Efficient broadband microwave absorption composite material as well as preparation method and application thereof

Abstract

The invention discloses a high-efficiency broadband microwave absorption composite material as well as a preparation method and application thereof, and belongs to the technical field of microwave absorption materials. The high-efficiency broadband microwave absorption composite material comprises a matrix and a conductive phase, the matrix is an amino-containing metal organic framework MIL-125-NH2 or a derivative thereof, the conductive phase is a double-walled carbon nanotube, the matrix is generated in situ on the double-walled carbon nanotube, the carbon nanotube is dispersed on the surface of the matrix and in a pore channel, a through multi-dimensional conductive network is formed, and the conductive phase is a metal organic framework MIL-125-NH2 or a derivative thereof. The mass fraction of the composite-walled carbon nanotubes in the composite material is 1-30 wt%. By adopting the broadband microwave absorption composite material as well as the preparation method and the application thereof, the problems of high density and narrow absorption frequency band of the existing microwave absorption composite material can be solved.

Description

Technical Field

[0001] This invention relates to the field of microwave absorbing materials technology, and in particular to a high-efficiency broadband microwave absorbing composite material, its preparation method, and its application. Background Technology

[0002] With the rapid development of 5G communication, automotive radar, IoT terminals, and high-density electronic packaging, electromagnetic radiation and crosstalk problems are becoming increasingly severe. While traditional metal plate or ferrite-based microwave absorbing materials possess some absorption capabilities, they generally suffer from high density, thick requirements, and narrow operating frequency bands, making it difficult to meet the engineering requirements of "thin, lightweight, wide-bandwidth, and strong absorption." Many existing systems require high filler volume fractions or large matching thicknesses to achieve sufficient reflection loss intensity, leading to increased overall device weight and difficulties in fabrication, thus limiting their application in aerospace, wearable electronics, and other fields.

[0003] Metal-organic frameworks (MOFs) are widely used as precursors or templates for microwave absorbing materials due to their designable composition, large specific surface area, rich pore structure, and tunable coordination environment. After high-temperature carbonization, MOFs can yield porous carbon / metal (or metal compound) composites with a certain pore structure, introducing numerous defect sites and multiphase interfaces at the microscale, which is beneficial for multiple reflections, scattering, and polarization losses of electromagnetic waves. In recent years, researchers have achieved significant dielectric loss enhancement and impedance matching optimization by compositing MOFs with conductive phases such as carbon nanotubes, graphene, and nitrogen-doped carbon to construct multidimensional conductive networks and multi-level heterogeneous interfaces.

[0004] However, existing MOF-derived microwave absorbing materials still have the following common problems: (1) Frequency band selectivity and bandwidth limitation: Many MOF-derived absorbing materials perform well in specific frequency bands, but their effective absorption bandwidth is limited and it is difficult to cover the 2–18 GHz wideband range.

[0005] (2) Density and filler ratio issues: Some materials have high metal content or require a higher filler ratio, which increases the overall density and is not conducive to lightweight design.

[0006] (3) The preparation route is complex and the process window is narrow: strategies such as metal-polyphenol network and MOF-on-MOF heterostructure usually involve multi-step synthesis or strict lattice matching control, which are complex and difficult to scale up.

[0007] Especially for Ti-based MOF materials, such as NH2-MIL-125, on the one hand, their amino ligands can introduce fixed dipole and defect sites, which is beneficial to dipole polarization and interfacial polarization; on the other hand, they are non-magnetic centers, and usually require the introduction of additional magnetic components or the construction of complex magneto-electric synergistic structures to obtain ideal impedance matching and microwave absorption performance. Therefore, how to simplify the process to construct multi-level conductive / polarization networks and achieve wide-bandwidth, highly efficient, and tunable microwave absorption performance while maintaining the advantages of Ti-based MOFs' lightweight and porous nature is an urgent technical problem to be solved. Summary of the Invention

[0008] The purpose of this invention is to provide a high-efficiency broadband microwave absorbing composite material, its preparation method, and its application, thereby solving the problems of high density and narrow absorption frequency band in existing microwave absorbing composite materials.

[0009] To achieve the above objectives, the present invention provides a high-efficiency broadband microwave absorbing composite material, comprising a matrix and a conductive phase. The matrix is ​​an amino-metal-organic framework MIL-125-NH2 or its derivative, and the conductive phase is multi-walled carbon nanotubes. The matrix is ​​generated in situ on the multi-walled carbon nanotubes, so that the multi-walled carbon nanotubes are dispersed on the surface and in the pores of the matrix, forming a through-hole multidimensional conductive network. The multi-walled carbon nanotubes account for 1wt%-30wt% of the mass fraction of the composite material.

[0010] Preferably, the composite carbon nanotubes account for 5wt%-20wt% of the mass fraction of the composite material; the diameter of the composite carbon nanotubes is 20 nm-30 nm and the length is 10 μm-30 μm.

[0011] The preparation method of the above-mentioned broadband microwave absorbing composite material includes the following steps: S1. Weigh 2-aminoterephthalic acid and add it to the solvent. Stir magnetically at room temperature for 5-15 minutes to obtain the precursor solution. S2. Add isopropyl titanate dropwise to the precursor solution to introduce the metal precursor, stir evenly to obtain a mixed solution; S3. Weigh the pre-dispersed composite wall carbon nanotubes in the solvent, add the composite wall carbon nanotube dispersion to the mixed solution, and ultrasonically disperse it evenly to obtain a homogeneous solution. S4. Transfer the homogeneous solution to a polytetrafluoroethylene-lined stainless steel reactor for solvothermal synthesis to complete the crystallization of MIL-125-NH2 and the in-situ loading of multi-walled carbon nanotubes on its surface and inside. After the reaction is completed, cool naturally to room temperature to obtain a suspension. S5. Centrifuge the suspension to separate the solid and liquid components, wash the separated solid with deionized water, and dry the solid to obtain the precursor; S6. The precursor is placed in a tube furnace and carbonized under an inert atmosphere, then naturally cooled to room temperature to obtain a microwave absorbing composite material.

[0012] Preferably, in S1, the solvent is a mixture of N,N-dimethylformamide and methanol in a volume ratio of 1:1.

[0013] Preferably, in S2, the molar ratio of isopropyl titanate to 2-aminoterephthalic acid is 2:1; in S3, multi-walled carbon nanotubes account for 5wt%-20wt% of the total mass of the precursor.

[0014] Preferably, in step S4, the solvothermal synthesis is carried out by holding the temperature at 140℃-160℃ for 8h-12h.

[0015] Preferably, in step S5, the drying temperature is 60°C and the drying time is 12 hours.

[0016] Preferably, in step S6, the carbonization process involves heating to 600℃–900℃ at a heating rate of 2℃ / min–10℃ / min and holding at that temperature for 1h–4h.

[0017] Preferably, the microwave absorbing composite material has a minimum reflection loss |RL| in the 2GHz–18GHz frequency band. min |≥20dB and effective absorption bandwidth EAB≥4.0 GHz.

[0018] The preparation method of broadband microwave absorbing composite material is used in the preparation of electromagnetic wave absorbing materials or electromagnetic shielding materials in the 2GHz–18GHz frequency band.

[0019] The advantages and positive effects of the broadband microwave absorbing composite material, its preparation method, and its applications described in this invention are as follows: This invention utilizes a synergistic strategy of amino-based Ti MOF templates, in-situ introduction of MWCNTs, and tunable carbonization treatment to construct a lightweight microwave absorbing composite material with both multi-level conductive networks and multi-scale polarization networks without introducing additional complex magnetic components or multi-step surface engineering. This facilitates a reduction in the density of the composite material and achieves high reflection loss and a wide effective absorption bandwidth in the 2–18 GHz frequency band. Compared with existing MOF-derived absorbing materials, this material has significant advantages in terms of process flow, structural design, and performance control, aligning with the overall trend of MOF-based absorbing materials towards "designable structure, explainable mechanism, and predictable performance." Furthermore, by changing the MWCNT content and carbonization temperature, the absorption band, peak position, and matching thickness can be customized according to application requirements, making it suitable for various scenarios such as radar stealth materials, electromagnetic shielding structures, electromagnetic leakage prevention packaging, and 5G base station perimeter protection.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 The flowchart of an embodiment of the present invention is as follows: Figure 2 The images are SEM images of Examples 1-3 and Comparative Examples 1-3 of the present invention; (a) is the SEM image of Comparative Example 1, (b) is the SEM image of Comparative Example 2, (c) is the SEM image of Comparative Example 3, (d) is the SEM image of Example 1, (e) is the SEM image of Example 2, and (f) is the SEM image of Example 3. Figure 3 The images are XRD patterns of Examples 1-3 and Comparative Examples 1-3 of the present invention; (a) is the XRD pattern of Comparative Examples 1-3, and (b) is the XRD pattern of Examples 1-3. Figure 4 The graphs are of the ε′, ε″, and tanε as a function of frequency for Examples 1-3 and Comparative Examples 1-3 of the present invention; (a) is a graph of ε′ as a function of frequency for Comparative Examples 1-3, (b) is a graph of ε′′ as a function of frequency for Comparative Examples 1-3, (c) is a tangent graph of ohmic loss for Comparative Examples 1-3, (d) is a graph of ε′′ as a function of frequency for Examples 1-3, (e) is a graph of ε′′′ as a function of frequency for Examples 1-3, and (f) is a tangent graph of ohmic loss for Examples 1-3. Figure 5 The graphs show the reflection loss (RL) curves of Comparative Example 2 and Examples 1 and 2 in the range of 2–18 GHz; a1-a3 represent the three-dimensional and two-dimensional RL graphs of Comparative Example 2, respectively; b1-b3 represent the three-dimensional and two-dimensional RL graphs of Example 1, respectively; and c1-c3 represent the three-dimensional and two-dimensional RL graphs of Example 2, respectively. Figure 6 This is a diagram illustrating the microwave absorption mechanism of the present invention. Detailed Implementation

[0022] In this application, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. In case of any inconsistency, the meaning set forth in this specification or derived from the content described herein shall prevail. Furthermore, the terminology used herein is for the purpose of describing embodiments of this application only and is not intended to limit the scope of this application.

[0023] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0024] A broadband microwave absorbing composite material includes a matrix and a conductive phase. The matrix is ​​an amino-containing metal-organic framework MIL-125-NH2 or its carbide. The conductive phase is multi-walled carbon nanotubes (MWCNTs). The multi-walled carbon nanotubes are introduced in situ and dispersed on the surface and in the pores of the matrix during the matrix formation process, forming a through-hole multidimensional conductive network. The multi-walled carbon nanotubes account for 1wt%-30wt% of the mass fraction of the composite material.

[0025] The mass fraction of the composite material is 5wt%-20wt%; the diameter of the composite carbon nanotubes is 20nm-30nm and the length is 10μm-30μm.

[0026] like Figure 1 As shown, the preparation method of the above-mentioned high-efficiency broadband microwave absorbing composite material includes the following steps: S1. Weigh 2-aminoterephthalic acid and add it to the solvent. Stir magnetically at room temperature for 5-15 minutes to obtain the precursor solution.

[0027] The solvent is a mixture of N,N-dimethylformamide and methanol in a volume ratio of 1:1.

[0028] S2. Add isopropyl titanate dropwise to the precursor solution to introduce the metal precursor, stir until homogeneous, and obtain a mixed solution.

[0029] The molar ratio of isopropyl titanate to 2-aminoterephthalic acid is 2:1.

[0030] S3. Weigh the pre-dispersed composite carbon nanotubes in the solvent, add the composite carbon nanotube dispersion to the mixed solution, and ultrasonically disperse until uniform solution is obtained.

[0031] Multi-walled carbon nanotubes account for 5wt%-20wt% of the total mass of the precursor.

[0032] Multi-walled carbon nanotubes (MWCNTs) were introduced in situ and uniformly loaded onto the sheet-like / polyhedral surface and around the pores during the growth of MIL-125-NH2.

[0033] S4. The homogeneous solution is transferred to a polytetrafluoroethylene-lined stainless steel reactor for solvothermal synthesis to complete the crystallization of MIL-125-NH2 and the in-situ loading of multi-walled carbon nanotubes on its surface and inside. After the reaction is completed, the solution is naturally cooled to room temperature to obtain a suspension.

[0034] Solvent-thermal synthesis involves holding the temperature at 140℃-160℃ for 8-12 hours.

[0035] S5. Centrifuge the suspension to separate the solid and liquid components, wash the separated solid with deionized water, and dry the solid to obtain the precursor.

[0036] The drying temperature is 60℃ and the drying time is 12 hours.

[0037] Using MIL-125-NH2 (metal-organic framework) formed by amino-containing organic ligand 2-aminoterephthalic acid and isopropyl titanate as the matrix, this MOF has a regular polyhedral / plate-like morphology and an ordered pore structure. The amino ligand can serve as a local nitrogen source and induce the formation of defects and dipole centers during the subsequent carbonization process, providing a structural basis for constructing a carbon-based framework with high polarization loss.

[0038] S6. The precursor is placed in a tube furnace and carbonized under a nitrogen, argon, or nitrogen-argon mixed inert atmosphere. It is then naturally cooled to room temperature to obtain a microwave absorbing composite material.

[0039] The carbonization process involves heating to 600℃–900℃ at a rate of 2℃ / min–10℃ / min and holding at that temperature for 1h–4h.

[0040] The matrix contains uniformly distributed Ti-O clusters and amino functional groups, which, after carbonization, form a composite structure containing TiO2 nanoparticles and a nitrogen-doped porous carbon framework. During MOF nucleation and growth, MWCNTs are dispersed in situ and fixed on the framework surface and near the pores, forming a continuous multidimensional conductive network.

[0041] The principle of the high-efficiency wide-band microwave absorbing composite material described in this invention is as follows: (1) Conductive loss caused by three-dimensional conductive network. After carbonization in an inert atmosphere, the amino-containing MIL-125-NH2 precursor is transformed into a porous carbon framework mainly composed of TiO2 and amorphous carbon / partially graphitized carbon. MWCNTs uniformly penetrate and bridge different particles to construct a continuous three-dimensional conductive network. The interconnected carbon nanotube network can significantly improve the conductivity and intrinsic attenuation coefficient of the composite material, thereby enhancing the ohmic loss and multiple reflection path length of electromagnetic waves inside the material.

[0042] (2) Polarization loss induced by defects and multiphase interfaces. On the one hand, amino ligands are prone to forming abundant nitrogen doping sites and structural defects during carbonization. These asymmetric charge distribution regions can serve as dipole polarization centers and undergo orientation polarization under alternating electromagnetic fields. On the other hand, a large number of heterogeneous interfaces are formed between TiO2 / carbon, carbon / carbon nanotubes and different carbon phases (graphitized carbon / amorphous carbon). Under the action of an external electric field, charge carriers on both sides of the interface accumulate and relax, resulting in a significant interface polarization effect. Lattice distortion and atomic-scale defects can also further enhance the polarization loss capability.

[0043] (3) Porous structure and "multiple reflection / scattering" effect. The MOF-derived porous carbon framework provides complex reflection and diffraction paths for electromagnetic waves to propagate within the material. The repeated reflection and scattering between the pore walls significantly increases the propagation distance and residence time of the wave in the material, which is conducive to the full dissipation of electromagnetic energy in conductivity loss and polarization loss. Compared with dense bulk materials, porous structures can further reduce the bulk density of the material while ensuring absorption performance, thus achieving lightweight design.

[0044] (4) Synergistic optimization of impedance matching and attenuation capability. Excellent microwave absorption performance usually comes from the synergistic effect of "good impedance matching + sufficient attenuation capability". By adjusting the MWCNT content and carbonization temperature, the real dielectric constant ε′ and imaginary dielectric constant ε″ of the material can be finely adjusted within a certain range. Under the premise of ensuring sufficient attenuation constant, the normalized input impedance of the composite material is made close to the free space impedance, thereby significantly reducing interface reflection and achieving high reflection loss depth and wideband effective absorption.

[0045] Example 1 S1. Weigh 0.66 g of 2-aminoterephthalic acid and add it to 40–50 mL of DMF / methanol (volume ratio 1:1) mixed solvent. Stir magnetically for 10 min at room temperature.

[0046] S2. Add 0.55 mL of isopropyl titanate and continue stirring / ultrasonicating until the solution is transparent and homogeneous to obtain a mixed solution.

[0047] S3. Weigh MWCNTs, which are 10 wt% of the total precursor mass, add them to a small amount of DMF and sonicate for 30 min to form a uniform dispersion; add the dispersion to the mixed solution and sonicate for 20–40 min using an ultrasonic cell disruptor to disperse evenly and obtain a homogeneous solution.

[0048] S4. Transfer the homogeneous solution into a 50 mL PTFE-lined stainless steel reactor and react at 150°C for 10 h. After the reaction is complete, allow it to cool naturally to room temperature.

[0049] S5. The suspension was centrifuged at 7000 rpm for 3 min to separate the solid, washed 3 times with deionized water, and dried in an oven at 60 ℃ for 12 h to obtain the precursor MIL-125-NH2(Ti) powder.

[0050] S6. Take 1.0 g–2.0 g of the precursor sample, place it in a quartz boat, and put it into a tube furnace. Purge with nitrogen gas at a flow rate of 100–200 mL / min, and hold at room temperature for 30 min to displace the air. Increase the temperature to 600 ℃ at a rate of 5 ℃ / min, hold for 2 h, and then allow to cool naturally to room temperature; the carbonized MWCNTs@TiO2 / C microwave absorbing composite material is obtained.

[0051] Example 2 The difference between this embodiment and Embodiment 1 is that the sintering temperature in this embodiment is 700 ℃.

[0052] Example 3 The difference between this embodiment and Embodiment 1 is that the sintering temperature in this embodiment is 800 ℃.

[0053] Comparative Example 1 The difference between this comparative example and Example 1 is that no MWCNTs were added in this comparative example. The mixed solution from S2 was directly transferred into the reactor and reacted at 150°C for 10 h. After the reaction, it was naturally cooled to room temperature. The suspension was centrifuged at 7000 rpm for 3 min to separate the solid, washed three times with deionized water, and dried in a 60°C oven for 12 h to obtain MIL-125-NH2(Ti) powder.

[0054] Comparative Example 2 The difference between this comparative example and Comparative Example 1 is that MWCNTs in this comparative example are 10 wt% of the total mass of the precursor, and the difference between this comparative example and Example 1 is that no carbonization process is performed in this comparative example.

[0055] Comparative Example 3 The difference between this comparative example and Comparative Example 1 is that MWCNTs in this comparative example are 20 wt% of the total mass of the precursor, and no carbonization process is performed in this comparative example.

[0056] Scanning electron microscopy (SEM) was performed on the microwave-absorbing composite materials obtained in Examples 1-3 and Comparative Examples 1-3 to demonstrate the distribution morphology of MWCNTs on the MOF matrix, such as... Figure 2 As shown. Figure 2 Image a shows the electron microscope image of Comparative Example 1 without the addition of MWCNTs, in which MIL-125-NH2(Ti) appears as an irregular pie shape. Figure 2 b and 2c show electron micrographs of Comparative Examples 2 and 3 after the addition of 10 wt% and 20 wt% MWCNTs, respectively. It can be seen from the figures that the MOF(Ti) still maintains its original shape, while the MWCNTs are distributed in a network on the surface and channels of MIL-125-NH2(Ti). Figure 2 df is an electron microscope image of the samples in Examples 1-3. It can be seen from the image that while the samples maintain their original shape, as the temperature increases, particulate substances gradually precipitate out on the surface of MIL-125-NH2(Ti).

[0057] XRD analysis was performed on the MIL-125-NH2(Ti) powder prepared in Comparative Example 1 and the microwave absorbing composite materials prepared in Comparative Examples 2 and 3. The results are as follows: Figure 3 As shown in Figure a, the peaks of the prepared MIL-125-NH2(Ti) are basically consistent with those of the standard card, indicating that the MIL-125-NH2(Ti) material was successfully synthesized. Furthermore, the figure shows that the synthesis of the MWCNTs@MIL-125-NH2(Ti) composite material did not change the crystal structure of MIL-125-NH2(Ti). Simultaneously, the electron micrographs of Comparative Example 2 (1-MT) and Comparative Example 3 (2-MT) ​​further confirm that MIL-125-NH2(Ti) retains its original disk shape. XRD analysis was performed on the MWCNTs@TiO2 / composite materials prepared in Examples 1-3, as shown... Figure 3 Figure b shows the XRD patterns of the samples at different carbonization temperatures. Characteristic peaks of TiO2 appear at 2θ = 27.46°, 35.99°, 39.23°, 41.17°, and 44.22°, corresponding to the (110), (101), (200), (111), and (210) crystal planes, respectively. This demonstrates that a phase transition occurs in the MWCNTs@MIL-125-NH2(Ti) material during carbonization, further verifying the appearance of TiO2 particles on the material surface with increasing carbonization temperature. Furthermore, a characteristic peak representing the (002) crystal plane of graphitized carbon appears near 25.5°, and the intensity of the diffraction peak increases with increasing carbonization temperature.

[0058] The microwave absorbing composite materials prepared in Examples 1-3 and Comparative Examples 1-3 were mixed with paraffin wax at a mass ratio of 1:3, ground uniformly, and then pressed into coaxial ring samples with an inner diameter of 3.04 mm and an outer diameter of 7.00 mm using a mold. The complex dielectric constant (ε′, ε″) and dielectric loss (tanε) of the samples were measured in the range of 2–18 GHz using a vector network analyzer (e.g., Agilent PNA N5224A). Figure 4 As shown. Figure 4 Figure a shows the frequency-dependent ε' curves for Comparative Example 1 (MIL-125-NH2(Ti)), Comparative Example 2 (1-MT), and Comparative Example 3 (2-MT). The graph shows that the ε' value of Comparative Example 1 fluctuates around 3, indicating a low energy storage capacity for the electric field. The ε' values ​​of Comparative Examples 2 and 3 fluctuate within the ranges of (7.3~5.9) and (8.4~4.6), respectively. The imaginary part (ε'') of the complex permittivity represents the strength of the material's ability to dissipate energy from the electric field. Figure 4b shows the ε'' curves of the three materials as a function of frequency. The graph shows that Comparative Example 1 also exhibits low loss capability to the electric field. The ε'' values ​​of Comparative Examples 2 and 3 fluctuate within the ranges of (1.5~2.0) and (1.5~3.5), respectively. Comparing the ε' and ε'' data, Comparative Example 1 has a very low complex permittivity, indicating insufficient dielectric response and ultimately poor electromagnetic wave attenuation characteristics. The complex permittivity values ​​of the composite materials prepared in Examples 2 and 3 are both greater than those in Example 1. This can be attributed to the bridging effect between MWCNTs and MIL-125-NH2(Ti). The MWCNTs interweave and significantly extend the electron migration path, which should also be the reason for the higher conductivity of MWCNTs@MIL-125-NH2(Ti). The ohmic loss tangent (tanδε=ε'' / ε') represents the dielectric loss attenuation mechanism, such as... Figure 4 As shown in Figure c, the tanδε values ​​of the composite materials prepared in Comparative Examples 2 and 3 are both greater than those in Comparative Example 1, indicating that the addition of MWCNTs improves the dielectric loss of the materials. Figure 4 Figures d-4f show the electromagnetic parameters of samples 1-3, including ε', ε'', and tanδε. The ε' value exhibits typical dispersion characteristics, which is due to the increased hysteresis between dielectric polarization and alternating electromagnetic waves in the microwave frequency range. In this temperature range, the conductivity of amorphous carbon changes gradually; the MWCNTs conductive network is basically formed but does not show rapid enhancement at high temperatures; the oxygen vacancy concentration of TiO2 increases only slightly; and the interface polarization process is consistent with the Debye relaxation mechanism. Therefore, the combined effect of conductive loss and polarization loss causes the dielectric response of both to exhibit highly similar dispersion characteristics across the entire frequency band. Sample 3 exhibits a high ε' value, which gradually decreases with increasing frequency. Under these conditions, the graphitization of the sample is excessive, resulting in an overly dense conductive network between MWCNTs. The excessive number of free electrons leads to a surge in ε″. While a high ε″ indicates strong loss capability, it severely disrupts impedance matching, causing electromagnetic waves to be reflected at the incident interface. The dielectric loss tangent (tanδε=ε′′ / ε′) is widely accepted to evaluate the dielectric loss capability of materials. As can be observed from the figure, multiple relaxation peaks appeared in the sample in the frequency range of 2-18 GHz, revealing the possible existence of multiple polarization behavior.

[0059] The curve of reflection loss RL as a function of frequency is calculated based on transmission line theory, as follows: Figure 5 As shown, three-dimensional and two-dimensional RL plots of the composite materials of Comparative Example 2 and Examples 1 and 2 are presented. As predicted, in Figure 5In diagrams a1-a3, we can observe that when the paraffin ring thickness is 3.16 mm, the RLmin reaches -53.39 dB, indicating that the composite material in Comparative Example 2 has good electromagnetic wave absorption performance. The maximum EAB corresponding to a thickness of 2.68 mm is 5.68 GHz. This fully demonstrates that adding a certain amount of MWCNTs is beneficial to improving the conductive polarization network of the material, thereby enhancing the microwave absorption performance of the composite material. Figure 5 The electromagnetic wave absorption performance of the composite materials of Examples 1 and 2 is represented by three-dimensional and two-dimensional RL plots (b1-5b3 and 5c1-5c3). The RL values ​​of the composite materials of Examples 1 and 2 remained below -10 dB in the frequency range of 3.9-18 GHz, indicating that the samples exhibited good electromagnetic wave absorption performance. Specifically, the composite material of Example 1 had an RLmin of -53.96 dB at a thickness of 2.18 mm and an EAB of 4.85 GHz at a thickness of 1.76 mm. The composite material of Example 2 only achieved an RLmin of -57.85 dB at a thickness of 1.7 mm and a bandwidth of 4.89 GHz at a thickness of only 1.7 mm. In contrast, the sample of Example 3, due to its excessively high carbonization temperature, resulted in excessive precipitation and accumulation of TiO2 particles, even destroying the material's framework structure, leading to poor microwave absorption performance. This demonstrates that selecting a suitable carbonization temperature can effectively increase the dielectric loss of the material and thus improve its microwave absorption performance.

[0060] The microwave absorption mechanism of this invention is as follows: Figure 6 As shown, firstly, the three-dimensional interconnected conductive network formed by the intertwined MWCNTs significantly enhances the conductive loss of the composite material. Secondly, there are numerous defects and heterogeneous / homogeneous interfaces between pyrolytic carbon and graphite, which promote dipole and interfacial polarization in the alternating electromagnetic field, resulting in significant dielectric loss capacity.

[0061] Therefore, the high-efficiency broadband microwave absorbing composite material, its preparation method, and its application described in this invention can solve the problems of high density and narrow absorption frequency band of existing microwave absorbing composite materials.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A high-efficiency broadband microwave absorbing composite material, characterized in that: The composite material comprises a base and a conductive phase, the base is MIL-125-NH2 or its derivative, the conductive phase is multi-walled carbon nanotubes, the base is grown in-situ on the multi-walled carbon nanotubes, the multi-walled carbon nanotubes are dispersed on the surface and in the pores of the base, and a through multi-dimensional conductive network is formed, and the mass fraction of the multi-walled carbon nanotubes in the composite material is 1wt%-30wt%.

2. The high-efficiency broadband microwave absorbing composite material according to claim 1, characterized in that: The mass fraction of the multi-walled carbon nanotubes in the composite material is 5wt%-20wt%, the diameter of the multi-walled carbon nanotubes is 20nm-30nm, and the length of the multi-walled carbon nanotubes is 10μm-30μm.

3. A method of producing the high-efficiency broadband microwave absorbing composite material according to claim 1 or 2, characterized by, The method comprises the following steps: S1, weighing 2-amino terephthalic acid and adding it into a solvent, and stirring magnetically at room temperature for 5min-15min to obtain a precursor solution; S2, adding titanium isopropyl titanate dropwise into the precursor solution to introduce a metal precursor, and stirring uniformly to obtain a mixed solution; S3, weighing multi-walled carbon nanotubes pre-dispersed in a solvent, adding the multi-walled carbon nanotube dispersion into the mixed solution, and ultrasonic dispersing uniformly to obtain a uniform solution; S4, transferring the uniform solution into a polytetrafluoroethylene-lined stainless steel reaction kettle, and performing solvothermal synthesis to complete the crystallization of MIL-125-NH2 and the in-situ loading of multi-walled carbon nanotubes on the surface and inside of MIL-125-NH2, and naturally cooling to room temperature after the reaction to obtain a suspension; S5, centrifuging the suspension to separate the solid and the liquid, washing the separated solid with deionized water, and drying the solid to obtain a precursor; S6, placing the precursor in a tube furnace, and performing carbonization treatment under an inert atmosphere, and naturally cooling to room temperature to obtain a microwave absorption composite material.

4. The method for preparing a high-efficiency broadband microwave absorbing composite material according to claim 3, characterized in that: In the S1, the solvent is a mixture of N,N-dimethylformamide and methanol, and the volume ratio is 1:

1.

5. The method for preparing a high-efficiency broadband microwave absorbing composite material according to claim 3, characterized in that: In the S2, the molar ratio of titanium isopropyl titanate to 2-amino terephthalic acid is 2:1; and in the S3, the mass fraction of the multi-walled carbon nanotubes in the total mass of the precursor is 5wt%-20wt%.

6. The method for preparing a high-efficiency broadband microwave absorbing composite material according to claim 3, characterized in that: In the S4, the solvothermal synthesis is performed at 140℃-160℃ for 8h-12h.

7. The method for preparing a high-efficiency broadband microwave absorbing composite material according to claim 3, characterized in that: In the S5, the drying temperature is 60℃, and the drying time is 12h.

8. The method for preparing a high-efficiency broadband microwave absorbing composite material according to claim 3, characterized in that: In the S6, the carbonization treatment is heating to 600℃-900℃ at a heating rate of 2℃ / min-10℃ / min, and the holding time is 1h-4h.

9. The method for preparing a high-efficiency broadband microwave absorbing composite material according to claim 3, characterized in that: The microwave absorbing composite has a minimum reflection loss |RL min ≥20 dB and an effective absorption bandwidth EAB≥4.0 GHz.

10. Application of a microwave absorption composite material prepared by the method of any one of claims 4-9 in the preparation of a 2GHz-18GHz frequency band electromagnetic wave absorption material or electromagnetic shielding material.

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