Preparation method of silicon carbide

Abstract

The invention provides a preparation method of silicon carbide, which comprises the following steps: pretreatment: mixing silicon powder with a carbon source, adding absolute ethyl alcohol, dispersing, and ball-milling; plasma activation: placing the pretreated mixture in a plasma reactor, and introducing argon to activate surface functional groups; adding a compounding agent: adding the compounding agent into an activated mixture obtained by plasma activation; gradient sintering; and post-treatment: ultrasonically cleaning the sintered product with hydrofluoric acid for 20-60 minutes, and drying to obtain a silicon carbide finished product. Through the synergistic effect of plasma activation, compounding agent optimization and gradient sintering, the problems of high energy consumption, many defects, insufficient purity and the like in a traditional method are solved. Comparison between data of the embodiment and a contrast verifies the effectiveness of the technical scheme, especially the key indexes such as density, hardness, high-temperature strength and the like are outstanding, and the method is suitable for the fields of high-power semiconductor devices and precision machining.

Description

Technical Field

[0001] The present invention belongs to the technical field of material preparation, and in particular relates to a method for preparing silicon carbide. Background Art

[0002] Silicon carbide (SiC) is a high-performance ceramic material that combines high hardness, high thermal conductivity, high-temperature resistance, and chemical stability. It is widely used in semiconductor devices, high-temperature structural materials, refractory materials, and electronic packaging. Existing technologies mainly rely on high-temperature sintering such as chemical vapor deposition or liquid phase exfoliation methods to prepare silicon carbide. However, these methods generally have the following problems: Traditional high-temperature sintering requires temperatures exceeding 2000°C, consumes enormous amounts of energy, and places stringent demands on equipment. For example, the heating costs of graphite electrodes are high. While chemical vapor deposition (CVD) can produce high-quality SiC crystals, the equipment is complex and the production cycle is long. SiC produced using existing methods often contains crystal defects such as micropipes and dislocations, which affect the performance of electronic devices. Liquid-phase exfoliation methods are prone to introducing impurities, resulting in insufficient product purity and a high defect rate. Combustion synthesis methods can rapidly produce SiC aerogels, but controlling the product morphology is difficult. The sol-gel method is a complex process, making it difficult to scale up industrially and limiting large-scale production.

[0003] Existing improvement plans alleviate the above problems by adding compounding agents or optimizing process parameters: using liquid phase method to replace vapor deposition. The traditional PVT method requires high temperature of more than 2300℃, which is prone to micropipe defects and difficult to expand. The liquid phase method uses metal silicon melt as a solvent to grow crystals through a dissolution-precipitation mechanism at conditions below 2000℃, significantly reducing the dislocation density and achieving ingot diameter expansion. For example, Tianyue Advanced has successfully produced low-defect 8-inch silicon carbide crystals and optimized the interface stability through thermal field design and solution control. A two-stage gradient sintering process is also used, with the first stage at 1500-1800℃ promoting carbon thermal reduction and the second stage at 2000-2200℃ high pressure densification. Combined with a vacuum environment and an alumina coating, the relative density reaches more than 98.5%, and the grain size is controlled below 1.2μm. The alumina layer acts as a stress buffer layer, which can increase the flexural strength retention rate at 1600℃ by 60%. A SiC-BN layered structure was adopted, and the stacking ratio was optimized through a genetic algorithm to achieve a bending strength of 434.5MPa while maintaining defect insensitivity. A thick hard layer (SiC) was used in the compressive zone, and a soft layer (BN) was distributed in the tensile zone, effectively improving toughness. Although the improved scheme significantly improved the performance of silicon carbide, problems still existed, such as insufficient dispersion uniformity of the compounding agent and defects in the liquid phase flux encapsulation. In addition, the process parameter window was narrow. For example, the gradient sintering temperature range needed to be accurate to 10°C, and the stability of large-scale production still needed to be improved. Therefore, it was necessary to design a preparation method for silicon carbide. Summary of the Invention

[0004] In order to overcome the defects in the prior art, a method for preparing silicon carbide is provided.

[0005] In order to achieve the above object, the present invention provides the following technical solutions: A method for preparing silicon carbide, the method comprising the following steps: Pretreatment: Mix silicon powder and carbon source in a mass ratio of 1: (1.2-1.8), add anhydrous ethanol to disperse and then ball mill for 4-8 hours; Plasma activation: Place the pre-treated mixture in a plasma reactor, introduce argon at a flow rate of 50-200 sccm, and treat at a power of 300-600 W for 10-30 minutes to activate surface functional groups; Adding compounding agent: adding compounding agent to the activated mixture obtained by plasma activation, wherein the total amount of compounding agent added is 5-15% of the mass of silicon powder; Gradient sintering: Place the product after adding the compounding agent in a vacuum sintering furnace, raise the temperature to 1500-1800℃ at a heating rate of 10-20℃ / min, and keep it warm for 1-3 hours; then raise the temperature to 2000-2200℃ at a heating rate of 5-10℃ / min, keep it warm for 0.5-2 hours, and cool the furnace to room temperature; Post-processing: The sintered product is ultrasonically cleaned with hydrofluoric acid for 20-60 minutes and dried to obtain the finished silicon carbide product.

[0006] In the pretreatment, the purity of the silicon powder is ≥99.5% and the particle size is 2-4 μm.

[0007] The carbon source includes petroleum coke and graphite powder, and the mass ratio of the petroleum coke to the graphite powder is (3-5):1.

[0008] The carbon source further comprises carbon nanotubes, and the added amount of the carbon nanotubes is 0.1-1% of the total mass of the carbon source.

[0009] The carbon nanotubes have a diameter of 10-50 nm and a length of 1-10 μm.

[0010] The carbon nanotubes are acidified with a mixed solution for 1-3 hours and then washed with water until neutral. The mixed solution is concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1. The carbon nanotubes have a diameter of 10-50 nm and a length of 1-10 μm.

[0011] The compounding agent is composed of nano-tungsten carbide with a particle size of 50-100 nm and polyethylene glycol in a mass ratio of 1:(0.5-1.5).

[0012] The surface of the nano-tungsten carbide is coated with an aluminum oxide layer, and the thickness of the aluminum oxide layer is 5-20 nm.

[0013] The aluminum oxide layer is coated on the surface of the nano-tungsten carbide by the following steps: dispersing the nano-tungsten carbide in isopropyl alcohol, adding aluminum chloride and 5-15% ammonia water, stirring and reacting at 60-80° C. for 2-4 hours, and then calcining at 500-800° C. for 1-3 hours.

[0014] The vacuum degree of the vacuum sintering furnace in the gradient sintering is ≤1×10 -3 Pa, and the sintering pressure is 0.5-2 MPa.

[0015] Compared with the prior art, the advantages and beneficial effects of the present invention are: 1. The silicon carbide preparation method of the present application significantly improves the preparation efficiency and product performance through the synergistic effect of multi-stage process optimization and composite materials. In the pretreatment stage, petroleum coke, graphite powder and acidified carbon nanotubes are used to compound the carbon source. The high specific surface area of ​​the nano-scale carbon source is used to enhance the reaction contact interface. Combined with the ball milling process, the uniformity of the silicon-carbon mixture is improved to a submicron-level dispersion state, effectively avoiding the risk of metal impurities introduced by mechanical crushing in the traditional solid-phase method. The acidification treatment of carbon nanotubes selectively removes the surface amorphous carbon layer through a sulfuric acid-nitric acid mixture, exposing the high conductivity characteristics of its tubular structure, and forming a three-dimensional conductive network in the subsequent sintering to promote the electron transfer efficiency of the carbon thermal reduction reaction, thereby reducing the activation energy of the synthesis reaction by about 15-20%.

[0016] 2. The plasma activation technology used in this application utilizes active particles generated by a high-frequency electric field in an argon medium to etch and modify the raw material surface with functional groups, forming a nanoscale pit structure on the silicon powder surface and generating a Si-OC transition layer. This transition layer serves as a rapid pathway for carbon diffusion during the subsequent sintering stage, lowering the starting temperature of the carbothermal reduction reaction from 1400°C in conventional processes to approximately 1250°C. This surface modification mechanism, in synergy with the nano-tungsten carbide / polyethylene glycol compound, causes the tungsten carbide particles to preferentially form heterogeneous nucleation sites at the silicon-carbon interface during sintering, inhibiting lattice distortion during the α-SiC to β-SiC phase transition and controlling dislocation density.

[0017] 3. The gradient sintering process achieves precise control of the densification process through a two-stage temperature control strategy: the first stage is a 1500-1800°C insulation to promote the complete carbon nanotube-assisted carbothermal reduction reaction, and the second stage is a 2000-2200°C high-pressure sintering to fill the grain boundary pores through the plastic flow of the alumina coating, so that the relative density of the final product reaches over 98.5% and the Vickers hardness is increased to 32 GPa. The vacuum environment combined with the 2 MPa pressure sintering effectively inhibits the decomposition and volatilization of SiC at high temperatures, controls the grain growth rate to below 0.5 μm / h, and obtains an equiaxed crystal structure with an average grain size of 1.2 μm. Compared with traditional processes, the method of this application can reduce energy consumption and meet the requirements of green manufacturing.

[0018] 4. In the post-processing stage, hydrofluoric acid ultrasonic cleaning combined with surfactant dispersion technology selectively removes unreacted free silicon and metallic impurities, achieving a product purity of 99.9%. Specifically, the interface of the aluminum oxide coating on the surface of the nano-tungsten carbide is optimized. Through directional adsorption in the isopropyl alcohol dispersion system, a uniform passivation layer is formed. During high-temperature sintering, this layer acts as a diffusion barrier to inhibit abnormal grain growth and as a stress buffer to reduce the probability of microcracks caused by thermal expansion coefficient mismatch. This structural design improves the material's flexural strength retention at 1600°C by 60% compared to conventional products, making it particularly suitable for the manufacture of packaging substrates for high-power semiconductor devices.

[0019] 5. This preparation method reduces the total process time to less than 24 hours by matching the timing of plasma activation and gradient sintering. The compounding agent uses readily available industrial nano-tungsten carbide and polyethylene glycol, eliminating the sol-gel method's reliance on specialized precursors. Experiments have shown that the resulting silicon carbide material, when used in single-crystal silicon cutting applications in the photovoltaic industry, extends tool life by more than three times and reduces wafer surface roughness to Ra 0.05 μm, meeting the ultra-precision machining requirements for substrate materials required for third-generation semiconductor devices. DETAILED DESCRIPTION

[0020] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0021] In this application, the models of various raw materials are briefly described as follows: Silicon powder: purchased from Xuzhou Lingyun Silicon Industry Co., Ltd., model 3.5N high-purity silicon powder; petroleum coke: purchased from Hebei Tanran Company, model low-sulfur CPC calcined petroleum coke; graphite powder: purchased from Sinopharm Group, analytical grade; carbon nanotubes: purchased from Shenzhen Better New Materials Co., Ltd., model NT-001 (purity before acidification ≥ 95%); nano-tungsten carbide: purchased from Shanghai Naio Nano Technology Co., Ltd., model NO-C-005-1; polyethylene glycol (PEG-4000): purchased from Hai'an Guoli Chemical Co., Ltd., model PEG-4000 (molecular weight 3800-4200); hydrofluoric acid: purchased from Sinopharm Group, analytical grade (concentration 40%, diluted to 5-15% for use in this application); aluminum chloride: purchased from Sinopharm Group, analytical grade (purity ≥99%); ammonia: purchased from Sinopharm Group, analytical grade (concentration 5-15%); concentrated sulfuric acid / concentrated nitric acid: purchased from Sinopharm Group, analytical grade; isopropyl alcohol: purchased from Sinopharm Group, analytical grade (purity ≥99.8%).

[0022] A method for preparing silicon carbide, the method comprising the following steps: Pretreatment: Mix silicon powder and carbon source in a mass ratio of 1: (1.2-1.8), add anhydrous ethanol to disperse and then ball mill for 4-8 hours; Plasma activation: Place the pre-treated mixture in a plasma reactor, introduce argon at a flow rate of 50-200 sccm, and treat at a power of 300-600 W for 10-30 minutes to activate surface functional groups; Adding compounding agent: adding compounding agent to the activated mixture obtained by plasma activation, wherein the total amount of compounding agent added is 5-15% of the mass of silicon powder; Gradient sintering: Place the product after adding the compounding agent in a vacuum sintering furnace, raise the temperature to 1500-1800℃ at a heating rate of 10-20℃ / min, and keep it warm for 1-3 hours; then raise the temperature to 2000-2200℃ at a heating rate of 5-10℃ / min, keep it warm for 0.5-2 hours, and cool the furnace to room temperature; Post-processing: The sintered product is ultrasonically cleaned with hydrofluoric acid for 20-60 minutes and dried to obtain the finished silicon carbide product.

[0023] In the pretreatment, the purity of the silicon powder is ≥99.5% and the particle size is 2-4 μm.

[0024] The carbon source includes petroleum coke and graphite powder, and the mass ratio of the petroleum coke to the graphite powder is (3-5):1.

[0025] The carbon source further comprises carbon nanotubes, and the added amount of the carbon nanotubes is 0.1-1% of the total mass of the carbon source.

[0026] The carbon nanotubes have a diameter of 10-50 nm and a length of 1-10 μm.

[0027] The carbon nanotubes are acidified with a mixed solution for 1-3 hours and then washed with water until neutral. The mixed solution is concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:1. The carbon nanotubes have a diameter of 10-50 nm and a length of 1-10 μm.

[0028] The compounding agent is composed of nano-tungsten carbide with a particle size of 50-100 nm and polyethylene glycol in a mass ratio of 1:(0.5-1.5).

[0029] The surface of the nano-tungsten carbide is coated with an aluminum oxide layer, and the thickness of the aluminum oxide layer is 5-20 nm.

[0030] The aluminum oxide layer is coated on the surface of the nano-tungsten carbide by the following steps: dispersing the nano-tungsten carbide in isopropyl alcohol, adding aluminum chloride and 5-15% ammonia water, stirring and reacting at 60-80° C. for 2-4 hours, and then calcining at 500-800° C. for 1-3 hours.

[0031] The vacuum degree of the vacuum sintering furnace in the gradient sintering is ≤1×10 -3 Pa, and the sintering pressure is 0.5-2 MPa.

[0032] The technical solution of the present invention is further illustrated by the following examples and comparative examples, but the protection scope of the present invention is not limited thereto.

[0033] Example 1 Silicon powder with a purity of 99.5% and a particle size of 4 μm was mixed with a carbon source in a mass ratio of 1:1.8. The carbon source consisted of petroleum coke and graphite powder in a mass ratio of 5:1. Carbon nanotubes (50 nm diameter, 10 μm length) were added at a concentration of 1% by weight of the total carbon source. The carbon nanotubes were acidified with a 3:1 (volume) mixture of concentrated sulfuric acid and nitric acid for 3 hours and washed with water until neutral. The mixture was dispersed in anhydrous ethanol and ball milled for 6 hours to obtain a homogeneous slurry. The slurry was placed in a plasma reactor, flowed with argon at a flow rate of 200 sccm and a power of 450 W for 10 minutes. The activated mixture was then added with a compounding agent (nano-tungsten carbide and polyethylene glycol in a mass ratio of 1:1.5) at a concentration of 15% by weight of the silicon powder. The tungsten carbide nanoparticles were coated with a 20 nm alumina layer (prepared by adding aluminum chloride and 15% ammonia aqueous solution to an isopropyl alcohol dispersion, reacting at 80°C for 4 hours, and calcining at 800°C for 3 hours). The mixed material is placed in a vacuum sintering furnace (vacuum degree ≤ 1×10 -3 The furnace was heated at 20°C / min to 1800°C and held for 3 hours. The temperature was then raised to 2200°C at 10°C / min and held for 2 hours. The furnace was then cooled. The sintered product was ultrasonically cleaned with hydrofluoric acid for 60 minutes and dried to obtain the finished silicon carbide.

[0034] Example 2 In this embodiment, the same points as in Example 1 are not repeated here, and the differences are as follows: Silicon powder with a purity of 99.5% and a particle size of 2μm was mixed with a carbon source in a ratio of 1:1.2. The carbon source was a 3:1 ratio of petroleum coke to graphite powder, with 0.1% carbon nanotubes (10nm diameter, 1μm length) added, and acidified for 1 hour. After ball milling for 4 hours, it was activated for 30 minutes at 50sccm of argon and 600W of power. The compounding agent was added in an amount of 5% (nano-tungsten carbide and polyethylene glycol in a ratio of 1:0.5), and the alumina layer thickness was 5nm (reaction at 60°C for 2 hours, calcination at 500°C for 1 hour). The sintering procedure was as follows: heating at 10°C / min to 1500°C and holding for 1 hour, then heating at 5°C / min to 2000°C and holding for 0.5 hour. The product was cleaned with hydrofluoric acid for 20 minutes.

[0035] Example 3 In this embodiment, the same points as in Example 1 are not repeated here, and the differences are as follows: Silicon powder with a particle size of 3 μm was mixed with a carbon source in a ratio of 1:1.5. A petroleum coke and graphite powder were mixed in a ratio of 4:1, with 0.5% carbon nanotubes (30 nm diameter, 5 μm length) added, and acidified for 2 hours. After ball milling for 6 hours, the mixture was treated at 125 sccm of argon and 300 W of power for 20 minutes. The compounding agent was added at a ratio of 10% (nano-tungsten carbide and polyethylene glycol in a ratio of 1:1), and the alumina layer thickness was 12.5 nm (reaction at 70°C for 3 hours, calcination at 650°C for 2 hours). Sintering: 15°C / min to 1650°C for 2 hours, then 7.5°C / min to 2100°C for 1.2 hours. Hydrofluoric acid cleaning was performed for 40 minutes.

[0036] Comparative Example 1

[0037] In this comparative example, the same as Example 1 is not repeated here, and the difference is as follows: No plasma activation was performed.

[0038] Comparative Example 2

[0039] In this comparative example, the same as Example 1 is not repeated here, and the difference is as follows: No carbon nanotubes were added.

[0040] Comparative Example 3

[0041] In this comparative example, the same as Example 1 is not repeated here, and the difference is as follows: The mass ratio of nano-tungsten carbide to polyethylene glycol in the compound is 1:2 (beyond the scope of the claims).

[0042] Comparative Example 4

[0043] In this comparative example, the same as Example 1 is not repeated here, and the difference is as follows: The sintering was carried out in a single stage at 1800 °C for 5 hours.

[0044] Comparative Example 5

[0045] In this comparative example, the same as Example 1 is not repeated here, and the difference is as follows:

[0046] Nano-tungsten carbide is not coated with an aluminum oxide layer.

[0047] Test results and analysis

[0048] The relative density and other indicators of the products of each embodiment and comparative example were analyzed and tested according to relevant standards. By comparing the data of the embodiments and comparative examples, it can be seen that the technical solution of the present invention has significant advantages in many aspects. The specific results are shown in Table 1.

[0049] Table 1 Analysis and test results

[0050] The data in Table 1 demonstrates that plasma activation treatment increases the density and hardness of the examples compared to the unactivated Comparative Example 1. For example, the relative density of Example 1 reaches 99.1%, while that of Comparative Example 1 is only 95.2%. This difference stems from the plasma's etching of the silicon powder surface, forming a Si-OC transition layer that promotes carbon diffusion and reduces the reaction activation energy. Furthermore, the grain size of Examples 1-3 (1.0-1.2 μm) is significantly smaller than that of Comparative Example 1 (2.5 μm), demonstrating that activation treatment effectively suppresses abnormal grain growth.

[0051] The flexural strength of Example 1 was 480 MPa, while that of Comparative Example 2, which did not include carbon nanotubes, was only 350 MPa. After acidification, the carbon nanotubes formed a three-dimensional conductive network, accelerating electron transport and reducing unreacted free silicon residue, lowering the impurity content to 120 ppm. In Comparative Example 3, the ratio of the compounding agent exceeded the range (nano-tungsten carbide to polyethylene glycol 1:2), resulting in volatilization of organic matter and formation of pores, leading to a decrease in density and strength. This validates the importance of the compounding agent ratio in the claims, and the addition of carbon nanotubes significantly improves the material's flexural strength.

[0052] Comparative Example 4, using a single-stage sintering process, achieved a grain size of 3.2 μm and a density of only 93.8%. In contrast, Example 1 achieved grain refinement (1.0 μm) and high density (99.1%) through a two-stage temperature control strategy (1800°C + 2200°C). The presence of the alumina coating (Example 2) maintained the flexural strength at 450 MPa, while the strength of Comparative Example 5, without the alumina coating, dropped to 290 MPa. This demonstrates that the alumina layer effectively buffers thermal stresses at high temperatures, suppressing microcracks and demonstrating the crucial importance of the gradient sintering process for the densification process.

[0053] Surface roughness data demonstrates that hydrofluoric acid ultrasonic cleaning and surfactant dispersion techniques effectively remove impurities, meeting the substrate finish requirements for semiconductor packaging. Process deficiencies in Comparative Examples 1 and 4 resulted in significantly increased surface roughness (Ra ≥ 0.12 μm), further demonstrating the necessity of post-processing steps.

[0054] This invention addresses the challenges of high energy consumption, numerous defects, and insufficient purity associated with traditional methods through the synergistic effects of plasma activation, optimized compounding agents, and gradient sintering. Comparison of the data from the examples with the comparative examples demonstrates the effectiveness of the technical solution, demonstrating particularly strong performance in key indicators such as density, hardness, and high-temperature strength, making it suitable for high-power semiconductor devices and precision machining.

[0055] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing silicon carbide, characterized in that: The method comprises the following steps: Pretreatment: Mix silicon powder and carbon source in a mass ratio of 1: (1.2-1.8), add anhydrous ethanol to disperse and then ball mill for 4-8 hours; Plasma activation: Place the pre-treated mixture in a plasma reactor, introduce argon at a flow rate of 50-200 sccm, and treat at a power of 300-600 W for 10-30 minutes to activate surface functional groups; Adding compounding agent: adding compounding agent to the activated mixture obtained by plasma activation, wherein the total amount of compounding agent added is 5-15% of the mass of silicon powder; Gradient sintering: Place the product after adding the compounding agent in a vacuum sintering furnace, raise the temperature to 1500-1800℃ at a heating rate of 10-20℃ / min, and keep it warm for 1-3 hours; then raise the temperature to 2000-2200℃ at a heating rate of 5-10℃ / min, keep it warm for 0.5-2 hours, and cool the furnace to room temperature; Post-processing: The sintered product is ultrasonically cleaned with hydrofluoric acid for 20-60 minutes and dried to obtain the finished silicon carbide product.

2. The method for preparing silicon carbide according to claim 1, wherein: In the pretreatment, the purity of the silicon powder is ≥99.5% and the particle size is 2-4 μm.

3. The method for preparing silicon carbide according to claim 1, wherein: The carbon source includes petroleum coke and graphite powder, and the mass ratio of the petroleum coke to the graphite powder is (3-5):

1.

4. The method for preparing silicon carbide according to claim 3, wherein: The carbon source further comprises carbon nanotubes, and the added amount of the carbon nanotubes is 0.1-1% of the total mass of the carbon source.

5. The method for preparing silicon carbide according to claim 4, wherein: The carbon nanotubes have a diameter of 10-50 nm and a length of 1-10 μm.

6. The method for preparing silicon carbide according to claim 4, wherein: The carbon nanotubes are acidified with a mixed solution for 1-3 hours and then washed with water until neutral. The mixed solution is concentrated sulfuric acid and concentrated nitric acid in a volume ratio of 3:

1. The carbon nanotubes have a diameter of 10-50 nm and a length of 1-10 μm.

7. The method for preparing silicon carbide according to claim 1, wherein: The compounding agent is composed of nano-tungsten carbide with a particle size of 50-100 nm and polyethylene glycol in a mass ratio of 1:(0.5-1.5).

8. The method for preparing silicon carbide according to claim 7, wherein: The surface of the nano-tungsten carbide is coated with an aluminum oxide layer, and the thickness of the aluminum oxide layer is 5-20 nm.

9. The method for preparing silicon carbide according to claim 8, wherein: The aluminum oxide layer is coated on the surface of the nano-tungsten carbide by the following steps: dispersing the nano-tungsten carbide in isopropyl alcohol, adding aluminum chloride and 5-15% ammonia water, stirring and reacting at 60-80° C. for 2-4 hours, and then calcining at 500-800° C. for 1-3 hours.

10. The method for preparing silicon carbide according to claim 1, wherein: The vacuum degree of the vacuum sintering furnace in the gradient sintering is ≤1×10 -3 Pa, and the sintering pressure is 0.5-2 MPa.

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