A method for producing a modified methylsilane-based silicon carbide fiber

Abstract

This invention belongs to the field of carbon fiber composite technology, specifically relating to a method for preparing silicon carbide fibers based on modified methylsilane. The method includes: low-temperature copolymerization of polymethylsilane, divinylbenzene, and phenyltrichlorosilane under a platinum-based catalyst to obtain a crosslinked modified precursor; melt spinning the precursor to obtain a precursor fiber; subjecting the precursor fiber to gradient-temperature ammonolysis in a mixed atmosphere of ammonia and inert gas, achieving curing through a dual mechanism of surface chemical ammonolysis and internal thermal crosslinking; and heating the cured fiber in an inert atmosphere to 1200°C, then switching to a nitrogen-containing atmosphere, and continuing to heat to 1200–1600°C for high-temperature crystallization sintering. This invention avoids the high-temperature performance degradation problem caused by the introduction of oxygen elements in traditional air crosslinking, while simultaneously generating a silicon nitride protective layer during high-temperature pyrolysis, improving the fiber's thermal shock resistance, oxidation resistance, and high-temperature mechanical stability.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber composite material technology, specifically relating to a method for preparing silicon carbide fibers based on modified methylsilane. Background Technology

[0002] Silicon carbide (SiC) fibers are widely used in aero-engine hot-end components, spacecraft thermal protection structures, and as reinforcements for high-temperature ceramic matrix composites due to their comprehensive properties, including high specific strength, high specific modulus, excellent high-temperature resistance, oxidation resistance, and thermal shock resistance. Currently, the industrial production of silicon carbide fibers mainly relies on the organic precursor conversion method, which involves converting organosilicon polymers into inorganic silicon carbide fibers through spinning, cross-linking and curing (non-melting treatment), and high-temperature pyrolysis ceramicization.

[0003] Commonly used organosilicon precursors in existing technologies include polycarbosilane (PCS), polysilazane, or polymethylsilane (PMS). In traditional manufacturing processes, the non-melting treatment of precursor fibers typically employs thermal oxidative crosslinking (air crosslinking) or high-energy electron beam irradiation crosslinking. However, thermal oxidative crosslinking inevitably introduces oxygen into the fiber, leading to a carbothermic reduction reaction during subsequent high-temperature (above 1200°C) use. This reaction generates silicon monoxide and carbon monoxide gases, resulting in fiber grain coarsening, increased porosity, and decreased mechanical properties, thus limiting the temperature resistance limit of silicon carbide fibers. On the other hand, while high-energy electron beam irradiation crosslinking can achieve oxygen-free non-melting, it requires significant equipment investment, has high operating costs, and is difficult to scale up for continuous production.

[0004] Furthermore, to improve the oxidation resistance and compatibility of silicon carbide fibers with the composite matrix, a protective layer such as silicon nitride or pyrolytic carbon is usually applied to the fiber surface. Traditional coating processes (such as chemical vapor deposition) are not only complex, but also have a clear physical interface between the coating and the fiber matrix. When subjected to high-temperature thermal shock, the mismatch in the coefficients of thermal expansion can easily generate interfacial stress, leading to coating peeling or fiber damage.

[0005] Therefore, there is a need in the field for a method for preparing silicon carbide fibers that has good process continuity, can achieve oxygen-free non-melting, and can simultaneously improve the high-temperature stability and antioxidant properties of the fibers. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing silicon carbide fibers based on modified methylsilane. This invention involves introducing a modifier into polymethylsilane for low-temperature catalytic copolymerization, followed by a gradient ammonolysis process in an anaerobic environment to achieve curing, ultimately generating high-temperature resistant silicon carbide fibers through high-temperature pyrolysis.

[0007] This invention provides a method for preparing silicon carbide fibers based on modified methylsilane, comprising the following steps:

[0008] Step 1: Mix polymethylsilane, divinylbenzene and phenyltrichlorosilane evenly, add platinum catalyst, and react at a constant temperature of 80-120℃ under a protective atmosphere to obtain a crosslinked modified precursor.

[0009] Step 2: After cooling, solidifying and pulverizing the crosslinked modified precursor, melt spinning is performed, with the spinning temperature controlled at 130-160℃, and the precursor fibers are collected.

[0010] Step 3: Place the raw fiber in a reactor and introduce a mixture of ammonia and inert gas for gradient heating ammonolysis treatment. The gradient heating ammonolysis treatment successively passes through gradient heat preservation stages of room temperature, 150°C and 250°C, so that the raw fiber and ammonia gas undergo chemical ammonolysis reaction to form an infusible shell layer. At the same time, thermal cross-linking reaction occurs inside the raw fiber to obtain cured fiber.

[0011] Step 4: The cured fiber is heated to 1200°C in an inert atmosphere or an ammonia atmosphere, and then switched to a nitrogen-containing atmosphere to continue heating to 1200-1600°C for high-temperature crystallization sintering to obtain silicon carbide fiber product.

[0012] Further, in step one, the mass fraction of the divinylbenzene is 5% to 8% of the polymethylsilane, and the mass fraction of the phenyltrichlorosilane is 10% to 15% of the polymethylsilane.

[0013] Furthermore, in step one, the amount of platinum-based catalyst added is 5–20 ppm, and the isothermal reaction time is 1–3 h.

[0014] Further, in step one, the platinum-based catalyst is chloroplatinic acid or an isopropanol solution of chloroplatinic acid, or a xylene solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex or platinum-divinyltetramethyldisiloxane complex.

[0015] Furthermore, in step two, the pressure of melt spinning is 200-300 kPa, and the traction speed is 200-300 m / min.

[0016] Furthermore, in step three, the specific procedure for the gradient heating ammonolysis treatment is as follows: ventilate at room temperature for 0.5–1 h, heat to 150°C and hold for 1–2 h, then heat to 250°C and hold for 1–2 h.

[0017] Furthermore, in step three, the exhaust pipeline of the reactor is heated to above 350°C throughout its entire length, and its end is connected to a water washing tower.

[0018] Furthermore, in step four, the nitrogen-containing atmosphere is pure nitrogen or a mixture of nitrogen and argon.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] (1) In this invention, phenyltrichlorosilane is introduced into the precursor as a modifier, and its inherent Si-Cl bond is used to undergo a gas-phase ammonolysis reaction with ammonia in an oxygen-free environment. This mechanism avoids oxygen crosslinking in traditional processes, reduces the introduction of oxygen into the fiber, and improves the high-temperature resistance limit of silicon carbide fiber.

[0021] (2) By controlling the copolymerization temperature at 80-120℃ and introducing a platinum-based catalyst, a highly selective hydrosilylation reaction between divinylbenzene and polymethylsilane was achieved, which improved the molecular weight and melt viscoelastic properties of the precursor and enabled continuous and stable spinning at 130-160℃, while retaining the Si-Cl bond on phenyltrichlorosilane.

[0022] (3) In the non-melting stage, the present invention adopts a curing mechanism of surface chemical ammonolysis and internal thermal cross-linking. By gradually increasing the temperature at room temperature, 150°C and 250°C, the melting and collapse of the fiber during the heating process is avoided. At the same time, the engineering design of full-process heat tracing of the exhaust pipeline prevents the blockage problem caused by the deposition of by-product ammonium chloride.

[0023] (4) During the high-temperature pyrolysis stage, the nitrogen-containing network on the surface is transformed in situ into a silicon nitride protective layer, while the interior is transformed into silicon carbide. This in-situ generated structure reduces the physical interface and thermal stress caused by traditional coating processes, and improves the fiber's thermal shock resistance, oxidation resistance and high-temperature mechanical stability. Detailed Implementation

[0024] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] It should be noted that the amount of platinum-based catalysts added in this invention is expressed in parts per million (ppm) relative to the total mass of the reaction system, based on the mass of platinum metal (Pt). In actual production operations, those skilled in the art can convert this amount based on the actual platinum content of the selected commercial catalyst solution. For example, when the total mass of the reaction system is 1000 g and the target platinum content is 10 ppm, if a chloroplatinic acid isopropanol solution with a platinum content of 2 wt% is used, i.e., a commercially available Karstedt catalyst solution with a platinum content of 2 wt%, 0.5 g of this catalyst solution actually needs to be added.

[0026] Example 1

[0027] This embodiment provides a method for preparing silicon carbide fibers based on modified methylsilane, and the specific production operation steps are as follows:

[0028] Step 1: Raw material preparation and precursor synthesis.

[0029] (1) Weighing raw materials: In a glove box filled with high-purity nitrogen (oxygen content <5 ppm, moisture <5 ppm), accurately weigh 1000 g of polymethylsilane, 50 g of divinylbenzene and 100 g of phenyltrichlorosilane.

[0030] (2) Equipment preparation and feeding: The weighed raw materials are sequentially fed into a 2L double-layered glass reactor equipped with a mechanical stirring device, a reflux condenser and a temperature control system. The stirring is turned on and the speed is set to 150 rpm to ensure that the materials are mixed evenly.

[0031] (3) Catalyst addition and reaction: 0.5 g of commercially available Karstedt catalyst solution was added to the system. The xylene solution contained 2 wt% platinum, which is equivalent to 10 ppm of platinum metal. Subsequently, heat transfer oil was introduced into the jacket of the reactor to heat the material inside the reactor to 100°C at a rate of 2°C / min.

[0032] (4) Isothermal copolymerization: The reaction was carried out at 100℃ with stirring for 2 hours. During this process, the viscosity of the reaction system gradually increased and the color gradually changed from light yellow to dark yellow and transparent. In this stage, the vinyl group of divinylbenzene undergoes a highly selective hydrosilylation reaction with the silane bonds of polymethylsilane, achieving mild chain extension and viscosity enhancement, while retaining the Si-Cl bond on phenyltrichlorosilane.

[0033] (5) Discharge and curing: After the reaction is completed, stop heating and discharge the hot, viscous liquid in the reactor into a receiving tray lined with polytetrafluoroethylene while it is still hot. Under nitrogen protection, cool naturally to room temperature to obtain a pale yellow, brittle solid crosslinked modified precursor.

[0034] Step 2: Melt spinning.

[0035] (1) Crushing treatment: The solid precursor obtained in step one is placed in a mechanical crusher and crushed into particles with a particle size of less than 2 mm to facilitate subsequent feeding.

[0036] (2) Equipment preheating: Turn on the single screw melt spinning machine, set the heating zone temperature of the barrel to 145℃, set the spinneret assembly temperature to 150℃, and keep it warm for 1 hour to ensure uniform temperature.

[0037] (3) Spinning operation: The precursor particles are added to the hopper of the spinning machine. At 145°C, the precursor melts and exhibits good fluidity and shear thinning properties. The extrusion screw is turned on, and the melt extrusion pressure (spinning pressure) is controlled to be stable at 250 kPa. After the melt is extruded through the spinneret, it is cooled and solidified into filaments in the air.

[0038] (4) Winding and collecting: Turn on the take-up drum and adjust the traction speed to 250 m / min to continuously wind and collect the extruded fibers. The spinning process is continuous and stable, with no fiber breakage. The resulting raw filament has a smooth surface and a uniform diameter distribution, mainly concentrated in the range of 13 to 15 µm.

[0039] Step 3: Ammonolysis and solidification.

[0040] (1) Scavenging: Place the graphite tool with the original wire wound into the tubular atmosphere reactor. After sealing the furnace, introduce a large flow of a mixture of ammonia and argon for scavenging for 30 minutes to completely replace the air in the furnace. The volume ratio of ammonia to argon is 1:1, and the total flow rate of the mixture of ammonia and argon is 1000 mL / min.

[0041] (2) Preheating of the exhaust system: Turn on the heat tracing cable of the reactor exhaust pipeline to heat the pipeline throughout and maintain it above 350°C. Connect the exhaust end to a water washing tower containing dilute hydrochloric acid solution to absorb unreacted ammonia and prevent the byproduct ammonium chloride from crystallizing and clogging the pipeline.

[0042] (3) Gradient heating ammonolysis.

[0043] Room temperature permeation stage: Continue ventilation at room temperature for 1 hour to allow ammonia to fully permeate into the surface of the precursor fiber.

[0044] Low-temperature ammonolysis stage: The temperature is increased to 150℃ at a rate of 2℃ / min and held for 2 hours. At this time, the Si-Cl bonds on the surface of the precursor fiber undergo a chemical ammonolysis reaction with ammonia gas to generate a Si-NH-Si cross-linked network, forming a rigid, non-melting shell.

[0045] High-temperature thermal crosslinking stage: The temperature is further increased to 250℃ at a rate of 1℃ / min and held for 2 hours. At this temperature, the unreacted polymethylsilane inside the precursor fiber undergoes a thermal crosslinking reaction, achieving dual curing of the fiber inside and out. After the treatment, the fiber is cooled to room temperature in the furnace to obtain a non-melting cured fiber.

[0046] Step 4: Pyrolysis and Crystallization

[0047] (1) Low-temperature pyrolysis: The cured fiber is transferred to a high-temperature carbon tube furnace. Under the protection of pure argon atmosphere, the temperature is increased from room temperature to 1200℃ at a rate of 5℃ / min. During this stage, the fiber undergoes a violent organic-inorganic transformation, releasing small molecule gases such as hydrogen and methane, and forming an amorphous silicon carbide skeleton.

[0048] (2) High-temperature crystallization and in-situ phase formation: When the temperature reaches 1200℃, the atmosphere in the furnace is switched to pure nitrogen. Then, the temperature is increased to 1500℃ at a rate of 2℃ / min, and held at this temperature for 1 hour for high-temperature crystallization sintering. Under the protection of the nitrogen-containing atmosphere, the nitrogen-rich network on the surface of the fiber is transformed in-situ into the thermodynamically stable Si3N4 phase, and the interior is transformed into the dense SiC phase.

[0049] (3) Cooling and unloading: After the heat preservation is completed, the furnace is naturally cooled to room temperature under nitrogen protection. The final product is a gradient structure silicon carbide fiber with a dark gray surface and a metallic luster.

[0050] Example 2

[0051] The main difference between this embodiment and Embodiment 1 lies in the precursor synthesis and the adjustment of spinning parameters. The specific operations are as follows:

[0052] (1) Precursor synthesis: Weigh 1000 g of polymethylsilane, 80 g of divinylbenzene and 150 g of phenyltrichlorosilane. Add 1.0 g of Karstedt catalyst solution with a platinum content of 2 wt% (equivalent to 20 ppm of platinum metal). Set the reaction temperature to 80℃ and extend the isothermal reaction time to 3 hours.

[0053] (2) Melt spinning: Due to slight differences in the degree of crosslinking of the precursors, the temperature of the spinning machine barrel was adjusted to 130℃ and the temperature of the spinneret was adjusted to 135℃. The spinning pressure was controlled at 300 kPa and the traction speed at 200 m / min. The diameter of the obtained precursor fiber was approximately 15–18 µm.

[0054] The operation and parameters of the remaining steps (anaerobic gradient ammonolysis and high-temperature pyrolysis) are exactly the same as those in Example 1.

[0055] Example 3

[0056] The main difference between this embodiment and Embodiment 1 lies in the adjustment of the ammonolysis and cracking parameters. The specific operations are as follows:

[0057] (1) Anaerobic gradient ammonolysis without melting: The gradient heating program is adjusted as follows: room temperature ventilation for 0.5 hours; heating to 150℃ at 3℃ / min and holding for 1 hour; heating to 250℃ at 2℃ / min and holding for 1 hour.

[0058] (2) High-temperature pyrolysis and in-situ ceramization: When switching the atmosphere at 1200℃, a mixture of nitrogen and argon (volume ratio 1:1) is introduced. The maximum sintering temperature is set to 1600℃, and the holding time is extended to 2 hours.

[0059] The remaining steps (precursor synthesis and melt spinning) were performed using the same parameters as in Example 1. The fibers obtained under these conditions maintained good structural integrity even at 1600°C.

[0060] Comparative Example 1

[0061] Operating Procedures: Mix 1000g of polymethylsilane, 50g of divinylbenzene, and 100g of phenyltrichlorosilane without adding a platinum catalyst. Under nitrogen protection, directly heat the reactor to 200℃ and maintain a constant temperature with stirring. Observations and Results: Approximately 1.5 hours after reaching 200℃, the stirring resistance inside the reactor increased sharply, causing the motor to overload. Upon opening the reactor, it was found that the material had gelled and solidified. After cooling and pulverizing the gel, it was added to a spinning machine, but it could not be melted and extruded at both 145℃ and 195℃, resulting in a complete failure of the spinning process.

[0062] Comparative Example 2

[0063] Operating steps: Use the raw silk obtained in steps one and two of Example 1.

[0064] In step three, instead of ammonolysis, the precursor fiber is placed in an air-circulating oven and heated to 180°C at an extremely slow rate of 10°C / h, and held at that temperature for 4 hours to perform thermal oxidative crosslinking.

[0065] In step four, sintering is carried out at 1500°C under an argon atmosphere throughout the process.

[0066] Phenomena and Results: Due to the introduction of oxygen into the fiber through air crosslinking, a violent carbothermic reduction reaction occurred inside the fiber during sintering at 1500℃, resulting in the escape of a large amount of gas. The resulting fiber has a rough, foamy surface, contains numerous micropores, and is extremely brittle, crumbling upon slight contact and completely losing its mechanical strength as a structural material.

[0067] In conclusion, through the actual operation comparison of the above embodiments and comparative examples, it can be seen that the present invention successfully avoids the gelation dead hole of the precursor at high temperature by low-temperature catalytic modification at 80-120℃, thus ensuring excellent spinning performance; at the same time, through the synergistic effect of gradient ammonolysis and high-temperature pyrolysis in a nitrogen-containing atmosphere, the oxygen introduction defect caused by traditional air crosslinking is eliminated.

[0068] To further verify the excellent performance of the silicon carbide fiber prepared by the present invention in terms of thermal shock resistance, oxidation resistance and high temperature mechanical stability, the silicon carbide fibers prepared in Examples 1-3 above, the fiber prepared in Comparative Example 2, and commercially available conventional first-generation silicon carbide fibers (similar to Nicalon CG level) and second-generation low-oxygen silicon carbide fibers (similar to Hi-Nicalon level) were subjected to performance tests and comparisons.

[0069] 1. Test Methods and Metric Definitions

[0070] (1) Oxygen content (wt%): The mass fraction of oxygen element inside the fiber was determined using an oxygen-nitrogen analyzer (such as LECO TC-436). Oxygen content is a key internal factor that determines the high-temperature stability of the fiber.

[0071] (2) Tensile strength at room temperature (GPa) and modulus of elasticity (GPa): Determined at room temperature using a monofilament tensile testing machine according to the monofilament tensile test standard (ASTM C1557), with a gauge length of 25 mm.

[0072] (3) High-temperature mechanical stability (strength retention rate at 1500℃): The fiber was heated to 1500℃ in a pure argon atmosphere and held for 1 hour. After cooling in the furnace, its room temperature tensile strength was tested, and the percentage of its initial room temperature tensile strength was calculated. This index reflects the fiber's resistance to grain coarsening and thermal degradation in a high-temperature inert environment.

[0073] (4) Antioxidant properties (strength retention rate after oxidation at 1200℃): The fiber was heated to 1200℃ in air and kept at that temperature for 10 hours. After cooling, its room temperature tensile strength retention rate was tested. This indicator directly reflects the fiber surface's ability to resist oxygen erosion.

[0074] (5) Thermal shock resistance test: After the fiber is kept in a high temperature furnace at 1200℃ for 15 minutes, it is quickly taken out and quenched in room temperature air. The cycle is repeated 10 times, the surface morphology of the fiber is observed and the strength retention rate is tested.

[0075] Test data table

[0076] sample Oxygen content (wt%) Tensile strength at room temperature (GPa) Room temperature elastic modulus (GPa) High temperature mechanical stability Antioxidant properties thermal shock resistance Example 1 0.85 2.95 265 92.5% 88.4% 90.2% Example 2 0.92 2.88 258 91.0% 86.5% 89.5% Example 3 0.76 3.12 275 95.2% 91.3% 93.1% Comparative Example 2 11.5 1.25 145 < 10% (Severe powdering) < 20% (severely oxidized) < 15% (brittle fracture) Conventional first-generation SiC fibers (refer to Nicalon CG) 10.0～12.0 2.5～3.0 180～220 < 30% < 40% < 30% Conventional second-generation SiC fibers (refer to Hi-Nicalon) < 1.0 2.8～3.0 270 80%～85% 70%～75% 75%～80%

[0077] 3. Data Analysis and Conclusions.

[0078] (1) High-temperature mechanical stability analysis: As can be seen from the table, the conventional air-crosslinked Comparative Example 2 and the conventional first-generation SiC fiber, due to their high internal oxygen content of over 10%, underwent a violent carbothermic reduction reaction (releasing CO and SiO gases) at 1500℃, resulting in fiber structure destruction and extremely low strength retention (<30%). However, Examples 1-3 of this invention, through gradient ammonolysis technology, strictly control the oxygen content to below 1.0%, and their strength retention at 1500℃ all exceed 90%, which is significantly better than the first-generation fiber and slightly better than the second-generation low-oxygen fiber.

[0079] (2) Analysis of oxidation resistance and thermal shock resistance: Although conventional second-generation SiC fibers (such as Hi-Nicalon) also have low-oxygen characteristics, their surface is a pure SiC or C-rich structure, which is easily oxidized in air at 1200℃ and prone to microcracks during thermal shock cycling, resulting in limited retention rates of oxidation resistance (70%–75%) and thermal shock resistance (75%–80%). In contrast, Examples 1-3 of the present invention generate a protective layer rich in silicon nitride (Si3N4) during ammonolysis and high-temperature pyrolysis. This protective layer not only has excellent anti-oxidation barrier properties, but also, due to its in-situ gradient generation, eliminates the physical interface of traditional coatings, greatly alleviating thermal stress. Therefore, the fiber of the present invention achieves a significant improvement in both the retention rate of oxidation resistance (>86%) and the retention rate of thermal shock resistance (>89%) compared to existing high-performance SiC fibers.

[0080] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing silicon carbide fibers based on modified methylsilane, characterized in that, Includes the following steps: Step 1: Mix polymethylsilane, divinylbenzene and phenyltrichlorosilane evenly, add platinum catalyst, and react at a constant temperature of 80-120℃ under a protective atmosphere to obtain a crosslinked modified precursor. Step 2: After cooling, solidifying and pulverizing the crosslinked modified precursor, melt spinning is performed, with the spinning temperature controlled at 130-160℃, and the precursor fibers are collected. Step 3: Place the raw fiber in a reactor and introduce a mixture of ammonia and inert gas for gradient heating ammonolysis treatment. The gradient heating ammonolysis treatment successively passes through gradient heat preservation stages of room temperature, 150°C and 250°C, so that the raw fiber and ammonia gas undergo chemical ammonolysis reaction to form an infusible shell layer. At the same time, thermal cross-linking reaction occurs inside the raw fiber to obtain cured fiber. Step 4: The cured fiber is heated to 1200°C in an inert atmosphere or an ammonia atmosphere, and then switched to a nitrogen-containing atmosphere to continue heating to 1200-1600°C for high-temperature crystallization sintering to obtain silicon carbide fiber product.

2. The method for preparing silicon carbide fibers based on modified methylsilane according to claim 1, characterized in that, In step one, the mass fraction of the divinylbenzene is 5% to 8% of the polymethylsilane, and the mass fraction of the phenyltrichlorosilane is 10% to 15% of the polymethylsilane.

3. The method for preparing silicon carbide fibers based on modified methylsilane according to claim 1, characterized in that, In step one, the amount of platinum-based catalyst added is 5–20 ppm, and the isothermal reaction time is 1–3 h.

4. A method for preparing silicon carbide fibers based on modified methylsilane according to claim 1 or 3, characterized in that, In step one, the platinum-based catalyst is chloroplatinic acid or an isopropanol solution of chloroplatinic acid, or a xylene solution of platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex or platinum-divinyltetramethyldisiloxane complex.

5. The method for preparing silicon carbide fibers based on modified methylsilane according to claim 1, characterized in that, In step two, the pressure of melt spinning is 200-300 kPa, and the traction speed is 200-300 m / min.

6. The method for preparing silicon carbide fibers based on modified methylsilane according to claim 1, characterized in that, In step three, the specific procedure for the gradient heating ammonia hydrolysis treatment is as follows: ventilate at room temperature for 0.5 to 1 hour, heat to 150°C and hold for 1 to 2 hours, then heat to 250°C and hold for 1 to 2 hours.

7. The method for preparing silicon carbide fibers based on modified methylsilane according to claim 1, characterized in that, In step three, the exhaust pipeline of the reactor is heated to above 350°C throughout its entire length, and its end is connected to a water washing tower.

8. The method for preparing silicon carbide fibers based on modified methylsilane according to claim 1, characterized in that, In step four, the nitrogen-containing atmosphere is pure nitrogen or a mixture of nitrogen and argon.