High-efficiency electromagnetic wave absorbing material, preparation method and application thereof

Abstract

The application belongs to the technical field of electromagnetic wave absorption, and discloses a kind of high-efficiency electromagnetic wave absorption material and its preparation method and application.The steps of the method are as follows:(1) Fe3O4 nanoparticles are synthesized using a solvothermal method;(2) thiolacetic acid / ethanol solution is mixed with the Fe3O4 nanoparticles, and the mixture is oscillated to modify the surface of the Fe3O4 nanoparticles with thiolacetic acid molecules (Fe3O4@MAA);(3) after mixing zinc acetate / aqueous solution with Fe3O4@MAA, uniform benzenetricarboxylic acid / ethanol solution is slowly added to the mixed solution to construct Zn-MOF on the surface of Fe3O4;(4) Fe3O4@MOF is pyrolyzed at high temperature to prepare multi-core-single-shell Fe3O4@Fe@C.The obtained multi-core-single-shell Fe3O4@Fe@C has super-strong electromagnetic wave absorption performance.

Description

Technical Field

[0001] This invention belongs to the technical field of electromagnetic wave absorption, specifically relating to a high-efficiency electromagnetic wave absorbing material, its preparation method, and its application. Background Technology

[0002] Due to advancements in electronic technology, excessive artificial electromagnetic radiation is harmful to human health and may interfere with the operation of military equipment. Therefore, the development of high-performance microwave absorbing materials is of great significance. As typical traditional microwave absorbing materials, ferromagnetic materials exhibit excellent electromagnetic wave absorption strength, including magnetic oxides, magnetic metals, and their related alloys. However, the widespread application of ferromagnetic materials is limited by their high density, susceptibility to corrosion, and narrow absorption bandwidth.

[0003] Carbon materials, as excellent dielectric loss materials, are promising candidates for microwave absorbers due to their low density, high chemical stability, and good electrical conductivity. Notably, metal-organic frameworks (MOFs) offer diverse metal-organic ligand combinations and abundant synthetic pathways, and their morphology, large specific surface area, unique pore structure, and pore size can be designed according to specific needs. Therefore, MOFs hold great promise as precursors for constructing porous carbon-based materials. Despite the significant structural advantages exhibited by MOFs, improvements and optimizations in constructing multiple heterointerfaces and achieving magneto-dielectric synergy still face limitations.

[0004] It is worth noting that core-shell structures are commonly used composite structures for fabricating high-performance electromagnetic wave absorbers because they possess various electromagnetic wave dissipation mechanisms, large interfaces, strong magnetic-dielectric synergy, and multiple electromagnetic wave propagation pathways. Therefore, combining the structural advantages of MOFs with the advantages of core-shell materials is a promising direction for further improving the absorption performance of electromagnetic wave absorbers. Summary of the Invention

[0005] To address the shortcomings and deficiencies of existing technologies, the primary objective of this invention is to provide a highly efficient electromagnetic wave absorbing material.

[0006] Another objective of this invention is to provide a method for preparing the aforementioned high-efficiency electromagnetic wave absorbing material. This invention constructs MOF-derived porous carbon on the surface of Fe3O4 using a solvothermal method and an in-situ pyrolysis method. The porous carbon can reduce the surface of Fe3O4 to Fe, forming a unique Fe3O4 / Fe / C multi-heterogeneous interface, thereby significantly enhancing interfacial polarization and improving the electromagnetic wave absorption performance of the composite material. Simultaneously, the Zn-MOF-derived porous carbon provides a large specific surface area, multiple electromagnetic wave reflection sites, and excellent dielectric loss. The multi-core / single-shell Fe3O4@Fe@C composite material, possessing the advantages of both core-shell structure and MOF-derived carbon materials, exhibits superior electromagnetic wave absorption performance.

[0007] Another object of the present invention is to provide applications of the above-mentioned high-efficiency electromagnetic wave absorbing material.

[0008] The objective of this invention is achieved through the following technical solution:

[0009] A method for preparing a highly efficient electromagnetic wave absorber includes the following steps:

[0010] (1) Fe3O4 nanoparticles were synthesized by solvothermal method using ferric chloride hexahydrate (FeCl3·6H2O) and sodium acetate (NaAc) as raw materials;

[0011] (2) Surface modification of Fe3O4 nanoparticles with thioglycolic acid molecules

[0012] Fe3O4 nanoparticles were added to an ethanol solution containing mercaptoacetic acid (MAA), and the reaction was shaken. After the reaction was completed, Fe3O4@MAA was obtained.

[0013] (3) Construction of Zn-MOF on Fe3O4 surface

[0014] Fe3O4@MAA was ultrasonically dispersed in water, and then zinc acetate (ZnAc) was added and stirred to obtain solution A; trimesic acid (H3BTC) was dissolved in anhydrous ethanol to obtain solution B; then, under vigorous stirring, solution B was slowly injected into solution A over 0.5-4 hours, and the collected brown powder was Fe3O4@Zn-MOF.

[0015] (4) Preparation of polynuclear-single-shell Fe3O4@Fe@C

[0016] Polynuclear-monoshell Fe3O4@Fe@C was prepared by pyrolyzing Fe3O4@Zn-MOF at 300-1100℃ for 0.5-4 hours under nitrogen or inert atmosphere.

[0017] The specific steps of step (1) are as follows: under stirring, ferric chloride hexahydrate (FeCl3·6H2O) and sodium acetate (NaAc) are dissolved in ethylene glycol respectively. Then, the two solutions are stirred and mixed. The resulting light brown solution is transferred to a stainless steel autoclave with a polytetrafluoroethylene liner, sealed, and heated at 200°C for 20 hours. The resulting black magnetite particles are washed and dried to obtain Fe3O4 nanoparticles.

[0018] The mass fractions of ferric chloride hexahydrate and sodium acetate are 5.4 parts and 8 parts, respectively; the washing refers to washing several times with distilled water and anhydrous ethanol, respectively; the drying refers to vacuum drying at 50°C for 6 hours.

[0019] In step (2), the ratio of Fe3O4 nanoparticles to mercaptoacetic acid is 0.1-0.3g:0.2mmol-1.5mmol, preferably 0.2g:1.0mmol; the shaking reaction time is 16-32 hours, preferably 24 hours, and the reaction temperature is 10-40℃.

[0020] The ratio of Fe3O4@MAA, ZnAc and H3BTC used in step (3) is (0.1g~0.3g):(0.5mmol~6mmol):(0.5mmol~6mmol). In a more preferred scheme, the amounts of ZnAc and H3BTC are the same.

[0021] In step (3), solution B is preferably slowly injected into solution A within 2 hours; the collected brown powder is vacuum dried at 30-60°C (preferably 45°C); the stirring speed in step (3) is 300 rpm to 400 rpm.

[0022] In step (4), the temperature is heated to the pyrolysis temperature at a rate of 2-10°C / min, preferably 5°C / min.

[0023] In step (4), the pyrolysis temperature is preferably 500-800℃, and most preferably 700℃; the pyrolysis time is preferably 2h.

[0024] The above method yields a multi-core-single-shell Fe3O4@Fe@C composite material, which is a columnar cucumber-like micron-scale composite material. A large number of Fe3O4 nanoparticles are embedded inside rod-shaped Zn-MOF to form a multi-core-shell precursor material structure. The precursor material surface has a rough and porous microstructure after pyrolysis. The Fe3O4@Fe@C formed after pyrolysis has abundant heterogeneous interfaces, such as Fe3O4\Fe and Fe\C.

[0025] The multi-core-single-shell Fe3O4@Fe@C composite material described in this invention can achieve efficient electromagnetic wave absorption.

[0026] This invention synthesizes a cucumber-shaped multi-core / monoshell Fe3O4@Fe@C composite material with superior electromagnetic wave absorption capabilities via solvothermal method and in-situ pyrolysis. By introducing Fe3O4 into columnar Zn-MOF, the original shape and structure of Zn-MOF can be maintained while forming a multi-core / monoshell structure, and the advantages of core-shell structure and MOF can be fully combined. The porous carbon derived from Zn-MOF provides a large specific surface area, multiple electromagnetic wave reflection sites, and excellent dielectric loss. The Fe3O4 / Fe and Fe / C interfaces brought about by the core-shell structure provide more non-uniform material interfaces, enhancing interfacial polarization loss. Combined with the magnetic loss of Fe3O4@Fe, the multi-core / monoshell MOF-derived composite material achieves excellent impedance matching. Therefore, the multi-core / monoshell Fe3O4@Fe@C composite material exhibits superior electromagnetic wave absorption performance, with a minimum reflection loss of -59.1 dB at a frequency of 13.36 GHz and an effective electromagnetic wave absorption bandwidth of 5.36 GHz. This invention innovatively designs an electromagnetic wave absorbing material that combines the advantages of core-shell and MOF structures, and the process is stable, exhibiting strong electromagnetic wave absorption performance.

[0027] Compared with existing electromagnetic wave absorbing materials, the present invention has the following advantages and beneficial effects:

[0028] (1) It has a lower density and when used as a coating, it has considerable microwave absorption performance and a matching thickness as low as 1.85 mm.

[0029] (2) The electromagnetic wave absorption intensity is extremely strong, with a minimum reflection loss value of -59.1dB and an effective absorption bandwidth of 5.36GHz, covering almost the entire Ku band.

[0030] (3) The graphitized MOF has higher chemical stability, which can protect the magnetic nanoparticles contained inside from oxidation. Attached Figure Description

[0031] Figure 1 This is a schematic diagram illustrating the preparation of the multi-core-single-shell Fe3O4@Fe@C described in this application.

[0032] Figure 2 High-resolution scanning electron images of Fe3O4 nanoparticles (left) and multi-core-shell Fe3O4@Zn-MOF (right) prepared in Example 1.

[0033] Figure 3The images shown are high-resolution scanning electron images of Fe3O4@Zn-MOF prepared in Examples 1-4 after pyrolysis at different temperatures. a) is Fe3O4@Fe@C obtained after pyrolysis at 500℃; b) is Fe3O4@Fe@C obtained after pyrolysis at 600℃; c) is Fe3O4@Fe@C obtained after pyrolysis at 700℃; and d) is Fe3O4@Fe@C obtained after pyrolysis at 800℃.

[0034] Figure 4 The images show the XRD patterns of Fe3O4@Fe@CX obtained after pyrolysis at different temperatures in Examples 1-4, where X represents the pyrolysis temperature and a is the standard XRD diffraction pattern of Fe, Fe3O4, and ZnO.

[0035] Figure 5 The images show the Raman spectra of Fe3O4@Fe@CX obtained after pyrolysis at different temperatures in Examples 1-4, where X represents the pyrolysis temperature.

[0036] Figure 6 The above are the XPS total spectra of Fe3O4@Fe@CX obtained after pyrolysis at different temperatures in Examples 1-4, where X represents the pyrolysis temperature.

[0037] Figure 7 The figures show the electromagnetic parameters of Fe3O4@Fe@CX obtained after pyrolysis at different temperatures in Examples 1-4, where X represents the pyrolysis temperature. In the figures, a is a graph showing the complex permittivity as a function of frequency; b is a graph showing the real part of the complex permeability as a function of frequency.

[0038] Figure 8 The graphs show the reflection loss values ​​of Fe3O4@Fe@CX obtained after pyrolysis at different temperatures in Examples 1-4, as a function of frequency and thickness. Specifically, a represents the reflection loss value of Fe3O4@Fe@C obtained after pyrolysis at 500℃; b represents the reflection loss value of Fe3O4@Fe@C obtained after pyrolysis at 600℃; c represents the reflection loss value of Fe3O4@Fe@C obtained after pyrolysis at 700℃; and d represents the reflection loss value of Fe3O4@Fe@C obtained after pyrolysis at 800℃.

[0039] Figure 9 The electromagnetic parameters of Fe3O4@Fe@CY obtained by pyrolysis at 700°C using different amounts of ZnAc and H3BTC in Examples 3 and 5-7 are shown, where Y represents the amount of ZnAc and H3BTC used (mmol). In the graphs, a is the curve of complex permittivity versus frequency; b is the curve of complex permeability versus frequency.

[0040] Figure 10The graphs show the reflection loss values ​​of Fe3O4@Fe@C obtained after pyrolysis at 700°C using different amounts of ZnAc and H3BTC in Examples 3 and 5-7, as a function of frequency and thickness. In the graphs, a represents the reflection loss value of Fe3O4; b represents the reflection loss value of Fe3O4@Fe@C obtained with 1 mmol of ZnAc and H3BTC; c represents the reflection loss value of Fe3O4@Fe@C obtained with 2 mmol of ZnAc and H3BTC; d represents the reflection loss value of Fe3O4@Fe@C obtained with 3 mmol of ZnAc and H3BTC; and e represents the reflection loss value of Fe3O4@Fe@C obtained with 4 mmol of ZnAc and H3BTC. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to embodiments and accompanying drawings, but the embodiments of the present invention are not limited thereto. All raw materials involved in the present invention can be purchased directly from the market. For process parameters not specifically specified, conventional techniques can be referred to.

[0042] The following are some of the performance testing methods for the multi-core-single-shell Fe3O4@Fe@C prepared in this invention. Other testing methods not specifically mentioned shall be performed in accordance with conventional methods in the art:

[0043] The obtained multi-core-monoshell Fe3O4@Fe@C was mixed with paraffin at a 1:1 mass ratio and pressed into a ring with an outer diameter of 7 mm, an inner diameter of 3 mm, and a thickness of 2 mm for electromagnetic parameter testing. The complex permittivity and complex permeability of the ring in the 2-18 GHz frequency range were measured using a microwave vector network analyzer (AV3629D, China 41st Research Institute). Transmission line theory was applied to calculate the obtained complex permittivity and complex permeability, yielding curves showing the reflection loss of the sample as a function of frequency and thickness. Figure 8 and Figure 10 ).

[0044] Example 1

[0045] (1) Under magnetic stirring, 5.4 g FeCl3·6H2O and 8.0 g NaAc were dissolved in 140 mL and 60 mL of ethylene glycol, respectively. The solutions were then mixed in a 250 mL beaker and stirred for 0.5 hours. The resulting light brown solution was transferred to a stainless steel autoclave lined with polytetrafluoroethylene, sealed, and heated at 200 °C for 20 hours. The resulting black magnetite particles were washed three times with distilled water and anhydrous ethanol, and then vacuum dried at 50 °C for 6 hours. The dried black powder was collected using a magnet, which was the synthesized Fe3O4 nanoparticles.

[0046] (2) To graft MAA onto the surface of Fe3O4, 0.2 g of Fe3O4 nanoparticles were added to 40 mL of ethanol solution containing MAA (1.0 mmol) and shaken for 24 hours. Fe3O4@MAA was separated by a magnet and washed three times with ethanol and water to obtain Fe3O4 nanoparticles with surface-modified mercaptoacetic acid molecules.

[0047] (3) 0.2 g Fe3O4@MAA was dispersed in 40 ml of distilled water by ultrasonication. Then, ZnAc (2.0 mmol) was added to the above solution and stirred for 10 minutes to obtain solution A. Solution B was obtained by dissolving H3BTC (2.0 mmol) in 20 ml of anhydrous ethanol. Then, over 2 hours, solution B was slowly injected into solution A through a constant pressure dropping funnel under vigorous stirring. The brown powder was collected by magnet and vacuum dried at 45 °C to obtain Fe3O4@Zn-MOF.

[0048] (4) Finally, Fe3O4@Zn-MOF was heated at a rate of 5℃ / min and pyrolyzed at 500℃ for 2 hours under a nitrogen atmosphere to obtain multinucleated-monoshell Fe3O4@Fe@C, denoted as Fe3O4@Fe@C-500.

[0049] Example 2

[0050] Multinucleated-single-shell Fe3O4@Fe@C was prepared according to the steps of Example 1. The difference from Example 1 is that the pyrolysis temperature in step (4) is 600℃. The final material is denoted as Fe3O4@Fe@C-600.

[0051] Example 3

[0052] Multinucleated-single-shell Fe3O4@Fe@C was prepared according to the steps of Example 1. The difference from Example 1 is that the pyrolysis temperature in step (4) is 700℃. The final material is recorded as Fe3O4@Fe@C-700 or Fe3O4@Fe@C-2.

[0053] Example 4

[0054] Multinucleated-single-shell Fe3O4@Fe@C was prepared according to the steps of Example 1. The difference from Example 1 is that the pyrolysis temperature of step (4) is 800℃. The final material is denoted as Fe3O4@Fe@C-800.

[0055] A schematic diagram of the preparation of multi-core-single-shell Fe3O4@Fe@C according to the present invention is shown below. Figure 1 As shown. Figure 2 Scanning electron microscope images of Fe3O4 nanoparticles and Fe3O4@Zn-MOF prepared in Example 1. Figure 2As shown, the obtained Fe3O4@MOF structure exhibits a unique cucumber-like morphology with a diameter of approximately 8 μm. Figure 2 (b)

[0056] Figure 3 Multinuclear-monoshell Fe3O4@Fe@C with a porous structure was obtained by direct pyrolysis of Fe3O4@MOF at different temperatures in flowing nitrogen. The pyrolyzed samples well inherited the morphology of Fe3O4-MOF, and due to the loss of organic components in Fe3O4 / MOF, Fe3O4@Fe@C exhibited a significantly rough and porous surface. Figure 4 The XRD pattern showed that the ZnO crystal peaks disappeared when the pyrolysis temperature reached 800℃, which is related to the reduction of ZnO to Zn by carbon and the further sublimation of Zn. Furthermore, in Figure 6 The disappearance of Zn elements can also be observed in the XPS overall spectrum.

[0057] Figure 5 These are Raman images of multinucleated-monoshell Fe3O4@Fe@C at different temperatures. The two peaks in the images are the defect peak and characteristic peak of graphite, respectively. As the temperature increases, the intensity of the defect peak of graphite increases, indicating that the degree of graphitization of the material increases.

[0058] Figure 7 The electromagnetic parameters of multinucleated-monoshell Fe3O4@Fe@C after pyrolysis at different temperatures are given. Figure 7 The highest imaginary part of the complex permittivity in Fe3O4@Fe@C-700 reflects the material's outstanding dielectric loss performance; however, in comparison... Figure 7 The lower imaginary part of the permeability in b indicates that dielectric loss is an important electromagnetic wave loss mechanism of Fe3O4@Fe@C.

[0059] Figure 8 The electromagnetic wave loss performance of the material at different pyrolysis temperatures was demonstrated. The material exhibited the best absorption performance at a pyrolysis temperature of 700℃, with a minimum reflection loss value of -59.1dB.

[0060] In the materials prepared in Examples 1-4, the electromagnetic wave absorption performance of the materials first increases and then decreases with the increase of pyrolysis temperature, and the specific relationship is shown in Table 1.

[0061] Table 1: Relationship between electromagnetic wave absorption performance and pyrolysis temperature

[0062] Pyrolysis temperature / °C 500 600 700 800 Maximum reflection loss value / dB -7.7 -38.6 -59.1 -29.3 Maximum effective absorption bandwidth / GHz - 4.24 4.72 3.76

[0063] Example 5

[0064] (1) Under magnetic stirring, 5.4 g FeCl3·6H2O and 8.0 g NaAc were dissolved in 140 mL and 60 mL of ethylene glycol, respectively. The solutions were then mixed in a 250 mL beaker and stirred for 0.5 hours. The resulting light brown solution was transferred to a PTFE-lined stainless steel autoclave, sealed, and heated at 200 °C for 20 hours. The resulting black magnetite particles were washed three times with distilled water and anhydrous ethanol, and then vacuum dried at 50 °C for 6 hours. The dried black Fe3O4 nanoparticles were collected.

[0065] (2) To graft MAA onto the surface of Fe3O4, 0.2 g of Fe3O4 nanoparticles were added to 40 mL of ethanol solution containing MAA (1.0 mmol) and shaken for 24 hours. Fe3O4@MAA was separated by a magnet and washed three times with ethanol and water to obtain Fe3O4 nanoparticles with surface-modified mercaptoacetic acid molecules.

[0066] (3) 0.2 g Fe3O4@MAA was dispersed in 40 ml of distilled water by ultrasonication. Then, ZnAc (1.0 mmol) was added to the above solution and stirred for 10 minutes to obtain solution A. Solution B was obtained by dissolving H3BTC (1.0 mmol) in 20 ml of anhydrous ethanol. Then, over 2 hours, solution B was slowly injected into solution A through a constant pressure dropping funnel under vigorous stirring. The brown powder was collected by magnet and vacuum dried at 45 °C to obtain Fe3O4@Zn-MOF.

[0067] (4) Finally, Fe3O4@Zn-MOF was heated at a rate of 5℃ / min and pyrolyzed at 700℃ for 2 hours under a nitrogen atmosphere to obtain multinucleated-monoshell Fe3O4@Fe@C, denoted as Fe3O4@Fe@C-1.

[0068] Example 6

[0069] Multinucleated-single-shell Fe3O4@Fe@C was prepared according to the steps of Example 5. The difference from Example 5 is that the amount of ZnAc and H3BTC added in step (3) is 3 mmol. The final material is denoted as Fe3O4@Fe@C-3.

[0070] Example 7

[0071] Multinucleated-single-shell Fe3O4@Fe@C was prepared according to the steps of Example 5. The difference from Example 5 is that the amount of ZnAc and H3BTC added in step (3) is 4 mmol. The final material is denoted as Fe3O4@Fe@C-4.

[0072] The composite materials prepared in Examples 3 and 5-7 have similar structures to those in Example 1, and the properties obtained are shown in Table 2. Figure 9 The electromagnetic parameters are those of the materials prepared in Examples 3 and 5-7. Figure 10 The graph shows the distribution of reflection loss values ​​of the materials prepared in Examples 3 and 5-7 as a function of frequency and thickness.

[0073] Table 2: Relationship between electromagnetic wave absorption performance and pyrolysis temperature

[0074]

[0075] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a highly efficient electromagnetic wave absorber, characterized in that, Includes the following steps: (1) Fe3O4 nanoparticles were synthesized by solvothermal method using ferric chloride hexahydrate and sodium acetate as raw materials; (2) Add Fe3O4 nanoparticles to an ethanol solution containing mercaptoacetic acid and shake to react; (3) Disperse the product obtained from the oscillation reaction in step (2) in water by ultrasonication, then add zinc acetate and stir to obtain solution A; dissolve pyromellitic acid in anhydrous ethanol to obtain solution B; then, under stirring, inject solution B into solution A over 0.5-4 hours, and the collected brown powder is Fe3O4@Zn-MOF; (4) Pyrolyze Fe3O4@Zn-MOF at 300-1100℃ for 0.5-4 hours under nitrogen or inert atmosphere to prepare multinucleated-monoshell Fe3O4@Fe@C.

2. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 1, characterized in that, The specific steps of step (1) are as follows: under stirring, ferric chloride hexahydrate and sodium acetate are dissolved in ethylene glycol respectively. Then, the two solutions are stirred and mixed. The resulting light brown solution is transferred to a stainless steel autoclave with a polytetrafluoroethylene liner, sealed, and heated at 200°C for 20 hours. The resulting black magnetite particles are washed and dried to obtain Fe3O4 nanoparticles.

3. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 2, characterized in that, The mass fractions of ferric chloride hexahydrate and sodium acetate are 5.4 parts and 8 parts, respectively; the washing refers to washing several times with distilled water and anhydrous ethanol, respectively; the drying refers to vacuum drying at 50°C for 6 hours.

4. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 1, characterized in that, In step (2), the ratio of Fe3O4 nanoparticles to mercaptoacetic acid is 0.1~0.3 g: 0.2 mmol~1.5 mmol; the shaking reaction time is 16-32 hours.

5. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 4, characterized in that, The ratio of Fe3O4 nanoparticles to mercaptoacetic acid was 0.2 g: 1.0 mmol; the shaking reaction time was 24 hours.

6. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 1, characterized in that, In step (3), the ratio of the product obtained from the oscillating reaction in step (2), ZnAc and H3BTC is (0.1 g ~ 0.3 g): (0.5 mmol ~ 6 mmol): (0.5 mmol ~ 6 mmol).

7. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 1, characterized in that, In step (3), solution B is slowly injected into solution A within 2 hours; the collected brown powder is vacuum dried at 30-60°C.

8. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 1, characterized in that, In step (4), the temperature is heated to the pyrolysis temperature at a rate of 2-10℃ / min.

9. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 1, characterized in that, In step (4), the pyrolysis temperature is 500-800℃ and the pyrolysis time is 2h.

10. The method for preparing a high-efficiency electromagnetic wave absorber according to claim 9, characterized in that, In step (4), the pyrolysis temperature is 700℃.

11. A multi-core-single-shell Fe3O4@Fe@C composite material prepared by the method according to any one of claims 1-10.

12. The application of the multi-core-single-shell Fe3O4@Fe@C composite material of claim 11 as an electromagnetic wave absorber.

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