Graphene-coated silicon carbide hollow microsphere composite heat-conducting filler as well as preparation method and application of graphene-coated silicon carbide hollow microsphere composite heat-conducting filler

Abstract

The invention belongs to the technical field of functional composite materials, and particularly relates to a graphene-coated silicon carbide hollow microsphere composite heat-conducting filler and a preparation method and application thereof.The graphene-coated silicon carbide hollow microsphere composite heat-conducting filler comprises silicon carbide hollow microspheres and graphene layers evenly coated on the surfaces of the silicon carbide hollow microspheres, the particle size of the silicon carbide hollow microspheres is 15-30 micrometers, the wall thickness is 1.0-2.2 micrometers, and the graphene layers are evenly coated on the surfaces of the silicon carbide hollow microspheres. The hollow rate is 60%-75%, the number of the graphene layers is 2-5, and the density of the composite heat-conducting filler is 0.8-1.2 g / cm < 3 >; the silicon carbide hollow microspheres are of a beta-phase cubic system structure, the grain size is 50-80 nanometers, the density of a shell layer is 95% or above, the density of the silicon carbide hollow microspheres is only 0.8-1.2 g / cm < 3 > and is reduced by 62.5%-75% compared with 3.2 g / cm < 3 > of solid silicon carbide, and the weight of the composite material can be reduced by 38%-45% under the same heat-conducting property.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of functional composite materials, and particularly relates to a graphene-coated silicon carbide hollow microsphere composite heat-conducting filler, a preparation method and applications thereof. The composite heat-conducting filler realizes perfect combination of ultra-high heat-conducting performance and light weight through hollow structure design and surface graphene coating, and is particularly suitable for high-end application fields sensitive to weight such as aviation electronic equipment, portable electronic equipment and wearable electronic equipment. BACKGROUND

[0002] With the continuous improvement of the power density of electronic devices, thermal management has become a key bottleneck restricting the performance and reliability of electronic products. Although traditional heat-conducting fillers such as alumina, boron nitride and silicon carbide have good heat-conducting performance, their high-density characteristics result in excessive weight of the composite material, which is difficult to meet the light-weight requirements of aerospace and portable devices. How to significantly reduce the material density while maintaining high heat-conducting performance has become a technical problem to be solved in the field of heat-conducting materials.

[0003] Chinese patent CN106634047A discloses a composite electronic heat-conducting and heat-dissipating material containing graphene and a preparation method thereof. In this technology, inorganic nanoparticles are used to grow in situ on the surface of graphene, so that the precursors of silicon dioxide, alumina or titanium dioxide are nucleated and grown on the surface of graphene in an alkaline environment of an alcohol solution, forming a core-shell composite structure of graphene and oxide. This scheme realizes certain heat-dissipating effect through the high heat conductivity of graphene and the high thermal emissivity of oxide, and the heat-conducting coefficient can reach 0.3 to 5 watts per meter kelvin. However, this technical solution has obvious limitations. First, the oxide particles are solid, and the overall density of the material is high, which is difficult to meet the light-weight application requirements. Second, the main function of the oxide is to improve the thermal emissivity rather than the heat conductivity, and the heat-conducting coefficient of the oxide itself is much lower than that of ceramic materials such as silicon carbide. Third, this scheme uses graphene as the core and oxide as the shell, and the dispersion of the oxide particles on the surface of graphene is in a discrete zero-dimensional form, which cannot form a continuous heat-conducting network, limiting the further improvement of the heat-conducting performance.

[0004] In the prior art, some researchers have also tried to reduce the density of the filler by using hollow microspheres, but these hollow microspheres are usually polymers or inorganic oxide materials, which have poor heat-conducting performance and lack effective heat-conducting network construction on the surface. At the same time, growing graphene on the surface of hollow ceramic microspheres involves complex processes such as catalyst deposition and low-temperature vapor deposition, and the existing technology does not provide an effective solution to how to uniformly coat graphene without damaging the hollow structure.

[0005] Therefore, developing a novel composite thermally conductive filler that simultaneously possesses high thermal conductivity and low density is of significant theoretical and practical value for promoting the lightweighting and high performance of electronic devices. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a graphene-coated silicon carbide hollow microsphere composite thermally conductive filler, its preparation method and its application. This composite filler achieves significant material weight reduction while maintaining excellent thermal conductivity through innovative hollow structure design and surface graphene coating technology.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows.

[0008] The graphene-coated silicon carbide hollow microsphere composite thermally conductive filler provided by this invention comprises silicon carbide hollow microspheres and a graphene layer uniformly coated on the surface of the silicon carbide hollow microspheres. The silicon carbide hollow microspheres have a particle size of 15 to 30 micrometers, a wall thickness of 1.0 to 2.2 micrometers, and a hollowness of 60% to 75%. This carefully designed hollow structure ensures that the material achieves a significant reduction in density while maintaining sufficient mechanical strength. The silicon carbide hollow microspheres have a β-phase cubic crystal structure with a grain size of 50 to 80 nanometers and a shell density of over 95%. The high-density shell ensures the structural stability of the microspheres during subsequent processing and use. The graphene layer consists of 2 to 5 layers, which are in close contact with the surface of the silicon carbide hollow microspheres, with an interlayer spacing of 0.34 nanometers. This close interfacial contact minimizes interfacial thermal resistance. The overall density of the composite thermally conductive filler is 0.8 to 1.2 grams per cubic centimeter, which is more than 60% lower than the 3.2 grams per cubic centimeter of the traditional solid silicon carbide filler.

[0009] Preferably, the composite thermally conductive filler further includes synergistic fillers, which include vapor-grown carbon fibers and silver nanoparticles. Vapor-grown carbon fibers possess excellent one-dimensional thermal conductivity, with a diameter of 150 nm, a length of 10 to 20 μm, and a thermal conductivity as high as 1950 W / m Kelvin. In the composite material, they act as a bridge between graphene-coated microspheres, constructing a three-dimensional thermally conductive framework. The addition amount is 2% to 5% of the mass of the graphene-coated silicon carbide hollow microspheres. Silver nanoparticles, as a zero-dimensional thermally conductive phase, fill the gaps between the microspheres, effectively reducing the contact thermal resistance between particles. Their particle size is 50 to 80 nm, and their surface is modified with oleic acid to prevent aggregation and oxidation. The addition amount is 0.5% to 1.5% of the mass of the graphene-coated silicon carbide hollow microspheres.

[0010] Furthermore, the graphene layer surface is modified with a silane coupling agent, namely γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane, at an amount of 1% to 3% of the mass of the graphene-coated silicon carbide hollow microspheres. The alkoxy groups of the silane coupling agent form Si-OC covalent bonds with the oxygen-containing groups on the graphene surface, while the amino or epoxy groups can chemically react with the polymer matrix. This bidirectional bonding significantly enhances the interfacial bonding between the filler and the matrix, reduces the interfacial thermal resistance, and improves the mechanical properties of the composite material.

[0011] In a preferred embodiment, the silicon carbide hollow microspheres employ a bimodal particle size distribution design. The large-diameter component has a particle size range of 25 to 30 micrometers, accounting for 60% to 70% of the total mass of the silicon carbide hollow microspheres, while the small-diameter component has a particle size range of 15 to 20 micrometers, accounting for 30% to 40% of the total mass of the silicon carbide hollow microspheres. This bimodal distribution design is based on particle packing theory. The small-diameter microspheres fill the gaps between the large-diameter microspheres, increasing the overall packing density of the filler and reducing porosity, thereby achieving a higher thermal conductivity with the same filler content.

[0012] The present invention also provides a method for preparing the above-mentioned graphene-coated silicon carbide hollow microsphere composite thermally conductive filler, which includes three core steps.

[0013] The first step is to prepare hollow silicon carbide microspheres. Polycarbosilane with a molecular weight of 1000 to 3000 is dissolved in tetrahydrofuran to prepare a solution with a mass concentration of 15% to 25%. A polycarbosilane coating is then formed on the surface of a porous silica microsphere template using a spray drying process. The selection of the porous silica microsphere template is crucial; its diameter should be 20 to 35 micrometers to ensure the final hollow silicon carbide microspheres meet the particle size requirements, and its porosity should be 40% to 55% to ensure complete removal later. The parameters of the spray drying process need to be precisely controlled: the atomizer speed is 18,000 to 22,000 rpm to produce uniform droplets; the inlet air temperature is 180 to 220 degrees Celsius to rapidly evaporate the solvent while avoiding premature cross-linking of the polycarbosilane; and the number of sprays is 2 to 4 to precisely control the coating thickness and obtain the ideal wall thickness. The coated microspheres were subjected to oxidative crosslinking treatment at 200°C for 2 to 4 hours to form a crosslinked network between the polycarbosilane molecular chains, improving shape retention during subsequent pyrolysis. The pyrolysis process was carried out under argon protection with a programmed temperature increase, rising slowly at a rate of 2°C per minute to 800°C and holding for 1 hour to complete pre-ceramization. During this stage, the organic groups in the polycarbosilane gradually decomposed, and the silicon-carbon framework began to form. The temperature was further increased to 1450 to 1550°C and held for 2 to 3 hours to complete the full ceramization transformation. At this point, silicon carbide crystals grew sufficiently to form a high-density ceramic shell. The argon flow rate was maintained at 200 to 300 mL per minute to avoid oxidation. The silica template was removed by stirring at 60°C for 12 hours using a 40% hydrofluoric acid solution. After repeated washing with deionized water and anhydrous ethanol, the microspheres were vacuum-dried at 80°C for 24 hours to obtain hollow silicon carbide microspheres with precise wall thickness and void ratio.

[0014] The second step is the deposition of a nickel catalyst layer. Silicon carbide surfaces are highly inert and lack the catalytic activity required for graphene growth; therefore, a thin layer of metal catalyst needs to be pre-deposited. Hollow silicon carbide microspheres are placed in a fluidized bed. The fluidized bed design ensures that the microspheres are suspended and tumble in the gas flow, achieving uniform treatment on all surfaces. First, the surface is activated by bombarding with argon plasma at a power of 200 watts and a pressure of 50 Pa for 5 minutes. The high-energy particles in the plasma remove organic contaminants from the surface and generate dangling bonds, enhancing surface activity. Subsequently, a nickel catalyst layer with a thickness of 2 to 5 nanometers is deposited on the microsphere surface by magnetron sputtering. The thickness of the nickel layer needs precise control; if it is too thin, the catalytic activity is insufficient; if it is too thick, it is prone to agglomeration into large particles during subsequent heating, losing its catalytic effect. The magnetron sputtering process parameters include a target power of 80 watts, an argon flow rate of 30 standard cubic centimeters per minute, a working pressure of 0.8 Pa, and a deposition time adjusted to 15 minutes per gram of microspheres based on the microsphere loading.

[0015] The third step is the growth of graphene via chemical vapor deposition. Silicon carbide microspheres treated with a catalyst layer are placed in a quartz boat and pushed into the central isothermal zone of a vertical hot-wall chemical vapor deposition reactor. Graphene growth employs low-temperature plasma-enhanced chemical vapor deposition technology, one of the key innovations of this invention. The temperature is raised to 650 degrees Celsius and held for 20 minutes in a hydrogen atmosphere with a flow rate of 100 standard cubic centimeters per minute. During this stage, hydrogen reduces surface oxides, and the nickel catalyst layer recrystallizes to form nanocrystals and active sites, providing active centers for the adsorption and diffusion of carbon atoms. A mixture of methane and hydrogen as the carbon source gases is introduced, with a volume ratio of methane to hydrogen of 1:4 and a total flow rate of 200 standard cubic centimeters per minute. The presence of hydrogen inhibits the deposition of amorphous carbon and promotes the ordered growth of graphene. The reaction temperature is controlled between 680 and 720 degrees Celsius, which is much lower than the 1000 degrees Celsius or higher of traditional chemical vapor deposition, preventing the hollow microspheres from collapsing due to thermal stress. The growth time is 20 to 40 minutes, and 2 to 5 layers of graphene can be obtained by precisely controlling the reaction time. Simultaneously, a 150-watt radio frequency plasma is applied for assistance. The active particles in the plasma lower the decomposition temperature of methane, increase the diffusion rate of carbon atoms, and promote the formation of high-quality graphene. After growth, the graphene is cooled to below 400 degrees Celsius in hydrogen gas before being removed from the furnace to prevent oxidation during the cooling process.

[0016] In a preferred embodiment of the preparation method, a surface functionalization modification step is included after the third step. The prepared graphene-coated silicon carbide hollow microspheres are dispersed in anhydrous ethanol with a solid content of 5% to 10%. γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane is added in an amount of 1% to 3% of the microsphere mass. The mixture is ultrasonically treated at 65°C for 30 minutes. The cavitation effect of the ultrasound promotes the uniform dispersion of silane molecules on the graphene surface and induces the hydrolysis and preliminary condensation of silanes. The mixture is then refluxed for 4 hours to fully construct the siloxane network on the graphene surface, forming a stable organic-inorganic hybrid layer. After centrifugation, washing, and drying, the surface-functionalized composite filler is obtained.

[0017] This invention also provides the application of the above-mentioned graphene-coated silicon carbide hollow microsphere composite thermally conductive filler in the preparation of high thermal conductivity composite materials. The composite thermally conductive filler is combined with an epoxy resin or thermoplastic polymer matrix, and the filler addition amount is 40% to 60%. When the addition amount is 40% in the epoxy resin system, the thermal conductivity reaches 5.8 W / m Kelvin, and when the addition amount is 60%, the thermal conductivity reaches 8.2 W / m Kelvin. The density of the composite material is reduced by 38% to 45% compared with the use of solid silicon carbide filler. It is applied to heat dissipation structural components of avionics, portable electronic devices, or wearable electronic devices.

[0018] The beneficial effects of this invention are as follows.

[0019] First, this invention achieves lightweighting of the thermally conductive filler through a hollow structure design. The density of the hollow silicon carbide microspheres is only 0.8 to 1.2 grams per cubic centimeter, a reduction of 62.5% to 75% compared to the 3.2 grams per cubic centimeter of solid silicon carbide. Under the same thermal conductivity, the weight of the composite material can be reduced by 38% to 45%, greatly expanding the application range of thermally conductive materials in weight-sensitive fields such as aerospace. The wall thickness of the hollow structure has been carefully optimized, within the range of 1.0 to 2.2 micrometers, ensuring both the mechanical strength of the microspheres and maximizing the hollowness, achieving an optimal balance between strength and density.

[0020] Secondly, the graphene coating constructs a highly efficient two-dimensional thermally conductive network. Graphene possesses an extremely high intrinsic thermal conductivity, with its sp... 2 The hybrid carbon atom lattice provides an ideal channel for phonon transport, with a mean free path of up to 775 nanometers. Two to five layers of graphene form a continuous coating on the surface of silicon carbide microspheres, allowing adjacent microspheres to directly contact or couple at close range through the graphene layers. Phonons can be efficiently transported within the graphene network without passing through high thermal resistance interfaces. This is completely different from the point contact between traditional filler particles, significantly reducing contact thermal resistance and improving the overall thermal conductivity of the composite material. In epoxy resin systems, a 40% addition achieves a thermal conductivity of 5.8 W / m Kelvin, and a 60% addition reaches 8.2 W / m Kelvin, significantly outperforming similar products.

[0021] Furthermore, the preparation method of this invention exhibits excellent controllability and reproducibility. The polymer precursor conversion method combined with template sacrificial technology provides precise structural control for the preparation of hollow silicon carbide microspheres. Wall thickness and void ratio can be accurately controlled by adjusting the number of spray cycles and the pyrolysis process. Low-temperature plasma-enhanced chemical vapor deposition overcomes the problem of damage to hollow structures caused by traditional high-temperature processes, achieving controllable growth of high-quality graphene at temperatures between 680 and 720 degrees Celsius. The number of graphene layers can be precisely controlled by the growth time. The uniform thickness of the nickel catalyst layer deposited by magnetron sputtering ensures uniform coating of graphene on the microsphere surface.

[0022] Furthermore, the composite of synergistic fillers further optimized the construction of the thermally conductive network. Vapor-grown carbon fibers, acting as a one-dimensional thermally conductive phase, formed a bridging structure between graphene-coated microspheres, extending the two-dimensional thermally conductive network into a three-dimensional thermally conductive framework, significantly improving the continuity and effectiveness of the thermal conduction path. Silver nanoparticles filled the gaps between the microspheres, utilizing their excellent thermal conductivity and deformability to reduce the contact thermal resistance between particles. This multi-scale synergistic construction of zero-dimensional, one-dimensional, two-dimensional, and three-dimensional fillers created a highly efficient composite thermally conductive network, achieving a synergistic effect greater than the sum of its parts.

[0023] Surface modification with silane coupling agents significantly improves the interfacial compatibility between the filler and the matrix. The bifunctional structure of the silane molecule establishes a chemical bridge between the filler surface and the polymer matrix, transforming the interface from simple physical contact to chemical bonding, and reducing the interfacial thermal resistance from 8.5 × 10⁻⁻⁻⁶. 8 The Kelvin per square meter per watt has decreased to 2.1 × 10⁻ ... 8 The per square meter Kelvin decreased by 75%. At the same time, the chemical bonding at the interface also improved the mechanical properties and long-term stability of the composite material, and the filler was less prone to debonding and agglomeration during thermal cycling. Detailed Implementation

[0024] The present invention will be further described in detail below with reference to specific embodiments. It should be noted that the following embodiments are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention.

[0025] Example 1 In this embodiment, a graphene-coated silicon carbide hollow microsphere composite thermally conductive filler with a particle size of 15 micrometers and a wall thickness of 1.0 micrometers was prepared, with two graphene layers.

[0026] The preparation method includes the following steps. First, hollow silicon carbide microspheres are prepared by dissolving polycarbosilane with a molecular weight of 1000 in tetrahydrofuran to prepare a 15% (w / w) solution. A porous silica microsphere template with a diameter of 20 μm and a porosity of 40% is suspended in deionized water to prepare a 10% suspension. Polycarbosilane is then coated onto the template surface by spray drying at 18000 rpm and an inlet air temperature of 180°C, with two spray cycles to obtain a thin coating. The coated composite microspheres are then oxidatively crosslinked in a muffle furnace at 200°C for 2 hours. Crosslinking forms Si-O-Si and Si-C-Si bridges between the polycarbosilane molecular chains, improving thermal stability. The crosslinked microspheres are then transferred to a tube furnace and heated to 800°C at a rate of 2°C per minute under argon protection, holding for 1 hour to complete pre-ceramization. During this stage, organic groups are gradually removed, and the inorganic framework is initially formed. The temperature was further increased to 1450°C and held for 2 hours to complete ceramization and ensure full growth of the β-silicon carbide crystal phase. The argon flow rate was maintained at 200 mL / min throughout the pyrolysis process. After cooling, the product was placed in a polytetrafluoroethylene beaker, and a 40% hydrofluoric acid solution was added. The mixture was stirred in a 60°C water bath for 12 hours until the silica template was completely dissolved. The product was washed with deionized water until neutral, then washed three times with anhydrous ethanol, and dried in an 80°C vacuum oven for 24 hours to obtain hollow silicon carbide microspheres with a particle size of approximately 15 μm and a wall thickness of approximately 1.0 μm, a hollowness of approximately 60%, and a shell density exceeding 95%.

[0027] Next, a nickel catalyst layer was deposited. Five grams of hollow silicon carbide microspheres were loaded into a fluidized bed reactor, and argon gas was introduced to fluidize the microspheres. An RF power supply was turned on to generate argon plasma at a power of 200 watts and a pressure of 50 Pa for 5 minutes to activate the microsphere surface, remove adsorbed organic matter, and generate active sites. Immediately after the plasma was turned off, magnetron sputtering was performed using a 99.99% pure nickel target at a power of 80 watts, an argon flow rate of 30 standard cubic centimeters per minute, and a working pressure of 0.8 Pa. The fluidized bed was continuously agitated to ensure uniform sputtering on all surfaces of the microspheres. The deposition time was 75 minutes, forming a uniform nickel catalyst layer approximately 2 nanometers thick on the microsphere surface.

[0028] Then, graphene was grown by chemical vapor deposition. Nickel-plated silicon carbide microspheres were uniformly placed in a quartz boat and pushed into the isothermal zone of a vertical hot-wall chemical vapor deposition furnace. High-purity hydrogen gas at a flow rate of 100 standard cubic centimeters per minute was first introduced, and the temperature was increased to 650 degrees Celsius at a rate of 5 degrees Celsius per minute and held for 20 minutes. The hydrogen gas reduced the surface oxides, and the nickel layer recrystallized to form nanocrystals. After the temperature stabilized, carbon source gases were introduced, with methane at a flow rate of 40 standard cubic centimeters per minute and hydrogen at a flow rate of 160 standard cubic centimeters per minute, with a volume ratio of 1:4. At the same time, the temperature was increased to 680 degrees Celsius, and a 150-watt plasma-assisted plasma was applied by turning on the radio frequency power supply. The graphene growth time was controlled to 20 minutes. Under the catalysis of nickel and the activation of plasma, carbon atoms generated by the decomposition of methane were adsorbed, diffused, and arranged in an orderly manner on the surface of the microspheres, forming two layers of high-quality graphene. After growth is complete, the methane is shut off, and the furnace is cooled to below 400 degrees Celsius while maintaining a hydrogen atmosphere to obtain graphene-coated silicon carbide hollow microspheres.

[0029] Finally, surface functionalization modification was performed. 10 g of graphene-coated silicon carbide hollow microspheres were weighed and dispersed in 100 mL of anhydrous ethanol, and ultrasonically dispersed for 10 minutes. 0.1 g of γ-aminopropyltriethoxysilane was added, representing 1% of the microsphere mass. The mixture was ultrasonically treated in a 65°C oil bath for 30 minutes, causing the silane to hydrolyze and undergo preliminary condensation. The mixture was transferred to a three-necked flask equipped with a reflux condenser and refluxed at 65°C for 4 hours. The siloxane network was fully constructed on the graphene surface. The ethoxy groups of the silane condensed with the carboxyl and hydroxyl groups at the graphene edges to form Si-OC bonds, with the aminopropyl groups facing the solution side. After the reaction, the mixture was centrifuged, washed three times with anhydrous ethanol to remove unreacted silane, and vacuum dried at 80°C for 12 hours to obtain the surface-functionalized graphene-coated silicon carbide hollow microsphere composite thermally conductive filler. The filler contains 97% silicon carbide hollow microspheres and 3% graphene coating, with an overall density of 0.8 g / cm³.

[0030] Example 2 In this embodiment, a graphene-coated silicon carbide hollow microsphere composite thermally conductive filler with a particle size of 20 micrometers and a wall thickness of 1.5 micrometers was prepared. The graphene layer has 3 layers, and synergistic fillers were added.

[0031] Silicon carbide hollow microspheres were prepared using polycarbosilane with a molecular weight of 2000, formulated into a 20% (w / w) tetrahydrofuran solution. A porous silica microsphere template with a diameter of 25 μm and a porosity of 48% was selected. The spray drying atomizer rotated at 20,000 rpm, the inlet air temperature was 200°C, and three sprays were performed to obtain a medium-thickness coating. Oxidative crosslinking was carried out at 200°C for 3 hours. The pyrolysis process involved pre-ceramization at 800°C for 1 hour under argon protection, followed by further heating to 1500°C and holding for 2.5 hours to complete ceramization. The argon flow rate was 250 mL / min. The template removal, washing, and drying steps were the same as in Example 1, yielding silicon carbide hollow microspheres with a particle size of approximately 20 μm, a wall thickness of approximately 1.5 μm, and a hollowness of approximately 68%.

[0032] The nickel catalyst layer was deposited using the same plasma activation parameters as in Example 1, with the magnetron sputtering deposition time adjusted to 15 minutes per gram of microspheres, forming a nickel layer approximately 3 nanometers thick. The graphene growth temperature was increased to 700 degrees Celsius, the methane to hydrogen flow rate ratio was maintained at 1:4, and the growth time was extended to 30 minutes, resulting in a 3-layer graphene coating. Surface functionalization was performed using γ-glycidoxypropyltrimethoxysilane at 2% of the microsphere mass, using the same treatment method as in Example 1.

[0033] Synergistic fillers were incorporated into functionalized graphene-coated silicon carbide hollow microspheres. 100g of the main filler was weighed, and 3g of vapor-grown carbon fiber (150nm diameter, 15µm length, thermal conductivity 1950 W / m Kelvin, manufactured by Showa Denko Corporation) was added. Simultaneously, 1g of oleic acid-modified silver nanoparticles (65nm diameter, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.) were added. The mixture was stirred in a V-type mixer for 30 minutes to ensure uniform dispersion of the synergistic and main fillers, resulting in a composite thermally conductive filler with an overall density of 0.9g / cm³. The carbon fiber forms a bridging structure connecting adjacent graphene-coated microspheres in the filler system, while the silver nanoparticles fill the gaps to reduce contact thermal resistance. The synergistic effect of the multi-scale fillers constructs a highly efficient three-dimensional thermally conductive network.

[0034] Example 3 In this embodiment, a graphene-coated silicon carbide hollow microsphere composite thermally conductive filler with a particle size of 25 micrometers and a wall thickness of 2.0 micrometers was prepared. The graphene layer has 4 layers and adopts a bimodal particle size distribution design.

[0035] First, two types of hollow silicon carbide microspheres with different particle sizes were prepared. For the large-particle-size component, a 30-micrometer diameter silica template was used, and a 22% (w / w) solution of polycarbosilane with a molecular weight of 2500 was prepared. The spray drying process was carried out using an atomizer at 21,000 rpm, with an inlet air temperature of 210°C, four spray cycles, followed by oxidative crosslinking for 3.5 hours, and then pyrolysis at 1520°C for 2.8 hours, yielding large microspheres with a particle size of approximately 25 micrometers and a wall thickness of approximately 2.0 micrometers. For the small-particle-size component, a 22-micrometer diameter silica template was used, with an 18% polycarbosilane solution concentration. The process involved three spray cycles and pyrolysis at 1480°C, yielding small microspheres with a particle size of approximately 18 micrometers and a wall thickness of approximately 1.6 micrometers.

[0036] Nickel catalyst layers were deposited on both large and small microspheres, with a nickel layer thickness of 4 nm. Graphene was grown at 710°C for 35 minutes, yielding four layers of graphene. Surface functionalization was performed using γ-aminopropyltriethoxysilane at a dosage of 2.5%. A bimodal particle size distribution was achieved by mixing the large-particle-size component and the small-particle-size component at a mass ratio of 7:3, with the small microspheres filling the gaps between the large microspheres, thus increasing the packing density of the filler. 3.5% vapor-grown carbon fibers and 1.2% silver nanoparticles were added as synergistic fillers, and the mixture was stirred in a V-mixer for 40 minutes to obtain a composite thermally conductive filler with a density of 1.0 g / cm³. The bimodal distribution design increased the effective filling rate of the filler in the composite material, reduced porosity, and created a more continuous heat conduction path.

[0037] Example 4 In this embodiment, a graphene-coated silicon carbide hollow microsphere composite thermally conductive filler with a particle size of 30 micrometers and a wall thickness of 2.2 micrometers was prepared. The number of graphene layers was 5, and the amount of synergistic filler added reached the upper limit.

[0038] Silicon carbide hollow microspheres were prepared using a 35-micrometer diameter silica template with a porosity of 55% and a 25% (w / w) solution of polycarbosilane with a molecular weight of 3000. The spray drying atomizer rotated at 22,000 rpm, the inlet air temperature was 220°C, and the spray was performed four times to obtain the maximum wall thickness. Oxidative crosslinking was carried out at 200°C for 4 hours. Pyrolysis was performed by heating to 1550°C and holding for 3 hours with an argon flow rate of 300 mL / min, yielding silicon carbide hollow microspheres with a particle size of approximately 30 micrometers and a wall thickness of approximately 2.2 micrometers, resulting in a hollowness of approximately 75%. The increased shell thickness improved the mechanical strength and thermal stability of the microspheres.

[0039] The nickel catalyst layer, with a thickness of 5 nanometers, is close to the upper limit, providing ample catalytic active sites. The graphene growth temperature is 720 degrees Celsius, and the growth time is extended to 40 minutes, resulting in 5 layers of graphene, the highest number of layers within the scope of this invention, further improving the integrity of the thermally conductive network. Surface functionalization utilizes γ-glycidyl etheroxypropyltrimethoxysilane at a dosage of 3%, reaching the upper limit, maximizing interfacial compatibility.

[0040] Adding 5% vapor-grown carbon fiber and 1.5% silver nanoparticles to the functionalized main filler, the addition amounts of both synergistic fillers reached their upper limits. The vapor-grown carbon fiber has an aspect ratio as high as 133, forming a dense bridging network in the filler system, significantly improving the thermal conductivity continuity between filler particles. The high addition amount of silver nanoparticles ensures that all microsphere gaps are effectively filled, minimizing contact thermal resistance. The overall density of this composite thermally conductive filler is 1.2 g / cm³, which is the upper limit within the scope of this invention, but still far lower than the density of solid fillers, achieving lightweight while ensuring the highest thermal conductivity.

[0041] Example 5 In this embodiment, a graphene-coated silicon carbide hollow microsphere composite thermally conductive filler for epoxy resin composites was prepared. Intermediate parameter values ​​were used to prepare a complete composite material and its performance was tested.

[0042] The hollow silicon carbide microspheres exhibit a bimodal distribution, with 65% being 27 μm in diameter and 35% being 17 μm in diameter. The polycarbosilane has a molecular weight of 2200 and a solution concentration of 21%. The spray drying atomizer speed is 20500 rpm, and the inlet air temperature is 205°C. Large microspheres were sprayed four times to achieve a wall thickness of 1.9 μm, while small microspheres were sprayed three times to achieve a wall thickness of 1.4 μm. The pyrolysis temperature was 1510°C, the holding time was 2.6 hours, and the argon flow rate was 270 mL / min. The nickel catalyst layer thickness was 3.5 nm, and the graphene growth temperature was 705°C for 32 minutes, yielding 3.5 layers of graphene. Surface functionalization was performed using γ-aminopropyltriethoxysilane at a dosage of 2.2%. 4% vapor-grown carbon fibers and 1.1% silver nanoparticles were added, resulting in a composite thermally conductive filler density of 1.05 g / cm³.

[0043] Epoxy resin composite material was prepared. Bisphenol A type epoxy resin with an epoxy value of 0.51 equivalents per 100 grams, commercially known as E-51, was purchased from Lanxing New Materials Wuxi Resin Factory. 200 grams of epoxy resin were heated at 80°C until the viscosity decreased to 3000 mPa·s. 3 grams of polyether-modified polydimethylsiloxane dispersant, with a polyether value of 700 and a viscosity of 800 centistokes, commercially known as BYK-333, manufactured by BYK Chemical Company, was added. The mixture was stirred at high speed (1500 rpm) for 10 minutes to completely dissolve the dispersant. The first batch consisted of 40 grams of composite thermally conductive filler, stirred for 15 minutes to fully wet the filler. The second batch consisted of 67 grams, stirred for 20 minutes. Finally, the remaining 27 grams were added, and stirred for 30 minutes. The total filler content was 134 grams, accounting for 40% of the mixture. The mixture was transferred to a three-roll mill with roller gaps of 50 micrometers for the first pass, 30 micrometers for the second pass, and 15 micrometers for the third pass. The milling speed was 80 rpm for the front roller, 120 rpm for the middle roller, and 180 rpm for the rear roller, and the milling cycle was repeated four times. After milling, the mixture was degassed in a vacuum degassing machine at -0.09 MPa for 30 minutes to remove entrained air bubbles. 115 g of methyltetrahydrophthalic anhydride curing agent (0.85 equivalents of epoxy resin) and 1.5 g of 2-ethyl-4-methylimidazolium accelerator (0.75 phr) were added. The mixture was then mixed in a planetary mixer at 200 rpm and 400 rpm for 10 minutes. The mixture was poured into a mold preheated to 60°C, pre-cured at 80°C for 2 hours, cured at 120°C for 3 hours, and post-cured at 150°C for 2 hours. After cooling to room temperature in the oven, the mixture was demolded to obtain a composite material sheet with a thickness of 2 mm.

[0044] Comparative Example 1 This comparative study prepared uncoated silicon carbide hollow microsphere fillers to demonstrate the crucial role of the graphene coating layer.

[0045] The preparation of silicon carbide hollow microspheres was exactly the same as in Example 5, with identical particle size and wall thickness parameters, but without the nickel catalyst deposition and graphene growth steps. After removing the template with hydrofluoric acid, washing, and drying, the silicon carbide hollow microspheres underwent direct silane coupling agent surface treatment, using the same amount and method as in Example 5. Due to the lack of a graphene coating, the microsphere surface only had a silanized layer, preventing the formation of a continuous two-dimensional thermally conductive network. After adding the same synergistic filler as in Example 5, epoxy resin composite materials were prepared using the same process. This comparative example was designed to demonstrate the irreplaceable role of the graphene coating in constructing an efficient thermally conductive network.

[0046] Comparative Example 2 This comparative example uses solid silicon carbide particles as fillers, with a particle size distribution consistent with the hollow silicon carbide microspheres of Example 5, to compare the lightweight advantages of hollow structures.

[0047] Solid silicon carbide particles, in the β phase with a grain size of 60 nm, were purchased from Weifang Kaihua Silicon Carbide Micropowder Co., Ltd. The particles were separated into two groups by airflow classification: 65% were large (27 μm) and 35% were small (17 μm), consistent with the bimodal distribution in Example 5. The solid silicon carbide particles were coated with graphene using the same nickel-catalyzed chemical vapor deposition process as in Example 5, growing 3.5 layers of graphene. Surface functionalization and synergistic filler addition were also the same as in Example 5. Epoxy resin composites were prepared using the same process, but due to the density of the solid silicon carbide (3.2 g / cm³) and the overall density of the filler (2.8 g / cm³), the density of the composite material was significantly higher than in Example 5. This comparative example demonstrates the crucial role of hollow structure design in achieving lightweighting.

[0048] Comparative Example 3 This comparative example does not add any synergistic fillers; it only uses graphene-coated silicon carbide hollow microspheres to verify the synergistic effect of the synergistic fillers.

[0049] The preparation of graphene-coated silicon carbide hollow microspheres was exactly the same as in Example 5, including bimodal particle size distribution, wall thickness, number of graphene layers, and surface functionalization, but without the addition of vapor-grown carbon fibers and silver nanoparticles. Epoxy resin composites were prepared using the same process, with the filler content remaining at 40%. Due to the lack of one-dimensional carbon fiber bridging and zero-dimensional silver particle interstitial filling, the thermal conductivity paths between filler particles were not continuous, resulting in higher contact thermal resistance and a lower thermal conductivity of the composite material compared to Example 5. This comparative example demonstrates the importance of multi-scale synergistic fillers in constructing a three-dimensional thermally conductive network.

[0050] Comparative Example 4 This comparative example uses the technical solution disclosed in prior art document CN106634047A to prepare graphene-supported silica composite filler, which is used to demonstrate the significant progress of the present invention.

[0051] 5 g of graphene oxide was dispersed in 50 mL of isopropanol and sonicated for 30 minutes. 2.5 g of tetraethyl orthosilicate and 25 mL of deionized water were added and stirred to form a sol. Ammonia was added dropwise to adjust the pH to 8.5, and the reaction was carried out at 50°C with stirring for 3 hours. The silica precursor hydrolyzed and condensed on the graphene surface to form nanoparticles. The product was washed, dried, and reduced in nitrogen at 500°C for 2 hours to obtain a graphene-supported silica composite filler. The silica particles in this filler were discretely distributed, unable to form a continuous thermally conductive network, and had a solid structure with a density as high as 2.1 g / cm³. Epoxy resin composites were prepared using the same process for performance comparison.

[0052] The composite materials prepared in all examples and comparative examples were subjected to systematic performance tests, and the test methods and conditions are as follows.

[0053] Thermal conductivity was tested using the laser flare method with a Netzsch LFA 467 laser thermal conductivity meter. The composite material sheet was cut into circular pieces with a diameter of 12.7 mm and a thickness of 2 mm, and a graphite layer was uniformly sprayed onto the surface to improve laser absorption. The test temperature was 25 degrees Celsius, and each sample was tested five times, with the average value taken. After measuring the thermal diffusivity using the laser flare method, the thermal conductivity was calculated in conjunction with the material density and specific heat capacity. Density was determined by the water displacement method, and specific heat capacity was determined by differential scanning calorimetry.

[0054] Mechanical property tests, including flexural strength and flexural modulus, were performed according to the national standard GB / T 2567-2008 using a CMT5105 electronic universal testing machine from Shenzhen Xin Sansi Co., Ltd. The spline dimensions were 80 mm x 10 mm x 4 mm, with a span of 64 mm and a loading rate of 2 mm / min. Impact strength was tested according to GB / T 1043-2008 using a simply supported beam impact method with a notch depth of 2 mm.

[0055] Thermal stability was characterized by thermogravimetric analysis using a TA Instruments Q500 thermogravimetric analyzer. The sample mass was approximately 10 mg. The temperature was increased from room temperature to 600°C per minute at a rate of 10°C per minute under a nitrogen atmosphere. Mass loss curves were recorded, and the 5% thermogravimetric temperature was determined.

[0056] Thermal reliability was evaluated through cyclic thermal shock testing using a high and low temperature shock chamber. Samples were cycled between -55°C and +125°C, with each temperature held for 30 minutes, and the thermal conductivity retention was tested after 500 cycles.

[0057] The test results are summarized in Table 1.

[0058] Table 1 Performance test results of composite materials in each embodiment and comparative example Sample Filler density (g / cm 3 )]> Composite density (g / cm 3 ) Thermal conductivity (W / (m K)) Flexural strength (MPa) Flexural modulus (GPa) Td5 (°C) Thermal conductivity retention rate after thermal shock (%) Example 1 0.8 1.28 4.2 92 3.5 380 94 Example 2 0.9 1.34 5.1 94 3.6 382 95 Example 3 1 1.4 6.3 95 3.7 384 95 Example 4 1.2 1.48 8.2 97 3.9 387 96 Example 5 1.05 1.42 5.8 95 3.8 385 96 Comparative Example 1 1.05 1.42 2.8 88 3.2 378 89 Comparative Example 2 2.8 2.12 5.9 91 3.7 383 95 Comparative Example 3 1.05 1.42 4.1 93 3.6 383 93 Comparative Example 4 2.1 1.86 1.9 82 3 372 85 Pure resin - 1.18 0.2 85 2.9 350 - As can be seen from the data in Table 1, Examples 1 to 5 of the present invention exhibit significant advantages over the comparative examples. Example 5, with a filler content of 40%, achieves a thermal conductivity of 5.8 W / m Kelvin, which is 29 times that of pure epoxy resin, while the composite material density is only 1.42 g / cm³. Comparative Example 1, without graphene coating, has a thermal conductivity of only 2.8 W / m Kelvin, a reduction of 52%, demonstrating that the two-dimensional thermally conductive network constructed by the graphene coating layer is crucial for improving thermal conductivity. Comparative Example 2 uses solid silicon carbide; although the thermal conductivity is close to that of Example 5, the composite material density is as high as 2.12 g / cm³, a 49% increase in weight compared to Example 5, failing to meet the lightweight requirements. Comparative Example 3, without the addition of synergistic fillers, achieves a thermal conductivity of 4.1 W / m Kelvin, 29% lower than Example 5, indicating that the multi-scale synergy of vapor-grown carbon fibers and silver nanoparticles significantly improves the effectiveness of the thermally conductive network. Comparative Example 4 uses the technical solution from the prior art, with a thermal conductivity of only 1.9 W / m Kelvin and a density as high as 1.86 g / cm³, which is significantly inferior to the present invention in both thermal conductivity and lightweighting.

[0059] Example 4 exhibits the best performance, achieving a thermal conductivity of 8.2 W / m Kelvin when the filler content reaches 60%. This is attributed to the maximum wall thickness providing higher mechanical strength, the 5-layer graphene constructing the most complete thermally conductive network, and the synergistic filler reaching its maximum capacity providing the densest three-dimensional thermally conductive framework. Example 1, although having the lowest filler density of only 0.8 g / cm³, achieves a thermal conductivity of 4.2 W / m Kelvin due to its thinner wall thickness and fewer graphene layers, striking a balance between lightweight design and thermal conductivity, making it suitable for applications with extremely stringent weight requirements.

[0060] In terms of mechanical properties, the flexural strength of all embodiments was higher than that of pure resin, thanks to the high-strength shell of silicon carbide hollow microspheres and the reinforcement effect of graphene. The flexural modulus increased with the increase of filler content, reaching 3.9 GPa in Example 4. The mechanical properties of Comparative Examples 1 and 4 were poor; the former lacked the reinforcement of graphene, while the latter had low silica strength and weak interfacial bonding with the matrix.

[0061] Thermal stability tests showed that the 5% thermal weight loss temperature of all examples was higher than that of pure resin, increasing by 30 to 37 degrees Celsius. This is because the addition of fillers increased the heat capacity of the composite material and hindered the diffusion of thermal degradation products. The highest Td5 of Example 4 reached 387 degrees Celsius, attributed to the largest filler addition and the most complete thermally conductive network, which accelerated heat transfer and reduced local overheating.

[0062] In thermal cycling reliability testing, all examples maintained a thermal conductivity retention rate of over 94% after 500 shocks ranging from -55°C to +125°C, with Examples 4 and 5 reaching 96%, demonstrating excellent thermal cycling stability. This is attributed to the surface modification of the silane coupling agent establishing chemical bonds between the filler and the matrix, making the interface less prone to debonding under thermal stress. Comparative Example 1 showed a retention rate of only 89% because the lack of graphene coating meant the interfacial bonding between silicon carbide and resin relied entirely on physical adsorption and silanization, making it prone to failure during thermal cycling. Comparative Example 4 had the lowest retention rate at 85%, indicating a more fragile interface between silicon dioxide and resin.

[0063] The mechanism by which this invention achieves superior performance involves synergistic effects at multiple levels. From a molecular perspective, the sp... 2 Hybridized carbon atoms form a perfect two-dimensional honeycomb lattice with a carbon-carbon bond length of only 0.142 nanometers and a bond energy as high as 518 kilojoules per mole. This strong covalent bond network provides an extremely low scattering channel for phonon transport. The group velocity of phonons in the graphene lattice can reach 21 kilometers per second, and the mean free path is 775 nanometers, far exceeding the several nanometers to tens of nanometers of ordinary materials. When 2 to 5 layers of graphene are coated on the surface of silicon carbide hollow microspheres, a continuous two-dimensional thermally conductive channel is formed. Adjacent microspheres achieve rapid phonon transport through van der Waals contacts or direct overlap of the graphene layers, avoiding the high interfacial thermal resistance caused by point contacts between traditional filler particles.

[0064] Silicon carbide, as the framework material for hollow microspheres, is also an excellent thermally conductive ceramic, with a thermal conductivity of 120 W / m Kelvin. More importantly, silicon carbide and graphene are both carbon-silicon systems, sharing a certain degree of lattice constant matching. While the lattice parameter of β-silicon carbide is 0.436 nm, differing from graphene's 0.246 nm, the presence of a nickel catalyst layer during chemical vapor deposition mitigates the lattice mismatch, allowing graphene to grow epitaxially on the silicon carbide surface and form a tightly bonded interface. The continuous arrangement of carbon atoms at the interface reduces phonon scattering during cross-interface transport, resulting in an interfacial thermal conductivity of 4.7 × 10⁻⁶. 7 The Kelvin per square meter is tens of times that of a typical filler-matrix interface.

[0065] The hollow structure design is based on a delicate balance between lightweight and thermal conductivity. Increasing the hollowness directly reduces the filler density, but excessively high hollowness leads to excessively thin walls, insufficient mechanical strength, and easy breakage during composite material processing. This invention controls the hollowness between 60% and 75% and the wall thickness between 1.0 and 2.2 micrometers, a range optimized through extensive experimentation. At a wall thickness of 1.0 micrometer, the hollowness can reach 75%, and the density drops to 0.8 g / cm³, but the strength is lower, suitable for low-stress applications. At a wall thickness of 2.2 micrometers, the strength is significantly improved, capable of withstanding strong shearing processes such as three-roll milling, while maintaining a hollowness of 60% and a density of 1.2 g / cm³, achieving the optimal balance between strength and lightweight. The key lies in the high strength of silicon carbide ceramic itself, with a flexural strength of up to 400 MPa, ensuring sufficient load-bearing capacity even in the formation of a thin-walled hollow structure.

[0066] The multi-scale composite of synergistic fillers constructs a three-dimensional thermally conductive network. Vapor-grown carbon fibers, with aspect ratios ranging from 67 to 133, can form a percolation network in the filler system with minimal addition. At 2%, the percolation threshold is approached, and at 3% to 5%, the percolation network is fully developed. The one-dimensional structure of the carbon fibers spans multiple graphene-coated microspheres, connecting the originally dispersed two-dimensional thermally conductive islands into a three-dimensional thermally conductive continent. Phonons can be transported long distances along the graphite lattice of the carbon fibers without frequent interface crossings. Although silver nanoparticles are zero-dimensional, their excellent thermal conductivity and good deformability make them ideal interstitial fillers. Silver has a thermal conductivity of 429 W / m Kelvin and, as a metal, provides additional channels for electronic heat transfer. The nano-sized silver particles can slightly deform under pressure during composite curing, increasing the contact area with the filler and matrix, forming good thermal contact. The oleic acid modification layer, only about 2 nm thick, prevents the oxidation and agglomeration of silver particles without significantly increasing interfacial thermal resistance.

[0067] Interfacial modification with silane coupling agents improves the compatibility between fillers and the matrix at the molecular level. The three ethoxy groups of γ-aminopropyltriethoxysilane, after hydrolysis, generate silanol groups, which undergo condensation reactions with carboxyl and hydroxyl groups at the edges and defects of graphene, forming Si-OC covalent bonds and firmly anchoring the silane molecules to the graphene surface. The aminopropyl groups face the resin side; during epoxy resin curing, the epoxy groups undergo ring-opening addition reactions with amino groups to generate hydroxyl-amino bonds, establishing a chemical bridge between the filler, silane, and resin. This bidirectional chemical bonding transforms the interface from simple physical adsorption to chemical connection, increasing the interfacial bonding strength by 3 to 5 times and reducing the interfacial thermal conductivity from the typical 2 × 10⁻⁶. 6 The number of tiles per square meter of Kelvin has been increased to 8×10. 6 The thermal conductivity is above Kelvin per square meter. During thermal cycling, the chemically bonded interfaces can withstand greater thermal stress without debonding, ensuring the long-term stability of thermal conductivity.

[0068] By combining the above multi-level mechanisms, this invention achieves an organic unity of material structure design, interface engineering, and multi-scale synergy, and has achieved excellent levels in multiple dimensions such as thermal conductivity, lightweight, mechanical properties, and thermal stability, providing a revolutionary material solution for the thermal management of electronic devices.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A graphene-coated silicon carbide hollow microsphere composite thermally conductive filler, characterized in that, The composite thermally conductive filler comprises hollow silicon carbide microspheres and a graphene layer uniformly coated on the surface of the hollow silicon carbide microspheres. The hollow silicon carbide microspheres have a particle size of 15 to 30 micrometers, a wall thickness of 1.0 to 2.2 micrometers, and a hollowness of 60% to 75%. The graphene layer has 2 to 5 layers. The density of the composite thermally conductive filler is 0.8 to 1.2 g per cubic centimeter. The hollow silicon carbide microspheres have a β-phase cubic crystal structure, a grain size of 50 to 80 nanometers, and a shell density of over 95%.

2. The graphene-coated silicon carbide hollow microsphere composite thermally conductive filler according to claim 1, characterized in that, It also includes synergistic fillers, which include vapor-grown carbon fibers and silver nanoparticles; the vapor-grown carbon fibers have a diameter of 150 nanometers, a length of 10 to 20 micrometers, a thermal conductivity of 1950 W / m Kelvin, and are added at 2% to 5% of the mass of graphene-coated silicon carbide hollow microspheres; the silver nanoparticles have a particle size of 50 to 80 nanometers, are surface-modified with oleic acid, and are added at 0.5% to 1.5% of the mass of graphene-coated silicon carbide hollow microspheres.

3. The graphene-coated silicon carbide hollow microsphere composite thermally conductive filler according to claim 1, characterized in that, The surface of the graphene layer is modified with a silane coupling agent, which is γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane. The amount of silane coupling agent used is 1% to 3% of the mass of the graphene-coated silicon carbide hollow microspheres. The alkoxy groups of the silane coupling agent form Si-OC covalent bonds with the oxygen-containing groups on the graphene surface.

4. The graphene-coated silicon carbide hollow microsphere composite thermally conductive filler according to claim 1, characterized in that, The silicon carbide hollow microspheres are designed with a bimodal particle size distribution. The particle size of the large particle size component ranges from 25 to 30 micrometers and accounts for 60% to 70% of the total mass of the silicon carbide hollow microspheres. The particle size of the small particle size component ranges from 15 to 20 micrometers and accounts for 30% to 40% of the total mass of the silicon carbide hollow microspheres.

5. The graphene-coated silicon carbide hollow microsphere composite thermally conductive filler according to claim 1, characterized in that, The graphene coating layer accounts for 3% to 8% of the total mass of the composite thermally conductive filler, and the silicon carbide hollow microspheres account for 92% to 97% of the total mass of the composite thermally conductive filler. The mass fraction of graphene is determined based on theoretical calculations and measured carbon content.

6. A method for preparing a graphene-coated silicon carbide hollow microsphere composite thermally conductive filler according to any one of claims 1 to 5, characterized in that, Includes the following steps: The first step is to prepare hollow silicon carbide microspheres: Polycarbosilane with a molecular weight of 1000 to 3000 is dissolved in tetrahydrofuran to prepare a solution with a mass concentration of 15% to 25%. A polycarbosilane coating is formed on the surface of a porous silica microsphere template by spray drying. The porous silica microsphere template has a diameter of 20 to 35 micrometers and a porosity of 40% to 55%. The atomizer speed of the spray drying process is 18000 to 22000 rpm, the inlet air temperature is 180 to 220 degrees Celsius, and the number of sprays is 2 to 4 to control the coating thickness. The coated microspheres were subjected to oxidative crosslinking treatment at 200°C for 2 to 4 hours, followed by programmed temperature pyrolysis under argon protection. The temperature was increased to 800°C at a rate of 2°C per minute and held for 1 hour to complete pre-ceramization. The temperature was then increased to 1450 to 1550°C and held for 2 to 3 hours to complete the full ceramization transformation. The argon flow rate was maintained at 200 to 300 mL per minute. The silica template was removed by stirring at 60°C for 12 hours with a 40% hydrofluoric acid solution. After repeated washing with deionized water and anhydrous ethanol, the microspheres were vacuum dried at 80°C for 24 hours to obtain hollow silicon carbide microspheres. The second step is to deposit a nickel catalyst layer: The hollow silicon carbide microspheres are placed in a fluidized bed and bombarded with argon plasma with a power of 200 watts and a pressure of 50 Pa for 5 minutes to activate the surface. Then, a nickel catalyst layer with a thickness of 2 to 5 nanometers is deposited on the surface of the microspheres by magnetron sputtering. The magnetron sputtering target power is 80 watts, the argon flow rate is 30 standard cubic centimeters per minute, the working pressure is 0.8 Pa, and the deposition time is adjusted to 15 minutes per gram of microspheres according to the microsphere loading amount. The third step is the chemical vapor deposition (CVD) growth of graphene: Silicon carbide microspheres treated with a catalyst layer are placed in a quartz boat and pushed into the central isothermal zone of a vertical hot-wall CVD reactor. The temperature is raised to 650 degrees Celsius and held for 20 minutes in a hydrogen atmosphere with a flow rate of 100 standard cubic centimeters per minute, allowing the nickel catalyst layer to form nanocrystals and active sites. A mixture of carbon source gas methane and hydrogen is introduced, with a volume ratio of methane to hydrogen of 1:4 and a total flow rate of 200 standard cubic centimeters per minute. The reaction temperature is controlled at 680 to 720 degrees Celsius, and the growth time is 20 to 40 minutes. Simultaneously, a 150-watt radio frequency plasma is applied to assist in the controlled growth of 2 to 5 layers of graphene. After growth, the graphene is cooled to below 400 degrees Celsius in hydrogen and removed from the reactor.

7. The preparation method according to claim 6, characterized in that, The third step is followed by a surface functionalization modification step: the prepared graphene-coated silicon carbide hollow microspheres are dispersed in anhydrous ethanol with a solid content of 5% to 10%, and γ-aminopropyltriethoxysilane or γ-glycidoxypropyltrimethoxysilane is added in an amount of 1% to 3% of the mass of the microspheres. The mixture is ultrasonically treated at 65 degrees Celsius for 30 minutes to promote the hydrolysis and condensation of silanes. Then, the mixture is refluxed for 4 hours to completely construct the siloxane network on the graphene surface. After centrifugation, washing, and drying, the surface-functionalized composite filler is obtained.

8. The preparation method according to claim 6, characterized in that, The thickness of the polycarbosilane coating is precisely controlled by the number of sprays: 2 sprays correspond to a wall thickness of 1.0 micrometer, 3 sprays correspond to a wall thickness of 1.5 micrometer, and 4 sprays correspond to a wall thickness of 2.2 micrometer. The heating rate of the pyrolysis process must be controlled at 2 degrees Celsius per minute to prevent severe weight loss that could cause the shell to crack.

9. The preparation method according to claim 6, characterized in that, The prepared graphene-coated silicon carbide hollow microspheres were subjected to airflow classification at a speed of 3000 to 5000 rpm to separate the filler into a large particle size component of 25 to 30 micrometers and a small particle size component of 15 to 20 micrometers. Before use, the filler was treated in an ultraviolet ozone cleaner for 10 minutes to remove organic pollutants and activate the surface.

10. The application of the graphene-coated silicon carbide hollow microsphere composite thermally conductive filler according to any one of claims 1 to 5 in the preparation of high thermal conductivity composite materials, characterized in that, The composite thermally conductive filler is combined with an epoxy resin or thermoplastic polymer matrix, with a filler content of 40% to 60%, and is used in heat dissipation structural components for avionics, portable electronic devices, or wearable electronic devices.