Preparation method of pressureless sintering extrusion type silicon carbide ceramic

Abstract

The invention relates to the technical field of silicon carbide ceramics, in particular to a preparation method of pressureless sintering extrusion type silicon carbide ceramics, which comprises the following steps: putting deionized water, silicon carbide micro powder, hexagonal boron nitride, aggregate, graphite, a dispersing agent, a sintering aid and a binder into a stirring kettle; vacuumizing with a vacuum pump, and mixing and stirring to obtain slurry; filtering the slurry, and preparing granulated micro powder through a spray drying method; preparing the granulated micro powder into a green body through a powder extrusion printing technology; feeding the green body into drying equipment for drying; the dried green body is wrapped with a layer of graphite felt, and the green body and the graphite felt are sent into a sintering furnace; and after sintering, cooling to room temperature to obtain the silicon carbide ceramic product. According to the method, the pressureless sintering technology is adopted, external pressure does not need to be applied, and the production cost and the equipment complexity are reduced; the green body is prepared through the powder extrusion printing technology, and the precision and shape complexity of the product are improved.

Description

Technical Field

[0001] The invention relates to the technical field of silicon carbide ceramics, and in particular to a method for preparing pressureless sintering extruded silicon carbide ceramics. 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. Therefore, they are processed into high-temperature resistant components such as silicon carbide flamethrowers, burner sleeves, square beams, rollers, preheating tubes, etc., and are widely used in the fields of electricity, steel, petroleum, chemical industry, machinery manufacturing, etc.

[0003] The Chinese invention patent with publication number CN115010505A discloses a pressureless sintered high thermal shock resistant silicon carbide ceramic heat exchanger tube and its processing method. The patent improves the thermal shock resistance by adding beneficial components such as titanium diboride and zirconium oxide powder, thereby extending the service life.

[0004] However, the patent obtains ceramic greenware by extrusion molding with an extruder, which improves production efficiency to a certain extent, but cannot obtain ceramic greenware with complex shapes. In addition, the sintering shrinkage rate of the ceramic material is large in this pressureless sintering process, and it does not have the near-net-size molding advantage of reaction sintering. Summary of the invention

[0005] In view of the defects mentioned in the background technology, the present invention provides a method for preparing pressureless sintered extruded silicon carbide ceramics, aiming to achieve the precision and shape complexity of the product.

[0006] In order to achieve the above-mentioned object of the invention, a method for preparing a pressureless sintered extruded silicon carbide ceramic of the present invention comprises the following steps: S1. By weight percentage, 60-75wt% of silicon carbide powder, 2-4wt% of hexagonal boron nitride, 15-20wt% of aggregate, 2-5wt% of graphite, 0.5-1.5wt% of dispersant, 5-8wt% of sintering aid and 0.5-1.5wt% of binder are put into a stirring kettle, and an appropriate amount of deionized water is added, and stirred until the material forms a muddy state; S2. Use a vacuum pump to extract the air in the stirring tank, and mix and stir the materials in a vacuum environment of 0.04MPa for 1.5h. The stirring intensity is measured by the velocity gradient, G = 1000s -1 , a slurry with a water content of 32% is obtained; S3, filtering the slurry to remove large particles, and preparing granulated micropowder with a water content of 6.5% by spray drying the filtered slurry; S4, forming a green body by using a powder extrusion printing technology to form the granulated micro powder; S5, sending the green body to a drying device for preliminary dehydration, the drying temperature is set to 55°C, and the drying time is 2.5h; S6. The dried green body is coated with a layer of graphite felt with a thickness of 6.5 mm and sent into a sintering furnace together. Nitrogen is introduced into the sintering furnace as a protective atmosphere for sintering. The sintering temperature is set to 2000°C. S7. After sintering, the heat preservation time is 2.5 hours. After cooling to room temperature, the graphite felt is removed to obtain a silicon carbide ceramic product.

[0007] Preferably, the sintering aid is a metal oxide.

[0008] Preferably, the metal oxide is a mixture of aluminum oxide, magnesium oxide and yttrium oxide, and the weight ratio of the aluminum oxide, magnesium oxide and yttrium oxide is 2:3:1.

[0009] Preferably, the purity of the silicon carbide powder is ≥99% and the particle size is 3-5 μm.

[0010] Preferably, the aggregate is obtained by grinding and pulverizing waste silicon carbide ceramic products, and the particle size of the aggregate is 8 to 15 μm.

[0011] Preferably, the binder is a mixture of polyvinyl alcohol and carboxymethyl cellulose, and the weight ratio of the polyvinyl alcohol to the carboxymethyl cellulose is 3:2.

[0012] Preferably, the carbon content of the graphite is ≥90%, and the particle size of the graphite is 2-5 μm.

[0013] Preferably, the dispersant is polyvinyl pyrrolidone.

[0014] Preferably, the sintering furnace uses a programmed temperature method to gradually increase the furnace chamber temperature to 2000°C; increase the temperature from room temperature to 625°C at a heating rate of 8°C / min; increase the temperature from 625°C to 1150°C at a heating rate of 12°C / min; increase the temperature from 1150°C to 2000°C at a heating rate of 15°C / min, and finally maintain it at 2000°C until the sintering is completed.

[0015] After adopting the above technical solution, the beneficial effects of the present invention are: This method uses pressureless sintering technology, which does not require external pressure, reducing production costs and equipment complexity; the green body is prepared through powder extrusion printing technology, which improves the accuracy and shape complexity of the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 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.

[0017] Figure 1 This is a microscopic metallographic picture of the silicon carbide ceramic in Example 1; Figure 2 This is a microscopic metallographic picture of the silicon carbide ceramic in Example 2; Figure 3 This is a microscopic metallographic picture of the silicon carbide ceramic in Example 3; Figure 4 This is a microscopic metallographic image of the silicon carbide ceramic in Example 4. DETAILED DESCRIPTION

[0018] 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.

[0019] Example 1 S1. Raw material preparation and mixing Prepare the following raw materials according to weight percentage: 60wt% silicon carbide powder, 20wt% aggregate, 5wt% graphite, 1.5wt% polyvinyl pyrrolidone, 4wt% hexagonal boron nitride, 8wt% sintering aid and 1.5wt% binder. Put the above raw materials into a stirring kettle, add an appropriate amount of deionized water, and stir until the material forms a muddy state.

[0020] As the main raw material of silicon carbide ceramics, the purity of silicon carbide powder needs to be ≥99%, and the particle size is controlled at 3-5μm, with 3.5μm being the best, to ensure the quality and performance of the final silicon carbide ceramic products.

[0021] Aggregates mainly come from waste materials in the production process of silicon carbide ceramic products, such as cracked products, products with large size differences or other defects. These products cannot be sold or used as finished products due to defects. In order to improve the utilization rate of resources, the classified waste materials are sent to grinders or crushers for grinding or crushing. The particle size is controlled at 8-15μm, with 12μm being the best, so that they can be converted into valuable aggregates.

[0022] The added aggregate, as a barren raw material, can effectively reduce the plasticity of the green body and help reduce the shrinkage of the green body during drying and sintering, thereby ensuring the near-net-size molding advantage. Aggregates can also serve as a skeleton to fill the gaps between silicon carbide micropowders, increase the density and strength of the green body, and thus improve the thermal stability and mechanical strength of silicon carbide ceramic products.

[0023] The carbon content of the added graphite is ≥90%, and the particle size is controlled at 2-5μm, with 3.5μm being the best.

[0024] Graphite and hexagonal boron nitride, as lubricants, 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 powder extrusion printing process, the powder material 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. Graphite is an allotrope of elemental carbon. In the later sintering process, the free silicon in the green body will react with graphite at high temperature to form β-silicon carbide, which improves the density and strength of the green body and reduces the presence of free silicon.

[0025] During the sintering process, hexagonal boron nitride can form a solid solution with silicon carbide, resulting in an increase in the concentration of point defects. The increase in the concentration of point defects will promote the diffusion of ions, thereby accelerating the transmission and rearrangement of substances in the sintered body, which is beneficial to the densification of the sintered body.

[0026] As a dispersant, polyvinyl pyrrolidone can be adsorbed on the surface of particles, reduce the surface tension between particles, promote the dispersion and stability of particles in the solvent, and improve the dispersibility and fluidity of the slurry.

[0027] The sintering aid is a mixture of three metal oxides: aluminum oxide, magnesium oxide, and yttrium oxide, with a weight ratio of 2:3:1. The high hardness and high strength characteristics of aluminum oxide enable it to enhance the mechanical properties of silicon carbide ceramics, such as hardness and flexural strength. Magnesium oxide can effectively inhibit the oxidation reaction of silicon carbide ceramics at high temperatures, thereby improving its thermal stability. Magnesium oxide can also promote the crystallization of silicon carbide ceramic grain boundaries, thereby improving grain boundary strength and hardness, and enhancing the mechanical properties and wear resistance of the material. Yttrium oxide can effectively adjust the crystal structure of silicon ceramics, making them more stable, and improve their wear resistance and heat resistance, which helps silicon ceramics maintain stable performance in harsh environments such as high temperature and high pressure.

[0028] Graphite and silicon carbide powder have different thermal expansion coefficients and expansion rates, which will cause pores inside the green body during the sintering process. The volatilization or decomposition of water and binders at high temperatures will also cause pores, thus affecting the density and mechanical properties of silicon carbide ceramic products. Sintering aids such as alumina, magnesium oxide, and yttrium oxide form a liquid phase at high temperatures, which fills into the pores inside the green body 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.

[0029] The binder is a mixture of polyvinyl alcohol and carboxymethyl cellulose, with a weight ratio of 3:2, which 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.

[0030] S2, Vacuum mixing Use a vacuum pump to extract the air in the stirring kettle to create a vacuum environment. The vacuum degree is controlled at 0.04MPa, which helps to remove bubbles in the material and reduce the porosity in the later sintering process. In this vacuum environment, the materials are mixed and stirred for 1.5 hours. The stirring intensity is measured by the speed gradient, G = 1000s -1 . High stirring intensity may cause excessive shearing or damage of materials, while too low stirring intensity may not achieve good mixing effect. Within this velocity gradient range, the particles or molecules inside the fluid will be subjected to sufficient shearing and collision forces, thereby achieving good mixing and dispersion effects. Stir until the water content of the slurry reaches 32% to ensure the fluidity and plasticity of the slurry.

[0031] S3. Slurry treatment The obtained slurry is filtered to remove large particles to ensure the fineness of the slurry. The filtered slurry is spray-dried to form a granulated micropowder with a water content of 6.5%, converting the slurry into a granular material that is easy to extrude and print.

[0032] S4, body forming Granulated micro powder is made into a green body through powder extrusion printing technology (PEP). PEP technology does not require the use of molds, thereby reducing development and production costs. PEP technology can obtain high-precision and high-quality green bodies by extruding materials and printing layer by layer. PEP technology can also achieve high-degree-of-freedom design and can produce complex shapes and structures that are difficult to achieve with traditional processes.

[0033] S5. Drying The formed green body is sent to the drying equipment, and the drying temperature is set to 55°C and the drying time is set to 2.5 hours. During the drying process, it is necessary to ensure that the green body is evenly heated to avoid cracking or deformation.

[0034] S6. Sintering A layer of graphite felt with a thickness of 6.5 mm is wrapped on the outside of the green body to enhance its thermal stability and prevent contamination during sintering. The green body wrapped with graphite felt is sent to the sintering furnace, the sintering temperature is set to 2000°C, and nitrogen is introduced as a protective atmosphere to prevent the occurrence of oxidation reaction.

[0035] Graphite felt can ensure that the green body is sintered under a stable temperature and environment, effectively prevent heat loss, ensure that the green body can be stably sintered within the required temperature range, prevent the green body from generating thermal stress due to excessive temperature gradient, and reduce cracking and deformation caused by temperature fluctuations.

[0036] The sintering furnace uses a programmed temperature method to gradually increase the furnace chamber temperature to 2000°C. The heating process is divided into three stages: from room temperature to 625°C, the heating rate is 8°C / min; from 625°C to 1150°C, the heating rate is 12°C / min; from 1150°C to 2000°C, the heating rate is 15°C / min.

[0037] S7, insulation After the firing is completed, the heat preservation treatment is carried out. The temperature is reduced from 2000℃ during sintering to 1650℃, and then to 650℃, and the total heat preservation time is 2.5 hours to ensure the complete sintering of the ceramic product and the stability of the internal structure. After the heat preservation is completed, the furnace door is opened, the graphite felt is removed, and the ceramic product is allowed to cool naturally to room temperature. At this point, the finished silicon carbide ceramic product can be obtained.

[0038] The thermal expansion coefficient of the silicon carbide ceramic product sample was measured to be 4.3×10 -6 / ℃. According to GB / T 16555.3-1996 "Chemical analysis methods for silicon carbide refractory materials - Determination of free silicon content by gas volumetric method", the free silicon content of silicon carbide ceramic products was measured to be 2.8%.

[0039] Example 2 Different from Example 1, the material ratio is: 75wt% silicon carbide powder, 2wt% hexagonal boron nitride, 15wt% aggregate, 2wt% graphite, 0.5wt% polyvinyl pyrrolidone, 5% sintering aid and 0.5wt% binder.

[0040] The thermal expansion coefficient of the silicon carbide ceramic product sample was measured to be 4.2×10 -6 / ℃. According to GB / T 16555.3-1996 "Chemical analysis methods for silicon carbide refractory materials - Determination of free silicon content by gas volumetric method", the free silicon content of silicon carbide ceramic products was measured to be 3.1%.

[0041] Example 3 Different from Example 1, the material ratio is: 65wt% silicon carbide powder, 3.5wt% hexagonal boron nitride, 18wt% aggregate, 4.5wt% graphite, 1wt% polyvinyl pyrrolidone, 7% sintering aid and 1wt% binder.

[0042] The thermal expansion coefficient of the silicon carbide ceramic product sample was measured to be 4.4×10 -6 / ℃. According to GB / T 16555.3-1996 "Chemical analysis methods for silicon carbide refractory materials - Determination of free silicon content by gas volumetric method", the free silicon content of silicon carbide ceramic products was measured to be 2.8%.

[0043] Example 4 Different from Example 1, the material ratio is: 70wt% silicon carbide powder, 2.5wt% hexagonal boron nitride, 16.5wt% aggregate, 2.5wt% graphite, 1wt% polyvinyl pyrrolidone, 6.5% sintering aid and 1wt% binder.

[0044] The thermal expansion coefficient of the silicon carbide ceramic product sample was measured to be 4.3×10 -6 / ℃. According to GB / T 16555.3-1996 "Chemical analysis methods for silicon carbide refractory materials - Determination of free silicon content by gas volumetric method", the free silicon content of silicon carbide ceramic products was measured to be 2.9%.

[0045] 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 pressureless sintered extruded silicon carbide ceramics, characterized in that: The steps include: S1. By weight percentage, 60-75wt% of silicon carbide powder, 2-4wt% of hexagonal boron nitride, 15-20wt% of aggregate, 2-5wt% of graphite, 0.5-1.5wt% of dispersant, 5-8wt% of sintering aid and 0.5-1.5wt% of binder are put into a stirring kettle, and an appropriate amount of deionized water is added, and stirred until the material forms a muddy state; S2. Use a vacuum pump to extract the air in the stirring tank, and mix and stir the materials in a vacuum environment of 0.04MPa for 1.5h. The stirring intensity is measured by the velocity gradient, G = 1000s -1 , a slurry with a water content of 32% is obtained; S3, filtering the slurry to remove large particles, and preparing granulated micropowder with a water content of 6.5% by spray drying the filtered slurry; S4, forming a green body by using a powder extrusion printing technology to form the granulated micro powder; S5, sending the green body to a drying device for preliminary dehydration, the drying temperature is set to 55°C, and the drying time is 2.5h; S6. The dried green body is coated with a layer of graphite felt with a thickness of 6.5 mm and sent into a sintering furnace together. Nitrogen is introduced into the sintering furnace as a protective atmosphere for sintering. The sintering temperature is set to 2000°C. S7. After sintering, the heat preservation time is 2.5 hours. After cooling to room temperature, the graphite felt is removed to obtain a silicon carbide ceramic product.

2. The method for preparing pressureless sintered extruded silicon carbide ceramics according to claim 1, characterized in that: The sintering aid is a metal oxide.

3. The method for preparing pressureless sintered extruded silicon carbide ceramic according to claim 2, characterized in that: The metal oxide is a mixture of aluminum oxide, magnesium oxide and yttrium oxide, and the weight ratio of the aluminum oxide, magnesium oxide and yttrium oxide is 2:3:

1.

4. The method for preparing pressureless sintered extruded silicon carbide ceramics according to claim 1, characterized in that: The purity of the silicon carbide micropowder is ≥99%, and the particle size is 3-5 μm.

5. The method for preparing pressureless sintered extruded silicon carbide ceramics according to claim 1, characterized in that: The aggregate is obtained by grinding and pulverizing waste silicon carbide ceramic products, and the particle size of the aggregate is 8 to 15 μm.

6. The method for preparing pressureless sintered extruded silicon carbide ceramics according to claim 1, characterized in that: The binder is a mixture of polyvinyl alcohol and carboxymethyl cellulose, and the weight ratio of the polyvinyl alcohol to the carboxymethyl cellulose is 3:

2.

7. The method for preparing pressureless sintered extruded silicon carbide ceramics according to claim 1, characterized in that: The carbon content of the graphite is ≥90%, and the particle size of the graphite is 2-5 μm.

8. The method for preparing pressureless sintered extruded silicon carbide ceramics according to claim 1, characterized in that: The dispersant is polyvinyl pyrrolidone.

9. The method for preparing pressureless sintered extruded silicon carbide ceramics according to claim 1, characterized in that: The sintering furnace adopts a programmed temperature rising method to gradually increase the furnace chamber temperature to 2000°C; the temperature is raised from room temperature to 625°C at a heating rate of 8°C / min; the temperature is raised from 625°C to 1150°C at a heating rate of 12°C / min; the temperature is raised from 1150°C to 2000°C at a heating rate of 15°C / min, and finally maintained at 2000°C until the sintering is completed.

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