Composite carbon nanofiber wave-absorbing material, preparation method and application thereof

Abstract

This invention belongs to the field of microwave absorbing materials technology, specifically relating to a composite carbon nanofiber microwave absorbing material, its preparation method, and its applications. The composite carbon nanofiber microwave absorbing material is obtained by electrospinning and heat treatment using a polynuclear pentameric acid metal complex as a precursor and polyacrylonitrile as a carbon source. The polynuclear pentameric acid metal complex is any one of trinuclear iron pentameric acid, dinuclear cobalt pentameric acid, and dinuclear nickel pentameric acid. The excellent microwave absorption performance of the composite carbon nanofiber microwave absorbing material in this invention achieves uniform loading of the nanophase, increases the number of heterogeneous interfaces, and enhances dissipation processes such as interfacial polarization, providing a feasible path to solve the common problems of particle agglomeration and impedance mismatch in composite systems.

Description

Technical Field

[0001] This invention belongs to the field of microwave absorbing materials technology, specifically relating to a composite carbon nanofiber microwave absorbing material, its preparation method, and its application. Background Technology

[0002] In existing microwave absorbing material systems, traditional magnetic materials such as ferrites, while possessing certain magnetic loss capabilities, generally suffer from high density, complex molding processes, and limited flexible fabrication. In contrast, carbon-based materials, with their advantages of low density, good chemical stability, tunable conductivity, and excellent designability, have become a better choice for constructing lightweight microwave absorbing materials. In particular, carbon nanofiber networks prepared through electrospinning, with their three-dimensional continuous fiber skeleton, abundant pore channels, and high specific surface area, are not only conducive to constructing lightweight and flexible microwave absorbing structures, but also enable multiple scattering and extended propagation paths through structural design. Furthermore, they provide an ideal platform for defect control and interface construction. This system has a natural advantage in thin-layer microwave absorption, but its performance limits are often constrained by the controllability of electromagnetic parameters and the stable distribution of composite components. In practical preparation, relying solely on a single carbon phase can easily lead to problems such as an overly strong conductive network resulting in poor matching, or insufficient dissipation channels leading to inadequate absorption. Introducing a magnetic or dielectric phase to enhance dissipation and improve matching often faces challenges such as insufficient precursor compatibility, component migration and phase separation during fiber formation, and particle migration, sintering, and agglomeration during carbonization. This results in uncontrolled particle size, decreased interfacial density, and unstable electromagnetic response, ultimately limiting further improvements in the broadband absorption performance of thin-layer composites. Furthermore, most existing electrospun composite systems use traditional inorganic metal salts or MOFs as spinning precursors. However, inorganic metal salts, due to their strong ionicity, have limited compatibility with polymer systems and are prone to migration, sintering, and agglomeration during carbonization. While MOFs possess a certain confinement effect, their synthesis process is cumbersome and difficult to scale up. Summary of the Invention

[0003] To address the aforementioned problems, this invention provides a multinucleopentanoic acid (MPiv) complex, its preparation method, and its applications. By introducing MPiv metal complexes as a source of magnetic components, and utilizing their excellent organic compatibility and the confinement effect of the ligand structure, uniform loading of the nanophase is achieved, increasing the number of heterogeneous interfaces and enhancing dissipation processes such as interfacial polarization. This provides a feasible approach to solving the common problems of particle agglomeration and impedance mismatch in composite systems.

[0004] To achieve the above objectives, the technical solution of the present invention is as follows.

[0005] The first aspect of this invention provides a composite carbon nanofiber microwave absorbing material, characterized in that it is obtained by electrospinning and heat treatment using a polynuclear pentanoic acid metal complex as a precursor and polyacrylonitrile as a carbon source. The polynuclear pentovalinate metal complex is any one of trinuclear iron pentovalinate, dinuclear cobalt pentovalinate, and dinuclear nickel pentovalinate. These three polynuclear pentovalinate metal complexes serve as electrospinning precursors primarily due to the material advantages conferred by their unique molecular structures: on the one hand, the large tert-butyl groups in the pentovalinate anion (piv−) enhance the organic compatibility of the precursor, resulting in excellent solubility and stability in N,N-dimethylformamide, thus enabling relatively uniform mixing and dispersion with the polyacrylonitrile matrix in the spinning solution; on the other hand, the large steric hindrance of the peripheral ligands slows down the migration and aggregation of the central metal atoms during pre-oxidation and high-temperature carbonization (i.e., heat treatment), thereby generating fine and uniformly dispersed magnetic nanoparticles in the carbon fiber skeleton, providing a foundation for the subsequent construction of rich heterogeneous interfaces and multi-channel loss. This invention proposes using polynuclear pentovalinate metal complexes (MPiv) as a novel source of magnetic components to prepare composite carbon nanofiber microwave absorbing materials. Unlike traditional inorganic metal salts, MPiv complexes possess well-defined composition and stable multinuclear coordination characteristics. Their metal centers exhibit definite stoichiometry and relative positions within the complex, providing a structural basis for the uniform spatial distribution of metal components at the molecular scale. Simultaneously, the carboxylic acid organic ligands on the exterior of the MPiv complex endow it with relatively good organic compatibility, enabling a more uniform dispersion in the spinning solution and reducing the risk of localized enrichment. During high-temperature carbonization, the gradual decomposition and carbonization of the sterically hindered pentavonic acid ligands can restrict the migration of the metal centers, facilitating the formation of finer and more uniformly distributed metal nanoparticles. This also introduces abundant heterointerfaces, thereby enhancing polarization loss. Furthermore, the magnetic nanoparticles generated by pyrolysis reduction can contribute to magnetic loss and synergistically improve impedance matching.

[0006] In another preferred embodiment, the mass ratio of the polynuclear pentovalinic acid metal complex to the polyacrylonitrile is 1 mmol to 3 mmol: 1 g.

[0007] A second aspect of this invention provides a method for preparing the aforementioned composite carbon nanofiber microwave absorbing material, comprising the following steps: Polyacrylonitrile was mixed with polynuclear terpentine metal complex in an organic environment to obtain an electrospinning precursor solution. The electrospinning precursor solution was prepared at an operating voltage of 13.00 kV to 13.6 kV, a distance of 15 cm to 20 cm between the spinning needle and the receiving roller, and a feed rate of 0.60 mL / h. −1 ~0.70 mL·h −1The drum rotation speed is 130 r / min. −1 r·min −1 The translation distance of the propulsion device is 90mm~100mm, and the translation speed is 150mm·min. −1 Electrospinning was performed under specific conditions to obtain the product; the product was then pre-oxidized and then carbonized to obtain a composite carbon nanofiber microwave absorbing material.

[0008] Specifically, the preparation process of the polynuclear tervastatin metal complex is as follows: Synthesis of trinuclear ferric pivalate: Pivalic acid and sodium carbonate are reacted in acetonitrile solvent at 80°C to produce sodium pivalate. Then, ferric nitrate nonahydrate is added, and the iron ions coordinate with the pivalate ions through reflux to form a trinuclear ferric oxide cluster. After the reaction solution is filtered to remove insoluble matter, the filtrate is evaporated to obtain a brownish-red microcrystalline solid, which is the synthesis of trinuclear ferric pivalate. Synthesis of dinuclear cobalt pivalate and dinuclear nickel pivalate: Using basic cobalt carbonate or basic nickel carbonate as the metal source, an excess of pivalic acid was refluxed at 160°C. The high-temperature melting reaction facilitated coordination of the metal ions with the pivalate anion, forming a dinuclear metal complex. After the reaction was complete, the mixture was cooled, and diethyl ether was added and stirred to remove unreacted pivalic acid and low-boiling impurities. After filtration, the diethyl ether was evaporated by heating, and acetonitrile was added and stirred overnight to promote crystallization, yielding dinuclear cobalt pivalate and dinuclear nickel pivalate, respectively.

[0009] In another preferred embodiment, the carbonization treatment is carried out at a temperature of 700°C for 120 minutes.

[0010] In another preferred embodiment, the pre-oxidation treatment refers to holding at 280°C for 120 minutes.

[0011] In another preferred embodiment, the heating rate of the carbonization treatment is 5°C·min. −1 The heating rate for the pre-oxidation treatment is 1℃·min. −1 .

[0012] In another preferred embodiment, the reagent used in the organic environment is N,N-dimethylformamide.

[0013] The third aspect of this invention provides the application of the aforementioned composite carbon nanofiber absorbing material in the preparation of absorbing materials.

[0014] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes polynuclear pentameric acid metal complexes (MPiv) as the magnetic phase precursor and polyacrylonitrile (PAN) as the carbon source, employing electrospinning technology and high-temperature carbonization processes to successfully prepare composite carbon nanofiber microwave absorbing materials with different metal compositions and feed amounts. The excellent organic compatibility of the MPiv precursor facilitates its uniform dispersion in the spinning solution, while the dual confinement effect of the polynuclear coordination structure and the fiber skeleton effectively inhibits the migration and aggregation of metal components during high-temperature carbonization. This provides a feasible precursor design approach for constructing lightweight, porous, and heterogeneous interface-rich carbon-based composite microwave absorbing materials. The type of metal in this invention significantly affects the phase composition, carbon skeleton structure, and electromagnetic loss mechanism of the composite material. The cobalt system mainly forms metallic cobalt, with a relatively smooth fiber surface. The high degree of graphitization weakens defect dipole polarization to some extent. In addition, the limited heterostructures restrict polarization relaxation, resulting in a weak overall attenuation capability. The nickel system is more likely to form a high-density, fine nickel particle cover with the increase of precursor feed. Although it has strong conduction loss, the high-density continuous distribution can easily cause local electric field shielding, making it more dependent on a larger material thickness to achieve better matching and deep absorption. In contrast, the iron system generates an in-situ Fe / Fe3C composite phase and exhibits hierarchical particle distribution characteristics. This multiphase coexistence structure not only catalyzes the construction of a conductive network with moderate defects, but also introduces extremely rich multiple heterostructures, thereby achieving better synergy between conduction loss, interface polarization relaxation and impedance matching. The superior microwave absorption performance of the composite carbon nanofiber absorbing material in this invention mainly comes from the synergistic effect of the continuous conductive pathway constructed by the three-dimensional carbon nanofibers and the multi-scale loss mechanism: the multiple reflections and scattering brought about by the interwoven fiber network extend the propagation path, and the conductive pathway of the carbon skeleton provides conduction loss; the rich heterogeneous interfaces and defect sites between the metal / metal carbide and the carbon matrix induce interface polarization and dipole polarization, while the weak magnetic response of the metal nanoparticles plays a key auxiliary role in optimizing impedance matching and broadening the effective frequency band. Attached Figure Description

[0015] Figure 1 The structural analysis results of Fe-2 / PANF-280, Co-2 / PANF-280 and Ni-2 / PANF-280 are shown in (a) schematic diagram of the molecular structure of Fe-2 / PANF-280, (b) schematic diagram of the molecular structure of Co-2 / PANF-280, (c) schematic diagram of the molecular structure of Ni-2 / PANF-280, and (d) XRD spectra of the three samples from testing and simulation.

[0016] Figure 2Thermogravimetric and differential thermogravimetric curves of Fe-2 / PANF-280, Co-2 / PANF-280 and Ni-2 / PANF-280 are shown; (a) Fe-2 / PANF-280, (b) Co-2 / PANF-280, (c) Ni-2 / PANF-280; (d) Comparison of residual mass fraction of the three groups of samples at each pyrolysis stage and at 1000℃.

[0017] Figure 3 X-ray diffraction patterns of different metal systems and feed amounts are shown; (a) is Fe-x / CNF, (b) Co-x / CNF, and (c) Ni-x / CN.

[0018] Figure 4 Laser Raman spectra of different metal systems and feed amounts and various samples I D / I G Line graph showing the change of value with feed amount x; where (a) is the laser Raman spectrum of Fe-x / CNF, (b) is the laser Raman spectrum of Co-x / CNF, and (c) is the laser Raman spectrum of Ni-x / CNF. x =1,2,3); (d) represents each sample I D / I G Line graph showing the change in value as the amount of material fed x.

[0019] Figure 5 The images show SEM images, fiber diameter distribution, and macroscopic photographs of the Fe-2 system at different process stages; (a) SEM image of Fe-2 / PANF, (b) SEM image of Fe-2 / PANF-280, (c) SEM image of Fe-2 / CNF, and (d) macroscopic photographs of fiber membranes at each stage.

[0020] Figure 6 Scanning electron microscope (SEM) images of different metal systems and feed amounts: (a) Fe-1 / CNF, (b) Fe-2 / CNF, (c) Fe-3 / CNF, (d) Co-1 / CNF, (e) Co-2 / CNF, (f) Co-3 / CNF, (g) Ni-1 / CNF, (h) Ni-2 / CNF, and (i) Ni-3 / CNF.

[0021] Figure 7 SEM images of different metal systems: (a) Fe-2 / CNF, (b) Co-2 / CNF, (c) Ni-2 / CNF.

[0022] Figure 8For different metal systems and feed amounts, Mx / CNF (M=Fe,Co,Ni); x The results of the complex permittivity and dielectric loss behavior for (=1,2,3) are shown in the figure; (a), (d), and (g) are the real parts of the complex permittivity. ε′ The resulting graphs; (b), (e), and (h) are the imaginary parts. ε′′ The results are shown in the figure; (c), (f), and (i) are the dielectric loss tangent tanδ. ε Curve showing the change with frequency.

[0023] Figure 9 Cole-cole for different metal systems and feed amounts ( ε′′ - ε′ The graph shows the curve direction from 2GHz to 18GHz, with dashed arrows indicating the curve direction. (a) is Fe-1 / CNF, (b) is Fe-2 / CNF, (c) is Fe-3 / CNF, (d) is Co-1 / CNF, (e) is Co-2 / CNF, (f) is Co-3 / CNF, (g) is Ni-1 / CNF, (h) is Ni-2 / CNF, and (i) is Ni-3 / CNF.

[0024] Figure 10 The graph shows the results of complex permeability and magnetic loss for different metal systems and feed amounts Mx / CNF (M=Fe,Co,Ni; x=1,2,3); (a), (d), and (g) are the real parts of complex permeability. μ′ (b), (e), and (h) are the imaginary parts of the complex permeability. μ′′ (c), (f), (i) Magnetic loss tangent tanδ μ Curve showing variation with frequency.

[0025] Figure 11 Eddy current coefficients for different metal systems and feed amounts C 0 as a function of frequency; (a) Fe-x / CNF, (b) Co-x / CNF, (c) Ni-x / CNF; (x=1,2,3).

[0026] Figure 12 Figure 1 shows the results of electromagnetic wave absorption performance analysis of the iron system; (a) and (b) are the three-dimensional and two-dimensional reflection loss diagrams of Fe-1 / CNF, respectively; (d) and (e) are the three-dimensional and two-dimensional reflection loss diagrams of Fe-2 / CNF, respectively; (g) and (h) are the three-dimensional and two-dimensional reflection loss diagrams of Fe-3 / CNF, respectively; (c) Fe-1 / CNF, (f) Fe-2 / CNF, and (i) Fe-3 / CNF are the reflection loss curves under different matching thicknesses, where the thickness indicated by the blue line is t. EAB The thickness shown by the red line is t. RL .

[0027] Figure 13 Figure 1 shows the electromagnetic wave absorption performance analysis results of the cobalt system; (a) and (b) are the three-dimensional and two-dimensional reflection loss diagrams of Co-1 / CNF, respectively; (d) and (e) are the three-dimensional and two-dimensional reflection loss diagrams of Co-2 / CNF, respectively; (g) and (h) are the three-dimensional and two-dimensional reflection loss diagrams of Co-3 / CNF, respectively; (c) Co-1 / CNF, (f) Co-2 / CNF, and (i) Co-3 / CNF are the reflection loss curves under different matching thicknesses, where the thickness indicated by the blue line is t. EAB The thickness shown by the red line is t. RL .

[0028] Figure 14 Figure 1 shows the electromagnetic wave absorption performance analysis results of the nickel-based system; (a) and (b) are the three-dimensional and two-dimensional reflection loss diagrams of Ni-1 / CNF, respectively; (d) and (e) are the three-dimensional and two-dimensional reflection loss diagrams of Ni-2 / CNF, respectively; (g) and (h) are the three-dimensional and two-dimensional reflection loss diagrams of Ni-3 / CNF, respectively; (c) Ni-1 / CNF, (f) Ni-2 / CNF, and (i) Ni-3 / CNF are the reflection loss curves under different matching thicknesses, where the thickness indicated by the blue line is t. EAB The thickness shown by the red line is t. RL .

[0029] Figure 15 Figure 1 shows the absorption performance analysis results of Mx / CNF (M=Fe, Co, Ni; x=1,2,3); (a) shows the minimum reflection loss RL. min (a) shows the matching thickness result; (b) shows the EAB. max The scatter plot shows the distribution of the matching thickness, with the yellow dashed arrows pointing to the region of the ideal absorbing material (strong, thin, wide).

[0030] Figure 16 The following are the results of impedance matching and attenuation capabilities analysis of different composite carbon nanofiber absorbing materials at different thicknesses; (a) Fe-2 / CNF, (b) Co-2 / CNF, (c) Ni-2 / CNF; where the thickness indicated by the blue line is t. EAB The thickness shown by the red line is t. RL The dashed line represents | Z in / Z 0|=1 ideal matching line; (d) is the curve of the attenuation constant α of M-2 / CNF (M=Fe,Co,Ni) as a function of frequency.

[0031] Figure 17This is a schematic diagram of the electromagnetic wave absorption mechanism of M-2 / CNF (M=Fe,Co,Ni) composite carbon nanofiber materials. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0033] Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0034] The following is a detailed description of a composite carbon nanofiber microwave absorbing material, its preparation method, and its applications.

[0035] Example 1: A method for preparing a composite carbon nanofiber microwave absorbing material, comprising the following steps: S1. In a round-bottom flask, add 20 mL of acetonitrile, 0.82 g (8 mmol) of pivalic acid, and 0.67 g (8 mmol) of sodium carbonate powder. Heat at 80 °C with stirring until no more bubbles are produced in the solution. Then add 1.62 g (4 mmol) of ferric nitrate nonahydrate and heat under reflux for 8 hours to allow the reaction to proceed completely. After the reaction is complete, cool the solution and filter to remove unreacted precipitates and insoluble impurities. Place the filtrate in a clean beaker, seal it with a sealing film and poke a hole in the film. Allow the solvent to evaporate slowly at room temperature, precipitating a brownish-red microcrystalline solid of ferric pivalate [Fe3O(OOCCMe3)6(EtOH)]. 3] (NO3), abbreviated as FePiv, is collected by filtration and thoroughly dried. The molecular structure of FePiv is as follows: Figure 1 As shown in (a).

[0036] S2. Weigh 1.00 g of polyacrylonitrile (PAN, Mw85000) and add it to 10.00 mL of N,N-dimethylformamide (DMF) solvent while stirring. Sonicate for 10 min to ensure complete dissolution of PAN, obtaining a homogeneous and transparent polymer matrix solution, denoted as PAN / DMF solution, and keep it for later use while stirring. To ensure comparability between different metal systems, the sample feed amount in this example is... x The amount of substance of the metal element M (M = Fe, Co, Ni) added to the system, for a pivalate metal complex with a nucleus number of z, is represented by the amount of substance x and the mass of MPiv. The following relationship must be satisfied:

[0037] ; In the formula: - Relative molecular mass of pivalic acid metal complex / g·mol−1 .

[0038] In this embodiment, the following were slowly added to the stirred PAN / DMF solution: x =1 mmol of FePiv precursor, corresponding to the mass weighed The sample was 0.3298 g, and ultrasonically treated for 10 min to promote the full dispersion of FePiv and its interaction with PAN. The mixed solution was then continuously stirred for more than 12 h to obtain a uniform brownish-yellow electrospinning precursor solution.

[0039] S3. The above electrospinning precursor solution is injected into a 10mL syringe, and a 19G metal electrospinning needle is fitted. Release paper is pre-covered on the surface of the receiving roller to collect the fiber film. The electrospinning equipment parameters are fixed as follows: operating voltage +13.60kV, distance from the spinning needle to the receiving roller 15cm, and feed rate 0.70mL·h. −1 The drum speed is 130 r·min −1 The translation distance of the propulsion device is 100 mm, and the translation speed is 150 mm·min. −1 After spinning is completed, the metal is processed according to the type of metal and the amount of material fed, using MPiv- x The name is derived using the / PANF (M=Fe, Co, Ni) method, and in this embodiment it is FePiv-1 / PANF.

[0040] S4. The FePiv-1 / PANF prepared by electrospinning was cut to the set size and sandwiched between two corundum plates in a muffle furnace for pre-oxidation treatment at 1℃·min. −1 The temperature was increased from room temperature to 280℃ and held for 120 min, then decreased to room temperature at a rate of 1℃ / min. The composite carbon nanofiber membrane changed from its original light color to dark brown. According to M- x The name is / PANF-280 (M=Fe, Co, Ni), and in this example it is Fe-1 / PANF-280; after pre-oxidation, it is heated in a tube furnace at 5℃·min. −1 The composite carbon nanofiber material was obtained by carbonizing at 700℃ for 120 min under N2 atmosphere and heating rate of M- x The naming convention is / CNF (M=Fe, Co, Ni), and in this embodiment it is Fe-1 / CNF.

[0041] Example 2: A method for preparing a composite carbon nanofiber microwave absorbing material, only the amount of material fed... xExcept for the difference between Example 1 and Example 2, which uses 2 mmol of FePiv precursor (i.e., 0.6597 g of FePiv precursor), the other steps are the same as in Example 1. After spinning, FePiv-2 / PANF is obtained; after pre-oxidation treatment, Fe-2 / PANF-280 is obtained. The composite carbon nanofiber material is denoted as Fe-2 / CNF.

[0042] Example 3: A method for preparing a composite carbon nanofiber microwave absorbing material, only the amount of material used... x Except for the difference between Example 1 and Example 2, the FePiv precursor of 3 mmol (i.e., 0.9895 g of FePiv precursor) was used. All other steps were the same as in Example 1. After spinning, FePiv-3 / PANF was obtained. After pre-oxidation treatment, Fe-3 / PANF-280 was obtained. The final composite carbon nanofiber material was denoted as Fe-3 / CNF.

[0043] Example 4: A method for preparing a composite carbon nanofiber microwave absorbing material, comprising the following steps: S1. In a round-bottom flask, add 2.58 g (5 mmol) of basic cobalt carbonate and excess 25.50 g (250 mmol) of terpentine sequentially. Heat under reflux at 160 °C for 8 h to promote a complete reaction, and the solution turns deep purple. Stir and cool to room temperature, then add 50 mL of diethyl ether and stir for 30 min. After filtering out impurities, heat to allow the diethyl ether to evaporate completely, then cool to room temperature and add 50 mL of acetonitrile. Stir overnight to precipitate purple cobalt terpentine crystals [Co2( μ [-OH2)(O2CCM3)4(HO2CCMe3)4], abbreviated as CoPiv, is washed and dried repeatedly with acetonitrile.

[0044] S2. Weigh 1.00 g of polyacrylonitrile (PAN, Mw85000) and add it to 10.00 mL of N,N-dimethylformamide (DMF) solvent while stirring. Sonicate for 10 min to ensure the PAN is fully dissolved, obtaining a homogeneous and transparent polymer matrix solution, denoted as the PAN / DMF solution, and set aside while stirring. Slowly add the following to the stirred PAN / DMF solution: x =1 mmol of CoPiv precursor was sonicated for 10 min to promote full dispersion of CoPiv and interaction with PAN. The mixed solution was then stirred continuously for more than 12 h to obtain a uniform brownish-yellow electrospinning precursor solution.

[0045] S3. The above electrospinning precursor solution is injected into a 10mL syringe, and a 19G metal electrospinning needle is fitted. Release paper is pre-covered on the surface of the receiving roller to collect the fiber film. The electrospinning equipment parameters are fixed as follows: operating voltage +13.60kV, distance from the spinning needle to the receiving roller 15cm, and feed rate 0.70mL·h.−1 The drum speed is 130 r·min −1 The translation distance of the propulsion device is 100 mm, and the translation speed is 150 mm·min. −1 After spinning is completed, the metal is processed according to the type of metal and the amount of material fed, using MPiv- x The name is derived using the / PANF (M=Fe, Co, Ni) method, and in this embodiment it is CoPiv-1 / PANF.

[0046] S4. CoPiv-1 / PANF prepared by electrospinning was cut to the set size and sandwiched between two corundum plates in a muffle furnace for pre-oxidation treatment at 1℃·min. −1 The temperature was increased from room temperature to 280℃ and held for 120 min, then decreased to room temperature at a rate of 1℃ / min. The composite carbon nanofiber membrane changed from its original light color to dark brown. According to M- x The name is / PANF-280 (M=Fe, Co, Ni), and in this example it is Co-1 / PANF-280; after pre-oxidation, it is heated in a tube furnace at 5℃·min. −1 The composite carbon nanofiber material was obtained by carbonizing at 700℃ for 120 min under N2 atmosphere and heating rate of M- x The naming convention is / CNF (M=Fe, Co, Ni), and in this example it is Co-1 / CNF. Figure 1 As shown in (b).

[0047] Example 5: A method for preparing a composite carbon nanofiber microwave absorbing material, only the amount of material used... x Except for the 2 mmol CoPiv precursor, which differs from Example 4, the other steps are the same as in Example 4. After spinning, CoPiv-2 / PANF is obtained; after pre-oxidation treatment, Co-2 / PANF-280 is obtained, and the final composite carbon nanofiber material is denoted as Co-2 / CNF.

[0048] Example 6: A method for preparing a composite carbon nanofiber microwave absorbing material, only the amount of material used... x Except for the 3 mmol CoPiv precursor, which is different from that in Example 4, all other steps are the same as in Example 4. After spinning, CoPiv-3 / PANF is obtained; after pre-oxidation treatment, Co-3 / PANF-280 is obtained, and the final composite carbon nanofiber material is denoted as Co-3 / CNF.

[0049] Example 7: A method for preparing a composite carbon nanofiber microwave absorbing material, comprising the following steps: S1. In a round-bottom flask, add 1.88 g (5 mmol) of basic nickel carbonate and excess 15.30 g (150 mmol) of pentanoic acid sequentially. Heat under reflux at 160 °C for 8 h to promote a complete reaction, and the solution turns deep purple. Stir and cool to room temperature, then add 50 mL of diethyl ether and stir for 30 min. After filtering out impurities, heat to allow the diethyl ether to evaporate completely, then cool to room temperature and add 50 mL of acetonitrile and stir overnight to obtain green crystalline nickel pentanoate [Ni2( μ The molecular structure of NiPiv is as follows: [-OH2)(O2CCM3)4(HO2CCMe3)4]. Figure 1 As shown in (c).

[0050] S2. Weigh 1.00 g of polyacrylonitrile (PAN, Mw85000) and add it to 10.00 mL of N,N-dimethylformamide (DMF) solvent while stirring. Sonicate for 10 min to ensure the PAN is fully dissolved, obtaining a homogeneous and transparent polymer matrix solution, denoted as the PAN / DMF solution, and set aside while stirring. Slowly add the following to the stirred PAN / DMF solution: x =1 mmol of NiPiv precursor was sonicated for 10 min to promote the full dispersion of NiPiv and its interaction with PAN. The mixed solution was then continuously stirred for more than 12 h to obtain a uniform brownish-yellow electrospinning precursor solution.

[0051] S3. The above electrospinning precursor solution is injected into a 10mL syringe, and a 19G metal electrospinning needle is fitted. Release paper is pre-covered on the surface of the receiving roller to collect the fiber film. The electrospinning equipment parameters are fixed as follows: operating voltage +13.60kV, distance from the spinning needle to the receiving roller 15cm, and feed rate 0.70mL·h. −1 The drum speed is 130 r·min −1 The translation distance of the propulsion device is 100 mm, and the translation speed is 150 mm·min. −1 After spinning is completed, the metal is processed according to the type of metal and the amount of material fed, using MPiv- x The name is derived using the / PANF (M=Fe, Co, Ni) method, and in this embodiment it is NiPiv-1 / PANF.

[0052] S4. The NiPiv-1 / PANF prepared by electrospinning was cut to the set size and sandwiched between two corundum plates in a muffle furnace for pre-oxidation treatment at 1℃·min. −1 The temperature was increased from room temperature to 280℃ and held for 120 min, then decreased to room temperature at a rate of 1℃ / min. The composite carbon nanofiber membrane changed from its original light color to dark brown. According to M- xThe name is specified as / PANF-280 (M=Fe, Co, Ni), and in this example it is Ni-1 / PANF-280; after pre-oxidation, it is heated in a tube furnace at 5℃·min. −1 The composite carbon nanofiber material was obtained by carbonizing at 700℃ for 120 min under N2 atmosphere and heating rate of M- x The naming convention is / CNF (M=Fe, Co, Ni), and in this embodiment it is Ni-1 / CNF.

[0053] Example 8: A method for preparing a composite carbon nanofiber microwave absorbing material, only the amount of material used... x Except for the 2 mmol NiPiv precursor, which differs from Example 7, the other steps are the same as in Example 7. After spinning, NiPiv-2 / PANF is obtained; after pre-oxidation treatment, Ni-2 / PANF-280 is obtained, and the final composite carbon nanofiber material is denoted as Ni-2 / CNF.

[0054] Example 9: A method for preparing a composite carbon nanofiber microwave absorbing material, only the amount of material used... x Except for the 3 mmol NiPiv precursor, which differs from Example 7, the other steps are the same as in Example 7. After spinning, NiPiv-3 / PANF is obtained; after pre-oxidation treatment, Ni-3 / PANF-280 is obtained, and the final composite carbon nanofiber material is denoted as Ni-3 / CNF.

[0055] The single-crystal structures of FePiv, CoPiv, and NiPiv prepared in Examples 1, 4, and 7 were analyzed, and the results are as follows: Figure 1 As shown in (d), standard simulated XRD pattern cards were exported using the crystallography software Mercury based on the single-crystal structure data of FePiv, CoPiv, and NiPiv. Comparison with the test data of the actual synthesized samples revealed that the diffraction peak positions and relative peak intensities of the three samples were highly consistent with the simulated patterns, indicating that the prepared samples matched the target crystal structure and possessed high purity. Samples with a feed amount of x = 2 mmol for each metal system, i.e., Fe-2 / PANF-280, Co-2 / PANF-280, and Ni-2 / PANF-280 from Examples 2, 5, and 7, were subjected to thermogravimetric (TG) and differential thermogravimetric (DTG) analysis. The results are as follows: Figure 2As shown. Based on the weight loss characteristics of the TG curves and the analysis of the DTG peak positions, the pyrolysis process of the three samples can be roughly divided into the following four stages. Stage I (room temperature – 135℃) mainly corresponds to the removal of physically adsorbed water and a small amount of low-boiling-point residual solvent in the sample. Stage II (135℃ – 350℃) mainly involves the further cracking and dehydrogenation reaction of residual unstable groups in the sample, accompanied by the breaking of some metal-carboxylic acid coordination bonds. Since the material has been fully pre-oxidized at 280℃ for two hours, the PAN molecular chain underwent a significant cyclization reaction in the air atmosphere and formed a conjugated ladder structure, which significantly improved the thermal stability of the fiber film. Therefore, the mass loss of the three groups of samples in this stage was not significant. Stage III (350℃ – 800℃) is the key stage that determines the final microstructure of the carbon material. In this stage, the polymer skeleton undergoes violent dehydrogenation, denitrification, and aromatization reactions, while the metal precursor is completely decomposed and reduced in situ to metal, metal oxide, or metal carbide. In stage IV (800℃–1000℃), the three groups of samples showed only minor weight loss of 2.8%–4.2%, indicating that the pyrolysis and carbonization processes of the three groups of samples were basically completed after 800℃, and the mass tended to stabilize. The temperature rise in this stage mainly caused further rearrangement of the carbon structure and growth of metal nanoparticles, without being accompanied by significant mass changes. The DTG curves reflected the characteristic temperatures at which the sample decomposition rate reached its maximum. The main peak temperature of Co-2 / PANF-280 was the lowest at 697℃, the main peak temperature of Fe-2 / PANF-280 was between the two at 719℃, while the main peak temperature of Ni-2 / PANF-280 was relatively the highest at 735℃, indicating that the key decomposition and transformation processes of the nickel system required higher temperatures to activate. Based on the above TG and DTG results, 700℃ was finally selected as the carbonization temperature for all samples (N2 atmosphere, holding for 120 min) to control variables and ensure comparability between the systems.

[0056] The structural characterization of the composite carbon nanofiber microwave absorbing materials prepared in Examples 1 to 9 was performed, and the specific results are as follows: 1) X-ray diffraction analysis Figure 3The XRD diffraction peaks of the F-composite carbon nanofiber microwave absorbing materials in Examples 1 to 9 are shown. No characteristic diffraction peaks of the PAN matrix were observed in any of the samples near 2θ ≈ 16.5°, while broad and diffuse diffraction peaks appeared near 24°–26°, corresponding to the (002) crystal plane diffraction characteristics of carbon materials. These characteristic peaks showed significant broadening and weak intensity, indicating that after carbonization treatment, the PAN matrix of all materials was transformed into a carbon material dominated by amorphous or disordered carbon, and a highly crystalline graphitic carbon structure had not yet been formed. In the iron-based samples, apart from the broad and diffuse diffraction peaks corresponding to the carbon matrix, no other distinguishable diffraction peaks of metals or metal carbides were observed in the XRD patterns of Fe-1 / CNF, suggesting that the iron phase may mainly exist in a small-sized, low-crystallinity, or highly dispersed nanoscale form. With increasing feed rate, diffraction peaks at 2θ≈44.7°, 65.0°, and 82.3° can be observed in Fe-2 / CNF and Fe-3 / CNF, corresponding to body-centered cubic metallic Fe (PDF#06-0696). In addition, diffraction characteristic peaks corresponding to Fe3C (PDF#35-0772) can be observed near 45°. This indicates that FePiv underwent pyrolysis reduction and carbonization reactions, forming a composite phase in which Fe and Fe3C coexist. The formation of Fe3C mainly stems from the high reactivity of Fe with carbon and its high solubility of carbon atoms at high temperatures. At the same time, the appearance of the Fe / Fe3C composite phase means additional heterogeneous interfaces and phase boundaries, providing more possibilities for subsequent electromagnetic losses. The XRD patterns of the cobalt system samples showed obvious diffraction peaks around 2θ≈44.2°, 51.5°, and 75.8°, which belong to the (111), (200), and (220) crystal planes of metallic Co (PDF#15-0806), indicating that CoPiv was mainly converted into elemental cobalt during carbonization. Compared with the iron and nickel systems, the diffraction peaks of metallic cobalt in Co-x / CNF were relatively lower in intensity and wider, indicating that the crystallinity of cobalt was relatively lower or the grain size was relatively smaller. With the increase of feed amount, the diffraction peaks of metallic cobalt gradually increased, but no sharp diffraction peaks appeared. The nickel system sample showed clear characteristic diffraction peaks near 2θ≈44.5°, 51.8°, and 76.4°, corresponding to the (111), (200), and (220) crystal planes (PDF#04-0850) of the face-centered cubic structure of metallic Ni. No diffraction peaks of NiO or other nickel oxides were observed, indicating that NiPiv was mainly reduced to metallic nickel during the carbonization process. Moreover, with the increase of NiPiv feed amount, the intensity of the diffraction peaks of metallic nickel gradually increased, and the peak shape became slightly sharper, indicating that the amount of metallic nickel generated and the degree of crystallization increased with the increase of NiPiv feed amount.

[0057] 2) Raman analysis like Figure 4As shown in (a) to (c), all samples were at 1350 cm⁻¹. −1 and 1580cm −1 The presence of distinct D and G peaks indicates that after carbonization treatment, the PAN precursor has been successfully transformed into a carbon structure dominated by sp2 carbon while still retaining certain defects, exhibiting characteristics of amorphous or disordered carbon. The ratio of the D and G peak intensities for each sample was further calculated. I D / I G ), its changing trend Figure 4 As shown in (d), I D / I G A lower value indicates fewer defects and a higher degree of graphitization in the carbon material. For nickel-based samples, as the NiPiv feed amount increases, I D / I G The value decreased slightly from 1.057 to 1.050, a small change, indicating that Ni's catalytic promoting effect on the ordered carbon structure under these carbonization conditions is relatively limited; in contrast, the cobalt system sample... I D / I G The value showed a more significant decreasing trend with the increase of CoPiv feed amount, decreasing from 1.063 to 0.919, indicating that increasing the CoPiv feed amount can significantly reduce the disorder of the carbon structure; while the iron system sample I D / I G The value decreased from 1.042 to 0.977 with increasing FePiv, showing a certain trend towards catalytic graphitization, but its rate of decrease and final order were not as good as those of the cobalt system. Under carbonization conditions of 700℃, the effects of different metal systems and feed amounts on the disorder of the PAN-derived composite carbon nanofiber structure varied significantly, but all samples showed similar results. I D / I G The values ​​are all close to 1. Combined with the results of the broadening of carbon (002) diffraction peaks and the absence of sharp graphite peaks in the previous XRD analysis, it can be confirmed that the carbon material obtained at this temperature is still mainly composed of amorphous carbon or disordered layered carbon structures, and has not yet formed a highly graphitized ordered structure. This defect-rich carbon network structure provides a suitable structural basis for the subsequent material interface-related properties.

[0058] 3) Microscopic morphology and elemental distribution analysis Microscopic and macroscopic morphological changes during spinning, pre-oxidation and carbonization processes The spun Fe-2 / PANF was cut into 2.0cm × 2.0cm squares and then subjected to subsequent pre-oxidation and carbonization treatments to obtain... Figure 5 (d) Macro photographs of each stage are shown. For example... Figure 5 As shown in (a), the spun Fe-2 / PANF fibers exhibit a continuous cylindrical structure with slight grooved texture on the surface, and an average diameter of 398±112 nm, with a relatively wide distribution. From Figure 5 (b) It can be seen that the microfiber structure has a more regular outline and a smoother surface, and the fibers remain independent without any melting, adhesion, or structural collapse. The average fiber diameter has significantly decreased to 237±60 nm, and the dispersion has become smaller. In terms of macroscopic morphology, the side length has decreased from 2.0 cm to about 1.7 cm, and the area has shrunk by about 28%. This is mainly due to the cyclization, dehydrogenation, and cross-linking reactions that occur in the PAN matrix during the pre-oxidation process, forming a thermally stable trapezoidal structure accompanied by the removal of volatile components, thereby improving structural stability and causing size shrinkage, and providing a structural basis for maintaining the integrity of the skeleton in the subsequent high-temperature carbonization stage. After carbonization at 700℃ for 2 hours, the sample has completely turned black, the side length has further shrunk to about 1.50 cm, and the area shrinkage rate is 44%. The microscopic morphology is as follows. Figure 5 As shown in (c), the fiber network structure of the sample remains continuous, but the surface changes from relatively smooth to significantly rough, with a large number of bright granular protrusions appearing. This indicates that in addition to the formation of the carbon skeleton, the carbonization process is accompanied by the in-situ reduction and precipitation of metal species, forming particles loaded on the fiber surface. It can be inferred that these bright particles are Fe / Fe3C. The apparent diameter of the carbonized fiber is statistically 290±70 nm, which is higher than that of the pre-oxidized sample, and the dispersion is increased.

[0059] SEM morphology comparison of Mx / CNF SEM analysis, such as Figure 6 As shown, the composite carbon nanofiber absorbing materials in Examples 1 to 9 all exhibit a continuous interwoven carbon nanofiber network, without large-area melting and adhesion or collapse of the skeletal network. This indicates that in the pivalic acid polynuclear metal complex precursor system, the spinning and heat treatment process parameters can maintain the basic morphological integrity of the carbon nanofiber network within different metal types and feed amounts, demonstrating process universality. Figure 6 As shown in (a) to (c), the Fe-1 / CNF fiber network is generally continuous, and many fine particle protrusions are visible on the surface; the Fe-2 / CNF fiber diameter is slightly increased, the surface particle size is larger and the outline is clearer, exhibiting a more uniform loading characteristic; a small number of breaks and slight fiber thickening are observed in the Fe-3 / CNF fiber network. Cobalt system samples are shown below. Figure 6As shown in (d) to (f), the fiber networks in Examples 4 to 6 maintained a good interweaving structure. Compared with the other two systems, the fiber surface was smoother and flatter overall, with only a few small bright spot particles. Furthermore, with the increase of CoPiv feed amount, the surface roughness only increased mildly within each group, and the morphological differences within the group were relatively small. SEM images of the nickel system samples are shown below. Figure 6 As shown in (g) to (i), the fiber surfaces of Ni-1 / CNF and Ni-2 / CNF samples remain relatively smooth, accompanied by a gradual increase in the number of fine particle bright spots. With further increases in NiPiv feed amount, Ni-3 / CNF exhibits high-density particles that are continuously distributed on the fiber surface, showing characteristics similar to a coating layer. Further magnified observation was conducted on three samples: Fe-2 / CNF, Co-2 / CNF, and Ni-2 / CNF. The results are as follows... Figure 7 As shown, the surface of Fe-2 / CNF fibers exhibits a hierarchical scale characteristic, mainly composed of fine particles with a small amount of larger particles mixed in, resulting in a more significant roughness on the fiber surface. The surface of Co-2 / CNF fibers is smoother overall, with almost no obvious nanoparticle protrusions observed, and only relatively sparse and finer bright particles distributed on its surface. Ni-2 / CNF, on the other hand, exhibits a higher density of fine particles distributed on the fiber surface, with a more uniform and continuous overall particle coverage, representing another type of granulation characteristic.

[0060] Electrical experiments were conducted on the composite carbon nanofiber microwave absorbing materials prepared in Examples 1 through 9, and the specific results are as follows: 1) Analysis of complex permittivity and dielectric loss behavior The real part of the complex permittivity of the composite carbon nanofiber absorbing materials prepared in Examples 1 to 9 in the 2 GHz to 18 GHz frequency band. ε ′、Imaginary part ε '′ and dielectric loss tangent tanδ ε The data were systematically compared, such as Figure 8 As shown. Examples 1 to 9 ε 'and ε The values ​​of '′′ gradually decrease with increasing frequency, exhibiting obvious dispersion characteristics, which reflects the weakening of the polarization process's response to high-frequency external fields; simultaneously, the presence of certain fluctuations in the high-frequency band indicates that the material may exhibit a superposition response of multi-scale polarization processes in this frequency range. For example... Figure 8 As shown in (a) to (c), the iron system samples ε 'and ε The distribution roughly follows a hierarchical pattern: Fe⁻² / CNF > Fe⁻³ / CNF >> Fe⁻¹ / CNF. Fe⁻¹ / CNF... ε 'and εThe fact that the value of '′′ remains at its minimum and varies little with frequency indicates that when the FePiv feed amount is low, there are insufficient effective units in the system capable of participating in the polarization response. (Fe-2 / CNF) ε 'and ε The FePiv flux remained the highest across the entire tested frequency range and showed a significant improvement in dielectric parameters compared to Fe-1 / CNF, indicating a substantial enhancement in polarization loss capability. This is partly due to the abundant heterostructures introduced by the generated Fe / Fe3C, which significantly enhanced interfacial polarization; and partly due to the fact that moderate graphitization can improve charge migration capability, thereby increasing the contribution to conduction loss. However, when the FePiv feed amount was further increased, the Fe-3 / CNF flux... ε 'and ε The tanδ values ​​of Fe-2 / CNF all showed varying degrees of decrease, which may be related to the localized fractures of the fiber network observed in the SEM images, weakening the effective interfacial polarization sites and dielectric dissipation capacity. Furthermore, the tanδ values ​​of Fe-2 / CNF also showed a decrease. ε It also ranked highest across the entire tested frequency range, further demonstrating its superior dielectric loss capability. The dielectric parameters of the cobalt system sample are compared to... Figure 8 As shown in (d) to (f), all values ​​are significantly lower than those of the iron system, and the differences within the system are relatively small. The Co-3 / CNF samples in... ε ′、 ε ′′ and tanδ ε The values ​​shown above are all at their lowest, indicating that excessive CoPiv feed did not lead to a stronger dielectric response. Combined with the Raman test results above, Co-3 / CNF exhibits the highest degree of graphitization, with a more ordered carbon skeleton, which to some extent reduces the defect dipole polarization of the material, leading to... ε ′′ and tanδ ε Lower. For example... Figure 8 As shown in (g) to (i), Ni-2 / CNF and Ni-3 / CNF ε The curves highly overlap and are much larger than Ni-1 / CNF, indicating that the amount of NiPiv fed into this system is from... x =1mmol increased to x At 2 mmol, the effective polarization capacity is significantly enhanced, and further increased to x However, it tends to saturate at 3 mmol / L; εThe curves generally show a trend of Ni-2 / CNF > Ni-3 / CNF > Ni-1 / CNF. In the nickel system samples, with increasing NiPiv feed amount, the fiber surface particles change from sparse to high-density coverage; Ni-3 / CNF exhibits high-density particles with continuous coverage. In contrast, Ni-2 / CNF maintains high conductivity while avoiding excessive metal particle accumulation, exhibiting relatively superior overall dielectric properties. The tanδ of the nickel system... ε The curves intersect; the tanδ of Ni-1 / CNF ε The value reaches a maximum of 0.42 in the low-frequency range, then decreases to around 0.30. Ni-2 / CNF is next, fluctuating steadily between 0.32 and 0.35, while Ni-3 / CNF is the lowest. This is due to tanδ... ε As a ratio indicator, it will be affected by the denominator. ε The effect of size, although Ni-1 / CNF ε ′′ lowest, but ε The lower ' makes tanδ ε The frequency is too high in the low-frequency range, so the Ni system relies solely on tanδ ε Judging its overall loss efficiency by this method may lead to bias. ε ′、 ε ′′ and tanδ ε In the nickel system, Ni-2 / CNF exhibits relatively superior dielectric response and dissipation balance.

[0061] The Cole-cole curves of the iron system samples are as follows: Figure 9 As shown in (a) to (c), in comparison, although Fe-3 / CNF still exhibits a semi-circular arc and linear tail, its overall size and arc complexity are lower than Fe-2 / CNF, resulting in a weaker overall dielectric loss capability. Cobalt system samples are shown below. Figure 9 As shown in (d) to (f), the overall trajectory scale is relatively small, the semi-circular arc structure is dominant, and the linear tail extension is relatively limited, indicating that the system is more inclined towards polarization relaxation loss, and the contribution of conduction loss is not prominent. However, as... Figure 9 The nickel system samples shown in (g) to (i) exhibit a change from being dominated by conduction loss to a combination of polarization relaxation and conduction loss as the amount of precursor metal added increases. Combined analysis of the complex permittivity and Cole-Cole curves reveals that the Fe system, due to the coexistence of Fe / Fe3C multiphases and a richer heterostructure interface, achieves higher conductivity under moderate conductivity pathways. ε′′ and tanδ ε Although the semi-circular relaxation characteristic of the Co system exists, its overall scale is small, and its contribution to conduction loss is limited, leading to... ε′′ and tanδ εThe dielectric response of the Ni system is relatively low, while that of the other two systems falls between the two. All three systems exhibit superior dielectric response when the precursor metal dosage is 2 mmol, indicating that this dosage maximizes the abundance of effective heterointerfaces in the material while ensuring the conductive pathway of the carbon framework.

[0062] 2) Analysis of the mechanism of complex permeability and magnetic loss like Figure 10 The real part of the complex permeability of the composite carbon nanofiber absorbing materials prepared in Examples 1 to 9 after high-temperature carbonization is given in the frequency range of 2 GHz to 18 GHz. μ′ virtual part μ′′ and magnetic loss tangent tanδ μ The curve showing the change with frequency, where, μ′ and μ′ '' reflects the material's ability to store and dissipate magnetic field energy, respectively, tanδ μ This reflects the strength of the material's magnetic loss relative to its magnetic response. Due to the limited content of magnetic components in this carbon nanofiber composite material, the μ′ values ​​of all samples are concentrated in the range of 0.8–1.1. μ′′ All values ​​were below 0.45, indicating that the overall magnetic response and magnetic loss contributions of this system were relatively weak. Its electromagnetic attenuation mechanism is more likely dominated by dielectric loss, while magnetic loss typically contributes more to impedance matching and broadband response. All samples... μ′ , μ′ and tanδ μ The values ​​decrease with increasing frequency and are accompanied by certain periodic fluctuations, indicating that there may be a superposition of multi-scale magnetic response processes in the material within this frequency band, but the overall strength is limited. Figure 10 (a) Fe-2 / CNF and Figure 10 Real part of relative permeability of Ni-2 / CNF in (g) μ′ The magnetic response of the material was significantly higher than that of the other two feed ratios in the same system, indicating that the material with a feed ratio of 2 mmol had the strongest magnetic response in the Fe and Ni system; and its corresponding μ′′ Figure 10 (b), (h) and tanδ μ Figure 10 The curves (c) to (i) almost overlap at different feed amounts, indicating that the high magnetic response of Fe-2 / CNF and Ni-2 / CNF does not correspond to high magnetic loss capability. However, under certain conditions, the impedance matching characteristics of the material can be adjusted to create conditions for electromagnetic waves to enter the material and cause dielectric loss. For the Co system, Figure 10 Samples with different feed amounts shown in (d) to (f) μ′ , μ′ and tanδ μThe overall differences are limited, indicating that the magnetic loss modulation range of the Co system is relatively weak.

[0063] like Figure 11 As shown in (a) to (c), all samples C The 0-curve decreases significantly with increasing frequency in the 2GHz–6GHz band, accompanied by obvious periodic fluctuations, indicating that the magnetic loss mechanism in this band is likely a resonant magnetic response, while at lower frequencies it is usually a natural resonance. (Nine samples) μ′ They are all concentrated in the range of 0.8 to 1.1. μ′′ All values ​​were below 0.45, indicating a weak overall magnetic response and a relatively limited contribution from magnetic loss. The magnetic loss in all samples was more likely due to a combination of competing mechanisms, including natural resonance, exchange resonance, and eddy current loss, rather than a single dominant mechanism like eddy current loss. Therefore, the electromagnetic attenuation of the system in this invention should be primarily explained by dielectric loss, while the magnetic component plays a supporting role by providing a certain magnetic response, optimizing material impedance matching, and synergistically broadening the effective bandwidth. This clarifies the boundaries for subsequent discussions on the material's microwave absorption performance mechanism.

[0064] 3) Study on electromagnetic wave absorption performance To systematically investigate the effect of iron content on the electromagnetic wave absorption performance of Fe-x / CNF composite nanofiber films, such as... Figure 12 As shown. Figure 12 (a) to (c) show that when the content of ferric pentovalerate is low, the absorption performance of the Fe-1 / CNF samples is poor. The three-dimensional reflection loss diagram shows that the reflection loss values ​​all fail to reach −10 dB. min The absorption loss is only −9.13 dB (6 mm), indicating that this material is difficult to achieve effective electromagnetic wave absorption within the test range. Simultaneously, the low content of magnetic iron in the sample results in limited magnetic loss, making it difficult to produce a significant magnetic response to electromagnetic waves, ultimately limiting the improvement of its absorption performance. With the increase of the precursor iron pivalate content, such as… Figure 12 As shown in (d) to (f), the Fe-2 / CNF sample exhibits the best broadband absorption performance, and the position of the absorption peak gradually shifts to lower frequencies with increasing thickness. At a thickness of 2.48 mm, the sample achieves an RLmin of −64.48 dB at 9.90 GHz, with an effective absorption bandwidth of 3.21 GHz; when the thickness is 1.73 mm, its EAB... maxReaching 4.91 GHz and covering the frequency range of 12.9 GHz to 17.86 GHz, the reflection loss at this thickness is −34.72 dB. Further observation from the two-dimensional reflection loss map reveals that Fe-2 / CNF forms a continuous banded effective absorption region below −10 dB, extending significantly towards thinner thicknesses, indicating stable electromagnetic wave absorption capability within a relatively thin matched thickness range. Simultaneously, the reflection loss curves at different matched thicknesses all exhibit obvious absorption peaks, demonstrating the material's excellent thickness-tuning characteristics. The significant improvement in the absorption performance of the Fe-2 / CNF sample is mainly due to the synergistic optimization of electromagnetic parameters: an appropriate amount of Fe catalyzes the rearrangement of the PAN matrix and enhances its graphitization degree, constructing a continuous and uniform carbon fiber conductive network, thus enhancing conduction loss; the formation and uniform dispersion of the Fe / Fe3C composite phase introduces abundant heterogeneous interfaces, inducing stronger interfacial polarization loss and multi-scale relaxation; Fe-2 / CNF has a relatively slightly higher... μ′ and μ′′ This helps optimize the impedance matching of materials, allowing more electromagnetic waves to enter the material and be dissipated. For example... Figure 12 As shown in (g) to (i), when the iron content further increases to 3 mmol, the microwave absorption performance of the Fe-3 / CNF sample shows a significant decrease, RL min The effective absorption bandwidth is -59.83 dB, corresponding to a matching thickness of 4.26 mm; the optimal effective absorption bandwidth is EAB. max At 4.01 GHz, the matching thickness was 1.86 mm. Based on the combined structure and electromagnetic parameters, it can be inferred that excessive iron pivalate precursors tend to agglomerate during pyrolysis and carbonization, disrupting the continuous conductive network structure of the carbon fibers, weakening conduction loss and interfacial polarization relaxation loss, leading to a decrease in overall microwave absorption performance. The electromagnetic wave absorption performance of the Fe-x / CNF system samples showed a significant pattern of first increasing and then decreasing with increasing iron pivalate precursor content. The Fe-2 / CNF sample exhibited the best microwave absorption performance, where the Fe / Fe3C magnetic nanoparticles achieved the optimal balance in inducing interfacial polarization, catalyzing the construction of a highly efficient conductive network, and optimizing impedance matching.

[0065] Analysis of the microwave absorption properties of Co-x / CNFs composite nanofibers, such as Figure 13 The study presents three-dimensional reflection loss distribution maps, two-dimensional mapping maps, and reflection loss curves at different matching thicknesses for Co-1 / CNF, Co-2 / CNF, and Co-3 / CNF in the 2GHz–18GHz range. Overall, the cobalt system samples also exhibit a characteristic of the absorption peak shifting to lower frequencies with increasing thickness. Figure 13The electromagnetic wave absorption energy and effective bandwidth of Co-1 / CNF shown in (a) to (c) are relatively limited. Its lowest reflection loss occurs at a matching thickness of 5.31 mm, RL min It is −31.23dB; its maximum effective absorption bandwidth is EAB. max The absorption band at 3.07 GHz (4.51 mm) is not continuous but rather a superposition of two frequency bands, indicating that the effective absorption of Co-1 / CNF is more sensitive to the matching of thickness and frequency, making it difficult to form a stable and continuous broadband absorption region. Co-2 / CNF exhibits significantly enhanced absorption performance and performs best in the cobalt system, achieving an RL at 7.46 GHz and 3.41 mm. min The absorption band is -56.95 dB, and even at a relatively thin thickness of 1.82 mm, the effective absorption bandwidth can reach 4.04 GHz, exhibiting a continuous banded distribution and demonstrating more stable broadband absorption capability. However, the complex dielectric parameter of the entire Co system is lower than that of Fe and Ni, which actually helps to avoid impedance mismatch caused by excessively high dielectric parameters, allowing Co-2 / CNF to achieve strong absorption and a continuous effective absorption bandwidth at a relatively thin thickness. In contrast, the electromagnetic wave absorption performance of Co-3 / CNF is significantly reduced, although its EAB... max It can reach 4.23 GHz, but its matching thickness is relatively thick at 5.76 mm, and it still does not show a continuous effective absorption bandwidth, indicating that its wider effective absorption is achieved at the expense of thickness. The cobalt system relies more on optimizing polarization relaxation loss and impedance matching to achieve effective absorption, and overall, the matching thickness of the complex system is also significantly thicker than that of the iron system.

[0066] Microwave absorption performance analysis of Ni-x / CNFs (x=1,2,3) composite nanofibers, such as Figure 14 The Ni-1 / CNF shown in (a) to (c) has relatively weak absorption performance, and its RL min At 7.21 GHz, it exhibits a value of −30.11 dB, corresponding to a matching thickness of 5.44 mm. (EAB) max At 3.73 mm, the GHz frequency reached 4.29 GHz. From the two-dimensional mapping, the strong absorption region shown in blue-green is relatively limited in distribution and does not form a significant banded structure. Ni-2 / CNF exhibits significant improvements in both strong absorption and bandwidth, such as... Figure 14 The samples shown in (d) to (f) achieved the highest absorption intensity RL among the nine samples at a thickness of 4.31 mm and a GHz frequency of 6.18 GHz. min =−69.83dB; Simultaneously, with an ultra-thin thickness of 1.90mm, a maximum effective absorption bandwidth of 4.49GHz was achieved, continuously covering 13.28GHz–17.78GHz. Ni-3 / CNF at 5.39mm RL min It reaches −66.54 dB, and its EABmax The corresponding matching thickness is 1.97 mm, with a wavelength of 4.08 GHz. However, the continuity of the strong absorption region in the two-dimensional mapping is relatively weakened. The absorption performance of the nickel system generally shows a trend of first increasing and then slightly decreasing with the increase of the amount of material. Among them, Ni-2 / CNF achieves a better synergy between attenuation capability and impedance matching, thus achieving strong absorption at low frequencies with a larger matching thickness and continuous broadband effective absorption at high frequencies with a smaller matching thickness.

[0067] 4) Absorption performance Based on the core indicators of absorbing materials in engineering applications—strong absorption, thinness, wide bandwidth, and lightweight—the RL values ​​of Examples 1 to 9 were statistically analyzed. min and its matching thickness and EAB max And its matching thickness. For example... Figure 15 As shown in (a), it is clear that the sample with low feed amount is located in the weak absorption region with a relatively thick material thickness in the lower left corner of the figure. Fe-1 / CNF does not achieve effective absorption across the entire thickness and frequency range, indicating that its electromagnetic wave attenuation path is insufficient and it is difficult to form an effective absorption peak. Although Co-1 / CNF and Ni-1 / CNF show certain reflection loss absorption peaks, the corresponding matching thicknesses are all above 5 mm, requiring a relatively thick material accumulation to achieve a certain absorption performance. In contrast, x The three samples with a concentration of 2 mmol all exhibited stronger reflection loss absorption peaks and reduced matching thickness, concentrated in the upper right corner of the figure, indicating their advantages of strong absorption and thin thickness. However, when the feed concentration was further increased to 3 mmol, Fe-3 / CNF and Ni-3 / CNF still showed lower RL values. min The values ​​all shift to the left, meaning their matching thickness increases, while the RL of Co-3 / CNF... min Then it becomes much shallower. Figure 15 (b) The three samples with a 2mmol feed amount shown also appear in the upper right corner, and their EAB max The frequencies are concentrated in the 4GHz to 5GHz range, and the corresponding matching thicknesses are all in the thin-layer range of approximately 2mm. In summary, in composite carbon nanofiber materials, x =2mmol represents the optimal precursor feed amount for strong absorption, thinness, and wide bandwidth comprehensive absorption performance obtained from different metal systems. Therefore, subsequent analysis will focus on three samples, M-2 / CNF (M=Fe,Co,Ni), to further analyze their electromagnetic wave absorption mechanism. The comparison results with previously disclosed similar transition metal / carbon-based composite nanofiber absorbing materials are shown in Table 1.

[0068] Table 1. Performance comparison of the sample in this invention with transition metal / carbon-based composite nanofiber microwave absorbing materials. Note: “—” indicates that this item was not detected. 1 is Guo YY, Zhang M, Cheng TT, et al. Enhancing electromagnetic wave absorption in carbon fiber using FeS2 nanoparticles[J]. Nano Res, 2023, 16(7): 9591–9601; 2 is Sun Y, Wang YJ, Ma HJ, et al. Fe3C nanocrystals encapsulated in N-doped carbon nanofibers as high-efficient microwave absorbers with superior oxidation / corrosion resistance[J]. Carbon, 2021, 178: 515–527; 3 is Guo RD, Su D, Chen F, et al. Hollow beaded Fe3C / N-doped carbon fibers toward broadband microwave absorption[J]. ACS Appl Mater Interfaces, 2022, 14(2): 3084–3094; 4 is Cao XH, Wu XY, Wang X, et al. Mechanistic insights into the structural evolution of ZIF-67 via electrospinning strategy toward high electromagnetic wave absorption performance of ZIF-67-derived carbon nanofibers[J]. Adv Sci, 2025, 12(26): 2502560; 5 is An B, Wu M, Yang XH, et al. Lightweight Co3O4 / CC composites with high microwave absorption performance[J]. Nanomaterials, 2023, 13(13): 1903; 6 is Shi SH, Mou PP, Wang D, et al.Diam Relat Mater, 2025,155: 112362; 8 is Yang JN, Guan GG, Xiang J, et al. Electrospinning fabrication and enhanced microwave absorption properties of nickel porous nanofibers[J]. for enhanced microwaveabsorption[J]. ACS Appl Nano Mater, 2024, 7(18): 22177–22188. .

[0069] 5) Impedance matching characteristics and attenuation capability analysis like Figure 16 The normalized input impedance magnitudes of three representative samples, Fe₂ / CNF, Co₂ / CNF, and Ni₂ / CNF, are shown. Z in / Z The curves showing the variation of 0| and the attenuation constant α with frequency.

[0070] like Figure 16 As shown in (a) to (c), the impedance matching curves at different matching thicknesses all exhibit matching peaks that vary with frequency. Furthermore, the position of the matching peak shifts towards lower frequencies as the sample matching thickness increases, indicating that all three exhibit typical thickness-tuning characteristics. The Fe-2 / CNF impedance matching curves at thickness t (shown by the blue line) show these peaks.EAB And the thickness t shown by the red line RL The average is closer | Z in / Z The dashed line represents 0|=1, and the impedance matching curves of different thicknesses are most densely distributed at the dashed line, indicating that it has excellent impedance matching characteristics over a wide range of matching thicknesses. At the same time, its attenuation constant is the highest in the entire test frequency band, indicating that its dielectric loss mechanism is richer, and electromagnetic waves entering the material can be effectively dissipated and converted into other forms of energy. This is also the reason why its overall electromagnetic wave absorption capability is the strongest.

[0071] 6) Absorption Mechanism The electromagnetic wave absorption mechanisms of Fe-2 / CNF, Co-2 / CNF and Ni-2 / CNF are as follows: Figure 17 As shown, the one-dimensional carbon nanofibers constructed by electrospinning interweave to form a three-dimensional continuous conductive framework and porous network structure. On the one hand, this causes the incident electromagnetic waves to undergo multiple reflections and scatterings between the porous fiber networks, as shown by the red broken line in the figure, significantly extending the propagation path of the electromagnetic waves. On the other hand, the graphitized carbon framework formed after high-temperature carbonization communicates with the loaded metal or metal carbide nanoparticles to jointly construct a highly efficient conductive network. Secondly, multi-scale polarization relaxation is the main pathway for electromagnetic wave dissipation. Due to the general difference in dielectric properties between metal or metal carbide nanoparticles and carbon fibers, charge carriers are hindered when migrating across heterogeneous interfaces, leading to the accumulation and uneven distribution of charges near the interface, thereby inducing a strong interfacial polarization effect. In particular, the presence of composite phases of Fe and Fe3C coexisting and hierarchical particles of different sizes in Fe-2 / CNF provides a richer heterogeneous interface, which is conducive to further enhancing interfacial polarization loss. Furthermore, the rearrangement of the carbon framework structure and the deoxygenation and dehydrogenation of functional groups during pre-oxidation and carbonization introduce a large number of defects, providing dipole polarization loss. Finally, the magnetic Fe / Fe3C, Co, and Ni nanoparticles dispersed on the carbon fiber surface provide a certain amount of magnetic loss. Although relatively limited, the eddy current loss, natural resonance, and exchange resonance introduced by the magnetic particles effectively improve the complex permeability of the material. While contributing to the magnetic loss, they also effectively improve the impedance matching characteristics of the material and broaden its effective absorption bandwidth. In summary, the electromagnetic wave absorption of M-2 / CNF is mainly due to the synergistic effect of multiple reflections and scattering, conduction loss, interface polarization relaxation loss, dipole polarization, and relatively weak magnetic response loss. By using multinucleated pentanoic acid metal complex precursors and PAN, the metal components can be uniformly introduced during the spinning solution stage, and a stable and continuous conductive framework and different nanoparticle loading morphologies can be formed after carbonization. This overcomes the bottleneck of impedance mismatch in single carbon materials and provides a structural and theoretical basis for designing lightweight, broadband, and efficient microwave absorbing materials.

[0072] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A composite carbon nanofiber microwave absorbing material, characterized in that, It was obtained by electrospinning and heat treatment using polynuclear pentanoic acid metal complexes as precursors and polyacrylonitrile as carbon source. The polynuclear pentamate metal complex is any one of trinuclear iron pentamate, dinuclear cobalt pentamate, and dinuclear nickel pentamate.

2. The composite carbon nanofiber microwave absorbing material according to claim 1, characterized in that, The molar mass ratio of the metal in the polynuclear pentovalinic acid metal complex to the polyacrylonitrile is 1 mmol to 3 mmol: 1 g.

3. A method for preparing the composite carbon nanofiber microwave absorbing material according to claim 2, characterized in that, Includes the following steps: Polyacrylonitrile was mixed with polynuclear terpentine metal complex in an organic environment to obtain an electrospinning precursor solution. The electrospinning precursor solution was prepared at an operating voltage of 13.00 kV to 13.6 kV, a distance of 15 cm to 20 cm between the spinning needle and the receiving roller, and a feed rate of 0.60 mL / h. −1 ~0.70 mL·h −1 The drum speed is 130 r·min −1 The translation distance of the propulsion device is 90mm~100mm, and the translation speed is 150mm·min. −1 Electrospinning was performed under the specified conditions to obtain the product; After pre-oxidation treatment, the product is then carbonized to obtain a composite carbon nanofiber microwave absorbing material.

4. The method for preparing the composite carbon nanofiber microwave absorbing material according to claim 3, characterized in that, The carbonization process was carried out at a temperature of 700°C for 120 minutes.

5. The method for preparing the composite carbon nanofiber microwave absorbing material according to claim 3, characterized in that, The pre-oxidation treatment refers to holding at 280℃ for 120 minutes.

6. The method for preparing the composite carbon nanofiber microwave absorbing material according to claim 3, characterized in that, The reagent used in the organic environment is N,N-dimethylformamide.

7. The method for preparing the composite carbon nanofiber microwave absorbing material according to claim 3, characterized in that, The heating rate for carbonization is 5℃·min. −1 The heating rate for the pre-oxidation treatment is 1℃·min. −1 .

8. The application of the composite carbon nanofiber absorbing material according to claim 2 in the preparation of absorbing materials.

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