High-thermal-conductivity polyimide-based dielectric composite film as well as preparation method and application thereof

Abstract

The invention relates to a high-thermal-conductivity polyimide-based dielectric composite film and a preparation method and application thereof.The preparation method includes preparation of functional filler and preparation of the composite film, and the preparation method of the functional filler includes preparation of graphene oxide (GO) through an improved Hummers method and high-temperature oxidation treatment. Hydroxylated boron nitride nanosheets (BNNS-OH) are simply and efficiently prepared through a liquid-phase ultrasonic stripping and graded centrifugation three-step method, active groups are introduced to the surfaces of the boron nitride nanosheets (BNNS) through hydroxylation modification, the filler-filler and filler-matrix interface interaction is enhanced, and the internal defects of the composite membrane are reduced to a great extent. The preparation method of the composite membrane comprises the following steps: carrying out polymerization reaction by adopting a low-temperature in-situ method, and obtaining the composite membrane with a compact nacre-like layered structure by adopting a blade coating method and a gradient thermal imidization method. The dielectric film with excellent flexibility, high heat-conducting property and energy storage property is obtained, and an effective solution is provided for application of the dielectric film at high temperature.

Description

Technical Field

[0001] The present invention relates to the technical fields of heat-conducting composite materials and new materials, and particularly relates to a high heat-conducting polyimide-based dielectric composite film, a preparation method thereof, and an application thereof. Background Art

[0002] With the continuous growth of the power of electronic devices and electrical equipment, the continuous improvement of the electrification level, and the increasing compactness of the volume in the fields of power systems, new energy electric vehicles, wearable electronics, etc., higher requirements are put forward for the performance of electronic and electrical materials.

[0003] Two inherent disadvantages of polymers directly limit the application of polymer dielectric capacitors. First, the intrinsic thermal conductivity of polymers is relatively low. The thermal conductivity of most polymers is about 0.2 W / (m·K). Under high temperature and high electric field, the electric field can cause the thermal vibration of polymer molecular chains to be converted into heat energy, and it is difficult for the system to dissipate heat in time, resulting in the continuous accumulation of heat in the polymer dielectric, which directly threatens the stability and safety of the entire system. Another inherent disadvantage of polymer dielectric capacitors is that polymers themselves have only a very low dielectric constant, which directly leads to the difficulty for polymer dielectrics to have a high energy storage density. The above aspects are difficult to meet the performance requirements of polymer dielectric materials in practical applications. In order to overcome these disadvantages, the present invention uses PI as the polymer matrix material, adds two functional fillers, namely hydroxylated boron nitride nanosheets (BNNS) and trace amounts of graphene oxide (GO) obtained by exfoliation modification, and uses an in-situ polymerization method to obtain a multi-functionalized PI-based nanocomposite. Cubic boron nitride (h-BN) is an electrical insulator with an intrinsic wide bandgap and high breakdown strength. Its intrinsic thermal conductivity is second only to diamond, and it can obtain ideal electrical insulation and thermal conductivity at the same time. The GO nanosheet has a high aspect ratio in structure, so the percolation threshold is low, which improves the dielectric constant of the composite material at low filler loading. The multi-functional nanocomposite has excellent thermal conductivity and dielectric properties while maintaining the excellent flexibility and mechanical properties of the PI matrix itself. This makes the nanocomposite promising to become a flexible dielectric capacitor with excellent heat dissipation ability in advanced electronics and electrical power. Summary of the Invention

[0004] To solve the problems existing in the prior art, the present invention provides a highly thermally conductive polyimide-based dielectric composite film, its preparation method and application, so as to solve the problems of poor thermal conductivity and low dielectric constant of the existing polyimide-based dielectric film, improve the heat dissipation capacity and energy storage density of the dielectric capacitor during operation, and provide a new way to broaden the application range of polymer dielectric capacitors and improve their performance and thermal stability. The preparation method of the present invention includes the preparation of functional fillers and the preparation of composite films. The preparation method of functional fillers includes: preparing graphene oxide (GO) using the modified Hummers method, and preparing hydroxylated boron nitride nanosheets (BNNS-OH) by a simple and efficient three-step method of high-temperature oxidation treatment, liquid-phase ultrasonic exfoliation and fractional centrifugation. The hydroxylation modification introduces active groups on the surface of h-BNNS, strengthening the interfacial interaction between fillers and between fillers and the matrix, and greatly reducing the internal defects of the composite film. The preparation of the composite film includes: to solve the problem of filler dispersion, a low-temperature in-situ polymerization reaction is used, and a composite film with a dense nacre-like layered structure is obtained by the doctor blade method and the gradient thermal imidization method. Large-size, ultrathin BNNS-OH can effectively reinforce the polymer and delay the propagation of the breakdown path; the conductive reduced graphene oxide (rGO) slightly lower than the percolation threshold content after imidization increases the dielectric constant. The two are reasonably assembled and arranged alternately to form a thermal conduction network, which improves the thermal conductivity of the composite film. The addition of double two-dimensional fillers realizes a significant improvement in the performance of the composite film at a low filler content, and a dielectric film with excellent flexibility, high thermal conductivity and energy storage performance is obtained, providing an effective solution for the application of dielectric films at high temperatures.

[0005] More specifically, to achieve the object of the invention, the present invention adopts the following technical scheme: a highly thermally conductive polyimide-based dielectric composite film, the composite film includes a polyimide matrix, and two fillers, namely graphene oxide (GO) and hydroxylated boron nitride nanosheets (BNNS-OH); the synergistic effect of the double fillers is reflected in the structure that the composite film has a dense nacre-like layered structure.

[0006] Preferably, the in-plane thermal conductivity of the composite film is ≥10 W / (m·K), the discharge energy density is ≥3 J / cm -3 , the efficiency is ≥90%, and it has excellent flexibility and can be stably used in a high-temperature environment of 150 °C; more preferably, the excellent flexibility of the composite film is reflected in that the radius of curvature of the composite film is <8 mm.

[0007] Preferably, the synthesis monomers of the polyimide are 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA), and their rich benzene ring structure provides excellent mechanical properties and high-temperature resistance for the composite film.

[0008] Preferably, the hydroxylated boron nitride nanosheets are obtained by subjecting boron nitride to oxidation treatment at 1000 °C, liquid-phase ultrasonic exfoliation, and fractional centrifugation for exfoliation modification; the hydroxylated boron nitride nanosheets are large-sized and ultrathin, and are more beneficial to the establishment of the heat conduction path and the improvement of the breakdown strength of the composite film at a lower content.

[0009] Preferably, the unilateral size of the hydroxylated boron nitride nanosheets is between 1 and 3 μm, and the thickness is between 2 and 4 nm; the unilateral size of the graphene oxide nanosheets is between 3 and 5 μm, and the thickness is between 1 and 3 nm. Large-sized ultrathin two-dimensional nanosheets have a lower percolation threshold in the polymer matrix, which is more conducive to reducing phonon scattering and defects.

[0010] Preferably, the content of the hydroxylated boron nitride nanosheets in the composite film is not less than 20 wt%; more preferably 20-40 wt%; the content of the graphene oxide nanosheets in the composite film shall not exceed 1.5 wt%; more preferably 0.75-1.25 wt%.

[0011] The high thermal conductivity of the composite film is due to the addition of BNNS-OH and a small amount of graphene oxide with high phonon matching degree in the matrix, which reduces the thermal resistance at the interface between the polymer matrix and the nano-fillers and between two adjacent nano-fillers, and the high thermal conductivity two-dimensional nanosheets are oriented to create a heat conduction network for efficient phonon transport, which is conducive to smooth heat transfer in the composite material. During the thermal imidization process, the graphene oxide undergoes a thermal reduction reaction, and the conductivity difference between the graphene oxide and the polymer matrix increases the dielectric constant of the composite film through interfacial polarization. Using the hydroxylated boron nitride nanosheets co-assembled and orderly arranged with graphene as an insulating layer to isolate the graphene, reducing the tunneling current between adjacent graphene sheets, reducing the dielectric loss of the composite material, improving the breakdown strength, and finally improving the energy storage density and efficiency of the dielectric film.

[0012] The present invention also protects a method for preparing the high thermal conductivity polyimide-based dielectric composite film, comprising the following steps:

[0013] (1) Placing hexagonal boron nitride (h-BN) powder in a tubular furnace, heating it to 1000 °C at a heating rate of 10 °C / min in an air atmosphere, and holding it for 3 h for high-temperature treatment to obtain oxygen-intercalated boron nitride (BN-O);

[0014] (2) Dispersing the oxygen-intercalated boron nitride in a mixed solvent of water and isopropanol, and ultrasonically treating it for 6-8 h to obtain a hydroxylated boron nitride nanosheet dispersion;

[0015] (3) Subjecting the hydroxylated boron nitride nanosheet dispersion to fractional centrifugation, and freeze-drying the obtained precipitate for 24-36 h to obtain hydroxylated boron nitride nanosheets (BNNS-OH);

[0016] (4) Disperse graphene oxide (GO) nanosheets and hydroxylated boron nitride nanosheets (BNNS-OH) into N,N-dimethylacetamide (DMAC) to obtain a GO / BNNS-OH dispersion;

[0017] (5) Add the GO / BNNS-OH dispersion into a three-necked flask and conduct mechanical stirring;

[0018] (6) Add the monomer 4,4'-diaminodiphenyl ether (ODA) into the three-necked flask and stir until completely dissolved;

[0019] (7) Add pyromellitic dianhydride (PMDA) into the three-necked flask and continuously react for 6 - 8 hours;

[0020] (8) Gradiently heat and imidize the obtained dispersion by the casting method to obtain a polyimide-based composite film.

[0021] In a preferred embodiment of the present invention, in step (1), the particle size of the hexagonal boron nitride raw material used is 5 - 10 μm; the high-temperature treatment is carried out at 1000 °C.

[0022] In a preferred embodiment of the present invention, in step (2), the volume ratio of water to isopropanol is 1:1, and the content of isopropanol can be appropriately increased to reduce the ultrasonic time;

[0023] In a preferred embodiment of the present invention, in step (3), the fractional centrifugation is specifically as follows: First, set the rotation speed at 4000 - 5000 rpm and the time at 10 - 15 minutes for centrifugation, then take the supernatant. Centrifuge the supernatant again at a rotation speed of 8000 - 10000 rpm and a time of 10 - 15 minutes, and then take the lower precipitate for freeze-drying.

[0024] In a preferred embodiment of the present invention, in step (4), the unilateral size of the graphene oxide (GO) is between 3 - 5 μm and the thickness is between 2 - 3 nm; the graphene oxide (GO) nanosheets and hydroxylated boron nitride nanosheets (BNNS-OH) need to be dried in an oven; more preferably, dried in a forced-air oven at 60 - 70 °C for at least 24 h; the N,N-dimethylacetamide (DMAC) needs to be dehydrated with a molecular sieve.

[0025] In a preferred embodiment of the present invention, in step (5), the three-necked flask needs to be kept dry.

[0026] In a preferred embodiment of the present invention, in step (6), the 4,4'-diaminodiphenyl ether (ODA) needs to be dried in an oven; more preferably, dried in a forced-air oven at 60 - 70 °C for at least 24 h; the three-necked flask needs to be placed in an ice-water bath and nitrogen gas is introduced.

[0027] In a preferred embodiment of the present invention, in step (7), the pyromellitic dianhydride (PMDA) needs to be added in 5 - 6 batches; among them, 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) need to be strictly in a ratio of 1:1 - 1.02 equivalents.

[0028] In a preferred embodiment of the present invention, in step (8), the gradient temperature - rising imidization is as follows: in the first stage, keep the temperature at 100 °C for 60 - 80 min to remove the solvent; in the second stage, keep the temperature at 200 °C for 60 - 80 min for preliminary imidization; in the third stage, keep the temperature at 300 °C for 60 - 80 min for complete imidization.

[0029] The present invention also protects the use of the above - mentioned high - thermal - conductivity polyimide - based dielectric composite film for energy storage at high temperatures (120 - 150 °C).

[0030] Compared with the prior art, the present invention uses high - temperature heat treatment, liquid - phase ultrasonic treatment, and fractional centrifugation methods to modify and exfoliate hexagonal boron nitride. The method is milder, the obtained boron nitride nanosheets have larger sizes, relatively thinner thicknesses, lower percolation thresholds and fewer defects, which is more conducive to reducing phonon scattering. The second filler component, GO nanosheets, is added to bridge BNNS - OH. The interaction between BNNS - OH and GO nanosheets is strong, and the phonon matching degree is relatively high, which improves the thermal conductivity of the composite film. BNNS - OH acts as an insulating layer to isolate GO nanosheets, reducing the tunneling current between adjacent GO nanosheets, reducing the dielectric loss of the composite film, and increasing the breakdown strength, thereby increasing the energy storage density. While having excellent thermal conductivity and dielectric properties, this dielectric composite film maintains the excellent flexibility and mechanical properties of the PI matrix itself. This makes the nanocomposite promising to be a flexible dielectric capacitor with excellent heat dissipation ability in advanced electronics and electrical power.

[0031] The present invention also has the following advantages:

[0032] (1) The BNNS - OH single sheets prepared by the present invention have large sizes, thin thicknesses, and have been surface - functionalized, with lower interfacial thermal resistance at low contents, which is conducive to the rapid transfer of heat. At the same time, the interfacial defects are reduced, and the breakdown strength of the composite film is increased.

[0033] (2) The present invention adds GO nanosheets to the system, and uses the polarization effect of GO under the electric field and the interfacial polarization effect induced by the difference in dielectric constants between GO and BNNS - OH to improve the dielectric properties of the polymer composite film.

[0034] (3) The present invention adds a double - filler component of BNNS - OH and GO to the system, achieving the multifunctionality of the composite film at low filler contents while maintaining the flexibility and mechanical properties of the polymer composite film itself.

[0035] (4) The present invention co-assembles BNNS-OH and GO nanosheets to synergistically construct a high thermal conductivity network, improving the in-plane thermal conductivity of the dielectric film.

[0036] (5) The present invention co-assembles BNNS-OH and GO nanosheets, achieving the decoupling of dielectric properties and breakdown properties, and their mutual synergistic effect can achieve a high energy storage density of the dielectric film. Description of the Drawings

[0037] The following is further described in conjunction with the drawings.

[0038] Figure 1 It is a preparation flow chart of BNNS-OH prepared in Example 1.

[0039] Figure 2 a is the scanning electron microscope (SEM) image and transmission electron microscope (TEM) image of BNNS-OH prepared in Example 1; Figure 2 b is the atomic force microscope (AFM) image of BNNS-OH prepared in Example 1.

[0040] Figure 3 a is the scanning electron microscope (SEM) image of GO nanosheets prepared in Example 2; Figure 3 b is the atomic force microscope (AFM) image of GO nanosheets prepared in Example 2.

[0041] Figure 4 It is a preparation flow chart of the dielectric composite film prepared in Example 3.

[0042] Figure 5 a is the scanning electron microscope (SEM) image of the plane of the dielectric composite film prepared in Example 3; Figure 5 b is the scanning electron microscope (SEM) image of the cross-section of the dielectric composite film prepared in Example 3.

[0043] Figure 6 It is a bar chart of the thermal conductivity of the dielectric composite film prepared in Example 3.

[0044] Figure 7 It is an infrared image of heat dissipation of the dielectric composite film prepared in Example 3.

[0045] Figure 8 It is a schematic diagram of the high thermal conductivity mechanism of the dielectric composite film prepared in Example 3.

[0046] Figure 9 It is a schematic diagram of the high dielectric performance mechanism of the dielectric composite film prepared in Example 3.

[0047] Figure 10 It is a photo of the dielectric composite film prepared in Example 3.

[0048] Figure 11 It is a graph of the energy storage density and efficiency of the dielectric composite film prepared in Example 3. Detailed implementation manners

[0049] The embodiments of the present invention will be further described below with reference to the accompanying drawings. The following embodiments are implemented on the premise of the technical solution of the present invention, but the protection scope of the present invention is not limited to the following embodiments. The methods are conventional methods unless otherwise specified, and the raw materials can be obtained from public commercial channels unless otherwise specified.

[0050] Example 1

[0051] As Figure 1 shown, BNNS-OH is obtained by three-step method of high-temperature treatment of hexagonal boron nitride, liquid-phase ultrasonic exfoliation and fractional centrifugation. Specifically, the preparation method of BNNS-OH is as follows:

[0052] (1) Pour 30 g of hexagonal boron nitride with a particle size of 10 μm into a corundum boat with a bottom surface of 5 * 10 cm. Push the corundum boat into the tubular furnace, seal the furnace plug, and keep it at 1000 °C for 4 h to obtain BN-O powder.

[0053] (2) Divide the BN-O powder obtained in step (1) into three parts and add them to a solvent with a volume ratio of 1:1 of 500 ml of water and isopropanol respectively. Perform ultrasonic treatment for 8 - 10 hours, collect the upper layer liquid, and obtain a dispersion of BN-OH.

[0054] (3) Perform fractional centrifugation on the dispersion of BN-OH obtained in step (2). First, centrifuge at a speed of 4000 rpm for 10 minutes, take the supernatant, then centrifuge the supernatant again at a speed of 8000 rpm for 10 minutes, take the lower layer precipitate and perform freeze-drying to obtain BNNS-OH powder.

[0055] Figure 2 a is the SEM image of BNNS-OH prepared in this example, Figure 2 b is the AFM image of BNNS-OH prepared in this example. It can be seen that the prepared BNNS-OH has a relatively large single-side size and a relatively thin thickness.

[0056] Example 2

[0057] As Figure 3 shown, graphene oxide is prepared by the modified Hummer method. Specifically, the preparation method is as follows:

[0058] (1) Weigh 46 mL of concentrated sulfuric acid, add 3.2 g of sodium nitrate (NaNO₃) to the concentrated sulfuric acid. After it dissolves, place the beaker containing the reactants in an ice bath and apply continuous magnetic stirring. Then add 2 g of flake graphite and continue stirring for 45 minutes.

[0059] (2) Slowly add KMnO₄ (6 g), always keeping the temperature of the reaction system below 5 °C. After adding, the reaction system becomes a purple-red oily mixture. Continue stirring continuously in the ice bath for 60 minutes, then transfer the reaction vessel from the ice bath to a 40 °C oil bath and continue stirring for 90 minutes.

[0060] (3) Slowly drip 92 mL of deionized water into the reaction system. The temperature of the system rises continuously as the distilled water is added. After adding the water, transfer it to a 100 °C oil bath and continue the reaction for 15 minutes.

[0061] (4) Take it out and let it stand. When the temperature drops to about 60 °C, slowly add distilled water for 180 minutes. The temperature of the system starts to drop, and then slowly drip about 30 mL of hydrogen peroxide solution (drip until no bubbles are generated in the solution and the reaction solution gradually changes from brown-yellow to golden-yellow), and the reaction terminates.

[0062] (5) Let it stand for 12 hours and then centrifuge (4500 rpm) to obtain a graphite oxide filter cake. Wash it with 5% hydrochloric acid solution - centrifuge 5 times and then wash it with distilled water 5 times until the pH of the solution = 7. Perform freeze-drying to obtain GO nanosheets.

[0063] Figure 3 a is the scanning electron microscope (SEM) image of the GO nanosheets prepared in this example; Figure 3 b is the atomic force microscope (AFM) image of the GO nanosheets prepared in this example. It can be seen that the prepared GO nanosheets have a relatively large single-sided size and a relatively thin thickness.

[0064] Example 3

[0065] As Figure 4 shown, the BNNS-OH prepared in Example 1 and the GO nanosheets prepared in Example 2 are used as fillers and added to the polyimide matrix by in-situ polymerization method to be compounded with polyimide, and a high thermal conductivity dielectric composite film can be obtained. Specifically, the preparation method of the dielectric composite film is as follows:

[0066] (1) Dry BNNS-OH, GO nanosheets, a three-necked flask, 4,4'-diaminodiphenyl ether (ODA), pyromellitic dianhydride (PMDA) and a glass plate in an oven at 70 °C for 24 hours to remove moisture.

[0067] (2) Treat N,N-dimethylacetamide (DMAC) with molecular sieves to remove excess moisture.

[0068] (3) Disperse 0.04 g of GO nanosheets and 1 g of BNNS-OH into 25 mL of N,N-dimethylacetamide (DMAC), and ultrasonicate for 15 minutes to obtain a GO / BNNS-OH dispersion.

[0069] (4) Add the GO / BNNS-OH dispersion into a three-necked flask and conduct mechanical stirring. Place the three-necked flask in an ice-water bath and introduce nitrogen for protection.

[0070] (5) Add 1.124 g of ODA monomer into the three-necked flask and stir until completely dissolved.

[0071] (6) Add 1.27 g of PMDA into the three-necked flask in 6 - 8 equal portions, and continuously react for 6 - 8 hours.

[0072] (7) Cast the obtained dispersion onto a glass plate to form a film by the casting method, and place it in a normal-temperature and windless environment for solvent evaporation.

[0073] (8) Place the film into a tubular furnace, set a gradient heating program for thermal imidization. In the first stage, keep it at 100 °C for 1 hour to remove the solvent; in the second stage, keep it at 200 °C for 1 hour for preliminary imidization; in the third stage, keep it at 300 °C for 1 hour for complete imidization. A polyimide-based composite film is obtained.

[0074] Figure 5 a is the scanning electron microscope (SEM) image of the plane of the dielectric composite film prepared in this example; Figure 5 b is the scanning electron microscope (SEM) image of the cross-section of the dielectric composite film prepared in this example. It can be seen that there is no obvious filler accumulation on the surface of the dielectric composite film, and the cross-section presents a dense nacreous layered structure.

[0075] Figure 6 It is the histogram of the thermal conductivity of the dielectric composite film prepared in Example 3. It can be seen that as the content of BNNS-OH increases, the thermal conductivity of the composite film gradually increases. Keeping the content of BNNS-OH unchanged and adding GO nanosheets, as the content of GO increases, the thermal conductivity of the composite film shows an increasing trend, indicating that the mixed filler has a better enhancing effect on the thermal conductivity of the composite film than a single component. At this time, BNNS and GO are not fully "cross-linked", and there are redundant BNNS / BNNS interfaces that are not bridged by GO, resulting in a smaller improvement in thermal conductivity. When the GO content is 1.25 wt%, the thermal conductivity of the composite film reaches the highest value of 12.03 W / (m K). When the GO content increases further, the excessive accumulation of graphene oxide will lead to a lower heat transfer efficiency between adjacent BNNS, so the thermal conductivity of the composite film also decreases.

[0076] Figure 7The infrared thermal image of the heat dissipation of the dielectric composite film prepared in Example 3. It can be seen that when the polyimide-based dielectric film is attached to the back of the heating ceramic plate, the surface temperature is only 103 °C at 60 seconds, rapidly cools to 43.2 °C after 45 seconds, and cools to 34.8 °C after 80 seconds, approaching room temperature. This experiment verifies the great potential of the PI / BNNS / GO film in the high-temperature thermal management of electronic devices.

[0077] Figure 8 Schematic diagram of the high thermal conductivity mechanism of the dielectric composite film prepared in Example 3. It can be seen that the flexible GO nanosheets will fill the gaps between adjacent BNNS-OH. Due to the strong interaction between BNNS-OH and GO nanosheets, the number of effective phonon scattering centers is reduced, thereby reducing the interfacial thermal resistance. When there is only a single BNNS filler in the system, the heat transfer mode is "line" to "surface". Compared with the "surface" to "surface" heat transfer mode bridged by flexible GO nanosheets, the heat transfer contact area is reduced, the risk of phonon scattering is increased, and the heat transfer efficiency is reduced.

[0078] Figure 9 Schematic diagram of the high dielectric performance mechanism of the dielectric composite film prepared in Example 3. A large number of components on both sides of the interface formed between the two fillers and between the filler and the polymer molecular chain have different polarities and conductivities. Under the action of an electric field, a large number of charge carriers are blocked on the micro-interface, the interfacial polarization is enhanced at low frequencies, and the stored electrostatic energy is increased, thereby increasing the dielectric constant.

[0079] Figure 10 Optical photograph of the dielectric composite film prepared in Example 3. It can be seen that the composite film has excellent flexibility and high tensile strength.

[0080] Figure 11 Energy storage density and efficiency diagram of the dielectric composite film prepared in Example 3. It can be seen that the discharge energy density of the dielectric composite film is ≥3 J cm -3 , and the efficiency can be maintained ≥90%.

[0081] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A high thermal conductivity polyimide-based dielectric composite film, characterized in that, The composite film described above includes a polyimide matrix, and two fillers, namely graphene oxide (GO) and hydroxylated boron nitride nanosheets (BNNS-OH). The synergistic effect of the dual fillers is structurally reflected in the fact that the composite film has a dense nacre-like layered structure.

2. The high thermal conductivity polyimide-based dielectric composite film according to claim 1, wherein The unilateral size of the hydroxylated boron nitride nanosheets is between 1 - 3 μm, and the thickness is between 2 - 4 nm; the unilateral size of the graphene oxide nanosheets is between 3 - 5 μm, and the thickness is between 1 - 3 nm; the synthesis monomers of the polyimide are 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA), and their rich benzene ring structure provides excellent mechanical properties and high-temperature resistance for the composite film.

3. The high thermal conductivity polyimide-based dielectric composite film according to claim 1, wherein The content of the hydroxylated boron nitride nanosheets in the composite film is not less than 20 wt%; more preferably 20 - 40 wt%; the content of the graphene oxide nanosheets in the composite film shall not exceed 1.5 wt%; more preferably 0.75 - 1.25 wt%.

4. The high thermal conductivity polyimide-based dielectric composite film according to any one of claims 1-3, characterized in that, The described hydroxylated boron nitride nanosheets are obtained by subjecting boron nitride to three-step exfoliation modification, namely, oxidation treatment at 1000 °C, liquid-phase ultrasonic exfoliation, and fractional centrifugation; the in-plane thermal conductivity of the composite film is ≥10 W / (m·K), the discharge energy density is ≥3 J / cm -3 , the efficiency is ≥90%, and it has excellent flexibility and can be stably used in a high-temperature environment of 150 °C; more preferably, the excellent flexibility of the composite film is reflected in that the radius of curvature of the composite film is <8 mm.

5. The preparation method of the high thermal conductivity polyimide-based dielectric composite film according to any one of claims 1-4, characterized in that, It includes the following steps: (1) Put hexagonal boron nitride (h-BN) powder into a tubular furnace, heat it up to 1000 °C at a heating rate of 10 °C / min in an air atmosphere, and keep it at this temperature for 3 h for high-temperature treatment to obtain oxygen-intercalated boron nitride (BN-O); (2) Disperse the oxygen-intercalated boron nitride in a mixed solvent of water and isopropanol, and ultrasonicate for 6 - 8 h to obtain a dispersion of hydroxylated boron nitride nanosheets; (3) Perform fractional centrifugation on the dispersion of hydroxylated boron nitride nanosheets, and freeze-dry the obtained precipitate for 24 - 36 h to obtain hydroxylated boron nitride nanosheets (BNNS-OH); (4) Disperse graphene oxide (GO) nanosheets and hydroxylated boron nitride nanosheets (BNNS-OH) into N,N-dimethylacetamide (DMAC) to obtain a GO / BNNS-OH dispersion; (5) Add the GO / BNNS-OH dispersion into a three-necked flask and carry out mechanical stirring; (6) Add the monomer 4,4'-diaminodiphenyl ether (ODA) into the three-necked flask and stir until it is completely dissolved; (7) Add pyromellitic dianhydride (PMDA) into the three-necked flask and continuously react for 6 - 8 hours; (8) Gradiently heat and imidize the obtained dispersion by the casting method to obtain a polyimide-based composite film.

6. According to the preparation method described in claim 5, characterized in that, In step (1), the particle size of the hexagonal boron nitride raw material used is 5 - 10 μm; the high-temperature treatment is carried out at 1000 °C; More preferably, in step (2), the volume ratio of water to isopropanol is 1:1, and the content of isopropanol can be appropriately increased to reduce the ultrasonic time; More preferably, in step (3), the fractional centrifugation is specifically as follows: first set the rotation speed at 4000 - 5000 rpm and the time at 10 - 15 minutes for centrifugation, take the supernatant, and then centrifuge the supernatant again at a rotation speed of 8000 - 10000 rpm and a time of 10 - 15 minutes, and take the lower layer precipitate for freeze-drying.

7. According to the preparation method described in claim 5, characterized in that, In step (4), the unilateral size of the graphene oxide (GO) is between 3 - 5 μm, and the thickness is between 2 - 3 nm; the graphene oxide (GO) nanosheets and hydroxylated boron nitride nanosheets (BNNS-OH) need to be dried in an oven; more preferably, dried in a forced-air oven at 60 - 70 °C for at least 24 h, and the N,N-dimethylacetamide (DMAC) needs to be dehydrated with molecular sieves. More preferably, in step (5), the three-necked flask needs to be kept dry. More preferably, in step (6), the 4,4'-diaminodiphenyl ether (ODA) needs to be dried in an oven; more preferably, dried in a forced-air oven at 60 - 70 °C for at least 24 h; the three-necked flask needs to be placed in an ice-water bath and nitrogen gas is introduced.

8. The preparation method according to claim 5, characterized in that, In step (7), the pyromellitic dianhydride (PMDA) needs to be added in 5 - 6 batches; among them, 4,4'-diaminodiphenyl ether (ODA) and pyromellitic dianhydride (PMDA) need to be strictly in a molar ratio of 1:1 - 1.02 equivalents.

9. The preparation method according to claim 5, characterized in that, In step (8), the gradient temperature imidization is as follows: in the first stage, keep at 100 °C for 60 - 80 min to remove the solvent; in the second stage, keep at 200 °C for 60 - 80 min for preliminary imidization; in the third stage, keep at 300 °C for 60 - 80 min for complete imidization.

10. The high thermal conductivity polyimide-based dielectric composite film described in claims 1-4 or the high thermal conductivity polyimide-based dielectric composite film prepared by the preparation method described in claims 5-9 is used for energy storage at high temperatures, characterized in that, The high temperature is 120 - 150 °C.

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