Method for preparing a heat-conducting composite material from ac electric field-induced oriented surface-modified boron nitride fibers

Abstract

A method for preparing thermally conductive composite materials using surface-modified boron nitride fibers induced by an alternating current electric field is disclosed. This invention aims to address the technical problem of poor conductivity and weak electric field response of boron nitride fibers, leading to poor orientation effects. In this invention, low-conductivity inorganic ceramic boron nitride fibers are coated with a thin carbon layer at high temperature to achieve surface modification. The fibers are then dispersed in a dielectric solution, and even with a relatively low electric field, significant orientation effects are still observed. Furthermore, the oriented fibers exhibit significant end-to-end overlap, forming a highly efficient thermally conductive pathway structure composed of fibers. This invention features a simple process and can prepare high-temperature-conductivity fiber / polymer thermally conductive composite materials with high orientation and continuous thermal pathways while maintaining good performance.

Description

Technical Field

[0001] This invention relates to a method for preparing thermally conductive composite materials using surface-modified boron nitride fibers. Background Technology

[0002] In recent years, with the rapid development of AI and chip computing power, thermal interface materials used to conduct heat accumulated between chips and heat sinks have received widespread attention. Polymer-based thermally conductive composite materials are composed of a polymer matrix and a high thermal conductivity filler loaded within it. When heat flows through the composite material, the thermally conductive filler within the matrix becomes a fast channel for heat conduction, thus obtaining a thermal interface material that has both high thermal conductivity and excellent mechanical properties of the polymer matrix. Currently, traditional metallic fillers can no longer meet the requirements of high thermal conductivity, good insulation, and good mechanical properties for service environments, while carbon fillers cannot avoid insulation problems. Inorganic ceramics have become the mainstream load filler for thermal interface materials. Taking boron nitride (BN) as an example, boron nitride polymer-based composite thermally conductive materials have great application potential in the field of electronic packaging heat dissipation due to their good thermal conductivity, corrosion resistance, oxidation resistance, strong surface adaptability, good mechanical behavior, and price that is much lower than graphene.

[0003] There are various methods for preparing polymer-based thermally conductive composites. For example, field-induced orientation, such as electric field-induced fiber orientation, is commonly used to prepare thermally conductive composites containing oriented fillers. This method is based on the polarization of fibers in an alternating electric field, which generates a dipole moment. Under the influence of the electric field, the fibers undergo polarization deflection along the direction of the electric field, resulting in a uniform orientation. This method offers significant orientation effects, is simple to operate, and can be used to develop samples with high thermal conductivity. However, this method has high requirements for the fibers. For example, boron nitride fibers in inorganic ceramic fillers have poor electrical conductivity and weak electric field response, which may lead to poor orientation effects. Even if some ceramic fibers can be oriented, under low filler loading, the fillers are isolated and do not form contact overlaps, making it difficult to form efficient thermal conduction pathways, which is still detrimental to improving the material's thermal conductivity. In previous studies, filler modification has become a common method to improve its response under an electric field and has developed rapidly. This method is beneficial for significantly orienting ceramic fibers under an electric field and achieving fiber end-to-end overlap to form efficient thermal conduction pathways. Currently, when preparing thermally conductive composite materials using the electric field-induced fiber orientation method, it is generally required that the filler fibers have strong electrical conductivity and polarization response capabilities, which has two limitations: First, very high field strengths are usually required for fibers with poor conductivity, resulting in high energy consumption, and the orientation of insulating fiber fillers such as electrically neutral inorganic ceramic fibers is almost ineffective; second, for insulating inorganic ceramic fibers, it is difficult to automatically construct an efficient end-to-end overlapping path structure under the action of an electric field, which increases phonon scattering and is therefore not conducive to continuous heat transfer over relatively long distances. Summary of the Invention

[0004] The present invention aims to solve the technical problem of poor conductivity and weak electric field response of boron nitride fibers, resulting in poor orientation effect, and provides a method for preparing thermally conductive composite materials by inducing orientation of surface-modified boron nitride fibers with AC electric field.

[0005] The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers is carried out according to the following steps:

[0006] I. Length of chopped fibers:

[0007] The dispersibility of boron nitride chopped fibers in a dispersion medium directly depends on the length of the chopped fibers. Theoretically, the shorter the fiber length, the less likely the fibers are to entangle and hinder each other in the dispersion medium, and the better the uniformity of the prepared slurry. However, if the fiber length is too short, the polarization torque generated under the electric field is insufficient, the orientation effect is poor, and it has an adverse effect on the thermal conductivity. In addition, the fibers are not easy to overlap in the dispersion medium, making the resulting oriented fiber preform loose and difficult to maintain orientation. On the other hand, if the fiber length is too long, the increased mass will accelerate the sedimentation of boron nitride fibers, thereby reducing the orientation time and also leading to a decrease in thermal conductivity. Therefore, it is necessary to select chopped fibers of appropriate length. Based on the characteristics of the selected raw materials, this invention first uses scissors to cut the boron nitride fiber bundles to 1mm~5mm, then puts the cut boron nitride fibers and water into a soymilk maker for cutting, and then filters and dries them through fine gauze to obtain chopped boron nitride fibers.

[0008] II. Carbon coating of chopped fibers:

[0009] The chopped boron nitride fibers and carbon source prepared in step one are placed in two crucibles respectively. The two crucibles are placed in the same tube furnace, with the carbon source near the gas inlet and the chopped boron nitride fibers near the gas outlet. High-temperature carbon coating treatment is carried out under a protective atmosphere. The carbon source is pyrolyzed to generate carbon-containing gas, which is transported to the surface of the boron nitride fibers by the protective gas flow for vapor deposition. Then, the furnace is naturally cooled to obtain chopped BN fibers with carbon coating on the surface.

[0010] 3. Disperse the carbon-coated short BN fibers from step 2 into a mixed solution composed of deionized water and silica sol. The silica sol plays a dispersing role. After stirring and mixing, a slurry is formed.

[0011] IV. Preparation of fiber preforms of different densities by combining electric field orientation with pressurized drainage:

[0012] The orientation mold is a cuboid sample cell made of acrylic sheets with an open top. A graphite electrode plate is fixedly attached to each of the two longer and opposite inner walls of the cell. The graphite electrode plates are parallel to the two longer inner walls of the cell. The two graphite electrode plates are connected to the positive and negative terminals of an AC power supply by wires respectively.

[0013] The slurry prepared in step three is poured into the orientation mold and brought into contact with two graphite electrode plates. An AC power supply is turned on, and the fibers are oriented under a sinusoidal electric field. By adjusting the electric field parameters, the fiber orientation and overlapping behavior in the electric field are controlled. During this process, after the BN fibers complete polarization deflection, the ends of each fiber approach each other, gradually forming an overlapping structure. After the electric field is applied for 3-8 seconds, the fiber orientation and overlapping are basically completed. After the slurry in the orientation mold stabilizes, the electric field is turned off. A draining device wrapped with fine yarn is placed in the orientation mold and above the slurry. A certain pressure is applied to press the fibers to a certain depth. This process controls the green body height to obtain green bodies of different densities while preserving the electric field orientation structure. Excess liquid will be separated from the fibers through the draining device. The water above the fine yarn is removed by a dropper, and a layer of binder is added to stabilize the orientation structure of the dry fiber body after subsequent freeze-drying, preventing the fibers from becoming loose. Pressure is applied and held for 30 seconds to obtain a single-layer wet preform of the material.

[0014] The drainage device for wrapping fine gauze consists of a perforated pressure plate with a rectangular parallelepiped structure and fine gauze. The fine gauze is wrapped around the outside of the perforated pressure plate, and the drainage device for wrapping fine gauze is tightly fitted to the inner wall of the directional mold.

[0015] The perforated pressure plate has multiple vertical through holes evenly distributed on it, with a hole diameter of 1mm to 3mm;

[0016] V. Freezing and Drying of Slurry

[0017] The orientation mold is placed in a cold trap and frozen until the single-layer wet preform is completely frozen. Then, the orientation mold is placed in a freeze dryer for freeze drying to ensure that all ice crystals sublimate, thus obtaining a single-layer fiber preform.

[0018] VI. Impregnation and Composite of Green Fabrics

[0019] The freeze-dried single-layer fiber preforms are removed from the mold, and multiple single-layer fiber preforms are stacked to ensure that the fiber orientation of each layer is parallel. Then, they are vacuum impregnated with PDMS and finally cured to obtain a fiber / polymer thermally conductive composite material.

[0020] This invention improves the electric field-induced orientation efficiency of ceramic fibers through surface carbon coating modification and combines it with electric field induction to construct a continuous end-to-end overlapping structure of oriented fibers, developing a fiber / polymer thermally conductive composite material with high orientation, continuous thermal conductivity, and high thermal conductivity. In this process, low-conductivity inorganic boron nitride ceramic fibers are coated with a thin carbon layer at high temperature to achieve surface modification. The fibers are then dispersed in a dielectric solution, and even with a relatively low electric field, they still exhibit significant orientation effects. Furthermore, after orientation, the fibers achieve significant end-to-end overlapping, forming a highly efficient thermally conductive pathway structure composed of fibers.

[0021] This invention employs a vapor deposition method to coat the fiber surface with a carbon layer, enhancing its polarization response under an electric field. This achieves high orientation even under low electric field strength without compromising its intrinsic properties, forming an efficient continuous overlap structure. By controlling the volume of the green body under pressure, monolayer green bodies of different densities are obtained. Subsequently, these monolayers are stacked, assembled, and impregnated to obtain materials with different loading capacities. Finally, performance characterization establishes the relationship between the fabrication process and performance, enabling the preparation of samples with high orientation, long-distance continuous fiber end-to-end connection, and high thermal conductivity. The process of this invention is simple and can prepare high-orientation, high-thermal-conductivity fiber / polymer thermally conductive composite materials with continuous thermal conductivity pathways while maintaining high performance. Attached Figure Description

[0022] Figure 1 SEM images of the fibers before and after modification in Experiment 1;

[0023] Figure 2 Optical microscope images of the fiber before and after applying the electric field in step four of Experiment 1;

[0024] Figure 3 SEM images of the monolayer fiber preform prepared in step five of Experiment 1 (Fig. a) and the unoriented fiber preform (Fig. b, the product of step five of the comparative experiment);

[0025] Figure 4 A graph showing the thermal conductivity data of fiber / polymer thermally conductive composites with different fiber loadings prepared in Experiment 1;

[0026] Figure 5 A photograph of the physical sample prepared during Experiment 1;

[0027] Figure 6 This is a schematic diagram of the orientation mold in step four of Experiment 1. Detailed Implementation

[0028] Specific Implementation Method 1: This implementation method is a method for preparing thermally conductive composite materials using AC electric field-induced directional surface-modified boron nitride fibers, specifically carried out according to the following steps:

[0029] I. Length of chopped fibers:

[0030] Use scissors to cut the boron nitride fiber bundles to 1mm~5mm, then put the cut boron nitride fibers and water into a soymilk maker for cutting, then filter and dry through fine gauze to obtain short-cut boron nitride fibers;

[0031] II. Carbon coating of chopped fibers:

[0032] The chopped boron nitride fibers and carbon source prepared in step one are placed in two crucibles respectively. The two crucibles are placed in the same tube furnace, with the carbon source near the gas inlet and the chopped boron nitride fibers near the gas outlet. High-temperature carbon coating treatment is carried out under a protective atmosphere. The carbon source is pyrolyzed to generate carbon-containing gas, which is transported to the surface of the boron nitride fibers by the protective gas flow for vapor deposition. Then, the furnace is naturally cooled to obtain chopped BN fibers with carbon coating on the surface.

[0033] 3. Disperse the carbon-coated short BN fibers from step 2 into a mixed solution composed of deionized water and silica sol. The silica sol plays a dispersing role. After stirring and mixing, a slurry is formed.

[0034] IV. Preparation of fiber preforms of different densities by combining electric field orientation with pressurized drainage:

[0035] The orientation mold is a cuboid sample cell made of acrylic sheets with an open top. A graphite electrode plate is fixedly attached to each of the two longer and opposite inner walls of the cell. The graphite electrode plates are parallel to the two longer inner walls of the cell. The two graphite electrode plates are connected to the positive and negative terminals of an AC power supply by wires respectively.

[0036] The slurry prepared in step three is poured into the orientation mold and brought into contact with two graphite electrode plates. The AC power is turned on, and the orientation is performed under the action of a sinusoidal electric field. By adjusting the electric field parameters, the orientation and overlapping behavior of the fibers in the electric field is controlled. During this process, after the BN fibers complete polarization deflection, the ends of the fibers approach each other and gradually form an overlapping structure. After the slurry in the orientation mold stabilizes, the electric field is turned off. A draining device wrapped with fine yarn is placed in the orientation mold and above the slurry. A certain pressure is applied to press the fibers to a certain depth. This process obtains green bodies of different densities by controlling the height of the green body and retains the electric field orientation structure. Excess liquid will be separated from the fibers through the draining device. The water above the fine yarn is removed by a dropper, and a layer of binder is added to stabilize the orientation structure of the dry fiber body after subsequent freeze-drying, so that the fibers do not become loose. Pressure is applied and held for 30 seconds to obtain a single-layer wet body of the material.

[0037] The drainage device for wrapping fine gauze consists of a perforated pressure plate with a rectangular parallelepiped structure and fine gauze. The fine gauze is wrapped around the outside of the perforated pressure plate, and the drainage device for wrapping fine gauze is tightly fitted to the inner wall of the directional mold.

[0038] The perforated pressure plate has multiple vertical through holes evenly distributed on it, with a hole diameter of 1mm to 3mm;

[0039] V. Freezing and Drying of Slurry

[0040] The orientation mold is placed in a cold trap and frozen until the single-layer wet preform is completely frozen. Then, the orientation mold is placed in a freeze dryer for freeze drying to ensure that all ice crystals sublimate, thus obtaining a single-layer fiber preform.

[0041] VI. Impregnation and Composite of Green Fabrics

[0042] The freeze-dried single-layer fiber preforms are removed from the mold, and multiple single-layer fiber preforms are stacked to ensure that the fiber orientation of each layer is parallel. Then, they are vacuum impregnated with PDMS and finally cured to obtain a fiber / polymer thermally conductive composite material.

[0043] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the mass ratio of the shortened boron nitride fiber to water in step one is 1:100, and the fine yarn is 300 mesh. Everything else is the same as in Specific Implementation Method One.

[0044] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the rotation speed of the soymilk maker in step one is 10000 r / min, and the shearing time is 5 minutes. Everything else is the same as in Specific Implementation Method One or Two.

[0045] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that: the carbon source in step two is PVP, and the mass ratio of chopped boron nitride fiber to PVP is 4:5; the protective gas is argon, and the argon flow rate is 50 mL / min to 200 mL / min. Everything else is the same as in Specific Implementation Methods One to Three.

[0046] Specific Implementation Method Five: This implementation method differs from Specific Implementation Method Four in that the parameters for the high-temperature carbon coating treatment in step two are: increasing the temperature from room temperature to 600°C at a rate of 3°C / min and holding for 2 hours, then increasing the temperature to 900°C at a rate of 5°C / min and holding for 5 hours. Everything else is the same as in Specific Implementation Method Four.

[0047] Specific Implementation Method Six: This implementation method differs from Specific Implementation Method Five in that the electric field parameters mentioned in step four are: field strength of 40Vpp~180Vpp and frequency of 100Hz~10kHz; the fine yarn mentioned in step four is 300 mesh. Everything else is the same as in Specific Implementation Method Five.

[0048] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Six in that the preparation method of the adhesive in step four is as follows: carboxymethyl cellulose is dissolved in deionized water, stirred with a magnetic stir bar, and heated in a water bath at 80°C until completely dissolved to obtain the adhesive; the mass ratio of carboxymethyl cellulose to deionized water is 3:997. Everything else is the same as in Specific Implementation Method Six.

[0049] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Seven in that the temperature of the cold trap described in step five is -20°C. Everything else is the same as in Specific Implementation Method Seven.

[0050] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Eight in that the temperature of the freeze dryer described in step five is -60℃, and the vacuum degree is 150Pa~200Pa. Everything else is the same as in Specific Implementation Method Eight.

[0051] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Method Nine in that the curing temperature in step six is ​​65°C and the time is 2 hours. Everything else is the same as in Specific Implementation Method Nine.

[0052] The invention was verified using the following experiments:

[0053] Experiment 1: This experiment demonstrates a method for preparing thermally conductive composite materials using AC electric field-induced directional surface-modified boron nitride fibers. The specific steps are as follows:

[0054] I. Length of chopped fibers:

[0055] Use scissors to cut the boron nitride fiber bundles to 1mm~5mm, then put the cut boron nitride fibers and water into a soymilk maker for 5 minutes to cut, then filter and dry through fine gauze to obtain short boron nitride fibers;

[0056] The mass ratio of the shortened boron nitride fiber to water is 1:100, and the fine yarn is 300 mesh.

[0057] The speed of the soymilk maker is 10000 r / min;

[0058] II. Carbon coating of chopped fibers:

[0059] The chopped boron nitride fibers and carbon source prepared in step one are placed in two crucibles respectively. The two crucibles are placed in the same tube furnace, with the carbon source near the gas inlet and the chopped boron nitride fibers near the gas outlet. High-temperature carbon coating treatment is carried out under a protective atmosphere. The carbon source is pyrolyzed to generate carbon-containing gas, which is transported to the surface of the boron nitride fibers by the protective gas flow for vapor deposition. Then, the furnace is naturally cooled to obtain chopped BN fibers with carbon coating on the surface.

[0060] The carbon source is PVP, and the mass ratio of chopped boron nitride fiber to PVP is 4:5.

[0061] The protective gas is argon, and the argon flow rate is 200 mL / min;

[0062] The parameters for the high-temperature carbon coating treatment are: increasing the temperature from room temperature to 600℃ at a rate of 3℃ / min and holding for 2 hours, then increasing the temperature to 900℃ at a rate of 5℃ / min and holding for 5 hours.

[0063] 3. Disperse the carbon-coated short BN fibers from step 2 into a mixed solution composed of deionized water and silica sol. The silica sol plays a dispersing role. After stirring for 30 minutes, the mixture is uniformly mixed to form a slurry.

[0064] The mass ratio of the silica sol to deionized water is 1:20;

[0065] The method for preparing the silica sol is as follows:

[0066] First, tetraethyl orthosilicate (TEOS) is poured into a beaker, followed by anhydrous ethanol, and then water is added to form a mixture. After this step, it is observed that the mixture does not show obvious stratification. Then, hydrochloric acid solution is added dropwise. The beaker is then placed in a 60°C water bath for 1 hour to obtain silica sol. The molar ratio of TEOS:H₂O:anhydrous ethanol:hydrochloric acid with hydrogen chloride is 1:4:4:7.5×10⁻⁶. -4 1g of chopped BN fibers corresponds to 70mL of silica sol;

[0067] IV. Preparation of fiber preforms of different densities by combining electric field orientation with pressurized drainage:

[0068] The orientation mold is a cuboid sample cell made of acrylic sheets, with an open top. A graphite electrode plate is fixedly attached to each of the two longer, opposite inner walls of the cell. The graphite electrode plates are parallel to the two longer inner walls of the cell, and the two graphite electrode plates are connected to the positive and negative terminals of an AC power supply by wires (e.g., ...). Figure 6 As shown, 1 is a graphite electrode plate, 2 is an acrylic plate, and 3 is an AC power supply.

[0069] The slurry prepared in step three is poured into the orientation mold and brought into contact with two graphite electrode plates. An AC power supply is turned on, and the fibers are oriented under a sinusoidal electric field. By adjusting the electric field parameters, the fiber orientation and overlapping behavior in the electric field are controlled. During this process, after the BN fibers complete polarization deflection, the ends of each fiber approach each other, gradually forming an overlapping structure. After 8 seconds of electric field application, the fibers are basically oriented and overlapping. Once the slurry in the orientation mold is stable, the electric field is turned off. A draining device wrapped with fine yarn is placed in the orientation mold above the slurry, and a certain pressure is applied to press the fibers to a certain depth. This process controls the green body height to obtain green bodies of different densities while preserving the electric field orientation structure. Excess liquid will be separated from the fibers through the draining device. The water above the fine yarn is removed by a dropper, and a layer of binder is added to stabilize the orientation structure of the dry fiber body after subsequent freeze-drying, preventing the fibers from becoming loose. Pressure is applied and held for 30 seconds to obtain a single-layer wet preform of the material.

[0070] The drainage device for wrapping fine gauze consists of a perforated pressure plate with a rectangular parallelepiped structure and fine gauze. The fine gauze is wrapped around the outside of the perforated pressure plate, and the drainage device for wrapping fine gauze is tightly fitted to the inner wall of the directional mold.

[0071] The perforated pressure plate has multiple vertical through holes evenly distributed on it, with a hole diameter of 2mm;

[0072] The electric field parameters are: field strength of 100Vpp and frequency of 100Hz; the fine yarn is 300 mesh.

[0073] The adhesive is prepared by dissolving carboxymethyl cellulose in deionized water, adding magnetic stirring, and heating in a water bath at 80°C until completely dissolved to obtain the adhesive; the mass ratio of carboxymethyl cellulose to deionized water is 3:997.

[0074] V. Freezing and Drying of Slurry

[0075] The orientation mold is placed in a cold trap and frozen until the single-layer wet preform is completely frozen. Then, the orientation mold is placed in a freeze dryer for freeze drying to ensure that all ice crystals sublimate, thus obtaining a single-layer fiber preform.

[0076] The temperature of the cold trap is -20°C;

[0077] The temperature of the freeze dryer is -60℃ and the vacuum degree is 150Pa~200Pa;

[0078] VI. Impregnation and Composite of Green Fabrics

[0079] The freeze-dried single-layer fiber preforms are taken out of the mold, and multiple single-layer fiber preforms are stacked to ensure that the fiber orientation of each layer is parallel. Then, they are vacuum impregnated with PDMS and finally cured to obtain fiber / polymer thermally conductive composite material.

[0080] The curing temperature is 65℃ and the time is 2 hours.

[0081] Figure 1 The images show SEM images of the fibers before and after modification in Experiment 1. Figure (a) shows the product before modification, i.e., step one, and Figure (b) shows the product after modification, i.e., step two. Comparing Figures (a) and (b), it can be seen that the surface morphology of the fibers did not change significantly before and after modification. This is because the carbon layer formed by vapor deposition only covers the fiber surface in a thin layer and does not significantly change the surface structure of the fibers.

[0082] Table 1 shows the carbon content and electrical conductivity data of the fibers before and after modification in Experiment 1 (test samples and...). Figure 1 As shown in Table 1, the carbon content on the surface of the modified fiber increased, and the electrical conductivity was significantly improved, indicating that the deposition of the carbon layer has a significant effect on improving the electrical conductivity of the fiber.

[0083] Table 1

[0084]

[0085] Figure 2 These are optical microscope images of the fiber before and after applying the electric field in step four of Experiment 1. Figure 2 (a) shows the fiber (slurry prepared in step 3) without an applied electric field, and it can be clearly seen that it exhibits a random orientation and disordered distribution. Figure 2 (b) This image shows the fiber after an electric field has been applied (the sample is a solution and has not yet been drained; the horizontal axis represents the duration of the electric field). It is clear that the fiber rapidly exhibits a high degree of orientation along the direction of the electric field under its influence, forming a distinct end-to-end overlap and creating a continuous pathway. This demonstrates that the modified fiber can complete orientation in a short time under an electric field, exhibiting high orientation efficiency and significantly improved overall orientation. Furthermore, it also shows a prominent end-to-end overlap characteristic.

[0086] Comparative Experiment: This experiment differs from Experiment 1 in that the electric field was not activated in step four, and step six was not performed. Everything else is the same as Experiment 1.

[0087] Figure 3 SEM images of the monolayer fiber preform (Fig. a) and the unoriented fiber preform (Fig. b, the product from step five of the comparative experiment) prepared in step five of Experiment 1, respectively. Figure 3 (a) It can be seen that after boron nitride fibers are dried and processed through a process of draining and freeze-drying under the influence of an applied electric field, the orientation and end-to-end overlap structure of the oriented sample are preserved. Figure 3 (b) It can be seen that the fibers of the unoriented sample did not show a clear uniform orientation, nor did they form a bridging structure. The fibers generally exhibited a random arrangement.

[0088] Figure 4The graph shows the thermal conductivity data of fiber / polymer thermally conductive composites prepared in Experiment 1 with different fiber loadings. The fiber loading is the product mass from step five divided by the product mass from step six. As can be seen from the graph, the thermal conductivity of the sample increased from 1.305 W / (m∙K) to 3.611 W / (m∙K) with increasing boron nitride fiber loading. This is because the increased density leads to more overlapping of oriented fibers, gradually forming continuous thermally conductive pathways throughout the material. With increasing fiber loading, more pathways are formed, causing the material to reach the percolation threshold, resulting in a significant increase in thermal conductivity. Furthermore, with increasing fiber loading, the overlapping pathways of the fibers tend to be more tightly packed. The close arrangement of thermally conductive fibers creates more hot spots per unit area of ​​the material surface. As the fiber proportion per unit area increases, the proportion of the low-thermal-conductivity matrix in heat conduction decreases, and the material's thermal conductivity also increases. In this experiment, the increase in the thermal conductivity of the sample corresponds to the increase in the fiber loading (12.57%~30.11%). The specific measure of thermal conductivity is the thermal conductivity of a material, and the magnitude of the thermal conductivity reflects the quality of the material's thermal conductivity. The high thermal conductivity of this sample indicates that it is a relatively good thermally conductive material.

[0089] Figure 5 Figure 1 shows the physical images of the samples prepared during Experiment 1. Figure 2 shows the physical image of the single-layer fiber preform (the product of step 5). Figure 3 shows the physical images of the composite material (left) and the thin sample (right) cut out after the fiber preforms are stacked, oriented and impregnated with PDMS in step 6.

Claims

1. A method for preparing thermally conductive composite materials by directional surface-modified boron nitride fibers induced by alternating electric field, characterized in that... The method is performed according to the following steps: I. Length of chopped fibers: Use scissors to cut the boron nitride fiber bundles to 1mm~5mm, then put the cut boron nitride fibers and water into a soymilk maker for cutting, then filter and dry through fine gauze to obtain short-cut boron nitride fibers; II. Carbon coating of chopped fibers: The chopped boron nitride fibers and carbon source prepared in step one are placed in two crucibles respectively. The two crucibles are placed in the same tube furnace, with the carbon source near the gas inlet and the chopped boron nitride fibers near the gas outlet. High-temperature carbon coating treatment is carried out under a protective atmosphere. The carbon source is pyrolyzed to generate carbon-containing gas, which is transported to the surface of the boron nitride fibers by the protective gas flow for vapor deposition. Then, the furnace is naturally cooled to obtain chopped BN fibers with carbon coating on the surface.

3. Disperse the carbon-coated short BN fibers from step 2 into a mixed solution composed of deionized water and silica sol. The silica sol plays a dispersing role. After stirring and mixing, a slurry is formed. IV. Preparation of fiber preforms of different densities by combining electric field orientation with pressurized drainage: The orientation mold is a cuboid sample cell made of acrylic sheets with an open top. A graphite electrode plate is fixedly attached to each of the two longer and opposite inner walls of the cell. The graphite electrode plates are parallel to the two longer inner walls of the cell. The two graphite electrode plates are connected to the positive and negative terminals of an AC power supply by wires respectively. The slurry prepared in step three is poured into the orientation mold and brought into contact with the two graphite electrode plates; Turn on the AC power supply and orient yourself under the influence of a sinusoidal electric field; After the slurry in the orientation mold stabilizes, the electric field is turned off. A draining device wrapped with fine yarn is placed in the orientation mold and above the slurry. Pressure is applied to press the fibers to a certain depth. This process controls the height of the green body to obtain green bodies of different densities while preserving the electric field orientation structure. Excess liquid will be separated from the fibers through the draining device. The water above the fine yarn is removed by a dropper, and a layer of binder is added to stabilize the orientation structure of the dry fiber body after subsequent freeze-drying, so that the fibers do not become loose. Pressure is applied and held for 30 seconds to obtain a single-layer wet body of the material. The drainage device for wrapping fine gauze consists of a perforated pressure plate with a rectangular parallelepiped structure and fine gauze. The fine gauze is wrapped around the outside of the perforated pressure plate, and the drainage device for wrapping fine gauze is tightly fitted to the inner wall of the directional mold. The perforated pressure plate has multiple vertical through holes evenly distributed on it, with a hole diameter of 1mm to 3mm; V. Freezing and Drying of Slurry The orientation mold is placed in a cold trap and frozen until the single-layer wet preform is completely frozen. Then, the orientation mold is placed in a freeze dryer for freeze drying to ensure that all ice crystals sublimate, thus obtaining a single-layer fiber preform. VI. Impregnation and Composite of Green Fabrics The freeze-dried single-layer fiber preforms are removed from the mold, and multiple single-layer fiber preforms are stacked to ensure that the fiber orientation of each layer is parallel. Then, they are vacuum impregnated with PDMS and finally cured to obtain a fiber / polymer thermally conductive composite material.

2. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 1, is characterized in that... The mass ratio of the shortened boron nitride fiber to water mentioned in step one is 1:100, and the fine yarn is 300 mesh.

3. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 1, is characterized in that... In step one, the speed of the soymilk maker is 10000 r / min, and the shearing time is 5 minutes.

4. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 1, is characterized in that... The carbon source mentioned in step two is PVP, and the mass ratio of chopped boron nitride fiber to PVP is 4:5; the protective gas is argon, and the argon flow rate is 50 mL / min to 200 mL / min.

5. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 1, is characterized in that... The parameters for the high-temperature carbon coating treatment in step two are: increasing the temperature from room temperature to 600℃ at a rate of 3℃ / min and holding for 2 hours, then increasing the temperature to 900℃ at a rate of 5℃ / min and holding for 5 hours.

6. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 1, is characterized in that... The electric field parameters mentioned in step four are: field strength of 40Vpp~180Vpp and frequency of 100Hz~10kHz; the fine yarn mentioned in step four is 300 mesh.

7. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 1, is characterized in that... The adhesive preparation method described in step four is as follows: carboxymethyl cellulose is dissolved in deionized water, stirred with a magnetic stir bar, and heated in a water bath at 80°C until completely dissolved to obtain the adhesive; the mass ratio of carboxymethyl cellulose to deionized water is 3:

997.

8. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 1, is characterized in that... The temperature of the cold trap mentioned in step five is -20°C.

9. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 8, is characterized in that... The freeze dryer described in step five has a temperature of -60℃ and a vacuum degree of 150Pa~200Pa.

10. The method for preparing thermally conductive composite materials by alternating current electric field-induced directional surface-modified boron nitride fibers, as described in claim 1, is characterized in that... The curing temperature in step six is ​​65°C, and the curing time is 2 hours.

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