Preparation method of carbon fiber hard felt with density gradient

Abstract

The invention discloses a preparation method of a carbon fiber hard felt with density gradient. The preparation method comprises the steps of carbon fiber pretreatment, carbon nanotube suspension preparation, isolation layer formation, slurry preparation, molding, drying, ceramic coating coating, curing, carbonization, post-treatment and the like. The gradient change of the density of the carbon fiber hard felt is realized, so that the carbon fiber hard felt shows excellent comprehensive performance in the aspects of heat insulation, mechanical property, chemical stability and the like. The preparation method provided by the invention solves the problem that a binder cannot be completely removed in the steps of curing and carbonizing the carbon fiber hard felt, and through the synergistic effect of the ceramic buffer layer and the carbon nanotube isolation layer, the direct impact of high temperature on carbon fibers can be reduced, the risk of carbon fiber damage is reduced, and the service life of the carbon fiber hard felt is prolonged. Therefore, the overall performance of the carbon fiber hard felt is maintained. Due to the use of the isolating layer, not only is the direct contact between the ceramic coating and the phenolic resin binder blocked, but also the binding force between the layers is enhanced, and the overall performance of the carbon fiber hard felt is improved.

Description

Technical Field

[0001] This invention relates to the field of carbon fiber material preparation technology, specifically to a method for preparing carbon fiber rigid felt with a density gradient. Background Technology

[0002] Carbon fiber rigid felt has wide applications in aerospace, high-temperature industrial furnaces, and other fields due to its excellent properties such as high strength, low density, and high temperature resistance. However, traditional carbon fiber rigid felt usually has a uniform density, which cannot fully meet the differentiated material performance requirements of different parts in practical applications.

[0003] In existing technologies, certain structural components in aerospace vehicles require materials with high strength and wear resistance near the outer layer to withstand high-speed airflow, while the inner layer prioritizes thermal insulation and lightweight design. Carbon fiber rigid felt with density gradients can exhibit different properties in different regions according to actual needs, thereby better leveraging the material's advantages and improving the overall performance and reliability of the product. However, in existing technologies, achieving density gradients often relies on complex in-situ foaming and gradient impregnation equipment, which is not only costly but also susceptible to factors such as solvent evaporation and pressure unevenness, making it difficult to achieve a continuous and controllable linear gradient. For example, traditional methods, involving multiple adjustments to slurry concentration during layering, are prone to interlayer concentration diffusion, leading to blurred gradient boundaries. These issues—complex processes, high costs, and difficulty in precisely controlling the density gradient—limit the large-scale application of carbon fiber rigid felt with density gradients.

[0004] Meanwhile, during the curing and carbonization steps in the manufacturing process of carbon fiber rigid felt, the binder cannot be completely removed, leading to the disruption of the density gradient distribution of the carbon fiber rigid felt, clogging of pores, and reduction in oxidation resistance, chemical corrosion resistance, thermal insulation, thermal stability, strength, modulus, toughness, and fatigue resistance. Increasing the carbonization temperature or extending the holding time directly causes high-temperature oxidative damage to the carbon fiber, reducing its mechanical properties. This contradiction becomes the core obstacle restricting performance improvement.

[0005] Therefore, it is necessary to provide a method for preparing carbon fiber rigid felt with a density gradient that can precisely control the density gradient, has a simple process, and can completely remove the binder to solve the above-mentioned technical problems. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing carbon fiber rigid felt with a density gradient, so as to solve the problems in existing carbon fiber rigid felt preparation methods, such as the difficulty in accurately controlling the density gradient and the inability to completely remove the binder during the curing and carbonization steps.

[0007] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing carbon fiber rigid felt with a density gradient, comprising the following steps: Step 1: Carbon fiber pretreatment The carbon fiber precursor is cleaned to remove surface impurities and oil; then dried to obtain a pretreated carbon fiber preform. Step 2: Preparation of carbon nanotube suspension First, add carbon nanotubes to ethanol and mix thoroughly to form a uniform carbon nanotube suspension with a carbon nanotube content of 0.1% to 0.5%. Step 3: Forming an isolation layer The carbon fiber preform obtained in step one is completely immersed in the carbon nanotube suspension in step two; after an isolation layer is formed on the surface of the carbon fiber preform, the carbon fiber preform is removed and the ethanol on the surface is removed. Step 4: Slurry preparation The carbon fiber preform with the isolation layer formed in step three is placed into the binder solution and the carbon fiber preform is uniformly dispersed to obtain carbon fiber slurry; the carbon fiber slurry is divided into multiple portions in an arithmetic progression, and the carbon fiber content in different portions of the slurry is different. Step 5: Shaping The slurry with different carbon fiber contents from step four is laid in the mold in a layer-by-layer manner. After each layer of slurry is laid, it is pressed and shaped, and the thickness of each layer is controlled to be basically consistent, with a thickness error of no more than ±0.1mm, to obtain the shaped blank. Step Six: Drying Remove the molded blank from the mold in step five and dry it to remove the solvent from the slurry and allow the binder to initially cure. Step 7: Apply ceramic coating The pre-prepared ceramic coating is uniformly applied to the surface of the blank after drying in step six. After coating, it is dried to allow the coating to initially cure. Step 8: Curing and Carbonization The dried blank from step seven is cured and carbonized to obtain carbon fiber rigid felt. Step Nine: Post-processing The surface of the carbonized carbon fiber hard felt in step eight is polished to obtain carbon fiber hard felt with density gradient.

[0008] Currently, those skilled in the art believe that achieving a linear density gradient requires high-end equipment such as in-situ gradient dispensing machines and real-time density monitoring instruments, which is unaffordable for small and medium-sized enterprises. This perception limits density gradient rigid felt to high-end laboratory preparation and prevents large-scale production. However, this invention achieves gradient control using only conventional equipment such as arithmetic sequence dispensing and ordinary hydraulic pressing. It maintains gradient continuity and minimizes product performance fluctuations. This process lowers the equipment barrier, making the industrialization of density gradient rigid felt possible and breaking the industry's conventional wisdom that high precision necessarily relies on high-cost equipment.

[0009] Those skilled in the art generally believe that carbonization temperatures exceeding 1000℃ or holding times exceeding 2 hours will lead to excessive graphitization and increased brittleness of carbon fibers. Therefore, to protect the fibers, the degree of carbonization is often reduced, tolerating binder residue. However, this invention, through the dual protection of ceramic coating insulation and carbon nanotube isolation, achieves complete pyrolysis of the binder while avoiding damage to the carbon fibers at high temperatures of 800–1200℃, breaking the perception that these two factors are irreconcilable.

[0010] Preferably, in step one, the fiber diameter of the carbon fiber precursor is in the range of 5-10 μm; the specific cleaning steps are as follows: ultrasonic cleaning is performed with an aqueous solution containing a surfactant, the ultrasonic frequency is 20-40 kHz, and the cleaning time is 30-60 min; the surfactant is sodium dodecylbenzenesulfonate, and its mass fraction in the aqueous solution is 1%-1.5%; after ultrasonic cleaning, it is rinsed with deionized water 3-5 times and then dried.

[0011] Preferably, in step two, the carbon nanotubes are single-walled or multi-walled carbon nanotubes with a diameter ranging from 1 to 50 nm and a length ranging from 1 to 10 μm; analytical grade ethanol with a purity of not less than 99.7% is used as the solvent; during preparation, the carbon nanotubes are first added to the ethanol and stirred with a magnetic stirrer at a speed of 300 to 500 r / min for 1 to 2 hours to initially disperse the carbon nanotubes; then ultrasonic oscillation is performed with an ultrasonic power of 200 to 400 W for 2 to 3 hours to form a uniform carbon nanotube suspension.

[0012] This invention uses carbon nanotubes as an insulating layer. Their nanoscale diameter of 1–50 nm allows them to penetrate into the gaps between carbon fibers, both preventing contact between the ceramic coating and the binder and enhancing interlayer bonding through nanoscale interfacial bonding. The compressive strength is not reduced by the addition of this functional layer, achieving a synergy between protection and adhesion.

[0013] Preferably, in step three, the carbon fiber preform is immersed in the carbon nanotube suspension for 1 to 2 hours; after the carbon fiber preform is removed, it is first allowed to evaporate naturally in a well-ventilated environment for 1 to 2 hours, and then dried at a low temperature in an oven at 50 to 55°C for 2 to 3 hours to remove the remaining ethanol.

[0014] Preferably, in step four, the binder is phenolic resin, with a mass fraction of 15%–25% in the mixed solution; the solvent is ethanol, with a mass fraction of 75%–85% in the mixed solution; during the preparation process, the carbon fibers that have formed the isolation layer are added to the mixed solution containing phenolic resin and solvent, and stirred for 2–3 hours at a speed of 800–1200 r / min using a high-speed mixer to ensure that the carbon fibers are uniformly dispersed in the slurry; the carbon fiber slurry is divided into 3–5 portions, and the carbon fiber content in each portion increases in an arithmetic progression, with the difference in carbon fiber content between two adjacent portions of slurry being 5%–10%.

[0015] Preferably, in step five, the inner wall of the mold is polished and coated with silicone oil as a release agent; the thickness of each layer of slurry is 1-5 mm; after each layer of slurry is laid, it is lightly pressed with a hydraulic press, the pressing pressure is precisely controlled at 0.2-0.3 MPa, and the pressing time is 6-8 min; after the top layer of slurry is laid, the slurry is leveled with a scraper.

[0016] Preferably, in step six, the drying temperature of the drying oven is 50-80°C, the drying time is 16-20 hours, and during the drying process, 2-3 temperature gradients are set, with each temperature increase being 10-15°C and the interval between each temperature increase being 2-3 hours.

[0017] Preferably, in step seven, the main material of the ceramic coating is alumina powder with a particle size ranging from 0.1 to 1 μm. The ceramic powder is mixed with an appropriate amount of binder II and solvent to prepare a uniform ceramic coating. The mass ratio of ceramic powder, binder II, and solvent is 5–7:1.5–2.5:1.5–2.5. The binder II is silica sol, and the solvent is water. The ceramic coating is uniformly applied to the surface of the dried green body by spraying or brushing. When spraying, the spraying pressure is 0.2–0.3 MPa, and the spraying distance is 10–15 cm. Alternatively, when brushing, the coating thickness is controlled at 50–100 μm. The drying process is as follows: the green body is first naturally dried at room temperature for 2–3 hours, and then dried in an oven at 80–100°C for 1–2 hours.

[0018] Preferably, in step eight, in a high-temperature furnace, firstly, under nitrogen protection, the temperature is raised to 150-200°C at a heating rate of 2-5°C / min and held for 2-4 hours to allow the adhesive to fully cure; then, the temperature is further raised to 800-1200°C at a heating rate of 5-10°C / min and held for 1-3 hours to perform carbonization treatment, thereby obtaining carbonized carbon fiber rigid felt.

[0019] Preferably, in step nine, the surface of the carbonized carbon fiber hard felt is polished with sandpaper or a grinding wheel, with the sandpaper grit gradually increasing from 200 grit to 800 grit to ensure that the surface flatness reaches Ra1.6~3.2μm; finally, the carbon fiber hard felt is cut into the required size using a cutting device, with the cutting accuracy controlled within ±0.5mm.

[0020] Compared with the prior art, the beneficial effects of the present invention are: This invention utilizes a simplified process of incrementally dividing carbon fiber slurry into equal layers and precisely pressing them layer by layer. This process controls the carbon fiber content difference within 5%–10%, combined with precise pressure application of 0.2–0.3 MPa and thickness error control of ±0.1 mm, achieving linear and predictable density gradient. Experimental data show that the thermal insulation performance of Example 3 reaches 0.032 W / (m·K), which is 23.8% higher than that of Comparative Example 3 without gradient design and 36% higher than that of traditional uniform density carbon fiber rigid felt. This demonstrates that the process simplifies operation and overcomes the bottleneck of gradient control precision.

[0021] In this invention, the alumina ceramic coating acts as a high-temperature barrier, reducing the direct impact of high temperatures on the carbon fibers during the 800–1200℃ carbonization process. The carbon nanotube isolation layer prevents direct contact between the ceramic coating and the phenolic resin, avoiding a reaction that could affect the adhesion. Furthermore, the stepped heating curing and carbonization process provides sufficient time for the pyrolysis of the binder. Experiments show that the compressive strength of Example 1 is 3.2 MPa, a 23% increase compared to Comparative Example 5 (which was not sufficiently carbonized), and no carbon fiber breakage occurred, completely resolving the contradiction between completely removing the binder and protecting the carbon fibers.

[0022] In summary, this invention increases the likelihood of binder removal by extending carbonization time, increasing carbonization temperature, and optimizing heating rate, thereby restoring and improving the density gradient distribution, pore structure, and various properties of the product. Simultaneously, a high-temperature resistant ceramic coating is used as a buffer layer to reduce the direct impact of high temperatures on the carbon fibers, ensuring their performance and maintaining overall performance. Furthermore, carbon nanotubes are used as an isolation layer to prevent chemical reactions between the buffer layer material and the binder, enhancing the bonding strength between layers and improving overall performance. Detailed Implementation

[0023] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1: This example provides a method for preparing carbon fiber rigid felt with a density gradient, including the following steps: Step 1: Carbon fiber pretreatment The carbon fiber precursor was ultrasonically cleaned in an aqueous solution containing a surfactant to remove surface impurities and oil. The cleaned carbon fiber precursor was rinsed with deionized water and then dried in an oven to obtain a pretreated carbon fiber preform. Step 2: Preparation of carbon nanotube suspension First, carbon nanotubes are added to ethanol and stirred with a magnetic stirrer, then subjected to ultrasonic oscillation to form a uniform carbon nanotube suspension. Step 3: Forming an isolation layer The carbon fiber preform obtained in step one is completely immersed in the carbon nanotube suspension in step two; after the carbon fiber preform is removed, it is first allowed to evaporate naturally in a well-ventilated environment, and then placed in a low-temperature oven to evaporate the remaining ethanol. Step 4: Slurry preparation The carbon fiber preform with the isolation layer formed in step three is placed into the adhesive solution one, which is phenolic resin. The mixture is stirred with a mixer to ensure that the carbon fiber is evenly dispersed in the slurry. The carbon fiber slurry is divided into multiple portions in an arithmetic progression, with different carbon fiber contents in different portions. Step 5: Shaping The slurry with different carbon fiber contents from step four was laid in the mold in a layer-by-layer manner. After each layer of slurry was laid, it was lightly pressed with a hydraulic press. After the top layer of slurry was laid, the slurry was scraped flat. Step Six: Drying Remove the molded blank from the mold in step five and place it in a drying oven to remove the solvent from the slurry and allow the binder to initially cure. Step 7: Apply ceramic coating Ceramic powder is mixed with binder 2 and solvent to prepare a uniform ceramic coating. The mass ratio of ceramic powder, binder and solvent is 6:2:2. The binder 2 is silica sol. The ceramic coating is uniformly applied to the surface of the blank after drying in step six. After coating, the blank is first dried naturally at room temperature, and then dried in an oven to allow the coating to initially cure. Step 8: Curing and Carbonization The dried blank from step seven is placed in a high-temperature furnace for curing and carbonization. Step Nine: Post-processing The surface of the carbonized carbon fiber hard felt in step eight is polished, and finally the carbon fiber hard felt is cut into the required size to obtain carbon fiber hard felt with density gradient.

[0025] The carbon fiber rigid felt prepared by the density gradient carbon fiber rigid felt preparation method provided by this invention has the following performance characteristics: through the synergistic effect of density gradient design, carbon nanotube isolation layer and alumina ceramic coating, the thermal conductivity of the rigid felt is as low as 0.032 W / (m·K), the compressive strength is as high as 3.5 MPa, and the acid and alkali corrosion resistance time is as long as 130 h, achieving a comprehensive improvement in thermal, mechanical and chemical stability. In terms of process, by using an arithmetic sequence to set the carbon fiber content of the slurry and a strict molding process, the density gradient is precisely controlled, ensuring the repeatability and stability of the process, which is conducive to large-scale production. Post-processing involves sanding the rigid felt in stages to achieve a surface smoothness of Ra1.6-3.2 μm and a cutting accuracy controlled within ±0.5 mm, improving surface quality and performance.

[0026] This invention addresses the problems caused by the incomplete removal of binder during the curing and carbonization steps, which disrupts the density gradient distribution of carbon fiber rigid felt, clogs pores, and reduces its oxidation resistance, chemical corrosion resistance, thermal insulation, thermal stability, strength, modulus, toughness, and fatigue resistance. However, increasing the carbonization temperature or extending the carbonization time to remove the binder may damage the carbon fibers, affecting their performance. To solve this problem, this invention adds a buffer layer, such as the ceramic coating in this embodiment, to the surface of the carbon fiber rigid felt blank. This provides thermal insulation and protection for the carbon fibers. However, the buffer layer material may chemically react with the binder, affecting the binder's curing process and bonding performance, leading to a decrease in the bonding force between the carbon fibers. Simultaneously, by appropriately extending the holding time at 800-1200℃, more time can be provided for the binder's pyrolysis and the volatilization of organic components. The heating rate can also be appropriately slowed down to allow the binder more time for pyrolysis. Furthermore, an isolation layer with good chemical stability and barrier properties is placed between the buffer layer (coating material) and the binder to prevent direct contact while maintaining good bonding strength with both. In this embodiment, carbon nanotubes are used as an isolation layer. They are dispersed in a solvent to form a suspension, and the carbon nanotubes are attached to the surface of carbon fibers by impregnation.

[0027] Specifically, this embodiment increases the possibility of binder removal by extending the carbonization time, increasing the carbonization temperature, and optimizing the heating rate. This helps to restore and improve the original density gradient distribution, pore structure, oxidation resistance, chemical corrosion resistance, thermal insulation, thermal stability, and mechanical properties of the carbon fiber rigid felt, thereby improving product quality and performance. The application of a high-temperature resistant buffer layer (such as a ceramic coating) effectively reduces the direct impact of high temperatures on the carbon fiber, lowers the risk of carbon fiber damage, ensures the performance of the carbon fiber, and thus maintains the overall performance of the carbon fiber rigid felt. The design of the isolation layer and the adjustment of the binder formulation effectively solve the incompatibility problem between the buffer layer material and the binder, preventing chemical reactions between the two from affecting the bonding performance. The use of a carbon nanotube isolation layer not only blocks direct contact between the ceramic coating and the phenolic resin binder but also enhances the bonding force between the layers, improving the overall performance of the carbon fiber rigid felt.

[0028] More specifically, in step one, the fiber diameter of the carbon fiber precursor is in the range of 5 to 10 μm; the ultrasonic cleaning frequency is 30 kHz and the cleaning time is 50 min; the surfactant is sodium dodecylbenzenesulfonate, and its mass fraction in the aqueous solution is 1.2%; after cleaning, it is rinsed with deionized water 4 times.

[0029] This step effectively removes impurities and oil stains from the surface of carbon fibers, improves the bonding performance between carbon fibers and subsequent materials, and lays the foundation for the preparation of high-performance carbon fiber rigid felt.

[0030] More specifically, in step two, single-walled or multi-walled carbon nanotubes are selected, with a diameter ranging from 1 to 50 nm and a length ranging from 1 to 10 μm; analytical grade ethanol with a purity of not less than 99.7% is used as the solvent; during preparation, the carbon nanotubes are first added to the ethanol and stirred at a speed of 400 r / min for 1.5 h using a magnetic stirrer to initially disperse the carbon nanotubes; then, ultrasonic oscillation is performed with an ultrasonic power of 300 W for 2.5 h to form a uniform carbon nanotube suspension with a carbon nanotube content of 0.3%.

[0031] In this step, the addition of carbon nanotubes improves the performance of the insulating layer and enhances the electrical conductivity, mechanical properties, and thermal stability of the carbon fiber rigid felt.

[0032] More specifically, in step three, the carbon fiber preform is immersed in the carbon nanotube suspension for 1.5 hours; after the carbon fiber preform is removed, it is allowed to evaporate naturally for 1.5 hours, and then dried in a low-temperature oven at 50°C for 2.5 hours to remove the remaining ethanol.

[0033] In this step, the formation of the isolation layer protects the carbon fiber, reduces the direct erosion of the carbon fiber by binders and other agents in subsequent processes, and improves the interfacial bonding performance between the carbon fiber and the binder.

[0034] More specifically, in step four, the phenolic resin has a mass fraction of 20% in the mixed solution; the solvent is ethanol, which has a mass fraction of 80% in the mixed solution; during the preparation process, the carbon fibers that have formed the isolation layer are added to the mixed solution containing phenolic resin and solvent, and stirred at 1000 r / min for 2.5 h using a high-speed mixer to ensure that the carbon fibers are uniformly dispersed in the slurry; the carbon fiber slurry is divided into 4 parts, and the difference in carbon fiber content between two adjacent parts of the slurry is 6%.

[0035] In this step, by precisely controlling the gradient change of carbon fiber content, the carbon fiber rigid felt has different properties in different parts to meet specific application requirements.

[0036] More specifically, in step five, the inner wall of the mold is polished and coated with silicone oil as a release agent; after each layer of slurry is laid, it is pressed using a hydraulic press, with the pressing pressure precisely controlled at 0.2 to 0.3 MPa and the pressing time at 6 to 8 minutes, ensuring that the thickness of each layer of slurry is 3 mm and the thickness error of each layer of slurry does not exceed ±0.1 mm; after the top layer of slurry is laid, the slurry is leveled using a scraper.

[0037] This step enables precise control of the density gradient, enhances the bonding force between layers, and gives the carbon fiber rigid felt excellent integrity.

[0038] More specifically, in step six, the initial drying temperature is 55℃, and the temperature is maintained for 3 hours; then the temperature is increased to 70℃ and maintained for 3 hours; finally, the temperature is increased to 80℃ and maintained for 8 hours, with the total drying time being 17 hours. That is, during the drying process, two temperature gradients are set, each increasing by 10-15℃, with a 3-hour interval between each increase.

[0039] In this step, the drying process effectively removes the solvent from the slurry, allowing the binder to initially solidify and ensuring the strength and shape stability of the blank.

[0040] More specifically, in step seven, the main material of the ceramic coating is alumina, and the particle size of the ceramic powder ranges from 0.1 to 1 μm. The ceramic powder is mixed with an appropriate amount of silica sol and water to prepare a uniform ceramic coating. The mass ratio of ceramic powder, silica sol and water is 6:2:2. The ceramic coating is uniformly applied to the surface of the dried green body by spraying or brushing. The green body is naturally dried at room temperature for 2.5 hours, and then dried in an oven at 90°C for another 1.5 hours. During spraying, the spraying pressure is 0.2 MPa and the spraying distance is 10 cm. Alternatively, when brushing, the coating thickness should be controlled at around 80μm.

[0041] In this step, the ceramic coating improves the surface properties and service life of the carbon fiber rigid felt, and enhances its resistance to high temperatures, abrasion, and corrosion.

[0042] More specifically, in step eight, in a high-temperature furnace, firstly, under nitrogen protection, the temperature is raised to 180°C at a heating rate of 3°C / min and held for 3 hours to allow the adhesive to fully cure; then, the temperature is raised to 1000°C at a heating rate of 8°C / min and held for 2 hours to perform carbonization treatment, thereby obtaining carbonized carbon fiber rigid felt.

[0043] In this step, the curing and carbonization processes further enhance the bonding strength between carbon fibers and between carbon fibers and the ceramic coating, improving the overall strength and stability of the product.

[0044] More specifically, in step nine, the surface of the carbonized carbon fiber hard felt is polished using sandpaper or a grinding wheel, with the sandpaper grit gradually increasing from 200 grit to 800 grit to ensure that the surface flatness reaches Ra1.6~3.2μm; finally, the carbon fiber hard felt is cut into the required size using a cutting device, with the cutting accuracy controlled within ±0.5mm.

[0045] In this embodiment, the post-processing makes the surface of the carbon fiber rigid felt smoother and the dimensions more accurate, thereby improving the appearance quality and performance of the product.

[0046] Example 2: Basically the same as Example 1, except that:

[0047] In step one, the ultrasonic frequency is 20 kHz and the cleaning time is 60 min; the mass fraction of sodium dodecylbenzenesulfonate in the aqueous solution is 1%; after ultrasonic cleaning, it is rinsed 3 times with deionized water.

[0048] In step two, the carbon nanotubes were stirred at 300 r / min for 2 hours to initially disperse them; then, ultrasonic oscillation was performed with an ultrasonic power of 400 W for 2 hours; the carbon nanotube content was 0.1%.

[0049] In step three, the carbon fiber preform is immersed in the carbon nanotube suspension for 1 hour; the natural evaporation time is 1 hour; and it is dried at 55°C for 2 hours.

[0050] In step four, the mass fraction of phenolic resin in the mixed solution is 15%; the mass fraction of ethanol in the mixed solution is 85%; during the preparation process, the mixture is stirred at 800 r / min for 3 hours; the carbon fiber slurry is divided into 3 portions, and the difference in carbon fiber content between two adjacent portions is 10%.

[0051] In step five, the thickness of each layer of slurry is 5mm, and the thickness error does not exceed ±0.1mm.

[0052] In step six, the initial drying temperature is 50℃, and the temperature is maintained for 2 hours; then the temperature is increased to 65℃ and maintained for 3 hours; finally, the temperature is increased to 80℃ and maintained for 7 hours, with a total drying time of 16 hours. That is, during the drying process, two temperature gradients are set, each increasing by 15℃, with an interval of 2 or 3 hours between each increase.

[0053] In step seven, the mass ratio of ceramic powder, silica sol, and water is 5:2.5:2.5; the green body is naturally dried at room temperature for 2 hours, and then dried in an oven at 100°C for 1 hour; the ceramic coating is uniformly applied to the surface of the dried green body by spraying or brushing; when spraying, the spraying pressure is 0.3 MPa and the spraying distance is 15 cm; or when brushing, the coating thickness is controlled at about 50 μm.

[0054] In step eight, firstly, under nitrogen protection, the temperature is raised to 150℃ at a heating rate of 2℃ / min and held for 4 hours to allow the adhesive to fully cure; then, the temperature is raised to 800℃ at a heating rate of 5℃ / min and held for 3 hours to perform carbonization treatment.

[0055] Everything else is the same as in Example 1.

[0056] Example 3: Basically the same as Example 1, except that: In step one, the ultrasonic frequency was 40 kHz and the cleaning time was 30 min; the mass fraction of sodium dodecylbenzenesulfonate in the aqueous solution was 1.5%; after ultrasonic cleaning, it was rinsed 5 times with deionized water.

[0057] In step two, the carbon nanotubes are stirred at 500 r / min for 1 h to initially disperse them; then ultrasonic oscillation is performed with an ultrasonic power of 200 W for 3 h; the carbon nanotube content is 0.5%.

[0058] In step three, the carbon fiber preform is immersed in the carbon nanotube suspension for 2 hours; the natural evaporation time is 2 hours; and it is dried at 50°C for 3 hours.

[0059] In step four, the mass fraction of phenolic resin in the mixed solution is 25%; the mass fraction of ethanol in the mixed solution is 75%; during the preparation process, the mixture is stirred at 1200 r / min for 2 hours; the carbon fiber slurry is divided into 5 portions, and the difference in carbon fiber content between two adjacent portions is 5%.

[0060] In step five, the thickness of each layer of slurry is 1 mm, and the thickness error does not exceed ±0.1 mm.

[0061] In step six, the initial drying temperature is 50℃, and the temperature is maintained for 2 hours; then the temperature is increased to 60℃ and maintained for 2 hours; then the temperature is increased again to 70℃ and maintained for 2 hours; finally, the temperature is increased to 80℃ and maintained for 10 hours, with a total drying time of 20 hours. That is, during the drying process, three temperature gradients are set, each increasing by 10℃, with a 2-hour interval between each increase.

[0062] In step seven, the mass ratio of ceramic powder, silica sol, and water is 7:1.5:1.5; the green body is naturally dried at room temperature for 3 hours, and then dried in an oven at 80°C for another 2 hours; the ceramic coating is uniformly applied to the surface of the dried green body by spraying or brushing; when spraying, the spraying pressure is 0.3 MPa and the spraying distance is 12 cm; or when brushing, the coating thickness is controlled at about 100 μm.

[0063] In step eight, firstly, under nitrogen protection, the temperature is raised to 200℃ at a heating rate of 5℃ / min and held for 2 hours to allow the adhesive to fully cure; then, the temperature is raised to 1200℃ at a heating rate of 10℃ / min and held for 1 hour to perform carbonization treatment.

[0064] Everything else is the same as in Example 1.

[0065] Comparative Example 1: The traditional method of preparing carbon fiber rigid felt with uniform density is used, without density gradient control, carbon nanotube isolation layer, or ceramic coating.

[0066] Comparative Example 2: Without setting an isolation layer, the other steps are the same as in Example 1. That is, the pretreated carbon fibers are directly used for slurry preparation, skipping the steps of carbon nanotube suspension preparation and isolation layer formation.

[0067] Comparative Example 3: No density gradient is set. The only difference from Example 1 is that the carbon fiber slurry is not divided into multiple parts in step four; and the carbon fiber slurry is directly laid in the mold in step five. The rest is the same as in Example 1.

[0068] Comparative Example 4: Without applying a ceramic coating, step seven is omitted. The dried green body from step six is ​​directly cured and carbonized, and the rest is the same as in Example 1.

[0069] Comparative Example 5: The only difference from Example 1 is that during the carbonization process in step eight, the temperature is raised to 700°C and held for 40 minutes; otherwise, it is the same as Example 1.

[0070] Experimental tests were conducted on the above embodiments and comparative examples: 1. Thermal insulation performance test Test method: Thermal conductivity meter was used for measurement, in accordance with standard GB / T 10295-2008.

[0071] Sample preparation: The carbon fiber rigid felt was cut into samples with dimensions of 100mm×100mm×10mm, and the surface was flat and without defects.

[0072] Test conditions: room temperature (25±2℃), relative humidity 50±5%, heat flux density 50 W / m², test time 60 s.

[0073] 2. Compressive strength test Test method: Using a universal testing machine, according to standard GB / T 8489-2006.

[0074] Sample size: 20mm×20mm×20mm cube specimen.

[0075] Test conditions: Compression rate 1 mm / min until the sample is damaged or the deformation reaches 30% of the original height, and record the maximum compressive stress.

[0076] 3. Acid and alkali corrosion resistance test Test method: Refer to GB / T 1763-2021 and adjust according to the actual application environment.

[0077] Reagents: Acid solution: 10% H2SO4 solution; Alkaline solution: 10% NaOH solution.

[0078] Test procedure: The sample (50mm×50mm×5mm) is completely immersed in the reagent. Every 24 hours, it is taken out, rinsed, dried and weighed until the mass loss exceeds 5% or obvious corrosion occurs. The time is recorded.

[0079] 4. Surface flatness test Test method: Use a surface roughness tester, according to standard GB / T 3505-2009.

[0080] Sampling method: Five different locations were randomly selected on the surface of each sample for measurement, and the arithmetic mean was taken as the final Ra value.

[0081] The experimental data between the above embodiments and comparative examples are shown in Table 1: Table 1. Experimental data for the examples and comparative examples.

[0082] Based on Table 1, comparing Example 1 with Comparative Example 1, it can be seen that the data of Example 1 are all better than those of Comparative Example 1. This shows that the present invention completely solves the pain points of disordered pores, weak interface bonding, and lack of protection of traditional hard felt through the triple innovation of density gradient design, carbon nanotube isolation layer, and ceramic coating, and its performance completely surpasses the basic process.

[0083] Comparing Example 1 with Comparative Examples 2-5, it is evident that the data from Example 1 are superior to those from Comparative Examples 2-5. This demonstrates that the density gradient design, carbon nanotubes, ceramic coating, and precise carbonization are all indispensable features of this invention; the absence of any one of them will lead to significant performance degradation. Specifically, the ceramic coating is the core guarantee of corrosion resistance; the carbon nanotube isolation layer plays a crucial role in heat insulation and strength enhancement; the density gradient design is fundamental to controlling thermal stress distribution and maintaining flatness; sufficient carbonization is essential to avoid resin residue, thereby reducing thermal conductivity, enhancing interfacial bonding, improving strength, and reducing organic corrosion.

[0084] In summary, the carbon fiber rigid felt with density gradient provided by the present invention is significantly superior to the comparative example in terms of thermal insulation performance, compressive strength, chemical stability and surface smoothness, which fully demonstrates the effectiveness and superiority of the preparation method of the present invention.

[0085] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for producing a carbon fiber hard felt having a density gradient, characterized by, The method comprises the following steps: Step 1: carbon fiber pretreatment The carbon fiber raw yarn is cleaned to remove surface impurities and oil stains, and then dried to obtain a pretreated carbon fiber blank; Step 2: preparation of carbon nanotube suspension Carbon nanotubes are added to ethanol and mixed thoroughly to form a uniform carbon nanotube suspension, with a carbon nanotube content of 0.1% to 0.5%; Step 3: forming an isolation layer The carbon fiber blank obtained in step 1 is completely immersed in the carbon nanotube suspension in step 2; after the formation of an isolation layer on the surface of the carbon fiber blank, the carbon fiber blank is removed, and the surface ethanol is then removed; Step 4: slurry preparation The carbon fiber blank with the formed isolation layer in step 3 is placed in a binder solution, and the carbon fiber blank is uniformly dispersed to obtain a carbon fiber slurry; the carbon fiber slurry is divided into multiple portions in an increasing arithmetic sequence, and the carbon fiber content in different portions is different; Step 5: forming The slurry with different carbon fiber contents in step 4 is sequentially laid in the mold in a layer-by-layer manner, each layer of slurry is laid and then pressed to form, and the thickness of each layer is controlled to be substantially uniform, with a thickness error of not more than ±0.1 mm, to obtain a formed blank; Step 6: drying The formed blank in step 5 is taken out of the mold and dried to remove the solvent in the slurry and preliminarily solidify the binder; Step 7: coating a ceramic coating A previously prepared ceramic coating is uniformly coated on the surface of the dried blank in step 6, and after coating is completed, drying treatment is performed to preliminarily solidify the coating; Step 8: curing and carbonization The dried blank in step 7 is subjected to curing and carbonization treatment to obtain a carbon fiber hard felt; Step 9: post-treatment The surface of the carbonized carbon fiber hard felt in step 8 is polished to obtain a carbon fiber hard felt with a density gradient.

2. The method according to claim 1, wherein: In step 1, the fiber diameter of the carbon fiber raw yarn ranges from 5 to 10 μm; And / or, the specific steps of cleaning are: ultrasonic cleaning with a water solution containing a surfactant, the ultrasonic frequency is 20-40 kHz, and the cleaning time is 30-60 min; the surfactant is sodium dodecyl benzene sulfonate, and the mass fraction in the water solution is 1%-1.5%; And / or, after ultrasonic cleaning, rinse with deionized water for 3-5 times, and then dry.

3. The method according to claim 1, wherein: In step 2, the carbon nanotubes are single-walled or multi-walled carbon nanotubes with a tube diameter ranging from 1 to 50 nm and a length of 1 to 10 μm; And / or, after adding the carbon nanotubes to ethanol, first stir at a speed of 300-500 r / min for 1-2 h to preliminarily disperse the carbon nanotubes; then ultrasonic oscillation is performed, the ultrasonic power is 200-400 W, and the ultrasonic time is 2-3 h to form a uniform carbon nanotube suspension.

4. The method according to claim 1, wherein: In step 3, the immersion time of the carbon fiber blank in the carbon nanotube suspension is 1-2 h. And / or, after taking out the carbon fiber blank, first in a well-ventilated environment, natural evaporation, natural evaporation time is 1~2h; then low temperature drying, drying temperature of 50~55℃ 2~3h, to remove the remaining ethanol.

5. The preparation method of the carbon fiber hard felt with density gradient according to claim 1, characterized in that: In step four, the binder is phenolic resin, and the mass fraction of the phenolic resin in the mixed solution is 15%~25%; the solvent is ethanol, and the mass fraction of the ethanol in the mixed solution is 75%~85%; And / or, during the preparation process, stirring at a speed of 800~1200r / min for 2~3h to ensure that the carbon fibers are uniformly dispersed in the slurry; And / or, the carbon fiber slurry is divided into 3~5 parts, and the difference between the carbon fiber content of adjacent two parts is 5%~10%.

6. The preparation method of the carbon fiber hard felt with density gradient according to claim 1, characterized in that: In step five, the thickness of each layer of the slurry is 1~5mm; And / or, after laying each layer of the slurry, the pressing pressure is accurately controlled at 0.2~0.3MPa, and the pressing time is 6~8min.

7. The preparation method of the carbon fiber hard felt with density gradient according to claim 1, characterized in that: In step six, the drying temperature is 50~80℃, and the drying time is 16~20h, and during the drying process, 2~3 times of temperature gradient is set, each time the temperature is increased by 10~15℃, and each time the temperature is increased for 2~3h.

8. The preparation method of the carbon fiber hard felt with density gradient according to claim 1, characterized in that: In step seven, the main material of the ceramic coating is alumina powder, and the particle size range is 0.1~1μm; And / or, the ceramic coating is prepared by mixing the ceramic powder, the binder two and the solvent, and the mass ratio of the ceramic powder, the binder two and the solvent is 5~7:1.5~2.5:1.5~2.5, the binder two is silica sol, and the solvent is water; And / or, the drying treatment is: first, the blank is naturally dried at room temperature for 2~3h, and then it is continuously dried in an oven at 80~100℃ for 1~2h.

9. The preparation method of the carbon fiber hard felt with density gradient according to claim 1, characterized in that: In step eight, first, the temperature is increased to 150~200℃ at a heating rate of 2~5℃ / min under nitrogen protection, and the temperature is kept for 2~4h to make the binder completely cured; then, the temperature is continuously increased to 800~1200℃ at a heating rate of 5~10℃ / min, and the temperature is kept for 1~3h to perform carbonization treatment, and the carbonized carbon fiber hard felt is obtained.

10. The preparation method of the carbon fiber hard felt with density gradient according to claim 1, characterized in that: In step nine, the surface of the carbonized carbon fiber hard felt is polished by using sandpaper or a grinding wheel, and the mesh number of the sandpaper is gradually increased from 200 mesh to 800 mesh to ensure that the surface flatness reaches Ra1.6~3.2μm.

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