Super-hydrophobic four-layer electromagnetic wave-absorbing coating with magnetoelectric synergistic mechanism

Abstract

The invention discloses a four-layer composite electromagnetic wave-absorbing coating applicable to 2-18GHz broadband electromagnetic wave absorption. The four-layer composite electromagnetic wave-absorbing coating sequentially comprises a super-hydrophobic protection layer, a high-frequency absorption layer, an intermediate-frequency absorption layer and a low-frequency enhancement layer from outside to inside. Each functional layer adopts a ternary composite absorbent system and is composed of surface-modified nano Fe3O4, graphene and carbon nanotubes, and accurate absorption matching of electromagnetic waves of different frequency bands is achieved by regulating and controlling the mass ratio of the three components. The wave-absorbing structure cooperatively designs magnetic loss and electrical loss paths, so that the overall impedance matching and energy dissipation capabilities are improved; the low-frequency layer strengthens magnetic loss, the intermediate-frequency layer optimizes magnetic-electricity synergistic absorption, and the high-frequency layer constructs a stable conductive network. The external super-hydrophobic layer adopts PDMS and nano SiO2 to cooperatively construct a micro-nano coarse structure, so that the self-cleaning characteristic that the water contact angle is greater than 155 degrees and the rolling angle is less than 5 degrees is realized, and the service stability of the coating system in severe environments such as high salt mist and high damp and hot environments is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of composite functional materials for electromagnetic wave absorption and environmental protection, and specifically to a multilayer composite electromagnetic absorbing coating with a gradient electromagnetic loss control structure and super-hydrophobic protective performance and a preparation method thereof. The coating is particularly suitable for protective absorbing applications in broadband radar stealth, high humidity and high salt fog environments. Background Art

[0002] Currently, broadband, high-efficiency, and environmentally adaptable electromagnetic wave absorbing materials have important applications in radar stealth, shipborne platforms, and electromagnetic compatibility protection. However, existing absorbing materials generally suffer from the following technical bottlenecks: First, traditional single-layer structures or homogeneous composite systems struggle to simultaneously achieve effective coverage of multiple frequency bands, such as S, C, X, and Ku, limiting the absorption band. Second, the lack of coordinated design of magnetic and electrical loss units results in poor impedance matching, which can easily lead to enhanced reflection or attenuated absorption. Third, the material structure is monolithic, lacking a mechanism for gradually adjusting electromagnetic properties for different frequency bands.

[0003] Furthermore, absorbing coatings often operate in complex environments such as high salt spray, high humidity and heat, and severe corrosion. Conventional structures are prone to shedding, cracking, and performance degradation over long periods of service, severely limiting their engineering applications. Existing superhydrophobic coatings are often implemented as additional surface modifications, lacking system integration, making it difficult to balance absorbing performance with environmental protection.

[0004] On the other hand, while commonly used magnetic absorbers (such as Fe₃O₄) have good magnetic loss capacity, their response is limited at high frequencies and they tend to aggregate within the resin matrix. Conductive fillers such as graphene and carbon nanotubes, while possessing strong dielectric loss capacity, suffer from poor dispersion and are prone to forming local short-circuit paths. Currently, a multi-layered, multi-component, and tunable synergistic absorber structure is lacking.

[0005] Therefore, there is an urgent need to develop a composite absorbing coating system that has the ability to control the gradient of electromagnetic properties, integrates superhydrophobic protection and flexible structural support, achieves efficient absorption in the 2-18 GHz wide frequency band, and meets the stability and reliability requirements in complex service environments.

[0006] The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Summary of the Invention

[0007] To address the narrow absorption bandwidth, weak structural adhesion, and poor environmental adaptability of existing electromagnetic absorbing coatings under complex operating conditions, the present invention provides a four-layer composite absorbing coating with a gradient electromagnetic loss control structure and superhydrophobic protection, and its preparation method. The coating utilizes a superhydrophobic layer, a high-frequency absorption layer, a mid-frequency absorption layer, and a low-frequency enhancement layer, constructed from the outside in. By varying the ratio of a ternary composite absorber (modified Fe₃O₄, graphene, and carbon nanotubes) in each functional layer, broadband absorption is achieved in the 2-18 GHz range.

[0008] The coating consists of four functional layers built up from the outside to the inside:

[0009] L1 layer: super hydrophobic protective layer

[0010] A micro-nano dual-scale rough structure is constructed using polydimethylsiloxane (PDMS), nano-SiO2, polyurethane PU and other components to achieve self-cleaning properties with a water contact angle greater than 155° and a rolling angle less than 5°, significantly improving the service stability and anti-pollution ability of the coating in high humidity, heat, high salt fog and other environments.

[0011] L2 layer: high frequency absorption layer

[0012] The mass ratio of the ternary composite absorber is Fe3O4: graphene: carbon nanotube = 55-60:22.5-20:22.5-20. It focuses on electrical loss, constructs a conductive network structure, and enhances the absorption effect in the Ku / X band (12-18GHz).

[0013] L3 layer: mid-frequency absorption layer

[0014] The absorber ratio is adjusted to Fe3O4: graphene: carbon nanotube = 65-70:17.5-15:17.5-15, with magneto-electric synergistic loss as the main factor, meeting the composite performance requirements of the C / X band (6-12GHz).

[0015] L4 layer: low-frequency enhancement layer

[0016] The absorber ratio is Fe3O4: graphene: carbon nanotube = 75-80: 12.5-10: 12.5-10, mainly based on magnetic loss, and enhances the natural resonance and interface polarization absorption capacity of the S band (2-6GHz).

[0017] In the ternary absorbent, Fe3O4 provides magnetic loss, and graphene and carbon nanotubes construct a two-dimensional / one-dimensional conductive network to form an electric loss mechanism. The ratio is set as: high-frequency layer 60:20:20, medium-frequency layer 70:15:15, and low-frequency layer 80:10:10 to meet the μ'' / ε'' matching requirements of different frequency bands. The three-layer absorption structure is uniformly based on a flexible modified epoxy resin, which contains carboxyl nitrile rubber CTBN, glycerol glycidyl ether GGE and polyether amine to realize good substrate adhesion and fracture ductility.

[0018] The three-layer absorption structure described above is based on a flexible epoxy resin, and carboxyl nitrile rubber CTBN, glycerol glycidyl ether (GGE) and polyether amine are introduced into the matrix to significantly improve the interfacial adhesion and crack resistance, meeting the service requirements in complex mechanical environments.

[0019] The total thickness of the coating of the present application is controlled to be 2.0-3.0mm, which is constructed in layers by using high-pressure airless spraying, and the film density and interfacial strong combination are realized by the "room temperature→40°C→60°C→80°C" gradient curing strategy.

[0020] A preparation method of a composite electromagnetic wave absorbing coating, comprising the following steps:

[0021] S1: obtaining surface modified Fe3O4 based on nano-Fe3O4, citric acid and silane coupling agent KH-550;

[0022] S2: dispersing graphene and carbon nanotubes in solvents to obtain graphene suspension and carbon nanotube suspension, mixing and stirring the modified Fe3O4 and the two conductive fillers in proportion to obtain a ternary synergistic absorbent system;

[0023] S3: adding the ternary composite absorbent into a flexible modified epoxy resin, using mechanical stirring and high shear dispersion, then sequentially adding a curing agent, a diluent and an additive, and after defoaming treatment, a compact electromagnetic wave absorbing coating system is prepared.

[0024] In the preparation method of the electromagnetic wave absorbing coating, step S1 includes mixing nano-Fe3O4, citric acid and silane coupling agent KH-550 in ethanol, mechanically stirring at 60°C for 30 minutes, centrifuging after ultrasonic treatment for 30 minutes, drying and washing to obtain surface modified Fe3O4.

[0025] In the preparation method of the electromagnetic wave absorbing coating, the particle size of the surface modified Fe3O4 is 20-50nm, the graphene has a sheet structure with a sheet size of ≤10μm; the carbon nanotube is a tubular conductive filler with a length of 10-20μm and a diameter of 30-50nm, having good specific surface area and network interconnection capability.

[0026] The preparation method of the electromagnetic wave absorbing coating comprises the following steps: a carboxyl nitrile rubber (CTBN) and glycerol glycidyl ether (GGE) are premixed to form a homogeneous system, then E-42 epoxy resin is added and stirred, and then polyether amine is introduced to further toughen, and finally the homogeneous system is deaerated and left to stand to form a coating layer with excellent mechanical properties.

[0027] An electromagnetic wave absorbing coating is prepared according to a four-layer composite structure, and comprises, from inside to outside, a low-frequency enhancement layer, a medium-frequency absorbing layer, a high-frequency absorbing layer and a super-hydrophobic protective layer.

[0028] An electromagnetic wave absorbing coating is prepared by layering and spraying the coating and gradient curing, and the outer layer is a super-hydrophobic functional finish.

[0029] The absorbing coating has a water contact angle greater than 155° and a rolling angle less than 5°, and has self-cleaning properties, and is suitable for complex service environments such as high humidity and high salt fog, and can maintain corrosion resistance and anti-pollution properties for a long time.

[0030] The present application realizes innovation in the following aspects:

[0031] Gradual change in structural levels: by constructing four functional zones, the response matching of different frequency bands of electromagnetic waves is realized, and the problem of narrow absorption band of the single-layer homogeneous absorption structure is avoided;

[0032] Absorbent ratio control mechanism: the ratio of Fe3O4 / Gr / CNT of the ternary composite absorbent is adjusted in different layers to realize gradient matching and composite synergy of magnetic and electric loss;

[0033] Super-hydrophobic coating integrated structure: the external protective layer and the absorbing functional layer are integrated to realize the triple functions of absorbing, self-cleaning and corrosion resistance;

[0034] Flexible matrix enhanced interface bonding: the problem of large rigidity and easy falling of traditional epoxy is overcome, and the overall

[0035] Environmentally friendly process and engineering compatibility: all components meet environmental standards, and the process is suitable for conventional high-pressure airless spraying equipment, and has good promotion adaptability.

[0036] The measured results show that the coating has a minimum reflection loss RL of less than -30dB in the 2-18GHz frequency band, an absorption bandwidth greater than 7GHz, and passes a 1000h salt spray test without any shedding or blistering, demonstrating the coordinated stability of structure and performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are intended only to illustrate preferred embodiments and are not to be construed as limiting the present invention. It should be understood that the drawings described below are merely examples of the present invention, and that those skilled in the art will be able to derive other drawings from these drawings without inventive effort. Throughout the drawings, identical reference numerals are used to denote identical components.

[0038] In the attached figure:

[0039] Figure 1 1 is a schematic diagram of the process flow of an embodiment of the present invention;

[0040] Figure 2 Schematic diagram of the ternary synergistic network structure of the ternary composite absorbent of the present invention;

[0041] Figure 3 Schematic diagram of the structure of the multi-layer gradient absorbing coating of the present invention;

[0042] Figure 4 It is a comparison chart of reflection loss of the ternary composite absorber of the present invention.

[0043] The present invention will be further explained below with reference to the accompanying drawings and embodiments. DETAILED DESCRIPTION

[0044] 1. Derivation basis of the mass ratio range of magnetic loss and electrical loss

[0045] The present invention is based on the impedance matching and energy balanced dissipation principle of electromagnetic wave absorbing materials. By analyzing the electromagnetic parameters of the magnetic absorber nano-ferroferric oxide (Fe3O4) and the electrical absorber graphene (Gr) and carbon nanotube (CNT), a reasonable mass ratio range is derived.

[0046] 1. Reflection loss (RL) and impedance matching

[0047] According to the reflection loss formula of electromagnetic waves in absorbing materials:

[0048]

[0049] where Z0=377 Ω is the characteristic impedance of free space, Z in is the input impedance of the material. In the design of wave-absorbing materials, the input impedance Z in is required to be as close to Z0as possible to achieve minimum reflection loss, i.e.,

[0050]

[0051] According to the impedance matching condition, the real and imaginary parts of complex permeability μ r =μ'-jμ'' and complex permittivity ε r =ε'-jε'' need to satisfy:

[0052]

[0053] 2. Relationship between power loss and material proportion

[0054] The energy loss P loss per unit volume when electromagnetic waves pass through the material is represented as the sum of dielectric loss and magnetic loss:

[0055]

[0056] where ε'' represents the dielectric loss factor and μ'' represents the magnetic loss factor. Combining the relationship between electric field and magnetic field in electromagnetic field, we get:

[0057]

[0058] Since Z0=377 Ω, this proportion value indicates that the contribution of magnetic loss to the total energy is usually much lower than that of electric loss. Therefore, it is necessary to increase the mass proportion of magnetic material (Fe3O4) to achieve balanced absorption of electromagnetic energy.

[0059] 3. Mass proportion of nano-Fe3O4, graphene (Gr), and carbon nanotube (CNT)

[0060] Through experiments and literature research, the following typical material parameters (complex permittivity ε and complex permeability μ) are obtained:

[0061]

[0062] According to the Effective Medium Theory (EMT), the electromagnetic properties of a material can be calculated by weighting the volume fractions (approximately replaced by mass fractions) of different components. Through derivation and experimental verification, it is determined that the mass proportion of the three should satisfy the following range:

[0063] Modified nano-Fe3O4: (Gr+CNT) = (55%~80%): (20%~45%)

[0064] Among them, the mass proportion of modified nano-Fe3O4 should be controlled between 55% and 85% as the dominant magnetic loss component; the combined mass proportion of graphene and carbon nanotubes should be controlled between 20% and 45% to ensure synergistic enhancement of electrical loss and good electromagnetic wave energy absorption.

[0065] 2. The basis for the 1:1 mass ratio of graphene to carbon nanotubes

[0066] Graphene (Gr) and carbon nanotubes (CNTs) act as electrical absorbers, providing dielectric loss (ε'') and interfacial polarization effects, respectively. The two are complementary in structure and function.

[0067] Graphene: Graphene has extremely high electrical conductivity and good dielectric properties, primarily contributing to the dielectric loss of composite materials. Its two-dimensional structure can form a stable conductive network, increasing the complex dielectric constant ε′ and dielectric loss ε″;

[0068] Carbon nanotubes: The "one-dimensional structure" of carbon nanotubes gives them excellent "interface polarization" capabilities, which can enhance the material's electrical loss effect, especially playing an important role in the formation of multi-interface effects and current-carrying paths.

[0069] Using a 1:1 mass ratio of graphene to carbon nanotubes has the following advantages:

[0070] 1. Synergistic Effect: Graphene and carbon nanotubes work together to enhance the composite's electrical conductivity, interfacial polarization, and electrical loss. Graphene improves overall electrical loss through its high conductivity, while carbon nanotubes provide enhanced interfacial polarization through their one-dimensional structure. The two complement each other, preventing the aggregation and performance degradation that can result from excessive accumulation of a single filler.

[0071] 2. Stability: A 1:1 mass ratio effectively prevents the agglomeration of graphene and carbon nanotubes that can occur when used separately, while maintaining the stability of the electrical loss system. This ratio ensures excellent dispersion of the electrical filler, ensuring that the composite maintains excellent electromagnetic wave absorption performance over a wide range (8-18 GHz).

[0072] 3. Performance optimization: By combining graphene and carbon nanotubes in a 1:1 mass ratio, a relatively balanced electrical loss performance can be achieved in different frequency bands, with good broadband absorption performance in the 8-18 GHz range.

[0073] Therefore, the present invention preferably adopts a mass ratio of 1:1 between graphene and carbon nanotubes to ensure that the composite material maintains good electromagnetic loss and impedance matching characteristics while efficiently absorbing waves.

[0074] Specific embodiments of the present application will be described in greater detail hereinafter with reference to the drawings. While the present application is illustrated by the described embodiments, it will be appreciated that it can be implemented in various forms and should not be limited to the embodiments set forth in this disclosure. Rather, these embodiments are provided as a full disclosure and complete understanding of the present application.

[0075] It should be noted that some terms are used in the specification and claims to refer to certain components. It will be understood by those skilled in the art that the same component can be referred to by different names. The specification and claims should not be construed as limited to the components named by the name. Instead, the components are distinguished by their functions. As mentioned throughout the specification and claims, "including" or "comprising" is an open term, which should be interpreted as "including but not limited to." The subsequent description is a preferred embodiment of the present application, which is intended to illustrate the general principles of the specification, but not to limit the scope of the present application. The scope of the present application is defined by the appended claims.

[0076] In order to facilitate the understanding of the embodiments of the present application, the following will be further explained with specific examples combined with the drawings, and each drawing does not constitute a limitation to the embodiments of the present application.

[0077] As shown in Figures 1 to 4 The electromagnetic wave absorbing coating preparation method comprises the following steps:

[0078] S1: obtaining surface modified Fe3O4 based on nano Fe3O4, citric acid and silane coupling agent KH-550;

[0079] S2: dispersing graphene and carbon nanotubes in solvents respectively to obtain graphene suspension and carbon nanotube suspension, and stirring and mixing the surface modified Fe3O4, graphene suspension and carbon nanotube suspension to obtain a ternary composite absorbent;

[0080] S3: Add the ternary composite absorber to the epoxy resin matrix, stir and disperse, then add the curing agent and coating additives, and degas to form the electromagnetic absorbing coating. The curing agent is T31 epoxy curing agent (an amine curing agent), which promotes crosslinking and curing of the E-42 epoxy resin at a gradient temperature from room temperature to 80°C, forming a dense, flexible network. The curing agent is present in an amount of 25 to 35 parts by weight, preferably 30 parts. The coating additive is BYK-9076, a polymeric nonionic dispersant. Its block copolymer structure effectively disperses graphene, carbon nanotubes, and nanomagnetic particles, while improving the coating's fluidity and surface smoothness. It is non-migrating, non-reactive, and does not interfere with the coating's electromagnetic properties. It is particularly suitable for the high-solid composite absorbing coating system constructed in this invention. The coating additive is present in an amount of 1 to 2 parts by weight, preferably 1.5 parts.

[0081] In a preferred embodiment of the method for preparing an electromagnetic absorbing coating, step S1 includes mixing nano-Fe3O4 with citric acid and silane coupling agent KH-550 in ethanol, mechanically stirring at 60°C for 30 minutes, ultrasonicating for 30 minutes, then centrifuging, drying and washing to obtain surface-modified Fe3O4.

[0082] In a preferred embodiment of the method for preparing an electromagnetic absorbing coating, the ternary composite absorber is composed of a magnetic component (surface-modified Fe3O4) and an electrical loss component (graphene and carbon nanotubes). The mass ratio of these components is controlled in a layered gradient according to the functional layer requirements to construct a high-frequency-medium-frequency-low-frequency progressive absorption structure.

[0083] In the high-frequency absorption layer, the mass ratio of Fe3O4: graphene: carbon nanotubes is 55-60:22.5-20:22.5-20, mainly to build a dense conductive network and enhance the electrical loss effect in the Ku / X band;

[0084] In the mid-frequency absorption layer, the ratio is adjusted to 65-70:17.5-15:17.5-15, taking into account the dual magnetic and electric mechanisms and improving the coordinated loss capability of the C / X band;

[0085] In the low-frequency enhancement layer, the ratio is further optimized to 75-80:12.5-10:12.5-10, and the Fe3O4 content is enhanced to improve the natural resonance and interface polarization loss capabilities of the S band.

[0086] Among them, graphene serves as a two-dimensional flaky conductive filler, and carbon nanotubes are one-dimensional tubular conductive structures. The two work together to construct a three-dimensional conductive network and form a multi-scale heterogeneous interface with the modified Fe3O4, effectively improving the impedance matching degree and composite dissipation capacity, and constructing a stable and efficient magneto-electric cooperative absorption system.

[0087] In the preferred embodiment of the preparation method of the electromagnetic wave absorbing coating, the particle size of the surface-modified Fe3O4 is 20-50 nm, the graphene is a sheet structure with a sheet size of ≤10 μm, and the carbon nanotube is a tubular conductive filler with a length of 10-20 μm and a diameter of 30-50 nm.

[0088] In the preferred embodiment of the preparation method of the electromagnetic wave absorbing coating, the epoxy resin matrix is a flexible modified epoxy resin, carboxyl nitrile rubber CTBN and glycerol glycidyl ether GGE are mixed and stirred to form a homogeneous phase, then added to the epoxy resin and continue to stir, then polyether amine is added and stirred, and the flexible modified epoxy resin is formed by standing at room temperature and defoaming.

[0089] In the preferred embodiment of the preparation method of the electromagnetic wave absorbing coating, the ternary composite absorber accounts for 15-25 parts by weight of the total mass of the coating, the flexible modified epoxy resin accounts for 90-105 parts by weight, the curing agent accounts for 25-35 parts by weight, the diluent accounts for 15-25 parts by weight, and the coating additive accounts for 1-2 parts by weight. The diluent appears as a component alone, and its function is to adjust the viscosity of the system and facilitate construction (spraying). Preferably, ethyl acetate is used as the diluent.

[0090] The preparation method of the electromagnetic wave absorbing coating further includes the following steps:

[0091] S4: In the order of low-frequency enhancement layer, medium-frequency absorption layer, high-frequency absorption layer, and super-hydrophobic protective layer, each functional coating is applied to the surface of the substrate by high-pressure airless spraying to ensure that the interface is tightly combined and the interlayer transition is uniform.

[0092] S5: After completing the layered spraying, a multi-stage gradient curing program is implemented, and the temperature is gradually increased for curing at room temperature, 40°C, 60°C, and 80°C, respectively, to reduce internal stress, improve coating density and overall adhesion, and finally naturally cool to form a four-layer composite electromagnetic wave absorbing coating.

[0093] An electromagnetic wave absorbing coating is prepared according to the above preparation method, which builds a gradient absorption structure from low frequency to high frequency and integrates a super-hydrophobic surface protective layer, and exhibits excellent electromagnetic wave absorption performance in a wide frequency range of 2-18 GHz, with a minimum reflection loss of less than -30 dB and an effective absorption bandwidth of more than 7 GHz.

[0094] An electromagnetic wave absorbing coating is formed by spraying the electromagnetic wave absorbing coating, and a super-hydrophobic functional finish is built on the electromagnetic wave absorbing coating. In the coating, polydimethylsiloxane (PDMS) is used as the main hydrophobic agent, nano-SiO2 particles are added as the rough structure builder, polydimethylsiloxane (PDMS) curing agent is used, and polyurethane (PU) is used as the film-forming enhancement component, and ethyl acetate is used as the dispersion and dilution medium.

[0095] In a preferred embodiment of the electromagnetic absorbing coating, it has self-cleaning properties with a water contact angle greater than 155° and a rolling angle less than 5°.

[0096] In a preferred embodiment, the electromagnetic wave absorbing coating is a high-performance, broadband absorbing system based on a ternary composite absorber and a flexible, modified epoxy resin. The absorber is composed of surface-modified nano-ferroferric oxide (Fe₃O₄), graphene (Gr), and carbon nanotubes (CNTs). By regulating the mass ratio of the three, a magneto-electric synergistic loss network structure is constructed.

[0097] Among them, the Fe3O4 nanopowder was surface-modified through citric acid chelation and reaction with KH-550 silane coupling agent, significantly improving its dispersion stability and interfacial polarization ability in the resin matrix; graphene and CNT respectively formed two-dimensional lamellar and one-dimensional tubular conductive networks, effectively enhancing the electrical loss mechanism in the high-frequency band, and overall synergistically constructed a cross-band multi-scale energy dissipation path.

[0098] The matrix material is E-42 epoxy resin, which is blended and modified with flexible chain segments such as CTBN carboxylated nitrile rubber, glycerol glycidyl ether (GGE) and polyetheramine to form a flexible epoxy resin system with good fracture ductility and stress release ability, significantly improving the overall adhesion and crack resistance of the coating.

[0099] In the design, the absorber ratio for the L2 (high-frequency) layer was Fe₃O₄:Gr:CNT = 60:20:20; for the L3 (mid-frequency) layer, it was 70:15:15; and for the L4 (low-frequency enhancement) layer, it was 80:10:10. The composite absorbers were added to the flexible matrix to prepare three functional absorbing coatings.

[0100] During the construction process, a low-frequency reinforcement layer, a mid-frequency absorption layer, and a high-frequency absorption layer were applied layer by layer to the substrate surface using high-pressure airless spraying. The outermost layer was sprayed with a PDMS / SiO2 / PU super-hydrophobic functional topcoat, creating a four-layer gradient coating system. Each functional layer was cured in a staged process (room temperature → 40°C → 60°C → 80°C) to ensure a dense film and good interlayer bonding.

[0101] In one embodiment, a multi-component synergistic interface-regulated high-flexibility broadband electromagnetic absorbing composite coating comprises the following components:

[0102] Modified ferroferric oxide (Fe3O4) magnetic absorbent;

[0103] Graphene (Gr);

[0104] carbon nanotubes (CNTs);

[0105] Flexible modified epoxy resin matrix;

[0106] Curing agent;

[0107] Thinners and coating additives.

[0108] Among them, the modified Fe3O4 absorber is surface-modified by the synergistic action of citric acid and KH-550 silane coupling agent in ethanol, thereby improving its dispersibility and interfacial polarization performance; the graphene and CNT synergistically construct a two-dimensional / one-dimensional conductive network in a 1:1 mass ratio, and together with Fe3O4 form a magneto-electric composite loss path.

[0109] In this embodiment, the mass ratio of the magnetic component (modified Fe3O4) to the electrical component (Gr and CNT combined) in the ternary composite absorbent is 55-80%:20-45%; the total addition amount of the ternary composite absorbent accounts for 15-25wt% of the coating mass, preferably 20wt%.

[0110] To identify the optimal absorber composition ratios for different functional layers, a controlled variable design with a fixed total absorber addition of 20wt% was employed. Experiments were conducted to optimize the ternary composite absorber ratio for the high-frequency absorption layer, the mid-frequency absorption layer, and the low-frequency reinforcement layer. In each set of experiments, the flexible modified epoxy resin matrix, curing system, and spray coating process remained consistent, with only the mass ratios of modified Fe₃O₄, graphene, and carbon nanotubes being manipulated. The reflection loss performance in the Ku / X, C / X, and S bands was evaluated to determine the optimal ratio for each functional layer.

[0111] Among them, the test ratio of the high-frequency absorption layer includes:

[0112] Modified Fe3O4:Gr:CNT=55:22.5:22.5; 57.5:21.25:21.25; 60:20:20.

[0113] The test ratio of the medium frequency absorption layer includes:

[0114] Modified Fe3O4:Gr:CNT=65:17.5:17.5; 67.5:16.25:16.25; 70:15:15.

[0115] The test ratio of the low-frequency reinforcement layer includes:

[0116] Modified Fe3O4:Gr:CNT=75:12.5:12.5; 77.5:11.25:11.25; 80:10:10.

[0117] By systematically comparing the reflection loss (RL) values, effective bandwidth (≥-10dB) and impedance matching performance of samples with different ratios, we finally determined that three groups of optimal ratios are suitable for high-frequency, medium-frequency and low-frequency absorption layer structures, respectively, providing a basis for subsequent structure construction.

[0118] To further optimize the overall absorbing coating performance, after determining the optimal ternary ratio of each functional layer, the impact of the total absorber addition on the coating performance was evaluated. The experiment adopted the following variable control strategy:

[0119] The total amount of absorbent added was set to: 15wt%, 20wt%, 25wt%;

[0120] The ratio of the ternary composite absorbent remains unchanged, corresponding to the optimal ratio determined for each layer;

[0121] Each group of experimental samples maintains a consistent structure, divided into three-layer composite structures: L4 (low frequency) → L3 (medium frequency) → L2 (high frequency), with a total thickness of 2.0-3.0 mm;

[0122] The superhydrophobic coating (L1) remained consistent and did not participate in the variable changes.

[0123] The optimal absorbent addition amount was screened by comprehensively evaluating the overall RL performance (2-18 GHz), effective bandwidth, self-cleaning retention rate and adhesion index of the composite coating at different addition amounts.

[0124] The flexible epoxy resin matrix is ​​formed by blending E-42 epoxy resin with three flexible segments, namely CTBN carboxyl nitrile rubber, glycerol glycidyl ether (GGE) and polyetheramine. The mass ratio of the flexible additive is preferably 2:1:0.5, and the total flexible component accounts for 15-20wt% of the resin mass.

[0125] Among them, Fe3O4 is a magnetic nanoparticle with a particle size of 20-50nm; graphene is a lamellar structure with a sheet diameter of ≤10μm; CNT is a tubular structure with a length of 10-20μm and an outer diameter of 30-50nm. The three work together to construct a micro-nano dissipative network.

[0126] The recommended component ratio of the absorbing coating is: 15-25 parts of ternary composite absorber, 90-105 parts of flexible modified epoxy resin matrix, 25-35 parts of curing agent, 15-25 parts of diluent, and 1-2 parts of coating additive.

[0127] A method for preparing a multi-layer gradient structure absorbing coating comprises the following steps:

[0128] S1: Nano-Fe3O4, citric acid, and KH-550 were added to ethanol, mechanically stirred at 60°C for 30 min, then ultrasonicated for 30 min, centrifuged, dried, and washed to obtain surface-modified Fe3O4;

[0129] S2: Graphene and carbon nanotubes were ultrasonically dissolved in a diluent for 15 min to obtain a uniform dispersion;

[0130] S3: Add the modified Fe3O4, Gr, and CNT ternary absorbent to the flexible epoxy resin according to the target ratio, mechanically stir and high shear mix, add T31 curing agent, diluent and additives, and degas to obtain a spray coating;

[0131] S4: The paint is sprayed on the surface of the substrate using a high-pressure airless spraying process. The total coating thickness is controlled at about 2.5mm, of which the high-frequency layer is about 0.5mm, the medium-frequency layer is about 1.0mm, and the low-frequency layer is about 1.0-1.2mm.

[0132] S5: Curing according to the gradient temperature program: standing at room temperature for 12 hours → 40°C for 4 hours → 60°C for 2 hours → 80°C for 1 hour, forming a film after cooling to obtain the target electromagnetic absorbing coating.

[0133] To further enhance the protective function, a layer of super-hydrophobic functional topcoat is sprayed on the surface of the absorbing coating. The topcoat is composed of polydimethylsiloxane (PDMS), its special curing agent, polyurethane PU, surface-modified nano-SiO2 and ethyl acetate. It is constructed through a micro-nano dual-scale rough structure to achieve self-cleaning, anti-corrosion and anti-fouling capabilities.

[0134] Performance verification results show that the multi-layer gradient composite coating has a minimum reflection loss RL of less than -30dB in the 2-18GHz frequency band, an effective absorption bandwidth of more than 7GHz, and no failure phenomena such as blistering, shedding, and cracking after a 1000h salt spray test. It exhibits excellent broadband absorption performance and service stability, and is suitable for electromagnetic protection and stealth applications in complex environments such as shipborne platforms, radar covers, and electronic packaging.

[0135] In one embodiment, in order to improve the stability and durability of the absorbing coating in complex service environments such as high humidity, heat, and high salt spray, the present invention further proposes to construct a super-hydrophobic functional topcoat on its surface. The topcoat uses polydimethylsiloxane (PDMS) as the hydrophobic main agent, adds 0.1g of nano-SiO2 particles (particle size 20nm) as a rough structure builder, and is combined with 0.05g of PDMS special curing agent and 0.25g of polyurethane (PU) as a film-forming enhancing component, and uses 15mL of ethyl acetate as a dispersion and dilution medium. The resulting spray liquid is evenly sprayed on the surface of the absorbing coating to form a super-hydrophobic protective topcoat of about 25μm. The super-hydrophobic topcoat gives the overall structure excellent self-cleaning and anti-corrosion properties. Its static water contact angle is greater than 155 ° , the rolling angle is less than 5 °It showed good surface stability and anti-pollution ability in 1000 hours of salt spray and outdoor exposure tests, significantly improving the environmental adaptability and service life of the absorbing coating.

[0136] In one embodiment, the absorbing performance depends not only on the dielectric loss (ε″) and magnetic loss (μ″) of the material itself, but also on the degree of matching between the two. In the principle of electromagnetic collaborative design, the magnetic loss and electrical loss of an ideal absorbing material should be maintained in a proportional relationship of μ″ / ε″≈1. This matching condition helps to achieve good impedance matching, thereby minimizing the reflection of electromagnetic waves and enhancing their attenuation and dissipation in the coating. When μ″ / ε″>1, although the magnetic loss is strong, the conductive loss is insufficient, which can easily cause enhanced reflection; when μ″ / ε″<1, although the polarization ability is strong, the magnetic response is insufficient, and it is difficult to form an effective attenuation mechanism. The ternary composite absorber consists of nano-iron tetroxide (Fe₃O₄), graphene, and carbon nanotubes (CNTs), each of which performs distinct electromagnetic absorption functions. The Fe₃O₄ particles (approximately 20 nm) are magnetic, providing μ″ primarily through natural resonance and eddy current effects, absorbing the magnetic field component of electromagnetic waves. Graphene, a two-dimensional carbon material with a high specific surface area and excellent conductivity, enhances ε″ and introduces a large number of interfacial polarization and dipole polarization sites into the composite system. CNTs, a one-dimensional nanoconductive material, provide three-dimensional electron channels within the sheet-tube-sphere network, enhancing multipath scattering and collaborating with graphene to increase overall electrical loss. To achieve an electromagnetic-magnetic matching relationship of μ″ / ε″≈1, the present invention achieves synergistic enhancement of magnetic and electrical losses within the X-band and Ku-band frequency ranges by regulating the mass ratio of the three components. The preferred mass ratio of magnetic loss (modified Fe₃O₄) to electrical loss (graphene and CNT) is (55%-80%): (20%-45%). Graphene and CNTs effectively build a stable conductive network while maintaining good dispersion. Modified Fe₃O₄ can be adjusted to achieve μ″ regulation by adjusting its content, thereby adjusting the material's overall magnetic response.

[0137] Preparation Example 1:

[0138] A preparation method of a ternary composite coating is as follows:

[0139] S1: Weigh 11 g of nano-Fe3O4, add 0.33 g of citric acid and 0.11 g of KH-550 silane coupling agent, dissolve in 33 mL of anhydrous ethanol, mechanically stir at 60 ° C for 30 min, and further ultrasonicate for 30 min, then centrifuge, wash, and dry to obtain surface-modified Fe3O4 magnetic particles;

[0140] S2: Weigh 4.5 g of graphene and 4.5 g of carbon nanotubes, respectively, and ultrasonically disperse them in ethyl acetate for 15 min to prepare graphene suspension and carbon nanotube suspension, respectively;

[0141] S3: Modified Fe3O4, graphene and carbon nanotubes are mixed in a mass ratio of 55%:22.5%:22.5%, and uniformly compounded by mechanical stirring for 30 minutes to obtain a ternary synergistic composite absorber suitable for high-frequency absorption layer.

[0142] S4: Weigh 20g of the ternary composite absorbent and add it to the flexible modified epoxy resin matrix. Mechanically stir for 40 minutes. (The composite absorbent accounts for 20% of the total coating)

[0143] S5: Add curing agent T31 (28 g), diluent ethyl acetate (18 g), and coating additive 0.75 g, and continue stirring for 20 minutes to obtain a flexible composite absorbing coating with multi-component coordinated regulation.

[0144] Preparation Example 2:

[0145] The steps for preparing the high-frequency absorption layer ternary composite absorbent (ratio 57.5:21.25:21.25, addition amount 20%) are the same as those in Example 1, except that the mass ratio of the ternary absorbent is adjusted to 57.5:21.25:21.25, and other conditions remain unchanged.

[0146] Preparation Example 3:

[0147] The high-frequency absorption layer ternary composite absorbent (ratio 60:20:20, addition amount 20%) is the same as Example 1, and the mass ratio of the ternary absorbent is adjusted to 60:20:20.

[0148] Preparation Example 4:

[0149] The steps for preparing the ternary composite absorber for the medium frequency absorption layer (ratio 65:17.5:17.5, addition amount 20%) were the same as those in Example 1, except that the mass ratio of the ternary absorber was adjusted to 65:17.5:17.5. The obtained sample was used for the C / X band medium frequency absorption performance test.

[0150] Preparation Example 5:

[0151] The steps for preparing the medium frequency absorption layer ternary composite absorber (ratio 67.5:16.25:16.25, addition amount 20%) are the same as those in Example 1, and the mass ratio of the ternary absorber is adjusted to 67.5:16.25:16.25.

[0152] Preparation Example 6:

[0153] The steps for preparing the medium frequency absorption layer ternary composite absorbent (ratio 70:15:15, addition amount 20%) are the same as those in Example 1, and the mass ratio of the ternary absorbent is adjusted to 70:15:15.

[0154] Preparation Example 7:

[0155] The low-frequency absorption layer ternary composite absorber (ratio 75:12.5:12.5, addition amount 20%) was prepared in the same manner as in Example 1, except that the mass ratio of the ternary absorber was adjusted to 75:12.5:12.5. The obtained sample was used for S-band low-frequency absorption performance analysis.

[0156] Preparation Example 8:

[0157] The low-frequency absorption layer ternary composite absorber (ratio 77.5:11.25:11.25, addition amount 20%) is prepared in the same steps as in Example 1, and the mass ratio of the ternary absorber is adjusted to 77.5:11.25:11.25.

[0158] Preparation Example 9:

[0159] The steps for preparing the ternary composite absorbent for the low-frequency absorption layer (ratio 80:10:10, addition amount 20%) are the same as those in Example 1, except that the mass ratio of the ternary absorbent is adjusted to 80:10:10.

[0160] Preparation Example 10:

[0161] The high-frequency absorption layer, medium-frequency absorption layer and low-frequency reinforcement layer respectively use the optimal ternary composite absorber ratio screened in the first stage, among which:

[0162] The mass ratio of the ternary composite absorber in the high-frequency layer is Fe3O4:Gr:CNT=60:20:20;

[0163] The mid-frequency layer is 70:15:15;

[0164] The low frequency layer is 80:10:10.

[0165] Preparation Example 11:

[0166] A method for preparing a multi-layer gradient absorbing coating is as follows:

[0167] In the three-layer absorbent structure, the total amount of the ternary composite absorbent added is set to 15wt% of the coating mass. The coating preparation method is the same as the first stage, including the following steps:

[0168] S1: Prepare the ternary composite absorbent required for each layer separately: Modified Fe3O4 is mixed with citric acid and KH-550 in ethanol, stirred at 60°C for 30 minutes, ultrasonicated for 30 minutes, centrifuged and dried; Graphene and CNT are ultrasonically dispersed in the diluent for 15 minutes, mixed with modified Fe3O4 in the target ratio, and stirred to obtain the absorbent;

[0169] S2: adding 15 wt% of the ternary absorbent of each functional layer to the flexible modified epoxy resin, stirring and dispersing, and then sequentially adding a curing agent, a diluent, and a coating additive, and vacuum degassing to obtain a spray coating;

[0170] S3: Use high-pressure airless spraying method to spray three layers of low frequency → medium frequency → high frequency in sequence, among which the high frequency layer is about 0.5mm, the medium frequency layer is about 1.0mm, and the low frequency layer is about 1.0-1.2mm. The total coating thickness is controlled at about 2.5mm.

[0171] S4: A gradient thermal curing process of "room temperature for 12 hours → 40°C for 4 hours → 60°C for 2 hours → 80°C for 1 hour" is used to form a dense absorbing structure;

[0172] S5: Spray PDMS-nano-SiO2 super-hydrophobic topcoat on the surface with a thickness of about 25 μm to construct a self-cleaning protective layer.

[0173] Preparation Example 12:

[0174] The steps were the same as those in Preparation Example 11, except that the amount of absorbent added to each layer was adjusted to 20 wt % of the coating mass. The remaining component proportions, mix ratios, spraying, and curing processes remained unchanged.

[0175] Preparation Example 13:

[0176] This example follows the same procedures as in Preparation Example 11, except that the absorbent content is increased to 25 wt %. This is used to evaluate the balance between broadband absorption performance and coating density in a high-absorbent system.

[0177] Preparation Example 14:

[0178] A flexible modified epoxy resin, the preparation steps of which are as follows:

[0179] S1: CTBN carboxyl nitrile rubber (10 g) and glycerol glycidyl ether (5 g) were mixed and stirred at 60°C for 30 minutes to form a homogeneous phase.

[0180] S2: Add 100 g of epoxy resin (E-42) and continue stirring for 10 minutes.

[0181] S3: Add 2.5 g of polyetheramine and stir for 15 minutes.

[0182] S4: The mixture was allowed to stand at room temperature for 30 minutes to degas, thereby forming a flexible modified epoxy resin.

[0183] Example

[0184] Example 1-5: Ternary composite absorbent ratio screening experiment

[0185] Using a uniform 20wt% absorbent addition, the effects of different Fe3O4:Gr:CNT ratios on RL performance were tested using the following methods:

[0186] (1) Preparation of modified Fe3O4 suspension: Fe3O4 nanopowders of different proportions (see the table below) were weighed, citric acid and KH-550 were added, and the mixture was dispersed in ethanol. The mixture was mechanically stirred at 60°C for 30 minutes, ultrasonicated for 30 minutes, and centrifuged for washing and dried.

[0187] (2) Graphene and carbon nanotubes were dispersed in ethyl acetate in proportion and ultrasonicated for 15 minutes to obtain the corresponding suspension.

[0188] (3) Mix the three components and stir them evenly to obtain a ternary composite absorbent.

[0189] (4) The results are as follows:

[0190] Example 1: Fe3O4:Gr:CNT=55:22.5:22.5, RLmin=-30.8dB, absorption bandwidth 7.1GHz;

[0191] Example 2: Fe3O4:Gr:CNT=57.5:21.25:21.25, RLmin=-31.6dB, absorption bandwidth 7.8GHz;

[0192] Example 3: Fe3O4:Gr:CNT=60:20:20, RLmin=-32.4dB, absorption bandwidth 8.5GHz (optimal for high frequency);

[0193] Example 4: Fe3O4:Gr:CNT=65:17.5:17.5, RLmin=-31.2dB, absorption bandwidth 7.3GHz;

[0194] Example 5: Fe3O4:Gr:CNT=67.5:16.25:16.25, RLmin=-33.0dB, absorption bandwidth 7.9GHz;

[0195] Example 6: Fe3O4:Gr:CNT=70:15:15, RLmin=-33.7dB, absorption bandwidth 8.2GHz (optimal at mid-frequency);

[0196] Example 7: Fe3O4:Gr:CNT=75:12.5:12.5, RLmin=-30.1dB, absorption bandwidth 7.0GHz;

[0197] Example 8: Fe3O4:Gr:CNT=77.5:11.25:11.25, RLmin=-31.0dB, absorption bandwidth 7.4GHz;

[0198] Example 9: Fe3O4:Gr:CNT=80:10:10, RLmin=-31.6dB, absorption bandwidth 8.1GHz (optimal at low frequency);

[0199] Examples 10-18: Experiments on optimizing the amount of absorbent added

[0200] Based on the optimal ratio of Example 3 (60:20:20), the absorption performance at three different total addition amounts was tested.

[0201] Example 10: absorbent addition amount 15wt%, RLmin=-30.1dB, bandwidth about 7.1GHz; adhesion level 1, no abnormality after 1000h of salt spray;

[0202] Example 11: absorbent addition amount 20wt%, RLmin=-31.9dB, bandwidth about 8.4GHz; adhesion level 1, no abnormality after salt spray for 1000h (optimal);

[0203] Example 12: absorbent addition amount 25wt%, RLmin=-31.0dB, bandwidth about 7.7GHz; adhesion level 1, no abnormality after 1000h of salt spray;

[0204] Based on the optimal ratio of Example 6 (70:15:15), the absorption performance at three different total addition amounts was tested.

[0205] Example 13: absorbent addition amount 15wt%, RLmin=-30.3dB, bandwidth about 7.2GHz; adhesion level 1, no abnormality after 1000h of salt spray;

[0206] Example 14: absorbent addition amount 20wt%, RLmin=-32.7dB, bandwidth about 8.5GHz; adhesion level 1, no abnormality after salt spray for 1000h (optimal);

[0207] Example 15: absorbent addition amount 25wt%, RLmin=-31.6dB, bandwidth about 7.9GHz; adhesion level 1, no abnormality after 1000h of salt spray;

[0208] Based on the optimal ratio of Example 9 (80:10:10), the wave absorbing performance at three different total addition amounts was tested.

[0209] Example 16: absorbent addition amount 15wt%, RLmin=-30.2dB, bandwidth about 7.0GHz; adhesion level 1, no abnormality after 1000h of salt spray;

[0210] Example 17: absorbent addition amount 20wt%, RLmin=-31.5dB, bandwidth about 8.3GHz; adhesion level 1, no abnormality after salt spray for 1000h (optimal);

[0211] Example 18: absorbent addition amount 25wt%, RLmin=-30.7dB, bandwidth about 7.6GHz; adhesion level 1, no abnormality after 1000h of salt spray;

[0212] The above coatings are all applied by high-pressure airless spraying method and cured in a gradient of room temperature → 40°C → 60°C → 80°C.

[0213] Example 19: Preparation of super-hydrophobic functional topcoat

[0214] On the basis of the absorbing layer, a super hydrophobic functional topcoat is sprayed on its surface:

[0215] Formula: PDMS 0.5g, special curing agent 0.05g, PU 0.25g, nano-SiO2 0.1g, ethyl acetate 15mL;

[0216] Spraying parameters: 0.3MPa pressure, dry film thickness about 25μm;

[0217] Performance: Contact angle 156.8°, rolling angle 4.3°, no surface corrosion and no pollutant adhesion after 1000h of salt spray.

[0218] Comparative Example

[0219] Comparative Example 1: No flexible modification, only E-42 resin + 20% absorber, RLmin = -30.1dB, bandwidth 4.9GHz, adhesion level 3, severe surface cracking;

[0220] Comparative Example 2: Using only 20 g of Fe3O4 absorber, without Gr and CNT, Rlmin=-26.1 dB, bandwidth 2.4 GHz, adhesion level 2, and obvious corrosion spots.

[0221] Through controlled variable experiments, the optimal ternary composite absorbent ratios for the high-frequency, mid-frequency, and low-frequency layers were determined to be 60:20:20, 70:15:15, and 80:10:10, respectively; the optimal total absorbent dosage was 20 wt%. This invention maintains broadband absorption capacity while also balancing adhesion, self-cleaning properties, and environmental adaptability, offering excellent engineering application prospects.

[0222] The table comparison data is as follows:

[0223] Table 1 Comparison of the absorption performance of ternary composite absorbers under different ratios

[0224]

[0225] Table 2 Comparison of the absorbing performance and adhesion performance of absorbing coatings

[0226]

[0227] The reflection loss test results show that within the 2-18 GHz wide frequency band, the constructed ternary composite absorber system exhibits excellent electromagnetic wave absorption capabilities at different ratios:

[0228] Under the optimal ratio of the high-frequency layer (Fe3O4:Gr:CNT=60:20:20), the minimum reflection loss RL of the ternary composite absorber reaches -32.4dB, and the effective absorption bandwidth (RL≤-10dB) is 8.5GHz;

[0229] Under the optimal ratio of the mid-frequency layer (70:15:15), RL min It is -33.7dB and the bandwidth is 8.2GHz;

[0230] Under the optimal ratio of low-frequency layer (80:10:10), RL min It is -31.6dB and the bandwidth is 8.1GHz.

[0231] The further constructed flexible epoxy-based coating system (added at 20wt%) inherits the structural advantages of the above-mentioned absorber. Its overall minimum reflection loss is maintained between -31.5 and -32.7dB, and the effective absorption bandwidth is greater than 8GHz, fully covering the main frequency bands of S, C, X, and Ku.

[0232] As the control group, the absorption performance of Comparative Example 1 (no flexible chain segment) and Comparative Example 2 (containing only Fe3O4 single component) was significantly deteriorated, with RLmin of -27.6dB and -24.8dB respectively, the absorption bandwidth was lower than 5GHz, and there were failure phenomena such as poor adhesion and structural cracking.

[0233] The results show that the ternary composite absorber ratio optimization mechanism and the multi-layer gradient structure synergistic strategy in the present invention significantly improve the broadband electromagnetic wave absorption performance and environmental service stability, which is superior to traditional structures and is suitable for electromagnetic protection needs in harsh working conditions such as shipborne platforms and radar covers.

[0234] The surface modification of nano-Fe₃O₄ in this invention utilizes citric acid and KH-550 silane coupling agent. The nano-Fe₃O₄ is then mechanically stirred and ultrasonically treated in an ethanol solvent at 60°C. Since nanoparticles tend to agglomerate, citric acid prevents Fe₃O₄ aggregation through electrostatic repulsion and steric hindrance, thereby improving its uniform dispersion in the resin. Enhanced interfacial bonding: KH-550, with its organic (amino) and inorganic (siloxane) functional groups, forms a "bridge" between the Fe₃O₄ surface and the epoxy resin, strengthening the interfacial bonding between the filler and the matrix and improving mechanical properties and thermal stability. Improved magnetic loss capacity: Well-dispersed nano-Fe₃O₄ effectively utilizes its magnetic response characteristics, absorbing electromagnetic wave energy through natural resonance and eddy current effects.

[0235] Graphene and carbon nanotubes (CNTs) construct a two-dimensional / one-dimensional conductive network. Graphene is a lamellar structure (≤10μm), while CNTs are tubular structures (30-50nm diameter, 10-20μm length). The composite is used in a 1:1 mass ratio. Graphene has a high specific surface area and excellent conductivity, introducing numerous interfacial polarization sites, significantly increasing ε″ (dielectric loss factor) and enhancing dielectric loss. CNTs, acting as one-dimensional conductive pathways, collaborate with graphene to construct a three-dimensional conductive network. This promotes multiple reflections and scattering of electromagnetic waves within the coating, enhancing electromagnetic wave attenuation – a mechanism known as multipath scattering. The conductive network formed by graphene / CNTs works synergistically with the magnetic Fe₃O₄ component to achieve a balance between magnetic loss (μ″) and dielectric loss (ε″), achieving an ideal matching relationship of μ″ / ε″≈1. This optimizes impedance matching and reduces electromagnetic wave reflections, a phenomenon known as the magneto-electric synergistic effect.

[0236] The ternary composite absorber system (modified Fe3O4, graphene Gr, carbon nanotubes CNT) constructs a magneto-electric synergistic loss mechanism. By regulating the proportion of components, it realizes multi-mechanism gradient absorption in different frequency bands, effectively covering a wide frequency range of 2-18 GHz.

[0237] First, in terms of matching magnetic and electrical losses, the overall ratio of the ternary composite absorber is controlled to be: magnetic loss (modified nano-Fe₃O₄): electrical loss (Gr+CNT) = 55%-80%: 20%-45%. Fe₃O₄ provides natural magnetic resonance and eddy current loss, dominating magnetic loss in the low- and mid-frequency bands; while graphene and CNT form a two-dimensional / one-dimensional conductive network, enhancing dielectric loss and interfacial polarization in the high-frequency range.

[0238] Secondly, a high-frequency-mid-frequency-low-frequency gradient structure layered design is adopted to achieve broadband precise response matching:

[0239] In the high-frequency layer (Ku / X band), the absorber ratio is 60:20:20, focusing on building a conductive network and enhancing dielectric loss;

[0240] In the medium frequency layer (C / X band), the absorber ratio is 70:15:15, which creates a magnetic-electric synergistic loss zone;

[0241] In the low-frequency layer (S band), the absorber ratio is 80:10:10, which strengthens the magnetic loss mechanism and interface polarization path.

[0242] Furthermore, the gradient three-layer absorbing structure achieves "multi-interface polarization + impedance matching gradient," effectively reducing incident wave reflection and improving electromagnetic energy dissipation efficiency. The synergistic absorption and composite optimization of each layer of the overall coating ensures efficient absorption and conversion of electromagnetic waves into heat and other forms of energy at all frequencies, creating a high-efficiency absorbing platform across a continuous wide frequency band.

[0243] In summary, the present invention significantly improves the absorbing performance, structural stability and engineering adaptability through the control of component ratio + interlayer functional progression + interface synergy mechanism, and is particularly suitable for application requirements in multi-frequency composite environments such as shipboard, radar cover, and electromagnetic interference shielding.

[0244] Flexible modified epoxy resin matrix (CTBN+GGE+polyetheramine), CTBN rubber increases toughness; GGE acts as a plasticizer to improve resin fluidity; polyetheramine acts as an active agent to enhance crosslinking density and adhesion.

[0245] Improved coating flexibility and adhesion: Traditional epoxy resins are brittle and have poor impact resistance. The addition of flexible components like CTBN creates a microphase separation structure that absorbs stress, improving bending resistance and adhesion. Improved processability: GGE reduces viscosity, facilitating spray application; polyetheramine accelerates the curing reaction, shortening process cycles. Enhanced environmental adaptability: Flexible resins are more resistant to harsh environments such as temperature fluctuations, humidity, and salt spray, extending their service life.

[0246] The gradient curing process (room temperature → 40°C → 60°C → 80°C) cures in stages, avoiding stress concentration and residual bubbles caused by rapid temperature increases. Internal stress relief: Slow temperature increases release volumetric shrinkage stress generated during the curing process, preventing cracking. Improved density: Gradient curing facilitates bubble release, increasing coating density and mechanical strength. Enhanced interfacial bonding: Gradual temperature increases promote full contact and bonding between the resin, filler, and substrate.

[0247] Super-hydrophobic functional topcoat (PDMS + nano-SiO2 + PU + ethyl acetate): PDMS provides the hydrophobic backbone; nano-SiO2 creates a micro-nano rough structure; PU enhances film strength; and ethyl acetate serves as a diluent. Self-cleaning: The super-hydrophobic surface boasts a water contact angle >155° and a rolling angle <5°, allowing rainwater to carry away dust and keep the surface clean. Corrosion protection: The hydrophobic layer isolates moisture, oxygen, and corrosive media, protecting the underlying absorbing material from corrosion. Enhanced environmental stability: Suitable for humid, high-salt-fog environments such as shipboard platforms and radomes, significantly extending service life.

[0248] High-pressure airless spraying technology is suitable for the industrial construction of multi-layer composite structures, ensuring uniform coating and good adhesion on complex curved surfaces. To meet the broadband absorption requirements of low-frequency and high-frequency electromagnetic waves while balancing structural lightweighting and stability, the present invention designs a coating with a total thickness of approximately 2.5mm, including a high-frequency layer of approximately 0.5mm, a medium-frequency layer of approximately 1.0mm, and a low-frequency layer of approximately 1.0-1.2mm. The outermost layer is a super-hydrophobic protective topcoat of approximately 25μm. This thickness distribution optimizes the absorption path and impedance gradient structure, improving absorption efficiency across the entire frequency range of 2-18GHz, and providing good construction adaptability and engineering service reliability.

[0249] Although the embodiments of the present application have been described above with reference to the accompanying drawings, the present application is not limited to the above-described specific embodiments and areas of application, and the above-described specific embodiments are merely illustrative and instructive, but are not restrictive. Many modifications can be made by those skilled in the art under the teachings of the present specification and without departing from the scope of the present application as defined by the claims.

Claims

1. A broadband electromagnetic absorbing coating, characterized in that: From outside to inside, it includes the following four layers: Super hydrophobic protective layer L1; High frequency absorption layer L2; Mid-frequency absorption layer L3; Low frequency enhanced absorption layer L4; Among them, the high-frequency absorption layer L2, the medium-frequency absorption layer L3 to the low-frequency enhanced absorption layer L4 are all constructed with a ternary composite absorber, which includes surface-modified nano-Fe3O4, graphene and carbon nanotubes. By regulating the mass ratio of the three, selective absorption of electromagnetic waves in the 2-18GHz frequency band is achieved.

2. The broadband electromagnetic absorbing coating according to claim 1, characterized in that: Preferably, the super-hydrophobic protective layer L1 is composed of polydimethylsiloxane PDMS, nano-SiO2 particles, polyurethane PU and ethyl acetate, has a micro-nano structure and has surface self-cleaning properties with a water contact angle greater than 155° and a rolling angle less than 5°.

3. The broadband electromagnetic absorbing coating according to claim 1, characterized in that: The mass ratio of the ternary composite absorber in the high-frequency absorption layer L2 is modified Fe3O4: graphene: carbon nanotube = 55-60: 20-22.5: 20-22.5, which is used to enhance the electromagnetic wave absorption performance in the Ku / X band.

4. The broadband electromagnetic absorbing coating according to claim 1, wherein: The mass ratio of the ternary composite absorber in the intermediate frequency absorption layer L3 is modified Fe3O4: graphene: carbon nanotube = 65-70: 15-17.5: 15-17.5, which is used to enhance the C-band absorption performance.

5. The broadband electromagnetic absorbing coating according to claim 1, characterized in that: The mass ratio of the ternary composite absorber in the low-frequency enhancement layer L4 is modified Fe3O4: graphene: carbon nanotube = 75-80: 10-12.5: 10-12.5, which is used to enhance the S-band absorption capacity.

6. The broadband electromagnetic absorbing coating according to claim 1, characterized in that: In the ternary composite absorbent: The nano-Fe3O4 is modified by citric acid and KH-550 silane coupling agent, and the particle size is 20-50nm; The graphene has a sheet structure and a sheet diameter of no more than 10 μm; The carbon nanotubes are tubular conductive fillers with a diameter of 30-50 nm and a length of 10-20 μm.

7. The broadband electromagnetic absorbing coating according to claim 1, characterized in that: The L2 to L4 layers use a flexible modified epoxy resin as a matrix, and the modified epoxy resin is blended with carboxyl nitrile rubber CTBN, glycerol glycidyl ether GGE and polyetheramine and E-42 epoxy resin, wherein the flexible component accounts for 15-20wt% of the resin matrix.

8. The broadband electromagnetic absorbing coating according to claim 1, wherein: The total thickness of the coating is about 2.5mm, of which the high-frequency absorption layer L2 is 0.5mm, the medium-frequency absorption layer L3 is 1.0mm, the low-frequency enhancement layer L4 is 1.0-1.2mm, and the outermost layer is a 25μm superhydrophobic protective topcoat.

9. The broadband electromagnetic absorbing coating according to claim 1, characterized in that: The four-layer structure is obtained by layered spraying construction, and a gradient curing process is adopted, including room temperature pre-curing and heat curing steps of successively heating to 40°C, 60°C and 80°C, so as to achieve dense film formation of the coating and stable interface bonding between the multiple layers.

10. The broadband electromagnetic absorbing coating according to any one of claims 1 to 9, wherein the minimum reflection loss in the 2-18 GHz frequency band is no greater than -30 dB, and the effective absorption bandwidth is no less than 7 GHz.

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