Composite fiber absorbent and preparation method thereof

Abstract

The invention discloses a composite fiber absorbent and a preparation method thereof, and relates to the technical field of electromagnetic wave absorbing materials, the absorbent takes porous nitrogen-doped carbon fiber as a three-dimensional network skeleton, and porous high-entropy alloy nanoparticles are encapsulated in the porous nitrogen-doped carbon fiber and on the surface in situ. The porous high-entropy alloy nanoparticles are in a sheet shape; a flaky high-entropy alloy is designed to serve as a magnetic core, the Snoek limit is effectively broken through by means of the strong shape anisotropy of the flaky high-entropy alloy, and in order to obtain high magnetic conductivity and high magnetic loss under the low-frequency and thin-layer conditions, ultra-thin nitrogen-doped carbon fibers serve as a three-dimensional framework; on one hand, as a'magnetic conductivity protector ', metal particles are isolated, eddy current is inhibited, and the dielectric constant is reduced and accurately regulated; on the other hand, strong dielectric polarization loss is introduced by abundant heterogeneous interfaces and nitrogen doping defects; finally, the cooperation of magnetic loss and dielectric loss is realized, and the contradiction between impedance matching and high attenuation capability is solved.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic wave absorbing materials technology, specifically to a composite fiber absorber and its preparation method. Background Technology

[0002] With the rapid development of 5G communication, military stealth technology and 6G terahertz communication, the complex electromagnetic environment has placed more stringent requirements on absorbing materials. In response to these requirements, a variety of types of absorbers have been developed. However, traditional absorbers still face challenges in achieving the "thin, light, wide and strong" properties of absorbing materials. Ferrite absorbers are characterized by high high-frequency magnetic loss capability and low cost. CN202210866292.1 discloses a low-frequency radar absorber with strong weather resistance and its preparation method. This low-frequency radar absorber is composed of nickel-zinc ferrite as the main component, with small amounts of iron-silicon-aluminum and graphene doped. However, ferrite absorbers have the problem of high density, making it difficult to further improve the low-frequency magnetic permeability. Magnetic metal powders have high saturation magnetization and high magnetic permeability. CN201610965509.9 discloses an iron-silicon-aluminum alloy absorber, but its high dielectric constant leads to impedance mismatch, and it also has problems such as easy oxidation, high density, and poor corrosion resistance. Carbon material absorbers have the advantages of low density, adjustable conductivity, and easy formation of porous structures, but they lack intrinsic magnetic loss, and the excessively high dielectric constant leads to severe reflection. Furthermore, porous carbon materials such as carbon aerogels are difficult to apply to coatings because their own strength does not meet the requirements.

[0003] CN202210805672.4 and CN202411397694.7 disclose a core-shell structured absorbent, whose core is a sheet-like soft magnetic material, such as iron-silicon-aluminum, and whose outer shell is a metal oxide layer with a thickness of 200-300nm, or a composite layer of other materials. This patent still belongs to the modification of existing absorbents, and its metal oxide-carbon outer shell is prone to interfacial impedance mismatch.

[0004] To overcome the limitations of traditional materials, researchers have developed several novel absorbents, such as the FeCoNiCr absorber constructed by a research team at Dalian University of Technology through two-phase high-entropy engineering. 0.4 Cu 0.4 / FeCoNiCrCu3O4 composite material. Its structure is a composite of a magnetic high-entropy alloy and an insulating high-entropy oxide. While introducing magnetic loss, the oxide layer reduces the overall conductivity and improves impedance matching. The two phases generate double natural resonance peaks through different magnetocrystalline anisotropies, thus broadening and enhancing the magnetic loss capability. Research by Yancheng Institute of Technology and other institutions, through the design of DyFeMOFs precursors, derived Dy2O3 / Fe3C / N doped carbon composite materials, introducing the rare earth element Dy... 3+ Utilizing its high coordination properties and low electronegativity, Fe can optimize dielectric loss (promoting interfacial polarization and charge transfer) while simultaneously...3+ By ensuring magnetic loss, efficient synergy between the two was achieved. A strong absorption of 56.08 dB and a wide bandwidth of 5.12 GHz were achieved in a 1.76 mm thin layer. Patent CN120676613A discloses a FeCoNi-based high-entropy alloy electromagnetic wave absorber, which is a mixture of flake-like powders (FexCoyNizSiuTivCrw) with an aspect ratio greater than 20. Its dielectric constant has a real part <20 and an imaginary part <6, and its permeability has a real part >2.5 and an imaginary part >0.6, with an absorption peak at 0.63 GHz. This confirms the application potential of high-entropy alloys in the field of electromagnetic wave absorption. Patent CN118812921B discloses a FeNiHo / carbon nanofiber composite absorber, prepared by electrospinning combined with heat treatment. The FeNiHo alloy nanoparticles improve the impedance matching and electromagnetic wave attenuation capability of the carbon nanofibers. Existing research improves performance through morphology control. For example, using magnetron sputtering to prepare sheet-shaped absorbers with a large aspect ratio (up to 80), the SiO2 dielectric layer covering them helps to reduce the dielectric constant and improve impedance matching.

[0005] The University of Tokyo in Japan has developed a terahertz absorber with a thickness of only 48 micrometers using λTi3O5, which makes miniaturization of 6G communication equipment possible. In addition, smart absorbers and metamaterials are developing rapidly, but they still have difficulty solving practical problems in terms of wide-band response and ultra-thin design. At present, the main problem facing high-performance absorbers is to achieve wide-band absorption at low frequencies (such as 14GHz) with low thickness. According to the quarter-wavelength matching theory, this requires the material to have extremely high permeability at low frequencies, but traditional magnetic materials are limited by the Snoek limit and it is difficult to break through.

[0006] Based on the above problems, this invention proposes a composite fiber absorbent and its preparation method that can solve the above problems. Summary of the Invention

[0007] The purpose of this invention is to provide a composite fiber absorbent and its preparation method to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution:

[0009] A composite fiber absorbent, wherein the absorbent uses porous nitrogen-doped carbon fiber as a three-dimensional network framework, and porous high-entropy alloy nanoparticles are encapsulated in situ inside and on the surface of the porous nitrogen-doped carbon fiber, wherein the porous high-entropy alloy nanoparticles are in the form of sheets.

[0010] As a preferred technical solution, the porous high-entropy alloy nanoparticles are a pentagonal alloy system containing Fe, Co, Ni, Mn and Al; the molar ratio of its constituent elements is Fe:Co:Ni:Mn:Al=1-1.5:1-1.5:1-1.5:0.5-1:0.5-1.

[0011] As a preferred technical solution, the porous high-entropy alloy nanoparticles also contain rare earth elements accounting for 0.5%-3% of the molar number of their transition metals Fe+Co+Ni+Mn+Al, wherein the rare earth elements are at least one of La, Ce, and Y.

[0012] As a preferred technical solution, the aspect ratio of the high-entropy alloy nanoparticles is greater than 20, and the thickness is 20-100 nanometers.

[0013] As a preferred technical solution, the thickness of the nitrogen-doped carbon shell of the porous nitrogen-doped carbon fiber is 2-10 nanometers, and the nitrogen doping amount is 5-20 at.

[0014] A method for preparing a composite fiber absorbent is also provided, for preparing a composite fiber absorbent as described in any of the above claims, comprising the following steps:

[0015] S1. Preparation of sheet-like high-entropy alloy precursors: Sheet-like precursors composed of hydroxides or salts of Fe, Co, Ni, Mn, and Al are prepared by solvothermal method or coprecipitation method.

[0016] S2. Preparation of electrospinning precursor solution: The sheet-like precursor, polymer carbon source, and nitrogen source obtained in step S1 are dispersed in a solvent to form a uniform spinning solution.

[0017] S3. Electrospinning and pre-oxidation: The spinning solution obtained in step S2 is electrospinned to obtain a nanofiber membrane, which is then pre-oxidized and stabilized in an air atmosphere.

[0018] S4. High-temperature heat treatment: The pre-oxidized fiber membrane is subjected to high-temperature heat treatment in a protective atmosphere or an atmosphere containing a nitrogen source to complete carbonization, reduction and high-entropy alloying, and obtain the final product.

[0019] As a preferred technical solution, the solvothermal method includes the following steps:

[0020] (1) Raw material preparation:

[0021] Analytical grade metal chlorides were used as the metal source;

[0022] The metal sources include FeCl3·6H2O, CoCl2·6H2O, NiCl2·6H2O, MnCl2·4H2O, and AlCl3·6H2O. Each metal salt is accurately weighed according to the molar ratio of Fe:Co:Ni:Mn:Al = 1-1.5:1-1.5:1-1.5:0.5-1:0.5-1. The above metal salts are dissolved together in ethylene glycol or deionized water to prepare a mixed salt solution with a total metal ion concentration of 0.1-0.3 mol / L. In particular, using ethylene glycol as a solvent is more effective because ethylene glycol has a high boiling point and reducing properties, which is conducive to the formation of a plate-like precursor with good crystallinity and uniform morphology.

[0023] Polyvinylpyrrolidone (PVP) was added to the mixed salt solution as a morphology control agent. The mass ratio of PVP to total metal salt was 1:2-5. The solution was stirred vigorously until completely dissolved.

[0024] (2) Solvent thermal reaction: Transfer the mixed solution to a high-pressure reactor lined with polytetrafluoroethylene, with a filling degree of 70-80%. After sealing the reactor, place it in an oven and react at 160-200°C for 10-15 hours. After the reaction is completed, allow it to cool naturally to room temperature.

[0025] (3) Post-treatment: The reaction product is washed several times by alternating centrifugation with ethanol and deionized water until the supernatant is neutral to completely remove residual anions and polyvinylpyrrolidone. It is then dried in a vacuum drying oven at 60-80°C for 10-12 hours to obtain a sheet-like layered double hydroxide (LDH) or coprecipitated hydroxide precursor. This precursor has achieved uniform mixing of metal elements at the atomic scale.

[0026] As a preferred technical solution, the co-precipitation method includes the following steps:

[0027] (1) Raw material preparation:

[0028] Analytical grade metal chlorides were used as the metal source;

[0029] The metal sources include FeCl3·6H2O, CoCl2·6H2O, NiCl2·6H2O, MnCl2·4H2O, and AlCl3·6H2O. Each metal salt is accurately weighed according to the molar ratio of Fe:Co:Ni:Mn:Al = 1-1.5:1-1.5:1-1.5:0.5-1:0.5-1, and they are dissolved together in deionized water to prepare a mixed salt solution, solution A. A sodium carbonate or sodium hydroxide solution with a concentration of 1.0-2.0 mol / L is prepared as a precipitant, solution B.

[0030] (2) Co-precipitation: Under continuous stirring, solution A and solution B are simultaneously and slowly added dropwise to a reactor containing bottom water by double titration. The bottom water serves as the seed crystal medium. The pH value is controlled between 9 and 11 throughout the process, and the temperature is maintained at 50-70°C. After titration, the solution is aged for 5-10 hours under this temperature and stirring to ensure complete precipitate aging and crystallization.

[0031] (3) Post-processing: After the reaction is completed, the sample is filtered and washed with deionized water until no chloride ions are present. The sample is then dried at 80-100°C for 12-24 hours to obtain hydroxide or basic carbonate precursor. Chloride ions can be detected by AgNO3 solution.

[0032] As a preferred technical solution, rare earth elements are added in step S1, and the specific steps are as follows:

[0033] Weigh out rare earth metal salt Ce(NO3)3·6H2O according to the calculated amount of 0.5%-3% of the total transition metals Fe+Co+Ni+Mn+Al, dissolve it in deionized water to prepare a rare earth salt solution, and add the prepared rare earth salt solution to the prepared mixed salt solution of Fe, Co, Ni, Mn and Al.

[0034] Under the high temperature and high pressure environment of the solvothermal reaction in the solvothermal method, rare earth ions can be effectively embedded into the LDH layer or uniformly adsorbed on the surface of hydroxide particles.

[0035] Alternatively, during the coprecipitation process of the coprecipitation method, rare earth ions may coprecipitate with other metal ions or be adsorbed on the surface of newly generated hydroxide precipitates, achieving atomic-level uniform doping.

[0036] As a preferred technical solution, in step S2, the carbon source of the polymer is polyacrylonitrile, the nitrogen source is urea or melamine, and the mass ratio of the sheet-like precursor to PAN is (1-4:1).

[0037] As a preferred technical solution, in step S4, the high-temperature heat treatment temperature is 700℃-900℃, the heat treatment time is 13 hours, the protective atmosphere is argon or nitrogen, and the atmosphere containing the nitrogen source is ammonia or a mixture of argon and ammonia.

[0038] Compared with the prior art, the beneficial effects of the present invention are:

[0039] By designing sheet-like high-entropy alloys as magnetic cores and utilizing their strong shape anisotropy, the Snoek limit is effectively broken. To achieve high permeability and high magnetic loss under low-frequency and thin-layer conditions, ultrathin nitrogen-doped carbon fibers are used as a three-dimensional skeleton. On the one hand, it acts as a "permeability protector" to isolate metal particles, suppress eddy currents, and reduce and precisely control the dielectric constant. On the other hand, its rich heterogeneous interfaces and nitrogen-doped defects introduce strong dielectric polarization losses. Ultimately, the synergy between magnetic loss and dielectric loss is achieved, resolving the contradiction between impedance matching and high attenuation capability. Detailed Implementation

[0040] The following describes in detail a composite fiber absorbent and its preparation method according to embodiments of the present disclosure. To make the objectives, technical solutions, and advantages of the present disclosure clearer, the technical solutions in the embodiments of the present disclosure are described clearly and completely. Obviously, the described embodiments are only some embodiments of the present disclosure, and not all embodiments.

[0041] Therefore, the detailed description of embodiments of this disclosure provided below is not intended to limit the scope of the claimed disclosure, but merely represents selected embodiments of the disclosure. All other embodiments obtained by those skilled in the art based on the embodiments of this disclosure without inventive effort are within the scope of protection of this disclosure.

[0042] Example 1:

[0043] S1. Preparation of high-entropy alloy precursors via solvothermal method:

[0044] Using analytically pure metal chlorides as metal sources, including FeCl3·6H2O, CoCl2·6H2O, NiCl2·6H2O, MnCl2·4H2O, and AlCl3·6H2O, each metal salt was accurately weighed according to the molar ratio of Fe:Co:Ni:Mn:Al = 1:1:1:1:1. The above metal salts were dissolved together in ethylene glycol or deionized water to prepare a mixed salt solution with a total metal ion concentration of 0.25 mol / L. Polyvinylpyrrolidone (PVP) was added to the mixed salt solution as a morphology control agent, with a mass ratio of PVP to total metal salts of 1:3. The solution was stirred vigorously until completely dissolved.

[0045] The mixed solution was transferred to a polytetrafluoroethylene-lined high-pressure reactor with a filling degree of 75%. The reactor was sealed and placed in an oven. The reaction was carried out at 160°C for 12 hours. After the reaction was completed, the mixture was allowed to cool naturally to room temperature.

[0046] The reaction product was washed several times by alternating centrifugation with ethanol and deionized water until the supernatant was neutral to completely remove residual anions and polyvinylpyrrolidone. It was then dried in a vacuum drying oven at 80°C for 10 hours to obtain a sheet-like layered double hydroxide (LDH) or coprecipitated hydroxide precursor. This precursor has achieved a uniform mixing of metal elements at the atomic scale.

[0047] S2. Preparation of electrospinning precursor solution:

[0048] The obtained sheet-like precursor, polyacrylonitrile (PAN), and urea were dispersed in a DMF solution to form a uniform spinning solution. The mass ratio of the sheet-like precursor to polyacrylonitrile (PAN) was 3:2, and the amount of urea added was 10% of the mass of PAN.

[0049] S3, Electrospinning and Pre-oxidation:

[0050] Nanofiber membranes were obtained by electrospinning the spinning solution, and the nanofiber membranes were pre-oxidized and stabilized at 250℃.

[0051] S4. High-temperature heat treatment:

[0052] The pre-oxidized fiber membrane was subjected to high-temperature heat treatment, carbonization and reduction at 800℃ in a mixed atmosphere of argon and ammonia (95:5) for 2 hours to obtain the final product.

[0053] In the obtained product, the high-entropy alloy particles are in the form of plates and are uniformly embedded in nitrogen-doped carbon fibers with a fiber diameter of about 300 nm. The absorber exhibits excellent magnetic permeability and moderate dielectric constant at both 2 GHz and 10 GHz. When the filling amount is 30 wt%, the effective absorption bandwidth (RL≤10 dB) of 1 mm coating in the X-band (8-12 GHz) reaches 4.2 GHz.

[0054] Example 2:

[0055] S1. Preparation of high-entropy alloy precursors via co-precipitation:

[0056] Analytical-grade metal chlorides were used as metal sources, including FeCl3·6H2O, CoCl2·6H2O, NiCl2·6H2O, MnCl2·4H2O, and AlCl3·6H2O, in the formula Fe:Co:Ni:Mn:Al = Each metal salt was accurately weighed in a molar ratio of 1:1:1:0.5:1 and dissolved together in deionized water to prepare a mixed salt solution A with a total metal ion concentration of 0.1 mol / L. La(NO3)3·6H2O with a molar concentration of 0.002 mol / L and Dy(NO3)3·6H2O with a molar concentration of 0.001 mol / L were added, and a NaOH solution with a concentration of 1.1 mol / L was prepared as a precipitant solution B. Under continuous stirring, the precipitant solution B and the mixed salt solution A were simultaneously and slowly added dropwise to a reactor containing bottom water (equivalent to 20% of the volume of the mixed metal salt solution, as a seed medium) using a double titration method. The pH value was controlled between 9 and 11 throughout the process, and the temperature was maintained at 60 ± 5°C. After titration, aging was continued for 6 hours at this temperature and with stirring. After the reaction was completed, the solution was filtered and dried at 80°C for 12 hours to obtain the hydroxide precursor.

[0057] The other steps and conditions are the same as in Example 1.

[0058] Due to the reduced Mn content, the saturation magnetization of the alloy was improved. Compared with Example 1, the real part of the permeability (μ´) near 2 GHz increased from 5.1 to 5.5, but the dielectric loss at high frequencies (such as above 12 GHz) was reduced, and the effective absorption bandwidth of the 1 mm coating was 3.8 GHz.

[0059] Example 3:

[0060] Before preparing the precursor solution in step S1, Ce element, accounting for 2% of the total molar amount of transition metals, is introduced. Specifically, Ce(NO3)3·6H2O, a rare earth metal salt, is weighed and dissolved in deionized water according to a calculated amount of 2% of the total molar amount of transition metals Fe+Co+Ni+Mn+Al to prepare a rare earth salt solution. The prepared rare earth salt solution is then added to the prepared mixed salt solution of Fe, Co, Ni, Mn, and Al. Under the high temperature and high pressure environment of the solvothermal reaction in the solvothermal method, rare earth ions can be effectively embedded into the LDH layer or uniformly adsorbed on the surface of hydroxide particles. Other conditions are the same as in Example 1.

[0061] The introduction of rare earth element Ce refines the alloy grains and enhances the anisotropic field of the material. Although the saturation magnetization decreases slightly, the natural resonant frequency shifts to higher frequencies, and the magnetic loss (μ") in the Ku band (12-18 GHz) is increased by about 15%.

[0062] Example 4:

[0063] In step S1, the reaction was carried out at 200°C for 8 hours and stirred at 800 rpm to prepare a sheet-like precursor with an average thickness of about 50 nm and an aspect ratio of about 30; other conditions were the same as in Example 1.

[0064] The slightly larger sheet-like particles obtained in this embodiment maintain good shape anisotropy while their real permeability is increased by about 8% in the 2-6 GHz frequency band compared to the smaller particles.

[0065] Example 5:

[0066] In step S2, the amount of urea added is adjusted to 20% of the mass of PAN; other conditions are the same as in Example 1.

[0067] The higher nitrogen doping content introduces more defects and polarization centers into the carbon framework. Compared with Example 1 (10% urea), the real part of the dielectric constant (ε´) of the resulting absorber remains at the same low level (5-15), but the dielectric loss (ε"), especially the loss caused by polarization, is increased by about 20%-30% across the entire frequency band. This results in a better synergistic effect with magnetic loss, enhancing the absorption capacity of the 1 mm coating in the X-band and reducing the minimum reflection loss value.

[0068] Comparative Example 1

[0069] Except for not synthesizing the sheet-like precursor, but instead directly mixing the metal salt with the PAN solution and then electrospinning and heat treatment, the other conditions were the same as in Example 1.

[0070] In the final product, the high-entropy alloy particles are solid spheres with a relatively thick carbon layer; their magnetic permeability decreases significantly in the high-frequency band, and the effective absorption bandwidth at a thickness of 1 mm is only 1.5 GHz, which is far inferior to the embodiments of the present invention.

[0071] Please refer to Table 1 below for the key performance indicators of the 1mm coating of the obtained products in Examples 1-5 and Comparative Example 1.

[0072] Table 1

[0073]

[0074] By designing sheet-like high-entropy alloys as magnetic cores and utilizing their strong shape anisotropy, the Snoek limit is effectively broken. To achieve high permeability and high magnetic loss under low-frequency and thin-layer conditions, ultrathin nitrogen-doped carbon fibers are used as a three-dimensional skeleton. On the one hand, it acts as a "permeability protector" to isolate metal particles, suppress eddy currents, and reduce and precisely control the dielectric constant. On the other hand, its rich heterogeneous interfaces and nitrogen-doped defects introduce strong dielectric polarization losses. Ultimately, the synergy between magnetic loss and dielectric loss is achieved, resolving the contradiction between impedance matching and high attenuation capability.

[0075] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A composite fiber absorbent, characterized in that, The absorbent uses porous nitrogen-doped carbon fiber as a three-dimensional network framework, and porous high-entropy alloy nanoparticles are encapsulated in situ inside and on the surface of the porous nitrogen-doped carbon fiber. The porous high-entropy alloy nanoparticles are in the form of sheets. The porous high-entropy alloy nanoparticles are a pentagonal alloy system containing Fe, Co, Ni, Mn, and Al; the molar ratio of the constituent elements is Fe:Co:Ni:Mn:Al = 1-1.5:1-1.5:1-1.5:0.5-1:0.5-1. The porous high-entropy alloy nanoparticles also contain rare earth elements accounting for 0.5%-3% of the molar number of their transition metals Fe+Co+Ni+Mn+Al, wherein the rare earth elements are at least one of La, Ce, and Y.

2. The composite fiber absorbent according to claim 1, characterized in that, The high-entropy alloy nanoparticles have an aspect ratio greater than 20 and a thickness of 20-100 nanometers. The thickness of the nitrogen-doped carbon shell of the porous nitrogen-doped carbon fiber is 2-10 nanometers, and the nitrogen doping amount is 5 at%-20 at%.

3. A method for preparing a composite fiber absorbent, used to prepare a composite fiber absorbent as described in any one of claims 1-2, characterized in that, Includes the following steps: S1. Preparation of sheet-like high-entropy alloy precursors: Sheet-like precursors composed of hydroxides or salts of Fe, Co, Ni, Mn, and Al are prepared by solvothermal method or coprecipitation method. S2. Preparation of electrospinning precursor solution: The sheet-like precursor, polymer carbon source, and nitrogen source obtained in step S1 are dispersed in a solvent to form a uniform spinning solution. S3. Electrospinning and pre-oxidation: The spinning solution obtained in step S2 is electrospinned to obtain a nanofiber membrane, which is then pre-oxidized and stabilized in an air atmosphere. S4. High-temperature heat treatment: The pre-oxidized fiber membrane is subjected to high-temperature heat treatment in a protective atmosphere or an atmosphere containing a nitrogen source to complete carbonization, reduction and high-entropy alloying, and obtain the final product.

4. The method for preparing a composite fiber absorbent according to claim 3, characterized in that, The solvothermal method includes the following steps: (1) Raw material preparation: Analytical grade metal chlorides were used as the metal source; The metal sources include FeCl3·6H2O, CoCl2·6H2O, NiCl2·6H2O, MnCl2·4H2O, and AlCl3·6H2O. Each metal salt is accurately weighed according to the molar ratio of Fe:Co:Ni:Mn:Al = 1-1.5:1-1.5:1-1.5:0.5-1:0.5-1. The metal salts are then dissolved together in ethylene glycol or deionized water to prepare a mixed salt solution with a total metal ion concentration of 0.1-0.3 mol / L. Polyvinylpyrrolidone (PVP) is added to the mixed salt solution as a morphology control agent, with a mass ratio of PVP to total metal salts of 1:2-5. The solution is stirred vigorously until completely dissolved. (2) Solvent thermal reaction: Transfer the mixed solution to a high-pressure reactor lined with polytetrafluoroethylene, with a filling degree of 70-80%. After sealing the reactor, place it in an oven and react at 160-200°C for 10-15 hours. After the reaction is completed, allow it to cool naturally to room temperature. (3) Post-treatment: The reaction product is washed several times by alternating centrifugation with ethanol and deionized water until the supernatant is neutral to completely remove residual anions and polyvinylpyrrolidone. It is then dried in a vacuum drying oven at 60-80°C for 10-12 hours to obtain a sheet-like layered double hydroxide (LDH) or coprecipitated hydroxide precursor. This precursor has achieved uniform mixing of metal elements at the atomic scale.

5. The method for preparing a composite fiber absorbent according to claim 3, characterized in that, The coprecipitation method includes the following steps: (1) Raw material preparation: Analytical grade metal chlorides were used as the metal source; The metal sources include FeCl3·6H2O, CoCl2·6H2O, NiCl2·6H2O, MnCl2·4H2O, and AlCl3·6H2O. Each metal salt is accurately weighed according to the molar ratio of Fe:Co:Ni:Mn:Al = 1-1.5:1-1.5:1-1.5:0.5-1:0.5-1, and they are dissolved together in deionized water to prepare a mixed salt solution, solution A. A sodium carbonate or sodium hydroxide solution with a concentration of 1.0-2.0 mol / L is prepared as a precipitant, solution B. (2) Co-precipitation: Under continuous stirring, solution A and solution B are simultaneously and slowly added dropwise to a reactor containing bottom water by double titration. The bottom water serves as the seed crystal medium. The pH value is controlled between 9 and 11 throughout the process, and the temperature is maintained at 50-70°C. After titration, the solution is aged for 5-10 hours under this temperature and stirring to ensure complete precipitate aging and crystallization. (3) Post-processing: After the reaction is completed, the sample is filtered and washed with deionized water until no chloride ions are present. The sample is then dried at 80-100°C for 12-24 hours to obtain hydroxide or basic carbonate precursor. Chloride ions can be detected by AgNO3 solution.

6. A method for preparing a composite fiber absorbent according to claim 4 or 5, characterized in that, Rare earth elements are added in step S1, and the specific steps are as follows: Weigh out rare earth metal salt Ce(NO3)3·6H2O according to the calculated amount of 0.5%-3% of the total transition metals Fe+Co+Ni+Mn+Al, and dissolve it in deionized water to prepare a rare earth salt solution. Add the prepared rare earth salt solution to the prepared mixed salt solution of Fe, Co, Ni, Mn and Al. Under the high temperature and high pressure environment of the solvothermal reaction in the solvothermal method, rare earth ions can be effectively embedded into the LDH layer or uniformly adsorbed on the surface of hydroxide particles. Alternatively, during the coprecipitation process of the coprecipitation method, rare earth ions may coprecipitate with other metal ions or be adsorbed on the surface of newly generated hydroxide precipitates, achieving atomic-level uniform doping.

7. The method for preparing a composite fiber absorbent according to claim 3, characterized in that, In step S2, the carbon source of the polymer is polyacrylonitrile, the nitrogen source is urea or melamine, and the mass ratio of the sheet-like precursor to PAN is 1-4:

1.

8. The method for preparing a composite fiber absorbent according to claim 3, characterized in that, In step S4, the high-temperature heat treatment temperature is 700℃-900℃, the heat treatment time is 13 hours, the protective atmosphere is argon or nitrogen, and the atmosphere containing nitrogen source is ammonia or a mixture of argon and ammonia.

Images