CN113321844B - Graphene/polyimide composite foam wave-absorbing material with oriented pore structure and preparation method thereof - Google Patents

Graphene/polyimide composite foam wave-absorbing material with oriented pore structure and preparation method thereof Download PDF

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CN113321844B
CN113321844B CN202110615275.6A CN202110615275A CN113321844B CN 113321844 B CN113321844 B CN 113321844B CN 202110615275 A CN202110615275 A CN 202110615275A CN 113321844 B CN113321844 B CN 113321844B
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graphene
polyamic acid
composite foam
graphene oxide
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CN113321844A (en
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王子成
刘天西
张亚伟
李双双
唐新伟
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Jiangnan University
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    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • C08J9/28—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof by elimination of a liquid phase from a macromolecular composition or article, e.g. drying of coagulum
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    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2201/00—Foams characterised by the foaming process
    • C08J2201/04—Foams characterised by the foaming process characterised by the elimination of a liquid or solid component, e.g. precipitation, leaching out, evaporation
    • C08J2201/048—Elimination of a frozen liquid phase
    • C08J2201/0484—Elimination of a frozen liquid phase the liquid phase being aqueous
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2379/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing nitrogen with or without oxygen, or carbon only, not provided for in groups C08J2361/00 - C08J2377/00
    • C08J2379/04—Polycondensates having nitrogen-containing heterocyclic rings in the main chain; Polyhydrazides; Polyamide acids or similar polyimide precursors
    • C08J2379/08—Polyimides; Polyester-imides; Polyamide-imides; Polyamide acids or similar polyimide precursors
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/02—Elements
    • C08K3/04—Carbon
    • C08K3/042—Graphene or derivatives, e.g. graphene oxides

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Abstract

The invention discloses a graphene/polyimide composite foam wave-absorbing material with an oriented pore structure and a preparation method thereof, and belongs to the technical field of porous composite foam wave-absorbing materials and preparation thereof. Graphene oxide and polyamide acid are compounded, and the graphene/polyimide composite foam material with the oriented laminated pore structure is obtained through a bidirectional ice template method, a vacuum freeze drying process and a thermal imidization treatment process. The graphene/polyimide composite foam material with the oriented pore structure has excellent electromagnetic wave absorption performance and a wider absorption frequency range, is simple and easy to operate in the preparation process, is green and pollution-free, and is expected to be applied to the fields of aerospace, military equipment, civil electric appliances and the like.

Description

Graphene/polyimide composite foam wave-absorbing material with oriented pore structure and preparation method thereof
Technical Field
The invention relates to a graphene/polyimide composite foam wave-absorbing material with an oriented pore structure and a preparation method thereof, belonging to the technical field of porous composite foam wave-absorbing materials and preparation thereof.
Background
With the development of society and the progress of science and technology, various electronic and electrical equipment and communication equipment bring convenience to daily life of people, and simultaneously, a large amount of electromagnetic pollution is inevitably brought to the health and living environment of people. In recent years, research and development of high-performance electromagnetic wave absorbing materials with strong absorption capacity and wide absorption frequency band has become one of the hot spots in the field of material research. The traditional wave-absorbing material comprises ferrite, carbonyl iron and the like, and has the characteristics of large magnetic loss and wide absorption frequency band. But the defects of larger density, high processing cost, difficult processing, poor corrosion resistance and the like seriously hinder the popularization and the application of the alloy in the fields of aerospace, military equipment and civil electric appliances. Carbon foam has been widely noticed and studied as a porous material having a three-dimensional conductive network, which has advantages of low density, high workability, and good corrosion resistance. The special porous structure in the carbon foam material increases the specific surface area in the material, so that incident electromagnetic waves can perform energy dissipation in a multiple reflection/scattering mode among the porous structures in the material.
However, in conventional carbon foams, carbon/polymer syntactic foams have a large impedance mismatch at the surface of the syntactic foam due to the carbon-based conductive phase wrapping up on the surface of the polymer insulating phase during assembly, which eventually results in a large reflection of incident electromagnetic waves with only a small fraction of the waves able to enter the interior of the foam; the pore size structure of polymeric pyrolytic foam materials (e.g., wood, cotton, butts, bread, etc.) is strongly dependent on the source of the raw materials, is less programmable and controllable, and generally results in a random pore structure that is also isotropic, making precise design and controllable adjustment of the internal pore structure and pore size of the material on a microscopic scale difficult.
Disclosure of Invention
[ problem ] to provide a method for producing a semiconductor device
The carbon foam material has high conductivity due to the internal special structure, so that incident electromagnetic waves are reflected in a large quantity, impedance mismatching is generated between the incident electromagnetic waves and the wave-absorbing material, the incident electromagnetic waves are prevented from entering the material, and the electromagnetic waves cannot be effectively attenuated and dissipated. Most of conventionally obtained carbon foam materials are isotropic random porous structures, incident electromagnetic waves are insufficient in multiple reflection and scattering degrees in the materials, the energy attenuation degree is low, and the like, so that the optimization of structural design for enhancing the microwave absorption performance is urgently needed.
[ technical solution ] A
In order to solve the technical problems, the invention provides a polyimide/graphene composite foam wave-absorbing material with an oriented pore structure and a preparation method thereof, and aims to overcome the defects of small microwave absorption amount, low multiple reflection and scattering degree of incident electromagnetic waves in the material, low energy attenuation degree and the like caused by surface impedance mismatch of a graphene foam material.
The graphene/polyimide composite foam wave-absorbing material with the oriented porous structure is obtained by compounding graphene oxide and polyamide acid and by a bidirectional ice template method, a vacuum freeze-drying technology and a heat treatment process. The introduction of the polyimide can improve the mechanical strength of the composite foam wave-absorbing material on one hand; on the other hand, the problem of material surface impedance mismatch caused by high conductivity of the graphene material is solved, more electromagnetic waves are promoted to be incident into the material, and therefore the electromagnetic waves are attenuated and dissipated in the material. Under the influence of the oriented laminated pore structure in the material, on one hand, electromagnetic waves are subjected to multiple reflection and scattering to a greater extent between the laminated pore structures in the material after being incident on the material, and on the other hand, the propagation path of the electromagnetic waves after entering the material is increased, so that the maximum energy attenuation and dissipation of the incident electromagnetic waves between the laminated pore structures are realized.
Specifically, the invention firstly provides a preparation method of a graphene/polyimide composite foam wave-absorbing material with an oriented pore structure, and the method comprises the following steps: preparing a graphene oxide dispersion liquid with a certain concentration under an ultrasonic condition; preparing a polyamic acid/triethylamine aqueous solution in ultrapure water; mixing the graphene oxide dispersion liquid and the polyamic acid/triethylamine aqueous solution under magnetic stirring to obtain a graphene oxide/polyamic acid composite liquid, and obtaining graphene oxide/polyamic acid composite foam through a sol-gel method, a two-way ice template method and a freeze drying process; and then carrying out thermal imidization treatment on the graphene oxide/polyamide acid composite foam in an inert atmosphere to obtain the graphene/polyimide composite foam wave-absorbing material.
In one embodiment of the present invention, the mass ratio (concentration ratio) of the graphene oxide, the water-soluble polyamic acid, and the triethylamine is (0-20 mg/mL): (0-40 mg/mL): 0-20 mg/mL), wherein the three components are not 0 at the same time.
In one embodiment of the present invention, the concentration ratio of the water-soluble polyamic acid to triethylamine is preferably 2.
In one embodiment of the present invention, the raw flake graphite used for preparing graphene oxide has a size of 80 to 350 mesh, and is prepared by Hummers method.
In one embodiment of the present invention, the water-soluble polyamic acid is prepared by the following method: dissolving diamine monomer in polar solvent, adding dicarboxylic anhydride monomer, performing polymerization reaction in ice-water bath, adding organic amine, and reacting to obtain water-soluble polyamic acid sol; solvent exchange and freeze drying to obtain water soluble polyamic acid.
In one embodiment of the present invention, the diamine monomer includes any one of p-phenylenediamine or 4,4' -diaminodiphenyl ether; the binary anhydride monomer comprises any one of biphenyl tetracarboxylic dianhydride or pyromellitic dianhydride; the polar solvent includes any one of N-methylpyrrolidone, N '-dimethylacetamide, or N, N' -dimethylformamide; the organic amine includes any one of triethylamine or dipropylamine.
In one embodiment of the invention, the molar ratio of the diamine monomer, the dibasic anhydride monomer and the organic amine is (0-1): 0-1.
In one embodiment of the present invention, the sol-gel method, the two-way ice template method, and the vacuum freeze-drying comprise: pouring the composite liquid into a bidirectional freezing mould, wherein the sol-gel time is 12-15 h, and then carrying out vacuum freeze drying, wherein the freezing temperature is-50 to-120 ℃, and the freezing time is 6-8 h; the vacuum degree is 0.1 to 2Pa, and the drying time is 72 to 96 hours.
In one embodiment of the invention, the bottom two-way freezing mold is an angled wedge-shaped, open cube-structured mold.
In one embodiment of the present invention, the angle of the wedge shape made of the polydimethylsiloxane material used in the two-way ice templating method is in the range of 10 to 30 °.
In one embodiment of the present invention, the thermal imidization treatment process includes: heat treatment is carried out for 1 to 4 hours at the temperature of between 25 and 300 ℃ in an inert atmosphere; the inert atmosphere is any one of nitrogen, helium or argon.
In one embodiment of the present invention, the heat treatment procedure of the thermal imidization treatment process is preferably 80 to 120 ℃ for 1 to 3 hours, 180 to 220 ℃ for 1 to 3 hours, 280 to 320 ℃ for 1 to 3 hours, and most preferably 100 ℃ for 1 hour, 200 ℃ for 1 hour, and 300 ℃ for 1 hour.
The invention also provides the graphene/polyimide composite foam wave-absorbing material with the oriented pore structure prepared by the method.
Moreover, the invention also provides electronic equipment, military equipment and precise electronic instruments which contain the graphene/polyimide composite foam wave-absorbing material.
Finally, the invention provides the application of the preparation method or the graphene/polyimide composite foam wave-absorbing material in the fields of aerospace, military equipment, civil electric appliances and magnetic protection.
The invention has the beneficial effects that:
(1) The invention adopts a bidirectional ice template technology to successfully prepare the graphene/polyimide composite foam wave-absorbing material with the oriented pore structure, and can be used for the application of wide-frequency strong microwave absorption;
(2) According to the impedance matching principle in the electromagnetic wave absorption theory, the surface impedance matching of the composite foam wave-absorbing material is improved by introducing the high-impedance polyimide component, so that more electromagnetic waves are made to enter the material, and the attenuation and dissipation of the electromagnetic waves in the material are realized;
(3) Under the influence of the oriented laminated pore structure in the material, on one hand, multiple reflection and scattering of electromagnetic waves occur to a greater extent between the laminated pore structures in the material after the electromagnetic waves enter the material, and on the other hand, the propagation path of the electromagnetic waves after entering the material is increased, so that the maximum energy attenuation and dissipation of the incident electromagnetic waves between the laminated pore structures are realized.
(4) The introduction of the polyimide can improve the mechanical strength of the composite foam on one hand; on the other hand, the problem of material surface impedance mismatch caused by high conductivity of the graphene material is solved, more electromagnetic waves are promoted to be incident into the material, and therefore the electromagnetic waves are attenuated and dissipated in the material. Under the influence of the oriented layered pore structure in the material, on one hand, after the electromagnetic wave is incident to the material, multiple reflection and scattering occur to a greater extent between the layered pore structures in the material, and on the other hand, the propagation path of the electromagnetic wave after entering the material is increased, so that the maximum energy attenuation and dissipation of the incident electromagnetic wave between the layered pore structures are realized.
Drawings
FIG. 1 is a representation of the micro-morphology of the wave-absorbing material in examples 1-2: (a) A top view cross-section of a unidirectional graphene/polyimide syntactic foam; (b) A top view cross section of a bi-directional graphene/polyimide syntactic foam. .
Fig. 2 is a wave-absorbing effect diagram of the unidirectional graphene/polyimide composite foam prepared in example 2: (a) a three-dimensional reflection loss map; (b) effective absorption bandwidth map.
FIG. 3 is a wave-absorbing effect diagram parallel to the direction of the oriented lamellar pore structure in example 1: (a) a three-dimensional reflection loss map; (b) an effective absorption bandwidth map.
FIG. 4 is a wave-absorbing effect diagram perpendicular to the direction of the oriented lamellar pore structure in example 1: (a) a three-dimensional reflection loss map; (b) effective absorption bandwidth map.
Detailed Description
The invention will be further illustrated with reference to the following specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention. Further, it should be understood that various changes or modifications of the present invention can be made by those skilled in the art after reading the teaching of the present invention, and these equivalents also fall within the scope of the claims appended to the present application.
The method for measuring the electromagnetic wave absorbing performance of the graphene/polyimide composite foam comprises the following steps: and measuring the electromagnetic parameters of the wave-absorbing material by using an Agilent 8720ET vector network analyzer in a frequency range of 0.5-18 GHz through a coaxial line method.
The method for characterizing the internal micro-morphology of the graphene/polyimide composite foam comprises the following steps: the microscopic morphology of the wave-absorbing material is observed and researched by using a field emission scanning electron microscope (Hitachi S-4800) under the voltage of 5 kV.
Example 1
(1) Preparing graphene oxide: adding 2g of 325-mesh crystalline flake graphite, 240mL of concentrated sulfuric acid and 27mL of concentrated phosphoric acid into a 500mL three-neck flask, mechanically stirring for 30min, slowly adding 12g of potassium permanganate, controlling the temperature in a water bath at 50 ℃, continuously stirring at a constant speed for 12h, after the reaction is finished, slowly pouring the mixed solution into a beaker filled with 400mL of deionized water, and uniformly stirring with a glass rod. Aqueous hydrogen peroxide was added dropwise to the mixture, and stirred until the solution became golden yellow, and left to stand overnight. Centrifuging the mixed solution to remove residual acid, metal ions and the like in the mixed solution; and then, respectively washing with dilute HCl solution and deionized water for multiple times until the pH value is close to 5-6, and carrying out vacuum freeze drying on the obtained graphene oxide for later use.
(2) Preparation of Water-soluble Polyamic acid: n, N '-dimethylacetamide is used as a solvent, and a polyamic acid solution with the solid content of 15% is prepared by condensation polymerization reaction of 4,4' -diaminodiphenyl ether and pyromellitic dianhydride in an equal molar ratio in an ice-water bath. The specific process is as follows: 8.0096g of 4,4 '-diaminodiphenyl ether is dissolved in 95.57g of N, N' -dimethylacetamide, 8.7248g of pyromellitic dianhydride is added, and the mixture is reacted for 5 hours in an ice-water bath. Then, adding 4.0476g of triethylamine into the mixture, and continuing to react for 5 hours to obtain a water-soluble polyamic acid solution with the solid content of 15%; precipitating the prepared water-soluble polyamic acid by using deionized water, precipitating the prepared water-soluble polyamic acid in the deionized water by flowing the prepared water-soluble polyamic acid, and finally performing vacuum freeze drying (the temperature is-80 ℃, and the vacuum degree is below 10 Pa) to obtain the water-soluble polyamic acid for later use.
(3) Preparing a bidirectional freezing mold: a square copper sheet (with the side length of 2cm and the thickness of 1 mm) is used as a freezing surface, a silica gel square tube (with the inner side length of 2cm and the height of 2 cm) is adhered to the copper sheet, the copper sheet is placed on an inclined platform with the gradient of 15 degrees, then dimethyl siloxane (PDMS) precursor solution is poured until the surface of the whole copper sheet is just covered, and then the two-way freezing mold is obtained by curing at the temperature of 80 ℃ for 2 hours.
(4) Preparing graphene/polyimide composite foam: uniformly compounding the graphene oxide obtained in the step (1), the polyamic acid obtained in the step (2) and triethylamine according to a concentration ratio of 2. The specific operation is as follows: preparing graphene oxide dispersion liquid with the concentration of 10mg/mL under an ultrasonic condition; preparing a polyamic acid/triethylamine aqueous solution with the polyamic acid concentration of 10mg/mL and the concentration ratio of the polyamic acid to the triethylamine of 2; and mixing the graphene oxide dispersion liquid and the polyamic acid/triethylamine aqueous solution under magnetic stirring to obtain the graphene oxide/polyamic acid composite liquid. Pouring the mixture into a bidirectional freezing mold, freezing for 6 hours at the temperature of minus 70 ℃, and then carrying out vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 hours) to obtain the graphene oxide/polyamide acid composite foam. And then, carrying out heat treatment on the graphene oxide/polyamic acid composite foam in a nitrogen atmosphere, wherein the heat treatment procedures are as follows in sequence: the graphene/polyimide composite foam material with the oriented pore structure is finally obtained by 1h at 100 ℃, 1h at 200 ℃ and 1h at 300 ℃, and is named as rGO/PI.
Example 2
Preparing a one-way freezing mold: a square copper sheet (with the side length of 2cm and the thickness of 1 mm) is used as a freezing surface, and a silica gel square tube (with the inner side length of 2cm and the height of 2 cm) is stuck on the copper sheet, so that the unidirectional freezing mold can be obtained.
The present embodiment is different from embodiment 1 in that: and (3) pouring the graphene oxide and polyamic acid composite solution obtained in the step (4) in the embodiment 1 into a one-way freezing mold, and performing the same low-temperature freezing process (-70 ℃,6 h) and vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 h) to obtain the graphene oxide/polyamic acid composite foam. Then carrying out heat treatment on the composite foam material in a nitrogen atmosphere (1 h at 200 ℃), and finally obtaining the unidirectional graphene/polyimide composite foam material.
Micro-morphology characterization and microwave absorption performance test
The composite foams prepared in examples 1 to 2 were subjected to microscopic morphology characterization and microwave absorption property test, and the results are shown in FIGS. 1 to 4.
FIG. 1 is a representation of the microscopic topography of the syntactic foams of examples 1-2: (a) A top view cross-section of a unidirectional graphene/polyimide syntactic foam; (b) A top view cross section of a bi-directional graphene/polyimide syntactic foam. As shown in fig. 1 (a), the microstructure of the cross section of the unidirectional graphene/polyimide composite foam in a plan view is an isotropic porous structure, and has a honeycomb-like shape and a pore size of about 50 μm. However, the internal microstructure of the graphene/polyimide composite foam prepared by the bidirectional ice template method is an oriented layered structure, and shows obvious anisotropy, and the distance between lamellar pores is about 50 μm, so that the result proves the effectiveness of the bidirectional freezing method for successfully constructing the oriented layered pore structure.
Fig. 2 is a wave-absorbing effect diagram of the unidirectional graphene/polyimide composite foam prepared in example 2: (a) a three-dimensional reflection loss map; (b) effective absorption bandwidth map. Freezing with a one-way freezing mold at-70 deg.C for 6h; the addition concentration of the graphene oxide and the water-soluble polyamic acid is 10mg/mL, and the unidirectional graphene/polyimide composite foam material prepared by the method is 10mg/mL, and has weaker microwave absorption strength. As shown in FIG. 2 (a), when the sample thickness is 5.00mm and the frequency is 10.48GHz, the minimum value RL of the microwave reflection loss of the sample is min Only-9.37 dB.
FIG. 3 is a diagram of the wave-absorbing effect in the direction parallel to the oriented lamellar pore structure in example 1: (a) a three-dimensional reflection loss map; (b) an effective absorption bandwidth map. Freezing with a bidirectional freezing mold at-70 deg.C for 6h; the addition concentration of the graphene oxide and the water-soluble polyamic acid is 10mg/mL, and the bidirectional graphene/polyimide composite foam material prepared by 10mg/mL has weaker microwave absorption intensity in a direction parallel to the orientation pore structure. As shown in FIG. 3 (a), when the sample thickness is 1.8mm and the frequency is 16.25GHz, the minimum value RL of the microwave reflection loss of the sample is min At-8.48 dB, which also confirms that the attenuation of the incident electromagnetic wave parallel to the direction of the laminar pore structure is ineffective.
FIG. 4 is a wave-absorbing effect diagram perpendicular to the direction of the oriented lamellar pore structure in example 1: (a) a three-dimensional reflection loss map; (b) an effective absorption bandwidth map. Freezing with a one-way freezing mold at-70 deg.C for 6h; the unidirectional graphene/polyimide composite foam material prepared from the graphene oxide and the water-soluble polyamic acid with the addition concentration of 10mg/mL to 10mg/mL has the maximum microwave absorption intensity in the direction perpendicular to the oriented laminar pore structure andbandwidth is absorbed effectively. As shown in FIG. 4 (a), when the sample thickness is 4.75mm and the frequency is 9.25GHz, the minimum value RL of the microwave reflection loss of the sample is min Reaches-61.29 dB; at this time, the effective absorption bandwidth reaches 5.51GHz (7.06-12.57 GHz). In addition, as can be seen from fig. 4 (b), the maximum effective absorption bandwidth of the sample can reach 5.86GHz (9.25-15.11 GHz) at a sample thickness of 3.84mm, covering X and Ku bands. As described above, the great improvement of the microwave absorption performance of the bidirectional graphene/polyimide composite foam can be attributed to the reasonable construction of the oriented layered pore structure in the material, which can effectively promote the electromagnetic wave to generate a greater degree of multiple reflection and scattering between the layered pore structures after being incident into the material perpendicular to the oriented layered pore structure, and meanwhile, the successful construction of the oriented layered pore structure can also increase the propagation path of the electromagnetic wave after entering the material, thereby realizing the maximum layer-by-layer energy attenuation and dissipation of the incident electromagnetic wave between the layered pore structures.
Example 3
The present embodiment is different from embodiment 1 in that: uniformly compounding the graphene oxide obtained in the step (1), the polyamic acid obtained in the step (2) and triethylamine according to a concentration ratio of 1. The specific operation is as follows: preparing a graphene oxide dispersion liquid with the concentration of 10mg/mL under an ultrasonic condition; preparing a polyamic acid/triethylamine aqueous solution with the concentration of polyamic acid being 20mg/ml and the concentration ratio of polyamic acid to triethylamine being 2; and mixing the graphene oxide dispersion liquid and the polyamic acid/triethylamine aqueous solution under magnetic stirring to obtain the graphene oxide/polyamic acid composite liquid. Pouring the composite foam into a bidirectional freezing mold, and performing low-temperature freezing process (-70 ℃,6 h) and vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 h) to obtain the graphene oxide/polyamic acid composite foam. And then carrying out heat treatment on the graphene oxide/polyamic acid composite foam in a nitrogen atmosphere, wherein the heat treatment procedures are as follows in sequence: and (3) 1h at 100 ℃, 1h at 200 ℃ and 1h at 300 ℃ to finally obtain the graphene/polyimide composite foam material. Minimum value RL of reflection loss of the composite foam when the thickness is 4.99mm and the frequency is 8.725GHz min Is-35.91 dB, corresponding effective absorption bandwidth (RL)<-10 dB) of 5.25GHz (6.80-12.05GHz)。
Example 4
The present embodiment is different from embodiment 1 in that: graphene oxide, polyamic acid and triethylamine were uniformly compounded at a concentration ratio of 2. The specific operation is as follows: preparing a graphene oxide dispersion liquid with the concentration of 10mg/mL under an ultrasonic condition; preparing a polyamic acid/triethylamine aqueous solution with the polyamic acid concentration of 5mg/ml and the concentration ratio of the polyamic acid to the triethylamine of 2; and mixing the graphene oxide dispersion liquid and the polyamic acid/triethylamine aqueous solution under magnetic stirring to obtain the graphene oxide/polyamic acid composite liquid. Pouring the composite foam into a bidirectional freezing mold, and performing the same low-temperature freezing process (-70 ℃,6 h) and vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 h) to obtain the graphene oxide/polyamic acid composite foam. Then carrying out heat treatment on the composite material in a nitrogen atmosphere (1 h at 100 ℃ 1h and 200 ℃ 1h at 300 ℃), and finally obtaining the graphene/polyimide composite foam material. Minimum value RL of reflection loss of the composite foam when the thickness is 5mm and the frequency is 11.0GHz min Is-10.38 dB, corresponding effective absorption bandwidth (RL)<-10 dB) is 1.40GHz (10.21-11.61 GHz).
Example 5
The present embodiment is different from embodiment 1 in that: and (3) pouring the graphene oxide/polyamic acid composite solution obtained in the step (4) into a bidirectional freezing mold, and performing low-temperature freezing process (-50 ℃,6 h) and vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 h) to obtain the graphene oxide/polyamic acid composite foam. And then carrying out heat treatment on the graphene oxide/polyamic acid composite foam in a nitrogen atmosphere, wherein the heat treatment procedures are as follows in sequence: and (3) 1h at 100 ℃, 1h at 200 ℃ and 1h at 300 ℃ to finally obtain the graphene/polyimide composite foam material. Minimum value RL of reflection loss of the composite foam when the thickness is 5mm and the frequency is 9.43GHz min Is-18.45 dB, corresponding effective absorption bandwidth (RL)<-10 dB) of 4.92GHz (7.49-12.41 GHz).
Example 6
The present embodiment is different from embodiment 1 in that: pouring the graphene oxide/polyamide acid composite liquid into a bidirectional freezing mouldAnd carrying out low-temperature freezing process (-90 ℃,6 h) and vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 h) to obtain the graphene oxide/polyamic acid composite foam. And then carrying out heat treatment on the graphene oxide/polyamic acid composite foam in a nitrogen atmosphere, wherein the heat treatment procedures are as follows in sequence: and (3) 1h at 100 ℃, 1h at 200 ℃ and 1h at 300 ℃ to finally obtain the graphene/polyimide composite foam material. Minimum value RL of reflection loss of the composite foam when the thickness is 5mm and the frequency is 8.81GHz min Is-26.16 dB, corresponding effective absorption bandwidth (RL)<-10 dB) is 4.98GHz (6.80-11.78 GHz).
Example 7
The present embodiment is different from embodiment 1 in that: preparing a graphene oxide dispersion liquid with the concentration of 5mg/mL under an ultrasonic condition; preparing a polyamic acid/triethylamine aqueous solution with the polyamic acid concentration of 5mg/ml and the concentration ratio of the polyamic acid to the triethylamine of 2; and mixing the graphene oxide dispersion liquid and the polyamic acid/triethylamine aqueous solution under magnetic stirring to obtain the graphene oxide/polyamic acid composite liquid. Pouring the composite foam into a bidirectional freezing mold, and performing the same low-temperature freezing process (-70 ℃,6 h) and vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 h) to obtain the graphene oxide/polyamic acid composite foam. Then, carrying out heat treatment on the composite material in a nitrogen atmosphere (1 h at 200 ℃), and finally obtaining the graphene/polyimide composite foam material. The minimum value RL of the reflection loss of the composite foam is when the thickness is 3.34mm and the frequency is 17.74GHz min Is-11.99 dB, corresponding effective absorption bandwidth (RL)<-10 dB) is 0.61GHz (17.39-18.00 GHz).
Example 8
The present embodiment is different from embodiment 1 in that: preparing a graphene oxide dispersion liquid with the concentration of 20mg/mL under an ultrasonic condition; preparing a polyamic acid/triethylamine aqueous solution with the concentration of polyamic acid being 20mg/ml and the concentration ratio of polyamic acid to triethylamine being 2; and mixing the graphene oxide dispersion liquid and the polyamic acid/triethylamine aqueous solution under magnetic stirring to obtain the graphene oxide/polyamic acid composite liquid. Pouring into a bidirectional freezing mold, and freezing at the same low temperatureObtaining the graphene oxide/polyamic acid composite foam after the process of (-70 ℃,6 h) and vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 h). Then carrying out heat treatment on the composite material in a nitrogen atmosphere (1 h at 100 ℃ 1h and 200 ℃ 1h at 300 ℃), and finally obtaining the graphene/polyimide composite foam material. The minimum value RL of the reflection loss of the composite foam when the thickness is 2.23mm and the frequency is 18.0GHz min Is-28.40 dB, corresponding effective absorption bandwidth (RL)<-10 dB) is 3.41GHz (14.59-18.00 GHz).
Comparative example 1
This comparative example differs from example 1 in that: graphene oxide obtained in the step (1) in example 1 is placed in ultrapure water to prepare a graphene oxide dispersion liquid with a concentration of 10mg/mL under an ultrasonic condition, the graphene oxide dispersion liquid is poured into a bidirectional freezing mold, and graphene oxide foam is obtained after the same low-temperature freezing process (-70 ℃,6 h) and vacuum freeze drying (the vacuum degree is 0.1-2 Pa, and the drying time is 72 h). Then carrying out heat treatment on the foam material in a nitrogen atmosphere (1 h at 100 ℃ 1h and 200 ℃ 1h at 300 ℃), and finally obtaining the reduced graphene oxide foam material. The minimum value RL of the reflection loss of the foam when the thickness is 5mm and the frequency is 10.74GHz min Is-5.40 dB, corresponding effective absorption bandwidth (RL)<-10 dB) is 0.
Comparative example 2
This comparative example differs from example 1 in that: the water-soluble polyamic acid obtained in step (2) in example 1 and triethylamine were mixed at a concentration ratio of 2. The specific operation is as follows: preparing a polyamic acid/triethylamine aqueous solution with the polyamic acid concentration of 10mg/mL and the concentration ratio of the polyamic acid to the triethylamine of 2. Then, the mixture is subjected to heat treatment under the nitrogen atmosphere, and the heat treatment procedures are as follows: 1h at 100 ℃, 1h at 200 ℃ and 1h at 300 ℃ to finally obtain the polyimide foam material. Minimum value of reflection loss RL of the foam when the thickness is 3.8mm and the frequency is 16.51GHz min Is-1.96 dB, corresponding effective absorption bandwidth (RL)<-10 dB) is 0.
Although the present invention has been described with reference to the preferred embodiments, it should be understood that various changes and modifications can be made therein by those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims.

Claims (7)

1. A preparation method of a graphene/polyimide composite foam wave-absorbing material with an oriented pore structure is characterized by comprising the following steps: preparing a graphene oxide dispersion liquid with a certain concentration under an ultrasonic condition; preparing a polyamic acid/triethylamine aqueous solution in ultrapure water; mixing the graphene oxide dispersion liquid and the polyamic acid/triethylamine aqueous solution under magnetic stirring to obtain a graphene oxide/polyamic acid composite liquid, and obtaining graphene oxide/polyamic acid composite foam through a sol-gel method, a two-way ice template method and a freeze drying process; then carrying out thermal imidization treatment on the graphene oxide/polyamide acid composite foam in an inert atmosphere to obtain a graphene/polyimide composite foam wave-absorbing material; the mixed concentration ratio of the graphene oxide dispersion liquid, the aqueous polyamic acid and the triethylamine is 2; the concentration of the graphene oxide dispersion liquid is 10mg/mL; the concentration of the aqueous polyamic acid is 10mg/mL; the sol-gel method, the two-way ice template method and the vacuum freeze drying comprise the following steps: pouring the composite liquid into a bidirectional freezing mould, wherein the sol-gel time is 12-15 h, and then carrying out vacuum freeze drying, wherein the freezing temperature is-70 ℃ and the freezing time is 6h; the vacuum degree is 0.1-2 Pa, and the drying time is 72h; the wedge angle range in the bidirectional ice template method is 10-30 degrees.
2. The method according to claim 1, wherein the thermal imidization treatment process includes: heat treatment is carried out for 1 to 4 hours at the temperature of between 25 and 300 ℃ in an inert atmosphere; the inert atmosphere is any one of nitrogen, helium or argon.
3. The method according to claim 2, wherein the thermal imidization treatment is performed at 80-120 ℃ for 1-3 h, at 180-220 ℃ for 1-3 h, and at 280-320 ℃ for 1-3 h.
4. The production method according to any one of claims 1 to 3, wherein the water-soluble polyamic acid is produced by: firstly, dissolving a diamine monomer in a polar solvent, then adding a dicarboxylic anhydride monomer, carrying out polymerization reaction in an ice-water bath, then adding organic amine, and continuing the reaction to obtain a water-soluble polyamic acid sol; solvent exchange and freeze drying to obtain water soluble polyamic acid.
5. The graphene/polyimide composite foam wave-absorbing material with the oriented pore structure, which is prepared by the preparation method according to any one of claims 1 to 4.
6. Electronic equipment, military equipment and precise electronic instruments containing the graphene/polyimide composite foam wave-absorbing material with the oriented pore structure as claimed in claim 5.
7. The graphene/polyimide composite foam wave-absorbing material with the oriented pore structure in claim 5 is applied to the fields of aerospace, military equipment, civil electric appliances and magnetic protection.
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