Preparation and application method of asymmetric electromagnetic shielding nanofiber film

Abstract

The application relates to the technical field of electromagnetic shielding and functional film preparation, in particular to a preparation and application method of an asymmetric electromagnetic shielding nanofiber film, which comprises the following steps: (1) preparing SiO2 nanoparticles; (2) dissolving the SiO2 nanoparticles in an organic solvent to obtain a SiO2 nanoparticle dispersion liquid; adding a high-molecular polymer as a carbon source into the SiO2 nanoparticle dispersion liquid to obtain a spinning liquid; (3) performing electrostatic spinning operation on the spinning liquid to prepare a PAN / SiO2 nanofiber film; (4) carbonizing the polymer to form a carbon-based skeleton to obtain a carbon-based film; (5) making the SiO2 and the carbon-based film undergo a carbon thermal reaction to obtain an asymmetric electromagnetic shielding nanofiber film; the nanofiber base is constructed through the electrostatic spinning technology, the Joule heat technology and the Fick diffusion theory are combined, and the asymmetric structure design of electric conduction-dielectric is realized, so that the demand of modern electronic equipment for electromagnetic wave shielding materials, such as light weight, high efficiency and multifunction, is met.

Description

Technical Field

[0001] This invention relates to the technical field of electromagnetic shielding and functional film preparation, and in particular to a method for preparing and applying an asymmetric electromagnetic shielding nanofiber film. Background Technology

[0002] With the rapid development of wireless communication technology and electronic devices, electromagnetic pollution has become increasingly serious, placing higher demands on efficient and lightweight electromagnetic shielding materials. Currently, while traditional electromagnetic shielding materials (such as metal foils and carbon-based composites) possess high shielding effectiveness, they generally suffer from problems such as being heavy, lacking flexibility, and being costly. In recent years, lightweight, efficient, and stable electromagnetic shielding materials have gradually become a research hotspot. Improving adjustable multiple electromagnetic losses to achieve stable electromagnetic shielding performance is an urgent problem to be solved.

[0003] The fabrication of conductive materials with unique structures provides a key approach for optimizing the reflection and absorption of electromagnetic waves at the substrate interface. For example, porous structures, mesh structures, multilayer structures, honeycomb structures, and sandwich structures are all methods to improve multiple losses. Introducing polarized charge layers and conductive networks at the interfaces of different components can significantly enhance the reflection and absorption of electromagnetic waves. However, the strong reflection of incident electromagnetic waves by the uniform conductive system in traditional shielding networks leads to secondary pollution, becoming a bottleneck restricting their application. Asymmetric electromagnetic shielding films, due to their unique interface effects and multifunctional characteristics, can achieve excellent electromagnetic wave shielding performance and lightweight design. However, current fabrication technologies for asymmetric electromagnetic shielding films still suffer from high complexity and cost, making it difficult to meet practical application requirements. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for preparing asymmetric electromagnetic shielding nanofiber films. The method involves constructing a nanofiber substrate using electrospinning technology and combining Joule heating technology with Fick diffusion theory to achieve a conductive-dielectric asymmetric structure design. This meets the demands of modern electronic devices for lightweight, high-efficiency, and multifunctional electromagnetic shielding materials.

[0005] The present invention discloses a method for preparing an asymmetric electromagnetic shielding nanofiber thin film, comprising the following steps:

[0006] (1) Prepare SiO2 nanoparticles as silicon source; SiO2 nanoparticles are chosen because of their good thermal and chemical stability. In the subsequent high-temperature processing, SiO2 can react with carbon to generate silicon carbide (SiC). SiC is a material with high conductivity, high hardness and good corrosion resistance, which is very suitable for electromagnetic shielding.

[0007] (2) Dissolve SiO2 nanoparticles in an organic solvent and stir and sonicate to obtain a SiO2 nanoparticle dispersion; then add a polymer as a carbon source to the SiO2 nanoparticle dispersion and stir evenly to obtain a spinning solution; this process not only makes the nanoparticles uniformly distributed in the fiber, but also gives the material a large specific surface area and porosity, which is beneficial to improving the electromagnetic wave absorption efficiency.

[0008] (3) Transfer the spinning solution into the syringe, adjust the electrospinning parameters, and perform electrospinning on the spinning solution to prepare a PAN / SiO2 nanofiber film. Electrospinning can form a nanofiber network with high specific surface area and porous structure. This microstructure not only increases the interaction interface between the material and electromagnetic waves, but also promotes internal multiple reflections and scattering, further enhancing the electromagnetic wave absorption capacity. In addition, the nanoscale fiber structure also endows the material with excellent mechanical strength and flexibility.

[0009] (4) The PAN / SiO2 nanofiber film is pre-oxidized to carbonize the polymer and form a carbon-based skeleton to obtain a carbon-based film. The pre-oxidation treatment causes PAN (polyacrylonitrile) to undergo a chemical transformation and eventually carbonize to form a stable carbon-based structure. This transformation enhances the mechanical strength and durability of the material, enabling the nanofiber film to maintain its integrity when faced with external pressure or deformation.

[0010] (5) By utilizing the Joule heating effect, SiO2 and carbon-based thin films undergo a carbothermic reaction to generate silicon carbide. The Joule heating can rapidly initiate the reaction in a short time, causing silicon dioxide to rapidly react with the carbon-based film surface to generate SiC, significantly shortening the production cycle. Due to the existence of the Fick diffusion effect, the concentration gradient migration of SiO2 vapor is achieved under high temperature conditions. Through heat treatment, the content of SiC nanoparticles on the film surface is induced to change layer by layer, forming a multilayer interface structure with conductivity-dielectric gradient. Furthermore, the conductivity-dielectric gradient in the film optimizes the impedance matching of electromagnetic wave attenuation, enabling efficient electromagnetic interference shielding. At the same time, the asymmetric electromagnetic shielding nanofiber film after Joule heat treatment has stable structure and performance at higher temperatures. It also has good resistance to chemical corrosion in common acid and alkali environments, ensuring that the electromagnetic shielding performance of the film is not affected under different environments. It has high stability. After extreme low temperature storage and high temperature heating treatment, the stability rate of electromagnetic shielding performance remains above 90%, and the stability remains above 95% after acid and alkali treatment.

[0011] Traditional uniform conductive systems produce strong reflections of incident electromagnetic waves, leading to secondary pollution. However, asymmetric electromagnetic shielding nanofiber films have unique structural advantages. The fibers form a continuous conductive network, which helps reduce conduction losses, and the hierarchical porous interface structure is conducive to multiple reflections and absorption of electromagnetic waves.

[0012] Furthermore, in step (1), the SiO2 nanoparticles are generated from tetraethyl silicate via a sol-gel reaction.

[0013] SiO2 nanoparticles prepared by the sol-gel method typically exhibit good dispersibility and stability, and are not prone to agglomeration. This not only benefits the preparation of subsequent spinning solutions but also ensures the uniform distribution of nanoparticles throughout the system, thereby improving the overall performance of the material. The sol-gel method is usually carried out under relatively mild conditions, without the need for extreme high temperatures or high pressures, thus reducing the impact on the environment.

[0014] Furthermore, the volume ratio of tetraethyl silicate, ammonia, deionized water, and ethanol required for the sol-gel method is 3:4:15:55;

[0015] More specifically, the sol-gel method includes the following steps:

[0016] In a fume hood, 3 mL of tetraethyl orthosilicate was added to a mixed solution of 55 mL of anhydrous ethanol and 15 mL of deionized water and stirred rapidly. Adding tetraethyl orthosilicate to the mixed solution of anhydrous ethanol and deionized water under rapid stirring can promote its uniform dispersion and avoid agglomeration caused by excessive local concentration. This helps to generate nanoparticles with uniform size distribution and regular shape.

[0017] Then add 4 mL of ammonia water and continue stirring for 5 hours. Adding ammonia water as a catalyst can effectively adjust the pH value of the solution, accelerate the hydrolysis and condensation reaction of tetraethyl silicate, and at the same time maintain an appropriate reaction rate to prevent the formation of an uneven structure due to excessive speed or slowness. Stirring for 5 hours ensures that the reaction proceeds fully and improves the yield.

[0018] Centrifuge at 7000 r / min for 5 min for washing and separation. Repeat washing and centrifugation 5-6 times until the pH of the supernatant is close to neutral (pH=7). Use centrifugation to wash and separate to thoroughly remove unreacted raw materials and other impurities. This process not only improves the purity of nanoparticles, but also reduces the risk of contamination in subsequent processing steps.

[0019] Finally, the white precipitate was freeze-dried for 12 hours to obtain SiO2 nanoparticles. Freeze-drying is a mild and effective drying method that can remove moisture without damaging the nanoparticle structure, resulting in stable and well-dispersed dry powder SiO2 nanoparticles.

[0020] SiO2 nanoparticles were prepared by sol-gel method using tetraethyl silicate (TEOS), ammonia, deionized water and ethanol in a volume ratio of 3:4:15:55. The overall advantage of this specific ratio is that it optimizes the entire reaction system and ensures efficient, stable and high-quality nanoparticle generation. This ratio not only promotes uniform dispersion and stable nanostructure formation, but also improves yield and purity.

[0021] The reaction conditions at this ratio ensure sufficient hydrolysis and condensation while avoiding excessively rapid or runaway reactions, thereby preventing agglomeration and ensuring uniform nanoparticle size distribution. The large amount of ethanol dilutes the system and reduces viscosity, which helps to mix the components evenly and inhibits rapid polymerization. An appropriate amount of ammonia acts as a catalyst to adjust the pH value, accelerating the reaction without causing drastic changes, making the reaction rate moderate and controllable.

[0022] Furthermore, in step (2), the organic solvent is N,N-dimethylformamide (DMF), and the amount of SiO2 nanoparticles added is 5% of the weight of the organic solvent;

[0023] N,N-Dimethylformamide is a polar aprotic solvent with excellent solubility, effectively dissolving various polymers and inorganic nanoparticles. It helps ensure the uniform dispersion of SiO2 nanoparticles in solution, preventing agglomeration and thus improving the performance consistency of the final material. Simultaneously, N,N-Dimethylformamide also exhibits high chemical and thermal stability, remaining stable over a wide temperature range. This is beneficial for subsequent spinning and other high-temperature processing, as its stability reduces side reactions and ensures process controllability and repeatability.

[0024] An addition of 5% can provide sufficient filling density to enhance the mechanical strength and electromagnetic shielding performance of the material while ensuring that the nanoparticles are fully dispersed. Too many nanoparticles may cause agglomeration and affect the uniformity of the material, while too few will not significantly improve the performance. The viscosity of the spinning solution at this ratio is moderate, which ensures the fluidity of the solution, making it easy to electrospinning, and is not too thin to form a stable fiber structure.

[0025] Furthermore, the polymer in step (2) is PAN (polyacrylonitrile);

[0026] PAN is a polymer material with excellent thermal stability, and it is not easily decomposed or deteriorated under high temperature conditions; this makes it very suitable for subsequent high-temperature pre-oxidation and carbonization treatment, ensuring the stability of the material structure and the consistency of its performance.

[0027] Meanwhile, PAN has excellent spinnability and can form long and uniform nanofibers through electrospinning technology. This fiber-forming property is crucial for constructing electromagnetic shielding materials with high specific surface area and porous structure, which helps to improve the absorption and scattering effect of electromagnetic waves.

[0028] After high-temperature pre-oxidation and carbonization, PAN can be transformed into a stable carbon-based material. This transformation not only retains the original nanofiber morphology, but also endows the material with excellent conductivity and mechanical strength, providing an ideal carbon source for the final generation of C / SiC composite film.

[0029] PAN and SiO2 nanoparticles have good compatibility and can coexist in solution without agglomeration. This compatibility ensures the uniform dispersion of the two in the spinning solution and promotes the uniform formation of silicon carbide (SiC) in subsequent reactions.

[0030] The method for uniformly stirring the spinning solution in step (2) is as follows: after adding PAN to the SiO2 nanoparticle dispersion, stir and dissolve it at 60°C for 6-10 hours;

[0031] More specifically, the above stirring is carried out in an oil bath at 60°C for 6-10 hours to ensure that PAN is fully dissolved and uniformly mixed with SiO2 nanoparticles; the gentle heating conditions help to accelerate the dissolution process while avoiding damage to the material from excessively high temperatures.

[0032] In step (2), the spinning solution contains PAN with a mass fraction of 10%-13% and SiO2 nanoparticles with a mass fraction of 5%.

[0033] PAN mass fraction of 10%-13%: PAN content within this range ensures that the spinning solution has appropriate viscosity and fluidity, while avoiding the difficulty of spinning due to excessive concentration or the impact on fiber formation quality due to excessively low concentration.

[0034] The SiO2 nanoparticle mass fraction is 5%. The appropriate amount of SiO2 nanoparticles not only improves the mechanical strength and electromagnetic shielding performance of the material, but also optimizes the functional properties of the material without significantly increasing the cost.

[0035] Furthermore, the electrospinning parameters in (3) are:

[0036] Voltage 15kV-17kV, receiving distance 10-15cm, injection speed 10μL / min-14μL / min, spinning time 12h-24h, collecting device roller speed 300-500rpm, after spinning, place the PAN / SiO2 nanofiber film in a 60℃ oven to dry for 6-12h.

[0037] The voltage range of 15kV-17kV provides sufficient electric field strength to overcome surface tension, enabling the solution to stably form Taylor cones and eject slender fibers. At the same time, it avoids unstable ejection or excessive bead formation that may be caused by excessive voltage, ensuring the quality and uniformity of the fibers.

[0038] The receiving distance is 10-15cm. This distance range ensures that the fibers have enough flight time to stretch and solidify, while preventing excessive fiber dispersion due to excessive distance, which would affect the collection efficiency. A moderate receiving distance helps to obtain nanofiber films with consistent morphology and orderly arrangement.

[0039] Injection speed directly affects the supply of spinning solution and fiber diameter. An injection speed of 10μL / min-14μL / min can ensure continuous feeding while avoiding coarse fibers or breakage caused by excessive flow, thus ensuring the fineness and uniformity of the fibers.

[0040] The spinning time is 12h-24h. A longer spinning process can accumulate sufficient fiber layer thickness, thereby improving the mechanical strength and electromagnetic shielding performance of the final film. At the same time, long spinning time helps to achieve a more uniform fiber distribution and reduce defects and voids.

[0041] The rotation of the roller helps the fibers spread evenly, avoiding local accumulation or uneven distribution. Setting the rotation speed of the collecting device roller to 300-500 rpm can ensure effective fiber deposition without causing fiber breakage or deformation due to excessive rotation speed.

[0042] The drying process removes residual solvents, further enhances the cross-linking effect between fibers, and improves the overall stability and mechanical properties of the material. Drying at 60℃ for 6-12 hours ensures that the fiber structure is not damaged, while promoting the uniform curing of the material.

[0043] By selecting and optimizing electrospinning parameters, combined with subsequent drying treatment, the overall quality and performance of PAN / SiO2 nanofiber films were improved, ensuring an efficient, stable, and economical production process, and laying a solid foundation for the development of high-performance asymmetric electromagnetic shielding materials.

[0044] Furthermore, the pre-oxidation in step (4) is carried out in air at 200-250°C for 120-150 min;

[0045] Pre-oxidation treatment can gradually transform the cyano groups in the PAN molecular chain into cyclic structures (such as ladder polymers) with better heat resistance, thereby improving the thermal and chemical stability of the material.

[0046] The pre-oxidation stage provides the necessary structural transformation basis for subsequent high-temperature carbonization; by initially removing some volatile components and forming stable intermediates, the carbonization process becomes more efficient and stable, reducing possible decomposition or weight loss during the carbonization process.

[0047] The temperature range of 200-250℃ is high enough to initiate the conversion reaction of the PAN molecular chain, but not so high as to cause rapid decomposition or excessive cross-linking of the material. It ensures the effective implementation of the pre-oxidation process while maximizing the preservation of the material's processability and functionality.

[0048] The pre-oxidation time is 120-150 minutes. During this period, the PAN molecular chains have sufficient time to complete the transformation from a linear to a ladder-like structure, forming a stable intermediate. A longer time also contributes to uniform heating and a complete reaction, reducing the risk of localized overheating or incomplete conversion.

[0049] Pre-oxidation in air promotes the oxidative cross-linking of PAN molecular chains, further enhancing the material's thermal stability and mechanical properties. In addition, air, as a common oxidation medium, is simple to operate and low in cost, making it suitable for large-scale production.

[0050] Furthermore, the following treatment is performed before pre-oxidation in step (4):

[0051] The PAN / SiO2 nanofiber film was cut into appropriate sizes and placed on a ceramic plate in the central area of ​​the box furnace.

[0052] Cutting samples to a uniform size ensures more even heating of the samples in the box furnace. Samples of different sizes may cause uneven heating, affecting the pre-oxidation effect. Standardizing the size helps reduce this difference and ensures that all samples achieve the ideal pre-oxidation state.

[0053] Ceramic sheets have good high temperature resistance and chemical stability, and can maintain structural integrity in high temperature environments without reacting with samples or releasing harmful substances. They provide a stable support platform for nanofiber films, preventing samples from deforming or sticking to the furnace wall during heating.

[0054] The central area of ​​a box furnace is usually the most temperature-uniform area, which can avoid temperature inconsistencies caused by edge effects. This ensures that the entire sample can be heated evenly, reduces the risk of local overheating or underheating, and improves the effectiveness and consistency of pre-oxidation treatment.

[0055] Furthermore, the method for inducing a carbothermic reaction between SiO2 and the carbon-based thin film using the Joule heating effect in step (5) includes:

[0056] The process is carried out under argon protection. The Joule heating equipment adopts a programmable control mode, which controls the current and voltage to reach the temperature of 1700-1800℃ within 10 seconds and hold it for 10 seconds to form an asymmetric electromagnetic shielding nanofiber film with an interfacial asymmetric structure.

[0057] Conducting the reaction under argon protection effectively prevents oxygen and other impurities from entering the reaction system, avoiding unnecessary oxidation or other side reactions. This helps maintain the purity and performance stability of the material. The temperature is rapidly raised to 1700-1800℃ within 10 seconds. The high temperature of 1700-1800℃ provides sufficient energy for the carbothermic reaction between SiO2 and carbon-based materials, accelerating the formation of SiC. Similarly, the temperature is held up for a short time. This rapid heat treatment method can effectively activate the carbothermic reaction, while avoiding excessive impact on the material structure due to prolonged high temperature and reducing the formation of by-products.

[0058] The present invention relates to a method for applying the asymmetric electromagnetic shielding nanofiber film. The asymmetric electromagnetic shielding nanofiber film is used as an electromagnetic shielding material. The prepared asymmetric electromagnetic shielding nanofiber film optimizes the electromagnetic wave attenuation path through a conductive-dielectric degree layer, thereby improving the electromagnetic shielding performance. It also exhibits excellent resistance to extreme temperatures and acids and alkalis.

[0059] The electromagnetic shielding performance of the asymmetric electromagnetic shielding nanofiber film prepared by this invention was tested in solution environments with different temperatures and pH values.

[0060] The temperatures are divided into: low temperature (liquid nitrogen and refrigerator treatment), normal temperature (room temperature 25℃), and high temperature (100℃, 300℃ and 500℃);

[0061] pH levels are 3, 5, 9, and 11.

[0062] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0063] (1) By utilizing the Fick diffusion effect, the concentration gradient migration of SiO2 vapor is achieved under high temperature conditions. The content of SiC nanoparticles on the film surface is induced to change layer by layer through heat treatment, forming a multilayer interface structure with conductivity-dielectric gradient. Furthermore, the conductivity-dielectric gradient in the film optimizes the impedance matching of electromagnetic wave attenuation, which can achieve efficient electromagnetic interference shielding.

[0064] (2) The prepared asymmetric membrane has unique structural advantages. The continuous conductive network formed by the fibers helps to reduce conduction loss, and the hierarchical porous interface structure is conducive to multiple reflections and absorption of electromagnetic waves.

[0065] (3) The asymmetric electromagnetic shielding nanofiber film after Joule heat treatment has stable structure and performance at higher temperatures. At the same time, it has good resistance to chemical corrosion in common acid and alkali environments, ensuring that the electromagnetic shielding performance of the film is not affected in different environments. It has high stability. After extreme low temperature storage and high temperature heating treatment, the electromagnetic shielding performance stability rate remains above 90%, and the stability remains above 95% after acid and alkali treatment.

[0066] (4) This invention utilizes Joule heating to rapidly initiate a reaction in a short time, causing silicon dioxide and the surface of the carbon-based film to undergo a rapid carbothermic reaction to generate SiC, which significantly shortens the production cycle, improves production efficiency, and saves a lot of time and costs, which is in stark contrast to the traditional long-term high-temperature treatment method.

[0067] (5) By electrospinning, parameters such as the mixing ratio and injection speed of the solution can be precisely controlled, thereby precisely adjusting the ratio and distribution of components such as PAN and SiO2 in the nanofiber membrane, which in turn affects the electromagnetic shielding performance of the material. For example, by adjusting the spinning time to change the film thickness, the electromagnetic shielding performance can be precisely controlled. Attached Figure Description

[0068] Figure 1 SEM image of the PAN / SiO2 nanofiber film prepared by electrospinning in Example 3;

[0069] Figure 2 Here is a SEM image of the asymmetric C / SiC nanofiber film in Example 3, where a is the bottom, b is the middle, c is the top, and d is an XRD image.

[0070] Figure 3 SEM images of C / SiC nanofiber films with uniform structure prepared by a comparative tube furnace are shown, where a is the bottom, b is the middle, c is the top, and d is the XRD image.

[0071] Figure 4 Figures a and b show the electromagnetic shielding performance comparison between the uniform C / SiC nanofiber film prepared using a tube furnace and the asymmetric C / SiC nanofiber film prepared in Example 3.

[0072] Figure 5 This is a comparison chart of the electromagnetic shielding performance of Examples 1, 2, and 3;

[0073] Figure 6 The graph shows the performance of the asymmetric C / SiC thin film in Example 3 after treatment with different temperatures and acids / alkalis. Detailed Implementation

[0074] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0075] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0076] Secondly, the term "embodiment" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the present invention. The phrase "in one embodiment" appearing in different places throughout this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that is mutually exclusive with other embodiments.

[0077] Example 1:

[0078] A method for preparing an asymmetric electromagnetic shielding nanofiber thin film includes the following steps:

[0079] (1) Add 3 mL of tetraethyl orthosilicate to a mixed solution of 55 mL of anhydrous ethanol and 15 mL of deionized water, stir rapidly, then add 4 mL of ammonia water, continue stirring for 5 h, and wash and centrifuge at 7000 r / min for 5 min. Wash and centrifuge 6 times until the supernatant is neutral (pH=7). Freeze-dry the white precipitate for 12 h to obtain SiO2 nanoparticles.

[0080] (2) Dissolve 0.5g SiO2 nanoparticles in 10mL DMF solution and stir under ultrasonication for 1 hour. Then add 1g PAN and stir in an oil bath at 60℃ for 6 hours to obtain a uniform spinning solution. The spinning was carried out on an electrospinning machine with a high voltage of 17kV and a flow rate of 14μL / min. The distance between the needle and the collector was 15cm. All experiments were carried out at room temperature with a humidity of about 50%. The rotation speed of the collecting device roller was 500rpm and the spinning time was 6h. After spinning, the PAN / SiO2 nanofiber film was placed in a 60℃ oven for drying for 12h.

[0081] (3) Cut the PAN / SiO2 nanofiber film into 10cm×10cm squares, place them on ceramic plates in the central area of ​​the furnace, heat the precursor in air to 250℃ at a heating rate of 5℃ / min and keep it at that temperature for 2h, and then cut it into 3cm×1.5cm rectangles after pre-oxidation.

[0082] (4) The Joule heating device is operated by programming. The settings are divided into two stages. In the first stage, the current is set to 380A and the heating time is 10s. In the second stage, the current is set to 360A and the holding time is 10s. Finally, it is naturally cooled to room temperature, thus successfully preparing the asymmetric electromagnetic shielding nanofiber film of the present invention.

[0083] Example 2:

[0084] (1) The preparation of SiO2 nanoparticles and spinning solution is the same as in Example 1;

[0085] (2) The spinning time was increased to 12 hours, and other electrospinning parameters were the same as in Example 1;

[0086] (3) Pre-oxidation and Joule heat treatment are the same as in Example 1.

[0087] Example 3:

[0088] (1) The preparation of SiO2 nanoparticles and spinning solution were the same as in Example 1;

[0089] (2) The spinning time was increased to 24 hours, and other electrospinning parameters were the same as in Example 1;

[0090] (3) Pre-oxidation and Joule heat treatment are the same as in Example 1.

[0091] Comparative example:

[0092] This comparative example is a uniform electromagnetic shielding nanofiber film prepared using a traditional tube furnace.

[0093] (1) The preparation of SiO2 nanoparticles and spinning solution is the same as in Example 1;

[0094] (2) Place the 3cm×1.5cm pre-oxidized film in a tube furnace, heat it to 1500℃ in argon at a rate of 4℃ / min and hold for 2 hours, and then cool it naturally to room temperature to obtain a C / SiC nanofiber film with a uniform structure.

[0095] The asymmetric electromagnetic shielding nanofiber films prepared in Examples 1-3 and the uniform electromagnetic shielding nanofiber films prepared in the comparative examples were placed in solution environments with different temperatures and pH values ​​to test their electromagnetic shielding performance.

[0096] The temperatures are divided into: low temperature (liquid nitrogen and refrigerator treatment), normal temperature (room temperature 25℃), and high temperature (100℃, 300℃ and 500℃);

[0097] pH levels are 3, 5, 9, and 11.

[0098] Depend on Figure 1 It was found that PAN / SiO2 nanofiber films can be well prepared by electrospinning. PAN nanofibers exhibit an interwoven three-dimensional porous structure with a relatively smooth fiber surface. A large number of SiO2 nanoparticles are attached to the fiber surface. These particles are evenly distributed and spherical in shape. The SiO2 nanoparticles are embedded in the fiber surface, showing a strong bond between the particles and the substrate.

[0099] Depend on Figure 2 It was learned that Figure 2 The SEM images of the C / SiC nanofiber film prepared by Joule heating are shown in the image. The asymmetric electromagnetic shielding nanofiber film is divided into three parts: top, middle and top; where a is the bottom, b is the middle and c is the top. A large number of pores are generated on the surface of the carbon nanofibers. Different contents of SiC nanoparticles are loaded onto the surface of different layers, indicating that Joule heating has a significant impact on the gradient distribution of SiC content. Under high temperature, SiO2 will vaporize. According to Fick diffusion theory, its vapor can migrate from the bottom of the film to the top of the film under the action of high temperature and pressure, and the probability of SiC formation gradually increases, resulting in an asymmetric structure. Figure d shows X-ray diffraction (XRD), which is used to reveal the detailed crystal structure of the synthesized C / SiC film. All characteristic peaks can be indexed as PDF#29-1129 and PDF#41-1487, which means that a β-SiC and graphite carbon mixed phase with good crystal properties has been formed. The diffraction peaks at 35.5°, 41.0°, 59.9° and 71.7° can be well labeled to the crystal planes (111), (200), (220) and (311), respectively. The peak at 2θ = 33.6° corresponds to the stacking interleaving (SF) in the (111) plane of SiC. The increase in layer dislocation density is conducive to the promotion of dipole polarization.

[0100] Depend on Figure 3 It was learned that Figure 3 SEM images from Zhongac show C / SiC nanofiber films prepared using the conventional tube furnace method. Some pores appear on the surface of the carbon nanofibers, while some spheres remain intact. Each layer exhibits a similar morphology. Figure 3 As can be seen from d, the diffraction peaks further confirm the uniform distribution of SiC within the heating film of the tube furnace.

[0101] Depend on Figure 4 It was learned that Figure 4 Figure a shows a comparison of the shielding performance of uniformly structured C / SiC nanofiber films prepared using a tube furnace and asymmetric electromagnetic shielding nanofiber films prepared using Joule heating. The electromagnetic shielding performance in the X-band (8-12 GHz) remains at 17 dB. In contrast, the electromagnetic shielding performance with a conductive gradient structure reaches 47 dB using the rapid Joule heating method. Figure 4As shown in Figure b, the electromagnetic shielding parameters are total shielding effect (SET), shielding reflection loss (SER), and shielding absorption loss (SEA). Compared with the tube furnace method (SER = 2.8 dB), the SER of Joule heating is higher (13.7 dB). In addition, the SEA (33.7 dB) of the asymmetric C / SiC nanofiber film obtained by Joule heat treatment is attributed to the absorption-reflection-absorption interface in the gradient conductive structural framework. This high absorption is due to the impedance matching at the interface between the top layer and the transition layer. Therefore, a reasonable gradient distribution of the top (impedance matching) and bottom (impedance mismatch) layers is beneficial for achieving efficient absorption and reflection.

[0102] Depend on Figure 5 It has been found that the electromagnetic shielding performance of asymmetric electromagnetic shielding nanofiber films increases with the increase of film thickness. As the film thickness increases, the propagation path of electromagnetic waves inside the material becomes longer, and the material has more opportunities to absorb and reflect electromagnetic waves. At the same time, the increase in thickness means that more absorbing mediums interact with electromagnetic waves, thereby improving absorption efficiency. This also shows that adjusting the spinning time parameters in the electrospinning process can precisely control the electromagnetic shielding performance.

[0103] Depend on Figure 6 It was found that the asymmetric electromagnetic shielding nanofiber film possesses excellent high-temperature resistance and acid and alkali resistance. After extreme low-temperature storage and high-temperature heating treatment, the stability of its electromagnetic shielding performance remains above 90%. After low-temperature treatment, the electron transport rate and conductivity decrease, resulting in a reduction in the amount of reflected and absorbed electromagnetic waves. After high-temperature treatment, the electron transport rate and conductivity within the material increase, enhancing the electromagnetic wave shielding performance. Furthermore, the material maintains stable performance after acid and alkali treatment, with a stability of over 95%. This is attributed to the stability of SiC, which does not react with acids and alkalis, providing important protection for the stability of its performance.

[0104] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing an asymmetric electromagnetic shielding nanofiber thin film, characterized in that, Includes the following steps: (1) Preparation of SiO2 nanoparticles as silicon source; (2) Dissolve SiO2 nanoparticles in an organic solvent and stir and sonicate to obtain a SiO2 nanoparticle dispersion; then add a polymer as a carbon source to the SiO2 nanoparticle dispersion and stir evenly to obtain a spinning solution. (3) Transfer the spinning solution into the syringe, adjust the electrospinning parameters, and then perform electrospinning on the spinning solution to prepare PAN / SiO2 nanofiber film. (4) The PAN / SiO2 nanofiber film is pre-oxidized to carbonize the polymer and form a carbon-based skeleton to obtain a carbon-based film; (5) By utilizing the Joule heating effect, SiO2 and carbon-based thin films undergo a carbothermic reaction to generate silicon carbide. Under the conditions of Joule heat treatment, SiO2 vapor undergoes a concentration gradient migration based on the Fick diffusion effect. The content of SiC nanoparticles on the film surface is induced to change layer by layer through Joule heat treatment, forming a multilayer interface structure with conductivity-dielectric gradient, thereby constructing an interface asymmetric structure and obtaining an asymmetric electromagnetic shielding nanofiber film. The following treatment is performed before pre-oxidation in step (4): The PAN / SiO2 nanofiber film was cut into appropriate sizes and placed on a ceramic plate in the central area of ​​the box furnace. The method for inducing a carbothermic reaction between SiO2 and carbon-based thin films using the Joule heating effect in step (5) includes: The process is carried out under argon protection. The Joule heating equipment adopts a programmable control mode, which controls the current and voltage to reach the temperature of 1700-1800℃ within 10 seconds and hold it at that temperature for 10 seconds, forming an asymmetric electromagnetic shielding nanofiber film with an interfacial asymmetric structure.

2. The method for preparing the asymmetric electromagnetic shielding nanofiber thin film as described in claim 1, characterized in that, In step (1), the SiO2 nanoparticles are generated by the reaction of tetraethyl silicate via sol-gel method.

3. The method for preparing the asymmetric electromagnetic shielding nanofiber thin film as described in claim 2, characterized in that, The volume ratio of tetraethyl silicate, ammonia, deionized water, and ethanol required for the sol-gel method is 3:4:15:

55.

4. The method for preparing the asymmetric electromagnetic shielding nanofiber thin film as described in claim 1, characterized in that, In step (2), the organic solvent is N,N-dimethylformamide, and the amount of SiO2 nanoparticles added is 5% of the weight of the organic solvent.

5. The method for preparing the asymmetric electromagnetic shielding nanofiber thin film as described in claim 4, characterized in that, The polymer used in step (2) is PAN; The method for uniformly stirring the spinning solution in step (2) is as follows: after adding PAN to the SiO2 nanoparticle dispersion, stir and dissolve it at 60°C for 6-10 hours. In step (2), the spinning solution has a PAN mass fraction of 10%-13% and a SiO2 nanoparticle mass fraction of 5%.

6. The method for preparing the asymmetric electromagnetic shielding nanofiber thin film as described in claim 1, characterized in that, The electrospinning parameters in (3) are as follows: The voltage is 15kV-17kV, the receiving distance is 10-15cm, the injection speed is 10μL / min-14μL / min, the spinning time is 12h-24h, the rotation speed of the collecting device roller is 300-500rpm, and after spinning, the PAN / SiO2 nanofiber film is placed in a 60℃ oven to dry for 6-12h.

7. The method for preparing the asymmetric electromagnetic shielding nanofiber thin film as described in claim 1, characterized in that, In step (4), the pre-oxidation is carried out in air at 200-250°C for 120-150 minutes.

8. The method for applying the asymmetric electromagnetic shielding nanofiber thin film prepared by the preparation method of the asymmetric electromagnetic shielding nanofiber thin film according to any one of claims 1-7, characterized in that, The asymmetric electromagnetic shielding nanofiber film serves as the electromagnetic shielding material.

Images