Hexagonal boron nitride-based composite film with high thermal conductivity and preparation method thereof

Abstract

The invention belongs to the technical field of high-performance thermal management materials, and discloses a high-thermal-conductivity hexagonal boron nitride-based composite film and a preparation method thereof.The composite film is composed of a hexagonal boron nitride (h-BN) nanosheet layer and a graphitized carbonaceous layer formed after a polyhydroxy aromatic compound is subjected to heat treatment, the graphitized carbonaceous layer is bridged between the hexagonal boron nitride nanosheet layers. The preparation method comprises the following steps: mixing an h-BN nanosheet and a polyhydroxy aromatic compound in a solvent to form a mixed dispersion liquid, self-assembling the h-BN nanosheet and the polyhydroxy aromatic compound into a highly-oriented prefabricated film through a vacuum filtration technology, drying, and carrying out heat treatment in an inert atmosphere to graphitize the polyhydroxy aromatic compound to obtain the composite film. The in-plane heat conductivity coefficient of the film is larger than 200 W / (m.K), and the film has excellent flexibility and mechanical strength and is especially suitable for efficient heat dissipation of 5G communication, high-power chips and flexible electronic equipment.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high-performance thermal management materials, and particularly relates to a high-thermal-conductivity hexagonal boron nitride-based composite film and a preparation method thereof. BACKGROUND

[0002] With the rapid development of electronic devices towards miniaturization and high power density, the internal heat flux density of the devices increases sharply, which seriously affects the performance, service life and operation reliability of the devices. Under this background, thermal management has become a core technical bottleneck restricting the development of the electronic information industry. Therefore, developing high-efficiency high-performance thermal management materials is crucial to ensure the stable operation of electronic devices.

[0003] As a new type of two-dimensional layered material, h-BN nanosheet has a crystal structure similar to graphene, with an extremely high intrinsic thermal conductivity in the plane, a theoretical value of 200-2000 W / (m·K), excellent electrical insulation (band gap of about 5.9eV), good chemical stability and thermal stability, and stable performance even in harsh environments such as high temperature and high pressure. Based on the above characteristics, h-BN nanosheet is considered as an ideal new type of thermal conductive and insulating material, and has broad application prospects in the field of electronic device heat dissipation.

[0004] However, to prepare a macroscopic thermal conductive film material that can fully exert the intrinsic high thermal conductivity of h-BN nanosheet, there are still many technical challenges: first, it is difficult to achieve a highly ordered in-plane orientation arrangement of h-BN nanosheet during the preparation of a macroscopic thermal conductive film. Random stacking of nanosheets leads to a large number of interface scattering of phonons in the heat transfer path, and the interface thermal resistance increases significantly, which seriously weakens the thermal conductivity of the macroscopic thermal conductive film; second, when a high filling amount of h-BN nanosheet is used to improve the thermal conductivity, it often leads to insufficient internal structural density of the film, aggregation between the layers, and other problems, which in turn leads to increased brittleness and poor flexibility of the film, and easy cracking and breaking, which cannot meet the heat dissipation requirements of flexible electronic devices or dynamic working components; third, the existing traditional preparation processes such as blade coating and casting film forming have problems such as complex process, high operation difficulty, and difficulty in accurately controlling the micro arrangement of nanosheets, which leads to poor product performance consistency, and restricts the large-scale production and practical application; fourth, the dispersants or binders used in some modification methods have poor compatibility with h-BN nanosheets, which easily forms defects at the interface, not only affecting the thermal conductivity, but also possibly reducing the mechanical strength and long-term stability of the film.

[0005] In the prior art, relevant research attempts to improve the performance of h-BN heat-conducting films through surface modification, optimization of preparation process and the like, but there are still many deficiencies. For example, some methods introduce organic binders to improve the interlayer bonding force, but the organic binders themselves have low thermal conductivity, which will significantly increase the interfacial thermal resistance; another study uses high-temperature sintering and the like to improve the denseness of the film, but too high a temperature will cause the structure of the h-BN nanosheets to be damaged, and at the same time, the brittleness of the film is aggravated. Therefore, there is an urgent need in the field for a new material and a new process that can simultaneously solve the contradiction between high thermal conductivity, low interfacial thermal resistance and excellent flexibility and mechanical strength, in order to meet the increasingly stringent requirements of modern electronic equipment for high-performance thermal management materials. SUMMARY

[0006] The purpose of the present application is to overcome the deficiencies of the prior art, provide a high-thermal-conductivity hexagonal boron nitride-based composite film and a preparation method thereof, and realize the synergistic unity of high-thermal-conductivity, excellent flexibility and good mechanical strength of the boron nitride-based composite film through reasonable component design and optimized preparation process, so as to effectively solve the technical problems existing in traditional h-BN heat-conducting films and meet the heat dissipation requirements of modern electronic equipment.

[0007] To achieve the above-mentioned purpose, the following technical solutions are adopted in the present application: The present application provides, in a first aspect, a high-thermal-conductivity hexagonal boron nitride-based composite film, comprising hexagonal boron nitride (h-BN) nanosheet layers and a graphitized carbon layer formed after heat treatment of a polyhydroxy aromatic compound, wherein the graphitized carbon layer is bridged between the hexagonal boron nitride nanosheet layers.

[0008] Preferably, the in-plane thermal conductivity of the composite film is greater than 200 W / (m·K).

[0009] Preferably, the polyhydroxy aromatic compound is a polyhydroxy aromatic compound, and its specific structure includes but is not limited to 2,3,6,7,10,11-hexahydroxytriphenylene, , , , , , , , , , , , , , , , , , , , , , 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 、 The polyhydroxy aromatic compound is graphitized after heat treatment, and a graphene-like bridging structure is formed between the h-BN nanosheets. The graphitized polyhydroxy aromatic compound molecule has excellent thermal conductivity and structural stability, can effectively fill the interlayer space of the hexagonal boron nitride nanosheet, reduce the interfacial thermal resistance, and at the same time, enhance the bonding force between the layers, thereby synergistically improving the thermal conductivity and mechanical properties of the thermal conductivity film.

[0010] Preferably, the thickness of the composite film is 30 μm to 90 μm, which can ensure that the film has sufficient structural strength and thermal conductivity path, and at the same time, meet the requirements of the electronic equipment for the light and thin heat dissipation material.

[0011] Preferably, the average size of the hexagonal boron nitride (h-BN) nanosheet is 0.05 μm to 10 μm, and the sheet layer thickness is 0.5 nm to 500 nm. The h-BN nanosheet with the size in the range has good dispersibility and orientation arrangement capability, can form a continuous heat transfer path in the composite film, and at the same time, avoid the stacking and agglomeration problem caused by the too large size of the nanosheet.

[0012] The second aspect of the present application provides a preparation method of the above-mentioned high-thermal-conductivity hexagonal boron nitride-based composite film, comprising the following steps: S1: mixing the hexagonal boron nitride nanosheet, the polyhydroxy aromatic compound and the solvent to form a uniform mixed dispersion liquid; S2: vacuum suction filtering the mixed dispersion liquid, and the hexagonal boron nitride nanosheet is self-assembled into a prefabricated film with high orientation under the action of the polyhydroxy aromatic compound; S3: drying the prefabricated film to obtain a dried prefabricated film; S4: heat treating the dried prefabricated film in an inert atmosphere, and the polyhydroxy aromatic compound is graphitized in the process to obtain the high-thermal-conductivity hexagonal boron nitride-based composite film.

[0013] Preferably, in step S1, the concentration of the hexagonal boron nitride nanosheets in the mixed dispersion is 0.05-0.5 mg / mL. Too low a concentration will result in low film formation efficiency and difficulty in controlling the thickness of the thin film; too high a concentration will easily cause the nanosheets to agglomerate, affecting the uniformity of the dispersion.

[0014] Preferably, in step S1, the mass ratio of the hexagonal boron nitride nanosheets to the polyhydroxy aromatic compound is 10:1 to 1:1, more preferably 5:1, 3:1, 3:1.62 or 1:1. By adjusting the mass ratio of the two, the thermal conductivity, flexibility and mechanical strength of the thermal conductive film can be precisely controlled. When the mass ratio is too high, the content of the polyhydroxy aromatic compound is insufficient to effectively fill the interlayer voids and enhance the bonding force; when the mass ratio is too low, too much polyhydroxy aromatic compound will form a thermal conductive bottleneck in the film, reducing the overall thermal conductivity.

[0015] Preferably, in step S1, the solvent is water or isopropyl alcohol (IPA), and the purity of the solvent is ≥99.5% to avoid the adverse effects of impurities on the dispersion effect and film performance.

[0016] Preferably, in step S1, the mixing process is first stirred and mixed at 20-30 ℃, then ultrasonic dispersion mixing is performed, the stirring rate is 300-500 rpm, the stirring time is 1-2 h, the ultrasonic dispersion time is 30-60 min, and the ultrasonic power is 100-300 W. The ultrasonic action breaks up the agglomerates of the hexagonal boron nitride nanosheets, allowing them to be uniformly dispersed in the solvent. Through the synergistic action of mechanical stirring and ultrasonic dispersion, the hexagonal boron nitride nanosheets and the polyhydroxy aromatic compound are fully mixed and uniformly dispersed in the solvent.

[0017] Preferably, in step S2, the vacuum degree of the vacuum filtration is -0.08 MPa to -0.09 MPa, and the temperature of the dispersion is maintained at 20-25 ℃ during the filtration process. Under the action of the vacuum pressure, the hexagonal boron nitride nanosheets are highly oriented in the direction of their two-dimensional plane parallel to the surface of the film under the guidance of the π-π interaction and hydrogen bonding of the polyhydroxy aromatic compound molecules, and self-assemble to form a dense pre-film, laying a structural foundation for obtaining high thermal conductivity in the subsequent process.

[0018] Preferably, in step S3, the drying temperature is 50-70 ℃, the drying time is 30-60 min, and the drying method is air drying or vacuum drying. The solvent in the pre-film is removed by drying to avoid the formation of pores and defects in the film due to the residual solvent, affecting the structural density and performance stability.

[0019] Preferably, in step S4, the inert atmosphere is nitrogen or argon with a purity of ≥ 99.99%, preventing the oxidation of the hexagonal boron nitride nanosheets and the polyhydroxy aromatic compound during the heat treatment. The temperature of the heat treatment is 800-1200 ℃, the heating rate is 3-5 ℃ / min, the holding time is 3-8 h, and then the furnace is cooled to room temperature at a cooling rate of ≤ 10 ℃ / min. The suitable heat treatment temperature and heating / cooling rate can ensure the graphitization of the polyhydroxy aromatic compound, while avoiding the internal stress and cracking of the film caused by too fast temperature change.

[0020] Preferably, after step S4, a post-treatment step is further included: cleaning the high-thermal-conductivity hexagonal boron nitride-based composite film after heat treatment with anhydrous ethanol or deionized water to remove surface residual impurities, and then drying at 40-60 ℃ for 20-30 min to obtain the final product with a smooth surface and stable performance.

[0021] The composite film prepared by the present application realizes the synergistic improvement of high thermal conductivity, excellent flexibility and good mechanical strength, avoids the problem of deterioration of other performances caused by single performance optimization, and has excellent comprehensive performance, which can meet the heat dissipation needs of electronic devices in different scenarios. The fundamental reason is that the polyhydroxy aromatic compound is introduced as a dispersant and assembly directing agent, combined with vacuum filtration and heat treatment process control, the hexagonal boron nitride nanosheets are highly oriented and arranged in the direction of their two-dimensional plane parallel to the film surface under the guidance of π-π interaction and hydrogen bond interaction of the polyhydroxy aromatic compound molecules, and self-assembled to form a dense preformed film. In the subsequent heat treatment, the polyhydroxy aromatic compound is graphitized in situ, and a graphitized carbon layer is formed between the highly oriented and arranged hexagonal boron nitride nanosheet layers. The graphitized carbon layer acts as a "nanoscale adhesive" and a "phonon bridge", which on the one hand greatly enhances the mechanical interaction between the hexagonal boron nitride nanosheet layers, making the film change from "brittle" to "tough", and significantly improving the flexibility and mechanical strength; on the other hand, the graphitized carbon layer has excellent thermal conductivity, filling the phonon transmission gap between the layers, significantly reducing the interface thermal resistance, and thus realizing the simultaneous leap of thermal conductivity and mechanical properties.

[0022] Advantages and benefits of the present application: (1) Ultra-high in-plane thermal conductivity: The composite film prepared by the present application has highly oriented and arranged hexagonal boron nitride nanosheet layers and a graphitized carbon layer, and its in-plane thermal conductivity is greater than 200 W / (m·K), which is much better than that of traditional hexagonal boron nitride thermal conductive film.

[0023] (2) Excellent flexibility and mechanical toughness: The composite film prepared by the present application has excellent flexibility and mechanical strength while achieving high thermal conductivity, successfully solving the industry pain point of traditional high-filling thermal conductive film which is easy to break and has poor flexibility.

[0024] (3) Simple and controllable process, easy to scale production: the preparation method has simple process, does not need complex and expensive equipment, and can realize fine structure regulation in nanometer scale through self-assembly and vacuum filtration, the process parameters are easy to control, the product performance is good in consistency, and is suitable for large-scale industrial production.

[0025] (4) Strong stability, wide application range: the heat-conducting film prepared by the application has good chemical stability and thermal stability, can maintain excellent performance in harsh environments such as high temperature and high humidity, and has electrical insulation, can be widely used in 5G communication, artificial intelligence chips, high-power semiconductor devices, flexible electronic devices and various electronic devices for efficient heat dissipation, can be used as heat dissipation pads, packaging materials and the like in various electronic devices, effectively solves the heat management problem brought by the miniaturization and high power density development of electronic devices, and has wide application prospect. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is a schematic diagram of the crystal structure of hexagonal boron nitride.

[0027] Figure 2 It is a schematic diagram of the three-dimensional structure of HHTBP.

[0028] Figure 3 It is a schematic diagram of the self-assembly of HHTBP on the surface of hexagonal boron nitride nanosheet through π-π and hydrogen bond interaction.

[0029] Figure 4 It is a Raman spectrum diagram of h-BN nanosheet, HHTBP before calcination, HHTBP after calcination and composite film after calcination. DETAILED DESCRIPTION

[0030] The application will be described in detail below through specific examples, but the protection scope of the application is not limited to these embodiments.

[0031] The reagents used in the following examples and comparative examples and their sources are as follows: Hexagonal boron nitride nanosheet (h-BN nanosheet): average size 0.05-4 μm, sheet thickness 1-10 layers of BN nanosheet, purchased from Muke Nanometer Co., Ltd., product number: MK503.

[0032] 2,3,6,7,10,11-hexahydroxytriphenyl (HHTBP): purity >98%, purchased from Shanghai Bide Pharmaceutical Technology Co., Ltd., product number: BD65207-250mg.

[0033] Isopropyl alcohol (IPA): purity ≥99.5%, purchased from Psony Bio-Pharmaceutical Technology (Tianjin) Co., Ltd., product number: I811927-2.5L.

[0034] Example 1 A method for preparing a high-thermal-conductivity hexagonal boron nitride-based composite film, comprising the following steps: S1: preparing a dispersion liquid: 6 mL of a dispersion liquid of h-BN nanosheets in IPA with a concentration of 0.5 mg / mL is measured, and then 1.62 mg of HHTBP is weighed and dissolved in 54 mL of IPA to obtain an HHTBP solution. The dispersion liquid and the HHTBP solution are mixed, stirred at a speed of 400 rpm at 25°C for 1 h, and then ultrasonically dispersed at a power of 200 W for 45 min to form a uniform mixed dispersion liquid. The schematic diagram of the crystal structure of hexagonal boron nitride is shown in Figure 1 , and the chemical structural formula of HHTBP is , and the schematic diagram of the stereoscopic structure is shown in Figure 2 ; S2: vacuum filtration and self-assembly: the above mixed dispersion liquid is poured into a filtration device, and filtration is performed under a vacuum degree of -0.085 MPa, while keeping the temperature of the dispersion liquid at 20°C, so that the h-BN nanosheets self-assemble into a dense preformed film under the regulation of HHTBP. The schematic diagram of the self-assembly of HHTBP on the surface of hexagonal boron nitride nanosheets through π-π and hydrogen bond interactions is shown in Figure 3 ; S3: drying: the obtained preformed film together with the filter membrane is placed in a vacuum drying box, and dried at 60°C for 30 min to remove the solvent, thereby obtaining a dry preformed film; S4: heat treatment: the dry preformed film is carefully peeled off from the filter membrane, placed in a corundum mold, and placed in a tube furnace under nitrogen protection (nitrogen purity ≥ 99.99%), and heated to 1000°C at a heating rate of 5°C / min, and heat treated for 3 h, and then cooled to room temperature with the furnace. In this process, the HHTBP is completely graphitized to obtain a high-thermal-conductivity hexagonal boron nitride-based composite film; S5: cleaning the high-thermal-conductivity hexagonal boron nitride-based composite film with anhydrous ethanol, and then drying at 50°C for 25 min.

[0035] Example 2 A method for preparing a high-thermal-conductivity hexagonal boron nitride-based composite film, comprising the following steps: S1: Preparation of dispersion: Measure 6 mL of a 0.5 mg / mL h-BN nanosheet dispersion in IPA. Then weigh 1.62 mg of HHTBP (2,3,6,7,10,11-hexahedroxytriphenylene) and dissolve it in 54 mL of IPA to obtain an HHTBP solution. Mix the dispersion and HHTBP solution, stir at 400 rpm for 1 h at 25 ℃, and then ultrasonically disperse at 200 W for 45 min to form a homogeneous mixed dispersion. A schematic diagram of the crystal structure of hexagonal boron nitride is shown below. Figure 1 As shown, the chemical structural formula of HHTBP is Its three-dimensional structural diagram is as follows Figure 2 As shown; S2: Vacuum Filtration Self-Assembly: The above mixed dispersion was poured into a vacuum filtration device and filtered under a vacuum of -0.085 MPa, maintaining the dispersion temperature at 20 °C. This allowed the h-BN nanosheets to self-assemble under the control of HHTBP, forming a dense pre-film. A schematic diagram of the self-assembly of HHTBP on the surface of hexagonal boron nitride nanosheets through π-π and hydrogen bonding interactions is shown below. Figure 3 As shown; S3: Drying: Place the obtained pre-formed membrane together with the filter membrane into a vacuum drying oven and dry at 60 ℃ for 30 min to remove the solvent and obtain the dried pre-formed membrane; S4: Heat Treatment: The dried pre-formed membrane was carefully peeled off from the filter membrane, placed in a corundum mold, and then placed in a tube furnace. Under nitrogen protection (nitrogen purity ≥ 99.99%), the temperature was increased to 1100 ℃ at a rate of 5 ℃ / min and held for 6 h for heat treatment. Afterward, the furnace was cooled to room temperature. During this process, HHTBP was completely graphitized, resulting in a high thermal conductivity hexagonal boron nitride-based composite film. S5: Clean the high thermal conductivity hexagonal boron nitride-based composite film with anhydrous ethanol, and then dry it at 50 °C for 25 min.

[0036] Comparative Example 1 A method for preparing a pure h-BN membrane, comprising the following steps: S1: Preparation of dispersion: Measure 6 mL of 0.5 mg / mL h-BN nanosheet dispersion in IPA and drop it into a flask containing 54 mL IPA. Mix them and stir at 400 rpm for 1 h at 25 °C. Then, ultrasonically disperse at 200 W for 45 min to form a uniform mixed dispersion.

[0037] S2: Vacuum filtration self-assembly: Pour the above mixed dispersion into a vacuum filtration device and perform filtration under a vacuum of -0.085 MPa, keeping the dispersion temperature at 20 ℃ to form a pre-formed membrane.

[0038] S3: Drying: The obtained preformed film together with the filter membrane was placed in a vacuum drying oven and dried at 60 °C for 30 min to remove the solvent, obtaining a pure h-BN film.

[0039] Comparative Example 2 A method for preparing a composite film, the steps are as follows: S1: Preparation of dispersion: 6 mL of h-BN nanosheet dispersion in IPA with a concentration of 0.5 mg / mL was measured, then 1.62 mg of HHTBP was weighed and dissolved in 54 mL of IPA to obtain an HHTBP solution. The dispersion and HHTBP solution were mixed and stirred at 25 °C at a rate of 400 rpm for 1 h, then ultrasonically dispersed at a power of 200 W for 45 min to form a uniform mixed dispersion.

[0040] S2: Vacuum filtration self-assembly: The above mixed dispersion was poured into a filtration device and filtered under a vacuum of -0.085 MPa, keeping the dispersion temperature at 20 °C. The h-BN nanosheets self-assembled into a dense preformed film under the regulation of HHTBP.

[0041] S3: Drying: The obtained preformed film together with the filter membrane was placed in a vacuum drying oven and dried at 60 °C for 30 min to remove the solvent, obtaining a pure h-BN film.

[0042] Comparative Example 3 A method for preparing an h-BN composite film, the steps are as follows: S1: Preparation of dispersion: 6 mL of h-BN nanosheet dispersion in IPA with a concentration of 0.5 mg / mL was measured, then 1.62 mg of HHTBP was weighed and dissolved in 54 mL of IPA to obtain an HHTBP solution. The dispersion and HHTBP solution were mixed and stirred at 25 °C at a rate of 400 rpm for 1 h, then ultrasonically dispersed at a power of 200 W for 45 min to form a uniform mixed dispersion.

[0043] S2: Casting film formation and drying: The above mixed dispersion was poured into a casting machine tank and the casting knife was moved at a speed of 5 mm / s to uniformly spread the dispersion on a clean glass substrate in a constant temperature environment of 25 °C. Then the spread substrate was placed in a ventilated oven and kept at a constant temperature of 60 °C for 8 h to dry. After the solvent was completely volatilized, the continuous film formed on the surface of the substrate was peeled off, completing the casting film formation process.

[0044] Comparative Example 4 A method for preparing an h-BN composite film with an organic binder, the steps are as follows: S1: Preparation of dispersion liquid: 6 mL of h-BN nanosheet dispersion liquid with a concentration of 0.5 mg / mL in IPA (isopropyl alcohol) was measured; 1.62 mg of polyethylene glycol (PEG-6000, organic binder) was weighed and dissolved in 54 mL of IPA to obtain an organic binder solution; the h-BN nanosheet dispersion liquid and the organic binder solution were mixed, stirred at 25°C at a speed of 400 rpm for 1 h, and then ultrasonically dispersed at a power of 200 W for 45 min to form a uniform mixed dispersion liquid.

[0045] S2: Vacuum filtration self-assembly: the mixed dispersion liquid was poured into a filtration device and filtered under a vacuum of -0.085 MPa, while maintaining the temperature of the dispersion liquid at 20°C, so that the h-BN nanosheets self-assembled into a preformed film under the binding effect of the organic binder.

[0046] S3: Drying: the obtained preformed film together with the filter membrane was placed in a vacuum drying oven and dried at 60°C for 30 min to remove the solvent, thereby obtaining a dried preformed film.

[0047] S4: Heat treatment: the dried preformed film was carefully peeled off from the filter membrane and placed in a corundum mold, which was then placed in a tube furnace; under nitrogen protection (nitrogen purity ≥ 99.99%), the temperature was raised to 1000°C at a rate of 5°C / min, and heat treatment was performed for 6 h, followed by furnace cooling to room temperature; in this process, the organic binder was thermally decomposed to obtain an h-BN composite film.

[0048] S5: The h-BN composite film was cleaned with anhydrous ethanol for 5 min, and then dried at 50°C for 25 min to form a final h-BN / binder residual composite film (with a small amount of residual organic binder after thermal decomposition).

[0049] Performance test and analysis The in-plane thermal conductivity, bending radius, tensile strength, and elongation at break of the products prepared in the examples and comparative examples were tested, and the test results are shown in Table 1.

[0050] The in-plane thermal conductivity was tested using a thermal conductivity instrument from Faireite Instrument Manufacturing Co., Ltd., Germany. The bending radius was tested using the cylindrical bending method (Mandrel Bend Test).

[0051] Specific operation: a series of cylindrical mandrels with known diameters were prepared, and the film sample was attached or wrapped around the surface of the mandrel for bending. Starting from the largest diameter, the mandrel was gradually replaced until cracks or breaks appeared on the surface of the film. The smallest mandrel radius that did not cause damage was the minimum bending radius of the film.

[0052] The uniaxial tensile test was performed on a universal material testing machine to test the tensile strength and elongation at break.

[0053] Specific operation: cut the film sample into a standard dumbbell shape and clamp it in the upper and lower clamps of the testing machine. Stretch at a constant rate and record the stress-strain curve synchronously. The tensile strength is the maximum stress value on the curve. The elongation at break is calculated from the displacement at break and the original gauge length.

[0054] Table 1 Test results of products prepared in examples and comparative examples

[0055] According to the data in Table 1, it can be seen that the in-plane thermal conductivity of the hexagonal boron nitride-based composite film prepared in the examples is ≥200 W / (m·K), the bending radius is ≤3 mm, the elongation at break is ≥8%, the tensile strength is ≥25 MPa, and the tensile modulus is ≥6 GPa.

[0056] Compared with the h-BN thermal conductive film prepared in Comparative Example 1, the in-plane thermal conductivity of the composite film prepared in Example 1 is increased by more than 200%, the bending radius is reduced by more than 200%, and the elongation at break and the tensile strength are both increased by more than 50%; compared with Comparative Example 4, the in-plane thermal conductivity of the composite film of the application is increased by more than 300%, the thermal stability is significantly better than the former, and the mechanical properties are also more advantageous.

[0057] The thermal conductivity and mechanical properties of the pure h-BN film obtained in Comparative Example 1 are significantly worse than those of the examples. Among them, the in-plane thermal conductivity is ≤60 W / (m·K); the elongation at break is ≤5%, and the film is brittle and prone to fragmentation, which cannot meet the requirements of actual thermal management applications for material mechanical stability and thermal conductivity.

[0058] The thermal conductivity and mechanical properties of the composite film obtained in Comparative Example 2 are worse than those of the examples. The in-plane thermal conductivity is 60-70 W / (m·K); the elongation at break of the film is 6%-8%. The results show that, without HHTBP, the HHTBP cannot be converted into a "graphene-like bridging structure" if it is not subjected to high-temperature graphitization treatment, and can only play a limited role in dispersion and assembly, and cannot form an efficient thermal bridge and a stable mechanical reinforcement network between the h-BN nanosheet layers, so its enhancement effect on the film performance is significantly limited.

[0059] Comparative Example 3 uses a traditional flow casting film instead of vacuum filtration self-assembly to prepare h-BN composite film, the in-plane orientation degree of which is 0.3, which is inferior to the embodiment. The results show that the vacuum filtration process is the key to promoting the preferred orientation of h-BN nanosheets in the in-plane direction. If this process is missing, h-BN nanosheets are prone to random and disordered stacking state, which cannot form continuous in-plane orientation path, not only leading to a significant decrease in in-plane orientation degree, but also hindering the efficient transmission of phonons in the in-plane direction and the uniform dispersion of stress, thus significantly limiting the improvement effect of the in-plane core performance of the composite film.

[0060] The Raman spectra of the h-BN nanosheets of Example 1, the HHTBP before calcination, the HHTBP after calcination, and the composite film after calcination were detected, respectively. As shown in Figure 4 , the Raman spectrum of the original HHTBP shows a series of characteristic peaks at ~1284 cm -1 , ~1463 cm -1 , ~1510 cm -1 , ~1559 cm -1 and ~1614 cm -1 , which are respectively attributed to C-O stretching vibration, C-H in-plane bending vibration and C=C stretching vibration of benzene ring skeleton. After calcination at 1000 DEG C, these organic characteristic peaks completely disappear, and two broad peaks appear at ~1350 cm -1 and ~1580 cm -1 , which are the characteristic D peak and G peak of carbon material. The appearance of the D peak indicates that there are a large number of structural defects or grain boundaries in the generated carbon material, which is due to the disordered accumulation in the pyrolysis process of the precursor. The existence of the G peak confirms the formation of sp 2 hybrid graphite domain. In contrast, the Raman spectrum of the h-BN nanosheet only shows a sharp E2g characteristic peak at ~1367 cm -1 , which is completely different from the signal of carbon material. After calcination of the composite film, the characteristic peaks of h-BN and HHTBP after calcination coexist, indicating that after 1000 DEG C calcination of HHTBP, the graphitized carbon layer is bridged between the layers of the hexagonal boron nitride nanosheet.

[0061] The above only describes the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto. Any changes or replacements within the technical range disclosed by the present application can be easily thought of by those skilled in the art, which should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be defined by the claims.

Claims

1. A high thermal conductivity hexagonal boron nitride-based composite thin film, characterized in that, The composite film comprises hexagonal boron nitride nanosheets and a graphitized carbonaceous layer formed by heat treatment of a polyhydroxy aromatic compound, wherein the graphitized carbonaceous layer bridges the hexagonal boron nitride nanosheets.

2. The high thermal conductivity hexagonal boron nitride-based composite thin film according to claim 1, characterized in that, The hexagonal boron nitride nanosheets have an average size of 0.05 μm to 10 μm and a sheet thickness of 0.5 nm to 500 nm.

3. The high thermal conductivity hexagonal boron nitride-based composite thin film according to claim 1, characterized in that, The thickness of the membrane is 30 μm to 90 μm.

4. The high thermal conductivity hexagonal boron nitride-based composite thin film according to claim 1, characterized in that, The in-plane thermal conductivity of the composite film is greater than 200 W / (m·K).

5. The high thermal conductivity hexagonal boron nitride-based composite thin film according to claim 1, characterized in that, The polyhydroxy aromatic compound is selected from one or more of the following compounds: 2,3,6,7,10,11-hexahydroxytriphenylene, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , .

6. A method for preparing a high thermal conductivity hexagonal boron nitride-based composite thin film as described in any one of claims 1-5, characterized in that, Includes the following steps: S1: Hexagonal boron nitride nanosheets, polyhydroxy aromatic compounds and solvents are mixed to form a uniform mixed dispersion; S2: The mixed dispersion is vacuum filtered, and the hexagonal boron nitride nanosheets self-assemble into a highly oriented pre-film under the action of polyhydroxy aromatic compounds. S3: The preformed film is dried to obtain a dried preformed film; S4: The dried pre-formed film is heat-treated in an inert atmosphere, during which the polyhydroxy aromatic compound is graphitized to obtain a high thermal conductivity hexagonal boron nitride-based composite film.

7. The preparation method according to claim 6, characterized in that, In step S1, the concentration of the hexagonal boron nitride nanosheets in the mixed dispersion is 0.05-0.5 mg / mL; the mass ratio of the hexagonal boron nitride nanosheets to the polyhydroxy aromatic compound is 10:1 to 1:1; the solvent is water or isopropanol with a solvent purity ≥99.5%; the mixing is carried out by stirring at 20-30 °C and then by ultrasonic dispersion.

8. The preparation method according to claim 6, characterized in that, In step S2, the vacuum degree of the vacuum filtration is -0.08 MPa to -0.09 MPa, and the temperature of the mixed dispersion is maintained at 20-25 ℃ during the filtration process; in step S3, the drying temperature is 50-70 ℃, the time is 30-60 min, and the drying method is forced air drying or vacuum drying.

9. The preparation method according to claim 6, characterized in that, In step S4, the inert atmosphere is nitrogen or argon with a gas purity ≥99.99%, the heat treatment temperature is 800-1200 ℃, the heating rate is 3-5 ℃ / min, the holding time is 3-8 h, and then the furnace is cooled to room temperature.

10. The preparation method according to claim 6, characterized in that, Step S4 is followed by a post-processing step: the high thermal conductivity hexagonal boron nitride-based composite film after heat treatment is sequentially cleaned and dried.

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