As a traditional electromagnetic wave absorption material, ferrite has been proved to have good electromagnetic wave absorption properties and wide absorption frequency band because of its high permeability, but the high density is the main factor restricting its development, and the preparation of mesoporous or hollow ferrite is an effective strategy to solve this problem. Lin et al. (Table 1) first synthesized Bi_0.9La_0.1FeO₃ particles by molten salt method, and then prepared porous ferrite microspheres by one-step acid etching method, and studied the electromagnetic wave absorption properties of ferrite before and after acid etching^[40]. The results show that for the unetched Bi_0.9La_0.1FeO₃, the minimum RL value is-19.5 dB at 10.6 GHz, and the matching thickness is 2.5 mm. In contrast, the flower-like porous Bi_0.9La_0.1FeO₃ ferrite microspheres prepared by water bath heating and acid etching for 60 min have a large number of mesopores with an average pore diameter of 9.16 nm, and the specific surface area can reach 75.09 m²·g^-1, and their absorption properties for electromagnetic waves are significantly improved.When the thickness of the porous ferrite Bi_0.9La_0.1FeO₃ microspheres (60 vol%)/paraffin composite is 2. 9 mm, the minimum RL value can reach − 57.9 dB, and the effective absorption bandwidth is 2. 7 GHz (6. 1 – 8.9 GHz). A large amount of nano-sheet ferrite is generated in the etching process, so that the magnetization and the permeability of the material are increased, and the impedance matching is optimized; In addition, the pore structure of the prepared ferrite microspheres makes them have a large specific surface area, which leads to multiple scattering and absorption of electromagnetic waves.

Magnetic metals such as Fe, Co, Ni and their alloys are considered to be very promising electromagnetic wave absorbing materials due to their large saturation magnetization and high Snoek limit. However, due to the high conductivity of metal materials, the eddy current loss induced by electromagnetic waves is large, which makes the permeability of metal materials decrease sharply with the increase of frequency and weakens its electromagnetic wave absorption performance. The study shows that the design of porous micro-nano structure is an effective means to suppress the formation of eddy currents. Yao et al. (Table 1) prepared porous Fe-Ni powder, which not only reduced the density of the material, but also improved the electromagnetic wave absorption performance of the powder^[15]. They first prepared Fe_0.5Ni_0.5C₂O₄·2H₂O precursor particles with uniform particle size distribution and morphology by oxalate precipitation method, and then prepared porous Fe-Ni powder with particle size of about 1 μm by thermal decomposition in N₂/H₂ mixed atmosphere at 450 ℃/30 min. The results show that when the thickness of Fe-Ni (30 wt%)/paraffin composite is 2.0 mm, the effective bandwidth of RL < -10 dB reaches 2.57 GHz; When the composite thickness is 3 mm, the minimum value of RL is − 52.58 dB at 6.82 GHz. The electromagnetic wave loss of Fe-Ni powder mainly depends on the magnetic loss dominated by natural resonance. The porous structure can effectively suppress the impedance mismatch problem caused by eddy current loss, thereby improving the electromagnetic wave absorption performance of the powder.

Magnetic materials also have the limitations of low dielectric constant, high density and poor thermal stability. In recent years, dielectric materials such as carbon materials and polyaniline have been widely used to improve the dielectric properties of electromagnetic wave absorption materials because of their light weight, corrosion resistance, simple preparation and excellent dielectric properties. Zhang et al. (Table 1) adjusted the electromagnetic parameters of the material from the perspective of porosity, thereby increasing its electrical loss to electromagnetic waves^[41]. They first prepared three different kinds of porous carbon (PC) by using EDTA, EDTA-2Na and EDTA-4Na as precursors and heat treatment at 700 ℃ for 1 H in argon atmosphere, and then coated polyaniline on the porous carbon with aniline, hydrochloric acid, phytic acid and ammonium persulfate as raw materials to prepare three kinds of polyaniline-modified porous carbon composite powders, namely PC @ PANI-1, PC @ PANI-2 and PC @ PANI-. The results show that the pore size of porous carbon is in the range of 500 ~ 600 nm, and the surface of porous carbon is rougher and more porous due to the coating of polyaniline. When the thickness of the absorber is 2.6 mm, the minimum RL of PC @ PANI-2 prepared from EDTA-2Na can reach-72.16 dB, and the maximum effective absorption bandwidth can reach 6.64 GHz (10.16 ~ 16.8 GHz). The existence of the hole effectively enhances the dielectric loss capability and impedance matching of the electromagnetic wave absorbing material, and the interaction process between the hole and the electromagnetic wave is shown in Figure 2. The scattering and multiple reflection of electromagnetic waves in the PC @ PANI porous structure increase the transmission path of electromagnetic waves and enhance the absorption of electromagnetic waves; On the other hand, the movement of electrons in the conductive network provided by PC and PANI also converts electromagnetic waves into heat energy and consumes them. Thirdly, the defects and heteroatoms in the porous structure can act as polarization centers, which is beneficial to polarization loss, and the electrons gather at a large number of heterogeneous interfaces, which further improves the electromagnetic wave absorption performance of porous carbon.

Multi-component compounding of magnetic metals with non-conductive materials is also a feasible strategy to reduce eddy current loss and improve electromagnetic wave absorption performance. Qiu et al. (Table 1) used nickel acetate (Ni(CH₃COO)₂·4H₂O), methanol and pyrrole as raw materials to prepare the precursor by alcohol-thermal method at 180 ° C/4 H, and then prepared porous Ni/C composite microspheres by carbothermal reduction at 550 ° C/5 H, and studied the pore size distribution, magnetic properties and 2 – 18 GHz electromagnetic wave absorption properties of the composite microspheres^[42]. The results show that the pores of the synthesized Ni/C composite microspheres mainly come from the pores between the hollow carbon nanospheres and the nickel nanoparticles and the hollow structure of the carbon nanospheres, and the pore size of the synthesized Ni/C composite microspheres is between a few nanometers and 50 nm. The saturation magnetization (M_S) of the porous Ni/C composite microspheres is 53.5 emug^-1, and the coercivity (H_C) is 51.4 Oe. When the thickness of the electromagnetic wave absorbing material is 9. 5 mm, the minimum RL value of the epoxy resin composite containing 60% porous Ni/C microspheres can reach-44. 5 dB at 2. 6 GHz. The synergistic effect between the magnetic loss of nickel nanoparticles and the dielectric loss of carbon hollow nanospheres and the large number of nickel-carbon heterointerfaces endow the composite with excellent electromagnetic wave absorption properties. In addition, the porous structure in the sample also enhances the input and loss of electromagnetic waves.

Magnetic materials still generally have the problem of narrow absorption band. Metal-organic framework (MOF) composites prepared by carbonization have the advantages of low density, porosity, many polarization centers, and both magnetic loss and dielectric loss, which not only improve the impedance matching of electromagnetic wave absorbing materials, but also enhance their multiple reflection and scattering capabilities, and ultimately achieve the purpose of effectively broadening the absorbing frequency band^[51,52]. Wang et al. (Table 1) used graphene oxide, cobalt nitrate hexahydrate, ferrous sulfate heptahydrate, 2-methylimidazole and ascorbic acid as raw materials to prepare porous cocoon-like graphene oxide by mechanical stirring combined with freeze-drying, and then in-situ growth of iron-cobalt metal organic framework materials on the surface of porous cocoon-like graphene oxide.Finally, the Fe-Co/nitrogen-doped carbon (NC)/reduced graphene oxide (Fe-Co/NC/rGO) composite powder with hierarchical pore structure was prepared after carbonization at 500 ℃/2 H in argon atmosphere (Fig. 3), and the bonding between the components of the sample and its electromagnetic wave absorption properties were studied^[43]. The results show that the Fe-Co alloy nanoparticles are coated by the graphitized NC layer, and the Fe-Co/NC particles are uniformly distributed on the porous reduced graphene oxide.The sample has a multi-stage pore structure, including 0. 9-25 μm macropores, 2-25 nm mesopores and 0. 4-2 nm micropores. When the thickness of the absorber is 2. 5 mm, the minimum reflection loss is-43. 26 dB, and the effective absorption bandwidth (RL ≤ -10 dB) is 9. 12 GHz (8. 88 ~ 18 GHz). The excellent electromagnetic wave absorption performance of Fe-Co/NC/rGO can be mainly attributed to the following aspects (Fig. 4): firstly, the hierarchical pore structure of the powder improves its impedance matching with the electromagnetic wave, while the multiple reflection and scattering of the electromagnetic wave in the pore channel prolong its propagation path; Secondly, the doping of Fe and Co enhances the magnetic loss of the powder. Thirdly, the defects caused by doping Fe, Co and N and the interface caused by multi-component recombination enhance the dielectric loss of the powder; Finally, the capacitive-like conductive network formed by the overlapping of porous reduced graphene oxide sheets also leads to a larger resistive loss.

At present, the pore structure of zero-dimensional porous electromagnetic wave absorbing materials is generally controlled by high-temperature carbonization or etching, and the pore size is generally between a few nanometers and tens of microns. Compared with the traditional zero-dimensional powder, the introduction of the porous structure increases the surface/interface of the material itself, which is conducive to improving the interface polarization process of the material, thereby improving its electromagnetic wave absorption performance. At present, there is no obvious rule to follow between the pore size and the electromagnetic wave absorption performance, which should be the focus of further research on porous electromagnetic wave absorption materials in the future.

The porous structure of magnetic materials improves the electromagnetic wave absorption properties of the materials to a certain extent, but the minimum reflection loss value is still limited, and the effective absorption bandwidth is narrow. Therefore, researchers have tried to composite porous magnetic materials with carbon-based materials with strong dielectric loss, so as to improve the impedance matching performance of the composite materials, and ultimately effectively improve its electromagnetic wave absorption performance (as shown in Table 1).

MOF has the advantages of designable composition, adjustable pore structure, high crystallinity and large specific surface area, and has also been used as a template for the preparation of porous carbon composites. By controlling the pyrolysis process, the organic linker in the MOF was converted into nanoporous carbon, while its original porous framework was also preserved. In addition, the metal ions in the MOF framework are converted into metal composites such as metals, metal oxides and metal carbides during carbothermal reduction, which are uniformly embedded in the carbon framework, so that the derived carbon-based composites have abundant interfaces and defects, continuous conductive networks, and magnetic properties, which are helpful to enhance the attenuation of electromagnetic waves^{[53⇓⇓⇓⇓~58]}.

Zero-dimensional porous electromagnetic wave absorbing materials have become a widely studied and mature electromagnetic wave absorbing material because of their low preparation cost, simple process and convenient application. The electromagnetic wave absorbing coating prepared by zero-dimensional porous electromagnetic wave absorbing materials mixed with binder can be easily applied to the surface of the required equipment substrate. However, zero-dimensional electromagnetic wave absorbing materials also have the problem of poor dispersion and easy agglomeration, and the absorbing coating will increase the weight of the aircraft in the application process, and its bonding strength with the substrate is low, so it is easy to peel off or crack and needs frequent repair.

Graphene materials have attracted great attention in the field of electromagnetic wave absorption due to their low density, large specific surface area, large aspect ratio and strong conductivity^[67]. Zhang et al. (Table 2) used graphene oxide as raw material to prepare ultra-light and compressible graphene aerogel by solvothermal combined with freeze-drying method^[38]. The results show that the prepared graphene aerogel has a three-dimensional interconnected porous network, and the effective absorption bandwidth of the aerogel can reach 50. 5 GHz in the ranges of 2 ~ 18, 26.5 ~ 40, and 75 ~ 110 GHz. More importantly, the electromagnetic wave absorption properties of graphene aerogel can be effectively adjusted by compression. When the compression deformation is 90%, the effective absorption bandwidth of graphene aerogel is increased to 60.5 GHz. The excellent electromagnetic wave absorption properties and ultra-low density (14 mg·cm^-3) make the specific electromagnetic wave absorption capacity of graphene aerogel up to 2.2×10⁵dB·cm²·g^-1 at 90% compression deformation. The excellent electromagnetic wave absorption performance of the aerogel can be attributed to the existence of a three-dimensional crosslinked resistance-inductance-capacitance coupling network built by graphene, and the current induced by the electromagnetic wave is rapidly attenuated in the network, so that the energy of the electromagnetic wave is rapidly dissipated. However, due to the inherent electromagnetic properties of graphene, a single graphene material has the defects of interface impedance mismatch and limited loss mechanism.

Graphene is an ideal support for loading various nanomaterials. Therefore, researchers have tried to compound graphene with magnetic nanoparticles to give full play to the advantages of each component and improve the absorbing properties of electromagnetic wave absorbing materials. Zhang et al. (Table 2) first prepared graphene oxide by modified Hummers method, and then prepared rGO/α-Fe₂O₃ composite aerogel by hydrothermal self-assembly process^[64]. The results showed that rGO in the composite aerogel was interconnected to form a three-dimensional porous network with micron and submicron scales, and the nano-α-Fe₂O₃ was uniformly dispersed in the porous network and well combined with rGO. The rGO/α-Fe₂O₃ composite aerogel has excellent electromagnetic wave absorption properties, and compared with the reduced graphene oxide aerogel, the composite aerogel has a wider effective microwave absorption bandwidth and a stronger microwave absorption ability in the frequency range of 1 – 18 GHz. The minimum reflection loss of the rGO/α-Fe₂O₃/ paraffin absorber with a thickness of 5.0 mm is − 33.5 dB at 7.12 GHz, and the effective absorption bandwidth of the absorber with a thickness of 3.0 mm can reach 6.4 GHz(10.8~17.2 GHz),rGO/α-Fe₂O₃. The good electromagnetic wave absorption performance of the composite aerogel can be attributed to the improvement of its impedance matching. Compared with the single rGO,α-Fe₂O₃, the addition of the composite material effectively reduces the overall dielectric constant of the composite material, which is beneficial to the coordination of dielectric loss and magnetic loss, thereby improving the electromagnetic wave loss capability of the material. On the other hand, the unique three-dimensional porous network structure of composite aerogel not only improves the impedance matching characteristics between electromagnetic waves and materials, but also prolongs the interaction path between electromagnetic waves and materials, which is conducive to the absorption of electromagnetic waves.

Two-dimensional carbon nanosheets can also be used as supports and form unique three-dimensional structures through self-assembly. Wu et al. (Table 2) used ferric chloride, urea, ethylene glycol and pyrrole as raw materials to prepare three-dimensional porous flower-like ferroferric oxide/carbon (Fe₃O₄/C) composite by solvothermal combined with carbon reduction method^[65]. The results show that the :Fe₃O₄/C flower is composed of two-dimensional sheets and particles, and there are a large number of mesopores with a pore size of about 25 nm in the 20~150 nm;Fe₃O₄/C flower, and its specific surface area is 70.2 m²·g^-1. When the content of Fe₃O₄/C is 50 wt%, the lowest reflection loss of the composite is -54.6 dB at 5.7 GHz when the matching thickness of the composite is 4.27 mm; When the thickness of the absorber is 2. 1 mm, the effective absorption bandwidth of the electromagnetic wave can reach 6. 0 GHz. The combination of the Fe₃O₄ dominated by magnetic loss and the carbon dominated by dielectric loss effectively improves the impedance matching performance of the material, so that electromagnetic waves can easily enter the interior of the material; The inhomogeneous interface between the Fe₃O₄ and carbon and the defects in the material increase the polarization loss. The flower-like three-dimensional porous structure enhances the multiple reflection and scattering of the incident electromagnetic wave. The synergistic effect of the above loss mechanisms finally enables the three-dimensional porous flower-like Fe₃O₄/C composite material to have excellent electromagnetic wave absorption performance.

The induced current and the enhanced interfacial polarization of carbon nanotubes in the alternating electromagnetic field are helpful to the attenuation of electromagnetic waves, but they are easy to agglomerate and difficult to form an effective attenuation network, thus weakening their electromagnetic wave absorption properties. Chen et al. (Table 2) solved the problem of carbon nanotube agglomeration by dispersing carbon nanotubes into a three-dimensional graphene skeleton to form a composite network^[66]; They used graphene oxide and multi-walled carbon nanotubes as raw materials to prepare ultra-light multi-walled carbon nanotube/graphene composite aerogels (CGFs) by solvothermal self-assembly method, and studied their electromagnetic wave absorption properties. The results show that the composite aerogel has an irregular network pore structure, and the anisotropy of the pore structure becomes more obvious with the increase of the loading of carbon nanotubes. The complex dielectric constant, conductivity, and microstructure of the multi-walled carbon nanotube/graphene hybrid aerogel can be conveniently adjusted by changing the loading amount of carbon nanotubes and the solvothermal reduction temperature; The minimum reflection loss of the sample can reach -39.5 dB, and the average absorption intensity of the sample in C-band (4 ~ 8 GHz) and X-band (8 ~ 12 GHz) is more than -22.5 dB, and the effective absorption bandwidth with the reflection loss less than -10 dB can reach 16 GHz, covering the whole measurement range of 2 ~ 18 GHz. The synergistic effect of multi-walled carbon nanotubes and graphene and the three-dimensional network structure constructed by the two are the main reasons for the excellent electromagnetic wave absorption properties of CGFs. The addition of multi-walled carbon nanotubes effectively improves the dielectric properties of CGFs, thereby improving their impedance matching characteristics. The porous structure enhances the scattering of electromagnetic waves in the material, and the positive and negative charges accumulated on the pore wall enhance the polarization loss process. The three-dimensional cross-linked conductive network formed by multi-walled carbon nanotubes and graphene can respond to the electromagnetic field through the generation of induced current and lose the incident electromagnetic wave. The synergistic effect of these three factors improves the electromagnetic wave absorption properties of CGFs aerogels.

Using the excellent mechanical properties of carbon nanotubes to reinforce the electromagnetic wave absorbing materials with poor mechanical properties is an effective way to realize their multi-functionality. Zeng et al. (Table 2) prepared porous multi-walled carbon nanotube/waterborne polyurethane composites by freeze-drying method^[67]. The results show that the prepared composite exhibits an anisotropic lamellar macroporous structure on the micron scale due to the unidirectional growth of ice crystals, and a large number of small pores with a size of about 100 nm are formed by the interconnection of carbon nanotubes/waterborne polyurethane. The composites with densities of 126 mg·cm^-3 and 20 mg·cm^-3 have minimum reflection losses of − 50 dB and − 20 dB, respectively, in the X-band. The excellent electromagnetic wave absorption performance of the composite can be attributed to the high conductivity of the multi-walled carbon nanotubes, the anisotropic pore structure and the polarization loss of the multi-walled carbon nanotubes and the waterborne polyurethane under an alternating electromagnetic field. In addition, the composite material has good flexibility, which also broadens the application range of electromagnetic wave absorbing materials.

Carbon nanotubes can not only be used as additives or reinforcements in electromagnetic wave absorbing materials, but also directly form electromagnetic wave absorbing materials with three-dimensional porous network structure. Song et al. (Table 2) first used SiO₂ nanowire aerogel as a template to obtain a carbon nanotube skeleton by template-directed chemical vapor deposition, and then grew a layer of closely arranged multilayer graphene on the carbon nanotube by plasma-enhanced chemical vapor deposition to prepare a carbon nanotube-multilayer graphene core-shell hybrid aerogel with light weight, flexibility and good conductivity^[68]. The results show that carbon nanotubes form a three-dimensional framework with a large number of pores. The inner diameter of carbon nanotubes is 220 ~ 350 nm. Multilayer graphene with different sizes and orientations is grafted on the surface of carbon nanotubes, which forms more nanopores on the surface. Carbon nanotubes have high porosity (more than 99%) and good mechanical properties. When the matching thickness of the absorber is 1.6 mm, the average reflection loss of the absorber with density of 0.0058 g·cm^-3 and 0.0089 g·cm^-3 in X-band reaches − 38.4 and − 47.5 dB, respectively. The electrical loss caused by the multi-scale conductive network formed by the sheet graphene winding on the carbon nanotube, and the dielectric relaxation, tunneling effect and interface effect caused by the high-density defects such as dislocations or lattice deformation at the exposed edge of the graphene sheet and the interface, together improve the electromagnetic wave attenuation ability of the aerogel.

In addition to graphene and carbon nanotubes, biomass materials or carbon-containing porous waste in life can also be used as carbon precursors or templates to prepare porous carbon electromagnetic wave absorbing materials through treatment. Zhou et al. (Table 2) prepared three-dimensional co-doped carbon aerogels by hydrothermal and pyrolysis processes using fish skin as precursor^[69]. The results show that there are a large number of mesopores and micropores in the aerogel, and the interconnected irregular channels form a three-dimensional porous network.The fish skin-based carbon aerogel prepared at 650 ℃ has a 1369.3 m²/g specific surface area and a high porosity. The internal pores of carbon aerogel are dominated by the naturally occurring pores of fish skin, and the heteroatoms O and N in fish skin are uniformly dispersed in the carbon skeleton. When the matching thickness is 2. 6 mm and the frequency is 15. 8 GHz, the minimum reflection loss of the material pyrolyzed at 650 ℃ is-52. 6 dB. In addition, the maximum effective absorption bandwidth of the absorber with a thickness of 3.0 mm is 8.6 GHz (9.4 ~ 18 GHz), and the minimum RL value is -33.5 dB. The pores in the three-dimensional fish skin-based carbon aerogel effectively improve the impedance matching of the absorber, the pore wall generates multiple reflection and scattering on the incident electromagnetic wave, the high conductivity of the three-dimensional fish skin-based carbon aerogel enables the electromagnetic wave energy to be dissipated in the form of Joule heat more quickly, and the abundant solid-gas interface in the material increases the interface polarization relaxation loss process; The N and O atoms in the fish skin act as polarization centers in the carbon, which further enhances the polarization loss. The combined effect of the above mechanisms makes the carbon aerogel have good electromagnetic wave absorption properties (Fig. 5).

Wang et al. (Table 2) used cigarette filter and graphene oxide as raw materials to prepare graphene nanosheet-coated carbon fiber aerogel by dip-coating combined with thermal reduction^[70]. The results showed that the fibers of cigarette filter were crosslinked and twisted to form a three-dimensional porous network after carbonization, and the graphene nanosheets were attached to the carbon fiber to make its surface more rough and porous, and there were macropores with a pore size of 100 ~ 300 μm and mesopores with a pore size of 5 ~ 20 nm in the structure, and the density of the composite aerogel was only 7.6 mg·cm^-3, and its compressive strength was about 0. 07 MPa. When the thickness of the absorber is 1. 5 mm, the minimum reflection loss is -30. 53 dB at 14. 6 GHz, and the effective absorption bandwidth is 4. 1 GHz. The three-dimensional porous network of the composite aerogel makes the electromagnetic waves entering the interior of the material scatter and attenuate between the fibers until they are absorbed and dissipated in the form of heat. In order to further improve the absorbing properties of the material, they coated a layer of polypyrrole on the surface of the composite aerogel by chemical polymerization, and the results showed that the minimum reflection loss at 7. 9 GHz could be as low as -45. 12 dB when the thickness of the absorber was 2. 5 mm. The thickness of the polypyrrole layer is very important for the electromagnetic wave absorption performance of the composite aerogel. Too much polypyrrole coating will increase the conductivity of the composite aerogel and reduce its dielectric properties, thus weakening the impedance matching of the absorber, so that a large number of electromagnetic waves are reflected rather than absorbed, and ultimately the absorption of electromagnetic waves by the absorber will be weakened.

Liu et al. (Table 2) used fluidized coke with high carbon content (an intermediate product of petroleum processing) as raw material to prepare porous activated coke with high specific surface area by alkali activation process^[71]. The results showed that the activated fluidized char produced onion-like layered porous structure and a large number of pores with pore size ranging from 0 to 75 μm, which effectively increased the specific surface area (3320 m²·g^-1) of the material, reduced its density, and provided abundant attachment sites for Ni nanoparticles. The results of microwave absorbing properties show that the minimum reflection loss of activated coke is-20 dB, and the effective absorption bandwidth is 13. 2 GHz (4. 8 ~ 18 GHz). The porous carbon/nickel composite was prepared by adding nickel hydroxide (Ni(OH)₂) to the raw materials and then in-situ reduction growth of Ni nanoparticles by heat treatment. When the loading of porous carbon/nickel is 20 wt%, the lowest reflection loss value of the composite with paraffin can be further reduced to − 47 dB, and the effective electromagnetic wave absorption band covers a wide frequency band from 3.5 GHz to 18 GHz. The porous structure improves the impedance matching between the composite material and free space, and increases the reflection of incident electromagnetic waves in the porous structure of the material; The formation of a large number of interfaces between porous carbon and nickel particles enhances the interfacial polarization and relaxation loss process. In addition, the presence of a large number of voids significantly mitigates the density of the material. The development and utilization of biomass carbon materials and domestic/industrial waste carbon materials simplify the preparation process of porous carbon and reduce the cost of electromagnetic wave absorbing materials.

Li et al. (As shown in Table 2) used Ti₃AlC₂ powder, poly (methyl methacrylate) (PMMA) microspheres and graphene oxide as raw materials to prepare reduced graphene oxide (rGO)/Ti₃C₂T_x(MXene) composite aerogel with hollow core-shell structure and controllable dielectric constant through self-assembly and sacrificial template process^[72]. The results showed that the :Ti₃C₂T_x sheets were assembled with PMMA as a template, and the hollow spherical particles with a pore size of about 5 μm were formed after the removal of the PMMA template. The rGO was uniformly coated on the outer surface of the Ti₃C₂T_x hollow spheres and connected with the adjacent hollow spheres, forming a porous network with a core-shell structure. The porosity of the aerogel was more than 99%. The prepared rGO/Ti₃C₂T_x aerogel has excellent electromagnetic wave absorption properties, and the effective absorption bandwidth can cover the whole X-band (8 ~ 12 GHz) when the thickness is 3.2 mm; The density of the electromagnetic wave absorbing material is only 0.0033 g·cm^-3. The aerogel has good electromagnetic wave absorption performance and high porosity, so that the aerogel can be applied in the fields of energy storage, sensors, wearable electronics and the like.

Magnetic metals are both conductive and magnetic, which can absorb electromagnetic waves through electric loss and magnetic loss. However, due to the impedance mismatch caused by their high density and high conductivity, there are few reports on electromagnetic wave absorbing materials based on magnetic metals. Porous structure provides an effective way to solve this problem. Qin et al. (Table 2) used F127, nickel nitrate hexahydrate and ferric nitrate nonahydrate as raw materials to prepare NiFe₂O₄/NiO/Ni porous nickel-based aerogel by fermentation dough method combined with thermal reduction process^[35]. The results show that the three-dimensional porous network of the nickel-based aerogel is composed of chain-like frameworks with a diameter of about 3 μm, and the oxide particles generated on the surface make it more rough and porous. The aerogel has a low density (0.06 g·cm^-3), and the effective absorption bandwidth of the absorber can reach 14.24 GHz when the matching thickness is 0.6 mm. The porous structure of aerogel and the multi-component synergistic effect of NiFe₂O₄/NiO/Ni are the main reasons for the improvement of the electromagnetic wave absorption capacity of the material (as shown in Figure 6). The porous structure of aerogel not only makes the absorber have good impedance matching performance, but also makes more incident electromagnetic waves enter the material and reflect many times. The interfacial polarization and defects of NiFe₂O₄/NiO/Ni also play a positive role in the dissipation of electromagnetic wave energy. The Ni-based aerogel effectively solves the problems of skin effect and high density of a metal-based electromagnetic wave absorbing material; In addition, the electromagnetic wave absorbing material also has good high-temperature oxidation resistance and can be used under high-temperature extreme working conditions.

In addition to regulating the pore structure at the micro/nano level, the combination of microstructure and macrostructure is also an effective means to adjust the absorbing properties of electromagnetic wave absorbing materials. Mei et al. (As shown in Table 2) first used 3D printing technology to successfully prepare a Al₂O₃ porous material with an oblique honeycomb shape, and then used a direct chemical vapor deposition process to grow SiC whiskers in the material to prepare a Al₂O₃/SiC composite material^[73]. The results show that the pore diameter of the composite is about 500 μm, and the SiC whiskers grown on the pore wall make the surface of the composite more rough and porous. The electromagnetic wave absorption property of the Al₂O₃/SiC composite material with an inclined honeycomb structure with a variable angle is adjustable, and the impedance matching, the internal scattering and the dielectric loss of the material are effectively improved by the porous inclined honeycomb structure with a reasonable angle and the introduction of micrometer scale SiC, so that the material has good electromagnetic wave absorption property. The Al₂O₃/SiC composite with a honeycomb structure tilt angle of 30 ° has the best electromagnetic wave absorption performance, and its minimum reflection loss value is − 63.65 dB at 9.8 GHz when the absorber thickness is 3.5 mm, and its effective absorption bandwidth covers the entire X-band (8.2 – 12.4 GHz).

Luo et al. (As shown in Table 2) designed a new type of electromagnetic wave absorbing material with gradient porous structure, and used CST Micro wave Studio simulation to study the influence of surface resistance R, height H and wall thickness w on the electromagnetic wave absorption performance of the absorber^[74]. The results show that the effective absorption bandwidth of the composite can reach 14. 06 GHz in the frequency range of 1 ~ 18 GHz. At the same time, they also successfully prepared gradient porous electromagnetic wave absorption materials by 3D printing combined with impregnation technology, and the broadband electromagnetic wave absorption performance of the designed gradient porous materials was significantly improved, which can be attributed to the synergistic effect of structure and material properties. The structure increases the square aperture from bottom to top, improves the impedance matching between the surface of the gradient porous structure and the air, and reduces the reflection of electromagnetic waves. In addition, the increase of the propagation path of the electromagnetic wave in the porous electromagnetic wave absorbing material is also beneficial to the attenuation of the electromagnetic wave. The experimental results are in good agreement with the simulation results, and this study provides a new design strategy for the development of anti-radar detection technology and electromagnetic interference shielding materials.

Three-dimensional porous electromagnetic wave absorbing materials not only inherit the characteristics and advantages of zero-dimensional, one-dimensional and two-dimensional materials, but also can assemble three-dimensional crosslinked porous networks due to the diversity of preparation methods.It not only retains the loss effect of each component in the three-dimensional structure on electromagnetic waves, but also significantly optimizes the overall impedance matching of the material, which is conducive to the entry of electromagnetic waves into the material. The multiple reflection and scattering of electromagnetic waves by the three-dimensional porous structure also provide a longer path for the propagation of electromagnetic waves in its interior. For conductive materials, the porous network can also increase the transmission path of induced current in the material, thereby increasing the conductive loss of electromagnetic waves.

In a word, the electromagnetic wave absorbing material with three-dimensional porous structure can not only give full play to the performance advantages of electromagnetic wave absorbing components in other dimensions, but also give full play to the unique advantages of three-dimensional porous network in attenuating and losing electromagnetic waves. At the same time, due to the flexibility of material preparation on the three-dimensional scale, the three-dimensional porous material is easier to have electromagnetic wave absorption performance and other physical and chemical properties at the same time, thereby realizing the structural and functional integration of the electromagnetic wave absorption material; In addition, the raw material source of the three-dimensional porous material is wider, and the electromagnetic wave absorbing material with the porous structure can be prepared by using industrial or domestic waste, so that the preparation cost of the material is reduced.