A method for preparing graphene oxide for graphene membranes

Abstract

The application relates to a preparation method of graphene oxide for graphene film and belongs to the field of graphene preparation processes, and comprises the following steps: mixing graphite, concentrated sulfuric acid and potassium permanganate according to proportions, then performing primary oxidation intercalation on the graphite to obtain mixed slurry A; performing pressure filtration on part of the mixed slurry A to obtain filter cake A, and returning the filtrate back to a reaction kettle; respectively adding concentrated sulfuric acid and potassium permanganate to obtain mixed slurry B, oxidizing the mixed slurry B to obtain oxidized graphite slurry, performing pressure filtration on the oxidized graphite slurry to obtain oxidized graphite filter cake B; washing the filter cake A and the oxidized graphite filter cake B together to obtain a composite graphene oxide filter cake; the preparation of slightly oxidized intercalation material and secondary fully intercalation material is realized through a step-by-step oxidation method; the slightly oxidized intercalation filter cake A in the composite filter cake provides more carbon residual amount when graphene film is prepared; the same concentration of slurry is adopted, and a wet film with the same thickness is coated; and finally, a graphene film end product with a larger thickness can be obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of graphene preparation process, and particularly relates to a preparation method of graphene oxide for graphene film. BACKGROUND

[0002] Graphene is a new type of two-dimensional carbon material formed by carbon atoms in a hexagonal close-packed plane. As the thinnest substance in the world, graphene has attracted worldwide attention since its discovery in 2004 due to its unique and excellent physical and chemical properties. Graphene film can be widely used in heat conduction, electromagnetic shielding, water treatment, electric heating and other fields.

[0003] Currently, graphene film materials are mostly prepared by using graphene oxide film as a precursor, preparing the precursor into a certain concentration of aqueous slurry, and then coating, drying, carbonizing, graphitizing and calendering to prepare graphene film. There are the following problems in the process of preparing graphene film by this process: 1) the concentration of graphene oxide slurry is low, generally not higher than 6%, because graphene oxide contains rich hydrophilic functional groups, and the graphene oxide is peeled off by high-speed mechanical peeling with water as the solvent, and after peeling, the graphene oxide basically exists in the form of single layer or oligolayer, and forms rich hydrogen bond structure with the solvent water, resulting in high viscosity at low concentration. The viscosity is too high to be coated, so the viscosity is generally controlled by controlling the concentration; 2) due to the low concentration of the slurry during coating, the thickness of the graphene oxide film after drying is thin. Under certain conditions, the heat flux is proportional to the thickness of the film, so the heat conduction performance of thick film is better. However, the present process cannot prepare thick film in one step, and usually adopts the method of multiple film formation or several layers of thin film stacking to realize the preparation of thick film. The multiple film formation process of graphene film also faces the problem of large thermal resistance caused by the interface effect between the film layers, and the preparation process is complex and multiple drying will cause damage to the structure of the solidified film layer, and the energy consumption and time consumption are high; for the preparation of graphene thick film by stacking, as described in the patent "CN107140619A", heat pressing is required, the process is complex, and the production cost is increased, and the structure of the assembled graphene oxide film will be damaged during the compaction process; 3) the oxygen content of graphene oxide is generally about 40%, so the carbon residual amount of graphene film prepared by this process is generally less than 50%; 4) graphene oxide has rich functional groups. In order to prevent the decomposition of functional groups during heat treatment, it is necessary to dry the film at a lower temperature to ensure that the film structure has a relatively high density. Then heat treatment is carried out to prepare graphene film, so that the material preparation takes a long time, the production efficiency is low, and the cost is high. This problem limits the scale industrial application of graphene film, and is at a disadvantage in commercial competition.

[0004] Therefore, there is an urgent need for a one-step forming method for quickly obtaining a graphene film precursor with a dense structure to improve production efficiency and reduce production costs. SUMMARY

[0005] The present application provides a method for preparing graphene oxide for graphene film.

[0006] The technical scheme for solving the above technical problems is as follows: a method for preparing graphene oxide for graphene film, characterized by comprising the following steps:

[0007] S1: In a reaction kettle, mix graphite, concentrated sulfuric acid and potassium permanganate in a weight ratio of 1:2:0.05-1:12:0.2, then perform preliminary oxidation intercalation on the graphite to obtain mixed slurry A; in this step, the material ratio is controlled to slightly oxidize and intercalate the graphite, ensuring that the degree of oxidation intercalation can cause slight expansion and peeling of the graphite to obtain a single-layer graphene structure when heated in the later preparation of the heat-conducting film, so that a graphite structure with a staggered layer stack can still be obtained in the subsequent calendering process;

[0008] S2: Filter part of the mixed slurry A to obtain filter cake A, and return the filtrate to the reaction kettle;

[0009] S3: Calculate the mass ratio of graphite, concentrated sulfuric acid and potassium permanganate in the reaction kettle, and add concentrated sulfuric acid and potassium permanganate respectively to make the mass ratio of graphite: concentrated sulfuric acid: potassium permanganate reach 1:40:1-1:60:4, to obtain mixed slurry B; in this step, the remaining materials in the kettle continue to react according to the hummers method for secondary intercalation, which has better intercalation effect, is easier to peel off and has a higher peeling degree, which is beneficial to the comprehensive performance of the graphene film;

[0010] S4: Prepare oxidized graphite from the mixed slurry B according to the hummers method to obtain an oxidized graphite slurry;

[0011] S5: Filter the oxidized graphite slurry obtained in step S4 to obtain an oxidized graphite filter cake B; the filter cake B with sufficient intercalation provides film-forming properties, which, in combination with the filter cake A with preliminary intercalation, improves the conversion rate of oxidized graphene to graphene film;

[0012] S6: Wash the filter cake A obtained in step S2 and the oxidized graphite filter cake B obtained in step S5 together to obtain a composite oxidized graphene filter cake for preparing a graphene film.

[0013] Further, in step S1, first inject concentrated sulfuric acid into the reaction kettle, add graphite into the concentrated sulfuric acid under continuous stirring, mix uniformly, then slowly add potassium permanganate to the mixture to perform preliminary oxidation intercalation on the graphite.

[0014] Further, in step S1, the temperature of the process control system during the addition of potassium permanganate is controlled within 20 DEG C.

[0015] Further, in step S2, 5% to 25% of the mixed slurry A obtained in step S1 is subjected to pressure filtration, and the rest of the mixed slurry remains in the reaction kettle.

[0016] Further, in step S3, the additional amount of concentrated sulfuric acid is first injected into the reaction kettle and mixed uniformly, and then the additional amount of potassium permanganate is slowly added to the mixture.

[0017] Further, in step S3, the temperature of the process control system during the addition of potassium permanganate is controlled within 20 DEG C.

[0018] Further, the method for preparing a graphene film from the composite graphene oxide filter cake obtained in step S6 is as follows: the filter cake is diluted with water to a concentration of 4% to 8%, and a slurry is prepared by high-speed dispersion and homogenization; the slurry is coated, dried, subjected to low-temperature heat treatment, carbonization, graphitization, and calendering to prepare a graphene heat dissipation film.

[0019] 1) The graphene oxide cake is dispersed in deionized water to obtain a graphene oxide dispersion liquid with a weight percentage of 4% to 8%;

[0020] 2) Ammonia water is slowly poured into the graphene oxide dispersion liquid until the pH value of the graphene oxide dispersion liquid reaches 6 to 9, and then the graphene oxide dispersion liquid is subjected to homogenization 2 to 5 times under a pressure of 800 to 1200 bar and defoaming treatment to obtain a graphene oxide slurry with a viscosity of 15000 to 60000 mPa·s, and the particle size of the graphene oxide slurry is controlled within a range of 0.5 μm to 5 μm.

[0021] 3) The graphene oxide slurry is coated on a substrate by a coating machine, and the coating thickness is controlled to be 4 mm, and then the substrate is dried in an environment at 40 DEG C, and then the substrate is subjected to peeling and slitting treatment to obtain a pretreated graphene film.

[0022] 4) The pretreated graphene film is cut into a film with a size of 30*100 cm, and then the film is subjected to low-temperature treatment in an environment at 180 DEG C for 28 h, carbonization treatment in an environment at not higher than 1400 DEG C for 10 h, and graphitization treatment in an environment at not higher than 3200 DEG C for 10 h to obtain a graphene foam film.

[0023] 5) The graphene foam film is subjected to single-layer rolling or multi-layer rolling under the action of an extrusion roller with a pressure of 1T to obtain a graphene heat dissipation film.

[0024] The beneficial effects of the present application are as follows:

[0025] 1、The present application realizes the preparation of slightly oxidized intercalation material and secondary fully intercalated material by setting up a step-by-step oxidation method, and the slightly oxidized intercalation filter cake A and the fully intercalated filter cake B are combined and washed after the oxidation intercalation is completed, and the perfect mixing of the two is realized in the washing process, and a uniform composite filter cake is obtained; the slightly oxidized intercalation filter cake A in the composite filter cake provides more carbon residual amount when preparing graphene film, and compared with the prior art one-step intercalation process, the same concentration of slurry is used and the same thickness of wet film is coated, and finally a thicker graphene film end product can be obtained.

[0026] 2、The composite filter cake prepared by the process method of the present application has low overall oxygen content, so the viscosity is low when preparing the slurry, which can effectively improve the concentration of the mixed filter cake when preparing the slurry, which is beneficial to the preparation of thick film by one-step method and improves the production efficiency; at the same time, it is more moderate in decomposition during subsequent heat treatment process, which helps to improve the overall thermal stability and increase the yield of graphene film.

[0027] In summary, when preparing a heat-conducting film using the composite filter cake prepared by the present application, a graphene film product with higher carbon residual amount, i.e. higher yield, better thermal stability and unaffected film-forming property can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 is a cross-sectional view of the graphene heat dissipation film prepared in Example 3 of the present application;

[0029] Figure 2 is a cross-sectional view of the graphene heat dissipation film prepared in Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0030] The principles and features of the present application will be described below, and the examples are only used to explain the present application and are not used to limit the scope of the present application.

[0031] The main raw materials used in the following examples and comparative examples are as follows:

[0032] Graphite: particle size: 150 mesh; purity > 99%, purchased from Qingdao Tianshengda Graphite Co., Ltd.;

[0033] Potassium permanganate: purity > 99%, purchased from Changzhou Yanze Industry Co., Ltd.;

[0034] Concentrated sulfuric acid: concentration > 96%, purchased from Hebei Xulong Chemical Co., Ltd.

[0035] Example 1

[0036] The preparation method of graphene film of the present application is characterized in that it comprises the following steps:

[0037] S1: take graphite and concentrated sulfuric acid according to the mass ratio of graphite: concentrated sulfuric acid = 1:2. Pour concentrated sulfuric acid into the reaction kettle, and add graphite into the concentrated sulfuric acid under continuous stirring. Stir for 30 min to make the two mix well. Start the cold water circulation, slowly add potassium permanganate to the mixture, the amount is 0.05 times of the graphite, (the temperature of the system is controlled within 20℃ during the addition of potassium permanganate), and continue to react for 60 min after the completion of the feeding. The graphite is preliminarily oxidized and intercalated to obtain a mixed slurry A;

[0038] S2: filter 5% of the total mass of the mixed slurry A obtained in step S1 through a filter press to obtain a filter cake A, and the filtrate returns to the reaction kettle;

[0039] S3: recalculate the mass ratio of graphite, concentrated sulfuric acid and potassium permanganate in the reaction kettle, and add concentrated sulfuric acid and potassium permanganate to make the mass ratio of graphite: concentrated sulfuric acid: potassium permanganate reach 1:40:1. First, add the amount of concentrated sulfuric acid to the reaction kettle and mix well. Start the cold water circulation, slowly add the amount of potassium permanganate to the mixture, and control the temperature of the system within 20℃ during the addition of potassium permanganate, to obtain a mixed slurry B;

[0040] S4: prepare graphite oxide according to the hummers method with the mixed slurry B obtained in step S3 to obtain an oxidized graphite slurry;

[0041] S5: filter the oxidized graphite slurry obtained in step S4 through a filter press to obtain an oxidized graphite filter cake B;

[0042] S6: put the filter cake A obtained in step S2 and the oxidized graphite filter cake B obtained in step S5 into a washing kettle together, and wash repeatedly to obtain a composite oxidized graphene filter cake.

[0043] Prepare a graphene film from the composite oxidized graphene filter cake prepared in S6: 1) take the oxidized graphene cake and disperse it in deionized water to obtain an oxidized graphene dispersion with a weight percentage of 4%;

[0044] 2) take ammonia water and slowly pour it into the oxidized graphene dispersion until the pH value of the oxidized graphene dispersion reaches 6. Then homogenize 3 times under 800 bar pressure and perform defoaming treatment to obtain an oxidized graphene slurry with a viscosity of 20000 mPa·s. The particle size in the oxidized graphene slurry is controlled within 5μm.

[0045] 3) take a substrate and coat the oxidized graphene slurry on the substrate through a coating machine, the coating thickness is controlled at 4mm, then dry it in a 40℃ environment, and then perform peeling and strip opening treatment to obtain a pretreated graphene film.

[0046] 4) The pretreated graphene film is cut into a 30*100 cm film and is subjected to low-temperature treatment at 180°C for 28 h, carbonization treatment at 1200°C for 10 h, and graphitization treatment at 3000°C for 10 h to obtain a graphene foam film;

[0047] 5) The graphene foam film is subjected to single-layer rolling or multi-layer rolling under the action of an extrusion roller with a pressure of 1T to obtain a graphene heat dissipation film M1 with a film thickness of 43 μm.

[0048] Example 2

[0049] The graphene film of the present example is prepared from graphene oxide, and the preparation method comprises the following steps:

[0050] S1: Graphite and concentrated sulfuric acid are weighed according to the mass ratio of graphite: concentrated sulfuric acid = 1:6. In a reaction kettle, concentrated sulfuric acid is injected, and graphite is added to the concentrated sulfuric acid under continuous stirring for 45 min to mix them uniformly. Cold water circulation is started, and potassium permanganate is slowly added to the mixture, with the amount being 0.1 times that of graphite. The temperature of the system during the addition of potassium permanganate is controlled to be within 20°C. After the completion of the addition, the reaction is continued for 50 min to preliminarily oxidize and intercalate the graphite, and a mixed slurry A is obtained.

[0051] S2: 10% of the total mass of the mixed slurry A obtained in step S1 is filtered by a filter press to obtain a filter cake A, and the filtrate is returned to the reaction kettle.

[0052] S3: The mass ratio of graphite, concentrated sulfuric acid, and potassium permanganate in the reaction kettle is recalculated, and concentrated sulfuric acid and potassium permanganate are added to make the mass ratio of graphite: concentrated sulfuric acid: potassium permanganate reach 1:50:2. First, the added amount of concentrated sulfuric acid is injected into the reaction kettle and mixed uniformly. Cold water circulation is started, and the added amount of potassium permanganate is slowly added to the mixture. The temperature of the system during the addition of potassium permanganate is controlled to be within 20°C, and a mixed slurry B is obtained.

[0053] S4: The mixed slurry B obtained in step S3 is used to prepare graphene oxide according to the hummers method, and a graphene oxide slurry is obtained.

[0054] S5: The graphene oxide slurry obtained in step S4 is filtered by a filter press to obtain a graphene oxide filter cake B.

[0055] S6: The filter cake A obtained in step S2 and the graphene oxide filter cake B obtained in step S5 are transferred into a washing kettle together for repeated washing to obtain a composite graphene oxide filter cake.

[0056] The composite graphene oxide filter cake obtained in S6 is used to prepare a graphene film: 1) The graphene oxide cake is dispersed in deionized water to obtain a graphene oxide dispersion with a weight percentage of 8%.

[0057] 2) Take ammonia water, slowly pour into the graphene oxide dispersion liquid until the PH value of the graphene oxide dispersion liquid reaches 6, then homogenize 3 times under 800 bar pressure and perform defoaming treatment to obtain a graphene oxide slurry with a viscosity of 60,000 mPa·s, and the particle size in the graphene oxide slurry is controlled within 5 μm.

[0058] 3) Take the substrate, coat the graphene oxide slurry on the substrate by a coating machine, control the coating thickness to be 4 mm, then dry in a 40℃ environment, and then perform peeling and strip opening to obtain a pretreated graphene film.

[0059] 4) Cut the pretreated graphene film into a 30*100 cm film, and place it in an environment of 180℃ for low-temperature treatment for 28 h, then place it in an environment of 1200℃ for carbonization treatment for 10 h, and finally place it in an environment of 3000℃ for graphitization treatment for 10 h to obtain a graphene foam film;

[0060] 5) The graphene foam film is subjected to single-layer rolling or multi-layer rolling under the action of an extrusion roller with a pressure of 1T to obtain a graphene heat dissipation film M2 with a film thickness of 65 μm.

[0061] Example 3

[0062] The preparation method of the graphene film of the present embodiment, characterized in that it comprises the following steps:

[0063] S1: Take graphite and concentrated sulfuric acid according to the mass ratio of graphite: concentrated sulfuric acid = 1:12. In the reaction kettle, inject concentrated sulfuric acid into it, and add graphite into the concentrated sulfuric acid under continuous stirring for 60 min to make the two mixed uniformly. Open the cold water circulation, slowly add potassium permanganate to the mixture, and the addition amount is 0.2 times of the graphite, (the potassium permanganate addition process controls the system temperature within 20℃), and after the feeding is completed, continuously react for 30 min to preliminarily oxidize and intercalate the graphite to obtain a mixed slurry A;

[0064] S2: Filter 25% of the total mass of the mixed slurry A obtained in step S1 through a filter press to obtain a filter cake A, and the filtrate returns to the reaction kettle;

[0065] S3: Calculate the mass ratio of graphite, concentrated sulfuric acid, and potassium permanganate in the reaction kettle, and add concentrated sulfuric acid and potassium permanganate to make the mass ratio of graphite: concentrated sulfuric acid: potassium permanganate reach 1:60:4; first inject the added amount of concentrated sulfuric acid into the reaction kettle and mix uniformly; open the cold water circulation, slowly add the added amount of potassium permanganate to the mixture, and the addition process of potassium permanganate controls the system temperature within 20℃ to obtain a mixed slurry B;

[0066] S4: The mixed slurry B obtained in step S3 is used to prepare graphite oxide according to the hummers method, to obtain an oxidized graphite slurry;

[0067] S5: The oxidized graphite slurry obtained in step S4 is filtered by a filter press to obtain an oxidized graphite filter cake B;

[0068] S6: The filter cake A obtained in step S2 and the oxidized graphite filter cake B obtained in step S5 are transferred into a washing kettle to be repeatedly washed to obtain a composite oxidized graphene filter cake.

[0069] The composite oxidized graphene filter cake obtained in S6 is used to prepare a graphite film: 1) The oxidized graphene cake is dispersed in deionized water to obtain an oxidized graphene dispersion with a weight percentage of 6%;

[0070] 2) Ammonia is slowly poured into the oxidized graphene dispersion until the pH value of the oxidized graphene dispersion reaches 6, and then the oxidized graphene dispersion is homogenized for 3 times under a pressure of 800 bar and defoaming treatment is performed to obtain an oxidized graphene slurry with a viscosity of 45000 mPa·s, and the particle size in the oxidized graphene slurry is controlled within a range of 5 μm.

[0071] 3) The oxidized graphene slurry is coated on a substrate by a coating machine, the coating thickness is controlled to be 4 mm, and then the substrate is dried in an environment at 40℃, and then stripping and slitting treatment are performed to obtain a pretreated graphene film.

[0072] 4) The pretreated graphene film is cut into a film with a size of 30*100 cm, and is placed in an environment at 180℃ for low-temperature treatment for 28 h, and then is placed in an environment at 1200℃ for carbonization treatment for 10 h, and finally is placed in an environment at 3000℃ for graphitization treatment for 10 h to obtain a graphene foam film;

[0073] 5) The graphene foam film is subjected to single-layer rolling or multi-layer rolling under the action of an extrusion roller, the pressure of the extrusion roller is 1T, and a graphene heat dissipation film M3 with a film thickness of 54 μm is obtained.

[0074] Comparative Example 1

[0075] The raw materials and amounts used in the present comparative example are basically the same as those in Example 3, except that the present comparative example does not undergo two-step oxidation intercalation, but the raw material graphite directly undergoes one-step oxidation intercalation to obtain the corresponding graphite oxide, and the specific process includes the following steps:

[0076] S1: The graphite and concentrated sulfuric acid are weighed according to the mass ratio of graphite: concentrated sulfuric acid = 1:60. In a reaction kettle, the concentrated sulfuric acid is injected, and the graphite is added into the concentrated sulfuric acid under continuous stirring for 60 min to make the two mixed uniformly. The cold water circulation is started, and the potassium permanganate is slowly added into the mixture, and the amount of potassium permanganate is 4 times of the amount of graphite, (the addition of potassium permanganate is controlled to keep the temperature of the system within 20℃).

[0077] S2: preparing graphite oxide according to hummers method, obtaining graphite oxide slurry;

[0078] S3: filtering the graphite oxide slurry obtained in step S2 by filter press to obtain graphite oxide filter cake;

[0079] S4: putting the graphite oxide filter cake obtained in step S3 into a washing kettle for repeated washing to obtain finished graphite oxide filter cake.

[0080] Preparation of graphite film from the graphite oxide filter cake obtained in S4: 1) taking the graphite oxide cake and dispersing it in deionized water to obtain a graphite oxide dispersion with a weight percentage of 6%;

[0081] 2) taking ammonia water and slowly pouring it into the graphite oxide dispersion until the pH value of the graphite oxide dispersion reaches 6, then homogenizing 3 times under a pressure of 800 bar and performing defoaming treatment to obtain a graphite oxide slurry with a viscosity of 55000 mPa·s, the particle size of the graphite oxide slurry being controlled within 5 μm.

[0082] 3) taking a substrate and coating the graphite oxide slurry on the substrate by a coating machine, the coating thickness being controlled at 4 mm, then drying at 40℃, and then performing peeling and slitting to obtain a pretreated graphene film.

[0083] 4) cutting the pretreated graphene film into a film with a size of 30*100 cm and placing it in an environment at 180℃ for low-temperature treatment for 28 h, then placing it in an environment at 1200℃ for carbonization treatment for 10 h, and finally placing it in an environment at 3000℃ for graphitization treatment for 10 h to obtain a graphene foam film;

[0084] 5) performing single-layer or multi-layer rolling of the graphene foam film under the action of an extrusion roller with a pressure of 1T to obtain a graphene heat dissipation film D1 with a film thickness of 45 μm.

[0085] The graphene heat dissipation films M1-M3 and D1 obtained in the above examples and comparative examples were respectively tested for thermal diffusivity by laser flash method using a NETZSCHL flash method thermal conductivity tester FA467 from Germany. The test results are shown in Table 1.

[0086] Table 1 Thermal diffusivity test of graphene film

[0087] Sample Film thickness pm Thermal diffusivity mm2 / s 2 / s]] M1 43 725 M2 65 731 M3 54 729 D1 45 727

[0088] As can be seen from the data in Table 1, the sample M1 can obtain a graphene film with a film thickness of 43 μm by using a slurry with a concentration of 4% to coat the film; the sample M2 can obtain a graphene film with a film thickness of 65 μm by using a slurry with a concentration of 8% to coat the film; and the sample M3 can obtain a graphene film with a film thickness of 54 μm by using a slurry with a concentration of 6% to coat the film. It can be seen that the higher the concentration of graphene, the thicker the film thickness obtained under the same film coating process, and the thermal diffusion coefficient of the film obtained in the three examples can reach 725-731 mm 2 / s. Compared with the sample M3, the preparation method of the filter cake is different, and the same concentration (6%) of the same slurry is used in the preparation process, the slurry viscosity of Example 3 is 10000 mPa·s lower; the same film coating process is used for coating and film preparation, but the film thickness obtained is significantly different, and the film thickness of Example 3 is 9 μm thicker than that of the comparative example. That is to say, the graphene oxide filter cake prepared by the two-step oxidation method of the present application, compared with the graphene oxide filter cake prepared by the one-step oxidation method of the prior art, although it only contains part of the slightly intercalated graphene oxide, the thermal diffusion coefficient of the prepared graphene film does not decrease, and the process of the present application has obvious gain effect in improving the film thickness in the coating and film preparation (under the premise of the same concentration of the slurry).

[0089] In summary, the technical scheme of the present application has obvious advantages in improving the conversion rate of graphene oxide to graphene, preparing thick film and reducing cost.

Claims

1. A method for preparing graphene oxide for graphene film, characterized by, It comprises the following steps: S1: in the reaction kettle, the graphite, concentrated sulfuric acid, potassium permanganate are mixed in proportion, then the graphite is preliminarily oxidized and intercalated, the weight ratio of graphite: concentrated sulfuric acid: potassium permanganate is 1:2:0.05-1:12:0.2, and mixed slurry A is obtained; S2: part of the mixed slurry A is pressure filtered to obtain filter cake A, and the filtrate is returned to the reaction kettle; S3: the mass ratio of graphite, concentrated sulfuric acid and potassium permanganate in the reaction kettle is recalculated, and concentrated sulfuric acid and potassium permanganate are added respectively to make the mass ratio of graphite: concentrated sulfuric acid: potassium permanganate reach 1:40:1-1:60:4, and mixed slurry B is obtained; S4: the mixed slurry B is used to prepare graphite oxide according to hummers method, and graphite oxide slurry is obtained; S5: the graphite oxide slurry obtained in step S4 is pressure filtered to obtain graphite oxide filter cake B; S6: the filter cake A obtained in step S2 and the graphite oxide filter cake B obtained in step S5 are washed together to obtain a composite graphite oxide filter cake, which is used to prepare a graphene film.

2. The method for preparing graphene oxide for graphene film according to claim 1, characterized by, In step S1, concentrated sulfuric acid is first injected into the reaction kettle, graphite is added to the concentrated sulfuric acid under continuous stirring, mixed uniformly, and then potassium permanganate is slowly added to the mixture to preliminarily oxidize and intercalate the graphite.

3. The method for preparing graphene oxide for graphene films according to claim 2, characterized in that, In step S1, the temperature of the system is controlled within 20℃ during the addition of potassium permanganate.

4. The method for preparing graphene oxide for graphene film according to claim 1, characterized by, In step S2, 5%-25 % of the mixed slurry A obtained in S1 is pressure filtered, and the rest of the mixed material remains in the reaction kettle.

5. The method for preparing graphene oxide for graphene films according to claim 1, characterized in that, In step S3, the added amount of concentrated sulfuric acid is first injected into the reaction kettle, mixed uniformly, and then the added amount of potassium permanganate is slowly added to the mixture.

6. The method for preparing graphene oxide for graphene films according to claim 4, characterized in that, In step S3, the temperature of the system is controlled within 20℃ during the addition of potassium permanganate.

7. The method for preparing graphene oxide for graphene films according to claim 1, characterized in that, The method for preparing a graphene film from the composite graphite oxide filter cake prepared in step S6: take the composite graphite oxide filter cake, dilute it with water to a mass concentration of 4-8 %, prepare an oxidized graphite oxide slurry by high-speed dispersion and homogenization, coat the oxidized graphite oxide slurry, dry it, heat treat it, carbonize it, graphitize it, and calender it to prepare a graphene heat-conducting film.

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