Polyimide composite wave-absorbing aerogel as well as preparation method and application thereof

Abstract

The invention provides polyimide composite wave-absorbing aerogel as well as a preparation method and application thereof. The polyimide composite wave-absorbing aerogel is prepared by taking water-soluble polyamide acid, graphene oxide and 2, 6-diacetylpyridine as raw materials in a mode of combining directional freezing and freeze drying, so that the polyimide composite wave-absorbing aerogel has a honeycomb-shaped pore structure in the vertical direction; and a pore channel structure in oriented arrangement is arranged in the horizontal direction. Therefore, the polyimide composite wave-absorbing aerogel can form an orderly arranged porous structure in a directional freezing mode, electromagnetic waves can be promoted to be reflected and pass through in pore channels for multiple times, the propagation path is prolonged, and energy of the electromagnetic waves is effectively weakened through interaction of the electromagnetic waves and pore walls. Compared with an existing traditional wave-absorbing material, the prepared polyimide composite wave-absorbing aerogel with the adjustable frequency band further has good compressibility and elasticity, and electromagnetic parameters can be adjusted through strain, so that effective wave-absorbing bandwidth regulation and control are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of microwave absorbing materials, and particularly relates to a polyimide composite microwave absorbing aerogel, a preparation method and an application thereof, and more particularly to a polyimide composite microwave absorbing aerogel with adjustable frequency band, a preparation method and an application thereof. Background Art

[0002] At present, high-performance electromagnetic wave absorbing materials can not only effectively eliminate the increasingly serious electromagnetic radiation pollution, but also be used in military detection fields such as radars to help weapons effectively avoid radar detection. Among them, polyimide aerogel is widely used as an electromagnetic wave absorbing material due to its performance characteristics such as high porosity, ultra-low density, and low thermal conductivity. However, the currently developed electromagnetic wave absorbing materials usually only have strong absorption ability and can no longer meet the requirements in practical applications. For example, mobile communication devices require electromagnetic wave absorbing materials to support the transmission and processing of multi-frequency signals (such as 4G and 5G) at the same time, and the military also usually requires broadband electromagnetic wave absorbing materials to cope with the detection of broadband radars.

[0003] However, current scholars have limited research on broadband electromagnetic wave absorbing materials, and 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 achieve the absorption of electromagnetic waves in a fixed frequency band. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a polyimide composite microwave absorbing aerogel, a preparation method and an application thereof. The polyimide composite microwave absorbing aerogel can realize the regulation of the absorption frequency band and absorption intensity of the material.

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

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

[0007] The polyimide composite microwave 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 layers.

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

[0011] Mix a diamine, a dianhydride, and a polar solvent and carry out a polymerization reaction. React the resulting product with an organic amine, precipitate, and dry to obtain a water-soluble polyamic acid.

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

[0013] Preferably, the dianhydride 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 under ice-water bath conditions.

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

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

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

[0020] Carry out directional freezing, freeze-drying, and thermal amination treatment on the aqueous dispersion of water-soluble polyamic acid, graphene oxide, and 2,6-diacetylpyridine to obtain a polyimide composite absorbing aerogel.

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

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

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

[0024] Heat from room temperature to 60 °C and keep warm for 1 to 2 h;

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

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

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

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

[0029] Thirdly, the present invention provides an application of the above polyimide composite absorbing aerogel in the preparation of self-adaptive electromagnetic wave shielding and stealth materials.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0031] The present invention provides a frequency-band tunable polyimide composite absorbing aerogel prepared from water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine. By combining directional freezing and freeze-drying, the present invention uses water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine as raw materials to prepare the polyimide composite absorbing aerogel, so that it has a "honeycomb-like" pore structure in the vertical direction and an oriented pore structure in the horizontal direction. It can be seen that through the directional freezing method, the polyimide composite absorbing aerogel of the present invention can form an orderly arranged porous structure, which can promote the 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 the existing traditional absorbing materials, the frequency-band tunable polyimide composite absorbing aerogel prepared by the present invention also has good compressibility and elasticity, and can adjust the electromagnetic parameters through strain to realize the regulation of the effective absorption bandwidth.

[0032] In addition, the directional freezing method provided by the present invention, that is, the ice template method, has the advantages of simple process, low cost and environmental friendliness, and can construct a three-dimensional composite absorbing aerogel material with a high specific surface area and an oriented pore structure, and has good application prospects. Description of the Drawings

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

[0034] Among them, Figure 1 a in Figure 1 corresponds to the vertical direction,

[0035] Figure 2 are electromagnetic parameter curves of the frequency-band tunable polyimide composite absorbing aerogel prepared in Example 2 of the present invention under different compression degrees. Among them, the abscissa is the frequency and the ordinate is the real part of the dielectric constant;

[0036] Figure 3 are electromagnetic parameter curves of the frequency-band tunable polyimide composite absorbing aerogel prepared in Example 2 of the present invention under different compression degrees. Among them, the abscissa is the frequency and the ordinate is the imaginary part of the dielectric constant;

[0037] Figure 4 Absorbing performance graph of the frequency-band tunable polyimide composite absorbing aerogel prepared in Example 2 of the present invention under different compression degrees; wherein, the abscissa is frequency and the ordinate is reflection loss;

[0038] Figure 5 Stress-strain curve of the frequency-band tunable polyimide composite aerogel prepared in Example 2 of the present invention under different compression degrees; wherein, the abscissa is strain and the ordinate is stress;

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

[0040] Figure 7 Stress-strain curve of the polyimide composite aerogel prepared in Comparative Example 2 of the present invention under 60% compression degree; wherein, the abscissa is strain and the ordinate is stress. Detailed implementation manners

[0041] Next, the technical solutions of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0042] Since the absorbing performance of traditional absorbing materials is fixed after preparation, the materials themselves do not have the ability to adapt to different electromagnetic wave environments. Therefore, compared with existing traditional absorbing materials, the present invention provides a frequency-band tunable polyimide composite absorbing aerogel.

[0043] In the present invention, the frequency-band tunable polyimide composite absorbing aerogel 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).

[0044] In the present invention, during the preparation process of the water-soluble polyamic acid, after thermal amination treatment, polyimide can be obtained, which can endow the aerogel with excellent thermal stability, high mechanical strength, resilience and wide-frequency tunable absorbing performance, and is beneficial to applications in aspects such as high-temperature electromagnetic shielding and stealth, and adaptive radar absorbing structures.

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

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

[0047] A diamine, a dianhydride, and a polar solvent are mixed and subjected to a polymerization reaction. The resulting product is reacted with an organic amine, and after precipitation and drying, a water-soluble polyamic acid is obtained.

[0048] In the above technical solution, the present invention first mixes a diamine, a dianhydride, and a polar solvent, and performs a condensation polymerization reaction in an ice-water bath for 2 to 5 h, preferably 3 h. Among them, the diamine includes p-phenylenediamine and / or 4,4'-diaminodiphenyl ether; the dianhydride 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. By means of the polymerization reaction between the two, a polyamic acid solution can be obtained.

[0049] Then, according to the above technical solution, the resulting product is further reacted with an organic amine for 3 to 6 h, preferably 4 h. In this process, the organic amine plays a role in neutralizing carboxylic acid groups, adjusting the reaction environment, and assisting cross-linking. The polyamic acid undergoes an acid-base neutralization reaction with it to obtain a water-soluble polyamic acid salt (abbreviation: PAS) solution. The organic amine includes any one or more of triethylamine, trimethylamine, or pyridine, and preferably triethylamine with moderate alkalinity.

[0050] Finally, after the resulting product is precipitated and dried, a water-soluble polyamic acid is obtained. The present invention preferably uses acetone for precipitation because polyamic acid has good solubility in polar solvents but significantly reduced solubility in acetone. As an antisolvent, acetone can rapidly destroy the solvation layer of the polyamic acid molecular chain and induce it to precipitate from the solution to form a solid precipitate. At the same time, acetone has high volatility and can quickly volatilize after precipitation, reducing the interference of residual solvents on subsequent freeze-drying and thermal amination steps. The drying is not particularly limited and can be carried out by means well-known to those skilled in the art.

[0051] In some preferred embodiments of the present invention, there is also a washing step before the precipitation and drying, and the washing can be carried out by means well-known to those skilled in the art.

[0052] In the present invention, the graphene oxide has excellent electrical conductivity and dielectric loss characteristics. At the same time, its surface is rich in oxygen-containing functional groups such as hydroxyl and carboxyl groups, which can chemically react with the functional groups (such as hydroxyl and carboxyl groups) of polyamic acid. Combining with the cross-linking effect of 2,6-diacetylpyridine, a more stable three-dimensional network structure is formed, enhancing the mechanical strength and structural integrity of the aerogel. At the same time, during the thermal amination treatment of the graphene oxide, reduced graphene oxide is generated, thereby endowing the aerogel with the function of absorbing electromagnetic waves.

[0053] In some embodiments of the present invention, the graphene oxide is 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, and is easier to disperse compared with multi-layer graphene oxide, which can avoid aggregation, ensure the formation of a uniform conductive network structure in the system, and is conducive to the formation of an oriented pore structure in cooperation with the polyimide matrix during the directional freezing process.

[0054] In the present invention, 2,6-diacetylpyridine, as an organic compound, has two carbonyl groups and a pyridine ring structure. Among them, the acetyl group has good chemical reactivity and can chemically cross-link with the hydroxyl or carboxyl group of water-soluble polyamic acid, 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 interaction, enhancing the interfacial binding force between graphene oxide and polyimide, reducing interfacial defects, forming a more stable network structure, and further improving the uniformity and stability of the composite aerogel material.

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

[0056] The present invention also provides a preparation method of the above polyimide composite electromagnetic wave absorbing aerogel, comprising the following steps:

[0057] After subjecting the aqueous dispersion of water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine to directional freezing, freeze-drying and thermal amination treatment, a polyimide composite electromagnetic wave absorbing aerogel is obtained.

[0058] In the present invention, the aqueous dispersion is obtained by adding water-soluble polyamic acid, graphene oxide, and 2,6-diacetylpyridine to water, preferably deionized water, and stirring evenly. The mass concentration of the aqueous dispersion is 40 to 60 mg / mL, preferably 50 mg / mL. Among them, the graphene oxide is 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 the water-soluble polyamic acid to 10 to 30 phr, the wave absorption frequency band (8.2 to 18 GHz) can be adjusted to meet the broadband requirement. Exceeding 30 phr may cause aggregation 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 to 2 phr. The network stability and mechanical resilience can be enhanced through chemical crosslinking, and the material can maintain high resilience under a compressive strain of 20 to 60%.

[0061] According to the present invention, the aqueous dispersion is subjected to directional freezing and freeze-drying, and the directional freezing and freeze-drying can be carried out by means well-known to those skilled in the art. Exemplarily, in some embodiments of the present invention, the aqueous dispersion is poured into a mold and placed in a container filled with liquid nitrogen, and only the bottom of the mold is in contact with the liquid nitrogen for directional freezing. After the aqueous dispersion solidifies, freeze-drying is carried out. The temperature of the freeze-drying is -40 to -50 °C, preferably -50 °C; the time is 60 to 80 h, preferably 72 h.

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

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

[0064] In some embodiments of the present invention, the thermal amination treatment preferably raises the temperature according to the following procedure:

[0065] Raise the temperature from room temperature to 60 °C and keep it warm for 1 to 2 h;

[0066] Heat from 60 °C to 100 °C and then hold for 1 - 2 h;

[0067] Heat from 100 °C to 200 °C and then hold for 1 - 2 h;

[0068] Heat from 200 °C to 300 °C and then hold for 2 - 4 h;

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

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

[0071] Heat from room temperature to 60 °C and then hold for 1 h;

[0072] Heat from 60 °C to 100 °C and then hold for 1 h;

[0073] Heat from 100 °C to 200 °C and then hold for 1 h;

[0074] Heat from 200 °C to 300 °C and then hold for 2 h;

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

[0076] The above programmed heating includes four stages. Among them, in the first stage, the molecular chains are initially cross-linked by hydrogen bonds to stabilize the porous framework; in the second stage, the molecular chains are initially cross-linked to enhance the mechanical strength of the pore walls and inhibit the collapse of the pore channels formed by the ice-templating method during subsequent heating; in the third stage, a rigid polyimide main chain is formed, endowing the material with high-temperature resistance, with cross-linking points evenly distributed to improve compression resilience; in the fourth stage, the pore wall structure is completely cured, the oriented pore channels are retained, the interfacial combination between graphene oxide and polyimide is enhanced, and the conductive network is optimized.

[0077] The present invention sets the above heating program. By controlling the reaction process in segments, the oriented pore channels are retained, the uniformity of the cross-linking network is achieved, and at the same time, the polyimide is completely converted.

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

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

[0080] To further illustrate the present invention, the following detailed description is provided through the following examples. The experimental raw materials used in the following examples of the present invention are all general commercially available products. The graphene oxide involved below is graphene oxide with 4 - 7 layers.

[0081] Example 1

[0082] Using N,N-dimethylacetamide as a solvent, a polyamic acid with a solid content of 18% was prepared by condensation polymerization of 4,4'-diaminodiphenyl ether and pyromellitic dianhydride in an equimolar ratio in an ice-water bath. 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 reaction was carried out in an ice-water bath for 5 h. Then, 3.483 g of triethylamine was added and the reaction was continued for 5 h to prepare a water-soluble polyamic acid solution with a solid content of 18%. The prepared water-soluble polyamic acid was precipitated with acetone, and then washed and dried to obtain the water-soluble polyamic acid for use.

[0083] Take 0.14 g of graphene oxide, 0.86 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, ultrasonicate for 30 min, and stir magnetically for 6 h to dissolve it. Transfer it to a container with a diameter of 3 cm, place it in liquid nitrogen and only make the bottom of the mold contact with the liquid nitrogen to freeze it into a solid, and finally freeze-dry it in a freeze-dryer for 72 h to obtain graphene oxide / polyamic acid aerogel.

[0084] The obtained graphene oxide / polyamic acid aerogel was placed in a tube furnace, and the temperature was programmed to rise (heating rate: 5 °C / min) in a nitrogen atmosphere, that is, from room temperature to 60 °C and then held for 1 h, from 60 °C to 100 °C and then held for 1 h, from 100 °C to 200 °C and then held for 1 h, from 200 °C to 300 °C and then held for 2 h to obtain a polyimide composite absorbing aerogel with adjustable frequency bands.

[0085] Example 2

[0086] The steps for preparing the 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, ultrasonicate for 30 min, and stir magnetically for 6 h to dissolve it. The remaining steps are the same as those in Example 1 to obtain a polyimide composite absorbing aerogel with adjustable frequency bands.

[0088] Example 3

[0089] The steps for preparing the 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, ultrasonicate for 30 min, and stir magnetically for 6 h to dissolve. The remaining steps are the same as in Example 1, and a polyimide composite microwave-absorbing aerogel with adjustable frequency bands can be obtained.

[0091] Example 4

[0092] The steps for preparing the water-soluble polyamic acid are the same as 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, ultrasonicate for 30 min, and stir magnetically for 6 h to dissolve. The remaining steps are the same as in Example 1, and a polyimide composite microwave-absorbing aerogel with adjustable frequency bands can be obtained.

[0094] Comparative Example 1

[0095] 2,6-Diacetylpyridine was not added in this comparative example, and the steps are as follows:

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

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

[0098] Comparative Example 2

[0099] In this comparative example, 2,6-dimethylpyridine was used to replace 2,6-diacetylpyridine, and the steps are as follows:

[0100] The steps for preparing the water-soluble polyamic acid are the same as 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-dimethylpyridine, add 19 mL of deionized water and 0.5 mL of triethylamine, ultrasonicate for 30 min, and stir magnetically for 6 h to dissolve. The remaining steps are the same as in Example 1, and a polyimide composite microwave-absorbing aerogel can be obtained.

[0102] SEM Characterization

[0103] The vertically (parallel to the freezing direction) and horizontally (perpendicular to the freezing direction) of the frequency-tunable polyimide composite absorbing aerogel sample obtained in Example 1 were observed using a scanning electron microscope. The SEM images of the frequency-tunable polyimide composite absorbing aerogel obtained in Example 1 are as shown in Figure 1 shown, where Figure 1 a in Figure 1 corresponds to the vertical direction, and b in

[0104] corresponds to the horizontal direction. By comparison, it can be seen that when observed from the vertical direction, the aerogel sample exhibits a "honeycomb-like" pore structure; when observed from the horizontal direction, the sample exhibits an oriented pore channel structure, indicating that an orderly arranged pore structure can be formed by the directional freezing method.

[0104] Electromagnetic wave absorption performance test

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

[0106] Test method: The electromagnetic parameters of the sample materials were tested in the X-band (8.2 - 12.4 GHz) and Ku-band (12.4 - 18 GHz) using the waveguide method. Among them, the number of acquisition points was set to 421 in the X-band and 561 in the Ku-band. Sample size: 23×10 mm in the X-band; 16×8 mm in the Ku-band.

[0107] According to the electromagnetic parameters and the electromagnetic field transmission line theory, the wave absorption performance (minimum reflection loss (dB) and effective absorption bandwidth (≤ -10 dB)) of the frequency-tunable polyimide composite absorbing aerogels obtained in Examples 1 to 4 under different compression degrees was calculated by matlab simulation. Among them, the minimum reflection loss (dB) was obtained based on the simulation results, and the effective absorption bandwidth was found by plotting the curve of the reflection loss RL versus frequency and finding the continuous frequency range where RL ≤ -10 dB, and its width is the effective absorption bandwidth. The test results are shown in Table 1 below:

[0108] Table 1

[0109]

[0110] As can be seen from Table 1, for different embodiments under different compressive strains, the corresponding wave-absorbing frequency bands and minimum reflection losses are completely different, which proves that the wave-absorbing performance of the material can be effectively adjusted by compression. In addition, from the data in Table 1 above, it can also be concluded that the frequency-band tunable polyimide composite wave-absorbing aerogels obtained in Examples 1 to 4 all have good wave-absorbing performance under different compression conditions. Among them, the wave-absorbing performance of the frequency-band tunable polyimide composite wave-absorbing aerogel obtained in Example 2 is the most excellent. Under the condition of 40% compression, the minimum reflection loss is -70.64 dB, and the corresponding wave-absorbing bandwidth is 7.95 GHz.

[0111] Figure 2 and Figure 3 Figure 6 is the electromagnetic parameter curve of the frequency-band tunable polyimide composite wave-absorbing aerogel material obtained in Example 2 under different compression degrees. It can be seen that within a certain range, the increase in the compression degree leads to the gradual increase of both the real part and the imaginary part of the dielectric constant of the material. The increase in the real part of the dielectric constant indicates that the dielectric storage capacity of the specimen gradually increases with the compressive strain. This is mainly attributed to the formation of multiple micro-capacitors between the pore walls of the composite wave-absorbing aerogel. Under mechanical compressive strain, the pore wall spacing of the composite wave-absorbing aerogel decreases, resulting in an increase in the capacitance of the micro-capacitor and an increase in the dielectric storage capacity of the specimen. The main reason for the increase in the imaginary part of the dielectric constant is that the pore conductive structure of the specimen becomes closer during compression, resulting in a gradual increase in the conductivity and a gradual increase in the conductive loss of the material, which is beneficial to the conversion of electrical energy into heat energy.

[0112] Figure 4 Figure 7 is the wave-absorbing performance diagram of the frequency-band tunable polyimide composite wave-absorbing aerogel material obtained in Example 2 under different compression degrees. As Figure 4 can be seen, the material has good wave-absorbing performance under different compression degrees. When the compression degree is 0%, the minimum reflection loss of the material is -21.04 dB, and the corresponding effective absorption bandwidth is 5.00 GHz (8.20 - 13.20 GHz); when the compression degree is 20%, the minimum reflection loss of the material is -29.13 dB, and the corresponding effective absorption bandwidth is 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, and the corresponding effective absorption bandwidth is 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, and the corresponding effective absorption bandwidth is 3.59 GHz (14.41 - 18.00 GHz). In addition, as Figure 4 can also be concluded, with the increase in the compression degree, the wave-absorbing frequency of the material gradually shifts from low frequency to high frequency, which also proves that the material can achieve the regulation of the wave-absorbing efficiency and wave-absorbing frequency band through the adjustment of the compressive strain.

[0113] Thermal performance test

[0114] The band - adjustable polyimide composite absorbing - wave aerogel prepared in Example 1 was placed on a hot stage at 100 °C. At regular intervals, an infrared thermal imager was used to take pictures of the upper surface of the sample, and the temperature change of the upper surface of the sample at different times was recorded.

[0115] Table 2

[0116]

[0117] It can be seen from Table 2 that after the polyimide composite absorbing - wave aerogel was placed on the 100 °C hot stage for 1 min, the surface temperature of the specimen was about 48.21 °C; after 2 min, the surface temperature of the specimen rose to about 50.43 °C; after 10 min, the surface temperature of the specimen almost remained unchanged. The above results indicate that due to its loose porous structure and the low thermal conductivity of polyimide, which prevent heat transfer, the polyimide composite absorbing - wave aerogel has excellent heat - insulation performance and has a broader application prospect in dealing with high - temperature harsh environments.

[0118] Mechanical property test

[0119] A universal testing machine was used to test the compression - recovery performance of the band - adjustable polyimide composite absorbing - wave aerogel obtained in Example 2 and the polyimide composite absorbing - wave aerogels obtained in Comparative Examples 1 - 2 under different compression degrees.

[0120] The test results are as Figures 5 - 7 shown. Among them, Figure 5 corresponds to the band - adjustable polyimide composite aerogel prepared in Example 2 of the present invention, Figure 6 corresponds to the polyimide composite aerogel prepared in Comparative Example 1 of the present invention, Figure 7 corresponds to the polyimide composite aerogel prepared in Comparative Example 2 of the present invention. It can be seen from Figure 5 that after adding 2,6 - diacetylpyridine to the system, when the compression degree of the material is 20%, 40%, and 60%, its stress - strain curves are all smooth closed loops, which means that no structural damage occurred during the compression of the aerogel, and it can completely return to its original state after the external force is removed without energy loss. The stress - strain curve rises smoothly without sudden changes or breaks, indicating that the cross - linked network effectively disperses the stress and avoids local failure. In contrast, from 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 approaches 55%, there is a breakpoint in its stress-strain curve. The reason 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 damaged, resulting in structural collapse, irreversible damage, a rapid rise and then a sudden drop in stress with the increase of strain, indicating that the material is brittle and has poor elastic recovery ability. After the external force is unloaded, the residual strain is significant and it cannot return to its original state; Figure 7 It can be seen that when the compressive strain is 60%, in the composite aerogel with the introduction of other pyridine substances (2,6-dimethylpyridine), although there is no breakpoint in its curve, the rising slope of the curve is gentle, and the residual strain after the external force is unloaded is Figure 5 greater, indicating insufficient resilience of the composite aerogel. The results show that the addition of 2,6-diacetylpyridine significantly improves the compressive resilience and structural stability of the composite aerogel, enabling it to maintain integrity and elastic recovery ability within the strain range of 20% - 60%.

[0121] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to these embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A polyimide composite microwave 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 wave aerogel has an orderly arranged pore structure.

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

3. The polyimide composite microwave-absorbing aerogel according to claim 2, wherein, 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: Mix a diamine, a dianhydride and a polar solvent and carry out a polymerization reaction, react the obtained product with an organic amine, precipitate and dry to obtain the water-soluble polyamic acid.

5. The polyimide composite microwave absorbing aerogel according to claim 4, characterized in that The diamine includes p-phenylenediamine and / or 4,4'-diaminodiphenyl ether; The dianhydride 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 microwave-absorbing aerogel according to claim 4 or 5, characterized in that, The polymerization reaction is carried out under an ice-water bath condition; The time of the polymerization reaction is 2~5 h, and the reaction time of the product and the organic amine is 3~6 h; The precipitation is carried out using acetone.

7. A preparation method of the polyimide composite microwave-absorbing aerogel according to any one of claims 1 to 6, characterized in that, It includes the following steps: Carry out directional freezing, freeze-drying and thermal amination treatment on the aqueous dispersion of water-soluble polyamic acid, graphene oxide and 2,6-diacetylpyridine to obtain the polyimide composite absorbing wave aerogel.

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

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

10. The application of the polyimide composite absorbing wave aerogel according to any one of claims 1~6 or the polyimide composite absorbing wave aerogel prepared by the preparation method according to any one of 7~9 in the preparation of an adaptive electromagnetic wave shielding and stealth material.

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