Lightweight flexible three-dimensional stereoscopic cellulose aerogel wave-absorbing super composite material and application thereof

Abstract

The present application belongs to the technical field of electromagnetic wave absorbing materials, and particularly relates to a lightweight flexible three-dimensional cellulose aerogel absorbing wave super composite material and application thereof. The absorbing wave super composite material comprises a CNT / acrylic resin absorbing wave material layer, and absorbing wave structure units are arrayed on the CNT / acrylic resin absorbing wave material layer. The absorbing wave structure unit is a structure in which a carbon / cellulose absorbing wave aerogel is filled in a flexible resin shell. The uniform porous structure of the carbon / cellulose absorbing wave aerogel is combined with the multiple reflection and absorption performance of high conductivity, which can ensure the highest absorption performance and effective use of the conductive filler. Meanwhile, the optimization and adjustment of the geometric structure parameters of the absorbing wave super composite material make it have electromagnetic wave absorption in an ultra-wide band of 4.36-40 GHz, and the relative bandwidth reaches 160.7%. Meanwhile, the absorbing wave super composite material has large RCS attenuation under the conditions of bending and different electromagnetic wave incident angles, and can be applied to special application environments such as curved surface structures and electromagnetic wave oblique incidence.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic wave absorbing materials technology, specifically, it relates to a lightweight, flexible three-dimensional cellulose aerogel wave-absorbing supercomposite material and its applications. Background Technology

[0002] The market currently demands higher performance from electromagnetic wave absorbing materials with broadband absorption capabilities, particularly for lightweight materials and effective absorption bandwidth covering low frequencies. Introducing air into carbon-based absorbing agents to increase their specific surface area can effectively improve impedance matching between the absorbing agent and free space. Designing heterogeneous interfaces within carbon-based absorbing agents can effectively enhance their dielectric loss capacity. Increasing the proportion of interfacial polarization in the overall loss of carbon materials can effectively control the impedance mismatch caused by increasing conductivity with temperature. However, current research on carbon-based absorbing agents mainly focuses on the X-band (8.2–12.4 GHz), which cannot meet the requirements of high-performance absorbing materials for ultra-wideband absorption. Therefore, there is a need to develop a broadband or even ultra-wideband absorbing material that is lightweight, flexible, and multifunctional.

[0003] Aerogel materials possess advantages such as low density, high porosity, and large specific surface area. Cellulose, as a renewable and biodegradable biomass material, has become a popular raw material for preparing aerogel materials in recent years, and significant progress has been made in cellulose aerogel-based microwave absorbing materials. In the low-frequency band, electromagnetic resonance loss caused by the periodic macroscopic structure can effectively dissipate electromagnetic wave energy. In the high-frequency band, dielectric loss mechanisms such as dipole polarization, interfacial polarization, and conductivity loss mechanisms of carbon materials are needed to dissipate electromagnetic waves. Therefore, in order to achieve ultra-wideband electromagnetic wave absorption performance covering low to high frequencies (4–40 GHz), it is necessary to design a class of microwave absorbing materials with macroscopic periodic structures and microscopic porous structures to achieve broadband electromagnetic absorption by combining low-frequency electromagnetic resonance loss with high-frequency dielectric loss. Summary of the Invention

[0004] To address the problems existing in the prior art, the present invention aims to provide a lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material and its applications. The present invention constructs an electromagnetic wave absorbing supercomposite material by designing periodic microwave absorbing structural units and controlling the composition, structure, morphology, composition, and processing method of the microwave absorbing material in the structural units. This results in the present invention's microwave absorbing supercomposite material having high electromagnetic wave absorption intensity, wide effective absorption bandwidth, ultrathinness, light weight, and good stability and flexibility.

[0005] Based on the above objectives, the technical solution adopted by the present invention is as follows:

[0006] In a first aspect, the present invention provides a lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material, the microwave absorbing supercomposite material comprising a CNT / acrylic resin microwave absorbing material layer, wherein microwave absorbing structural units are arrayed on the CNT / acrylic resin microwave absorbing material layer; the microwave absorbing structural unit comprises a flexible resin shell, wherein the flexible resin shell is filled with carbon / cellulose microwave absorbing aerogel.

[0007] This invention employs a method of filling a flexible resin shell with carbon / cellulose absorbing aerogel and then arraying it on a CNT / acrylic resin absorbing material. By placing this invention's absorbing supercomposite material on a metal base plate, combined with the special structural design of the absorbing supercomposite material, a multi-scale synergistic effect of wave absorption can be generated to achieve ultra-wideband electromagnetic wave absorption. Moreover, the carbon / cellulose absorbing aerogel filled in the flexible resin shell serves as the upper absorption layer, and the CNT / acrylic resin absorbing material serves as the lower absorption layer. This double-layer structure can effectively improve the impedance matching of gradient electromagnetic performance. Combined with the periodic structural design, this invention's absorbing supercomposite material has the characteristics of broadband and even ultra-wideband electromagnetic wave absorption, and the absorption rate of electromagnetic waves is not less than 90%.

[0008] Preferably, the flexible resin shell consists of a square shell and a surrounding plate at the bottom of the square shell, and a round hole is provided at the top of the square shell.

[0009] The circular holes on the surface of the flexible resin housing can improve impedance matching.

[0010] Preferably, the flexible resin shell is made of flexible acrylic resin by photopolymerization 3D printing.

[0011] Preferably, the periodic dimension of the absorbing structure unit is 10-20 mm; the thickness of the CNT / acrylic resin absorbing material layer is 1.2-2.0 mm; the height of the carbon / cellulose absorbing aerogel filled in the flexible resin shell is 3-8 mm, and the width of the carbon / cellulose absorbing aerogel is 60%-75% of the periodic dimension of the absorbing structure unit.

[0012] Preferably, the wall thickness of the square shell is 1.5 to 2.0 mm, the thickness of the surrounding plate is 1.0 to 1.5 mm, and the radius of the circular hole is 35% to 45% of the width of the carbon / cellulose absorbing aerogel.

[0013] The equivalent electromagnetic performance of the periodic absorbing structure unit was adjusted by modifying its shape and corresponding geometric parameters. The optimized unit structure was simulated using the microwave simulation software CST studio. Based on the optimal electromagnetic absorption parameters, the optimal geometric parameters of the absorbing structure unit were determined. The optimal geometric parameters of the absorbing structure unit are as follows:

[0014] The periodic dimension of the absorbing structural unit is 10–20 mm; the thickness of the CNT / acrylic resin absorbing material layer is 1.2–2 mm; the height of the carbon / cellulose absorbing aerogel filled in the flexible resin shell is 4.5–5.5 mm, and the width of the carbon / cellulose absorbing aerogel is 6–15 mm; the wall thickness of the square shell is 1.5–2.0 mm, the thickness of the surrounding plate is 1.0–1.2 mm, and the radius of the circular hole is 3–5.5 mm.

[0015] Preferably, the density of the carbon / cellulose microwave absorbing aerogel is 0.05–0.08 g / cm³. 3 It can achieve a large range of adjustment at 10GHz, with the real part of the relative permittivity changing from 3.36 to 7.15, the imaginary part of the permittivity changing from 2.56 to 13.27, and the dielectric loss changing from 0.72 to 2.08.

[0016] Preferably, the carbon / cellulose absorbing aerogel is a carbon nanotube / cellulose absorbing aerogel or a graphene / cellulose absorbing aerogel.

[0017] Preferably, the carbon / cellulose microwave absorbing aerogel is prepared by the following method:

[0018] Carbon materials and cellulose are dispersed and mixed to form a carbon material / cellulose aqueous solution, which is then subjected to a gelation reaction to form a carbon material / cellulose hydrogel, and finally freeze-dried to produce a carbon / cellulose microwave absorbing aerogel.

[0019] Alternatively, the carbon / cellulose microwave-absorbing aerogel can be prepared by the following method:

[0020] A cellulose aqueous solution is gelled to form a cellulose hydrogel, which is then freeze-dried to form a cellulose aerogel. The cellulose aerogel is then immersed in an aqueous dispersion of carbon materials and freeze-dried to form a carbon / cellulose microwave absorbing aerogel.

[0021] Preferably, the carbon / cellulose microwave absorbing aerogel is prepared by the following method:

[0022] S1: Disperse carbon materials in water to form a carbon material aqueous dispersion, add urea and NaOH to the carbon material aqueous dispersion to form a complex solution;

[0023] S2: After pre-cooling the complex solution, cellulose is added to the complex solution to form a carbon material / cellulose aqueous solution;

[0024] S3: The carbon material / cellulose aqueous solution is gelled at 60-80℃ for 48-96 hours to form a carbon material / cellulose hydrogel;

[0025] S4: Carbon / cellulose aerogels are prepared by washing and solvent exchange of carbon materials / cellulose hydrogels and then freeze-drying them.

[0026] This invention uses cellulose as the matrix, dissolves cellulose using a NaOH / urea system as the solvent, and uses carbon materials as the microwave absorbing agent to prepare carbon / cellulose microwave absorbing aerogels under low temperature conditions.

[0027] This invention employs a method of pre-mixing a carbon material aqueous dispersion with urea and NaOH, and then adding cellulose. When cellulose is dissolved in a pre-cooled complex solution containing urea and NaOH, the NaOH hydrate interacts with the hydroxyl groups on the cellulose macromolecules, disrupting the intramolecular and intermolecular hydrogen bonds of the cellulose molecules. Simultaneously, in the carbon material / cellulose aqueous solution, urea and NaOH combine to form a worm-like inclusion complex with urea as a shell. Once the temperature rises, the inclusion complex decomposes, and the cellulose chains self-associate and entangle through hydrogen bonds. The cellulose molecular chains and carbon material are bound together by hydrogen bonds, easily undergoing a gelation reaction. Therefore, the carbon / cellulose aerogel prepared by the method of this invention has excellent electrical conductivity and electromagnetic wave absorption performance.

[0028] The method of this invention can effectively improve the dispersion uniformity of carbon materials in a cellulose matrix; this invention can regulate the microwave absorption properties of the aerogel by adjusting the carbon material content in the aerogel. Compared with the prior art, the carbon / cellulose microwave absorbing aerogel prepared by this invention does not require high-temperature carbonization treatment, and can obtain an aerogel with effective adjustment of the real and imaginary parts of the relative permittivity, and the aerogel can maintain high machinability.

[0029] Preferably, the carbon content in the carbon material / cellulose aqueous solution is 0.2-2 wt%; the weight ratio of carbon material to cellulose in the carbon material / cellulose aqueous solution is 0.16-0.3:1.

[0030] Preferably, the carbon material content in the carbon material aqueous dispersion is 0.75–2 wt%.

[0031] Preferably, the cellulose is short-fiber cellulose, and the carbon material is graphene and / or carbon nanotubes.

[0032] Preferably, the mass ratio of water, urea, and NaOH in the complex solution is 81:12:7.

[0033] Preferably, pre-cooling involves pre-cooling the complex solution to -12°C to -14°C; cellulose is added to the pre-cooled complex solution while it is being stirred at a speed of 3000 to 8000 r / min.

[0034] Preferably, washing involves immersing the carbon material / cellulose hydrogel in water until the pH of the water reaches 7, indicating that washing is complete; solvent exchange is performed by immersing the washed carbon material / cellulose hydrogel in a 5-20% tert-butanol solution.

[0035] The carbon / cellulose hydrogel was immersed in deionized water to wash away urea and NaOH. The carbon / cellulose hydrogel was also immersed in a tert-butanol solution for thorough solvent exchange. Tert-butanol promotes the growth rate of ice crystals during the freezing process, resulting in smaller ice crystals. This reduces the average pore size and alters the pore morphology of the carbon / cellulose aerogel, thereby enhancing its mechanical properties.

[0036] Secondly, the present invention provides the above-mentioned lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material as an application of electromagnetic wave absorption and lightweight, flexible, multifunctional microwave absorbing material. The microwave absorbing supercomposite material of the present invention can absorb more than 90% of electromagnetic waves in the ultra-wideband electromagnetic wave of 4.36 to 40 GHz, with a relative bandwidth of 160.7%; the bending diameter of the microwave absorbing supercomposite material of the present invention can reach 80 mm.

[0037] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0038] (1) This invention proposes a strategy that simultaneously improves the porous structure and adjusts the distribution of the absorbing agent, which can effectively control the microstructure and electromagnetic parameters of cellulose aerogel. The carbon / cellulose absorbing aerogel prepared by the method of this invention has a porous structure with a three-dimensional interconnected network and a large amount of air between its pore walls, resulting in excellent impedance matching of the carbon / cellulose absorbing aerogel. The absorbing agent is directly coated on the cellulose cell wall, which can more effectively improve the absorption performance of the composite material. The combination of uniform porous structure and high conductivity enables multiple reflections and absorptions, which can ensure the highest electromagnetic absorption performance and effective utilization of conductive fillers.

[0039] (2) This invention innovatively fills a 3D-printed flexible resin shell with carbon / cellulose absorbing aerogel by cutting, and encapsulates it with flexible resin absorbing material. Through periodic absorbing structural units and control of geometric parameters, a cellulose aerogel absorbing supercomposite material with excellent electromagnetic absorption performance and flexible lightweight properties is prepared. This achieves ultra-wideband electromagnetic wave absorption of 4.36–40 GHz with a relative bandwidth of 160.7%. Simultaneously, the supercomposite material exhibits large RCS attenuation under bending conditions and different electromagnetic wave incident angles, making it applicable to curved surfaces and special application environments with oblique electromagnetic wave incidence. Therefore, this invention has significant guiding significance for the development of lightweight, efficient, flexible, environmentally friendly, and high-performance absorbing materials. Attached Figure Description

[0040] Figure 1 This is a flowchart illustrating the preparation process of the microwave absorbing supercomposite material in Example 1;

[0041] Figure 2 Here is a SEM image of the CNT / cellulose absorbing aerogel in Example 1;

[0042] Figure 3 The dielectric properties of the CNT / cellulose absorbing aerogel in Example 1;

[0043] Figure 4 The microwave absorption performance curves of the microwave absorbing supercomposite material under different geometric parameters in Example 1 are shown.

[0044] Figure 5 The image shows the physical sample of the microwave absorbing supercomposite material prepared in Example 1 and its reflection loss curve measured at 4–40 GHz.

[0045] Figure 6 The RCS values ​​of the microwave absorbing supercomposite material and the same shaped metal in Example 1 under bending conditions at different angles;

[0046] Figure 7 Here is a SEM image of the GNS / cellulose absorbing aerogel in Example 2;

[0047] Figure 8 The dielectric properties of the GNS / cellulose microwave absorbing aerogel in Example 2 are shown in the figure.

[0048] Figure 9 This is a reflection loss curve of the GNS / cellulose absorbing aerogel in Example 2;

[0049] Figure 10 Dielectric properties diagram of GNS / cellulose microwave absorbing aerogel in Example 3;

[0050] Figure 11 The graph shows the reflection loss curve of the GNS / cellulose absorbing aerogel in Example 3. Detailed Implementation

[0051] To better illustrate the purpose, technical solution, and advantages of this invention, the invention will be further described below with reference to specific embodiments. Those skilled in the art should understand that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. Unless otherwise specified, the experimental methods used in the embodiments are conventional methods; the materials and reagents used, unless otherwise specified, are commercially available.

[0052] Example 1

[0053] This embodiment provides a lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material, such as... Figure 1As shown, it includes CNT / acrylic resin absorbing material laid flat on a metal base plate to form a CNT / acrylic resin absorbing material layer. The CNT / acrylic resin absorbing material layer has an array of absorbing structural units. The absorbing structural unit includes a flexible resin shell, which is filled with carbon / cellulose absorbing aerogel. The flexible resin shell is composed of a square shell and a baffle plate at the bottom of the square shell. A round hole is opened at the top of the square shell.

[0054] In this embodiment, the filling material, carbon / cellulose absorbing aerogel, is a carbon nanotube / cellulose absorbing aerogel. Its preparation method is as follows: under low temperature conditions, short-fiber cellulose is used as the matrix, and NaOH / urea system is used as the solvent to dissolve cellulose. A typical carbon material, namely carbon nanotubes (CNTs), is used as the absorbing agent. First, the CNT aqueous dispersion is added to the NaOH / urea solution, and then short-fiber cellulose is added to dissolve the cellulose. Then, the CNT / cellulose absorbing aerogel is prepared by gelation reaction and freeze drying.

[0055] The specific preparation process of CNT / cellulose absorbing aerogel is as follows: Figure 1 As shown, it includes the following steps:

[0056] S1: CNTs and dispersant cetyltrimethylammonium bromide (CTAB) are ultrasonically dispersed in deionized water for 30–60 min, with a CNT to CTAB weight ratio of 1:1, to obtain a CNT aqueous dispersion. Urea and NaOH are then directly dissolved in the CNT suspension to form a complex solution. The mass ratio of water, urea, and NaOH in the complex solution is 81:12:7. The NaOH / urea / aqueous solution system is used to rapidly dissolve cellulose.

[0057] S2: The obtained complex solution was pre-cooled to about -12°C in a low-temperature room. Then, at room temperature, 2-12g of cellulose was rapidly added to the pre-cooled complex solution using a constant-speed stirrer at a speed gradually increased to 8000r / min and stirred for 5-20min. As the cellulose dissolved, the carbon nanotubes were uniformly mixed with the cellulose at the molecular level to obtain a uniform CNT / cellulose aqueous solution. The mass ratio of CNT to cellulose in the CNT / cellulose aqueous solution was 0.3:1.

[0058] S3: Pour the CNT / cellulose aqueous solution into a centrifuge tube and centrifuge at 8000 r / min for 5–30 min to remove air bubbles. Cast the centrifuged CNT / cellulose aqueous solution into a custom-made polytetrafluoroethylene mold and place it in a 70℃ oven for 48–96 h to allow the CNT / cellulose aqueous solution to transform into a hydrogel state, forming a CNT / cellulose hydrogel.

[0059] S4: Due to the presence of numerous polar groups, cellulose exhibits excellent hydrophilicity. The resulting CNT / cellulose hydrogel is then immersed in a large volume of distilled water to wash away NaOH and urea until the pH of the distilled water reaches 7, indicating that washing is complete. The CNT / cellulose hydrogel is then immersed in a prepared 5–20% tert-butanol (TBA) aqueous solution for thorough solvent exchange to enhance its mechanical properties.

[0060] When cellulose is dissolved in a pre-cooled NaOH / urea solvent, the NaOH hydrate interacts with the hydroxyl groups on the cellulose macromolecules, disrupting the intramolecular and intermolecular hydrogen bonds. Simultaneously, in a homogeneous cellulose / NaOH / urea / CNT aqueous solution, urea combines with NaOH to form a urea-shell-like inclusion complex containing cellulose molecules in a worm-like form. Upon increasing temperature, the inclusion complex decomposes, allowing the cellulose chains to self-associate and entangle through hydrogen bonds. The cellulose molecular chains and CNTs are bound together by hydrogen bonds, readily undergoing a gelation process. Therefore, carbon / cellulose aerogels exhibit excellent electrical conductivity and electromagnetic wave absorption properties.

[0061] Subsequently, solvent exchange was performed using a TBA / water solution. The addition of TBA promoted the growth rate of ice crystals, resulting in smaller ice crystals, which ultimately led to a reduction in the average pore size and an alteration in the pore morphology of the cellulose aerogel. Therefore, by introducing CNTs into the cellulose carrier through the above preparation process, the multifunctionality of cellulose aerogels was achieved.

[0062] The washed and solvent-exchanged CNT / cellulose hydrogel was placed in a closed space for uniform freezing to minimize environmental interference. Finally, the frozen CNT / cellulose hydrogel was dried in a freeze dryer for 48 hours to remove the solvent without causing pore collapse, thus obtaining a CNT / cellulose microwave-absorbing aerogel material.

[0063] By controlling the CNT content in the CNT / cellulose aqueous solution to be 0.5 wt%, 1 wt%, and 2 wt%, respectively, CNT / cellulose aerogels with different CNT contents were prepared according to the above method of "dissolution, regeneration, solvent exchange, and freeze-drying". The CNT / cellulose aerogels prepared from the CNT / cellulose aqueous solutions with CNT contents of 0.5 wt%, 1 wt%, and 2 wt% were respectively designated as CNT-0.5 / cellulose microwave absorbing aerogel, CNT-1 / cellulose microwave absorbing aerogel, and CNT-2 / cellulose microwave absorbing aerogel. Simultaneously, a pure cellulose aerogel without CNTs was prepared as a control, designated as CNT-0 / cellulose microwave absorbing aerogel.

[0064] SEM images of pure cellulose aerogel and CNT / cellulose absorbing aerogels with different CNT contents are shown below. Figure 2 As shown, where Figure 2(a) is a pure cellulose aerogel without carbon nanotubes. Due to the poor conductivity of cellulose, electrons are difficult to transfer and tend to accumulate on the fiber surface, resulting in localized brightening in its SEM image and low strength of cellulose. Figure 2 (b) and Figure 2 (c) CNT / cellulose absorbing aerogel materials with CNT contents of 2wt% and 1wt% respectively showed significantly improved conductivity and more uniform pore distribution. Furthermore, when the CNT content was too high, CNT aggregation occurred, resulting in denser and thicker pore walls and uneven protrusions on the surface. Figure 2 (d) is Figure 2 (c) is a magnified view of a part of the porous wall surface, which is uniformly covered with interconnected CNTs, forming an interconnected conductive network with a large specific surface area and high porosity.

[0065] Depend on Figure 2 It is known that in aerogels with a CNT content of 0.5wt%–2wt%, CNTs are uniformly distributed within the cellulose matrix, and the aerogel contains numerous pores. The uniform distribution of CNTs within the cellulose aerogel matrix is ​​an effective method for controlling the microstructure of composite materials, which directly affects the construction of the internal conductive network and the attenuation performance of electromagnetic waves. When the CNT content in the CNT / cellulose aqueous solution is 1wt%, the CNT / cellulose absorbing aerogel obtained through the above preparation process exhibits an ideal three-dimensional structure and demonstrates relatively optimal electromagnetic wave absorption performance.

[0066] Dielectric properties of CNT / cellulose absorbing aerogels with different CNT contents, such as Figure 3 As shown, where Figure 3 (a) is the real part of the dielectric constant. Figure 3 (b) represents the imaginary part of the dielectric constant. The test results show that adjusting the CNT content effectively modulates the real and imaginary parts of the relative dielectric constant of the aerogel material, providing a solid foundation for the design and optimization of the microwave absorbing supercomposite material. The real part of the relative dielectric constant of air is 1, and the imaginary part is 0. The larger the real and imaginary parts of the relative dielectric constant of a material, the worse its impedance matching with air. The dielectric loss of a material, i.e., the ratio of the imaginary part to the real part, is also important; the higher the dielectric loss, the stronger its ability to dissipate electromagnetic waves. For microwave absorbing materials, both impedance matching and electromagnetic loss must be satisfied simultaneously; therefore, microwave absorbing materials typically need to have moderate real and imaginary dielectric constants.

[0067] according to Figure 3The relative permittivity of CNT / cellulose absorbing aerogels with different CNT contents shows that, with increasing CNT content, both the real and imaginary parts of the dielectric constant of the CNT / cellulose composite aerogel exhibit a significant upward trend. When the CNT mass content increases to 2%, the real part of the dielectric constant at 10 GHz increases from 3.36 to 5.55, the imaginary part from 4.03 to 11.55, and the dielectric loss from 1.21 to 2.08. A higher dielectric constant indicates a higher polarization capability in response to electromagnetic waves. A higher CNT content results in a more compact microstructure, which is beneficial for current conduction, thus exhibiting good conductivity. Furthermore, compared to aerogels with other contents, CNT-1 maintains a consistently high dielectric constant across different frequency bands, with a real part of 5.14, an imaginary part of 7.81, and a dielectric loss of 1.52. Therefore, CNT-1 / cellulose absorbing aerogel possesses moderate dielectric constant and dielectric loss, effectively balancing the impedance matching and electromagnetic loss characteristics required for microwave absorbing materials.

[0068] In this embodiment, the flexible resin shell is a photopolymer 3D printed flexible acrylic resin shell, and CNT-1 / cellulose microwave absorbing aerogel material is filled in the flexible acrylic resin shell. The density of CNT-1 / cellulose microwave absorbing aerogel is 0.06 g / cm³. 3 .

[0069] CNT / acrylic resin absorbing material is laid flat on a metal substrate to form a CNT / acrylic resin absorbing material layer. Periodic absorbing structural units are distributed on this layer. Each absorbing structural unit is a 3D-printed flexible acrylic resin shell formed by photopolymerization. The top of the square shell has a circular hole, and the bottom of the shell has surrounding plates. CNT-1 / cellulose aerogel is then filled into the square shell using a cutting method, resulting in a lightweight, flexible, three-dimensional cellulose aerogel absorbing supercomposite material. The use of a circular hole structure on the surface of the flexible acrylic resin shell improves impedance matching; the bottom of the absorbing structural unit is a continuous layer of CNT / acrylic resin absorbing material to enhance electromagnetic wave attenuation.

[0070] The geometric parameters involved in the microwave absorbing supercomposite material in this embodiment include: the periodic dimension p of the microwave absorbing structural unit, the height H1 of the bottom-connected CNT / acrylic resin microwave absorbing material layer, the height H2 of the CNT-1 / cellulose microwave absorbing aerogel layer, the width W of the CNT-1 / cellulose aerogel layer, the wall thickness d of the flexible resin shell, the thickness D of the flexible resin of the connecting part (i.e., the enclosure) of the microwave absorbing structural unit, the radius R of the top circular hole of the flexible resin shell, and the total thickness H = H1 + H2 + d of the microwave absorbing supercomposite material.

[0071] The structural design of the microwave absorbing supercomposite material in this embodiment can generate a synergistic effect of multi-scale microwave absorption to achieve ultra-wideband electromagnetic wave absorption. The CNT-1 / cellulose microwave absorbing aerogel filled in the flexible resin shell serves as the upper absorption layer, and the CNT / acrylic resin microwave absorbing material serves as the lower absorption layer. The double-layer structure material can effectively improve the impedance matching of gradient electromagnetic performance. The resin shell material with periodic circular hole pattern can adjust the equivalent electromagnetic performance by optimizing the shape of the structural unit and the corresponding geometric parameters.

[0072] Simulation calculations were performed in the microwave simulation software CST studio to optimize the design unit structure and determine the geometry and electromagnetic parameters of the designed metamaterial. The microwave absorption performance under different geometric parameters is shown below. Figure 4 As shown, the principle is to achieve the maximum absorption bandwidth (and satisfy greater than 90% electromagnetic wave absorption) with the minimum total thickness of the absorbing material. Figure 4 (a) It can be seen that the width W of the aerogel layer has a significant impact on the absorption performance. If the W value is too low, the absorption performance will not meet the expected requirements in the working frequency range. As the W value increases, the number and intensity of the absorption peaks increase significantly. However, when the W value is greater than 12 mm, the intensity of the absorption peaks decreases significantly, so the absorption performance at low and high frequencies decreases. Figure 4 (b) shows the effect of the aperture R of the circular hole on the absorption performance. As an impedance matching layer, the R value has no effect on the position of the first absorption peak. As the R value increases, the absorption peak intensity increases. However, the absorption performance is poor in the range of 8 to 16 GHz due to an excessively large R value. Figure 4 (c) indicates the effect of the height H2 of the absorbing aerogel layer on the absorption performance. Since the resonant thickness is one-quarter of the wavelength, the absorption peak changes significantly with different H2 values. Increasing H2 can significantly improve the low-frequency absorption performance, that is, the absorption peak shifts to lower frequencies, significantly broadening the effective absorption bandwidth. However, an excessively large H2 value will reduce the intensity of the resonant peak, affecting its absorption performance near 10 GHz, which does not meet the requirements. Figure 4 (d) indicates the effect of the height H1 of the bottom CNT / acrylic resin absorbing material layer on the absorbing performance. As H1 increases, the absorption peak shifts to lower frequencies and the intensity increases, but the absorbing performance near 10 GHz is reduced. Figure 4 (e) represents the thickness D of the flexible acrylic resin shell connecting part of the enclosure. Changing the value of D has almost no effect on the wave absorption performance. Considering the printing accuracy factor, the value of D is selected as 1mm. Figure 4 (f) represents the wall thickness d of the flexible acrylic resin shell. Too small a value of d will reduce the absorption performance at mid-to-high frequencies. Increasing the value of d will cause the position of the absorption peak to shift to lower frequencies, but will reduce the intensity of the absorption peak.

[0073] Considering the overall thickness of the absorbing metamaterial and the frequency range where the corresponding reflection loss RL is less than or equal to -10dB, the geometric parameters of the periodic absorbing structure unit selected in this embodiment are as follows: p = 18mm, H1 = 1.5mm, H2 = 5mm, W = 12mm, d = 2mm, D = 1.0mm, R = 5mm. At this point, the absorbing metamaterial can achieve effective electromagnetic wave absorption in the frequency range of 4.36–40GHz with a total thickness of only 8.5mm.

[0074] The physical image of the lightweight, flexible three-dimensional cellulose aerogel microwave absorbing supercomposite material prepared in this embodiment and the measured reflection loss curves at 4–40 GHz are shown below. Figure 5 As shown, the prepared microwave absorbing supercomposite material consists of 10×10 periodic structural units to meet the testing requirements. In practical applications, the number of periodic structural units is not limited to 10×10; the array number of periodic structural units depends on the specific application. The results show that the microwave absorbing supercomposite material in this embodiment achieves over 90% electromagnetic wave absorption in the 4.36–40 GHz range, with an effective absorption bandwidth of 35.64 GHz and a relative bandwidth of 160.7%.

[0075] The radar-absorbing supercomposite material was bent to a diameter of 80 mm. Its radar cross-section (RCS) at different angles was compared to that of a metal of the same shape. Figure 6 As shown. Compared with metals, the RCS of CNT / cellulose aerogel microwave absorbing supercomposite material is effectively attenuated at different angles. Therefore, it can be applied to electromagnetic wave incident at different angles and curved structures.

[0076] The CNT / cellulose aerogel absorbing metamaterial prepared in this embodiment achieves ultra-wideband electromagnetic absorption performance with a relatively small total thickness (8.5 mm). Moreover, the use of a flexible resin shell can combine flexibility and lightness, better meeting the requirements of "thin, light, wide, and strong" for absorbing materials.

[0077] Example 2

[0078] This embodiment provides a lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material. The carbon / cellulose microwave absorbing aerogel filled within the flexible resin shell of the supercomposite material is a graphene / cellulose microwave absorbing aerogel, specifically GNS / cellulose microwave absorbing aerogel, with a density of 0.08 g / cm³. 3 .

[0079] Using the method described in Example 1, GNS / cellulose aqueous solutions were prepared by adding cellulose to a composite solution containing different mass fractions of graphene. The weight ratio of GNS to cellulose was 0.3:1. After centrifugation to remove air bubbles, the mass fraction of GNS in the GNS / cellulose aqueous solution was 1.5 wt%. The corresponding GNS / cellulose microwave absorbing aerogels were then prepared through regeneration, solvent exchange, and freeze-drying. All other aspects except those described were the same as in Example 1. The SEM image of the GNS / cellulose microwave absorbing aerogels prepared in this example is shown below. Figure 7 As shown, its SEM structure is similar to that of CNT / cellulose absorbing aerogel, both showing a large number of pores inside the aerogel. GNS is similar to CNT, and can also form a uniform conductive network for electromagnetic wave loss and attenuation. The difference is that in this embodiment, cellulose and GNS are uniformly interwoven to form the pore walls of the aerogel.

[0080] The dielectric properties of the GNS / cellulose absorbing aerogel in this embodiment were tested as follows: Figure 8 As shown, the solid line represents the real part of the relative permittivity, and the dashed line represents the imaginary part of the permittivity. Compared with the CNT-1 / cellulose absorbing aerogel in Example 1, the GNS / cellulose absorbing aerogel has smaller real and imaginary parts of the relative permittivity. The ratio of the real to imaginary parts of the relative permittivity (i.e., dielectric loss) is 0.72. Therefore, the impedance matching of the GNS / cellulose absorbing aerogel in this example is better than that of the CNT-1 / cellulose absorbing aerogel in Example 1, but its electromagnetic loss capability is relatively worse.

[0081] A three-dimensional microwave absorbing structure was designed using CST simulation software based on the tested dielectric parameters. The geometric parameters of the microwave absorbing supercomposite material in this embodiment are: p = 10 mm, H1 = 1.2 mm, H2 = 5.5 mm, W = 6.5 mm, d = 1.5 mm, D = 1.0 mm, R = 3 mm, and the total thickness is 8.2 mm.

[0082] Finally, a lightweight, flexible GNS / cellulose aerogel microwave absorbing supercomposite material was prepared, and its microwave absorption performance curve is shown in the figure. Figure 9 As shown, the strongest reflection loss is -43.5dB, with more than 99.99% of electromagnetic waves being absorbed. In the range of 6.52-40GHz, the reflection loss is less than -8dB, with more than 84.2% of electromagnetic waves being absorbed. The effective absorption bandwidth is 29.3GHz, and the relative bandwidth of -8dB is 145.9%.

[0083] Example 3

[0084] This embodiment provides a lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material, wherein the microwave absorbing phase of the carbon / cellulose microwave absorbing aerogel is graphene (GNS), that is, the carbon / cellulose microwave absorbing aerogel filled in the flexible resin shell in this embodiment is GNS / cellulose microwave absorbing aerogel.

[0085] The GNS / cellulose microwave-absorbing aerogel filled in the flexible resin shell of this embodiment includes the following preparation steps: First, a cellulose aerogel (i.e., CNT-0 / cellulose microwave-absorbing aerogel) without added microwave-absorbing agent is prepared using the method in Example 1. Then, the CNT-0 / cellulose microwave-absorbing aerogel is impregnated with aqueous dispersions of different graphene concentrations, with the graphene mass fractions in the dispersions being 0.75 wt.%, 1.25 wt.%, and 2.0 wt.%, respectively. The graphene loading on the aerogel is controlled by adjusting the graphene concentration in the dispersion. The impregnation time is 1–5 min, and then the GNS / cellulose microwave-absorbing aerogel is obtained by freeze-drying, with densities of 0.05 g / cm³. 3 0.06g / cm 3 0.08g / cm 3 .

[0086] The dielectric properties of the GNS / cellulose microwave absorbing aerogel prepared in this embodiment were then tested, and the test results are as follows: Figure 10 As shown, the real part of the dielectric constant of the microwave absorbing aerogel in this embodiment ranges from 5.95 to 7.15, and the imaginary part ranges from 5.15 to 13.27, with a ratio of real to imaginary parts of 0.82 to 1.98. Compared with Example 1, the aerogel material obtained in this embodiment has slightly larger real and imaginary values ​​of relative dielectric constant, and the change in the imaginary part is more significant, indicating a larger variation in the material's electromagnetic loss capability. Furthermore, compared with Example 2, the GNS / cellulose microwave absorbing aerogel prepared by impregnating GNS microwave absorbing agent in this embodiment also has larger real and imaginary dielectric constants because GNS easily aggregates on the cellulose surface to form a conductive network.

[0087] Based on the tested dielectric parameters, a three-dimensional microwave absorbing structure was designed using CST simulation software, and the geometric parameters were optimized. Simulation calculations showed that the microwave absorbing supercomposite material exhibited the best performance when an aerogel with a GNS mass fraction of 1.25 wt.% was filled in a periodic resin shell, achieving both impedance matching and electromagnetic loss mitigation. Therefore, a GNS / cellulose microwave absorbing aerogel with a GNS mass fraction of 1.25 wt.% was selected as the filler for the 3D-printed flexible resin shell, with a GNS to cellulose weight ratio of 0.16:1. The geometric parameters of the microwave absorbing supercomposite material are: p = 20 mm, H1 = 2.0 mm, H2 = 4.5 mm, W = 13 mm, d = 1.8 mm, D = 1.2 mm, R = 5.5 mm, and a total thickness of 8.3 mm.

[0088] Finally, a lightweight, flexible GNS / cellulose aerogel microwave absorbing supercomposite material was prepared, and its reflection loss curve is shown in the figure. Figure 11 As shown, in the range of 4.64 to 40 GHz, the reflection loss is less than -10 dB, more than 90% of the electromagnetic waves are absorbed, the effective absorption bandwidth reaches 35.36 GHz, and the relative bandwidth is 158.4%.

Claims

1. A lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material, characterized in that, The microwave absorbing supercomposite material includes a CNT / acrylic resin microwave absorbing material layer, on which microwave absorbing structural units are arrayed; the microwave absorbing structural unit includes a flexible resin shell, which is filled with carbon / cellulose microwave absorbing aerogel; the flexible resin shell is composed of a square shell and a baffle plate at the bottom of the square shell, and a circular hole is opened at the top of the square shell; The periodic dimension of the microwave absorbing structural unit is 10–20 mm; the thickness of the CNT / acrylic resin microwave absorbing material layer is 1.2–2 mm; the height of the carbon / cellulose microwave absorbing aerogel filled in the flexible resin shell is 4.5–5.5 mm, and the width of the carbon / cellulose microwave absorbing aerogel is 6–13 mm; the wall thickness of the square shell is 1.5–2.0 mm, the thickness of the surrounding plate is 1.0–1.2 mm, and the radius of the circular hole is 3–5.5 mm; The carbon / cellulose microwave absorbing aerogel was prepared by the following method: Carbon materials and cellulose are dispersed and mixed to form a carbon material / cellulose aqueous solution, which is then subjected to a gelation reaction to form a carbon material / cellulose hydrogel, and finally freeze-dried to produce a carbon / cellulose microwave absorbing aerogel. Alternatively, the carbon / cellulose microwave absorbing aerogel can be prepared by the following method: A cellulose aqueous solution is gelled to form a cellulose hydrogel, which is then freeze-dried to form a cellulose aerogel. The cellulose aerogel is then immersed in an aqueous dispersion of carbon materials and freeze-dried to form a carbon / cellulose microwave absorbing aerogel. The carbon material content in the carbon / cellulose aqueous solution is 0.2–2 wt%; the weight ratio of carbon material to cellulose in the carbon / cellulose aqueous solution is 0.16–0.3:1; the carbon material content in the carbon material aqueous dispersion is 0.75–2 wt%. The cellulose is short-fiber cellulose, and the carbon material is graphene and / or carbon nanotubes.

2. The lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material according to claim 1, characterized in that, The flexible resin shell is made of flexible acrylic resin by photopolymerization 3D printing.

3. The lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material according to claim 1, characterized in that, The density of the carbon / cellulose absorbing aerogel is 0.05–0.08 g / cm³. 3 It can achieve a large range of adjustment at 10 GHz, with the real part of the relative permittivity changing from 3.36 to 7.15, the imaginary part of the permittivity changing from 2.56 to 13.27, and the dielectric loss changing from 0.72 to 2.

08.

4. The lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material according to claim 1, characterized in that, The carbon / cellulose absorbing aerogel is either a carbon nanotube / cellulose absorbing aerogel or a graphene / cellulose absorbing aerogel.

5. The application of the lightweight, flexible, three-dimensional cellulose aerogel microwave absorbing supercomposite material of claim 1 as a lightweight, flexible, multifunctional microwave absorbing material that also absorbs electromagnetic waves, characterized in that... The microwave absorbing supercomposite material can absorb more than 90% of electromagnetic waves in the 4.36–40 GHz ultra-wideband electromagnetic wave range, with a relative bandwidth of 160.7%; the bending diameter of the microwave absorbing supercomposite material can reach 80 mm.

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