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

Abstract

The invention discloses a preparation method of near-stoichiometric boron and aluminum doped silicon carbide fibers, which comprises the following steps: doping PMS with an aluminum element and a boron 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 boron and aluminum doped silicon carbide fibers with 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 particularly to a method for preparing a near-stoichiometric boron- and aluminum-doped silicon carbide fiber. 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 achieved 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, obtaining continuous PCS fibers through melt spinning, then subjecting them to oxidative cross-linking in air to form infusible fibers (referred to as non-melting treatment), and finally performing high-temperature firing under the protection of an inert atmosphere to obtain continuous SiC fibers. The two major foreign manufacturers, Nippon Carbon Co., Ltd. and Ube Industries, Ltd., 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 air oxidation non-melting treatment is used, 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 fibers. 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 method for preparing a near-stoichiometric boron- and aluminum-doped silicon carbide fiber. By introducing boron and aluminum elements to dope the silicon carbide fiber, a near-stoichiometric silicon carbide fiber is obtained, significantly improving the high-temperature resistance and oxidation resistance of the silicon carbide fiber, enabling it 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 a near-stoichiometric boron- and aluminum-doped silicon carbide fiber, comprising the following steps: Step 1: First, using PMS and aluminum acetylacetonate as raw materials, react to prepare aluminum-doped polymethylsilane (Al-PMS); Step 2: Then, under nitrogen protection, react PMS with polydiphenylborosiloxane (PPBSO) to obtain boron-doped polymethylsilane (B-PMS); Step 3: Then, under nitrogen protection, blend B-PMS, Al-PMS, and PMS at room temperature, and filter off the insoluble matter using ultra-precision filter paper to obtain a B-PMS / Al-PMS / PMS mixed solution; Step 4: Then, under nitrogen protection, put the B-PMS / Al-PMS / PMS mixed solution and refined PCS into a melt spinning machine according to the ratio, heat to 260-280°C under nitrogen protection, melt and stir evenly, perform degassing and removal of small molecules treatment, then lower the temperature to 230-250°C and keep it warm, and the molten melt is spun through a spinneret and wound by a winding machine to produce a boron- and aluminum-doped polycarbosilane precursor filament; Step 5: Then, use the low pre-oxidation-thermal crosslinking method to infusibilize the boron- and aluminum-doped polycarbosilane precursor filament to obtain a polycarbosilane crosslinked filament; Step 6: Then, under the protection of nitrogen, perform high-temperature pyrolysis on the polycarbosilane crosslinked filament to obtain a polycarbosilane pyrolysis filament; Step 7: Finally, under the protection of argon, perform high-temperature sintering on the polycarbosilane pyrolysis filament to obtain a near-stoichiometric silicon carbide fiber containing a small amount of boron and a small amount of aluminum, wherein the boron content accounts for 0.5 wt % - 0.7 wt % of the mass of the silicon carbide fiber, and the aluminum content accounts for 0.4 wt % - 0.8 wt % of the mass of the silicon carbide fiber; 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 the Al-PMS crude material; In Step 2, using PMS and PPBSO as raw materials, under the protection of nitrogen, condense and reflux at a temperature of 350~370°C, and then the product is successively dissolved in xylene, filtered, and refined by distillation to obtain the crude B-PMS.

[0006] In Step 1, the mass dosage of aluminum acetylacetonate is 9.5 wt %~10.5 wt % of the mass dosage of PMS. In Step 2, the mass dosage of PPBSO is 4 wt %~8 wt % of the mass dosage of PMS.

[0007] In Step 2, PPBSO is prepared by pre-reacting dichlorophenylsilane and boric acid at 100°C and then heating to 300°C for vacuum filtration.

[0008] In Step 3, before blending Al-PMS with PMS, it is necessary to dissolve, filter, and perform vacuum distillation on Al-PMS to remove low-molecular substances in Al-PMS to adjust the molecular weight distribution of Al-PMS and obtain a suitable precursor Al-PMS. Before blending B-PMS with PMS, it is also necessary to dissolve, filter, and perform vacuum distillation on B-PMS to remove low-molecular substances in B-PMS to adjust the molecular weight distribution of B-PMS and obtain a suitable precursor B-PMS.

[0009] In Step 3, the mass dosage ratio of B-PMS, Al-PMS, and PMS is 8:10:90 ~ 9:11:90.

[0010] In Step 4, the refined PCS is obtained by filtering insoluble substances from PCS using ultra-precision filter paper.

[0011] In Step 5, the process of the low pre-oxidation - thermal crosslinking method is as follows: at 190~210°C, the polycarbosilane precursor fiber is treated in an air atmosphere for 25~35 min to cause preliminary oxidative crosslinking of the fiber precursor. Subsequently, at 340~360°C, the polycarbosilane precursor fiber is heat-treated in an inert atmosphere for 25~35 min to convert the fiber molecules into an infusible network structure.

[0012] In Step 6, the process of the high-temperature pyrolysis is as follows: place the polycarbosilane crosslinked fiber on a quartz boat and put it into a quartz tube, evacuate, replace with nitrogen three times to remove air and impurities in the quartz tube. Under the protection of nitrogen, heat up to 1350°C at a rate of 100~200°C / h, keep the temperature for 30~60 min to further densify the fiber, and then cool to room temperature in the quartz tube to obtain the polycarbosilane pyrolysis fiber.

[0013] In Step 7, the process of high-temperature sintering is as follows: Place the polycarbosilane pyrolysis fiber in a graphite sintering furnace. Under argon protection, raise the temperature to 1800 - 2000 °C at a heating rate of 200 - 300 °C / h, and hold for 30 - 60 min for sintering treatment.

[0014] After adopting the above technical solution, the preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber of the present invention has the following beneficial effects: 1. Using existing equipment and optimizing raw materials: The present invention can use existing equipment for large-scale production without changing the original PCS production line, and optimize the raw materials. Simply using polycarbosilane (PCS) as the raw material, the pyrolysis product is still severely carbon-rich, and the C / Si ratio is generally 1.2 - 1.4. The SiC generated by the pyrolysis of polymethylsilane (PMS) is generally silicon-rich, and the Si / C ratio is 1.2 - 1.3. Therefore, adding PMS to PCS to make silicon carbide fibers reduces the surplus carbon in the obtained silicon carbide fibers, enhances the high-temperature performance, oxidation resistance, and creep resistance of the fibers, solves the problem of surplus carbon from the raw materials, and modifies PMS into B-PMS and Al-PMS, skillfully introducing B and Al elements. Since the obtained silicon carbide fibers contain boron and aluminum elements, the doping of boron forms B-C and B2O3 phases on the surface of β-SiC grains, and the doping of aluminum forms Al-C and Al2O3 phases on the surface of β-SiC grains, effectively inhibiting the growth of grains at high temperatures, enabling uniform control of the grain size and achieving the purpose of densifying the microstructure, and finally obtaining near-stoichiometric continuous silicon carbide fibers; the addition of aluminum is beneficial to the densification of the fibers, but it is difficult to obtain high-density fibers by introducing Al alone. When B and Al are added simultaneously, the fibers can be sintered to near full densification. The average crystal size of the sintered silicon carbide fibers is about 170 nm. Using two sintering elements simultaneously can further improve the density of the fibers, which is beneficial to maintaining the fiber morphology and improving the fiber properties; 2. Improve high-temperature resistance and oxidation resistance: By introducing boron and aluminum elements to dope 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 high strength and modulus, and extend the service life. Introducing a small amount of Al / B 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 crack and pore defects generated in the ceramics. Second, boron and aluminum can, to a certain extent, inhibit the rapid growth of SiC grains at high temperatures. Third, boron and aluminum can also remove excess carbon and oxygen in SiC fibers during the sintering process, improving the chemical stability, high-temperature resistance, and creep resistance of SiC fibers. At the same time, using two sintering elements can further improve the density of the fibers, which is beneficial to maintaining the fiber morphology and improving fiber performance; 3. Optimize the cross-linking method: Adopt a two-step method of low pre-oxidation - thermal cross-linking. Combining the functions of boron and aluminum in B-PMS and Al-PMS, boron and aluminum atoms can react with the polymer molecular chains in the precursor filaments to form a cross-linked 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 fiber quality; 4. Optimization of melt spinning: Performing degassing and removing small molecules before melt spinning can effectively increase the molecular weight and cross-linking degree of PMS, thereby increasing the fiber yield.

[0015] Furthermore, by controlling the dosage ratios of B-PMS, Al-PMS, PMS, and PCS, control the boron content in the silicon carbide fibers to be 0.5 wt % - 0.7 wt %, and control the aluminum content in the silicon carbide fibers 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; when the boron content in the silicon carbide fibers exceeds 1.0 wt %, it causes excessive cross-linking of the fibers, making it impossible to spin or the spinning is prone to breakage.

[0016] Furthermore, the high-temperature sintering temperature is above 1800 °C, making the silicon carbide fibers have good crystallinity and maintain good fiber performance at higher temperatures. Description of the Drawings

[0017] Figure 1 It is a flowchart of a preparation method of a near-stoichiometric boron and aluminum-doped silicon carbide fiber. Detailed Embodiments

[0018] To further explain the technical solution of the present invention, the present invention will be elaborated in detail through specific embodiments below.

[0019] Example 1 A preparation method of near-stoichiometric boron- and aluminum-doped silicon carbide fibers, 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 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; Step 2: Then pre-react 62 g of dichlorophenylsilane and 23 g of boric acid at 100°C, then heat up to 300°C for vacuum filtration to prepare PPBSO. Then mix 200 g of PMS and 16 g of PPBSO, and carry out condensation reflux at 350°C under the protection of nitrogen. Then the product is successively dissolved in xylene, filtered, and distilled and refined to obtain B-PMS crude material; Before performing Step 3, the Al-PMS crude material needs to go through the processes of dissolving in xylene, filtering, and 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; the B-PMS crude material also needs to go through the processes of dissolving in xylene, filtering, and vacuum distillation to remove low-molecular substances in B-PMS to adjust the molecular weight distribution of B-PMS, and obtain a precursor B-PMS with a molecular weight of 1400 - 1600; Step 3: Then, under the protection of nitrogen, mix B-PMS, Al-PMS, and PMS with a mass ratio of 8:10:90 at room temperature, and filter out insoluble substances using ultra-precision filter paper to obtain a B-PMS / Al-PMS / PMS mixed solution; Step 4: Then filter out insoluble substances in PCS using ultra-precision filter paper to obtain refined PCS. Under the protection of nitrogen, put the B-PMS / Al-PMS / PMS mixed solution and refined PCS into a melt spinning machine according to a mass ratio of 10:90, heat to 260°C under the protection of nitrogen, melt and stir evenly, carry out degassing and dehydrogenation heat cross-linking treatment, then lower the temperature to 240°C and keep warm. The molten melt is spun through a spinneret and wound by a winder to produce boron- and aluminum-doped polycarbosilane precursor filaments; Step 5: Then, at 200 °C, the boron- and aluminum-doped polycarbosilane precursor fibers are treated in an air atmosphere for 30 min to cause preliminary oxidative cross-linking of the fiber precursor. Subsequently, at 350 °C, the polycarbosilane precursor fibers are heat-treated in an inert atmosphere for 30 min to convert the fiber molecules into an infusible network structure, obtaining polycarbosilane cross-linked fibers; Step 6: Then, the polycarbosilane cross-linked fibers are placed on a quartz boat and placed into a quartz tube. The tube is evacuated, and nitrogen is replaced three times to remove air and impurities in the quartz tube. Under nitrogen protection, the temperature is raised to 1350 °C at a rate of 100 °C / h and held for 30 min to further densify the fibers. Then, it is cooled to room temperature in the quartz tube to obtain polycarbosilane pyrolytic fibers; Step 7: Finally, the polycarbosilane pyrolytic fibers are placed in a graphite sintering furnace. Under argon protection, the temperature is raised to 1800 °C at a heating rate of 300 °C / h and held for 30 min for sintering treatment. Sizing and winding are carried out to obtain silicon carbide fibers with a near stoichiometric ratio (C / Si ratio close to 1) containing a small amount of boron and a small amount of aluminum.

[0020] Example 2 A method for preparing near-stoichiometric boron- and aluminum-doped silicon carbide fibers, comprising the following steps: Step 1: First, using PMS and aluminum acetylacetonate with a mass ratio of 94:9 as raw materials, under nitrogen protection, aluminum acetylacetonate is placed at the bottom of a three-necked flask, and then PMS is covered on aluminum acetylacetonate to prevent uneven reaction caused by the sublimation of aluminum acetylacetonate. After mixing evenly, the temperature is slowly raised. First, toluene is distilled out at 120 °C, and then the temperature is quickly raised. It is slowly heated and reacted at 360 °C for 5 hours, with continuous stirring during the reaction. When the temperature of the cracking column rises above 460 °C, it is held for 9 hours and then cooled to room temperature to obtain Al-PMS crude material; Step 2: Then, 62 g of dichlorophenylsilane and 23 g of boric acid are pre-reacted at 100 °C, and then the temperature is raised to 300 °C for vacuum filtration to prepare PPBSO. Then, 200 g of PMS and 10 g of PPBSO are mixed, and condensed and refluxed at 360 °C under nitrogen protection. Then, the product is successively filtered after being dissolved in xylene and refined by distillation to obtain B-PMS crude material; Before performing Step 3, the Al-PMS crude material needs to undergo a process of dissolving in xylene, filtering, and 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; the B-PMS crude material also needs to undergo a process of dissolving in xylene, filtering, and vacuum distillation to remove low-molecular substances in B-PMS to adjust the molecular weight distribution of B-PMS, obtaining a precursor B-PMS with a molecular weight of 1400 - 1600; Step 3. Then, under nitrogen protection, B-PMS, Al-PMS, and PMS with a mass ratio of 9:11:90 were blended at room temperature, and the insoluble matters were filtered off using ultra-precision filter paper to obtain a B-PMS / Al-PMS / PMS mixture; Step 4. Then, the insoluble matters in PCS were filtered off using ultra-precision filter paper to obtain refined PCS. Under nitrogen protection, the B-PMS / Al-PMS / PMS mixture and the refined PCS were put into a melt spinning machine according to a mass ratio of 13:90. Under nitrogen protection, it was heated to 270 °C, melted and stirred evenly, and subjected to degassing, dehydrogenation, and thermal cross-linking treatment. Then the temperature was lowered to 230 °C and held for heat preservation. The molten melt was spun through a spinneret and wound by a winding machine to produce a boron- and aluminum-doped polycarbosilane precursor fiber; Step 5. Then, at 205 °C, the boron- and aluminum-doped polycarbosilane precursor fiber was 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 was 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 6. Then, the polycarbosilane cross-linked fiber was placed on a quartz boat and placed into a quartz tube. The vacuum was pumped, and nitrogen was replaced three times to remove the air and impurities in the quartz tube. Under nitrogen protection, it was heated to 1350 °C at a rate of 200 °C / h and held for 40 min to further densify the fiber. Then it was cooled to room temperature in the quartz tube to obtain a polycarbosilane pyrolysis fiber; Step 7. Finally, the polycarbosilane pyrolysis fiber was placed in a graphite sintering furnace. Under argon protection, it was heated to 2000 °C at a heating rate of 200 °C / h and held for 40 min for sintering treatment, sized and wound to obtain a silicon carbide fiber with a near stoichiometric ratio (C / Si ratio close to 1) containing a small amount of boron and a small amount of aluminum.

[0021] Example 3 A preparation method of a near stoichiometric ratio silicon carbide fiber doped with boron and aluminum, comprising the following steps: Step 1. First, using PMS and aluminum acetylacetonate with a mass ratio of 90:9 as raw materials, aluminum acetylacetonate was placed at the bottom of a three-necked flask under nitrogen protection, and then PMS was covered on aluminum acetylacetonate to prevent the sublimation of aluminum acetylacetonate and make the reaction uneven. After mixing evenly, the temperature was slowly raised. First, toluene was distilled out at 150 °C, and then the temperature was quickly raised, and it was slowly heated and reacted at 420 °C for 3 hours. Stirring was continued during the reaction. When the temperature of the cracking column rose above 480 °C and held for 8 hours, it was cooled to room temperature to obtain Al-PMS crude material; Step 2: Then, 62 g of dichlorophenylsilane and 23 g of boric acid are pre-reacted at 100 °C, and then the temperature is raised to 300 °C for vacuum filtration to prepare PPBSO. Then, 200 g of PMS and 12 g of PPBSO are mixed, and under the protection of nitrogen, condensation reflux is carried out at 350 °C. Then, the product is successively dissolved in xylene, filtered, and refined by distillation to obtain the crude B-PMS; Before performing Step 3, the Al-PMS crude material needs to undergo the processes of dissolution in xylene, filtration, and vacuum distillation to remove low-molecular substances in Al-PMS to adjust the molecular weight distribution of Al-PMS, obtaining the precursor Al-PMS with a molecular weight of 1600; the B-PMS crude material also needs to undergo the processes of dissolution in xylene, filtration, and vacuum distillation to remove low-molecular substances in B-PMS to adjust the molecular weight distribution of B-PMS, obtaining the precursor B-PMS with a molecular weight of 1400 - 1600; Step 3: Then, under the protection of nitrogen, B-PMS, Al-PMS, and PMS with a mass ratio of 9:10:90 are blended at room temperature, and the insoluble substances are filtered off using ultra-precision filter paper to obtain the B-PMS / Al-PMS / PMS mixed solution; Step 4: Then, the insoluble substances in PCS are filtered off using ultra-precision filter paper to obtain refined PCS. Under the protection of nitrogen, the B-PMS / Al-PMS / PMS mixed solution and refined PCS are put into a melt spinning machine according to a mass ratio of 10:90, heated to 280 °C under the protection of nitrogen, melted and stirred evenly, and subjected to degassing, dehydrogenation, and thermal cross-linking treatment. Then, the temperature is lowered to 250 °C and held, and the molten melt is spun through a spinneret and wound by a winding machine to produce boron- and aluminum-doped polycarbosilane precursor filaments; Step 5: Then, at 195 °C, the boron- and aluminum-doped polycarbosilane precursor filaments are treated in an air atmosphere for 35 min to cause preliminary oxidative cross-linking of the fiber precursor filaments. Subsequently, at 340 °C, the polycarbosilane precursor filaments are heat-treated in an inert atmosphere for 35 min to convert the fiber molecules into an infusible network structure, obtaining polycarbosilane cross-linked filaments; Step 6: Then, the polycarbosilane cross-linked filaments are placed on a quartz boat and placed into a quartz tube, evacuated, and replaced with nitrogen three times to remove the air and impurities in the quartz tube. Under the protection of nitrogen, the temperature is raised to 1350 °C at a rate of 150 °C / h and held for 60 min to further densify the fibers. Then, it is cooled to room temperature in the quartz tube to obtain polycarbosilane pyrolysis filaments; Step 7: Finally, the polycarbosilane pyrolysis filaments are placed in a graphite sintering furnace, and under the protection of argon, the temperature is raised to 1800 °C at a heating rate of 300 °C / h and held for 60 min for sintering treatment, sized and wound to obtain silicon carbide fibers with a near stoichiometric ratio (C / Si ratio close to 1) containing a small amount of boron and a small amount of aluminum.

[0022] In the above embodiments, the PCS used was produced by Fujian Liyaxin New Materials Co., Ltd., with a molecular weight of 1100 - 1300 and a softening point of 200 - 215 °C; the PMS used was 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%, with stable chemical properties and can be stored and used at room temperature.

[0023] The properties of the near-stoichiometric boron- and aluminum-doped silicon carbide fibers prepared in each embodiment and the undoped silicon carbide fibers (Liyaxin'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. The aluminum content and boron content were analyzed using a well-known elemental testing method (ICP-OES). The results are shown in Table 1.

[0024] Table 1 Performance data of silicon carbide fibers

[0025] Note: The strength retention rate was measured after heat treatment of the silicon carbide fibers in an argon atmosphere at 1800 °C for 1 h.

[0026] As can be seen from Table 1, by introducing boron and aluminum elements for doping, near-stoichiometric silicon carbide fibers containing a small amount of boron and a small amount of aluminum were obtained.

[0027] The C / Si atomic ratio of the silicon carbide fibers prepared in Examples 1 - 3 was closer to 1.0 (1.01 - 1.02), with less free carbon, significantly improved high-temperature stability, and a lower C / Si atomic ratio (1.05) than that of the undoped third-generation silicon carbide fibers in Comparative Example 1.

[0028] The silicon carbide fibers prepared in Examples 1 - 3 all contained boron (0.54 wt % - 0.65 wt %) and aluminum (0.64 wt % - 0.72 wt %). Using boron and aluminum as two sintering elements can further improve the fiber density, which is beneficial for maintaining the fiber morphology and improving the fiber properties.

[0029] The oxygen content of the silicon carbide fibers prepared in Examples 1 - 3 was between 0.32 wt % - 0.42 wt %, much lower than the oxygen content (0.74 wt %) of the undoped silicon carbide fibers in Comparative Example 1, which is beneficial for improving the high-temperature resistance and mechanical properties of the silicon carbide fibers.

[0030] In addition, the average grain size of the silicon carbide fibers prepared in Examples 1 to 3 is about 170 nm, which can effectively inhibit the grain coarsening of the silicon carbide fibers at high temperatures (grain coarsening will lead to a rapid decrease in fiber strength). The strength retention rate is greater than 92%, significantly superior to Comparative Example 1 without doping (the fibers were pulverized at 1800 °C and the strength retention rate could not be measured), enabling it to maintain excellent performance in extremely high-temperature and strong oxidation environments and meet the usage requirements of high-temperature structural components in high-temperature harsh environments such as the aerospace field.

[0031] The above embodiments 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 relevant technical field shall be regarded as not departing from the patent scope of the present invention.

Claims

1. A preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber, characterized in that: It includes the following steps: Step 1: First, use PMS and aluminum acetylacetonate as raw materials to react and prepare Al-PMS; Step 2: Then, under nitrogen protection, react PMS with PPBSO to obtain B-PMS; Step 3: Then, under nitrogen protection, blend B-PMS, Al-PMS, and PMS at room temperature, and filter out insoluble substances using ultra-precision filter paper to obtain a B-PMS / Al-PMS / PMS mixture; Step 4: Then, under nitrogen protection, put the B-PMS / Al-PMS / PMS mixture and refined PCS into a melt spinning machine according to the ratio, heat to 260 - 280 °C under nitrogen protection, melt and stir evenly, perform degassing and removal of small molecules treatment, then lower the temperature to 230 - 250 °C and keep it warm. The molten melt is spun through a spinneret and wound by a winding machine to produce a boron-aluminum doped polycarbosilane precursor fiber; Step 5: Then, use the low pre-oxidation - thermal cross-linking method to infusibilize the boron-aluminum doped polycarbosilane precursor fiber to obtain a polycarbosilane cross-linked fiber; Step 6: Then, pyrolyze the polycarbosilane cross-linked fiber at high temperature under nitrogen protection to obtain a polycarbosilane pyrolysis fiber; Step 7. Finally, under the protection of nitrogen, the polycarbosilane pyrolysis fiber is subjected to high-temperature sintering to obtain a near-stoichiometric silicon carbide fiber containing a small amount of boron and a small amount of aluminum, where the boron content accounts for 0.5 wt % to 0.7 wt %, and the aluminum content accounts for 0.4 wt % to 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, then cover an appropriate amount of PMS on aluminum acetylacetonate, mix evenly and then 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. Keep stirring 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; In Step 2, using PMS and PPBSO as raw materials, under nitrogen protection, condense and reflux at a temperature of 350 - 370 °C, and then the product is successively dissolved in xylene, filtered, and distilled and refined to obtain crude B-PMS.

2. The preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber according to claim 1, characterized in that: In Step 1, the mass dosage of aluminum acetylacetonate is 9.5 wt % to 10.5 wt %, and in Step 2, the mass dosage of PPBSO is 4 wt % to 8 wt %.

3. The preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber according to claim 1, characterized in that: In Step 2, PPBSO is prepared by pre-reacting dichlorophenylsilane and boric acid at 100 °C and then heating to 300 °C for vacuum filtration.

4. The preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber according to claim 1, characterized in that: In Step 3, before blending Al-PMS with PMS, it is necessary to dissolve in xylene, filter, and perform vacuum distillation to remove low-molecular substances in Al-PMS to adjust the molecular weight distribution of Al-PMS to obtain a suitable precursor Al-PMS. Before blending B-PMS with PMS, it is also necessary to dissolve in xylene, filter, and perform vacuum distillation to remove low-molecular substances in B-PMS to adjust the molecular weight distribution of B-PMS to obtain a suitable precursor B-PMS.

5. The preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber according to claim 1, characterized in that: In Step 3, the mass dosage ratio of the B-PMS, Al-PMS, and PMS is 8:10:90 - 9:11:

90.

6. The preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber according to claim 1, characterized in that: In Step 4, refined PCS is obtained by filtering out insoluble substances from PCS using ultra-precision filter paper.

7. The preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber according to claim 1, characterized in that: In Step 5, the process of the low pre-oxidation - thermal cross-linking method is as follows: at 190 - 210 °C, the polycarbosilane precursor filaments are treated in an air atmosphere for 25 - 35 min to cause preliminary oxidative cross-linking of the fiber precursor filaments. Subsequently, at 340 - 360 °C, the polycarbosilane precursor filaments are heat-treated in an inert atmosphere for 25 - 35 min to convert the fiber molecules into an infusible network structure.

8. The preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber according to claim 1, characterized in that: In Step 6, 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.

9. The preparation method of a near-stoichiometric boron- and aluminum-doped silicon carbide fiber according to claim 1, characterized in that: In Step 7, 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.

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