A temperature-sensitive composite binder suitable for dry pressing of silicon nitride and its use

Abstract

This invention discloses a temperature-sensitive composite binder suitable for silicon nitride dry pressing and its application. It comprises the following components by weight: 5-15 parts linear poly(N-isopropylacrylamide), 2-8 parts polyvinyl butyral, 3-8 parts plasticizer, 0.5-2 parts dispersant, and 65-85 parts solvent. The linear poly(N-isopropylacrylamide) has a minimum critical dissolution temperature of 32-42 degrees Celsius and exhibits temperature-sensitive thickening properties; polyvinyl butyral acts as an auxiliary binder to enhance the green body strength; both are low-carbon molecular structures. This invention also discloses the application of this binder in silicon nitride dry pressing. Through steps such as mixing and granulation, temperature-controlled compaction, cooling and demolding, segmented adhesive removal, and sintering, the binder addition amount is only 1-3% of the silicon nitride powder mass. This invention solves the problem of mold sticking in traditional dry pressing molding with binders. The green body has a bending strength of 7-10 MPa, a residual carbon content of ≤0.1% after adhesive removal, and a ceramic density of ≥96% and a bending strength of ≥800 MPa after sintering, making it suitable for large-scale production.

Description

Technical Field

[0001] This invention belongs to the field of silicon nitride ceramic molding technology, and particularly relates to a temperature-sensitive composite binder suitable for silicon nitride dry pressing molding and its application. Background Technology

[0002] Silicon nitride ceramics possess excellent properties such as high strength, high hardness, high temperature resistance, corrosion resistance, and low coefficient of thermal expansion, making them widely used in high-end equipment fields such as machinery, electronics, aerospace, and chemicals. Dry pressing is a common process for preparing silicon nitride green bodies. The principle involves uniformly mixing silicon nitride powder with a binder, then loading the mixture into a mold and applying pressure to form a green body with a certain strength. This process offers advantages such as simple operation, high production efficiency, uniform green body density, and low cost, making it suitable for mass production of various silicon nitride structural parts. The binder is a key component in silicon nitride dry pressing; its performance directly determines the powder flowability, demolding effect during molding, green body strength, and subsequent sintering performance, and is a crucial factor affecting the product yield and performance stability of silicon nitride ceramics.

[0003] Currently, the commonly used binders in silicon nitride dry pressing are mainly traditional polymer binders such as polyvinyl alcohol, polyvinyl butyral, and polyethylene glycol. However, these binders have many intractable defects in actual industrial applications: First, the viscosity of the binder is not significantly affected by temperature, making it difficult to precisely control and prone to sticking to the mold. This leads to difficulties in demolding the green body, surface damage, and dimensional accuracy deviations, which not only significantly reduces the product qualification rate but also exacerbates mold wear, shortens mold life, and increases production costs. Second, the strength of green bodies prepared with traditional binders is generally low; the flexural strength of silicon nitride green bodies prepared by traditional processes is only 1-5 MPa. First, low-strength green bodies are prone to breakage and flaking during demolding, transfer, and debinding. They are also susceptible to internal stress cracks due to insufficient structural stability, failing to provide a high-quality green body foundation for subsequent sintering. Second, incomplete decomposition of polymer binders during debinding leads to the reaction of residual carbon with silicon nitride to generate impurities, reducing the density and thermal conductivity of the ceramic and failing to meet the performance requirements of high-end equipment for silicon nitride ceramics. Third, poor compatibility with silicon nitride powder and commonly used sintering aids easily leads to powder agglomeration, affecting the density uniformity of the formed green body, and consequently causing uneven ceramic performance and poor stability after sintering, limiting its application in high-end fields.

[0004] The aforementioned problems result in low yield and unstable performance of silicon nitride ceramics, hindering their application in high-end equipment. Current technologies primarily focus on binder improvements in tape casting, injection molding, or additive manufacturing, with limited research on binders specifically for silicon nitride dry pressing. Furthermore, existing improvement schemes struggle to simultaneously address the core technical challenges of mold sticking, low green strength, high residual carbon content, high binder dosage, and poor molding stability. Therefore, developing a dedicated binder suitable for silicon nitride dry pressing processes, with controllable viscosity temperature, low residual carbon content, high green strength, reduced binder dosage, and no need for large-scale modifications to existing equipment, has become a pressing technical challenge in the silicon nitride ceramic molding field, and is of great significance for promoting the industrial upgrading of silicon nitride ceramics. Summary of the Invention

[0005] In view of this, the present invention provides a temperature-sensitive composite adhesive suitable for silicon nitride dry pressing and its application. Compared with traditional adhesives such as polyvinyl alcohol and polyvinyl butyral, the temperature-sensitive composite adhesive provided by the present invention has significant advantages such as controllable viscosity temperature, high green strength, low residual carbon content after adhesive removal, low addition amount, and no need to modify existing equipment.

[0006] To achieve the objectives of the invention described above, the present invention provides the following technical solution:

[0007] This invention provides a temperature-sensitive composite adhesive suitable for silicon nitride dry pressing, which, by weight, consists of the following components: 5-15 parts of linear poly(N-isopropylacrylamide), 2-8 parts of polyvinyl butyral, 3-8 parts of plasticizer, 0.5-2 parts of dispersant, and 65-85 parts of solvent.

[0008] The linear poly(N-isopropylacrylamide) has a number-average molecular weight of 20,000-80,000, is a non-crosslinked structure, and has a minimum critical dissolution temperature of 32-42℃.

[0009] The polyvinyl butyral has a number-average molecular weight of 40,000-80,000 and an acetal degree of 75-85%.

[0010] The plasticizer is one or two of glycerol and diglycerol mixed in any proportion;

[0011] The dispersant is one of ammonium polyacrylate and polyethylene glycol octylphenyl ether;

[0012] The solvent is anhydrous ethanol or 95% ethanol.

[0013] In one embodiment, the product comprises, by weight, 8-12 parts of linear poly(N-isopropylacrylamide), 4-6 parts of polyvinyl butyral, 5-7 parts of plasticizer, 1-1.5 parts of dispersant, and 70-80 parts of solvent.

[0014] In one embodiment, the linear poly(N-isopropylacrylamide) has a number-average molecular weight of 40,000-60,000 and a minimum critical dissolution temperature of 32-42°C.

[0015] In one embodiment, the degree of acetalization of the polyvinyl butyral is 78-82%.

[0016] On the other hand, the present invention provides an application of the temperature-sensitive composite adhesive described in any of the above claims in silicon nitride dry pressing, comprising the following steps:

[0017] S1. Add linear poly(N-isopropylacrylamide), polyvinyl butyral, plasticizer, and dispersant to the solvent in the following proportions by weight. Stir for 30-60 minutes at <32℃ and a stirring rate of 200-300 r / min to form a uniform and transparent binder solution.

[0018] S2. The silicon nitride powder and the binder solution prepared in step S1 are mixed at a mass ratio of 100:5-15. The mixture is placed in a mixer, and an appropriate amount of ethanol solvent is added to adjust the solid content of the slurry to 40%-50% by mass. The mixture is stirred and mixed at <32℃ for 20-40 minutes, and then spray-dried to obtain granulated powder. The moisture content of the granulated powder is controlled at 0.5-1.5%.

[0019] S3. Load the granulated powder into the dry pressing mold, adjust the mold temperature to 32-45℃, keep it at that temperature for 5-10 minutes, apply a pressure of 50-150MPa, and hold the pressure for 30-60 seconds.

[0020] After S4 molding, the mold temperature is reduced to below 32℃, and the temperature is maintained for 5-10 minutes before demolding to obtain silicon nitride green blank.

[0021] S5 performs segmented debinding on the demolded silicon nitride green blank: under a nitrogen atmosphere, the temperature is raised to 200-300℃ at a rate of 5-10℃ / min and held for 1-2 hours; then the temperature is raised to 400-500℃ at a rate of 3-5℃ / min and held for 2-3 hours; finally, the temperature is raised to 600-700℃ and held for 1 hour.

[0022] S6 places the debinding silicon nitride green blank in a sintering furnace. Under a nitrogen atmosphere, the furnace pressure is controlled at 1-4 MPa. The temperature is increased to 1600-1800℃ at a rate of 10-15℃ / min, held for 2-4 hours, and then sintered.

[0023] In one embodiment, in step S2, the silicon nitride powder has a particle size of 0.5-5 μm and a purity of ≥99%.

[0024] In one embodiment, in step S2, sintering aids Y2O3 and Al2O3 are also added, wherein the amount of sintering aids added is 3-8% of the mass of silicon nitride powder, and the mass ratio of Y2O3 to Al2O3 is 2:1-3:1.

[0025] In one embodiment, in step S3, the pressure is adjusted according to the thickness of the billet: for thin billets with a thickness ≤ 10 mm, the pressure is controlled at 50-80 MPa; for medium-thick billets with a thickness of 10-20 mm, the pressure is controlled at 80-120 MPa; for thick billets or irregularly shaped billets with a thickness > 20 mm, the pressure is controlled at 120-150 MPa.

[0026] In one embodiment, the silicon nitride green blank obtained by demolding in step S4 has a bending strength of 7-10 MPa.

[0027] In one embodiment, after the segmented debinding process in step S5, the residual carbon content after debinding is ≤0.1%.

[0028] Compared with the prior art, the present invention has at least the following beneficial effects:

[0029] 1. Controllable viscosity temperature of the binder solves the problem of sticking to the mold, balancing molding stability and environmental friendliness, and significantly improving production efficiency: This invention uses linear PNIPAm as the temperature-sensitive core component, combined with PVB to construct a composite binder system. Utilizing the LCST characteristic of PNIPAm, precise temperature control of viscosity is achieved, perfectly adapting to the entire dry pressing process: In the mixing stage (<32℃, below LCST), PNIPAm is fully dissolved, the binder viscosity is low, and the system uses ethanol as a solvent, exhibiting excellent compatibility with silicon nitride powder and sintering aids. There is no layering or precipitation during mixing, allowing for uniform coating of the powder and improving the flowability of the granulated powder (angle of repose ≤32°), facilitating mold loading and ensuring uniform green body density; In the compaction stage (32-45℃, LCST range), PNIPAm undergoes a temperature-sensitive phase transition, moderately increasing viscosity, enhancing the bonding force between powder particles, promoting compaction and density of the green body, and avoiding local porosity and performance inhomogeneity caused by powder agglomeration; In the demolding stage (<32℃, below LCST), PNIPAm… The viscosity is significantly reduced, weakening the interaction between the adhesive and the mold wall, enabling rapid and non-destructive demolding. This completely solves the problems of blank damage and dimensional deviations caused by traditional adhesive sticking to the mold, increasing the demolding qualification rate to over 98%. It also reduces mold wear and lowers production costs. Furthermore, the ethanol has moderate volatility, preventing rapid solvent escape during molding and thus eliminating blank defects. VOC emissions are low, meeting environmental protection requirements and making it suitable for industrial green production.

[0030] 2. Significantly improved green body strength and excellent structural stability, providing a high-quality green body foundation for sintering: Linear PNIPAm and PVB synergistically construct a high-strength bonding skeleton. Compared with green bodies prepared by traditional processes (flexural strength 1-5MPa), the flexural strength of the prepared silicon nitride green bodies can reach 7-10MPa. The high green body strength fundamentally solves the problems of cracking and slag shedding during demolding, transportation and debinding. Moreover, the green body has excellent structural stability, with no internal stress cracks. The integrity of the green body can be guaranteed without additional complicated treatment, greatly reducing production losses. It provides a homogeneous and defect-free high-quality green body foundation for subsequent sintering processes, further ensuring the performance stability of the sintered ceramic products.

[0031] 3. Low-carbon structure, achieving low residual carbon, suitable for dry pressing process and significantly improving the overall performance of ceramics: The linear PNIPAm and PVB selected in this invention have low-carbon molecular structures, with carbon content far lower than traditional PVA, CMC and other binders. Due to the significantly improved bonding efficiency, the amount of binder added is only 1-3% of the mass of silicon nitride powder, which is much less than the traditional 3-5% addition amount. Combined with the segmented debinding process, the binder can be completely decomposed and removed. The residual carbon content after debinding is ≤0.1%, avoiding the reaction of residual carbon with silicon nitride to form impurity phases, ensuring the excellent performance of silicon nitride ceramics. After sintering, the ceramic density is ≥96%, the flexural strength is ≥800MPa, and the mechanical and thermal properties are significantly improved compared with traditional products, which can meet the needs of high-end equipment. Meanwhile, this adhesive and its application method are specifically designed for silicon nitride dry pressing molding process. The process adjustment is simple and the operation is convenient. It eliminates the cumbersome drying steps, shortens the production cycle, and does not require large-scale modification of existing dry pressing equipment. Efficient molding can be achieved simply by precise control of temperature and pressure. The cost is controllable and it is suitable for the large-scale production of various silicon nitride structural parts (block, sheet, irregular shape). It has extremely high practical value and industrial promotion prospects. Attached Figure Description

[0032] Figure 1 This is a comparison diagram of silicon nitride green embryos of the embodiments and comparative examples of this application;

[0033] Figure 2 This is a comparison image of the preform before and after glue removal in an embodiment of this application;

[0034] Figure 3 This is a photograph of the sintered ceramic product according to an embodiment of this application. Detailed Implementation

[0035] This invention provides a temperature-sensitive composite adhesive suitable for silicon nitride dry pressing, prepared from the following components in parts by weight: 5-15 parts linear N-isopropylacrylamide, 2-8 parts polyvinyl butyral, 3-8 parts plasticizer, 0.5-2 parts dispersant, and 65-85 parts solvent. The linear N-isopropylacrylamide has a number-average molecular weight of 20,000-80,000, is non-crosslinked, and has a minimum critical dissolution temperature of 32-42 degrees Celsius; the polyvinyl butyral has a number-average molecular weight of 40,000-80,000 and an acetal degree of 75-85%; the plasticizer is one or a mixture of two of glycerol and diglycerol in any proportion; the dispersant is one of ammonium polyacrylate and polyethylene glycol octylphenyl ether; and the solvent is anhydrous ethanol or 95% ethanol by volume.

[0036] Unless otherwise specified, all raw materials used in this invention are commercially available.

[0037] The linear poly(N-isopropylacrylamide) in the temperature-sensitive composite adhesive provided by this invention comprises 5-15 parts by weight, preferably 8-12 parts. Linear poly(N-isopropylacrylamide) is a temperature-sensitive polymer containing hydrophilic amide groups and hydrophobic isopropyl groups on its molecular chain. Below the minimum critical dissolution temperature, the polymer chain exhibits an extended conformation, forming hydrogen bonds with water molecules, resulting in low adhesive viscosity. When the temperature rises above the minimum critical dissolution temperature, the hydrogen bonds are broken, the polymer chain collapses, and the viscosity moderately increases. By selecting uncrosslinked linear poly(N-isopropylacrylamide), this invention achieves precise temperature control of the adhesive viscosity: low viscosity during the mixing stage promotes uniform powder mixing; moderately increased viscosity during the compaction stage promotes densification of the preform; and significantly reduced viscosity during the demolding stage enables rapid and non-destructive demolding, completely solving the technical problem of traditional adhesives sticking to the mold. The molecular weight is controlled within the range of 20,000-80,000, preferably 40,000-60,000, which ensures good temperature-sensitive response speed and avoids insufficient bonding strength due to too low a molecular weight or difficulty in dissolution and dispersion due to too high a molecular weight.

[0038] By weight, the temperature-sensitive composite adhesive provided by this invention contains 2-8 parts of polyvinyl butyral, preferably 4-6 parts. The number-average molecular weight of polyvinyl butyral is 40,000-80,000, and the degree of acetalization is 75-85%, preferably 78-82%. As an auxiliary binder, polyvinyl butyral has a much lower carbon content in its molecular structure than traditional binders such as polyvinyl alcohol and sodium carboxymethyl cellulose. It can synergistically construct a high-strength adhesive skeleton with linear poly(N-isopropylacrylamide), significantly improving the green body strength while reducing residual carbon content. By selecting polyvinyl butyral with a degree of acetalization of 78-82%, this invention achieves optimal solubility in ethanol solvent and exhibits the strongest synergistic effect with linear poly(N-isopropylacrylamide) and plasticizers, thereby achieving a green body flexural strength of 7-10 MPa, an improvement of more than 40% compared to traditional processes.

[0039] The plasticizer in the temperature-sensitive composite adhesive provided by this invention comprises 3-8 parts by weight, preferably 5-7 parts. The plasticizer is one or a mixture of two of glycerol and diglycerol in any proportion. By adding the plasticizer, this invention can lower the glass transition temperature of the adhesive, increase the flexibility of the adhesive system, prevent cracks in the green body due to excessive brittleness during demolding and transfer, and help ensure the structural stability of the green body.

[0040] The dispersant in the temperature-sensitive composite binder provided by this invention comprises 0.5-2 parts by weight, preferably 1-1.5 parts. The dispersant is one of ammonium polyacrylate and polyethylene glycol octylphenyl ether. By adding a dispersant, this invention can improve the compatibility between the binder and silicon nitride powder and sintering aids, prevent powder agglomeration, ensure the uniformity and flowability of the granulated powder, and thus ensure the density uniformity of the molded green body.

[0041] The solvent in the temperature-sensitive composite adhesive provided by this invention comprises 65-85 parts by weight, preferably 70-80 parts. The solvent is anhydrous ethanol or ethanol with a volume fraction of 95%. The use of ethanol as a solvent in this invention demonstrates good compatibility with all components and moderate volatility, which facilitates compaction and demolding of the blank during dry pressing, while simultaneously reducing emissions of volatile organic compounds, thus meeting environmental protection production requirements.

[0042] This invention also provides the application of the above-mentioned temperature-sensitive composite adhesive in silicon nitride dry pressing, including the following steps:

[0043] S1. According to the weight parts, add linear poly(N-isopropylacrylamide), polyvinyl butyral, plasticizer and dispersant to the solvent in sequence, and stir for 30-60 minutes at a temperature below 32 degrees Celsius and a stirring speed of 200-300 rpm to form a uniform and transparent binder solution.

[0044] This invention employs low-temperature preparation of the binder solution, controlling the temperature below the minimum critical dissolution temperature of linear poly(N-isopropylacrylamide). This ensures the complete dissolution of the linear poly(N-isopropylacrylamide) and prevents premature phase transition and precipitation of the polymer due to excessively high temperatures. Stirring at a rate of 200-300 rpm for 30-60 minutes ensures that all components are fully dissolved and uniformly dispersed, forming a stable and transparent binder solution, laying the foundation for subsequent uniform mixing with the powder.

[0045] S2. Mix silicon nitride powder with the binder solution prepared in step S1 at a mass ratio of 100:5-15, place in a mixer, add an appropriate amount of ethanol solvent, adjust the solid content of the slurry to 40% to 50% by mass, stir and mix for 20-40 minutes at a temperature below 32 degrees Celsius, and spray dry to obtain granulated powder. The moisture content of the granulated powder is controlled at 0.5% to 1.5%.

[0046] This invention mixes silicon nitride powder and a binder solution at a low temperature below 32 degrees Celsius, avoiding agglomeration caused by increased binder viscosity during mixing and ensuring uniform coating of the powder surface. Spray drying is used to prepare granulated powder, resulting in well-flowing spherical particles that facilitate subsequent molding and compaction. Controlling the moisture content of the granulated powder to 0.5% to 1.5% avoids both delamination and cracking defects caused by excessive moisture content and decreased molding performance caused by excessively low moisture content. Preferably, the silicon nitride powder has a particle size of 0.5-5 micrometers and a purity greater than or equal to 99%, with the addition of sintering aids yttrium oxide and alumina. The amount of sintering aids added is 3-8% of the silicon nitride powder mass, and the mass ratio of yttrium oxide to alumina is 2:1 to 3:1, which effectively promotes the densification of silicon nitride during sintering, further improving the mechanical and thermal properties of the ceramic product.

[0047] S3. Load the granulated powder into the dry pressing mold, adjust the mold temperature to 32-45 degrees Celsius, keep it warm for 5-10 minutes, then apply a pressure of 50-150 MPa and hold the pressure for 30-60 seconds.

[0048] This invention adjusts the mold temperature to the lowest critical melting temperature range of linear poly(N-isopropylacrylamide), i.e., 32-45 degrees Celsius, and holds it at this temperature for 5-10 minutes to ensure that the mold and granulation powder reach this temperature uniformly. At this temperature, linear poly(N-isopropylacrylamide) undergoes a thermosensitive phase transition, resulting in a moderate increase in viscosity, which enhances the bonding force between powder particles and promotes compaction and densification of the green body. Applying a pressure of 50-150 MPa and holding it for 30-60 seconds ensures uniform compaction of the green body, free from internal porosity and delamination defects. Preferably, the pressure parameters are adjusted according to the green body thickness: for thin green bodies with a thickness of 10 mm or less, the pressure is controlled at 50-80 MPa; for medium-thick green bodies with a thickness of 10-20 mm, the pressure is controlled at 80-120 MPa; and for thick green bodies or irregularly shaped green bodies with a thickness greater than 20 mm, the pressure is controlled at 120-150 MPa, which can adapt to the molding requirements of green bodies of different specifications.

[0049] S4. After molding, lower the mold temperature to below 32 degrees Celsius, keep it warm for 5-10 minutes, and demold to obtain silicon nitride green blank.

[0050] This invention involves lowering the mold temperature to below 32 degrees Celsius after molding, which is below the minimum critical melting temperature of linear poly(N-isopropylacrylamide), and holding the temperature for 5-10 minutes before demolding. At this temperature, the viscosity of linear poly(N-isopropylacrylamide) is significantly reduced, and the force between the adhesive and the inner wall of the mold is weakened, enabling rapid and non-destructive demolding. This completely solves the problems of green body breakage and dimensional deviation caused by traditional adhesive sticking to the mold, and the demolding qualification rate can be increased to over 98%. The resulting silicon nitride green body has a bending strength of 7-10 MPa, which is more than 40% higher than the 1-5 MPa green body prepared by traditional processes, fundamentally solving the problems of cracking and flaking during demolding, transportation, and adhesive removal.

[0051] S5. Perform segmented debinding treatment on the demolded silicon nitride green blank: Under a nitrogen atmosphere, heat to 200-300 degrees Celsius at a rate of 5-10 degrees Celsius per minute and hold for 1-2 hours; then heat to 400-500 degrees Celsius at a rate of 3-5 degrees Celsius per minute and hold for 2-3 hours; finally heat to 600-700 degrees Celsius and hold for 1 hour.

[0052] This invention employs a segmented heating and adhesive removal process, enabling the complete decomposition and removal of the adhesive. The first stage involves heating to 200-300 degrees Celsius at a rate of 5-10 degrees Celsius per minute and holding for 1-2 hours to initially decompose organic components and remove most volatile products. The second stage involves heating to 400-500 degrees Celsius at a lower rate of 3-5 degrees Celsius per minute and holding for 2-3 hours to thoroughly remove residual organic matter. The third stage involves heating to 600-700 degrees Celsius and holding for 1 hour to ensure the complete decomposition of all carbides. Since the linear poly(N-isopropylacrylamide) and polyvinyl butyral selected in this invention are both low-carbon molecular structures, their carbon content is much lower than that of traditional binders such as polyvinyl alcohol and sodium carboxymethyl cellulose. Moreover, the amount of binder added is only 1-3% of the mass of silicon nitride powder, which is significantly reduced compared to the traditional 3-5% addition amount. Combined with the segmented debinding process, the residual carbon content after debinding can be less than or equal to 0.1%, avoiding the reaction of residual carbon with silicon nitride to generate impurity phases and ensuring the excellent performance of silicon nitride ceramics.

[0053] S6. Place the debinding silicon nitride green blank in a sintering furnace. Under a nitrogen atmosphere, control the furnace pressure at 1-4 MPa, raise the temperature to 1600-1800 degrees Celsius at a rate of 10-15 degrees Celsius per minute, hold for 2-4 hours, and sinter to form the desired shape.

[0054] This invention utilizes gas pressure sintering under a nitrogen atmosphere, with the furnace pressure controlled at 1-4 MPa, effectively suppressing the decomposition of silicon nitride at high temperatures. Heating to 1600-1800 degrees Celsius at a rate of 10-15 degrees Celsius per minute and holding for 2-4 hours achieves thorough densification of the silicon nitride ceramic. After sintering, the ceramic has a density greater than or equal to 96%, a flexural strength greater than or equal to 800 MPa, and significantly improved mechanical and thermal properties compared to traditional products, meeting the requirements of high-end equipment.

[0055] The present invention will be further described in detail below with reference to specific embodiments, but the embodiments listed herein do not limit the scope of protection of the present invention.

[0056] Example 1

[0057] A temperature-sensitive composite adhesive suitable for silicon nitride dry pressing, comprising the following components by weight:

[0058] 8 parts of linear PNIPAm (number average molecular weight 40,000, LCST 34℃), 5 parts of PVB (number average molecular weight 60,000, acetal degree 78%), 6 parts of glycerol, 1.5 parts of ammonium polyacrylate, and 79.5 parts of anhydrous ethanol.

[0059] The application method of the above-mentioned composite binder in silicon nitride dry pressing includes the following steps:

[0060] S1. According to the above weight proportions, add linear PNIPAm, PVB, glycerin and ammonium polyacrylate to anhydrous ethanol in sequence, and stir for 60 minutes at 25°C and stirring speed of 200 r / min to form a uniform and transparent binder solution.

[0061] S2. Take 100g of silicon nitride powder with a particle size of 0.5-2μm and a purity of 99.5%, add 4g of Y2O3 and 2g of Al2O3 (the total mass of sintering aid is 6% of the mass of silicon nitride powder, and the mass ratio of Y2O3 to Al2O3 is 2:1), mix with 8g of the binder solution prepared in step S1, place in a mixer, add an appropriate amount of ethanol solvent, adjust the slurry solid content to 45% (mass fraction), stir at 25℃ for 30min, and spray dry to obtain granulated powder with a moisture content of 0.8%;

[0062] S3. Load the granulated powder into a dry pressing mold (to prepare a sheet-like preform with a thickness of 8 mm), adjust the mold temperature to 35°C, hold for 8 min, then apply a pressure of 60 MPa and hold for 40 s;

[0063] S4. After molding, reduce the mold temperature to 23°C, hold for 8 minutes, and demold to obtain silicon nitride green blank;

[0064] S5. Segmented glue removal: Under nitrogen atmosphere, heat to 250℃ at a rate of 5℃ / min and hold for 1.5h; heat to 450℃ at a rate of 3℃ / min and hold for 2.5h; heat to 650℃ and hold for 1h.

[0065] S6. Under a nitrogen atmosphere, with a furnace pressure of 3.2 MPa, the temperature is increased to 1700℃ at a rate of 10℃ / min, held for 3 hours, and then sintered.

[0066] Testing showed that the silicon nitride green prepared in this embodiment was undamaged and crack-free, with a bending strength of 7.7 MPa; the residual carbon content after debinding was 0.06%; the ceramic density after sintering was 97.5%, the bending strength was 830 MPa, and the demolding qualification rate was 99%.

[0067] Example 2

[0068] A temperature-sensitive composite adhesive suitable for silicon nitride dry pressing, comprising the following components by weight:

[0069] 10 parts of linear PNIPAm (number average molecular weight 60,000, LCST 36℃), 5 parts of PVB (number average molecular weight 80,000, acetal degree 82%), 6 parts of diglycerides, 1.5 parts of OP-10, and 77.5 parts of 95% ethanol.

[0070] The application method of the above-mentioned composite binder in silicon nitride dry pressing includes the following steps:

[0071] S1. According to the above weight proportions, add linear PNIPAm, PVB, diglyceride and OP-10 to 95% ethanol in sequence, and stir for 30 min at 30℃ and 300 r / min to form a uniform and transparent binder solution.

[0072] S2. Take 100g of silicon nitride powder with a particle size of 2-5μm and a purity of 99%, add 5g of Y2O3 and 2g of Al2O3 (the total mass of sintering aid is 7% of the silicon nitride powder, and the mass ratio of Y2O3 to Al2O3 is 5:2), mix with 10g of the binder solution prepared in step S1, place in a mixer, add an appropriate amount of ethanol solvent, adjust the slurry solid content to 42% (mass fraction), stir at 22℃ for 25min, and spray dry to obtain granulated powder with a moisture content of 1.2%;

[0073] S3. Load the granulated powder into a dry pressing mold (to prepare a block blank with a thickness of 15 mm), adjust the mold temperature to 36℃, hold for 6 min, then apply a pressure of 100 MPa and hold for 50 s;

[0074] S4. After molding, reduce the mold temperature to 22℃, hold for 6 minutes, and demold to obtain silicon nitride green blank;

[0075] S5. Segmented glue removal: Under nitrogen atmosphere, heat to 300℃ at a rate of 10℃ / min and hold for 1 hour; heat to 500℃ at a rate of 5℃ / min and hold for 2 hours; heat to 700℃ and hold for 1 hour.

[0076] S6. Under a nitrogen atmosphere, with a furnace pressure of 3.6 MPa, the temperature is increased to 1800℃ at a rate of 15℃ / min, held for 2 hours, and then sintered.

[0077] Testing showed that the silicon nitride green prepared in this embodiment was undamaged and crack-free, with a bending strength of 9.1 MPa; the residual carbon content after debinding was 0.07%; the ceramic density after sintering was 97%, the bending strength was 870 MPa, and the demolding qualification rate was 98.5%.

[0078] Example 3

[0079] A temperature-sensitive composite adhesive suitable for silicon nitride dry pressing, comprising the following components by weight:

[0080] 11 parts of linear PNIPAm (number average molecular weight 50,000, LCST 35℃), 6.5 parts of PVB (number average molecular weight 50,000, acetal degree 80%), 7.3 parts of glycerol, 1.2 parts of ammonium polyacrylate, and 75 parts of anhydrous ethanol.

[0081] The application method of the above-mentioned composite binder in silicon nitride dry pressing includes the following steps:

[0082] S1. According to the above weight proportions, add linear PNIPAm, PVB, glycerin and ammonium polyacrylate to anhydrous ethanol in sequence, and stir for 45 minutes at 28°C and stirring speed of 250 r / min to form a uniform and transparent binder solution.

[0083] S2. Take 100g of silicon nitride powder with a particle size of 1-3μm and a purity of 99.2%, add 3g of Y2O3 and 1.5g of Al2O3 (the total mass of sintering aid is 4.5% of the silicon nitride powder, and the mass ratio of Y2O3 to Al2O3 is 2:1), mix with 13g of the binder solution prepared in step S1, place in a mixer, add an appropriate amount of ethanol solvent, adjust the slurry solid content to 47% (mass fraction), stir at 26℃ for 35min, and spray dry to obtain granulated powder with a moisture content of 1.0%;

[0084] S3. Load the granulated powder into a dry pressing mold (to prepare a thick block blank with a thickness of 25 mm), adjust the mold temperature to 35℃, keep it at the temperature for 10 min, first pre-press at low pressure of 25 MPa for 10 s, then increase to 140 MPa and keep for 60 s;

[0085] S4. After molding, reduce the mold temperature to 26°C, hold for 10 minutes, and demold to obtain silicon nitride green blank;

[0086] S5. Segmented glue removal: Under nitrogen atmosphere, heat to 280℃ at a rate of 8℃ / min and hold for 1.2h; heat to 480℃ at a rate of 4℃ / min and hold for 2.2h; heat to 680℃ and hold for 1h.

[0087] S6. Under a nitrogen atmosphere, with a furnace pressure of 3.4 MPa, the temperature is increased to 1750℃ at a rate of 12℃ / min, held for 3.5 hours, and then sintered.

[0088] Testing showed that the silicon nitride green prepared in this embodiment was free of damage and cracks, with a bending strength of 8.3 MPa; the residual carbon content after debinding was 0.09%; the ceramic density after sintering was 96.3%, the bending strength was 820 MPa, and the demolding qualification rate was 99%.

[0089] Comparative Example 1

[0090] A traditional single PVB adhesive was used, with the following components by weight: 12 parts PVB, 6 parts glycerin, 1.2 parts ammonium polyacrylate, and 80.8 parts anhydrous ethanol. The amount of adhesive added was 4.1% of the mass of silicon nitride powder. The remaining application methods were the same as in Example 3.

[0091] Testing revealed that the prepared silicon nitride green body exhibited significant sticking to the mold, with a surface breakage rate of 15% after demolding. The green body's bending strength was 4.2 MPa, and a small number of microcracks were present. The residual carbon content after debinding was 0.5%. After sintering, the ceramic density was 92%, the bending strength was 600 MPa, and the demolding qualification rate was only 82%.

[0092] Comparative Example 2

[0093] Without adding PNIPAm, only the PVB + glycerin system was used, with the following weight parts: 15 parts PVB, 6 parts glycerin, 1.2 parts ammonium polyacrylate, and 77.8 parts anhydrous ethanol. The amount of binder added was 5% of the mass of silicon nitride powder, and the rest of the application method was the same as in Example 3.

[0094] Testing revealed that the prepared silicon nitride green body exhibited severe sticking to the mold, making demolding difficult, with a green body breakage rate of 20%, a green body bending strength of 4.7 MPa, and a high crack incidence rate; the residual carbon content after debinding was 0.43%; the ceramic density after sintering was 93%, the bending strength was 620 MPa, and the demolding qualification rate was 85%.

[0095] Table 1 Performance Test Comparison Table .

[0096] A comparison of the performance data of Examples 1-3 with Comparative Examples 1-2 shows that the temperature-sensitive composite binder provided in this application achieves higher green body flexural strength with a lower binder addition amount, increasing the green body strength by more than 40%. Due to the use of a low-carbon molecular structure composed of linear poly(N-isopropylacrylamide) and polyvinyl butyral, combined with a segmented binder removal process, the residual carbon content after binder removal is significantly reduced, far lower than that of the comparative examples. The low residual carbon content effectively prevents the reaction between carbon and silicon nitride during high-temperature sintering to form impurity phases, resulting in significantly better ceramic density and flexural strength after sintering compared to the comparative examples. Furthermore, thanks to the controllable viscosity temperature of the temperature-sensitive binder, the demolding success rate is greatly improved, completely solving the problem of mold sticking in traditional binder dry pressing molding.

[0097] The above results show that this application achieves comprehensive technical effects by using the synergistic compounding of linear poly(N-isopropylacrylamide) and polyvinyl butyral to control the viscosity temperature through thermosensitive properties. This results in reduced binder addition, increased green strength, reduced residual carbon content, and improved demolding pass rate, significantly improving the performance of silicon nitride ceramic products. It has obvious technical advantages and industrial application value.

[0098] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A temperature-sensitive composite adhesive suitable for silicon nitride dry pressing, characterized in that, By weight, it consists of the following components: 5-15 parts linear poly(N-isopropylacrylamide), 2-8 parts polyvinyl butyral, 3-8 parts plasticizer, 0.5-2 parts dispersant, and 65-85 parts solvent. The linear poly(N-isopropylacrylamide) has a number-average molecular weight of 20,000-80,000, is a non-crosslinked structure, and has a minimum critical dissolution temperature of 32-42℃. The polyvinyl butyral has a number-average molecular weight of 40,000-80,000 and an acetal degree of 75-85%. The plasticizer is one or two of glycerol and diglycerol mixed in any proportion; The dispersant is one of ammonium polyacrylate and polyethylene glycol octylphenyl ether; The solvent is anhydrous ethanol or 95% ethanol.

2. The temperature-sensitive composite adhesive suitable for silicon nitride dry pressing according to claim 1, characterized in that, By weight, it consists of the following components: 8-12 parts linear poly(N-isopropylacrylamide), 4-6 parts polyvinyl butyral, 5-7 parts plasticizer, 1-1.5 parts dispersant, and 70-80 parts solvent.

3. The temperature-sensitive composite adhesive suitable for silicon nitride dry pressing according to claim 1, characterized in that, The linear poly(N-isopropylacrylamide) has a number-average molecular weight of 40,000-60,000 and a minimum critical dissolution temperature of 32-42°C.

4. The temperature-sensitive composite adhesive suitable for silicon nitride dry pressing according to claim 1, characterized in that, The degree of acetalization of the polyvinyl butyral is 78-82%.

5. The application of the temperature-sensitive composite adhesive according to any one of claims 1-4 in silicon nitride dry pressing, characterized in that, Includes the following steps: S1. Add linear poly(N-isopropylacrylamide), polyvinyl butyral, plasticizer, and dispersant to the solvent in the following proportions by weight. Stir for 30-60 minutes at <32℃ and a stirring rate of 200-300 r / min to form a uniform and transparent binder solution. S2. The silicon nitride powder and the binder solution prepared in step S1 are mixed at a mass ratio of 100:5-15. The mixture is placed in a mixer, and an appropriate amount of ethanol solvent is added to adjust the solid content of the slurry to 40%-50% by mass. The mixture is stirred and mixed at <32℃ for 20-40 minutes, and then spray-dried to obtain granulated powder. The moisture content of the granulated powder is controlled at 0.5-1.5%. S3. Load the granulated powder into the dry pressing mold, adjust the mold temperature to 32-45℃, keep it at that temperature for 5-10 minutes, apply a pressure of 50-150MPa, and hold the pressure for 30-60 seconds. After S4 molding, the mold temperature is reduced to below 32℃, and the temperature is maintained for 5-10 minutes before demolding to obtain silicon nitride green blank. S5 performs segmented debinding on the demolded silicon nitride green blank: under a nitrogen atmosphere, the temperature is raised to 200-300℃ at a rate of 5-10℃ / min and held for 1-2 hours; then the temperature is raised to 400-500℃ at a rate of 3-5℃ / min and held for 2-3 hours; finally, the temperature is raised to 600-700℃ and held for 1 hour. S6 places the debinding silicon nitride green blank in a sintering furnace. Under a nitrogen atmosphere, the furnace pressure is controlled at 1-4 MPa. The temperature is increased to 1600-1800℃ at a rate of 10-15℃ / min, held for 2-4 hours, and then sintered.

6. The application according to claim 5, characterized in that, In step S2, the silicon nitride powder has a particle size of 0.5-5 μm and a purity of ≥99%.

7. The application according to claim 5, characterized in that, In step S2, sintering aids Y2O3 and Al2O3 are also added. The amount of sintering aids added is 3-8% of the mass of silicon nitride powder, wherein the mass ratio of Y2O3 to Al2O3 is 2:1-3:

1.

8. The application according to claim 5, characterized in that, In step S3, the pressure is adjusted according to the thickness of the billet: for thin billets with a thickness ≤ 10 mm, the pressure is controlled at 50-80 MPa; for medium-thick billets with a thickness of 10-20 mm, the pressure is controlled at 80-120 MPa; for thick billets or irregularly shaped billets with a thickness > 20 mm, the pressure is controlled at 120-150 MPa.

9. The application according to claim 5, characterized in that, The silicon nitride green blank obtained by demolding in step S4 has a bending strength of 7-10 MPa.

10. The application according to claim 5, characterized in that, After the segmented glue removal process in step S5, the residual carbon content after glue removal is ≤0.1%.

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