A full spectrum stealth composite material for aircraft and a method of forming the same

Abstract

The application provides a full-spectrum stealth composite material for an aircraft, which is an advanced composite material exhibiting a three-phase three-dimensional interpenetrating network topology at micro-to-macro scales, and is formed by penetrating an insulating main load-bearing reinforcing phase into a rigid electromagnetic loss skeleton to form a mutually penetrating and interlocking structure, and filling all remaining spaces by a solidified matrix phase, so that the whole exhibits a mutually penetrating, interlocking and synergistic structure in three-dimensional space; an outer aerodynamic surface of the above composite material body member is in-situ constructed with a nano silicon-titanium-based magnetic composite functional coating by spraying, spin coating or other methods to form the full-spectrum stealth composite material for the aircraft. The nano silicon-titanium-based magnetic composite functional coating is provided with a ladder curing process combining "room temperature rapid pre-curing" and "controllable thermal densification"; the process ingeniously combines construction convenience and ultimate reliability.

Description

Technical Field

[0001] This invention belongs to the field of aerospace structure-function integration technology, specifically relating to a full-spectrum stealth composite material for aircraft and its molding method. Background Technology

[0002] Currently, the low observability (stealth) capability of advanced aircraft has become a core element determining their battlefield survivability and combat effectiveness. With the continuous development of detection technology, the requirement to reduce the radar cross section (RCS) of aircraft has expanded from traditional centimeter waves (such as X and Ku bands) to "full-spectrum, wide-angle" stealth covering meter waves, millimeter waves, and even infrared and visible light bands. However, existing stealth structural technology systems face profound and irreconcilable technical contradictions when addressing this challenge, particularly in solving the most threatening meter-wave (frequency 30MHz-3GHz, wavelength 0.1-10m) detection, specifically manifested in the following aspects: (1) The widely used "radar absorbing structure" (RAS) mainly follows the "sandwich sandwich" design paradigm; CN114030269A discloses a "graphene modified honeycomb core absorbing stealth composite material", which essentially gives the "absorbing" function to the non-load-bearing or secondary load-bearing core material; the core material with the best absorbing performance often has the worst mechanical properties, and pursuing mechanical performance will sacrifice stealth performance; in addition, the multi-layer interface (panel-film-core material-base plate) is prone to delamination failure under harsh environments such as aerodynamic heat and vibration fatigue, which seriously restricts the reliability of the structure; (2) The physical coupling advantage of the thickness required for meter-wave stealth and the load-bearing thickness of the aircraft structure has not been utilized; the main load-bearing structures such as the wing root, fuselage sidewall, and vertical tail of modern advanced aircraft themselves require sufficient thickness (10-30cm) to meet the requirements of bending stiffness, torsion resistance and equipment accommodation; existing technologies add an additional wave-absorbing layer in addition to the structural thickness, resulting in ineffective weight increase and aerodynamic deterioration. (3) Carbon fiber is the preferred reinforcing material for the main load-bearing structure of aircraft, but it is essentially a good conductor. It produces strong specular reflection and secondary radiation of incident electromagnetic waves, which seriously damages or even completely negates the design effect of the wave-absorbing filler or structure. The traditional solution is to avoid using continuous carbon fiber or cut it into short fibers, but this will greatly sacrifice the mechanical properties of the material. (4) Traditional processes such as prepreg lay-up and resin transfer molding (RTM) are prone to problems such as incomplete resin wetting, uneven fiber distribution, and concentrated heat release during curing when preparing thick components. This results in electromagnetic "blind spots" or mechanically weak areas inside the material.

[0003] (5) Room temperature or low temperature curing absorbing coatings are easy to apply, but these coatings have inherent defects in terms of hardness, adhesion, resistance to environmental aging (especially resistance to damp heat and ultraviolet radiation), and long-term electrical performance stability. Their curing process mainly relies on solvent evaporation or physical cross-linking, resulting in a film layer with low density and poor heat resistance (usually with a long-term operating temperature <100℃), which cannot meet the long-term service requirements of high-speed aircraft surfaces under harsh aerodynamic heat, rain erosion, sand erosion, and strong ultraviolet radiation environments. In addition, room temperature curing coatings are difficult to form stable chemical bonds with high-performance magnetic absorbing fillers (such as nano-carbonyl iron), and the fillers are prone to oxidation and migration, leading to a sharp decline in millimeter wave absorption performance over time. Therefore, developing a coating system that can achieve excellent application performance (such as room temperature surface drying) and must obtain intrinsic high performance through subsequent heat treatment is the key to achieving reliable full-spectrum stealth.

[0004] In summary, existing technologies are limited by a separation of structure and function, the inherent contradiction of high strength leading to high conductivity, bottlenecks in the manufacturing process of thick components, and the inadequacy of surface coating performance and durability. These limitations prevent them from meeting the urgent needs of future stealth aircraft for integrated components that are structurally sound, offer full-spectrum stealth, are lightweight and efficient, and have a long lifespan and high reliability. Therefore, there is an urgent need to develop a disruptive material system and manufacturing method that can completely break down the boundaries between structure and function, resolve the conductivity-stealth contradiction at the material element level, and simultaneously overcome the challenges of manufacturing thick components and integrating high-performance coatings. This would allow the main load-bearing structure of the aircraft to become a highly efficient broadband electromagnetic wave absorber. Summary of the Invention

[0005] In view of the above situation, the purpose of this invention is to provide a full-spectrum stealth composite material for aircraft and its molding method; by deeply interpenetrating and fusing a rigid electromagnetic loss skeleton, an insulating continuous fiber reinforcement and a liquid matrix material in a three-dimensional scale, a novel material is provided that has both excellent main load-bearing performance and broadband high-efficiency meter-wave absorption capability in its body, seamlessly integrating millimeter-wave, infrared and visible light stealth functions, and through integrated material-structure-function design and in-situ manufacturing, full-spectrum stealth, zero functional weight gain and high environmental durability of the main load-bearing components of the aircraft are achieved.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a full-spectrum stealth composite material for aircraft and its molding method are provided; the full-spectrum stealth composite material is an advanced composite material that exhibits a three-phase three-dimensional interpenetrating network topology structure at both the microscopic and macroscopic scales. Specifically, the insulating main load-bearing reinforcement phase is penetrated through the rigid electromagnetic loss skeleton to form an interpenetrating and interlocking structure, while the solidified matrix phase fills all the remaining space. The whole exhibits an interpenetrating, interlocking, and synergistic structure in three-dimensional space.

[0007] The insulating main load-bearing reinforcement phase is composed of continuous high-performance fibers with a dense nano-silicon-titanium ceramic insulating protective film on the surface; the rigid electromagnetic loss skeleton phase is a rigid porous body with a three-dimensional interconnected open pore network formed by a large number of hollow ceramic microspheres through a controllable low-temperature sintering process; the solidified matrix phase is formed by completely filling all the interconnected pores of the rigid skeleton and all the gaps between the insulating fiber reinforcements with liquid matrix material under vacuum, pressure or capillary force, and then solidifying it through physical or chemical changes.

[0008] On the outer aerodynamic surface of the above-mentioned full-spectrum stealth composite material body component, a nano-silicon titanium-based magnetic composite functional coating is constructed in situ by spraying, spin coating and other methods. The coating uses nano-SiO2-TiO2 sol as the matrix, in which one or more magnetic / conductive functional phases such as nano-ferrite, nano-carbonyl iron, graphene, and carbon nanotubes are uniformly dispersed.

[0009] The insulating main load-bearing reinforcement fiber is preferably carbon fiber; a continuous and dense amorphous SiO2-TiO2 ceramic film with a thickness of 100 nm to 1 μm is grown in situ on the surface of each fiber monofilament using a sol-gel process; this film reduces the bulk resistivity of the carbon fiber from 10... - The order of 3Ω·cm has been increased to at least 10¹ 0 The carbon fiber, with a density of Ω·cm, achieves complete insulation. The insulated carbon fiber serves solely as a mechanical load-bearing skeleton in the composite material, completely eliminating its contribution to electromagnetic wave reflection. The insulated fibers, arranged in predetermined layup directions (such as quasi-isotropic layups at 0°, ±45°, and 90°) and using methods such as three-dimensional weaving, multi-directional stitching, or Z-axis puncture, form a main load-bearing network that penetrates the entire thickness of the material. This is particularly suitable for thick-walled components to resist interlaminar shear forces.

[0010] The rigid electromagnetic loss skeleton phase undertakes the following core functions: ① Electromagnetic wave loss body: Utilizing the huge gas-solid dielectric constant contrast formed by the enclosed gas (dielectric constant ≈ 1) inside the microspheres and the ceramic shell wall (dielectric constant > 3), it generates strong multiple scattering, interference cancellation, and mode conversion of incident electromagnetic waves (especially low-frequency meter waves); ② Dielectric polarization loss source: The ceramic material itself generates significant polarization relaxation loss under high-frequency alternating electric field, converting electromagnetic energy into heat energy; ③ Initial mechanical support and fiber positioning: It provides basic shape retention and compressive stiffness for the entire material preform before infusion, and serves as a three-dimensional spatial grid to position and support the insulating fiber network; ④ Large thickness homogenization guarantee: The rigid skeleton provides stable support in the large thickness direction, preventing the reinforcing fibers from settling or shifting during the impregnation process of the liquid matrix, ensuring the performance consistency of the material in the thickness direction.

[0011] The core functions of the solidified matrix phase are: ① High-efficiency bonding and stress transfer: firmly bonding the rigid skeleton and insulating fibers into a whole, and transferring and distributing stress between them to achieve mechanical synergy; ② Environmental protection and interface optimization: protecting the fibers and ceramic microspheres from environmental erosion; ③ Impedance matching adjustment: the dielectric properties of the matrix material itself can be used as a key variable to adjust the overall equivalent impedance of the composite material.

[0012] In this invention, the liquid matrix material is universal and can be flexibly selected from thermosetting resins, thermoplastic resins, metal melts, ceramic precursors (sols) and their mixtures, depending on the requirements of the final application scenario for temperature resistance, thermal conductivity, density, etc.

[0013] The nano-silicon-titanium-based magnetic composite functional coating is specifically designed for high-frequency stealth: through a synergistic mechanism of hysteresis loss, eddy current loss, dielectric polarization loss, and interfacial polarization loss, it achieves strong absorption (≥10dB) in the millimeter-wave band (30-300GHz); simultaneously, the nano-silicon-titanium matrix possesses extremely low infrared emissivity (tunable to ≤0.3) and tunable matte coloring capability in the visible light band, thus integrating millimeter-wave, infrared, and visible light stealth. The surface layer and the bulk material form a strong chemical bond interface through subsequent heat treatment, ensuring robustness and reliability.

[0014] The preparation process of the full-spectrum stealth composite material: Step A: Carbon fiber insulation pretreatment A1. Preparation of nano-silicon-titanium sol: Dissolve TEOS and TBT in anhydrous ethanol, add appropriate amount of deionized water and ammonia catalyst (control the final pH=9.0), and carry out hydrolysis-condensation reaction by magnetic stirring in a constant temperature water bath to obtain a clear and transparent nano-silicon-titanium composite sol. Adjust its solid content to 10wt% and store at 4℃ for later use. A2. Continuous impregnation and gelation: T1100 carbon fiber filaments are drawn from the spindle and continuously passed through an impregnation tank containing the above-mentioned sol at a constant speed of 0.5 m / min through a tension control system to ensure that each monofilament is fully impregnated by the sol; then it enters the first stage drying channel, with the temperature set at 80℃ and the residence time at 3 minutes, so that the sol can be initially gelled on the fiber surface. A3. Heat treatment and curing: The fibers that have undergone preliminary gelation enter the second stage heat treatment channel and are treated in air at 180°C for 15 minutes to further dehydrate and densify the gel film, ultimately forming a uniform, continuous, and dense amorphous SiO2-TiO2 ceramic protective film on the carbon fiber surface. A4. Performance characterization: The treated fibers were wound up for later use. Random samples were taken for testing. The thickness of the ceramic film was approximately 500 ± 50 nm, and it was continuous without cracks. The resistivity of the single filament was increased to ≥ 2×10¹¹ Ω·cm, meeting the insulation requirements. The fiber strength retention rate was above 98%.

[0015] Step B: Prefabricate a rigid electromagnetic loss frame B1. Ingredients and Mixing: The hollow alumina microspheres treated with nano-silicon titanium dioxide and the nano-silica sol are mixed in a planetary mixer for 60 minutes to ensure that the sol evenly coats each microsphere; B2. High-pressure cold pressing: The uniformly mixed powder is filled into a steel mold and a two-way gradient pressurization technology is adopted. First, it is pre-pressed at 5 MPa for 2 minutes, and then the pressure is increased to 10 MPa and held for 10 minutes to ensure that the density of the blank is uniform in the thickness direction. B3. Atmosphere sintering: The green body is placed in a large atmosphere sintering furnace, and high-purity nitrogen is introduced at a flow rate of 10 L / min. The temperature is raised to 900 ℃ at a rate of 3 ℃ / min and held for 120 minutes. Then, the temperature is programmed to drop to below 200 ℃ at a rate of 2 ℃ / min and cooled to room temperature in the furnace. B4. Post-processing and inspection: After demolding, a rigid porous ceramic preform with an opening ratio of (52 ± 2)% and a thickness of up to 280 mm is obtained. The pores are uniformly distributed throughout the thickness range, and the microspheres form neck-shaped connections, with three-dimensional interconnection of pores.

[0016] Step C: Implanting insulating carbon fiber reinforcement C1. Layup and 3D Reinforcement Design: Using 3D weaving technology, a three-dimensional orthogonal fabric of T1100 carbon fiber is woven to match the shape of the prefabricated internal cavity. The fabric contains continuous fibers in the X, Y, and Z directions, with the Z-direction fiber volume content accounting for 5% of the total fiber volume. C2. Implantation: The three-dimensional fabric is implanted into the internal cavity of the ceramic preform, ensuring that the fabric fits tightly against the pore walls of the preform, and a highly breathable release cloth is laid on top.

[0017] Step D: Vacuum infusion of resin and gradient staged curing D1. Mold preparation and multi-port design: Using Invar alloy mold, three resin injection ports are set at 1 / 3, 1 / 2 and 2 / 3 of the thickness direction, the main vacuum outlet is set at the top and the auxiliary exhaust / observation port is set at the bottom; D2. Preheating: Push the entire mold into a preheated autoclave at 60°C and keep it at that temperature for 4 hours; D3. Segmented vacuum-assisted injection: Evacuate the mold to a high vacuum of -0.099 MPa, first open the middle injection port to inject resin, and when the resin front reaches the upper and lower injection port positions, open the auxiliary injection valve simultaneously. After the resin continues to flow out without bubbles, close the injection port and maintain pressure for 30 minutes to release the air. D4. Autoclave gradient staged curing: Pressurize and cure in stages at 100℃ for 4 hours, 140℃ for 6 hours, and 180℃ for 8 hours, then cool down to below 60℃ at a rate of ≤0.5℃ / min. D5. Demolding and post-processing: After opening the can and demolding, perform full-section ultrasonic C-scan and industrial CT inspection. If no defects are found, perform water jet cutting and milling finishing.

[0018] Step E: Preparation of Multifunctional Stealth Coating on Surface E1. Surface pretreatment: white corundum sandblasting, acetone ultrasonic cleaning, atmospheric plasma activation treatment; E2. Coating slurry preparation: Add nano carbonyl iron powder and multilayer graphene nanosheets to nano SiO2-TiO2 composite sol, add dispersant and photoinitiator, and ultrasonically disperse in ice water bath; E3. Spraying: High-pressure airless spraying using a six-axis robot in a clean room, with wet film thickness controlled at 200 ± 20 μm; E4. Stepped curing process: Pre-curing at room temperature for 15-30 minutes until surface dry; UV-LED surface light source irradiation for 60 seconds; heat treatment at 120℃-140℃ for 60-90 minutes, followed by slow cooling down to below 60℃.

[0019] Performance test and characterization results Mechanical properties: Three-point bending strength 1180 ± 40 MPa, bending modulus 95 ± 3 GPa, interlaminar shear strength 42 ± 3 MPa, longitudinal compressive strength 980 ± 35 MPa, and open-cell compressive strength 320 ± 20 MPa; Electrical and stealth properties: Bulk volume resistivity > 10¹ 0 Ω·cm, meter-wave reflectivity at 0.8GHz is -18.5dB, -10dB absorption bandwidth is 0.45-1.3GHz, average attenuation of 16dB in X-band and 21dB in Ka-band, infrared emissivity is 0.20, and the matte dark gray visible light camouflage effect is excellent. Environmental adaptability: Performance degradation is minimal after passing temperature shock, damp heat aging, and vibration fatigue tests; Weight reduction benefits: Weight reduction of approximately 45% compared to traditional solutions.

[0020] This invention is the first to systematically propose and realize the design philosophy of actively utilizing the inherent thickness (centimeter to decimeter level) of the main load-bearing structure of an aircraft as a broadband meter-wave absorber. It overturns the traditional mindset of "adding a wave-absorbing layer to the structure," recognizing that the physical fact that aircraft wings, fuselages, and other parts must be thick enough to meet stiffness requirements for meter-wave stealth, and the physical law that sufficient thickness is needed to effectively absorb meter-waves, are two aspects of the same physical reality. Through a unique three-phase three-dimensional interpenetrating network structure, this invention allows the thickness of the material bearing the main bending and shear loads to directly participate in and dominate the matching and dissipation process of meter-wave energy, transforming "structural burden" into "stealth advantage," achieving full-spectrum stealth with zero weight gain or even weight reduction.

[0021] This invention proposes a pretreatment technology for nano-silicon titanium sol insulation coating of continuous carbon fibers. This technology constructs an extremely thin (submicron-level) but dense and continuous ceramic insulating film on the surface of each carbon fiber monofilament, increasing its volume resistivity by more than eight orders of magnitude without damaging the fiber itself or reducing its mechanical properties. This fundamentally transforms it from an "electromagnetic wave reflection source" into a pure "mechanical reinforcement," completely solving the core contradiction that has long restricted the use of carbon fiber composite materials in broadband stealth main load-bearing structures.

[0022] Building upon insulating fibers, this invention further constructs a microstructure in which a "rigid ceramic loss skeleton - insulating continuous fiber reinforcement network - liquid matrix filling phase" interpenetrate, interlock, and synergistically bear load in three-dimensional space. This structure is particularly suitable for thick components: shear resistance and delamination prevention; the rigid skeleton and Z-direction through-fiber network (through 3D weaving or stitching) provide the material with extremely high interlaminar shear strength and damage tolerance, solving the delamination problem of traditional laminates at large thicknesses; thickness-direction uniformity of electromagnetic and mechanical properties; the three-phase components uniformly interpenetrate in three-dimensional space, ensuring that even for a 30cm thick component, the electromagnetic parameters and mechanical properties from the surface to the core are highly consistent, avoiding performance gradients; integrated load bearing and wave absorption; each layer of material in the thickness direction of the structure simultaneously contributes stiffness and wave absorption efficiency, achieving the theoretical limit of material utilization.

[0023] To address the industry challenges of resin impregnation difficulties, concentrated heat release during curing, and susceptibility to defects in thick composite material components, this invention proposes a systematic process: "fiber insulation → prefabricated skeleton → three-dimensional reinforcement → gradient pressure impregnation → segmented curing." By increasing the cold-pressing pressure (e.g., 8-10 MPa), uniform density of the bulk blank is ensured; vacuum-assisted resin transfer molding combined with segmented gradient injection technology ensures complete resin impregnation along the thickness direction; and a segmented curing regime is implemented to precisely control the heat release during curing, preventing internal overheating that could lead to performance degradation or cracking. This process ensures the reliability and consistency of manufacturing components from thin-walled to ultra-thick-walled profiles.

[0024] This invention establishes a scientific division of labor system where the inner body solves the meter wave problem and the surface coating solves the high frequency problem. The two are seamlessly connected through integrated design and in-situ manufacturing process, forming a strong interface bond, which completely eliminates the risk of traditional coating peeling off and significantly improves the stability of stealth performance and environmental durability throughout the entire life cycle.

[0025] The beneficial effects of this invention are as follows: The nano-silicon-titanium-based magnetic composite functional coating employs a stepped curing process combining "room temperature rapid pre-curing" and "controlled thermal densification." Room temperature / UV pre-curing, utilizing the excellent room temperature film-forming properties of nano-silicon-titanium sol and selectable photoinitiators, enables the coating to achieve rapid setting (surface drying) within seconds to minutes after application, preventing sagging and facilitating handling and transport. This stage forms a gel network primarily based on physical processes. Heat treatment and chemical curing are essential steps for achieving the final performance of the coating, achieved through controlled heat treatment at 80℃-250℃. This process initiates deep dehydration and condensation reactions between sol particles, forming a strong, three-dimensional Si-O-Si and Ti-O-Ti covalent network. This process densifies the coating, achieving a pencil hardness of ≥5H and excellent wear and impact resistance. It significantly improves adhesion, enabling the coating to form a chemically bonded interface with the composite material. It stabilizes functional fillers, firmly anchoring magnetic / conductive nanoparticles within the ceramic mesh, preventing oxidation and agglomeration, and ensuring long-term stability of millimeter-wave absorption performance. It also imparts weather resistance, allowing the coating to withstand temperature shocks from -55°C to +200°C, damp heat, salt spray, and UV aging. This ingenious process design combines ease of application with ultimate reliability, avoiding the industry-wide pitfall of sacrificing performance for ease of application. Attached Figure Description

[0026] Figure 1 This is a SEM image of the cross-sectional morphology of the three-phase composite material of the present invention; Figure 2 This is a SEM image of the surface morphology of the insulating carbon fiber monofilament of the present invention. Figure 3 This is a graph showing the reflectivity test results of the composite material in the meter-wave band of this invention. Figure 4 This is a test curve of the millimeter-wave band reflection attenuation of the composite material of the present invention; Figure 5 This is a graph showing the infrared emissivity test results for the stealth coating of this invention. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of this invention clearer, a non-limiting detailed description of the invention will be provided below with reference to the accompanying drawings and specific embodiments. It should be understood that the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without inventive effort are within the scope of protection of this invention.

[0028] This invention provides an integrated full-spectrum stealth composite material, whose three-phase three-dimensional interpenetrating network microstructure, such as... Figure 1 As shown, the microstructure of the material is given, specifically including a rigid electromagnetic loss skeleton formed by sintering hollow ceramic microspheres, an insulating continuous carbon fiber monofilament coated with a ceramic insulating film, and a solidified matrix phase filling the pores, which can be any one of resin, metal, and ceramic; wherein, the surface of the carbon fiber has a nano-SiO2-TiO2 ceramic insulating layer 102a; wherein the hollow ceramic microspheres form a neck-shaped sintered connection, the continuous carbon fiber runs through the rigid electromagnetic loss skeleton, and the solidified matrix phase fills all the remaining spaces, and the whole presents a three-way interpenetrating and interlocking structure. Example 1

[0029] Application Background and Performance Objectives: To fabricate a key main load-bearing beam at the wing root of a certain type of fighter jet. This structural region has a thickness of 280mm to withstand enormous bending and torsional moments. The beam is required to meet the strength and stiffness requirements under extreme aerodynamic loads, while utilizing its own thickness to achieve meter-wave stealth with a reflectivity ≤-10dB in the 0.3-1.5GHz frequency band, and to integrate stealth capabilities for other frequency bands.

[0030] Raw material details: Reinforcing fiber: Toray T1100 grade carbon fiber from Japan, 12K tow; Insulation treatment materials: Tetrabutyl orthosilicate (TEOS), tetrabutyl titanate (TBT), anhydrous ethanol, ammonia, deionized water; Hollow ceramic microspheres: High-purity alumina (α-Al2O3) hollow microspheres, average particle size: 100 ± 20 μm, single sphere hydrostatic crushing strength: ≥ 70 MPa, pretreated with nano-silica titanium sol before use; Inorganic binder: Nano-silica sol; Matrix resin: High-temperature curing, high-toughness, low-viscosity bismaleimide resin, grade: BMI-3701L of AVIC XX Institute (specifically optimized for thick components, room temperature viscosity 300 mPa·s); Surface coating materials: Nano-SiO2-TiO2 composite sol (solid content 15%, pH=9.5), flake-shaped nano-carbonyl iron powder (particle size 100-300 nm, coercivity Hc<100). A / m), multilayer graphene nanosheets (thickness 3-5 nm, sheet diameter 5-10 μm), dispersant polyethylene glycol octylphenyl ether (Triton X-100), and ultraviolet photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-propanone (Darocur 1173).

[0031] Preparation process: Step A: Carbon fiber insulation pretreatment A1. Preparation of nano-silicon-titanium sol: TEOS and TBT were dissolved in anhydrous ethanol at a molar ratio of 7:3. An appropriate amount of deionized water and ammonia catalyst were added (to control the final pH=9.0). The mixture was magnetically stirred in a constant temperature water bath at 60℃ for 24 hours to carry out the hydrolysis-condensation reaction and obtain a clear and transparent nano-silicon-titanium composite sol. Its solid content was adjusted to 10wt% and stored at 4℃ for later use. A2. Continuous Impregnation and Gelation: T1100 carbon fiber filaments are drawn from the spindle and continuously passed through an impregnation tank containing the aforementioned sol at a constant speed of 0.5 m / min using a tension control system, ensuring that each monofilament is fully impregnated by the sol. They then enter the first-stage drying channel, with the temperature set at 80℃ and a residence time of 3 minutes, allowing the sol to initially gel on the fiber surface. A3. Heat treatment and curing: The fibers that have undergone preliminary gelation enter the second stage heat treatment channel and are treated in air at 180°C for 15 minutes to further dehydrate and densify the gel film, ultimately forming a uniform, continuous, and dense amorphous SiO2-TiO2 ceramic protective film on the carbon fiber surface. A4. Performance characterization: The treated fibers were wound up for later use. Random samples were taken for testing. The thickness of the ceramic film was approximately 500 ± 50 nm, and it was continuous without cracks. The resistivity of the single filament was increased to ≥ 2×10¹¹ Ω·cm, meeting the insulation requirements. The fiber strength retention rate was above 98%.

[0032] Step B: Prefabricate a rigid electromagnetic loss frame (optimized for large thicknesses) B1. Ingredients and Mixing: Mix 100 parts by weight of nano-silicon titanium-treated alumina hollow microspheres and 5 parts by weight of nano-silica sol in a planetary mixer at a speed of 200 rpm for 60 minutes to ensure that the sol evenly coats each microsphere. B2. High-pressure cold pressing: The uniformly mixed powder is filled into a large "I"-shaped high-strength steel mold. The bidirectional gradient pressurization technology is used. First, the pressure is pre-pressed at 5 MPa for 2 minutes, and then increased to 10 MPa and held for 10 minutes to ensure that the density of the blank is uniform in the thickness direction (density deviation <2%). B3. Atmosphere sintering: The green body is placed in a large atmosphere sintering furnace, and high-purity nitrogen (purity ≥99.999%) is introduced at a flow rate of 10 L / min. The temperature is raised to 900 ℃ at a rate of 3 ℃ / min and held for 120 minutes. Then, the temperature is programmed to drop to below 200 ℃ at a rate of 2 ℃ / min and cooled to room temperature in the furnace. B4. Post-processing and inspection: After demolding, a rigid porous ceramic preform with an opening ratio of (52 ± 2)% and a thickness of up to 280 mm is obtained. The pores are uniformly distributed throughout the thickness range, and the microspheres form neck-shaped connections, with three-dimensional interconnection of pores.

[0033] Step C: Implanting insulating carbon fiber reinforcement (optimized for thick shear resistance) C1. Layup and 3D Reinforcement Design: Using 3D weaving technology, a three-dimensional orthogonal fabric of T1100 carbon fiber is woven to match the shape of the prefabricated internal cavity. The fabric contains continuous fibers in the X, Y, and Z directions, with the Z-direction fiber volume content accounting for 5% of the total fiber volume. C2. Implantation: The three-dimensional fabric is implanted into the internal cavity of the ceramic preform, ensuring that the fabric fits tightly against the pore walls of the preform, and a highly breathable release cloth is laid on top.

[0034] Step D: Vacuum infusion of resin and gradient segmented curing (core process for large thicknesses) D1. Mold preparation and multi-port design: Using Invar alloy mold, three resin injection ports are set at 1 / 3, 1 / 2 and 2 / 3 of the thickness direction, the main vacuum outlet is set at the top and the auxiliary exhaust / observation port is set at the bottom; D2. Preheating: Push the entire mold into a preheated autoclave at 60°C and keep it at that temperature for 4 hours; D3. Segmented vacuum-assisted injection: Evacuate the mold to a high vacuum of -0.099 MPa, first open the middle injection port to inject resin, and when the resin front reaches the upper and lower injection port positions, open the auxiliary injection valve simultaneously. After the resin continues to flow out without bubbles, close the injection port and maintain pressure for 30 minutes to release the air. D4. Autoclave gradient staged curing: Pressurize to 1.0 MPa, cure in stages at 100℃ for 4 hours, 140℃ for 6 hours, and 180℃ for 8 hours, and cool down to below 60℃ at a rate of ≤0.5℃ / min; D5. Demolding and post-processing: After opening the can and demolding, perform full-section ultrasonic C-scan and industrial CT inspection. If no defects are found, perform water jet cutting and milling finishing.

[0035] Step E: Preparation of Multifunctional Stealth Coating on Surface E1. Surface pretreatment: 80-mesh white corundum sandblasting, acetone ultrasonic cleaning, atmospheric plasma activation treatment; E2. Coating slurry preparation: Add nano carbonyl iron powder and multilayer graphene nanosheets to nano SiO2-TiO2 composite sol, add dispersant and photoinitiator, and ultrasonically disperse in ice water bath for 60 minutes; E3. Spraying: High-pressure airless spraying using a six-axis robot in a clean room, with wet film thickness controlled at 200 ± 20 μm; E4. Stepped curing process: Pre-curing at room temperature for 15-30 minutes until surface dry; UV-LED surface light source irradiation for 60 seconds; heat treatment at 120℃-140℃ for 60-90 minutes, followed by slow cooling down to below 60℃.

[0036] Performance test and characterization results Mechanical properties: Three-point bending strength 1180 ± 40 MPa, bending modulus 95 ± 3 GPa, interlaminar shear strength 42 ± 3 MPa, longitudinal compressive strength 980 ± 35 MPa, and open-cell compressive strength 320 ± 20 MPa; Electrical and stealth properties: Bulk volume resistivity > 10¹ 0 Ω·cm, meter-wave reflectivity at 0.8GHz is -18.5dB, -10dB absorption bandwidth is 0.45-1.3GHz, average attenuation of 16dB in X-band and 21dB in Ka-band, infrared emissivity is 0.20, and the matte dark gray visible light camouflage effect is excellent. Environmental adaptability: Performance degradation is minimal after passing temperature shock, damp heat aging, and vibration fatigue tests; Weight reduction benefits: Weight reduction of approximately 45% compared to traditional solutions. Example 2

[0037] Application Background: For advanced UAV fuselage panels with low cost, high production volume, and medium performance requirements, the thickness is 15mm, and the requirements are rapid prototyping, good impact resistance and stealth performance.

[0038] Material and process adjustments: The fiber used is the insulating carbon fiber of Example 1; the microspheres are replaced with borosilicate hollow glass microspheres; the matrix resin is PEEK thermoplastic resin; the sintering temperature is reduced to 720℃ in the process; the reinforcement is T800 carbon fiber plain weave fabric + Z-direction stitching; the infusion process is changed to hot pressing molding; and the coating adopts a 180℃ thermosetting silicon titanium resin system.

[0039] Performance results: flexural strength 920 MPa, flexural modulus 68 GPa, post-impact compressive strength 280 MPa; 1GHz meter wave reflectivity -14 dB, X-band average attenuation 15 dB, infrared emissivity 0.26; molding cycle shortened by about 40%, suitable for mass production. Example 3

[0040] Application background: Used in high-temperature parts such as the leading edge of the wing and the nose cone of hypersonic aircraft, requiring ultra-high temperature load-bearing capacity, efficient thermal protection and stealth capabilities.

[0041] Material and process adjustments: Insulated carbon fiber or silicon carbide fiber is used for the fiber; high-purity alumina hollow microspheres are used for the microspheres; the matrix is ​​replaced with aluminum-silicon alloy or gallium-indium-tin eutectic alloy; the process adopts vacuum pressure impregnation (VPI), and the surface coating is an ultra-high temperature resistant SiC-Si3N4 ceramic coating.

[0042] Performance characteristics: The thermal conductivity of the metal matrix composite material is >100 W / m·K, with excellent high-temperature strength, retaining the ability to absorb meter waves, and the infrared characteristics can be controlled by the surface coating, making it suitable for extreme thermal-mechanical-stealth coupling environments.

[0043] like Figure 3 As shown, reflectivity curves (dB vs. frequency) for the 0.3–1.5 GHz band are presented. The measured reflectivity curve of the 280 mm thick component of this invention, with a -10 dB reference line, shows an absorption bandwidth covering 0.45–1.3 GHz. The lowest reflectivity absorption peak curve reaches a minimum reflectivity of -18.5 dB at 0.8 GHz. This demonstrates that the material achieves efficient absorption of the meter-wave band using its own thickness, meeting design specifications. Figure 4 As shown, reflection attenuation curves (dB vs. frequency) for the 2–40 GHz band are presented, with an average attenuation of 16 dB in the X-band (8–12 GHz) and 21 dB in the Ka-band (33–37 GHz); Figure 5 As shown, 01 is the measured emissivity curve of the coating of the present invention, and 02 is the infrared low emissivity benchmark value. The average emissivity is 0.20, which meets the requirements of low infrared characteristics and can be used to prove the low emissivity characteristics of the coating in the infrared band, so as to achieve infrared stealth.

Claims

1. A full-spectrum stealth composite material for aircraft, characterized in that: The aforementioned full-spectrum stealth composite material is a composite material exhibiting a three-phase, three-dimensional interpenetrating network topology at both the microscopic and macroscopic scales. Specifically, it consists of an insulating main load-bearing reinforcement phase that penetrates a rigid electromagnetic loss skeleton, forming an interpenetrating and interlocking structure. Simultaneously, a cured matrix phase fills all remaining spaces, resulting in an overall interpenetrating, interlocking, and synergistic structure in three-dimensional space. Insulating main load-bearing reinforcement phase: composed of continuous high-performance fibers with a dense nano-silicon-titanium ceramic insulating protective film on the surface; Rigid electromagnetic loss framework phase: a rigid porous body with a three-dimensional interconnected open pore network and an open porosity of 30%-65%, formed by a controllable low-temperature sintering process of hollow ceramic microspheres. Cured matrix phase: formed by completely filling all the interconnected pores of the rigid electromagnetic loss skeleton and all the gaps between the insulating fiber reinforcements with liquid matrix material under vacuum, pressure or capillary force, and then curing it through physical or chemical changes. A nano-silicon-titanium-based magnetic composite functional coating is in situ constructed on the outer aerodynamic surface of the above-mentioned composite material body component by spraying, spin coating or other methods, and the thickness of the structure body is 10mm to 300mm.

2. The full-spectrum stealth composite material for aircraft according to claim 1, characterized in that: The insulating main load-bearing reinforcement fiber is preferably carbon fiber; through a unique sol-gel process, a continuous and dense amorphous SiO2-TiO2 ceramic film with a thickness of 100 nm to 1 μm is grown in situ on the surface of each fiber monofilament.

3. The full-spectrum stealth composite material for aircraft according to claim 2, characterized in that: The amorphous SiO2-TiO2 ceramic film reduces the bulk resistivity of the carbon fibers from 10... - The order of 3Ω·cm has been increased to at least 10¹ 0 Ω·cm; Insulated carbon fibers, in a predetermined layup direction and through three-dimensional weaving, multi-directional stitching or Z-direction puncture, form the main load-bearing network that runs through the entire thickness of the material.

4. The full-spectrum stealth composite material for aircraft according to claim 1, characterized in that: The rigid electromagnetic loss framework phase utilizes the huge gas-solid dielectric constant contrast formed by the enclosed gas inside the microspheres and the ceramic shell wall to generate strong multiple scattering, interference cancellation and mode conversion of incident electromagnetic waves; the ceramic material itself generates significant polarization relaxation loss under high-frequency alternating electric field, converting electromagnetic energy into heat energy.

5. The full-spectrum stealth composite material for aircraft according to claim 1, characterized in that: The solidified matrix phase is selected from liquid matrix materials that are any one or a mixture of thermosetting resins, thermoplastic resins, metal melts, and ceramic precursors.

6. The full-spectrum stealth composite material for aircraft according to claim 1, characterized in that: The nano-silicon-titanium-based magnetic composite functional coating uses nano-SiO2-TiO2 sol as a matrix, in which one or more magnetic / conductive functional phases selected from nano-ferrite, nano-carbonyl iron, graphene, and carbon nanotubes are uniformly dispersed.

7. The full-spectrum stealth composite material for aircraft according to claim 1, characterized in that: The preparation process of the full-spectrum stealth composite material: Step A: Carbon fiber insulation pretreatment A1. Preparation of nano-silicon-titanium sol: Dissolve TEOS and TBT in anhydrous ethanol, add appropriate amount of deionized water and ammonia catalyst, control the final pH=9.0, and carry out hydrolysis-condensation reaction by magnetic stirring in a constant temperature water bath to obtain a clear and transparent nano-silicon-titanium composite sol. Adjust its solid content to 10wt% and store at 4℃ for later use. A2. Continuous Impregnation and Gelation: T1100 carbon fiber filaments are drawn from the spindle and continuously passed through an impregnation tank containing the aforementioned sol at a constant speed of 0.5 m / min using a tension control system, ensuring that each monofilament is fully impregnated by the sol. They then enter the first-stage drying channel, with the temperature set at 80℃ and a residence time of 3 minutes, allowing the sol to initially gel on the fiber surface. A3. Heat treatment and curing: The fibers that have undergone preliminary gelation enter the second stage heat treatment channel and are treated in air at 180°C for 15 minutes to further dehydrate and densify the gel film, ultimately forming a uniform, continuous, and dense amorphous SiO2-TiO2 ceramic protective film on the carbon fiber surface. A4. Performance Characterization: The treated fibers were wound up for later use. Random samples were tested. The ceramic film thickness was approximately 500 ± 50 nm, with no cracks. The single filament resistivity was increased to ≥ 2×10¹¹ Ω·cm, meeting the insulation requirements. The fiber strength retention rate was above 98%. Step B: Prefabricate a rigid electromagnetic loss frame B1. Ingredients and Mixing: The hollow alumina microspheres treated with nano-silicon titanium dioxide and the nano-silica sol are mixed in a planetary mixer for 60 minutes to ensure that the sol evenly coats each microsphere; B2. High-pressure cold pressing: The uniformly mixed powder is filled into a steel mold and a two-way gradient pressurization technology is adopted. First, it is pre-pressed at 5MPa for 2 minutes, and then the pressure is increased to 10MPa and held for 10 minutes to ensure that the density of the blank is uniform in the thickness direction. B3. Atmosphere sintering: The green body is placed in a large atmosphere sintering furnace, and high-purity nitrogen is introduced at a flow rate of 10 L / min. The temperature is raised to 900 ℃ at a rate of 3℃ / min and held for 120 minutes. Then, the temperature is programmed to drop to below 200 ℃ at a rate of 2 ℃ / min and cooled to room temperature in the furnace. B4. Post-processing and inspection: After demolding, a rigid porous ceramic preform with an open area ratio of 52 ± 2% and a thickness of 280 mm is obtained. The pores are uniformly distributed throughout the thickness range, and the microspheres form neck-shaped connections, with three-dimensional interconnection of pores. Step C: Implanting insulating carbon fiber reinforcement C1. Layup and 3D Reinforcement Design: Using 3D weaving technology, a three-dimensional orthogonal fabric of T1100 carbon fiber is woven to match the shape of the prefabricated internal cavity. The fabric contains continuous fibers in the X, Y, and Z directions, with the Z-direction fiber volume content accounting for 5% of the total fiber volume. C2. Implantation: The three-dimensional fabric is implanted into the internal cavity of the ceramic preform, ensuring that the fabric is tightly attached to the pore walls of the preform, and a highly breathable release cloth is laid on top. Step D: Vacuum infusion of resin and gradient staged curing D1. Mold preparation and multi-port design: Using Invar alloy mold, three resin injection ports are set at 1 / 3, 1 / 2 and 2 / 3 of the thickness direction, the main vacuum outlet is set at the top and the auxiliary exhaust / observation port is set at the bottom; D2. Preheating: Push the entire mold into a preheated autoclave at 60°C and keep it at that temperature for 4 hours; D3. Segmented vacuum-assisted injection: Evacuate the mold to a high vacuum of -0.099 MPa, first open the middle injection port to inject resin, and when the resin front reaches the upper and lower injection port positions, open the auxiliary injection valve simultaneously. After the resin continues to flow out without bubbles, close the injection port and maintain pressure for 30 minutes to release the air. D4. Autoclave gradient staged curing: Pressurize and cure in stages at 100℃ for 4 hours, 140℃ for 6 hours, and 180℃ for 8 hours, then cool down to below 60℃ at a rate of ≤0.5℃ / min. D5. Demolding and post-processing: After opening the can and demolding, perform full-section ultrasonic C-scan and industrial CT inspection. After confirming there are no defects, perform water jet cutting and milling finishing. Step E: Preparation of Multifunctional Stealth Coating on Surface E1. Surface pretreatment: white corundum sandblasting, acetone ultrasonic cleaning, atmospheric plasma activation treatment; E2. Coating slurry preparation: Nano carbonyl iron powder and multilayer graphene nanosheets are added to nano SiO2-TiO2 composite sol, dispersant and photoinitiator are added, and ultrasonic dispersion is performed in an ice water bath; E3. Spraying: High-pressure airless spraying using a six-axis robot in a clean room, with wet film thickness controlled at 200 ± 20 μm; E4. Stepped curing process: Pre-curing at room temperature for 15-30 minutes until surface dry; UV-LED surface light source irradiation for 60 seconds; heat treatment at 120℃-140℃ for 60-90 minutes, followed by slow cooling down to below 60℃.

8. The full-spectrum stealth composite material for aircraft according to claim 1, characterized in that: The performance test of the full-spectrum stealth composite material is as follows: Mechanical properties: Three-point bending strength 1180 ± 40 MPa, bending modulus 95 ± 3 GPa, interlaminar shear strength 42 ± 3 MPa, longitudinal compressive strength 980 ± 35 MPa, and open-cell compressive strength 320 ± 20 MPa. Electrical and stealth properties: Bulk volume resistivity > 10¹ 0 Ω·cm, meter-wave reflectivity at 0.8 GHz -18.5 dB, -10 dB absorption bandwidth 0.45-1.3 GHz, average attenuation of 16 dB in X-band, average attenuation of 21 dB in Ka-band, infrared emissivity 0.20, and excellent matte dark gray visible light camouflage effect.

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