A heating frequency-selective wave-absorbing composite coating and a preparation method thereof

Abstract

This invention relates to the field of surface engineering technology, specifically to a heated frequency-selective absorbing composite coating and its preparation method. The heated frequency-selective absorbing composite coating comprises a radar absorbing layer and a perforated electrothermal layer stacked sequentially from bottom to top. The perforated electrothermal layer is a graphene heating film or a carbon fiber heating film and is used to allow the penetration of electromagnetic waves of a specific band under applied voltage. The perforated grid structure of the perforated electrothermal layer is used to reflect electromagnetic waves under unapplied voltage. The purpose of this heated frequency-selective absorbing composite coating and its preparation method is to solve the problem of poor compatibility between heat transfer and radar wave absorption when the heating film and the absorbing coating are combined.

Description

Technical Field

[0001] This invention relates to the field of surface engineering technology, specifically to a heating-selective frequency-absorbing composite coating and its preparation method. Background Technology

[0002] Radar wave absorption and shielding play a crucial role. By controlling the shielding and absorption conversion of electromagnetic waves within a specific operating frequency band, dynamic feedback of radar waves can be achieved, enabling control over the reception and shielding of electromagnetic waves in that band. This controllable adjustment significantly improves the stability and reliability of electromagnetic wave reception and transmission in aircraft communication processes within radar-illuminated environments. Therefore, electromagnetic window shielding and absorption conversion can adjust the channel switching during radar wave detection or transmission, resulting in controllable absorption and transmission of electromagnetic waves, thus enhancing environmental adaptability and effectiveness.

[0003] Meanwhile, the icing problem of electromagnetic windows in flight environments cannot be solved by current direct heating methods. The conductivity of the heating material will affect the functionality of the wave-absorbing and shielding coatings, thus causing the entire forward detection mission system to fail. Therefore, anti-icing and specific band electromagnetic wave absorption and shielding designs will greatly improve the current controllability of electromagnetic windows, forming controllable electromagnetic wave reflection and absorption, improving applicability in icing environments, and expanding applications to aerospace, automotive and other related fields.

[0004] Given the above background, current traditional heating methods based on resistance heating are energy-intensive, inefficient, and cannot effectively coordinate with frequency-selective absorption. The heating layer is typically made of metal or carbon-based materials with some electrical conductivity. When placed on the surface of absorbing materials or structures, the electromagnetic shielding effect causes reflection of electromagnetic waves across the entire frequency band, resulting in no electromagnetic wave absorption. When placed at the bottom of absorbing materials or structures, heat transfer requires penetration through the absorbing coating, leading to heat loss and significantly reducing the effectiveness of electrothermal heating. Therefore, in heating and / or radar compatibility research, the principles of electric heating and electromagnetic wave absorption are mutually influential. The contradiction between heat transfer and radar wave absorption when the heating film and absorbing coating are combined remains unresolved, resulting in poor compatibility.

[0005] Therefore, the inventors provide a heating-selective frequency-absorbing composite coating and its preparation method. Summary of the Invention

[0006] (1) Technical problems to be solved

[0007] This invention provides a heating-selective frequency-absorbing composite coating and its preparation method, which solves the technical problem of poor compatibility between heat transfer and radar wave absorption when the heating film and the absorbing coating are combined.

[0008] (2) Technical solution

[0009] This invention provides a heating-selective frequency-absorbing composite coating, comprising a radar absorbing layer and a hollowed-out electrothermal layer stacked and connected sequentially from bottom to top. The hollowed-out electrothermal layer is a graphene heating film or a carbon fiber heating film and is used to enable the penetration of electromagnetic waves of a specific band under applied voltage. The hollowed-out grid structure of the hollowed-out electrothermal layer is used to reflect electromagnetic waves under unapplied voltage.

[0010] Furthermore, the radar absorbing layer includes a wave-transparent matrix and a ferromagnetic absorbing agent filled within the wave-transparent matrix.

[0011] Furthermore, the ferromagnetic microwave absorbing agent includes layered carbonyl iron and spherical carbonyl iron.

[0012] Furthermore, the wave-transparent substrate is polyurethane.

[0013] Furthermore, the upper and lower surfaces of the hollowed-out heating layer are encapsulated by an insulating film and a heat-insulating film, respectively.

[0014] Furthermore, the heat insulation film is a graphene film or a carbon fiber film.

[0015] Furthermore, the heating frequency-selective absorbing composite coating also includes a surface functional layer, which is coated on the hollowed-out electrothermal layer. The surface functional layer is a superhydrophobic coating or an anti-corrosion coating.

[0016] Furthermore, the perforated grid of the perforated heating layer is elongated.

[0017] The present invention also provides a method for preparing the above-mentioned heating-selective absorbing composite coating, comprising the following steps:

[0018] A radar absorbing layer is prepared and then coated onto the surface of the workpiece.

[0019] A hollowed-out electrothermal layer is prepared, and the hollowed-out electrothermal layer is coated on the surface of the radar absorbing layer;

[0020] A surface functional layer is prepared and coated onto the surface of the hollowed-out electrothermal layer.

[0021] Furthermore, an adhesive is used to bond the radar absorbing layer to the perforated electrothermal layer.

[0022] (3) Beneficial effects

[0023] In summary, this invention utilizes graphene or carbon fiber as the heating film, leveraging the unique semiconductor properties of these materials. Through the principle of heatability and resistance changes in the Fermi level due to voltage variations, it achieves simultaneous changes in heating and electromagnetic wave penetration, as well as dynamic control of wave absorption and heating. Simultaneously, a perforated patterned design is employed, with a large-area perforated heating film forming electromagnetic wave shielding. Upon applying voltage, it generates a heating effect and allows electromagnetic waves to penetrate. The underlying radar absorbing layer uses a highly efficient absorber to effectively absorb electromagnetic waves at specific frequencies penetrating the interior. This dual-layer composite structure creates a surface structure that is heatable, selectively absorbs, and shields. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the structure of a heating-selective frequency-absorbing composite coating provided in an embodiment of the present invention;

[0026] Figure 2 This is a schematic diagram of the structure of a radar absorbing layer provided in an embodiment of the present invention;

[0027] Figure 3 This is a schematic diagram of the structure of a hollowed-out electrothermal layer provided in an embodiment of the present invention;

[0028] Figure 4 This is a schematic diagram of another heating-selective frequency-absorbing composite coating provided in an embodiment of the present invention;

[0029] Figure 5 This is a schematic diagram of the frequency selective absorption process of a heating frequency selective absorbing composite coating provided in an embodiment of the present invention;

[0030] Figure 6 This is a schematic diagram of the frequency selective absorption process of a heating frequency selective absorbing composite coating provided in an embodiment of the present invention;

[0031] Figure 7 This is a schematic flowchart of a method for preparing a heating-selective frequency-absorbing composite coating according to an embodiment of the present invention.

[0032] In the picture:

[0033] 1-Radar absorbing layer; 2-Perforated electrothermal layer; 3-Surface functional layer; 4-Heating positive electrode; 5-Heating negative electrode. Detailed Implementation

[0034] The embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. The following detailed description of the embodiments and the accompanying drawings are used to illustrate the principles of the present invention by way of example, but should not be used to limit the scope of the present invention. That is, the present invention is not limited to the described embodiments, and any modifications, substitutions and improvements to the parts, components and connection methods are covered without departing from the spirit of the present invention.

[0035] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. This application will now be described in detail with reference to the accompanying drawings and embodiments.

[0036] In the description of this invention, it should be understood that the terms "upper," "lower," "front," "rear," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. They are only used to facilitate the description of this invention and to simplify the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0037] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set" and "install" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0038] Figure 1 This is a schematic diagram of the structure of a heating-selective frequency-absorbing composite coating provided in an embodiment of the present invention, as shown below. Figure 1-3 As shown, the composite coating may include a radar absorbing layer 1, a hollowed-out electrothermal layer 2, and a surface functional layer 3 stacked and connected sequentially from bottom to top. The hollowed-out electrothermal layer 2 is a graphene heating film or a carbon fiber heating film and is used to enable the penetration of electromagnetic waves of a specific band under the applied voltage (i.e., heating positive electrode 4 and heating negative electrode 5 are respectively installed on both sides of the hollowed-out electrothermal layer 2). The hollowed-out grid structure of the hollowed-out electrothermal layer 2 is used to reflect electromagnetic waves under the unapplied voltage.

[0039] In the above embodiment, the perforated electrothermal layer 2 achieves electromagnetic wave shielding without power, realizing total reflection of electromagnetic waves on the surface. When a voltage is applied, it achieves electrothermal heating while allowing specific bands of electromagnetic waves to penetrate, reducing electromagnetic wave reflection on the surface. The radar absorbing layer 1 efficiently absorbs electromagnetic waves that enter the coating after a voltage is applied to the perforated electrothermal layer 2, forming frequency-selective absorption of electromagnetic waves in a specific band. The absorption peak value can be controlled by the voltage applied to the heating layer. Through the design of the perforated structure of the heating layer, the shielding function of the heating layer is solved. By adjusting the voltage, electromagnetic waves in the radar band can penetrate, and efficient absorption is achieved in a specific band through the absorbing coating. This forms electromagnetic wave shielding and absorption control within a controllable frequency band, and also heats the surface, reducing the problem of window icing.

[0040] The main principle of the perforated electrothermal layer 2 is to utilize the special electromagnetic properties of graphene and carbon fiber heating film materials, as well as the specific perforated structure and size, to achieve a change in the impedance of the electrothermal layer after the applied voltage is applied. This causes the applied electromagnetic field to change from dereflection to transmission. While forming efficient heating, the electromagnetic wave changes from a reflected state to a transmitted state. Combined with the wave-absorbing coating, it achieves the absorption of electromagnetic waves in a specific frequency band, solving the contradiction between electromagnetic reflection shielding and electromagnetic wave transmission absorption, and realizing the dynamic adjustment of the two states. The specific implementation process and principle are as follows: In the given electrothermal coating, the grid-shaped conductive network is a typical shielding structure that reflects electromagnetic waves. Figure 5 As shown, after applying voltage, the voltage changes the Fermi level of the graphene or carbon fiber in the heating film. Increasing the voltage raises the Fermi level, thereby increasing the resistance, making the resistance of the heating film closer to that of air, and thus the impedance closer to that of air. This allows more electromagnetic waves to enter the interior of the material, and then work together with the bottom coating to absorb the electromagnetic waves. This achieves the conversion between reflection and transmission of electromagnetic waves.

[0041] The shift in absorption peak frequency can be explained using an equivalent circuit model. The expression for the resonant frequency of the equivalent LC circuit is given by the following equation (1):

[0042]

[0043] f0 is the resonant frequency of the electromagnetic wave, π is pi, L is the equivalent inductance, and C is the equivalent capacitance. It can be seen that as the inductance or capacitance decreases, the absorption peak shifts to higher frequencies. The given perforated electrothermal layer's mesh structure is mainly elongated, resulting in a longer path for free charge movement; therefore, it can be considered equivalent to an inductor, while the capacitance is negligible. As mentioned above, when the voltage varies within a certain range, the material's impedance matching is good. When the electromagnetic wave is incident on the material's interior, the voltage's control over the free charges and the electromagnetic wave's electric field driving the free charges to generate inductance interact, causing the frequency to shift within a certain range, thus forming absorption at a specific frequency.

[0044] As an optional implementation, the radar absorbing layer 1 includes a wave-transparent matrix and a ferromagnetic absorbing agent filled within the wave-transparent matrix. The ferromagnetic absorbing agent includes layered carbonyl iron and spherical carbonyl iron, and the wave-transparent matrix is ​​polyurethane.

[0045] As an optional implementation, the upper and lower surfaces of the perforated heating layer 2 are encapsulated with an insulating film and a heat-insulating film, respectively. The bottom layer uses a heat-insulating film to reduce the thermal impact on the microwave-absorbing coating, while the upper layer uses an insulating film to prevent leakage during the electric heating process. Preferably, the heat-insulating film is made of graphene or carbon fiber film, and the insulating film is preferably made of polyimide film.

[0046] As an optional implementation method, such as Figure 4 As shown, the heating-selective frequency-absorbing composite coating also includes a surface functional layer 3, which is coated on the perforated electrothermal layer 2. The surface functional layer 3 is a superhydrophobic coating or an anti-corrosion coating. The surface functional layer 3 can achieve a synergistic effect on anti-icing and de-icing by using a coating of wave-transparent material such as a superhydrophobic coating or an anti-corrosion coating, including reducing ice thickness, delaying ice time, reducing ice intensity, and improving the adaptability and functional reliability of the electromagnetic window surface in complex electromagnetic environments.

[0047] In summary, compared with traditional shielding coatings or frequency-selective absorbing coatings, this invention resolves the contradiction between electromagnetic wave shielding and transmission. For example... Figure 6 As shown, when the absorbing coating is not combined with the heating coating, it exhibits an absorption effect of -6dB in the 6–12 GHz frequency range. After the absorbing coating surface is combined with the hollow heating layer, it exhibits a state of complete reflection. When a voltage below 15V is applied, no obvious absorption or heating effect is observed. When a voltage of 20V is applied, a heating effect of 46 degrees Celsius is achieved, and an absorption effect in the 10–16 GHz range is observed, with the absorption peak value reaching below 12dB. When a voltage of 25–27V is applied, the peak position does not change, but the peak value decreases. Therefore, it can be seen that under different applied voltages, the reflectivity of the heated frequency-selective absorbing composite coating for electromagnetic waves of different frequencies varies. When the applied voltage is 20–27V, the absorption effect for electromagnetic waves in the 10–16 GHz frequency band is optimal, achieving dynamic adjustment of shielding and frequency-selective absorption of electromagnetic waves in the 10–16 GHz frequency band. The conversion from shielding to transmission is achieved by applying a DC voltage to the hollow graphene / carbon fiber heating layer.

[0048] Figure 7 This is a schematic flowchart of a method for preparing the above-mentioned heating-selective frequency-absorbing composite coating according to an embodiment of the present invention. The method may include the following steps:

[0049] S100. Prepare radar absorbing layer 1 and coat radar absorbing layer 1 on the surface of the workpiece;

[0050] S200. Prepare the hollowed-out electrothermal layer 2 and coat the hollowed-out electrothermal layer 2 onto the surface of the radar absorbing layer 1.

[0051] In the above embodiments, in step S100, the ferromagnetic microwave absorbing agent is dispersed by ultrasonically vibrating and mechanically stirring one or more microwave absorbing agent powders to ensure uniform mixing of ferromagnetic absorbers of different sizes and shapes. Preferred ferromagnetic microwave absorbing agents include layered carbonyl iron and spherical carbonyl iron, with sizes ranging from a few micrometers to tens of micrometers. The microwave absorbing agent and microwave-transparent material are mixed by mixing: the preferred microwave absorbing agent is mixed with the filler and the microwave-transparent matrix, with the microwave absorbing agent accounting for 50% to 60% of the resin (by mass). Ultrasonic vibration and mechanical stirring are used, with the stirring rate, stirring time, and other parameters aimed at achieving uniform mixing. The preferred microwave-transparent matrix is ​​polyurethane. The microwave absorbing layer is coated or sprayed using a pneumatic spray gun or brush. The coating thickness is 1mm to 1.5mm. Pneumatic spraying involves multiple sprays to achieve the required thickness. The spray gun movement speed is adjusted according to the workpiece shape and the required thickness of each coating section. The coating thickness is about 120μm for each reciprocating spraying cycle. After 3 to 4 spraying cycles, the spraying stops for 3 minutes while the workpiece continues to rotate. This is beneficial for coating leveling and solvent evaporation. The spraying gas pressure is 0.2 to 0.4MPa.

[0052] In step S200, the perforated heating film is manufactured using manual or laser engraving techniques to create a perforated structure of graphene or carbon fiber heating film. The structural gap size is 25-30mm × 2-3mm, and the width of the connecting portion of the heating film is 3mm. The perforated heating film is then encapsulated using insulating and heat-insulating films. The bottom layer uses a heat-insulating film to reduce the thermal impact on the radar-absorbing coating, while the top layer uses an insulating film to prevent leakage during the electric heating process. Preferably, the heat-insulating film material is graphene or carbon fiber film, and the preferred insulating film material is polyimide film. Furthermore, an adhesive is used to bond the radar-absorbing layer 1 to the perforated electric heating layer 2.

[0053] In some optional embodiments, step S300 is further included: preparing the surface functional layer 3 and coating the surface functional layer 3 onto the surface of the perforated electrothermal layer 2. A functional layer coating (superhydrophobic coating or polymer anti-corrosion coating) is then applied to the encapsulated heating layer surface. The coating is applied using a pneumatic spray gun or brush, with a thickness of 0.1 mm to 0.8 mm. Pneumatic spraying involves multiple coats to achieve the required thickness. The spray gun movement speed is adjusted according to the workpiece shape and the required thickness of each coating segment. The coating thickness in each reciprocating spray cycle is approximately 120 μm. After 3 to 4 spray cycles, the spraying is paused for 3 minutes while the workpiece continues to rotate. This facilitates coating leveling and solvent evaporation. The spraying gas pressure is 0.2 to 0.4 MPa.

[0054] Example 1

[0055] Polyurethane-based planar absorbing layer with heatable frequency-selective absorbing surface

[0056] (1) Preparation of microwave absorbing coating

[0057] Flake and spherical (~50μm) ferromagnetic microwave absorbers were dispersed in a 1:1 ratio and subjected to ultrasonic oscillation and mechanical stirring to ensure uniform mixing of ferromagnetic absorbers of different sizes and shapes.

[0058] The microwave absorbing agent and microwave-transparent material are mixed by stirring. The preferred microwave absorbing agent and filler are mixed with the polyurethane microwave-transparent matrix. The microwave absorbing agent accounts for 50% to 60% of the polyurethane resin (by mass). Ultrasonic oscillation and mechanical stirring are used. The stirring rate, stirring time and other parameters are set to achieve uniform mixing.

[0059] The microwave absorbing layer is applied by spraying or using a pneumatic spray gun or brush. The coating thickness is 1 mm, and the coating thickness is about 120 μm per reciprocating spraying cycle. After 3 to 4 spraying cycles, the spraying is stopped for 3 minutes while the workpiece continues to rotate. This is beneficial for the leveling of the coating and the evaporation of the solvent. The spraying gas pressure is 0.2 MPa.

[0060] (2) Preparation of heating layer

[0061] The hollow heating film is manufactured using manual or laser engraving techniques to create a hollow structure for the graphene or carbon fiber heating film. The structural gap size is 25mm × 2mm, and the width of the connecting part of the heating film is 3mm.

[0062] The hollow heating film is encapsulated using polyimide insulating and heat-insulating film for both the top and bottom.

[0063] (3) Functional layer preparation

[0064] A functional layer (superhydrophobic coating or polymer anti-corrosion coating) is sprayed onto the surface of the encapsulated heating layer. The coating is applied using a pneumatic spray gun or brush, with a thickness of 0.2 mm.

[0065] (4) Composite preparation of microwave absorbing coating and heating functional layer

[0066] Polyurethane-based adhesives are used to bond the microwave absorbing coating to the heating functional layer, and the coating is bonded by brushing or spraying.

[0067] Example 2

[0068] Polyurethane-based structured absorbing layer with heatable frequency-selective absorbing surface

[0069] (1) Preparation of microwave absorbing coating

[0070] Flake and spherical (~50μm) ferromagnetic microwave absorbers were dispersed in a 1:1 ratio and subjected to ultrasonic oscillation and mechanical stirring to ensure uniform mixing of ferromagnetic absorbers of different sizes and shapes.

[0071] The microwave absorbing agent and microwave-transparent material are mixed by stirring. The preferred microwave absorbing agent and filler are mixed with the polyurethane microwave-transparent matrix. The microwave absorbing agent accounts for 50% to 60% of the polyurethane resin (by mass). Ultrasonic oscillation and mechanical stirring are used. The stirring rate, stirring time and other parameters are set to achieve uniform mixing.

[0072] The microwave absorbing layer is applied by spraying or using a pneumatic spray gun or brush. The coating thickness is 1 mm, and the coating thickness is about 120 μm per reciprocating spraying cycle. After 3 to 4 spraying cycles, the spraying is stopped for 3 minutes while the workpiece continues to rotate. This is beneficial for the leveling of the coating and the evaporation of the solvent. The spraying gas pressure is 0.2 MPa.

[0073] Discontinuous structure processing is carried out by mold imprinting. The mold surface is sprayed with polyurethane release agent. When the polyurethane resin microwave absorbing coating is semi-dry (2-4 hours), the imprinting is carried out. The microwave absorbing structure is hexagonal in shape and 5-13mm in size. After the mold is pressed into the coating, it is cured at room temperature for 24-36 hours until the resin is completely cured. The mold is then removed to complete the structure processing.

[0074] Polyurethane coating is applied using a pneumatic spray gun. The organic polyurethane coating is poured into the spray bottle, shaken evenly, and then sprayed with 0.3MPa compressed air. The coating is applied evenly back and forth multiple times, with a 3-minute pause between each spray and the workpiece is not stopped rotating. This facilitates the leveling of the coating and the evaporation of the solvent. After 2-4 coats to fill gaps and level the coating, it is cured at room temperature for 60 minutes.

[0075] (2) Preparation of heating layer

[0076] The hollow heating film is manufactured using manual or laser engraving techniques to create a hollow structure for the graphene or carbon fiber heating film. The structural gap size is 25mm × 2mm, and the width of the connecting part of the heating film is 3mm.

[0077] The hollow heating film is encapsulated using polyimide insulating and heat-insulating film for both the top and bottom.

[0078] (3) Functional layer preparation

[0079] A functional layer (superhydrophobic coating or polymer anti-corrosion coating) is sprayed onto the surface of the encapsulated heating layer. The coating is applied using a pneumatic spray gun or brush, with a thickness of 0.2 mm.

[0080] (4) Composite preparation of microwave absorbing coating and heating functional layer

[0081] Polyurethane-based adhesives are used to bond the microwave absorbing coating to the heating functional layer, and the coating is bonded by brushing or spraying.

[0082] It should be noted that the various embodiments in this specification are described in a progressive manner, and the same or similar parts between the various embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. The present invention is not limited to the specific steps and structures described above and shown in the figures. Furthermore, for the sake of brevity, detailed descriptions of known methods and techniques are omitted here.

[0083] The above are merely embodiments of this application and are not intended to limit the scope of this application. Various modifications and variations can be made to this application by those skilled in the art without departing from the scope of the invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principle of this application should be included within the scope of the claims of this application.

Claims

1. A heating-selective frequency-absorbing composite coating, characterized in that, The device comprises a radar absorbing layer (1) and a perforated electrothermal layer (2) stacked sequentially from bottom to top. The perforated electrothermal layer (2) is a graphene heating film or a carbon fiber heating film and is used to enable the penetration of electromagnetic waves of a specific band under applied voltage. The perforated grid structure of the perforated electrothermal layer (2) is used to reflect electromagnetic waves under unapplied voltage. The radar absorbing layer (1) includes a wave-transparent substrate and a ferromagnetic absorbing agent filled in the wave-transparent substrate. The perforated grid of the perforated electrothermal layer (2) is long and narrow. The perforated electrothermal layer (2) is processed by manual carving or laser carving to create a perforated structure of graphene or carbon fiber heating film. The structural gap size is 25-30mm × 2-3mm, and the width of the heating film connection part is 3mm.

2. The heating-selective frequency-absorbing composite coating according to claim 1, characterized in that, The ferromagnetic microwave absorber includes layered carbonyl iron and spherical carbonyl iron.

3. The heating-selective frequency-absorbing composite coating according to claim 1, characterized in that, The upper and lower surfaces of the hollowed-out electric heating layer (2) are respectively encapsulated by an insulating film and a heat-insulating film.

4. The heating-selective frequency-absorbing composite coating according to claim 3, characterized in that, The heat insulation film is a graphene film or a carbon fiber film.

5. The heating-selective frequency-absorbing composite coating according to claim 1, characterized in that, It also includes a surface functional layer (3), which is coated on the hollowed-out electrothermal layer (2) and is a superhydrophobic coating or an anti-corrosion coating.

6. A method for preparing a heating-selective frequency-absorbing composite coating as described in any one of claims 1-5, characterized in that, The method includes the following steps: A radar absorbing layer (1) is prepared and the radar absorbing layer (1) is coated on the surface of the workpiece; Prepare a hollowed-out electrothermal layer (2) and coat the hollowed-out electrothermal layer (2) onto the surface of the radar absorbing layer (1).

7. The method for preparing the heating-selective frequency-absorbing composite coating according to claim 6, characterized in that, Prepare a surface functional layer (3) and coat the surface functional layer (3) onto the surface of the hollowed-out electrothermal layer (2).

8. The method for preparing the heating-selective frequency-absorbing composite coating according to claim 6, characterized in that, The radar absorbing layer (1) and the hollowed-out electrothermal layer (2) are bonded together using an adhesive.

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