Preparation method of near-stoichiometric ZrB2 / ZrC doped SiC fiber

Abstract

The invention discloses a near stoichiometric ratio ZrB2 / ZrC doped SiC fiber preparation method, which comprises: carrying out a reaction on dimethylamine borane (DMAB) and polymethylsilane (PMS) to generate a high-dispersion and high-stability boron sol, blending the boron sol with polyzirconium silane (PZCS) to prepare precursor powder, and sequentially carrying out melt spinning, non-melting treatment, hydrogen high-temperature pyrolysis, high-temperature sintering and other processes to obtain the near stoichiometric ratio ZrB2 / ZrC doped SiC fiber. The ZrB2 / ZrC doped silicon carbide fiber with the C / Si atomic ratio close to an ideal value is obtained, the high temperature resistance and the oxidation resistance of the silicon carbide fiber are remarkably improved, and the silicon carbide fiber can keep excellent performance under the extremely high temperature and strong oxidation environment.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon carbide fibers, and specifically relates to a preparation method of a near-stoichiometric SiC fiber doped with ZrB2 / ZrC. Background Art

[0002] Silicon carbide (SiC) fibers have important application prospects in high-tech fields such as aviation, aerospace, nuclear industry, and weaponry due to their excellent properties such as high strength, high modulus, high temperature resistance, oxidation resistance, and corrosion resistance. Research institutions at home and abroad have achieved industrial production of continuous SiC fibers through the polycarbosilane (PCS) precursor 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 infusibilization 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 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 PCS precursor of this SiC fiber is rich in carbon and the air oxidation infusibilization 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, SiC fibers have gone through three generations of research and development, gradually evolving from a high-oxygen high-carbon structure to a low-oxygen high-carbon structure, and finally forming the third-generation continuous silicon carbide fibers with low oxygen and low carbon. Foreign third-generation SiC 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] At present, the research level of zirconium-containing silicon carbide fibers and zirconium boride-containing silicon carbide fibers in China is still limited. Only the mass production of doped second-generation silicon carbide fibers has been achieved. Next, it is necessary to find a more excellent method for preparing silicon carbide fibers, improve the oxidation resistance and creep resistance of the fibers under high-temperature conditions, and prepare high-performance third-generation zirconium-containing silicon carbide fibers or zirconium boride-containing silicon carbide fibers that can be industrially produced. Summary of the Invention

[0004] The object of the present invention is to provide a method for preparing near-stoichiometric SiC fibers doped with ZrB2 / ZrC. By introducing ZrB2 / ZrC to dope the SiC fibers, near-stoichiometric SiC fibers doped with ZrB2 / ZrC 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 SiC fibers doped with ZrB2 / ZrC, comprising the following steps: Step 1: First, under nitrogen protection, the polyzirconium carbosilane (PZCS) and polymethylsilane (PMS) are respectively treated by the atmospheric pressure high-temperature method to obtain refined PZCS and refined PMS; Step 2: Then, under nitrogen protection, using refined PMS and dimethylamine borane (DMAB) as raw materials, a reaction is carried out by the blending method, and then cooled to room temperature. After filtration, a highly dispersed and highly stable boron sol is obtained; Step 3: Then, under nitrogen protection, the refined PZCS and the boron sol are dissolved in xylene, the insoluble substances are filtered off, and then the xylene is removed by vacuum drying to obtain a boron-containing PZCS precursor powder; Step 4: Then, under nitrogen protection, the boron-containing PZCS precursor powder is put into a melt spinning machine, heated to 260-270 °C, melted and stirred evenly, and kept for 12-18 h for degassing and removing small molecules. Then the temperature is lowered to 180-190 °C and kept warm. The molten melt is spun through a spinneret and wound by a winding machine to produce a boron-containing polyzirconium carbosilane raw fiber; Step 5: Then, the boron-containing polyzirconium carbosilane raw fiber is infusibilized by the method of low-oxygen rapid irradiation cross-linking - heat treatment to obtain a boron-containing polyzirconium carbosilane cross-linked fiber; Step 6: Then, the boron-containing polyzirconium carbosilane cross-linked fiber is subjected to high-temperature pyrolysis decarburization in a hydrogen atmosphere to obtain a boron-containing polyzirconium carbosilane pyrolyzed fiber; Step 7: Finally, under the protection of nitrogen, tension is applied to the boron-containing polyzirconium carbosilane pyrolyzed fiber and then high-temperature sintering is carried out to obtain near-stoichiometric SiC fibers doped with ZrB2 / ZrC.

[0006] In Step 1, the atmospheric pressure high-temperature method for the refined PZCS is as follows: at room temperature, the xylene solution of PZCS is placed in a rotary evaporator, and the temperature is slowly raised to 160 °C under the protection of a high-purity nitrogen atmosphere. During the heating process, the solid substances are completely melted; after the solvent xylene is evaporated, the temperature is further raised to 300-360 °C and kept warm for 4-6 hours to obtain a black brittle PZCS precursor, in which the zirconium content is 2 wt%, and then dissolve the PZCS precursor in toluene solution, place it in an autoclave, introduce ammonia gas, heat up to 350 °C, maintain the pressure at 4 MPa, react for 5 hours. After the reaction ends, cool it down to room temperature, and filter the precipitate. Repeat the filtration three times to obtain refined PZCS. This can remove the macromolecular insoluble substances in PZCS and obtain refined PZCS, which is beneficial to improving the spinning and the final properties of the fiber.

[0007] In step 1, the atmospheric pressure high-temperature method of the refined PMS is as follows: Under nitrogen protection, dissolve PMS in xylene or 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 xylene or toluene is evaporated to dryness, slowly raise the temperature to 220 - 280 °C, keep it warm for 4 - 6 hours, and cool it to room temperature to obtain refined PMS. This can effectively remove the impurities in PMS, improve its purity, and achieve the purpose of adjusting the molecular weight distribution.

[0008] In step 2, the process of the blending method is as follows: Add the refined PMS and DMAB to the reactor at a mass ratio of 3:1 - 6:1. Under nitrogen protection, raise the reaction temperature to 350 - 400 °C, react for 8 - 10 h, then cool it to room temperature. After filtering the reaction product, a yellow transparent boron sol is obtained.

[0009] In step 3, the mass dosage ratio of the refined PZCS to the boron sol is 3:1 - 5:1.

[0010] In step 5, the cross-linking method of the low-oxygen rapid irradiation cross-linking - heat treatment is as follows: Under nitrogen: Under the atmosphere protection of a mixed gas composed of nitrogen and oxygen with a volume ratio of 99:1, irradiate the polycarbosilane raw fiber with a low irradiation dose of 4 - 6 mGy for 4 h to cross-link the fiber raw filament. Subsequently, at 340 - 360 °C, heat-treat the boron-containing polyzirconium carbon silicon raw fiber in an inert atmosphere for 1 h to complete annealing, inactivate the Si free radicals, and completely convert the fiber into a stable infusible network structure. Then cool it down to room temperature to obtain the boron-containing polyzirconium carbon silicon cross-linked fiber.

[0011] In step 6, the process of the high-temperature pyrolysis is as follows: Place the polycarbosilane cross-linked fiber on a quartz boat and place it in a quartz tube. Evacuate the air, change the nitrogen three times to remove the air and impurities in the quartz tube. Then fill hydrogen at a flow rate of 4 - 6 L / min, raise the temperature to 1200 - 1400 °C at a rate of 100 - 200 °C / h, keep it warm for 30 - 60 min to further densify the fiber. Then cool it down to room temperature in the quartz tube to obtain the boron-containing polyzirconium carbon silicon pyrolysis fiber. By changing the hydrogen concentration and sintering treatment temperature during the fiber pyrolysis process, ceramic fibers with different carbon contents can be prepared, making the composition of the fiber close to the stoichiometric ratio of silicon carbide.

[0012] In Step 7, the process of high-temperature sintering is as follows: Place the boron-containing polyzirconium carbon silane pyrolysis fiber in a graphite sintering furnace. Under argon protection, apply a tension of 0.45 - 0.5 N, and increase the temperature to 1800 - 1900 °C at a heating rate of 200 - 300 °C / h, hold for 20 - 30 min, and conduct sintering treatment to form a dense near-stoichiometric SiC fiber doped with ZrB2 / ZrC.

[0013] After adopting the above technical solution, the preparation method of a near-stoichiometric SiC fiber doped with ZrB2 / ZrC 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. Prepare borosol by the borosol blending method, and then blend it with polyzirconium silane (PZCS) to prepare a precursor. Combining chemical crosslinking and physical blending not only ensures the efficient introduction of boron elements but also is compatible with the existing spinning process, especially suitable for large-scale production; When using only polyzirconium silane (PZCS) 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, with an Si / C ratio of 1.2 - 1.3. Therefore, adding PMS to PZCS to make silicon carbide fibers reduces the surplus carbon in the obtained silicon carbide fibers, which can enhance the high-temperature performance, oxidation resistance, and creep resistance of the fibers, solve the problem of surplus carbon from the raw materials, and react PMS with dimethylamine borane (DMAB) using the blending method to cleverly introduce B elements. Since the obtained silicon carbide fibers contain boron elements, it can effectively inhibit the growth of grains at high temperatures, uniformly control the grain size, and achieve the purpose of densifying the microstructure, and finally obtain a near-stoichiometric SiC fiber doped with ZrB2 / ZrC.

[0014] 2. Improving high-temperature resistance and oxidation resistance: ZrB2 improves the high-temperature oxidation resistance of the fiber. The oxidation weight gain rate is lower than the sintering temperature of pure ZrC-SiC fibers. At a high temperature of 1800 °C, both ZrB2 (melting point about 3245 °C) and ZrC (melting point 3540 °C) can stably exist, and their lattice constants are similar (ZrB2: a = 3.17 Å, c = 3.53 Å; ZrC: a = 4.69 Å), and a coherent interface can be formed. When ZrB2 / ZrC is formed inside the SiC fiber, it can adjust and improve the SiC fiber from the microstructure and performance, enhance the density and mechanical properties of the fiber, and synergistically improve the oxidation resistance (ZrB2 generates a protective ZrO2-B2O3 layer) and high-temperature strength (supported by the high hardness of ZrC, and ZrC oxidizes to generate ZrO2) of the fiber.

[0015] 3. Optimize the cross-linking method: Adopt a two-step method of low-oxygen rapid cross-linking and heat treatment. Combining with the role of boron in the fiber, boron atoms can react with the polymer molecular chains in the precursor fiber to form a cross-linked structure, improving the thermal stability and mechanical strength of the precursor fiber, enabling it to resist high temperatures without melting at a lower oxygen weight gain rate, and enhancing the process controllability and fiber quality. The oxygen in SiC fibers is mainly introduced during the air stabilization process. Therefore, improving the cross-linking method can reduce the oxygen content in the fibers. The radiation cross-linking method is a hot topic in non-oxidative cross-linking research. Using the low-oxygen radiation cross-linking method can effectively reduce the oxygen content in the fibers and obtain SiC fibers with excellent properties.

[0016] 4. Increase the fiber yield: Performing degassing and removing small molecules before melt spinning can effectively increase the molecular weight and cross-linking degree of the mixed precursor, thereby increasing the fiber yield. 5. Optimize the control of carbon and oxygen content: Different carbon content ceramic fibers can be prepared by changing the hydrogen concentration and sintering treatment temperature during the fiber pyrolysis process. Once free carbon forms in the fiber, removing it will cause pores or defects in the fiber. Therefore, hydrogen is introduced during the formation of amorphous fibers in the present invention to reduce the carbon content in the fibers.

[0017] 6. During the high-temperature treatment process, due to the decomposition of the SiC x O y amorphous phase, the strength of Si-B-C-O fibers will significantly decrease. Subsequently, during the high-temperature sintering process, under the sintering effect of boron, the fibers gradually become dense and the strength begins to increase. The strength of the fibers has a great relationship with the defects existing in the fibers. However, due to the three-step decarbonization control of the free carbon in the fibers and the control of the oxygen content in the precursor preparation, fiber cross-linking, and pyrolysis steps, the influence on the original strength and properties of the fibers during removal is much smaller than that of the traditional high-temperature treatment method (CN 116639983 A, a high-temperature resistant near-stoichiometric continuous silicon carbide fiber and its preparation method). The lower oxygen content inside the fiber can reduce the total oxygen content of the fiber to a certain extent, and oxygen is mainly distributed in the outer layer, which is beneficial for oxygen to combine with other elements and escape from the system during high-temperature sintering.

[0018] 7. Using the method of generating boron sol by reacting dimethylamine borane (DMAB) with PMS is a better choice for boron introduction. The B-H bond of DMAB reacts with the Si-H bond of PMS to form a chemical cross-linked structure. The boron sol is evenly dispersed (no agglomeration), ensuring uniform distribution of boron elements in the fibers, and being compatible with the existing polycarbosilane spinning process, suitable for large-scale production.

[0019] 8. Introducing zirconium into silicon carbide fibers increases the decomposition temperature of the Si-O-C phase, enabling the silicon carbide fibers to be used at higher temperatures. However, when the temperature is above 1600 °C, the amorphous phase of the zirconium-containing silicon carbide fibers still decomposes, leading to a decrease in their mechanical strength. Therefore, boron is introduced into the fibers to further improve their high-temperature resistance.

[0020] Furthermore, the high-temperature sintering temperature is above 1800 °C, resulting in good crystallinity of the silicon carbide fibers and avoiding fiber embrittlement caused by excessive grain growth. Due to the sintering effect of boron, after being treated at 1800 °C, the structure of the ZrB2 / ZrC-doped SiC fibers is denser than that of the fibers treated at 1600 °C. Description of the Drawings

[0021] Figure 1 It is a flowchart of a preparation method of a near-stoichiometric ZrB2 / ZrC-doped SiC fiber. Detailed Embodiments

[0022] To further explain the technical solution of the present invention, the present invention will be elaborated in detail through specific embodiments below.

[0023] Example 1 At room temperature, the xylene solution of PZCS is placed in a rotary evaporator. The mass ratio of PZCS to xylene is 1:2. Under the protection of a high-purity nitrogen atmosphere, the temperature is slowly raised to 160 °C step by step. During the heating process, the solid substance completely melts; after the solvent xylene is distilled off, the temperature is further raised to 360 °C and kept warm for 6 hours to obtain a black brittle PZCS precursor, in which the zirconium content is 2 wt %.

[0024] Example 2 A preparation method of a near-stoichiometric ZrB2 / ZrC-doped SiC fiber, as Figure 1 shown, includes the following steps: Step 1. First, according to the mass ratio of PZCS precursor:toluene = 1:2, the toluene solution containing the PZCS precursor is placed in an autoclave, ammonia gas is introduced, the temperature is raised to 350 °C, and the pressure is maintained at 4 MPa. The reaction lasts for 5 hours. After the reaction, it is cooled to room temperature and the precipitate is filtered. This is repeated three times to obtain refined PZCS; Then, under the protection of nitrogen, according to the mass ratio of 1:2, PMS is dissolved in toluene, the macromolecular insoluble substances are filtered off to obtain a PMS mixture, which is then transferred to a three-necked flask and heated to 160 °C by a salt bath. After the toluene is evaporated to dryness, the temperature is slowly raised to 280 °C, kept warm for 6 hours, and cooled to room temperature to obtain refined PMS; Step 2: Then, under nitrogen protection, add refined PMS and DMAB into the reactor according to the mass ratio of 4:1. Raise the reaction temperature to 380 °C. After reacting for 10 h, cool it to room temperature. Filter the reaction product to obtain a yellow transparent boron sol; Step 3: Then, under nitrogen protection, dissolve refined PZCS and the boron sol in xylene according to the mass ratio of 4:1. Filter to remove insoluble substances, and then remove xylene by vacuum drying at 120 °C to obtain a boron-containing PZCS precursor powder; Step 4: Then, under nitrogen protection, put the boron-containing PZCS precursor powder into a melt spinning machine, heat it to 260 °C, melt and stir evenly, keep it for 12 h for degassing and removing small molecules, then lower the temperature to 180 °C and keep it warm. The molten melt is spun through a spinneret at 180 °C and wound by a winding machine to produce a boron-containing polyzirconium carbon silane precursor fiber; 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 polycarbon silicon precursor fiber with a low dose of 4 mGy for 4 h to crosslink the fiber precursor. Subsequently, at 350 °C, heat-treat the boron-containing polyzirconium carbon silane precursor fiber in an inert atmosphere for 1 h to complete annealing, and convert the Si radical-inactivated fiber into a stable infusible network structure completely. Then cool it to room temperature to obtain a boron-containing polyzirconium carbon silane crosslinked fiber; Step 6: Then place the polycarbon silicon crosslinked fiber on a quartz boat and put it into a quartz tube. Evacuate, change nitrogen three times to remove air and impurities in the quartz tube. Then fill hydrogen at a flow rate of 5 L / min and heat it to 1350 °C at a rate of 100 °C / h, keep it warm for 45 min to further densify the fiber, and then cool it to room temperature in the quartz tube to obtain a boron-containing polyzirconium carbon silane pyrolysis fiber; Step 7: Finally, place the boron-containing polyzirconium carbon silane pyrolysis fiber in a graphite sintering furnace. Under argon protection, apply a tension of 0.5 N and heat it to 1800 °C at a heating rate of 250 °C / h, keep it warm for 25 min for sintering treatment, size and wind it to form a dense near-stoichiometric SiC fiber doped with ZrB2 / ZrC.

[0025] Example 3 A method for preparing a near-stoichiometric SiC fiber doped with ZrB2 / ZrC includes the following steps: Step 1: First, according to the mass ratio of PZCS precursor:toluene = 1:2, place the toluene solution containing the PZCS precursor in an autoclave, introduce ammonia gas, raise the temperature to 350 °C, keep the pressure at 4 MPa, react for 5 h. After the reaction, lower the temperature to room temperature and filter the precipitate, repeat three times to obtain refined PZCS; Then, under nitrogen protection, PMS was dissolved in toluene according to the mass ratio of 1:2, and the macromolecular insoluble substances were removed by filtration to obtain a PMS mixture. Then it was transferred to a three-necked flask and heated to 160 °C by a salt bath. After the toluene was evaporated, the temperature was slowly raised to 220 °C and held for 4 hours, and then cooled to room temperature to obtain refined PMS; Step 2: Then, under nitrogen protection, the refined PMS and DMAB were added to the reactor according to the mass ratio of 5:1, the reaction temperature was raised to 380 °C, and after reacting for 10 h, it was cooled to room temperature. The reaction product was filtered to obtain a yellow transparent boron sol; Step 3: Then, under nitrogen protection, the refined PZCS and the boron sol were dissolved in xylene according to the mass ratio of 5:1, the insoluble substances were removed by filtration, and then dried in vacuum at 120 °C to remove xylene to obtain a boron-containing PZCS precursor powder; Step 4: Then, under nitrogen protection, the boron-containing PZCS precursor powder was put into a melt spinning machine, heated to 270 °C, melted and stirred evenly, and held for 15 h for degassing and removing small molecules. Then the temperature was lowered to 180 °C and held. The molten melt was spun through a spinneret at 180 °C and wound by a winding machine to produce a boron-containing polyzirconocarbosilane precursor fiber; Step 5: Then, under the atmosphere protection of a mixed gas composed of nitrogen and oxygen with a volume ratio of 99:1, the polycarbosilane precursor fiber was irradiated with a low dose of 6 mGy for 4 h to crosslink the fiber precursor. Subsequently, at 360 °C, the boron-containing polyzirconocarbosilane precursor fiber was heat-treated in an inert atmosphere for 1 h to complete annealing, and the Si radical-inactivated fiber was completely transformed into a stable infusible network structure, and then cooled to room temperature to obtain a boron-containing polyzirconocarbosilane crosslinked fiber; Step 6: Then the polycarbosilane crosslinked fiber was placed on a quartz boat and placed in a quartz tube, evacuated, and replaced with nitrogen three times to remove the air and impurities in the quartz tube. Then hydrogen was filled at a flow rate of 6 L / min, and the temperature was raised to 1350 °C at a rate of 100 °C / h and held for 45 min to further densify the fiber. Then it was cooled to room temperature in the quartz tube to obtain a boron-containing polyzirconocarbosilane pyrolytic fiber; Step 7: Finally, the boron-containing polyzirconocarbosilane pyrolytic fiber was placed in a graphite sintering furnace. Under argon protection, a tension of 0.48 N was applied, and the temperature was raised to 1850 °C at a heating rate of 250 °C / h and held for 25 min for sintering treatment, sizing and winding to form a dense near-stoichiometric ZrB2 / ZrC-doped SiC fiber.

[0026] Example 4 A method for preparing a near-stoichiometric ZrB2 / ZrC-doped SiC fiber, comprising the following steps: Step 1: First, according to the mass ratio of PZCS precursor:toluene = 1:3, place the toluene solution containing the PZCS precursor in an autoclave, introduce ammonia gas, raise the temperature to 350 °C, maintain the pressure at 4 MPa, and react for 5 hours. After the reaction, cool it to room temperature and filter the precipitate. Repeat this process three times to obtain refined PZCS; Then, under nitrogen protection, dissolve PMS in xylene according to the mass ratio of 1:3, filter to remove macromolecular insoluble substances to obtain a PMS mixture, and then transfer it to a three-necked flask. Heat it to 160 °C through a salt bath. After the xylene is evaporated, slowly raise the temperature to 250 °C and keep it warm for 5 hours. Cool it to room temperature to obtain refined PMS; Step 2: Then, under nitrogen protection, add refined PMS and DMAB to the reactor according to the mass ratio of 3:1, raise the reaction temperature to 400 °C, react for 10 h, cool it to room temperature, and filter the reaction product to obtain a yellow transparent boron sol; Step 3: Then, under nitrogen protection, dissolve refined PZCS and the boron sol in xylene according to the mass ratio of 3:1, filter to remove insoluble substances, and then remove xylene by vacuum drying at 120 °C to obtain a boron-containing PZCS precursor powder; Step 4: Then, under nitrogen protection, put the boron-containing PZCS precursor powder into a melt spinning machine, heat it to 270 °C, melt and stir evenly, keep it for 18 h for degassing and removing small molecules, then lower the temperature to 190 °C and keep it warm. The molten melt is spun through a spinneret at 190 °C and wound by a winding machine to produce a boron-containing polyzirconium carbon silane precursor fiber; 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 polycarbon silicon alkane precursor fiber with a low dose of 5 mGy for 4 h to crosslink the fiber precursor. Subsequently, at 340 °C, heat-treat the boron-containing polyzirconium carbon silane precursor fiber in an inert atmosphere for 1 h to complete annealing, so that the Si free radical-inactivated fiber is completely transformed into a stable infusible network structure, and then cool it to room temperature to obtain a boron-containing polyzirconium carbon silane crosslinked fiber; Step 6: Then place the polycarbon silicon alkane crosslinked fiber on a quartz boat and place it in a quartz tube. Evacuate the air, change nitrogen three times to remove the air and impurities in the quartz tube, then fill hydrogen at a flow rate of 4 L / min, and heat it to 1400 °C at a rate of 200 °C / h, keep it warm for 60 min to further densify the fiber, and then cool it to room temperature in the quartz tube to obtain a boron-containing polyzirconium carbon silane pyrolysis fiber; Step 7. Finally, place the boron-containing polyzirconium carbosilane pyrolysis fiber in a graphite sintering furnace. Under argon protection, apply a tension of 0.5 N, heat it to 1900 °C at a heating rate of 300 °C / h, hold for 30 min for sintering treatment, and then size and wind it to form a dense near-stoichiometric SiC fiber doped with ZrB2 / ZrC.

[0027] In the above embodiments, the polyzirconium carbosilane (PZCS) used can also be directly purchased from Fujian Liyaxin Materials Co., Ltd. The zirconium content is 2 wt %, the molecular weight is 1000 - 1500, and the softening point is 180 - 190 °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 its chemical properties are stable, and it can be stored and used at room temperature.

[0028] The properties of the near-stoichiometric SiC fibers doped with ZrB2 / ZrC prepared in Examples 2 - 4 and the undoped SiC fibers (Liyaxin's third-generation SiC fibers) in Comparative Example 1 were tested according to the national standard of SiC fibers GB / T43760 - 2024. The contents of ZrB2 and ZrC were analyzed and converted according to the well-known element testing method (ICP-OES). The results are shown in Table 1.

[0029] Table 1 Performance data of SiC fibers

[0030] Note: The strength retention rate was obtained by testing the SiC fiber after heat treatment in an argon atmosphere at 1500 °C for 1 h.

[0031] The tensile strength (3.6 GPa) and tensile modulus (355 GPa) of the SiC fibers in Comparative Example 1 were the highest. This is because the undoped SiC fibers (near-stoichiometric) have a purer SiC grain boundary and fewer heterogeneous interface defects, thus showing better mechanical properties at room temperature. Its strength retention rate in an argon atmosphere at 1500 °C is only 62.3%, indicating that the undoped SiC fibers have significantly deteriorated performance at high temperatures due to grain coarsening (pure SiC grains grow rapidly above 1400 °C) and oxidation damage (higher oxygen content).

[0032] In the ZrB2 / ZrC-doped SiC fibers prepared in Examples 2 - 4, the doped ZrB2 and ZrC nanoparticles will form an interface with the SiC matrix, resulting in local stress concentration, thereby causing a slight decrease in tensile strength and a decrease in tensile modulus.

[0033] Function of ZrB2: At high temperatures, ZrB2 oxidizes to form ZrO2 and a B2O3 glass phase. The latter fills microcracks and inhibits oxygen diffusion. Therefore, the ZrB2 content in Example 4 is the highest, and the strength retention rate reaches 88.6%.

[0034] Function of ZrC: The ZrC content in Example 3 is the highest. ZrC forms a dense oxide layer (ZrO2) at high temperatures and synergistically protects the matrix with B2O3, thus further enhancing the oxidation resistance.

[0035] Low oxygen content: The oxygen content of the ZrB2 / ZrC-doped silicon carbide fibers prepared in Examples 2 to 4 is significantly lower than that of the silicon carbide fibers in Comparative Example 1 (0.64%). This reduces the possibility of defects caused by high-temperature oxidation. Moreover, the C / Si ratio is close to 1.0. SiC with a stoichiometric ratio close to 1.0 (C / Si = 1.0) reduces the presence of free carbon, decreases the porosity caused by carbon oxidation at high temperatures, and improves the high-temperature stability.

[0036] Therefore, ZrB2 / ZrC doping significantly improves the high-temperature performance of silicon carbide fibers (the strength retention rate increases by 25% - 40%) through nanoparticle strengthening and high-temperature oxidation protection mechanisms. However, the negative impact of the doping amount on the room-temperature mechanical properties needs to be weighed. The silicon carbide fibers doped in Example 2 have the best comprehensive performance (tensile strength is 3.0 GPa, and the strength retention rate is 85.4%).

[0037] 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 preparation method of near-stoichiometric SiC fibers doped with ZrB2 / ZrC, characterized in that: It includes the following steps: Step 1: Under nitrogen protection, PZCS and PMS are respectively treated by the atmospheric pressure high-temperature method to obtain refined PZCS and refined PMS; Step 2: Then, under nitrogen protection, using refined PMS and DMAB as raw materials, a reaction is carried out by the blending method, and then cooled to room temperature. After filtration, a boron sol is obtained; Step 3: Then, under nitrogen protection, refined PZCS and the boron sol are dissolved in xylene, and the insoluble substances are filtered off. Then, xylene is removed by vacuum drying to obtain a boron-containing PZCS precursor powder; Step 4: Then, under nitrogen protection, the boron-containing PZCS precursor powder is put into a melt spinning machine, heated to 260-270 °C, melted and stirred evenly, and kept for 12-18 h for degassing and removing small molecules. Then, the temperature is lowered to 180-190 °C and kept warm. The molten melt is spun through a spinneret and wound by a winder to produce a boron-containing polyzirconium carbon silane precursor fiber; Step 5: Then, the boron-containing polyzirconium carbon silane precursor fiber is infusibilized by the method of low-oxygen rapid irradiation cross-linking - heat treatment to obtain a boron-containing polyzirconium carbon silane cross-linked fiber; Step 6: Then, the boron-containing polyzirconium carbon silane cross-linked fiber is subjected to high-temperature pyrolysis decarbonization in a hydrogen atmosphere to obtain a boron-containing polyzirconium carbon silane pyrolysis fiber; Step 7: Finally, under the protection of nitrogen, tension is applied to the boron-containing polyzirconium carbon silane pyrolysis fiber and then high-temperature sintering is carried out to obtain an SiC fiber doped with ZrB2 / ZrC with a near stoichiometric ratio.

2. The preparation method of a near-stoichiometric ZrB2 / ZrC-doped SiC fiber according to claim 1, characterized in that: In Step 1, the atmospheric pressure high-temperature method for the refined PZCS is as follows: at room temperature, place the xylene solution of PZCS in a rotary evaporator, and slowly raise the temperature to 160 °C under the protection of a high-purity nitrogen atmosphere. During the heating process, the solid substance completely melts. After the solvent xylene is distilled off, further raise the temperature to 300 - 360 °C and keep it warm for 4 - 6 hours to obtain a black brittle PZCS precursor with a zirconium content of 2 wt %, then dissolve the PZCS precursor in a toluene solution, place it in an autoclave, introduce ammonia gas, raise the temperature to 350 °C, keep the pressure at 4 MPa, and react for 5 hours. After the reaction is completed, cool it down to room temperature and filter the precipitate. Repeat the filtration three times to obtain the refined PZCS.

3. A method for preparing a near-stoichiometric SiC fiber doped with ZrB2 / ZrC according to claim 1, characterized in that: In Step 1, the atmospheric pressure high-temperature method for the refined PMS is as follows: Under nitrogen protection, PMS is dissolved in xylene or toluene, the macromolecular insoluble substances are filtered off to obtain a PMS mixed solution, and then transferred to a three-necked flask. It is heated to 160 °C by a salt bath. After the xylene or toluene is evaporated to dryness, the temperature is slowly raised to 220-280 °C, kept warm for 4-6 hours, and cooled to room temperature to obtain refined PMS.

4. A method for preparing a near-stoichiometric SiC fiber doped with ZrB2 / ZrC according to claim 1, characterized in that: In Step 2, the process of the blending method is as follows: Refined PMS and DMAB are added to the reactor according to a mass ratio of 3:1 - 6:

1. Under nitrogen protection, the reaction temperature is raised to 350-400 °C, and after reacting for 8-10 h, it is cooled to room temperature. After filtering the reaction product, a boron sol is obtained.

5. A method for preparing a near-stoichiometric SiC fiber doped with ZrB2 / ZrC according to claim 1, characterized in that: In Step 3, the mass ratio of refined PZCS to the boron sol is 3:1 - 5:

1.

6. A method for preparing a near-stoichiometric ZrB2 / ZrC-doped SiC fiber according to claim 1, characterized in that: In Step 5, the cross-linking method of the low-oxygen rapid irradiation cross-linking - heat treatment is as follows: Under nitrogen: Under the atmosphere protection of a mixed gas composed of nitrogen and oxygen with a volume ratio of 99:1, the polycarbon silicon alkane precursor fiber is irradiated with a low dose of 4-6 mGy for 4 h to cause cross-linking of the fiber precursor. Subsequently, at 340-360 °C, the boron-containing polyzirconium carbon silane precursor fiber is heat-treated in an inert atmosphere for 1 h to complete annealing, so that the Si free radical deactivated fiber is completely transformed into a stable infusible network structure, and then cooled to room temperature to obtain a boron-containing polyzirconium carbon silane cross-linked fiber.

7. A method for preparing a near-stoichiometric SiC fiber doped with ZrB2 / ZrC according to claim 1, characterized in that: In Step 6, the process of high-temperature pyrolysis is as follows: 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. Then, fill hydrogen into the tube at a flow rate of 4-6 L / min, and heat it up to 1200-1400 °C at a rate of 100-200 °C / h, and keep it at this temperature for 30-60 min to further densify the fibers. Then, cool it down to room temperature in the quartz tube to obtain boron-containing pyrolyzed polyzirconium carbosilane filaments.

8. A method for preparing a near-stoichiometric ZrB2 / ZrC-doped SiC fiber according to claim 1, characterized in that: In Step 7, the process of high-temperature sintering is as follows: Place the boron-containing pyrolyzed polyzirconium carbosilane filaments in a graphite sintering furnace. Under the protection of argon, apply a tension of 0.45-0.5 N, and heat it up to 1800-1900 °C at a heating rate of 200-300 °C / h, and keep it at this temperature for 20-30 min for sintering treatment to form dense near-stoichiometric SiC fibers doped with ZrB2 / ZrC.

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