A wave-absorbing honeycomb composite material with a spatial gradient distribution of absorbents and a preparation method thereof

Abstract

The present application relates to a kind of absorbing agent spatial gradient distribution wave-absorbing honeycomb composite material and its preparation method, belong to wave-absorbing material preparation technical field.The present application is combined with the characteristics of multilayer wave-absorbing honeycomb, according to electric performance design in semi-solid honeycomb core material cell, different electromagnetic characteristics wave-absorbing adhesive film layer is pressed, electromagnetic characteristics is gradient change along cell direction, impedance matching structure is obtained;Wave-absorbing honeycomb is pasted on the surface (curved surface) of product or mould and solidified, and the wave-absorbing honeycomb core material after heat setting is obtained;Upper and lower surface is respectively bonded with wave-transparent skin, and excellent performance wave-absorbing honeycomb composite material is obtained, and the stealth demand of curved surface product is met.The present application provides process easy to control, wave-absorbing performance, excellent mechanical property, can be used for curved surface forming, absorbing agent spatial gradient distribution wave-absorbing honeycomb composite material and its preparation method, with excellent wave-absorbing performance in ultra-wide frequency range.

Description

Technical Field

[0001] The invention relates to a wave-absorbing honeycomb composite material with spatially gradient distributed absorbent and a preparation method thereof, belonging to the technical field of wave-absorbing material preparation. Background Art

[0002] Aramid paper honeycomb is a new lightweight structural sandwich material with advantages such as low density, high mechanical properties, and excellent heat resistance. It is primarily used in lightweight sandwich structures and has found widespread application in aircraft. With the advancement of detection technology, aircraft are required to achieve stealth with a wider absorption bandwidth and enhanced absorption. While aramid paper honeycomb has excellent electromagnetic wave transmission properties, it lacks electromagnetic wave absorption properties and cannot be directly used in the absorption field. Conventional approaches have been to introduce electromagnetic wave absorbers into the walls of the aramid paper honeycomb.

[0003] At present, most existing aramid paper absorbing honeycombs are made by first preparing ordinary aramid paper without electromagnetic loss function into aramid paper transparent honeycombs, then impregnating the honeycomb walls of the aramid paper transparent honeycombs with electromagnetic wave absorbent glue, and finally curing and forming the honeycombs. For example, in patent CN114228266A, a gradient impregnation method is used to distribute the absorbent in a gradient along the honeycomb grid or in a direction perpendicular to the honeycomb grid to obtain an impedance matching structure and better absorbing performance. However, the absorbing honeycombs obtained by the impregnation method have absorbents attached to the pore walls, which are easy to fall off, and have poor environmental resistance. In addition, the impregnation process is difficult to control and has poor process stability. The impregnation process cannot be used with absorbents such as carbon fibers and magnetic fibers with large aspect ratios, resulting in low magnetic loss performance of the aramid paper absorbing honeycombs. In addition, the impregnation process is poorly controllable, resulting in low electromagnetic performance stability of the aramid paper absorbing honeycombs.

[0004] To avoid the shortcomings of impregnated absorbing honeycombs, the patent "Dielectric Loss Aramid Paper, Absorbing Honeycomb, and Preparation Method" (CN112553942A) proposes a method of adding a carbon fiber absorber to aramid paper to produce the absorbing aramid paper, which is then prepared into an absorbing honeycomb. This method has the advantages of good environmental resistance due to the absorber being incorporated into the aramid paper, the amount of absorber added can be precisely controlled, and the process is stable. However, due to the limitations of the papermaking process, only fiber-based absorbers can be added, and the amount added should not be too high. Consequently, the method lacks excellent absorbing performance over an ultra-wide frequency range and generally exhibits poor performance at low frequencies. Furthermore, using traditional honeycomb preparation processes, the properties and tension of different regions of the absorbing aramid paper vary, making it difficult to ensure uniform honeycomb cell structure. Another method involves applying an absorbent to aramid paper to produce absorbing aramid paper, which is then used to prepare an absorbing honeycomb, as described in patents CN109796624A, CN202111440881.5, and CN114214871A. This method allows for the addition of various types of absorbents, and large amounts can be added to meet varying absorption requirements. The absorbent is then applied to the surface of the aramid paper to produce the absorbing aramid paper, which is then prepared using traditional honeycomb preparation methods. However, this method requires stretching to form hexagonal cells. If the absorbent coating has insufficient adhesion, debonding can easily occur. Furthermore, the absorbent coating should not be too thick, and the absorbent content should not be high. Currently, both of these methods can produce absorbing honeycombs with stable performance and excellent environmental resistance. However, the absorbent content in the honeycomb is fixed and low, and no absorbing structure design is employed. The absorbing honeycomb exhibits a single electromagnetic characteristic overall, lacking impedance matching, making it difficult to achieve excellent ultra-wideband absorbing performance.

[0005] In order to obtain an impedance matching structure, the commonly used method currently is to introduce an absorbing layer between the honeycomb sheet layers, perform absorbing structure design, and obtain an absorbing honeycomb composite material with good absorbing performance, such as the paper "Design of A-type skin composite honeycomb structure and its absorbing performance" (Journal of Composite Materials, 2022, 39(3): 1180-1185). However, in this structure, stress concentration points will form at the intersection of the two layers of honeycomb sheets, which are extremely easy to damage, thereby reducing the compressive strength of the overall structure; and the large number of bonding surfaces between the honeycomb layers will also affect the reliability of the overall structure.

[0006] In the patent for an absorbing honeycomb with an impedance-matching structure and its preparation method (CN115891293A), an absorbent is coated on aramid paper and a template method is used to create the absorbing honeycomb, resulting in an impedance-matching structure in the cell direction. By stacking the cells, rods are inserted into the honeycomb cells to apply pressure to the cell walls, ensuring the bonding strength of the core adhesive. The preparation method described in this patent precludes the production of curved absorbing honeycombs using semi-cured absorbing honeycomb core material.

[0007] The resulting absorbing honeycombs mentioned in the aforementioned studies are all flat, making them unsuitable for curved components (such as air intakes). Furthermore, the cured honeycombs cannot be bent or reshaped. For uniform absorbing honeycombs, the desired curved shape can be achieved through surface machining. However, for impedance-matched absorbing structures, the thickness of each layer cannot be changed, making machining impossible and creating a curved outer contour impossible. Summary of the Invention

[0008] The purpose of the present invention is to overcome the differences in the existing technology and provide an absorbing honeycomb composite material and its preparation method that is easy to control the process, has excellent absorbing performance and mechanical properties, can be used for curved surface molding, and has a spatial gradient distribution of absorbent, and has excellent absorbing performance in an ultra-wide frequency range.

[0009] The present invention combines the characteristics of multi-layered absorbing honeycombs to propose an absorbing honeycomb composite material and preparation method for curved surface molding, featuring an impedance-matching structure and a spatially gradient distribution of absorbents. Based on the electrical performance design, absorbing adhesive film layers with different electromagnetic properties are pressed into the cells of the semi-cured honeycomb core material. The electromagnetic properties exhibit a gradient change along the cell direction, resulting in an impedance-matching structure. The absorbing honeycomb is applied to the surface (curved surface) of a product or mold and cured to obtain a heat-set absorbing honeycomb core material. The upper and lower surfaces are bonded to a transparent skin, respectively, to obtain an excellent absorbing honeycomb composite material that meets the stealth requirements of curved products.

[0010] The technical solution of the present invention:

[0011] A microwave-absorbing honeycomb composite material with a gradient distribution of absorbent comprises an upper skin, a lower skin, a honeycomb core material and a microwave-absorbing film layer; the honeycomb core material is formed by curing a semi-cured honeycomb core material; the microwave-absorbing film layer is located within the cells of the honeycomb core material, and the honeycomb core material containing the microwave-absorbing film layer is located between the upper skin and the lower skin; the microwave-absorbing film layer contains an absorbent, and the electromagnetic properties of the absorbent in each microwave-absorbing film layer are distributed in a gradient.

[0012] Furthermore, the absorbing honeycomb composite material is designed based on electrical properties. Absorbent film layers with different electromagnetic properties are pressed into different locations within the cells of the semi-cured honeycomb core, resulting in a gradient of electromagnetic properties along the cell direction, creating an impedance-matching structure. The semi-cured honeycomb core material, including the absorbing film layer, is then applied to the surface (curved surface) of a product or mold and cured to produce a heat-set absorbing honeycomb core material. The upper and lower surfaces of the absorbing honeycomb core material are bonded to a transparent skin, achieving excellent absorbing performance and meeting the stealth requirements of curved products.

[0013] Furthermore, the semi-cured honeycomb core material is aramid paper impregnated with phenolic resin, which is dried to remove the solvent; and then subjected to low-temperature and long-term treatment to adjust the phenolic resin to a semi-cured state, thereby ensuring that the honeycomb core material has a certain compressive resistance while retaining the heat setting ability of the honeycomb core material.

[0014] Furthermore, the low-temperature and long-term treatment has a treatment temperature of 70° C. to 90° C. and a treatment time of 6 h to 12 h. During the low-temperature treatment, it is also necessary to ensure that the phenolic resin on the honeycomb cell walls does not flow along the cell walls.

[0015] Furthermore, the wave-absorbing film layer is composed of an absorbent and an adhesive, and has a thickness of 0.5 mm to 3 mm.

[0016] Furthermore, the absorbent has electromagnetic loss capability and is one or more of carbonyl iron powder, sendust, iron-cobalt-nickel, ferrite, etc., with a content of 30 wt% to 85 wt%.

[0017] Furthermore, the adhesive is one of epoxy resin, phenolic resin and bismaleimide resin, preferably phenolic resin.

[0018] Furthermore, electrical performance design is carried out to determine the type of absorber; electromagnetic simulation software is used to calculate and optimize the optimal gradient absorbing structure, determine the number of absorbing film layers in the absorbing honeycomb, the position of the absorbing film layers in the honeycomb cells, the thickness of each absorbing film layer and the content of the absorber.

[0019] Furthermore, the upper and lower skins are made of a wave-transmitting resin-based composite material, comprising a reinforcement layer and a matrix, wherein the reinforcement layer is one of wave-transmitting fiber cloths such as quartz fiber cloth and glass fiber cloth, and the matrix is ​​one of epoxy resin, phenolic resin, and bismaleimide resin.

[0020] A microwave-absorbing honeycomb composite material with a spatially gradient distribution of absorbent and a preparation method thereof are achieved by the following steps:

[0021] The first step is to prepare a semi-cured honeycomb core material;

[0022] The second step is to prepare the wave absorbing adhesive film layer;

[0023] The third step is to press the absorbing film layer into the cells of the semi-cured honeycomb core material according to the electrical performance design to obtain the absorbing honeycomb core material;

[0024] The fourth step is to heat-set the wave-absorbing honeycomb core material;

[0025] The fifth step is to bond the wave-absorbing honeycomb core material to the wave-transmitting skin to obtain a wave-absorbing honeycomb composite material.

[0026] Furthermore, the semi-cured honeycomb core material is made of aramid paper impregnated with phenolic resin, which is then dried to remove the solvent. After a long, low-temperature treatment, the phenolic resin is in a semi-cured state, giving the honeycomb core material both a certain degree of compressive strength and thermal setting capabilities. The low-temperature treatment temperature is 70°C to 90°C, and the treatment time is 6-12 hours. During the low-temperature treatment, it is also necessary to ensure that the phenolic resin on the honeycomb cell walls does not flow along the cell walls.

[0027] Furthermore, the absorbing film layer is composed of an absorber and an adhesive. The absorber has magnetic loss capability and is one or more of carbonyl iron powder, sendustine, iron-cobalt-nickel, ferrite, etc. The adhesive is one of epoxy resin, phenolic resin, and bismaleimide resin.

[0028] Furthermore, the wave-absorbing adhesive film layer is prepared by calendaring or casting, and is in an uncured and flexible state, which is convenient for pressing into the honeycomb cells.

[0029] Furthermore, electrical performance design is carried out to determine the type of absorber; electromagnetic simulation software is used to calculate and optimize the optimal gradient absorbing structure, determine the number of absorbing film layers in the absorbing honeycomb and the position of the absorbing film layers in the honeycomb cells; and then determine the thickness of each absorbing film layer and the content of the absorber.

[0030] Furthermore, the aforementioned method of pressing the absorbing film layer into the honeycomb cells employs a two-step process. In the first step, the absorbing film is placed on a press and heated to 50°C to 80°C, then held at this temperature for 0.5 to 1 hour to soften the film. A semi-cured honeycomb core is placed on top of the film, and the press is used to apply pressure, pressing the absorbing film into the honeycomb cells until the film is completely embedded within the cells. In the second step, a rigid foam flat plate is placed beneath the honeycomb core, and the press is used to continue applying pressure to the honeycomb core. The honeycomb cell walls are cut and immersed into the rigid foam, which then enters the honeycomb cells, pushing the absorbing film into the cells. By controlling the press stroke, the position of the absorbing film within the honeycomb cells can be precisely controlled.

[0031] Furthermore, the fourth step, heat setting the absorbing honeycomb core material, involves placing the semi-cured absorbing honeycomb on the mold surface (curved surface) and curing it using a vacuum bag press or autoclave process. The curing temperature is 180°C to 200°C, and the curing time is 2 hours to 4 hours.

[0032] Furthermore, the absorbing honeycomb core is bonded to a wave-transmitting skin, wherein the skin is a wave-transmitting fiber cloth resin-based composite material. During bonding, the skin can be in a cured or uncured state. The skin and the absorbing honeycomb core are bonded using a plate-to-core adhesive.

[0033] The beneficial effects of the present invention compared with the prior art are as follows:

[0034] (1) The present invention pushes the absorbing film layer into the semi-cured honeycomb cells to form an impedance matching structure, thereby achieving precise control of the absorbing structure. The obtained absorbing honeycomb core material can be formed on a curved surface to meet the stealth requirements of curved components, thus overcoming the problem that the absorbing honeycomb of the traditional impedance matching structure cannot change its thickness and cannot be processed to obtain a curved surface.

[0035] (2) The present invention pushes the absorbing film layer into different positions in the honeycomb cells by physical methods, thereby obtaining an impedance matching structure while keeping the honeycomb cells intact, thus overcoming the shortcomings of the multi-layer absorbing honeycomb cell walls intersecting and insufficient compressive resistance.

[0036] (3) The present invention places the absorbing film into the honeycomb cells, and different types of absorbents can be added to meet different absorbing requirements; the absorbent can be added in large quantities, and has excellent absorbing effect in an ultra-wide frequency range, overcoming the shortcomings of the existing absorbent-doped aramid paper absorbing honeycomb, such as the limited absorbent types (only fibrous) and low absorbent content, and also overcomes the shortcomings of the thin absorbing layer thickness in the coated aramid paper absorbing honeycomb. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the structure of the semi-cured wave-absorbing honeycomb core material that can be used for curved surface molding according to the present invention;

[0038] Figure 2 This is a flow chart for preparing the wave-absorbing honeycomb composite material that can be used for curved surface molding according to the present invention;

[0039] Figure 3 Schematic diagram of the process of pushing the absorbing film into the honeycomb cells of the present invention. DETAILED DESCRIPTION

[0040] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to specific embodiments and accompanying drawings.

[0041] The present invention combines the characteristics of multi-layer absorbing honeycomb to propose an absorbing honeycomb composite material with an impedance matching structure and a spatial gradient distribution of absorbents and a preparation method. According to the electrical performance design, absorbing adhesive film layers with different electromagnetic properties are inserted at different positions in the cells of the semi-cured honeycomb core material. The electromagnetic properties change gradiently along the cell direction, resulting in an impedance matching structure. Figure 1 The absorbing honeycomb is applied to the surface (curved surface) of the product or mold and solidified to obtain a heat-set absorbing honeycomb core material; the upper and lower surfaces are bonded to the wave-transmitting skin respectively to obtain an excellent performance absorbing honeycomb composite material that meets the stealth requirements of curved products.

[0042] Based on the electrical performance design, the impedance matching structure of the absorbing film in the honeycomb core material is calculated and optimized. The type and content of the absorbent in the absorbing film layer, the thickness of the absorbing film layer, the number of absorbing film layers, and the position of the absorbing film layer within the honeycomb cells are determined. Absorbents of varying types and contents are mixed with an adhesive and then calendered or cast to form the absorbing film layer. The calendering or casting process is well known in the art, and specific process parameters are determined based on actual production needs.

[0043] The present invention also provides Figure 2 The microwave-absorbing honeycomb composite material that can be used for curved surface molding and its preparation method are shown, which are achieved by the following steps:

[0044] 1. Preparation of semi-cured honeycomb core material

[0045] The semi-cured honeycomb core material in this step is aramid paper impregnated with phenolic resin, and the solvent is removed after drying. After long-term treatment at low temperature, the phenolic resin is in a semi-cured state, and the honeycomb core material has both certain compressive resistance and thermal setting ability.

[0046] Low temperature and long time treatment, the treatment temperature is 70℃~90℃, the treatment time is 6h-12h. During the low temperature treatment process, it is also necessary to ensure that the phenolic resin on the honeycomb pore wall does not flow along the pore wall.

[0047] 2. Preparation of absorbing film layer

[0048] According to the electrical performance design, the impedance matching structure of the absorbing film in the honeycomb core material is calculated and optimized, and the type and content of the absorber in the absorbing film layer, the thickness of the absorbing film layer, the number of absorbing film layers, and the position of the absorbing film layer in the honeycomb cell are determined.

[0049] The absorbing film layer is composed of an absorber and an adhesive. The absorber has magnetic loss resistance and can be one or more of carbonyl iron powder, sendust, iron-cobalt-nickel, ferrite, etc. The adhesive can be one of epoxy resin, phenolic resin, or bismaleimide resin.

[0050] Based on electrical performance design, different absorbent types and contents are mixed with adhesives and then produced using a casting or calendaring process to create different absorbing film layers. The absorbent content in the absorbing film layers ranges from 30wt% to 85wt%, and the thickness ranges from 0.5mm to 3mm. The absorbing film layers with different electromagnetic properties, A1, A2, A3, A4,..., and An, are prepared with increasing absorbent content and electromagnetic parameters.

[0051] 3. Based on the electrical performance design, the semi-cured honeycomb core material cells are built with an absorbing film layer

[0052] The absorbing film is pressed into the honeycomb cells using a two-step method, such as Figure 3As shown. The first step is to place the absorbing film on a press and heat it to 50°C to 80°C. Keep it warm for 0.5 to 1 hour to soften the film. A semi-cured honeycomb core is placed on top of the film and pressurized with a press to press the absorbing film into the honeycomb cells until the film is completely embedded. The second step is to place a rigid foam flat sheet beneath the honeycomb core and continue pressurizing the core with a press. The honeycomb cell walls are cut and immersed in the rigid foam. The rigid foam enters the honeycomb cells, pushing the absorbing film into them. By controlling the press stroke, the position of the absorbing film within the honeycomb cells can be precisely controlled.

[0053] According to the designed order and position, push the absorbing film layer into the semi-cured honeycomb core material cells one by one, such as Figure 3 As shown in the second step, place the absorbing film layer A1 at the bottom of the honeycomb core cells. Place a rigid foam flat plate beneath the absorbing film layer. Apply pressure and push the film layer upward from bottom to the desired position. Sequentially place the A2, A3, and An film layers and push them to their corresponding positions, resulting in a semi-cured absorbing honeycomb core with an impedance-matching structure.

[0054] 4. Heat setting of wave-absorbing honeycomb core material

[0055] In this step, the absorbing honeycomb core material is heat-set by placing the semi-cured absorbing honeycomb on the mold surface (curved surface), with the lower surface of the honeycomb core material serving as the mold surface. Curing is performed using a vacuum bag press or autoclave process at a temperature of 180°C to 200°C for 2 to 4 hours.

[0056] During the heat setting process, the absorbing film layer also heats up and cures. It softens during this heating process. Due to the high proportion of magnetic absorber, the absorbing film is unlikely to flow along the honeycomb pore walls, maintaining an impedance-matching structure. The resin on both the absorbing film layer and the honeycomb core pore walls is not fully cured, allowing for better resin penetration during the heating and holding process. Fully cured ensures a strong bond between the two.

[0057] After the wave-absorbing honeycomb core material is cured, the shape of the designed curved surface is maintained, thereby obtaining a heat-set wave-absorbing honeycomb core material.

[0058] 5. The wave-absorbing honeycomb core material is bonded to the wave-transmitting skin to obtain a wave-absorbing honeycomb composite material.

[0059] The wave-transmitting skin in this step is a wave-transmitting fiber cloth resin-based composite material. The wave-transmitting fiber cloth can be quartz fiber cloth or glass fiber cloth, and the resin can be epoxy resin, phenolic resin, cyanate resin or bismaleimide resin.

[0060] The wave-transparent fiber / resin prepreg can be directly laid on the upper and lower surfaces of the wave-absorbing honeycomb core material and placed on a curved mold for curing; or the skin prepreg can be laid on the curved mold first, and the upper and lower skins are cured respectively, and then the upper and lower skins are laid on the upper and lower surfaces of the wave-absorbing honeycomb core material for bonding.

[0061] The skin and the wave-absorbing honeycomb core material are bonded by using a plate-core adhesive, and the bonding process is hot pressing or vacuum bag process, the bonding and curing are carried out on a curved mold to obtain the wave-absorbing honeycomb composite material. The curing temperature is 175-185℃, and the curing time is 2-4h.

[0062] The present application will be described in detail below in combination with the drawings and specific examples.

[0063] Example 1

[0064] Step 1: According to the actual demand, the total thickness of the wave-absorbing honeycomb composite material is determined to be 27mm, wherein the thickness of the wave-absorbing honeycomb core material is 25mm, the honeycomb cell shape is hexagonal, and the length is 2.77mm, and the upper and lower skins are both 1mm thick quartz fiber cloth / epoxy resin composite materials.

[0065] The semi-cured wave-absorbing honeycomb core material is prepared by impregnating aramid paper with phenolic resin, and the preparation process is aramid paper core strip adhesive, lamination, hot pressing, stretching, glue dipping, drying, and then low-temperature treatment at 80℃ for 8h. After low-temperature treatment, the semi-cured honeycomb core material is obtained, at this time the phenolic resin on the honeycomb cell wall is in a semi-cured state and has heat setting ability.

[0066] Step 2: According to the wave-absorbing requirement in the frequency band of 1GHz-18GHz, the electrical performance of the wave-absorbing honeycomb is designed. The optimal impedance matching wave-absorbing structure is calculated by using electromagnetic simulation software. The absorber in the wave-absorbing adhesive film layer is carbonyl iron powder, and the adhesive is epoxy resin. The thickness of the adhesive film layer is 2.0mm, and there are six layers, A1, A2, A3, A4, A5, and A6. The mass ratio of the absorber in the wave-absorbing adhesive film layers A1, A2, A3, A4, A5, and A6 is 30%, 40%, 50%, 60%, 70%, and 85% respectively. The positions of each wave-absorbing layer, i.e. the distance from the lower surface, are 22mm, 18mm, 14mm, 10mm, 6mm, and 1mm respectively. From the upper surface to the lower surface of the honeycomb core material (in the direction of the honeycomb thickness), the wave-absorbing adhesive film in the honeycomb cell is A1, A2, A3, A4, A5, and A6 in turn.

[0067] According to the designed absorber content, different contents of carbonyl iron powder and epoxy resin are mixed uniformly to prepare the wave-absorbing adhesive film by using calendering method, and the thickness is 2.0mm. The obtained wave-absorbing adhesive films are A1, A2, A3, A4, A5, and A6 respectively.

[0068] Step 3: Press the absorbing film into the honeycomb cells. First, place the absorbing film on a press and heat it to 60°C. Hold the temperature for 0.5 hours to soften it. Place a semi-cured honeycomb core material on top of the film and apply pressure with the press, forcing the absorbing film into the honeycomb cells until it is completely embedded. Second, place a rigid foam flat sheet beneath the honeycomb core material and continue applying pressure with the press. Cut the honeycomb cell walls and immerse them in the rigid foam. The rigid foam enters the honeycomb cells, pushing the absorbing film into them. By controlling the press stroke, the position of the absorbing film within the honeycomb cells can be precisely controlled.

[0069] First, the absorbing film A1 is pushed in from the bottom of the honeycomb core. Rigid foam is used to push the absorbing film A1 into the cells, controlling the insertion depth to 22mm to ensure the accurate positioning of the A1 layer within the honeycomb. Layers A2, A3, A4, A5, and A6 are then placed into the cells in sequence, and metal pillars are used to push the film into the designed positions, with insertion depths of 18mm, 14mm, 10mm, 6mm, and 1mm, respectively. The result is a semi-cured absorbing honeycomb core with impedance matching. The A1 and A6 absorbing film layers are each 1mm away from the upper and lower surfaces of the honeycomb core to increase the bonding strength between the honeycomb core and the skin during the plate-to-core adhesive bonding.

[0070] Step 4: Design a curved mold for honeycomb curing based on the actual product profile. Place the semi-cured absorbing honeycomb core material on the curved mold surface, with the lower surface of the honeycomb core material serving as the mold contact surface. Curing is performed using a vacuum bagging process at 180°C for 2 hours. After curing, remove the mold to obtain the heat-set absorbing honeycomb core material. Mark the upper and lower surfaces.

[0071] Step 5: Select quartz fiber cloth / epoxy resin as the skin material, with both upper and lower skins 1.0 mm thick. Apply the quartz fiber cloth / epoxy resin prepreg to the mold surface with a thickness of 1.0 mm. Curing is performed using a vacuum bagging process at 130°C for 2 hours to obtain the upper skin. The lower skin is prepared using the same method.

[0072] The lower skin, plate-core adhesive, absorbing honeycomb core material, plate-core adhesive, and upper skin are laid on the mold surface in this order, and cured by vacuum bag pressing process at a curing temperature of 180°C and a curing time of 3 hours to obtain an absorbing honeycomb composite material.

[0073] The absorbing honeycomb composite material has a smooth appearance and good bonding quality between the skin and the honeycomb core material; the profile is consistent with the theoretical curved surface, with a deviation of less than 0.2mm. The absorbing performance (reflectivity) of the curved absorbing honeycomb composite material cannot be tested. Using the same process, the absorbing honeycomb core material is cured on a flat mold, and then the absorbing honeycomb composite material (equivalent flat plate) is prepared. Its absorbing performance is 1GHz~2GHz reflectivity ≤-20dB, 2GHz~8GHz reflectivity ≤-25dB, and 8GHz~18GHz reflectivity ≤-30dB. The absorbing honeycomb has excellent absorbing effect within the required electromagnetic wave frequency band (1GHz-18GHz).

[0074] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0075] Example 2

[0076] The absorbing frequency of the absorbing honeycomb composite material is mainly 0.5GHz-8GHz, the absorber is sendust flake powder, and the mass ratios are 1%, 2%, 6%, 12%, 24% and 30% respectively. The rest are the same as in Example 1, and a absorbing honeycomb composite material is obtained.

[0077] This absorbing honeycomb composite material has a smooth and smooth appearance, with excellent bonding between the skin and the honeycomb core. The profile is consistent with the theoretical curve, with a deviation of less than 0.2mm. The absorbing honeycomb composite material (equivalent to a flat plate) exhibits a reflectivity of ≤-10dB from 0.5GHz to 1GHz, ≤-20dB from 1GHz to 2GHz, ≤-25dB from 2GHz to 8GHz, and ≤-20dB from 8GHz to 18GHz. The absorbing honeycomb exhibits excellent absorption within the required electromagnetic wave frequency band (0.5GHz-8GHz).

[0078] Example 3

[0079] The microwave-absorbing honeycomb composite material primarily absorbs waves with a frequency range of 0.5 GHz to 18 GHz. Its thickness is 32 mm, with the absorbing honeycomb core material being 30 mm thick and the upper and lower skins each 1 mm thick. The absorbents are carbonyl iron powder and sendust, each comprising 50% carbonyl iron powder and 50% sendust aluminum. The microwave-absorbing adhesive film is comprised of seven layers; each 1.5 mm thick, with absorbent content ratios of 30%, 35%, 45%, 55%, 65%, 75%, and 85% by weight, respectively. The layers are positioned at distances of 27 mm, 24 mm, 20 mm, 15 mm, 10 mm, 5 mm, and 1 mm from the lower surface, respectively. The remaining steps are the same as in Example 1, resulting in a microwave-absorbing honeycomb composite material.

[0080] This absorbing honeycomb composite material has a smooth and smooth appearance, with excellent bonding between the skin and the honeycomb core. The profile is consistent with the theoretical curve, with a deviation of less than 0.2mm. The absorbing honeycomb composite material (equivalent to a flat plate) exhibits a reflectivity of ≤-8dB from 0.5GHz to 1GHz, ≤-18dB from 1GHz to 2GHz, ≤-25dB from 2GHz to 8GHz, and ≤-28dB from 8GHz to 18GHz. The absorbing honeycomb exhibits excellent absorption within the required electromagnetic wave frequency band (0.5GHz-18GHz).

[0081] Other implementations:

[0082] Other implementation methods mainly change the thickness of the honeycomb core material, the type and content of the absorber, the thickness, number of layers and position of the absorbing film layer, which are essentially the same as the above three implementation methods, and the preparation method remains unchanged.

[0083] Comparative Example 1: The absorbent content in the absorbing film layer was 50%, and the remaining conditions were the same as in Example 1, resulting in an absorbing honeycomb composite material. This absorbing honeycomb composite material had a smooth and smooth appearance, good bonding quality between the skin and the honeycomb core, and a profile consistent with the theoretical curved surface, with a deviation of less than 0.2 mm. The absorbing performance of the absorbing honeycomb composite material (equivalent flat plate) was a reflectivity of ≤-10 dB from 1 GHz to 2 GHz, ≤-15 dB from 2 GHz to 8 GHz, and ≤-20 dB from 8 GHz to 18 GHz. The absorbing honeycomb exhibited poor absorbing performance within the required electromagnetic wave frequency band (1 GHz to 18 GHz).

[0084] Comparative Example 2: The honeycomb core material substrate is a cured honeycomb core material, and the rest is the same as Example 1, to obtain an absorbing honeycomb composite material. The absorbing honeycomb composite material cannot be heat-set, has a poor appearance, cracks in the honeycomb core, and poor bonding quality between the skin and the honeycomb core; the profile is very different from the theoretical curved surface, with a deviation greater than 5mm. The absorbing performance of the absorbing honeycomb composite material (equivalent flat plate) is a reflectivity of ≤-18dB from 1GHz to 2GHz, ≤-23dB from 2GHz to 8GHz, and ≤-28dB from 8GHz to 18GHz. The absorbing honeycomb has a good absorbing effect within the required electromagnetic wave frequency band (1GHz-18GHz). This is the absorbing performance tested under a flat plate. The absorbing honeycomb composite material of Comparative Example 2 cannot be heat-set and bent, will crack, and cannot be used on curved surfaces.

[0085] The specific embodiments and drawings of the present invention disclosed above are intended to facilitate understanding of the present invention and its implementation. Those skilled in the art will appreciate that various substitutions, changes, and modifications are possible without departing from the spirit and scope of the present invention. The present invention should not be limited to the embodiments and drawings disclosed in this specification. The scope of protection of the present invention shall be determined by the scope defined in the claims.

Claims

1. A microwave-absorbing honeycomb composite material with a spatially gradient distribution of absorbent, characterized in that: The invention comprises an upper skin, a lower skin, a honeycomb core material and an absorbing film layer; the honeycomb core material is formed by curing a semi-cured honeycomb core material; the absorbing film layer is located in the cells of the honeycomb core material, and the honeycomb core material containing the absorbing film layer is located between the upper skin and the lower skin; the absorbing film layer contains an absorbent, and the electromagnetic properties of the absorbent in each absorbing film layer are distributed in a gradient; the absorbing honeycomb composite material is prepared by the following steps: according to the electrical performance design, absorbing film layers with different electromagnetic properties are pressed into different positions in the cells of the semi-cured honeycomb core material, and the electromagnetic properties change in a gradient along the cell direction to obtain an impedance matching structure; the semi-cured honeycomb core material containing the absorbing film layer is applied to the surface of a mold and cured to obtain a heat-set absorbing honeycomb core material; the upper and lower surfaces of the absorbing honeycomb core material are bonded to the upper skin and the lower skin respectively; the content of the absorbent is 30wt% to 85wt%; The wave-absorbing honeycomb composite material with a spatially gradient distribution of absorbent is prepared by the following steps: preparing a semi-cured honeycomb core material; preparing a wave-absorbing adhesive film layer; According to the electrical performance design, the absorbing film layer is pressed into the cells of the semi-cured honeycomb core material to obtain the absorbing honeycomb core material; Heat-setting the wave-absorbing honeycomb core material; bonding the wave-absorbing honeycomb core material to the upper skin and the lower skin to obtain a wave-absorbing honeycomb composite material; The wave-absorbing adhesive film layer is in an uncured and flexible state; The absorbing film layer is pressed into the honeycomb cells in a two-step process: The first step is to place the absorbing film on a press and heat it to 50°C~80°C. Keep it warm for 0.5h~1h to soften the absorbing film. A semi-cured honeycomb core is placed on top of the film. The press is then used to apply pressure to press the absorbing film into the honeycomb cells until the film is completely inside the cells. In the second step, a hard foam flat plate is placed under the honeycomb core material, and the honeycomb core material is continued to be pressurized using a press. The honeycomb pore wall is cut and immersed in the hard foam. The hard foam enters the honeycomb pores and pushes the absorbing film into the honeycomb pores. The position of the absorbing film in the honeycomb pores is precisely controlled by controlling the stroke of the press.

2. The wave-absorbing honeycomb composite material according to claim 1, characterized in that: The semi-cured honeycomb core material is aramid paper impregnated with phenolic resin. After drying, the solvent is removed and then low-temperature and long-term treatment is performed to adjust the phenolic resin to a semi-cured state. The low-temperature and long-term treatment is performed at a temperature of 70°C to 90°C and for 6h to 12h. During the low-temperature treatment, it is ensured that the phenolic resin on the honeycomb cell walls does not flow along the cell walls.

3. The wave-absorbing honeycomb composite material according to claim 1, characterized in that: The absorbing film layer includes an absorber and an adhesive, and the thickness of the absorbing film layer is 0.5mm~3mm; the absorber has electromagnetic loss capability and is at least one of carbonyl iron powder, sendustine, iron cobalt nickel, and ferrite; the adhesive is one of epoxy resin, phenolic resin, and bismaleimide resin.

4. The wave-absorbing honeycomb composite material according to claim 1, characterized in that: The type of absorber is determined by conducting electrical performance design; the optimal gradient absorbing structure is calculated and optimized using electromagnetic simulation software, and the number of absorbing film layers in the absorbing honeycomb, the position of the absorbing film layers in the honeycomb cells, the thickness of each absorbing film layer and the content of the absorber are determined.

5. The wave-absorbing honeycomb composite material according to claim 1, characterized in that: The upper skin and the lower skin are wave-transmitting resin-based composite materials, including a reinforcement layer and a matrix, wherein the reinforcement is one of quartz fiber cloth and glass fiber cloth, and the matrix is ​​one of epoxy resin, phenolic resin, and bismaleimide resin.

6. A method for preparing the microwave-absorbing honeycomb composite material with a spatially gradient distribution of absorbent according to claim 1, characterized in that: The following steps are involved: preparing a semi-cured honeycomb core material; preparing a wave-absorbing adhesive film layer; According to the electrical performance design, the absorbing film layer is pressed into the cells of the semi-cured honeycomb core material to obtain the absorbing honeycomb core material; Heat-setting the wave-absorbing honeycomb core material; The wave-absorbing honeycomb core material is bonded to the upper skin and the lower skin to obtain a wave-absorbing honeycomb composite material.

7. The preparation method according to claim 6, characterized in that The wave-absorbing adhesive film layer is prepared by calendaring or casting, and is in an uncured and flexible state, and is easy to be pressed into the honeycomb cells.

8. The preparation method according to claim 6, characterized in that The absorbing film layer is pressed into the honeycomb cells in a two-step process: The first step is to place the absorbing film on a press and heat it to 50°C~80°C. Keep it warm for 0.5h~1h to soften the absorbing film. A semi-cured honeycomb core is placed on top of the film. The press is then used to apply pressure to press the absorbing film into the honeycomb cells until the film is completely inside the cells. In the second step, a hard foam flat plate is placed under the honeycomb core material, and the honeycomb core material is continued to be pressurized using a press. The honeycomb pore wall is cut and immersed in the hard foam. The hard foam enters the honeycomb pores and pushes the absorbing film into the honeycomb pores. The position of the absorbing film in the honeycomb pores is precisely controlled by controlling the stroke of the press.

9. The preparation method according to claim 6, characterized in that The heat setting of the wave-absorbing honeycomb core material is performed by placing the semi-cured wave-absorbing honeycomb on the mold surface and curing it by vacuum bag pressing or autoclave process, with the curing temperature being 180°C to 200°C and the curing time being 2h to 4h. The wave-absorbing honeycomb core material is bonded to the upper skin and the lower skin, wherein the skin is a wave-transmitting fiber cloth resin-based composite material, and the skin is in a cured state or an uncured state during bonding, and the skin and the wave-absorbing honeycomb core material are bonded using plate-core adhesive.

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