A method for preparing a multilayer concentration gradient graphene, polyimide fiber, and carbon nanotube composite aerogel for electromagnetic protection

Abstract

The present invention discloses a method for preparing a multilayer concentration-gradient graphene, polyimide fiber, and carbon nanotube composite aerogel for electromagnetic protection. The multilayer concentration-gradient graphene / polyimide fiber / carbon nanotube composite aerogel comprises a multilayer structure, each layer of which is formed by intertwining and overlapping polyimide fibers to form a three-dimensional support framework, which is then coated between graphene sheets. Carbon nanotubes are also evenly distributed within the graphene sheets, and the concentration of carbon nanotubes increases sequentially within the multilayer structure. The multilayer concentration-gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared by the present invention uses graphene as the main material, with polyimide fibers and carbon nanotubes evenly distributed within the graphene sheets, forming a unique structure similar to the main veins and capillary veins of a leaf, providing excellent support, excellent compression resilience, and excellent wave absorption performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite materials, and specifically relates to a method for preparing a multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel for electromagnetic protection. Background Art

[0002] Today, electromagnetic waves have occupied an absolute dominant position in the fields of wireless communication and detection, penetrating into all aspects of human life, and bringing revolutionary changes to human production and lifestyle. While benefiting mankind, the increasingly serious electromagnetic pollution has become the fifth largest source of pollution after water pollution, noise pollution, air pollution, and solid waste pollution. [1,2] Electromagnetic radiation not only interferes with the normal use of electronic devices, but also, because the contact distance between electronic products and the body is greatly shortened, electromagnetic radiation can cause damage to human organs and tissues. [3] To solve this problem, electromagnetic shielding materials have been recognized as an emerging material in the field of materials science.

[0003] Graphene is a two-dimensional material composed of a single layer of carbon atoms arranged in a hexagonal honeycomb pattern. It is one of the thinnest, strongest, and most conductive materials known. Graphene's unique structure and excellent properties make it show great potential in the field of electromagnetic wave absorption. Graphene's high conductivity and extremely thin thickness enable it to effectively couple and attenuate incident electromagnetic waves. [4] At the same time, the specific surface area of ​​graphene is very large (theoretical value is 2630m 2 / g), which can provide more active sites for absorbing waves, significantly improving absorption efficiency. In addition, graphene also has excellent chemical stability and environmental adaptability, making it suitable for use in various complex environments.

[0004] Patent application CN117551423A discloses an iron-loaded graphene aerogel absorbing material and its preparation method. The aerogel composite absorbing material prepared by self-assembly has low density and achieves an effective absorption bandwidth of 3.23 GHz at an ultra-thin matching thickness. [5] .

[0005] Patent application CN118184297A discloses a nano-ceramic / reduced graphene oxide composite aerogel absorbing material and its preparation method. The nano-barium titanate / reduced graphene oxide composite aerogel absorbing material was prepared by a hydrothermal method, achieving an effective absorption bandwidth of about 6.3 GHz. [6] .

[0006] Patent application CN118062833A discloses a nitrogen-doped graphene aerogel absorbing material and its preparation method. The nitrogen-doped graphene aerogel absorbing material is prepared by self-assembly and achieves an effective absorption bandwidth of about 5 GHz.[7] .

[0007] Patent application CN117638517A discloses a method for preparing a flexible three-dimensional polyimide fiber / MXene / graphene absorbing material. A flexible three-dimensional polyimide fiber / MXene / graphene absorbing material was prepared by a hydrothermal method, achieving an effective absorption bandwidth of approximately 3.5 GHz. [8] .

[0008] The above patent applications are all graphene-based composite aerogel absorbing materials, which are characterized by being homogeneous and having no obvious changes in internal impedance. This results in the material being able to achieve impedance matching only within a specific band, so it exhibits effective absorption within a narrower frequency band. Compared to single-layer homogeneous absorbing aerogel materials, multi-layer gradient composite materials have stronger selectivity in the function, component, and structural design of each layer, such as the three-layer gradient structure design of impedance matching layer, loss layer, and reflection layer, which exhibits greater advantages in both impedance matching and loss capacity. There are many methods for preparing related multi-layer gradient absorbing materials. [9-11] , such as 3D printing, layer-by-layer casting and multi-layer stacking, but the gradient structures of these methods are formed layer by layer, and the actual interface may not be evenly and completely connected. The non-uniformity of the connection at the interface may cause delamination during application, thereby affecting the mechanical and absorbing performance. Summary of the Invention

[0009] In response to the deficiencies in the prior art, the present invention aims to provide a multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel. The multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel is formed by overlapping polyimide fibers to form a three-dimensional support skeleton, which is wrapped between graphene sheets. The carbon nanotubes are also evenly distributed in the graphene sheets. Due to the difference in size, a pore wall structure is formed with polyimide fibers as the main veins, carbon nanotubes as capillary veins, and graphene as leaf veins, which is similar to leaf veins.

[0010] Another object of the present invention is to provide a method for preparing the above-mentioned multi-layer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel.

[0011] The purpose of the present invention is achieved through the following technical solutions.

[0012] A multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel comprises a multilayer structure, wherein each layer comprises polyimide fibers, graphene sheets, and carbon nanotubes. The polyimide fibers are overlapped and wound to form a continuous three-dimensional network skeleton, the graphene sheets encapsulate the three-dimensional network skeleton to form a continuous pore wall structure, and the carbon nanotubes are evenly dispersed in the pore wall structure, forming a leaf-like pore wall structure with the polyimide fibers as the main veins, the carbon nanotubes as capillary veins, and the graphene sheets as leaves.

[0013] The concentration of carbon nanotubes in the multilayer structure increases successively.

[0014] In the above technical solution, the thickness of the multi-layer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel is 4 to 10 mm, and the thickness of each layer structure is 1.5 to 2.5 mm.

[0015] A method for preparing a multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel comprises the following steps:

[0016] Step 1), adding a precursor solution in N times, freezing the precursor solution after each addition, so that an ice layer is formed after freezing the precursor solution each time, to obtain a precursor ice layer, wherein the precursor solution is a mixture of graphene oxide, polyimide fiber, carboxylated carbon nanotubes and a solvent, wherein the ratio of graphene oxide, polyimide fiber and carboxylated carbon nanotubes in the precursor solution is 1:(0.1-1):(0.1-1) by mass, and the concentration of carboxylated carbon nanotubes in the precursor solution added in N times increases successively;

[0017] In step 1), the thickness of the ice layer is 2.5 to 4.5 mm.

[0018] In the step 1), the freezing temperature is -50 to -30°C.

[0019] In the step 1), bubbles are removed from the precursor solution each time after the precursor solution is added, and the bubbles are removed by shaking.

[0020] In the step 1), N=2-5.

[0021] In the step 1), the solvent is a mixture of anhydrous ethanol and water, and the volume fraction of anhydrous ethanol in the solvent is 10-20%.

[0022] In the step 1), the ratio of the mass fraction of the graphene oxide to the volume fraction of the solvent is (0.001-0.01):1, the unit of the mass fraction is g, and the unit of the volume fraction is mL.

[0023] In the step 1), the precursor solution is obtained by uniformly mixing a graphene oxide dispersion, a polyimide fiber dispersion and a carboxylated carbon nanotube dispersion, wherein the graphene oxide dispersion is a mixture of graphene oxide and a first solvent, the polyimide fiber dispersion is a mixture of polyimide fibers and a second solvent, and the carboxylated carbon nanotube dispersion is a mixture of carboxylated carbon nanotubes and a third solvent. The first solvent, the second solvent and the third solvent are the same or different, so that the volume fraction of anhydrous ethanol in the solvent is 10 to 20%.

[0024] In the step 1), the concentration of graphene oxide in the graphene oxide dispersion is 1 to 10 mg / ml, the concentration of polyimide fiber in the polyimide fiber dispersion is 1 to 10 mg / ml, and the concentration of carboxylated carbon nanotubes in the carboxylated carbon nanotube dispersion is 1 to 10 mg / ml.

[0025] In the step 1), the uniform mixing includes: firstly ultrasonication and then oscillation.

[0026] Step 2) heating the precursor ice layer obtained in step 1) to 50-70°C at a heating rate of 3-5°C / min and maintaining it for 20-30 minutes, and then heating it to 100-180°C at a heating rate of 8-10°C / min and maintaining it for 10-12 hours to obtain a composite hydrogel;

[0027] Step 3) The composite hydrogel is first cooled and frozen, and then freeze-dried in a vacuum to obtain a composite aerogel.

[0028] In the step 3), the cooling and freezing temperature is -20 to -15°C, and the cooling and freezing time is 6 to 8 hours.

[0029] In the step 3), the vacuum freeze-drying temperature is -60 to -55°C, and the vacuum freeze-drying time is 2 to 4 days.

[0030] Step 4) heat-treating the composite aerogel obtained in step 3) at a high temperature to reduce the graphene oxide and the carboxylated carbon nanotubes, thereby obtaining a multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel.

[0031] In the step 4), the high-temperature heat treatment is carried out in a mixed gas atmosphere of argon and hydrogen, and the volume ratio of argon to hydrogen is (10-15):1.

[0032] In the step 4), the heating rate of the high temperature heat treatment is 5-10°C / min, the time of the high temperature heat treatment is 30-60min, and the temperature of the high temperature heat treatment is 400-600°C.

[0033] Compared with the prior art, the present invention has the following beneficial effects:

[0034] (1) The multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared by the present invention uses graphene as the main material, and polyimide fibers and carbon nanotubes are evenly distributed in the graphene sheets, forming a special structure similar to the main veins and capillary veins of leaves, which plays a good supporting role and has excellent compression resilience.

[0035] (2) The multilayer gradient structure of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared by the present invention is formed in one pot by hydrothermal heating. The aerogel is a whole and does not require secondary bonding and splicing after forming. This makes the interface connection between the aerogel structural layers with different concentrations more complete, which is beneficial to the transmission of electrons, thereby forming a more complete intra-layer and inter-layer support network and conductive network. The concentration of the conductive medium increases layer by layer from top to bottom. The change in conductivity leads to a change in the dielectric constant, and the impedance also shows a gradient enhancement, thereby optimizing the loss characteristics and impedance matching of the composite aerogel, showing excellent wave absorption performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a physical picture of multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel;

[0037] Figure 2 Compression-rebound diagram of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1, where (a) is before compression, (b) is the compressed state, and (c) is after rebound;

[0038] Figure 3 This is the stress-strain curve of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1;

[0039] Figure 4 This is the stress-strain curve of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 under ultra-low temperature environment (-130°C);

[0040] Figure 5 This is a scanning electron microscope image of the microscopic pores and pore wall structure of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1;

[0041] Figure 6The wave absorption performance tests of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogels prepared in Examples 1 to 5, wherein (a) is an electromagnetic wave reflection loss performance graph of Example 1, (b) is an electromagnetic wave reflection loss performance graph of Example 2, (c) is an electromagnetic wave reflection loss performance graph of Example 3, (d) is an electromagnetic wave reflection loss performance graph of Example 4, and (e) is an electromagnetic wave reflection loss performance graph of Example 5;

[0042] Figure 7 The wave absorption performance test of the graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 6 was conducted. DETAILED DESCRIPTION

[0043] The technical solution of the present invention is further described in detail below with reference to specific embodiments / drawings.

[0044] Examples 1 to 5

[0045] A method for preparing a multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel comprises the following steps:

[0046] Step 1) The precursor solution is added to the inner lining of the tetrafluoroethylene reactor in N times, and the volume of the precursor solution added each time is 5 mL (the bubbles are removed by oscillation for 30 minutes after each addition of the precursor solution). After each addition of the precursor solution, it is placed on a cold table pre-cooled with liquid nitrogen and frozen at -40°C for 3 minutes, so that each added precursor solution forms an ice layer with a thickness of 2.5 mm after freezing. N ice layers constitute the precursor ice layer, wherein the precursor solution is graphene oxide, polyimide fiber , a mixture of carboxylated carbon nanotubes and a solvent, the solvent being a mixture of anhydrous ethanol and water, the volume fraction of anhydrous ethanol in the solvent being 10%, the concentration of carboxylated carbon nanotubes in the precursor solution added in N times successively increasing, and the ratio of graphene oxide, polyimide fiber and carboxylated carbon nanotubes in the precursor solution added in N times being X in mass fraction; the ratio of the mass fraction of graphene oxide to the volume fraction of the solvent is 0.025:1, the unit of mass fraction is g, and the unit of volume fraction is mL.

[0047] A method for obtaining a precursor solution comprises mixing a graphene oxide dispersion, a polyimide fiber dispersion, and a carboxylated carbon nanotube dispersion, first sonicating for 30 minutes, and then oscillating for 30 minutes to achieve uniform mixing, thereby obtaining a precursor solution. The graphene oxide dispersion is a mixture of graphene oxide and a first solvent, the polyimide fiber dispersion is a mixture of polyimide fibers and a second solvent, and the carboxylated carbon nanotube dispersion is a mixture of carboxylated carbon nanotubes and a third solvent, wherein the first, second, and third solvents are identical, namely, a mixture of anhydrous ethanol and water in a volume ratio of 1:9. The concentration of graphene oxide in the graphene oxide dispersion is 2.5 mg / ml, the concentration of polyimide fibers in the polyimide fiber dispersion is 2 mg / ml, and the concentration of carboxylated carbon nanotubes in the carboxylated carbon nanotube dispersion is 1.5 mg / ml.

[0048] Step 2), placing the tetrafluoroethylene reactor liner containing the precursor ice layer obtained in step 1) into the reactor, heating it to T4°C at a heating rate of 3°C / min and maintaining it for 30 minutes (to allow the ice layer to melt slowly. At this time, there is a certain degree of fusion between the interfaces of the ice layers with different carboxylated carbon nanotube concentrations, but the overall carboxylated carbon nanotube concentration gradient in the precursor ice layer is still maintained.), then heating it to T1 at a heating rate of 10°C / min and maintaining it for 12 hours to obtain a composite hydrogel with a gradient change in the concentration of carboxylated carbon nanotubes;

[0049] In step 3), the composite hydrogel was first placed in a refrigerator and frozen at -20°C for 7 hours, and then transferred to a vacuum freeze dryer and vacuum freeze-dried at -60°C for T3 days to obtain a composite aerogel.

[0050] Step 4), placing the composite aerogel obtained in step 3) in a vacuum tube furnace, heating it to T2°C at 5°C / min and heat treating it at T2°C for 30 minutes to reduce the graphene oxide and the carboxylated carbon nanotubes to obtain a multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel, wherein the high-temperature heat treatment is carried out in a mixed gas atmosphere of argon and hydrogen, and the volume ratio of argon to hydrogen is 10:1.

[0051] T1 to T4, X and N are shown in Table 1.

[0052] Table 1

[0053] Example N X T1 T2 T3 T4 Example 1 3 1:0.4:0.2, 1:0.4:0.4, and 1:0.4:0.6 110 500 3 60 Example 2 3 1:0.2:0.2, 1:0.2:0.6, and 1:0.2:1 120 600 3 60 Example 3 3 1:0.6:0.6, 1:0.6:0.8, and 1:0.6:1 140 400 3 60 Example 4 3 1:0.8:0.2, 1:0.8:0.4, and 1:0.8:0.8 130 500 4 60 Example 5 4 1:0.8:0.2, 1:0.8:0.4, 1:0.8:0.6, and 1:0.8:0.8 120 600 4 70

[0054] Example 6

[0055] A graphene / polyimide fiber / carbon nanotube composite aerogel, the preparation method of which is basically the same as that of Example 1, except for step 1). In step 1) of this embodiment, 15 mL of a precursor solution is added to the inner liner of a tetrafluoroethylene reactor, the bubbles are removed by shaking for 30 minutes, and the solution is placed on a cold table pre-cooled with liquid nitrogen and frozen at -40°C for 3 minutes to obtain a precursor ice layer with a thickness of 7.5 mm. The precursor solution is the same as the precursor solution added for the third time in Example 1 (i.e., the mass ratio of graphene oxide, polyimide fiber, and carboxylated carbon nanotubes in the precursor solution in this embodiment is 1:0.4:0.6).

[0056] Figure 1 This is a photo of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1. Figure 1 It can be seen that the boundaries of different concentration layers of the multi-layer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 are clearly visible.

[0057] The compression-rebound properties of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 were tested using a universal testing machine. The test results are as follows: Figure 2 As shown, from Figure 2 It can be seen that the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 has excellent compression resilience. It was subjected to 1000 compression-rebound cycle tests under 90% strain at room temperature and the test results of the first, 500th and 1000th times were recorded. The test results are as follows: Figure 3 As shown by Figure 2 and Figure 3 It can be seen that the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 can still maintain a shape retention rate of more than 95% after the 1000th compression-rebound cycle test.

[0058] In order to test the rebound performance of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 at ultra-low temperature, 100 compression-rebound cycle tests were carried out at an ambient temperature of -130°C under 85% strain, and the test results of the 1st, 50th and 100th times were recorded. The test results are shown in FIG. Figure 4 As shown by Figure 4 It can be seen that, in an ultra-low temperature environment, the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 can still maintain very excellent compression rebound performance.

[0059] Figure 5The scanning electron microscope images of the microscopic pores and pore wall structure of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1. As the magnification of the electron microscope increases, (b) is an enlarged view of the circle in (a), and (c) and (d) are enlarged views of different positions in (b). Figure 5 (ad) As can be seen from the microstructure, wavy layers of pore walls can be observed. The pore walls are coated with polyimide fibers and carbon nanotubes, and the structure resembles the veins of a leaf, providing strong support. This is also the reason why the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 has excellent resilience. The continuous pore wall structure also provides a channel for electron migration. The thin pore walls also greatly reduce the skin effect, resulting in strong dielectric loss and good electromagnetic wave absorption.

[0060] The multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Examples 1 to 5 was tested for its wave absorption performance. The test results are as follows: Figure 6 As shown by Figure 6 As shown in (a), the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 1 can achieve broadband absorption. The thickness of each layer is 1.5 mm, and the thickness of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel is 4.5 mm, achieving an effective absorption bandwidth of more than 8 GHz (10 to 18 GHz). Figure 6 As shown in (b), the thickness of each layer of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 2 is 1.5 mm, and the thickness of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel is 4.5 mm, achieving an effective absorption bandwidth above 8 GHz (10 to 18 GHz). Figure 6 As shown in (c), the thickness of each layer of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 3 is 1.5 mm, and the thickness of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel is 4.5 mm, achieving an effective absorption bandwidth of more than 8 GHz (10 to 18 GHz). Figure 6 As shown in (d), the thickness of each layer of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 4 is 1.5 mm. When the thickness of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel is 4.5 mm, the effective absorption bandwidth is above 8 GHz (10 to 18 GHz). Figure 6(e) shows that the thickness of each layer of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 5 is 1.5 mm. When the thickness of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel is 6 mm, the effective absorption bandwidth is above 8 GHz (10 to 18 GHz).

[0061] like Figure 7 It can be seen that the graphene / polyimide fiber / carbon nanotube composite aerogel prepared in Example 6 is a single-layer homogeneous sample. When the thickness of the graphene / polyimide fiber / carbon nanotube composite aerogel is 4.5 mm, its total effective absorption bandwidth in the Ku band is less than 4 GHz. The above description of the present invention is exemplary. It should be noted that any simple variation, modification, or equivalent substitution that can be made by a person skilled in the art without inventive effort falls within the scope of protection of the present invention without departing from the core of the present invention.

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Claims

1. A multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel, characterized in that: include: A multi-layer structure, each layer of which includes polyimide fibers, graphene sheets, and carbon nanotubes. The polyimide fibers are overlapped and entangled to form a continuous three-dimensional network skeleton. The graphene sheets cover the three-dimensional network skeleton to form a continuous pore wall structure. The carbon nanotubes are also evenly dispersed in the pore wall structure, forming a leaf-like pore wall structure with polyimide fibers as the main veins, carbon nanotubes as capillary veins, and graphene sheets as leaves. The concentration of carbon nanotubes in the multilayer structure increases successively.

2. The multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel according to claim 1, characterized in that: The thickness of the multi-layer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel is 4 to 10 mm, and the thickness of each layer structure is 1.5 to 2.5 mm.

3. A method for preparing a multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel, characterized in that: The following steps are involved: Step 1), adding a precursor solution in N times, freezing the precursor solution after each addition, so that an ice layer is formed after freezing the precursor solution each time, to obtain a precursor ice layer, wherein the precursor solution is a mixture of graphene oxide, polyimide fiber, carboxylated carbon nanotubes and a solvent, wherein the ratio of graphene oxide, polyimide fiber and carboxylated carbon nanotubes in the precursor solution is 1:(0.1-1):(0.1-1) by mass, and the concentration of carboxylated carbon nanotubes in the precursor solution added in N times increases successively; Step 2) heating the precursor ice layer obtained in step 1) to 50-70°C at a heating rate of 3-5°C / min and maintaining it for 20-30 minutes, and then heating it to 100-180°C at a heating rate of 8-10°C / min and maintaining it for 10-12 hours to obtain a composite hydrogel; Step 3), cooling and freezing the composite hydrogel, and then freeze-drying the composite hydrogel under vacuum to obtain a composite aerogel; Step 4) heat-treating the composite aerogel obtained in step 3) at a high temperature to reduce the graphene oxide and the carboxylated carbon nanotubes, thereby obtaining a multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel.

4. The preparation method according to claim 3, characterized in that N＝2～5。 5. The preparation method according to claim 3, characterized in that The solvent is a mixture of anhydrous ethanol and water, and the volume fraction of the anhydrous ethanol in the solvent is 10-20%.

6. The preparation method according to claim 3, characterized in that In the step 1), the ratio of the mass fraction of the graphene oxide to the volume fraction of the solvent is (0.001-0.01):1, the unit of the mass fraction is g, and the unit of the volume fraction is mL.

7. The preparation method according to claim 3, characterized in that In the step 1), the thickness of the ice layer is 2.5 to 4.5 mm.

8. The preparation method according to claim 3, characterized in that In the step 4), the high-temperature heat treatment is carried out in a mixed gas atmosphere of argon and hydrogen, and the volume ratio of argon to hydrogen is (10-15):

1.

9. The preparation method according to claim 3, characterized in that In the step 4), the heating rate of the high temperature heat treatment is 5-10°C / min, the time of the high temperature heat treatment is 30-60min, and the temperature of the high temperature heat treatment is 400-600°C.

10. Use of the multilayer concentration gradient graphene / polyimide fiber / carbon nanotube composite aerogel according to claim 1 in improving wave absorbing performance.

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