A Co@CA microwave absorbing material, its preparation method and application

Abstract

This invention provides a Co@CA microwave absorbing material, its preparation method, and its application. The preparation method of the Co@CA microwave absorbing material is as follows: 1) After simple precipitation of Co salt and dimethylimidazole in methanol solution, the material is washed, dried, and ground to obtain MOF:ZIF-67 powder; 2) Resorcinol is dissolved in Na2CO3 aqueous solution, formaldehyde solution is added, and the resulting transparent solution is injected into a mold, sealed, and heated to solidify. Then, MOF:ZIF-67 powder is added, and the mixture is stirred thoroughly with a spatula. After solidification, the mixture is freeze-dried; 3) Finally, the material is calcined at high temperature under a nitrogen atmosphere to complete the preparation. This invention applies functional building blocks and spatial order to the design of microwave absorbing materials. Using metal / carbon as functional building blocks, the ordered construction of magnetic metal particles on carbon aerogel enhances its electromagnetic wave absorption performance, ultimately achieving the requirements of "wide, light, thin, and strong". The preparation process is simple and inexpensive, enabling large-scale mass production. The obtained microwave absorbing material has suitable electrical conductivity and excellent electromagnetic wave absorption intensity.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic wave absorption technology, specifically to a Co@CA absorbing material, its preparation method, and its applications. Background Technology

[0002] The use of electromagnetic wave absorbers to convert harmful electromagnetic waves into heat or other forms of energy for dissipation and absorption has become a widespread concern. In the military field, the demand for electromagnetic wave absorbing materials is extremely urgent for military equipment such as radar stealth aircraft, missiles, and combat command vehicles. MOFs (Metal-Oxide-Factory Materials) materials, due to their excellent properties, can be used to prepare absorbing materials with ultra-high porosity and large specific surface area. However, the electromagnetic wave reflection loss intensity and absorption bandwidth of MOF-derived materials are not ideal. Combining them with other materials to improve their electromagnetic wave absorption performance is an effective method. Carbon aerogel is a porous three-dimensional framework material with a gaseous dispersion medium, possessing ideal electrical conductivity, extremely high porosity (up to 99.8%), abundant pore structure (pore size distribution between 1–100 nm), and ultra-high specific surface area (up to 2000 m²). 2 ·g -1 Extremely low density (as low as 3 mg / cm³) -3 These excellent properties fully meet the requirements of "thin" and "light" for microwave absorbing materials. This invention applies the paradigm of functional building blocks and spatial order to the design of microwave absorbing materials. Using metal / carbon as functional building blocks, the electromagnetic wave absorption performance is improved through the orderly construction of magnetic metal particles on carbon aerogel, ultimately achieving the requirements of "wide, light, thin, and strong". Summary of the Invention

[0003] The purpose of this invention is to address the problems of low electromagnetic wave absorption intensity, narrow electromagnetic wave absorption bandwidth, complex synthesis process, and small scale of existing MOF-derived metal / carbon absorbing materials, and to provide a Co@CA absorbing material, its preparation method, and its applications. The Co@CA absorbing material of this invention has a simple and inexpensive preparation process, can be mass-produced, and the obtained absorbing material exhibits suitable conductivity and excellent electromagnetic wave absorption intensity.

[0004] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0005] The preparation method of the Co@CA microwave absorbing material includes the following steps:

[0006] 1) The ZIF-67 precursor was prepared by simple precipitation of Co salt and dimethylimidazole in methanol solution at room temperature, as follows: Co salt and dimethylimidazole were mixed in methanol and stirred at room temperature for 4-8 hours. The resulting purple precipitate was collected by centrifugation, washed several times with alcohol, dried and ground to obtain purple MOF: ZIF-67 material powder.

[0007] 2) Dissolve resorcinol in Na2CO3 aqueous solution, then add formaldehyde aqueous solution and stir. Pour the resulting transparent solution into the mold, seal it, heat and cure it. Then add the purple MOF: ZIF-67 material powder obtained in step 1) to the mold and stir the mixture thoroughly with a spatula until a uniform appearance is achieved. After curing at 303~353K for 5~10 days, freeze dry it in a freeze dryer.

[0008] 3) The dried composite material from step 2) is calcined at high temperature in a tube furnace under a nitrogen atmosphere to obtain a black material, which is the Co@CA microwave absorbing material.

[0009] The method for preparing a Co@CA microwave absorbing material is characterized in that, in step 1), the Co salt is cobalt nitrate hexahydrate, and the molar ratio of the Co salt to dimethylimidazole is 1:40~50, preferably 1:45.

[0010] The method for preparing a Co@CA microwave absorbing material is characterized in that, in step 2), the molar ratio of resorcinol to formaldehyde is 0.7~0.8:1, preferably 0.73:1.

[0011] The method for preparing a Co@CA microwave absorbing material is characterized in that, in step 2), the molar ratio of resorcinol to sodium carbonate is 900~1100:1, preferably 992:1.

[0012] The method for preparing a Co@CA microwave absorbing material is characterized in that, in step 2), the concentration of the Na2CO3 aqueous solution is 0.001~0.003 mol·L⁻¹. −1 Preferably, it is 0.0021 mol·L −1 The formaldehyde aqueous solution has a mass concentration of 20-30%, preferably 24%.

[0013] The method for preparing a Co@CA microwave absorbing material is characterized in that, in step 2), the heating and curing temperature is 323~363K, preferably 343K, and the heating and curing time is 60~100min, preferably 80min.

[0014] The method for preparing a Co@CA microwave absorbing material is characterized in that, in step 2), the freeze-drying time is 20~30h, preferably 24h, and the freezing temperature is -30~-50℃, preferably -40℃.

[0015] The method for preparing a Co@CA microwave absorbing material is characterized in that the calcination temperature in step 3) is 700~900℃, preferably 800℃, and the calcination time is 3~6h.

[0016] The Co@CA absorbing material provided by this invention can be well applied to the absorption of electromagnetic waves with a frequency range of 2 to 18 GHz.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0018] 1) This invention combines MOF (ZIF-67) with carbon aerogel to obtain materials with different properties at different calcination temperatures, revealing the electromagnetic wave loss mechanism of functional unit order, optimizing the process, and ultimately achieving the requirements of "wide, light, thin, and strong". "Wide" refers to a wide electromagnetic wave absorption frequency range (the frequency range occupied by RL < -10dB), "light" refers to the material's light weight, "thin" refers to the sample's thickness, and "strong" refers to high reflection loss intensity (electromagnetic wave absorption intensity). Specific data can be found in the data... Figure 4-8 This can be seen from the text.

[0019] 2) The preparation method of the present invention combines spatial ordered arrangement and functional unit design to construct Co@CA, thereby achieving the superposition of multiple loss mechanisms.

[0020] 3) The Co@CA electromagnetic wave absorbing material prepared by this invention is lighter and thinner than traditional absorbing materials, and has ideal reflection loss and absorption bandwidth.

[0021] 4) The Co@CA microwave absorbing material prepared by this invention has a unique coating structure, and the constructed conductive network can rapidly attenuate the incident electromagnetic waves and eventually convert them into heat energy dissipation.

[0022] The Co@CA absorbing material prepared by this invention exhibits excellent electromagnetic absorption performance. This material achieves its minimum reflection loss value (RL) through dielectric losses originating from the interface and dipole polarization. min It reached -59.98dB, which can absorb 99.999% of incident electromagnetic waves. Attached Figure Description

[0023] Figure 1 The image shows the X-ray electron diffraction (XRD) pattern of the Co@CA-800 microwave absorbing material prepared in Example 1.

[0024] Figure 2aThe image shows a SEM image of the 1-Co@CA-800 microwave absorbing material prepared in Example 1.

[0025] Figure 2b The image shows a SEM image of the 1-Co@CA-800 microwave absorbing material prepared in Example 1.

[0026] Figure 3 This is a transmission electron microscope (TEM) image of the 1-Co@CA-800 microwave absorbing material prepared in Example 1.

[0027] Figure 4 The reflection loss (RL) curve of the 1-Co@CA-800 absorbing material prepared in Example 1 is shown.

[0028] Figure 5 The reflection loss (RL) curve of the 1-Co@CA-700 absorbing material prepared in Example 2 is shown.

[0029] Figure 6 The reflection loss (RL) curve of the 1-Co@CA-900 absorbing material prepared in Example 3 is shown.

[0030] Figure 7 The reflection loss (RL) curve of the 2-Co@CA-800 absorbing material prepared in Example 4 is shown.

[0031] Figure 8 The reflection loss (RL) curve of the 3-Co@CA-800 absorbing material prepared in Example 5 is shown. Detailed Implementation

[0032] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto. Example 1

[0033] Step 1: The ZIF-67 precursor was prepared by simple precipitation of Co(NO3)2·6H2O and dimethylimidazole in methanol solution at room temperature. Specifically, 3 mL of Co(NO3)2·6H2O (0.50 mol·dm³) was used. -3 A methanol solution of ) and 20 mL of dimethylimidazole (3.40 mol·dm³) -3 The mixture was stirred at room temperature for 6 hours with a methanol solution of 1,2-ZIF-67. The resulting purple precipitate was collected by centrifugation, washed three times with alcohol, and dried at 80°C for 24 hours to obtain the purple MOF (ZIF-67).

[0034] Step 2: Dissolve 4.59 g of resorcinol (41.7 mmol) in 19.74 g of Na2CO3 aqueous solution (0.0021 mol·L⁻¹). -1; 0.042 mmol Na2CO3). Then add 7.14 g of formaldehyde aqueous solution (57.1 mmol; containing 24% formaldehyde) and stir the solution for 5 minutes. Pour the clear solution into a mold, seal it with several layers of polyethylene and aluminum foil, and cure at 343 K for 80 min. The heat treatment of the resorcinol-formaldehyde solution produced a higher viscosity. Then add 250 mg of well-ground ZIF-67 powder obtained in step one to the mold and stir the mixture thoroughly with a spatula until a uniform appearance is achieved. After curing at 333 K for 7 days, freeze-dry at -40 °C for 24 h using a freeze dryer.

[0035] Step 3: The dried composite material from Step 2 is calcined at high temperature in a tube furnace under a nitrogen atmosphere at 5°C·min. -1 Heat to 250°C at a rate of 5°C / min and maintain under a nitrogen atmosphere for 2 hours, then continue heating at 5°C / min. -1 The temperature was rapidly increased to 800°C, and the mixture was kept at 800°C under N2 for 4 hours. Finally, it was cooled to room temperature to obtain the Co@CA material.

[0036] X-ray electron diffraction (XRD) tests were performed on the Co@CA material prepared in Example 1, as follows: Figure 1 A weak, broad peak is observed at approximately 25°, which may be attributed to the (002) peak of the graphitic carbon material with a low degree of graphitization. In addition to this peak, two main peaks are observed at 44.2° and 51.5°, which are attributed to... fcc The structure of metallic Co (PDF no. 89−4307). In addition, some weak peaks of CoO were observed for this sample, which is attributed to the oxidation of small Co nanoparticles when the sample was exposed to air.

[0037] The Co@CA material prepared in Example 1 was subjected to SEM and TEM detection. Figure 2a SEM images of pure carbon alumina (CA) are shown, exhibiting its well-known pearl-string morphology and spherical aggregate structure. Larger gaps were found between the elongated and slender carbon bead clusters, easily leading to a loose network structure. Therefore, the porous structure of CA is conducive to electromagnetic wave propagation. Figure 2b In this process, the Co@CA hybrid inherits the porous structure of CA, with Co nanoparticles embedded in CA to form a large embedded body. The embedded structure is beneficial for increasing the specific surface area and interfacial polarization of the material, thereby enhancing the electromagnetic wave absorption performance of the material. Figure 3 The image shows a TEM image of the Co@CA hybrid. The TEM image reveals that the Co nanoparticles have agglomerated. The Co nanoparticle clusters are relatively large. Larger nanoparticles have a negative effect on the electromagnetic wave absorption performance of the material, so it is necessary to control the size of the nanoparticles.

[0038] The electromagnetic wave absorption performance of the Co@CA material prepared in Example 1 was tested. The reflection loss (RL) curves of the material at thicknesses of 1.0–5.5 mm and frequencies of 2–18 GHz are shown in the figure. Generally, an RL value below -10 dB indicates that 90% of the electromagnetic waves are absorbed. Figure 4 As shown, the RL of the Co@CA absorbing material min The absorption value is -59.98 dB, and the thickness is 2.5 mm. The effective absorption bandwidth (RL value < -10 dB) is 4.16 GHz. The excellent absorption performance of Co@CA material can be attributed to its unique embedded structure; suitable conductivity; dipole polarization relaxation loss caused by local defects and multifunctional surface groups; interfacial polarization loss between multiple interfaces; and good impedance matching. Therefore, this invention provides a new design concept and technical support for the preparation of lightweight, high-performance electromagnetic absorbing materials. Example 2

[0039] Step 1: The ZIF-67 precursor was prepared by simple precipitation of Co(NO3)2·6H2O and dimethylimidazole in methanol solution at room temperature. Specifically, 3 mL of Co(NO3)2·6H2O (0.50 mol·dm³) was used. -3 Methanol solution and 20 mL dimethylimidazole (3.40 mol·dm³) -3 The methanol solution was mixed and stirred at room temperature for 6 hours. The resulting purple precipitate was collected by centrifugation, washed three times with alcohol, and dried at 80°C for 24 hours to obtain a purple MOF (ZIF-67).

[0040] Step 2: Dissolve 4.59 g of resorcinol (41.7 mmol) in 19.74 g of Na2CO3 aqueous solution (0.0021 mol·L⁻¹). -1 ; 0.042 mmol Na2CO3). Then add 7.14 g of formaldehyde aqueous solution (57.1 mmol; containing 24% formaldehyde) and stir the solution for 5 minutes. Pour the clear solution into the mold, seal with several layers of polyethylene and aluminum foil, and cure at 343 K for 80 min. The heat treatment of the resorcinol-formaldehyde solution produced a higher viscosity. Then add 250 mg of well-ground ZIF-67 powder obtained in step one to the mold and stir the mixture thoroughly with a spatula until a uniform appearance is achieved. After curing at 333 K for 7 days, freeze-dry at -40 °C for 24 h using a freeze dryer.

[0041] Step 3: The dried composite material from Step 2 is calcined at high temperature in a tube furnace under a nitrogen atmosphere at 5°C·min. -1Heat to 250°C at a rate of 5°C / min and maintain under a nitrogen atmosphere for 2 hours, then continue heating at 5°C / min. -1 The temperature was rapidly increased to 700°C, and the mixture was kept at 700°C under N2 for 4 hours. Finally, it was cooled to room temperature to obtain the Co@CA material.

[0042] The electromagnetic wave absorption performance of the Co@CA material prepared in Example 2 was tested. The reflection loss (RL) curves of the material at thicknesses of 1.0–5.5 mm and frequencies of 2–18 GHz are shown in the figure. Generally, an RL value below -10 dB indicates that 90% of the electromagnetic waves are absorbed. Figure 5 As shown, the RL of the Co@CA absorbing material min The RL value is -30.19 dB, and the thickness is 1.9 mm. The effective absorption bandwidth (RL value < -10 dB) is 6.0 GHz. The excellent absorption performance of Co@CA material can be attributed to its unique embedded structure; suitable conductivity; dipole polarization relaxation loss caused by local defects and multifunctional surface groups; interfacial polarization loss between multiple interfaces; and good impedance matching. Therefore, this invention provides a new design concept and technical support for the preparation of lightweight, high-performance electromagnetic absorbing materials. Example 3

[0043] Step 1: The ZIF-67 precursor was prepared by simple precipitation of Co(NO3)2·6H2O and dimethylimidazole in methanol solution at room temperature. Specifically, 3 mL of Co(NO3)2·6H2O (0.50 mol·dm³) was used. -3 Methanol solution and 20 mL dimethylimidazole (3.40 mol·dm³) -3 The methanol solution was mixed and stirred at room temperature for 6 hours. The resulting purple precipitate was collected by centrifugation, washed three times with alcohol, and dried at 80°C for 24 hours to obtain a purple MOF (ZIF-67).

[0044] Step 2: Dissolve 4.59 g of resorcinol (41.7 mmol) in 19.74 g of Na2CO3 aqueous solution (0.0021 mol·L⁻¹). -1 ; 0.042 mmol Na2CO3). Then add 7.14 g of formaldehyde aqueous solution (57.1 mmol; containing 24% formaldehyde) and stir the solution for 5 minutes. Pour the clear solution into the mold, seal with several layers of polyethylene and aluminum foil, and cure at 343 K for 80 min. The heat treatment of the resorcinol-formaldehyde solution produced a higher viscosity. Then add 250 mg of well-ground ZIF-67 powder obtained in step one to the mold and stir the mixture thoroughly with a spatula until a uniform appearance is achieved. After curing at 333 K for 7 days, freeze-dry at -40 °C for 24 h using a freeze dryer.

[0045] Step 3: The dried composite material from Step 2 is calcined at high temperature in a tube furnace under a nitrogen atmosphere at 5°C·min. -1 Heat to 250°C at a rate of 5°C / min and maintain under a nitrogen atmosphere for 2 hours, then continue heating at 5°C / min. -1 The temperature was rapidly increased to 900°C, and the mixture was kept at 900°C under N2 for 4 hours. Finally, it was cooled to room temperature to obtain the Co@CA material.

[0046] The electromagnetic wave absorption performance of the Co@CA material prepared in Example 3 was tested. The reflection loss (RL) curves of the material at thicknesses of 1.0–5.5 mm and frequencies of 2–18 GHz are shown in the figure. Generally, an RL value below -10 dB indicates that 90% of the electromagnetic waves are absorbed. Figure 6 As shown, the RL of the Co@CA absorbing material min The RL value is -5.52 dB, and the thickness is 1.9 mm. The effective absorption bandwidth (RL value < -10 dB) is 0 GHz. The Co@CA material obtained by calcination at 900℃ has very poor electromagnetic wave absorption performance. Example 4

[0047] Step 1: The ZIF-67 precursor was prepared by simple precipitation of Co(NO3)2·6H2O and dimethylimidazole in methanol solution at room temperature. Specifically, 3 mL of Co(NO3)2·6H2O (0.50 mol·dm³) was used. -3 Methanol solution and 20 mL dimethylimidazole (3.40 mol·dm³) -3 The methanol solution was mixed and stirred at room temperature for 6 hours. The resulting purple precipitate was collected by centrifugation, washed three times with alcohol, and dried at 80°C for 24 hours to obtain a purple MOF (ZIF-67).

[0048] Step 2: Dissolve 4.59 g of resorcinol (41.7 mmol) in 19.74 g of Na2CO3 aqueous solution (0.0021 mol·L⁻¹). -1 ; 0.042 mmol Na2CO3). Then add 7.14 g of formaldehyde aqueous solution (57.1 mmol; containing 24% formaldehyde) and stir the solution for 5 minutes. Pour the clear solution into the mold, seal it with several layers of polyethylene and aluminum foil, and cure it at 343 K for 80 min. The heat treatment of the resorcinol-formaldehyde solution produced a higher viscosity. Then add 500 mg of well-ground ZIF-67 powder obtained in step one to the mold and stir the mixture thoroughly with a spatula until a uniform appearance is achieved. After curing at 333 K for 7 days, freeze-dry it at -40 °C for 24 h using a freeze dryer.

[0049] Step 3: The dried composite material from Step 2 is calcined at high temperature in a tube furnace under a nitrogen atmosphere at 5°C·min. -1 Heat to 250°C at a rate of 5°C / min and maintain under a nitrogen atmosphere for 2 hours, then continue heating at 5°C / min. -1 The temperature was rapidly increased to 800°C, and the mixture was kept at 800°C under N2 for 4 hours. Finally, it was cooled to room temperature to obtain the Co@CA material.

[0050] The electromagnetic wave absorption performance of the Co@CA material prepared in Example 4 was tested. The reflection loss (RL) curves of the material at thicknesses of 1.0–5.5 mm and frequencies of 2–18 GHz are shown in the figure. Generally, an RL value below -10 dB indicates that 90% of the electromagnetic waves are absorbed. Figure 7 As shown, the RL of the Co@CA absorbing material min The absorbing power is -24.32 dB, and the thickness is 3.0 mm. The effective absorption bandwidth (RL value < -10 dB) is 4.16 GHz. The excellent absorption performance of Co@CA material can be attributed to its unique embedded structure; suitable conductivity; dipole polarization relaxation loss caused by local defects and multifunctional surface groups; interfacial polarization loss between multiple interfaces; and good impedance matching. Therefore, this invention provides a new design concept and technical support for the preparation of lightweight, high-performance electromagnetic absorbing materials. Example 5

[0051] Step 1: The ZIF-67 precursor was prepared by simple precipitation of Co(NO3)2·6H2O and dimethylimidazole in methanol solution at room temperature. Specifically, 3 mL of Co(NO3)2·6H2O (0.50 mol·dm³) was used. -3 Methanol solution and 20 mL dimethylimidazole (3.40 mol·dm³) -3 The methanol solution was mixed and stirred at room temperature for 6 hours. The resulting purple precipitate was collected by centrifugation, washed three times with alcohol, and dried at 80°C for 24 hours to obtain a purple MOF (ZIF-67).

[0052] Step 2: Dissolve 4.59 g of resorcinol (41.7 mmol) in 19.74 g of Na2CO3 aqueous solution (0.0021 mol·L⁻¹). -1; 0.042 mmol Na2CO3). Then add 7.14 g of formaldehyde aqueous solution (57.1 mmol; containing 24% formaldehyde) and stir the solution for 5 minutes. Pour the clear solution into the mold, seal it with several layers of polyethylene and aluminum foil, and cure at 343 K for 80 min. The heat treatment of the resorcinol-formaldehyde solution produced a higher viscosity. Then add 750 mg of well-ground ZIF-67 powder obtained in step one to the mold and stir the mixture thoroughly with a spatula until a uniform appearance is achieved. After curing at 333 K for 7 days, freeze-dry at -40 °C for 24 h using a freeze dryer.

[0053] Step 3: The dried composite material from Step 2 is calcined at high temperature in a tube furnace under a nitrogen atmosphere at 5°C·min. -1 Heat to 250°C at a rate of 5°C / min and maintain under a nitrogen atmosphere for 2 hours, then continue heating at 5°C / min. -1 The temperature was rapidly increased to 800°C, and the mixture was kept at 800°C under N2 for 4 hours. Finally, it was cooled to room temperature to obtain the Co@CA material.

[0054] The electromagnetic wave absorption performance of the Co@CA material prepared in Example 5 was tested. The reflection loss (RL) curves of the material at thicknesses of 1.0–5.5 mm and frequencies of 2–18 GHz are shown in the figure. Generally, an RL value below -10 dB indicates that 90% of the electromagnetic waves are absorbed. Figure 8 As shown, the RL of Co@CA absorbing material min The RL value is -43.21 dB, and the thickness is 5.5 mm. The effective absorption bandwidth (RL value < -10 dB) is 1.6 GHz. The excellent absorption performance of Co@CA material can be attributed to its unique embedded structure; suitable conductivity; dipole polarization relaxation loss caused by local defects and multifunctional surface groups; interfacial polarization loss between multiple interfaces; and good impedance matching. Therefore, this invention provides a new design concept and technical support for the preparation of lightweight, high-performance electromagnetic absorbing materials.

[0055] The contents described in this specification are merely an enumeration of the implementation forms of the inventive concept, and the scope of protection of this invention should not be regarded as limited to the specific forms described in the embodiments.

Claims

1. A method for preparing a Co@CA microwave absorbing material, characterized in that... Includes the following steps: 1) The ZIF-67 precursor was prepared by simple precipitation of Co salt and dimethylimidazole in methanol solution at room temperature, as follows: Co salt and dimethylimidazole were mixed in methanol and stirred at room temperature for 4-8 hours. The resulting purple precipitate was collected by centrifugation, washed several times with alcohol, dried and ground to obtain purple MOF: ZIF-67 material powder. 2) Dissolve resorcinol in Na2CO3 aqueous solution, then add formaldehyde aqueous solution and stir. Pour the resulting transparent solution into the mold, seal it, heat and cure it. Then add the purple MOF: ZIF-67 material powder obtained in step 1) to the mold and stir the mixture thoroughly with a spatula until a uniform appearance is achieved. After curing at 303~353K for 5~10 days, freeze dry it in a freeze dryer. 3) The dried composite material from step 2) is calcined at high temperature in a tube furnace under a nitrogen atmosphere to obtain a black material, which is the Co@CA microwave absorbing material. In step 3), the calcination temperature is 700~800℃ and the calcination time is 3~6h.

2. The preparation method according to claim 1, characterized in that... In step 1), the Co salt is cobalt nitrate hexahydrate, and the molar ratio of the Co salt to dimethylimidazole is 1:40~50.

3. The preparation method according to claim 2, characterized in that... In step 1), the molar ratio of Co salt to dimethylimidazole is 1:

45.

4. The preparation method according to claim 1, characterized in that... In step 2), the molar ratio of resorcinol to formaldehyde is 0.7~0.8:

1.

5. The preparation method according to claim 4, characterized in that... In step 2), the molar ratio of resorcinol to formaldehyde is 0.73:

1.

6. The preparation method according to claim 1, characterized in that... In step 2), the molar ratio of resorcinol to sodium carbonate is 900~1100:

1.

7. The preparation method according to claim 6, characterized in that... In step 2), the molar ratio of resorcinol to sodium carbonate is 992:

1.

8. The preparation method according to claim 1, characterized in that... In step 2), the concentration of the Na₂CO₃ aqueous solution is 0.001~0.003 mol·L⁻¹. −1 The formaldehyde aqueous solution has a mass concentration of 20-30%.

9. The preparation method according to claim 8, characterized in that... In step 2), the concentration of the Na₂CO₃ aqueous solution is 0.0021 mol·L⁻¹. −1 The mass concentration of the formaldehyde aqueous solution is 24%.

10. The preparation method according to claim 1, characterized in that... In step 2), the heating curing temperature is 323~363K and the heating curing time is 60~100min.

11. The preparation method according to claim 10, characterized in that... In step 2), the heating curing temperature is 343K and the heating curing time is 80 min.

12. The preparation method according to claim 1, characterized in that... In step 2), the freeze-drying time is 20~30h and the freezing temperature is -30~-50℃.

13. The preparation method according to claim 12, characterized in that... In step 2), the freeze-drying time is 24 hours and the freezing temperature is -40℃.

14. The preparation method according to claim 1, characterized in that... The calcination temperature in step 3) is 800℃.

15. A Co@CA microwave absorbing material prepared by the method described in any one of claims 1-14.

16. The application of the Co@CA absorbing material as described in claim 15 in absorbing electromagnetic waves, characterized in that... The frequency range of electromagnetic waves is 2 to 18 GHz.

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