A structure / stealth integrated composite material and a preparation method thereof

Abstract

The present application relates to a kind of structure / stealth integrated composite material and its preparation method, the structure / stealth integrated composite material includes: at least two layers of resin-based composite material layer;At least one frequency selective surface layer is clamped between the at least two layers of resin-based composite material layer, the resin-based composite material layer and the frequency selective surface layer are alternately arranged;And the bottom layer metal reflection layer, the relative dielectric constant of the resin-based composite material constituting the resin-based composite material layer is 2-5 in 2-20 GHz frequency band, the frequency selective surface layer has two-dimensional periodic arrangement of conductive pattern, the size of adjacent conductive pattern and the distance between its geometric center are between 1 / 20 to 1 / 5 of the lowest limit corresponding wavelength of the wave absorption operating frequency band of the structure / stealth integrated composite material, the tensile strength of the structure / stealth integrated composite material is not less than 200MPa.

Description

Technical Field

[0001] This invention relates to the technical field of radar wave absorbing materials. More specifically, this invention relates to a structural / stealth integrated material that combines radar wave absorption capabilities with high mechanical strength. Background Technology

[0002] Drones play a significant role in various fields worldwide, and the development of next-generation stealth drones has become a global focus. Modern drone stealth encompasses two approaches: shape stealth and material stealth. The disadvantage of shape stealth technology is that it typically weakens the drone's aerodynamic performance. Further coating the drone's surface with radar-absorbing materials in areas with a large radar cross-section can effectively reduce the drone's radar cross-section. However, according to the working principle of radar-absorbing materials, the wider the absorption bandwidth, the greater the thickness of the material. Even in theoretical calculations, the minimum thickness of the non-magnetic radar-absorbing material with a reflection attenuation better than -10dB in the 2–18 GHz band added outside the skin is 9.7 mm. This results in problems such as heavy weight, large thickness, and high maintenance costs, imposing additional loads on the drone, which greatly limits the implementation and application of radar-absorbing materials. Summary of the Invention

[0003] The purpose of this invention is to provide a structural / stealth integrated composite material and its preparation method that can utilize the resonant effect of the thickness and frequency-selective surface of the resin-based composite material to achieve broadband radar absorption and suppress additional loads.

[0004] On one hand, one embodiment of the present invention provides a structural / stealth integrated composite material comprising: at least two resin-based composite material layers; at least one frequency-selective surface layer sandwiched between the at least two resin-based composite material layers, wherein the resin-based composite material layers and the frequency-selective surface layer are alternately arranged; and a bottom metal reflective layer. The resin-based composite material constituting the resin-based composite material layers has a relative permittivity of 2 to 5 in the 2–20 GHz frequency band. The frequency-selective surface layer has a two-dimensional periodically arranged conductive pattern, the size of adjacent conductive patterns and the spacing between their geometric centers being between 1 / 20 and 1 / 5 of the wavelength corresponding to the lowest absorption frequency band of the structural / stealth integrated composite material, and its tensile strength being not less than 200 MPa. According to the configuration of the present invention, broadband radar absorption can be achieved by utilizing the thickness of the resin-based composite material itself and the resonance effect of the frequency-selective surface. This material can be used directly as a skin structure material and can achieve broadband radar absorption without the need for additional radar-absorbing coating. It effectively resolves the contradiction between the thickness of the radar-absorbing material and the radar-absorbing bandwidth, reduces the difficulty of shape design for stealth drones, ships, etc., and lowers the maintenance cost of stealth radar-absorbing materials such as radar-absorbing materials for stealth aircraft.

[0005] The microwave absorption operating frequency band of the integrated structure / stealth composite material can fall within the range of 2 to 20 GHz.

[0006] The surface sheet resistance of the conductive pattern can be between 0.01 Ω / sq and 1000 Ω / sq.

[0007] Preferably, the relative permittivity of the resin-based composite material in the resin-based composite material layer is between 2 and 5 in the 2-18 GHz frequency band.

[0008] The resin-based composite material can be a fiber-reinforced composite material. The reinforcing fibers used can be at least one of glass fiber, quartz fiber, aramid fiber, and carbon fiber. Preferably, the material of the resin-based composite material layer is at least one of glass fiber reinforced epoxy resin, glass fiber reinforced polyetheretherketone, and aramid fiber reinforced epoxy resin.

[0009] The conductive pattern can be made of copper foil, aluminum foil, conductive carbon film, or ITO conductive film. The conductive pattern can be one of a square, a circle, a square ring, or a hexagonal ring.

[0010] The thickness of each resin-based composite material layer can be less than 4 mm; the thickness of each frequency-selective surface layer can be less than 0.2 mm; and the thickness of the metal reflective layer can be less than 0.15 mm. The total thickness of the integrated structure / stealth composite material can be less than 20 mm, and the absorption band can be 2–20 GHz.

[0011] The sheet resistance of the metal reflective layer can be no greater than 15Ω / sq.

[0012] When the resin-based composite material layer is a fiber-reinforced composite material layer, it may include multiple layers, such as one or more of the following: -45° fiber layer, +45° fiber layer, 0° fiber layer, and 90° fiber layer. The direction of the fibers in the 0° fiber layer is perpendicular to the direction of the fibers in the 90° fiber layer. The angles between the fibers in the 45° fiber layer, -45° fiber layer and 0° fiber layer are 45° and -45°, respectively.

[0013] The aforementioned integrated structural / stealth composite material can be applied to the skin structure of aircraft and ships.

[0014] On the other hand, a method for preparing a structural / stealth integrated composite material according to one aspect of the present invention includes:

[0015] Conductive patterns are created based on the set dimensions and the spacing between geometric centers to form a frequency-selective surface;

[0016] Prepreg is obtained by impregnating fiber cloth or fiber bundles in resin slurry.

[0017] The prepreg is laid in a mold in a predetermined order and placed on a frequency-selective surface for hot pressing; and after the hot pressing is completed, a metal reflective layer is formed on the bottom layer using a conductive paste or conductive film.

[0018] Preferably, a base film is prepared using polyimide or polyethylene terephthalate as the frequency-selective surface. The areas of the base film not covering the frequency-selective surface are activated by treating with a sodium hydroxide solution with a concentration between 0.8 and 2 mol / L at room temperature for 0.5 to 1.5 hours, or by using an ammonia solution with a mass percentage of 25% to 33% and holding at 60 to 90°C for 4 to 7 hours. This configuration further enhances the mechanical strength of the interface between the frequency-selective surface and the composite material, ensuring a good combination of mechanical strength and microwave absorption performance.

[0019] Based on the structure of the present invention described above, it is possible to provide a structural / stealth integrated composite material and its preparation method that can ensure mechanical strength while possessing excellent wave absorption performance and suppressing additional loads. Attached Figure Description

[0020] Figure 1 : Schematic diagram of the integrated structural / stealth composite material of implementation form 1;

[0021] Figure 2 Example 1: Schematic diagram (left) and top view of the integrated structural / stealth composite material and frequency selective surface (1 is glass fiber reinforced epoxy resin, 2 is frequency selective surface, 3 is conductive copper ring, d1 = 2mm, d2 = 2mm, p = 14.4mm, r1 = 7mm, r2 = 4.4mm);

[0022] Figure 3 Example 1: Actual test results of absorption performance;

[0023] Figure 4 Example 2: Schematic diagram (left) and top view of the integrated structure / stealth composite material and frequency selective surface (1 is glass fiber reinforced epoxy resin, 2 is frequency selective surface, 3 is conductive copper ring, d1 = 2.5mm, d2 = 2.5mm, p = 6.8mm, a1 = 6mm, a2 = 2mm);

[0024] Figure 5 Example 2: Actual test results of absorption performance;

[0025] Figure 6 This shows the variation of the material's microwave absorption performance with the arrangement period p in Example 2-2;

[0026] Figure 7 The variation of the material's microwave absorption performance with the outer diameter a1 of the square ring is shown in Examples 2-3;

[0027] Figure 8 The variation of the material's microwave absorption performance with the ring width (ring width = (a1-a2) / 2) in Examples 2-4 is shown. Detailed Implementation

[0028] The present invention will be further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0029] This disclosure relates to a structural / stealth integrated composite material (hereinafter, sometimes referred to as "structural / stealth integrated material") and its preparation method. This structural / stealth integrated material is formed by in-situ implanting at least one layer of frequency-selective surface within a resin-based composite material, such as a fiber-reinforced composite material, creating a layered structure with alternating layers of resin-based material and frequency-selective surface, with a metal reflective layer at the bottom. Broadband microwave absorption is achieved by utilizing the thickness of the composite material itself to achieve resonance with the frequency-selective surface, and high mechanical strength is achieved by utilizing the composition of the composite material itself. This invention achieves high mechanical strength and microwave absorption, meaning that a portion of the microwave energy is absorbed, and the remainder is reflected.

[0030] The structural / stealth integrated composite material disclosed herein includes: at least one frequency selective surface layer between at least two resin-based composite material layers, wherein the resin-based composite material layers and the frequency selective surface layer are alternately arranged; and a bottom metal reflective layer, wherein the resin-based composite material constituting the resin-based composite material layers has a relative permittivity of 2 to 5 in the 2 to 20 GHz frequency band, and the frequency selective surface layer has a two-dimensional periodically arranged conductive pattern, wherein the size of adjacent conductive patterns and the spacing between their geometric centers are between 1 / 20 and 1 / 5 of the wavelength corresponding to the lowest limit of the absorption operating frequency band of the structural / stealth integrated composite material.

[0031] Implementation Form 1

[0032] Figure 1 This is a schematic diagram of the integrated structural / stealth composite material of Embodiment 1. The integrated structural / stealth composite material 10 of Embodiment 1 includes: a multilayer resin-based composite material layer 1 (in this embodiment, it is a fiber-reinforced resin-based composite material layer, where d1 and d2 represent the thickness range of the fiber-reinforced resin-based composite material); and a frequency-selective surface layer 2 sandwiched between the resin-based composite material layers. Furthermore, a metal reflective layer is located at the bottom layer, and the first layer defining the side where electromagnetic waves are incident is the top layer. In this embodiment, the frequency-selective surface layer is a single layer, but the number of frequency-selective surface layers is not limited to this; it can also be multiple layers, such as 1 to 3 layers.

[0033] The frequency selective surface layer has a two-dimensional periodic arrangement of conductive patterns for forming the frequency selective surface (frequency selective plane). In this embodiment, the frequency selective surface layer has a conductive metal sheet or conductive film for forming the conductive pattern, and a substrate. Alternatively, a substrate-free conductive metal sheet or conductive film may also be used.

[0034] Conductive metal sheets can be, for example, copper foil or aluminum foil. Conductive films can be, for example, conductive carbon films or ITO conductive films. Substrates can be, for example, polyimide films, PET films, polycarbonate, or polymethyl methacrylate. The surface sheet resistance of the conductive metal sheet or conductive film ranges from 0.01 Ω / sq to 1000 Ω / sq. On the other hand, the sheet resistance of the conductive metal sheet / conductive film is determined by the thickness of the conductive film; generally, the greater the thickness, the smaller the sheet resistance. This thickness is on the order of approximately hundreds of nanometers, which is negligible relative to the substrate thickness. The substrate thickness is between 0.05 and 0.2 mm, and the substrate serves to support the conductive film (support). This thickness range facilitates good film flexibility while suppressing the impedance matching design that would affect the absorption performance if the thickness is too great.

[0035] Conductive patterns include, but are not limited to, square sheets, circular rings, square loops, hexagonal loops, etc. The geometric dimensions of adjacent conductive patterns and the spacing between their geometric centers (the periodicity of the conductive pattern arrangement) are between 1 / 20 and 1 / 5 of the wavelength corresponding to the lowest absorption frequency band of the integrated structural / stealth composite material, preferably between 1 / 15 and 1 / 5 of the wavelength corresponding to the lowest absorption frequency band of the integrated structural / stealth composite material. As an example, in Embodiment 2 described later, the integrated structural / stealth composite material with a reflection attenuation better than -10dB across the entire 5–21 GHz frequency band has a wavelength corresponding to the lowest absorption frequency band of 60 mm (the wavelength corresponding to a 5 GHz radar wave) multiplied by (1 / 20–1 / 5), i.e., 3–12 mm. Furthermore, the longitudinal dimension of each conductive pattern is between 1 / 20 and 1 / 5 of the wavelength corresponding to the lowest absorption frequency band of the integrated structural / stealth composite material. The subwavelength periodic unit structure can resonate with electromagnetic waves. The same composite material can contain the same pattern or a combination of different patterns. The sheet resistance of the frequency-selective surface (two-dimensional periodically arranged conductive patterns) is between 0.01 Ω / sq and 1000 Ω / sq, which can achieve loss of incident electromagnetic waves. Here, sheet resistance refers to the sheet resistance of the conductive pattern. When there is a substrate, the substrate is insulating, so the sheet resistance of the conductive pattern is the same as that of the conductive pattern + substrate. The sheet resistance value of this invention is designed based on the impedance matching principle, and the design of the sheet resistance value is coordinated with other structural parameters.

[0036] The resin-based composite material layer 1 may include fiber-reinforced composite materials. The reinforcing fibers are selected from non-conductive fibers such as glass fiber, quartz fiber, and aramid fiber, or conductive fibers such as carbon fiber may be used. One type may be used, or a mixture of two may be used to balance mechanical properties and cost. For example, fibers with high mechanical strength and high cost may be mixed with fibers with low mechanical strength and low cost. Long fibers (not short fibers) are preferred. When using conductive fibers such as carbon fiber, the proportion of uninsulated conductive fibers added is generally no more than 15% when the fiber surface is insulated or mixed with other fibers. For example, the resin-based composite material layer 1 may include glass fiber reinforced epoxy resin, glass fiber reinforced polyetheretherketone, or aramid fiber reinforced epoxy resin, which can further ensure good mechanical strength. The relative permittivity of the resin-based composite material in the 2-20 GHz frequency band is preferably between 2 and 5. Controlling the permittivity within this range further facilitates broadband absorption design. The resin-based composite material layer 1 may have a 1-4 layer structure. In this embodiment, on both sides of the frequency-selective surface layer 2 ( Figure 1 The resin-based composite material is symmetrically arranged in the middle (top and bottom sides) to protect the frequency selective layer inside and prevent environmental erosion.

[0037] Furthermore, when the resin-based composite material layer is a fiber-reinforced composite material layer, it may include multiple layers of prepreg or fibers, and each layer of prepreg or reinforcing fiber may be arranged in a unidirectional direction. The multilayer prepreg or fiber structure may include one or more of the following: -45° fiber layer, +45° fiber layer, 0° fiber layer, and 90° fiber layer. The direction of the fibers in the 0° fiber layer is perpendicular to the direction of the fibers in the 90° fiber layer. The angles between the fibers in the 45° fiber layer, the -45° fiber layer, and the 0° fiber layer are 45° and -45°, respectively, and the direction of the fibers in the 45° fiber layer is perpendicular to the direction of the fibers in the -45° fiber layer. For example, the arrangement sequence (independent of the frequency selection surface location) cycles through 0° / 45° / 90° / 90° / -45° / 0°, 0° / 45° / -45° / 0°, and 45° / 90° / 90° / -45°. The fiber arrangement (layout) satisfies quasi-isotropy (quasi-isotropy in resin-based composites), ensuring uniformity of material properties, such as consistent tensile strength in all directions. Planar woven fibers can be used. The thickness of each resin-based composite layer can be 2–4 mm. The total thickness of the structural / stealth integrated composite material can be no more than 20 mm.

[0038] The thickness of the bottom metal reflective layer (not shown) of the integrated structural / stealth composite material 10 can be 0.05-0.15 mm, or less than 0.1 mm. The sheet resistance of the bottom metal reflective layer can be set to no more than 15 Ω / sq.

[0039] The structural / stealth integrated composite material of this invention incorporates a frequency-selective surface layer within a resin-based composite material. Utilizing the inherent thickness of the resin-based composite material and the resonant effect of the frequency-selective surface, broadband radar absorption is achieved. This invention can be applied to the skin structures of UAVs and ship decks. The aforementioned resin-based composite material with the embedded frequency-selective surface can both bear loads and absorb radar waves, achieving broadband radar absorption without the need for an additional radar-absorbing coating. This effectively resolves the contradiction between the thickness of the absorbing material and the absorption bandwidth (suppressing additional loads), reduces the design complexity of stealth UAVs, and lowers the maintenance costs of stealth radar-absorbing materials.

[0040] The following exemplifies the preparation method of the integrated structure / stealth composite material of the present invention.

[0041] Taking the integrated structural / stealth composite material 10 of Embodiment 1 as an example, the inventors used the coaxial transmission line method to measure the relative permittivity and relative permeability of resin-based composite materials, such as fiber-reinforced resin-based composite materials, in the 2-20 GHz frequency band. They then used high-frequency electromagnetic simulation software to import the measured relative permittivity and relative permeability to design the integrated structural / stealth material. The design parameters included: the number of layers, the thickness of each layer of resin-based composite material, and the shape, size, arrangement period, and surface sheet resistance of the periodic frequency selection surface between each layer of resin-based composite material.

[0042] First, a conductive pattern is created from copper foil, aluminum foil, conductive carbon film, or ITO conductive film to obtain a frequency-selective surface. This forms a frequency-selective surface with a specified sheet resistance value, for example, controlled by oxygen partial pressure. The conductive metal sheet or conductive film with a substrate (base film) can be engraved. The conductive pattern on the frequency-selective surface can be engraved using photolithography, laser processing, or an engraving machine. When the gaps between the periodic patterns are large enough, holes are drilled at the gaps in the periodically arranged conductive patterns to remove part of the resistive film substrate, resulting in the frequency-selective surface. By drilling, the upper and lower layers of adhesive can flow, further improving the bonding strength. Alternatively, a conductive metal sheet or conductive film can be deposited on the substrate using magnetron sputtering or similar processes. Furthermore, the substrate side of the resistive film can be sandblasted before engraving or coating to increase the surface roughness of the resistive film. The sand used for sandblasting is between 200 and 300 mesh, and the sandblasting pressure is no greater than 12.5 psi. The purpose of sandblasting is to enhance the roughness of the frequency-selective surface and strengthen the interface between the frequency-selective surface and the resin-based composite material.

[0043] Furthermore, the implantation of a frequency-selective surface layer within the resin-based composite material may reduce its mechanical strength to some extent. To address this, the present invention preferably activates the areas of the base film not covered by the conductive film to further ensure the mechanical strength of the multilayer microwave absorbing material of the present invention. For example, a base film (with a conductive pattern applied) is prepared using polyimide, polyethylene terephthalate, polycarbonate, or polymethyl methacrylate as the frequency-selective surface. The areas of the base film not covered by the frequency-selective surface are treated with a sodium hydroxide solution with a concentration between 0.8 and 2 mol / L at room temperature for 0.5 to 1.5 hours for surface activation, or with an ammonia solution with a mass fraction of 25 to 33% at 60 to 90°C for 4 to 7 hours for surface activation. By utilizing the inherent mechanical properties of the resin-based composite material, such as fiber-reinforced composites, and employing the surface treatment process of the frequency-selective film, the fiber-reinforced composite material with the implanted frequency-selective surface is further guaranteed to maintain good mechanical strength. This approach imparts excellent microwave absorption performance to the resin-based composite material without significantly reducing its mechanical strength. The tensile strength of this integrated structure / stealth composite material is over 200 MPa.

[0044] Furthermore, when using substrate-free conductive films such as copper foil, aluminum foil, and conductive carbon film to create conductive patterns, a weakly adhesive acrylic glue or water-based polyurethane glue can be used to coat the unprocessed substrate-free conductive film onto the surface of oil paper or PET. After the pattern is processed, the pattern is transferred to the surface of the resin-based composite material and then the oil paper or PET is removed. At this time, there is no need to perform sandblasting and hole drilling operations.

[0045] Next, a prepreg is prepared for forming the resin-based composite material layer. For example, a fiber cloth or fiber bundle is impregnated in a specified resin slurry to obtain the prepreg.

[0046] Next, prepreg is laid layer by layer in the mold. Once the prepreg reaches the designed thickness, the corresponding frequency-selective surface is placed, followed by the laying of subsequent prepregs and frequency-selective surfaces. The prepreg corresponding to the bottom fiber-reinforced composite layer can be laid in the mold first, followed by the laying of the prepregs corresponding to the remaining fiber-reinforced composite layers or the placement of the frequency-selective surface layer in a predetermined order. After laying the prepreg corresponding to the top fiber-reinforced composite layer, curing is performed. The curing temperature can be 80–200°C; the pressure (hot pressing pressure) can be 1–5 MPa, or vacuum curing can be used; the time can be 8–12 hours. Depending on the specific shrinkage of the composite substrate, the thickness of the prepreg can be increased by 0.2% to 5% compared to the specified value (corresponding to the resin-based composite layer).

[0047] Next, conductive paste or conductive film is applied to one side of the resin-based composite material. Figure 1A metallic reflective layer is formed on the outer surface of the material (one side of the top and bottom sides). The cured material can be polished to remove excess resin-based composite material around the edges, and a conductive paste or conductive film can be sprayed onto the bottom layer to obtain the metallic reflective layer, resulting in a structural / stealth integrated material. The raw materials for the metallic reflective layer can include conductive pastes such as conductive carbon paste, copper-coated polyimide, and conductive ITO. The material can be cured at 80–100°C after applying the conductive paste.

[0048] Based on the above structure, the various design parameters coordinate with each other to form an organic whole. This ensures that the equivalent dielectric constant and equivalent permeability of the integrated structural / stealth material are approximately equal within the absorption frequency band, allowing the integrated structural / stealth material to match the impedance of free space and minimizing reflection. According to the basic principles of microwave transmission lines, due to the presence of the metal reflective layer, electromagnetic waves will be completely reflected when they reach the metal backplate, resulting in zero transmission. The reflected electromagnetic waves and the incident electromagnetic waves superimpose to form a standing wave. If a surface resistance of R is introduced at the point of strongest electric field at this time... s The frequency-selective surface will generate a surface current under external field excitation, and the electromagnetic energy will be completely dissipated by the lossy surface and converted into heat energy. In this invention, the integrated structural / stealth composite material can achieve radar wave absorption effect with reflection attenuation better than -7dB in the 2-18GHz frequency band.

[0049] The structural / stealth integrated composite material of this invention combines the mechanical advantages of resin-based composite materials with the design concept of artificial resonant structures (i.e., frequency-selective surfaces). On the one hand, it retains the excellent mechanical properties of resin-based composite materials, and by implanting at least one layer of frequency-selective surface in situ within the resin-based composite material, it endows it with excellent microwave absorption performance without significantly reducing the mechanical strength of the resin-based composite material. Simultaneously, the artificial resonant structure is protected within the resin-based composite material, resulting in low maintenance costs and greatly facilitating the design of stealth UAVs. It also suppresses the additional load imposed on the UAV by the stealth material, effectively resolving the contradiction between the thickness and bandwidth of the microwave absorbing material.

[0050] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention fall within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0051] In the following embodiments, unless otherwise specified, the reagents, materials and instruments used are all conventional reagents, materials and instruments, and are commercially available. The reagents involved can also be synthesized by conventional synthesis methods.

[0052] Example 1:

[0053] This embodiment provides a structural / stealth integrated composite material that operates in the X-band;

[0054] In this embodiment, glass fiber reinforced epoxy resin (brand name CYDBN-240) is used as the fiber-reinforced composite material. Its relative permittivity in the 2–18 GHz frequency band was measured using the coaxial line method to be 3.8, its loss tangent was 0.1, and it was non-magnetic.

[0055] The frequency selection surface is designed as a periodically arranged conductive ring. The specific design parameters are: the spacing between adjacent rings is 14.4 mm, the outer diameter of the ring is 7 mm, the inner diameter is 4.4 mm, the surface sheet resistance of the ring is 2 Ω / sq, and the thickness of the upper and lower glass fiber reinforced epoxy resin boards on the frequency selection surface is 2 mm.

[0056] A frequency selective surface was fabricated using copper foil with a thickness of 0.017 mm. The copper foil was then coated onto a 0.1 mm thick polyimide film, and a periodically arranged array of rings was created using a laser engraving machine. The polyimide film surface was then sandblasted to improve the interfacial bonding strength between the frequency selective surface and the fiber-reinforced composite material. The sand used was 300 mesh abrasive at a pressure of 10 psi. Simultaneously, the polyimide film was immersed in a 1.5 mol / L sodium hydroxide solution at room temperature for 0.5 hours. Then, holes with a diameter of 2 mm were drilled in the center area of ​​four adjacent conductive rings on the polyimide film.

[0057] Epoxy resin was applied to a 0.6mm thick woven fiberglass cloth. Four layers of fiberglass cloth were then laid into a 350mm×350mm mold at 0° / 45° / -45° / 0° and pressed firmly, with a layer thickness of approximately 2.2mm. The prepared frequency-selective surface was then placed in the center of the mold, and epoxy resin-impregnated fiberglass cloth was laid in layers at 0° / 45° / -45° / 0°. An appropriate amount of epoxy resin was added to the mold until it just covered the fiberglass cloth, and the mold was then closed. The mold was then pressure-cured at 80℃ and 2MPa for 8 hours.

[0058] After curing, remove the material, trim off any excess, and sand the surrounding area. Then, use a grinding machine to finish the glass fiber reinforced epoxy resin on both sides, controlling the thickness to 2±0.1mm.

[0059] Finally, a layer of conductive carbon paste with a sheet resistance of about 15Ω / sq is sprayed onto the underside of the above material and cured in an oven at 80°C for 12 hours to obtain a structural / stealth integrated composite material.

[0060] Example 2:

[0061] This embodiment provides a structural / stealth integrated composite material with reflection attenuation better than -10dB across the entire 5–21 GHz frequency band; in this embodiment, glass fiber reinforced epoxy resin (grade CYDHD-240) is used as the fiber-reinforced composite material. Its relative permittivity in the 2–25 GHz frequency band was measured using the coaxial line method to be 4.4, its loss tangent was 0.02, and it is non-magnetic.

[0062] The frequency selection surface is designed as a periodically arranged conductive square ring. The specific design parameters are as follows: the ring arrangement period is 6.8 mm, the outer diameter of the square ring is 6 mm, the ring width is 2 mm (i.e., the side length of the central hollow square is 2 mm), the surface sheet resistance of the square ring is 70 Ω / sq, and the thickness of the upper and lower glass fiber reinforced epoxy resin boards on the frequency selection surface is 2.5 mm.

[0063] A 0.05 mm thick PET film was used as the conductive film substrate. The surface of the PET film was sandblasted with 300-mesh sand at a pressure of 10 psi to increase its roughness. Simultaneously, the PET film was immersed in 33% ammonia water at 80°C for 5 hours to improve the interfacial bonding strength between the frequency-selective surface and the fiber-reinforced composite material. Then, a 70 nm thick ITO film was deposited on the treated PET film using magnetron sputtering. Finally, a periodically arranged square ring array was processed using a laser engraving machine.

[0064] Epoxy resin was applied to a 0.6mm thick woven fiberglass cloth. Four layers of fiberglass cloth were then laid into a 350mm×350mm mold at 45° / 90° / 90° / -45° and pressed firmly, with a layer thickness of approximately 2.7mm. The prepared frequency-selective surface was then placed in the center of the mold, and epoxy resin-impregnated fiberglass cloth was laid in layers at 45° / 90° / 90° / -45°. An appropriate amount of epoxy resin was added to the mold until it just covered the fiberglass cloth. The mold was then closed and hot-pressed in an 80℃ oven for 8 hours at a pressure of 2MPa.

[0065] After curing, remove the material, trim off any excess, and sand the surrounding area. Then, use a grinding machine to finish the glass fiber reinforced epoxy resin on both sides, controlling the thickness to 2.5±0.2mm.

[0066] Finally, a layer of conductive carbon paste with a sheet resistance of about 15Ω / sq is sprayed onto the underside of the above material and cured in an oven at 80°C for 12 hours to obtain a structural / stealth integrated composite material.

[0067] The composite materials obtained in the examples and comparative examples were tested:

[0068] Mechanical tensile strength test: The tensile strength of the examples and comparative examples was tested using a universal tensile testing machine according to the method specified in standard GB / T 1447-2005 "Test Method for Tensile Properties of Fiber Reinforced Plastics". Example 1 showed a tensile strength of 348 MPa, and Example 2 showed a tensile strength of 337 MPa. Comparative Examples 1 and 2 were composite materials without frequency-selective surfaces. Comparative Example 1 showed a tensile strength of 362 MPa, and Comparative Example 2 showed a tensile strength of 346 MPa. The mechanical strength of Examples 1 and 2 is comparable to that of their corresponding Comparative Examples 1 and 2.

[0069] Radar reflection attenuation performance test: Following the method specified in standard GJB2023-2011 "Test Method for Reflectivity of Radar Absorbing Materials", an arched method test system including a vector network analyzer was used to test the radar reflection attenuation of the embodiments and comparative examples. Embodiment 1 achieved a reflection attenuation better than -27.5dB at 12GHz, and Embodiment 2 achieved a reflection attenuation better than -10dB in the 5-21GHz frequency band. The comparative example did not show any radar wave attenuation effect. Embodiments 1 and 2 not only achieve radar reflection attenuation in specific frequency bands but also ensure good mechanical strength.

[0070] Table 1

[0071]

[0072] Table 2

[0073]

[0074] As shown in Table 1-2, surface treatment of the frequency-selective surface of the plastic film substrate with a certain concentration of sodium hydroxide and ammonia solution can improve the bonding strength between the frequency-selective surface and the composite material. Ideally, the sodium hydroxide solution concentration should be no less than 0.8 mol / L to avoid insufficient promotion of the formation of activating groups on the polyimide surface in a short time, thus promoting the bonding between the film and the composite material. When treating the PET substrate with ammonia, ideally, its mass percentage should be greater than 25%, and the treatment should be carried out at a temperature no lower than 60°C for at least 4-6 hours. This further ensures that the composite material after implantation of the frequency-selective surface possesses good mechanical strength.

[0075] Figure 6The variation of the material's microwave absorption performance with the arrangement period p is shown in Example 2-2. It can be seen that when the periodic unit is between 1 / 20 and 1 / 5 of the wavelength corresponding to the lowest limit of the microwave absorption operating frequency band, it exhibits better microwave absorption performance.

[0076] Figure 7 The diagram illustrates the variation of the material's microwave absorption performance with the outer diameter a1 of the square ring in Examples 2-3. It can be seen that as a1 increases, the absorption peak at the high-frequency end gradually shifts towards higher frequencies; the position of the high-frequency absorption peak can be adjusted by regulating a1.

[0077] Figure 8 The diagram illustrates the variation of the material's microwave absorption performance with the ring width (ring width = (a1-a2) / 2) in Examples 2-4. It can be seen that as the ring width increases, the overall frequency shifts towards lower frequencies; therefore, the overall position of the absorption frequency can be adjusted by regulating the ring width.

Claims

1. A structural / stealth integrated composite material, characterized in that, Structural / stealth integrated composite materials include: At least two layers of resin-based composite material; the relative permittivity of the resin-based composite material in the 2–18 GHz frequency band is between 2 and 5; the thickness of each resin-based composite material layer is less than 4 mm; At least one frequency-selective surface layer is sandwiched between at least two resin-based composite material layers, wherein the resin-based composite material layers and the frequency-selective surface layer are arranged alternately; the frequency-selective surface layer has a two-dimensional periodically arranged conductive pattern; the frequency-selective surface is a periodically arranged conductive ring, the spacing between adjacent rings is 14.4 mm, the outer diameter of the ring is 7 mm, the inner diameter is 4.4 mm, and the surface sheet resistance of the ring is 2 Ω / sq; the conductive pattern is made of copper foil, aluminum foil, conductive carbon film, or ITO conductive film; the thickness of each frequency-selective surface layer is less than 0.2 mm; And the bottom layer of metal reflective layer; the sheet resistance of the metal reflective layer is not greater than 15Ω / sq; the thickness of the metal reflective layer is less than 0.15 mm; The total thickness of the integrated structure / stealth composite material is less than 20 mm, the absorption band is 2 to 18 GHz, the reflection attenuation in the 2 to 18 GHz frequency band is better than -7 dB, and the tensile strength is not less than 200 MPa.

2. The integrated structural / stealth composite material according to claim 1, characterized in that, The resin-based composite material is a fiber-reinforced composite material, and the reinforcing fiber is at least one of glass fiber, quartz fiber, aramid fiber, and carbon fiber.

3. The integrated structural / stealth composite material according to claim 2, characterized in that, The resin-based composite material is at least one of glass fiber reinforced epoxy resin, glass fiber reinforced polyether ether ketone, and aramid fiber reinforced epoxy resin.

4. The application of a structural / stealth integrated composite material according to any one of claims 1 to 3 in aircraft skin or shipboard structure.

5. A method for preparing a structural / stealth integrated composite material according to any one of claims 1 to 3, characterized in that, include: Conductive patterns are created based on the set dimensions and the spacing between geometric centers to form a frequency-selective surface; Prepreg is obtained by impregnating fiber cloth or fiber bundles in resin slurry. The prepreg is laid in a mold in a predetermined order and placed on a frequency-selective surface, then hot-pressed; and After the hot pressing is completed, a metal reflective layer is formed on the bottom layer using conductive paste or conductive film. The base film is prepared using polyimide, polyethylene terephthalate, polycarbonate, or polymethyl methacrylate as the frequency selective surface. The areas of the base film not covering the frequency selective surface are treated with a sodium hydroxide solution with a concentration between 0.8 and 2 mol / L at room temperature for 0.5 to 1.5 hours to activate the surface, or a 25% to 33% ammonia solution is used to keep the surface at 60 to 90°C for 4 to 7 hours to activate the surface.

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