Alumina / polyimide composite thermal insulation material and preparation method thereof

Abstract

The invention provides an alumina / polyimide composite thermal insulation material. The alumina / polyimide composite thermal insulation material comprises alumina fiber meshes and polyimide nanofiber membranes which are alternately stacked, the aluminum oxide fiber net is prepared through a foam forming method, the polyimide nanofiber layer is deposited on the surface of the aluminum oxide fiber net through electrostatic spinning, the aluminum oxide fiber net and the polyimide nanofiber layer are reinforced and compounded through needling, and a precursor spinning solution for polyimide electrostatic spinning is prepared firstly; the preparation method comprises the following steps: spinning a precursor polyamide acid nanofiber membrane by adopting electrostatic spinning equipment, then preparing alumina fiber meshes with specific pore diameters by adopting a foam forming method, fixing the prepared alumina fiber meshes on two surfaces of the precursor polyamide acid nanofiber membrane, and alternating for 10-20 times, so as to obtain the composite material. And performing programmed thermal imidization treatment, and finally performing needling reinforcement to obtain the aluminum oxide / polyimide composite fiber thermal insulation material. The material has high heat barrier efficiency, excellent flame retardant property and mechanical flexibility.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to an alumina / polyimide composite thermal insulation material and its preparation method. Background Technology

[0002] The electric vehicle industry is developing rapidly, and the safety of power batteries has become a core concern. According to the mandatory "Safety Requirements for Power Batteries for Electric Vehicles" (GB38031-2025), which will be implemented on July 1, 2026, battery systems must meet the mandatory requirement of not catching fire or exploding for at least two hours after a single cell experiences thermal runaway. This places unprecedentedly high standards on the thermal insulation materials within the battery pack: not only do they need extremely low thermal conductivity to slow heat diffusion, but they also need excellent flame retardant properties and structural integrity to withstand extreme conditions such as bottom impacts. Currently, materials such as aerogel are used for battery insulation, but their cost is high; while traditional polyimide or alumina materials, when used alone, often fail to fully meet the stringent requirements of the new national standard in terms of thermal insulation efficiency, mechanical strength, or cost. Therefore, developing a new type of composite thermal insulation material that can efficiently delay thermal runaway, possesses excellent flame retardancy and mechanical properties, and is cost-effective has become an urgent need for the industry. Summary of the Invention

[0003] To achieve the aforementioned objectives, this invention provides an alumina / polyimide composite thermal insulation material and its preparation method. This material, achieved by combining an alumina fiber mesh with a polyimide nanofiber membrane using a specific process, aims to achieve extremely high heat barrier efficiency, excellent flame retardant properties, and good mechanical flexibility. It is particularly designed to meet the requirements of GB38031-2025 standard regarding preventing fire and explosion within two hours after thermal runaway of a battery system, and thus has broad application prospects.

[0004] To achieve the above-mentioned objectives, the present invention provides an alumina / polyimide composite thermal insulation material, comprising at least alternating layers of alumina fiber mesh and polyimide nanofiber membrane;

[0005] The alumina fiber mesh is prepared by foam molding, and the polyimide nanofiber layer is deposited on the surface of the alumina fiber mesh by electrospinning. The alumina fiber mesh and the polyimide nanofiber layer are reinforced by needle punching. By using micron-sized alumina fibers and nano-sized PI fibers to form a multi-level thermal insulation network with "micro-nano synergy", the thermal resistance is maximized, and efficient thermal insulation is achieved by suppressing convective heat transfer and solid heat conduction.

[0006] The alumina / polyimide composite fiber membrane insulation material has a thickness of 0.5-5 mm, 10-20 layers, and a thermal conductivity of 0.03-0.07 W / m·k.

[0007] In some technical solutions of this invention, the alumina fibers are 1-3 cm long and 10-20 μm in diameter. Long fibers within this range are easy to overlap to form a stable and continuous three-dimensional network, providing a structural basis for obtaining uniform pore size; while fibers within this diameter range ensure the rigidity of the skeleton while avoiding the problem of excessively large pores due to excessively thick fibers.

[0008] In some technical solutions of this invention, the alumina fiber mesh has a pore size of 10-30 μm and a single-layer thickness of 0.1-0.5 mm. This range effectively limits air convection heat transfer, which is key to achieving a low thermal conductivity.

[0009] In some technical solutions of the present invention, the polyimide nanofibers have a diameter of 200-800 nm and a single-layer thickness of 10-50 μm.

[0010] This invention also provides a method for preparing an alumina / polyimide composite thermal insulation material, comprising at least the following steps:

[0011] Step 1: Dissolve 4,4'-diaminodiphenyl ether in N,N-dimethylformamide, and add pyromellitic anhydride to the mixed solution in 3-5 portions to prepare the precursor spinning solution for polyimide electrospinning;

[0012] Step 2: Use a syringe to extract the precursor spinning solution and use an electrospinning device to spin the precursor polyamic acid nanofiber membrane.

[0013] Step 3: Prepare alumina fiber mesh with specific pore sizes using a foam molding method;

[0014] Step 4: Fix the alumina fiber mesh prepared in step 3 to the precursor polyamic acid nanofiber membrane prepared in step 2, and repeat step 2 to deposit the precursor polyamic acid nanofiber membrane on the other side of the alumina fiber mesh.

[0015] Step 5: Repeat steps 3 and 4, alternating 10-20 times;

[0016] Step 6: Remove the prepared fiber web and dry it at 45-90℃ for 1-2 hours to remove residual solvent. Then perform programmed thermal imidization treatment: raise the temperature from room temperature to 280-350℃ at a rate of 2-5℃ / min, and hold at this temperature for 1-2 hours to fully convert the precursor polyamic acid nanofiber membrane into polyimide;

[0017] Step 7: The heat-treated material is needle-punched for reinforcement to obtain the alumina / polyimide composite fiber membrane thermal insulation material.

[0018] In some technical solutions of the present invention, the mass ratio of 4,4′-diaminodiphenyl ether to pyromellitic anhydride in step 1 is 1:1-2:1.

[0019] In some technical solutions of the present invention, the mass concentration of the precursor spinning solution in step 1 is 14-20 wt%.

[0020] In some technical solutions of the present invention, the electrospinning parameters in step 2 are set as follows: voltage 15-20kV, spinning distance 10-20cm, and feed speed 1-2mL / h.

[0021] In some technical solutions of the present invention, the pretreatment of alumina fibers in step 3 specifically includes the following operations: First, the alumina fibers are cut into lengths of 1-3 cm; then, the fibers are placed in a mixed solution of hydrochloric acid and sulfuric acid with pH=1-2, and subjected to ultrasonic treatment for 5-15 min, followed by water bath heating at 45-90℃ for 1-2 h; finally, after ultrasonic treatment for 5-15 min, the fibers are washed, and then dried at 45-90℃. Preferably, the volume ratio of hydrochloric acid to sulfuric acid in the mixed solution of hydrochloric acid and sulfuric acid in step 3 is 1:1-1:3.

[0022] In some technical solutions of the present invention, the preparation method of the alumina fiber mesh in step 3 specifically includes the following steps:

[0023] Step 3.1: Slurry preparation. After pretreatment of alumina fibers, alumina fibers, polyethylene oxide, amide-based gemini quaternary ammonium salt, and deionized water are mixed to prepare a slurry, and polyvinyl alcohol is added as a pre-binder. Step 3.2: Stirring and foaming. The slurry is stirred at 1000-2000 rpm for 5-15 minutes to foam. Step 3.3: Molding and drying. The foamed slurry is filtered to form a fiber web and dried at 100-150℃.

[0024] Specifically, in step 3.2, the stirring speed has a significant impact on the pore size of the alumina fiber mesh. Higher speeds help to reduce the pore size, while lower speeds tend to create larger pores. Smaller pore sizes effectively restrict macroscopic airflow, thereby cutting off the main path for heat transfer via air convection and thus reducing the thermal conductivity of the material.

[0025] In some technical solutions of the present invention, the slurry in step 3.1 comprises, by mass fraction, 0.3%-0.5% alumina fiber, 0.02%-0.05% polyethylene oxide, 0.03%-0.05% amide-based gemini quaternary ammonium salt, and 99.4%-99.7% deionized water.

[0026] In some technical solutions of the present invention, the volume ratio of polyvinyl alcohol to deionized water in step 3.1 is 1:80-1:100.

[0027] In some technical solutions of the present invention, the needle punching density in step 7 is 70~95 c / min, the needle punching depth is 5~20 mm, the input speed is 0.5~1.5 m / min, the output speed is 0.5~1.5 m / min, and the stretching ratio is 0%; the needle punching process only requires needle punching and does not use pressure rollers for pressure.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] 1. The alternating composite structure of micron-sized alumina fiber mesh and nano-sized polyimide nanofiber membrane achieves synergistic thermal insulation through optimized thickness and multi-layered alternating structure. It maximizes thermal resistance within a limited space, exhibits low thermal conductivity, and significantly delays vertical heat transfer through the "path effect," forming the structural basis for meeting the national standard's 2-hour insulation requirement. Specifically, firstly, an alumina fiber mesh with optimized pore sizes of 10-30 μm is prepared using a foam molding method with coordinated control of fiber size, slurry, and foaming parameters to effectively limit air convection heat transfer. Secondly, polyimide nanofibers with diameters of 200-800 nm are deposited on the fiber mesh surface using electrospinning technology. Utilizing their large specific surface area and interface effect, these nanofibers significantly scatter and hinder solid-state heat conduction. Finally, the alternating stacked structure of 10-20 layers and needle-punching reinforcement further create numerous solid-gas interfaces, complicating the heat flow path and enhancing the scattering of heat radiation. These three synergistic effects result in the material's superior thermal insulation performance.

[0030] 2. Since the main components are heat-resistant and inherently flame-retardant polyimide and inorganic alumina, the toxicity of the flue gas generated during thermal runaway is low, which helps the battery system meet the new national standard for flue gas safety regulations.

[0031] 3. Through needle punching reinforcement and interlocking fiber network structure, the material maintains its flexibility while enhancing its resistance to compression and impact, which helps the battery pack maintain the integrity of its protective structure when facing conditions such as bottom impact.

[0032] 4. Battery systems using the alumina / polyimide composite thermal insulation material provided by this invention, in accordance with GB38031-2025 standard, can effectively delay the propagation of thermal runaway and help the system meet the technical requirement of not catching fire or exploding within an observation period of at least 2 hours after triggering single-cell thermal runaway. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 A schematic diagram of the structure of the present invention. Detailed Implementation

[0035] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. These described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0036] Example 1

[0037] An alumina / polyimide composite thermal insulation material is composed of alternating layers of alumina fiber mesh and polyimide nanofiber membrane. The alumina fiber mesh is prepared by a foam forming method, with an average fiber length of 2.1 cm, an average diameter of 16.4 μm, an average pore size of 15.7 μm, and a thickness of 0.36 mm. The polyimide nanofiber layer is directly deposited onto the surface of the alumina fiber mesh via electrospinning, with an average fiber diameter of 357 nm and a single-layer thickness of 15.9 μm. The composite fiber membrane insulation material has a thickness of 2.7 mm and a total of 15 layers. The thermal conductivity is 0.0524 W / m². -1 k -1 Thermal runaway propagation time can be extended by 1528 seconds.

[0038] A method for preparing an alumina / polyimide composite thermal insulation material includes the following steps:

[0039] Step 1: Dissolve 4,4'-diaminodiphenyl ether in N,N-dimethylformamide, and add pyromellitic anhydride to the mixed solution in multiple portions. The mass ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride is 1:1. Prepare a precursor spinning solution for polyimide electrospinning with a spinning solution concentration of 14wt%.

[0040] Step 2: Use a syringe to extract the precursor spinning solution and use an electrospinning device to spin the precursor polyamic acid nanofiber membrane. The electrospinning parameters are set as follows: voltage 20kV, spinning distance 20cm, and feed speed 2mL / h.

[0041] Step 3: Pretreatment of alumina fibers: Cut alumina fibers into 2cm lengths. Place the fibers in a HCl and H2SO4 solution with a pH=1 concentration (HCl to H2SO4 volume ratio 1:1), and sonicate for 10 min. Then heat in a 40℃ water bath for 2 h, sonicate again for 10 min, and wash. Remove the alumina fibers and dry them at 80℃. Prepare a slurry by mixing the treated alumina short fibers, polyethylene oxide, amide-based gemini quaternary ammonium salt, and deionized water. Add polyvinyl alcohol as a pre-binder. The mass proportions of alumina fibers are 0.3%, polyethylene oxide 0.02%, amide-based gemini quaternary ammonium salt 0.03%, and deionized water 99.65%. The ratio of polyvinyl alcohol to deionized water is 1:80. Stir at 1200 rpm for 15 min using an electric stirrer. Pour the stirred solution into a vacuum filter. After filtration, a fiber web of a certain thickness is formed and dried at 120℃ for later use.

[0042] Step 4: Fix the prepared alumina fiber web onto the precursor polyamic acid nanofiber membrane spun in step 2, and repeat step 2 to deposit the precursor polyamic acid nanofiber membrane directly onto the alumina fiber web.

[0043] Step 5: Repeat steps 3 and 4. The electrospinning distance of each polyamic acid nanofiber membrane gradually decreases, and the diameter of the spun fibers gradually increases, forming a trapezoidal structure. The total number of layers in the composite material is 15.

[0044] Step 6: Remove the prepared fiber web and dry it at 60°C for 2 hours to evaporate excess N,N-dimethylformamide solvent. Then, transfer the composite fiber membrane into a muffle furnace and perform gradient heating with a temperature gradient of 100°C for 2 hours.

[0045] Step 7: The calcined material is needle-punched to obtain an alumina / polyimide composite fiber membrane insulation material. The needle-punching density is 70 c / min, the needle-punching depth is 10 mm, the input speed is 0.5 m / min, the output speed is 0.5 m / min, and the stretching ratio is 0%. The needle-punching process only requires needle-punching and does not use pressure rollers.

[0046] Example 2

[0047] An alumina / polyimide composite thermal insulation material is composed of alternating layers of alumina fiber mesh and polyimide nanofiber membrane. The alumina fiber mesh is prepared by a foam forming method, with an average fiber length of 1.9 cm, an average diameter of 17.6 μm, an average pore size of 18.3 μm, and a thickness of 0.28 mm. The polyimide nanofiber layer is directly deposited onto the surface of the alumina fiber mesh via electrospinning, with an average fiber diameter of 569 nm and a single-layer thickness of 17.3 μm. The composite fiber membrane insulation material has a thickness of 3.2 mm and a total of 13 layers. The thermal conductivity is 0.0674 W / m². -1 k -1 Thermal runaway propagation time can be extended by 1280 seconds.

[0048] A method for preparing an alumina / polyimide composite thermal insulation material includes the following steps:

[0049] Step 1: Dissolve 4,4'-diaminodiphenyl ether in N,N-dimethylformamide, and add pyromellitic anhydride to the mixed solution in multiple portions. The mass ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride is 1.5:1. Prepare a precursor spinning solution for polyimide electrospinning with a spinning solution concentration of 16wt%.

[0050] Step 2: Use a syringe to extract the precursor spinning solution and use an electrospinning device to spin the precursor polyamic acid nanofiber membrane. The electrospinning parameters are set as follows: voltage 17kV, spinning distance 15cm, and feed speed 2mL / h.

[0051] Step 3: Pretreatment of alumina fibers: Cut alumina fibers into lengths of 1-3 cm. Place the fibers in a solution of HCl and H2SO4 at pH=1 (HCl to H2SO4 volume ratio 1:2), sonicate for 10 min, then heat in a 60℃ water bath for 1 h, sonicate again for 10 min, wash, and dry at 90℃. Prepare a slurry by mixing the treated alumina short fibers, polyethylene oxide, amide-based gemini quaternary ammonium salt, and deionized water, and add polyvinyl alcohol as a pre-binder. The mass proportions of alumina fibers are 0.4%, polyethylene oxide 0.03%, amide-based gemini quaternary ammonium salt 0.05%, and deionized water 99.52%, with a polyvinyl alcohol to deionized water ratio of 1:90. Stir at 1500 rpm for 10 min using an electric stirrer. Pour the stirred solution into a vacuum filter, filter to form a fiber web of a certain thickness, and dry at 100℃ for later use.

[0052] Step 4: Fix the prepared alumina fiber web onto the precursor polyamic acid nanofiber membrane spun in step 2, and repeat step 2 to deposit the precursor polyamic acid nanofiber membrane directly onto the alumina fiber web.

[0053] Step 5: Repeat steps 3 and 4. The electrospinning distance of each polyamic acid nanofiber membrane gradually decreases, and the diameter of the spun fibers gradually increases, forming a trapezoidal structure. The total number of layers in the composite material is 13.

[0054] Step 6: Remove the prepared fiber web and dry it at 60°C for 2 hours to evaporate excess N,N-dimethylformamide solvent. Then, transfer the composite fiber membrane into a muffle furnace and perform gradient heating at a temperature gradient of 150°C for 1 hour.

[0055] Step 7: The calcined material is needle-punched to obtain an alumina / polyimide composite fiber membrane insulation material. The needle-punching density is 80 c / min, the needle-punching depth is 15 mm, the input speed is 1 m / min, the output speed is 1 m / min, and the stretching ratio is 0%. The needle-punching process only requires needle-punching and does not use pressure rollers.

[0056] Example 3

[0057] An alumina / polyimide composite thermal insulation material is composed of alternating layers of alumina fiber mesh and polyimide nanofiber membrane. The alumina fiber mesh is prepared by a foam forming method, with an average fiber length of 2.3 cm, an average diameter of 18.1 μm, an average pore size of 16.4 μm, and a thickness of 0.35 mm. The polyimide nanofiber layer is directly deposited onto the surface of the alumina fiber mesh via electrospinning, with an average fiber diameter of 673 nm and a single-layer thickness of 21.5 μm. The composite fiber membrane insulation material has a thickness of 4.3 mm and a total of 17 layers. The thermal conductivity is 0.0793 W / m². -1 k -1 Thermal runaway propagation time can be extended by 1675 seconds.

[0058] A method for preparing an alumina / polyimide composite thermal insulation material includes the following steps:

[0059] Step 1: Dissolve 4,4'-diaminodiphenyl ether in N,N-dimethylformamide, and add pyromellitic anhydride to the mixed solution in multiple portions. The mass ratio of 4,4'-diaminodiphenyl ether to pyromellitic anhydride is 2:1. Prepare a precursor spinning solution for polyimide electrospinning with a spinning solution concentration of 18wt%.

[0060] Step 2: Use a syringe to extract the precursor spinning solution and use an electrospinning device to spin the precursor polyamic acid nanofiber membrane. The electrospinning parameters are set as follows: voltage 15kV, spinning distance 10cm, and feed speed 1mL / h.

[0061] Step 3: Pretreatment of alumina fibers: Cut alumina fibers into lengths of 1-3 cm. Place the fibers in a solution of HCl and H2SO4 at pH=2 (HCl to H2SO4 volume ratio 1:3), and sonicate for 15 min. Then heat in a 60℃ water bath for 2 h, sonicate again for 15 min, and wash. Remove the alumina fibers and dry them at 60℃. Prepare a slurry by mixing the treated alumina short fibers, polyethylene oxide, amide-based gemini quaternary ammonium salt, and deionized water. Add polyvinyl alcohol as a pre-binder. The mass proportions of alumina fibers are 0.5%, polyethylene oxide 0.05%, amide-based gemini quaternary ammonium salt 0.05%, and deionized water 99.4%. The ratio of polyvinyl alcohol to deionized water is 1:100. Stir at 1800 rpm for 10 min using an electric stirrer. Pour the stirred solution into a vacuum filter. After filtration, a fiber web of a certain thickness is formed and dried at 150℃ for later use.

[0062] Step 4: Fix the prepared alumina fiber web onto the precursor polyamic acid nanofiber membrane spun in step 2, and repeat step 2 to deposit the precursor polyamic acid nanofiber membrane directly onto the alumina fiber web.

[0063] Step 5: Repeat steps 3 and 4. The electrospinning distance of each polyamic acid nanofiber membrane gradually decreases, and the diameter of the spun fibers gradually increases, forming a trapezoidal structure. The total number of layers in the composite material is 17.

[0064] Step 6: Remove the prepared fiber web and dry it at 80°C for 1 hour to evaporate excess N,N-dimethylformamide solvent. Then, transfer the composite fiber membrane into a muffle furnace and perform gradient heating at a temperature gradient of 200°C for 1 hour.

[0065] Step 7: The calcined material is needle-punched to obtain an alumina / polyimide composite fiber membrane insulation material. The needle-punching density is 90 c / min, the needle-punching depth is 20 mm, the input speed is 1.5 m / min, the output speed is 1.5 m / min, and the stretching ratio is 0%. The needle-punching process only requires needle-punching and does not use pressure rollers.

[0066] Performance testing:

[0067] 1. Thermal conductivity test: The thermal conductivity of the sample was tested in accordance with GB / T 10295-2008.

[0068] 2. Thermal runaway propagation test: The test shall be conducted in accordance with GB 38031-2025.

[0069] 3. Vertical burning test of materials: The test shall be conducted in accordance with GB / T 2408-2021.

[0070] Performance tests on the alumina / polyimide composite thermal insulation material prepared in this embodiment show that it has excellent comprehensive performance and fully meets the safety requirements for power battery applications. The thermal conductivity of this embodiment is as low as 0.0793 W·m⁻¹·K⁻¹, demonstrating its superior basic thermal insulation capability. It significantly extends the propagation of thermal runaway, far exceeding the safe escape time, and provides a crucial guarantee for the core system to meet the national standard's 2-hour flame retardancy requirement. Furthermore, the vertical combustion tests of Examples 1-3 rated it at UL-94 V-0, the highest flame retardancy level, ensuring its high safety in open flame environments. The above data collectively verify that this material, through the synergistic effect of multi-stage thermal insulation and highly efficient flame retardancy, provides a reliable solution for achieving high safety in power battery systems.

[0071] Finally, it should be noted that although the present invention has been described in detail above with general descriptions and specific embodiments, the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An alumina / polyimide composite thermal insulation material, characterized in that, It includes at least alternating layers of alumina fiber mesh and polyimide nanofiber membrane; The alumina fiber mesh is prepared by foam molding, and the polyimide nanofiber layer is deposited on the surface of the alumina fiber mesh by electrospinning. The alumina fiber mesh and the polyimide nanofiber layer are reinforced by needle punching. The combination of micron-sized alumina fibers and nano-sized PI fibers forms a multi-level thermal insulation network with "micro-nano synergy".

2. The alumina / polyimide composite thermal insulation material according to claim 1, characterized in that, The thickness of the alumina / polyimide composite fiber membrane insulation material is 0.5-5mm, and the number of layers is 10-20.

3. The alumina / polyimide composite thermal insulation material according to claim 1, characterized in that, The alumina fibers are 1-3 cm long and 10-20 μm in diameter; the mesh size of the alumina fibers is 10-30 μm and the thickness of a single layer is 0.1-0.5 mm.

4. The alumina / polyimide composite thermal insulation material according to claim 1, characterized in that, The polyimide nanofibers have a diameter of 200-800 nm and a single-layer thickness of 10-50 μm.

5. A method for preparing an alumina / polyimide composite thermal insulation material, comprising at least the following steps: Step 1: Dissolve 4,4'-diaminodiphenyl ether in N,N-dimethylformamide, and add pyromellitic anhydride to the mixed solution in 3-5 portions to prepare the precursor spinning solution for polyimide electrospinning; Step 2: Use a syringe to extract the precursor spinning solution and use an electrospinning device to spin the precursor polyamic acid nanofiber membrane. Step 3: Prepare alumina fiber mesh with specific pore sizes using a foam molding method; Step 4: Fix the alumina fiber mesh prepared in step 3 to the precursor polyamic acid nanofiber membrane prepared in step 2, and repeat step 2 to deposit the precursor polyamic acid nanofiber membrane on the other side of the alumina fiber mesh. Step 5: Repeat steps 3 and 4, alternating 10-20 times; Step 6: Remove the prepared fiber web and dry it at 45-90℃ for 1-2 hours to remove residual solvent. Then perform programmed thermal imidization treatment: raise the temperature from room temperature to 280-350℃ at a rate of 2-5℃ / min, and hold at this temperature for 1-2 hours to fully convert the precursor polyamic acid nanofiber membrane into polyimide; Step 7: The heat-treated material is needle-punched for reinforcement to obtain the alumina / polyimide composite fiber membrane thermal insulation material.

6. The method for preparing an alumina / polyimide composite thermal insulation material according to claim 5, characterized in that, The mass ratio of 4,4′-diaminodiphenyl ether to pyromellitic anhydride in step 1 is 1:1 to 2:

1.

7. The method for preparing an alumina / polyimide composite thermal insulation material according to claim 5, characterized in that, The pretreatment of alumina fibers in step 3 specifically includes the following operations: First, the alumina fibers are cut into lengths of 1-3 cm; then, the fibers are placed in a mixed solution of hydrochloric acid and sulfuric acid with a pH of 1-2, and subjected to ultrasonic treatment for 5-15 minutes, followed by heating in a water bath at 45-90°C for 1-2 hours; finally, after ultrasonic treatment for 5-15 minutes, the fibers are washed, removed, and dried at 45-90°C. Preferably, the volume ratio of hydrochloric acid to sulfuric acid in the mixed solution of hydrochloric acid and sulfuric acid in step 3 is 1:1-1:

3.

8. The method for preparing an alumina / polyimide composite thermal insulation material according to claim 5, characterized in that, The preparation method of the alumina fiber mesh in step 3 specifically includes the following steps: Step 3.1: Slurry preparation. After pretreating the alumina fiber, the alumina fiber, polyethylene oxide, amide-based gemini quaternary ammonium salt and deionized water are mixed to prepare a slurry, and polyvinyl alcohol is added as a pre-bonding agent. Step 3.2: Stirring and foaming. Stir the slurry at 1000-2000 rpm for 5-15 minutes to foam it. Step 3.3: Molding and drying. The foamed slurry is filtered to form a fiber web and dried at 100-150℃. In some technical solutions of the present invention, the slurry in step 3.1 comprises, by mass fraction, 0.3%-0.5% alumina fiber, 0.02%-0.05% polyethylene oxide, 0.03%-0.05% amide-based gemini quaternary ammonium salt, and 99.4%-99.7% deionized water.

9. The method for preparing an alumina / polyimide composite thermal insulation material according to claim 5, characterized in that, The volume ratio of polyvinyl alcohol to deionized water in step 3.1 is 1:80-1:

100.

10. The method for preparing an alumina / polyimide composite thermal insulation material according to claim 5, characterized in that, In step 7, the needle punching density is 70~95 c / min, the needle punching depth is 5~20 mm, the input speed is 0.5~1.5 m / min, the output speed is 0.5~1.5 m / min, and the stretching ratio is 0%; the needle punching process only requires needle punching and does not utilize pressure rollers for pressurization.

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