Electromagnetic shielding composite material and electromagnetic shielding sheet

Abstract

The invention discloses an electromagnetic shielding composite material and an electromagnetic shielding sheet, and belongs to the technical field of electromagnetic functional materials.The composite material comprises liquid metal, magnetic nanocrystalline fluid and a conductive reinforcing phase, magnetic particles are distributed in the liquid metal in a gradient mode through vortex mixing, and the magnetic particles are enriched on the outer layer to enhance absorption loss; the inner layer is enriched with a conductive reinforcement phase to improve reflection loss. The electromagnetic shielding sheet comprises a flexible substrate with periodic grid grooves and a flexible electromagnetic shielding composite material which is filled in the flexible substrate and based on magnetic nanocrystalline fluid and liquid metal, and is covered with an upper protective layer. The composite material has the characteristics of high conductivity, magnetic responsiveness and flexibility, can be applied to the fields of electromagnetic shielding, wearable electronics, aerospace equipment and the like, is particularly suitable for electromagnetic wave shielding of different frequency bands such as WiFi and 5G millimeter waves, realizes resonance absorption shielding of a specific frequency band by regulating and controlling the grid spacing and the groove size, and improves the local shielding effectiveness.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic functional materials technology, specifically relating to a flexible electromagnetic shielding composite material and electromagnetic shielding sheet based on magnetic nanocrystalline fluid and liquid metal. Background Technology

[0002] With the integration and miniaturization of electronic devices and the rapid development of emerging technologies such as 5G communication and the Internet of Things, electromagnetic interference (EMI) has become an increasingly prominent problem. As a key means of solving EMI, electromagnetic shielding materials are experiencing continuous market demand growth. Traditional EMI shielding materials, such as metal sheets and metal foams, while offering good shielding effects, suffer from problems such as secondary reflection pollution, heavy weight, poor processability, high cost, susceptibility to corrosion, and limited functionality. These limitations make it difficult to meet the requirements of modern electronic devices for lightweight, flexible, high-performance materials with adaptability to complex shapes.

[0003] To overcome the limitations of traditional materials, research has shifted its focus to novel composite materials with high conductivity, flexibility, and functional integration. Liquid metals, with their high conductivity, good fluidity, and self-healing properties, can form a continuous conductive network within a matrix, effectively reflecting and absorbing electromagnetic waves, and are considered ideal candidates for next-generation flexible shielding materials.

[0004] However, single liquid metal composite materials still primarily suffer from conductivity loss and reflection, and the problem of secondary reflection remains unresolved. Magnetic nanocrystalline fluids, on the other hand, are magnetic nanofluids whose internal magnetic nanoparticles can generate magnetic loss on electromagnetic waves. This reduces the electromagnetic waves reflected back into the environment by highly reflective shielding materials (such as metal foil), preventing the accumulation of electromagnetic energy in space and interference with other sensitive equipment, thus contributing to further improvements in electromagnetic shielding effectiveness. Therefore, incorporating magnetic nanocrystalline fluids as a magnetic loss component can be considered. Through natural resonance and hysteresis loss mechanisms, electromagnetic wave energy can be converted into heat energy, effectively enhancing absorption performance and reducing reflection pollution. By combining liquid metal with magnetic nanocrystalline fluids, the synergistic effect of the two can be fully utilized to overcome the performance bottlenecks of traditional electromagnetic shielding materials, achieving a balance between reflection and absorption. This is expected to yield high shielding effectiveness and low reflection characteristics over a wide frequency range, aligning with the development direction of green shielding.

[0005] However, the development of this composite material still faces several technical challenges: (1) Challenges of structural uniformity and stability: The high surface tension of liquid metal and the tendency of magnetic nanoparticles to agglomerate make it difficult for the two phases to be uniformly dispersed, and phase separation is likely to occur, affecting the synergistic effect; (2) Difficulty in balancing flexibility and functionality: High filler content can easily lead to a decline in the mechanical properties of the matrix, resulting in embrittlement or cracking, which restricts its application in dynamic scenarios; (3) Insufficient long-term reliability: Under cyclic deformation or temperature changes, problems such as weakening of interfacial bonding, oxidation of nanoparticles and leakage of liquid metal may cause the shielding performance to degrade. (4) The scalability of the process needs to be improved: existing methods are mostly limited to the laboratory scale and lack stable and low-cost preparation technologies suitable for large-scale production.

[0006] In summary, while current technologies have demonstrated the potential of combining liquid metals with magnetic nanocrystalline fluids at the theoretical level, significant bottlenecks remain in areas such as material stability, structural design, and process maturity. There is an urgent need to develop a new generation of electromagnetic shielding materials that can balance high conductivity, strong magnetic response, excellent flexibility, and environmental stability. Summary of the Invention

[0007] In view of the above, the purpose of this invention is to provide a flexible electromagnetic shielding composite material and electromagnetic shielding sheet based on magnetic nanocrystalline fluid and liquid metal, which achieves adjustment of the electromagnetic wave shielding frequency band through material composite system design and structural design. In the material composite system, the conductivity and permeability of the composite material are adjusted by changing the ratio of magnetic nanocrystalline fluid to liquid metal. In terms of structural design, the prepared electromagnetic shielding sheet consists of a three-layer structure: a bottom laser-patterned layer, a middle electromagnetic shielding composite material coating layer, and an upper protective layer. The bottom flexible substrate is laser-processed into groove structures with different patterns. By adjusting the pattern of the groove structure, strong electromagnetic wave shielding of a specific frequency band is achieved. Subsequently, the electromagnetic shielding composite material is injected into the grooves, and finally, the protective layer is attached to bond the three layers together to form a flexible electromagnetic shielding composite material. This achieves a combination of high conductivity, magnetic responsiveness, and flexibility, making it suitable for electromagnetic shielding, wearable sensors, lightweight airborne aerospace equipment, implantable electronic devices, adaptability to bending and folding conditions, and potential expansion into emerging fields such as smart skins and soft robots.

[0008] To achieve the above-mentioned objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, comprising the following components and mass percentages: liquid metal 70%-95%, magnetic nanocrystalline fluid 5%-30%, and conductive reinforcing phase 0%-5%. Through vortex mixing, the magnetic nanoparticles in the magnetic nanocrystalline fluid form a gradient distribution in the liquid metal, that is, the outer layer is enriched with magnetic nanoparticles to enhance absorption loss (SEA), and the inner layer is enriched with conductive reinforcing phase to improve reflection loss (SER).

[0009] Preferably, the magnetic nanocrystalline fluid comprises magnetic nanoparticles, a surfactant, and a dispersion medium, wherein the magnetic nanoparticles are selected from one or more of iron-based nanocrystals, cobalt-based amorphous nanocrystals, nickel-based amorphous nanocrystals, Fe3O4 nanoparticles, or γ-Fe2O3; the surfactant is oleic acid or sodium dodecylbenzenesulfonate (SDBS); and the dispersion medium is ethylene glycol, deionized water, alkanes, or esters.

[0010] Preferably, the liquid metal is selected from one of gallium indium eutectic alloy (EGaIn), gallium indium tin eutectic alloy (Galinstan), or gallium indium tin zinc alloy (GaInSnZn).

[0011] Preferably, the conductive enhancement phase is MXene nanosheets Ti3C2T x Its thickness is 1-5 nm.

[0012] Secondly, embodiments of the present invention also provide a method for preparing the flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal as described above, comprising the following steps: (1) Preparation of magnetic nanocrystalline fluid: magnetic nanoparticles, surfactants and dispersion media are mixed and ball-milled, then magnetically separated, washed and redispersed, and then emulsified by high-speed shearing and vacuum degassing to obtain a stable magnetic nanocrystalline fluid; (2) Preparation of composite material: Liquid metal and ethanol are mixed and ultrasonically broken to obtain nanodroplets. Citric acid is added to inhibit oxidation. Then, magnetic nanocrystalline fluid, conductive reinforcing phase and silane coupling agent are added in batches. After vortex mixing, a gradient distribution is formed. Finally, ethanol is removed by vacuum to obtain a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal.

[0013] Thirdly, embodiments of the present invention also provide an electromagnetic shielding sheet prepared using the above-mentioned flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, comprising: a flexible substrate having periodic grid grooves and a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal filled therein, and covered with a protective layer with a total thickness of 0.125-0.6 mm.

[0014] Preferably, the flexible substrate is PET, PI or PDMS with a thickness of 0.1-0.5 mm; the protective layer is a fluorinated ethylene propylene copolymer film with a thickness of 25-100 μm.

[0015] Preferably, the pattern of the periodic grid grooves is spiral, honeycomb, or arrayed, adapted to the electromagnetic wave resonant wavelength λ / 4. The arrayed grid grooves have a width of 2-10 mm, a depth of 0.1-0.5 mm, and row and column spacing of 1-5 mm.

[0016] Fourthly, embodiments of the present invention also provide a method for preparing the electromagnetic shielding sheet as described above, comprising the following steps: (1) Preparation of composite materials: A radial gradient conductive reinforced liquid metal encapsulated magnetic nanocrystalline fluid structure was prepared by vortex mixing; (2) Mesh design and fabrication: Periodic mesh grooves are processed on a flexible substrate using laser, and the mesh row spacing, column spacing, and groove width and depth are controlled; (3) Composite material filling: A flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal is filled into the groove with a periodic grid. After cooling, it is laminated with the adhesive-coated upper protective layer and hot-pressed into an integral sheet.

[0017] Fifthly, embodiments of the present invention also provide an application of the electromagnetic shielding sheet described above in electromagnetic shielding, applied to electromagnetic shielding of different frequency bands of WiFi and 5G millimeter waves, achieving resonant absorption of specific frequency bands by adjusting the size of periodic grid grooves, thereby improving local shielding effectiveness.

[0018] Compared with the prior art, the beneficial effects of the present invention include at least the following: (1) Multifunctional synergistic shielding, breaking through performance bottlenecks: This invention establishes a reflection-absorption synergistic mechanism. By combining magnetic nanocrystalline fluid (magnetic loss) with liquid metal (high conductivity) and gradient distribution structure (outer layer magnetic particles enhance absorption, inner layer high conductivity enhances reflection), efficient shielding is achieved.

[0019] (2) Flexible and lightweight design to adapt to complex working conditions: This invention achieves lightweighting based on periodic grid design, with a total material thickness as low as 0.125 mm and a density of <0.3 g / cm³. 3 (Traditional metal foil >2 g / cm) 3 Even with a bending radius of <3 mm, the SE attenuation remains <5%, meeting the dynamic deformation requirements of wearable devices and folding applications. The liquid metal network can self-repair after scratches (24h recovery rate >90%), extending its service life.

[0020] (3) Adjustable frequency band and precise shielding: This invention achieves resonance effect and reduces transmittance by adjusting the width and depth of the periodic grid-like grooves, as well as the row and column spacing of the grid. It is compatible with different frequency bands such as 5G millimeter wave (24.25-52.6 GHz) and WiFi 2.45 / 5.8GHz. Combined with the micro-macro structural design, the local shielding effectiveness is improved by 15-20 dB. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art 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.

[0022] Figure 1 This is a schematic diagram of the structure of a battery shielding sheet prepared using a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, as provided in an embodiment of the present invention. Figure 2 This is a schematic diagram of the preparation method of the battery shielding sheet made of a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal provided in the embodiments of the present invention; Figure 3 The electromagnetic shielding performance at 2.45 GHz of the battery shielding sheet prepared using a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, as provided in Embodiment 1 of the present invention; Figure 4 The electromagnetic shielding performance at 5.8 GHz of the battery shielding sheet prepared using a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, as provided in Embodiment 2 of the present invention; Figure 5 The electromagnetic shielding performance at 24GHz of the battery shielding sheet prepared using a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, as provided in Embodiment 3 of the present invention; Figure 6 The electromagnetic shielding performance at 31 GHz of the battery shielding sheet prepared using a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, as provided in Embodiment 4 of the present invention. Figure 7 The electromagnetic shielding effectiveness of the electromagnetic shielding sheet prepared using a flexible electromagnetic shielding composite material based on patternless magnetic nanocrystalline fluid and liquid metal, as provided in Comparative Example 1 of this invention. Figure 8 This is the electromagnetic shielding effectiveness of the electromagnetic shielding sheet prepared using a flexible electromagnetic shielding material based on liquid metal, as provided in Comparative Example 2 of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the scope of protection of this invention.

[0024] The inventive concept of this invention is as follows: Addressing the shortcomings of existing electromagnetic shielding materials based on liquid metal and magnetic nanocrystalline fluid in terms of material stability, structural design, and process maturity, this invention provides a flexible electromagnetic shielding composite material and electromagnetic shielding sheet based on magnetic nanocrystalline fluid and liquid metal. During vortex mixing, the centrifugal force generated by high-speed rotation forces heavier particles to migrate to the outer periphery, forming a radial concentration gradient of conductive-enhanced liquid metal encapsulating magnetic nanocrystalline fluid. This achieves a gradient distribution of magnetic particles in the liquid metal—the outer layer enriches magnetic particles to enhance absorption loss (SEA), while the inner layer enriches conductive-enhanced liquid metal to improve reflection loss (SER). Furthermore, periodic grid grooves are laser-processed on a flexible substrate. By controlling the grid row spacing, column spacing, and groove width and depth, the grid shape of the filled magnetic nanocrystalline fluid / liquid metal composite material is controlled, thereby achieving strong absorption of electromagnetic waves in a specific frequency band. Its core lies in combining micro- and macro-level structural design to significantly improve electromagnetic wave shielding in a specific frequency band while achieving flexibility and lightweight design.

[0025] like Figure 1 As shown, the embodiment provides an electromagnetic shielding sheet prepared using a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, comprising: a bottom laser pattern processing layer (flexible substrate), a middle electromagnetic shielding composite material coating layer (flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal), and an upper protective layer, wherein the flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal is filled in periodic grid grooves on the flexible substrate.

[0026] The flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal comprises: 70%-95% liquid metal, 5%-30% magnetic nanocrystalline fluid, and 0%-5% conductive reinforcing phase. Through vortex mixing, the magnetic nanoparticles of the magnetic nanocrystalline fluid form a gradient distribution in the liquid metal; that is, the outer layer is enriched with magnetic nanoparticles to enhance absorption loss, while the inner layer is enriched with the conductive reinforcing phase to improve reflection loss. Its preparation method includes the following steps: (1) Preparation of magnetic nanocrystalline fluid: Magnetic nanoparticles were mixed with a surfactant (3 wt%) and an appropriate amount of dispersion medium and ball-milled for 2 hours. Then, the prepared magnetic nanocrystalline fluid was placed on a magnetic separator to separate the magnetic nanoparticles from the dispersion medium under the action of a magnetic field. The supernatant was discarded, and the magnetic particle precipitate was retained. The magnetic particles were washed 2-3 times with the dispersion medium to remove excess surfactant and impurity ions. After that, the washed magnetic particles were redispersed in an appropriate amount of dispersion medium and sheared and dispersed at a speed of 5000-10000 r / min for 10-15 minutes using a high-speed shear emulsifier. Finally, the magnetic nanocrystalline fluid was placed in a vacuum drying oven and degassed under vacuum at 50-60℃ for 1-2 hours to remove air bubbles and obtain a stable magnetic nanocrystalline fluid.

[0027] (2) Preparation of composite material: Liquid metal and ethanol (volume ratio 1:5) were mixed and ultrasonically broken up (power 500 W, 30 minutes) to obtain nanodroplets with a particle size of 100-300 nm. The surface was coated with citric acid (0.5 wt%) to inhibit oxidation. Magnetic nanocrystalline fluid, MXene nanosheets and silane coupling agent (KH550, 0.5 wt%) were added in portions and vortexed (2000-5000 rpm) to form a gradient distribution. Finally, the magnetic nanocrystalline fluid was placed in a vacuum drying oven and degassed under vacuum at 50-60℃ for 1-2 hours to remove ethanol and bubbles, obtaining a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal.

[0028] like Figure 2 As shown in the embodiment, a method for preparing an electromagnetic shielding sheet using a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal is also provided, including the following steps: S1, Composite material preparation: A radially gradient conductive reinforced liquid metal-encapsulated magnetic nanocrystalline fluid structure was prepared by vortex mixing.

[0029] Specifically, when using vortex mixing, it's important to note that the entire magnetic nanocrystalline fluid and conductive reinforcing phase are not added to the liquid metal at once, but rather in multiple batches. After each addition, the mixture is allowed to vortex for a period of time, allowing the magnetic nanocrystalline fluid to first form a relatively uniform initial distribution in the liquid metal before adding the next batch. By controlling the amount of magnetic nanocrystalline fluid added each time and the mixing time, a gradient distribution of the magnetic nanocrystalline fluid can be gradually formed in the liquid metal. During vortex mixing, the centrifugal force generated by the high-speed rotation (2000-5000 rpm) forces heavier particles to migrate to the outer periphery, forming a radial concentration gradient.

[0030] S2, Mesh Design and Fabrication: Periodic mesh grooves are processed on a flexible substrate using laser, controlling the mesh row spacing, column spacing, and groove width and depth.

[0031] Specifically, select a suitable substrate and use a CO2 laser engraving machine. The recommended laser power is approximately 10-50W, frequency 5-30kHz, duty cycle 20%-50%, and scanning speed approximately 300-1500mm / min. Use laser processing software to draw an array of periodic grid-like grooves 2-10mm wide and 0.1-0.5mm deep, and convert it into a processing path format recognizable by the laser equipment. Set the grid's row spacing, column spacing, and other parameters. Fix the substrate on the laser processing machine's worktable, ensuring the surface is flat and perpendicular to the laser beam. Adjust the laser's focus position so that it is on the substrate surface, start the laser processing machine, and process according to the preset parameters and path. During processing, observe the laser beam trajectory and processing effect, and adjust the parameters as needed.

[0032] S3, Composite Material Filling: A flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal is filled into grooves with a periodic grid. After cooling, it is laminated with an adhesive-coated upper protective layer and hot-pressed into an integral sheet.

[0033] Specifically, the laser-processed grooves were treated with oxygen plasma (50 W, 60 s) to enhance surface wettability. A pre-prepared flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal was then precisely filled into the grooves using a scraper, followed by slurry morphology fixation at 0-4℃. A 25-100 μm thick fluorinated ethylene propylene copolymer (FEP) film was used as the upper protective layer, with an acrylic adhesive layer on its lower surface, stacked with the bottom substrate (including the filling composite material). The layers were placed in a hot press (130℃, 0.5 MPa, 5 minutes) to achieve interfacial molecular diffusion bonding. A UV-curable adhesive (LOCTITE 4305) was applied around the perimeter, followed by UV irradiation (365 nm, 100 mW / cm²). 2 (30 seconds) to form an airtight seal.

[0034] The following examples are in conjunction with Examples 1-4, Comparative Examples 1-2, and others. Figure 3-8 The electromagnetic shielding effectiveness curves shown further verify the electromagnetic shielding effectiveness of the electromagnetic shielding sheet prepared using a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal provided in the embodiments of the present invention.

[0035] Example 1 S101, Preparation of magnetic nanocrystalline fluid: Iron-based nanocrystalline particles (20 g) and oleic acid (3 wt%) were added to ethylene glycol (200 mL) and ball-milled for 2 hours (300 rpm). The supernatant was removed by magnetic separation, and the mixture was washed three times with ethylene glycol and redispersed in ethylene glycol. High-speed shear emulsification was then performed (8000 rpm, 12 min). The mixture was then degassed under vacuum (55 °C, 1.5 h) to obtain a stable magnetic iron-based nanocrystalline fluid.

[0036] S102, Preparation of magnetic nanocrystalline fluid / liquid metal composite materials: Gallium indium tin eutectic alloy liquid metal (85 g) was mixed with ethanol (425 mL) and ultrasonically broken up (500 W, 30 min) to obtain iron-based nanocrystalline droplets with a particle size of ~200 nm. Citric acid (0.5 wt%) was added to inhibit oxidation. Magnetic nanocrystalline fluid (total 15 g) and MXene (0.5 g) were added in three portions, vortexed after each addition (4000 rpm, 5 min). The final slurry viscosity was 35 Pa·s (25 °C), and the magnetic nanocrystalline particles exhibited a radial gradient distribution. Ethanol was removed under vacuum (60 °C, 2 h) to obtain a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, with a mass ratio of magnetic nanocrystalline fluid to liquid metal of 15:85.

[0037] S103, Laser patterning of grooves on the substrate: A 0.3 mm thick PI substrate was selected. The laser power was set to 30 W, the frequency to 20 kHz, and the scanning speed to 800 mm / min. The focal point was located on the PI surface. An array of grid-like grooves was machined on the PI, with a groove width of 5.7 mm and a depth of 0.2 mm. The row and column spacing of the grid were both 4.3 mm. Subsequently, the PI surface was subjected to oxygen plasma treatment (50 W, 60 s) to reduce the contact angle of the groove surface.

[0038] S104, Composite Material Filling and Encapsulation: The flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, prepared by S102, was precisely filled into the groove using a scraper and cooled to 4°C for pre-curing. A 50 μm thick FEP film (pre-coated with acrylic adhesive, 5 μm thick) was selected as the upper protective layer. The adhesive-coated side was placed face down and stacked with the PI matrix (containing the filled composite material), and then placed in a hot press (temperature 130°C, pressure 0.5 MPa, time 5 minutes) to achieve interfacial molecular diffusion bonding. The edges were sealed with UV-curable adhesive and irradiated at 365 nm for 30 seconds. Finally, an electromagnetic shielding sheet with an electromagnetic shielding frequency band of 2.45 GHz and a total thickness of 0.35 mm was obtained. Its electromagnetic shielding effectiveness is as follows: Figure 3 As shown.

[0039] Example 2 S101, Preparation of magnetic nanocrystalline fluid: Cobalt-based nanocrystalline nanoparticles (20 g) and oleic acid (3 wt%) were added to ethylene glycol (200 mL) and ball-milled for 2 hours (300 rpm). The supernatant was removed by magnetic separation, washed three times with ethylene glycol, and redispersed in ethylene glycol. High-speed shear emulsification was then performed (8000 rpm, 12 min). Vacuum degassing (55 °C, 1.5 h) was then performed to obtain a stable magnetic cobalt-based nanocrystalline fluid.

[0040] S102, Preparation of magnetic nanocrystalline fluid / liquid metal composite materials: 90 g of gallium indium tin eutectic alloy liquid metal was mixed with ethanol (425 mL) and ultrasonically broken up (500 W, 30 min) to obtain nanodroplets with a particle size of ~200 nm. Citric acid (0.5 wt%) was added to inhibit oxidation. Magnetic nanocrystalline fluid (total 10 g) and MXene (0.5 g) were added in three portions, vortexed after each addition (4000 rpm, 5 min). The final slurry viscosity was 35 Pa·s (25 °C), and the magnetic particles exhibited a radial gradient distribution. Ethanol was removed under vacuum (60 °C, 2 h) to obtain a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, with a mass ratio of magnetic nanocrystalline fluid to liquid metal of 10:90.

[0041] S103, Laser patterning of grooves on the substrate: A 0.3 mm thick PI substrate was selected. The laser power was set to 30 W, the frequency to 20 kHz, and the scanning speed to 800 mm / min. The focal point was located on the PI surface. An array of grid-like grooves was machined on the PI, with a groove width of 4.5 mm and a depth of 0.2 mm. The row and column spacing of the grid were both 2.5 mm. Subsequently, the PI surface was subjected to oxygen plasma treatment (50 W, 60 s) to reduce the contact angle of the groove surface.

[0042] S104, Composite Material Filling and Encapsulation: The flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, prepared by S102, was precisely filled into the groove using a scraper and pre-cured at 4°C. A 50 μm thick FEP film (pre-coated with acrylic adhesive, 5 μm thick) was selected as the upper protective layer. The adhesive-coated side was placed face down and stacked with the PI matrix (containing the filled composite material), and then placed in a hot press (temperature 130°C, pressure 0.5 MPa, time 5 minutes) to achieve interfacial molecular diffusion bonding. The edges were sealed with UV-curable adhesive and irradiated at 365 nm for 30 seconds. Finally, an electromagnetic shielding sheet with an electromagnetic shielding frequency band of 5.8 GHz and a total thickness of 0.35 mm was obtained. Its electromagnetic shielding effectiveness is as follows: Figure 4 As shown.

[0043] Example 3 S101, Preparation of magnetic nanocrystalline fluid: Fe3O4 nanoparticles (20 g) and oleic acid (3 wt%) were added to ethylene glycol (200 mL) and ball-milled for 2 hours (300 rpm). The supernatant was removed by magnetic separation, washed three times with ethylene glycol, and redispersed in ethylene glycol. High-speed shear emulsification was then performed (8000 rpm, 12 min). Vacuum degassing (55 °C, 1.5 h) was then performed to obtain a stable Fe3O4 magnetic nanocrystalline fluid.

[0044] S102, Preparation of magnetic nanocrystalline fluid / liquid metal composite materials: Gallium indium tin eutectic alloy liquid metal (95 g) was mixed with ethanol (425 mL) and ultrasonically broken up (500 W, 30 min) to obtain nanodroplets with a particle size of ~200 nm. Citric acid (0.5 wt%) was added to inhibit oxidation. Magnetic nanocrystalline fluid (total 5 g) and MXene (0.5 g) were added in three portions, vortexed after each addition (4000 rpm, 5 min). The final slurry viscosity was 35 Pa·s (25 °C), and the magnetic particles exhibited a radial gradient distribution. Ethanol was removed under vacuum (60 °C, 2 h) to obtain a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, with a mass ratio of magnetic nanocrystalline fluid to liquid metal of 5:95.

[0045] S103, Laser patterning of grooves on the substrate: A 0.3 mm thick PI substrate was selected. The laser power was set to 30 W, the frequency to 20 kHz, and the scanning speed to 800 mm / min. The focal point was located on the PI surface. An array of grid-like grooves was machined on the PI, with a groove width of 4.8 mm and a depth of 0.2 mm. The row and column spacing of the grid were both 1.2 mm. Subsequently, the PI surface was subjected to oxygen plasma treatment (50 W, 60 s) to reduce the contact angle of the groove surface.

[0046] S104, Composite Material Filling and Encapsulation: The flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, prepared by S102, was precisely filled into the groove using a scraper and pre-cured at 4°C. A 50 μm thick FEP film (pre-coated with acrylic adhesive, 5 μm thick) was selected as the upper protective layer. The adhesive-coated side was placed face down and stacked with the PI matrix (containing the filled composite material), and then placed in a hot press (temperature 130°C, pressure 0.5 MPa, time 5 minutes) to achieve interfacial molecular diffusion bonding. The edges were sealed with UV-curable adhesive and irradiated at 365 nm for 30 seconds. Finally, an electromagnetic shielding sheet with an electromagnetic shielding frequency band of 24 GHz and a total thickness of 0.35 mm was obtained. Its electromagnetic shielding effectiveness is as follows: Figure 5 As shown.

[0047] Example 4 Unlike Example 3, in S103, the groove width is 4mm, the depth is 0.2mm, and the row and column spacing of the grid are both 1mm. The electromagnetic shielding effectiveness is as follows: Figure 6 As shown.

[0048] Comparative Example 1 Under the same conditions of using the flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal, unlike Example 1, in S103, it is not processed into a specific pattern but is a single piece of flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal. The electromagnetic shielding effectiveness is as follows: Figure 7 As shown.

[0049] Comparative Example 2 Unlike Example 1, only liquid metal is used without magnetic nanocrystals, and the electromagnetic shielding effectiveness is as follows: Figure 8 As shown.

[0050] The specific embodiments described above illustrate the technical solution and beneficial effects of the present invention in detail. It should be understood that the above description is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An electromagnetic shielding composite material, characterized in that, It includes the following components and mass percentages: liquid metal 70%-95%, magnetic nanocrystalline fluid 5%-30%, and conductive reinforcing phase 0%-5%. Through vortex mixing, the magnetic nanoparticles in the magnetic nanocrystalline fluid form a gradient distribution in the liquid metal, that is, the outer layer is enriched with magnetic nanoparticles to enhance absorption loss, and the inner layer is enriched with conductive reinforcing phase to improve reflection loss.

2. The electromagnetic shielding composite material according to claim 1, characterized in that, The magnetic nanocrystalline fluid comprises magnetic nanoparticles, a surfactant, and a dispersion medium. The magnetic nanoparticles are selected from one or more of iron-based nanocrystals, cobalt-based amorphous nanocrystals, nickel-based amorphous nanocrystals, Fe3O4 nanoparticles, or γ-Fe2O3. The surfactant is oleic acid or sodium dodecylbenzenesulfonate. The dispersion medium is ethylene glycol, deionized water, alkanes, or esters.

3. The electromagnetic shielding composite material according to claim 1, characterized in that, The liquid metal is selected from one of EGaIn alloy, Galinstan alloy, or GaInSnZn alloy.

4. The electromagnetic shielding composite material according to claim 1, characterized in that, The conductive reinforcement phase is MXene nanosheets Ti3C2T. x Its thickness is 1-5 nm.

5. A method for preparing an electromagnetic shielding composite material as described in any one of claims 1-4, characterized in that, Includes the following steps: (1) Preparation of magnetic nanocrystalline fluid: magnetic nanoparticles, surfactants and dispersion media are mixed and ball-milled, then magnetically separated, washed and redispersed, and then emulsified by high-speed shearing and vacuum degassing to obtain a stable magnetic nanocrystalline fluid; (2) Preparation of composite material: Liquid metal and ethanol are mixed and ultrasonically broken to obtain nanodroplets. Citric acid is added to inhibit oxidation. Then, magnetic nanocrystalline fluid, conductive reinforcing phase and silane coupling agent are added in batches. After vortex mixing, a gradient distribution is formed. Finally, ethanol is removed by vacuum to obtain a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal.

6. An electromagnetic shielding sheet prepared using the electromagnetic shielding composite material according to any one of claims 1-4, characterized in that, include: A flexible matrix with periodic grid grooves and a flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal filled therein, covered with a protective layer, with a total thickness of 0.125-0.6 mm.

7. The electromagnetic shielding sheet according to claim 6, characterized in that, The flexible substrate is PET, PI, or PDMS with a thickness of 0.1-0.5 mm; the protective layer is a fluorinated ethylene propylene copolymer film with a thickness of 25-100 μm.

8. The electromagnetic shielding sheet according to claim 6, characterized in that, The periodic grid grooves have a spiral, honeycomb, or array grid pattern, designed to match the electromagnetic wave resonant wavelength λ / 4.

9. A method for preparing an electromagnetic shielding sheet as described in any one of claims 6-8, characterized in that, Includes the following steps: (1) Preparation of composite materials: A radial gradient conductive reinforced liquid metal encapsulated magnetic nanocrystalline fluid structure was prepared by vortex mixing; (2) Mesh design and fabrication: Periodic mesh grooves are processed on a flexible substrate using laser, and the mesh row spacing, column spacing, and groove width and depth are controlled; (3) Composite material filling: A flexible electromagnetic shielding composite material based on magnetic nanocrystalline fluid and liquid metal is filled into the groove with a periodic grid. After cooling, it is laminated with the adhesive-coated upper protective layer and hot-pressed into an integral sheet.

10. An application of the electromagnetic shielding sheet as described in any one of claims 6-8 in electromagnetic shielding, characterized in that, Electromagnetic shielding for different frequency bands of WiFi and 5G millimeter waves can achieve resonant absorption of specific frequency bands by adjusting the size of periodic grid grooves, thereby improving local shielding effectiveness.

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