A composite filler for heat-conducting gel and a preparation method thereof

Abstract

The application belongs to the field of heat-conducting materials, and provides a composite filler for heat-conducting gel and a preparation method thereof. The application adopts a core-shell composite particle design, uses aluminum oxide particles as a core layer and hexagonal boron nitride as a shell layer, and constructs an interface connection layer through the amino-epoxy covalent coupling of 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane, so that the two fillers are stably combined, have the synergistic advantages of high filling density of aluminum oxide and high heat conduction of the plane of hexagonal boron nitride, maintain low viscosity and a wide processing window under high filling conditions, endow the system with low contact thermal resistance and thermal cycle structure stability, solve the core problems that the high filling heat-conducting network construction and the system processability are difficult to be considered together and the introduction of low-thermal-conductivity phase by interface organicization restricts the heat conduction efficiency, and have wide application value in the fields of heat-conducting gel, electronic packaging and thermal management.

Description

Technical Field

[0001] This invention relates to the field of thermal interface materials technology, specifically to a composite filler for thermally conductive gel and its preparation method. Background Technology

[0002] With the continuous evolution of electronic packaging technology, the power density of chips has increased significantly and the system size has continued to shrink, placing more stringent demands on the thermal management performance of thermal interface materials and thermally conductive gels. Thermally conductive gels, as a type of phase-change thermal interface material that combines compressibility and flowability, are widely used at the thermal coupling interface between chips and heat sinks to fill microscopic roughness gaps, reduce contact thermal resistance, and achieve efficient heat conduction. In practical applications, excellent thermally conductive gels not only need to possess high thermal conductivity to support stable heat dissipation of high-power devices, but also need to maintain suitable rheological properties and application window at high filler volumes to meet the operability requirements of automated dispensing processes. Simultaneously, under thermal cycling and mechanical shock loads during service, the gel system must maintain a stable microstructure and uniform filler dispersion to ensure long-term reliability. Therefore, how to comprehensively balance high thermal conductivity, excellent processability, and structural stability has become a core issue in the research of next-generation high-performance thermally conductive gel filler systems, and promoting technological progress in this field has significant engineering value and industrial significance.

[0003] Currently, researchers are attempting to improve the thermal conductivity of thermally conductive ceramic fillers by modifying the surface with organosilane coupling agents or by compounding multiple thermally conductive fillers. However, all these approaches have significant limitations. For example, Chinese patent CN112778704B discloses an epoxy molding compound with a highly efficient continuous thermally conductive network and its preparation method. It uses a coupling agent to adsorb thermally conductive nanoparticles onto the surface of reinforcing fibers, which together with spherical fillers construct a continuous thermally conductive network in the matrix. However, it suffers from drawbacks such as the introduction of an organic interfacial layer by the coupling agent treatment, increased system viscosity under high filling conditions, and deterioration of processing and construction performance. Another example is Chinese patent CN109880297A, which discloses a thermally conductive and insulating epoxy resin composite material and its preparation method. Boron nitride is used to fill the polymer to construct thermally conductive pathways. However, it suffers from drawbacks such as the lack of chemical bonding interfaces between fillers in different dimensions due to only physical blending, easy agglomeration and phase separation of fillers under thermal cycling and mechanical impact loads, insufficient structural stability, complex rheological behavior of the thermally conductive system under high filling conditions, and limited processability. Summary of the Invention

[0004] The purpose of this invention is to provide a composite filler for thermally conductive gels and its preparation method, which solves the problems of current thermally conductive gel filler systems, such as the difficulty in balancing the construction of high-filling thermally conductive networks with the rheological processability of the system, the coupling contradiction between high modulus stability and low contact thermal resistance and compliant compressibility, and the introduction of low thermal conductivity organic phases that restrict thermal conductivity when achieving dispersion stability through interface organicification.

[0005] This invention organically integrates the high-density volumetric filling advantage of alumina particles with the high in-plane thermal conductivity of a hexagonal boron nitride shell through a unique core-shell composite particle structure design. The chemically stable composite of the two is achieved with an extremely thin amino-epoxy covalent interface layer, enabling the composite filler to simultaneously achieve high thermal conductivity, low interfacial thermal resistance, and a wide processing window. This results in a synergistic improvement in multiple properties that is difficult to achieve with a single component, fundamentally breaking through the ternary contradiction of mutual constraints between thermal conductivity, processability, and interfacial reliability in high-performance thermally conductive gel filler systems.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A composite filler for thermally conductive gels is a core-shell composite particle, comprising: alumina particles as the core layer, a hexagonal boron nitride shell as the shell layer, and an interface bonding layer between the alumina particles and the hexagonal boron nitride shell layer; wherein the interface bonding layer comprises siloxane bonds formed by the condensation of 3-glycidoxypropyltrimethoxysilane with hydroxyl groups on the surface of the alumina particles, and covalent bonds formed by the reaction of amino groups introduced by 3-aminopropyltriethoxysilane with epoxy groups of 3-glycidoxypropyltrimethoxysilane; The composite filler for thermally conductive gel is obtained by reacting epoxy-silanized alumina with amino-silanized hexagonal boron nitride. The mass fraction of alumina in the composite filler for thermally conductive gel is 60wt% to 89wt%, the mass fraction of hexagonal boron nitride is 10wt% to 39wt%, and the sum of the mass fractions of alumina and hexagonal boron nitride is not greater than 99.7wt%. The balance is organic components introduced by the interface bonding layer and unavoidable impurities and volatiles.

[0007] Furthermore, aminosilanized hexagonal boron nitride is prepared via the following steps: A1. Raw material preparation: 100 parts by weight of hexagonal boron nitride, 200 to 800 parts by weight of hydrogen peroxide aqueous solution, 5 to 50 parts by weight of sodium hydroxide, and 200 to 1000 parts by weight of deionized water; wherein the mass fraction of hydrogen peroxide in the hydrogen peroxide aqueous solution is 10 wt% to 35 wt%. A2. Alkaline oxidation: After thoroughly mixing the system obtained in step A1, the pH is measured. If the pH does not reach 11 to 13, 10 wt% to 30 wt% sodium hydroxide aqueous solution is added until the pH of the system is 11 to 13. Then, the reaction is carried out at 60℃ to 90℃ for 1 to 6 hours to obtain hexagonal boron nitride treated with alkaline oxidation. A3. Washing and drying: The product obtained in step A2 is separated into solid and liquid phases and washed with deionized water until the pH of the washing solution is 6 to 8; then dried at 60°C to 120°C for 4 to 16 hours to obtain dried hexagonal boron nitride with oxidation treatment. A4. Silanization reaction: The product obtained in step A3 is dispersed in a mixed solvent of ethanol and deionized water. Based on 100 parts by mass of the product obtained in step A3, the amount of ethanol used is 150 to 700 parts by mass, and the amount of deionized water used is 30 to 150 parts by mass, with a mass ratio of ethanol to deionized water of 70:30 to 95:5. Glacial acetic acid is added to adjust the pH of the system to 4 to 6. The pH value is determined by pH meter after mixing a sample of the system with deionized water at a mass ratio of 1:1 at 25°C. Then, 3-aminopropyltriethoxysilane is added in an amount of 0.5 to 5.0 parts by mass per 100 parts by mass of hexagonal boron nitride. The reaction is carried out at 30°C to 70°C for 1 to 8 hours under a nitrogen atmosphere. A5. Post-processing and quality control: The product obtained in step A4 is separated into solid and liquid phases by centrifugation or vacuum filtration; it is washed with ethanol and deionized water 2 to 5 times, with each washing solution being 2 to 10 times the mass of the solid, until the conductivity of the filtrate is no greater than 50 μS / cm; then it is dried at a pressure of 0.00005 MPa to 0.01 MPa and a temperature of 60°C to 120°C for 4 to 16 hours, and the constant weight endpoint is determined by the mass difference between two consecutive weighings not exceeding 0.1% of the mass of the previous weighing, with an interval of 1 to 2 hours between the two consecutive weighings. Before each weighing, the sample is placed in a desiccator and cooled to room temperature before weighing, and the volatile content is controlled to be no higher than 1.0 wt%, to obtain aminosilanized hexagonal boron nitride.

[0008] Furthermore, epoxy-silanized alumina is prepared via the following steps: B1. Raw material preparation: 100 parts by weight of alumina, 0.5 to 5.0 parts by weight of 3-glycidoxypropyltrimethoxysilane, 200 to 800 parts by weight of ethanol, 10 to 200 parts by weight of deionized water, and 0.1 to 5.0 parts by weight of glacial acetic acid; B2. Solvent premixing: Mix the ethanol, deionized water and glacial acetic acid from step B1 and adjust the pH to 4 to 6. The pH is the value measured by a pH meter after mixing the system sample with deionized water at a mass ratio of 1:1 at 25°C; keep warm at 20°C to 40°C for 0.5 h to 2 h. B3. Silanization reaction: Alumina and 3-glycidoxypropyltrimethoxysilane were added to the system obtained in step B2, and the reaction was carried out at 30°C to 70°C for 1 to 6 hours under a nitrogen atmosphere; B4. Post-processing and quality control: The product obtained in step B3 is separated into solid and liquid phases by centrifugation or vacuum filtration; it is washed with ethanol and deionized water 2 to 5 times, with each washing solution being 2 to 10 times the mass of the solid, until the conductivity of the filtrate is no greater than 50 μS / cm; then it is dried at a pressure of 0.00005 MPa to 0.01 MPa and a temperature of 60°C to 120°C for 4 to 16 hours, and the constant weight endpoint is determined by the mass difference between two consecutive weighings not exceeding 0.1% of the mass of the previous weighing, with an interval of 1 to 2 hours between the two consecutive weighings. Before each weighing, the sample is placed in a desiccator and cooled to room temperature before weighing, and the volatile content is controlled to be no higher than 1.0 wt%, to obtain epoxy-silanized alumina.

[0009] Furthermore, the composite filler for the thermally conductive gel is subjected to spray drying granulation, which is achieved through the following steps: D1. Disperse the thermally conductive gel with composite filler in a mixed solvent of ethanol and deionized water, wherein the mass ratio of ethanol to deionized water is 70:30 to 95:5, so that the solid content is 20wt% to 60wt%. D2. Spray drying: Spray drying is carried out under a nitrogen atmosphere, with an inlet temperature of 120℃ to 200℃ and an outlet temperature of 60℃ to 110℃; an airflow atomizer is used in the spray drying process, with an atomization pressure of 0.1MPa to 0.4MPa and a feed rate of 5mL / min to 30mL / min; D3. Obtain granulated particles and control the median particle size D50 of the granulated particles to be between 50 μm and 200 μm.

[0010] Furthermore, the median particle size D50 of the alumina particles is 5 μm to 40 μm, and the median particle size D50 of the composite filler for the thermally conductive gel is not less than the median particle size D50 of the alumina particles, and is 6 μm to 60 μm; wherein, the median particle size D50 of the alumina particles, the composite filler for the thermally conductive gel, and the granulated particles is determined by laser diffraction in a deionized water dispersion system, and the dispersion is ultrasonically treated for 30 s to 120 s before measurement; the thickness of the hexagonal boron nitride shell is 5 nm to 50 nm, and the thickness of the hexagonal boron nitride shell is statistically determined by transmission electron microscopy cross-sectional analysis on no less than 20 particles, and the average value is taken; the coating rate of the hexagonal boron nitride shell on the outer surface of the alumina particles is 70% to 100%, wherein the coating rate is the hexagonal boron nitride shell. The coating rate is calculated as the ratio of the area of ​​the covered outer surface to the total area of ​​the outer surface of the alumina particles, and the coating rate is calculated by analyzing and statistically processing images of no less than 50 particles using scanning electron microscopy backscattered electron imaging, using the ratio of the area covered by the hexagonal boron nitride shell to the total area of ​​the outer surface of the alumina particles; furthermore, the mass fraction of the organic components introduced by 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane in the composite filler for the thermally conductive gel is 0.3wt% to 2.0wt%, and the mass fraction of the organic components is determined by thermogravimetric analysis under nitrogen atmosphere and a heating rate of 10℃ / min. The mass loss in the range of 200℃ to 800℃ is taken and the corresponding mass loss of blank alumina and blank hexagonal boron nitride in the same range is deducted before calculation.

[0011] As a concept of this invention, the core-shell composite particle design is mainly used to enhance the overall thermal conductivity and processability of composite fillers for thermally conductive gels. Alumina particles, with their excellent volumetric packing density and high intrinsic thermal conductivity, serve as the framework for the thermally conductive network in the core-shell structure. Their spherical morphology helps achieve close packing at high solid content, reducing system viscosity. The hexagonal boron nitride shell, with its layered crystal structure and ultra-high in-plane thermal conductivity, forms a high-thermal-conductivity planar pathway on the outer surface of the alumina particles. Together, they form a synergistic thermal transfer channel at the particle scale, significantly improving thermal conductivity efficiency compared to using either component alone. The interface bonding layer is constructed by chemical covalent bonds between 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane at the interface between alumina and hexagonal boron nitride. This achieves a strong bond between the two fillers in an extremely thin organic phase, which not only ensures the structural stability of the core-shell particles, but also controls the thickness of the organic interface layer to a minimum, so that the mass fraction of the organic component is only 0.3wt% to 2.0wt%. This significantly reduces the additional thermal resistance at the interface, thereby achieving dispersion stability and insulation reliability without sacrificing thermal conductivity.

[0012] This invention also discloses a method for preparing a composite filler for thermally conductive gel, comprising the following steps: S1. Provides aminosilanized hexagonal boron nitride, which is prepared by sequentially subjecting hexagonal boron nitride to alkaline oxidation and aminosilanization. S2. Provides epoxy-silanized alumina, which is prepared by epoxy-silanizing alumina; S3. The aminosilane-modified hexagonal boron nitride provided in step S1 and the epoxy-modified alumina provided in step S2 are mixed in ethanol. The total solid content is the sum of the masses of aminosilane-modified hexagonal boron nitride and epoxy-modified alumina, so that the mass fraction of hexagonal boron nitride in the total solid content is 10wt% to 39wt%, and the solid content of the dispersion system is 20wt% to 60wt%, wherein the solid content is the mass percentage of the total solid content to the total mass of the dispersion system. The reaction is carried out at 50℃ to 75℃ for 1h to 8h under a nitrogen atmosphere. The reaction is carried out under mechanical stirring at a stirring speed of 50rpm to 500rpm, so that the amino and epoxy groups react to form covalent bonds to obtain core-shell composite particles. S4. The core-shell composite particles obtained in step S3 are separated into solid and liquid phases by centrifugation or vacuum filtration. The particles are washed with ethanol 2 to 5 times, with each washing solution being 2 to 10 times the mass of the solid, until the conductivity of the filtrate is no greater than 50 μS / cm. The particles are then dried at a pressure of 0.00005 MPa to 0.01 MPa and a temperature of 60°C to 120°C for 4 to 16 hours. The constant weight endpoint is determined by the mass difference between two consecutive weighings being no greater than 0.1% of the mass of the previous weighing. The interval between two consecutive weighings is 1 to 2 hours. Before each weighing, the sample is cooled to room temperature in a desiccator. The volatile content is controlled to be no higher than 1.0 wt%, thus obtaining the composite filler for thermally conductive gel. S5. Granulation: The thermally conductive gel obtained in step S4 is spray-dried and granulated using composite filler. The spray drying granulation adopts an airflow atomizer with an atomization pressure of 0.1MPa to 0.4MPa and a feed rate of 5mL / min to 30mL / min to obtain granulated particles with a median particle size D50 of 50μm to 200μm.

[0013] Furthermore, in the preparation of aminosilanized hexagonal boron nitride provided in step S1, the mass fraction of hydrogen peroxide in the hydrogen peroxide aqueous solution is 10wt% to 35wt%, and the amount of 3-aminopropyltriethoxysilane is 0.5 parts by mass to 5.0 parts by mass per 100 parts by mass of hexagonal boron nitride; in the preparation of epoxysilanized alumina provided in step S2, the amount of 3-glycidoxypropyltrimethoxysilane is 0.5 parts by mass to 5.0 parts by mass per 100 parts by mass of alumina.

[0014] Further, in step S5, the thermally conductive gel obtained in step S4 is dispersed in a mixed solvent of ethanol and deionized water, with the mass ratio of ethanol to deionized water being 70:30 to 95:5, so that the solid content is 20wt% to 60wt%; and spray-dried and granulated under a nitrogen atmosphere, with an inlet temperature of 120℃ to 200℃ and an outlet temperature of 60℃ to 110℃.

[0015] Furthermore, in the composite filler for the thermally conductive gel obtained in step S4, the total residual amount of free 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane is not higher than 0.05 wt%; wherein the determination method for the total residual amount of free silane is as follows: take 5 g of sample, add 25 mL of methanol, extract by ultrasonication at 50 °C for 30 min, centrifuge, take the supernatant and compare it with the monomer standards of 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane by high performance liquid chromatography or gas chromatography-mass spectrometry, and convert it into the sample mass fraction.

[0016] Further, in step S1, the preparation of aminosilanized hexagonal boron nitride includes: mixing hexagonal boron nitride with an aqueous solution of hydrogen peroxide and sodium hydroxide, adjusting the pH to 11 to 13, performing alkaline oxidation treatment at 60°C to 90°C for 1 to 6 hours, separating the solid and liquid, washing with deionized water until the pH of the washing solution is 6 to 8, and drying; then dispersing the dried hexagonal boron nitride in a mixed solvent of ethanol and deionized water, adding glacial acetic acid to adjust the pH of the system to 4 to 6, adding 3-aminopropyltriethoxysilane, reacting at 30°C to 70°C for 1 to 8 hours under a nitrogen atmosphere, separating the solid and liquid, washing, and drying to obtain aminosilanized hexagonal boron nitride.

[0017] Furthermore, the median particle size D50 of the hexagonal boron nitride raw material is 0.3 μm to 5.0 μm, and the sheet thickness is 5 nm to 80 nm.

[0018] Furthermore, in step A2, the alkaline oxidation is carried out under mechanical stirring at a speed of 100 rpm to 500 rpm.

[0019] Furthermore, in step A4, after adding 3-aminopropyltriethoxysilane to the mixed solvent, a silanization reaction is carried out under mechanical stirring at a speed of 50 rpm to 500 rpm.

[0020] Furthermore, in step B3, the silanization reaction is carried out under mechanical stirring at a speed of 100 rpm to 500 rpm.

[0021] Furthermore, in step B1, 3-glycidoxypropyltrimethoxysilane is pre-added to a mixture of ethanol, deionized water and glacial acetic acid in step B2, and hydrolyzed in an alcohol-water system with pH 4 to 6 for 0.5 h to 2 h. In step B3, only alumina is added to the hydrolysis product obtained in step B2 to carry out the silanization reaction.

[0022] Further, in step S3, after adding epoxy-silanized alumina and amino-silanized hexagonal boron nitride to ethanol, they are dispersed evenly under mechanical stirring at a speed of 50 rpm to 500 rpm and a dispersion time of 10 min to 60 min.

[0023] Further, in step D1, the thermally conductive gel composite filler is added to a mixed solvent of ethanol and deionized water and then uniformly dispersed under mechanical stirring at a speed of 100 rpm to 500 rpm for a dispersion time of 10 min to 60 min.

[0024] Furthermore, the covalent coupling reaction system in step S3 is equipped with a reflux condenser, and the reaction temperature does not exceed 75°C.

[0025] Further, in step S5, the thermally conductive gel obtained in step S4 is dispersed in a mixed solvent of ethanol and deionized water, with the mass ratio of ethanol to deionized water being 70:30 to 95:5, so that the solid content is 20wt% to 60wt%. Spray drying and granulation are carried out under a nitrogen atmosphere, with an inlet temperature of 120℃ to 200℃ and an outlet temperature of 60℃ to 110℃.

[0026] Furthermore, in steps A5, B4, and S4, the volatile matter is determined by the mass loss rate after drying to constant weight at a constant temperature of 120°C.

[0027] As another aspect of this invention, a three-step tandem preparation process of alkaline oxidation activation, silane coupling modification, and amino-epoxy covalent coupling is employed. This process is primarily used to enhance the interfacial bonding strength and controllability of the core-shell structure of composite fillers for thermally conductive gels. Hexagonal boron nitride raw material is activated by alkaline hydrogen peroxide oxidation, introducing abundant oxygen-containing functional groups onto its inert basal surface, providing reaction sites for subsequent graft coupling with 3-aminopropyltriethoxysilane. Alumina particles are modified with 3-glycidoxypropyltrimethoxysilane, introducing epoxy functional groups onto their surface. The two functionalized products undergo amino-epoxy ring-opening covalent coupling in ethanol medium under mild conditions, achieving the coating of the alumina core by the hexagonal boron nitride shell, forming a chemically bonded interfacial bonding layer. The preparation process strictly controls the solid-liquid separation and washing process, reduced pressure and low temperature drying conditions, and constant weight endpoint criteria to reduce the free silane residue to below 0.05 wt% and the volatile matter to below 1.0 wt%, ensuring the consistency and reproducibility of product quality. Then, the granulation is carried out by airflow atomization spray drying, and the median particle size D50 of the granulated particles is 50 μm to 200 μm, which takes into account both the engineering operability of batch preparation and the flowability of the final product.

[0028] In the core-shell composite particle system of this invention, alumina particles and hexagonal boron nitride play distinct yet complementary functional roles. Regarding the mechanism of improved thermal conductivity, alumina particles, in a high-density spherical packing configuration, construct a volumetric thermally conductive network framework within the matrix. Their high intrinsic thermal conductivity ensures the basic heat transfer capacity of this network framework. Meanwhile, the hexagonal boron nitride sheets, with their extremely high phonon conduction efficiency on the crystal plane, form high-speed thermal conduction pathways on the surface of the alumina particles. Together, they form a composite thermally conductive topology at the particle scale, combining a volumetric low-thermal-resistance framework with a surface-type high-thermal-conductivity pathway. This achieves efficient heat flow convergence and conduction, resulting in a lower effective thermal percolation threshold and higher final thermal conductivity compared to single-component systems. In terms of improving processability and structural stability, the spherical alumina core reduces the disordered stacking tendency of the hexagonal boron nitride sheets and reduces interparticle rheological resistance; the chemically bonded interface layer locks the two components into integrated particles, preventing phase separation during high-shear processing or thermal cycling, and maintaining the uniform dispersion and long-term structural integrity of the filler system; the synergistic effect of the two enables the composite filler to exhibit the comprehensive advantages of low viscosity, wide processing window and anti-pumping in high-filling systems, breaking through the bottleneck of single fillers being unable to achieve both high thermal conductivity and good processability.

[0029] Beneficial technical effects 1. Through a core-shell composite particle design, alumina particles are used as the core and hexagonal boron nitride as the shell. Stable chemical bonding is achieved through an amino-epoxy covalent bond interface layer. Alumina provides a high-density volume-filling substrate, and the hexagonal boron nitride shell layer establishes a high thermal conductivity planar pathway. The two form a synergistic thermal conduction pathway at the interface. The organic interface layer is extremely thin, with an organic component mass fraction of only 0.3wt% to 2.0wt%. While ensuring stable bonding between fillers, the contribution of the organic phase thermal resistance is minimized, achieving a synergistic improvement in thermal conductivity and interface reliability.

[0030] 2. Core-shell composite particles anchor hexagonal boron nitride sheets to the surface of alumina spherical particles through chemical bonding, avoiding disordered stacking and agglomeration of hexagonal boron nitride sheets in high-filler systems and effectively reducing the interparticle friction coefficient; maintaining low system viscosity and good construction fluidity under high solid content conditions, broadening the processability window of thermally conductive gel, and effectively alleviating the industry pain point of poor processability of high thermal conductivity filler systems.

[0031] 3. The amino-epoxy covalent bonds provided by the interface bonding layer give the core-shell composite particles a stable interface for chemical bonding. During service, the filler components do not undergo phase separation or shell detachment under thermal cycling and mechanical shock, thus maintaining the integrity of the microstructure. Combined with the moderate compressibility provided by the hexagonal boron nitride shell, the composite particles can adapt to the micro-roughness of the substrate when bonded at the interface, taking into account both low contact thermal resistance and structural stability, and reducing the risk of pump-out.

[0032] 4. The preparation method is based on a three-step chemical reaction: alkaline oxidation activation, silane coupling modification, and amino-epoxy covalent coupling. The process is clear and the conditions are mild. The consistency of product batches is ensured by double protection through the detection of free silane residue (≤0.05wt%) and constant weight drying quality control indicators. The particles obtained after spray drying and granulation have good flowability (granulated particle D50 is 50μm to 200μm), which is convenient for storage, transportation and subsequent mixing, and is suitable for large-scale engineering production. Attached Figure Description

[0033] Figure 1 The FTIR spectra of Example 1 and Comparative Example 4 are used to compare evidence of core-shell covalent bonds.

[0034] Figure 2 This is a schematic diagram of XPS narrow spectrum comparison of Si2p peaks in Example 1 and Comparative Example 1.

[0035] Figure 3 This is a schematic diagram of the N1s peak fitting for the XPS narrow spectrum comparison between Example 1 and Comparative Example 1.

[0036] Figure 4 This is a superimposed graph of thermogravimetric analysis curves, comparing the quantitative content of organic components in Examples 1 to 4 with that in Comparative Example 6.

[0037] Figure 5 The XPS N1s spectra and peak fitting diagrams for Example 1 and Comparative Example 5 are shown.

[0038] Figure 6 Core-shell composite filler for thermally conductive gel in Example 1 ( Macroscopic optical photographs of the spray-dried granules.

[0039] Figure 7 Example 1 Scanning electron microscope image of the core-shell composite packing material.

[0040] Figure 8 This is a transmission electron microscopy (TEM) characterization image of the composite filler used in the thermally conductive gel of Example 1. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Example 1

[0042] This embodiment provides a composite filler for thermally conductive gel and its preparation method.

[0043] In this embodiment, the thermally conductive gel uses a core-shell composite filler, comprising: alumina particles as the core layer, a hexagonal boron nitride shell as the shell layer, and an interface bonding layer between the alumina particles and the hexagonal boron nitride shell. The interface bonding layer in this embodiment includes siloxane bonds formed by the condensation of 3-glycidoxypropyltrimethoxysilane with hydroxyl groups on the surface of the alumina particles, and covalent bonds formed by the reaction of amino groups introduced by 3-aminopropyltriethoxysilane with the epoxy groups of 3-glycidoxypropyltrimethoxysilane.

[0044] In this embodiment, the composite filler for the thermally conductive gel contains 75.0 wt% alumina and 24.0 wt% hexagonal boron nitride, with a total mass fraction of 99.0 wt%. The balance consists of organic components introduced through the interface bonding layer, unavoidable impurities, and volatiles. The organic components introduced by 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane in the composite filler for the thermally conductive gel in this embodiment have a mass fraction of 1.0 wt%.

[0045] In this embodiment, the median particle size D50 of the alumina particles is 20.0 μm, and the median particle size D50 of the composite filler for the thermally conductive gel is 22.0 μm. The thickness of the hexagonal boron nitride shell in this embodiment is 25 nm, and the coverage rate of the hexagonal boron nitride shell on the outer surface of the alumina particles is 85%.

[0046] The preparation method of the composite filler for thermally conductive gel in this embodiment includes the following steps: Step S1: Preparation of aminosilanized hexagonal boron nitride A1. Raw material preparation: Take 100 parts by weight of hexagonal boron nitride with a median particle size D50 of 2.0 μm and a sheet thickness of 40 nm, 500 parts by weight of hydrogen peroxide aqueous solution with a mass fraction of 25 wt%, 25 parts by weight of sodium hydroxide, and 600 parts by weight of deionized water, and mix thoroughly.

[0047] A2. Alkaline oxidation: Add 20 wt% sodium hydroxide aqueous solution to the mixture obtained in step A1 to adjust the pH of the system to 12. React at 75°C for 3.5 h with mechanical stirring at a stirring speed of 300 rpm to obtain alkaline-oxidized hexagonal boron nitride.

[0048] A3. Washing and Drying: The product obtained in step A2 was subjected to solid-liquid separation by vacuum filtration, and washed with deionized water until the pH of the washing solution reached 7. It was then dried at 90°C for 10 hours to obtain dried hexagonal boron nitride treated with oxidation.

[0049] A4. Silanization reaction: 100 parts by mass of the product obtained in step A3 were dispersed in a mixed solvent of 400 parts by mass of ethanol and 85 parts by mass of deionized water, with a mass ratio of ethanol to deionized water of 82.5:17.5. Glacial acetic acid was added to adjust the pH of the system to 5. 2.5 parts by mass of 3-aminopropyltriethoxysilane were added under mechanical stirring at a speed of 250 rpm. The reaction was carried out at 50°C for 4.5 h under a nitrogen atmosphere.

[0050] A5. Post-processing and quality control: The product obtained in step A4 was subjected to solid-liquid separation by vacuum filtration, and washed three times with ethanol and deionized water, each time using 6 times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, it was dried for 10 h at a pressure of 0.002 MPa and a temperature of 90 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 1.5 h. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, and the volatile content was controlled to be no higher than 1.0 wt%, to obtain aminosilanized hexagonal boron nitride.

[0051] Step S2: Preparation of epoxy-silanized alumina B1. Raw material preparation: Take 100 parts by weight of alumina with a median particle size D50 of 20.0 μm, 2.5 parts by weight of 3-glycidoxypropyltrimethoxysilane, 500 parts by weight of ethanol, 100 parts by weight of deionized water, and 2.5 parts by weight of glacial acetic acid.

[0052] B2. Solvent premixing: Mix the ethanol, deionized water, and glacial acetic acid from step B1 and adjust the pH to 5. Incubate at 30°C for 1 hour.

[0053] B3. Silanization reaction: Alumina and 3-glycidoxypropyltrimethoxysilane were added to the system obtained in step B2, and the reaction was carried out under mechanical stirring at a speed of 300 rpm. The reaction was carried out at 50 °C for 3.5 h under a nitrogen atmosphere.

[0054] B4. Post-processing and quality control: The product obtained in step B3 was subjected to solid-liquid separation by vacuum filtration, and washed three times with ethanol and deionized water, each time using 6 times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, it was dried for 10 h at a pressure of 0.002 MPa and a temperature of 90 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 1.5 h. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, and the volatile content was controlled to be no higher than 1.0 wt%, to obtain epoxy-silanized alumina.

[0055] Step S3: Covalent Coupling Reaction 24.5 parts by mass of aminosilanized hexagonal boron nitride prepared in step S1 and 75.5 parts by mass of epoxy-silanized alumina prepared in step S2 were added to ethanol, with a total solid content of 100 parts by mass and a hexagonal boron nitride mass fraction of 24.5 wt% in the total solid content. The mixture was dispersed uniformly under mechanical stirring at 250 rpm for 30 min, resulting in a solid content of 40 wt% and 150 parts by mass of ethanol. The system was equipped with a reflux condenser and reacted at 65°C for 4.5 h under a nitrogen atmosphere to allow the amino and epoxy groups to react and form covalent bonds, yielding core-shell composite particles.

[0056] Step S4: Post-processing The core-shell composite particles obtained in step S3 were subjected to solid-liquid separation by vacuum filtration, and washed three times with ethanol, each time using 6 times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, the particles were dried for 10 h at a pressure of 0.002 MPa and a temperature of 90 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 1.5 h. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, controlling the volatile matter content to be no higher than 1.0 wt%, thus obtaining the composite filler for thermally conductive gel. The total residual amount of free 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane was determined to be 0.03 wt%.

[0057] Step S5: Spray drying granulation 100 parts by weight of the thermally conductive gel composite filler obtained in step S4 were dispersed in a mixed solvent of 120 parts by weight of ethanol and 30 parts by weight of deionized water, with a mass ratio of ethanol to deionized water of 80:20 and a solid content of 40 wt%. The dispersion was carried out uniformly under mechanical stirring at a speed of 300 rpm for 30 min. Spray drying granulation was then performed under a nitrogen atmosphere using an airflow atomizer with an inlet temperature of 160℃, an outlet temperature of 85℃, an atomization pressure of 0.25 MPa, and a feed rate of 15 mL / min, resulting in granulated particles with a median particle size D50 of 120 μm.

[0058] Features of Example 1: The process parameters selected in this example are robust and the proportions of each component are coordinated. It is suitable for mid-to-high-end thermal conductive gel applications that require a balance of comprehensive performance. It is particularly suitable for scenarios that require both thermal conductivity and dielectric properties, such as electronic device packaging, heat dissipation of 5G communication equipment, and thermal management of new energy vehicle batteries. Example 2

[0059] This embodiment provides a composite filler for thermally conductive gel and its preparation method.

[0060] In this embodiment, the thermally conductive gel uses a core-shell composite filler, comprising: alumina particles as the core layer, a hexagonal boron nitride shell as the shell layer, and an interface bonding layer between the alumina particles and the hexagonal boron nitride shell. The interface bonding layer in this embodiment includes siloxane bonds formed by the condensation of 3-glycidoxypropyltrimethoxysilane with hydroxyl groups on the surface of the alumina particles, and covalent bonds formed by the reaction of amino groups introduced by 3-aminopropyltriethoxysilane with the epoxy groups of 3-glycidoxypropyltrimethoxysilane.

[0061] In this embodiment, the composite filler for the thermally conductive gel contains 82.0 wt% alumina and 17.0 wt% hexagonal boron nitride, with a total mass fraction of 99.0 wt%. The balance consists of organic components introduced by the interface bonding layer, unavoidable impurities, and volatiles. The organic components introduced by 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane in the composite filler for the thermally conductive gel in this embodiment have a mass fraction of 0.8 wt%.

[0062] In this embodiment, the median particle size D50 of the alumina particles is 28.0 μm, and the median particle size D50 of the composite filler for the thermally conductive gel is 32.0 μm. The thickness of the hexagonal boron nitride shell in this embodiment is 18 nm, and the coverage rate of the hexagonal boron nitride shell on the outer surface of the alumina particles is 75%.

[0063] The preparation method of the composite filler for thermally conductive gel in this embodiment includes the following steps: Step S1: Preparation of aminosilanized hexagonal boron nitride A1. Raw material preparation: Take 100 parts by weight of hexagonal boron nitride with a median particle size D50 of 1.2 μm and a sheet thickness of 28 nm, 350 parts by weight of hydrogen peroxide aqueous solution with a mass fraction of 18 wt%, 18 parts by weight of sodium hydroxide, and 450 parts by weight of deionized water, and mix thoroughly.

[0064] A2. Alkaline oxidation: Add 15 wt% sodium hydroxide aqueous solution to the mixture obtained in step A1 to adjust the pH of the system to 11.5. React at 68℃ for 2.5 h with mechanical stirring at a stirring speed of 220 rpm to obtain alkaline-oxidized hexagonal boron nitride.

[0065] A3. Washing and Drying: The product obtained in step A2 was subjected to solid-liquid separation by centrifugation, and washed with deionized water until the pH of the washing solution reached 6.5. Subsequently, it was dried at 80°C for 12 hours to obtain dried hexagonal boron nitride treated with oxidation.

[0066] A4. Silanization reaction: 100 parts by mass of the product obtained in step A3 were dispersed in a mixed solvent of 320 parts by mass of ethanol and 55 parts by mass of deionized water, with a mass ratio of ethanol to deionized water of 85.3:14.7. Glacial acetic acid was added to adjust the pH of the system to 4.5. 1.8 parts by mass of 3-aminopropyltriethoxysilane were added under mechanical stirring at a speed of 180 rpm. The reaction was carried out at 42°C for 3.0 h under a nitrogen atmosphere.

[0067] A5. Post-processing and quality control: The product obtained in step A4 was subjected to solid-liquid separation by centrifugation, and washed four times with ethanol and deionized water, each time using five times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, it was dried for 11 h at a pressure of 0.005 MPa and a temperature of 85 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 1.5 h. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, and the volatile content was controlled to be no higher than 1.0 wt%, to obtain aminosilanized hexagonal boron nitride.

[0068] Step S2: Preparation of epoxy-silanized alumina B1. Raw material preparation: Take 100 parts by weight of alumina with a median particle size D50 of 28.0 μm, 1.8 parts by weight of 3-glycidoxypropyltrimethoxysilane, 420 parts by weight of ethanol, 65 parts by weight of deionized water, and 1.5 parts by weight of glacial acetic acid.

[0069] B2. Solvent Premixing: Mix the ethanol, deionized water, glacial acetic acid, and 3-glycidoxypropyltrimethoxysilane from step B1 and adjust the pH to 4.5. Incubate at 28°C for 1.2 hours to allow the silane agent to fully hydrolyze.

[0070] B3. Silanization reaction: Alumina was added to the hydrolysis system obtained in step B2, and the reaction was carried out under mechanical stirring at a speed of 250 rpm. The reaction was carried out at 45°C for 2.8 h under a nitrogen atmosphere.

[0071] B4. Post-processing and quality control: The product obtained in step B3 was subjected to solid-liquid separation by centrifugation, and washed four times with ethanol and deionized water, each time using five times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, it was dried for 11 hours at a pressure of 0.005 MPa and a temperature of 85 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 1.5 hours. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, controlling the volatile content to be no higher than 1.0 wt%, thus obtaining epoxy-silanized alumina.

[0072] Step S3: Covalent Coupling Reaction 17.2 parts by mass of aminosilanized hexagonal boron nitride prepared in step S1 and 82.8 parts by mass of epoxy-silanized alumina prepared in step S2 were added to ethanol, with a total solid content of 100 parts by mass and a hexagonal boron nitride mass fraction of 17.2 wt% in the total solid content. The mixture was dispersed uniformly under mechanical stirring at 200 rpm for 25 min, resulting in a solid content of 48 wt% and 108 parts by mass of ethanol. The system was equipped with a reflux condenser, and the reaction was carried out at 58 °C for 3.2 h under a nitrogen atmosphere, allowing the amino and epoxy groups to react and form covalent bonds, yielding core-shell composite particles.

[0073] Step S4: Post-processing The core-shell composite particles obtained in step S3 were subjected to solid-liquid separation by centrifugation, and washed four times with ethanol, each time using five times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, the particles were dried at 0.005 MPa and 85 °C for 11 h. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, with an interval of 1.5 h between weighings. Before each weighing, the sample was cooled to room temperature in a desiccator, and the volatile content was controlled to be no higher than 1.0 wt%, yielding a composite filler for thermally conductive gel. The total residual amount of free 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane was determined to be 0.02 wt%.

[0074] Step S5: Spray drying granulation 100 parts by weight of the thermally conductive gel composite filler obtained in step S4 were dispersed in a mixed solvent of 140 parts by weight of ethanol and 22 parts by weight of deionized water, with a mass ratio of ethanol to deionized water of 86.4:13.6 and a solid content of 38 wt%. The dispersion was carried out uniformly under mechanical stirring at a speed of 250 rpm for 25 min. Spray drying granulation was performed under a nitrogen atmosphere using an airflow atomizer with an inlet temperature of 145℃, an outlet temperature of 75℃, an atomization pressure of 0.18 MPa, and a feed rate of 12 mL / min, resulting in granulated particles with a median particle size D50 of 95 μm.

[0075] Features of Example 2: This example focuses on improving the thermal conductivity and thermal conductivity of the composite filler by increasing the alumina content and using a larger particle size substrate, while reducing the amount of organic components introduced to optimize thermal stability. It is particularly suitable for applications with extremely high requirements for thermal conductivity, such as heat dissipation of high power density devices, high power LED lighting modules, and server chip heat dissipation. Example 3

[0076] This embodiment provides a composite filler for thermally conductive gel and its preparation method.

[0077] In this embodiment, the thermally conductive gel uses a core-shell composite filler, comprising: alumina particles as the core layer, a hexagonal boron nitride shell as the shell layer, and an interface bonding layer between the alumina particles and the hexagonal boron nitride shell. The interface bonding layer in this embodiment includes siloxane bonds formed by the condensation of 3-glycidoxypropyltrimethoxysilane with hydroxyl groups on the surface of the alumina particles, and covalent bonds formed by the reaction of amino groups introduced by 3-aminopropyltriethoxysilane with the epoxy groups of 3-glycidoxypropyltrimethoxysilane.

[0078] In this embodiment, the composite filler for the thermally conductive gel contains 65.0 wt% alumina and 33.5 wt% hexagonal boron nitride, with a total mass fraction of 98.5 wt%. The balance consists of organic components introduced through the interface bonding layer, unavoidable impurities, and volatiles. The organic components introduced by 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane in the composite filler for the thermally conductive gel in this embodiment have a mass fraction of 1.5 wt%.

[0079] In this embodiment, the median particle size D50 of the alumina particles is 12.0 μm, and the median particle size D50 of the composite filler for the thermally conductive gel is 15.0 μm. The thickness of the hexagonal boron nitride shell in this embodiment is 38 nm, and the coverage rate of the hexagonal boron nitride shell on the outer surface of the alumina particles is 92%.

[0080] The preparation method of the composite filler for thermally conductive gel in this embodiment includes the following steps: Step S1: Preparation of aminosilanized hexagonal boron nitride A1. Raw material preparation: Take 100 parts by weight of hexagonal boron nitride with a median particle size D50 of 3.5 μm and a sheet thickness of 58 nm, 650 parts by weight of hydrogen peroxide aqueous solution with a mass fraction of 30 wt%, 38 parts by weight of sodium hydroxide, and 850 parts by weight of deionized water, and mix thoroughly.

[0081] A2. Alkaline oxidation: Add 25 wt% sodium hydroxide aqueous solution to the mixture obtained in step A1 to adjust the pH of the system to 12.5. React at 82℃ for 5.0 h with mechanical stirring at a stirring speed of 420 rpm to obtain alkaline-oxidized hexagonal boron nitride.

[0082] A3. Washing and Drying: The product obtained in step A2 was subjected to solid-liquid separation by vacuum filtration, and washed with deionized water until the pH of the washing solution reached 7.5. Subsequently, it was dried at 105℃ for 8 hours to obtain dried hexagonal boron nitride treated with oxidation.

[0083] A4. Silanization reaction: 100 parts by mass of the product obtained in step A3 were dispersed in a mixed solvent of 580 parts by mass of ethanol and 120 parts by mass of deionized water, with a mass ratio of ethanol to deionized water of 82.9:17.1. Glacial acetic acid was added to adjust the pH of the system to 5.5. 3.8 parts by mass of 3-aminopropyltriethoxysilane were added under mechanical stirring at a speed of 380 rpm. The reaction was carried out at 62°C for 6.5 h under a nitrogen atmosphere.

[0084] A5. Post-processing and quality control: The product obtained in step A4 was subjected to solid-liquid separation by vacuum filtration, and washed three times with ethanol and deionized water, each time using 7 times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, it was dried for 9 h at a pressure of 0.0008 MPa and a temperature of 102 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 1.5 h. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, and the volatile content was controlled to be no higher than 1.0 wt%, to obtain aminosilanized hexagonal boron nitride.

[0085] Step S2: Preparation of epoxy-silanized alumina B1. Raw material preparation: Take 100 parts by weight of alumina with a median particle size D50 of 12.0 μm, 3.8 parts by weight of 3-glycidoxypropyltrimethoxysilane, 650 parts by weight of ethanol, 145 parts by weight of deionized water, and 3.8 parts by weight of glacial acetic acid.

[0086] B2. Solvent premixing: Mix the ethanol, deionized water, and glacial acetic acid from step B1 and adjust the pH to 5.5. Incubate at 35°C for 1.5 hours.

[0087] B3. Silanization reaction: Alumina and 3-glycidoxypropyltrimethoxysilane were added to the system obtained in step B2, and the reaction was carried out under mechanical stirring at a speed of 380 rpm. The reaction was carried out at 62 °C for 4.8 h under a nitrogen atmosphere.

[0088] B4. Post-processing and quality control: The product obtained in step B3 was subjected to solid-liquid separation by vacuum filtration, and washed three times with ethanol and deionized water, each time using 7 times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, it was dried for 9 h at a pressure of 0.0008 MPa and a temperature of 102 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 1.5 h. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, and the volatile matter was controlled to be no higher than 1.0 wt%, to obtain epoxy-silanized alumina.

[0089] Step S3: Covalent Coupling Reaction

[0090] 34.5 parts by mass of aminosilanized hexagonal boron nitride prepared in step S1 and 65.5 parts by mass of epoxy-silanized alumina prepared in step S2 were added to ethanol, with a total solid content of 100 parts by mass and a hexagonal boron nitride mass fraction of 34.5 wt% in the total solid content. The mixture was dispersed uniformly under mechanical stirring at 350 rpm for 45 min, resulting in a solid content of 32 wt% and 212 parts by mass of ethanol. The system was equipped with a reflux condenser and reacted at 72°C for 6.5 h under a nitrogen atmosphere to allow the amino and epoxy groups to react and form covalent bonds, yielding core-shell composite particles.

[0091] Step S4: Post-processing

[0092] The core-shell composite particles obtained in step S3 were subjected to solid-liquid separation by vacuum filtration, and washed three times with ethanol, each time using 7 times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, the particles were dried for 9 hours at a pressure of 0.0008 MPa and a temperature of 102 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 1.5 hours. Before each weighing, the sample was placed in a desiccator and cooled to room temperature, controlling the volatile content to be no higher than 1.0 wt%, thus obtaining the composite filler for thermally conductive gel. The total residual amount of free 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane was determined to be 0.04 wt%.

[0093] Step S5: Spray drying granulation

[0094] 100 parts by weight of the thermally conductive gel composite filler obtained in step S4 were dispersed in a mixed solvent of 95 parts by weight of ethanol and 40 parts by weight of deionized water, with a mass ratio of ethanol to deionized water of 70.4:29.6 and a solid content of 43 wt%. The dispersion was carried out uniformly under mechanical stirring at a speed of 380 rpm for 45 min. Spray drying granulation was then performed under a nitrogen atmosphere using an airflow atomizer with an inlet temperature of 175℃, an outlet temperature of 95℃, an atomization pressure of 0.32 MPa, and a feed rate of 22 mL / min, resulting in granulated particles with a median particle size D50 of 158 μm.

[0095] Features of Example 3:

[0096] This embodiment focuses on reducing the dielectric constant and dielectric loss of the composite filler by increasing the hexagonal boron nitride content and shell thickness, while improving insulation performance. It is particularly suitable for applications with strict requirements for low dielectric properties and high insulation, such as high-frequency communication devices, radio frequency modules, microwave devices, and high-voltage insulation and heat dissipation. Example 4

[0097] This embodiment provides a composite filler for thermally conductive gel and its preparation method.

[0098] In this embodiment, the thermally conductive gel uses a core-shell composite filler, comprising: alumina particles as the core layer, a hexagonal boron nitride shell as the shell layer, and an interface bonding layer between the alumina particles and the hexagonal boron nitride shell. The interface bonding layer in this embodiment includes siloxane bonds formed by the condensation of 3-glycidoxypropyltrimethoxysilane with hydroxyl groups on the surface of the alumina particles, and covalent bonds formed by the reaction of amino groups introduced by 3-aminopropyltriethoxysilane with the epoxy groups of 3-glycidoxypropyltrimethoxysilane.

[0099] In this embodiment, the composite filler for the thermally conductive gel contains 87.0 wt% alumina and 12.0 wt% hexagonal boron nitride, with a total mass fraction of 99.0 wt%. The balance consists of organic components introduced through the interface bonding layer, unavoidable impurities, and volatiles. The organic components introduced by 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane in the composite filler for the thermally conductive gel in this embodiment have a mass fraction of 0.5 wt%.

[0100] In this embodiment, the median particle size D50 of the alumina particles is 36.0 μm, and the median particle size D50 of the composite filler for the thermally conductive gel is 40.0 μm. The thickness of the hexagonal boron nitride shell in this embodiment is 10 nm, and the coverage rate of the hexagonal boron nitride shell on the outer surface of the alumina particles is 78%.

[0101] The preparation method of the composite filler for thermally conductive gel in this embodiment includes the following steps: Step S1: Preparation of aminosilanized hexagonal boron nitride A1. Raw material preparation: Take 100 parts by weight of hexagonal boron nitride with a median particle size D50 of 0.5 μm and a sheet thickness of 12 nm, 260 parts by weight of hydrogen peroxide aqueous solution with a mass fraction of 12 wt%, 8 parts by weight of sodium hydroxide, and 280 parts by weight of deionized water, and mix thoroughly.

[0102] A2. Alkaline oxidation: Add 12 wt% sodium hydroxide aqueous solution to the mixture obtained in step A1 to adjust the pH of the system to 11.2. React at 64℃ for 1.5 h with mechanical stirring at a stirring speed of 150 rpm to obtain alkaline-oxidized hexagonal boron nitride.

[0103] A3. Washing and Drying: The product obtained in step A2 was subjected to solid-liquid separation by centrifugation, and washed with deionized water until the pH of the washing solution reached 6.2. Subsequently, it was dried at 70°C for 14 hours to obtain dried hexagonal boron nitride treated with oxidation.

[0104] A4. Silanization reaction: 100 parts by mass of the product obtained in step A3 were dispersed in a mixed solvent of 220 parts by mass of ethanol and 38 parts by mass of deionized water, with a mass ratio of ethanol to deionized water of 85.3:14.7. Glacial acetic acid was added to adjust the pH of the system to 4.2. 0.8 parts by mass of 3-aminopropyltriethoxysilane were added under mechanical stirring at a speed of 120 rpm. The reaction was carried out at 35°C for 1.8 h under a nitrogen atmosphere.

[0105] A5. Post-processing and quality control: The product obtained in step A4 was subjected to solid-liquid separation by centrifugation, and washed twice with ethanol and deionized water, with the washing solution volume being 4 times the mass of the solid each time, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, it was dried at a pressure of 0.008 MPa and a temperature of 75℃ for 13 h. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing mass, and the interval between two consecutive weighings was 2 h. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, and the volatile content was controlled to be no higher than 1.0 wt%, to obtain aminosilanized hexagonal boron nitride.

[0106] Step S2: Preparation of epoxy-silanized alumina B1. Raw material preparation: Take 100 parts by weight of alumina with a median particle size D50 of 36.0 μm, 0.8 parts by weight of 3-glycidoxypropyltrimethoxysilane, 280 parts by weight of ethanol, 28 parts by weight of deionized water, and 0.5 parts by weight of glacial acetic acid.

[0107] B2. Solvent Premixing: Mix the ethanol, deionized water, glacial acetic acid, and 3-glycidoxypropyltrimethoxysilane from step B1 and adjust the pH to 4.2. Incubate at 22°C for 0.8 h to allow the silane agent to fully hydrolyze.

[0108] B3. Silanization reaction: Alumina was added to the hydrolysis system obtained in step B2, and the reaction was carried out under mechanical stirring at a speed of 150 rpm. The reaction was carried out at 35°C for 1.5 h under a nitrogen atmosphere.

[0109] B4. Post-processing and quality control: The product obtained in step B3 was subjected to solid-liquid separation by centrifugation, and washed twice with ethanol and deionized water, each time using 4 times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, it was dried for 13 h at a pressure of 0.008 MPa and a temperature of 75 °C. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, and the interval between two consecutive weighings was 2 h. Before each weighing, the sample was placed in a desiccator and cooled to room temperature before weighing, and the volatile content was controlled to be no higher than 1.0 wt%, to obtain epoxy-silanized alumina.

[0110] Step S3: Covalent Coupling Reaction

[0111] 12.1 parts by mass of aminosilanized hexagonal boron nitride prepared in step S1 and 87.9 parts by mass of epoxy-silanized alumina prepared in step S2 were added to ethanol, with a total solid content of 100 parts by mass and a hexagonal boron nitride mass fraction of 12.1 wt% in the total solid content. The mixture was dispersed uniformly under mechanical stirring at 120 rpm for 18 min, resulting in a solid content of 52 wt% and 92 parts by mass of ethanol. The system was equipped with a reflux condenser and reacted at 54 °C for 2.2 h under a nitrogen atmosphere to allow the amino and epoxy groups to react and form covalent bonds, yielding core-shell composite particles.

[0112] Step S4: Post-processing

[0113] The core-shell composite particles obtained in step S3 were subjected to solid-liquid separation by centrifugation, and washed twice with ethanol, each time using four times the mass of the solid, until the conductivity of the filtrate was no greater than 50 μS / cm. Subsequently, the particles were dried at 0.008 MPa and 75℃ for 13 h. The mass difference between two consecutive weighings was no greater than 0.1% of the previous weighing, with a 2-h interval between weighings. Before each weighing, the sample was cooled to room temperature in a desiccator, and the volatile content was controlled to be no higher than 1.0 wt%, yielding a composite filler for thermally conductive gel. The total residual amount of free 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane was determined to be 0.01 wt%.

[0114] Step S5: Spray drying granulation

[0115] 100 parts by weight of the thermally conductive gel composite filler obtained in step S4 were dispersed in a mixed solvent of 156 parts by weight of ethanol and 19 parts by weight of deionized water, with a mass ratio of ethanol to deionized water of 89.1:10.9 and a solid content of 36 wt%. The dispersion was carried out uniformly under mechanical stirring at a speed of 180 rpm for 18 min. Spray drying granulation was performed under a nitrogen atmosphere using an airflow atomizer with an inlet temperature of 135℃, an outlet temperature of 68℃, an atomization pressure of 0.15 MPa, and a feed rate of 8 mL / min, yielding granulated particles with a median particle size D50 of 72 μm.

[0116] Features of Example 4: This example uses alumina as the main body and hexagonal boron nitride as a thin layer coating, which maximizes the retention of the high thermal conductivity of alumina, while providing basic surface modification and insulation protection through the boron nitride thin layer. It is particularly suitable for applications such as heat dissipation of high-power power electronic devices, industrial-grade high-power frequency converters, and large motors, where the requirements for thermal conductivity are extremely high and the requirements for dielectric properties are relatively relaxed.

[0117] Comparative Example 1: It is basically the same as Example 1, except that step A2 alkaline oxidation treatment is omitted. That is, hexagonal boron nitride is not activated by alkaline oxidation with hydrogen oxide and sodium hydroxide. The mixed system obtained in step A1 is directly subjected to step A3 solid-liquid separation, washing and drying before entering step A4 aminosilanization reaction. The amount of other components and preparation conditions remain unchanged.

[0118] Comparative Example 2: It is basically the same as Example 1, except that in step S3, the mass fraction of hexagonal boron nitride in the total solids is 43 wt%, and the mass fraction of alumina is adjusted to 57 wt% accordingly. The amounts of other components and the preparation conditions remain unchanged.

[0119] Comparative Example 3: It is basically the same as Example 1, except that in step S3, the mass fraction of hexagonal boron nitride in the total solids is 7 wt%, and the mass fraction of alumina is adjusted to 93 wt%, while the amounts of other components and preparation conditions remain unchanged.

[0120] Comparative Example 4: Basically the same as Example 1, except that all silanization treatments in steps S1 and S2 and the covalent coupling reaction in step S3 are omitted. 75.5 parts by mass of raw alumina without any silanization treatment and 24.5 parts by mass of raw hexagonal boron nitride are directly added to 150 parts by mass of ethanol. After being dispersed and mixed under mechanical stirring (250 rpm, 30 min), the post-treatment in step S4 and spray drying granulation in step S5 are carried out directly, with other conditions remaining unchanged.

[0121] Comparative Example 5: It is basically the same as Example 1, except that the amount of 3-aminopropyltriethoxysilane used in step A4 is 0.2 parts by mass (per 100 parts by mass of hexagonal boron nitride), while the amounts of other components and preparation conditions remain unchanged.

[0122] Comparative Example 6: It is basically the same as Example 1, except that the amount of 3-glycidoxypropyltrimethoxysilane used in steps B1 and B3 is 6.5 parts by mass (per 100 parts by mass of alumina), while the amounts of other components and preparation conditions remain unchanged.

[0123] Comparative Example 7: It is basically the same as Example 1, except that step S5 spray drying granulation is omitted, and the ungranulated thermal conductive gel composite filler powder obtained in step S4 is used directly as the final product, while other conditions remain unchanged.

[0124] Comparative Example 8: It is basically the same as Example 1, except that the covalent coupling reaction temperature in step S3 is adjusted to 40°C, while the amount of other components and preparation conditions remain unchanged.

[0125] Performance testing: Experiment 1: Determination of thermal conductivity of composite filler-gel system by laser scintillation method The composite filler (75 wt%) of each embodiment and comparative example was mixed with vinyl silicone oil (kinematic viscosity 500 cSt) at a mass ratio of 3:1, and then pressed into test samples with a diameter of 25.4 mm and a thickness of 3.0 ± 0.1 mm under a uniaxial pressure of 200 MPa. One side of the sample was irradiated with a laser pulse, and the temperature response curve on the opposite side was measured. The thermal diffusivity α was fitted, and the thermal conductivity was calculated by combining the density ρ and specific heat capacity Cp. Quantitative evaluation of the effect of composite fillers on the thermal conductivity of thermally conductive gel systems The contribution of the core-shell structure was directly verified to reduce the interfacial thermal resistance. Tests were conducted under a nitrogen atmosphere (flow rate 200 mL / min) at 25 ± 2 °C, with a laser energy of 8 J and a pulse width of 0.4 ms. Each sample was repeated 5 times. Time-temperature rise CSV curves were exported, and the λ value was reported as mean ± standard deviation (n=5).

[0126] Experiment 2: Determination of viscosity and yield stress of thermally conductive gel system using rotational rheometer The composite fillers (75 wt% solids content) of each embodiment and comparative example were dispersed in the thermally conductive gel system obtained by vinyl silicone oil. Rheological tests were performed at 25 ± 0.5 °C using a flat plate fixture (25 mm diameter, 1.0 mm gap), with the samples equilibrated between the fixtures for 300 s before testing. Steady-state shear was then used to measure the rheological properties of the samples. The viscosity-shear rate curve was obtained to characterize the system viscosity η (Pa·s); the storage modulus G' and loss modulus G'' were obtained by oscillatory shear (strain scan 1–1000%, frequency 1Hz), and the stress corresponding to the intersection of G'=G'' was taken as the yield stress. (Pa), the core challenge of balancing thermal conductivity and rheological processability. Export CSV data of shear rate-viscosity and strain-G' / G'', and take... Report the viscosity values, with the mean ± standard deviation.

[0127] Experiment 3: Determination of Organic Component Content in Composite Fillers by Thermogravimetric Analysis Approximately 10 mg of composite filler powder for the thermally conductive gels in each embodiment and comparative example was placed in a platinum crucible and heated from 30 °C to 800 °C at a rate of 10 °C / min under a nitrogen atmosphere (flow rate 50 mL / min). The organic components underwent thermal decomposition and volatilization within the 200–800 °C range. After deducting the mass losses of blank alumina and blank hexagonal boron nitride within the same range (weighted average), the mass fraction w_org (wt%) of the organic components introduced by 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane was quantitatively determined, directly quantifying the contribution of the amount of interfacial organic phase introduced to the interfacial thermal resistance. A temperature-mass % CSV curve was derived, and the mass loss rate in the 200–800 °C range was calculated.

[0128] Experiment 4: AC Breakdown Strength Test The composite filler (75wt%) of each embodiment and comparative example was mixed with vinyl silicone oil at a mass ratio of 3:1 and cast into uniform thin sheets with a thickness of 0.50±0.02mm. After standing naturally for 24 hours, the sheets were used for testing. A ball-plate electrode (6.35mm diameter) was used, and an AC high voltage was applied at a step-up rate of 1kV / s at 25℃ / (50±5)%RH. When the applied field strength reached the critical value, the insulating medium broke down. The breakdown strength U_B (kV / mm) was defined by dividing the breakdown voltage by the sample thickness. The insulation performance imparted by the hexagonal boron nitride shell was evaluated, and the effect of the core-shell structure on improving the insulation reliability of the thermally conductive gel was verified. Five different locations were tested for breakdown at each sample, and U_B was calculated by breakdown voltage / thickness. The mean ± standard deviation was reported.

[0129] Experiment 5: X-ray photoelectron spectroscopy analysis of interfacial chemical states A composite filler powder consisting of aminosilanized hexagonal boron nitride, epoxy-silanized alumina, and thermally conductive gel is pressed into tablets and fixed on a sample stage under vacuum. The photoelectrons at the characteristic binding energies of each element on the excited surface (5–10 nm depth) were irradiated with monochromatic AlKα rays (1486.6 eV). Charge effects were corrected with C1s 284.8 eV, with a broad pass energy of 100 eV, a narrow pass energy of 20 eV, a step size of 0.05 eV, and an analysis area of ​​approximately 300 μm². Peak shifts and area ratios of N1s (101–103 eV range), N1s (399–402 eV range), and C1s, characterizing The formation of covalent bonds, CN bonds, and the N1s chemical state directly proves the successful construction of covalent bonds at the bifunctional silane interface. The binding energy-count rate CSV was exported, and Gaussian-Lorentz function peak fitting was performed to calculate the area percentage of each chemical state.

[0130] Experiment 6: Characterization of Fourier Transform Infrared Spectroscopy Coupled Branches Original hexagonal boron nitride, aminosilanized hexagonal boron nitride, epoxysilanized alumina, and composite filler powders for thermally conductive gels were ground and compressed with KBr at a mass ratio of 1:100. Using pure KBr tablets as a background reference, the samples were scanned at 25°C and <30%RH. Range, resolution A total of 32 scans were performed. Characteristic molecular groups absorb infrared radiation at specific wavenumbers, tracking the CO stretching vibration of the epoxy ring (approximately...). disappear), Stretch ( Enhanced) and CN bond (approximately The intensity change of the generated peak () characterizes the grafting state of the silane coupling agent and the degree of amino-epoxy covalent reaction. The effectiveness of the interfacial coupling is qualitatively and quantitatively verified by the disappearance of the epoxy ring-opening peak and the formation of new covalent bonds. Wavenumber-absorbance CSV is exported to... The peak area integral is used to calculate the epoxy group consumption rate.

[0131] Figure 1 The FTIR spectra of Example 1 and Comparative Example 4 are used to compare evidence of core-shell covalent bond formation. The basic parameters are a test wavenumber range of 400 to 4000 per centimeter with local magnification of 850 to 1250 per centimeter. The variable parameters are the sample type of Example 1 and Comparative Example 4 and the intensity changes of characteristic peaks of functional groups. The results show that Example 1 has a more significant response in the silicon-oxygen bond-related absorption region and the organic group-related absorption region, while Comparative Example 4 is relatively weaker. The conclusion proves that more stable core-shell chemical connections can be formed through surface modification and interfacial reactions, thus providing a reasonable structural basis for subsequent performance improvement.

[0132] Figure 2 This is a schematic diagram of XPS narrow spectrum comparison of Si2p peaks in Example 1 and Comparative Example 1. The basic parameters are the binding energy window from 100.0 to 104.5 eV, and the background and component peaks of the spectra are fitted. The variable parameters are the sample type, Example 1 and Comparative Example 1, and the relative peak area and peak position of free and covalent silicon chemical states. The results show that the covalent state-related components in Example 1 contribute more and the peak structure is more consistent with the characteristics of chemical bonding. The conclusion proves that this scheme can effectively promote silicon-related interface bonding and chemical state stabilization, which is reasonable.

[0133] Figure 3This is a schematic diagram of XPS narrow spectrum comparison of N1s peak fitting between Example 1 and Comparative Example 1. The basic parameters are the binding energy window from 398.5 to 402.5 eV, and the nitrogen chemical state is analyzed by background and component peak fitting. The variable parameters are the sample type of Example 1 and Comparative Example 1, and the relative proportion of amino-related and higher binding energy nitrogen species. The results show that the nitrogen chemical state composition of Example 1 is tilted towards covalently bound components, while Comparative Example 1 is mainly contributed by weakly bound or non-covalent components. The conclusion proves that the nitrogen-containing functional groups of Example 1 are more likely to form stable interfacial bonds, thus supporting the correctness of the structural design.

[0134] Figure 4 The thermogravimetric analysis curves are overlaid to quantitatively compare the organic component content of Examples 1 to 4 and Comparative Example 6. The basic parameter is the temperature range of 30 to 800 degrees Celsius, and the thermogravimetric behavior is compared with mass percentage as the vertical axis. The variable parameters are the sample formulations of Examples 1 to 4 and Comparative Example 6 and the corresponding organic component content levels. The results show that there are regular differences in the mass loss of different samples in the range of 200 to 800 degrees Celsius, and the higher organic content of Comparative Example 6 leads to more obvious weight loss. The conclusion proves that the scheme can achieve controllable loading of organic components and consistent with thermal stability by adjusting the amount of modification introduced, indicating that the design is reasonable and controllable.

[0135] Figure 5 The XPS N1s spectra and peak fitting results for Example 1 and Comparative Example 5 are shown. The basic parameters are the binding energy window of 398.5 to 402.5 eV, and background subtraction and component peak fitting are performed to analyze the N1s chemical state composition. The variable parameters are the sample type of Example 1 and Comparative Example 5, and the difference in the peak area ratio of amino-related and covalent-related components. The results show that the overall signal of Example 1 is stronger and the covalent-related components are more prominent, while the signal of Comparative Example 5 is weaker and mainly contributed by non-covalent components. The conclusion is that Example 1 has a higher introduction of nitrogen-containing functional groups and more complete chemical binding, providing direct evidence for the interface construction path.

[0136] Figure 6 Core-shell composite filler for thermally conductive gel in Example 1 ( Macroscopic optical photographs of the core-shell composite material and its spray-dried granules are shown. The samples all exhibit a milky white matte appearance, good drying flowability, and no agglomeration, demonstrating that the core-shell composite process and subsequent granulation treatment successfully prepared stable, dried fillers that meet application requirements.

[0137] Figure 7 shows Example 1 Scanning electron microscope (SEM) images of core-shell composite packing materials are used to display particle morphology characteristics and surface morphology information.

[0138] Figure 8This is a transmission electron microscope (TEM) characterization image of the composite filler used in the thermally conductive gel of Example 1. The basic parameters are: the characterization method is bright-field and high-resolution observation of the edge region of the core-shell composite particles using TEM, combined with selected area electron diffraction analysis of crystallographic information; the variable parameters are: the thickness of the hexagonal boron nitride shell is 25 nm; and the interface connection layer consists of siloxane bonds formed by the condensation of 3-glycidoxypropyltrimethoxysilane with hydroxyl groups on the surface of alumina particles, and covalent bonds formed by the reaction of amino groups introduced by 3-aminopropyltriethoxysilane with epoxy groups of 3-glycidoxypropyltrimethoxysilane. The conclusion is that a continuous or quasi-continuous thin-layer shell structure can be distinguished at the outer edge of the alumina particles, and the hexagonal boron nitride layered lattice stripe characteristics are observed under high resolution. There are no obvious debonding voids at the interface, and a nanoscale transition layer contrast is shown. This proves that the hexagonal boron nitride is not simply physically adsorbed, but achieves a stable interface connection through silane coupling, thereby verifying the correctness and rationality of the core-shell structure construction path.

[0139] Table 1 Summary of performance comparisons between each embodiment and the comparative example

[0140] Note: Thermal conductivity λ is the value measured by laser flare method (ASTM E1461-13); viscosity η is the steady-state shear value measured by rotational rheometer. The measured values ​​are as follows: breakdown strength U_B is the result of AC breakdown test (GB / T1408.1-2016); worg is determined by thermogravimetric analysis (ISO11358-1:2014, 200–800℃ range); wfree is determined by high performance liquid chromatography according to the method described in the claims; all data are expressed as mean ± standard deviation (n≥3).

[0141] As can be seen from the performance of the examples and comparative examples in Table 1, omitting the alkaline oxidation activation treatment of hexagonal boron nitride (Comparative Example 1) results in a severely insufficient hydroxyl density on the hBN surface, a significant decrease in the 3-aminopropyltriethoxysilane grafting density, uneven hexagonal boron nitride shell coating with weak bonding, and a significant increase in interfacial thermal resistance, causing the thermal conductivity to decrease to [value missing]. Viscosity increased to The breakdown strength decreased to 12.5 kV / mm, while the free silane residue was as high as 0.080 wt%, indicating that alkaline oxidation pretreatment is a prerequisite for forming a high-quality covalently bonded core-shell structure. When the mass fraction of hexagonal boron nitride exceeded the preferred range and increased to 43 wt% (Comparative Example 2), the excess hexagonal boron nitride had too much relative surface area to the alumina core, making it impossible to effectively form a complete core-shell coating structure. A large number of free hexagonal boron nitride sheets agglomerated in the system, and the thermal conductivity decreased to The viscosity increased to Processing performance and thermal conductivity deteriorate simultaneously; when the amount of hexagonal boron nitride is too small (comparative examples 3, 7 wt%), the shell continuity is severely insufficient, and the insulation breakdown strength drops sharply to 11.2 kV / mm. At the same time, due to the large amount of exposed alumina surface, the interparticle contact thermal resistance cannot be effectively controlled by the hexagonal boron nitride shell, and the thermal conductivity is also only [missing information]. The high alumina content failed to demonstrate the expected thermal conductivity advantage; the physical mixing scheme that completely omitted silanization treatment (Comparative Example 4) had the worst performance across all indicators, with a thermal conductivity of only [missing value]. Viscosity as high as Breakdown strength is only This fully demonstrates that the covalently coupled core-shell structure of bifunctional silanes is the core mechanism for achieving synergistic enhancement of multiple properties. When the amount of 3-aminopropyltriethoxysilane is severely insufficient (Comparative Example 5, 0.2 parts by mass), the amino group supply is extremely small, the degree of covalent reaction with the epoxy group is extremely low, the core-shell interface bonding is poor, and the thermal conductivity drops to a low level. Viscosity as high as Furthermore, the residual amount of free silane increased to 0.060 wt%; when the amount of 3-glycidoxypropyltrimethoxysilane was too high (Comparative Example 6, 6.5 parts by mass), the content of organic components exceeded 2.50 wt%. The excessive introduction of the low thermal conductivity organic phase significantly increased the interfacial thermal resistance, and the thermal conductivity decreased to The free silane residue was as high as 0.120 wt%, far exceeding the control index specified in this invention, indicating that the adverse effect of excessive silanization on thermal management performance cannot be ignored; omitting the spray drying granulation step (Comparative Example 7) resulted in the composite filler powder being difficult to disperse uniformly in the gel matrix, causing particle agglomeration, reducing the efficiency of thermal network formation, and increasing the viscosity. The thermal conductivity decreased to This demonstrates the significant contribution of granulation technology to filler dispersibility and processing window; when the covalent coupling reaction temperature is too low (Comparative Example 8, 40℃), the ring-opening addition reaction kinetics of amino and epoxy groups are insufficient, the interfacial covalent conversion rate is low, and the core-shell structure is incomplete, resulting in a decrease in thermal conductivity to [missing value]. The breakdown strength dropped to 13.8 kV / mm, indicating a significant deterioration in overall performance. Overall, the four embodiments showed significantly better performance than all comparative examples in core properties such as thermal conductivity, viscosity control, and insulation breakdown strength. This fully verifies the significant superiority of the present invention's technical solution for constructing an alumina-hexagonal boron nitride core-shell structure through bifunctional silane covalent coupling in terms of synergistic optimization of multiple properties including thermal conductivity, rheological processability, and insulation.

[0142] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that any equivalent structural transformations made under the concept of the present invention and using the contents of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A composite filler for thermally conductive gels, characterized in that, The composite filler for the thermally conductive gel is a core-shell composite particle, which includes: alumina particles as the core layer, a hexagonal boron nitride shell layer as the shell layer, and an interface bonding layer between the alumina particles and the hexagonal boron nitride shell layer; wherein, the interface bonding layer includes siloxane bonds formed by the condensation of 3-glycidoxypropyltrimethoxysilane with hydroxyl groups on the surface of alumina particles, and covalent bonds formed by the reaction of amino groups introduced by 3-aminopropyltriethoxysilane with epoxy groups of 3-glycidoxypropyltrimethoxysilane; The composite filler for thermally conductive gel is obtained by reacting epoxy-silanized alumina with amino-silanized hexagonal boron nitride. The mass fraction of alumina in the composite filler for thermally conductive gel is 60wt% to 89wt%, the mass fraction of hexagonal boron nitride is 10wt% to 39wt%, and the sum of the mass fractions of alumina and hexagonal boron nitride is not greater than 99.7wt%. The balance is organic components introduced by the interface bonding layer and unavoidable impurities and volatiles.

2. The composite filler for thermally conductive gel according to claim 1, characterized in that, Aminosilanized hexagonal boron nitride is prepared by the following steps: A1. Raw material preparation: 100 parts by weight of hexagonal boron nitride, 200 to 800 parts by weight of hydrogen peroxide aqueous solution, 5 to 50 parts by weight of sodium hydroxide, and 200 to 1000 parts by weight of deionized water; wherein the mass fraction of hydrogen peroxide in the hydrogen peroxide aqueous solution is 10 wt% to 35 wt%. A2. Alkaline oxidation: After thoroughly mixing the system obtained in step A1, the pH is measured. If the pH does not reach 11 to 13, 10 wt% to 30 wt% sodium hydroxide aqueous solution is added until the pH of the system is 11 to 13. Then, the reaction is carried out at 60℃ to 90℃ for 1 to 6 hours to obtain hexagonal boron nitride treated with alkaline oxidation. A3. Washing and drying: The product obtained in step A2 is separated into solid and liquid phases and washed with deionized water until the pH of the washing solution is 6 to 8; then dried at 60°C to 120°C for 4 to 16 hours to obtain dried hexagonal boron nitride with oxidation treatment. A4. Silanization reaction: The product obtained in step A3 is dispersed in a mixed solvent of ethanol and deionized water. Based on 100 parts by mass of the product obtained in step A3, the amount of ethanol used is 150 to 700 parts by mass, and the amount of deionized water used is 30 to 150 parts by mass, with a mass ratio of ethanol to deionized water of 70:30 to 95:

5. Glacial acetic acid is added to adjust the pH of the system to 4 to 6. The pH value is determined by pH meter after mixing a sample of the system with deionized water at a mass ratio of 1:1 at 25°C. Then, 3-aminopropyltriethoxysilane is added in an amount of 0.5 to 5.0 parts by mass per 100 parts by mass of hexagonal boron nitride. The reaction is carried out at 30°C to 70°C for 1 to 8 hours under a nitrogen atmosphere. A5. Post-processing and quality control: The product obtained in step A4 is separated into solid and liquid phases by centrifugation or vacuum filtration; it is washed with ethanol and deionized water 2 to 5 times, with each washing solution being 2 to 10 times the mass of the solid, until the conductivity of the filtrate is no greater than 50 μS / cm; then it is dried at a pressure of 0.00005 MPa to 0.01 MPa and a temperature of 60°C to 120°C for 4 to 16 hours, and the constant weight endpoint is determined by the mass difference between two consecutive weighings not exceeding 0.1% of the mass of the previous weighing, with an interval of 1 to 2 hours between the two consecutive weighings. Before each weighing, the sample is placed in a desiccator and cooled to room temperature before weighing, and the volatile content is controlled to be no higher than 1.0 wt%, to obtain aminosilanized hexagonal boron nitride.

3. The composite filler for thermally conductive gel according to claim 1, characterized in that, Epoxy-silanized alumina is prepared by the following steps: B1. Raw material preparation: 100 parts by weight of alumina, 0.5 to 5.0 parts by weight of 3-glycidoxypropyltrimethoxysilane, 200 to 800 parts by weight of ethanol, 10 to 200 parts by weight of deionized water, and 0.1 to 5.0 parts by weight of glacial acetic acid; B2. Solvent premixing: Mix the ethanol, deionized water and glacial acetic acid from step B1 and adjust the pH to 4 to 6. The pH is the value measured by a pH meter after mixing the system sample with deionized water at a mass ratio of 1:1 at 25°C; keep warm at 20°C to 40°C for 0.5 h to 2 h. B3. Silanization reaction: Alumina and 3-glycidoxypropyltrimethoxysilane were added to the system obtained in step B2, and the reaction was carried out at 30°C to 70°C for 1 to 6 hours under a nitrogen atmosphere; B4. Post-processing and quality control: Separate the solid and liquid components of the product obtained in step B3 by centrifugation or vacuum filtration. Wash with ethanol and deionized water 2 to 5 times, each time using 2 to 10 times the mass of the solid, until the conductivity of the filtrate is no greater than 50 μS / cm; then dry at a pressure of 0.00005 MPa to 0.01 MPa and a temperature of 60°C to 120°C for 4 to 16 hours. The constant weight endpoint is determined by the mass difference between two consecutive weighings not exceeding 0.1% of the mass of the previous weighing, with an interval of 1 to 2 hours between the two consecutive weighings. Before each weighing, the sample is placed in a desiccator and cooled to room temperature before weighing, and the volatile content is controlled to be no higher than 1.0 wt%, to obtain epoxy-silanized alumina.

4. The composite filler for thermally conductive gel according to claim 1, characterized in that, The composite filler for thermally conductive gel is subjected to spray drying granulation, which is achieved through the following steps: D1. The thermally conductive gel is dispersed in a mixed solvent of ethanol and deionized water in a mass ratio of ethanol to deionized water of 70:30 to 95:5, so that the solid content is 20wt% to 60wt%. D2. Spray drying: Spray drying is carried out under a nitrogen atmosphere, with an inlet temperature of 120℃ to 200℃ and an outlet temperature of 60℃ to 110℃; an airflow atomizer is used in the spray drying process, with an atomization pressure of 0.1MPa to 0.4MPa and a feed rate of 5mL / min to 30mL / min; D3. Obtain granulated particles and control the median particle size D50 of the granulated particles to be between 50 μm and 200 μm.

5. The composite filler for thermally conductive gel according to claim 1, characterized in that, The median particle size D50 of the alumina particles is 5 μm to 40 μm, and the median particle size D50 of the composite filler for the thermally conductive gel is not less than the median particle size D50 of the alumina particles, and is 6 μm to 60 μm. The median particle size D50 of the alumina particles, the composite filler for the thermally conductive gel, and the granulated particles is determined by laser diffraction in a deionized water dispersion system. Before measurement, the dispersion is ultrasonically treated for 30 to 120 seconds. The thickness of the hexagonal boron nitride shell is 5 nm to 50 nm. The thickness of the hexagonal boron nitride shell is statistically determined by transmission electron microscopy cross-sectional analysis on no fewer than 20 particles, and the average value is taken. The coating rate of the hexagonal boron nitride shell on the outer surface of the alumina particles is 70% to 100%, where the coating rate is the coverage of the hexagonal boron nitride shell. The ratio of the outer surface area of ​​the alumina particles to the total outer surface area of ​​the alumina particles is used, and the coating rate is calculated by analyzing and statistically processing images of no less than 50 particles using scanning electron microscopy backscattered electron imaging, based on the ratio of the hexagonal boron nitride shell coverage area to the total outer surface area of ​​the alumina particles; furthermore, the mass fraction of the organic components introduced by 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane in the composite filler for the thermally conductive gel is 0.3wt% to 2.0wt%, and the mass fraction of the organic components is determined by thermogravimetric analysis under nitrogen atmosphere and a heating rate of 10℃ / min. The mass loss in the range of 200℃ to 800℃ is taken and the corresponding mass loss of blank alumina and blank hexagonal boron nitride in the same range is deducted before calculation.

6. A method for preparing a composite filler for thermally conductive gel as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Provides aminosilanized hexagonal boron nitride, which is prepared by sequentially subjecting hexagonal boron nitride to alkaline oxidation and aminosilanization. S2. Provides epoxy-silanized alumina, which is prepared by epoxy-silanizing alumina; S3. The aminosilane-modified hexagonal boron nitride provided in step S1 and the epoxy-modified alumina provided in step S2 are mixed in ethanol. The total solid content is the sum of the masses of aminosilane-modified hexagonal boron nitride and epoxy-modified alumina, so that the mass fraction of hexagonal boron nitride in the total solid content is 10wt% to 39wt%, and the solid content of the dispersion system is 20wt% to 60wt%, wherein the solid content is the mass percentage of the total solid content to the total mass of the dispersion system. The reaction is carried out at 50℃ to 75℃ for 1h to 8h under a nitrogen atmosphere. The reaction is carried out under mechanical stirring at a stirring speed of 50rpm to 500rpm, so that the amino and epoxy groups react to form covalent bonds to obtain core-shell composite particles. S4. Separate the core-shell composite particles obtained in step S3 into solid and liquid phases, wherein the solid-liquid separation is performed by centrifugation or vacuum filtration. Wash with ethanol 2 to 5 times, each time using 2 to 10 times the mass of the solid, until the conductivity of the filtrate is no greater than 50 μS / cm; then dry at a pressure of 0.00005 MPa to 0.01 MPa and a temperature of 60℃ to 120℃ for 4 to 16 hours. The constant weight endpoint is determined by the mass difference between two consecutive weighings not exceeding 0.1% of the mass of the previous weighing, with an interval of 1 to 2 hours between the two consecutive weighings. Before each weighing, the sample is placed in a desiccator and cooled to room temperature before weighing, and the volatile content is controlled to be no higher than 1.0 wt%, thus obtaining the composite filler for thermally conductive gel. S5. Granulation: The thermally conductive gel obtained in step S4 is spray-dried and granulated using composite filler. The spray drying granulation adopts an airflow atomizer with an atomization pressure of 0.1MPa to 0.4MPa and a feed rate of 5mL / min to 30mL / min to obtain granulated particles with a median particle size D50 of 50μm to 200μm.

7. The preparation method according to claim 6, characterized in that, In step S1, the preparation of aminosilanized hexagonal boron nitride involves hydrogen peroxide in an aqueous solution with a mass fraction of 10 wt% to 35 wt%, and the amount of 3-aminopropyltriethoxysilane used is 0.5 to 5.0 parts by mass per 100 parts by mass of hexagonal boron nitride. In step S2, the preparation of epoxysilanized alumina involves 3-glycidoxypropyltrimethoxysilane used with a mass fraction of 0.5 to 5.0 parts by mass per 100 parts by mass of alumina.

8. The preparation method according to claim 6, characterized in that, In step S5, the thermally conductive gel obtained in step S4 is dispersed in a mixed solvent of ethanol and deionized water with a mass ratio of ethanol to deionized water of 70:30 to 95:5, so that the solid content is 20wt% to 60wt%; the gel is spray-dried and granulated under a nitrogen atmosphere with an inlet temperature of 120℃ to 200℃ and an outlet temperature of 60℃ to 110℃.

9. The preparation method according to claim 6, characterized in that, In the composite filler for the thermally conductive gel obtained in step S4, the total residual amount of free 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane is not higher than 0.05 wt%. The determination method for the total residual amount of free silane is as follows: take 5 g of sample, add 25 mL of methanol, extract by ultrasonication at 50 °C for 30 min, centrifuge, take the supernatant and quantify it by comparison with the monomer standards of 3-glycidoxypropyltrimethoxysilane and 3-aminopropyltriethoxysilane using high performance liquid chromatography or gas chromatography-mass spectrometry, and convert it into the sample mass fraction.

10. The preparation method according to claim 6, characterized in that, In step S1, the preparation of aminosilanized hexagonal boron nitride includes: mixing hexagonal boron nitride with an aqueous solution of hydrogen peroxide and sodium hydroxide, adjusting the pH to 11 to 13, and performing alkaline oxidation treatment at 60°C to 90°C for 1 to 6 hours. After solid-liquid separation, the mixture is washed with deionized water until the pH of the washing solution is 6 to 8 and then dried. The dried hexagonal boron nitride is then dispersed in a mixed solvent of ethanol and deionized water, glacial acetic acid is added to adjust the pH of the system to 4 to 6, 3-aminopropyltriethoxysilane is added, and the mixture is reacted at 30°C to 70°C for 1 to 8 hours under a nitrogen atmosphere. After solid-liquid separation, washing, and drying, aminosilanized hexagonal boron nitride is obtained.

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