A wave-absorbing concealing skin material and a preparation method thereof

Abstract

The application relates to the technical field of layered products, and particularly discloses a skin material for wave absorption and hiding and a preparation method. The skin material has a layered structure and comprises, from top to bottom, a biomimetic micro-forest wave-transparent layer, a gradient transition layer, a wave-absorbing layer with a'skeleton-matrix' double-network structure, a reflecting layer and a heat-insulating layer; the biomimetic micro-forest wave-transparent layer is provided with a micro-pyramid array on the surface, so that wide-frequency impedance matching is realized; the wave-absorbing layer comprises a three-dimensional ceramic skeleton and a magnetic-dielectric composite matrix filled in the pores of the skeleton, and the magnetic absorbent in the wave-absorbing layer is arranged in the layer plane direction through magnetic field auxiliary technology. The skin material prepared by the application has excellent comprehensive properties of super-wide-frequency wave absorption, high mechanical strength, light weight and high-temperature resistance, and is particularly suitable for stealth requirements in the field of aviation and aerospace.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of layered products, more particularly to a skin material for wave absorption and concealment and a preparation method thereof. BACKGROUND

[0002] With the rapid development of modern detection technology, the stealth performance of aircraft, ships and other equipment has become a key indicator of their combat effectiveness. As one of the key technologies for achieving stealth, wave-absorbing materials can effectively absorb electromagnetic waves and reduce the detectability of targets.

[0003] Currently, commonly used wave-absorbing materials are mostly of layered structure, which realizes the absorption of electromagnetic waves through the combination of impedance matching layers, wave-absorbing layers, and reflective layers. For example, patent CN117087247B discloses a wave-absorbing composite material comprising an outer skin layer, a honeycomb core layer, and an inner skin layer; and patent CN116812137A discloses a polyimide fiber composite stealth sandwich structure comprising a high-transmission skin, a wave-absorbing honeycomb core layer, and a carbon fiber reflective plate. However, the existing technology has the following problems:

[0004] The impedance matching layer is mostly a uniform coating or a simple gradient layer, which is difficult to achieve good impedance matching in a wide frequency band, resulting in a narrow wave-absorbing frequency band, especially poor performance in the low frequency band.

[0005] The wave-absorbing layer usually uses a simple mixture of magnetic and dielectric absorbents, which lacks a synergistic enhancement mechanism between the two, and the high filler content leads to high brittleness and poor mechanical properties of the material.

[0006] The interlayer interface has low bonding strength and is prone to delamination under thermal stress, affecting the service life and reliability of the material.

[0007] Traditional preparation processes are difficult to achieve complex microstructure control, which cannot meet the demand for high-performance wave-absorbing materials.

[0008] Therefore, it is of great significance to develop a skin material with wideband wave absorption, high mechanical strength, and good environmental adaptability. SUMMARY

[0009] The purpose of the present application is to provide a skin material for wave absorption and concealment and a preparation method thereof to solve the problems raised in the background technology.

[0010] To achieve the above purpose, on the one hand, the present application provides a skin material for wave absorption and concealment, which comprises, from top to bottom, a biomimetic micro-forest wave-transparent layer, a gradient transition layer, a wave-absorbing layer, a reflective layer, and a thermal insulation layer.

[0011] The biomimetic micro-forest wave-transparent layer is provided with a micro-pyramid array on the surface, the height of the micro-pyramid is 150-300 μm, and the ratio of the base diameter to the height of the pyramid is 1:(1.5-3).

[0012] The wave-absorbing layer comprises a three-dimensional ceramic skeleton and a magnetic-dielectric composite matrix filled in the pores of the three-dimensional ceramic skeleton, and the magnetic absorbing agent in the magnetic-dielectric composite matrix is arranged in the layer plane direction.

[0013] The biomimetic micro-forest wave-transparent layer simulates the structure of moth eyes, realizes a continuous gradient transition of the dielectric constant from air to the interior of the material through a conical array, and solves the problem of impedance matching at a wide frequency band and a large incident angle from the structural level, thereby maximizing the efficiency of electromagnetic wave incidence. The "skeleton-matrix" dual-network wave-absorbing layer is a core innovation, in which the three-dimensional ceramic skeleton bears mechanical load and prolongs the electromagnetic wave propagation path, and the magnetic-dielectric composite matrix converts electromagnetic energy into heat energy through the synergistic effect of magnetic loss and dielectric loss; in particular, the magnetic domain is arranged in a direction through magnetic field induction, thereby producing anisotropic magnetic permeability, significantly enhancing low-frequency magnetic resonance loss, and solving the contradiction between wide frequency (especially low frequency) and high efficiency absorption and material mechanical strength. The reflection layer reflects the electromagnetic waves that are not absorbed back to the wave-absorbing layer for secondary absorption, thereby improving the single absorption efficiency. The thermal insulation layer blocks the transmission of external high temperature to the inside through super thermal insulation materials such as aerogel, thereby ensuring the performance stability of the wave-absorbing functional layer in a wide temperature range. The structure realizes the full-path optimization of efficient electromagnetic wave incidence → gradient matching → synergistic loss → secondary absorption → thermal protection.

[0014] Preferably, the micro-cone array is a hexagonal close-packed structure, the cone angle of the micro-cone is 30°-60°, and the center distance between adjacent micro-cones is 150-400 μm. The hexagonal close-packed structure is one of the most effective close-packed structures in nature, which can arrange the most micro-cone units in a unit area, realize the most smooth and continuous equivalent dielectric constant gradient, and ensure that the reflection of electromagnetic waves on the side wall of the cone is suppressed to the lowest level. The optimized center distance (100-500 μm) avoids unnecessary electromagnetic coupling resonance between adjacent micro-cones, thereby ensuring stable and excellent wave-transparent and impedance matching performance in the frequency band of 2-18 GHz or even higher.

[0015] Preferably, the porosity of the gradient transition layer linearly increases from the side close to the biomimetic micro-forest wave-transparent layer to the side close to the wave-absorbing layer, and the thickness is 0.1-0.5 mm. The gradient transition layer establishes a buffer zone in mechanics and electromagnetics between the wave-transparent layer (low dielectric constant) and the wave-absorbing layer (high dielectric constant). From the perspective of electromagnetics, the continuous change of porosity essentially constitutes a gradient transition of dielectric constant, further smoothing the wave impedance and reducing the interlayer reflection caused by the sudden change of dielectric constant. From the perspective of mechanics, the porous structure can effectively absorb and disperse the interlayer stress, inhibit the delamination or cracking caused by the mismatch of thermal expansion coefficients, and greatly improve the structural reliability and service life of the skin material.

[0016] Preferably, the three-dimensional ceramic framework is built by silica fibers, and the porosity is 40%-70%; the magnetic-dielectric composite matrix comprises a magnetic absorber and a dielectric absorber, the magnetic absorber is flaky carbonyl iron powder with an aspect ratio of (5-20):1, and the dielectric absorber is a graphene-coated ceramic microsphere, wherein the particle size of the ceramic microsphere is 10-100 μm, and the thickness of the graphene coating layer is 1-5 nm. The three-dimensional framework built by silica fibers not only provides excellent heat resistance and mechanical strength, but also creates a maze effect of multiple reflections and scattering of electromagnetic waves, thereby increasing the absorption path. The flaky carbonyl iron powder (high aspect ratio) is more easily oriented under a magnetic field, and the shape anisotropy greatly enhances the natural resonance frequency, which is the key to improving the low-frequency wave absorption performance. The graphene-coated ceramic microsphere ingeniously introduces a large number of interface polarization centers: graphene as a good conductor produces conductive loss, and the heterogeneous interface formed by graphene, ceramic core and surrounding matrix will produce strong interface polarization loss, while the ceramic microsphere itself plays a role in adjusting the dielectric constant and preventing excessive conduction. The three work together to form a perfect complement of magnetic loss and dielectric loss.

[0017] Preferably, the mass ratio of the silica fibers, the flaky carbonyl iron powder and the graphene-coated ceramic microsphere is (2-4):(3-5):3.

[0018] Preferably, the reflective layer is a metal mesh or a metal film, when the reflective layer is a metal mesh, the mesh opening rate is 40%-60%, and the mesh line width is 10-30 μm; the thermal insulation layer is an aerogel reinforced ceramic fiber felt with a thermal conductivity of not higher than 0.05 W / (m·K).

[0019] In another aspect, the application also discloses a preparation method of the above-mentioned wave-absorbing and concealing skin material, comprising the following steps:

[0020] Preparation of the bionic micro-forest wave-transparent layer: forming a micro-cone array prototype on the surface of a quartz fiber reinforced epoxy resin prepreg, and then performing surface finishing through reactive ion etching to obtain the bionic micro-forest wave-transparent layer after curing;

[0021] Preparation of the gradient transition layer: depositing glass microbeads and aerogel slurry on the bionic micro-forest wave-transparent layer in a gradient distribution of porosity through layer-by-layer spraying to form the gradient transition layer after curing;

[0022] Preparation of the wave-absorbing layer:

[0023] Dispersing the silica fibers, the flaky carbonyl iron powder and the graphene-coated ceramic microsphere in a solvent, adding a dispersing agent and a binder to obtain a mixed and uniform wave-absorbing layer slurry;

[0024] The wave-absorbing layer slurry is cast into a film under an alternating magnetic field, so that the flaky carbonyl iron powder is arranged in a directional manner along the layer plane direction, and then directional freezing is performed, and after freeze-drying and sintering, the wave-absorbing layer with a three-dimensional ceramic skeleton and a magnetodielectric composite matrix arranged in a directional manner is obtained;

[0025] Laminated composite: according to the order of the bionic micro-forest wave-transparent layer, the gradient transition layer, the wave-absorbing layer, the reflecting layer and the heat-insulating layer, laminated and placed into a hot press;

[0026] Curing: hot-pressing curing is performed by adopting a stepwise temperature rise and a gradient pressure (firstly, maintaining at 80-120 DEG C, 2-5 MPa for 20-40 min, and then maintaining at 150-180 DEG C, 12-20 MPa for 40-90 min), and after cooling and demolding, surface hydrophobic treatment is performed.

[0027] The combination of mould pressing and reactive ion etching can efficiently and accurately prepare the micro-cone array with nanoscale roughness, which is the process basis for the bionic structure to play the efficiency. The magnetic field assisted casting ensures the preferential orientation of the flaky magnetic absorber, forming anisotropic magnetic loss characteristics. The directional growth of ice crystals is used in the freeze injection molding technology to expel solid materials and self-assemble to form a through three-dimensional ceramic skeleton, which is the core process of building the "skeleton-matrix" structure. The variable temperature and variable pressure co-curing process ensures the resin to be fully impregnated and bubble-free in the low temperature and low pressure stage, and realizes the mutual fusion and high strength crosslinking of the resin between the layers in the high temperature and high pressure stage, which is an effective means to solve the interface bonding problem in the composite of multi-layer heterogeneous materials.

[0028] Preferably, the gas of the reactive ion etching is a mixed gas of CF4 and O2, the flow rate of CF4 is 30-50 sccm, the flow rate of O2 is 10-20 sccm, the etching power is 100-200 W, and the etching time is 5-15 min. The mixed gas of CF4 and O2 generates plasma, CF4 is responsible for anisotropic etching of inorganic components such as quartz fibers, and O2 is responsible for bombarding and removing organic matter, and the synergistic effect of the two can refine the ideal geometry on the micro-cone prototype and introduce nanoscale roughness. This nanoscale roughness can further scatter electromagnetic waves and optimize the surface impedance.

[0029] Preferably, the magnetic field strength of the alternating magnetic field is 0.5-1.5 T, and the frequency is 50-200 Hz; the freezing temperature of the directional freezing is-30 DEG C to-50 DEG C, and the freezing direction is perpendicular to the layer plane.

[0030] Preferably, the surface hydrophobic treatment adopts fluorosilane solution spraying, the coating thickness is 5-10 microns, and the contact angle is greater than 150 DEG.

[0031] The present application has the advantages that:

[0032] 1. Ultra-wideband wave-absorbing performance: The bionic micro-forest wave-transparent layer realizes gradient change of dielectric constant through micro-pyramid array, achieving excellent impedance matching in a wide frequency band of 2-40 GHz, especially solving the problem of poor low-frequency wave-absorbing performance of traditional materials.

[0033] 2. Excellent mechanical properties: The wave-absorbing layer adopts a "skeleton-matrix" double network structure, and the three-dimensional ceramic skeleton provides mechanical support, avoiding the brittleness problem caused by high filler content in traditional wave-absorbing layers. The tensile strength of the material can reach more than 75 MPa.

[0034] 3. Excellent thermal stability: The unique three-dimensional ceramic skeleton structure and gradient transition layer design enable the material to maintain stable wave-absorbing performance and mechanical properties in high-temperature environments (250°C).

[0035] 4. Innovative preparation process: Combining magnetic field assisted directional technology and freeze gelation process, the material realizes controllable arrangement of wave-absorbing agents and precise construction of complex microstructure, solving the problem of complex structure that traditional processes cannot achieve.

[0036] 5. Good environmental adaptability: Surface hydrophobic treatment endows the material with self-cleaning function, with a contact angle greater than 150°, improving the service life of the material in complex environments. DETAILED DESCRIPTION

[0037] The specific embodiments of the present application are described in detail below. It should be understood that the specific embodiments described herein are only for illustration and explanation of the present application, and are not intended to limit the present application.

[0038] It should be noted that all reagents and raw materials in the present application are commercially available, and the purity of the reagents is analytical pure.

[0039] The aerogel reinforced ceramic fiber felt is purchased from Qingdao Jiujiang Thermal Insulation and Energy Saving Materials Co., Ltd., and the product name is ceramic fiber aerogel.

[0040] The latent curing agent is derived from the German Alzchem (Alzchem) Group, and the model number is DYHARD® 100S.

[0041] Perfluorodecyltriethoxysilane is derived from Wuhan Prolif Biological Technology Co., Ltd., and the product name is silane coupling agent AC-FAS.

[0042] Example 1

[0043] A wave-absorbing hidden skin material is prepared according to the following steps:

[0044] (1) Preparation of bionic micro-forest wave-transparent layer

[0045] Raw materials: quartz fiber cloth (thickness 0.2mm, areal density 180g / m²) and epoxy resin (E-51), curing agent 4-methyltetrahydrophthalic anhydride;

[0046] Mold pressing: the quartz fiber cloth was immersed in the glue solution mixed by epoxy resin and curing agent with a mass ratio of 1:1, and then was pressed into a mold with a micro-taper array at 80℃ and 0.5MPa, the height of the micro-taper was 150μm, the ratio of the base diameter to the height of the taper was 1:2, the hexagonal close-packed structure was adopted, the taper angle of the micro-taper was 45°, and the center distance was 200μm;

[0047] Reaction ion etching: the preform after mold pressing was put into the RIE equipment, CF4(40sccm) and O2(15sccm) were introduced, and etching was carried out at 150W power for 10min to trim the micro-taper shape and form a nano-scale roughness;

[0048] Curing: curing at 120℃ for 1h to obtain the biomimetic micro-forest wave-transparent layer.

[0049] (2) Preparation of gradient transition layer

[0050] Slurry A: glass microbeads 42wt%, aerogel slurry 15wt%, epoxy resin 20wt%, latent curing agent 2wt%, deionized water 21wt%;

[0051] Slurry B: glass microbeads 18wt%, aerogel slurry 39wt%, epoxy resin 20wt%, latent curing agent 2wt%, deionized water 21wt%.

[0052] The prepared slurry A and slurry B were respectively loaded into the A and B tanks of the double-channel precision spraying system, the nozzle diameter was set to 0.8mm, the spraying pressure was set to 0.3MPa, the nozzle moving speed was set to 10mm / s, the spraying distance was set to 150mm, and the total spraying time T was set to 10min in the control system;

[0053] Linear flow change program was compiled: the flow of slurry A was linearly decreased from 100% to 0%, while the flow of slurry B was linearly increased from 0% to 100%;

[0054] Spraying: the biomimetic micro-forest wave-transparent layer preform was fixed on the spraying platform, the system was started, and the nozzle began to move according to the preset path, the double-channel output slurry according to the program, after the completion of the first layer spraying, the workpiece was immediately transferred to the 80℃ air drying oven and stood for 5min;

[0055] The workpiece was taken out, and the next layer was sprayed by repeating the spraying step. The cycle was repeated for a total of 5 layers, and a gradient transition layer green body with a total thickness of about 0.2mm was obtained.

[0056] (3) Preparation of wave-absorbing layer

[0057] Raw materials: silica fibers (average diameter 5 μm, average length 150 μm), flaky carbonyl iron powder (aspect ratio 8-10:1, average particle size 8 μm), graphene-coated alumina microspheres (alumina microspheres average particle size 50 μm, graphene coating layer thickness about 2 nm);

[0058] Slurry preparation: silica fibers, flaky carbonyl iron powder, graphene-coated alumina microspheres were dispersed in deionized water at a mass ratio of 3:4:3, with a solid content of 40%, to obtain a crude slurry, and 0.5% of the total mass of the slurry silane coupling agent KH-560 was dissolved in an ethanol-water solution (volume ratio of ethanol to water 1:0.1) 8 times its mass, stirred for 30 min to obtain a hydrolysis mixture, then the hydrolysis mixture was added to the crude slurry, and ball-milled for 2 h to obtain the slurry;

[0059] Magnetic field assisted casting: the slurry was cast into a film while an alternating magnetic field (magnetic field strength 1.0 T, frequency 100 Hz) was applied to make the flaky carbonyl iron powder directional arrange along the layer plane direction;

[0060] Freeze gelation: the cast film was quickly moved to a cold table at -40℃ for unidirectional freezing, with the freezing direction perpendicular to the layer plane, followed by freeze drying for 24 h;

[0061] Sintering: the freeze gelation film was sintered at 800℃ for 2 h to form a three-dimensional ceramic skeleton, obtaining an absorbing layer with a thickness of 2.0 mm.

[0062] (4) Laminating and curing

[0063] Sequentially laminating: biomimetic micro-forest wave-transparent layer, gradient transition layer, absorbing layer, reflective layer (aluminum grid, opening rate 50%, line width 20 μm), thermal insulation layer (aerogel reinforced ceramic fiber felt, thickness 1.0 mm, thermal conductivity coefficient 0.04 W / (m·K)) were sequentially laminated to obtain a laminated product;

[0064] Variable temperature and pressure co-curing: the laminated product was placed in a hot press, first at 100℃, 3 MPa for 30 min, then at 160℃, 1 MPa for 60 min, to obtain a crude product.

[0065] (5) Post-processing

[0066] After the crude product was cooled and demolded, perfluorodecyltriethoxysilane was added to isopropanol solvent to prepare a fluorosilane solution with a mass fraction of 1.5% for surface spraying, and the spraying amount was 8 g / m 2 , and cured at 120℃ for 30 min to obtain a super-hydrophobic surface with a contact angle of 152°, i.e. an absorbing and hiding skin material.

[0067] Example 2

[0068] A wave-absorbing and concealing skin material was prepared in the following steps:

[0069] (1) Preparation of a biomimetic micro-forest wave-transparent layer:

[0070] Raw materials: quartz fiber cloth (thickness 0.2 mm, areal density 180 g / m²) and epoxy resin (E-51), curing agent 4-methyltetrahydrophthalic anhydride;

[0071] Mold pressing: the quartz fiber cloth was immersed in a glue solution prepared by mixing epoxy resin and curing agent at a mass ratio of 1:1, and was pressed and formed at 80°C and 0.5 MPa by a mold with a micro-taper array, the micro-taper height was 250 μm, the ratio of the taper base diameter to the taper height was 1:1.5, a hexagonal close-packed structure was adopted, the taper angle of the micro-taper was 30°, and the center distance was 400 μm;

[0072] Reaction ion etching: the preform after mold pressing was placed in an RIE device, CF4 (30 sccm) and O2 (10 sccm) were introduced, and etching was carried out at 100 W power for 5 min to trim the micro-taper shape and form a nano-scale roughness;

[0073] Curing: curing at 120°C for 1 h to obtain the biomimetic micro-forest wave-transparent layer.

[0074] (2) Preparation of a gradient transition layer (thickness about 0.1 mm), which was the same as in Example 1.

[0075] (3) Preparation of a wave-absorbing layer:

[0076] Raw materials: silica fiber (average diameter 5 μm, average length 100 μm), flaky carbonyl iron powder (aspect ratio 5-8:1, average particle size 8 μm), graphene-coated alumina microspheres (alumina microspheres average particle size 10 μm, graphene coating layer thickness about 1 nm);

[0077] Slurry preparation: the silica fiber, flaky carbonyl iron powder, and graphene-coated alumina microspheres were dispersed in deionized water at a mass ratio of 2:5:3, with a solid content of 40%, to obtain a coarse slurry, 0.5% of silane coupling agent KH-560 based on the total mass of the slurry was dissolved in an ethanol-water solution (volume ratio of ethanol to water 1:1) with a mass of 8 times that of the slurry, and stirring was carried out for 30 min to obtain a hydrolysis mixture, then the hydrolysis mixture was added to the coarse slurry, and ball milling was carried out for 2 h to obtain the slurry;

[0078] Magnetic field assisted casting: the slurry was cast into a film while an alternating magnetic field (magnetic field strength 1.5 T, frequency 150 Hz) was applied to make the flaky carbonyl iron powder directionally arrange along the layer plane direction;

[0079] Freeze-gelation: the casted film was quickly moved to a cold stage at -50℃ for unidirectional freezing, with the freezing direction perpendicular to the layer plane, followed by freeze-drying for 24h;

[0080] Sintering: the freeze-gelated film was sintered at 800℃ for 2h to form a three-dimensional ceramic skeleton, obtaining the wave-absorbing layer, with a thickness of 2.0mm.

[0081] (4) Laminating and curing

[0082] Sequentially laminating: the biomimetic micro-forest wave-transparent layer, the gradient transition layer, the wave-absorbing layer, the reflective layer (aluminum grid, opening rate 60%, line width 10μm), and the thermal insulation layer (aerogel reinforced ceramic fiber felt, thickness 1.0mm, thermal conductivity 0.03W / (m·K)) were sequentially laminated to obtain a laminate.

[0083] Variable-temperature and variable-pressure co-curing: the laminate was placed in a hot press, first kept at 80℃, 2MPa for 40min, and then kept at 180℃, 2MPa for 40min to obtain a crude product.

[0084] (5) Post-processing

[0085] After the crude product was cooled and demolded, perfluorodecyltriethoxysilane was added to isopropanol solvent to prepare a fluorosilane solution with a mass fraction of 1% for surface spraying, with a spraying amount of 9g / m 2 , and cured at 120℃ for 30min to obtain a super-hydrophobic surface with a contact angle of 154°, thereby obtaining the wave-absorbing and concealing skin material.

[0086] Example 3

[0087] A wave-absorbing and concealing skin material was prepared by the following steps:

[0088] (1) Preparation of a biomimetic micro-forest wave-transparent layer:

[0089] Raw materials: quartz fiber cloth (thickness 0.2mm, areal density 180g / m²) and epoxy resin (E-51), curing agent 4-methyltetrahydrophthalic anhydride;

[0090] Mold pressing: the quartz fiber cloth was immersed in a glue solution prepared by mixing the epoxy resin and the curing agent at a mass ratio of 1:1, and was pressed and formed by a mold with a micro-pyramid array at 80℃, 0.5MPa, with a micro-pyramid height of 300μm, a ratio of the base diameter to the height of 1:3, a hexagonal close-packed structure, and a micro-pyramid angle of 60°, with a center distance of 150μm;

[0091] Reactive ion etching: the molded preform was placed in an RIE device, CF4(50sccm) and O2(20sccm) were introduced, and etching was carried out at 200W power for 15min to trim the micro-pyramid shape and form a nano-scale roughness;

[0092] Curing: curing at 120℃ for 1h to obtain the biomimetic micro-forest wave-transparent layer.

[0093] (2) Preparation of the gradient transition layer (thickness of about 0.5mm), which is the same as that in Example 1.

[0094] (3) Preparation of the wave-absorbing layer:

[0095] Raw materials: silica fibers (average diameter of 5μm, average length of 100μm), flaky carbonyl iron powder (aspect ratio of 18-20:1, average particle size of 8μm), graphene-coated alumina microspheres (average particle size of alumina microspheres of 100μm, thickness of graphene coating layer of about 5nm);

[0096] Preparation of the slurry: silica fibers, flaky carbonyl iron powder, graphene-coated alumina microspheres were dispersed in deionized water at a mass ratio of 4:3:3, and the solid content was 40% to obtain a crude slurry. 0.5% of silane coupling agent KH-560 based on the total mass of the slurry was dissolved in an ethanol-water solution (volume ratio of ethanol to water was 1:1) with a mass of 8 times that of the slurry, and stirring was performed for pre-hydrolysis treatment for 30min to obtain a hydrolysis mixture. Then the hydrolysis mixture was added to the crude slurry, and ball milling was performed for 2h to obtain the slurry;

[0097] Magnetic field assisted casting: the slurry was cast into a film while an alternating magnetic field (magnetic field strength of 0.5T, frequency of 50Hz) was applied to make the flaky carbonyl iron powder directionally arrange along the layer plane direction;

[0098] Freeze gelation: the cast film was quickly moved to a cold table at-30℃ for unidirectional freezing, and the freezing direction was perpendicular to the layer plane, followed by freeze drying for 24h;

[0099] Sintering: the freeze gelation film was sintered at 800℃ for 2h to form a three-dimensional ceramic skeleton, and the wave-absorbing layer was obtained, with a thickness of 3.0mm.

[0100] (4) Laminating and curing

[0101] Sequentially laminating: biomimetic micro-forest wave-transparent layer, gradient transition layer, wave-absorbing layer, reflective layer (aluminum grid, opening rate of 40%, line width of 30μm), thermal insulation layer (aerogel reinforced ceramic fiber felt, thickness of 1.0mm, thermal conductivity of 0.035W / (m·K)) were sequentially laminated to obtain a laminated product;

[0102] Variable temperature and pressure co-curing: the laminated product was placed in a hot press, first at 120℃, 5MPa for 20min, and then at 150℃, 1MPa for 90min to obtain a crude product.

[0103] (5) Post-treatment

[0104] After the rough product was cooled and demolded, perfluorodecyltriethoxysilane was added into isopropanol solvent to prepare a fluorosilane solution with a mass fraction of 2% for surface spraying, and the spraying amount was 10 g / m 2 Curing at 120℃ for 30 min, an ultrahydrophobic surface with a contact angle of 153° was obtained, which was a skin material for wave absorption and hiding.

[0105] Example 4

[0106] A skin material for wave absorption and hiding was prepared by the following steps:

[0107] (1) Preparation of a biomimetic micro-forest wave-transparent layer:

[0108] Raw materials: quartz fiber cloth (thickness 0.2 mm, areal density 180 g / m²) and epoxy resin (E-51), curing agent 4-methyltetrahydrophthalic anhydride;

[0109] Mold pressing: The quartz fiber cloth was immersed in a glue solution prepared by mixing epoxy resin and curing agent at a mass ratio of 1:1, and was pressed and formed at 80℃ and 0.5 MPa by a mold with a micro-pyramid array, the micro-pyramid height was 180 μm, the ratio of the base diameter to the height of the micro-pyramid was 1:2.5, a hexagonal close-packed structure was adopted, the cone angle of the micro-pyramid was 40°, and the center distance was 300 μm;

[0110] Reaction ion etching: The preform after mold pressing was placed in the RIE equipment, CF4 (35 sccm) and O2 (18 sccm) were introduced, and etching was carried out at 200 W power for 15 min to trim the micro-pyramid shape and form a nano-scale roughness;

[0111] Curing: Curing at 120℃ for 1h, a biomimetic micro-forest wave-transparent layer was obtained.

[0112] (2) Preparation of a gradient transition layer (thickness about 0.4 mm), which was the same as in Example 1.

[0113] (3) Preparation of a wave-absorbing layer:

[0114] Raw materials: silicon oxide fiber (average diameter 5 μm, average length 100 μm), flaky carbonyl iron powder (aspect ratio 12-15:1, average particle size 6 μm), graphene-coated alumina microspheres (alumina microspheres average particle size 40 μm, graphene coating layer thickness about 3 nm);

[0115] Slurry preparation: disperse silica fibers, flaky carbonyl iron powder, graphene-coated alumina microspheres in deionized water at a mass ratio of 4:3:3, with a solid content of 40%, to obtain a coarse slurry, and then add 0.5% of silane coupling agent KH-560 in the total mass of the slurry into an ethanol-water solution (volume ratio of ethanol to water is 1:1) with 8 times the mass of the slurry, stir for 30 min to obtain a hydrolysis mixture, then add the hydrolysis mixture into the coarse slurry, and ball mill for 2 h to obtain the slurry;

[0116] Magnetic field assisted casting: cast the slurry into a film while applying an alternating magnetic field (magnetic field strength 1.0T, frequency 100Hz) to make the flaky carbonyl iron powder directional arrange along the layer plane direction;

[0117] Freeze gelation: quickly move the cast film to a cold table at-35℃ for unidirectional freezing, with the freezing direction perpendicular to the layer plane, and then freeze dry for 24h;

[0118] Sintering: sinter the freeze gelation film at 800℃ for 2h to form a three-dimensional ceramic skeleton, and obtain the wave-absorbing layer with a thickness of 3.0mm.

[0119] (4) Laminating and curing

[0120] Sequentially laminating: biomimetic micro-forest wave-transparent layer, gradient transition layer, wave-absorbing layer, reflective layer (copper film, thickness 0.02mm), thermal insulation layer (aerogel reinforced ceramic fiber felt, thickness 1.0mm, thermal conductivity 0.035W / (m·K)) are sequentially laminated to obtain a laminated product.

[0121] Variable temperature and pressure co-curing: place the laminated product in a hot press, first at 90℃, 4MPa for 25min, then at 170℃, 1.5MPa for 60min to obtain a crude product.

[0122] (5) Post-processing

[0123] After the crude product is cooled and demolded, add perfluorodecyltriethoxysilane into isopropanol solvent to prepare a fluorosilane solution with a mass fraction of 2% for surface spraying, and the spraying amount is 12g / m 2 , and then cure at 120℃ for 30min to obtain a super-hydrophobic surface with a contact angle of 153°, which is the wave-absorbing hidden skin material.

[0124] Comparative Example 1

[0125] The same as Example 1, but the wave-transparent layer has a flat surface without micro-pyramid arrays.

[0126] Comparative Example 2

[0127] The same as Example 1, but no magnetic field is applied in the wave-absorbing layer, and the flaky carbonyl iron powder is not directional arranged.

[0128] Comparative Example 3

[0129] The same as Example 1, but no silica fiber was added in the wave-absorbing layer, i.e. no three-dimensional ceramic skeleton.

[0130] Comparative Example 4

[0131] The same as Example 1, but no gradient transition layer, the wave-transparent layer was directly combined with the wave-absorbing layer.

[0132] Comparative Example 5

[0133] The same as Example 1, but a conventional hot-pressing process (constant temperature 150℃, constant pressure 10MPa, time 60min) was used instead of the variable-temperature and variable-pressure process.

[0134] The skin materials in Example 1-4 and Comparative Examples 1-5 were tested for relevant properties, and the test standards and methods are as follows:

[0135] 1. Electromagnetic wave reflectivity: GJB 2038A-2011 “Radar Wave-Absorbing Material Reflectivity Test Method”

[0136] This test was carried out in a microwave anechoic chamber to eliminate environmental electromagnetic interference. The core of the test system was a large metal arch, on which a vector network analyzer, a transmitting antenna and a receiving antenna were installed. During testing, the backscattering power of the sample covered on the metal plate in a specific frequency band (such as 2-18GHz and 26.5-40GHz) was first measured, and then the sample was removed to measure the reflection power of the metal plate itself as a reference. The reflectivity result was expressed in decibels (dB), calculated by the formula , where is the sample reflection power, is the metal plate reflection power. “Reflectivity (2-18GHz minimum value, dB)” is the minimum value extracted from the curve, representing the strongest absorption capacity of the material; “Reflectivity (8-12GHz average value, dB)” and “Reflectivity (26.5-40GHz average value, dB)” are the arithmetic mean values of the reflectivity of all sampling points in the X-band (8-12GHz) and Ka-band (26.5-40GHz) respectively, which are used to evaluate the overall absorption level of the material in the key application frequency band.

[0137] 2. Tensile strength: GB / T 1040.3-2006 “Determination of tensile properties of plastics – Part 3: test conditions for films and sheets”

[0138] This test is performed on a universal material testing machine. First, the material is made into a standard dumbbell-shaped specimen, and its two ends are firmly clamped in the upper and lower clamps of the testing machine. The testing machine stretches the specimen at a constant displacement speed (such as 5 mm / min), while the force sensor and extensometer record the load and deformation in real time, respectively. The test continues until the specimen breaks. The tensile strength (unit: MPa) is calculated by dividing the maximum load (Fmax) borne by the specimen before it breaks by the original cross-sectional area (A0) of the specimen, i.e. . .

[0139] 3. Compression strength: GB / T 1041-2008 "Determination of the compression properties of plastics"

[0140] This test also uses a universal material testing machine. The material is processed into a regular-shaped specimen (such as a cube or a cylinder) and placed between the two parallel compression plates of the testing machine. The testing machine applies an axial compression load to the specimen at a constant speed (such as 2 mm / min). The compression strength (unit: MPa) is calculated by dividing the maximum compression load (Fmax) borne by the specimen when it breaks or reaches a certain strain by the cross-sectional area (A) of the specimen, i.e. . .

[0141] 4. Thermal conductivity: GB / T 10297-2015 "Determination of thermal conductivity of non-metallic solid materials - Hot-wire method"

[0142] The core device of this test is a thin metal hot-wire that serves as a heat source and temperature sensor. During testing, the hot-wire is clamped between two flat samples to be tested, or embedded inside the sample. A constant heating power is applied to the hot-wire, and its own resistance (i.e. temperature) is monitored over time. Since the temperature rise rate directly depends on the thermal conductivity of the surrounding material, by analyzing the temperature rise-time logarithmic relationship within a certain time window, the thermal conductivity λ of the material can be directly calculated according to the mathematical model, with the unit being W / (m·K).

[0143] 5. High temperature stability

[0144] ​​This test is used to evaluate the attenuation of key performance of materials after long-term high-temperature exposure. First, the initial electromagnetic wave reflectivity of the sample at room temperature is tested according to the aforementioned standard method (such as GJB 2038A-2011), and its reflectivity curve in the entire frequency band (such as 2-18GHz) is obtained as a benchmark. Subsequently, the sample is placed in a precisely temperature-controlled high-temperature oven at a set temperature (such as 250°C) and atmospheric environment for a specified period of time (such as 100 hours). After the time has elapsed, the sample is removed and cooled in a standard laboratory environment. Finally, its electromagnetic wave reflectivity is measured again under the same conditions. High-temperature stability is evaluated by comparing the changes in reflectivity curves before and after aging, specifically quantified as the difference (ΔR, unit dB) in reflectivity values at a specific frequency point (such as the lowest reflectivity frequency point) or a specific frequency band (such as the X-band).

[0145] The performance test results are as follows:

[0146] Sample Reflectivity (2-18 GHz min, dB) Reflectivity (8-12 GHz mean, dB) Reflectivity (26.5-40 GHz mean, dB) Tensile strength (MPa) Compressive strength (MPa) Areal density (kg / m2) Thermal conductivity (W / m-K) ΔR (dB) Example 1 -25.2 -18.5 -16.8 75 285 4.5 0.42 +0.5 Example 2 -27.5 -20.3 -18.2 70 272 4.7 0.45 +0.6 Example 3 -23.8 -16.7 -15.5 78 296 4.3 0.38 +0.4 Example 4 -24.5 -17.8 -16.1 72 281 4.8 0.40 +0.5 Comparative Example 1 -15.3 -10.2 -8.5 64 253 4.4 0.43 +1.2 Comparative Example 2 -19.1 -13.4 -11.8 68 264 4.5 0.44 +1.6 Comparative Example 3 -17.2 -11.6 -9.7 46 188 4.2 0.55 +2.8 Comparative Example 4 -20.6 -14.2 -12.4 57 233 4.3 0.46 +1.7 Comparative Example 5 -22.1 -15.9 -13.9 62 245 4.6 0.41 +1.1

[0147] The reflectivity of Comparative Example 1 is only -15.3dB at the lowest reflectivity, with averages of -10.2dB and -8.5dB in the X-band and Ka-band, respectively, which is far inferior to -25.2dB, -18.5dB and -16.8dB of Example 1, indicating that its wave absorption performance has significantly decreased. The main reason is: Comparative Example 1 removes the impedance matching layer, causing a sharp change in wave impedance when the electromagnetic wave enters the high-dielectric-constant wave-absorbing layer directly from the wave-transparent layer, thereby producing strong Fresnel reflection at the interface, and a large amount of electromagnetic wave is reflected back before entering the material interior, which cannot be effectively consumed by the wave-absorbing layer. This proves the key role of the impedance matching layer in smoothing the impedance transition and promoting the penetration of electromagnetic waves into the material interior.

[0148] The reflectivity of Comparative Example 2 (-19.1dB, -13.4dB, -11.8dB) is better than that of Comparative Example 1, but still significantly lower than that of Example 1. Its high-temperature stability (reflectivity change +1.6dB) is also poorer than that of Example 1 (+0.5dB). The main reason is: Comparative Example 2 only uses magnetic absorbent, lacks dielectric absorbent, and destroys the synergistic mechanism of magnetic loss and dielectric loss in Example 1. This leads to insufficient absorption of high-frequency electromagnetic waves (mainly relying on dielectric loss) by the material, resulting in higher average reflectivity in the X-band and Ka-band. At the same time, the single-component system is more prone to performance degradation at high temperatures, while the multiple absorbents in Example 1 can maintain stability through different mechanisms at different temperature ranges, thereby exhibiting better environmental adaptability.

[0149] The wave absorption performance (-17.2 dB, -11.6 dB, -9.7 dB) and mechanical performance (tensile 46 MPa, compressive 188 MPa) of Comparative Example 3 are overall declined, and the thermal conductivity is increased (0.55 W / m·K), and the high-temperature stability is the worst (+2.8 dB). The main reason is that: the lack of a reflection layer makes the electromagnetic waves not completely absorbed by the wave absorption layer directly penetrate the material and escape, which loses the opportunity to be reflected back for secondary absorption, reducing the single absorption efficiency. The lack of a thermal insulation layer causes the heat to quickly pass into the interior of the material in a hot environment, not only causing the performance of the wave absorption agent to degrade, but also more seriously causing the resin matrix to overheat and degrade, resulting in a sharp decline in overall mechanical strength (especially compressive strength) and severe performance degradation at high temperatures.

[0150] The reflectivity (-20.6 dB, -14.2 dB, -12.4 dB) and mechanical performance (tensile 57 MPa, compressive 233 MPa) of Comparative Example 4 are inferior to those of Example 1, and the high-temperature stability (+1.8 dB) is also poor. The main reason is that: Comparative Example 4 removes the gradient transition layer, resulting in a direct combination between the wave transmission layer and the wave absorption layer. This not only forms a clear electromagnetic impedance mutation interface between the two layers, increasing interlayer reflection, but also generates a large internal stress due to the difference in thermal expansion coefficient between the two materials during hot pressing and high-temperature use. This stress causes the interlayer bonding force to weaken, the mechanical performance to decline, and more obvious interface separation or microcracks to appear after high-temperature aging, causing performance degradation.

[0151] The wave absorption performance (-22.1 dB, -15.9 dB, -13.9 dB) and mechanical performance (tensile 62 MPa, compressive 245 MPa) of Comparative Example 5 are both significantly lower than those of Example 1, and the high-temperature stability (+1.1 dB) is also significantly worse. The reason is that: the excessively low hot pressing temperature, pressure, and time result in the resin not being able to fully melt, flow, and solidify. As a result, the interlayer bonding force is weak, and there are a large number of interface defects and micro-pores. These defects not only severely damage the mechanical integrity of the material, but also form countless electromagnetic wave reflection interfaces between the layers, disrupting the carefully designed electromagnetic wave transmission and loss path, resulting in a decrease in wave absorption performance. At the same time, the unstable interface becomes the source of performance deterioration at high temperatures, causing a large shift in reflectivity.

[0152] The above describes the preferred embodiments of the present application, but the present application is not limited to the specific details in the above embodiments, and various simple modifications can be made to the technical solutions of the present application within the technical concept of the present application, and these simple modifications all belong to the protection scope of the present application. In addition, it should be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction, and in order to avoid unnecessary repetition, the present application will not further describe various possible combinations.

[0153] Furthermore, the various embodiments can also be combined, if not in contradiction, as long as they do not deviate from the spirit of the present application, which should be considered as disclosed.

Claims

1. A wave-absorbing and concealing skin material, characterized in that, From top to bottom, it includes: a biomimetic micro-forest wave-transparent layer, a gradient transition layer, a wave-absorbing layer, a reflective layer, and a heat-insulating layer; The surface of the biomimetic microforest wave-transparent layer is provided with a microcone array, the height of the microcone is 150-300μm, the ratio of the diameter of the cone base to the cone height is 1:(1.5-3), the microcone array is a hexagonal close-packed structure, the cone angle of the microcone is 30°-60°, and the center distance between adjacent microcones is 150-400μm; The porosity of the gradient transition layer increases linearly from the wave-transparent layer side near the biomimetic micro-forest to the wave-absorbing layer side, and the thickness is 0.1-0.5 mm; The absorbing layer comprises a three-dimensional ceramic skeleton and a magnetic-dielectric composite matrix filling the pores of the three-dimensional ceramic skeleton. The magnetic absorber in the magnetic-dielectric composite matrix is ​​oriented along the plane of the layer. The three-dimensional ceramic skeleton is constructed of silicon oxide fibers with a porosity of 40%-70%. The magnetic-dielectric composite matrix comprises a magnetic absorber and a dielectric absorber. The magnetic absorber is flake-shaped carbonyl iron powder with an aspect ratio of (5-20):

1. The dielectric absorber is graphene-coated ceramic microspheres with a particle size of 10-100 μm and a graphene coating thickness of 1-5 nm.

2. The wave-absorbing concealment skin material according to claim 1, characterized in that, The insulation layer is an aerogel-reinforced ceramic fiber felt with a thermal conductivity of no more than 0.05 W / (m·K).

3. The wave-absorbing concealment skin material according to claim 1, characterized in that, The mass ratio of the silica fiber, the sheet-like carbonyl iron powder, and the graphene-coated ceramic microspheres is (2-4):(3-5):

3.

4. The wave-absorbing concealment skin material according to claim 1, characterized in that, The reflective layer is a metal mesh or a metal film. When the reflective layer is a metal mesh, the mesh aperture ratio is 40%-60% and the mesh line width is 10-30μm.

5. A method for preparing a wave-absorbing concealment skin material according to any one of claims 1-4, characterized in that, Includes the following steps: Preparation of biomimetic micro-forest wave-transparent layer: A micro-cone array prototype is formed on the surface of quartz fiber reinforced epoxy resin prepreg, and then the surface is refined by reactive ion etching. After curing, the biomimetic micro-forest wave-transparent layer is obtained. Preparation of gradient transition layer: Glass microspheres and aerogel slurry are deposited on the biomimetic micro-forest wave-transparent layer by layer spraying according to the porosity gradient, and then solidified to form gradient transition layer; Fabrication of the absorbing layer: Silica fibers, sheet-like carbonyl iron powder, and graphene-coated ceramic microspheres are dispersed in a solvent, and a dispersant and binder are added to obtain a uniformly mixed microwave absorbing layer slurry. The microwave absorbing layer slurry is cast into a film under an alternating magnetic field, so that the sheet-like carbonyl iron powder is oriented along the plane of the layer. Then, it is orientedly frozen, freeze-dried and sintered to obtain a microwave absorbing layer with a three-dimensional ceramic skeleton and an oriented magnetic-dielectric composite matrix. The layers are laid out in the following order: biomimetic micro-forest wave-transparent layer, gradient transition layer, wave-absorbing layer, reflective layer, and heat insulation layer, and then placed in a hot press. The product is cured by hot pressing using a stepped heating and gradient pressure method. After cooling and demolding, the surface is treated with a hydrophobic coating.

6. The method for preparing the wave-absorbing concealment skin material according to claim 5, characterized in that, The hot-press curing process using stepped heating and gradient pressure includes: first maintaining the temperature at 80-120℃ and 2-5MPa for 20-40 minutes, and then maintaining the temperature at 150-180℃ and 12-20MPa for 40-90 minutes.