A process for producing silicon carbide

Abstract

This invention discloses a novel silicon carbide production process, comprising the following steps: S1: placing a carbon source and sodium chloride at the bottom of a synthesis reaction vessel to remove some impurities; S2: placing the pretreated silicon source at the bottom of a purification vessel and gradually heating it to 2250°C to remove low-boiling-point impurities; S3: introducing an inert gas into the bottom of the purification vessel in step S2 to further remove low-boiling-point impurities, then introducing it into a synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the carbon source in the reactor for 10-20 hours, yielding crude silicon carbide; S4: purifying the crude silicon carbide obtained in step S3 to achieve a purity >99.999%. This invention provides a simple and low-cost preparation method that reduces CO2 emissions and produces silicon carbide with higher purity (greater than 99.993%), suitable for use in electronic products and chips.

Description

Technical Field

[0001] This invention relates to the field of silicon carbide technology, and more specifically to a new silicon carbide production process. Background Technology

[0002] Silicon carbide (SiC) is an inorganic non-metallic material with extremely high hardness and good thermal and chemical stability. It belongs to the wide-bandgap semiconductor material category and has the following main applications:

[0003] 1. Abrasives and grinding materials: Due to its high hardness (Mohs hardness 9.5, second only to diamond) and strong wear resistance, silicon carbide is often used to manufacture grinding wheels, sandpaper, grinding paste, etc., and is suitable for precision grinding and cutting of hard materials such as metals, stone and ceramics.

[0004] 2. Refractory materials and ceramics: Silicon carbide has excellent high-temperature resistance and good thermal shock stability, making it an ideal material for manufacturing high-temperature refractory components such as high-temperature furnaces, kiln furniture, heat exchanger linings, and rocket nozzles.

[0005] 3. Metallurgical industry: Silicon carbide is used as a wear-resistant lining material in metallurgical processes such as mineral processing and steel manufacturing, such as pump rooms, impellers, wear-resistant pipes and hydrocyclones, and its wear resistance far exceeds that of rubber and cast iron.

[0006] 4. Electronic and Semiconductor Devices: As a semiconductor material, silicon carbide is used to manufacture electronic devices with high power, high frequency, high temperature and excellent radiation resistance, such as power electronic switches, high temperature sensors, light-emitting diodes (LEDs), etc.

[0007] 5. Aerospace: Due to its lightweight, high strength and high temperature resistance, silicon carbide composites are used to manufacture structural components, braking systems and heat protection systems for aircraft and spacecraft.

[0008] 6. Automotive Industry: In electric vehicles, silicon carbide-based semiconductor devices are used for power conversion and control, which can effectively improve energy efficiency and reduce energy consumption.

[0009] 7. Nuclear Energy: Silicon carbide has good radiation resistance and can be used as a structural and protective material for nuclear reactors.

[0010] Due to its wide range of applications, its synthesis methods have been extensively studied, mainly including the following methods:

[0011] 1. Acheson Process: This is the earliest and most traditional industrial method for synthesizing silicon carbide, invented by Edward G. Acheson in 1891. The method is carried out in a resistance furnace filled with silica sand (SiO2) as the silicon source, graphite as the carbon source, and a small amount of sawdust or sodium chloride (NaCl, used only in the production of green silicon carbide). At high temperatures (approximately 2000-2500°C), the silica sand is reduced to silicon, which then reacts with carbon to form silicon carbide. This process involves complex chemical reactions, including the reduction of silica sand, the oxidation of graphite, and the formation of silicon carbide. The final product is granular silicon carbide, which needs to be screened, crushed, and graded before use.

[0012] 2. Chemical Vapor Deposition (CVD): In the CVD process, silicon and carbon sources are introduced into a high-temperature reaction chamber in gaseous form, where a chemical reaction occurs on the substrate surface to directly deposit a silicon carbide thin film. This method allows for precise control of the film thickness and composition, making it suitable for manufacturing high-quality silicon carbide coatings or ultrathin layers in electronic devices.

[0013] 3. Physical vapor transport (PVT): Similar to the Acheson method, but with a greater focus on preparing high-purity and large single-crystal silicon carbide. In this method, silicon carbide raw material is sublimated into a gaseous state at high temperatures (above approximately 2000°C) within a sealed chamber. Then, under the influence of a temperature gradient, the gaseous material is transported and deposited on a seed crystal in a low-temperature region, gradually growing into single-crystal silicon carbide.

[0014] 4. Solution growth method: Utilizing the solubility properties of silicon carbide, silicon carbide raw materials are dissolved in a specific solvent, and then silicon carbide crystallizes out by slow cooling or adding seed crystals to form single crystals or polycrystalline materials of a specific shape.

[0015] The existing production process uses SiO2 raw material (99.3% purity) with carbon materials and salt to generate silicon carbide at high temperature. In the process of generating silicon carbide, carbon is also needed to replace the oxygen in silicon dioxide, which increases the amount of carbon materials used and also increases the amount of carbon dioxide emitted.

[0016] CN 113788480 B discloses a method for preparing high-purity silicon carbide, including the following steps: S1, placing high-purity carbon powder in a graphite crucible and purifying the high-purity carbon powder using a high-temperature vacuum method; S2, placing bulk silicon in the graphite crucible, heating to melt the bulk silicon, rotating the graphite crucible to mix the high-purity carbon powder and bulk silicon evenly, and reacting to synthesize high-purity silicon carbide. This application uses bulk silicon instead of silicon powder, and the purity of bulk silicon is much higher than that of silicon powder. After the bulk silicon melts into liquid silicon, high-speed rotation of the graphite crucible causes convection within the silicon solution, allowing the high-purity carbon powder to be evenly distributed in the silicon solution. After cooling, high-purity, high-synthesis-degree high-purity silicon carbide can be formed, with a purity generally higher than 99.999%. However, the preparation cost is too high. Summary of the Invention

[0017] To address the shortcomings of existing technologies, this invention provides a novel silicon carbide production process, comprising the following steps:

[0018] S1: Place the carbon source and sodium chloride at the bottom of the synthesis reaction vessel, introduce inert gas, preheat to 700~750℃, and remove some impurities again. The temperature in step S1 must be controlled below 800℃ to prevent sodium chloride from melting and blocking the active sites of the carbon source.

[0019] S2: Place the pretreated silicon source at the bottom of the purification container and gradually raise the temperature to 2250℃ in steps to remove low-boiling-point impurities. The temperature in step S2 must be controlled below 2300℃ to prevent silicon vapor from volatilizing during the impurity removal process and wasting raw materials.

[0020] S3: Inert gas is introduced into the bottom of the purification container in step S2, and the temperature is raised to 2350~2400℃. The inert gas carries the gaseous silicon from the purification container into the buffer tank. The temperature of the buffer tank is 2250~2350℃, which further removes low-boiling-point impurities. Then it enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the carbon source in the synthesis reactor to 1900~2200℃. At the same time, the synthesis reactor starts to heat up and maintains the reaction temperature at 1900~2200℃ for 10~20 hours to obtain crude silicon carbide. In step S3, the purity of the silicon source is further improved through the further purification in the buffer tank, so that high-purity silicon carbide can be synthesized.

[0021] S4: Cool the crude silicon carbide obtained in step S3 to 700~750℃, ultrasonically crush it, and then purify it again by introducing oxygen. After purification, cool it down, wash it with water, and dry it to obtain pure silicon carbide with a purity of >99.99%. Ultrasonic crushing in step S4 is necessary to remove excess carbon source and sodium chloride in subsequent purification steps.

[0022] Furthermore, in step S2, the silicon source is silicon sludge waste of about 2 micrometers produced by photovoltaics, with a silicon content of more than 80%. The pretreatment method of the silicon source is as follows: the silicon sludge waste is heated to 2200°C in an intermediate frequency furnace, and then slowly heated to 2350°C for 3 hours to melt it into blocks, remove impurities from the silicon sludge, and increase the content to more than 99.9%. Then, the block silicon material with a content of more than 99.9% is crushed into particles of 0~20mm. Only by slowly heating can the impurities in the silicon source be completely removed and the purity of the silicon source be improved.

[0023] Furthermore, in step S1, the carbon source is at least one of petroleum coke, anthracite, the bottom material in the distillation vessel after distilling the crude 2-ethylanthraquinone in the process of producing 2-ethylanthraquinone, and fullerene. When the carbon source is fullerene, the purity of the prepared product is the highest because silicon atoms can penetrate deep into the fullerene to react, the reaction is complete, and the purity of the prepared product is also higher.

[0024] Furthermore, in step S1, the mass ratio of carbon source to sodium chloride is 6~9:1, and in step S3, the molar ratio of carbon to silicon in the synthesis reaction is 1~1.05:1.

[0025] Furthermore, in step S1, when the carbon source is petroleum coke, anthracite, or the bottom material in the distillation vessel after distilling crude 2-ethylanthraquinone in the production process of 2-ethylanthraquinone, the carbon source needs to be pretreated. The pretreatment method includes the following steps:

[0026] S21: The carbon source is crushed, then washed with ethanol and water to obtain the washed carbon source;

[0027] S22: The washed carbon source obtained in step S21 is activated by soaking in a 3wt% sodium hydroxide solution for 2-3 hours, and then filtered and washed to obtain the alkaline-washed carbon source.

[0028] S23: The carbon source washed with alkali in step S22 is soaked in an acid solution at a temperature between 20 and 60°C for 3 hours, and then filtered, washed, neutralized and dried to obtain the pretreated carbon source.

[0029] Furthermore, in step S23, the acid solution contains 0.5 wt% hydrofluoric acid and 1 wt% hydrochloric acid.

[0030] Furthermore, the inert gas is either argon or helium. Helium is more effective because helium molecules are smaller and can provide a deeper inert environment.

[0031] Furthermore, in step S2, the heating process involves first heating to 750°C and holding for 2 hours, then heating to 1600°C and holding for 1 hour, and finally heating to 2250°C and holding for 3 hours. By distributing the temperature, impurities can be separated step by step, thereby improving the purity of the silicon source.

[0032] The preparation method of this invention is simple, low-cost, reduces CO2 emissions, and can produce silicon carbide with higher purity, with the prepared silicon carbide having a purity greater than 99.993%, which can be used in electronic products and chips and other fields.

[0033] Specific implementation party

[0034] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0036] Example 1

[0037] A new silicon carbide production process includes the following steps:

[0038] S1: Place 120g of petroleum coke and 18g of sodium chloride at the bottom of the synthesis reaction vessel, introduce argon gas, preheat to 700~750℃, and remove some impurities again.

[0039] The petroleum coke pretreatment method includes the following steps:

[0040] S21: Crush the petroleum coke, then wash it with ethanol and water to obtain washed petroleum coke;

[0041] S22: The washed petroleum coke obtained in step S21 is soaked and activated in a 3wt% sodium hydroxide solution for 2-3 hours, and then filtered and washed to obtain alkaline washed petroleum coke.

[0042] S23: The petroleum coke washed with alkali in step S22 is soaked in an acid solution at a temperature between 20 and 30°C for 3 hours, then filtered, washed, neutralized, and dried to obtain pretreated petroleum coke. The acid solution contains 0.5 wt% hydrofluoric acid and 1 wt% hydrochloric acid.

[0043] S2: Place 275g of pretreated silicon source at the bottom of the purification container and gradually heat it to 2250℃ in steps. The specific heating process is as follows: first heat to 750℃ and hold for 2 hours, then heat to 1600℃ and hold for 1 hour, and finally heat to 2250℃ and hold for 3 hours to remove low-boiling-point impurities. The silicon source before pretreatment is silicon sludge waste of about 2 microns generated by photovoltaics, with a silicon content of more than 80%. The pretreatment method of the silicon source is to heat the silicon sludge waste to 2200℃ in a medium frequency furnace, and then slowly heat it to 2350℃ for 3 hours to melt it into blocks, remove impurities in the silicon sludge, and increase the content to more than 99.9%. Then, the block silicon material with a content of more than 99.9% is crushed into particles of 0~20mm.

[0044] S3: Argon gas is introduced into the bottom of the purification container in step S2, and the temperature is raised to 2350~2400℃. The argon gas carries the gaseous silicon from the purification container into the buffer tank. The temperature of the buffer tank is 2250~2350℃, which further removes low-boiling point impurities. Then it enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the petroleum coke in the synthesis reactor to 1900~2000℃. At the same time, the synthesis reactor starts to heat up and maintains the reaction temperature at 1900~2000℃ for 15 hours to obtain crude silicon carbide.

[0045] S4: Cool the crude silicon carbide obtained in step S3 to 700~750℃, ultrasonically crush it, and then purify it again by introducing oxygen. After purification, cool it down, wash it with water, and dry it to obtain pure silicon carbide with a purity of 99.996%.

[0046] Example 2

[0047] A new silicon carbide production process includes the following steps:

[0048] S1: Place 120g of the bottom material from the distillation vessel after refining the crude 2-ethylanthraquinone (hereinafter referred to as the distillation bottom material) and 18g of sodium chloride at the bottom of the synthesis reaction vessel, introduce argon gas, preheat to 700~750℃, and remove some impurities again.

[0049] The pretreatment of the distillation feedstock includes the following steps:

[0050] S21: Crush the distillation bottom material, then wash it with ethanol and water to obtain the washed distillation bottom material;

[0051] S22: The washed distillation bottom material obtained in step S21 is soaked and activated in a 3wt% sodium hydroxide solution for 2-3 hours, and then filtered and washed to obtain the alkaline washed distillation bottom material;

[0052] S23: The distillation bottom material after alkaline washing in step S22 is soaked in an acid solution at a temperature between 40 and 50°C for 3 hours, then filtered, washed, neutralized, and dried to obtain the pretreated distillation bottom material. The acid solution contains 0.5 wt% hydrofluoric acid and 1 wt% hydrochloric acid.

[0053] S2: Place 275g of pretreated silicon source at the bottom of the purification container and gradually heat it to 2250℃ in steps. The specific heating process is as follows: first heat to 750℃ and hold for 2 hours, then heat to 1600℃ and hold for 1 hour, and finally heat to 2250℃ and hold for 3 hours to remove low-boiling-point impurities. The silicon source before pretreatment is silicon sludge waste of about 2 microns generated by photovoltaics, with a silicon content of more than 80%. The pretreatment method of the silicon source is to heat the silicon sludge waste to 2200℃ in a medium frequency furnace, and then slowly heat it to 2350℃ for 3 hours to melt it into blocks, remove impurities in the silicon sludge, and increase the content to more than 99.9%. Then, the block silicon material with a content of more than 99.9% is crushed into particles of 0~20mm.

[0054] S3: Helium gas is introduced into the bottom of the purification container in step S2, and the temperature is raised to 2350~2400℃. The helium gas carries the gaseous silicon from the purification container into the buffer tank, where the temperature is 2250~2350℃, to further remove low-boiling-point impurities. Then it enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the distillation bottom material in the synthesis reactor to 2100~2200℃. At the same time, the synthesis reactor starts heating and maintains the reaction temperature at 2100~2200℃ for 18 hours to obtain crude silicon carbide.

[0055] S4: Cool the crude silicon carbide obtained in step S3 to 700~750℃, ultrasonically crush it, and then purify it again by introducing oxygen. After purification, cool it down, wash it with water, and dry it to obtain pure silicon carbide with a purity of 99.999%.

[0056] Example 3

[0057] A new silicon carbide production process includes the following steps:

[0058] S1: Place 120g of fullerene and 18g of sodium chloride at the bottom of the synthesis reaction vessel, purge with argon gas, preheat to 700~750℃, and remove some impurities again.

[0059] The pretreatment of fullerene includes the following steps:

[0060] S21: Crush the fullerene, then wash it with ethanol and water to obtain the washed fullerene;

[0061] S22: The washed fullerene obtained in step S21 is activated by soaking in a 3wt% sodium hydroxide solution for 2-3 hours, and then filtered and washed to obtain alkaline-washed fullerene.

[0062] S23: The fullerene washed with alkali in step S22 is soaked in an acid solution at a temperature between 20 and 30°C for 3 hours, then filtered, washed, neutralized, and dried to obtain pretreated fullerene. The acid solution contains 0.5 wt% hydrofluoric acid and 1 wt% hydrochloric acid.

[0063] S2: Place 275g of pretreated silicon source at the bottom of the purification container and gradually heat it to 2250℃ in steps. The specific heating process is as follows: first heat to 750℃ and hold for 2 hours, then heat to 1600℃ and hold for 1 hour, and finally heat to 2250℃ and hold for 3 hours to remove low-boiling-point impurities. The silicon source before pretreatment is silicon sludge waste of about 2 microns generated by photovoltaics, with a silicon content of more than 80%. The pretreatment method of the silicon source is to heat the silicon sludge waste to 2200℃ in a medium frequency furnace, and then slowly heat it to 2350℃ for 3 hours to melt it into blocks, remove impurities in the silicon sludge, and increase the content to more than 99.9%. Then, the block silicon material with a content of more than 99.9% is crushed into particles of 0~20mm.

[0064] S3: Argon gas is introduced into the bottom of the purification container in step S2, and the temperature is raised to 2350~2400℃. The argon gas carries the gaseous silicon from the purification container into the buffer tank. The temperature of the buffer tank is 2250~2350℃ to further remove low-boiling-point impurities. Then it enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the fullerene in the synthesis reactor to 1900~2000℃. At the same time, the synthesis reactor starts to heat up and maintains the reaction temperature at 1900~2000℃ for 15 hours to obtain crude silicon carbide.

[0065] S4: Cool the crude silicon carbide obtained in step S3 to 700~750℃, ultrasonically crush it, and then purify it again by introducing oxygen. After purification, cool it down, wash it with water, and dry it to obtain pure silicon carbide with a purity of 99.9996%.

[0066] Example 4

[0067] A new silicon carbide production process includes the following steps:

[0068] S1: Place 120g of anthracite and 18g of sodium chloride at the bottom of the synthesis reaction vessel, introduce argon gas, preheat to 700~750℃, and remove some impurities again.

[0069] The anthracite pretreatment method includes the following steps:

[0070] S21: The anthracite is crushed, then washed with ethanol and water to obtain washed anthracite;

[0071] S22: The washed anthracite obtained in step S21 is soaked and activated in a 3wt% sodium hydroxide solution for 2-3 hours, then filtered and washed to obtain alkaline washed anthracite.

[0072] S23: The anthracite washed with alkali in step S22 is soaked in an acid solution at a temperature between 20 and 30°C for 3 hours, then filtered, washed, neutralized, and dried to obtain pretreated anthracite. The acid solution contains 0.5 wt% hydrofluoric acid and 1 wt% hydrochloric acid.

[0073] S2: Place 275g of pretreated silicon source at the bottom of the purification container and gradually heat it to 2250℃ in steps. The specific heating process is as follows: first heat to 750℃ and hold for 2 hours, then heat to 1600℃ and hold for 1 hour, and finally heat to 2250℃ and hold for 3 hours to remove low-boiling-point impurities. The silicon source before pretreatment is silicon sludge waste of about 2 microns generated by photovoltaics, with a silicon content of more than 80%. The pretreatment method of the silicon source is to heat the silicon sludge waste to 2200℃ in a medium frequency furnace, and then slowly heat it to 2350℃ for 3 hours to melt it into blocks, remove impurities in the silicon sludge, and increase the content to more than 99.9%. Then, the block silicon material with a content of more than 99.9% is crushed into particles of 0~20mm.

[0074] S3: Argon gas is introduced into the bottom of the purification container in step S2, and the temperature is raised to 2350~2400℃. The argon gas carries the gaseous silicon from the purification container into the buffer tank. The temperature of the buffer tank is 2250~2350℃, which further removes low-boiling-point impurities. Then it enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the anthracite in the synthesis reactor to 1900~2000℃. At the same time, the synthesis reactor starts to heat up and maintains the reaction temperature at 1900~2000℃ for 15 hours to obtain crude silicon carbide.

[0075] S4: Cool the crude silicon carbide obtained in step S3 to 700~750℃, ultrasonically crush it, and then purify it again by introducing oxygen. After purification, cool it down, wash it with water, and dry it to obtain pure silicon carbide with a purity of 99.994%.

[0076] Comparative Example 1

[0077] In Example 1, the petroleum coke in step S1 was used directly without pretreatment, and the rest was the same as in Example 1. The final silicon carbide produced had a purity of only 99.1%, which will not be elaborated further. This shows that the pretreatment of the carbon source has a significant technical effect.

[0078] Comparison 2

[0079] Step S2 of Example 1 is omitted, and the rest is the same as in Example 1. The purity of the final prepared silicon carbide is only 99.93%, which will not be described again. This shows that S2 is effective in improving the purity of the product.

[0080] Comparison 3

[0081] The ultrasonic crushing step S4 in Example 1 is omitted, and the rest is the same as in Example 1. The purity of the final silicon carbide is only 99.98%, which will not be repeated here. This shows that the ultrasonic crushing in step S4 is effective in improving the purity of the product.

Claims

1. A process for the production of silicon carbide, characterized in that, Includes the following steps: S1: Place the carbon source and sodium chloride at the bottom of the synthesis reaction vessel, introduce inert gas, preheat to 700~750℃, and remove some impurities again; S2: Place the pretreated silicon source at the bottom of the purification container and gradually heat it to 2250℃ in steps to remove low-boiling-point impurities. S3: Inert gas is introduced into the bottom of the purification container in step S2, and the temperature is raised to 2350~2400℃. The inert gas carries the gaseous silicon from the purification container into the buffer tank. The temperature of the buffer tank is 2250~2350℃, which further removes low-boiling-point impurities. Then it enters the synthesis reactor. The gaseous silicon entering the synthesis reactor gradually heats the temperature of the carbon source in the synthesis reactor to 1900~2200℃. At the same time, the synthesis reactor starts to heat up and maintains the reaction temperature at 1900~2200℃ for 10~20 hours to obtain crude silicon carbide. S4: Cool the crude silicon carbide obtained in step S3 to 700~750℃, ultrasonically crush it, and then purify it again by introducing oxygen. After purification, cool it down, wash it with water, and dry it to obtain pure silicon carbide with a purity >99.99%. In step S2, the silicon source is 2-micron silicon sludge waste generated by photovoltaics, with a silicon content greater than 80%. The pretreatment method of the silicon source is as follows: the silicon sludge waste is first heated to 2200℃ in an intermediate frequency furnace, and then slowly heated to 2350℃ for 3 hours to melt it into blocks, remove impurities from the silicon sludge, and increase the content to more than 99.9%. Then, the block silicon material with a content of more than 99.9% is crushed into particles of 0~20mm. In step S1, the carbon source is at least one of petroleum coke, anthracite, the bottom material in the distillation vessel after the crude 2-ethylanthraquinone is distilled in the process of producing 2-ethylanthraquinone, and fullerene.

2. The silicon carbide production process of claim 1, wherein, In step S1, the mass ratio of carbon source to sodium chloride is 6~9:1, and in step S3, the molar ratio of carbon to silicon in the synthesis reaction is 1~1.05:

1.

3. The silicon carbide production process of claim 1, wherein, In step S1, the carbon source is petroleum coke, anthracite, or the bottom material in the distillation kettle after distilling crude 2-ethylanthraquinone in the process of producing 2-ethylanthraquinone. The carbon source needs to be pretreated, and the pretreatment method includes the following steps: S21: The carbon source is crushed, then washed with ethanol and water to obtain the washed carbon source; S22: The washed carbon source obtained in step S21 is activated by soaking in a 3wt% sodium hydroxide solution for 2-3 hours, and then filtered and washed to obtain the alkaline-washed carbon source. S23: The carbon source washed with alkali in step S22 is soaked in an acid solution at a temperature between 20 and 60°C for 3 hours, and then filtered, washed, neutralized and dried to obtain the pretreated carbon source.

4. The silicon carbide production process of claim 3, wherein, In step S23, the acid solution contains 0.5 wt% hydrofluoric acid and 1 wt% hydrochloric acid.

5. The silicon carbide production process of claim 1 wherein, The inert gas is either argon or helium.

6. The silicon carbide production process of claim 1 wherein, The heating process in step S2 is as follows: first, the temperature is raised to 750°C and held for 2 hours; then, the temperature is raised to 1600°C and held for 1 hour; finally, the temperature is raised to 2250°C and held for 3 hours.