Anisotropic heterogeneous pore double-layer composite aerogel and a preparation method thereof

Abstract

This invention relates to the field of multifunctional electromagnetic shielding materials, specifically an anisotropic heteroporous porous bilayer composite aerogel and its preparation method. The anisotropic heteroporous porous bilayer composite aerogel is an in-situ bonded, integrated, interface-free bilayer structure. The upper layer is a vertically oriented porous magnetic coupling impedance matching layer, composed of a WPU / BC matrix loaded with an MXene@hollow Fe3O4 core-shell heterostructure; the lower layer is a disordered, random porous, high-dielectric-loss layer, composed of a WPU / BC matrix loaded with an MXene / CNTs coaxial conductive network. The prepared bilayer composite aerogel exhibits high shielding effectiveness and low reflectivity, and can be used in ultra-low reflectivity electromagnetic shielding, radar-infrared dual-compatible stealth, flexible wearable electromagnetic protection, and integrated aerospace thermal management applications.

Description

Technical Field

[0001] This invention relates to the field of multifunctional electromagnetic protection materials technology, specifically an anisotropic heteroporous porous bilayer composite aerogel and its preparation method. Background Technology

[0002] With the development of high-frequency communication and aerospace stealth technology, low-reflection, lightweight, infrared-compatible, flexible, and reliable electromagnetic shielding materials have become a core requirement. Traditional shielding materials suffer from drawbacks such as excessive reflection, impedance mismatch, and limited functionality; existing patented technologies also have significant limitations. Patent CN202510375099.1 describes the preparation of concentration-pore dual gradient aerogels using 3D printing, which relies on the CNFs / MXene / PEDOT:PSS system. The equipment cost is high, but the reflectivity still reaches 0.42. Pure dielectric loss system has a high reflectivity, no magnetic loss and infrared stealth function. Patent CN202310336923.3 describes the preparation of ANF / PI-based aerogels using vacuum filtration, multilayer stacking, and high-temperature imidization. However, this method requires 300℃ heat treatment, suffers from easy interface delamination due to multilayer stacking, impedance mismatch, poor reflection suppression, and lacks anisotropic porous structure design.

[0003] None of the above technologies have achieved the in-situ bonded heterostructure of "vertical oriented aperture + disordered random aperture", have adopted hollow core-shell magnetic-dielectric heterojunction, or have a step-by-step in-situ coupling cryogenic green process, and therefore cannot meet the integrated requirements of "ultra-low reflection + infrared stealth + anisotropic thermal management". Summary of the Invention

[0004] The purpose of this invention is to provide an anisotropic heteroporous porous bilayer composite aerogel and its preparation method, so as to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the present invention provides the following technical solution: An anisotropic heteroporous porous bilayer composite aerogel is an in-situ bonded, integrated bilayer structure without interface gaps. The upper layer is a vertically oriented porous magnetic coupling impedance matching layer, composed of a WPU / BC matrix loaded with an MXene@hollow Fe3O4 core-shell heterostructure. The lower layer is a disordered, random porous, high dielectric loss layer, composed of a WPU / BC matrix loaded with an MXene / CNTs coaxial conductive network.

[0006] Furthermore, the upper layer has vertically oriented holes with a diameter of 15~35μm and continuously oriented hole walls; the lower layer has disordered random holes with a diameter of 5~20μm and randomly intersecting channels; the upper layer accounts for 62~68% of the thickness, and the lower layer accounts for 32~38%.

[0007] A method for preparing anisotropic heteroporous porous bilayer composite aerogel, the method comprising the following steps: S1. Preparation of MXene@hollow Fe3O4 core-shell heterostructure by solvothermal method; S2. Prepare the lower layer precursor solution and the upper layer precursor solution separately; S3, Step 1: Disorderly freezing: Inject the lower layer precursor liquid into the mold and freeze at -20~-25℃ for 3~4.5h to form a disordered random hole strong dielectric loss layer; S4. Second step: In-situ directional freezing: Maintain a low temperature below -10℃, inject the upper liquid while the lower layer is frozen but not dried, and use liquid nitrogen-copper plate bottom thermal gradient directional freezing for 30~60min to form a vertically oriented hole magnetic coupling impedance matching layer. S5. Freeze-dry at -70℃ and <2Pa for 48h to obtain in-situ bonded anisotropic heteroporous bilayer aerogel.

[0008] Furthermore, the lower precursor solution includes MXene, CNTs and WPU / BC aqueous solution, and the mass ratio of MXene, CNTs and WPU / BC aqueous solution is 80:5~15:5~15.

[0009] Furthermore, the upper precursor solution includes an MXene@hollow Fe3O4 core-shell heterostructure and a WPU / BC aqueous solution, with a mass ratio of 4:6 between the MXene@hollow Fe3O4 core-shell heterostructure and the WPU / BC aqueous solution.

[0010] Furthermore, the WPU / BC aqueous solutions of the upper and lower precursor solutions contain WPU and BC at mass ratios of 32-35 wt% and 0.6-1.0 wt%, respectively.

[0011] Compared with the prior art, the beneficial effects of the present invention are: The bilayer composite aerogel prepared by this invention has a reflectance coefficient ≤0.15 and an absorption ratio >85%, completely suppressing secondary pollution. Through anisotropic heteropores and in-situ bonding interfaces, the prepared material achieves high shielding (greater than 80 dB), low infrared emissivity, lightweight flexibility, and cycle stability. At the same time, the preparation method is simple, fully water-based, low-temperature molding, requires no high-end equipment, and can be mass-produced. It also meets the requirements of electromagnetic protection, infrared stealth, thermal management, and flexible wearables. Attached Figure Description

[0012] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 A schematic diagram of the structure of anisotropic heteroporous bilayer composite aerogel; Figure 2 This is a cross-sectional SEM image of the anisotropic heteroporous porous bilayer composite aerogel prepared in Example 1; Figure 3 The electromagnetic shielding effectiveness of the anisotropic heteroporous porous bilayer composite aerogel prepared in Example 1 after 50 cycles of cyclic compression is shown in the figure. Figure 4 The conductivity diagram is shown for the anisotropic heteroporous porous bilayer composite aerogel prepared in Example 1. Figure 5 This is a process flow diagram for preparing anisotropic heteroporous porous bilayer composite aerogels in this invention. Detailed Implementation

[0014] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0015] like Figure 1-5 As shown, the present invention provides: Example 1 An anisotropic heteroporous porous bilayer composite aerogel is an in-situ bonded, integrated bilayer structure without interface gaps. The upper layer is a vertically oriented porous magnetic coupling impedance matching layer, composed of a WPU / BC matrix loaded with an MXene@hollow Fe3O4 core-shell heterostructure. The lower layer is a disordered, random porous, high dielectric loss layer, composed of a WPU / BC matrix loaded with an MXene / CNTs coaxial conductive network.

[0016] The present invention prepares anisotropic heteroporous porous bilayer composite aerogels through the following embodiments: Example 1 S1. Preparation of MXene@hollow Fe3O4 core-shell heterostructure by solvothermal method; S2. Prepare the lower and upper precursor solutions separately. The lower precursor solution includes MXene, CNTs, and WPU / BC aqueous solution, with the mass percentages of MXene, CNTs, and WPU / BC aqueous solution being 80wt%, 5wt%, and 15wt%, respectively. The mass percentages of BC and WPU in the WPU / BC aqueous solution are 0.8wt% and 34wt%, respectively. The upper precursor solution includes MXene@hollow Fe3O4 core-shell heterostructure and WPU / BC aqueous solution, with the mass percentages of MXene@hollow Fe3O4 core-shell heterostructure and WPU / BC aqueous solution being 40wt% and 60wt%, respectively. The mass percentages of BC and WPU in the WPU / BC aqueous solution are 0.8wt% and 34wt%, respectively. S3, Step 1: Disordered freezing: Inject the lower layer precursor liquid into the mold and freeze at -22℃ for 3.5h to form a disordered random hole strong dielectric loss layer (lower layer). S4. Second step: In-situ directional freezing: Maintain a low temperature below -10℃, add precursor liquid to the upper layer, and use liquid nitrogen-copper plate bottom thermal gradient directional freezing for 40 minutes to form a vertically oriented hole magnetic coupling impedance matching layer (upper layer). S5. After freeze-drying at -70℃ and <2Pa for 48 hours, a bilayer composite aerogel was obtained.

[0017] Example 2 S1. Preparation of MXene@hollow Fe3O4 core-shell heterostructure by solvothermal method; S2. Prepare the lower and upper precursor solutions separately. The lower precursor solution includes MXene, CNTs, and WPU / BC aqueous solution, with the mass percentages of MXene, CNTs, and WPU / BC aqueous solution being 80wt%, 10wt%, and 10wt%, respectively. The mass percentages of BC and WPU in the WPU / BC aqueous solution are 0.8wt% and 34wt%, respectively. The upper precursor solution includes MXene@hollow Fe3O4 core-shell heterostructure and WPU / BC aqueous solution, with the mass percentages of MXene@hollow Fe3O4 core-shell heterostructure and WPU / BC aqueous solution being 40wt% and 60wt%, respectively. The mass percentages of BC and WPU in the WPU / BC aqueous solution are 0.8wt% and 34wt%, respectively. S3, Step 1 Disorderly Freezing: Inject the lower layer precursor liquid into the mold and freeze at -20℃ for 3 hours to form a disordered random hole strong dielectric loss layer. S4. Second step: In-situ directional freezing: Maintain a low temperature below -10℃, add precursor liquid to the upper layer, and use liquid nitrogen-copper plate bottom thermal gradient directional freezing for 30 minutes to form a vertically oriented hole magnetic coupling impedance matching layer. S5. After freeze-drying at -70℃ and <2Pa for 48 hours, a bilayer composite aerogel was obtained.

[0018] Example 3 S1. Preparation of MXene@hollow Fe3O4 core-shell heterostructure by solvothermal method; S2. Prepare the lower and upper precursor solutions separately. The lower precursor solution includes MXene, CNTs, and WPU / BC aqueous solution, with the mass percentages of MXene, CNTs, and WPU / BC aqueous solution being 80wt%, 15wt%, and 5wt%, respectively. The mass percentages of BC and WPU in the WPU / BC aqueous solution are 0.8wt% and 34wt%, respectively. The upper precursor solution includes MXene@hollow Fe3O4 core-shell heterostructure and WPU / BC aqueous solution, with the mass percentages of MXene@hollow Fe3O4 core-shell heterostructure and WPU / BC aqueous solution being 40wt% and 60wt%, respectively. The mass percentages of BC and WPU in the WPU / BC aqueous solution are 0.8wt% and 34wt%, respectively. S3, Step 1: Disordered freezing: Inject the lower layer precursor liquid into the mold and freeze at -25℃ for 4.5h to form a disordered random hole strong dielectric loss layer. S4. Second step: In-situ directional freezing: Maintain a low temperature below -10℃, add precursor liquid to the upper layer, and use liquid nitrogen-copper plate bottom thermal gradient directional freezing for 60 minutes to form a vertically oriented hole magnetic coupling impedance matching layer. S5. After freeze-drying at -70℃ and <2Pa for 48 hours, a bilayer composite aerogel was obtained.

[0019] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that the in-situ directional freezing of the upper layer in step S4 is changed to disordered freezing, while the remaining steps are exactly the same as in Example 1.

[0020] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that the disordered freezing of the lower layer in step S3 is changed to in-situ directional freezing, while the rest of the steps are exactly the same as in Example 1.

[0021] In the above embodiments, the mass ratio of WPU and BC in the WPU / BC aqueous solution of the upper and lower precursor solutions can be 32~35wt% and 0.6~1.0wt%, respectively. In the specific embodiments of this application, they are specifically selected as 34wt% and 0.8wt%.

[0022] The disordered freezing in this invention specifically involves: injecting the lower layer of precursor liquid into a flat-bottomed mold and placing it in a conventional low-temperature environment of -20~-25℃ for static freezing for 3~4.5h; during this process, there is no external directional thermal gradient, and ice crystals randomly nucleate and grow disorderedly inside the precursor liquid, with the ice crystal skeleton intertwined, ultimately forming a disordered random porous strong dielectric loss layer. The pores are distributed in a three-dimensional interlaced disorder, providing strong dielectric loss and multiple scattering sites for electromagnetic waves; In-situ directional freezing specifically involves: maintaining the overall ambient temperature of the mold no higher than -10℃ to ensure that the lower disordered frozen skeleton does not melt or collapse; injecting the upper precursor liquid in situ above the lower skeleton; attaching a low-temperature copper plate cooled by liquid nitrogen to the bottom of the mold to create a vertical thermal gradient along the thickness direction inside the sample (the bottom temperature is extremely low, and the upper temperature is relatively high). Driven by the thermal gradient, ice crystals grow vertically and oriented along the heat flow direction and are regularly arranged. The oriented ice crystals compress the precursor liquid components to form continuous directional pore walls, ultimately obtaining a vertically oriented pore magnetic coupling impedance matching layer; the upper precursor liquid and the lower frozen skeleton are in-situ contacted and solidified at low temperature, with no adhesive or gaps at the interface between the two layers, achieving in-situ bonding and integration, fundamentally avoiding the interface delamination and impedance mismatch problems caused by multi-layer stacking.

[0023] In Examples 1-3 of this invention, the upper layer has vertically oriented pores with a diameter of 15-35 μm and continuously oriented pore walls; the lower layer has disordered random pores with a diameter of 5-20 μm and randomly intersecting channels; the upper layer accounts for 62-68% of the thickness, the lower layer accounts for 32-38%, the upper layer accounts for 62%, the lower layer accounts for 38%, and the total thickness of the aerogel is 5.2 mm (the same thickness test was selected for the comparative example and the examples).

[0024] The aerogels prepared in Examples 1-3 of this invention have densities of 0.25~0.28 g / cm³. 3 The radial thermal conductivity is 0.14~0.16 W / (m·K). After 50 cycles of 40% strain compression, the shielding effectiveness retention rate is ≥98.5%, and the absorption ratio is >85%. Density testing was conducted according to GB / T 6342-1996 using the volumetric weighing method. The sample was made into a regular cuboid, the dimensions were measured to calculate the volume V, the mass m was weighed using an electronic balance, and the density was calculated using ρ=m / V. Radial thermal conductivity testing was conducted according to GB / T 3399-1982 using the transient plane heat source method (Hot Disk) at room temperature. The 40% strain cyclic compression test was conducted according to GB / T 8813-2008 on a universal testing machine, with a strain of 40%, a compression rate of 10 mm / min, and 50 cycles. The absorption ratio is the absorption loss SE. A SE as a percentage of total shielding loss TThe proportion >85% means that more than 85% of the shielding loss is due to absorption, with less than 15% due to surface reflection; this proves that the shielding mechanism is mainly absorption-based, with almost no secondary electromagnetic pollution; Test method: The aerogel sample is processed into a standard specimen matching the waveguide size and placed in a waveguide fixture; the S-parameters (S11, S21) of the sample are tested in the X-band (8.2~12.4 GHz) using a vector network analyzer; the total electromagnetic shielding effectiveness SE is calculated from S21. T = -20lg|S21|;Calculate the reflection coefficient R = |S11| from S11 2 And calculate the reflection loss SE R = -10lg(1-R); Absorption loss: SE A = SE T - SE R Absorption ratio = (SE) A / SE T The shielding effectiveness and retention rate were tested according to GB / T 30142-2013, using the waveguide method to test the electromagnetic shielding effectiveness in the X-band (8.2-12.4 GHz). The total shielding effectiveness SE0 and SE100% were tested before and after cyclic compression. 50 The shielding effectiveness retention rate is calculated using the following formula: Shielding effectiveness retention rate = (SE 50 / SE0) × 100% is used to evaluate the structural stability and mechanical durability of the in-situ bonded interface.

[0025] The MXene@hollow Fe3O4 core-shell heterostructures in various specific embodiments of the present invention are prepared using the following methods: FeCl3·6H2O, sodium citrate, and urea were dissolved in water, MXene nanosheets were added, ultrasonically dispersed, solvothermal at 200℃ for 12 h, washed and dried to obtain a core-shell heterostructure (where MXene: hollow Fe3O4 = 1:2~1:4, and a ratio of 1:3 was used in the example). The components are: FeCl3·6H2O: 0.80~0.90 g, sodium citrate: 1.80~1.95 g, urea: 0.50~0.60 g, MXene nanosheets: 0.075~0.090 g, and deionized water: 80-120 mL.

[0026] Taking a 1:3 ratio as an example, the specific dosage is as follows: 0.8646g FeCl3·6H2O, 1.8816g sodium citrate, and 0.56g urea are dissolved in deionized water, and 0.0823g MXene nanosheets are added. The mixture is ultrasonically dispersed for 30~60 min, solvothermal at 200℃ for 12 h, washed and dried to obtain a core-shell heterostructure of MXene: hollow Fe3O4 = 1:3.

[0027] The total shielding effectiveness and reflection coefficient of the five groups of bilayer composite aerogels prepared in Examples 1-3 and Comparative Examples 1-2 were tested, and the test results are shown in Table 1 below: The test was conducted according to GB / T 30142-2013, using the waveguide method in the X-band (8.2~12.4 GHz). The test instrument was a vector network analyzer paired with a waveguide test system. The composite aerogel prepared in the example was processed into a standard sample that matched the waveguide fixture and conformed to the size of the test cavity (e.g., in rectangular waveguide testing, the X-band sample size is usually 22.86 mm * 10.16 mm).

[0028] The overall shielding effectiveness (SE) was calculated by testing the S-parameters (S11 and S21) of the samples. T ), reflection loss (SE) R ) and absorption loss (SE) A ).

[0029] Table 1: Performance test results of bilayer composite aerogels prepared in Examples 1-3 and Comparative Examples 1-2 Table 1 shows that the electromagnetic shielding effectiveness of the bilayer composite aerogels prepared in Examples 1-3 is significantly improved, which is better than that of the composite aerogels with uniform structure prepared in Comparative Examples 1-2. They have good electromagnetic shielding effectiveness and low reflection coefficient.

[0030] Figure 1 This is a schematic diagram of an anisotropic heteroporous porous bilayer composite aerogel. Through a stepwise in-situ coupling freezing process, an integrated bilayer structure is formed, consisting of an upper vertically oriented porous magnetic coupling impedance matching layer and a lower disordered random porous strong dielectric loss layer.

[0031] Figure 2 The image shows a cross-sectional SEM image of an anisotropic heteroporous bilayer composite aerogel, which corresponds to the structural schematic diagram in Figure 1. The upper layer consists of vertically oriented channels, and the lower layer consists of disordered random channels, thus realizing an anisotropic heteroporous structure and constructing a gradient impedance electromagnetic wave loss network.

[0032] Figure 3The electromagnetic shielding effectiveness diagram is shown for anisotropic heteroporous double-layer composite aerogel after 50 cycles of compression. The optimal embodiment has a total shielding effectiveness (SE) of over 80 dB and a reflection coefficient as low as below 0.15, achieving ultra-low reflection electromagnetic shielding. Moreover, the performance retention rate is still over 98.5% after 50 cycles, demonstrating excellent mechanical properties and recyclability.

[0033] Figure 4 The conductivity diagram shows that the conductivity of the upper and lower layers of the aerogel differs by more than three orders of magnitude, further verifying the effective formation of the anisotropic heteroporous bilayer gradient structure.

[0034] The first-ever in-situ bonded double-layer structure of "vertical oriented aperture + disordered random aperture" is different from all gradient structures and stacked structures. It completely avoids existing patents in terms of aperture morphology and achieves dual synergy of impedance gradient and loss gradient. The MXene@hollow Fe3O4 hollow core-shell structure is adopted. The hollow structure enhances multiple scattering and magnetic components introduce magnetic loss, which breaks through the limitation of pure MXene only having dielectric loss. The reflection coefficient is reduced to below 0.15, which is far superior to existing patents (R≥0.38). WPU (waterborne polyurethane) provides flexibility, while BC (bacterial cellulose) provides skeletal rigidity. This organic-inorganic hybrid matrix, combined with in-situ bonding interfaces, solves the problems of fragility and delamination in aerogels. WPU / BC serves as the matrix, and BC is not only the matrix, but its "nano-network interweaving" in WPU is also the core reason why the aerogel can withstand 50 cycles of compression at 40% strain without collapsing.

[0035] Using a disordered freezing → in-situ directional freezing coupling process, there is no 3D printing, no filtration, no high-temperature imidization, no binder, and it is a fully water-based low-temperature molding process. The process is green and extremely low-cost, and it is completely different from existing patented processes. Achieving synergistic effects of ultra-low reflection electromagnetic shielding, infrared-compatible stealth, anisotropic thermal insulation, and flexible mechanics, filling the gap in materials for "low radar and infrared detection" in the field of aerospace stealth. It can be used in ultra-low reflection electromagnetic shielding, radar-infrared dual-compatible stealth, flexible wearable electromagnetic protection, and integrated aerospace thermal management scenarios.

[0036] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An anisotropic heteroporous porous bilayer composite aerogel, characterized in that, The anisotropic heteroporous porous bilayer composite aerogel is an in-situ bonded integrated bilayer structure without interface gaps. The upper layer is a vertically oriented porous magnetic coupling impedance matching layer, which is composed of a WPU / BC matrix loaded with an MXene@hollow Fe3O4 core-shell heterostructure. The lower layer is a disordered random porous strong dielectric loss layer, which is composed of a WPU / BC matrix loaded with an MXene / CNTs coaxial conductive network.

2. The anisotropic heteroporous porous bilayer composite aerogel according to claim 1, characterized in that, The upper layer has vertically oriented holes with a diameter of 15~35μm and continuously oriented hole walls; the lower layer has disordered random holes with a diameter of 5~20μm and randomly intersecting channels; the upper layer accounts for 62~68% of the thickness, and the lower layer accounts for 32~38%.

3. A method for preparing anisotropic heteroporous porous bilayer composite aerogel as described in any one of claims 1-2, characterized in that, The preparation method includes the following steps: S1. Preparation of MXene@hollow Fe3O4 core-shell heterostructure by solvothermal method; S2. Prepare the lower layer precursor solution and the upper layer precursor solution separately; S3, Step 1: Disorderly freezing: Inject the lower layer precursor liquid into the mold and freeze at -20~-25℃ for 3~4.5h to form a disordered random hole strong dielectric loss layer; S4. Second step: In-situ directional freezing: Maintain a low temperature below -10℃, inject the upper liquid while the lower layer is frozen but not dried, and use liquid nitrogen-copper plate bottom thermal gradient directional freezing for 30~60min to form a vertically oriented hole magnetic coupling impedance matching layer. S5. Freeze-dry at -70℃ and <2Pa for 48h to obtain in-situ bonded anisotropic heteroporous bilayer aerogel.

4. The method for preparing anisotropic heteroporous porous bilayer composite aerogel according to claim 3, characterized in that, The lower precursor solution includes MXene, CNTs and WPU / BC aqueous solution, and the mass ratio of MXene, CNTs and WPU / BC aqueous solution is 80:5~15:5~15.

5. The method for preparing anisotropic heteroporous porous bilayer composite aerogel according to claim 3, characterized in that, The upper precursor solution includes an MXene@hollow Fe3O4 core-shell heterostructure and a WPU / BC aqueous solution, with a mass ratio of 4:6 between the MXene@hollow Fe3O4 core-shell heterostructure and the WPU / BC aqueous solution.

6. The method for preparing anisotropic heteroporous porous bilayer composite aerogel according to claim 3, characterized in that, The WPU / BC aqueous solutions of the upper and lower precursor solutions contain WPU and BC at mass ratios of 32-35 wt% and 0.6-1.0 wt%, respectively.

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