A polyimide composite wave-absorbing aerogel and its preparation method and application

Abstract

The present invention provides a polyimide composite absorbing aerogel, and its preparation method and application. The present invention prepares the polyimide composite absorbing aerogel using water-soluble polyamic acid, graphene oxide, and 2,6-diacetylpyridine as raw materials through a combination of directional freezing and freeze-drying, so that the polyimide composite absorbing aerogel has a "honeycomb" pore structure in the vertical direction and an oriented pore structure in the horizontal direction. It can be seen that the present invention can form an orderly arranged porous structure of the polyimide composite absorbing aerogel through directional freezing, which can promote multiple reflections and passages of electromagnetic waves in the pores, extend the propagation path, and the interaction between the electromagnetic waves and the pore walls effectively weakens the energy of the electromagnetic waves. Compared with existing traditional absorbing materials, the band-adjustable polyimide composite absorbing aerogel prepared by the present invention also has good compressibility and elasticity, and can adjust the electromagnetic parameters by strain to achieve regulation of the effective absorbing bandwidth.

Description

Technical Field

[0001] The present invention belongs to the technical field of absorbing materials, and specifically relates to a polyimide composite absorbing aerogel, a preparation method and applications thereof, and more particularly to a band-adjustable polyimide composite absorbing aerogel, a preparation method and applications thereof. Background Art

[0002] Currently, high-performance electromagnetic wave absorbing materials can not only effectively eliminate the increasingly serious electromagnetic radiation pollution, but can also be used in military detection fields such as radar, helping weapons effectively avoid radar detection. Among them, polyimide aerogel is widely used as an electromagnetic wave absorbing material due to its high porosity, ultra-low density, and low thermal conductivity. However, the electromagnetic wave absorbing materials currently under development generally only have strong absorption capabilities and are no longer able to meet the needs of practical applications. For example, mobile communication devices require electromagnetic wave absorbing materials that can simultaneously support the transmission and processing of multi-frequency signals (such as 4G and 5G). Military applications also generally require broadband electromagnetic wave absorbing materials to cope with broadband radar detection.

[0003] However, current research on broadband electromagnetic wave absorbing materials is limited. Due to the limitations of the inherent properties of broadband electromagnetic wave absorbing materials, they do not have the ability to adapt to different electromagnetic wave environments and can only absorb electromagnetic waves in a fixed frequency band. Summary of the Invention

[0004] In view of this, the present invention aims to provide a polyimide composite absorbing aerogel and its preparation method and application. The polyimide composite absorbing aerogel can achieve the regulation of the material's absorbing frequency band and absorbing intensity.

[0005] To achieve this object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a polyimide composite absorbing aerogel prepared from water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine;

[0007] The polyimide composite wave-absorbing aerogel has an orderly arranged pore structure.

[0008] Preferably, the mass ratio of the water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine is 100:(10-30):(0.5-2).

[0009] Preferably, the graphene oxide is few-layer graphene oxide, and the number of layers of the few-layer graphene oxide is less than 10.

[0010] Preferably, the water-soluble polyamic acid is prepared according to the following method:

[0011] The diamine, dibasic anhydride and polar solvent are mixed and polymerized, and the obtained product is reacted with an organic amine, and then precipitated and dried to obtain a water-soluble polyamic acid.

[0012] Preferably, the diamine comprises p-phenylenediamine and / or 4,4'-diaminodiphenyl ether.

[0013] Preferably, the dibasic anhydride includes any one or more of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride or diphenyl ether tetracarboxylic dianhydride.

[0014] Preferably, the polar solvent includes any one or more of N,N-dimethylacetamide, N-methylpyrrolidone or N,N-dimethylformamide.

[0015] Preferably, the organic amine includes any one or more of triethylamine, trimethylamine or pyridine.

[0016] Preferably, the polymerization reaction is carried out in an ice-water bath.

[0017] Preferably, the polymerization reaction time is 2 to 5 hours, and the reaction time of the product and the organic amine is 3 to 6 hours.

[0018] Preferably, the precipitation is carried out using acetone.

[0019] In a second aspect, the present invention provides a method for preparing the above-mentioned polyimide composite absorbing aerogel, comprising the following steps:

[0020] The polyimide composite microwave-absorbing aerogel is obtained by subjecting an aqueous dispersion of water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine to directional freezing, freeze drying and thermal amination treatment.

[0021] Preferably, the mass concentration of the aqueous dispersion is 40-60 mg / mL.

[0022] Preferably, the freeze-drying temperature is -40 to -50°C, and the time is 60 to 80 hours.

[0023] Preferably, the thermal amination treatment is heated according to the following procedure:

[0024] Heat the room temperature to 60°C and keep warm for 1-2 hours;

[0025] Heat from 60°C to 100°C and keep warm for 1-2 hours;

[0026] Heat from 100°C to 200°C and keep warm for 1-2 hours;

[0027] Heat from 200°C to 300°C and keep warm for 2-4 hours;

[0028] The heating rate of the heating is 3-8°C / min.

[0029] In a third aspect, the present invention provides an application of the above-mentioned polyimide composite absorbing aerogel in the preparation of adaptive electromagnetic wave shielding and stealth materials.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The present invention provides a band-tunable polyimide composite absorbing aerogel prepared from water-soluble polyamic acid, graphene oxide, and 2,6-diacetylpyridine. The polyimide composite absorbing aerogel is prepared using water-soluble polyamic acid, graphene oxide, and 2,6-diacetylpyridine through a combination of directional freezing and freeze-drying. The resulting aerogel exhibits a "honeycomb" pore structure in the vertical direction and an oriented pore structure in the horizontal direction. This directional freezing method enables the polyimide composite absorbing aerogel to form an orderly porous structure, which promotes multiple reflections and passages of electromagnetic waves within the pores, extending the propagation path. The interaction between the electromagnetic waves and the pore walls effectively weakens the electromagnetic wave energy. Compared to existing traditional absorbing materials, the band-tunable polyimide composite absorbing aerogel prepared by the present invention also exhibits excellent compressibility and elasticity, allowing the effective absorption bandwidth to be controlled by adjusting electromagnetic parameters through strain.

[0032] In addition, the directional freezing method provided by the present invention, namely the ice template method, has the advantages of simple process, low cost and green environmental protection. It can construct a three-dimensional composite absorbing aerogel material with a high specific surface area and a directional pore structure, and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 SEM images of the band-tunable polyimide composite microwave-absorbing aerogel prepared in Example 1 of the present invention observed in vertical and horizontal directions;

[0034] in, Figure 1 The a in corresponds to the vertical direction, Figure 1 The b in corresponds to the horizontal direction;

[0035] Figure 2 This is a graph of electromagnetic parameters of the frequency-tunable polyimide composite absorbing aerogel prepared in Example 2 of the present invention at different compression levels, where the abscissa is the frequency and the ordinate is the real part of the dielectric constant;

[0036] Figure 3 This is a graph of electromagnetic parameters of the frequency-tunable polyimide composite absorbing aerogel prepared in Example 2 of the present invention at different compression levels, where the abscissa is the frequency and the ordinate is the imaginary part of the dielectric constant;

[0037] Figure 4 This is a graph showing the absorbing performance of the frequency-tunable polyimide composite absorbing aerogel prepared in Example 2 of the present invention at different compression levels; wherein the abscissa is frequency and the ordinate is reflection loss;

[0038] Figure 5 The stress-strain curves of the band-tunable polyimide composite aerogel prepared in Example 2 of the present invention at different compression levels; wherein the abscissa is strain and the ordinate is stress;

[0039] Figure 6 This is the stress-strain curve of the polyimide composite aerogel prepared in Comparative Example 1 of the present invention at a compression degree of 60%; wherein the abscissa is strain and the ordinate is stress;

[0040] Figure 7 This is the stress-strain curve of the polyimide composite aerogel prepared in Comparative Example 2 of the present invention at a compression degree of 60%, wherein the abscissa is strain and the ordinate is stress. DETAILED DESCRIPTION

[0041] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0042] Because traditional absorbing materials have fixed absorbing properties after preparation and are unable to adapt to different electromagnetic wave environments, the present invention provides a band-tunable polyimide composite absorbing aerogel, compared to existing traditional absorbing materials.

[0043] In the present invention, the band-tunable polyimide composite microwave-absorbing aerogel is prepared from water-soluble polyamic acid, graphene oxide, and 2,6-diacetylpyridine in a mass ratio of 100:(10-30):(0.5-2).

[0044] In the present invention, the water-soluble polyamic acid is subjected to thermal amination treatment during the preparation process to obtain polyimide, which can give the aerogel excellent thermal stability, high mechanical strength, resilience and broadband adjustable wave absorption performance, which is beneficial to the application of high-temperature electromagnetic shielding and stealth, adaptive radar absorbing structures, etc.

[0045] In the present invention, the water-soluble polyamic acid has excellent ability to dissolve in water. Compared with the traditional polyamic acid that is insoluble in water, it is not only more environmentally friendly but also has low preparation cost.

[0046] In some embodiments of the present invention, the water-soluble polyamic acid is prepared according to the following method:

[0047] The diamine, dibasic anhydride and polar solvent are mixed and polymerized, and the obtained product is reacted with an organic amine, and then precipitated and dried to obtain a water-soluble polyamic acid.

[0048] In the above technical solution, the present invention preferably mixes a diamine, a dibasic anhydride, and a polar solvent and conducts a condensation polymerization reaction in an ice-water bath for 2-5 hours, preferably 3 hours. The diamine includes p-phenylenediamine and / or 4,4'-diaminodiphenyl ether; the dibasic anhydride includes any one or more of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, or diphenyl ether tetracarboxylic dianhydride; and the polar solvent includes any one or more of N,N-dimethylacetamide, N-methylpyrrolidone, or N,N-dimethylformamide. Through this polymerization reaction, the present invention can produce a polyamic acid solution.

[0049] The resulting product is then reacted with an organic amine for a further 3-6 hours, preferably 4 hours, according to the above technical scheme. During this process, the organic amine neutralizes the carboxylic acid groups, regulates the reaction environment, and aids crosslinking. The polyamic acid reacts with the organic amine to produce a water-soluble polyamic acid salt (PAS) solution. The organic amine can include any one or more of triethylamine, trimethylamine, or pyridine, with triethylamine, which has a moderate alkalinity, being preferred.

[0050] Finally, the obtained product is precipitated and dried to obtain a water-soluble polyamic acid. The present invention preferably uses acetone to precipitate, because polyamic acid has good solubility in polar solvents, but solubility is significantly reduced in acetone. Acetone, as an anti-solvent, can rapidly destroy the solvation layer of the polyamic acid molecular chain, induce it to separate out from the solution, and form a solid precipitate. Simultaneously, acetone has high volatility and can evaporate quickly after precipitation, reducing the interference of residual solvent to subsequent freeze drying and thermal amination steps. The drying is not particularly limited and is carried out according to means well known to those skilled in the art.

[0051] In some preferred embodiments of the present invention, a washing step is further performed before the precipitation and drying, and the washing can be performed by methods well known to those skilled in the art.

[0052] In this invention, the graphene oxide exhibits excellent electrical conductivity and dielectric loss characteristics. Its surface is rich in oxygen-containing functional groups, such as hydroxyl and carboxyl groups, which react chemically with the functional groups (such as hydroxyl and carboxyl) of polyamic acid. Combined with the cross-linking effect of 2,6-diacetylpyridine, this forms a more stable three-dimensional network structure, enhancing the mechanical strength and structural integrity of the aerogel. Furthermore, during thermal amination, the graphene oxide generates reduced graphene oxide, which imparts microwave absorption properties to the aerogel.

[0053] In some embodiments of the present invention, the graphene oxide is a few-layer graphene oxide, and the number of layers of the few-layer graphene oxide is less than 10 layers, preferably less than 7 layers. It has a larger specific surface area and more exposed surface functional groups. Compared with multi-layer graphene oxide, it is easier to disperse and avoid agglomeration, ensuring the formation of a uniform conductive network structure in the system, and is conducive to the coordinated formation of an oriented pore structure with the polyimide matrix during the directional freezing process.

[0054] In the present invention, the 2,6-diacetylpyridine is an organic compound having two carbonyl groups and a pyridine ring structure. Among them, the acetyl group has good chemical reactivity and can undergo a chemical cross-linking reaction with the hydroxyl or carboxyl group of the water-soluble polyamic acid, thereby improving the cross-linking degree of the system, thereby further enhancing the mechanical strength and thermal stability of the composite aerogel material and improving the structural integrity of the aerogel. In addition, the conjugated structure of the pyridine ring can, to a certain extent, combine with graphene oxide through π-π interactions, thereby enhancing the interfacial bonding between graphene oxide and polyimide, reducing interfacial defects, forming a more stable network structure, and thus improving the uniformity and stability of the composite aerogel material.

[0055] The polyimide composite absorbing aerogel provided by the present invention has an orderly arranged pore structure, and the electromagnetic parameters of the composite absorbing aerogel can be adjusted by simple mechanical compression, thereby achieving the regulation of the absorbing frequency band and absorbing intensity of the composite absorbing aerogel.

[0056] The present invention also provides a method for preparing the polyimide composite absorbing aerogel, comprising the following steps:

[0057] The polyimide composite microwave-absorbing aerogel is obtained by subjecting an aqueous dispersion of water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine to directional freezing, freeze drying and thermal amination treatment.

[0058] In the present invention, the aqueous dispersion is prepared by adding water, preferably deionized water, to a water-soluble polyamic acid, graphene oxide, and 2,6-diacetylpyridine, and stirring until uniform. The aqueous dispersion has a mass concentration of 40 to 60 mg / mL, preferably 50 mg / mL. The graphene oxide is a few-layer graphene oxide powder, the content of the graphene oxide relative to the water-soluble polyamic acid is 10 to 30 phr, preferably 14 to 26 phr, and the content of the 2,6-diacetylpyridine relative to the water-soluble polyamic acid is 0.5 to 2 phr, preferably 1 phr.

[0059] In the present invention, by adjusting the content of graphene oxide relative to water-soluble polyamic acid to 10-30 phr, the absorption frequency band (8.2-18 GHz) can be adjusted to meet broadband requirements. Exceeding 30 phr may cause agglomeration and affect the dispersibility and mechanical properties.

[0060] In the present invention, the content of the crosslinking agent (2,6-diacetylpyridine) relative to the water-soluble polyamic acid is 0.5-2 phr, which can enhance the network stability and mechanical resilience through chemical crosslinking, and maintain high resilience of the material under a compressive strain of 20-60%.

[0061] According to the present invention, the aqueous dispersion is subjected to directionally freezing and freeze-drying, and the directionally freezing and freeze-drying can be carried out according to methods well known to those skilled in the art. For example, in some embodiments of the present invention, the aqueous dispersion is poured into a mold, which is then placed in a container filled with liquid nitrogen, with only the bottom of the mold in contact with the liquid nitrogen, and directionally frozen. After the aqueous dispersion solidifies, freeze-drying is performed. The freeze-drying temperature is -40 to -50°C, preferably -50°C, and the time is 60 to 80 hours, preferably 72 hours.

[0062] In the present invention, during the directional freezing process, an ordered pore skeleton is formed inside the aerogel through the physical phase change and directional growth of the ice crystal template; during the freeze-drying process, the pore structure inside the aerogel is retained by ice crystal sublimation to avoid physical collapse.

[0063] After the freeze-drying is completed, according to the present invention, the obtained product is subjected to a thermal amination treatment. During this process, the polyamic acid forms a rigid polyimide backbone through dehydration cyclization, and the crosslinking agent (2,6-diacetylpyridine) forms a three-dimensional network through an amidation reaction, ultimately obtaining a band-tunable polyimide composite absorbing aerogel.

[0064] In some embodiments of the present invention, the thermal amination treatment is preferably heated according to the following program:

[0065] Heat the room temperature to 60°C and keep warm for 1-2 hours;

[0066] Heat from 60°C to 100°C and keep warm for 1-2 hours;

[0067] Heat from 100°C to 200°C and keep warm for 1-2 hours;

[0068] Heat from 200°C to 300°C and keep warm for 2-4 hours;

[0069] The heating rate of the heating is 3-8°C / min.

[0070] In a certain embodiment of the present invention, the temperature programming is set as follows:

[0071] The room temperature was raised to 60°C and kept warm for 1 h;

[0072] Heat from 60°C to 100°C and keep warm for 1 h;

[0073] Heat from 100°C to 200°C and keep warm for 1 h;

[0074] Heat from 200°C to 300°C and keep warm for 2 h;

[0075] The heating rate of the heating is 5°C / min.

[0076] The above-mentioned programmed temperature rise includes four stages. In the first stage, the molecular chains are initially cross-linked through hydrogen bonds to stabilize the porous skeleton; in the second stage, the molecular chains are initially cross-linked to enhance the mechanical strength of the pore wall and inhibit the collapse of the pores formed by the ice template method during subsequent heating; in the third stage, a rigid polyimide main chain is formed to give the material high temperature resistance, the cross-linking points are evenly distributed, and the compression resilience is improved; in the fourth stage, the pore wall structure is completely solidified, the directional pores are retained, the interface bonding between graphene oxide and polyimide is enhanced, and the conductive network is optimized.

[0077] The present invention sets the above-mentioned temperature rising program and controls the reaction process in sections, thereby retaining the directional channels, achieving the uniformity of the cross-linked network, and achieving complete conversion of the polyimide.

[0078] The preparation method provided by the present invention has simple steps, low cost, is environmentally friendly, and is conducive to large-scale production or industrial production.

[0079] The present invention also provides an application of the polyimide composite absorbing aerogel in the preparation of adaptive electromagnetic shielding and stealth materials.

[0080] To further illustrate the present invention, the following examples are provided for detailed description. The experimental raw materials used in the following examples are all commercially available products. The graphene oxide described below has 4 to 7 layers.

[0081] Example 1

[0082] Using N,N-dimethylacetamide as the solvent, a polyamic acid with an 18% solid content was prepared by condensation polymerization of 4,4'-diaminodiphenyl ether and pyromellitic dianhydride in an ice-water bath at equal molar ratios. The specific process is as follows: 3.446 g of 4,4'-diaminodiphenyl ether was dissolved in 32.8 g of N,N-dimethylacetamide, 3.754 g of pyromellitic dianhydride was added, and the mixture was reacted in an ice-water bath for 5 hours. Then, 3.483 g of triethylamine was added, and the reaction was continued for another 5 hours to prepare a water-soluble polyamic acid solution with an 18% solid content. The prepared water-soluble polyamic acid was precipitated with acetone, washed, and dried to obtain the water-soluble polyamic acid for future use.

[0083] 0.14 g of graphene oxide, 0.86 g of water-soluble polyamic acid, and 0.01 g of 2,6-diacetylpyridine were added to 19 mL of deionized water and 0.5 mL of triethylamine. The mixture was ultrasonicated for 30 minutes and magnetically stirred for 6 hours to dissolve. The mixture was then transferred to a 3 cm diameter container and placed in liquid nitrogen, with only the bottom of the mold in contact with the liquid nitrogen, to freeze it solid. Finally, the container was freeze-dried in a freeze dryer for 72 hours to obtain the graphene oxide / polyamic acid aerogel.

[0084] The obtained graphene oxide / polyamic acid aerogel was placed in a tubular furnace under a nitrogen atmosphere and subjected to programmed temperature increase (heating rate of 5 °C / min), i.e., the temperature was increased from room temperature to 60 °C and then kept for 1 h, from 60 °C to 100 °C and then kept for 1 h, from 100 °C to 200 °C and then kept for 1 h, and from 200 °C to 300 °C and then kept for 2 h, thereby obtaining a band-tunable polyimide composite absorbing aerogel.

[0085] Example 2

[0086] The steps for preparing water-soluble polyamic acid are the same as those in Example 1.

[0087] Take 0.18 g of graphene oxide, 0.82 g of water-soluble polyamic acid, and 0.01 g of 2,6-diacetylpyridine, add 19 mL of deionized water and 0.5 mL of triethylamine, sonicate for 30 minutes, and magnetically stir for 6 hours to dissolve. Follow the remaining steps as in Example 1 to obtain a band-tunable polyimide composite microwave-absorbing aerogel.

[0088] Example 3

[0089] The steps for preparing water-soluble polyamic acid are the same as those in Example 1.

[0090] Take 0.22 g of graphene oxide, 0.78 g of water-soluble polyamic acid, and 0.01 g of 2,6-diacetylpyridine, add 19 mL of deionized water and 0.5 mL of triethylamine, sonicate for 30 minutes, and magnetically stir for 6 hours to dissolve. Follow the remaining steps as in Example 1 to obtain a band-tunable polyimide composite microwave-absorbing aerogel.

[0091] Example 4

[0092] The steps for preparing water-soluble polyamic acid are the same as those in Example 1.

[0093] Take 0.26 g of graphene oxide, 0.74 g of water-soluble polyamic acid, and 0.01 g of 2,6-diacetylpyridine, add 19 mL of deionized water and 0.5 mL of triethylamine, sonicate for 30 minutes, and magnetically stir for 6 hours to dissolve. Follow the remaining steps as in Example 1 to obtain a band-tunable polyimide composite microwave-absorbing aerogel.

[0094] Comparative Example 1

[0095] This comparative example does not add 2,6-diacetylpyridine, and comprises the following steps:

[0096] The steps for preparing water-soluble polyamic acid are the same as those in Example 1.

[0097] Take 0.26 g of graphene oxide and 0.74 g of water-soluble polyamic acid, add 19 mL of deionized water and 0.5 mL of triethylamine, sonicate for 30 minutes, and magnetically stir for 6 hours to dissolve. The remaining steps are the same as in Example 1 to obtain a polyimide composite absorbing aerogel.

[0098] Comparative Example 2

[0099] This comparative example uses 2,6-lutidine to replace 2,6-diacetylpyridine, comprising the following steps:

[0100] The steps for preparing water-soluble polyamic acid are the same as those in Example 1.

[0101] Take 0.26 g of graphene oxide, 0.74 g of water-soluble polyamic acid, and 0.01 g of 2,6-lutidine, add 19 mL of deionized water and 0.5 mL of triethylamine, sonicate for 30 minutes, and magnetically stir for 6 hours to dissolve. Follow the remaining steps as in Example 1 to obtain a polyimide composite microwave-absorbing aerogel.

[0102] SEM characterization

[0103] The band-tunable polyimide composite microwave-absorbing aerogel sample obtained in Example 1 was observed using a scanning electron microscope in the vertical direction (parallel to the freezing direction) and the horizontal direction (perpendicular to the freezing direction). The SEM image of the band-tunable polyimide composite microwave-absorbing aerogel obtained in Example 1 is as follows: Figure 1 As shown, Figure 1 The a in corresponds to the vertical direction, Figure 1 The b in the figure corresponds to the horizontal direction. A comparison shows that when observed vertically, the aerogel sample exhibits a "honeycomb" pore structure; when observed horizontally, the sample exhibits an oriented pore structure, indicating that an ordered pore structure can be formed through directional freezing.

[0104] Electromagnetic wave absorption performance test

[0105] The electromagnetic parameters of the frequency-band-adjustable polyimide composite absorbing aerogels obtained in Examples 1 to 4, which were pre-compressed at different compression ratios, were tested using a vector network analyzer, with a frequency range of 8.2 to 18 GHz.

[0106] Test Method: The electromagnetic parameters of the sample material were measured using the waveguide method in the X-band (8.2-12.4 GHz) and Ku-band (12.4-18 GHz). The number of acquisition points was set to 421 for the X-band and 561 for the Ku-band. Sample dimensions: 23 × 10 mm for the X-band and 16 × 8 mm for the Ku-band.

[0107] The absorbing performance (minimum reflection loss (dB) and effective absorption bandwidth (≤-10 dB)) of the band-adjustable polyimide composite absorbing aerogel obtained in Examples 1 to 4 at different compression levels were calculated by Matlab simulation based on electromagnetic parameters and electromagnetic field transmission line theory. The minimum reflection loss (dB) was obtained based on the simulation calculation results, and the effective absorption bandwidth was obtained by plotting the curve of reflection loss RL versus frequency to find the continuous frequency range of RL ≤-10 dB. The width of the effective absorption bandwidth is the effective absorption bandwidth. The test results are shown in Table 1 below:

[0108] Table 1

[0109]

[0110] As shown in Table 1, the corresponding absorption frequency bands and minimum reflection losses for different examples under different compression strains are completely different, demonstrating that compression can effectively adjust the material's absorption properties. Furthermore, the data in Table 1 also show that the band-tunable polyimide composite absorbing aerogels obtained in Examples 1-4 all exhibit excellent absorption performance under different compression conditions. Among them, the band-tunable polyimide composite absorbing aerogel obtained in Example 2 exhibits the most excellent absorption performance, achieving a minimum reflection loss of -70.64 dB at a compression level of 40%, corresponding to an absorption bandwidth of 7.95 GHz.

[0111] Figure 2 and Figure 3 The electromagnetic parameter curves of the band-tunable polyimide composite absorbing aerogel material obtained in Example 2 at different compression levels show that, within a certain range, increasing compression leads to a gradual increase in both the real and imaginary parts of the material's dielectric constant. The increase in the real part of the dielectric constant indicates that the dielectric storage capacity of the sample gradually improves with compressive strain. This is primarily due to the formation of multiple microcapacitors between the pore walls of the composite absorbing aerogel. Under mechanical compressive strain, the spacing between the pore walls of the composite absorbing aerogel decreases, increasing the capacitance of the microcapacitors and leading to an increase in the dielectric storage capacity of the sample. The main reason for the increase in the imaginary part of the dielectric constant is that the sample's pore conductive structure becomes tighter during compression, resulting in a gradual increase in conductivity and a gradual increase in the material's conductive losses, which facilitates the conversion of electrical energy into thermal energy.

[0112] Figure 4 The graph of the absorbing performance of the band-adjustable polyimide composite absorbing aerogel material obtained in Example 2 at different compression levels is shown in FIG. Figure 4 It can be seen that the material has good wave absorption performance under different compression degrees. When the compression degree is 0%, the minimum reflection loss of the material is -21.04 dB, corresponding to an effective absorption bandwidth of 5.00 GHz (8.20~13.20GHz); when the compression degree is 20%, the minimum reflection loss of the material is -29.13 dB, corresponding to an effective absorption bandwidth of 5.39 GHz (8.20~13.59 GHz); when the compression degree is 40%, the minimum reflection loss of the material is -70.64 dB, corresponding to an effective absorption bandwidth of 7.95 GHz (9.53~17.48 GHz). When the compression degree is 60%, the minimum reflection loss of the material is -21.71 dB, corresponding to an effective absorption bandwidth of 3.59 GHz (14.41~18.00 GHz). In addition, by Figure 4 A pattern can also be drawn. As the degree of compression increases, the material's absorption frequency gradually shifts from low frequency to high frequency, which also proves that the material can achieve regulation of absorption efficiency and absorption frequency band by adjusting the compression strain.

[0113] Thermal performance test

[0114] The band-tunable polyimide composite absorbing aerogel prepared in Example 1 was placed on a hot plate at 100° C. The upper surface of the sample was photographed with an infrared thermal imager at regular intervals to record the temperature changes of the upper surface of the sample at different times.

[0115] Table 2

[0116]

[0117] Table 2 shows that after 1 minute on a 100°C heating platform, the surface temperature of the polyimide composite absorbing aerogel sample reached approximately 48.21°C. After 2 minutes, the surface temperature rose to approximately 50.43°C, and after 10 minutes, the surface temperature remained virtually unchanged. These results demonstrate that the polyimide composite absorbing aerogel exhibits excellent thermal insulation properties, as its loose porous structure and the low thermal conductivity of polyimide prevent heat transfer. This suggests that the polyimide composite absorbing aerogel has broad application prospects in high-temperature and harsh environments.

[0118] Mechanical properties testing

[0119] The compression rebound properties of the frequency-band-adjustable polyimide composite absorbing aerogel obtained in Example 2 and the polyimide composite absorbing aerogels obtained in Comparative Examples 1 and 2 at different compression levels were tested using a universal testing machine.

[0120] The test results are as follows Figures 5-7 As shown. Among them, Figure 5 Corresponding to the band-tunable polyimide composite aerogel prepared in Example 2 of the present invention, Figure 6 Corresponding to the polyimide composite aerogel prepared in Comparative Example 1 of the present invention, Figure 7 The polyimide composite aerogel prepared in Comparative Example 2 of the present invention is obtained. Figure 5 It can be seen that after adding 2,6-diacetylpyridine to the system, the stress-strain curves of the material at compression levels of 20%, 40%, and 60% are all smooth closed loops, which means that the aerogel has no structural damage during the compression process and can completely recover to its original state after the external force is removed, without causing energy loss. The stress-strain curve rises smoothly without mutations or breakpoints, indicating that the cross-linked network effectively disperses stress and avoids local failure. Figure 6It can be found that when the compressive strain is 60%, in the composite aerogel without the introduction of 2,6-diacetylpyridine, when the compressive strain is close to 55%, there is a breakpoint in its stress-strain curve. The reason for this is that the mechanical properties of the composite aerogel are poor. During the compression process, as the stress increases, the internal pores of the composite aerogel are destroyed, resulting in structural collapse and irreversible damage. The stress rises rapidly with the strain and then drops suddenly. The reaction material is brittle, indicating that its elastic recovery ability is poor. After the external force is unloaded, the residual strain is significant and cannot be restored to its original state. Figure 7 It can be seen that when the compressive strain is 60%, the curve of the composite aerogel with other pyridine substances (2,6-dimethylpyridine) has no breakpoints, but the slope of the curve rises gently, and the residual strain after external force unloading is relatively large. Figure 5 The results show that the addition of 2,6-diacetylpyridine significantly improves the compressive resilience and structural stability of the composite aerogel, allowing it to maintain integrity and elastic recovery within the strain range of 20% to 60%.

[0121] The above description of the disclosed embodiments will enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is to be construed in the widest manner consistent with the principles and novel features disclosed herein.

Claims

1. A polyimide composite absorbing aerogel, characterized in that: It is prepared from water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine; The mass ratio of the water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine is 100:(10-30):(0.5-2); The polyimide composite absorbing aerogel has an orderly arranged pore structure; The polyimide composite absorbing aerogel is prepared according to the following method: The polyimide composite microwave-absorbing aerogel is obtained by subjecting an aqueous dispersion of water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine to directional freezing, freeze drying and thermal amination treatment.

2. The polyimide composite absorbing aerogel according to claim 1, characterized in that: The graphene oxide is few-layer graphene oxide.

3. The polyimide composite absorbing aerogel according to claim 2, characterized in that: The number of layers of the few-layer graphene oxide is less than 10 layers.

4. The polyimide composite absorbing aerogel according to claim 1, characterized in that: The water-soluble polyamic acid is prepared according to the following method: The diamine, dibasic anhydride and polar solvent are mixed and polymerized, and the obtained product is reacted with an organic amine, and then precipitated and dried to obtain a water-soluble polyamic acid.

5. The polyimide composite absorbing aerogel according to claim 4, characterized in that: The diamine includes p-phenylenediamine and / or 4,4'-diaminodiphenyl ether; The dibasic anhydride includes any one or more of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride or diphenyl ether tetracarboxylic dianhydride; The polar solvent includes any one or more of N,N-dimethylacetamide, N-methylpyrrolidone or N,N-dimethylformamide; The organic amine includes any one or more of triethylamine, trimethylamine or pyridine.

6. The polyimide composite absorbing aerogel according to claim 4 or 5, characterized in that: The polymerization reaction is carried out in an ice-water bath; The polymerization reaction time is 2 to 5 hours, and the reaction time of the product and the organic amine is 3 to 6 hours; The precipitation is carried out using acetone.

7. A method for preparing the polyimide composite absorbing aerogel according to any one of claims 1 to 6, characterized in that: The following steps are involved: The polyimide composite microwave-absorbing aerogel is obtained by subjecting an aqueous dispersion of water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine to directional freezing, freeze drying and thermal amination treatment.

8. The preparation method according to claim 7, characterized in that The mass concentration of the aqueous dispersion is 40-60 mg / mL; The freeze-drying temperature is -40 to -50°C, and the time is 60 to 80 hours.

9. The preparation method according to claim 7 or 8, characterized in that The thermal amination treatment is heated according to the following procedure: Heat the room temperature to 60°C and keep warm for 1-2 hours; Heat from 60°C to 100°C and keep warm for 1-2 hours; Heat from 100°C to 200°C and keep warm for 1-2 hours; Heat from 200°C to 300°C and keep warm for 2-4 hours; The heating rate of the heating is 3-8°C / min.

10. Use of the polyimide composite absorbing aerogel according to any one of claims 1 to 6 or the polyimide composite absorbing aerogel prepared according to any one of claims 7 to 9 in preparing adaptive electromagnetic wave shielding and stealth materials.

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