Highly heat-conductive graphite film, and preparation method and application thereof

Abstract

This invention belongs to the field of thermally conductive graphite materials, specifically relating to a high thermal conductivity graphite film prepared by high-temperature carbonization and graphitization processes, its preparation method, and its application. This invention provides a method for preparing a graphite film: first, a wet film of modified polyarylene oxadiazole is prepared; then, the obtained wet film is biaxially stretched and dried to obtain a dry modified polyarylene oxadiazole film; finally, the dry modified polyarylene oxadiazole film is subjected to high-temperature carbonization and graphitization treatment to obtain the graphite film; wherein, the modified polyarylene oxadiazole has the following structural formula, where x:y:z = (0.95~0.55):(0.05~0.45):(0.01~0.10). The graphite film prepared by this invention has a thickness of 20-200 μm and a density of 1.85-2.2 g / cm³. 3 Its thermal conductivity is 1400-1800 W / (mK).

Description

Technical Field

[0001] This invention belongs to the field of thermally conductive graphite materials, specifically relating to a high thermal conductivity graphite film prepared by high-temperature carbonization and graphitization processes, its preparation method, and its application. Background Technology

[0002] With the widespread application of 5G technology, the increased operating frequency of electronic devices has led to a significant increase in data processing speed, accompanied by a substantial rise in heat generation. This poses new challenges to the stability and lifespan of critical electronic devices such as smartphones, communication base stations, and data centers. Therefore, developing efficient thermal management technologies is crucial for ensuring the reliable operation and extending the lifespan of these devices. Among various potential materials, graphene-based thermal conductive films are considered ideal for addressing the thermal management challenges of 5G devices due to their excellent thermal conductivity and good electrical properties.

[0003] Currently, the carbonization-graphitization method, represented by polyimide (PI), has attracted widespread attention in graphite film preparation technology, mainly due to its ability to produce graphite films with high thermal conductivity, high crystallinity, and high orientation. This method uses PI-based polymer precursors, and through precisely controlled preforming and carbonization processes, followed by a high-temperature graphitization process, graphene thermal conductive sheets or fibers are produced. These materials not only have high thermal conductivity, but their optimized crystallinity and orientation are also crucial for the thinning and efficient heat dissipation of 5G communication devices. Summary of the Invention

[0004] This invention provides a novel high thermal conductivity graphite film. The graphite film is prepared by first obtaining a wet poly(arylexadiazole) (POD) film, then biaxially stretching and drying the wet POD film to obtain a dry POD film. The resulting POD film exhibits high density and high orientation. Finally, the dry POD film is subjected to high-temperature carbonization-graphitization treatment to obtain a high-performance graphite film. The prepared graphite film has a thickness of 20–200 μm and a density of 2.0–2.2 g / cm³. 3 Its thermal conductivity is 1400–1700 W / (mK).

[0005] The technical solution of the present invention:

[0006] The first technical problem to be solved by the present invention is to provide a method for preparing a graphite film, wherein the preparation method is as follows:

[0007] First, a wet film of modified polyaromatic oxadiazole (POD) is prepared;

[0008] The obtained wet film is then biaxially stretched and dried to obtain a dry POD film;

[0009] Finally, the dry POD film was subjected to high-temperature carbonization and graphitization to obtain the graphite film.

[0010] The modified polyarylene oxadiazole has the following structural formula:

[0011]

[0012] Where x:y:z = (0.95~0.55):(0.05~0.45):(0.01~0.10).

[0013] Furthermore, the wet membrane of the modified polyarylexadiazole has a water content of 70-80 vol%.

[0014] Furthermore, in the above-mentioned method for preparing graphite film, during the biaxial stretching process, the stretching rate is 1-5% / s and the stretching ratio is 1.2-1.7 times.

[0015] Furthermore, in the above-mentioned method for preparing graphite film, the stretching temperature is 10–60°C.

[0016] Furthermore, in the above-mentioned method for preparing graphite films, after biaxial stretching, annealing is performed at 100–150°C for 10–60 minutes to obtain a dry biaxially stretched POD film. This allows for rapid solvent evaporation, thereby quickly fixing the stretched orientation structure and obtaining a dense, non-porous film structure under the influence of tensile internal stress.

[0017] Furthermore, in the above-mentioned method for preparing graphite films, the method for preparing the wet film of modified polyarylene diazole (POD) is as follows:

[0018] 1) Preparation of the stock solution: First, a modified polyarylene oxadiazole (POD) stock solution was prepared using a solution mixing copolymerization method;

[0019] 2) Wet film forming: The modified POD stock solution is then solidified by extrusion molding to obtain a water-containing POD wet film; during this process, the bidirectional diffusion effect of the solvent in the coagulation bath gives the wet film the necessary lubricity.

[0020] Specifically, the method for preparing the wet membrane of modified polyarylene oxadiazole (POD) is as follows:

[0021] 1) Synthesis of the stock solution: Terephthalic acid, isophthalic acid, azobenzene compound, and hydrazine salt (hydrazine sulfate or hydrazine hydrochloride) are reacted at 80–90℃ for 0.5–1 h in the presence of fuming sulfuric acid, followed by heating to 120–135℃ and reacting for 2–5 h. The reaction is terminated with benzoic acid, and the reaction is continued at 120–135℃ for another 0.5–1 h. Vacuum degassing is then performed to obtain the POD stock solution. The molar ratio of isophthalic acid to terephthalic acid is 0.05–0.55:0.95–0.45, and the mass ratio of the azobenzene compound is: isophthalic acid to terephthalic acid: The total mass ratio of phthalic acid and terephthalic acid is 0.01–0.09:1; the molar addition of hydrazine salt to the total molar ratio of isophthalic acid and terephthalic acid is 1.02–1.10:1, i.e., hydrazine salt needs to be in excess by 0.02–0.10 (preferably 0.05); the solid content of the reaction system is 5–25%; in the reaction system of this invention, the solid mass is the total mass of terephthalic acid, isophthalic acid, azobenzene compound, hydrazine salt, and benzoic acid; fuming sulfuric acid not only acts as a solvent but also as a dehydrating agent, promoting the polymerization and cyclization reactions of monomers;

[0022] 2) Wet film forming: The obtained modified POD stock solution is extruded through a die at 60-120℃ to form a film, and gradient coagulation is carried out to obtain a wet film of modified POD (the water volume ratio of the wet film is 70-80%).

[0023] Furthermore, the azobenzene compound is selected from: 4,4'-dicarboxylated azobenzene (azoBDC), 4,4'-dihydroxyazobenzene (azoBDH), 4,4'-diaminoazobenzene (azoBDA), 3,3'-dicarboxylated azobenzene, 3,3'-dihydroxyazobenzene, 3,3'-diaminoazobenzene, 2,4'-dicarboxylated azobenzene, 2,4'-dihydroxyazobenzene, or 2,4'-diaminoazobenzene.

[0024] Furthermore, the molar ratio of the terminator benzoic acid to the excess hydrazine salt is 1:1.

[0025] Furthermore, the sulfur trioxide content in the fuming sulfuric acid is 20-40 wt% (preferably 25 wt%).

[0026] Furthermore, in the above-mentioned method for preparing graphite film, the carbonization temperature is 1200-1500℃ (preferably 1400℃), and the graphitization temperature is 2600-3000℃ (preferably 2900℃).

[0027] Preferably, in the above-mentioned method for preparing graphite film, the carbonization process is as follows: under inert gas protection, the dry POD film is gradually heated from room temperature to 400-500°C at a heating rate of 2-5°C / min, and held for 0.5-2 hours; then the heating rate is reduced to 0.5-2°C / min and the temperature is increased to 500-600°C, and held for 0.5-2 hours; then the heating rate is increased to 2-5°C / min and the temperature is increased to 1200-1500°C (preferably 1400°C) and held for 0.5-5 hours.

[0028] Preferably, in the above-mentioned method for preparing graphite film, the graphitization process is as follows: heating to 2200-2300°C at a heating rate of 3-10°C / min, holding at that temperature for 0.5-2 hours, then reducing the heating rate to 0.5-2°C / min and heating to 2400-2500°C, holding at that temperature for 0.5-2 hours; then increasing the heating rate to 3-10°C / min and heating to 2600-3000°C and holding at that temperature for 0.5-2 hours, followed by natural cooling to room temperature to obtain the final graphite film.

[0029] Furthermore, the graphitized graphite film can be cold-pressed to improve its density and performance. This involves placing the graphite film between two smooth tungsten sheets and pressing it with a pressure of 100 MPa for 0.5 hours, 200 MPa for 1 hour, and finally 300 MPa for 2 hours to obtain a flexible and dense graphite film.

[0030] The second technical problem to be solved by the present invention is to provide a graphite film prepared by the above-described preparation method.

[0031] Furthermore, the thickness of the graphite film is 20–200 μm.

[0032] Furthermore, the density of the graphite film is 1.85–2.2 g / cm³. 3 .

[0033] Furthermore, the in-plane thermal conductivity of the graphite film is 1400–1800 W / (mK).

[0034] Furthermore, the conductivity of the graphite film is 0.8–2.2 × 10⁻⁶. 6 S / m.

[0035] Furthermore, the graphite film has an elongation at break of 5-15% and a tensile strength of 5-10 MPa.

[0036] Furthermore, the graphite film has a graphitization degree of 95-100%.

[0037] Furthermore, the lattice spacing of the graphite film is 0.335–0.350 nm.

[0038] The third technical problem to be solved by this invention is to improve the in-plane thermal conductivity of a polyarylene oxadiazole-based film. The method involves: after preparing a wet film using a modified polyarylene oxadiazole stock solution, biaxial stretching is introduced to biaxially stretch the wet film, followed by drying to obtain a dry film, and finally carbonization and graphitization to obtain a graphite film. During the biaxial stretching process, the stretching rate is 1–5% / s, and the stretching ratio is 1.2–1.7 times. The structural formula of the modified polyarylene oxadiazole is as follows:

[0039]

[0040] Wherein, x:y:z = (0.95~0.55):(0.05~0.45):(0.01~0.10). Preferably, the stretching rate is 1~5% / s, and the stretching ratio is 1.3~1.7 times.

[0041] Preferably, in the above method for improving the in-plane thermal conductivity of POD film, the carbonization process is as follows: under inert gas protection, the dry POD film is gradually heated from room temperature to 400-500°C at a heating rate of 2-5°C / min, and held for 0.5-2 hours; then the heating rate is reduced to 0.5-2°C / min and the temperature is increased to 500-600°C, and held for 0.5-2 hours; then the heating rate is increased to 2-5°C / min and the temperature is increased to 1200-1500°C and held for 0.5-5 hours.

[0042] Preferably, in the above method for improving the in-plane thermal conductivity of the POD film, the graphitization process is as follows: heating to 2200-2300°C at a heating rate of 3-10°C / min, holding at that temperature for 0.5-2 hours, then reducing the heating rate to 0.5-2°C / min and heating to 2400-2500°C, holding at that temperature for 0.5-2 hours; then increasing the heating rate to 3-10°C / min and heating to 2600-3000°C and holding at that temperature for 0.5-2 hours, followed by natural cooling to room temperature to obtain the final graphite film.

[0043] The beneficial effects of this invention are:

[0044] This invention utilizes cost-effective raw materials and a simplified processing technology, effectively reducing production costs. The biaxial stretching process, achieved through simple operation, not only improves the processing efficiency of POD wet films but also ensures their high quality. Furthermore, this invention, through optimization of the biaxial stretching process for wet films, significantly improves the density and in-plane orientation of organic precursor films such as POD, demonstrating its broad application potential.

[0045] The graphite film obtained by this invention has a 10-20% increase in density and a 15-25% increase in thermal conductivity; this improvement is particularly important in thermal management applications for high-performance electronic devices. The resulting graphite film can be used in fields such as electronics manufacturing, energy storage, and aerospace. Attached Figure Description

[0046] Figure 1 In the figure, a is a graph showing the change in thickness and density of the base film during the biaxial stretching process, b is a graph comparing the tensile properties of the unstretched wet film and the dry film, and cf is a graph comparing the tensile properties of the dry film obtained under different stretching ratios and stretching speeds. This figure shows a comparison of the tensile strength and elongation at break of the materials obtained in Examples 1, 2, and 3 of this invention and the unstretched POD base film.

[0047] Figure 2 a) is a two-dimensional small-angle X-ray diffraction (SAXS) pattern of the materials obtained in Examples 1, 2, and 3 of the present invention; b) is an X-ray diffraction (XRD) pattern of the materials obtained in Examples 1, 2, and 3 of the present invention and the unstretched POD base film.

[0048] Figure 3 a is a two-dimensional wide-angle X-ray diffraction (2D-GIXRD) pattern of the materials obtained in Examples 1, 2, and 3 of the invention and the unstretched POD base film; b and c are in-plane and out-of-plane GIXRD patterns of the materials obtained in Examples 1, 2, and 3 of the invention and the unstretched POD base film.

[0049] Figure 4 The following are macroscopic morphological comparisons of the materials obtained in Examples 1, 2, and 3 of this invention, as well as the unstretched POD base film and the corresponding graphite film, and illustrations of the changes in residual carbon rate, shrinkage rate, density, and thickness: a is a macroscopic morphology diagram, b is a comparison of dimensional shrinkage rate and residual carbon rate, and c is a comparison of density and thickness.

[0050] Figure 5 The surface morphology of the materials obtained in Examples 1, 2, and 3 of this invention and the corresponding graphite films of the unstretched POD base film were compared.

[0051] Figure 6 The images show a scanning electron microscope (SEM) comparison of the surface and cross-section of the materials obtained in Examples 1, 2, and 3 of this invention and the graphite film corresponding to the unstretched POD base film.

[0052] Figure 7 The following are transmission electron microscope (TEM) comparison images of the material obtained in Example 3 of the present invention and the graphite film corresponding to the unstretched POD base film: a is the cross-sectional lattice diagram of Example 3, b and c are the surface lattice diagrams of Example 3 of the present invention, d is the cross-sectional lattice diagram of the unstretched sample, and e and f are the surface lattice diagrams of the unstretched sample.

[0053] Figure 8The X-ray diffraction (XRD) comparison diagrams (Fig. a) and Raman spectrum comparison diagrams (Fig. b surface, Fig. c cross section) of the materials obtained in Examples 1, 2 and 3 of the present invention and the graphite film corresponding to the unstretched POD base film are compared.

[0054] Figure 9 The thermal conductivity (a) and electrical conductivity (b) of the materials obtained in Examples 1, 2, and 3 of this invention and the graphite film corresponding to the unstretched POD base film were compared.

[0055] Figure 10 The invention presents a comparison of the heat dissipation performance of graphite films obtained in Examples 1, 2, and 3 of this invention with that of commercial graphite films: ad represents the comparison of graphite films prepared by 1.2x stretching, 1.3x stretching, 1.4x stretching, and 1.5x stretching with commercial graphite films. Detailed Implementation

[0056] Generally, as the film thickness increases, the in-plane orientation of the film spontaneously decreases, leading to molecular chain entanglement and the formation of three-dimensional ordered crystals. This phenomenon is detrimental to improving the degree of graphitization during the high-temperature carbonization-graphitization process. To address this deficiency, this invention uses POD as the film substrate. A wet POD film is first prepared, and then a dry POD film is obtained through biaxial stretching and drying. This method effectively improves the in-plane orientation and compactness of the POD film. This modification process optimizes the film's microstructure, thereby enhancing the degree of graphitization during the high-temperature carbonization-graphitization process. Finally, the dry POD film undergoes high-temperature carbonization and graphitization treatment to obtain a POD graphite film, which exhibits excellent thermal conductivity.

[0057] This invention uses a POD film as the base film and introduces a biaxial stretching process to prepare a graphite film with high thermal conductivity. The specific implementation steps include:

[0058] Preparation of the stock solution: Polyarylene oxadiazole (POD) polymer stock solution was prepared by solution mixing copolymerization. In this step, fuming sulfuric acid not only acts as a solvent but also as a dehydrating agent to promote the polymerization and cyclization reaction of the monomers.

[0059] Wet film forming: The above-mentioned raw solution is solidified by extrusion molding technology to obtain a moisture-containing POD wet film. During this process, the bidirectional diffusion effect of the solvent in the coagulation bath imparts the necessary lubricity to the wet film.

[0060] Biaxial stretching and heat setting: Biaxial stretching of the wet film effectively improves the film's density and in-plane orientation, and further enhances its quality during the subsequent heat setting process. This treatment disrupts the three-dimensional crystalline structure of the POD film, enhancing its orientation. The key to preparing a dense, highly oriented POD film in this invention lies in employing rapid biaxial stretching at a speed close to the critical value, followed by rapid evaporation of the solvent water at 120°C. This allows the stretched orientation structure to be quickly fixed, resulting in a dense, non-porous film structure under the influence of tensile internal stress. The stretching ratio can be controlled, thereby allowing for the adjustment of the POD film thickness.

[0061] High-temperature carbonization-graphitization treatment: Under inert gas protection, the dry POD film is gradually heated from room temperature to 1200-1500℃ at a heating rate of 1-5℃ / min, and held at this temperature for 0.5-2 hours. When the temperature rises to the range of 400-600℃, due to the rapid loss of POD mass, it is preferable to set a holding-deceleration zone, holding at 400℃ and 600℃ for 0.5-2 hours respectively, with a heating rate of 0.5-2℃ / min in each zone, to ensure a stable carbonization process and make the POD film smoother and flatter after carbonization. The graphitization process is as follows: under inert gas protection, the temperature is raised to 2600-3000℃ at a heating rate of 1-10℃ / min and held at this temperature for 0.5-2h; and in the 2200-2500℃ range, since this is the key stage for the formation of the graphene sheet stacking structure, a slow-down holding zone is preferably set, holding at 2200℃ and 2500℃ for 0.5-1h (preferably 0.5h) respectively, with a heating rate of 0.5-2℃ / min (preferably 1℃ / min), and then naturally cooled to room temperature to obtain the final graphite film.

[0062] High-temperature treatment and cold pressing: After high-temperature carbonization-graphitization treatment and subsequent cold pressing process, the final graphite film has significantly improved graphitization degree and thermal conductivity.

[0063] The embodiments of the present invention will further illustrate the above-described implementation schemes. It should be emphasized that these embodiments are only used to illustrate the process and effects of the present invention and do not limit the scope of application of the present invention. Unless otherwise specified, the conditions, reagents, or equipment used in the experiments follow industry standards or manufacturer-recommended specifications.

[0064] In this embodiment of the invention, all raw materials are directly purchased industrial-grade raw materials.

[0065] Example 1

[0066] Preparation of biaxially stretched POD membranes:

[0067] 1) Synthesis of raw material: POD raw material was synthesized by reacting terephthalic acid, isophthalic acid, 4,4'-dicarboxylated azobenzene (azoBDC) and hydrazine sulfate in 20% fuming sulfuric acid (diluted with concentrated sulfuric acid from 50% fuming acid). First, the raw materials were reacted at 85°C for 0.5 hours according to the proportions shown in Table 1. Then, the temperature was raised to 120°C and reacted for 3 hours. The reaction was terminated with benzoic acid (the amount of which was equal to the molar amount of excess hydrazine sulfate). The reaction was then continued at 120°C for another 0.5 hours, and a vacuum degassing treatment was performed for 12 hours to obtain POD raw material.

[0068] Table 1. Proportions of each raw material

[0069] raw material Quality fraction (%) terephthalic acid 5.3972 isophthalic acid 0.9524 4,4'-Dicarboxylated azobenzene 0.1905 hydrazine sulfate 5.3182 concentrated sulfuric acid 37.3865 Fuming sulfuric acid (50%) 50.5175 benzoic acid 0.2377 / 100

[0070] 2) Wet film forming: POD stock solution is extruded through a mold and subjected to gradient condensation process (the gradient condensation process involves passing the stock solution through 60wt% sulfuric acid solution, 30wt% sulfuric acid solution, water and sodium bicarbonate aqueous solution (mass concentration of 35wt%) in sequence, with a residence time of 10min for each step) to obtain a POD wet film with a water content of 80vol% and a thickness of 1000μm (the dry film thickness after drying is 200μm, and the final graphite film thickness is 98μm).

[0071] 3) Stretching and heat setting: Biaxial stretching was performed using a laboratory-grade biaxial stretching apparatus at stretching rates of 1%, 2%, and 3% / s and a stretching ratio of 1.3 times. Subsequently, the wet film was dried at 120°C for 0.5 hours to obtain the dry POD film, which were denoted as 1.3-1, 1.3-2, and 1.3-3, respectively.

[0072] 4) Carbonization and graphitization of biaxially stretched POD films:

[0073] Carbonization treatment: carried out in a graphite furnace; under argon protection at a rate of 5 L / min, first heat to 400℃ at a rate of 2.5℃ / min, hold for 0.5h, then adjust the heating rate to 1℃ / min to 600℃, hold for 0.5h; then adjust the heating rate to 2.5℃ / min to 1400℃ and hold for 1 hour;

[0074] Graphitization treatment: First, heat to 2200℃ at a rate of 5℃ / min and hold for 0.5h. Then, adjust the heating rate to 1℃ / min to 2500℃ and hold for 0.5h. Next, adjust the heating rate to 5℃ / min to 2900℃ and hold for 1 hour. Then, allow to cool naturally to room temperature to obtain the final graphite film. For easy distinction, the obtained graphite film is named as stretching ratio-stretching speed-g, such as 1.3-1-g, which is a graphite film obtained by carbonization and graphitization of POD with a stretching ratio of 1.3 and a stretching speed of 1% / s.

[0075] 5) Cold pressing treatment

[0076] Cold pressing: Each graphite film obtained is placed in a smooth tungsten sheet and pressed at 100 MPa for 0.5 hours, then at 200 MPa for 1 hour, and finally at 300 MPa for 2 hours to gradually increase the density and obtain a flexible and dense graphite film.

[0077] In addition, comparative experiments were conducted in this invention: Using the same stock solution preparation method as in Example 1, a 700 μm thick POD wet film was directly prepared by wet film forming (the dry film thickness after drying was 140 μm, and the final graphite film thickness was 70 μm). Then, without biaxial stretching, a POD dry film was directly dried. This dry film was then processed through the same carbonization and graphitization processes as in Example 1, and the resulting film was designated as origin-70. Alternatively, a 700 μm thick POD wet film was directly prepared by wet film forming (the dry film thickness after drying was 140 μm, and the final graphite film thickness was 70 μm). Afterward, without biaxial stretching, and during the carbonization and graphitization process, a conventional heating process was used, with a heating rate of 10 °C / min throughout, and holding at 1400 °C and 2900 °C for 1 hour. The resulting film was designated as origin-70 rapid heating.

[0078] Thermal conductivity test: The thermal diffusivity of the graphene film was tested using Netzsch LFA 467. The test temperature was set at room temperature (25℃), and the test sample size was a circular piece with a diameter of 2.5cm. The test results of thermal conductivity are shown in Table 2.

[0079] Results analysis:

[0080] Figure 1-3 The results show improvements in density and three-dimensional crystallization disruption, as well as increased orientation, achieved by 1.3x stretched POD films. Figure 4-6 This demonstrates the improvement in surface smoothness, internal density, and graphitized structure of graphite films prepared by sintering 1.3x biaxially stretched POD films. Figure 7-8 This study demonstrates the improvement in graphitization degree and graphene sheet stacking structure of graphite films prepared by 1.3x biaxially stretched POD films. Figure 9-10 The results showed that the graphite film prepared by 1.3 times biaxially stretched POD film has an improved thermal conductivity of about 7% compared with the unstretched POD film of the same thickness, and the heat dissipation effect is also better in practical applications.

[0081] Example 2

[0082] The operation method of this embodiment is the same as that of embodiment 1, the main difference being that the stretching ratio is adjusted to 1.4 times.

[0083] Performance results analysis:

[0084] Figure 1-3The results showed that the POD film stretched by 1.4 times was improved in terms of density and three-dimensional crystallization disruption compared to the film stretched by 1.3 times, and the orientation degree was also improved. Figure 4-6 This indicates that the graphite film prepared by sintering a POD film stretched by 1.4 times exhibits improvements in surface smoothness, internal density, and graphitization structure compared to that stretched by 1.3 times. Figure 7-8 This study demonstrates the improvement in graphitization degree and graphene sheet stacking structure of graphite films prepared by POD films with 1.4x biaxial stretching. Figure 9-10 This study demonstrates that graphite films prepared by 1.4 times biaxially stretched POD films exhibit approximately 15% improved thermal conductivity compared to unstretched POD films of the same thickness, resulting in better heat dissipation in practical applications.

[0085] In addition, comparative experiments were conducted in this invention: Using the same stock solution as in Example 2, a POD wet film with a thickness of 550 μm (110 μm after drying, and 55 μm for the final graphite film) was directly prepared by wet film forming. Then, without biaxial stretching, a POD dry film was directly dried. This dry film was then processed through the same carbonization and graphitization processes as in Example 1, and the resulting film was designated as origin-55. Alternatively, a POD wet film with a thickness of 550 μm (110 μm after drying, and 55 μm for the final graphite film) was directly prepared by wet film forming. Without biaxial stretching, and during the carbonization and graphitization process, a conventional heating process was used, with a heating rate of 10 °C / min throughout, and holding at 1400 °C and 2900 °C for 1 hour. The resulting film was designated as origin-55 rapid heating.

[0086] Example 3

[0087] This embodiment follows the operation method in Embodiment 1, with the main difference being that the stretching ratio of the POD film is increased to 1.5 times.

[0088] Results analysis:

[0089] Figure 1-3 The results showed that the POD film stretched by 1.5 times had significant improvements in density and three-dimensional crystallization disruption compared to the films stretched by 1.3 times and 1.4 times, and the orientation degree was also further improved. Figure 4-6 This indicates that graphite films prepared by sintering POD films stretched by 1.5 times exhibit more significant improvements in surface smoothness, internal density, and graphitized structure compared to those prepared by lower stretching ratios. Figure 7-8 This study demonstrates further improvements in graphite degree and graphene sheet stacking structure in graphite films prepared by 1.5x biaxial stretching of POD films. Figure 9-10The results showed that graphite films prepared by 1.5 times biaxial stretching of POD film improved thermal conductivity by about 25% compared to unstretched POD film of the same thickness, and exhibited better heat dissipation performance in practical applications.

[0090] In addition, comparative experiments were conducted in this invention: Using the same stock solution preparation method as in Example 1, a POD wet film with a thickness of 400 μm (80 μm after drying, and 40 μm for the final graphite film) was directly prepared by wet film forming. Then, without biaxial stretching, a POD dry film was directly dried. This dry film was then processed through the same carbonization and graphitization processes as in Example 1, and the resulting film was designated as origin-40. Alternatively, a POD wet film with a thickness of 400 μm (80 μm after drying, and 40 μm for the final graphite film) was directly prepared by wet film forming. Afterward, without biaxial stretching, and during the carbonization and graphitization process, a conventional heating process was used, with a heating rate of 10 °C / min throughout, and holding at 1400 °C and 2900 °C for 1 hour. The resulting film was designated as origin-40 rapid heating.

[0091] Example 4

[0092] In this embodiment, by changing the mold width during the wet film preparation process, a larger thickness of wet film (1700μm, 2250, 2800μm) was obtained. Then, biaxial stretching was performed with stretching parameters of 1.5 times the stretching ratio and 3% / s stretching speed. The heating process of this invention (graphitization temperature of 2900℃) was used to finally obtain graphite films of 75μm, 100μm, and 125μm, named 1.5-3-g-75, 1.5-3-g-100, and 1.5-3-g-125.

[0093] Comparative Example 1

[0094] The preparation process is the same as in Example 1, except that biaxial stretching is not performed. That is, the POD wet film with a thickness of 1000 μm (the dry film thickness after drying is 200 μm and the graphite film thickness is 98 μm) obtained in Example 1 is directly dried to obtain a POD dry film, and then subjected to the carbonization and graphitization treatments as described in Example 1.

[0095] Comparative Example 2

[0096] The preparation process is the same as in Example 1, except that: the biaxial stretching process was not performed (the POD wet film with a thickness of 500 μm (98 μm after drying) obtained in Example 1 was directly dried to obtain the POD dry film; and during the carbonization and graphitization process, a conventional heating process was used, with a heating rate of 10℃ / min throughout, and the temperature was maintained at 1400℃ and 2900℃ for 1 hour.

[0097] Table 2 Performance of the samples obtained from each embodiment

[0098]

[0099] In Table 2, α ∥ - In-plane thermal diffusivity, α ⊥ -Vertical thermal diffusivity, C p -Specific heat capacity, ρ -Density, λ ∥ - In-plane thermal conductivity, λ ⊥ - Vertical thermal conductivity.

[0100] As shown in Table 2, the heating process of this invention can effectively improve the density and thermal conductivity of the graphite film compared to the rapid heating process. In addition, the larger the stretching ratio, the faster the stretching speed. At the same time, the heating process of this invention can effectively improve the density and thermal conductivity in the carbonization and graphitization process. Furthermore, the biaxial stretching process of this invention can effectively mitigate the problem of the gradual decrease in thermal conductivity caused by the increase in graphite film thickness, and maintain good thermal conductivity even at large thicknesses.

Claims

1. A method for preparing a graphite film, characterized in that, The preparation method is as follows: First prepare a wet film of modified polyaromatic oxadiazole; The obtained wet membrane was then biaxially stretched and dried to obtain a dry modified polyarylexadiazole film; Finally, the dry-modified polyarylene diazole film was subjected to high-temperature carbonization and graphitization to obtain the graphite film. The modified polyarylene oxadiazole has the following structural formula: ； Where x:y:z = (0.95~0.55):(0.05~0.45):(0.01~0.10); During the biaxial stretching process, the stretching rate is 1–5% / s, and the stretching ratio is 1.3–1.7 times; the density of the graphite film is 1.87–2.2 g / cm³. 3 .

2. The method for preparing a graphite film according to claim 1, characterized in that, The stretching temperature is 10–60°C.

3. The method for preparing a graphite film according to claim 1 or 2, characterized in that, The wet membrane of the modified polyarylexadiazole has a water content of 70-80 vol.

4. The method for preparing a graphite film according to claim 1 or 2, characterized in that, After biaxial stretching, the film is annealed at 100-150℃ for 10-60 minutes to obtain a dry-state modified polyarylexadiazole film after biaxial stretching.

5. The method for preparing a graphite film according to any one of claims 1 to 2, characterized in that, The method for preparing the wet membrane of modified polyarylene oxadiazole is as follows: 1) Preparation of the stock solution: First, a modified polyarylene oxadiazole stock solution was prepared using a solution mixing copolymerization method; 2) Wet film forming: The obtained modified polyarylene oxadiazole stock solution is then solidified by extrusion molding to obtain a wet film containing water-modified polyarylene oxadiazole.

6. The method for preparing a graphite film according to claim 5, characterized in that, The modified polyarylene oxadiazole stock solution is prepared by the following method: terephthalic acid, isophthalic acid, azobenzene compound, and hydrazine salt are reacted at 80–90°C for 0.5–1 h under the action of fuming sulfuric acid, followed by heating to 120–135°C and reacting for 2–5 h; the reaction is terminated with benzoic acid, and the reaction is continued at 120–135°C for another 0.5–1 h, followed by vacuum degassing to obtain the modified polyarylene oxadiazole stock solution; wherein, the molar ratio of isophthalic acid to terephthalic acid is 0.05–0.55:0.95–0.45, the mass of azobenzene compound: the total mass of isophthalic acid and terephthalic acid = 0.01–0.09:1; the molar addition of hydrazine salt: the total molar amount of isophthalic acid and terephthalic acid = 1.02–1.10:1; the solid content of the reaction system is 5–25%.

7. The method for preparing a graphite film according to claim 6, characterized in that, The azobenzene compound is selected from: 4,4'-dicarboxylated azobenzene, 4,4'-dihydroxylated azobenzene, 4,4'-diaminolated azobenzene, 3,3'-dicarboxylated azobenzene, 3,3'-dihydroxylated azobenzene, 3,3'-diaminolated azobenzene, 2,4'-dicarboxylated azobenzene, 2,4'-dihydroxylated azobenzene, or 2,4'-diaminolated azobenzene.

8. The method for preparing a graphite film according to claim 1 or 2, characterized in that, The carbonization temperature is 1200–1500℃, and the graphitization temperature is 2600–3000℃.

9. The method for preparing a graphite film according to claim 7, characterized in that, The carbonization process is as follows: under inert gas protection, the dry modified polyarylene diazole film is gradually heated from room temperature to 400-500°C at a heating rate of 2-5°C / min, and held at that temperature for 0.5-2 hours. Then, the heating rate is reduced to 0.5-2°C / min and the temperature is increased to 500-600°C, and held at that temperature for 0.5-2 hours. Then, the heating rate is increased to 2-5°C / min and the temperature is increased to 1200-1500°C and held for 0.5-5 hours.

10. The method for preparing a graphite film according to claim 7, characterized in that, The graphitization process is as follows: heating to 2200-2300℃ at a heating rate of 3-10℃ / min, holding at that temperature for 0.5h-2h, then reducing the heating rate to 0.5-2℃ / min and heating to 2400-2500℃, holding at that temperature for 0.5h-2h; then increasing the heating rate to 3-10℃ / min and heating to 2600-3000℃ and holding at that temperature for 0.5h-2h, followed by natural cooling to room temperature to obtain the graphite film.

11. A graphite film, characterized in that, The graphite film is prepared by any one of claims 1 to 9.

12. The graphite film according to claim 11, characterized in that, The thickness of the graphite film is 20–200 μm.

13. The graphite film according to claim 11 or 12, characterized in that, The in-plane thermal conductivity of the graphite film is 1400–1800 W / (mK).

14. The graphite film according to claim 11 or 12, characterized in that, The conductivity of the graphite film is 0.8–2.2 × 10⁻⁶. 6 S / m.

15. The graphite film according to claim 11 or 12, characterized in that, The graphite film has an elongation at break of 5–15% and a tensile strength of 5–10 MPa.

16. The graphite film according to claim 11 or 12, characterized in that, The graphite film has a graphitization degree of 95-100%.

17. The graphite film according to claim 11 or 12, characterized in that, The lattice spacing of the graphite film is 0.335–0.350 nm.

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