Electromagnetic shielding material with porous gradient structure and preparation method thereof

Abstract

The invention discloses a porous gradient structure electromagnetic shielding material and a preparation method thereof, and belongs to the technical field of electromagnetic shielding materials. The preparation method comprises the following steps: directionally arranging electromagnetic shielding foam slurry with different carbon fiber contents according to a gradient sequence (the carbon fiber contents are from low to high or from high to low), and carrying out multi-stage suction filtration-drying treatment to prepare the carbon fiber composite foam material with continuous gradient distribution. According to the material, a dielectric layer-loss layer and reflecting layer sandwich structure is constructed through collaborative design of a porous structure and concentration gradient, the dielectric layer is composed of low-carbon-fiber-content foam, electromagnetic wave impedance matching is achieved, the loss layer has gradient increasing carbon fiber content, and electromagnetic waves are dissipated through collaborative design of dielectric loss and conductive loss; the reflecting layer is foam with high carbon fiber content and reflects residual electromagnetic waves back to the loss layer for secondary absorption.

Description

Technical Field

[0001] The invention belongs to the technical field of electromagnetic shielding materials and relates to an electromagnetic shielding material with a porous gradient structure and a preparation method thereof. Background Art

[0002] Information technologies such as 5G, artificial intelligence, the Internet of Things, and big data are not only profoundly impacting the development of human society but have also become key hallmarks of modern civilization. As the fundamental carriers of these technologies, electromagnetic waves inevitably generate electromagnetic leakage and pollution during their transmission. Electromagnetic pollution not only poses a potential hazard to human health but also interferes with the normal operation of electronic equipment. Therefore, the development of high-performance electromagnetic shielding materials has become a key approach to blocking electromagnetic interference, with lightweight, efficient, and flexible electromagnetic shielding materials being a key research priority.

[0003] Currently, improving material conductivity remains the primary method for achieving effective electromagnetic shielding. However, these highly conductive materials rely primarily on their strong electromagnetic wave reflection properties to achieve their shielding effectiveness, which can lead to severe secondary electromagnetic pollution and fail to fundamentally address the hazards of electromagnetic radiation. Therefore, developing electromagnetic shielding materials with low reflection and high absorption properties through precise material component design and multi-scale structural control is an effective way to address the problem of electromagnetic pollution.

[0004] The design of high-performance, low-reflection electromagnetic shielding materials requires two key conditions: first, the material should possess excellent impedance matching properties to maximize the penetration of electromagnetic waves into the material, thereby reducing surface reflections; second, the material must possess excellent electromagnetic wave attenuation capabilities, effectively dissipating electromagnetic waves that enter the material through the synergistic effect of multiple loss mechanisms. To achieve this goal, structural optimization and multi-component composites have become important material design strategies. Specifically, by constructing asymmetric layered structures, porous structures, or isolated structures, and precisely controlling relevant parameters, electromagnetic shielding materials with excellent performance (such as lightness, low density, broadband, and strong absorption) and low reflection characteristics can be obtained. Currently, research on high-performance, low-reflection electromagnetic shielding materials mainly focuses on three major material systems: fiber-reinforced composites, composite conductive films, and integral foam materials. In contrast, there are few reports on paper-based electromagnetic shielding materials with low reflection properties.

[0005] As a high-performance fiber with light weight, high strength and high modulus and excellent electrical and thermal conductivity, carbon fiber has been widely used in the field of fiber-reinforced composite materials and electromagnetic shielding. Through the wet forming process, chopped carbon fibers can be processed into carbon fiber paper-based materials. This material not only inherits the lightweight characteristics and outstanding electrical and thermal conductivity of carbon fibers, but also exhibits good electromagnetic shielding properties. However, carbon fiber paper still has obvious limitations as an electromagnetic shielding material: from a performance point of view, high conductivity leads to excessive reflectivity of electromagnetic waves, which can easily cause secondary electromagnetic pollution; from a microscopic morphology point of view, due to the low polarity and smoothness of the carbon fiber surface, the interfacial bonding performance between fibers is poor, which not only affects the molding and processing performance of the material, but also restricts the improvement of its comprehensive performance and the expansion of its application range. Summary of the Invention

[0006] This invention aims to overcome the shortcomings of existing technologies by providing an electromagnetic shielding material with a porous gradient structure and a method for its preparation. This invention primarily addresses the following technical issues: existing carbon fiber paper has poor electromagnetic wave absorption, which can easily lead to secondary electromagnetic pollution; and poor interfacial bonding between fibers, which affects the overall performance of the material.

[0007] Compared with the prior art, the present invention has the following beneficial effects: A method for preparing an electromagnetic shielding material having a porous gradient structure, comprising the following steps: S1, placing sodium lauryl sulfate, deionized water, and carbon fibers in a reaction vessel, mechanically stirring to obtain a mixed system, and vacuum filtering the mixed system; repeatedly washing the filtered product with deionized water, and drying to obtain surface-modified hydrophilic carbon fibers; S2, mechanically dispersing the aramid fibrid in deionized water until it is completely dispersed, mixing it with the hydrophilic carbon fiber prepared in S1 according to a predetermined mass ratio, and performing secondary dispersing to obtain a uniform fiber mixed material; adding sodium lauryl sulfate, and stirring at high speed to obtain a carbon fiber foam slurry; S3, repeating S2, preparing foam slurries with different carbon fiber contents by adjusting the mass ratio of hydrophilic carbon fiber to aramid fibrils; then, pouring the foam slurries with different components into a forming mold in a predetermined gradient order of carbon fiber content, to form a composite carbon fiber foam material with a continuous gradient structure; S4, performing vacuum filtration and drying treatment on the composite carbon fiber foam with a gradient structure for several times, and finally preparing an electromagnetic shielding material with a porous gradient structure.

[0008] A further improvement of the present invention is: Preferably, the mixing mass ratio of sodium lauryl sulfate, deionized water and carbon fiber in S1 is (0.1-3): (300-3000): (2-30).

[0009] Preferably, the rotation speed of the mechanical stirring in S1 is 500~5000 r / min, and the stirring time is 0.5~6 h.

[0010] Preferably, in S1, the drying treatment temperature is 60-180°C, and the drying time is 2-48 h.

[0011] Preferably, in S2, the absolute dry mass ratio of carbon fiber to aramid fibrid is (5-80):(95-20).

[0012] Preferably, in S2, the specific process for obtaining a uniform fiber mixture material is as follows: the aramid precipitated fibers are subjected to a deionized water treatment at a rotation speed of 10,000-30,000 r / min for 10-30 minutes; then, the hydrophilic carbon fibers are added and the decomposition treatment is continued at the same rotation speed for 10-30 minutes to obtain a uniformly dispersed fiber mixture slurry.

[0013] Preferably, in S2, the concentration of sodium lauryl sulfate added to the carbon fiber foam slurry is 0.2-2.3 g / L.

[0014] Preferably, in S4, the gradient structure composite carbon fiber foam material is dried at 40-180° C. for 2-24 hours.

[0015] An electromagnetic shielding material with a porous gradient structure obtained by any of the above preparation methods, comprising a dielectric layer, a loss layer and a reflective layer stacked in sequence; wherein the dielectric layer, the loss layer and the reflective layer are each composed of one or more layers of carbon fiber foam, and the carbon fiber content in each layer of carbon fiber foam is distributed in a gradient increasing manner from the dielectric layer to the reflective layer.

[0016] Preferably, the content of carbon fibers in the dielectric layer is 5-30 wt%, the content of carbon fibers in the lossy layer is 30-70 wt%, and the content of carbon fibers in the reflective layer is 70-80 wt%.

[0017] The present invention provides a method for preparing a porous gradient structure electromagnetic shielding material based on a foam molding method, the method comprising the following steps: (1) using aramid precipitated fibers as an interface enhancer and carbon fibers as a structural skeleton, and adopting a foam molding process to prepare electromagnetic shielding foam materials with different carbon fiber contents; (2) arranging the foam materials with different carbon fiber contents prepared above in a preset gradient order, wherein the carbon fiber content is continuously and gradually distributed from high to low or from low to high along the thickness direction; (3) subjecting the gradient-arranged foam material to multiple filtration and drying treatments, and finally obtaining an electromagnetic shielding material with a porous gradient structure. The preparation method of the present invention has the following advantages: (1) Surface modification of carbon fibers with sodium dodecyl sulfate can effectively improve the water phase dispersion problem of carbon fibers caused by their hydrophobicity. After modification, the dispersion uniformity of carbon fibers in the foam system is significantly improved. At the same time, the composite material exhibits excellent electromagnetic wave absorption properties, which can meet the performance requirements of various electromagnetic shielding application scenarios.

[0018] (2) By constructing a porous structure through a foaming process and combining it with multiple filtration techniques to achieve a concentration gradient design, the high reflection and low loss of electromagnetic waves, a problem that exists in traditional highly conductive materials, is effectively solved. At the same time, the introduction of aramid precipitated fibers improves the overall performance of the material, including excellent high-temperature resistance, wear resistance, and high strength and high modulus properties. This preparation method is simple, and the resulting electromagnetic shielding material maintains excellent performance and durability in harsh environments such as high temperature and high pressure, and has important engineering application value.

[0019] This invention provides an electromagnetic shielding material with a porous gradient structure. This material utilizes a porous structure and a concentration gradient design to construct a multilayered structure consisting of a dielectric layer, a lossy layer, and a reflective layer. Each functional layer is composed of one or more layers of carbon fiber foam slurry. The dielectric layer (5-30 wt% carbon fiber) achieves impedance matching, facilitating the entry of electromagnetic waves into the material. The gradient-varying lossy layer (30-70 wt% carbon fiber) continuously dissipates electromagnetic energy through a synergistic mechanism of dielectric and conductive loss. The reflective layer (70-80 wt% carbon fiber) reflects residual electromagnetic waves back to the lossy layer for secondary absorption. This structure implements an "absorption-reflection-reabsorption" mechanism for electromagnetic waves, ultimately resulting in a gradient porous electromagnetic shielding material with both low reflection and high absorptivity. This porous gradient structure enables efficient absorption and conversion of electromagnetic energy when incident from a low-gradient direction. Furthermore, this porous gradient foam material exhibits excellent thermal insulation properties, showing broad application prospects in various fields, including electromagnetic shielding and thermal protection. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 It is a preparation process flow chart of the present invention; Figure 2 The following are the preparation flow chart and finished product diagram of Example 1 of the present invention; Figure 3 These are front and side images of carbon fiber foams with different carbon fiber contents according to Example 1 of the present invention; Figure 4 Graph showing the electrical conductivity of carbon fiber foams Fx with different carbon fiber contents according to Example 1 of the present invention; Figure 5 This is a diagram of shielding effectiveness of different carbon fiber contents according to Example 1 of the present invention; Figure 6 This is a power coefficient diagram of different carbon fiber contents in Example 1 of the present invention; Figure 7 This is a graph of thermal conductivity coefficients for different carbon fiber contents according to Example 1 of the present invention; Figure 8 This is a thermogravimetric graph of different carbon fiber contents in Example 1 of the present invention; Figure 9 Graph showing the total shielding performance and absorption and reflection power coefficient of the foams in Examples 3, 6, and 7. DETAILED DESCRIPTION

[0021] The present invention is described in further detail below with reference to the accompanying drawings: To facilitate understanding of the features and effects of the present invention by those skilled in the art, the following provides a general description and definition of the terms and expressions used in the specification and claims. Unless otherwise indicated, all technical and scientific terms used herein have the ordinary meanings as understood by those skilled in the art regarding the present invention. In the event of conflict, the definitions in this specification shall prevail.

[0022] In this document, unless otherwise specified, “include,” “including,” “contains,” “has” or similar terms cover the meanings of “consisting of” and “mainly consisting of,” for example, “A includes a” covers the meanings of “A includes a and other” and “A only includes a.”

[0023] Below in conjunction with specific embodiment, further set forth the present invention.Should be understood that these embodiments are only used to illustrate the present invention and are not used in limiting the scope of the present invention.In addition, should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall equally within the scope limited by the appended claims of the application.

[0024] The following examples were prepared using conventional instruments and equipment in the art. Experimental methods in the following examples, where specific conditions are not specified, were generally performed under conventional conditions or according to the conditions recommended by the manufacturer. The various raw materials used in the following examples, unless otherwise specified, were conventional commercially available products, with specifications conventional in the art. In the present specification and the following examples, unless otherwise specified, "%" indicates percentage by weight, "part" indicates parts by weight, and "ratio" indicates weight ratio.

[0025] The first aspect of the present invention discloses a method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate (SDS), deionized water, and carbon fibers (CF) were added to a reaction vessel at a mass ratio of (0.1-3): (300-3000): (2-30), and mechanically stirred at 500-5000 r / min for 0.5-6 h. The mixture was vacuum filtered, washed three times with deionized water, and dried at 60-180°C for 2-48 h to obtain surface-modified hydrophilic carbon fibers.

[0026] In this process, sodium dodecyl sulfate acts as an anionic surfactant, which removes the hydrophobic components on the surface of carbon fiber and exposes the intrinsic hydrophilic surface of carbon fiber, thereby significantly improving its dispersibility in the aqueous phase system.

[0027] (2) Preparation of foam slurries with different carbon fiber contents: First, aramid precipitated fibers (AF) were dispersed in deionized water at a speed of 20,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added at an absolute dry mass ratio of 95:5 to 20:80, and the mixture was dispersed at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium dodecyl sulfate was added to each slurry to a final concentration of 0.2~2.3 g / L, and finally, the mixture was stirred at a high speed of 1000~6000 r / min to obtain a series of carbon fiber foam slurries with a gradient of carbon fiber content (5.0%-80.0%).

[0028] (3) Adjust the content of hydrophilic carbon fiber in the mixed material and repeat step (2) to prepare carbon fiber foam slurries of different components. Subsequently, each slurry is placed in a container in sequence according to the carbon fiber content gradient (from low to high or from high to low) to obtain a composite carbon fiber foam with a concentration gradient.

[0029] (4) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (3) is vacuum filtered 1 to 10 times and dried at 40 to 180°C for 2 to 24 hours to finally obtain a carbon fiber composite electromagnetic shielding foam material with a porous gradient structure.

[0030] A second aspect of the present invention discloses a porous gradient structure electromagnetic shielding material prepared using a foam molding method, obtained by the aforementioned method. The composite foam material has a multilayer structure, comprising a dielectric layer, a lossy layer, and a reflective layer arranged in sequence. Each functional layer is composed of a single or multiple layers of carbon fiber foam, and the carbon fiber content distribution varies between functional layers. The carbon fiber content increases in a gradient from the dielectric layer to the reflective layer, thereby achieving electromagnetic wave absorption.

[0031] The content of carbon fiber in the dielectric layer is 5-30 wt%, the content of carbon fiber in the lossy layer is 30-70 wt%, and the content of carbon fiber in the reflective layer is 70-80 wt%.

[0032] In a specific embodiment, the entire electromagnetic shielding material includes five layers, wherein the carbon fiber contents are 10%, 30%, 40%, 50% and 70% respectively.

[0033] The three-dimensional porous gradient foam material prepared by this invention overcomes the limitations of traditional two-dimensional shielding films and exhibits the following technical advantages: First, its unique three-dimensional porous structure enables the material to achieve higher overall shielding effectiveness and electromagnetic wave absorptivity; second, its gradient pore structure imparts excellent thermal insulation properties, achieving synergistic optimization of electromagnetic shielding and thermal protection. This integrated design effectively overcomes the technical bottleneck of traditional materials with a single function and has important application value in fields such as aerospace and electronic equipment, where multifunctional composite materials are in urgent need.

[0034] The present invention is described in further detail below with reference to the embodiments.

[0035] Example 1 See also Figure 2 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method comprises the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 0.42:1000:10, and mechanically stirred at 1500 r / min for 1 h. The mixed system was vacuum filtered, washed three times with deionized water, and dried at 105°C for 12 h to obtain surface-modified hydrophilic carbon fibers.

[0036] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 20,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added according to the absolute dry mass ratios of 1:9 (10.0%), 3:7 (30.0%), 4:6 (40.0%), 5:5 (50.0%) and 7:3 (70.0%), and the dispersion was continued at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium lauryl sulfate was added to each slurry to a final concentration of 0.6 g / L, and finally, the mixture was stirred at a high speed of 5,000 r / min to obtain a series of carbon fiber foam slurries with a carbon fiber content gradient (10.0%-70.0%).

[0037] The carbon fiber foam slurry was stacked in a container in the order of carbon fiber content of 10%, 30%, 40%, 50% and 70%.

[0038] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered five times and then dried at 150°C for 12 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0039] See also Figure 3 It can be seen that all foam materials exhibit fluffy and porous properties. As the carbon fiber content increases, the color of the single-layer carbon fiber / aramid composite foam gradually deepens, and the foam height gradually increases due to the gradual strengthening of the carbon fiber's support for the foam. Multi-layer carbon fiber / aramid composite foam (GF) also exhibits a distinct porous, layered, and gradient structure, which can further improve impedance mismatch, reduce interfacial electromagnetic reflection, and enhance internal electromagnetic absorption.

[0040] See also Figure 4 It can be seen that as the carbon fiber content increases from 10% to 70%, the conductivity of the carbon fiber / aramid fiber composite foam increases from 1.6 mS / m to 17.9 mS / m. The significant increase in conductivity can also be visually verified by the brightness of the small light bulb.

[0041] See also Figure 5 and Figure 6 As can be seen, F-10 has a total shielding effectiveness of 74 dB and an absorption coefficient of 52%, significantly superior to a paper-based material with a carbon fiber content of 10 wt%, indicating that the porous foam structure significantly improves the material's electromagnetic shielding effectiveness. The conductivity of the F-30, F-40, F-50, and F-70 composite foams is further improved, with their total shielding effectiveness exceeding 100 dB. However, their absorption coefficients gradually decrease, indicating that the high resistance leads to impedance mismatch on the material surface, resulting in a large amount of electromagnetic wave reflection, which is not conducive to electromagnetic wave absorption.

[0042] See also Figure 7 It can be seen that the thermal conductivity of composite foam is very low, ranging from 35 to 38 mW / m·K.

[0043] Figure 8 The carbon fiber / aramid composite foam prepared in this embodiment is stable within 350°C and can be applied to most electromagnetic shielding, heat insulation, and sound absorption fields.

[0044] Example 2 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 0.1:500:5, and mechanically stirred at 1200 r / min for 0.5 h. The mixed system was vacuum filtered, washed three times with deionized water, and dried at 150°C for 4 h to obtain surface-modified hydrophilic carbon fibers.

[0045] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 20,000 r / min until they were completely dispersed; then, hydrophilic carbon fibers were added at an absolute dry mass ratio of 1:1 (50.0%) and 2:1 (66.7%), respectively, and the dispersion was continued at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium dodecyl sulfate was added to each slurry to a final concentration of 0.3 g / L, and finally, the mixture was stirred at a high speed of 4,000 r / min to obtain a series of carbon fiber foam slurries with carbon fiber contents of 50.0% and 66.7%, respectively.

[0046] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered twice and then dried at 105°C for 4 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0047] Example 3 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 0.42:1000:10, and mechanically stirred at 1500 r / min for 1 h. The mixed system was vacuum filtered, washed three times with deionized water, and dried at 105°C for 12 h to obtain surface-modified hydrophilic carbon fibers.

[0048] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 20,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added at an absolute dry mass ratio of 1:1 (50.0%), 2:1 (66.7%), and 3:1 (75.0%), respectively, and the dispersion was continued at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium lauryl sulfate was added to each slurry to a final concentration of 0.6 g / L, and finally, the mixture was stirred at a high speed of 5,000 r / min to obtain a series of carbon fiber foam slurries with gradient carbon fiber contents (50.0%-75.0%).

[0049] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered three times and then dried at 150°C for 12 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0050] Example 4 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 1:3000:20, and mechanically stirred at a speed of 2000 r / min for 2 h. The mixed system was vacuum filtered, washed repeatedly with deionized water for 3 times, and dried at 105°C for 24 h to obtain surface-modified hydrophilic carbon fibers.

[0051] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 20,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added according to the absolute dry mass ratio of 1:2 (33.3%), 2:3 (40.0%), 1:1 (50.0%), and 3:2 (60.0%), and the dispersion was continued at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium lauryl sulfate was added to each slurry to a final concentration of 1.2 g / L, and finally, the slurries were stirred at a high speed of 4,000 r / min to prepare a series of carbon fiber foam slurries with gradient carbon fiber contents (33.3%-60.0%).

[0052] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered four times and then dried at 80°C for 24 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0053] Example 5 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 0.1:500:10. The mixture was mechanically stirred at 1000 r / min for 1 h. The mixed system was vacuum filtered, washed three times with deionized water, and dried at 80°C for 12 h to obtain surface-modified hydrophilic carbon fibers.

[0054] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 20,000 r / min until they were completely dispersed; then, hydrophilic carbon fibers were added at an absolute dry mass ratio of 1:2 (33.3%) and 4:5 (44.4%), respectively, and the dispersion was continued at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium dodecyl sulfate was added to each slurry to a final concentration of 1.2 g / L, and finally, the mixture was stirred at a high speed of 4,000 r / min to obtain a series of carbon fiber foam slurries with gradient carbon fiber contents (33.3%-44.4%).

[0055] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered twice and then dried at 80°C for 4 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0056] Example 6 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 0.42:2000:20. The mixture was mechanically stirred at 1000 r / min for 1 h. The mixture was vacuum filtered, washed three times with deionized water, and dried at 105°C for 12 h to obtain surface-modified hydrophilic carbon fibers.

[0057] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 20,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added according to the absolute dry mass ratios of 1:1 (50%), 2:3 (40%), 4:5 (44%), 1:5 (20%) and 1:6 (16.7%), and the dispersion was continued at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium lauryl sulfate was added to each slurry to a final concentration of 1.2 g / L, and finally, the slurries were stirred at a high speed of 3,000 r / min to obtain a series of carbon fiber foam slurries with gradient carbon fiber contents (16.7%-50%).

[0058] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered 5 times and then dried at 105°C for 4 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0059] Example 7 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 1:2000:10. The mixture was mechanically stirred at 2000 r / min for 2 h. The mixed system was vacuum filtered, washed three times with deionized water, and dried at 80°C for 24 h to obtain surface-modified hydrophilic carbon fibers.

[0060] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 20,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added according to the absolute dry mass ratios of 9:1 (90%), 1:1 (50%), 2:3 (40%), 1:2 (33%), 1:3 (25%), 1:4 (20%), 1:5 (16.7%), 1:6 (14.3%) and 1:7 (12.5%), and the mixture was dispersed at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium lauryl sulfate was added to each slurry to a final concentration of 0.6 g / L, and finally, the mixture was stirred at a high speed of 5,000 r / min to obtain a series of carbon fiber foam slurries with gradient carbon fiber contents (12.5%-90%).

[0061] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered 9 times and then dried at 150°C for 12 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0062] Example 8 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 0.42:1000:5. The mixture was mechanically stirred at 1500 r / min for 1 h. The mixed system was vacuum filtered, washed three times with deionized water, and dried at 80°C for 12 h to obtain surface-modified hydrophilic carbon fibers.

[0063] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 20,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added according to the absolute dry mass ratios of 9:1 (90%), 9:2 (81.8%), 3:1 (75%), 1:1 (50%) and 1:2 (33%), and the dispersion was continued at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium lauryl sulfate was added to each slurry to a final concentration of 1.2 g / L, and finally, the slurries were stirred at a high speed of 4,000 r / min to obtain a series of carbon fiber foam slurries with gradient carbon fiber contents (33%-90%).

[0064] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered five times and then dried at 80°C for 24 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0065] Taking implementation cases 3, 6, and 7 as examples, the performance of the electromagnetic shielding materials with porous gradient structures obtained by the present invention was tested. Their total electromagnetic shielding effectiveness exceeded 50 dB, and the absorption coefficient was as high as 67%, exceeding that of similar composite foams.

[0066] Example 9 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 0.1:300:2. The mixture was mechanically stirred at 500 r / min for 0.5 h. The mixture was vacuum filtered, washed three times with deionized water, and dried at 60°C for 48 h to obtain surface-modified hydrophilic carbon fibers.

[0067] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 10,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added according to the absolute dry mass ratios of 9:1 (90%), 1:1 (50%), 2:3 (40%), 1:2 (33%), 1:3 (25%), 1:4 (20%), 1:5 (16.7%), 1:6 (14.3%) and 1:7 (12.5%), and the mixture was dispersed at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium lauryl sulfate was added to each slurry to a final concentration of 0.2 g / L, and finally, the mixture was stirred at a high speed of 5,000 r / min to obtain a series of carbon fiber foam slurries with gradient carbon fiber contents (12.5%-90%).

[0068] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered 9 times and then dried at 40°C for 24 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0069] Example 10 A method for preparing an electromagnetic shielding material having a porous gradient structure by a foam molding method, comprising the following steps: (1) Preparation of hydrophilic carbon fibers: Sodium dodecyl sulfate, deionized water, and carbon fibers were added to a reaction vessel at a mass ratio of 3:3000:30, and mechanically stirred at 5000 r / min for 0.5 h. The mixed system was vacuum filtered, washed three times with deionized water, and dried at 180°C for 2 h to obtain surface-modified hydrophilic carbon fibers.

[0070] (2) Preparation of foam slurries with different carbon fiber contents: First, the aramid precipitated fibers were dispersed in deionized water at a speed of 30,000 r / min until completely dispersed; then, hydrophilic carbon fibers were added according to the absolute dry mass ratios of 9:1 (90%), 1:1 (50%), 2:3 (40%), 1:2 (33%), 1:3 (25%), 1:4 (20%), 1:5 (16.7%), 1:6 (14.3%) and 1:7 (12.5%), and the mixture was dispersed at the same speed to obtain a uniformly dispersed mixed slurry; then, sodium lauryl sulfate was added to each slurry to a final concentration of 2.3 g / L, and finally, the mixture was stirred at a high speed of 5,000 r / min to obtain a series of carbon fiber foam slurries with gradient carbon fiber contents (12.5%-90%).

[0071] (3) Preparation of porous gradient structure carbon fiber composite foam: The foam slurry with different carbon fiber contents obtained in step (2) was filtered 9 times and then dried at 180°C for 2 h to prepare a porous gradient structure carbon fiber electromagnetic shielding foam.

[0072] This invention successfully fabricates a carbon fiber composite foam material with a multi-layered, porous gradient structure by using a foam molding process to create a rich cellular structure, combined with a concentration gradient design. This material achieves efficient electromagnetic wave attenuation through the following methods: First, a foam with a low carbon fiber content is used as a dielectric layer to effectively reduce electromagnetic wave reflection from the material surface; second, a conductive foam with a positive conductivity gradient is used as a loss absorption layer to maximize electromagnetic wave absorption and loss; finally, a foam reflective layer with a high carbon fiber content reflects any residual electromagnetic waves back to the loss layer for secondary absorption. This simple operation process and low reflection properties offer a wide range of potential applications.

[0073] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing an electromagnetic shielding material having a porous gradient structure, characterized in that: The following steps are involved: S1, placing sodium lauryl sulfate, deionized water, and carbon fibers in a reaction vessel, mechanically stirring to obtain a mixed system, and vacuum filtering the mixed system; repeatedly washing the filtered product with deionized water, and drying to obtain surface-modified hydrophilic carbon fibers; S2, mechanically dispersing the aramid fibrid in deionized water until it is completely dispersed, mixing it with the hydrophilic carbon fiber prepared in S1 according to a predetermined mass ratio, and performing secondary dispersing to obtain a uniform fiber mixed material; adding sodium lauryl sulfate, and stirring at high speed to obtain a carbon fiber foam slurry; S3, repeating S2, preparing foam slurries with different carbon fiber contents by adjusting the mass ratio of hydrophilic carbon fiber to aramid fibrils; then, pouring the foam slurries with different components into a forming mold in a predetermined gradient order of carbon fiber content, to form a composite carbon fiber foam material with a continuous gradient structure; S4, performing vacuum filtration and drying treatment on the composite carbon fiber foam with a gradient structure for several times, and finally preparing an electromagnetic shielding material with a porous gradient structure.

2. The preparation method according to claim 1, characterized in that The mixing mass ratio of sodium lauryl sulfate, deionized water and carbon fiber described in S1 is (0.1~3): (300~3000): (2~30).

3. The preparation method according to claim 1, characterized in that The mechanical stirring speed in S1 is 500-5000 r / min, and the stirring time is 0.5-6 h.

4. The method for preparing an electromagnetic shielding material having a porous gradient structure according to claim 1, characterized in that: In S1, the drying treatment temperature is 60-180°C, and the drying time is 2-48 h.

5. The method for preparing an electromagnetic shielding material having a porous gradient structure according to claim 1, characterized in that: In S2, the absolute dry mass ratio of carbon fiber to aramid fibril is (5~80):(95~20).

6. The method for preparing an electromagnetic shielding material having a porous gradient structure according to claim 1, characterized in that: In S2, the specific process of obtaining a uniform fiber mixture material is as follows: the aramid precipitated fibers are subjected to a deionized water treatment at a rotation speed of 10,000-30,000 r / min for 10-30 minutes; then, the hydrophilic carbon fibers are added and the decomposition treatment is continued at the same rotation speed for 10-30 minutes to obtain a uniformly dispersed fiber mixture slurry.

7. The method for preparing an electromagnetic shielding material having a porous gradient structure according to claim 1, characterized in that: In S2, the addition concentration of sodium lauryl sulfate in the carbon fiber foam slurry is 0.2~2.3 g / L.

8. The method for preparing an electromagnetic shielding material having a porous gradient structure according to claim 1, characterized in that: In S4, the gradient structure composite carbon fiber foam material is dried at 40-180° C. for 2-24 hours.

9. An electromagnetic shielding material having a porous gradient structure prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The invention comprises a dielectric layer, a lossy layer and a reflective layer stacked in sequence; wherein the dielectric layer, the lossy layer and the reflective layer are each composed of one or more layers of carbon fiber foam, and the carbon fiber content in each layer of carbon fiber foam is distributed in a gradient increasing manner from the dielectric layer to the reflective layer.

10. The method for preparing an electromagnetic shielding material having a porous gradient structure according to claim 9, characterized in that: The content of carbon fiber in the dielectric layer is 5-30 wt %, the content of carbon fiber in the lossy layer is 30-70 wt %, and the content of carbon fiber in the reflective layer is 70-80 wt %.

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