Preparation method of a porous silicon nitride ceramic

Abstract

The present invention provides a method for preparing porous silicon nitride ceramics, belonging to the technical field of porous ceramics; the preparation method includes steps of preparing a reinforcing phase, compounding, mixing, and post-treatment; the compounding step is to add N,N-dimethylformamide to a reaction vessel, then add the reinforcing phase and modified silicon carbide, stir evenly, add triethylamine, introduce nitrogen for high-temperature reaction, the reaction temperature is 117-122 °C, the reaction time is 5.8-6.2 h, after the reaction is completed, filter, wash, and dry to obtain a composite; the porous silicon nitride ceramics prepared by the method of the present invention, while improving the porosity and the uniformity of pore distribution, ensure excellent mechanical properties and toughness, and enhance the high-temperature resistance.

Description

Technical Field

[0001] The present invention belongs to the technical field of porous ceramics, and particularly relates to a preparation method of porous silicon nitride ceramics. Background Art

[0002] Silicon nitride ceramics have the advantages of high strength, high temperature resistance, wear resistance, etc. They have good thermal conductivity and good high-frequency electromagnetic wave transmission performance, and have broad application prospects in the fields of high-temperature gas filtration, catalyst carriers, sensor development, and radomes;

[0003] Porous silicon nitride ceramics, in addition to possessing the excellent properties of silicon nitride ceramics, also have a small bulk density, a large specific surface area, and unique physical properties. A higher porosity can improve the lightweight, heat insulation, and wave transmission performance of the material, promote the integration, miniaturization, and high-performance development of devices, and good mechanical properties and toughness are the basis for the practical application of porous silicon nitride ceramics.

[0004] For the preparation methods of porous silicon nitride ceramics, they mainly include the pore-forming agent method and the gel-casting method; the gel-casting method has a high production cost and limited part shapes, and it is impossible to realize the preparation of complex shapes; the pore-forming agent method is to add a certain amount of pore-forming agent to the silicon nitride ceramic powder. During the sintering process, the pore-forming agent will be discharged, thereby leaving pores inside the ceramic matrix to form a porous structure of the silicon nitride ceramic product.

[0005] Mechanical properties and porosity are the key performance indicators of porous silicon nitride ceramic products. The pore structure has an important influence on the product performance. Porous ceramics with dense and strong pore walls and porous ceramics with loose and weak pore walls have excellent mechanical properties;

[0006] However, during the preparation process, on the one hand, the mechanical properties and porosity can vary within a large range, but the contradiction between the two is difficult to reconcile. With the increase of porosity, the mechanical properties of porous silicon nitride ceramics will be reduced, while too low porosity will limit the application range of the product;

[0007] On the other hand, the volatilization of the pore-forming agent at a low temperature leads to stress changes in the surrounding silicon nitride particles. After high-temperature sintering, the silicon nitride particles form particle clusters inside the pores, and there are defects such as poor uniformity of pore distribution and low porosity. It not only affects the porosity of the porous ceramic product, but also affects the mechanical properties of the material;

[0008] On the other hand, the silicon nitride ceramics prepared by the prior art are brittle. During the sintering process, microdefects such as cracks are likely to form inside the ceramics. When subjected to external forces, cracks are likely to form at the defect sites and rapidly expand, ultimately leading to material fracture. The prior art usually adds a second phase for toughening, specifically introducing reinforcing phases into the matrix, such as fibers, whiskers, and particles. However, the fibers, whiskers, and particles are prone to agglomeration, and the interfacial bonding force between them and the silicon nitride ceramic matrix is poor, unable to fully exert the toughening effect, ultimately affecting the mechanical properties of the porous ceramic products.

[0009] The porous silicon nitride ceramics prepared by the prior art also have the defect of poor high-temperature resistance, and their mechanical properties decrease significantly in a high-temperature environment.

[0010] Therefore, providing a preparation method for porous silicon nitride ceramics, while increasing the porosity and the uniformity of pore distribution, enhancing the mechanical properties and toughness, and improving the high-temperature resistance is a technical problem urgently to be solved in the prior art. Summary of the Invention

[0011] To solve the technical problems existing in the prior art, the present invention provides a preparation method for porous silicon nitride ceramics, which can increase the porosity and the uniformity of pore distribution while ensuring excellent mechanical properties and toughness and enhancing the high-temperature resistance.

[0012] For the above technical problems, the present invention adopts the following technical solutions:

[0013] A preparation method for porous silicon nitride ceramics includes steps of preparing a reinforcing phase, compounding, mixing, and post-treatment, and the specific operations are as follows:

[0014] 1. Preparing a reinforcing phase

[0015] (1) Pretreatment

[0016] Place silicon nitride fibers in acetone with a mass 6 - 8 times that of the fibers, stir evenly, then raise the temperature to 70 - 75 °C for a reflux reaction. The reflux reaction time is 5.7 - 6.3 h. After the reflux reaction ends, filter and dry to obtain pretreated silicon nitride fibers;

[0017] The silicon nitride fibers have a diameter of 90 - 110 nm and a length of 0.1 - 0.3 μm;

[0018] (2) Modification treatment

[0019] Place the pretreated silicon nitride fibers in the pre-modifying solution, heat it to 60 - 65°C at a rate of 1.8 - 2.2°C / min, keep it warm and stir for 1.4 - 1.7 h. After stirring, filter it out and put it into absolute ethanol. Add the modifying solution, raise the temperature to 56 - 60°C, and stir and react for 0.7 - 1.2 h. After the reaction, filter, wash and dry to obtain modified silicon nitride fibers;

[0020] The mass ratio of the pretreated silicon nitride fibers, the pre-modifying solution, absolute ethanol, and the modifying solution is 9.7 - 10.2:57 - 63:63 - 68:76 - 85;

[0021] The pre-modifying solution is a mixture of deionized water and cyclohexanehexol, and the mass ratio of deionized water and cyclohexanehexol is 100:8.4 - 9.0;

[0022] The modifying solution is a mixture of deionized water, polydopamine, and poly-L-methionine, and the mass ratio of deionized water, polydopamine, and poly-L-methionine is 100:7.2 - 7.6:5.0 - 5.3;

[0023] (3)Carboxylation

[0024] Add the modified silicon nitride fibers to N,N-dimethylformamide, stir evenly, then add 2-octenyl succinic anhydride for ultrasonic reaction. The ultrasonic temperature is 37 - 42°C, the ultrasonic time is 1.8 - 2.3 h, the ultrasonic power is 107 - 115 W, and the ultrasonic frequency is 28 - 34 kHz. After the ultrasonic reaction, centrifuge, wash and dry to obtain the reinforcing phase;

[0025] The mass ratio of the modified silicon nitride fibers, N,N-dimethylformamide, and 2-octenyl succinic anhydride is 9.2 - 9.6:98 - 104:3.0 - 3.5.

[0026] 2. Composite

[0027] Add N,N-dimethylformamide to the reaction vessel, then add the reinforcing phase and modified silicon carbide, stir evenly, add triethylamine, and introduce nitrogen for high-temperature reaction. The reaction temperature is 117 - 122°C, the reaction time is 5.8 - 6.2 h. After the reaction, filter, wash and dry to obtain the composite;

[0028] The mass ratio of N,N-dimethylformamide, the reinforcing phase, modified silicon carbide, and triethylamine is 495 - 510:6.0 - 6.4:3.6 - 4.0:1.2 - 1.5;

[0029] The preparation method of the modified silicon carbide is as follows: place silicon carbide in an ethanol solution, stir evenly, then add γ-glycidoxypropylmethyldiethoxysilane and γ-ureidopropyltriethoxysilane for ball milling. The ball milling time is 2.0 - 2.5 h, the ball milling speed is 112 - 126 rpm, and the ball-to-material ratio is 2 - 4:1. After ball milling, filter, wash, and dry to obtain the modified silicon carbide;

[0030] The particle size of the silicon carbide is 150 - 180 nm;

[0031] The mass ratio of the silicon carbide, ethanol solution, γ-glycidoxypropylmethyldiethoxysilane, and γ-ureidopropyltriethoxysilane is 7.7 - 8.2:86 - 95:0.2 - 0.4:0.2 - 0.5;

[0032] The mass concentration of the ethanol solution is 26 - 30%.

[0033] 3. Mixing

[0034] Add silicon nitride, the composite, and the sintering aid to the mixed solution, and perform ultrasonic treatment. The ultrasonic time is 28 - 32 min, the ultrasonic power is 172 - 186 W, and the ultrasonic frequency is 24 - 30 kHz. After ultrasonic treatment, add the pore former and the stabilizer, raise the temperature to 62 - 67 °C, and continue ultrasonic treatment for 0.8 - 1.2 h, keeping the power and frequency of ultrasonic treatment unchanged. After ultrasonic treatment, dry to obtain the mixed material;

[0035] The particle size of the silicon nitride is 240 - 260 nm;

[0036] The mass ratio of the silicon nitride, the composite, the sintering aid, the mixed solution, the pore former, and the stabilizer is 47 - 52:4.5 - 5.0:2.2 - 2.5:994 - 1010:20.4 - 21.1:2.0 - 2.5;

[0037] The mixed solution is a mixture of a 58 - 63 wt% ethanol solution and vinyltrimethoxysilane, and the mass ratio of the 58 - 63 wt% ethanol solution to vinyltrimethoxysilane is 90:1.0 - 1.5;

[0038] The sintering aid is a mixture of magnesium oxide and yttrium oxide, and the mass ratio of magnesium oxide to yttrium oxide is 1:0.7 - 1.2;

[0039] The pore former is polymethylmethacrylate microspheres with a particle size of 4.8 - 5.3 μm;

[0040] The stabilizer is a mixture of antioxidant 1010 and polyvinyl alcohol, and the mass ratio of antioxidant 1010 to polyvinyl alcohol is 1:0.8 - 1.3.

[0041] 4. Post-treatment

[0042] Place the mixture in a mold for cold isostatic pressing, control the pressure at 80 - 110 MPa and the time at 28 - 34 min. Then, in an air atmosphere, heat it at a rate of 1.7 - 2.3 °C / min to 175 - 185 °C, hold for 18 - 23 min, then heat it at a rate of 1.0 - 1.5 °C / min to 320 - 360 °C, hold for 25 - 35 min, and finally heat it at a rate of 0.4 - 0.6 °C / min to 650 - 670 °C, hold for 1.8 - 2.2 h to obtain a silicon nitride ceramic green body; place the silicon nitride ceramic green body in a nitrogen atmosphere, heat it at a rate of 3.0 - 4.0 °C / min to 840 - 860 °C, hold for 18 - 25 min, then heat it at a rate of 1.8 - 2.2 °C / min to 1715 - 1724 °C, hold for 0.8 - 1.2 h to obtain porous silicon nitride ceramics.

[0043] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0044] 1. The present invention uses silicon nitride fibers as the reinforcing phase and modifies the silicon nitride fibers. Specifically, cyclohexanehexol with a large number of hydroxyl groups is first used as a pre-modifying solution, so that the surface of the silicon nitride fibers contains a large number of hydroxyl groups. Then, an amino compound is added, and the amino group of the amino compound combines with the hydroxyl groups on the surface of the silicon nitride fibers, so that the surface of the modified silicon nitride fibers contains a large number of amino groups. Then, an acid anhydride group is introduced, so that the surface of the silicon nitride fibers is rich in carboxyl groups; in the compounding step, modified silicon carbide is introduced. Specifically, a ureido group and an epoxy group are introduced on the surface of the silicon carbide. The epoxy group can react with the carboxyl group and the ureido group, so that the modified silicon carbide and the silicon nitride fibers are crosslinked to form a stable three-dimensional network structure. Moreover, after the silicon carbide is modified, it has good dispersibility and can be uniformly compounded with the silicon nitride fibers to obtain a composite. The composite has good compatibility with silicon nitride particles and can be uniformly dispersed, and then the toughening performance of the composite can be fully exerted, improving the brittleness of the silicon nitride ceramic. When it is used in the silicon nitride ceramic, through crack deflection, pulling out and bridging effects, the further expansion of cracks is prevented, the toughness of the silicon nitride ceramic is improved, and the mechanical properties and high-temperature oxidation resistance of the silicon nitride ceramic can be improved, further enhancing the comprehensive performance; in the mixing step, a specific mixed solution is added, which contains a vinyl silane coupling agent. The hydroxyl group at one end of the silane coupling agent can combine with groups such as the hydroxyl groups on the surface of the composite and the silicon nitride particles, and the other end has good compatibility with the pore-forming agent. The materials can be uniformly dispersed, and the obtained mixed material has good dispersibility and excellent homogeneity. During the high-temperature treatment in the post-treatment process, as the temperature rises, the pore-forming agent decomposes and volatilizes. The vinyl silane coupling agent can effectively prevent the collapse of the pores left after volatilization as a support, avoiding the entry and aggregation of silicon nitride particles into the pores, ensuring the uniform dispersion of the pores, and thus achieving the improvement of the porosity and the uniformity of the pore distribution while ensuring the mechanical properties and high-temperature resistance of the ceramic product and enhancing the toughness;

[0045] 2. The porous silicon nitride ceramic prepared by the method of the present invention has a density of 1.3 - 1.5 g / cm 3 , and a porosity of 50.3 - 55.2%;

[0046] 3. The porous silicon nitride ceramic prepared by the method of the present invention has a fracture toughness of 1.0 - 2.1 Mpa·m 1 / 2 , and a flexural strength of 110 - 150 MPa; when heated to 1500 °C at a rate of 20 °C / min and held for 1000 s, after the holding treatment is completed, it is cooled to room temperature at a rate of 50 °C / min, and the flexural strength is measured again to be 80 - 120 MPa;

[0047] 4. The porous silicon nitride ceramic prepared by the method of the present invention has a linear shrinkage rate of 0.05 - 0.5%;

[0048] 5. The porous silicon nitride ceramic prepared by the method of the present invention has a dielectric constant of 3.10 - 3.38, a loss tangent value of 0.75×10 -3 -5.6×10 -3 at 25°C, and a dielectric loss tangent value of 1×10 -3 -8×10 -3 . Detailed implementation manners

[0049] For a clearer understanding of the technical features, objectives, and effects of the present invention, the detailed implementation manners of the present invention are described below.

[0050] Example 1

[0051] 1. Preparation of the reinforcing phase

[0052] (1) Pretreatment

[0053] Put the silicon nitride fibers into acetone with a mass 7 times that of the fibers. After stirring evenly, raise the temperature to 73°C for reflux reaction. The reflux reaction time is 6.0 h. After the reflux reaction ends, filter and dry to obtain pretreated silicon nitride fibers;

[0054] The silicon nitride fibers have a diameter of 100 nm and a length of 0.2 μm;

[0055] (2) Modification treatment

[0056] Put 10.0 g of pretreated silicon nitride fibers into 60 g of pre-modification solution, raise the temperature to 62°C at a rate of 2.0°C / min, keep warm and stir for 1.5 h. After stirring ends, filter and put them into 65 g of absolute ethanol. Add 80 g of modification solution, raise the temperature to 58°C, and stir and react for 1.0 h. After the reaction ends, filter, wash, and dry to obtain modified silicon nitride fibers;

[0057] The pre-modification solution is a mixture of deionized water and cyclohexanehexol, and the mass ratio of deionized water to cyclohexanehexol is 100:8.7;

[0058] The modification solution is a mixture of deionized water, polydopamine, and poly-L-methionine, and the mass ratio of deionized water, polydopamine, and poly-L-methionine is 100:7.4:5.1;

[0059] (3) Carboxylation

[0060] Add 9.4 g of modified silicon nitride fibers to 100 g of N,N-dimethylformamide. After stirring evenly, add 3.3 g of 2-octenyl succinic anhydride for ultrasonic reaction. The ultrasonic temperature is 40°C, the ultrasonic time is 2.0 h, the ultrasonic power is 110 W, and the ultrasonic frequency is 30 kHz. After the ultrasonic reaction ends, centrifuge, wash, and dry to obtain the reinforcing phase.

[0061] 2. Composite

[0062] Add 500 g of N,N - dimethylformamide into the reaction vessel, then add 6.2 g of the reinforcing phase and 3.7 g of modified silicon carbide. After stirring evenly, add 1.4 g of triethylamine, and introduce nitrogen for high - temperature reaction. The reaction temperature is 120 °C, and the reaction time is 6.0 h. After the reaction, filter, wash, and dry to obtain the composite;

[0063] The preparation method of the modified silicon carbide is as follows: Place 8.0 g of silicon carbide in 90 g of 28 wt% ethanol solution. After stirring evenly, add 0.3 g of γ - glycidoxypropylmethyldiethoxysilane and 0.3 g of γ - ureidopropyltriethoxysilane for ball - milling treatment. The ball - milling time is 2.2 h, the ball - milling speed is 120 rpm, and the ball - to - material ratio is 3:1. After ball - milling, filter, wash, and dry to obtain the modified silicon carbide;

[0064] The particle size of the silicon carbide is 160 nm.

[0065] 3. Mixing

[0066] Add 50 g of silicon nitride, 4.7 g of the composite, and 2.4 g of sintering aid into 1000 g of the mixed solution, and perform ultrasonic treatment. The ultrasonic time is 30 min, the ultrasonic power is 180 W, and the ultrasonic frequency is 26 kHz. After ultrasonic treatment, add 20.8 g of pore - forming agent and 2.2 g of stabilizer, raise the temperature to 64 °C, and continue ultrasonic treatment for 1.0 h while keeping the power and frequency of ultrasonic treatment unchanged. After ultrasonic treatment, dry to obtain the mixed material;

[0067] The particle size of the silicon nitride is 250 nm;

[0068] The mixed solution is a mixture of 60 wt% ethanol solution and vinyltrimethoxysilane, and the mass ratio of the 60 wt% ethanol solution to vinyltrimethoxysilane is 90:1.3;

[0069] The sintering aid is a mixture of magnesium oxide and yttrium oxide, and the mass ratio of magnesium oxide to yttrium oxide is 1:1;

[0070] The pore - forming agent is polymethylmethacrylate microspheres with a particle size of 5.0 μm;

[0071] The stabilizer is a mixture of antioxidant 1010 and polyvinyl alcohol, and the mass ratio of antioxidant 1010 to polyvinyl alcohol is 1:1.

[0072] 4. Post - treatment

[0073] The mixture is placed in a mold for cold isostatic pressing. The pressure is controlled at 100 MPa and the time is 30 min. Then, in an air atmosphere, it is heated to 180 °C at a rate of 2.0 °C / min and held for 20 min. Next, it is heated to 340 °C at a rate of 1.2 °C / min and held for 30 min. Finally, it is heated to 660 °C at a rate of 0.5 °C / min and held for 2.0 h to obtain a silicon nitride ceramic green body. The silicon nitride ceramic green body is placed in a nitrogen atmosphere and heated to 850 °C at a rate of 3.5 °C / min and held for 20 min. Then, it is heated to 1720 °C at a rate of 2.0 °C / min and held for 1.0 h to obtain porous silicon nitride ceramics.

[0074] Example 2

[0075] 1. Preparation of reinforcing phase

[0076] (1)Pretreatment

[0077] The silicon nitride fibers are placed in acetone with a mass 6 times that of the fibers. After stirring evenly, the temperature is raised to 70 °C for reflux reaction. The reflux reaction time is 5.7 h. After the reflux reaction ends, it is filtered and dried to obtain pretreated silicon nitride fibers.

[0078] The silicon nitride fibers have a diameter of 90 nm and a length of 0.1 μm.

[0079] (2)Modification treatment

[0080] 9.7 g of pretreated silicon nitride fibers are placed in 57 g of pre-modification solution. The temperature is raised to 60 °C at a rate of 1.8 °C / min and stirred and held for 1.4 h. After stirring ends, it is filtered out and put into 63 g of absolute ethanol. 76 g of modification solution is added, and the temperature is raised to 56 °C and stirred and reacted for 0.7 h. After the reaction ends, it is filtered, washed, and dried to obtain modified silicon nitride fibers.

[0081] The pre-modification solution is a mixture of deionized water and cyclohexanehexol, and the mass ratio of deionized water to cyclohexanehexol is 100:8.4.

[0082] The modification solution is a mixture of deionized water, polydopamine, and poly-L-methionine, and the mass ratio of deionized water, polydopamine, and poly-L-methionine is 100:7.2:5.0.

[0083] (3)Carboxylation

[0084] 9.2 g of modified silicon nitride fibers were added to 98 g of N,N-dimethylformamide. After stirring evenly, 3.0 g of 2-octenyl succinic anhydride was added for ultrasonic reaction. The ultrasonic temperature was 37 °C, the ultrasonic time was 1.8 h, the ultrasonic power was 107 W, and the ultrasonic frequency was 28 kHz. After the ultrasonic reaction ended, it was centrifuged, washed, and dried to obtain the reinforcing phase.

[0085] 2. Composite

[0086] 495 g of N,N-dimethylformamide was added to the reaction vessel, then 6.0 g of the reinforcing phase and 3.6 g of modified silicon carbide were added. After stirring evenly, 1.2 g of triethylamine was added, and nitrogen was introduced for high-temperature reaction. The reaction temperature was 117 °C, the reaction time was 5.8 h. After the reaction ended, it was filtered, washed, and dried to obtain the composite;

[0087] The preparation method of the modified silicon carbide was as follows: 7.7 g of silicon carbide was placed in 86 g of 26 wt% ethanol solution. After stirring evenly, 0.2 g of γ-glycidoxypropylmethyldiethoxysilane and 0.5 g of γ-ureidopropyltriethoxysilane were added for ball milling. The ball milling time was 2.0 h, the ball milling speed was 112 rpm, the ball-to-material ratio was 2:1. After the ball milling ended, it was filtered, washed, and dried to obtain the modified silicon carbide;

[0088] The particle size of the silicon carbide was 150 nm.

[0089] 3. Mixing

[0090] 47 g of silicon nitride, 4.5 g of the composite, and 2.2 g of sintering aids were added to 994 g of the mixed solution for ultrasonic treatment. The ultrasonic time was 28 min, the ultrasonic power was 172 W, and the ultrasonic frequency was 24 kHz. After the ultrasonic treatment ended, 20.4 g of pore-forming agent and 2.0 g of stabilizer were added, and the temperature was raised to 62 °C. Ultrasonic treatment was continued for 0.8 h, controlling the power and frequency of the ultrasonic treatment to remain unchanged. After the ultrasonic treatment ended, it was dried to obtain the mixed material;

[0091] The particle size of the silicon nitride was 240 nm;

[0092] The mixed solution was a mixture of 58 wt% ethanol solution and vinyltrimethoxysilane, and the mass ratio of the 58 wt% ethanol solution to vinyltrimethoxysilane was 90:1.0;

[0093] The sintering aids were a mixture of magnesium oxide and yttrium oxide, and the mass ratio of magnesium oxide to yttrium oxide was 1:0.7;

[0094] The pore-forming agent was polymethyl methacrylate microspheres with a particle size of 4.8 μm;

[0095] The stabilizer is a mixture of antioxidant 1010 and polyvinyl alcohol, and the mass ratio of antioxidant 1010 to polyvinyl alcohol is 1:1.3.

[0096] 4. Post-treatment

[0097] The mixture is placed in a mold for cold isostatic pressing, with the pressure controlled at 80 MPa and the time at 34 min. Then, in an air atmosphere, it is heated to 175 °C at a rate of 1.7 °C / min and held for 23 min, then heated to 320 °C at a rate of 1.0 °C / min and held for 35 min, and finally heated to 650 °C at a rate of 0.4 °C / min and held for 2.2 h to obtain a silicon nitride ceramic green body; the silicon nitride ceramic green body is placed in a nitrogen atmosphere, heated to 840 °C at a rate of 3.0 °C / min and held for 25 min, then heated to 1715 °C at a rate of 1.8 °C / min and held for 1.2 h to obtain porous silicon nitride ceramics.

[0098] Example 3

[0099] 1. Preparation of the reinforcing phase

[0100] (1) Pretreatment

[0101] The silicon nitride fibers are placed in acetone with a mass 8 times that of the fibers, stirred evenly, then the temperature is raised to 75 °C for reflux reaction. The reflux reaction time is 6.3 h. After the reflux reaction ends, they are filtered and dried to obtain pretreated silicon nitride fibers;

[0102] The silicon nitride fibers have a diameter of 110 nm and a length of 0.3 μm;

[0103] (2) Modification treatment

[0104] 10.2 g of pretreated silicon nitride fibers are placed in 63 g of pre-modification solution, heated to 65 °C at a rate of 2.2 °C / min, and kept stirring for 1.7 h. After stirring ends, they are filtered out and put into 68 g of absolute ethanol. 85 g of modification solution is added, the temperature is raised to 60 °C, and the reaction is stirred for 1.2 h. After the reaction ends, they are filtered, washed, and dried to obtain modified silicon nitride fibers;

[0105] The pre-modification solution is a mixture of deionized water and cyclohexanehexol, and the mass ratio of deionized water to cyclohexanehexol is 100:9.0;

[0106] The modification solution is a mixture of deionized water, polydopamine, and poly-L-methionine, and the mass ratio of deionized water, polydopamine, and poly-L-methionine is 100:7.6:5.3;

[0107] (3) Carboxylation

[0108] 9.6 g of modified silicon nitride fibers were added to 104 g of N,N-dimethylformamide. After stirring evenly, 3.5 g of 2-octenyl succinic anhydride was added for ultrasonic reaction. The ultrasonic temperature was 42 °C, the ultrasonic time was 2.3 h, the ultrasonic power was 115 W, and the ultrasonic frequency was 34 kHz. After the ultrasonic reaction ended, it was centrifuged, washed, and dried to obtain the reinforcing phase.

[0109] 2. Composite

[0110] 510 g of N,N-dimethylformamide was added to the reaction vessel, then 6.4 g of the reinforcing phase and 4.0 g of modified silicon carbide were added. After stirring evenly, 1.5 g of triethylamine was added, and nitrogen was introduced for high-temperature reaction. The reaction temperature was 122 °C, the reaction time was 6.2 h. After the reaction ended, it was filtered, washed, and dried to obtain the composite;

[0111] The preparation method of the modified silicon carbide was as follows: 8.2 g of silicon carbide was placed in 95 g of 30 wt% ethanol solution. After stirring evenly, 0.4 g of γ-glycidoxypropylmethyldiethoxysilane and 0.2 g of γ-ureidopropyltriethoxysilane were added for ball milling. The ball milling time was 2.5 h, the ball milling speed was 126 rpm, and the ball-to-material ratio was 4:1. After the ball milling ended, it was filtered, washed, and dried to obtain the modified silicon carbide;

[0112] The particle size of the silicon carbide was 180 nm.

[0113] 3. Mixing

[0114] 52 g of silicon nitride, 5.0 g of the composite, and 2.5 g of sintering aids were added to 1010 g of the mixed solution for ultrasonic treatment. The ultrasonic time was 32 min, the ultrasonic power was 186 W, and the ultrasonic frequency was 30 kHz. After the ultrasonic treatment ended, 21.1 g of pore-forming agent and 2.5 g of stabilizer were added, and the temperature was raised to 67 °C. Ultrasonic treatment was continued for 1.2 h, controlling the power and frequency of the ultrasonic treatment to remain unchanged. After the ultrasonic treatment ended, it was dried to obtain the mixed material;

[0115] The particle size of the silicon nitride was 260 nm;

[0116] The mixed solution was a mixture of 63 wt% ethanol solution and vinyltrimethoxysilane, and the mass ratio of the 58 wt% ethanol solution to vinyltrimethoxysilane was 90:1.5;

[0117] The sintering aids were a mixture of magnesium oxide and yttrium oxide, and the mass ratio of magnesium oxide to yttrium oxide was 1:1.2;

[0118] The pore-forming agent was polymethyl methacrylate microspheres with a particle size of 5.3 μm;

[0119] The stabilizer is a mixture of antioxidant 1010 and polyvinyl alcohol, and the mass ratio of antioxidant 1010 to polyvinyl alcohol is 1:0.8.

[0120] 4. Post-treatment

[0121] The mixed material is placed in a mold for cold isostatic pressing, with the pressure controlled at 110 MPa and the time at 28 min. Then, in an air atmosphere, it is heated to 185 °C at a rate of 2.3 °C / min, held for 18 min, further heated to 360 °C at a rate of 1.5 °C / min, held for 25 min, and finally heated to 670 °C at a rate of 0.6 °C / min, held for 1.8 h to obtain a silicon nitride ceramic green body; the silicon nitride ceramic green body is placed in a nitrogen atmosphere, heated to 860 °C at a rate of 4.0 °C / min, held for 18 min, and then heated to 1724 °C at a rate of 2.2 °C / min, held for 0.8 h to obtain porous silicon nitride ceramic.

[0122] Comparative Example 1

[0123] Based on Example 1, the differences are as follows:

[0124] In the step of preparing the reinforcing phase, the modification treatment and carboxylation step are omitted;

[0125] In the compounding step, the reinforcing phase is replaced with pretreated silicon nitride fibers in equal amounts;

[0126] The remaining operations are the same.

[0127] Comparative Example 2

[0128] Based on Example 1, the differences are as follows:

[0129] (1) The compounding step is as follows: 500 g of N,N-dimethylformamide is added to a reaction vessel, then 6.2 g of the reinforcing phase and 3.7 g of silicon carbide are added, stirred evenly, filtered, washed, and dried to obtain a composite;

[0130] The particle size of the silicon carbide is 160 nm

[0131] (2) In the mixing step, the mixed liquid is replaced with deionized water in equal amounts;

[0132] The remaining operations are the same.

[0133] Performance test

[0134] The porous silicon nitride ceramics prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to performance tests, as follows:

[0135]

[0136] Among them, for the high-temperature resistance performance, the porous silicon nitride ceramics prepared in Examples 1-3 and Comparative Examples 1-2 were heated to 1500 °C at a rate of 20 °C / min, held for 1000 s, and after the holding treatment was completed, cooled to room temperature at a rate of 50 °C / min, and the fracture toughness and flexural strength were measured again;

[0137] The present invention uses silicon nitride fibers as the reinforcing phase and modifies the silicon nitride fibers. Specifically, cyclohexanehexol with more hydroxyl groups is first used as the pre-modifying solution, so that the surface of the silicon nitride fibers contains more hydroxyl groups, and then an amino compound is added, so that the amino group of the amino compound combines with the hydroxyl group on the surface of the silicon nitride fibers, so that the surface of the modified silicon nitride fibers contains more amino groups, and then an anhydride group is introduced, so that the surface of the silicon nitride fibers is rich in carboxyl groups; in the composite step, modified silicon carbide is introduced. Specifically, ureido groups and epoxy groups are introduced on the surface of the silicon carbide. The epoxy group can react with the carboxyl group and the ureido group, so that the modified silicon carbide and the silicon nitride fibers are crosslinked to form a stable three-dimensional network structure. Moreover, after the silicon carbide is modified, it has good dispersibility and can be uniformly compounded with the silicon nitride fibers to obtain a composite. The composite has good compatibility with the silicon nitride particles and can be uniformly dispersed, so that the toughening performance of the composite can be fully exerted, the brittleness of the silicon nitride ceramics can be improved, and when it is used in the silicon nitride ceramics, through crack deflection, pulling out and bridging effects, the further propagation of cracks can be prevented, the toughness of the silicon nitride ceramics can be improved, and the mechanical properties and high-temperature oxidation resistance of the silicon nitride ceramics can be improved, further enhancing the comprehensive performance; in the mixing step, a specific mixed solution containing vinyl silane coupling agent is added. One end of the silane coupling agent has a hydroxyl group that can combine with groups such as the hydroxyl groups on the surface of the composite and the silicon nitride particles, and the other end has good compatibility with the pore-forming agent, and the materials can be uniformly dispersed, and the obtained mixed material has good dispersibility and excellent homogeneity. During the high-temperature treatment in the post-treatment process, as the temperature rises, the pore-forming agent decomposes and volatilizes, and the vinyl silane coupling agent can effectively prevent the collapse of the pores left after volatilization as a support, avoiding the entry and aggregation of silicon nitride particles into the pores, ensuring the uniform dispersion of the pores, so that while increasing the porosity and the uniformity of the pore distribution, the mechanical properties and high-temperature resistance of the ceramic product are ensured, and the toughness is enhanced.

[0138] In Comparative Example 1, in the composite step, the reinforcing phase was replaced with pretreated silicon nitride fibers, and the modification treatment and carboxylation step were omitted. The dispersibility of the pretreated silicon nitride fibers was poor, and the interfacial bonding with the silicon carbide matrix was not good. Uniform dispersion could not be achieved between the two, and it could not fully exert the strengthening and toughening properties of the short silicon nitride fibers. Moreover, the crosslinking with the modified silicon nitride was unstable, reducing the strength, toughness and high-temperature resistance of the ceramic product, and making its dielectric loss more at high temperatures and the dielectric properties poor;

[0139] In Comparative Example 2, in the compounding step, the preparation of the modified silicon carbide was omitted, and silicon carbide and the reinforcing phase were directly mixed. The aggregation effect of silicon carbide was strong, and uniform dispersion could not be achieved. A stable cross-linked network structure could not be formed between silicon carbide and the reinforcing phase either, thus weakening the reinforcing performance of the reinforcing phase. Moreover, the components of the mixed solution were omitted, resulting in poor compatibility with other components, uneven pore formation, and poor homogeneity, ultimately reducing the comprehensive performance of the product.

[0140] Unless otherwise specified, the percentages used in the present invention are all mass percentages.

[0141] Finally, it should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or perform equivalent replacements for some of the technical features. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a porous silicon nitride ceramic, characterized in that, It includes steps of preparing reinforcing phase, compounding, mixing and post-treatment; The preparation of the reinforcing phase includes pre-treatment, modification treatment and carboxylation steps; In the modification treatment step, the pre-treated silicon nitride fiber is placed in a pre-modification solution, stirred at 60 - 65 °C for 1.4 - 1.7 h, then put into absolute ethanol, the modification solution is added, and stirred at 56 - 60 °C for 0.7 - 1.2 h to obtain modified silicon nitride fiber; The pre-modification solution is a mixture of deionized water and cyclohexanehexol, and the modification solution is a mixture of deionized water, polydopamine and poly-L-methionine; In the carboxylation step, the modified silicon nitride fiber is added to N,N-dimethylformamide, 2-octenyl succinic anhydride is added for ultrasonic reaction to obtain the reinforcing phase; In the compounding step, N,N-dimethylformamide is added to the reaction vessel, the reinforcing phase and modified silicon carbide are added, triethylamine is added, and nitrogen is introduced for high-temperature reaction to obtain a composite; The preparation method of the modified silicon carbide is to place silicon carbide in an ethanol solution, add γ-glycidoxypropylmethyldiethoxysilane and γ-ureidopropyltriethoxysilane for ball milling treatment to obtain modified silicon carbide; In the mixing step, silicon nitride, the composite, a sintering aid are added to the mixed solution, ultrasonic treatment is carried out, the ultrasonic time is 28 - 32 min, the ultrasonic power is 172 - 186 W, the ultrasonic frequency is 24 - 30 kHz. After the ultrasonic treatment, a pore-forming agent and a stabilizer are added, the temperature is raised to 62 - 67 °C, and ultrasonic treatment is continued for 0.8 - 1.2 h, controlling the power and frequency of the ultrasonic treatment to be unchanged. After the ultrasonic treatment is completed, it is dried to obtain a mixed material; The mass ratio of the silicon nitride, the composite, the sintering aid, the mixed solution, the pore-forming agent, and the stabilizer is 47 - 52:4.5 - 5.0:2.2 - 2.5:994 - 1010:20.4 - 21.1:2.0 - 2.5; The mixed solution is a mixture of a 58 - 63 wt% ethanol solution and vinyltrimethoxysilane; The pore-forming agent is polymethylmethacrylate microspheres.

2. The preparation method of a porous silicon nitride ceramic according to claim 1, characterized in that, In the pre-treatment step, the silicon nitride fiber is placed in acetone with a mass 6 - 8 times that of the fiber, stirred evenly, then the temperature is raised to 70 - 75 °C for reflux reaction, the reflux reaction time is 5.7 - 6.3 h. After the reflux reaction is completed, it is filtered and dried to obtain pre-treated silicon nitride fiber; The silicon nitride fiber has a diameter of 90 - 110 nm and a length of 0.1 - 0.3 μm.

3. The preparation method of a porous silicon nitride ceramic according to claim 1, characterized in that, In the modification treatment step, the pre-treated silicon nitride fiber is placed in the pre-modification solution, heated to 60 - 65 °C at a rate of 1.8 - 2.2 °C / min, kept warm and stirred for 1.4 - 1.7 h. After the stirring is completed, it is filtered out, put into absolute ethanol, the modification solution is added, the temperature is raised to 56 - 60 °C, and stirred and reacted for 0.7 - 1.2 h. After the reaction is completed, it is filtered, washed and dried to obtain modified silicon nitride fiber; The mass ratio of the pretreated silicon nitride fiber, pre-modifying solution, absolute ethanol, and modifying solution is 9.7 - 10.2:57 - 63:63 - 68:76 - 85.

4. The preparation method of a porous silicon nitride ceramic according to claim 1, characterized in that In the pre-modifying solution, the mass ratio of deionized water to cyclohexanehexol is 100:8.4 - 9.0; In the modifying solution, the mass ratio of deionized water, polydopamine, and poly-L-methionine is 100:7.2 - 7.6:5.0 - 5.

3.

5. The preparation method of a porous silicon nitride ceramic according to claim 1, characterized in that In the carboxylation step, for the ultrasonic reaction, the ultrasonic temperature is 37 - 42°C, the ultrasonic time is 1.8 - 2.3 h, the ultrasonic power is 107 - 115 W, and the ultrasonic frequency is 28 - 34 kHz; The mass ratio of the modified silicon nitride fiber, N,N-dimethylformamide, and 2-octenyl succinic anhydride is 9.2 - 9.6:98 - 104:3.0 - 3.

5.

6. The preparation method of a porous silicon nitride ceramic according to claim 1, characterized in that The compounding step is as follows: Add N,N-dimethylformamide to a reaction vessel, then add the reinforcing phase and modified silicon carbide, stir evenly, add triethylamine, and introduce nitrogen for high-temperature reaction. The reaction temperature is 117 - 122°C, the reaction time is 5.8 - 6.2 h. After the reaction ends, filter, wash, and dry to obtain the composite; The mass ratio of N,N-dimethylformamide, reinforcing phase, modified silicon carbide, and triethylamine is 495 - 510:6.0 - 6.4:3.6 - 4.0:1.2 - 1.

5.

7. The preparation method of a porous silicon nitride ceramic according to claim 1, characterized in that In the preparation method of the modified silicon carbide, for the ball milling treatment, the ball milling time is 2.0 - 2.5 h, the ball milling speed is 112 - 126 rpm, and the ball-to-material ratio is 2 - 4:1; The particle size of the silicon carbide is 150 - 180 nm; The mass ratio of the silicon carbide, ethanol solution, γ-glycidoxypropylmethyldiethoxysilane, and γ-ureidopropyltriethoxysilane is 7.7 - 8.2:86 - 95:0.2 - 0.4:0.2 - 0.5; The mass concentration of the ethanol solution is 26 - 30%.

8. The preparation method of a porous silicon nitride ceramic according to claim 1, characterized in that In the mixing step, the particle size of the silicon nitride is 240 - 260 nm; In the mixed solution, the mass ratio of the 58 - 63 wt% ethanol solution to vinyltrimethoxysilane is 90:1.0 - 1.5; The sintering aid is a mixture of magnesium oxide and yttrium oxide, and the mass ratio of magnesium oxide to yttrium oxide is 1:0.7 - 1.2; The particle size of the pore-forming agent is 4.8 - 5.3 μm; The stabilizer is a mixture of antioxidant 1010 and polyvinyl alcohol, and the mass ratio of antioxidant 1010 to polyvinyl alcohol is 1:0.8 - 1.

3.

9. The preparation method of a porous silicon nitride ceramic according to claim 1, wherein the post-treatment step is as follows: placing the mixture in a mold for cold isostatic pressing, controlling the pressure at 80-110 MPa and the time at 28-34 min, then in an air atmosphere, heating it at a rate of 1.7-2.3 °C / min to 175-185 °C, holding for 18-23 min, then heating it at a rate of 1.0-1.5 °C / min to 320-360 °C, holding for 25-35 min, and finally heating it at a rate of 0.4-0.6 °C / min to 650-670 °C, holding for 1.8-2.2 h to obtain a silicon nitride ceramic green body; placing the silicon nitride ceramic green body in a nitrogen atmosphere, heating it at a rate of 3.0-4.0 °C / min to 840-860 °C, holding for 18-25 min, and then heating it at a rate of 1.8-2.2 °C / min to 1715-1724 °C, holding for 0.8-1.2 h to obtain a porous silicon nitride ceramic.

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