Boron nitride heterostructure heat-conducting filler, heat-conducting composite material containing same and preparation method of heat-conducting composite material

Abstract

The invention relates to the technical field of boron nitride heat-conducting materials, and discloses a boron nitride heterostructure heat-conducting filler, a heat-conducting composite material containing the same and a preparation method of the heat-conducting composite material. The boron nitride heterostructure heat-conducting filler is prepared by directionally growing a one-dimensional boron nitride nanotube on the surface of a two-dimensional boron nitride nanosheet through chemical vapor deposition; the material has a covalent bond connected boron nitride heterostructure. A one-dimensional boron nitride nanotube (BNNT) is grown on the surface of a two-dimensional boron nitride nanosheet (BNNS) in an oriented manner through a chemical vapor deposition method, and a boron nitride heterostructure with covalent bond connection is formed. The unique morphology not only enhances the phonon coupling efficiency between BNNS and BNNT, but also reduces the percolation threshold of the filler in a polymer matrix, and improves the dispersibility of the filler in the polymer matrix. The heat-conducting composite material containing the boron nitride heterostructure heat-conducting filler has low interface thermal resistance, high compatibility and high heat-conducting property.

Description

Technical Field

[0001] The present application relates to the technical field of boron nitride thermal conductive materials, and in particular to a boron nitride heterostructure thermal conductive filler, a thermal conductive composite material containing the same, and a preparation method thereof. Background Art

[0002] As electronic devices develop towards miniaturization, high integration, and high power density, the problem of internal heat accumulation is becoming increasingly prominent. If the heat cannot be dissipated in a timely manner, it will cause local temperature rise in the device, significantly reducing operational stability and service life. Boron nitride (BN) is considered an ideal thermally conductive filler due to its high thermal conductivity, insulation, and chemical stability. However, traditional BN fillers have high interfacial thermal resistance and are prone to agglomeration in the polymer matrix, resulting in discontinuous thermal paths and reduced thermal conductivity.

[0003] Currently, filler dispersibility can be partially improved through heterostructure design. However, the weak interface between the filler and the matrix still limits the improvement of thermal conductivity, and the balance between mechanical properties and thermal conductivity (trade-off effect) needs to be solved urgently. Ionic liquids (ILs) provide a new approach for interface modification due to their unique cation-π interaction and designability. By modifying the surface of boron nitride heterostructures with functionalized ILs, the filler-matrix interface bonding can be enhanced, the interfacial thermal resistance can be reduced, and the intrinsic thermal conductivity of the filler can be maintained. However, existing methods mostly use blending modification, which makes it difficult to achieve in-situ directional regulation.

[0004] Based on this, it is of great significance to develop a boron nitride-based thermal conductive filler with efficient interface function in combination with three-dimensional orientation structure design. Summary of the Invention

[0005] The present application provides a boron nitride heterostructure thermal conductive filler, a thermal conductive composite material containing the same, and a preparation method thereof, aiming to solve the technical problems of low thermal conductivity caused by high filler-matrix interface thermal resistance and poor dispersibility of existing boron nitride fillers.

[0006] In order to achieve the above objectives, this application adopts the following technical solutions.

[0007] In a first aspect of the present application, a boron nitride heterostructure thermal conductive filler is provided, which is prepared by directionally growing one-dimensional boron nitride nanotubes on the surface of two-dimensional boron nitride nanosheets by chemical vapor deposition; the filler has a covalently bonded boron nitride heterostructure.

[0008] Another aspect of the present application provides a method for preparing the above-mentioned boron nitride heterostructure thermal conductive filler, comprising:

[0009] Urea, boric acid and boron nitride nanosheets are mixed and ball-milled to obtain a precursor powder;

[0010] The precursor powder is annealed at high temperature under nitrogen protection to obtain the product.

[0011] Preferably, the ball milling treatment specifically refers to:

[0012] Urea, boric acid and boron nitride nanosheets are ball milled in a ball milling tank at a speed of 400-500 rpm for 6-8 h; wherein the mass ratio of urea, boric acid and boron nitride nanosheets is 10:1:0.2.

[0013] and / or,

[0014] The high-temperature annealing specifically refers to: increasing the temperature to 1000-1100℃ at a rate of 2-4℃ / min, and keeping the temperature for 8-10 h.

[0015] and / or,

[0016] The boron nitride nanosheets are prepared by the following method:

[0017] Hexagonal boron nitride and zirconium oxide milling beads are added to a mixed solvent of deionized water and isopropanol in a volume ratio of 1:1, and ultrasonic treatment is performed for 18-24 h; then the solid phase precipitate is collected by centrifugation twice, washed, and freeze-dried to obtain boron nitride nanosheets; wherein the size of the hexagonal boron nitride is 1-10 μm; the diameter of the milling beads is 0.15-0.3 mm.

[0018] In another aspect of the present application, a preparation method of an ionic liquid modified boron nitride heterostructure heat conducting filler is provided, comprising:

[0019] The above-mentioned boron nitride heterostructure heat conducting filler is added to an aqueous solution of 1-butyl-3-methylimidazolium acetate, ultrasonic treatment is performed, and then condensation reflux is performed at 80-90℃ to obtain a reaction liquid.

[0020] The solid phase in the reaction liquid is collected by centrifugation, washed, and then freeze-dried to obtain the ionic liquid modified boron nitride heterostructure heat conducting filler.

[0021] Preferably, the mass ratio of the boron nitride heterostructure heat conducting filler and 1-butyl-3-methylimidazolium acetate is 0.1:1.

[0022] The concentration of the aqueous solution of 1-butyl-3-methylimidazolium acetate is 10 wt%.

[0023] In another aspect of the present application, the ionic liquid modified boron nitride heterostructure heat conducting filler prepared by the above-mentioned preparation method is provided.

[0024] In another aspect of the present application, a preparation method of a heat conducting composite material is provided, comprising:

[0025] S1, dissolving polyamide acid and triethylamine in deionized water to obtain a polyamide acid solution; adding the ion liquid modified boron nitride heterostructure heat-conducting filler of claim 7 into the polyamide acid solution, uniformly dispersing by ultrasonic to obtain a dispersion liquid;

[0026] S2, transferring the dispersion liquid into a mold, and obtaining a porous aerogel with a three-dimensional through skeleton structure by freeze-drying;

[0027] S3, performing a stepwise heat pressing treatment on the porous aerogel to convert the polyamide acid into polyimide, and obtaining a heat-conducting composite material.

[0028] Preferably, the polyamide acid is prepared by the following method:

[0029] 4,4'-diamino diphenyl ether is dissolved in N,N-dimethylacetamide, and 1,2,4,5-benzene tetracarboxylic dianhydride is added and stirred to react;

[0030] Excess deionized water is added to the reaction solution to precipitate a precipitate, and the precipitate is filtered, washed and dried to obtain a solid polyamide acid.

[0031] Preferably, the polyamide acid solution has a ratio of 0.8g-1.0g of polyamide acid, 0.48g of triethylamine, and 18.52ml-23.52ml of deionized water.

[0032] The amount of the ion liquid modified boron nitride heterostructure heat-conducting filler is 5-20wt% of the amount of polyamide acid.

[0033] and / or,

[0034] The freeze-drying includes: immersing the bottom of the mold into liquid nitrogen for directional freezing, and then drying the frozen sample;

[0035] and / or,

[0036] The stepwise heat pressing treatment specifically includes: heat pressing at 10-15MPa, 100-120℃, 180-200℃ and 210-230℃ respectively for 0.5-1.5h.

[0037] In another aspect of the present application, a heat-conducting composite material prepared by the above preparation method is provided.

[0038] Compared with the prior art, the present application has the following beneficial effects:

[0039] The present application significantly reduces the filler-matrix interface thermal resistance through the multi-level branch structure of the bionic coral and the synergistic effect of the heterogeneous components, constructs a three-dimensional efficient heat conduction path, and finally realizes a breakthrough improvement in the heat conduction performance of the composite material.

[0040] Firstly, the application grows one-dimensional boron nitride nanotubes (BNNT) on the surface of two-dimensional boron nitride nanosheets (BNNS) by chemical vapor deposition to form a boron nitride heterostructure with covalent bond connection. This unique morphology not only enhances the phonon coupling efficiency between BNNS and BNNT, but also reduces the percolation threshold of fillers in the polymer matrix and improves the dispersibility of fillers in the polymer matrix.

[0041] Secondly, the application further functionalizes the surface of BNNS-BNNT through the cation-π interaction of a-IL (1-butyl-3-methylimidazolium acetate) to significantly enhance the interfacial bonding strength between the filler and the matrix, reduce the interfacial thermal resistance, and significantly improve the interfacial compatibility between the filler and the polyimide matrix.

[0042] Thirdly, the three-dimensional hierarchical characteristics of the boron nitride structure in the filler of the application form a three-dimensional interpenetrating network in the composite material: BNNS as the main trunk provides an in-plane high thermal conductivity channel, and BNNT as the branch realizes three-dimensional bridging between fillers; and the a-IL establishes a continuous heat transfer path through synergistic effect, enhances the heat transfer performance between the boron nitride nanotube and the boron nitride nanosheet and the polyimide, and improves the thermal conductivity of the PI composite material, thereby significantly improving the thermal conductivity of the polyimide material at a low filler content. BRIEF DESCRIPTION OF DRAWINGS

[0043] In order to more clearly illustrate the technical solutions of the embodiments of the application, the drawings needed in the embodiment description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments described in the application, and those skilled in the art can also obtain other drawings according to these drawings without creating any creative labor.

[0044] Figure 1 The scanning electron microscope image of BNNS-BNNT prepared for Example 1;

[0045] Figure 2 The scanning electron microscope image of the existing two-dimensional boron nitride nanosheet;

[0046] Figure 3 The scanning electron microscope image of a-IL@BNNS-BNNT prepared for Example 1;

[0047] Figure 4 The transmission electron microscope image of a-IL@BNNS-BNNT prepared for Example 1;

[0048] Figure 5 The scanning electron microscope image of a-IL@BNNS-BNNT / PAA aerogel prepared for Example 1;

[0049] Figure 6A scanning electron microscope image of the a-IL@BNNS-BNNT / PI heat-conducting composite material prepared for Example 1;

[0050] Figure 7 A graph of the thermal conductivity test results of the heat-conducting composite materials prepared for Examples and Comparative Examples. DETAILED DESCRIPTION

[0051] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the scope of protection of the present application.

[0052] In the following description of the present embodiments, the terms "comprise", "contain", "have" and "include" and the like are open terms, that is, they mean including but not limited to.

[0053] In the following description of the present embodiments, the term "and / or" is used to describe the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B can represent the following three cases: A exists alone, B exists alone, and A and B exist at the same time. Wherein A and B can be singular or plural. The character " / " generally represents an "or" relationship between the front and rear associated objects.

[0054] In the following description of the present embodiments, the term "at least one" means one or more, and "multiple" means two or more. "At least one of the following" or similar expressions means any combination of these items, including any combination of single item or multiple items. For example, "at least one of a, b or c", or "at least one of a, b and c", can represent a, b, c, a-b (i.e. a and b), a-c, b-c, or a-b-c, wherein a, b, and c can be single or multiple.

[0055] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0056] Those skilled in the art should understand that in the following description of the embodiments of the present application, the order of the serial numbers does not mean the order of execution, and some or all steps can be executed in parallel or in sequence, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0057] It is understood by those skilled in the art that the numerical ranges in the embodiments of the present application should be understood to include each intermediate value between the upper and lower limits of the range. Each intermediate value within any stated value or stated range, as well as any other stated value or intermediate value within the stated range, is included in the present application. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0058] Unless otherwise defined, technical / scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In case of conflict between the content of the specification and that of any document incorporated herein by reference, the content of the specification prevails.

[0059] In a first aspect, the present application provides a boron nitride heterostructure heat-conducting filler, which is prepared by chemical vapor deposition of one-dimensional boron nitride nanotubes on the surface of two-dimensional boron nitride nanosheets; and has a covalently bonded boron nitride heterostructure.

[0060] The present application forms a boron nitride heterostructure with covalent bonding by chemical vapor deposition of one-dimensional boron nitride nanotubes on the surface of two-dimensional boron nitride nanosheets (BNNS). This unique morphology not only enhances the phonon coupling efficiency between BNNS and BNNT, but also reduces the percolation threshold of the filler in the polymer matrix, and improves the dispersibility of the filler in the polymer matrix.

[0061] The preparation method of the boron nitride heterostructure heat-conducting filler comprises:

[0062] Urea, boric acid and boron nitride nanosheets are mixed and then treated by ball milling to obtain a precursor powder; specifically, urea, boric acid and boron nitride nanosheets are ball milled in a ball milling tank at a speed of 400-500 rpm for 6-8 hours to obtain a uniform white precursor powder; wherein the mass ratio of urea, boric acid and boron nitride nanosheets is preferably 10:1:0.2.

[0063] The precursor powder is annealed under nitrogen protection, and the boron nitride heterostructure heat-conducting filler is obtained. Specifically, in a tube furnace, nitrogen is introduced at a flow rate of 50-100 mL / min, the precursor powder is heated to 1000-1100℃ at a heating rate of 2-4℃ / min for gas deposition reaction, and the temperature is kept for 8-10 h to form the boron nitride heterostructure heat-conducting filler by self-assembly.

[0064] In the present application, the boron nitride nanosheet is prepared by the following method:

[0065] In a mixed solution of water and isopropyl alcohol with a volume ratio of 1:1 as a dispersion medium, hexagonal boron nitride and zirconia milling beads are added, and ultrasonic treatment is performed in an ultrasonic machine for 18-24 hours; the suspension after ultrasonic treatment is centrifuged to obtain a precipitate, the centrifugal speed is 8000-10000 rpm, and the centrifugal time is 10 minutes; the precipitate is washed with deionized water for 3-5 times, and then freeze-dried at-50℃ for 24 hours to obtain boron nitride nanosheets. The mass ratio of hexagonal boron nitride to zirconia milling beads is preferably 5:1; the size of the hexagonal boron nitride is 1-10 μm, preferably 10 μm; the diameter of the zirconia milling beads is 0.15-0.3 mm, and the milling beads with a diameter of 0.15 mm and a diameter of 0.3 mm are graded according to a mass ratio of 1:1.

[0066] In a second aspect, the application provides a preparation method of an ionic liquid modified boron nitride heterostructure heat-conducting filler, comprising:

[0067] The above boron nitride heterostructure heat-conducting filler is added into a water solution of 1-butyl-3-methylimidazolium acetate, ultrasonic treatment is performed, and then condensation reflux is performed at 80-90℃ to obtain a reaction liquid.

[0068] In the application, the mass ratio of the boron nitride heterostructure heat-conducting filler to 1-butyl-3-methylimidazolium acetate is preferably 0.1:1; the concentration of the water solution of 1-butyl-3-methylimidazolium acetate is preferably 10wt%.

[0069] In the application, the ultrasonic treatment is performed at room temperature, and the ultrasonic treatment time is 30-60 minutes; the condensation reflux adopts oil bath and magnetic stirring, and the oil bath temperature is preferably 80℃.

[0070] The solid phase in the reaction liquid is collected by centrifugation, washed, and then freeze-dried to obtain the ionic liquid modified boron nitride heterostructure heat-conducting filler.

[0071] In the application, the solid phase in the reaction liquid is collected by centrifugation, the centrifugal speed is preferably 9500-10000 rpm, and the centrifugal time is preferably 15-20 minutes; the free 1-butyl-3-methylimidazolium acetate is removed by water washing. The freeze-drying temperature is preferably-50℃, and the pressure is preferably 25 Pa.

[0072] The ionic liquid modified boron nitride heterostructure heat-conducting filler prepared in the application significantly enhances the interfacial bonding strength between the filler and the matrix, reduces the interfacial thermal resistance, and significantly improves the interfacial compatibility between the filler and the polyimide matrix through the cation-π interaction functionalization of BNNS-BNNT surface by a-IL.

[0073] In a third aspect, the application provides a preparation method of a heat-conducting composite material, comprising:

[0074] S1, dissolving polyamide acid and triethylamine in deionized water to obtain a polyamide acid solution; adding the above-mentioned ionic liquid modified boron nitride heterogeneous structure heat-conducting filler into the polyamide acid solution, uniformly dispersing by ultrasonic to obtain a dispersion liquid;

[0075] The ratio of the polyamide acid solution is: polyamide acid 0.8g-1.0g, triethylamine 0.48g, deionized water 18.52ml-23.52ml; preferably the content of polyamide acid in the polyamide acid solution is 4wt%.

[0076] The amount of the ionic liquid modified boron nitride heterogeneous structure heat-conducting filler is 5-20wt% of the amount of polyamide acid;

[0077] In the present application, the polyamide acid is prepared by the following method:

[0078] Dissolving 4,4'-diamino diphenyl ether in N,N-dimethylacetamide, then adding 1,2,4,5-benzene tetracarboxylic dianhydride and stirring to react; wherein the mass ratio of 4,4'-diamino diphenyl ether to benzene tetracarboxylic dianhydride is 1:(0.8-1);

[0079] Excess deionized water is added to the reaction solution to precipitate the precipitate, and the precipitate is filtered, washed and dried to obtain solid polyamide acid.

[0080] S2, transferring the dispersion liquid to a mold, and obtaining a porous aerogel with a three-dimensional network skeleton structure by freeze-drying;

[0081] Specifically, the dispersion liquid is transferred to a special mold, and directional freezing is realized by controlling the contact depth of the bottom of the mold with liquid nitrogen to achieve a temperature gradient in the vertical direction, and the ice crystals are induced to grow in the axial direction. Subsequently, the sample is placed in a freeze dryer and treated at a temperature of-50℃ and a pressure of 25Pa for 24h, and finally a porous aerogel with a three-dimensional through skeleton structure is obtained. The mold is a polytetrafluoroethylene lining / copper base composite structure.

[0082] S3, performing a stepwise hot pressing treatment on the porous aerogel to convert the polyamide acid into polyimide, and obtaining a heat-conducting composite material.

[0083] In the present application, the stepwise hot pressing treatment is specifically: under a pressure of 10-15MPa, hot pressing at 100-120℃, 180-200℃ and 210-230℃ respectively for 0.5-1.5h. After the stepwise hot pressing treatment, the polyamide acid is converted into polyimide, and a heat-conducting composite material is obtained.

[0084] The three-dimensional hierarchical characteristics of the boron nitride structure in the filler of the present application form a three-dimensional interpenetrating network in the composite material: the BNNS provides an in-plane high thermal conduction channel as the trunk, and the BNNT realizes three-dimensional bridging between fillers as the branch; and the continuous heat transfer path is established by the synergistic effect of a-IL, the heat transfer performance between the boron nitride nanotube and the boron nitride nanosheet and the polyimide is enhanced, and the thermal conductivity of the PI-based polymer composite material is improved, thereby significantly improving the thermal conductivity of the polyimide-based polymer composite material at a low filler content.

[0085] The present application is further illustrated by the following examples.

[0086] Example 1

[0087] The present embodiment provides a preparation method of a boron nitride heterostructure thermal conductive filler, an ionic liquid modified boron nitride heterostructure thermal conductive filler and a thermal conductive composite material, comprising:

[0088] Step 1, preparation of a boron nitride heterostructure thermal conductive filler

[0089] At room temperature, 5g of high-purity urea, 0.5g of boric acid and 0.1g of boron nitride nanosheet (BNNT) are uniformly mixed; the mixture is placed in a ball mill tank and ball milled at a speed of 500rpm for 8h to obtain a uniform white precursor powder;

[0090] The white precursor powder is placed in a tube furnace and heated to 1000℃ at a rate of 2℃ / min under a nitrogen protective atmosphere, and gas deposition is carried out for 10h to obtain a boron nitride heterostructure thermal conductive filler, denoted as BNNS-BNNT.

[0091] Step 2, preparation of an ionic liquid modified boron nitride heterostructure thermal conductive filler

[0092] The boron nitride heterostructure thermal conductive filler obtained in step 1 is added to a 1-butyl-3-methylimidazole acetate (c-IL) aqueous solution, and ultrasonic treatment is carried out at room temperature for 30-60min to obtain a mixed solution; the mixed solution is poured into a three-necked flask and magnetically stirred, and an oil bath is used for condensation reflux at 80℃ for 6h to carry out the reaction;

[0093] The mixed solution after the reaction is centrifuged at a speed of 10000rpm for 15min, and the solid phase is collected; the solid phase is washed with water for 3 times to remove free 1-butyl-3-methylimidazole acetate, and then freeze-dried at a temperature of-50℃ and a pressure of 25Pa for 24h to obtain an ionic liquid modified boron nitride heterostructure thermal conductive filler, denoted as a-IL@BNNS-BNNT.

[0094] Step 3, preparation of a thermal conductive composite material

[0095] S1, 1.0 g of 4,4'-diaminodiphenyl ether was dissolved in 11.8 g of anhydrous N,N-dimethylacetamide, after complete dissolution, 0.9 g of pyromellitic dianhydride was added in batches, the reaction temperature was controlled at 0°C, and mechanical stirring was continued at a speed of 300 rpm for 2 h to obtain a viscous polyamic acid prepolymer solution;

[0096] The polyamic acid prepolymer solution was slowly immersed in excess deionized water to precipitate a light yellow fibrous precipitate, which was washed with deionized water for 3 h, and then vacuum dried at 60°C for 12 h to obtain a solid polyamic acid, denoted as PAA;

[0097] S2, 1.0 g of PAA solid and 0.48 g of triethylamine were dissolved in 23.52 mL of deionized water, and magnetically stirred at 500 rpm for 5 h at 25°C to obtain a polyamic acid solution with a concentration of 4 wt%;

[0098] 0.021 g of a-IL@BNNS-BNNT was added to 12.5 g of polyamic acid solution, and ultrasonic treatment was carried out under the conditions of a frequency of 40 kHz and a power of 300 W for 30 min to break the agglomeration of the fillers and make them uniformly dispersed, to obtain a-IL@BNNS-BNNT / PAA dispersion;

[0099] S3, the a-IL@BNNS-BNNT / PAA dispersion was injected into a mold with a copper base and a polytetrafluoroethylene liner, the contact depth of the mold bottom with liquid nitrogen was controlled at 5±0.5 cm, an axial temperature gradient was established, and ice crystals were induced to grow vertically. After complete freezing, the sample was immediately transferred to a freeze dryer and treated at a temperature of -50°C and a pressure of 25 Pa for 24 h to obtain a-IL@BNNS-BNNT / PAA aerogel with a three-dimensional skeleton structure;

[0100] S4, the a-IL@BNNS-BNNT / PAA aerogel was placed in a flat plate hot press, and was heated at 120°C, 180°C and 220°C for 1 h respectively under a pressure of 10-15 MPa to realize the complete conversion of PAA to PI, and a 5 wt% a-IL@BNNS-BNNT / PI thermal conductive composite material was obtained.

[0101] Example 2

[0102] Example 2 differs from Example 1 in that in S2 of step 3 for preparing the thermal conductive composite material, the amount of a-IL@BNNS-BNNT is changed to 0.044 g, and the rest is the same as Example 1. The thermal conductive composite material prepared in Example 2 is denoted as 10 wt% a-IL@BNNS-BNNT / PI.

[0103] Example 3

[0104] The difference between Example 3 and Example 1 is that in S2 of Step 3 for preparing the thermally conductive composite material, the amount of a-IL@BNNS-BNNT is changed to 0.071 g, and the rest is the same as Example 1. The thermally conductive composite material prepared in Example 2 is denoted as 15wt% a-IL@BNNS-BNNT / PI.

[0105] Example 4

[0106] The difference between Example 4 and Example 1 is that in S2 of Step 3 for preparing the thermally conductive composite material, the amount of a-IL@BNNS-BNNT is changed to 0.1 g, and the rest is the same as Example 1. The thermally conductive composite material prepared in Example 2 is denoted as 20wt% a-IL@BNNS-BNNT / PI.

[0107] Comparative Example 1

[0108] Comparative Example 1 does not use an ionic liquid to modify the BNNS-BNNT compared with Example 1. The preparation method comprises:

[0109] Step 1, preparation of boron nitride heterostructure thermally conductive filler

[0110] At room temperature, 5 g of high-purity urea, 0.5 g of boric acid and 0.1 g of boron nitride nanosheet are uniformly mixed; the mixture is placed in a ball mill tank and ball milled at a speed of 500 rpm for 8 h to obtain a uniform white precursor powder;

[0111] The white precursor powder is placed in a tube furnace and heated to 1000℃ at a rate of 2℃ / min under a nitrogen protective atmosphere, and gas deposition is carried out for 10 h to obtain a boron nitride heterostructure thermally conductive filler, denoted as BNNS-BNNT.

[0112] Step 2, preparation of a thermally conductive composite material

[0113] S1, 1.0 g of 4,4'-diamino diphenyl ether is dissolved in 11.8 g of anhydrous N,N-dimethylacetamide, and after complete dissolution, 0.9 g of pyromellitic dianhydride is added in batches, the reaction temperature is controlled at 0℃, and mechanical stirring is continued at a speed of 300 rpm for 2 h to obtain a viscous polyamic acid prepolymer solution;

[0114] The polyamic acid prepolymer solution is slowly immersed in excess deionized water to precipitate a light yellow fibrous precipitate, which is washed with deionized water for 3 times, and then vacuum dried at 60℃ for 12 h to obtain a solid polyamic acid, denoted as PAA;

[0115] S2, 1.0 g of PAA solid and 0.48 g of triethylamine are dissolved in 23.52 mL of deionized water, and magnetic stirring is carried out at a speed of 500 rpm for 5 h at 25℃ to obtain a polyamic acid solution with a concentration of 4wt%;

[0116] 0.021 g of BNNS-BNNT was added into 12.5 g of polyamic acid solution, and the filler agglomeration was broken and uniformly dispersed by ultrasonic treatment at a frequency of 40 kHz and a power of 300 W for 30 min to obtain a BNNS-BNNT / PAA dispersion;

[0117] S3, the BNNS-BNNT / PAA dispersion was injected into a mold with a copper substrate and a polytetrafluoroethylene liner, the contact depth of the bottom of the mold with liquid nitrogen was controlled to be 5±0.5 cm, an axial temperature gradient was established, and ice crystals were induced to grow vertically. After complete freezing, the sample was immediately transferred to a freeze dryer and treated at a temperature of -50°C and a pressure of 25 Pa for 24 h to obtain a BNNS-BNNT / PAA aerogel with a three-dimensional skeleton structure;

[0118] S4, the BNNS-BNNT / PAA aerogel was placed in a flat plate hot press, and was sequentially hot pressed at 120°C, 180°C and 220°C for 1 h under a pressure of 10-15 MPa to realize complete conversion of PAA to PI, and obtain a 5wt% BNNS-BNNT / PI thermal conductive composite material.

[0119] Comparative Example 2

[0120] Comparative Example 2 differs from Comparative Example 1 in that in S2 of step 2, the amount of BNNS-BNNT is changed to 0.044 g, and the rest is the same as Comparative Example 1. The thermal conductive composite material prepared in Comparative Example 2 is denoted as 10wt% BNNS-BNNT / PI.

[0121] Comparative Example 3

[0122] Comparative Example 3 differs from Comparative Example 1 in that in S2 of step 2, the amount of BNNS-BNNT is changed to 0.071 g, and the rest is the same as Comparative Example 1. The thermal conductive composite material prepared in Comparative Example 3 is denoted as 15wt% BNNS-BNNT / PI.

[0123] Comparative Example 4

[0124] Comparative Example 4 differs from Comparative Example 1 in that in S2 of step 2, the amount of BNNS-BNNT is changed to 0.1 g, and the rest is the same as Comparative Example 1. The thermal conductive composite material prepared in Comparative Example 4 is denoted as 20wt% BNNS-BNNT / PI.

[0125] Comparative Example 5

[0126] Comparative Example 5 differs from Example 1 in that two-dimensional boron nitride nanosheets are used instead of boron nitride heterostructure thermal conductive fillers, and no ionic liquid modification is performed. The preparation method comprises:

[0127] S1, 1.0 g of 4,4'-diaminodiphenyl ether was dissolved in 11.8 g of anhydrous N,N-dimethylacetamide, after complete dissolution, 0.9 g of pyromellitic dianhydride was added in batches, the reaction temperature was controlled at 0°C, and mechanical stirring was continued at a speed of 300 rpm for 2 h to obtain a viscous polyamic acid prepolymer solution;

[0128] The polyamic acid prepolymer solution was slowly immersed in excess deionized water to precipitate a light yellow fibrous precipitate, which was washed with deionized water for 3 times, and then vacuum dried at 60°C for 12 h to obtain a solid polyamic acid, denoted as PAA;

[0129] S2, 1.0 g of PAA solid and 0.48 g of triethylamine were dissolved in 23.52 mL of deionized water, and magnetically stirred at a speed of 500 rpm for 5 h at 25°C to obtain a polyamic acid solution with a concentration of 4 wt%;

[0130] 0.021 g of two-dimensional boron nitride nanosheet BNNS was added to 12.5 g of polyamic acid solution, and ultrasonic treatment was carried out under the condition of a frequency of 40 kHz and a power of 300 W for 30 min to break the agglomeration of the filler and make it uniformly dispersed, to obtain a BNNS / PAA dispersion;

[0131] S3, the BNNS / PAA dispersion was injected into a mold with a copper base and a polytetrafluoroethylene liner, the contact depth of the mold bottom with liquid nitrogen was controlled at 5±0.5 cm, an axial temperature gradient was established to induce the directional growth of ice crystals in the vertical direction. After complete freezing, the sample was immediately transferred to a freeze dryer and treated at a temperature of -50°C and a pressure of 25 Pa for 24 h to obtain a BNNS / PAA aerogel;

[0132] S4, the BNNS / PAA aerogel was placed in a flat plate hot press, and was sequentially hot pressed at 120°C, 180°C and 220°C for 1 h under a pressure of 10-15 MPa to realize the complete conversion of PAA to PI, and a 5 wt% BNNS / PI thermal conductive composite material was obtained.

[0133] Comparative Example 6

[0134] Comparative Example 6 and Comparative Example 5 differ in that in step S2, the amount of BNNS is changed to 0.044 g, and the rest is the same as Comparative Example 5. The thermal conductive composite material prepared in Comparative Example 6 is denoted as 10 wt% BNNS / PI.

[0135] Comparative Example 7

[0136] Comparative Example 7 and Comparative Example 5 differ in that in step S2, the amount of BNNS is changed to 0.070 g, and the rest is the same as Comparative Example 5. The thermal conductive composite material prepared in Comparative Example 6 is denoted as 15 wt% BNNS / PI.

[0137] Comparative Example 8

[0138] The difference between Comparative Example 8 and Comparative Example 5 is that the amount of BNNS in step S2 is changed to 0.1 g, and the rest is the same as Comparative Example 5. The thermal conductive composite prepared in Comparative Example 6 is denoted as 20wt% BNNS / PI.

[0139] The scanning electron microscope image of the BNNS-BNNT prepared in Example 1 is shown in Figure 1 It can be seen from Figure 1 that the boron nitride nanotubes grow on the boron nitride nanosheets to form a boron nitride heterostructure, and there is no obvious separation interface between the BNNS and the BNNT, which is conducive to the effective construction of the multi-level thermal conduction path. The existing

[0140] The scanning electron microscope image of the two-dimensional boron nitride nanosheet is shown in Figure 2 It can be seen that the boron nitride is in the form of a sheet, does not have a boron nitride heterostructure, and does not have a multi-level thermal conduction path.

[0141] The scanning electron microscope image of the a-IL@BNNS-BNNT prepared in Example 1 is shown in Figure 3 It can be seen from Figure 3 that the surface of the boron nitride nanotubes and the boron nitride nanosheets becomes rough, and there is no obvious separation interface between the a-IL and the BNNS-BNNT in the structure, which is conducive to the effective construction of the multi-level thermal conduction path.

[0142] The transmission electron microscope image of the a-IL@BNNS-BNNT prepared in Example 1 is shown in Figure 4 It can be seen from Figure 4 that the boron nitride nanotubes grow on the boron nitride nanosheets to form a boron nitride heterostructure, and the a-IL acts on the BNNS-BNNT, and there is no obvious separation interface between the a-IL and the BNNS-BNNT in the structure, which is conducive to the effective construction of the multi-level thermal conduction path.

[0143] The scanning electron microscope image of the a-IL@BNNS-BNNT / PAA aerogel prepared in Example 1 is shown in Figure 5 It can be seen from Figure 5 that the BNNS-BNNT is arranged in an ordered layer-by-layer manner to form a long-range ordered orientation structure, which is conducive to the effective construction of the thermal conduction path.

[0144] The scanning electron microscope image of the a-IL@BNNS-BNNT / PI thermal conductive composite prepared in Example 1 is shown in Figure 6 It can be seen from Figure 6 that the skeleton structure of the a-IL@BNNS-BNNT / PI obtained after heat treatment becomes densified, and the densified structure significantly improves the thermal conduction performance, providing an efficient path for heat transfer in its interior.

[0145] The thermal conductivity of the thermal conductive composite prepared in Examples 1-4 and Comparative Examples 1-8 was tested, and the thermal conductivity of each thermal conductive composite was tested 3 times, and the test results are shown in Table 1 and Table 2. Among them, PI film was used as a control. Figure 7

[0146] Table 1 Thermal conductivity test values of thermal conductive composite of examples and comparative examples

[0147]

[0148]

[0149] From Figure 7 As can be seen from Table 1 and Table 2, the thermal conductivity of PI is greatly improved after being compounded with the thermal conductive filler; with the increase of the filling amount, the thermal conductivity of the thermal conductive composite increases.

[0150] Among them, under the same filling amount, the thermal conductivity of a-IL@BNNS-BNNT / PI is the highest, the thermal conductivity of BNNS-BNNT / PI is the second, and the thermal conductivity of BNNS / PI is the lowest. That is to say, under the same filling amount, compared with Comparative Example 1 and Comparative Example 5, the thermal conductivity of the thermal conductive composite of Example 1 is significantly improved, which shows that after the ionic liquid is modified to the boron nitride heterostructure thermal conductive filler of the present application, the thermal conductivity of the thermal conductive composite formed by the ionic liquid and PI can be significantly improved.

[0151] Although the present application has been described in detail in the specification and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements can be made on the basis of the present application. Therefore, these modifications or improvements made on the basis of not deviating from the spirit of the present application, all belong to the scope of protection claimed by the present application.​

Claims

1. A boron nitride heterostructure thermal conductive filler, characterized in that: It is prepared by directionally growing one-dimensional boron nitride nanotubes on the surface of two-dimensional boron nitride nanosheets through chemical vapor deposition; it has a covalently bonded boron nitride heterostructure.

2. The method for preparing the boron nitride heterostructure thermal conductive filler according to claim 1, characterized in that: include: Urea, boric acid and boron nitride nanosheets are mixed and ball-milled to obtain a precursor powder; The precursor powder is annealed at high temperature under nitrogen protection to obtain the product.

3. The preparation method according to claim 2, characterized in that The ball milling process is specifically as follows: The urea, boric acid and boron nitride nanosheets were ball-milled in a ball mill at a speed of 400-500 rpm for 6-8 hours; wherein the mass ratio of urea, boric acid and boron nitride nanosheets was 10:1:0.2; and / or, The high temperature annealing is specifically as follows: heating to 1000-1100°C at a heating rate of 2-4°C / min and keeping the temperature for 8-10 hours; and / or, The boron nitride nanosheets are prepared by the following method: Hexagonal boron nitride and zirconium oxide ball milling beads are added to a mixed solvent of deionized water and isopropanol in a volume ratio of 1:1 and ultrasonically treated for 18 to 24 hours. The solid phase precipitate is then collected by two centrifugations, washed, and freeze-dried to obtain boron nitride nanosheets. The size of the hexagonal boron nitride is 1 to 10 μm, and the diameter of the ball milling beads is 0.15-0.3 mm.

4. A method for preparing an ionic liquid modified boron nitride heterostructure thermal conductive filler, characterized in that: include: adding the boron nitride heterostructure thermal conductive filler according to claim 1 to an aqueous solution of 1-butyl-3-methylimidazolium acetate, performing ultrasonic treatment, and then condensing and refluxing at 80-90° C. to obtain a reaction solution; The solid phase in the reaction solution is collected by centrifugation, washed, and then freeze-dried to obtain an ionic liquid modified boron nitride heterostructure thermal conductive filler.

5. The preparation method according to claim 4, characterized in that The mass ratio of the boron nitride heterostructure thermal conductive filler to 1-butyl-3-methylimidazolium acetate is 0.1:1; The concentration of the aqueous solution of 1-butyl-3-methylimidazolium acetate is 10 wt %.

6. The ionic liquid modified boron nitride heterostructure thermal conductive filler prepared by the preparation method according to claim 4 or 5.

7. A method for preparing a thermally conductive composite material, characterized in that: include: S1, dissolving polyamic acid and triethylamine in deionized water to obtain a polyamic acid solution; adding the ionic liquid modified boron nitride heterostructure thermal conductive filler according to claim 6 to the polyamic acid solution, and uniformly dispersing the mixture by ultrasonication to obtain a dispersion; S2, transferring the dispersion into a mold and freeze-drying the mold to obtain a porous aerogel having a three-dimensional network skeleton structure; S3, performing a stepwise hot pressing process on the porous aerogel to convert the polyamic acid into polyimide to obtain a thermally conductive composite material.

8. The preparation method according to claim 7, characterized in that The polyamic acid is prepared by the following method: Dissolve 4,4'-diaminodiphenyl ether in N,N-dimethylacetamide, then add 1,2,4,5-pyromellitic dianhydride and stir thoroughly to react; Excess deionized water is added to the reaction solution to precipitate a precipitate, and the precipitate is filtered, washed, and dried to obtain a solid polyamic acid.

9. The preparation method according to claim 7, characterized in that The ratio of the polyamic acid solution is: polyamic acid 0.8g-1.0g, triethylamine 0.48g, deionized water 18.52ml-23.52ml; The amount of the ionic liquid modified boron nitride heterostructure thermal conductive filler is 5 to 20 wt% of the amount of the polyamic acid; and / or, The freeze drying comprises: immersing the bottom of the mold in liquid nitrogen for directional freezing, and then drying the frozen sample; and / or, The stepwise hot pressing treatment is specifically as follows: hot pressing at 100-120° C., 180-200° C. and 210-230° C. for 0.5-1.5 h respectively under a pressure of 10-15 MPa.

10. The thermally conductive composite material prepared by the preparation method according to any one of claims 7 to 9.

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