A method for loading a low-boron content reaction sintered silicon carbide
By coating carbon black and Si3N4 fine powder onto the surface of elemental silicon particles and using Si3N4-bonded SiC refractory products as load-bearing components, the problem of high boron content in RB-SiC was solved, and the preparation of low-boron RB-SiC materials was realized, thus improving the performance stability of the materials.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- SINOSTEEL LUOYANG INSTITUTE OF REFRACTORIES RESEARCH CO LTD
- Filing Date
- 2024-05-23
- Publication Date
- 2026-06-09
AI Technical Summary
In the current RB-SiC preparation process, the boron content is too high, which affects the consistency and reliability of material performance, especially in semiconductor and photovoltaic cell manufacturing, leading to unstable performance.
Carbon black and Si3N4 fine powder are coated on the surface of elemental silicon particles. Si3N4 combined with SiC refractory products are used as the load-bearing components. The use of BN coating containing boron is avoided. The boron content of RB-SiC is reduced by controlling the boron content in the raw materials.
It effectively reduces the boron content in RB-SiC materials, improves the material's performance consistency and reliability, and is suitable for high-temperature, high-strength, and highly corrosive environments.
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Figure CN118580082B_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of silicon carbide ceramic technology and relates to a furnace charging method for reaction-sintered silicon carbide with low boron content. Background Technology
[0002] Reaction-bonded silicon carbide ceramics (RB-SiC) are a type of engineering ceramic material with excellent properties and wide applications. The preparation process involves: first, preparing a porous green body containing silicon carbide and carbon; then, embedding the green body with silicon particles or powder; during high-temperature sintering, silicon diffuses into the porous green body in liquid or gaseous form, reacting with the carbon in the green body to generate new silicon carbide; the remaining pores are filled with silicon, ultimately forming a dense sintered body. This material possesses a series of excellent properties, including high strength, high hardness, wear resistance, corrosion resistance, good thermal shock stability, high thermal conductivity, low coefficient of thermal expansion, and strong oxidation resistance. It has been widely used in ceramics, photovoltaics, semiconductors, and machinery industries. Currently, my country ranks first in the world in both RB-SiC production and consumption.
[0003] The furnace charging method for RB-SiC is a crucial fundamental technology in the RB-SiC sintering process. Elemental silicon particles are required during charging, with silicon filling the area around the billet. During sintering, the silicon permeates into the billet. The amount, chemical composition, and surface treatment of the silicon particles significantly impact the impurity content and raw material cost of the RB-SiC material. The selection and surface treatment of the firing plate material are also critical. During high-temperature sintering, the silicon particles melt, and silicon permeates into the firing plate in liquid or gaseous form, potentially corroding the plate. Currently, most RB-SiC manufacturers use graphite materials coated with fine BN powder. The installation method of the firing plate and the treatment and use of silicon particles directly affect the RB-SiC firing efficiency, impurity content, and production cost.
[0004] In certain application areas, such as RB-SiC functional components used in semiconductor and photovoltaic cell manufacturing processes, there are strict requirements for impurity elements such as boron. Excessive boron content can affect the performance and reliability of the electronic devices it supports, and also lead to inconsistencies in the performance of reaction-sintered silicon carbide materials, resulting in differences in mechanical strength, conductivity, and thermal shock stability. This reduces the reliability of RB-SiC and limits its application in high-temperature, high-strength, and highly corrosive environments.
[0005] The main raw materials used in the preparation of RB-SiC are silicon carbide, silicon granules or silicon powder, carbon black, and graphite. These are all industrially synthesized raw materials, and the boron content in these raw materials is generally less than 0.01 wt%. The boron content in RB-SiC materials is mainly introduced by sintering. Industrial sintering furnace linings often use fine BN powder as a protective coating, silicon granules and silicon powder usually use fine BN powder as an additive, and the sintering components (plates, crucibles, saggers, etc., mainly made of graphite) usually use BN as a coating. During high-temperature sintering, the fine BN powder used, especially the BN additives in silicon granules and silicon powder, and the BN coating in the sintering plate, are in direct contact with the green body. During high-temperature sintering, the boron in BN will penetrate into the green body with the silicon solution or silicon vapor, resulting in the boron content in RB-SiC materials being much higher than the boron content in the raw materials.
[0006] Chinese patent "CN 116013822 A A Purification Method for High-Purity Silicon Carbide Crystal Boat, High-Purity Silica-Coated Silicon Carbide Crystal Boat and Its Production Process" mentions that the boron content in industrial RB-SiC materials is generally 0.1wt%-0.2wt% (1000-2000ppm). The technical approach to reducing the boron content in this patent is to subject the sintered RB-SiC crystal boat substrate to multiple subsequent processing steps, including high-temperature initial purification, halide treatment, high-temperature post-purification, and acid washing and soaking, thereby obtaining a high-purity RB-SiC crystal boat, where the boron content can be reduced from the initial 1000-2000ppm to 500ppm-600ppm. Chinese patent "CN 117700234 A" A method for preparing ultra-low boron content silicon carbide by reaction sintering discloses a method for preparing RB-SiC material with a boron content ≤80ppm. The dimensions of the sintering plate are: length 100-1000mm, width 100-600mm, and thickness 10-100mm. The material is one of SiC, BN, or graphite. The first coating of boron nitride on the sintering plate is 0.1-2mm thick, and the second and third coatings are 0.3-3mm thick. BN is not completely avoided in the coating of the sintering plate. The patent does not provide a chemical analysis report of RB-SiC. Chinese patent "CN 108706977 B" In the "A Method for Charging Reaction-Sintered Silicon Carbide", the elemental silicon particles were surface-treated, and carbon black and BN powder were uniformly attached to the surface of the elemental silicon particles. The ratio of silicon particles: carbon black: BN powder = (80-95): (1-3): (0.1-1) (mass ratio). The sintering plate used was an uncoated graphite plate. The patent did not give the B content in the prepared RB-SiC material. In the paper (He Guoxu et al. Study on the preparation of silicon carbide ceramics by reaction sintering and its properties [J]. Refractory Materials, 2022, 56(2): 146-149.), the silicon particles used for RB-SiC sintering were surface-treated with BN. The weight ratio of BN, carbon black and silicon particles was 1:100:2000. The paper did not give the chemical analysis data of B in RB-SiC. In the paper (Deng Mingjin et al. Influence of phenolic resin on the microstructure and properties of reaction-sintered silicon carbide [J]. Refractory Materials, 2008, 42(4): 267-270.), a SiC crucible with BN coating was used as the sintering component for RB-SiC sintering. The paper did not provide chemical analysis data of B in RB-SiC. In the paper (Wu Qide et al. Study on the preparation of reaction-sintered silicon carbide by silicon infiltration of pure carbon blank [J]. Journal of Wuhan University of Technology, 23(6): 1-3.), a graphite crucible with BN coating was used as the sintering component for RB-SiC sintering. The paper did not provide chemical analysis data of B in RB-SiC.In the paper (Wu Hongyan et al. Effect of sintering temperature on microstructure and properties of reaction-sintered silicon carbide [J]. Powder Metallurgy Technology, 2017, 35(5): 342-346.), BN-coated graphite plates were used for RB-SiC sintering, but no chemical analysis data of B in RB-SiC were given. Based on a comprehensive analysis of the comparative documents, some producers use BN as an additive for silicon raw materials when loading RB-SiC into the furnace, and BN is usually used as a coating material for the sintering components. There are almost no research reports on the effect of BN use on the B content in RB-SiC materials during sintering. If BN or other B-containing raw materials are not used during the furnace loading process, the B content in RB-SiC materials after sintering comes only from the raw materials used. Controlling the B content in the raw materials used can completely control the B content in the initial RB-SiC materials after sintering. Summary of the Invention
[0007] The purpose of this invention is to provide a furnace charging method for reaction-sintered silicon carbide with low boron content, which can solve the technical problem of high boron content in the existing RB-SiC preparation technology.
[0008] To achieve the above objectives, the present invention adopts the following technical solution:
[0009] A furnace charging method for low-boron-content reactive sintered silicon carbide, the specific steps of which are as follows:
[0010] 1) Coating treatment of the surface of elemental silicon particles: Carbon black and Si3N4 fine powder are uniformly attached to the surface of elemental silicon particles to obtain elemental silicon particles coated with carbon black and Si3N4. The coating treatment process of elemental silicon particles is as follows: elemental silicon particles, carbon black and Si3N4 fine powder are mixed evenly, an appropriate amount of liquid binder solution is added, mixed evenly, and then dried at a temperature of ≥100℃ to make the carbon black and Si3N4 fine powder firmly adhere to the surface of elemental Si particles, forming elemental silicon particles with a coating layer.
[0011] 2) The supporting components for the sintering of the green body are Si3N4 combined with SiC refractory products. The supporting components are selected according to the shape of the green body, such as flat plates, crucibles, saggers, hollow rings and other refractory products.
[0012] 3) The billet is buried with elemental silicon particles with a coating layer. For flat billets, the elemental silicon particles are laid on top of the billet. For large-sized annular billets, the elemental silicon particles are filled in the space between the hollow annular support component and the inner or outer side of the billet.
[0013] The assembly consisting of a blank, a Si3N4 bonded silicon carbide support component, and coated silicon particles is loaded into or moved into a vacuum furnace for high-temperature sintering. After cooling, the support component is removed and the silicon particle residue is cleaned to obtain a low-boron content reaction sintered silicon carbide material.
[0014] During furnace loading, based on the shape of the billet, a Si3N4-bonded SiC firing plate is first selected, and the billet is placed on the firing plate. Coated Si particles are then filled on top of or around the product. For flat RB-SiC products, a Si3N4-bonded SiC flat plate, larger in both length and width than the RB-SiC product, is used as the firing plate. The billet is placed flat on the firing plate, and then the coated silicon particles are laid on top of the billet. For large-sized annular billets, a Si3N4-bonded SiC flat plate is selected, and the billet is placed on the plate. A cylindrical or hollow annular Si3N4-bonded SiC product with an outer diameter smaller than the inner diameter of the billet is placed inside the inner hole of the billet. Coated silicon particles are then filled into the space between the outer wall of the Si3N4-bonded SiC product and the inner hole wall of the billet. For large-sized annular blanks, after the blank is placed on a Si3N4-bonded SiC plate, a hollow annular Si3N4-bonded SiC product with an outer diameter larger than that of the blank can also be placed around the blank. Then, silicon particles with a coating layer are filled in the space between the inner side of the Si3N4-bonded SiC product and the outer wall of the blank.
[0015] During the surface coating treatment of elemental silicon particles, the weight ratio of Si3N4 fine powder in elemental silicon particles, carbon black, and Si3N4 fine powder is not less than 0.2wt%, and the optimal weight ratio of Si3N4 is 0.3wt%; the average particle size of Si3N4 fine powder is less than 50µm, and the optimal particle size range is 0.5-10µm.
[0016] The Si3N4-bonded SiC refractory product serves as a load-bearing component, wherein Si3N4 ≥ 12wt% and Fe2O3 ≤ 0.1wt%.
[0017] The weight of the coated silicon particles used for filling should not be less than 0.5 times the weight of the billet, and the optimal dosage range is 0.6-0.7 times the weight of the billet.
[0018] The liquid binder is one of polyvinyl alcohol (PVA), hydroxymethyl cellulose (CMC), or dextrin solution, and its function is to adhere carbon black and Si3N4 fine powder together to the surface of elemental Si particles; the drying treatment is to evaporate the water in the organic binder solution, so that carbon black and Si3N4 fine powder are firmly adhered to the surface of elemental Si particles.
[0019] Si3N4-bonded SiC refractories are a technologically mature and historically used non-oxide composite shaped refractory material with Si3N4 as the main bonding phase. This refractory material has high temperature resistance, high high temperature strength, and good high temperature stability. Because the bonding phase of this material is Si3N4, it is difficult for liquid or gaseous Si to penetrate into the Si3N4-bonded SiC refractories during high-temperature sintering. When used as a support component, Si3N4-bonded SiC refractories will not introduce boron and can be reused repeatedly.
[0020] In this method, the Si3N4 content in the Si3N4-bonded SiC refractory material is required to be ≥12wt%.
[0021] The Si3N4-combined SiC refractory material used as a load-bearing component can be shaped into different products such as flat plates, crucibles, saggers, and hollow rings, depending on the shape of the billet.
[0022] This invention proposes a furnace charging method for low-boron-content reaction-sintered silicon carbide. Compared with the prior art, this invention does not introduce B-containing fine powders such as BN onto the surface of silicon particles, nor does it use B-containing coatings such as BN on the surface of the sintering components. This avoids the possibility of introducing B-containing substances into the silicon particles and sintering components that come into contact with the green body. The B content in the RB-SiC material obtained after green body sintering depends on the B content in the green body. By controlling the B content in the raw materials SiC, carbon black, graphite, and Si powder used, the B content in the green body can be completely controlled, thereby obtaining a low-B-content reaction-sintered SiC material. Attached Figure Description
[0023] Figure 1 A schematic diagram of the cross-section of a plate-shaped reactive sintered silicon carbide product loaded into the furnace.
[0024] Figure 2 A schematic diagram of the cross-section of a hollow, bottom-shaped, annular reactive sintered silicon carbide product loaded into the furnace.
[0025] Figure 3 A schematic diagram of the cross-section of a circular reaction-sintered silicon carbide crucible product loaded into the furnace.
[0026] In the figure: 1 is the blank, 2 is the elemental Si particle with a coating layer, 3 is the Si3N4 bonded SiC plate, and 4 is the Si3N4 bonded SiC cylinder. Detailed Implementation
[0027] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. Example 1
[0028] Purchase high-purity elemental silicon particles (particle size 2-5mm, Si content 99.9wt%, B element content 0.005wt%), carbon black (average particle size 0.05µm, C content 99.1wt%, B element content 0.004%), and Si3N4 fine powder (average particle size 5µm, Si3N4 content 99.0wt%, Fe2O3 content 0.02wt%, B element content 0.006wt%). Weigh these three raw materials according to the weight ratio of elemental silicon particles: carbon black: Si3N4 fine powder = 87:2.8:0.2. Prepare an 8wt% concentration PVA aqueous solution. Mix the elemental silicon particles, carbon black, and Si3N4 evenly, add the PVA aqueous solution, stir evenly, and dry at 120℃ for 6 hours. After cooling, obtain elemental silicon particles with a coating layer. High-purity SiC fine powder (0.1-0.3 mm, SiC content 99.6 wt%, B content 0.008 wt%), high-purity SiC micro powder (average particle size 3 µm, SiC content 99.4 wt%, B content 0.005 wt%), and carbon black (average particle size 0.05 µm, C content 99.1 wt%, B element content 0.004 wt%) were mixed in a SiC fine powder: SiC micro powder: carbon black ratio of 50:45:5 (by weight). A 5% PVA solution was added, and the mixture was pressed into a flat blank of 200 × 200 × 6 mm. The blank was then dried at 130℃ for 6 hours. The density of the dried blank was 2.4 g•cm³. -3 A Si3N4-bonded SiC plate was purchased from the market. The Si3N4-bonded SiC plate measures 250×250×10mm, and the bulk density of the material is 2.68g•cm³. -3 The SiC content is 87.0 wt%, the Si3N4 content is 12 wt%, the Fe2O3 content is 0.01 wt%, and the B content is 0.008 wt%. (According to...) Figure 1 Furnace loading method: The billet is placed on a Si3N4-bonded SiC plate, and coated elemental silicon particles are laid on top of the billet. The billet weighs 576g, and the amount of coated elemental silicon particles is 288g. The loaded plate is transferred into an electrically heated sintering furnace, the furnace door is sealed, a vacuum is drawn, the temperature is raised to 1680℃, and held for 2 hours. After holding, it is cooled under a nitrogen atmosphere to obtain the RB-SiC product. Physicochemical analysis of the obtained material shows that its bulk density is 3.03 g•cm³. -3 The room temperature flexural strength is 300 MPa, the SiC content is 88.2 wt%, the Si content is 11.7 wt%, and the B content is 62 ppm. Example 2
[0029] Purchase high-purity elemental silicon particles (particle size 3-4 mm, Si content 99.9 wt%, B element content 0.012 wt%), carbon black (average particle size 0.05 µm, C content 99.1%, B element content 0.005%), and Si3N4 fine powder (average particle size 5 µm, Si3N4 content 99.5 wt%, Fe2O3 content 0.01 wt%, B element content 0.005%). Weigh these three raw materials according to the weight ratio of elemental silicon particles: carbon black: Si3N4 fine powder = 86:35:0.5. Prepare a 5% concentration PVA aqueous solution. Mix the elemental silicon particles, carbon black, and Si3N4 evenly, add the PVA aqueous solution, stir evenly, and dry at 110℃ for 10 hours. After cooling, obtain elemental silicon particles with a coating layer. High-purity SiC fine powder (0.1-0.3 mm, SiC content 99.5%, B content 0.010%), high-purity SiC micro powder (average particle size 3.5 µm, SiC content 99.3%, B content 0.02%), carbon black (average particle size 0.05 µm, C content 99.0%, B element content 0.008%), and graphite fine powder (average particle size 15 µm, C content 99.0%, B element content 0.01%) were mixed in a weight ratio of SiC fine powder: SiC micro powder: carbon black: graphite = 60:32:5:3. A 6% PVA solution was added, and a hollow ring-shaped blank with an inner diameter of 1000 mm, a wall thickness of 25 mm, and a height of 920 mm was prepared by isostatic pressing. The blank was dried at 120℃ for 12 hours, and the density of the dried blank was 2.26 g•cm³. -3 The billet was cold-worked to obtain a hollow annular billet with precise dimensions of 1000 mm inner diameter, 18 mm wall thickness, and 900 mm height; a Si3N4-bonded SiC plate was purchased, with dimensions of 1100 × 1100 × 20 mm and a material bulk density of 2.60 g•cm³. -3 The composition of the product is as follows: SiC content 75.0 wt%, Si3N4 content 24.5 wt%, Fe2O3 content 0.01 wt%, and B content 0.015 wt%. Additionally, one hollow cylindrical product combining Si3N4 and SiC with an outer diameter of 800 mm, a wall thickness of 25 mm, and a height of 900 mm is purchased. The product's bulk density is 2.58 g·cm³. -3 The content of SiC is 74.0 wt%, Si3N4 is 25.2 wt%, Fe2O3 is 0.01 wt%, and B is 0.01 wt%. Figure 2Furnace loading method: A Si3N4-bonded SiC plate is placed in an electrically heated sintering furnace. The billet is placed on the Si3N4-bonded SiC plate, and a hollow Si3N4-bonded SiC cylinder is placed in the center of the inner hole of the billet. Elemental silicon particles with a coating layer are filled between the inner wall of the billet and the outer surface of the hollow Si3N4-bonded SiC cylinder. The billet weighs 117 kg, and the amount of coated elemental silicon particles is 70.2 kg. The furnace door is sealed, a vacuum is drawn, the temperature is raised to 1550℃, and held for 5 hours. After holding, the material is cooled under an argon atmosphere to obtain the RB-SiC product. Physicochemical analysis of the obtained material shows a bulk density of 2.98 g·cm³. -3 The room temperature flexural strength is 250 MPa, the SiC content is 71.1 wt%, the Si content is 28.8 wt%, and the B content is 150 ppm. Example 3
[0030] Purchase high-purity elemental silicon particles (particle size 3-5mm, Si content 99.5wt%, B element content 0.016wt%), carbon black (average particle size 0.06µm, C content 99.2%, B element content 0.018%), and Si3N4 fine powder (average particle size 5µm, Si3N4 content 99.5wt%, Fe2O3 content 0.01wt%, B element content 0.01wt%). Weigh these three raw materials according to the weight ratio of elemental silicon particles: carbon black: Si3N4 fine powder = 85:4:1. Prepare a 6wt% CMC aqueous solution. Mix the elemental silicon particles, carbon black, and Si3N4 evenly, add the CMC aqueous solution, stir evenly, and dry at 120℃ for 8 hours. After cooling, obtain elemental silicon particles with a coating layer. High-purity SiC fine powder (0.1-0.4 mm, SiC content 99.5 wt%, B content 0.015 wt%), high-purity SiC micro powder (average particle size 2.5 µm, SiC content 99.2 wt%, B content 0.018 wt%), carbon black (average particle size 0.035 µm, C content 99.2 wt%, B element content 0.009 wt%), and graphite fine powder (average particle size 20 µm, C content 99.2 wt%, B element content 0.012 wt%) were mixed in a SiC fine powder: SiC micro powder: carbon black: graphite ratio of 50:40:5:5 (weight ratio). An 8 wt% PVA solution was added, and a hollow cylindrical blank with an outer diameter of 600 mm, a wall thickness of 25 mm, and a height of 1550 mm was prepared by isostatic pressing. The blank was dried at 130℃ for 10 hours, and the density of the dried blank was 2.20 g•cm³. -3 The billet was cold-worked to obtain a hollow cylindrical billet with precise dimensions of 600 mm outer diameter, 20 mm wall thickness, and 1500 mm height. One Si3N4-bonded SiC plate was purchased, with dimensions of 850 × 850 × 25 mm and a bulk density of 2.65 g·cm³.-3 The composition of the product is as follows: SiC content 84.0 wt%, Si3N4 content 30 wt%, Fe2O3 content 0.01 wt%, and B content 0.012 wt%. Additionally, purchase one hollow cylindrical product made of Si3N4 combined with SiC, with an inner diameter of 800 mm, a wall thickness of 20 mm, and a height of 1500 mm. The product's bulk density is 2.55 g·cm³. -3 (SiC content 78.0 wt%, Si3N4 content 21.2 wt%, Fe2O3 content 0.01 wt%, B content 0.01 wt%); according to Figure 3 Furnace loading method: A Si3N4-bonded SiC plate is placed in an electrically heated sintering furnace. The billet is placed on the Si3N4-bonded SiC plate, and a hollow Si3N4-bonded SiC cylinder is placed inside the billet. Elemental silicon particles with a coating layer are filled between the inner wall of the Si3N4-bonded SiC cylinder and the outer wall of the billet. The billet weighs 170 kg, and the amount of coated elemental silicon particles is 119 kg. The furnace door is sealed, a vacuum is drawn, the temperature is raised to 1600℃, and held for 4 hours. After holding, the material is cooled under a nitrogen atmosphere to obtain the RB-SiC product. Physicochemical analysis of the obtained material shows a bulk density of 2.95 g·cm³. -3 The room temperature flexural strength is 230 MPa, the SiC content is 79.0 wt%, the Si content is 29.8 wt%, and the B content is 180 ppm.
Claims
1. A furnace charging method for low-boron-content reaction-sintered silicon carbide, characterized in that: 1) Coating treatment of elemental silicon particles: Carbon black and Si3N4 fine powder are uniformly attached to the surface of elemental silicon particles to obtain silicon particles coated with carbon black and Si3N4. The silicon particle coating treatment process is as follows: elemental silicon particles, carbon black, and Si3N4 fine powder are mixed evenly, an appropriate amount of liquid binder solution is added, mixed evenly, and then dried to make the carbon black and Si3N4 fine powder firmly adhere to the surface of elemental silicon particles, forming elemental silicon particles with a coating layer. During the surface coating treatment of elemental silicon particles, the weight ratio of Si3N4 fine powder in elemental silicon particles, carbon black, and Si3N4 fine powder is not less than 0.2wt%, and the average particle size of Si3N4 fine powder is less than 50µm. 2) The supporting components for the sintering of the green body are Si3N4 combined with SiC refractory products. The supporting components are selected according to the shape of the green body, including refractory products of different shapes such as flat plates, crucibles, saggers, and hollow rings. 3) The billet is buried with elemental silicon particles with a coating layer. For flat billets, the elemental silicon particles are laid on top of the billet. For large-sized annular billets, the elemental silicon particles are filled in the space between the hollow annular support component and the inner or outer side of the billet.
2. The furnace charging method for low-boron-content reactive sintered silicon carbide according to claim 1, characterized in that: The weight ratio of Si3N4 is 0.3wt%; the particle size range of Si3N4 fine powder is 0.5-10µm.
3. The furnace charging method for low-boron-content reactive sintered silicon carbide according to claim 1, characterized in that: The Si3N4-bonded SiC refractory product serves as a load-bearing component, wherein Si3N4 ≥ 12wt% and Fe2O3 ≤ 0.1wt%.
4. The furnace charging method for low-boron-content reactive sintered silicon carbide according to claim 1, characterized in that: The weight of the coated elemental silicon particles used for filling shall not be less than 0.5 times the weight of the blank.
5. The furnace charging method for low-boron-content reactive sintered silicon carbide according to claim 4, characterized in that: The amount of coated elemental silicon particles used for filling ranges from 0.6 to 0.7 times the weight of the blank.