Preparation method of double-layer flexible wave-absorbing coating based on Ag-plated three-dimensional graphene

Abstract

The application discloses a preparation method of a double-layer flexible wave-absorbing coating based on Ag-plated three-dimensional graphene, and specifically comprises the following steps: sequentially performing alkali washing, sensitization, water washing and drying on three-dimensional graphene powder to obtain pretreated three-dimensional graphene powder; preparing a silver-ammonia solution and a reducing solution; performing silver-ammonia plating on the pretreated three-dimensional graphene powder by using the silver-ammonia solution and the reducing solution to obtain silver-plated three-dimensional graphene powder; mixing the silver-plated three-dimensional graphene with a polyurethane resin matrix to obtain a flexible fabric microwave-absorbing coating; and preparing a frequency selective surface with a periodic structure on the surface of the coating to obtain a double-layer flexible wave-absorbing coating. The Ag-plated three-dimensional graphene powder prepared by the application enhances the polarization loss capacity of the coating, and the enhancement of the conductivity is helpful to improve the electric conduction loss capacity of the coating, so that the introduction of Ag is helpful to improve the loss capacity of the material to electromagnetic waves.

Description

Technical Field

[0001] This invention belongs to the technical field of electromagnetic wave absorber preparation methods, specifically relating to a method for preparing a double-layer flexible wave-absorbing coating based on Ag-plated three-dimensional graphene. Background Technology

[0002] With the rapid development of electronic technology, modern radio technology, and radar detection systems, electromagnetic radiation and pollution have become serious problems threatening human life and health, and pose a significant threat to the survival of aircraft equipment. Electromagnetic wave absorbing materials can block and eliminate electromagnetic waves, providing an effective solution to these problems. Researchers have been dedicated to exploring materials with excellent microwave absorption properties. In the past few decades, the most widely studied absorbers have been carbon materials (such as graphite, conductive carbon black, carbon nanotubes, and graphene), ferrites, magnetic powders, and conductive polymers. However, with increasing application demands, the shortcomings of microwave absorbing materials, such as narrow effective absorption bandwidth and high density, have become increasingly apparent. Therefore, researchers have begun to explore microwave absorbing materials that are lightweight, have a wide bandwidth, strong absorption performance, and are thin.

[0003] Graphene is a material composed of carbon atoms with a novel two-dimensional (2D) hexagonal structure. It possesses excellent mechanical properties, high thermal conductivity, and a large specific surface area. Its unique electronic structure gives it extremely high carrier mobility. More importantly, graphene also exhibits the room-temperature quantum Hall effect and tunneling effect. Its band structure allows for the existence of independent holes and electrons, thus enabling electron conduction. The inherent defects in its structure not only improve its impedance matching characteristics but also provide the energy for transitions between adjacent energy levels to the Fermi level, and introduce defect polarization and dipole relaxation, further enhancing its ability to absorb electromagnetic waves. These electromagnetic properties, unlike those of traditional materials, make it a potential next-generation microwave absorbing material. However, when used alone as a microwave absorber, its application is limited by its high dielectric loss and tendency to aggregate. Therefore, combining graphene with other absorbers based on loss mechanisms has become an important method for obtaining high-performance microwave absorbing materials.

[0004] Currently, graphene-based composite materials, including graphene / polymers, graphene / magnetic metals, graphene / ferrites, graphene / ceramics, and graphene / multi-component composites, have been studied. Research on graphene-based composites has primarily focused on 2D graphene and reduced graphene oxide. However, the poor dispersion, high interfacial contact resistance, and dielectric constant deviation of 2D graphene limit its application in microwave absorbing materials. Furthermore, the strong π-π interactions between graphene sheets lead to graphene aggregation and stacking, reducing its surface area and severely impairing its excellent electrical and microwave absorption properties. Therefore, researchers have begun designing and modifying the structure of graphene and its composites to improve their microwave absorption performance.

[0005] Three-dimensional (3D) graphene not only possesses the inherent properties of 2D graphene but also exhibits excellent mechanical properties, electrical conductivity, and a large specific surface area, making it exceptionally promising for applications in crucial cutting-edge fields such as sensors, supercapacitors, and catalytic electrodes. To date, there are few reports in the literature regarding the microwave absorption performance of 3D graphene-based composite materials. Therefore, designing and preparing 3D graphene-based microwave absorbing materials with good absorption properties presents both broad research prospects and significant challenges. Furthermore, next-generation unmanned intelligent deformable aircraft, as a new concept in contemporary air weaponry, can flexibly perform various combat missions and play a crucial role in future system-of-systems warfare. Compared to the fixed, rigid structure of traditional aircraft, intelligent deformable aircraft extensively utilize flexible structures or materials, requiring stealth materials loaded onto the aircraft surface to possess flexible characteristics. Therefore, the preparation of 3D graphene-based flexible microwave absorbing composite materials is of great significance.

[0006] Besides focusing on absorbers to improve the performance of flexible fabric microwave absorbing materials, research on coating structure adjustment is also an effective approach. Single-layer coatings have narrow absorption bandwidths, leading to the development of magnetoelectric composite double-layer or multi-layer microwave absorbing coatings. Numerous studies have shown that composite double-layer or multi-layer microwave absorbing coatings offer superior absorption performance and bandwidth compared to single-layer coatings. Introducing certain shapes or structures into the coating, such as circular or rectangular periodic structures and discontinuous structures, can further enhance absorption performance and broaden the frequency band. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene. This coating has excellent microwave absorption performance and good flexibility, and can effectively absorb electromagnetic waves incident over a wide range of angles.

[0008] The technical solution adopted in this invention is a method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene, which is implemented according to the following steps:

[0009] Step 1: Pre-treat the three-dimensional graphene powder;

[0010] Step 2: Prepare silver ammonia solution and reducing solution;

[0011] Step 3: Use the silver ammonia solution and reducing solution obtained in Step 2 to perform silver ammonia plating on the pretreated three-dimensional graphene powder obtained in Step 1 to obtain silver-plated three-dimensional graphene powder.

[0012] Step 4: Mix silver-plated three-dimensional graphene with a polyurethane resin matrix to prepare a flexible fabric microwave absorbing coating.

[0013] Step 5: Prepare a frequency-selective surface with a periodic structure on the coating surface to obtain a double-layer flexible microwave absorbing coating.

[0014] The invention is further characterized in that,

[0015] Step 1 specifically involves:

[0016] Step 1.1: Mix three-dimensional graphene powder with an average particle size of 50 nm and a purity greater than 98% with NaOH aqueous solution and stir and wash with alkali.

[0017] Step 1.2: Mix hydrochloric acid and SnCl2 aqueous solution at a volume ratio of 1-3:1 to form a sensitization treatment solution. Pour the three-dimensional graphene powder obtained in step 1.1 into the sensitization treatment solution for sensitization treatment. Wash with water and filter until the filtrate is used with Ag. + No white precipitate forms after the solution is added dropwise. Dry and store for later use.

[0018] Step 2 specifically involves:

[0019] Step 2.1: Prepare an AgNO3 solution with a mass concentration of 5-20 g / L, gradually add ammonia water until the solution is clear, then add a sodium dodecylbenzenesulfonate solution with a mass concentration of 1-1.5 g / L, ethylene glycol, and PVP with a mass concentration of 2-4 g / L. Stir at 40-60℃ for 20-30 min at a stirring speed of 800-1000 rpm until completely dissolved to obtain a silver ammonia solution.

[0020] Step 2.2: Mix glucose solution and tartaric acid solution in a mass ratio of 4 to 8:1, heat and boil for 5 to 10 minutes to obtain the reducing solution.

[0021] Step 3 specifically involves:

[0022] Step 3.1: Add the three-dimensional graphene powder pretreated in Step 1 to the silver ammonia solution in Step 2.1 and disperse it by ultrasonic vibration for 10-15 minutes to form a mixed dispersion.

[0023] Step 3.2: Slowly pour the reducing solution into the mixed dispersion from Step 3.1, and ultrasonically stir at 40–80°C for 10–30 min. After stirring, add NaOH solution with a mass concentration of 10–20 g / L dropwise over a period of 40–60 min. After the addition is complete, continue ultrasonic stirring for 40–60 min at a speed of 800–1000 rpm. Wash with water until neutral, filter, and dry to obtain silver-plated three-dimensional graphene.

[0024] In step 4, specifically: silver-plated three-dimensional graphene powder is added to the fully stirred polyurethane resin, followed by the addition of silane coupling agent and defoamer. The mixture is stirred for 10-30 minutes to form a mixed slurry. The mixed slurry is then uniformly coated onto the flexible fabric substrate using a coating machine and cured to obtain a flexible fabric microwave absorbing coating.

[0025] The curing time is 24–72 hours, and the curing temperature is 25–80℃.

[0026] The mass of the silver-plated three-dimensional graphene powder is 0.5 to 2.5% of the mass of the polyurethane resin.

[0027] During the coating process, the scraper runs at a speed of 2–4 mm / s, and the coating thickness ranges from 0.3 mm to 0.6 mm.

[0028] In step 5, specifically: the HFSS software is used to simulate the periodic structure frequency selective surface design of the flexible fabric microwave absorbing coating. Based on the simulation results, a hollow mold with the same periodic structure is prepared and attached to the surface of the flexible microwave absorbing coating. Then, an Al metal coating with a periodic structure is sputtered on the surface of the flexible fabric microwave absorbing coating using a magnetron sputtering coating process, thus obtaining a double-layer flexible microwave absorbing coating with a periodic structure frequency selective surface.

[0029] The beneficial effects of this invention are as follows:

[0030] (1) This invention pre-treats spherical graphene, which facilitates the unfolding of graphene, increases the surface area of ​​graphene, and allows for better deposition of nano-Ag. After graphene is combined with Ag particles, on the one hand, the increased interface improves the polarization effect of graphene powder; on the other hand, the introduction of Ag improves the conductivity of three-dimensional graphene powder.

[0031] (2) The Ag-plated three-dimensional graphene powder prepared by the present invention enhances the polarization loss capability of the coating. The enhanced conductivity helps to improve the conductivity loss capability of the coating. Therefore, the introduction of Ag helps to improve the material's ability to lose electromagnetic waves.

[0032] (3) By preparing a periodic structure frequency selective surface on the coating, the present invention can greatly optimize the wave absorption performance of the coating, which can not only broaden the absorption frequency band of the coating, but also enable the flexible coating to absorb electromagnetic waves in a wider range of incident angles. Attached Figure Description

[0033] Figure 1 This is the XRD pattern of Ag-coated three-dimensional graphene powder prepared by the method of this invention.

[0034] Figure 2 This is a TEM image of pure spherical graphene;

[0035] Figure 3 This is a TEM image (I) of Ag-coated three-dimensional graphene powder;

[0036] Figure 4 This is a TEM image (II) of Ag-coated three-dimensional graphene powder;

[0037] Figure 5 This is a diagram showing the microwave absorption performance of a flexible microwave absorbing coating containing pure spherical graphene.

[0038] Figure 6 This is a diagram showing the microwave absorption performance of a flexible microwave-absorbing coating made of Ag three-dimensional graphene powder. Detailed Implementation

[0039] The present invention will now be described in detail with reference to specific embodiments.

[0040] This invention relates to a method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene, which is specifically implemented according to the following steps:

[0041] Step 1: The three-dimensional graphene powder is subjected to alkali washing, sensitization, water washing, and drying in sequence to obtain pretreated three-dimensional graphene powder; specifically:

[0042] Step 1.1: Mix three-dimensional graphene powder with an average particle size of 50 nm and a purity greater than 98% with NaOH aqueous solution, so that the three-dimensional graphene powder is completely immersed in NaOH aqueous solution, and stir and wash with alkali for 10 to 30 minutes.

[0043] The mass fraction of the NaOH aqueous solution is 1.0–1.5%.

[0044] Step 1.2: Mix hydrochloric acid and SnCl2 aqueous solution at a volume ratio of 1-3:1 to form a sensitization treatment solution. Pour the three-dimensional graphene powder obtained in Step 1.1 into the sensitization treatment solution for sensitization treatment, ensuring the three-dimensional graphene powder is completely submerged in the sensitization treatment solution. The sensitization treatment time is 10-20 minutes. Wash with water and filter until the filtrate is used with Ag. + No white precipitate forms after the solution is added dropwise. Dry at 80–120°C for later use.

[0045] Step 2: Prepare silver ammonia solution and reducing solution; specifically:

[0046] Step 2.1: Prepare an AgNO3 solution with a mass concentration of 5-20 g / L, gradually add ammonia water until the solution is clear, then add a sodium dodecylbenzenesulfonate solution with a mass concentration of 1-1.5 g / L, ethylene glycol, and PVP with a mass concentration of 2-4 g / L. Stir at 40-60℃ for 20-30 min at a stirring speed of 800-1000 rpm until completely dissolved to obtain a silver ammonia solution.

[0047] Step 2.2: Mix glucose solution and tartaric acid solution in a mass ratio of 4 to 8:1, heat and boil for 5 to 10 minutes to obtain the reducing solution;

[0048] Step 3: Using the silver ammonia solution and reducing solution obtained in Step 2, perform silver ammonia plating on the pretreated three-dimensional graphene powder obtained in Step 1 to obtain silver-plated three-dimensional graphene powder; specifically:

[0049] Step 3.1: Add the three-dimensional graphene powder pretreated in Step 1 to the silver ammonia solution in Step 2.1 and disperse it by ultrasonic vibration to form a mixed dispersion.

[0050] The ultrasonic oscillation time is 10–15 min;

[0051] By attaching active groups to the surface of graphene, a repulsive force is generated, which makes the curled spherical graphene loose and facilitates the unfolding of graphene, increases the surface area of ​​the powder, and thus improves the adhesion rate of Ag atoms on the graphene surface.

[0052] The combined effect of additives and high-speed stirring is more conducive to the unfolding of spherical graphene, increasing the coating surface area of ​​graphene and facilitating the attachment of Ag atoms; while the addition of polyvinylpyrrolidone (PVP) is to prevent Ag from agglomerating after reduction, inhibiting the nucleation and growth of Ag particles, thereby avoiding the formation of coarse, continuous Ag particles.

[0053] Step 3.2: Slowly pour the reducing solution into the mixed dispersion from Step 3.1, and ultrasonically stir at 40–80°C for 10–30 min. After the stirring is complete, add a NaOH solution with a mass concentration of 10–20 g / L dropwise over a period of 40–60 min. After the addition is complete, continue ultrasonic stirring for 40–60 min at a speed of 800–1000 rpm. Wash with water until neutral, filter, and dry at 80°C–120°C to obtain silver-plated three-dimensional graphene.

[0054] The combined action of stirring and ultrasound not only facilitates the unfolding of graphene and the adhesion of Ag atoms to the graphene surface, but also prevents graphene from floating on the solution surface, achieving a uniform coating effect and avoiding the separate precipitation and sedimentation of Ag particles from the graphene.

[0055] Step 4: Mix silver-plated three-dimensional graphene with a polyurethane resin matrix to prepare a flexible fabric microwave absorbing coating.

[0056] Specifically, silver-plated three-dimensional graphene powder is added to a fully stirred polyurethane resin, followed by the addition of a silane coupling agent and a defoamer. The mixture is stirred for 10–30 minutes to form a slurry. The slurry is then uniformly coated onto a flexible fabric substrate using a coating machine with a blade speed of 2–4 mm / s and a coating thickness of 0.3 mm–0.6 mm. The coating is then cured to obtain a flexible fabric microwave absorbing coating.

[0057] The curing time is 24–72 hours, and the curing temperature is 25–80℃.

[0058] The mass of the silver-plated three-dimensional graphene powder is 0.5% to 2.5% of the mass of the polyurethane resin;

[0059] Step 5: Prepare a frequency-selective surface with a periodic structure on the coating surface to obtain a high-performance double-layer flexible microwave absorbing coating.

[0060] Specifically, the design of a periodic structure frequency-selective surface for the flexible fabric microwave absorbing coating is simulated using HFSS software. Based on the simulation results, a hollow mold with the same periodic structure is prepared and attached to the surface of the flexible microwave absorbing coating. Then, an Al metal coating with a periodic structure is sputtered onto the surface of the flexible fabric microwave absorbing coating using a magnetron sputtering process, thus obtaining a double-layer flexible microwave absorbing coating with a periodic frequency-selective surface.

[0061] The magnetron sputtering power is 100–120 W, the gas pressure is 0.3–0.5 Pa, and the magnetron sputtering time is 15–60 min.

[0062] The Ag-plated three-dimensional graphene prepared in this invention forms numerous interfaces between Ag and SG particles after Ag particles are deposited on the SG surface. Introducing Ag@SG into the resin matrix also increases the number of interfaces between the filler and the matrix and enhances the interfacial polarization effect. Furthermore, the introduction of Ag helps increase the concentration of free electrons, thereby generating a stronger relaxation polarization effect under an alternating electric field. Therefore, the enhancement of interfaces and the improvement of relaxation polarization effect are beneficial to improving the real part of the Ag@SG particles. As a folded three-dimensional nanomaterial, SG has a large specific surface area. When SG powder is combined with Ag particles, more interfaces are formed, which provides a higher real part of the complex permittivity for the Ag@SG powder.

[0063] Figure 1 This is the XRD pattern of silver-plated three-dimensional graphene powder, from... Figure 1 As can be seen from the data, in addition to the diffraction peaks of graphene, the diffraction peaks of Ag can also be clearly observed, indicating that silver-plated three-dimensional graphene powder has been successfully prepared.

[0064] Figure 2 and Figure 3 , Figure 4 These are TEM images of pure spherical graphene and Ag-coated three-dimensional graphene powder. From... Figure 2 It can be seen that the pure graphene particles before pretreatment are distinctly spherical, with some exhibiting severe agglomeration. The average particle size is approximately 50 nm, and the spherical particles are formed from rolled graphene sheets. Figure 3 As can be seen, the pretreated graphene effectively unfolds its sheets, increasing its surface area and making it more conducive to the adhesion of nano-Ag particles. From Figure 4 It can be seen that Ag particles are attached to the surface of spherical graphene, and the nano Ag particles have a wide range of particle sizes from 10 to 80 nm, indicating that the silver powder has been successfully attached to the spherical graphene.

[0065] Figure 5 and Figure 6 It refers to the microwave absorption performance of a flexible microwave-absorbing coating containing pure spherical graphene and Ag-coated three-dimensional graphene powder. For example... Figure 5 As shown, pure graphene has very poor wave absorption performance, with reflectivity above -5dB; after being combined with Ag, the wave absorption performance of graphene is greatly improved, with reflectivity below -10dB in a considerable frequency range.

[0066] Example 1

[0067] Take three-dimensional graphene powder with an average particle size of 20-100 nm and a purity greater than 98%. Pour the three-dimensional graphene powder into a NaOH aqueous solution, ensuring the powder is completely submerged. Stir with a stirring rod and perform alkaline washing on the powder using the NaOH aqueous solution for 10 minutes (NaOH aqueous solution mass fraction is 1.0%). Mix hydrochloric acid and SnCl2 aqueous solution at a volume ratio of 1:1, add 30 ml of deionized water for dilution, and stir for 10 minutes to form a sensitization treatment solution. Pour the alkaline-washed three-dimensional graphene powder into the sensitization treatment solution, ensuring the powder is completely submerged. Sensitize the powder using the solution for 15 minutes at room temperature with stirring. Wash with water and filter until neutral.

[0068] Prepare a 10 g / L AgNO3 solution, then gradually add ammonia water until the solution is clear. Then add a 1 g / L sodium dodecylbenzenesulfonate solution, 5 ml ethylene glycol, and a 3.0 g / L PVP solution. Stir at 40 °C for 20 min to obtain a silver ammonia solution.

[0069] A solution of glucose and tartaric acid in a mass ratio of 4:1 was heated to boiling for 5 minutes to obtain a reducing solution.

[0070] The treated three-dimensional graphene was added to a silver ammonia solution and ultrasonically dispersed for 10 min to form a mixed dispersion. The reducing solution was slowly poured into the mixed dispersion from step 3.1 and ultrasonically stirred at 40°C for 10 min. After the stirring was completed, a 10 g / L NaOH solution was added dropwise over a period of 40 min. After the addition was completed, ultrasonic stirring was continued for another 40 min at a speed of 800 rpm. The mixture was washed with water until neutral, filtered, and dried to obtain silver-plated three-dimensional graphene.

[0071] Add 0.5 wt.% silver-plated three-dimensional graphene powder to a fully stirred polyurethane resin, then add silane coupling agent and defoamer, stir for 20 min to form a mixed slurry, and use a coating machine with a scraper speed of 2 mm / s and a thickness range of 0.3 mm to uniformly coat the mixed slurry onto a flexible fabric substrate, and cure at 25℃ for 24 h to obtain a flexible fabric microwave absorbing coating.

[0072] The periodic structure frequency-selective surface design of the flexible fabric microwave absorbing coating was simulated using HFSS software. Based on the simulation results, a hollow mold with the same periodic structure was prepared and attached to the surface of the flexible microwave absorbing coating. Then, an Al metal coating with a periodic structure was sputtered on the surface of the flexible fabric microwave absorbing coating using a magnetron sputtering process, thus obtaining a double-layer flexible microwave absorbing coating with a periodic structure frequency-selective surface.

[0073] The magnetron sputtering power was 100W, the gas pressure was 0.3Pa, and the magnetron sputtering time was 15min.

[0074] Example 2

[0075] Take three-dimensional graphene powder with an average particle size of 20 nm and a purity greater than 98%. Pour the three-dimensional graphene powder into a NaOH aqueous solution, ensuring the powder is completely submerged. Stir with a stirring rod and perform alkaline washing on the powder using the NaOH aqueous solution for 15 minutes (NaOH aqueous solution mass fraction is 1.2%). Mix hydrochloric acid and SnCl2 aqueous solution at a volume ratio of 2:1, dilute with 40 ml of deionized water, and stir for 15 minutes to form a sensitization treatment solution. Pour the alkaline-washed three-dimensional graphene powder into the sensitization treatment solution, ensuring the powder is completely submerged. Sensitize the powder using the solution for 20 minutes at room temperature with stirring. Wash with water and filter until neutral.

[0076] Prepare an AgNO3 solution with a mass concentration of 15 g / L, then gradually add ammonia water until the solution is clear. Then add a sodium dodecylbenzenesulfonate solution with a mass concentration of 1.5 g / L, 8 ml of ethanol, and PVP with a mass concentration of 3.2 g / L. Stir at 50 °C for 25 min until completely dissolved to obtain a silver ammonia solution.

[0077] A solution of glucose and tartaric acid in a mass ratio of 6:1 was heated to boiling for 6 minutes to obtain a reducing solution.

[0078] The pretreated three-dimensional graphene powder was added to a silver ammonia solution and ultrasonically dispersed for 15 minutes to form a mixed dispersion.

[0079] The reducing solution was slowly poured into the mixed dispersion and ultrasonically stirred at 60°C for 20 min. After that, a NaOH solution with a mass concentration of 15 g / L was added dropwise over a time of 50 min. After the addition was completed, ultrasonic stirring was continued for another 50 min at a speed of 900 rpm. The mixture was washed with water until neutral, filtered, and dried to obtain silver-plated three-dimensional graphene.

[0080] Add 1 wt.% silver-plated three-dimensional graphene powder to a fully stirred polyurethane resin, then add silane coupling agent and defoamer, stir for 25 min to form a mixed slurry, and use a coating machine to uniformly coat the mixed slurry onto a flexible fabric substrate, and cure at 45℃ for 30 h to obtain a flexible fabric microwave absorbing coating.

[0081] The periodic structure design of the flexible fabric microwave absorbing coating surface was simulated using HFSS software. Based on the simulation results, a hollow mold with the same periodic structure frequency-selective surface was prepared and attached to the surface of the flexible microwave absorbing coating. Then, an Al metal coating with a periodic structure was sputtered on the surface of the flexible fabric microwave absorbing coating using a magnetron sputtering process, thus obtaining a double-layer flexible microwave absorbing coating with a periodic structure frequency-selective surface.

[0082] The magnetron sputtering power was 110 W, the gas pressure was 0.4 Pa, and the magnetron sputtering time was 30 min; Example 3

[0083] Take three-dimensional graphene powder with an average particle size of 100 nm and a purity greater than 98%. Pour the three-dimensional graphene powder into a NaOH aqueous solution, ensuring the powder is completely submerged. Stir with a stirring rod and perform alkaline washing on the powder using the NaOH aqueous solution for 30 minutes (NaOH aqueous solution mass fraction is 1.5%). Mix hydrochloric acid and SnCl2 aqueous solution at a volume ratio of 3:1, dilute with 50 ml of deionized water, and stir for 20 minutes to form a sensitization treatment solution. Pour the alkaline-washed three-dimensional graphene powder into the sensitization treatment solution, ensuring the powder is completely submerged. Sensitize the powder using the solution for 30 minutes at room temperature with stirring. Wash with water and filter until neutral.

[0084] Prepare an AgNO3 solution with a mass concentration of 20 g / L, then gradually add ammonia water until the solution is clear. Then add a sodium dodecylbenzenesulfonate solution with a mass concentration of 1.5 g / L, 10 ml of ethanol, and PVP with a mass concentration of 3.5 g / L. Stir at 60 °C for 30 min until completely dissolved to obtain a silver ammonia solution.

[0085] A solution of glucose and tartaric acid in a mass ratio of 8:1 was heated to boiling for 10 minutes to obtain a reducing solution.

[0086] The pretreated three-dimensional graphene powder was added to a silver ammonia solution and ultrasonically dispersed for 15 minutes to form a mixed dispersion.

[0087] The reducing solution was slowly poured into the mixed dispersion and ultrasonically stirred at 80°C for 30 min. After that, a NaOH solution with a mass concentration of 20 g / L was added dropwise over a time of 60 min. After the addition was completed, ultrasonic stirring was continued for another 60 min at a speed of 1000 rpm. The mixture was washed with water until neutral, filtered, and dried to obtain silver-plated three-dimensional graphene.

[0088] Add 2.5 wt.% silver-plated three-dimensional graphene powder to a fully stirred polyurethane resin, then add silane coupling agent and defoamer, stir for 30 min to form a mixed slurry, and use a coating machine with a scraper coating method (scraper running speed of 4 mm / s and coating thickness of 0.6 mm) to uniformly coat the mixed slurry onto a flexible fabric substrate and cure it to obtain a flexible fabric microwave absorbing coating.

[0089] The periodic structure frequency-selective surface design of the flexible fabric microwave absorbing coating was simulated using HFSS software. Based on the simulation results, a hollow mold with the same periodic structure was prepared and attached to the surface of the flexible microwave absorbing coating. Then, an Al metal coating with a periodic structure was sputtered on the surface of the flexible fabric microwave absorbing coating using a magnetron sputtering process, thus obtaining a double-layer flexible microwave absorbing coating with a periodic structure frequency-selective surface.

[0090] The magnetron sputtering power was 120W, the gas pressure was 0.5Pa, and the magnetron sputtering time was 60min.

[0091] The present invention provides silver-plated three-dimensional graphene powder prepared by chemical plating. While retaining the excellent properties of graphene, the introduction of silver can effectively improve the conductivity of the powder, thereby greatly improving its dielectric and microwave absorption properties.

[0092] This invention prepared silver-plated three-dimensional graphene powder and added it to polyurethane resin to prepare a flexible fabric microwave absorbing coating. The phase composition and morphology of the Ag@SG powder were characterized by XRD and TEM. After being compounded with Ag, the complex dielectric constant of SG was enhanced, increasing with increasing Ag content. The coating containing 20Ag@SG exhibited the highest dielectric constant and the best microwave absorption performance. For a coating with a thickness of 3.0 mm, the minimum reflection loss was -15 dB at 10.2 GHz, and the effective absorption bandwidth was 9.08-11.52 GHz. Combining the flexible fabric microwave absorbing coating of this invention with an FSS patch significantly improved the microwave absorption performance. Furthermore, the flexible fabric microwave absorbing coating possesses an effective absorption bandwidth over a wide angular range.

Claims

1. A method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene, characterized in that, The specific steps are as follows: Step 1: Pre-treat the three-dimensional graphene powder; specifically: Step 1.1: Mix three-dimensional graphene powder with an average particle size of 50nm and a purity greater than 98% with NaOH aqueous solution and stir and wash with alkali. Step 1.2: Mix hydrochloric acid and SnCl2 aqueous solution at a volume ratio of 1~3:1 to form a sensitization treatment solution. Pour the three-dimensional graphene powder obtained in step 1.1 into the sensitization treatment solution for sensitization treatment. Wash with water and filter until the filtrate is used with Ag. + No white precipitate formed after the solution was added dropwise; dry and set aside. Step 2: Prepare silver ammonia solution and reducing solution; Step 3: Use the silver ammonia solution and reducing solution obtained in Step 2 to perform silver ammonia plating on the pretreated three-dimensional graphene powder obtained in Step 1 to obtain silver-plated three-dimensional graphene powder. Step 4: Mix silver-plated three-dimensional graphene with a polyurethane resin matrix to prepare a flexible fabric microwave absorbing coating. Step 5: Prepare a frequency-selective surface with a periodic structure on the coating surface to obtain a double-layer flexible microwave absorbing coating.

2. The method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene according to claim 1, characterized in that, Step 2 specifically involves: Step 2.1: Prepare an AgNO3 solution with a mass concentration of 5~20 g / L, gradually add ammonia water until the solution is clear, then add a sodium dodecylbenzenesulfonate solution with a mass concentration of 1~1.5 g / L, ethylene glycol, and PVP with a mass concentration of 2~4 g / L. Stir at 40~60℃ for 20~30 min at a stirring speed of 800~1000 rpm until completely dissolved to obtain a silver ammonia solution. Step 2.2: Mix glucose solution and tartaric acid solution in a mass ratio of 4 to 8:1, heat and boil for 5 to 10 minutes to obtain the reducing solution.

3. The method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene according to claim 1, characterized in that, Step 3 specifically involves: Step 3.1: Add the three-dimensional graphene powder pretreated in Step 1 to the silver ammonia solution in Step 2.1 and disperse it by ultrasonic vibration for 10-15 min to form a mixed dispersion. Step 3.2: Slowly pour the reducing solution into the mixed dispersion from Step 3.1, and ultrasonically stir at 40-80℃ for 10-30 min. After the stirring is complete, add a NaOH solution with a mass concentration of 10-20 g / L dropwise over a period of 40-60 min. After the addition is complete, continue ultrasonic stirring for 40-60 min at a speed of 800-1000 rpm. Wash with water until neutral, filter, and dry to obtain silver-plated three-dimensional graphene.

4. The method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene according to claim 1, characterized in that, In step 4, specifically: silver-plated three-dimensional graphene powder is added to the fully stirred polyurethane resin, followed by the addition of silane coupling agent and defoamer. The mixture is stirred for 10-30 minutes to form a mixed slurry. The mixed slurry is then uniformly coated onto the flexible fabric substrate using a coating machine and cured to obtain a flexible fabric microwave absorbing coating.

5. The method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene according to claim 4, characterized in that, The curing time is 24~72h, and the curing temperature is 25~80℃.

6. The method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene according to claim 4, characterized in that, The mass of the silver-plated three-dimensional graphene powder is 0.5 to 2.5% of the mass of the polyurethane resin.

7. The method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene according to claim 4, characterized in that, During the coating process, the scraper speed is 2~4 mm / s, and the coating thickness ranges from 0.3mm to 0.6mm.

8. The method for preparing a double-layer flexible microwave absorbing coating based on Ag-plated three-dimensional graphene according to claim 1, characterized in that, In step 5, specifically: the periodic structure frequency selective surface design of the flexible fabric microwave absorbing coating surface is simulated using HFSS software. Based on the simulation results, a hollow mold with the same periodic structure is prepared and attached to the surface of the flexible microwave absorbing coating. Then, an Al metal coating with a periodic structure is sputtered on the surface of the flexible fabric microwave absorbing coating using a magnetron sputtering coating process, thereby obtaining a double-layer flexible microwave absorbing coating with a periodic structure frequency selective surface.

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