A magnetic polyimide composite foam with hierarchical impedance structure, its preparation method and application in radar stealth

Abstract

The application discloses a magnetic polyimide composite foam material with a hierarchical impedance structure, and a preparation method thereof. The preparation method comprises the following steps: (1) preparing a polyimide foam; (2) preparing a carbon nanotube polyimide composite foam; (3) preparing a nickel-iron magnetic polyimide composite foam; and (4) bonding the carbon nanotube polyimide composite foam prepared in the step (2) and the nickel-iron magnetic polyimide composite foam prepared in the step (3) through strong glue, so as to obtain the magnetic polyimide composite foam material with the hierarchical impedance structure. In the application, the polyimide foam is used as a framework, and the electrically-conductive magnetic filler is loaded on the framework through chemical plating and impregnation, so that the attenuation form of the porous foam material is enriched, the dissipation of electromagnetic waves is enhanced, the introduction of the anisotropic carbonyl iron can further enhance the magnetic loss capacity of the composite material, and the multiple loss forms are combined to realize the excellent radar stealth performance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of composite foam materials, in particular to a magnetic polyimide composite foam material with hierarchical impedance structure and its preparation method and application in radar stealth. BACKGROUND

[0002] With the continuous development of science and technology, a large number of new wireless connection devices, especially wearable electronic devices and microwave sensors, have completely changed the society and paved the way for the arrival of the intelligent era. Although the rapid development of wireless technology has improved our quality of life, it has inevitably led to serious electromagnetic pollution, which can affect human health and interfere with the operation of nearby electronic devices, resulting in signal loss or interruption. Therefore, it is urgent to study a high-performance radar stealth material with strong absorption capacity and wide absorption frequency band to solve these problems.

[0003] As a kind of three-dimensional conductive network porous material, polyimide foam has gradually become a highly competitive microwave absorbing material and received more and more attention due to its low density, easy processing, corrosion resistance, high temperature resistance and other characteristics. However, the internal loss mechanism of pure polyimide foam material is single, and the electromagnetic wave cannot be effectively attenuated after entering the material.

[0004] The porous framework of polyimide foam material enables the conductive filler to form a connected conductive network structure at a low addition amount, but the internal loss mechanism of the porous foam material is single, and the impedance matching structure is not perfect, and the loss form is limited to dielectric loss, resulting in weak absorption intensity, narrow frequency band, single frequency band and other problems, making it difficult to achieve efficient attenuation of electromagnetic waves with electromagnetic multiple loss forms. At the same time, the interface bonding force between the functional filler and the foam is relatively poor, which is also one of the difficult problems to be solved. Therefore, it is urgent to optimize the structure design and method selection to construct the impedance matching structure, increase the interface bonding force between the functional filler and the matrix while introducing the magnetic component, so as to enhance the radar stealth performance. SUMMARY

[0005] In view of the above problems existing in the prior art, the present application provides a magnetic polyimide composite foam material with hierarchical impedance structure and its preparation method and application in radar stealth. The present application uses polyimide foam as the skeleton, loads conductive magnetic filler through chemical plating, dipping and other methods, enriches the attenuation form of porous foam material, and enhances the dissipation of electromagnetic waves. At the same time, the introduction of anisotropic carbonyl iron can further enhance the magnetic loss capacity of the composite material, and the combination of multiple loss forms can achieve excellent radar stealth performance.

[0006] The technical scheme of the present application is as follows:

[0007] The first object of the present application is to provide a magnetic polyimide composite foam with a hierarchical impedance structure, which is composed of a carbon nanotube polyimide composite foam layer and a nickel-iron magnetic polyimide composite foam layer; the carbon nanotube polyimide composite foam layer and the nickel-iron magnetic polyimide composite foam layer are bonded by strong glue.

[0008] The thickness of the carbon nanotube polyimide composite foam layer is 1-5 mm, and the loading amount of carbon nanotubes is 20-40 wt%.

[0009] The thickness of the nickel-iron magnetic polyimide composite foam layer is 2-6 mm, and the loading amount of carbonyl iron is 40-80 wt%.

[0010] In an embodiment of the present application, the foam average pore size of the carbon nanotube polyimide composite foam layer is 300 μm.

[0011] In an embodiment of the present application, the diameter of carbon nanotubes in the carbon nanotube polyimide composite foam layer is 20-40 nm.

[0012] In an embodiment of the present application, the foam average pore size of the nickel-iron magnetic polyimide composite foam layer is 300 μm.

[0013] The second object of the present application is to provide a preparation method of a magnetic polyimide composite foam, which comprises the following steps:

[0014] (1) preparing a polyimide foam;

[0015] In the condensation polymerization product of the dianhydride monomer and the alcohol monomer, a stabilizer, a catalyst and water are added to obtain a foaming precursor, which is then mixed with a foaming agent to foam, and then subjected to thermal imidization to obtain the polyimide foam;

[0016] (2) preparing a carbon nanotube polyimide composite foam;

[0017] The polyimide foam prepared in step (1) is immersed in a carbon nanotube dispersion solution to obtain a carbon nanotube polyimide composite foam, which is then dried for standby use;

[0018] (3) preparing a nickel-iron magnetic polyimide composite foam;

[0019] The polyimide foam prepared in step (1) is roughened in a roughening solution, then activated in an activation solution, and then plated in a plating solution to obtain a polyimide foam loaded with nickel, which is finally immersed in a flaky carbonyl iron dispersion solution to obtain the nickel-iron magnetic polyimide composite foam;

[0020] (4) The carbon nanotube polyimide composite foam prepared in step (2) is adhered to the nickel-iron magnetic polyimide composite foam prepared in step (3) by strong glue to obtain the magnetic polyimide composite foam material with a hierarchical impedance structure.

[0021] In an embodiment of the present application, in step (1), the dianhydride monomer is one or more of pyromellitic dianhydride (PMDA), biphenyl tetracarboxylic dianhydride (BPDA), 3,3',4,4'-benzophenone tetracarboxylic dianhydride (BTDA), hexafluoro dianhydride (6FDA), bisphenol A type diether dianhydride (BPADA), and 4,4'-oxydiphthalic anhydride (ODPA).

[0022] In an embodiment of the present application, the alcohol monomer is one or more of methanol (MT), ethanol (ET), and polyethylene glycol (PEG).

[0023] In an embodiment of the present application, the stabilizer is one or more of DC-193, DC-5098, and AK-8818.

[0024] In an embodiment of the present application, the catalyst is one or more of triethanolamine (TEOA), dibutyltin dilaurate (DBTDL), and Dabco 33-LV.

[0025] In an embodiment of the present application, the foaming agent is one or more of diphenylmethane diisocyanate, polyphenyl polymethylene polyisocyanate, and triphenylmethane triisocyanate.

[0026] In an embodiment of the present application, in step (1), the mass ratio of the dianhydride monomer to the alcohol monomer is 1-10:1.

[0027] In an embodiment of the present application, in step (1), the amount of the stabilizer is 5-30% of the total mass of the dianhydride monomer and the alcohol monomer.

[0028] In an embodiment of the present application, in step (1), the amount of the catalyst is 0.5-10% of the total mass of the dianhydride monomer and the alcohol monomer.

[0029] In an embodiment of the present application, in step (1), the solvent used in the polycondensation reaction is N,N-dimethylformamide.

[0030] In an embodiment of the present application, in step (1), the polycondensation reaction is performed at a temperature of 50-100℃ for 2-5h.

[0031] In an embodiment of the present application, in step (1), the mass ratio of the amount of the foaming agent to the total amount of the dianhydride monomer and the alcohol monomer is 1-5:1.

[0032] In one embodiment of the present application, in step (1), the foaming is carried out at room temperature; and the imidization treatment method is: incubation at 130℃ for 2-6h, incubation at 160℃ for 2-6h, and incubation at 220℃ for 3-10h.

[0033] In one embodiment of the present application, in step (2), the preparation method of the carbon nanotube dispersion liquid is: dispersing the carbon nanotubes and the polyamide acid in water to obtain the carbon nanotube dispersion liquid.

[0034] In one embodiment of the present application, the content of the carbon nanotubes in the carbon nanotube dispersion liquid is 2-5wt%.

[0035] In one embodiment of the present application, the content of the polyamide acid in the carbon nanotube dispersion liquid is 1-5wt%.

[0036] In one embodiment of the present application, in step (2), the polyimide foam is immersed in the carbon nanotube dispersion liquid by using an assisted impregnation method.

[0037] The assisted impregnation converts the closed pores of the foam structure into open pores by continuously pressing the foam, so that more functional fillers in the impregnation liquid can enter the interior of the material, thereby realizing the functionalization of the material.

[0038] In one embodiment of the present application, in step (2), the polyimide foam prepared in step (1) is cut into a size of 4cm*4cm in length and width and 2-6mm in thickness, is immersed in 10-15mL of the carbon nanotube dispersion liquid for 8-15min, to obtain a carbon nanotube polyimide composite foam, and then is dried for standby use.

[0039] In one embodiment of the present application, in step (3), the solute of the roughening solution is one or more of dopamine, citric acid, polyphenylamine, and tannic acid; the solvent is water; and the concentration of the roughening solution is 1-6g / L.

[0040] In one embodiment of the present application, the solute of the activation liquid is tin dichloride and palladium chloride; the solvent is water; the concentration of the tin dichloride is 10-50g / L, and the concentration of the palladium chloride is 0.5-1g / L.

[0041] In one embodiment of the present application, the plating solution comprises a metal salt, a complexing agent, a reducing agent, water, and a pH adjuster.

[0042] In one embodiment of the present application, the metal salt is one or more of nickel sulfate, nickel chloride, and nickel nitrate, and the concentration is 8-80g / L.

[0043] Further, the nickel sulfate is nickel sulfate hexahydrate.

[0044] In one embodiment of the present application, the complexing agent is one or more of lactic acid, sodium citrate, thiourea, sodium acetate, malic acid, ammonium chloride, and tartaric acid, and the concentration is 5-40 g / L.

[0045] Further, the sodium citrate is trisodium citrate dihydrate.

[0046] In one embodiment of the present application, the reducing agent is one or more of dimethylamine borane, sodium hypophosphite, and sodium borohydride, and the concentration is 5-20 g / L.

[0047] In one embodiment of the present application, the pH adjusting agent is one or more of ammonia, sodium hydroxide, potassium hydroxide, hydrochloric acid, and sulfuric acid, and the amount is such that the pH of the plating solution is 5-11.

[0048] In one embodiment of the present application, in step (3), the roughening treatment conditions are: roughening at room temperature for 12-48 h.

[0049] The activation treatment conditions are: activation at room temperature for 0.5-12 h.

[0050] In one embodiment of the present application, the plating treatment conditions are: temperature of 10-30 DEG C, and time of 1-5 min.

[0051] The flaky carbonyl iron dispersion liquid is composed of flaky carbonyl iron, polyamide acid, and water; the content of the flaky carbonyl iron is 1-90 wt%, and the content of the polyamide acid is 1-5 wt%.

[0052] In one embodiment of the present application, after the plating treatment, cleaning and drying treatment are performed; the drying temperature is 40-60 DEG C, and the time is 1-2 h.

[0053] A third object of the present application is to provide an application of the magnetic polyimide composite foam material with hierarchical impedance structure in radar stealth.

[0054] Further, the application includes the application in the electromagnetic protection field of aerospace, precision electronic instruments, and the like.

[0055] The present application has the beneficial technical effects of:

[0056] The present application successfully prepares a magnetic polyimide composite foam material with hierarchical impedance structure by chemical plating and auxiliary impregnation, and has excellent radar stealth effect; the present application is simple to operate and green and environmentally friendly.

[0057] The present application enhances the interface bonding force between the filler and the material matrix by roughening, and disperses the filler by using the polyimide precursor to obtain an impregnation liquid, and after impregnation, imidization is carried out, so that the adhesion between the filler and the matrix is stronger, and thus both of the two methods are beneficial to the interface bonding force between the functional filler and the matrix.

[0058] According to the electromagnetic wave impedance matching principle, the present application loads the conductive magnetic filler in the form of chemical plating, impregnation and the like, the dissipation layer loads the magnetic nickel on the polyimide foam material through the adjustment of the chemical plating nickel time, then the impregnation of iron carbonyl is carried out in the polyimide foam plated with nickel, the introduction layer loads the carbon nanotube through impregnation, and finally the two are combined through the physical adhesion. The chemical plating nickel and the impregnation anisotropic iron carbonyl not only construct the connected conductive network in the dissipation layer, but also enrich the attenuation characteristics due to the high magnetic loss of the introduced iron and nickel, and the polyimide foam material impregnated with the carbon nanotube of the introduction layer and the foam material constructing the complete conductive network of the dissipation layer constitute a hierarchical impedance structure, which is beneficial to the introduction and attenuation of the electromagnetic wave and further enhances the radar stealth performance of the composite foam. BRIEF DESCRIPTION OF DRAWINGS

[0059] Figure 1 It is the scanning electron microscope photo of the magnetic polyimide composite foam material prepared in the present application embodiment 1.

[0060] Figure 2 It is the electromagnetic parameter and reflectivity of the magnetic polyimide composite foam material prepared in the present application embodiment 1.

[0061] Figure 3 It is the electromagnetic parameter and reflectivity image of the PCP-PNFP of the magnetic polyimide composite foam material prepared in the present application embodiments 9-11.

[0062] Figure 4 It is the electromagnetic parameter and reflectivity image of the PCP-PNFP of the magnetic polyimide composite foam material prepared in the present application comparative examples 3 and 4.

[0063] Figure 5 It is the reflectivity contrast image of the composite foam material prepared in the present application embodiment 1 and comparative examples 1 and 2.

[0064] Figure 6 It is the SE and reflectivity image of the PCP-PNFP of the magnetic polyimide composite foam material prepared in the present application embodiment 1 after being treated at-80℃ and 200℃ for 12h. T DETAILED DESCRIPTION

[0065] ​The application will be further described in conjunction with specific examples. It should be understood that these examples are only used to illustrate but not to limit the scope of the application. Furthermore, it should be understood that those skilled in the art can make various modifications or changes to the application after reading the content of the application, and these equivalent forms also fall within the scope defined by the appended claims.

[0066] The theoretical loading of carbon nanotubes in the carbon nanotube polyimide composite foam = mass of carbon nanotubes / mass of carbon nanotube polyimide composite foam x 100%;

[0067] The theoretical loading of carbonyl iron in the nickel-iron magnetic polyimide composite foam = mass of carbonyl iron / mass of nickel-iron magnetic polyimide composite foam x 100%.

[0068] Example 1

[0069] A preparation method of a magnetic polyimide composite foam material with a hierarchical impedance structure, the method comprising the following steps:

[0070] (1) preparing a polyimide foam;

[0071] Add 44 g of DMF to a three-necked flask, slowly add 65.8 g of PMDA, and when the temperature of the water bath kettle is raised to 90°C, add 10.3 g of methanol, and the solution becomes clear quickly, then add 4 g of polyethylene glycol, and carry out a polycondensation reaction. After the reaction is completed, reduce the temperature to room temperature, and add 20 g of DC-193, 24 g of water, 4.2 g of triethanolamine (TEOA), and 3.1 g of dibutyltin dilaurate (DBTDL), and stir for 1 h to obtain a precursor solution. Stir and mix the precursor solution with 132 g of polyphenyl polymethylene polyisocyanate uniformly, and place it in a cuboid mold with a size of 4 cm*4 cm*2 cm, so that it is fully reacted. After the foaming is completed, the foam is taken out of the mold, and placed in an oven for heat preservation at 130°C for 4 h, at 160°C for 4 h, and at 220°C for 4 h. After the temperature is reduced to room temperature, the polyimide foam is taken out, and is named as PI.

[0072] (2) preparing a carbon nanotube polyimide composite foam;

[0073] The polyimide foam (4 cm*4 cm*4 mm) prepared in step (1) is immersed in 10 mL of a carbon nanotube dispersion solution (carbon nanotube content of 3 wt%, and polyamide acid content of 4 wt%) in an assisted impregnation manner, and is impregnated for 10 min to obtain a carbon nanotube polyimide composite foam, which is then dried for standby use (loading of carbon nanotubes of 25 wt%); and is named as PCP.

[0074] (3) preparing a nickel-iron magnetic polyimide composite foam;

[0075] The polyimide foam (4 cm*4 cm*4 mm) prepared in step (1) is placed in a tris solution with pH = 9, and ultrasonic treatment is performed for 5 min to allow the polyimide foam to be fully infiltrated, and then 2.0 g of dopamine is added, and stirring is continuously performed under magnetic stirring for 36 h; after the end, the polyimide foam is washed with ultrapure water for 2-3 times, ultrasonic washing is performed for 5 min, and then the polyimide foam is washed with anhydrous ethanol, and finally dried in a 60°C oven for 1 h to obtain a roughened polyimide foam; the roughened polyimide foam is first placed in 100 mL of a 15 g / L tin dichloride solution, and magnetic stirring is performed for 0.5 h, and then the polyimide foam is washed with ultrapure water and anhydrous ethanol, and then dried in a 60°C oven for 1 h, and then the preliminary activated foam obtained in the first step is placed in 100 mL of a 0.5 g / L palladium chloride solution, and magnetic stirring is continuously performed for 0.5 h, and then the polyimide foam is washed with ultrapure water and anhydrous ethanol, and then dried in a 60°C oven for 1 h to obtain an activated polyimide foam; the activated polyimide foam is placed in 150 mL of a plating solution (the concentration of nickel sulfate hexahydrate is 80 g / L, the concentration of trisodium citrate dihydrate is 40 g / L, the concentration of lactic acid is 20 g / L, and the concentration of dimethylamine borane is 10 g / L, and the pH is adjusted to 8 with ammonia water), and plating is performed at 25°C for 2 min, and then the polyimide foam is taken out, washed with ultrapure water and anhydrous ethanol, and finally dried in a 60°C oven for 1 h to obtain a polyimide foam loaded with nickel; the polyimide foam loaded with nickel is immersed in 100 mL of a flaky carbonyl iron dispersion liquid (the content of flaky carbonyl iron is 15 wt%, and the content of polyamide acid is 4 wt%), and a nickel-iron magnetic polyimide composite foam (the loading amount of flaky carbonyl iron is 60 wt%) is obtained, which is named as PNFP;

[0076] (4) Preparation of a magnetic polyimide composite foam material with a hierarchical impedance structure;

[0077] The PCP prepared in step (2) and the PNFP prepared in step (3) are combined by physical adhesion to obtain a magnetic polyimide composite foam material with a hierarchical impedance structure, which is named as PCP-PNFP.

[0078] The scanning electron microscope photos of the magnetic polyimide composite foam material prepared in this example are shown in FIG. 1, wherein Figure 1 a is a polyimide foam, Figure 1 b is a nickel-iron magnetic polyimide composite foam, Figure 1 c is a carbon nanotube polyimide composite foam, Figure 1 d is an enlarged image of the nickel-iron magnetic polyimide composite foam. Figure 1 As can be seen from FIG. 1, nickel and iron are successfully loaded on the polyimide foam, and there is interface polarization between the interfaces of nickel and iron, which enriches the attenuation forms and increases the electromagnetic wave attenuation ability. Figure 1

[0079] ​Example 2

[0080] Referring to Example 1, the difference is that the content of flaky carbonyl iron in the flaky carbonyl iron dispersion liquid in step (3) of Example 1 is adjusted to 25wt%, and other conditions are the same as Example 1, to obtain a magnetic polyimide composite foam material with hierarchical impedance structure.

[0081] Example 3

[0082] Referring to Example 1, the difference is that the content of flaky carbonyl iron in the flaky carbonyl iron dispersion liquid in step (3) of Example 1 is adjusted to 35wt%, and other conditions are the same as Example 1, to obtain a magnetic polyimide composite foam material with hierarchical impedance structure.

[0083] Example 4

[0084] Referring to Example 1, the difference is that the content of flaky carbonyl iron in the flaky carbonyl iron dispersion liquid in step (3) of Example 1 is adjusted to 45wt%, and other conditions are the same as Example 1, to obtain a magnetic polyimide composite foam material with hierarchical impedance structure.

[0085] Example 5

[0086] Referring to Example 1, the difference is that the content of flaky carbonyl iron in the flaky carbonyl iron dispersion liquid in step (3) of Example 1 is adjusted to 55wt%, and other conditions are the same as Example 1, to obtain a magnetic polyimide composite foam material with hierarchical impedance structure.

[0087] Example 6

[0088] Referring to Example 1, the difference is that the content of flaky carbonyl iron in the flaky carbonyl iron dispersion liquid in step (3) of Example 1 is adjusted to 65wt%, and other conditions are the same as Example 1, to obtain a magnetic polyimide composite foam material with hierarchical impedance structure.

[0089] Example 7

[0090] Referring to Example 1, the difference is that the content of flaky carbonyl iron in the flaky carbonyl iron dispersion liquid in step (3) of Example 1 is adjusted to 75wt%, and other conditions are the same as Example 1, to obtain a magnetic polyimide composite foam material with hierarchical impedance structure.

[0091] Example 8

[0092] Referring to Example 1, the difference is that the content of flaky carbonyl iron in the flaky carbonyl iron dispersion liquid in step (3) of Example 1 is adjusted to 85wt%, and other conditions are the same as Example 1, to obtain a magnetic polyimide composite foam material with hierarchical impedance structure.

[0093] Example 9

[0094] Referring to Example 1, the difference is that the content of carbon nanotubes in the carbon nanotube dispersion liquid in step (2) of Example 1 is adjusted to 2wt%, and other conditions are the same as those in Example 1, to obtain a magnetic polyimide composite foam material with a hierarchical impedance structure.

[0095] Example 10

[0096] Referring to Example 1, the difference is that the content of carbon nanotubes in the carbon nanotube dispersion liquid in step (2) of Example 1 is adjusted to 4wt%, and other conditions are the same as those in Example 1, to obtain a magnetic polyimide composite foam material with a hierarchical impedance structure.

[0097] Example 11

[0098] Referring to Example 1, the difference is that the content of carbon nanotubes in the carbon nanotube dispersion liquid in step (2) of Example 1 is adjusted to 5wt%, and other conditions are the same as those in Example 1, to obtain a magnetic polyimide composite foam material with a hierarchical impedance structure.

[0099] Example 12

[0100] Referring to Example 1, the difference is that the reducing agent in step (3) of Example 1 is sodium borohydride, and the concentration is still 10g / L, and other conditions are the same as those in Example 1, to obtain a magnetic polyimide composite foam material with a hierarchical impedance structure.

[0101] Comparative Example 1

[0102] A method for preparing a polyimide composite foam material, the method comprising the following steps:

[0103] (1) preparing a polyimide foam;

[0104] A three-necked flask is added with 44g of DMF, and 65.8g of PMDA is slowly added. When the water bath temperature is raised to 90°C, the solution is particularly thick due to the high content of PMDA, and after the addition of 10.3g of methanol, the solution quickly becomes clear. Then 4g of polyethylene glycol is added, and the condensation reaction is carried out. After the reaction is completed, the temperature is lowered to room temperature, and 20g of silicone oil DC-193 and 24g of water, 4.2g of catalyst triethanolamine (TEOA) and 3.1g of dibutyltin dilaurate (DBTDL) are added. After stirring for 1h, a precursor solution is obtained. The precursor solution is uniformly mixed with 132g of polyphenyl polymethylene polyisocyanate, and the mixture is allowed to react sufficiently. After the foaming is completed, the foam is removed from the mold and placed in an oven at 130°C for 4h, at 160°C for 4h, and at 220°C for 4h. After the temperature is lowered to room temperature, the polyimide foam is taken out and named PI.

[0105] (2) Preparation of carbon nanotube carbonyl iron polyimide composite foam;

[0106] The carbon nanotubes, flaky carbonyl iron and polyamide acid were dispersed in 100 g of ultrapure water to obtain a dispersion liquid with a carbon nanotube solid content of 3 wt% and a carbonyl iron solid content of 15 wt%. The dispersion liquid of carbon nanotubes and carbonyl iron was immersed into the polyimide foam (4 cm*4 cm*4 mm) prepared in step (1) by assisted immersion, and then imidization was performed to obtain a carbon nanotube carbonyl iron polyimide composite foam. Then, drying was performed to obtain a carbon nanotube carbonyl iron polyimide composite foam, which was named PCFP;

[0107] (3) Preparation of polyimide foam loaded with nickel;

[0108] The polyimide foam (4 cm*4 cm*4 mm) prepared in step (1) was placed in a tris solution with a pH of 9, and ultrasonic treatment was performed for 5 min to ensure that the polyimide foam was fully infiltrated. Then, 2.0 g of dopamine was added, and magnetic stirring was continuously performed for 36 h. After the end of the reaction, the polyimide foam was washed with ultrapure water for 2-3 times, ultrasonic treatment was performed for 5 min, and then the polyimide foam was washed with anhydrous ethanol. Finally, drying was performed in a 60°C oven for 1 h to obtain a roughened polyimide foam. The roughened polyimide foam was first placed in 100 mL of a 15 g / L tin dichloride solution, and magnetic stirring was performed for 0.5 h to perform preliminary activation. After washing with ultrapure water and anhydrous ethanol, drying was performed in a 60°C oven for 1 h. Then, the preliminarily activated foam was placed in 100 mL of a 0.5 g / L palladium chloride solution, and magnetic stirring was continuously performed for 0.5 h. After washing with ultrapure water and anhydrous ethanol, drying was performed in a 60°C oven for 1 h to obtain an activated polyimide foam. The activated polyimide foam was placed in 150 mL of a plating solution (the concentration of nickel sulfate hexahydrate was 80 g / L, the concentration of sodium citrate dihydrate was 40 g / L, the concentration of lactic acid was 20 g / L, and the concentration of dimethylamine borane was 10 g / L, and the pH was adjusted to 8 by using ammonia water), and plating was performed at 25°C for 10 min. After the end of the plating, the polyimide foam was taken out, washed with ultrapure water and anhydrous ethanol, and finally dried in a 60°C oven for 1 h to obtain a polyimide foam loaded with nickel, which was named PN.

[0109] (4) Preparation of a polyimide composite foam material;

[0110] The PCFP prepared in step (2) and the PN prepared in step (3) were combined by physical adhesion to obtain a polyimide composite foam material, which was named PCFP-PN.

[0111] Comparative Example 2

[0112] A method for preparing a conductive and magnetic polyimide composite foam material, the method comprising the following steps:

[0113] (1) Preparation of a polyimide foam;

[0114] In a three-necked flask, 44 g of DMF was added, and 65.8 g of PMDA was slowly added. When the water bath temperature was raised to 90°C, the solution became particularly viscous due to the high PMDA content. After adding 10.3 g of methanol, the solution quickly became clear. Then 4 g of polyethylene glycol was added, and the polycondensation reaction was carried out. After the reaction was completed, the temperature was lowered to room temperature, and 20 g of silicone oil DC-193 and 24 g of water were added. Then 4.2 g of catalyst triethanolamine (TEOA) and 3.1 g of dibutyltin dilaurate (DBTDL) were added, and the mixture was stirred for 1 h to obtain a precursor solution. The precursor solution was then mixed with 132 g of polyphenyl polymethylene polyisocyanate, and the mixture was stirred to ensure uniformity. The mixture was then allowed to react for a sufficient period of time. After the foaming was completed, the foam was removed from the mold and placed in an oven at 130°C for 4 h, at 160°C for 4 h, and at 220°C for 4 h. After the temperature was lowered to room temperature, the polyimide foam was removed and named PI.

[0115] (2) Preparation of conductive magnetic polyimide composite foam material;

[0116] The polyimide foam (4 cm*4 cm*8 mm) prepared in step (1) was placed in a tris solution with a pH of 9 and ultrasonicated for 5 min to ensure that the polyimide foam was fully immersed. Then 2.0 g of dopamine was added, and the mixture was continuously stirred under magnetic stirring for 36 h. After the reaction was completed, the polyimide foam was washed with ultrapure water for 2-3 times, ultrasonicated for 5 min, and then washed with anhydrous ethanol. Finally, the polyimide foam was dried in an oven at 60°C for 1 h to obtain a roughened polyimide foam.

[0117] The roughened polyimide foam was first placed in a 100 mL solution of 15 g / L tin dichloride and stirred magnetically for 0.5 h to perform preliminary activation. After washing with ultrapure water and anhydrous ethanol, the foam was dried in an oven at 60°C for 1 h. Then the preliminarily activated foam was placed in a 100 mL solution of 0.5 g / L palladium chloride and continuously stirred magnetically for 0.5 h. After washing with ultrapure water and anhydrous ethanol, the foam was dried in an oven at 60°C for 1 h to obtain an activated polyimide foam.

[0118] The activated polyimide foam was placed in a 150 mL plating solution (80 g / L of nickel sulfate hexahydrate, 40 g / L of sodium citrate dihydrate, 20 g / L of lactic acid, and 10 g / L of dimethylamine borane, and the pH was adjusted to 8 with ammonia water) and plated for 10 min at 25°C. After plating, the foam was removed, washed with ultrapure water and anhydrous ethanol, and finally dried in an oven at 60°C for 1 h to obtain a nickel-loaded polyimide foam.

[0119] The flaky carbonyl iron, carbon nanotubes and polyamide acid were dispersed in 100 ml ultrapure water to obtain a dispersion liquid with a carbonyl iron solid content of 15 wt% and a carbon nanotube solid content of 3 wt%. The dispersion liquid of the carbonyl iron and the carbon nanotubes was immersed into the polyimide foam loaded with nickel by an assisted immersion method, and then imidization was performed to obtain a carbon nanotube nickel-iron conductive magnetic composite polyimide composite foam, which was named as PNCFP.

[0120] Comparative Example 3

[0121] With reference to Example 1, the difference lies in that the concentration of the palladium chloride solution in step (3) of Example 1 is adjusted to 0.1 g / L; other conditions are the same as those in Example 1. Due to the decrease in the concentration of the palladium chloride, the reaction rate is relatively reduced, and a magnetic polyimide composite foam material with a hierarchical impedance structure is obtained.

[0122] Comparative Example 4

[0123] With reference to Example 1, the difference lies in that the plating time in step (3) of Example 1 is adjusted to 10 min; other conditions are the same as those in Example 1. A magnetic polyimide composite foam material with a hierarchical impedance structure is obtained.

[0124] Comparative Example 5

[0125] With reference to Example 1, the difference lies in that the plating time in step (3) of Example 1 is adjusted to 20 min; other conditions are the same as those in Example 1. A magnetic polyimide composite foam material with a hierarchical impedance structure is obtained.

[0126] Test Example:

[0127] (1) Radar stealth performance

[0128] The S parameters of the composite film were measured by a waveguide method in the frequency range of 8.2-40 GHz using an Agilent E5063A vector network analyzer, and the reflectivity was derived from the S parameters.

[0129] The magnetic polyimide composite foam material with a hierarchical impedance structure prepared in Example 1 was subjected to radar stealth performance test, and the test results are shown in Figure 2 Figure 2 a is the electromagnetic parameter of PCP-PNFP, Figure 2 b is the reflectivity of PCP-PNFP; from the figure we can see that the final sample has an SE T value greater than 20 dB, and its reflectivity at 8.2-40 GHz is less than -10 dB, which can absorb 90% of the electromagnetic waves in the range of 8.2-40 GHz, thus having good radar stealth performance in the range of 8.2-40 GHz.

[0130] ​The magnetic polyimide composite foams with hierarchical impedance structure prepared in Examples 11-13 were tested for radar stealth performance, and the test results are shown in Figure 3 wherein Figure 3 a, 3c, 3e are the electromagnetic parameters of the composite foams prepared in Examples 11-13, respectively, Figure 3 b, 3d, 3f are the reflectivity of the composite foams prepared in Examples 11-13, respectively; and Figure 2 It can be seen from the comparison that when the carbon nanotube content of the carbon nanotube dispersion is 3wt%, the radar stealth has excellent effect in the range of 8.2-40GHz.

[0131] The magnetic polyimide composite foams with hierarchical impedance structure prepared in Examples 9-11 were tested for radar stealth performance, and the test results are shown in Figure 3 wherein Figure 3 a, 3c, 3e are the electromagnetic parameters of the composite foams prepared in Examples 9-11, respectively, Figure 3 b, 3d, 3f are the reflectivity of the composite foams prepared in Examples 9-11, respectively; it can be seen from the figure that when the solid content of the carbon nanotube dispersion is 2-5wt%, the reflectivity is below -9dB in the range of 8.2-40GHz, and has good radar stealth performance, and Figure 2 It can be seen from the comparison that when the solid content of the carbon nanotube dispersion is 3wt%, the radar stealth is below -10dB in the range of 8.2-40GHz, and it can absorb more than 90% of the electromagnetic waves, and has better radar stealth effect than others.

[0132] The magnetic polyimide composite foams with hierarchical impedance structure prepared in Comparative Examples 4, 5 were tested for radar stealth performance, and the test results are shown in Figure 4 wherein Figure 4 a, Figure 4 c are the electromagnetic parameters of the composite foams prepared in Comparative Examples 4, 5, respectively, Figure 4 b, Figure 4 d are the reflectivity of the composite foams prepared in Comparative Examples 4, 5, respectively; and Figure 2 It can be found from the comparison that as the nickel plating time increases, the SE T value will increase accordingly, but the reflectivity will show a result of more than -10dB in most wave bands, and the radar stealth effect is much worse than Figure 2 Therefore, it can be seen that as the nickel plating time increases, the shielding effect will continuously increase, but the absorption effect of the electromagnetic waves will become worse.

[0133] The reflectivity of the composite foams prepared in Example 1 and Comparative Examples 1-2 is shown in Figure 5As shown in the figure, the reflectivity of the composite foam with carbon nanotubes and carbonyl iron as the introductory layer and nickel as the dissipation layer is above -10 dB in the R-band, while the reflectivity of the single-layer carbon nanotube-nickel-iron conductive and magnetic composite foam is above -10 dB across the entire wavelength range. Figure 2 Compared to other materials, the magnetic polyimide composite foam material with a hierarchical impedance structure exhibits poor radar stealth performance across the entire band below -10dB. Therefore, the magnetic polyimide composite foam material of this invention has the best radar stealth performance.

[0134] (2) High and low temperature resistance

[0135] After treating the composite thin film for 12 hours each in a low-temperature reaction bath at -80℃ (Hangzhou David Science and Education Company) and an electric thermostatic drying oven at 200℃ (Shanghai Senxin Company), the S-parameters of the composite thin film were measured using the waveguide method in the frequency range of 8.2-40GHz using an Agilent E5063A vector network analyzer. The reflectivity was then calculated from the S-parameters.

[0136] The magnetic polyimide composite foam material with a hierarchical impedance structure prepared in Example 1 was treated at -80°C and 200°C for 12 h, and the SE of PCP-PNFP was measured. T and reflectivity; results as follows Figure 6 As shown. By Figure 6 It can be seen that its ability to attenuate electromagnetic waves remains basically unchanged before and after the treatment. Therefore, it can be seen that the magnetic polyimide composite foam material prepared by the present invention has good high and low temperature resistance.

[0137] The magnetic composite foam material prepared by the method of this invention has broadband microwave absorption properties and can be used as a good radar stealth material.

[0138] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A magnetic polyimide composite foam material with a hierarchical impedance structure, characterized in that, The magnetic polyimide composite foam material is composed of a carbon nanotube polyimide composite foam layer and a nickel-iron magnetic polyimide composite foam layer; the carbon nanotube polyimide composite foam layer and the nickel-iron magnetic polyimide composite foam layer are bonded together with a strong adhesive. The thickness of the carbon nanotube-polyimide composite foam layer is 1-5 mm, and the loading of carbon nanotubes is 20-40 wt%. The thickness of the nickel-iron magnetic polyimide composite foam layer is 2-6 mm, and the carbonyl iron loading is 40-80 wt%. The preparation method of the magnetic polyimide composite foam material includes the following steps: (1) Preparation of polyimide foam; Stabilizer, catalyst and water are added to the polycondensation product of diacid monomer and alcohol monomer to obtain a foaming precursor. Then it is mixed with a foaming agent and foamed. After thermal imidization, polyimide foam is obtained. (2) Preparation of carbon nanotube polyimide composite foam; The polyimide foam obtained in step (1) was immersed in carbon nanotube dispersion to obtain carbon nanotube polyimide composite foam, and then dried for later use. The polyimide foam was immersed in carbon nanotube dispersion by an auxiliary impregnation method. The auxiliary impregnation changed the pore structure of the foam from closed pores to open pores by continuously pressing the foam. (3) Preparation of nickel-iron magnetic polyimide composite foam; The polyimide foam obtained in step (1) is roughened in a roughening solution, then activated in an activation solution, and then plated in a plating solution to obtain a nickel-loaded polyimide foam. Finally, it is immersed in a flake carbonyl iron dispersion to obtain the nickel-iron magnetic polyimide composite foam. The solute in the roughening solution is one or more of dopamine, citric acid, polyphenol amine, and tannic acid. The solutes in the activation solution are tin dichloride and palladium chloride; the concentration of palladium chloride is 0.5-1 g / L. The plating solution includes metal salts, complexing agents, reducing agents, and pH adjusters; The metal salt is one or more of nickel sulfate, nickel chloride, and nickel nitrate; The complexing agent is one or more of the following: lactic acid, sodium citrate, thiourea, sodium acetate, malic acid, ammonium chloride, and tartaric acid. The reducing agent is one or more of dimethylamineborane, sodium hypophosphite, and sodium borohydride; The pH adjuster is one or more of ammonia, sodium hydroxide, potassium hydroxide, hydrochloric acid, and sulfuric acid; (4) The carbon nanotube polyimide composite foam obtained in step (2) and the nickel-iron magnetic polyimide composite foam obtained in step (3) are bonded together with strong adhesive to obtain the magnetic polyimide composite foam material with hierarchical impedance structure.

2. A method for preparing the magnetic polyimide composite foam material according to claim 1, characterized in that, The preparation method includes the following steps: (1) Preparation of polyimide foam; Stabilizer, catalyst and water are added to the polycondensation product of diacid monomer and alcohol monomer to obtain a foaming precursor. Then it is mixed with a foaming agent and foamed. After thermal imidization, polyimide foam is obtained. (2) Preparation of carbon nanotube polyimide composite foam; The polyimide foam obtained in step (1) was immersed in carbon nanotube dispersion to obtain carbon nanotube polyimide composite foam, and then dried for later use. The polyimide foam was immersed in carbon nanotube dispersion by an auxiliary impregnation method. The auxiliary impregnation changed the pore structure of the foam from closed pores to open pores by continuously pressing the foam. (3) Preparation of nickel-iron magnetic polyimide composite foam; The polyimide foam obtained in step (1) is roughened in a roughening solution, then activated in an activation solution, and then plated in a plating solution to obtain a nickel-loaded polyimide foam. Finally, it is immersed in a flake carbonyl iron dispersion to obtain the nickel-iron magnetic polyimide composite foam. The solute in the roughening solution is one or more of dopamine, citric acid, polyphenol amine, and tannic acid. The solutes in the activation solution are tin dichloride and palladium chloride; the concentration of palladium chloride is 0.5-1 g / L. The plating solution includes metal salts, complexing agents, reducing agents, and pH adjusters; The metal salt is one or more of nickel sulfate, nickel chloride, and nickel nitrate; The complexing agent is one or more of the following: lactic acid, sodium citrate, thiourea, sodium acetate, malic acid, ammonium chloride, and tartaric acid. The reducing agent is one or more of dimethylamineborane, sodium hypophosphite, and sodium borohydride; The pH adjuster is one or more of ammonia, sodium hydroxide, potassium hydroxide, hydrochloric acid, and sulfuric acid; (4) The carbon nanotube polyimide composite foam obtained in step (2) and the nickel-iron magnetic polyimide composite foam obtained in step (3) are bonded together with strong adhesive to obtain the magnetic polyimide composite foam material with hierarchical impedance structure.

3. The preparation method according to claim 2, characterized in that, In step (1), the dihydric anhydride monomer is one or more of the following: pyromellitic dianhydride, biphenyl dianhydride, 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride, hexafluoro dianhydride, bisphenol A type diether dianhydride, and 4,4'-oxophthalic anhydride. The alcohol monomer is one or more of methanol, ethanol, and polyethylene glycol; The stabilizer is one or more of DC-193, DC-5098, and AK-8818; The catalyst is one or more of triethanolamine, dibutyltin dilaurate, and Dabco33-LV; The foaming agent is one or more of diphenylmethane diisocyanate, polyphenylmethylene polyisocyanate, and triphenylmethane triisocyanate.

4. The preparation method according to claim 2, characterized in that, In step (1), the mass ratio of the diacid monomer to the alcohol monomer is 1-10:1; the amount of stabilizer is 5-30% of the total mass of the diacid monomer and the alcohol monomer; and the amount of catalyst is 0.5-10% of the total mass of the diacid monomer and the alcohol monomer.

5. The preparation method according to claim 2, characterized in that, In step (1), the conditions for the polycondensation reaction are: temperature of 50-100 ℃ and time of 2-5 h; foaming is carried out at room temperature; the imidization treatment method is: heat preservation at 130℃ for 2-6 h, heat preservation at 160℃ for 2-6 h, and heat preservation at 220℃ for 3-10 h.

6. The preparation method according to claim 2, characterized in that, In step (2), the carbon nanotube dispersion is prepared by dispersing carbon nanotubes and polyamic acid in water to obtain a carbon nanotube dispersion. The carbon nanotube content in the carbon nanotube dispersion is 2-5 wt%.

7. The preparation method according to claim 2, characterized in that, In step (3), the roughening treatment conditions are: roughening at room temperature for 12-48 hours; The activation conditions were: activation at room temperature for 0.5-12 hours; The plating conditions are: temperature 10-30℃, time 1-5min.

8. An application of the magnetic polyimide composite foam material with a hierarchical impedance structure as described in claim 1, characterized in that, Used for radar stealth.

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