Melamine foam with broadband wave absorbing performance and preparation of structure / wave absorbing integrated composite material thereof

Abstract

The application provides melamine foam with wideband wave-absorbing performance and a structure / wave-absorbing integrated composite material prepared from the melamine foam, and belongs to the technical field of advanced materials. The melamine foam is prepared by immersing melamine foam in graphene oxide dispersion liquid, drying, immersing in a dispersion liquid containing a reducing agent, reducing and foaming, and then annealing. The application adopts a specific preparation method, and successfully obtains RGO / MF foam with wideband wave-absorbing performance. In addition, the application fills the RGO / MF foam in aramid honeycomb by means of liquid nitrogen and rapid pressurization, and the composite material obtained by filling the RGO / MF foam in the aramid honeycomb has excellent wideband wave-absorbing performance without affecting the mechanical property of the aramid honeycomb, and can realize ultra-wide effective absorption (<-10dB) in the range of 3-18GHz. The RGO / MF foam prepared by the application and the composite material obtained by filling the RGO / MF foam in the aramid honeycomb have excellent wave-absorbing performance, and show wide application prospects in the stealth field of aerospace.

Description

Technical Field

[0001] This invention belongs to the field of advanced materials technology, specifically relating to a melamine foam with broadband microwave absorption properties and the structural / microwave absorption integrated composite material prepared therefrom. Background Technology

[0002] With the rapid development of electronic technology, especially the explosive growth of multi-band and broadband electronic instruments, there is a strong demand in both civilian and military fields for high-performance microwave electromagnetic (MA) materials with broadband absorption (MA) capabilities, as they can effectively convert the energy of incident electromagnetic waves into heat loss. Compared with other types of MA materials, carbon-based materials exhibit many superior properties, including good corrosion resistance, lightweight, a wide MA frequency range, and high thermal stability, making them a strong candidate material for lightweight and broadband MA materials.

[0003] Carbon-based materials such as carbon black, graphene, carbon nanotubes (CNTs), and their composites hold a superior position in the field of high-frequency electromagnetic wave absorption. Graphene not only possesses a stable structure but also exhibits a high specific surface area and excellent electronic conductivity, making it highly advantageous in aerospace, automotive, and miniaturized electronics applications. Furthermore, reduced graphene oxide (RGO), derived from graphene oxide (GO), possesses numerous defect sites and functional groups during the reduction process, providing more polarization sites and conductive loss mechanisms, leading to improved impedance matching and MA performance. Unlike electromagnetic interference (EMI) materials that only consider improving conductivity, MA materials require moderate loss characteristics so that electromagnetic waves can enter and generate losses. However, due to the π-π stacking and van der Waals forces between graphene sheets, untreated RGO easily self-aggregates, maintaining high conductivity. This necessitates the development of three-dimensional structures (introducing air) or composite materials to improve its impedance matching and MA performance.

[0004] The porous structure gives foamed MA materials a larger surface area and significantly more reflections of incident electromagnetic waves, which allows the foam to better absorb and attenuate microwaves. To achieve the dispersion of RGO from a two-dimensional planar structure to a three-dimensional network, commercial melamine foam (MF) can be used as an active carrier for conductive fillers. It has gained popularity due to its high strength, high porosity (over 99%), low density, good elasticity, and low cost. Furthermore, the surface of MF is hydrophilic, which facilitates GO distribution. Under the same conditions, the MA performance of RGO sponge with a porous framework structure is much better than that of RGO powder. Patent application CN111944196A discloses a method for preparing a smart microwave absorbing foam material, including immersing melamine foam in a graphene oxide solution followed by vacuum treatment to uniformly disperse the graphene oxide on the surface of the melamine foam framework, and then performing a hydrothermal reduction treatment using a reducing agent to reduce the graphene oxide. The microwave absorbing foam material obtained by this patent has good microwave absorption performance, but the effective absorption bandwidth is narrow, failing to achieve broadband microwave absorption. Therefore, it is necessary to improve and provide a wave-absorbing foam material with better wave-absorbing effect.

[0005] Furthermore, to better utilize microwave absorbing foam materials in the aerospace field, strengthening the structure of the foam material is also crucial. Aramid honeycomb (hexagonal mesh) structures have attracted widespread attention due to their high strength, low weight, and excellent thermal insulation properties, and can serve as active carriers for MA materials. However, filling honeycomb structures with MA materials while ensuring that the weight gain does not exceed 30% remains a significant challenge. In existing technologies, filling aramid honeycomb with MA materials is difficult. Most methods employ epoxy resin coatings or use MA materials coated on glass cloth to construct composite materials for the honeycomb. However, these methods lead to low utilization of the honeycomb pores and problems such as reflection loss and mismatch between the coating and the air interface. Simultaneously, due to aircraft weight limitations, this approach inevitably results in excessive weight and low utilization. Compared to traditional MA coatings, filling aramid honeycomb with lightweight microwave absorbing foam will significantly promote aircraft weight reduction. However, due to the difficulty of honeycomb filling and thickness limitations, there is currently no research on the preparation of integrated microwave absorbing structures using lightweight foam and aramid honeycomb composites. Effectively utilizing the internal space of aramid honeycomb and fully filling the honeycomb with lightweight, broadband MA material remains a huge challenge. Summary of the Invention

[0006] The purpose of this invention is to provide a melamine foam with broadband microwave absorption properties and the structural / microwave absorption integrated composite material prepared therefrom.

[0007] This invention provides a reduced graphene oxide / melamine foam, which is prepared by wetting and drying melamine foam in a graphene oxide dispersion, then immersing it in a dispersion containing a reducing agent for reduction foaming, and finally annealing it.

[0008] Furthermore, the concentration of the graphene oxide dispersion is 0.1% to 1%.

[0009] And / or, the concentration of the reducing agent in the dispersion containing the reducing agent is 1-80%;

[0010] And / or, the number of times the immersion and drying is performed is one or more;

[0011] Preferably,

[0012] The concentration of the graphene oxide dispersion is 0.2-0.5%;

[0013] And / or, the concentration of the reducing agent in the dispersion containing the reducing agent is 10-40%;

[0014] And / or, the immersion and drying are performed at least twice;

[0015] More preferably,

[0016] The method of impregnation and drying is as follows: melamine foam is fully impregnated in graphene oxide dispersion and then dried at 60-120°C.

[0017] Furthermore, the dispersion medium of the graphene oxide dispersion is one or more of water, ethanol, methanol, and glycerol;

[0018] And / or, the reducing agent is one or more of vitamin C, sodium bicarbonate, sodium borohydride, hydrazine hydrate, and hydroquinone;

[0019] And / or, the dispersion medium of the dispersion containing the reducing agent is one or more of water, ethanol, methanol, and glycerol.

[0020] Furthermore, the method of reducing foaming is as follows: the foam after being soaked and dried is immersed in a dispersion containing a reducing agent and kept at 40-80°C for 1-60 minutes, and then the foam is dried at 80-90°C for 6-18 hours;

[0021] And / or, the annealing method is as follows: after reduction foaming, the temperature is raised to 100-200℃ and held for 2-8 hours.

[0022] Furthermore, the thickness of the melamine foam is 10–50 mm;

[0023] Preferably, the thickness of the melamine foam is 12-30 mm.

[0024] The present invention also provides a method for preparing the aforementioned reduced graphene oxide / melamine foam, which includes the following steps:

[0025] (1) Melamine foam was soaked in graphene oxide dispersion and then dried;

[0026] (2) Then the foam is immersed in a dispersion containing a reducing agent for reduction foaming;

[0027] (3) Finally, the foam is annealed and cooled to obtain the final product;

[0028] Preferably,

[0029] In step (1), the concentration of the graphene oxide dispersion is 0.1-1%;

[0030] And / or, in step (1), the number of times the immersion and drying is performed is one or more;

[0031] And / or, in step (2), the concentration of the reducing agent in the dispersion containing the reducing agent is 1-80%;

[0032] And / or, in step (2), the method of reducing foaming is as follows: immerse the soaked and dried foam in a dispersion containing a reducing agent and keep it at 40-80℃ for 1-60 minutes, and then put the foam in 80-90℃ to dry for 6-18 hours;

[0033] And / or, in step (3), the annealing method is: after reduction foaming, the temperature is raised to 100-200℃ and held for 2-8 hours;

[0034] More preferably,

[0035] In step (1), the concentration of the graphene oxide dispersion is 0.2-0.5%;

[0036] And / or, in step (1), the method of impregnation and drying is: immersing melamine foam in graphene oxide dispersion for full impregnation, and then drying at 60-120°C;

[0037] And / or, in step (2), the concentration of the reducing agent in the dispersion containing the reducing agent is 10-40%.

[0038] The present invention also provides the use of the aforementioned reduced graphene oxide / melamine foam in the preparation of integrated structural / wave-absorbing composite materials.

[0039] The present invention also provides a structural / wave-absorbing integrated composite material, which is obtained by filling aramid honeycomb with the aforementioned reduced graphene oxide / melamine foam;

[0040] Preferably, the aramid honeycomb lattice cell has a side length of 2.85 mm and a density of 80 kg·m³.-3 The wall thickness is 0.15mm.

[0041] The present invention also provides a method for preparing the aforementioned integrated structural / wave-absorbing composite material, which includes the following steps:

[0042] 1) Immerse the aforementioned reduced graphene oxide / melamine foam in liquid nitrogen and freeze;

[0043] 2) Place the frozen reduced graphene oxide / melamine foam on the surface of the aramid honeycomb and press it into the aramid honeycomb quickly with external force to completely fill the aramid honeycomb.

[0044] Preferably, in step 1), the freezing time is 1 to 60 minutes;

[0045] And / or, in step 2), the magnitude of the external force is 0.05 to 0.5 MPa;

[0046] And / or, in step 2), the reduced graphene oxide / melamine foam pressed into the aramid honeycomb can be one or more layers, enough to fully fill the interior of the aramid honeycomb.

[0047] The present invention also provides the use of the aforementioned reduced graphene oxide / melamine foam, or the aforementioned integrated structural / wave-absorbing composite material, in the preparation of stealth materials;

[0048] Preferably, the stealth material is a stealth material used in the aerospace field.

[0049] Graphene materials have attracted widespread attention for electromagnetic wave suppression due to their numerous remarkable properties, including low density, high specific surface area, large aspect ratio, and versatility in processing. However, single graphene materials exhibit poor dispersion in the matrix, interfacial impedance mismatch due to unsuitable conductivity, and limited loss mechanisms. To date, all reported graphene-based MA composites have only achieved absorption bandwidths of a few GHz with reflection losses (RL) less than -10 dB.

[0050] Developing lightweight, efficient, and high-load-bearing microwave absorbing materials is a challenge in the aerospace field. Without a reliable mechanical carrier, it is difficult to apply microwave absorbing materials to real-world aerospace scenarios. Based on this, this invention prepares reduced graphene oxide / melamine foam (RGO / MF) with broadband microwave absorption properties, and then, for the first time, fills it into aramid honeycomb to obtain an integrated structural / microwave-absorbing composite material.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] (1) This invention employs a specific RGO / MF foam preparation method. MF achieves the three-dimensional macroscopic dispersion of RGO, while chemical reduction foaming achieves the microscopic three-dimensional separation of RGO, synergistically enhancing the dispersion of RGO. This successfully yields an ultra-low density RGO / MF foam with broadband microwave absorption properties. The RGO / MF foam prepared by this invention exhibits excellent microwave absorption performance.

[0053] (2) This invention completely fills the aramid honeycomb with RGO / MF foam using liquid nitrogen and rapid pressurization. The resulting composite material achieves ultrawide effective absorption in the 3-18 GHz range without affecting the mechanical properties of the aramid honeycomb. The microwave absorption performance of the composite material originates from the electric field loss of RGO / MF. The cavity of the hexagonal honeycomb allows microwaves to propagate inside the honeycomb, resulting in sufficient microwave matching depth and multiple losses.

[0054] (3) The lightweight broadband structure / wave-absorbing integrated composite material prepared by this invention ensures its tolerance and stability in harsh environments due to the high-temperature preparation process. The composite material prepared based on hydrophobic RGO foam can still maintain stable and reliable performance in high-temperature and humid environments.

[0055] In summary, this invention employs a specific RGO / MF foam preparation method to successfully obtain an RGO / MF foam with broadband microwave absorption properties. Furthermore, this invention, through liquid nitrogen and rapid pressurization, completely fills the aramid honeycomb with RGO / MF foam, resulting in a composite material that also exhibits excellent broadband microwave absorption performance without affecting the mechanical properties of the aramid honeycomb, achieving ultra-wide effective absorption (<-10dB) in the 3-18GHz range. The RGO / MF foam prepared by this invention, and the composite material obtained by filling aramid honeycomb with RGO / MF foam, demonstrate excellent microwave absorption performance, showing broad application prospects in the stealth field of aerospace.

[0056] All RGO / MF foams prepared by this invention can rapidly fill honeycomb structures of various thicknesses. In the standardized preparation process, firstly, absorbing foams with specific optimized electromagnetic parameters are selected, and then uniformly cut into 10mm thick pieces for later use. Depending on the honeycomb thickness, the 10mm thick standard foam pieces are layered and filled until the honeycomb is completely filled. At this point, the absorption performance of the honeycomb is the same as the foam absorption performance corresponding to the honeycomb thickness. For example, in a 30mm honeycomb, three layers of 10mm thick absorbing foam are used for layered filling. When all three layers of foam are filled into the honeycomb, the absorption performance of the honeycomb is the same as that of the 30mm thick foam.

[0057] Obviously, based on the above description of the present invention, and according to common technical knowledge and conventional methods in the field, various other modifications, substitutions or alterations can be made without departing from the basic technical concept of the present invention.

[0058] The following detailed embodiments further illustrate the above-described content of the present invention. However, this should not be construed as limiting the scope of the present invention to the following examples. All technologies implemented based on the above-described content of the present invention fall within the scope of the present invention. Attached Figure Description

[0059] Figure 1 The following are schematic diagrams illustrating the synthesis of RGO / MF foam and RGO / MF foam-filled aramid honeycomb according to the present invention: where a is a schematic diagram illustrating the synthesis of RGO / MF foam; and b is a schematic diagram illustrating the synthesis of RGO / MF foam-filled aramid honeycomb.

[0060] Figure 2 This is a schematic diagram of the microwave absorption of the dual three-dimensional composite microwave absorbing foam filled with aramid honeycomb and an overview of the electromagnetic absorption mechanism of the present invention.

[0061] Figure 3 The results of the study on the microwave absorption performance of the dual-three-dimensional composite microwave absorbing foam filled with aramid honeycomb are shown in this invention; where a is a simulation diagram and b is the microwave absorption performance of the composite material.

[0062] Figure 4 The microwave absorption properties of 0.2% D1-RA foams with thicknesses of 15 mm and 30 mm prepared for Comparative Example 1.

[0063] Figure 5 Microwave absorption properties and macroscopic images of a 15 mm thick 0.4% D1-RA foam prepared for Comparative Example 2.

[0064] Figure 6 The images shown are scanning electron microscope images of each group of foams and the schematic diagram of D2-RA: where a is a scanning electron microscope image and b is the schematic diagram.

[0065] Figure 7 The results are Raman spectroscopy findings for each group of foams.

[0066] Figure 8 Electromagnetic parameters of D2-RA foam prepared with 0.2% GO concentration were obtained using a vector network analyzer and the coaxial circular ring test method (inner diameter 3.04 mm, outer diameter 7 mm).

[0067] Figure 9 The microwave absorption properties of RGO / MF foam with a thickness of 10-30 mm and a GO concentration of 0.2% are given; where a represents the microwave absorption properties of D2-RA foam and b represents the microwave absorption properties of D3-RA foam.

[0068] Figure 10 The compressive strength of D2-RA foam (RGO / MF in the figure), aramid honeycomb panel, and composite aramid honeycomb panel filled with D2-RA foam, all with a thickness of 30 mm and a GO concentration of 0.2%, is measured. Detailed Implementation

[0069] The raw materials and equipment used in the specific embodiments of the present invention are all known products, obtained by purchasing commercially available products.

[0070] A 1% (w / v) graphene oxide (GO) aqueous dispersion was provided by Sichuan Xitan Technology Co., Ltd. (Deyang, Sichuan), with a solid content of 1.0±0.1%, GO monolayer ratio >50%, average thickness <1.5nm, and sheet diameter D50 ≤4.0μm. The density of the melamine (MF) foam was 10 kg·m³. -3 The product is from Zhengzhou Fengtai Nanomaterials Co., Ltd. (Zhengzhou, China). 80% (v / v) hydrazine hydrate and solid paraffin are both from Aladdin Co., Ltd. (Shanghai, China). The para-aramid honeycomb material provided by AVIC Composite Materials Technology Co., Ltd. (Beijing) has a lattice cell edge length of 2.85 mm and a density of 80 kg·m³. -3 The wall thickness was 0.15 mm. Liquid nitrogen was provided by the Materials Supply Department of Sichuan University. All reagents were analytical grade and used without purification. Deionized water was used for preparation and characterization in the experiment.

[0071] The relevant characteristics involved in the specific embodiments of the present invention are as follows:

[0072] Thermogravimetric analysis (TGA) was performed using a thermogravimetric analyzer (TG 209F1, Netzsch Ltd., Germany) under nitrogen atmosphere at a temperature range of 30–800 °C and a heating rate of 5 °C / min. Functional group variations in the samples were studied using an FTIR spectrophotometer (Nicolet 570, USA). The phase composition of the samples was characterized using a Cu-Kα radiation X-ray diffractometer (XRD, DY-1291, Philips, Netherlands).

[0073] The 500 cm⁻¹ range was studied using an Ar ion laser (λ = 532 nm) on a confocal Raman spectroscopy system (Raman, Dilor, France). -1 up to 3000cm -1 Raman spectra.

[0074] The water contact angle (WCA) values ​​of the samples were measured using a WCA analyzer (OCA20, Data Physics, Germany). The combination of X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher) was investigated using advanced XPS, with high-resolution C1s elemental spectra fitted.

[0075] The structure and surface morphology of the samples were observed using a high-field emission scanning electron microscope (SEM, Quanta 250, FEI Co. Ltd, USA).

[0076] The foam was cut into ring-shaped samples with an outer diameter of 7 mm and an inner diameter of 3.04 mm. The real and imaginary parts of the dielectric constant and magnetic permeability of the foam were measured using a coaxial circular test method connected to a two-port waveguide of a vector network analyzer (VNA, Agilent N5230, USA).

[0077] To obtain the electromagnetic wave loss performance of the samples, the bow-shaped method of the Naval Research Laboratory (NRL) was used, and the same VNA was used to measure the cellular composite material according to the national standard GJB2038A-2011. Two horn antennas connected to the VNA were used to transmit and receive microwave signals in the frequency range of 2-18 GHz.

[0078] The mechanical test was conducted using 1000 kg SANS according to ISO 844:2004 standard, with foam dimensions of 50 mm × 50 mm × 50 mm and a loading speed of 10 mm / min. Honeycomb samples were prepared with dimensions of 50 mm × 50 mm × 30 mm.

[0079] Figure 1 This is a schematic diagram illustrating the synthesis of RGO / MF foam and the filling of aramid honeycomb with RGO / MF foam according to the present invention.

[0080] Example 1: Preparation of RGO / MF foam and preparation of composite materials by filling aramid honeycomb with RGO / MF foam.

[0081] 1. Preparation of reduced graphene oxide / melamine (RGO / MF) foam

[0082] Cut the MF foam into 10mm thick sheets.

[0083] The 1% GO dispersion was diluted with water to a concentration of 0.2%, meaning 0.2g of GO per 100ml of dispersion. Then, the MF foam was fully impregnated in the GO dispersion using a foam extrusion and re-absorption process. The fully impregnated MF foam was removed from the GO dispersion and dried in an oven at 80℃. The dried MF foam was then subjected to the same impregnation and drying process once more to obtain MF foam that had been impregnated and dried twice, i.e., D2 foam.

[0084] 80% hydrazine hydrate was mixed with water and stirred thoroughly to obtain a 10% hydrazine hydrate dispersion. The dispersion was placed in a water bath and slowly heated to 50°C for later use. D2 foam was fully immersed in the 10% hydrazine hydrate dispersion and reduced in a 50°C water bath for 30 minutes. The foam was then placed in an oven at 80°C for 6 hours to obtain a reduced D2 foam sample, named D2-R foam. The oven temperature was then further increased to 180–182°C and held for 2 hours for annealing to remove residual reducing agent. After cooling, D2-RA foam (0.2% D2-RA), i.e., RGO / MF foam, was obtained.

[0085] 2. Preparation of composite materials by filling aramid honeycomb with RGO / MF foam.

[0086] The RGO / MF foam prepared above was completely immersed in liquid nitrogen and frozen for 1 minute. Then, it was placed on the surface of the aramid honeycomb material, and the RGO / MF foam was rapidly pressed into the honeycomb using a uniform external force (which can be applied by an iron block, iron plate, or steel plate, with a force of 0.2 MPa). In this embodiment, the aramid honeycomb thickness is 30 mm, therefore, three layers of 10 mm thick RGO / MF foam were used for filling, with each filling method described above. This completes the filling of the aramid honeycomb material, resulting in a dual-three-dimensional composite microwave-absorbing foam-filled aramid honeycomb composite material (RGO / MF honeycomb composite material).

[0087] The electromagnetic absorption principle of the RGO / MF honeycomb composite material of this invention is as follows: Figure 2As shown: Based on the microwave absorption properties of the prepared composite material, a possible microwave absorption (MA) mechanism of RGO / MF is proposed: (i) Aramid honeycomb, as a wave-transparent material, can improve impedance and promote the incidence of electromagnetic waves. The wave-transparent material of aramid honeycomb improves the impedance of the cell relative to air, generating standing waves in the structure and the adjacent space. (ii) At the macroscopic scale, the three-dimensional macroscopic dispersion of RGO provided by MF can introduce air between the foam skeletons, inducing multipolarization at the heterogeneous interface. Therefore, due to the introduction of air, multiple reflections may occur between the foam skeletons, inducing MA performance and further converting electromagnetic energy into thermal energy. (iii) At the microscopic scale, as a two-dimensional crystalline material, RGO can act as a capacitor between two sheets, generating charge polarization between RGO sheets. When the polarization process cannot keep up with the change of electric field, interfacial polarization relaxation occurs. RGO / MF has different defects with the introduction of hybrid N atoms, which may lead to more polarization relaxation and EMW absorption processes, increasing MA intensity. In addition, RGO allows more carrier migration to occur, which enhances conduction loss. When electrons migrate through RGO atoms, electromagnetic energy is converted into thermal energy through collisions with the lattice. During this process, a macroscopic dipole moment of Debye relaxation is also generated to dissipate electromagnetic waves. In summary, RGO / MF honeycomb composite materials were successfully fabricated using the aforementioned multi-scale microwave loss mechanism, achieving ultra-wideband and high-density microwave (MA) performance.

[0088] The wave absorption properties of honeycomb composite materials, such as Figure 3 As shown in Figure a, the simulated reflectivity of the D2-RA filled honeycomb composite material is similar to the absorption performance (RL) value of the D2-RA foam. The small fluctuation in RL is likely due to the induction of Nomex honeycomb pores, which, as a wave-transmitting material, partially reduces the RL value of the composite material. Furthermore, as... Figure 3 As shown in b, the experimental results generally agree with the simulation results. The experimental reflectivity achieved an absorptivity of -10 dB in the 2–18 GHz range and <-20 dB above 10 GHz. The compression deformation of the D2-RA foam during lamination is likely the reason for the improved actual performance—achieving an indirect gradient parameter change. Due to the excellent properties of D2-RA foam and honeycomb in promoting electromagnetic wave incidence, the fabricated honeycomb composite achieved ultrawide effective absorption in the 3–18 GHz range. These results validate the application of MA foam in aramid honeycomb to construct lightweight broadband structures / wave-absorbing integrated components.

[0089] Comparative Example 1: Preparation of other RGO / MF foams

[0090] Cut MF foam into sheets with thicknesses of 15mm and 30mm.

[0091] The 1% GO dispersion was diluted with water to a concentration of 0.2%, meaning 0.2g of GO per 100ml of dispersion. Then, the MF foam was fully impregnated into the GO dispersion using a foam extrusion and re-absorption process to ensure complete impregnation. The fully impregnated MF foam was removed from the GO dispersion and dried in an oven at 80℃ to obtain MF foam that has been impregnated and dried once, i.e., D1 foam.

[0092] 80% hydrazine hydrate was mixed with water and stirred thoroughly to obtain a 10% hydrazine hydrate dispersion. The dispersion was placed in a water bath and slowly heated to 50°C for later use. D1 foam was fully immersed in the 10% hydrazine hydrate dispersion and reduced in a 50°C water bath for 30 minutes. The foam was then placed in an oven and dried at 80°C for 6 hours. The oven temperature was then increased to 180–182°C and maintained for 2 hours for annealing to remove residual reducing agent. After cooling, D1-RA foam (0.2% D1-RA), i.e., RGO / MF foam, was obtained.

[0093] The microwave absorption properties of D1-RA foams with thicknesses of 15 mm and 30 mm prepared by the above method were tested respectively, and the results are as follows: Figure 4 As shown. By Figure 4 It is evident that RGO / MF foam obtained using only a single impregnation method cannot achieve excellent microwave absorption performance at thicknesses of 15mm or 30mm, and cannot achieve broadband microwave absorption. Reliable absorption of -10dB cannot be achieved in the 2-18GHz range.

[0094] Comparative Example 2: Preparation of other RGO / MF foams

[0095] Cut the MF foam into 15mm thick sheets.

[0096] The 1% GO dispersion was diluted with water to a concentration of 0.4%, meaning 0.4 g of GO was present per 100 ml of dispersion. Then, the MF foam was fully impregnated into the GO dispersion using a foam extrusion and re-absorption process to ensure complete impregnation. The fully impregnated MF foam was removed from the GO dispersion and dried in an oven at 80°C to obtain MF foam that had undergone one impregnation and drying process, i.e., D1 foam.

[0097] 80% hydrazine hydrate was mixed with water and stirred thoroughly to obtain a 10% hydrazine hydrate dispersion. The dispersion was placed in a water bath and slowly heated to 50°C for later use. D1 foam was fully immersed in the 10% hydrazine hydrate dispersion and kept at 50°C for 30 minutes during reduction in the water bath. The foam was then placed in an oven and dried at 80°C for 6 hours. The oven temperature was then increased to 180–182°C and held for 2 hours for annealing to remove residual reducing agent. After cooling, D1-RA foam (0.4% D1-RA), i.e., RGO / MF foam, was obtained.

[0098] The microwave absorption properties of the D1-RA foam prepared by the above method were tested. The results are as follows: Figure 5 As shown. By Figure 5 It can be seen that due to the high GO impregnation concentration in the sample, a skin forms on the surface during the drying process, preventing electromagnetic waves from penetrating and producing a significant skin reflection effect. Therefore, the resulting RGO / MF foam cannot achieve excellent microwave absorption performance.

[0099] The following specific experimental examples demonstrate the beneficial effects of the present invention.

[0100] Experimental Example 1: SEM observation of the RGO / MF foam of the present invention

[0101] 1. Experimental Methods

[0102] Sample sections were prepared using foam with a thickness of 10 mm.

[0103] D2 foam, D2-R foam, and D2-RA foam were prepared according to the method described in Example 1 and observed by SEM. Simultaneously, D2-A foam was also observed by SEM.

[0104] The preparation method of D2-A foam is as follows: MF foam is cut into thin sheets with a thickness of 10 mm. A 1% GO dispersion is diluted with water to a concentration of 0.2%, i.e., 0.2 g of GO per 100 ml of dispersion. Then, the MF foam is fully impregnated into the GO dispersion using a foam extrusion and re-absorption process to ensure complete impregnation. The fully impregnated MF foam is removed from the GO dispersion and dried in an oven at 80°C. The dried MF foam is then subjected to the same impregnation and drying process once more to obtain MF foam that has been impregnated and dried twice, i.e., D2 foam. The D2 foam is then annealed by heating it to 180–182°C and holding it at that temperature for 6 hours to obtain annealed D2-A foam.

[0105] 2. Experimental Results

[0106] To investigate the microstructure and adhesion of GO / RGO in MF, this invention presents SEM images of D2 foam, D2-R foam, D2-RA foam, and D2-A foam, such as... Figure 6 As shown in Figure a: Both D2 and D2-A foams exhibit a tight layered structure without interlayer delamination or loose surface wrinkles. In contrast, the D2-R and D2-RA samples prepared by chemical reduction show a sparse layered structure with obvious wrinkles on the film surface. This is due to the escape of the volatile atmosphere during the reduction process. Figure 6As shown in b, after hydrazine hydrate reacts with the carboxyl and hydroxyl groups in the GO film, the resulting N2 and water vapor diffuse from the GO film interlayer to the external environment. This generates sufficient internal pressure to overcome the van der Waals forces that hold the GO layers together. Compared to the D2-R sample, more undulating waves and a fluffy appearance were observed in D2-RA, which is related to the reduction and increased gas volatilization rate during annealing. The above results indicate that a fluffy RGO was successfully prepared, exhibiting good adhesion to MF. Compared to other foams, D2-RA foam has more significant stratification and dispersion effects, achieving spacing and stratification at the microscopic level, which will contribute to the generation of multiple reflections of electromagnetic waves and interfacial polarization at the microscopic level.

[0107] Experimental Example 2: Raman Detection of RGO / MF Foam of the Present Invention

[0108] 1. Experimental Methods

[0109] Raman spectroscopy was performed on D2 foam, D2-R foam, D2-RA foam, and D2-A foam. The results for D2 foam, D2-R foam, D2-RA foam, and D2-A foam were the same as in Test Example 1.

[0110] 2. Experimental Results

[0111] Figure 7 The images show the Raman spectroscopy results for each group of foams. Raman spectroscopy is used to study the bonding state of carbon atoms, which has a significant impact on the reduction degree of GO. For example... Figure 7 As shown, all samples have similar spectra with two distinct peaks, located in the D band (near 1360 cm⁻¹). -1 ) and G band (nearly 1600cm) -1 ), representing the vibrational modes of disordered graphitic carbon atoms and the stretching vibrational modes of sp2 hybridized carbon atoms, respectively. Generally, I D / I G The ratio of D2, D2-A, D2-R, and D2-RA can be used to represent the degree of disorder in the carbon structure. D / I G The ratios were 0.99, 1.00, and 1.06, respectively, which may be due to the increase in disordered defects or edges during reduction and / or annealing. Similarly, sp was found in D2-RA compared to D2-R. 2 The reduction in domains indicates a decrease in oxygen-containing groups and the formation of RGO. These results suggest that micron-sized RGOs contain more nanocrystalline regions and more disordered carbon structures, potentially generating more dipole centers during microwave transmission due to their varying electrophilic properties.

[0112] Experimental Example 3: Wave Absorption Performance of the RGO / MF Foam of the Present Invention

[0113] 1. Experimental Methods

[0114] RGO / MF foam was prepared using the method described in Example 1, except that the MF-cut foam was cut into sheets with thicknesses of 12, 14, 16, 18, 20, 22, 24, 26, 28, and 30 mm. The microwave absorption properties of each D2-RA and D3-RA foam were tested. The difference between D3-RA foam and D2-RA foam is that the impregnation and drying process was performed three times.

[0115] 2. Experimental Results

[0116] Figure 8 Electromagnetic parameters of D2-RA foam prepared with a GO concentration of 0.2% were obtained using a vector network analyzer and the coaxial circular ring test method (inner diameter 3.04 mm, outer diameter 7 mm). Figure 8 As shown, the real and imaginary parts of the dielectric constant of the D2-RA foam sample exhibit a clear, smooth downward trend, remaining at a low level. This is beneficial for impedance matching of electromagnetic wave incidence and for conducting electromagnetic waves through electrical conduction in thick foam. The foam has no magnetic properties; both the real and imaginary parts of its permeability are 0. The above results characterize the fundamental electromagnetic parameters of the D2-RA sample. Under these optimized parameters, D2-RA foam can achieve broadband wave absorption performance in thicknesses ranging from 12 to 30 mm.

[0117] Figure 9 The different absorption properties of D2-RA and D3-RA samples with a GO concentration of 0.2% at thicknesses of 12-30 mm are shown in the figure. Both D2-RA and D3-RA samples exhibit broadband RL characteristics. The D3-RA sample, with a thickness of 12 mm, achieves a wide EAB of 11.15 GHz, realizing broadband coverage in both high (Ku band) and low (SC band) frequency bands. The microscopic distribution of the reduced graphene oxide layer separated by chemical reduction foaming and the mesoscopic uniform distribution of reduced graphene oxide in the matrix ensure rapid response and occurrence of conductive loss. Based on this, the 30 mm thickness of D2-RA ( Figure 9 a) It exhibits the widest effective absorption band, achieving effective absorption of <-10dB in the 3-18GHz range. These results demonstrate the significant advantages of this process for D2-RA and D3-RA foams in broadband absorption performance.

[0118] Example 4: Mechanical properties of composite materials prepared by filling aramid honeycomb with RGO / MF foam according to the present invention.

[0119] 1. Experimental Methods

[0120] Following the method described in Example 1, the thickness of the RGO / MF foam was reduced to 30 mm to prepare a 30 mm thick D2-RA foam with a GO concentration of 0.2%. An aramid honeycomb panel filled with the 30 mm thick D2-RA foam with a GO concentration of 0.2% was also prepared following the method described in Example 1. Compressive strength tests were performed on the D2-RA foam, the D2-RA foam-filled aramid honeycomb panel, and the unfilled aramid honeycomb panel.

[0121] 2. Experimental Results

[0122] The results of the compressive strength test are as follows Figure 10 As shown: Figure 10 The compressive properties of the three samples were shown. Compared with pure honeycomb, the compressive properties of RGO / MF were negligible, with a difference of three orders of magnitude. This invention found that the compressive stress of the RGO / MF honeycomb composite material was not significantly different from that of pure honeycomb, both reaching a compressive strength of 3.13 MPa. These results indicate that honeycomb filled with RGO / MF can achieve broadband microwave absorption performance without affecting its mechanical properties.

[0123] In summary, this invention employs a specific RGO / MF foam preparation method to successfully obtain an RGO / MF foam with broadband microwave absorption properties. Furthermore, this invention, through liquid nitrogen and rapid pressurization, completely fills the aramid honeycomb with RGO / MF foam, resulting in a composite material that also exhibits excellent broadband microwave absorption performance without affecting the mechanical properties of the aramid honeycomb, achieving ultra-wide effective absorption (< -10dB) in the 3-18GHz range. The RGO / MF foam prepared by this invention, and the composite material obtained by filling aramid honeycomb with RGO / MF foam, demonstrate excellent microwave absorption performance, showing broad application prospects in the stealth field of aerospace.

Claims

1. A reduced graphene oxide / melamine foam, characterized in that: It is prepared by wetting and drying melamine foam in a graphene oxide dispersion, then immersing it in a dispersion containing a reducing agent for reduction foaming, and finally annealing it. The concentration of the graphene oxide dispersion is 0.1~0.2%; The soaking and drying process is repeated at least twice; The concentration of the reducing agent in the dispersion is 1-80%; The reducing agent is hydrazine hydrate; The method of reducing foaming is as follows: the foam after wetting and drying is immersed in a dispersion containing a reducing agent and kept at 40~80℃ for 1~60 minutes, and then the foam is dried at 80~90℃ for 6~18 hours. The annealing method is as follows: after reduction foaming, the temperature is raised to 100~200℃ and held for 2~8 hours.

2. The reduced graphene oxide / melamine foam according to claim 1, characterized in that: The concentration of the reducing agent in the dispersion containing the reducing agent is 10-40%.

3. The reduced graphene oxide / melamine foam according to claim 2, characterized in that: The method of impregnation and drying is as follows: melamine foam is fully impregnated in graphene oxide dispersion and then dried at 60~120℃.

4. The reduced graphene oxide / melamine foam according to claim 1, characterized in that: The dispersion medium of the graphene oxide dispersion is one or more of water, ethanol, methanol, and glycerol. And / or, the dispersion medium of the dispersion containing the reducing agent is one or more of water, ethanol, methanol, and glycerol.

5. The reduced graphene oxide / melamine foam according to any one of claims 1 to 4, characterized in that: The thickness of the melamine foam is 10~50mm.

6. The reduced graphene oxide / melamine foam according to claim 5, characterized in that: The thickness of the melamine foam is 12~30mm.

7. The method for preparing reduced graphene oxide / melamine foam according to any one of claims 1 to 6, characterized in that: It includes the following steps: (1) The melamine foam was soaked in the graphene oxide dispersion and then dried. (2) Then the foam is immersed in a dispersion containing a reducing agent for reduction foaming; (3) Finally, the foam is annealed and cooled to obtain the final product; In step (1), the concentration of the graphene oxide dispersion is 0.1~0.2%; In step (1), the soaking and drying process is performed at least twice; In step (2), the concentration of the reducing agent in the dispersion containing the reducing agent is 1-80%; In step (2), the method of reducing foaming is as follows: the foam after soaking and drying is immersed in a dispersion containing reducing agent and kept at 40~80℃ for 1~60min, and then the foam is placed at 80~90℃ to dry for 6~18h; In step (3), the annealing method is as follows: after reduction foaming, the temperature is raised to 100~200℃ and held for 2~8 hours.

8. The preparation method according to claim 7, characterized in that: In step (1), the method of impregnation and drying is as follows: after fully impregnating the melamine foam in the graphene oxide dispersion, it is dried at 60~120℃. And / or, in step (2), the concentration of reducing agent in the dispersion containing reducing agent is 10~40%.

9. The use of the reduced graphene oxide / melamine foam according to any one of claims 1 to 6 in the preparation of integrated structural / wave-absorbing composite materials.

10. A structural / wave-absorbing integrated composite material, characterized in that: It is obtained by filling aramid honeycomb with the reduced graphene oxide / melamine foam described in any one of claims 1 to 6.

11. The integrated structural / wave-absorbing composite material according to claim 10, characterized in that: The aramid honeycomb lattice cells have a side length of 2.85 mm and a density of 80 kg·m³. -3 The wall thickness is 0.15 mm.

12. The method for preparing the integrated structural / wave-absorbing composite material according to claim 10 or 11, characterized in that: It includes the following steps: 1) Immerse the reduced graphene oxide / melamine foam according to any one of claims 1 to 6 in liquid nitrogen and freeze it; 2) The frozen reduced graphene oxide / melamine foam is placed on the surface of the aramid honeycomb and then quickly pressed into the aramid honeycomb with external force to completely fill the aramid honeycomb.

13. The preparation method according to claim 12, characterized in that: In step 1), the freezing time is 1~60 min; And / or, in step 2), the magnitude of the external force is 0.05~0.5MPa; And / or, in step 2), the reduced graphene oxide / melamine foam pressed into the aramid honeycomb can be one or more layers, enough to fully fill the interior of the aramid honeycomb.

14. Use of the reduced graphene oxide / melamine foam according to any one of claims 1 to 6 or the integrated structure / wave-absorbing composite material according to claim 10 or 11 in the preparation of stealth materials.

15. The use according to claim 14, characterized in that: The stealth material is a stealth material used in the aerospace field.

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