Polyimide-based graphene film and preparation method thereof

Abstract

The invention belongs to the field of graphene film materials, and particularly relates to a polyimide-based graphene film and a preparation method thereof. According to the preparation method, the PAA modified g-C3N4 is adopted as an initiation raw material to prepare the polyimide-based graphene film with medium thickness, and the polyimide-based graphene film has relatively high thermal conductivity.

Description

Technical Field

[0001] The invention belongs to the field of graphene film materials, and in particular relates to a polyimide-based graphene film induced by two-dimensional graphite phase carbon nitride and a preparation method thereof. Background Art

[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 is 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 film has a wide adjustable range of heat transfer. According to the selection of raw materials and the carbonization process, graphite films with different properties can be prepared. The in-plane thermal conductivity of the graphite film material determines the heat dissipation capacity of the film material to the greatest extent. The in-plane thermal conductivity of traditional graphite films ranges from 200W / m K to 2000W / m K, but in graphite film materials with a thickness greater than 20 microns, heat transfer is usually limited to about 1500W / m K. At present, there are two main types of traditional methods for preparing high thermal conductivity graphite films. One is to prepare high thermal conductivity graphite films by carbonization of inorganic graphene oxide, and the other is to prepare high thermal conductivity graphite films by carbonization of organic polyimide. The preparation of graphite films by inorganic graphene oxide requires very precise processing technology. Graphene oxide itself contains a large number of oxygen-containing functional groups. The presence of these functional groups, on the one hand, ensures the orderly overlap of the carbonized precursor sheets, but on the other hand, the small molecules generated by a large number of oxygen-containing functional groups at high temperatures will break through the intrinsic rigid molecular layer of graphite materials to a certain extent, which will further cause defects. Therefore, graphene oxide films require more precise carbonization processes to reduce such defects. At the same time, because of its intrinsic rigid sheets, graphene oxide is easier to process into graphite film materials within a certain thickness range.

[0004] Polyimide (PI) is a typical representative of aromatic polyamides that can be carbonized and graphitized. Polyimides, led by Kapton, have been widely used as mature industrial products. Its main characteristic is that the orderliness of the carbonization process of polyimide gives it a high degree of carbonization and graphitization: the imide ring is effectively attacked and cyclized as a two-dimensional active site, and the polyimide is also effectively cyclized while removing heteroatoms, which makes the prepared graphene have a higher crystal size. However, the degree of planar orientation of polyimide is limited by the orientation degree of the precursor polyamic acid (PAA). Traditional scraping and casting processes are difficult to prepare 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 the orientation of membrane molecules is an important means. There are literature reports on the introduction of two-dimensional graphene oxide nanosheets into organic precursors to increase their carbonization and graphitization. For example, Gao Chao's team reported the introduction of graphene oxide nanosheets into polyacrylonitrile fibers and membrane materials to increase their orientation and carbonization 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 be used as an induction factor, but its rigid structure and compatibility with organic precursors and mismatch problems will also affect the intrinsic cyclization structure of the organic precursor. Usually, two-dimensional nanomaterials require high filling to achieve a good auxiliary carbonization effect.

[0005] At present, there are no reports at home and abroad on the use of low-filled two-dimensional non-graphite nanofillers to induce aromatic organic precursors to prepare medium-thickness graphite films. Summary of the invention

[0006] The purpose of the present invention is to provide a polyimide-based graphene film prepared by induction of graphite phase carbon nitride (g-C3N4) modified with polyamic acid (PAA), which has greatly improved thermal conductivity.

[0007] The technical solution of the present invention:

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

[0009] (1) adding anhydride and diamine to the nano-scale g-C3N4 dispersion to react, thereby obtaining a PAA-grafted g-C3N4 dispersion;

[0010] (2) uniformly mixing the PAA-grafted g-C3N4 dispersion obtained in step (1) and an organic solvent containing a diamine, adding anhydride, and reacting to obtain a g-C3N4 / PAA slurry;

[0011] (3) coating the g-C3N4 / PAA slurry obtained in step (2) on a substrate, and drying to obtain a PAA composite film; and then sintering and cooling the PAA composite film 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 preparation method of the above-mentioned polyimide-based graphene film, in step (1), the nanoscale g-C3N4 is obtained by ultrasonic exfoliation using graphite phase carbon nitride (g-C3N4) as a raw material. Furthermore, the power of the ultrasound is 100 to 600 W. The preferred power is 600 W. The ultrasound time is 8 to 16 hours. The solvent used in the ultrasound is selected from any one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. The preferred solvent is N,N-dimethylacetamide (DMAC). The ultrasound uses probe ultrasound.

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

[0015] Preferably, in the method for preparing the polyimide-based graphene film, 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 the polyimide-based graphene film, in step (1), the acid anhydride is selected from any one or more combinations of pyromellitic dianhydride (PMDA), 4,4-oxydiphthalic anhydride (ODPA), and 4,4'-diphthalic anhydride (BPDA).

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

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

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

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

[0021] Preferably, in the above-mentioned method for preparing the 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 the polyimide-based graphene film, in step (2), the acid anhydride is selected from any one or more combinations of pyromellitic dianhydride (PMDA), 4,4-oxydiphthalic anhydride (ODPA), and 4,4'-diphthalic anhydride (BPDA).

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

[0024] Preferably, in the above-mentioned method for preparing the 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 method for preparing the above-mentioned polyimide-based graphene film, in step (2), the PAA-grafted g-C3N4 dispersion and the organic solvent containing the diamine are mixed for 30 minutes to 1 hour before adding the anhydride.

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

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

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

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

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

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

[0032] Preferably, in the preparation method of the above-mentioned polyimide-based graphene film, in step (3), the sintering is to increase the temperature to 260-280°C at a rate of 1-5°C / min and keep the temperature constant for 1-3h, and then increase the temperature to 300-320°C at a rate of 1-5°C / min and keep the temperature constant for 1-3h.

[0033] Preferably, in the preparation method of the above-mentioned polyimide-based graphene film, in step (4), the three stages of heating are: first stage: heating to 500-700°C at 1-5°C / min and keeping the temperature constant for 0.5-2h; second stage: heating to 1450-1550°C (preferably 1500°C) at 1-3°C / min and keeping the temperature constant for 0.5-2h; third stage: heating to 2950-3050°C (preferably 3000°C) at 3-5°C / min and keeping the temperature constant for 0.5-2h.

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

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

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

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

[0038] The present invention uses PAA-modified g-C3N4 nanosheets to assist in preparing a thick and highly oriented graphene film. The prepared thick polyimide-based film has a higher molecular chain orientation in the thickness direction, so that the sintered graphene film obtains better graphite microcrystal growth in the thickness direction. The thermal conductivity of the 18μm thick graphene film is 1797W / m K, and the thermal conductivity of the 58μm thick single film is 1557W / m K, which effectively improves the thermal conductivity of the polyimide-based graphene film in preparing a single thick film. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 The scanning electron microscope morphology images of the graphite phase carbon nitride obtained in Example 1 of the present invention before and after ultrasonic exfoliation. a) Graphite phase carbon nitride coarse particles; b) exfoliated graphite phase carbon nitride nanosheets; c) nanosheet particle size distribution.

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

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

[0042] Figure 4 Comparison of thermal conductivity of graphene films obtained in Example 2 and the comparative example.

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

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

[0045] The invention provides a polyimide-based graphene film induced by graphite phase carbon nitride and a preparation method thereof, wherein the method effectively improves the thermal conductivity of the polyimide-based graphene film.

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

[0047] Specifically, the method for preparing the polyimide-based graphene film of the present invention comprises the following steps:

[0048] (1) adding anhydride and diamine to the nano-scale g-C3N4 dispersion to react, thereby obtaining a PAA-grafted g-C3N4 dispersion;

[0049] (2) uniformly mixing the PAA-grafted g-C3N4 dispersion obtained in step (1) and an organic solvent containing a diamine, adding anhydride, and reacting to obtain a g-C3N4 / PAA slurry;

[0050] (3) coating the g-C3N4 / PAA slurry obtained in step (2) on a substrate, and drying to obtain a PAA composite film; and then sintering and cooling the PAA composite film 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 the present invention, nano-scale g-C3N4 is used to better disperse between the molecular chains of polyamino acid (PAA), and the thinner size can provide a better plane-induced orientation effect between the molecular chains. Nano-scale g-C3N4 can be obtained by ultrasonic stripping using graphite phase carbon nitride as a raw material. Furthermore, the ultrasonic stripping power is 100 to 600 W. The preferred power is 600 W. The ultrasonic time is 8 to 16 hours. The solvent used in the ultrasound is selected from one of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone, and dimethyl sulfoxide. The preferred solvent is N,N-dimethylacetamide. The ultrasound uses probe ultrasound. The size of the nano-scale g-C3N4 is 300 to 500 nm and the thickness is 3 to 5 nm.

[0053] In step (1) of the method of the present invention, since there are amino groups on the surface of g-C3N4, anhydride is first added to react with the amino groups, and then diamine is added to react with the grafted anhydride, so that g-C3N4 is grafted with PAA. Preferably, the interval between the addition of anhydride and diamine is 1 to 2 hours.

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

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

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

[0057] In the method of the present invention, step (1) is to modify graphite phase carbon nitride nanosheets with PAA, the purpose of which is to induce further growth of the newly added PAA raw material molecular chain in step (2), thereby forming more PAA main chains that grow in two dimensions, and inducing the two-dimensional orientation of the molecular chain during the coating process, thereby promoting the 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 to imidize, so that PAA is converted into polyimide. Step (4) polyimide composite film is prepared by slowly heating gradient constant temperature to prepare a high-crystallinity graphene film. Under the first gradient constant temperature, it is the process of polyimide removing heteroatoms and cyclizing, and the temperature is maintained at 200 to 400° C. above the imidization temperature for a period of time to ensure the slow removal of heteroatoms. In the second and third stages, under the constant temperature of high temperature gradient, there is the process of orderly carbonization and graphitization of macromolecules. These two processes are respectively the process of rearrangement of carbon bonds of macromolecules to form preliminary graphite structure and the process of repairing graphite defects at high temperature.

[0058] The embodiments of the present invention will be described in detail below in conjunction with the examples, but it will be appreciated by those skilled in the art that the following examples are only used to illustrate the present invention and should not be construed as limiting the scope of the present invention. If specific conditions are not specified in the examples, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0059] Example 1

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

[0061] 10 g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500 mL of N,N-dimethylacetamide solution, placed in a high-power cell sonicator at 600 W for 8 h, allowed to stand for 18 h, 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 placed in 10 g of N,N-dimethylacetamide and ultrasonically stirred for 2 h. Then 123.9 mg of pyromellitic dianhydride was added and stirred for 2 h to allow the surface of the nanosheets to be fully connected with the dianhydride monomer. Then 113.7 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified N,N-dimethylacetamide dispersion of g-C3N4 nanosheets.

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

[0065] In a reactor in an inert gas atmosphere, 9.8 g of diaminodiphenyl ether was added to 108 g of N,N-dimethylacetamide. The mixture was stirred evenly until the diaminodiphenyl ether was completely dissolved, and then the dispersion obtained in step (2) was added, stirred for 30 min, and then 10.90 g of pyromellitic anhydride was added in two portions, added to the reactor at an interval of 1 h, and reacted for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.25%, where the fraction means the mass fraction of g-C3N4 in the total g-C3N4 / PAA solute, the same below. The film was scraped on a clean glass plate, and the scraping thickness was controlled to be 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 kept at this temperature for 1 h, and then the temperature was increased to 300 ° C at the same heating rate and kept at this temperature for 1 h, and then slowly cooled to obtain a g-C3N4 / PI composite film (polyimide composite film).

[0066] (4) Preparation of graphene film

[0067] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min and kept at this temperature for 2 hours; then heated to 1500°C at 1°C / min and kept at this temperature for 2 hours; then heated to 3000°C at 5°C / min and kept at this temperature for 2 hours, then slowly cooled to room temperature and the sample was taken out. The finished product was then pressed at 300Mpa for 1 hour, and the thickness of the finished product was about 18μm.

[0068] Example 2

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

[0070] 10 g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500 mL of N,N-dimethylacetamide solution, placed in a high-power cell sonicator at 600 W for 8 h, allowed to stand for 18 h, 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 placed in 10 g of N,N-dimethylacetamide and ultrasonically stirred for 2 h. Then 247.8 mg of pyromellitic dianhydride was added and stirred for 2 h to allow the surface of the nanosheets to be fully connected with the dianhydride monomer. Then 227.3 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified N,N-dimethylacetamide dispersion of g-C3N4 nanosheets.

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

[0074] In a reactor under an inert gas atmosphere, add 9.7 g of diaminodiphenyl ether to 108 g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, then add the dispersion obtained in step (2), stir for 30 min, then add 10.6 g of pyromellitic anhydride, add two portions, add them to the reactor at an interval of 1 h, react for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. Apply on a clean glass plate. The film is dried at 80°C for 4 h. Then heat to 280°C at 5°C / min and keep at this temperature for 1 h, then heat to 300°C at the same heating rate and keep at this temperature for 1 h, and slowly cool to obtain a g-C3N4 / PI composite film.

[0075] (4) Preparation of graphene film

[0076] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min for 2 hours; then heated to 1500°C at 1°C / min for 2 hours; then heated to 3000°C at 5°C / min for 2 hours, slowly cooled to room temperature, and the sample was taken out. After pressing at 300Mpa for 1 hour, the finished product was obtained, and the thickness of the finished product was about 18μm. The obtained graphene film was recorded as Gg-C3N4 / PI-a.

[0077] In order to reflect the inductive effect of PAA-modified g-C3N4 on the thickness of polyimide-based graphene film, we scraped the rubber in Example 2 into films of different thicknesses, and finally prepared graphene films of 18μm, 25μm, 42μm, and 60μm. The specific thermal conductivity is as follows: Figure 5 As shown in b.

[0078] The morphology of g-C3N4 nanosheets before and after ultrasonic exfoliation was measured using a JSM-7500F scanning electron microscope. Figure 1 It can be seen that after strong ultrasonic exfoliation, g-C3N4 is transformed from agglomerated blocks into nanosheets, and the size of the flaky graphite phase carbon nitride is about 500nm. Figure 2 The dispersion of g-C3N4 nanosheets is shown. It can be seen that g-C3N4 after exfoliation and PAA modification can be stably present in DMAC, which is conducive to further initiating the polymerization of the PAA main chain and promoting the dispersion of g-C3N4 in PAA. The thermal conductivity of the graphite film was tested using the NETZSCH LFA467 laser thermal conductivity meter. Figure 4The thermal conductivity of graphite films prepared under different preparation processes is shown. It can be seen that g-C3N4 modified with PAA can effectively promote the graphitization process of polyimide, and the thermal conductivity is improved by 14.8% compared with the unmodified g-C3N4-induced polyimide-based graphene film (Comparative Example 2) (where the thermal conductivity obtained in Example 2 is 1797W / mK, and the thermal conductivity of the graphene film obtained in Comparative Example 2 is 1562W / m K). At the same time, the thermal conductivity of the polyimide-based graphite film prepared by g-C3N4 modified with PAA at different filling contents is shown in the figure below. Figure 5 As shown. It can be seen that g-C3N4 has a certain promoting effect on the graphitization of PI under a small amount of filling, but limited by the heterogeneity of nanosheets, the induced graphitization effect is best at 0.5% mass fraction, with higher thermal conductivity, and a certain downward trend at higher filling. At the same time, the graphite film prepared at this content can maintain a certain high thermal conductivity at a certain thickness, which is 52.6% higher than the thermal conductivity of the pure PI-based graphene film (Comparative Example 4) at the same thickness of about 60μm.

[0079] Example 3

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

[0081] 10 g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500 mL of N,N-dimethylacetamide solution, placed in a high-power cell sonicator at 600 W for 8 h, allowed to stand for 18 h, 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 placed in 10 g of N,N-dimethylacetamide and ultrasonically stirred for 2 h. Then 255 mg of pyromellitic dianhydride was added and stirred for 2 h to allow the surface of the nanosheets to be fully connected to the dianhydride monomer. Then 341 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified N,N-dimethylacetamide dispersion of g-C3N4 nanosheets.

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

[0085] In a reactor under an inert gas atmosphere, add 9.5 g of diaminodiphenyl ether to 108 g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, then add the dispersion obtained in step (2), stir for 30 min, then add 10.4 g of pyromellitic anhydride, add two portions, add them to the reactor at an interval of 1 h, react for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.75%. Apply the coating on a clean glass plate, and the coating thickness is controlled at 0.5 mm. The film is dried at 80 ° C for 4 h. Then heat to 280 ° C at 5 ° C / min and keep the temperature constant for 1 h, then heat to 300 ° C at the same heating rate and keep the temperature constant for 1 h, and slowly cool to obtain a g-C3N4 / PI composite film.

[0086] (4) Preparation of graphene film

[0087] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min and kept at this temperature for 2h; then heated to 1500°C at 1°C / min and kept at this temperature for 2h; then heated to 3000°C at 5°C / min and kept at this temperature for 2h, then slowly cooled to room temperature and the sample was taken out. The finished product was then pressed at 300Mpa for 1h, and the thickness of the finished film was about 18μm.

[0088] Example 4

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

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

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

[0092] Take 210 mg of g-C3N4 nanosheets and ultrasonically stir them in 10 g of N,N-dimethylacetamide for 2 hours. Then add 340.5 mg of pyromellitic dianhydride and stir for 2 hours to allow the surface of the nanosheets to be fully connected with the dianhydride monomer. Then add 454.6 mg of diaminodiphenyl ether and stir for 2 hours to obtain N,N-dimethylacetamide dispersion of PAA-modified g-C3N4 nanosheets.

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

[0094] In a reactor under an inert gas atmosphere, add 9.3 g of diaminodiphenyl ether to 108 g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, then add the dispersion obtained in step (2), stir for 30 min, then add 10.1 g of pyromellitic anhydride, add two portions, add them to the reactor at an interval of 1 h, react for 24 h to obtain a composite slurry with a g-C3N4 mass content of 1%. Apply the coating on a clean glass plate, and the coating thickness is controlled at 0.5 mm. The film is dried at 80 ° C for 4 h. Then heat to 280 ° C at 5 ° C / min and keep the temperature constant for 1 h, then heat to 300 ° C at the same heating rate and keep the temperature constant for 1 h, and slowly cool to obtain a g-C3N4 / PI composite film.

[0095] (4) Preparation of graphene film

[0096] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min and kept at this temperature for 2h; then heated to 1500°C at 1°C / min and kept at this temperature for 2h; then heated to 3000°C at 5°C / min and kept at this temperature for 2h, then slowly cooled to room temperature and the sample was taken out. The finished product was then pressed at 300Mpa for 1h, and the thickness of the finished film was about 18μm.

[0097] Example 5

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

[0099] 10 g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500 mL of N,N-dimethylacetamide solution, placed in a high-power cell sonicator at 600 W for 8 h, allowed to stand for 18 h, 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 placed in 10 g of N,N-dimethylacetamide and ultrasonically stirred for 2 h. Then 334.2 mg of 4,4'-diphthalic anhydride was added and stirred for 2 h to allow the surface of the nanosheets to be fully connected to the dianhydride monomer. Then 227.3 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified N,N-dimethylacetamide dispersion of g-C3N4 nanosheets.

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

[0103] In a reactor under an inert gas atmosphere, add 9.8 g of diaminodiphenyl ether to 108 g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, then add the dispersion obtained in step (2), stir for 30 min, then add 14.7 g of 4,4'-diphthalic anhydride, add two portions, add to the reactor at an interval of 1 h, react for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. Apply the coating on a clean glass plate, and the coating thickness is controlled at 0.5 mm. The film is dried at 80 ° C for 4 h. Then heat to 280 ° C at 5 ° C / min and keep at this temperature for 1 h, then heat to 300 ° C at the same heating rate and keep at this temperature for 1 h, and slowly cool to obtain a g-C3N4 / PI composite film.

[0104] (4) Preparation of graphene film

[0105] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min for 2 hours; then heated to 1500°C at 1°C / min for 2 hours; then heated to 3000°C at 5°C / min for 2 hours, slowly cooled to room temperature, and the sample was taken out. After pressing at 300Mpa for 1 hour, the finished product was obtained, and the thickness of the finished film was about 18μm. The obtained graphene film was recorded as Gg-C3N4 / PI-b.

[0106] Example 6

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

[0108] 10 g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500 mL of N,N-dimethylacetamide solution, placed in a high-power cell sonicator at 600 W for 8 h, allowed to stand for 18 h, 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 placed in 10 g of N,N-dimethylacetamide and ultrasonically stirred for 2 h. Then 352.4 mg of 4,4-oxydiphthalic anhydride was added and stirred for 2 h to allow the surface of the nanosheets to be fully connected to the dianhydride monomer. Then 227.3 mg of diaminodiphenyl ether was added and stirred for 4 h to obtain a PAA-modified N,N-dimethylacetamide dispersion of g-C3N4 nanosheets.

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

[0112] In a reactor under an inert gas atmosphere, add 6.53 g of diaminodiphenyl ether to 108 g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, then add the dispersion obtained in step (2), stir for 30 min, then add 10.3 g of 4,4-oxydiphthalic anhydride, add two portions, add to the reactor at an interval of 1 h, react for 24 h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. Apply the coating on a clean glass plate, and the coating thickness is controlled at 0.5 mm. The film is dried at 80 ° C for 4 h. Then heat to 280 ° C at 5 ° C / min and keep at this temperature for 1 h, then heat to 300 ° C at the same heating rate and keep at this temperature for 1 h, and slowly cool to obtain a g-C3N4 / PI composite film.

[0113] (4) Preparation of graphene film

[0114] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min for 2 hours; then heated to 1500°C at 1°C / min for 2 hours; then heated to 3000°C at 5°C / min for 2 hours, slowly cooled to room temperature, and the sample was taken out. After pressing at 300Mpa for 1 hour, the finished product was obtained, and the thickness of the finished product was about 18μm. The obtained graphene film was recorded as Gg-C3N4 / PI-c.

[0115] Example 7

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

[0117] 10 g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500 mL of N,N-dimethylacetamide solution, placed in a high-power cell sonicator at 600 W for 8 h, allowed to stand for 18 h, 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 placed in 10 g of N,N-dimethylacetamide and ultrasonically stirred for 2 h. Then 352.4 mg of 4,4-oxydiphthalic anhydride was added and stirred for 2 h to allow the surface of the nanosheets to be fully connected with the dianhydride monomer. 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 a reactor under an inert gas atmosphere, add 5.29g of p-phenylenediamine to 108g of N,N-dimethylacetamide. Stir evenly until p-phenylenediamine is completely dissolved, then add the dispersion obtained in step (2), stir for 30min, then add 15.5g of 4,4-oxydiphthalic anhydride, add two portions, add to the reactor at an interval of 1h, react for 24h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. Apply the coating on a clean glass plate, and the coating thickness is controlled at 0.5mm. The film is dried at 80℃ for 4h. Then heat to 280℃ at 5℃ / min and keep at this temperature for 1h, then heat to 300℃ at the same heating rate and keep at this temperature for 1h, and slowly cool to obtain a g-C3N4 / PI composite film.

[0122] (4) Preparation of graphene film

[0123] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min for 2h; then heated to 1500°C at 1°C / min for 2h; then heated to 3000°C at 5°C / min for 2h, slowly cooled to room temperature, and the sample was taken out. After pressing at 300Mpa for 1h, the finished product was obtained, and the thickness of the finished film was about 18μm. The obtained graphene film was recorded as Gg-C3N4 / PI-d.

[0124] Example 8

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

[0126] 10 g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were added to 500 mL of N,N-dimethylacetamide solution, placed in a high-power cell sonicator at 600 W for 8 h, allowed to stand for 18 h, 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 placed in 10 g of N,N-dimethylacetamide and ultrasonically stirred for 2 h. Then 247.8 mg of pyromellitic dianhydride was added and stirred for 2 h to allow the surface of the nanosheets to be fully connected with the dianhydride monomer. Then 210 mg of p-phenylenediamine was added and stirred for 4 h to obtain an N,N-dimethylacetamide dispersion of PAA-modified g-C3N4 nanosheets.

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

[0130] In a reactor under an inert gas atmosphere, add 5.29g of p-phenylenediamine to 108g of N,N-dimethylacetamide. Stir evenly until p-phenylenediamine is completely dissolved, then add the dispersion obtained in step (2), stir for 30min, then add 10.9g of pyromellitic anhydride, add two portions, add them to the reactor at an interval of 1h, react for 24h to obtain a composite slurry with a g-C3N4 mass content of 0.5%. Apply the coating on a clean glass plate, and the coating thickness is controlled at 0.5mm. The film is dried at 80℃ for 4h. Then heat to 280℃ at 5℃ / min and keep the temperature constant for 1h, then heat to 300℃ at the same heating rate and keep the temperature constant for 1h, and slowly cool to obtain a g-C3N4 / PI composite film.

[0131] (4) Preparation of graphene film

[0132] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min for 2 hours; then heated to 1500°C at 1°C / min for 2 hours; then heated to 3000°C at 5°C / min for 2 hours, slowly cooled to room temperature, and the sample was taken out. After pressing at 300Mpa for 1 hour, the finished product was obtained, and the thickness of the finished film was about 18μm. The obtained graphene film was recorded as Gg-C3N4 / PI-e.

[0133] Comparative Example 1

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

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

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

[0137] Take 105 mg of g-C3N4 and stir it ultrasonically in 10 g of N,N-dimethylacetamide for 2 hours. Then add 247.8 mg of pyromellitic dianhydride and stir it for 2 hours to allow the surface of the nanosheets to be fully connected with the dianhydride monomer. Then add 227.3 mg of diaminodiphenyl ether and stir it for 4 hours to obtain N,N-dimethylacetamide dispersion of PAA-modified g-C3N4 nanosheets.

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

[0139] In a reactor under an inert gas atmosphere, add 9.8 g of diaminodiphenyl ether to 108 g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, then add 10.90 g of pyromellitic anhydride in two portions, add them to the reactor at an interval of 1 hour. React for 24 hours to obtain PAA slurry, then add the dispersion obtained in step (2) and stir for 10 minutes to obtain a composite slurry with a g-C3N4 mass content of 0.5%. Apply the slurry on a clean glass plate, and the thickness of the slurry is controlled at 0.5 mm. The film is dried at 80°C for 4 hours. Then, the temperature is increased to 280°C at 5°C / min and kept constant for 1 hour, then the temperature is increased to 300°C at the same heating rate and kept constant for 1 hour, and then slowly cooled to obtain a g-C3N4 / PI composite film.

[0140] (4) Preparation of graphite film

[0141] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min for 2h; then heated to 1500°C at 1°C / min for 2h; then heated to 3000°C at 5°C / min for 2h, slowly cooled to room temperature, and the sample was taken out. After pressing at 300Mpa for 1h, the finished product was obtained, and the thickness of the finished film was about 18μm. The obtained graphene film was recorded as Gg-C3N4 / PI-f.

[0142] Comparative Example 2

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

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

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

[0146] 105 mg of g-C3N4 nanosheets were placed in 10 g of N,N-dimethylacetamide and ultrasonically stirred for 2 h.

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

[0148] In a reactor under an inert gas atmosphere, add 10 g of diaminodiphenyl ether to 108 g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, then add the dispersion obtained in step (2) and stir for 10 minutes, then add 11.12 g of pyromellitic anhydride in two portions, add them to the reactor at an interval of 1 hour. Then add the dispersion obtained in step (2) and stir for 10 minutes to obtain a composite slurry with a g-C3N4 mass content of 0.5%. Apply the slurry on a clean glass plate. The film is dried at 80°C for 4 hours, heated to 280°C at 5°C / min and kept at this temperature for 1 hour, then heated to 300°C at the same heating rate and kept at this temperature for 1 hour, and slowly cooled to obtain a g-C3N4 / PI composite film.

[0149] (4) Preparation of graphite film

[0150] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min for 2 hours; then heated to 1500°C at 1°C / min for 2 hours; then heated to 3000°C at 5°C / min for 2 hours, slowly cooled to room temperature, and the sample was taken out. After pressing at 300Mpa for 1 hour, the finished product was obtained, and the thickness of the finished film was about 18μm. The obtained graphene film was recorded as Gg-C3N4 / PI-g.

[0151] In order to compare the thickness of polyimide-based graphene film with that of g-C3N4 modified with PAA, we scraped the rubber in Comparative Example 2 into films of different thicknesses, and finally prepared graphene films of 18 μm, 26 μm, 41 μm, and 58 μm. The specific thermal conductivity is as follows: Figure 5 As shown in b.

[0152] Due to its intrinsic two-dimensional rigidity, the unmodified carbon nitride nanosheets still have an inducing carbonization effect on PI, but compared with the PI-based graphene film prepared by in-situ doping of PAA-modified carbon nitride nanosheets, its inducing effect in the thickness direction tends to decrease, such as Figure 5 This may be related to its dispersion.

[0153] Comparative Example 3

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

[0155] 10 g of g-C3N4 coarse particles prepared by thermal polymerization of melamine were ground and added to 500 mL of N,N-dimethylacetamide solution, placed in a high-power cell sonicator for 1 h at 100 W, filtered, washed, and dried to obtain g-C3N4 powder.

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

[0157] Take 105 mg of g-C3N4 and stir it ultrasonically in 10 g of N,N-dimethylacetamide for 2 hours. Then add 247.8 mg of pyromellitic dianhydride and stir it for 2 hours to allow the surface of the nanosheets to be fully connected with the dianhydride monomer. Then add 227.3 mg of diaminodiphenyl ether and stir it for 4 hours to obtain the N,N-dimethylacetamide dispersion of PAA-modified g-C3N4 coarse particles.

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

[0159] In a reactor under an inert gas atmosphere, add 10 g of diaminodiphenyl ether to 108 g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, then add the dispersion obtained in step (2), stir for 10 min, then add 11.12 g of pyromellitic anhydride, add two portions, add them to the reactor at an interval of 1 h, and react to obtain a composite slurry. Apply the coating on a clean glass plate, and the coating thickness is controlled to be 0.5 mm. The film is dried at 80°C for 4 h, heated to 280°C at 5°C / min and kept at this temperature for 1 h, then heated to 300°C at the same heating rate and kept at this temperature for 1 h, and slowly cooled to obtain a g-C3N4 / PI composite film.

[0160] (4) Preparation of graphite film

[0161] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min for 2 hours; then heated to 1500°C at 1°C / min for 2 hours; then heated to 3000°C at 5°C / min for 2 hours, slowly cooled to room temperature, and the sample was taken out. After pressing at 300Mpa for 1 hour, the finished product was obtained, and the thickness of the finished film was about 18μm. The obtained graphene film was recorded as Gg-C3N4 / PI-h.

[0162] Comparative Example 4

[0163] (1) Preparation of PI film

[0164] In a reactor with an inert gas atmosphere, add 10g of diaminodiphenyl ether to 108g of N,N-dimethylacetamide. Stir evenly until the diaminodiphenyl ether is completely dissolved, stir for 10 minutes, then add 11.12g of pyromellitic anhydride in two portions, add them to the reactor at an interval of 1 hour, and react to obtain a composite slurry. Apply the coating on a clean glass plate. The film is dried at 80℃ for 4 hours, heated to 280℃ at 5℃ / min and kept at this temperature for 1 hour, then heated to 300℃ at the same heating rate and kept at this temperature for 1 hour, and slowly cooled to obtain a polyimide film.

[0165] (2) Preparation of graphite film

[0166] The composite film was placed in a graphite furnace and heated to 700°C at 1°C / min and kept at this temperature for 2h; then heated to 1500°C at 1°C / min and kept at this temperature for 2h; then heated to 3000°C at 5°C / min and kept at this temperature for 2h, then slowly cooled to room temperature and the sample was taken out. The finished product was obtained by pressing at 300Mpa for 1h. The obtained graphene film was recorded as G-PI.

[0167] In order to compare the thickness of polyimide-based graphene film with that of g-C3N4 modified with PAA, we scraped the rubber in Comparative Example 4 into films of different thicknesses, and finally prepared graphene films of 23 μm, 26 μm, 40 μm, and 60 μm. The specific thermal conductivity is as follows: Figure 5 As shown in b.

[0168] In the thermal conductivity test, Figure 4 It can be seen that the degree of PI graphitization and thermal conductivity induced by unpeeled g-C3N4 (Comparative Example 3) and peeled unmodified g-C3N4 (Comparative Example 2) are lower than those of g-C3N4 modified with PAA, but the g-C3N4 nanosheets as a whole promote the graphitization of PI. The graphitized graphite film was tested using a Rigaku Ultima IVdiffractometer X-ray diffractometer, and a regular (002) crystal plane diffraction peak was obtained. Figure 6 As shown, the interlayer spacing of graphite flakes is calculated by the Bragg equation: 2dsinθ=nλ, and then the graphitization degree of the graphite film is calculated by the empirical formula: γ=((0.3440-d(002)) / 0.0086)*100%. From the X-ray diffraction pattern, it can be seen that PAA-modified g-C3N4 has a good graphitization-promoting effect on PI, and the diffraction peak shows a sharp peak at 26.54°. The degree of graphitization is improved by 3.34%. It is more intuitive to illustrate the promoting effect of PAA-modified g-C3N4 on PI graphitization.

Claims

1. A method for preparing a polyimide-based graphene film, characterized in that: The following steps are involved: (1) adding anhydride and diamine to the nano-scale g-C3N4 dispersion to react, thereby obtaining a PAA-grafted g-C3N4 dispersion; (2) uniformly mixing the PAA-grafted g-C3N4 dispersion obtained in step (1) and an organic solvent containing a diamine, adding anhydride, and reacting to obtain a g-C3N4 / PAA slurry; (3) coating the g-C3N4 / PAA slurry obtained in step (2) on a substrate, and drying to obtain a PAA composite film; and then sintering and cooling the PAA composite film 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 a polyimide-based graphene film according to claim 1, wherein: In step (1), the size of the nanoscale g-C3N4 is 300 to 500 nm, and the thickness is 3 to 5 nm; 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 N,N-dimethylformamide or N,N-dimethylacetamide; more preferably N,N-dimethylacetamide.

3. The method for preparing a 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 more combinations of pyromellitic anhydride, 4,4-oxydiphthalic anhydride, and 4,4'-diphthalic anhydride; the diamine is selected from any one or a combination of two of diaminodiphenyl ether and p-phenylenediamine; the mass ratio of g-C3N4, acid anhydride, and diamine is 25-50:80-150:80-160.

4. The method for preparing a polyimide-based graphene film according to any one of claims 1 to 3, characterized in that: In step (1), the interval between the steps is 1 to 2 hours; and the reaction time is 2 to 8 hours.

5. The method for preparing a polyimide-based graphene film according to any one of claims 1 to 4, characterized in that: In step (2), the acid anhydride is selected from any one or more combinations of pyromellitic anhydride, 4,4-oxydiphthalic anhydride, and 4,4'-diphthalic anhydride; the diamine is selected from any one or a combination of two of diaminodiphenyl ether and p-phenylenediamine; the mass ratio of g-C3N4, diamine and acid anhydride is 0.5-3:100-150:110-160.

6. The method for preparing a polyimide-based graphene film according to any one of claims 1 to 5, characterized in that: In step (2), at least one of the following conditions is met: The reaction is carried out under an inert atmosphere; The PAA-grafted g-C3N4 dispersion and the organic solvent containing diamine are mixed for 30 min-1 h and then anhydride is added; preferably, the organic solvent containing diamine is selected from any one or more of N,N-dimethylformamide, N,N-dimethylacetamide, N-methylpyrrolidone and dimethyl sulfoxide; The anhydride is added in two portions, with an interval of 0.5 to 2 hours; The reaction time is 12 to 24 hours; The mass fraction of g-C3N4 / PAA in the g-C3N4 / PAA slurry is 8-20%.

7. The method for preparing a polyimide-based graphene film according to any one of claims 1 to 6, characterized in that: In step (3), the sintering is performed by heating the temperature to 260-280° C. at a rate of 1-5° C. / min and maintaining the temperature for 1-3 hours, and then heating the temperature to 300-320° C. at a rate of 1-5° C. / min and maintaining the temperature for 1-3 hours.

8. The method for preparing a polyimide-based graphene film according to any one of claims 1 to 7, characterized in that: In step (4), the three-stage heating is as follows: the first stage: heating at 1-5°C / min to 500-700°C and maintaining the temperature for 0.5-2h; The second stage: heating to 1450-1550°C at 1-3°C / min and maintaining the temperature for 0.5-2h, preferably 1500°C; the third stage: heating to 2950-3050°C at 3-5°C / min and maintaining the temperature for 0.5-2h, preferably 3000°C.

9. A polyimide-based graphene film prepared by the method for preparing a polyimide-based graphene film according to any one of claims 1 to 8.

10. The polyimide-based graphene film according to claim 9, characterized in that: The thickness of the polyimide-based graphite film is 10 to 60 μm.

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