Additive manufacturing composite material with high thermal conductivity and low expansion coefficient and preparation method thereof

Abstract

The invention discloses an additive manufacturing composite material with high thermal conductivity and low expansion coefficient and a preparation method thereof, and belongs to the technical field of composite materials, and the additive manufacturing composite material is a carbon fiber / carbon nanotube / aluminum oxide composite material. The preparation method comprises the following steps: enriching nano aluminum oxide on the surface of an aminated carbon nanotube, subsequently depositing the carbon nanotube on the surface of carbon fiber subjected to acid etching treatment, and extruding and granulating the treated carbon fiber and a resin matrix through a twin-screw extruder to obtain the composite material with low anisotropy and low thermal expansion coefficient.

Description

Technical Field

[0001] The present invention belongs to the technical field of composite materials, and in particular relates to an additively manufactured composite material with high thermal conductivity and low expansion coefficient and a preparation method thereof. Background Art

[0002] Thermoplastic composites offer excellent heat resistance, damage tolerance, and impact resistance, along with short molding cycles and high production efficiency, promising broad applications in the aerospace sector. 3D printing technology, through the "layer-by-layer" accumulation of materials, enables the direct molding of complex, special-shaped structures. The use of 3D printing to produce thermoplastic composite molds for aviation applications offers significant advantages over existing mold technologies in terms of small batch sizes, low cost, and rapid development. Consequently, this technology has become a research hotspot in the mold industry.

[0003] However, the thermal expansion coefficient of molds formed using the FDM printing process has significant anisotropy. Testing of printed samples revealed that the average linear expansion coefficient along the print layer height and print line width is large, far exceeding the thermal expansion coefficient of Q235A material, and cannot meet the requirements for mold use. To address this issue, researchers have attempted to solve the problems of excessively high thermal expansion coefficients of resin-based materials and anisotropy of additively manufactured parts by designing three-dimensional lattice structures with low thermal expansion coefficients and adding polymer composites with low thermal expansion coefficients and low dissipation factors to composite materials. However, these methods are relatively complex, and in the actual part production process, the mold and part structure are often limited, making it impossible to change the structural model. In addition, during the additive manufacturing process, the internal polymer material exhibits obvious orientation due to the shear force at the print nozzle, and its ability to solve anisotropy problems is slightly insufficient.

[0004] Our research revealed that when additively manufacturing chopped carbon fiber-reinforced resin-based composites, the layer stacking and fiber orientation result in significant anisotropy in the linear thermal expansion coefficient of the final part. Consequently, when preparing composite molds, the parts expand irregularly after heating, affecting their proper function. To address this issue, researchers have explored various approaches to address the high thermal expansion coefficient of resin-based materials and the anisotropy of additively manufactured parts.

[0005] For example, the patents of Huazhong University of Science and Technology [CN110125406A] and National University of Defense Technology [CNIO7643552A] both achieve the purpose of reducing the thermal expansion coefficient by designing a three-dimensional space lattice structure with a low thermal expansion coefficient. However, these methods are relatively complicated, and in the actual part production process, the mold and part structure are often limited, and the structural model cannot be changed. Therefore, it is necessary to solve the problem from the material itself. Patent [CN109867982A] achieves the purpose of reducing the thermal expansion coefficient by adding a liquid crystal polymer composite material with a low thermal expansion coefficient and a low dissipation factor to the composite material. However, when the composite material prepared by this method undergoes the additive manufacturing process, the internal polymer material shows obvious orientation due to the shear force at the printing nozzle, and its ability to solve the anisotropy problem is slightly insufficient. Patent [CN104549146A] discloses an alumina-modified multi-walled carbon nanotube nanocomposite. This method produces the CNT nanocomposite through an impregnation process. However, scanning electron microscopy reveals that the carbon nanotubes and alumina produced by this method intermingle and aggregate, making them unsuitable for additive manufacturing (AM) applications and prone to clogging and carbonization of printheads. Therefore, it is crucial to design a method for preparing a composite material with low anisotropy and a low thermal expansion coefficient suitable for AM. Summary of the Invention

[0006] In order to solve the above technical problems, the present invention provides an additive manufacturing composite material with high thermal conductivity and low thermal expansion coefficient and a preparation method thereof. The process involves enriching nano-alumina on the surface of amino-treated carbon nanotubes, subsequently depositing the carbon nanotubes on the surface of acid-etched carbon fibers, and granulating the treated carbon fiber filaments and resin matrix through twin-screw extrusion to obtain a composite material with low anisotropy and low thermal expansion coefficient.

[0007] The purpose of the present invention is achieved through the following technical solutions: An additive manufacturing composite material with high thermal conductivity and low expansion coefficient is a carbon fiber / carbon nanotube / aluminum oxide composite material.

[0008] A method for preparing an additive manufacturing composite material with high thermal conductivity and low expansion coefficient comprises the following steps: hydroxylating carbon nanotubes with a hydroxyl compound, partially aminated the hydroxylated carbon nanotubes with a silane coupling agent, fixing aluminum ions to the surface of the carbon nanotubes through the coordination of active amino functional groups, and obtaining carbon nanotubes loaded with nano-alumina after drying and high-temperature heating. Subsequently, carbon fibers are placed in a concentrated nitric acid solution, heated and stirred for a certain period of time, taken out and dried, and the treated carbon fibers and the carbon nanotubes still carrying some hydroxyl functional groups are placed in concentrated sulfuric acid and stirred so that the carbon nanotubes can be smoothly deposited on the surface of the carbon fibers. After removal, the carbon fibers are washed and dried, and heat-treated in a tubular furnace for a certain period of time to obtain a CFs-CNTs-Al2O3 sample. The sample is then mechanically mixed with polycarbonate resin powder and extruded and granulated through a twin-screw extruder to obtain an additive manufacturing composite material with high thermal conductivity and low expansion coefficient.

[0009] Preferably, the method specifically includes the following steps: Step 1: Weigh an appropriate amount of hydroxyl compound and carbon nanotubes, place them in anhydrous ethanol and perform ultrasonic stirring, then continue to heat in a water bath for reaction, filter, rinse repeatedly with deionized water and dry to obtain hydroxylated carbon nanotubes; Step 2: The product obtained in step 1 is placed in N,N-dimethylformamide solvent and ultrasonically dispersed uniformly, then heated in a water bath and an appropriate amount of silane coupling agent is added, refluxed and stirred to react, filtered and dried to obtain semi-hydroxyl and semi-amino carbon nanotubes; Step 3: Place the product obtained in step 2 in deionized water and ultrasonically disperse it uniformly. Then, add an appropriate amount of aluminum nitrate solution and an equal amount of ammonia solution and ultrasonically stir. After the reaction is completed, filter, rinse, and dry. Step 4: placing the product obtained in step 3 in a tube furnace and performing high-temperature heat treatment under a protective gas atmosphere to obtain carbon nanotube powder with nano-alumina uniformly distributed on the surface; Step 5: Take the chopped carbon fibers and completely immerse them in a concentrated nitric acid solution. After heating and stirring for a certain period of time, take them out and repeatedly wash them with deionized water until they are neutral. After drying, obtain the acid-etched carbon fibers. Step 6: Take the product obtained in step 4 and the product obtained in step 5, put them into concentrated sulfuric acid and completely immerse them, stir them for a certain period of time, take them out, wash them repeatedly with deionized water until they are neutral, and dry them to obtain a carbon fiber / carbon nanotube / alumina composite material; Step 7: Take the product obtained in step 5, preliminarily blend it with the base resin powder, and then transfer it to a grinder. Use the grinder to stir it at high speed for thorough physical mixing. Finally, place the mixed raw materials in a high-temperature drying oven to fully dry; Step 8: The product in step 6 is manually added to the twin-screw extruder through the feed port. After sufficient blending, shearing, and melting, a semi-molten composite wire is extruded to obtain the semi-molten wire. The semi-molten wire is cooled and solidified in a water cooling tank. Finally, the solidified composite wire is granulated by a pelletizer to obtain composite pellets of relatively uniform size.

[0010] Preferably, in step 1, the hydroxy compound is potassium hydroxide, the length of the carbon nanotubes is 30-50 μm, and the weight ratio of the hydroxy compound to the carbon nanotubes is ≥8.

[0011] Preferably, in step 1, the ultrasonic stirring time is 45-60 min; the water bath heating temperature is 90-100° C., the heating time is ≥10 h; the drying temperature is 75-85° C., and the drying time is ≥12 h.

[0012] Preferably, in the step 2, the ultrasonic dispersion time is 45-60 min; the silane coupling agent is an aminosilane coupling agent, the reflux stirring time is ≥12 h, the drying temperature is 75-85° C., and the drying time is ≥12 h.

[0013] Preferably, in step three, the ultrasonic dispersion time is 20-30 min, the reaction time after adding the aluminum nitrate solution is 30 min, the amount of ammonia solution added is 1.0-1.1 times the concentration of aluminum nitrate; the drying temperature is 75-85° C., and the drying time is ≥12 h.

[0014] Preferably, in step 4, during the heat treatment: heat up to 360°C at 10°C / min, keep warm for 2 hours; continue to heat up to 600°C at 10°C / min, keep warm for 3 hours; continue to heat up to 1200°C at 10°C / min, keep warm for 2 hours; and cool to room temperature.

[0015] Preferably, in step five, the length of the chopped carbon fibers is 3-5 mm; the concentration of concentrated nitric acid is 16 mol / L, the heating temperature is ≥80° C., and the heating time is ≥180 min.

[0016] Preferably, in step six, the concentration of concentrated sulfuric acid is 18 mol / L, and the stirring sedimentation time is ≥8 h.

[0017] Preferably, in step seven, the matrix resin is polycarbonate, polylactic acid or polyphenylene sulfide, the rotation speed of the pulverizer is 35000 r / min, and the stirring time is 1 min.

[0018] Preferably, in step eight, the head temperature of the twin-screw equipment is the melting point of the resin matrix T m +50℃, the temperature of zone 6-11 is T m +50℃, the temperature of zone 4-5 is T m +45℃, the temperature of zone 3 is T m+35℃, the temperature of zone 1-2 is T m +25℃.

[0019] The beneficial effects of this technical solution are as follows: 1. The present invention provides an additive manufacturing composite material with high thermal conductivity and low expansion coefficient. The thermal conductivity of conventional composite materials is about 0.4W / (m·K), while the thermal conductivity of the composite material is about 1.5-3.0W / (m·K), which is increased by 3.75-7.5 times; and the expansion coefficient and anisotropy are significantly reduced: the linear thermal expansion coefficient XY-CTE of conventional composite materials is 8ppm, and the Z-CTE is 30ppm. The linear thermal expansion coefficient XY-CTE of the composite material provided by the present invention is 3.4ppm, and the Z-CTE is 12.2ppm, which are reduced by 57.5% and 59.3%, respectively.

[0020] 2. The present invention provides a method for preparing a composite material for additive manufacturing with high thermal conductivity and low expansion coefficient. An aminosilane coupling agent is used to control the hydroxylation and transamination of carbon nanotubes, and the amount of aminosilane coupling agent is controlled to control the partial amination of carbon nanotubes, which can not only provide coordination sites for the enrichment of aluminum ions, but also provide sites for the deposition of carbon nanotubes on the surface of carbon fibers; nano-alumina is in an amorphous state and has a lower aspect ratio than carbon fibers, carbon nanotubes, etc., which can effectively reduce the anisotropy occurring in the additive manufacturing process; carbon nanotubes play a connecting role in the structure of the composite material, and the carbon fibers, carbon nanotubes, and nano-alumina are combined through chemical modification methods to give the composite material the characteristics of a filler: the mechanical properties are improved due to the increase of carbon fibers, the thermal conductivity is improved due to the increase of carbon nanotubes and nano-alumina, and the linear expansion coefficient is increased due to the increase of carbon fibers and nano-alumina. The anisotropy of the material is alleviated due to the increase of nano-alumina; carbon nanotubes are deposited on the surface of carbon fiber, and the overall structure is multi-branched. During the heat transfer process, the heat can be evenly diffused to the surroundings along the multi-branched structure, which can not only improve the thermal conductivity of the composite material, but also make the heat transfer more uniform, and reduce the stress problem caused by uneven heating during the use of parts; due to the presence of various active groups and nano-alumina on the surface of carbon nanotubes, the specific surface area is increased, and the dispersion of the material is also increased, making it difficult for the composite material filler to agglomerate during the preparation of the final product, and also plays a decisive role in maintaining various properties; the effect of the acid etching route on the carbon fiber surface can effectively increase its surface roughness, which can not only promote a stronger mechanical meshing between the additive and the resin, but also make the carbon nanotubes and carbon fibers more closely combined, further improving the dispersion of the additive and mechanical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a flow chart of the enrichment of nano-alumina on the surface of partially amination carbon nanotubes in the present invention; Figure 2 Schematic diagram of the synthesis of carbon fiber / carbon nanotube / alumina composite material in the present invention. DETAILED DESCRIPTION

[0022] The present invention will be further described in detail below with reference to the examples, but the embodiments of the present invention are not limited thereto.

[0023] Example 1 A method for preparing an additive manufacturing composite material with high thermal conductivity and low expansion coefficient, comprising: hydroxylating carbon nanotubes with a hydroxy compound, partially aminoating the hydroxylated carbon nanotubes with a silane coupling agent, and then fixing aluminum ions on the surface of the carbon nanotubes through the coordination of active amino functional groups. After drying and high-temperature heating, carbon nanotubes loaded with nano-alumina are obtained. Figure 1 As shown; the carbon fiber is subsequently placed in a concentrated nitric acid solution and heated and stirred for a certain period of time, then taken out and dried, and the treated carbon fiber and the carbon nanotubes still carrying some hydroxyl functional groups are placed in concentrated sulfuric acid and stirred so that the carbon nanotubes can be smoothly deposited on the surface of the carbon fiber. After being taken out, they are washed and dried, and after heat treatment in a tube furnace for a certain period of time, a CFs-CNTs-Al2O3 sample is obtained, as shown Figure 2 As shown, the above sample is then mechanically mixed with polycarbonate resin powder and extruded into granules through a twin-screw extruder to obtain an additive manufacturing composite material with high thermal conductivity and low expansion coefficient.

[0024] Specifically, the following steps are included: Step 1: Weigh an appropriate amount of hydroxyl compound and carbon nanotubes, place them in anhydrous ethanol and perform ultrasonic stirring, then continue to heat in a water bath for reaction, filter, rinse repeatedly with deionized water and dry to obtain hydroxylated carbon nanotubes; Step 2: The product obtained in step 1 is placed in N,N-dimethylformamide solvent and ultrasonically dispersed uniformly, then heated in a water bath and an appropriate amount of silane coupling agent is added, refluxed and stirred to react, filtered and dried to obtain semi-hydroxyl and semi-amino carbon nanotubes; Step 3: Place the product obtained in step 2 in deionized water and ultrasonically disperse it uniformly. Then, add an appropriate amount of aluminum nitrate solution and an equal amount of ammonia solution and ultrasonically stir. After the reaction is completed, filter, rinse, and dry. Step 4: placing the product obtained in step 3 in a tube furnace and performing high-temperature heat treatment under a protective gas atmosphere to obtain carbon nanotube powder with nano-alumina uniformly distributed on the surface; Step 5: Take the chopped carbon fibers and completely immerse them in a concentrated nitric acid solution. After heating and stirring for a certain period of time, take them out and repeatedly wash them with deionized water until they are neutral. After drying, obtain the acid-etched carbon fibers. Step 6: Take the product obtained in step 4 and the product obtained in step 5, put them into concentrated sulfuric acid and completely immerse them, stir them for a certain period of time, take them out, wash them repeatedly with deionized water until they are neutral, and dry them to obtain a carbon fiber / carbon nanotube / alumina composite material; Step 7: Take the product obtained in step 5, preliminarily blend it with the base resin powder, and then transfer it to a grinder. Use the grinder to stir it at high speed for thorough physical mixing. Finally, place the mixed raw materials in a high-temperature drying oven to fully dry; Step 8: The product in step 6 is manually added to the twin-screw extruder through the feed port. After sufficient blending, shearing, and melting, a semi-molten composite wire is extruded to obtain the semi-molten wire. The semi-molten wire is cooled and solidified in a water cooling tank. Finally, the solidified composite wire is granulated by a pelletizer to obtain composite pellets of relatively uniform size.

[0025] Wherein, in the step 1, the hydroxy compound is potassium hydroxide, the length of the carbon nanotubes is 30-50 μm, and the weight ratio of the hydroxy compound to the carbon nanotubes is ≥8.

[0026] Wherein, in the step 1, the ultrasonic stirring time is 45-60 minutes; the water bath heating temperature is 90-100° C., the heating time is ≥10 hours, the drying temperature is 75-85° C., and the drying time is ≥12 hours.

[0027] In the step 2, the ultrasonic dispersion time is 45-60 minutes; the silane coupling agent is an aminosilane coupling agent composed of a primary amino group (-NH2), a secondary amino group (-NH-) and Si-OR, preferably 3-triethoxysilyl-1-propylamine. The coupling agent is weighed and compared during addition to ensure that the hydroxyl group is partially converted; the reflux stirring time is ≥12 hours, the drying temperature is 75-85°C, and the drying time is ≥12 hours.

[0028] Among them, in the step three, the filtration and rinsing process must ensure that there are no residual aluminum ions, the ultrasonic dispersion time is 20-30 minutes, the reaction time after adding the aluminum nitrate solution is 30 minutes, and the amount of ammonia solution added is 1.0-1.1 times the concentration of aluminum nitrate; the drying temperature is 75-85°C, and the drying time is ≥12 hours.

[0029] Wherein, in the step 4, during the heat treatment: the temperature is raised to 360°C at 10°C / min and kept warm for 2 hours; the temperature is further raised to 600°C at 10°C / min and kept warm for 3 hours; the temperature is further raised to 1200°C at 10°C / min and kept warm for 2 hours; and the temperature is cooled to room temperature.

[0030] Wherein, in the step 5, the length of the chopped carbon fibers is 3-5 mm; the concentration of concentrated nitric acid is 16 mol / L, the heating temperature is ≥80° C., and the heating time is ≥180 min.

[0031] Wherein, in step six, the concentration of concentrated sulfuric acid is 18 mol / L, and the stirring sedimentation time is ≥8 h.

[0032] Wherein, in the step seven, the base resin is polycarbonate, polylactic acid or polyphenylene sulfide, the rotation speed of the crusher is 35000 r / min, and the stirring time is 1 min.

[0033] Wherein, in said step eight, the head temperature of the twin-screw equipment is the melting point T of the resin matrix. m +50℃, the temperature of zone 6-11 is T m +50℃, the temperature of zone 4-5 is T m +45℃, the temperature of zone 3 is T m +35℃, the temperature of zone 1-2 is T m +25℃ Example 2 An additively manufactured composite material with high thermal conductivity and low expansion coefficient comprises the following steps: 1) Preparation of hydroxylated carbon nanotubes, the steps are as follows: (1) weighing 5 g of carbon nanotubes and 40 g of potassium hydroxide, placing them in anhydrous ethanol and ultrasonically stirring for 60 min; (2) heating the obtained product in a water bath to 90°C and stirring for 12 h; (3) cooling to room temperature and filtering to obtain the reaction product, washing it with deionized water, and placing it in an oven at 85°C for 14 h; 2) Preparation of partially amino-modified carbon nanotubes, the steps are as follows: (1) weighing 5 g of the product obtained from the reaction in step 1, placing it in N,N-dimethylformamide solvent and ultrasonically dispersing it for 60 min; (2) taking 20 ml of 30 wt% γ-aminopropyltriethoxysilane solution, dropping it into the obtained solution, placing it in a water bath, heating it to 100°C, and reflux stirring for 12 h; (3) cooling it to room temperature, filtering the reaction product, washing it with deionized water, and placing it in an oven at 85°C for 14 h; 3) Preparation of carbon nanotube / nanoalumina composite material, the steps are as follows: (1) weighing 5g of the product obtained in the reaction of step 2, placing it in deionized water for ultrasonic dispersion for 60min; (2) taking 10g of aluminum nitrate and dissolving it in the solution, stirring it for 30min; (3) taking 100ml of 10% ammonia water and adding it dropwise into the solution, and continuing to stir for 60min to complete the reaction; (4) filtering the reaction product, washing it with deionized water, and placing it in an oven at 60℃ for 8h; (5) taking the product after heat preservation and drying, placing it in a tube furnace, and heat treating it in an argon atmosphere: heating it to 360℃ at 10℃ / min, keeping it warm for 2h; continuing to heating it to 600℃ at 10℃ / min, keeping it warm for 3h; continuing to heating it to 1200℃ at 10℃ / min, keeping it warm for 2h; cooling it to room temperature; 4) Preparation of acid-etched carbon fiber, the steps are as follows: (1) Take T300 short-cut carbon fiber with a length of 5 mm and place it in 16 mol / L concentrated nitric acid solution to completely immerse it; (2) Heat it to 80℃, keep it warm and stir it for 180 minutes; (3) Take out the carbon fiber, wash it repeatedly with deionized water until it is neutral, and dry it at 60℃ for 12 hours to remove moisture from the fiber surface.

[0034] 5) Depositing carbon nanotubes to prepare carbon fiber / carbon nanotube / nanoalumina composite materials, the steps are as follows: (1) taking 5 g of the product obtained in step 3 and 100 g of the product obtained in step 4 and placing them in 18 mol / L concentrated sulfuric acid to completely immerse them; (2) stirring for 8 h, taking them out, repeatedly washing them with deionized water until neutral, and drying them at 60 °C for 12 h to remove moisture from the fiber surface.

[0035] 6) Preparation of composite materials for additive manufacturing, the steps are as follows: (1) 100 g of the product obtained in step 5 is taken, preliminarily blended with 200 g of polycarbonate resin powder, and then transferred to a crusher, and fully physically mixed by high-speed stirring in the crusher (speed 35,000 r / min, 1 min), and finally the mixed raw materials are placed in a high-temperature drying oven for full drying (130°C, 4 h); (2) The pretreated dry mixture is manually added to a twin-screw extruder through the feed port, and after being fully blended, sheared, and melted in screw zones 1-11, a semi-molten composite wire is extruded to obtain a semi-molten composite wire. The semi-molten wire is cooled and solidified in a water cooling tank, and finally the solidified composite wire is granulated by a pelletizer to obtain composite pellets of relatively uniform size. The temperatures of each zone are: 255°C for zone 2, 265°C for zone 3, 275°C for zones 4-5, 280°C for zones 6-11, and 280°C for the die head.

[0036] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification or equivalent change made to the above embodiment based on the technical essence of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A composite material for additive manufacturing with high thermal conductivity and low coefficient of expansion, characterized by: The additive manufacturing composite material is a carbon fiber / carbon nanotube / aluminum oxide composite material.

2. A method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing, characterized by: The carbon nanotubes are hydroxylated by a hydroxy compound, and then partially amino-treated by a silane coupling agent. The aluminum ions are then fixed to the surface of the carbon nanotubes by the coordination of the active amino functional groups. After drying and high-temperature heating, carbon nanotubes loaded with nano-alumina are obtained. Subsequently, the carbon fiber is placed in a concentrated nitric acid solution, heated and stirred for a certain period of time, then taken out and dried. The treated carbon fiber and the carbon nanotubes still carrying some hydroxyl functional groups are then placed in concentrated sulfuric acid and stirred so that the carbon nanotubes can be smoothly deposited on the surface of the carbon fiber. After being taken out, they are washed and dried. After heat treatment in a tubular furnace for a certain period of time, a CFs-CNTs-Al2O3 sample is obtained. The above sample is then mechanically mixed with polycarbonate resin powder and extruded into granules through a twin-screw extruder to obtain an additive manufacturing composite material with high thermal conductivity and low expansion coefficient.

3. The method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing according to claim 2, characterized in that: The specific steps include: Step 1: Weigh an appropriate amount of hydroxyl compound and carbon nanotubes, place them in anhydrous ethanol and perform ultrasonic stirring, then continue to heat in a water bath for reaction, filter, rinse repeatedly with deionized water and dry to obtain hydroxylated carbon nanotubes; Step 2: The product obtained in step 1 is placed in N,N-dimethylformamide solvent and ultrasonically dispersed uniformly, then heated in a water bath and an appropriate amount of silane coupling agent is added, refluxed and stirred to react, filtered and dried to obtain semi-hydroxyl and semi-amino carbon nanotubes; Step 3: Place the product obtained in step 2 in deionized water and ultrasonically disperse it uniformly. Then, add an appropriate amount of aluminum nitrate solution and an equal amount of ammonia solution and ultrasonically stir. After the reaction is completed, filter, rinse, and dry. Step 4: placing the product obtained in step 3 in a tube furnace and performing high-temperature heat treatment under a protective gas atmosphere to obtain carbon nanotube powder with nano-alumina uniformly distributed on the surface; Step 5: Take the chopped carbon fibers and completely immerse them in a concentrated nitric acid solution. After heating and stirring for a certain period of time, take them out and repeatedly wash them with deionized water until they are neutral. After drying, obtain the acid-etched carbon fibers. Step 6: Take the product obtained in step 4 and the product obtained in step 5, put them into concentrated sulfuric acid and completely immerse them, stir them for a certain period of time, take them out, wash them repeatedly with deionized water until they are neutral, and dry them to obtain a carbon fiber / carbon nanotube / alumina composite material; Step 7: Take the product obtained in step 5, preliminarily blend it with the base resin powder, and then transfer it to a grinder. Use the grinder to stir it at high speed for thorough physical mixing. Finally, place the mixed raw materials in a high-temperature drying oven to fully dry; Step 8: The product in step 6 is manually added to the twin-screw extruder through the feed port. After sufficient blending, shearing, and melting, a semi-molten composite wire is extruded to obtain the semi-molten wire. The semi-molten wire is cooled and solidified in a water cooling tank. Finally, the solidified composite wire is granulated by a pelletizer to obtain composite pellets of relatively uniform size.

4. The method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing according to claim 3, characterized in that: In step 1, the hydroxyl compound is potassium hydroxide, the length of the carbon nanotubes is 30-50 μm, and the weight ratio of the hydroxyl compound to the carbon nanotubes is ≥8; the ultrasonic stirring time is 45-60 minutes; the water bath heating temperature is 90-100° C., the heating time is ≥10 hours, and the drying temperature is 75-85° C., and the drying time is ≥12 hours.

5. The method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing according to claim 4, characterized in that: In the step 2, the ultrasonic dispersion time is 45-60 minutes; the silane coupling agent is an aminosilane coupling agent, the reflux stirring time is ≥12 hours, the drying temperature is 75-85° C., and the drying time is ≥12 hours.

6. The method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing according to claim 5, characterized in that: In the step 3, the ultrasonic dispersion time is 20-30 minutes, the reaction time after adding the aluminum nitrate solution is 30 minutes, the amount of ammonia solution added is 1.0-1.1 times the concentration of aluminum nitrate; the drying temperature is 75-85° C., and the drying time is ≥12 hours.

7. The method for preparing a composite material with high thermal conductivity and low expansion coefficient by additive manufacturing according to claim 6, characterized in that: In the step 4, during the heat treatment: the temperature is raised to 360°C at 10°C / min and kept at this temperature for 2 hours; the temperature is further raised to 600°C at 10°C / min and kept at this temperature for 3 hours; the temperature is further raised to 1200°C at 10°C / min and kept at this temperature for 2 hours; and the temperature is cooled to room temperature.

8. The method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing according to claim 7, characterized in that: In the step 5, the length of the chopped carbon fibers is 3-5 mm; the concentration of concentrated nitric acid is 16 mol / L, the heating temperature is ≥80° C., and the heating time is ≥180 min.

9. The method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing according to claim 8, characterized in that: In step 6, the concentration of concentrated sulfuric acid is 18 mol / L, and the stirring sedimentation time is ≥ 8 h.

10. The method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing according to claim 9, characterized in that: In the step seven, the matrix resin is polycarbonate, polylactic acid or polyphenylene sulfide, the rotation speed of the pulverizer is 35000 r / min, and the stirring time is 1 min.

11. The method for preparing a composite material with high thermal conductivity and low expansion coefficient for additive manufacturing according to claim 10, characterized in that: In the step 8, the head temperature of the twin-screw equipment is the melting point of the resin matrix T m +50℃, the temperature of zone 6-11 is T m +50℃, the temperature of zone 4-5 is T m +45℃, the temperature of zone 3 is T m +35℃, the temperature of zone 1-2 is T m +25℃.

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