A flexible multifunctional composite film integrating heat dissipation and flame retardation and a preparation method and application thereof

Abstract

This invention discloses a flexible multifunctional composite film integrating heat dissipation and flame retardancy, its preparation method, and its application, belonging to the technical field of multifunctional composite film materials. The composite film is composed of MXene, graphene oxide (GO), and carbon nanotubes (CNTs) in a specific mass ratio, prepared by magnetic stirring, ultrasonic dispersion in a cold water bath, pulsed / gradient negative pressure vacuum filtration, and freeze-drying. The film thickness is 20-30 μm. In the layered structure of the film, GO sheets are inserted between MXene layers, and CNTs bridge the layers to form a three-dimensional thermally conductive network, achieving high in-plane thermal conductivity (≥4 W·m). ‑1 ·K ‑1 It also possesses excellent flame retardancy (limiting oxygen index LOI ≥ 28%, UL-94V-0 rated) and excellent mechanical flexibility. This film can be used as an integrated heat dissipation-flame retardant interface layer in high-power density electronic devices, electric vehicles, and other fields to address risks such as thermal runaway and fire.

Description

Technical Field

[0001] This invention belongs to the field of multifunctional composite film technology, and relates to a flexible multifunctional composite film integrating heat dissipation and flame retardancy, its preparation method and application, especially a layered composite film constructed with two-dimensional MXene sheets as the main body and synergistic graphene oxide and carbon nanotubes. Background Technology

[0002] With the continuous increase in power density of power batteries, flexible electronics, and semiconductor devices, the operating heat flux density of core components is rapidly rising. There is an urgent need for lightweight interface materials that combine high in-plane thermal conductivity, low radial thermal conductivity, flame retardancy, and mechanical flexibility to avoid localized overheating, thermal runaway, and the resulting safety risks. However, existing heat dissipation and flame retardant solutions mainly rely on metal heat sinks, phase change materials, or coated flame retardants: metal layers are heavy and difficult to bend, phase change materials are often irreversible and pose leakage risks, and single flame-retardant coatings are difficult to quickly dissipate heat, making it difficult to simultaneously meet the requirements of thinness, flexibility, and multifunctionality. Therefore, a multifunctional thin film with heat dissipation, flame retardancy, and flexibility is needed to meet the requirements of next-generation highly integrated, lightweight, and safe energy storage systems.

[0003] MXene (in Ti3C2T) x MXene (represented by MXene) is a class of two-dimensional transition metal carbide / nitride materials. Its intralayer covalent-metallic bond network provides metallic phonon transport channels and has high mechanical strength. At the same time, MXene can be water-dispersible and contains functional groups such as –O, –OH, and –F on its surface, which facilitates interfacial coupling with other components. It is an ideal material for constructing thin, lightweight, and highly thermally conductive layered films.

[0004] Chinese patent CN119795691A discloses an ultra-high anisotropic thermal conductivity MXene thin film and its preparation method. The method employs a layer-by-layer coating process, alternately depositing MXene and montmorillonite (MMT) nanosheets and cross-linking them with a silane coupling agent, resulting in a significantly improved thermal anisotropy of the film. However, the MMT in this patent exhibits low thermal conductivity and is prone to moisture absorption and brittleness, easily introducing additional interfacial thermal resistance. Furthermore, the dense layered structure formed by the tight stacking of MXene and MMT leads to increased longitudinal thermal resistance and decreased mechanical toughness.

[0005] Chinese patent CN119136504A discloses a graphene composite film with high thermal conductivity, its preparation method and application. The graphene composite film is prepared by CVD growth of monolayer graphene on a copper-nickel alloy substrate and construction of a pagoda-shaped core, followed by coating with graphene oxide slurry and multi-stage carbonization-graphitization and calendering. Although the thermal conductivity of the graphene composite film is significantly improved, it still has defects such as extremely high process temperature, high energy consumption, complex and easy cracking of transfer and 100 MPa-level calendering processes, and graphitization embrittlement leading to insufficient flexibility-bending life. It is difficult to meet the requirements of flexible, lightweight and flame-retardant thermal management films.

[0006] Chinese patent CN119175418A discloses a high thermal conductivity carbon material reinforced metal matrix composite and its preparation method. The carbon material reinforced metal matrix composite is formed by embedding a metallized high thermal conductivity graphite film, carbon nanotube film or graphene film stack within a metal shell such as aluminum / copper, and then brazing it under argon protection at 100-1000℃ and 0.1-300MPa. It can be used for heat dissipation substrates, cold plates and chassis shells. However, this solution still has defects such as high energy consumption of high temperature and high pressure brazing, the need for vacuum and inert environment, relatively complex process, and high density and complete loss of flexibility due to metal shell.

[0007] Chinese patent CN119100379A discloses a graphene thermally conductive film, its preparation method, and its applications. The method involves introducing graphene microspheres obtained through spray drying-microwave / thermal reduction treatment into a graphene oxide slurry, and then blending them with a first graphene oxide slurry to form a film. After pre-curing, carbonization, and graphitization, the film is calendered into sheets. This utilizes the sliding and resilient hollow framework structure of the microspheres to reduce the Young's modulus of the graphene thermally conductive film, overcoming the problem of excessive brittleness and inflexibility caused by the dense interlayer stacking of traditional high-temperature graphitized films. However, this approach suffers from the sensitivity of microsphere size distribution to thermo-mechanical properties and a narrow process window. Furthermore, the improvement in thermal conductivity mainly stems from increased graphitization rather than interfacial coupling design, making it difficult to achieve a balance between lightweight, low-energy-consumption fabrication and multifunctional requirements.

[0008] Therefore, this invention develops a layered composite thin film with a clear structural hierarchy and synergistic complementary components, achieving good thermal conductivity, excellent bending cycle life and excellent flame retardancy, providing an integrated interface film layer with both heat dissipation and flame retardancy functions for high power density flexible devices and energy storage systems. Summary of the Invention

[0009] To address the aforementioned technical problems, this invention provides a flexible multifunctional composite film integrating heat dissipation and flame retardancy, and its preparation method, which is particularly suitable for high-power-density batteries, flexible electronic devices, and other fields requiring heat dissipation, flame retardancy, and flexibility.

[0010] The technical solution provided by this invention is as follows:

[0011] This invention also provides a method for preparing a flexible multifunctional composite film integrating heat dissipation and flame retardancy, comprising the following steps: (1) Mix appropriate amounts of MXene, graphene oxide (GO) and carbon nanotubes (CNT) aqueous dispersions with magnetic stirring at room temperature. After stirring at a medium speed for a certain period of time, further mix by ultrasonic dispersion in a cold water bath. (2) The ultrasonically mixed solution was filtered by pulse / gradient negative pressure vacuum, and after further drying, the MXene-graphene oxide-carbon nanotube self-supporting layered composite film was obtained by peeling.

[0012] Furthermore, the MXene mass percentage is 50%-55%, the graphene oxide mass percentage is 35%-40%, and the carbon nanotube mass percentage is 10%-15%, all based on the mass of the composite film.

[0013] Furthermore, the concentration of the Mxene aqueous dispersion is 4-10 mg / ml, the number of Mxene layers is 1-10, and the sheet diameter is 0.2-5 µm. The Mxene can be selected from Ti3C2T. x Ti2CT x Nb2CT x V2CT x Mo2CT x Ta4C3T x At least one of them, wherein T x It has surface functional groups such as –O, –OH, –F, and –Cl.

[0014] Furthermore, the concentration of the graphene oxide aqueous dispersion is 2-5 mg / ml, the number of graphene oxide layers is single layer, and the sheet diameter is 200-500 nm.

[0015] Furthermore, the concentration of the carbon nanotube aqueous dispersion is 1-4 mg / ml, the diameter of the carbon nanotubes is 1-2 nm, and the length is 5-30 µm.

[0016] Furthermore, in step (1), it is preferable to first mix the GO dispersion and CNT dispersion to form a GO / CNT pre-dispersion, and then add the MXene dispersion to the GO / CNT pre-dispersion for mixing. This order of addition can improve the dispersion stability and film uniformity of the system, and reduce agglomeration and local defects; compared with adding MXene first or mixing the three at the same time, it is more conducive to forming a stable "sheet-sheet-wire" synergistic network.

[0017] Furthermore, in step (1), the magnetic stirrer speed is 300-600 rpm, the temperature is room temperature, and the time is 45-70 min.

[0018] Furthermore, the ultrasonic dispersion time in step (1) is 30-45 min. During the ultrasonic process, the water bath temperature is controlled not to exceed 10℃.

[0019] Furthermore, in the vacuum filtration step (2), a "pulse / gradient negative pressure control mode" is introduced: the filtration process is divided into n stages (n≥3), and p1 is applied sequentially. <p2<…<p n The multi-stage increasing pressure difference is preferred, with p1 being 0.05–0.07 MPa. n The pressure difference is 0.10–0.12 MPa, the pressure difference increment Δp between adjacent stages is 0.01–0.03 MPa, and each stage lasts 30–70 min; periodic pulses are superimposed within each stage (achieved by short-term opening / closing of proportional valves / solenoid valves), with a pulse frequency of 0.5–1.5 Hz, a duty cycle of 15%–85%, and a single pulse duration of 0.5–2 s.

[0020] Furthermore, in step (2), the drying can be freeze drying or vacuum drying.

[0021] Freeze-drying involves placing the composite membrane, along with the filter membrane, obtained through pulse / gradient negative pressure vacuum filtration into a freeze dryer, first at -40°C. Pre-freeze at -45℃ for 2-3 hours, then freeze at -50℃. Dry at -60 ℃ and 10-15 Pa for 18-23 h; then drop 2-5 ml of ethanol onto the back of the filter membrane to wet it along the periphery of the filter membrane, and slowly peel it off along the edge of the filter membrane to obtain a self-supporting composite film.

[0022] Vacuum drying involves placing the composite membrane obtained through vacuum filtration, along with the filter membrane, into a vacuum oven and drying it at 50-80℃ and 50-200 Pa for 12-24 hours to allow the solvent to gradually evaporate and be discharged. After drying is complete and the membrane is cooled to room temperature, 2-5 ml of ethanol can be dropped onto the back of the filter membrane to wet it, similar to the process of freeze-drying a sample. The membrane is then slowly peeled off along its edge to obtain a self-supporting composite film.

[0023] The flexible multifunctional composite film with integrated heat dissipation and flame retardancy prepared by the above method has a layered structure in which GO sheets are alternately inserted between MXene layers, and a gradient pore structure is formed along the thickness direction. CNTs are distributed across layers to bridge the heat conduction network and form a self-supporting layered structure. The MXene sheets and graphene oxide sheets are arranged obliquely or perpendicularly with carbon nanotubes.

[0024] The composite film satisfies the following characteristics: (1) The composite film has a thickness of 20-30µm, an in-plane thermal conductivity ≥4W / m·K, and a thickness-direction thermal conductivity not higher than 0.5 W·m. -1 ·K-1 The in-plane / thickness thermal anisotropy coefficient is not less than 8.

[0025] (2) The composite film was subjected to more than 1,000 reciprocating 180° bending under the condition of bending radius ≤2mm, and no visible cracks, delamination or peeling appeared.

[0026] (3) The limiting oxygen index (LOI) of the composite film is ≥28%, and it passes the UL-94 vertical burning test to reach the V-0 level.

[0027] The present invention also provides the application of the above-mentioned thin film as an integrated heat dissipation-flame retardant interface layer, which is disposed between the heat-generating functional unit and the external package to reduce the maximum operating temperature of the device and improve the overall flame retardant safety level.

[0028] Compared with existing technical solutions, the technical solution of the present invention has the following advantages and beneficial effects: (1) In this invention, the composite of MXene, graphene oxide, and carbon nanotubes forms a three-dimensional synergistic network of "sheet-sheet-wire" (MXene sheet + GO intercalation + CNT interlayer bridge) to provide a continuous high-speed channel for phonons and electrons, enabling the in-plane thermal conductivity of the 20-30µm thick film to reach 4-5W·m. -1 ·K -1 This value is significantly higher than that of common polymer or MMT layered flame retardant films (<1 W·m). -1 ·K -1 It also represents a further improvement over low-load systems in ordinary MXene single-layer membranes.

[0029] (2) In this invention, flame retardancy and heat dissipation work together. When GO is burned, it is charred in situ and oxidized with MXene to form a complex carbon layer, forming a dense oxygen-barrier shell. MXene itself is non-flammable, and CNT forms a continuous thermally conductive skeleton and a conductive carbon layer at high temperature.

[0030] (3) The flexibility and folding life of the present invention are significantly improved. The "cross-layer bridge" of CNT can effectively disperse bending strain; the hydrogen bonding / electrostatic interaction between GO-MXene improves interlayer adhesion. The film has no cracks after multiple 180° reciprocations with a bending radius ≤2mm. In contrast, graphite paper and metal foil will show cracks or fatigue failure within 200 cycles.

[0031] (4) The present invention adopts a pulse / gradient negative pressure vacuum filtration process. Compared with the ordinary vacuum filtration process, the pressure and pulse frequency can be adjusted to control the microstructure of the membrane, thereby optimizing its thermal management, mechanical properties and other functions.

[0032] (5) This invention designs and prepares a flexible multifunctional composite film integrating heat dissipation and flame retardancy, which combines high in-plane thermal conductivity, flame retardancy, safety, and mechanical flexibility. The film thickness is 20-30 µm, and the in-plane thermal conductivity is not less than 4 W·m. -1 ·K -1 It has a limiting oxygen index (LOI) of not less than 28% and achieves V-0 rating in the UL-94 vertical burning test; it can achieve more than 1,000 180° reciprocating bends without visible cracks, delamination or peeling under a bending radius of ≤2 mm, meeting the interface integration requirements of thin and flexible devices. Attached Figure Description

[0033] Figure 1 The flowchart shows the pulse / gradient negative pressure process device for the heat dissipation-flame retardant integrated flexible multifunctional composite film prepared in Example 1.

[0034] Figure 2 Scanning electron microscope (SEM) image of the heat dissipation-flame retardant integrated flexible multifunctional composite film prepared in Example 1.

[0035] Figure 3 The image shows the XRD pattern of the flexible multifunctional composite film that integrates heat dissipation and flame retardancy prepared in Example 1.

[0036] Figure 4 The image shows the energy dissipation (EDS) results of the flexible multifunctional composite thin film with integrated heat dissipation and flame retardancy prepared in Example 1.

[0037] Figure 5 The image shows a vertical combustion test of the flexible multifunctional composite film that integrates heat dissipation and flame retardancy prepared in Example 1. Detailed Implementation

[0038] Example 1 A method for preparing a flexible multifunctional composite film integrating heat dissipation and flame retardancy includes the following steps: (1) Preparation of MXene-graphene oxide-carbon nanotube mixed solution 2.1 ml of graphene oxide dispersion with a concentration of 2 mg / ml was slowly added dropwise to 1.8 ml of carbon nanotube dispersion with a concentration of 1 mg / ml. The mixture was stirred at 300 rpm for 15 min at room temperature using a magnetic stirrer to allow the GO and CNT to be initially mixed.

[0039] Increase the stirring speed to 600 rpm and continue stirring for 35 min to obtain a uniform GO / CNT predispersant. Then add 1.2 ml of MXene dispersion with a concentration of 5 mg / ml to the predispersant and stir at 600 rpm for 20 min to ensure that the MXene sheets, GO sheets and CNT network are in full contact and evenly distributed.

[0040] After stirring, the resulting dispersion was transferred to an ultrasonic cleaner containing ice water and ultrasonically dispersed under ice water bath conditions for 40 minutes. During the ultrasonic process, the water bath temperature was controlled not to exceed 10°C to slow down MXene oxidation and obtain a uniform and stable MXene-GO-CNT mixed dispersion.

[0041] (2) Pretreatment of filter membrane and device: A φ50mm aqueous mixed cellulose filter membrane with a pore size of 0.22µm was selected. Before use, 8ml of deionized water and 5ml of ethanol were added dropwise to the surface of the filter membrane to completely wet it. The filter membrane swelled slightly during the liquid absorption process. After wetting, the filter membrane was allowed to stand at room temperature for 3 minutes to allow any residual air bubbles in the filter membrane pores to be fully expelled, ensuring unobstructed liquid channels during the subsequent filtration process.

[0042] Complete the pipeline connection in the following order: vacuum pump → buffer tank → proportional valve → pressure sensor → sand core filter; check all joint seals to ensure there is no obvious air leakage in the system, and calibrate the zero point of the pressure sensor. Load the preset differential pressure / valve timing program.

[0043] (3) Preparation of MXene-graphene oxide-carbon nanotube composite film: prepared using pulse / gradient negative pressure vacuum filtration process device.

[0044] Pre-lay stage: After adding the MXene-GO-CNT mixed dispersion to the filter cup, an initial pressure difference of p0=0.05Mpa was applied and maintained for 20min to allow the initial layer to spread evenly.

[0045] After the pre-lay stage, the filtration process is divided into 6 incremental pressure differentials, each embedded with a periodic pulse: p1 = 0.05 MPa, 40 min (the main valve is closed with a 1-second pulse every 5 min). p2 = 0.07 MPa, 45 min (the main valve is closed with a 1-second pulse every 5 min). p3 = 0.09 MPa, 50 min (1 Hz, 50% duty cycle high-frequency PWM pulses are executed continuously for 10 min in the middle of the segment) p4 = 0.10 MPa, 55 min (1 s pulse every 4 min) p5 = 0.11 MPa, 60 min (1 Hz high-frequency pulses are executed again for 10 min at the end of the segment) p6 = 0.12 MPa, 70 min (only the gradient is maintained, no more high-frequency pulses are used to prevent membrane wrinkling) (4) Freeze-drying and self-supporting film peeling The entire filter membrane with the wet film is removed from the filtration device and placed in a freeze dryer. Pre-freezing at 40℃ for 2 hours allows the internal moisture of the membrane to fully freeze and fix its porous structure. Then, switching to freeze-drying mode... The ice crystals were dried at 50℃ and a vacuum of about 10 Pa for 20 hours to sublimate and obtain a stable porous layered structure.

[0046] After freeze-drying and returning to room temperature, approximately 3 ml of ethanol was evenly dropped onto the back of the filter membrane, allowing the ethanol to gradually wet the interface between the membrane layer and the filter membrane along the pores, thus weakening the interaction between them. After standing for about 2 minutes, the membrane was slowly peeled off from the edge to obtain a self-supporting layered composite film of MXene-graphene oxide-carbon nanotubes with a thickness of approximately 20 µm, which is the aforementioned flexible multifunctional composite film integrating heat dissipation and flame retardancy.

[0047] (5) Characterization of membrane structure and properties Figure 1 This is a schematic diagram of the pulse / gradient negative pressure vacuum filtration process apparatus in this embodiment. This apparatus enables segmented control of the pressure difference and periodic opening and closing of valves, thereby constructing a thickness-oriented gradient pore structure and a heat-conducting network during the filtration process.

[0048] Figure 2 The image shows a scanning electron microscope (SEM) image of the heat dissipation-flame retardant integrated flexible multifunctional composite film prepared in Example 1. As can be seen from the image, the film exhibits a typical layered stacked structure: MXene / GO sheets are oriented in the in-plane direction, and CNTs are distributed across the interlayer layers, acting as "bridging" or "cables," constructing a continuous three-dimensional thermally conductive network and enhancing bending crack resistance. Based on the thickness measurement results from Example 1, the film thickness is approximately 20 µm (adjustable within the range of 20-30 µm).

[0049] Figure 3 The XRD pattern of the flexible multifunctional composite film integrating heat dissipation and flame retardancy prepared in Example 1 shows that it has a typical MXene layered structure with varying interlayer spacing. The relatively high intensity of the diffraction peak indicates that MXene is the dominant component.

[0050] Figure 4 The energy dissipation (EDS) analysis results of the flexible multifunctional composite film with integrated heat dissipation and flame retardancy prepared in Example 1 show that the various elements (Ti, O, C, N, P, etc.) are uniformly distributed in the composite film, indicating that this process can obtain a multiphase composite material with uniform component distribution.

[0051] Figure 5Vertical combustion tests were conducted on the flexible multifunctional composite film with integrated heat dissipation and flame retardancy prepared in Example 1. Within 0s-30s, it can be seen that the composite film not only extinguishes rapidly after the fire source is removed, but also does not drip any burning material during the test, effectively preventing the spread of fire and demonstrating strong safety and reliability.

[0052] Example 2 The difference between this embodiment and Embodiment 1 is that the concentration and amount of each component dispersion were adjusted, and the gradient negative pressure parameters and pulse frequency during the filtration process were optimized, thereby obtaining composite films with different thicknesses and microstructures.

[0053] (1) Preparation of MXene-graphene oxide-carbon nanotube mixed solution 2.45 ml of graphene oxide dispersion with a concentration of 2 mg / ml was slowly added dropwise to 2.1 ml of carbon nanotube dispersion with a concentration of 1 mg / ml. The mixture was stirred at 350 rpm for 15 min at room temperature using a magnetic stirrer to allow the GO and CNT to be initially mixed.

[0054] Increase the stirring speed to 650 rpm and continue stirring for 30 min to obtain a homogeneous GO / CNT premixed dispersion. Then add 1.4 ml of MXene dispersion with a concentration of 5 mg / ml to the premixed dispersion and stir at 600 rpm for 15 min. This ensures that the MXene sheets, GO sheets, and CNT network are fully contacted and dispersed.

[0055] After stirring, the dispersion was transferred to an ice-water bath and ultrasonically dispersed using an ultrasonic cleaner for 40 minutes. During the ultrasonication process, the water bath temperature was controlled to not exceed 10°C to slow down MXene oxidation. A homogeneous and stable MXene-GO-CNT mixed dispersion was obtained.

[0056] (2) Filter membrane pretreatment and vacuum filtration device setup A water-based mixed cellulose filter membrane with a diameter of φ50mm and a pore size of 0.22µm was selected. Before use, 8ml of deionized water and 5ml of ethanol were added to the surface of the filter membrane to completely wet it, and it was allowed to stand at room temperature for 3 minutes to remove any residual air bubbles in the filter membrane pores.

[0057] The filtration unit is connected in the following order: vacuum pump - buffer tank - proportional valve - pressure sensor - sand core filtration unit. The sealing of each interface is checked, and the pressure sensor is zero-point calibrated. Then, a pulse / gradient negative pressure program is preset in the control system.

[0058] (3) Pulse / gradient negative pressure vacuum filtration membrane formation Pre-lay stage: After adding the MXene-graphene oxide-carbon nanotube mixed solution to the filter cup, an initial pressure difference of p0=0.05Mpa was applied and maintained for 20min to allow the initial layer to spread uniformly.

[0059] After the pre-lay stage, the filtration process is divided into 6 incremental pressure differentials, each embedded with a periodic pulse: p1 = 0.05 MPa, 40 min (the main valve is closed with a 1-second pulse every 5 min). p2 = 0.07 MPa, 45 min (the main valve is closed with a 1-second pulse every 5 min). p3 = 0.09 MPa, 50 min (1 Hz, 50% duty cycle high-frequency PWM pulses are executed continuously for 10 min in the middle of the segment) p4 = 0.10 MPa, 55 min (1 s pulse every 4 min) p5 = 0.11 MPa, 60 min (1 Hz high-frequency pulses are executed again for 10 min at the end of the segment) p6 = 0.12 MPa, 70 min (only the gradient is maintained, no more high-frequency pulses are used to prevent membrane wrinkling) (4) Freeze-drying and self-supporting film peeling The entire filter membrane with the wet film is removed from the filtration device and placed in a freeze dryer. Pre-freezing at 40℃ for 2 hours allows the internal moisture of the membrane to fully freeze and fix its porous structure. Then, switching to freeze-drying mode... The ice crystals were dried at 50°C and a vacuum of approximately 10 Pa for 20 hours to sublimate and obtain a stable porous layered structure.

[0060] After freeze-drying and returning to room temperature, approximately 3 ml of ethanol was evenly dropped onto the back of the filter membrane, allowing the ethanol to gradually wet the interface between the membrane layer and the filter membrane along the pores, thus weakening the interaction between them. After standing for about 2 minutes, the membrane was slowly peeled off from the edge to obtain a self-supporting layered composite film of MXene-graphene oxide-carbon nanotubes with a thickness of approximately 22 µm, which is the aforementioned flexible multifunctional composite film integrating heat dissipation and flame retardancy.

[0061] Example 3 The preparation steps in this embodiment are basically the same as those in Example 1, except that the filter membrane material and pre-wetting conditions are changed to examine the effects of different filter membranes on the membrane structure and performance.

[0062] (1) Preparation of mixed dispersion Add 2.1 ml of graphene oxide dispersion with a concentration of 2 mg / ml and 1.8 ml of carbon nanotube dispersion with a concentration of 1 mg / ml to a beaker. Stir at 300 rpm for 15 min under magnetic stirring, and then increase the speed to 600 rpm and continue stirring for 35 min to obtain GO / CNT premixed dispersion.

[0063] Add 1.2 ml of MXene dispersion with a concentration of 5 mg / ml to the premixed dispersion and stir at 600 rpm for 20 min to ensure thorough mixing of the three components. Then, sonicate the dispersion in an ice-water bath for 40 min, controlling the water bath temperature at 5–10 °C.

[0064] (2) CA filter membrane pretreatment and device setup A cellulose acetate (CA) filter membrane with a diameter of φ50 mm and a pore size of 0.22 µm was selected. Before use, 3 ml of deionized water was dropped onto the surface of the filter membrane to completely wet the membrane and allow it to expand slightly. The membrane was then allowed to stand for 2 minutes. Next, 2 ml of ethanol was dropped onto the surface of the filter membrane to further reduce the surface tension and accelerate the expulsion of air bubbles. The membrane was then allowed to stand at room temperature for 5 minutes to ensure that the gas inside the filter membrane pores was completely expelled.

[0065] The filtration device is connected in the order of vacuum pump - buffer tank - proportional valve - pressure sensor - sand core filtration device. The sealing check and zero point calibration of the pressure sensor are completed, and the same pulse / gradient negative pressure program as in Example 1 is input into the control system.

[0066] (3) Pulse / gradient negative pressure filtration membrane formation The MXene-GO-CNT mixed dispersion was poured into a filter cup containing a CA filter membrane. An initial pressure difference of p0 = 0.05 MPa was first applied, and pre-spreading was performed for 20 min. Subsequently, vacuum filtration was carried out according to the procedure of pressure difference, time, and pulse frequency for each stage as described in Example 1. After vacuum filtration, the thickness of the wet membrane obtained was approximately 20–25 µm.

[0067] (4) Freeze-drying and film peeling Place the membrane filter in a freeze dryer, Pre-freeze at 40℃ for 2 hours, then... The membrane was dried at 50°C and a vacuum of approximately 10 Pa for 20 hours. After drying, approximately 3 ml of ethanol was dropped from the back of the filter membrane along its perimeter, allowing the ethanol to permeate through the pores of the filter membrane to the interface. After standing for approximately 2 minutes, the membrane was slowly peeled off along the edge to obtain a complete MXene-GO-CNT self-supporting composite film with a thickness of approximately 21 µm, which is the aforementioned flexible multifunctional composite film integrating heat dissipation and flame retardancy.

[0068] Test results show that the in-plane thermal conductivity of the thin film obtained in this embodiment is approximately 4.0–4.3 W·m. -1 ·K -1 It has a limiting oxygen index of not less than 28%, can reach V-0 rating in UL-94 vertical burning test, and has good bending performance.

[0069] Example 4 The preparation steps in this embodiment are basically the same as those in Example 1. The difference is that the number of gradient negative pressure segments, the time of each segment, and the pressure difference are changed during the filtration process in order to study the influence of different gradient strategies on the membrane structure.

[0070] (1) Preparation of mixed dispersion The MXene-GO-CNT mixed dispersion was prepared according to the method in Example 1, namely: 2.1 ml of GO dispersion with a concentration of 2 mg / ml was added to 1.8 ml of CNT dispersion with a concentration of 1 mg / ml, stirred at 300 rpm for 15 min, then stirred at 600 rpm for 35 min, then 1.2 ml of MXene dispersion with a concentration of 5 mg / ml was added, stirred at 600 rpm for 20 min, and finally sonicated in an ice-water bath for 40 min to obtain a uniform and stable mixed dispersion.

[0071] (2) Filter membrane pretreatment and vacuum filtration device setup A 50mm aqueous mixed cellulose filter membrane with a pore size of 0.22µm was selected. 8ml of deionized water and 5ml of ethanol were dropped onto the surface of the filter membrane in sequence. After complete wetting, the membrane was allowed to stand at room temperature for 3 minutes to release the gas.

[0072] The filtration device is connected in the following order: vacuum pump - buffer tank - proportional valve - pressure sensor - sand core filtration device. It is used after the sealing and pressure sensor are calibrated.

[0073] (3) Adjusted pulse / gradient negative pressure filtration process After adding the MXene-graphene oxide-carbon nanotube mixed solution to the filter cup, an initial pressure difference of p0=0.05Mpa was applied and maintained for 15min to allow the initial layer to spread uniformly.

[0074] Pulsed / gradient negative pressure divides the filtration process into 5 progressively increasing pressure differentials, each segment containing a periodic pulse: p1 = 0.06 MPa, 30 min (the main valve is closed with a 1-second pulse every 6 min). p2 = 0.07 MPa, 55 min (the main valve is closed with a 1-second pulse every 5 min). p3 = 0.09MPa, 50min (1Hz, 50% duty cycle high-frequency PWM pulses are executed continuously for 10min in the middle of the segment) P4 = 0.10 MPa, 60 min (1 s pulse every 5 min) p5 = 0.11 MPa, 50 min (1 Hz high-frequency pulse for 10 min at the end of the segment) p6 = 0.12 MPa, 20 min (maintain only the gradient, no more high-frequency pulses, to prevent membrane wrinkling) (4) Freeze-drying and film peeling Place the wet filter membrane in a freeze dryer, Pre-freeze at 40℃ for 2 hours, then... The membrane was dried at 50°C and a vacuum of approximately 10 Pa for 20 hours. After drying, approximately 3 ml of ethanol was added dropwise from the back of the filter membrane. After standing for 2–3 minutes, the membrane was slowly peeled off along the edge to obtain a self-supporting layered composite film of MXene-graphene oxide-carbon nanotubes with a thickness of approximately 25 µm, which is the aforementioned flexible multifunctional composite film integrating heat dissipation and flame retardancy.

[0075] Example 5 The preparation steps in this embodiment are basically the same as those in Example 1, except that constant pressure difference filtration is used.

[0076] (1) Preparation of mixed dispersion The MXene-GO-CNT mixed dispersion was prepared using the same method as in Example 1: 2.1 ml of GO dispersion at 2 mg / ml and 1.8 ml of CNT dispersion at 1 mg / ml were added to a beaker, stirred at 300 rpm for 15 min, then increased to 600 rpm and stirred for 35 min. Then 1.2 ml of MXene dispersion at 5 mg / ml was added, stirred at 600 rpm for 20 min, and then sonicated in an ice-water bath for 40 min.

[0077] (2) Membrane pretreatment and vacuum filtration device Select a 50mm aqueous mixed cellulose filter membrane with a pore size of 0.22µm, add 8ml of deionized water and 5ml of ethanol dropwise to completely wet it, and let it stand at room temperature for 3 minutes to release the gas.

[0078] The filtration device is simplified to a connection structure of vacuum pump - pressure sensor - sand core filtration device, that is, without buffer tank and proportional valve, the total pressure difference of the system is adjusted only by vacuum pump. It is used after completing the sealing check and pressure sensor calibration.

[0079] (3) Constant pressure filtration to form a membrane Pour the mixed dispersion into a filter cup, start the vacuum pump to establish a pressure difference p = 0.10 MPa, and continuously filter under this constant pressure difference for 5 hours without segmented gradient adjustment or superimposed pulse signals.

[0080] Under constant pressure, MXene, GO and CNT are mainly deposited on the filter membrane surface by pressure difference and gravity. The pore gradient in the thickness direction of the membrane layer is weak, and the wet membrane thickness is about 20~25µm after filtration.

[0081] (4) Freeze-drying and film peeling Place the wet filter membrane in a freeze dryer, Pre-freeze at 40℃ for 2 hours, then... The membrane was dried at 50°C and a vacuum of approximately 10 Pa for 20 hours. After drying, approximately 3 ml of ethanol was added dropwise from the back of the filter membrane. After standing for approximately 2 minutes, the membrane was slowly peeled off along the edge to obtain a self-supporting layered composite film of MXene-graphene oxide-carbon nanotubes.

[0082] Test results show that the in-plane thermal conductivity of this thin film is approximately 3.8–4.0 W·m. -1 ·K -1 The thermal conductivity in the thickness direction is approximately 0.5 W·m. -1 ·K -1 The limiting oxygen index is approximately 28%, and it achieves a V-0 rating in the UL-94 vertical burning test. Compared to films prepared using a pulse / gradient negative pressure process, films prepared using a constant pressure difference filtration method exhibit slightly lower thermal anisotropy, and reduced energy dissipation and crack passivation capabilities during bending.

[0083] Example 6 This embodiment, based on the constant pressure filtration of Example 5, further increases the filtration pressure difference and uses vacuum oven drying instead of freeze drying to compare the effects of different drying methods on membrane structure and performance.

[0084] (1) Preparation of mixed dispersion The MXene-GO-CNT mixed dispersion was prepared according to the same proportions as in Example 1, namely: 2.1 ml of GO dispersion with a concentration of 2 mg / ml; 1.8 ml of CNT dispersion with a concentration of 1 mg / ml; and 1.2 ml of MXene dispersion with a concentration of 5 mg / ml.

[0085] After mixing the GO and CNT dispersions, the mixture was stirred at 300 rpm for 15 min, then at 600 rpm for 35 min. After adding the MXene dispersion, the mixture was stirred at 600 rpm for 20 min, and then sonicated in an ice-water bath for 40 min.

[0086] (2) Filter membrane pretreatment and vacuum filtration device setup A 50mm aqueous mixed cellulose filter membrane with a pore size of 0.22µm was selected. Before use, 8ml of deionized water and 5ml of ethanol were added sequentially to wet the membrane, and it was allowed to stand at room temperature for 3 minutes to release the gas.

[0087] The filtration device adopts a simplified structure of vacuum pump-pressure sensor-sand core filtration device, and is put into use after checking and confirming that the sealing performance is good.

[0088] (3) Constant pressure filtration The above-mentioned mixed dispersion was poured into a filter cup, and a vacuum pump was started to establish a pressure difference p = 0.11 MPa. Filtration was carried out continuously for 4.8 hours under this constant pressure difference. Because the pressure difference was slightly higher than in Example 5, the solvent discharge rate was faster during filtration, resulting in a denser overall membrane. After filtration, the wet membrane thickness was approximately 20-24 µm.

[0089] (4) Vacuum oven drying and film peeling After filtration, the wet filter membrane is removed and placed in a vacuum oven. It is dried for 20 hours at 60°C and a vacuum of about 50-80 Pa, allowing the moisture to gradually evaporate and be discharged. During the drying process, the membrane undergoes a certain degree of shrinkage and secondary densification.

[0090] After drying and cooling to room temperature, 3-4 ml of ethanol is dropped onto the back of the filter membrane along the periphery, allowing the ethanol to penetrate along the pores of the filter membrane to the membrane-filter interface. After standing for about 2 minutes, it is slowly peeled off along the edge to obtain a self-supporting layered composite film of MXene-graphene oxide-carbon nanotubes with a thickness of about 21 µm, which is the flexible multifunctional composite film integrating heat dissipation and flame retardancy.

[0091] Tests show that the in-plane thermal conductivity of this thin film is approximately 4.0–4.2 W·m. -1 ·K -1 The thermal conductivity in the thickness direction is approximately 0.5 W·m. -1 ·K -1 The limiting oxygen index is not less than 28%, and the UL-94 vertical burning test achieves a V-0 rating. Compared with the freeze-dried sample, the film obtained in this embodiment is more dense overall, resulting in slightly higher stiffness. The film layer may crack or be damaged when bent or stretched.

[0092] The embodiments disclosed in this invention are merely illustrative examples of technical solutions. Any equivalent modifications, functional substitutions, combined optimizations, or partial improvements made by those skilled in the art based on this specification and claims without departing from the core ideas and essence of this invention should be considered within the scope of protection of this invention.

Claims

1. A flexible, multifunctional composite film integrating heat dissipation and flame retardancy, characterized in that, A layered composite film of MXene-graphene oxide-carbon nanotubes was prepared by sequentially using three aqueous dispersions of MXene, graphene oxide, and carbon nanotubes through magnetic stirring, ultrasonic dispersion, pulse / gradient negative pressure vacuum filtration, and freeze drying. This film is the flexible multifunctional composite film that integrates heat dissipation and flame retardancy.

2. The flexible multifunctional composite film integrating heat dissipation and flame retardancy according to claim 1, characterized in that, The MXene mentioned is Ti3C2T x The number of layers is 1-10, the sheet diameter is 0.2-5µm, and the surface contains –O, –OH, –F, and –Cl functional groups.

3. The flexible multifunctional composite film integrating heat dissipation and flame retardancy according to claim 1, characterized in that, The graphene oxide is a single-layer graphene oxide with a sheet diameter of 200-500 nm; the carbon nanotubes are single-walled carbon nanotubes with a diameter of 1-4 nm and a length of 5-30 µm.

4. The flexible multifunctional composite film integrating heat dissipation and flame retardancy according to claim 1, characterized in that, The composite film has a layered structure in which GO sheets are alternately inserted between MXene layers, and has a gradient pore structure along the thickness direction. CNTs are distributed across layers to bridge the thermally conductive network, forming a self-supporting layered structure. The MXene sheets and graphene oxide sheets are arranged obliquely or perpendicularly to the carbon nanotubes.

5. The method for preparing the heat dissipation-flame retardant integrated flexible multifunctional composite film according to any one of claims 1-4, characterized in that, Includes the following steps: Step 1: Mix MXene dispersion, graphene oxide dispersion and carbon nanotube dispersion at a mass ratio of 50%-55%: 35%-40%: 10%-15%, based on the mass of the composite film; Step 2: Stir the resulting mixture magnetically for 40-70 minutes; Step 3: Disperse the stirred mixture in a cold water bath using ultrasonication for 30-45 minutes; Step 4: Perform pulse / gradient negative pressure vacuum filtration for 4.5-5.5 hours; Step 5: After drying following vacuum filtration, peel off the self-supporting layered film.

6. The preparation method according to claim 5, characterized in that, The pulse / gradient negative pressure vacuum filtration adopts a multi-stage incremental pressure difference control mode: the filtration process is divided into n stages (n≥3), and p1 is applied sequentially. <p2<…<p n The pressure difference, where p1 is 0.05–0.07 MPa, p n The pressure difference is 0.10–0.12 MPa, and the pressure difference increment Δp between adjacent stages is 0.01–0.03 MPa; each stage lasts 30–70 min, and periodic pulses are superimposed within each stage. The pulse frequency is 0.5–1.5 Hz, the duty cycle is 15%–85%, and the duration of a single pulse is 0.5–2 s.

7. The preparation method according to claim 5, characterized in that, Step 5 describes freeze drying: the composite membrane obtained by pulse / gradient negative pressure vacuum filtration, along with the filter membrane as a whole, is placed into a freeze dryer and first dried at -40°C. Pre-freeze at -45℃ for 2-3 hours, then freeze at -50℃. Dry at -60℃ and 10-15 Pa for 18-23 h; then add ethanol dropwise to the back of the filter membrane to wet the periphery of the filter membrane, and slowly peel it off along the edge of the filter membrane to obtain a self-supporting composite film.

8. The flexible multifunctional composite film integrating heat dissipation and flame retardancy according to any one of claims 1-7, characterized in that, The composite film has a thickness of 20-30 µm, an in-plane thermal conductivity ≥4 W / m·K, and a thickness-direction thermal conductivity not higher than 0.5 W·m. -1 ·K -1 The in-plane / thickness thermal anisotropy coefficient is not less than 8.

9. The flexible multifunctional composite film integrating heat dissipation and flame retardancy according to any one of claims 1-8, characterized in that, The composite film was subjected to more than 1,000 reciprocating 180° folds with a bending radius of ≤2mm without any visible cracks, delamination, or peeling.

10. The flexible multifunctional composite film integrating heat dissipation and flame retardancy according to any one of claims 1-9, characterized in that, It has a limiting oxygen index (LOI) of ≥28% and passes the UL-94 vertical burning test to achieve a V-0 rating.

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