Preparation method of low-cost in-situ doped continuous silicon carbide fiber

Abstract

The invention discloses a preparation method of low-cost in-situ doped continuous silicon carbide fiber, which comprises the following steps: by taking vinyl liquid polycarbosilane as a precursor, adding a boron monomer, an organic solvent and metallocene, and doping to prepare a doped polycarbosilane precursor; carrying out melt spinning on the doped precursor to obtain a doped precursor; placing the doped precursor in a mixed gas of an inert atmosphere and a boron-containing active atmosphere, and carrying out thermal crosslinking to obtain a doped crosslinked filament; and finally, pyrolyzing in an ammonia atmosphere to complete partial inorganic transformation of the doped cross-linked wire, then completing inorganic transformation in a hydrogen atmosphere, and finally performing high-temperature heat treatment on the doped cross-linked wire in an inert atmosphere to obtain the in-situ doped continuous silicon carbide fiber. Therefore, the problem of non-uniform coating of a ceramic-based composite material prefabricated body with a complex structure is solved, meanwhile, high cost and large damage to fibers of a secondary coating are avoided, and the composite material preparation technological process is simplified.

Description

Technical Field

[0001] The present invention relates to the technical field of preparation of silicon carbide fibers, and in particular to a method for preparing low-cost in-situ doped continuous silicon carbide fibers. Background Art

[0002] With the development of aerospace technology, the new generation of weapons and equipment is rapidly developing towards higher Mach numbers, greater thrust-to-weight ratios, and higher temperature resistance. Continuous silicon carbide fibers are widely used in high-temperature structural parts in the fields of aviation, aerospace, nuclear energy, etc. They are ideal reinforcement materials for the next generation of engines and can increase the operating temperature of the engine to above 1650°C. It is reported that the doped third-generation silicon carbide fibers TyrannoSA, KD-SA and Sylramic-iBN have excellent temperature resistance and can meet the higher temperature requirements of ceramic-based composite materials. Doped silicon carbide fibers are mainly prepared by introducing doping elements in the preparation process of the precursor and then carrying out subsequent process. They have excellent wave absorption, high temperature resistance, creep resistance and other properties.

[0003] Although the existing technology can realize the preparation of various doped silicon carbide fibers, the application process of the prepared silicon carbide fibers still needs to go through the weaving of the fiber preform and the preparation of the preform coating before the densification process of the ceramic matrix composite material can be carried out. The coating is a transitional area between the fiber and the ceramic matrix, and is a key factor in determining the mechanical properties of the ceramic matrix composite material. BN coating is currently a hot spot and focus in the research and application of silicon carbide fiber reinforced ceramic matrix composites, and is mainly obtained by depositing the fiber preform by CVD method. However, as the structural design of the preform becomes more and more complex, the process of depositing BN coating by CVD method is difficult to ensure that the BN coating can be evenly prepared on the surface of the inner and outer fibers of the preform, and as the external BN coating becomes thicker and covers the surface of the preform, it will be difficult for the internal fibers to obtain the same thickness of BN coating. In addition, the BN coating needs to be above 1200°C to obtain highly crystalline and highly stable hexagonal boron nitride, but the preparation process of the coating at this high temperature will also cause certain damage to the mechanical properties and structure of the silicon carbide fiber. Therefore, the use of uncoated silicon carbide fibers for the preparation of composite materials not only makes it difficult to obtain a BN coating with uniform thickness and high crystallinity, but is also detrimental to the mechanical properties of the fiber preform.

[0004] Therefore, in order to solve the problem of uneven coating of complex structure preforms of ceramic-based composite materials, avoid the high cost of secondary coating and severe damage to fibers, and simplify the process of composite material preparation, the inventors of this case conducted in-depth research, which led to the creation of this case. Summary of the invention

[0005] The purpose of the present invention is to provide a low-cost method for preparing in-situ doped continuous silicon carbide fibers to solve the problem of uneven coating of complex structure preforms of ceramic-based composite materials, while avoiding the problems of high cost of secondary coating and severe damage to the fibers.

[0006] In order to achieve the above object, the technical solution of the present invention is: A method for preparing low-cost in-situ doped continuous silicon carbide fiber, characterized by comprising the following steps: Step 1: Synthesis of doped polycarbosilane precursor Vinyl liquid polycarbosilane, boron monomer, organic solvent and metallocene are mixed and stirred under a nitrogen atmosphere and kept at 40-100° C. for 12 h-48 h, and a doped polycarbosilane precursor is obtained by vacuum distillation; Step 2: Melt Spinning The doped polycarbosilane precursor is placed in a melt spinning system, heated to 180°C~240°C for melting under nitrogen protection and kept warm for 12h~24h, and the doped raw yarn is obtained by spinning.

[0007] Step 3: Boron-containing active atmosphere assisted thermal crosslinking The doped raw silk is moved into a cross-linking furnace, and a mixed gas of an inert atmosphere and a boron-containing active atmosphere with a volume ratio of 5 to 20:1 is introduced, and thermal cross-linking is completed at 250°C to 400°C for 8h to 24h to obtain the doped cross-linked silk. Step 4: High temperature firing The doped cross-linked wire is heated to 800-1000°C for pyrolysis in an ammonia atmosphere to complete a partial inorganic transformation of the doped cross-linked wire, and then the inorganic transformation is completed in a hydrogen atmosphere; the above-mentioned ammonia pyrolysis to partially inorganic transformation of the cross-linked wire is achieved by controlling the ammonia pyrolysis time, the ammonia pyrolysis time is 10min-60min, the boron on the surface layer of the doped cross-linked wire reacts with the ammonia, the boron-containing active atmosphere introduced during the cross-linking process combines with the active sites on the fiber surface of the doped cross-linked wire or is directly deposited on the fiber surface of the doped cross-linked wire, and then reacts with the ammonia during the high-temperature sintering process so that the boron element on the fiber surface of the doped cross-linked wire reacts with the ammonia to form a boron nitride surface coating; Finally, the doped cross-linked fibers are heat treated in an inert atmosphere to obtain in-situ doped continuous silicon carbide fibers, and the heat treatment temperature is 1600°C to 1900°C.

[0008] Furthermore, the molecular weight of the vinyl liquid polycarbosilane in step 1 is 950-1300, the viscosity at 25° C. is ≤30 mPa.s, the ceramic yield is greater than 55%, and it has self-crosslinking properties.

[0009] Furthermore, the boron monomer in step one is a solid or liquid boron monomer, including at least one of borane, tris(dimethylamino)borane, dimethylaminoborane, borane-amine complex, borane-pyridine complex and tetrahydrofuran-borane complex.

[0010] Furthermore, the organic solvent in step 1 is toluene, xylene, tetrahydrofuran or divinylbenzene; The inert atmosphere in step 3 is nitrogen, argon or helium. The inert atmosphere in step 4 is nitrogen, argon or helium.

[0011] Furthermore, the metallocene in step 1 is a metal coordination compound composed of a transition metal and cyclopentadiene, including at least one of titanocene dichloride, hafnocenene dichloride and zirconocene dichloride.

[0012] Furthermore, in the step 1, the mass ratio of the vinyl liquid polycarbosilane, the boron monomer, the organic solvent and the metallocene is 60-80:2-10:8-15:5-20.

[0013] Furthermore, the boron-containing active atmosphere in step three is one of the boric acids containing carbon-carbon double bonds or carbon-carbon triple bonds, or the boron-containing active atmosphere in step three is a mixture of multiple gases in boric acids containing carbon-carbon double bonds or carbon-carbon triple bonds, and the multiple gases are a combination of at least two gases.

[0014] Furthermore, the boron-containing active atmosphere in step three is one of 2,2-methylvinylboronic acid, 2,4,6-trivinylboroxine, 4-ethynylphenylboronic acid, 2,2-dimethylethynylboronic acid and 1-pentenylboronic acid.

[0015] Furthermore, the boron-containing active atmosphere in step three is heated and gasified by a gasification device before being introduced.

[0016] Furthermore, the partial inorganic transformation of the cross-linked fibers by the ammonia pyrolysis in step 4 is achieved by controlling the ammonia pyrolysis time, and the ammonia pyrolysis time is 10 min to 60 min.

[0017] After adopting the above technical solution, the preparation method of in-situ doped continuous silicon carbide fiber of the present invention has the following beneficial effects: 1) Doped silicon carbide fibers containing boron, titanium, hafnium, zirconium and other elements were prepared by precursor doping. Titanium boride (TiB2), hafnium boride (HfB2) and zirconium boride (ZrB2) are all ultra-high temperature ceramic materials (melting point > 2600°C). After in-situ doping with silicon carbide fibers, the temperature resistance of silicon carbide fibers was further improved; 2) Since metallocene contains carbon-carbon double bonds and liquid polycarbosilane itself contains vinyl groups and has self-crosslinking properties, the precursor in the present invention can be heated with active gas to make the original filament infusible, thus avoiding the high cost of crosslinking by electron beam irradiation.

[0018] 3) The boron-containing active gas introduced during the crosslinking process of the present invention can combine with the active sites on the surface of the original silk fiber or directly deposit on the fiber surface, and then react with ammonia during the high-temperature sintering process to react the surface boron element with ammonia to form a boron nitride (BN) surface coating. After high-temperature heat treatment at 1200~1900℃, the crystallinity of the boron nitride coating is simultaneously improved. The coating thickness reaches 150~200nm, and the thickness is uniform, which solves the problem of uneven coating of the preform.

[0019] 4) The silicon carbide fiber of the present invention has an in-situ coating, and the interface preparation step between the fiber and the matrix can be omitted in the subsequent preparation of the composite material, thereby simplifying the composite material preparation process. In addition, the conventional coating process is carried out after the fiber is woven into a preform, and the inner layer of the preform is difficult to deposit onto the coating or the thickness is insufficient. The surface coating of the fiber of the present invention is prepared directly by reaction on the fiber surface, so the fiber of the present invention can provide an interface coating with a more uniform thickness for the composite material.

[0020] 5) In step 4, the ammonia pyrolysis time is controlled within 10 to 60 minutes to ensure that only the boron on the surface layer reacts with the ammonia. If the time is too long, the carbon inside the fiber will be replaced by the nitrogen in the ammonia, forming silicon nitride fiber instead of silicon carbide fiber. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a SEM image of the cross section and surface morphology of the doped silicon carbide fiber prepared in Example 2 of the present invention; Figure 2 This is the infrared diffraction spectrum of the doped silicon carbide fiber prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0022] In order to further explain the technical solution of the present invention, the present invention is described in detail below through specific embodiments.

[0023] 1. Preparation of Silicon Carbide Fiber Example 1 The present invention provides a method for preparing in-situ doped continuous silicon carbide fibers, comprising the following steps: Step 1: Synthesis of doped polycarbosilane precursor Vinyl liquid polycarbosilane, borane, toluene and dichlorotitanocene in a mass ratio of 80:2:8:10 were mixed and stirred in a nitrogen atmosphere, and kept at 100° C. for 48 hours, and a doped polycarbosilane precursor was obtained by reduced pressure distillation.

[0024] Step 2: Melt Spinning The doped polycarbosilane precursor was placed in a melt spinning system, heated to 180°C for melting under nitrogen protection and kept warm for 12 hours, and the doped raw fibers were obtained by spinning.

[0025] Step 3: Boron-containing active atmosphere assisted thermal crosslinking The doped raw silk was moved into a cross-linking furnace, and the 2,2-dimethylvinylboric acid was gasified by a gasification device at 200°C, mixed with nitrogen and passed into the cross-linking furnace. The volume ratio of nitrogen to 2,2-dimethylvinylboric acid was 5:1. Thermal cross-linking was completed at 250°C for 8 hours to obtain the doped cross-linked silk.

[0026] Step 4: High temperature firing The doped cross-linked fibers were heated to 1000°C for pyrolysis in an ammonia atmosphere for 10 min to complete a partial inorganic transformation of the doped cross-linked fibers, and then the inorganic transformation was completed in a hydrogen atmosphere. Finally, the doped cross-linked fibers were subjected to a high-temperature heat treatment at 1600°C in an argon atmosphere to obtain in-situ doped continuous silicon carbide fibers.

[0027] The yield of the vinyl liquid polycarbosilane ceramic in step 1 is 60.32%. In the present invention, the ceramic is heated to 900°C in a nitrogen atmosphere and kept for 120 minutes, and the mass left after firing is divided by the initial mass to obtain the ceramic yield.

[0028] In the present invention, the partial inorganic transformation of the cross-linked silk by pyrolysis of ammonia is achieved by controlling the pyrolysis time of ammonia, the boron in the surface layer of the doped cross-linked silk reacts with ammonia, the boron-containing active atmosphere introduced during the cross-linking process combines with the active sites on the fiber surface of the doped cross-linked silk or is directly deposited on the fiber surface of the doped cross-linked silk, and then reacts with ammonia during the high-temperature sintering process so that the boron element on the fiber surface of the doped cross-linked silk reacts with ammonia to form a boron nitride surface coating.

[0029] Example 2 The present invention provides a method for preparing in-situ doped continuous silicon carbide fibers, comprising the following steps: Step 1: Synthesis of doped polycarbosilane precursor Vinyl liquid polycarbosilane, tri(dimethylamino)borane, xylene and hafnocene dichloride were mixed and stirred in a nitrogen atmosphere at a mass ratio of 70:6:9:15 and kept at 80° C. for 36 hours, and a doped polycarbosilane precursor was obtained by vacuum distillation.

[0030] Step 2: Melt Spinning The doped polycarbosilane precursor was placed in a melt spinning system, heated to 200°C for melting under nitrogen protection and kept warm for 16 hours, and the doped raw yarn was obtained by spinning.

[0031] Step 3: Boron-containing active atmosphere assisted thermal crosslinking The doped raw silk was moved into a cross-linking furnace, and 2,4,6-trivinyl cycloboroxine was gasified at 120°C by a gasification device, mixed with argon and introduced into the cross-linking furnace. The volume ratio of argon to 2,4,6-trivinyl cycloboroxine was 10:1. Thermal cross-linking was completed at 300°C for 12 hours to obtain doped cross-linked silk. Step 4: High temperature firing The doped cross-linked fibers were partially inorganically transformed by pyrolysis at 950°C in an ammonia atmosphere for 20 min, and then inorganically transformed by pyrolysis in a hydrogen atmosphere. Finally, the doped cross-linked fibers were subjected to a high-temperature heat treatment at 1700°C in an argon atmosphere to obtain in-situ doped continuous silicon carbide fibers.

[0032] The yield of the vinyl liquid polycarbosilane ceramic in the step 1 is 55.96%.

[0033] The cross section and surface morphology of the prepared in-situ doped continuous silicon carbide fiber are as follows: Figure 1 As shown, the infrared diffraction spectrum is as follows Figure 2 shown.

[0034] Depend on Figure 1 It can be seen that the fiber surface coating is tightly combined with the fiber, has a dense structure, a thickness of 168nm, and a dense surface without defects such as protrusions.

[0035] Depend on Figure 2 It can be seen that: at 1345cm -1 and 867cm -1 The in-plane stretching vibration peak and out-of-plane bending vibration peak of the h-BN coating appeared respectively, proving that the surface coating is hexagonal boron nitride.

[0036] Example 3 The present invention provides a method for preparing in-situ doped continuous silicon carbide fibers, comprising the following steps: Step 1: Synthesis of doped polycarbosilane precursor Vinyl liquid polycarbosilane, dimethylaminoborane, tetrahydrofuran and zirconocene dichloride were mixed and stirred in a nitrogen atmosphere at a mass ratio of 60:8:12:20 and kept at 40° C. for 24 hours, and a doped polycarbosilane precursor was obtained by vacuum distillation.

[0037] Step 2: Melt Spinning The doped polycarbosilane precursor was placed in a melt spinning system, heated to 240°C for melting under nitrogen protection and kept warm for 20 hours, and the doped raw yarn was obtained by spinning.

[0038] Step 3: Boron-containing active atmosphere assisted thermal crosslinking The doped raw silk was moved into a cross-linking furnace, and 4-ethynylphenylboric acid was gasified at 300°C by a gasification device, mixed with helium and passed into the cross-linking furnace. The volume ratio of helium to 4-ethynylphenylboric acid was 20:1. Thermal cross-linking was completed at 400°C for 24 hours to obtain doped cross-linked silk. Step 4: High temperature firing The doped cross-linked fibers were partially inorganically transformed by pyrolysis at 900°C in an ammonia atmosphere for 30 minutes, and then inorganically transformed in a hydrogen atmosphere. Finally, the doped cross-linked fibers were subjected to a high-temperature heat treatment at 1800°C in a helium atmosphere to obtain in-situ doped continuous silicon carbide fibers.

[0039] The yield of the vinyl liquid polycarbosilane ceramic in the step 1 is 65.46%.

[0040] Example 4 The present invention provides a method for preparing in-situ doped continuous silicon carbide fibers, comprising the following steps: Step 1: Synthesis of doped polycarbosilane precursor Vinyl liquid polycarbosilane, borane ammonia complex, tetrahydrofuran and zirconocene dichloride were mixed and stirred in a nitrogen atmosphere at a mass ratio of 70:10:15:5 and kept at 40° C. for 12 h, and a doped polycarbosilane precursor was obtained by vacuum distillation.

[0041] Step 2: Melt Spinning The doped polycarbosilane precursor was placed in a melt spinning system, heated to 240°C for melting under nitrogen protection and kept warm for 20 hours, and the doped raw yarn was obtained by spinning.

[0042] Step 3: Boron-containing active atmosphere assisted thermal crosslinking The doped raw silk was moved into a cross-linking furnace, and the 2,2-dimethylethynylboric acid was gasified by a gasification device at 230°C, mixed with nitrogen and passed into the cross-linking furnace. The volume ratio of nitrogen to 2,2-dimethylethynylboric acid was 5:1. Thermal cross-linking was completed at 280°C for 12 hours to obtain the doped cross-linked silk. Step 4: High temperature firing The doped cross-linked fibers were partially inorganically transformed by pyrolysis at 850°C in an ammonia atmosphere for 40 min, and then inorganically transformed in a hydrogen atmosphere. Finally, the doped cross-linked fibers were subjected to a high-temperature heat treatment at 1900°C in an argon atmosphere to obtain in-situ doped continuous silicon carbide fibers.

[0043] The yield of the vinyl liquid polycarbosilane ceramic in the step 1 is 62.58%.

[0044] Example 5 The present invention provides a method for preparing in-situ doped continuous silicon carbide fibers, comprising the following steps: Step 1: Synthesis of doped polycarbosilane precursor Vinyl liquid polycarbosilane, borane pyridine complex, divinylbenzene and dichlorotitanocene were mixed and stirred in a nitrogen atmosphere at a mass ratio of 70:10:15:5 and kept at 50° C. for 24 hours, and a doped polycarbosilane precursor was obtained by vacuum distillation.

[0045] Step 2: Melt Spinning The doped polycarbosilane precursor was placed in a melt spinning system, heated to 200°C for melting under nitrogen protection and kept warm for 24 hours, and the doped raw fibers were obtained by spinning.

[0046] Step 3: Boron-containing active atmosphere assisted thermal crosslinking The doped raw silk was moved into a cross-linking furnace, and the 1-pentenylboric acid was gasified by a gasification device at 230°C, mixed with helium and passed into the cross-linking furnace. The volume ratio of helium to 1-pentenylboric acid was 15:1. Thermal cross-linking was completed at 260°C for 16 hours to obtain the doped cross-linked silk. Step 4: High temperature firing The doped cross-linked fibers were partially inorganically transformed by pyrolysis at 800°C in an ammonia atmosphere for 60 minutes, and then inorganically transformed in a hydrogen atmosphere. Finally, the doped cross-linked fibers were subjected to a high-temperature heat treatment at 1600°C in a nitrogen atmosphere to obtain in-situ doped continuous silicon carbide fibers.

[0047] The yield of the vinyl liquid polycarbosilane ceramic in the step 1 is 58.78%.

[0048] 2. Performance Test (1) The test results of the above embodiments under the industry standard test method are shown in the following table:

[0049] It can be seen from the above table that the volume density of the in-situ doped silicon carbide fiber examples prepared by the method of the present invention is greater than 3.1 g.cm -3 , oxygen content is less than 0.5%, and the single-filament tensile strength is above 3.5 GPa. After heat treatment at 1600°C in argon atmosphere for 1 hour, the single-filament tensile strength of the fiber is higher than 3.0 GPa. After heat treatment at 1600°C in air atmosphere for 1 hour, the single-filament tensile strength of the fiber is higher than 2.7 GPa. Example 2 using hafnium element has higher fiber volume density, single-filament tensile strength and single-filament tensile strength after high temperature, which is related to the excellent high temperature resistance of hafnium boride (HfB2).

[0050] The above embodiments and drawings do not limit the product form and style of the present invention. Any appropriate changes or modifications made thereto by ordinary technicians in the relevant technical field should be deemed to be within the patent scope of the present invention.

Claims

1. A method for preparing low-cost in-situ doped continuous silicon carbide fibers, characterized in that: The following steps are involved: Step 1: Synthesis of doped polycarbosilane precursor Vinyl liquid polycarbosilane, boron monomer, organic solvent and metallocene are mixed and stirred under a nitrogen atmosphere and kept at 40-100° C. for 12 h-48 h, and a doped polycarbosilane precursor is obtained by vacuum distillation; Step 2: Melt Spinning The doped polycarbosilane precursor is placed in a melt spinning system, heated to 180°C to 240°C for melting under nitrogen protection and kept warm for 12h to 24h, and the doped raw yarn is obtained by spinning; Step 3: Boron-containing active atmosphere assisted thermal crosslinking The doped raw silk is moved into a cross-linking furnace, and a mixed gas of an inert atmosphere and a boron-containing active atmosphere with a volume ratio of 5 to 20:1 is introduced, and thermal cross-linking is completed at 250°C to 400°C for 8h to 24h to obtain the doped cross-linked silk; Step 4: High temperature firing The doped cross-linked wire is heated to 800-1000°C for pyrolysis in an ammonia atmosphere to complete a partial inorganic transformation of the doped cross-linked wire, and then the inorganic transformation is completed in a hydrogen atmosphere; the above-mentioned ammonia pyrolysis to partially inorganic transformation of the cross-linked wire is achieved by controlling the ammonia pyrolysis time, the ammonia pyrolysis time is 10min-60min, the boron on the surface layer of the doped cross-linked wire reacts with the ammonia, the boron-containing active atmosphere introduced during the cross-linking process combines with the active sites on the fiber surface of the doped cross-linked wire or is directly deposited on the fiber surface of the doped cross-linked wire, and then reacts with the ammonia during the high-temperature sintering process so that the boron element on the fiber surface of the doped cross-linked wire reacts with the ammonia to form a boron nitride surface coating; Finally, the doped cross-linked fibers are heat treated in an inert atmosphere to obtain in-situ doped continuous silicon carbide fibers, and the heat treatment temperature is 1600°C to 1900°C.

2. The method for preparing a low-cost in-situ doped continuous silicon carbide fiber according to claim 1, comprising the following steps: The molecular weight of the vinyl liquid polycarbosilane in step 1 is 950-1300, the viscosity at 25° C. is ≤30 mPa.s, the ceramic yield is greater than 55%, and it has self-crosslinking properties.

3. The method for preparing low-cost in-situ doped continuous silicon carbide fiber according to claim 1, characterized in that: The boron monomer in step 1 is a solid or liquid boron monomer, including at least one of borane, tris(dimethylamino)borane, dimethylaminoborane, borane-amine complex, borane-pyridine complex and tetrahydrofuran-borane complex.

4. The method for preparing low-cost in-situ doped continuous silicon carbide fiber according to claim 1, characterized in that: The organic solvent in step 1 is toluene, xylene, tetrahydrofuran or divinylbenzene; The inert atmosphere in step 3 is nitrogen, argon or helium. The inert atmosphere in step 4 is nitrogen, argon or helium.

5. The method for preparing low-cost in-situ doped continuous silicon carbide fiber according to claim 1, characterized in that: The metallocene in step 1 is a metal coordination compound composed of a transition metal and cyclopentadiene, including at least one of titanocene dichloride, hafnocenene dichloride and zirconocene dichloride.

6. The method for preparing low-cost in-situ doped continuous silicon carbide fiber according to claim 1, characterized in that: The mass ratio of the vinyl liquid polycarbosilane, the boron monomer, the organic solvent and the metallocene in the step 1 is 60-80:2-10:8-15:5-20.

7. The method for preparing low-cost in-situ doped continuous silicon carbide fiber according to claim 1, characterized in that: The boron-containing active atmosphere in step three is one of the boric acids containing carbon-carbon double bonds or carbon-carbon triple bonds, or the boron-containing active atmosphere in step three is a mixture of multiple gases of boric acids containing carbon-carbon double bonds or carbon-carbon triple bonds, and the multiple gases are a combination of at least two gases.

8. A method for preparing low-cost in-situ doped continuous silicon carbide fibers as claimed in claim 7, characterized in that: The boron-containing active atmosphere in step three is one of 2,2-methylvinylboronic acid, 2,4,6-trivinylboroxine, 4-ethynylphenylboronic acid, 2,2-dimethylethynylboronic acid and 1-pentenylboronic acid.

9. The method for preparing low-cost in-situ doped continuous silicon carbide fiber according to claim 1, characterized in that: The boron-containing active atmosphere in step three is heated and gasified by a gasification device before being introduced.

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