Preparation method of near-stoichiometric aluminum-doped silicon carbide fiber

Abstract

The invention discloses a preparation method of a near-stoichiometric aluminum-doped silicon carbide fiber, which comprises the following steps: doping PMS with an aluminum element, blending with PCS, and sequentially carrying out melt spinning, non-melting treatment, high-temperature pyrolysis, high-temperature sintering and the like to obtain the aluminum-doped silicon carbide fiber with a C / Si atomic ratio close to an ideal value. And the near-stoichiometric ratio regulation and control of the silicon carbide fiber components are realized.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon carbide fibers, and specifically relates to a preparation method of near-stoichiometric aluminum-doped silicon carbide fibers. Background Art

[0002] Silicon carbide (SiC) fibers have excellent properties such as high strength, high modulus, high temperature resistance, oxidation resistance, and corrosion resistance, and have important application prospects in high-tech fields such as aviation, aerospace, nuclear industry, and weaponry. Research institutions at home and abroad have realized the industrial production of continuous SiC fibers through the polycarbosilane (PCS) conversion method. The commonly used preparation process is as follows: using polycarbosilane with a suitable softening point as the precursor, continuous PCS fibers are obtained through melt spinning, and then placed in air for oxidative cross-linking to form infusible fibers (referred to as infusibilization treatment), and finally high-temperature firing is carried out under the protection of an inert atmosphere to obtain continuous SiC fibers. The two major foreign manufacturers, Nippon Carbon Company and Ube Industries, have used this method to produce continuous SiC fiber products under the trade names of "Nicalon" and "Tyranno". The National University of Defense Technology in China has also used this method to prepare "KD-I" type continuous SiC fibers and established a pilot production line. However, due to the fact that the precursor PCS of this SiC fiber is rich in carbon itself and the air oxidation infusibilization treatment is adopted, the prepared SiC fibers have problems such as carbon enrichment, high oxygen content, and non-stoichiometry. When the use temperature is higher than 1200 °C, the impurity SiC x O y phase undergoes violent thermal decomposition, generating a large amount of gaseous CO and SiO, resulting in a large number of defects and forming a loose structure, greatly reducing the mechanical strength of the fiber. Therefore, the first-generation SiC fibers can only be used in an environment below 1050 °C. Since then, the research and development of silicon carbide fibers have gone through three generations, gradually evolving from a high-oxygen and high-carbon structure to a low-oxygen and high-carbon structure, and finally forming the third-generation continuous silicon carbide fibers with a near-stoichiometric low-oxygen and low-carbon structure. Foreign third-generation silicon carbide fibers such as Hi-Nicalon S, Tyranno SA, and Sylramic fibers use methods such as hydrogen decarburization and oxidative decarburization, but the fibers themselves have a large carbon content, and a large number of pores are easily generated after decarburization, resulting in a sharp drop in fiber strength.

[0003] In recent years, the preparation of ultra-near-stoichiometric SiC fibers has become the focus of research and development. Summary of the Invention

[0004] The purpose of the present invention is to provide a preparation method of near-stoichiometric aluminum-doped silicon carbide fibers. By introducing aluminum elements to dope the silicon carbide fibers, near-stoichiometric silicon carbide fibers are obtained, significantly improving the high-temperature resistance and oxidation resistance of the silicon carbide fibers, enabling them to maintain excellent properties in extremely high-temperature and strong oxidation environments.

[0005] To achieve the above object, the solution of the present invention is as follows: A method for preparing near-stoichiometric aluminum-doped silicon carbide fibers, comprising the following steps: Step 1: First, use polymethylsilane (PMS) and aluminum acetylacetonate as raw materials to react to prepare aluminum-doped polymethylsilane (Al-PMS); Step 2: Then, under nitrogen protection, blend Al-PMS and PMS at room temperature, and filter off the insoluble matter using ultra-precision filter paper to obtain an Al-PMS / PMS mixed solution; Step 3: Then, under nitrogen protection, put the Al-PMS / PMS mixed solution and refined PCS into a melt spinning machine according to the ratio, heat to 240 - 270 °C under nitrogen protection, melt and stir evenly, perform degassing and removal of small molecules treatment, then lower the temperature to 220 - 230 °C and keep it warm. The molten melt is extruded through a spinneret and wound by a winding machine to produce aluminum-containing polycarbosilane precursor filaments; Step 4: Then, use the low pre-oxidation - thermal crosslinking method to infusibilize the aluminum-containing polycarbosilane precursor filaments to obtain polycarbosilane crosslinked filaments; Step 5: Then, under nitrogen protection, perform high-temperature pyrolysis on the polycarbosilane crosslinked filaments to obtain polycarbosilane pyrolysis filaments; Step 6: Finally, under argon protection, perform high-temperature sintering on the polycarbosilane pyrolysis filaments to obtain near-stoichiometric (C / Si ratio close to 1) silicon carbide fibers containing a small amount of aluminum and a small amount of oxygen, where the aluminum content accounts for 0.4 wt % - 0.8 wt %; In Step 1, using PMS and aluminum acetylacetonate as raw materials, place aluminum acetylacetonate at the bottom of a three-necked flask under nitrogen protection, and then cover an appropriate amount of PMS on aluminum acetylacetonate to prevent the sublimation of aluminum acetylacetonate and make the reaction uneven. After mixing evenly, start to slowly heat up. First, distill out toluene at 120 - 150 °C, then quickly heat up, and slowly heat and react at 360 - 420 °C for 3 - 5 hours. Continuously stir during the reaction. When the temperature of the cracking column rises above 460 - 500 °C, keep it warm for 7 - 9 hours and then cool to room temperature to obtain crude Al-PMS.

[0006] In Step 1, the mass dosage of aluminum acetylacetonate is 9.5 wt % - 10.5 wt %.

[0007] In Step 2, before blending Al-PMS with PMS, low-molecular substances in Al-PMS need to be removed through the processes of dissolution in xylene, filtration, and vacuum distillation to adjust the molecular weight distribution of Al-PMS and obtain a suitable precursor Al-PMS. The mass ratio of Al-PMS to PMS is 2 - 3:18.

[0008] In Step 3, the mass dosage of the Al-PMS / PMS mixture is 10 wt % - 15 wt % of the mass dosage of the refined PCS. When the mass dosage of the Al-PMS / PMS mixture accounts for less than 10 wt % of the mass dosage of the refined PCS, the fibers will fuse together after high-temperature pyrolysis, indicating insufficient cross-linking degree of the fibers. When the mass dosage of the Al-PMS / PMS mixture accounts for more than 15 wt % of the mass dosage of the refined PCS, it is difficult to spin the fibers during the melt spinning process.

[0009] In Step 3, the refined PCS is obtained by filtering off the insoluble substances from PCS using ultra-precision filter paper.

[0010] In Step 4, the process of the low pre-oxidation - thermal cross-linking method is as follows: at 195 - 205 °C, the polycarbosilane raw filaments are treated in an air atmosphere for 25 - 35 min to cause preliminary oxidative cross-linking of the fiber raw filaments. Subsequently, at 340 - 360 °C, the polycarbosilane raw filaments are heat-treated in an inert atmosphere for 25 - 35 min to convert the fiber molecules into an infusible network structure.

[0011] In Step 5, the process of the high-temperature pyrolysis is as follows: the polycarbosilane cross-linked filaments are placed on a quartz boat and placed into a quartz tube. The air is evacuated, and nitrogen is replaced three times to remove the air and impurities in the quartz tube. Under nitrogen protection, the temperature is raised to 1350 °C at a rate of 100 - 200 °C / h and held for 30 - 60 min to further densify the fibers. Then, it is cooled to room temperature in the quartz tube to obtain polycarbosilane pyrolysis filaments.

[0012] In Step 6, the process of the high-temperature sintering is as follows: the polycarbosilane pyrolysis filaments are placed in a graphite sintering furnace. Under argon protection, the temperature is raised to 1800 - 2000 °C at a heating rate of 200 - 300 °C / h and held for 30 - 60 min for sintering treatment.

[0013] After adopting the above technical solution, the preparation method of a near-stoichiometric aluminum-doped silicon carbide fiber of the present invention has the following beneficial effects: 1. The present invention can be used for large-scale production with existing equipment. Without changing the original PCS production line, the raw materials are optimized. By simply using polycarbosilane (PCS) as the raw material, the pyrolysis products are still severely carbon-rich, and the C / Si ratio is generally 1.2 - 1.4. The SiC produced by the pyrolysis of polymethylsilane (PMS) is generally silicon-rich, with an Si / C ratio of 1.2 - 1.3. Therefore, adding PMS to PCS to produce silicon carbide fibers can reduce the surplus carbon in the obtained silicon carbide fibers, enhance the high-temperature performance, oxidation resistance, and creep resistance of the fibers, solve the problem of surplus carbon from the raw materials, and modify PMS to Al-PMS, cleverly introducing the Al element. Since the obtained silicon carbide fibers contain aluminum elements, the doping of aluminum forms Al-C and Al2O3 phases on the surface of β-SiC grains, effectively inhibiting the grain growth at high temperatures, enabling uniform control of the grain size and achieving the purpose of densifying the microstructure, and finally obtaining continuous silicon carbide fibers with a near stoichiometric ratio; 2. By doping aluminum elements into silicon carbide fibers, the high-temperature resistance and oxidation resistance of silicon carbide fibers are further significantly improved. The prepared fibers can still maintain good performance in high-temperature environments above 1800 °C, can maintain relatively high strength and modulus, and extend the service life; Introducing a small amount of Al into silicon carbide fibers is beneficial to the high-temperature stability of silicon carbide fibers, which is mainly manifested in the following three aspects. First, it densifies the ceramics during high-temperature sintering, reducing the crack and pore defects generated in the ceramics; Second, aluminum can, to a certain extent, inhibit the rapid growth of SiC grains at high temperatures; Third, aluminum can also remove the surplus carbon and oxygen in SiC fibers during the sintering process, improving the chemical stability, high-temperature resistance, and creep resistance of SiC fibers; 3. Adopting a two-step method of low pre-oxidation - thermal crosslinking, combined with the role of aluminum in Al-PMS, aluminum atoms can react with the polymer molecular chains in the precursor filaments to form a crosslinked structure, improving the thermal stability and mechanical strength of the precursor filaments, enabling them to resist high temperatures without melting at a lower oxygen weight gain rate, improving the process controllability and the quality of the fibers; 4. Before melt spinning, degassing and removing small molecules can effectively increase the molecular weight and crosslinking degree of PMS, thereby increasing the fiber yield.

[0014] Furthermore, by controlling the dosage ratios of Al-PMS, PMS, and PCS, the aluminum content in the silicon carbide fibers is controlled to be 0.4 wt % - 0.8 wt %, which significantly improves the high-temperature resistance, oxidation resistance, and mechanical properties. When the aluminum content in the silicon carbide fibers exceeds 1.0 wt %, it will increase the probability of intergranular fracture behavior of the fibers and reduce the fiber strength.

[0015] Further, the temperature of high-temperature sintering is above 1800 °C, which makes the crystallinity of the silicon carbide fiber good and enables better fiber properties to be maintained at higher temperatures. Description of the Drawings

[0016] Figure 1 It is a flowchart of a preparation method of an aluminum-doped silicon carbide fiber with a near stoichiometric ratio. Detailed Embodiments

[0017] In order to further explain the technical solution of the present invention, the present invention will be elaborated in detail below through specific embodiments.

[0018] Example 1 A preparation method of an aluminum-doped silicon carbide fiber with a near stoichiometric ratio, as Figure 1 shown, includes the following steps: Step 1: First, using PMS and aluminum acetylacetonate with a mass ratio of 90:9 as raw materials, place aluminum acetylacetonate at the bottom of a three-necked flask under the protection of nitrogen, and then cover PMS on aluminum acetylacetonate to prevent uneven reaction caused by the sublimation of aluminum acetylacetonate. After mixing evenly, start to slowly heat up. First, distill toluene at 150 °C, then quickly heat up, and slowly heat and react at 420 °C for 3 hours. Keep stirring continuously during the reaction. When the temperature of the cracking column rises above 480 °C, keep warm for 8 hours and then cool to room temperature to obtain Al-PMS crude material; Before proceeding to Step 2, the Al-PMS crude material needs to be dissolved in xylene, filtered, and subjected to vacuum distillation to remove low-molecular substances in Al-PMS to adjust the molecular weight distribution of Al-PMS, and a precursor Al-PMS with a molecular weight of 1600 is obtained; Step 2: Then, under the protection of nitrogen, blend Al-PMS and PMS with a mass ratio of 10:90 at room temperature, and filter out insoluble substances using ultra-precision filter paper to obtain an Al-PMS / PMS mixed solution; Step 3: Then, filter out insoluble substances in PCS using ultra-precision filter paper to obtain refined PCS. Under the protection of nitrogen, put the Al-PMS / PMS mixed solution and refined PCS into a melt spinning machine according to a mass ratio of 10:90, heat to 240 °C under the protection of nitrogen, melt and stir evenly, perform degassing and dehydrogenation heat cross-linking treatment, then lower the temperature to 220 °C and keep warm. The molten melt is extruded through a spinneret and wound by a winder to produce an aluminum-containing polycarbosilane raw fiber; Step 4: Then, at 200 °C, treat the aluminum-containing polycarbosilane raw fiber in an air atmosphere for 30 min to cause preliminary oxidative cross-linking of the fiber raw material. Subsequently, at 350 °C, heat-treat the polycarbosilane raw fiber in an inert atmosphere for 30 min to convert the fiber molecules into an infusible network structure to obtain a polycarbosilane cross-linked fiber; Step 5. Then place the polycarbosilane cross-linked filaments on a quartz boat and put them into a quartz tube. Evacuate the air, and replace the air with nitrogen three times to remove the air and impurities in the quartz tube. Under nitrogen protection, heat the filaments to 1350 °C at a rate of 100 °C / h, hold the temperature for 30 min to further densify the fibers, and then cool them to room temperature in the quartz tube to obtain polycarbosilane pyrolysis filaments. Step 6. Finally, place the polycarbosilane pyrolysis filaments in a graphite sintering furnace. Under argon protection, heat the filaments to 1800 °C at a heating rate of 300 °C / h, hold the temperature for 30 min for sintering treatment, apply sizing and wind the filaments to obtain silicon carbide fibers with a near stoichiometric ratio (C / Si ratio close to 1) containing a small amount of aluminum and a small amount of oxygen.

[0019] Example 2 A method for preparing near-stoichiometric aluminum-doped silicon carbide fibers, as Figure 1 shown, includes the following steps: Step 1. First, use PMS and aluminum acetylacetonate with a mass ratio of 94:9 as raw materials. Under nitrogen protection, place aluminum acetylacetonate at the bottom of a three-necked flask, and then cover PMS on aluminum acetylacetonate to prevent the sublimation of aluminum acetylacetonate and make the reaction uneven. After mixing evenly, start to slowly heat up. First, distill toluene at 120 °C, then quickly heat up, and slowly heat and react at 360 °C for 5 hours. Keep stirring during the reaction. When the temperature of the cracking column rises above 460 °C, hold the temperature for 9 hours and then cool to room temperature to obtain Al-PMS crude material. Before performing Step 2, the Al-PMS crude material needs to be dissolved in xylene, filtered, and subjected to vacuum distillation to remove low-molecular substances in Al-PMS to adjust the molecular weight distribution of Al-PMS and obtain a precursor Al-PMS with a molecular weight of 1600. Step 2. Then, under nitrogen protection, blend Al-PMS and PMS with a mass ratio of 13:90 at room temperature, and filter out the insoluble substances using ultra-precision filter paper to obtain an Al-PMS / PMS mixture. Step 3. Then filter out the insoluble substances in PCS using ultra-precision filter paper to obtain refined PCS. Under nitrogen protection, put the Al-PMS / PMS mixture and refined PCS into a melt spinning machine at a mass ratio of 13:90. Under nitrogen protection, heat to 270 °C, melt and stir evenly, perform degassing and dehydrogenation thermal cross-linking treatment, then lower the temperature to 230 °C and hold the temperature. The molten melt is extruded through a spinneret and wound by a winding machine to produce aluminum-containing polycarbosilane precursor filaments. Step 4: Then, at 205 °C, the aluminum-containing polycarbosilane precursor fiber is treated in an air atmosphere for 25 min to cause preliminary oxidative cross-linking of the fiber precursor. Subsequently, at 360 °C, the polycarbosilane precursor fiber is heat-treated in an inert atmosphere for 25 min to convert the fiber molecules into an infusible network structure, obtaining a polycarbosilane cross-linked fiber. Step 5: Then, place the polycarbosilane cross-linked fiber on a quartz boat and put it into a quartz tube. Evacuate the air, and replace the gas with nitrogen three times to remove the air and impurities in the quartz tube. Under nitrogen protection, heat it to 1350 °C at a rate of 200 °C / h and hold for 40 min to further densify the fiber. Then, cool it to room temperature in the quartz tube to obtain a polycarbosilane pyrolysis fiber. Step 6: Finally, place the polycarbosilane pyrolysis fiber in a graphite sintering furnace. Under argon protection, heat it to 2000 °C at a heating rate of 200 °C / h, hold for 40 min for sintering treatment, size and wind it to obtain a silicon carbide fiber with a near stoichiometric ratio (C / Si ratio close to 1) containing a small amount of aluminum and a small amount of oxygen.

[0020] Example 3 A preparation method of a near-stoichiometric aluminum-doped silicon carbide fiber, as Figure 1 shown, includes the following steps: Step 1: First, using PMS and aluminum acetylacetonate with a mass ratio of 90:9 as raw materials, place aluminum acetylacetonate at the bottom of a three-necked flask under nitrogen protection, and then cover PMS on aluminum acetylacetonate to prevent uneven reaction caused by the sublimation of aluminum acetylacetonate. After mixing evenly, start to slowly heat up. First, distill toluene at 150 °C, then quickly heat up, and slowly heat and react at 420 °C for 3 hours. Continuously stir during the reaction. When the temperature of the cracking column rises above 480 °C, hold for 8 hours and then cool to room temperature to obtain Al-PMS crude material. Before performing Step 2, the Al-PMS crude material needs to be dissolved in xylene, filtered, and subjected to vacuum distillation to remove low-molecular substances in Al-PMS to adjust the molecular weight distribution of Al-PMS, obtaining a precursor Al-PMS with a molecular weight of 1600. Step 2: Then, under nitrogen protection, blend Al-PMS and PMS with a mass ratio of 10:90 at room temperature, and filter out insoluble substances using ultra-precision filter paper to obtain an Al-PMS / PMS mixed solution. Step 3: Then, use ultra-precision filter paper to filter out the insoluble substances in PCS to obtain refined PCS. Under nitrogen protection, put the Al-PMS / PMS mixture and refined PCS into a melt spinning machine at a mass ratio of 10:90. Under nitrogen protection, heat to 240 °C, melt and stir evenly, conduct degassing, dehydrogenation, and thermal cross-linking treatment, then lower the temperature to 220 °C and keep it warm. The molten melt is extruded through a spinneret and wound by a winder to produce aluminum-containing polycarbosilane precursor filaments. Step 4: Then, at 195 °C, treat the aluminum-containing polycarbosilane precursor filaments in an air atmosphere for 35 min to cause preliminary oxidative cross-linking of the fiber precursor filaments. Subsequently, at 340 °C, heat-treat the polycarbosilane precursor filaments in an inert atmosphere for 35 min to convert the fiber molecules into an infusible network structure, obtaining polycarbosilane cross-linked filaments. Step 5: Then, place the polycarbosilane cross-linked filaments on a quartz boat and put them into a quartz tube. Evacuate, change nitrogen three times to remove the air and impurities in the quartz tube. Under nitrogen protection, heat to 1350 °C at a rate of 150 °C / h, keep warm for 60 min to further densify the fibers, and then cool to room temperature in the quartz tube to obtain polycarbosilane pyrolysis filaments. Step 6: Finally, place the polycarbosilane pyrolysis filaments in a graphite sintering furnace. Under argon protection, heat to 1800 °C at a heating rate of 300 °C / h, keep warm for 60 min for sintering treatment, size and wind to obtain silicon carbide fibers with a near stoichiometric ratio (C / Si ratio close to 1) containing a small amount of aluminum and a small amount of oxygen.

[0021] In the above embodiments, the PCS used is produced by Fujian Liya New Materials Co., Ltd., with a molecular weight of 1100 - 1300 and a softening point of 200 - 215 °C; the PMS used is produced by Hunan Bowang Carbon Ceramics Co., Ltd., with a molecular weight of about 1500, a melting point of 30 °C, and a density of 0.95 g / cm 3 , the ceramic yield > 65%, and the chemical properties are stable, and it can be stored and used at room temperature.

[0022] The properties of the near-stoichiometric aluminum-doped silicon carbide fibers prepared in each example and the undoped silicon carbide fibers (Liya's third-generation silicon carbide fibers) in Comparative Example 1 were tested in accordance with the national standard for silicon carbide fibers GB / T43760-2024, and the aluminum content was analyzed by a well-known elemental test method (ICP-OES). The results are shown in Table 1.

[0023] Table 1 Performance data of silicon carbide fibers

[0024] Note: The strength retention rate was measured after heat-treating the silicon carbide fibers in an argon atmosphere at 1900 °C for 1 h.

[0025] As can be seen from Table 1, by introducing aluminum element doping, silicon carbide fibers with a near stoichiometric ratio (C / Si ratio close to 1) containing a small amount of aluminum and a small amount of oxygen are obtained.

[0026] The tensile strength of the silicon carbide fibers prepared in Examples 1 to 3 is between 2.3 and 2.8 GPa, which is lower than that of the third-generation silicon carbide fibers (3.4 GPa) in Comparative Example 1. This may be because the doping grain coarsening leads to a reduction in the number of grain boundaries and a weakening of the grain boundary strengthening effect. At the same time, the phase formed by aluminum at the grain boundary may reduce the bonding strength of the grain boundary. The tensile strength of the silicon carbide fibers prepared in Example 3 is relatively high, reaching 2.8 GPa, indicating that at an appropriate aluminum content, the strength loss caused by grain coarsening can be balanced to a certain extent.

[0027] The C / Si atomic ratio of the silicon carbide fibers prepared in Examples 1 to 3 is between 1.01 and 1.02, close to the ideal stoichiometric ratio of 1:1, while the C / Si atom of the third-generation silicon carbide fibers in Comparative Example 1 is 1.05. This is because the examples successfully reduce the residual free carbon and improve the high-temperature oxidation resistance of the fibers. In addition, the oxygen content (0.25 wt % - 0.47 wt %) of the silicon carbide fibers prepared in Examples 1 to 3 is significantly lower than that of Comparative Example 1 (0.64 wt %), reducing the formation of the SiC x O y amorphous phase and reducing the performance degradation caused by fiber decomposition at high temperatures; finally, the strength retention rate of the aluminum-doped silicon carbide fibers after heat treatment at 1900 °C exceeds 90%, which is significantly better than that of undoped Comparative Example 1 (the fibers are pulverized at 1900 °C and the strength retention rate cannot be measured). This is because after the aluminum element enters the SiC lattice, it can inhibit grain growth in a high-temperature environment, hinder grain boundary migration at high temperatures, reduce structural deterioration, and retain the tensile strength of the fibers.

[0028] In summary, the aluminum-doped silicon carbide fibers significantly improve the high-temperature resistance of silicon carbide fibers by reducing the oxygen content, near stoichiometric ratio, and inhibiting grain growth. Among them, the comprehensive performance of the silicon carbide fibers (0.68 wt % Al) prepared in Example 3 is the best, meeting the use requirements of high-temperature resistant structural parts in extreme environments.

[0029] The above examples and diagrams do not limit the product form and style of the present invention. Any appropriate changes or modifications made by those of ordinary skill in the art shall be regarded as not departing from the patent scope of the present invention.

Claims

1. A method for preparing aluminum-doped silicon carbide fibers in a near-stoichiometric ratio, characterized in that: The following steps are involved: Step 1, firstly using PMS and aluminum acetylacetonate as raw materials to react and prepare Al-PMS; Step 2: Then, under nitrogen protection, Al-PMS and PMS are blended at room temperature, and insoluble matter is filtered out using ultra-precision filter paper to obtain an Al-PMS / PMS mixed solution; Step 3, then under nitrogen protection, put the Al-PMS / PMS mixed solution and refined PCS into a melt spinning machine according to the ratio, heat to 240-270°C under nitrogen protection, melt and stir evenly, perform degassing and small molecule removal treatment, then reduce the temperature to 220-230°C and keep warm, spin the molten melt through a spinneret and wind it through a winder to produce aluminum-containing polycarbosilane precursor; Step 4, then using a low pre-oxidation-thermal cross-linking method to perform an infusible treatment on the aluminum-containing polycarbosilane raw yarn to obtain a polycarbosilane cross-linked yarn; Step 5, then pyrolyzing the polycarbosilane cross-linked wire at high temperature under the protection of nitrogen to obtain polycarbosilane pyrolysis wire; Step 6: Finally, the polycarbosilane pyrolysis filaments are sintered at high temperature under the protection of argon to obtain silicon carbide fibers with a near stoichiometric ratio containing a small amount of aluminum and a small amount of oxygen, wherein the aluminum content accounts for 0.4% of the mass of the silicon carbide fiber. wt %~0.8 wt %; In step 1, PMS and aluminum acetylacetonate are used as raw materials. Under the protection of nitrogen, aluminum acetylacetonate is placed at the bottom of a three-necked flask, and then an appropriate amount of PMS is covered on the aluminum acetylacetonate to prevent the sublimation of aluminum acetylacetonate from causing uneven reaction. After mixing evenly, the temperature is slowly raised. Toluene is first distilled out at 120-150° C., and then the temperature is quickly raised. The reaction is slowly heated at 360-420° C. for 3-5 hours. Stirring is continued during the reaction. When the temperature of the cracking column rises to above 460-500° C., it is kept warm for 7-9 hours and then cooled to room temperature to obtain Al-PMS crude material.

2. The method for preparing a silicon carbide fiber doped with aluminum in a near stoichiometric ratio according to claim 1, characterized in that: In step 1, the mass amount of aluminum acetylacetonate is 9.5 of the mass amount of PMS. wt %~10.5 wt %.

3. The method for preparing silicon carbide fiber doped with aluminum in a near stoichiometric ratio according to claim 1, characterized in that: In step 2, before Al-PMS and PMS are blended, low molecular weight substances in Al-PMS need to be removed by dissolving in xylene, filtering and distilling under reduced pressure to adjust the molecular weight distribution of Al-PMS to obtain a suitable precursor Al-PMS. The mass ratio of Al-PMS to PMS is 2~3:

18.

4. The method for preparing silicon carbide fiber doped with aluminum in a near stoichiometric ratio according to claim 1, characterized in that: In step 3, the mass dosage of Al-PMS / PMS mixed solution is 10% of the mass dosage of refined PCS. wt %~15 wt %.

5. The method for preparing silicon carbide fiber doped with aluminum in a near stoichiometric ratio according to claim 1, characterized in that: In step 3, refined PCS is obtained by filtering out insoluble matter from PCS using ultra-precision filter paper.

6. The method for preparing silicon carbide fiber doped with aluminum in a near stoichiometric ratio according to claim 1, characterized in that: In step 4, the process of the low pre-oxidation-thermal crosslinking method is: at 195-205°C, the polycarbosilane precursor is treated in an air atmosphere for 25-35 min to allow the fiber precursor to undergo initial oxidation crosslinking, and then at 340-360°C, the polycarbosilane precursor is heat treated in an inert atmosphere for 25-35 min to convert the fiber molecules into an infusible network structure.

7. The method for preparing silicon carbide fiber doped with aluminum in a near stoichiometric ratio according to claim 1, characterized in that: In step 5, the high-temperature pyrolysis process is as follows: placing the polycarbosilane cross-linked fibers on a quartz boat and placing it in a quartz tube, evacuating the tube, replacing the nitrogen three times, removing the air and impurities in the tube, and heating the tube at a rate of 100-200°C / h to 1350°C under nitrogen protection, keeping the temperature for 30-60 min to further densify the fibers, and then cooling the tube to room temperature to obtain polycarbosilane pyrolysis fibers.

8. The method for preparing aluminum-doped silicon carbide fibers in a near-stoichiometric ratio according to claim 1, characterized in that: In step 6, the high temperature sintering process is: placing the polycarbosilane pyrolytic wire in a graphite sintering furnace, under argon protection, heating the temperature to 1800-2000° C. at a heating rate of 200-300° C. / h, and keeping the temperature for 30-60 min for sintering.

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