A stretchable electromagnetic shielding film and a method of manufacturing the same

Abstract

The application provides a stretchable electromagnetic shielding film and a preparation method thereof, and the electromagnetic shielding film comprises a substrate layer and a liquid metal conductive layer, and the liquid metal conductive layer is formed by mechanically sintering after gradient ultrasonic treatment liquid metal dispersion liquid is sprayed on the substrate layer through ultrasonic spraying. The application forms liquid metal droplets with hierarchical sizes through gradient ultrasonic treatment, and controls the sintering process to break large droplets to form a conductive network, and to form cracks on medium droplets. In the stretching process of the electromagnetic shielding film, the outflow of the liquid in the medium droplets and the conductive fluid of the large droplets realize the maintenance of high electromagnetic shielding efficiency, greatly improve the shielding efficiency maintenance rate of the shielding film in the stretching process, and have wide application prospect and economic value in the field of variable electronic products.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic shielding materials, and in particular relates to a stretchable electromagnetic shielding film and its preparation method. Background Technology

[0002] The development of electromagnetic technology has placed more functional demands on shielding materials. In particular, modern electronic products such as flexible electronics and wearable devices require electromagnetic shielding materials to not only meet performance requirements but also possess excellent bending performance, stretchability, and conformal bonding capabilities.

[0003] There are several existing methods for preparing flexible electromagnetic shielding films: one is to directly apply a slurry containing conductive fillers to a flexible substrate through surface spraying or coating. This method has the advantage of perfectly conforming to various irregular curved surfaces, but the adhesion between the formed conductive layer and the flexible substrate is low, making it prone to detachment. Another method is to deposit a dense metal layer on the surface of a thin film or fabric through chemical plating, electroplating, or vapor deposition. This method has the advantages of uniform thickness, excellent conductivity, and high electromagnetic shielding effectiveness, but the metal layer is prone to breakage during bending, thus losing its shielding performance. Another method is to weave or blend conductive metal wires, interweaving copper, stainless steel, and other metal fibers with ordinary textile fibers to form conductive fabrics. These materials have high structural strength, bend resistance, and good air permeability, but their shielding effectiveness is relatively low and they cannot be stretched. Therefore, developing flexible, especially stretchable, electromagnetic shielding materials, and constructing shielding materials with high shielding performance that are bendable and stretchable, is essential to meeting the shielding requirements of next-generation deformable electronic devices.

[0004] Among various flexible electromagnetic shielding materials, liquid metal-based electromagnetic shielding materials exhibit the best ductility, combining the high fluidity of liquids with the high electrical conductivity of metals. Therefore, coating a flexible substrate with liquid metal can form a high-performance, ultra-elastic electromagnetic shielding film. For example, Chinese invention patent CN121060787A discloses an ultrasonically sprayed liquid metal elastomer film with high shielding effectiveness and its preparation method. The method involves ultrasonically dispersing liquid metal in a solvent to obtain a uniform and stable dispersion containing liquid metal droplets, and then ultrasonically spraying the liquid metal dispersion onto a thermoplastic polyurethane substrate, providing a large-scale preparation scheme for this type of film. However, the shielding performance of this type of film decreases under heavy stretching, limiting its application. Therefore, how to form a uniform and stable film that maintains high shielding performance under tensile stress is the main challenge facing liquid metal electromagnetic shielding films. Summary of the Invention

[0005] In view of this, the present invention aims to provide a stretchable electromagnetic shielding film and its preparation method to overcome the problem of decreased shielding performance of electromagnetic shielding films after stretching in the prior art.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows: In a first aspect, the present invention provides a stretchable electromagnetic shielding film, the electromagnetic shielding film comprising a substrate layer and a liquid metal conductive layer, wherein the liquid metal conductive layer is formed by ultrasonically spraying a liquid metal dispersion treated with gradient ultrasonication onto the substrate layer and then mechanically sintering it.

[0007] Furthermore, the liquid metal droplets in the liquid metal conductive layer include small droplets, medium droplets, and large droplets. The small droplets have a particle size of 100-200 nm, the medium droplets have a particle size of 200-300 nm, and the large droplets have a particle size of 300-500 nm. During the mechanical sintering process: the large droplets completely break down, and the liquid metal flows out to fill the gaps, forming a complete and continuous conductive thin layer; the medium droplets form cracks, retaining part of their shape, and the liquid metal is connected to the external liquid metal, with some internal liquid flowing out during the stretching process; the small droplets retain their original shape, serving as internal support, connecting the external liquid metal, reducing the fluidity of the liquid, and preventing the film from breaking. Preferably, the ratio of large droplets, medium droplets, and small droplets is 21%~25%: 31.5%~32.5%: 31%~38%.

[0008] Furthermore, the gradient ultrasonic treatment includes performing a first-stage ultrasonic treatment and a second-stage ultrasonic treatment sequentially under water bath conditions. The power of the first-stage ultrasonic treatment is 600~750W, for example, any one or any two values ​​of 600W, 650W, 700W, and 750W, and the time is 15~30min, for example, any one or any two values ​​of 15min, 20min, 25min, and 30min. The power of the second-stage ultrasonic treatment is 800~1200W, for example, any one or any two values ​​of 800W, 850W, 900W, 950W, 1000W, 1050W, 1100W, 1150W, and 1200W, and the time is 15~30min, for example, any one or any two values ​​of 15min, 20min, 25min, and 30min.

[0009] Preferably, the thickness of the liquid metal conductive layer is 1~8μm, for example, it can be any one of 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm or a range between any two values.

[0010] Furthermore, the electromagnetic shielding film retains ≥90% of its electromagnetic shielding effectiveness even when the strain is 200%.

[0011] Furthermore, the base layer is a thermoplastic polyurethane base.

[0012] In a second aspect, the present invention provides a method for preparing a stretchable electromagnetic shielding film as described in the first aspect, the method comprising the following steps: S1. Prepare a liquid metal dispersion and subject the liquid metal dispersion to gradient ultrasonic treatment; S2. The liquid metal dispersion treated with gradient ultrasound is ultrasonically sprayed onto the substrate to form a liquid metal layer; S3. Mechanically sinter the liquid metal layer to form a liquid metal conductive layer, thereby obtaining the stretchable electromagnetic shielding film.

[0013] Further, in step S1, the concentration of liquid metal in the liquid metal dispersion is 4~6 mg / mL; Preferably, the liquid metal is a GaIn binary eutectic alloy; Preferably, the solvent for the liquid metal dispersion is anhydrous ethanol or deionized water.

[0014] Further, the gradient ultrasonic treatment includes sequentially performing a first-stage ultrasonic treatment and a second-stage ultrasonic treatment under water bath conditions. The power of the first-stage ultrasonic treatment is 600~750W, for example, any one or any two values ​​from 600W, 650W, 700W, and 750W, or a range between any two values, and the time is 15~30min, for example, any one or any two values ​​from 15min, 20min, 25min, and 30min, or a range between any two values. The power of the second-stage ultrasonic treatment is 800~1200W, for example, any one or any two values ​​from 800W, 850W, 900W, 950W, 1000W, 1050W, 1100W, 1150W, and 1200W, or a range between any two values, and the time is 15~30min, for example, 15min, 20min, 25 ... The inventors discovered that ultrasonic power has a significant impact on the distribution of droplets of different sizes. When the power is too low, the proportion of large droplets increases and the proportion of small droplets decreases, resulting in more liquid metal produced during sintering. This liquid metal has high fluidity and lacks the support of small droplets, making it prone to breakage under surface tension during film stretching. When the power is too high, the proportion of small droplets increases and the proportion of large droplets decreases, resulting in less free-flowing liquid metal between small droplets. This makes the film prone to breakage due to the lack of adhesion from the flowing liquid metal during film stretching. Through experiments, the present invention uses a first-stage ultrasonic treatment with a power of 700W for 20 minutes and a second-stage ultrasonic treatment with a power of 800W for 20 minutes to obtain the optimal droplet distribution ratio, which still maintains a high shielding effect after stretching. Furthermore, under the action of ultrasound, a solid oxide layer (Ga2O3) is formed in the part of the liquid metal that comes into contact with the air, which plays a supporting role.

[0015] Preferably, the gradient ultrasound processing sequence is 3 seconds on and 3 seconds off; Preferably, the water bath temperature is 35°C.

[0016] Further, in step S2, the carrier gas pressure of the ultrasonic spray nozzle is 0.04 MPa, the spraying height is 50 mm, the spraying spacing is 2 mm, and the liquid supply rate is 0.1~0.3 mL / min. Preferably, the number of ultrasonic spraying layers is 150 to 1200; Preferably, when the solvent of the liquid metal dispersion is anhydrous ethanol, the ultrasonic spraying temperature is room temperature; when the solvent of the liquid metal dispersion is deionized water, the ultrasonic spraying temperature is 80°C.

[0017] Furthermore, in step S3, the mechanical sintering is performed using roll pressing sintering; Preferably, the rolling speed of the roll pressing sintering is 5 mm / s; Preferably, the pressure of the roll pressing sintering is 93~122N, for example, it can be any one value or a range between any two values ​​of 93N, 95N, 98N, 100N, 103N, 105N, 108N, 110N, 113N, 115N, 118N, 120N, 122N.

[0018] Under the premise of graded droplet size, by controlling the sintering pressure, the liquid metal flowing out of the large droplets is broken up and fills the gaps between the medium and small droplets, forming a conductive network. Under the above sintering pressure, the medium droplets can maintain their shape and develop cracks; furthermore, the small droplets can maintain their original shape. During the stretching process, the conductive fluid formed by the large droplets and the supporting effect formed by the small droplets are reasonably distributed. During the stretching of the film, the liquid metal inside the medium droplets flows out, which helps to maintain the conductive network. Experiments have shown that even when the strain reaches 200%, it can still maintain 90% of the electromagnetic shielding performance.

[0019] Thirdly, the present invention provides the application of the stretchable electromagnetic shielding film according to the first aspect or the electromagnetic shielding film prepared by the preparation method described in the second aspect in the preparation of electromagnetic shielding components for electronic products.

[0020] Compared with existing technologies, the stretchable electromagnetic shielding film and its preparation method described in this invention have the following advantages: This invention uses gradient ultrasonic treatment to form liquid metal droplets of graded sizes, which are then sprayed onto a substrate. Simultaneously, the sintering process is controlled to break up large droplets into a conductive network and create cracks in medium droplets. During the stretching process of the electromagnetic shielding film, the outflow of liquid from the medium droplets and the conductive fluid from the large droplets maintain a high electromagnetic shielding efficiency, greatly improving the tensile deformation resistance. This invention has broad application prospects and economic value in the field of electronic products. Attached Figure Description

[0021] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is an electron microscope image of the liquid metal droplets after spraying in the example; Figure 2 This is an electron microscope image of the liquid metal conductive layer after mechanical sintering in the embodiment; Figure 3 The particle size distribution diagrams of liquid metal droplets after ultrasonic treatment are shown in the examples and comparative examples. Figure 4 The electromagnetic shielding performance curves before stretching are shown for the examples and comparative examples. Figure 5 The figures show the electromagnetic shielding performance curves after stretching in both the example and comparative cases. Detailed Implementation

[0022] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

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

[0024] Example

[0025] The method for preparing the stretchable electromagnetic shielding film in this embodiment includes the following steps: 1. Preparation of dispersion: Weigh 0.1g of liquid metal (GaIn binary eutectic alloy) and disperse it in 25mL of anhydrous ethanol. Set the ultrasonic power to 700W, the ultrasonic time sequence to 3s on, 3s off, and maintain the water bath temperature at 35℃ for 20min. Change the ultrasonic power to 800W, the ultrasonic time sequence to 3s on, 3s off, and maintain the water bath temperature at 35℃ for 20min to obtain a liquid metal dispersion. Figure 1 As shown, the liquid metal droplets are mainly large droplets (300~500nm), medium droplets (200~300nm) and small droplets (100~200nm).

[0026] 2. Ultrasonic spraying: Liquid metal dispersion was atomized using ultrasonic spraying technology and deposited onto a thermoplastic polyurethane substrate to form a uniform and dense liquid metal layer. During ultrasonic spraying, the carrier gas pressure of the nozzle was 0.04 MPa; the ultrasonic spraying height was 50 mm; the spraying spacing was 2.0 mm; the ultrasonic spraying liquid supply rate was 0.1 mL / min; the ultrasonic spraying temperature was room temperature; the number of ultrasonic spraying layers was 900; and the thickness of the formed liquid metal layer was approximately 6 μm.

[0027] 3. Mechanical sintering: The rolling speed was set to 5 mm / s, and a constant pressure of 100 N was applied to obtain an electromagnetic shielding film. Through mechanical sintering, large droplets were broken up, and the resulting liquid metal filled the gaps between small and medium-sized droplets, forming a conductive network. The medium-sized droplets maintained their shape and developed cracks, while the small droplets retained their original shape. This composite structure ensures that during the film stretching process, the conductive fluid formed by the large droplets and the supporting effect of the small droplets are rationally distributed, and the liquid metal inside the medium droplets flows out, thus maintaining the conductive network.

[0028] Comparative Example 1 The difference from the embodiment is that in step 1, the ultrasonic power is 700W, the ultrasonic timing is 3s on, 3s off, the water bath temperature is maintained at 35℃, and the ultrasonic dispersion is 40min. The remaining steps are the same as in the embodiment.

[0029] Comparative Example 2 The difference from the embodiment is that in step 1, the ultrasonic power is 800W, the ultrasonic timing is 3s on, 3s off, the water bath temperature is maintained at 35℃, and the ultrasonic dispersion is 40min. The remaining steps are the same as in the embodiment.

[0030] Test Example 1: Particle Size Distribution The particle size distribution of the liquid metal dispersions prepared in the examples and comparative examples was detected by dynamic light scattering method, and the results are as follows: Figure 3As shown in the figure, Comparative Example 1, treated with a single 700W ultrasonic treatment for 40 minutes, showed a significant bias in the particle size distribution peak towards the large particle size range (above 300nm), indicating an excessively high proportion of large droplets in the system. Comparative Example 2, treated with a single 800W ultrasonic treatment for 40 minutes, exhibited the opposite trend, with the particle size distribution peak biased towards the small particle size range (below 200nm), indicating an excessively high proportion of small droplets. The particle size distribution curves obtained from the two-step gradient ultrasonic treatment in the examples showed obvious bimodal or multimodal distribution characteristics. Within the range of 100nm to 500nm, small droplets (100~200nm), medium droplets (200~300nm), and large droplets (300~500nm) all occupied reasonable proportions. Based on the above results, it can be concluded that single-power ultrasonic treatment can only obtain a single-peak normal distribution and cannot simultaneously meet the functional requirements of droplets of all three sizes. Regardless of whether the bias is towards large or small particle sizes, performance shortcomings will occur under large strain tensile stress. The two-step gradient ultrasonic treatment can regulate the particle size distribution and form a three-phase structure with linkage function: large droplets provide the material source for subsequent sintering to form a conductive backbone, medium droplets serve as a reservoir of liquid metal during stretching, and small droplets act as mechanical anchoring points to suppress excessive liquid flow.

[0031] Test Example 2 Tensile Test The electromagnetic shielding performance of the electromagnetic shielding films prepared in the examples and comparative examples before and after stretching (tensile strain 200%) was measured using a dynamic vector analyzer. The results are as follows: Figure 4-5 As shown.

[0032] according to Figure 4 The results show that the electromagnetic shielding films prepared in the examples and comparative examples all exhibited good electromagnetic shielding effects before stretching, and there was no significant difference. This proves that regardless of whether single-power ultrasound or two-step gradient ultrasound is used, as long as the liquid metal droplet coverage and the conductive path formed after sintering are basically intact, the electromagnetic shielding film can achieve good electromagnetic shielding performance in the unstretched state.

[0033] according to Figure 5The results show that under 200% strain, the shielding performance of the electromagnetic shielding films prepared under all three conditions decreased to varying degrees. Comparative Example 1, using a single 700W ultrasonic treatment, and Comparative Example 2, using a single 800W ultrasonic treatment, showed significant decreases in shielding effectiveness, demonstrating that the conductive network suffered considerable damage under large deformation. In contrast, the electromagnetic shielding film prepared using the two-step gradient ultrasonic treatment in the example only experienced a decrease in shielding effectiveness of about 10%, maintaining 90% of its original shielding efficiency. Under 200% strain, the shielding effectiveness of the electromagnetic shielding film prepared in the example was about 5 dB higher than that of Comparative Examples 1-2. Since decibels are logarithmic units, for every 10 dB increase in shielding effectiveness, the transmitted electromagnetic wave power is reduced to one-tenth of its original value. Therefore, a 5 dB increase means that the electromagnetic wave energy transmitted by the electromagnetic shielding film prepared in the example after stretching is only about one-third of that in Comparative Examples 1-2, meaning that under large stretching conditions, the shielding effect of the electromagnetic shielding film prepared in the example is three times that of Comparative Examples 1-2.

[0034] The above results directly demonstrate that the graded droplet structure obtained by the present invention through two-step gradient ultrasonic treatment forms a three-phase functional structure after precisely controlled mechanical sintering, in which large droplets break up to form the conductive backbone, medium droplets serve as fluid reserves, and small droplets act as mechanical anchors. When the film is stretched to 200%, the liquid metal flowing out from the cracks in the medium droplets can promptly fill the conductive breaks caused by deformation, while the solid oxide layer structure of the small droplets effectively suppresses the excessive flow of liquid metal. The combined effect of these two factors ensures that the conductive network remains intact under large strain, thereby achieving stable maintenance of shielding performance.

[0035] The embodiments described above are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

Claims

1. A stretchable electromagnetic shielding film, characterized in that: The electromagnetic shielding film includes a base layer and a liquid metal conductive layer. The liquid metal conductive layer is formed by ultrasonically spraying a liquid metal dispersion treated with gradient ultrasound onto the base layer and then mechanically sintering it.

2. The stretchable electromagnetic shielding film according to claim 1, characterized in that, The liquid metal droplets in the liquid metal conductive layer include small droplets, medium droplets and large droplets. The small droplets have a particle size of 100~200nm, the medium droplets have a particle size of 200~300nm, and the large droplets have a particle size of 300~500nm. Preferably, the thickness of the liquid metal conductive layer is 1~8μm.

3. The stretchable electromagnetic shielding film according to claim 1, characterized in that, The electromagnetic shielding film retains an electromagnetic shielding effectiveness of ≥90% when the strain is 200%.

4. The stretchable electromagnetic shielding film according to claim 1, characterized in that, The base layer is a thermoplastic polyurethane base.

5. The method for preparing the stretchable electromagnetic shielding film according to any one of claims 1-4, characterized in that, The preparation method includes the following steps: S1. Prepare a liquid metal dispersion and subject the liquid metal dispersion to gradient ultrasonic treatment; S2. The liquid metal dispersion treated with gradient ultrasound is ultrasonically sprayed onto the substrate to form a liquid metal layer; S3. Mechanically sinter the liquid metal layer to form a liquid metal conductive layer, thereby obtaining the stretchable electromagnetic shielding film.

6. The preparation method according to claim 5, characterized in that: In step S1, the concentration of liquid metal in the liquid metal dispersion is 4~6 mg / mL; Preferably, the liquid metal is a GaIn binary eutectic alloy; Preferably, the solvent for the liquid metal dispersion is anhydrous ethanol or deionized water.

7. The preparation method according to claim 5, characterized in that, The gradient ultrasonic treatment includes performing a first-level ultrasonic treatment and a second-level ultrasonic treatment sequentially under water bath conditions. The power of the first-level ultrasonic treatment is 600~750W and the time is 15~30min. The power of the second-level ultrasonic treatment is 800~1200W and the time is 15~30min. Preferably, the gradient ultrasound processing sequence is 3 seconds on and 3 seconds off; Preferably, the water bath temperature is 35°C.

8. The preparation method according to claim 5, characterized in that: In step S2, the ultrasonic spraying nozzle has a carrier gas pressure of 0.04 MPa, a spraying height of 50 mm, a spraying spacing of 2 mm, and a liquid supply rate of 0.1~0.3 mL / min. Preferably, the number of ultrasonic spraying layers is 150 to 1200; Preferably, when the solvent of the liquid metal dispersion is anhydrous ethanol, the ultrasonic spraying temperature is room temperature; when the solvent of the liquid metal dispersion is deionized water, the ultrasonic spraying temperature is 80°C.

9. The preparation method according to claim 5, characterized in that: In step S3, the mechanical sintering is performed using roll pressing sintering; Preferably, the rolling speed of the roll pressing sintering is 5 mm / s; Preferably, the pressure of the roll pressing sintering is 93~122N.

10. The application of the stretchable electromagnetic shielding film according to any one of claims 1-4 or the electromagnetic shielding film prepared by the preparation method according to any one of claims 5-9 in the preparation of electromagnetic shielding components for electronic products.

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