A method for preparing a rare earth nitrate spray-dried method silicon nitride powder surface coating film

Abstract

This invention discloses a method for preparing a surface coating of silicon nitride powder using rare earth nitrate spray drying, belonging to the field of silicon nitride powder modification technology. The method uses rare earth nitrate as a sintering aid precursor and deionized water as a dispersion medium. After adding a dispersant, it is mixed with silicon nitride powder to prepare a stable suspension slurry. The slurry is atomized by an atomizer and then rapidly dehydrated by contact with a hot inert gas flow in a spray drying tower under an inert atmosphere, causing the rare earth nitrate to deposit in situ and coat the surface of the silicon nitride powder, obtaining a coated powder. Then, it undergoes gradient heating and calcination to decompose the rare earth nitrate into rare earth oxides, finally obtaining Si3N4@RE2O3 core-shell structure coated silicon nitride powder. This invention achieves nanoscale uniform coating of rare earth sintering aids on the surface of silicon nitride powder. The process is continuous and controllable, with no impurity residue, significantly improving the sintering densification degree, mechanical properties, and batch stability of silicon nitride ceramics, making it suitable for the large-scale production of high-end silicon nitride ceramics.

Description

Technical Field

[0001] This invention relates to the field of silicon nitride powder modification technology, and more particularly to a method for preparing a surface coating of silicon nitride powder by rare earth nitrate spray drying. Background Technology

[0002] Silicon nitride (Si3N4) ceramics possess excellent properties such as high strength, high hardness, high temperature resistance, corrosion resistance, and thermal shock resistance, making them irreplaceable in high-end fields such as machinery, aerospace, electronics, metallurgy, and chemical engineering. Due to the high bond energy and low grain boundary diffusion coefficient of silicon nitride, solid-state sintering is difficult to achieve densification. Industrially, rare earth oxides (such as Y2O3 and La2O3) are typically added as liquid-phase sintering aids. These aids promote particle rearrangement, dissolution, and diffusion by forming a low-melting-point glassy phase, thereby improving the sintering densification and mechanical properties of the ceramics.

[0003] Currently, the main methods for mixing rare earth oxide sintering aids with silicon nitride powder are dry mixing or wet ball milling. These methods have significant technical drawbacks: on the one hand, the density and surface energy of rare earth oxide powder and silicon nitride powder differ greatly, making it easy to form micron-sized agglomerates during mixing, making it difficult to achieve molecular or nanoscale uniform dispersion; on the other hand, uneven mixing leads to localized enrichment of aids and abrupt changes in liquid phase during sintering, which in turn causes abnormal growth of silicon nitride grains and uneven distribution of the glassy phase at grain boundaries. Ultimately, this results in large fluctuations in the mechanical properties of ceramic products and poor batch stability, severely restricting the industrial application of high-end silicon nitride ceramics.

[0004] To address the issue of homogenization of sintering aids, various improvement schemes have been proposed in existing technologies. For example, CN103420678A discloses a method for preparing SiAlON ceramic materials using a heterogeneous precipitation method. This method uses soluble aluminum salts and rare earth salts as precursors, which are precipitated on the surface of Si3N4 particles and then calcined to form an Al2O3-rare earth oxide coating layer. While this method can achieve coating to a certain extent, the chemical precipitation reaction requires strict control of parameters such as pH and water bath temperature, making process control difficult and prone to introducing impurity ions that affect powder purity. Furthermore, the co-precipitation method is an intermittent process with low production efficiency, making it difficult to adapt to large-scale continuous production.

[0005] JP03069546A discloses a ceramic sintered body and its manufacturing method. This technology involves mixing yttrium nitrate and aluminum nitrate solutions with Si3N4 powder, spray drying to ensure uniform nitrate adhesion, followed by thermal decomposition and calcination to fix oxide additives onto the surface of the Si3N4 particles, and finally shaping and sintering. Further analysis reveals the following unresolved technical problems in JP03069546A:

[0006] First, the patent fails to control the atmosphere during the spray drying process. Silicon nitride powder is highly susceptible to oxidation at high temperatures. Without an inert atmosphere, a SiO2 layer will form on the powder surface. This severely hinders the interfacial bonding between rare earth oxide additives and silicon nitride during subsequent sintering, leading to coating failure, decreased sintering activity, and ultimately unstable ceramic mechanical properties. The patent does not mention any anti-oxidation measures, making it difficult to control the oxygen content in its products to an ideal level.

[0007] Second, the patent fails to optimize and control the solid content, type and amount of dispersant, and degree of grinding of the slurry. The stability of the slurry directly affects the uniformity of the coating after spray drying: too low a solid content results in high drying energy consumption and low yield; too high a solid content results in excessive slurry viscosity and difficulty in atomization; insufficient or excessive dispersant will lead to powder agglomeration. The patent only describes the mixing operation in a general way and does not provide the key process parameters for achieving a stable suspension slurry. In fact, without precise control over the rheological properties of the slurry, the rare earth nitrates in the coated powder obtained by spray drying do not adhere in the form of a continuous and uniform nanolayer, but exist in the form of discontinuous islands or agglomerates, resulting in uneven distribution of the additives after calcination.

[0008] Third, this patent involves direct calcination (thermal decomposition) after spray drying, but without a gradient heating process. Rare earth nitrates release large amounts of NO2 and O2 gases during rapid decomposition. If the heating rate is too fast, the rapid escape of these gases can damage the integrity of the coating layer, causing cracking, peeling, or even detachment, exposing parts of the silicon nitride surface. This patent employs a one-step thermal decomposition method, which cannot effectively control the gas release rate and makes it difficult to ensure a continuous and dense coating layer.

[0009] Fourth, the patent does not cover the post-processing of the calcined powder (such as air jet milling, sieving, etc.), resulting in a wide particle size distribution and poor flowability of the powder, which is not conducive to subsequent molding processes. Furthermore, the coating layer is partially peeled off due to sintering and agglomeration, affecting batch stability.

[0010] In summary, existing technologies have failed to simultaneously address the issues of uniformity, continuity, and process controllability in rare earth additive coating of silicon nitride powder surfaces, and have not yet achieved large-scale stable production of nanoscale coating layers. Summary of the Invention

[0011] This invention aims to solve the technical problems in existing technologies, such as uneven mixing of rare earth sintering aids and silicon nitride powder, complex co-precipitation processes, and discontinuous coatings, easy oxidation of powders, and unstable ceramic properties due to lack of atmosphere protection and parameter optimization in spray drying. It provides a method for preparing a surface coating of silicon nitride powder using rare earth nitrate spray drying. This invention uses rare earth nitrates as a precursor, utilizing their molecular-level solubility in water to obtain a highly stable suspension through optimized slurry formulation; spray drying is performed under an inert atmosphere to isolate oxygen and prevent Si3N4 oxidation; precise inlet / outlet air temperature control ensures rapid solvent evaporation while preventing premature nitrate decomposition; finally, gradient heating calcination controls the gas release rate to ensure that the rare earth nitrates decompose in situ into oxides and form a continuous, dense, nanoscale coating layer.

[0012] The technical means employed in this invention are as follows: A method for preparing a surface coating of silicon nitride powder by rare earth nitrate spray drying includes the following steps: S1. Rare earth nitrates are dissolved in deionized water to form a precursor solution; silicon nitride powder and dispersant are added to the precursor solution, and after dispersion and grinding, a stable suspension slurry is obtained. S2. The slurry obtained in S1 is fed into a closed-loop spray dryer, atomized under an inert atmosphere, and dried by contact with a hot inert airflow, so that rare earth nitrates are deposited and coated on the surface of silicon nitride powder to obtain rare earth nitrate coated silicon nitride powder. S3. The coated powder obtained in S2 is placed in an atmosphere furnace and calcined to decompose the rare earth nitrates into rare earth oxides. After post-processing, Si3N4@RE2O3 core-shell structure coated silicon nitride powder is obtained.

[0013] Furthermore, the rare earth nitrate in S1 is selected from one or a combination of two of Y(NO3)3·6H2O and La(NO3)3·6H2O, with a purity ≥99.99%; the rare earth nitrate, calculated as rare earth oxide RE2O3, is added in an amount of 5~10wt% of the silicon nitride powder.

[0014] Furthermore, the rare earth nitrate is a composite system of Y(NO3)3·6H2O and La(NO3)3·6H2O, wherein the mass ratio of Y2O3 to La2O3 is 3:2.

[0015] Furthermore, the silicon nitride powder in S1 is mainly composed of α-Si3N4, with a purity ≥99.5%, a median particle size D50 = 0.5~1.0μm, and an oxygen content ≤1.5wt%; the amount of deionized water added is 100~150wt% of the mass of the silicon nitride powder, and the solid content of the slurry is 25~35wt%.

[0016] Furthermore, the dispersant in S1 is selected from ammonium polyacrylate or polyethylene glycol, and the amount added is 0.5~1.0wt% of the silicon nitride powder mass; the grinding and refining process uses zirconia beads as the medium and grinds until the slurry particle size D90≤2μm.

[0017] Furthermore, the inert atmosphere described in S2 is high-purity nitrogen with an oxygen content ≤100ppm; the atomization uses a centrifugal atomizer with a rotation speed of 20000~25000rpm or a pressure atomizer with a pressure of 0.4~0.8MPa; the inlet air temperature for drying is 180~220℃, and the outlet air temperature is 90~110℃.

[0018] Furthermore, the calcination described in S3 is a gradient heating calcination: the temperature is increased to 200°C at a heating rate of 1°C / min and held for 1 hour; then the temperature is increased to 500°C at a heating rate of 2°C / min and held for 2-3 hours; the calcination atmosphere is air or high-purity nitrogen.

[0019] Furthermore, the post-processing described in S3 includes air jet milling and vibrating screen sieving, and the median particle size D50 of the resulting coated silicon nitride powder is 50~100μm; in the Si3N4@RE2O3 core-shell structure, the thickness of the rare earth oxide coating layer is 10~50nm, and the coating layer is continuous and undamaged.

[0020] Furthermore, S1 also contains a binder, which is polyvinyl alcohol, and the amount added is 0.5~1.0 wt% of the silicon nitride powder.

[0021] A Si3N4@RE2O3 core-shell structured coated silicon nitride powder prepared by the above method.

[0022] Compared with the prior art, the present invention has the following advantages: 1. This invention uses rare earth nitrates as precursors. Their molecular-level dissolution in water ensures the atomic / ionic uniform distribution of additives in the liquid phase. Combined with the in-situ deposition mechanism of spray drying, it achieves nanoscale uniform coating of rare earth oxides on the surface of silicon nitride powder, with a coating thickness of 10-50 nm, which is far superior to the micron-level agglomeration distribution of mechanical mixing. The ceramics prepared in this way exhibit simultaneous and uniform formation of additives in the liquid phase during sintering, with uniform grain size, consistent grain boundary phase distribution, and significantly improved mechanical properties.

[0023] 2. This invention introduces an inert atmosphere during the spray drying process, controlling the oxygen content to ≤100ppm, effectively preventing high-temperature oxidation of silicon nitride and ensuring a pure interface between the coating layer and the substrate. Simultaneously, by optimizing the slurry's solid content, viscosity, and grinding particle size, slurry stability and atomization uniformity are guaranteed, allowing nitrate droplets to uniformly adhere to the surface of each silicon nitride particle, forming a continuous coating rather than an island-like dispersion.

[0024] 3. This invention employs a two-stage calcination process: initial heating at 200℃ for dehydration, followed by a slow heating to 500℃ at a rate of 2℃ / min for decomposition. This ensures that nitrate decomposition gases escape at a uniform rate, preventing cracks, bubbles, or peeling of the coating layer. By controlling the heating rate and using segmented heating, this invention achieves high integrity of the coating layer.

[0025] 4. Due to the achievement of nanoscale uniform coating of the additives, the silicon nitride ceramics prepared using the powder of this invention have a flexural strength ≥900MPa and a fracture toughness ≥8MPa·m. 1 / 2 The coefficient of variation (CV) for this process is ≤3%, while that for the traditional dry-mixing process is ≥8%. This improvement in performance stability is of great significance for the engineering applications of high-end ceramics. Detailed Implementation

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0027] Example 1 This embodiment provides a method for preparing a coating on the surface of silicon nitride powder by rare earth nitrate spray drying, which specifically includes the following steps.

[0028] S1, Slurry Preparation Weigh out rare earth nitrates with a purity of 99.99%. In this embodiment, Y(NO3)3·6H2O (calculated as Y2O3) is selected as the sintering aid precursor, and its addition amount is 5~10wt% of the silicon nitride powder mass (preferably 8wt% in this embodiment). Add it to deionized water with a resistivity of 18.2 MΩ·cm, and stir at 300~500 r / min for 30~60 min at room temperature until completely dissolved to form a clear and transparent precursor solution. The amount of deionized water added is 100~150wt% of the silicon nitride powder mass (preferably 120wt% in this embodiment).

[0029] α-Si3N4 powder (purity ≥99.5%, median particle size D50 = 0.5~1.0 μm, oxygen content ≤1.5 wt%, in this embodiment D50 = 0.8 μm, oxygen content 1.2 wt%) and dispersant ammonium polyacrylate (PAA-NH4) were added to the above precursor solution. The amount of dispersant added was 0.5~1.0 wt% of the silicon nitride powder mass (preferably 0.8 wt% in this embodiment). Pre-dispersion was performed by high-speed stirring at 1000~2000 rpm (1500 rpm in this embodiment) for 30 min to fully wet the powder and preliminarily depolymerize it. In some optional embodiments, a binder, namely polyvinyl alcohol, may also be added, with an addition amount of 0.5~1.0 wt% of the silicon nitride powder mass.

[0030] The pre-dispersed slurry was transferred to a sand mill and ground for 2-4 hours (3 hours in this example) using zirconia beads with a diameter of 0.8-1.2 mm (1.0 mm in this example) as the grinding medium. Particle size was measured periodically until the D90 of the particles in the slurry was ≤2 μm. The purpose of grinding was to break up soft agglomerates in the powder, fully exposing the surface of each silicon nitride particle to provide conditions for subsequent uniform coating. After grinding, the slurry was degassed under vacuum for 30 minutes to remove air bubbles entrained during stirring and grinding, preventing droplet bursting or the formation of hollow particles during spray drying. It was then allowed to stand for 1 hour to allow the components in the slurry to fully balance, ultimately obtaining a stable and uniform suspension slurry. The slurry was tested and found to have a solid content of 25-35 wt% (approximately 30 wt% in this example) and a room temperature viscosity of 50-200 mPa·s (approximately 120 mPa·s in this example). After standing for 24 hours, there was no obvious stratification or sedimentation, indicating that it has good suspension stability and flowability, and meets the requirements for spray drying feed.

[0031] S2, Spray drying coating The stable suspension slurry prepared by S1 was fed into a closed-loop spray dryer via a peristaltic pump. Before starting the feed, high-purity nitrogen (purity ≥99.999%) was introduced into the drying system to replace the oxygen concentration in the system to ≤100ppm, forming an inert protective atmosphere. This step is crucial: silicon nitride readily reacts with oxygen at high temperatures to form SiO2 (Si3N4+3O2→3SiO2+2N2), and the formation of the surface oxide layer will hinder the subsequent interfacial bonding between rare earth oxides and silicon nitride; controlling the oxygen content below 100ppm can effectively inhibit oxidation and ensure the purity and adhesion of the coating layer.

[0032] A centrifugal atomizer was used, with a rotation speed set at 20,000~25,000 rpm (22,000 rpm in this example) to atomize the slurry into micron-sized droplets of 10~100 μm. Simultaneously, the inlet temperature of the hot nitrogen was controlled at 180~220℃ (200℃ in this example), the outlet temperature at 90~110℃ (100℃ in this example), and the feed rate at 5~15 mL / min (laboratory scale) or adjusted according to equipment capacity. After the droplets came into countercurrent contact with the hot nitrogen, the water evaporated rapidly within 0.1~1 s, and the rare earth nitrates dissolved in the water crystallized in situ due to supersaturation, depositing and coating the surface of the silicon nitride particles to form a uniform coating layer. During this process, the inlet air temperature must be controlled between 180 and 220℃: if the temperature is too low (<180℃), moisture evaporation will be incomplete, the powder will easily stick to the wall, and the moisture content of the coating layer will be too high, causing water vapor to escape during subsequent calcination and damaging the integrity of the coating layer; if the temperature is too high (>220℃), rare earth nitrates may decompose prematurely or cause localized oxidation of the silicon nitride surface. Maintaining the outlet air temperature at 90~110℃ can ensure that the moisture content of the dried powder is ≤0.5wt%, which serves as the criterion for the drying endpoint.

[0033] The dried powder enters a cyclone separator and a bag filter along with the exhaust gas. After two-stage separation and collection, rare earth nitrate-coated silicon nitrate powder is obtained with a yield of ≥99%. Water vapor in the exhaust gas is condensed and recovered (recovery rate ≥98%), and non-condensable gases (mainly nitrogen) are purified and recycled, achieving energy saving and consumption reduction.

[0034] S3, Calcination Conversion and Post-treatment The rare earth nitrate-coated silicon nitride powder obtained in S2 was placed in an atmosphere furnace and calcined under a gradient temperature rise in air or high-purity nitrogen atmosphere. In this embodiment, a high-purity nitrogen atmosphere was selected to further prevent oxidation of the silicon nitride surface. The specific heating program was as follows: first, the temperature was increased from room temperature to 200°C at a heating rate of 1°C / min and held for 1 hour. This stage was used to remove residual adsorbed water and crystal water in the powder to avoid bubbling of the coating layer due to violent vaporization of water vapor during subsequent heating. Then, the temperature was increased to 500°C at a heating rate of 2°C / min and held for 2-3 hours (2.5 hours in this embodiment). Within this temperature range, the rare earth nitrate was completely decomposed: 2RE(NO3)3 → RE2O3 + 6NO2↑ + 3O2↑. A slow heating rate of 2℃ / min allows the gases (NO2, O2) produced during decomposition to escape at a gradual pace, preventing the coating layer from cracking or peeling due to rapid gas release. This ensures that the rare earth oxide coating layer formed after decomposition remains continuous, dense, and crack-free. If the heating rate is too fast (e.g., >5℃ / min), a large amount of gas will be generated in a short time, making the coating layer prone to cracking and peeling, resulting in partial exposure of the silicon nitride surface and loss of uniform coating effect.

[0035] After calcination, the powder was allowed to cool naturally to room temperature and then removed. Since slight adhesion or soft agglomeration may occur between particles during calcination, post-processing was necessary: ​​the powder was fed into an air jet mill and lightly crushed with compressed air at 0.3–0.6 MPa to break up the agglomerates; then it was sieved through a vibrating screen (80–250 mesh, 150 mesh in this example) to control the median particle size D50 of the powder to 50–100 μm. After sieving, Si3N4@Y2O3 coated silicon nitride powder with a core-shell structure was obtained. Transmission electron microscopy and energy dispersive spectroscopy analysis showed that the Y2O3 coating was continuous, uniform, and undamaged, with a coverage of nearly 100% and a thickness of approximately 10–50 nm (approximately 20 nm in this example), tightly adhering to the surface of the silicon nitride particles.

[0036] Using this coated powder in the subsequent forming and sintering of silicon nitride ceramics (such as dry pressing, cold isostatic pressing, hot pressing, or gas pressure sintering) can significantly improve the densification behavior and mechanical properties of the ceramics. Testing showed that the silicon nitride ceramics prepared using the powder from this embodiment exhibited a flexural strength ≥900 MPa and a fracture toughness ≥8 MPa·m. 1 / 2 Furthermore, the coefficient of variation (CV) between batches is ≤3%, which is far superior to the traditional wet ball milling mixing process (CV≥8%), indicating that the method of the present invention has made substantial progress in terms of coating uniformity and product stability.

[0037] Example 2 The preparation process of this embodiment is basically the same as that of Example 1. The difference lies in the type and amount of rare earth nitrates used, as well as the adjustment of some process parameters, in order to demonstrate the good adaptability of the method of the present invention to different rare earth additive systems.

[0038] S1, Slurry Preparation Weigh out La(NO3)3·6H2O with a purity of 99.99% as a rare earth nitrate. The amount added, calculated as La2O3, is 5-10 wt% of the silicon nitride powder (preferably 8 wt% in this embodiment). Add La(NO3)3·6H2O to deionized water with a resistivity of 18.3 MΩ·cm, and stir at 300-500 r / min for 45 min at room temperature until completely dissolved, forming a clear and transparent precursor solution. The amount of deionized water added is 100-150 wt% of the silicon nitride powder (preferably 120 wt% in this embodiment).

[0039] α-Si3N4 powder (purity ≥99.5%, median particle size D50 = 0.5~1.0 μm, oxygen content ≤1.5 wt%, in this embodiment D50 = 0.8 μm, oxygen content 1.3 wt%) and dispersant polyethylene glycol (PEG, molecular weight 400) were added to the precursor solution. The amount of dispersant added was 0.5~1.0 wt% of the mass of silicon nitride powder (preferably 0.8 wt% in this embodiment). The mixture was first pre-dispersed by high-speed stirring at 1000~2000 rpm (1500 rpm in this embodiment) for 30 min, then transferred to a sand mill and ground for 3 h using φ0.8~1.2 mm zirconia beads as the grinding medium until the slurry D90 ≤ 2 μm. After grinding, vacuum degassing was performed for 30 min, followed by aging for 1 h to obtain a stable suspension slurry. The slurry was tested and found to have a solid content of 25-35 wt% (approximately 30 wt% in this example), a room temperature viscosity of 50-200 mPa·s (approximately 110 mPa·s in this example), and no stratification or sedimentation after standing for 24 hours, thus meeting the requirements for spray drying.

[0040] S2, Spray drying coating The slurry was fed into a closed-loop spray dryer at a feed rate of 10 mL / min. High-purity nitrogen (≥99.999%) was pre-purified into the system to control the oxygen content to ≤100 ppm. A centrifugal atomizer was used at 22000 rpm to atomize the slurry into droplets of 10–80 μm. The inlet air temperature was set to 200℃, and the outlet air temperature was controlled to 100℃. The droplets were rapidly dried in the hot nitrogen stream. After moisture evaporation, La(NO3)3 was deposited in situ and uniformly coated on the surface of Si3N4 particles. The dried powder was collected by a cyclone separator and a bag filter, with a yield ≥99% and a powder moisture content ≤0.5 wt%.

[0041] S3, Calcination Conversion and Post-treatment The collected La(NO3)3-coated Si3N4 powder was placed in an atmosphere furnace and subjected to gradient calcination under high-purity nitrogen protection. The heating program was as follows: the temperature was increased to 200℃ at a rate of 1℃ / min and held for 1 hour (to remove adsorbed water and water of crystallization); then the temperature was increased to 500℃ at a rate of 2℃ / min and held for 2.5 hours to completely decompose La(NO3)3 into La2O3 (2La(NO3)3→La2O3+6NO2↑+3O2↑). Slow heating was used to ensure stable gas escape and to avoid cracking of the coating layer. After calcination, the powder was allowed to cool naturally to room temperature.

[0042] The calcined powder was fed into an air jet mill and lightly crushed with compressed air at 0.3–0.6 MPa to break up soft agglomerates. It was then sieved through a 150-mesh vibrating screen to control the median particle size D50 to be 50–100 μm (approximately 80 μm in this example), yielding Si3N4@La2O3 core-shell coated powder. Transmission electron microscopy showed that the La2O3 coating layer was continuous, uniform, and dense, with a thickness of approximately 20–40 nm (approximately 30 nm in this example), tightly adhering to the surface of the silicon nitride particles. The silicon nitride ceramic prepared using the powder from this example exhibited a flexural strength ≥930 MPa and a fracture toughness ≥8.3 MPa·m. 1 / 2 The batch performance coefficient of variation (CV) is ≤2.8%.

[0043] Example 3 This embodiment uses a composite system of Y(NO3)3·6H2O and La(NO3)3·6H2O, and adjusts the spray drying and calcination parameters to demonstrate the synergistic effect of composite rare earth oxide coating on ceramic performance.

[0044] S1, Slurry Preparation Weigh out Y(NO3)3·6H2O and La(NO3)3·6H2O with a purity of 99.99%, and compound them at a Y2O3 to La2O3 mass ratio of 3:2. The total amount of the two nitrates added is 5-10 wt% of the mass of the silicon nitride powder (preferably 10 wt% in this embodiment), based on the composite rare earth oxides. Add the above rare earth nitrates to deionized water with a resistivity of 18.5 MΩ·cm, and stir at 400 r / min for 60 min at room temperature until completely dissolved to form a clear and transparent precursor solution. The amount of deionized water added is 100-150 wt% of the mass of the silicon nitride powder (preferably 150 wt% in this embodiment).

[0045] α-Si3N4 powder (purity ≥99.5%, median particle size D50 = 0.5~1.0 μm, oxygen content ≤1.5 wt%, in this embodiment D50 = 1.0 μm, oxygen content 1.4 wt%) and dispersant ammonium polyacrylate (PAA-NH4) were added to the precursor solution at an amount of 0.5~1.0 wt% of the silicon nitride powder mass (preferably 1.0 wt% in this embodiment). The mixture was first pre-dispersed by high-speed stirring at 2000 rpm for 30 min, then transferred to a sand mill and ground for 4 h using φ1.2 mm zirconia beads as the grinding medium until the slurry D90 ≤ 2 μm. After grinding, the mixture was vacuum degassed for 30 min and aged for 1 h to obtain a stable suspension slurry. The slurry was tested and found to have a solid content of 25-35 wt% (approximately 35 wt% in this example), a room temperature viscosity of 50-200 mPa·s (approximately 180 mPa·s in this example), and no stratification or sedimentation was observed after standing for 24 hours.

[0046] S2, Spray drying coating The slurry was fed into a closed-loop spray dryer at a feed rate of 15 mL / min. High-purity nitrogen was used to purge the oxygen content to ≤100 ppm. A centrifugal atomizer was used at 25,000 rpm to atomize the slurry into droplets of 10–100 μm. The inlet air temperature was set to 220℃, and the outlet air temperature was controlled at 110℃. The droplets were rapidly dried in the hot nitrogen stream. After moisture evaporation, composite rare earth nitrates were deposited in situ and uniformly coated on the surface of Si3N4 particles. The dried powder was collected by a cyclone separator and a bag filter, with a yield ≥99% and a powder moisture content ≤0.5 wt%.

[0047] S3, Calcination Conversion and Post-treatment The collected composite rare earth nitrate-coated Si3N4 powder was placed in an atmosphere furnace and subjected to gradient calcination in air (calcination in air promotes complete decomposition of organic matter, and the oxidation of the silicon nitride matrix is ​​negligible due to the dense coating layer). The heating program was as follows: heating to 200℃ at 1℃ / min and holding for 1 hour (to remove adsorbed water and water of crystallization); then heating to 500℃ at 2℃ / min and holding for 3 hours (to ensure complete decomposition of the two nitrates, generating Y2O3 and La2O3). The gradient heating ensured the smooth escape of gaseous products and the integrity of the coating layer. After calcination, the powder was naturally cooled to room temperature.

[0048] The calcined powder was fed into an air jet mill and lightly crushed with 0.6 MPa compressed air to break up soft agglomerates. It was then sieved through a 150-mesh vibrating screen to control the median particle size D50 to be 50-100 μm (approximately 100 μm in this example), yielding Si3N4@(Y2O3-La2O3) core-shell structured coated powder. Transmission electron microscopy showed that the composite rare earth oxide coating was continuous, uniform, and dense, with a thickness of approximately 35-50 nm (approximately 45 nm in this example), and the coating layer bonded well to the substrate. The silicon nitride ceramic prepared using the powder from this example exhibited a flexural strength ≥980 MPa and a fracture toughness ≥8.8 MPa·m. 1 / 2 The batch performance coefficient of variation (CV) is ≤2.5%. Compared with single Y2O3 or La2O3 coating, the composite rare earth oxide system further optimizes the grain boundary phase composition and significantly improves the mechanical properties of ceramics.

[0049] Comparative Example 1 (Traditional wet ball milling mixing method) α-Si3N4 powder and La2O3 powder (8wt%) of the same specifications as in Example 2 were taken, and deionized water and dispersant PEG (0.8wt%) were added. The mixture was then wet-milled (φ1.0mm zirconia beads) for 3 hours to obtain a slurry. The slurry was dried, crushed, and sieved to obtain a mixed powder. This powder was used for sintering silicon nitride ceramics, and the resulting ceramics had a flexural strength of 750 MPa and a fracture toughness of 7.0 MPa·m. 1 / 2 The coefficient of variation (CV) for performance is 8.5%.

[0050] Comparative Example 2 (Coprecipitation Coating) Following the method disclosed in CN116217244A, aluminum nitrate, yttrium nitrate, and ferric chloride (molar ratio 5:2.5:0.01) were added to deionized water to form a precursor solution. Urea (molar ratio to the precursor mixture 20:1) was added, followed by α-Si3N4 powder of the same specifications as in Example 2 (molar ratio 28:5.56). After ultrasonic dispersion, the mixture was kept in a water bath at 80°C for 3 hours. The powder was then centrifuged, washed twice with water, and centrifuged twice with alcohol. After drying at 60°C, the powder was calcined at 1150°C to obtain the coated powder. This powder was used for sintering silicon nitride ceramics, resulting in a ceramic with a flexural strength of 880 MPa and a fracture toughness of 8.0 MPa·m. 1 / 2 The coefficient of variation (CV) was 4.2%, and the process control was complex, with production efficiency only 60% of that of Example 2.

[0051] As can be seen from the comparison between Examples 1-3 and Comparative Examples 1-2, the coated silicon nitride powder prepared by the method of the present invention has significantly better mechanical properties and batch stability than the traditional wet ball milling mixing method and co-precipitation method when used for ceramic sintering. Moreover, the process is simpler, the production efficiency is higher, there are no impurity residues, and it is more suitable for large-scale industrial applications.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a surface coating of silicon nitride powder by rare earth nitrate spray drying, characterized in that, Includes the following steps: S1. Add rare earth nitrates to deionized water to dissolve them and form a precursor solution; Silicon nitride powder and dispersant are added to the precursor solution, and after dispersion and grinding, a stable suspension slurry is obtained. S2. The slurry obtained in S1 is fed into a closed-loop spray dryer, atomized under an inert atmosphere, and dried by contact with a hot inert airflow, so that rare earth nitrates are deposited and coated on the surface of silicon nitride powder to obtain rare earth nitrate coated silicon nitride powder. S3. The coated powder obtained in S2 is placed in an atmosphere furnace and calcined to decompose the rare earth nitrates into rare earth oxides. After post-processing, Si3N4@RE2O3 core-shell structure coated silicon nitride powder is obtained.

2. The preparation method according to claim 1, characterized in that, The rare earth nitrates in S1 are selected from one or a combination of two of Y(NO3)3·6H2O and La(NO3)3·6H2O, with a purity ≥99.99%; the rare earth nitrates are calculated as rare earth oxides RE2O3, and the amount added is 5~10wt% of the mass of silicon nitride powder.

3. The preparation method according to claim 2, characterized in that, The rare earth nitrate is a composite system of Y(NO3)3·6H2O and La(NO3)3·6H2O, wherein the mass ratio of Y2O3 to La2O3 is 3:

2.

4. The preparation method according to claim 1, characterized in that, The silicon nitride powder in S1 is mainly composed of α-Si3N4, with a purity ≥99.5%, median particle size D50 = 0.5~1.0μm, and oxygen content ≤1.5wt%; the amount of deionized water added is 100~150wt% of the mass of silicon nitride powder, and the solid content of the slurry is 25~35wt%.

5. The preparation method according to claim 1, characterized in that, The dispersant mentioned in S1 is selected from ammonium polyacrylate or polyethylene glycol, and the amount added is 0.5~1.0wt% of the silicon nitride powder mass; the grinding and refining process uses zirconia beads as the medium and grinds until the slurry particle size D90≤2μm.

6. The preparation method according to claim 1, characterized in that, The inert atmosphere described in S2 is high-purity nitrogen with an oxygen content ≤100ppm; the atomization uses a centrifugal atomizer with a rotation speed of 20000~25000rpm or a pressure atomizer with a pressure of 0.4~0.8MPa; the inlet air temperature for drying is 180~220℃ and the outlet air temperature is 90~110℃.

7. The preparation method according to claim 1, characterized in that, The calcination described in S3 is a gradient heating calcination: the temperature is increased to 200℃ at a heating rate of 1℃ / min and held for 1h; then the temperature is increased to 500℃ at a heating rate of 2℃ / min and held for 2~3h; the calcination atmosphere is air or high-purity nitrogen.

8. The preparation method according to claim 1, characterized in that, The post-processing described in S3 includes air jet milling and vibrating screen sieving, and the median particle size D50 of the resulting coated silicon nitride powder is 50~100μm; in the Si3N4@RE2O3 core-shell structure, the thickness of the rare earth oxide coating layer is 10~50nm, and the coating layer is continuous and undamaged.

9. The preparation method according to claim 1, characterized in that, S1 also contains a binder, which is polyvinyl alcohol, and the amount added is 0.5~1.0 wt% of the silicon nitride powder.

10. A Si3N4@RE2O3 core-shell structured coated silicon nitride powder prepared by the method according to any one of claims 1 to 9.