A method for preparing a porous silicon nitride ceramic sintered body

Abstract

The present invention belongs to the technical field of porous ceramics, and in particular to a preparation method of a porous silicon nitride ceramic sintered body. The present invention mixes the hydrolysis solution of a silane coupling agent with Si3N4 powder, a sintering aid and a pore-forming agent, and dry pressing is performed after removing the solvent. The obtained silicon nitride green body is subjected to binder removal and sintering to obtain a porous silicon nitride ceramic sintered body. The present invention connects silicon nitride powder and polymethyl methacrylate microspheres by a silane coupling agent. When the PMMA microspheres volatilize, the intermediate layer KH-570 can play a part of the supporting role, effectively preventing silicon nitride particles from collapsing into the pores. Thus achieving the purpose of suppressing the abnormal growth of silicon nitride clusters inside the porous ceramic pores, the inhibitory effect is obvious. This method allows each pore to maintain a complete spherical structure, thereby further improving the porosity and strength of porous silicon nitride ceramics.

Description

Technical Field

[0001] The invention belongs to the technical field of porous ceramics, and in particular relates to a method for preparing a porous silicon nitride ceramic sintered body. Background Art

[0002] Porous silicon nitride ceramics have excellent dielectric properties, corrosion resistance, thermal stability, and chemical stability. Therefore, porous silicon nitride ceramics have become one of the most promising materials in the industrial field, such as dielectric resonators, catalyst supports, and bone scaffolds.

[0003] Research has shown that pore structure significantly influences material properties. At the same porosity level, porous ceramics with dense, strong pore walls exhibit higher strength than those with loose, weak pore walls. Spherical pore structures are considered ideal for material strength. Therefore, it is necessary to produce spherical structures with strong pore walls to improve the strength of porous ceramics. Compared with other processing methods, the pore-forming agent addition method allows the pore structure of the material to be regulated by controlling the particle size and dosage of the pore-forming agent. Numerous researchers have used this pore-forming agent addition method to prepare porous silicon nitride ceramics. These materials are widely used in radome components, achieving integrated load-bearing, heat-resistant, and wave-transmitting properties. Li et al. (Permeability of the porous Al2O3 ceramic with bimodal pore size distribution. Ceram Int. 2019. 45: p. 5952-5957.) used monodisperse polymethyl methacrylate (PMMA) microspheres approximately 2 μm in size as pore-forming agents to achieve a bimodal pore size distribution in porous ceramics. Subsequently, Li et al. (Mechanical property and gas filtration behavior of porous reaction bonded Si3N4 with monodispersed PMMA as a pore former. Int J Appl Ceram Tec, 2022. 19(6): p. 3127-3138.) successfully prepared porous silicon nitride ceramics using monodispersed PMMA microspheres as pore formers. The incorporation of prismatic β-Si3N4 grains and circular pores enhanced the ceramic strength. However, it was also found that the structure of the pores did not completely inherit the structure of the template. It either showed a core-shell structure (Effect of porosity on the effective electrical conductivity of different ceramic membranes used as separators in eletrochemical reactors. J Membrane Sci, 2006. 280(1): p. 536-544.), or the pores were filled with abnormally grown silicon nitride clusters (Microstructural comparison of porous oxide ceramics from the system Al2O3-ZrO2 prepared with starch as apore-forming agent. J Eur Ceram Soc, 2012. 32(10): p. 2163-2172.).This is because the volatilization of the pore-forming agent at relatively low temperatures causes stress changes in the surrounding silicon nitride particles, squeezing some of the silicon nitride particles into the spaces created by the volatilization of the pore-forming agent. Ultimately, after high-temperature sintering, clusters of silicon nitride particles form within the pores. The emergence of the above problem seriously affects the porosity level of porous ceramics, resulting in the pore structure not achieving the ideal spherical structure, and also affecting the mechanical properties of the material. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for preparing a porous silicon nitride ceramic sintered body. The porous silicon nitride ceramic sintered body prepared by the present invention has a spherical pore structure and a high porosity, which further increases the porosity of the silicon nitride ceramic while ensuring that the material has excellent mechanical properties.

[0005] In order to achieve the above object, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing a porous silicon nitride ceramic sintered body, comprising the following steps:

[0007] (1) subjecting a solution of a silane coupling agent to a hydrolysis reaction to obtain a hydrolyzed solution;

[0008] (2) mixing Si3N4 powder, a sintering aid, the hydrolysis solution and a pore-forming agent to obtain a mixed slurry, wherein the sintering aid includes a main group metal oxide and a rare earth oxide, and the pore-forming agent is an organic polymer microsphere;

[0009] (3) removing the solvent from the mixed slurry and sieving it to obtain a mixed powder;

[0010] (4) dry-pressing the mixed powder to obtain a silicon nitride green body;

[0011] (5) Debinding and sintering the silicon nitride green body in sequence to obtain a porous silicon nitride ceramic sintered body.

[0012] Preferably, the mass of the α-Si3N4 powder accounts for 85-95% of the total mass of the Si3N4 powder and the sintering aid.

[0013] Preferably, the silane coupling agent is γ-methacryloxypropyltrimethoxysilane; the mass of the silane coupling agent accounts for 10 to 30% of the total mass of the Si3N4 powder, sintering aid and pore-forming agent.

[0014] Preferably, the main group metal oxide includes MgO and / or Al2O3; the rare earth metal oxide includes one or more of Y2O3, La2O3 and Ce2O3; and the mass ratio of the main group metal oxide to the rare earth oxide is 1-2:1-3.

[0015] Preferably, the pore-forming agent is polymethyl methacrylate microspheres, and the particle size of the pore-forming agent is 10 to 50 μm; the mass of the pore-forming agent accounts for 10 to 30% of the total mass of the Si3N4 powder and the sintering aid.

[0016] Preferably, the solution of the silane coupling agent comprises a silane coupling agent and a solvent, the solvent is ethanol and water, and the volume content of ethanol in the solvent is 75-90%; the hydrolysis reaction is carried out under acidic conditions, and the pH value of the hydrolysis reaction is 3-4.

[0017] Preferably, in step (2), the mixing includes the following steps: premixing Si3N4 powder, sintering aid and the hydrolysis solution to obtain a premixed slurry; heating and mixing the premixed slurry and the pore-forming agent to obtain a mixed slurry; the heating and mixing temperature is 60 to 80°C and the time is 1 to 2 hours.

[0018] Preferably, in step (4), the dry pressing pressure is 20 to 200 MPa, and the holding time is 1 to 3 minutes.

[0019] Preferably, in step (5), the debinding comprises the following steps: heating the silicon nitride green body to a first temperature at a first heating rate and performing a first heat preservation; then heating the green body from the first temperature to a second temperature at a second heating rate and performing a second heat preservation to obtain a silicon nitride green body; the debinding is carried out in a circulating air atmosphere, the first heating rate and the second heating rate are independently 1 to 5°C / min, the first temperature is 100 to 120°C, the first heat preservation time is 30 to 40min, the second temperature is 600 to 800°C, and the second heat preservation time is 1 to 2h.

[0020] Preferably, in step (5), the sintering comprises the following steps: heating the silicon nitride body obtained by the debinding to a third temperature at a third heating rate; then heating from the third temperature to a fourth temperature at a fourth heating rate and sintering at a holding temperature to obtain a porous silicon nitride ceramic sintered body; the third heating rate and the fourth heating rate are independently 3 to 6°C / min, the third temperature is 1000 to 1100°C, the fourth temperature is 1650 to 1750°C, and the holding temperature sintering time is 1 to 2 hours;

[0021] The sintering method includes gas pressure sintering, spark plasma sintering or pressureless sintering; the sintering pressure of the gas pressure sintering or spark plasma sintering is independently 20-30 MPa; the sintering pressure of the pressureless sintering is normal pressure; the sintering is carried out in a protective gas atmosphere, and the protective gas is nitrogen.

[0022] The present invention provides a method for preparing a porous silicon nitride ceramic sintered body, comprising the following steps:

[0023] (1) subjecting a solution of a silane coupling agent to a hydrolysis reaction to obtain a hydrolysis solution; (2) mixing Si3N4 powder, a sintering aid, the hydrolysis solution and a pore-forming agent to obtain a mixed slurry, wherein the sintering aid comprises a main group metal oxide and a rare earth oxide, and the pore-forming agent is an organic polymer microsphere; (3) removing the solvent from the mixed slurry and sieving it to obtain a mixed powder; (4) dry-pressing the mixed powder to obtain a silicon nitride green body; and (5) sequentially debinding and sintering the silicon nitride green body to obtain a porous silicon nitride ceramic sintered body. The present invention hydrolyzes a silane coupling agent and mixes it with Si3N4 powder, a sintering aid and a pore-forming agent. A silane coupling agent is used to connect Si3N4 powder and a pore-forming agent (organic polymer microspheres). A silane coupling agent layer is formed near the core of the organic polymer microspheres. A raw material layer composed of Si3N4 powder and a sintering aid is formed in the outer layer of the silane coupling agent layer. When the organic polymer microspheres decompose and volatilize during binder removal and sintering, the intermediate layer silane coupling agent can play a part of the supporting role, effectively preventing Si3N4 particles from collapsing into the pore structure, thereby achieving the purpose of suppressing the abnormal growth of silicon nitride clusters in the porous ceramic pores, and the inhibitory effect is obvious. Thus, the porous silicon nitride ceramic sintered body pore structure obtained by the preparation method provided by the present invention is spherical and has a high porosity. While further increasing the porosity of the silicon nitride ceramic, it is ensured that the material has excellent mechanical properties.

[0024] Furthermore, the silane coupling agent is γ-methacryloxypropyltrimethoxysilane; the mass of the silane coupling agent accounts for 10-30% of the total mass of the Si3N4 powder, sintering aid, and pore-forming agent; the pore-forming agent is polymethyl methacrylate microspheres; the mass of the pore-forming agent accounts for 10-30% of the total mass of the Si3N4 powder and sintering aid. The present invention provides a method for controlling the mass ratio of the silane coupling agent and the pore-forming agent to the sintering raw materials (Si3N4 powder and sintering aid) of the silicon nitride ceramic to form an effective silane coupling agent protective layer on the surface of the pore-forming agent, thereby effectively preventing pore structure collapse caused by volatilization of the pore-forming agent and abnormal production of silicon nitride clusters within the pore structure. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a SEM image of the fracture of the porous silicon nitride ceramic material prepared in Comparative Example 1 of the present invention;

[0026] Figure 2 This is a SEM image of the fracture of the porous silicon nitride ceramic material prepared in Comparative Example 2 of the present invention;

[0027] Figure 3 This is a SEM image of the fracture of the porous silicon nitride ceramic material prepared in Comparative Example 3 of the present invention;

[0028] Figure 4This is a SEM image of the fracture of the porous silicon nitride ceramic material prepared in Example 1 of the present invention;

[0029] Figure 5 This is a SEM image of the fracture of the porous silicon nitride ceramic material prepared in Example 2 of the present invention;

[0030] Figure 6 This is a SEM image of the fracture of the porous silicon nitride ceramic material prepared in Example 3 of the present invention. DETAILED DESCRIPTION

[0031] The present invention provides a method for preparing a porous silicon nitride ceramic sintered body, comprising the following steps:

[0032] (1) subjecting a solution of a silane coupling agent to a hydrolysis reaction to obtain a hydrolyzed solution;

[0033] (2) mixing Si3N4 powder, a sintering aid, the hydrolysis solution and a pore-forming agent to obtain a mixed slurry, wherein the sintering aid includes a main group metal oxide and a rare earth oxide, and the pore-forming agent is an organic polymer microsphere;

[0034] (3) removing the solvent from the mixed slurry and sieving it to obtain a mixed powder;

[0035] (4) dry-pressing the mixed powder to obtain a silicon nitride green body;

[0036] (5) Debinding and sintering the silicon nitride green body in sequence to obtain a porous silicon nitride ceramic sintered body.

[0037] In the present invention, unless otherwise specified, all preparation raw materials / components are commercially available products well known to those skilled in the art.

[0038] The present invention hydrolyzes a silane coupling agent solution to obtain a hydrolyzed solution. In the present invention, the silane coupling agent is easily hydrolyzed and self-agglomerated in water. The hydrolyzed solution obtained by the hydrolysis reaction is preferably used for a subsequent coating reaction, thereby forming a uniform silane coupling agent layer on the surface of the organic polymer microspheres.

[0039] In the present invention, the silane coupling agent solution preferably comprises a silane coupling agent and a solvent. The silane coupling agent is preferably γ-methacryloxypropyltrimethoxysilane (KH-570). The mass of the silane coupling agent relative to the total mass of the Si3N4 powder, sintering aid, and sintering aid is preferably 10-30%, more preferably 15-25%. The solvent is preferably ethanol and water. The water is preferably deionized water. The volume content of ethanol in the solvent is preferably 75-90%, preferably 80-85%. The volume ratio of the silane coupling agent to ethanol is preferably (3-6):(110-130), more preferably 5:120. The hydrolysis reaction is preferably carried out under acidic conditions, and the pH value of the hydrolysis reaction is preferably 3-4. In the present invention, the hydrolysis reaction is preferably carried out after adding an acid to the silane coupling agent solution to adjust the pH. The acid is preferably acetic acid. The hydrolysis reaction is carried out with stirring, preferably at room temperature, and for 20-40 minutes.

[0040] After obtaining the hydrolysis solution, the present invention mixes Si3N4 powder, a sintering aid, the hydrolysis solution and a pore-forming agent to obtain a mixed slurry, wherein the sintering aid includes main group metal oxides and rare earth oxides, and the pore-forming agent is organic polymer microspheres.

[0041] In the present invention, the Si3N4 powder is preferably α-Si3N4 powder. The main group metal oxide preferably includes MgO and / or Al2O3, more preferably Al2O3. The rare earth metal oxide preferably includes one or more of Y2O3, La2O3, and Ce2O3, more preferably Y2O3. The mass ratio of the main group metal oxide to the rare earth oxide is preferably 1-2:1-3, more preferably 2:3. In the present invention, the sintering aid is preferably Al2O3 and Y2O3. The mass ratio of Al2O3 to Y2O3 is preferably 1-2:1-3, more preferably 2:3. The mass of the α-Si3N4 powder accounts for 85-95% of the total mass of the α-Si3N4 powder and the sintering aid, more preferably 90-95%. In the present invention, the pore-forming agent is preferably polymethyl methacrylate (PMMA) microspheres. The particle size of the pore-forming agent is preferably 10 to 50 μm, more preferably 15 to 45 μm, and even more preferably 20 to 40 μm. The pore-forming agent is preferably spherical PMMA microspheres. While acting as a pore-forming agent, the spherical structure can improve the fluidity of the slurry. The mass of the pore-forming agent accounts for preferably 10 to 30% of the total mass of the Si3N4 powder and sintering aid, and more preferably 15 to 25%.

[0042] In the present invention, the mixing preferably includes the following steps: premixing Si3N4 powder, sintering aid and the hydrolysis solution to obtain a premixed slurry; heating and mixing the premixed slurry and the pore-forming agent to obtain a mixed slurry. The premixing is preferably carried out under stirring conditions, and the stirring method is preferably one or more of manual stirring, mechanical stirring, and magnetic stirring, more preferably magnetic stirring. The temperature of the premixing is preferably room temperature, and the time is preferably 20 to 40 minutes. The temperature of the heating and mixing is preferably 60 to 80°C, more preferably 65 to 75°C; the time is preferably 1 to 2 hours. The heating and mixing is carried out under stirring conditions, and the stirring method is preferably one or more of manual stirring, mechanical stirring and magnetic stirring, more preferably magnetic stirring. After adding the pore-forming agent, the present invention heats and mixes the mixture under stirring conditions. The heating + stirring mixing method not only enables the pore-forming agent, α-Si3N4 powder and sintering aid to be evenly mixed, and the pore-forming agent to be evenly distributed in the Si3N4 powder, but also protects the structure of the organic polymer microspheres to achieve the purpose of uniform pore diameter, further increases the porosity, and ensures the mechanical properties of the silicon nitride ceramic material.

[0043] After obtaining the mixed slurry, the present invention removes the solvent from the mixed slurry and then sieves it to obtain a mixed powder.

[0044] In the present invention, the desolventizing method is preferably evaporative drying, and the desolventizing is preferably carried out in a rotary evaporator. The temperature of the desolventizing is preferably 60 to 80°C, and the time is preferably 20 to 40 minutes. The present invention preferably sieves the dried material to obtain a mixed powder. The mesh size of the sieve used for the sieving is preferably 100 mesh. The present invention preferably uses rotary evaporation to remove the solvent, and controls the temperature of the desolventizing to be 60 to 80°C, so that the silane coupling agent can form a uniform coating layer on the surface of the organic polymer microspheres during the evaporation of the solvent, and through the action of the silane coupling agent, the polymer microspheres with the silane coupling agent coating layer are uniformly dispersed in the mixed raw material of α-Si3N4 powder and sintering aid.

[0045] After obtaining the mixed powder, the present invention dry-presses the mixed powder to obtain a silicon nitride green body. In the present invention, the dry pressing pressure is preferably 20 to 200 MPa, more preferably 50 to 150 MPa. The holding time is preferably 1 to 3 minutes. The silicon nitride green body obtained by the dry pressing is preferably cylindrical in shape, preferably with a diameter of 20 to 36 mm and a height of 3 to 5 mm.

[0046] After obtaining the silicon nitride green body, the present invention sequentially performs debinding and sintering on the silicon nitride green body to obtain a porous silicon nitride ceramic sintered body.

[0047] In the present invention, the debinding process preferably includes the following steps: heating the silicon nitride green body to a first temperature at a first heating rate and performing a first heat preservation; then heating the green body from the first temperature to a second temperature at a second heating rate and performing a second heat preservation to obtain a silicon nitride green body. The debinding process is preferably carried out in a circulating air atmosphere, and the first heating rate is preferably 1 to 5°C / min, more preferably 2 to 4°C / min. The first temperature is preferably 100 to 120°C. The first heat preservation time is preferably 30 to 40 minutes. The second heating rate is preferably 1 to 5°C / min, more preferably 1 to 3°C / min. The second temperature is preferably 600 to 800°C, more preferably 650 to 750°C. The second heat preservation time is preferably 1 to 2 hours.

[0048] In the present invention, the silicon nitride green body is sequentially debinded to obtain a silicon nitride body. In the present invention, the silicon nitride body is preferably sintered to obtain a porous silicon nitride ceramic sintered body. The initial sintering temperature is preferably the second temperature set for debinding. The sintering is preferably performed in a graphite crucible. The sintering preferably includes the following steps: heating the silicon nitride body obtained by debinding to a third temperature at a third heating rate; then heating from the third temperature to a fourth temperature at a fourth heating rate and holding the temperature to obtain a porous silicon nitride ceramic sintered body. The third heating rate is preferably 3-6°C / min, more preferably 4-5°C / min. The third temperature is preferably 1000-1100°C, more preferably 1000°C. The fourth heating rate is preferably 3-6°C / min, more preferably 3-4°C / min. The fourth temperature is preferably 1650-1750°C, more preferably 1650-1700°C. The holding sintering time is preferably 1-2 hours. The sintering method preferably includes gas pressure sintering, spark plasma sintering, or pressureless sintering. The sintering pressure of the gas pressure sintering is preferably 20-30 MPa, more preferably 22-27 MPa. The sintering pressure of the spark plasma sintering is preferably 20-30 MPa, more preferably 22-27 MPa. The sintering pressure of the pressureless sintering is preferably atmospheric pressure. The sintering is preferably carried out in a protective gas atmosphere, preferably nitrogen.

[0049] The preparation method provided by the present invention connects silicon nitride powder (Si3N4) and polymethyl methacrylate microspheres (PMMA) through a silane coupling agent (KH-570), and the coupling agent is wrapped around the outside of the PMMA microspheres. When the PMMA microspheres evaporate, the intermediate layer KH-570 can play a partial supporting role, effectively suppressing the abnormal growth of silicon nitride clusters inside the pores of porous ceramics, and the inhibitory effect is very obvious. The present invention adopts the method of magnetic stirring (preparation of mixed slurry) + rotary evaporation (solvent removal) when preparing the mixed powder, which can not only make the pore-forming agent uniformly distributed in the powder, but also protect the structure of the microspheres to achieve the purpose of uniform pore size. The preparation method provided by the present invention further increases the porosity while ensuring the mechanical properties of the material, and ultimately achieves the regulation of the porosity and pore structure of porous silicon nitride ceramics.

[0050] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0051] Example 1: Adding silane coupling agent / 10 μm / 30 wt%

[0052] Step 1, hydrolysis of silane coupling agent

[0053] An ethanol-water solution was prepared for the hydrolysis of KH-570. The volume ratio of ethanol to water was 9:1, and the amount of KH-570 added was 15% of the total mass of α-Si₃N₄, Al₂O₃, Y₂O₃, and PMMA microspheres. The volume ratio of KH-570 to ethanol was 5:120. 3.9 g of KH-570, 93.6 g of ethanol, and 10.4 g of water were added. Acetic acid was added to adjust the pH of the solution to 4, and the KH-570 was hydrolyzed under magnetic stirring. The hydrolysis was continued for 30 minutes at room temperature. This resulted in Solution A.

[0054] Step 2: Prepare water-based slurry

[0055] 19g of α-Si3N4, 0.4g of Al2O3, and 0.6g of Y2O3 were added to Solution A and reacted with the hydrolyzed KH-570 under magnetic stirring to produce mixed slurry A. The weight percentages of the raw materials were: 95% α-Si3N4, 2% Al2O3, and 3% Y2O3. After 20 minutes of reaction, 6g of a pore-forming agent was added. Stirring was continued while the temperature was raised to 70°C and maintained for 2 hours to allow for full reaction between the KH-570 and the pore-forming agent. The pore-forming agent was PMMA microspheres with a particle size of 10μm. The amount of pore-forming agent added was 30% of the total weight of α-Si3N4, Al2O3, and Y2O3. This resulted in mixed slurry B.

[0056] Step 3, rotary evaporation drying - powder sieving

[0057] The mixed slurry B was placed in a rotary evaporator for evaporation and drying at a drying temperature of 60°C and a drying time of 40 min to obtain a mixed powder A. The powder was then sieved with a 100-mesh sieve to obtain a mixed powder B.

[0058] Step 4: Dry pressing

[0059] The mixed powder B was pressed into a cylinder with a diameter of 36 mm and a height of 3 mm at a pressure of 20 MPa and a holding time of 1 min.

[0060] Step 5: Debinding and sintering

[0061] (1) Debinding the silicon nitride green body

[0062] Debinding was carried out in a circulating air atmosphere, and the debinding system was as follows: raising the temperature to 100°C at a rate of 2°C / min and keeping it warm for 30 minutes; raising the temperature to 600°C at a rate of 1°C / min and keeping it warm for 2 hours to obtain a silicon nitride blank.

[0063] (2) Atmospheric pressure sintering

[0064] The debinded silicon nitride body was placed in a graphite crucible for sintering. The sintering temperature was raised to 1000°C at a rate of 5°C / min, then to 1700°C at a rate of 3°C / min, and held for 1 hour. The sintering atmosphere was nitrogen, resulting in a porous silicon nitride ceramic sintered body.

[0065] Example 2: Adding silane coupling agent / 20μm / 10wt%

[0066] The particle size and addition amount of the pore-forming agent PMMA in step 2 of Example 1 are adjusted. Specifically, the pore-forming agent PMMA is added, the PMMA particle size is 20 μm, and the addition amount is 10% of the total mass of α-Si3N4, Al2O3 and Y2O3. The remaining steps are the same as Example 1.

[0067] Example 3: Adding silane coupling agent / 50μm / 10wt%

[0068] The particle size and addition amount of the pore-forming agent PMMA in step 2 of Example 1 are adjusted. Specifically, the pore-forming agent PMMA is added, the PMMA particle size is 50 μm, and the addition amount is 10% of the total mass of α-Si3N4, Al2O3 and Y2O3. The remaining steps are the same as Example 1.

[0069] Comparative Example 1

[0070] Step 1: Prepare an ethanol-water solution by mixing 93.6 g of ethanol and 10.4 g of water in a 9:1 volume ratio. Add acetic acid to adjust the pH of the solution to 4 and stir under magnetic stirring for 30 minutes at room temperature. This yields Solution A.

[0071] Step 2: Prepare water-based slurry

[0072] α-Si₃N₄, Al₂O₃, and Y₂O₃ were added to Solution A under magnetic stirring to prepare mixed slurry A. The weight percentages of the raw materials were: 95% α-Si₃N₄, 2% Al₂O₃, and 3% Y₂O₃. After 20 minutes of reaction, a pore-forming agent was added. Stirring was continued while the temperature was raised to 70°C and maintained for 2 hours. The pore-forming agent was PMMA microspheres with a particle size of 10 μm. The amount of pore-forming agent added was 20% of the total weight of α-Si₃N₄, Al₂O₃, and Y₂O₃. Finally, mixed slurry B was obtained.

[0073] Steps 3 to 5 are the same as those in Example 1.

[0074] Comparative Example 2

[0075] Step 1: Prepare an ethanol-water solution by mixing 93.6 g of ethanol and 10.4 g of water in a 9:1 volume ratio. Add acetic acid to adjust the pH of the solution to 4 and stir under magnetic stirring for 30 minutes at room temperature. This yields Solution A.

[0076] Step 2: Prepare water-based slurry

[0077] α-Si₃N₄, Al₂O₃, and Y₂O₃ were added to Solution A under magnetic stirring to prepare mixed slurry A. The weight percentages of the raw materials were: 95% α-Si₃N₄, 2% Al₂O₃, and 3% Y₂O₃. After 20 minutes of reaction, a pore-forming agent was added. Stirring was continued while the temperature was raised to 70°C and maintained for 2 hours. The pore-forming agent was PMMA microspheres with a particle size of 20 μm. The amount of pore-forming agent added was 10% of the total weight of α-Si₃N₄, Al₂O₃, and Y₂O₃. Finally, mixed slurry B was obtained.

[0078] Steps 3 to 5 are the same as those in Example 2.

[0079] Comparative Example 3

[0080] Step 1: Prepare an ethanol-water solution by mixing 93.6 g of ethanol and 10.4 g of water in a 9:1 volume ratio. Add acetic acid to adjust the pH of the solution to 4 and stir under magnetic stirring for 30 minutes at room temperature. This yields Solution A.

[0081] Step 2: Prepare water-based slurry

[0082] α-Si₃N₄, Al₂O₃, and Y₂O₃ were added to Solution A under magnetic stirring to prepare mixed slurry A. The weight percentages of the raw materials were: 95% α-Si₃N₄, 2% Al₂O₃, and 3% Y₂O₃. After 20 minutes of reaction, a pore-forming agent was added. Stirring was continued while the temperature was raised to 70°C and maintained for 2 hours. The pore-forming agent was PMMA microspheres with a particle size of 50 μm. The amount of pore-forming agent added was 10% of the total weight of α-Si₃N₄, Al₂O₃, and Y₂O₃. Finally, mixed slurry B was obtained.

[0083] Steps 3 to 5 are the same as those in Example 3.

[0084] Figures 1 to 6 The scanning electron microscope photos of the ceramic products prepared in Examples 1 to 3 and Comparative Examples 1 to 3 are shown in FIG. Figures 1 to 3 Without adding KH-570( Figure 1 This is the SEM image of the fracture surface of the product prepared in Comparative Example 1. Figure 2 This is the SEM image of the fracture surface of the product prepared in Comparative Example 2. Figure 3 The product fracture SEM picture prepared in Comparative Example 3) and Figures 4 to 6 The porous silicon nitride ceramics prepared by adding KH-570 ( Figure 4 This is the SEM image of the fracture of the product prepared in Example 1. Figure 5 This is the SEM image of the fracture of the product prepared in Example 2. Figure 6 The fracture SEM picture of the product prepared in Example 3). Figures 1 to 3 It can be seen that the porous silicon nitride ceramics prepared in Comparative Examples 1 to 3 without adding KH-570 have pore structures that show bridge structures, or the pores are filled with abnormally grown silicon nitride clusters. Figures 4 to 6 It can be seen that the pores of the ceramic products prepared in Examples 1 to 3 maintain a relatively complete and independent structure, which is consistent with the structure of PMMA microspheres before burning out. Figure 1 Especially when comparing Example 1 with Comparative Example 1, the content of PMMA microspheres added in Example 1 is higher than that in Comparative Example 1. Relatively speaking, it is more difficult to create a complete pore structure. Figure 4 It can be seen that Example 1 successfully prepared a ceramic product with intact and independent pores. Figures 1 to 6 It can be shown that the present invention can effectively prevent silicon nitride particles from collapsing into the pores by adding KH-570, thereby achieving the purpose of inhibiting the abnormal growth of silicon nitride particle clusters inside the pores of porous ceramics, and the inhibitory effect is obvious.

[0085] Although the above embodiment provides a detailed description of the present invention, it is only a part of the embodiments of the present invention, not all of the embodiments. Other embodiments can be obtained based on this embodiment without creativity, and these embodiments all fall within the scope of protection of the present invention.

Claims

1. A method for preparing a porous silicon nitride ceramic sintered body, characterized in that: The following steps are involved: (1) subjecting a solution of a silane coupling agent to a hydrolysis reaction to obtain a hydrolyzed solution, wherein the silane coupling agent is γ-methacryloxypropyltrimethoxysilane; the mass of the silane coupling agent accounts for 10 to 30% of the total mass of the Si3N4 powder, the sintering aid and the pore-forming agent; (2) Premixing Si3N4 powder, a sintering aid and the hydrolysis solution to obtain a premixed slurry; heating and mixing the premixed slurry and a pore-forming agent to obtain a mixed slurry; the heating and mixing temperature is 60-80°C, and the heating and mixing is carried out under stirring conditions, and the stirring method is one or more of manual stirring, mechanical stirring and magnetic stirring; the sintering aid includes a main group metal oxide and a rare earth oxide, and the pore-forming agent is polymethyl methacrylate microspheres; the mass of the pore-forming agent accounts for 10-30% of the total mass of the Si3N4 powder and the sintering aid; (3) removing the solvent from the mixed slurry and then sieving to obtain a mixed powder, wherein the solvent removal method is rotary evaporation and the solvent removal temperature is 60-80 °C; (4) dry-pressing the mixed powder to obtain a silicon nitride green body; (5) Debinding and sintering the silicon nitride green body in sequence to obtain a porous silicon nitride ceramic sintered body.

2. The preparation method according to claim 1, characterized in that The mass of the Si3N4 powder accounts for 85-95% of the total mass of the Si3N4 powder and the sintering aid.

3. The preparation method according to claim 1 or 2, characterized in that The main group metal oxide includes MgO and / or Al2O3; the rare earth oxide includes one or more of Y2O3, La2O3 and Ce2O3; the mass ratio of the main group metal oxide to the rare earth oxide is 1~2:1~3.

4. The preparation method according to claim 1 or 2, characterized in that The particle size of the pore-forming agent is 10-50 μm.

5. The preparation method according to claim 1, characterized in that The silane coupling agent solution comprises a silane coupling agent and a solvent, wherein the solvent is ethanol and water, and the volume content of ethanol in the solvent is 75-90%. The hydrolysis reaction is carried out under acidic conditions, and the pH value of the hydrolysis reaction is 3-4.

6. The preparation method according to claim 1 or 5, characterized in that In step (2), the heating and mixing time is 1 to 2 hours.

7. The preparation method according to claim 1, characterized in that In step (4), the dry pressing pressure is 20-200 MPa, and the holding time is 1-3 min.

8. The preparation method according to claim 1, characterized in that In step (5), the debinding includes the following steps: heating the silicon nitride green body to a first temperature at a first heating rate and performing a first heat preservation; then heating the green body from the first temperature to a second temperature at a second heating rate and performing a second heat preservation to obtain a silicon nitride green body; the debinding is carried out in a circulating air atmosphere, the first heating rate and the second heating rate are independently 1~5 ℃ / min, the first temperature is 100~120 ℃, the first heat preservation time is 30~40 min, the second temperature is 600~800 ℃, and the second heat preservation time is 1~2 h.

9. The preparation method according to claim 1 or 8, characterized in that In step (5), the sintering comprises the following steps: heating the silicon nitride body obtained by the debinding to a third temperature at a third heating rate; then heating the body from the third temperature to a fourth temperature at a fourth heating rate for heat preservation and sintering to obtain a porous silicon nitride ceramic sintered body; the third heating rate and the fourth heating rate are independently 3-6 °C / min, the third temperature is 1000-1100 °C, the fourth temperature is 1650-1750 °C, and the heat preservation and sintering time is 1-2 h; The sintering method includes gas pressure sintering, spark plasma sintering or pressureless sintering; the sintering pressure of the gas pressure sintering or spark plasma sintering is independently 20~30 MPa; the sintering pressure of the pressureless sintering is normal pressure; the sintering is carried out in a protective gas atmosphere, and the protective gas is nitrogen.

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