A doped inorganic nanoparticle polyimide composite film and a preparation method thereof

Abstract

The application discloses a kind of doped inorganic nanoparticle polyimide composite film and preparation method thereof, by polyimide resin, AlOOH inorganic nanoparticles and polar proton organic solvent are fully mixed, coating and drying are obtained after.The AlOOH inorganic nanoparticles of the present application is introduced into polyimide resin as filler, and the polyimide nanocomposite film with excellent comprehensive performance is prepared, specifically, high transparency, high thermodynamic performance, good thermal conductivity, low thermal expansion coefficient;Compared with the film without adding AlOOH inorganic nanoparticles, the doped inorganic nanoparticle polyimide composite film of the present application is not dissolved in N, N-dimethylacetamide (DMAC), it is the performance of the enhanced molecular binding force, can enhance the solvent resistance of film, expand the application of polyimide film.

Description

Technical Field

[0001] This invention belongs to the field of polyimide composite film technology, specifically relating to a polyimide composite film doped with inorganic nanoparticles and its preparation method. Background Technology

[0002] As a substrate material for flexible displays, it needs to possess an excellent combination of properties, including high transparency, excellent mechanical properties, and outstanding thermal properties. Polyimide films, as polymer substrate materials, have many advantages, such as high mechanical strength, excellent flexibility, and excellent electrical properties and chemical resistance. However, their low transparency and large coefficient of linear thermal expansion (CTE) limit their further application development.

[0003] In traditional polyimide synthesis processes, the raw materials diamine and dianhydride monomers form electron-transfer complexes during polymerization, causing severe coloration of the film. Furthermore, using commercially available aromatic diamines and dianhydrides to prepare polyimides appears to offer limited protection against CTE (color transfer effect). Polyimide films themselves generally also have poor thermal conductivity. To meet the demands of novel flexible displays, polyimide composite films with superior properties have received considerable attention and development in recent years. In the prior art, CN107383873A discloses a graphene-polyimide composite film with high thermal conductivity, insulation properties, and a light transmittance close to 0%, exhibiting shielding performance; CN115960460A discloses a flame-retardant conductive polyimide composite film, wherein the first filler is a carbon-based conductive filler with excellent conductivity, and the second filler is a phosphorus-based flame-retardant material with excellent flame retardant properties, so that the material as a whole has excellent conductivity, while the limiting oxygen index of the film reaches 66.2%, and it also has excellent flame-retardant properties; CN110372895A discloses a SiO₂ with a low coefficient of thermal expansion. 2 / Polyimide composite films possess high thermal stability and a low coefficient of thermal expansion, which can well meet the thermal matching requirements of silicon-based materials in integrated circuit and chip packaging technologies. CN106380844A discloses a high thermal conductivity and high insulation polyimide composite film co-doped with boron nitride and aluminum nitride, which significantly improves insulation resistance and tensile strength. CN111269571A discloses a high-strength and high thermal conductivity polyimide composite film, which, due to the doping of carbon nanotubes and sheet-like hexagonal boron nitride, has excellent mechanical strength and thermal conductivity, effectively improving the performance and application range of polyimide films. However, some of the above-mentioned existing technologies suffer from complex preparation processes and high equipment requirements, while others only improve a single performance. Currently, there is still a lack of high-performance polyimide composite films on the market that combine high transparency, high thermal conductivity, and a low coefficient of thermal expansion.

[0004] The interactions between polymer chains are mainly van der Waals forces. These weak van der Waals forces cannot effectively inhibit chain segment movement with increasing temperature, hence most polyimides have a high coefficient of thermal expansion. Combining linear or rod-shaped fillers with polyimide resins is expected to more effectively restrict the thermal movement of polymer molecules, resulting in a lower coefficient of thermal expansion. Related literature reports (Transparent poly(ether sulfone) nanocomposite film with low thermal expansion coefficient for flexible display substrates) that nanocomposite films formed by adding rod-shaped AlOOH inorganic nanoparticles as fillers to polyether sulfone exhibit high transparency, low coefficient of thermal expansion, high thermal stability, and dimensional stability. However, a drawback is the poor compatibility between polyether sulfone and AlOOH, requiring sulfonation of the polyether sulfone to enhance its compatibility with the AlOOH filler. However, the sulfonation process is quite difficult in practical industrial applications, limiting the mass production of composite films.

[0005] Substrate materials used in actual devices not only need short-term heat resistance and dimensional stability, but also excellent optical transparency. Some polyimide films already possess high transparency. Related literature reports that inorganic particles such as ZnS (reducing the coefficient of thermal expansion of polyimide films in microelectronics processing using ZnS particles at low concentrations) and ZrW2P8 (Zirconium tungstate (ZrW2O8) / polyimide nanocomposites exhibiting a reduced coefficient of thermal expansion) are used as fillers added to polyimide resins to lower the material's coefficient of thermal expansion. However, the transparency and light transmittance of the resulting nanocomposite films often cannot be maintained. If the size of the filler is kept below the visible light wavenumber range, then the high transparency of the material itself can be preserved.

[0006] High-frequency applications such as mechanical devices require substrate materials with high thermal conductivity, but the poor thermal conductivity of polyimide films themselves severely limits their development. Some literature (Flexible and quasi-isotropically thermoconductive polyimide films by guided assembly of boron nitridenanoplate / boron nitride flakes for microelectronic applications) has explored using graphene, carbon nitride nanosheets, and hexagonal boron nitride nanosheets, which inherently possess good thermal conductivity, as fillers combined with polyimide resins to improve the material's thermal conductivity. However, the results show that this deteriorates other properties such as electrical insulation and dielectric properties, limiting the material's specific applications.

[0007] AlOOH itself is a pure white hydrated alumina powder, exhibiting morphologies such as nanotubes, needles, sea urchins, nanorods, fibers, and hexagonal plates. Rod-shaped AlOOH, when used as a filler, can overlap to form a continuous thermally conductive network, significantly reducing in-plane thermal resistance and improving thermal conductivity. Furthermore, its combination with polyimide resin is expected to more effectively restrict the thermal motion of polymer molecules, resulting in a lower coefficient of thermal expansion. The synthesis process of AlOOH is relatively mature, and the size of rod-shaped nanoparticles is easily controlled during synthesis, making rod-shaped AlOOH a good choice as a filler for polyimide resins. Moreover, the market still lacks polyimide films with a superior combination of properties, leading to a surge in demand for polyimide composite films that combine high transparency, high thermal conductivity, and a low coefficient of thermal expansion. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a polyimide composite film doped with inorganic nanoparticles.

[0009] Another object of the present invention is to provide a method for preparing the above-mentioned doped inorganic nanoparticle polyimide composite film.

[0010] The technical solution of the present invention is as follows:

[0011] A polyimide composite film doped with inorganic nanoparticles is obtained by thoroughly mixing polyimide resin, AlOOH inorganic nanoparticles, and a polar protonic organic solvent, followed by coating and drying.

[0012] The structural formula of polyimide resin is .

[0013] In a preferred embodiment of the present invention, the method for preparing the polyimide resin includes:

[0014] (1) 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB) and 4,4'-(hexafluoroisopropylidene) phthalic anhydride (6FDA) were added to a polar aprotic organic solvent and mixed evenly to carry out the polymerization reaction to obtain a polyamic acid solution.

[0015] (2) Add a catalyst and a dehydrating agent dropwise to the above polyamic acid solution for chemical imidization, then add a precipitant for precipitation, and wash and dry the precipitate in sequence to obtain the final product.

[0016] More preferably, the molar ratio of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFMB) to 4,4'-(hexafluoroisopropylidene) phthalic anhydride (6FDA) is 1:1; the polar aprotic organic solvent is selected from N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), N-ethyl-2-pyrrolidone (NEP), N-methyl-2-pyrrolidone (NMP), and dimethyl sulfoxide (DMSO); the catalyst is selected from pyridine, dimethylpyridine, p-pyrrolidone, trimethylpyridine, triethylamine, and quinoline; the dehydrating agent is selected from acetic anhydride, trifluoroacetic anhydride, and propionic anhydride; and the precipitant is selected from methanol and ethanol.

[0017] More preferably, the solid content of the polyamic acid solution obtained in step (1) is 5-30 wt%.

[0018] More preferably, the solid content of the polyamic acid solution obtained in step (1) is 12-16 wt%.

[0019] In a preferred embodiment of the present invention, the method for preparing AlOOH inorganic nanoparticles includes:

[0020] (1) At room temperature, aluminum nitrate nonahydrate (Al(NO3)3·9H2O) is fully dissolved in distilled water to obtain an aluminum nitrate solution. Then, NH3·H2O solution is added dropwise to the aluminum nitrate solution to adjust it to a weakly acidic state.

[0021] (2) Centrifuge the material obtained in step (1), collect the precipitate, and wash it thoroughly with distilled water;

[0022] (3) Disperse the material obtained in step (2) evenly in distilled water, hydrothermally treat it at 210 °C for 24 h, then cool it naturally to room temperature, centrifuge to obtain a precipitate, wash the precipitate thoroughly with distilled water and ethanol, and then dry it to obtain the final product.

[0023] More preferably, the concentration of the aluminum nitrate solution is 1-1.5 mol / L, the concentration of the NH3·H2O solution is 3.0 mol / L, and the pH of the weakly acidic solution is 4.

[0024] In a preferred embodiment of the present invention, the polar aprotic organic solvent is selected from N,N-dimethylacetamide (DMAc), N,N-dimethylformamide (DMF), N-ethyl-2-pyrrolidone (NEP), N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO).

[0025] The preparation method of the above-mentioned doped inorganic nanoparticle polyimide composite film includes the following steps:

[0026] (1) AlOOH inorganic nanoparticles were uniformly dispersed in a polar aprotic organic solvent to obtain a dispersion with a solid content of 5-30 wt%.

[0027] (2) Dissolve the polyimide resin completely in the dispersion obtained in step (1);

[0028] (3) The material obtained in step (2) is uniformly coated on the load substrate and heated and dried to obtain a polyimide nanocomposite film.

[0029] In a preferred embodiment of the present invention, the heating and drying method is a stepped incremental heating and drying method, with the programmed temperature rise as follows: 50-70℃, 80-100℃, 110-130℃, 140-160℃, 170-190℃, and 200-220℃, each for 0.5-1.5 hours. When the drying temperature exceeds 100℃, the material is transferred to a vacuum drying oven for further drying to prevent it from being oxidized in the air, which would increase its yellowness.

[0030] The beneficial effects of this invention are:

[0031] 1. This invention introduces AlOOH inorganic nanoparticles as fillers into polyimide resin to prepare polyimide nanocomposite films with excellent comprehensive properties, specifically high transparency, high thermodynamic properties, good thermal conductivity, and low coefficient of thermal expansion. The chemical and physical interaction between polyimide and AlOOH inorganic nanoparticles breaks down the interface between organic and inorganic materials, realizing the combination of inorganic and organic materials.

[0032] 2. The polyimide resin selected in this invention is fluorinated polyimide, which contains a large number of fluorine bonds, providing a structural basis for the formation of hydrogen bonds. The AlOOH inorganic nanoparticles are nanoscale, with a particle length between 250-450 nm and a width between 15-50 nm. The particle size is smaller than the wavelength of visible light. When added to polyimide as a filler, it will not affect its total transmittance. At the same time, it forms hydrogen bonds with the organic matrix, enhancing the compatibility between the two and improving the mechanical and thermal properties of the composite film.

[0033] 3. The AlOOH inorganic nanoparticles in this invention have a high thermal conductivity. When added to polyimide resin as a filler, they can form a continuous "thermal bridge", which greatly reduces the in-plane thermal resistance, improves its thermal conductivity, and enhances its thermal conductivity performance.

[0034] 4. The hydrogen bonds and physical cross-linking between the polyimide resin and AlOOH inorganic nanoparticles in this invention effectively restrict the thermal motion of polymer molecules, reduce the linear thermal expansion coefficient of the film, and provide application prospects for flexible display substrates and optoelectronic devices.

[0035] 5. Compared with films without fillers, the doped inorganic nanoparticle polyimide composite film of the present invention is insoluble in N,N-dimethylacetamide (DMAC), which is a manifestation of enhanced intermolecular bonding, which can enhance the solvent resistance of the film and expand the application of polyimide films. Attached Figure Description

[0036] Figure 1 This is a scanning electron microscope image of the AlOOH inorganic nanoparticles prepared in Example 1 of this invention.

[0037] Figure 2 Transmission electron microscopy (TEM) image of AlOOH inorganic nanoparticles prepared in Example 1 of this invention.

[0038] Figure 3 Thermogravimetric analysis data of AlOOH inorganic nanoparticles prepared in Example 1 of this invention. Detailed Implementation

[0039] The technical solution of the present invention will be further explained and described below with reference to specific embodiments and figures.

[0040] Example 1

[0041] (1) Weigh 3 g of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) into a dry three-necked flask, and measure 40.6 mL of N,N-dimethylacetamide (DMAc) into the three-necked flask. Under a nitrogen atmosphere and with mechanical stirring, TFDB is completely dissolved in the solvent with an initial solid content of 15%. After 30 min, add 4.16 g of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), react at room temperature for 20 min, and then react at 0 ℃ in an ice bath for 3 h. Remove the ice bath and react at room temperature for 9 h to obtain a polyamic acid solution. Add 8.79 mL of acetic anhydride and 3.77 mL of pyridine to the above polyamic acid solution, and react at room temperature for 14 h to obtain a polyimide solution; add the above polyimide solution dropwise to a stirred ethanol solution, precipitating a white fibrous polymer. Wash the precipitate repeatedly with ethanol three times, and wash the final precipitate with methanol. Dry under vacuum at 65 °C to obtain polyimide resin, the structural formula of which is:

[0042]

[0043] (2) At room temperature, weigh 15 g of aluminum nitrate nonahydrate (Al(NO3)3·9H2O) and dissolve it completely in 35 mL of distilled water. Add 3.0 mol / L NH3·H2O solution dropwise to adjust the final solution pH to 4. Centrifuge the solution, collect the precipitate, and wash it with a large amount of distilled water. Dissolve the washed precipitate again in 30 mL of distilled water, disperse it evenly, transfer it to an autoclave, and hydrothermally treat it at 210 ℃ for 24 h. After the hot water reaction, allow it to cool naturally to room temperature, centrifuge the precipitate, wash it with a large amount of distilled water and ethanol, and dry it at 80 ℃ for 8 h to obtain the solution as shown below. Figures 1 to 3 The AlOOH inorganic nanoparticles shown are shown.

[0044] (3) Weigh 0.042 g of AlOOH inorganic nanoparticles into a serum bottle, and use a syringe to draw 9 mL of N,N-dimethylacetamide (DMAc) into the serum bottle. Seal the bottle opening with sealing film to ensure a tight seal, and sonicate for 3 h to form a uniformly dispersed system. Weigh 1 g of polyimide resin into a beaker, pour the well-dispersed solution into the beaker, seal the beaker opening with plastic wrap, and tie it tightly with a rubber band. The solid content is 10%, and the concentration of AlOOH inorganic nanoparticles is 4 wt%. Dissolve the solution under magnetic stirring for 12 h.

[0045] (4) The material obtained in step (3) is evenly coated onto a clean glass plate. A film coating machine is used to control the thickness of the film on the glass plate before drying to 1200 μm. The film is placed in an oven and heated to a specific temperature of 60 ℃, 80 ℃, 120 ℃, 150 ℃, 180 ℃, and 210 ℃ for 1 h each. To prevent the film from being oxidized in the air under high temperature conditions, the film is transferred to a vacuum oven for drying when the temperature exceeds 150 ℃. The film is then allowed to cool naturally to room temperature. The unpeeled film is then annealed under vacuum at 210 ℃ for 1 h to remove residual stress. The film is then peeled off from the glass substrate to obtain the doped inorganic nanoparticle polyimide composite film.

[0046] Example 2

[0047] (1) Preparation of polyimide resin: Same as in Example 1.

[0048] (2) Preparation of AlOOH inorganic nanoparticle filler: Same as in Example 1.

[0049] (3) Weigh 0.0638 g of AlOOH inorganic nanoparticles into a serum bottle, and use a syringe to draw 9 mL of N,N-dimethylacetamide (DMAc) into the serum bottle. Seal the bottle opening with sealing film to ensure a tight seal, and sonicate for 3 h to form a uniformly dispersed system. Weigh 1 g of polyimide resin into a beaker, pour the well-dispersed solution into the beaker, seal the beaker opening with plastic wrap, and tie it tightly with a rubber band. The solid content is 10%, and the concentration of AlOOH inorganic nanoparticles is 6 wt%. Dissolve the solution under magnetic stirring for 12 h.

[0050] (4) Preparation of polyimide composite film doped with inorganic nanoparticles: Same as in Example 1.

[0051] Example 3

[0052] (1) Preparation of polyimide resin: Same as in Example 1.

[0053] (2) Preparation of inorganic AlOOH inorganic nanoparticle filler: Same as in Example 1.

[0054] (3) Weigh 0.087 g of AlOOH inorganic nanoparticles into a serum bottle, and use a syringe to draw 9 mL of N,N-dimethylacetamide (DMAc) into the serum bottle. Seal the bottle mouth with sealing film to ensure a tight seal, and sonicate for 3 h to form a uniformly dispersed system. Weigh 1 g of polyimide resin into a beaker, pour the well-dispersed solution into the beaker, seal the beaker mouth with plastic wrap, and tie it tightly with a rubber band. The solid content is 10%, and the concentration of AlOOH inorganic nanoparticles is 8 wt%. Dissolve for 12 h under magnetic stirring.

[0055] (4) Preparation of polyimide composite film doped with inorganic nanoparticles: Same as in Example 1.

[0056] Example 4

[0057] (1) Preparation of polyimide resin: Same as in Example 1.

[0058] (2) Preparation of AlOOH inorganic nanoparticle filler: Same as in Example 1.

[0059] (3) Weigh 0.111 g of AlOOH inorganic nanoparticles into a serum bottle, and use a syringe to draw 9 mL of N,N-dimethylacetamide (DMAc) into the serum bottle. Seal the bottle opening with sealing film to ensure a tight seal, and sonicate for 3 h to form a uniformly dispersed system. Weigh 1 g of polyimide resin into a beaker, pour the well-dispersed solution into the beaker, seal the beaker opening with plastic wrap, and tie it tightly with a rubber band. The solid content is 10%, and the concentration of AlOOH inorganic nanoparticles is 10 wt%. Dissolve the solution under magnetic stirring for 12 h.

[0060] (4) Preparation of polyimide composite film doped with inorganic nanoparticles: Same as in Example 1.

[0061] Example 5

[0062] (1) Preparation of polyimide resin: Same as in Example 1.

[0063] (2) Preparation of inorganic AlOOH inorganic nanoparticle filler: Same as in Example 1.

[0064] (3) Weigh 0.136 g of AlOOH inorganic nanoparticles into a serum bottle, and use a syringe to draw 9 mL of N,N-dimethylacetamide (DMAc) into the serum bottle. Seal the bottle opening with sealing film to ensure a tight seal, and sonicate for 3 h to form a uniformly dispersed system. Weigh 1 g of polyimide resin into a beaker, pour the well-dispersed solution into the beaker, seal the beaker opening with plastic wrap, and tie it tightly with a rubber band. The solid content is 10%, and the concentration of AlOOH inorganic nanoparticles is 12 wt%. Dissolve the solution under magnetic stirring for 12 h.

[0065] (4) Preparation of polyimide composite film doped with inorganic nanoparticles: Same as in Example 1.

[0066] Comparative Example 1

[0067] (1) Preparation of polyimide resin: Same as in Example 1.

[0068] (2) Weigh 1 g of polyimide resin into a beaker, use a syringe to draw 9 mL of N,N-dimethylacetamide (DMAc) into the beaker, seal the mouth of the beaker with plastic wrap and tie it tightly with a rubber band, the solid content is 10%, and dissolve for 12 h under magnetic stirring.

[0069] (3) Same as Example 1.

[0070] (4) The material obtained in step (3) is evenly coated onto a clean glass plate. A film coating machine is used to control the thickness of the film on the glass plate to 1200 μm before drying. The film is placed in an oven and heated to a specific temperature of 60 ℃, 80 ℃, 120 ℃, 150 ℃, 180 ℃, and 210 ℃ for 1 h each. To prevent the film from being oxidized in the air under high temperature conditions, the film is transferred to a vacuum oven to dry when the temperature exceeds 150 ℃. The film is then allowed to cool naturally to room temperature. The unpeeled film is then annealed under vacuum at 210 ℃ for 1 h to remove residual stress. The film is then peeled off from the glass substrate to obtain the comparative composite film.

[0071] The glass transition temperature (T0) of the doped inorganic nanoparticle polyimide composite films prepared in the above embodiments and comparative examples is consistent with that of the films. g The in-plane thermal conductivity (Tc), linear thermal expansion coefficient (CTE), light transmittance, thermogravimetric analysis (5%), and tensile strength are shown in Table 1 below.

[0072] Table 1

[0073] Example 1 320.2 3.761 32.18 89.755 510.21 126.58 Example 2 322 4.691 29.91 89.362 509.45 126.82 Example 3 324.7 5.367 20.34 88.541 491.62 131.07 Example 4 328 5.853 15.37 88.538 507.32 135.73 Example 5 317.2 6.238 31.52 88.503 506.88 143.02 Comparative Example 1 323.8 1.205 54.41 89.895 524.57 124.09

[0074] The above description is merely a preferred embodiment of the present invention, and therefore should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made in accordance with the scope of the patent and the contents of the specification should still fall within the scope of the present invention.

Claims

1. A polyimide composite film doped with inorganic nanoparticles, characterized in that: It is obtained by thoroughly mixing polyimide resin, AlOOH inorganic nanoparticles, and a polar protonic organic solvent, followed by coating and drying. The structural formula of polyimide resin is ； The preparation methods of the above-mentioned AlOOH inorganic nanoparticles include: (1) At room temperature, aluminum nitrate nonahydrate is fully dissolved in distilled water to obtain aluminum nitrate solution. Then, NH3·H2O solution is added dropwise to the aluminum nitrate solution to adjust it to weak acidity. (2) Centrifuge the material obtained in step (1), collect the precipitate, and wash it thoroughly with distilled water; (3) Disperse the material obtained in step (2) evenly in distilled water, hydrothermally treat it at 210 °C for 24 h, then cool it naturally to room temperature, centrifuge to obtain a precipitate, wash the precipitate thoroughly with distilled water and ethanol, and then dry it to obtain the final product.

2. The doped inorganic nanoparticle polyimide composite film as described in claim 1, characterized in that: The preparation method of the polyimide resin includes: (1) 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride were added to a polar aprotic organic solvent and mixed evenly to carry out a polymerization reaction to obtain a polyamic acid solution; (2) Add a catalyst and a dehydrating agent dropwise to the above polyamic acid solution for chemical imidization, then add a precipitant for precipitation, and wash and dry the precipitate in sequence to obtain the final product.

3. The doped inorganic nanoparticle polyimide composite film as described in claim 2, characterized in that: The molar ratio of 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl to 4,4'-(hexafluoroisopropylidene)diphthalic anhydride is 1:1; the polar aprotic organic solvent is selected from N,N-dimethylacetamide, N,N-dimethylformamide, N-ethyl-2-pyrrolidone, N-methyl-2-pyrrolidone, and dimethyl sulfoxide; the catalyst is selected from pyridine, dimethylpyridine, p-pyrrolidone, trimethylpyridine, triethylamine, and quinoline; the dehydrating agent is selected from acetic anhydride, trifluoroacetic anhydride, and propionic anhydride; and the precipitant is selected from methanol and ethanol.

4. The doped inorganic nanoparticle polyimide composite film as described in claim 2, characterized in that: The solid content of the polyamic acid solution obtained in step (1) is 5-30 wt%.

5. The doped inorganic nanoparticle polyimide composite film as described in claim 2, characterized in that: The solid content of the polyamic acid solution obtained in step (1) is 12-16 wt%.

6. The doped inorganic nanoparticle polyimide composite film as described in claim 1, characterized in that: The concentration of the aluminum nitrate solution is 1-1.5 mol / L, the concentration of the NH3·H2O solution is 3.0 mol / L, and the pH of the weakly acidic solution is 4.

7. The doped inorganic nanoparticle polyimide composite film as described in claim 1, characterized in that: The polar aprotic organic solvent is selected from N,N-dimethylacetamide, N,N-dimethylformamide, N-ethyl-2-pyrrolidone, N-methyl-2-pyrrolidone, and dimethyl sulfoxide.

8. The method for preparing the doped inorganic nanoparticle polyimide composite film according to any one of claims 1 to 7, characterized in that: The steps include the following: (1) The AlOOH inorganic nanoparticles were uniformly dispersed in a polar aprotic organic solvent to obtain a dispersion with a solid content of 5-30 wt%. (2) Dissolve the polyimide resin completely in the dispersion obtained in step (1); (3) The material obtained in step (2) is uniformly coated onto the load substrate and heated and dried to obtain a polyimide nanocomposite film. In this step, the coating film is formed by blade coating and the wet film thickness is controlled to be 1200 μm.

9. The preparation method according to claim 8, characterized in that: The heating and drying method is a stepped incremental heating and drying process, with the following programmed temperatures: 50-70℃, 80-100℃, 110-130℃, 140-160℃, 170-190℃, and 200-220℃, each for 0.5-1.5 hours. When the drying temperature exceeds 100℃, the material is transferred to a vacuum drying oven for further drying to prevent oxidation in the air, which would increase its yellowness.

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