Graphene-coated silicon carbide hollow microsphere composite heat-conducting filler, preparation method and application thereof

Abstract

The 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 application thereof, which comprises silicon carbide hollow microspheres and a graphene layer uniformly coated on the surfaces of the silicon carbide hollow microspheres, the particle size of the silicon carbide hollow microspheres is 15-30 microns, the wall thickness is 1.0-2.2 microns, the hollow rate is 60%-75%, the number of layers of the graphene layer is 2-5 layers, and the density of the composite heat-conducting filler is 0.8-1.2 g / cm3; the silicon carbide hollow microspheres have a beta-phase cubic crystal structure, the grain size is 50-80 nm, the shell layer density is more than 95%, the density of the silicon carbide hollow microspheres is only 0.8-1.2 g / cm3, which is reduced by 62.5%-75% compared with 3.2 g / cm3 of solid silicon carbide, and the weight of the composite material can be reduced by 38%-45% under the same heat-conducting performance.

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 new type of composite thermal conductive filler with high thermal conductivity and low density has important theoretical value and practical significance for promoting the lightweight and high performance of electronic devices. SUMMARY

[0006] The present application aims to overcome the shortcomings of the prior art, and provides a graphene-coated silicon carbide hollow microsphere composite thermal conductive filler, a preparation method and applications thereof. The composite filler, through innovative hollow structure design and surface graphene coating technology, realizes significant lightweight of the material while maintaining excellent thermal conductivity.

[0007] To achieve the above-mentioned purposes, the technical solutions adopted by the present application are as follows.

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

[0009] Preferably, the composite thermal conductive filler further comprises a synergistic filler, which comprises vapor-grown carbon fibers and silver nanoparticles. The vapor-grown carbon fibers have excellent one-dimensional thermal conductivity characteristics, with a diameter of 150 nanometers and a length of 10 to 20 microns, and a thermal conductivity coefficient as high as 1950 watts per meter kelvin. They play a role in bridging the graphene-coated microspheres in the composite material, constructing a three-dimensional thermal conduction skeleton, and the addition amount is 2% to 5% of the mass of the graphene-coated silicon carbide hollow microspheres. Silver nanoparticles, as zero-dimensional thermal phase, are filled in the interstitial space of the microspheres, effectively reducing the contact thermal resistance between the particles. The particle size is 50 to 80 nanometers, and the surface is modified with oleic acid to prevent agglomeration and oxidation. The addition amount is 0.5% to 1.5% of the mass of the graphene-coated silicon carbide hollow microspheres.

[0010] Further, the surface of the graphene layer is modified by a silane coupling agent, which is γ-aminopropyl triethoxysilane or γ-glycidyl ether oxygen propyl trimethoxysilane, and the amount of the silane coupling agent is 1% to 3% of the mass of the graphene-coated silicon carbide hollow microspheres. The alkoxy group of the silane coupling agent forms a Si-O-C covalent bond with the oxygen-containing group on the surface of the graphene, and the amino group or the epoxy group can chemically react with the polymer matrix. This bidirectional bonding significantly enhances the interface bonding between the filler and the matrix, reduces the interface thermal resistance, and improves the mechanical properties of the composite material.

[0011] In a preferred embodiment, the silicon carbide hollow microspheres have a bimodal particle size distribution design. The particle size of the large particle size component is in the range of 25 to 30 microns, and the large particle size component accounts for 60% to 70% of the total mass of the silicon carbide hollow microspheres. The particle size of the small particle size component is in the range of 15 to 20 microns, and the small particle size component accounts for 30% to 40% of the total mass of the silicon carbide hollow microspheres. This bimodal distribution design is based on the particle packing theory. The small particle size microspheres are filled in the gaps of the large particle size microspheres, which improves the overall packing density of the filler and reduces the porosity, thereby obtaining a higher thermal conductivity at the same filler content.

[0012] The application also provides a preparation method of the graphene-coated silicon carbide hollow microsphere composite thermal conductive filler described above. The method includes three core steps.

[0013] The first step is to prepare the silicon carbide hollow microspheres. Polycarbosilane with molecular weight of 1000 to 3000 is dissolved in tetrahydrofuran to prepare a solution with mass concentration of 15% to 25%, and a polycarbosilane coating is formed on the surface of porous silica microsphere templates by a spray drying process. The selection of the porous silica microsphere template is crucial, and its diameter should be 20 to 35 microns to ensure that the final silicon carbide hollow microspheres meet the particle size requirements, and the 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 18000 to 22000 revolutions per minute to produce uniform droplets, the inlet air temperature is 180 to 220 degrees Celsius to quickly evaporate the solvent while avoiding premature crosslinking of the polycarbosilane, and the spraying frequency is 2 to 4 times to accurately control the coating thickness to obtain the ideal wall thickness. The coated microspheres are subjected to oxidative crosslinking treatment at 200 degrees Celsius for 2 to 4 hours to form a crosslinked network between the polycarbosilane molecular chains, improving the shape retention ability during the subsequent pyrolysis process. The pyrolysis process is carried out under argon protection with programmed temperature rising, and the temperature is slowly raised to 800 degrees Celsius at a rate of 2 degrees Celsius per minute and held for 1 hour to complete the pre-ceramic process. At this stage, the organic groups in the polycarbosilane gradually decompose, and the silicon-carbon skeleton begins to form. Continue to heat to 1450 to 1550 degrees Celsius and hold for 2 to 3 hours to complete the complete ceramic transformation, at which time the silicon carbide crystals grow fully, forming a high-density ceramic shell layer. The argon flow rate is maintained at 200 to 300 milliliters per minute to avoid oxidation. Use a 40% mass fraction hydrofluoric acid solution to remove the silica template at 60 degrees Celsius for 12 hours, and then wash repeatedly with deionized water and anhydrous ethanol, and then vacuum dry at 80 degrees Celsius for 24 hours to obtain silicon carbide hollow microspheres with precise wall thickness and hollow rate.

[0014] The second step is to deposit a nickel catalyst layer. Silicon carbide has high surface inertness and lacks the catalytic activity required for graphene growth, so a thin layer of metal catalyst needs to be deposited in advance. The silicon carbide hollow microspheres are placed in a fluidized bed, and the design of the fluidized bed ensures that the microspheres are suspended and rolled in the gas flow, achieving uniform treatment of all surfaces. First, use argon plasma with a power of 200 watts and a pressure of 50 pascals to bombard for 5 minutes to activate the surface. High-energy particles in the plasma bombard to remove organic contaminants on the surface and create dangling bonds, enhancing surface activity. Subsequently, a 2 to 5 nanometer thick nickel catalyst layer is deposited on the surface of the microspheres by magnetron sputtering. The thickness of the nickel layer needs to be precisely controlled, too thin and the catalytic activity will be insufficient, too thick and it will easily agglomerate into large particles and lose its catalytic effect during the subsequent heating process. The process parameters of magnetron sputtering include a target material power of 80 watts, an argon flow rate of 30 standard cubic centimeters per minute, a working pressure of 0.8 pascals, and a deposition time of 15 minutes per gram of microspheres adjusted according to the amount of microsphere loading.

[0015] The third step is chemical vapor deposition growth of graphene. The carbonized silicon microspheres treated by the catalytic layer are placed in a quartz boat and pushed into the center constant temperature zone of a vertical hot-wall chemical vapor deposition reactor. The growth of graphene uses low-temperature plasma enhanced chemical vapor deposition technology, which is one of the key innovations of the present application. In a hydrogen atmosphere with a flow rate of 100 standard cubic centimeters per minute, the temperature is raised to 650 degrees Celsius and kept for 20 minutes. In this stage, hydrogen reduces the surface oxides, and the nickel catalytic layer recrystallizes to form nanocrystalline grains and active sites, providing active centers for the adsorption and diffusion of carbon atoms. Carbon source gas methane is introduced into the mixed gas with hydrogen, and the volume ratio of methane to hydrogen is 1 to 4, and the total flow rate is 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 at 680 to 720 degrees Celsius, which is much lower than the traditional chemical vapor deposition temperature of more than 1000 degrees Celsius, avoiding the collapse of the hollow microspheres due to thermal stress. The growth time is 20 to 40 minutes, and by accurately controlling the reaction time, 2 to 5 layers of graphene can be obtained. At the same time, plasma assistance with a radio frequency power of 150 watts is applied. The active particles in the plasma reduce the decomposition temperature of methane, increase the diffusion rate of carbon atoms, and promote the formation of high-quality graphene. After the growth is completed, the temperature is lowered to below 400 degrees Celsius in hydrogen to avoid oxidation of the graphene during cooling.

[0016] In a preferred embodiment of the preparation method, the third step is followed by a surface functionalization modification step. The prepared graphene-coated carbonized silicon hollow microspheres are dispersed in anhydrous ethanol with a solid content of 5% to 10%, and γ-aminopropyl triethoxysilane or γ-glycidyl ether oxypropyl trimethoxysilane is added in an amount of 1% to 3% of the mass of the microspheres. Ultrasonic treatment is carried out at 65 degrees Celsius for 30 minutes. The cavitation effect of ultrasonic waves promotes the uniform dispersion of silane molecules on the surface of graphene, and at the same time induces the hydrolysis and preliminary condensation of silane. Subsequently, reflux reaction is carried out for 4 hours to allow the siloxane network to be fully constructed on the surface of graphene, forming a stable organic-inorganic hybrid layer. After centrifugal separation, washing, and drying, the surface functionalized composite filler is obtained.

[0017] The present application also provides the use of the above-mentioned graphene-coated carbonized silicon hollow microsphere composite thermal conductive filler in the preparation of high thermal conductive composite materials. The composite thermal conductive filler is compounded with an epoxy resin or a thermoplastic polymer matrix, and the filler addition amount is 40% to 60%. In the epoxy resin system, the thermal conductivity reaches 5.8 watts per meter kelvin when the addition amount is 40%, and the thermal conductivity reaches 8.2 watts per meter kelvin when the addition amount is 60%. The density of the composite material is reduced by 38% to 45% compared with the use of solid carbonized silicon filler, and the composite material is applied to the heat dissipation structure of aviation electronic equipment, portable electronic equipment, or wearable electronic equipment.

[0018] The beneficial effects of the present application are as follows.

[0019] Firstly, the present application realizes the lightweight of the thermal conductive filler through the hollow structure design. The density of the silicon carbide hollow microspheres is only 0.8 to 1.2 grams per cubic centimeter, which is reduced by 62.5% to 75% compared with 3.2 grams per cubic centimeter of solid silicon carbide. The weight of the composite material can be reduced by 38% to 45% under the same thermal conductivity, which greatly expands the application range of the thermal conductive material in the weight-sensitive field such as aerospace. The wall thickness of the hollow structure is carefully optimized to be in the range of 1.0 to 2.2 microns, which not only ensures the mechanical strength of the microspheres, but also maximizes the hollow rate, achieving the best balance between strength and density.

[0020] Secondly, the graphene coating layer constructs a high-efficiency two-dimensional thermal conduction network. Graphene has an extremely high intrinsic thermal conductivity, and its sp 2 The hybrid carbon atomic lattice provides an ideal channel for phonon transmission, with an average free path of 775 nanometers. Two to five layers of graphene form a continuous coating layer on the surface of the silicon carbide microspheres, and adjacent microspheres are in direct contact or close coupling through the graphene layer. Phonons can efficiently transfer in the graphene network without passing through high thermal resistance interfaces. This is completely different from the point contact between traditional filler particles, greatly reducing the contact thermal resistance and improving the overall thermal conductivity of the composite material. In the epoxy resin system, a 40% addition amount can achieve a thermal conductivity of 5.8 watts per meter kelvin, and a 60% addition amount can reach 8.2 watts per meter kelvin, which is significantly better than similar products.

[0021] Thirdly, the preparation method of the present application has good controllability and repeatability. The polymer precursor conversion method combined with the template sacrifice technology provides precise structural control means for the preparation of silicon carbide hollow microspheres. By adjusting the spraying frequency and pyrolysis process, the wall thickness and hollow rate can be accurately controlled. The low-temperature plasma enhanced chemical vapor deposition technology breaks through the problem of damage to the hollow structure in traditional high-temperature processes, and realizes the controllable growth of high-quality graphene at a temperature of 680 to 720 degrees Celsius. The number of graphene layers can be accurately controlled by the growth time. The thickness of the nickel catalytic layer deposited by magnetron sputtering is uniform, ensuring uniform coating of graphene on the surface of the microspheres.

[0022] In addition, the compounding of the synergistic filler further optimizes the construction of the thermal conduction network. Vapor grown carbon fibers form a bridging structure between the graphene coated microspheres, expanding the two-dimensional thermal conduction network to a three-dimensional thermal skeleton, significantly improving the continuity and effectiveness of the thermal conduction path. Silver nanoparticles are filled in the interstitial space between the microspheres, utilizing their excellent thermal conductivity and deformation ability to reduce the contact thermal resistance between the particles. This multi-scale synergistic construction of zero-dimensional, one-dimensional, two-dimensional, and three-dimensional fillers constructs a high-efficiency composite thermal conduction network, achieving a synergistic effect of 1 plus 1 greater than 2.

[0023] The surface modification of silane coupling agent significantly improves the interface compatibility between filler and matrix. The bifunctional structure of silane molecule establishes a chemical bridge between the filler surface and the polymer matrix, and the interface changes from simple physical contact to chemical bonding. The interface thermal resistance decreases from 8.5 × 10⁻ 8 square meters kelvin per watt to 2.1 × 10⁻ 8 square meters kelvin, with a decrease of 75%. At the same time, the chemical bonding of the interface also improves the mechanical properties and long-term stability of the composite material, and the filler is not easy to debond and aggregate during thermal cycling. DETAILED DESCRIPTION

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

[0025] Example 1

[0026] In this embodiment, a graphene-coated silicon carbide hollow microsphere composite thermal conductive filler with a particle size of 15 microns and a wall thickness of 1.0 microns is prepared, and the number of graphene layers is 2.

[0027] The preparation method includes the following steps. First, prepare the silicon carbide hollow microspheres, select polycarbosilane with a molecular weight of 1000 to be dissolved in tetrahydrofuran to prepare a solution with a mass concentration of 15%. The porous silica microsphere template with a diameter of 20 microns and a porosity of 40% is suspended in deionized water to prepare a 10% suspension. The polycarbosilane is coated on the surface of the template by spray drying, the rotation speed of the atomizer is 18000 revolutions per minute, the inlet temperature is 180 degrees Celsius, and the coating is performed twice to obtain a thinner coating. The coated composite microspheres are oxidized and crosslinked in a muffle furnace at 200 degrees Celsius for 2 hours. Crosslinking forms Si-O-Si and Si-C-Si bridges between polycarbosilane molecular chains, improving thermal stability. The crosslinked microspheres are transferred to a tube furnace, heated to 800 degrees Celsius at a rate of 2 degrees Celsius per minute under argon protection, and held for 1 hour to complete pre-ceramization. In this stage, organic groups are gradually removed, and an inorganic skeleton is initially formed. Continue to heat to 1450 degrees Celsius and hold for 2 hours to complete complete ceramization, and β-silicon carbide crystal phase grows fully. The argon flow rate during the entire pyrolysis process is maintained at 200 milliliters per minute. After cooling, the product is placed in a polytetrafluoroethylene beaker, a 40% mass fraction hydrofluoric acid solution is added, and stirring is performed in a 60 degrees Celsius water bath for 12 hours. The silica template is completely dissolved. The product is washed with deionized water until neutral, then washed with anhydrous ethanol three times, and dried in a 80 degrees Celsius vacuum oven for 24 hours to obtain silicon carbide hollow microspheres with a particle size of about 15 microns, a wall thickness of about 1.0 microns, a hollow rate of about 60%, and a shell layer density of more than 95%.

[0028] Secondly, a nickel catalytic layer is deposited. 5 grams of hollow silicon carbide microspheres are loaded into a fluidized bed reactor, and argon is introduced to make the microspheres in a fluidized state. A radio frequency power supply is turned on to generate argon plasma, with a power of 200 watts, a pressure of 50 pascals, and a treatment time of 5 minutes to activate the surface of the microspheres, remove adsorbed organic matter, and produce active sites. After the plasma is turned off, magnetron sputtering is immediately performed, using a nickel target with a purity of 99.99%, with a target power of 80 watts, an argon flow rate of 30 standard cubic centimeters per minute, and a working pressure of 0.8 pascals. The fluidized bed is continuously stirred to ensure that the microspheres receive uniform sputtering on all sides. The deposition time is 75 minutes, and a uniform nickel catalytic layer with a thickness of about 2 nanometers is formed on the surface of the microspheres.

[0029] Then, chemical vapor deposition is performed to grow graphene. The nickel-coated silicon carbide microspheres are uniformly spread in a quartz boat and pushed into the constant temperature zone of a vertical hot-wall chemical vapor deposition furnace. High-purity hydrogen gas with a flow rate of 100 standard cubic centimeters per minute is first introduced, and the temperature is raised to 650 degrees Celsius at a rate of 5 degrees Celsius per minute and held for 20 minutes to reduce the surface oxides and recrystallize the nickel layer into nanocrystalline grains. After the temperature is stabilized, carbon source gas is introduced, with a methane flow rate of 40 standard cubic centimeters per minute and a hydrogen flow rate of 160 standard cubic centimeters per minute, for a volume ratio of 1 to 4. At the same time, the temperature is raised to 680 degrees Celsius, and a plasma auxiliary with a power of 150 watts is applied by turning on the radio frequency power supply. The graphene growth time is controlled to be 20 minutes. Under the catalytic action of nickel and the activation of plasma, carbon atoms produced by the decomposition of methane are adsorbed, diffused, and orderly arranged on the surface of the microspheres to form 2 layers of high-quality graphene. After the growth is completed, the methane is turned off, the hydrogen atmosphere is maintained, the temperature is lowered to below 400 degrees Celsius, and the furnace is taken out to obtain graphene-coated silicon carbide hollow microspheres.

[0030] Finally, surface functionalization and modification are performed. 10 grams of graphene-coated silicon carbide hollow microspheres are dispersed in 100 milliliters of anhydrous ethanol, and ultrasonic dispersion is performed for 10 minutes. 0.1 grams of γ-aminopropyl triethoxysilane is added, with a dosage of 1% of the mass of the microspheres. Ultrasonic treatment is performed in an oil bath at 65 degrees Celsius for 30 minutes, and the silane is hydrolyzed and initially condensed. It is transferred to a three-necked flask equipped with a reflux condenser, and refluxed at 65 degrees Celsius for 4 hours. The siloxane network is completely constructed on the surface of the graphene, the ethoxy groups of the silane react with the carboxyl and hydroxyl groups on the edges of the graphene to form Si-O-C bonds, and the aminopropyl groups face the solution side. After the reaction is completed, centrifugal separation is performed, and the unreacted silane is removed by washing three times with anhydrous ethanol. Vacuum drying is performed at 80 degrees Celsius for 12 hours to obtain surface-functionalized graphene-coated silicon carbide hollow microsphere composite heat-conducting fillers. The silicon carbide hollow microspheres in the fillers account for 97%, and the graphene coating layer accounts for 3%. The overall density is 0.8 grams per cubic centimeter.

[0031] Example 2

[0032] The graphene-coated silicon carbide hollow microsphere composite thermal conductive filler with a particle size of 20 microns and a wall thickness of 1.5 microns is prepared, the number of graphene layers is 3, and a synergistic filler is added.

[0033] The silicon carbide hollow microspheres are prepared by using polycarbosilane with a molecular weight of 2000 to prepare a tetrahydrofuran solution with a mass concentration of 20%. Porous silica microspheres with a diameter of 25 microns and a porosity of 48% are selected, the atomizer speed of the spray drying is 20000 revolutions per minute, the inlet air temperature is 200 degrees Celsius, and the spray is performed 3 times to obtain a medium thickness coating. Oxidation and crosslinking are performed at 200 degrees Celsius for 3 hours. The pyrolysis process is to preceramic at 800 degrees Celsius for 1 hour under argon protection, and to complete the ceramization at 1500 degrees Celsius for 2.5 hours, with an argon flow rate of 250 milliliters per minute. The template removal, washing and drying steps are the same as in Example 1, and silicon carbide hollow microspheres with a particle size of about 20 microns, a wall thickness of about 1.5 microns and a hollow rate of about 68% are obtained.

[0034] The nickel catalytic layer deposition uses the same plasma activation parameters as in Example 1, the magnetron sputtering deposition time is adjusted to 15 minutes per gram of microspheres, and a nickel layer with a thickness of about 3 nanometers is formed. The graphene growth temperature is increased to 700 degrees Celsius, the methane to hydrogen flow ratio is kept at 1 to 4, and the growth time is extended to 30 minutes to obtain 3 layers of graphene coating. Surface functionalization uses γ-glycidoxypropyltrimethoxysilane, the amount is 2% of the mass of the microspheres, and the treatment method is the same as in Example 1.

[0035] The synergistic filler is compounded in the graphene-coated silicon carbide hollow microspheres after functionalization. 100 grams of main filler is weighed, 3 grams of vapor grown carbon fiber is added, the carbon fiber has a diameter of 150 nanometers, a length of 15 microns, and a thermal conductivity of 1950 watts per meter kelvin, and is produced by Showa Denko K.K. 1 gram of silver nanoparticles with the surface modified by oleic acid is also added, the particle size is 65 nanometers, and is purchased from Shanghai Aladdin Biochem Technology Co., Ltd. The mixture is mixed in a V-type mixer for 30 minutes to ensure uniform dispersion of the synergistic filler and the main filler, and a composite thermal conductive filler is obtained, with a bulk density of 0.9 grams per cubic centimeter. The carbon fiber forms a bridging structure to connect adjacent graphene-coated microspheres in the filler system, and the silver nanoparticles fill the gaps to reduce the thermal contact resistance, and the synergistic effect of the multi-scale filler constructs an efficient three-dimensional thermal conduction network.

[0036] Example 3

[0037] The graphene-coated silicon carbide hollow microsphere composite thermal conductive filler with a particle size of 25 microns and a wall thickness of 2.0 microns is prepared, the number of graphene layers is 4, and a bimodal particle size distribution design is used.

[0038] First, two kinds of silicon carbide hollow microspheres with different particle sizes were prepared. The large particle size component used a silica template with a diameter of 30 microns, a polycarbosilane solution with a molecular weight of 2500 was prepared with a mass concentration of 22%, the spray dryer rotation speed was 21000 revolutions per minute, the inlet air temperature was 210 degrees Celsius, the spray was 4 times, the oxidation crosslinking was 3.5 hours, the pyrolysis was heated to 1520 degrees Celsius for 2.8 hours, and the large microspheres with a particle size of about 25 microns and a wall thickness of about 2.0 microns were obtained. The small particle size component used a silica template with a diameter of 22 microns, the polycarbosilane solution concentration was 18%, the spray was 3 times, and the pyrolysis temperature was 1480 degrees Celsius, and the small microspheres with a particle size of about 18 microns and a wall thickness of about 1.6 microns were obtained.

[0039] The large and small microspheres were subjected to nickel catalytic layer deposition, and the nickel layer thickness was 4 nanometers. The graphene growth temperature was 710 degrees Celsius, and the growth time was 35 minutes, and 4 layers of graphene were obtained. The surface functionalization used γ-aminopropyl triethoxysilane, and the amount was 2.5%. The large particle size component and the small particle size component were mixed in a mass ratio of 7 to 3 to achieve a bimodal particle size distribution, and the small microspheres were filled in the gaps of the large microspheres to improve the packing density of the filler. Add 3.5% of vapor grown carbon fiber and 1.2% of silver nanoparticles as synergistic fillers, and mix in a V-type mixer for 40 minutes to obtain a composite thermal conductive filler with a density of 1.0 grams per cubic centimeter. The bimodal distribution design increases the effective filling rate of the filler in the composite material, reduces the porosity, and makes the thermal conduction path more continuous.

[0040] Example 4

[0041] In this example, a graphene-coated silicon carbide hollow microsphere composite thermal conductive filler with a particle size of 30 microns and a wall thickness of 2.2 microns was prepared, the number of graphene layers was 5, and the amount of synergistic filler added reached the upper limit.

[0042] The silicon carbide hollow microspheres used a silica template with a diameter of 35 microns and a porosity of 55%, a polycarbosilane solution with a molecular weight of 3000 was prepared with a mass concentration of 25%, the spray dryer rotation speed was 22000 revolutions per minute, the inlet air temperature was 220 degrees Celsius, and the spray was 4 times to obtain the maximum wall thickness. Oxidation crosslinking was carried out at 200 degrees Celsius for 4 hours. The pyrolysis was heated to 1550 degrees Celsius for 3 hours, and the argon flow rate was 300 milliliters per minute, and the silicon carbide hollow microspheres with a particle size of about 30 microns and a wall thickness of about 2.2 microns were obtained. The hollow rate was about 75%, and the increase in shell thickness improved the mechanical strength and thermal stability of the microspheres.

[0043] The thickness of the nickel catalytic layer is 5 nanometers, close to the upper limit, which provides sufficient catalytic active sites. The graphene growth temperature is 720 degrees Celsius, and the growth time is extended to 40 minutes to obtain 5 layers of graphene, which is the maximum number of layers within the scope of the application, further improving the integrity of the heat conduction network. The surface functionalization uses γ-glycidyl ether propyl trimethoxysilane, and the amount is 3%, reaching the upper limit, which maximizes the improvement of the interface compatibility.

[0044] In the functionalized primary filler, 5% of vapor grown carbon fiber and 1.5% of silver nanoparticles are added, and the amount of the added filler reaches the upper limit. The aspect ratio of the vapor grown carbon fiber is as high as 133, forming a dense bridging network in the filler system, significantly improving the thermal conductivity continuity between the filler particles. The high amount of silver nanoparticles ensures that all the gaps between the microspheres are effectively filled, and the contact thermal resistance is minimized. The overall density of the composite thermal filler is 1.2 grams per cubic centimeter, which is the upper limit within the scope of the application, but is still much lower than the density of the solid filler, achieving lightweight while ensuring the highest thermal conductivity performance.

[0045] Example 5

[0046] This example prepares a graphene-coated silicon carbide hollow microsphere composite thermal filler for epoxy resin composites, using intermediate parameter values, preparing a complete composite material and testing its performance.

[0047] The silicon carbide hollow microspheres have a bimodal distribution, with large particle sizes of 27 microns accounting for 65% and small particle sizes of 17 microns accounting for 35%. The polycarbosilane has a molecular weight of 2200, a solution concentration of 21%, and a spray drying atomizer speed of 20500 revolutions per minute. The inlet air temperature is 205 degrees Celsius. The large microspheres are sprayed 4 times to obtain a wall thickness of 1.9 microns, and the small microspheres are sprayed 3 times to obtain a wall thickness of 1.4 microns. The pyrolysis temperature is 1510 degrees Celsius, the holding time is 2.6 hours, and the argon flow rate is 270 milliliters per minute. The thickness of the nickel catalytic layer is 3.5 nanometers, the graphene growth temperature is 705 degrees Celsius, and the time is 32 minutes to obtain 3.5 layers of graphene. The surface functionalization uses γ-aminopropyl triethoxysilane, and the amount is 2.2%. The addition of 4% vapor grown carbon fiber and 1.1% silver nanoparticles results in a composite thermal filler density of 1.05 grams per cubic centimeter.

[0048] An epoxy resin composite was prepared. Bisphenol A type epoxy resin was chosen, with an epoxy value of 0.51 equivalent per 100 grams, and a trade name of E-51, purchased from Bluestar New Material Wuxi Resin Factory. 200 grams of epoxy resin was heated to 80 degrees Celsius until the viscosity dropped to 3000 millipascal seconds, and 3 grams of polyether-modified polydimethylsiloxane dispersant was added, with a polyether value of 700 and a viscosity of 800 centistokes, and a trade name of BYK-333, produced by BYK-Chemie. The dispersant was completely dissolved by high-speed stirring at 1500 revolutions per minute for 10 minutes. The first batch of 40 grams of composite thermal conductive filler was added, and stirred for 15 minutes to fully wet the surface, the second batch of 67 grams was added, and stirred for 20 minutes, and finally the remaining 27 grams was added, and stirred for 30 minutes. The total amount of filler added was 134 grams, accounting for 40% of the mixture. The mixture was transferred to a three-roll mill, with a first pass gap of 50 microns, a second pass gap of 30 microns, and a third pass gap of 15 microns, and a grinding speed of 80 revolutions per minute for the front roller, 120 revolutions per minute for the middle roller, and 180 revolutions per minute for the rear roller, with 4 cycles of grinding. After grinding, the mixture was degassed in a vacuum degassing machine at -0.09 megapascals for 30 minutes to remove air bubbles. 115 grams of methyltetrahydrophthalic anhydride curing agent was added, with a dosage of 0.85 equivalent of epoxy resin, and 1.5 grams of 2-ethyl-4-methylimidazole accelerator, with a dosage of 0.75 phr. The mixture was mixed in a planetary mixer at a revolution speed of 200 revolutions per minute and a rotation speed of 400 revolutions per minute for 10 minutes. The mixture was poured into a mold preheated to 60 degrees Celsius, pre-cured at 80 degrees Celsius for 2 hours, cured at 120 degrees Celsius for 3 hours, post-cured at 150 degrees Celsius for 2 hours, and demolded after cooling to room temperature in the furnace, resulting in a composite material plate with a thickness of 2 millimeters.

[0049] Comparative Example 1

[0050] This comparative example prepared a silicon carbide hollow microsphere filler without graphene coating, to demonstrate the key role of the graphene coating layer.

[0051] The preparation of silicon carbide hollow microspheres was exactly the same as in Example 5, with consistent particle size and wall thickness parameters, but without the nickel catalyst layer deposition and graphene growth steps. After the silicon carbide hollow microspheres were removed from the template with hydrofluoric acid, washed and dried, they were directly treated with a silane coupling agent, with the same dosage and method as in Example 5. Due to the lack of a graphene coating layer, the microsphere surface only has a silanization layer, which cannot form a continuous two-dimensional thermal conduction network. After adding the same synergistic filler as in Example 5, an epoxy resin composite was prepared according to the same process. The design of this comparative example aims to demonstrate the irreplaceability of the graphene coating layer in building an efficient thermal conduction network.

[0052] Comparative Example 2

[0053] This comparative example used solid silicon carbide particles as fillers, with a particle size distribution consistent with the silicon carbide hollow microspheres of Example 5, to compare the lightweight advantage of the hollow structure.

[0054] The solid silicon carbide particles are β phase with a grain size of 60 nm, purchased from Weifang Kaixia Silicon Carbide Powder Co., Ltd. The particles are classified by airflow into two groups, with a large particle size of 27 microns accounting for 65% and a small particle size of 17 microns accounting for 35%, consistent with the bimodal distribution of Example 5. The surface of the solid silicon carbide particles is coated with graphene using the same nickel catalysis and chemical vapor deposition process as in Example 5, growing 3.5 layers of graphene. The surface functionalization and synergistic filler addition are also the same as in Example 5. The epoxy resin composites are prepared according to the same process, but due to the density of the solid silicon carbide being 3.2 grams per cubic centimeter, the overall density of the filler reaches 2.8 grams per cubic centimeter, resulting in a significantly higher density of the composite material than in Example 5. This comparative example demonstrates the key role of hollow structure design in achieving lightweight.

[0055] Comparative Example 3

[0056] This comparative example does not add a synergistic filler, only using graphene-coated silicon carbide hollow microspheres, to verify the synergistic effect of the synergistic filler.

[0057] The preparation of graphene-coated silicon carbide hollow microspheres is 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. The epoxy resin composites are prepared according to the same process, and the filler addition amount is still 40%. Due to the lack of bridging by one-dimensional carbon fibers and gap filling by zero-dimensional silver particles, the thermal conduction path between the filler particles is not continuous enough, and the contact thermal resistance is high, resulting in a lower thermal conductivity of the composite material than in Example 5. This comparative example demonstrates the importance of multi-scale synergistic fillers in constructing a three-dimensional thermal conduction network.

[0058] Comparative Example 4

[0059] This comparative example uses the technical solution disclosed in Comparative Document CN106634047A to prepare a graphene-loaded silica composite filler, to demonstrate the significant progress of the present application.

[0060] Disperse 5 grams of graphene oxide in 50 milliliters of isopropyl alcohol and ultrasonically treat for 30 minutes. Add 2.5 grams of tetraethyl orthosilicate and 25 milliliters of deionized water, and stir to form a sol. Add ammonia water to adjust the pH value to 8.5, and stir at 50 degrees Celsius for 3 hours to hydrolyze and condense the silica precursor on the surface of the graphene to form nanoparticles. The product is washed, dried, and reduced in a nitrogen atmosphere at 500 degrees Celsius for 2 hours to obtain a graphene-loaded silica composite filler. The silica particles in this filler are in discrete distribution and cannot form a continuous thermal conduction network, and are in a solid structure with a density as high as 2.1 grams per cubic centimeter. The epoxy resin composites are prepared according to the same process for performance comparison.

[0061] Systematic performance tests are conducted on the composites prepared in all examples and comparative examples, with the test methods and conditions as follows.

[0062] Thermal conductivity was tested by laser flash method using LFA 467 laser thermal conductivity instrument from NETZSCH. The composite panel was cut into a 12.7 mm diameter and 2 mm thick disc, and the surface was evenly sprayed with a graphite layer to improve the laser absorption rate. The test temperature was 25 degrees Celsius, and each sample was tested 5 times to take the average value. After the thermal diffusivity was measured by the laser flash method, the thermal conductivity was calculated in combination with the material density and specific heat capacity. The density was determined by the drainage method, and the specific heat capacity was determined by differential scanning calorimetry.

[0063] Mechanical property tests included flexural strength and flexural modulus, which were performed according to GB / T 2567-2008, using a CMT5105 electronic universal testing machine from Xin San Si Company in Shenzhen. The sample size was 80 mm x 10 mm x 4 mm, with a span of 64 mm and a loading rate of 2 mm per minute. Impact strength was tested according to GB / T 1043-2008 using a simply supported beam impact method with a notch depth of 2 mm.

[0064] Thermal stability was characterized by thermogravimetric analysis using a Q500 thermogravimetric analyzer from TA Instruments. The sample mass was about 10 mg, and the temperature was raised from room temperature to 600 degrees Celsius at a rate of 10 degrees Celsius per minute under a nitrogen atmosphere. The mass loss curve was recorded to determine the 5% thermal weight loss temperature.

[0065] Thermal reliability was evaluated by cyclic thermal shock testing using a high-low temperature shock test chamber. The sample was cycled between -55 degrees Celsius and +125 degrees Celsius, with each temperature maintained for 30 minutes. The thermal conductivity retention rate was tested after 500 cycles.

[0066] The test results are summarized in Table 1.

[0067] Table 1 Performance test results of composite materials of each example and comparative example

[0068] 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 -

[0069] From the data in Table 1, it can be seen that Examples 1-5 of the present application exhibit significant advantages over Comparative Examples. Example 5 achieves a thermal conductivity of 5.8 W / mK at a 40% filler loading, which is 29 times that of pure epoxy resin, while the composite density is only 1.42 g / cm3. Comparative Example 1, which lacks graphene coating, has a thermal conductivity of only 2.8 W / mK, a 52% reduction, demonstrating that the two-dimensional thermal conduction network constructed by the graphene coating layer is crucial to improving thermal conductivity. Comparative Example 2 uses solid silicon carbide, although the thermal conductivity is close to that of Example 5, the composite density is as high as 2.12 g / cm3, an increase of 49% compared to Example 5, which cannot meet the lightweight requirement. Comparative Example 3 does not add synergistic fillers, and the thermal conductivity decreases to 4.1 W / mK, which is 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 thermal conduction network. Comparative Example 4 uses the technical solution of the comparative document, with a thermal conductivity of only 1.9 W / mK and a density of 1.86 g / cm3, which is significantly inferior to the present application in both thermal conductivity and lightweight.

[0070] Example 4 has the most excellent performance, with a thermal conductivity of 8.2 W / mK at a filler loading of 60%, which is due to the largest wall thickness providing higher mechanical strength, the 5-layer graphene constructing the most complete thermal conduction network, and the synergistic fillers reaching the upper limit to provide the most dense three-dimensional thermal conduction skeleton. Although Example 1 has the lowest filler density of only 0.8 g / cm3, due to the thinner wall thickness and fewer layers of graphene, the thermal conductivity is 4.2 W / mK, achieving a balance between lightweight and thermal performance, making it suitable for applications with extremely stringent weight requirements.

[0071] In terms of mechanical properties, the bending strength of all examples is higher than that of pure resin, which is due to the high strength shell of the hollow microspheres of silicon carbide and the reinforcing effect of graphene. The bending modulus increases with the increase of filler loading, and Example 4 reaches 3.9 GPa. Comparative Examples 1 and 4 have poor mechanical properties, the former lacks the reinforcement of graphene, and the latter has low strength and weak interface bonding with the matrix.

[0072] Thermal stability tests show that the 5% thermal weight loss temperature of all examples is higher than that of pure resin, with an increase of 30 to 37 degrees Celsius, because the addition of fillers increases the heat capacity of the composite and hinders the diffusion of thermal degradation products. The Td5 of Example 4 is as high as 387 degrees Celsius, due to the largest filler loading and the most complete thermal conduction network, which accelerates heat transfer and reduces local overheating.

[0073] In thermal cycle reliability test, all examples maintained thermal conductivity retention rate above 94% after 500 times of -55 to 125 Celsius degree impact, examples 4 and 5 reached 96%, showing excellent thermal cycle stability. This is attributed to the surface modification of silane coupling agent which established chemical bonding between filler and matrix, the interface is not easy to debond under thermal stress. The retention rate of comparative example 1 was only 89%, because of the lack of graphene coating, the interface bonding between silicon carbide and resin completely depends on physical adsorption and silanization, which is easy to fail in thermal cycle. The retention rate of comparative example 4 was the lowest, 85%, the interface between silica and resin is more fragile.

[0074] The mechanism of realizing excellent performance of the present application involves multiple levels of synergies. From the molecular level, the sp 2 Hybrid carbon atoms form a perfect two-dimensional honeycomb lattice, with a carbon-carbon bond length of only 0.142 nanometers and a bond energy of up to 518 kilojoules per mole. This strong covalent bond network provides extremely low scattering channels for phonon transport. The group velocity of phonons in the graphene lattice can reach 21 kilometers per second, and the mean free path is as large as 775 nanometers, much larger than the several nanometers to several tens of nanometers of ordinary materials. When 2 to 5 layers of graphene are coated on the surface of the hollow silicon carbide microspheres, a continuous two-dimensional heat conduction channel is formed, and the adjacent microspheres are in van der Waals contact or directly overlapped through the graphene layer, realizing rapid phonon transfer, avoiding the high interfacial thermal resistance caused by point contact between traditional filler particles.

[0075] Silicon carbide as a hollow microsphere skeleton material is also an excellent heat-conducting ceramic with a thermal conductivity of 120 watts per meter kelvin. More importantly, silicon carbide and graphene are both carbon-silicon systems, and the lattice constants have certain matching properties. The lattice parameter of beta-silicon carbide is 0.436 nanometers, although there is a difference from the 0.246 nanometers of graphene, but in the chemical vapor deposition process, the presence of the nickel catalytic layer relieves the lattice mismatch, allowing graphene to grow epitaxially on the surface of silicon carbide and form a tightly bonded interface. The continuous arrangement of carbon atoms at the interface reduces the scattering of phonons when transferring across the interface, and the interfacial thermal conductivity reaches 4.7 x 10 7 Watts per square meter kelvin, which is tens of times that of the general filler-matrix interface.

[0076] The design of the hollow structure is based on the delicate balance between light weight and heat conduction. The increase of hollow rate directly reduces the density of fillers, but too high hollow rate will lead to too thin wall thickness, insufficient mechanical strength, and easy to break in the process of composite material processing. The hollow rate is controlled in 60% to 75%, and the wall thickness is 1.0 to 2.2 microns, which is optimized through a large number of experiments. When the wall thickness is 1.0 micron, the hollow rate can reach 75%, the density is reduced to 0.8 grams per cubic centimeter, but the strength is low, suitable for low stress application. When the wall thickness is 2.2 microns, the strength is significantly improved, can withstand strong shear processing such as three roll grinding, the hollow rate remains 60%, the density is 1.2 grams per cubic centimeter, and the best balance between strength and light weight is achieved. The key lies in the high strength of silicon carbide ceramics itself, which can reach 400 megapascals, even if it forms a thin-walled hollow structure, it still has enough carrying capacity.

[0077] The multi-scale complex of synergistic fillers constructs a three-dimensional heat conduction network. The aspect ratio of vapor grown carbon fiber is as high as 67 to 133, which can form a percolation network in the filler system with a small amount of addition. When the addition amount reaches 2%, it is close to the percolation threshold, and when it is 3% to 5%, the percolation network is fully developed. The one-dimensional structure of carbon fiber spans multiple graphene coated microspheres, connecting the originally dispersed two-dimensional heat conduction islands into three-dimensional heat conduction continents. Phonons can be transmitted along the graphite lattice of carbon fiber for a long distance without frequently crossing the interface. Although silver nanoparticles are zero-dimensional structures, their excellent heat conduction performance and good deformation ability make them ideal interstitial fillers. The thermal conductivity of silver is 429 watts per meter kelvin, and as a metal, it has an additional channel for electronic heat transfer. Nanoscale silver particles can deform slightly under pressure during the curing of the composite material, increasing the contact area with the filler and the matrix, and forming good thermal contact. The thickness of the oleic acid modification layer is only about 2 nanometers, which prevents the oxidation and agglomeration of silver particles, and does not significantly increase the interfacial thermal resistance.

[0078] The interface modification of silane coupling agent improves the compatibility of fillers and matrix at the molecular level. The three ethoxyl groups of γ-aminopropyl triethoxysilane generate silanol groups after hydrolysis, which condenses with the carboxyl and hydroxyl groups on the edges and defects of graphene, forming Si-O-C covalent bonds, and anchoring the silane molecules firmly on the surface of graphene. The aminopropyl group is directed to the resin side, and during the curing process of the epoxy resin, the epoxy group reacts with the amino group to form a hydroxyl amino bond, establishing a chemical bridge between the filler-silane-resin. This two-way chemical bonding changes the interface from simple physical adsorption to chemical connection, and the interfacial bonding strength is increased by 3 to 5 times, and the interfacial thermal conductivity is improved from the general 2×10 6 to 8×10 6 Watts per square meter kelvin or more. During the thermal cycle process, the chemically bonded interface can withstand greater thermal stress without debonding, ensuring the long-term stability of the heat conduction performance.

[0079] In combination with the above multi-level mechanisms, the application realizes organic unity of material structure design, interface engineering and multi-scale coordination, and achieves excellent levels in multiple dimensions such as heat conduction performance, lightweight, mechanical performance and thermal stability, thereby providing a revolutionary material solution for thermal management of electronic devices.

[0080] The above merely describes preferred embodiments of the application and is not intended to limit the application, and any modifications, equivalent replacements and improvements made within the spirit and principle of the application shall be included in the protection scope of the application.

Claims

1. A graphene-coated silicon carbide hollow microsphere composite heat-conducting filler, characterized in that, The composite thermal conductive filler comprises silicon carbide hollow microspheres and a graphene layer uniformly coated on the surface of the silicon carbide hollow microspheres, the particle size of the silicon carbide hollow microspheres is 15-30 microns, the wall thickness is 1.0-2.2 microns, the hollow rate is 60%-75%, the number of layers of the graphene layer is 2-5 layers, and the density of the composite thermal conductive filler is 0.8-1.2 g / cm3; the silicon carbide hollow microspheres have a beta-phase cubic crystal structure, and the grain size is 50-80 nm, and the shell wall density of the silicon carbide hollow microspheres is more than 95%; The composite thermal conductive filler further comprises a synergistic filler, the synergistic filler comprises vapor-grown carbon fibers and silver nanoparticles; the vapor-grown carbon fibers have a diameter of 150 nm, a length of 10-20 microns, and a thermal conductivity of 1950 W / (m·K), and the addition amount is 2%-5% of the mass of the graphene-coated silicon carbide hollow microspheres; the silver nanoparticles have a particle size of 50-80 nm, and the surface is modified by oleic acid, and the addition amount is 0.5%-1.5% of the mass of the graphene-coated silicon carbide hollow microspheres; The surface of the graphene layer is modified by a silane coupling agent, the silane coupling agent is γ-aminopropyl triethoxysilane or γ-glycidyl ether propyl trimethoxysilane, and the amount of the silane coupling agent is 1%-3% of the mass of the graphene-coated silicon carbide hollow microspheres; the alkoxy group of the silane coupling agent forms a Si-O-C covalent bond with the oxygen-containing group on the surface of the graphene; The silicon carbide hollow microspheres adopt a bimodal particle size distribution design, the particle size of the large particle size component is 25-30 microns, and the large particle size component accounts for 60%-70% of the total mass of the silicon carbide hollow microspheres; the particle size of the small particle size component is 15-20 microns, and the small particle size component accounts for 30%-40% of the total mass of the silicon carbide hollow microspheres.

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

3. A method for preparing the graphene-coated silicon carbide hollow microsphere composite heat-conducting filler according to claim 1 or 2, characterized in that, The method comprises the following steps: In a first step, silicon carbide hollow microspheres are prepared: polycarbosilane with a molecular weight of 1000-3000 is dissolved in tetrahydrofuran to prepare a solution with a mass concentration of 15%-25%, a polycarbosilane coating layer is formed on the surface of a porous silica microsphere template through a spray drying process, the diameter of the porous silica microsphere template is 20-35 microns, the porosity is 40%-55%, the rotation speed of the atomizer of the spray drying process is 18,000-22,000 rpm, the inlet air temperature is 180-220°C, and the spraying frequency is 2-4 times to control the coating thickness; After coating, the microspheres are subjected to oxidative crosslinking treatment at 200 degrees Celsius for 2 to 4 hours, and then subjected to programmed temperature pyrolysis under argon protection, with the temperature raised to 800 degrees Celsius at a rate of 2 degrees Celsius per minute and maintained for 1 hour to complete preceramization, and then the temperature is continuously raised to 1450 to 1550 degrees Celsius and maintained for 2 to 3 hours to complete complete ceramization conversion, with the argon flow maintained at 200 to 300 milliliters per minute; the silica template is removed using a 40% mass fraction hydrofluoric acid solution at 60 degrees Celsius with stirring for 12 hours, and then repeatedly washed with deionized water and anhydrous ethanol, and then vacuum dried at 80 degrees Celsius for 24 hours to obtain silicon carbide hollow microspheres; In the second step, a nickel catalytic layer is deposited: the silicon carbide hollow microspheres are placed in a fluidized bed, the surface is activated by argon plasma bombardment at a power of 200 watts and a pressure of 50 pascals for 5 minutes, and then a nickel catalytic layer with a thickness of 2 to 5 nanometers is deposited on the surface of the microspheres by magnetron sputtering, with the target material power of the magnetron sputtering being 80 watts, the argon flow being 30 standard cubic centimeters per minute, the working pressure being 0.8 pascals, and the deposition time being adjusted to 15 minutes per gram of microspheres according to the microsphere loading; In the third step, graphene is grown by chemical vapor deposition: the silicon carbide microspheres treated with the catalytic layer are placed in a quartz boat and pushed into the center of the constant temperature zone of a vertical hot-wall chemical vapor deposition reactor, the temperature is raised to 650 degrees Celsius under a hydrogen atmosphere with a flow of 100 standard cubic centimeters per minute and maintained for 20 minutes to form nanocrystalline grains and active sites in the nickel catalytic layer; a carbon source gas mixture of methane and hydrogen is introduced, with the volume ratio of methane to hydrogen being 1 to 4 and the total flow being 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, while a plasma auxiliary with a radio frequency power of 150 watts is applied to achieve controllable growth of 2 to 5 layers of graphene; after the growth is completed, the temperature is lowered to below 400 degrees Celsius in hydrogen and the reactor is discharged.

4. The preparation method according to claim 3, characterized in that, After the third step, a surface functionalization modification step is also included: 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-glycidyl ether propyltrimethoxysilane is added, with a dosage of 1% to 3% of the mass of the microspheres, and ultrasonic treatment is performed at 65 degrees Celsius for 30 minutes to promote the hydrolysis and condensation of the silane, and then reflux reaction is performed for 4 hours to completely build a siloxane network on the surface of the graphene, and after centrifugal separation, washing, and drying, a surface-functionalized composite filler is obtained.

5. The preparation method according to claim 3, characterized in that, The thickness of the polycarbosilane coating is precisely controlled by the number of spraying times, with 2 spraying times corresponding to a wall thickness of 1.0 micrometers, 3 spraying times corresponding to a wall thickness of 1.5 micrometers, and 4 spraying times corresponding to a wall thickness of 2.2 micrometers; the heating rate of the pyrolysis process must be controlled at 2 degrees Celsius per minute to prevent cracking of the shell layer due to severe weight loss.

6. The preparation method according to claim 3, characterized in that, The prepared graphene-coated silicon carbide hollow microspheres are subjected to airflow classification treatment, the classification rotation speed is 3000-5000 rpm, the filler is separated into a large particle size component 25-30 μm and a small particle size component 15-20 μm, and is treated in a UV-ozone cleaning machine for 10 minutes before use to remove organic contaminants and activate the surface.

7. The use of the graphene-coated silicon carbide hollow microsphere composite thermal conductive filler according to claim 1 or 2 in the preparation of high thermal conductive composites, characterized in that, The composite heat-conducting filler is compounded with an epoxy resin or a thermoplastic polymer matrix, the filler addition amount is 40%-60%, and is applied to a heat dissipation structural member of an avionics device, a portable electronic device or a wearable electronic device.