A low-frequency flexible wave-absorbing composite material based on a localized field enhancement effect and a preparation method thereof

Abstract

This invention discloses a low-frequency flexible microwave absorbing composite material based on the localized field enhancement effect and its preparation method, relating to the field of electromagnetic protection technology. It aims to solve the problems of large thickness and difficult conformal design of complex curved surfaces in traditional microwave absorbing structures at low frequencies. The composite material comprises, from top to bottom: a flexible metasurface patterned layer with specific sheet resistance and topological array, a flexible polymer dielectric layer dispersed with modified magnetic filler, and a bottom total reflection layer. This invention utilizes the strong localized electromagnetic field excited by the metasurface to focus and penetrate into the dielectric layer, amplifying the dissipation potential of the magnetic filler; simultaneously, the bulk loss of the dielectric layer generates reverse damping on the metasurface resonance, thus broadening the bandwidth. This synergistic mechanism breaks the quarter-wavelength limitation. The preparation method includes pattern layer molding, dielectric layer mixing and curing, and overall composite molding. This invention achieves high-efficiency low-frequency broadband absorption at a single-layer subwavelength thickness, while also possessing excellent flexibility, making it highly suitable for stealth on complex curved surfaces with total reflection boundaries.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic protection technology, and provides a low-frequency flexible microwave absorbing composite material based on the local field enhancement effect and its preparation method. Background Technology

[0002] Low-frequency absorbing materials are core functional materials in fields such as anti-stealth detection, electromagnetic compatibility, and civilian electromagnetic protection, possessing irreplaceable strategic and application value. With the rapid development of low-frequency detection technologies such as meter-wave radar, the development of broadband absorbing materials suitable for low-frequency bands such as L and S has become a key challenge. Simultaneously, modern stealth platforms (such as UAV wings and missile radomes) often feature complex irregular curved surfaces, requiring absorbing materials to possess excellent flexibility and conformal bonding capabilities.

[0003] For a long time, the development of low-frequency absorbing materials has been constrained by the physical contradiction of "thickness-bandwidth". Traditional absorbers based on the principle of destructive interference need to meet a thickness of one-quarter wavelength, which results in extremely large material sizes in the low-frequency band, seriously disrupting the aerodynamic layout of equipment. On the other hand, traditional magnetic absorption layers based on high-permeability materials (such as ferrite or iron carbonyl) are limited by the Snoek limit and skin effect, making it difficult to simultaneously achieve "thinness" and "bandwidth". To overcome the physical limits, metasurface structures have been introduced in recent years. However, traditional metasurfaces are mostly based on rigid printed circuit board (PCB) technology, and their inherent rigidity makes them unsuitable for complex curved surfaces. Even some attempts at flexible metasurfaces often use indium tin oxide (ITO) or silver paste to construct their conductive pattern layers: the former has extremely poor flexibility and is easily brittle, while the latter has significant shortcomings in long-term thermal stability, making it difficult to meet the long-term reliability requirements in complex environments. In addition, in pursuit of high loss, traditional composite materials often blindly increase the mass fraction of magnetic fillers (such as iron carbonyl), but this leads to a sharp deterioration in the mechanical properties of the composite matrix, causing it to completely lose its flexibility.

[0004] To address the bottleneck of low-frequency thickness, cutting-edge technologies have proposed a "cascadeable" metasurface absorbing scheme. This involves removing the underlying reflective backplate and utilizing a metasurface to achieve partial transmission and multi-layer cascaded absorption of electromagnetic waves. However, in practical stealth applications for most high-value equipment, such as aerospace equipment, absorbing materials must typically be directly laid on the equipment's metal outer shell or fuselage skin. This metal substrate itself constitutes an impenetrable "total reflection boundary." Under this closed physical system, electromagnetic waves cannot be transmitted, rendering the aforementioned semi-open "cascadeable" design completely ineffective. Under total reflection boundary conditions, once a single-layer absorbing material is compressed to a subwavelength thickness, it faces extremely strong interfacial impedance mismatch and insufficient loss paths, resulting in strong specular reflection of low-frequency electromagnetic waves.

[0005] In summary, there is an urgent need in this field for an innovative single-layer flexible microwave absorption solution for total internal reflection boundary conditions. The key challenges are: how to abandon traditional ITO or silver paste processes and find or develop novel flexible conductive materials that combine high electrical conductivity, excellent flexibility, and superior thermal stability to construct the top-layer metasurface; how to overcome the mechanical degradation caused by high-concentration magnetic fillers and explore the optimal dielectric layer ratio with the best mechanical and electromagnetic balance; and ultimately, how to fully stimulate the magnetic loss potential of the flexible dielectric layer through a top-layer local electromagnetic field enhancement mechanism, thereby achieving efficient low-frequency broadband absorption under extreme single-layer thickness. These are the core technological challenges that urgently need to be addressed to achieve lightweight, conformal, and low-frequency stealth capabilities for equipment. Summary of the Invention

[0006] The purpose of this invention is to solve the problems of prominent thickness-bandwidth contradiction, interface impedance mismatch and insufficient loss path in traditional absorbing structures, as well as the poor flexibility and thermal stability of existing flexible metasurface conductive layers and the mechanical degradation of the matrix caused by high filler ratio magnetic fillers, which make it difficult to achieve ultra-thin flexible broadband absorbing and conformal bonding of complex curved surfaces in the low frequency band.

[0007] To achieve the above objectives, the present invention employs the following technical means:

[0008] This invention provides a low-frequency flexible microwave absorbing composite material based on the local field enhancement effect, comprising, from top to bottom, the following layers bonded together: a flexible metasurface pattern layer, a flexible high-loss magnetic medium layer, and a bottom total reflection layer;

[0009] The flexible metasurface patterned layer includes a periodic array made of conductive material; the topology of the periodic array is coordinated with its sheet resistance and configured to generate electromagnetic resonance under the excitation of incident electromagnetic waves, thereby stimulating the enhancement of local electromagnetic fields.

[0010] The flexible high-loss magnetic dielectric layer comprises a flexible polymer matrix and modified magnetic fillers dispersed therein.

[0011] The flexible metasurface patterned layer, the flexible high-loss magnetic medium layer, and the bottom total reflection layer are sequentially stacked and composited.

[0012] The enhanced local electromagnetic field excited by the flexible metasurface patterned layer penetrates into the interior of the flexible high-loss magnetic medium layer and undergoes strong electromagnetic coupling with the flexible high-loss magnetic medium layer to amplify the energy dissipation of the medium. Under the condition that the composite material has a total reflection boundary on the bottom surface, it can achieve low-frequency broadband electromagnetic wave absorption within the subwavelength thickness.

[0013] In the above scheme, the structure of the periodic array is selected from cross-shaped, square or circular, and its structural parameters include the side length P of the periodic unit and the characteristic size L.

[0014] In the above scheme, the conductive material of the flexible metasurface patterned layer is selected from PEDOT:PSS or nickel-plated PET, and the sheet resistance R of the flexible metasurface patterned layer is... s < 10.0 Ω / sq.

[0015] In the above scheme, the flexible polymer matrix is ​​PDMS, the modified magnetic filler is FCI surface-treated with KH-560 silane coupling agent, and the mass ratio of FCI to the total mass of the PDMS main agent and curing agent is 3:1.

[0016] In the above scheme, the total thickness of the composite material is 3.39 to 5.96 mm, and the EAB of the composite material falls within the 0.5 to 6.0 GHz frequency band.

[0017] This invention also provides a method for preparing a low-frequency flexible microwave absorbing composite material based on the local field enhancement effect, comprising the following steps:

[0018] S1. Fabrication of a flexible metasurface patterned layer: A periodic array with a specific topological structure is formed on a flexible substrate using a conductive material, and the sheet resistance R of the flexible metasurface patterned layer is controlled. s ;

[0019] S2. Preparation of flexible high-loss magnetic dielectric layer: After the magnetic filler is modified with KH-560 silane coupling agent, it is mixed evenly with flexible polymer matrix and curing agent, and then cured after degassing treatment.

[0020] S3. Composite molding: A flexible conductive material with microwave total internal reflection characteristics is introduced as the bottom total internal reflection layer; the flexible metasurface pattern layer obtained in step S1, the flexible high-loss magnetic medium layer obtained in step S2, and the bottom total internal reflection layer are sequentially bonded and laid from top to bottom, pressure is applied, and composite molding is performed to obtain the microwave absorbing composite material.

[0021] In the above preparation method, in step S1, the flexible metasurface patterned layer is prepared by one of the following methods: screen printing with PEDOT:PSS conductive ink with a solid content of 5% to 10% at a screen mesh of 150 to 250, printing 1 to 5 layers, and then drying at 50 to 80°C for 3 to 5 hours to achieve full drying or 5 to 8 minutes to achieve surface drying; or

[0022] A nickel-plated PET film is obtained by laser etching, wherein the PET film has a thickness of 50–200 μm and the nickel layer has a thickness of 100–250 nm.

[0023] In the above preparation method, step S2, the modification of the silane coupling agent specifically involves: adding 25-35 parts by mass of KH-560 silane coupling agent, 5-10 parts by mass of deionized water, and 400-600 parts by volume of anhydrous ethanol to form a mixture based on 1000 parts by mass of flake carbonyl iron FCI; adjusting the pH to 3.5-5.0 with glacial acetic acid; ultrasonically treating the mixture; mechanically stirring for 2-6 hours; allowing it to stand; washing it 3-5 times with anhydrous ethanol; centrifuging the mixture; washing it; drying it at 80°C for 12 hours; and finally drying it to obtain the modified FCI.

[0024] In the above preparation method, step S2, the uniform mixing specifically involves: adding the modified FCI to the PDMS main agent at a mass ratio of 3:1 and mechanically stirring for 30 min; then adding the curing agent at a mass ratio of 10:1 and continuing to stir for 5 min; and finally, after the mixture is degassed by shaking or ultrasonication, curing it at room temperature for more than 24 h.

[0025] In the above preparation method, in step S3, the composite molding specifically involves: cutting the bottom total reflection layer, the flexible high-loss magnetic medium layer, and the flexible metasurface pattern layer prepared in step S1 to match the mold size, and laying them sequentially from bottom to top in the polytetrafluoroethylene mold; applying pressure to the upper surface of the flexible metasurface pattern layer for pressure bonding, curing at room temperature for more than 24 hours, and then allowing it to stand for more than 7 days to stabilize its performance.

[0026] Because the present invention employs the above-mentioned technical means, it has the following beneficial effects:

[0027] 1. Achieving low-frequency broadband absorption at subwavelength thickness: Traditional absorption schemes, under total internal reflection metal boundaries, are prone to strong interface impedance mismatch and insufficient loss paths due to the inability of electromagnetic waves to transmit and the limitation of single-layer thickness on the subwavelength scale. This invention completely eliminates the semi-open design relying on transmission or cascading by directly bonding and spatially coupling a flexible metasurface patterned layer with a flexible high-loss magnetic dielectric layer. Example test data shows that, at the same physical thickness, after introducing the metasurface structure of this invention, PDMS / FCI... 75 The composite material's EAB produces a leapfrog improvement. The cross-shaped structure expands the absorption range from 1.14 GHz to 1.76–1.82 GHz; the square structure expands it from 0.65 GHz to 1.44–1.46 GHz; and the ring structure enables the substrate, which originally had no effective absorption in the 1–6 GHz frequency band, to achieve effective absorption in the 0.84–1.05 GHz band.

[0028] 2. The local electromagnetic field enhancement mechanism is deeply coupled with the energy dissipation of the lossy dielectric layer, resulting in a significant synergistic broadband broadening effect. This invention creatively utilizes the specific pattern topology of the metasurface array (such as cross-shaped, square, or ring-shaped) and its optimized sheet resistance (configured to a specific finite resistance value to balance resonant intensity and ohmic loss). Under low-frequency incident electromagnetic wave excitation, this structure induces a strong local electromagnetic field enhancement effect at the pattern gaps and edges (mechanism see...). Figure 17 This mechanism highly confines and focuses microwave energy into a subwavelength-thickness interface region, penetrating deeply and acting directly on the underlying flexible dielectric layer. This fully excites the polarization and hysteresis loss potential of the high-concentration magnetic filler within it. Simultaneously, the distributed bulk dissipation in the flexible dielectric layer exerts a strong reverse damping effect on the inherent high quality factor (high Q value) narrowband resonance of the metasurface, significantly reducing the system's resonant Q value and effectively suppressing the strong reflection peaks and narrowband limitations that are prone to occur in single metasurface structures. These two elements form a deep physical coupling of "field-enhanced focusing excitation loss – dielectric loss reverse damping broadening," achieving efficient low-frequency broadband absorption at an extremely thin single-layer thickness. This synergistic mechanism cannot be achieved independently by a single metasurface or a single absorbing rubber, nor can it be easily foreseen by those skilled in the art through conventional physical thickening or simple material compounding. It possesses outstanding substantial characteristics and represents significant progress.

[0029] 3. Synergistic effect of interface-modified high-magnetic filler and flexible matrix: Balancing high electromagnetic loss with excellent conformal mechanical properties. Addressing the technical bottleneck of traditional microwave absorbing materials that blindly increase the mass fraction of magnetic filler in pursuit of high loss, leading to a sharp deterioration in matrix mechanical properties and loss of flexibility, this invention uses KH-560 silane coupling agent to modify the surface of FCI and then composites it with a PDMS matrix at an optimized mass ratio. This modification effectively improves the interfacial bonding between high-concentration filler (e.g., 75% FCI in the example) and the polymer matrix, exploring the optimal mechanical and electromagnetic balance point. Figure 18 Mechanical testing results show that the composite material maintains excellent low-frequency wave absorption performance while exhibiting good tensile and compressive strength (such as compressive performance at 25% strain). This achieves a synergistic unity of "high magnetic permeability loss" and "flexible conformal bonding" at the material level, meeting the stringent requirements of modern irregularly shaped surface stealth equipment for long-term reliability.

[0030] 4. A novel flexible conductive material system replaces traditional processes, improving environmental adaptability and structural robustness. This invention abandons the ITO or silver paste processes used in traditional flexible metasurfaces, instead employing PEDOT:PSS conductive ink screen printing or nickel-plated PET film laser etching to construct the top layer metasurface. This material system not only possesses excellent electrical conductivity but also fundamentally solves the inherent defects of ITO (poor flexibility and brittleness) and silver paste (insufficient long-term thermal stability). Comparative verification in Examples 1-6 shows that regardless of whether PEDOT:PSS ink printing or nickel-plated film etching is used, a highly consistent microwave absorption synergy effect can be achieved under the same structural parameters, demonstrating the compatibility of the top conductive material selection and the robustness of the structural design, providing reliable assurance for long-term application under complex temperature and deformation environments. Attached Figure Description

[0031] Figure 1 : Schematic diagram of the design of a metasurface absorbing composite material based on a cross-shaped structure. In the figure, 1 is the reflective layer, 2 is the PDMS / FCI composite material, 3 is the PET film, and 4 is the conductive material.

[0032] Figure 2 Parameter design of microwave absorbing composite materials based on cross-shaped structure: (a) 3D view; (b) Top view;

[0033] Figure 3 Schematic diagram of a periodic structure of a metasurface absorbing composite material based on a cross-shaped structure;

[0034] Figure 4 Parameter design of microwave absorbing composite material based on square structure: (a) 3D view; (b) Top view;

[0035] Figure 5 Schematic diagram of a periodic structure of a metasurface absorbing composite material based on a square structure;

[0036] Figure 6 Parameter design of microwave absorbing composite materials based on a circular ring structure: (a) 3D view; (b) Top view;

[0037] Figure 7 Schematic diagram of a periodic structure of a metasurface absorbing composite material based on a ring structure;

[0038] Figure 8 Image of a cross-shaped metasurface layer fabricated using PEDOT:PSS conductive ink screen printing;

[0039] Figure 9 :R s =6.23 Ω / sq corresponds to the presence or absence of a cross-shaped metasurface PDMS / FCI 75 Comparison of reflection loss curves for composite materials;

[0040] Figure 10 Image of a cross-shaped metasurface structure fabricated by laser etching of nickel-plated PET film;

[0041] Figure 11 : Does the PET nickel-plated film correspond to a cross-shaped metasurface PDMS / FCI? 75 Comparison of reflection loss curves for composite materials;

[0042] Figure 12 :R s =5.96 Ω / sq corresponds to the presence or absence of square metasurface PDMS / FCI 75 Comparison of reflection loss curves for composite materials;

[0043] Figure 13 : Does the PET nickel-plated film correspond to square metasurface PDMS / FCI? 75 Comparison of reflection loss curves for composite materials;

[0044] Figure 14 :R s =6.82 Ω / sq corresponds to the presence or absence of a toroidal metasurface PDMS / FCI 75 Comparison of reflection loss curves for composite materials;

[0045] Figure 15 : PET nickel-plated film with or without ring-shaped metasurface PDMS / FCI 75 Comparison of reflection loss curves for composite materials;

[0046] Figure 16 PDMS / FCI 75 Electromagnetic parameters of composite materials: (a) dielectric constant; (b) magnetic permeability;

[0047] Figure 17 : Mechanism diagram of the local electromagnetic field enhancement mechanism based on cross-shaped metasurface absorbing composite materials;

[0048] Figure 18 PDMS / FCI 75 Composite materials: (a) tensile properties; (b) compressive properties (at 25% strain). Detailed Implementation

[0049] The embodiments of the present invention will be described in detail below. Although the present invention will be described and illustrated in conjunction with some specific embodiments, it should be noted that the present invention is not limited to these embodiments. On the contrary, any modifications or equivalent substitutions made to the present invention should be covered within the scope of the claims of the present invention.

[0050] Furthermore, to better illustrate the present invention, numerous specific details are set forth in the following detailed embodiments. Those skilled in the art will understand that the present invention can be practiced without these specific details.

[0051] To facilitate a better understanding of the technical solution of this invention by those skilled in the art, the following specific response is provided:

[0052] I. Preparation Process

[0053] 1.1 PEDOT: PSS conductive ink

[0054] Take 200 mL of PEDOT:PSS (PHV500) dispersion (solid content 1.0%-1.3%), add approximately 2 g of superabsorbent polymer beads, let stand at room temperature for 1 h with occasional stirring, then remove the beads, pre-freeze in a refrigerator for 3 h, and finally freeze-dry in a freeze dryer for 72 h to obtain solid PEDOT:PSS. Take 2 g of solid PEDOT:PSS with 6.06 mL of dimethyl sulfoxide (DMSO) and 14.16 mL of deionized water, and manually push the mixture back and forth using two syringes until completely homogeneous to obtain PEDOT:PSS conductive ink with a solid content of 9%.

[0055] (Note: There are no specific requirements for the supplier and solid content of the PEDOT:PSS dispersion. The ratio of solid PEDOT:PSS to DMSO and deionized PEDOT:PSS can be adjusted within a certain range according to the sheet resistance of the desired metasurface pattern. It is recommended that the volume ratio of DMSO should not exceed 50% of the total volume. The viscosity of the conductive ink prepared by controlling the solid content of PEDOT:PSS to 5% to 10% is more suitable. On this basis, the higher the conductivity of the PEDOT:PSS conductive ink, the better.)

[0056] 1.2 Fabrication of metasurface layers (two optional methods provided)

[0057] (1) Screen printing

[0058] The PET film was ultrasonically cleaned in acetone solution for 2 hours, then dried in an oven at 60°C for 1 hour to ensure thorough removal of impurities from the film surface. The desired metasurface pattern array was then fabricated on the treated PET film using PEDOT:PSS conductive ink via screen printing, and the film was placed in an oven at 60°C for 5 hours.

[0059] (Note: The mesh count of the screen printing plate is 150-250, and the number of printing layers is 1-5. When stacking printing layers, it is divided into full drying and surface drying. In the drying process, the full drying temperature is 50-80 ℃ and the drying time is 3-5 h, while the surface drying temperature is 50-80 ℃ and the drying time is 5-8 min.)

[0060] (2) Laser etching

[0061] The target parameter pattern is etched into a nickel-plated PET film using laser etching.

[0062] (Note: Nickel-plated PET film can be purchased directly. The thickness of the PET film is 50-200 μm, and the thickness of the nickel layer is 100-250 nm. If the nickel layer thickness is less than 100 nm, the sheet resistance will be too high, and if it is too thick, the heat accumulation during the laser etching process will be severe, causing the film to burn out.)

[0063] 1.3 Surface Modification of FCI

[0064] Measure 500 mL of anhydrous ethanol, 6.9 g of deionized water, and 30 g of KH-560 silane coupling agent. Add glacial acetic acid to adjust the pH to 3.5–5.0, sonicate for 5 min, then add 1000 g of FCI, mechanically stir for 2 h, and let stand for 6 h. Wash 3–5 times with anhydrous ethanol, separating the anhydrous ethanol and FCI by centrifugation at 9000 rpm for 8 min each time. Finally, place in an oven at 80 ℃ for 12 h to obtain the modified FCI.

[0065] 1.4 Preparation of PDMS / FCI composite material

[0066] The defective substrate layer is a PDMS / FCI composite material. First, according to a 3:1 ratio of modified FCI to (PDMS main agent + curing agent) total mass, the corresponding mass of modified FCI is weighed and added, and mechanically stirred for 30 min. Then, PDMS curing agent (main agent: curing agent = 10:1) is added at a 10:1 mass ratio, and mechanically stirred for 5 min to mix thoroughly. The mixture is then poured into a custom-made PTFE mold, vibrated and sonicated 3-5 times to remove air bubbles, and cured at room temperature (23 ℃) for at least 24 h. Afterward, it is left to stand for at least 7 days to stabilize its properties, resulting in the PDMS / FCI composite. 75 Composite materials.

[0067] 1.5 Preparation of Metasurface Absorbing Composite Materials

[0068] First, clean the polytetrafluoroethylene mold with anhydrous ethanol, and then clean each component (bottom total reflective layer, PDMS / FCI). 75The composite material (and metasurface patterned layer) was cut to the size of the mold, and the prepared PDMS / FCI was sanded with sandpaper. 75 The top and bottom surfaces of the composite material are polished smooth. Then, from bottom to top, the bottom total reflective layer and the polished PDMS / FCI are applied sequentially. 75 Composite materials and metasurface patterned layers are laid in the mold (see reference). Figure 1 (The order of layer stacking). Apply a certain pressure to the top layer for pressure bonding, keep the composite in this state at room temperature (23 ℃) for more than 24 hours to allow it to cure as a whole, and then leave it for more than 7 days to allow the interface bonding and overall performance to stabilize, thus obtaining the microwave absorbing composite material.

[0069] II. Design of Low-Frequency Ultrathin Flexible Absorbing Composite Material Structure

[0070] 2.1 Metasurface Absorbing Composite Materials Based on Cross-Shaped Structure

[0071] This invention employs a metasurface array based on a cross-shaped structure, which offers strong design flexibility and includes five parameters: the side length P of the periodic unit, the metasurface-related structural parameters L1 and L2, and the sheet resistance R of the metasurface patch. s And the three-dimensional and top views of the metasurface absorbing composite material with a defective substrate thickness h, as shown in the figure. Figure 2 As shown. The frequency domain solver of the electromagnetic simulation software CST Microwave Studio was used to optimize the design of the metamaterial absorbing composite. The flexible high-loss dielectric layer used in the simulation was PDMS / FCI. 75 The composite material was fabricated into a coaxial ring with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. Its electromagnetic parameters were extracted using the coaxial method, and the results are as follows: Figure 16 As shown, the absorbers are periodically distributed in the xy-plane and Z-plane. min The direction is set to electric (Et=0) boundary, Z max The direction is set to open (add space) boundary, and the simulation frequency is 0.5 to 6.0 GHz.

[0072] The optimal metasurface structure parameters for microwave absorption performance were designed using the optimization algorithm of CST simulation software, where P = 27.23 mm, L1 = 11.24 mm, and L2 = 4.02 mm; the sheet resistance R of the metasurface patch was determined. s The closer to a perfect electrical conductor (PEC), the better. The lossy substrate thickness h = 3.39 mm. A schematic diagram of a metasurface periodic array based on a cross-shaped structure is shown below. Figure 3 As shown.

[0073] 2.2 Metasurface Absorbing Composite Materials Based on Square Structure

[0074] This invention employs a metasurface array based on a square structure, which offers strong design flexibility and includes four parameters: the side length P of the periodic unit, the metasurface-related structural parameters L1, and the sheet resistance R of the metasurface patch. s And the three-dimensional and top views of the metasurface absorbing composite material with a defective substrate thickness h, as shown in the figure. Figure 4 As shown. The frequency domain solver of the electromagnetic simulation software CST Microwave Studio was used to optimize the design of the metamaterial absorbing composite. The flexible high-loss dielectric layer used in the simulation was PDMS / FCI. 75 The composite material was fabricated into a coaxial ring with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. Its electromagnetic parameters were extracted using the coaxial method, and the results are as follows: Figure 16 As shown, the absorbers are periodically distributed in the xy-plane and Z-plane. min The direction is set to electric (Et=0) boundary, Z max The direction is set to open (add space) boundary, and the simulation frequency is 0.5 to 6.0 GHz.

[0075] The optimal metasurface structure parameters for microwave absorption performance were designed using the optimization algorithm of CST simulation software, where P = 32.07 mm, L1 = 11.53 mm, and the metasurface patch R... s The closer the sheet resistance is to PEC, the better. The lossy substrate thickness h = 4.36 mm. A schematic diagram of a metasurface periodic array based on a square structure is shown below. Figure 5 As shown.

[0076] 2.3 Metasurface Absorbing Composite Materials Based on Ring Structure

[0077] This study employs a metasurface array based on a ring structure, which offers strong design flexibility and includes four parameters: the side length P of the periodic unit, the metasurface-related structural parameters L1 and L2, and the sheet resistance R of the metasurface patch. s And the three-dimensional and top views of the metasurface absorbing composite material with a defective substrate thickness h, as shown in the figure. Figure 6 As shown. The frequency domain solver of the electromagnetic simulation software CST Microwave Studio was used to optimize the design of the metamaterial absorbing composite. The flexible high-loss dielectric layer used in the simulation was PDMS / FCI. 75 The composite material was fabricated into a coaxial ring with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. Its electromagnetic parameters were extracted using the coaxial method, and the results are as follows: Figure 16 As shown, the absorbers are periodically distributed in the xy-plane and Z-plane. min The direction is set to electric (Et=0) boundary, Z max The direction is set to open (add space) boundary, and the simulation frequency is 0.5 to 6.0 GHz.

[0078] The optimal metasurface structure parameters for microwave absorption performance were designed using the optimization algorithm of CST simulation software, where P = 37.70 mm, L1 = 17.16 mm, L2 = 13.54 mm, and the metasurface patch R... s The closer the sheet resistance is to PEC, the better. The lossy substrate thickness is h=5.96 mm. A schematic diagram of a metasurface periodic array based on a ring structure is shown below. Figure 7 As shown.

[0079] Example 1

[0080] (1) Preparation of PEDOT:PSS conductive ink

[0081] Take 200 mL of PEDOT:PSS dispersion, add approximately 2 g of superabsorbent polymer beads, let stand at room temperature for 1 h with occasional stirring, then remove the beads, pre-freeze in a refrigerator for 3 h, and finally freeze-dry in a freeze dryer for 72 h to obtain solid PEDOT:PSS. Take 2 g of solid PEDOT:PSS and mix it with 6.06 mL of DMSO and 14.16 mL of deionized water, and manually push the mixture back and forth using two syringes until it is completely mixed to obtain PEDOT:PSS conductive ink with a solid content of 9%.

[0082] (2) Preparation of metasurface layer

[0083] The PET film was ultrasonically cleaned in acetone solution for 2 hours, then dried in an oven at 60°C for 1 hour to ensure thorough removal of impurities from the film surface. A cross-shaped metasurface pattern array was fabricated on the treated PET film using screen printing with 9% solids content PEDOT:PSS conductive ink, with specific structural parameters as described in section 3.1. The film was then placed in an oven at 60°C for 5 hours. The screen printing process used a 200-mesh screen and 5 printing layers (4 passes for surface drying, 1 pass for full drying). The drying process included a full drying temperature of 60°C for 5 hours and a surface drying temperature of 60°C for 8 minutes. The sheet resistance R of the PEDOT:PSS film was measured using the four-probe method. s =6.23 Ω / sq. The actual image of the fabricated metasurface layer is shown below. Figure 8 As shown.

[0084] (3) PDMS / FCI 75 Preparation of composite materials

[0085] The damaged substrate is PDMS / FCI 75Composite materials. First, weigh a certain amount of PDMS main agent and add it to a beaker. Then, add modified FCI at a mass ratio of 3:1 (FCI: PDMS main agent + curing agent = 3:1), and mechanically stir for 30 min. Finally, add a certain amount of PDMS curing agent (m... 主剂 :m 固化剂 =10:1), mechanically stir for 5 min to mix evenly. Then pour into a custom PTFE mold, vibrate and sonicate 3-5 times to remove air bubbles, and let it cure at room temperature (23 ℃) for more than 24 h, and then let it stand for more than 7 days to stabilize its properties, to obtain PDMS / FCI. 75 Composite materials.

[0086] (4) Preparation of metasurface absorbing composite materials

[0087] First, clean the polytetrafluoroethylene mold with ethanol, cut each component to the size of the mold, and then use sandpaper to polish the prepared PDMS / FCI. 75 The top and bottom surfaces of the composite material are ground smooth, and then proceeded from bottom to top according to... Figure 1 After laying the materials in sequence and flattening them, place them into the mold. Apply a certain pressure to the top layer and let it sit at room temperature (23 ℃) for more than 24 hours to allow it to solidify as a whole. Then let it sit for more than 7 days to allow its performance to stabilize, thus obtaining the supersurface microwave absorbing composite material.

[0088] The absorption performance was tested using the bow-shaped method. The sample size was 500*500*3.4 mm. 3 The result is as follows Figure 9 As shown, the introduction of a cross-shaped metasurface layer significantly improves the overall microwave absorption performance of the composite material. At the same thickness, PDMS / FCI... 75 The EAB of the composite material is 1.14 GHz, and the EAB of the metasurface absorbing composite material based on the cross-shaped structure is 1.76 GHz.

[0089] Example 2

[0090] (1) Preparation of metasurface layer

[0091] A cross-shaped metasurface pattern array was etched from a nickel-plated PET film using laser etching. Specific structural parameters are described in section "2.1 Metasurface Absorbing Composite Material Based on Cross-Shaped Structure," where the PET film thickness is 50 μm and the nickel layer thickness is 200 nm. The resulting metasurface layer is shown in the image below. Figure 10 As shown.

[0092] (2) PDMS / FCI 75 The preparation process of the composite material is the same as in Example 1.

[0093] (3) The preparation process of the metasurface absorbing composite material is the same as in Example 1.

[0094] The absorption performance was tested using the bow-shaped method. The sample size was 500*500*3.4 mm. 3 The result is as follows Figure 11 As shown, the introduction of a cross-shaped metasurface layer significantly improves the overall microwave absorption performance of the composite material. At the same thickness, PDMS / FCI... 75 The composite material has an EAB of 1.14 GHz, while the metasurface absorbing composite material based on a cross-shaped structure has an EAB of 1.82 GHz.

[0095] Example 3

[0096] (1) PEDOT: The preparation of PSS conductive ink is the same as in Example 1.

[0097] (2) Preparation of metasurface layer

[0098] The preparation process is the same as in Example 1, and the specific structural parameters are described in section "2.2 Metasurface Microwave Absorbing Composite Material Based on Square Structure". The sheet resistance R of the PEDOT:PSS thin film was tested using the four-probe method. s =5.96 Ω / sq.

[0099] (3) PDMS / FCI 75 The preparation of the composite material is the same as in Example 1.

[0100] (4) The preparation of the metasurface absorbing composite material is the same as in Example 1.

[0101] The absorption performance was tested using the bow-shaped method. The sample size was 500*500*4.4 mm. 3 The result is as follows Figure 12 As shown, the introduction of a metasurface layer based on a square structure significantly improves the overall microwave absorption performance of the composite material. At the same thickness, PDMS / FCI... 75 The composite material has an EAB of 0.65 GHz, while the metasurface absorbing composite material based on a square structure has an EAB of 1.46 GHz.

[0102] Example 4

[0103] (1) Preparation of metasurface layer

[0104] A square metasurface pattern array was etched into a PET nickel-plated film using laser etching. The specific structural parameters are described in Section 3.2, where the PET film thickness is 50 μm and the nickel layer thickness is 200 nm.

[0105] (2) PDMS / FCI 75 The preparation process of the composite material is the same as in Example 1.

[0106] (3) The preparation process of the metasurface absorbing composite material is the same as in Example 1.

[0107] The absorption performance was tested using the bow-shaped method. The sample size was 500*500*4.4 mm. 3 The result is as follows Figure 13 As shown, the introduction of a metasurface layer based on a square structure significantly improves the overall microwave absorption performance of the composite material. At the same thickness, PDMS / FCI... 75 The composite material has an EAB of 0.65 GHz, while the metasurface absorbing composite material based on a square structure has an EAB of 1.44 GHz.

[0108] Example 5

[0109] (1) PEDOT: The preparation of PSS conductive ink is the same as in Example 1.

[0110] (2) Preparation of metasurface layer

[0111] The preparation process is the same as in Example 1, and the specific structural parameters are as described in section "2.3 Metasurface Absorbing Composite Material Based on Ring Structure". The sheet resistance R of the PEDOT:PSS thin film was tested using the four-probe method. s =6.82 Ω / sq.

[0112] (3) PDMS / FCI 75 The preparation of the composite material is the same as in Example 1;

[0113] (4) The preparation of the metasurface absorbing composite material is the same as in Example 1.

[0114] The absorption performance was tested using the bow-shaped method. The sample size was 500*500*6.0 mm. 3 The result is as follows Figure 14 As shown, the introduction of a metasurface layer based on a ring structure significantly improves the overall microwave absorption performance of the composite material. At the same thickness, PDMS / FCI... 75 The composite material has no effective absorption frequency band in the range of 1 to 6 GHz, and the metasurface absorbing composite material based on the ring structure has an EAB value of 1.05 GHz.

[0115] Example 6

[0116] (1) Preparation of metasurface layer

[0117] A ring-shaped metasurface pattern array was etched into a PET nickel-plated film using laser etching. The specific structural parameters are described in Section 3.3, where the PET film thickness is 50 μm and the nickel layer thickness is 200 nm.

[0118] (2) PDMS / FCI 75 The preparation process of the composite material is the same as in Example 1.

[0119] (3) The preparation process of the metasurface absorbing composite material is the same as in Example 1.

[0120] The absorption performance was tested using the bow-shaped method. The sample size was 500*500*6.0 mm. 3 The result is as follows Figure 15 As shown, the introduction of a metasurface layer based on a ring structure significantly improves the overall microwave absorption performance of the composite material. At the same thickness, PDMS / FCI... 75 The composite material has no effective absorption frequency band, and the EAB of the metasurface absorbing composite material based on the ring structure is 0.84 GHz.

Claims

1. A low-frequency flexible microwave absorbing composite material based on localized field enhancement effect, characterized in that, It includes, from top to bottom, the following layers bonded together: a flexible metasurface pattern layer, a flexible high-loss magnetic dielectric layer, and a bottom total reflection layer; The flexible metasurface patterned layer includes a periodic array made of conductive material; the topology of the periodic array is coordinated with its sheet resistance and configured to generate electromagnetic resonance under the excitation of incident electromagnetic waves, thereby stimulating the enhancement of local electromagnetic fields. The flexible high-loss magnetic dielectric layer comprises a flexible polymer matrix and modified magnetic fillers dispersed therein. The flexible metasurface patterned layer, the flexible high-loss magnetic medium layer, and the bottom total reflection layer are sequentially stacked and composited. The enhanced local electromagnetic field excited by the flexible metasurface patterned layer penetrates into the interior of the flexible high-loss magnetic medium layer and undergoes strong electromagnetic coupling to amplify the energy dissipation of the medium. This allows the composite material to achieve low-frequency broadband electromagnetic wave absorption within a subwavelength thickness under the condition that a total reflection boundary is set on the bottom surface.

2. The single-layer flexible metasurface microwave absorbing composite material according to claim 1, characterized in that, The structure of the periodic array is selected from cross-shaped, square, or circular shapes, and its structural parameters include the side length P of the periodic unit and the characteristic dimension L.

3. The low-frequency flexible absorbing composite material based on localized field enhancement effect according to claim 1, characterized in that, The conductive material of the flexible metasurface patterned layer is selected from poly(3,4-ethylenedioxythiophene):polystyrene sulfonate PEDOT:PSS or nickel-plated polyethylene terephthalate (PET), and the sheet resistance R of the flexible metasurface patterned layer is... s < 10.0 Ω / sq.

4. The low-frequency flexible absorbing composite material based on localized field enhancement effect according to claim 1, characterized in that, The flexible polymer matrix is ​​polydimethylsiloxane (PDMS), and the modified magnetic filler is flake-shaped carbonyl iron (FCI) surface-treated with KH-560 silane coupling agent. The mass ratio of FCI to the total mass of the PDMS main agent and curing agent is 3:

1.

5. The low-frequency flexible absorbing composite material based on localized field enhancement effect according to claim 1, characterized in that, The total thickness of the composite material is 3.39–5.96 mm, and the effective absorption bandwidth of the composite material falls within the 0.5–6.0 GHz frequency band, with a reflection loss ≤-10 dB.

6. A method for preparing a low-frequency flexible microwave absorbing composite material based on localized field enhancement effect as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Fabrication of a flexible metasurface patterned layer: A periodic array with a specific topological structure is formed on a flexible substrate using conductive materials, and the sheet resistance of the flexible metasurface patterned layer is controlled; S2. Preparation of flexible high-loss magnetic dielectric layer: After the magnetic filler is modified with KH-560 silane coupling agent, it is mixed evenly with flexible polymer matrix and curing agent, and then cured after degassing treatment. S3. Introduction and composite molding of total reflection layer: A flexible conductive material with total reflection properties is introduced as the bottom total reflection layer; the flexible metasurface pattern layer obtained in step S1, the flexible high-loss magnetic medium layer obtained in step S2, and the bottom total reflection layer are sequentially bonded and laid from top to bottom, pressure is applied and composite molding is performed to obtain the microwave absorbing composite material.

7. The preparation method according to claim 6, characterized in that, In step S1, the flexible metasurface patterned layer is prepared by one of the following methods: Screen printing is performed using PEDOT:PSS conductive ink with a solid content of 5%–10%, with a screen mesh of 150–250 mesh. After printing 1–5 layers, the ink is dried at 50–80℃ for 3–5 hours until fully dry or 5–8 minutes until surface dry; or A nickel-plated PET film is obtained by laser etching, wherein the PET film has a thickness of 50–200 μm and the nickel layer has a thickness of 100–250 nm.

8. The preparation method according to claim 6, characterized in that, In step S2, the silane coupling agent modification is specifically performed as follows: based on 1000 parts by weight of flake carbonyl iron FCI, 25-35 parts by weight of KH-560 silane coupling agent, 5-10 parts by weight of deionized water, and 400-600 parts by volume of anhydrous ethanol are added to form a mixture. The pH is adjusted to 3.5-5.0 with glacial acetic acid, ultrasonically treated, and mechanically stirred for 2-6 hours. After standing, it is washed 3-5 times with anhydrous ethanol, centrifuged, washed, and dried at 80°C for 12 hours. Finally, the modified FCI is obtained by drying.

9. The preparation method according to claim 6, characterized in that, In step S2, the mixing is specifically as follows: the modified FCI is added to the PDMS main agent at a mass ratio of 3:1 and mechanically stirred for 30 min, then the curing agent is added at a mass ratio of 10:1 to the main agent and stirred for another 5 min. After the mixture is degassed by shaking or ultrasonication, it is cured at room temperature for more than 24 h.

10. The preparation method according to claim 6, characterized in that, In step S3, the composite molding specifically involves cutting the bottom total reflection layer, the flexible high-loss magnetic medium layer, and the flexible metasurface pattern layer prepared in step S1 to the size of the matching mold, and laying them sequentially from bottom to top in the polytetrafluoroethylene mold. Pressure is applied to the upper surface of the flexible metasurface patterned layer, and the mixture is cured at room temperature for more than 24 hours, followed by standing for more than 7 days to stabilize its performance.

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