Highly oriented, densely packed carbon fiber heat conducting composite material and method of making same

Abstract

The application relates to the technical field of heat-conducting composite materials, in particular to a high-orientation and close-arrangement carbon fiber heat-conducting composite material and a preparation method thereof. The preparation method comprises the following steps: dispersing solventophilic modified pitch-based short carbon fibers in a polymer solution to form a mixed solution, pouring the mixed solution into a mold provided with a permanent magnetic field, the magnetic field direction of the permanent magnetic field is parallel to the direction of gravity, simultaneously applying circumferential ultrasonic vibration and torsional vibration to the mold provided with the permanent magnetic field to obtain pitch-based short carbon fiber precipitation with high-order arrangement, then evaporating the solvent in the polymer solution, and solidifying the polymer, and the high-orientation and close-arrangement carbon fiber heat-conducting composite material is obtained. The carbon fiber heat-conducting composite material prepared by the preparation method has excellent vertical heat conduction performance, and the equipment is simple, the production cycle is short, and the process cost is low.

Description

Technical Field

[0001] This invention relates to the field of thermally conductive composite materials technology, and in particular to a highly oriented, densely packed carbon fiber thermally conductive composite material and its preparation method. Background Technology

[0002] The information disclosed in the background section of this invention is intended only to enhance the understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] With the development of semiconductor and 5G technologies, advanced electronic products are becoming more efficient, highly integrated, and multifunctional. During operation, they generate significant amounts of heat, which not only reduces device efficiency but also impacts the safety and lifespan of electronic devices. Efficient thermal management technologies have attracted increasing attention from researchers. Thermal interface materials, as a crucial component of thermal management technology, act as a bridge between electronic devices and heat sinks, filling the gap between two rigid, rough surfaces and creating a good contact surface that effectively controls the temperature of the operating device. Current traditional thermal interface materials are typically made by combining thermally conductive fillers with a polymer matrix; however, the disordered distribution of fillers makes it difficult to form efficient thermal conduction pathways and achieve the desired thermal conductivity, failing to meet the ever-increasing heat dissipation demands.

[0004] Compared to traditional methods for preparing thermally conductive interface materials, constructing directional thermally conductive pathways within composite materials using anisotropic thermally conductive fillers such as graphene and boron nitride nanosheets is an effective way to achieve high thermal conductivity values. Because pitch-based carbon fibers possess anisotropic thermal conductivity, their axial thermal conductivity can reach over 800 W / mK. Furthermore, compared to the challenges in preparing graphene or boron nitride nanosheets, pitch-based carbon fibers have already achieved industrial-scale production. Therefore, many researchers are dedicated to employing various methods to obtain the orientation of carbon fibers in composite materials.

[0005] Patent CN 111909520A (publication date: 2020.11.10) discloses a low-density oriented high thermal conductivity pad and its preparation method. It involves oxidizing and magnetically plating carbon fibers, which then oriented the carbon fibers in a magnetic field. However, the oxidizing and magnetically plating process not only increases the difficulty and cost of preparation, but also easily introduces defects into the carbon fibers, leading to a decrease in their thermal conductivity.

[0006] Patent CN 112976438A (publication date: June 18, 2021) discloses a method for preparing a high thermal conductivity interface material with directional interconnection and a product. The method involves placing nickel-plated carbon fibers in a mold and orienting them in a magnetic field environment with permanent magnets placed above and below. Then, a polymer colloid is injected into the mold, and the mixture is vacuumed and heated to cure. However, the carbon fibers in this patent require nickel plating. When the nickel content is below 60%, the magnetic induction of the nickel-plated carbon fibers under the influence of an external magnetic field is too low, making it difficult for the nickel-plated carbon fibers to achieve directional interconnection under the influence of the external magnetic field. This results in a decrease in the orientation degree of the nickel-plated carbon fibers and an inability to form good thermal conductivity channels.

[0007] Patent CN 115594941A (publication date: January 13, 2023) discloses a method for preparing a carbon fiber oriented skeleton. This method first orients carbon fibers in a dilute phenolic resin solution using a magnetic field and horizontal vibration, followed by curing and drying. The phenolic resin is then carbonized and graphitized to obtain an oriented carbon fiber skeleton with a certain mechanical strength. Finally, a high-viscosity polymer is infused into the skeleton, and high-temperature curing yields the target composite material. However, this method has a lengthy and complex preparation process, and the high-temperature treatment increases the preparation cost. Although the horizontal vibration method makes the carbon fibers more densely arranged, forming more thermal conductivity pathways, the horizontal vibration also affects the degree of carbon fiber orientation, resulting in uneven carbon fiber arrangement and affecting the material's thermal conductivity.

[0008] Therefore, existing methods for preparing composite materials using magnetic pitch-based carbon fibers with directional alignment generally suffer from complex processes and high production costs. Furthermore, the thermal conductivity of these composites largely depends on the orderly arrangement of the carbon fibers within the composite material. Finding a low-cost, easily industrialized method to achieve a highly oriented, densely packed structure of carbon fibers during composite molding is a pressing issue. Summary of the Invention

[0009] In view of this, the present invention provides a highly oriented, densely packed carbon fiber thermally conductive composite material and its preparation method. The equipment is simple, the production cycle is short, and the process cost is low. Moreover, the carbon fiber thermally conductive composite material prepared by the present invention has excellent vertical thermal conductivity and has good application prospects.

[0010] In a first aspect, the present invention provides a method for preparing a highly oriented, closely packed carbon fiber thermally conductive composite material, comprising the following steps:

[0011] A solvation-modified pitch-based chopped carbon fiber is dispersed in a polymer solution to form a mixed solution. The mixed solution is then poured into a mold equipped with a permanent magnet field, the magnetic field direction of which is parallel to the direction of gravity. Circumferential ultrasonic vibration and torsional vibration are simultaneously applied to the mold to obtain a highly ordered pitch-based chopped carbon fiber precipitate. The solvent in the polymer solution is then evaporated, and the polymer is solidified to obtain the final product.

[0012] Preferably, the length of the pitch-based short-cut carbon fiber is 0.1-2 mm, and more preferably 0.15-0.25 mm.

[0013] Preferably, the solvation modification method includes chemical grafting, oxidative etching, or surface deposition, with chemical grafting being the most preferred.

[0014] Furthermore, the preparation method of the solvation-modified pitch-based chopped carbon fiber is as follows: using a silane coupling agent as a chemical grafting agent, surface grafting is performed on the surface of the chopped carbon fiber; preferably, the silane coupling agent is selected from one of (3-mercaptopropyl)trimethoxysilane, γ-aminopropyltriethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane or γ-glycidoxypropyltrimethoxysilane.

[0015] Preferably, the method for dispersing pitch-based short-cut carbon fibers in a polymer solution includes one or more of ultrasound, mechanical stirring, or heating, with ultrasound and mechanical stirring being preferred.

[0016] Preferably, the polymer is a flexible polymer material, and more preferably, the polymer is selected from one or more of silicone rubber, polyurethane, polydimethylsiloxane, polyvinyl alcohol, polyvinylpyrrolidone, and epoxy resin.

[0017] Preferably, the solvent of the polymer solution is a volatile solvent selected from one or more of deionized water, ethanol, methanol, solvent oil, white oil, n-hexane, and petroleum ether.

[0018] Preferably, in the polymer solution, the mass ratio of polymer to solvent is 0.02-0.08:1, more preferably 0.03-0.05:1.

[0019] Preferably, the density of the polymer solution is less than 2.2 g / mL.

[0020] Preferably, in the mixed solution, the mass ratio of pitch-based short-cut carbon fibers to solvent is 0.02-0.15:1, more preferably 0.075-0.125:1.

[0021] Preferably, the permanent magnet field is generated by iron-neodymium-boron permanent magnets arranged on the upper and lower sides of the mold, and the magnetic field strength is 0.2-1T.

[0022] Preferably, the circumferential ultrasonic vibration frequency is 40-120kHz and the power is 15-300W.

[0023] Preferably, the frequency of the torsional vibration is 20-50Hz and the vertical amplitude is 0.1-0.5mm.

[0024] Preferably, after simultaneously applying circumferential ultrasonic vibration and torsional vibration for 15-30 seconds, the circumferential ultrasonic vibration is stopped, and the torsional vibration is maintained for 4-10 minutes.

[0025] Preferably, after the polymer curing step, a surface treatment step is further included, wherein the surface treatment step is selected from any one of cutting, dissolution etching or polishing, and polishing is preferred.

[0026] Secondly, the present invention provides a highly oriented, densely packed carbon fiber thermally conductive composite material, which is prepared by the preparation method described in the above technical solution.

[0027] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0028] (1) The present invention uses a magnetic field parallel to the direction of gravity to cause the pitch-based short carbon fibers in the mixed solution to settle in the direction of gravity. The circumferential ultrasonic vibration can break up the fiber overlap and agglomeration problem that occurs when the carbon fibers are oriented (rotated in situ), and redistribute the overlapped and agglomerated carbon fibers. The torsional vibration can cause the carbon fibers that have settled to the bottom and have achieved a certain orientation or the carbon fibers that have been bridged but have not achieved the target orientation to jump, thereby rearranging the carbon fibers until they are completely aligned with the direction of the magnetic field line. Finally, a highly vertically oriented and tightly packed carbon fiber thermal conductive composite material can be obtained.

[0029] (2) The preparation method provided by the present invention can form the mixed solution in a mold in one step, which is simple, has a short production cycle and low process cost. At the same time, the carbon fiber thermally conductive composite material prepared by the present invention has excellent vertical thermal conductivity, with a thermal conductivity of up to 16 W / mK or more, and has excellent application prospects. Attached Figure Description

[0030] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation thereof. Obviously, those skilled in the art can obtain other drawings based on these drawings without any inventive effort.

[0031] Figure 1 This is a schematic diagram of the preparation equipment according to a specific embodiment of the present invention;

[0032] Figure 2 An optical photograph of the carbon fiber thermally conductive composite material prepared in Example 1 of the present invention;

[0033] Figure 3 This is a cross-sectional scanning electron microscope image of the carbon fiber thermally conductive composite material prepared in Example 1 of the present invention;

[0034] Figure 4 This is a scanning electron microscope image of the surface of the carbon fiber thermally conductive composite material prepared in Example 1 of the present invention after polishing.

[0035] Figure 5 This is a cross-sectional scanning electron microscope image of the carbon fiber thermally conductive composite material prepared in Example 4 of the present invention;

[0036] Figure 6 This is a cross-sectional scanning electron microscope image of the carbon fiber thermally conductive composite material prepared in Comparative Example 1 of the present invention. Detailed Implementation

[0037] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0038] As described in the background section, existing methods for preparing carbon fiber thermally conductive composite materials using magnetic fields suffer from complex procedures and high costs. Therefore, this invention provides a method for preparing highly oriented, closely packed carbon fiber thermally conductive composite materials, comprising the following steps:

[0039] A solvation-modified pitch-based chopped carbon fiber is dispersed in a polymer solution to form a mixed solution. The mixed solution is then poured into a mold equipped with a permanent magnet field, the magnetic field direction of which is parallel to the direction of gravity. Circumferential ultrasonic vibration and torsional vibration are simultaneously applied to the mold to obtain a highly ordered pitch-based chopped carbon fiber precipitate. The solvent in the polymer solution is then evaporated, and the polymer is solidified to obtain the final product.

[0040] The short-cut carbon fibers used in this invention to prepare highly oriented, closely packed carbon fiber thermally conductive composite materials are highly thermally conductive pitch-based carbon fibers with a highly graphitized structure. The highly graphitized structure gives them a high anisotropic magnetic susceptibility, which is sufficient to provide them with a magnetic torque for deflection in a low magnetic field. Therefore, there is no need to perform nickel plating or magnetic plating treatment on the short-cut carbon fibers, as they can be oriented in a magnetic field.

[0041] This invention directly mixes modified pitch-based chopped carbon fibers with a polymer solution to obtain a co-dispersion system of modified pitch-based chopped carbon fibers and polymer, forming a mixed solution. The mixed solution is then immediately placed in a vertical magnetic field. The anisotropic diamagnetic properties of the modified pitch-based chopped carbon fibers generate a torsional magnetic torque. Because ultrasonic vibration can improve fiber dispersibility and flowability, even in a low-intensity magnetic field environment, the torsional magnetic torque is still greater than the resistance torque exerted by the surrounding solution, causing the modified pitch-based chopped carbon fibers to tend to align along the magnetic field direction. Furthermore, ultrasound can break up fiber agglomeration that occurs during the orientation movement (in-situ rotation) of the modified pitch-based chopped carbon fibers, allowing the agglomerated modified pitch-based chopped carbon fibers to redisperse and orient themselves. The modified pitch-based chopped carbon fibers then gradually settle to the bottom of the mold. However, during the settling process, fiber bridging, i.e., the "powder bridge" effect, is highly likely to occur. Once the "powder bridge" is overlapped, it becomes quite stable, hindering the orientation movement (in-situ rotation) and densification (vertical displacement) of the modified pitch-based chopped carbon fibers in contact with the "powder bridge." To address this issue, this invention utilizes torsional vibration during the carbon fiber settling process to induce a jumping motion in the carbon fibers that have settled to the bottom and achieved a certain orientation, or in the carbon fibers that have overlapped but not yet achieved the target orientation. During this jumping motion, the carbon fibers are subjected to magnetic force and reorient themselves along the magnetic field lines. Subsequently, they settle to the bottom of the mold. In this process, the alignment direction of the carbon fibers continuously converges with the direction of the magnetic field lines until they are completely aligned, thereby giving the chopped carbon fibers a highly vertically oriented and tightly packed structure.

[0042] The present invention does not impose any special restrictions on the length of pitch-based short-cut carbon fibers. The length of pitch-based short-cut carbon fibers commonly used in the art can be used, preferably 0.1-2 mm, and more preferably 0.15-0.25 mm.

[0043] This invention does not impose any special limitations on the method of solvation modification of pitch-based chopped carbon fibers. Conventional solvation modification methods can be used, including but not limited to chemical grafting, oxidative etching, or surface deposition. Solvation modification is to ensure that the pitch-based chopped carbon fibers can be uniformly dispersed in the polymer solution to obtain a uniformly dispersed mixed solution, which is beneficial for subsequent orientation processes. This invention preferably uses chemical grafting, specifically: using a silane coupling agent as a chemical grafting agent to perform surface grafting on the surface of the chopped carbon fibers to increase the number of polar or non-polar groups on the surface of the chopped carbon fibers. This invention does not impose any special limitations on the type of silane coupling agent; commonly used silane coupling agents in the art can be used. The silane coupling agent includes, but is not limited to, one of (3-mercaptopropyl)trimethoxysilane, γ-aminopropyltriethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane, or γ-glycidoxypropyltrimethoxysilane. Those skilled in the art can select the appropriate agent based on the type of solvent.

[0044] The present invention does not impose any particular limitation on the method of dispersing pitch-based short-cut carbon fibers in a polymer solution. Any dispersion method commonly used in the art can be used, including one or more of ultrasonication, mechanical stirring or heating, preferably a combination of ultrasonication and mechanical stirring.

[0045] Since the thermal interface material needs to have a certain degree of flexibility in order to better fit the heat-generating chip and the heat sink and reduce the thermal resistance of the interface at the contact point between the material and the chip, the polymer described in this invention is a flexible polymer material. It can be any conventional flexible polymer material in the field, including but not limited to one or more of silicone rubber, polyurethane, polydimethylsiloxane, polyvinyl alcohol, polyvinylpyrrolidone, and epoxy resin.

[0046] The present invention does not impose special restrictions on the solvent of the polymer solution, as long as it meets the conditions of being able to dissolve the polymer and being easily volatile. The solvent includes, but is not limited to, one or more of deionized water, ethanol, methanol, solvent oil, white oil, n-hexane, and petroleum ether.

[0047] In the polymer solution of this invention, the mass ratio of polymer to solvent is 0.02-0.08:1, preferably 0.03-0.05:1. A suitable polymer concentration ensures the polymer solution has a suitable viscosity, which is beneficial for the directional movement of pitch-based chopped carbon fibers. Excessive solvent not only leads to solvent waste but also increases energy consumption during subsequent solvent evaporation. The density of the polymer solution of this invention is preferably less than 2.2 g / mL to facilitate the smooth settling of the pitch-based chopped carbon fibers.

[0048] In the mixed solution of this invention, the mass ratio of pitch-based chopped carbon fibers to solvent is 0.02-0.15:1, preferably 0.075-0.125:1. The amount of pitch-based chopped carbon fibers added affects their directional movement in a magnetic field, and also affects the carbon fiber density of the final carbon fiber conductive composite material. If the amount of pitch-based chopped carbon fibers added is too small, the thermal conductive pathways formed by carbon fibers inside the composite material will be few or unable to form complete thermal conductive pathways, thus failing to effectively improve the thermal conductivity of the composite material; if the amount of pitch-based chopped carbon fibers added is too large, the carbon fibers will be unevenly dispersed in the solution and the viscosity will increase, making it impossible to achieve high carbon fiber orientation.

[0049] The permanent magnet field of this invention is generated by neodymium iron boron permanent magnets arranged on the upper and lower sides of the mold, with a magnetic field strength of 0.2-1T, preferably 0.5T. The magnetic field strength can be changed by altering the distance between the neodymium iron boron permanent magnets on the upper and lower sides, eliminating the need for complex methods of generating strong magnetic fields, thus improving operational simplicity and reducing equipment costs. Since the direction of the permanent magnet field of this invention is parallel to the direction of gravity, the pitch-based short-cut carbon fibers of this invention will undergo downward directional sedimentation under the influence of the magnetic field and gravity.

[0050] The circumferential ultrasonic vibration and torsional vibration described in this invention are generated by a ring ultrasonic vibration table, as shown in the schematic diagram below. Figure 1 As shown. The circumferential ultrasonic vibration frequency of this invention is 40-120kHz, and the power is 15-300W. Circumferential ultrasonic vibration is used to break up the fiber overlap and agglomeration that occurs during the orientation movement of carbon fibers, allowing the overlapped and agglomerated carbon fibers to redistribute and orient themselves. The torsional vibration frequency of this invention is 20-50Hz, and the vertical amplitude is 0.1-0.5mm. The high-frequency, low-intensity torsional force causes the carbon fibers to rearrange, effectively overcoming the "powder bridge" problem generated during the orientation movement of carbon fibers.

[0051] This invention applies circumferential ultrasonic vibration and torsional vibration simultaneously for 15-30 seconds, then stops the circumferential ultrasonic vibration and continues to maintain torsional vibration for 4-10 minutes. Applying appropriate circumferential ultrasound in the early stage of carbon fiber settling can break up the agglomeration generated during the carbon fiber orientation movement (in-situ rotation) and promote the carbon fiber orientation movement (in-situ rotation). However, prolonged ultrasound will hinder the uniform and dense settling of carbon fibers.

[0052] In this invention, the temperature during solvent evaporation is below the boiling point of the solvent used, and the curing temperature is 40-100℃. This invention does not impose special restrictions on the heating method used during solvent evaporation and polymer curing; commonly used heating methods are acceptable, including but not limited to one or more of the following: oven heating, infrared lamp heating, and heating plate heating.

[0053] The present invention further includes a surface treatment step after the polymer curing step. The surface treatment step is selected from any one of cutting, dissolution etching, or polishing, preferably polishing. Surface treatment can make the surface smooth, while simultaneously exposing the pitch-based short-cut carbon fibers.

[0054] This invention provides a highly oriented, densely packed carbon fiber thermally conductive composite material, which is prepared by the preparation method described in the above technical solution.

[0055] The technical solution of the present invention will be further described below with reference to specific embodiments. In the following embodiments, the pitch-based chopped carbon fibers with a length of 0.15 mm were purchased from Japan Graphite Fiber Co., Ltd., and the pitch-based chopped carbon fibers with a length of 0.25 mm were purchased from Hunan Dongying Carbon Materials Co., Ltd.

[0056] Example 1

[0057] In this embodiment, a method for preparing a highly oriented, closely packed carbon fiber thermally conductive composite material includes the following steps:

[0058] (1) Modified pitch-based short-cut carbon fiber modification

[0059] Prepare 200g of 80wt% ethanol aqueous solution, add 20g of short-cut carbon fibers to 5g of (3-mercaptopropyl)trimethoxysilane, stir for 1h, then centrifuge and wash the carbon fibers, and dry to obtain modified pitch-based short-cut carbon fibers.

[0060] (2) Modified pitch-based short-cut carbon fibers co-dispersed with polymers:

[0061] Weigh 0.3g of raw silicone rubber and disperse it in 10g of n-hexane to form a solution. Then weigh 1g of 0.15mm modified pitch-based short-cut carbon fiber and add it to the solution to form a mixed solution. Perform mechanical stirring under ultrasound for 5 minutes. Then add 15wt% of silicone rubber curing agent to the raw silicone rubber and stir to disperse evenly to obtain a mixed solution.

[0062] (3) Highly oriented carbon fibers with annular ultrasonic vibration and torsional vibration

[0063] The mixed solution was introduced into a tooling mold under a magnetic field environment. While the short-cut carbon fibers were settling, ultrasonic waves with a frequency of 40kHz and a power of 30W and torsional vibrations with a frequency of 40Hz and a vertical amplitude of 0.5mm were applied to vertically orient and densify the short-cut carbon fibers. The ultrasonic waves were stopped after 20 seconds, and the torsional vibrations were continued for 9 minutes.

[0064] (4) Composite material molding

[0065] After the orientation is completed, the mixed solution and the magnetic field device are placed in a 50°C oven and kept at that temperature for 2 hours to allow the solvent n-hexane to evaporate completely. Then, the oven temperature is raised to 80°C and kept at that temperature for 4 hours to allow the polymer to solidify completely.

[0066] (5) Grinding the material surface

[0067] The composite material is removed from the mold and slowly placed on a copper plate immersed in liquid nitrogen. It is then frozen and hardened for 10 seconds, and the surface of the composite material is polished smooth with sandpaper.

[0068] Figure 2 The image shows an optical photograph of the carbon fiber thermally conductive composite material prepared in Example 1. It can be seen that the composite material remains intact even when bent at a large angle, proving that it has good flexibility and the ability to adhere to the heating chip and the heat sink. Figure 3The image shown is a cross-sectional scanning electron microscope image of the carbon fiber thermally conductive composite material prepared in Example 1. It can be seen that this example prepared a highly vertically oriented and closely packed carbon fiber thermally conductive composite material. Figure 4 The image shown is a scanning electron microscope (SEM) image of the surface of the carbon fiber thermally conductive composite material prepared in Example 1 of this invention after polishing. It can be seen that the carbon fiber is exposed on the surface of the material.

[0069] Example 2

[0070] Compared with Example 1, the difference is that in step (2), the amount of 0.15mm modified pitch-based short-cut carbon fiber added is 0.5g.

[0071] Example 3

[0072] Compared with Example 1, the difference is that in step (2), the amount of 0.15mm modified pitch-based short-cut carbon fiber added is 0.75g.

[0073] Example 4

[0074] Compared with Example 1, the difference is that in step (2), the amount of 0.15mm modified pitch-based short-cut carbon fiber added is 1.25g.

[0075] Figure 5 The image shows a cross-sectional scanning electron microscope image of the carbon fiber thermally conductive composite material prepared in Example 4. It can be seen that the carbon fibers in the composite material have a vertical orientation. However, due to the large content of carbon fibers, the vertical orientation effect of the carbon fibers is not ideal, and some carbon fibers exhibit random orientation.

[0076] Example 5

[0077] Compared with Example 1, the difference is that the silicone rubber raw material in step (2) of Example 1 is changed to polydimethylsiloxane (PDMS), and the corresponding silicone rubber curing agent is changed to a special curing agent for polydimethylsiloxane, with a mass fraction of 10wt%.

[0078] Example 6

[0079] Compared with Example 1, the difference is as follows: Step (2) is as follows: Weigh 0.35g of water-soluble polyurethane, disperse it in 10g of deionized water to form a solution, then weigh 1g of 0.15mm short-cut carbon fiber, add it to the solution to form a dispersion, and mechanically stir it in ultrasound for 5min. After the dispersion is uniform, a mixed solution is obtained.

[0080] Step (4) is as follows: After the orientation is completed, place the mixed solution together with the magnetic field device in an 80°C oven for 12 hours to allow the solvent water to evaporate completely and the polymer to solidify completely.

[0081] The remaining steps are the same as in Example 1.

[0082] Example 7

[0083] Compared with Example 1, the difference is that in step (2), the 0.15mm modified pitch-based short-cut carbon fiber is replaced with 0.25mm carbon fiber.

[0084] Comparative Example 1

[0085] The ultrasonic and torsional vibrations in step (3) of Example 1 are removed, and the other steps are the same as in Example 1.

[0086] Figure 6 The cross-sectional scanning electron microscope image of the carbon fiber thermally conductive composite material prepared in Comparative Example 1 shows that, due to the agglomeration of carbon fibers and the powder bridging phenomenon, the composite material does not achieve a highly vertically oriented, densely arranged structure, but rather exhibits an inclined arrangement structure.

[0087] Comparative Example 2

[0088] (1) Modified pitch-based short-cut carbon fibers co-dispersed with polymers:

[0089] Weigh 0.3g of raw silicone rubber and disperse it in 10g of n-hexane to form a solution. Then weigh 1g of 0.15mm modified pitch-based short-cut carbon fiber and add it to the solution to form a mixed solution. Perform mechanical stirring under ultrasound for 5 minutes, and then add 15% of the weight of the raw silicone rubber as a curing agent and stir to disperse evenly.

[0090] (2) Composite material molding

[0091] The mixed solution was poured into a polytetrafluoroethylene mold. After the short-cut carbon fibers were completely precipitated, the mixed solution was placed in a 40°C oven and kept at that temperature for 4 hours to allow the solvent to evaporate completely. Then, the oven temperature was raised to 60°C and kept at that temperature for 8 hours to allow the polymer to cure completely.

[0092] (3) Grinding the material surface

[0093] The composite material was removed from the polytetrafluoroethylene mold and slowly placed on a copper plate immersed in liquid nitrogen. It was then frozen and hardened for 10 seconds, and the surface of the composite material was smoothed with sandpaper.

[0094] Test case

[0095] The thermal conductivity of the composite material was tested using a DRL-V type heat flow thermal conductivity meter manufactured by Hunan Xiangtan Instrument Co., Ltd., according to ASTM D 5470-06 standard. The test results are shown in Table 1.

[0096] Table 1 Comparison of thermal conductivity of composite materials prepared in each example / comparative example

[0097]

[0098]

[0099] Table 1 shows the thermal conductivity of each embodiment and comparative example. Examples 1 to 7 can all prepare carbon fiber thermally conductive composite materials with highly vertically oriented and closely packed carbon fibers and a thermal conductivity of more than 4 W / mK. The carbon fiber thermally conductive composite material prepared in Example 1 has the best thermal conductivity.

[0100] Compared to Example 1, Examples 2 and 3 use less carbon fiber, resulting in less dense thermal conductive pathways formed by the short-cut carbon fibers in the composite material. Therefore, their thermal conductivity is lower than that of Example 1.

[0101] Compared to Example 1, Example 4 used a larger amount of chopped carbon fiber, resulting in an excessively high concentration of chopped carbon fiber in the mixed solution. This caused it to settle very easily, and the chopped carbon fiber at the bottom of the solution had not yet achieved a stable directional arrangement before settling to the bottom of the mold, forming a "powder bridge" structure that was accumulated at the very bottom. Even with the assistance of a twisting feeding platform, it was difficult to resolve this most stable "powder bridge" structure that was accumulated at the very bottom. Figure 5 As shown, compared to the highly oriented short-cut carbon fiber arrangement in the middle part of the composite material, the short-cut carbon fibers at the bottom of the mold exhibit a clear non-oriented distribution. This structure is not conducive to the high axial thermal conductivity of the short-cut carbon fibers, so the thermal conductivity of the composite material is lower than that of Example 1.

[0102] Compared to Example 1, the polymers in Examples 5 and 6 were changed to PDMS and polyurethane, and the solvent in Example 6 was changed to deionized water with the corresponding polarity to polyurethane. Due to the different polymer matrix, the interfacial thermal resistance between the pitch-based short carbon fiber and the matrix increased, which reduced the thermal conductivity of the composite material. In addition, PDMS and polyurethane have higher hardness and flexural modulus, which increased the contact thermal resistance between the composite material and the hot and cold electrodes in the heat flow method test. Therefore, its thermal conductivity decreased compared to Example 1.

[0103] Compared to Example 1, Example 7 used 0.25 mm modified pitch-based short-cut carbon fibers. This may be because the short-cut carbon fibers used in Comparative Example 3 were longer and the overall length distribution was not uniform, resulting in the short-cut carbon fibers in the composite material not being arranged densely enough. Therefore, its thermal conductivity was lower than that of Example 1.

[0104] In Comparative Example 1, ultrasonic and torsional vibration were removed. Without the auxiliary alignment effects of ultrasonic and torsional vibration, the orientation of the short-cut carbon fibers in the composite material was incomplete, and they easily overlapped to form "powder bridges," resulting in a less compact arrangement and a high likelihood of tilting. Figure 4 As shown, compared to the vertically arranged short-cut carbon fiber structure of Example 1, the carbon fiber arrangement in Comparative Example 1 has an inclined angle and is not a highly vertically oriented structure, resulting in a decrease in the thermal conductivity of Comparative Example 1 compared to Example 1. It can be seen that circumferential ultrasonic vibration and torsional vibration can ensure a well-oriented, tightly packed structure. Compared to Comparative Example 1, the thermal conductivity of Example 1 is improved by nearly 40%.

[0105] In Comparative Example 2, the short-cut carbon fibers in the composite material are distributed in a completely disordered manner, which prevents the high axial conductivity of the short-cut carbon fibers from being effectively utilized. Therefore, it has the lowest thermal conductivity among all embodiments and comparative examples.

[0106] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a highly oriented, closely packed carbon fiber thermally conductive composite material, characterized in that, The process includes the following steps: dispersing solvation-modified pitch-based chopped carbon fibers in a polymer solution to form a mixed solution; pouring the mixed solution into a mold equipped with a permanent magnet field, the magnetic field direction of which is parallel to the direction of gravity; simultaneously applying circumferential ultrasonic vibration and torsional vibration to the mold equipped with the permanent magnet field to obtain a highly ordered pitch-based chopped carbon fiber precipitate; then evaporating the solvent in the polymer solution to solidify the polymer, thus obtaining the final product. The circumferential ultrasonic vibration frequency is 40-120kHz, and the power is 15-300W; the torsional vibration frequency is 20-50Hz, and the vertical amplitude is 0.1-0.5mm. Simultaneously apply circumferential ultrasonic vibration and torsional vibration for 15-30 seconds, then stop the circumferential ultrasonic vibration and continue torsion vibration for 4-10 minutes.

2. The preparation method according to claim 1, characterized in that, The length of the pitch-based short-cut carbon fiber is 0.1-2 mm.

3. The preparation method according to claim 2, characterized in that, The length of the pitch-based short-cut carbon fiber is 0.15-0.25 mm.

4. The preparation method according to claim 1, characterized in that, The methods for solvation modification include chemical grafting, oxidative etching, or surface deposition; Alternatively, the preparation method of the solvation-modified pitch-based chopped carbon fiber is as follows: using a silane coupling agent as a chemical grafting agent, surface grafting is performed on the surface of the chopped carbon fiber; the silane coupling agent is selected from one of (3-mercaptopropyl)trimethoxysilane, γ-aminopropyltriethoxysilane, γ-(methacryloyloxy)propyltrimethoxysilane or γ-glycidoxypropyltrimethoxysilane.

5. The preparation method according to claim 1, characterized in that, The polymer is selected from one or more of silicone rubber, polyurethane, polydimethylsiloxane, polyvinyl alcohol, polyvinylpyrrolidone, and epoxy resin; the solvent of the polymer solution is selected from one or more of deionized water, ethanol, methanol, solvent oil, white oil, n-hexane, and petroleum ether.

6. The preparation method according to claim 1, characterized in that, In the polymer solution, the mass ratio of polymer to solvent is 0.02-0.08:1; The density of the polymer solution is less than 2.2 g / mL; In the mixed solution, the mass ratio of pitch-based short-cut carbon fibers to solvent is 0.02-0.15:

1.

7. The preparation method according to claim 6, characterized in that, The mass ratio of polymer to solvent is 0.03-0.05:

1.

8. The preparation method according to claim 6, characterized in that, The mass ratio of pitch-based chopped carbon fiber to solvent is 0.075-0.125:

1.

9. The preparation method according to claim 1, characterized in that, The permanent magnet field is generated by iron-neodymium-boron permanent magnets set on the upper and lower sides of the mold, and the magnetic field strength is 0.2-1T.

10. The preparation method according to claim 1, characterized in that, include: Following the polymer curing step, a surface treatment step is also included, wherein the surface treatment step is selected from any one of cutting, dissolution etching, or polishing.

11. A highly oriented, densely packed carbon fiber thermally conductive composite material, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 10.

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