Photocurable silicon nitride ceramic slurry, silicon nitride ceramic and method for manufacturing the same

Abstract

This invention belongs to the field of ceramic materials technology, specifically relating to photocurable silicon nitride ceramic slurry, silicon nitride ceramics, and their preparation methods. The invention involves sequentially subjecting silicon nitride powder to surface oxidation modification and sintering aid coating modification to obtain a silicon nitride composite powder with a silicon nitride core, a silicon dioxide intermediate layer, and a sintering aid outer shell. This composite powder is then uniformly mixed with a photocurable resin, a photoinitiator, and a dispersant to prepare a photocurable silicon nitride ceramic slurry. The slurry is then photocured and 3D printed to obtain a silicon nitride ceramic green body, which is subsequently subjected to thermal debinding and pressure sintering to obtain the photocurable silicon nitride ceramic. This invention, through two-step chemical modification, simultaneously optimizes the optical properties and sintering activity of silicon nitride powder, thereby improving the light transmittance of the silicon nitride ceramic slurry and enhancing the mechanical properties of the silicon nitride ceramic.

Description

Technical Field

[0001] This invention belongs to the field of ceramic materials technology, specifically relating to photocurable silicon nitride ceramic slurry, silicon nitride ceramics and their preparation methods. Background Technology

[0002] Silicon nitride (Si3N4) ceramics have broad application prospects in aerospace, military, electronics and biomedical fields due to their high strength, high hardness, good wear resistance, thermal shock resistance and dielectric properties.

[0003] Photopolymer 3D printing technologies (such as DLP and SLA) can efficiently fabricate ceramic parts with complex structures, but their application is severely limited by the optical properties of the ceramic materials themselves. Silicon nitride powder has an extremely high refractive index (n≈2.1), which differs significantly from commonly used photopolymer resins (n≈1.5). This results in severe light scattering in the slurry, significantly reducing the depth of light transmission (Cd), thus limiting the printed layer thickness, decreasing accuracy, and even preventing the formation of the final product. The traditional solution is to add a high-refractive-index reactive diluent to the slurry, but this has limited effectiveness and may sacrifice the slurry's rheological properties.

[0004] Existing technologies also include studies on reducing the refractive index difference between silicon nitride powder and photocurable resin by modifying the powder or optimizing the resin. For example, Chinese invention patent application CN114524676A utilizes a high-refractive-index resin to reduce the difference in optical refractive index between silicon nitride powder and resin, thereby increasing the curing depth of silicon nitride ceramic slurry. However, this method places high demands on the photocurable resin and has a limited range for reducing the difference in optical refractive index, thus limiting the improvement in light transmission depth. The Chinese invention patent application with publication number CN118479887A modifies the surface of silicon nitride powder by high-temperature oxidation and hydrogen peroxide modification, thereby partially or completely coating the surface of silicon nitride powder with silicon dioxide to reduce the surface refractive index of silicon nitride powder. However, the silicon dioxide film formed by high-temperature oxidation has defects such as unevenness and non-density, so the effect of reducing the surface refractive index of silicon nitride powder is not optimal. Moreover, its secondary modification method is too cumbersome and generates a large amount of skeleton waste during the modification process, which is not suitable for industrial production.

[0005] On the other hand, silicon nitride is a strong covalent compound, which is difficult to sinter into a dense state. Sintering aids (such as Y2O3, Al2O3, etc.) must be added. Traditional mechanical mixing methods are difficult to achieve nanoscale uniform distribution of aids among Si3N4 powders, which can easily lead to uneven microstructure after sintering and performance defects.

[0006] Therefore, developing a technology that can simultaneously solve the problems of optical properties of slurry and uniform distribution of sintering aids is the key to achieving high-quality photopolymerization molding of silicon nitride ceramics. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the present invention aims to provide a photocurable silicon nitride ceramic slurry, silicon nitride ceramics, and a method for preparing the same. Through two-step chemical modification, the optical properties and sintering activity of the powder are optimized simultaneously, thereby improving the light transmittance of the silicon nitride ceramic slurry and enhancing the mechanical properties of the silicon nitride ceramics.

[0008] One objective of this invention is to provide a method for preparing photocurable silicon nitride ceramic slurry, comprising the following steps: (1) Surface oxidation modification: The silicon nitride powder is first heated to 950-1050℃ in an oxygen atmosphere and kept at that temperature for 1-2 hours. Then it is heated to 1200-1300℃ in an air or oxygen atmosphere and kept at that temperature for 0.5-1 hours. Then it is cooled to obtain silicon nitride powder with a silicon dioxide film coating on the surface. (2) Sintering aid coating: After dispersing silicon nitride powder coated with a silicon dioxide film on the surface in water, it is mixed with an aqueous solution of sintering aid precursor, and then a precipitant is added to form a co-precipitation system. The co-precipitation reaction is carried out to obtain a silicon nitride composite powder with silicon nitride as the core, silicon dioxide as the middle layer and sintering aid as the outer shell. (3) Preparation of ceramic slurry: The silicon nitride composite powder obtained in step (2) is mixed evenly with photocurable resin, photoinitiator and dispersant to prepare photocurable silicon nitride ceramic slurry.

[0009] In step (1), before modification, the average particle size of the silicon nitride powder is 0.5-5 μm; preferably 0.5-2 μm.

[0010] In step (1), the heating rate is 3-5℃ / min for both heating cycles.

[0011] In step (1), cooling is achieved by introducing nitrogen or argon gas to lower the temperature.

[0012] In step (1), the silicon nitride powder with a silicon dioxide film coating on its surface has a silicon dioxide film thickness of 60-120 nm.

[0013] This invention employs a segmented oxidation process, controlling the density and thickness of the SiO2 film by adjusting the oxidation temperature and time. In the initial stage of oxidation, a high oxygen concentration helps to increase the initial oxidation rate, while the SiO2 film formed under low temperature conditions is more uniform, denser, and has fewer defects. Oxidation under high oxygen concentration and low temperature conditions can form a uniform and dense SiO2 film on the surface of silicon nitride powder, reducing coating defects. In the later stage of oxidation, high temperature oxidation helps to increase the diffusion rate of oxygen in the SiO2 film. Oxidation under high temperature conditions can increase the thickness of the SiO2 film in a shorter time, allowing it to grow rapidly to 60-120 nm. SiO2 films within this thickness range can balance refractive index regulation with subsequent sintering performance. On one hand, the refractive index of SiO2 (n≈1.46) is much lower than that of Si3N4. Experiments have shown that SiO2 films thicker than 60 nm can effectively reduce the overall equivalent refractive index of silicon nitride powder, decreasing the refractive index difference between it and the resin, thereby significantly suppressing light scattering and improving the transmittance of the slurry. On the other hand, SiO2 can serve as heterogeneous nucleation sites, which is beneficial for subsequent sintering aid coating. Furthermore, excessive SiO2 film thickness can hinder liquid phase diffusion during sintering, affecting densification. Controlling its thickness to within 120 nm can avoid its impact on the sintering performance of silicon nitride ceramics.

[0014] In step (2), when the silicon nitride powder with a silicon dioxide film coating is dispersed in water, the mass-volume ratio of the silicon nitride powder with a silicon dioxide film coating to water is 1g:(3-10)mL.

[0015] In step (2), the sintering aid precursor is Y(NO3)3·6H2O, Al(NO3)3·9H2O, or Mg(NO3)2. 6H2O, La(NO3)3 6H2O, Ce(NO3)3 At least one of 6H2O.

[0016] The mass of the sintering aid precursor is calculated as 4-8 wt.% of the final sintering aid in the silicon nitride composite powder.

[0017] The total concentration of the sintering aid precursor in the aqueous solution is 0.5-1.5 mol / L.

[0018] In step (2), the precipitant is one of urea, ammonia, or ammonium carbonate.

[0019] The pH value of the coprecipitation system is controlled between 9 and 11.

[0020] In step (2), after the coprecipitation reaction is completed, the coprecipitation system is centrifuged, and the centrifuged powder is washed with water and dried to obtain silicon nitride composite powder.

[0021] During the co-precipitation process, the sintering aid precursor undergoes directional deposition on the SiO2 layer surface. Unlike random deposition on the silicon nitride surface or spontaneous nucleation in solution, this interface-induced deposition mechanism effectively avoids the problem of uneven aid distribution in traditional mechanical mixing or direct co-precipitation methods, significantly improving the uniformity and firmness of the sintering aid coating. This greatly enhances the effect of the sintering aid, improving ceramic sintering performance while reducing the amount of sintering aid required. This invention controls the sintering aid mass to between 3-6 wt.% of the silicon nitride composite powder mass, ensuring ceramic sintering performance while significantly reducing the coating thickness to below 20 nm. This film thickness has minimal impact on optical properties and does not affect the refractive index of the SiO2 film.

[0022] In step (3), the photocurable resin is at least one of acrylate, epoxy acrylate, and polyurethane acrylate.

[0023] The volume ratio of the photocurable resin to the silicon nitride composite powder is (40-60):(60-40).

[0024] In step (3), the photoinitiator is at least one of 2,4,6-trimethylbenzoyl-diphenylphosphine oxide (TPO), 2-isopropylthioxanthone (ITX), and phenyl bis(2,4,6-trimethylbenzoyl)phosphine oxide (BAPO).

[0025] The amount of photoinitiator added is 2-3 wt. of the mass of the photocurable resin.

[0026] In step (3), the dispersant is at least one of BYK-111, BYK-W969, ammonium polycarboxylate, and phosphate ester dispersants.

[0027] The amount of dispersant added is 0.5-1 wt. of the silicon nitride composite powder.

[0028] In step (3), when mixing the silicon nitride composite powder with the photocurable resin, photoinitiator and dispersant, it is preferable to use ball milling or planetary stirring.

[0029] The second objective of this invention is to provide a photocurable silicon nitride ceramic slurry, prepared by the above-mentioned method, which has a light transmittance of not less than 80 μm at a wavelength of 405 nm.

[0030] The third objective of this invention is to provide a method for preparing photocurable silicon nitride ceramics, comprising the following steps: Photocurable silicon nitride ceramic slurry is photocured and 3D printed to obtain silicon nitride ceramic green body, which is then subjected to thermal degreasing and gas pressure sintering to obtain photocurable silicon nitride ceramic.

[0031] The photopolymer 3D printing molding preferably employs surface exposure or digital light processing (DLP) technology.

[0032] The conditions for the thermal degreasing treatment are as follows: the temperature is increased at a rate of 1-1.5℃ / min under a flowing nitrogen atmosphere, and the temperature is maintained for 2-5 hours at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ respectively.

[0033] The conditions for pressure sintering are: nitrogen pressure 0.5-6 MPa, sintering temperature 1700-1850℃, and holding time 2-4 hours.

[0034] The fourth objective of this invention is to provide a photocurable silicon nitride ceramic, prepared by the above-mentioned method, which has a uniform microstructure, high density, and excellent mechanical properties.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention modifies silicon nitride powder in two steps. First, a dense, uniform SiO2 film with a thickness between 60-120 nm is generated on its surface through segmented temperature-controlled oxidation. Then, a uniform sintering aid coating layer with a thickness of less than 15 nm is formed on the surface of the SiO2 film through a co-precipitation reaction. The SiO2 film layer not only reduces the overall equivalent refractive index of the silicon nitride powder and improves the light transmittance of the slurry, but also provides a better reaction interface for the uniform coating of the sintering aid. Compared with the method of directly mixing silicon nitride powder, sintering aid, photocurable resin, dispersant and photoinitiator together, this invention can complete the surface functionalization of silicon nitride powder before the addition of photocurable resin, reduce the random dispersion and agglomeration of sintering aid in slurry, and reduce the amount of sintering aid used. 2. This invention improves the uniformity and density of SiO2 film coating by controlling the surface oxidation modification process parameters. In the early stage of oxidation treatment, high oxygen concentration and low temperature conditions are used. High oxygen concentration helps to increase the initial oxidation rate, and low temperature conditions are conducive to the formation of a more uniform and denser SiO2 film layer and reduce coating defects. In the later stage of oxidation treatment, high temperature conditions are used to help increase the diffusion rate of oxygen in the SiO2 film layer, thereby increasing the thickness of the SiO2 film layer in a short time, making its thickness grow rapidly to 60-120nm, and thus achieving the control of the equivalent refractive index of the powder. 3. This invention achieves molecular-level uniform coating of sintering aids on the surface of ceramic particles through chemical co-precipitation, solving the problem of uneven mechanical mixing and avoiding the problems of local enrichment, agglomeration and interfacial instability caused by the simultaneous mixing of aids and organic system during direct blending. This is beneficial for obtaining sintered bodies with fine grains and excellent mechanical properties. At the same time, it can reduce the amount of sintering aids used, making its coating thickness less than 20nm, thus avoiding its adverse effect on the equivalent refractive index. 4. This invention achieves a synergistic match between the SiO2 film thickness and the sintering aid layer thickness by controlling oxygen concentration, employing segmented oxidation, and using confined co-precipitation coating processes. This allows the composite powder to maintain excellent sintering activity while still possessing high light transmittance. The prepared photocurable silicon nitride ceramic slurry has strong process compatibility and can be perfectly integrated with existing DLP / SLA printing technology, debinding, and sintering processes. The light transmittance depth at a wavelength of 405nm is 80-90μm. The prepared silicon nitride ceramic has high density, high toughness, and high strength. Detailed Implementation

[0036] The present invention will be further described below with reference to embodiments, but is not limited to the specific embodiments listed herein. Unless otherwise specified, the process methods used in the embodiments are conventional methods in the art. Unless otherwise specified, the raw materials used in the embodiments are commercially available conventional raw materials, or can be prepared using existing technologies.

[0037] Example 1 Photocurable silicon nitride ceramic slurry and photocurable silicon nitride ceramic were prepared according to the following method: (1) Surface oxidation modification: α-Si3N4 powder with an average particle size of 0.5 μm was placed in a tube furnace, oxygen was introduced, and the temperature was first raised to 950℃ at 3℃ / min and held for 1 h; then the temperature was raised to 1200℃ at 3℃ / min and held for 0.5 h; then argon was introduced and the furnace was cooled down to obtain silicon nitride powder with a silicon dioxide film on the surface. The depth analysis of the silicon nitride powder by transmission electron microscopy (TEM) combined with XPS showed that an amorphous SiO2 film with a thickness of about 60 nm was formed on the surface of the silicon nitride powder. (2) Sintering aid coating: Silicon nitride powder coated with a silica film was dispersed in deionized water at a mass-to-volume ratio of 1 g: 3 mL to prepare a suspension; Y(NO3)3·6H2O and Mg(NO3)2 were added to the above suspension based on the final silicon nitride composite powder containing 5 wt.% Y2O3 and 3 wt.% MgO. An aqueous solution of 6H2O was prepared, with a total concentration of two metal salts of 1.5 mol / L. Under continuous stirring and at 80°C, ammonia was slowly added dropwise as a precipitant to form a coprecipitation system with a pH of 11. The coprecipitation reaction was carried out for 6 hours, followed by centrifugation, washing with water, and drying to finally obtain a silicon nitride composite powder with a core of Si3N4, a middle layer of SiO2, and a shell of Y2O3 / MgO. (3) Preparation of ceramic slurry: Mix 50 vol.% silicon nitride composite powder with 50 vol.% acrylate, add 2.5 wt.% photoinitiator ITX and 0.8 wt.% dispersant BYK-W969 according to the mass of acrylate, and stir at 1500 rpm for 60 min with a planetary mixer to prepare photocurable silicon nitride ceramic slurry; (4) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated in a fluid nitrogen atmosphere at a heating rate of 1.5℃ / min. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 3h each to remove the grease. Then they were placed in a graphite crucible and sintered at 1700℃ for 4h under a nitrogen pressure of 3MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0038] Example 2 Photocurable silicon nitride ceramic slurry and photocurable silicon nitride ceramic were prepared according to the following method: (1) Surface oxidation modification: α-Si3N4 powder with an average particle size of 1μm was placed in a tube furnace. Oxygen was first introduced and the temperature was raised to 1000℃ at 5℃ / min and held for 2h. Then air was introduced and the temperature was raised to 1250℃ at 5℃ / min and held for 1h. Then nitrogen was introduced and the furnace was cooled down to obtain silicon nitride powder with a silicon dioxide film on the surface. The amorphous SiO2 film with a thickness of about 95nm was generated on the surface of the silicon nitride powder by transmission electron microscopy (TEM) combined with XPS depth analysis. (2) Sintering aid coating: The silicon nitride powder coated with a silicon dioxide film was dispersed in deionized water at a mass-volume ratio of 1g:5mL to prepare a suspension. Based on the final silicon nitride composite powder containing 4wt.% Y2O3 and 2wt.% Al2O3, an aqueous solution of Y(NO3)3·6H2O and Al(NO3)3·9H2O was added to the above suspension, with a total concentration of the two metal salts of 0.8mol / L. Under continuous stirring and 80℃ conditions, a urea solution (concentration 1.2mol / L) was slowly added as a precipitant to form a coprecipitation system with a pH of 10. The coprecipitation reaction was carried out for 6h. Then, after centrifugation, washing with water and drying, a silicon nitride composite powder with a core of Si3N4, an intermediate layer of SiO2 and an outer shell of Y2O3 / Al2O3 was finally obtained. (3) Preparation of ceramic slurry: Mix 55 vol.% silicon nitride composite powder with 45 vol.% polyurethane acrylate, add 2 wt.% photoinitiator TPO and 0.5 wt.% dispersant BYK-111 according to the mass of polyurethane acrylate, and stir at 1500 rpm for 60 min with a planetary mixer to prepare photocurable silicon nitride ceramic slurry; (4) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated at a heating rate of 1℃ / min under a fluid nitrogen atmosphere. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 2h each to remove the grease. Then they were placed in a graphite crucible and sintered at 1750℃ for 3h under a nitrogen pressure of 0.5MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0039] Example 3 Photocurable silicon nitride ceramic slurry and photocurable silicon nitride ceramic were prepared according to the following method: (1) Surface oxidation modification: α-Si3N4 powder with an average particle size of 2μm was placed in a tube furnace. Oxygen was first introduced and the temperature was raised to 1050℃ at 4℃ / min and held for 1h. Then air was introduced and the temperature was raised to 1300℃ at 4℃ / min and held for 1h. Then nitrogen was introduced and the furnace was cooled down to obtain silicon nitride powder with a silicon dioxide film on the surface. The depth analysis of transmission electron microscopy (TEM) combined with XPS showed that an amorphous SiO2 film with a thickness of about 120nm was formed on the surface of the silicon nitride powder. (2) Sintering aid coating: The silicon nitride powder coated with a silica film was dispersed in deionized water at a mass-to-volume ratio of 1g:10mL to prepare a suspension; La(NO3)3 was added to the above suspension based on the final silicon nitride composite powder containing 2wt.% La2O3 and 2wt.% CeO2. 6H2O and Ce(NO3)3 An aqueous solution of 6H2O was prepared, with a total concentration of two metal salts of 0.8 mol / L. Under continuous stirring and at 80°C, ammonium carbonate solution (concentration 1.0 mol / L) was slowly added dropwise as a precipitant to form a coprecipitation system with a pH of 9. The coprecipitation reaction was carried out for 6 hours, followed by centrifugation, washing with water, and drying to finally obtain a silicon nitride composite powder with a core of Si3N4, a middle layer of SiO2, and a shell of La2O3 / CeO2. (3) Preparation of ceramic slurry: 60 vol.% of silicon nitride composite powder and 40 vol.% of epoxy acrylate are mixed, and 3 wt.% of photoinitiator BAPO and 1 wt.% of dispersant ammonium polycarboxylate are added. The mixture is stirred at 1500 rpm for 60 min by a planetary mixer to prepare photocurable silicon nitride ceramic slurry. (3) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated at a heating rate of 1℃ / min under a fluid nitrogen atmosphere. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 2 hours each to remove the grease. Then they were placed in a graphite crucible and sintered at 1850℃ for 2 hours under a nitrogen pressure of 6MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0040] Comparative Example 1 The only difference between this comparative example and Example 2 is that the silicon nitride powder is not surface-oxidized; instead, it is directly coated with sintering aids. The specific steps are as follows: (1) Sintering aid coating: α-Si3N4 powder with an average particle size of 1μm was dispersed in deionized water at a mass-volume ratio of 1g:5mL to prepare a suspension; based on the final silicon nitride composite powder containing 4wt.% Y2O3 and 2wt.% Al2O3, aqueous solutions of Y(NO3)3·6H2O and Al(NO3)3·9H2O were added to the above suspension, with a total concentration of the two metal salts of 0.8mol / L; under continuous stirring and 80℃ conditions, urea solution (concentration 1.2mol / L) was slowly added as a precipitant to form a coprecipitation system with a pH of 10. The coprecipitation reaction was carried out for 6h, and then centrifuged, washed with water and dried to finally obtain silicon nitride composite powder with Si3N4 core and Y2O3 / Al2O3 shell; (2) Preparation of ceramic slurry: Mix 55 vol.% silicon nitride composite powder with 45 vol.% polyurethane acrylate, add 2 wt.% photoinitiator TPO and 0.5 wt.% dispersant BYK-111 according to the mass of polyurethane acrylate, and stir at 1500 rpm for 60 min with a planetary mixer to prepare photocurable silicon nitride ceramic slurry; (3) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated at a heating rate of 1℃ / min under a fluid nitrogen atmosphere. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 2h each to remove the grease. Then they were placed in a graphite crucible and sintered at 1750℃ for 3h under a nitrogen pressure of 0.5MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0041] Comparative Example 2 The only difference between this comparative example and Example 2 is that the surface oxidation modification conditions were adjusted to make the SiO2 film thickness less than 60 nm. The specific steps are as follows: (1) Surface oxidation modification: α-Si3N4 powder with an average particle size of 1μm was placed in a tube furnace, oxygen was introduced, and the temperature was raised to 1000℃ at 5℃ / min and held for 2h; then air was introduced and the temperature was raised to 1250℃ at 5℃ / min and held for 20min; then nitrogen was introduced and the furnace was cooled down to obtain silicon nitride powder with a silicon dioxide film on the surface. The depth analysis by transmission electron microscopy (TEM) combined with XPS showed that an amorphous SiO2 film with a thickness of about 40nm was formed on the surface of the silicon nitride powder. (2) Sintering aid coating: The silicon nitride powder coated with a silicon dioxide film was dispersed in deionized water at a mass-volume ratio of 1g:5mL to prepare a suspension. Based on the final silicon nitride composite powder containing 4wt.% Y2O3 and 2wt.% Al2O3, an aqueous solution of Y(NO3)3·6H2O and Al(NO3)3·9H2O was added to the above suspension, with a total concentration of the two metal salts of 0.8mol / L. Under continuous stirring and 80℃ conditions, a urea solution (concentration 1.2mol / L) was slowly added as a precipitant to form a coprecipitation system with a pH of 10. The coprecipitation reaction was carried out for 6h. Then, after centrifugation, washing with water and drying, a silicon nitride composite powder with a core of Si3N4, an intermediate layer of SiO2 and an outer shell of Y2O3 / Al2O3 was finally obtained. (3) Preparation of ceramic slurry: Mix 55 vol.% silicon nitride composite powder with 45 vol.% polyurethane acrylate, add 2 wt.% photoinitiator TPO and 0.5 wt.% dispersant BYK-111 according to the mass of polyurethane acrylate, and stir at 1500 rpm for 60 min with a planetary mixer to prepare photocurable silicon nitride ceramic slurry; (4) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated at a heating rate of 1℃ / min under a fluid nitrogen atmosphere. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 2h each to remove the grease. Then they were placed in a graphite crucible and sintered at 1750℃ for 3h under a nitrogen pressure of 0.5MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0042] Comparative Example 3 The only difference between this comparative example and Example 2 is that the surface oxidation modification conditions were adjusted to make the SiO2 film thickness greater than 120 nm. The specific steps are as follows: (1) Surface oxidation modification: α-Si3N4 powder with an average particle size of 1μm was placed in a tube furnace, oxygen was introduced, and the temperature was raised to 1000℃ at 5℃ / min and held for 2h; then air was introduced and the temperature was raised to 1250℃ at 5℃ / min and held for 2h; then nitrogen was introduced and the furnace was cooled down to obtain silicon nitride powder with a silicon dioxide film on the surface. The amorphous SiO2 film with a thickness of about 150nm was generated on the surface of the silicon nitride powder by transmission electron microscopy (TEM) combined with XPS depth analysis. (2) Sintering aid coating: The silicon nitride powder coated with a silicon dioxide film was dispersed in deionized water at a mass-volume ratio of 1g:5mL to prepare a suspension. Based on the final silicon nitride composite powder containing 4wt.% Y2O3 and 2wt.% Al2O3, an aqueous solution of Y(NO3)3·6H2O and Al(NO3)3·9H2O was added to the above suspension, with a total concentration of the two metal salts of 0.8mol / L. Under continuous stirring and 80℃ conditions, a urea solution (concentration 1.2mol / L) was slowly added as a precipitant to form a coprecipitation system with a pH of 10. The coprecipitation reaction was carried out for 6h. Then, after centrifugation, washing with water and drying, a silicon nitride composite powder with a core of Si3N4, an intermediate layer of SiO2 and an outer shell of Y2O3 / Al2O3 was finally obtained. (3) Preparation of ceramic slurry: Mix 55 vol.% silicon nitride composite powder with 45 vol.% polyurethane acrylate, add 2 wt.% photoinitiator TPO and 0.5 wt.% dispersant BYK-111 according to the mass of polyurethane acrylate, and stir at 1500 rpm for 60 min with a planetary mixer to prepare photocurable silicon nitride ceramic slurry; (4) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated at a heating rate of 1℃ / min under a fluid nitrogen atmosphere. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 2h each to remove the grease. Then they were placed in a graphite crucible and sintered at 1750℃ for 3h under a nitrogen pressure of 0.5MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0043] Comparative Example 4 The only difference between this comparative example and Example 2 is that the silicon nitride powder is surface-oxidized using a conventional high-temperature oxidation method. The specific steps are as follows: (1) Surface oxidation modification: α-Si3N4 powder with an average particle size of 1μm was placed in a tube furnace, air was introduced, and the temperature was raised to 1250℃ at 5℃ / min and held for 1h; then nitrogen was introduced and the furnace was cooled down to obtain silicon nitride powder with a silicon dioxide film on the surface. The depth analysis by transmission electron microscopy (TEM) combined with XPS showed that an amorphous SiO2 film with a thickness of about 80nm was formed on the surface of the silicon nitride powder. (2) Sintering aid coating: The silicon nitride powder coated with a silicon dioxide film was dispersed in deionized water at a mass-volume ratio of 1g:5mL to prepare a suspension. Based on the final silicon nitride composite powder containing 4wt.% Y2O3 and 2wt.% Al2O3, an aqueous solution of Y(NO3)3·6H2O and Al(NO3)3·9H2O was added to the above suspension, with a total concentration of the two metal salts of 0.8mol / L. Under continuous stirring and 80℃ conditions, a urea solution (concentration 1.2mol / L) was slowly added as a precipitant to form a coprecipitation system with a pH of 10. The coprecipitation reaction was carried out for 6h. Then, after centrifugation, washing with water and drying, a silicon nitride composite powder with a core of Si3N4, an intermediate layer of SiO2 and an outer shell of Y2O3 / Al2O3 was finally obtained. (3) Preparation of ceramic slurry: Mix 55 vol.% silicon nitride composite powder with 45 vol.% polyurethane acrylate, add 2 wt.% photoinitiator TPO and 0.5 wt.% dispersant BYK-111 according to the mass of polyurethane acrylate, and stir at 1500 rpm for 60 min with a planetary mixer to prepare photocurable silicon nitride ceramic slurry; (4) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated at a heating rate of 1℃ / min under a fluid nitrogen atmosphere. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 2h each to remove the grease. Then they were placed in a graphite crucible and sintered at 1750℃ for 3h under a nitrogen pressure of 0.5MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0044] Comparative Example 5 The only difference between this comparative example and Example 2 is that the silicon nitride powder is not coated with sintering aids. The specific steps are as follows: (1) Surface oxidation modification: α-Si3N4 powder with an average particle size of 1μm was placed in a tube furnace. Oxygen was first introduced and the temperature was raised to 1000℃ at 5℃ / min and held for 2h. Then air was introduced and the temperature was raised to 1250℃ at 5℃ / min and held for 1h. Then nitrogen was introduced and the furnace was cooled down to obtain silicon nitride powder with a silicon dioxide film coating on the surface. (2) Sintering aid mixing: Based on the final silicon nitride composite powder containing 8 wt.% Y2O3 and 4 wt.% Al2O3, the silicon nitride powder with a silicon dioxide film coating on the surface is mixed with Y2O3 powder and Al2O3 powder and ball-milled for 12 hours to finally obtain silicon nitride mixed powder; (3) Preparation of ceramic slurry: Mix 55 vol.% silicon nitride mixed powder with 45 vol.% polyurethane acrylate, add 2 wt.% photoinitiator TPO and 0.5 wt.% dispersant BYK-111 according to the mass of polyurethane acrylate, and stir at 1500 rpm for 60 min with a planetary mixer to prepare photocurable silicon nitride ceramic slurry; (4) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated at a heating rate of 1℃ / min under a fluid nitrogen atmosphere. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 2h each to remove the grease. Then they were placed in a graphite crucible and sintered at 1750℃ for 3h under a nitrogen pressure of 0.5MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0045] Comparative Example 6 The only difference between this comparative example and Example 2 is that the coating amount of the sintering aid is increased. The specific steps are as follows: (1) Surface oxidation modification: α-Si3N4 powder with an average particle size of 1μm was placed in a tube furnace. Oxygen was first introduced and the temperature was raised to 1000℃ at 5℃ / min and held for 2h. Then air was introduced and the temperature was raised to 1250℃ at 5℃ / min and held for 1h. Then nitrogen was introduced and the furnace was cooled down to obtain silicon nitride powder with a silicon dioxide film on the surface. The amorphous SiO2 film with a thickness of about 95nm was generated on the surface of the silicon nitride powder by transmission electron microscopy (TEM) combined with XPS depth analysis. (2) Sintering aid coating: The silicon nitride powder coated with a silicon dioxide film was dispersed in deionized water at a mass-volume ratio of 1g:5mL to prepare a suspension. Based on the final silicon nitride composite powder containing 8wt.% Y2O3 and 4wt.% Al2O3, an aqueous solution of Y(NO3)3·6H2O and Al(NO3)3·9H2O was added to the above suspension, with a total concentration of the two metal salts of 0.8mol / L. Under continuous stirring and 80℃ conditions, a urea solution (concentration 1.2mol / L) was slowly added as a precipitant to form a coprecipitation system with a pH of 10. The coprecipitation reaction was carried out for 6h. Then, after centrifugation, washing with water and drying, a silicon nitride composite powder with a core of Si3N4, an intermediate layer of SiO2 and an outer shell of Y2O3 / Al2O3 was finally obtained. (3) Preparation of ceramic slurry: Mix 55 vol.% silicon nitride composite powder with 45 vol.% polyurethane acrylate, add 2 wt.% photoinitiator TPO and 0.5 wt.% dispersant BYK-111 according to the mass of polyurethane acrylate, and stir at 1500 rpm for 60 min with a planetary mixer to prepare photocurable silicon nitride ceramic slurry; (4) 3D printing molding and sintering: The above photocurable silicon nitride ceramic slurry was printed into strip-shaped silicon nitride ceramic green bodies using a DLP / SLA printer (405nm wavelength). The printed silicon nitride ceramic green bodies were heated at a heating rate of 1℃ / min under a fluid nitrogen atmosphere. They were held at 100℃, 200℃, 300℃, 400℃, 500℃ and 600℃ for 2h each to remove the grease. Then they were placed in a graphite crucible and sintered at 1750℃ for 3h under a nitrogen pressure of 0.5MPa. The furnace was then cooled to obtain photocurable silicon nitride ceramics.

[0046] The performance of the photocurable silicon nitride ceramics prepared in each embodiment and comparative example was tested, and the results are shown in Table 1.

[0047] Table 1. Photocurable silicon nitride ceramic slurries and ceramic performance indicators prepared in each embodiment and comparative example.

[0048] As shown in Table 1, the silicon nitride ceramic slurries prepared in Examples 1 to 3 all had a light transmittance of no less than 80 μm at a wavelength of 405 nm, which was significantly higher than that of the comparative example. Furthermore, the density and mechanical properties of the cured silicon nitride ceramics were also higher than those of the comparative example. This indicates that the segmented oxidation and confined co-precipitation process can not only effectively improve the refractive index matching of the silicon nitride composite powder, but also contribute to the improvement of sintering performance. Meanwhile, Examples 1 to 3 investigated the modification effect of silicon nitride powders with different particle sizes. The results showed that when the SiO2 film thickness on the surface of the silicon nitride powder was in the range of 60-120 nm, and the sintering aid coating thickness was controlled within 20 nm, a significant improvement in light transmittance could be achieved for silicon nitride powders of different particle sizes. Moreover, the light transmittance increased slightly with the increase of silicon nitride powder particle size and SiO2 film thickness.

[0049] Comparative Examples 1, 2, 3, and 2 investigated the effects of SiO2 film thickness on the light transmission depth and ceramic properties of silicon nitride. The results showed that as the SiO2 film thickness increased, the light transmission depth also gradually increased, thanks to the reduction in the overall equivalent refractive index of the silicon nitride powder by the SiO2 film. In Comparative Example 1, no SiO2 transition film was formed through oxidation, making it difficult for sintering aids to form a uniform deposition on the surface of Si3N4 particles. This resulted in poor refractive index matching within the slurry, enhanced light scattering, and a shallow light transmission depth. Furthermore, the lack of reaction between SiO2 and the sintering aids at the sintering temperature hindered liquid-phase sintering, leading to localized pores and abnormal grains within the sintered body, and a decrease in the density and mechanical properties of the silicon nitride ceramic. The SiO2 film thickness in Comparative Example 2 is relatively small, limiting its improvement in light transmission depth. However, the dense SiO2 film provides a better reaction interface for the uniform coating of subsequent sintering aids and provides sufficient conditions for liquid-phase sintering. Therefore, the silicon nitride ceramic exhibits higher density and mechanical properties. The SiO2 film thickness in Comparative Example 3 is larger, with the same light transmission depth as Comparative Example 2. Considering the lack of significant differences in light transmission depth between Examples 1 and 3, it can be concluded that when the SiO2 film thickness is ≥60nm, it can significantly reduce the overall equivalent refractive index of the silicon nitride powder. However, an excessively thick SiO2 film will generate an excessive glass phase during sintering, making the silicon nitride ceramic porous and with coarse grains, resulting in a significant reduction in its density and mechanical properties.

[0050] Comparative Example 4 uses a conventional high-temperature oxidation method. The SiO2 film layer formed under high-temperature conditions is uneven in thickness, resulting in some parts of the silicon nitride powder surface not being coated with SiO2 film layer. On the one hand, this weakens the effect of reducing the overall equivalent refractive index of the silicon nitride powder, and on the other hand, it also affects the uniformity of the coating of subsequent sintering aids. As a result, the light transmission depth of the silicon nitride ceramic slurry is reduced compared with Example 2, and the density and mechanical properties of the silicon nitride ceramic are also slightly reduced.

[0051] Comparative Example 5 uses mechanical mixing to add sintering aids. In order to achieve the expected sintering effect, the amount of sintering aids is appropriately increased. The mechanical mixing method causes the sintering aids to agglomerate locally in the slurry. As a result, the internal refractive index of the composite silicon nitride powder fluctuates greatly, the light transmission depth is reduced, and the liquid phase distribution is uneven during the sintering process. This leads to inconsistent grain size distribution inside the sintered body and the occurrence of local abnormal grain growth. The density and mechanical properties of silicon nitride ceramics are also significantly reduced.

[0052] Comparative Example 6 increased the amount of sintering aid (the same as Comparative Example 5). The increase in the thickness of the sintering aid film significantly improved its influence on optical properties. Since the refractive index of the sintering aid is higher than that of SiO2, the equivalent refractive index of the final silicon nitride composite powder is higher than that of Example 2, resulting in a decrease in light transmission depth. In addition, the density and mechanical properties of the silicon nitride ceramics of Comparative Example 6 and Example 2 are basically the same, indicating that the present invention uses a segmented oxidation and confined co-precipitation coating process to achieve molecular-level uniform coating of the sintering aid on the surface of ceramic particles, which can reduce the amount of sintering aid used.

Claims

1. A method for preparing a photocurable silicon nitride ceramic slurry, characterized in that: Includes the following steps: (1) Surface oxidation modification: The silicon nitride powder is first heated to 950-1050℃ in an oxygen atmosphere and kept at that temperature for 1-2 hours. Then it is heated to 1200-1300℃ in an air or oxygen atmosphere and kept at that temperature for 0.5-1 hours. Then it is cooled to obtain silicon nitride powder with a silicon dioxide film coating on the surface. (2) Sintering aid coating: After dispersing silicon nitride powder coated with a silicon dioxide film on the surface in water, it is mixed with an aqueous solution of sintering aid precursor, and then a precipitant is added to form a co-precipitation system. The co-precipitation reaction is carried out to obtain a silicon nitride composite powder with silicon nitride as the core, silicon dioxide as the middle layer and sintering aid as the outer shell. The mass of the sintering aid precursor is calculated as 4-8 wt.% of the final sintering aid in the silicon nitride composite powder. (3) Preparation of ceramic slurry: The silicon nitride composite powder obtained in step (2) is mixed evenly with photocurable resin, photoinitiator and dispersant to prepare photocurable silicon nitride ceramic slurry.

2. The method for preparing photocurable silicon nitride ceramic slurry according to claim 1, characterized in that: In step (1), before modification, the average particle size of silicon nitride powder is 0.5-5 μm.

3. The method for preparing photocurable silicon nitride ceramic slurry according to claim 1, characterized in that: In step (1), the heating rate is 3-5℃ / min for both heating cycles.

4. The method for preparing photocurable silicon nitride ceramic slurry according to claim 1, characterized in that: In step (1), the silicon nitride powder with a silicon dioxide film coating on its surface has a silicon dioxide film thickness of 60-120 nm.

5. The method for preparing photocurable silicon nitride ceramic slurry according to claim 1, characterized in that: In step (2), when the silicon nitride powder with a silicon dioxide film coating is dispersed in water, the mass-volume ratio of the silicon nitride powder with a silicon dioxide film coating to water is 1g:(3-10)mL.

6. The method for preparing photocurable silicon nitride ceramic slurry according to claim 1, characterized in that: In step (2), the sintering aid precursors are Y(NO3)3·6H2O, Al(NO3)3·9H2O, and Mg(NO3)2. 6H2O, La(NO3)3 6H2O, Ce(NO3)3 At least one of 6H2O; the total concentration of the sintering aid precursor in the aqueous solution is 0.5-1.5 mol / L; The precipitant is one of urea, ammonia, or ammonium carbonate; the pH value of the co-precipitation system is controlled between 9 and 11.

7. The method for preparing photocurable silicon nitride ceramic slurry according to claim 1, characterized in that: In step (3), the photocurable resin is at least one of acrylate, epoxy acrylate, and polyurethane acrylate; the volume ratio of the photocurable resin to the silicon nitride composite powder is (40-60):(60-40). The photoinitiator is at least one selected from 2,4,6-trimethylbenzoyl-diphenylphosphine oxide, 2-isopropylthioxanthone, and phenylbis(2,4,6-trimethylbenzoyl)phosphine oxide; the amount of the photoinitiator added is 2-3 wt.% of the mass of the photocurable resin. The dispersant is at least one of BYK-111, BYK-W969, ammonium polycarboxylate, and phosphate ester dispersants; the amount of the dispersant added is 0.5-1 wt. of the mass of the silicon nitride composite powder.

8. A photocurable silicon nitride ceramic slurry, characterized in that: The photocurable silicon nitride ceramic slurry prepared by any one of claims 1-7 has a light transmittance of 80-90 μm at a wavelength of 405 nm.

9. A method for preparing photocurable silicon nitride ceramics, characterized in that: Includes the following steps: The photocurable silicon nitride ceramic slurry described in claim 8 is photocured and 3D printed to obtain a silicon nitride ceramic green body, which is then subjected to thermal degreasing and gas pressure sintering to obtain a photocurable silicon nitride ceramic.

10. A photocurable silicon nitride ceramic, characterized in that: It is prepared by the method for preparing photocurable silicon nitride ceramic as described in claim 9.

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