A heat-insulating and wave-absorbing skin material, its preparation method and application

Abstract

This invention relates to the field of layered product technology, specifically disclosing a heat-insulating and microwave-absorbing skin material, its preparation method, and its application. The heat-insulating and microwave-absorbing skin material is an integrated layered structure based on an alumina-mullite system, comprising a functionally gradient layer with dual gradients in fiber density and pore size, an impedance matching layer with a gradient in dielectric constant, a resonant microwave-absorbing layer composed of a periodic lattice of silicon carbide@alumina core-shell microspheres, and a heat-insulating layer with a nano- to micron dual-scale pore structure. The preparation method mainly involves constructing a green body through casting and screen printing, followed by co-sintering to form an integrated structure. The material of this invention possesses excellent properties such as high temperature resistance, toughness, high-efficiency heat insulation, and broadband microwave absorption, making it suitable for extreme environments such as hypersonic vehicle skins.

Description

Technical Field

[0001] This invention relates to the field of layered product technology, and more specifically to a heat-insulating and wave-absorbing skin material, its preparation method, and its application. Background Technology

[0002] With the development of aerospace technology, hypersonic vehicles and other equipment have placed unprecedented demands on skin materials: they must withstand aerodynamic heat of thousands of degrees Celsius while effectively absorbing radar waves to achieve stealth. In existing technologies, thermal insulation materials (such as ceramic tiles) and radar-absorbing materials (such as ferrite coatings) are often separate components. Simple mechanical combinations can lead to interface mismatches, delamination under thermal shock conditions, and bulky systems. More importantly, traditional magnetic radar-absorbing materials (such as barium ferrite) have low Curie temperatures (approximately 450°C) and completely lose their radar-absorbing properties at high temperatures; while high-temperature resistant dielectric materials (such as alumina) have low radar-absorbing efficiency. Furthermore, the mismatch in thermal expansion coefficients of the layers in a layered structure, as well as chemical incompatibility at high temperatures, are the main causes of material cracking and failure during thermal cycling. Therefore, developing an integrated material that can operate stably at high temperatures for extended periods, combining strength, thermal insulation, and radar absorption, has become an urgent need. Summary of the Invention

[0003] The purpose of this invention is to provide a heat-insulating and wave-absorbing skin material, its preparation method, and its application, so as to solve the problems mentioned in the background art.

[0004] To achieve the above objectives, on the one hand, the present invention provides a heat-insulating and microwave-absorbing skin material, comprising a functional gradient layer, an impedance matching layer, a resonant microwave-absorbing layer and a heat-insulating layer, which are arranged sequentially from the outside to the inside and formed by co-sintering and chemically bonded;

[0005] The functionally graded layer is an alumina fiber-reinforced mullite-based composite material, and its structure has a fiber density gradient and a pore size gradient along the thickness direction.

[0006] The impedance matching layer is a rare earth hexaaluminate ceramic layer, and its dielectric constant varies with a gradient along the thickness direction.

[0007] The resonant absorbing layer is a periodic lattice structure composed of silicon carbide@alumina core-shell microspheres;

[0008] The thermal insulation layer is an alumina-mullite aerogel with a nano-micro dual-scale pore structure.

[0009] Based on the chemically compatible alumina-mullite system, a chemically bonded integrated layered structure is formed through co-sintering, fundamentally solving the high-temperature delamination and failure problems caused by thermal expansion coefficient mismatch and interfacial reactions in traditional multilayer materials. The functionally graded layer bears the main load and thermal shock; the impedance matching layer enables efficient electromagnetic wave transmission; the resonant absorbing layer converts electromagnetic energy into heat energy; and the thermal insulation layer minimizes heat conduction. This four-in-one design ensures the material's comprehensive performance and reliability under extreme environments.

[0010] Preferably, the mass fraction of alumina fibers in the functionally graded layer decreases from 45-50% on the outer surface to 5-10% on the inner surface; its pore size increases from 1 μm on the outer surface to 40-50 μm on the inner surface. The decreasing fiber density and increasing pore size from the outside to the inside constitute a mechanical buffer zone with a rigid-flexible transition and an efficient thermal barrier. The dense region on the outer surface (30% fiber, 100 nm pores) provides excellent erosion resistance and mechanical strength; the loose region on the inside (5% fiber, 50 μm pores) utilizes micron-sized pores to scatter infrared radiation and effectively insulate heat. This dual-gradient design perfectly balances the contradictory requirements of material strength and thermal insulation.

[0011] Preferably, the rare earth hexaaluminate in the impedance matching layer is LaMgAl. 11 O 19 The gradient change of its dielectric constant is achieved by controlling the porosity within the layer, increasing from 3-8% near the functionally graded layer to 30-40% near the resonant absorbing layer. Rare-earth hexaaluminate (LaMgAl) is selected as it exhibits no phase change at high temperatures and structural stability. 11 O 19 The dielectric constant changes continuously through its porosity gradient (5%-40%). This allows for a smooth transition in wave impedance as electromagnetic waves pass through this layer, significantly reducing interface reflections caused by impedance abrupt changes. This creates crucial silent entry conditions for the subsequent absorbing layer to efficiently absorb electromagnetic waves.

[0012] Preferably, the silicon carbide@alumina core-shell microspheres in the resonant absorbing layer have a silicon carbide core with a particle size of 1-3 μm and an alumina shell thickness of 0.5-1 μm; the lattice structure is a tetragonal array with a period of 5-15 mm. The silicon carbide core provides stable dielectric polarization loss at high temperatures; the complete alumina coating effectively prevents oxidation of silicon carbide during high-temperature preparation and service, and avoids adverse reactions between silicon carbide and the substrate. Arranging these core-shell microspheres into a periodic lattice can excite strong local surface wave resonance and interference effects, thereby efficiently absorbing incident electromagnetic waves over a wide frequency band, with performance stable up to 1000℃ and above.

[0013] Preferably, in the thermal insulation layer, the nanopores have a pore size of 10-50 nm and are formed by aerogel; the micropores have a pore size of 1-10 μm and are formed by the decomposition of a pore-forming agent; and the mass fraction of alumina fibers is 10-30%. The nanopores (10-50 nm) greatly suppress heat conduction by gas molecules using the Knudsen effect; the micropores (1-10 μm) serve as performance redundancy, effectively blocking infrared radiation heat transfer and maintaining excellent thermal insulation performance even when the nanopores may undergo high-temperature sintering. The introduction of alumina fibers (10%-30%) acts as a reinforcing skeleton, preventing the aerogel from cracking during preparation and use, and ensuring structural integrity.

[0014] On the other hand, the present invention provides a method for preparing a heat-insulating and wave-absorbing skin material, comprising the following steps:

[0015] Functional gradient layer green body preparation: Mullite-based slurries with different alumina fiber contents and pore-forming agent contents are sequentially stacked through tape casting to prepare a green body with fiber density and pore size gradient.

[0016] Impedance matching layer preparation: A layer of rare earth hexaaluminate slurry containing a pore-forming agent is cast onto the surface of the functionally graded layer green body;

[0017] Resonant absorbing layer preparation: Using screen printing, periodic dot pattern of silicon carbide@alumina core-shell microsphere paste is printed on the surface of impedance matching layer to form a three-layer structure;

[0018] Thermal insulation layer composite: Alumina-mullite slurry containing alumina fibers and dual-scale pore-forming agent is cast onto the back of the three-layer structure to obtain a multi-layer green body;

[0019] Co-sintering: The multi-layer green body is held at 1600-1800℃ for 2-4 hours to achieve co-sintering densification.

[0020] Employing a fully paste-based casting and printing technology, precise construction and integrated molding of multi-layer structures are achieved. This method boasts strong process compatibility and is suitable for complex curved surfaces and large-area fabrication. Most importantly, through the final "co-sintering" step, strong chemical bonds are formed between the layers through ion diffusion and chemical reactions. The bonding strength is far superior to physical adhesion, fundamentally ensuring the excellent stability of the interlayer under thermal cycling.

[0021] Preferably, the mullite-based slurry achieves a fiber density gradient by sequentially casting mullite-based slurries with fiber contents of 45-50%, 20-35%, 10-20%, and 5-10%. By sequentially casting slurries with different fiber contents, a gradient structure with continuously varying fiber density can be precisely and repeatably constructed. This method is simple and effective, providing a reliable process path for achieving the dual-gradient performance of functionally graded layers.

[0022] Preferably, the co-sintering process is carried out in an air atmosphere, with a heating rate of 1-3℃ / min, and holding at 500℃ and 1000℃ for 30 minutes respectively to remove organic matter and complete crystal transformation. The slow heating rate (1-3℃ / min) and the staged holding at 500℃ and 1000℃ ensure the safe and complete removal of organic matter from the green body and complete the full transformation of crystalline phases such as mullite, avoiding defects such as cracking and blistering caused by rapid heating or poor venting, thereby obtaining a dense and uniform sintered body.

[0023] Preferably, the silicon carbide@alumina core-shell microspheres are prepared by sol-gel coating, with the alumina shell completely covering the silicon carbide core. The sol-gel method can form a complete, uniform, and controllable thickness alumina protective shell on the surface of the silicon carbide microspheres. This dense armor is the core component that ensures the long-term stable operation of the microwave absorber in high-temperature environments, allowing it to maintain its performance without degradation in harsh chemical and thermophysical environments.

[0024] Furthermore, this invention provides the application of the aforementioned heat-insulating and radar-absorbing skin materials in hypersonic vehicle skins, heat shields for engine hot-end components, or shipborne radar stealth structures. The materials of this invention possess resistance to extreme high temperatures, high-efficiency heat insulation, broadband radar absorption, and excellent thermal shock resistance, enabling them to meet the stringent requirements for multifunctional integration in applications such as hypersonic vehicle skins, engine hot-end components, and shipborne stealth structures. This solves the technical bottleneck of traditional materials being unable to simultaneously achieve multiple performance characteristics in these fields.

[0025] The beneficial effects of this invention are as follows:

[0026] This invention constructs an integrated layered structure based on alumina-mullite, with each layer chemically compatible and formed by co-sintering to create a strong chemical bond. It utilizes the dual-gradient design of the functionally graded layer to balance mechanical and thermal insulation performance, achieves efficient transmission of broadband electromagnetic waves through the porosity gradient of the impedance matching layer, excites resonance loss by the periodic lattice of the core-shell structure absorbing elements to achieve stable high-temperature wave absorption, and maximizes thermal resistance by relying on the dual-scale pore structure of the thermal insulation layer. Thus, the material synergistically maintains excellent structural integrity, high-efficiency thermal insulation (thermal conductivity 0.030-0.042 W / (m·K)) and broadband electromagnetic wave absorption capability (average reflection loss ≤-25 dB for 2-18 GHz) even under extreme high-temperature environments. It successfully solves the technical challenge of traditional materials being unable to simultaneously achieve high temperature, toughness, thermal insulation, and wave absorption in a multifunctional integrated manner. Detailed Implementation

[0027] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0028] It should be noted that all reagents and raw materials used in this invention are commercially available, and the reagents are of analytical grade.

[0029] Polyvinyl alcohol is sourced from Jiangsu Pules Biotechnology Co., Ltd., with CAS number 9002-89-5.

[0030] The high-temperature adhesive is sourced from Jinan Huolong Thermal Ceramics Co., Ltd., and its model number is HLHG-26.

[0031] The acrylic resin adhesive is sourced from Zibo Jiafeng Chemical Technology Development Co., Ltd., and its model number is JF1530.

[0032] Terpineol is sourced from Hunan Songyuan Biotechnology Co., Ltd., with CAS number 8000-41-7.

[0033] Ethyl cellulose is sourced from Hubei Tuoyuan Fine Chemical Co., Ltd., with CAS number 9004-57-3.

[0034] Example 1

[0035] 1. Preparation of PMMA microspheres

[0036] Oil phase: Polymethyl methacrylate (PMMA) was dissolved in dichloromethane to prepare a 5% (w / v, g / mL) PMMA / dichloromethane solution;

[0037] Aqueous phase: Polyvinyl alcohol (PVA) was dissolved in deionized water to prepare a 1% (w / v, g / mL) PVA / water solution as an emulsifier.

[0038] step:

[0039] While stirring at 400 rpm, the oil phase was slowly poured into the aqueous phase (oil-water volume ratio 1:5) to form a crude emulsion;

[0040] The crude emulsion was transferred to a high-speed homogenizer and homogenized at 8000 rpm for 5 min to form a stable O / W emulsion.

[0041] The emulsion was transferred to a three-necked flask and reacted in a 25°C water bath for 4 hours with stirring at 300 rpm. During this process, dichloromethane in the oil phase was effectively removed by dissolving and diffusing into the aqueous phase, resulting in supersaturation and solidification of polymethyl methacrylate (PMMA) to obtain solid microspheres.

[0042] After the reaction was completed, the solid microspheres were collected by centrifugation, washed three times with deionized water to remove PVA, and finally dried in a vacuum drying oven at 40°C for 12 hours to obtain PMMA microspheres with smooth surface and good monodispersity.

[0043] 2. Preparation of Functionally Graded Layers

[0044] Preparation of mullite sol:

[0045] Step 1: Preparation of aluminum sol

[0046] Dissolve 0.5 mol of aluminum chloride hexahydrate in 300 mL of anhydrous ethanol and stir in a 40 °C water bath until completely clear to obtain an aluminum source. Slowly add 100 mL of deionized water to the aluminum source, and then add 0.1 mol / L HCl to adjust the pH to 3.0. Continue stirring at 60 °C for 2 h until the solution is clear and transparent to obtain a stable acidic aluminum sol. Cool to room temperature for later use.

[0047] Step 2: Preparation of silica sol

[0048] Mix 0.5 mol of tetraethyl orthosilicate with 200 mL of anhydrous ethanol, then add an acidic aqueous solution consisting of 36 mL of deionized water and 0.5 mL of concentrated hydrochloric acid (37 wt%). Stir at a constant speed at 60 °C for 4 h to complete the pre-hydrolysis of TEOS and obtain a transparent silica sol.

[0049] Step 3: Mixing and Reaction

[0050] The silica sol prepared in step two was transferred into a constant pressure dropping funnel and added dropwise to the aluminum sol prepared in step one within 30 minutes while stirring at 400 rpm. After the addition was complete, the mixed solution was placed in a constant temperature water bath at 80°C and stirred at 300 rpm for reflux reaction for 6 hours.

[0051] Step Four: Aging and Adjustment

[0052] After the reaction was complete, the sol was sealed and allowed to stand at room temperature for 36 hours. The pH of the sol was slowly adjusted back to 4.8 using dilute ammonia (concentration 0.1mol / L). Finally, the sol was filtered through a 400-mesh sieve to obtain a clear, homogeneous mullite sol with a solid content of 30%.

[0053] Slurry preparation:

[0054] Basic formula: mullite sol;

[0055] Reinforcing phase: alumina fiber (average length 75 μm, average diameter 8 μm);

[0056] Pore-forming agent: PMMA microspheres were used, and four sizes were prepared: PM-1 (D50=1μm), PM-5 (D50=5μm), PM-20 (D50=20μm) and PM-50 (D50=50μm). Different proportions were used in different layers to construct a pore size gradient.

[0057] Dispersant: Ammonium polyacrylate dispersant;

[0058] Gradient design: Four different proportions of slurry were prepared (calculated based on a total solid mass of 100 grams), as shown in Table 1:

[0059] Table 1. Slurry Proportioning Table for Functionally Graded Layers

[0060] raw material First layer (outer) Second floor Third layer Fourth floor (inner) Alumina fiber (g) 45 32 20 10 PM-1 microspheres (g) 5 2 0 0 PM-5 microspheres (g) 0 6 5 0 PM-20 microspheres (g) 0 0 13 18 PM-50 microspheres (g) 0 0 0 19 Mullite sol (g) 166.7 200.0 206.7 176.7 Mullite solid (g) 50 60 62 53 Ammonium polyacrylate dispersant 0.5 0.5 0.5 0.5 Target fiber mass fraction 45% 32% 18% 9% Target aperture 1μm 5μm 20μm 50μm

[0061] Casting:

[0062] In a constant temperature and humidity workshop (temperature 25℃, relative humidity 40%), the above four slurries are sequentially cast onto the base belt in the order of the first layer to the fourth layer using a casting machine;

[0063] The casting gaps were set to 0.35mm, 0.33mm, 0.32mm, and 0.30mm, respectively, and the belt speed was controlled at 0.8m / min.

[0064] After each layer is cast, it is dried in an oven at 80°C for 15 minutes before casting the next layer, ultimately forming a green embryo with a total thickness of 1.2 mm.

[0065] High-temperature sintering:

[0066] The green blanks are placed in a high-temperature sintering furnace and subjected to a specific sintering process:

[0067] Increase the temperature from room temperature to 500℃ at a rate of 1℃ / min, hold for 30 minutes, and remove organic matter.

[0068] The temperature was increased to 1000℃ at a rate of 2℃ / min and held for 30 min to complete the crystal transformation and form mullite crystal nuclei.

[0069] Heat to 1700℃ at 3℃ / min, hold for 2 hours, and then cool to room temperature in the furnace.

[0070] After sintering, a functional gradient layer with a dual gradient structure of fiber density and pore size with a total thickness of 0.8 mm is obtained.

[0071] 3. Impedance matching layer fabrication

[0072] Slurry preparation:

[0073] La(CH3COO)3, MgO, and AlCl3·6H2O were mixed according to the formula LaMgAl 11 O 19 Weigh the stoichiometric ratios and dissolve them together in deionized water to prepare a solution with a total metal ion concentration of 0.8 mol / L.

[0074] Add an appropriate amount of EDTA (the amount added is based on the metal ions (La) in the solution). 3+ Mg 2+ Al3+ The total molar amount of the metal ions (which is 1.1 times the total molar amount of the metal ions) was continuously stirred in an 80°C water bath to form a stable yellow transparent solution.

[0075] The yellow transparent solution was evaporated at 120°C to form a wet gel, and then dried at 250°C for 2.5 h to obtain a dark brown solid precursor.

[0076] The precursor was calcined at 1200℃ for 4 h to obtain pure phase LaMgAl. 11 O 19 Powder (D50=0.8μm);

[0077] LaMgAl 11 O 19 Three slurries were prepared by mixing the powder with PMMA microspheres (PM-5, D50=5μm) in different proportions, as shown in Table 2:

[0078] Table 2 Impedance Matching Layer Slurry Proportioning Table

[0079] raw material Slurry A (bottom layer) Slurry B (intermediate) Slurry C (top layer) <![CDATA[LaMgAl 11 O 19 Powder (g) 95 80 65 PM-5 microspheres (g) 5 20 35 Acrylic resin adhesive (g) 4 4 4 Dioctyl phthalate (g) 2 2 2 Ammonium polyacrylate dispersant (g) 1.5 1.5 1.5 Ethyl 3-ethoxypropionate (g) 32.5 32.5 32.5 Target porosity 5% 20% 35%

[0080] The above components were ball-milled for 24 hours to form a uniform slurry.

[0081] Flow-deposition:

[0082] On the dense surface of the functional gradient layer, slurry A, B, and C are sequentially cast using a precision casting machine. After each layer is cast, it is dried for 30 minutes at 40°C and 50% relative humidity.

[0083] After all the castings are completed, the components are dried for 24 hours under the same conditions to obtain a green blank with an impedance matching layer.

[0084] Co-sintering:

[0085] The component with the impedance matching layer green blank is co-sintered:

[0086] Increase the temperature to 500°C at a rate of 1°C / min and hold for 60 minutes;

[0087] Increase the temperature to 1200°C at a rate of 2°C / min and hold for 120 min;

[0088] As the furnace cools, an impedance matching layer with a thickness of 0.2 mm is finally formed.

[0089] 4. Fabrication of Resonant Absorbing Layer

[0090] Preparation of core-shell microspheres:

[0091] Silicon carbide microspheres (D50=2μm) were dispersed in an ethanol solution of aluminum isopropoxide (concentration 0.5mol / L) and stirred in a water bath at 60℃ for 4h.

[0092] After centrifugation and washing, the microspheres were calcined at 800℃ for 2 hours to form a complete alumina coating with a thickness of 0.8 μm, thus obtaining core-shell microspheres.

[0093] Patterned printing:

[0094] Patterned printing paste for resonant absorbing layer: 65wt% core-shell microspheres, 6wt% ethyl cellulose binder, 20wt% terpineol, 8.5% anhydrous ethanol, and 0.5wt% dibutyl phthalate.

[0095] Ethyl cellulose binder, terpineol, anhydrous ethanol, and dibutyl phthalate were placed in a capped glass bottle in proportion. The glass bottle was placed in a water bath at 60°C and stirred continuously at 200 rpm until the ethyl cellulose was completely dissolved. The mixture was then cooled to room temperature and allowed to stand for 16 hours to defoam, thus obtaining the organic carrier.

[0096] Add 0.5% of the mass of the patterned printing paste for the resonant absorbing layer to the core-shell microspheres as an ammonium polyacrylate dispersant, and add 1 / 3 of the mass of the organic carrier as an organic carrier. Premix to form a viscous paste.

[0097] Transfer the paste to the hopper of a planetary centrifugal mixer and add all remaining organic carriers. Mix at 800 rpm for 5 min to ensure all components are initially and evenly mixed. Add zirconia grinding balls (average diameter 4 mm) and disperse at 2000 rpm for 20 min. Then degas under a vacuum of -0.1 MPa for 10 min until no obvious bubbles overflow from the surface of the slurry.

[0098] The patterned printing paste for the resonant absorbing layer was obtained by filtering with a 400-mesh stainless steel wire mesh. A square dot matrix with a period of 10 mm and a thickness of 0.25 mm was printed on the surface of the impedance matching layer.

[0099] After printing, the material is first dried at 80℃ for 1 hour, and then preheated at 600℃ for 1 hour at a rate of 2℃ / min to remove organic matter, forming a three-layer structure with a resonant absorbing layer.

[0100] 5. Preparation of thermal insulation layer

[0101] Preparation of thermal insulation slurry:

[0102] 20wt% of alumina short chopped fibers (average length 150μm), 10wt% of nano-sized silica aerogel powder (average pore size 25nm), 20wt% of starch pore-forming agent (D50=5μm), 49.5wt% of mullite sol, and 0.5wt% of ammonium polyacrylate dispersant.

[0103] Short alumina fibers, ammonium polyacrylate dispersant, and 40% of the total mullite sol were mixed and ball-milled at 150 rpm for 30 min to obtain a premix.

[0104] The premix was mixed with the remaining mullite sol, nano-sized silica aerogel powder and starch pore-forming agent, and stirred at 400 rpm for 60 min. Then, it was stirred at 200 rpm for 30 min at -0.1 MPa to obtain the heat insulation layer slurry.

[0105] Casting composite:

[0106] The three-layer back-side heat insulation layer slurry prepared in step 4 has a thickness of 2 mm and a casting gap of 1.8 mm.

[0107] Multilayer embryos were obtained by slow drying at 30℃ and 60% relative humidity for 48 hours.

[0108] 6. Co-sintering process

[0109] The complete multi-layered green body was placed in a high-temperature sintering furnace, and the optimized sintering curve was used:

[0110] Heat from room temperature to 500℃ at a rate of 1℃ / min and hold for 60 minutes (to completely remove the adhesive).

[0111] The temperature was increased from 500℃ to 1000℃ at a rate of 2℃ / min and held for 60min (mullite and hexaaluminate crystallization).

[0112] The temperature was increased from 1000℃ to 1700℃ at a rate of 3℃ / min and held for 3 hours (densification).

[0113] The skin material was obtained by cooling it to room temperature in the furnace at a rate of 2℃ / min.

[0114] The entire sintering process is carried out in an air atmosphere at atmospheric pressure.

[0115] Example 2

[0116] 1. Preparation of PMMA microspheres, same as in Example 1;

[0117] 2. Preparation of functionally graded layers: The preparation steps are the same as in Example 1, except that the proportions of the raw materials for different layers are different, as detailed below:

[0118] First layer (outer): 48g alumina fiber, 3g PM-1 (D50=1μm), 49g mullite solid;

[0119] Second layer: 35g alumina fiber, 2g PM-1 (D50=1μm), 6g PM-5 (D50=5μm), 55g mullite solid;

[0120] Third layer: 15g alumina fiber, 15g PM-20 (D50=20μm), 70g mullite solid;

[0121] Fourth layer: 5g alumina fiber, 15g PM-20 (D50=20μm), 20g PM-40 (D50=40μm), 60g mullite solid;

[0122] 3. The preparation of the impedance matching layer is largely the same as in Example 1, except for the raw material ratios for different layers and the LaMgAl content. 11 O 19 The sintering temperatures differ, as detailed below:

[0123] LaMgAl 11 O 19 The sintering temperature is 1180℃;

[0124] Slurry A (bottom layer): LaMgAl 11 O 19 97g of powder, 3g of PM-5 (D50=5μm);

[0125] Slurry B (intermediate): LaMgAl 11 O 19 85g of powder, 15g of PM-5 (D50=5μm);

[0126] Slurry C (top layer): LaMgAl 11 O 19 70g of powder, 30g of PM-5 (D50=5μm).

[0127] 4. The preparation of the resonant absorption layer is roughly the same as in Example 1, except that the lattice period is 5 mm, the silicon nitride microspheres D50=1 μm, and the alumina shell thickness is 0.5 μm.

[0128] 5. The preparation of the thermal insulation layer is largely the same as in Example 1, except that:

[0129] Thermal insulation layer slurry: 10wt% alumina short-cut fiber (average length 150μm), 10wt% nano-sized silica aerogel powder (average pore size 10nm), 20wt% starch pore-forming agent (D50=1μm), 59.5wt% mullite sol, and 0.5wt% ammonium polyacrylate dispersant.

[0130] 6. The co-sintering process is largely the same as in Example 1, except that the maximum co-sintering temperature is 1600℃ and the holding time at the maximum temperature is 2h.

[0131] Example 3

[0132] 1. Preparation of PMMA microspheres, same as in Example 1;

[0133] 2. Preparation of functionally graded layers: The preparation steps are the same as in Example 1, except that the proportions of the raw materials for different layers are different, as detailed below:

[0134] First layer (outer): 50g alumina fiber, 3g PM-1 (D50=1μm), 2g PM-5 (D50=5μm), 45g mullite solid;

[0135] Second layer: 20g alumina fiber, 10g PM-5 (D50=5μm), 70g mullite solid;

[0136] Third layer: 10g alumina fiber, 20g PM-20 (D50=20μm), 70g mullite solid;

[0137] Fourth layer: 8g alumina fiber, 42g PM-45 (D50=45μm), 50g mullite solid;

[0138] 3. The preparation of the impedance matching layer is largely the same as in Example 1, except for the raw material ratios for different layers and the LaMgAl content. 11 O 19 The sintering temperatures differ, as detailed below:

[0139] LaMgAl 11 O 19 The sintering temperature is 1220℃;

[0140] Slurry A (bottom layer): LaMgAl 11 O 19 92g of powder, 8g of PM-5 (D50=5μm);

[0141] Slurry B (intermediate): LaMgAl 11 O 19 75g of powder, 25g of PM-5 (D50=5μm);

[0142] Slurry C (top layer): LaMgAl 11 O 19 60g of powder, 40g of PM-5 (D50=5μm).

[0143] 4. The preparation of the resonant absorption layer is roughly the same as in Example 1, except that the lattice period is 15 mm, the silicon nitride microspheres D50 is 3 μm, and the alumina shell thickness is 1 μm.

[0144] 5. The preparation of the thermal insulation layer is largely the same as in Example 1, except that:

[0145] Thermal insulation layer slurry: 30wt% alumina short-cut fiber (average length 150μm), 10wt% nano-sized silica aerogel powder (average pore size 50nm), 20wt% starch pore-forming agent (D50=10μm), 39.5wt% mullite sol, and 0.5wt% ammonium polyacrylate dispersant.

[0146] 6. The co-sintering process is largely the same as in Example 1, except that the maximum co-sintering temperature is 1750℃ and the holding time at the maximum temperature is 2.5h.

[0147] Example 4

[0148] 1. Preparation of PMMA microspheres, same as in Example 1;

[0149] 2. Preparation of functionally graded layers: The preparation steps are the same as in Example 1, except that the proportions of the raw materials for different layers are different, as detailed below:

[0150] First layer (outer): 46g alumina fiber, 4g PM-1 (D50=1μm), 1g PM-5 (D50=5μm), 49g mullite solid;

[0151] Second layer: 25g alumina fiber, 8g PM-5 (D50=5μm), 67g mullite solid;

[0152] Third layer: 12g alumina fiber, 18g PM-20 (D50=20μm), 70g mullite solid;

[0153] Fourth layer: 6g alumina fiber, 14g PM-20 (D50=20μm), 30g PM-50 (D50=50μm), 50g mullite solid.

[0154] 3. The preparation of the impedance matching layer is largely the same as in Example 1, except for the raw material ratios for different layers and the LaMgAl content. 11 O 19 The sintering temperatures differ, as detailed below:

[0155] LaMgAl 11 O 19 The sintering temperature is 1200℃;

[0156] Slurry A (bottom layer): LaMgAl11 O 19 95g of powder, 5g of PM-5 (D50=5μm);

[0157] Slurry B (intermediate): LaMgAl 11 O 19 80g of powder, 20g of PM-5 (D50=5μm);

[0158] Slurry C (top layer): LaMgAl 11 O 19 65g of powder, 35g of PM-5 (D50=5μm).

[0159] 4. The preparation of the resonant absorption layer is roughly the same as in Example 1, except that the lattice period is 12 mm, the silicon nitride microspheres D50 is 2 μm, and the alumina shell thickness is 0.7 μm.

[0160] 5. The preparation of the thermal insulation layer is largely the same as in Example 1, except that:

[0161] Thermal insulation layer slurry: 25wt% alumina short-cut fiber (average length 150μm), 10wt% nano-sized silica aerogel powder (average pore size 20nm), 20wt% starch pore-forming agent (D50=6μm), 44.5wt% mullite sol, and 0.5wt% ammonium polyacrylate dispersant.

[0162] 6. The co-sintering process is largely the same as in Example 1, except that the maximum co-sintering temperature is 1800℃ and the holding time at the maximum temperature is 4h.

[0163] Comparative Example 1

[0164] After the functionally graded layer is prepared, no LaMgAl is deposited. 11 O 19 The resonant absorbing layer is directly printed on the surface of the functionally graded layer, and the remaining steps are exactly the same as in Example 1.

[0165] Comparative Example 2

[0166] The functional gradient layer is a single-layer casting, with the alumina fiber content in the slurry being constant at 15 vol% and the PMMA pore-forming agent content being constant at 20%. The total thickness is consistent with that of Example 1, and the remaining steps are the same as in Example 1.

[0167] Comparative Example 3

[0168] The resonant absorbing layer slurry directly uses uncoated pure silicon carbide microspheres (D50=2μm), omitting the alumina coating step, and the remaining steps are the same as in Example 1.

[0169] Comparative Example 4

[0170] The four functional layers were independently prepared and sintered according to the method in Example 1. Then, they were bonded together at room temperature using a high-temperature adhesive with a bonding pressure of 0.5 MPa and curing conditions: first cured at 120°C for 2 hours, and then cured at 200°C for 1 hour.

[0171] Comparative Example 5

[0172] The thermal insulation layer was replaced with a commercial microporous calcium silicate board (density 950kg / m³, thickness 1.5mm), which was bonded to the resonant absorbing layer with a high-temperature adhesive. The remaining steps were the same as in Example 1.

[0173] The products from Examples 1-4 and Comparative Examples 1-5 were subjected to relevant performance tests, and the test standards and methods are as follows:

[0174] 1. Density (GB / T 1463-2005 "Test Methods for Density and Relative Density of Fiber Reinforced Plastics")

[0175] This standard uses the hydrostatic balance method for determination. The core method is as follows: first, weigh the sample in air; then, immerse it in distilled or deionized water and weigh the buoyant force it experiences in the liquid. Based on Archimedes' principle, calculate the volume of liquid displaced by the sample using the mass difference between the air and liquid states. Finally, divide the sample's mass by its volume to obtain the sample's density.

[0176] 2. Thermal conductivity (GB / T 10297-2015 Determination of thermal conductivity of non-metallic solid materials - hot wire method)

[0177] This standard uses the hot-wire method for measurement. Its core principle is to embed a thin, long hot wire (which also serves as a heater and temperature sensor) into the material being tested or place it between two pieces of material. A constant heating power is applied to the hot wire, causing its temperature to rise. By recording the temperature rise of the hot wire itself or a point nearby over time, the thermal conductivity of the material can be calculated.

[0178] 3.2-18GHz Average Reflection Loss (GB / T 17626.21-2014 Electromagnetic Compatibility Testing and Measurement Techniques - Mixing Chamber Test Method)

[0179] This standard involves testing in a reverberation chamber. The core method is as follows: the sample is placed in the test window of the reverberation chamber, and a vector network analyzer transmits a swept-frequency signal of 2-18 GHz into the chamber. A stirrer inside the chamber continuously rotates, randomizing the internal electromagnetic field environment and creating a statistically uniform field. By comparing the power measured by the receiving antenna before and after the sample is placed in the chamber, and through a series of calculations, the reflection loss of the sample at each frequency point across the entire frequency band can be obtained. Finally, the average value is taken as the final result.

[0180] 4. Interlaminar shear strength (GB / T 1450.1-2005 "Test Method for Interlaminar Shear Strength of Fiber Reinforced Plastics")

[0181] This standard employs the short-beam shear method for determination. The core method involves placing the specimen as a simply supported beam on two support rollers, with the specimen's longitudinal direction parallel to the rollers. A loading roller applies a constant downward load at the center of the specimen until failure. Due to the short span (typically controlling the span-to-thickness ratio at 5:1), the specimen primarily bears shear stress, and its failure mode is interlaminar shear failure. By recording the maximum load and substituting it into a specific formula, the interlaminar shear strength can be calculated.

[0182] 5. Maximum operating temperature

[0183] The core of this test method is to place the sample in a high-temperature furnace and continuously heat it in air at a specified heating rate (e.g., 10°C / min). The temperature at which structural cracking, delamination, or blistering occurs during heating is recorded through observation or instrument monitoring, while mass loss is monitored via thermogravimetric analysis. The highest temperature at which the material structure remains intact and mass loss does not exceed 5% is defined as its maximum service temperature.

[0184] The test results are shown in Table 3:

[0185] Table 3 Performance Test Results

[0186]

[0187] Compared to Example 1, Comparative Example 1 exhibits significantly inferior absorption performance (-15dB) compared to Example 1 (-28dB). The reason is that Comparative Example 1 lacks an impedance matching layer, resulting in strong Fresnel reflection when the electromagnetic wave encounters the first interface (air / functionally graded layer) along its propagation path. This invention utilizes LaMgAl... 11 O 19 The dielectric constant gradient of the layer enables a smooth transition of wave impedance from free space to the absorbing layer, much like laying a reflection-free channel for electromagnetic waves, allowing them to efficiently enter the material and be consumed. Data from Comparative Example 1 demonstrates that without this gradient layer, even with an excellent resonant absorbing layer, the overall absorption performance deteriorates drastically.

[0188] Compared to Example 1, Comparative Example 2 exhibits a significantly higher thermal conductivity (0.058) and a substantial decrease in interlaminar shear strength (25 MPa). The reason for this is that homogeneous structures cannot resolve the inherent contradiction between strength and thermal insulation. Insufficient strength on the outer surface leads to low thermal insulation performance on the inner surface. The gradient design of this invention prioritizes the bearing of mechanical loads on the outer surface (high fiber, low porosity), while maximizing the thermal insulation effect on the inner surface (low fiber, high porosity). Simultaneously, the gradient structure avoids abrupt changes in interlaminar performance and effectively disperses interfacial stress, thus achieving a balance between high strength and low thermal conductivity. The data from Comparative Example 2 confirms that simple homogeneous materials cannot achieve the performance balance achieved by the gradient design of this invention.

[0189] Compared to Example 1, Comparative Example 3 showed a significant degradation in microwave absorption performance after high-temperature testing, and its maximum operating temperature was limited. The reason for this is that in co-sintering and high-temperature service environments, the uncoated silicon carbide undergoes severe oxidation (SiC + O2 → SiO2 + CO) and interfacial reactions with the substrate, leading to damage and performance failure of the microwave absorbing element. The alumina shell of this invention acts as a dense diffusion barrier, physically isolating oxygen from the silicon carbide core at high temperatures and inhibiting elemental interdiffusion with the substrate, thereby ensuring the chemical stability and functional durability of the microwave absorbing element under extreme environments.

[0190] Compared to Example 1, Comparative Example 4 exhibits extremely low interlaminar shear strength (16 MPa) and is prone to delamination during thermal cycling tests. The reason is that physical adhesion relies on the mechanical interlocking and van der Waals forces between the adhesive and the substrate, resulting in low bonding strength, and the adhesive is prone to aging and decomposition at high temperatures. In contrast, the co-sintering process of this invention enables each layer to form a strong chemical bond (ionic and covalent) interface at the atomic scale through ion diffusion and solid-state reactions. The bonding energy of this interface is far higher than that of physical adhesion, and its strength is close to that of the material bulk, thus ensuring the integrity of the multilayer structure under harsh thermo-mechanical coupling environments.

[0191] Compared with Example 1, Comparative Example 5 density (1100 kg / m³) 3 The thermal conductivity (0.085) is too high. Analysis of the cause: Traditional microporous calcium silicate boards have multiple solid-state heat transfer paths and lack effective suppression of gaseous and solid-state heat transfer at the nanoscale. The dual-scale aerogel of this invention constructs an extremely low-density thermal resistance network through the synergy of nanopores (Knudsen effect suppressing gas heat transfer) and micropores (scattering infrared radiation), as well as the nano-reinforcement of the alumina fiber skeleton. This allows the invention to achieve the same thermal insulation effect while being lighter, which is of decisive significance for weight reduction requirements in the aerospace field.

[0192] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the specific details of the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, and these simple modifications all fall within the protection scope of the present invention. Furthermore, it should be noted that the various specific technical features described in the above embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not further describe the various possible combinations.

[0193] Furthermore, various different embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention, they should also be regarded as the content disclosed by the present invention.

Claims

1. A thermally insulating, wave-absorbing skin material, characterized in that The functional gradient layer, the impedance matching layer, the resonant wave absorbing layer and the thermal insulation layer are sequentially arranged from outside to inside and are chemically bonded by co-sintering. The functional gradient layer is an alumina fiber reinforced mullite-based composite material, and has a fiber density gradient and a pore size gradient along the thickness direction. The pore size of the functional gradient layer increases from 1 μm at the outer surface to 40-50 μm at the inner surface. The impedance matching layer is a rare earth hexaaluminate ceramic layer, the dielectric constant of which changes in gradient along the thickness direction, the rare earth hexaaluminate in the impedance matching layer is LaMgAl 11 O 19 The porosity increases from 3-8% close to the functional gradient layer to 30-40% close to the resonant wave absorbing layer. The resonant wave absorbing layer is a periodic lattice structure composed of silicon carbide@alumina core-shell microspheres. The thermal insulation layer is an alumina-mullite aerogel having a nano-micro dual-scale pore structure.

2. The thermally insulating, wave-absorbing skin material of claim 1, wherein, The silicon carbide@alumina core-shell microspheres in the resonant wave absorbing layer have a silicon carbide core with a particle size of 1-3 μm and an alumina shell with a thickness of 0.5-1 μm.

3. The thermally insulating, wave-absorbing skin material of claim 1, wherein, The lattice structure is a square array with a period of 5-15 mm.

4. A method for the production of a thermally and acoustically insulating, wave-absorbing skin material according to any one of claims 1 to 3, characterized in that In the thermal insulation layer, the nano-pores have a pore size of 10-50 nm and are formed by the aerogel, and the micro-pores have a pore size of 1-10 μm and are formed by the pore-forming agent, and the alumina fiber has a mass fraction of 10-30%. The method comprises the following steps: Functional gradient layer green body preparation: mullite-based slurries with different alumina fiber contents and pore-forming agent contents are sequentially stacked by tape casting to prepare a green body with a fiber density and pore size gradient; Impedance matching layer preparation: a rare earth hexaaluminate slurry containing a pore-forming agent is tape cast on the surface of the functional gradient layer green body; Resonant wave absorbing layer preparation: a silicon carbide@alumina core-shell microsphere slurry is screen printed on the surface of the impedance matching layer to form a periodic lattice pattern, thereby forming a three-layer structure; Thermal insulation layer compounding: an alumina-mullite slurry containing alumina fibers and a dual-scale pore-forming agent is tape cast on the back of the three-layer structure to obtain a multi-layer green body; 5. The method of claim 4, wherein the method further comprises, Co-sintering: the multi-layer green body is integrally sintered at 1600-1800 ℃ for 2-4 h to densify.

6. The method of claim 4, wherein the method further comprises, The mullite-based slurry realizes the fiber density gradient by sequentially tape casting mullite-based slurries with fiber contents of 45-50%, 20-35%, 10-20% and 5-10%.

7. The method of claim 4, wherein the microwave absorbing, thermally insulating, skin material is prepared by the steps of: The co-sintering process is carried out in an air atmosphere, and the heating rate is 1-3 ℃ / min, and the temperature is kept at 500 ℃ and 1000 ℃ for 30 min respectively to remove organic matter and complete crystal transformation. The silicon carbide@alumina core-shell microspheres are prepared by sol-gel coating, and the alumina shell completely covers the silicon carbide core.

8. Use of the thermal insulation and wave absorbing skin material according to any one of claims 1-3 in the skin of a hypersonic aircraft, a thermal insulation screen of a hot end part of an engine or a shipborne radar stealth structure.

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