Composite material for improving longitudinal heat-conducting property of polyimide film

Abstract

The invention discloses a composite material for improving the longitudinal heat-conducting property of a polyimide film. The composite material comprises the following components in parts by mass: 60-80 parts of polyimide resin; 20 to 30 parts of modified boron nitride; 5 to 15 parts of carbon nitride nanosheets; 5 to 15 parts of modified graphene; 2 to 10 parts of hexagonal boron nitride nanotubes; 3-5 parts of a thermal crosslinking agent; according to the composite material disclosed by the invention, by introducing the modified boron nitride, the carbon nitride nanosheets, the modified graphene, the hexagonal boron nitride nanotubes and the thermal cross-linking agent, the longitudinal heat-conducting property of the polyimide film is remarkably improved, and meanwhile, the original excellent physical and mechanical properties of the polyimide film are kept; the composite material has a wide application prospect in the fields of aerospace, electronics and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and specifically relates to a composite material for improving the longitudinal thermal conductivity of polyimide films. Background Art

[0002] As a polymer material with excellent thermal stability, mechanical properties and dielectric properties, polyimide is widely used in microelectronic devices, electronic packaging, aerospace and other fields. However, its poor thermal conductivity limits its use in some high-performance applications. At present, existing technologies mainly improve the thermal conductivity of polyimide by adding nano-fillers such as boron nitride, alumina, carbon nanotubes, etc., but these methods have limitations such as affecting mechanical properties or complex preparation processes.

[0003] Based on this, a composite material for improving the longitudinal thermal conductivity of polyimide films is designed. Summary of the Invention

[0004] In view of the above situation, in order to overcome the defects of the prior art, the present invention provides a composite material for improving the longitudinal thermal conductivity of polyimide films, effectively solving the problems raised in the above background.

[0005] To achieve the above object, the present invention provides the following technical solution: A composite material for improving the longitudinal thermal conductivity of polyimide films, comprising the following components in parts by mass:

[0006] Polyimide resin: 60 - 80 parts;

[0007] Modified boron nitride: 20 - 30 parts;

[0008] Carbon nitride nanosheets: 5 - 15 parts;

[0009] Modified graphene: 5 - 15 parts;

[0010] Hexagonal boron nitride nanotubes: 2 - 10 parts;

[0011] Thermal crosslinking agent: 3 - 5 parts.

[0012] Preferably, the preparation method of the modified boron nitride is as follows:

[0013] S1. Select high-purity boron nitride powder with an average particle size of 50 nm;

[0014] S2. Disperse the boron nitride powder in ethanol, add an appropriate amount of silane coupling agent, stir evenly, and then reflux and react at 80 °C for 4 h to cause chemical bonding between the silane coupling agent and the surface of the boron nitride, improving its compatibility with polyimide;

[0015] S3. Dry the reacted mixture, and then grind it to an average particle size of 40 nm to obtain modified boron nitride.

[0016] Preferably, the preparation method of carbon nitride nanosheets is as follows:

[0017] S1. Weigh melamine and place it in a round crucible. Then, place the treated nickel foam flat on top of the melamine, cover the crucible lid, and place it flat in a muffle furnace.

[0018] S2. The initial temperature is 50 °C. Heat the muffle furnace to 500 - 550 °C at a heating rate of 2.3 °C / min, and react at this temperature for 4 h.

[0019] S3. After natural cooling to room temperature, obtain the upper-layer black nickel foam and the corresponding lower-layer carbon nitride nanosheets.

[0020] Preferably, the preparation method of modified graphene is as follows:

[0021] S1. Oxidize graphite to obtain graphite oxide.

[0022] S2. Disperse the graphite oxide in deionized water and ultrasonicate for 1 - 4 h to obtain a first dispersion with a concentration of 0.5 - 1.0 g / L.

[0023] S3. Add hydroiodic acid to the first dispersion to obtain a second dispersion, such that the concentration of hydroiodic acid in the second dispersion is 0.05 - 0.5 g / L. Stir, filter, wash the filtrate, and dry to obtain graphene.

[0024] S4. Place the graphene in an environment of inert gas and ethanol vapor, heat it to 400 - 800 °C and keep it warm for 0.5 - 2 h, and then cool it to room temperature to obtain modified graphene.

[0025] Preferably, the preparation method of hexagonal boron nitride nanotubes is as follows;

[0026] S1. In nitrogen or ammonia, ball-mill the raw material containing boron powder into a powder.

[0027] S2. In nitrogen, mix the powder with a solvent to obtain a slurry.

[0028] S3. In nitrogen, spray the slurry onto the surface of a substrate to obtain a substrate.

[0029] S4. Consecutively transport multiple substrates to a high-temperature device for heat treatment, which is carried out in a mixed gas of nitrogen and hydrogen to achieve continuous heat treatment, and obtain a substrate with hexagonal boron nitride nanotubes grown on it.

[0030] S5. After collecting the hexagonal boron nitride nanotubes grown on the substrate, carry out purification and drying.

[0031] Preferably, in the preparation of the carbon nitride nanosheets, the nickel foam has a specification of 3 cm × 3 cm × 1 mm. The treatment of the nickel foam includes ultrasonic washing with 6 mol / L hydrochloric acid for 30 min, followed by washing three times with water and ethanol respectively, and then vacuum drying at 60 °C for 2 h.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] By introducing modified boron nitride, carbon nitride nanosheets, modified graphene, hexagonal boron nitride nanotubes and a thermal crosslinking agent component, the composite material of the present invention significantly improves the longitudinal thermal conductivity of the polyimide film while maintaining its original excellent physical and mechanical properties. The composite material has broad application prospects in the fields of aerospace, electronics and electrical appliances. Specific embodiments

[0034] Next, specific embodiments of the present invention will be used to clearly and completely describe the technical solutions of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments; based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work belong to the scope of protection of the present invention.

[0035] The present invention provides a composite material for improving the longitudinal thermal conductivity of a polyimide film, which comprises the following components in parts by mass:

[0036] Polyimide resin: 60 - 80 parts;

[0037] Modified boron nitride: 20 - 30 parts;

[0038] Carbon nitride nanosheets: 5 - 15 parts;

[0039] Modified graphene: 5 - 15 parts;

[0040] Hexagonal boron nitride nanotubes: 2 - 10 parts;

[0041] Thermal crosslinking agent: 3 - 5 parts.

[0042] The modified boron nitride improves the interfacial compatibility with the polyimide through surface treatment, enhancing the heat transfer efficiency;

[0043] The carbon nitride nanosheets utilize their good graphite-like structure and insulating properties to construct an efficient heat channel in the polyimide matrix, significantly improving the longitudinal thermal conductivity;

[0044] The modified graphene not only improves the thermal conductivity of graphene through the carbon nitride coating layer grown in-situ on the graphene, but also hinders the formation of the graphene conductive network, enabling the composite material to have good insulating properties while maintaining high thermal conductivity;

[0045] Hexagonal boron nitride nanotubes are inorganic nanotube materials with high thermal conductivity and low density, which can further enhance the longitudinal thermal conductivity of composite materials while maintaining the lightweight characteristics of the materials;

[0046] The thermal crosslinking agent enhances the chemical bonding between polyimide molecules through a thermal crosslinking reaction, further improving the thermal conduction efficiency.

[0047] The preparation method of the modified boron nitride in this embodiment is as follows:

[0048] S1. Select high-purity boron nitride powder with an average particle size of 50 nm;

[0049] S2. Disperse the boron nitride powder in ethanol, add an appropriate amount of silane coupling agent, stir evenly, and then reflux at 80 °C for 4 h to cause chemical bonding between the silane coupling agent and the surface of boron nitride, improving its compatibility with polyimide;

[0050] S3. Dry the reacted mixture, and then grind it to an average particle size of 40 nm to obtain modified boron nitride.

[0051] The preparation method of the carbon nitride nanosheets in this embodiment is as follows:

[0052] S1. Weigh melamine and place it in a round crucible, then place the treated nickel foam flat on top of the melamine, cover the crucible lid, and place it flat in a muffle furnace;

[0053] S2. The initial temperature is 50 °C, and the muffle furnace is heated to 500 - 550 °C at a heating rate of 2.3 °C / min, and react at this temperature for 4 h;

[0054] S3. After natural cooling to room temperature, obtain the upper black nickel foam and the corresponding carbon nitride nanosheets in the lower layer.

[0055] The preparation method of the modified graphene in this embodiment is as follows:

[0056] S1. Oxidize graphite to obtain graphite oxide;

[0057] S2. Disperse the graphite oxide in deionized water and ultrasonicate for 1 - 4 h to obtain a first dispersion with a concentration of 0.5 - 1.0 g / L;

[0058] S3. Add hydroiodic acid to the first dispersion to obtain a second dispersion, so that the concentration of hydroiodic acid in the second dispersion is 0.05 - 0.5 g / L, stir, filter, wash the filtrate, and dry to obtain graphene.

[0059] S4. Place the graphene in an environment of inert gas and ethanol vapor, heat it to 400 - 800 °C and keep it warm for 0.5 - 2 h, and then cool it to room temperature to obtain modified graphene.

[0060] The preparation method of the hexagonal boron nitride nanotubes in this example is as follows;

[0061] S1. In nitrogen or ammonia gas, ball-mill the raw material containing boron powder into a powder;

[0062] S2. In nitrogen gas, mix the powder with a solvent to obtain a slurry;

[0063] S3. In nitrogen gas, spray the slurry onto the surface of the substrate to obtain a substrate;

[0064] S4. Consecutively transport multiple substrates to a high-temperature device for heat treatment, which is carried out in a mixed gas of nitrogen and hydrogen to achieve continuous heat treatment, and obtain a substrate with hexagonal boron nitride nanotubes grown thereon;

[0065] S5. After collecting the hexagonal boron nitride nanotubes grown on the substrate, carry out purification and drying.

[0066] In the preparation of the carbon nitride nanosheets in this example, the specifications of the nickel foam are 3 cm × 3 cm × 1 mm. The treatment of the nickel foam includes ultrasonic washing with 6 mol / L hydrochloric acid for 30 min, then washing three times with water and ethanol respectively, and then vacuum drying at 60 °C for 2 h.

[0067] Example 1:

[0068] A composite material for improving the longitudinal thermal conductivity of a polyimide film, comprising the following components by mass:

[0069] Polyimide resin: 60 parts;

[0070] Modified boron nitride: 20 parts;

[0071] Carbon nitride nanosheets: 5 parts;

[0072] Modified graphene: 5 parts;

[0073] Hexagonal boron nitride nanotubes: 2 parts;

[0074] Thermal crosslinking agent: 3 parts.

[0075] The preparation method of the modified boron nitride in this example is as follows:

[0076] S1. Select high-purity boron nitride powder with an average particle size of 50 nm;

[0077] S2. Disperse boron nitride powder in ethanol, add an appropriate amount of silane coupling agent, stir evenly, and then reflux and react at 80 °C for 4 h to cause chemical bonding between the silane coupling agent and the surface of boron nitride, thereby improving its compatibility with polyimide.

[0078] S3. Dry the reacted mixture, and then grind it to an average particle size of 40 nm to obtain modified boron nitride.

[0079] The preparation method of the carbon nitride nanosheets in this example is as follows:

[0080] S1. Weigh melamine and place it in a round crucible. Then, place the treated nickel foam flat on top of the melamine, cover the crucible lid, and place it flat in a muffle furnace.

[0081] S2. The initial temperature is 50 °C. Heat the muffle furnace to 500 °C at a heating rate of 2.3 °C / min, and react at this temperature for 4 h.

[0082] S3. After naturally cooling to room temperature, obtain the upper black nickel foam and the corresponding carbon nitride nanosheets in the lower layer.

[0083] The preparation method of the modified graphene in this example is as follows:

[0084] S1. Oxidize graphite to obtain graphite oxide.

[0085] S2. Disperse graphite oxide in deionized water and sonicate for 1 h to obtain a first dispersion with a concentration of 0.5 g / L.

[0086] S3. Add hydroiodic acid to the first dispersion to obtain a second dispersion, such that the concentration of hydroiodic acid in the second dispersion is 0.05 g / L. Stir, filter, wash the filter cake, and dry it to obtain graphene.

[0087] S4. Place the graphene in an environment of inert gas and ethanol vapor, heat it to 400 °C and hold for 0.5 h, and then cool it to room temperature to obtain modified graphene.

[0088] The preparation method of the hexagonal boron nitride nanotubes in this example is as follows;

[0089] S1. In nitrogen or ammonia gas, ball-mill the raw material containing boron powder into a powder.

[0090] S2. In nitrogen gas, mix the powder with a solvent to obtain a slurry.

[0091] S3. In nitrogen gas, spray the slurry onto the surface of a substrate to obtain a substrate.

[0092] S4. Consecutively convey multiple substrates to a high-temperature device for heat treatment in a mixed gas of nitrogen and hydrogen to achieve continuous heat treatment and obtain substrates with hexagonal boron nitride nanotubes grown thereon.

[0093] S5. After collecting the hexagonal boron nitride nanotubes grown on the substrates, perform purification and drying.

[0094] In the preparation of the carbon nitride nanosheets in this embodiment, the nickel foam has a specification of 3 cm × 3 cm × 1 mm. The treatment of the nickel foam includes ultrasonic washing with 6 mol / L hydrochloric acid for 30 min, then washing three times with water and ethanol respectively, and then vacuum drying at 60 °C for 2 h.

[0095] Example 2:

[0096] A composite material for improving the longitudinal thermal conductivity of a polyimide film, comprising the following components by mass fraction:

[0097] Polyimide resin: 80 parts;

[0098] Modified boron nitride: 30 parts;

[0099] Carbon nitride nanosheets: 15 parts;

[0100] Modified graphene: 15 parts;

[0101] Hexagonal boron nitride nanotubes: 10 parts;

[0102] Thermal crosslinking agent: 5 parts.

[0103] The preparation method of the modified boron nitride in this embodiment is as follows:

[0104] S1. Select high-purity boron nitride powder with an average particle size of 50 nm;

[0105] S2. Disperse the boron nitride powder in ethanol, add an appropriate amount of silane coupling agent, stir evenly, and then reflux and react at 80 °C for 4 h to cause chemical bonding between the silane coupling agent and the surface of the boron nitride, improving its compatibility with polyimide;

[0106] S3. Dry the reaction mixture and then grind it to an average particle size of 40 nm to obtain modified boron nitride.

[0107] The preparation method of the carbon nitride nanosheets in this embodiment is as follows:

[0108] S1. Weigh melamine and place it in a round crucible, then place the treated nickel foam flat on top of the melamine, cover the crucible lid, and place it flat in a muffle furnace;

[0109] S2. The initial temperature is 50 °C. The muffle furnace is heated to 550 °C at a heating rate of 2.3 °C / min and reacted at this temperature for 4 h;

[0110] S3. After natural cooling to room temperature, upper-layer black nickel foam and lower-layer corresponding carbon nitride nanosheets are obtained.

[0111] The preparation method of the modified graphene in this example is as follows:

[0112] S1. Graphite is oxidized to obtain graphite oxide;

[0113] S2. The graphite oxide is dispersed in deionized water and ultrasonicated for 4 h to obtain a first dispersion with a concentration of 1.0 g / L;

[0114] S3. Hydroiodic acid is added to the first dispersion to obtain a second dispersion, so that the concentration of hydroiodic acid in the second dispersion is 0.5 g / L. The mixture is stirred, filtered, the filtrate is washed, and then dried to obtain graphene.

[0115] S4. The graphene is placed in an environment of inert gas and ethanol vapor, heated to 800 °C and held for 2 h, and then cooled to room temperature to obtain modified graphene.

[0116] The preparation method of the hexagonal boron nitride nanotubes in this example is as follows;

[0117] S1. In nitrogen or ammonia, the raw material containing boron powder is ball-milled into powder;

[0118] S2. In nitrogen, the powder is mixed with a solvent to obtain a slurry;

[0119] S3. In nitrogen, the slurry is sprayed on the surface of the substrate to obtain a substrate;

[0120] S4. Multiple substrates are sequentially transported to a high-temperature device for heat treatment, which is carried out in a mixed gas of nitrogen and hydrogen to achieve continuous heat treatment, and a substrate with hexagonal boron nitride nanotubes grown thereon is obtained;

[0121] S5. After the hexagonal boron nitride nanotubes grown on the substrate are collected, they are purified and dried.

[0122] In the preparation of the carbon nitride nanosheets in this example, the nickel foam has a specification of 3 cm × 3 cm × 1 mm. The treatment of the nickel foam includes ultrasonic washing with 6 mol / L hydrochloric acid for 30 min, then washing three times with water and ethanol respectively, and then vacuum drying at 60 °C for 2 h.

[0123] Example 3:

[0124] A composite material for improving the longitudinal thermal conductivity of a polyimide film, comprising the following components by mass:

[0125] Polyimide resin: 70 parts;

[0126] Modified boron nitride: 25 parts;

[0127] Carbon nitride nanosheets: 10 parts;

[0128] Modified graphene: 10 parts;

[0129] Hexagonal boron nitride nanotubes: 6 parts;

[0130] Thermal crosslinking agent: 4 parts.

[0131] The preparation method of the modified boron nitride in this embodiment is as follows:

[0132] S1. Select high-purity boron nitride powder with an average particle size of 50 nm;

[0133] S2. Disperse the boron nitride powder in ethanol, add an appropriate amount of silane coupling agent, stir evenly, and then reflux and react at 80 °C for 4 h to make the silane coupling agent chemically bond with the surface of boron nitride, improving its compatibility with polyimide;

[0134] S3. Dry the reacted mixture, and then grind it to an average particle size of 40 nm to obtain modified boron nitride.

[0135] The preparation method of the carbon nitride nanosheets in this embodiment is as follows:

[0136] S1. Weigh melamine and place it in a round crucible, then place the treated nickel foam flat on top of the melamine, cover the crucible lid, and place it flat in a muffle furnace;

[0137] S2. The initial temperature is 50 °C, and the muffle furnace is heated to 525 °C at a heating rate of 2.3 °C / min and reacted at this temperature for 4 h;

[0138] S3. After natural cooling to room temperature, the upper black nickel foam and the corresponding lower layer of carbon nitride nanosheets are obtained.

[0139] The preparation method of the modified graphene in this embodiment is as follows:

[0140] S1. Oxidize graphite to obtain graphite oxide;

[0141] S2. Disperse the graphite oxide in deionized water and ultrasonicate for 2.5 h to obtain a first dispersion with a concentration of 0.75 g / L;

[0142] S3. Add hydroiodic acid to the first dispersion to obtain a second dispersion, so that the concentration of hydroiodic acid in the second dispersion is 0.35 g / L, stir, filter, wash the filter residue, and dry to obtain graphene.

[0143] S4. Place the graphene in an inert gas and ethanol vapor environment, heat it to 600 °C, hold for 1.5 h, and then cool it to room temperature to obtain modified graphene.

[0144] The preparation method of the hexagonal boron nitride nanotubes in this embodiment is as follows:

[0145] S1. In nitrogen or ammonia gas, ball-mill the raw material containing boron powder into a powder.

[0146] S2. In nitrogen gas, mix the powder with a solvent to obtain a slurry.

[0147] S3. In nitrogen gas, spray the slurry onto the surface of a substrate to obtain a matrix.

[0148] S4. Sequentially transport multiple matrices to a high-temperature device for heat treatment, which is carried out in a mixed gas of nitrogen and hydrogen to achieve continuous heat treatment, and obtain a matrix with hexagonal boron nitride nanotubes grown on it.

[0149] S5. After collecting the hexagonal boron nitride nanotubes grown on the matrix, carry out purification and drying.

[0150] In the preparation of the carbon nitride nanosheets in this embodiment, the specifications of the nickel foam are 3 cm × 3 cm × 1 mm. The treatment of the nickel foam includes ultrasonic washing with 6 mol / L hydrochloric acid for 30 min, then washing three times with water and ethanol respectively, and then vacuum drying at 60 °C for 2 h.

[0151] Example 4:

[0152] A composite material for improving the longitudinal thermal conductivity of a polyimide film, comprising the following components by mass:

[0153] Polyimide resin: 65 parts;

[0154] Modified boron nitride: 23 parts;

[0155] Carbon nitride nanosheets: 7 parts;

[0156] Modified graphene: 7 parts;

[0157] Hexagonal boron nitride nanotubes: 4 parts;

[0158] Thermal cross-linking agent: 3.5 parts.

[0159] The preparation method of the modified boron nitride in this embodiment is as follows:

[0160] S1. Select high-purity boron nitride powder with an average particle size of 50 nm.

[0161] S2. Disperse boron nitride powder in ethanol, add an appropriate amount of silane coupling agent, stir evenly, and then reflux and react at 80 °C for 4 h to cause chemical bonding between the silane coupling agent and the surface of boron nitride, thereby improving its compatibility with polyimide;

[0162] S3. Dry the reacted mixture, and then grind it to an average particle size of 40 nm to obtain modified boron nitride.

[0163] The preparation method of the carbon nitride nanosheets in this example is as follows:

[0164] S1. Weigh melamine and place it in a round crucible. Then, place the treated nickel foam flat on top of the melamine, cover the crucible lid, and place it flat in a muffle furnace;

[0165] S2. The initial temperature is 50 °C. Heat the muffle furnace to 500 - 550 °C at a heating rate of 2.3 °C / min, and react at this temperature for 4 h;

[0166] S3. After naturally cooling to room temperature, obtain the upper - layer black nickel foam and the corresponding lower - layer carbon nitride nanosheets.

[0167] The preparation method of the modified graphene in this example is as follows:

[0168] S1. Oxidize graphite to obtain graphite oxide;

[0169] S2. Disperse graphite oxide in deionized water and ultrasonicate for 2 h to obtain a first dispersion with a concentration of 0.6 g / L;

[0170] S3. Add hydroiodic acid to the first dispersion to obtain a second dispersion, such that the concentration of hydroiodic acid in the second dispersion is 0.25 g / L. Stir, filter, wash the filtrate, and dry to obtain graphene.

[0171] S4. Place the graphene in an environment of inert gas and ethanol vapor, heat it to 500 °C and hold for 1 h, and then cool to room temperature to obtain modified graphene.

[0172] The preparation method of the hexagonal boron nitride nanotubes in this example is as follows;

[0173] S1. In nitrogen or ammonia, ball - mill the raw material containing boron powder into a powder;

[0174] S2. In nitrogen, mix the powder with a solvent to obtain a slurry;

[0175] S3. In nitrogen, spray the slurry onto the surface of a substrate to obtain a substrate;

[0176] S4. Sequentially convey multiple substrates to a high-temperature device for heat treatment in a mixed gas of nitrogen and hydrogen to achieve continuous heat treatment, and obtain substrates with hexagonal boron nitride nanotubes grown thereon.

[0177] S5. After collecting the hexagonal boron nitride nanotubes grown on the substrates, perform purification and drying.

[0178] In the preparation of the carbon nitride nanosheets in this embodiment, the specifications of the nickel foam are 3 cm × 3 cm × 1 mm. The treatment of the nickel foam includes ultrasonic washing with 6 mol / L hydrochloric acid for 30 min, then washing three times with water and ethanol respectively, and then vacuum drying at 60 °C for 2 h.

[0179] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0180] Although the embodiments of the present invention have been shown and described, those of ordinary skill in the art can understand that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A composite material for improving the longitudinal thermal conductivity of polyimide film, characterized in that, It includes the following components in parts by mass: Polyimide resin: 60 - 80 parts; Modified boron nitride: 20 - 30 parts; Carbon nitride nanosheets: 5 - 15 parts; Modified graphene: 5 - 15 parts; Hexagonal boron nitride nanotubes: 2 - 10 parts; Thermal crosslinking agent: 3 - 5 parts.

2. A composite material for improving the longitudinal thermal conductivity of a polyimide film according to claim 1, wherein, The preparation method of modified boron nitride is as follows: S1. Select high-purity boron nitride powder with an average particle size of 50 nm; S2. Disperse the boron nitride powder in ethanol, add an appropriate amount of silane coupling agent, stir evenly, and then reflux at 80 °C for 4 h to cause chemical bonding between the silane coupling agent and the surface of boron nitride, improving its compatibility with polyimide; S3. Dry the reaction mixture, and then grind it to an average particle size of 40 nm to obtain modified boron nitride.

3. A composite material for improving the longitudinal thermal conductivity of a polyimide film according to claim 1, characterized in that, The preparation method of carbon nitride nanosheets is as follows: S1. Weigh melamine and place it in a round crucible, then place the treated nickel foam flat on the melamine, cover the crucible lid, and place it flat in a muffle furnace; S2. The initial temperature is 50 °C, and the muffle furnace is heated to 500 - 550 °C at a heating rate of 2.3 °C / min and react at this temperature for 4 h; S3. After natural cooling to room temperature, obtain the upper-layer black nickel foam and the corresponding lower-layer carbon nitride nanosheets.

4. A composite material for improving the longitudinal thermal conductivity of a polyimide film according to claim 1, characterized in that, The preparation method of modified graphene is as follows: S1. Oxidize graphite to obtain graphite oxide; S2. Disperse the graphite oxide in deionized water and ultrasonicate for 1 - 4 h to obtain a first dispersion with a concentration of 0.5 - 1.0 g / L; S3. Add hydroiodic acid to the first dispersion to obtain a second dispersion, so that the concentration of hydroiodic acid in the second dispersion is 0.05 - 0.5 g / L, stir, filter, wash the filter cake, and dry to obtain graphene. S4. Place the graphene in an environment of inert gas and ethanol vapor, heat it to 400 - 800 °C and keep it warm for 0.5 - 2 h, and then cool it to room temperature to obtain modified graphene.

5. A composite material for improving the longitudinal thermal conductivity of a polyimide film according to claim 1, characterized in that, The preparation method of hexagonal boron nitride nanotubes is as follows; S1. In nitrogen or ammonia, ball-mill the raw material containing boron powder into a powder; S2. In nitrogen, mix the powder with a solvent to obtain a slurry; S3. In nitrogen, spray the slurry onto the surface of a substrate to obtain a substrate; S4. Send multiple substrates to a high-temperature device for heat treatment in turn, which is carried out in a mixed gas of nitrogen and hydrogen to achieve continuous heat treatment to obtain a substrate with hexagonal boron nitride nanotubes grown on it; S5. After collecting the hexagonal boron nitride nanotubes grown on the substrate, carry out purification and drying.

6. A composite material for improving the longitudinal thermal conductivity of a polyimide film according to claim 3, characterized in that, In the preparation of carbon nitride nanosheets, the nickel foam has a specification of 3 cm × 3 cm × 1 mm, and the treatment of the nickel foam includes ultrasonic washing with 6 mol / L hydrochloric acid for 30 min, then washing three times with water and ethanol respectively, and then vacuum drying at 60 °C for 2 h.