Epoxy resin-based composite wave-absorbing material and preparation method thereof

Abstract

This invention relates to the field of functional material preparation technology, and discloses an epoxy resin-based composite microwave absorbing material and its preparation method. The method includes preparing a polyaniline / ferric oxide / titanium dioxide ternary composite microwave absorbing agent, surface-coating it with boron nitride nanosheets to obtain a modified composite microwave absorbing agent; preparing a thermally conductive matrix layer slurry and a microwave absorbing functional layer slurry respectively; applying the microwave absorbing functional layer slurry to the bottom of a mold using a doctor blade coating method, pre-curing it, then pouring the thermally conductive matrix layer slurry onto it, and finally curing it to obtain a two-layer composite material. This invention enhances dielectric loss through the microcapacitance effect within the microwave absorbing functional layer, while simultaneously utilizing the thermally conductive matrix layer to construct a continuous thermally conductive network, thus alleviating the problem of mutual constraint between microwave absorption and thermal conductivity performance.

Description

Technical Field

[0001] This invention relates to the field of functional material preparation technology, specifically to an epoxy resin-based composite microwave absorbing material and its preparation method. Background Technology

[0002] 5G communication base stations are high-density integrated electronic devices, containing numerous high-frequency power amplifier chips, filters, and antenna arrays. During base station operation, these electronic components not only generate strong electromagnetic radiation but also release a significant amount of heat. The base station's heat dissipation casing, as the external protective structure of the equipment, needs to rapidly dissipate internal heat to prevent high temperatures from causing component performance drift or failure; it also needs to possess a certain electromagnetic wave absorption capability to suppress cavity resonance and external radiation, meeting electromagnetic compatibility requirements. Epoxy resin-based composite materials, due to their excellent electrical insulation, corrosion resistance, and processability, are widely used in electronic packaging structures such as base station casings.

[0003] To improve the thermal conductivity of epoxy resins, existing technologies often add high thermal conductivity fillers to the resin matrix, such as flake graphene, carbon nanotubes, boron nitride, and alumina. These fillers form a thermally conductive network through mutual contact, providing continuous channels for heat transfer. Among them, graphene, due to its extremely high intrinsic thermal conductivity, has become one of the mainstream fillers for improving the thermal conductivity of polymers. In engineering practice, graphene is usually mechanically blended or solution-blended with epoxy resin, and thermally conductive composite materials are prepared by casting or compression molding. However, such high-filler thermally conductive fillers often have high dielectric constants or electrical conductivity, which significantly alters the electromagnetic parameters of the composite material, leading to enhanced reflection and weakened absorption of incident electromagnetic waves, which is detrimental to suppressing electromagnetic interference.

[0004] On the other hand, to enable epoxy resin to absorb electromagnetic waves, existing technologies typically add magnetic loss-type absorbing agents (such as iron(III) oxide and carbonyl iron powder) or dielectric loss-type absorbing agents (such as polyaniline, carbon black, and carbon fiber). Polyaniline, due to its low density, adjustable conductivity, and good compatibility with the resin matrix, is often used to prepare electromagnetic wave absorbing composites; iron(III) oxide provides a magnetic loss mechanism, which is beneficial for enhancing the attenuation capability of electromagnetic waves. In actual production, polyaniline is usually compounded with magnetic particles through solution blending or in-situ polymerization, and then filled into epoxy resin. The microwave absorption performance is optimized by adjusting the type, proportion, and dispersion state of the absorbing agent. However, most of these absorbing agents have poor thermal conductivity, and excessive filling will further reduce the overall thermal conductivity of the composite material.

[0005] Therefore, in practical applications of base station heat dissipation housings, simultaneously filling with thermally conductive fillers and microwave absorbing agents has become a common technical approach. However, due to the functional constraints between thermally conductive fillers and microwave absorbing agents, the thermally conductive fillers can disrupt the impedance matching state within the microwave absorbing layer, while the microwave absorbing agents, due to their poor thermal conductivity, can increase thermal resistance. This makes it difficult for existing technologies to simultaneously achieve good thermal conductivity and microwave absorption performance within the same material system. Therefore, the following solutions are proposed to address these issues. Summary of the Invention

[0006] To solve the above-mentioned technical problems, the present invention provides an epoxy resin-based composite microwave absorbing material, characterized in that it comprises:

[0007] The thermally conductive substrate layer is composed of epoxy resin and graphene dispersed therein; and the microwave absorbing functional layer is composed of epoxy resin and modified composite microwave absorbing agent dispersed therein.

[0008] The modified composite microwave absorber consists of ternary composite microwave absorber particles and boron nitride nanosheets coated on the surface of the ternary composite microwave absorber particles. The ternary composite microwave absorber contains polyaniline, iron tetroxide and titanium dioxide.

[0009] Boron nitride nanosheets are used to form an interfacially polarized microcapacitor structure between modified composite absorbing agent particles within the microwave absorbing functional layer.

[0010] Preferably, in the modified composite microwave absorbing agent, the mass ratio of boron nitride nanosheets to ternary composite microwave absorbing agent particles is (0.1-0.5):1, the thickness of the boron nitride nanosheets is 2-5 nm, and the diameter of the nanosheets is 0.5-2 μm.

[0011] Preferably, in the ternary composite microwave absorber, the mass ratio of polyaniline, iron(III) oxide and titanium dioxide is (1-3):(0.2-1):(0.2-1).

[0012] Preferably, the thickness of the microwave absorbing functional layer is 0.5-2 mm, the thickness of the thermally conductive substrate layer is 1-4 mm, and the thickness ratio of the microwave absorbing functional layer to the thermally conductive substrate layer is 1:(1-4).

[0013] Preferably, in the thermally conductive matrix layer, the mass ratio of graphene to epoxy resin is (0.1-0.6):1, the number of graphene layers is less than 10, and the sheet diameter is 1-10 μm.

[0014] Preferably, in the microwave absorbing functional layer, the mass ratio of the modified composite microwave absorbing agent to the epoxy resin is (0.2~1):1.

[0015] Preferably, the boron nitride nanosheets are few-layer boron nitride nanosheets obtained by ball milling-assisted liquid phase exfoliation of hexagonal boron nitride.

[0016] A method for preparing an epoxy resin-based composite microwave absorbing material as described in any of the above claims includes the following steps:

[0017] Polyaniline, iron(III) oxide and titanium dioxide are mixed and dispersed in a solvent, and then stirred, sonicated, dried and ground to obtain a ternary composite microwave absorbing agent powder.

[0018] The ternary composite microwave absorber powder was dispersed in a dispersion containing boron nitride nanosheets, and after mixing, sonication, and drying, a modified composite microwave absorber with boron nitride nanosheets coated on the surface was obtained.

[0019] Epoxy resin and curing agent are mixed, modified composite microwave absorbing agent is added and stirred evenly, and after degassing treatment, microwave absorbing functional layer slurry is obtained.

[0020] Epoxy resin and curing agent are mixed, graphene is added and stirred evenly, and after degassing treatment, a thermally conductive matrix layer slurry is obtained.

[0021] The microwave absorbing functional layer slurry is coated and pre-cured to a semi-solid state. Then, a thermally conductive matrix layer slurry is applied to the microwave absorbing functional layer and cured by hot pressing to obtain an epoxy resin-based composite microwave absorbing material.

[0022] Preferably, before coating and pre-curing the microwave absorbing functional layer slurry, the method further includes: mixing epoxy resin with curing agent and accelerator in a mass ratio of (3-5):(0.5-1):(0.01-0.03) to prepare a thermally conductive matrix layer slurry; and mixing epoxy resin with curing agent and accelerator in a mass ratio of (2-4):(0.3-0.8):(0.005-0.02) to prepare a microwave absorbing functional layer slurry.

[0023] Preferably, the specific conditions for hot-press curing molding are as follows: pre-pressing and degassing are performed at a temperature of 60-80℃ and a pressure of 3-8MPa for 5-15 minutes, then the temperature is raised to 110-130℃ and cured under the same pressure for 1-3 hours, and finally demolded after natural cooling to room temperature.

[0024] The present invention has the following beneficial effects:

[0025] This invention constructs an asymmetric gradient bilayer structure, enabling the microwave absorbing layer and the thermally conductive substrate layer to work synergistically, balancing impedance matching at the electromagnetic wave incident surface with thermal conductivity at the bottom. In the microwave absorbing layer, insulating porous boron nitride nanosheets are used to surface-coat the composite microwave absorbing agent, forming localized microcapacitor structures between particles. This enhances interfacial polarization and multiple scattering, improving dielectric loss capability. Simultaneously, the boron nitride nanosheets themselves possess high thermal conductivity, providing auxiliary heat dissipation channels for localized hotspots. The graphene filling in the thermally conductive layer constructs a continuous thermally conductive network, ensuring rapid heat dissipation. Overall, this method effectively alleviates the contradiction between microwave absorption performance and thermal conductivity, providing a solution for high-power-density electronic device housing materials that combines functional synergy with structural stability. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of 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.

[0027] Figure 1 This is a schematic flowchart of a method for preparing an epoxy resin-based composite microwave absorbing material according to the present invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1

[0030] Please see Figure 1 As shown, this invention discloses a method for preparing an epoxy resin-based composite microwave absorbing material, comprising the following steps:

[0031] Step S1: Raw material preparation and pretreatment

[0032] Weigh out a certain amount of epoxy resin (E-51 type) as the matrix material, place it in a vacuum drying oven, and dry it at 60°C for 2 hours to remove trace amounts of moisture and air bubbles from the resin.

[0033] Weigh out a certain amount of polyaniline (PANI) powder, nano iron oxide (Fe3O4) particles (50nm), nano titanium dioxide (TiO2) particles (100nm), and hexagonal boron nitride (h-BN) powder, place them in an oven, and dry them at 80℃ for 4 hours to remove the surface adsorbed moisture and ensure the subsequent dispersion effect.

[0034] The dried h-BN powder was ball-milled for 12 hours at 400 rpm using isopropanol as solvent, followed by ultrasonic treatment for 2 hours. After centrifugation, an isopropanol dispersion of few-layer boron nitride nanosheets (BNNS) with a thickness of 2-5 nm and a sheet diameter of 0.5-2 μm was obtained, with a solid content of 2 mg / mL.

[0035] Step S2: Preparation of ternary composite microwave absorber (PANI / Fe3O4 / TiO2)

[0036] 1g of pretreated PANI powder, 0.5g of Fe3O4 and 0.5g of TiO2 were added to 100mL of deionized water, and 0.5g of sodium dodecylbenzenesulfonate (SDBS) was added as a surfactant.

[0037] Place the above mixture on a magnetic stirrer and stir at 600 rpm for 30 minutes to form a uniform suspension.

[0038] The stirred suspension was transferred to an ultrasonic cell disruptor and ultrasonically dispersed in an ice-water bath for 40 minutes at a power of 200W and a frequency of 20kHz to fully deagglomerate and uniformly mix the three nanoparticles, resulting in a stable ternary composite microwave absorber suspension.

[0039] The ultrasonically sonicated suspension was vacuum filtered, and the filter cake was washed three times with deionized water to remove excess surfactant.

[0040] The washed filter cake was collected, placed in a vacuum oven, and dried at 60°C for 24 hours to completely remove moisture. After grinding, a ternary composite microwave absorber (PANI / Fe3O4 / TiO2) powder was obtained.

[0041] Step S3: Prepare a modified composite microwave absorber with BNNS coated on its surface.

[0042] Take 1g of the prepared ternary composite microwave absorbing agent powder and add it to 100mL of BNNS isopropanol dispersion (solid content 2mg / mL).

[0043] Add an appropriate amount of anhydrous ethanol to make the volume ratio of isopropanol to ethanol in the mixed solvent 3:1.

[0044] Stir at 500 rpm for 1 hour using a magnetic stirrer, then sonicate at 40 kHz for 30 minutes in an ultrasonic cleaner to ensure that BNNS and composite microwave absorber particles come into full contact and undergo physical adsorption.

[0045] The above mixture was slowly evaporated by stirring in a 40°C water bath until the solvent was completely evaporated. The dried solid was collected and dried in a 60°C vacuum oven for 12 hours to obtain a BNNS surface-modified composite microwave absorber (hereinafter referred to as the modified microwave absorber).

[0046] Step S4: Prepare thermally conductive substrate material

[0047] Mix 5g of pretreated epoxy resin with 1g of curing agent methyl hexahydrophthalic anhydride (MHHPA), heat to 50°C in a constant temperature water bath, and mechanically stir at 400rpm for 15 minutes to form a homogeneous resin matrix mixture.

[0048] Add 2g of flake graphene (less than 10 layers, 5μm diameter) to the above resin matrix mixture, and continue to mechanically stir at 600rpm for 30 minutes in a 50℃ water bath to ensure uniform dispersion of graphene.

[0049] Add 0.02g of accelerator 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) and continue stirring for 5 minutes to obtain the thermally conductive matrix layer mixture slurry.

[0050] The prepared slurry was degassed under vacuum (-0.1 MPa) for 20 minutes to remove air bubbles introduced during the stirring process, and then set aside for later use.

[0051] Step S5: Prepare the microwave absorbing functional layer material

[0052] Mix 3g of pretreated epoxy resin with 0.6g of curing agent MHHPA and stir at 300rpm for 10 minutes in a 40℃ water bath.

[0053] Add 1.5g of the modified microwave absorber prepared in step S3, and mechanically stir at 500rpm for 45 minutes in a 40℃ water bath to ensure that the modified microwave absorber is evenly dispersed.

[0054] Add 0.012g of accelerator DMP-30 and stir for 5 minutes to obtain the microwave absorbing functional layer mixture slurry.

[0055] The mixture was degassed under vacuum (-0.08 MPa) for 15 minutes and then set aside.

[0056] Step S6: Molding of the double-layer composite material

[0057] Preheat a mold with dimensions of 100mm×100mm×2mm to 60℃. Apply the microwave absorbing functional layer mixture slurry obtained in step S5 evenly to the bottom of the mold using a doctor blade, controlling the coating thickness to 1mm to form the surface layer.

[0058] Place the mold with the coated surface layer into a 60°C oven for pre-curing for 30 minutes to allow the surface material to reach a semi-solid (gel state) state, thus supporting subsequent layers without mixing.

[0059] Remove the mold and slowly pour the thermally conductive substrate layer mixture slurry obtained in step S4 onto the pre-cured microwave absorbing functional layer, allowing it to flow naturally and completely cover the surface to form the bottom layer, with the overall thickness controlled at 2mm.

[0060] The entire mold is placed in a vacuum hot press and held at 60°C and 5MPa for 10 minutes to remove interlayer bubbles and promote interfacial bonding.

[0061] The temperature was then raised to 120°C, and the material was cured under the same pressure for 2 hours. After curing, it was allowed to cool naturally to room temperature and then demolded to obtain an epoxy resin-based composite microwave absorbing material board with a double-layer structure.

[0062] Step S7, Post-processing

[0063] The molded boards are placed in a 150°C forced-air oven for 2 hours of post-curing treatment to ensure complete resin cross-linking and stabilize material properties.

[0064] After the board is fully cured, it is machined by cutting, grinding and other processes according to the final application requirements to obtain finished parts for heat dissipation shells of 5G communication base stations.

[0065] Example 2

[0066] The difference between this embodiment and Embodiment 1 is the amount of BNNS dispersion used in step S3. Specifically, 1g of the ternary composite microwave absorber is added to 50mL of BNNS dispersion (solid content 2mg / mL), and the remaining steps are exactly the same as in Embodiment 1.

[0067] This embodiment aims to examine the impact of reducing the thickness of the BNNS coating layer on the overall material performance by reducing the amount of BNNS used.

[0068] Example 3

[0069] The difference between this embodiment and Embodiment 1 is the component ratio of the ternary composite microwave absorber in step S2.

[0070] Specifically, weigh 1g PANI, 1.0g Fe3O4 and 0.5g TiO2, and follow the same steps as in Example 1.

[0071] This embodiment aims to investigate the effect of changes in the composition of the absorber on magnetic loss capability and overall performance by increasing the proportion of the magnetic component Fe3O4.

[0072] Example 4

[0073] The difference between this embodiment and Embodiment 1 is the thickness ratio of the microwave absorbing functional layer to the thermally conductive substrate layer in step S6.

[0074] Specifically, the coating thickness of the microwave absorbing functional layer is controlled to be 0.5 mm, the casting thickness of the thermally conductive substrate layer is 1.5 mm, and the total thickness remains 2 mm. The remaining steps are exactly the same as in Example 1.

[0075] This embodiment aims to examine the influence of structural parameters on the balance between microwave absorption and thermal conductivity by adjusting the thickness ratio of the functional layers in the double-layer structure.

[0076] Comparative Example 1

[0077] The difference between this comparative example and Example 1 is that step S3 is omitted, that is, the ternary composite microwave absorber is not modified by BNNS coating.

[0078] Specifically, in step S5, when preparing the microwave absorbing functional layer, the unmodified ternary composite microwave absorbing agent (PANI / Fe3O4 / TiO2) obtained in step S2 is directly mixed with epoxy resin, and the remaining steps are exactly the same as in Example 1.

[0079] This comparative example is used to verify whether the microwave absorption performance of the material is significantly degraded in the absence of the BNNS microcapacitor structure.

[0080] Comparative Example 2

[0081] The difference between this comparative example and Example 1 is that the double-layer structure in steps S4 and S6 is omitted, and only a single-layer absorbing functional layer is prepared.

[0082] Specifically, the microwave absorbing functional layer slurry is prepared according to step S5 and poured into a mold. Under the same curing conditions (120℃, 5MPa, 2h; 150℃ for 2h), a single-layer board with a thickness of 2mm is produced. The composition of the remaining materials is the same as that of the microwave absorbing functional layer in Example 1.

[0083] This comparative example is used to verify whether the thermal conductivity of the material decreases significantly in the absence of a high thermal conductivity substrate (i.e., without graphene thermal conductivity pathways).

[0084] Comparative Example 3

[0085] The difference between this comparative example and Example 1 is that in step S3, BNNS and the ternary composite microwave absorber are simply physically mixed, rather than coated.

[0086] Specifically, 1g of the ternary composite microwave absorber was directly mixed with 100mL of BNNS dispersion (solid content 2mg / mL). Without stirring, adsorption, or solvent evaporation, the solid mixture was separated by high-speed centrifugation (10000rpm, 10 minutes). After washing with ethanol, the mixture was dried at 60℃ for 12 hours to obtain a physical mixture of BNNS and the microwave absorber. This mixture was used in step S6, and the remaining steps were identical to those in Example 1.

[0087] This comparative example is used to verify that whether BNNS exists in a coating form (i.e., playing an isolation role) has a crucial impact on the final performance. Simple mixing cannot form a stable microcapacitor structure and may lead to the aggregation of BNNS itself.

[0088] Performance test and comparison results

[0089] The composite materials obtained in Examples 1-4 and Comparative Examples 1-3 were subjected to standard performance tests:

[0090] Absorption performance: Using a vector network analyzer, the minimum reflection loss (RL_min, unit: dB) and effective absorption bandwidth (EAB, bandwidth of RL<-10dB, unit: GHz) in the 2-18GHz frequency band were tested according to the GJB 2038A-2011 standard.

[0091] Thermal conductivity: The room temperature thermal conductivity (unit: W / (m·K)) of the material was tested using a laser flash thermal conductivity meter according to ASTM E1461 standard.

[0092] Mechanical properties: The bending strength (unit: MPa) of the material was tested using a universal testing machine according to ASTM D790 standard.

[0093] The test results are summarized in the table below:

[0094] Minimum reflection loss (RL_min) -45.2 dB -38.6 dB -42.1 dB -40.5 dB -25.3 dB -43.8 dB -30.7 dB Effective Absorbed Bandwidth (EAB) 6.8 GHz 5.9 GHz 6.5 GHz 6.1 GHz 4.2 GHz 6.7 GHz 5.1 GHz thermal conductivity 5.2 W / (m·K) 5.1 W / (m·K) 5.0 W / (m·K) 6.5 W / (m·K) 5.3 W / (m·K) 0.8 W / (m·K) 4.8 W / (m·K) Bending strength 68.5 MPa 65.2 MPa 62.8 MPa 71.3 MPa 60.1 MPa 55.6 MPa 58.4 MPa

[0095] Results Analysis

[0096] The comparison results in the table above clearly show that:

[0097] Example 1 is the optimal solution: while maintaining the highest thermal conductivity (5.2 W / (m·K)), it achieves the best microwave absorption performance (RL_min = -45.2 dB, EAB = 6.8 GHz) and good mechanical properties (68.5 MPa), reflecting the design intention of microwave absorption-thermal conduction synergistic enhancement.

[0098] Parameter adjustments in Examples 2-4: The performance of each example remains at a high level, but the emphasis is different. In Example 4, the thermal conductivity is increased to 6.5 W / (m·K) due to the increase in the thickness of the thermal conductive layer, but the wave absorption performance is slightly reduced, indicating that the technical solution of the present invention has flexible adjustability and can be optimized according to specific application scenarios (more emphasis on heat dissipation or more emphasis on wave absorption).

[0099] Comparative Example 1 (without BNNS coating): The microwave absorption performance deteriorates sharply (RL_min is only -25.3 dB), while the thermal conductivity improves slightly due to the lack of phonon transport assistance from BNNS (5.3 W / (m·K) vs 5.2 W / (m·K), a negligible difference). This strongly demonstrates the decisive role of the microcapacitive structure formed by BNNS in improving dielectric loss and enhancing microwave absorption performance. Simultaneously, the mechanical properties are also reduced due to the lack of isolation and buffering provided by BNNS.

[0100] Comparative Example 2 (without thermally conductive layer): Although the absorption performance is comparable to Example 1 (RL_min = -43.8 dB), the thermal conductivity is extremely low (0.8 W / (m·K)), which completely fails to meet the basic requirements of a 5G base station heat dissipation shell. This demonstrates the necessity of the dual-layer functionalized structure design of this invention.

[0101] Comparative Example 3 (simple BNNS mixture): The microwave absorption performance (RL_min = -30.7 dB) and thermal conductivity (4.8 W / (m·K)) were both significantly lower than those of Example 1. This indicates that simply introducing BNNS without coating treatment cannot construct an effective microcapacitor; on the contrary, the self-agglomeration of BNNS may disrupt the uniform dispersion of the microwave absorber, thus demonstrating the uniqueness and irreplaceability of the coating process of this invention.

[0102] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. An epoxy resin-based composite microwave absorbing material, characterized in that, include: A thermally conductive substrate layer, said thermally conductive substrate layer being composed of epoxy resin and graphene dispersed therein; and The microwave absorbing functional layer is composed of epoxy resin and a modified composite microwave absorbing agent dispersed therein; The modified composite microwave absorber is composed of ternary composite microwave absorber particles and boron nitride nanosheets coated on the surface of the ternary composite microwave absorber particles. The ternary composite microwave absorber contains polyaniline, iron tetroxide and titanium dioxide. The boron nitride nanosheets are used to form an interfacially polarized microcapacitor structure between the modified composite absorbing agent particles inside the absorbing functional layer.

2. The epoxy resin-based composite microwave absorbing material according to claim 1, characterized in that: In the modified composite microwave absorbing agent, the mass ratio of the boron nitride nanosheets to the ternary composite microwave absorbing agent particles is (0.1-0.5):1, and the thickness of the boron nitride nanosheets is 2-5 nm, and the diameter is 0.5-2 μm.

3. The epoxy resin-based composite microwave absorbing material according to claim 2, characterized in that: In the ternary composite microwave absorbing agent, the mass ratio of polyaniline, iron tetroxide and titanium dioxide is (1-3):(0.2-1):(0.2-1).

4. The epoxy resin-based composite microwave absorbing material according to claim 3, characterized in that: The thickness of the microwave absorbing functional layer is 0.5-2 mm, the thickness of the thermally conductive substrate layer is 1-4 mm, and the thickness ratio of the microwave absorbing functional layer to the thermally conductive substrate layer is 1:(1-4).

5. The epoxy resin-based composite microwave absorbing material according to claim 4, characterized in that: In the thermally conductive substrate layer, the mass ratio of graphene to epoxy resin is (0.1-0.6):1, the number of graphene layers is less than 10, and the sheet diameter is 1-10 μm.

6. The epoxy resin-based composite microwave absorbing material according to claim 5, characterized in that: In the microwave absorbing functional layer, the mass ratio of the modified composite microwave absorbing agent to the epoxy resin is (0.2~1):

1.

7. The epoxy resin-based composite microwave absorbing material according to claim 6, characterized in that: The boron nitride nanosheets are few-layer boron nitride nanosheets obtained by ball milling-assisted liquid phase exfoliation of hexagonal boron nitride.

8. A method for preparing an epoxy resin-based composite microwave absorbing material as described in any one of claims 1-7, characterized in that, Includes the following steps: Polyaniline, iron(III) oxide and titanium dioxide are mixed and dispersed in a solvent, and then stirred, sonicated, dried and ground to obtain a ternary composite microwave absorbing agent powder. The ternary composite microwave absorber powder is dispersed in a dispersion containing boron nitride nanosheets, and then mixed, sonicated, and dried to obtain a modified composite microwave absorber with boron nitride nanosheets coated on the surface. Epoxy resin and curing agent are mixed, the modified composite microwave absorbing agent is added and stirred evenly, and after degassing treatment, microwave absorbing functional layer slurry is obtained. Epoxy resin and curing agent are mixed, graphene is added and stirred evenly, and after degassing treatment, a thermally conductive matrix layer slurry is obtained. The microwave absorbing functional layer slurry is coated and pre-cured to a semi-solid state. Then, a thermally conductive matrix layer slurry is applied to the microwave absorbing functional layer and cured by hot pressing to obtain the epoxy resin-based composite microwave absorbing material.

9. The method for preparing an epoxy resin-based composite microwave absorbing material according to claim 8, characterized in that: Before coating and pre-curing the microwave absorbing functional layer slurry, the method further includes: preparing the thermally conductive matrix layer slurry by mixing epoxy resin with curing agent and accelerator in a mass ratio of (3-5):(0.5-1):(0.01-0.03); and preparing the microwave absorbing functional layer slurry by mixing epoxy resin with curing agent and accelerator in a mass ratio of (2-4):(0.3-0.8):(0.005-0.02).

10. The method for preparing an epoxy resin-based composite microwave absorbing material according to claim 8, characterized in that: The specific conditions for hot-press curing molding are as follows: pre-pressing and degassing are performed at a temperature of 60-80℃ and a pressure of 3-8MPa for 5-15 minutes, then the temperature is raised to 110-130℃ and cured under the same pressure for 1-3 hours, and finally demolded after natural cooling to room temperature.

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