Graphene / nickel-cobalt double-metal oxide / PMI foam composite wave-absorbing material and preparation method and repairing method thereof

Abstract

The application belongs to the technical field of wave-absorbing materials, and particularly relates to a graphene / nickel-cobalt bimetal oxide / PMI foam composite wave-absorbing material and a preparation method and a repairing method thereof. The preparation method of the graphene / nickel-cobalt bimetal oxide / PMI foam composite wave-absorbing material comprises the following steps: dispersing nickel-cobalt bimetal oxide nanoparticles and graphene oxide in a solvent, adding a silane coupling agent, immersing PMI foam in the mixture, performing magnetic field treatment, performing vacuum freeze-drying heat treatment, and obtaining the composite wave-absorbing material. The graphene / nickel-cobalt bimetal oxide / PMI foam composite wave-absorbing material and the preparation method thereof are simple to operate, and the preparation method is mild and easy to implement. The composite wave-absorbing material improves the impedance matching characteristics of the material in different frequency bands, improves the wave-absorbing performance retention ability of the material in a damaged state, and has a simple recovery method and a high recovery rate.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorbing materials technology, specifically relating to graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing materials and their preparation and repair methods. Background Technology

[0002] Modern radar detection technology is developing towards multi-band, high resolution, and strong anti-interference, which puts forward higher requirements for the performance of electromagnetic stealth materials. Traditional absorbing materials designed for single or narrow frequency bands are difficult to adapt to the application needs of complex electromagnetic environments.

[0003] Existing electromagnetic absorbing materials mainly include ferrite materials, metallic magnetic particles, and carbon-based materials. While ferrite materials possess a certain magnetic loss capability, their high density and limited low-frequency absorption performance restrict their use in weight-sensitive applications such as aerospace. Metallic magnetic particles suffer from easy oxidation and insufficient environmental stability. Carbon-based materials, such as carbon black, carbon nanotubes, and graphene, primarily rely on dielectric loss for electromagnetic absorption; when used alone, their impedance matching capability in the low-frequency range is weak, making it difficult to achieve broadband and efficient absorption. Therefore, combining dielectric loss materials with magnetic or quasi-magnetic functional components and achieving synergistic effects of multiple loss mechanisms through structural design is considered an effective way to improve the performance of electromagnetic absorbing materials.

[0004] Polymethacrylimide (PMI) foam, characterized by low density, high specific strength, and excellent resistance to high and low temperatures, has been widely used as a lightweight core material for aerospace structural components. In recent years, PMI foam-based microwave absorbing composites have attracted increasing attention. By introducing microwave absorbing functional components into the foam matrix, electromagnetic absorption functions can be imparted while maintaining structural performance. However, existing technologies often introduce microwave absorbing components through uniform filling or surface coating, which can easily lead to a uniform distribution of functional components within the foam pores. This makes it difficult to simultaneously meet the impedance matching requirements of electromagnetic waves in different frequency bands, and the absorption bandwidth remains limited. For example, Chinese patent CN115891204A discloses a method for preparing a honeycomb foam composite microwave absorbing material based on PMI foam. This material broadens the absorption frequency band to a certain extent through structural design and possesses certain mechanical properties. However, its microwave absorbing functional components are mainly uniformly distributed, lacking effective control over the spatial distribution of components. Furthermore, if microcracks or interface damage occur during service, the microwave absorption performance may irreversibly decline, indicating insufficient consideration for performance durability in the design. Furthermore, Chinese patent CN117621580A discloses a lightweight broadband microwave-absorbing PMI foam sandwich composite material. By combining foam foaming and fiber curing processes, it achieves an integrated design of structure and microwave absorption performance, improving low-frequency absorption to some extent and reducing overall density. However, this technology relies on multi-layer fiber cloth or sandwich structure to improve performance, making the preparation process relatively complex. Moreover, the microwave absorption function is still mainly achieved through uniformly distributed microwave-absorbing fillers, resulting in problems such as single distribution or local enrichment of functional components. In addition, it does not specifically address the recovery of microwave absorption performance after material damage.

[0005] Therefore, in existing technologies, whether based on carbon foam, carbon-based porous materials, or PMI foam, the microwave-absorbing functional components are mostly introduced through uniform filling, surface coating, or multilayer stacking. The spatial distribution of these components within the foam has low controllability, making it difficult to form a stable gradient structure along the thickness direction. Furthermore, existing methods fail to fully utilize the directional migration characteristics of magnetic microwave-absorbing components under an applied magnetic field. The magnetic field is often used only as an auxiliary processing method, rather than as a core driving factor in constructing the material's microstructure. This results in limited control over the material's microwave absorption performance, and the absorption performance is prone to irreversible degradation after microscopic damage to the material. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material and its preparation method. The preparation method is simple to operate, mild and easy to implement. The graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention improves the impedance matching characteristics of the material in different frequency bands, enhances the ability of the material to maintain its microwave absorption performance under damaged conditions, and has a simple recovery method and a high recovery rate.

[0007] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: dispersing nickel-cobalt bimetallic oxide nanoparticles and graphene oxide in anhydrous ethanol, adding a silane coupling agent, and mixing to obtain a dispersion; immersing PMI (polymethacrylimide) foam in the dispersion and treating it under an external directional magnetic field; subjecting the treated PMI foam to alternating magnetic field treatment and then vacuum freeze-drying; and heat-treating the freeze-dried material to obtain the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material. Anhydrous ethanol is preferred as the solvent, and the silane coupling agent is preferably 5-10% of the ethanol mass. In the addition of raw materials, the mass ratio of nickel-cobalt bimetallic oxide nanoparticles to graphene oxide is 10:1 to 10:3.

[0008] The total loading of graphene and nickel-cobalt bimetallic oxides in the resulting composite microwave absorbing material is 5-15% of the mass of the PMI foam matrix.

[0009] The magnetic induction intensity of the directional magnetic field is 0.2-0.8T, and the direction of the magnetic field is consistent with the thickness direction of the PMI foam.

[0010] The frequency of the alternating magnetic field is 100-500kHz, and the duration of action is 2-8 minutes.

[0011] The heat treatment is performed in a protective atmosphere at 150-250°C. The protective gas is preferably an inert gas, and the heat treatment time is preferably 2-3 hours.

[0012] The nickel-cobalt bimetallic oxide nanoparticles are spinel structure NiCo2O4 with a particle size of 10-20 nm.

[0013] The PMI foam has a pore size of 10-50 μm and is an insulating porous polymer foam.

[0014] The silane coupling agent is one of KH550, KH792 or OFS-6011.

[0015] A graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material is prepared by the aforementioned method.

[0016] The repair method for the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material is as follows: after the microwave absorbing material absorbs microwaves, it is treated in an alternating magnetic field of 100-500kHz to repair the microwave absorbing material.

[0017] In a preferred embodiment of the present invention, the preparation method is as follows: nickel cobalt bimetallic oxide nanoparticles and graphene oxide are dispersed in a solvent, a silane coupling agent is added, and ultrasonic mixing is performed to obtain a dispersion; PMI foam is immersed in the dispersion and treated under an external directional magnetic field; the treated PMI foam is first subjected to alternating magnetic field treatment and then vacuum freeze-dried; the freeze-dried material is heat-treated to obtain a graphene / nickel cobalt bimetallic oxide / PMI foam composite microwave absorbing material.

[0018] The preferred second technical solution: Based on the first technical solution, the total loading of graphene and nickel-cobalt bimetallic oxide in the obtained composite microwave absorbing material is 5-15% of the mass of the PMI foam matrix.

[0019] The preferred third technical solution: Based on the second technical solution, the magnetic induction intensity of the directional magnetic field is 0.2-0.8T, and the direction of the magnetic field is consistent with the thickness direction of the PMI foam.

[0020] The preferred fourth technical solution is as follows: Based on the third technical solution, the frequency of the alternating magnetic field is 100-500kHz, and the action time is 2-8min.

[0021] The preferred fifth technical solution: Based on the fourth technical solution, the heat treatment is carried out in a protective atmosphere at 150~250°C. The protective gas is preferably nitrogen or an inert gas, and the inert gas is He or Ar.

[0022] Among the five preferred technical solutions above, the selection of materials such as nickel-cobalt bimetallic oxide nanoparticles, PMI foam, silane coupling agents, and solvents can be randomly adapted.

[0023] In this invention, for the preferred microwave absorbing material, the PMI foam is an insulating porous polymer foam with a pore size of 10-50 μm. Its pore walls exhibit swelling characteristics under the action of solvents, enabling magnetic NiCo2O4 nanoparticles to achieve spatial redistribution under the synergistic effect of magnetic field and solvent migration, thereby constructing the gradient structure. The silane coupling agent is selected from KH550, KH792, or OFS-6011 and is used to enhance the interfacial bonding ability between the inorganic functional components and the PMI foam matrix.

[0024] This invention relates to microwave absorbing composite materials: any of the above-mentioned technical solutions can be used to prepare the material. The prepared microwave absorbing material can undergo an electromagnetic response under the action of an alternating magnetic field, causing an overall temperature increase. As the temperature rises, the weak interactions between the graphene sheets, residual oxygen-containing functional groups, and silane coupling agents are enhanced, which is beneficial for the closure of microcracks within the material and the restoration of the interface structure. Therefore, after being damaged and treated with an alternating magnetic field, the electromagnetic absorption performance of the material is restored.

[0025] The microwave absorbing material of this invention differs from conductive carbon foam or carbon-based porous materials. PMI foam is an insulating polymer foam, and its pore walls exhibit certain swelling and polarity characteristics under the action of solvents. Under an applied magnetic field, it is more conducive to the spatial redistribution of magnetic NiCo2O4 nanoparticles as they migrate with the solvent. Due to the high rigidity and poor swelling properties of carbon foam materials, magnetic particles mainly exhibit surface adsorption or random distribution within them, making it difficult to form a stable gradient structure under the action of a magnetic field. Therefore, PMI foam is irreplaceable in this invention for constructing gradient microwave absorbing structures.

[0026] One specific technical solution of this invention is a method for preparing a graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material, comprising the following steps:

[0027] (1) Preparation of dispersion: Nickel-cobalt bimetallic oxide (NiCo2O4) nanoparticles and graphene oxide were dispersed in 100 mL of ethanol solvent at a mass ratio of 10:1-10:3. A silane coupling agent was added and the dispersion was ultrasonically dispersed to obtain a stable dispersion.

[0028] (2) Treatment of PMI foam: Immerse the PMI foam matrix in the dispersion. Under the condition of an external magnetic field with a fixed direction, the magnetic induction intensity is 0.2-0.8T and the magnetic field direction is consistent with the thickness direction of the PMI foam. Let it stand for 30-60 minutes, so that the NiCo2O4 nanoparticles migrate in a directional manner along the thickness direction of the PMI foam under the action of the magnetic field force, and gradually accumulate in the surface area of ​​the foam during the solvent migration process, forming an initial spatial distribution difference. The amount of PMI foam added can be added according to the theoretically designed loading amount: the total loading amount of graphene and nickel cobalt bimetallic oxide in the PMI foam is 5-15% of the mass of the PMI foam matrix.

[0029] (3) The treated PMI foam is first subjected to an alternating magnetic field at a frequency of 100-500kHz for 2-8 minutes to induce the NiCo2O4 nanoparticles to migrate and rearrange secondaryly within the foam pores, thereby stabilizing and forming a gradient distribution structure along the foam thickness direction. Then, it is freeze-dried under vacuum for 12-24 hours with the following parameters: vacuum degree ≤10Pa, temperature -40~-60℃. Subsequently, it is kept at 150-250℃ for 2-3 hours in an inert atmosphere to reduce the graphene oxide to graphene, thus obtaining the composite microwave absorbing material. After oxidation-reduction, the lateral dimension of the graphene sheets in the composite microwave absorbing material is 1-5μm.

[0030] The present invention presents a gradient distribution structure of NiCo2O4 nanoparticles and graphene in PMI foam pores, which gradually decreases from the surface to the interior along the thickness direction of the foam, and this gradient structure remains stable after heat treatment.

[0031] Compared with the prior art, the beneficial effects of the present invention are:

[0032] (1) By introducing a directional magnetic field and an alternating magnetic field during the preparation process, the present invention induces magnetic nickel-cobalt bimetallic oxide nanoparticles to migrate and rearrange in porous PMI foam in a synergistic effect, thereby constructing a gradient distribution structure that exists stably along the thickness direction of the foam, thereby improving the impedance matching characteristics of the material in different frequency bands and broadening the effective absorption band of the material.

[0033] (2) This invention utilizes nickel-cobalt bimetallic oxide and graphene to form a magnetic-electric coupling microwave absorption system, which generates a magnetic response temperature rise under the action of an alternating magnetic field, plays a positive role in the recovery of micro-cracks and interface structures inside the material, and improves the material's ability to maintain microwave absorption performance under damaged conditions.

[0034] (3) The present invention uses PMI foam as the matrix, which achieves the synergy of wave absorption performance and structural buffering performance while ensuring the lightweight and structural stability of the material. It is suitable for application scenarios with strict requirements on weight and electromagnetic performance.

[0035] (4) The preparation method of the present invention is reasonably designed and the conditions are mild. Attached Figure Description

[0036] Figure 1 The image shows a transmission electron microscope (TEM) image of the composite absorbing material (1.5 mm thick) prepared in Example 1.

[0037] Figure 2 The image shows the X-ray diffraction (XRD) pattern of the composite absorbing material (1.5 mm thick) prepared in Example 1.

[0038] Figure 3 The graph shows the reflection loss-frequency curves of composite absorbing material samples of different thicknesses prepared in Example 1. Detailed Implementation

[0039] The present invention will be further described below with reference to specific embodiments.

[0040] Unless otherwise specified, all raw materials used in this invention are commercially available.

[0041] The nickel-cobalt bimetallic oxide nanoparticles acquired were spinel-structured NiCo2O4 with a particle size of 10-20 nm.

[0042] Example 1

[0043] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: NiCo2O4 nanoparticles and graphene oxide are dispersed in 100mL of ethanol at a mass ratio of 10:2 (total mass 1g), and KH792 with 7.5% ethanol mass is added. The dispersion is prepared by ultrasonic dispersion for 30min; a PMI foam matrix with a pore size of 10μm is immersed in the dispersion, and under a fixed magnetic field of 0.5T, it is left to stand for 45min to allow the NiCo2O4 nanoparticles to migrate directionally along the thickness direction of the PMI foam. Then, an alternating magnetic field with a frequency of 300kHz is applied for 2min to stabilize and form a gradient distribution structure. Subsequently, it is freeze-dried in vacuum at a temperature of -40~-60℃ for 18h with a vacuum degree ≤10Pa, and then kept at 150℃ for 2.5h under a nitrogen atmosphere to reduce the graphene oxide to graphene, thereby obtaining the composite microwave absorbing material.

[0044] Transmission electron microscopy (TEM) image of the prepared composite absorbing material is shown below. Figure 1 As shown, NiCo2O4 nanoparticles are uniformly dispersed on the surface of graphene sheets and within the pores of PMI foam, exhibiting a clear gradient distribution along the thickness direction of the material. The surface particles are enriched, while the internal particle density gradually decreases, without any agglomeration. The transverse dimensions of the graphene sheets are 1-5 μm, forming a tight interfacial bond with the NiCo2O4 nanoparticles.

[0045] The X-ray diffraction (XRD) spectrum of the prepared composite absorbing material is shown below. Figure 2 As shown, the spectrum shows characteristic diffraction peaks of spinel structure NiCo2O4 (corresponding to crystal planes (111), (220), (311), (400), (511), etc.), proving that NiCo2O4 phase is pure and has good crystallinity; at the same time, characteristic diffraction peaks of graphene (corresponding to crystal plane (002)) are also present, indicating that graphene oxide was successfully reduced to graphene after heat treatment; no impurity diffraction peaks are present, indicating that the silane coupling agent did not introduce new phases, and the interfaces of each component are stable with no obvious phase separation phenomenon.

[0046] The composite absorbing material obtained in Example 1 was cut along the thickness direction using a precision cutting machine to obtain 10 samples with thicknesses of 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, 3.0 mm, 3.5 mm, 4.0 mm, 4.5 mm, 5.0 mm, and 5.5 mm. The curves showing the variation of reflection loss of the products with different thicknesses as a function of frequency are shown below. Figure 3As shown, samples of different thicknesses exhibit excellent absorption performance in the 2-40GHz frequency band. Among them, the sample with a thickness of 1.5mm has the widest effective absorption frequency band (RL<-10dB), reaching 4.2-32.5GHz, with a minimum reflection loss value as low as -37.4dB. As the sample thickness increases, the effective absorption frequency band shifts towards lower frequencies, but the overall absorption bandwidth remains above 32GHz, verifying the optimization effect of the gradient structure of this invention on impedance matching in different frequency bands and achieving wideband high-efficiency absorption.

[0047] Example 2

[0048] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: NiCo2O4 nanoparticles and graphene oxide are dispersed in 100mL of ethanol at a mass ratio of 10:1 (total mass 3g), and 5% KH550 by mass of ethanol is added. The dispersion is prepared by ultrasonic dispersion for 30min; a PMI foam matrix with a pore size of 30μm is immersed in the dispersion, and under a fixed magnetic field of 0.2T, it is left to stand for 30min to allow the NiCo2O4 nanoparticles to migrate directionally along the thickness direction of the PMI foam. An alternating magnetic field with a frequency of 500kHz is applied and treated for 5min to stabilize and form a gradient distribution structure. Then, the material is freeze-dried in vacuum at a vacuum degree ≤10Pa and a temperature of -40~-60℃ for 12h, and then kept at 250℃ for 2h in a nitrogen atmosphere to reduce the graphene oxide to graphene, thereby obtaining the composite microwave absorbing material.

[0049] Example 3

[0050] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: NiCo2O4 nanoparticles and graphene oxide are dispersed in 100mL of ethanol at a mass ratio of 10:3 (total mass 1g), and OFS-6011 with 10% ethanol mass is added. The dispersion is prepared by ultrasonic dispersion for 30min; a PMI foam matrix with a pore size of 50μm is immersed in the dispersion, and under a fixed magnetic field of 0.8T, it is left to stand for 60min to allow the NiCo2O4 nanoparticles to migrate directionally along the thickness direction of the PMI foam. An alternating magnetic field with a frequency of 100kHz is applied and treated for 8min to stabilize and form a gradient distribution structure. Then, it is freeze-dried in vacuum for 24h and kept at 200℃ for 3h under a nitrogen atmosphere to reduce the graphene oxide to graphene, thereby obtaining the composite microwave absorbing material.

[0051] Example 4

[0052] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: NiCo2O4 nanoparticles and graphene oxide are dispersed in 100mL of ethanol at a mass ratio of 10:3 (total mass 2g), and 5% KH550 by mass of ethanol is added. The dispersion is prepared by ultrasonic dispersion for 30min; a PMI foam matrix with a pore size of 10μm is immersed in the dispersion, and under a fixed magnetic field of 0.5T, it is left to stand for 30min to allow the NiCo2O4 nanoparticles to migrate directionally along the thickness direction of the PMI foam. An alternating magnetic field with a frequency of 300kHz is applied for 5min to stabilize and form a gradient distribution structure. Then, the material is freeze-dried in vacuum at a vacuum degree ≤10Pa and a temperature of -40~-60℃ for 12h, and then kept at 150℃ for 2h in a nitrogen atmosphere to reduce the graphene oxide to graphene, thereby obtaining the composite microwave absorbing material.

[0053] Example 5

[0054] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: NiCo2O4 nanoparticles and graphene oxide are dispersed in 100mL of ethanol at a mass ratio of 10:2 (total mass 3g), and OFS-6011 with 10% ethanol mass is added. The dispersion is prepared by ultrasonic dispersion for 30min; a PMI foam matrix with a pore size of 30μm is immersed in the dispersion, and under a fixed magnetic field of 0.2T, it is left to stand for 60min to allow the NiCo2O4 nanoparticles to migrate directionally along the thickness direction of the PMI foam. An alternating magnetic field with a frequency of 500kHz is applied for 2min to stabilize and form a gradient distribution structure. Then, the material is freeze-dried in vacuum at a vacuum degree ≤10Pa and a temperature of -40~-60℃ for 24h, and then kept at 250℃ for 3h in a nitrogen atmosphere to reduce the graphene oxide to graphene, thereby obtaining the composite microwave absorbing material.

[0055] Example 6

[0056] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: NiCo2O4 nanoparticles and graphene oxide are dispersed in 100mL of ethanol at a mass ratio of 10:1 (total mass 1g), and KH792 with 7.5% ethanol mass is added. The dispersion is prepared by ultrasonic dispersion for 30min; a PMI foam matrix with a pore size of 50μm is immersed in the dispersion, and under a fixed magnetic field of 0.8T, it is left to stand for 45min to allow the NiCo2O4 nanoparticles to migrate directionally along the thickness direction of the PMI foam. An alternating magnetic field with a frequency of 100kHz is applied and treated for 8min to stabilize and form a gradient distribution structure. Then, the material is freeze-dried in vacuum at a vacuum degree ≤10Pa and a temperature of -40~-60℃ for 18h, and then kept at 200℃ for 2.5h in a nitrogen atmosphere to reduce the graphene oxide to graphene, thereby obtaining the composite microwave absorbing material.

[0057] Example 7

[0058] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: NiCo2O4 nanoparticles and graphene oxide are dispersed in 100mL of ethanol at a mass ratio of 10:2 (total mass 2g), and OFS-6011 with 10% ethanol mass is added. The dispersion is prepared by ultrasonic dispersion for 30min; a PMI foam matrix with a pore size of 10μm is immersed in the dispersion, and under a fixed magnetic field of 0.5T, it is left to stand for 45min to allow the NiCo2O4 nanoparticles to migrate directionally along the thickness direction of the PMI foam. An alternating magnetic field with a frequency of 300kHz is applied for 5min to stabilize and form a gradient distribution structure. Then, the material is freeze-dried in vacuum at a vacuum degree ≤10Pa and a temperature of -40~-60℃ for 18h, and then kept at 150℃ for 2.5h in a nitrogen atmosphere to reduce the graphene oxide to graphene, thereby obtaining the composite microwave absorbing material.

[0059] Example 8

[0060] The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material of the present invention includes the following steps: NiCo2O4 nanoparticles and graphene oxide are dispersed in 100mL of ethanol at a mass ratio of 10:3 (total mass 3g), and KH792 with 7.5% ethanol mass is added. The dispersion is prepared by ultrasonic dispersion for 30min; a PMI foam matrix with a pore size of 30μm is immersed in the dispersion, and under a fixed magnetic field of 0.2T, it is left to stand for 45min to allow the NiCo2O4 nanoparticles to migrate directionally along the thickness direction of the PMI foam. An alternating magnetic field with a frequency of 500kHz is applied for 8min to stabilize and form a gradient distribution structure. Then, the material is freeze-dried in vacuum at a vacuum degree ≤10Pa and a temperature of -40~-60℃ for 24h, and then kept at 250℃ for 2h in a nitrogen atmosphere to reduce the graphene oxide to graphene, thereby obtaining the composite microwave absorbing material.

[0061] Comparative Example 1

[0062] In this comparative example, no NiCo2O4 nanoparticles were introduced during the preparation process. The remaining preparation steps were the same as in Example 1, as follows: 1g of graphene oxide was dispersed in 100mL of ethanol, and 7.5% KH792 by mass of ethanol was added. The dispersion was prepared by ultrasonic dispersion for 30min. A PMI foam matrix with a pore size of 10μm was immersed in the dispersion. Under a fixed magnetic field of 0.5T, it was allowed to stand for 45min and then treated with an alternating magnetic field with a frequency of 300kHz for 5min. Subsequently, it was freeze-dried in vacuum for 18h and kept at 150℃ for 2.5h under a nitrogen atmosphere to reduce graphene oxide to graphene, thus obtaining a graphene / PMI composite microwave absorbing material.

[0063] Since magnetic nickel-cobalt bimetallic oxide nanoparticles were not introduced in this comparative example, even if an external magnetic field was applied during the preparation process, it was difficult to induce effective directional migration and rearrangement of functional components within the PMI foam pores. As a result, the functional components inside the material mainly exhibited an approximately uniform distribution.

[0064] Comparative Example 2

[0065] In this comparative example, no graphene oxide was introduced during the preparation process. The remaining preparation steps were the same as in Example 1 of this invention, as follows: 1g of NiCo2O4 nanoparticles were dispersed in 100mL of ethanol, and 7.5% KH792 by mass of ethanol was added. The dispersion was prepared by ultrasonic dispersion for 30min. A PMI foam matrix with a pore size of 10μm was immersed in the dispersion. Under a fixed magnetic field of 0.5T, it was allowed to stand for 45min and then treated with an alternating magnetic field with a frequency of 300kHz for 5min. Subsequently, it was freeze-dried in vacuum for 18h and kept at 150℃ for 2.5h under a nitrogen atmosphere to obtain the NiCo2O4 / PMI composite microwave absorbing material.

[0066] Since graphene was not introduced in this comparative example, the composite system lacked an effective dielectric loss component. Although the magnetic NiCo2O4 nanoparticles could migrate to some extent under the influence of a magnetic field, it was difficult to form a synergistic effect with the dielectric loss mechanism, thus limiting the broadband microwave absorption performance of the material. Furthermore, after the material was subjected to microscopic damage, even with alternating magnetic field treatment, the lack of interfacial flexibility and thermal conduction pathways provided by graphene sheets and their residual functional groups resulted in limited recovery of the microwave absorption performance.

[0067] Comparative Example 3

[0068] In this comparative example, no fixed magnetic field was used during the preparation process; only mechanical mixing was performed. The remaining preparation steps were the same as in Example 1 of this invention, as follows: NiCo2O4 nanoparticles and graphene oxide were dispersed in 100 mL of ethanol at a mass ratio of 10:2 (total mass 1 g), and 7.5% KH792 (by mass of ethanol) was added. The mixture was ultrasonically dispersed for 30 min to obtain a dispersion. A PMI foam matrix with a pore size of 10 μm was immersed in the dispersion and mechanically stirred for 45 min. Then, an alternating magnetic field with a frequency of 300 kHz was applied for 2 min, followed by vacuum freeze-drying for 18 h and heat treatment at 150 °C for 2.5 h under a nitrogen atmosphere to reduce graphene oxide to graphene, thus obtaining a composite microwave absorbing material.

[0069] Comparative Example 4

[0070] In this comparative example, no alternating magnetic field treatment was performed during the preparation process; only mechanical mixing was conducted. The remaining preparation steps were the same as in Example 1 of this invention, as follows: NiCo2O4 nanoparticles and graphene oxide were dispersed in 100mL of ethanol at a mass ratio of 10:2 (total mass 1g), and 7.5% KH792 by mass of ethanol was added. The dispersion was prepared by ultrasonic dispersion for 30min. A PMI foam matrix with a pore size of 10μm was immersed in the dispersion and allowed to stand for 45min under a fixed magnetic field of 0.5T. Then, it was mechanically stirred for 2min, followed by vacuum freeze-drying for 18h and heat treatment at 150℃ for 2.5h under a nitrogen atmosphere to reduce graphene oxide to graphene, thus obtaining a composite microwave absorbing material.

[0071] Comparative Example 5

[0072] In this comparative example, no fixed magnetic field treatment or alternating magnetic field treatment was performed during the preparation process; only mechanical mixing was performed. The remaining preparation steps were the same as in Example 1 of this invention, as follows: NiCo2O4 nanoparticles and graphene oxide were dispersed in 100mL of ethanol at a mass ratio of 10:2 (total mass 1g), and 7.5% KH792 by mass of ethanol was added. The dispersion was prepared by ultrasonic dispersion for 30min. A PMI foam matrix with a pore size of 10μm was immersed in the dispersion and mechanically stirred for 47min. Then, it was freeze-dried under vacuum for 18h and kept at 150℃ for 2.5h under a nitrogen atmosphere to reduce graphene oxide to graphene, thus obtaining a composite microwave absorbing material.

[0073] Comparative Example 6

[0074] In this comparative example, the PMI foam matrix was replaced with porous carbon foam (10 μm pore size) during the preparation process. The remaining preparation steps were the same as in Example 1 of this invention, as follows: NiCo2O4 nanoparticles and graphene oxide were dispersed in 100 mL of ethanol at a mass ratio of 10:2 (total mass 1 g), and 7.5% KH792 by mass of ethanol was added. The dispersion was prepared by ultrasonic dispersion for 30 min. The porous carbon foam matrix with a pore size of 10 μm was immersed in the dispersion and allowed to stand for 45 min under a fixed magnetic field of 0.5 T. Then, an alternating magnetic field with a frequency of 300 kHz was applied for 2 min. Subsequently, it was freeze-dried in vacuum for 18 h and kept at 150 °C for 2.5 h under a nitrogen atmosphere to reduce graphene oxide to graphene, thus obtaining the composite microwave absorbing material.

[0075] The microwave absorbing materials prepared in the above embodiments and comparative examples were tested for microwave absorption performance and microwave absorption recovery rate after damage.

[0076] Radar absorption recovery rate after damage: Reflection loss value test: According to GJB2038A-2011 "Test Method for Reflectivity of Radar Absorbing Materials", the sample to be tested was processed into a square sample of 200mm×200mm (thickness is the target thickness prepared in each embodiment / comparative example). With a standard good conductor metal plate of the same size as a reference, the return power of the sample was measured in the 2-40GHz frequency band. The minimum reflection loss value (RLmin) and effective absorption frequency band (RL<-10dB) of the material were obtained by comparison and calculation. During the test, each group of samples was tested repeatedly for no less than 5 times, and the average value of the minimum reflection loss value in the 5 tests was taken as the final result (the effective absorption frequency band is the intersection range of the 5 tests).

[0077] Reflection loss value: The same standard (GJB2038A-2011) was used for the reflection loss value test. First, the minimum reflection loss value (RL damage) of the material after artificially simulating micro-damage (slightly polishing the surface with sandpaper to create micro-cracks at the interface) was measured. Then, after being treated with an alternating magnetic field for 5 minutes in the preparation parameters of the corresponding example / comparative example, the minimum reflection loss value (RL recovery) was measured. The recovery rate was calculated as follows: recovery rate (%) = (RL recovery - RL damage) / (RL initial - RL damage) × 100%, where RL initial is the minimum reflection loss value of the material in the undamaged state.

[0078] The results are shown in Table 1.

[0079] Table 1. Comparison of electromagnetic properties and structural stability of composite absorbing materials in different embodiments and comparative examples.

[0080]

[0081] As can be seen from the above, Comparative Examples 1, 2, 3, and 5 lack gradient structures and have no recovery ability after damage. Comparative Example 4 has a low recovery rate and an unstable gradient structure. Comparative Example 6, with its carbon foam, lacks swelling properties, cannot form a gradient structure, and has a low recovery rate. This invention, by simultaneously introducing graphene and magnetic nickel-cobalt bimetallic oxides and inducing their directional migration and rearrangement within the foam pores using a magnetic field during preparation, forms a gradient distribution structure that continuously varies along the material thickness direction, thereby achieving a synergistic effect between dielectric loss and magnetic response loss. This synergistic effect cannot be achieved by a single functional component or a simple parallel combination.

Claims

1. A method for preparing a graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material, characterized in that, Includes the following steps: Nickel-cobalt bimetallic oxide nanoparticles and graphene oxide were dispersed in ethanol, and a silane coupling agent was added. The mixture was then treated to obtain a dispersion. PMI foam was immersed in the dispersion and treated under an external directional magnetic field. The treated PMI foam was then subjected to an alternating magnetic field and then freeze-dried in vacuum. The freeze-dried material is heat-treated to obtain a graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material; the magnetic induction intensity of the directional magnetic field is 0.2-0.8T, and the direction of the magnetic field is consistent with the thickness direction of the PMI foam; the frequency of the alternating magnetic field is 100-500kHz.

2. The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material according to claim 1, characterized in that, The total loading of graphene and nickel-cobalt bimetallic oxides in the resulting composite microwave absorbing material is 5-15% of the mass of the PMI foam matrix.

3. The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material according to claim 1, characterized in that, The alternating magnetic field is applied for 2-8 minutes.

4. The preparation method of the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material according to claim 3, characterized in that, The heat treatment is performed in a protective atmosphere at 150~250°C.

5. The method for preparing the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material according to any one of claims 1-4, characterized in that, The nickel-cobalt bimetallic oxide nanoparticles are spinel structure NiCo2O4 with a particle size of 10-20 nm.

6. The method for preparing the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material according to any one of claims 1-4, characterized in that, The pore size of PMI foam is 10-50 μm.

7. The method for preparing the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material according to any one of claims 1-4, characterized in that, The silane coupling agent is one of KH550, KH792 or OFS-6011.

8. A graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material, characterized in that: It is prepared by the method of graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material according to claim 1.

9. A method for repairing the graphene / nickel-cobalt bimetallic oxide / PMI foam composite microwave absorbing material as described in claim 8, characterized in that: After the absorbing material absorbs waves, it is repaired by being treated in an alternating magnetic field of 100-500kHz.

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