A method for preparing a band-tunable CoFe@C / ZnO coral-structured lightweight microwave absorbing material

Abstract

This invention discloses a method for preparing a band-tunable CoFe@C / ZnO coral-structured lightweight microwave absorbing material, belonging to the field of electromagnetic functional materials. The invention first obtains a coral-like porous carbon matrix embedded with CoFe nanocrystals through solvothermal and high-temperature heat treatment. Subsequently, based on interface modulation, ZnO quantum dots rich in oxygen vacancies are in situ loaded on the matrix surface, ultimately obtaining a semiconductor quantum dot-modified CoFe@C / ZnO composite material. Based on a lightweight coral-like CoFe@C matrix, this invention introduces semiconductor quantum dot modification through interface modulation to optimize input impedance and improve electromagnetic wave incident rate. Simultaneously, relying on the constructed multiphase heterogeneous interface to enhance synergistic loss, the material achieves efficient and band-tunable electromagnetic wave absorption performance in the 2-18 GHz frequency band, while maintaining its overall lightweight advantage.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic functional materials, specifically relating to a method for preparing a band-tunable CoFe@C / ZnO coral-structured lightweight microwave absorbing material. Background Technology

[0002] With the rapid development of electronic technology and communication tools, the development of a new generation of microwave absorbing materials with comprehensive properties of "thin, light, wide, and strong" has become an industry trend. Metal-organic frameworks (MOFs) are crystalline porous materials formed by the self-assembly of metal nodes and organic ligands through coordination bonds. They possess significant structural advantages, such as large specific surface area, high porosity, ordered pores, and highly tunable chemical and topological structures. Through controlled pyrolysis, MOFs can be transformed into magnetic / carbon-based composite materials with rich heterogeneous interfaces, geometric structures, and tunable electromagnetic parameters, providing a highly promising platform for constructing lightweight and efficient microwave absorbing materials. For example, patent application CN202310103356.7 discloses a nano-ZrO2 / C heat-resistant microwave absorbing material derived from UiO-66MOF, and patent application CN201910106896.4 discloses a CoNi / C composite material based on the pyrolysis of CoNi-MOF, its preparation method, and applications, etc.

[0003] However, the electromagnetic properties of single MOF-derived carbon materials are often constrained by the inherent composition and configuration of the precursor, resulting in an imbalance between intrinsic permittivity and permeability. This leads to severe impedance mismatch, causing incident microwaves to be reflected at the material's outer interface and unable to penetrate the interior. This has become a core bottleneck restricting the performance improvement of MOF-based microwave absorbing materials. Existing patents mainly focus on two main lines: magnetic composite and dielectric nano-coating. However, both have inherent defects that make it difficult to balance impedance matching and electromagnetic loss. Magnetic / MOF derivative composites have limited ability to improve magnetic loss due to the small size of the magnetic domains, while dielectric nano-coating causes a significant sacrifice in intrinsic conductivity and polarization loss, resulting in overall microwave absorption performance that fails to meet practical application requirements. Summary of the Invention

[0004] This invention provides a band-tunable lightweight CoFe@C / ZnO coral-structured microwave absorbing material and its preparation method. The band-tunable lightweight CoFe@C / ZnO quantum dot electromagnetic wave absorbing composite material is prepared using a liquid-phase reflux strategy. Through the lightweight interface modification of quantum dots, the contradiction between the high loss of the highly conductive carbon skeleton and impedance matching is effectively resolved.

[0005] Zero-dimensional semiconductor quantum dots exhibit enormous potential for interface dielectric manipulation due to their quantum confinement effect, abundant surface defects, and extremely high specific surface area. In-situ growth of semiconductor quantum dots on the surface of MOF-derived coral-like porous carbon frameworks can, on the one hand, physically block the continuous conductive network formed by high-temperature pyrolysis in the carbon matrix, fundamentally improving surface eddy currents; on the other hand, their huge specific surface area allows for the construction of rich multiphase heterogeneous interfaces at the microscale, introducing additional interface polarization and defect losses. This effectively achieves the synergy of spatial impedance optimization and multiple dissipation mechanisms, improving the overall absorption efficiency and performance of the composite system.

[0006] This invention first synthesizes a coral-like metal-organic framework (CoFe-MOF) via a solvothermal method, and then obtains a coral-like porous carbon matrix (CoFe@C) embedded with CoFe nanocrystals through high-temperature heat treatment. This coral-like matrix is ​​composed of numerous interwoven and assembled flower-like porous carbon units, where the uniformly embedded CoFe nanocrystals contribute strong intrinsic magnetic losses, while the interwoven petal-like porous carbon framework constructs a rich mesoporous structure and scattering interfaces. Next, a low-temperature reflux strategy based on triethylene glycol (TEG) solvent is used to in-situ load semiconductor ZnO quantum dots onto the surface of the coral-like CoFe@C. This method utilizes the reducing and strong chelating properties of TEG to achieve controllable nucleation of zinc ion precursors, introducing abundant oxygen vacancies into the ZnO lattice while completely preserving the fine porous framework of the matrix. With the in-situ attachment of ZnO quantum dots, the macroscopic electrical conductance loss of the system is moderately reduced, the external interface impedance of the composite material is significantly optimized, and the incident electromagnetic waves can be transmitted into the porous skeleton. Through multiple reflections and scatterings by the coral-like structure, the electromagnetic wave absorption performance of the composite system is greatly improved in conjunction with multiple mechanisms such as interface polarization.

[0007] The present invention discloses a method for preparing a band-tunable CoFe@C / ZnO coral-structured lightweight microwave absorbing material, comprising the following steps:

[0008] Step 1: Cobalt salt, iron salt and organic ligand are dissolved in an organic solvent to synthesize a coral-like metal-organic framework CoFe-MOF precursor by a solvothermal method;

[0009] Step 2: The coral-like metal-organic framework CoFe-MOF precursor obtained in Step 1 is subjected to high-temperature heat treatment under an inert atmosphere to carbonize and reduce it in situ, thereby obtaining a coral-like porous carbon matrix embedded with CoFe nanocrystals.

[0010] Step 3: The coral-like porous carbon matrix with CoFe nanocrystals embedded in Step 2 is dispersed in triethylene glycol (TEG) solvent, a zinc source and an alkaline substance are added, and a liquid-phase reaction is carried out under heating and reflux conditions to allow the zinc ion precursor to nucleate and crystallize. ZnO quantum dots are then grown in situ on the surface of the porous carbon matrix to obtain a coral-like CoFe@C / ZnO quantum dot composite microwave absorbing material.

[0011] further:

[0012] In step 1, the cobalt salt is cobalt nitrate hexahydrate (Co(NO3)2·6H2O), the iron salt is ferric chloride hexahydrate (FeCl3·6H2O), and the organic ligand is 2,5-dihydroxyterephthalic acid (DHTA).

[0013] In step 1, the reaction temperature of the solvothermal method is 100℃-120℃, and the reaction time is 6-8 h.

[0014] In step 2, the high-temperature heat treatment temperature is 750-850℃, and the holding time is 2-3 hours.

[0015] In step 3, the zinc source is anhydrous zinc chloride; the alkaline substance is sodium hydroxide.

[0016] The specific steps include:

[0017] In step 1, 150 mg of cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 140 mg of ferric chloride hexahydrate (FeCl3·6H2O), and 100 mg of 2,5-dihydroxyterephthalic acid (DHTA) were dissolved in 60 mL of N,N-dimethylformamide (DMF) and stirred continuously for 30 min to ensure homogeneity. The mixture was then transferred to a polytetrafluoroethylene-lined high-pressure reactor and subjected to a solvothermal reaction at 100℃-120℃ for 6-8 h. After natural cooling, the mixture was washed and vacuum dried to obtain the coral-like metal-organic framework CoFe-MOF precursor.

[0018] In step 2, 0.75-1.5g of CoFe-MOF powder is placed in a crucible and heated to 750-850℃ at a rate of 2℃ / min under nitrogen atmosphere protection, and held for 2-3 h to obtain a coral-like porous carbon matrix embedded with CoFe nanocrystals.

[0019] In step 3, 160-320 mg of coral-like porous carbon matrix embedded with CoFe nanocrystals was weighed and added to 30 mL of triethylene glycol (TEG), and sonicated for 30 min to ensure sufficient dispersion. Subsequently, 60-120 mg of anhydrous zinc chloride and 40-80 mg of sodium hydroxide were added to the mixed solution, and the mixture was stirred for 5 min before being transferred to a three-necked flask. The liquid phase system was refluxed under normal pressure and heated to 80-90 °C and kept at that temperature for 3-4 h. Then, the temperature was raised to 120 °C and heated for another 1-2 h to promote crystallization and control the growth of ZnO crystals within the quantum dot scale range. After natural cooling, the mixture was magnetically separated, washed with deionized water and anhydrous ethanol, and vacuum dried to obtain the coral-like CoFe@C / ZnO quantum dot composite microwave absorbing material.

[0020] This invention first synthesizes a coral-like metal-organic framework (CoFe-MOF) via a solvothermal method, and then obtains a porous carbon derivative (CoFe@C) that retains the coral-like precursor skeleton and contains embedded CoFe magnetic nanocrystals through high-temperature heat treatment. Next, a liquid-phase reflux method is used to guide the controlled nucleation of zinc ion precursors using the strong chelating and reducing properties of triethylene glycol (TEG), allowing semiconductor ZnO quantum dots to be attached in situ to the surface of the coral-like carbon skeleton. This multiphase composite of a lightweight porous coral-like matrix and surface-dispersed semiconductor quantum dots gives it the characteristics of a lightweight and highly efficient microwave absorbing material with precisely tunable surface impedance and highly synergistic multiple loss mechanisms.

[0021] The beneficial effects of this invention are reflected in:

[0022] 1. This invention effectively optimizes the impedance matching characteristics of composite materials. The in-situ grown semiconductor ZnO quantum dots form a physical barrier to the three-dimensional highly graphitized carbon framework. This steric hindrance effect greatly restricts the long-range migration of free electrons, fundamentally suppressing electromagnetic wave reflection caused by the skin effect.

[0023] 2. This invention introduces a multi-polarization loss mechanism. Although the conductivity decreases, the discrete ZnO quantum dots construct abundant multiphase heterogeneous interfaces (CoFe / C and ZnO / C), inducing strong Maxwell-Wagner interface polarization; at the same time, the large number of oxygen vacancies generated during ZnO growth serve as dipole centers, effectively compensating for dielectric loss and forming tunable high and low absorption bands.

[0024] 3. The preparation process of this invention is controllable. The TEG liquid-phase reflux method avoids the damage to the internal CoFe magnetic nanocrystals and porous carbon framework structure caused by secondary high-temperature calcination, thus inheriting the synergistic dielectric / magnetic loss characteristics of the matrix. Under optimal control, the material exhibits a wide effective absorption bandwidth and strong absorption depth in both high and low frequency bands. Attached Figure Description

[0025] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a SEM image of CoFe-MOF in Example 1.

[0027] Figure 2 This is a SEM image of CoFe@C from Example 1.

[0028] Figure 3 This is a TEM image of the CoFe@C / ZnO quantum dot composite material in Example 1.

[0029] Figure 4 This is an HRTEM image of the interface of the CoFe@C / ZnO quantum dot composite material in Example 1.

[0030] Figure 5 The image shows the reflection loss (RL) curves of samples with different thicknesses in Example 1 at 2-18 GHz.

[0031] Figure 6 The XRD patterns of the samples obtained in Comparative Examples 1-3 are shown.

[0032] Figure 7 The image shows the reflection loss (RL) curves of samples with different thicknesses in Comparative Example 1 at 2-18 GHz.

[0033] Figure 8 This is an HRTEM image of the interface of the CoFe@C / ZnO quantum dot composite material in Example 2.

[0034] Figure 9 The image shows the reflection loss (RL) curves of samples with different thicknesses in Example 2 at 2-18 GHz.

[0035] Figure 10 This is a SEM image of CoFe@C in Example 3.

[0036] Figure 11 The image shows the reflection loss (RL) curves of samples with different thicknesses in Example 3 at 2-18 GHz.

[0037] Figure 12 This is a SEM image of the CoFe@C / ZnO composite material in Example 4.

[0038] Figure 13 The image shows the reflection loss (RL) curves of samples with different thicknesses in Example 4 at 2-18 GHz. Detailed Implementation

[0039] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. Obviously, the embodiments shown in this invention are only some embodiments of the invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of this invention.

[0040] Example 1:

[0041] The preparation method of the CoFe@C / ZnO coral-structured lightweight microwave absorbing material in this embodiment includes the following steps:

[0042] 1. Weigh 150 mg of cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 140 mg of ferric chloride hexahydrate (FeCl3·6H2O), and 100 mg of 2,5-dihydroxyterephthalic acid (DHTA) into 60 mL of N,N-dimethylformamide (DMF) and stir continuously for 30 min to mix evenly. Transfer the mixture to a high-pressure reactor lined with polytetrafluoroethylene and carry out a solvothermal reaction at 110 °C for 6 h. After natural cooling, wash and vacuum dry to obtain the coral-like metal-organic framework CoFe-MOF precursor.

[0043] 2. Take 1.0 g of the above coral-like metal-organic framework CoFe-MOF precursor powder and place it in a crucible. Under nitrogen atmosphere protection, heat it to 800℃ at a rate of 2℃ / min and hold it for 2 h to obtain a black coral-like porous carbon matrix (CoFe@C) embedded with CoFe nanocrystals.

[0044] 3. Weigh 320 mg of the CoFe@C powder obtained in step 2 and add it to 30 mL of triethylene glycol (TEG). Sonicate the mixture for 30 min to ensure thorough dispersion. Then, add 60 mg of anhydrous zinc chloride and 40 mg of sodium hydroxide to the mixture. Stir continuously for 5 min and transfer to a three-necked flask. Reflux the liquid phase system under normal pressure and heat to 90 °C. Maintain the temperature for 3 h and then raise the temperature to 120 °C and continue heating for 1 h. After natural cooling, separate the components magnetically and wash with deionized water and anhydrous ethanol. After vacuum drying, obtain the coral-like CoFe@C / ZnO quantum dot composite microwave absorbing material.

[0045] Figure 1 and Figure 2 The images show SEM images of the CoFe-MOF precursor and CoFe@C powder in this embodiment, respectively, demonstrating that the CoFe@C powder prepared in this embodiment inherits the coral-like morphology of the CoFe-MOF precursor.

[0046] Figure 3 and Figure 4 These are TEM and HRTEM images of the interface of the CoFe@C / ZnO quantum dots in this embodiment. The interface image shows that 4-6 nm ZnO quantum dots are uniformly grown on the outer interface of the three-dimensional coral-like derivative, and the stripe pattern of the quantum dots indicates that the loading is ZnO.

[0047] Figure 5 The image shows the reflection loss (RL) curves of a paraffin sample containing 30 wt% CoFe@C / ZnO quantum dots from this embodiment at 2-18 GHz. It can be seen that the CoFe@C / ZnO quantum dot composite material achieves a minimum RL value of -56.20 dB at a high frequency of 14.16 GHz with a thickness of 1.8 mm, and possesses a wide absorption band of 5.2 GHz, achieving effective coverage of most areas of the Ku-band.

[0048] Comparative Example 1:

[0049] The preparation steps of this comparative example are basically the same as those of Example 1, except that step 3 is not included, i.e., ZnO quantum dots are not loaded on the substrate surface. The resulting pure substrate is labeled CF-800.

[0050] Comparative Example 2:

[0051] The preparation steps of this comparative example are basically the same as those of comparative example 1, except that in step 2, the high-temperature heat treatment temperature is 700℃, marked as CF-700.

[0052] Comparative Example 3:

[0053] The preparation steps of this comparative example are basically the same as those of comparative example 1, except that in step 2, the high-temperature heat treatment temperature is 600℃, marked as CF-600.

[0054] Figure 6 The XRD patterns of the materials obtained in Comparative Examples 1, 2, and 3 are shown. Under high-temperature reduction at 800℃, the metal source in the MOF was completely reduced to CoFe magnetic nanocrystals, and the interface between the carbon skeleton and CoFe was partially graphitized under catalysis. However, at lower pyrolysis temperatures, such as 600℃ and 700℃, due to insufficient carbon reduction capacity, the product was mainly a mixed phase of Fe3O4 and CoFe, and the transformation to the CoFe alloy phase was not completely completed.

[0055] Figure 7The image shows the reflection loss (RL) curves of a paraffin sample containing 30 wt% CF-800 prepared in Comparative Example 1 from 2 to 18 GHz. Its minimum RL is only -10.74 dB at a thickness of 1.275 mm and a frequency of 16.96 GHz. This poor absorption performance indicates that the excessively high intrinsic dielectric constant of this graphitized carbon matrix induces a severe surface impedance mismatch.

[0056] Example 2:

[0057] The preparation method of the CoFe@C / ZnO coral-structured lightweight microwave absorbing material in this embodiment includes the following steps:

[0058] 1. Weigh 150 mg of cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 140 mg of ferric chloride hexahydrate (FeCl3·6H2O), and 100 mg of 2,5-dihydroxyterephthalic acid (DHTA) into 60 mL of N,N-dimethylformamide (DMF) and stir continuously for 30 min to mix evenly. Transfer the mixture to a high-pressure reactor lined with polytetrafluoroethylene and carry out a solvothermal reaction at 110 °C for 6 h. After natural cooling, wash and vacuum dry to obtain the coral-like metal-organic framework CoFe-MOF precursor.

[0059] 2. Take 1.0 g of the above coral-like metal-organic framework CoFe-MOF precursor powder and place it in a crucible. Under nitrogen atmosphere protection, heat it to 800℃ at a rate of 2℃ / min and hold it for 2 h to obtain a black coral-like porous carbon matrix (CoFe@C) embedded with CoFe nanocrystals.

[0060] 3. Weigh 160 mg of the CoFe@C powder obtained in step 2 and add it to 30 mL of triethylene glycol (TEG). Sonicate the mixture for 30 min to ensure thorough dispersion. Then, add 60 mg of anhydrous zinc chloride and 40 mg of sodium hydroxide to the mixture. Stir continuously for 5 min and transfer to a three-necked flask. Reflux the liquid phase system under normal pressure and heat to 90 °C for 3 h. Then, raise the temperature to 120 °C and continue heating for 1 h. After natural cooling, separate the components magnetically and wash with deionized water and anhydrous ethanol. After vacuum drying, obtain the coral-like CoFe@C / ZnO quantum dot composite microwave absorbing material.

[0061] Figure 8 This is an HRTEM image of the interface of the CoFe@C / ZnO quantum dots in this embodiment. The interface image shows that the ZnO quantum dots begin to aggregate due to dipole-dipole interactions.

[0062] Figure 9The image shows the reflection loss (RL) curves of a paraffin sample containing 30 wt% CoFe@C / ZnO quantum dots from this embodiment in the 2-18 GHz range. It can be seen that the CoFe@C / ZnO quantum dot composite material achieves a maximum effective absorption bandwidth of 4.92 GHz at a thickness of 1.775 mm. At a thickness of 6.45 mm, it achieves a minimum RL value of -62.07 dB in the low-frequency S-band at 3.48 GHz, with an effective absorption bandwidth of 1.2 GHz. Compared to Example 1, the mass percentage of the carbon matrix is ​​reduced in this embodiment, resulting in a decrease in the overall intrinsic dielectric loss capability of the composite material. This change in dielectric parameters causes the effective impedance matching of the material to shift towards the larger thickness range, thereby achieving excellent low-frequency electromagnetic wave absorption characteristics at greater thicknesses.

[0063] Example 3:

[0064] The preparation method of the CoFe@C / ZnO coral-structured lightweight microwave absorbing material in this embodiment includes the following steps:

[0065] 1. Weigh 150 mg of cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 140 mg of ferric chloride hexahydrate (FeCl3·6H2O), and 100 mg of 2,5-dihydroxyterephthalic acid (DHTA) into 60 mL of N,N-dimethylformamide (DMF) and stir continuously for 30 min to mix evenly. Transfer the mixture to a high-pressure reactor lined with polytetrafluoroethylene and carry out a solvothermal reaction at 150 °C for 6 h. After natural cooling, wash and vacuum dry to obtain the metal-organic framework CoFe-MOF precursor.

[0066] 2. Take 1.0 g of the above metal-organic framework CoFe-MOF precursor powder and place it in a crucible. Under nitrogen atmosphere protection, heat it to 800℃ at a rate of 2℃ / min and hold it for 2 h to obtain a black coral-like porous carbon matrix (CoFe@C) embedded with CoFe nanocrystals.

[0067] 3. Weigh 160 mg of the CoFe@C powder obtained in step 2 and add it to 30 mL of triethylene glycol (TEG). Sonicate the mixture for 30 min to ensure thorough dispersion. Then, add 60 mg of anhydrous zinc chloride and 40 mg of sodium hydroxide to the mixture. Stir continuously for 5 min and transfer to a three-necked flask. Reflux the liquid phase system under normal pressure and heat to 90 °C for 3 h. Then, raise the temperature to 120 °C and continue heating for 1 h. After natural cooling, separate the components magnetically and wash with deionized water and anhydrous ethanol. After vacuum drying, obtain the CoFe@C / ZnO quantum dot composite microwave absorbing material.

[0068] Figure 10This is a SEM image of CoFe@C after pyrolysis treatment with increased hydrothermal temperature. It can be seen that the high temperature caused the collapse of the three-dimensional coral-like structure.

[0069] Figure 11 The image shows the reflection loss (RL) curves of a paraffin sample containing 30 wt% CoFe@C / ZnO quantum dots from this embodiment in the 2-18 GHz range. It can be seen that the CoFe@C / ZnO quantum dot composite material achieves a maximum effective absorption bandwidth of 4.52 GHz at a thickness of 1.775 mm. At a thickness of 7.55 mm, it achieves a minimum RL value of -21.86 dB in the S-band at 3 GHz, with an effective absorption bandwidth of 0.76 GHz. The collapse of the three-dimensional coral-like framework of the matrix drastically reduces the effective specific surface area and surface defect sites, leading to further aggregation of quantum dots and completely destroying the material's originally excellent surface impedance matching and multipolarization mechanism, thus causing a significant degradation in absorption performance.

[0070] Example 4:

[0071] The preparation method of the CoFe@C / ZnO coral-structured lightweight microwave absorbing material in this embodiment includes the following steps:

[0072] 1. Weigh 150 mg of cobalt nitrate hexahydrate (Co(NO3)2·6H2O), 140 mg of ferric chloride hexahydrate (FeCl3·6H2O), and 100 mg of 2,5-dihydroxyterephthalic acid (DHTA) into 60 mL of N,N-dimethylformamide (DMF) and stir continuously for 30 min to mix evenly. Transfer the mixture to a high-pressure reactor lined with polytetrafluoroethylene and carry out a solvothermal reaction at 110 °C for 6 h. After natural cooling, wash and vacuum dry to obtain the coral-like metal-organic framework CoFe-MOF precursor.

[0073] 2. Take 1.0 g of the above coral-like metal-organic framework CoFe-MOF precursor powder and place it in a crucible. Under nitrogen atmosphere protection, heat it to 800℃ at a rate of 2℃ / min and hold it for 2 h to obtain a black coral-like porous carbon matrix (CoFe@C) embedded with CoFe nanocrystals.

[0074] 3. Weigh 320 mg of the CoFe@C powder obtained in step 2 and add it to 30 mL of triethylene glycol (TEG). Sonicate the mixture for 30 min to ensure thorough dispersion. Then, add 120 mg of anhydrous zinc chloride and 80 mg of sodium hydroxide to the mixture. Stir continuously for 5 min and transfer to a three-necked flask. Reflux the liquid phase system under normal pressure and heat to 90 °C for 3 h. Then, raise the temperature to 120 °C and continue heating for 4 h. After natural cooling, separate the components magnetically and wash with deionized water and anhydrous ethanol. After vacuum drying, obtain the coral-like CoFe@C / ZnO quantum dot composite microwave absorbing material.

[0075] Figure 12 This is a SEM image of the CoFe@C / ZnO composite material in this embodiment. After increasing the amount of zinc source and alkali source and extending the growth time, ZnO evolved into nanoparticles with a size of 20-40 nm.

[0076] Figure 13 The image shows the reflection loss (RL) curves of the paraffin sample containing 30 wt% CoFe@C / ZnO composite material from this embodiment at 2-18 GHz. It can be seen that the CoFe@C / ZnO composite material achieved a minimum RL value of -25.11 dB at a thickness of 1.45 mm. Compared to Example 1, the higher zinc precursor concentration and longer growth time in this embodiment synergistically led to overgrowth of ZnO crystals, significantly weakening the size effect and quantum confinement effect unique to zero-dimensional quantum dots. The sharp decrease in specific surface area resulted in a significant reduction in surface defect polarization centers such as oxygen vacancies, and it was also impossible to finely control the surface impedance like quantum dots.

[0077] In summary, by changing the synthesis temperature of the MOF precursor, the carbonization pyrolysis temperature, the matrix-zinc source ratio, and the solvothermal growth time, the zero-dimensional / three-dimensional heterostructure, surface impedance, high and low frequency absorption bands, and microwave electromagnetic loss performance of the three-dimensional coral-like CoFe@C / ZnO composite system can be precisely modulated. This strategy effectively achieves flexible adjustment of electromagnetic wave absorption performance and effective absorption bandwidth, and has good application value. The above embodiments are typical embodiments of the present invention and are not intended to limit the present invention in any way. For example, hydrothermal temperature, hydrothermal time, heat treatment temperature, heat treatment time, material ratio and content, etc., can be further adjusted. Therefore, based on the overall concept of the present invention, any adjustments and modifications to the process parameters described by those skilled in the art, as long as they do not deviate from the inventive concept or exceed the scope defined in the claims, should fall within the protection scope of the present invention.

[0078] The above description is merely an example and illustration of the structure of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described, or use similar methods to replace them, as long as they do not deviate from the structure of the present invention or exceed the scope defined in the claims, all of which should fall within the protection scope of the present invention.

Claims

1. A method for preparing a band-tunable CoFe@C / ZnO coral-structured lightweight microwave absorbing material, characterized in that: First, a coral-like metal-organic framework (CoFe-MOF) precursor was synthesized via a solvothermal method and then subjected to high-temperature heat treatment for carbonization and in-situ reduction to obtain a coral-like porous carbon matrix embedded with CoFe nanocrystals. Next, ZnO quantum dots were grown in-situ on the outer surface of the porous carbon framework using a liquid-phase reflux method. The reducing properties, strong chelating effect, and high boiling point of triethylene glycol solvent were utilized to guide the controllable nucleation of the zinc ion precursor, thereby achieving in-situ attachment of ZnO quantum dots to the carbon framework. Finally, a coral-like CoFe@C / ZnO quantum dot composite microwave absorbing material with complete morphology and synergistic multi-loss characteristics was obtained.

2. The preparation method according to claim 1, characterized in that... Includes the following steps: Step 1: Cobalt salt, iron salt and organic ligand are dissolved in an organic solvent to synthesize a coral-like metal-organic framework CoFe-MOF precursor by a solvothermal method; Step 2: The coral-like metal-organic framework CoFe-MOF precursor obtained in Step 1 is subjected to high-temperature heat treatment under an inert atmosphere to carbonize and reduce it in situ, thereby obtaining a coral-like porous carbon matrix embedded with CoFe nanocrystals. Step 3: The coral-like porous carbon matrix with CoFe nanocrystals embedded in Step 2 is dispersed in triethylene glycol solvent, a zinc source and an alkaline substance are added, and a liquid-phase reaction is carried out under heating and reflux conditions to allow the zinc ion precursor to nucleate and crystallize, and ZnO quantum dots are grown in situ on the surface of the porous carbon matrix to obtain a coral-like CoFe@C / ZnO quantum dot composite microwave absorbing material.

3. The preparation method according to claim 2, characterized in that: In step 1, the cobalt salt is cobalt nitrate hexahydrate, the iron salt is ferric chloride hexahydrate, and the organic ligand is 2,5-dihydroxyterephthalic acid.

4. The preparation method according to claim 2, characterized in that: In step 1, the reaction temperature of the solvothermal method is 100℃-120℃, and the reaction time is 6-8 h.

5. The preparation method according to claim 2, 3 or 4, characterized in that: In step 1, 150 mg of cobalt nitrate hexahydrate, 140 mg of ferric chloride hexahydrate, and 100 mg of 2,5-dihydroxyterephthalic acid were dissolved in N,N-dimethylformamide and stirred until homogeneous. The mixture was then transferred to a high-pressure reactor lined with polytetrafluoroethylene and subjected to a solvothermal reaction at 100℃-120℃ for 6-8 h. After natural cooling, the mixture was washed and vacuum dried to obtain the coral-like metal-organic framework CoFe-MOF precursor.

6. The preparation method according to claim 2, characterized in that: In step 2, the high-temperature heat treatment temperature is 750-850℃, and the holding time is 2-3 hours.

7. The preparation method according to claim 6, characterized in that: Under nitrogen atmosphere protection, the temperature is increased to 750-850℃ at a rate of 2℃ / min and held for 2-3 hours.

8. The preparation method according to claim 2, characterized in that: In step 3, the zinc source is anhydrous zinc chloride; the alkaline substance is sodium hydroxide.

9. The preparation method according to claim 8, characterized in that: In step 3, 160-320 mg of coral-like porous carbon matrix embedded with CoFe nanocrystals was weighed and added to triethylene glycol and ultrasonically dispersed evenly. Subsequently, 60-120 mg of anhydrous zinc chloride and 40-80 mg of sodium hydroxide were added to the mixed solution. The liquid phase system was refluxed under normal pressure and heated to 80-90°C and kept at that temperature for 3-4 h. Then, the temperature was raised to 120°C and heated further to promote crystallization. After natural cooling, the materials are magnetically separated, cleaned with deionized water and anhydrous ethanol, and then vacuum dried to obtain a coral-like CoFe@C / ZnO quantum dot composite microwave absorbing material.

10. The preparation method according to claim 9, characterized in that: Increase the temperature to 120℃ and continue heating for 1-2 hours to promote crystallization.

Images