B),the synthesized composite(3D CoNi/N-GCT)has a unique bamboo-like three-dimensional structure,a large specific surface area,and a variety of heterogeneous interfaces,which promote the absorption of electromagnetic waves.in order to verify the effect of the content of carbon nanotubes in the absorbing material on the electromagnetic wave absorption properties,Li et al.Constructed a new type of three-dimensional carbon nanotube network composite absorbing material by phenolic resin template method,and explored the effect of different carbon nanotube content on the absorbing properties^[81]。 the composites prepared With different amounts of CNTs and resin were denoted as CNTx/Epoxy,and the conductivity of the samples increased steadily with the increase of CNTs content,as shown in Fig.3(C).the microwave absorbing properties were tested by coaxial method,and the results showed that the microwave absorbing properties first decreased and then increased with the increase of CNTs content,and moved to the low frequency direction with the increase of thickness.When the thickness is 2.9 mm,the CNT5/Epoxy composite has a reflection loss peak of-42.13 dB at 6.4 GHz and an effective absorption bandwidth of 1.6 GHz.with the increase of carbon nanotube content,the electromagnetic wave absorption properties of CNT10/Epoxy composites show a trend to move to the low frequency at the same thickness,but the maximum absorption peak decreases.When the thickness is 3.6 mm,the reflection loss peak is-38.53 dB at 6.2 GHz,and the effective absorption bandwidth is 1.68 GHz,as shown in Fig.3(d,e).This may be due to the high content of carbon nanotubes,which leads to the high surface conductivity of the sample and the skin effect,thus reducing the absorbing properties of the material.On the other hand,the increase of carbon nanotube content leads to the gradual reduction of defects at the edge of the material,which reduces the relaxation polarization of defective carbon atoms。

Zhao et al.Prepared carbon nanopolyhedrons(CNPs)-reduced graphene oxide(CNPs/rGO)composites by in-situ pyrolysis using metal-organic framework/graphene oxide hybrids as raw materials^[82]。 the electromagnetic parameters of the material can be adjusted according to the amount of GO added.When 32 mL of GO dispersion was added,the microwave absorbing property of the material was the best.When the thickness of the composite is 2.89 mm,the reflection loss peak is-66.2 dB at 6.2 GHz,and the effective absorption bandwidth is 2.2 GHz.Lv et al.Synthesized graphene/carbon nanosphere composite absorbing materials by liquid phase method^[83]。 the electrical resistivity of the carbon nanospheres is changed by adjusting the annealing temperature,thereby changing the electromagnetic parameters of the composite.when the annealing temperature is 800℃,the composite has the best absorbing properties,the reflection loss peak is-28.1 dB and the effective absorption bandwidth is 5.7 GHz When the sample thickness is 1.5 mm。

Qiang et al.Prepared carbon microsphere@yolk-like carbon shell composite by"coating-coating etching"method^[84]。 The microsphere consists of a core with a diameter of 300 nm,a shell with a thickness of 20 nm,and a gap between the core and the shell.As shown in Fig.4(a,B),the material possesses a large specific surface area of 629 m²·g⁻¹and a high porosity of 0.8 cm³·g⁻¹.The microwave absorbing properties of 50 wt%sample mixed with paraffin were tested,and when the thickness was 2 mm,the reflection loss peak reached-34.8 dB at 15 GHz,and the effective absorption bandwidth was 5.4 GHz.Fig.4(e,f)shows the comparison of the microwave absorption performance between the carbon microsphere and the carbon microsphere@yolk-shaped carbon shell.It can be seen that the addition of the yolk-shaped carbon shell structure significantly enhances the electromagnetic wave absorption performance.This is due to the fact that the impedance of the hollow carbon shell matches the impedance of free space,and the electromagnetic wave is easier to enter the interior of the material,and multiple scattering and reflection between the core and shell lead to increased electromagnetic wave loss.On the other hand,as shown in Fig.4(C,d),the relative complex permittivity of carbon microsphere is much higher than that of carbon microsphere@yolk-like carbon shell,while the complex permeability of the two materials is almost equal.However,the large difference between complex permittivity and complex permeability is not conducive to the impedance matching of the material,which leads to strong reflection of electromagnetic waves on the surface of the absorbing material.Therefore,the microwave absorbing properties of carbon microspheres are poor 。

to sum up,carbon-carbon composite radar absorbing material is a composite radar absorbing material with large specific surface area,high porosity and hollow structure,which is composed of two carbon materials with different dimensions by in-situ growth,high temperature carbonization and hydrothermal method.Uch as bamboo-shaped three-dimensional carbon nanotube arrays,yolk-shell-shaped carbon microspheres and the like.as a double dielectric system,the conduction loss becomes the main reason for the dissipation of electromagnetic energy,and the dipole polarization and interface polarization provide auxiliary contributions to the consumption of electromagnetic energy.electromagnetic waves are consumed by compounding two carbon materials to form a porous structure,introducing defects,and constructing a three-dimensional conductive network.the porous three-dimensional structure makes the electromagnetic wave reflected and scattered many times in the material,which leads to the enhancement of electromagnetic wave loss.Secondly,the abundant defects and functional groups On the surface produce more dipole polarization and interfacial polarization.on the other hand,the unique three-dimensional conductive network promotes the conductive loss of electromagnetic waves.However,despite the excellent characteristics of carbon-carbon composites,carbon materials still face problems such as high dielectric constant,poor impedance matching,and narrow effective bandwidth of absorption.Compositing carbon materials with magnetic materials is an effective way to solve the above problems^[85]。

magnetic materials with low coercivity and high saturation magnetization can produce characteristic Magnetic interaction with electromagnetic waves,resulting in excellent attenuation of electromagnetic waves^[86]。 magnetic materials such as iron,cobalt,nickel and their oxides are compounded with carbon nanomaterials,which can effectively introduce Magnetic loss and improve the permeability of carbon materials,thus improving the absorbing properties of composite materials。

Hou et al.Prepared MWCNTs/Fe₃O₄composite by hydrothermal method,and studied the effect of Fe²⁺and Fe³⁺concentration of the composite in X-band on its microwave absorbing properties^[87]。 When the concentrations of Fe²⁺and Fe³⁺are 0.02 and 0.04 mol·L⁻¹,respectively,the MWCNT/Fe₃O₄composite shows better absorbing properties with a reflection loss peak of−18.22 dB at 12.05 GHz.Zhu et al.Used a hydrothermal method to embed Fe₃O₄submicrospheres into rGO to form Fe₃O₄/rGO composites,and the dielectric properties of the composites were regulated by changing the amount of rGO addition^[88]。 When the addition of rGO is 4 wt%,the balance between permittivity and permeability of the material reaches the best level,and the impedance matching is improved.When the thickness is 3.5 mm,the reflection loss peak of the material reaches−45 dB,and the effective absorption bandwidth is 3.2 GHz.Wu et al.Prepared a novel amorphous iron/graphene oxide nanocomposite by one-step chemical reduction method,and the preparation process is shown in Fig.5(a)^[89]。 When Fe²⁺ions and GO are simultaneously reduced to amorphous Fe and rGO during the reduction process,the rGO flakes can provide abundant nucleation conditions and growth sites for Fe,resulting in heterogeneous nucleation of Fe nanoparticles on the rGO surface.The formed amorphous iron particles were uniformly dispersed and firmly embedded into the reduced graphene oxide surface,as shown in Fig.5(B).When the addition of graphene oxide is 5 wt%,the composite has excellent microwave absorption properties,as shown in Fig.5(C).When the thickness is 3.26 mm,the reflection loss peak is as high as−72.8 dB at 7.28 GHz,and the effective absorption bandwidth is 5.9 GHz.Because reduced graphene oxide is a non-magnetic material,the saturation magnetization and coercivity of the composite decrease with the increase of reduced graphene oxide content.The reduction of coercivity is beneficial to the improvement of absorbing performance,and its hysteresis loop is shown in Fig.5(d).The excellent microwave absorbing properties of the composite are due to its low coercivity and high saturation magnetization,which maximize the magnetic loss.Secondly,the combination of reduced graphene oxide with excellent dielectric properties makes the magnetic loss and dielectric loss in the composite material play a synergistic role,so that the material has excellent impedance matching and electromagnetic wave attenuation ability.Lv et al.Prepared layered hollow carbon nanospheres@Fe@Fe₃O₄composites by template method and thermal cracking method,and the microwave absorbing properties of the composites could be controlled by adjusting the thickness of the hollow carbon nanospheres^[90]。 the material has excellent microwave absorbing properties due to the interfacial polarization generated by a variety of heterogeneous interfaces.When the thickness is 1.5 mm,the reflection loss peak is−40 dB,and the effective absorption bandwidth is 5.2 GHz.Kim et al.Designed a new electroless plating method without heat treatment to prepare FeCoNi@graphene composite absorbing film^[91]。 Due to the good impedance matching of the composite,the peak reflection loss is as high as−68 dB at 8.4 GHz when the thickness is 2.33 mm.Ding et al.Mixed Co₃O₄nanosheets prepared by hydrothermal method with rGO to obtain Co₃O₄/rGO composite^[92]。 Due to the excellent dielectric properties and polarization ability of rGO and the excellent magnetic loss of the Co₃O₄sheet,the material exhibits a reflection loss peak of−45.15 dB and an effective absorption bandwidth of up to 7.14 GHz at a thickness of 3.6 mm.Chen et al.Used a solvothermal method to load Fe₃O₄on sulfur-doped graphene,which provided a good condition for material dipole polarization because the surface of the material was rich in polar bonds C-S^[93]。 In addition,due to the improvement of impedance matching ability and the introduction of magnetic loss,the composite has excellent microwave absorbing properties.At a thickness of 2 mm,the reflection loss peak is−41 dB,and the effective absorption bandwidth is 5.3 GHz。

metal-organic frameworks(MOFs)are a class of crystalline porous materials composed of Metal nodes joined together by organic ligands through strong coordination bonds,and they are also considered to be excellent precursors for various carbon-based functional materials due to the pyrolysis of organic ligands at high temperature in an inert atmosphere^[94⇓~96]。 MOF has many excellent features that make it an excellent precursor for high-performance microwave absorbers.Qiang et al.Took the lead in synthesizing uniform Fe/C composites by direct pyrolysis of organic framework Prussian blue nanocubes,and found that the high dispersion of Fe nanoparticles is conducive to multiple dielectric and magnetic resonance,resulting in excellent electromagnetic absorption properties and wide response bandwidth,with a maximum effective absorption bandwidth of 7.2 GHz^[97]。 Yin et al.prepared dodecahedral porous carbon nanotube/cobalt nanoparticle composite(CNTs/Co)by in-situ pyrolysis method using cobalt-based zeolite imidazole ester framework(ZIF-67)as precursor,and explored The effect of pyrolysis temperature on the morphology and microstructure of the composite.the preparation process is shown in Fig.6(a)^[98]。 as shown in Fig.6(B),the surface of the dodecahedron is distributed with high density of short and curved carbon nanotubes.when the pyrolysis temperature increases to 900℃,the amount of Co nanoparticles attached increases and the dispersion is good.with the further increase of pyrolysis temperature,although the growth of carbon nanotubes and the graphitization degree of the material can be promoted,the Co nanoparticles will reaggregate and seriously damage the magnetic loss.Therefore,when the pyrolysis temperature is 900℃,the microwave absorbing property of the composite is the best.As shown in Fig.6(C,d),when the thickness is 1.81 mm,the reflection loss peak at 15 GHz is−60.4 dB,the effective absorption bandwidth is 5.2 GHz,and the position of the absorption peak is consistent with the quarter-wavelength cancellation law.Fig.6(d)is a schematic diagram of the absorbing mechanism of the CNTs/Co composite,and the reasons for its excellent absorbing performance are As follows:(1)due to the multi-plane and large specific surface area of the material,electromagnetic waves are scattered and reflected multiple times inside the material^[99]; (2)that surface of the carbon nanotube contain a large number of defect and oxygen-containing functional groups to promote dipole polarization;(3)interfacial polarization between Co particles and carbon nanotubes;(4)that carbon nanotube are mutually contacted to form a conductive network to generate resistance type Los;(5)that transition of electron between different interfaces further consume electromagnetic wave energy;(6)Co particles produce excellent magnetic losses。

ferrite absorbing material is a kind of material based on the principle of absorbing electromagnetic wave by magnetic loss.Compared with other radar absorbing materials,it has the advantages of high frequency,wide frequency band and thin coating.Commonly used ferrite soft magnetic materials are manganese-zinc,nickel-copper-zinc,magnesium-copper-zinc,nickel-magnesium-zinc and ultra-high frequency soft magnetic ferrite of planar hexagonal system^[100⇓~102]。 However,its high density and poor oxidation resistance have seriously hindered the wide application of the material.To overcome these problems,composites with carbon materials are commonly performed。

Zhang et al.Used solvothermal method to attach CoFe₂O₄nanoparticles on carbon nanotubes to synthesize CoFe₂O₄/CNTs composite^[103]。 Cobalt ferrite nanoparticles not only have the characteristics of traditional microwave absorbing materials,but also have high permeability at high frequency,which can improve the microwave absorbing properties to a certain extent by combining with CNTs to form composites.Wang et al.Synthesized a CoFe₂O₄/nitrogen-doped redox graphene aerogel(CNGA)composite with a three-dimensional porous structure by solvothermal method and freeze-drying,the synthesis process is shown in Fig.7(a),and explored the effect of different CoFe₂O₄additions on the absorbing properties of the composite^[104]。 The addition of CoFe₂O₄in the three samples showed an increasing relationship,which was marked as CNGA-1,CNGA-2,CNGA-3.Interestingly,as shown in Fig.7(B),the C₀of the three samples do not change in the frequency range of 5~18 GHz,which proves that the eddy current loss is the main magnetic loss in this frequency band^[106]。 With the increase of CoFe₂O₄content,the attenuation ability of the material to electromagnetic wave is weakened,which is attributed to the re-agglomeration of magnetic CoFe₂O₄particles and the decrease of magnetic loss ability of the material.In addition,due to the appropriate ratio of CoFe₂O₄and rGO,CNGA-2 among the three samples shows excellent impedance matching,and its absorbing properties are shown in Fig.7(C,d).When the sample filling amount is 20 wt%and the thickness is 2.1 mm,the reflection loss peak is as high as−60.4 dB,and the effective absorption bandwidth is 6.48 GHz.At the same time,when its thickness is 3 mm,it can fully absorb the X-band.The absorbing mechanism of the composite is shown in Fig.7(e).The three-dimensional porous structure causes multiple reflection and scattering of electromagnetic waves,which is beneficial to the attenuation of electromagnetic waves.Aerogel with high specific surface area produces rich dipole polarization,interface polarization and defect polarization,thus increasing the polarization loss of the material;At the same time,the three-dimensional conductive network promotes electron transmission and transition,and improves the resistance loss of the material;In addition,the eddy current loss generated by the CoFe₂O₄nanoparticles enhances the magnetic loss of the material.Zong et al.Synthesized rGO/CoFe₂O₄composites by hydrothermal method,and proved that different reduction degrees of graphene oxide would change the electromagnetic parameters of the composites,thus obtaining different microwave absorbing properties^[107]。 Graphene oxide was reduced by two different reducing agents to obtain rGO composites with different degrees of reduction.By changing the C/O ratio and the number of oxygen-containing functional groups on the surface of the material,the electromagnetic parameters of the composite are changed,thus changing its absorbing properties.He et al.Used the same method to prepare NiFe₂O₄/rGO composites by uniformly depositing magnetic nickel ferrite nanoparticles on reduced graphene oxide nanosheets,and explored the effects of NiFe₂O₄in the composites^[108]。 The NiFe₂O₄induces a variety of polarization relaxation and natural resonance phenomena,which change the permeability and permittivity of the composite,thus improving the absorbing properties of the material.When the thickness is 5 mm,the reflection loss peak of the material is-42 dB,and the effective absorption bandwidth is 5.3 GHz.Shu et al.Prepared multiwalled carbon nanotube/zinc ferrite composite(MWCNTs/ZnFe₂O₄)by solvothermal method^[109]。 The ZnFe₂O₄microsphere is fixed on the surface of the MWCNTs,and the ZnFe₂O₄microsphere has high saturation magnetization and low coercive force,so that the magnetic loss is introduced,and the dielectric loss of the composite material is increased.When the sample thickness is 1.5 mm,the reflection loss peak reaches−55.5 dB at 13.4 GHz,and the effective absorption bandwidth is 3.6 GHz.Wang et al.Combined spray drying,solvothermal and calcination to prepare a flower-like CoFe₂O₄@graphene composite with gossamer-like graphene wrapping CoFe₂O₄microspheres^[110]。 This unique structure can effectively reduce the stacking of graphene,thereby improving the electromagnetic wave absorption performance of the composite.When the thickness is 2 mm,the reflection loss peak is-42 dB at 12.9 GHz,and the effective absorption bandwidth is 4.59 GHz.Lin et al.Prepared three-dimensional porous Bi₂Fe₄O₉microspheres/reduced graphene oxide(Bi₂Fe₄O₉/rGO)composite using a one-step etching method with flaky Bi₂Fe₄O₉as the precursor^[111]。 As shown in Fig.8(a~C),the Bi₂Fe₄O₉microspheres with an average diameter of 500 nm were uniformly grown on the graphene sheet.In addition,it can be seen from Fig.8(C)that after etching,a large number of pores appear on the surface of the Bi₂Fe₄O₉,the specific surface area of the composite is significantly increased,and the electromagnetic wave can produce more scattering and reflection,thus enhancing the absorbing performance of the composite.The porous Bi₂Fe₄O₉microspheres improve the dielectric loss and magnetic loss of the composite,as shown in Fig.8(d),when the sample thickness is 2 mm,the reflection loss peak is as high as−71.88 dB at 13.8 GHz,the effective absorption bandwidth is 4.24 GHz,and the absorption peak position follows the quarter-wave cancellation law.Fig.8(e)is a schematic diagram of the absorbing mechanism of the composite.Because that composite material has high-conductivity graphene and high-porosity Bi₂Fe₄O₉microsphere,electromagnetic waves are subject to multiple scattering and reflection in the composite material;The porous structure of Bi₂Fe₄O₉and the functional groups in rGO will break the charge distribution balance and cause dipole polarization and relaxation process.The interface polarization between two phases plays an important role in the absorption of electromagnetic waves 。

Similarly,Gao et al.Prepared three-dimensional porous BiFeO₃microspheres/reduced graphene oxide（BiFeO₃/rGO）composite by the same method using BiFeO₃particles and graphene oxide as precursors^[112]。 The specific process is as follows:the precursor and the graphene oxide are subjected to dissolution,crystallization,re-reduction and the like to generate a large number of small lamellar BiFeO₃and reduced graphene oxide,and finally the three-dimensional porous structure composite radar absorbing material is formed through self-assembly.When the thickness of the composite is 1.8 mm,the peak reflection loss is-46.7 dB,and the effective absorption bandwidth is 4.7 GHz.Zhao et al.Prepared carbon nanotube-linked expanded graphite sandwich composite(CNT/EG/BF)with BaFe₁₂O₁₉metal particles in the middle by sol-gel method^[113]。 CNTs make the BaFe₁₂O₁₉metal particles and expanded graphite interconnected and form a special three-dimensional conductive network,which increases the conductivity and dielectric loss of the composite,and the sandwich structure is conducive to the multiple scattering and reflection of electromagnetic waves.When the thickness of the sample is 1 mm,the peak reflection loss is-45.8 dB at 14.2 GHz,and the effective absorption bandwidth is 2.3 GHz 。

In addition to the design of carbon nanomaterials/ferrite composite structure to improve its absorbing properties,the ferrite can also be doped to improve the dielectric loss and magnetic loss of the ferrite.For example,Bibi et al.Synthesized copper-doped ferrite/multi-walled carbon nanotube(Cu_0.25Ni_0.25Zn_0.5Fe₂O₄/MWCNTs)composite by co-precipitation method^[114]。 Copper doping improves the saturation magnetization and conductivity of ferrite,and forms a good conductive network with MWCNTs.When the sample thickness is 2.5 mm,the reflection loss peak is-37.7 dB at 10.2 GHz,and the effective absorption bandwidth is 4.1 GHz.Liu et al.Prepared a novel cobalt-doped nickel-zinc ferrite/graphene composite（Co_0.2Ni_0.4Zn_0.4Fe₂O₄/GN）B y hydrothermal method^[115]。 The Co_0.2Ni_0.4Zn_0.4Fe₂O₄nanoparticles were uniformly covered by graphene,thus preventing the agglomeration of magnetic particles.In addition,with the increase of the graphene content,the conductivity of the composite material is improved,and the dielectric loss capability of the composite material for electromagnetic waves is further enhanced.When the sample thickness is 5.5 mm,two absorption peaks appear at 9.6 GHz and 5.2 GHz,and the corresponding reflection loss peaks are-53.5 dB and-58.3 dB,respectively,and the effective absorption bandwidth is up to 14.8 GHz.Shu et al.Synthesized nitrogen-doped reduced graphene oxide/nickel-zinc ferrite(N-rGO/Ni_0.5Zn_0.5Fe₂O₄)composite by a two-step method using graphene oxide as a template^[116]。 When the sample thickness is 2.91 mm,the reflection loss peak is-63.2 dB,and the effective absorption bandwidth is 5.4 GHz。

Recently,carbon nanohorns have aroused great interest of many scholars,and our research group has been studying in the field of carbon nanohorns for many years,and has verified their excellent microwave absorbing properties.Single-wall carbon nanohorns(SWCNHs)are closed hollow pyramidal structures composed of carbon atoms bonded by wall carbon nanohorns,with a diameter of 2~5 nm and a length of 40~50 nm.They cannot exist alone under normal conditions,and most of them exist in the form of spherical aggregates aggregated by thousands of SWCNHs^[117]。 Because of its low density,large specific surface area,excellent electrical and thermal conductivity,it has been widely used in electrochemistry,sensors,metal batteries,medical treatment and catalysts^[118,119]。 At the same time,compared with other carbon materials,SWCNHs have the following advantages:(1)the preparation process does not need a metal catalyst and has higher purity^[120]; (2)It is highly modifiable and can provide more surface active sites^[121]; (3)the spherical aggregate structure with a diameter of 50~100 nm makes SWCNHs have a high specific surface area;(4)the SWCNHs aggregate has a large internal space,allowing the encapsulated molecules to move freely.in addition,SWCNHs also have Many advantages in electromagnetic wave absorption:the urchin-like spherical aggregate of SWCNHs is beneficial to multiple scattering and reflection of electromagnetic waves,and the unique conical structure is beneficial to impedance matching of the material;More surface active sites are beneficial to the composite of other materials.many advantages make SWCNHs have great potential in the field of microwave absorption.For example,Zhang et al.Synthesized N-doped carbon nanohorns coated with Co particles(Co@N-CNHs)by DC arc plasma^[122]。 in the process of arc plasma discharge,the content of metal Co in the composite material can be changed by evaporating the carbon rod anode with Co wires inserted in the middle and adjusting the number of Co wires in the carbon rod.Due to the unique conical structure of carbon nanohorns and magnetic cobalt particles,the composite material has excellent impedance matching,and electromagnetic waves can better enter the material,so that its wave absorption capacity can be improved.When the sample thickness is 2.5 mm,the reflection loss peak is-41.39 dB at 7.3 GHz.Zhang et al.Prepared N-doped carbon nanohorn coated Fe nanoparticle composite(Fe@N-CNHs)by the same method,and changed the loading of Fe in carbon nanohorns by adjusting the number of Fe wires,and the preparation process is shown in Fig.9(a)^[123]。 the"dahlia-like"carbon nanohorn aggregates prepared by direct arc current plasma uniformly wrap the Fe nanoparticles,showing excellent microwave absorbing properties,as shown in Fig.9(B,C).When the sample thickness is 1.6 mm,the maximum reflection loss peak is−44.52 dB at 10.86 GHz,and the maximum effective absorption bandwidth is 3.23 GHz.Fig.9(d)shows the absorbing mechanism of the material.the unique conical structure of the carbon nanohorn makes it have a large specific surface area,which is conducive to multiple scattering and reflection of electromagnetic waves and enhances the attenuation of electromagnetic waves.Secondly,the dielectric loss of the composite is enhanced due to the dipole polarization produced by the doping of N atoms in the CNHs lattice.in addition,the interfacial polarization can be effectively controlled by adjusting the content of Fe nanoparticles encapsulated in CNHs,and the magnetic loss is enhanced by the introduction of magnetic particles。

In a word,carbon-metal/metal oxide composite absorbing materials have become the focus of absorbing research because of their excellent absorbing properties and multi-loss mechanism.the carbon-metal/metal oxide composite absorbing material is finally formed by coating a carbon layer on the surface of the metal/metal oxide by a solvothermal method,a sol-gel method,a hydrothermal method and the like,or loading a ferrite on a carbon material by electrostatic adsorption,physical mixing and other means.carbon coating with high chemical stability is beneficial to improve the oxidation resistance and corrosion resistance of metal materials,and improve the service life of composite materials.Secondly,the carbon material loaded with magnetic metal can effectively prevent the agglomeration of magnetic metal and give full play to the magnetic loss of magnetic metal,and the addition of carbon material effectively reduces the overall quality of the composite material,which meets the lightweight requirements of absorbing materials。

with the introduction of magnetic metals,carbon-based composite absorbing materials will produce a variety of loss mechanisms,such as dielectric loss,conductive loss and magnetic loss.the good magnetic properties of magnetic metals contribute to excellent magnetic losses,which promote the dissipation of electromagnetic waves through a variety of interactions with electromagnetic fields,such as natural resonance,exchange resonance,and eddy current loss;On the other hand,by controlling the morphology and doping other metals to change the composition and structure of the ferrite,such as flaky magnetic metal,flower-like aggregate,copper ferrite,zinc ferrite and the like,the saturation magnetization and permeability of the magnetic metal can be effectively improved,so that the composite absorbing material with excellent electromagnetic wave absorption performance can be obtained。

high temperature environment is one of the important application scenarios of radar absorbing materials,and ceramic materials have good high temperature resistance.Compositing ceramics with carbon materials can not only obtain composite materials with high temperature resistance,wear resistance and corrosion resistance,but also combine dielectric loss with resistive loss to improve the absorbing properties of composite materials。

Han et al.Prepared a graphene/silicon carbide nanowire ceramic composite by polymer pyrolysis process^[124]。 Graphene promotes the heterogeneous nucleation of silicon carbide nanowires at low temperatures,and the two form a layered structure through self-assembly.When the thickness of the sample is 2.35 mm and the addition of graphene is 3 wt%,the peak reflection loss is-69.3 dB at 10.55 GHz.In addition,the resistivity and dielectric constant of the material increase with the increase of temperature,and the absorbing properties of the material are improved.When the temperature is 673 K and the thickness of the sample is 2.3 mm,the composite shows the best absorbing properties,and the maximum reflection loss peak is-50.13 dB at 11.67 GHz.Wen et al.Prepared MWCNTs/SiO₂composite powders by mixing 2 wt%,5 wt%,10 wt%MWCNTs with silica,grinding and sintering using ultrasonic method^[125]。 When the content of MWCNTs is 5 wt%and the thickness of the sample is 3.5 mm,the maximum reflection loss peak of the composite powder is-60.54 dB at 573 K,and the effective absorption bandwidth is 4.2 GHz.Cao et al.Mixed rGO with SiO₂powder and prepared the composite by cold briquetting^[126]。 Combining the rGO conductivity and SiO₂dielectric properties,the composite radar absorber with 2.1 mm thickness was obtained,and the reflection loss peak-42 dB at 413 K temperature and the effective absorption bandwidth of 4.2 GHz were obtained.Lu et al.Used the solution method to adsorb ZnO nanocrystals on the surface of multi-walled carbon nanotubes and added SiO₂to form the composite^[127]。 At the same time,the high temperature dielectric properties and electromagnetic wave absorption properties of the material in the range of 373~673 K were studied.The addition of the ZnO crystal improves the dielectric property of the carbon material,and the SiO₂improves the thermal stability of the composite material,so that the material still has excellent microwave absorbing property at high temperature.Wang et al.Used phosphomolybdic acid/polypyrrole（PMo₁₂/PPy）nanospheres simply polymerized at room temperature as a precursor to successfully construct pomegranate-shaped Mo₂C@C nanospheres by high temperature pyrolysis under inert atmosphere,and the tiny Mo₂C nanoparticles were coated with ultrathin carbon shells and uniformly dispersed in the carbon matrix^[128]。 Both of them are dielectric materials,and the conduction loss is the main reason for the electromagnetic energy dissipation of Mo₂C@C nanospheres,and the dipole polarization and interface polarization provide auxiliary contributions in this process.Lian et al.Successfully prepared tungsten carbide/carbon composites by using a solid mixture of dicyandiamide(DCA)and ammonium metatungstate(AM)as precursors,in which ultrafine cubic tungsten carbide nanoparticles were grown in situ and uniformly dispersed on carbon nanosheets during high temperature pyrolysis^[129]。 When the ratio of DCA to AM is 6:1,the two samples show good microwave absorption properties,the strongest reflection peak value is-55.6 dB,and the qualified absorption bandwidth covers the frequency range of 3.6~18.0 GHz。

To sum up,carbon-ceramic composite radar absorbing materials have excellent radar absorbing properties at high temperature.carbon materials and resistive materials,such as silicon carbide nanowires and silicon dioxide powder,were compounded by high temperature pyrolysis,ultrasonic mixing,mechanical cold pressing and other methods,and finally carbon-ceramic composite absorbing materials were prepared.On the one hand,the conductive loss and dielectric loss capabilities of the composite absorbing material are enhanced,and the absorbing properties of the composite material can be regulated and controlled by adjusting the content of the ceramic powder;On the other hand,ceramic materials have the characteristics of strong thermal stability,high hardness,corrosion resistance,wear resistance and high temperature resistance,which greatly enhance the application of carbon-ceramic composite absorbing materials in extreme environments and prolong the service life of materials.Although carbon-ceramic composite radar absorbing materials have many excellent characteristics,the weight of ceramic materials can not be ignored,which also affects the requirement of lightweight radar absorbing materials and seriously limits their practical application。

Finally,different types of carbon-based composite absorbing materials formed by structure design,composition design,introduction of magnetic metal and ceramic powder show excellent absorbing properties.As shown in Table 2,the synthesis methods,reflection loss peak,and effective absorption bandwidth of some carbon-based composite absorbing materials are listed。