Preparation method of wave-absorbing aerogel and wave-absorbing aerogel

Abstract

The invention discloses a preparation method of wave-absorbing aerogel and the wave-absorbing aerogel, and the preparation method comprises the following steps: dispersing polymethyl methacrylate in water, adding sodium citrate, and carrying out surface negative electrochemical treatment to obtain a polymethyl methacrylate dispersion liquid; the preparation method comprises the following steps: adding an iron source into a polymethyl methacrylate dispersion liquid, carrying out a co-precipitation reaction under an alkaline condition and an inert gas environment to obtain a first precipitate, and washing and drying the first precipitate to obtain the composite microspheres. The preparation method comprises the following steps: reacting p-phenylenediamine, 1, 2-dimethylimidazole and 3, 3 ', 4, 4'-biphenyl tetracarboxylic dianhydride in a hydrogen peroxide aqueous solution to obtain polyamide acid salt; providing a graphene solution, mixing polyamic acid salt with the graphene solution, then adding the composite microspheres, and carrying out ultrasonic dispersion and freeze-drying molding to obtain an aerogel matrix; and carrying out high-temperature heat treatment on the aerogel matrix by a pyrolysis method to obtain the wave-absorbing aerogel. The wave-absorbing aerogel prepared in the invention has multiple losses on electromagnetic waves, and also has good flexibility and deformation resistance.

Description

Technical Field

[0001] This application relates to the field of electromagnetic absorbing aerogel technology, and in particular to a method for preparing an absorbing aerogel and the absorbing aerogel itself. Background Technology

[0002] In recent years, the rapid development of electronic and communication technologies has brought convenience but also led to increasingly serious electromagnetic pollution problems. Electromagnetic interference not only affects the normal operation of precision electronic equipment and may even cause damage to critical equipment, but also poses potential risks to human health. The use of electromagnetic shielding and microwave absorbing materials is a major technical approach to controlling electromagnetic pollution. However, while traditional metal-based electromagnetic shielding materials have high conductivity, they are susceptible to interference and corrosion.

[0003] Conductive polymer composites (CPCs) offer advantages such as low density, corrosion resistance, and ease of processing, gradually replacing traditional metal-based electromagnetic interference shielding materials. However, to improve absorption performance, existing CPCs-type absorbing materials often require increasing the content of conductive fillers. However, increasing the amount of conductive fillers can impair the material's deformability and toughness. Excessive conductive fillers can also enhance the rigidity of CPCs-type absorbing materials, making them prone to fracture under external forces and unable to withstand cyclic deformation. This limits the application of CPCs-type absorbing materials in fields requiring high toughness, such as flexible electronics and wearable devices. Summary of the Invention

[0004] This application aims to provide a method for preparing microwave absorbing aerogel and the microwave absorbing aerogel, with the goal of improving the deformation ability and toughness of microwave absorbing materials.

[0005] In a first aspect, embodiments of this application provide a method for preparing a microwave-absorbing aerogel, comprising: dispersing polymethyl methacrylate in water; adding sodium citrate for surface negative electrochemical treatment to obtain a polymethyl methacrylate dispersion; adding an iron source to the polymethyl methacrylate dispersion and performing a co-precipitation reaction under alkaline conditions and an inert gas environment to obtain a first precipitate; washing and drying the first precipitate to obtain composite microspheres; reacting p-phenylenediamine, 1,2-dimethylimidazole, and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride in an aqueous hydrogen peroxide solution to obtain a polyamic acid salt; dissolving the polyamic acid salt in a first solvent to obtain a polyamic acid solution; providing a graphene solution; mixing the polyamic acid solution with the graphene solution; adding the composite microspheres; and after ultrasonic dispersion and freeze-drying to obtain an aerogel matrix; and subjecting the aerogel matrix to high-temperature heat treatment via pyrolysis to obtain a microwave-absorbing aerogel.

[0006] In some embodiments, the iron source is a mixture of ferric chloride hexahydrate and ferrous chloride tetrahydrate, wherein the molar ratio of ferric chloride hexahydrate to ferrous chloride tetrahydrate is 2:1.

[0007] In some embodiments, the coprecipitation reaction under alkaline conditions and an inert gas environment includes: adding ammonia to the polymethyl methacrylate dispersion, adjusting the pH to 8.5-9.5, and carrying out the coprecipitation reaction in an inert gas environment. The temperature of the coprecipitation reaction is 60°C to 80°C, and the duration is 0.5h to 2h. The inert gas includes at least one of nitrogen, helium, and argon.

[0008] In some embodiments, the molar ratio of p-phenylenediamine, 1,2-dimethylimidazole and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride is 1:(2~3):1, and the concentration of the hydrogen peroxide aqueous solution is 25% to 35%.

[0009] In some embodiments, the step of washing and drying the first precipitate to obtain composite microspheres includes: washing the first precipitate at least once with anhydrous ethanol, then washing it at least once with deionized water, placing the washed first precipitate in a vacuum drying oven at a temperature of 70°C to 80°C, and drying it for 4 to 6 hours to obtain the composite microspheres.

[0010] In some embodiments, the concentration of the polyamic acid solution is from 0.08 g / ml to 0.12 g / ml.

[0011] In some embodiments, the concentration of the graphene solution is from 0.03 g / ml to 0.08 g / ml.

[0012] In some embodiments, the high-temperature heat treatment of the aerogel matrix by pyrolysis includes: placing the aerogel matrix in a tube furnace at a temperature of 400°C to 500°C for heat treatment for a duration of 0.5 h to 2 h.

[0013] In some embodiments, the volume ratio of the polyamic acid solution to the graphene solution is 1:(1.5~4).

[0014] Secondly, embodiments of this application also provide a microwave absorbing aerogel, which is prepared using the microwave absorbing aerogel preparation method described in any embodiment of the first aspect.

[0015] This application provides a method for preparing a microwave-absorbing aerogel, comprising dispersing polymethyl methacrylate (PMMA) in water, adding sodium citrate for surface negative electrochemical treatment to obtain a PMMA dispersion. Adding an iron source to the PMMA dispersion and performing a co-precipitation reaction under alkaline conditions and an inert gas environment to obtain a first precipitate, which is then washed and dried to obtain composite microspheres. Reacting p-phenylenediamine, 1,2-dimethylimidazole, and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride in an aqueous hydrogen peroxide solution to obtain a polyamic acid salt. Providing a graphene solution, mixing the polyamic acid salt with the graphene solution, adding the composite microspheres, and then ultrasonically dispersing and freeze-drying to obtain an aerogel matrix. Finally, subjecting the aerogel matrix to high-temperature heat treatment via pyrolysis to obtain the microwave-absorbing aerogel. The aerogel matrix undergoes high-temperature heat treatment to form a porous structure of hollow Fe3O4 microspheres. Combined with the hierarchical pore structure formed during the freeze-drying process, the pore walls of the aerogel deform under pressure, and the pore structure recovers autonomously after the pressure is removed. This gives the aerogel a certain degree of compressibility and allows it to withstand cyclic deformation. Polyamic acid is converted into polyimide through high-temperature heat treatment. Polyimide, as the binder phase of the aerogel, can encapsulate the hollow Fe3O4 microspheres, integrating them with graphene and providing rigid support for the overall structure. The polyimide also includes flexible connecting segments. When the aerogel is subjected to bending, folding, or other external forces, these flexible connecting segments can disperse stress, reduce localized stress, and decrease the likelihood of aerogel breakage.

[0016] Furthermore, the hollow Fe3O4 microspheres are uniformly dispersed in the binder phase formed by polyimide, and deform synergistically with the polyimide skeleton, improving the overall flexibility of the microwave-absorbing aerogel. The Fe3O4 within the hollow Fe3O4 microspheres can undergo magnetic loss, converting electromagnetic wave energy into heat. The hollow Fe3O4 microspheres will cause multiple reflections and scattering of electromagnetic waves. After entering the hollow cavity, the electromagnetic waves will repeatedly bounce between the cavity walls and the shell, thereby lengthening the propagation path and improving the energy conversion efficiency. The microwave-absorbing aerogel contains a hierarchical porous structure, which generates numerous heterogeneous interfaces, resulting in polarization loss. Furthermore, the hierarchical porous structure further increases the propagation path of electromagnetic waves, allowing polarization loss, magnetic loss, and conductivity loss to be fully utilized, thus consuming electromagnetic waves to a greater extent.

[0017] Additional aspects and advantages of the embodiments of this application will be described, shown, or illustrated in part by way of implementation of the embodiments of this application in the following description. Attached Figure Description

[0018] One or more embodiments are illustrated by way of example with reference to the accompanying drawings, which are not intended to limit the embodiments, and elements having the same reference numerals in the drawings are designated as similar elements.

[0019] Figure 1 A flowchart illustrating a method for preparing microwave-absorbing aerogels according to some embodiments of this application. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly described below with reference to the accompanying drawings. Obviously, the described embodiments are some embodiments of this application, but not all embodiments.

[0021] In this application, the reference to "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment that is mutually exclusive with other embodiments.

[0022] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0023] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0024] The technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0025] Firstly, this application proposes a method for preparing microwave-absorbing aerogel, please refer to... Figure 1 The preparation method includes the following steps: Step S1: Polymethyl methacrylate is dispersed in water, and sodium citrate is added for surface negative electrochemical treatment to obtain a polymethyl methacrylate dispersion.

[0026] Polymethyl methacrylate (PMMA) is a high molecular weight polymer with good chemical stability and moldability, as well as high transparency, low price, and easy processing. PMMA serves as the basic framework for preparing composite microspheres, acting as a carrier for loading other functional components and providing fundamental structural support for the formation of composite microspheres.

[0027] Sodium citrate is a surface modifier containing carboxyl groups (-COOH) in its molecular structure. In water, it ionizes into negatively charged ions, which adsorb onto the surface of PMMA particles, thus imparting a negative charge to the PMMA surface and achieving negative electrostatic treatment. The negatively charged PMMA particles generate electrostatic repulsion, ensuring stable dispersion in water and reducing aggregation. Furthermore, the negative charge on the PMMA particle surface can adsorb positively charged metal cations, providing active sites for these cations and enabling magnetic nanoparticles to be loaded onto the PMMA surface.

[0028] Step S2: Add an iron source to the polymethyl methacrylate dispersion and carry out a co-precipitation reaction under alkaline conditions and an inert gas environment to obtain the first precipitate. After washing and drying the first precipitate, composite microspheres are obtained.

[0029] An iron source is a compound that provides iron in a chemical reaction. For example, an iron source is a mixture of ferric chloride hexahydrate (FeCl3·6H2O) and ferrous chloride tetrahydrate (FeCl2·4H2O). The molar ratio of ferric chloride hexahydrate to ferrous chloride tetrahydrate is 2:1.

[0030] Alkaline conditions can promote the precipitation and formation of Fe3O4 magnetic nanoparticles. Inert gases can create an oxygen-free environment, reducing the oxidation of ferrous ions and resulting in purer Fe3O4 magnetic nanoparticles.

[0031] The composite microspheres obtained in this application are polymethyl methacrylate microspheres coated with iron oxide (PMMA@Fe3O4 microspheres). The Fe3O4 magnetic nanoparticles are a magnetic loss agent, which can absorb more electromagnetic waves and reduce electromagnetic wave reflection through magnetic loss capability.

[0032] In some embodiments, the coprecipitation reaction is carried out under alkaline conditions and an inert gas environment, including adding ammonia to a polymethyl methacrylate dispersion, adjusting the pH to 8.5-9.5, and carrying out the coprecipitation reaction in an inert gas environment. The coprecipitation reaction temperature is 60°C to 80°C, and the duration is 0.5 h to 2 h. The inert gas includes at least one of nitrogen, helium, and argon.

[0033] In some embodiments, the composite microspheres are obtained by washing and drying the first precipitate, including washing the first precipitate at least once with anhydrous ethanol, then washing it at least once with deionized water, and placing the washed first precipitate in a vacuum drying oven at a temperature of 70°C to 80°C for 4 to 6 hours to obtain the composite microspheres.

[0034] Step S3: Reaction of p-phenylenediamine, 1,2-dimethylimidazole and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride in aqueous hydrogen peroxide solution to obtain polyamic acid salt. Dissolving polyamic acid salt in a first solvent to obtain polyamic acid solution.

[0035] Polyamic acid salts (PAAS) are synthesized from monomers p-phenylenediamine, 1,2-dimethylimidazole, and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (BPDA) via a condensation reaction. Hydrogen peroxide aqueous solution is a mild oxidant that reduces the oxidation of p-phenylenediamine by atmospheric oxygen during the reaction and maintains the purity of the polyamic acid salt.

[0036] In some embodiments, the first solvent includes at least one of N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone.

[0037] Step S4: Provide a graphene solution, mix polyamic acid salt with the graphene solution, add composite microspheres, and after ultrasonic dispersion and freeze-drying, obtain an aerogel matrix.

[0038] Graphene solution serves as the primary conductive medium and framework material in aerogel matrices, facilitating the construction of three-dimensional porous network structures. Graphene possesses a large specific surface area and excellent conductivity, enabling it to generate significant conductive losses. Furthermore, graphene can form numerous interfaces with other components, leading to interfacial polarization.

[0039] Freeze-drying can solidify a mixed solution of polyamic acid salts, graphene solution, and composite microspheres into a three-dimensional porous aerogel structure. Freeze-drying fixes the positions of each component and enriches the pore structure of the aerogel. These pore structures not only make the material lighter but also extend the propagation path of electromagnetic waves, dissipating them through multiple reflections and scattering.

[0040] In some embodiments, after ultrasonically dispersing polyamic acid salt and graphene solution, composite microspheres are added, and ultrasonic dispersion is performed again for 10 to 15 minutes. The mixed solution is then poured into a specific silicone mold and placed in a freeze dryer at a temperature of -36°C to -48°C for 24 to 30 hours.

[0041] Step S5: The aerogel matrix is ​​subjected to high-temperature heat treatment by pyrolysis to obtain microwave absorbing aerogel.

[0042] During the high-temperature heat treatment of the aerogel matrix, PMMA completely decomposes and volatilizes, leaving a large number of cavities inside the composite microspheres, forming a porous structure of hollow Fe3O4 microspheres. Combined with the multi-level pore structure formed by the aerogel matrix during freeze-drying, the pore walls of the microwave absorbing aerogel will deform when subjected to pressure. After the pressure is removed, the pore structure will recover autonomously, giving the microwave absorbing aerogel a certain compressibility and the ability to withstand cyclic deformation.

[0043] Polyamates undergo cyclization and dehydration through high-temperature heat treatment, transforming into polyimide. Polyimide is a polymer material possessing both rigidity and toughness. As a binder phase in microwave-absorbing aerogels, polyimide can encapsulate hollow Fe3O4 microspheres, integrating them with graphene and providing rigid support for the overall structure. The methylene chains within the polyimide molecular chain are flexible connecting segments with excellent toughness. When the microwave-absorbing aerogel is subjected to bending, folding, or other external forces, these flexible connecting segments can disperse stress, reducing localized stress and minimizing the likelihood of fracture. Furthermore, the hollow Fe3O4 microspheres are uniformly dispersed within the polyimide binder phase, deforming synergistically with the polyimide skeleton, thus enhancing the overall flexibility of the microwave-absorbing aerogel.

[0044] The Fe3O4 in hollow Fe3O4 microspheres can undergo magnetic loss, converting electromagnetic wave energy into heat. The hollow Fe3O4 microspheres cause multiple reflections and scattering of electromagnetic waves. After entering the hollow cavity, the electromagnetic waves repeatedly bounce between the cavity walls and the shell, lengthening the propagation path and increasing energy conversion. The microwave-absorbing aerogel contains a hierarchical porous structure, which generates numerous heterogeneous interfaces, resulting in polarization loss. Furthermore, the hierarchical porous structure increases the propagation path of the electromagnetic waves, allowing polarization loss, magnetic loss, and conductivity loss to fully play their role, thus consuming electromagnetic waves to a greater extent.

[0045] In some embodiments, the aerogel matrix is ​​subjected to high-temperature heat treatment by pyrolysis, including placing the aerogel matrix in a tube furnace at a temperature of 400°C to 500°C for 0.5 h to 2 h.

[0046] This application provides a method for preparing a microwave-absorbing aerogel, comprising dispersing polymethyl methacrylate (PMMA) in water, adding sodium citrate for surface negative electrochemical treatment to obtain a PMMA dispersion. Adding an iron source to the PMMA dispersion and performing a co-precipitation reaction under alkaline conditions and an inert gas environment to obtain a first precipitate, which is then washed and dried to obtain composite microspheres. Reacting p-phenylenediamine, 1,2-dimethylimidazole, and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride in an aqueous hydrogen peroxide solution to obtain a polyamic acid salt. Providing a graphene solution, mixing the polyamic acid salt with the graphene solution, adding the composite microspheres, and then ultrasonically dispersing and freeze-drying to obtain an aerogel matrix. Finally, subjecting the aerogel matrix to high-temperature heat treatment via pyrolysis to obtain the microwave-absorbing aerogel. The aerogel matrix undergoes high-temperature heat treatment to form a porous structure of hollow Fe3O4 microspheres. Combined with the hierarchical pore structure formed during the freeze-drying process, the pore walls of the aerogel deform under pressure, and the pore structure recovers autonomously after the pressure is removed. This gives the aerogel a certain degree of compressibility and allows it to withstand cyclic deformation. Polyamic acid is converted into polyimide through high-temperature heat treatment. Polyimide, as the binder phase of the aerogel, can encapsulate the hollow Fe3O4 microspheres, integrating them with graphene and providing rigid support for the overall structure. The polyimide also includes flexible connecting segments. When the aerogel is subjected to bending, folding, or other external forces, these flexible connecting segments can disperse stress, reduce localized stress, and decrease the likelihood of aerogel breakage.

[0047] Furthermore, the hollow Fe3O4 microspheres are uniformly dispersed in the binder phase formed by polyimide, and deform synergistically with the polyimide skeleton, improving the overall flexibility of the microwave-absorbing aerogel. The Fe3O4 within the hollow Fe3O4 microspheres can undergo magnetic loss, converting electromagnetic wave energy into heat. The hollow Fe3O4 microspheres will cause multiple reflections and scattering of electromagnetic waves. After entering the hollow cavity, the electromagnetic waves will repeatedly bounce between the cavity walls and the shell, thereby lengthening the propagation path and improving the energy conversion efficiency. The microwave-absorbing aerogel contains a hierarchical porous structure, which generates numerous heterogeneous interfaces, resulting in polarization loss. Furthermore, the hierarchical porous structure further increases the propagation path of electromagnetic waves, allowing polarization loss, magnetic loss, and conductivity loss to be fully utilized, thus consuming electromagnetic waves to a greater extent.

[0048] In some embodiments, the molar ratio of p-phenylenediamine, 1,2-dimethylimidazole, and 3,3',4,4'-biphenyltetracarboxylic dianhydride is 1:(2~3):1, and the concentration of the hydrogen peroxide aqueous solution is 25% to 35%. It is understood that if the 1,2-dimethylimidazole content is too low, the nitrogen content in the absorbing aerogel will be low, the dipole polarization effect will be weakened, and the absorption performance will decrease. If the 1,2-dimethylimidazole content is too high, it will hinder the normal condensation reaction between p-phenylenediamine and 3,3',4,4'-biphenyltetracarboxylic dianhydride, resulting in a polymer with a low molecular weight and an incomplete network structure. An appropriate amount of 1,2-dimethylimidazole can enhance dipole polarization loss and improve electromagnetic wave absorption performance. In some embodiments, the concentration of the polyamic acid solution is from 0.08 g / ml to 0.12 g / ml, the concentration of the graphene solution is from 0.03 g / ml to 0.08 g / ml, and the volume ratio of the polyamic acid solution to the graphene solution is 1:(1.5~4).

[0049] The polyamic acid solution concentration, ranging from 0.08 g / ml to 0.12 g / ml, provides sufficient polymer chains to form a stable aerogel structure after freeze-drying and high-temperature heat treatment, reducing the risk of microwave absorbing aerogel collapse. It also encapsulates and connects graphene and composite microspheres, preventing excessive viscosity from affecting the uniformity of graphene and composite microsphere dispersion. If the polyamic acid solution concentration is too high, the mixture becomes too viscous, easily forming unevenly mixed clumps. Excessive polyamic acid can also clog the pores between graphene sheets, leading to increased density, reduced pore size, and decreased microwave absorption performance in the aerogel. Conversely, if the polyamic acid solution concentration is too low, insufficient polymer chains result in a microwave absorbing aerogel prone to collapse.

[0050] If the graphene solution concentration is too low, the amount of graphene will be insufficient, which is not conducive to the formation of a continuous conductive path, resulting in poor overall conductivity and insufficient conductivity loss performance of the absorbing aerogel, thus reducing its microwave absorption performance. If the graphene solution concentration is too high, the van der Waals forces between the graphene sheets will be enhanced, making the graphene sheets prone to stacking into clumps, reducing the specific surface area, and weakening the interfacial polarization. A graphene solution concentration of 0.03 g / ml to 0.08 g / ml is conducive to the formation of a continuous conductive path, reduces the stacking between graphene sheets, and maximizes the surface area of ​​the graphene.

[0051] Secondly, embodiments of this application also provide a microwave absorbing aerogel, which is prepared using the microwave absorbing aerogel preparation method as described in any embodiment of the first aspect.

[0052] The preparation method of microwave absorbing aerogel is described below with reference to specific embodiments: Example 1 (1) Disperse 1g of PMMA microspheres in 50 mL of water, add sodium citrate, and sonicate for 30 min to obtain a PMMA dispersion. Add a mixture of ferric chloride hexahydrate and ferrous chloride tetrahydrate to the PMMA dispersion, wherein the molar ratio of ferric chloride hexahydrate to ferrous chloride tetrahydrate is 2:1. Adjust the pH of the solution to about 9 with ammonia, stir at 60 °C for 1 h under N2 protection, remove the first precipitate and wash it three times in anhydrous ethanol, then wash it three times with deionized water, and bake it in a vacuum oven at 80 °C for 5 h to obtain PMMA@Fe3O4 microspheres.

[0053] (2) Add 28 mmol of p-phenylenediamine to 100 mL of 30% hydrogen peroxide, then add 70 mmol of 1,2-dimethylimidazole, stir at room temperature for 10 min, then add 28 mmol of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, stir at 70 °C for 12 hours to obtain PAAS.

[0054] (3) PAAS was prepared into a solution with a concentration of 0.1 g / mL, and graphene was prepared into a dispersion with a concentration of 0.05 g / mL. 1 mL of PAAS solution and 4 mL of graphene solution were ultrasonically dispersed for 10 min, and then 0.5 g of PMMA@Fe3O4 microspheres were added. After ultrasonication for 10 min, the mixed solution was poured into a 4×4×1 cm silicone mold and freeze-dried at -48℃ for 24 h to obtain an aerogel matrix. The aerogel matrix was heated in a tube furnace at 450 ℃ for 1 h to obtain a microwave-absorbing aerogel.

[0055] Example 2 (1) Disperse 1g of PMMA microspheres in 50 mL of water, add sodium citrate, and sonicate for 30 min to obtain a PMMA dispersion. Add a mixture of ferric chloride hexahydrate and ferrous chloride tetrahydrate to the PMMA dispersion, wherein the molar ratio of ferric chloride hexahydrate to ferrous chloride tetrahydrate is 2:1. Adjust the pH of the solution to about 9 with ammonia, stir at 60 °C for 1 h under N2 protection, remove the first precipitate and wash it three times in anhydrous ethanol, then wash it three times with deionized water, and bake it in a vacuum oven at 80 °C for 5 h to obtain PMMA@Fe3O4 microspheres.

[0056] (2) Add 35 mmol of p-phenylenediamine to 120 mL of 30% hydrogen peroxide, then add 50 mmol of 1,2-dimethylimidazole, stir at room temperature for 10 min, then add 35 mmol of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, stir at 70 °C for 12 hours to obtain PAAS.

[0057] (3) PAAS was prepared into a solution with a concentration of 0.1 g / mL, and graphene was prepared into a dispersion with a concentration of 0.05 g / mL. 2 mL of PAAS solution and 3 mL of graphene solution were ultrasonically dispersed for 10 min, and then 0.5 g of PMMA@Fe3O4 microspheres were added. After ultrasonication for 10 min, the mixed solution was poured into a 4×4×1 cm silicone mold and freeze-dried at -48 ℃ for 48 h to obtain an aerogel matrix. The aerogel matrix was heated in a tube furnace at 500 ℃ for 1 h to obtain a microwave-absorbing aerogel.

[0058] The microwave absorption performance and pressure resistance of the microwave-absorbing aerogels prepared in Examples 1 and 2 were tested respectively. The microwave absorption performance and pressure resistance of the different examples are shown in Table 1.

[0059] The method for testing the absorption performance is as follows: the electromagnetic wave absorption performance is tested in the 2.0 GHz to 18.0 GHz band using a vector network analyzer.

[0060] The method for testing compressive strength is as follows: a universal testing machine is used to conduct a compression test at a compression speed of 50 mm / min. The aerogel is subjected to 500 cycles of compression, and the compressive strength is recorded as the first cycle compression strength. The compressive strength is also recorded as the second cycle compression strength after 1000 cycles of compression.

[0061] Table 1

[0062] According to the microwave absorption performance test results, the absorption loss of the microwave absorbing aerogel in Example 1 was -52.25 dB. The absorption loss of the microwave absorbing aerogel in Example 2 was -49.55 dB. This indicates that the microwave absorbing aerogels prepared in Examples 1 and 2 have good microwave absorption performance.

[0063] According to the pressure resistance test results, the initial compressive strength of the microwave absorbing aerogel in Example 1 was 105.5 MPa. After 500 cycles of compression, the compressive strength was 100.2 MPa, a decrease of only 5.02% compared to the initial compressive strength. After 1000 cycles of compression, the compressive strength was 94.95 MPa, a decrease of 10% compared to the initial compressive strength. The initial compressive strength of the microwave absorbing aerogel in Example 2 was 113.2 MPa. After 500 cycles of compression, the compressive strength was 104.1 MPa, a decrease of only 8.04% compared to the initial compressive strength. After 1000 cycles of compression, the compressive strength was 97.35 MPa, a decrease of 14% compared to the initial compressive strength. This indicates that the microwave absorbing aerogels prepared in Examples 1 and 2 possess certain flexibility, strong compression resistance, and can withstand cyclic deformation.

[0064] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them; under the concept of this application, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of different aspects of this application as described above, which are not provided in detail for the sake of brevity; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for preparing a microwave-absorbing aerogel, characterized in that, include: Polymethyl methacrylate (PMMA) was dispersed in water, and sodium citrate was added for surface negative electrochemical treatment to obtain a PMMA dispersion. The polymethyl methacrylate dispersion is added to an iron source and undergoes a co-precipitation reaction under alkaline conditions and an inert gas environment to obtain a first precipitate. After washing and drying the first precipitate, composite microspheres are obtained. p-phenylenediamine, 1,2-dimethylimidazole and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride were reacted in an aqueous hydrogen peroxide solution to obtain a polyamic acid salt. The polyamic acid salt was dissolved in a first solvent to obtain a polyamic acid solution. A graphene solution is provided, the polyamic acid solution is mixed with the graphene solution, and then the composite microspheres are added. After ultrasonic dispersion and freeze-drying, an aerogel matrix is ​​obtained. The aerogel matrix is ​​subjected to high-temperature heat treatment by pyrolysis to obtain microwave absorbing aerogel.

2. The preparation method according to claim 1, characterized in that, The iron source is a mixture of ferric chloride hexahydrate and ferrous chloride tetrahydrate, with a molar ratio of ferric chloride hexahydrate to ferrous chloride tetrahydrate of 2:

1.

3. The preparation method according to claim 1, characterized in that, The coprecipitation reaction under alkaline conditions and an inert gas environment includes: Ammonia water is added to the polymethyl methacrylate dispersion to adjust the pH to 8.5-9.

5. The coprecipitation reaction is carried out in an inert gas environment at a temperature of 60°C to 80°C for 0.5-2 hours. The inert gas includes at least one of nitrogen, helium, and argon.

4. The preparation method according to claim 1, characterized in that, The molar ratio of p-phenylenediamine, 1,2-dimethylimidazole and 3,3',4,4'-biphenyltetracarboxylic acid dianhydride is 1:(2~3):1, and the concentration of the hydrogen peroxide aqueous solution is 25% to 35%.

5. The preparation method according to claim 1, characterized in that, The process of washing and drying the first precipitate to obtain composite microspheres includes: The first precipitate was washed at least once with anhydrous ethanol and then at least once with deionized water. The washed first precipitate was placed in a vacuum drying oven at a temperature of 70°C to 80°C and dried for 4 to 6 hours to obtain the composite microspheres.

6. The preparation method according to claim 1, characterized in that, The concentration of the polyamic acid solution is from 0.08 g / ml to 0.12 g / ml.

7. The preparation method according to claim 1, characterized in that, The concentration of the graphene solution is from 0.03 g / ml to 0.08 g / ml.

8. The preparation method according to claim 1, characterized in that, The process of subjecting the aerogel matrix to high-temperature heat treatment via pyrolysis includes: The aerogel matrix is ​​heat-treated in a tube furnace at a temperature of 400°C to 500°C for 0.5 to 2 hours.

9. The preparation method according to claim 1, characterized in that, The volume ratio of the polyamic acid solution to the graphene solution is 1:(1.5~4).

10. A microwave-absorbing aerogel, characterized in that, It is prepared by the method for preparing microwave absorbing aerogel as described in any one of claims 1 to 9.

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