A method for preparing pressureless sintered silicon carbide ceramics by gel casting

Abstract

The present application relates to a method for preparing pressureless sintered silicon carbide ceramics by gel injection molding, and relates to the field of ceramic technology, comprising the following steps: adding an organic phase to water for stirring and dissolving to obtain an organic phase solution, adding a crosslinking agent, a dispersant, a defoaming agent and a suspending agent to the organic phase solution and stirring to obtain a premixed liquid; adding a powder to the premixed liquid and stirring to obtain a slurry; ball milling the slurry, degassing the slurry after ball milling, and vibrating the slurry during degassing to obtain a degassed slurry; adding a catalyst and an initiator to the degassed slurry, stirring and pouring it into a mold, and obtaining a green body after molding, drying the green body according to a drying system to obtain a dried green body, and pressureless sintering the dried green body to obtain a silicon carbide ceramic product. The present application has the effect of improving the density, hardness and drying performance of silicon carbide ceramics, and can effectively improve the strength of the green body and reduce the occurrence of cracking and deformation.

Description

Technical Field

[0001] The present application relates to the technical field of ceramics, and particularly to a method for preparing pressureless sintered silicon carbide ceramics by gel casting. Background Art

[0002] Silicon carbide ceramics not only have excellent room-temperature mechanical properties, such as high flexural strength, excellent oxidation resistance, good corrosion resistance, high wear resistance, and low friction coefficient, etc., but also have the best high-temperature mechanical properties (strength, creep resistance, etc.) among known ceramic materials. The high-temperature strength of silicon carbide ceramic materials prepared by hot pressing sintering, pressureless sintering, and hot isostatic pressing sintering can be maintained up to 1600 °C, which is the best among ceramic materials in terms of high-temperature strength. Therefore, its products are widely used in the fields of heating and heat exchange industries. In recent years, with the development of the sintering theory of silicon carbide products, the improvement of the performance of fine powders, the diversification and in-depth research of sintering aids, the process of sintering high-performance silicon carbide products by pressureless sintering process has begun to develop and improve. Its advantages lie in the relatively low production cost, no limitation on the shape and size of products. Especially, the solid-phase sintered SiC ceramics have high density, uniform microstructure, and excellent comprehensive properties of materials.

[0003] In recent years, new colloidal forming such as pressure filtration forming, gel casting molding, and direct coagulation casting molding and other in-situ curing technologies are effective methods for preparing highly reliable and complex-shaped ceramic components. Gel casting molding has been widely studied in the fields of porous materials, composite materials, and functional materials, but it is rarely used in actual industrial production. The biggest reason is that microcracks and deformations often occur in gel casting, and the yield rate is relatively low. Delving into the reasons, it is because the gel drying conditions are relatively harsh, and gel fracture is likely to occur, ultimately leading to the cracking of the green body. Therefore, it is crucial to improve the strength of the gel to resist compressive stress and tensile stress.

[0004] In order to sinter ceramics densely, ultrafine powders are commonly used as the main raw materials. It is difficult to obtain slurries with a high solid content of ultrafine powders. For example, pressureless sintered silicon carbide is basically ultrafine powder. Even after powder modification, the solid content can only be about 55% (VOL). The strength of the wet green body is very low, and cracking is likely to occur during the drying process, so it needs to be improved. Summary of the Invention

[0005] In order to reduce the cracking and deformation phenomena of the silicon carbide ceramic green body during the drying process, the present application provides a method for preparing pressureless sintered silicon carbide ceramics by gel casting.

[0006] A method for preparing pressureless sintered silicon carbide ceramics by gel casting provided by the present application adopts the following technical scheme: A method for preparing pressureless sintered silicon carbide ceramics by gel casting, comprising the following steps:

[0007] (1) adding an organic phase to water and stirring to dissolve to obtain an organic phase solution, adding a crosslinking agent, a dispersant, a defoaming agent and a suspending agent to the organic phase solution and stirring to obtain a premixed solution;

[0008] (2) adding the powder to the premixed liquid and stirring to obtain a slurry;

[0009] (3) ball milling the slurry, degassing the slurry after ball milling, and vibrating the slurry during degassing to obtain a degassed slurry;

[0010] (4) Adding a catalyst and an initiator to the degassed slurry, stirring and pouring the mixture into a mold, and obtaining a green body after forming. Drying the green body according to a drying system to obtain a dried green body, and pressurelessly sintering the dried green body to obtain a silicon carbide ceramic product.

[0011] The slurry prepared by organic phase, cross-linking agent, dispersant, suspending agent, defoaming agent and powder has high solid content and low viscosity. The green body obtained after gel injection molding has high strength, which reduces the occurrence of cracks and deformation during the drying process and improves the density and hardness of silicon carbide ceramic products.

[0012] Preferably, the organic phase comprises acrylamide, polysiloxane and tannic acid.

[0013] The in-situ cross-linking polymerization of acrylamide and polysiloxane obtains a high-toughness and high-strength gel network, which can effectively improve the strength of the ceramic body and reduce cracking and deformation during the drying process, thereby improving the density and hardness of silicon carbide ceramics; tannic acid has good bonding properties and can enhance the bonding force between the polymer organic phase and the powder inorganic phase, thereby improving the compatibility of the system; tannic acid has abundant catechol and pyrogallol groups and has strong reactivity, and can react with acrylamide and polysiloxane to enhance the cross-linking structure of the gel network and increase the cross-linking density, thereby improving the strength of the body, making the silicon carbide ceramic structure compact, and improving the density and hardness of the ceramic product.

[0014] Preferably, the raw materials for preparing the polysiloxane include N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, glycidol and allyltrimethoxysilane, and the mass ratio of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, glycidol and allyltrimethoxysilane is 1:0.685:(0.15-0.3).

[0015] N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane has an active amino group and can react with glycidol to introduce alcoholic hydroxyl groups on the silane to obtain modified silane, thereby improving the hydrophilicity of the modified silane; the modified silane is co-condensed with allyltrimethoxysilane to form a polysiloxane with a nanocage structure, and the rigid cage structure can limit the chain movement between polymer molecules and improve the strength and toughness of the blank; the polysiloxane has abundant alcoholic hydroxyl groups and allyl groups, has good reaction performance and hydrophilicity, can react with acrylamide to form a covalent bond connection, enhance the strength and stability of the gel network, improve the strength of the blank, and reduce the occurrence of cracking and deformation.

[0016] Preferably, the polysiloxane is prepared by the following steps:

[0017] N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane is added to glycidol under ice bath conditions, and stirred while adding, to obtain modified silane after reaction; the modified silane prepared above and allyltrimethoxysilane are mixed, added to methanol and hydrofluoric acid solution, stirred for reaction, and then rotary evaporated to obtain polysiloxane.

[0018] The polysiloxane prepared according to the above steps has good reactivity and can effectively improve the structural strength and stability of the gel network.

[0019] Preferably, the premixed liquid further comprises chemical additives and aluminum chloride.

[0020] Chemical additives can inhibit the hydrolysis of alcohol salts in the silicon carbide ceramic system and increase the polymerization rate, thereby increasing the strength of the gel network; chemical additives can also make the pore size distribution of the gel uniform, thereby reducing uneven stress during drying, shortening the drying time, and reducing the occurrence of cracking and deformation of the green body; the aluminum ions in aluminum chloride have a high charge density and can coordinate with the amide bonds in acrylamide and the active sites on polysiloxane, promoting the cross-linking reaction of the gel network, making the three-dimensional gel network more stable, thereby improving the strength and stability of the green body.

[0021] Preferably, the mass of the chemical additive accounts for 2-5% of the mass of the powder.

[0022] Adding chemical additives according to the above content can effectively improve the strength of the gel network and optimize the pore size distribution, thereby improving the drying performance of the green body.

[0023] Preferably, the chemical additives include formamide, dimethylformamide and oxalic acid.

[0024] The chemical additive of formamide, dimethylformamide and oxalic acid can help the powder to be evenly dispersed in the slurry, reduce the occurrence of agglomeration, inhibit the hydrolysis of alcohol salts, and improve the polymerization efficiency, thereby improving the performance of the slurry, improving the stability and strength of the green body, reducing the occurrence of cracking and deformation, and improving the density and hardness of silicon carbide ceramics.

[0025] Preferably, the powder comprises ceramic powder and a sintering aid, and the sintering aid comprises carbon black and boron carbide.

[0026] Preferably, the ceramic powder includes ultrafine silicon carbide powder, β-silicon carbide whiskers and tungsten carbide.

[0027] β-Silicon carbide whiskers have high strength and can improve the overall strength and toughness of silicon carbide ceramics, reinforce structural defects inside ceramics, improve the strength of the gel network and the strength of the green body, reduce the occurrence of cracking and deformation, and thus improve the density and hardness of silicon carbide ceramics; tungsten carbide has high hardness and can work synergistically with sintering aids during the sintering process to further reduce the generation of pores and improve the densification of silicon carbide ceramics, thereby improving the density and hardness of silicon carbide ceramics.

[0028] Preferably, the mass ratio of the silicon carbide ultrafine powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide is (86.4-91.4): (5-10): 1:2:0.6.

[0029] The powder configured according to the above mass ratio has good sintering performance and can effectively improve the density and hardness of silicon carbide ceramics.

[0030] In summary, the present application includes at least one of the following beneficial technical effects:

[0031] 1. The slurry prepared by organic phase, cross-linking agent, dispersant, suspending agent, defoaming agent and powder has high solid content and low viscosity. The green body obtained after gel casting has high strength, which reduces the occurrence of cracks and deformation during the drying process and improves the density and hardness of silicon carbide ceramic products.

[0032] 2. N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane has an active amino group and can react with propylene oxide to introduce alcoholic hydroxyl groups on the silane to obtain modified silane, thereby improving the hydrophilicity of the modified silane; the modified silane is co-condensed with allyltrimethoxysilane to form a polysiloxane with a nanocage structure. The rigid cage structure can limit the chain movement between polymer molecules and improve the strength and toughness of the blank; the polysiloxane has abundant alcoholic hydroxyl groups and allyl groups, has good reactivity and hydrophilicity, can react with acrylamide to form covalent bonds, enhance the strength and stability of the gel network, improve the strength of the blank, and reduce the occurrence of cracking and deformation.

[0033] 3. Chemical additives can inhibit the hydrolysis of alcohol salts in the silicon carbide ceramic system and increase the polymerization rate, thereby increasing the strength of the gel network; chemical additives can also make the pore size distribution of the gel uniform, thereby reducing uneven stress during drying, shortening the drying time, and reducing the occurrence of cracking and deformation of the green body; the aluminum ions in aluminum chloride have a high charge density and can coordinate with the amide bonds in acrylamide and the active sites on polysiloxane, promoting the cross-linking reaction of the gel network, making the three-dimensional gel network more stable, thereby improving the strength and stability of the green body. DETAILED DESCRIPTION

[0034] The present application discloses a method for preparing pressureless sintered silicon carbide ceramics by gel casting. The raw materials used in the present application can be obtained from commercially available raw materials unless otherwise specified. The present application is further described in detail in conjunction with the embodiments below: Raw materials description: N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane (CAS No.: 1760-24-3), glycidol (CAS No.: 556-52-5), allyltrimethoxysilane (CAS No.: 2551-83-9), acrylamide (CAS No.: 79-06-1), tannic acid (CAS No.: 1401-55-4), and the cross-linking agent is N, N'-methylenebisacrylamide (CAS No.: 110- 26-9), dispersant CE64 purchased from Qiren Chemical, defoamer sodium dodecyl sulfate (CAS No.: 151-21-3), suspending agent sodium carboxymethyl cellulose (CAS No.: 9004-32-4), catalyst tetramethylethylenediamine (CAS No.: 110-18-9), initiator ammonium persulfate (CAS No.: 7727-54-0) and potassium persulfate (CAS No.: 7727-21-1), formamide (CAS No.: 75-12-7), dimethylformamide (CAS No.: 68-12-2), oxalic acid (CAS No.: 144-62-7), β-silicon carbide whiskers purchased from Bohuasi Nanotechnology (Ningbo) Co., Ltd.

[0035] Example 1

[0036] Preparation of polysiloxane: 5.45 kg of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane was added to 3.73 kg of glycidol under ice bath conditions within 2 hours. The mixture was stirred at 200 rpm and reacted for 1 hour after the addition was completed to obtain modified silane; the modified silane prepared above was mixed with 0.82 kg of allyltrimethoxysilane, added to 10 L of 50% methanol by mass and 0.5 mol / L hydrofluoric acid aqueous solution, stirred at 200 rpm at 25°C for 2 hours, and polysiloxane was obtained after rotary evaporation.

[0037] Preparation of Pressureless Sintered Silicon Carbide Ceramics by Gel Casting

[0038] (1) dissolving acrylamide in deionized water to obtain an acrylamide aqueous solution, adding polysiloxane to the acrylamide aqueous solution, stirring at a speed of 200 rpm in a 60° C. water bath until dissolved, adding tannic acid, stirring at a speed of 200 rpm for 15 min, and obtaining an organic phase solution; adding a crosslinking agent, a dispersant, a defoaming agent, and a suspending agent to the organic phase solution and stirring to obtain a preliminary premix solution; adding chemical additives and aluminum chloride hexahydrate to the preliminary premix solution, stirring at a speed of 500 rpm for 0.5 h, and obtaining a premix solution; The liquid is prepared from a slurry of claim 1, wherein the chemical additives include formamide, dimethylformamide and oxalic acid, and the mass ratio of formamide, dimethylformamide and oxalic acid is 1:1:1; wherein acrylamide accounts for 1% of the mass of the slurry, polysiloxane accounts for 0.8% of the mass of the slurry, tannic acid accounts for 0.1% of the mass of the slurry, a cross-linking agent accounts for 0.1% of the mass of the slurry, a dispersant accounts for 1.2% of the mass of the slurry, a defoamer accounts for 0.3% of the mass of the slurry, a suspending agent accounts for 0.5% of the mass of the slurry, chemical additives account for 2% of the mass of the powder, and aluminum chloride hexahydrate accounts for 0.05% of the mass of the slurry.

[0039] (2) Adding powders into the premixed liquid in the order of ultrafine silicon carbide powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide, stirring at a speed of 5000 rpm for 15 minutes to obtain a slurry, wherein the mass ratio of the ultrafine silicon carbide powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide is 86.4:10:1:2:0.6, the powder accounts for 60% of the slurry mass, and the remainder is deionized water; the particle size D50 of the β-silicon carbide whisker is 1.5-2 μm, and the aspect ratio is (10-15):1.

[0040] (3) The slurry was subjected to ball milling treatment, the ball milling time was 12 hours, the ball milling speed was 30 rpm, and after ball milling, it was subjected to degassing treatment, vacuum degassing was performed for 10 minutes, and vibration treatment was performed during degassing, and the vibration frequency was 5 Hz to obtain the degassed slurry.

[0041] (4) Adding a catalyst and an initiator to the degassed slurry, the catalyst accounting for 0.03% of the slurry mass, the initiator accounting for 0.03% of the slurry mass, the mass ratio of ammonium persulfate to potassium persulfate in the initiator being 1:1, stirring at a speed of 5000 rpm for 3 min at 17°C, pouring into a mold after stirring, and obtaining a green body after forming. According to the drying system in Table 1, the green body is dried from low temperature and high humidity to low humidity and high temperature to obtain a dried green body, and the dried green body is pressurelessly sintered, the temperature is increased to 2100°C at a rate of 8°C / min, and the temperature is kept for 5 hours to obtain a silicon carbide ceramic product.

[0042] Table 1 Drying system of silicon carbide ceramics

[0043] Temperature ℃ 30 40 50 50 60 70 90 100 100 70 30 humidity% 75 70 60 50 40 20 0 0 0 0 0 Time 1 20 20 2 10 6 10 5 10 10 4

[0044] Example 2

[0045] Preparation of polysiloxane: 5.04 kg of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane was added to 3.45 kg of glycidol in an ice bath, and the addition was completed within 2 hours. The mixture was stirred at a speed of 200 rpm. After the addition was completed, the mixture was reacted for 1 hour to obtain modified silane; the modified silane prepared above was mixed with 1.51 kg of allyltrimethoxysilane, added to 10 L of 50% methanol by mass and 0.5 mol / L hydrofluoric acid aqueous solution, stirred at 200 rpm at 25°C for 2 hours, and polysiloxane was obtained after rotary evaporation.

[0046] Preparation of Pressureless Sintered Silicon Carbide Ceramics by Gel Casting

[0047] (1) Dissolving acrylamide in deionized water to obtain an acrylamide aqueous solution, adding polysiloxane to the acrylamide aqueous solution, stirring at a speed of 200 rpm in a 60° C. water bath until dissolved, adding tannic acid, stirring at a speed of 200 rpm for 15 min, and obtaining an organic phase solution; adding a crosslinking agent, a dispersant, a defoaming agent, and a suspending agent to the organic phase solution and stirring to obtain a preliminary premixed solution; adding chemical additives and aluminum chloride hexahydrate to the preliminary premixed solution, stirring at a speed of 500 rpm for 0.5 h, and obtaining a premixed solution. The above-mentioned chemical additives include formamide, dimethylformamide and oxalic acid, and the mass ratio of formamide, dimethylformamide and oxalic acid is 1:1:1; among them, acrylamide accounts for 1.5% of the mass of the slurry, polysiloxane accounts for 1.2% of the mass of the slurry, tannic acid accounts for 0.2% of the mass of the slurry, cross-linking agent accounts for 0.1% of the mass of the slurry, dispersant accounts for 1.4% of the mass of the slurry, defoaming agent accounts for 0.5% of the mass of the slurry, suspending agent accounts for 0.7% of the mass of the slurry, chemical additives account for 5% of the mass of the powder, and aluminum chloride hexahydrate accounts for 0.07% of the mass of the slurry.

[0048] (2) Adding powders into the premixed liquid in the order of ultrafine silicon carbide powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide, stirring at a speed of 5000 rpm for 15 minutes to obtain a slurry, wherein the mass ratio of the ultrafine silicon carbide powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide is 91.4:5:1:2:0.6, the powder accounts for 68% of the mass of the slurry, and the remainder is deionized water; the particle size D50 of the β-silicon carbide whisker is 1.5-2 μm, and the aspect ratio is (10-15):1.

[0049] (3) The slurry was subjected to ball milling treatment, the ball milling time was 16 hours, the ball milling speed was 50 rpm, and after ball milling, it was subjected to degassing treatment, vacuum degassing was performed for 10 minutes, and vibration treatment was performed during degassing, and the vibration frequency was 10 Hz to obtain a degassed slurry.

[0050] (4) Adding a catalyst and an initiator to the degassed slurry, the catalyst accounting for 0.05% of the slurry mass, the initiator accounting for 0.05% of the slurry mass, the mass ratio of ammonium persulfate to potassium persulfate in the initiator being 1:1, stirring at a speed of 5000 rpm for 3 min at 20°C, pouring into a mold after stirring, and obtaining a green body after forming. According to the drying system in Table 1, the green body is dried from low temperature and high humidity to low humidity and high temperature to obtain a dried green body, and the dried green body is pressurelessly sintered, and the temperature is increased to 2200°C at a rate of 12°C / min, and kept warm for 2h to obtain a silicon carbide ceramic product.

[0051] Table 1 Drying system of silicon carbide ceramics

[0052] Temperature ℃ 30 40 50 50 60 70 90 100 100 70 30 humidity% 75 70 60 50 40 20 0 0 0 0 0 Time 1 20 20 2 10 6 10 5 10 10 4

[0053] Example 3

[0054] Preparation of polysiloxane: 5.23 kg of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane was added to 3.59 kg of glycidol under ice bath conditions within 2 hours. The mixture was stirred at 200 rpm and reacted for 1 hour after the addition was completed to obtain modified silane; the modified silane prepared above was mixed with 1.18 kg of allyltrimethoxysilane, added to 10 L of 50% methanol by mass and 0.5 mol / L hydrofluoric acid aqueous solution, stirred at 200 rpm at 25°C for 2 hours, and polysiloxane was obtained after rotary evaporation.

[0055] Preparation of Pressureless Sintered Silicon Carbide Ceramics by Gel Casting

[0056] (1) dissolving acrylamide in deionized water to obtain an acrylamide aqueous solution, adding polysiloxane to the acrylamide aqueous solution, stirring at a speed of 200 rpm in a 60° C. water bath until dissolved, adding tannic acid, stirring at a speed of 200 rpm for 15 min, and obtaining an organic phase solution; adding a crosslinking agent, a dispersant, a defoaming agent, and a suspending agent to the organic phase solution and stirring to obtain a preliminary premixed solution; adding chemical additives and aluminum chloride hexahydrate to the preliminary premixed solution, stirring at a speed of 500 rpm for 0.5 h, and obtaining a premixed solution; The above-mentioned chemical additives include formamide, dimethylformamide and oxalic acid, and the mass ratio of formamide, dimethylformamide and oxalic acid is 1:1:1; among them, acrylamide accounts for 1.25% of the mass of the slurry, polysiloxane accounts for 1% of the mass of the slurry, tannic acid accounts for 0.15% of the mass of the slurry, cross-linking agent accounts for 0.1% of the mass of the slurry, dispersant accounts for 1.3% of the mass of the slurry, defoaming agent accounts for 0.4% of the mass of the slurry, suspending agent accounts for 0.6% of the mass of the slurry, chemical additives account for 3.5% of the mass of the powder, and aluminum chloride hexahydrate accounts for 0.06% of the mass of the slurry.

[0057] (2) Adding powders into the premixed liquid in the order of ultrafine silicon carbide powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide, stirring at a speed of 5000 rpm for 15 minutes to obtain a slurry, wherein the mass ratio of the ultrafine silicon carbide powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide is 88.9:7.5:1:2:0.6, the powder accounts for 64% of the mass of the slurry, and the remainder is deionized water; the particle size D50 of the β-silicon carbide whisker is 1.5-2 μm, and the aspect ratio is (10-15):1.

[0058] (3) The slurry was subjected to ball milling treatment, the ball milling time was 14 hours, the ball milling speed was 40 rpm, and after ball milling, it was subjected to degassing treatment, vacuum degassing was performed for 10 minutes, and vibration treatment was performed during degassing, and the vibration frequency was 7.5 Hz to obtain a degassed slurry.

[0059] (4) Adding a catalyst and an initiator to the degassed slurry, the catalyst accounting for 0.05% of the slurry mass, the initiator accounting for 0.05% of the slurry mass, the mass ratio of ammonium persulfate to potassium persulfate in the initiator being 1:1, stirring at a speed of 5000 rpm for 3 min at 18.5°C, pouring into a mold after stirring, and obtaining a green body after molding. According to the drying system in Table 1, the green body is dried from low temperature and high humidity to low humidity and high temperature to obtain a dried green body, and the dried green body is pressurelessly sintered, and the temperature is increased to 2150°C at a rate of 10°C / min, and the temperature is kept for 3.5 hours to obtain a silicon carbide ceramic product.

[0060] Table 1 Drying system of silicon carbide ceramics

[0061] Temperature ℃ 30 40 50 50 60 70 90 100 100 70 30 humidity% 75 70 60 50 40 20 0 0 0 0 0 Time 1 20 20 2 10 6 10 5 10 10 4

[0062] Example 4

[0063] Example 4 is based on Example 3. The only difference between Example 4 and Example 3 is that in Example 4, the amount of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane is 5.76 kg, the amount of glycidol is 3.95 kg, and the amount of allyltrimethoxysilane is 0.29 kg.

[0064] Example 5

[0065] Example 5 is based on Example 3. The only difference between Example 5 and Example 3 is that in Example 5, the amount of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane is 4.8 kg, the amount of glycidol is 3.28 kg, and the amount of allyltrimethoxysilane is 1.92 kg.

[0066] Example 6

[0067] Example 6 is based on Example 3. The only difference between Example 6 and Example 3 is that tannic acid is not added to the premix in Example 6.

[0068] Example 7

[0069] Example 7 is based on Example 3, and the only difference between Example 7 and Example 3 is that in Example 7, aluminum chloride is not added to the premixed solution.

[0070] Example 8

[0071] Example 8 is based on Example 3. The only difference between Example 8 and Example 3 is that the mass of the chemical additive in the premix in Example 8 accounts for 1% of the mass of the powder.

[0072] Example 9

[0073] Example 9 is based on Example 3. The only difference between Example 9 and Example 3 is that the mass of the chemical additive in the premix in Example 9 accounts for 7% of the mass of the powder.

[0074] Example 10

[0075] Example 10 is based on Example 3. The only difference between Example 10 and Example 3 is that in Example 10, no dimethylformamide is added to the chemical additives.

[0076] Embodiment 11

[0077] Example 11 is based on Example 3. The only difference between Example 11 and Example 3 is that in Example 11, formamide and oxalic acid are not added to the chemical additives.

[0078] Example 12

[0079] Example 12 is based on Example 3. The only difference between Example 12 and Example 3 is that in Example 12, the mass ratio of silicon carbide ultrafine powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide is 81.4:15:1:2:0.6.

[0080] Example 13

[0081] Example 13 is based on Example 3. The only difference between Example 13 and Example 3 is that in Example 13, the mass ratio of silicon carbide ultrafine powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide is 96.4:0:1:2:0.6.

[0082] Embodiment 14

[0083] Example 14 is based on Example 3. The only difference between Example 14 and Example 3 is that in Example 14, the mass ratio of silicon carbide ultrafine powder, β-silicon carbide whisker, tungsten carbide, carbon black and boron carbide is 89.9:7.5:0:2:0.6.

[0084] Performance testing

[0085] (1) Density test: The sample was prepared into a cubic silicon carbide ceramic specimen of 100 mm*200 mm*20 mm. The density was calculated based on the volume and mass. Each sample was tested three times and the average value was taken after measurement. The results are recorded in Table 2.

[0086] (2) Select GB 16534-2009 Test method for room temperature hardness of fine ceramics as the standard, use a Vickers hardness tester to test, calculate the Vickers hardness of the sample, test each sample three times, take the average value after measurement, and record the results in Table 2.

[0087] (3) Drying performance test: The wet blank was dried and the condition of the dry blank after drying was observed and recorded. The results are recorded in Table 2.

[0088] Table 2 Test results of density, hardness and drying performance of silicon carbide ceramics

[0089] Test results <![CDATA[Density (g / cm 3 )]]> Hardness(GPa) Whether there is cracking or deformation after drying Example 1 3.19 26 No cracking or deformation Example 2 3.21 27 No cracking or deformation Example 3 3.21 28 No cracking or deformation Example 4 3.16 24 No cracking or deformation Example 5 3.17 25 No cracking or deformation Example 6 3.15 23 Slight cracking and deformation Example 7 3.14 23 Slight cracking and deformation Example 8 3.16 24 No cracking or deformation Example 9 3.16 23 No cracking or deformation Example 10 3.15 22 No cracking or deformation Embodiment 11 3.13 20 Slight cracking and deformation Example 12 3.15 21 No cracking or deformation Example 13 3.16 20 No cracking or deformation Embodiment 14 3.19 18 No cracking or deformation

[0090] As shown in Table 2, the density of Examples 1-3 is greater than 3.19 g / cm 3 , the hardness is greater than 26 GPa, and there is no cracking in the green body after drying, which shows that the silicon carbide ceramic product prepared in this application has high density, high hardness and good drying performance.

[0091] As can be seen from Table 2, the only difference between Examples 4 and 5 and Example 3 is that in Example 4, the mass ratio of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, glycidol and allyltrimethoxysilane is 1:0.685:0.05, and in Example 5, the mass ratio of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, glycidol and allyltrimethoxysilane is 1:0.685:0.4. Compared with Example 3, the density and hardness of Examples 4 and 5 are reduced; this is because the mass ratio of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, glycidol and allyltrimethoxysilane is not within the specified range. Too much or too little allyltrimethoxysilane will affect the reaction performance and system compatibility of polysiloxane, thereby reducing the stability, compatibility and strength of the three-dimensional gel network, and then the density and hardness of the silicon carbide ceramic are reduced.

[0092] As can be seen from Table 2, the only difference between Examples 6 and 7 and Example 3 is that no tannic acid is added to the premix in Example 6, and no aluminum chloride is added to the premix in Example 7. Compared with Example 3, the density, hardness and drying performance of Examples 6 and 7 are reduced. This is because without adding tannic acid or aluminum chloride, the cross-linking structure in the gel network is reduced, the stability and strength of the gel network are reduced, and the compatibility of the system is deteriorated, which leads to a decrease in the density and hardness of the silicon carbide ceramic and a deterioration in the drying performance.

[0093] As can be seen from Table 2, the only difference between Examples 8, 9, 10, and 11 and Example 3 is that the mass of the chemical additive in the premix in Example 8 accounts for 1% of the mass of the powder, the mass of the chemical additive in the premix in Example 9 accounts for 7% of the mass of the powder, no dimethylformamide is added to the chemical additive in Example 10, and no formamide and oxalic acid are added to the chemical additive in Example 11. Compared with Example 3, the density and hardness of Examples 8 and 9 are reduced, and the drying performance is deteriorated; this is because the amount and composition of the chemical additives are adjusted, which affects the stability and cross-linking performance and rate of the slurry, thereby affecting the strength and stability of the green body, resulting in a decrease in the density and hardness of the silicon carbide ceramic and a deterioration in the drying performance.

[0094] As can be seen from Table 2, the only difference between Examples 12, 13, and 14 and Example 3 is that the mass ratio of silicon carbide ultrafine powder, β-silicon carbide whisker, tungsten carbide, carbon black, and boron carbide in Example 12 is 81.4:15:1:2:0.6, the mass ratio of silicon carbide ultrafine powder, β-silicon carbide whisker, tungsten carbide, carbon black, and boron carbide in Example 13 is 96.4:0:1:2:0.6, and the mass ratio of silicon carbide ultrafine powder, β-silicon carbide whisker, tungsten carbide, carbon black, and boron carbide in Example 14 is 96.4:0:1:2:0.6. The mass ratio of silicon whiskers, tungsten carbide, carbon black and boron carbide is 89.9:7.5:0:2:0.6. Compared with Example 3, the density and hardness of Examples 12, 13 and 14 are reduced. This is because the ratio of the substances in the powder is adjusted. Too much or too little silicon carbide whiskers will affect the strength and sintering density. If tungsten carbide is not added, it will directly affect the hardness of silicon carbide ceramics, thereby reducing the density and hardness of silicon carbide ceramic products.

[0095] This specific embodiment is only an explanation of the present application, and it is not a limitation of the present application. Through the above description, relevant staff can make various changes and modifications without deviating from the technical idea of ​​the present application. The technical scope of the present application is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A method for preparing pressureless sintered silicon carbide ceramics by gel casting, characterized in that: The steps include: (1) adding the organic phase into water and stirring to dissolve to obtain an organic phase solution, adding a crosslinking agent, a dispersant, a defoaming agent and a suspending agent into the organic phase solution and stirring to obtain a premixed solution; (2) Add the powder to the premixed liquid and stir to obtain a slurry; (3) ball milling the slurry, degassing the slurry after ball milling, and vibrating the slurry during degassing to obtain a degassed slurry; (4) adding a catalyst and an initiator to the degassed slurry, stirring and pouring the mixture into a mold, and obtaining a green body after forming. Drying the green body according to a drying system to obtain a dried green body, and pressurelessly sintering the dried green body to obtain a silicon carbide ceramic product; The organic phase includes acrylamide, polysiloxane and tannic acid; The raw materials for preparing the polysiloxane include N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, glycidol and allyltrimethoxysilane, and the mass ratio of N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane, glycidol and allyltrimethoxysilane is 1:0.685:(0.15-0.3); The polysiloxane is prepared by the following steps: N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane is added to propylene oxide under ice bath conditions, and stirred while adding, and modified silane is obtained after reaction; the modified silane prepared above and allyltrimethoxysilane are mixed, added to methanol and hydrofluoric acid solution, stirred for reaction, and then rotary evaporated to obtain polysiloxane.

2. The method for preparing pressureless sintered silicon carbide ceramics by gel casting according to claim 1, characterized in that: The premix solution also includes chemical additives and aluminum chloride.

3. The method for preparing pressureless sintered silicon carbide ceramics by gel casting according to claim 2, characterized in that: The weight of the chemical additives accounts for 2-5% of the weight of the powder.

4. The method for preparing pressureless sintered silicon carbide ceramics by gel casting according to claim 3, characterized in that: The chemical additives include formamide, dimethylformamide and oxalic acid.

5. The method for preparing pressureless sintered silicon carbide ceramics by gel casting according to claim 1, characterized in that: The powder comprises ceramic powder and a sintering aid, and the sintering aid comprises carbon black and boron carbide.

6. The method for preparing pressureless sintered silicon carbide ceramics by gel casting according to claim 5, characterized in that: The ceramic powder comprises ultrafine silicon carbide powder, beta-silicon carbide whisker and tungsten carbide.

7. The method for preparing pressureless sintered silicon carbide ceramics by gel casting according to claim 6, characterized in that: The mass ratio of the silicon carbide ultrafine powder, beta-silicon carbide whisker, tungsten carbide, carbon black and boron carbide is (86.4-91.4): (5-10): 1:2:0.6.