Composite microwave absorbing material and preparation method and application thereof

Abstract

The present application relates to a kind of composite microwave absorbing material and its preparation method and application, comprising the following steps: manganese salt, nickel salt, terephthalic acid and first crystal control agent are added to first solvent, mixed processing is obtained mixed liquor A;Mixed liquor A is separated and dried by hydrothermal reaction, and obtains NiMn-MOF;2-dimethylimidazole, second crystal control agent and NiMn-MOF are dispersed in second solvent, and mixed liquor B is obtained;Cobalt salt solution is added to mixed liquor B, after mixing uniformly, by stationary reaction, separation and drying, and NiMn-MOF@ZIF-67 composite material is obtained;Under protective atmosphere, NiMn-MOF@ZIF-67 composite material and dicyan diamine particles are annealed, and composite microwave absorbing material is obtained.The composite microwave absorbing material of the present application has abundant hetero-interface, has excellent electromagnetic wave absorption performance, and the addition amount is low when being made into electromagnetic wave absorbing agent.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorbing materials technology, specifically relating to a composite microwave absorbing material, its preparation method, and its application. Background Technology

[0002] With the widespread application of technologies such as 5G communication, radar detection, and smart terminals, the impact of electromagnetic radiation on the ecological environment and human health has received increasing attention. To reduce electromagnetic pollution and ensure the stable operation of electronic systems, the development of efficient electromagnetic wave absorbing materials is particularly important. However, traditional metal absorber materials suffer from problems such as high density, high dosage requirements, narrow effective absorption bandwidth, and poor controllability. Therefore, the ideal absorbing material should not only possess excellent electromagnetic wave absorption capabilities but also meet the comprehensive performance requirements of being lightweight, thin-layered, and having a wide bandwidth.

[0003] Compared to the problems of high conductivity and simple morphology caused by the dense conductive network of 1D and 2D microwave absorbing materials, 3D microwave absorbing materials theoretically have advantages such as adjustable morphology, high specific surface area, high porosity and low additive amount. However, the current process methods still cannot solve the problem of high additive amount in the microwave absorbing materials.

[0004] The patent "A Core-Shell Ni / Co Alloy@Nitrogen-Doped Carbon-Based Microwave Absorbing Composite Material and Its Preparation Method" (CN202111309243.X) uses a hydrothermal method to generate a columnar core-shell structure Ni / Co alloy@nitrogen-doped carbon-based composite microwave absorbing material, achieving an optimal RL value of -71.9 dB at 8.9 GHz. However, the amount of microwave absorbing material added reaches 40%, which is difficult to meet the low-density requirements of microwave absorbing materials. The patent "A Core-Shell Structure Composite Microwave Absorbing Material" (CN202010027635.6) prepares a precursor via hydrothermal method followed by pyrolysis to obtain nanoporous carbon. Chiral polySchiff base iron salt is then in-situ composited in a solvent to obtain a core-shell structure nanoporous carbon@chiral polySchiff base iron salt composite microwave absorbing material, but the amount added is also as high as 50%. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned technical deficiencies and provide a composite microwave absorbing material, its preparation method and application, thereby solving the technical problem that the high addition amount of microwave absorbing materials in the prior art makes it difficult to achieve lightweight.

[0006] To achieve the above-mentioned technical objectives, the technical solution provided by this invention is as follows: In a first aspect, the present invention provides a method for preparing a composite microwave absorbing material derived from a porous heterostructure MOF, comprising the following steps: S1, adding manganese salt, nickel salt, terephthalic acid and a first crystal regulator to a first solvent, and mixing to obtain a mixture A; S2, subjecting mixture A to hydrothermal reaction and separation drying to obtain NiMn-MOF; S3, dispersing 2-dimethylimidazole, a second crystal regulator and NiMn-MOF in a second solvent to obtain a mixture B; adding a cobalt salt solution to mixture B, mixing evenly, and then subjecting to static reaction and separation drying to obtain a NiMn-MOF@ZIF-67 composite material; S4, annealing the NiMn-MOF@ZIF-67 composite material and dicyandiamide particles under a protective atmosphere to obtain a composite microwave absorbing material.

[0007] Secondly, the present invention provides a composite microwave absorbing material prepared by the above-described preparation method.

[0008] Thirdly, the present invention provides an electromagnetic wave absorber comprising, by mass percentage, 22-28% of the above-mentioned composite microwave absorbing material and 72-78% of paraffin.

[0009] Compared with the prior art, the beneficial effects of the present invention include: The composite microwave absorbing material of this invention uses a micron-sized porous flower-shaped NiMn bimetallic MOF as a substrate, with ZIF-67 particles uniformly attached to its layered surface. Carbon nanotubes are generated through high-temperature catalysis, forming a large number of Ni, Co nanoscale metal particles and Mn. 0.7 Ni 0.3 The alloy particles have abundant heterogeneous interfaces and excellent electromagnetic wave absorption performance. When made into an electromagnetic wave absorber, it has rich loss mechanisms and features low absorption material addition (only 22-28%), wide effective absorption bandwidth (maximum absorption bandwidth can reach 5.53GHz), and high absorption intensity (strongest reflection loss value can reach -61.41dB). Attached Figure Description

[0010] Figure 1 SEM image of the NiMn MOF-derived composite microwave absorbing material prepared in Example 3; Figure 2 for Figure 1 SEM image of NiMn-MOF@ZIF-67 composite microwave absorbing material after the porous heterostructure NiMn-MOF is combined with ZIF-67. Figure 3 SEM image of the porous heterostructure MOF composite-derived microwave absorbing material prepared in Example 2; Figure 4X-ray crystal diffraction patterns of microwave absorbing materials derived from porous heterostructure MOF composites in Examples 1, 2, and 3; Figure 5 TEM image of the microwave absorbing material derived from the porous heterostructure MOF composite in Example 3; Figure 6 TEM image of the microwave absorbing material derived from the porous heterostructure MOF composite in Example 3; Figure 7 This is a two-dimensional diagram of the reflection loss of the microwave absorbing material derived from the porous heterostructure MOF composite in Example 3. Detailed Implementation

[0011] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0012] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the invention, are intended to cover non-exclusive inclusion.

[0013] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of the invention. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0014] To address the current limitations of high-addition-content microwave absorbing materials that hinder lightweighting, this invention provides a composite microwave absorbing material, its preparation method, and its applications. The MOF (Metal-Organic Framework) is a 3D organic metal framework composed of metal atoms and high-specific-surface-area organic ligands. A second-phase MOF is precisely generated in situ using a solvent method, enabling highly tunable electromagnetic parameters. During high-temperature annealing, uniformly distributed nano-metal particles, nano-metal oxide particles, and nano-alloys are formed within the carbon framework. These components combine with magnetic and dielectric losses to create a synergistic effect. Furthermore, by adjusting the microstructure, multiple loss mechanisms are formed, effectively improving the material's impedance matching characteristics, enhancing electromagnetic wave absorption performance, and increasing dielectric loss performance, while maintaining low density and low addition amount.

[0015] In a first aspect, the present invention provides a method for preparing a composite microwave absorbing material derived from a porous heterostructure MOF, comprising the following steps: S1, manganese salt, nickel salt, terephthalic acid and the first crystal regulator are added to the first solvent and mixed to obtain mixture A; S2, mixture A is subjected to hydrothermal reaction and separation drying to obtain NiMn-MOF; S3, 2-dimethylimidazole, a second crystal regulator and NiMn-MOF are dispersed in a second solvent to obtain a mixture B; a cobalt salt solution is added to the mixture B, and after mixing evenly, the mixture is allowed to stand for reaction, separated and dried to obtain the NiMn-MOF@ZIF-67 composite material; S4. Under a protective atmosphere, the NiMn-MOF@ZIF-67 composite material and dicyandiamide particles are annealed to obtain a composite microwave absorbing material.

[0016] In some embodiments, in step S1, the manganese salt includes manganese chloride or manganese nitrate.

[0017] In some embodiments, in step S1, the nickel salt includes nickel chloride or nickel nitrate.

[0018] In some embodiments, in step S1, the molar ratio of manganese salt, nickel salt and terephthalic acid (PTA) is 1:2:(2.2 to 2.6).

[0019] In some embodiments, in step S1, the first solvent is a mixture of N,N-dimethylformamide (DMF), anhydrous ethanol, and ultrapure water.

[0020] Furthermore, the volume ratio of N,N-dimethylformamide (DMF), anhydrous ethanol, and ultrapure water is (10-16):1:1, more preferably 12:1:1. The first solvent of the present invention has good compatibility and can adjust the polarity of the organic ligand (PTA) and the ionic salt, resulting in more uniform coordination.

[0021] In some embodiments, in step S1, the first crystal regulator includes polyvinylpyrrolidone; the ratio between the nickel salt and the first crystal regulator is (4.5-5.0) mmol: (100-120) mg; and the ratio between the first crystal regulator and the first solvent is (0.3-0.7) mg: 1 mL.

[0022] In some embodiments, in step S1, the mixing process is performed by stirring under ultrasonic conditions for 10 to 20 minutes.

[0023] In some embodiments, the conditions for the hydrothermal reaction in step S2 include: a temperature of 130–140°C and a time of 12–24 h.

[0024] In some embodiments, in step S3, the second crystal regulator includes polyvinylpyrrolidone; the cobalt salt includes cobalt nitrate hexahydrate; the mass ratio of 2-dimethylimidazolium, the second crystal regulator, and NiMn-MOF is (7-8):(0.08-0.12):(0.16-0.24); and the ratio between NiMn-MOF and cobalt salt is (0.16-0.24) g:1 mmol.

[0025] In some embodiments, in step S3, the second solvent includes deionized water or ultrapure water.

[0026] In some embodiments, in step S3, the concentration of the cobalt salt solution is 0.05–0.15 mmol / mL.

[0027] In some embodiments, in step S3, the volume ratio of the second solvent to the solvent in the cobalt salt solution is (8-10):1.

[0028] In some embodiments, in step S3, the static reaction is carried out at 10–30°C for 1.5–2.5 h.

[0029] In some embodiments, in step S4, the mass ratio of NiMn-MOF@ZIF-67 composite material to dicyandiamide particles is 1:(1.5~2.5).

[0030] In some embodiments, in step S4, the annealing temperature is 600–800°C and the time is 2–3 hours.

[0031] Secondly, the present invention provides a composite microwave absorbing material prepared by the above-described preparation method.

[0032] The composite microwave absorbing material of this invention uses micron-sized porous flower-shaped NiMn-MOF (nickel-manganese bimetallic MOF) as a substrate, with ZIF-67 particles uniformly attached to its layered surface. Carbon nanotubes are generated through high-temperature catalysis, forming a large number of Ni, Co nanoscale metal particles and Mn. 0.7 Ni 0.3 Alloy particles provide abundant heterogeneous interfaces, enabling them to achieve rich loss mechanisms, low absorption material addition, wide effective absorption bandwidth, and high absorption intensity, as well as excellent electromagnetic wave absorption performance, especially electromagnetic microwave absorption performance.

[0033] In some embodiments, the diameter of the micron-sized porous flower-shaped NiMn-MOF is 8–20 μm, and the diameter of the ZIF-67 particles is 200–500 nm.

[0034] Thirdly, the present invention provides an electromagnetic wave absorber comprising, by mass percentage, 22-28% of the above-mentioned composite microwave absorbing material and 72-78% of paraffin.

[0035] In some embodiments, the electromagnetic wave absorber preferably contains 25% composite microwave absorbing material.

[0036] The main mechanism of action and advantages of this invention are as follows: (1) First, a large-sized flower-shaped substrate NiMn-MOF with a porous framework was synthesized by hydrothermal method. The high specific surface area and complex structure provided sufficient attachment sites for the second phase MOF. Then, a large number of ZIF-67 particles were attached to the substrate MOF sheet structure by solution reaction. Under the catalysis of dicyandiamide, carbon nanotubes, as well as nano-magnetic metal Ni, Co and Mn particles were generated on the spherical surface by high temperature catalysis. 0.7 Ni 0.3 Alloy phase.

[0037] (2) This composite process not only creates a rich heterogeneous interface and improves dielectric polarization by increasing interface polarization, but also forms nano-magnetic metal Ni and Co particles. The Ni and Co nano-metal particles form a magnetic loss effect. In addition, the porous framework with dense carbon nanotubes forms a porous structure that causes multiple reflection and scattering losses. The multiple loss mechanisms work together. The strongest reflection loss value can reach -61.41dB and the maximum absorption bandwidth is 5.53GHz.

[0038] (3) The composite microwave absorbing material obtained by the present invention comprises metallic nickel, metallic cobalt, nickel-manganese alloy and carbon; it has a flower-shaped porous structure, and carbon nanotubes, nano-metals and nano-alloys are uniformly distributed on the surface of the porous structure, so that the composite microwave absorbing material has a micron-level 3D porous network conductive structure, which can form a larger volume network with a lower addition amount, construct a densely connected conductive network, improve dielectric loss performance, have a lower density and addition amount, and can be used for microwave absorption in the frequency range of 2 to 18 GHz.

[0039] The present invention will be further described in detail below through specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments used that do not specify the manufacturer are all conventional products that can be obtained commercially.

[0040] Example 1 A porous heterostructure MOF composite-derived microwave absorbing material and its preparation method. The preparation method described in this specific embodiment includes the following steps: S1, under ultrasonic conditions, the following solutes were added in the order of MnCl2·4H2O, NiCl2·6H2O, terephthalic acid (PTA), and polyvinylpyrrolidone (PVP) to a mixed reaction solvent of N,N-dimethylformamide (DMF), anhydrous ethanol, and ultrapure water in a volume ratio of 12:1:1 (total solvent volume: 224 mL). The mixture was magnetically stirred and ultrasonically treated for 15 mins to obtain mixture A. The molar ratio of MnCl2·4H2O, NiCl2·6H2O, and terephthalic acid was 1:2:2.5. The amount of NiCl2·6H2O used in mixture A was 1146.0 mg (4.8 mmol), and the amount of polyvinylpyrrolidone (PVP) added was 112 mg (concentration of 0.5 mg / mL in mixture A).

[0041] S2, the obtained mixture A is transferred to a reaction vessel with a polytetrafluoroethylene liner, the reaction vessel is sealed and transferred to an oven, the temperature is raised to 130℃, and a hydrothermal reaction is carried out for 12 hours; after the reaction is completed, the solid sample is collected by centrifugation, washed and dried to obtain light green NiMn-MOF.

[0042] In step S3, 7.62 g of 2-dimethylimidazole, 100 mg of PVP, and 0.2 g of NiMn-MOF obtained in step S2 were dissolved and dispersed in 90 mL of deionized water to obtain mixture B. 0.297 g (1 mmol) of cobalt nitrate hexahydrate was dissolved in 10 mL of deionized water to obtain a cobalt nitrate solution, meaning the ratio of deionized water in mixture B to cobalt nitrate solution was 9:1. Subsequently, the cobalt nitrate solution was slowly added dropwise to mixture B under continuous mechanical stirring at 25 °C. After mixing, the resulting suspension was mechanically stirred for 15 mins, and then allowed to stand for 2 h under sealed conditions. The reaction solution was then centrifuged three times with anhydrous ethanol and dried to obtain the purple NiMn-MOF@ZIF-67 composite material.

[0043] S4. The NiMn-MOF@ZIF-67 composite material described in S3 is placed in alumina crucible A, and dicyandiamide particles are placed in alumina crucible B. The mass ratio of NiMn-MOF@ZIF-67 composite material to dicyandiamide particles is 1:2. Alumina crucible B is placed upstream of the gas flow of alumina crucible A. Under a nitrogen atmosphere, the temperature is raised to 600℃ at a heating rate of 5℃ / min for annealing treatment. The holding time is 2h. After natural cooling, the porous heterostructure MOF-derived composite microwave absorbing material is obtained.

[0044] An electromagnetic wave absorber: a composite microwave absorbing material derived from porous heterostructure MOF is mixed with paraffin in a ratio of 1:3 (i.e., the content of the composite microwave absorbing material is 25%).

[0045] Example 2 Compared with Example 1, the only difference is that the annealing temperature in step S4 is 700°C, while the other steps and conditions are the same as in Example 1.

[0046] Example 3 Compared with Example 1, the only difference is that the annealing temperature in step S4 is 800°C, while the other steps and conditions are the same as in Example 1.

[0047] Comparative Example 1 Compared with Example 2, the only difference is that the ratio of the microwave absorbing material derived from the porous heterostructure MOF composite to paraffin is 3:7 (the content of the composite microwave absorbing material is 30%), and the other steps and conditions are the same as in Example 2.

[0048] The results showed that the excessive microwave absorbing material formed a conductive network, resulting in excessively high conductivity and impedance mismatch, and no -10dB absorption bandwidth.

[0049] Comparative Example 2 Compared with Example 3, the only difference is that the ratio of the microwave absorbing material derived from the porous heterostructure MOF composite to paraffin is 1:4 (the content of the composite microwave absorbing material is 20%), and the other steps and conditions are the same as in Example 3.

[0050] The results showed that due to insufficient microwave absorbing material, there was too much microwave transmitting material and too little microwave absorbing component, resulting in a narrow -10dB absorption bandwidth.

[0051] Performance testing (1) The prepared sample was scanned by electron microscopy, and the results are as follows: Figures 1 to 3 As shown. Among them. Figure 1 SEM image of NiMn-MOF (product of step S2) prepared in Example 3; Figure 2 SEM image of NiMn-MOF@ZIF-67 composite material (product of step S3) prepared in Example 3 after combining porous heterostructure NiMn-MOF with ZIF-67; Figure 3 The image shows a SEM image of the porous heterostructure MOF-derived composite microwave absorbing material (product of step S4) prepared in Example 3.

[0052] from Figure 1 It can be seen that the prepared NiMn-MOF surface forms a petal-like lamellar structure, and the overall structure is spherical. The irregular combination of nanoscale thick lamellar structures forms a porous spherical structure, indicating that the material has the characteristics of high specific surface area and low density, which meets the performance requirements of low addition amount. Under the same mass, it can form a large volume network in the matrix material.

[0053] from Figure 2It can be seen that the NiMn-MOF@ZIF-67 composite material synthesized in situ with solvent still has a clear petal-like spherical structure and a large number of ZIF-67 particles uniformly attached to the surface. This indicates that the NiMn-MOF@ZIF-67 composite material synthesized in situ with solvent still has a good petal-like lamellar structure and exhibits good stability.

[0054] from Figure 3 It can be seen that the spherical surface of the composite microwave absorbing material prepared in Example 3 is completely covered by a large number of carbon nanotubes, and some metal particle agglomerates are generated, with the diameter of the carbon nanotubes ranging from 20 nm to 100 nm.

[0055] (2) Figure 4 The X-ray crystal diffraction patterns of the composite microwave absorbing materials of Examples 1, 2 and 3 are shown, corresponding to NMCNT-600, NMCNT-700 and NMCNT-800 respectively.

[0056] from Figure 4 It can be seen that after annealing at 600℃~800℃, the peak areas of Ni and Co elemental metals in (111), (200), and (220) gradually increase and the peak shapes gradually become sharper. Comparison confirms that the synthesized alloy phase is Mn. 0.7 Ni 0.3 Furthermore, the peak positions show the same trend, indicating that as the annealing temperature increases, the crystallinity of the metal increases and the grains become larger.

[0057] (3) Figure 5 and Figure 6 The images shown are TEM images of the composite microwave absorbing material of Example 3 at different magnifications.

[0058] from Figure 5 It can be seen from the interplanar spacing analysis by TEM that Mn is present in the particles. 0.7 Ni 0.3 The presence of a heterogeneous interface with the Co nanoparticles confirms the successful introduction of various polarization losses. Furthermore, the carbon layer coating on the nanoparticles prevents impedance matching failure due to oxidation upon contact with air, thus improving the material's stability.

[0059] from Figure 6 It can be seen that, through TEM interplanar spacing analysis, there is a heterogeneous interface between Co nanoparticles and Ni nanoparticles in the sample, indicating that the metal nanoparticles form a magnetic coupling structure under the carbon layer, constituting a surface magnetic network, and are stably doped by magnetic particles.

[0060] (4) Figure 7 This is a two-dimensional diagram of the reflection loss of the electromagnetic wave absorber in Example 3 at different thicknesses.

[0061] from Figure 7 It can be seen that when the composite microwave absorbing material prepared in Example 3 is mixed with paraffin at a ratio of 1:3 to obtain an electromagnetic wave absorber with a thickness of 4.2 mm and a frequency of 4.82 GHz, the maximum reflection loss intensity can be reached as low as -61.41 dB. Furthermore, with a thickness of 1.65 mm, the maximum effective absorption bandwidth can be reached as high as 5.53 GHz, indicating that the composite microwave absorbing material has the characteristic of having a wide effective absorption bandwidth.

[0062] In summary, this invention involves placing hydrothermally synthesized NiMn-MOF in a solvent to attach a large number of ZIF-67 particles, then placing it in an inert atmosphere and using dicyandiamide high-temperature catalysis to grow a large number of carbon nanotube particles, nano-metal particles, and nano-alloy particles on the surface of the spheres. The abundant ZIF-67 particles create rich heterogeneous interfaces after high-temperature catalysis, providing a large number of magnetic nanoparticles, improving the polarization loss and impedance matching of the material, and forming a 3D material structure with a high specific surface area porous heterogeneous structure. The composite microwave absorbing material obtained by this invention exhibits a petal-shaped spherical porous structure. Specifically, it uses micron-sized flower-shaped NiMn-MOF synthesized by hydrothermal method as a substrate, with ZIF-67 particles attached to the petal-like structure. Furthermore, under the catalysis of dicyandiamide, the MOF and ZIF composite structure generates Ni and Co catalytic carbon nanotubes that coat the substrate surface. Therefore, the composite microwave absorbing material obtained by this invention has a porous heterogeneous structure, high specific surface area, and low density, exhibiting excellent microwave absorption performance and achieving superior microwave absorption characteristics with a relatively low addition amount.

[0063] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. A method for preparing a composite microwave absorbing material derived from a porous heterostructure MOF, characterized in that, Includes the following steps: S1, manganese salt, nickel salt, terephthalic acid and the first crystal regulator are added to the first solvent and mixed to obtain mixture A; S2, mixture A is subjected to hydrothermal reaction and separation drying to obtain NiMn-MOF; S3, 2-dimethylimidazole, a second crystal regulator and NiMn-MOF are dispersed in a second solvent to obtain a mixture B; a cobalt salt solution is added to the mixture B, and after mixing evenly, the mixture is allowed to stand for reaction, separated and dried to obtain the NiMn-MOF@ZIF-67 composite material; S4. Under a protective atmosphere, the NiMn-MOF@ZIF-67 composite material and dicyandiamide particles are annealed to obtain a composite microwave absorbing material.

2. The method for preparing the composite microwave absorbing material derived from porous heterostructure MOF according to claim 1, characterized in that, In step S1, the manganese salt includes manganese chloride or manganese nitrate; The nickel salt includes nickel chloride or nickel nitrate; The molar ratio of the manganese salt, nickel salt and terephthalic acid is 1:2:(2.2-2.6).

3. The method for preparing the composite microwave absorbing material derived from porous heterostructure MOF according to claim 1, characterized in that, In step S1, the first solvent is a mixture of N,N-dimethylformamide, anhydrous ethanol, and ultrapure water; The first crystal regulator comprises polyvinylpyrrolidone; the ratio between the nickel salt and the first crystal regulator is (4.5-5.0) mmol: (100-120) mg; the ratio between the first crystal regulator and the first solvent is (0.3-0.7) mg: 1 mL.

4. The method for preparing the composite microwave absorbing material derived from porous heterostructure MOF according to claim 1, characterized in that, In step S1, the mixing process is carried out under ultrasonic conditions, with stirring for 10 to 20 minutes.

5. The method for preparing the composite microwave absorbing material derived from porous heterostructure MOF according to claim 1, characterized in that, In step S2, the conditions for the hydrothermal reaction include: a temperature of 130–140°C and a time of 12–24 hours.

6. The method for preparing the composite microwave absorbing material derived from porous heterostructure MOF according to claim 1, characterized in that, In step S3, the second crystal regulator includes polyvinylpyrrolidone; The cobalt salt includes cobalt nitrate or cobalt chloride; The mass ratio of 2-dimethylimidazolium, the second crystal regulator, and NiMn-MOF is (7-8):(0.08-0.12):(0.16-0.24). The ratio between NiMn-MOF and cobalt salt is (0.16–0.24) g: 1 mmol.

7. The method for preparing the composite microwave absorbing material derived from porous heterostructure MOF according to claim 1, characterized in that, In step S3, the second solvent includes deionized water or ultrapure water; The concentration of the cobalt salt solution is 0.05–0.15 mmol / mL; The volume ratio of the second solvent to the solvent in the cobalt salt solution is (8-10):1; The static reaction is carried out at 10–30°C for 1.5–2.5 hours.

8. The method for preparing the composite microwave absorbing material derived from porous heterostructure MOF according to claim 1, characterized in that, In step S4, the mass ratio of the NiMn-MOF@ZIF-67 composite material to dicyandiamide particles is 1:(1.5~2.5). The annealing process is performed at a temperature of 600–800°C for 2–3 hours.

9. The composite microwave absorbing material prepared by the preparation method according to any one of claims 1-8.

10. An electromagnetic wave absorber, characterized in that, It comprises, by weight percentage, 22-28% of the composite microwave absorbing material as described in claim 9 and 72-78% paraffin.

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