Silicon carbide gel blank drying method and application thereof

Abstract

The invention relates to a silicon carbide gel blank drying method and application thereof. The drying method comprises the following steps that an alcohol-based silicon carbide gel blank is placed in a supercritical carbon dioxide system, absolute ethyl alcohol is used for immersing the alcohol-based silicon carbide gel blank, then carbon dioxide is introduced for drying, and drying comprises a supercritical carbon dioxide flushing drying stage and a supercritical carbon dioxide static drying stage. The technical problem to be solved is how to provide the supercritical carbon dioxide drying method for the alcohol-based silicon carbide gel green body, and the problems of shrinkage, structure collapse, cracking and the like of the alcohol-based silicon carbide gel green body due to liquid-gas interfacial tension and uneven and excessive drying of an inner layer and an outer layer in the supercritical carbon dioxide drying process are solved.

Description

Technical Field

[0001] The present invention belongs to the field of silicon carbide, and particularly relates to a method for drying a silicon carbide gel green body and its application. Background Art

[0002] The silicon carbide gel casting molding technology combines the principles of polymer chemistry and rheology, and is a low-cost, near-net-size molding method, which is suitable for preparing silicon carbide ceramic products with complex shapes and high performance.

[0003] The drying of the silicon carbide gel green body is an important link in the preparation of high-performance silicon carbide ceramics by the silicon carbide gel casting molding technology. The purpose is to remove the residual solvents and other volatile substances in the gel green body, and at the same time, try to maintain the structural integrity and shape stability of the green body. However, during the drying process of the silicon carbide gel green body, especially for large-sized green bodies, due to the high solid content of the gel in the green body and the complex internal network structure, a series of problems are often caused, such as green body cracking, non-uniform shrinkage, and microstructural damage. These problems mainly come from the capillary action and thermal stress caused by the solvent volatilization during the drying process, as well as the stress concentration caused by the mismatch between the solvent diffusion and evaporation rates inside the gel green body.

[0004] Some literature first places the silicon carbide gel green body in absolute ethanol for complete solvent replacement to form an alcohol-based silicon carbide gel green body, and then places the alcohol-based silicon carbide gel green body in a carbon dioxide environment for continuous static drying. This method reduces the generation of capillary pressure to a certain extent, and thus reduces the volume mutation or stress concentration caused by phase change, can protect the microstructure of the gel green body, and effectively prevent the green body from cracking, shrinking or deforming. However, the drying method of the silicon carbide gel green body disclosed in the aforementioned literature still has problems such as excessive shrinkage, structural collapse, and cracking caused by liquid-gas interfacial tension and uneven drying of the inner and outer layers during the drying process. Summary of the Invention

[0005] The main purpose of the present invention is to provide a method for drying a silicon carbide gel green body and its application. The technical problem to be solved is how to provide a supercritical carbon dioxide drying method for an alcohol-based silicon carbide gel green body to avoid problems such as excessive shrinkage, structural collapse, and cracking caused by liquid-gas interfacial tension and uneven drying of the inner and outer layers during the supercritical carbon dioxide drying process of the alcohol-based silicon carbide gel green body.

[0006] The object of the present invention and the technical problem to be solved are achieved by the following technical solutions. A method for drying a silicon carbide gel green body according to the present invention includes the following steps: Place the alcohol-based silicon carbide gel green body in a supercritical carbon dioxide system, immerse the alcohol-based silicon carbide gel green body with absolute ethanol, and then introduce carbon dioxide for drying. The drying includes a supercritical carbon dioxide flushing and drying stage and a supercritical carbon dioxide static drying stage.

[0007] The object of the present invention and the technical problems to be solved can be further realized by the following technical measures.

[0008] Preferably, in the aforementioned drying method, the supercritical carbon dioxide flushing and drying includes: flushing by introducing 1 - 1.5 L / min of carbon dioxide into each liter of the supercritical carbon dioxide system under the conditions of a temperature of 50 - 60 °C and a pressure of 10 - 11 MPa.

[0009] Preferably, in the aforementioned drying method, the temperature of the supercritical carbon dioxide static drying is 50 - 60 °C and the pressure is 11 - 12 MPa.

[0010] Preferably, in the aforementioned drying method, it further includes the following steps: Completely replace the solvent of the water-based silicon carbide gel green body with an alcohol solvent displacement liquid of equal gradient concentration to obtain the alcohol-based silicon carbide gel green body; the alcohol solvent displacement liquid is methanol and / or ethanol.

[0011] Preferably, in the aforementioned drying method, the alcohol solvent displacement liquid of equal gradient concentration successively includes ethanol aqueous solutions with volume concentrations of 25%, 50%, 75%, and 100%.

[0012] Preferably, in the aforementioned drying method, during the process of completely replacing the solvent of the water-based silicon carbide gel green body with an alcohol solvent displacement liquid of equal gradient concentration, the water-based silicon carbide gel green body is soaked in the alcohol solvent displacement liquid of each concentration for 4 - 6 h.

[0013] Preferably, in the aforementioned drying method, the alcohol solvent displacement liquid further includes glycerol with a volume concentration of 0 - 10%.

[0014] Preferably, in the aforementioned drying method, the supercritical carbon dioxide flushing and drying stage and the supercritical carbon dioxide static drying stage are alternately cycled; the supercritical carbon dioxide flushing and drying stage lasts for 25 - 45 min each time; the supercritical carbon dioxide static drying stage lasts for at least 60 min each time.

[0015] Preferably, in the aforementioned drying method, the water content of the water-based silicon carbide gel green body is 12 - 20 wt%.

[0016] The object of the present invention and the technical problems to be solved thereof are also achieved by the following technical solutions. A porous silicon carbide ceramic according to the present invention, the silicon carbide ceramic gel green body is dried by the drying method described in the foregoing solution.

[0017] By means of the above technical solutions, a drying method and application of a silicon carbide gel green body proposed by the present invention have at least the following advantages: A drying method of a silicon carbide gel green body provided by the present invention, during the drying process of placing an alcohol-based silicon carbide gel green body in a supercritical carbon dioxide system, before introducing carbon dioxide, the alcohol-based silicon carbide gel green body is immersed in anhydrous ethanol, and then supercritical carbon dioxide flushing drying and supercritical carbon dioxide static drying are carried out in sequence, so as to avoid the generation of liquid-gas interfacial tension during the drying process of the alcohol-based silicon carbide gel green body and control the uniform drying of the inner and outer layers of the alcohol-based silicon carbide gel green body, thereby avoiding the problems of structural collapse and cracking of the alcohol-based silicon carbide gel green body.

[0018] The above description is only an overview of the technical solutions of the present invention. In order to be able to more clearly understand the technical means of the present invention and implement it in accordance with the content of the specification, the following takes the preferred embodiments of the present invention and combines with the accompanying drawings to describe in detail as follows. Brief Description of the Drawings

[0019] Figure 1 SEM image of the silicon carbide gel green body prepared in Example 1; Figure 2 SEM image of the silicon carbide gel green body prepared in Comparative Example 1. Detailed Description of the Invention

[0020] To further elaborate on the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following combines the accompanying drawings and preferred embodiments to describe in detail a drying method and application of a silicon carbide gel green body according to the present invention. Its specific implementation manner, structure, characteristics and effects are described in detail as follows.

[0021] The applicant found through research that directly placing the alcohol-based silicon carbide gel green body in a static carbon dioxide environment for supercritical drying is prone to cracks. This is because the alcohol-based silicon carbide gel green body is prone to generate liquid-gas interfacial tension during static drying, and during the drying process, carbon dioxide penetrates from the outside to the inside of the green body, and uneven drying of the inner and outer layers is prone to occur when the green body dries from the outside to the inside.

[0022] In order to prevent problems such as premature drying of the surface causing cracks and uneven drying of the inner and outer layers, in the present invention, after the alcohol-based silicon carbide gel green body is placed in a supercritical carbon dioxide system, the green body is immersed in anhydrous ethanol, and then carbon dioxide is introduced for supercritical drying to avoid the generation of liquid-gas interfacial tension during the drying process. However, during the research process, the applicant found that if only supercritical carbon dioxide static drying is used, during the pressure reduction process, residual ethanol vapor is likely to liquefy and penetrate into the green body again, so that the alcohol solvent inside the green body is not completely dried, affecting the quality of the subsequent prepared porous silicon carbide. The applicant further studied and found that before the static supercritical carbon dioxide green body, the green body is rinsed with supercritical carbon dioxide to remove the excess alcohol solvent on the surface of the green body, and the excess alcohol solvent is separated from the system along with the carbon dioxide fluid during the rinsing process. By combining rinsing and static drying, the applicant not only reduces or even avoids the problem of alcohol solvent residue after the alcohol-based silicon carbide gel green body is dried by supercritical carbon dioxide, but also solves the problems of liquid-gas interfacial tension generated in the green body during the drying process and uneven drying of the inner and outer layers due to too fast drying of the green body surface. The formed silicon carbide gel green body after drying is as Figure 1 shown, and there are no problems such as structural collapse in the green body.

[0023] In view of the above, the present invention provides a method for drying a silicon carbide gel green body, which includes the following steps: Place the alcohol-based silicon carbide gel green body in a supercritical carbon dioxide system, immerse the alcohol-based silicon carbide gel green body in anhydrous ethanol, and then introduce carbon dioxide for drying. The drying includes a supercritical carbon dioxide rinsing and drying stage and a supercritical carbon dioxide static drying stage.

[0024] In the foregoing solution, the supercritical carbon dioxide rinsing and drying stage refers to continuously introducing carbon dioxide gas into the supercritical carbon dioxide system where the alcohol-based silicon carbide gel green body is placed, and the mixed fluid (supercritical carbon dioxide and ethanol solvent) in the supercritical carbon dioxide system can flow out from the outlet.

[0025] In the foregoing solution, the supercritical carbon dioxide static drying stage means that the supercritical carbon dioxide system where the alcohol-based silicon carbide gel green body is placed is in a closed state, and the supercritical carbon dioxide system is filled with carbon dioxide.

[0026] In order to more effectively remove the anhydrous ethanol on the surface of the alcohol-based silicon carbide gel green body through supercritical carbon dioxide rinsing and drying, and better ensure the structural integrity of the alcohol-based silicon carbide gel green body during the supercritical carbon dioxide rinsing and drying stage, in some embodiments of the present invention, during the supercritical carbon dioxide rinsing and drying process, the temperature of the supercritical carbon dioxide system is 50-60 °C, the pressure is 10-11 MPa, and 1-1.5 L / min of carbon dioxide is introduced into each liter of the supercritical carbon dioxide system for rinsing.

[0027] In order to more effectively displace the alcohol solvent inside the alcohol-based silicon carbide gel green body through the supercritical carbon dioxide static drying, in some embodiments of the present invention, the supercritical carbon dioxide static drying temperature is 50-60°C and the pressure is 11-12 MPa.

[0028] In the foregoing solution, during the process of forming an alcohol-based silicon carbide gel green body by solvent displacement of the water-based silicon carbide gel green body, in order to avoid excessive changes in the surface tension of the solvent causing changes in the microstructure of the water-based silicon carbide gel green body, preferably, in some embodiments, the water-based silicon carbide gel green body is completely displaced with an alcohol solvent displacement liquid having an equal gradient concentration to obtain the alcohol-based silicon carbide gel green body; the alcohol solvent displacement liquid described in the present invention refers to an alcohol solvent displacement liquid containing a certain mass percentage, and the mass percentage of the alcohol organic solvent in the alcohol solvent displacement liquid is any value within the range of 0-100%. In other embodiments, the alcohol solvent displacement liquid is methanol and / or ethanol.

[0029] In the foregoing solution, in some embodiments, further preferably, the alcohol solvent displacement liquid with an equal gradient concentration sequentially includes ethanol aqueous solutions with volume concentrations of 25%, 50%, 75%, and 100%. Under this condition, the integrity of the microstructure of the alcohol-based silicon carbide gel green body can be better protected.

[0030] In the foregoing solution, in some embodiments, further preferably, during the process of completely displacing the solvent of the water-based silicon carbide gel green body with an alcohol solvent displacement liquid having an equal gradient concentration, the water-based silicon carbide gel green body is soaked in the alcohol solvent displacement liquid at each concentration for 4-6 h. The present invention controls the soaking time of the water-based silicon carbide gel green body in the solvent displacement liquid at each concentration, which not only ensures complete solvent displacement but also avoids the influence of too long soaking time on the microstructure of the alcohol-based silicon carbide gel green body.

[0031] In the foregoing solution, in some embodiments, further preferably, the alcohol solvent displacement liquid further includes glycerol with a volume concentration of 0-10%. This enables the water-based silicon carbide gel green body to be quickly shaped during the solvent displacement process, thereby making the microstructure integrity of the formed alcohol-based silicon carbide gel green body better.

[0032] In the foregoing solution, in order to better achieve the design purpose of this solution, the supercritical carbon dioxide rinsing and drying stage and the supercritical carbon dioxide static drying stage are alternately cycled; each supercritical carbon dioxide rinsing and drying stage lasts for 25 - 45 minutes; each supercritical carbon dioxide static drying stage lasts for at least 60 minutes. Preferably, in some embodiments, the supercritical carbon dioxide rinsing and drying stage and the supercritical carbon dioxide static drying stage are carried out at least 3 times, which can not only gradually and steadily displace the solvent, but also better ensure the integrity of the microstructure of the silicon carbide gel green body.

[0033] In the foregoing solution, in some embodiments, the water-based silicon carbide gel green body has a moisture content of 12 - 20 wt%. Under this condition, the solvent in the silicon carbide gel green body obtained by the solvent displacement and supercritical carbon dioxide drying method of the technical solution of the present invention is completely dried, and the microstructure has good integrity.

[0034] In the foregoing solution, in some embodiments, after the silicon carbide gel green body is completed with supercritical carbon dioxide drying, during the pressure reduction process, in order to avoid the collapse of the gel structure caused by too rapid pressure change, preferably, the pressure reduction rate is controlled at 0.3 - 0.5 MPa / min.

[0035] In the foregoing solution, in some embodiments, the preparation method of the water-based silicon carbide gel green body is as follows: S1 Mix methylacrylamide monomer, methylene bisacrylamide cross-linking monomer, ammonium polyacrylate and deionized water in a mass ratio of 45 - 50:2 - 4:1:45 - 50 to form a slightly yellow transparent premixed liquid; S2 Mix silicon carbide powder and the premixed liquid in a mass ratio of 1.8 - 2.0:1, and grind evenly to obtain a black opaque silicon carbide slurry; in some embodiments, the purity of the silicon carbide powder is ≥95%; S3 Mix the silicon carbide slurry, ammonium persulfate solution and tetramethylethylenediamine solution and grind evenly to form a slurry to be cast. In terms of mass percentage content, in some embodiments, the ammonium persulfate in the slurry to be cast is 0.02 - 0.03%, and the tetramethylethylenediamine is 0.01 - 0.02%; preferably, in some other embodiments, the mass ratio of ammonium persulfate to tetramethylethylenediamine in the slurry to be cast is 2 - 2.5:1. In some other embodiments, the mass concentrations of the ammonium persulfate solution and the tetramethylethylenediamine solution in the slurry to be cast are both 8 - 12%.

[0036] In the foregoing solution, the water-based silicon carbide gel green body is subjected to replacement with an alcohol-containing solvent and supercritical carbon dioxide drying to form a dried green body. The dried green body is placed in a vacuum drying oven and dried at 70-80 °C for 16-24 h to finally obtain a completely dried porous silicon carbide gel green body.

[0037] The present invention also provides a porous silicon carbide ceramic, and the silicon carbide ceramic gel green body is dried by the drying method described in the foregoing solution.

[0038] The present invention will be further described below in conjunction with specific embodiments, but it should not be construed as a limitation on the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art to the present invention based on the above content of the present invention still fall within the protection scope of the present invention.

[0039] Unless otherwise specified, the materials, reagents, etc. involved below are all commercially available products well-known to those skilled in the art; unless otherwise specified, the methods are all well-known methods in the art. Unless otherwise defined, the technical terms or scientific terms used should have the ordinary meaning understood by those of ordinary skill in the art to which the present invention belongs.

[0040] Example 1 The present invention prepares a silicon carbide gel green body, and the specific method is as follows: 1) Place 94 g of deionized water and 94 g of methylacrylamide monomer in a ball milling tank, add a magnetic stirrer and stir evenly. Then add 6 g of methylene bisacrylamide crosslinking monomer and dissolve and stir evenly. Further add 2 g of ammonium polyacrylate and dissolve and stir evenly to obtain a slightly yellow transparent premixed liquid.

[0041] 2) Add 390 g of silicon carbide powder to the ball milling tank containing the premixed solution, add silicon carbide grinding balls, and place it in a planetary ball mill for sufficient grinding to obtain a black opaque silicon carbide slurry.

[0042] 3) Add 1.5 mL of ammonium persulfate solution with a mass concentration of 10% and 0.75 mL of tetramethylethylenediamine solution with a mass concentration of 10% to the silicon carbide slurry, mix evenly by ball milling, remove the grinding balls, obtain the slurry to be cast, after vacuum degassing treatment, carry out casting treatment, and then let it stand for 16 h to achieve complete gelation, to obtain a water-based silicon carbide gel green body with a diameter of 80 mm and a height of 40 mm, and the water content of the water-based silicon carbide gel green body is 16 wt%.

[0043] The water-based silicon carbide gel green body prepared in this example is subjected to supercritical carbon dioxide drying, specifically as follows: Prepare a series of concentration replacement solutions: Mix absolute ethanol, deionized water, and glycerol as shown in Table 1 to prepare gradient concentration replacement solutions with ethanol volume ratios of 25%, 50%, 75%, and 100%.

[0044] Table 1 Component content of gradient concentration replacement solutions Soak the water-based silicon carbide gel green body in replacement solution 1, replacement solution 2, replacement solution 3, and replacement solution 4 for 4 hours each in sequence, and finally obtain a completely replaced ethanol-based silicon carbide gel green body.

[0045] Place the ethanol-based silicon carbide gel green body in a carbon dioxide supercritical drying container, and add absolute ethanol to the container to submerge the surface of the ethanol-based silicon carbide gel green body.

[0046] Close the outlet valve of the supercritical container, start the preheating device, heat the drying container to 50 °C, start the pressurizing device, slowly pump carbon dioxide into the drying container, and pressurize it to 12 MPa to make carbon dioxide enter the supercritical state.

[0047] Open the high-pressure regulating valve at the lower end of the drying container, continuously supply carbon dioxide by the high-pressure pump and keep the internal pressure of the container stable at 10 MPa, so that the mixed fluid (supercritical carbon dioxide and ethanol solvent) is discharged from the lower outlet, and continuously rinse for 30 minutes. The flow rate of carbon dioxide introduced per liter of volume during the rinsing stage is 1 L / min.

[0048] Close the carbon dioxide inlet valve and outlet valve of the drying container, keep the internal pressure of the container at 12 MPa, and be in a static supercritical state to promote the diffusion of the internal carbon dioxide solvent to the surface and reach an equilibrium state. The static supercritical state is maintained for 60 minutes.

[0049] Continuously circulate supercritical carbon dioxide rinsing and drying and supercritical carbon dioxide static drying 3 times until no liquid flows out of the drying container.

[0050] Open the carbon dioxide outlet valve, gradually reduce the pressure in the drying container to atmospheric pressure, and control the pressure reduction rate to 0.4 MPa / min.

[0051] Take out the green body after carbon dioxide supercritical drying, place it in a vacuum drying oven and dry it at 80 °C for 16 hours to finally obtain a completely dry porous silicon carbide gel green body.

[0052] Index detection: Perform the following detections on the completely dry silicon carbide gel green body: Compressive strength: Sample preparation: Cut the sample into a cube with a side length of 10 mm ± 0.2 mm, and the parallelism deviation of the upper and lower surfaces is less than 0.02 mm (Experimental method for room temperature compressive strength of porous ceramics - GB / T 1964-2023).

[0053] Testing: Ten test samples are used. The testing instrument is an electronic universal testing machine. The moving speed of the crossbeam of the testing machine is 0.5 mm / min until the sample is damaged and the maximum load at the time of damage is recorded. The final result is the average compressive strength.

[0054] Pore size and porosity: Use the mercury intrusion method (Determination of pore size distribution and porosity of solid materials by mercury intrusion method and gas adsorption method - Part 1: Mercury intrusion method - GB / T 21650.1-2008).

[0055] Appearance: Observe the cracks and collapse of the silicon carbide gel green body.

[0056] Microscopic morphology of the porous silicon carbide gel green body: Observe using a scanning electron microscope.

[0057] The test results are shown in Table 2, and the microscopic morphology of the porous silicon carbide gel green body is shown in the appendix Figure 1 .

[0058] Example 2 This example is basically the same as Example 1, except that the process conditions are different during the supercritical carbon dioxide drying process of the water-based silicon carbide gel green body, as follows: Immerse the water-based silicon carbide gel green body in displacement solution 1, displacement solution 2, displacement solution 3, and displacement solution 4 for 5 hours each, and finally obtain a completely replaced ethanol-based silicon carbide gel green body.

[0059] Place the ethanol-based silicon carbide gel green body in a supercritical carbon dioxide drying container, and add anhydrous ethanol to the container to submerge the surface of the ethanol-based silicon carbide gel green body.

[0060] During the supercritical carbon dioxide flushing and drying process, the internal pressure of the container is stabilized at 10.5 MPa, the continuous flushing process lasts for 25 minutes, and the flow rate of carbon dioxide introduced per liter of volume during the flushing stage is 1.2 L / min.

[0061] During the supercritical carbon dioxide static drying process, the internal pressure of the container is maintained at 11.5 MPa, and the static supercritical state is maintained for 65 minutes.

[0062] Continuously and cyclically perform supercritical carbon dioxide flushing and drying and supercritical carbon dioxide static drying 4 times until no liquid flows out of the drying container. Open the carbon dioxide outlet valve to reduce the pressure, and control the pressure reduction speed to 0.3 MPa / min.

[0063] Take out the green body after supercritical drying of carbon dioxide, place it in a vacuum drying oven and dry it at 75 °C for 20 h to finally obtain a completely dry porous silicon carbide gel green body.

[0064] Index detection: Perform compressive strength, pore size and porosity, and appearance detection on the completely dry silicon carbide gel green body. The specific method is as in Example 1, and the detection results are shown in Table 2.

[0065] Example 3 This example is basically the same as Example 1, except that the process conditions are different during the supercritical carbon dioxide drying of the water-based silicon carbide gel green body, as follows: Soak the water-based silicon carbide gel green body in replacement solution 1, replacement solution 2, replacement solution 3, and replacement solution 4 for 6 h each in sequence to finally obtain a completely replaced ethanol-based silicon carbide gel green body.

[0066] Place the ethanol-based silicon carbide gel green body in a carbon dioxide supercritical drying container, and add anhydrous ethanol to the container to submerge the surface of the ethanol-based silicon carbide gel green body.

[0067] During the supercritical carbon dioxide flushing and drying process, the internal pressure of the container is stabilized at 11 MPa, the continuous flushing process lasts for 45 min, and the flow rate of carbon dioxide introduced per liter of volume during the flushing stage is 1 L / min.

[0068] During the supercritical carbon dioxide static drying process, the internal pressure of the container is maintained at 11 MPa, and the static supercritical state is maintained for 70 min.

[0069] Open the carbon dioxide outlet valve to reduce the pressure, and control the pressure reduction speed to 0.5 MPa / min.

[0070] Take out the green body after supercritical drying of carbon dioxide, place it in a vacuum drying oven and dry it at 70 °C for 24 h to finally obtain a completely dry porous silicon carbide gel green body.

[0071] Index detection: Perform compressive strength, pore size and porosity, and appearance detection on the completely dry silicon carbide gel green body. The specific method is as in Example 1, and the detection results are shown in Table 2.

[0072] Example 4 This example is basically the same as Example 1, and the differences are as follows: Soak the water-based silicon carbide gel green body directly in anhydrous ethanol containing 10% glycerol for 16 h.

[0073] The remaining processes and steps are the same as in Example 1.

[0074] Index detection: The compressive strength, pore size and porosity, and appearance of the completely dry silicon carbide gel green body were detected. The specific method was as in Example 1, and the detection results are shown in Table 2.

[0075] Example 5 This example is basically the same as Example 1, and the differences are as follows: During the preparation of the gradient concentration displacement liquid with ethanol volume ratios of 25%, 50%, 75%, and 100%, glycerol was not added.

[0076] The remaining processes and steps were the same as those in Example 1.

[0077] Index detection: The compressive strength, pore size and porosity, and appearance of the completely dry silicon carbide gel green body were detected. The specific method was as in Example 1, and the detection results are shown in Table 2.

[0078] Example 6 This example is basically the same as Example 1, and the differences are as follows: During the preparation of the displacement liquid, methanol was used to replace ethanol.

[0079] The remaining processes and steps were the same as those in Example 1.

[0080] Index detection: The compressive strength, pore size and porosity, and appearance of the completely dry silicon carbide gel green body were detected. The specific method was as in Example 1, and the detection results are shown in Table 2.

[0081] Example 7 This example is basically the same as Example 1, and the difference is that: During the preparation of the water-based silicon carbide gel green body, by adjusting the mass ratio of methylacrylamide monomer, methylene bisacrylamide cross-linking monomer, ammonium polyacrylate, and deionized water, the water content of the prepared water-based silicon carbide gel green body was 12 wt%.

[0082] Index detection: The compressive strength, pore size and porosity, and appearance of the completely dry silicon carbide gel green body were detected. The specific method was as in Example 1, and the detection results are shown in Table 2.

[0083] Example 8 This example is basically the same as Example 1, and the difference is that: During the preparation of the water-based silicon carbide gel green body, by adjusting the mass ratio of methylacrylamide monomer, methylene bisacrylamide cross-linking monomer, ammonium polyacrylate, and deionized water, the water content of the prepared water-based silicon carbide gel green body was 20 wt%.

[0084] Index detection: The compressive strength, pore size and porosity, and appearance of the completely dried silicon carbide gel green body were detected. The specific method was as in Example 1, and the detection results are shown in Table 2.

[0085] Comparative Example 1 The difference between this comparative example and Example 1 is as follows: During the supercritical drying process of the prepared ethanol-based silicon carbide gel green body, the ethanol-based silicon carbide gel green body was placed in a carbon dioxide supercritical drying container, and anhydrous ethanol was added to the container to submerge the surface of the ethanol-based silicon carbide gel green body.

[0086] Close the outlet valve of the supercritical container, start the preheating device, heat the drying container to 50 °C, start the pressurizing device, slowly pump carbon dioxide into the drying container, and pressurize it to 12 MPa to make carbon dioxide enter the supercritical state. Close the carbon dioxide inlet valve of the drying container to keep the internal pressure of the container at 12 MPa and be in a static supercritical state. The static supercritical state is maintained for 4.5 h.

[0087] Open the carbon dioxide outlet valve and gradually reduce the pressure in the drying container to atmospheric pressure. The pressure reduction rate is controlled at 0.4 MPa / min.

[0088] Take out the green body after carbon dioxide supercritical drying, place it in a vacuum drying oven and dry it at 80 °C for 16 h to finally obtain a completely dried porous silicon carbide gel green body.

[0089] Index detection: The compressive strength, pore size and porosity, appearance, and microscopic morphology of the completely dried silicon carbide gel green body were detected. The specific method was as in Example 1, and the detection results are shown in Table 2. The microscopic morphology of the porous silicon carbide gel green body in this comparative example is shown in the appendix Figure 2 .

[0090] Table 2 Detection results of each example and comparative example From the detection results and drawings of the above examples and comparative examples, it can be seen that the silicon carbide gel green body obtained by using the drying method of the silicon carbide gel green body provided by the present invention has good structural integrity, good compressive strength, small pore size and appropriate porosity.

[0091] The technical features in the claims and / or the specification of the present invention can be combined, and the combination method is not limited to the combination obtained through the citation relationship in the claims. The technical solutions obtained by combining the technical features in the claims and / or the specification are also within the protection scope of the present invention.

[0092] The above are only the preferred embodiments of the present invention, and do not impose any form of limitation on the present invention. Any simple modifications, equivalent changes and decorations made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A method for drying a silicon carbide gel green body, characterized in that, It includes the following steps: Place the alcohol-based silicon carbide gel green body in a supercritical carbon dioxide system, immerse the alcohol-based silicon carbide gel green body with absolute ethanol, and then introduce carbon dioxide for drying. The drying includes a supercritical carbon dioxide flushing and drying stage and a supercritical carbon dioxide static drying stage.

2. The drying method according to claim 1, wherein The supercritical carbon dioxide flushing and drying includes: flushing by introducing 1-1.5 L / min of carbon dioxide into each liter of the supercritical carbon dioxide system under the conditions of a temperature of 50-60 °C and a pressure of 10-11 MPa.

3. The drying method according to claim 2, wherein The temperature of the supercritical carbon dioxide static drying is 50-60 °C and the pressure is 11-12 MPa.

4. In the drying method according to claim 1, characterized in that, It further includes the following steps: Completely replace the solvent of the water-based silicon carbide gel green body with an alcohol solvent replacement liquid having an equal gradient concentration to obtain the alcohol-based silicon carbide gel green body; the alcohol solvent replacement liquid is methanol and / or ethanol.

5. The drying method according to claim 4, wherein The alcohol solvent replacement liquid with an equal gradient concentration successively includes ethanol aqueous solutions with volume concentrations of 25%, 50%, 75%, and 100%.

6. The drying method according to claim 5, characterized in that, During the process of completely replacing the solvent of the water-based silicon carbide gel green body with the alcohol solvent replacement liquid with an equal gradient concentration, the water-based silicon carbide gel green body is soaked in the alcohol solvent replacement liquid at each concentration for 4-6 h.

7. The drying method according to any one of claims 4 to 6, characterized in that The alcohol solvent replacement liquid also includes glycerol with a volume concentration of 0-10%.

8. The drying method according to any one of claims 1 to 6, characterized in that The supercritical carbon dioxide flushing and drying stage and the supercritical carbon dioxide static drying stage are alternately cycled; the supercritical carbon dioxide flushing and drying stage lasts for 25-45 min each time; the supercritical carbon dioxide static drying stage lasts for at least 60 min each time.

9. The drying method according to claim 5 or 6, characterized in that The water content of the water-based silicon carbide gel green body is 12-20 wt%.

10. A porous silicon carbide ceramic, characterized in that, The silicon carbide ceramic gel green body is dried by the drying method according to any one of claims 1 to 9.

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