Carbon fiber-based wave-absorbing agent, preparation method thereof and wave-absorbing material

Abstract

The invention discloses a carbon fiber-based wave-absorbing agent, a preparation method thereof and a wave-absorbing material. The carbon fiber-based wave absorbing agent comprises carbon fibers and cobalt ferrite nanosheets, wherein the cobalt ferrite nanosheets are arranged on the surfaces of the carbon fibers. Compared with a single-component carbon fiber or ferrite wave-absorbing material, the carbon fiber-based wave-absorbing agent disclosed by the invention realizes uniform compounding and synergistic interaction of magnetic-dielectric components, effectively solves the problems of poor interface bonding, non-uniform dispersion of magnetic components and the like in a traditional carbon-based wave-absorbing material, simplifies the preparation process while improving the electromagnetic wave absorption performance, and is suitable for industrial production. The production cost is reduced, and the method has a good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorbing materials technology. Specifically, this invention relates to a carbon fiber-based microwave absorbing agent, its preparation method, and microwave absorbing materials. Background Technology

[0002] Carbon fiber and its composites, due to their lightweight, excellent mechanical properties, high corrosion resistance, and high electrical conductivity, can achieve integrated wave absorption and load-bearing, making them an important research direction in the field of wave-absorbing materials and possessing great application prospects. However, the impedance mismatch caused by the high dielectric constant of carbon fiber itself and the single electromagnetic wave loss mechanism result in weak wave absorption performance and a narrow effective absorption bandwidth for carbon fiber-based wave-absorbing materials, making them unsuitable for long-term service in complex electromagnetic environments for aerospace equipment. Therefore, improving the impedance matching characteristics of carbon fiber and introducing multiple electromagnetic wave loss mechanisms are key research focuses for overcoming the current performance bottlenecks of carbon fiber-based wave-absorbing materials. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention provides a carbon fiber-based microwave absorbing agent, its preparation method, and the microwave absorbing material, which solves the problems of impedance matching failure caused by the high dielectric constant of carbon fibers and the single electromagnetic wave loss mechanism.

[0004] The first aspect of the present invention provides a carbon fiber-based microwave absorbing agent, comprising carbon fiber and cobalt ferrite nanosheets, wherein the cobalt ferrite nanosheets are disposed on the surface of the carbon fiber.

[0005] Furthermore, the mass ratio of the carbon fiber to the cobalt ferrite nanosheet is 1:(0.2-0.8).

[0006] Furthermore, the molar ratio of cobalt to iron in the cobalt ferrite nanosheet is (2-8):1.

[0007] Furthermore, the cobalt ferrite nanosheets have a thickness of 20nm-60nm and a lateral dimension of 0.2μm-3μm.

[0008] Furthermore, the BET specific surface area of ​​the carbon fiber-based microwave absorbing agent is 5 m². 2 / g-30 m 2 / g.

[0009] A second aspect of this invention provides a method for preparing a carbon fiber-based microwave absorber, comprising: (1) Immerse the carbon fiber in a metal salt mixture containing soluble cobalt salt and soluble iron salt, take it out and dry it to obtain carbon fiber with cobalt and iron active sites on the surface. (2) Mix the metal salt mixture described in step (1) with an alkaline solution to obtain a cobalt-iron layered bimetallic hydroxide seed crystal mixture; (3) The carbon fibers with cobalt and iron active sites on their surface are immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture to obtain a carbon fiber / cobalt-iron layered bimetallic hydroxide composite material. (4) The carbon fiber / cobalt iron layered bimetallic hydroxide composite material is heat-treated in air atmosphere to obtain the carbon fiber-based microwave absorber.

[0010] Further, in step (1), the concentration of soluble cobalt salt in the metal salt mixture is 0.025-0.07 mol / L, and the concentration of soluble iron salt is 0.008-0.05 mol / L; and / or the drying and dehydration temperature is not lower than 65 ℃; and / or the carbon fiber is pre-anodized before immersing it in the metal salt mixture containing soluble cobalt salt and soluble iron salt.

[0011] Further, in step (2), the concentration of the alkaline solution is 0.3-0.6 mol / L; and / or the alkaline solution includes at least one of dimethylimidazole, ammonia, sodium hydroxide, and sodium bicarbonate.

[0012] Furthermore, in step (3), the carbon fiber with cobalt and iron active sites on its surface is immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture in a sealed hydrothermal environment with a hydrothermal temperature of 35℃-95℃ and an immersion time of 6-24 h.

[0013] Furthermore, in step (4), the heat treatment temperature is 400-600℃, the heating rate is 2-10℃ / min, and the holding time is 1-2h.

[0014] A third aspect of the present invention provides a microwave absorbing material, wherein the microwave absorbing material comprises the carbon fiber-based microwave absorbing agent described in the first aspect of the present invention or the carbon fiber-based microwave absorbing agent obtained by the method described in the second aspect of the present invention.

[0015] Compared with existing technologies, the carbon fiber-based microwave absorbing agent of this invention comprises carbon fiber and cobalt ferrite nanosheets. The cobalt ferrite nanosheets are disposed on the surface of the carbon fiber. The cobalt ferrite nanosheets have high porosity and a large specific surface area. Combining them with carbon fiber can generate dipole polarization and defect polarization, which is beneficial for electromagnetic wave absorption and can effectively improve the impedance matching characteristics of the absorbing agent. The successful construction of cobalt ferrite nanosheets on the carbon fiber surface introduces magnetic loss. Under the synergistic effect of magnetic and dielectric losses, the electromagnetic wave absorption capacity of the absorbing agent is significantly improved. Compared with single-component carbon fiber or ferrite microwave absorbing materials, the carbon fiber-based microwave absorbing agent of this invention achieves uniform composite and synergistic effect of magnetic and dielectric components, effectively solving the problems of poor interfacial bonding and uneven dispersion of magnetic components in traditional carbon-based microwave absorbing materials. While improving electromagnetic wave absorption performance, it simplifies the preparation process, reduces production costs, and has good application prospects. Attached Figure Description

[0016] Figure 1 This is a SEM image of the carbon fiber-based microwave absorber from Example 1; Figure 2 The image shows the SEM image of the microwave absorber obtained in Comparative Example 1. Figure 3 The image shows the SEM image of the microwave absorber obtained in Comparative Example 2. Figure 4 These are the XRD patterns of the microwave absorbers obtained in Examples 1-5; Figure 5 The XRD patterns of the microwave absorbers obtained in Comparative Examples 1-2 are shown. Figure 6 These are the impedance matching characteristics of the absorbing agents obtained in Examples 1-5 and Comparative Examples 1-2. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. The following examples are merely descriptive and not limiting, and should not be used to limit the scope of protection of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0018] The first aspect of the present invention provides a carbon fiber-based microwave absorbing agent, the carbon fiber-based microwave absorbing agent comprising carbon fibers and cobalt ferrite nanosheets, wherein the cobalt ferrite nanosheets are disposed on the surface of the carbon fibers.

[0019] The cobalt ferrite nanosheets described in this invention are disposed on the surface of the carbon fiber. The cobalt ferrite nanosheets have the characteristics of high porosity and large specific surface area. When combined with carbon fiber, they form a rich heterogeneous interface and porous structure, thereby inducing dipole polarization and defect polarization, and enhancing the dielectric loss of the material. In addition, the introduction of magnetic cobalt ferrite effectively modulates the overall electromagnetic parameters of the composite material, improves the impedance matching characteristics, and allows more incident electromagnetic waves to enter the interior of the material instead of being reflected by the surface.

[0020] Compared to traditional single iron oxide and cobalt oxide microwave absorbers, the cobalt ferrite composite structure of this invention exhibits significant advantages. While single iron oxide possesses high saturation magnetization, its excessively high dielectric constant easily leads to impedance mismatch, and its low magnetocrystalline anisotropy limits magnetic loss capability. Single cobalt oxide, although possessing extremely high magnetocrystalline anisotropy, has very low saturation magnetization at room temperature, relying primarily on dielectric loss to contribute to absorption efficiency, making it difficult to achieve high-efficiency magnetic loss. The spinel-type cobalt ferrite used in this invention integrates both iron and cobalt ions in its lattice, forming a natural magneto-dielectric synergistic system. Specifically, cobalt ions contribute strong magnetocrystalline anisotropy, providing a powerful driving force for magnetic loss; while iron ions provide high saturation magnetization, ensuring sufficient magnetic moment response. Furthermore, the electronic transition processes between multivalent ions in the spinel lattice also contribute additional dielectric relaxation loss. Therefore, the cobalt ferrite achieves a balance between the two loss mechanisms, which is the fundamental reason why its absorption performance is superior to that of single cobalt oxide and iron oxide.

[0021] According to an embodiment of the present invention, the mass ratio of the carbon fiber to the cobalt ferrite nanosheets in the above-mentioned microwave absorbing agent is 1:(0.2-0.8). The inventors have found that if the content of cobalt ferrite nanosheets is too high, it will lead to excessive stacking of the nanosheets, destroying their original high specific surface area, resulting in a sharp drop in polarization loss capability and potentially causing an imbalance in electromagnetic parameters; if the content of cobalt ferrite nanosheets is too low, the contribution of the microwave absorbing agent to magnetic loss and polarization loss is weak, making it difficult for electromagnetic waves to be effectively attenuated. Therefore, by using the mass ratio of carbon fiber to cobalt ferrite nanosheets within the above-mentioned range in the microwave absorbing agent of the present invention, the impedance matching characteristics of the microwave absorbing agent can be improved while simultaneously increasing magnetic loss, thereby enhancing the electromagnetic wave absorption capability of the microwave absorbing agent.

[0022] According to an embodiment of the present invention, the molar ratio of cobalt to iron in the above-mentioned cobalt ferrite nanosheets is (2-8):1. In this invention, the cobalt ferrite nanosheets are obtained by heat treatment conversion of cobalt-iron layered bimetallic hydroxide. The molar ratio of cobalt to iron has a decisive influence on the structural stability and thermal conversion morphology of the cobalt-iron layered bimetallic hydroxide precursor, thereby precisely controlling the microstructure and electromagnetic properties of the final cobalt ferrite nanosheets.

[0023] When the cobalt content is too high, the lamellar structure of cobalt-iron layered bimetallic hydroxides becomes unstable, easily forming non-layered hydroxide impurities. This disrupts the high aspect ratio nanosheet morphology of the precursor, leading to uneven coverage on the carbon fiber surface and inducing crystallinity degradation and particle agglomeration during heat treatment, forming non-ideal dense aggregates and thus losing the polarization loss advantage brought by the high specific surface area. When the iron content in the precursor is too high, the structural stability of cobalt-iron layered bimetallic hydroxides decreases, potentially leading to the formation of impurities such as ferrite or amorphous iron hydroxides. In subsequent heat treatment, due to insufficient cobalt content, the magnetocrystalline anisotropy of the final spinel-type cobalt ferrite is significantly weakened, resulting in insufficient magnetic loss capability. Simultaneously, electromagnetic parameter imbalance severely damages impedance matching characteristics.

[0024] According to embodiments of the present invention, the thickness of the carbon fiber and the cobalt ferrite nanosheets in the above-mentioned microwave absorber is 20-60 nm, and the lateral dimension is 0.2-3 μm. The inventors have found that if the layer thickness is too small, the nanosheet structure is prone to curling or fusion during heat treatment, leading to a decrease in specific surface area and a significant reduction in interfacial polarization sites; if the layer thickness is too large, the rigidity of the nanosheets increases, resulting in poor adhesion stability on the carbon fiber surface, making them prone to peeling during service, and excessively thick sheets are not conducive to forming an effective electromagnetic wave multiple reflection channel. On the other hand, if the lateral dimension is too small, it is difficult to form a continuous coverage on the carbon fiber surface, with some carbon fibers directly exposed to the electromagnetic field, causing impedance mismatch and easy surface reflection of incident waves; if the lateral dimension is too large, the stress concentration at the nanosheet edges intensifies, making them prone to detachment from the matrix, and excessively large sheets are prone to stacking and agglomeration, hindering electromagnetic waves from entering the material interior. Therefore, by controlling the thickness and lateral dimensions of cobalt ferrite nanosheets within the aforementioned range, the unique interfacial polarization and multiple scattering effects of two-dimensional nanostructures can be fully utilized while maintaining their structural stability and high specific surface area, thereby synergistically optimizing the impedance matching characteristics and electromagnetic wave attenuation capability of the absorber.

[0025] Furthermore, the BET specific surface area of ​​the carbon fiber-based microwave absorbing agent is 5 m². 2 / g-30 m 2 / g. Therefore, by using cobalt ferrite nanosheets with a large specific surface area, dipole polarization and defect polarization can be generated, which is beneficial for electromagnetic wave absorption and can effectively improve the impedance matching characteristics of the absorber.

[0026] A second aspect of the present invention provides a method for preparing the above-mentioned carbon fiber-based microwave absorbing agent. According to an embodiment of the present invention, the method includes: S1: Immerse the carbon fiber in a mixture of metal salts containing soluble cobalt salt and soluble iron salt. In this step, soluble cobalt salt and soluble iron salt are dissolved in deionized water and stirred until homogeneous to obtain a clear metal salt mixture. Carbon fibers are then immersed in this mixture, removed, and dried to obtain carbon fibers with a surface rich in cobalt and iron active sites. Specifically, soluble cobalt salt and soluble iron salt are dissolved in deionized water. The solution is sealed and stirred thoroughly at room temperature to obtain a transparent and clear metal salt mixture. Carbon fibers are then immersed in the metal salt mixture and allowed to stand for 2-4 hours, allowing metal ions to diffuse and adsorb onto the carbon fiber surface through electrostatic adsorption. The carbon fibers are then removed and dried in an oven to obtain carbon fibers with a surface rich in cobalt and iron active sites.

[0027] Furthermore, the concentration of soluble cobalt salt in the metal salt mixture is 0.025-0.07 mol / L, and the concentration of soluble iron salt is 0.008-0.05 mol / L.

[0028] According to an embodiment of the present invention, before immersing the carbon fiber in the metal salt mixture, the carbon fiber is pre-anodized to introduce active functional groups on the surface of the carbon fiber and further increase the surface roughness, thereby enhancing the anchoring effect of metal ions on the fiber surface and the bonding force between cobalt ferrite nanosheets and carbon fiber. For example, using continuous carbon fiber (T300) as the anode, a graphite plate as the cathode, and an ammonium bicarbonate aqueous solution as the electrolyte, an electric field is applied to the carbon fiber for electrolytic oxidation treatment. The electrolysis time is 70-150 s, and the current density is 1-3 A / m. 2 .

[0029] According to an embodiment of the present invention, the drying temperature in the oven is not lower than 65 °C, which can effectively remove moisture from the surface of the carbon fiber and further enhance the anchoring effect between the carbon fiber and metal ions, providing active reaction sites for the subsequent heterogeneous nucleation and epitaxial growth of cobalt-iron layered bimetallic hydroxide nanosheets.

[0030] It should be noted that those skilled in the art can select soluble cobalt salts and soluble iron salts according to actual needs. For example, soluble cobalt salts include at least one of cobalt nitrate hexahydrate, cobalt sulfate heptahydrate, and cobalt acetate tetrahydrate; soluble iron salts include at least one of ferrous sulfate heptahydrate and ferrous chloride tetrahydrate.

[0031] S2: Mix the metal salt mixture from step S1 with the alkaline solution. According to an embodiment of the present invention, the alkali is dissolved in deionized water and stirred evenly. The metal salt mixed solution is poured into the obtained alkali solution, and the stirring speed is controlled at 500-850 r / min and the stirring time is 5-15 min. After thorough stirring, a mixed solution containing cobalt-iron layered bimetallic hydroxide seed crystals is obtained.

[0032] According to an embodiment of the present invention, the concentration of the alkaline solution is 0.3-0.6 mol / L. Therefore, using a metal salt mixture of this concentration can effectively control the size of cobalt-iron layered bimetallic hydroxide nanosheets, thereby facilitating the extension of the electromagnetic wave transmission path in the absorbing agent.

[0033] Furthermore, the alkaline solution comprises at least one of dimethylimidazole, ammonia, sodium hydroxide, and sodium bicarbonate.

[0034] S3: Immerse carbon fibers with cobalt and iron active sites on their surface into a mixture of cobalt-iron layered bimetallic hydroxide seed crystals. In this step, the carbon fibers with cobalt and iron active sites on the surface obtained in step S1 are immersed in a mixture of cobalt-iron layered bimetallic hydroxide seed crystals, the container is sealed and left to stand, and cobalt-iron layered bimetallic hydroxide nanosheets are grown on the surface of the carbon fibers. After the reaction, the carbon fibers are washed and dried to obtain a carbon fiber / cobalt-iron layered bimetallic hydroxide composite material.

[0035] Taking an alkaline solution of dimethylimidazole aqueous solution as an example, dimethylimidazole undergoes a proton transfer reaction in aqueous solution and gradually releases OH-. - This provides an alkaline environment for the growth of cobalt-iron layered bimetallic hydroxide nanosheets, with Co in aqueous solution... 2+ and Fe 2+ In OH - Under the influence of the action, a hydrolysis reaction occurs, during which some Fe... 2+ Gradually oxidized to Fe 3+ The hydroxides Co(OH)2 and Fe(OH)3 generated by the hydrolysis of metal ions on the carbon fiber surface undergo a co-precipitation reaction and are transmitted through the shared OH groups at the edges. - The six-membered ring structure initially forms the LDH laminations. The nitrogen-containing groups of dimethylimidazole coordinate with the metal ions in the laminations, regulating the charge density. Simultaneously, its molecular size may partially embed into the interlayer, interacting with water molecules and SO42-. 2- The LDH structure is stabilized by the co-stabilization process, and the resulting LDH layers are further expanded into two-dimensional nanosheet structures through the Ostwald ripening process.

[0036] The specific reaction process is as follows: C5H8N2+H2O [C5H7N2] + +OH - 4Fe 2+ +O2+2H2O+8OH - →4Fe(OH)3↓ Co 2+ +2OH - →Co(OH)2↓ {(1-x)[Co(OH)6]4- +x[Fe(OH)6] 3-}+2xSO4 2- +mH2O→ [Co 2+ 1-x Fe 3+ x (OH)2] x+ ·[2xSO4 2- ]·mH2O+4OH - According to an embodiment of the present invention, the carbon fibers with cobalt and iron active sites on their surface are immersed in a mixture of cobalt-iron layered bimetallic hydroxide seed crystals in a sealed hydrothermal environment at a temperature of 35 ℃-95 ℃ for 6-24 h. This effectively controls the degree of crystallinity and microstructure of the cobalt-iron layered bimetallic hydroxide nanosheets.

[0037] S4: Heat-treat the carbon fiber / cobalt-iron layered bimetallic hydroxide composite material in air atmosphere. In this step, carbon fiber / cobalt-iron layered bimetallic hydroxide composite material (i.e., carbon fiber with cobalt-iron layered bimetallic hydroxide nanosheets loaded on its surface) is heat-treated in an oxygen atmosphere to obtain cobalt ferrite nanosheets. Specifically, the carbon fiber with cobalt-iron layered bimetallic hydroxide nanosheets loaded on its surface, obtained after dehydration and drying, is placed in a muffle furnace and heat-treated in an air atmosphere to cause a topological transformation of the cobalt-iron layered bimetallic hydroxide nanosheets to generate cobalt ferrite nanosheets.

[0038] According to an embodiment of the present invention, the dehydrated and dried carbon fibers with cobalt-iron layered bimetallic hydroxide nanosheets loaded on their surface are laid flat in a quartz boat and placed in a muffle furnace for heat treatment in an air atmosphere. Specifically, the heat treatment temperature in an air atmosphere is 350-600 °C, the heating rate is 2-10 °C / min, and the holding time is 1-2 h. Thus, this heat treatment stage mainly involves the removal of adsorbed water and interlayer water, as well as the condensation of some hydroxyl groups. Under these heat treatment conditions, the cobalt-iron layered bimetallic hydroxide nanosheets can be completely transformed into cobalt ferrite nanosheets with a spinel structure, and the sintering, agglomeration, or structural collapse of the nanosheet layers can be avoided, which may damage their two-dimensional microstructure.

[0039] According to the method of this invention, not only can structurally stable and controllable cobalt-iron layered bimetallic hydroxide nanosheets be prepared simply and efficiently, but magnetically tunable cobalt ferrite nanosheets can also be obtained through a simple one-step heat treatment method. This effectively solves the problem of poor microwave absorption performance caused by the high conductivity and impedance mismatch of carbon fibers. Specifically, the microwave absorbing agent of this invention can effectively introduce magnetic loss and balance the contributions of magnetic loss and dielectric loss within the absorbing agent. The cobalt ferrite nanosheets with high porosity and large specific surface area can extend the transmission path of electromagnetic waves within the absorbing agent and optimize the impedance matching degree of the absorbing agent. The minimum reflection loss value of the microwave absorbing agent of this invention can reach -66.84 dB, and the maximum effective absorption bandwidth is 4.60 GHz, thus demonstrating that the absorbing agent has excellent microwave absorption performance.

[0040] A third aspect of this invention provides a microwave absorbing material, comprising the carbon fiber-based microwave absorbing agent described in the first aspect of this invention or a carbon fiber-based microwave absorbing agent obtained by the method described in the second aspect of this invention. It should be noted that the features and advantages described above for the carbon fiber-based microwave absorbing agent and its preparation method also apply to this microwave absorbing material, and will not be repeated here.

[0041] The present invention will now be described with reference to specific embodiments. It should be noted that these embodiments are merely descriptive and do not limit the present invention in any way.

[0042] Example 1 (1) Dissolve 1.25 mmol of ferrous sulfate heptahydrate and 2.5 mmol of cobalt nitrate hexahydrate in 50 mL of deionized water. Stir the metal salts until dissolved to obtain a metal salt mixture. Anodized T300 carbon fiber (anode is T300 carbon fiber, cathode is graphite plate, electrolyte solution is ammonium bicarbonate (7 wt%), electrolysis time is 100 s, current density is 1.5 A / m 2 The carbon fibers are immersed in a metal salt mixture, the container is sealed, and left to stand at room temperature for 2 hours. After the carbon fibers are removed, they are dried and dehydrated in an oven at a temperature of not less than 65°C to obtain carbon fibers with cobalt and iron active sites on the surface. (2) Dissolve 30 mmol of dimethylimidazole in 50 ml of deionized water. After stirring and dissolving, slowly pour the metal salt mixture from step (1) into the dimethylimidazole aqueous solution and stir. The stirring speed is 800 r / min and the stirring time is 10 min to obtain a mixture containing cobalt-iron layered bimetallic hydroxide seeds. (3) The carbon fiber with cobalt and iron active sites on the surface obtained in step (1) was immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture obtained in step (2). Then the mixture and carbon fiber were transferred to a round bottom flask and heated in an oil bath at 95 °C for 10 h. Then the carbon fiber was taken out and rinsed three times with deionized water and anhydrous ethanol respectively. After dehydration and drying, carbon fiber with cobalt-iron layered bimetallic hydroxide nanosheets loaded on the surface was obtained. (4) The carbon fibers with cobalt-iron layered bimetallic hydroxide nanosheets obtained in step (3) are placed in a muffle furnace and heat-treated in an air atmosphere. The temperature is increased to 400 ℃ at a heating rate of 5 ℃ / min, and then kept at this temperature for 1 h to obtain the microwave absorber.

[0043] Example 2 (1) Dissolve 0.75 mmol of ferrous sulfate heptahydrate and 3 mmol of cobalt nitrate hexahydrate in 50 mL of deionized water. Stir the metal salts until dissolved to obtain a metal salt mixture. Anodized T300 carbon fiber (anode is T300 carbon fiber, cathode is graphite plate, electrolyte solution is ammonium bicarbonate (concentration is 7 wt%), electrolysis time is 100 s, current density is 1.5 A / m 2 The carbon fibers are immersed in a metal salt mixture, the container is sealed, and left to stand at room temperature for 2 hours. After the carbon fibers are removed, they are dried and dehydrated in an oven at a temperature of not less than 65°C to obtain carbon fibers with cobalt and iron active sites on the surface. (2) Dissolve 30 mmol of dimethylimidazole in 50 ml of deionized water. After stirring and dissolving, slowly pour the metal mixture from step (1) into the dimethylimidazole aqueous solution and stir. The stirring speed is 750 r / min and the stirring time is 15 min to obtain a mixture containing cobalt-iron layered bimetallic hydroxide seeds. (3) The carbon fiber with cobalt and iron active sites on the surface obtained in step (1) was immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture obtained in step (2). Then the mixture and carbon fiber were transferred to a round bottom flask and heated in an oil bath at 95 °C for 6 h. Then the carbon fiber was taken out and rinsed three times with deionized water and anhydrous ethanol respectively. After dehydration and drying, carbon fiber with cobalt-iron layered bimetallic hydroxide nanosheets loaded on the surface was obtained. (4) The carbon fibers with cobalt-iron layered bimetallic hydroxide nanosheets obtained in step (3) are placed in a muffle furnace and heat-treated in an air atmosphere. The temperature is increased to 450 °C at a heating rate of 10 °C / min, and then kept at this temperature for 1 h to obtain the microwave absorber. Example 3 (1) Dissolve 0.54 mmol of ferrous sulfate heptahydrate and 3.21 mmol of cobalt nitrate hexahydrate in 50 mL of deionized water. Stir the metal salts until dissolved to obtain a metal salt mixture. Anodized T300 carbon fiber (anode is T300 carbon fiber, cathode is graphite plate, electrolyte solution is ammonium bicarbonate (concentration is 7 wt%), electrolysis time is 100 s, current density is 1.5 A / m 2 The carbon fibers are immersed in a metal salt mixture, the container is sealed, and left to stand at room temperature for 1 hour. After the carbon fibers are removed, they are dried and dehydrated in an oven at a temperature of not less than 65°C to obtain carbon fibers with cobalt and iron active sites on the surface. (2) Dissolve 30 mmol of dimethylimidazole in 50 ml of deionized water. After stirring and dissolving, slowly pour the metal salt mixture from step (1) into the dimethylimidazole aqueous solution and stir. The stirring speed is 700 r / min and the stirring time is 10 min to obtain a mixture containing cobalt-iron layered bimetallic hydroxide seeds. (3) The carbon fiber with cobalt and iron active sites on the surface obtained in step (1) was immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture obtained in step (2). Then the mixture and carbon fiber were transferred to a round bottom flask and heated in an oil bath at 95 °C for 24 h. The carbon fiber was then removed and rinsed three times with deionized water and anhydrous ethanol. After dehydration and drying, carbon fiber with cobalt-iron layered bimetallic hydroxide nanosheets loaded on the surface was obtained. (4) The carbon fibers with cobalt-iron layered bimetallic hydroxide nanosheets obtained in step (3) are placed in a muffle furnace and heat-treated in an air atmosphere. The temperature is increased to 500 °C at a heating rate of 2 °C / min, and then kept at this temperature for 1 h to obtain the microwave absorber. Example 4 (1) Dissolve 1.25 mmol of ferrous sulfate heptahydrate and 2.5 mmol of cobalt nitrate hexahydrate in 50 mL of deionized water. Stir the metal salts until dissolved to obtain a metal salt mixture. Anodized T300 carbon fiber (anode is T300 carbon fiber, cathode is graphite plate, electrolyte solution is ammonium bicarbonate (concentration is 7 wt%), electrolysis time is 100 s, current density is 1.5 A / m 2 The carbon fibers are immersed in a metal salt mixture, the container is sealed, and left to stand at room temperature for 2 hours. After the carbon fibers are removed, they are dried and dehydrated in an oven at a temperature of not less than 65°C to obtain carbon fibers with cobalt and iron active sites on the surface. (2) Dissolve 30 mmol of dimethylimidazole in 30 ml of deionized water. After stirring and dissolving, slowly pour the metal salt solution from step (1) into the dimethylimidazole aqueous solution and mix and stir. The stirring speed is 750 r / min and the stirring time is 10 min to obtain a mixed solution containing cobalt-iron layered bimetallic hydroxide seeds. (3) The carbon fiber with cobalt and iron active sites on the surface obtained in step (1) was immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture obtained in step (2). Then the mixture and carbon fiber were transferred to a round bottom flask and heated in an oil bath at 65 °C for 10 h. Then the carbon fiber was taken out and rinsed three times with deionized water and anhydrous ethanol respectively. After dehydration and drying, carbon fiber with cobalt-iron layered bimetallic hydroxide nanosheets loaded on the surface was obtained. (4) The carbon fibers with cobalt-iron layered bimetallic hydroxide nanosheets obtained in step (3) are placed in a muffle furnace and heat-treated in an air atmosphere. The temperature is increased to 400 °C at a heating rate of 2 °C / min, and then kept at this temperature for 2 h to obtain the microwave absorber. Example 5 (1) Dissolve 1.25 mmol of ferrous sulfate heptahydrate and 2.5 mmol of cobalt nitrate hexahydrate in 50 mL of deionized water. Stir the metal salts until dissolved to obtain a metal salt mixture. Anodized T300 carbon fiber (anode is T300 carbon fiber, cathode is graphite plate, electrolyte solution is ammonium bicarbonate (concentration is 7 wt%), electrolysis time is 100 s, current density is 1.5 A / m 2 The carbon fibers are immersed in a metal salt mixture, the container is sealed, and left to stand at room temperature for 2 hours. After the carbon fibers are removed, they are dried and dehydrated in an oven at a temperature of not less than 65°C to obtain carbon fibers with cobalt and iron active sites on the surface. (2) Dissolve 30 mmol of dimethylimidazole in 30 ml of deionized water. After stirring and dissolving, slowly pour the metal salt mixture from step (1) into the dimethylimidazole aqueous solution and stir. The stirring speed is 750 r / min and the stirring time is 10 min to obtain a mixture containing cobalt-iron layered bimetallic hydroxide seeds. (3) The carbon fiber with cobalt and iron active sites on the surface obtained in step (1) was immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture obtained in step (2). Then the mixture and carbon fiber were placed in a 35 ℃ shaking water bath and heated for 10 h. Then the carbon fiber was taken out and rinsed three times with deionized water and anhydrous ethanol respectively. After dehydration and drying, carbon fiber with cobalt-iron layered bimetallic hydroxide nanosheets loaded on the surface was obtained. (4) The carbon fibers with cobalt-iron layered bimetallic hydroxide nanosheets obtained in step (3) are placed in a muffle furnace and heat-treated in an air atmosphere. The temperature is increased to 400 ℃ at a heating rate of 5 ℃ / min, and then kept at this temperature for 1 h to obtain the microwave absorber.

[0044] Comparative Example 1 (1) 3.75 mmol of cobalt nitrate hexahydrate was dissolved in 50 mL of deionized water. The metal salt was stirred until dissolved to obtain a metal salt mixture. Anodized T300 carbon fiber (anode: T300 carbon fiber; cathode: graphite plate; electrolyte solution: ammonium bicarbonate (concentration: 7 wt%)) was then electrolyzed for 100 s at a current density of 1.5 A / m 2 The carbon fibers are immersed in a metal salt mixture, the container is sealed, and left to stand at room temperature for 2 hours. After the carbon fibers are removed, they are dried and dehydrated in an oven at a temperature of not less than 65°C to obtain carbon fibers with cobalt-rich active sites on the surface. (2) Dissolve 30 mmol of dimethylimidazole in 50 ml of deionized water. After stirring and dissolving, slowly pour the metal salt solution from step (1) into the dimethylimidazole aqueous solution and mix and stir. The stirring speed is 850 r / min and the stirring time is 10 min to obtain a mixed solution containing cobalt hydroxide seeds. (3) The carbon fiber with cobalt active sites on the surface obtained in step (1) was immersed in the cobalt hydroxide seed mixture obtained in step (2). Then the mixture and carbon fiber were transferred to a round bottom flask and heated in an oil bath at 95 °C for 10 h. Then the carbon fiber was taken out and rinsed three times with deionized water and anhydrous ethanol respectively. After dehydration and drying, carbon fiber with cobalt hydroxide loaded on the surface was obtained. (4) The carbon fibers with cobalt hydroxide loaded on the surface obtained in step (3) are placed in a muffle furnace and heat-treated in an air atmosphere. The temperature is increased to 450 °C at a heating rate of 5 °C / min, and then kept at this temperature for 1 h to obtain the microwave absorber. Comparative Example 2 (1) 3.75 mmol of ferrous sulfate heptahydrate was dissolved in 50 mL of deionized water. The metal salt was stirred until dissolved to obtain a metal salt mixture. Anodized T300 carbon fiber (anode: T300 carbon fiber; cathode: graphite plate; electrolyte solution: ammonium bicarbonate (concentration: 7 wt%)) was then used for electrolysis for 100 s at a current density of 1.5 A / m 2 The carbon fibers are immersed in a metal salt mixture, the container is sealed, and left to stand at room temperature for 2 hours. After the carbon fibers are taken out, they are dried and dehydrated in an oven at a temperature of not less than 65°C to obtain carbon fibers with iron-rich active sites on the surface. (2) Dissolve 30 mmol of dimethylimidazole in 50 ml of deionized water. After stirring and dissolving, slowly pour the metal salt solution from step (1) into the dimethylimidazole aqueous solution and mix and stir. The stirring speed is 850 r / min and the stirring time is 10 min to obtain a mixed solution containing iron hydroxide seeds. (3) The carbon fiber with iron active sites on the surface obtained in step (1) was immersed in the iron hydroxide seed mixture obtained in step (2). Then the mixture and carbon fiber were transferred to a round bottom flask and heated in an oil bath at 95 °C for 10 h. Then the carbon fiber was taken out and rinsed three times with deionized water and anhydrous ethanol respectively. After dehydration and drying, carbon fiber with iron hydroxide loaded on the surface was obtained. The carbon fibers with iron hydroxide loaded on the surface obtained in step (3) were placed in a muffle furnace and heat-treated in an air atmosphere. The temperature was increased to 450 °C at a heating rate of 5 °C / min, and then held at this temperature for 1 h to obtain the microwave absorber.

[0045] The thickness, lateral dimensions, and specific surface area of ​​the cobalt ferrite nanosheets in the microwave absorbers of Examples 1-5 were characterized, and the test data are shown in Table 1. Methods for testing the thickness and lateral dimensions of cobalt ferrite nanosheets: Nanosheet dimensions were measured using ImageJ software on SEM images of the samples: First, the SEM images were calibrated. The "Line Tool" was selected to draw a line segment along the clear edge of the nanosheet to measure its thickness; the "Line Segment Tool" or "Freeform Tool" was selected to measure the transverse diameter along the direction of maximum extension of the nanosheet. At least 20 nanosheets at random locations were measured for each dimension parameter, and the results were recorded as "mean ± standard deviation".

[0046] Specific surface area testing method for carbon fiber-based microwave absorbers: The specific surface area of ​​the samples was analyzed using a specific surface area and pore size analyzer (BET, ASAP2460). Experimental conditions included N2 adsorption, degassing temperature of 120 ℃, and degassing time of 8 h.

[0047] SEM testing was performed on the microwave absorber. Figure 1 The image shows the SEM image of the microwave absorber obtained in Example 1, which shows that a nanosheet structure is formed on its surface. Figure 2 Here is a SEM image of the microwave absorber obtained in Comparative Example 1. Figure 3 The image shows the SEM image of the absorbing agent obtained in Comparative Example 2. Figure 2 and 3 It can be seen that the absorbers obtained in Comparative Example 1 and Comparative Example 2 exhibited severe agglomeration on their surfaces, making it impossible to form a layered structure.

[0048] Table 1

[0049] The microwave absorbers obtained in Examples 1-5 and Comparative Examples 1-2 were subjected to XRD tests, and the test data are as follows: Figure 4 and 5 As shown: Depend on Figure 4 It can be seen that the (002) diffraction peak of carbon fiber can be observed at 2θ≈25.4° in all samples of Examples 1-5. In addition, characteristic diffraction peaks belonging to cobalt ferrite appear at 2θ≈35.5° and 43.1°, corresponding to its (311) and (400) crystal planes respectively, indicating that the cobalt-iron layered bimetallic hydroxide precursor has been transformed into cobalt ferrite through topological transformation. However, since carbon fiber is a strong scattering matrix, its broad and strong diffraction background masks the weak diffraction signals of other crystal planes of CoFe2O4, resulting in some characteristic peaks not being fully displayed. Figure 5 It can be seen that the sample of Comparative Example 1 exhibits XRD diffraction peaks of cobalt oxide at 2θ≈31.3°(220); 2θ≈36.8°(311); 2θ≈44.9°(400); 2θ≈59.6°(511); 2θ≈65.4°(440); Figure 5 It can be seen that the sample of Comparative Example 2 has XRD diffraction peaks of iron oxide at 2θ≈18.4°(111); 2θ≈30.1°(220); 2θ≈35.5°(311); 2θ≈43.1°(400); 2θ≈53.5°(422); 2θ≈57.3°(511); 2θ≈62.7°(440); ≈18.4°(111).

[0050] The microwave absorption performance and electromagnetic wave dissipation performance of the microwave absorbers obtained in Examples 1-5 and Comparative Examples 1-2 are characterized in Table 2.

[0051] Characterization methods for the reflection loss and maximum absorption bandwidth of the absorber: Measurements were performed using a vector network analyzer (VNA, Agilent E5071C) in the range of 2.0 ~ 18.0 GHz, employing the coaxial method. The test sample was prepared by uniformly mixing the absorber at 5 wt% in a paraffin matrix and compacting it into a coaxial ring with an outer diameter of 7.00 mm, an inner diameter of 3.04 mm, and a thickness of 2 mm.

[0052] The reflection loss (RL) of the absorbing material is

[0053] Among them, Z in Z0 and Z can represent the input impedance and the characteristic resistance of free space, respectively; f is the electromagnetic wave frequency; d is the thickness of the absorbing material; c is the propagation speed of the electromagnetic wave; ε r μ is the complex permittivity of the material. r denoted as ρ, where ρ is the complex permeability of the material.

[0054] RL is less than The frequency range corresponding to 10 dB is called the effective absorption bandwidth (EAB).

[0055] Table 2

[0056] The impedance matching characteristics of the absorbers obtained in Examples 1-5 and Comparative Examples 1-2 are characterized as follows: Figure 6 As shown.

[0057] Based on transmission line theory, the normalized input impedance (Z-parameters) of the composite material at a specific frequency and matching thickness is calculated using the scattering parameters (S-parameters) measured by a network vector analyzer. in The calculation formula is shown in Equation 2. The impedance matching capability is obtained by calculating its magnitude. The closer the Z value is to 1, the better the impedance matching characteristics and the less electromagnetic wave reflection. Generally, a Z value between 0.8 and 1.2 is considered to indicate that the material has good impedance matching characteristics.

[0058] In this invention, Examples 1-5 and Comparative Examples 1-2 all used the frequency and matching thickness corresponding to the minimum reflection loss value to calculate the impedance matching modulus of each sample. The pink shaded area represents impedance matching modulus values ​​between 0.8 and 1.2. The more the curve is surrounded by the pink area, the better the impedance matching characteristics of the sample. Figure 6 It can be seen that the graphs of the microwave absorbing agents in Examples 1-5 are surrounded by more pink areas than those in Comparative Examples 1-2, thus indicating that the carbon fiber-based microwave absorbing agent of the present invention has excellent impedance matching characteristics.

[0059] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A carbon fiber-based microwave absorbing agent, characterized in that, It includes carbon fibers and cobalt ferrite nanosheets, wherein the cobalt ferrite nanosheets are disposed on the surface of the carbon fibers.

2. The carbon fiber-based microwave absorber according to claim 1, characterized in that, The mass ratio of the carbon fiber to the cobalt ferrite nanosheet is 1:(0.2-0.8).

3. The carbon fiber-based microwave absorber according to claim 1 or 2, characterized in that, The molar ratio of cobalt to iron in the cobalt ferrite nanosheets is (2-8):

1.

4. The carbon fiber-based microwave absorbing agent according to claim 1 or 2, characterized in that, The cobalt ferrite nanosheets have a thickness of 20 nm-60 nm and a lateral dimension of 0.2 μm-3 μm; and / or The BET specific surface area of ​​the carbon fiber-based microwave absorber is 5 m². 2 / g-30 m 2 / g.

5. A method for preparing the carbon fiber-based microwave absorbing agent according to any one of claims 1-4, characterized in that, include: (1) Immerse the carbon fiber in a metal salt mixture containing soluble cobalt salt and soluble iron salt, take it out and dry it to obtain carbon fiber with cobalt and iron active sites on the surface. (2) Mix the metal salt mixture described in step (1) with an alkaline solution to obtain a cobalt-iron layered bimetallic hydroxide seed crystal mixture; (3) The carbon fibers with cobalt and iron active sites on their surface are immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture to obtain a carbon fiber / cobalt-iron layered bimetallic hydroxide composite material. (4) The carbon fiber / cobalt iron layered bimetallic hydroxide composite material is heat-treated in air atmosphere to obtain the carbon fiber-based microwave absorber.

6. The method according to claim 5, characterized in that, In step (1), the concentration of soluble cobalt salt in the metal salt mixture is 0.025-0.07 mol / L, and the concentration of soluble iron salt is 0.008-0.05 mol / L; and / or The drying and dehydration temperature shall not be lower than 65°C; and / or The carbon fibers are pre-anodized before being immersed in the metal salt mixture containing soluble cobalt salt and soluble iron salt.

7. The method according to claim 5 or 6, characterized in that, In step (2), the concentration of the alkaline solution is 0.3-0.6 mol / L; and / or The alkaline solution includes at least one of dimethylimidazole, ammonia, sodium hydroxide, and sodium bicarbonate.

8. The method according to claim 5 or 6, characterized in that, In step (3), the carbon fiber with cobalt and iron active sites on its surface is immersed in the cobalt-iron layered bimetallic hydroxide seed crystal mixture in a sealed hydrothermal environment with a temperature of 35℃-95℃ and an immersion time of 6-24 h.

9. The method according to claim 5 or 6, characterized in that, In step (4), the heat treatment temperature is 400-600℃, the heating rate is 2-10℃ / min, and the holding time is 1-2h.

10. A microwave absorbing material, characterized in that, The microwave absorbing material includes any one of the carbon fiber-based microwave absorbing agents according to claims 1-4 or carbon fiber-based microwave absorbing agents obtained by the method described in any one of claims 5-9.

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