A method for preparing high-purity silicon carbide powder

Abstract

The application discloses a preparation method of high-purity silicon carbide powder, which comprises the following steps: mixing carbon source raw materials and silicon source raw materials, then directly synthesizing the silicon carbide powder by microwave heating solid-phase reaction, cooling the silicon carbide powder, crushing the powder, purifying the powder, and then treating the high-purity silicon carbide powder; a step of continuously rotating the mixed raw materials is arranged in the microwave heating process; the molar ratio of the carbon source raw materials to the silicon source raw materials is 0.8:1-1.1:1; the microwave heating process is that a 300kW tunnel microwave oven is used to microwave heat the mixed carbon source raw materials and silicon source raw materials; the microwave frequency is 915MHz, and an argon protective atmosphere is used during the microwave heating. The application dynamically changes the field distribution, ensures uniform microwave field distribution, ensures that the reaction materials are uniformly heated, and provides a good balanced thermal field for the sample.

Description

Technical Field

[0001] This invention relates to advanced inorganic non-metallic materials, prepared using silicon, and specifically to a method for preparing high-purity silicon carbide powder. Background Technology

[0002] Silicon carbide (SiC) ceramics possess excellent mechanical properties, oxidation resistance, wear resistance, thermal stability, thermal shock resistance, and chemical corrosion resistance, along with a low coefficient of thermal expansion and high thermal conductivity. Their excellent high-temperature strength, wear and corrosion resistance, and thermal shock resistance have led to their increasingly widespread application and their growing importance in the materials field. Therefore, further research into methods for preparing SiC materials is urgently needed to continuously improve their superior properties while reducing production costs, simplifying production processes, and promoting the industrialization of SiC ceramic products. Hao Bin's paper, "Research on the Preparation of Silicon Carbide Powder by Microwave Sintering," published in the March 2015 issue of *Bulletin of the Chinese Ceramic Society* (Vol. 34, No. 3), mentions that the current industrial production method for SiC powder involves multiple mechanical crushings of SiC blocks, particle classification via air separation or water flotation, followed by chemical purification. This process consumes a large amount of energy, and the resulting SiC particles are of poor quality, making them unsuitable for precision machining. Both current industrial production technologies and new synthesis methods suffer from low heating efficiency and high energy consumption in the synthesis of SiC micron powder. Therefore, the primary issue to address is reducing energy consumption during the heating process. One solution is to change the heating method and adopt high-efficiency heating technology to achieve the reaction. Secondly, it's crucial to rationally select reactants, refine raw material particles, lower the reaction temperature, and accelerate the reaction rate. The method of directly heating acetylene black and silicon powder to obtain silicon carbide powder is a direct synthesis method, a solid-state reaction method. However, in large-scale microwave heating for silicon carbide (SiC) powder production, uneven microwave field distribution can lead to uneven heating of the reactants, resulting in problems such as localized overheating or incomplete reaction. Existing technologies typically employ multi-mode cavity designs to optimize the microwave field distribution to address this issue. However, optimizing the cavity shape, structure, and dimensions requires calculation and design, increasing the workload of early-stage design and development. Summary of the Invention

[0003] The purpose of this invention is to overcome the deficiencies in the existing technology and provide a method for preparing high-purity silicon carbide powder, which dynamically changes the field distribution and optimizes the microwave feeding method. The rotating sample support system can dynamically change the field distribution, ensuring uniform microwave field distribution and uniform heating of the reactants. Combined with its three-layer insulation structure, it can also provide a good and balanced thermal field for the sample.

[0004] To achieve the above objectives, the technical solution of this invention is to design a method for preparing high-purity silicon carbide powder, comprising the following preparation steps: mixing carbon source raw materials and silicon source raw materials, and then directly synthesizing silicon carbide powder by microwave heating solid-state reaction; after obtaining the silicon carbide powder, cooling, crushing, and purifying the powder to obtain high-purity silicon carbide powder; the aforementioned microwave heating process also includes a step of continuously rotating the mixed raw materials; the molar ratio of carbon source raw materials to silicon source raw materials is 0.8:1 to 1.1:1; the microwave heating process is as follows: using a 300kW tunnel microwave oven to microwave heat the mixed carbon source raw materials and silicon source raw materials; wherein, the microwave frequency is 915MHz, and an argon protective atmosphere is used during microwave heating.

[0005] The field distribution is dynamically changed, optimizing the microwave feeding method. According to existing technology, "Research on Microwave Sintering for the Preparation of Silicon Carbide Powder," the sample needs a well-balanced thermal field to eliminate the temperature gradient between the center and surface of the sample. Therefore, the rotating sample-bearing system can dynamically change the field distribution, ensuring uniform microwave field distribution and uniform heating of the reactants. Combined with its three-layer insulation structure, it can also provide a well-balanced thermal field for the sample.

[0006] A further technical solution is that the carbon source material is graphite powder, carbon black, or activated carbon; and the silicon source material is silicon powder or silica.

[0007] A further technical solution is that the mixing process is as follows: the carbon source material and the silicon source material are mixed and then placed in the sample support system; the step of continuously rotating the mixed material is as follows: the sample support system is rotated by a rotation drive mechanism in the tunnel microwave oven.

[0008] A further technical solution is that the sample support system consists of an outer insulation layer made of alumina bricks, an intermediate insulation layer made of high-alumina fibers, an inner insulation layer made of alumina bricks, and an alumina crucible on the inner insulation layer, with an opening on the inner insulation layer that is compatible with the alumina crucible.

[0009] A further technical solution is that the rotary drive mechanism is a geared motor fixedly connected to the conveyor belt inside the tunnel microwave oven, and the output shaft of the geared motor is fixedly connected to the outer insulation layer.

[0010] Another technical solution is that the rotary drive mechanism is a geared motor fixedly connected to the conveyor belt inside the tunnel microwave oven; a pin is fixedly installed under the outer insulation layer, and a rectangular hole is provided on the pin. The output shaft of the geared motor is fixedly connected to a protrusion that matches the aforementioned rectangular hole.

[0011] The rotating sample carrier system (sample carrier and pin integrated) can be detachably connected and inserted into different microwave devices, improving adaptability and application range.

[0012] A further technical solution is that the tunnel of the tunnel-type microwave oven is 15-20 meters long; the post-purification treatment includes the following steps:

[0013] Acid washing and purification: The carbon-removed powder is heated and stirred in a polytetrafluoroethylene container with a mixture of HCl and HF to remove metal impurities and SiO2; it is then repeatedly centrifuged and washed with high-purity water until the conductivity of the supernatant is close to that of high-purity water.

[0014] Alkaline washing to remove silicon: The acid-washed powder is washed with deionized water and then placed in a boiling and stirred alkaline solution to react for 1-4 hours to remove residual free silicon.

[0015] Decarbonization calcination: Place the pulverized powder in a crucible, put the crucible into a box-type muffle furnace or bell furnace, and calcine at 650~700℃ for 1~2 hours in a circulating air atmosphere;

[0016] Washing, drying and classification: The powder is repeatedly washed with deionized water until the filtrate is neutral to ensure that there are no chloride or fluoride ions remaining; the washed slurry is filtered and dried by vacuum drying oven or spray dryer to prevent agglomeration; finally, high-purity SiC powder with uniform particle size distribution is obtained by air classification or sieving.

[0017] A further technical solution is that, in the acid washing and purification step, the volume ratio of HCl:HF:H2O in the mixed acid is (2~4):1:(5~7); the heating temperature is 40~60℃; the stirring time is 2~4 hours; and the stirring speed is 100~400r / min.

[0018] In the alkaline washing and desiliconization step, the alkaline solution is sodium hydroxide with a mass concentration of 20%~40%; the solid-liquid ratio of powder to alkaline solution is 1:5~1:10; and the stirring speed is 200~400 r / min.

[0019] In the water washing step, the solid-liquid ratio is 1:5 to 1:10; the stirring speed is 200 to 400 r / min; and the stirring time is 15 to 30 minutes each time.

[0020] During the filtration and drying step, the vacuum degree is -0.08 ~ -0.095 MPa; drying continues until there are no obvious water marks on the surface of the filter cake and it forms a cracked appearance. The drying time is 30 ~ 60 minutes, and the powder moisture content is reduced to 15% ~ 25%.

[0021] In the drying step, a core-type or pressure-type spray dryer is used for spray drying to obtain spherical agglomerates with good flowability; the dried slurry is then re-dispersed and prepared into a uniform slurry with a solid content of 20% to 30%; the inlet temperature is 200 to 250°C and the outlet temperature is 90 to 110°C.

[0022] In the air classification step, an air classifier and an air classifier are used to classify the dried powder by air classification; the feeding speed is 5 ~ 20 kg / h; the classifier wheel speed is 3000 ~ 6000 r / min; and the air pressure is 0.6 ~ 0.8 MPa.

[0023] The sample support system consists of a porous graphite crucible and a rotating shaft connected to the porous graphite crucible; the porous graphite crucible is rotatably connected to the rotating shaft.

[0024] The rotary drive mechanism consists of a geared motor fixedly connected to the conveyor belt inside the tunnel microwave oven, a turntable fixedly connected to the output shaft of the geared motor, a connecting rod fixedly mounted on the turntable, a blind-hole-shaped insertion hole on the connecting rod that is adapted to the rotating shaft, and several argon gas pipes spaced apart on the top wall inside the tunnel microwave oven; the connecting rod is inclined to the counterweight line and there is a gap between the connecting rod and the center of the turntable.

[0025] The conveyor belt is a quartz fiber woven mesh belt or a silicon carbide fiber woven mesh belt.

[0026] The sample support system consists of a porous graphite crucible and a rotating shaft connected to the porous graphite crucible; the porous graphite crucible is rotatably connected to the rotating shaft and is spherical.

[0027] The rotary drive mechanism consists of several trays spaced apart on the surface of the conveyor belt inside the tunnel microwave oven, connecting rods fixedly connected to the trays, pointed blocks fixedly installed on the upper surface of the conveyor belt as fulcrums for the trays, and wedge structures on the frame along one side of the conveyor belt's length. The number of wedge structures is the same as the number of trays. Each wedge structure consists of a wedge-shaped block fixedly connected to the top of the frame and a sponge layer fixedly connected to the upper surface of the wedge block. The horizontal plane at the highest point of the wedge block is flush with the horizontal plane at the highest point of the pointed block. The center of the lower surface of the tray is hinged to the upper end of the pointed block, and the rotating shaft is hinged to the connecting rod, which is located at the center of the tray. The dimension of the tray along the width of the conveyor belt is larger than the width of the conveyor belt, and the wedge structures are located outside the conveyor belt but do not extend beyond the side of the tray along the width of the conveyor belt.

[0028] The porous graphite crucible has an opening at its top for adding raw material mixtures. A porous ceramic lid, fitted to the opening, covers the top of the crucible. The lid has 0.5mm pores, allowing argon gas to permeate but blocking powder, ensuring an argon atmosphere during the reaction while preventing the argon gas flow from blowing the raw materials inside the crucible away. The lid is spherical in shape. The porous ceramic lid and the porous graphite crucible are secured together by a high-temperature spring and bolts (the spring material can be molybdenum or tungsten alloy; that is, after the porous ceramic lid is placed over the opening at the top of the crucible, it is connected to the crucible by bolts; and a high-temperature spring is fitted onto the bolts, which both connects the lid to the crucible and presses the lid to ensure a complete seal between the lid and the top opening of the crucible). This ensures a tight seal under high temperature and rotation conditions.

[0029] When the wind is strong enough (argon flow velocity 1~5m) 3 A flow rate of 10~20m / h is insufficient to drive the crucible to rotate. 3 The sample-bearing system, driven by a rotating airflow (which can be driven by a rotating airflow), rotates around the connecting rod (simultaneously, it also allows the crucible to rotate around the axis, enabling the crucible to rotate around both the connecting rod and the axis even at high argon flow rates). When the airflow is low, the sample-bearing system is positioned on a pallet, causing one end of the pallet to droop. Combined with the conveyor belt movement, the pallet passes over a wedge structure. This ensures that regardless of which end of the pallet droops, the drooping end is always lifted by the wedge structure, thus disrupting the pallet's balance. The sample-bearing system then rotates around the connecting rod, allowing the crucible to rotate around both the connecting rod and the axis even at low argon flow rates. This rotating sample-bearing system effectively rotates 360°, essentially operating without a fixed axis of rotation. This rotation method greatly satisfies the need for dynamically changing the field distribution, ensuring the uniformity of the microwave field distribution, guaranteeing uniform heating of the reactants, and providing a well-balanced thermal field for the sample.

[0030] Argon variable flow rate gas supply is based on periodic process requirements: the flow rate is increased to flush out residual air during material batch intervals, and the flow rate is reduced during processing. This is combined with the structural design of the sample carrying system and the rotary drive mechanism to achieve 360° rotation of the sample carrying system. This greatly satisfies the need to dynamically change the field distribution, ensure the uniformity of the microwave field distribution, ensure uniform heating of the reactants, and provide a good and balanced thermal field for the sample.

[0031] In the acid washing and purification process, the acid washing equipment consists of a polytetrafluoroethylene tank, a PTFE stirring blade installed inside the tank, a flue pipe connected to the tank, a metal jacket installed outside the tank, and a geared motor fixedly installed outside the tank; the output shaft of the geared motor passes through the tank and is connected to the stirring blade.

[0032] The exhaust pipe outlet is located inside a high-efficiency fume hood; the stirring speed of the agitator blades is 100~300 r / min.

[0033] The exhaust pipe is a PTFE hose, which is connected to the tank via a PTFE flange.

[0034] The exhaust pipe consists of a section of PTFE corrugated pipe, PTFE flanges connected to both ends of the PTFE corrugated pipe, and a rigid PTFE pipe connected to one of the PTFE flanges; the other PTFE flange is connected to the tank body.

[0035] The PTFE tank is fitted with a metal jacket on the outside, with some areas not fitted (specifically, the upper part of the tank, especially near the exhaust pipe / vent pipe, where the exhaust pipe / vent pipe is soft or easily deformable, allowing it to deform to withstand sudden pressure increases in the tank while still providing protection, and allowing operators to operate without splash-proof goggles and masks, thus providing convenience for operators). The PTFE hose is connected to the tank via a PTFE flange (or a PTFE threaded fitting or PFA union is used).

[0036] The exhaust pipe outlet is located inside a high-efficiency fume hood; this utilizes the tank to prevent acid splashing, while the generated HCl acid mist is drawn away from the tank and sucked away by the high-efficiency fume hood, allowing operators to operate without wearing splash-proof goggles and masks.

[0037] The exterior of the PTFE container can be fitted with a metal jacket for support and fixation to prevent slow deformation under continuous pressure; a heating jacket is used for water bath or oil bath heating to avoid localized overheating of the PTFE container. PTFE paddle mechanical stirring is used at a moderate speed to ensure sufficient contact between the powder and the acid solution while preventing excessively vigorous stirring that could lead to acid splashing or powder loss.

[0038] Currently, the acid washing and purification process requires operators to wear splash-proof goggles and face shields for double protection against acid splashes into their eyes and face, but this does cause inconvenience for the operators.

[0039] In the acid washing and purification process, the current order of acid addition is generally as follows: first add high-purity water, then add hydrochloric acid (HCl), and finally slowly add hydrofluoric acid (HF). This avoids excessively vigorous exothermic reactions and splashing. Furthermore, all operations involving mixed acids are currently conducted in a high-efficiency fume hood to ensure that fumes (heating produces a large amount of highly irritating HCl fumes) are completely removed.

[0040] The advantages and beneficial effects of this invention are as follows: it dynamically changes the field distribution and optimizes the microwave feeding method. The rotating sample-bearing system can dynamically change the field distribution, ensuring uniform microwave field distribution and uniform heating of the reactants. Combined with its three-layer insulation structure, it can also provide a good and balanced thermal field for the sample.

[0041] The rotating sample carrier system (sample carrier and pin integrated) can be detachably connected and inserted into different microwave devices, improving adaptability and application range.

[0042] A higher liquid-to-solid ratio helps to dilute and remove impurity ions, thus improving washing efficiency.

[0043] Conductive sealant (such as silver-filled epoxy resin) is used at the connection between the pipes and the furnace wall to ensure electrical continuity. Argon gas, with a higher density than air, is introduced from the top and vented from the bottom, utilizing gravity to naturally displace the air, resulting in higher air displacement efficiency. A geared motor drives the turntable to rotate, causing the shaft inserted into the connecting rod to rotate with the turntable. The porous graphite crucible on the shaft rotates around the shaft due to the argon gas flow from the argon pipe, achieving almost 360° rotation of the sample-bearing system (the porous graphite crucible rotates around the shaft on one hand, and around the center of the turntable on the other). This greatly satisfies the need for dynamically changing the field distribution, ensuring the uniformity of the microwave field distribution, ensuring uniform heating of the reactants, and providing a good and balanced thermal field for the sample. The conveyor belt uses a mesh belt to facilitate the flow of argon gas and quickly displace the air inside the tunnel microwave oven to form an argon protective atmosphere more rapidly.

[0044] The minimum argon flow rate of 2.4 m³ / h (100 m / s per nozzle) can drive a 1.5 kg porous graphite crucible to rotate at 5 r / min. A better approach is to use two nozzles (1.2 m³ / h each) to improve stability, with a total flow rate of 2.4 m³ / h. The pressure is 0.1~0.2 bar, with low-friction bearings (μ≤0.05). This is within the standard argon flow rate range required for the argon atmosphere in tunnel microwave ovens: 1~5 m³ / h for the maintenance phase (adjusted according to sealing and production capacity), and a short period of high flow rate (10~20 m³ / h) for initial purging. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of a tunnel microwave oven in Example 1 of the preparation method of high-purity silicon carbide powder of the present invention;

[0046] Figure 2 yes Figure 1 The main view;

[0047] Figure 3 yes Figure 2A schematic diagram showing the tunnel microwave oven after part of its outer wall has been removed.

[0048] Figure 4 yes Figure 3 Schematic diagram of the sample support system;

[0049] Figure 5 This is an exploded view of the sample support system in Embodiment 2 of the present invention;

[0050] Figure 6 yes Figure 5 A diagram showing the installation process;

[0051] Figure 7 yes Figure 5 A schematic diagram of the center pin;

[0052] Figure 8 yes Figure 7 A three-dimensional schematic diagram;

[0053] Figure 9 This is a schematic diagram of Embodiment 3 of the present invention;

[0054] Figure 10 yes Figure 9 A partially enlarged schematic diagram of the right end;

[0055] Figure 11 yes Figure 10 Enlarged schematic diagram of a porous graphite crucible and a rotating shaft;

[0056] Figure 12 This is a schematic diagram of Embodiment 4 of the present invention;

[0057] Figure 13 yes Figure 12 A partially enlarged schematic diagram of the right end;

[0058] Figure 14 yes Figure 13 A schematic diagram of one of the wedge structures;

[0059] Figure 15 yes Figure 13 Top view;

[0060] Figure 16 yes Figure 15 A magnified view of the middle section;

[0061] Figure 17 This is a schematic diagram of the pickling equipment in Embodiment 5 of the present invention;

[0062] Figure 18 yes Figure 17 Side view;

[0063] Figure 19 yes Figure 18Sectional view along axis AA;

[0064] Figure 20 yes Figure 17 Rear view.

[0065] In the diagram: 1. Outer insulation layer; 2. Middle insulation layer; 3. Inner insulation layer; 4. Alumina crucible; 5. Tunnel microwave oven; 6. Conveyor belt; 7. Gear motor; 8. Pin; 9. Rectangular hole; 10. Protrusion; 11. Porous graphite crucible; 12. Shaft; 13. Turntable; 14. Connecting rod; 15. Argon gas pipeline; 16. Support plate; 17. Connecting rod; 18. Spike; 19. Frame; 20. Wedge block; 21. Sponge layer; 22. Shaft; 23. Tank body; 24. Stirring blade; 25. Gear motor; 26. PTFE bellows; 27. PTFE flange; 28. Rigid PTFE pipeline. Detailed Implementation

[0066] The specific embodiments of the present invention will be further described below with reference to the accompanying drawings and examples. The following examples are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0067] Example 1: As Figures 1 to 4 As shown, this invention discloses a method for preparing high-purity silicon carbide powder, comprising the following preparation steps: mixing carbon source material and silicon source material, and then directly synthesizing them by microwave heating solid-state reaction; the aforementioned microwave heating process also includes a step of continuously rotating the mixed raw materials; the molar ratio of carbon source material to silicon source material is 0.8:1 to 1.1:1; the microwave heating process is as follows: using a 300kW tunnel microwave oven to microwave heat the mixed carbon source material and silicon source material; wherein, the microwave frequency is 915MHz, and an argon protective atmosphere is used during microwave heating.

[0068] The carbon source material is graphite powder, carbon black, or activated carbon; the silicon source material is silicon powder or silica.

[0069] The mixing process is as follows: the carbon source material and the silicon source material are mixed and then placed in the sample support system; the step of continuous rotation of the mixed material is as follows: the tunnel microwave oven is equipped with a rotation drive mechanism for the sample support system.

[0070] The sample support system consists of an outer insulation layer 1 made of alumina bricks, an intermediate insulation layer 2 made of high-alumina fibers, an inner insulation layer 3 made of alumina bricks, and an alumina crucible 4 on the inner insulation layer. An opening adapted to the alumina crucible 4 is provided on the inner insulation layer 3.

[0071] The rotary drive mechanism is a geared motor 7 fixedly connected to the conveyor belt 6 inside the tunnel microwave oven 5, and the output shaft of the geared motor is fixedly connected to the outer insulation layer 1.

[0072] The geared motor is either a brushless DC motor with an all-metal casing or an AC geared motor with an all-metal casing. The motor casing is grounded and uses a double-layer shielding layer (such as an aluminum casing + internal copper mesh) to prevent microwave leakage from interfering with the circuit. A waveguide shield is installed outside the motor. The conveyor belt shaft, gears, etc., are made of microwave inert materials (such as stainless steel, PTFE) to avoid absorbing microwave heat or releasing harmful substances. The alumina crucible has an opening at the top for adding raw material mixtures, and a porous ceramic lid is fitted to the opening (to cover the top of the porous graphite crucible; the opening diameter of the porous ceramic lid is 0.5mm, allowing argon gas to permeate but blocking powder, thus ensuring an argon atmosphere during the reaction process, but preventing the argon gas flow from blowing the raw materials in the crucible up); the porous ceramic lid is tightened to the alumina crucible by a high-temperature spring and bolts.

[0073] Example 2: The difference from Example 1 is that, as shown in Example 2... Figures 5 to 8 As shown, the rotary drive mechanism is a geared motor 7 fixedly connected to the conveyor belt inside the tunnel microwave oven; a pin 8 is fixedly installed under the outer insulation layer 1, and a rectangular hole 9 is provided on the pin. The output shaft of the geared motor is fixedly connected to a protrusion 10 that matches the aforementioned rectangular hole.

[0074] Example 3: The difference from Example 1 is that, as shown in Example 3... Figures 9 to 11 As shown (for ease of illustration), Figure 9 (Only a portion of the argon gas pipe is shown, only one turntable is shown, and part of the outer wall of the tunnel microwave oven has been removed.) The sample carrying system consists of a porous graphite crucible 11 and a rotating shaft 12 connected to the porous graphite crucible; the porous graphite crucible is rotatably connected to the rotating shaft.

[0075] The rotary drive mechanism consists of a geared motor fixedly connected to the conveyor belt 6 inside the tunnel microwave oven, a turntable 13 fixedly connected to the output shaft of the geared motor 7, a connecting rod 14 fixedly installed on the turntable, a blind hole-shaped insertion hole on the connecting rod that is adapted to the rotating shaft, and several argon gas pipes 15 spaced apart on the top wall inside the tunnel microwave oven; the connecting rod 14 is inclined to the line of the counterweight and there is a gap between the connecting rod and the center of the turntable.

[0076] The conveyor belt is a quartz fiber woven mesh belt or a silicon carbide fiber woven mesh belt.

[0077] The porous graphite crucible has an opening at its top for adding raw material mixtures. A porous ceramic lid, fitted to the opening, covers the top of the crucible. The pores on the ceramic lid are 0.5mm in diameter, allowing argon gas to permeate but blocking powder, thus ensuring an argon atmosphere during the reaction while preventing the argon gas flow from blowing the raw materials inside the crucible away. The porous ceramic lid is secured to the porous graphite crucible by a high-temperature spring and bolts (the spring material can be molybdenum or tungsten alloy; that is, after the porous ceramic lid is placed over the opening at the top of the crucible, it is then secured with bolts). The lid is connected to the porous graphite crucible; a high-temperature spring is fitted onto the bolt, which, on the one hand, connects the porous ceramic lid to the porous graphite crucible via the bolt, and on the other hand, the spring presses the lid to ensure that the porous ceramic lid completely seals the upper opening of the porous graphite crucible, ensuring sealing under high temperature and rotation. The argon gas pipeline uses a polytetrafluoroethylene (PTFE) argon gas delivery pipe; conductive sealant is adhered at the connection between the pipeline and the tunnel microwave oven wall; microwave-transparent materials (such as PTFE, ceramic, or quartz) are used as the argon gas delivery pipe to avoid interference from the microwave field by metal parts. Conductive sealant (such as silver-filled epoxy resin) is used at the connection between the pipeline and the oven wall to ensure electrical continuity. Argon gas is denser than air, so it enters from the top and has an exhaust port at the bottom, utilizing gravity to naturally displace the air, resulting in higher air displacement efficiency. A geared motor drives the turntable to rotate, causing the shaft inserted on the connecting rod to rotate with the turntable. The porous graphite crucible on the shaft rotates around the shaft due to the argon gas flow from the argon gas pipeline, achieving almost 360° rotation of the sample-bearing system (the porous graphite crucible rotates around the shaft on one hand, and around the center of the turntable on the other). This greatly satisfies the need for dynamically changing the field distribution, ensuring the uniformity of the microwave field distribution, guaranteeing uniform heating of the reactants, and providing a good and balanced thermal field for the sample. The conveyor belt uses a mesh belt to facilitate the flow of argon gas and quickly replace the air inside the tunnel microwave oven to more rapidly form an argon protective atmosphere.

[0078] Industrial scale: A continuously fed tunnel microwave oven can process 10 kg per hour (10 kg at a time, with 10 crucibles spaced apart on the conveyor belt of the tunnel microwave oven, each crucible containing 1 kg of raw material mixture); the crucibles are removed from the tunnel microwave oven at the end of the conveyor belt. Raw material mixture is added manually or mechanically to empty porous graphite crucibles at the beginning of the conveyor belt.

[0079] Minimum argon flow rate: 2.4 m 3 A single nozzle speed of 100 m / s can drive a 1.5 kg porous graphite crucible to rotate at 5 r / min. A better solution is to use two nozzles (each 1.2 m). 3 / h), improving stability, total flow rate 2.4m 3 / h. Pressure 0.1~0.2 bar, with low-friction bearings (μ≤0.05). This is consistent with the standard argon flow rate required for the argon atmosphere in a tunnel microwave oven: 1~5 m / s during the maintenance phase. 3 / h (adjusted according to sealing and capacity), initial replacement requires a short period of high flow (10~20 m³ / h). 3 Compared to ( / h), it falls within this flow rate range; the density of porous graphite crucibles can be as low as 0.5~1.0 g / cm³. 3 To prevent graphite from reacting with silicon to form SiC, a coating or isolation layer can be applied to the surface of the porous graphite crucible.

[0080] Example 4: The difference from Example 3 is that, as shown in Example 4... Figures 12 to 16 As shown (for ease of illustration), Figure 12 (Only a portion of the argon gas pipeline, only two wedge structures, two support plates, and part of the outer wall of the tunnel microwave oven are shown.) The sample carrying system consists of a porous graphite crucible 11 and a rotating shaft 22 connected to the porous graphite crucible; the porous graphite crucible is rotatably connected to the rotating shaft and is spherical.

[0081] The rotary drive mechanism consists of several trays 16 spaced apart on the surface of the conveyor belt 6 inside the tunnel microwave oven, connecting rods 17 fixedly connected to the trays, pointed blocks 18 fixedly installed on the upper surface of the conveyor belt as support points for the trays, and a wedge structure on the frame on one side of the conveyor belt along its length. The number of wedge structures is the same as the number of trays 16. The wedge structure consists of a wedge-shaped block 20 fixedly connected to the top of the frame 19 and a sponge layer 21 fixedly connected to the upper surface of the wedge block. The horizontal plane at the highest point of the wedge block is flush with the horizontal plane at the highest point of the pointed block 18. The center of the lower surface of the tray is hinged to the upper end of the pointed block 18, and the rotating shaft 22 is hinged to the connecting rod 17. The connecting rod 17 is located at the center of the tray 16. The dimension of the tray along the width direction of the conveyor belt 6 is larger than the width of the conveyor belt. The wedge structure is located outside the conveyor belt 6 but does not extend beyond the side of the tray 16 along the width direction of the conveyor belt.

[0082] Example 5: The difference from Example 1 is that, as shown in Example 5... Figures 17 to 20 As shown (for ease of illustration), Figure 17 (PTFE bellows and PTFE flange not shown) In the acid washing and purification process, the acid washing equipment consists of a polytetrafluoroethylene tank 23, a PTFE stirring blade 24 installed inside the tank, a flue pipe connected to the tank, a metal jacket installed outside the tank, and a geared motor 25 fixedly installed outside the tank; the output shaft of the geared motor passes through the tank and is connected to the stirring blade 24.

[0083] The exhaust pipe outlet is located inside a high-efficiency fume hood; the stirring speed of the agitator blades is 100~300 r / min.

[0084] The exhaust pipe is a PTFE hose, which is connected to the tank via a PTFE flange.

[0085] The exhaust pipe consists of a PTFE corrugated pipe 26, PTFE flanges 27 connecting both ends of the PTFE corrugated pipe, and a rigid PTFE pipe 28 connected to one of the PTFE flanges; the other PTFE flange is connected to the tank body.

[0086] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing high-purity silicon carbide powder, characterized in that, The preparation process consists of the following steps: mixing carbon source materials and silicon source materials, then directly synthesizing silicon carbide powder through a solid-state reaction using microwave heating; cooling, crushing, and purifying the silicon carbide powder to obtain high-purity silicon carbide powder; the microwave heating process also includes a step of continuous rotation of the mixed materials; the molar ratio of carbon source materials to silicon source materials is 0.8:1 to 1.1:1; the microwave heating process is as follows: using a 300kW tunnel microwave oven to microwave heat the mixed carbon source materials and silicon source materials; wherein, the microwave frequency is 915MHz, and an argon protective atmosphere is used during microwave heating; The mixing process is as follows: carbon source material and silicon source material are mixed and then placed in the sample support system; the step of continuous rotation of the mixed material is as follows: the sample support system is rotated by a rotation drive mechanism in the tunnel microwave oven; The sample support system consists of a porous graphite crucible and a rotating shaft connected to the porous graphite crucible; the porous graphite crucible is rotatably connected to the rotating shaft. The rotary drive mechanism consists of a geared motor fixedly connected to the conveyor belt inside the tunnel microwave oven, a turntable fixedly connected to the output shaft of the geared motor, a connecting rod fixedly mounted on the turntable, a blind hole-shaped insertion hole on the connecting rod that is adapted to the rotating shaft, and several argon gas pipes spaced apart on the top wall inside the tunnel microwave oven; the connecting rod is inclined to the line of the counterweight and there is a gap between the position of the connecting rod and the center of the turntable. The conveyor belt is a quartz fiber woven mesh belt or a silicon carbide fiber woven mesh belt.

2. The method for preparing high-purity silicon carbide powder according to claim 1, characterized in that, The carbon source material is graphite powder, carbon black, or activated carbon; the silicon source material is silicon powder or silica.

3. The method for preparing high-purity silicon carbide powder according to claim 2, characterized in that, The tunnel of the tunnel-type microwave oven is 15-20 meters long; the post-purification treatment includes the following steps: Acid washing and purification: The carbon-removed powder is heated and stirred in a polytetrafluoroethylene container with a mixture of HCl and HF to remove metal impurities and SiO2; it is then repeatedly centrifuged and washed with high-purity water until the conductivity of the supernatant is close to that of high-purity water. Alkaline washing to remove silicon: The acid-washed powder is washed with deionized water and then placed in a boiling and stirred alkaline solution to react for 1-4 hours to remove residual free silicon. Decarbonization calcination: Place the pulverized powder in a crucible, put the crucible into a box-type muffle furnace or bell furnace, and calcine at 650~700℃ for 1~2 hours in a circulating air atmosphere; Washing, drying and classification: The powder is repeatedly washed with deionized water until the filtrate is neutral to ensure that there are no chloride or fluoride ions remaining; the washed slurry is filtered and dried by vacuum drying oven or spray dryer to prevent agglomeration; finally, high-purity SiC powder with uniform particle size distribution is obtained by air classification or sieving.

4. The method for preparing high-purity silicon carbide powder according to claim 3, characterized in that, In the acid washing and purification step, the volume ratio of the mixed acid HCl:HF:H2O is (2~4):1:(5~7); the heating temperature is 40~60℃; the stirring time is 2~4 hours and the stirring speed is 100~400r / min.

5. The method for preparing high-purity silicon carbide powder according to claim 4, characterized in that, In the alkaline washing and desiliconization step, the alkaline solution is sodium hydroxide with a mass concentration of 20%~40%; the solid-liquid ratio of powder to alkaline solution is 1:5~1:10; and the stirring speed is 200~400 r / min.

6. A method for preparing high-purity silicon carbide powder according to claim 5, characterized in that, In the water washing step, the solid-liquid ratio is 1:5 to 1:10; the stirring speed is 200 to 400 r / min; and the stirring time is 15 to 30 minutes each time.

7. The method for preparing high-purity silicon carbide powder according to claim 6, characterized in that, In the filtration and drying step, the vacuum degree is -0.08 to -0.095 MPa; the filter cake is dried until there are no obvious water marks on the surface and it forms a cracked shape. The drying time is 30 to 60 minutes, and the powder moisture content is reduced to 15% to 25%.

8. The method for preparing high-purity silicon carbide powder according to claim 7, characterized in that, In the drying step, a core-type or pressure-type spray dryer is used for spray drying to obtain spherical agglomerates with good flowability; the dried slurry is re-dispersed and prepared into a uniform slurry with a solid content of 20%~30%; the inlet temperature is 200~250℃ and the outlet temperature is 90~110℃.

9. The method for preparing high-purity silicon carbide powder according to claim 8, characterized in that, In the air classification step, an air classifier and an air classifier are used to classify the dried powder by air classification; the feeding speed is 5~20 kg / h; the classifier wheel speed is 3000~6000 r / min; and the air pressure is 0.6~0.8 MPa.

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