A honeycomb sandwich structure wave-absorbing material and a preparation method thereof

Abstract

The application belongs to the field of functional composite materials and stealth technology, and particularly relates to a honeycomb sandwich structure wave-absorbing material and a preparation method thereof. The wave-absorbing material comprises a honeycomb core material and a plurality of magnetic wave-absorbing units. The magnetic wave-absorbing units are filled in the interior of a plurality of cells of the honeycomb core material and are fixed in the cells by an adhesive. The magnetic wave-absorbing units fill all or part of the cells of the honeycomb core material. The honeycomb core material has a plurality of through cells. The magnetic wave-absorbing units are block-shaped geometric bodies formed by cutting sintered magnetic materials, and the cross-sectional shape of the block-shaped geometric bodies is consistent with the cross-sectional shape of the inner wall of the honeycomb cells. The wave-absorbing material has excellent low-frequency wave-absorbing performance, high specific strength and high designability of the block-shaped magnetic material, and realizes curved surface conformation. The problem of large brittleness of the block-shaped magnetic material and difficulty in direct application as a structural component is overcome, and the problem of powderization of the magnetic material for processability in the prior art, which leads to loss of low-frequency wave-absorbing capacity, is solved.

Description

Technical Field

[0001] This invention specifically relates to a honeycomb sandwich structure microwave absorbing material and its preparation method, belonging to the field of functional composite materials and stealth technology. Background Technology

[0002] As stealth technology expands into lower frequency bands, radar absorption in the P-band (especially 300MHz-800MHz) has become a research hotspot. However, due to the long wavelength of electromagnetic waves in the P-band, effective absorption in this band often requires the use of thick absorbing honeycomb or thick layers of magnetic absorbing materials, resulting in significant costs in terms of thickness and weight.

[0003] Currently, traditional microwave absorbing materials in this band are mainly divided into two categories: coating type and structural type. Coating type microwave absorbing materials typically involve mixing magnetic powder with a polymer matrix and coating it onto the target surface. However, when magnetic materials are pulverized into powder, their microstructure undergoes a fundamental change: from a blocky, continuous multi-domain structure to an isolated single-domain or confined multi-domain structure. Domain wall resonance is the main mechanism by which magnetic materials achieve efficient microwave absorption in the MHz low-frequency band, but powdering causes the domain wall resonance mechanism to fail, resulting in a significant decrease in the absorption performance of powder-based microwave absorbing materials in the P-band. Structural microwave absorbing materials, such as honeycomb sandwich structures, typically achieve their absorption function by coating the honeycomb walls with an absorbing coating or filling them with absorbing foam. However, its low-frequency absorption performance is limited by the coating thickness and the intrinsic properties of the material, making it difficult to meet the deep attenuation requirements of the P-band. For example, patent CN112126114B discloses a microwave-absorbing honeycomb / rigid foam composite material and its preparation method, mainly used to improve the absorption and load-bearing capacity of the honeycomb while avoiding excessive weight increase. However, it does not solve the molding process problem of the magnetic block or improve its impact resistance; at the same time, it is also difficult to solve the low-frequency absorption problem.

[0004] Bulk magnetic materials (such as sintered ferrites and magnetic metal composites) possess excellent low-frequency absorption properties, achieving significant absorption in the P-band even at a thickness of 3 mm. However, bulk magnetic materials are essentially ceramic materials, characterized by high brittleness and poor toughness, making them unsuitable for direct use as structural components and difficult to process into complex curved shapes, severely limiting their engineering applications.

[0005] Therefore, how to combine the excellent low-frequency absorption performance of bulk magnetic materials with the good mechanical properties and formability of engineering structural components has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by providing a honeycomb sandwich structure microwave absorbing material and its preparation method. This material possesses the excellent low-frequency microwave absorption performance of bulk magnetic materials, while also exhibiting high specific strength and high designability, enabling conformal surface design. It overcomes the problems of the high brittleness of bulk magnetic materials, making them difficult to use directly as structural components, and solves the problem in existing technologies where the magnetic material is generally powdered for processability, leading to a loss of low-frequency microwave absorption capability.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: a honeycomb sandwich structure microwave absorbing material, the material comprising a honeycomb core material and a plurality of magnetic microwave absorbing units; the magnetic microwave absorbing units are filled inside a plurality of pores of the honeycomb core material and fixed in the pores by an adhesive; the magnetic microwave absorbing units fill all the pores of the honeycomb core material, or fill part of the pores of the honeycomb core material; the honeycomb core material has a plurality of through pores; the magnetic microwave absorbing unit is a block geometry formed by cutting sintered magnetic material; the cross-sectional shape of the magnetic microwave absorbing unit is consistent with the cross-sectional shape of the inner wall of the honeycomb pores.

[0008] Furthermore, the pore size of the honeycomb is 1~50mm and the thickness is 1~80mm; the thickness of the magnetic absorbing unit is 1~10mm and the gap between it and the pore wall of the honeycomb is 0.05~0.2mm.

[0009] Furthermore, the inner wall shape of the honeycomb lattice and the geometry of the magnetic absorbing unit are one of hexagonal prisms, cylinders, or square prisms. Preferably, the geometry of the honeycomb lattice is a regular hexagon; and the geometry of the magnetic absorbing unit is a regular hexagonal prism.

[0010] Furthermore, the honeycomb core material is aramid paper honeycomb core material or fiberglass honeycomb core material; the magnetic absorbing unit is a block material that maintains a continuous multi-domain structure and has a domain wall resonance absorbing mechanism; the adhesive is epoxy resin or cyanate ester resin, wherein the elongation at break of the adhesive is 4~15% and the modulus is 0.5GPa~3GPa.

[0011] Furthermore, the imaginary part μ″ of the complex permeability of the magnetic absorbing unit in the frequency range of 0.3 GHz to 1 GHz is ≥1.5.

[0012] Furthermore, the magnetic absorbing unit is a sintered body of nickel-zinc ferrite, manganese-zinc ferrite, or carbonyl iron.

[0013] Furthermore, the minimum viscosity in the viscosity-temperature curve of the adhesive is 1 Pa·s to 25 Pa·s.

[0014] This invention also provides a method for preparing a honeycomb sandwich structure microwave absorbing material, the method comprising the following steps: S1. Preparation of bulk magnetic materials: Magnetic powder is sintered to prepare dense bulk magnetic materials, wherein the bulk magnetic materials have a continuous multi-domain structure; S2. Preparation of honeycomb core material: Use honeycomb core material impregnated with resin, wherein the honeycomb core material is either uncured or cured; S3. Unit cutting: Cut the block magnetic material prepared in step S1 into magnetic absorbing units that are consistent with the shape and type of the honeycomb core material lattice. Control the size of the magnetic absorbing units so that the gap between them and the lattice wall is 0.05mm~0.2mm after being filled into the honeycomb lattice. S4. Honeycomb filling: Apply adhesive to the inner wall of the honeycomb cell or the surface of the magnetic absorbing unit obtained in step S2, and fill the magnetic absorbing unit into all or part of the cells of the honeycomb core material. One magnetic absorbing unit is embedded in each cell. S5. Curing and Shaping: The filled honeycomb core material is heated and cured to fix the magnetic wave absorbing unit to the honeycomb core material, thus obtaining a honeycomb sandwich structure wave absorbing material.

[0015] Furthermore, in step S3, the cutting method is one of wire cutting, laser cutting, or water jet cutting, and the dimensional accuracy of the magnetic absorbing unit is controlled within ±0.1mm.

[0016] Furthermore, in step S4, the filling method is either automated cell-by-cell filling or press-in whole-plate filling.

[0017] This invention also provides a method for preparing a conformal honeycomb sandwich structure microwave absorbing material, the method comprising the following steps: (1) Using the method of steps S1 to S4 in the preparation method of a honeycomb sandwich structure absorbing material provided by the present invention, a flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units is obtained. (2) Place the flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units on the curved mold, apply pressure to make the honeycomb core material bend and deform to fit the mold surface, so as to adapt to the curved shape and maintain the fit. (3) Heating to cure the honeycomb core material and / or adhesive, and fixing the magnetic wave absorbing unit to the honeycomb core material; (4) Cool to room temperature under pressure and demold to obtain a honeycomb sandwich structure microwave absorbing material with curved surface conformal.

[0018] The beneficial effects of this invention are: (1) The honeycomb sandwich structure absorbing material provided by the present invention achieves high efficiency absorption in the low frequency band. The present invention uses block sintered magnetic material as the absorbing unit and combines it with honeycomb as a sandwich structure absorbing material. It retains the continuous multi-domain structure of the block magnetic material, which can give full play to its domain wall resonance mechanism and achieve high efficiency absorption in the P band (especially 300MHz-1GHz).

[0019] (2) The honeycomb sandwich structure microwave absorbing material provided by the present invention uses honeycomb core material as a discretization carrier, discretizes brittle block material into small units of macroscopic size and fills them in the honeycomb lattice. The honeycomb core material can bear the structural load, and the thin adhesive layer and honeycomb wall layer serve as bonding buffer areas to form an integral sandwich structure. As a result, the sandwich material prepared by the honeycomb sandwich structure has good bending and shear properties, thus overcoming the defect that block magnetic materials cannot be directly used as structural components, and realizing the integration of microwave absorption and structure.

[0020] (3) The honeycomb sandwich structure absorbing material provided by the present invention has the ability to conform to curved surfaces: by utilizing the deformable characteristics of the honeycomb core material, the filled composite structure can be further heated and bent to realize the manufacturing of complex curved surface parts and meet the conformal design requirements of aircraft surfaces.

[0021] (4) The honeycomb sandwich structure absorbing material provided by the present invention has controllable thickness and flexible design: the absorbing performance can be flexibly adjusted by adjusting the thickness of the magnetic absorbing unit and the height of the honeycomb core material; different frequency band requirements can also be achieved by changing the magnetic material formula and changing the filling method of the magnetic absorbing unit.

[0022] (5) The honeycomb sandwich structure absorbing material provided by the present invention has both excellent low-frequency absorbing performance and good mechanical properties. It can be widely used in aircraft stealth skin, ship superstructure, ground equipment stealth cover and other scenarios, and is especially suitable for P-band radar stealth modification. Attached Figure Description

[0023] Figure 1 The electromagnetic parameter test curves of the bulk magnetic material are shown, where (a) is the real part of the dielectric constant, (b) is the imaginary part of the dielectric constant, (c) is the real part of the permeability, and (d) is the imaginary part of the permeability. Figure 2 Reflectance calculation curves for bulk magnetic materials of different thicknesses; Figure 3 The measured reflectance curves of bulk magnetic materials of different thicknesses in the L-band (sample 300mm×300mm). Figure 4 The measured reflectance curve of a 3mm thick block magnetic material in the P-band (sample 1000mm×1000mm). Figure 5This is a schematic diagram of the structure of block magnetic absorbing material filled into the honeycomb lattice in Example 1; Figure 6 This is a photograph of the test piece used to verify the curved surface structure process in Example 2. Detailed Implementation

[0024] The specific embodiments of the present invention will be described in detail below. The present invention can be implemented in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0025] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used is for describing particular embodiments only and is not intended to limit the invention.

[0026] Bulk magnetic materials (such as sintered ferrites and magnetic metal composites) inherently possess excellent low-frequency wave absorption properties. For example... Figure 1-4As shown in the measured data, bulk magnetic materials can achieve significant absorption in the P-band at a thickness of 3 mm. The reflectivity of 3 mm thick bulk material in the P-band is below -10 dB, significantly better than traditional powder-based absorbing materials. However, bulk magnetic materials are essentially ceramic materials, characterized by high brittleness and poor toughness. These mechanical disadvantages prevent them from being directly used in composite structures, and their limited processing capabilities make it difficult to process them into complex curved shapes, failing to meet the molding requirements of complex structural components, severely restricting their engineering applications. Therefore, it is difficult to directly use large blocks of magnetic materials in composite structural components, let alone integrally mold them with composite materials into structural components with complex curved shapes. Consequently, in conventional thinking, bulk magnetic absorbing materials are generally not directly used in composite structures. The common practice is to make the magnetic material into a powder, then bind it with resin or polymer materials to prepare a deformable absorbing material with the required processing capabilities. This can meet the manufacturing process requirements of composite structural components, including magnetic coatings and magnetic fillers. However, powdering causes magnetic materials to lose their strong absorption capabilities based on domain wall resonance, resulting in a significant decrease in their low-frequency absorption capacity. Hybrid honeycombs can be fabricated by filling the honeycomb with other materials. However, since honeycombs are used as lightweight materials, the prevailing thinking in existing hybrid honeycomb technology, driven by weight reduction, primarily involves filling the honeycomb cells with lightweight foam or other auxiliary absorbing materials. This is mainly to improve the honeycomb's absorption and load-bearing capacity while avoiding excessive weight increase. The idea of ​​filling with high-density bulk materials, or using the honeycomb as an auxiliary distribution structure, deformation control structure, and load-bearing structure for high-density bulk functional materials, is not considered. This would help to facilitate the small-scale distribution of magnetic blocks, addressing the molding process and structural mechanics issues of magnetic blocks. Furthermore, existing technologies also struggle to solve the low-frequency absorption problem.

[0027] This invention provides a honeycomb sandwich structure microwave absorbing material, the material comprising a honeycomb core and multiple magnetic microwave absorbing units; the magnetic microwave absorbing units fill the interior of several pores of the honeycomb core and are fixed to the pores by an adhesive; the magnetic microwave absorbing units fill all or part of the pores of the honeycomb core; the honeycomb core has multiple through pores; the magnetic microwave absorbing units are block-shaped geometric bodies formed by cutting sintered magnetic materials; the cross-sectional shape of the magnetic microwave absorbing units is consistent with the cross-sectional shape of the inner wall of the honeycomb pores.

[0028] Understandably, this invention sintersects and miniaturizes magnetic materials to create a continuous multi-domain structure, enabling efficient P-band absorption through domain wall resonance. The blocky magnetic material is cut into micro-units that match the honeycomb lattice structure and filled within the lattice of the honeycomb core material. The porous structure of the honeycomb fixes the miniaturized magnetic material, fully utilizing the structural load-bearing capacity, deformation capacity, and curing ability of the honeycomb core material. This results in a honeycomb sandwich structure absorbing material with excellent structural manufacturing performance, achieving integrated low-frequency absorption and structural technology. The honeycomb-magnetic material adhesive structure also effectively reduces the fragility of the magnetic material. The absorbing material obtained by this technology also possesses the processability to be manufactured into curved conformal components.

[0029] In this invention, instead of preparing the magnetic material into powder or using large blocks of magnetic material, the magnetic material is prepared into small blocks, somewhere between powder and large blocks. The refined size of these magnetic blocks significantly enhances their resistance to breakage while maintaining the overall size of the macroscopic structure and a good domain wall resonance absorption mechanism. These blocks are then prepared to fit the dimensions of the honeycomb cells. The honeycomb core material and the blocky magnetic material are then composited using a relatively flexible adhesive. This honeycomb wall-adhesive-magnetic block structure further improves crack resistance, prevents honeycomb wall collapse, and maintains the bending strength of the honeycomb core material prepared from this honeycomb sandwich absorbing material. This results in a honeycomb sandwich structure absorbing material that retains the domain wall resonance absorption mechanism. It not only possesses the excellent properties of the honeycomb core structure, such as good co-curing performance and interfacial bonding with composite materials, but also exhibits the excellent low-frequency absorption capability of magnetic blocks, while also possessing deformability. This achieves a novel absorbing material with low-frequency high absorption and good processability.

[0030] Specifically, the honeycomb pore size is 1-50 mm, and the thickness is 1-80 mm; the thickness of the magnetic absorbing unit is 1-10 mm, and the gap between the magnetic absorbing unit and the pore wall of the honeycomb is 0.05-0.2 mm. The size and shape of the magnetic absorbing unit are indirectly controlled by the size and shape of the honeycomb pores, and the gap between the magnetic absorbing unit and the pore wall of the honeycomb is well controlled, ensuring that the magnetic block of the present invention can maintain the domain wall resonance absorption characteristics. In this invention, on the one hand, the miniaturized size of the magnetic absorbing unit makes the honeycomb sandwich structure less prone to breakage during the molding and pressure process in the subsequent preparation of composite material structural parts, thus giving it excellent low-frequency absorption performance, processability, and good mechanical properties of engineering structural parts; on the other hand, controlling the size of the pores and magnetic absorbing units, as well as the thickness of the honeycomb, gives the honeycomb sandwich structure absorbing material a certain degree of deformability, avoiding cracking of the honeycomb wall during molding, while also meeting the requirements for preparing honeycomb sandwich structure absorbing materials with curved surfaces; in terms of improving structural load-bearing capacity, excessively large magnetic absorbing units are prone to stress cracking; thus achieving the requirements of structural load-bearing capacity and molding. The gap between the magnetic absorbing unit and the wall of the honeycomb hole is 0.05~0.2mm. This is to prevent the honeycomb wall from cracking due to compression during bending and molding of the honeycomb filled with magnetic units. At the same time, the adhesive layer of a certain thickness can effectively buffer the impact of external stress and thermal cycling on the magnetic block, while avoiding electromagnetic wave leakage due to excessive gaps, which would reduce the absorption performance. For structures with the same height of honeycomb and magnetic absorbing unit, it has a high structural load-bearing capacity. The thickness of the magnetic absorbing unit is 1~10mm, which meets the absorption performance requirements of the p-band (reflectivity ≤-10dB).

[0031] It should be noted that the thickness of the honeycomb core material and the thickness of the magnetic absorbing unit can meet the requirements of this invention. In some embodiments, the thickness of the magnetic absorbing unit can be less than the thickness of the honeycomb core material. In this case, the magnetic absorbing unit is located inside the honeycomb core material and does not exceed the upper or lower cross-section of the honeycomb core material. Preferably, the lower surface of the magnetic absorbing unit is flush with the lower end surface of the honeycomb core material.

[0032] Preferably, the thickness of the magnetic absorbing unit is equal to the thickness of the honeycomb core material, and the upper and lower surfaces of the magnetic absorbing unit are flush with the upper and lower end faces of the honeycomb core material, respectively. More preferably, the difference between the thickness of the magnetic absorbing unit and the thickness of the honeycomb core material is controlled to be less than ±0.2 mm. This achieves a special form of this honeycomb sandwich structure absorbing material. The honeycomb sandwich material prepared with this structure has excellent mechanical properties, with a compressive strength much higher than that of conventional honeycomb sandwich structures, a flexural strength similar to that of conventional honeycomb sandwich structures, and, at the same thickness, a much higher absorption performance in the P-band than that of magnetic absorbing layers prepared with conventional magnetic powder.

[0033] In this invention, the inner wall shape of the honeycomb lattice and the geometry of the magnetic absorbing unit are one of hexagonal prisms, cylinders, or square prisms. The shape of the magnetic absorbing unit matches the characteristics of existing honeycomb structures, thereby preventing large gaps from appearing in the sandwich structure formed by the honeycomb and magnetic units due to shape mismatch, and meeting the design requirements for absorbing performance.

[0034] Preferably, the honeycomb core material is aramid paper honeycomb core material or fiberglass honeycomb core material, and the geometry of the honeycomb cells is regular hexagonal; the geometry of the magnetic absorbing unit is regular hexagonal prism. This matches the structural types widely used in the existing aerospace field. Here, "regular hexagonal" and "regular hexagonal prism" refer to the designed structure being regular hexagonal; the actual honeycomb cell shape may undergo slight deformation due to process and material factors.

[0035] Preferably, the magnetic absorbing unit is a bulk material that maintains a continuous multi-domain structure and has a domain wall resonance absorption mechanism. The imaginary part μ″ of the complex permeability of the magnetic absorbing unit is ≥1.5 in the frequency range of 0.3GHz to 1GHz. By controlling the parameters of the magnetic absorbing unit, this sandwich absorbing structure can achieve effective absorption in the P-band.

[0036] Preferably, the magnetic absorbing unit is a sintered body of nickel-zinc ferrite, manganese-zinc ferrite, or carbonyl iron; these three types of magnetic absorbing units have better multi-domain structure after being miniaturized, have a high imaginary part of magnetic permeability, and achieve good P-band absorption effect.

[0037] Specifically, the adhesive is epoxy resin or cyanate ester resin, with an elongation at break of 4-15% and a modulus of 0.5 GPa-3 GPa. This not only matches the composite process for further applications of the material, but its relatively low modulus (lower than the honeycomb matrix resin) and high elongation at break ensure that the honeycomb-sandwich microwave absorbing structure exhibits a good hard-soft-confined surface-soft-hard impact-resistant structure in further applications. This improves the crack resistance of the bonding surface between the adhesive layer and the honeycomb wall, while also preventing the magnetic block from easily fracturing under pressure, thus avoiding structural instability and collapse under pressure and impact. Furthermore, controlling the modulus above 0.5 GPa is to avoid deteriorating the bonding interface between the honeycomb sandwich microwave absorbing material and the co-cured composite material. Therefore, by controlling the parameter range of the adhesive, the flexural strength and interlaminar shear strength of the sandwich structure prepared from this microwave absorbing material reach levels comparable to those of pure honeycomb sandwich composite materials, providing a basis for its application as a structural material.

[0038] More specifically, the minimum viscosity in the viscosity-temperature curve of the adhesive is 1 Pa·s to 25 Pa·s, which allows the adhesive to flow in a short distance in the gaps between the magnetic unit blocks and the honeycomb walls, maintaining effective and uniform adhesive storage, and forming good bonding.

[0039] This invention also provides a method for preparing a honeycomb sandwich structure microwave absorbing material, the method comprising the following steps: S1. Preparation of bulk magnetic materials: Magnetic powder is sintered to prepare dense bulk magnetic materials, wherein the bulk magnetic materials have a continuous multi-domain structure; S2. Preparation of honeycomb core material: Use honeycomb core material impregnated with resin, wherein the honeycomb core material is either uncured or cured; S3. Unit cutting: Cut the block magnetic material prepared in step S1 into magnetic absorbing units that are consistent with the shape and type of the honeycomb core material lattice. Control the size of the magnetic absorbing units so that the gap between them and the lattice wall is 0.05mm~0.2mm after being filled into the honeycomb lattice. S4. Honeycomb filling: Apply adhesive to the inner wall of the honeycomb cell or the surface of the magnetic absorbing unit obtained in step S2, and fill the magnetic absorbing unit into all or part of the cells of the honeycomb core material. One magnetic absorbing unit is embedded in each cell. S5. Curing and Shaping: The filled honeycomb core material is heated and cured to fix the magnetic wave absorbing unit to the honeycomb core material, thus obtaining a honeycomb sandwich structure wave absorbing material.

[0040] In this invention, the specific operation in step S1 is as follows: various components of the magnetic powder are selected, thoroughly mixed by ball milling, and then sintered at a high temperature of 800~1500℃ for 1~6 hours to obtain a dense blocky magnetic material. Through ball milling and high-temperature melting and sintering, the magnetic material forms a dense multi-domain structure block, giving it a wave absorption mechanism of domain wall resonance.

[0041] It is understood that the ball milling process used in step S1 is a conventional technique in this field.

[0042] Specifically, in step S3, the cutting method is one of wire cutting, laser cutting, or water jet cutting, and the dimensional accuracy of the magnetic absorbing unit is controlled within ±0.1mm. Controlling the size and dimensional accuracy of the magnetic absorbing unit ensures it matches the honeycomb lattice size and meets the material's structural design requirements. This invention provides the buffering effect of the 0.05mm~0.2mm gap in the adhesive layer, the deformability of the honeycomb-magnetic material block, and prevents electromagnetic defects caused by excessively large gaps.

[0043] In this invention, in step S4, an adhesive is coated on the surface of the magnetic absorbing unit and embedded into the honeycomb holes. The adhesive forms an adhesive layer between the magnetic unit and the honeycomb surface and initially fixes its position. In a system where all holes are embedded, the honeycomb sandwich structure absorbing material has good P-band absorption capability. In a partial hole embedding scheme, other types of absorbing materials can be further embedded in other holes to meet special absorption needs, such as a honeycomb sandwich structure absorbing material that can be used for broadband absorption and also meets P-band absorption requirements.

[0044] Preferably, the filling method in step S4 is automated cell-by-cell filling or press-in whole-plate filling; the minimum viscosity in the viscosity-temperature curve of the adhesive is 1~25 Pa·s, which meets the short-range flow requirements of the adhesive in the gap of the honeycomb-magnetic wave-absorbing unit, thereby improving effective bonding.

[0045] In step S5, the pre-impregnated honeycomb resin and the adhesive are cured sequentially according to their curing temperatures.

[0046] This invention also provides a method for preparing a conformal honeycomb sandwich structure microwave absorbing material, the method comprising the following steps: (1) Using the method of steps S1 to S4 in the preparation method of a honeycomb sandwich structure absorbing material provided by the present invention, a flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units is obtained. (2) Place the flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units on the curved mold, apply pressure to make the honeycomb core material bend and deform to fit the mold surface, so as to adapt to the curved shape and maintain the fit. (3) Heating to cure the honeycomb core material and / or adhesive, and fixing the magnetic wave absorbing unit to the honeycomb core material; (4) Cool to room temperature under pressure and demold to obtain a honeycomb sandwich structure microwave absorbing material with curved surface conformal.

[0047] In this invention, the honeycomb core material is either an uncured honeycomb core material or a cured honeycomb core material. The uncured honeycomb core material is at least one of completely uncured honeycomb core material (referring to the state after impregnation and drying) and moderately cured honeycomb core material (after moderate pre-curing treatment).

[0048] In the process of preparing conformal honeycomb sandwich structure microwave absorbing material, in steps (2) and (3), in order to achieve effective bending deformation, maintain the conformal surface after curing, and prevent honeycomb cracking, collapse and adhesive extrusion, the pressure deformation operation and curing operation are different depending on the type of honeycomb core material: In some embodiments, when using uncured honeycomb core material, the resin in the honeycomb core material softens after heating (i.e., the resin becomes viscous or highly elastic), and the softening temperature does not exceed 100°C. In step (2), the temperature should be raised to 50~100°C first to soften the honeycomb, and then a pressure not exceeding 0.3MPa is applied to control the deformation. The heating temperature is preferably controlled to be at least 50°C lower than the curing temperature. The honeycomb is softened within this temperature range, and the pressure is applied to make the honeycomb core material bend and deform to fit the mold surface to adapt to the curved shape and maintain the fit. In step (3), heating continues, and the honeycomb pre-impregnated resin and adhesive are cured sequentially according to their curing temperatures.

[0049] In some embodiments, when using a cured honeycomb core material, in step (2), pressure is applied according to the required degree of surface deformation, not exceeding 1.0 MPa, to control the deformation, so that the honeycomb core material is bent and deformed to adapt to the surface shape and maintain the fit; in step (3), the adhesive is heated to the curing temperature of the adhesive to cure the adhesive.

[0050] The conformal honeycomb sandwich structure microwave absorbing material prepared using uncured core material has lower stress and is particularly suitable for the bending and forming process of sandwich structures with magnetic unit height less than 5 mm of honeycomb height.

[0051] This invention does not simply fill a honeycomb grid with magnetic blocks, but rather systematically solves the contradiction between achieving low-frequency absorption and lightweight structure through multi-scale collaborative design of "electromagnetic function, structural mechanics, and material processing." First, at the level of electromagnetic function, the sintering process preserves the continuous multi-domain structure of the magnetic material, providing a physical basis for domain wall resonance in the low-frequency band. This is a fundamental prerequisite for achieving efficient P-band absorption. If powder is used instead, domain wall resonance disappears, and the low-frequency performance does not meet the requirements. Second, at the level of structural load-bearing, the honeycomb grid array evenly distributes the macroscopic load to the local constraints of each cell, while the adhesive layer acts as a mechanical buffer bridge between the rigid and brittle magnetic units and the brittle honeycomb walls. The precise matching of its modulus (0.5~3GPa) and elongation at break (4%~15%) allows it to both transmit stress and absorb energy through viscoelastic deformation, overcoming the problem that bulk ceramic materials are fragile and cannot be directly applied in structures, and also avoiding local stress concentration in the honeycomb walls that could lead to wall collapse. If the modulus is too high, such as exceeding that of the cured honeycomb resin (typically 3 GPa), the honeycomb wall will be easily crushed under stress, resulting in decreased flexural strength after being fabricated into a sandwich structure. If the modulus is too low, the cured adhesive adhering to the honeycomb wall and bottom will have poor bonding strength with the upper and lower panels after being fabricated into a sandwich structure, leading to a decrease in the overall flexural and shear strength of the structure. Furthermore, by controlling the gap to 0.05~0.2 mm, the flexible adhesive layer can provide redundant space for thermal expansion and contraction, and also act as a buffer between the rigid magnetic block and the honeycomb wall, preventing the two rigid structures from being crushed and failing under stress, while also preventing electromagnetic defects caused by excessively large gaps. Finally, at the process molding level, by utilizing the compressive deformation capability of the honeycomb core material and the good fluidity of the adhesive in the lowest viscosity range (1~25 Pa·s), the rigid magnetic absorbing unit undergoes slight translation or deflection under the lubrication of the adhesive layer to adapt to the geometric distortion of the honeycomb wall, and is cured in situ after deformation, thus bypassing the physical bottleneck of the inability of ceramic materials to bend and achieving curved surface co-forming. In summary, the sintered block ensures the electromagnetic function of domain wall resonance, the honeycomb skeleton bears the structural load, the flexible adhesive layer buffers stress and fixes the unit, and the honeycomb and low-viscosity adhesive work together to achieve curved surface follow-up molding. These four links are interlinked and indispensable, and together they achieve strong P-band absorption, structural formability, and good mechanical properties.

[0052] In some embodiments, the phenolic resin selected in this invention is sourced from the Beijing Institute of Aeronautical Materials and is a special phenolic resin for impregnating honeycomb cores.

[0053] Medium-temperature curing epoxy resin adhesive (single component, tested by viscosity-temperature curve method, it has a minimum viscosity of 4 Pa·s at 85℃, cured modulus of 1.53 GPa, and elongation at break of 7.2%): provided by Heilongjiang Petrochemical Research Institute.

[0054] High-temperature curing epoxy resin adhesive (single component, tested by viscosity-temperature curve method, it has a minimum viscosity of 5.5 Pa·s at 120℃, a cured modulus of 2.12 GPa, and an elongation at break of 6.1%): provided by Heilongjiang Petrochemical Research Institute.

[0055] Medium-temperature curing epoxy resin adhesive (single component, tested by viscosity-temperature curve method, it has a minimum viscosity of 15 Pa·s at 85℃, cured modulus of 0.95 GPa, and elongation at break of 10.3%): provided by Heilongjiang Petrochemical Research Institute.

[0056] Bisphenol A type epoxy resin (E54): Shanghai Huayi Resin Co., Ltd.

[0057] To better illustrate the embodiments of the present invention, the present invention will be further described in detail below through specific examples.

[0058] Example 1 The preparation of a honeycomb sandwich structure microwave absorbing material includes the following steps: S1. Preparation of bulk magnetic materials: Nickel-zinc ferrite magnetic powder (mainly composed of Ni) is prepared... 0.2 Zn 0.8 Fe2O4 was ball-milled and then sintered at 1200℃ for 4 hours to prepare a dense, bulk nickel-zinc ferrite magnetic material. Testing showed that the material exhibited a complex permeability imaginary part μ″≥2 in the frequency range of 0.3GHz to 1GHz, displaying typical domain wall resonance characteristics.

[0059] S2. Preparation of honeycomb core material: Select aramid paper honeycomb core material (the geometric shape of the honeycomb cells is regular hexagonal, the side length of the cells is 4.5mm, the calculated cell diameter is 9mm, the honeycomb thickness is 3mm, and the density is 48kg / m³). 3 The impregnating resin is phenolic resin and is a cured honeycomb core material.

[0060] S3. Unit cutting: The block magnetic material prepared in step S1 is cut into magnetic absorbing units with the same shape and type as the honeycomb core material using a diamond wire cutter. The size of the magnetic absorbing unit is controlled so that the magnetic absorbing unit is a regular hexagonal prism with a side length of 4.4 mm and a thickness of 3.0 mm, and the dimensional accuracy is controlled within ±0.05 mm.

[0061] S4. Honeycomb Filling: A medium-temperature curing epoxy resin adhesive is uniformly coated on the inner wall surface of the honeycomb cell (tested by the viscosity-temperature curve method, it has a minimum viscosity of 4 Pa·s at 85℃, a cured modulus of 1.53 GPa, and an elongation at break of 7.2%). Using an automated cell-by-cell filling device, the cut regular hexagonal prism magnetic absorbing units are pressed one by one into all the cells of the honeycomb core material. Each cell is densely filled, and the upper and lower surfaces of the magnetic absorbing units are flush with the upper and lower end faces of the honeycomb core material, respectively.

[0062] S5. Curing and Shaping: Place the filled honeycomb in an oven and heat it at 120℃ for 2 hours to fix the magnetic wave-absorbing unit to the honeycomb core material, thus obtaining a honeycomb sandwich structure wave-absorbing material.

[0063] A schematic diagram of the structure of the blocky magnetic absorbing material filled into the honeycomb grid in this embodiment is shown below. Figure 5 As shown.

[0064] In this embodiment, in step S2, the resin can also be an uncured honeycomb core material. In step S5, after heating and curing at 120°C for 2 hours, the temperature needs to be raised to 180°C for another 2 hours to complete the curing of the honeycomb core material impregnated with resin.

[0065] The reflectivity of the absorbing material prepared in this embodiment was tested (arch method, sample size 300mm×300mm). The results showed that the reflectivity was less than -12dB in the 300MHz-800MHz frequency band, and the peak absorption reached -18dB, indicating excellent absorbing performance.

[0066] The performance of the sandwich structure prepared using this microwave absorbing material was tested. The tensile strength was 383 MPa, the flexural strength was 430 MPa, and the interlaminar shear strength was 34.7 MPa, which is comparable to that of conventional honeycomb sandwich structures. This indicates that the built-in magnetic microwave absorbing unit structure of the honeycomb sandwich structure did not cause deterioration of the bonding interface between the core layer and the panel, did not cause load-bearing failure of the honeycomb wall due to the mutual compression of the magnetic microwave absorbing units, and did not cause a significant decrease in flexural strength due to the extremely poor deformability and brittleness of the magnetic microwave absorbing units. The main mechanical properties of the honeycomb sandwich structure are determined by the panel and the honeycomb sandwich, demonstrating good structural performance and matching existing structural materials.

[0067] Example 2

[0068] The preparation of a conformal honeycomb sandwich structure microwave absorbing material includes the following steps: (1) In this embodiment, the same method as steps S1 to S4 of embodiment 1 is used to obtain a flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units. The honeycomb material used is the same as in embodiment 1, but the honeycomb core material is not cured. At this time, the resin on the honeycomb core material is a hard solid at room temperature, and softens and exhibits a viscous flow state at ≥50℃. The honeycomb thickness is 2mm, the thickness of the regular hexagonal prism magnetic block is 2mm, and the side length is 4.9mm.

[0069] (2) Heat the flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units to 65°C, place it in a single-curvature curved surface mold (curvature radius is 120mm), apply a pressure of 0.05MPa or 0.1MPa, keep it warm and pressurized for 30min, so that the honeycomb will bend and deform and fit into the mold surface to adapt to the curved surface shape and maintain the fit.

[0070] (3) Then depressurize to 0.05MPa, continue heating to 120℃ for 2 hours to cure the medium-temperature curing adhesive, and then heat to 180℃ for 2 hours to complete the curing of the honeycomb core material impregnated with resin, so that the magnetic wave absorbing unit is fixedly connected to the honeycomb core material.

[0071] (4) Cool to room temperature under pressure, then depressurize and demold to obtain a honeycomb sandwich structure microwave absorbing material with curved surface conformal.

[0072] In this embodiment, the honeycomb core material in step (1) can also be a pre-cured honeycomb core material, and the operations of steps (2) to (4) are as follows: (2) The flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units is heated to 65°C, placed in a single-curvature curved surface mold (curvature radius is 120mm), and a pressure of 0.5MPa is applied to make the honeycomb bend and deform, and fit into the mold surface to adapt to the curved surface shape and maintain the fit.

[0073] (3) Maintain pressure and continue heating to 120°C for 2 hours to cure the adhesive.

[0074] (4) Cool to room temperature under pressure and demold to obtain a honeycomb sandwich structure microwave absorbing material with curved surface conformal.

[0075] The curved surface structure of the conformal honeycomb sandwich structure absorbing material obtained in this embodiment is subjected to visual inspection, such as... Figure 6 As shown, the curved surface is well formed, the honeycomb structure does not collapse, and the magnetic units do not fall off or crack, verifying that the structure has the capability for conformal manufacturing of curved surfaces.

[0076] Example 3 The preparation of a P-band honeycomb sandwich structure microwave absorbing material includes the following steps: S1. Preparation of bulk magnetic materials: Manganese-zinc ferrite magnetic powder (mainly composed of Mn) is prepared... 0.25 Zn 0.75 After being ball-milled and uniformly mixed, Fe2O4 was sintered at 1250℃ for 3 hours to prepare a dense, bulk manganese-zinc ferrite magnetic material. Testing showed that the material exhibited a complex permeability imaginary part μ″ ≥ 3.2 in the frequency range of 0.3 GHz to 1 GHz, displaying typical domain wall resonance characteristics.

[0077] S2. Preparation of honeycomb core material: Select fiberglass honeycomb core material (the geometric shape of the honeycomb cells is a regular square, the side length of the cells is 10mm, the calculated pore size is 14mm, the honeycomb thickness is 30mm, and the density is 32kg / m³). 3 The impregnating resin is phenolic resin and is a cured honeycomb core material.

[0078] S3. Unit cutting: The block magnetic material prepared in step S1 is cut into magnetic absorbing units with the same shape and type as the honeycomb core material using a water jet cutting machine. The size of the magnetic absorbing unit is controlled so that the magnetic absorbing unit is a regular square prism with a side length of 9.8 mm and a thickness of 3 mm, and the dimensional accuracy is controlled within ±0.05 mm.

[0079] S4. Honeycomb Filling: A high-temperature curing epoxy resin adhesive is uniformly coated on the surface of the magnetic absorbing unit (tested by the viscosity-temperature curve method, it has a minimum viscosity of 5.5 Pa·s at 120℃, a cured modulus of 2.12 GPa, and an elongation at break of 6.1%). Using an automated cell-by-cell filling device, the cut square prism magnetic absorbing units are pressed one by one into all the cells of the honeycomb core material. Each cell is filled densely, and the lower surface of the magnetic absorbing unit is flush with the lower end face of the honeycomb core material.

[0080] S5. Curing and Shaping: Place the filled honeycomb in an oven and heat it at 180℃ for 2 hours to fix the magnetic wave-absorbing unit to the honeycomb core material, thus obtaining a honeycomb sandwich structure wave-absorbing material.

[0081] The reflectivity of the absorbing material prepared in this embodiment was tested (arch method, sample size 300mm×300mm). The results showed that in the 300MHz-800MHz frequency band, the reflectivity was less than -13dB, and the peak absorption reached -20dB, demonstrating excellent absorption performance. The honeycomb sandwich structure prepared using this absorbing material was firmly bonded without visible cracks, and the honeycomb walls were firmly bonded to the prepreg.

[0082] Example 4 The preparation of a honeycomb sandwich structure microwave absorbing material suitable for broadband microwave absorption includes the following steps: S1. Preparation of bulk magnetic materials: Nickel-zinc ferrite magnetic powder (mainly composed of Ni) is prepared... 0.2Zn 0.8 Fe2O4 was ball-milled and then sintered at 1000℃ for 5 hours to prepare a dense, bulk nickel-zinc ferrite magnetic material. Testing showed that the material exhibited a complex permeability imaginary part μ″ ≥ 2.2 in the frequency range of 0.3 GHz to 1 GHz, displaying typical domain wall resonance characteristics.

[0083] S2. Preparation of honeycomb core material: Select aramid paper honeycomb core material (the geometric shape of the honeycomb cells is regular hexagonal, the side length of the cells is 2.83mm, the calculated pore size is 5.66mm, the honeycomb thickness is 5mm, and the density is 48kg / m³). 3 The impregnating resin is phenolic resin and is a cured honeycomb core material.

[0084] S3. Unit cutting: The block magnetic material prepared in step S1 is cut into magnetic absorbing units with the same shape and type as the honeycomb core material using a diamond wire cutter. The size of the magnetic absorbing unit is controlled so that the magnetic absorbing unit is a regular hexagonal prism with a side length of 2.75 mm and a thickness of 4.9 mm, and the dimensional accuracy is controlled within ±0.05 mm.

[0085] S4. Honeycomb Filling: A medium-temperature curing epoxy resin adhesive is uniformly coated on the inner wall surface of the honeycomb cell (tested by the viscosity-temperature curve method, it has a minimum viscosity of 4 Pa·s at 85℃, a cured modulus of 1.53 GPa, and an elongation at break of 7.2%). An automated cell-by-cell filling device is used to press the cut regular hexagonal prism magnetic absorbing units one by one into the cells of the honeycomb core material. The filling method is as follows: the middle cell of each hexagonal unit formed by 7 cells is not filled, and the other 6 cells are filled, ensuring that each cell is filled densely. The lower surface of the magnetic absorbing unit is flush with the lower end face of the honeycomb core material.

[0086] S5. Curing and Shaping: Place the filled honeycomb in an oven and heat it at 120℃ for 2 hours to fix the magnetic wave-absorbing unit to the honeycomb core material, thus obtaining a honeycomb sandwich structure wave-absorbing material.

[0087] In step S4 of this embodiment, the filling material of the middle pore of each hexagonal unit formed by 7 pores is not limited. It can be filled with other materials with microwave absorption properties in the field, such as carbon nanotube aerogel, according to design needs, so as to achieve absorption in the whole band.

[0088] The reflectivity of the absorbing material prepared in this embodiment was tested (arch method, sample size 300mm×300mm). The results showed that in the 300MHz-1GHz frequency band, the reflectivity was below -6.5dB, and the peak absorption reached -7.1dB (the reflectivity can be further reduced after filling the unfilled pores, depending on the design requirements). The performance of the honeycomb sandwich structure was tested, and the bending strength was 432MPa and the interlaminar shear strength was 38.1MPa.

[0089] Example 5 The preparation of a P-band honeycomb sandwich structure microwave absorbing material includes the following steps: S1. Preparation of bulk magnetic materials: Nickel-zinc ferrite magnetic powder (mainly composed of Ni) is prepared... 0.25 Zn 0.75 Fe2O4 was ball-milled and then sintered at 1280℃ for 3 hours to prepare a dense, bulk nickel-zinc ferrite magnetic material. Testing showed that the material exhibited a complex permeability imaginary part μ″ ≥ 2.4 in the frequency range of 0.3 GHz to 1 GHz, displaying typical domain wall resonance characteristics.

[0090] S2. Preparation of honeycomb core material: Select aramid paper honeycomb core material (the geometric shape of the honeycomb cells is regular hexagonal, the side length of the cells is 20mm, the calculated cell diameter is 40mm, the honeycomb thickness is 55mm, and the density is 15kg / m³). 3 The impregnating resin is phenolic resin and is a cured honeycomb core material.

[0091] S3. Unit cutting: The block magnetic material prepared in step S1 is cut into magnetic absorbing units with the same shape and type as the honeycomb core material using a diamond wire cutter. The size of the magnetic absorbing unit is controlled so that the magnetic absorbing unit is a regular hexagonal prism with a side length of 19.95mm and a thickness of 2.0mm, and the dimensional accuracy is controlled within ±0.05mm.

[0092] S4. Honeycomb Filling: A medium-temperature curing epoxy resin adhesive is uniformly coated on the inner wall surface of the honeycomb cell (tested by the viscosity-temperature curve method, it has a minimum viscosity of 15 Pa·s at 85℃, a cured modulus of 0.95 GPa, and an elongation at break of 10.3%). Using a manual cell-by-cell filling device, the cut regular hexagonal prism magnetic absorbing units are pressed one by one into the cells of the honeycomb core material. The filling method is: full filling, ensuring that each cell is densely filled. The lower surface of the magnetic absorbing unit is 0.5 mm away from the lower end face of the honeycomb core material.

[0093] S5. Curing and Shaping: Place the filled honeycomb in an oven and heat it at 120℃ for 2 hours to fix the magnetic wave-absorbing unit to the honeycomb core material, thus obtaining a honeycomb sandwich structure wave-absorbing material.

[0094] Comparative Example 1 The traditional powder absorbing coating process is adopted: nickel-zinc ferrite powder with the same composition as in Example 1 is mixed with epoxy resin (100 parts by mass of bisphenol A type epoxy resin (E54) and 34 parts by mass of 4,4'-diaminodiphenyl sulfone (DDS) are mixed evenly) at a mass ratio of 4:1 and coated on the surface of aluminum plate with a coating thickness of 3mm.

[0095] The reflectivity of this material in the P-band was tested, and the results showed that the reflectivity was only -3dB to -5dB in the 300MHz-800MHz frequency band, and the absorption performance was far lower than that of Example 1. It was then combined with a wave-transparent prepreg to form a coated sandwich structure, and its mechanical properties were tested. The bending strength was 285MPa, and the interlaminar shear strength was 21.5MPa.

[0096] Comparative Example 2 Nickel-zinc ferrite magnetic blocks were prepared using the same method as in Example 1. They were cut into block magnetic material plates with a thickness of 3 mm and a size of 300 mm * 300 mm as sandwich absorbing structures. The reflectivity of these blocks in the 300 MHz-800 MHz frequency band was tested. The results showed that the average reflectivity in the 300 MHz-800 MHz frequency band was less than -15 dB.

[0097] Attempt to co-lay with QW280 / 5224 prepreg on a single curved surface (layout method is [0 f ]4. The magnetic block is placed in the middle layer (the radius of curvature of the single curved surface is 780mm). After heating to 135℃ and applying pressure of 0.6MPa, the magnetic plate breaks into several pieces and cannot be fitted into the molded surface to obtain the composite material part.

[0098] An attempt was made to prepare a sandwich structure test piece using nickel-zinc ferrite magnetic bulk material as the core layer. When the shear strength and bending strength were tested by three-point bending, the results showed that the material underwent brittle fracture when the bending displacement was only 0.5 mm. The tested shear strength was only 5.6 MPa and the bending strength was only 49 MPa. The main failure mode was core layer fracture.

[0099] Comparative Example 3 This comparative example uses the same method as Example 1 to prepare a honeycomb sandwich structure absorbing material, the difference being that: the honeycomb cells are filled with nickel-zinc ferrite magnetic powder, which is not sintered into blocks, but bonded together using an adhesive, specifically: In step S1, nickel-zinc ferrite magnetic powder (mainly composed of Ni) is used. 0.2 Zn 0.8 Fe2O4 does not need to be sintered into blocks after being ball-milled and mixed evenly.

[0100] In step S4, the nickel-zinc ferrite magnetic powder that was mixed evenly in step S1 is mixed with the medium-temperature curing epoxy resin adhesive at a mass ratio of 4:1, and then filled into the honeycomb cells to a height of 3mm.

[0101] The final p-band reflectivity performance of the honeycomb sandwich structure absorbing material was significantly worse than that of Example 1, and even worse than that of Comparative Example 1.

[0102] Comparative Example 4 This comparative example uses the same method as Example 1 to prepare a honeycomb sandwich structure microwave absorbing material, the difference being that the filling adhesive is different, specifically: In step S4, Henkel Loctite® 3565 adhesive (cured modulus of adhesive is 3.45 GPa) is uniformly coated on the inner wall surface of the honeycomb lattice.

[0103] The resulting honeycomb sandwich structure was prepared into a composite sandwich structure by combining the microwave absorbing material and the microwave-transparent prepreg. The bending strength was tested and found to be 366 MPa, which was significantly lower than that of Example 1. The failure characteristics were that the honeycomb wall and the adhesive were fractured and failed due to local stress concentration. The interlaminar shear strength was slightly lower than that of Example 1.

[0104] The mechanical property testing of the honeycomb sandwich structure microwave absorbing material obtained in the embodiments and comparative examples of this invention involves the following standards: (1) Tensile strength: GB / T 1452-2005, test speed 1mm / min.

[0105] (2) Shear strength: GB / T 1455-2005, test speed 1mm / min.

[0106] (3) Bending strength: GB / T 1456-2005, test speed 1mm / min, span 120mm.

[0107] The bonding interface of the honeycomb sandwich structure obtained by bonding the above-mentioned honeycomb sandwich structure absorbing material and composite material wave-transparent panel was evaluated by shear strength test, and the overall mechanical properties of the structure were evaluated by flexural strength test, thus evaluating the mechanical compatibility between the honeycomb sandwich structure absorbing material and commonly used wave-transparent panels. All mechanical tests were conducted at 25±2℃. The sandwich structure used was obtained by co-laying the core layer (including the honeycomb sandwich structure in the examples and other materials used as core layers in the comparative examples) with QW280 / 5224 quartz fiber fabric reinforced epoxy resin prepreg on a plane, wherein the laying method was [0 f ]4-J116B-core layer-J116B film-[0 f 4. The curing conditions are 135℃ / 1h + 180℃ / 2h, and the curing pressure is 0.4MPa.

[0108] The performance test data of the microwave absorbing materials obtained in the examples and comparative examples are shown in Table 1.

[0109] Table 1. Performance test data of the microwave absorbing materials obtained from the examples and comparative examples.

[0110] Table 2 shows a comparison of the performance of the absorbing materials obtained in Example 1 and the comparative example.

[0111] Table 2. Performance comparison of the absorbing materials obtained in Example 1 and the comparative example.

[0112] Based on the data in Tables 1 and 2, a comparison between Example 1 and Comparative Examples 1 and 2 shows that, compared to the bulk magnetic absorber in Comparative Example 2, the reflectivity of Comparative Example 1, using a powdered magnetic absorber, increases significantly by 10-12 dB in the 0.3-0.8 GHz range, which is far from meeting the requirements. In contrast, Example 1, by miniaturizing the magnetic absorbing unit, only increases the reflectivity by 3 dB compared to the bulk magnetic absorber, thus meeting the requirements for this frequency band. The honeycomb sandwich structure absorbing material provided by this invention maintains good low-frequency absorption capability, achieves effective absorption in the P-band, significantly increases structural load-bearing capacity, and obtains a curved surface forming capability not possessed by bulk magnetic absorbing materials.

[0113] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0114] For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, and these modifications and improvements are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. A honeycomb sandwich structure microwave absorbing material, characterized in that, The material includes a honeycomb core and multiple magnetic wave-absorbing units; Magnetic wave-absorbing units are filled inside several pores of the honeycomb core material and fixed in the pores by adhesive. Magnetic absorbing units fill all or part of the pores of the honeycomb core material. The honeycomb core material has multiple through-holes; the magnetic absorbing unit is a block geometry formed by cutting sintered magnetic material. The cross-sectional shape of the magnetic absorbing unit is consistent with the cross-sectional shape of the inner wall of the honeycomb lattice.

2. The honeycomb sandwich structure microwave absorbing material according to claim 1, characterized in that, The honeycomb pore size is 1~50mm, and the thickness is 1~80mm; The thickness of the magnetic absorbing unit is 1~10mm, and the gap between it and the wall of the honeycomb hole is 0.05~0.2mm.

3. The honeycomb sandwich structure microwave absorbing material according to claim 1, characterized in that, The inner wall shape of the honeycomb lattice and the geometry of the magnetic wave-absorbing unit are one of hexagonal prisms, cylinders, or square prisms.

4. The honeycomb sandwich structure microwave absorbing material according to claim 1, characterized in that, The honeycomb core material is aramid paper honeycomb core material or fiberglass honeycomb core material; The magnetic absorbing unit is a blocky material that maintains a continuous multi-domain structure and has a domain wall resonance absorbing mechanism. The adhesive is an epoxy resin or a cyanate ester resin, wherein the elongation at break of the adhesive is 4~15% and the modulus is 0.5GPa~3GPa.

5. The honeycomb sandwich structure microwave absorbing material according to claim 4, characterized in that, The magnetic absorbing unit has an imaginary part μ″ of complex permeability ≥ 1.5 in the frequency range of 0.3 GHz to 1 GHz.

6. The honeycomb sandwich structure microwave absorbing material according to claim 5, characterized in that, The magnetic absorbing unit is a sintered body of nickel-zinc ferrite, manganese-zinc ferrite, or carbonyl iron.

7. The honeycomb sandwich structure microwave absorbing material according to claim 4, characterized in that, The minimum viscosity in the viscosity-temperature curve of the adhesive is 1 Pa·s to 25 Pa·s.

8. A method for preparing a honeycomb sandwich structure microwave absorbing material according to any one of claims 1-7, characterized in that, The preparation method includes the following steps: S1. Preparation of bulk magnetic materials: Magnetic powder is sintered to prepare dense bulk magnetic materials, wherein the bulk magnetic materials have a continuous multi-domain structure; S2. Preparation of honeycomb core material: Use honeycomb core material impregnated with resin, wherein the honeycomb core material is either uncured or cured; S3. Unit cutting: Cut the block magnetic material prepared in step S1 into magnetic absorbing units that are consistent with the shape and type of the honeycomb core material lattice. Control the size of the magnetic absorbing units so that the gap between them and the lattice wall is 0.05mm~0.2mm after being filled into the honeycomb lattice. S4. Honeycomb filling: Apply adhesive to the inner wall of the honeycomb cell or the surface of the magnetic absorbing unit obtained in step S2, and fill the magnetic absorbing unit into all or part of the cells of the honeycomb core material. One magnetic absorbing unit is embedded in each cell. S5. Curing and Shaping: The filled honeycomb core material is heated and cured to fix the magnetic wave absorbing unit to the honeycomb core material, thus obtaining a honeycomb sandwich structure wave absorbing material.

9. The method for preparing a honeycomb sandwich structure microwave absorbing material according to claim 8, characterized in that, In step S3, the cutting method is one of wire cutting, laser cutting or water jet cutting, and the dimensional accuracy of the magnetic absorbing unit is controlled within ±0.1mm; In step S4, the filling method is either automated cell-by-cell filling or press-in whole-plate filling.

10. A method for preparing a conformal honeycomb sandwich structure microwave absorbing material, characterized in that, The preparation method includes the following steps: (1) Using the method of steps S1 to S4 in claim 8, a flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units is obtained; (2) Place the flat honeycomb sandwich structure absorbing material filled with magnetic absorbing units on the curved mold, apply pressure to make the honeycomb core material bend and deform to fit the mold surface, so as to adapt to the curved shape and maintain the fit. (3) Heating to cure the honeycomb core material and / or adhesive, and fixing the magnetic wave absorbing unit to the honeycomb core material; (4) Cool to room temperature under pressure and demold to obtain a honeycomb sandwich structure microwave absorbing material with curved surface conformal.

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