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

Abstract

The invention discloses a preparation method of a near-stoichiometric boron-doped silicon carbide fiber, which comprises the following steps: doping PMS with a boron element, blending with PCS, and sequentially carrying out melt spinning, non-melting treatment, high-temperature pyrolysis, high-temperature sintering and other processes to obtain the boron-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 method for preparing a near-stoichiometric boron-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 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, 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 obtaining continuous SiC fibers through high-temperature firing under the protection of an inert atmosphere. The two major foreign manufacturers, Nippon Carbon Co., Ltd. and Ube Industries, Ltd., have both 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 non-melting 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. Foreign third-generation silicon carbide fibers such as Hi-NicBon 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-doped silicon carbide fiber. By introducing boron 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-doped silicon carbide fiber, comprising the following steps: Step 1: First, under nitrogen protection, using polymethylsilane (PMS) as a raw material, the PMS is treated by an atmospheric pressure high-temperature method to obtain refined PMS; Step 2: Then, under nitrogen protection, the refined PMS and a boron-containing monomer are subjected to a copolymerization reaction to obtain boron-doped polymethylsilane (B-PMS); Step 3: Then, under nitrogen protection, B-PMS and refined PMS are blended at room temperature, and insoluble substances are filtered off using ultra-precision filter paper to obtain a B-PMS / PMS mixture; Step 4: Then, under nitrogen protection, the B-PMS / PMS mixture and refined PCS are put into a melt spinning machine according to a ratio, heated to 240-270 °C under nitrogen protection, melted and stirred evenly, subjected to degassing and dehydrogenation heat cross-linking treatment, and then the temperature is lowered to 220-230 °C and held, and the molten melt is spun through a spinneret and wound by a winding machine to produce a boron-containing polycarbosilane precursor filament; Step 5: Then, the boron-containing polycarbosilane precursor filament is subjected to infusibilization treatment by a low-oxygen rapid irradiation cross-linking-thermal treatment method to obtain a polycarbosilane cross-linked filament; Step 6: Then, the polycarbosilane cross-linked filament is subjected to high-temperature thermal decarburization in a hydrogen atmosphere to obtain a polycarbosilane pyrolysis filament; Step 7: Finally, the polycarbosilane pyrolysis filament is subjected to high-temperature sintering under argon protection to obtain a near-stoichiometric silicon carbide fiber containing a small amount of boron and a small amount of oxygen, wherein the boron content accounts for 0.4 wt % - 0.8 wt %; In Step 2, the boron-containing monomer is one of trimethyl borate, triethyl borate, and triphenyl borate.

[0006] In Step 1, the process of the atmospheric pressure high-temperature method is as follows: Under nitrogen protection, PMS is dissolved in xylene or toluene, and macromolecular insoluble substances are filtered off to obtain a PMS mixture, which is then transferred to a three-necked flask, heated to 160 °C by a salt bath, and after the xylene or toluene is removed, the temperature is slowly raised to 220-280 °C, held for 4-6 hours, and cooled to room temperature to obtain refined PMS, which can effectively remove the impurities in PMS, improve its purity, and achieve the purpose of adjusting the molecular weight distribution.

[0007] In Step 2, the copolymerization reaction is carried out under nitrogen protection. The refined PMS is added to the boron-containing monomer and dissolved in an organic solvent to form a homogeneous solution. The molar ratio of boron to silicon is controlled to be 1:5 to 1:10. Then, water or an aqueous solution containing a catalyst is slowly added under stirring to initiate a hydrolysis reaction. The reaction temperature is controlled at 30 to 60 °C, and the reaction time is 2 to 8 h. After the reaction is completed, the generated by-products are removed by vacuum distillation or rotary evaporation, and an inert gas is used for purging to obtain a precursor viscous liquid.

[0008] In Step 2, the molar ratio of boron to silicon is controlled. If the molar ratio of boron to silicon is too high, boron enrichment will occur, forming a brittle B4C phase; if it is too low, boron doping will be insufficient, affecting the antioxidant property and high-temperature resistance of the fiber.

[0009] In Step 2, the mass dosage of the boron-containing monomer is 7.5 wt % to 11.5 wt % of the mass dosage of the refined PMS. The catalyst is hydrochloric acid or ammonia water, and the pH of the reaction solution is 2 to 5 or 8 to 10.

[0010] In Step 3, the mass dosage ratio of B-PMS to refined PMS is 1 to 3:1 to 10.

[0011] In Step 4, the mass dosage of the B-PMS / PMS mixed solution is 10 wt % to 15 wt % of the mass dosage of the refined PCS. When the mass dosage of the B-PMS / PMS mixed solution accounts for less than 10 wt % of the mass dosage of the refined PCS, the fibers will be filament-welded after high-temperature pyrolysis, indicating insufficient fiber cross-linking degree. When the mass dosage of the B-PMS / PMS mixed solution accounts for more than 15 wt % of the mass dosage of the refined PCS, filament-welding will occur during the cross-linking process, making it difficult to spin.

[0012] In Step 4, the refined PCS is obtained by filtering out insoluble substances from PCS using ultra-precision filter paper.

[0013] In Step 5, the process of the low-oxygen rapid irradiation cross-linking - heat treatment method is as follows: Under the atmosphere protection of a mixed gas composed of nitrogen and oxygen with a volume ratio of 99:1, the polycarbosilane raw filaments are irradiated with a low irradiation dose of 2.0 mGy for 4 h to cross-link the fiber raw filaments. Subsequently, at 340 to 360 °C, the polycarbosilane raw filaments are heat-treated in an inert atmosphere for 1 h to complete annealing, enabling the Si radicals to inactivate and the fiber to be completely transformed into a stable infusible network structure. Then, it is cooled to room temperature to obtain polycarbosilane cross-linked filaments.

[0014] In Step 6, the process of high-temperature thermal decarbonization is as follows: Place the polycarbosilane cross-linked fiber on a quartz boat and put it into a quartz tube. Evacuate the air, replace it with nitrogen three times to remove the air and impurities in the quartz tube. Then, introduce hydrogen at a flow rate of 50 m 3 / h. Under the protection of hydrogen, heat it up to 1350 °C at a rate of 100 - 200 °C / h, keep it warm for 30 - 60 min to further densify the fiber, and then cool it to room temperature in the quartz tube to obtain the polycarbosilane pyrolysis fiber. By setting the hydrogen concentration and sintering temperature during the fiber pyrolysis process, the composition of the fiber can be made close to the stoichiometric ratio of silicon carbide.

[0015] In Step 7, the process of high-temperature sintering is as follows: Place the polycarbosilane pyrolysis fiber in a graphite sintering furnace. Under the protection of argon, heat it up to 1800 - 2000 °C at a heating rate of 200 - 300 °C / h and keep it warm for 20 - 40 min for sintering treatment.

[0016] After adopting the above technical solution, the preparation method of a near-stoichiometric boron-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. Optimize the raw materials. When using only polycarbosilane (PCS) as the raw material, the pyrolysis product is still seriously carbon-rich, and the C / Si ratio is generally 1.2 - 1.4. When using polymethylsilane (PMS) to pyrolyze to produce SiC, it is generally silicon-rich, and the Si / C ratio is 1.2 - 1.3. Therefore, adding PMS to PCS to make 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 B-PMS to cleverly introduce the B element. Since the obtained silicon carbide fibers contain boron, the doping of boron forms B-C and B2O3 phases on the surface of β - SiC grains, effectively inhibiting the growth of grains at high temperatures, being able to uniformly control the grain size and achieve the purpose of densifying the microstructure, and finally obtaining near-stoichiometric continuous silicon carbide fibers; 2. Improve high-temperature resistance and oxidation resistance: By doping silicon carbide fibers with boron elements, 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 of 1800 °C and above, can maintain high strength and modulus, and extend service life; Introducing a small amount of 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 ceramic during high-temperature sintering, reducing crack and pore defects generated in the ceramic; Second, boron can inhibit the rapid growth of SiC grains at high temperatures to a certain extent; Third, boron 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; 3. Optimize the cross-linking method: Adopt a two-step method of low-oxygen rapid irradiation cross-linking and heat treatment. Combining with the role of boron in B-PMS, boron atoms can react with polymer molecular chains in the polycarbosilane precursor to form a cross-linked structure, improving the thermal stability and mechanical strength of the precursor, enabling it to resist high temperatures without melting at a lower oxygen weight gain rate, improving the process controllability and fiber quality; The oxygen in SiC fibers is mainly introduced during the air non-melting treatment process. Therefore, improving the cross-linking method can reduce the oxygen content in the fibers. The radiation cross-linking method is a research hotspot in non-oxidative cross-linking. Using the low-oxygen radiation cross-linking method can effectively reduce the oxygen content in the fibers and obtain SiC fibers with excellent performance; 4. Increase fiber yield and precisely control boron content: Dewaxing and removing small molecules before melt spinning can effectively increase the molecular weight and cross-linking degree of PMS, thereby increasing the fiber yield. Further, by controlling the dosage ratio of B-PMS, PMS and PCS, the boron content in 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 boron content in silicon carbide fibers exceeds 1.0 wt%, the fibers are over-cross-linked and cannot be spun or are prone to breakage during spinning; 5. Optimize carbon and oxygen content control: Different carbon content ceramic fibers can be prepared by changing the hydrogen concentration and sintering temperature during the fiber pyrolysis process. Once free carbon forms in the fibers, removing it will cause pores or defects in the fibers. Therefore, this patent introduces hydrogen during the formation of amorphous fibers to reduce the carbon content in the fibers; 6. Remove excess carbon and oxygen elements by the decomposition of SiC x O y at high temperature: During the high-temperature pyrolysis treatment process, due to SiC x O yWhen the amorphous phase decomposes, the strength of the Si-B-C-O fiber will decrease significantly. During the subsequent high-temperature sintering process, under the sintering effect of boron, the fiber gradually becomes dense and its strength begins to increase. The strength of the fiber has a great relationship with the defects existing in the fiber. However, due to the three-step decarbonization control of the free carbon in the fiber and the control of the oxygen content during the preparation of the precursor, fiber crosslinking, and pyrolysis processes, the influence on the original strength and performance of the fiber during the removal is much smaller than that of conventional methods (such as CN 116639983 A, a method for preparing a high-temperature-resistant near-stoichiometric continuous silicon carbide fiber). The lower oxygen content inside the fiber can reduce the total oxygen content of the fiber to a certain extent, and the oxygen is mainly distributed in the outer layer, which is beneficial to the escape of oxygen combined with other elements from the system during high-temperature sintering. Brief Description of the Drawings

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

[0018] In order 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 method for preparing a near-stoichiometric boron-doped silicon carbide fiber, as Figure 1 shown, includes the following steps: Step 1: First, under nitrogen protection, dissolve 1200 g of polymethylsilane (PMS) in 3600 mL of xylene, filter to remove macromolecular insoluble substances to obtain a PMS mixed solution, then transfer it to a three-necked flask, heat it to 160 °C through a salt bath, and after the xylene is evaporated to dryness, slowly raise the temperature to 260 °C, keep it warm for 5 hours, and cool to room temperature to obtain refined PMS; Step 2: Then, under nitrogen protection, place 80 g of triethyl borate at the bottom of the three-necked flask, then cover 1000 g of refined PMS on the triethyl borate, add 2160 mL of toluene and mix evenly, then start to slowly stir and raise the temperature to 50 °C and keep it. Add an aqueous solution containing hydrochloric acid (adjust the pH = 3) to initiate the hydrolysis and polycondensation reaction, react for 5 h, end the reaction, filter to obtain a filtrate, carry out vacuum distillation at 300 °C for 1 h to remove toluene and the generated ethanol, then quickly raise the temperature, slowly heat and react at 420 °C for 3 h, continuously stir during the reaction. When the temperature of the cracking column rises above 460 °C, keep it warm for 8 hours and then cool to room temperature, purge with nitrogen, and then remove the low-molecular substances in B-PMS through the processes of toluene dissolution, filtration, and vacuum distillation to adjust the molecular weight distribution of B-PMS, and obtain B-PMS with a molecular weight of about 1600; Step 3: Then, under nitrogen protection, blend B-PMS and refined PMS with a mass ratio of 1:1 at room temperature, and filter out the insoluble substances using ultra-precision filter paper to obtain a B-PMS / PMS mixture. Step 4: Then, filter out the insoluble substances in PCS using ultra-precision filter paper to obtain refined PCS. Under nitrogen protection, put the B-PMS / PMS mixture and refined PCS into a melt spinning machine according to the ratio (the mass dosage of the B-PMS / PMS mixture is 12 wt % of the mass dosage of refined PCS). Under nitrogen protection, heat to 250 °C, melt and stir evenly, perform degassing, dehydrogenation, and thermal cross-linking treatment, then lower the temperature to 225 °C and keep it warm. The molten melt is extruded through a spinneret and wound by a winding machine to produce boron-containing polycarbosilane precursor filaments. Step 5: Then, under the atmosphere protection of a mixed gas composed of nitrogen and oxygen with a volume ratio of 99:1, irradiate the boron-containing polycarbosilane precursor filaments with a low dose of 2.0 mGy for 4 h to cause cross-linking of the fiber precursor filaments. Subsequently, at 350 °C, heat-treat the polycarbosilane precursor filaments in a nitrogen atmosphere for 1 h to complete annealing, so that the Si radical-inactivated fibers are completely transformed into a stable infusible network structure, and then cool to room temperature to obtain polycarbosilane cross-linked filaments. Step 6: 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, and introduce hydrogen at a flow rate of 50 m 3 / h. Under hydrogen protection, heat to 1350 °C at a rate of 100 °C / h, keep it warm for 45 min to further densify the fibers, and then cool to room temperature in the quartz tube to obtain polycarbosilane pyrolysis filaments. Step 7: Finally, place the polycarbosilane pyrolysis filaments in a graphite sintering furnace. Under argon protection, heat to 1900 °C at a heating rate of 250 °C / h, keep it warm for 30 min for sintering treatment, size and wind to obtain near-stoichiometric boron-doped silicon carbide fibers.

[0020] Example 2 A method for preparing near-stoichiometric boron-doped silicon carbide fibers, comprising the following steps: Step 1: First, under nitrogen protection, dissolve 1500 g of PMS in 3000 mL of xylene, filter out the macromolecular insoluble substances to obtain a PMS mixture, then transfer it to a three-necked flask, heat to 160 °C through a salt bath, and after the xylene is evaporated to dryness, slowly raise the temperature to 220 °C, keep it warm for 4 hours, and cool to room temperature to obtain refined PMS. Step 2: Then, under the protection of nitrogen, place 100 g of trimethyl borate at the bottom of a three-necked flask, and then cover the trimethyl borate with 1200 g of refined PMS. Add 2600 mL of toluene and mix evenly, then start to slowly stir and heat up to 35 °C and maintain. Add an aqueous solution containing ammonia (adjust the pH = 9) to initiate the hydrolysis and polycondensation reaction. React for 6 h. After the reaction ends, filter to obtain the filtrate. Distill under reduced pressure at 300 °C for 1 h to remove toluene and the generated methanol. Then quickly heat up and slowly heat and react at 420 °C for 3 h. Continuously stir during the reaction. When the temperature of the cracking column rises above 460 °C, keep warm for 8 hours and then cool to room temperature. Purge with nitrogen, and then remove the low-molecular substances in B-PMS through the processes of dissolution in toluene, filtration, and distillation under reduced pressure to adjust the molecular weight distribution of B-PMS, and obtain B-PMS with a molecular weight of about 1600; Step 3: Then, under the protection of nitrogen, blend B-PMS and refined PMS with a mass ratio of 1:5 at room temperature, and filter off the insoluble substances using ultra-precision filter paper to obtain a B-PMS / PMS mixed solution; Step 4: Then filter off the insoluble substances in PCS using ultra-precision filter paper to obtain refined PCS. Under the protection of nitrogen, put the B-PMS / PMS mixed solution and refined PCS into a melt spinning machine according to the ratio (the mass dosage of the B-PMS / PMS mixed solution is 10 wt % of the mass dosage of refined PCS). Under the protection of nitrogen, heat to 240 °C, melt and stir evenly, carry out degassing, dehydrogenation, and thermal cross-linking treatment, then lower the temperature to 220 °C and keep warm. The molten melt is spun through a spinneret and wound by a winding machine to produce boron-containing polycarbosilane precursor filaments; Step 5: Then, under the atmosphere protection of a mixed gas composed of nitrogen and oxygen with a volume ratio of 99:1, irradiate the boron-containing polycarbosilane precursor filaments with a low dose of 2.0 mGy for 4 h to cross-link the fiber precursor filaments. Subsequently, at 350 °C, heat-treat the polycarbosilane precursor filaments in a nitrogen atmosphere for 1 h to complete annealing, so that the Si free radical-inactivated fibers are completely transformed into a stable infusible network structure, and then cool to room temperature to obtain polycarbosilane cross-linked filaments; Step 6: Then place the polycarbosilane cross-linked filaments on a quartz boat and place them in a quartz tube. Evacuate, change nitrogen three times to remove the air and impurities in the quartz tube. Pass hydrogen at a flow rate of 50 m 3 / h, and heat up to 1350 °C at a rate of 100 °C / h under the protection of hydrogen, keep warm for 45 min to further densify the fibers, and then cool to room temperature in the quartz tube to obtain polycarbosilane pyrolysis filaments; Step 7. Finally, place the polycarbosilane pyrolysis fiber in a graphite sintering furnace. Under argon protection, heat it to 1800 °C at a heating rate of 200 °C / h, hold for 40 min for sintering treatment, size and wind it to obtain boron-doped silicon carbide fibers with a near stoichiometric ratio.

[0021] Example 3 A preparation method of boron-doped silicon carbide fibers with a near stoichiometric ratio, comprising the following steps: Step 1. First, under nitrogen protection, dissolve 1200 g of PMS in 2400 mL of toluene, filter to remove macromolecular insoluble substances to obtain a PMS mixture, then transfer it to a three-necked flask, heat it to 160 °C through a salt bath. After the toluene is evaporated, slowly raise the temperature to 280 °C, hold for 6 hours, cool to room temperature to obtain refined PMS; Step 2. Then, under nitrogen protection, place 90 g of triphenyl borate at the bottom of a three-necked flask, then cover 800 g of refined PMS on the benzyl borate, add 1780 mL of toluene and mix evenly, then start to slowly stir and raise the temperature to 50 °C and hold. Add an aqueous solution containing hydrochloric acid (adjust the pH = 4) to initiate the hydrolysis polycondensation reaction, react for 5 h, end the reaction, filter to obtain a filtrate, carry out vacuum distillation at 300 °C for 1 h to remove toluene and the generated phenol, then quickly raise the temperature, slowly heat and react at 420 °C for 3 h, continuously stir during the reaction. When the temperature of the cracking column rises above 460 °C, hold for 8 hours and then cool to room temperature, purge with nitrogen, and then remove the low-molecular substances in B-PMS through the processes of toluene dissolution, filtration, and vacuum distillation to adjust the molecular weight distribution of B-PMS to obtain B-PMS with a molecular weight of about 1600; Step 3. Then, under nitrogen protection, blend B-PMS and refined PMS with a mass ratio of 2:3 at room temperature, filter off the insoluble substances with ultra-precision filter paper to obtain a B-PMS / PMS mixture; Step 4. Then, filter off the insoluble substances in PCS with ultra-precision filter paper to obtain refined PCS. Under nitrogen protection, put the B-PMS / PMS mixture and refined PCS in a melt spinning machine according to the ratio (the mass dosage of the B-PMS / PMS mixture is 15 wt %) Heat to 270 °C under nitrogen protection, melt and stir evenly, carry out degassing and dehydrogenation heat cross-linking treatment, then lower the temperature to 230 °C and hold. The molten melt is spun through a spinneret and wound by a winding machine to produce boron-containing polycarbosilane raw filaments; Step 5: Then, under the atmosphere protection of a mixed gas composed of nitrogen and oxygen with a volume ratio of 99:1, irradiate the boron-containing polycarbosilane precursor fiber with a low dose of 2.0 mGy for 4 h to cause cross-linking of the fiber precursor. Subsequently, at 350 °C, heat-treat the polycarbosilane precursor fiber in a nitrogen atmosphere for 1 h to complete annealing, so that the Si radical-inactivated fiber is completely transformed into a stable infusible network structure. Then cool it to room temperature to obtain polycarbosilane cross-linked fiber; Step 6: Then place the polycarbosilane cross-linked fiber on a quartz boat and put it into a quartz tube. Evacuate the air, and replace it with nitrogen three times to remove the air and impurities in the quartz tube. Pass hydrogen into the tube at a flow rate of 50 m 3 / h, and heat it up to 1350 °C at a rate of 100 °C / h under hydrogen protection, and keep it at this temperature for 45 min to further densify the fiber. Then cool it to room temperature in the quartz tube to obtain polycarbosilane pyrolyzed fiber; Step 7: Finally, place the polycarbosilane pyrolyzed fiber in a graphite sintering furnace, heat it up to 1850 °C at a heating rate of 200 °C / h under argon protection, and keep it at this temperature for 40 min for sintering treatment. Then size and wind it to obtain boron-doped silicon carbide fiber with a near stoichiometric ratio.

[0022] Example 4 A method for preparing boron-doped silicon carbide fiber with a near stoichiometric ratio, comprising the following steps: Step 1: First, under nitrogen protection, dissolve 1500 g of PMS in 3000 mL of toluene, filter to remove macromolecular insoluble substances to obtain a PMS mixed solution, then transfer it to a three-necked flask, heat it to 160 °C through a salt bath. After the toluene is evaporated, slowly raise the temperature to 240 °C, keep it warm for 5 hours, and cool it to room temperature to obtain refined PMS; Step 2: Then, under the protection of nitrogen, place 120 g of trimethyl borate at the bottom of a three-necked flask, then cover 1100 g of refined PMS on the trimethyl borate, add 2440 mL of toluene and mix evenly, then start to slowly stir and heat up to 60 °C and keep it. Add an aqueous solution containing hydrochloric acid (adjust the pH = 5) to initiate the hydrolysis and polycondensation reaction, react for 6 h, end the reaction, filter to obtain a filtrate, carry out vacuum distillation at 300 °C for 1 h to remove toluene and the generated methanol, then quickly raise the temperature, slowly heat and react at 420 °C for 3 h, continuously stir during the reaction. When the temperature of the cracking column rises above 460 °C, keep it warm for 8 hours and then cool it to room temperature, purge with nitrogen, and then remove the low-molecular substances in B-PMS through the processes of toluene dissolution, filtration and vacuum distillation to adjust the molecular weight distribution of B-PMS, and obtain B-PMS with a molecular weight of about 1600; Step 3: Then, under nitrogen protection, blend B-PMS and refined PMS with a mass ratio of 3:7 at room temperature, and filter out the insoluble substances using ultra-precision filter paper to obtain a B-PMS / PMS mixture; Step 4: Then, filter out the insoluble substances in PCS using ultra-precision filter paper to obtain refined PCS. Under nitrogen protection, put the B-PMS / PMS mixture and refined PCS into a melt spinning machine according to the ratio (the mass dosage of the B-PMS / PMS mixture is 14 wt % of the mass dosage of refined PCS). Under nitrogen protection, heat to 255 °C, melt and stir evenly, perform degassing, dehydrogenation, and thermal cross-linking treatment, then lower the temperature to 223 °C and keep it warm. The molten melt is spun through a spinneret and wound by a winding machine to produce boron-containing polycarbosilane precursor filaments; Step 5: Then, under the atmosphere protection of a mixed gas composed of nitrogen and oxygen with a volume ratio of 99:1, irradiate the boron-containing polycarbosilane precursor filaments with a low dose of 2.0 mGy for 4 h to cross-link the fiber precursor filaments. Subsequently, at 350 °C, heat-treat the polycarbosilane precursor filaments in a nitrogen atmosphere for 1 h to complete annealing, deactivate the Si free radicals, and completely transform the fibers into a stable infusible network structure. Then cool to room temperature to obtain polycarbosilane cross-linked filaments; Step 6: Then place the polycarbosilane cross-linked filaments on a quartz boat and put them into a quartz tube. Evacuate the air, change nitrogen three times to remove the air and impurities in the quartz tube. Introduce hydrogen at a flow rate of 50 m 3 / h, and heat up to 1350 °C at a rate of 120 °C / h under hydrogen protection, keep it warm for 45 min to further densify the fibers, and then cool to room temperature in the quartz tube to obtain polycarbosilane pyrolysis filaments; Step 7: Finally, place the polycarbosilane pyrolysis filaments in a graphite sintering furnace, heat up to 1850 °C at a heating rate of 220 °C / h under argon protection, keep it warm for 50 min for sintering treatment, size and wind to obtain near-stoichiometric boron-doped silicon carbide fibers.

[0023] In the above embodiments, the PCS used is produced by Fujian Liyaxin 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 , a ceramic yield > 65%, chemically stable, and can be stored and used at room temperature.

[0024] The properties of the near-stoichiometric boron-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 / T 43760-2024. The boron content was analyzed using a well-known elemental testing method (ICP-OES). The results are shown in Table 1. Table 1 Performance data of silicon carbide fibers

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

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

[0027] The C / Si atomic ratio of the silicon carbide fibers prepared in Examples 1-4 was close to 1 (1.01-1.02), which was lower than that of the third-generation silicon carbide fibers in Comparative Example 1 (1.06). This was because the residual free carbon was reduced, improving the high-temperature stability.

[0028] The oxygen content of the boron-doped silicon carbide fibers was low (0.32 wt % - 0.48 wt %), reducing the formation of the SiC x O y amorphous phase and avoiding performance degradation caused by high-temperature decomposition. This significantly improved the high-temperature resistance and mechanical properties of the silicon carbide fibers. After introducing boron, the SiC grain size was significantly controlled at high temperatures, and its average grain size was between 39 and 48 nm, enabling it to maintain excellent performance in extremely high-temperature and strong oxidation environments. In contrast, traditional fine-grained silicon carbide fibers (the grain size of Comparative Example 1 was 8.8 nm) were prone to coarsening at high temperatures, resulting in a rapid decrease in fiber strength and a low strength retention rate.

[0029] The silicon carbide fibers prepared in Example 2 (containing 0.72 wt %) had the best comprehensive performance. The strength was 2.8 GPa, only 17.6% lower than that of the undoped silicon carbide fibers (20% - 32% lower than those prepared in other examples). The grain size was the smallest (39 nm), and the grain boundary strengthening effect still existed. The tensile modulus (363 GPa) was 3% higher than that of the silicon carbide fibers in the comparative example, and the strength retention rate was 93%. The oxygen content was moderate (0.42 wt %), indicating that the "grain boundary regulation" and "oxidation protection" effects of boron reached a balance at this doping level, making it suitable for hot-end components of aeroengines, etc.

[0030] 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 art shall be regarded as not departing from the patent scope of the present invention.

Claims

1. A method for preparing boron-doped silicon carbide fibers in a near-stoichiometric ratio, characterized in that: The following steps are involved: Step 1: first, under nitrogen protection, using PMS as a raw material, treating PMS using a normal pressure and high temperature method to obtain refined PMS; Step 2, then copolymerizing the refined PMS with the boron-containing monomer under nitrogen protection to obtain B-PMS; Step 3, then under nitrogen protection, blending B-PMS and refined PMS at room temperature, filtering out insoluble matter with ultra-precision filter paper to obtain a B-PMS / PMS mixed solution; Step 4, then under nitrogen protection, the B-PMS / PMS mixed solution and the refined PCS are put into a melt spinning machine according to the ratio, heated to 240-270°C under nitrogen protection, melted and stirred evenly, degassing, dehydrogenation and thermal cross-linking treatment are carried out, and then the temperature is reduced to 220-230°C and kept warm, the molten melt is spun through a spinneret and wound by a winder to produce a boron-containing polycarbosilane precursor; Step 5, then using a low-oxygen rapid radiation cross-linking-heat treatment method to perform an infusible treatment on the boron-containing polycarbosilane raw silk to obtain a polycarbosilane cross-linked silk; Step 6, then decarburizing the polycarbosilane cross-linked wire by high-temperature pyrolysis in a hydrogen atmosphere to obtain polycarbosilane pyrolysis wire; Step 7: 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 of a small amount of boron and a small amount of oxygen, wherein the boron content accounts for 0.4% of the mass of the silicon carbide fiber. wt %~0.8 wt %%; In step 2, the boron-containing monomer is one of trimethyl borate, triethyl borate and triphenyl borate.

2. The method for preparing a nearly stoichiometric boron-doped silicon carbide fiber according to claim 1, characterized in that: In step 2, the copolymerization reaction is carried out under nitrogen protection, the refined PMS is added to the boron-containing monomer and dissolved in an organic solvent to form a homogeneous solution, the molar ratio of boron to silicon is controlled to be 1:5~1:10, and then water or an aqueous solution containing a catalyst is slowly added under stirring conditions to initiate a hydrolysis reaction, the reaction temperature is controlled at 30~60°C, the reaction time is 2~8 h, and after the reaction is completed, the generated by-products are removed by reduced pressure distillation or rotary evaporation, and an inert gas is used for purging to obtain a precursor viscous liquid.

3. The method for preparing a silicon carbide fiber doped with boron in a near stoichiometric ratio according to claim 1, characterized in that: In step 2, the mass amount of the boron-containing monomer is 7.5 of the mass amount of the refined PMS. wt %~11.5 wt %, the catalyst is hydrochloric acid or ammonia water, and the reaction solution pH = 2~5 or 8~10.

4. The method for preparing a silicon carbide fiber doped with boron in a near stoichiometric ratio according to claim 1, characterized in that: In step 3, the mass ratio of B-PMS to refined PMS is 1~3:1~10.

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

6. The method for preparing a silicon carbide fiber doped with boron in a near stoichiometric ratio according to claim 1, characterized in that: In step 4, refined PCS is obtained by filtering out insoluble matter from PCS using ultra-precision filter paper.

7. The method for preparing a silicon carbide fiber doped with boron in a near stoichiometric ratio according to claim 1, characterized in that: In step 5, the process of the low-oxygen rapid radiation cross-linking-heat treatment method is as follows: under the protection of an atmosphere of a mixed gas composed of nitrogen and oxygen in a volume ratio of 99:1, the polycarbosilane precursor is irradiated with a low dose of 2.0 mGy for 4 hours to cross-link the fiber precursors, and then the polycarbosilane precursor is heat-treated in an inert atmosphere at 340~360°C for 1 hour to complete annealing, so that the Si free radical inactivated fiber is completely converted into a stable infusible network structure, and then cooled to room temperature to obtain polycarbosilane cross-linked fibers.

8. The method for preparing a silicon carbide fiber doped with boron in a near stoichiometric ratio according to claim 1, characterized in that: In step 6, the high temperature pyrolysis decarburization process is as follows: placing the polycarbosilane cross-linked wire 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 quartz tube, and heating the tube at 50 m 3 Hydrogen is introduced at a flow rate of 100-200°C / h under hydrogen protection, and the temperature is raised to 1350°C at a rate of 100-200°C / h and kept warm for 30-60 min to further densify the fiber. The fiber is then cooled to room temperature in a quartz tube to obtain polycarbosilane pyrolysis fibers.

9. The method for preparing a silicon carbide fiber doped with boron in a near stoichiometric ratio according to claim 1, characterized in that: In step 7, the high temperature sintering process is as follows: placing the polycarbosilane pyrolytic wire in a graphite sintering furnace, heating the temperature to 1800-2000° C. at a heating rate of 200-300° C. / h under argon protection, and keeping the temperature for 20-40 min for sintering.

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