Electromagnetic shielding foam and method of making same

Abstract

The application relates to the technical field of electromagnetic shielding materials, in particular to an electromagnetic shielding foam and a preparation method thereof. Raw materials comprise silicon dioxide, carbon nanotubes, silver nanowires, polyimide and cellulose nanofibers; the electromagnetic shielding foam is divided into an upper layer, a middle layer and a lower layer, the upper layer of silicon dioxide / polyimide is an electromagnetic wave incidence layer, the middle layer of carbon nanotube / polyimide is an electromagnetic wave dissipation layer, and the bottom layer of silver nanowire / cellulose nanofiber film serves as an electromagnetic wave reflection layer and is prepared by adopting a split type conductive modularization. According to the use requirement, the electromagnetic module is reasonably assembled, and the high unification of electromagnetic wave absorption and shielding is realized. The electromagnetic shielding foam with a gradual long channel structure has good impedance matching characteristics and provides excellent electromagnetic wave absorption capacity.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic shielding materials technology, and more specifically, to an electromagnetic shielding foam and its preparation method. Background Technology

[0002] Electromagnetic waves, as carriers of modern communication information, generate unwanted electromagnetic radiation pollution while transmitting signals. Developing electromagnetic functional materials that can effectively block electromagnetic radiation is of great practical significance for solving this scientific and engineering problem of radiation pollution. Polymer materials, with their advantages of corrosion resistance, low density, and ease of processing, are widely used in the field of electromagnetic interference (EMI) shielding. Previous work has focused on constructing conductive networks within polymer matrices, and then using the principle of impedance mismatch to improve the electromagnetic shielding performance of materials. However, there are limitations to improving the electromagnetic shielding performance of materials by constructing a perfect and uniform conductive network within the polymer matrix. Most current shielding materials are designed based on single-band electromagnetic wave principles, which cannot meet the dual requirements of radar stealth and infrared stealth for sensitive electronic equipment. Materials with porous structures such as aerogels, foams, and sponges can achieve infrared camouflage of target objects by suppressing heat conduction and convection. However, most porous materials prepared based on thermal insulation technology cannot adapt to dynamic environmental changes and cannot autonomously switch between infrared camouflage and infrared response. Summary of the Invention

[0003] The present invention aims to at least solve one of the technical problems existing in the prior art. To this end, one aspect of the present invention is to provide an electromagnetic shielding foam, characterized in that: the raw materials of the electromagnetic shielding foam include: silica, carbon nanotubes, silver nanowires, polyimide and cellulose nanofibers; the electromagnetic shielding foam is divided into three layers: the upper silica / polyimide layer is the electromagnetic wave incident layer, the middle carbon nanotube / polyimide layer is the electromagnetic wave dissipation layer, and the bottom silver nanowire / cellulose nanofiber film serves as the electromagnetic wave reflecting layer, and it is prepared by a split conductive modular design.

[0004] Preferably, the mass ratio of silica, carbon nanotubes, silver nanowires, polyimide and cellulose nanofibers is 20:13:1.25 to 7.5:20:5.

[0005] Another objective of this invention is to provide a method for preparing electromagnetic shielding foam, the specific steps of which are as follows:

[0006] S1. Synthesis of polyamic acid: 4,4'-diaminodiphenyl ether was completely dissolved in a solvent, and aromatic dianhydride was added in four separate portions, stirring continuously until completely dissolved. Then, triethylamine was slowly added to the mixture and stirred until homogeneous to obtain a polyamic acid solution. For easy storage, the obtained polyamic acid solution was poured into deionized water for solvent exchange, and then freeze-dried to obtain water-soluble polyamic acid filaments. The mass ratio of deionized water to polyamic acid solution was 2000~5000:118, the freeze-drying time was 72 hours, and the temperature was -65℃.

[0007] S2. Preparation of silica / carbon nanotube / polyimide foam: Carbon nanotubes and silica are added to a polyamic acid solution to prepare a mixture of carbon nanotubes and polyamic acid and a mixture of silica and polyamic acid. The above mixtures are sequentially added to a polytetrafluoroethylene mold for unidirectional freeze-drying to obtain a silica / carbon nanotube / polyamic acid foam precursor. The silica / carbon nanotube / polyamic acid foam precursor is then thermally imidized at a gradient temperature to obtain silica / carbon nanotube / polyimide foam.

[0008] S3. Preparation of silver nanowire / cellulose nanofiber film: Silver nanowires and cellulose dispersion were mixed and magnetically stirred to prepare a uniform dispersion. The mixture was then poured into a filter flask, vacuum filtered, and then dried in an oven to obtain silver nanowire / cellulose nanofiber film.

[0009] S4. Preparation of electromagnetic shielding foam: Based on the principle of modular design of split conductive material, thermally imidized silica / carbon nanotube / polyimide foam is bonded to silver nanowire / cellulose nanofiber film with polyamic acid solution to obtain silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam.

[0010] Preferably, the mass ratio of 4,4'-diaminodiphenyl ether to solvent in S1 is 8.62~17.24:100; the solvent is N,N-dimethylacetamide; and the interval time is 30 min.

[0011] Preferably, in step S1, the mass ratio of aromatic dianhydride to 4,4'-diaminodiphenyl ether added four times is 0.35:1, 0.29:1, 0.23:1, and 0.22:1, respectively; the aromatic dianhydride is one or more combinations of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, hexafluorodianhydride, diphenyl ether dianhydride, and benzophenone dianhydride; the mass ratio of triethylamine to 4,4'-diaminodiphenyl ether is 0.51:1; and the stirring time is 3 hours.

[0012] Preferably, the carbon nanotubes in S2 are acidified carbon nanotubes, prepared by adding carbon nanotubes to a mixed acid solution of sulfuric acid and nitric acid at a mass ratio of 3:1 and oxidizing at 60°C for 2 hours; the mass ratio of carbon nanotubes to polyamic acid solution is 1.3:2, and the mass ratio of silica to polyamic acid solution is 1:1, resulting in a 40wt% carbon nanotube and polyamic acid mixture and a 50wt% silica and polyamic acid mixture. The thickness of the carbon nanotube / polyimide foam can be adjusted by changing the volume of the carbon nanotube / polyamic acid dispersion.

[0013] Preferably, in step S2, the unidirectional freeze-drying time is 72 hours, the temperature is -65°C, the gradient temperature is 100°C, 200°C and 250°C, the thermal imidization time is 1 hour, the thickness of silica / polyimide in the silica / carbon nanotube / polyimide foam is 1 mm, and the thickness of carbon nanotube / polyimide foam is 1 mm to 9 mm.

[0014] Preferably, the silver nanowires in S3 are prepared by dissolving silver nitrate in ethylene glycol at a mass ratio of 1:53 and stirring magnetically to form solution I, dissolving ferric chloride hexahydrate and polyvinylpyrrolidone in ethylene glycol at a mass ratio of 1:1:52 and stirring magnetically to form solution II, slowly adding solution I to solution II, and then transferring the mixed solution to an autoclave and heating at 160°C for 150 min.

[0015] Preferably, in S3, the mass ratio of silver nanowires to cellulose dispersion is 1-5:5-9, the mass fraction of cellulose dispersion is 2%, the magnetic stirring time is 30 minutes, the vacuum degree of vacuum filtration is -0.1 MPa, the oven drying time is 12-24 hours, the temperature is 60°C, and the mass fraction of silver nanowires in the silver nanowire / cellulose nanofiber film is 20wt%-60wt%.

[0016] Preferably, in the S4 mixture of silver nanowires and cellulose nanofibers, the mass ratio of silver nanowires to cellulose nanofibers is 1.25 to 7.5:5; the thickness of the silver nanowire / cellulose nanofiber film is 120 μm; and the silver nanowire / cellulose nanofiber film is bonded to one side of the carbon nanotube / polyimide foam using a 4 wt% polyamic acid solution.

[0017] The beneficial effects of this invention are as follows:

[0018] This invention, based on a modular, split-type conductive design principle, prepares a composite foam of silica / carbon nanotubes / polyimide-silver nanowires / cellulose nanofibers. According to usage requirements, the electromagnetic modules are rationally assembled, thereby achieving a high degree of unity between electromagnetic wave absorption and shielding. The electromagnetic shielding foam with a progressive long-channel structure exhibits excellent impedance matching characteristics and provides superior electromagnetic wave absorption capabilities.

[0019] The silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention not only possesses electromagnetic shielding properties primarily based on absorption, but also exhibits an autonomous switching function between infrared camouflage and infrared response. This is due to the low infrared emissivity and Joule heating effect of the silver nanowire / cellulose nanofiber film, as well as the thermal insulation properties of the silica / carbon nanotube / polyimide foam. This invention will stimulate the adaptive application of silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam in radar stealth and infrared stealth engineering environments.

[0020] Additional aspects and advantages of the invention will become apparent from the description which follows, or may be learned by practice of the invention. Attached Figure Description

[0021] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0022] Figure 1 This is a schematic diagram of the process for preparing the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam according to the present invention.

[0023] Figure 2 This is a scanning electron microscope image at 5µm of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention.

[0024] Figure 3 This is a scanning electron microscope image of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention at 10µm.

[0025] Figure 4 This is a shielding effectiveness diagram of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention;

[0026] Figure 5 This is a diagram showing the reflection / absorption power coefficient of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention;

[0027] Figure 6This is a schematic diagram of the two-dimensional radar cross-section of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention at different angles.

[0028] Figure 7 Infrared camouflage image of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention;

[0029] Figure 8 This is the infrared thermal response diagram of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention.

[0030] Figure 9 This is a diagram illustrating the electromagnetic wave dissipation mechanism of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention. Detailed Implementation

[0031] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0032] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0033] The preparation process of Examples 1-5 is as follows: Figure 1 As shown.

[0034] Example 1

[0035] S1. Synthesis of polyamic acid: 8.62 g of 4,4'-diaminodiphenyl ether was completely dissolved in 100 g of N,N-dimethylacetamide, and 3 g, 2.5 g, 2 g, and 1.9 g of 4,4'-oxophthalic anhydride were added in four portions at 30 min intervals, respectively, and the mixture was stirred continuously until completely dissolved. Then, 4.4 g of triethylamine was slowly added to the mixed solution and stirred for 3 hours to obtain a polyamic acid solution.

[0036] S2. Preparation of silica / carbon nanotube / polyimide foam: Carbon nanotubes (acidified carbon nanotubes, obtained by adding carbon nanotubes to a mixed acid solution of sulfuric acid and nitric acid at a mass ratio of 3:1 and oxidizing at 60°C for 2 hours) and silica were added separately to a polyamic acid solution. The mass ratio of carbon nanotubes to polyamic acid solution was 1.3:2, and the mass ratio of silica to polyamic acid solution was 1:1, thus obtaining a 40wt% silica / polyamic acid foam. A mixture of silica and polyamic acid at a concentration of 50 wt% was prepared and then sequentially added to a polytetrafluoroethylene mold for unidirectional freeze-drying at -65°C for 72 hours, yielding a silica / carbon nanotube / polyamic acid foam precursor. This precursor was then thermally imidized at gradient temperatures of 100°C, 200°C, and 250°C for 1 hour to obtain a silica / carbon nanotube / polyimide foam. The silica / polyimide layer in the silica / carbon nanotube / polyimide foam has a thickness of 1 mm, while the carbon nanotube / polyimide foam has a thickness of 8 mm. The thickness of the carbon nanotube / polyimide foam can be adjusted by changing the volume of the carbon nanotube / polyamic acid dispersion.

[0037] S3. Preparation of silver nanowire / cellulose nanofiber film: 0.68 g of silver nitrate was dissolved in 35.94 g of ethylene glycol and magnetically stirred to form solution I. 0.69 mg of ferric chloride hexahydrate and 0.71 g of polyvinylpyrrolidone were dissolved in another 35.94 g of ethylene glycol and magnetically stirred to form solution II. Solution I was slowly added to solution II, and then the mixed solution was transferred to an autoclave and heated at 160 °C for 150 min to obtain silver nanowires. 0.0125 g of silver nanowires and 2.5 g of cellulose dispersion (2% by mass) were mixed and magnetically stirred for 30 min to prepare a homogeneous dispersion. The mixture was then poured into a filter flask, vacuum filtered at a vacuum degree of -0.1 MPa, and then dried in an oven at 60 °C for 14 hours to obtain a silver nanowire / cellulose nanofiber film. The silver nanowire content in the prepared silver nanowire / cellulose nanofiber film was 20 wt%.

[0038] S4. Preparation of electromagnetic shielding foam: Based on the principle of modular conductive design, thermally imidized silica / carbon nanotube / polyimide foam and a silver nanowire / cellulose nanofiber film with a silver nanowire content of 20wt% are bonded together with a 4wt% polyamic acid solution. The thickness of the silver nanowire / cellulose nanofiber film is 120μm. The silver nanowire / cellulose nanofiber film is bonded to one side of the carbon nanotube / polyimide foam to obtain silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam.

[0039] Example 2

[0040] S1. Synthesis of polyamic acid: 8.62 g of 4,4'-diaminodiphenyl ether was completely dissolved in 100 g of N,N-dimethylacetamide, and 3 g, 2.5 g, 2 g, and 1.9 g of 4,4'-oxophthalic anhydride were added in four portions at 30 min intervals, respectively, and the mixture was stirred continuously until completely dissolved. Then, 4.4 g of triethylamine was slowly added to the mixed solution and stirred for 3 hours to obtain a polyamic acid solution.

[0041] S2. Preparation of silica / carbon nanotube / polyimide foam: Carbon nanotubes (acidified carbon nanotubes, obtained by adding carbon nanotubes to a mixed acid solution of sulfuric acid and nitric acid at a mass ratio of 3:1 and oxidizing at 60°C for 2 hours) and silica were added separately to a polyamic acid solution. The mass ratio of carbon nanotubes to polyamic acid solution was 1.3:2, and the mass ratio of silica to polyamic acid solution was 1:1, thus obtaining a 40wt% silica / polyamic acid foam. A mixture of silica and polyamic acid at a concentration of 50 wt% was prepared and then sequentially added to a polytetrafluoroethylene mold for unidirectional freeze-drying at -65°C for 72 hours, yielding a silica / carbon nanotube / polyamic acid foam precursor. This precursor was then thermally imidized at gradient temperatures of 100°C, 200°C, and 250°C for 1 hour to obtain a silica / carbon nanotube / polyimide foam. The silica / polyimide layer in the silica / carbon nanotube / polyimide foam has a thickness of 1 mm, while the carbon nanotube / polyimide foam has a thickness of 8 mm. The thickness of the carbon nanotube / polyimide foam can be adjusted by changing the volume of the carbon nanotube / polyamic acid dispersion.

[0042] S3. Preparation of silver nanowire / cellulose nanofiber film: 0.68 g of silver nitrate was dissolved in 35.94 g of ethylene glycol and magnetically stirred to form solution I. 0.69 mg of ferric chloride hexahydrate and 0.71 g of polyvinylpyrrolidone were dissolved in another 35.94 g of ethylene glycol and magnetically stirred to form solution II. Solution I was slowly added to solution II, and then the mixed solution was transferred to an autoclave and heated at 160 °C for 150 min to obtain silver nanowires. 0.0214 g of silver nanowires and 2.5 g of cellulose dispersion (2% by mass) were mixed and magnetically stirred for 30 min to prepare a homogeneous dispersion. The mixture was then poured into a filter flask, vacuum filtered at -0.1 MPa, and then dried in an oven at 60 °C for 14 hours to obtain a silver nanowire / cellulose nanofiber film. The silver nanowire content in the prepared silver nanowire / cellulose nanofiber film was 30 wt%.

[0043] S4. Preparation of electromagnetic shielding foam: Based on the principle of modular conductive design, thermally imidized silica / carbon nanotube / polyimide foam and a silver nanowire / cellulose nanofiber film with a silver nanowire content of 30wt% are bonded together with a 4wt% polyamic acid solution. The thickness of the silver nanowire / cellulose nanofiber film is 120μm. The silver nanowire / cellulose nanofiber film is bonded to one side of the carbon nanotube / polyimide foam to obtain silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam.

[0044] Example 3

[0045] S1. Synthesis of polyamic acid: 8.62 g of 4,4'-diaminodiphenyl ether was completely dissolved in 100 g of N,N-dimethylacetamide, and 3 g, 2.5 g, 2 g, and 1.9 g of 4,4'-oxophthalic anhydride were added in four portions at 30 min intervals, respectively, and the mixture was stirred continuously until completely dissolved. Then, 4.4 g of triethylamine was slowly added to the mixed solution and stirred for 3 hours to obtain a polyamic acid solution.

[0046] S2. Preparation of silica / carbon nanotube / polyimide foam: Carbon nanotubes (acidified carbon nanotubes, obtained by adding carbon nanotubes to a mixed acid solution of sulfuric acid and nitric acid at a mass ratio of 3:1 and oxidizing at 60°C for 2 hours) and silica were added separately to a polyamic acid solution. The mass ratio of carbon nanotubes to polyamic acid solution was 1.3:2, and the mass ratio of silica to polyamic acid solution was 1:1, thus obtaining a 40wt% silica / polyamic acid foam. A mixture of silica and polyamic acid at a concentration of 50 wt% was prepared and then sequentially added to a polytetrafluoroethylene mold for unidirectional freeze-drying at -65°C for 72 hours, yielding a silica / carbon nanotube / polyamic acid foam precursor. This precursor was then thermally imidized at gradient temperatures of 100°C, 200°C, and 250°C for 1 hour to obtain a silica / carbon nanotube / polyimide foam. The silica / polyimide layer in the silica / carbon nanotube / polyimide foam has a thickness of 1 mm, while the carbon nanotube / polyimide foam has a thickness of 8 mm. The thickness of the carbon nanotube / polyimide foam can be adjusted by changing the volume of the carbon nanotube / polyamic acid dispersion.

[0047] S3. Preparation of silver nanowire / cellulose nanofiber film: 0.68 g of silver nitrate was dissolved in 35.94 g of ethylene glycol and magnetically stirred to form solution I. 0.69 mg of ferric chloride hexahydrate and 0.71 g of polyvinylpyrrolidone were dissolved in another 35.94 g of ethylene glycol and magnetically stirred to form solution II. Solution I was slowly added to solution II, and then the mixed solution was transferred to an autoclave and heated at 160 °C for 150 min to obtain silver nanowires. 0.0333 g of silver nanowires and 2.5 g of cellulose dispersion (2% by mass) were mixed and magnetically stirred for 30 min to prepare a homogeneous dispersion. The mixture was then poured into a filter flask, vacuum filtered at a vacuum degree of -0.1 MPa, and then dried in an oven at 60 °C for 14 hours to obtain a silver nanowire / cellulose nanofiber film. The silver nanowire content in the prepared silver nanowire / cellulose nanofiber film was 40 wt%.

[0048] S4. Preparation of electromagnetic shielding foam: Based on the principle of modular conductive design, thermally imidized silica / carbon nanotube / polyimide foam and a silver nanowire / cellulose nanofiber film with a silver nanowire content of 40wt% are bonded together with a 4wt% polyamic acid solution. The thickness of the silver nanowire / cellulose nanofiber film is 120μm. The silver nanowire / cellulose nanofiber film is bonded to one side of the carbon nanotube / polyimide foam to obtain silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam.

[0049] Example 4

[0050] S1. Synthesis of polyamic acid: 8.62 g of 4,4'-diaminodiphenyl ether was completely dissolved in 100 g of N,N-dimethylacetamide, and 3 g, 2.5 g, 2 g, and 1.9 g of 4,4'-oxophthalic anhydride were added in four portions at 30 min intervals, respectively, and the mixture was stirred continuously until completely dissolved. Then, 4.4 g of triethylamine was slowly added to the mixed solution and stirred for 3 hours to obtain a polyamic acid solution.

[0051] S2. Preparation of silica / carbon nanotube / polyimide foam: Carbon nanotubes (acidified carbon nanotubes, obtained by adding carbon nanotubes to a mixed acid solution of sulfuric acid and nitric acid at a mass ratio of 3:1 and oxidizing at 60°C for 2 hours) and silica were added separately to a polyamic acid solution. The mass ratio of carbon nanotubes to polyamic acid solution was 1.3:2, and the mass ratio of silica to polyamic acid solution was 1:1, thus obtaining a 40wt% silica / polyamic acid foam. A mixture of silica and polyamic acid at a concentration of 50 wt% was prepared and then sequentially added to a polytetrafluoroethylene mold for unidirectional freeze-drying at -65°C for 72 hours, yielding a silica / carbon nanotube / polyamic acid foam precursor. This precursor was then thermally imidized at gradient temperatures of 100°C, 200°C, and 250°C for 1 hour to obtain a silica / carbon nanotube / polyimide foam. The silica / polyimide layer in the silica / carbon nanotube / polyimide foam has a thickness of 1 mm, while the carbon nanotube / polyimide foam has a thickness of 8 mm. The thickness of the carbon nanotube / polyimide foam can be adjusted by changing the volume of the carbon nanotube / polyamic acid dispersion.

[0052] S3. Preparation of silver nanowire / cellulose nanofiber film: 0.68 g of silver nitrate was dissolved in 35.94 g of ethylene glycol and magnetically stirred to form solution I. 0.69 mg of ferric chloride hexahydrate and 0.71 g of polyvinylpyrrolidone were dissolved in another 35.94 g of ethylene glycol and magnetically stirred to form solution II. Solution I was slowly added to solution II, and then the mixed solution was transferred to an autoclave and heated at 160 °C for 150 min to obtain silver nanowires. 0.05 g of silver nanowires and 2.5 g of cellulose dispersion (2% by mass) were mixed and magnetically stirred for 30 min to prepare a homogeneous dispersion. The mixture was then poured into a filter flask, vacuum filtered at a vacuum degree of -0.1 MPa, and then dried in an oven at 60 °C for 14 hours to obtain a silver nanowire / cellulose nanofiber film. The silver nanowire content in the prepared silver nanowire / cellulose nanofiber film was 50 wt%.

[0053] S4. Preparation of electromagnetic shielding foam: Based on the principle of modular conductive design, thermally imidized silica / carbon nanotube / polyimide foam and a silver nanowire / cellulose nanofiber film with a silver nanowire content of 50wt% are bonded together with a 4wt% polyamic acid solution. The thickness of the silver nanowire / cellulose nanofiber film is 120μm. The silver nanowire / cellulose nanofiber film is bonded to one side of the carbon nanotube / polyimide foam to obtain silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam.

[0054] Example 5

[0055] S1. Synthesis of polyamic acid: 8.62 g of 4,4'-diaminodiphenyl ether was completely dissolved in 100 g of N,N-dimethylacetamide, and 3 g, 2.5 g, 2 g, and 1.9 g of 4,4'-oxophthalic anhydride were added in four portions at 30 min intervals, respectively, and the mixture was stirred continuously until completely dissolved. Then, 4.4 g of triethylamine was slowly added to the mixed solution and stirred for 3 hours to obtain a polyamic acid solution.

[0056] S2. Preparation of silica / carbon nanotube / polyimide foam: Carbon nanotubes (acidified carbon nanotubes, obtained by adding carbon nanotubes to a mixed acid solution of sulfuric acid and nitric acid at a mass ratio of 3:1 and oxidizing at 60°C for 2 hours) and silica were added separately to a polyamic acid solution. The mass ratio of carbon nanotubes to polyamic acid solution was 1.3:2, and the mass ratio of silica to polyamic acid solution was 1:1, thus obtaining a 40wt% silica / polyamic acid foam. A mixture of silica and polyamic acid at a concentration of 50 wt% was prepared and then sequentially added to a polytetrafluoroethylene mold for unidirectional freeze-drying at -65°C for 72 hours, yielding a silica / carbon nanotube / polyamic acid foam precursor. This precursor was then thermally imidized at gradient temperatures of 100°C, 200°C, and 250°C for 1 hour to obtain a silica / carbon nanotube / polyimide foam. The silica / polyimide layer in the silica / carbon nanotube / polyimide foam has a thickness of 1 mm, while the carbon nanotube / polyimide foam has a thickness of 8 mm. The thickness of the carbon nanotube / polyimide foam can be adjusted by changing the volume of the carbon nanotube / polyamic acid dispersion.

[0057] S3. Preparation of silver nanowire / cellulose nanofiber film: 0.68 g of silver nitrate was dissolved in 35.94 g of ethylene glycol and magnetically stirred to form solution I. 0.69 mg of ferric chloride hexahydrate and 0.71 g of polyvinylpyrrolidone were dissolved in another 35.94 g of ethylene glycol and magnetically stirred to form solution II. Solution I was slowly added to solution II, and then the mixed solution was transferred to an autoclave and heated at 160 °C for 150 min to obtain silver nanowires. 0.075 g of silver nanowires and 2.5 g of cellulose dispersion (2% by mass) were mixed and magnetically stirred for 30 min to prepare a homogeneous dispersion. The mixture was then poured into a filter flask, vacuum filtered at a vacuum degree of -0.1 MPa, and then dried in an oven at 60 °C for 14 hours to obtain a silver nanowire / cellulose nanofiber film. The silver nanowire content in the prepared silver nanowire / cellulose nanofiber film was 60 wt%.

[0058] S4. Preparation of electromagnetic shielding foam: Based on the principle of modular conductive design, thermally imidized silica / carbon nanotube / polyimide foam and a silver nanowire / cellulose nanofiber film with a silver nanowire content of 60wt% are bonded together with a 4wt% polyamic acid solution. The thickness of the silver nanowire / cellulose nanofiber film is 120μm. The silver nanowire / cellulose nanofiber film is bonded to one side of the carbon nanotube / polyimide foam to obtain silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam.

[0059] Compare with Example 1

[0060] Preparation of silver nanowire / cellulose nanofiber films: 0.68 g of silver nitrate was dissolved in 35.94 g of ethylene glycol and magnetically stirred to form solution I. 0.69 mg of ferric chloride hexahydrate and 0.71 g of polyvinylpyrrolidone were dissolved in another 35.94 g of ethylene glycol and magnetically stirred to form solution II. Solution I was slowly added to solution II, and then the mixture was transferred to an autoclave and heated at 160 °C for 150 min to obtain silver nanowires. 0.075 g of silver nanowires and 2.5 g of cellulose dispersion (2% by mass) were mixed and magnetically stirred for 30 min to prepare a homogeneous dispersion. The mixture was then poured into a filter flask and vacuum filtered at -0.1 MPa. The filtration was then carried out in an oven for 14 hours at 60 °C to obtain a silver nanowire / cellulose nanofiber film. The silver nanowire content in the prepared silver nanowire / cellulose nanofiber film was 60 wt%.

[0061] Compare with Example 2

[0062] S1. Synthesis of polyamic acid: 8.62 g of 4,4'-diaminodiphenyl ether was completely dissolved in 100 g of N,N-dimethylacetamide, and 3 g, 2.5 g, 2 g, and 1.9 g of 4,4'-oxophthalic anhydride were added in four portions at 30 min intervals, respectively, and the mixture was stirred continuously until completely dissolved. Then, 4.4 g of triethylamine was slowly added to the mixed solution and stirred for 3 hours to obtain a polyamic acid solution.

[0063] S2. Preparation of silica / carbon nanotube / polyimide foam: Carbon nanotubes (acidified carbon nanotubes, obtained by adding carbon nanotubes to a mixed acid solution of sulfuric acid and nitric acid at a mass ratio of 3:1 and oxidizing at 60°C for 2 hours) and silica were added separately to a polyamic acid solution. The mass ratio of carbon nanotubes to polyamic acid solution was 1.3:2, and the mass ratio of silica to polyamic acid solution was 1:1, thus obtaining a 40wt% silica / polyamic acid foam. A mixture of silica and polyamic acid with a concentration of 50 wt% was prepared. This mixture was then sequentially added to a polytetrafluoroethylene mold and subjected to unidirectional freeze-drying for 72 hours at -65°C to obtain a silica / carbon nanotube / polyamic acid foam precursor. This precursor was then thermally imidized at gradient temperatures of 100°C, 200°C, and 250°C for 1 hour to obtain a silica / carbon nanotube / polyimide foam. The silica / polyimide layer in the silica / carbon nanotube / polyimide foam was 1 mm thick, and the carbon nanotube / polyimide layer was 8 mm thick.

[0064] The above Examples 1-5 and Comparative Examples 1 and 2 were tested, and the test results are shown in Table 1 below:

[0065] Table 1. Detection performance results of Examples 1 to 5 and Comparative Examples 1 and 2

[0066] silicon dioxide

[0067] / wt%

[0068] Carbon nanotubes / wt%

[0069] polyimide / g

[0070] Silver nanowires

[0071] / wt%

[0072] Cellulose

[0073] Nanofibers / g

[0074] shield

[0075] efficacy

[0076] dB

[0077] Reflection power coefficient / R

[0078] Decrease temperature / °C

[0079] Example 1 50 40 0.2 20 0.05 36.84 0.016

[0087] -

[0088] Example 2 50 40 0.2 30 0.05 60.39 0.014

[0096] -

[0097] Example 3 50 40 0.2 40 0.05 80.25 0.0077

[0105] -

[0106] Example 4 50 40 0.2 50 0.05 94.26 0.0041

[0114] -

[0115] Example 5 50 40 0.2 60 0.05 115.50 0.0044 86.9

[0124] Comparative Example 1

[0125] -

[0126] -

[0127] - 60 0.05 102.82 0.99

[0132] -

[0133] Comparative Example 2 50 40 0.2

[0137] -

[0138] - 4.50 0.0051 68.7

[0142] The scanning electron microscope (SEM) image of the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam prepared in this invention at 5 µm is shown below. Figure 2 As shown, the scanning electron microscope image at 10µm is as follows. Figure 3 As shown, the shielding effectiveness diagram is as follows: Figure 4 As shown in the figure, the reflection / absorption power coefficient diagram is as follows: Figure 5 As shown in the figure, the two-dimensional results of the radar cross-section at different angles are illustrated in the figure. Figure 6 As shown, the infrared camouflage pattern is as follows: Figure 7 As shown, the infrared thermal response diagram is as follows: Figure 8 As shown;

[0143] Examples 1-5 are composite materials prepared by assembling silica / carbon nanotube / polyimide foam with silver nanowire / cellulose nanofiber film. The upper silica / polyimide layer is the electromagnetic wave incident layer, the middle carbon nanotube / polyimide layer is the electromagnetic wave dissipation layer, and the bottom silver nanowire / cellulose nanofiber film serves as the electromagnetic wave reflective layer. The assembled composite materials of Examples 1-5 have an electromagnetic wave incident layer and a dissipation layer. Due to the good impedance matching between the incident electromagnetic wave and the material, the vast majority of the electromagnetic wave enters the middle electromagnetic wave dissipation layer. The dielectric loss and conductivity loss in the dissipation layer partially attenuate the electromagnetic wave. The remaining electromagnetic wave reaches the silver nanowire / cellulose nanofiber film reflective layer, where it is reflected back to the middle dissipation layer and attenuated again, undergoing a dissipation process of "absorption-reflection-reabsorption" (the dissipation mechanism is as follows). Figure 9 As shown in the figure, the electromagnetic energy reflected back to free space is significantly reduced, and the reflection coefficient is significantly smaller. Comparative Example 1 has only a lower layer of silver nanowire / cellulose nanofiber film electromagnetic wave reflective layer. Due to the high conductivity of the silver nanowire / cellulose nanofiber film, it is mismatched with the air impedance, and the incident electromagnetic waves cannot fully penetrate into the interior of the material, causing most of the electromagnetic waves to be reflected back to free space, resulting in a high reflection coefficient (R). Comparative Example 2 is a foam assembled from silica / polyimide and carbon nanotube / polyimide, which lacks a lower shielding reflective layer. The electromagnetic wave dissipation layer has a certain absorption and dissipation capacity for electromagnetic waves, but compared with Examples 1 to 5, due to the lack of a reflective layer, some electromagnetic waves pass through the material, the electromagnetic wave dissipation path is significantly reduced, and the shielding effectiveness is lower.

[0144] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. An electromagnetic shielding foam, characterized in that: The electromagnetic shielding foam raw materials include: silica, carbon nanotubes, silver nanowires, polyimide, and cellulose nanofibers. The electromagnetic shielding foam has a three-layer structure: the upper silica / polyimide layer is the electromagnetic wave incident layer, the middle carbon nanotube / polyimide layer is the electromagnetic wave dissipation layer, and the bottom silver nanowire / cellulose nanofiber film serves as the electromagnetic wave reflecting layer. It is prepared using a split conductive modular design. Based on the split conductive modular design principle, the thermally imidized silica / carbon nanotube / polyimide foam and the silver nanowire / cellulose nanofiber film are bonded together with a polyamic acid solution. The silver nanowire / cellulose nanofiber film is bonded to one side of the carbon nanotube / polyimide foam to obtain the silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam.

2. The electromagnetic shielding foam according to claim 1, characterized in that: The mass ratio of silica, carbon nanotubes, silver nanowires, polyimide and cellulose nanofibers is 20:13:1.25~7.5:20:

5.

3. A method for preparing electromagnetic shielding foam, characterized in that: The specific steps of the preparation method are as follows: S1. Synthesis of polyamic acid: 4,4'-diaminodiphenyl ether was completely dissolved in a solvent, and aromatic dianhydride was added in 4 portions at intervals while stirring continuously until completely dissolved. Then, triethylamine was slowly added to the mixed solution and stirred until homogeneous to obtain a polyamic acid solution. S2. Preparation of silica / carbon nanotube / polyimide foam: Carbon nanotubes and silica are added to a polyamic acid solution to prepare a mixture of carbon nanotubes and polyamic acid and a mixture of silica and polyamic acid. The above mixtures are sequentially added to a polytetrafluoroethylene mold for unidirectional freeze-drying to obtain a silica / carbon nanotube / polyamic acid foam precursor. The silica / carbon nanotube / polyamic acid foam precursor is then thermally imidized at a gradient temperature to obtain silica / carbon nanotube / polyimide foam. S3. Preparation of silver nanowire / cellulose nanofiber film: Silver nanowires and cellulose dispersion were mixed and magnetically stirred to prepare a uniform dispersion. The mixture was then poured into a filter flask, vacuum filtered, and then dried in an oven to obtain silver nanowire / cellulose nanofiber film. S4. Preparation of electromagnetic shielding foam: Based on the principle of modular design of split conductive material, thermally imidized silica / carbon nanotube / polyimide foam is bonded to silver nanowire / cellulose nanofiber film with polyamic acid solution. The silver nanowire / cellulose nanofiber film is bonded to one side of the carbon nanotube / polyimide foam to obtain silica / carbon nanotube / polyimide-silver nanowire / cellulose nanofiber electromagnetic shielding foam.

4. The method for preparing electromagnetic shielding foam according to claim 3, characterized in that: The mass ratio of 4,4'-diaminodiphenyl ether to solvent in S1 is 8.62~17.24:100; the solvent is N,N-dimethylacetamide; the interval time is 30 min.

5. The method for preparing electromagnetic shielding foam according to claim 3, characterized in that: In step S1, the mass ratios of aromatic dianhydride and 4,4'-diaminodiphenyl ether added four times are 0.35:1, 0.29:1, 0.23:1, and 0.22:1, respectively. The aromatic dianhydride is one or more combinations of pyromellitic dianhydride, biphenyltetracarboxylic dianhydride, hexafluorodianhydride, diphenyl ether dianhydride, and benzophenone dianhydride. The mass ratio of triethylamine to 4,4'-diaminodiphenyl ether is 0.51:1, and the stirring time is 3 hours.

6. The method for preparing electromagnetic shielding foam according to claim 3, characterized in that: The carbon nanotubes in S2 are acidified carbon nanotubes, which are added to a mixture of sulfuric acid and nitric acid in a mass ratio of 3:1 and oxidized at 60°C for 2 hours. The mass ratio of carbon nanotubes to polyamic acid solution is 1.3:2, and the mass ratio of silica to polyamic acid solution is 1:1, to obtain a 40wt% carbon nanotube and polyamic acid mixture and a 50wt% silica and polyamic acid mixture.

7. The method for preparing electromagnetic shielding foam according to claim 3, characterized in that: In S2, the unidirectional freeze-drying time is 72 hours and the temperature is -65℃; the gradient temperature is 100℃, 200℃ and 250℃, the thermal imidization time is 1 hour, the thickness of silica / polyimide in silica / carbon nanotube / polyimide foam is 1mm, and the thickness of carbon nanotube / polyimide foam is 1mm to 9mm.

8. The method for preparing electromagnetic shielding foam according to claim 3, characterized in that: The silver nanowires in S3 are prepared by dissolving silver nitrate in ethylene glycol at a mass ratio of 1:53 and stirring magnetically to form solution I. Ferric chloride hexahydrate and polyvinylpyrrolidone are dissolved in ethylene glycol at a mass ratio of 1:1:52 and stirred magnetically to form solution II. Solution I is slowly added to solution II, and then the mixed solution is transferred to an autoclave and heated at 160°C for 150 min.

9. The method for preparing electromagnetic shielding foam according to claim 3, characterized in that: In the S3, the mass ratio of silver nanowires to cellulose dispersion is 1-5:5-9, the mass fraction of cellulose dispersion is 2%, the magnetic stirring time is 30 minutes, the vacuum degree of vacuum filtration is -0.1 MPa, the oven drying time is 12-24 hours, and the temperature is 60℃. The mass fraction of silver nanowires in the silver nanowire / cellulose nanofiber film is 20wt%-60wt%.

10. A method for preparing electromagnetic shielding foam according to claim 3, characterized in that: In the S4 mixture of silver nanowires and cellulose nanofibers, the mass ratio of silver nanowires to cellulose nanofibers is 1.25–7.5:5; the thickness of the silver nanowire / cellulose nanofiber film is 120 μm; and it is bonded with a 4 wt% polyamic acid solution.

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