Preparation method of low-cost in-situ doped silicon nitride fiber

Abstract

The invention discloses a preparation method of a low-cost in-situ doped silicon nitride fiber, which comprises the following steps: taking one or two of vinyl liquid polycarbosilane and vinyl polysilazane as a precursor, adding a boron monomer, an organic solvent and metallocene, and doping to prepare a doped silicon nitride fiber containing boron, titanium, hafnium, zirconium and other elements; carrying out melt spinning on the doped precursor to obtain a doped precursor; then boron-containing active gas introduced in the crosslinking process can be combined with active sites on the surface of the protofilament fiber or directly deposited on the surface of the fiber; and finally, the boron-containing active gas reacts with ammonia gas in the high-temperature sintering process to generate a boron nitride (BN) surface coating. Thus, the temperature resistance of the silicon nitride fiber is improved, the problem that a coating of a prefabricated body of a silicon nitride fiber ceramic matrix composite material with a complex structure is not uniform is solved, meanwhile, high cost of a secondary coating and large damage to the fiber are avoided, the composite material preparation process flow is simplified, and the invention provides the low-cost preparation method for in-situ doping of the silicon nitride fiber.

Description

Technical Field

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

[0002] Wave-transmitting continuous silicon nitride fiber has excellent properties such as high strength and high modulus, low density, ablation resistance, low dielectric constant, and low dielectric loss. It is currently the key new material with the best overall performance, integrating load-bearing, wave-transmitting, ablation resistance and other multi-functions. Silicon nitride fiber can be used for aircraft radomes and antenna windows. Compared with the commonly used quartz fiber with an operating temperature of 800°C, silicon nitride fiber can work stably for a long time at 1400°C. It is the preferred material for radomes of future high Mach number aircraft, space shuttles, etc., and has strategic significance for national scientific research and national defense development.

[0003] The wave-transmitting continuous silicon nitride fiber converted by the polycarbosilane precursor method is amorphous and mainly composed of SiN4 structural units and SiNxOy phases, with a density of less than 2.3 g.cm -3 , and the theoretical density of silicon nitride is 3.12 g.cm -3 The phase difference is large, and there are many voids inside the fiber, which is not conducive to the fiber strength. In addition, the SiNxOy phase and free carbon elements in the fiber will decompose under high temperature environment to produce holes. After high temperature heat treatment above 1450℃, the fiber will transform from amorphous to crystalline, affecting the high temperature mechanical properties and wave transmission performance of the fiber.

[0004] The key to Si3N4 fiber's reinforcing effect in wave-transmitting ceramic-based composites lies in the transition area between the fiber and the matrix - the interface phase. As the link between the fiber and the matrix, the interface phase not only has the functions of "mechanical fuse" and "load transfer", but also has the role of "buffer", and its structure and performance directly affect the strength and toughness of the composite material.

[0005] BN coating is the hot spot and focus of the current research and application of silicon nitride fiber reinforced ceramic matrix composites. It is mainly obtained by deposition on 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℃ 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 nitride fiber. Therefore, the use of uncoated silicon nitride fiber for the preparation of composite materials is not only difficult to obtain a uniform thickness and high crystallinity BN coating, but also unfavorable to the mechanical properties of the fiber preform.

[0006] In view of this, the inventor of this case conducted in-depth research, which resulted in the creation of this case. Summary of the invention

[0007] The purpose of the present invention is to provide a low-cost preparation method for in-situ doped silicon nitride fibers, improve the temperature resistance of silicon nitride fibers, solve the problem of uneven coating of complex structure preforms of silicon nitride fiber ceramic-based composite materials, and avoid the problems of high cost of secondary coating and severe damage to the fibers. In order to achieve the above object, the technical solution of the present invention is: A method for preparing low-cost in-situ doped silicon nitride fibers comprises the following steps: Step 1: Synthesize doping precursor Vinyl liquid polycarbosilane and / or vinyl polysilazane is used as a vinyl precursor, a boron monomer, an organic solvent and a metallocene are added, and the mixture is mixed and stirred under a nitrogen atmosphere, and is kept at 40-150° C. for 12 h-36 h, and a doped precursor is obtained by vacuum distillation; Step 2: Melt Spinning The doped precursor is placed in a melt spinning system, heated to 180°C to 240°C under nitrogen protection to melt and kept warm for 12h to 24h, and then the doped raw silk 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 fibers are heated to 850°C-1000°C and kept warm, and pyrolyzed in an ammonia atmosphere to complete the inorganic transformation of the doped cross-linked fibers. The temperature is then raised to 1300-1400°C and heat-treated in an inert atmosphere to obtain in-situ doped continuous silicon nitride fibers.

[0008] Furthermore, the molecular weight of the vinyl liquid polycarbosilane is 950-1300, and the ceramic yield is greater than 55%; the molecular weight of the vinyl polysilazane is greater than 2000, and the ceramic yield is greater than 65%.

[0009] Furthermore, 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, tetrahydrofuran-borane complex and borane-pyridine complex.

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

[0011] Furthermore, the metallocene in step 1 refers to 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 precursor, 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 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.

[0014] Further, the boron-containing active atmosphere 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] After adopting the above technical solution, the low-cost in-situ doped silicon nitride fiber preparation method of the present invention has the following beneficial effects: 1) Doped silicon nitride 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 nitride fibers, the temperature resistance of silicon nitride fibers is further improved. In addition, boron, titanium, hafnium, zirconium, and tungsten can also combine with nitrogen atoms to form hafnium nitride (HfN), titanium nitride (TiN), and zirconium nitride (ZrN) high temperature ceramic materials to jointly enhance the temperature resistance of silicon nitride fibers.

[0017] 2) Since metallocene contains carbon-carbon double bonds, and vinyl liquid polycarbosilane and vinyl polysilazane themselves contain vinyl groups and have self-crosslinking properties, the precursor in the present invention is highly active and can be heated with active gas to render 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 form a boron nitride (BN) surface coating. After high-temperature heat treatment at 1300~1400℃, 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 nitride 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The cross-section and surface morphology SEM image of the doped silicon nitride fiber prepared in Example 2 of the present invention; Figure 2 This is an infrared diffraction spectrum of the doped silicon nitride fiber prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0021] In order to further explain the technical solution of the present invention, the present invention is described in detail below through specific embodiments.

[0022] 1. Preparation of Silicon Nitride Fiber Example 1 The present invention provides a low-cost method for preparing in-situ doped continuous silicon nitride fibers, comprising the following steps: Step 1: Synthesize doping precursor Vinyl liquid polycarbosilane, borane, toluene and dichlorotitanocene were mixed and stirred in a nitrogen atmosphere at a mass ratio of 60:10:10:20, and kept at 80° C. for 36 hours, and a doped precursor was obtained by vacuum distillation.

[0023] Step 2: Melt Spinning The doped 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.

[0024] 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 10 hours to obtain the doped cross-linked silk. Step 4: High temperature firing The doped cross-linked fibers are heated to 850° C. and inorganically transformed in an ammonia atmosphere, and then the temperature is raised to 1300° C. for high-temperature heat treatment to obtain in-situ doped continuous silicon nitride fibers.

[0025] The yield of the vinyl liquid polycarbosilane ceramic in step 1 is 64.85%. 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.

[0026] Example 2 The present invention provides a low-cost method for preparing in-situ doped continuous silicon nitride fibers, comprising the following steps: Step 1: Synthesize doping precursor Vinyl liquid polycarbosilane, tri(dimethylamino)borane, xylene and hafnocene dichloride were mixed and stirred in a mass ratio of 65:8:15:12 under a nitrogen atmosphere and kept at 120° C. for 24 hours, and a doped precursor was obtained by vacuum distillation.

[0027] Step 2: Melt Spinning The doped precursor was placed in a melt spinning system, heated to 220°C for melting under nitrogen protection and kept warm for 24 hours, and the doped raw fibers were obtained by spinning.

[0028] 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 140°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 16 hours to obtain doped cross-linked silk. Step 4: High temperature firing The doped cross-linked fibers were placed in a push plate furnace and heated to 900°C in an ammonia atmosphere to complete the inorganic transformation of the doped cross-linked fibers. The temperature was then raised to 1330°C for high temperature heat treatment to obtain in-situ doped continuous silicon nitride fibers.

[0029] The yield of the vinyl liquid polycarbosilane ceramic in the step 1 is 62.46%.

[0030] The cross section and surface morphology of the prepared in-situ doped continuous silicon nitride fiber are as follows: Figure 1 As shown; the infrared diffraction spectrum of the in-situ doped continuous silicon nitride fiber is shown Figure 2 shown.

[0031] 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 189nm, and a dense surface without defects such as protrusions.

[0032] Depend on Figure 2 It can be seen that: at 1336cm -1 and 780cm -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.

[0033] Example 3 The present invention provides a low-cost method for preparing in-situ doped continuous silicon nitride fibers, comprising the following steps: Step 1: Synthesize doping precursor Vinyl polysilazane, dimethylamino borane, tetrahydrofuran and zirconocene dichloride were mixed and stirred in a mass ratio of 70:8:8:14 under a nitrogen atmosphere and kept at 40° C. for 12 h, and a doped precursor was obtained by vacuum distillation.

[0034] Step 2: Melt Spinning The doped precursor was placed in a melt spinning system, heated to 240°C for melting under nitrogen protection and kept warm for 16 hours, and the doped raw fibers were obtained by spinning.

[0035] 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 310°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 are heated to 950° C. in an ammonia atmosphere to complete the inorganic transformation, and then the temperature is raised to 1350° C. for high-temperature heat treatment to obtain in-situ doped continuous silicon nitride fibers.

[0036] The yield of the vinyl liquid polycarbosilane ceramic in the step 1 is 63.40%.

[0037] Example 4 The present invention provides a low-cost method for preparing in-situ doped continuous silicon nitride fibers, comprising the following steps: Step 1: Synthesize doping precursor Vinyl polysilazane, borane amine complex, divinylbenzene and hafnocene dichloride were mixed and stirred in a mass ratio of 75:6:9:10 under a nitrogen atmosphere and kept at 60° C. for 12 h, and a doped precursor was obtained by vacuum distillation.

[0038] Step 2: Melt Spinning The doped 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.

[0039] 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 are heated to 1000° C. in an ammonia atmosphere to complete the inorganic transformation, and then the temperature is raised to 1400° C. for high-temperature heat treatment to obtain in-situ doped continuous silicon nitride fibers.

[0040] The yield of the vinyl liquid polycarbosilane ceramic in the step 1 is 58.32%.

[0041] Example 5 The present invention provides a low-cost method for preparing in-situ doped continuous silicon nitride fibers, comprising the following steps: Step 1: Synthesize doping precursor A mixture of vinyl liquid polycarbosilane and vinyl polysilazane in a mass ratio of 1:1 is used as the main material, and then mixed with tetrahydrofuran borane complex, toluene and zirconocene dichloride in a mass ratio of 80:2:8:10 in a nitrogen atmosphere and kept at 80°C for 24 hours, and the doped precursor is obtained by reduced pressure distillation.

[0042] Step 2: Melt Spinning The doped 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.

[0043] Step 3: Boron-containing active atmosphere assisted thermal crosslinking 1-pentenylboric acid was gasified at 230°C in a gasification furnace, mixed with helium and passed into a cross-linking furnace with a volume ratio of helium to 1-pentenylboric acid of 15:1. Thermal cross-linking was completed at 260°C for 16 hours to obtain doped cross-linked wire. Step 4: High temperature firing The doped cross-linked fibers are heated to 900° C. in an ammonia atmosphere to complete the inorganic transformation, and then the temperature is raised to 1350° C. for high-temperature heat treatment to obtain in-situ doped continuous silicon nitride fibers.

[0044] The yield of the vinyl liquid polycarbosilane ceramic in the step 1 is 55.98%.

[0045] The beneficial effects of the present invention are as follows: 1) Doped silicon nitride 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 nitride fibers, the temperature resistance of silicon nitride fibers is further improved. In addition, boron, titanium, hafnium, zirconium, and tungsten can also combine with nitrogen atoms to form hafnium nitride (HfN), titanium nitride (TiN), and zirconium nitride (ZrN) high temperature ceramic materials to jointly enhance the temperature resistance of silicon nitride fibers.

[0046] 2) Since metallocene contains carbon-carbon double bonds, and vinyl liquid polycarbosilane and vinyl polysilazane themselves contain vinyl groups and have self-crosslinking properties, the precursor in the present invention is highly active and can be heated with active gas to render the original filament infusible, thus avoiding the high cost of crosslinking by electron beam irradiation.

[0047] 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 form a boron nitride (BN) surface coating. After high-temperature heat treatment at 1300~1400℃, 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.

[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 nitride fiber examples prepared by the method of the present invention is greater than 2.5 g.cm -3 , oxygen content is less than 0.8%, and the single fiber tensile strength is above 1.8 GPa. After heat treatment at 1200℃ in argon atmosphere for 1h, the single fiber tensile strength of the fiber is higher than 1.7 GPa. After heat treatment at 1200℃ in air atmosphere for 1h, the single fiber tensile strength of the fiber is higher than 1.5 GPa. The fiber strength of Examples 2 and 4 using hafnium elements is relatively higher, which is related to the excellent high temperature resistance of hafnium boride (HfB2) and hafnium nitride (HfN).

[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 silicon nitride fibers, characterized in that: The following steps are involved: Step 1: Synthesize doping precursor Vinyl liquid polycarbosilane and / or vinyl polysilazane is used as a vinyl precursor, a boron monomer, an organic solvent and a metallocene are added, and the mixture is mixed and stirred under a nitrogen atmosphere, and is kept at 40-150° C. for 12 h-36 h, and a doped precursor is obtained by vacuum distillation; Step 2: Melt Spinning The doped precursor is placed in a melt spinning system, heated to 180°C to 240°C under nitrogen protection to melt and kept warm for 12h to 24h, and then the doped raw silk 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 fibers are heated to 850°C-1000°C and kept warm, and pyrolyzed in an ammonia atmosphere to complete the inorganic transformation of the doped cross-linked fibers. The temperature is then raised to 1300-1400°C and heat-treated in an inert atmosphere to obtain in-situ doped continuous silicon nitride fibers.

2. A method for preparing low-cost in-situ doped silicon nitride fiber according to claim 1, characterized in that: The molecular weight of vinyl liquid polycarbosilane is 950~1300, and the ceramic yield is greater than 55%; the molecular weight of vinyl polysilazane is greater than 2000, and the ceramic yield is greater than 65%.

3. The method for preparing low-cost in-situ doped silicon nitride 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-ammine complex, tetrahydrofuran-borane complex and borane-pyridine complex.

4. The method for preparing low-cost in-situ doped silicon nitride fiber according to claim 1, characterized in that: The organic solvent in step 1 is toluene, xylene or tetrahydrofuran; 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 silicon nitride 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 silicon nitride fiber according to claim 1, characterized in that: The mass ratio of the vinyl precursor, 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 silicon nitride 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 silicon nitride fiber according to 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 silicon nitride 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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