Design and preparation method of high-temperature-resistant wave-absorbing material based on alumina fiber

Abstract

The invention belongs to the technical field of radar stealth materials and high-temperature electromagnetic protection, and particularly relates to design and a preparation method of a high-temperature-resistant wave-absorbing material based on alumina fibers. According to the material, a high-temperature-resistant ceramic precursor serves as a base body, a reinforcing phase is formed by alternately stacking alumina fiber cloth and an alumina fiber web tire containing chopped silicon carbide fibers, and a three-dimensional reinforcing structure is formed through a needling technology; the chopped silicon carbide fibers are directionally distributed in the net tire in a non-uniform mode, an enhanced area array with the local geometric regular pattern is constructed, the size and the arrangement period change in a gradient mode in the thickness direction, a vertical dielectric gradient structure is formed in the material, and multi-path propagation, reflection and polarization loss of electromagnetic waves are improved; the surface of alumina fiber cloth is coated with a ceramic interface layer, so that the interface bonding strength and thermal stability are improved; the ceramic matrix is densified for multiple rounds, so that the structure is compact; the material has excellent electromagnetic absorption performance, heat resistance and mechanical strength, and is suitable for high-temperature electromagnetic environments such as aerospace, missile bay sections and the like.

Description

Technical Field

[0001] This invention relates to the design and preparation method of a high-temperature resistant microwave absorbing material based on alumina fiber, belonging to the field of radar stealth materials and high-temperature electromagnetic protection technology. Background Technology

[0002] With the development of radar stealth technology, electromagnetic compatibility and electromagnetic interference resistance, higher requirements have been placed on absorbing materials, especially the performance of absorbing materials that work in high-temperature environments.

[0003] Most existing microwave absorbing materials cannot simultaneously achieve high-temperature stability and broadband absorption performance. Especially in high-temperature and complex environments, conventional polymer-based microwave absorbing materials are prone to thermal degradation, which reduces the performance of the matrix and leads to the overall failure of the microwave absorbing material. On the other hand, when the temperature reaches the glass transition temperature or melting point of the polymer matrix, the structure of the matrix will change, and the mechanical properties will be greatly reduced, causing the structure of the microwave absorbing material to collapse. In addition, high-temperature environments will also destroy the interfacial bonding force between the polymer matrix and the microwave absorbing agent, destroying the synergistic effect within the material and ultimately affecting the microwave absorption performance.

[0004] Chinese Patent CN214938226U discloses a device for uniformly distributing silicon carbide chopped strands. This device uses a dispersing roller with needles evenly distributed on its circumferential surface to uniformly disperse the silicon carbide chopped strands, and then uses an air blowing device and multiple dispersing chambers to achieve uniform distribution. However, this device lacks anti-accumulation capability; the high-loft chopped strands accumulate at the inlet, requiring frequent shutdowns for cleaning. Furthermore, the air blowing device for uniform distribution has significant limitations; the air outlet is prone to blockage, causing localized fiber clumps to fall and affecting the final distribution effect. Chinese Patent CN112621996A discloses a device and method for preparing oriented distributed chopped silicon carbide fiber reinforced composite materials. This method uses a stirring and hydraulic extrusion process to push a ceramic powder slurry containing chopped silicon carbide fibers into oriented holes matching the fiber length, ensuring that the silicon carbide fibers in the outflowing ceramic powder slurry are uniformly oriented. While this approach can achieve good directional results in a single direction, it has the following drawbacks: First, it relies on the mechanical constraints of slurry flow, resulting in complex equipment, limited aperture, and extremely low design freedom, making it difficult to achieve complex path orientation in multiple regions and directions; second, the process has high requirements for slurry viscosity and dispersibility, making it unsuitable for dry laying and semi-dry processes; and third, the complex extrusion equipment and molds also limit its application in the manufacturing of large and complex interlayer reinforcement structures.

[0005] Therefore, how to achieve the directional distribution of short-cut silicon carbide fibers in microwave absorbing materials is a problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes a high-temperature resistant microwave absorbing material based on alumina fibers and its preparation method. This microwave absorbing material uses a high-temperature resistant ceramic as the matrix, with the reinforcing phase consisting of alternating layers of alumina fiber cloth and an alumina fiber mesh containing short-cut silicon carbide fibers. A three-dimensional reinforcing structure is formed through a needle-punching process. Combined with interfacial ceramic coating and multiple rounds of ceramic matrix densification treatment, a dense and high-strength microwave absorbing material is obtained. The high-temperature resistant ceramic matrix is ​​prepared from a ceramic precursor through a conversion process.

[0007] The first objective of this invention is to provide a method for preparing a high-temperature resistant microwave absorbing material based on alumina fibers, the method comprising:

[0008] Step 1: Raw material preparation; Obtain short-cut silicon carbide fibers, alumina fiber cloth, alumina fiber mesh, high-temperature resistant ceramic matrix precursor, interface layer material, and sacrificial template material;

[0009] Step 2: A preform in which chopped silicon carbide fibers are oriented and distributed in an alumina fiber mesh layer is prepared by using chopped silicon carbide fibers, sacrificial template material, and alumina fiber mesh.

[0010] Step 3: Perform interface treatment on the alumina fiber cloth;

[0011] Step 4: Prepare a three-dimensional reinforced preform based on the preform obtained in Step 2 and the alumina fiber cloth obtained in Step 3;

[0012] Step 5: Matrix introduction and ceramization; The three-dimensional reinforced preform obtained in Step 4 is impregnated using an introduction method that matches the high-temperature resistant ceramic matrix precursor, and then ceramized through corresponding curing, pyrolysis and / or sintering processes to obtain the final alumina fiber microwave absorbing material.

[0013] In step one:

[0014] The alumina fiber cloth is made of high-purity alumina fiber cloth with plain weave (Al2O3 content >99%) and an areal density of 200 g / m². 2 ;

[0015] Alumina fiber mesh is made from chopped alumina fibers using an air-flow forming process, with a surface density of 150 g / m². 2 ;

[0016] Short-cut silicon carbide fibers with a length of 3 mm and an axial resistivity range of 1 to 100 Ω·cm are selected.

[0017] High-temperature resistant ceramic matrix precursors are mainly prepared from ceramic precursors with low dielectric loss characteristics through a conversion process; depending on the properties of the precursors and the introduction method, they may include, but are not limited to, at least one of the following:

[0018] (1) Polymer precursor: selected from at least one of polysiloxane (for forming SiOC or SiO2 matrix), polysilazane (for forming Si3N4 or SiCN matrix), polyborosilicate (for forming SiBCN matrix), or modified thereof; such precursor can be introduced by polymer impregnation pyrolysis (PIP).

[0019] (2) Sol-gel precursor: selected from at least one of silica sol (for forming SiO2 matrix), aluminum sol (for forming Al2O3 matrix), or mixed sol that can form low-loss composite oxide ceramics such as mullite; such precursor can be introduced by the sol-gel method.

[0020] (3) Powder slurry precursor: a slurry made by mixing low-loss ceramic powders such as ultrafine silicon dioxide (SiO2), silicon nitride (Si3N4), and boron nitride (BN) with dispersants and binders; such precursors can be introduced by slurry impregnation and sintering.

[0021] The interface layer material is selected from at least one of boron nitride (BN), silicon carbide (SiC), pyrolytic carbon (PyC), titanium nitride (TiN), zirconium carbide (ZrC), and graphene, or any combination thereof to form a composite coating.

[0022] The sacrificial template material is selected from thermally decomposable polymer materials or fusible low-melting-point materials; the thermally decomposable polymer materials are selected from polyvinyl alcohol (PVA), polylactic acid (PLA), polyethylene glycol (PEG) or any combination thereof;

[0023] Step two includes:

[0024] Step S1: Functional wire preparation; Short-cut silicon carbide fibers are combined with sacrificial template material to prepare a thermoplastic functional wire, wherein the short-cut silicon carbide fibers are oriented along the axial direction of the wire.

[0025] Step S2: Path design; According to the preset reinforcement requirements, multiple two-dimensional laying paths are planned on the alumina fiber mesh layer. The paths form geometric patterns such as rectangles, rings, radials, squares or arbitrary curves to form reinforcement areas.

[0026] Step S3: Using an automated wire laying head integrated into a three-axis or multi-axis CNC system, functional wires are precisely laid in multiple designated areas of the alumina fiber mesh layer according to a preset path; single or multiple layers of functional wires can be laid as needed to achieve the designed areal density.

[0027] Step S4: Template removal and structural curing; The preform obtained in step S3 is heat-treated. After removing the sacrificial template material, the alumina fiber mesh layer and the chopped silicon carbide fibers are physically interlocked, and finally a preform with chopped silicon carbide fibers having a directional distribution structure in the alumina fiber mesh layer is obtained.

[0028] Furthermore, the chopped silicon carbide fibers form a stable nested structure within the reinforcement region along each layer;

[0029] Furthermore, the preparation method of the functional wire is one of melt spinning, solution spinning or co-extrusion.

[0030] Furthermore, the laying device is an automated filament laying head, which is integrated into a three-axis or multi-axis CNC motion system;

[0031] Furthermore, the laying paths form geometric patterns in the form of rectangles, rings, radial patterns, squares, or arbitrary curves, and are arranged in arrays.

[0032] Furthermore, different layers of chopped silicon carbide fibers form a filling structure with spatial gradient design variations;

[0033] The size of the enhanced region changes layer by layer from top to bottom along the direction of the electromagnetic wave incident surface. The key is that the size (L) and arrangement period (P) of the silicon carbide fiber pattern on each functionalized mesh layer change in a gradient along the material thickness direction (from the electromagnetic wave incident surface to the bottom surface), following the design trend of "size increasing - period decreasing".

[0034] To ensure the clarity and completeness of this technical solution, the concept of gradient design will be explained first:

[0035] Electromagnetic wave incident surface: refers to the top layer of the prefabricated structure, which is the surface on which the electromagnetic wave first comes into contact.

[0036] Size increment: From the top layer to the bottom layer, the characteristic dimensions of the silicon carbide fiber pattern (such as the side length L of the square) increase layer by layer.

[0037] Decreasing period: From the bottom layer to the top layer, the distribution center of silicon carbide fibers is consistent, but the distance between adjacent borders of adjacent patterns in each layer (period P) increases layer by layer, which means that the pattern arrangement is more dispersed.

[0038] Furthermore, in the direction perpendicular to the electromagnetic wave incident surface (last layer) to the incident surface (top layer), an optimized filling structure for chopped silicon carbide fibers is as follows:

[0039] The first layer of mesh: a square array of periodic units with a side length L = 60 mm and a period of 12 mm.

[0040] The second layer of mesh: a square array of periodic units with a side length L = 50 mm and a period of 22 mm.

[0041] The third layer of mesh: a square array of periodic units with a side length L = 40 mm and a period of 32 mm.

[0042] The fourth layer of mesh: a square array of periodic units with a side length L = 30mm and a period of 42mm.

[0043] The fifth layer of mesh: a square array of periodic units with a side length L = 20mm and a period of 52mm.

[0044] The 6th layer of mesh: a square array of periodic units with a side length L = 10mm and a period of 62mm.

[0045] Furthermore, the heat treatment steps in step S4 include annealing, quenching, or tempering in an inert atmosphere or vacuum environment.

[0046] In step three: the high-purity alumina fiber cloth undergoes interface treatment, and a uniform boron nitride (BN) interface layer is generated in situ on its surface using a precursor conversion method; the detailed preparation process is as follows:

[0047] a) Immersion Coating: Using the final concentration control method, anhydrous toluene or xylene is used as a solvent to dilute commercially available polyboron azela solution to a low viscosity solution with a mass fraction of 5% to 15%, and set aside. The cut alumina fiber cloth is completely immersed in the prepared diluted polyboron azela solution and kept immersed for 10 to 30 minutes to ensure that the precursor solution fully penetrates and wets each alumina fiber. Subsequently, the fiber cloth is slowly lifted out of the solution, and excess liquid on the surface is removed by natural dripping or slow lifting to ensure the uniformity of the coating.

[0048] b) Drying and curing: The impregnated fibers are placed in a fume hood and dried at room temperature for 30-60 minutes to evaporate most of the solvent. Then, they are transferred to a tube furnace and subjected to stepwise heat treatment under an inert atmosphere such as nitrogen or argon: First, the temperature is raised to 80-120°C at a rate of 1-5°C / min and held for 1 hour to completely remove residual solvent. Then, the temperature is raised to 200-300°C at a rate of 1-5°C / min and held for 1-2 hours to allow the polyborazine precursor to undergo a cross-linking reaction and solidify into an infusible and insoluble solid layer.

[0049] c) High-temperature pyrolysis: After curing, continue heating under an inert atmosphere to 1000–1200°C at a rate of 2–5°C / min (25°C / min), and hold for 1–2 hours. At this high temperature, the cured polyboronazine precursor pyrolyzes, releasing small molecule gases, and its framework structure rearranges to transform into amorphous or low-crystallinity boron nitride (BN) ceramics, thereby forming a uniform and dense BN interface layer on the surface of the alumina fibers. After treatment, cool to room temperature in the furnace and remove.

[0050] d) Cyclic thickening (preferred): The impregnation-curing-pyrolysis cycle described in a) to c) above can be repeated 2 to 4 times to obtain a denser BN interface layer with a thickness of about 200-600 nm.

[0051] Optionally, the interface treatment method may be one of the following: precursor conversion method, chemical vapor deposition (CVD) method, sol-gel method, or dip coating method.

[0052] Furthermore, the surface of the alumina fiber cloth is coated with a ceramic interface layer to optimize the fiber-matrix interface bonding strength and thermal stability.

[0053] In step four: a non-uniform gradient reinforcement region is constructed using the alumina fiber cloth obtained through interface treatment in step three and the alumina fiber mesh layer obtained in step two; the specific process is as follows:

[0054] S1: Gradient Structure Design and Lamination: Functionalized mesh and interface-modified fabric are alternately stacked in the order of "fabric-mesh-fabric-mesh...". The key is that the size (L) and arrangement period (P) of the silicon carbide fiber pattern on each functionalized mesh layer change gradient along the material thickness direction (from the electromagnetic wave incident surface to the bottom surface), following a design trend of "increasing size - decreasing period". This design allows the content of the absorbing phase to gradually increase from the surface to the bottom layer, forming a gradient impedance structure to achieve broadband and efficient absorption.

[0055] The following describes the specific laying process of a 13-layer three-dimensional gradient prefabricated structure consisting of 6 layers of functional mesh and 7 layers of structural fabric, laid vertically from the electromagnetic wave incident surface (last layer) to the incident surface (top layer):

[0056] Preparation: Prepare 6 pieces of alumina fiber mesh fabric with a specific silicon carbide fiber square array pattern prepared according to step 2 (referred to as mesh fabric 1 to mesh fabric 6); prepare 7 pieces of alumina fiber cloth coated with BN interface layer prepared according to step 3 (referred to as cloth 1 to cloth 7).

[0057] Alternating layer laying process:

[0058] Layer 1 (bottom layer, opposite direction of electromagnetic wave incident surface): Take a layer of cloth 1 (alumina fiber cloth with BN interface treatment) and lay it flat on the bottom of the mold or worktable.

[0059] Layer 2 (bottom layer, opposite direction of electromagnetic wave incident surface): Take mesh 1, on which short-cut silicon carbide fibers are laid;

[0060] Layer 3: Place a layer of cloth 2 on top of the mesh 1;

[0061] 4th layer: On top of fabric 2, stack mesh 2, making sure to align the center of mesh 2 with the center of mesh 1 below;

[0062] 5th layer: Place a layer of cloth 3 on top of the mesh 2;

[0063] Layer 6: Place the mesh 3 on top of the fabric 3, aligning it with the lower layer;

[0064] Layer 7: Place a layer of cloth 4 on top of the mesh tire 3;

[0065] Layer 8: Place the mesh 4 on top of the fabric 4, aligning it with the layer below;

[0066] Layer 9: Place a layer of cloth 5 on top of the mesh tire 4;

[0067] Layer 10: Place the mesh 5 on top of the fabric 5 and align it with the lower layer;

[0068] 11th layer: On top of the mesh 5, place a layer of cloth 6;

[0069] Layer 12: Place the mesh 6 on top of the fabric 6 and align it with the lower layer;

[0070] Layer 13: Place a layer of cloth 7 on top of the mesh tire 6;

[0071] The 13 layers of material that are stacked alternately are subjected to slight pressure preforming treatment, for example, by applying a pressure of 0.01 to 0.1 MPa through flat plate pressing or vacuum bag process, so that the layers are tightly bonded together.

[0072] S2: Overall Needling Reinforcement: The 13 layers of material, which are stacked alternately as described above, are fed into a needle punching machine and needled in both longitudinal and transverse directions using triangular needles. Needling causes some fibers to rearrange in the thickness (Z-direction) direction, firmly connecting the layers into a whole three-dimensional reinforced preform.

[0073] Preferably, during the laying process, each layer of mesh can be needled in both longitudinal and transverse directions once, and finally the whole layer can be needled.

[0074] S3: Template Removal and Preforming: The needle-punched preform is placed in a vacuum oven and heat-treated in an inert atmosphere or vacuum environment, heated above the decomposition temperature of the sacrificial template material (such as PVA) to completely thermally decompose and extract it. At this time, the chopped silicon carbide fibers are stabilized in the preset array position due to the physical interlocking of the alumina fiber mesh. Afterwards, slight flat-plate pressure or vacuum bag preforming can be performed to form a structurally stable three-dimensional gradient-reinforced preform.

[0075] In step five: matrix introduction and ceramization; the three-dimensional reinforced preform obtained in step four is subjected to repeated polymer impregnation and pyrolysis (PIP) processes for matrix introduction and densification to prepare the final composite material.

[0076] Polysiloxane (PSO) was selected as the precursor to form a silicon carbide (SiOC) ceramic matrix with excellent high-temperature resistance and low dielectric loss. The specific process flow is as follows:

[0077] a) Vacuum impregnation: Place the preform in a container, evacuate to below 10 Pa and maintain for 15-30 minutes, introduce PSO impregnation solution under vacuum to completely immerse the preform; after continuous impregnation for 30-60 minutes, restore normal pressure and apply a positive pressure of 0.2-1.0 MPa, maintain pressure for impregnation for 30-60 minutes to ensure full penetration;

[0078] b) Curing treatment: The impregnated preform is heated to 150-250°C at a rate of 1-3°C / min in air or an inert atmosphere and held for 1-2 hours to allow the PSO precursor to crosslink and cure.

[0079] c) Pyrolysis ceramization: The cured preform is heated to 1000-1200℃ at a rate of 2-5℃ / min under an inert atmosphere and held for 1-3 hours to cause PSO to pyrolyze and transform into an amorphous SiOC ceramic matrix.

[0080] d) Repeated impregnation-pyrolysis cycle: To fill the pores created by pyrolysis and improve the material density, repeat the above cycles of a) impregnation, b) curing, and c) pyrolysis 5-10 times. The densification process is considered complete when the mass increase rate is less than 0.5% and the porosity is less than 10% after two consecutive cycles.

[0081] e) Obtaining the final material: After completing the final pyrolysis cycle and cooling, the final, dense, gradient-structured high-temperature absorbing material based on an alumina fiber-reinforced SiOC ceramic matrix is ​​obtained.

[0082] Preferably, the matrix introduction method includes polymer impregnation pyrolysis (PIP), sol-gel, or slurry impregnation sintering.

[0083] Preferably, the ceramicization process can be repeated multiple times to fill the pores generated in the matrix during the conversion process, thereby improving the density and wave transmission performance of the material.

[0084] The second objective of this invention is to provide a high-temperature resistant microwave absorbing material based on alumina fiber, the material comprising a ceramic matrix formed with polysiloxane as a precursor, and a reinforcing phase composed of alternating layers of alumina fiber cloth and an alumina fiber mesh containing short-cut silicon carbide fibers, the laminated structure being formed into a three-dimensional reinforced preform by a needle punching process;

[0085] The short-cut silicon carbide fibers are distributed locally in a non-uniform manner along each layer of the mesh, forming an electromagnetic wave dielectric loss region within a defined area. The reinforcement region is a geometrically regular shape, including but not limited to square, circular, and rectangular shapes, with its size and arrangement period varying gradient along the thickness direction to construct a dielectric response structure with vertical spatial variation in the composite material, thereby enhancing the multipath propagation and loss of electromagnetic waves.

[0086] The beneficial effects of this invention are:

[0087] This invention provides a design and preparation method for a microwave absorbing material based on alumina fibers. By guiding short-cut silicon carbide fibers to be distributed locally and non-uniformly within a mesh, an array of reinforcing regions with multi-layered size variations and periodic changes (such as rectangular, ring-shaped, radial, or arbitrary curved geometric patterns) is formed. This constructs a vertically intermediate gradient structure within the material, effectively enhancing multipath reflection and polarization loss of electromagnetic waves. This structure combines the anti-delamination capability of needle-punched reinforcements with the in-plane support advantages of fiber cloth, achieving structural-performance synergy. The gradient structure design allows the top layer of the preform to act as an impedance matching layer, minimizing electromagnetic wave reflection at the material surface and guiding it into the material's interior. The gradient structure gradually and efficiently converts electromagnetic wave energy into heat energy for dissipation, thus achieving wide-bandwidth, high-efficiency microwave absorption performance. The use of alumina fiber cloth as the spacer and main structural reinforcement layer ensures the overall mechanical properties and high-temperature stability of the material.

[0088] The method provided by this invention, through interface treatment, suppresses interfacial diffusion and chemical reactions between alumina fibers and the high-temperature resistant ceramic matrix, avoiding fiber strength degradation caused by the formation of brittle phases and ensuring the performance of the reinforcement. Simultaneously, the bonding interface formed by the interface layer induces cracks to deflect and propagate along the fiber / matrix interface when the material is subjected to external force, rather than directly penetrating the high-strength alumina fibers. This greatly consumes fracture energy, achieving material toughening and preventing catastrophic brittle fracture, thereby significantly improving the damage tolerance and structural reliability of the final composite material. Furthermore, by reducing electromagnetic wave reflection at the fiber-matrix micro-interface, the efficiency of electromagnetic waves entering the material's interior is improved, thus enhancing absorption efficiency. Increasing the number of heterogeneous interfaces within the material induces more interfacial polarization and related relaxation losses, which helps to finely control the overall dielectric loss characteristics of the material, further optimizing its absorption performance in the high-frequency band.

[0089] The method of this invention, through digital path planning, can precisely control the geometry, size, and arrangement density of the silicon carbide fiber absorbing phase in each layer. This means that the optimal gradient structure can be designed in reverse according to different application scenarios (such as different target frequency bands and operating temperatures), achieving "tailor-made" absorption performance and demonstrating significant advantages not possessed by traditional homogeneous absorbing materials. The fabrication process of this invention is simple and suitable for low-cost, short-cycle mass production. The material prepared by the method proposed in this invention exhibits a reflectivity of less than -8dB in the 2-18GHz frequency band at a thickness of 4.0mm, demonstrating excellent broadband absorption performance, thermal stability, and mechanical strength, making it suitable for high-temperature electromagnetic environments such as aerospace and missile compartments. Attached Figure Description

[0090] To facilitate understanding of the present invention, the present invention will be described more fully and in detail below with reference to the accompanying drawings and preferred embodiments, but the scope of protection of the present invention is not limited to the following specific embodiments.

[0091] Figure 1 This is a schematic diagram of the distribution shape of short-cut silicon carbide fibers in the design and preparation method of a high-temperature resistant microwave absorbing material based on alumina fibers provided in Embodiment 1 of the present invention.

[0092] Figure 2 This is a schematic diagram of the preform structure stacking and the short-cut silicon carbide fiber gradient structure in the design and preparation method of a high-temperature resistant microwave absorbing material based on alumina fiber provided in Embodiment 1 of the present invention.

[0093] Figure 3 This is a schematic diagram of the ceramic matrix composite material in the design and preparation method of a high-temperature resistant microwave absorbing material based on alumina fiber provided in Embodiment 1 of the present invention;

[0094] Figure 4 This is a schematic diagram of the electromagnetic wave absorption performance test results in the design and preparation method of a high-temperature resistant microwave absorbing material based on alumina fiber provided in Embodiment 1 of the present invention.

[0095] Figure 5 This is a schematic diagram of the distribution shape of short-cut silicon carbide fibers in the design and preparation method of a high-temperature resistant microwave absorbing material based on alumina fibers provided in Embodiment 2 of the present invention.

[0096] Figure 6 This is a schematic diagram of the electromagnetic wave absorption performance test results in the design and preparation method of a high-temperature resistant microwave absorbing material based on alumina fiber provided in Embodiment 2 of the present invention. Detailed Implementation

[0097] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0098] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0099] Unless otherwise specified, all reagents and raw materials used in this invention are commercially available products or products that can be prepared by known methods.

[0100] Example 1

[0101] This embodiment provides a design and preparation method for a high-temperature resistant microwave absorbing material based on alumina fiber, the method comprising:

[0102] Step 1: Raw material preparation; Obtain chopped silicon carbide fibers, alumina fiber cloth, alumina fiber mesh, polysiloxane precursor, interface layer precursor, and sacrificial template material;

[0103] Step 2: A preform of alumina fiber mesh layer with directional distribution of short-cut silicon carbide fibers is prepared by using short-cut silicon carbide fibers, sacrificial template material, and alumina fiber mesh.

[0104] Step 3: Perform interface treatment on the alumina fiber cloth;

[0105] Step 4: Construct a three-dimensional alumina fiber-reinforced preform with a non-uniform gradient reinforcement region using the materials obtained in Steps 2 and 3;

[0106] Step 5: Introduce a polysiloxane precursor into the three-dimensional reinforced preform obtained in Step 4 and ceramicize it to obtain the final alumina fiber microwave absorbing material.

[0107] Step 1: Raw material preparation;

[0108] Alumina fiber cloth: High-purity alumina fiber cloth with plain weave (Al2O3 content >99%), with an areal density of 200g / m². 2 .

[0109] Alumina fiber mesh: A nonwoven fiber mesh made from chopped alumina fibers using an air-flow forming process, with a surface density of 150 g / m². 2 .

[0110] Short-cut silicon carbide fiber: The selected length is 3mm.

[0111] Sacrificial template material: Polyvinyl alcohol (PVA) powder is selected.

[0112] The interface layer is boron nitride (BN), and its preparation process is as follows:

[0113] a) Immersion Coating: Using the final concentration control method, anhydrous toluene or xylene is used as a solvent to dilute commercially available polyboron azela solution to a low viscosity solution with a mass fraction of 5% to 15%, and set aside. The cut alumina fiber cloth is completely immersed in the prepared diluted polyboron azela solution and kept immersed for 10 to 30 minutes to ensure that the precursor solution fully penetrates and wets each alumina fiber. Subsequently, the fiber cloth is slowly lifted out of the solution, and excess liquid on the surface is removed by natural dripping or slow lifting to ensure the uniformity of the coating.

[0114] b) Drying and curing: The impregnated fibers are placed in a fume hood and dried at room temperature for 30-60 minutes to evaporate most of the solvent. Then, they are transferred to a tube furnace and subjected to stepwise heat treatment under an inert atmosphere such as nitrogen or argon: First, the temperature is raised to 80-120°C at a rate of 1-5°C / min and held for 1 hour to completely remove residual solvent. Then, the temperature is raised to 200-300°C at a rate of 1-5°C / min and held for 1-2 hours to allow the polyborazine precursor to undergo a cross-linking reaction and solidify into an infusible and insoluble solid layer.

[0115] c) High-temperature pyrolysis: After curing, continue heating under an inert atmosphere to 1000–1200°C at a rate of 2–5°C / min (25°C / min), and hold for 1–2 hours. At this high temperature, the cured polyboronazine precursor pyrolyzes, releasing small molecule gases, and its framework structure rearranges to transform into amorphous or low-crystallinity boron nitride (BN) ceramics, thereby forming a uniform and dense BN interface layer on the surface of the alumina fibers. After treatment, cool to room temperature in the furnace and remove.

[0116] d) Cyclic thickening (preferred): The impregnation-curing-pyrolysis cycle described in a) to c) above can be repeated 2 to 4 times to obtain a denser BN interface layer with a thickness of about 200-600 nm.

[0117] The high-temperature resistant ceramic matrix precursor is a polysiloxane (PSO) with the following structure: Figure 3 As shown, its preparation process is as follows:

[0118] Commercially available methyl polysiloxane (PSO) was selected and diluted with toluene as a solvent. The mass ratio of PSO to toluene was adjusted to 1:0.8 to 1:2, and the viscosity was monitored using a rotational viscometer. The final viscosity was 20 to 80 mPa·s.

[0119] The prepared impregnation solution was placed in a vacuum drying oven and subjected to vacuum degassing treatment at room temperature and a vacuum degree of -0.08 MPa for 10–30 minutes to remove dissolved gases and volatile small molecules from the solution, preventing defects during subsequent curing and pyrolysis. The treated impregnation solution was then sealed and refrigerated at 4°C for later use.

[0120] Step two includes: preparing a preform of alumina fiber mesh layer with directional distribution of short-cut silicon carbide fibers by using short-cut silicon carbide fibers, sacrificial template material, and alumina fiber mesh;

[0121] Step S1: Functional wire preparation; Short-cut silicon carbide fibers are combined with sacrificial template material to prepare a thermoplastic functional wire, wherein the short-cut silicon carbide fibers are oriented along the axial direction of the wire;

[0122] A co-extrusion method was used to mix and melt 3mm long chopped silicon carbide fibers and PVA powder at a mass ratio of 1:2 in a twin-screw extruder. The mixture was then extruded through a 0.5mm diameter spinneret at 190°C, and after cooling and traction, formed a "silicon carbide fiber-PVA" functional wire with a diameter of approximately 0.5mm. In this wire, the long axis of the chopped silicon carbide fibers was substantially aligned with the wire's axial direction.

[0123] Step S2: Path design; According to the preset reinforcement requirements, multiple two-dimensional laying paths are planned on the alumina fiber mesh layer. The paths form a square geometric pattern to form the reinforcement area.

[0124] Step S3: Using an automated wire laying head integrated into a three-axis or multi-axis CNC system, functional wires are precisely laid in multiple designated areas of the alumina fiber mesh layer according to a preset path; single or multiple layers of functional wires can be laid as needed to achieve the designed areal density;

[0125] Step S4: Template removal and structural curing; The preform obtained in step S3 is heat-treated. After removing the sacrificial template material, the alumina fiber mesh layer and the chopped silicon carbide fibers are physically interlocked, and finally a preform with chopped silicon carbide fibers having a directional distribution structure in the alumina fiber mesh layer is obtained.

[0126] Geometric patterns composed of short-cut silicon carbide fibers, such as Figure 1 As shown, chopped silicon carbide fibers form a stable nested structure within the reinforcing region along each layer; chopped silicon carbide fibers in different layers form a filling structure with spatially gradient design variations, such as... Figure 2 As shown;

[0127] The size of the enhanced region changes layer by layer from top to bottom along the direction of the electromagnetic wave incident surface. The key is that the size (L) and arrangement period (P) of the silicon carbide fiber pattern on each functionalized mesh layer change in a gradient along the material thickness direction (from the electromagnetic wave incident surface to the bottom surface), following the design trend of "size increasing - period decreasing".

[0128] To ensure the clarity and completeness of this technical solution, the concept of gradient design will be explained first:

[0129] Electromagnetic wave incident surface: refers to the top layer of the prefabricated structure, which is the surface on which the electromagnetic wave first comes into contact.

[0130] Size increment: From the top layer to the bottom layer, the characteristic dimensions of the silicon carbide fiber pattern (such as the side length L of the square) increase layer by layer.

[0131] Decreasing period: From the top layer to the bottom layer, the distance between the center points of adjacent patterns (period P) decreases layer by layer, which means that the patterns are arranged more closely.

[0132] Automated Laying: Using an automated fiber placement head integrated into a three-axis CNC system, the aforementioned functional wires are precisely laid onto six independent alumina fiber meshes according to a preset gradient path, resulting in six functional meshes with different silicon carbide fiber patterns (denoted as mesh 1 to mesh 6). The laying paths are all square arrays, with the specific side length (L) and period (P) parameters as follows. This design follows the principle of "increasing size - decreasing period" from the top layer (incident surface) to the bottom layer. The specific filling structure of the chopped silicon carbide fibers is as follows:

[0133] The first layer (bottom layer) of mesh: a square array of periodic units with a side length L = 60 mm and a period of 12 mm.

[0134] The second layer of mesh: a square array of periodic units with a side length L = 50 mm and a period of 22 mm.

[0135] The third layer of mesh: a square array of periodic units with a side length L = 40 mm and a period of 32 mm.

[0136] The fourth layer of mesh: a square array of periodic units with a side length L = 30mm and a period of 42mm.

[0137] The fifth layer of mesh: a square array of periodic units with a side length L = 20mm and a period of 52mm.

[0138] The 6th (top) mesh layer: a square array of periodic units with a side length L = 10mm and a period of 62mm.

[0139] Furthermore, the heat treatment steps in step S4 include annealing, quenching, or tempering in an inert atmosphere or vacuum environment.

[0140] Step 3: Perform interface treatment on the alumina fiber cloth;

[0141] a) Immersion Coating: Using the final concentration control method, anhydrous toluene or xylene is used as a solvent to dilute commercially available polyboron azela solution to a low viscosity solution with a mass fraction of 5% to 15%, and set aside. The cut alumina fiber cloth is completely immersed in the prepared diluted polyboron azela solution and kept immersed for 10 to 30 minutes to ensure that the precursor solution fully penetrates and wets each alumina fiber. Subsequently, the fiber cloth is slowly lifted out of the solution, and excess liquid on the surface is removed by natural dripping or slow lifting to ensure the uniformity of the coating.

[0142] b) Drying and curing: The impregnated fibers are placed in a fume hood and dried at room temperature for 30-60 minutes to evaporate most of the solvent. Then, they are transferred to a tube furnace and subjected to stepwise heat treatment under an inert atmosphere such as nitrogen or argon: First, the temperature is raised to 80-120°C at a rate of 1-5°C / min and held for 1 hour to completely remove residual solvent. Then, the temperature is raised to 200-300°C at a rate of 1-5°C / min and held for 1-2 hours to allow the polyborazine precursor to undergo a cross-linking reaction and solidify into an infusible and insoluble solid layer.

[0143] c) High-temperature pyrolysis: After curing, continue heating under an inert atmosphere to 1000–1200°C at a rate of 2–5°C / min (25°C / min), and hold for 1–2 hours. At this high temperature, the cured polyboronazine precursor pyrolyzes, releasing small molecule gases, and its framework structure rearranges to transform into amorphous or low-crystallinity boron nitride (BN) ceramics, thereby forming a uniform and dense BN interface layer on the surface of the alumina fibers. After treatment, cool to room temperature in the furnace and remove.

[0144] d) Cyclic thickening: Repeat the impregnation-curing-pyrolysis cycle from a) to c) above 2 to 4 times to obtain a dense BN interface layer with a thickness of approximately 200-600 nm.

[0145] Step 4: Construct a three-dimensional alumina fiber-reinforced preform with a non-uniform gradient reinforcement region using the materials obtained in Steps 2 and 3;

[0146] S1: Gradient Structure Design and Lamination: Functionalized mesh and interface-modified fabric are alternately stacked in the order of "fabric-mesh-fabric-mesh...". The key is that the size (L) and arrangement period (P) of the silicon carbide fiber pattern on each functionalized mesh layer change gradient along the material thickness direction (from the electromagnetic wave incident surface to the bottom surface), following a design trend of "increasing size - decreasing period". This design allows the content of the absorbing phase to gradually increase from the surface to the bottom layer, forming a gradient impedance structure to achieve broadband and efficient absorption.

[0147] The following describes the specific laying process of a 13-layer three-dimensional gradient prefabricated structure consisting of 6 layers of functional mesh and 7 layers of structural fabric, laid vertically from the electromagnetic wave incident surface (last layer) to the incident surface (top layer):

[0148] Preparation: Prepare 6 pieces of alumina fiber mesh fabric with a specific silicon carbide fiber square array pattern prepared according to step 2 (referred to as mesh fabric 1 to mesh fabric 6); prepare 7 pieces of alumina fiber cloth coated with BN interface layer prepared according to step 3 (referred to as cloth 1 to cloth 7).

[0149] Alternating layer laying process:

[0150] Layer 1 (bottom layer, opposite direction of electromagnetic wave incident surface): Take a layer of cloth 1 (alumina fiber cloth with BN interface treatment) and lay it flat on the bottom of the mold or worktable.

[0151] Layer 2 (bottom layer, opposite direction of electromagnetic wave incident surface): Take mesh 1, on which short-cut silicon carbide fibers are laid;

[0152] Layer 3: Place a layer of cloth 2 on top of the mesh 1;

[0153] 4th layer: On top of fabric 2, stack mesh 2, making sure to align the center of mesh 2 with the center of mesh 1 below;

[0154] 5th layer: Place a layer of cloth 3 on top of the mesh 2;

[0155] Layer 6: Place the mesh 3 on top of the fabric 3, aligning it with the lower layer;

[0156] Layer 7: Place a layer of cloth 4 on top of the mesh tire 3;

[0157] Layer 8: Place the mesh 4 on top of the fabric 4, aligning it with the layer below;

[0158] Layer 9: Place a layer of cloth 5 on top of the mesh tire 4;

[0159] Layer 10: Place the mesh 5 on top of the fabric 5 and align it with the lower layer;

[0160] 11th layer: On top of the mesh 5, place a layer of cloth 6;

[0161] Layer 12: Place the mesh 6 on top of the fabric 6 and align it with the lower layer;

[0162] Layer 13: Place a layer of cloth 7 on top of the mesh tire 6;

[0163] The 13 layers of material that are stacked alternately are subjected to slight pressure preforming treatment, for example, by applying a pressure of 0.01 to 0.1 MPa through flat plate pressing or vacuum bag process, so that the layers are tightly bonded together.

[0164] S2: Overall Needling Reinforcement: The 13 layers of material, which are stacked alternately as described above, are fed into a needle punching machine and needled in both longitudinal and transverse directions using triangular needles. Needling causes some fibers to rearrange in the thickness (Z-direction) direction, firmly connecting each layer into a whole three-dimensional reinforced prefabricated structure. During the laying process, a longitudinal and transverse needle punching operation is performed after each layer of mesh is laid, and finally, overall needle punching is performed.

[0165] S3: Template Removal and Preforming: The needle-punched preform is placed in a vacuum oven and, under nitrogen atmosphere protection, slowly heated to 400°C at a rate of 5°C / min, and held at that temperature for 2 hours to allow the PVA sacrificial template to completely decompose and be extracted. At this point, the chopped silicon carbide fibers are stabilized in the preset array positions due to the physical interlocking of the alumina fiber mesh. Afterwards, slight flatbed pressure or vacuum bag preforming can be performed to obtain a structurally complete three-dimensional gradient-reinforced preform in which the chopped silicon carbide fibers are stably interlocked in a gradient array.

[0166] Step 5: Introduce a polysiloxane precursor into the three-dimensional reinforced preform obtained in Step 4 and ceramicize it to obtain the final alumina fiber microwave absorbing material.

[0167] Polysiloxane (PSO) was selected as the precursor to form a silicon carbide (SiOC) ceramic matrix with excellent high-temperature resistance and low dielectric loss. The specific process flow is as follows:

[0168] a) Vacuum impregnation: Place the preform in an impregnation container, evacuate to -0.09 MPa and maintain for 30 minutes. Under vacuum, slowly pour in PSO impregnation solution with a viscosity of 50 mPa·s until the preform is completely submerged. Continue vacuum impregnation for 30 minutes, then restore atmospheric pressure and continue impregnation for 2 hours.

[0169] b) Curing treatment: The impregnated preform is heated to 200°C at a rate of 3°C / min in air or an inert atmosphere and held at that temperature for 1 hour to allow the PSO precursor to crosslink and cure.

[0170] c) Pyrolysis Ceramicization: The solidified preform was heated to 1100℃ at a rate of 3℃ / min under an inert atmosphere and held for 2 hours to complete the pyrolysis transformation of the PSO precursor into SiOC ceramic. After the treatment, the preform was cooled to room temperature in the furnace.

[0171] d) Repeated impregnation-pyrolysis cycle: To fill the pores generated by pyrolysis and improve the material density, repeat the above cycle of a) impregnation, b) curing and c) pyrolysis 7 times; when the mass increase rate after two consecutive cycles is less than 0.5% and the porosity is less than 10%, the densification of the material is completed.

[0172] e) Obtaining the final material: The densified composite material block is cut and polished using a diamond grinding wheel to prepare a high-temperature absorbing material sample with precise dimensions, a smooth surface, and a final thickness of 4.0 mm.

[0173] After the final pyrolysis cycle is completed and cooled, the final, dense, gradient-structured high-temperature absorbing material based on an alumina fiber-reinforced SiOC ceramic matrix is ​​obtained.

[0174] The obtained absorbing material was subjected to performance testing. Its electromagnetic reflectivity was measured using a vector network analyzer in the 2-18 GHz frequency band via the waveguide method. The test results are as follows: Figure 4 As shown, the electromagnetic reflectivity curve has an RL ≤ -5dB in the range of 2-18GHz, with an effective bandwidth of 8GHz; the lower the peak value, the stronger the absorption capability, and a deeper absorption peak appears at the 12GHz frequency point, with a peak loss of -15.3dB.

[0175] In addition, the mechanical properties of the absorbing material were tested through a three-point bending test, and its bending strength was 214 MPa, showing good stability.

[0176] Example 2

[0177] This embodiment provides an alternative implementation of the design and preparation method of a high-temperature resistant microwave absorbing material based on alumina fiber. This method aims to demonstrate the flexibility and versatility of the present invention by changing the gradient design details and matrix preparation process.

[0178] Step 1: Raw material preparation

[0179] Reinforcing fiber: Same as in Example 1, with an areal density of 200 g / m². 2 High-purity alumina fiber cloth with a surface density of 150 g / m 2Alumina fiber mesh;

[0180] Short-cut silicon carbide fibers: 5mm in length are selected;

[0181] Sacrificial template material: Polylactic acid (PLA) particles with a melting point of approximately 180℃ are selected;

[0182] Interface layer precursor: Polycarbosilane (PCS) that can form a silicon carbide (SiC) interface is selected; it is dissolved in anhydrous xylene to prepare an impregnation solution with a mass fraction of 8%;

[0183] Ceramic matrix precursor: Silica sol prepared by the sol-gel method; specifically, commercially available nano silica sol with a solid content of 30%, deionized water as solvent, and pH adjusted to 3-4 to maintain stability;

[0184] Step 2: Preparation of functionalized alumina fiber mesh;

[0185] Functional wire preparation: Short-cut silicon carbide fibers with a length of 5 mm and PLA particles are mixed at a mass ratio of 1:5 and melt spinning is used to prepare "silicon carbide fiber-PLA" functional wires with a diameter of about 0.6 mm.

[0186] Automated Laying: Using an automated wire laying head, functional wires are laid out in a rectangular array with different geometric parameters on six independent alumina fiber meshes. This design also follows the principle of "increasing size - decreasing cycle" from the bottom layer (incident surface) to the top layer, with the distribution of the geometric pattern as follows: Figure 5 As shown, the specific parameters are as follows:

[0187] Mesh 1 (for the bottom layer): A square array of periodic units with an outer side length L = 14mm, an inner side length of 13m, and a period P = 4mm.

[0188] Net 2: Laying a square array of periodic units with an outer side length L = 12mm, an inner side length 11mm, and a period P = 6mm.

[0189] Netting 3: A square array of periodic units with an outer side length L = 10mm, an inner side length of 9mm, and a period P = 8mm.

[0190] Net 4: Laying a square array of periodic units with an outer side length L = 8mm, an inner side length 7mm, and a period P = 10mm.

[0191] Netting 5: A square array of periodic units with an outer side length L = 6mm, an inner side length 5mm, and a period P = 12mm.

[0192] Mesh 6 (for the top layer): Lay a square array of periodic units with an outer side length L = 4mm, an inner side length 3mm, and a period P = 14mm.

[0193] Step 3: Interface treatment of alumina fiber cloth

[0194] SiC interface layer preparation was performed on 7 pieces of cut alumina fiber cloth:

[0195] Impregnation: Immerse the alumina fiber cloth completely in an 8% polycarbosilane (PCS) solution for 20 minutes.

[0196] Drying and curing: Remove the fabric, air dry it in a fume hood, then transfer it to an oven and heat treat it at 200°C for 1 hour to allow the PCS to crosslink and cure.

[0197] High-temperature pyrolysis: The cured fiber cloth is heated to 1200℃ at a rate of 3℃ / min under the protection of high-purity argon gas and held at that temperature for 2 hours. This process causes PCS to pyrolyze and transform into SiC ceramic.

[0198] Cyclic thickening: The above impregnation-curing-pyrolysis process is repeated twice, and finally a SiC interface layer with a thickness of about 300nm is formed on the fiber surface.

[0199] Step 4: Construction of 3D Gradient-Enhanced Prefabricated Structure

[0200] Alternating Layering: Prepare the 7 functionalized mesh sheets obtained in step two and the 8 interface-modified fabric sheets obtained in step three. Alternately layer them in the order of "fabric-mesh-fabric-mesh..." to form a 15-layer stack. The laying sequence from the top layer (incident surface) to the bottom layer is as follows:

[0201] Fabric 1 → Mesh 1 → Fabric 2 → Mesh 2 → ... → Fabric 6 → Mesh 6 → Fabric 7

[0202] Integral needle puncture: The entire laminated body is needled in both longitudinal and transverse directions to form a three-dimensional integrated structure.

[0203] Template removal: The needled preform is placed in a vacuum oven and heat-treated at 450°C for 2 hours to completely decompose and remove the PLA sacrificial template, thus obtaining the final three-dimensional gradient-reinforced preform.

[0204] Step 5: Matrix Introduction and Ceramicization (Sol-Gel Method)

[0205] The matrix of the preform is introduced and densified by multiple sol-gel impregnation and sintering processes.

[0206] Vacuum impregnation: Place the preform in a container, evacuate to 1 Pa, and maintain for 30 minutes. Slowly introduce the prepared silica sol under vacuum until completely submerged. Continue vacuum impregnation for 1 hour, then restore atmospheric pressure and apply a positive pressure of 0.5 MPa, maintaining the pressure for 2 hours to promote sol penetration.

[0207] Gelping and drying: The impregnated preform was removed and placed in a constant temperature and humidity chamber at 60℃ and 80% relative humidity for 24 hours for gelation and aging. Subsequently, it was dried in an oven at 80℃ for 48 hours to obtain a composite preform containing SiO2 dry gel.

[0208] High-temperature sintering: The green body is placed in a sintering furnace and heated to 1200℃ at a rate of 2℃ / min under air atmosphere, and held at that temperature for 2 hours. This process densifies the amorphous SiO2 dry gel by sintering, forming a SiO2 ceramic matrix.

[0209] Repeated densification: The "impregnation-gelation-drying-sintering" cycle of steps 1 to 3 was repeated. In this embodiment, a total of 5 cycles were performed until the porosity of the composite material was less than 15%, which was considered to have reached a highly dense state.

[0210] Final processing: The densified composite material block is cut and polished to prepare a high-temperature absorbing material sample with a final thickness of 4.0 mm.

[0211] This embodiment demonstrates the preparation of a high-temperature absorbing material with a more complex gradient design, employing a SiC interface layer and a SiO2 ceramic matrix, thus verifying the broad applicability of the method of this invention in material systems and structural design.

[0212] The performance of the obtained high-temperature resistant microwave absorbing material based on alumina fiber was tested. The electromagnetic reflectivity was measured using a vector network analyzer in the 2-18 GHz frequency band via the waveguide method. The test results are as follows: Figure 6 As shown, the electromagnetic reflectivity curve has an RL ≤ -8dB in the range of 6–18GHz, with an effective bandwidth of 6GHz; the lower the peak value, the stronger the absorption capability, and a deeper absorption peak appears at the 15GHz frequency point, with a peak loss of -16dB.

[0213] In addition, the mechanical properties of the absorbing material were tested through a three-point bending test. Its bending strength was 278 MPa, which showed excellent mechanical strength. Furthermore, the structure remained intact at a high temperature of 1000℃, demonstrating its good thermal stability.

[0214] Some steps in the embodiments of the present invention can be implemented using software, and the corresponding software program can be stored in a readable storage medium, such as an optical disc or a hard disk.

[0215] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A design and preparation method of a microwave absorbing material based on alumina fiber, characterized in that, The method includes: Step 1: Raw material preparation; Obtain chopped silicon carbide fibers, alumina fiber cloth, alumina fiber mesh, ceramic matrix precursor, interface layer material, and sacrificial template material; Step 2: A preform in which chopped silicon carbide fibers are oriented and distributed in an alumina fiber mesh layer is prepared by using chopped silicon carbide fibers, sacrificial template material, and alumina fiber mesh. Step 3: Perform interface treatment on the alumina fiber cloth; Step 4: Prepare a three-dimensional reinforced preform based on the preform obtained in Step 2 and the alumina fiber cloth obtained in Step 3; Step 5: Matrix introduction and ceramization; The three-dimensional reinforced preform obtained in Step 4 is impregnated using an introduction method that matches the ceramic matrix precursor, and then ceramized to obtain the final alumina fiber microwave absorbing material.

2. The method according to claim 1, characterized in that, The short-cut silicon carbide fibers in step one are 2-5 mm in length and form a nested structure along each layer within the laying area. The short-cut silicon carbide fibers are arranged in an array of one or a combination of square, rectangular, circular, rectangular or radial patterns, and are embedded in an alumina fiber mesh layer. The alumina fiber cloth is made of plain-weave alumina fiber cloth with an Al2O3 content of 99% and an areal density of 200 g / m³. 2 ; The alumina fiber mesh is a fiber mesh made of chopped alumina fibers through an air-flow forming process, with a surface density of 150 g / m². 2 ; The sacrificial template material is a thermally decomposable polymer material or a fusible low-melting-point material; The thermally decomposable polymer material is selected from polyvinyl alcohol, polylactic acid, polyethylene glycol, or any combination thereof.

3. The method according to claim 2, characterized in that, Step two includes: Step S1: Functional wire preparation; Short-cut silicon carbide fibers are combined with sacrificial template material to prepare functional wires, the axial direction of the functional wires being consistent with the long axis direction of the internal short-cut silicon carbide fibers; Step S2: Path design; Based on the preset reinforcement requirements, multiple two-dimensional laying paths are planned on several alumina fiber mesh layers; The paths form geometric patterns such as rectangles, rings, radials, squares or arbitrary curves to form reinforcement areas. Step S3: Selective laying; Using an automated laying device, the functional wires are laid in multiple designated areas of different alumina fiber mesh layers according to the laying path in step S2, and several layers are laid to obtain a preform. Step S4: Template removal and structure solidification; The preform obtained in step S3 is subjected to heat treatment to remove the sacrificial template material, so that the alumina fiber mesh layer and the chopped silicon carbide fiber are physically bonded together to obtain a preform with chopped silicon carbide fiber having a directional distribution structure in the alumina fiber mesh layer.

4. The method according to claim 3, characterized in that, In step S2, the reinforced region is distributed in a gradient from bottom to top along the thickness direction. The structure follows the design law of increasing size and decreasing period. Specifically, the gradient arrangement of the short-cut silicon carbide fibers is as follows: from the electromagnetic wave incident surface of the preform to the bottom surface away from the incident surface, the characteristic size of the geometric pattern increases layer by layer, and the arrangement period of the geometric pattern decreases layer by layer.

5. The method according to claim 4, characterized in that, The arrangement structure of the enhanced regions is as follows: The first layer of mesh: a square array of periodic units with a side length L = 60 mm and a period of 12 mm. The second layer of mesh: a square array of periodic units with a side length L = 50 mm and a period of 22 mm; The third layer of mesh: a square array of periodic units with a side length L = 40 mm and a period of 32 mm; The fourth layer of mesh: a square array of periodic units with a side length L = 30mm and a period of 42mm; The fifth layer of mesh: a square array of periodic units with a side length L = 20mm and a period of 52mm; The 6th layer of mesh: a square array of periodic units with a side length L = 10mm and a period of 62mm.

6. The method according to claim 5, characterized in that, The material used for interface treatment in step three is at least one of boron nitride, silicon carbide, pyrolytic carbon, titanium nitride, zirconium carbide, or graphene, or a composite coating formed by any combination thereof. Different introduction methods are selected based on the different precursors: The polymer precursor is at least one of polysiloxane, polysilazane, polyborosilicate, or a modified form thereof; the method for introducing the polymer precursor is polymer impregnation pyrolysis. The sol-gel precursor is at least one of silica sol, alumina sol, or mullite; the method for introducing the sol-gel precursor is the sol-gel method. The powder slurry precursor is a slurry made by mixing silicon dioxide, silicon nitride, or boron nitride with a dispersant and a binder; the method for introducing the powder slurry precursor is the slurry impregnation and sintering method.

7. The method according to claim 6, characterized in that, Step four includes: Step T1: Gradient structure design and layering: Following the sequence of "fabric-mesh-fabric-mesh...", functionalized mesh and interface-modified fabric are alternately layered; the alternating layering process is as follows: the first layer is a layer of alumina fiber cloth that has undergone interface treatment; the second layer is a layer of alumina fiber mesh obtained from step two; several layers are laid in sequence, ensuring that the centers of the alumina fiber cloth and the alumina mesh are aligned during the laying process; Step T2: Overall needle punching reinforcement; The material obtained in step T1 is fed into a needle punching machine and needle punched in both longitudinal and transverse directions using triangular needles; Needling causes some fibers to rearrange in the thickness direction, connecting the layers into an integral three-dimensional reinforced preform; Step T3: Template removal and preforming; the needled preform is placed in a vacuum oven and heat-treated in an inert atmosphere or vacuum environment, heated to above the decomposition temperature of the sacrificial template material, so that it is completely thermally decomposed and extracted; then flat plate pressure or vacuum bag preforming is performed to obtain a three-dimensional gradient reinforced preform.

8. The method according to claim 7, characterized in that, Step five includes: Step M1: Vacuum impregnation; Place the preform in a container, evacuate to below 10 Pa and maintain for 15-30 minutes, introduce PSO impregnation solution under vacuum to completely immerse the preform; after continuous impregnation for 30-60 minutes, restore normal pressure and apply a positive pressure of 0.2-1.0 MPa, maintain pressure for impregnation for 30-60 minutes to ensure full penetration; Step M2: Curing treatment; The preform after vacuum impregnation is heated to 150-250°C at a rate of 1-3°C / min in air or an inert atmosphere and held for 1-2 hours to allow the PSO precursor to crosslink and cure. Step M3: Pyrolysis Ceramicization; The cured preform is heated to 1000-1200℃ at a rate of 2-5℃ / min under an inert atmosphere and held for 1-3 hours to cause PSO to pyrolyze and transform into an amorphous SiOC ceramic matrix. Step M4: Repeat the impregnation-pyrolysis cycle; to fill the pores generated by pyrolysis and improve the material density, repeat the above steps M1 to M3 cycles 5-10 times; when the mass increase rate after two consecutive cycles is less than 0.5% and the porosity is less than 10%, the densification process is considered complete. Step M5: Obtain the final material; after completing the last pyrolysis cycle and cooling, the final high-temperature absorbing material with a gradient structure based on an alumina fiber reinforced SiOC ceramic matrix is ​​obtained.

9. A microwave absorbing material based on alumina fibers, characterized in that, The absorbing material is implemented based on the method described in any one of claims 1-8.

10. A radar stealth material, characterized in that, The material is prepared based on the alumina fiber-based microwave absorbing material as described in claim 9.

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