Polyimide-based graphene film and method for preparing the same

Abstract

The application belongs to the field of graphene film materials, and particularly relates to a polyimide-based graphene film and a preparation method thereof. The polyimide-based graphene film with a medium thickness is prepared by using PAA modified g-C3N4 as an initiating raw material, and has high thermal conductivity.

Description

Technical Field

[0001] This invention belongs to the field of graphene film materials, specifically relating to a polyimide-based graphene film induced by two-dimensional graphitic carbon nitride and its preparation method. Background Technology

[0002] Graphene thermal conductive film is a very important thermal management material in recent years. Based on its excellent heat transfer properties and low temperature resistance, it has been widely used in electronics, communications, aerospace and other fields. It is an excellent heat dissipation material in microelectronics, integrated circuits and other components.

[0003] Graphite films possess a wide adjustable range of heat transfer properties, and different properties can be produced depending on the selection of raw materials and the carbonization process. The in-plane thermal conductivity of the graphite film material largely determines its heat dissipation capacity. Traditional graphite films exhibit in-plane thermal conductivity ranging from 200 W / m K to 2000 W / m K, but for graphite films thicker than 20 micrometers, heat transfer is typically limited to around 1500 W / m K. Currently, there are two main methods for preparing high thermal conductivity graphite films: one is through the carbonization of inorganic graphene oxide, and the other is through the carbonization of organic polyimide. The method using inorganic graphene oxide requires very precise processing techniques. Graphene oxide contains a large number of oxygen-containing functional groups. These functional groups ensure the orderly stacking of the carbonized precursor sheets. However, the small molecules generated by these oxygen-containing functional groups at high temperatures can, to some extent, break through the inherent rigidity of the graphite material's molecular layers, further causing defects. Therefore, graphene oxide films require more precise carbonization processes to mitigate these defects. Simultaneously, the inherent rigidity of graphene oxide sheets makes it easier to process into graphite film materials within a certain thickness range.

[0004] Polyimide (PI) is a typical example of an aromatic polyamide capable of graphitization. Polyimides, spearheaded by Kapton, are widely used as mature industrial products. Their main characteristic lies in the ordered nature of the polyimide carbonization process, which endows them with a high degree of graphitization: the imine ring, acting as a two-dimensional active site, is effectively attacked and cyclized. While removing heteroatoms, the polyimide also undergoes effective cyclization, resulting in graphene with a higher crystal size. However, the planar orientation of polyimide is limited by the orientation of the precursor polyamic acid (PAA). Traditional blade coating and casting processes are difficult to use for preparing highly oriented films of a certain thickness. Therefore, higher requirements are placed on the structural design of the precursor and the carbonization process. Using two-dimensional nanomaterials to induce membrane molecular orientation is an important approach. Previous literature has reported methods for introducing two-dimensional graphene oxide nanosheets into organic precursors to increase their graphitization. For example, Gao Chao's team reported introducing graphene oxide nanosheets into polyacrylonitrile fibers and membrane materials to increase their orientation and graphitization capabilities. Alternatively, Yu Zhongzhen's team introduced two-dimensional graphene oxide into polyimide foam to increase the crystallinity of the material. Graphene oxide can act as an inducing factor, but its rigid structure and compatibility with organic precursors, as well as mismatches, can affect the intrinsic cyclization structure of organic precursors. Typically, two-dimensional nanomaterials require high filling levels to achieve good assisted carbonization effects.

[0005] Currently, there are no reports, either domestically or internationally, on the preparation of medium-thickness graphite films using low-filling two-dimensional non-graphite nanofillers to induce aromatic organic precursors. Summary of the Invention

[0006] The purpose of this invention is to propose a polyimide-based graphene film prepared by polyamic acid (PAA)-modified graphitic carbon nitride (g-C3N4) induced by polyimide, which has significantly improved thermal conductivity.

[0007] The technical solution of the present invention:

[0008] This invention provides a method for preparing a polyimide-based graphene film. The preparation method includes the following steps:

[0009] (1) Acid anhydride and diamine were added to the nano-sized g-C3N4 dispersion in sequence to react and obtain PAA-grafted g-C3N4 dispersion.

[0010] (2) Mix the PAA-grafted g-C3N4 dispersion obtained in step (1) with an organic solvent containing diamine, add acid anhydride, and react to obtain g-C3N4 / PAA slurry.

[0011] (3) The g-C3N4 / PAA slurry obtained in step (2) is coated on the substrate and dried to obtain a PAA composite film; then the PAA composite film is sintered and cooled to obtain a g-C3N4 / PI composite film.

[0012] (4) The g-C3N4 / PI composite film obtained in step (3) is subjected to three stages of heating, cooling and pressing to obtain a polyimide-based graphene film.

[0013] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (1), the nano-sized g-C3N4 is obtained by ultrasonic exfoliation using graphitic carbon nitride (g-C3N4) as raw material. Further, the ultrasonic power is 100–600 W. Preferably, the power is 600 W. The ultrasonic time is 8–16 h. The solvent used for ultrasonication is selected from any one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. Preferably, the solvent is N,N-dimethylacetamide (DMAC). The ultrasonication is performed using a probe-type ultrasonic method.

[0014] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (1), the size of the nanoscale g-C3N4 is 300-500 nm and the thickness is 3-5 nm.

[0015] Preferably, in the above-mentioned method for preparing polyimide-based graphene films, in step (1), the organic solvent used in the nanoscale g-C3N4 dispersion is selected from any one or more combinations of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. Preferably, it is N,N-dimethylformamide or N,N-dimethylacetamide. More preferably, it is N,N-dimethylacetamide.

[0016] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (1), the anhydride is selected from any one or a combination of pyromellitic dianhydride (PMDA), 4,4-oxobisphthalic anhydride (ODPA), and 4,4'-diphthalic anhydride (BPDA).

[0017] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (1), the diamine is selected from any one or a combination of two of diaminodiphenyl ether (ODA) and p-phenylenediamine (PPD).

[0018] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (1), the mass ratio of g-C3N4, acid anhydride, and diamine is 25-50:80-150:80-160.

[0019] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, step (1) is performed with an interval of 1 to 2 hours.

[0020] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, the reaction time in step (1) is 2 to 8 hours.

[0021] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (2), the reaction is carried out under an inert atmosphere.

[0022] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (2), the anhydride is selected from any one or a combination of pyromellitic dianhydride (PMDA), 4,4-oxobisphthalic anhydride (ODPA), and 4,4'-diphthalic anhydride (BPDA).

[0023] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (2), the diamine is selected from any one or a combination of two of diaminodiphenyl ether (ODA) and p-phenylenediamine (PPD).

[0024] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (2), the organic solvent for dissolving the diamine is selected from any one or more combinations of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

[0025] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (2), the PAA-grafted g-C3N4 dispersion and the organic solvent containing diamine are mixed for 30 min-1 h before adding acid anhydride.

[0026] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (2), the acid anhydride is added in two portions with an interval of 0.5 to 2 hours.

[0027] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (2), the mass ratio of g-C3N4, diamine and acid anhydride is 0.5-3:100-150:110-160.

[0028] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, the reaction time in step (2) is 12-24 h.

[0029] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (2), the mass fraction of g-C3N4 / PAA in the g-C3N4 / PAA slurry is 8-20%.

[0030] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (3), the coating thickness is 0.5 to 2 mm.

[0031] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (3), the drying temperature is 60–100°C, and the drying time is 2–8 hours.

[0032] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (3), the sintering is performed by heating at 1-5℃ / min to 260-280℃ and holding at that temperature for 1-3 hours, and then heating at 1-5℃ / min to 300-320℃ and holding at that temperature for 1-3 hours.

[0033] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (4), the three-stage heating is as follows: the first stage: heating at 1-5℃ / min to 500-700℃ and holding at that temperature for 0.5-2h; the second stage: heating at 1-3℃ / min to 1450-1550℃ (preferably 1500℃) and holding at that temperature for 0.5-2h; the third stage: heating at 3-5℃ / min to 2950-3050℃ (preferably 3000℃) and holding at that temperature for 0.5-2h.

[0034] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (4), the pressing pressure is 250-350 MPa.

[0035] Preferably, in the above-mentioned method for preparing polyimide-based graphene film, in step (4), the thickness of the polyimide-based graphene film is 10-60 μm.

[0036] The present invention also provides a polyimide-based graphene film prepared by the above-described method for preparing polyimide-based graphene films.

[0037] Preferably, the thickness of the polyimide-based graphene film is 10–60 μm.

[0038] This invention utilizes PAA-modified g-C3N4 nanosheets to facilitate the preparation of thick, highly oriented graphene films. The prepared thick polyimide-based films exhibit higher molecular chain orientation along their thickness direction, resulting in better graphite crystallite growth in the sintered graphene films. The thermal conductivity of an 18 μm thick graphene film is 1797 W / m K, and that of a 58 μm thick monolithic film is 1557 W / m K, effectively improving the thermal conductivity of thick monolithic polyimide-based graphene films. Attached Figure Description

[0039] Figure 1 These are scanning electron microscope (SEM) images of the graphitic carbon nitride obtained in Example 1 of this invention before and after ultrasonic exfoliation. a) Coarse graphitic carbon nitride particles; b) Exfoliated graphitic carbon nitride nanosheets; c) Nanosheet particle size distribution.

[0040] Figure 2This is an organic dispersion of PAA-modified graphitic carbon nitride obtained in Example 1 of the present invention. a) PAA-modified g-C3N4 was stably stored in DMAC for 24 h; b) The stratification phenomenon of pure g-C3N4 after being stored in DMAC for 24 h.

[0041] Figure 3 The thermal conductivity is the value of the graphene film obtained in the embodiments of the present invention.

[0042] Figure 4 This is a comparison of the thermal conductivity of the graphene films obtained in Example 2 and the comparative example.

[0043] Figure 5 The effect of graphitic carbon nitride content on the thermal conductivity of polyimide-based graphite films; where a is the thermal conductivity of PI-based graphite films with different g-C3N4 contents, and b is the thermal conductivity of graphite films with different thicknesses.

[0044] Figure 6 Comparison of the effects of graphitic carbon nitride on the crystal structure of polyimide-based graphite films; where a is the XRD image of PI film before and after PAA modification and graphitized pure PI, and b is a comparison of the degree of graphitization of the three. Detailed Implementation

[0045] This invention provides a polyimide-based graphene film induced by graphitic carbon nitride and its preparation method, which effectively improves the thermal conductivity of the polyimide-based graphene film.

[0046] The preparation method of polyimide-based graphene film of the present invention mainly includes the following steps: Step (1) using high-power cell ultrasound to effectively peel off coarse particles of graphitic carbon nitride prepared by pyrolysis into nanosheets, and then mixing a certain amount of PAA monomer with graphitic carbon nitride nanosheets to prepare graphitic carbon nitride nanosheets with PAA surface modification; Step (2) the PAA-modified graphitic carbon nitride nanosheets further participate in the synthesis of PAA main chain to obtain composite slurry; Step (3) the composite slurry is coated, dried and imidized to obtain polyimide composite film; Step (4) the polyimide composite film is gradient carbonized and graphitized to form a polyimide-based graphene film with high orientation and high graphitization.

[0047] Specifically, the preparation method of the polyimide-based graphene film of the present invention includes the following steps:

[0048] (1) Acid anhydride and diamine were added to the nano-sized g-C3N4 dispersion in sequence to react and obtain PAA-grafted g-C3N4 dispersion.

[0049] (2) Mix the PAA-grafted g-C3N4 dispersion obtained in step (1) with an organic solvent containing diamine, add acid anhydride, and react to obtain g-C3N4 / PAA slurry.

[0050] (3) The g-C3N4 / PAA slurry obtained in step (2) is coated on the substrate and dried to obtain a PAA composite film; then the PAA composite film is sintered and cooled to obtain a g-C3N4 / PI composite film.

[0051] (4) The g-C3N4 / PI composite film obtained in step (3) is subjected to three stages of heating, cooling and pressing to obtain a polyimide-based graphene film.

[0052] In step (1) of the method of this invention, nano-sized g-C3N4 is used to better disperse within the molecular chains of polyacrylamide (PAA), and its thinner size provides better planar orientation induction between the molecular chains. Nano-sized g-C3N4 can be obtained from graphitic carbon nitride using ultrasonic exfoliation. Further, the ultrasonic exfoliation power is 100–600 W, preferably 600 W. The ultrasonic time is 8–16 h. The solvent used for ultrasonic exfoliation is selected from N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. N,N-dimethylacetamide is preferred. The ultrasonic exfoliation is performed using a probe-type ultrasonic transducer. The nano-sized g-C3N4 has a size of 300–500 nm and a thickness of 3–5 nm.

[0053] In step (1) of the method of the present invention, since there is an amino group on the surface of g-C3N4, an acid anhydride is added first to react with the amino group, and then a diamine is added to react with the attached acid anhydride, thereby grafting PAA onto g-C3N4. Preferably, the interval between the addition of the acid anhydride and the diamine is 1 to 2 hours.

[0054] In step (1) of the method of the present invention, the PAA-modified g-C3N4 has better dispersibility in organic reagents, thereby resulting in a higher degree of two-dimensional orientation of the prepared polyamic acid film. Therefore, in step (1), the preferred mass ratio of g-C3N4, acid anhydride, and diamine is 25-50:80-150:80-160.

[0055] In step (2) of the method of the present invention, since the reaction between acid anhydride and diamine is relatively vigorous, in order to ensure the degree of polymerization, it is preferable to control the acid anhydride to be added in two parts. The interval is 0.5 to 2 hours.

[0056] In step (2) of the method of the present invention, in order to ensure the viscosity required for film coating, it is preferable to control the mass fraction of g-C3N4 / PAA in the g-C3N4 / PAA slurry to be 8-20%.

[0057] In the method of this invention, step (1) involves modifying graphitic carbon nitride nanosheets with PAA. The purpose is to induce the further growth of the newly added PAA raw material molecular chains in step (2), thereby forming more extended two-dimensional PAA main chains. During the coating process, the two-dimensional orientation of the molecular chains is induced, promoting imidization and carbonization cyclization between molecules. Step (1) is to grow PAA chains on the surface of g-C3N4 filler, and step (2) is to prepare a polyamic acid (PAA) matrix. Step (3) drying is to remove the solvent, and sintering is for imidization, converting PAA into polyimide. Step (4) the polyimide composite film is prepared into a highly crystalline graphene film by a slow temperature gradient isothermal method. In the first stage of gradient isothermal, the polyimide undergoes heteroatom cyclization, and the temperature is maintained at 200-400℃ higher than the imidization temperature for a period of time to ensure the slow removal of heteroatoms. In the second and third sections, under the high temperature gradient isothermal conditions, there are processes of ordered carbonization and graphitization of macromolecules. These two processes are respectively the process of rearranging carbon bonds in macromolecules to form a preliminary graphite structure and the process of repairing graphite defects at high temperatures.

[0058] The embodiments of the present invention will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer are followed. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0059] Example 1

[0060] (1) Preparation of nanoscale g-C3N4 nanosheets

[0061] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0062] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0063] 52.5 mg of g-C3N4 nanosheets were ultrasonically stirred in 10 g of N,N-dimethylacetamide for 2 h. Then, 123.9 mg of pyromellitic dianhydride was added and stirred for 2 h to fully attach the dianhydride monomer to the surface of the nanosheets. Then, 113.7 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0064] (3) Preparation of g-C3N4 / PI composite membrane

[0065] In an inert gas atmosphere reactor, 9.8 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved. Then, the dispersion obtained in step (2) was added and stirred for 30 min. Then, 10.90 g of pyromellitic dianhydride was added in two portions, one after the other at a 1-hour interval. The mixture was reacted for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.25%. This fraction means that g-C3N4 accounts for the mass fraction of g-C3N4 / PAA solute in the total mass. The same applies below. The film was coated on a clean glass plate with a thickness controlled at 0.5 mm. The film was dried at 80 °C for 4 h. Then, the temperature was increased to 280 °C at 5 °C / min and held for 1 h. Then, the temperature was increased to 300 °C at the same rate and held for 1 h. The film was then slowly cooled to obtain a g-C3N4 / PI composite film (polyimide composite film).

[0066] (4) Preparation of graphene film

[0067] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min and held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min and held at that temperature for 2 hours; then heated to 3000℃ at a rate of 5℃ / min and held at that temperature for 2 hours. The membrane was then slowly cooled to room temperature, and the sample was removed. After pressing at 300 MPa for 1 hour, the finished product was obtained, with a thickness of approximately 18 μm.

[0068] Example 2

[0069] (1) Preparation of nanoscale g-C3N4 nanosheets

[0070] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0071] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0072] 105 mg of g-C3N4 nanosheets were ultrasonically stirred in 10 g of N,N-dimethylacetamide for 2 h. Then, 247.8 mg of pyromellitic dianhydride was added and stirred for 2 h to fully attach the dianhydride monomer to the surface of the nanosheets. Then, 227.3 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0073] (3) Preparation of g-C3N4 / PI composite membrane

[0074] In an inert gas atmosphere reactor, 9.7 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 30 min. Next, 10.6 g of pyromellitic dianhydride was added in two portions, with a 1-hour interval between additions. The reaction was carried out for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. The slurry was then coated onto a clean glass plate. The film was dried at 80 °C for 4 h. Then, the temperature was increased to 280 °C at a rate of 5 °C / min and held for 1 h. The temperature was then increased to 300 °C at the same rate and held for 1 h. Finally, the film was slowly cooled to obtain the g-C3N4 / PI composite film.

[0075] (4) Preparation of graphene film

[0076] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min, held at that temperature for 2 hours; finally, heated to 3000℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then slowly cooled to room temperature. The sample was then removed. It was then pressed at 300 MPa for 1 hour to obtain the final product, which had a thickness of approximately 18 μm. The resulting graphene membrane is designated as Gg-C3N4 / PI-a.

[0077] To demonstrate the inductive effect of PAA-modified g-C3N4 on thick polyimide-based graphene films, we coated films of different thicknesses using the adhesive from Example 2, ultimately obtaining graphene films of 18 μm, 25 μm, 42 μm, and 60 μm. Specific thermal conductivity properties are as follows: Figure 5 As shown in b.

[0078] The morphology of g-C3N4 nanosheets before and after ultrasonic exfoliation was tested using a JSM-7500F scanning electron microscope. Figure 1 As can be seen, after strong ultrasonic ablation, g-C3N4 transforms from aggregated bulk material into nanosheets, with the size of the sheet-like graphitic carbon nitride being around 500 nm. Figure 2 The dispersion of g-C3N4 nanosheets was shown. It can be seen that the exfoliated and PAA-modified g-C3N4 nanosheets are stable in DMAC, which is beneficial for further initiating PAA backbone polymerization and promoting the dispersion of g-C3N4 in PAA. The thermal conductivity of the graphite film was tested using a Netzsch LFA467 laser thermal conductivity meter. Figure 4The thermal conductivity of graphite films prepared using different processes is shown. It can be seen that PAA-modified g-C3N4 effectively promotes the graphitization process of polyimide, with a 14.8% increase in thermal conductivity compared to the unmodified g-C3N4-induced polyimide-based graphene film (Comparative Example 2) (where the thermal conductivity obtained in Example 2 is 1797 W / mK, and the thermal conductivity of the graphene film obtained in Comparative Example 2 is 1562 W / mK). Furthermore, the changes in thermal conductivity of polyimide-based graphite films prepared with PAA-modified g-C3N4 at different filler contents are shown below. Figure 5 As shown, g-C3N4 promotes the graphitization of PI even with small amounts of filler. However, due to the heterogeneity of the nanosheets, the induced graphitization effect is best at a mass fraction of 0.5%, resulting in higher thermal conductivity. Higher filler concentrations show a decreasing trend. Furthermore, the graphite film prepared at this content maintains high thermal conductivity at a certain thickness, which is 52.6% higher than that of pure PI-based graphene film (Comparative Example 4) at the same thickness of approximately 60 μm.

[0079] Example 3

[0080] (1) Preparation of nanoscale g-C3N4 nanosheets

[0081] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0082] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0083] 157.5 mg of g-C3N4 nanosheets were ultrasonically stirred in 10 g of N,N-dimethylacetamide for 2 h. Then, 255 mg of pyromellitic dianhydride was added and stirred for 2 h to fully attach the dianhydride monomer to the surface of the nanosheets. Then, 341 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0084] (3) Preparation of g-C3N4 / PI composite membrane

[0085] In an inert gas atmosphere reactor, 9.5 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 30 min. Next, 10.4 g of pyromellitic dianhydride was added in two portions, one after the other at a 1-hour interval. The reaction was carried out for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.75%. The slurry was coated onto a clean glass plate, with the coating thickness controlled at 0.5 mm. The film was dried at 80 °C for 4 h. Then, the temperature was increased to 280 °C at a rate of 5 °C / min and held for 1 h. The temperature was then increased to 300 °C at the same rate and held for 1 h. The film was then slowly cooled to obtain the g-C3N4 / PI composite film.

[0086] (4) Preparation of graphene film

[0087] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min and held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min and held at that temperature for 2 hours; then heated to 3000℃ at a rate of 5℃ / min and held at that temperature for 2 hours. The membrane was then slowly cooled to room temperature, and the sample was removed. After pressing at 300 MPa for 1 hour, the finished product was obtained, with a membrane thickness of approximately 18 μm.

[0088] Example 4

[0089] (1) Preparation of nanoscale g-C3N4 nanosheets

[0090] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0091] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0092] 210 mg of g-C3N4 nanosheets were ultrasonically stirred in 10 g of N,N-dimethylacetamide for 2 h. Then, 340.5 mg of pyromellitic dianhydride was added and stirred for 2 h to fully attach the dianhydride monomer to the surface of the nanosheets. Then, 454.6 mg of diaminodiphenyl ether was added and stirred for 2 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0093] (3) Preparation of g-C3N4 / PI composite membrane

[0094] In an inert gas atmosphere reactor, 9.3 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 30 min. Next, 10.1 g of pyromellitic dianhydride was added in two portions, with a 1-hour interval between additions. The reaction was carried out for 24 h to obtain a composite slurry with a g-C3N4 mass content of 1%. The slurry was coated onto a clean glass plate, with the coating thickness controlled at 0.5 mm. The film was dried at 80 °C for 4 h. Then, the temperature was increased to 280 °C at a rate of 5 °C / min and held for 1 h. The temperature was then increased to 300 °C at the same rate and held for 1 h. The film was then slowly cooled to obtain the g-C3N4 / PI composite film.

[0095] (4) Preparation of graphene film

[0096] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min and held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min and held at that temperature for 2 hours; then heated to 3000℃ at a rate of 5℃ / min and held at that temperature for 2 hours. The membrane was then slowly cooled to room temperature, and the sample was removed. After pressing at 300 MPa for 1 hour, the finished product was obtained, with a membrane thickness of approximately 18 μm.

[0097] Example 5

[0098] (1) Preparation of nanoscale g-C3N4 nanosheets

[0099] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0100] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0101] 105 mg of g-C3N4 nanosheets were ultrasonically stirred in 10 g of N,N-dimethylacetamide for 2 h. Then, 334.2 mg of 4,4'-diphthalic anhydride was added and stirred for 2 h to allow the dianhydride monomer to be fully attached to the surface of the nanosheets. Then, 227.3 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0102] (3) Preparation of g-C3N4 / PI composite membrane

[0103] In an inert gas atmosphere reactor, 9.8 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 30 min. Next, 14.7 g of 4,4'-phthalic anhydride was added in two portions, with a 1-hour interval between additions. The reaction was carried out for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. The slurry was coated onto a clean glass plate, with the coating thickness controlled at 0.5 mm. The film was dried at 80 °C for 4 h. Then, the temperature was increased to 280 °C at a rate of 5 °C / min and held for 1 h. The temperature was then increased to 300 °C at the same rate and held for 1 h. The film was then slowly cooled to obtain the g-C3N4 / PI composite film.

[0104] (4) Preparation of graphene film

[0105] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min, held at that temperature for 2 hours; finally, heated to 3000℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then slowly cooled to room temperature. The sample was then removed. After pressing at 300 MPa for 1 hour, the final product was obtained, with a membrane thickness of approximately 18 μm. The obtained graphene membrane is designated as Gg-C3N4 / PI-b.

[0106] Example 6

[0107] (1) Preparation of nanoscale g-C3N4 nanosheets

[0108] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0109] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0110] 105 mg of g-C3N4 nanosheets were ultrasonically stirred in 10 g of N,N-dimethylacetamide for 2 h. Then, 352.4 mg of 4,4-oxophthalic anhydride was added and stirred for 2 h to allow the dianhydride monomer to be fully attached to the surface of the nanosheets. Then, 227.3 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0111] (3) Preparation of g-C3N4 / PI composite membrane

[0112] In an inert gas atmosphere reactor, 6.53 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 30 min. Next, 10.3 g of 4,4-oxophthalic anhydride was added in two portions, with a 1-hour interval between additions. The reaction was carried out for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. The slurry was coated onto a clean glass plate, with the coating thickness controlled at 0.5 mm. The film was dried at 80 °C for 4 h. Then, the temperature was increased to 280 °C at a rate of 5 °C / min and held for 1 h. The temperature was then increased to 300 °C at the same rate and held for 1 h. The film was then slowly cooled to obtain the g-C3N4 / PI composite film.

[0113] (4) Preparation of graphene film

[0114] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min, held at that temperature for 2 hours; finally, heated to 3000℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then slowly cooled to room temperature. The sample was then removed. It was then pressed at 300 MPa for 1 hour to obtain the final product, which had a thickness of approximately 18 μm. The resulting graphene membrane is denoted as Gg-C3N4 / PI-c.

[0115] Example 7

[0116] (1) Preparation of nanoscale g-C3N4 nanosheets

[0117] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0118] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0119] 105 mg of g-C3N4 nanosheets were ultrasonically stirred in 10 g of N,N-dimethylacetamide for 2 h. Then, 352.4 mg of 4,4-oxophthalic anhydride was added and stirred for 2 h to allow the dianhydride monomer to be fully attached to the surface of the nanosheets. Then, 210 mg of p-phenylenediamine was added and stirred for 4 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0120] (3) Preparation of g-C3N4 / PI composite membrane

[0121] In an inert gas atmosphere reactor, 5.29 g of p-phenylenediamine was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the p-phenylenediamine was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 30 min. Next, 15.5 g of 4,4-oxophthalic anhydride was added in two portions, with a 1-hour interval between additions. The reaction was carried out for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. The slurry was coated onto a clean glass plate, with the coating thickness controlled at 0.5 mm. The film was dried at 80 °C for 4 h. Then, the temperature was increased to 280 °C at a rate of 5 °C / min and held for 1 h. The temperature was then increased to 300 °C at the same rate and held for 1 h. The film was then slowly cooled to obtain the g-C3N4 / PI composite film.

[0122] (4) Preparation of graphene film

[0123] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min, held at that temperature for 2 hours; finally, heated to 3000℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then slowly cooled to room temperature. The sample was then removed. After pressing at 300 MPa for 1 hour, the final product was obtained, with a membrane thickness of approximately 18 μm. The obtained graphene membrane is denoted as Gg-C3N4 / PI-d.

[0124] Example 8

[0125] (1) Preparation of nanoscale g-C3N4 nanosheets

[0126] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0127] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0128] 105 mg of g-C3N4 nanosheets were ultrasonically stirred in 10 g of N,N-dimethylacetamide for 2 h. Then, 247.8 mg of pyromellitic dianhydride was added and stirred for 2 h to fully attach the dianhydride monomer to the surface of the nanosheets. Then, 210 mg of p-phenylenediamine was added and stirred for 4 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0129] (3) Preparation of g-C3N4 / PI composite membrane

[0130] In an inert gas atmosphere reactor, 5.29 g of p-phenylenediamine was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the p-phenylenediamine was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 30 min. Next, 10.9 g of pyromellitic dianhydride was added in two portions, with a 1-hour interval between additions. The reaction was carried out for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. The slurry was coated onto a clean glass plate, with the coating thickness controlled at 0.5 mm. The film was dried at 80 °C for 4 h. Then, the temperature was increased to 280 °C at a rate of 5 °C / min and held for 1 h, followed by a further increase to 300 °C at the same rate and held for 1 h. Slow cooling was then performed to obtain the g-C3N4 / PI composite film.

[0131] (4) Preparation of graphene film

[0132] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min, held at that temperature for 2 hours; finally, heated to 3000℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then slowly cooled to room temperature. The sample was then removed. After pressing at 300 MPa for 1 hour, the final product was obtained, with a membrane thickness of approximately 18 μm. The resulting graphene membrane is designated as Gg-C3N4 / PI-e.

[0133] Comparative Example 1

[0134] (1) Preparation of nanoscale g-C3N4 nanosheets

[0135] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0136] (2) Preparation of PAA-modified g-C3N4 nanosheets

[0137] 105 mg g-C3N4 was ultrasonically stirred in 10 g N,N-dimethylacetamide for 2 h, then 247.8 mg pyromellitic dianhydride was added and stirred for 2 h to fully attach the dianhydride monomer to the surface of the nanosheets. Then 227.3 mg diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified g-C3N4 nanosheet N,N-dimethylacetamide dispersion.

[0138] (3) Preparation of g-C3N4 / PI composite membrane

[0139] In an inert gas atmosphere reactor, 9.8 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved, and then 10.90 g of pyromellitic dianhydride was added in two portions, one after the other, with a 1-hour interval between additions. After reacting for 24 hours, a PAA slurry was obtained. The dispersion obtained in step (2) was then added to this slurry, and the mixture was stirred for 10 minutes to obtain a composite slurry with a g-C3N4 mass content of 0.5%. The slurry was then coated onto a clean glass plate, with the coating thickness controlled at 0.5 mm. The film was dried at 80°C for 4 hours. Then, the temperature was increased to 280°C at a rate of 5°C / min and held for 1 hour, followed by a further increase to 300°C at the same rate and held for 1 hour. The film was then slowly cooled to obtain a g-C3N4 / PI composite film.

[0140] (4) Preparation of graphite film

[0141] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 3000℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and slowly cooled to room temperature. The sample was then removed. It was then pressed at 300 MPa for 1 hour to obtain the final product, which had a thickness of approximately 18 μm. The obtained graphene membrane is denoted as Gg-C3N4 / PI-f.

[0142] Comparative Example 2

[0143] (1) Preparation of nanoscale g-C3N4 nanosheets

[0144] 10g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500mL of N,N-dimethylacetamide solution, and sonicated at 600W for 8h in a high-power cell sonicator. After standing for 18h, the upper dispersion was filtered, washed, and dried to obtain g-C3N4 nanosheets.

[0145] (2) Preparation of g-C3N4 nanosheet dispersion

[0146] Take 105 mg g-C3N4 nanosheets and ultrasonically stir them in 10 g N,N-dimethylacetamide for 2 h.

[0147] (3) Preparation of g-C3N4 / PI composite membrane

[0148] In an inert gas atmosphere reactor, 10g of diaminodiphenyl ether was added to 108g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 10 minutes. Next, 11.12g of pyromellitic dianhydride was added, divided into two portions, and added to the reactor one hour apart. The dispersion obtained in step (2) was then added to the mixture, and the mixture was stirred for 10 minutes to obtain a composite slurry with a g-C3N4 mass content of 0.5%. This slurry was then coated onto a clean glass plate. The film was dried at 80℃ for 4 hours, then heated to 280℃ at a rate of 5℃ / min and held at that temperature for 1 hour. The temperature was then increased to 300℃ at the same rate and held at that temperature for 1 hour. Finally, the film was slowly cooled to obtain the g-C3N4 / PI composite film.

[0149] (4) Preparation of graphite film

[0150] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min, held at that temperature for 2 hours; finally, heated to 3000℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then slowly cooled to room temperature. The sample was then removed. After pressing at 300 MPa for 1 hour, the final product was obtained, with a membrane thickness of approximately 18 μm. The obtained graphene membrane is denoted as Gg-C3N4 / PI-g.

[0151] To compare the thickness of polyimide-based graphene films with that of PAA-modified g-C3N4, we coated films of different thicknesses using the adhesive in Comparative Example 2, ultimately obtaining graphene films with thicknesses of 18 μm, 26 μm, 41 μm, and 58 μm. Specific thermal conductivity properties are as follows: Figure 5 As shown in b.

[0152] Unmodified carbon nitride nanosheets, due to their inherent two-dimensional rigidity, still induce carbonization of PI. However, compared to PI-based graphene films prepared by in-situ doping of PAA-modified carbon nitride nanosheets, the inductive effect in the thickness direction tends to decrease. Figure 5 As shown, this may be related to its dispersion.

[0153] Comparative Example 3

[0154] (1) Preparation of coarse-grained g-C3N4

[0155] 10g of coarse g-C3N4 particles prepared by thermal polymerization of melamine were crushed and added to 500mL of N,N-dimethylacetamide solution. The mixture was then placed in a high-power cell sonicator and sonicated at 100W for 1h. After filtration, washing, and drying, g-C3N4 powder was obtained.

[0156] (2) Preparation of g-C3N4 nanosheet dispersion

[0157] 105 mg g-C3N4 was ultrasonically stirred in 10 g N,N-dimethylacetamide for 2 h, then 247.8 mg pyromellitic dianhydride was added and stirred for 2 h to fully attach the dianhydride monomer to the surface of the nanosheets. Then 227.3 mg diaminodiphenyl ether was added and stirred for 4 h to obtain a dispersion of PAA-modified g-C3N4 coarse particles in N,N-dimethylacetamide.

[0158] (3) Preparation of g-C3N4 / PI composite membrane

[0159] In an inert gas atmosphere reactor, 10g of diaminodiphenyl ether was added to 108g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved. Then, the dispersion obtained in step (2) was added, and the mixture was stirred for 10 minutes. Next, 11.12g of pyromellitic dianhydride was added in two portions, with a 1-hour interval between additions, to the reactor. The resulting composite slurry was coated onto a clean glass plate, with the coating thickness controlled at 0.5mm. The film was dried at 80℃ for 4 hours, then heated to 280℃ at a rate of 5℃ / min and held at that temperature for 1 hour. The temperature was then increased to 300℃ at the same rate and held at that temperature for 1 hour. Finally, the film was slowly cooled to obtain the g-C3N4 / PI composite membrane.

[0160] (4) Preparation of graphite film

[0161] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 1500℃ at a rate of 1℃ / min, held at that temperature for 2 hours; then heated to 3000℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and slowly cooled to room temperature. The sample was then removed. It was then pressed at 300 MPa for 1 hour to obtain the final product, which had a thickness of approximately 18 μm. The obtained graphene membrane is denoted as Gg-C3N4 / PI-h.

[0162] Comparative Example 4

[0163] (1) Preparation of PI film

[0164] In an inert gas atmosphere reactor, 10 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred until the diaminodiphenyl ether was completely dissolved, and stirred for 10 min. Then, 11.12 g of pyromellitic dianhydride was added in two portions, with a 1-hour interval between additions, to the reactor to obtain a composite slurry. This slurry was then coated onto a clean glass plate. The film was dried at 80°C for 4 h, then heated to 280°C at a rate of 5°C / min and held at that temperature for 1 h. The temperature was then increased to 300°C at the same rate and held at that temperature for 1 h, followed by slow cooling to obtain a polyimide film.

[0165] (2) Preparation of graphite film

[0166] The composite membrane was placed in a graphite furnace and heated to 700℃ at a rate of 1℃ / min, then held at that temperature for 2 hours; subsequently, the temperature was increased to 1500℃ at a rate of 1℃ / min and held for 2 hours; then, the temperature was increased to 3000℃ at a rate of 5℃ / min and held for 2 hours. The membrane was then slowly cooled to room temperature, and the sample was removed. It was then pressed at 300 MPa for 1 hour to obtain the final product. The resulting graphene membrane is designated G-PI.

[0167] To compare the thickness of polyimide-based graphene films with that of PAA-modified g-C3N4, we coated films of different thicknesses using the adhesive in Comparative Example 4, ultimately obtaining graphene films with thicknesses of 23 μm, 26 μm, 40 μm, and 60 μm. Specific thermal conductivity properties are as follows: Figure 5 As shown in b.

[0168] In the thermal conductivity test, from Figure 4 As can be seen, the degree of graphitization and thermal conductivity of unexfoliated g-C3N4 (Comparative Example 3) and exfoliated unmodified g-C3N4 (Comparative Example 2) are somewhat lower than those of PAA-modified g-C3N4, but the g-C3N4 nanosheets as a whole promote the graphitization of PI. The graphitized graphite film was tested using a Rigaku Ultima IV diffractometer X-ray diffractometer, and regular (002) crystal plane diffraction peaks were obtained. Figure 6 As shown, the interlayer spacing of graphite lamellars was calculated using the Bragg equation: 2dsinθ=nλ. Then, the degree of graphitization of the graphite film was calculated using the empirical formula: γ=((0.3440-d(002)) / 0.0086)*100%. The X-ray diffraction pattern shows that PAA-modified g-C3N4 has a significant promoting effect on the graphitization of PI, with a sharp peak at 26.54°. The degree of graphitization was increased by 3.34%. This more intuitively illustrates the promoting effect of PAA-modified g-C3N4 on the graphitization of PI.

Claims

1. A method for preparing a polyimide-based graphene film, characterized in that: Includes the following steps: (1) An acid anhydride and a diamine were added to a nano-sized g-C3N4 dispersion to react and obtain a PAA-grafted g-C3N4 dispersion; wherein the mass ratio of g-C3N4, acid anhydride and diamine was 25~50 : 80~150 : 80~160. (2) Mix the PAA-grafted g-C3N4 dispersion obtained in step (1) with an organic solvent containing diamine, add acid anhydride, and react to obtain g-C3N4 / PAA slurry; wherein the mass ratio of g-C3N4, diamine and acid anhydride is 0.5~3 : 100~150 : 110~160; (3) The g-C3N4 / PAA slurry obtained in step (2) is coated on the substrate and dried to obtain a PAA composite film; then the PAA composite film is sintered and cooled to obtain a g-C3N4 / PI composite film. (4) The g-C3N4 / PI composite film obtained in step (3) is subjected to three stages of heating, cooling and pressing to obtain a polyimide-based graphene film.

2. The method for preparing the polyimide-based graphene film according to claim 1, characterized in that: In step (1), the size of the nanoscale g-C3N4 is 300~500 nm and the thickness is 3~5 nm.

3. The method for preparing the polyimide-based graphene film according to claim 1 or 2, characterized in that: In step (1), the organic solvent used in the nano-scale g-C3N4 dispersion is selected from any one or a combination of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

4. The method for preparing the polyimide-based graphene film according to claim 3, characterized in that: The organic solvent is N,N-dimethylformamide or N,N-dimethylacetamide.

5. The method for preparing the polyimide-based graphene film according to claim 4, characterized in that: The organic solvent is N,N-dimethylacetamide.

6. The method for preparing the polyimide-based graphene film according to claim 1 or 2, characterized in that: In step (1), the acid anhydride is selected from any one or a combination of pyromellitic dianhydride, 4,4-oxobisphthalic anhydride, and 4,4'-diphthalic anhydride; the diamine is selected from any one or a combination of two of diaminodiphenyl ether and p-phenylenediamine.

7. The method for preparing the polyimide-based graphene film according to claim 1 or 2, characterized in that: In step (1), the order is 1 to 2 hours apart.

8. The method for preparing the polyimide-based graphene film according to claim 1 or 2, characterized in that: In step (1), the reaction time is 2 to 8 hours.

9. The method for preparing the polyimide-based graphene film according to claim 1 or 2, characterized in that: In step (2), the acid anhydride is selected from any one or a combination of pyromellitic dianhydride, 4,4-oxobisphthalic anhydride, and 4,4'-diphthalic anhydride; the diamine is selected from any one or a combination of two of diaminodiphenyl ether and p-phenylenediamine.

10. The method for preparing the polyimide-based graphene film according to claim 1 or 2, characterized in that: In step (2), at least one of the following conditions must be met: The reaction is carried out in an inert atmosphere; The PAA-grafted g-C3N4 dispersion was mixed with an organic solvent containing diamine for 30 min-1 h before adding acid anhydride. The acid anhydride was added in two portions, with an interval of 0.5 to 2 hours. The reaction time is 12-24 h; The mass fraction of g-C3N4 / PAA in the g-C3N4 / PAA slurry is 8~20%.

11. The method for preparing the polyimide-based graphene film according to claim 10, characterized in that: In step (2), the organic solvent for dissolving the diamine is selected from any one or a combination of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide.

12. The method for preparing the polyimide-based graphene film according to claim 1 or 2, characterized in that: In step (3), the sintering is to raise the temperature to 260-280℃ at 1-5℃ / min and hold it at 260-280℃ for 1-3 hours, and then raise the temperature to 300-320℃ at 1-5℃ / min and hold it at 300-320℃ for 1-3 hours.

13. The method for preparing the polyimide-based graphene film according to claim 1 or 2, characterized in that: In step (4), the three-stage heating is as follows: First stage: heating at 1~5℃ / min to 500~700℃ and holding at that temperature for 0.5~2 h; Second stage: Increase the temperature at 1~3℃ / min to 1450~1550℃ and hold for 0.5~2 h; Third stage: Increase the temperature at 3~5℃ / min to 2950~3050℃ and hold for 0.5~2 h.

14. The method for preparing the polyimide-based graphene film according to claim 13, characterized in that: In step (4), the second section is heated to 1500℃ at a rate of 1~3℃ / min and held at that temperature for 0.5~2 h.

15. The method for preparing the polyimide-based graphene film according to claim 13, characterized in that: In step (4), the third section is heated to 3000℃ at a rate of 3~5℃ / min and held at that temperature for 0.5~2 h.

16. A polyimide-based graphene film prepared by the method for preparing a polyimide-based graphene film according to any one of claims 1-15.

17. The polyimide-based graphene film according to claim 16, characterized in that: The thickness of the polyimide-based graphene film is 10~60μm.

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