A method for preparing high-strength continuous silicon nitride fiber

Abstract

The present invention discloses a method for preparing high-strength continuous silicon nitride fiber, which relates to the technical field of silicon nitride fibers. The method uses small-molecule liquid polycarbosilane waste generated in a spinning and degassing process and solid high-molecular-weight polycarbosilane as spinning raw materials. The method undergoes melt spinning, thermal crosslinking, low-irradiation-dose crosslinking, nitriding and decarburization, high-temperature pyrolysis, and high-temperature deoxidation to produce a high-strength continuous silicon nitride fiber. The method rationally utilizes the small-molecule liquid polycarbosilane waste generated in the existing spinning and degassing process, improves the strength of the prepared continuous silicon nitride fiber, increases the density of the fiber, reduces the oxygen content of the fiber, and improves the weaving adaptability of the fiber.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon nitride fibers, and in particular to a method for preparing high-strength continuous silicon nitride fibers. Background Art

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

[0003] Continuous silicon nitride fibers are mainly prepared by the precursor method, and the precursors used mainly include: hydrogenated polysilazane, perhydropolysilazane, polycarbosilazane, polycarbosilane, polysilazane and other polymer compounds. At present, there are two main ideas for the preparation of silicon nitride fibers: one is to use polysilazane as a precursor, and the other is to use polycarbosilane as a precursor. The polysilazane precursor is similar to the fiber composition, making it easier to obtain ceramic fibers with high strength and few defects; while the polycarbosilane precursor does not contain nitrogen atoms and needs to be treated by nitridation and decarburization to form a silicon nitride structure. The resulting ceramic fibers are bound to have many defects, but its advantage is that it avoids the process difficulties caused by the high activity of polysilazane. Therefore, the polycarbosilane precursor conversion method for preparing continuous silicon nitride fibers is currently the most commonly used preparation method in industry.

[0004] The Japan Atomic Energy Research Institute uses polycarbosilane for melt spinning and electron beam cross-linking in a helium atmosphere to obtain an infusible raw fiber. Subsequently, it is nitrided in an ammonia flow at 500-1000°C and sintered in a nitrogen flow at 1000-1400°C to produce silicon nitride fiber. The fiber contains 58% Si, 35% N, 4% C, and 3% O, with a fiber density of 2.3g / cm -3 , fiber diameter 15μm, tensile strength 2.0GPa, Young's modulus 120GPa.

[0005] In China, continuous silicon nitride fibers are mainly prepared by the polycarbosilane precursor method, and the main research institutions include Xiamen University, National University of Defense Technology, and Chongqing University. At present, the only company in China that has achieved industrialization of continuous silicon nitride fibers is Fujian Jianli Asia New Materials Co., Ltd., whose fibers contain O < 3%, C < 0.9%, and have a fiber density of (2.15-2.35) g / cm -3, tensile strength ≥ 1.6GPa, tensile modulus ≥ 140GPa. The continuous silicon nitride fiber KD-SN reported by the National University of Defense Technology contains 58.6% Si, 38.3% N, 0.6% C, 2.5% O, and a fiber density of 2.3g / cm -3 , the tensile modulus is between 150 and 160 GPa, and the tensile strength is 1.5 GPa.

[0006] The wave-transmitting continuous silicon nitride fiber converted by polycarbosilane precursor method is amorphous and mainly composed of SiN4 structural units and SiN x O y Phase composition, there is a trace amount of carbon element, and the density is generally less than 2.3g / cm -3 , and the theoretical density of silicon nitride is 3.12g / cm -3 The difference is large, and there are more voids inside the fiber, which is not conducive to the fiber strength. In addition, the SiN x O y Phase and free carbon elements will decompose in high temperature environments, forming holes and affecting the high-temperature mechanical properties of the fiber. The low strength of the fiber will easily lead to fiber accumulation and breakage in the subsequent weaving process, especially the weaving process of complex structures, which will eventually lead to the inability to complete the weaving or the woven components not meeting the requirements.

[0007] During the melt-spinning stage of producing continuous silicon carbide and silicon nitride fibers using the polycarbosilane precursor process, vacuum degassing is required after the polycarbosilane is heated and melted. Otherwise, the resulting polycarbosilane precursor will break due to bubbles, resulting in unspinnable fibers. However, a portion of the low-molecule liquid polycarbosilane is easily distilled out of the melt tank under reduced pressure in a vacuum environment, forming spinning waste. This waste contains a high number of Si-H bonds, resulting in high reactivity and flammability, posing safety risks for waste disposal and storage. However, Si-H bonds are active bonds in polycarbosilane. The higher the Si-H bond content, the more reactive the polycarbosilane. During cleavage, Si-H bonds are more likely to break, forming free radicals that react with ammonia to form Si-N bonds. Therefore, recycling this spinning waste and taking measures to prevent its volatilization during distillation would not only facilitate the production of continuous silicon nitride fibers but also address the waste disposal challenges faced by fiber manufacturers. Summary of the Invention

[0008] The purpose of the present invention is to provide a method for preparing high-strength continuous silicon nitride fibers, to rationally utilize the small molecule liquid polycarbosilane waste in the existing spinning and degassing process, and to improve the strength of the prepared continuous silicon nitride fibers, increase the density of the fibers, reduce the oxygen content of the fibers, and improve the weaving adaptability of the fibers.

[0009] In order to achieve the above object, the solution of the present invention is:

[0010] A method for preparing high-strength continuous silicon nitride fiber comprises the following steps:

[0011] Step 1, melt spinning:

[0012] The low-molecule liquid polycarbosilane waste generated in the spinning degassing process and the solid high-molecular polycarbosilane are used as spinning raw materials, mixed in a ratio of 5-30:95-70, melted at 170-200° C., a nitrogen-containing crosslinking agent is added, and stirring is continued for 1-3 hours. After vacuum degassing for 18-30 hours, melt spinning is performed to obtain polycarbosilane precursor yarn;

[0013] The mass amount of the nitrogen-containing cross-linking agent accounts for 0.5% to 5% of the mass of the spinning raw material;

[0014] Step 2: Thermal cross-linking:

[0015] Then, the polycarbosilane precursor obtained in step 1 is placed in a heat treatment furnace, filled with protective gas, and thermally crosslinked at 250-350° C. for 0.5-2 hours to obtain thermally crosslinked yarn;

[0016] Step 3: Low radiation dose cross-linking:

[0017] The obtained thermally cross-linked filaments are then placed in an irradiation chamber for low-dose cross-linking, with the irradiation dose being 3 to 8 MGy;

[0018] Step 4: Nitriding and decarburization:

[0019] Then, the fiber cross-linked by low radiation dose is placed in a high-temperature furnace, heated to 500-800°C under the protection of protective gas, ammonia is introduced, and kept warm for 4-5 hours for nitriding and decarburization treatment;

[0020] Step 5: High temperature pyrolysis:

[0021] Then, the protective gas is continuously introduced and the temperature is raised to 1000-1200°C for high-temperature pyrolysis;

[0022] Step 6: High temperature deoxidation:

[0023] Finally, the temperature is raised to 1300-1400° C. and heat treated for 20-40 minutes to obtain the high-strength continuous silicon nitride fiber.

[0024] In step 1, the molecular weight of the solid polymer polycarbosilane is in the range of 1000 to 2000.

[0025] In step 1, the nitrogen-containing crosslinking agent is one of vinylsilazane, trimethyltrivinyl cyclic trisilazane and vinyl polysilazane.

[0026] The protective gas in steps 2, 4 and 5 is nitrogen.

[0027] After adopting the above technical solution, the method for preparing high-strength continuous silicon nitride fiber of the present invention has the following beneficial effects:

[0028] 1. First, the present invention recycles a certain proportion of the small molecule liquid polycarbosilane waste generated in the spinning degassing process during the melt spinning stage. On the one hand, it solves the potential safety hazards of this type of waste in the discharge and storage. On the other hand, because the spinning waste contains more Si-H bonds and has greater reaction activity, it can react with a nitrogen-containing cross-linking agent during the spinning process to generate a large molecule polycarbosilane containing Si-N-Si bonds, thereby avoiding the escape of small molecule polycarbosilane in the subsequent spinning degassing process, and realizing the recycling of small molecule liquid polycarbosilane waste.

[0029] 2. Secondly, the thermal cross-linking reaction allows the nitrogen-containing cross-linking agent to fully react with the Si-H bonds in the small molecule liquid polycarbosilane raw material, thereby improving the cross-linking degree of the fiber, thereby reducing the radiation dose of subsequent oxygen-free electron beam cross-linking to a certain extent. Compared with the current radiation dose generally higher than 10MGy, the present invention only requires a radiation dose of 2 to 6MGy after thermal cross-linking.

[0030] 3. Compared with the common process, the present invention adds a high temperature deoxidation process after high temperature pyrolysis to make the SiN in the fiber x O y The further decomposition not only further improves the density of the fiber, but also makes the final oxygen content of the silicon nitride fiber less than 0.5%, further enhancing the effect of oxygen-free electron beam cross-linking.

[0031] 4. The carbon content of the continuous silicon nitride fiber prepared by the polycarbosilane precursor method of the present invention is less than 0.8%, the oxygen content is less than 0.5%, and the density is greater than 2.40g / cm -3 The single-filament tensile strength is greater than 2.6 GPa, the tensile modulus is greater than 170 GPa, the strength retention rate is greater than 90% after treatment at 1200°C for 1 hour in nitrogen, and the strength retention rate is greater than 80% after treatment at 1200°C for 1 hour in air, and the elongation at break is greater than 1.4%. The silicon nitride fiber prepared by the present invention has few pores, high density, high tensile strength, high elongation at break, low oxygen content on the fiber surface, and few defects. In the subsequent weaving process, problems such as broken wire and hairy wire accumulation are not likely to occur, and it can be adapted to the weaving of complex preforms. DETAILED DESCRIPTION

[0032] In order to further explain the technical solution of the present invention, the present invention is described in detail below through specific embodiments.

[0033] Example 1

[0034] Step 1, melt spinning:

[0035] The low-molecule liquid polycarbosilane waste generated during the spinning degassing process and the solid high-molecular polycarbosilane with a molecular weight of 1000 to 2000 are used as spinning raw materials. The mixture is mixed in a ratio of 5:95 and melted at 200°C. 0.5% of vinylsilazane, which accounts for 0.5% of the mass of the spinning raw material, is added and stirred evenly for 1 hour. After vacuum degassing for 30 hours, the mixture is melt-spun to obtain polycarbosilane precursor.

[0036] Step 2: Thermal cross-linking:

[0037] Then, the polycarbosilane precursor obtained in step 1 is placed in a heat treatment furnace, filled with nitrogen, and thermally cross-linked at 350° C. for 2 hours to obtain thermally cross-linked yarn;

[0038] Step 3: Low radiation dose cross-linking:

[0039] The obtained thermally cross-linked filaments were then placed in an irradiation chamber for low-dose cross-linking, with an irradiation dose of 8 MGy.

[0040] Step 4: Nitriding and decarburization:

[0041] The fibers cross-linked with low radiation doses were then placed in a high-temperature furnace, heated to 800°C under nitrogen protection, introduced with ammonia, and kept at this temperature for 5 hours for nitriding and decarburization treatment.

[0042] Step 5: High temperature pyrolysis:

[0043] Then, nitrogen is continuously introduced and the temperature is raised to 1200°C for high-temperature pyrolysis;

[0044] Step 6: High temperature deoxidation:

[0045] Finally, the temperature was raised to 1300°C and heat treated for 40 minutes to obtain high-strength continuous silicon nitride fibers.

[0046] The vinylsilazane in this embodiment was purchased from Sinopharm Reagent Network.

[0047] Example 2

[0048] Step 1, melt spinning:

[0049] The low-molecule liquid polycarbosilane waste generated during the spinning degassing process and the solid high-molecular polycarbosilane with a molecular weight of 1000 to 2000 are used as spinning raw materials, which are mixed in a ratio of 30:70 and melted at 170° C., and trimethyltrivinyl cyclic trisilazane accounting for 5% of the mass of the spinning raw material is added. The mixture is stirred evenly for 3 hours, and then vacuum degassing is carried out for 18 hours before melt spinning to obtain polycarbosilane precursor.

[0050] Step 2: Thermal cross-linking:

[0051] Then, the polycarbosilane precursor obtained in step 1 is placed in a heat treatment furnace, filled with nitrogen, and thermally cross-linked at 250° C. for 0.5 h to obtain thermally cross-linked yarn;

[0052] Step 3: Low radiation dose cross-linking:

[0053] The obtained thermally cross-linked filaments were then placed in an irradiation chamber for low-dose cross-linking, with an irradiation dose of 3 MGy.

[0054] Step 4: Nitriding and decarburization:

[0055] The fibers cross-linked with low radiation doses were then placed in a high-temperature furnace, heated to 500°C under nitrogen protection, ammonia introduced, and kept at this temperature for 4 hours for nitriding and decarburization treatment.

[0056] Step 5: High temperature pyrolysis:

[0057] Then continue to introduce nitrogen and raise the temperature to 1000℃ for high-temperature pyrolysis;

[0058] Step 6: High temperature deoxidation:

[0059] Finally, the temperature was raised to 1400°C and heat treated for 20 minutes to obtain high-strength continuous silicon nitride fibers.

[0060] The trimethyltrivinyl cyclic trisilazane in this embodiment was purchased from the Sinopharm Reagent Network.

[0061] Example 3

[0062] Step 1, melt spinning:

[0063] The low-molecule liquid polycarbosilane waste generated during the spinning degassing process and the solid high-molecular polycarbosilane with a molecular weight of 1000 to 2000 are used as spinning raw materials, which are mixed in a ratio of 15:85 and melted at 180° C., and 3% of the weight of the spinning raw material is added with vinyl polysilazane. The mixture is stirred evenly for 2 hours, and then vacuum degassed for 25 hours before melt spinning to obtain polycarbosilane precursor.

[0064] Step 2: Thermal cross-linking:

[0065] Then, the polycarbosilane precursor obtained in step 1 is placed in a heat treatment furnace, filled with nitrogen, and thermally cross-linked at 300° C. for 1 hour to obtain thermally cross-linked yarn;

[0066] Step 3: Low radiation dose cross-linking:

[0067] The obtained thermally cross-linked filaments were then placed in an irradiation chamber for low-dose cross-linking, with an irradiation dose of 5.5 MGy.

[0068] Step 4: Nitriding and decarburization:

[0069] The fibers cross-linked with low radiation doses were then placed in a high-temperature furnace, heated to 650°C under nitrogen protection, introduced with ammonia, and kept at this temperature for 4.5 hours for nitriding and decarburization treatment.

[0070] Step 5: High temperature pyrolysis:

[0071] Then, nitrogen is continuously introduced and the temperature is raised to 1100°C for high-temperature pyrolysis;

[0072] Step 6: High temperature deoxidation:

[0073] Finally, the temperature was raised to 1350°C and heat treated for 30 minutes to obtain high-strength continuous silicon nitride fibers.

[0074] The vinyl polysilazane in this embodiment was purchased from Hangzhou Qingci New Material Technology Co., Ltd.

[0075] Comparative Example 1

[0076] The difference from Example 3 is that Comparative Example 1 does not perform the high-temperature deoxidation step.

[0077] Comparative Example 2

[0078] The difference from Example 2 is that in Comparative Example 2, the ratio of small molecule liquid polycarbosilane waste to solid polycarbosilane is 35:65, and the added amount of trimethyltrivinyl cyclic trisilazane is 6%.

[0079] The performance data of the silicon nitride fibers prepared in the examples and comparative examples are shown in Table 1. The test methods for each performance are all well-known test methods in the art.

[0080] Table 1 Performance results of silicon nitride fibers

[0081]

[0082]

[0083] Note: Strength retention rate in the table ① Strength retention rate of the fiber after being treated at 1200℃ for 1h under nitrogen; Strength retention rate ② It is the strength retention rate of the fiber after being treated at 1200℃ for 1h in air.

[0084] From the data in Table 1, it can be seen that the carbon content of the continuous silicon nitride fiber prepared by the present invention is less than 0.8%, the oxygen content is less than 0.5%, and the density is greater than 2.40 g / cm -3The tensile strength of the single fiber is greater than 2.6GPa, the tensile modulus is greater than 170GPa, the strength retention rate is greater than 90% after being treated at 1200℃ for 1h under nitrogen, the strength retention rate is greater than 80% after being treated at 1200℃ for 1h under air, and the elongation at break is greater than 1.4%.

[0085] By comparing the data of Example 3 and Comparative Example 1, it can be found that the oxygen content of Example 3, which has undergone high-temperature deoxidation, is 0.44%, which is much lower than the oxygen content of Comparative Example 1 of 1.15%, and various mechanical properties are also better than those of Comparative Example 1.

[0086] By comparing Example 2 and Comparative Example 2, it can be seen that after adding 35% of small molecule liquid polycarbosilane waste, due to the excessive amount of liquid small molecule polycarbosilane and the excessive reaction activity, it undergoes excessive thermal decomposition and volatilization in the spinning stage and the cross-linking stage, resulting in reduced density and various mechanical properties, which is not conducive to the preparation of high-strength continuous silicon nitride fibers.

[0087] Therefore, the silicon nitride fiber prepared by the present invention has few pores, high density, high tensile strength, high elongation at break, low oxygen content and few defects on the fiber surface. In the subsequent weaving process, it is not easy to have problems such as broken wires and accumulation of hair fibers, and can adapt to the weaving of complex preforms.

[0088] The above embodiments do not limit the product form and style of the present invention. Any appropriate changes or modifications made 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 high-strength continuous silicon nitride fiber, characterized by: The following steps are involved: Step 1, melt spinning: The low-molecule liquid polycarbosilane waste generated during the spinning degassing process and solid high-molecular polycarbosilane were used as spinning raw materials. They were mixed in a ratio of 5-30:95-70 and melted at 170-200°C. A nitrogen-containing crosslinking agent was added and stirred for 1-3 hours. After vacuum degassing for 18-30 hours, the mixture was melt-spun to obtain polycarbosilane precursor fibers. The mass amount of the nitrogen-containing cross-linking agent accounts for 0.5% to 5% of the mass of the spinning raw material; Step 2: Thermal cross-linking: Then, the polycarbosilane precursor obtained in step 1 is placed in a heat treatment furnace, filled with protective gas, and thermally cross-linked at 250-350° C. for 0.5-2 h to obtain a thermally cross-linked filament; Step 3: Low radiation dose cross-linking: The obtained thermally cross-linked filaments are then placed in an irradiation chamber for low-dose cross-linking, with an irradiation dose of 3 to 8 MGy. Step 4: Nitriding and decarburization: The fiber cross-linked by low radiation dose is then placed in a high-temperature furnace, heated to 500-800°C under the protection of protective gas, ammonia is introduced, and the temperature is kept for 4-5 hours for nitriding and decarburization treatment; Step 5: High temperature pyrolysis: Then, the protective gas is continuously introduced and the temperature is raised to 1000-1200°C for high-temperature pyrolysis; Step 6: High temperature deoxidation: Finally, the temperature is raised to 1300-1400° C. and heat treated for 20-40 minutes to obtain the high-strength continuous silicon nitride fiber; In step 1, the nitrogen-containing crosslinking agent is one of vinylsilazane, trimethyltrivinyl cyclic trisilazane and vinyl polysilazane.

2. The method for preparing high-strength continuous silicon nitride fiber according to claim 1, wherein: In step 1, the molecular weight of the solid polymer polycarbosilane is in the range of 1000 to 2000.

3. The method for preparing high-strength continuous silicon nitride fiber according to claim 1, wherein: The protective gas in steps 2, 4 and 5 is nitrogen.

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