Fe3o4-ccnts / pi composite film, and preparation method and application thereof

Abstract

This invention discloses a Fe3O4-CCNTs / PI composite film, its preparation method, and its applications. The composite film has a bilayer gradient filler structure, comprising a first layer and a second layer stacked together. Each layer consists of a polyimide matrix and dispersed Fe3O4 nanoparticles and carboxylated carbon nanotubes, with the total mass fraction of filler in the second layer being higher than that in the first layer. During preparation, two polyamic acid precursor solutions with different filler contents are prepared, and then coated and freeze-set sequentially to form a bilayer wet film. A thermal imidization treatment is then performed to obtain a self-supporting composite film with absorption as the primary shielding mechanism. This invention effectively improves impedance matching, enhances electromagnetic wave absorption efficiency, significantly reduces secondary pollution, and simultaneously achieves multifunctional integration such as Joule heating, high temperature resistance, and high flame retardancy. It achieves a shielding effectiveness of 36.8 dB in the X-band with an absorption ratio of approximately 70%, making it suitable for flexible electronics and communication equipment.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic shielding materials technology, specifically to a Fe3O4-CCNTs / PI composite film, its preparation method, and its application. Background Technology

[0002] With the rapid development of 5G communication, the Internet of Things, and flexible electronics, electronic devices are evolving towards higher integration, higher frequency, and miniaturization, leading to increasingly serious electromagnetic interference problems. To ensure the normal operation and communication quality of electronic devices, there is an urgent need for electromagnetic shielding materials that are lightweight, flexible, efficient, heat-resistant, and environmentally stable. Polyimide (PI) has become an ideal substrate for electromagnetic shielding due to its excellent thermal stability, mechanical properties, flame retardancy, and environmental resistance. Carboxylated carbon nanotubes (CCNTs), with their high conductivity, high aspect ratio, and good dispersibility, can efficiently construct conductive networks, significantly improving the electromagnetic shielding performance of materials. However, single conductive filler systems often suffer from severe impedance mismatch due to excessively high conductivity, resulting in a large amount of electromagnetic waves being reflected from the material surface. This not only leads to low absorption and attenuation efficiency but also causes serious secondary electromagnetic pollution. Furthermore, high filler content can cause carbon nanotubes to aggregate, resulting in decreased film mechanical properties and unstable shielding performance.

[0003] To improve impedance matching, some existing technologies attempt to simultaneously introduce magnetic nanoparticles, such as Fe3O4 nanoparticles, into the polyimide matrix to supplement the electromagnetic wave attenuation path through magnetic loss mechanisms. However, most existing solutions employ a single-layer homogeneous blend structure, with magnetic particles and conductive fillers randomly distributed within the same layer. This results in problems such as uneven filler dispersion, discontinuities between the conductive and magnetic loss networks, and weak interfacial bonding, limiting the improvement in absorption ratio and making it difficult to achieve truly efficient absorption-type electromagnetic shielding. Other existing technologies use multi-layer alternating structures to increase loss by enhancing interlayer interface reflection. However, such multi-layer designs often rely on vacuum filtration or layer-by-layer self-assembly processes, which suffer from complex processes, poor interlayer bonding, easy delamination, and difficulty in large-scale production. Furthermore, the aforementioned existing technologies generally only possess a single electromagnetic shielding function, lacking integrated properties such as Joule heating and efficient flame retardancy. Their performance degrades significantly under extreme environments such as acids and alkalis, high and low temperatures, and repeated bending, failing to meet the diverse application requirements of high-end flexible electronics and communication equipment. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a Fe3O4-CCNTs / PI composite film, its preparation method and application.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A CCNTs / PIFe3O4-CCNTs / PI composite film includes a first film layer and a second film layer stacked together. The first film layer comprises a first polyimide matrix and Fe3O4 nanoparticles and carboxylated carbon nanotubes dispersed in the first polyimide matrix. The second film layer comprises a second polyimide matrix and Fe3O4 nanoparticles and carboxylated carbon nanotubes dispersed in the second polyimide matrix. The total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the first film layer is lower than the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the second film layer.

[0006] In the above technical solution of the present invention, by designing the composite film as a double-layer gradient filler structure, the total mass fraction of filler in the second film layer is higher than that in the first film layer, forming a filler content gradient from the surface to the interior. When electromagnetic waves are incident on the film surface, the first film layer, due to its relatively low filler content, has a surface impedance closer to its free space impedance, effectively reducing direct reflection of electromagnetic waves on the surface and allowing more electromagnetic waves to enter the interior of the film. After entering the film, the electromagnetic waves encounter the second film layer with a higher filler content. The highly efficient magnetoelectric synergistic loss network composed of Fe3O4 nanoparticles and carboxylated carbon nanotubes in this layer can strongly absorb and attenuate the electromagnetic waves. At the same time, the interface between the two films can also provide additional interface reflection and interface polarization loss, further extending the transmission path of electromagnetic waves inside the material and enhancing the absorption effect; Fe3 O4 nanoparticles provide magnetic and eddy current losses, while carboxylated carbon nanotubes provide high conductivity losses. The synergistic effect of the two significantly increases the proportion of absorption loss in the total shielding effectiveness of the film, avoiding impedance mismatch and secondary pollution problems caused by a single conductive filler. This structural design enables the composite film of this invention to use absorption as the main shielding mechanism, with excellent electromagnetic shielding effectiveness and low reflection loss. It fundamentally solves the technical problems of serious impedance mismatch, high reflection ratio, and serious secondary electromagnetic pollution in the prior art. At the same time, the material has multifunctional integrated properties such as Joule heating, high temperature resistance, and flame retardancy, and its environmental stability is significantly improved.

[0007] In a preferred embodiment of the present invention, the mass ratio of Fe3O4 nanoparticles to carboxylated carbon nanotubes in both the first and second films is 1:3; this ratio was determined through experimental optimization, and under this ratio, magnetic loss and dielectric loss achieve a better synergistic effect.

[0008] In a preferred embodiment of the present invention, the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the first film layer is 20% to 30%, and the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the second film layer is 25% to 35%. More preferably, the total mass fraction of the first film layer is 25%, and the total mass fraction of the second film layer is 30%.

[0009] In a preferred embodiment of the present invention, the Fe3O4-CCNTs / PI composite film is a self-supporting film.

[0010] In a preferred embodiment of the present invention, the thickness of the Fe3O4-CCNTs / PI composite film is 50-100 μm, the electromagnetic shielding effectiveness in the X-band is ≥35dB, the absorption loss accounts for ≥65% of the total shielding effectiveness, and the limiting oxygen index is ≥38%.

[0011] The present invention also provides a method for preparing the above-mentioned Fe3O4-CCNTs / PI composite film, comprising the following steps: S1. Prepare a first precursor solution and a second precursor solution. The first precursor solution contains polyamic acid, Fe3O4 nanoparticles, and carboxylated carbon nanotubes, wherein the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes is C1. The second precursor solution contains polyamic acid, Fe3O4 nanoparticles, and carboxylated carbon nanotubes, wherein the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes is C2, and C2 > C1. S2. The first precursor solution is coated onto the substrate to form a first precursor wet film; S3. The first precursor wet film is subjected to freezing treatment to fix its shape, thereby obtaining the first film layer after freezing and fixing; S4. Coat the second precursor solution onto the first film layer after freeze-setting to form a second precursor wet film, and obtain a double-layer precursor film; S5. The bilayer precursor film is subjected to thermal imidization treatment to convert polyamic acid into polyimide, thereby obtaining the Fe3O4-CCNTs / PI composite film.

[0012] In the above preparation method, by combining layer-by-layer coating with intermediate freeze-setting, the redissolution and diffusion of the first layer filler caused by solvent penetration during the coating of the second layer are effectively avoided, ensuring clear interlayer boundaries and stable filler gradient distribution in the double-layer gradient structure. At the same time, during the thermal imidization process, the polyamic acid molecular chains of the two precursor films diffuse and entangle with each other at the interface, and after cyclization and dehydration, they form an integrated polyimide structure with interpenetrating molecular chains, giving the double films excellent interlayer bonding force and overcoming the technical defects of poor interlayer bonding and easy delamination in existing multilayer composite processes. The entire preparation method adopts a blade coating process using conventional equipment, which is simple to operate, process controllable, and easy to scale up for production.

[0013] In a preferred embodiment of the present invention, the steps of preparing the first precursor solution and the second precursor solution in step S1 each include: dispersing Fe3O4 nanoparticles and carboxylated carbon nanotubes in an organic solvent, adding an aromatic diamine monomer and stirring to dissolve, and then adding an aromatic dianhydride monomer under ice-water bath cooling to carry out a polymerization reaction to obtain a precursor solution containing polyamic acid; the in-situ polymerization method allows the polyamic acid to be generated in situ on the surface of the filler, which is beneficial to the uniform dispersion of the filler and the strengthening of the interfacial bonding.

[0014] In a preferred embodiment of the present invention, the aromatic diamine is 4,4'-diaminodiphenyl ether, the aromatic dianhydride is pyromellitic dianhydride, and the organic solvent is dimethylacetamide; the molar ratio of the aromatic dianhydride to the aromatic diamine is 1.02:1, and the slightly excess dianhydride can compensate for the consumption of anhydride groups caused by trace amounts of water and other factors during the reaction, ensuring that the polyamic acid has an appropriate molecular weight; the solid content of the precursor solution is 8% to 12%.

[0015] In a preferred embodiment of the present invention, the coating in steps S2 and S4 is by blade coating; the thickness of the blade coating in step S2 is 300-500 μm, and the thickness of the blade coating in step S4 is 600-900 μm; the freezing treatment temperature in step S3 is -20℃ to 0℃, and the time is 1-5 hours; the thermal imidization treatment in step S5 includes staged heating, and the final imidization temperature is 250-400℃; after step S5, the method further includes: immersing the thermally imidized film with the substrate in water to peel off the film and obtain the self-supporting Fe3O4-CCNTs / PI composite film. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the preparation process of Fe3O4-CCNTs / PI composite films. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0018] Furthermore, in this invention, an element referred to as fixed to or disposed on another element may be directly disposed on the other element, or there may be an intermediate element. When an element is considered to be connected to another element, it may be directly connected to the other element, or there may be an intermediate element present simultaneously. The terms vertical, horizontal, left, right, and similar expressions used herein are for illustrative purposes only and do not represent the only possible implementation.

[0019] Example 1 This embodiment provides a method for preparing Fe3O4-CCNTs / PI composite films; First, a precursor solution was prepared: 0.4 g of Fe3O4 nanoparticles and 1.2 g of carboxylated carbon nanotubes (CCNTs) were weighed at a mass ratio of 1:3 and added to 40.00 g of dimethylacetamide (DMAc) solvent. The mixture was ultrasonically treated for 2 hours to achieve uniform dispersion. During ultrasonic treatment, high-frequency sound waves generated cavitation in the solvent, effectively overcoming the van der Waals forces between the nanoparticles, thus forming a stable suspension dispersion system of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the solvent. Subsequently, the ultrasonically dispersed mixture was transferred to a three-necked caustic soda. In a reaction vessel, under the protective condition of purging high-purity nitrogen to purge air from the system, 2.27 g (11.35 mmol) of 4,4'-diaminodiphenyl ether (ODA) monomer was added. Mechanical stirring was started and continued for 20 minutes to ensure that ODA was fully dissolved and uniformly distributed in the system. Nitrogen protection effectively prevents side reactions between moisture in the air and the subsequently added anhydride monomer, ensuring the smooth progress of the polymerization reaction. Under an ice-water bath cooling environment, 2.53 g (11.58 mmol) of pyromellitic dianhydride (PMDA) was dispersed in multiple... The solution is slowly added to the reaction system. An ice-water bath can promptly remove the heat released during polymerization, preventing localized overheating that could lead to uneven reaction or side reactions. The molar ratio of ODA to PMDA is set at 1:1.02; a slight excess of PMDA compensates for the consumption of anhydride groups caused by trace amounts of moisture in the system during the reaction, ensuring that the resulting polyamic acid has appropriate molecular weight and viscosity. Subsequently, 17.60 g of DMAc solvent is added to the system to adjust the solid content of the entire reaction system to 10%. After stirring for another 4 hours, a brownish-black viscous solution is obtained. A precursor solution, namely the Fe3O4-CCNTs / PAA composite precursor solution, is prepared in which the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes is 25%. During this in-situ polymerization process, ODA and PMDA undergo a condensation reaction on the filler surface to generate polyamic acid (PAA) molecular chains. Some PAA molecular chains form chemical bonds or strong physical adsorption through the carboxyl groups on the surface of carboxylated carbon nanotubes, realizing the effective combination of the polymer matrix and the filler at the molecular level. This lays the structural foundation for the subsequent formation of a uniformly dispersed composite film with strong interfacial bonding.

[0020] Following the same preparation process parameters as the first precursor solution, the amounts of Fe3O4 nanoparticles and carboxylated carbon nanotubes were adjusted to make the total mass fraction of filler 30%, while keeping the mass ratio of Fe3O4 nanoparticles to carboxylated carbon nanotubes constant at 1:3, thus preparing the second precursor solution.

[0021] After the precursor solution was prepared, a bilayer precursor film was prepared. The prepared first precursor solution was uniformly poured onto the cleaned flat glass substrate surface. The coating thickness parameter of the scraper was adjusted to 350 μm, and the scraping operation was performed at a uniform speed and force to form a smooth first precursor wet film. The glass plate with the wet film was then transferred to a refrigerator and frozen at approximately -18°C for 3 hours. The freezing treatment has multiple effects: on the one hand, the low temperature significantly slows down the molecular motion of the solvent DMAc, effectively inhibiting the activity of the polyamic acid molecular chains, causing the wet film to change from a fluid state to a solid state, and achieving preliminary shaping. On the other hand, freezing can prevent the two solutions from dissolving or exchanging components during the subsequent coating of the second layer, thereby ensuring the clarity of the interlayer interface and the stability of the filler gradient distribution in the bilayer structure. Then, the second precursor solution is uniformly poured onto the first film layer after freezing and setting, and the film scraper thickness is set to 700 μm for a second film scraping to form a second precursor wet film, resulting in a bilayer precursor film composed of the first film layer and the second film layer. In this bilayer precursor film, the total mass fraction of filler in the first film layer is 25%, and the total mass fraction of filler in the second film layer is 30%, which meets the design requirement that the filler content in the first film layer is lower than that in the second film layer.

[0022] After the preparation of the bilayer precursor film, it is subjected to thermal imidization and peeling. The glass substrate with the bilayer precursor film is placed in an oven for thermal imidization, with a staged heating program. During thermal imidization, the polyamic acid in the precursor undergoes a cyclization and dehydration reaction, and the amic acid groups in the molecular chain condense with the carboxyl groups to form a stable imide ring structure, transforming into polyimide. The PAA molecular chains in the first and second film layers undergo cyclization transformation simultaneously during imidization. The molecular chains of the two layers diffuse and entangle with each other at the interface, ultimately forming a molecular... The interpenetrating polyimide structure endows the bilayer film with excellent interlayer bonding force, which essentially avoids the problem of easy delamination of traditional multilayer films. After the oven cools naturally to room temperature, the glass substrate is taken out of the oven and immersed in deionized water for 2 hours. Water molecules gradually penetrate along the edge of the interface between the film and the glass, and the capillary action of water breaks the physical adsorption force between the film and the glass, so that the film and the glass plate are automatically separated. The peeled composite film is gently lifted to obtain the formed self-supporting bilayer structure Fe3O4-CCNTs / PI composite film product.

[0023] The structure and properties of the prepared composite film were characterized. Scanning electron microscopy revealed that the composite film exhibited a clear bilayer structure with a tight interface between the first and second layers, free from defects such as delamination, cracks, and pores. Fe3O4 nanoparticles and carboxylated carbon nanotubes were uniformly dispersed in both layers without significant agglomeration. The carboxylated carbon nanotubes formed a three-dimensional continuous conductive network structure, in which Fe3O4 nanoparticles were uniformly embedded.

[0024] Testing revealed that the composite film prepared in this embodiment has a thickness of approximately 70 μm and exhibits excellent overall performance. Regarding electromagnetic shielding performance, the film achieves an electromagnetic shielding effectiveness of 36.8 dB in the X-band (8.2-12.4 GHz), blocking over 99.9% of electromagnetic waves. Absorption loss accounts for approximately 70% of the total shielding effectiveness, while reflection loss is only about 30%, indicating that its shielding mechanism is primarily absorption-based, effectively reducing the risk of secondary electromagnetic pollution. The specific shielding effectiveness calculated per unit thickness is as high as 525.7 dB / mm, significantly superior to most similar films. This polyimide-based electromagnetic shielding material exhibits superior performance primarily due to: the low filler content and good impedance-free space matching of the first film layer, reducing direct reflection of electromagnetic waves at the surface; the high filler content of the second film layer, providing a robust magnetoelectric synergistic loss network; the bilayer interface providing additional reflection and interfacial polarization losses; and the synergistic effect of the magnetic and eddy current losses of Fe3O4 and the dielectric losses of carboxylated carbon nanotubes, achieving a multiple attenuation mechanism for electromagnetic waves. Regarding Joule heating performance, thanks to the continuous conductive network formed by the carboxylated carbon nanotubes, the film maintains high performance under 6V DC voltage. It can rapidly heat up to 145.7℃ in just 15 seconds, demonstrating fast response and high heating efficiency. It can operate stably for extended periods at a low voltage of 3V, maintaining a constant temperature, making it suitable for applications such as flexible wearable heating devices, defrosting, and temperature control chips. Regarding thermal stability and flame retardancy, the film's thermal decomposition temperature is above 500℃, with no significant thermal weight loss below 500℃. Its limiting oxygen index reaches 39.1%, far exceeding the minimum oxygen concentration required for combustion in air (approximately 21%), achieving a high flame retardancy rating. Fe3O4 nanoparticles and carboxylated carbon nanotubes can promote polymerization at high temperatures. The imide matrix forms a dense carbon layer, which acts as a physical barrier, delaying further decomposition of the internal materials and synergistically improving flame retardancy. In terms of environmental stability, the film can still maintain its complete structure and appearance after being treated in liquid nitrogen at -196℃ and high temperature environments at 300℃. After being immersed in strong acid and strong alkali solutions, the electromagnetic shielding performance retention rate is ≥95.5%. After repeated bending tests, the film does not crack or delaminate, and the performance degradation is minimal, exhibiting excellent high and low temperature resistance, acid and alkali corrosion resistance and mechanical flexibility, which can meet the requirements for long-term stable service in extremely harsh environments.

[0025] Example 2 The preparation method of this embodiment is basically the same as that of Example 1, except that the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the first film layer is 20%, and the total mass fraction in the second film layer is 25%. The composite film obtained in this embodiment has a double-layer gradient structure similar to that in Example 1. Due to the overall reduction in filler content, the electromagnetic shielding effectiveness of the film is slightly reduced, but its surface impedance is closer to the free space impedance, the surface reflection of electromagnetic waves is further reduced, and the absorption ratio is higher than that in Example 1. At the same time, the flexibility and light transmittance of the film are improved, making it suitable for application scenarios with moderate requirements for electromagnetic shielding effectiveness but higher requirements for flexibility and optical performance.

[0026] Example 3 The preparation method of this embodiment is basically the same as that of Example 1, except that the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the first film layer is 30%, and the total mass fraction in the second film layer is 35%. The composite film obtained in this embodiment has a more compact conductive network due to the overall increase in filler content, and its electromagnetic shielding effectiveness is further improved compared with Example 1, but its flexibility is reduced. This film is suitable for application scenarios with extremely high shielding performance requirements and relatively low flexibility requirements.

[0027] Example 4 The preparation method of this embodiment is basically the same as that of Example 1, except that the thickness of the first film layer is 300 μm and the thickness of the second film layer is 600 μm. The thickness of the resulting composite film is about 50 μm, which maintains a good double-layer gradient structure and electromagnetic shielding performance. At the same time, due to its thinner thickness and lighter weight, it has advantages in weight-sensitive applications such as aerospace and portable electronic devices.

[0028] Example 5 The preparation method of this embodiment is basically the same as that of Example 1, except that the thickness of the first film layer is 500 μm and the thickness of the second film layer is 900 μm; the thickness of the resulting composite film is about 100 μm. Due to the increase in thickness, the transmission path of electromagnetic waves inside the material is extended, the absorption loss is more complete, and the electromagnetic shielding effectiveness is further improved.

[0029] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0030] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A Fe3O4-CCNTs / PI composite film, characterized in that, It includes a first film layer and a second film layer stacked together; The first film layer comprises a first polyimide matrix and Fe3O4 nanoparticles and carboxylated carbon nanotubes dispersed in the first polyimide matrix; The second film layer comprises a second polyimide matrix and Fe3O4 nanoparticles and carboxylated carbon nanotubes dispersed in the second polyimide matrix; The total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the first film layer is lower than that in the second film layer.

2. The Fe3O4-CCNTs / PI composite film according to claim 1, characterized in that, In both the first and second films, the mass ratio of Fe3O4 nanoparticles to carboxylated carbon nanotubes is 1:

3.

3. The Fe3O4-CCNTs / PI composite film according to claim 1, characterized in that, The total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the first film layer is 20% to 30%. The total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes in the second film layer is 25% to 35%.

4. The Fe3O4-CCNTs / PI composite film according to claim 3, characterized in that, The total mass fraction of the first film layer is 25%, and the total mass fraction of the second film layer is 30%.

5. The Fe3O4-CCNTs / PI composite film according to claim 1, characterized in that, The Fe3O4-CCNTs / PI composite film is a self-supporting film.

6. The Fe3O4-CCNTs / PI composite film according to any one of claims 1 to 5, characterized in that, The thickness of the Fe3O4-CCNTs / PI composite film is 50-100 μm; The electromagnetic shielding effectiveness in the X-band is ≥35dB, the absorption loss accounts for ≥65% of the total shielding effectiveness, and the limiting oxygen index is ≥38%.

7. A method for preparing the Fe3O4-CCNTs / PI composite film according to any one of claims 1-6, comprising the following steps: S1. Prepare a first precursor solution and a second precursor solution. The first precursor solution contains polyamic acid, Fe3O4 nanoparticles, and carboxylated carbon nanotubes, wherein the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes is C1. The second precursor solution contains polyamic acid, Fe3O4 nanoparticles, and carboxylated carbon nanotubes, wherein the total mass fraction of Fe3O4 nanoparticles and carboxylated carbon nanotubes is C2, and C2 > C1. S2. The first precursor solution is coated onto the substrate to form a first precursor wet film; S3. The first precursor wet film is subjected to freezing treatment to fix its shape, thereby obtaining the first film layer after freezing and fixing; S4. Coat the second precursor solution onto the first film layer after freeze-setting to form a second precursor wet film, and obtain a double-layer precursor film; S5. The bilayer precursor film is subjected to thermal imidization treatment to convert polyamic acid into polyimide, thereby obtaining the Fe3O4-CCNTs / PI composite film.

8. The method according to claim 7, characterized in that, In step S1, the steps of preparing the first precursor solution and the second precursor solution each include: Fe3O4 nanoparticles and carboxylated carbon nanotubes were dispersed in an organic solvent, and an aromatic diamine monomer was added and stirred to dissolve. Then, an aromatic dianhydride monomer was added under ice-water bath cooling to carry out a polymerization reaction to obtain a precursor solution containing polyamic acid.

9. The method according to claim 8, characterized in that, The aromatic diamine is 4,4'-diaminodiphenyl ether, the aromatic dianhydride is pyromellitic dianhydride, and the organic solvent is dimethylacetamide; The molar ratio of the aromatic dianhydride to the aromatic diamine is 1.02:1; the solid content of the precursor solution is 8% to 12%.

10. The method according to claim 7, characterized in that, The coating in steps S2 and S4 is applied by scraping. In step S2, the coating thickness is 300–500 μm; in step S4, the coating thickness is 600–900 μm; in step S3, the freezing temperature is -20°C to 0°C and the time is 1–5 hours; in step S5, the thermal imidization treatment includes staged heating, and the final imidization temperature is 250–400°C. Step S5 is followed by immersing the thermally imidized film with the substrate in water to peel off the film and obtain the self-supporting Fe3O4-CCNTs / PI composite film.

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