Preparation method and sintering device of large-size pressureless sintered silicon carbide ceramic pipe

Abstract

The invention relates to the technical field of silicon carbide ceramics, in particular to a preparation method and a sintering device of a large-sized pressureless sintered silicon carbide ceramic tubing, and the preparation method comprises the following steps: uniformly mixing beta-type silicon carbide whiskers, graphite powder, boron carbide, a sintering aid, a dispersing agent, a binder and silicon carbide micro powder, then adding water, and stirring to prepare pug with the water content of 32-38%; pugging is conducted till pug with the water content being 8%-11% is obtained, and then the pug obtained after pugging is sealed and aged at the room temperature; carrying out extrusion molding on the aged pug through a vacuum extruder to obtain a ceramic tube blank; the ceramic blank pipe is fed into drying equipment to be dried; and sintering the ceramic blank tube in a nitrogen atmosphere at 1900-2150 DEG C to obtain the silicon carbide ceramic tube. By adding the beta-type silicon carbide whiskers, the silicon carbide micro powder, the sintering aid and other substances, the toughness and strength of the silicon carbide ceramic pipe can be enhanced, the wall thickness of the pipe can be reduced on the premise of keeping the same strength level, thermal resistance can be reduced, and the heat exchange efficiency of a heat exchanger can be improved.

Description

Technical Field

[0001] The invention relates to the technical field of silicon carbide ceramics, and in particular to a preparation method and a sintering device for a large-scale pressureless sintered silicon carbide ceramic pipe. Background Art

[0002] Silicon carbide ceramics are a high-tech ceramic material with the advantages of high high-temperature strength, strong oxidation resistance, strong acid and alkali corrosion resistance, good thermal stability, and high thermal conductivity. Heat exchange tubes made of silicon carbide ceramics are widely used in heat exchange systems in harsh environments such as high temperature and corrosion.

[0003] The Chinese invention patent with publication number N108395248B discloses a method for preparing a silicon carbide ceramic heat exchange tube. The patent uses a specific preparation process, including mixing raw materials, extrusion molding, drying, sintering and other steps, to prepare a silicon carbide ceramic heat exchange tube with good roundness, high verticality, no collapse, and no breakage. Among them, the addition of binder and water reducer and the synergistic effect of various process parameters in the extrusion molding process play a key role.

[0004] However, during the high-temperature sintering process, the volatilization of additives such as water, water reducers, and binders will cause tiny pores in the pipe. These pores will reduce the overall strength of the material because the pores are places where stress is concentrated and are prone to fracture. Summary of the invention

[0005] In response to the defects mentioned in the background technology, the present invention provides a preparation method and a sintering device for large-sized pressureless sintered silicon carbide ceramic pipes, aiming to reduce the generation of pores and improve the strength and toughness of large-sized ceramic pipes.

[0006] On the one hand, in order to solve the above technical problems, the present invention provides a method for preparing a large-scale pressureless sintered silicon carbide ceramic tube, comprising the following steps: S1. By weight, 12 to 16 parts of β-type silicon carbide whiskers, 3 to 5 parts of graphite powder, 6 to 8 parts of sintering aids, 3 to 5 parts of boron carbide, 0.5 to 1.8 parts of dispersant, 1.2 to 1.6 parts of binder and 86 to 98 parts of silicon carbide powder are mixed evenly, and then water is added and stirred to obtain a mud material with a moisture content of 32 to 38%; S2, repeatedly kneading the clay at room temperature and a vacuum degree of 0.02-0.04 MPa until the clay with a moisture content of 8-11% is obtained, and then sealing and aging the kneaded clay at room temperature for 26-38 hours; S3, extruding the aged clay material through a vacuum extruder to obtain a ceramic blank tube; S4, sending the ceramic tube to a drying device for drying, the drying temperature is set to 45-48°C, and the drying time is 10-18h; S5. The ceramic tube is sintered at 1900-2150° C. in a nitrogen atmosphere. After sintering, the tube is kept warm for 2-2.5 hours and cooled naturally to obtain a silicon carbide ceramic tube.

[0007] Preferably, the purity of the silicon carbide powder is ≥99% and the particle size is 8 to 46 μm.

[0008] Preferably, the silicon carbide powder is a mixture of powders of three different particle sizes, with powders with a particle size of 8 to 16 μm accounting for 30 wt%, powders with a particle size of 16 to 34 μm accounting for 50 wt%, and powders with a particle size of 34 to 46 μm accounting for 20 wt%.

[0009] Preferably, the purity of the β-type silicon carbide whisker is ≥98%, and the aspect ratio is 60-80.

[0010] Preferably, the sintering aid is composed of aluminum oxide, magnesium oxide and yttrium oxide, and the weight ratio of aluminum oxide, magnesium oxide and yttrium oxide is 12:5:3.

[0011] Preferably, the carbon content of the graphite powder is ≥92% and the particle size is 8-10 μm.

[0012] Preferably, the particle size of the boron carbide is 10 to 14 μm.

[0013] Preferably, the binder is composed of phenolic resin, polyvinyl alcohol and carboxymethyl cellulose, and the weight ratio of the phenolic resin, polyvinyl alcohol and carboxymethyl cellulose is 5:3:2.

[0014] Preferably, the dispersant is polyvinyl pyrrolidone.

[0015] On the other hand, the present invention also provides a sintering device for sintering the above-mentioned silicon carbide ceramic tube, comprising a sintering furnace, a graphite frame is arranged inside the furnace chamber of the sintering furnace, a plurality of groups of ceramic rollers for supporting the graphite frame are arranged at the bottom of the furnace chamber of the sintering furnace, a plurality of groups of through holes are arranged on the outer walls around the graphite frame, a plurality of layers of graphite supporting plates are arranged in the graphite frame, a plurality of groups of vertically arranged card grooves for overlapping the graphite supporting plates are arranged on the inner walls on both sides of the graphite frame, and a plurality of groups of V-shaped grooves are arranged on the upper end surface of the graphite supporting plate.

[0016] After adopting the above technical solution, the beneficial effects of the present invention are: The addition of β-type silicon carbide whiskers, silicon carbide powder, sintering aids and other substances can enhance the toughness and strength of ceramic pipes. Under the premise of maintaining the same strength level, the wall thickness of the pipe can be reduced, thereby achieving thin-walled, reducing material costs, helping to reduce thermal resistance, and improving the heat exchange efficiency of the heat exchanger. Sintering aids can form a liquid phase at high temperatures and fill the pores inside the pipe through capillary action, reducing the porosity and increasing the density of the ceramic. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative labor.

[0018] Figure 1 It is the overall structural schematic diagram of the sintering device; Figure 2 It is a schematic diagram of the internal structure of the sintering device; Figure 3 Schematic diagram of the structure of the graphite frame.

[0019] Figure numerals: 1-sintering furnace, 2-graphite frame, 3-ceramic roller, 4-through hole, 5-graphite bearing plate, 6-slot, 7-V-groove. DETAILED DESCRIPTION

[0020] The features and exemplary embodiments of various aspects of the present invention will be described in detail below. In order to make the purpose, technical solutions and advantages of the present invention clearer, the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the specific embodiments described herein are only configured to explain the present invention and are not configured to limit the present invention. For those skilled in the art, the present invention can be implemented without the need for some of these specific details. The following description of the embodiments is only to provide a better understanding of the present invention by illustrating examples of the present invention.

[0021] A sintering device specially designed for high temperature sintering of silicon carbide ceramic tubes. Figure 1-3 As shown, the sintering device includes a sintering furnace 1, and a graphite frame 2 is arranged inside the furnace of the sintering furnace 1 for carrying the ceramic tube to be sintered. A plurality of ceramic rollers 3 are arranged at the bottom of the furnace of the sintering furnace 1 for carrying and conveniently moving the graphite frame 2 so as to load and unload the ceramic tube. A plurality of through holes 4 are arranged on the outer walls of the graphite frame 2 to help evenly transfer the heat in the furnace to the inside of the graphite frame 2, so as to ensure even heating of the ceramic tube.

[0022] The graphite frame 2 is provided with multiple layers of graphite support plates 5 with flexible height adjustment, and the upper end surface of the graphite support plates 5 is provided with multiple groups of V-shaped grooves 7. The inner walls on both sides of the graphite frame 1 are provided with multiple groups of vertically arranged clamping grooves 6 for overlapping the graphite support plates 5. The position of the graphite support plates 5 on the clamping grooves 6 is adjusted according to the size of the ceramic blank tube, and the spacing between the graphite support plates 5 is adjusted to maximize the use of the space in the graphite frame 2, and multiple ceramic blank tubes can be sintered at the same time, thereby improving production efficiency.

[0023] In the traditional sintering stacking method, ceramic tubes are stacked layer by layer. Due to the gravity of the upper tubes, the bottom layer of ceramic tubes may be subjected to excessive pressure, causing them to deform or be damaged during the sintering process. When the number of stacked layers is large, the bottom layer of tubes may be insufficiently sintered due to the obstruction of heat transfer, while the upper layer of tubes may be deformed due to overheating. The design of the V-groove 7 and the graphite support plate 5 allows each ceramic tube to be placed separately and can accommodate ceramic tubes of different sizes without generating direct stacking pressure between each other, which helps to optimize the heat distribution. Heat can be transferred to each tube more evenly, thereby ensuring the uniformity and consistency of the sintering process.

[0024] Example 1 A method for preparing a large-scale pressureless sintered silicon carbide ceramic pipe comprises the following steps: S1. Raw material preparation and mixing Prepare 86 parts of silicon carbide powder, 12 parts of β-silicon carbide whiskers, 3 parts of graphite powder, 3 parts of boron carbide, 6 parts of sintering aids, 0.5 parts of polyvinyl pyrrolidone and 1.2 parts of binder by weight. Mix the above raw materials evenly according to the ratio, add water and stir to obtain a mud material with a moisture content of 32%.

[0025] The purity of the silicon carbide powder used is ≥99%, and the particle size should not be less than 8um and not more than 46μm. Silicon carbide particles that are too small or too large may increase the difficulty in the mixing and molding process: fine particles are easy to form agglomerations due to their large specific surface area and high surface energy, resulting in uneven distribution during mixing and difficulty in forming tight stacking, which affects the strength and stability of the product; large gaps are easily formed between large particles, affecting the density and mechanical properties after sintering.

[0026] This method divides silicon carbide powder into three different particle size grades with 16μm, 34μm and 46μm as the boundaries: the small particle size silicon carbide powder has an upper limit of 16μm, and the small particle size (8-16μm) silicon carbide powder accounts for 30wt%; the medium particle size silicon carbide powder has an upper limit of 34μm, and the medium particle size (16-34μm) silicon carbide powder accounts for 50wt%; the large particle size silicon carbide powder has an upper limit of 46μm, and the large particle size (34-46μm) silicon carbide powder accounts for 20wt%.

[0027] The sintering performance of silicon carbide ceramic tubes can be optimized through reasonable particle grading. Particles of different particle sizes can complement each other during the sintering process to form a more uniform and dense microstructure. This helps to reduce shrinkage and porosity during the sintering process and improve the dimensional accuracy and performance stability of the product.

[0028] The purity of the β-type silicon carbide whisker is ≥92%, and the aspect ratio is 60. When the β-type silicon carbide whisker is uniformly dispersed in the silicon carbide ceramic tube matrix, it can form an effective reinforcement network and significantly improve the toughness and strength of the silicon carbide ceramic tube.

[0029] When the crack expands in the silicon carbide ceramic tube matrix, it will deflect or be pulled out of the matrix when encountering β-type silicon carbide whiskers, changing the crack expansion path, thereby absorbing or consuming the energy of crack expansion, preventing further expansion of the crack, and preventing further expansion of the crack, thereby improving the toughness of the material. β-type silicon carbide whiskers can also span across the two sides of the crack to form a bridging structure, dispersing the stress generated during the crack expansion process, thereby reducing the destructive effect of the crack on the material.

[0030] The silicon carbide ceramic tube prepared by this method can significantly reduce the wall thickness while meeting the large tube diameter. This is because, in the silicon carbide ceramic tube matrix, the β-type silicon carbide whiskers can support the entire structure like a skeleton, improve the compression, bending and torsional strength of the tube, and avoid the collapse of the silicon carbide ceramic tube in the blank stage. Therefore, under the premise of maintaining the same strength level, this method can reduce the thickness of the silicon carbide ceramic tube, achieve thin wall, reduce material costs, help reduce thermal resistance, and improve the heat exchange efficiency of the heat exchanger.

[0031] The particle size of boron carbide is controlled at 10μm. As a high-performance ceramic material additive, boron carbide can significantly improve the hardness of silicon carbide ceramics. Boron carbide can also be used as a crystal nucleation promoter to help the growth and arrangement of silicon carbide crystals, thereby improving the microstructure and performance of ceramics.

[0032] The carbon content of graphite powder is ≥98%, and the particle size is controlled at 8μm. Graphite powder provides a rich carbon source, which helps to remove silicon oxide impurities on the surface of silicon carbide micropowder, improve surface energy, thereby improving powder activity, and is conducive to the sintering and densification of silicon carbide. At the same time, graphite powder can reduce the surface energy of the material, thereby reducing the friction coefficient and improving the self-lubricating properties of the material. In the later extrusion molding process, the ceramic blank tube is easier to be extruded, which helps to reduce scratches and damage on the surface of the product and improve the surface quality and precision of the product.

[0033] The sintering aid is composed of alumina, magnesium oxide and yttrium oxide, and the weight ratio of alumina, magnesium oxide and yttrium oxide is 12:5:3. Alumina has excellent physical properties. By adding alumina, the hardness and strength of silicon carbide ceramic pipes can be significantly improved, making them more wear-resistant and high-temperature resistant, which helps silicon carbide ceramic pipes maintain stable performance in harsh environments such as high temperature and high pressure. An appropriate amount of magnesium oxide can effectively inhibit the oxidation reaction of silicon carbide ceramic pipes at high temperatures and improve their thermal stability. It can also promote the crystallization of silicon carbide ceramic grain boundaries, improve grain boundary strength and hardness, enhance the mechanical properties and wear resistance of the material, and improve the processing performance of silicon carbide ceramic pipes. Yttrium oxide can be added to silicon carbide ceramic pipes as a stabilizer, which helps to stabilize the crystal phase structure of silicon carbide and prevent it from undergoing adverse phase changes at high temperatures.

[0034] Alumina, magnesium oxide, and yttrium oxide will form a liquid phase at high temperatures, and part of the liquid phase will fill the pores inside the ceramic tube through capillary action, reducing the porosity and increasing the density of the ceramic. At the same time, the presence of the liquid phase also promotes the diffusion and bonding between silicon carbide particles, accelerating the densification process. In addition, at high temperatures, alumina will react with yttrium oxide to form yttrium aluminum garnet (YAG). When the temperature is higher than 1400°C, the ions in alumina and yttrium oxide obtain enough energy to overcome the potential barrier, so that they diffuse and react with each other to form the YAG phase. With the formation of the YAG phase, the number of pores in the ceramic tube gradually decreases, because the YAG phase can fill the pores between the particles (the gaps between the particles and the pores generated after the high-temperature decomposition of the dispersant, binder, etc.). By promoting densification, the formation of the YAG phase helps to improve the mechanical properties (such as hardness, compressive strength, etc.) as well as the thermal and chemical stability of the ceramic material.

[0035] As a dispersant, polyvinyl pyrrolidone can be adsorbed on the surface of particles, reduce the surface tension between particles, and improve the dispersibility and fluidity of the mud.

[0036] The binder is composed of phenolic resin, polyvinyl alcohol and carboxymethyl cellulose, and the weight ratio of phenolic resin, polyvinyl alcohol and carboxymethyl cellulose is 5:3:2. It is used to improve the adhesion and plasticity of the slurry and ensure that the green body after extrusion molding has a certain strength and stability.

[0037] S2, stale clay The clay is repeatedly kneaded at room temperature and a vacuum of 0.02MPa until a clay with a moisture content of 8% is obtained, and then the kneaded clay is sealed and aged at room temperature for 26 hours. In a vacuum environment, the moisture in the clay will be partially extracted due to the vacuum effect, thereby reducing the moisture content of the clay. At the same time, vacuum kneading can also improve the uniformity and plasticity of the clay, laying a good foundation for subsequent molding and firing. Through aging, moisture further penetrates and diffuses, making the clay more delicate and uniform, improving its plasticity, improving the viscosity of the mixed clay, and improving molding performance.

[0038] S3, Extrusion molding The aged clay material is extruded through a vacuum extruder to obtain a ceramic blank tube.

[0039] S4. Drying The ceramic tube is sent to the drying equipment for drying, and the drying temperature is set to 45°C and the drying time is 10 hours. During the drying process, it is necessary to ensure that the ceramic tube is evenly heated to avoid cracking or deformation.

[0040] S5. Loading the furnace Open the door of the sintering furnace 1, pull out the graphite frame 2 in the furnace, and adjust the height of each layer of graphite support plate 5 in the graphite frame 2 according to the diameter of the ceramic tube. Lay a layer of graphite felt with a thickness of 5 mm on each layer of graphite support plate 5, put the dried ceramic tubes into the graphite support plate 5 one by one, then push the graphite frame 2 back into the furnace, and close the furnace door.

[0041] S6. Sintering The ceramic tube is sintered at 1900°C in a nitrogen atmosphere and kept at this temperature for 2 hours. After natural cooling, the door of the sintering furnace 1 is opened, the graphite frame 2 in the furnace is pulled out, and the products are taken out one by one to obtain silicon carbide ceramic tubes.

[0042] Example 2 S1. Raw material preparation and mixing Prepare 88 parts of silicon carbide powder, 15 parts of β-silicon carbide whiskers (aspect ratio of 70), 4.5 parts of graphite powder (particle size of 8.5μm), 4.5 parts of boron carbide (particle size of 10.5μm), 7 parts of sintering aids, 1 part of polyvinyl pyrrolidone and 1.25 parts of binder by weight. Mix the above raw materials evenly according to the proportion, add water and stir to obtain a mud with a water content of 33%.

[0043] S2, stale clay The clay was repeatedly kneaded at room temperature and a vacuum degree of 0.025 MPa until the clay had a moisture content of 11%, and then the kneaded clay was sealed and aged at room temperature for 28 hours.

[0044] S3, Extrusion molding The aged clay material is extruded through a vacuum extruder to obtain a ceramic blank tube.

[0045] S4. Drying The ceramic tube is sent to the drying equipment for drying. The drying temperature is set to 48°C and the drying time is 12 hours.

[0046] S5. Loading the furnace Open the door of the sintering furnace 1, pull out the graphite frame 2 in the furnace, and adjust the height of each layer of graphite support plate 5 in the graphite frame 2 according to the diameter of the ceramic tube. Lay a layer of graphite felt with a thickness of 5 mm on each layer of graphite support plate 5, put the dried ceramic tubes into the graphite support plate 5 one by one, then push the graphite frame 2 back into the furnace, and close the furnace door.

[0047] S6. Sintering The ceramic tube is sintered at 2050°C in a nitrogen atmosphere and kept at this temperature for 2.2 hours. After natural cooling, the door of the sintering furnace 1 is opened, the graphite frame 2 in the furnace is pulled out, and the products are taken out one by one to obtain silicon carbide ceramic tubes.

[0048] Example 3 S1. Raw material preparation and mixing Prepare 90 parts of silicon carbide powder, 15 parts of β-silicon carbide whiskers (aspect ratio of 70), 3 parts of graphite powder (particle size of 9μm), 3 parts of boron carbide (particle size of 13μm), 7.5 parts of sintering aid, 1.5 parts of polyvinyl pyrrolidone and 1.5 parts of binder by weight. Mix the above raw materials evenly according to the ratio, add water and stir to obtain a mud with a water content of 36%.

[0049] S2, stale clay The clay was repeatedly kneaded at room temperature and a vacuum degree of 0.03 MPa until the clay had a moisture content of 10%, and then the kneaded clay was sealed and aged at room temperature for 32 hours.

[0050] S3, Extrusion molding The aged clay material is extruded through a vacuum extruder to obtain a ceramic blank tube.

[0051] S4. Drying The ceramic tube is sent to the drying equipment for drying. The drying temperature is set to 47°C and the drying time is 18 hours.

[0052] S5. Loading the furnace Open the door of the sintering furnace 1, pull out the graphite frame 2 in the furnace, and adjust the height of each layer of graphite support plate 5 in the graphite frame 2 according to the diameter of the ceramic tube. Lay a layer of graphite felt with a thickness of 5 mm on each layer of graphite support plate 5, put the dried ceramic tubes into the graphite support plate 5 one by one, then push the graphite frame 2 back into the furnace, and close the furnace door.

[0053] S6. Sintering The ceramic tube is sintered at 2000°C in a nitrogen atmosphere and kept at this temperature for 2.2 hours. After natural cooling, the door of the sintering furnace 1 is opened, the graphite frame 2 in the furnace is pulled out, and the products are taken out one by one to obtain silicon carbide ceramic tubes.

[0054] Example 4 S1. Raw material preparation and mixing Prepare 98 parts of silicon carbide powder, 16 parts of β-silicon carbide whiskers (aspect ratio of 80), 5 parts of graphite powder (particle size of 10μm), 5 parts of boron carbide (particle size of 14μm), 8 parts of sintering aid, 1.8 parts of polyvinyl pyrrolidone and 1.6 parts of binder by weight. Mix the above raw materials evenly according to the ratio, add water and stir to obtain a mud with a water content of 38%.

[0055] S2, stale clay The clay was repeatedly kneaded at room temperature and a vacuum degree of 0.04 MPa until the clay had a moisture content of 9%, and then the kneaded clay was sealed and aged at room temperature for 38 hours.

[0056] S3, Extrusion molding The aged clay material is extruded through a vacuum extruder to obtain a ceramic blank tube.

[0057] S4. Drying The ceramic tube is sent to the drying equipment for drying. The drying temperature is set to 46°C and the drying time is 12 hours.

[0058] S5. Loading the furnace Open the door of the sintering furnace 1, pull out the graphite frame 2 in the furnace, and adjust the height of each layer of graphite support plate 5 in the graphite frame 2 according to the diameter of the ceramic tube. Lay a layer of graphite felt with a thickness of 5 mm on each layer of graphite support plate 5, put the dried ceramic tubes into the graphite support plate 5 one by one, then push the graphite frame 2 back into the furnace, and close the furnace door.

[0059] S6. Sintering The ceramic tube is sintered at 2150°C in a nitrogen atmosphere and kept at this temperature for 2.5 hours. After natural cooling, the door of the sintering furnace 1 is opened, the graphite frame 2 in the furnace is pulled out, and the products are taken out one by one to obtain silicon carbide ceramic tubes.

[0060] Four groups of silicon carbide ceramic tubes with a wall thickness of 2.25 mm and a nominal diameter of 19 mm were prepared by the preparation methods of the above Examples 1-4, and were tested. The results were referred to JC / T 2212-2014 "Silicon carbide ceramic heat exchange tubes sintered at normal pressure solid phase". The results are shown in the following table.

[0061]

[0062] It can be seen from the table that the ceramic pipes prepared by this method are far superior to various performance indicators.

[0063] According to the embodiments of the present invention as described above, these embodiments do not describe all the details in detail, nor do they limit the invention to specific embodiments. Obviously, many modifications and changes can be made based on the above description. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can make good use of the present invention and the modified use based on the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a large-size pressureless sintered silicon carbide ceramic tube, comprising the following steps: S1. By weight, 12 to 16 parts of β-type silicon carbide whiskers, 3 to 5 parts of graphite powder, 6 to 8 parts of sintering aids, 3 to 5 parts of boron carbide, 0.5 to 1.8 parts of dispersant, 1.2 to 1.6 parts of binder and 86 to 98 parts of silicon carbide powder are mixed evenly, and then water is added and stirred to obtain a mud material with a moisture content of 32 to 38%; S2, repeatedly kneading the clay at room temperature and a vacuum degree of 0.02-0.04 MPa until the clay with a moisture content of 8-11% is obtained, and then sealing and aging the kneaded clay at room temperature for 26-38 hours; S3, extruding the aged clay material through a vacuum extruder to obtain a ceramic blank tube; S4, sending the ceramic tube to a drying device for drying, the drying temperature is set to 45-48°C, and the drying time is 10-18h; S5, opening the furnace door of the sintering furnace (1), pulling out the graphite frame (2) in the furnace, adjusting the height of each layer of graphite support plates (5) in the graphite frame (2) according to the diameter of the ceramic tube, placing the dried ceramic tubes one by one into the graphite support plates (5), and then pushing the graphite frame (2) back into the furnace, and closing the furnace door; S6, the ceramic tube is sintered at 1900-2150° C. in a nitrogen atmosphere, and kept warm for 2-2.5 hours after sintering, and then cooled naturally to obtain a silicon carbide ceramic tube; In step S6, a plurality of groups of ceramic rollers (3) are arranged at the bottom of the furnace chamber of the sintering furnace (1), a plurality of groups of through holes (4) are arranged on the outer walls around the graphite frame (2), a plurality of groups of vertically arranged slots (6) are arranged on the inner walls on both sides of the graphite frame (2), and a plurality of groups of V-shaped grooves (7) are arranged on the upper end surface of the graphite support plate (5).

2. The method for preparing a large-sized pressureless sintered silicon carbide ceramic pipe according to claim 1, characterized in that: The purity of the silicon carbide micropowder is ≥99%, and the particle size is 8-46 μm.

3. The method for preparing a large-sized pressureless sintered silicon carbide ceramic pipe according to claim 2, characterized in that: The silicon carbide powder is a mixture of powders of three different particle sizes, wherein the powder with a particle size of 8 to 16 μm accounts for 30 wt%, the powder with a particle size of 16 to 34 μm accounts for 50 wt%, and the powder with a particle size of 34 to 46 μm accounts for 20 wt%.

4. The method for preparing a large-sized pressureless sintered silicon carbide ceramic pipe according to claim 1, characterized in that: The purity of the beta-type silicon carbide whisker is ≥92%, and the aspect ratio is 60-80.

5. The method for preparing a large-sized pressureless sintered silicon carbide ceramic pipe according to claim 1, characterized in that: The sintering aid is composed of aluminum oxide, magnesium oxide and yttrium oxide, and the weight ratio of aluminum oxide, magnesium oxide and yttrium oxide is 12:5:

3.

6. The method for preparing a large-sized pressureless sintered silicon carbide ceramic tube according to claim 1, characterized in that: The carbon content of the graphite powder is ≥98%, and the particle size is 8-10 μm.

7. The method for preparing a large-sized pressureless sintered silicon carbide ceramic tube according to claim 1, characterized in that: The particle size of the boron carbide is 10-14 μm.

8. The method for preparing a large-sized pressureless sintered silicon carbide ceramic tube according to claim 1, characterized in that: The binder consists of phenolic resin, polyvinyl alcohol and carboxymethyl cellulose, and the weight ratio of the phenolic resin, polyvinyl alcohol and carboxymethyl cellulose is 5:3:

2.

9. The method for preparing a large-sized pressureless sintered silicon carbide ceramic tube according to claim 1, characterized in that: The dispersant is polyvinyl pyrrolidone.

Images