A POD precursor film, a POD-based graphite film and a preparation method thereof

Abstract

The present invention relates to the technical field of heat-conducting graphite materials, and particularly relates to a POD precursor film, a POD-based graphite film and a preparation method. The preparation method is as follows: terephthalic acid, isophthalic acid, hydrazine salt and boron arene compounds are added into a fuming sulfuric acid system, and a polycondensation copolymerization reaction is carried out under heating conditions, and benzoic acid is added as a capping agent. After the reaction is completed, an antioxidant-modified polyarylene oxadiazole polymer solution is obtained; the antioxidant-modified polyarylene oxadiazole polymer solution is coated and scraped into a film, and then solidified and formed in a multi-stage coagulation bath, and finally neutralized, washed and dried to obtain a POD precursor film; the coagulation bath is an aqueous solution of sulfuric acid, boron compounds and sulfuric acid aqueous solution. The POD-based graphite film is obtained after the POD precursor film is subjected to carbonization treatment and graphitization treatment. The present invention realizes the significant improvement of the antioxidant performance of the graphite film while maintaining high thermal conductivity by synergistically introducing boron elements inside and outside the POD precursor film.

Description

Technical Field

[0001] The present invention relates to a POD precursor film, a POD-based graphite film and a preparation method thereof, belonging to the technical field of heat-conducting graphite materials. Background Art

[0002] Graphite films have excellent thermal conductivity and structural stability, can effectively dissipate heat and withstand high-temperature loads, and can be widely used as heat management materials in high-temperature environments, especially in industries with extremely strict heat management requirements such as aerospace, electronic devices, and new energy vehicles, playing an important role.

[0003] Conventional industrialized graphite films are mainly prepared by carbonizing and graphitizing polyimide (PI) films. The technical process is relatively mature. However, with the continuous improvement of material performance requirements, PI-based graphite films gradually show limitations in thermal conductivity, conversion rate, etc. In contrast, POD (aromatic polyoxadiazole) has significant advantages as a precursor film in preparing graphite films. The high aromaticity and stable chemical structure in POD molecules enable it to have a higher carbon residue rate during the carbonization and graphitization processes, reducing the generation of volatile by-products and ensuring a higher material utilization rate. In addition, the POD film has a lower volume shrinkage rate, can effectively maintain its thickness, and with the high density and thickness adjustability of its initial film structure, the thickness of the finally prepared graphite film is significantly better than that of the graphite film with PI as the precursor film, and thus can exhibit better heat management capabilities.

[0004] However, in the actual application process, the stability of graphite films in high-temperature environments is crucial. Although graphite materials themselves have relatively high thermal conductivity, in an oxidizing atmosphere, their antioxidant performance is poor and they are prone to oxidation decomposition, thereby affecting their long-term use stability. In many application scenarios, such as aero-engine components, heat dissipation films for electronic devices, etc., graphite films are easily exposed to high-temperature oxidation environments and must have excellent antioxidant performance to ensure stable structure and performance at extreme temperatures. The antioxidant ability of POD-based graphite films in high-temperature oxidation environments is relatively weak, which limits their wide application under high-temperature and high-pressure conditions, especially their stability under long-term operation or in a high-oxygen atmosphere. Patent applications with publication numbers CN118419923A, CN117645296A, and CN117945758A have all disclosed preparation methods of related graphite films. These graphite films all have good thermal conductivity, but the antioxidant performance is still a technical problem that needs to be solved in this field. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the present invention provides a POD precursor film, a POD-based graphite film and a preparation method thereof. By introducing an appropriate amount of boron aromatic compounds during the POD polymerization process, a copolymerization reaction occurs with the molecular chain of the POD precursor to obtain an antioxidant-modified polymer. Secondly, during the coagulation and forming process of the film, a certain proportion of boron compounds is added to the coagulation bath, and boron elements are further introduced onto the film surface through a solvent replacement process, thereby obtaining a POD precursor film with internal and external synergistic antioxidant functions. Subsequently, through carbonization and graphitization treatments, the POD precursor film is transformed into a POD-based graphite film with internal and external synergistic antioxidant properties, and the POD-based graphite film has good thermal conductivity and antioxidant properties.

[0006] The technical solution of the present invention to solve the above technical problems is as follows: A POD precursor film is prepared by first preparing an antioxidant-modified poly(arylene oxydiazole) polymer solution, then further introducing the antioxidant element boron onto the film surface during the solution wet film-forming process, and finally obtaining the POD precursor film through neutralization, washing, and drying. The structural formula of the antioxidant-modified poly(arylene oxydiazole) polymer is:

[0007] ;

[0008] The viscosity-average molecular weight of the antioxidant-modified poly(arylene oxydiazole) polymer is 20,000 - 80,000 g / mol; x:y:z = (0.9 - 0.6):(0.1 - 0.5):(0.01 - 0.1).

[0009] The present invention also discloses a preparation method of a POD precursor film, and the preparation method is as follows:

[0010] S1. Preparation of an antioxidant-modified poly(arylene oxydiazole) polymer solution:

[0011] Add terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic compounds into a fuming sulfuric acid system, carry out a polycondensation copolymerization reaction under heating conditions, and add benzoic acid as a capping agent. After the reaction ends, an antioxidant-modified poly(arylene oxydiazole) polymer solution is obtained;

[0012] S2. Preparation of the POD precursor film:

[0013] Use the antioxidant-modified poly(arylene oxydiazole) polymer solution prepared in step S1 for coating and doctor blading, then coagulate and form in a multi-stage coagulation bath, and finally obtain the POD precursor film through neutralization, washing, and drying;

[0014] The coagulation bath is an aqueous solution of sulfuric acid, boron compounds and sulfuric acid aqueous solution, and the boron compounds are at least one of boric acid and boron salts.

[0015] Furthermore, the hydrazine salt is selected from at least one of hydrazine sulfate and hydrazine hydrochloride;

[0016] The boron arene compound is 3,5-dicarboxyltriphenylboron;

[0017] The boron salt is selected from at least one of borax, sodium metaborate, potassium borate, lithium borate, and sodium 3,5-dicarboxyltriphenylboron.

[0018] Further, in step S1, the molar ratio of terephthalic acid, isophthalic acid, hydrazine salt, and boron arene compound is (5-8):(0.5-1.2):(1-2):(0.1-0.5);

[0019] The ratio of the total molar number of terephthalic acid, isophthalic acid, hydrazine salt, and boron arene compound to the molar number of benzoic acid is 1:(0.009-0.030).

[0020] Further, in step S1, the specific preparation process of the antioxidant-modified poly(arylene oxadiazole) polymer solution is as follows: First, fuming sulfuric acid is added to the reactor, and terephthalic acid, isophthalic acid, hydrazine salt, and boron arene compound are added in sequence under stirring conditions. After complete dispersion, the temperature is raised to 70-90°C and stirred for 1-2 hours, then the temperature is further raised to 120-140°C and stirred for 2-4 hours. Benzoic acid is added as a capping agent, and then the temperature is raised to 140-170°C and stirred for 0.5-2 hours, and then vacuum degassing is carried out at 75-85°C to obtain the antioxidant-modified poly(arylene oxadiazole) polymer solution.

[0021] Further, in step S2, the coagulation bath is a three-stage coagulation bath:

[0022] The first-stage coagulation bath is an aqueous solution of sulfuric acid and boron compound. The total mass content of boron compound and sulfuric acid in the first-stage coagulation bath is 35-45%, and the mass ratio of sulfuric acid to boron compound is (4-20):1;

[0023] The second-stage coagulation bath is an aqueous solution of sulfuric acid and boron compound. The total mass content of boron compound and sulfuric acid in the second-stage coagulation bath is 20-30%, and the mass ratio of sulfuric acid to boron compound is (4-25):1;

[0024] The third-stage coagulation bath is an aqueous solution of sulfuric acid. The mass content of sulfuric acid in the third-stage coagulation bath is 5-15%.

[0025] Further, in step S2, the coagulation bath is a three-stage coagulation bath: The difference in the mass content of boron compound between adjacent two-stage coagulation baths is not more than 5%; the difference in the mass content of sulfuric acid between adjacent two-stage coagulation baths is not more than 15%.

[0026] The present invention also discloses a POD-based graphite film, which is obtained by carbonizing and graphitizing the POD precursor film of the present invention.

[0027] The present invention also discloses a preparation method of a POD-based graphite film, and the preparation method is as follows:

[0028] Carbonization process: Under argon protection, place the POD precursor film in a carbonization furnace, heat it from room temperature to 200°C - 300°C at a heating rate of 3 - 10°C / min for pre-oxidation treatment, and keep it warm for 1 - 3 h; then heat it to 500 - 600°C at a heating rate of 2 - 5°C / min for low-temperature decomposition treatment, and keep it warm for 1 - 3 h; then heat it to 1200 - 1500°C at a heating rate of 2 - 5°C / min for carbonization, and keep it warm for 1 - 3 h to obtain a POD carbonized film;

[0029] Graphitization process: Under argon protection, heat the POD carbonized film to 2000 - 2200°C at a heating rate of 3 - 10°C / min, keep it warm for 1 - 3 h, then heat it to 2400 - 2500°C at a heating rate of 2 - 5°C / min, and keep it warm for 1 - 3 h; then heat it to 2600 - 3000°C at a heating rate of 2 - 5°C / min and hold it for 1 - 3 h, and then slowly cool it to room temperature at a cooling rate of 3 - 10°C / min. During the cooling process, dope 1 - 5% mass concentration of oxygen in the argon atmosphere to obtain the final POD-based graphite film.

[0030] The beneficial effects of the present invention are:

[0031] By synergistically introducing boron elements inside and outside the POD precursor film, the present invention achieves a significant improvement in the antioxidant performance of the graphite film while maintaining high thermal conductivity. This performance improvement is mainly attributed to multiple effects induced by boron atom doping: on the one hand, the introduction of boron atoms changes the electron cloud distribution on the surface of the graphite crystal, reducing the adsorption activity of oxygen molecules on the graphite surface, thereby effectively slowing down the oxidation reaction rate; on the other hand, during the graphitization process, the B-C covalent bonds formed by the combination of boron atoms and carbon atoms can further generate a stable boron oxide film at high temperatures. This film forms a physical barrier on the surface of the graphite film, effectively isolating oxygen and thus enhancing the antioxidant performance. In addition, boron doping optimizes the microstructure of the graphite film, such as reducing crystal defects and increasing lattice order, decreasing the scattering probability of phonons at lattice boundaries and defects, improving the phonon conduction efficiency. At the same time, the introduction of B-C covalent bonds further enhances the overall stability of the carbon atom network, reduces the non-uniformity of crystal vibration modes, and improves the phonon conduction path, thereby promoting the efficient propagation of thermal energy in the film plane direction. To ensure the effect of boron doping, the present invention adopts a strategy of internal and external synergistic doping. The internal boron element is introduced through copolymerization reaction, with a small doping amount and strong binding, which can significantly optimize the microstructure of the graphite film and improve its thermal conductivity; the external boron element is introduced through solvent replacement, mainly distributed on the film surface, and forms a boron oxide film with antioxidant effect after graphitization, thereby further enhancing the antioxidant performance. At the same time, the present invention strictly controls the doping concentration and method of boron elements to avoid excessive or improper doping from destroying the lattice symmetry of the graphite film, resulting in increased phonon scattering and decreased thermal conductivity. Under this scientific design, the graphite film of the present invention achieves a good balance between high thermal conductivity and antioxidant performance, and has important application value.

[0032] The POD-based graphite film prepared by the present invention has excellent performance. Its thickness is 50 - 200 μm, density is 1.9 - 2.2 g / cm³, thermal conductivity reaches 1300 - 1800 W / (m·K), thermal diffusivity is 800 - 1000 mm² / s, and the oxidation induction time is 40 - 60 minutes at 200 °C, showing excellent antioxidant ability and thermal management performance.

[0033] The POD-based graphite film described in the present invention can be applied in more severe environments. By introducing antioxidant elements on the surface and inside the film, the antioxidant ability of the graphite film can be effectively enhanced, its service life can be extended, while not affecting its excellent thermal conductivity and high conversion rate. Moreover, the film surface is uniform and flat, and there will be no powder falling problem. The antioxidant performance of the POD-based graphite film prepared by the preparation method described in the present invention is improved, which can not only enhance its stability in high-temperature environments, but also promote its applications in multiple industries such as aerospace, electronic heat dissipation, and energy conversion, providing a new research direction and technological breakthrough point for the development of the next generation of high-performance thermal management materials. Detailed Embodiments

[0034] The following provides a detailed description of the specific embodiments of the present invention. The present invention can be implemented in many other ways different from those described herein. Those skilled in the art can make similar improvements without departing from the connotation of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein.

[0035] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used are only for describing the specific embodiments and do not limit the present invention.

[0036] A POD precursor film is prepared by first preparing an antioxidant-modified poly(arylene oxadiazole) polymer solution, then further introducing the antioxidant element boron onto the film surface during the solution wet film-forming process, and finally obtaining the POD precursor film through neutralization, washing, and drying. The structural formula of the antioxidant-modified poly(arylene oxadiazole) polymer is:

[0037] ;

[0038] The viscosity-average molecular weight of the antioxidant-modified poly(arylene oxadiazole) polymer is 20,000 - 80,000 g / mol; x:y:z = (0.9 - 0.6):(0.1 - 0.5):(0.01 - 0.1).

[0039] The present invention also discloses a method for preparing a POD precursor film. The preparation method is as follows:

[0040] S1. Preparation of an antioxidant-modified poly(arylene oxadiazole) polymer (POD) solution:

[0041] Add terephthalic acid, isophthalic acid, hydrazine salt, and boron aromatic compounds into a fuming sulfuric acid system, carry out polycondensation copolymerization reaction under heating conditions, and add benzoic acid as a capping agent. After the reaction ends, obtain an antioxidant-modified poly(arylene oxadiazole) polymer solution;

[0042] S2. Preparation of the POD precursor film:

[0043] Use the antioxidant-modified poly(arylene oxadiazole) polymer solution prepared in step S1 for coating and doctor blading, then solidify and form in a multi-stage coagulation bath, and finally obtain the POD precursor film through neutralization, washing, and drying;

[0044] The coagulation bath is an aqueous solution of sulfuric acid, boron compounds, and sulfuric acid aqueous solution, and the boron compounds are at least one of boric acid and borate salts.

[0045] Specifically, the hydrazine salt is selected from at least one of hydrazine sulfate and hydrazine hydrochloride;

[0046] The boron aromatic compound is 3,5-dicarboxytriphenylborane;

[0047] The boron salt is selected from at least one of borax, sodium metaborate, potassium borate, lithium borate and 3,5-dicarboxytriphenylborate sodium.

[0048] More specifically, the structural formula of 3,5-dicarboxytriphenylborane is: .

[0049] The 3,5-dicarboxy triphenylborane is self-made, and the self-made method is as follows: (1) Add 3,5-dibromotoluene, tetrabutylammonium bromide (catalyst) and toluene (solvent) into a three-necked flask, stir and dissolve; add dimethyl sulfate dropwise, heat to 80°C, slowly add NaOH aqueous solution (provide the alkaline conditions necessary for the reaction); reflux for 12 hours; separate the liquids after cooling, dry the organic phase with anhydrous Na2SO4, and remove the solvent by rotary evaporation; purify by column chromatography to obtain 3,5-dibromo-p-xylene. (2) Dissolve 3,5-dibromo-p-xylene in concentrated H2SO4, add KMnO4 in batches under stirring, and control the temperature to <40°C; heat to 100°C, reflux for 6 hours until the purple color fades (generating MnO2 precipitate); filter while hot, cool the filtrate to room temperature, adjust the pH to 1 with concentrated hydrochloric acid, and precipitate a white precipitate; filter, wash with water, and recrystallize from ethanol / water (volume ratio 1:1) to obtain 3,5-dibromophthalic acid. (3) Dissolve 3,5-dibromophthalic acid in anhydrous THF, cool to -78°C (dry ice / acetone bath), slowly add n-BuLi dropwise, keep the temperature <-70°C, stir for 2 hours to obtain a deep red solution; add BF3·OEt2 dropwise, slowly warm to room temperature, stir for 12 hours, add dilute hydrochloric acid to quench the reaction, and extract with ethyl acetate; combine the organic phases, dry over anhydrous Na2SO4, and rotary evaporate to obtain a crude product of 3,5-dicarboxytriphenylborane; dissolve the crude product in THF / water (volume ratio 1:1), add dilute hydrochloric acid to pH = 2, precipitate, filter, and vacuum dry (40°C, 12 h) to obtain 3,5-dicarboxytriphenylborane.

[0050] Specifically, in step S1, the molar ratio of terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic compound is (5-8): (0.5-1.2): (1-2): (0.1-0.5);

[0051] The ratio of the total mole number of terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic hydrocarbon compounds to the mole number of benzoic acid is 1: (0.009-0.030).

[0052] More specifically, the equivalent concentration of the oleum is 115%, and the mass ratio of the total mass of terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic hydrocarbon compounds to oleum is (0.05-0.15): (0.85-0.95).

[0053] Specifically, in step S1, the specific preparation process of the antioxidant-modified poly(arylene oxadiazole) polymer solution is as follows: First, fuming sulfuric acid is added to a reactor, and terephthalic acid, isophthalic acid, hydrazine salt, and borarene compound are sequentially added under stirring conditions. After complete dispersion, the temperature is raised to 70 - 90 °C and stirred for 1 - 2 h, then the temperature is further raised to 120 - 140 °C and stirred for 2 - 4 h. Benzoic acid is added as a capping agent, and then the temperature is raised to 140 - 170 °C and stirred for 0.5 - 2 h. Then, vacuum degassing is carried out at 75 - 85 °C to obtain the antioxidant-modified poly(arylene oxadiazole) polymer solution.

[0054] Specifically, in step S2, the coagulation bath is a three-stage coagulation bath:

[0055] The first-stage coagulation bath is an aqueous solution of sulfuric acid and boride compound. The total mass content of the boride compound and sulfuric acid in the first-stage coagulation bath is 35 - 45%, and the mass ratio of sulfuric acid to boride compound is (4 - 20):1;

[0056] The second-stage coagulation bath is an aqueous solution of sulfuric acid and boride compound. The total mass content of the boride compound and sulfuric acid in the second-stage coagulation bath is 20 - 30%, and the mass ratio of sulfuric acid to boride compound is (4 - 25):1;

[0057] The third-stage coagulation bath is an aqueous solution of sulfuric acid. The mass content of sulfuric acid in the third-stage coagulation bath is 5 - 15%.

[0058] Specifically, in step S2, the coagulation bath is a three-stage coagulation bath: The difference in the mass content of the boride compound between adjacent two-stage coagulation baths is not more than 5%; the difference in the mass content of sulfuric acid between adjacent two-stage coagulation baths is not more than 15%.

[0059] More specifically, the poly(arylene oxadiazole) polymer solution prepared in step S1 is poured onto a hot stage glass plate at 80 - 120 °C and coated and scraped with a certain thickness. The wet film thickness is adjusted by adjusting the distance between the scraper and the glass plate. The thickness of the scraper coating is 800 - 1500 mm; the coated wet film and the glass plate are placed in the first-stage coagulation bath for 5 - 10 min to solidify and form; the wet film is peeled off from the glass plate and placed in the second-stage coagulation bath for 10 - 20 min; the wet film is then placed in the third-stage coagulation bath for 10 - 20 min; the completely solidified wet film is placed in an alkali solution for 20 - 40 min to neutralize acidic ions, and finally the wet film is placed in deionized water and soaked for more than 8 h to remove the alkali solution. The obtained wet film is dried at 80 - 120 °C for 30 - 60 min to obtain a POD precursor film with internal and external synergistic antioxidant properties.

[0060] More specifically, the alkali solution is an aqueous sodium hydroxide solution with a mass concentration of 1 - 5%.

[0061] The present invention also discloses a POD-based graphite film, which is obtained by carbonizing and graphitizing the POD precursor film of the present invention.

[0062] The present invention also discloses a preparation method of a POD-based graphite film. The preparation method is as follows:

[0063] Carbonization process: Under argon protection, place the POD precursor film in a carbonization furnace, heat it from room temperature to 200°C - 300°C at a heating rate of 3 - 10°C / min for pre-oxidation treatment, and keep it warm for 1 - 3 h; then heat it to 500 - 600°C at a heating rate of 2 - 5°C / min for low-temperature decomposition treatment, and keep it warm for 1 - 3 h; then heat it to 1200 - 1500°C at a heating rate of 2 - 5°C / min for carbonization, and keep it warm for 1 - 3 h. At this time, the polymer decomposition is completed, forming a preliminary skeleton structure mainly composed of amorphous carbon. At the same time, the doped boron element begins to react with the carbon-based material initially, and a POD carbonized film is obtained.

[0064] Graphitization process: Under argon protection, heat the POD carbonized film to 2000 - 2200°C at a heating rate of 3 - 10°C / min, keep it warm for 1 - 3 h, then heat it to 2400 - 2500°C at a heating rate of 2 - 5°C / min, and keep it warm for 1 - 3 h; then heat it to 2600 - 3000°C at a heating rate of 2 - 5°C / min and keep it for 1 - 3 h. Subsequently, slowly cool it to room temperature at a cooling rate of 3 - 10°C / min. During the cooling process, dope 1 - 5% mass concentration of oxygen in the argon atmosphere to promote the combination of boron element on the film surface with oxygen to form boron oxide, forming a protective oxide film, and obtaining the final POD-based graphite film with high thermal conductivity and internal and external synergistic antioxidant properties.

[0065] Example 1

[0066] S1: Preparation of POD solution:

[0067] Disperse and dissolve terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dicarboxyltriphenylboron in 115% effective concentration fuming sulfuric acid. After each monomer is dissolved, raise the temperature to 80°C and stir for 1 h, then continue to raise the temperature to 120°C and stir for 2 h. Add benzoic acid as a capping agent, then raise the temperature to 150°C and stir for 1 h, and then cool to 80°C for vacuum degassing for 2 h to obtain an antioxidant-modified POD solution; wherein, the molar ratio of terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dicarboxyltriphenylboron is 7.5:1.2:1:0.3, the ratio of the total molar number of terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dicarboxyltriphenylboron to the molar number of benzoic acid is 1:0.009, and the mass ratio of the total mass of terephthalic acid, isophthalic acid, hydrazine salt, and boron aromatic compounds to fuming sulfuric acid is 0.10:0.90.

[0068] S2: Preparation of POD precursor film:

[0069] Pour the POD solution prepared in step S1 onto a hot stage glass plate at 80 °C and coat and scrape the film with a thickness of 1000 mm. Place the coated wet film and the glass plate in the first-stage coagulation bath (mixed aqueous solution of 32% mass concentration sulfuric acid and 8% mass concentration boric acid) for 8 min to solidify and form; peel the wet film from the glass plate and place the wet film in the second-stage coagulation bath (mixed aqueous solution of 20% mass concentration sulfuric acid and 5% mass concentration boric acid) for 10 min; then place the wet film in the third-stage coagulation bath (10% mass concentration sulfuric acid solution) for 10 min; place the completely solidified wet film in 1% alkali solution for 30 min to neutralize acidic ions, and finally place the wet film in deionized water and soak for more than 8 h to remove the alkali solution. The obtained wet film is dried at 100 °C for 40 min to obtain the POD film.

[0070] S3: Preparation of graphite film:

[0071] Prepare a graphite film from the POD precursor film prepared in step S2 through a carbonization and graphitization process.

[0072] The carbonization process is as follows: Under argon protection, place the POD precursor film in a carbonization furnace, heat it from room temperature to 300 °C at a heating rate of 6 °C / min for pre-oxidation treatment, and keep it warm for 2 h; then heat it to 600 °C at a heating rate of 3 °C / min for low-temperature decomposition treatment, and keep it warm for 2 h; then heat it to 1500 °C at a heating rate of 3 °C / min for carbonization, and keep it warm for 2 h.

[0073] The graphitization process is as follows: Under argon protection, heat the POD carbonized film to 2200 °C at a heating rate of 10 °C / min, keep it warm for 3 h, then heat it to 2500 °C at a heating rate of 5 °C / min, keep it warm for 3 h; then heat it to 3000 °C at a heating rate of 5 °C / min and keep it for 3 h, and then slowly cool it to room temperature at a cooling rate of 10 °C / min. During the cooling process, dope 2% concentration of oxygen atmosphere in the argon atmosphere to finally obtain the POD-based graphite film.

[0074] Example 2

[0075] The preparation method is the same as that of Example 1. Only in the preparation stage of the POD solution, part of the monomer ratio is changed, and the ingredients are prepared according to the recalculated mass of each monomer. Among them, the molar ratio of terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dicarboxyltriphenylboron is 7.0:0.9:1.5:0.4, the ratio of the total molar number of terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dicarboxyltriphenylboron to the molar number of benzoic acid is 1:0.015, and the mass ratio of the total mass of terephthalic acid, isophthalic acid, hydrazine salt, and boron aromatic compounds to the mass of fuming sulfuric acid is 0.15:0.95.

[0076] Example 3

[0077] The preparation method is the same as that of Example 1. Only in the preparation stage of the POD solution, part of the monomer ratio is changed, and the ingredients are prepared according to the recalculated mass of each monomer. Among them, the molar ratio of terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dimethyl-3,5-dicarboxyltriphenylboron is 6.5:0.8:1:0.4, the ratio of the total molar number of terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dicarboxyltriphenylboron to the molar number of benzoic acid is 1:0.012, and the mass ratio of the total mass of terephthalic acid, isophthalic acid, hydrazine salt, and boron aromatic compounds to the mass of fuming sulfuric acid is 0.10:0.90.

[0078] Example 4

[0079] The preparation method is the same as that of Example 1. Only in the preparation stage of the POD solution, part of the monomer ratio is changed, and the ingredients are prepared according to the recalculated mass of each monomer. Among them, the molar ratio of terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dicarboxyltriphenylboron is 7.5:1:1:0.5, the ratio of the total molar number of terephthalic acid, isophthalic acid, hydrazine sulfate, and 3,5-dicarboxyltriphenylboron to the molar number of benzoic acid is 1:0.009, and the mass ratio of the total mass of terephthalic acid, isophthalic acid, hydrazine salt, and boron aromatic compounds to the mass of fuming sulfuric acid is 0.10:0.90.

[0080] Example 5

[0081] S1: The preparation method is the same as that of Example 1.

[0082] S2: Preparation of the POD precursor film:

[0083] Pour the POD solution prepared in step S1 onto a hot stage glass plate at 80 °C and coat and scrape the film with a thickness of 1000 mm. Place the coated wet film and the glass plate in the first-stage coagulation bath (a mixed aqueous solution of sulfuric acid with a mass concentration of 38% and boric acid with a mass concentration of 2%) for 8 min to solidify and form. Peel the wet film from the glass plate and place the wet film in the second-stage coagulation bath (a mixed aqueous solution of sulfuric acid with a mass concentration of 24% and boric acid with a mass concentration of 1%) for 10 min. Then place the wet film in the third-stage coagulation bath (sulfuric acid solution with a mass concentration of 10%) for 10 min. Place the completely solidified wet film in 1% alkali solution for 30 min to neutralize acidic ions. Finally, soak the wet film in deionized water for more than 8 h to remove the alkali solution. The obtained wet film is dried at 100 °C for 40 min to obtain the POD film.

[0084] S3: The preparation method is the same as that in Example 1.

[0085] Example 6

[0086] S1: Preparation of POD solution:

[0087] Disperse and dissolve terephthalic acid, isophthalic acid, hydrazine hydrochloride, and 3,5-dicarboxyltriphenylboron in fuming sulfuric acid with an effective concentration of 115%. After each monomer is dissolved, raise the temperature to 70 °C and stir for 2 h, then continue to raise the temperature to 140 °C and stir for 3 h. Add benzoic acid as a capping agent, and then raise the temperature to 170 °C and stir for 0.5 h. Then cool down to 85 °C and carry out vacuum degassing for 2 h to obtain an antioxidant-modified POD solution. Among them, the molar ratio of terephthalic acid, isophthalic acid, hydrazine hydrochloride, and 3,5-dicarboxyltriphenylboron is 8:0.5:2:0.1, the ratio of the total molar number of terephthalic acid, isophthalic acid, hydrazine hydrochloride, and 3,5-dicarboxyltriphenylboron to the molar number of benzoic acid is 1:0.030, and the mass ratio of the total mass of terephthalic acid, isophthalic acid, hydrazine salt, and boron aromatic compounds to fuming sulfuric acid is 0.15:0.85.

[0088] S2: Preparation of POD precursor film:

[0089] Pour the POD solution prepared in step S1 onto a hot stage glass plate at 120 °C and coat and scrape the film with a thickness of 1000 mm. Place the coated wet film and the glass plate in a first-stage coagulation bath (a mixed aqueous solution of 40% mass concentration sulfuric acid and 5% mass concentration potassium borate) for 10 min to solidify and form a shape; peel the wet film from the glass plate and place the wet film in a second-stage coagulation bath (a mixed aqueous solution of 28% mass concentration sulfuric acid and 2% mass concentration potassium borate) for 15 min; then place the wet film in a third-stage coagulation bath (15% mass concentration sulfuric acid solution) for 15 min; place the completely solidified wet film in 1% alkali solution for 30 min to neutralize acidic ions, and finally place the wet film in deionized water and soak for more than 8 h to remove the alkali solution. The obtained wet film is dried at 100 °C for 40 min to obtain a POD film.

[0090] S3: Preparation of graphite film:

[0091] The POD precursor film prepared in step S2 is prepared into a graphite film through a carbonization and graphitization process.

[0092] The carbonization process is as follows: Under argon protection, place the POD precursor film in a carbonization furnace, heat it from room temperature to 300 °C at a heating rate of 10 °C / min for pre-oxidation treatment, and keep it warm for 3 h; then heat it to 500 °C at a heating rate of 2 °C / min for low-temperature decomposition treatment, and keep it warm for 3 h; then heat it to 1200 °C at a heating rate of 5 °C / min for carbonization, and keep it warm for 3 h.

[0093] The graphitization process is as follows: Under argon protection, heat the POD carbonized film to 2100 °C at a heating rate of 5 °C / min, keep it warm for 2 h, then heat it to 2400 °C at a heating rate of 2 °C / min, keep it warm for 3 h; then heat it to 2600 °C at a heating rate of 3 °C / min and keep it for 3 h, and then slowly cool it to room temperature at a cooling rate of 5 °C / min. During the cooling process, dope 1% concentration of oxygen atmosphere in the argon atmosphere to finally obtain a POD-based graphite film.

[0094] Example 7

[0095] S1: Preparation of POD solution:

[0096] Dissolve terephthalic acid, isophthalic acid, hydrazine hydrochloride and 3,5-dicarboxytriphenylboron in fuming sulfuric acid with an effective concentration of 115%. After each monomer is dissolved, raise the temperature to 90 °C and stir for 1 h, then continue to raise the temperature to 120 °C and stir for 4 h. Add benzoic acid as a capping agent, and then raise the temperature to 140 °C and stir for 3 h. Then cool down to 75 °C and carry out vacuum degassing for 2 h to obtain an antioxidant-modified POD solution. Among them, the molar ratio of terephthalic acid, isophthalic acid, hydrazine hydrochloride and 3,5-dicarboxytriphenylboron is 5:1.2:1.5:0.2, the ratio of the total molar number of terephthalic acid, isophthalic acid, hydrazine hydrochloride and 3,5-dicarboxytriphenylboron to the molar number of benzoic acid is 1:0.020, and the ratio of the total mass of terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic compound to the mass of fuming sulfuric acid is 0.05:0.95.

[0097] S2: Preparation of POD precursor film:

[0098] Pour the POD solution prepared in step S1 onto a hot stage glass plate at 120 °C and coat and scrape the film with a thickness of 1000 mm. The coated wet film and the glass plate are placed in a first-stage coagulation bath (a mixed aqueous solution of sulfuric acid with a mass concentration of 30% and sodium metaborate with a mass concentration of 5%) for 10 min to solidify and form. Peel the wet film from the glass plate, and place the wet film in a second-stage coagulation bath (a mixed aqueous solution of sulfuric acid with a mass concentration of 17% and sodium metaborate with a mass concentration of 3%) for 15 min; the wet film is then placed in a third-stage coagulation bath (sulfuric acid solution with a mass concentration of 5%) for 20 min; the completely solidified wet film is placed in 1% alkali solution for 30 min to neutralize acidic ions. Finally, the wet film is placed in deionized water and soaked for more than 8 h to remove the alkali solution. The obtained wet film is dried at 80 °C for 60 min to obtain a POD film.

[0099] S3: Preparation of graphite film:

[0100] The POD precursor film prepared in step S2 is prepared into a graphite film through a carbonization and graphitization process.

[0101] The carbonization process is as follows: Under argon protection, place the POD precursor film in a carbonization furnace, heat it from room temperature to 250 °C at a heating rate of 3 °C / min for pre-oxidation treatment, and keep it warm for 1 h; then heat it to 550 °C at a heating rate of 5 °C / min for low-temperature decomposition treatment, and keep it warm for 1 h; then heat it to 1400 °C at a heating rate of 2 °C / min for carbonization, and keep it warm for 1 h.

[0102] The graphitization process is as follows: Under argon protection, the POD carbonized film is heated to 2000 °C at a heating rate of 3 °C / min, held for 1 h, then heated to 2500 °C at a heating rate of 3 °C / min and held for 1 h; then heated to 2800 °C at a heating rate of 4 °C / min and held for 2 h, and subsequently slowly cooled to room temperature at a cooling rate of 3 °C / min. During the cooling process, an oxygen atmosphere with a concentration of 5% is doped in the argon atmosphere, and finally a POD-based graphite film is obtained.

[0103] Comparative Example 1

[0104] S1: The preparation method is the same as that in Example 1, except that: 3,5-dicarboxyltriphenylboron is not added, and the quality of other raw materials remains unchanged.

[0105] S2: The preparation method is the same as that in Example 1, except that: only sulfuric acid is used in the coagulation bath, and boric acid is not added. The specific process is as follows:

[0106] The POD solution prepared in step S1 is poured onto a hot stage glass plate at 80 °C and coated and scraped with a thickness of 1000 mm. The coated wet film and the glass plate are placed in the first-stage coagulation bath (40% sulfuric acid solution) for 8 min to solidify and form; the wet film is peeled off from the glass plate, and the wet film is placed in the second-stage coagulation bath (25% sulfuric acid) for 10 min; the wet film is then placed in the third-stage coagulation bath (10% sulfuric acid solution) for 10 min; the completely solidified wet film is placed in 1% alkali solution for 30 min to neutralize acidic ions, and finally the wet film is placed in deionized water and soaked for more than 8 h to remove the alkali solution. The obtained wet film is dried at 100 °C for 40 min to obtain a POD film.

[0107] S3: The preparation method is the same as that in Example 1.

[0108] Comparative Example 2

[0109] S1: The preparation method is the same as that in Example 1, except that: 3,5-dicarboxyltriphenylboron is not added, and the quality of other raw materials remains unchanged.

[0110] S2: The preparation method is the same as that in Example 1.

[0111] S3: The preparation method is the same as that in Example 1.

[0112] Comparative Example 3

[0113] S1: The preparation method is the same as that in Example 1.

[0114] S2: Preparation of the POD precursor film:

[0115] Pour the POD solution prepared in step S1 onto a hot stage glass plate at 80 °C and coat and scrape the film with a thickness of 1000 mm. The coated wet film and the glass plate are placed in a first-stage coagulation bath (40% concentrated sulfuric acid solution) for 8 min to solidify and form a shape; peel the wet film from the glass plate, and place the wet film in a second-stage coagulation bath (25% concentrated sulfuric acid) for 10 min; then place the wet film in a third-stage coagulation bath (10% concentrated sulfuric acid solution) for 10 min; the completely solidified wet film is placed in 1% alkali solution for 30 min to neutralize acidic ions, and finally the wet film is placed in deionized water and soaked for more than 8 h to remove the alkali solution. The obtained wet film is dried at 100 °C for 40 min to obtain a POD film.

[0116] S3: The preparation method is the same as that in Example 1.

[0117] Comparative Example 4

[0118] S1: The preparation method is the same as that in Example 1, except that: in this Comparative Example 4, the addition ratio of 3,5-dicarboxyltriphenylboron is increased. In this Comparative Example 4, the molar ratio of terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic compound is 7.5:1.2:1:0.8; the amounts of other materials used are the same as those in Example 1.

[0119] S2: The preparation method is the same as that in Example 1.

[0120] S3: The preparation method is the same as that in Example 1.

[0121] Comparative Example 5

[0122] S1: The preparation method is the same as that in Example 1.

[0123] S2: The preparation method is the same as that in Example 1, except that: in this Comparative Example 5, only a first-stage coagulation bath is used, and the specific process is as follows:

[0124] Pour the POD solution prepared in step S1 onto a hot stage glass plate at 80 °C and coat and scrape the film with a thickness of 1000 mm. The coated wet film and the glass plate are placed in a coagulation bath (mixed aqueous solution of 20% mass concentration sulfuric acid and 5% mass concentration boric acid) for 28 min; the completely solidified wet film is placed in 1% alkali solution for 30 min to neutralize acidic ions, and finally the wet film is placed in deionized water and soaked for more than 8 h to remove the alkali solution. The obtained wet film is dried at 100 °C for 40 min to obtain a POD film.

[0125] S3: The preparation method is the same as that in Example 1.

[0126] Comparative Example 6

[0127] S1: The preparation method is the same as that in Example 1.

[0128] S2: The preparation method is the same as that in Example 1, with the difference that in this Comparative Example 6, the concentration difference of boric acid between adjacent coagulation baths is increased. The specific process is as follows:

[0129] Pour the POD solution prepared in step S1 onto a hot stage glass plate at 80 °C and coat and scrape the film with a thickness of 1000 mm. The coated wet film and the glass plate are placed in a first-stage coagulation bath (a mixed aqueous solution of 30% mass concentration sulfuric acid and 10% mass concentration boric acid) for 8 min to solidify and form; peel the wet film from the glass plate, and place the wet film in a second-stage coagulation bath (a mixed aqueous solution of 22% mass concentration sulfuric acid and 3% mass concentration boric acid) for 10 min; then place the wet film in a third-stage coagulation bath (10% mass concentration sulfuric acid solution) for 10 min; the completely solidified wet film is placed in 1% alkali solution for 30 min to neutralize acidic ions, and finally the wet film is placed in deionized water and soaked for more than 8 h to remove the alkali solution. The obtained wet film is dried at 100 °C for 40 min to obtain a POD film.

[0130] S3: The preparation method is the same as that in Example 1.

[0131] Comparative Example 7

[0132] S1: The preparation method is the same as that in Example 1.

[0133] S2: The preparation method is the same as that in Example 1.

[0134] S3: The preparation method is the same as that in Example 1, with the difference that during the carbonization process, the temperature of the low-temperature decomposition treatment is increased. The specific process is as follows:

[0135] The carbonization process is as follows: Under argon protection, place the POD precursor film in a carbonization furnace, heat it from room temperature to 300 °C at a heating rate of 6 °C / min for pre-oxidation treatment, and keep it warm for 2 h; then heat it to 700 °C at a heating rate of 3 °C / min for low-temperature decomposition treatment, and keep it warm for 2 h; then heat it to 1500 °C at a heating rate of 3 °C / min for carbonization, and keep it warm for 2 h.

[0136] The graphitization process is as follows: Under argon protection, heat the POD carbonized film to 2200 °C at a heating rate of 10 °C / min, keep it warm for 3 h, then heat it to 2500 °C at a heating rate of 5 °C / min, keep it warm for 3 h; then heat it to 3000 °C at a heating rate of 5 °C / min and keep it for 3 h, and then slowly cool it to room temperature at a cooling rate of 10 °C / min. During the cooling process, dope 2% concentration of oxygen atmosphere in the argon atmosphere, and finally obtain a POD-based graphite film.

[0137] Perform performance tests on the POD-based graphite films prepared in the above examples and comparative examples. The specific test results are shown in Table 1 below. The test methods involved are:

[0138] (1)Thermal conductivity test: The steady-state hot plate method was used to test the thermal conductivity and thermal diffusivity of each sample.

[0139] (2)Oxidation induction time: Under the condition of 200 °C, the oxidation induction time of each sample was tested using a pure oxygen atmosphere.

[0140] Table 1 Test results of the performance of POD-based graphite films

[0141]

[0142] It can be seen from the above table data that: For the POD-based graphite films prepared by the preparation method described in the present invention in Examples 1-7, the antioxidant performance of the POD-based graphite films is improved through internal and external synergy in the preparation method, so that the POD-based graphite films have good antioxidant performance while having high thermal conductivity and diffusivity. In addition, by matching appropriate coagulation bath conditions, the POD-based graphite films have good antioxidant performance, smooth and flat surfaces, and no powder falling problems, thereby further improving the comprehensive performance of the POD-based graphite films.

[0143] It can be seen from the comparison of the experimental results between Comparative Example 1 and Example 1 that: If boron aromatic compounds and boron compounds are not added during the preparation of the POD-based graphite film, the antioxidant performance of the POD-based graphite film will decrease significantly.

[0144] It can be seen from the comparison of the experimental results between Comparative Example 2, Comparative Example 3 and Example 1 that: If boron aromatic compounds are not added during the preparation of the POD-based graphite film or boron compounds are not added during the coagulation and forming process, the antioxidant properties of the POD-based graphite film will both decrease significantly. Therefore, introducing boron elements internally and externally in the POD precursor film is more conducive to significantly improving the antioxidant performance while maintaining high thermal conductivity of the graphite film.

[0145] It can be seen from the comparison of the experimental results between Comparative Example 4 and Example 1 that: If the addition ratio of 3,5-dicarboxyltriphenylboron is increased, too much boron element is introduced into the POD polymer, destroying the original structure of the polymer and resulting in a decrease in thermal conductivity.

[0146] It can be seen from the comparison of the experimental results between Comparative Example 5 and Example 1 that: If only the primary coagulation bath is used, the phase transition during the coagulation process of the polymerization solution is reduced, the phase transition reaction is intense, resulting in an increase in internal defects of the film, and thus powder falling occurs during the carbonization and graphitization process.

[0147] It can be seen from the comparison of the experimental results between Comparative Example 6 and Example 1 that: If the concentration difference of boric acid between adjacent coagulation baths is increased, it slightly affects the performance of the final POD-based graphite film. Therefore, using the coagulation bath conditions defined in the present invention is more conducive to obtaining POD-based graphite film products with more excellent quality.

[0148] It can be seen from the comparison of the experimental results of Comparative Example 7 and Example 1 that: during the carbonization process, if the temperature of the low-temperature decomposition treatment is increased, boric acid or boron-containing groups are easily dehydrated and accelerated to generate B2O3. However, the vapor pressure of B2O3 increases significantly at high temperatures, resulting in the loss of boron atoms through the gas phase, ultimately affecting the antioxidant performance of the POD-based graphite film. Therefore, the cooperation between the POD precursor film and the carbonization process conditions of the present invention is more conducive to obtaining a POD-based graphite film product with excellent comprehensive performance.

[0149] The technical features of the above-described embodiments can be combined arbitrarily. For the sake of brevity of description, all possible combinations of the various technical features in the above embodiments are not exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, it should be considered to be within the scope described in this specification.

[0150] For those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. The protection scope of the present invention is subject to the appended claims.

Claims

1. A method for preparing a POD precursor film, characterized in that: The preparation method is: S1. Preparation of antioxidant modified polyaromatic oxadiazole polymer solution: Add terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic hydrocarbon compound into fuming sulfuric acid system, carry out polycondensation copolymerization reaction under heating condition, add benzoic acid as end-capping agent, and obtain antioxidant modified polyaromatic oxadiazole polymer solution after the reaction. The molar ratio of terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic hydrocarbon compound is (5-8): (0.5-1.2): (1-2): (0.1-0.5); S2. Preparation of POD precursor film: The antioxidant modified polyaromatic oxadiazole polymer solution prepared in step S1 is used to coat the scraping film, and then solidified in a multi-stage solidification bath, and finally neutralized, washed, and dried to obtain a POD precursor film; In step S2, the coagulation bath is a three-stage coagulation bath: The first coagulation bath is an aqueous solution of sulfuric acid and a boron compound. The total mass content of the boron compound and sulfuric acid in the first coagulation bath is 35-45%, and the mass ratio of sulfuric acid to the boron compound is (4-20):1; The second coagulation bath is an aqueous solution of sulfuric acid and boron compounds. The total mass content of the boron compounds and sulfuric acid in the second coagulation bath is 20-30%, and the mass ratio of sulfuric acid to the boron compounds is (4-25):1; The third coagulation bath is an aqueous solution of sulfuric acid, and the mass content of sulfuric acid in the third coagulation bath is 5-15%; The difference in mass content of boron compounds in two adjacent coagulation baths shall not exceed 5%; the difference in mass content of sulfuric acid in two adjacent coagulation baths shall not exceed 15%; The boron compound is at least one of boric acid and boron salt.

2. The method for preparing a POD precursor film according to claim 1, characterized in that: The hydrazine salt is selected from at least one of hydrazine sulfate and hydrazine hydrochloride; The boron aromatic compound is 3,5-dicarboxytriphenylborane; The boron salt is selected from at least one of borax, sodium metaborate, potassium borate, lithium borate, and 3,5-dicarboxytriphenylborate sodium.

3. The method for preparing a POD precursor film according to claim 1, characterized in that: In step S1, the ratio of the total mole number of terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic hydrocarbon compounds to the mole number of benzoic acid is 1: (0.009-0.030).

4. The method for preparing a POD precursor film according to claim 1, characterized in that: In step S1, the specific preparation process of the antioxidant modified polyaromatic oxadiazole polymer solution is as follows: first, fuming sulfuric acid is added to the reactor, and terephthalic acid, isophthalic acid, hydrazine salt and boron aromatic compounds are added in sequence under stirring conditions. After complete dispersion, the temperature is raised to 70-90°C and stirred for 1h-2h, and then the temperature is continued to be raised to 120-140°C and stirred for 2-4h, benzoic acid is added as a capping agent, and the temperature is raised to 140-170°C and stirred for 0.5-2h, and then vacuum degassing is performed at 75-85°C to obtain an antioxidant modified polyaromatic oxadiazole polymer solution.

5. A POD-based graphite film, characterized in that: The POD-based graphite film is obtained by subjecting the POD precursor film to carbonization and graphitization; during the carbonization process, a low-temperature decomposition treatment is performed at 500-600° C.; The POD precursor film is prepared according to the preparation method according to any one of claims 1 to 4.

6. A POD-based graphite film according to claim 5, characterized in that: The POD-based graphite film has a thickness of 50-200 μm, a density of 1.9-2.2 g / cm³, a thermal conductivity of 1300-1800 W / (m·K), and a thermal diffusion coefficient of 800-1000 mm² / s.

7. A method for preparing a POD-based graphite film according to claim 5 or 6, characterized in that: The preparation method is: Carbonization treatment process: under argon protection, the POD precursor film is placed in a carbonization furnace, heated from room temperature to 200-300°C at a heating rate of 3-10°C / min for pre-oxidation treatment, and kept warm for 1-3h; then heated to 500-600°C at a heating rate of 2-5°C / min for low-temperature decomposition treatment, and kept warm for 1-3h; then heated to 1200-1500°C at a heating rate of 2-5°C / min for carbonization, and kept warm for 1-3h to obtain a POD carbonized film; The graphitization treatment process is as follows: under argon protection, the POD carbonized film is heated to 2000-2200°C at a heating rate of 3-10°C / min, kept warm for 1-3h, then heated to 2400-2500°C at a heating rate of 2-5°C / min, kept warm for 1-3h; then heated to 2600-3000°C at a heating rate of 2-5°C / min and kept warm for 1~3h, then slowly cooled to room temperature at a cooling rate of 3-10°C / min. During the cooling process, 1-5% mass concentration of oxygen is doped into the argon atmosphere to obtain the final POD-based graphite film.

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