Method for constructing an equivalent electromagnetic parameter calculation model for non-woven / honeycomb absorbing structures

Abstract

This invention discloses a method for constructing an equivalent electromagnetic parameter calculation model for a nonwoven / honeycomb absorbing structure, which includes the following steps: S1, obtaining the dielectric constant ε of the honeycomb core material and honeycomb wall. h The dielectric constant ε of CNTs / CB / RGO / PU impregnation layer i By adding a polarization correction factor M and a structure correction factor N to the BG formula, ε h and ε i Establish an equivalent homogeneous BG model as shown in Equation (Ⅰ), and obtain the dielectric constant ε of the graphene / nonwoven composite material in S2. fill The model is then combined with the equivalent homogeneous model obtained in step S1 to establish the MG model, resulting in the homogeneous absorbing model shown in formula (II). This invention uses the BG and MG formulas to establish an equivalent electromagnetic parameter calculation model for the nonwoven / honeycomb absorbing structure. Based on the feedback comparison between the simulated and measured values ​​of the electromagnetic parameters of the honeycomb structure, a correction factor is innovatively introduced into the BG formula to improve the consistency between the calculated and actual values ​​of the equivalent electromagnetic parameters of the honeycomb structure.

Description

Technical Field

[0001] This invention relates to an electromagnetic parameter calculation model, specifically, to a method for constructing an equivalent electromagnetic parameter calculation model for a nonwoven / honeycomb absorbing structure. Background Technology

[0002] Electromagnetic parameters are crucial for the design and electromagnetic response behavior analysis of nonwoven / honeycomb absorbing materials. However, currently, the electromagnetic parameters of structural absorbing materials can only be measured using a limited number of methods, such as the free-space method and waveguide method. These methods are highly susceptible to errors due to factors such as sample fabrication and testing angle, leading to significant random errors and inconvenience in measurement. Therefore, in the design of structural absorbing materials, researchers tend to measure the electromagnetic parameters of certain easily measurable and accurate single media, and then calculate the equivalent electromagnetic parameters using effective medium theory. This allows for the optimization of structural material design and analysis of absorbing behavior.

[0003] Classical effective dielectric theories include Maxwell-Garnett (MG), Bruggeman (BG), Looyenga, and Monecke, which are primarily used to predict the effective electromagnetic parameters of binary or multi-component composites. Among these, the Maxwell-Garnett theory posits that the dielectric constant of a composite material is determined by the dielectric constant and volume percentage of each component. The composite material consists of spherical particles with an average radius of r, uniformly distributed throughout the matrix medium. The polarization of each sphere is assumed to be caused solely by the presence of an external field, neglecting multipolar effects caused by adjacent particles. Therefore, when the filler content in the matrix is ​​high, the spheres are no longer isolated, and interfacial polarization or the formation of a conductive network occurs, rendering the original MG theory inapplicable. Summary of the Invention

[0004] To address the problem that the effective electromagnetic parameters cannot be calculated using the MG model when the filler content in the matrix is ​​high, this invention provides a method for constructing an equivalent electromagnetic parameter calculation model for a nonwoven / honeycomb absorbing structure. This method constructs a uniform porous honeycomb model (BG model) as a prerequisite for MG formula calculation, while also solving the influence of the polarization anisotropy of carbon-based absorbers in the matrix and quantifying the gain of dielectric properties due to the unique hexagonal structure of the honeycomb.

[0005] To achieve the above objectives, the present invention provides a method for constructing an equivalent electromagnetic parameter calculation model of a nonwoven / honeycomb absorbing structure, wherein the nonwoven / honeycomb absorbing structure includes a honeycomb core material, a CNTs / CB / RGO / PU impregnation layer formed on the honeycomb wall of the honeycomb core material, and a graphene / nonwoven composite material filled in the honeycomb pores of the honeycomb core material.

[0006] The method for constructing the computational model includes the following steps:

[0007] S1. Obtain the dielectric constant ε of the honeycomb core material and honeycomb wall. h The dielectric constant ε of CNTs / CB / RGO / PU impregnation layer i By adding a polarization correction factor M and a structure correction factor N to the BG formula, ε h and ε i An equivalent homogeneous BG model is established as shown in Equation (Ⅰ).

[0008]

[0009] Where, ε h ε i and ε BG These refer to the dielectric constants of the honeycomb wall, the impregnated layer, and the BG model, respectively. h is the volume ratio of the cell wall in the BG model, M is the polarization correction factor, and N is the structure correction factor;

[0010] S2. Obtain the dielectric constant ε of the graphene / nonwoven composite material. fill This model is then combined with the equivalent homogeneous model obtained in step S1 to establish the MG model, resulting in the homogeneous absorbing model shown in formula (II).

[0011]

[0012] Where, ε BG ε fill and ε eff It is the dielectric constant of the BG model, the cell filler, and the MG equivalent dielectric, f. BG It is the volume ratio of the BG model in the cell.

[0013] Specifically, in step S1, the polarization correction factor M and the structure correction factor N are obtained by measuring the dielectric constant of the honeycomb wall and the impregnation layer, substituting it into formula (Ⅰ), and performing inverse calculation to obtain the values ​​of M and N.

[0014] Preferably, the dielectric constant of the honeycomb wall and the impregnation layer is measured by the following method: using the free space method, the upper and lower surfaces of the nonwoven fabric / honeycomb absorbing structure are taken as the incident ports, and the average value is taken after multiple tests to obtain the dielectric constant of the honeycomb wall and the impregnation layer.

[0015] Preferably, in step S1, the CNTs / CB / RGO / PU in the impregnation layer are regarded as individual dielectric loss particles, and the inner wall of the honeycomb core is equivalent to a uniform matrix, and the CNTs / CB / RGO / PU electromagnetic wave loss particles are uniformly dispersed on the inner wall of the honeycomb core.

[0016] Preferably, in the graphene / nonwoven composite material, graphene is uniformly dispersed in the nonwoven fabric and fills the gaps between the nonwoven fibers. The graphene attached to different fibers are interconnected to form a three-dimensional graphene structure, which can greatly reduce the aggregation of graphene and improve the dispersibility of graphene in the matrix material.

[0017] In the above technical solution, a porous graphene aerogel composite material is prepared by distributing graphene in a nonwoven fabric fiber matrix using nonwoven fabric as a template. The connections between graphene atoms can form a wide conductive network, and the resulting three-dimensional graphene structure will help the graphene "confining effect" influence electromagnetic waves.

[0018] Preferably, the graphene / nonwoven composite material is prepared by the following method: impregnating nonwoven fabric in a graphene oxide solution to uniformly disperse graphene oxide in the nonwoven fabric, and then reducing the graphene oxide in the nonwoven fabric to graphene.

[0019] Because graphene is generally insoluble in water and tends to aggregate in water, it is difficult to directly disperse graphene in nonwoven fabrics to form an ideal graphene / nonwoven fabric composite material. Therefore, graphene oxide (GO), which has good solubility, is used. The nonwoven fabric is impregnated in a graphene oxide solution to uniformly disperse the graphene oxide within the fabric. Then, the graphene oxide in the nonwoven fabric is reduced to water-insoluble graphene to obtain the graphene / nonwoven fabric composite material.

[0020] Graphene oxide (GO) was prepared using the Hummers method. During this process, negatively charged functional groups with phenolic and carboxyl groups are formed on the GO surface, inhibiting GO aggregation and allowing for relatively uniform dispersion in water. The hydrophilic nature of GO means that water molecules can more easily penetrate between the layers, widening the interlayer spacing to 0.6–1.2 nm. After stirring and sonicating in solution, individual GO sheets were exfoliated. Purified nonwoven fabric was then impregnated in the GO solution, ensuring uniform dispersion of GO within the fabric. Finally, the GO in the nonwoven fabric was reduced to water-insoluble graphene, and after washing and drying, the graphene / nonwoven fabric composite material was obtained.

[0021] Preferably, the concentration of the graphene oxide solution is 3–9 mg / mL.

[0022] The concentration of graphene in graphene / nonwoven materials is positively correlated with the concentration of GO solution. Under the same conditions, the higher the concentration of GO solution used in preparation, the higher the concentration of graphene in the graphene / nonwoven material. Therefore, the concentration of graphene in graphene / nonwoven composite materials can be adjusted by the concentration of GO solution during the preparation process to obtain good wave absorption performance.

[0023] Preferably, the thickness of the CNTs / CB / RGO / PU impregnation layer is 30–38 μm.

[0024] Preferably, the mass ratio of CNTs to CB / RGO in the CNTs / CB / RGO / PU impregnation layer is (1-4):1.

[0025] The electromagnetic wave loss mechanisms of the CNTs / CB / RGO / PU impregnation layer are mainly polarization loss and conduction loss, with CNTs-dominated conduction loss playing the most critical attenuation role. Analysis of the absorption performance of the absorbing honeycomb core revealed that the absorption performance of a single-layer absorbing honeycomb core first increases and then decreases with increasing CNT concentration. The optimal absorption performance, with an effective bandwidth of 7.4 GHz, is achieved when the CNTs:CB / RGO ratio is 3:1.

[0026] Preferably, the carbon nanotubes in the CNTs / CB / RGO / PU impregnation layer are multi-walled carbon nanotubes modified by strong acid oxidation. Short, straight CNTs exhibit better dispersibility, facilitating the control of impedance matching and dielectric properties of the composite material. Simultaneously, the surface of the strongly acid-oxidized CNTs contains numerous fine flocculent substances. This is due to the strong acid causing the C-C bonds on the outer layer of the multi-walled carbon nanotubes to break during the acidification process, forming graphene flakes. The presence of these graphene flakes may lead to multiple scattering of electromagnetic waves at this location, enhancing the electromagnetic wave attenuation capability of the CNTs absorber.

[0027] Short-cut CNTs formed after acid modification can exhibit good dispersibility through ultrasonic oscillation. This allows CNTs / CB / RGO / PU composites to leverage the advantages of multi-scale carbon-based materials, creating microstructural conditions for electromagnetic wave propagation loss and polarization loss.

[0028] In the CNTs / CB / RGO / PU impregnation layer, CBs are grafted onto RGO, forming stable chemical bonds between CBs and RGO. The smaller CBs adhere tightly to the RGO surface, exhibiting some degree of aggregation, but the aggregation sites are interconnected, forming multiple conductive links. Furthermore, the tight bonding of CBs to the RGO surface creates numerous heterogeneous interfaces, laying the structural foundation for interfacial polarization.

[0029] Preferably, the nonwoven / honeycomb absorbing structure has multiple layers, and from high to low, the mass ratio of CNTs to CB / RGO in the CNTs / CB / RGO / PU impregnation layer gradually increases, and the graphene content in the graphene / nonwoven composite material gradually increases.

[0030] Based on the Jaumamn layer structure design concept, a multilayer nonwoven / honeycomb composite material composed of a honeycomb absorbing structure was designed. This multilayer nonwoven / honeycomb composite material can be viewed as a multilayer structure consisting of an upper impedance matching layer and a lower strong absorption layer, but it is also an organic whole, a typical multi-scale absorbing material. Its excellent absorption performance is generated by the synergistic cooperation of multiple absorption modes at the micro, meso, and macro scales and from zero to three dimensions. By optimizing the concentration gradient combination of the nonwoven / honeycomb composite material and the carbon-doped honeycomb absorbing material in the multilayer absorbing structure, absorption across the entire wavelength range of 2–18 GHz can be achieved. Furthermore, the nonwoven filling enhances the compressive strength of the nonwoven / honeycomb composite material compared to the honeycomb absorbing material, resulting in excellent compressive strength.

[0031] Through the above technical solution, the present invention achieves the following beneficial effects:

[0032] This invention establishes an equivalent electromagnetic parameter calculation model for nonwoven / honeycomb absorbing structures using the Bruggeman and Maxwell-Garnett formulas. Based on the feedback comparison between the simulated and measured values ​​of the electromagnetic parameters of the honeycomb structure, a correction factor is innovatively introduced into the Bruggeman formula to address the influence of the polarization anisotropy of the carbon-based absorber in the matrix. The gain of dielectric properties due to the unique hexagonal structure of the honeycomb is quantified, thereby improving the consistency between the calculated and actual values ​​of the equivalent electromagnetic parameters of the honeycomb structure. Attached Figure Description

[0033] Figure 1 The equivalent dielectric constants of the 16 combinations of nonwoven / honeycomb absorbing materials prepared in Example 1 of this invention, with (a, c, e, f) real parts and (b, d, f, h) imaginary parts;

[0034] Figure 2 The equivalent dielectric constants of the four combinations of nonwoven / honeycomb absorbing materials prepared in Example 1 of this invention, (a) the real part of the dielectric constant, (b) the imaginary part of the dielectric constant;

[0035] Figure 3 yes Figure 2 Electromagnetic parameters of materials in the four samples: (a) Cloe-Cloe ring, (b) dielectric loss tangent.

[0036] Figure 4 RL is a single-layer nonwoven / honeycomb absorbing material, a absorbing honeycomb core and a graphene / nonwoven composite material, (ad) nonwoven / honeycomb absorbing material, (e) absorbing honeycomb core, (f) graphene / nonwoven composite material;

[0037] Figure 5It is a simulation model of non-woven fabric / honeycomb absorbing material, (a) unit structure, (b) periodic structure;

[0038] Figure 6 yes Figure 5 Simulation boundary conditions;

[0039] Figure 7 These are simulation results of the nonwoven / honeycomb absorbing material model;

[0040] Figure 8 These are the power-loss field distribution and equivalent impedance matching coefficients of the nonwoven / honeycomb absorbing material model, (ab)1-1, (cd)2-2, (ef)3-3, (gh)4-4;

[0041] Figure 9 The image shows the RL measurement of nonwoven / honeycomb composite materials. (a) Schematic diagram of nonwoven / honeycomb composite material preparation, (b) RL measurement using the bow method, and (c) RL test results.

[0042] Figure 10 The absorption performance of the double-layer nonwoven / honeycomb composite material is shown in Figure 1, where I represents the absorption peak frequencies (a) and (b) of the matching layer, and II and III represent the absorption peak frequencies (c) and (d) of the matching layer and their bandwidths.

[0043] Figure 11 The comparison of the microwave absorption performance of non-woven / honeycomb microwave absorbing materials and their sandwich structures is shown in (a) single layer and (b) double layer.

[0044] Figure 12 The comparison shows the compressive strength of nonwoven / honeycomb composite sandwich structures: (a) load-deformation curve of single-layer structure, (b) load-deformation curve of double-layer structure, (c) compressive strength of single-layer structure, and (d) compressive strength of double-layer structure.

[0045] Figure 13 This is a schematic diagram of the nonwoven / honeycomb absorbing structure described in this invention;

[0046] Figure 14 The microstructure of the absorbing honeycomb core prepared in Example 1 of this invention is shown in (ad) as the surface of the impregnation layer and (ef) as the cross-section. Detailed Implementation

[0047] The specific embodiments of the present invention will be described in detail below with reference to examples. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0048] like Figure 13As shown, the nonwoven / honeycomb microwave absorbing structure of the present invention includes a honeycomb core material 1, a CNTs / CB / RGO / PU impregnation layer 2 formed on the honeycomb wall of the honeycomb core material 1, and a graphene / nonwoven composite material 3 filled in the honeycomb pores of the honeycomb core material 1.

[0049] In the following embodiments of the present invention, the equivalent electromagnetic parameter is the equivalent dielectric constant.

[0050] Example 1

[0051] The fabrication method for nonwoven / honeycomb absorbing structures includes the following steps:

[0052] Step 1: Preparation of graphene / nonwoven composite material

[0053] (1) Preparation of graphene oxide solution

[0054] Graphene oxide (GO) solution was prepared using the Hummers method. Graphite powder (6 g) and 1.5 g NaNO3 were added to concentrated H2SO4 (360 mL, 70%) and stirred in an ice-water bath for 1 h. Then, 18 g KMnO4 powder was slowly added. Due to the significant heat release during stirring, the addition rate had to be slow and the temperature kept below 5 °C. After stirring for two hours, the mixture was dark green. The solution was then heated to 30 °C and stirred for another 2 h. Deionized water (600 mL) was added to the solution and stirred for 0.5 h, followed by the addition of H2O2 (5%, 150 mL) and stirring for another 0.5 h. After precipitation for 12 h, the supernatant was collected, and the precipitate was washed with hydrochloric acid solution and deionized water, centrifuged, and washed six times. Finally, the precipitate was dissolved in deionized water and stirred until homogeneous to obtain the GO solution.

[0055] (2) Nonwoven fabric sample

[0056] Cut the nonwoven fabric to size 180×180×5mm and soak it in anhydrous ethanol for 1 hour. Then place it in a constant temperature drying oven and keep it at 80℃ for 2 hours before taking it out for use.

[0057] (3) GO concentration measurement

[0058] Fold aluminum foil into a square foil trough, weigh it, and record the initial mass. Place 5 mL of GO solution into the foil trough, allow it to air dry, and then weigh it. Calculate the GO solution concentration based on the mass difference.

[0059] (4) Preparation of graphene / nonwoven composite material

[0060] Take appropriate amounts of graphene oxide solution and adjust the concentration to 3, 5, 7, and 9 mg / mL with deionized water. After uniform stirring, add hydroquinone at a graphene-to-hydroquinone mass ratio of 1:5 and stir for another 30 minutes. Immerse the nonwoven fabric in the mixture for 1.5 hours. Then seal with plastic wrap and transparent tape, place in a constant temperature drying oven, and keep at 100℃ for 10 hours. After cooling, remove the nonwoven fabric and immerse it in deionized water for 5 hours to dissolve the residual hydroquinone. Finally, dry the sample to obtain the graphene / nonwoven fabric composite material.

[0061] Step 2: Fabrication of Multi-Scale Carbon-Based Absorbing Cellular Cores

[0062] (1) Acidification modification of CNTs

[0063] A mixed strong acid was prepared by mixing concentrated H₂SO₄ and HNO₃ in a volume ratio of 3:1. CNT powder at a mass ratio of 1:80 to the strong acid mixture was added. After stirring in an ice bath for 0.5 hours, KMnO₄ powder was slowly added to the solution, maintaining the solution temperature below 5°C. The solution was then placed in a vacuum furnace set to 30°C and maintained for 4 hours. After removal, deionized water and 5% H₂O₂ were added, and the mixture was slowly stirred. The solution was then subjected to multiple high-speed centrifugations and deionized washings until neutral. Finally, vacuum freeze-drying was performed to obtain fluffy modified CNT powder.

[0064] (2) Preparation of CB / RGO complex

[0065] In the graphene oxide solution preparation process, graphite powder (6g) and NaNO3 (1.5g) were first added to concentrated H2SO4 (360mL) and stirred in an ice-water bath for 1 hour. Then, 18g of KMnO4 powder was slowly added. Due to the large amount of heat released during stirring, the addition rate had to be slow and the temperature kept below 5°C. After stirring for two hours, the mixture turned a deep greenish-black color. The solution was then heated to 30°C and stirred for another two hours. Deionized water (600mL) was added to the solution and stirred for 0.5 hours, followed by the addition of H2O2 (150mL, 5%) and stirring for another 0.5 hours. After precipitation for 12 hours, the supernatant was discarded, and the precipitate was washed with hydrochloric acid solution and deionized water. The precipitate was obtained by centrifugation, and the washing was repeated six times. Finally, the precipitate was dissolved in deionized water and stirred to prepare a 500mL GO solution.

[0066] CB / RGO was prepared using a liquid-phase reduction method, where GO was reduced to RGO while CB was grafted onto the RGO. First, 2.4 g of CB powder and 1.2 g of CTAB (hexadecyltrimethylammonium bromide) were placed in 500 mL of deionized water and stirred with a magnetic stirrer for 30 min followed by ultrasonic agitation for 1 h to uniformly disperse the CB and CTAB. Then, 100 mL of a homogeneous GO solution was added to the CB and CTAB mixture, and ultrasonic agitation was continued for 1 h. Hydroquinone was then added at a GO to hydroquinone mass ratio of 1:5, and the mixture was stirred for another 30 min. The beaker was then sealed with plastic wrap. The resulting mixture was transferred to a drying oven and heated at 100 °C for 12 h, followed by high-speed centrifugation and washing four times with deionized water. Finally, the mixture was freeze-dried under vacuum to obtain the CB / RGO composite material.

[0067] (3) Preparation of carbon-based absorbing honeycomb core

[0068] The honeycomb core has dimensions of 180×180×5mm, a pore size of 2.75mm, and a wall thickness of 0.1mm.

[0069] Add CB / RGO and CNTs to 200 mL of waterborne polyurethane in the specified ratio, then add deionized water to adjust the solution to 800 mL. Place the beaker on a magnetic stirrer and stir in a 30°C water bath. Add 10 g of defoamer and 10 g of dispersant sequentially. Use a high-speed mixer to thoroughly mix CNTs and CB / RGO. After 40 min, remove the CNTs / CB / RGO mixture and sonicate it for 30 min to obtain a CNTs / CB / RGO solution, forming a homogeneous slurry. To avoid the influence of gravity causing resin flow and thickness gradients on the honeycomb core wall, place the top and bottom sides of the honeycomb core into the slurry for 5 min each, then vertically pull it out and place it in a constant temperature drying oven. Pre-curing is performed at 120°C for 10 min, during which the honeycomb core is rotated 3–5 times. Finally, the CNTs / CB / RGO honeycomb microwave absorbing composite material is cured at 100°C for 30 min. Repeat the impregnation process to obtain the designed coating thickness. The filling ratio of CNTs / CB / RGO to PU is 4.7%, and the mass ratio of CNTs to CB / RGO is 1:1, 2:1, 3:1 and 4:1.

[0070] The surface and cross-section of the impregnation layer of the absorbing honeycomb core were characterized using scanning electron microscopy. Figure 14(ad) shows the surface state of the impregnated layer. While the spherical CB and the delicate RGO film are difficult to discern from the image, the tubular CNTs on the surface of the impregnated layer are easily distinguishable. Furthermore, as the proportion of CNTs increases, the number of CNTs on the surface of the impregnated layer gradually increases, resulting in a rougher surface morphology. Typically, nanoparticles with high surface energy are prone to aggregation, especially tubular nanomaterials like CNTs. In the microscopic state, the interaction forces between particles can cause entanglement, leading to uneven dispersion in the matrix and difficulty in forming a continuous and stable conductive network, thus affecting their electromagnetic wave absorption performance. However, the surface state of the impregnated layer shows that even when the mass ratio of CNTs to CB / RGO reaches 4:1, the dispersion of CNTs remains excellent. This indicates that the short-cut CNTs formed after acid modification can achieve good dispersion through ultrasonic oscillation. This is beneficial for the CNT / CB / RGO / PU composite material to leverage the advantages of multi-scale carbon-based materials, creating the microstructural conditions necessary for electromagnetic wave transmission and polarization losses. Figure 14 (ef) shows a cross-sectional morphology of the honeycomb impregnation layer, which can be seen to show that the impregnation layer is tightly attached to the honeycomb core, with a thickness ranging from 30 μm to 38 μm.

[0071] Step 3: Preparation of multi-scale carbon-based nonwoven / honeycomb microwave absorbing materials

[0072] The graphene / nonwoven composite material obtained in step one is punched into hexagons with the same pore size as the honeycomb core in step two, and then filled into the honeycomb pores of the carbon-based microwave absorbing honeycomb core obtained in step two to obtain a multi-scale carbon-based nonwoven / honeycomb microwave absorbing material.

[0073] There are 16 combinations of multi-scale carbon-based nonwoven / honeycomb microwave absorbing materials, which are composed of 4 types of honeycomb cores prepared in step 2 and 4 types of nonwoven composite materials prepared in step 1 (as shown in Table 1).

[0074] Table 1. Design Details of Nonwoven Fabric / Honeycomb Absorbing Structure

[0075]

[0076]

[0077] Example 2: Calculation of the dielectric constant of nonwoven / honeycomb absorbing material

[0078] Using a carbon-based microwave absorbing honeycomb core with a CNTs to CB / RGO mass ratio of 2:1 as the test sample, and based on the known structural parameters of the honeycomb core (side length 2.75 mm, thickness 100 μm, average impregnation layer thickness 34 μm), f can be calculated. h and f BG The volume ratios are 0.561 and 0.146.

[0079] The dielectric constant of the honeycomb wall of the test sample was measured using the free space method. The real part was 1.5 and the real part was 0. The dielectric constant of the impregnated layer in the frequency range of 2 to 18 GHz is shown in Table 2.

[0080] Table 2. Results of dielectric constant determination of the impregnated layer

[0081]

[0082]

[0083] Substituting the above dielectric constant into formula (Ⅰ), we can complete ε. eff The inverse operation yields the values ​​of M and N as 7.2 and 0.65, respectively.

[0084]

[0085] Where, ε h ε i and ε BG These refer to the dielectric constants of the honeycomb wall, the impregnated layer, and the BG model, respectively. h is the volume ratio of the cell wall in the BG model, M is the polarization correction factor, and N is the structure correction factor.

[0086] The equivalent dielectric constants of the 16 combinations of nonwoven / honeycomb absorbing materials in Table 1 are calculated using formula (II). Figure 1 As shown in (ah), the dielectric constant of the nonwoven / honeycomb absorbing material generally increases with the increase of the concentration of loss particles in the honeycomb and nonwoven fabrics. The dielectric constant of 1-1, with the lowest carbon nanoparticle absorber concentration, has a real part between 9.1 and 4.5, and an imaginary part between 4.3 and 2.6; while the dielectric constant of 4-4, with the highest carbon nanoparticle absorber concentration, has a real part between 14.8 and 6.3, and an imaginary part between 8.3 and 4.7. The dielectric constant values ​​are significantly higher than those of honeycomb structure absorbing materials and nonwoven composite materials, indicating a substantial improvement in dielectric performance when combined.

[0087]

[0088] Where, ε BG ε fill and ε eff It is the dielectric constant of the BG model, the cell filler, and the MG equivalent dielectric, f. BG It is the volume ratio of the BG model in the cell.

[0089] Example 3: Verification of the correctness of the equivalent dielectric constant calculation

[0090] To verify the accuracy of the equivalent dielectric constant calculation and to clarify the electromagnetic wave loss mode and absorption mechanism of nonwoven / honeycomb absorbing materials, further theoretical analysis, simulation analysis and sample measurement analysis were conducted on samples 1-1, 2-2, 3-3 and 4-4.

[0091] Figure 2 The theoretical dielectric constants of the four samples, calculated using the above formula, are given by the formula for the frequency range of 2–18 GHz. At 2 GHz, the ε' and ε" of the four samples, in order of increasing concentration, are 1-1, 2-2, 3-3, and 4-4, respectively. It can be seen that the dielectric constants of the four samples do not increase uniformly, but rather increase in a gradient manner. This phenomenon indicates that as the concentration of loss particles increases, the gain of the resonant cavity formed by the cellular unit structure on the dielectric performance continuously increases. Even the dielectric constant of sample 4-4, which decreases to its lowest value at 18 GHz due to polarization relaxation, is still much higher than the maximum values ​​of samples 1-1 and 2-2. The real part ε' ranges from 6.2 to 14, and the imaginary part ε" ranges from 8.3 to 4.6. Such excessively high dielectric performance is obviously unsuitable for structural absorbing materials. According to the formula ε=ε'+iσω, a high ε" of dielectric material means a strong conductivity σ, which leads to a more obvious skin effect of the material. Electromagnetic waves cannot enter the interior of the structural material and are reflected in large quantities at a lower thickness, so the structural absorbing material cannot play its role as a resonant structure, thus reducing the absorbing performance of the material.

[0092] Figure 3 The ε'-ε" vector curves are for the four samples. According to Debye relaxation theory, the relationship between ε' and ε" can be described by formula (Ⅲ), where ε s and ε ∞ These represent the static dielectric constant and the dielectric constant at infinite frequency, respectively. Therefore, the semicircles in the vector diagrams of ε' and ε" can signify the existence of the polarization relaxation process; the more semicircles, the stronger the polarization relaxation, and the longer the line tail, the stronger the conduction loss. This provides a theoretical basis for further understanding the polarization behavior of nonwoven / honeycomb absorbing materials during electromagnetic absorption. Figure 3 As shown, semicircles appear in all four images, proving that polarization occurs in all four samples during electromagnetic wave loss. From... Figure 3 As shown in (a), the number of CLEE rings in 1-1 and 2-2 remains consistent at 3. However, as the concentration of loss particles increases, the number of CLEE rings in 3-3 and 4-4 decreases to 2 and 1 respectively, and the thread tails become longer. This indicates that an excessively high concentration of loss particles in the nonwoven / honeycomb absorbing material weakens its polarization loss and enhances its conduction loss.

[0093]

[0094] Figure 3(b) shows that the dielectric loss intensities of the samples, from low to high, are 1-1, 2-2, 3-3, and 4-4. Since the increased concentration of loss particles weakens the polarization loss of the material, the enhancement of dielectric loss can only originate from conduction loss. This phenomenon may be caused by the increased graphene concentration in the graphene / nonwoven composite material. While enhancing the dielectric properties of the graphene / nonwoven composite material, it also reduces the material's transmittance, making it difficult for electromagnetic waves to penetrate the nonwoven fabric and reach the honeycomb wall. The electromagnetic wave loss function of the honeycomb structure's absorbing material cannot be fully utilized, which will simultaneously affect the gain of the resonant cavity on electromagnetic wave loss. Furthermore, the conductive network formed by the interconnection of graphene nanomaterials exhibits good conductivity under the quantum tunneling effect, and its metallic properties cause a large amount of electromagnetic waves to be reflected from the surface of the nonwoven / honeycomb absorbing material.

[0095] (I) Absorption performance of single-layer non-woven fabric / honeycomb absorbing structure

[0096] According to transmission line theory, the electromagnetic wave absorption performance of an absorber can be calculated using its complex permittivity and complex permeability. The relevant formulas are as follows:

[0097]

[0098]

[0099] Z in the formula in This refers to the input impedance of the absorber, where f represents the frequency of the electromagnetic wave, d is the thickness of the absorber, and c is the velocity of the electromagnetic wave in a vacuum. The complex permittivity ε of the absorber... r =ε'+jε" can be calculated from the equivalent permittivity. Since carbon-based materials are considered to be completely dielectric loss materials and do not have magnetic loss on electromagnetic waves, the influence of permeability is not discussed in this paper. It is assumed that the real part of the permeability μ' = 1 and the imaginary part μ" = 0, and the complex permeability u r =μ'+jμ"=1.

[0100] Figure 4 (ad) shows the RL curves of 16 nonwoven / honeycomb absorbing materials in the range of 2–18 GHz. Figure 4Figures (e) and (f) show the RL curves of four single-layer honeycomb structure absorbing materials and four single-layer graphene / nonwoven fabric materials, which comprise these 16 materials. The figures show that when the honeycomb structure absorbing material is selected and the graphene / nonwoven fabric filling is adjusted, the RL peak of the sample shifts to lower frequencies as the graphene concentration in the nonwoven fabric filling the honeycomb increases. Simultaneously, the absorption peaks of the nonwoven fabric / honeycomb absorbing materials are located between the peaks of the honeycomb structure absorbing material and the nonwoven fabric material, without exhibiting a double-peak phenomenon due to the combination of the two materials. This indicates that the nonwoven fabric / honeycomb absorbing material is not a simple superposition of two composite materials, but rather forms an organic whole.

[0101] like Figure 4 As shown in (a), the absorption performance of nonwoven / honeycomb absorbing materials 1-1 to 1-4 exhibits a trend of first increasing and then decreasing. Sample 1-1 has a peak absorption resistance (RL) of -35 dB and an effective absorption bandwidth of 9.3 GHz (6.4–15.7 GHz). Sample 1-2 has a RL peak value 2 dB lower than 1-1, with an RL of -33 dB at 10.0 GHz, and its absorption bandwidth of 11.8 GHz has increased by 2.5 GHz. Samples 1-3 and 1-4 have absorption bandwidths of 8.8 GHz and 6.2 GHz, respectively, with absorption peak values ​​of -23 dB and -17 dB, showing a significant decrease in absorption performance compared to sample 1-2. This phenomenon indicates that the dielectric properties of 1-3 and 1-4 are too high, resulting in a low impedance matching degree and thus a decrease in absorption performance. Figure 4 (b) shows that, with honeycomb sample 2 as the external structure, the absorption effect of the sample gradually decreases as the dielectric properties of the graphene / non-woven fabric filling the honeycomb cells gradually increase. Sample 2-1 exhibits the best absorption effect, with a maximum RL intensity of -36 dB and an absorption bandwidth of 12.2 GHz. Furthermore, the trend of the RL value of sample 2-1 at 18 GHz (-16 dB) and the curve indicates that this sample still maintains good absorption performance at electromagnetic wave frequencies exceeding 18 GHz. Figure 4 As shown in (c) and (d), when honeycomb samples 3 and 4 are combined with nonwoven fabric, the microwave absorption performance is not significantly improved compared to honeycomb structure microwave absorbing materials and nonwoven fabric composites. In fact, for samples 3-3, 3-4, 4-3, and 4-4, the microwave absorption performance is even lower than that of their constituent materials. In summary, single-layer nonwoven fabric / honeycomb microwave absorbing materials are suitable for low-filling carbon-based honeycomb cores and graphene / nonwoven fabric composites. Excessive filling can easily lead to a low impedance matching degree, affecting the microwave absorption performance.

[0102] To verify the correctness of the equivalent calculation and to explore the electromagnetic wave loss mechanism of the nonwoven / honeycomb absorbing material, CST (CST MICROWAVE STUDIO·2016) software was used for modeling and simulation. The structure of a single nonwoven / honeycomb unit includes a honeycomb core, impregnated inner / outer layers, and graphene / nonwoven material. For example... Figure 5 As shown in (a), a hexagonal structure is constructed in a cylindrical manner. Let the side length of the honeycomb core be *a* and the thickness be *d1*. Then the outer diameter of the honeycomb core is equal to 2*a*, and the inner diameter is... If the thickness of the inner and outer impregnation layers is the same as d2, then the outer diameter of the inner impregnation layer is... Inner diameter is The outer diameter and inner diameter of the outer layer are respectively And a; the outer diameter of the graphene / nonwoven fabric is The inner diameter is 0. As mentioned earlier, the honeycomb core side length a = 2.75 mm, thickness d1 = 100 μm, and the average thickness d2 of the impregnated layer is 34 μm. Substituting these parameters, the nonwoven / honeycomb unit structure can be established. The unit structure is then copied and cut to obtain the model structure used for simulation. Figure 5 (b)).

[0103] Nonwoven / honeycomb absorbing material models 1-1, 2-2, 3-3, and 4-4, each consisting of a honeycomb sample (H1, H2, H3, H4) and a nonwoven fabric sample (N1, N2, N3, N4), were selected as the research objects. The dielectric constants of four CNTs / CB / RGO / PU impregnation layer materials, graphene / nonwoven fabric, and aramid paper fiber were loaded into the corresponding structures. Then, as follows... Figure 6 The boundary conditions are set in the X, Y, and Z axes as shown. The positive and negative X and Y axes are set to Unit cell, the negative Z axis to electric (Et=0), and the positive Z axis to Open (add space). Therefore, during the simulation, the nonwoven / honeycomb model is infinitely replicated to the electromagnetic field boundary in the X and Y axes. The excitation port of the electromagnetic wave is Z... max Electromagnetic waves are incident at 15° above the Z-axis of the model, and propagate at a loss within the model. min Completely reflected. S in the scattering (S) parameter. Zmax(1),Zmax(1) This means that from Z max The difference in electromagnetic wave intensity between the incident and emitted waves at the port is the reflection loss RL of the entire model.

[0104] Simulation results ( Figure 7The simulation results show that Model 1-1 has an absorption bandwidth of 11.8 GHz (6.2–18 GHz), with an absorption peak at 10.1 GHz and a peak value of -31 dB. Model 2-2 has an absorption bandwidth of 12.1 GHz (5.9–18 GHz), with an absorption peak at 9.3 GHz and a peak value of -42 dB. It can be seen that the simulated absorption bandwidth, absorption intensity, and frequency of the absorption peak of these two models are highly consistent with the equivalent calculation results. The absorption bandwidths of Models 3-3 and 4-4 are 7.4 GHz (6.2–13.6 GHz) and 4.6 GHz (5.8–9.4 GHz), respectively. However, the calculation results show that the absorption bandwidths of 3-3 and 4-4 are 7.8 GHz (6.3–14.1 GHz) and 4.0 GHz (6.2–10.2 GHz), differing by 0.4 GHz and 0.6 GHz, respectively. This shows that the error between the calculated and simulated results increases with the increase of the absorber concentration in the honeycomb structure absorbing material and the graphene / non-woven composite material. This is related to the inherent characteristics of the MG formula; as the filler content in the composite matrix increases and the interparticle spacing decreases, the multipole effect becomes more pronounced, leading to an increase in the error of the equivalent calculation. However, the calculated and simulated results show that the absorption performance of the samples, from best to worst, is 2-2, 1-1, 3-3, and 4-4, with a completely consistent overall trend.

[0105] To further summarize the response characteristics between nonwoven / honeycomb absorbing materials and electromagnetic waves, and to analyze the absorption mechanism of nonwoven / honeycomb absorbing materials, a "power loss" field monitor was set at the frequency points where the absorption peaks of the four models appeared (10.1 GHz, 9.3 GHz, 8.6 GHz, and 7.4 GHz). The equivalent impedance matching Z-axis of each model in the range of 2–18 GHz was also analyzed. eff Perform the calculation.

[0106] As shown in equation (4.8)

[0107]

[0108] Wherein, S 11 =S Zmax(1)，Zmax(1) S 21 =S Zmin(1)，Zmax(1) Since the boundary in the negative Y-axis direction is set to electric (Et = 0), S 21 =0, equation (4.9) can be simplified to:

[0109]

[0110] Ideal impedance matching is a necessary condition for an absorber to have good absorption performance. Specifically, Z eff The real part should be close to 1, and the imaginary part should be close to zero. The closer the imaginary part is to zero, the better the impedance matching.

[0111] Figure 8 (a, c, e, g) shows the intensity of electromagnetic field energy loss at various parts of the model at a specific frequency; the darker the color, the higher the energy loss intensity. Figure 8 As shown in (a), at 10.1 GHz, the surface color of the nonwoven / honeycomb absorbing material model 1-1 is lighter than the internal color, indicating that the energy loss intensity on its surface is very low. Most of the electromagnetic waves pass through the model surface and enter the interior, where they are lost. Simultaneously, it can be noted that the energy loss of the nonwoven fabric in the nonwoven / honeycomb absorbing material is relatively weak, and the electromagnetic field energy loss is mainly concentrated on the honeycomb walls. Combined with... Figure 8 (b) Z eff The curves show that Model 1-1 has good impedance matching performance in the frequency range of 2–18 GHz, especially around 10 GHz. eff The real and imaginary parts are both approximately 0.5, which leads to the appearance of the absorption peak.

[0112] Figure 8 (c) shows the electromagnetic wave energy loss distribution of Model 2-2 at 9.3 GHz. Compared with Model 1-1, the concentration of electrically dissipating particles on both the honeycomb and nonwoven fabric in Model 2-2 increases simultaneously, and the electromagnetic field energy loss intensity of the nonwoven fabric increases significantly. However, the electromagnetic field energy loss of the honeycomb structure absorbing material decreases. This may be because the dielectric properties of the nonwoven composite material are enhanced, which increases the dielectric loss capability of the nonwoven fabric while reducing the transmittance, thus reducing the electromagnetic waves propagating to the honeycomb wall. Furthermore, from... Figure 8 As shown in (d), the impedance matching Z of Model 2-2 is... eff The impedance matching performance remained within a good range of 2–18 GHz and was not reduced due to the increase in electrical loss particles.

[0113] Figure 8 (e) shows the electromagnetic wave energy loss distribution of Model 3-3 at 8.6 GHz. At this point, the energy loss is mainly concentrated on the nonwoven composite material, with no significant energy loss on the honeycomb walls. It can be inferred that this phenomenon is due to the excessively strong dielectric properties of the nonwoven composite material, causing the electromagnetic field energy loss to be mainly concentrated on the outer layer of the nonwoven / honeycomb absorbing material and the nonwoven composite material. The honeycomb structure absorbing material has difficulty attenuating electromagnetic wave energy, which can also be seen from... Figure 3 The reduction in the Cole-Cloe ring is evident in (a). Figure 8 As can be seen in (f), compared with 2-2, the impedance matching curve of 3-3 fluctuates more significantly, the impedance matching performance is poorer, and the difficulty for electromagnetic waves to enter the model is increased. According to Figure 3(b) The dielectric loss tangent curve of the composite material shows that the dielectric loss capability of the nonwoven / honeycomb absorbing material increases with the increase of the loss particle concentration in the honeycomb structure absorbing material and the nonwoven composite material. As the model with the strongest dielectric loss capability, Model 4-4 is limited by impedance matching, and its energy loss can only exist on the surface of the model.

[0114] like Figure 9 As shown in (a), to further verify the correctness of the equivalent calculation and simulation results, four types of nonwoven / honeycomb absorbing materials, namely 1-1, 2-2, 3-3, and 4-4, were prepared in this paper. The results were then analyzed using a vector network analyzer (…). Figure 9 (b) The microwave absorption performance of four nonwoven / honeycomb absorbing materials was tested. Figure 9 (c) shows that the best microwave absorption performance is achieved by non-woven / honeycomb microwave absorbing materials 2-2 and 1-1, followed by 3-3 and 4-4, which is completely consistent with the results of equivalent calculations and simulations. Regarding the microwave absorption performance of specific samples, excluding limitations in the fabrication process and testing errors, the results are basically consistent with the calculation and simulation results.

[0115] In summary, nonwoven / honeycomb absorbing materials composed of honeycomb structure absorbing materials with relatively weak dielectric properties and nonwoven composite materials can possess excellent electromagnetic wave absorption performance. In particular, sample 2-2, prepared from a nonwoven fabric with a CNTs:CB / RGO ratio of 2:1 and a GO concentration of 5 mg / mL, exhibits an absorption bandwidth of 12.3 GHz and a reflection loss intensity of -37 dB. Furthermore, the trend of the curves shows that it still maintains good absorption performance in the frequency range exceeding 18 GHz, making it a novel structural material with great research and application potential. On the other hand, the comparison between simulation, measured, and calculated results demonstrates that the modified MG model established in this invention can effectively calculate the equivalent electromagnetic parameters of honeycomb composite materials.

[0116] (II) Double-layer carbon-based nonwoven fabric / honeycomb absorbing material

[0117] Based on the Jaumamn layer structure design concept, four types of nonwoven / honeycomb absorbing materials were stacked according to a concentration gradient to prepare a double-layer nonwoven / honeycomb absorbing material. Samples 1-1, 2-2, 3-3, and 4-4 were named I, II, III, and IV, respectively, and the stacking scheme is shown in Table 3.

[0118] Table 3. Double-layer non-woven fabric / honeycomb absorbing material stacking scheme

[0119]

[0120]

[0121] The microwave absorption performance of double-layer nonwoven / honeycomb absorbing materials in the frequency range of 2–18 GHz was tested using the bow-shaped method. Figure 10 (a) and (c) respectively show the RL of double-layer nonwoven / honeycomb absorbing materials samples I-II, I-III, I-IV and II-III, II-IV, III-IV. Figure 10 As shown in (a), all three samples achieved full-frequency absorption within the 2–18 GHz range, and still exhibited an electromagnetic wave loss intensity of approximately -18 dB at 18 GHz, showing a trend towards higher frequencies. Samples I-II demonstrated the best absorption performance within the 2–18 GHz range, possessing the highest average reflection loss among the three, with a maximum reflection loss of -22 dB. Simultaneously, it was observed that as the dielectric properties of the second layer of nonwoven / honeycomb absorbing material increased, the absorption peak shifted to lower frequencies, and a triple-peak pattern appeared when the second layer was nonwoven / honeycomb absorbing material IV. Furthermore, as... Figure 10 (b) The peak frequencies of samples I-II are 4.2 GHz and 13.9 GHz, respectively; the peak frequencies of samples I-III are 3.8 GHz and 12.8 GHz, respectively; and the peak frequencies of samples I-IV are 3.1 GHz, 10.4 GHz, and 16.7 GHz, respectively. There is an approximately three-fold relationship between the frequencies of the first and second peaks, while the frequency of the third peak is approximately five-fold relative to the frequency of the first peak. This shift of peaks to lower frequencies and the frequency relationships between peaks perfectly conform to the relevant descriptions of the quarter-wavelength theory.

[0122] Figure 10 (c) The reflection loss curves of double-layer nonwoven / honeycomb absorbing material samples II-III, II-IV, and III-IV in the frequency range of 2–18 GHz are shown. It can be seen from the figure that when II and III are used as the first layer of the double-layer nonwoven / honeycomb absorbing material, the overall absorption performance decreases. Figure 10As shown in (d), only sample II-III maintained full-band absorption from 2 to 18 GHz, but its absorption intensity was significantly reduced. Samples II-IV and III-IV had effective absorption bandwidths of only 9.3 GHz and 5.0 GHz, respectively. This demonstrates that the dielectric properties of the upper nonwoven / honeycomb absorbing material have a significant impact on its overall absorption performance. An upper material with excessively strong dielectric properties exhibits poor impedance matching with air, increasing the difficulty for electromagnetic waves to penetrate the material and thus reducing absorption performance. The decrease in absorption performance from sample II-III to II-IV also suggests that changing the lower layer while keeping the upper layer constant also affects impedance matching, causing some electromagnetic waves that have passed through the upper layer to be reflected again and not adequately attenuated when reaching the lower layer. This analytical method is only suitable for qualitative analysis of structurally similar Jaumamn layer structures; it cannot separate two completely Jaumamn layer structures as independent entities. For example, sample III-IV, composed of a combination of single-layer nonwoven / honeycomb absorbing materials III and IV, exhibits a completely different absorption performance compared to III, even showing a decrease compared to III. However, if the two layers are independent entities, the upper layer absorbs electromagnetic waves first, and after the electromagnetic waves pass through the upper layer, the lower layer absorbs them. In this case, the absorption performance of the double-layer III-IV is definitely stronger than that of the single-layer sample III. Therefore, it can be concluded that the double-layer nonwoven / honeycomb absorbing material can be viewed as a double-layer structure composed of an upper impedance matching layer and a lower strong absorption layer, but it is also an organic whole.

[0123] Based on the Jaumamn layer structure design concept, the double-layer nonwoven / honeycomb absorbing material, composed of honeycomb structure absorbing material and graphene / nonwoven composite material, is a typical multi-scale structure absorbing material. Its excellent absorbing performance is generated by the synergistic cooperation of multiple absorbing modes at three scales (micro, meso, and macro) and multiple dimensions (from zero to three).

[0124] (1) The conductive network constructed by the three carbon-based materials at the microscopic level provides the conditions for the conduction loss of electromagnetic waves in the impregnation layer and non-woven fabric. The graphene fragments on the surface of the modified carbon nanotubes and the porous three-dimensional structure formed between the non-woven fabric and graphene cause multiple scattering of electromagnetic waves.

[0125] (2) Structural defects and functional groups on the surfaces of acidified carbon nanotubes and graphene become polarization centers under the influence of an electromagnetic field, leading to dipole polarization. Simultaneously, heterogeneous interfaces formed by the contact between the three carbon-based materials can also exhibit interfacial polarization under the influence of an electromagnetic field. Furthermore, interfacial polarization can occur at points where there are changes in the medium between nonwoven fabric and honeycomb walls, or between layers. The three-dimensional structure constructed from nonwoven fabric and graphene can locally create a "confining effect," trapping electromagnetic waves within the structure and causing them to be reflected multiple times until they are completely converted into heat and dissipated.

[0126] (3) The non-woven / honeycomb absorbing material can be regarded as an absorber composed of multiple honeycomb structure absorbing material units and graphene / non-woven particles filled in them. The impregnation layer on the surface of the honeycomb structure absorbing material unit is equivalent to a conductive wall surrounding the graphene / non-woven particles, and the graphene / non-woven material adds a "porous cover" to the top and bottom of this surrounding conductive wall, thus forming a semi-closed resonant cavity. The electromagnetic waves in the resonant cavity undergo a series of actions such as reflection, refraction and scattering between the impregnation layer and the graphene / non-woven fabric. During this process, a large amount of electromagnetic waves are lost. Except for some "escaped" electromagnetic waves that are reflected back to the air, the electromagnetic waves that are not lost will enter the next layer and be further lost.

[0127] (4) Following the Jaumamn layer structure design concept, a gradient dielectric impedance structure is adopted to meet the characteristic of continuous change of dielectric layer impedance and increase of loss, thereby realizing the resonant absorption of the structural absorbing material in a wider frequency range. The good impedance matching between the upper layer and the air allows more electromagnetic waves to enter the interior of the material. After initial loss, they enter the lower layer with stronger dielectric properties. The electromagnetic waves reflected in the lower layer are lost by the upper layer or reflected back to the lower layer again. This cycle continues until the electromagnetic waves entering are completely lost.

[0128] (5) Quarter-wavelength cancellation is also an important method for reducing electromagnetic wave loss in non-woven / honeycomb absorbing materials. When the thickness and the wavelength of the incident electromagnetic wave satisfy the following relationship: When the electromagnetic wave that enters the material and is reflected is in phase with the electromagnetic wave that is reflected directly on the surface, the phase difference is exactly 180 degrees. Due to wave interference, these two waves will completely dissipate, thus greatly reducing the total reflected wave.

[0129] In summary, it is the synergistic effect of the various electromagnetic wave loss mechanisms mentioned above that gives nonwoven / honeycomb absorbing materials their excellent electromagnetic wave absorption performance.

[0130] (III) Non-woven fabric / honeycomb sandwich structure

[0131] Single-layer nonwoven / honeycomb absorbing material 2-2 and double-layer nonwoven / honeycomb absorbing material I-II, both with excellent microwave absorption properties, were used to prepare a nonwoven / honeycomb sandwich structure microwave absorber using a vacuum bag pressing method. In the preparation process, a release agent was first coated onto an aluminum plate, followed by an epoxy resin adhesive coating onto the nonwoven / honeycomb absorbing structure. Then, a fiberglass board, the nonwoven / honeycomb absorbing structure, a carbon fiber board, a separation membrane, a ventilation felt, and a vacuum bag were sequentially stacked on the aluminum plate. The vacuum bag was sealed to the aluminum plate using high-temperature adhesive. A silicone tube was inserted into the vacuum bag and connected to a vacuum pump, with the vacuum level controlled between 0.09 MPa and 0.1 MPa. Finally, the entire system was placed in an oven at 100°C and baked for 1.5 hours for curing, completing the preparation of the nonwoven / honeycomb sandwich structure.

[0132] The S-shape of a single-layer nonwoven fabric / honeycomb sandwich structure material was measured using the arc method. 2-2 and double-layer single-layer non-woven fabric / honeycomb sandwich structure material S Ⅰ-Ⅱ Wave absorption performance in the frequency range of 2 to 18 GHz. Figure 11 This is a comparison of the RL curves of single / double-layer nonwoven / honeycomb absorbing materials 2-2 and I-II with their corresponding sandwich structures. (Example) Figure 11 As shown in (a), S 2-2 It is basically the same as the RL curve in 2-2, the difference being S 2-2 The absorption bandwidth is 12.8 GHz (5.2–18 GHz), and the peak intensity is -28 dB, which is 0.5 GHz wider than the absorption bandwidth of 2-2, while the peak intensity is reduced by 10 dB. For Figure 11 (b) I-II and S shown Ⅰ-Ⅱ The RL performance of both samples shows that their absorption bandwidths can fully cover 2–18 GHz. Ⅰ-Ⅱ The first absorption peak appears at a frequency of 3.7 GHz, and the second peak is at 12.1 GHz, both of which are shifted to lower frequencies compared to the absorption peaks of I-II.

[0133] Therefore, it can be concluded that the nonwoven / honeycomb sandwich composite material prepared by vacuum bagging exhibits a wider absorption bandwidth, lower peak frequency, and reduced average absorption intensity. According to the quarter-wavelength theory, this phenomenon may be due to the addition of glass fiber and carbon fiber plates, which increases the overall material thickness (d), allowing longer wavelength electromagnetic waves to enter the material and undergo loss. Consequently, the wavelength of the electromagnetic wave satisfying the 180° phase difference between the wave reflected from the base plate and the surface reflected wave increases, i.e., the electromagnetic wave frequency decreases. The decrease in average absorption intensity is attributed to the addition of prepreg during the vacuum bagging process, which solidifies part of the carbon nanomaterial absorber, weakening the resonance between the absorber and the electromagnetic wave.

[0134] A 60×60mm honeycomb sandwich composite material sample was prepared. The compressive strength of the nonwoven fabric / honeycomb sandwich structure was tested using an electronic universal testing machine with a uniform pressure of 2mm / min. The load-deformation curve was recorded and compared with the compressive strength of the honeycomb sandwich structure. Figure 12 (a) and (c) show the load-deformation curves and compressive strength diagrams of a single-layer honeycomb sandwich composite material and a non-woven / honeycomb sandwich structure, respectively. It can be seen that the honeycomb sandwich composite material S3 fractures at a compression of 1.3 mm, with a force of 7103 N. The non-woven / honeycomb sandwich structure fractures at a compression of 1.4 mm, with a force of 7316 N. Furthermore, the compressive strength of the former is 2.00 MPa, while that of the latter is 2.03 MPa, representing an average increase of 1.5% in compressive strength. From... Figure 12 (b) and (d) show the load-deformation curves and compressive strength diagrams of the double-layer honeycomb sandwich composite material and the non-woven / honeycomb sandwich structure. It can be seen that the sample S 2-4 The maximum pressure it can withstand is 8012 N, and the material fractures when the deformation is 1.8 mm; Sample S Ⅰ-Ⅱ The maximum pressure it can withstand is 8327 N, and the material fractures when the deformation is 2.2 mm. The maximum compressive stress of I-II is 2.31 MPa, which is 4.0% higher than the former's 2.22 MPa. In summary, the compressive strength of the non-woven fabric / honeycomb absorbing structure mainly comes from the carbon-based absorbing honeycomb core. The non-woven fabric filling improves the compressive strength of the non-woven fabric / honeycomb absorbing material compared to honeycomb structure absorbing materials.

[0135] The preferred embodiments of the present invention have been described in detail above with reference to the examples. However, the present invention is not limited to the specific details in the above embodiments. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

[0136] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not describe the various possible combinations separately.

[0137] 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 method for constructing a non-woven fabric / honeycomb wave-absorbing structure equivalent electromagnetic parameter calculation model, characterized in that, The non-woven fabric / honeycomb wave-absorbing structure comprises a honeycomb core material (1), a CNTs / CB / RGO / PU impregnated layer (2) formed on the honeycomb wall of the honeycomb core material (1), and a graphene / non-woven fabric composite material (3) filled in the honeycomb hole of the honeycomb core material (1). The construction method of the calculation model comprises the following steps: S1, obtaining the dielectric constant ε of the honeycomb wall of the honeycomb core material (1) h and the dielectric constant ε of the CNTs / CB / RGO / PU impregnated layer (2) i , by adding the polarization correction factor M and the structure correction factor N in the BG formula, ε h and ε i is established as an equivalent homogeneous BG model as shown in formula (I), where ε h , ε i and ε BG are the dielectric constants of the honeycomb wall, the impregnated layer and the BG model, respectively, f h is the volume fraction of the honeycomb wall in the BG model, M is the polarization correction factor, and N is the structure correction factor; S2, obtaining the dielectric constant ε of the graphene / non-woven fabric composite material (3) fill and combining it with the equivalent homogeneous model obtained in step S1, an MG model is established, and a homogeneous wave-absorbing model as shown in formula (II) is obtained, where ε BG , ε fill , and ε eff are the dielectric constants of the BG model, the honeycomb filler, and the MG equivalent medium, respectively, and f BG is the volume fraction of the BG model in the honeycomb.

2. The method according to claim 1, wherein In step S1, the obtaining method of the polarization correction factor M and the structure correction factor N is as follows: after measuring the dielectric constants of the honeycomb wall and the impregnated layer, the formula (I) is substituted, and the inverse operation is performed to obtain the values of M and N.

3. The method according to claim 2, wherein The measurement method of the dielectric constants of the honeycomb wall and the impregnated layer is as follows: the free space method is adopted, the upper and lower surfaces of the non-woven fabric / honeycomb wave-absorbing structure are taken as the incident ports, and the average value is obtained after multiple tests to obtain the dielectric constants of the honeycomb wall and the impregnated layer.

4. The method according to claim 1, wherein In step S1, the CNTs / CB / RGO / PU in the impregnated layer is regarded as a single dielectric loss particle, and the inner wall of the honeycomb core is equivalent to a uniform matrix, and the CNTs / CB / RGO / PU electromagnetic wave loss particles are uniformly dispersed on the inner wall of the honeycomb core.

5. The method according to any one of claims 1 to 4, wherein The graphene in the graphene / non-woven fabric composite material (3) is uniformly dispersed in the non-woven fabric, and is filled in the gap between the non-woven fabric fibers, the graphenes attached to different fibers are connected with each other, and a three-dimensional graphene structure is formed.

6. The method according to claim 5, wherein The graphene / non-woven fabric composite material (3) is prepared by the following method: the non-woven fabric is immersed in the graphene oxide solution, the graphene oxide is uniformly dispersed in the non-woven fabric, and then the graphene oxide in the non-woven fabric is reduced to graphene to obtain the graphene / non-woven fabric composite material (3).

7. The method according to any one of claims 1 to 4, wherein The carbon nanotubes in the CNTs / CB / RGO / PU impregnated layer (2) are multi-walled carbon nanotubes modified by strong acid oxidation; and / or The CB in the CNTs / CB / RGO / PU impregnated layer (2) is grafted on the RGO.

8. The method according to any one of claims 1 to 4, wherein the method is characterized by, The non-woven fabric / honeycomb wave-absorbing structure has multiple layers, and from high to low, the mass ratio of CNTs to CB / RGO in the CNTs / CB / RGO / PU impregnated layer (2) gradually increases, and the content of graphene in the graphene / non-woven fabric composite material (3) gradually increases.

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