A cobalt / cobalt oxide / reduced graphene oxide composite material, a preparation method thereof, and a microwave absorber

Abstract

The invention discloses a cobalt / cobalt oxide / reduced graphene oxide composite material, a preparation method thereof, and a microwave absorber, relating to the technical field of microwave absorption materials. The cobalt / cobalt oxide / reduced graphene oxide composite material comprises microspheres and a shell coated on the surface of the microspheres. Among them, the material of the microspheres comprises cobalt and cobalt oxide, and the material of the shell comprises cobalt, cobalt oxide, and reduced graphene oxide. By compounding cobalt, cobalt oxide, and RGO to form the core-shell structure, the impedance matching and attenuation ability can be effectively adjusted. Through the synergistic effect of dielectric loss and magnetic loss, a composite material with strong reflection loss and an ultra-wide bandwidth covering the entire X-band is obtained, enabling it to be used as a low-frequency microwave absorber and applicable to aspects such as radar stealth.

Description

Technical Field

[0001] The present invention relates to the technical field of microwave absorption materials, and particularly relates to a cobalt / cobalt oxide / reduced graphene oxide composite material, a preparation method thereof, and a microwave absorber. Background Art

[0002] With the rapid development of information technology, electromagnetic waves have been widely used in fields such as communication, the national defense industry, and big data transmission. However, the resulting electromagnetic interference will have an adverse impact on military security. Therefore, researchers are committed to designing microwave absorption materials with "strong absorption, thin thickness, light mass, and wide frequency band" for electronic security and defense stealth technology. The performance of microwave absorption materials depends to a large extent on the synergistic effect of impedance matching, attenuation ability, and electromagnetic loss.

[0003] In recent years, due to the unique 2D structure, low density, high specific surface area, high dielectric constant, rich surface defects, and high electrical conductivity of reduced graphene oxide (RGO), it has attracted wide attention in the field of microwave absorption. The skin effect caused by impedance mismatch and high electrical conductivity of RGO has hindered its further development in the practical application of the microwave absorption field.

[0004] Generally, ferromagnetic materials include magnetic oxides (such as Fe3O4, CoO, NiO, etc.), magnetic metals (such as Fe, Co, Ni, etc.), and magnetic alloys (such as FeCo, CoNi, etc.). Due to their excellent magnetic loss characteristics, they are considered promising microwave absorption materials. Among them, cobalt and its corresponding cobalt oxides have been widely studied and reported in the field of microwave absorption due to their unique physical and chemical properties. However, due to their high density, impedance mismatch, easy corrosion, and narrow absorption bandwidth, they cannot be practically applied in the microwave absorption field. Summary of the Invention

[0005] The main object of the present invention is to propose a cobalt / cobalt oxide / reduced graphene oxide composite material, a preparation method thereof, and a microwave absorber, aiming to provide a microwave absorption material with strong absorption and wide frequency band.

[0006] To achieve the above object, the present invention proposes a cobalt / cobalt oxide / reduced graphene oxide composite material, including microspheres and a shell coated on the surface of the microspheres. Among them, the material of the microspheres includes cobalt and cobalt oxide, and the material of the shell includes cobalt, cobalt oxide, and reduced graphene oxide.

[0007] Optionally, the reduced graphene oxide is in a sheet shape. Taking the sheet-shaped reduced graphene oxide as a carrier, the cobalt and cobalt oxide are loaded on the reduced graphene oxide to form the shell.

[0008] Optionally, the particle size of the cobalt / cobalt oxide / reduced graphene oxide composite material is 5 - 10 μm.

[0009] Based on the above object, the present invention also provides a preparation method of the cobalt / cobalt oxide / reduced graphene oxide composite material as described above, and the preparation method includes the following steps:

[0010] Disperse graphene oxide in an organic solvent to obtain a graphene solution;

[0011] Add a cobalt salt precursor, 2-aminoterephthalic acid, and polyvinylpyrrolidone to the graphene solution, and stir until completely dissolved and uniformly dispersed to obtain a mixed solution;

[0012] React the mixed solution at 153 - 160 °C for 7.5 - 9 h, then cool and perform solid-liquid separation. Wash and dry the separated solid product, and then perform high-temperature treatment under a protective gas atmosphere to obtain the cobalt / cobalt oxide / reduced graphene oxide composite material.

[0013] Optionally, the organic solvent includes N,N-dimethylformamide; and / or,

[0014] The cobalt salt precursor includes cobalt nitrate hexahydrate.

[0015] Optionally, the mass ratio of the graphene oxide to the volume of the organic solvent is 0.01 - 0.05 g:35 mL.

[0016] Optionally, the mass ratio of the graphene oxide, cobalt nitrate hexahydrate, 2-aminoterephthalic acid, and polyvinylpyrrolidone is (0.01 - 0.05):(1.44 - 1.46):(0.90 - 0.92):(1.05 - 1.10).

[0017] Optionally, the temperature of the high-temperature treatment is 490 - 510 °C, and the time is 1 - 3 h.

[0018] Optionally, the protective gas includes a mixed gas of hydrogen and argon.

[0019] In addition, the present invention also provides a microwave absorber, and the microwave absorber includes the cobalt / cobalt oxide / reduced graphene oxide composite material as described above.

[0020] In the technical solution provided by the present invention, the cobalt / cobalt oxide / reduced graphene oxide composite material has a core-shell structure, and the microsphere made of cobalt and cobalt oxide is used as the core, and the outer shell is made of cobalt, cobalt oxide and RGO. By compounding cobalt, cobalt oxide and RGO to form the core-shell structure, the impedance matching and attenuation ability can be effectively adjusted. Through the synergistic effect of dielectric loss and magnetic loss, a composite material with strong reflection loss and an ultra-wide bandwidth covering the entire X-band is obtained, enabling it to be used as a low-frequency microwave absorber and can be applied to radar stealth and other aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0022] Figure 1 Schematic diagram of the preparation process of an embodiment of the preparation method of the cobalt / cobalt oxide / reduced graphene oxide composite material provided by the present invention;

[0023] Figure 2 Schematic diagram of the microwave absorption mechanism of the CCOR composite material prepared in the embodiment of the present invention;

[0024] Figure 3 Electron holographic image and reconstruction diagram of the CCOR composite material prepared by the present invention;

[0025] Figure 4 Electron holographic image of the CCOR composite material prepared by the present invention and the corresponding magnetic field line reconstruction diagram;

[0026] Figure 5 FESEM image of the CCOR-1 composite material prepared in Example 1 of the present invention;

[0027] Figure 6 FESEM image of the CCOR-2 composite material prepared in Example 2 of the present invention;

[0028] Figure 7 FESEM image of the CCOR-3 composite material prepared in Example 3 of the present invention;

[0029] Figure 8 FESEM image of the CCOR-4 composite material prepared in Example 4 of the present invention;

[0030] Figure 9 FESEM image of the CCOR-5 composite material prepared in Example 5 of the present invention;

[0031] Figure 10 FESEM image of Co / CoO microspheres prepared in Comparative Example 1 of the present invention;

[0032] Figure 11 FESEM image of Co / CoO / RGO composite prepared in Comparative Example 2 of the present invention;

[0033] Figure 12 FESEM image and TEM image of CCOR-3 composite material prepared in Example 3 of the present invention;

[0034] Figure 13 Energy spectrum diagrams of CO, O, and C of the CCOR-3 composite material prepared in Example 3 of the present invention;

[0035] Figure 14 XRD patterns of CCOR composite materials prepared in Examples 1-5 of the present invention, Co / CoO microspheres prepared in Comparative Example 1, and RGO;

[0036] Figure 15 Raman spectra of CCOR composite materials prepared in Examples 1-5 of the present invention and Co / CoO microspheres prepared in Comparative Example 1;

[0037] Figure 16 VSM test diagrams of CCOR composite materials prepared in Examples 1-5 of the present invention and Co / CoO microspheres prepared in Comparative Example 1;

[0038] Figure 17 Real part ε' diagram of complex dielectric constant of CCOR composite materials prepared in Examples 1-5 of the present invention and Co / CoO microspheres prepared in Comparative Example 1 in the 2-18 GHz frequency band;

[0039] Figure 18 Imaginary part ε'' diagram of complex dielectric constant of CCOR composite materials prepared in Examples 1-5 of the present invention and Co / CoO microspheres prepared in Comparative Example 1 in the 2-18 GHz frequency band;

[0040] Figure 19 Dielectric loss tangent value tanδ of CCOR composite materials prepared in Examples 1-5 of the present invention and Co / CoO microspheres prepared in Comparative Example 1 ε Figure and magnetic loss tangent value tanδ μ Figure;

[0041] Figure 20 Electromagnetic parameter curve diagrams of CCOR composite materials prepared in Examples 1-5 of the present invention and Co / CoO microspheres prepared in Comparative Example 1 in the 2-18 GHz frequency band;

[0042] Figure 21 It is the spectrogram of the microwave reflection loss of the CCOR composite material prepared in Examples 1-2 of the present invention varying with frequency at different matching thicknesses;

[0043] Figure 22 It is the spectrogram of the microwave reflection loss of the CCOR composite material prepared in Example 3 of the present invention varying with frequency at different matching thicknesses;

[0044] Figure 23 It is the spectrogram of the microwave reflection loss of the CCOR composite material prepared in Examples 4-5 of the present invention varying with frequency at different matching thicknesses;

[0045] Figure 24 It is the widest effective absorption bandwidth of the CCOR composite prepared in Examples 1-4 of the present invention.

[0046] The realization, functional features and advantages of the objectives of the present invention will be further described with reference to the embodiments and the accompanying drawings. Detailed implementation manners

[0047] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Those not specified in the embodiments shall be carried out according to the conventional conditions or the conditions recommended by the manufacturer. Those reagents or instruments not specified by the manufacturer can all be obtained as conventional products through commercial purchase.

[0048] In addition, the meaning of "and / or" appearing throughout the text includes three parallel solutions. Taking "A and / or B" as an example, it includes Solution A, or Solution B, or the solution where both A and B are satisfied simultaneously. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of those of ordinary skill in the art to implement. When the combination of technical solutions results in contradictions or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0049] The impedance mismatch of RGO and the skin effect caused by its high conductivity have hindered the further development of its practical application in the field of microwave absorption. Cobalt and its corresponding cobalt oxides have been widely studied and reported in the field of microwave absorption due to their unique physical and chemical properties. However, due to their high density, impedance mismatch, easy corrosion and narrow absorption bandwidth and other characteristics, they cannot be practically applied in the field of microwave absorption.

[0050] In view of this, the present invention provides a cobalt / cobalt oxide / reduced graphene oxide composite material. In one embodiment, the cobalt / cobalt oxide / reduced graphene oxide composite material comprises microspheres and a shell coated on the surface of the microspheres. Among them, the material of the microspheres comprises cobalt and cobalt oxide, and the material of the shell comprises cobalt, cobalt oxide and reduced graphene oxide.

[0051] Therefore, the cobalt / cobalt oxide / reduced graphene oxide composite material can be represented by Co / CoO@Co / CoO / RGO (CCOR) composite material. It should be noted that for the convenience of description, hereinafter Co / CoO@Co / CoO / RGO will be simply referred to as CCOR.

[0052] In the technical solution provided by the present invention, the cobalt / cobalt oxide / reduced graphene oxide composite material has a core-shell structure, and the microspheres made of cobalt and cobalt oxide are used as the core, and the shell made of cobalt, cobalt oxide and reduced graphene oxide. By compounding cobalt, cobalt oxide and RGO to form the core-shell structure, the impedance matching and attenuation ability can be effectively adjusted, and through the synergistic effect of dielectric loss and magnetic loss, a composite material with strong reflection loss and an ultra-wide bandwidth covering the entire X-band (8 - 12.5 GHz) is obtained, so that it can be used as a low-frequency microwave absorber and can be applied to radar stealth and other aspects.

[0053] Furthermore, in this embodiment, the reduced graphene oxide is in a sheet shape. Taking the sheet-shaped reduced graphene oxide as a carrier, the cobalt and cobalt oxide are compounded with the reduced graphene oxide to form the shell. This unique shell structure helps to form a 3D conductive network and can better optimize the impedance matching and attenuation ability. The CCOR composite material is specifically a core-shell structure in which the sheets compounded by Co / CoO / RGO are intertwined to form a shell, wrapping the microspheres assembled by Co / CoO nanoparticles.

[0054] In order to make the microwave absorption performance of the composite material better, in this embodiment, the particle size of the cobalt / cobalt oxide / reduced graphene oxide composite material is 5 - 10 μm.

[0055] Based on the above purpose, the present invention also provides a preparation method of the cobalt / cobalt oxide / reduced graphene oxide composite material as described above. In one embodiment, the preparation method comprises the following steps:

[0056] Step S10: Disperse graphene oxide in an organic solvent to obtain a graphene solution;

[0057] Specifically, add graphene oxide (GO) into the organic solvent and use ultrasonic dispersion for 0.5 - 1.5 h to make it evenly dispersed to obtain a graphene solution.

[0058] The present invention does not limit the specific type of the organic solvent, which can be conventional organic solvents such as acetone and tetrahydrofuran. In this embodiment, the organic solvent is N,N-dimethylformamide (DMF). In order to achieve a better dispersion effect of the graphene oxide, in this embodiment, the mass ratio of the graphene oxide to the volume of the organic solvent is 0.01~0.05 g: 35 mL.

[0059] Step S20: Add a cobalt salt precursor, 2-aminoterephthalic acid, and polyvinylpyrrolidone to the graphene solution, and stir until completely dissolved and evenly dispersed to obtain a mixed solution;

[0060] Among them, the cobalt salt precursor includes cobalt nitrate hexahydrate. The mass ratio of the graphene oxide, cobalt nitrate hexahydrate, 2-aminoterephthalic acid, and polyvinylpyrrolidone (PVP) is 0.01~0.05: 1.44~1.46: 0.90~0.92: 1.05~1.10. In order to save labor, in this embodiment, the stirring is magnetic stirring, and the stirring time is 1.5~2.5 h.

[0061] Step S30: React the mixed solution at 153~160 °C for 7.5~9 h, then cool it, perform solid-liquid separation, wash and dry the separated solid product, and then perform high-temperature treatment in a protective gas atmosphere to obtain a cobalt / cobalt oxide / reduced graphene oxide composite material.

[0062] Specifically in implementation, step S30 includes: Transfer the mixed solution to a hydrothermal autoclave, react it at 153~160 °C in a constant-temperature oven for 7.5~9 h. After the reaction is completed, naturally cool the hydrothermal autoclave to room temperature, then take out the polytetrafluoroethylene inner liner, and centrifuge to separate the solid product. The separated solid product is first centrifugally washed once with N,N-dimethylformamide solvent, and then centrifugally washed multiple times with absolute ethanol solvent. Then, the washed solid product is placed in a vacuum oven and vacuum dried overnight at 50~70 °C to obtain a purple precursor powder. Finally, the obtained precursor powder is placed in a tube furnace in a protective gas atmosphere and kept at 490~510 °C for 1~3 h, where the heating rate is 1~3 °C / min, to obtain a black cobalt / cobalt oxide / reduced graphene oxide composite material.

[0063] Preferably, react the mixed solution at 155 °C for 8 h, then cool it, perform solid-liquid separation, wash the separated solid product, vacuum dry it overnight at 60 °C, and then keep it at 500~502 °C for 2 h in a protective gas atmosphere, where the heating rate is 2 °C / min, to obtain a black cobalt / cobalt oxide / reduced graphene oxide composite material.

[0064] Among them, the protective gas includes a mixed gas of hydrogen and argon. Further, in the mixed gas, the volume of hydrogen is 5% of the total volume of the mixed gas. In this way, the protective gas provides a reducing atmosphere to reduce graphene oxide to reduced graphene oxide.

[0065] In this embodiment, the Co / CoO@Co / CoO / RGO composite material is prepared by a simple solvothermal method, and the core-shell microspheres are synthesized by a cooperative self-assembly strategy. Specifically, please refer to Figure 1 , first, graphene oxide (GO) is uniformly dispersed in N, N-dimethylformamide solvent (DMF) by ultrasonic treatment to obtain a graphene solution; secondly, a cobalt salt precursor, polyvinylpyrrolidone (PVP), and 2-aminoterephthalic acid are successively added to the graphene solution. The CCOR composite material with a core-shell structure is synthesized under the guidance of a cooperative self-assembly strategy. Among them, Co in cobalt nitrate hexahydrate 2+ plays two roles in the formation of the CCOR composite material. First, Co 2+ forms a conductive outer shell through electrostatic adsorption with GO sheets having abundant oxygen-containing groups (-OH, -COOH). In addition, Co 2+ ions participate in the self-assembly to form microspheres as the core. Finally, through the cooperative self-assembly effect, a CCOR composite material with a 3D multi-level core-shell structure is successfully obtained under the synergistic action of the reactants.

[0066] In the present invention, by designing the preparation steps and process parameters, the morphology, microstructure, and components of the CCOR composite material are controlled, and the magnetic Co / CoO and dielectric RGO sheets are well combined together to form magnetoelectric synergy, thereby obtaining a composite material with strong absorption and an ultra-wide effective absorption bandwidth.

[0067] Specifically, the CCOR composite material prepared in the present invention is a valuable microwave absorption material with excellent reflection loss ability and an ultra-wide bandwidth covering the entire X-band. Here, the microwave loss of the CCOR composite material mainly comes from dielectric loss and magnetic loss. The ingenious combination of the RGO dielectric component and the Co / CoO magnetic component and its unique 3D conductive network shell layer are all factors affecting the microwave absorption performance of the CCOR composite microspheres. In order to clearly understand the microwave absorption mechanism of the composite material, we analyzed the propagation and attenuation process of microwaves. Among them, the microwave absorption mechanism of the composite material includes the following aspects, such as Figure 2 shown:

[0068] (a) Polarization loss. Polarization loss can be divided into dipole polarization and interfacial polarization. For ε″ (the imaginary part of the complex permittivity), the relaxation peaks are located at ~8 GHz, ~12 GHz, and ~14−16 GHz, which are attributed to dipole polarization and interfacial polarization, respectively. Under the induction of an alternating electromagnetic field, a large number of residual functional groups and structural defects on RGO can cause a large amount of dipole polarization. Interfacial polarization mainly comes from the uneven distribution of charges at the heterojunction interface or grain boundaries. There are several interfaces in the CCOR composite: the interface between Co / CoO nanoparticles, and the interface between RGO and Co / CoO nanoparticles. Due to the different intrinsic conductivities of RGO and Co / CoO nanoparticles, when charges encounter Co / CoO nanoparticles during the transfer process, the charges will accumulate around the Co / CoO nanoparticles, which will induce rich interfacial polarization. To deeply clarify the interfacial polarization, we carried out off-axis electron holographic analysis, please refer to Figure 3 , in which, the electron holograms of the CCOR composite prepared in this invention are shown in Figure 3 a and 3d, Figure 3 b and 3e are the reconstructed diagrams of the holographic images, Figure 3 c and 3f are respectively Figure 3 the interfacial charge density distribution diagrams of the connected parts in Figure 3 b and 3e. As shown in Figure 3 c, 3f, the distribution of positive and negative charges around the Co / CoO nanoparticles and the interface between RGO and Co / CoO nanoparticles can be seen (the corresponding areas are indicated by black rectangles in Figure 3 b, 3e), showing an obvious interfacial polarization phenomenon. Therefore, this composite material with a unique 3D core-shell structure, with microspheres assembled by Co / CoO nanoparticles as the core and a conductive shell layer composed of RGO and Co / CoO nanoparticles as the shell, will generate rich interfacial polarization, greatly enhancing the dielectric loss ability. The phase boundary interface between RGO and Co / CoO nanoparticles can form a capacitor-like structure. Therefore, when high-density charges accumulate around these special interfaces, a large amount of dipole polarization will be caused. These polarizations will enhance the dielectric loss, making the CCOR composite material have excellent microwave loss ability.

[0069] (b) Conduction loss. ε' (the real part of the complex permittivity) is proportional to the conductivity based on the free electron theory. Due to the high conductivity of RGO, the conductivity of the CCOR composite increases with the increase of the GO content (please refer to Figure 17 and 18). The single-component RGO will cause the skin effect due to its high conductivity, resulting in poor microwave dissipation ability. Here, in order to obtain appropriate attenuation ability, Co / CoO nanoparticles are introduced to be compounded with RGO. At the same time, the impedance matching closely related to the MA characteristics is also appropriately optimized. In addition, the shell formed by the combination of RGO sheets and Co / CoO nanoparticles will form a conductive network structure, which is beneficial to enhancing the conduction loss and microwave attenuation ability.

[0070] (c) Magnetic loss. The CCOR composite also has magnetic properties and magnetic coupling effects, which will cause magnetic loss. In order to intuitively observe the inherent magnetic properties of the CCOR microspheres, off-axis electron holography is used to visually demonstrate the internal connection between magnetic resonance and magnetic coupling. Please refer to Figure 4 , Figure 4 which shows the stray field distribution in different regions of the CCOR microspheres. In Figure 4 a2, a high-density surrounding magnetic flux line originating from the magnetic Co / CoO nanoparticles can be clearly observed. The characteristic of magnetic materials is that the magnetic field lines can propagate to infinity, which far exceeds the range of the material itself. Therefore, the CCOR composite not only exhibits magnetic loss in the microsphere structure, but also expands the magnetic response to a certain extent, thus confirming its advantage as a microwave absorption material. As Figure 4 shown in b1-d2, the density of the magnetic field lines emitted by the shell composed of RGO sheets and Co / CoO nanoparticles ( Figure 4 b1, b2) and the clusters assembled by Co / CoO nanoparticles ( Figure 4 d1, d2) is higher than that of the Co / CoO nanoparticles ( Figure 4 a1, a2), which results from the magnetic coupling effect. Therefore, the obvious magnetic coupling effect can enhance the attenuation of electromagnetic energy, thus improving the magnetic loss and wave absorption performance. At the same time, as Figure 4 shown in c1, c2, due to the existence of the magnetic coupling effect, even if the clusters do not contact each other, the magnetic intensity between the clusters can still be enhanced. Therefore, the strong magnetic response ability and magnetic coupling effect of the core-shell structured CCOR composite are helpful to improve its magnetic loss ability and further improve the microwave attenuation ability.

[0071] In summary, the excellent wave absorption performance of the CCOR composite is closely related to its morphology, components, and micro-nano structure.

[0072] In addition, the present invention also provides a microwave absorber, which comprises the CCOR composite material as described above. Preferably, the CCOR composite material is prepared by the preparation method of the CCOR composite material as described above. The CCOR composite material has the characteristics of strong absorption and ultra-wide bandwidth. Therefore, when it is applied to the microwave absorber, the performance of the microwave absorber is better. It can be understood that the CCOR composite material can be directly used as a microwave absorber, or used as one of the components to make a microwave absorber together with other components.

[0073] The technical solutions of the present invention will be further described in detail below in conjunction with specific embodiments and the accompanying drawings. It should be understood that the following embodiments are only used to explain the present invention and are not used to limit the present invention.

[0074] Example 1

[0075] (1) Add 0.01 g of graphene oxide to 35 mL of DMF, and ultrasonically treat for 1 h to uniformly disperse the graphene oxide in DMF to obtain a graphene solution;

[0076] (2) Add 1.45 g of cobalt nitrate hexahydrate, 0.91 g of 2-aminoterephthalic acid, and 1.05 g of polyvinylpyrrolidone to the above graphene solution, and magnetically stir for 2 h until completely dissolved and uniformly dispersed to obtain a mixed solution;

[0077] (3) Transfer the above mixed solution to a 50 mL hydrothermal reactor, react at 155 °C in a constant temperature oven for 8 h. After the reaction, naturally cool the hydrothermal reactor to room temperature, then take out the polytetrafluoroethylene liner and centrifuge to separate the solid product. Wash the separated solid product once by centrifugation with DMF, and then wash it several times by centrifugation with anhydrous ethanol solvent. Then put the solid product into a vacuum oven and dry it overnight at 60 °C under vacuum to obtain a purple precursor powder. Finally, put the obtained precursor powder into a tubular furnace and keep it at 500 °C for 2 h in a mixed gas of 5% H2 / Ar, where the heating rate is 2 °C / min to obtain a black cobalt / cobalt oxide / reduced graphene oxide composite material, which is named CCOR-1 composite material, and its FESEM image is as Figure 5 shown.

[0078] Example 2

[0079] (1) Add 0.02 g of graphene oxide to 35 mL of DMF, and ultrasonically treat for 1 h to uniformly disperse the graphene oxide in DMF to obtain a graphene solution;

[0080] (2) Add 1.45 g of cobalt nitrate hexahydrate, 0.91 g of 2-aminoterephthalic acid, and 1.05 g of polyvinylpyrrolidone to the above graphene solution, and magnetically stir for 2 h until completely dissolved and uniformly dispersed to obtain a mixed solution;

[0081] (3) Transfer the above mixed solution to a 50 mL hydrothermal reactor, react at 155 °C in a constant temperature oven for 8 h. After the reaction, naturally cool the hydrothermal reactor to room temperature, then take out the PTFE liner and centrifuge to separate the solid product. Wash the separated solid product by centrifugation with DMF once, and then wash it by centrifugation with anhydrous ethanol solvent multiple times. Then place the solid product in a vacuum oven and dry it under vacuum at 60 °C overnight to obtain purple precursor powder. Finally, put the obtained precursor powder into a tube furnace and keep it at 500 °C for 2 h in a mixed gas of 5% H2 / Ar, where the heating rate is 2 °C / min to obtain black cobalt / cobalt oxide / reduced graphene oxide composite material, which is named CCOR-2 composite material, and its FESEM image is as Figure 6 shown.

[0082] Example 3

[0083] (1) Add 0.03 g of graphene oxide to 35 mL of DMF, and ultrasonicate for 1 h to uniformly disperse the graphene oxide in DMF to obtain a graphene solution;

[0084] (2) Add 1.45 g of cobalt nitrate hexahydrate, 0.91 g of 2-aminoterephthalic acid, and 1.05 g of polyvinylpyrrolidone to the above graphene solution, and stir magnetically for 2 h until completely dissolved and uniformly dispersed to obtain a mixed solution;

[0085] (3) Transfer the above mixed solution to a 50 mL hydrothermal reactor, react at 155 °C in a constant temperature oven for 8 h. After the reaction, naturally cool the hydrothermal reactor to room temperature, then take out the PTFE liner and centrifuge to separate the solid product. Wash the separated solid product by centrifugation with DMF once, and then wash it by centrifugation with anhydrous ethanol solvent multiple times. Then place the solid product in a vacuum oven and dry it under vacuum at 60 °C overnight to obtain purple precursor powder. Finally, put the obtained precursor powder into a tube furnace and keep it at 500 °C for 2 h in a mixed gas of 5% H2 / Ar, where the heating rate is 2 °C / min to obtain black cobalt / cobalt oxide / reduced graphene oxide composite material, which is named CCOR-3 composite material, and its FESEM image is as Figure 7 shown.

[0086] Example 4

[0087] (1) Add 0.04 g of graphene oxide to 35 mL of DMF, and ultrasonicate for 1 h to uniformly disperse the graphene oxide in DMF to obtain a graphene solution;

[0088] (2) Add 1.45 g of cobalt nitrate hexahydrate, 0.91 g of 2-aminoterephthalic acid, and 1.05 g of polyvinylpyrrolidone to the above graphene solution, and magnetically stir for 2 h until completely dissolved and evenly dispersed to obtain a mixed solution;

[0089] (3) Transfer the above mixed solution to a 50 mL hydrothermal reactor, react at 155 °C in a constant-temperature oven for 8 h. After the reaction, naturally cool the hydrothermal reactor to room temperature, then take out the polytetrafluoroethylene liner and centrifuge to separate the solid product. Wash the separated solid product once by centrifugation with DMF, then wash it multiple times by centrifugation with anhydrous ethanol solvent. Then place the solid product in a vacuum oven and dry it overnight at 60 °C under vacuum to obtain a purple precursor powder. Finally, put the obtained precursor powder into a tubular furnace and keep it at 500 °C for 2 h in a mixed gas of 5% H2 / Ar. Among them, the heating rate is 2 °C / min to obtain a black cobalt / cobalt oxide / reduced graphene oxide composite material, which is named CCOR-4 composite material. Its FESEM image is as Figure 8 shown.

[0090] Example 5

[0091] (1) Add 0.05 g of graphene oxide to 35 mL of DMF, and ultrasonically stir for 1 h to uniformly disperse the graphene oxide in DMF to obtain a graphene solution;

[0092] (2) Add 1.45 g of cobalt nitrate hexahydrate, 0.91 g of 2-aminoterephthalic acid, and 1.05 g of polyvinylpyrrolidone to the above graphene solution, and magnetically stir for 2 h until completely dissolved and evenly dispersed to obtain a mixed solution;

[0093] (3) Transfer the above mixed solution to a 50 mL hydrothermal reactor, react at 155 °C in a constant-temperature oven for 8 h. After the reaction, naturally cool the hydrothermal reactor to room temperature, then take out the polytetrafluoroethylene liner and centrifuge to separate the solid product. Wash the separated solid product once by centrifugation with DMF, then wash it multiple times by centrifugation with anhydrous ethanol solvent. Then place the solid product in a vacuum oven and dry it overnight at 60 °C under vacuum to obtain a purple precursor powder. Finally, put the obtained precursor powder into a tubular furnace and keep it at 500 °C for 2 h in a mixed gas of 5% H2 / Ar. Among them, the heating rate is 2 °C / min to obtain a black cobalt / cobalt oxide / reduced graphene oxide composite material, which is named CCOR-5 composite material. Its FESEM image is as Figure 9 shown.

[0094] Example 6

[0095] (1) Add 0.03 g of graphene oxide to 35 mL of DMF, and ultrasonically stir for 1 h to uniformly disperse the graphene oxide in DMF to obtain a graphene solution;

[0096] (2) Add 1.44 g of cobalt nitrate hexahydrate, 0.90 g of 2-aminoterephthalic acid, and 1.1 g of polyvinylpyrrolidone to the above graphene solution, and stir magnetically for 2.5 h until completely dissolved and evenly dispersed to obtain a mixed solution;

[0097] (3) Transfer the above mixed solution to a 50 mL hydrothermal reactor, react at 153 °C in a constant temperature oven for 9 h. After the reaction, let the hydrothermal reactor cool naturally to room temperature, then take out the polytetrafluoroethylene liner, centrifuge to separate the solid product. Wash the separated solid product by centrifugation with DMF once, and then wash it by centrifugation with anhydrous ethanol solvent multiple times. Then put the solid product into a vacuum oven and dry it under vacuum at 70 °C overnight to obtain a purple precursor powder. Finally, put the obtained precursor powder into a tubular furnace and keep it at 490 °C for 3 h in a mixed gas of 5% H2 / Ar, where the heating rate is 1 °C / min to obtain a black cobalt / cobalt oxide / reduced graphene oxide composite material.

[0098] Example 7

[0099] (1) Add 0.03 g of graphene oxide to 35 mL of DMF, and ultrasonicate for 1 h to evenly disperse the graphene oxide in DMF to obtain a graphene solution;

[0100] (2) Add 1.46 g of cobalt nitrate hexahydrate, 0.92 g of 2-aminoterephthalic acid, and 1.07 g of polyvinylpyrrolidone to the above graphene solution, and stir magnetically for 1.5 h until completely dissolved and evenly dispersed to obtain a mixed solution;

[0101] (3) Transfer the above mixed solution to a 50 mL hydrothermal reactor, react at 160 °C in a constant temperature oven for 7.5 h. After the reaction, let the hydrothermal reactor cool naturally to room temperature, then take out the polytetrafluoroethylene liner, centrifuge to separate the solid product. Wash the separated solid product by centrifugation with DMF once, and then wash it by centrifugation with anhydrous ethanol solvent multiple times. Then put the solid product into a vacuum oven and dry it under vacuum at 50 °C overnight to obtain a purple precursor powder. Finally, put the obtained precursor powder into a tubular furnace and keep it at 510 °C for 1 h in a mixed gas of 5% H2 / Ar, where the heating rate is 3 °C / min to obtain a black cobalt / cobalt oxide / reduced graphene oxide composite material.

[0102] Comparative Example 1

[0103] (1) Add 1.45 g of cobalt nitrate hexahydrate, 0.91 g of 2-aminoterephthalic acid, and 1.05 g of polyvinylpyrrolidone to DMF, and stir magnetically for 2 h until completely dissolved and evenly dispersed to obtain a homogeneous transparent solution;

[0104] (2) Transfer the above transparent solution into a 50 mL hydrothermal reactor, react at 155 °C in a constant temperature oven for 8 h. After the reaction, let the hydrothermal reactor cool naturally to room temperature, then take out the polytetrafluoroethylene liner and centrifuge to separate the solid product. Wash the separated solid product once by centrifugation with DMF, then wash it several times by centrifugation with anhydrous ethanol solvent. Then put the solid product into a vacuum oven and dry it under vacuum at 60 °C overnight to obtain purple precursor powder. Finally, put the obtained precursor powder into a tube furnace and keep it at 500 °C for 2 h in a mixed gas of 5% H2 / Ar, where the heating rate is 2 °C / min to obtain Co / CoO microspheres assembled by nanosheets (abbreviated as CCO), with a size of about 5 μm, and its FESEM image is as Figure 10 shown, Figure 10 Figures a and 10b are pictures at different resolutions.

[0105] Comparative Example 2

[0106] (1) Add 0.03 g of graphene oxide to 35 mL of DMF, and ultrasonicate for 1 h to uniformly disperse the graphene oxide in DMF to obtain a graphene solution;

[0107] (2) Add 1.45 g of cobalt nitrate hexahydrate, 0.91 g of 2-aminoterephthalic acid, and 1.05 g of polyvinylpyrrolidone to the above graphene solution, and stir magnetically for 2 h until completely dissolved and uniformly dispersed to obtain a mixed solution;

[0108] (3) Transfer the above mixed solution into a 50 mL hydrothermal reactor, react at 150 °C in a constant temperature oven for 6 h. After the reaction, let the hydrothermal reactor cool naturally to room temperature, then take out the polytetrafluoroethylene liner and centrifuge to separate the solid product. Wash the separated solid product once by centrifugation with DMF, then wash it several times by centrifugation with anhydrous ethanol solvent. Then put the solid product into a vacuum oven and dry it under vacuum at 60 °C overnight to obtain purple precursor powder. Finally, put the obtained precursor powder into a tube furnace and keep it at 500 °C for 2 h in a mixed gas of 5% H2 / Ar, where the heating rate is 2 °C / min to obtain a Co / CoO / RGO composite, and its FESEM image is as Figure 11 shown, and the Co / CoO / RGO composite has a layered structure.

[0109] 1. FESEM Characterization of CCOR Composite Materials

[0110] Figures 5 to 9 are the field emission scanning electron microscope (FESEM) images of the CCOR composite materials prepared in Examples 1 - 5. Among them, Figure 5 a1 and a2 in are CCOR-1 at different resolutions. It can be understood that, Figures 6 - 9The two figures are also the same product at different resolutions.

[0111] As can be seen from Figures 5 to 9 the surface of the microspheres is wrapped by a conductive outer shell composed of Co / CoO / RGO composite. As can be seen from Figures 5 to 7 when the input amounts of GO are 0.01 g, 0.02 g, and 0.03 g (i.e., the mass ratio of GO to cobalt nitrate hexahydrate is 0.01 - 0.03:1.45), the microspheres (cores) are mostly covered by the conductive outer shell composed of Co / CoO / RGO composite. As can be seen from Figure 8 and Figure 9 when the amount of GO is adjusted to 0.04 g and 0.05 g, the microspheres (cores) are completely covered by the conductive outer shell composed of Co / CoO / RGO composite. Such a difference proves that the amount of GO plays an important role in the construction of the 3D core - shell structure microspheres.

[0112] 2. TEM Characterization of CCOR Composite

[0113] To further observe the morphology and microstructure of the CCOR composite, the prepared CCOR composite was characterized by transmission electron microscopy (TEM). Figure 12 Figure 19 shows the FESEM image and TEM images of the CCOR - 3 composite prepared in Example 3 of the present invention. Among them, a is the FESEM image of the CCOR - 3 composite, b and c are the TEM images of the CCOR - 3 composite at different fields of view, and d - f are the high - resolution TEM (HRTEM) images of the CCOR - 3 composite at different fields of view.

[0114] As can be clearly observed from Figure 12 a, two - dimensional RGO thin distribution sheets are around the surface of the 3D microspheres. As can be clearly seen from Figure 12 b and Figure 12 c, RGO flakes covering the microspheres as the outer shell layer can be clearly seen. In addition, since RGO contains rich oxygen - containing groups, Co / CoO nanoparticles are anchored on the RGO sheets. This unique shell structure helps the formation of a 3D conductive network and can also better optimize the impedance matching and attenuation ability.

[0115] Figure 12 d - f show that the lattice spacing distribution of the nanoparticles (0.34 nm, 0.18 nm, 0.20 nm, 0.21 nm) can match the (200) plane of graphitized carbon, the (111) and (200) planes of metallic Co, and the (111) and (200) planes of CoO. And from Figure 12Significant lattice wrinkles of Co / CoO nanoparticles can often be observed from d-f. Such a special lattice structure may induce lattice defects and promote the occurrence of dipole / relaxation polarization. Therefore, Co / CoO nanoparticles with lattice defects are uniformly dispersed on RGO sheets and then co-assembled into 3D core-shell structure microspheres through synergy, which is very beneficial to enhancing the wave absorption performance.

[0116] 3. EDS Analysis of CCOR Composite Materials

[0117] Figure 13 This is the X-ray photoelectron spectroscopy (EDS) analysis diagram of the CCOR-3 composite material prepared in Example 3 of the present invention. Among them, Figure 13 g is the CCOR-3 composite material to be analyzed, Figure 13 h is the energy spectrum diagram of Co in the CCOR-3 composite material, Figure 13 i is the energy spectrum diagram of O in the CCOR-3 composite material, Figure 13 j is the energy spectrum diagram of C in the CCOR-3 composite material.

[0118] It can be seen from Figure 13 that the C, O, and Co elements in the CCOR-3 composite material are uniformly distributed.

[0119] 4. XRD Analysis

[0120] Figure 14 This is the XRD (X-ray diffraction) pattern of the CCOR composite materials prepared in Examples 1-5 of the present invention, the Co / CoO microspheres prepared in Comparative Example 1, and RGO.

[0121] It can be seen from Figure 14 that the XRD pattern of RGO has a sharp peak at 2 θ = 11.2°, which indicates that GO is converted into reduced graphene oxide after being treated in a reducing atmosphere and is marked as RGO, and this peak does not appear in the XRD spectrum of the CCOR composite material, which indicates that Co / CoO nanoparticles are uniformly distributed on RGO sheets; both the Co / CoO microspheres and the CCOR composite materials contain the phase components of metallic Co (JCPDS No. 15-0806) and CoO (JCPDS No. 48-1718). The signal peak of CoO is very weak, which may be formed by the inevitable surface oxidation of metallic Co.

[0122] 5. Microstructure Analysis

[0123] Figure 15 This is the Raman spectrum diagram of the CCOR composite materials prepared in Examples 1-5 of the present invention and the Co / CoO microspheres prepared in Comparative Example 1.

[0124] It can be seen from Figure 15It can be seen that the peaks in the range of 400 - 700 cm -1 correspond to the metal bond peaks of Co - O. The characteristic peaks of the RGO spectrum include the D peak at 1335 cm -1 and the G peak at 1590 cm -1 . The D peak is usually attributed to the lattice defects of carbon atoms, while the G peak is the in - plane stretching vibration of the sp 2 hybridization of carbon atoms. The defect concentration of carbon materials can be determined by the integral area ratio of the D peak and the G peak (I D / I G ). Affected by the microstructure of the CCOR composite material, I D / I G varies. A large number of defects will induce charge polarization, which is closely related to the wave - absorbing performance.

[0125] 6. Magnetic analysis

[0126] Figure 16 These are the magnetic property diagrams of the CCOR composite materials prepared in Examples 1 - 5 of the present invention and the Co / CoO microspheres prepared in Comparative Example 1 tested by VSM (Vibrating Sample Magnetometer). Among them, Figure 16 a and Figure 16 b are essentially the same, except that Figure 16 the graduation value of X in

[0127] b is smaller. Figure 16 It can be seen from

[0128] that the saturation magnetization intensities (Ms) of the Co / CoO microspheres, CCOR - 1~CCOR - 5 are 48 emu / g, 71 emu / g, 65 emu / g, 114 emu / g, 44 emu / g and 83 emu / g respectively. It can be seen from this that the amount of GO has an obvious influence on the magnetism of the samples, which is closely related to different microstructures and chemical compositions. Generally, Ms is closely related to magnetic loss. The coercivity (Hc) also has a great influence on magnetic loss. Hc is directly affected by the size of nanoparticles and microspheres. The Hc values of the Co / CoO microspheres, CCOR - 1~CCOR - 5 are 299 Oe, 399 Oe, 449 Oe, 445 Oe, 399 Oe and 399 Oe respectively. These magnetic parameters are greatly related to the magnetic loss and wave - absorbing performance of the materials.

[0129] The complex permittivity (ε r ) and the complex permeability (μ r ) are closely related to the performance of wave - absorbing materials, while ε r and μ rIt is affected by the morphology, chemical composition, and microstructure of the material. According to Debye's theory, the real parts (ε', μ') and imaginary parts (ε'', μ'') of the complex permittivity and complex permeability represent the storage and dissipation capabilities of incident electromagnetic energy, respectively. Figure 17 、 18 and 19 are the real part ε' diagram, imaginary part ε'' diagram, and dielectric loss tangent value tanδ of the complex permittivity of the CCOR composite materials prepared in Examples 1-5 of the present invention and the Co / CoO microspheres (CCO in the figure) prepared in Comparative Example 1 in the 2-18 GHz frequency band. ε Figure, Figure 20 is the electromagnetic parameter curve diagram of the CCOR composite materials prepared in Examples 1-5 of the present invention in the 2-18 GHz frequency band.

[0130] From Figure 17 and Figure 18 it can be seen that the ε' and ε'' values of the Co / CoO microspheres are very small and almost remain constant in the 2-18 GHz frequency range, indicating that its dielectric loss ability is very poor; as the GO input amount increases, the ε' and ε'' values of the CCOR composite materials become larger, that is, the dielectric properties of the CCOR composite materials are significantly optimized; the ε' and ε'' of all CCOR samples decrease with the increase of frequency, showing a typical dispersion effect.

[0131] Figure 20 In, the ε' value of the Co / CoO microspheres decreases from 3.683 to 3.626 ( Figure 20 a), the ε' value of CCOR-1 decreases from 5.22 to 3.52 ( Figure 20 b), CCOR-2 ( Figure 20 c) decreases from 7.26 to 4.20, CCOR-3 ( Figure 20 d) decreases from 7.90 to 4.59, CCOR-4 ( Figure 20 e) decreases from 11.39 to 6.52, and CCOR-5 decreases from 12.52 to 5.89 ( Figure 20 f). From this, it can be seen that the complex permittivity of the CCOR composite materials increases with the increase of RGO. And from Figure 17 and Figure 18 it can be seen that the complex permittivity of CCOR-5 fluctuates greatly with the increase of frequency, which means that its response to microwaves is more obvious. The complex permeability (μ', μ'') of the CCOR composite materials is as shown in Figure 20 a-f. Compared with ε' and ε'' in the corresponding test frequency range, the μ' and μ'' values of the composite materials are relatively low, and the μ' value changes irregularly with the increase of GO. However, it is highly consistent with the change trend of M s value ( Figure 16 a).

[0132] The microwave absorption performance is jointly affected by dielectric loss and magnetic loss. To intuitively compare dielectric loss and magnetic loss, we introduce the dielectric loss tangent (tanδ ε =ε'' / ε') ( Figure 19 a) and the magnetic loss tangent (tanδ μ =μ'' / μ') ( Figure 19 b) as the basis. The tanδ ε =ε'' / ε' value of CCOR-5 is larger than that of other samples, indicating that it has a strong dielectric loss ability. The tanδ μ =μ'' / μ' value of CCOR-2 is larger than that of other samples, meaning that it has a strong magnetic loss ability. From the perspective of tanδ ε =ε'' / ε', the improvement of dielectric loss ability is closely related to the amount of GO, while the change trend of tanδ μ =μ'' / μ' is irregular. The effects of tanδ ε and tanδ μ on the microwave absorption performance of CCOR composites were comprehensively analyzed, and the conclusion was that due to the synergistic effect of dielectric loss and magnetic loss, CCOR-3 exhibited the best microwave absorption performance.

[0133] 8. Microwave Absorption Performance Analysis

[0134] Based on the electromagnetic wave transmission line theory, the microwave absorption performance of the 3D CCOR core-shell structure composite was calculated. This theory highly depends on εr and μr, and the corresponding reflection loss (RL) formula is shown as follows:

[0135]

[0136] where Z in , Z0, f, c, and d are the input impedance of the microwave absorption material, the impedance of free space, the test frequency, the speed of light, and the thickness of the microwave absorption material, respectively.

[0137] Figures 21 - 23 This is the spectrogram of the microwave reflection loss varying with frequency of the CCOR composites prepared in Examples 1-5 of the present invention at different matching thicknesses. Among them, Figure 21 a represents the spectrogram of CCOR-1, Figure 21 b represents the spectrogram of CCOR-2, Figure 22 represents the spectrogram of CCOR-3, Figure 23 a represents the spectrogram of CCOR-4, Figure 23 b represents the spectrogram of CCOR-5.

[0138] Figures 21 - 23shows the RL of the CCOR composite microspheres and their corresponding RL max are as follows: For CCOR-1 at 8.64 GHz, the RL max is -18.7 dB; for CCOR-2 at 9.36 GHz, the RL max is -30.4 dB; for CCOR-3 at 5.6 GHz, the RL max is -50.1 dB; for CCOR-4 at 4.42 GHz, the RL max is -41.7 dB; for CCOR-5 at 15.36 GHz, the RL max is -25.2 dB. That is, as the input amount of GO gradually increases from 0.01 g to 0.02 g and 0.03 g, the RL of the CCOR composite material max can change from -18.7 dB to -30.4 dB and -50.1 dB. Then, when the GO amount is gradually adjusted from 0.03 g to 0.04 g and 0.05 g, the RL of CCOR max changes from -50.1 dB to -41.7 dB and -25.2 dB. Therefore, it can be confirmed that the input amount of GO has an important influence on the wave absorption performance, which is attributed to the influence of GO on the morphology, micro-nano structure and its inherent properties of the composite material.

[0139] In addition, the bandwidth with an RL value less than -10 dB is usually defined as the effective absorption bandwidth, which is also an important factor for evaluating the wave absorption performance. It can be Figures 21 - 24 seen that the optimal effective absorption bandwidths of the CCOR-1~CCOR-5 composite materials are as follows: for CCOR-1 it is 4.62 GHz (8.64 - 13.28 GHz), for CCOR-2 it is 6.56 GHz (7.52 - 14.08 GHz), for CCOR-3 it is 6.4 GHz (8.16 - 14.56 GHz), and for CCOR-4 it is 3.68 GHz (9.24 - 12.92 GHz). That is, it can be seen that the CCOR composite material prepared in the embodiments of the present invention has excellent wave absorption performance in the X-band (8 - 12.5 GHz) range.

[0140] It should be noted that since the core-shell structured CCOR composite materials can be successfully prepared in Embodiments 1 - 7 of the present invention and the principles are similar, the test results of Embodiments 6 and 7 are similar to those of Embodiments 1 - 5.

[0141] In summary, through the design of the preparation steps and process parameters, the present invention controls the morphology, microstructure, and components of the CCOR composite material, combines the magnetic Co / CoO and dielectric RGO sheets well together to form magnetoelectric synergy, thereby obtaining a composite material with strong reflection loss and an ultra-wide bandwidth covering the entire X-band, enabling it to be used as a low-frequency microwave absorber and applicable to radar stealth and other aspects.

[0142] The above are only the preferred embodiments of the present invention, and thus do not limit the patent scope of the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the patent protection scope of the present invention.

Claims

1. A cobalt / cobalt oxide / reduced graphene oxide composite material, characterized in that, The cobalt / cobalt oxide / reduced graphene oxide composite material comprises microspheres and a shell coated on the surface of the microspheres. Among them, the material of the microspheres comprises cobalt and cobalt oxide, and the material of the shell comprises cobalt, cobalt oxide and reduced graphene oxide; The reduced graphene oxide is in a sheet shape. Taking the sheet-shaped reduced graphene oxide as a carrier, the cobalt and cobalt oxide are loaded on the reduced graphene oxide to form the shell.

2. The cobalt / cobalt oxide / reduced graphene oxide composite material according to claim 1, wherein The particle size of the cobalt / cobalt oxide / reduced graphene oxide composite material is 5-10 μm.

3. A method for preparing a cobalt / cobalt oxide / reduced graphene oxide composite material according to any one of claims 1 to 2, characterized in that, It includes the following steps: Disperse graphene oxide in an organic solvent to obtain a graphene solution; Add a cobalt salt precursor, 2-aminoterephthalic acid and polyvinylpyrrolidone into the graphene solution, and stir until completely dissolved and uniformly dispersed to obtain a mixed solution; React the mixed solution at 153-160 °C for 7.5-9 h, then cool and perform solid-liquid separation. Wash and dry the separated solid product, and then perform high-temperature treatment in a protective gas atmosphere to obtain the cobalt / cobalt oxide / reduced graphene oxide composite material.

4. The preparation method of the cobalt / cobalt oxide / reduced graphene oxide composite material according to claim 3, characterized in that, The organic solvent includes N,N-dimethylformamide; and / or, the cobalt salt precursor includes cobalt nitrate hexahydrate.

5. The preparation method of the cobalt / cobalt oxide / reduced graphene oxide composite material according to claim 3, characterized in that, The mass ratio of the graphene oxide to the volume of the organic solvent is 0.01-0.05 g:35 mL.

6. The preparation method of the cobalt / cobalt oxide / reduced graphene oxide composite material according to claim 3, characterized in that, The mass ratio of the graphene oxide, cobalt nitrate hexahydrate, 2-aminoterephthalic acid and polyvinylpyrrolidone is 0.01-0.05:1.44-1.46:0.90-0.92:1.05-1.

10.

7. The preparation method of the cobalt / cobalt oxide / reduced graphene oxide composite material according to claim 3, wherein, The temperature of the high-temperature treatment is 490-510 °C, and the time is 1-3 h.

8. The preparation method of the cobalt / cobalt oxide / reduced graphene oxide composite material according to claim 3, characterized in that, The protective gas includes a mixed gas of hydrogen and argon.

9. A microwave absorber, characterized in that, It includes the cobalt / cobalt oxide / reduced graphene oxide composite material according to any one of claims 1 to 2.

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