A variable porosity silicon nitride ceramic component and method of making the same

Abstract

The present application relates to a kind of variable porosity silicon nitride ceramic components and its preparation method, which comprises the following steps: S1, gel injection molding: preparation of different porosity silicon nitride ceramic blank and corresponding sintering aid slurry;S2, blank bonding: different porosity silicon nitride ceramic blank is connected and sintering aid slurry is applied to the joint to form a gel thin layer, to obtain variable porosity silicon nitride ceramic component precursor;S3, degumming sintering: after excluding the organic matter in variable porosity silicon nitride ceramic component precursor, sintering is carried out to obtain variable porosity silicon nitride ceramic component. Compared with prior art, the present application obtains defect-free, internal stress-free, crack-free silicon nitride ceramic component.

Description

Technical Field

[0001] This invention belongs to the field of ceramic material preparation technology, and in particular relates to a variable porosity silicon nitride ceramic component and its preparation method. Background Technology

[0002] High-density silicon nitride ceramics are high-performance structural ceramics with advantages such as good mechanical properties, low coefficient of thermal expansion, and good thermal stability. They are often used to make bearing balls, cutting tools, IGBT substrates, and light emitters, and have wide applications in chemical, electronic information, mechanical, and military fields. Low-density silicon nitride ceramics, also known as porous silicon nitride ceramics, have advantages such as low dielectric constant, stable high-frequency dielectric properties, and low coefficient of thermal expansion. They are often used in wave-transparent components and loop heat pipes. To meet specific requirements, silicon nitride ceramics with different porosities are often combined. For example, in aerospace radomes, variable porosity silicon nitride ceramic components can effectively combine load-bearing, wave transmission, and thermal insulation, and can further improve thermal shock resistance, achieving broadband wave transmission for the radome.

[0003] Gel casting can produce green bodies with complex shapes at net dimensions. For ceramic components that cannot be machined or modified, gel casting combined with silicon nitride ceramic green bodies of different porosities is an effective preparation method at the green body stage.

[0004] Chinese patent CN104924412A provides a method for bonding SiC preforms using gel, but this method does not take into account deformation during curing. More importantly, it is only applicable to reaction sintering, a sintering method with low net size and internal stress. However, silicon nitride preforms with different porosities will experience significant shrinkage of 5% to 20% after pressure sintering. Chinese patent CN110483091A provides a method for joining porous silicon nitride ceramics, which uses tape casting with α-Si3N4 powder as a connecting layer. However, it still has the problem of direct connection between silicon nitride preforms. During sintering, due to the lack of "lubrication" between preforms, there are often differences in dimensional shrinkage, which can cause stress imbalance at the joint, resulting in cracking, peeling, and other problems. Therefore, this method is no longer applicable.

[0005] Based on this, this application explores a more scientific and practical thin-layer gel bonding technology for sintering aids, so that the bonding layer of the sintered green body is free of cracks, defects and stress, thereby realizing the preparation of silicon nitride ceramic components with variable porosity. Summary of the Invention

[0006] The purpose of this invention is to overcome the defects of the prior art by providing a variable porosity silicon nitride ceramic component and its preparation method. This application uses a sintering aid thin layer as a medium to connect two silicon nitride ceramics with different porosities. During sintering, the sintering aid thin layer reacts with the silicon dioxide on the surface of the silicon nitride blank to form a liquid phase. The low viscosity liquid phase can "lubricate" the bonding surface of the silicon nitride blank, allowing the blank to have a certain shrinkage. In addition, the generated liquid phase can also promote the growth and development of grains at the bonding surface. The intersecting of long columnar silicon nitride grains can make the bonding layer tighter, thereby obtaining a variable porosity silicon nitride ceramic component without defects, internal stress, or cracks.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] This invention provides a method for preparing silicon nitride ceramic components with variable porosity, comprising the following steps:

[0009] S1, Gel casting: Preparation of silicon nitride ceramic blanks with different porosities and corresponding sintering aid slurries;

[0010] S2, Green body bonding: Connect silicon nitride ceramic green bodies with different porosities and apply sintering aid slurry to the joint to form a gel thin layer, thereby obtaining a precursor of silicon nitride ceramic components with variable porosity;

[0011] S3, Debonding and Sintering: After removing organic matter from the precursor of variable porosity silicon nitride ceramic components, sintering is performed to obtain variable porosity silicon nitride ceramic components.

[0012] Furthermore, in step S1, the specific method for preparing the silicon nitride ceramic green body is as follows:

[0013] S11, prepare a polymer premix: a mixture of monomers, crosslinking agent, polymerization inhibitor, pH agent, dispersant, and water;

[0014] S12, ceramic powder formulation: a mixture of silicon nitride powder, yttrium oxide powder, and magnesium oxide powder;

[0015] S13, preparing ceramic slurry: mixing polymer premix and ceramic powder, and ball milling to disperse to obtain ceramic slurry;

[0016] S14, Polymerization: Initiator and catalyst are added to ceramic slurry, and the reaction proceeds.

[0017] S15, Casting: The slurry after S14 polymerization is vacuum defoamed and poured into a mold to form a gel preform;

[0018] S16, Demolding and Moisturizing: Remove the gel blank, wash it, wrap it in plastic wrap and store it.

[0019] Furthermore, in step S12, the mass fractions of silicon nitride powder, yttrium oxide powder, and magnesium oxide powder are 96-99%, 0.5-2.5%, and 0.5-1.5%, respectively.

[0020] In step S13, the solid content of the ceramic slurry is 20%-60%, and the ceramic slurry comprises the following components by mass fraction: 10-50% deionized water, 5-15% monomer, 0.5%-1.5% crosslinking agent, 0.08-0.23% polymerization inhibitor, 0.05-0.2% dispersant, 0.1-0.3% pH agent, and the balance being ceramic powder.

[0021] Furthermore, the monomer is acrylamide, the crosslinking agent is N,N'-methylenebisacrylamide, the polymerization inhibitor is catechol or phenothiazine, the pH agent is ammonia or acetic acid, and the dispersant is one of triethylhexylphosphate, sodium dodecyl sulfate, ammonium polyacrylate, and tetramethylammonium hydroxide;

[0022] The median particle size of the silicon nitride powder is 0.5-1.5 μm, and the median particle size of the yttrium oxide powder and magnesium oxide powder is 5-20 nm.

[0023] Furthermore, in step S14, the initiator is one of the following: a solution containing hydrogen peroxide, a solution containing ammonium persulfate, or a solution containing benzoyl peroxide; and the catalyst is tetramethylethylenediamine.

[0024] The added initiator and catalyst account for 0.4-0.8% and 0.8-1.6% of the monomer by mass, respectively.

[0025] Furthermore, in step S1, the sintering aid slurry is prepared by mixing and ball milling the sintering aid powder and the polymer premix.

[0026] Furthermore, the sintering aid powder is one or more of yttrium oxide, magnesium oxide, aluminum oxide, and magnesium silicon nitride.

[0027] Furthermore, in step S2, the thickness of the gel thin layer is 0.5-1.5 mm, and the solid content is 20%-60%.

[0028] Furthermore, in step S3, the removal of organic matter from the precursor of the silicon nitride ceramic component with variable porosity specifically involves: heating the substrate to 400-800℃ in an air environment and holding it at that temperature for 2-6 hours to eliminate the organic matter in the precursor of the silicon nitride ceramic component.

[0029] The sintering is gas pressure sintering, with a sintering temperature of 1600-1700℃. The heating rate is 0.5-2℃ / min for 400-800℃, 3-6℃ / min for 800-1200℃, and 5-10℃ / min for 1200-1700℃. The holding time is 1-2 hours.

[0030] The present invention further provides a silicon nitride ceramic component with variable porosity obtained by the preparation method of the silicon nitride ceramic component with variable porosity as described above.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] 1. This invention utilizes a gel thin layer formed by sintering aids as a bonding layer. During sintering, the low-viscosity liquid phase not only lubricates the bonding surface of the silicon nitride preform, ensuring a defect-free, stress-free, and crack-free bonding joint, but also promotes grain growth and development at the bonding surface. The intersecting long columnar silicon nitride grains further strengthen the bonding layer. This technology enables the integrated sintering of silicon nitride ceramic components, overcoming problems such as cracking and defects in the bonding layer that exist in conventional ceramic composites.

[0033] 2. The process used in this invention is gel casting molding process. This process is simple to prepare and has low equipment cost. It can obtain high-strength and high-precision green blanks, and after sintering, silicon nitride ceramics with high porosity and uniform pore size can be obtained.

[0034] 3. The variable porosity silicon nitride ceramic component prepared by the present invention can solve the problem of difficult connection of current gradient density silicon nitride ceramics and obtain a ceramic component that takes into account both structural and functional performance. Attached Figure Description

[0035] Figure 1 This is a flowchart of the preparation of a variable porosity silicon nitride ceramic component in Embodiment 1 of the present invention.

[0036] Figure 2 This is a schematic diagram of the structure of the variable porosity silicon nitride ceramic component prepared in Example 1 of the present invention.

[0037] Figure 3 This is a SEM image of the variable porosity silicon nitride ceramic component prepared in Example 1 of the present invention.

[0038] Figure 4 This is a macroscopic photograph of the variable porosity silicon nitride ceramic component prepared in Example 1 of the present invention.

[0039] Numbering on the map:

[0040] 1-First silicon nitride layer, 2-Gel thin layer, 3-Second silicon nitride layer. Detailed Implementation

[0041] To facilitate understanding of this application, it will be described more fully below through embodiments. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0042] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly stated; and any lower limit can be combined with other lower limits to form a range not explicitly stated, just as any upper limit can be combined with any other upper limit to form a range not explicitly stated. Furthermore, although not explicitly stated, every point or individual value between the endpoints of the range is included within that range. Therefore, each point or individual value can be used as its own lower or upper limit and combined with any other point or individual value or with other lower or upper limits to form a range not explicitly stated.

[0043] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. It should be noted that, unless otherwise stated, the term "and / or" as used herein includes any and all combinations of one or more of the associated listed items, "above," "below," includes the stated number, and "one or more" with "multiple" means two or more.

[0044] Variable porosity silicon nitride ceramic components can effectively combine load-bearing, wave transmission, and heat insulation, and have a wide range of applications, such as in aerospace radomes. In the existing technology, gel casting molding process is often used to combine silicon nitride ceramic blanks with different porosities in the blank stage. However, research has found that when silicon nitride blanks are directly connected to each other for sintering, it often leads to dimensional shrinkage, which in turn causes stress imbalance at the joint, resulting in cracking, peeling and other problems. Furthermore, the connecting layer used in the existing technology cannot overcome the above problems.

[0045] Based on this, the present application proposes the following technical solution.

[0046] The first aspect of this application provides a method for preparing a silicon nitride ceramic component with variable porosity, comprising the following steps:

[0047] S1, Gel casting: Preparation of silicon nitride ceramic blanks with different porosities and corresponding sintering aid slurries;

[0048] Gel injection molding technology can form complex-shaped ceramic blanks in net dimensions, reducing the subsequent processing cost of ceramic components. It can also reduce or eliminate defects such as agglomeration of ceramic particles, macroscopic pores, and cracks in the material, thereby improving the reliability of the material. For ceramic components that have been illegally processed or modified, this technology can be used for mechanical pre-processing in the blank stage.

[0049] In one embodiment, the specific method for preparing the silicon nitride ceramic green body in step S1 is as follows:

[0050] S11, prepare a polymer premix: a mixture of monomers, crosslinking agent, polymerization inhibitor, pH agent, dispersant, and water;

[0051] It should be noted that the system used in this application for preparing silicon nitride ceramic green bodies by gel casting is a water-based gel system. The water-based gel system uses water as a solvent, which replaces the toxic solvents used in traditional silicon nitride ceramic green bodies. It is harmless to the human body and the environment and is conducive to industrialization.

[0052] S12, ceramic powder formulation: a mixture of silicon nitride powder, yttrium oxide powder, and magnesium oxide powder;

[0053] In one embodiment, the mass fractions of silicon nitride powder, yttrium oxide powder, and magnesium oxide powder are 96-99%, 0.5-2.5%, and 0.5-1.5%, respectively. For example, the mass fraction of silicon nitride powder can be 96%, 97%, 98%, 99%, or any value within the above ranges. For example, the mass fraction of yttrium oxide powder can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.5%, 2.0%, 2.5%, or any value within the above ranges. For example, the mass fraction of magnesium oxide powder can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.5%, or any value within the above ranges. Of course, those skilled in the art should understand that any combination of the above ranges can be used, but the sum must equal 100%. Further examples will not be provided here.

[0054] Preferably, the mass fractions of silicon nitride powder, yttrium oxide powder, and magnesium oxide powder are 97%, 1%, and 2%, respectively.

[0055] In one embodiment, the median particle size of the silicon nitride powder is 0.5-1.5 μm. For example, the median particle size of the silicon nitride powder can be 0.5 μm, 0.6 μm, 0.7 μm, 1.0 μm, 1.2 μm, 1.5 μm, or any value within the above range, preferably 0.5 μm. The smaller the particle size of the powder, the better the sintering activity. The median particle size of the yttrium oxide powder and magnesium oxide powder is 5-20 nm. For example, the median particle size of the yttrium oxide powder and magnesium oxide powder can be 5 nm, 6 nm, 7 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm, or any value within the above range, preferably 10 nm.

[0056] Furthermore, when determining the silicon nitride ceramic green body with different solid phase contents, the solid phase content and water content in the silicon nitride ceramic green body should be determined first. For example, when determining the solid phase content, the solid phase content is calculated as: mass of each substance / specific gravity of each substance / (mass of each substance / specific gravity of each substance + mass of water). Once the mass of water is confirmed, the mass of the other additives can also be confirmed.

[0057] S13, preparing ceramic slurry: mixing polymer premix and ceramic powder, and ball milling to disperse to obtain ceramic slurry;

[0058] Specifically, after mixing, the mixture is placed in a silicon nitride container and ball-milled and stirred to disperse it. The ball milling speed is 50-400 r / min, and the time is 2-24 hours.

[0059] Preferably, the ball milling speed is 100-350 r / min and the time is 8-24 hours;

[0060] Alternatively, the ball milling speed is 300 r / min and the time is 12 hours.

[0061] In some embodiments, the solid content of the ceramic slurry is 20%-60%. Specifically, when the solid content in the slurry is too high, the fluidity is poor, and when the solid content is too low, it cannot be molded. The ceramic slurry comprises the following components by mass fraction: 10-50% deionized water (e.g., the mass fraction of deionized water can be 10%, 20%, 30%, 40%, 50%, or any value within the above range), 5-15% monomer (e.g., the mass fraction of monomer can be 5%, 6%, 7%, 8%, 9%, 10%, 12%, 15%, or any value within the above range), 0.5%-1.5% crosslinking agent (e.g., the mass fraction of crosslinking agent can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.2%, 1.5%, or any value within the above range), and 0.08-0.23% polymerization inhibitor (e.g., the mass fraction of polymerization inhibitor can be 0.08%, 0.09%, 0.10%, 0.11%). The following components are included: 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.20%, 0.23%, or any value within the above range); 0.05-0.2% dispersant (e.g., the mass fraction of the dispersant can be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, 0.15%, 0.20%, or any value within the above range); 0.1-0.3% pH agent (e.g., the mass fraction of the pH agent can be 0.1%, 0.2%, 0.3%, or any value within the above range); and the balance is ceramic powder. Of course, those skilled in the art should understand that any combination of the above range values ​​is possible, but the sum must equal 100%. Further examples will not be provided here.

[0062] In some embodiments, the monomer is acrylamide;

[0063] The crosslinking agent is N,N'-methylenebisacrylamide, which is used to transform linear or slightly branched macromolecules into a three-dimensional network structure, thereby improving properties such as strength, heat resistance, wear resistance, and solvent resistance.

[0064] The polymerization inhibitor is catechol or phenothiazine, preferably catechol. The role of the polymerization inhibitor is to prevent the gelation reaction from occurring too quickly and to ensure that the slurry does not solidify when vacuuming.

[0065] The pH agent is ammonia or acetic acid. The function of the pH agent is to adsorb an electron layer on the surface of the powder, so that the particles do not agglomerate through electrostatic repulsion.

[0066] The dispersant is one of triethylhexylphosphate, sodium dodecyl sulfate, ammonium polyacrylate, and tetramethylammonium hydroxide, preferably ammonium polyacrylate (PAA-NH4). Its main function is to disperse the powder in water so that it does not agglomerate. It has higher dispersibility than general water-resistant dispersants, so it is used in smaller quantities and can be industrialized on a large scale.

[0067] S14, Polymerization: Initiator and catalyst are added to ceramic slurry, and the reaction proceeds.

[0068] Specifically, initiators and catalysts can initiate and promote monomer polymerization.

[0069] In some embodiments, the initiator is one of a hydrogen peroxide solution, an ammonium persulfate solution, or a benzoyl peroxide solution, preferably an ammonium persulfate solution, and the catalyst is tetramethylethylenediamine.

[0070] In some embodiments, the added initiator and catalyst account for 0.4-0.8% and 0.8-1.6% of the monomer by mass, respectively. For example, the initiator can account for 0.4%, 0.5%, 0.6%, 0.7%, 0.8% of the monomer by mass, or any value within the above range, preferably 0.45%. For example, the catalyst can account for 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6% of the monomer by mass, or any value within the above range, preferably 1.25%. The role of the initiator is to guide free radical polymerization. If too little is added, the gelation time will be too long, resulting in an excessively thick inhibitory layer. If too much is added, the gelation time will be too fast, making it impossible to conduct subsequent experimental operations. The catalyst is mainly used to accelerate the free radical polymerization process.

[0071] S15, Casting: The slurry after S14 polymerization is vacuum defoamed and poured into a mold to form a gel preform;

[0072] S16, Demolding and Moisturizing: Remove the gel blank, wash it, wrap it in plastic wrap and store it.

[0073] Specifically, the gel blank is removed from the mold, the surface is washed, and then the blank is wrapped with plastic wrap to prevent moisture evaporation.

[0074] In one embodiment, in step S1, the sintering aid slurry is prepared by mixing and ball milling the sintering aid powder and the polymer premix.

[0075] It should be noted that the polymer premix used here is prepared in step S11.

[0076] In some embodiments, the sintering aid powder is one or more of yttrium oxide, magnesium oxide, aluminum oxide, and magnesium silicon nitride, preferably magnesium oxide. The role of the sintering aid is to form a liquid phase with silicon nitride, so that elements such as Si and N dissolve in the liquid phase, accelerate the mass transfer process between silicon nitride particles, and allow silicon nitride to achieve density even at low sintering temperatures.

[0077] The reason for choosing these is that the oxides and non-oxides of Y, Mg, and Al are typical sintering aids that can significantly reduce the sintering temperature and promote densification. S2, Green body bonding: Connect silicon nitride ceramic green bodies with different porosities and apply sintering aid slurry to the joint to form a gel thin layer, thereby obtaining a precursor of silicon nitride ceramic components with variable porosity;

[0078] It should be noted that there may be two, three, or four different porosities, or preparations may be made according to the actual situation. A sintering aid slurry is applied between the two connections to form a gel thin layer.

[0079] In some embodiments, in step S2, the thickness of the gel layer is 0.5-1.5 mm. The thickness of the gel layer can be 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm, 1.0 mm, 1.1 mm, 1.2 mm, 1.3 mm, 1.5 mm, or any value within the above range. As the thickness of the gel layer gradually increases, the porosity of the sintered component gradually decreases. The solid content is 20%-60%, which can be 20%, 30%, 40%, 50%, 60%, or any value within the above range, preferably 50%. This is because if the content is too low (below 50%), there will be too many pores after bonding, resulting in low bonding strength; if the content is too high (above 60%), the slurry is not easily dispersed, resulting in high viscosity and uneven layer thickness. A thickness of 50% and bonding strength are both most suitable.

[0080] S3, Debonding and Sintering: After removing organic matter from the precursor of variable porosity silicon nitride ceramic components, sintering is performed to obtain variable porosity silicon nitride ceramic components.

[0081] In some embodiments, in step S3, removing organic matter from the precursor of the silicon nitride ceramic component with variable porosity specifically involves heating the material to 400-800°C in an air environment and holding it at that temperature for 2-6 hours to eliminate organic matter from the precursor of the silicon nitride ceramic component.

[0082] Preferably, the temperature is raised to 500-700℃ in an air environment and held for 3-5 hours to eliminate organic matter in the precursor of silicon nitride ceramic components;

[0083] Alternatively, the temperature is raised to 600°C in an air environment and held for 3 hours to eliminate organic matter in the precursor of the silicon nitride ceramic component. Under these conditions, all organic matter in the precursor of the silicon nitride ceramic component can be completely removed, avoiding the situation where organic matter cannot volatilize in a short time at high temperature during sintering, resulting in residual C inside and thus reducing structural performance.

[0084] In some embodiments, in step S3, sintering is gas pressure sintering, the sintering temperature is 1600-1700℃, the heating rate is 0.5-2℃ / min for 400-800℃, the heating rate is 3-6℃ / min for 800-1200℃, the heating rate is 5-10℃ / min for 1200-1700℃, and the holding time is 1-2h.

[0085] Preferably, the sintering temperature is 1700℃, wherein the heating rate is 2℃ / min for 400-800℃, 3℃ / min for 800-1200℃, and 5℃ / min for 1200-1700℃, and the holding time is 2h.

[0086] Alternatively, the sintering temperature is 1650℃, with a heating rate of 2℃ / min for 400-800℃, a heating rate of 3℃ / min for 800-1200℃, a heating rate of 5℃ / min for 1200-1700℃, and a holding time of 2h.

[0087] Alternatively, the sintering temperature is 1600℃, with a heating rate of 2℃ / min for 400-800℃, a heating rate of 3℃ / min for 800-1200℃, a heating rate of 5℃ / min for 1200-1700℃, and a holding time of 2h.

[0088] Specifically, during the sintering process, the thin layer of sintering aid reacts with the SiO2 layer on the silicon nitride surface to form a low-viscosity liquid phase, which "lubricates" the bonding surface of the silicon nitride blank and promotes grain growth and development at the bonding surface, making the bonding layer more tightly connected. This technology can be used to sinter silicon nitride ceramic components in an integrated manner, making up for the performance instability problem of conventional ceramic composites.

[0089] The second aspect of this application provides a variable porosity silicon nitride ceramic component obtained by the preparation method of the variable porosity silicon nitride ceramic component as described above. This variable porosity silicon nitride ceramic component can solve the problem of difficult connection of current gradient density silicon nitride ceramics, and obtain a ceramic component that takes into account both structural and functional performance, with high porosity and uniform pore size.

[0090] The above implementation methods will be described in more detail below with reference to specific embodiments.

[0091] The reagents involved in the following examples are shown below:

[0092] Raw materials used in the experiment

[0093] silicon nitride <![CDATA[α-Si3N4]]> ＞95％ Nippon Kosan Industrial Co., Ltd. Alumina <![CDATA[Al2O3]]> 99.99％ Shanghai McLean Biochemical Technology Co., Ltd. Magnesium nitride <![CDATA[MgSiN2]]> 99.99％ Qilu Institute of Optical Physics and Engineering Technology magnesium oxide MgO 98％ Shanghai McLean Biochemical Technology Co., Ltd. Yttrium oxide <![CDATA[Y2O3]]> 99.9％ Shanghai McLean Biochemical Technology Co., Ltd.

[0094] Reagents used in the experiment

[0095] Acrylamide AM Chemically pure Shanghai Sinopharm Chemical Reagent Co., Ltd. N,N'-methylenebisacrylamide MBAM Chemically pure Shanghai Sinopharm Chemical Reagent Co., Ltd. ammonium persulfate APS Chemically pure Shanghai Sinopharm Chemical Reagent Co., Ltd. catechol <![CDATA[C6H6O2]]> Chemically pure Shanghai McLean Biochemical Technology Co., Ltd. N,N,N',N'-Tetramethylethylenediamine TEMED Chemically pure Shanghai Sinopharm Chemical Reagent Co., Ltd. ammonia <![CDATA[NH4·H2O]]> 25％～28％ Shanghai Sinopharm Chemical Reagent Co., Ltd. ammonium polyacrylate PAA-NH4 42wt% Aisen Flocculant Co., Ltd.

[0096] Preparation Example

[0097] Example 1

[0098] This embodiment describes a method for preparing a silicon nitride ceramic component with variable porosity. The process can be found in [link to relevant documentation]. Figure 1 As shown, it includes the following steps:

[0099] S1. Ingredients: A silicon nitride ceramic green body with a solid content of 30% is prepared, wherein the silicon nitride powder weighs 97g, the alumina powder weighs 1g, the magnesium oxide powder weighs 2g, and the water weighs 73.6g respectively; A silicon nitride green body with a solid content of 45% is prepared, wherein the silicon nitride powder weighs 97g, the magnesium oxide powder weighs 1g, the yttrium oxide powder weighs 2g, and the water weighs 38.3g respectively.

[0100] S2. Preparation of the premix: Dissolve the monomer acrylamide (AM) and the crosslinking agent N,N'-methylenebisacrylamide (MBAM) in water, adding them at 10% and 1% of the water weight, respectively. Add the dispersant ammonium polyacrylate (PAA-NH4) at 0.3% of the weight of the silicon nitride ceramic green body; add the polymerization inhibitor catechol at 2% of the weight of the silicon nitride ceramic green body; add the pH agent ammonia water (NH3·H2O) at 0.2% of the weight of the silicon nitride ceramic green body.

[0101] S3. Preparation of slurry: Add the ingredients from S1 to the premixed liquid according to a solid content of 30% and ball mill to obtain a slurry. First, add the initiator ammonium persulfate (APS) and the catalyst N,N,N',N'-tetramethylethylenediamine (TEMED) to the slurries with solid contents of 30% and 45%, respectively, at amounts of 0.45% and 1.25% of the monomer weight. After gelation, green preforms with solid contents of 30% and 45% are obtained.

[0102] S4. Green Body Bonding: A gel thin layer with a solid content of 50% sintering aid is prepared, comprising 25g of magnesium oxide powder and 7g of water. The premix is ​​prepared in S2. Ammonium persulfate (APS) initiator and N,N,N',N'-tetramethylethylenediamine (TEMED) catalyst are added to the sintering aid slurry with a solid content of 50%, at amounts of 0.45% and 1.25% of the monomer weight, respectively. After thorough mixing, the mixture is uniformly coated onto the surface of silicon nitride ceramics with two different porosities, resulting in a gel thickness of 0.5mm. The silicon nitride ceramic green body is bonded by the gel of the sintering aid slurry. Its cross-section is shown below. Figure 2 As shown, a gel thin layer 2 formed by a sintering aid is connected between the first silicon nitride layer 1 and the second silicon nitride layer 3.

[0103] S5. Degumming and Sintering: First, place the green blank in an air environment and heat it to 600℃. Hold it for 3 hours to remove organic matter from the green blank. Then, place it in a sintering furnace with nitrogen as a protective gas. Sinter it under the conditions of 5MPa pressure and sintering temperature of 1600℃. Hold it for 2 hours to obtain silicon nitride ceramic components with variable porosity.

[0104] Example 2

[0105] The process is largely the same as in Example 1, except that in step S4, the gel thickness is changed to 1.0 mm.

[0106] Example 3

[0107] The process is largely the same as in Example 1, except that in step S4, the gel thickness is changed to 1.5 mm.

[0108] Example 4

[0109] The process is largely the same as in Example 1, except that the sintering temperature in step S5 is changed to 1650°C.

[0110] Example 5

[0111] Compared with Example 1, it is mostly the same, except that the sintering temperature in step S5 is changed to 1650℃ and the gel thickness in step S4 is changed to 1.0mm.

[0112] Example 6

[0113] Compared with Example 1, it is mostly the same, except that the sintering temperature in step S5 is changed to 1650℃ and the gel thickness in step S4 is changed to 1.5mm.

[0114] Example 7

[0115] It is largely the same as Example 1, except that the sintering temperature in step S5 is changed to 1700°C.

[0116] Example 8

[0117] Compared with Example 1, it is mostly the same, except that the sintering temperature in step S5 is changed to 1700℃ and the gel thickness in step S4 is changed to 1.0mm.

[0118] Example 9

[0119] Compared with Example 1, it is mostly the same, except that the sintering temperature in step S5 is changed to 1700℃ and the gel thickness in step S4 is changed to 1.5mm.

[0120] Example 10

[0121] Compared with Example 1, most of the contents are the same, except that in step S1, the weight of silicon nitride powder (97g), alumina powder (1g), and yttrium oxide (2g) is changed to 99g of silicon nitride powder, 0.5g of alumina powder, and 0.5g of yttrium oxide.

[0122] Example 11

[0123] Compared with Example 1, most of the contents are the same, except that in step S1, the weight of silicon nitride powder (97g), alumina powder (1g), and yttrium oxide (2g) is changed to 96g of silicon nitride powder, 2.5g of alumina powder, and 1.5g of yttrium oxide.

[0124] Comparative Example 1

[0125] Compared with Example 1, it is almost identical, except that the thickness of the gel layer is changed to 0 mm, that is, the two materials are directly connected.

[0126] Comparative Example 2

[0127] The results are largely the same as in Example 1, except that the thickness of the gel layer is changed to 0.2 mm.

[0128] Comparative Example 3

[0129] It is almost identical to Example 1, except that the thickness of the gel layer is changed to 2 mm.

[0130] Comparative Example 4

[0131] The process is largely the same as in Example 1, except that the sintering aid in the gel thin layer is replaced with pure α-Si3N4 powder.

[0132] Comparative Example 5

[0133] The process is largely the same as in Example 1, except that the sintering aid in the gel thin layer is replaced with pure β-Si3N4 powder.

[0134] Comparative Example 6

[0135] The majority of the steps are the same as in Example 1, except that step S4 is changed to:

[0136] S4. Green body bonding: Apply magnesium oxide powder evenly to the surface of silicon nitride ceramics with two different pore sizes, with a coating thickness of 0.5 mm.

[0137] Characterization example

[0138] 1) Mechanical properties and dielectric constant testing

[0139] The test methods and instruments used for mechanical property and dielectric constant testing are as follows: the complex dielectric constant was tested using an E8362B network analyzer (frequency 8.2–12.4 GHz), and the porosity of the material was measured using the Archimede method.

[0140]

[0141]

[0142] 2) SEM testing and macroscopic photographs

[0143] SEM images were scanned and viewed using a TESCAN VEGA3 SUB device.

[0144] See Figure 3 As shown, a bonding layer can be clearly seen between the two porous silicon nitride layers.

[0145] See Figure 4 As shown in the macroscopic photograph, the connection is good, with no defects, no internal stress, and no cracks.

[0146] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing a silicon nitride ceramic component with variable porosity, characterized in that, Includes the following steps: S1, Gel casting: Preparation of silicon nitride ceramic blanks with different porosities and corresponding sintering aid slurries; S2, Green body bonding: Connect silicon nitride ceramic green bodies with different porosities and apply a sintering aid slurry, initiator and catalyst mixed evenly at the joint to form a gel thin layer to obtain a silicon nitride ceramic component precursor with variable porosity. S3, Debonding and Sintering: After removing organic matter from the precursor of variable porosity silicon nitride ceramic components, sintering is performed to obtain variable porosity silicon nitride ceramic components. In step S1, the sintering aid slurry is prepared by mixing and ball milling sintering aid powder and polymer premixed liquid, wherein the sintering aid powder is one or more of yttrium oxide, magnesium oxide, aluminum oxide and magnesium silicon nitride. The polymer premix is ​​prepared by mixing monomers, crosslinking agents, polymerization inhibitors, pH agents, dispersants, and water. The thickness of the gel layer is 0.5-1.5 mm, and the solid content is 20%-60%.

2. The method for preparing a variable porosity silicon nitride ceramic component according to claim 1, characterized in that, In step S3, the organic matter in the precursor of the silicon nitride ceramic component with variable porosity is removed by heating the temperature to 400-800℃ in an air environment and holding it for 2-6 hours to eliminate the organic matter in the precursor of the silicon nitride ceramic component. The sintering is gas pressure sintering, with a sintering temperature of 1600-1700℃. The heating rate is 0.5-2℃ / min for 400-800℃, 3-6℃ / min for 800-1200℃, and 5-10℃ / min for 1200-1700℃. The holding time is 1-2 hours.

3. A silicon nitride ceramic component with variable porosity obtained by the preparation method of the silicon nitride ceramic component with variable porosity as described in claim 1 or 2.

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