Preparation method of porous silicon carbide ceramic

Abstract

The invention relates to the technical field of silicon carbide ceramic preparation, and particularly discloses a preparation method of porous silicon carbide ceramic. The method comprises the following steps: S1, uniformly stirring starch, a silicon source and pure water, heating to obtain wet gel, freeze-drying the wet gel to obtain dry gel, and ball-milling the dry gel to obtain a silicon-containing starch composite dry gel pore-forming agent; and S2, mixing the xerogel pore-forming agent with the silicon carbide powder, carrying out dry pressing molding to prepare a green body, and sintering the green body in an argon atmosphere to obtain the porous silicon carbide ceramic. The silicon-containing starch composite xerogel has the characteristics of light weight, porosity, higher hardness and the like, so that higher porosity can be obtained only by using a small amount of pore-forming agent, the shape and pore structure of a sample can be well maintained in the forming process, and the sample is not easy to deform or collapse, thereby improving the porosity of the material. By optimizing the dosage of the pore-forming agent, the porosity of the porous silicon carbide ceramic can be effectively improved, and the application prospect is good.

Description

Technical Field

[0001] The invention belongs to the technical field of silicon carbide ceramic preparation, and specifically relates to a method for preparing porous silicon carbide ceramic. Background Art

[0002] Porous silicon carbide ceramics have high porosity, as well as the advantages of acid and alkali corrosion resistance and high temperature resistance. They are currently widely used in many fields such as chemical industry, energy, metallurgy, biomedicine, aerospace, environmental protection, etc. They are known as a new generation of high-performance ceramic membranes. However, large-scale silicon carbide flat ceramics are difficult to form and are prone to deformation when extruded. Pressing can effectively reduce the deformation problem of ceramic products, but the high-pressure pressing process may cause the porosity of the ceramic body to decrease. For some ceramic products, such as filters or some porous ceramics, higher porosity is necessary. In order to obtain higher porosity, pore-forming agents are generally added to increase the porosity of porous ceramic materials prepared by pressing. Starch, as a commonly used pore-forming agent, is widely used in the preparation of porous ceramics. Using starch as a pore-forming agent is an effective and environmentally friendly method for preparing porous ceramics. This method mainly utilizes the combustion or decomposition of starch during heat treatment to leave pores, thereby forming ceramics with a porous structure. However, for the high-temperature sintering process of silicon carbide ceramics, starch may not be completely decomposed, leaving residues to form residual carbon, which affects the purity and performance of the sample; in addition, starch particles have a low hardness at room temperature and may not provide sufficient support during the molding process. They are easily crushed or deformed, making it difficult to maintain the desired pore structure, resulting in unsatisfactory porosity of porous silicon carbide ceramic products. Summary of the invention

[0003] The purpose of the present invention is to address the above-mentioned deficiencies in the prior art and provide a novel method for preparing porous silicon carbide ceramics. Silicon-containing starch composite dry gel is used as a pore-forming agent. The freeze-drying technology used in the preparation of porous silicon carbide ceramics is beneficial to maintaining the lightweight and porous properties of the gel, improving the hardness of the dry gel, and enhancing the structural stability, so that it can better support the pore structure during the pressing process, thereby increasing the porosity of the material. A porous ceramic product with high porosity can be obtained using a small amount of pore-forming agent.

[0004] To achieve the above object, the present invention adopts the following technical solution:

[0005] The first object of the present invention is to provide a method for preparing porous silicon carbide ceramics, comprising the following steps: S1, weighing starch, silicon source and pure water in a certain proportion, stirring evenly, heating to obtain a wet gel, freeze-drying the wet gel to obtain a dry gel, grinding the obtained dry gel with a mortar, and putting it into a planetary ball mill for ball milling to obtain a silicon-containing starch composite dry gel pore-forming agent.

[0006] S2. Weigh the pore-forming agent and silicon carbide powder in S1 according to a certain ratio, mix them, and prepare a green body by dry pressing. The green body is sintered in an argon atmosphere. Then, the prepared green body sample is placed in a silicon carbide sintering furnace and sintered in an argon atmosphere to obtain a porous silicon carbide ceramic.

[0007] Furthermore, in step S1, the mass ratio of starch: silicon source: pure water is 1:1:5 to 1:3:10, and the silicon source includes any one of silica sol, tetraethyl orthosilicate or nano silicon dioxide.

[0008] Furthermore, in step S1, the heating temperature is 60°C to 100°C, and the heating time is 1 to 3 hours.

[0009] Furthermore, in step S1, the rotation speed of the ball mill is 100 to 500 rpm, and the ball milling time is 2 to 5 hours.

[0010] Furthermore, in step S2, the mass ratio of dry gel pore former to silicon carbide powder is 1:100 to 50:100.

[0011] Furthermore, in step S2, the ball mill speed is 100-500 rpm, and the ball milling time is 1-5 h.

[0012] Furthermore, in step S2, the high temperature sintering is performed by raising the temperature to 1600°C at a rate of 15°C / min and keeping the temperature for 2 to 2.5 hours to perform a carbon thermal reaction.

[0013] Furthermore, in step S2, after the carbothermal reaction, the temperature is raised to 2000-2200°C at a rate of 10°C / min, and recrystallization sintering is performed for 1-2 hours.

[0014] Furthermore, the pore size of the porous silicon carbide ceramic ranges from 0.32 to 36.7 μm.

[0015] Furthermore, the porosity of the porous silicon carbide ceramic is 56% to 67%.

[0016] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0017] (1) The xerogel pore former provided by the present invention has light and porous characteristics, and a porous ceramic product with high porosity can be obtained by using a small amount of xerogel pore former; the hardness and structural stability of the composite xerogel pore former are enhanced, and the pore structure can be better supported during the pressing process, thereby increasing the porosity of the material. This method provides an effective way to prepare porous materials with developed pore structures.

[0018] (2) In the composite dry gel prepared with starch as the carbon source and silica sol or ethyl orthosilicate or nano-silicon dioxide as the silicon source, due to the macromolecular characteristics of starch, the dry gel and silicon carbide powder are well adhered together after mixing. During the pressurization process, the silicon carbide powder will be "embedded" into the porous gel network of the pore-forming agent, so that the pore-forming agent and the raw material powder are evenly mixed. After the starch is carbonized by heat, C-coated SiO 2 The structure of C and SiO 2 The reaction generates SiC particles and gaseous CO. The escape of CO can increase the porosity of the sample. At the same time, the temperature is raised to above 2000℃ for recrystallization sintering. During the recrystallization sintering process, the newly generated SiC particles evaporate and condense between the silicon carbide powder particles as a sintering aid, forming a good sintering neck, promoting the growth of the sample's bending strength. At the same time, the fine powder is consumed, increasing the porosity. The composite pore-forming agent can also play the role of a sintering aid, effectively eliminating residual carbon and improving the purity and strength of the sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a SEM image of the porous silicon carbide ceramic sample prepared in Example 3;

[0020] Figure 2 This is the XRD pattern of the porous silicon carbide ceramic sample prepared in Example 3. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical scheme and advantages of the present invention clearer, the specific embodiments of the present invention are further described in detail below in conjunction with specific examples and drawings. If no specific test method, instrument or condition is specified in the examples, it is carried out according to the technology or condition described in the literature in this field or according to the product instructions. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be obtained commercially.

[0022] In order to eliminate the residual carbon problem caused by starch, the preparation method provided by the present invention adds a silicon source to the raw material to make the silicon source react with the residual carbon to generate silicon carbide, effectively eliminating the residual carbon and forming high-purity silicon carbide ceramics. Starch is used as the carbon source, and silica sol or ethyl orthosilicate or nano silicon dioxide is used as the silicon source. The gelling properties of starch are used to convert the nano-SiO2 in the silicon source into 2 Evenly dispersed in starch gel, due to the light and porous characteristics of starch gel, a small amount of pore-forming agent can produce porous ceramic products with high porosity. 2 The structure of C and SiO 2 The reaction produces particulate SiC and gaseous CO, as shown in Equation 1, which increases the porosity.

[0023] SiO 2(s)+3C(s)→SiC(S)+2CO(g) Equation 1

[0024] In order to achieve the above object, the present invention adopts the following technical solutions:

[0025] (1) Raw material selection: Starch (corn starch, potato starch, wheat starch, rice starch, etc.) is used as the carbon source, silica sol or tetraethyl orthosilicate or nano-silicon dioxide is used as the silicon source, and high-purity silicon carbide powder is selected as the raw material.

[0026] (2) Preparation of composite dry gel pore former: Starch and silicon source are first mixed with water in a certain proportion and heated and stirred until gelatinization to obtain a wet gel. The wet gel is dried (freeze drying technology, supercritical drying) to prepare a dry gel, and then ball milled and sifted to obtain a silicon-containing starch composite dry gel pore former.

[0027] (3) Molding: The silicon-containing starch composite dry gel pore-forming agent and silicon carbide powder are ball-milled and mixed in a certain proportion and then pressed to prepare a silicon carbide ceramic green body.

[0028] (4) Sintering: The molded sample is first subjected to carbon thermal reaction sintering at a low temperature stage, and then the temperature is continued to be raised for recrystallization sintering to obtain a porous silicon carbide ceramic sample.

[0029] The following is a detailed description with a specific implementation example. The silicon source used in the implementation is the silica sol, which is Gulf Group's JN-30 alkaline silica sol, containing 30% silicon oxide, ethyl orthosilicate is a common commercial product, and the nano-silica powder has a particle size of 1nm to 20nm.

[0030] Example 1

[0031] The present embodiment provides a method for preparing porous silicon carbide ceramics using a silicon-containing starch composite xerogel as a pore-forming agent. The specific steps are as follows:

[0032] Step S1, starch: silica sol: pure water are mixed in a beaker according to a formula of 1:3:6 by mass, the beaker is placed in a heat-collecting constant temperature magnetic stirrer, heated to 80°C and stirred for 2 hours to obtain a wet gel, the wet gel is taken out, placed in a freeze dryer, and freeze-dried at -75°C for 24 hours to obtain a dry gel. The obtained dry gel sample is ground with a mortar and placed in a planetary ball mill, and ball-milled at a speed of 300 rpm for 2 hours to obtain a silicon-containing starch composite dry gel pore-forming agent.

[0033] Step S2, mixing the dry gel pore former prepared in S1 with silicon carbide powder (particle size 1 μm, 50 μm, 100 μm), wherein the ratio of the pore former to the silicon carbide powder is 1:100, and mixing is performed in a ball mill at a speed of 100 rpm for 0.5 h. The particle size of the powder after mixing is 1 to 100 μm. Weighing the uniform powder after ball milling, dry pressing and molding, and preparing a silicon carbide ceramic green body.

[0034] Step S3, placing the prepared sample into a silicon carbide sintering furnace and sintering it in an argon atmosphere. The sintering temperature is 2100°C, and the specific sintering operation is: heating to 1600°C at 15°C / min, keeping warm for 2h, and performing a carbon thermal reaction. Then, heating to 2100°C at 10°C / min, performing recrystallization sintering for 1h, and preparing a porous silicon carbide ceramic sample. The average pore sizes of the corresponding porous silicon carbide ceramic samples are 0.32μm, 16.8μm and 35.2μm, respectively, and the porosity is 56.1-57.4%.

[0035] Example 2

[0036] This embodiment provides a method for preparing porous silicon carbide ceramics using silicon-containing starch composite dry gel as a pore-forming agent.

[0037] The specific steps are as follows:

[0038] Step S1, starch: silica sol: pure water are mixed in a beaker according to a formula of 1:3:6 by mass, the beaker is placed in a heat-collecting constant temperature magnetic stirrer, heated to 80°C and stirred for 2 hours to obtain a wet gel, the wet gel is taken out, placed in a freeze dryer, and freeze-dried at -75°C for 24 hours to obtain a dry gel. The obtained dry gel sample is ground with a mortar and placed in a planetary ball mill, and ball-milled at a speed of 300 rpm for 2 hours to obtain a silicon-containing starch composite dry gel pore-forming agent.

[0039] Step S2, mixing the dry gel pore former prepared in S1 with silicon carbide powder (particle size 1 μm, 50 μm, 100 μm), wherein the ratio of the pore former to the silicon carbide powder is 5:100, and mixing is performed in a ball mill at a speed of 100 rpm for 0.5 h. The particle size of the powder after mixing is 1 to 100 μm. Weighing the uniform powder after ball milling, dry pressing and molding, and preparing a silicon carbide ceramic green body.

[0040] Step S3, placing the prepared sample into a silicon carbide sintering furnace and sintering it in an argon atmosphere. The sintering temperature is 2100°C, and the specific sintering operation is: heating to 1600°C at 15°C / min, keeping warm for 2h, and performing a carbon thermal reaction. Then, heating to 2100°C at 10°C / min, performing recrystallization sintering for 1h, and preparing a porous silicon carbide ceramic sample. The average pore sizes of the corresponding porous silicon carbide ceramic samples are 0.36μm, 17.3μm and 35.9μm, respectively, and the porosity is 60.2-61.5%.

[0041] Example 3

[0042] This embodiment provides a method for preparing porous silicon carbide ceramics using silicon-containing starch composite dry gel as a pore-forming agent.

[0043] The specific steps are as follows:

[0044] Step S1, starch: silica sol: pure water are mixed in a beaker according to a formula of 1:3:6 by mass, the beaker is placed in a heat-collecting constant temperature magnetic stirrer, heated to 80°C and stirred for 2 hours to obtain a wet gel, the wet gel is taken out, placed in a freeze dryer, and freeze-dried at -75°C for 24 hours to obtain a dry gel. The obtained dry gel sample is ground with a mortar and placed in a planetary ball mill, and ball-milled at a speed of 300 rpm for 2 hours to obtain a silicon-containing starch composite dry gel pore-forming agent.

[0045] Step S2, mixing the dry gel pore former prepared in S1 with silicon carbide powder (particle size 1 μm, 50 μm, 100 μm), wherein the ratio of the pore former to the silicon carbide powder is 10:100, and mixing is performed in a ball mill at a speed of 100 rpm for 0.5 h. The particle size of the powder after mixing is 1 to 100 μm. Weighing the uniform powder after ball milling, dry pressing and molding, and preparing a silicon carbide ceramic green body.

[0046] Step S3, placing the prepared sample into a silicon carbide sintering furnace and sintering it in an argon atmosphere. The sintering temperature is 2100°C, and the specific sintering operation is: heating to 1600°C at 15°C / min, keeping warm for 2h, and performing a carbon thermal reaction. Then, heating to 2100°C at 10°C / min, performing recrystallization sintering for 1h, and obtaining a porous silicon carbide ceramic sample. The average pore sizes of the corresponding porous silicon carbide ceramic samples are 0.45μm, 17.9μm and 36.7μm, respectively, and the porosity is 65.1-66.9%.

[0047] Example 4

[0048] This embodiment provides a method for preparing porous silicon carbide ceramics using silicon-containing starch composite dry gel as a pore-forming agent.

[0049] The specific steps are basically the same as those in Example 1, except that the silicon source is ethyl orthosilicate. The porosity of the prepared porous silicon carbide ceramic sample ranges from 56% to 67%.

[0050] Example 5

[0051] This embodiment provides a method for preparing porous silicon carbide ceramics using silicon-containing starch composite dry gel as a pore-forming agent.

[0052] The specific steps are basically the same as those in Example 1, except that the silicon source is nano-silicon dioxide powder. The porosity of the prepared porous silicon carbide ceramic sample ranges from 56% to 67%.

[0053] The porous silicon carbide ceramic samples prepared in Examples 1 to 5 were characterized and found to have similar microscopic morphologies. The following is a detailed description using Example 3 as a representative:

[0054] refer to Figure 1 , is the SEM image of the porous silicon carbide ceramic sample. It can be seen that the silicon carbide particles form a porous structure with interconnected pores, uniform pore distribution, good sintering neck development, and can maintain the strength of the material.

[0055] refer to Figure 2 , which is the XRD diagram of the porous silicon carbide ceramic sample. It can be seen that the obtained porous silicon carbide ceramic sample phase is all 6H-SiC (α-SiC), which is a pure porous silicon carbide ceramic sample.

[0056] The porous silicon carbide ceramic sample provided by the present invention has an open porosity of up to 56.1-66.9%, which is higher than the porosity of mullite-bonded silicon carbide ceramic membranes (34-64.7%), oxide-bonded silicon carbide ceramic membranes (30%), silicon carbide ceramic membranes prepared using zeolite as raw material (46%), and pure silicon carbide ceramic membranes (-50%) reported in existing literature.

[0057] Example 6

[0058] The porous silicon carbide ceramic samples prepared in Example 3 had average pore sizes of 0.45 μm, 17.9 μm and 36.7 μm, respectively. In the high-temperature (500°C) filtration and dust removal experiment, the removal rates of solid particles of 0.1 μm, 1 μm and 2.5 μm were as high as over 99%. The results show that the prepared porous silicon carbide ceramics can remove dust from gases at high temperatures.

[0059] Comparative Example 1

[0060] The invention provides a method for preparing porous silicon carbide ceramics by using silicon-containing starch composite dry gel as a pore-forming agent.

[0061] The specific steps are as follows:

[0062] Step S1, weighing according to the formula of starch: silicon source mass ratio of 1:3, using a ball mill to mix, the ball mill speed is 300 rpm, and the mixture is ball milled for 2 hours to obtain a composite pore-forming agent.

[0063] Step S2, the composite pore-forming agent prepared in S1 is mixed with silicon carbide powder (particle size 1 μm, 50 μm, 100 μm), wherein the ratio of the pore-forming agent to the silicon carbide powder is 10:100, and the mixing is performed in a ball mill, the speed of the ball mill is 100 rpm, and the ball milling is performed for 0.5 h. The particle size of the powder after mixing is 1 to 100 μm. The uniform powder after ball milling is weighed and dry-pressed to prepare a silicon carbide ceramic green body.

[0064] Step S3, placing the prepared sample into a silicon carbide sintering furnace and sintering it under an argon atmosphere. The sintering temperature is 2100°C, and the specific sintering operation is: heating to 1600°C at 15°C / min, keeping warm for 2h, and performing a carbon thermal reaction. Then, heating to 2100°C at 10°C / min, performing recrystallization sintering for 1h, and preparing a porous silicon carbide ceramic sample. The average pore sizes of the prepared porous silicon carbide ceramic samples are 0.30μm, 16.2μm and 35.0μm, respectively, and the porosity is 50.5-51.2%. The average porosity is lower than the porosity of the porous silicon carbide ceramic sample prepared by adding silicon-containing starch composite dry gel as a pore-forming agent in Example 1-3.

[0065] In the absence of conflict, the above embodiments and features in the embodiments may be combined with each other.

[0066] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing porous silicon carbide ceramics, characterized in that: The following steps are involved: S1, weighing starch, silicon source and pure water in a certain proportion, stirring evenly, heating to obtain a wet gel, freeze-drying the wet gel to obtain a dry gel, and putting the obtained dry gel into a planetary ball mill to obtain a silicon-containing starch composite dry gel pore-forming agent; S2. Weigh the pore-forming agent and silicon carbide powder in S1 according to a certain ratio, mix them, and prepare a green body by dry pressing. The green body is sintered in an argon atmosphere. Then, the prepared green body sample is placed in a silicon carbide sintering furnace and sintered in an argon atmosphere to obtain a porous silicon carbide ceramic.

2. The preparation method according to claim 1, characterized in that In step S1, the mass ratio of starch: silicon source: pure water is 1:1:5 to 1:3:10, and the silicon source includes any one of silica sol, tetraethyl orthosilicate or nano silicon dioxide.

3. The preparation method according to claim 1, characterized in that: In step S1, the heating temperature is 60°C to 100°C, and the heating time is 1 to 3 hours.

4. The preparation method according to claim 1, characterized in that: In step S1, the rotation speed of the ball mill is 100 to 500 rpm, and the ball milling time is 2 to 5 hours.

5. The preparation method according to claim 1, characterized in that: In step S2, the mass ratio of dry gel pore former to silicon carbide powder is 1:100 to 50:

100.

6. The preparation method according to claim 1, characterized in that: In step S2, the ball mill speed is 100-500 rpm, and the ball milling time is 1-5 hours.

7. The preparation method according to claim 1, characterized in that: In step S2, the high temperature sintering is carried out by raising the temperature to 1600°C at a rate of 15°C / min and keeping the temperature for 2 to 2.5 hours to perform a carbon thermal reaction.

8. The preparation method according to claim 1, characterized in that: In step S2, after the carbothermal reaction, the temperature is raised to 2000-2200°C at a rate of 10°C / min, and recrystallization sintering is performed for 1-2 hours.

9. The preparation method according to claim 1, characterized in that: The pore size of the porous silicon carbide ceramic ranges from 0.32 to 36.7 μm.

10. The preparation method according to claim 1, characterized in that: The porosity of the porous silicon carbide ceramic is 56% to 67%.

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