High-purity silicon hollow microbeads and a method for preparing the same

Abstract

This invention discloses a high-purity silicon hollow microsphere and its preparation method, belonging to the field of high-performance ceramic material preparation technology. High-purity silicon powder is modified by coating with rare earth nitrates to form a composite silicon powder with rare earth elements coated on the surface. Organic polymer microspheres are used as pore-forming agents, and the coated silicon powder is uniformly coated onto the surface of the pore-forming agent through a sphere-to-sphere process, forming a composite microsphere green body with a core-shell structure. After cold isostatic pressing for pre-densification, the green body is placed in a hot isostatic pressing sintering furnace, where a gradient temperature core-removal process is used to remove the pore-forming agent. Simultaneously, silicon powder nitriding and shell densification are completed under a high-temperature, high-pressure nitrogen atmosphere to obtain hollow silicon nitride microspheres. This invention integrates the three steps of pore-forming agent removal, silicon powder nitriding, and shell densification into the same HIP sintering process, solving the problems of complex traditional hollow microsphere preparation processes, incomplete nitriding, and low shell density. The apparent density of the product is 0.8~1.2 g / cm³. 3 Compressive strength ≥600MPa, shell density ≥99.5%.

Description

Technical Field

[0001] This invention relates to the field of high-performance ceramic material preparation technology, and more particularly to a high-purity silicon hollow microsphere and its preparation method. Background Technology

[0002] Hollow silicon nitride microspheres, due to their excellent properties such as low density, high strength, high temperature resistance, and corrosion resistance, have broad application prospects in aerospace lightweight structural materials, high-temperature thermal insulation materials, catalyst supports, and high-performance composite materials. Traditional methods for preparing hollow ceramic microspheres mainly include template methods, spray drying methods, and self-propagating reaction methods.

[0003] In the prior art, CN119977623A discloses a method for preparing porous silicon nitride ceramic materials with large pore size and high porosity. This method uses α-Si3N4 powder as a matrix, polymethyl methacrylate microspheres as a pore-forming agent, and an alcohol solution of phenolic resin as a binder. Silicon nitride powder is coated onto the surface of the pore-forming agent using a rolling mill method, followed by curing, carbonization, debinding, and gas pressure sintering to obtain porous silicon nitride ceramics. While this technology can obtain a regular spherical pore structure, it has the following drawbacks: First, the raw material is pre-formed α-Si3N4 powder, not silicon powder, resulting in higher costs; second, the sintering method is gas pressure sintering rather than hot isostatic pressing, leading to limited shell density and difficulty in obtaining high-strength hollow microspheres; third, the sintering activity of silicon nitride powder is low, requiring the addition of a large amount of sintering aids.

[0004] CN101215183A discloses a method for preparing porous silicon nitride ceramics, using fly ash hollow microspheres as pore-forming agents and sintering aids. These microspheres are mixed with composite silicon nitride powder and then subjected to dry pressing, cold isostatic pressing, and atmosphere sintering to prepare porous silicon nitride ceramics. Although this technology utilizes natural hollow microspheres for pore formation, the pore-forming agent itself is hollow, making it impossible to flexibly control the size and shell thickness of the hollow cavities. Furthermore, the fly ash hollow microspheres contain numerous impurities, affecting product purity and performance consistency.

[0005] CN104326766A discloses a method for preparing porous silicon nitride ceramic materials with spherical pore structures. The method involves using a spray drying process to prepare composite microspheres of silicon nitride powder and PMMA pore-forming agent, followed by cold isostatic pressing and debinding sintering to obtain the porous silicon nitride material. While this technique can achieve a spherical pore structure, the pore-forming agent is uniformly distributed throughout the entire preform, resulting in interconnected or semi-interconnected porous structures rather than independent, sealed hollow cavities. Furthermore, the sintering method is gas pressure sintering rather than hot isostatic pressing, making it difficult to obtain high-density pore walls.

[0006] In addition, the above-mentioned existing technologies all have the following common problems: First, the cost of silicon nitride powder raw materials is high, which is not conducive to large-scale application; second, the mixing method of sintering aid and matrix powder is simple, making it difficult to achieve uniform dispersion at the nanoscale; third, the removal of pore-forming agent and densification of matrix are carried out in steps, which is complex and prone to defects; fourth, it is impossible to accurately control the size, shell thickness and airtightness of hollow cavity.

[0007] Therefore, developing a preparation method that can simultaneously achieve pore-forming agent removal, silicon powder nitriding, and shell densification using low-cost, high-purity silicon powder as raw material, and can precisely control the hollow structure, has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0008] To address the aforementioned technical problems, this invention provides a method for preparing high-purity silicon hollow microspheres. The invention uses high-purity silicon powder as raw material and achieves nanoscale uniform pre-distribution of rare earth sintering aids through rare earth nitrate coating modification. Organic polymer microspheres are used as pore-forming agents, and a sphere-to-sphere process is employed to form a composite microsphere green body with the pore-forming agent as the core and the coated silicon powder as the shell. After cold isostatic pressing pre-densification, three consecutive processes—gradient heating core removal, silicon powder nitriding, and shell densification—are sequentially completed in the same HIP sintering process, ultimately yielding silicon nitride hollow microspheres with a single sealed hollow cavity.

[0009] The technical means employed in this invention are as follows:

[0010] A method for preparing high-purity silicon hollow microspheres includes the following steps: S1: High-purity silicon powder is modified by coating with rare earth nitrates to obtain modified silicon powder with a rare earth element coating layer on the surface. S2: The modified silicon powder obtained in step S1 is mixed with a pore-forming agent, and the modified silicon powder is uniformly coated on the surface of the pore-forming agent through a sphere-forming process to form a composite microsphere green body with a core-shell structure; the pore-forming agent is an organic polymer microsphere that can be completely removed during sintering. S3: The composite microsphere green body obtained in step S2 is subjected to cold isostatic pressing to obtain a pre-densified green body; S4: The pre-densified green blank obtained in step S3 is placed in a hot isostatic pressing sintering furnace and sintered in an integrated manner under a nitrogen-containing atmosphere and a high-pressure inert atmosphere. The sintering process includes a gradient heating and core removal stage, a nitriding reaction stage and a densification sintering stage, to obtain high-purity silicon hollow microspheres.

[0011] Further, the high-purity silicon powder in step S1 has a purity of ≥99.99% and a median particle size D50 of 0.3~0.5μm; the rare earth nitrate is selected from one or a mixture of two of Y(NO3)3·6H2O or La(NO3)3·6H2O, and the amount added is 5~8wt% of the mass of the high-purity silicon powder.

[0012] Further, the coating modification process in step S1 specifically includes: dissolving rare earth nitrates in deionized water to form a rare earth nitrate solution, adding high-purity silicon powder and dispersant, forming a uniform slurry after high-speed stirring and ultrasonic dispersion, and then spray drying to obtain coated modified silicon powder.

[0013] Further, the pore-forming agent mentioned in step S2 is one or more of polymethyl methacrylate microspheres, polystyrene microspheres or starch microspheres, with a particle size D50 of 50~200μm and a sphericity ≥0.95; the amount of pore-forming agent added is 20~40wt% of the modified silica powder mass.

[0014] Furthermore, the ball forming process in step S2 adopts a rolling ball forming machine with a turntable speed of 140~180 r / min and a liquid spraying volume of 4~7 kg / h. A binder is added during the forming process, which is an ethanol solution of phenolic resin with a mass concentration of 50~70%.

[0015] Furthermore, in step S3, the cold isostatic pressing treatment pressure is 100~130MPa, the holding time is 30~35min, the pressurization rate is 3~4MPa / min, and the depressurization rate is 2~3MPa / min.

[0016] Furthermore, the specific process of the gradient heating and core removal stage in step S4 is as follows: the temperature is raised from room temperature to 200℃ at a heating rate of 1~3℃ / min and held for 1~2h, then raised to 400℃ at a heating rate of 2~4℃ / min and held for 2~3h, and then raised to 600℃ at a heating rate of 3~5℃ / min and held for 2~3h; the pressure inside the furnace during the core removal stage is controlled at 0.05~0.1MPa, and inert gas is continuously introduced.

[0017] Further, the process conditions for the nitriding reaction stage in step S4 are as follows: heating from 600℃ to 1100-1200℃ at a heating rate of 3-5℃ / min, nitrogen pressure in the furnace is 0.1-0.2MPa, and holding for 5-7h; the process conditions for the densification sintering stage are as follows: heating from 1100-1200℃ to 1700-1800℃ at a heating rate of 2-4℃ / min, argon pressure in the furnace is 90-130MPa, and holding for 4-5h.

[0018] Furthermore, the cooling process after sintering adopts a gradient cooling: the cooling rate is 1.5~2℃ / min in the 1700~1200℃ range, the cooling rate is 2~3℃ / min in the 1200~800℃ range, and the cooling rate from 800℃ to room temperature is 3~5℃ / min; during the cooling process, the furnace pressure is maintained at 90~130MPa until the temperature drops to 800℃, and then the pressure is gradually released.

[0019] On the other hand, the present invention also discloses high-purity silicon hollow microspheres prepared by the above method, wherein the hollow microspheres have a single closed hollow cavity, an outer shell density ≥99.5%, and an apparent density of 0.8~1.2 g / cm³. 3 Compressive strength ≥600MPa, hardness ≥88HRA, fracture toughness ≥6.5MPa·m 1 / 2 The sphericity is ≥0.9, and the ratio of shell thickness to inner diameter is 0.15~0.3.

[0020] Compared with the prior art, the present invention has the following advantages: 1. This invention integrates three core steps—gradual removal of pore-forming agent, in-situ nitriding of silicon powder, and HIP densification of the outer shell—into a single hot isostatic pressing furnace cavity for continuous completion. This eliminates the need for intermediate transfer of the billet, significantly simplifying the process and avoiding billet damage and contamination caused by transfer.

[0021] 2. This invention uses high-purity silicon powder (4N grade) as raw material, instead of the α-Si3N4 powder used in the prior art. The price of silicon powder is much lower than that of silicon nitride powder, and the silicon powder can be completely nitrided by catalyzing the nitriding reaction with rare earth nitrate coating, which greatly reduces the cost of raw materials while ensuring product performance.

[0022] 3. Unlike existing technologies that use ball milling to mechanically mix sintering aids with matrix powder, this invention uses a rare earth nitrate solution coating-spray drying process to form a uniform nanoscale coating layer on the silicon powder surface. This coating layer decomposes in situ into rare earth oxides during nitriding, achieving atomically uniform distribution of the sintering aids on the silicon nitride particle surface, far superior to mechanical mixing methods. This contributes to the formation of a uniform grain boundary phase, improving the density and mechanical properties of the outer shell.

[0023] 4. This invention employs hot isostatic pressing (HIP) sintering with high-pressure argon gas as the pressurizing medium. The preform is isotropically pressurized at 1700-1800℃ and 90-130MPa, achieving a shell density of over 99.5%. This differs from existing technologies that use gas pressure sintering, which operates at only 0.1-2MPa and struggles to achieve the same pore density. The highly dense shell imparts excellent mechanical properties to the hollow microspheres.

[0024] 5. This invention addresses the thermal decomposition characteristics of pore-forming agents by designing a three-stage gradient heat preservation and core-removal process at 200℃, 400℃, and 600℃. The slow heating allows the pore-forming agent to soften, decompose, and volatilize in stages, ensuring stable gas escape and effectively preventing shell rupture due to rapid gas release. Simultaneously, a slight positive pressure and continuous inert gas purging are maintained during the core-removal stage to promptly expel decomposed gases, further guaranteeing the integrity of the hollow structure.

[0025] 6. By adjusting the particle size and dosage of the pore-forming agent, hollow microspheres of different sizes and shell thicknesses can be prepared, with the shell thickness / inner diameter ratio controlled within the range of 0.15~0.3. The apparent density of the product is 0.8~1.2 g / cm³. 3 Compressive strength ≥ 600 MPa, fracture toughness ≥ 6.5 MPa·m 1 / 2 With a sphericity ≥0.9, it meets the application requirements of high-end fields such as aerospace lightweight structural materials and high-temperature thermal insulation materials. 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 example provides a method for preparing high-purity silicon hollow microspheres. The method uses high-purity silicon powder as raw material and involves steps such as rare earth nitrate coating modification, spheroidization, cold isostatic pressing pre-densification, and HIP integrated sintering to prepare silicon nitride hollow microspheres with a single sealed hollow cavity. The specific implementation steps are as follows.

[0028] S1: High-purity silicon powder is modified by coating with rare earth nitrates to obtain modified silicon powder with a rare earth element coating layer on the surface. High-purity silicon powder is selected as the raw material, with a purity of not less than 99.99%, a median particle size D50 of 0.3~0.5μm (preferably 0.4μm in this embodiment), and a specific surface area of ​​15~25m². 2 / g, loose bulk density 0.8~1.0g / cm³ 3 The fluidity is ≥15s / 50g. Rare earth nitrates are selected from a mixture of Y(NO3)3·6H2O and La(NO3)3·6H2O (mass ratio 1:1), with a purity ≥99.99%, and the total addition amount is 5~8wt% of the high-purity silicon powder (preferably 6.5wt% in this embodiment). The dispersant is polyethylene glycol (PEG-6000), added at 0.3~0.5% of the high-purity silicon powder (0.4% in this embodiment). The pore-forming agent is polymethyl methacrylate microspheres, with a median particle size D50 of 50~200μm (preferably 100μm in this embodiment), sphericity ≥0.95, thermal decomposition temperature range of 200~600℃, and decomposition residue ≤0.1%. The binder is an ethanol solution of phenolic resin with a mass concentration of 50-70% (preferably 60% in this embodiment), and the amount added is 15-20 wt% of the modified silica powder (18 wt% in this embodiment). The pore-forming agent and dispersant is sodium dodecylbenzenesulfonate, and the amount added is 0.5-1.0% of the pore-forming agent (0.8% in this embodiment).

[0029] The rare earth nitrates were dissolved in deionized water in a certain proportion (the mass ratio of rare earth nitrates to deionized water was 1:10 to 1:15, 1:12 in this example), and a dispersant was added. The mixture was stirred at 50 to 60°C (55°C in this example) and 300 to 500 r / min (400 r / min in this example) for 45 to 60 minutes until completely dissolved, forming a transparent and uniform rare earth nitrate solution.

[0030] High-purity silicon powder was added to the above solution (the mass ratio of silicon powder to rare earth nitrate solution was 2:1 to 3:1, 2.5:1 in this example), and high-speed stirring was started (800 to 1000 r / min, 900 r / min in this example) for 60 to 90 minutes (75 minutes in this example). Simultaneously, ultrasonic dispersion (500 W power, 20 kHz frequency) was activated for 30 minutes to assist dispersion. The solid content of the slurry was adjusted to 40 to 50% (45% in this example), and filtered through a 100-mesh sieve to obtain a uniform and stable coating slurry with a viscosity of 500 to 800 mPa·s (approximately 650 mPa·s in this example).

[0031] The above slurry was fed into a centrifugal spray dryer for drying. The equipment parameters were as follows: inlet air temperature 200~220℃ (210℃ in this example), outlet air temperature 80~100℃ (90℃ in this example), feed rate 10~15kg / h (12kg / h in this example), atomization pressure 0.3~0.5MPa (0.4MPa in this example), and atomizer speed 20000~25000r / min (22000r / min in this example). After drying, modified silicon powder with rare earth nitrate coating was obtained, with a coating thickness of 20~50nm (approximately 35nm in this example), particle size of 1~5mm, bulk density of 0.68~0.87g / cm³, and porosity of 50~70%. The modified silicon powder was vacuum dried at 100℃ for 1h and then sealed with nitrogen for later use.

[0032] S2: The modified silicon powder obtained in step S1 is mixed with a pore-forming agent, and the modified silicon powder is uniformly coated on the surface of the pore-forming agent through a sphere-forming process to form a composite microsphere green body with a core-shell structure; the pore-forming agent is an organic polymer microsphere that can be completely removed during sintering. The modified silicon powder and pore-forming agent were mixed at a mass ratio (modified silicon powder:pore-forming agent = 60:40~80:20, 70:30 in this example). The pore-forming agent was pre-dispersed: the pore-forming agent was placed in a forced-air drying oven and dried at 60°C for 1 hour, then deionized water and pore-forming agent dispersing aid (SDBS) were added, and ultrasonically dispersed at a frequency of 40kHz and a power of 200W for 15~20 minutes to prepare a pore-forming agent dispersion, ensuring that the pore-forming agent was uniformly dispersed and free from agglomeration.

[0033] A rolling ball-forming machine was used, with a turntable speed of 140-180 r / min (160 r / min in this example). The pore-forming agent was placed in the turntable, and the turntable was started to atomize and spray the binder (an ethanol solution of phenolic resin) onto the surface of the pore-forming agent, ensuring uniform wetting. Then, modified silica powder was sprinkled in, and the machine was rolled to evenly coat the surface of the pore-forming agent with the silica powder. This process of spraying the binder and sprinkling the silica powder was repeated, with each spray using 8-12% (10% in this example) of the total binder amount, until all the modified silica powder was coated, forming a composite microsphere green body with a pore-forming agent core and a coated silica powder shell structure.

[0034] The molded composite microspheres were dried using a gradient drying method: 60℃ for 2 hours → 80℃ for 2 hours → 90℃ for 2 hours, for a total drying time of 6 hours. After drying, the microspheres were passed through a double-layer sieve of 100 mesh and 500 mesh to screen for green microspheres with a particle size of 120~520μm, allowing for 10~15% sintering shrinkage, and discarding damaged, deformed, or pore-forming microspheres.

[0035] S3: The composite microsphere green body obtained in step S2 is subjected to cold isostatic pressing to obtain a pre-densified green body; The graded composite microsphere preforms were packed into polyurethane sealed bags (bag thickness 0.15~0.25mm, 0.2mm in this example), and evenly spread to a thickness ≤50mm. The sealed bags were then filled with inert gas (argon) at a pressure of 0.05~0.1MPa (0.08MPa in this example), and then evacuated to -0.09MPa before sealing.

[0036] The sealed bag was placed in a wet cold isostatic press, with an emulsion as the working medium. The pressurization rate was 3~4 MPa / min (3.5 MPa / min in this example), increasing the pressure to 100~130 MPa (115 MPa in this example), and holding the pressure for 30~35 min (32 min in this example). The depressurization rate was 2~3 MPa / min (2.5 MPa / min in this example), reducing the pressure to atmospheric pressure. The pre-densified green body was removed, and the surface residual emulsion was washed off with deionized water. It was then placed in a vacuum drying oven and dried for 2 hours at 100℃ and a vacuum degree ≤-0.09 MPa to remove surface moisture. The density of the pre-densified green body was measured to be 1.8~2.0 g / cm³ (1.9 g / cm³ in this example). 3 The outer shell has a density of ≥85%, with no cracks or leakage of pore-forming agent.

[0037] S4: The pre-densified green blank obtained in step S3 is placed in a hot isostatic pressing sintering furnace and sintered in an inert atmosphere and under high temperature and high pressure conditions. The sintering process includes a gradient heating core removal stage, a nitriding reaction stage and a densification sintering stage to obtain high-purity silicon hollow microspheres. The pre-densified green body is evenly spread in a graphite crucible with a thickness of ≤40mm. The inner wall of the crucible is pre-coated with high-purity boron nitride (BN) powder with a thickness of 1.5~2.5mm (2mm in this example) to prevent sticking. The crucible is placed in a high-temperature and high-pressure HIP sintering furnace, and the furnace door is closed.

[0038] (1) Gradient heating and core removal stage: Start the vacuum system and evacuate the vacuum in the furnace to ≤5×10 -4 High-purity nitrogen (purity ≥99.999%) is introduced, and the furnace pressure is controlled at 0.05~0.1MPa (0.08MPa in this example). The temperature is increased from room temperature to 200℃ at a rate of 1~3℃ / min (2℃ / min in this example), and held for 1~2h (1.5h in this example) to allow the pore-forming agent to soften initially; the temperature is increased to 400℃ at a rate of 2~4℃ / min (3℃ / min in this example), and held for 2~3h (2.5h in this example) to allow a large amount of pore-forming agent to decompose and volatilize; the temperature is increased to 600℃ at a rate of 3~5℃ / min (4℃ / min in this example), and held for 2~3h (2.5h in this example) to completely remove the pore-forming agent. Nitrogen gas is continuously introduced throughout the core removal process to promptly discharge the decomposed gas from the furnace and prevent it from accumulating inside the microspheres and causing the outer shell to crack.

[0039] (2) Nitriding reaction stage: After core removal, maintain a nitrogen atmosphere and adjust the furnace pressure to 0.1~0.2MPa (0.15MPa in this example). Increase the temperature from 600℃ to 1100~1200℃ (1150℃ in this example) at a rate of 3~5℃ / min (4℃ / min in this example), and hold for 5~7h (6h in this example). During this stage, silicon powder reacts with nitrogen to undergo a nitriding reaction: 3Si + 2N2 → Si3N4. Simultaneously, rare earth nitrates thermally decompose into rare earth oxides: 2RE(NO3)3 → RE2O3 + 6NO2↑ + 3O2↑, uniformly adhering to the surface of the silicon nitride particles in situ. Monitor the furnace atmosphere in real time during the reaction to ensure complete nitriding.

[0040] (3) Densification sintering stage: After nitriding is completed, nitrogen gas is turned off and high-purity argon gas (purity ≥99.999%) is introduced to raise the furnace pressure to 90~130MPa (110MPa in this example). The temperature is raised from 1150℃ to 1700~1800℃ (1750℃ in this example) at a heating rate of 2~4℃ / min (3℃ / min in this example) and held for 4~5h (4.5h in this example). Under high temperature and high pressure, silicon nitride particles rearrange and diffuse, achieving deep densification of the outer shell. At the same time, rare earth oxides act as sintering aids to promote grain boundary bonding and inhibit abnormal grain growth.

[0041] (4) Gradient cooling: After sintering, heating is stopped, and the argon pressure in the furnace is maintained at 90~130MPa (110MPa in this example) until the temperature drops to 800℃. The cooling rate is controlled in segments: the cooling rate in the 1700~1200℃ range is 1.5~2℃ / min (1.8℃ / min in this example), the cooling rate in the 1200~800℃ range is 2~3℃ / min (2.5℃ / min in this example), and the cooling rate in the 800℃ to room temperature range is 3~5℃ / min (4℃ / min in this example). After the temperature drops to 800℃, the pressure is gradually reduced to atmospheric pressure, and the furnace is cooled to room temperature.

[0042] The sintered hollow microspheres were removed and placed in an ultrasonic cleaner with deionized water. The cleaning process was carried out at a frequency of 40 kHz and a power of 250 W for 30-40 minutes (35 minutes in this example) to remove residual BN powder from the surface. After cleaning, the microspheres were first dried in a forced-air drying oven at 80°C for 2 hours, and then transferred to a vacuum drying oven at 120°C for 2 hours.

[0043] The dried hollow microspheres were classified using a vibrating sieve with a vibration frequency controlled to ≤50Hz. The microspheres were then passed sequentially through 50-mesh, 100-mesh, 200-mesh, and 500-mesh sieves, and products of each particle size range were collected. Simultaneously, samples were taken to check the proportion of broken or collapsed microspheres, and defective products were discarded. The final product was high-purity silicon hollow microspheres.

[0044] Testing revealed that the hollow microspheres prepared in this embodiment possess a single, sealed hollow cavity, a sphericity ≥0.92, a shell thickness to inner diameter ratio of 0.2~0.25, and an apparent density of 0.9~1.1 g / cm³. 3 The shell has a density ≥99.5%, compressive strength ≥620MPa, hardness ≥89HRA, and fracture toughness ≥6.8MPa·m. 1 / 2 Silicon nitride purity ≥99%, β phase content ≥92%, residual silicon ≤0.3%. Product particle size is mainly distributed in the range of 120~500μm, and the performance variation coefficient between batches is ≤4%. The product has a regular morphology, without cracks, collapses, or interconnected pores.

[0045] 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 nitrate, the particle size of the pore-forming agent, the cold isostatic pressing pressure and the selection of HIP sintering parameters. The specific implementation is as follows. If there are any technical parameters not mentioned, the scheme of Example 1 will continue.

[0046] High-purity silica powder with a purity ≥99.99% and a median particle size D50 of 0.3~0.5μm (0.4μm in this example) was selected. The rare earth nitrate used was Y(NO3)3·6H2O (purity ≥99.99%), added at 5wt% of the silica powder mass. Polyethylene glycol (PEG-6000) was used as the dispersant at 0.4wt%. Polymethyl methacrylate microspheres were used as the pore-forming agent with a median particle size D50 of 200μm and a sphericity ≥0.95. The binder was an ethanol solution of phenolic resin with a mass concentration of 60%, added at 15wt% of the modified silica powder mass. Sodium dodecylbenzenesulfonate, a pore-forming dispersant, was added at 0.8% of the pore-forming agent mass.

[0047] Y(NO3)3·6H2O was dissolved in deionized water at a mass ratio of 1:12 and stirred at 55℃ and 400 rpm for 45 min until completely dissolved. High-purity silicon powder (silicon powder to rare earth nitrate solution mass ratio 2.5:1) was added, and the mixture was stirred at 900 rpm for 75 min, followed by simultaneous ultrasonic dispersion (500W, 20kHz) for 30 min. The solid content of the slurry was adjusted to 45%, and then filtered through a 100-mesh sieve. Spray drying parameters: inlet air temperature 210℃, outlet air temperature 90℃, feed rate 12 kg / h, atomization pressure 0.4 MPa, and atomizer speed 22000 rpm. Coated modified silicon powder was obtained, with a coating thickness of approximately 30 nm and a particle size of 1–5 mm.

[0048] Modified silica powder and pore-forming agent were mixed at a mass ratio of 75:25. The pore-forming agent was pre-dispersed (dried at 60℃ for 1 h, then ultrasonically dispersed for 15 min at 40 kHz and 200 W with the addition of deionized water and SDBS). A rolling ball-forming machine was used with a turntable speed of 150 r / min. The binder was atomized and sprayed onto the surface of the pore-forming agent, and the modified silica powder was sprinkled in. This process was repeated until all silica powder was coated. The total amount of binder added was 15% of the mass of the modified silica powder. The composite microspheres were subjected to gradient drying (60℃ / 2 h → 80℃ / 2 h → 90℃ / 2 h), and passed through 100-mesh and 500-mesh sieves to obtain green bodies with a particle size of 120~520 μm.

[0049] The green compact was placed in a polyurethane sealed bag (0.2 mm thick), filled with argon gas to 0.08 MPa, and then vacuum-sealed. Cold isostatic pressing was applied at 100 MPa, with a pressurization rate of 3.5 MPa / min, a holding time of 30 min, and a depressurization rate of 2.5 MPa / min. After pre-densification, the green compact was vacuum-dried at 100℃ for 2 h, resulting in a green compact density of approximately 1.85 g / cm³. 3 The outer shell has a density of ≥85% and is free of cracks.

[0050] The pre-densified green body was placed in a graphite crucible (with the inner wall coated with 2 mm of BN powder) and then placed in a HIP sintering furnace.

[0051] (1) Gradient heating and core removal stage: Evacuate to ≤5×10 -4 High-purity nitrogen gas is introduced at a pressure of 0.08 MPa. The heating program is as follows: 1.5℃ / min from room temperature to 200℃, hold for 1.5 h; 2.5℃ / min to 400℃, hold for 2.5 h; 3.5℃ / min to 600℃, hold for 2.5 h. Nitrogen gas is continuously introduced.

[0052] (2) Nitriding reaction stage: Maintain a nitrogen atmosphere and a pressure of 0.15 MPa. Increase the temperature from 600℃ to 1120℃ at a rate of 4℃ / min and hold for 6 hours to complete the nitriding.

[0053] (3) Densification sintering stage: switch to high-purity argon gas and pressurize to 90MPa. Increase to 1700℃ at 3℃ / min and hold for 4h.

[0054] (4) Gradual cooling: Maintain pressure at 90 MPa up to 800℃. Cooling rate: 2℃ / min for 1700~1200℃, 2.5℃ / min for 1200~800℃, and 4℃ / min for below 800℃. Gradually depressurize after 800℃.

[0055] Ultrasonic cleaning (40kHz, 250W, 35min), forced air drying at 80℃ for 2h, vacuum drying at 120℃ for 2h. Grading with a vibrating screen to remove damaged products.

[0056] Testing revealed that the hollow microspheres had a sphericity ≥0.92, a shell thickness / inner diameter ratio of 0.18~0.25, and an apparent density of 1.0~1.2 g / cm³. 3 The shell has a density ≥99.5%, compressive strength ≥610MPa, hardness ≥88HRA, and fracture toughness ≥6.6MPa·m. 1 / 2 Silicon nitride purity ≥99%, β phase content ≥91%, residual silicon ≤0.4%. Product particle size is mainly distributed in 120~500μm, and the performance variation coefficient between batches is ≤5%.

[0057] Example 3 The preparation process of this embodiment is basically the same as that of Embodiment 1. If there are any unmentioned technical parameters, the scheme of Embodiment 1 shall be continued. High-purity silicon powder with a purity ≥99.99% and a median particle size D50 of 0.3~0.5μm (0.45μm in this example) was selected. Rare earth nitrates were selected from a composite system of Y(NO3)3·6H2O and La(NO3)3·6H2O (mass ratio 1:1), with a total addition amount of 8wt% of the high-purity silicon powder. A dispersant was added at 0.5wt%. Polymethyl methacrylate microspheres were selected as the pore-forming agent, with a median particle size D50 of 50μm and a sphericity ≥0.95. The binder had a mass concentration of 70% and was added at 20wt% of the modified silicon powder. The pore-forming agent dispersant was added at 1.0% of the pore-forming agent's mass.

[0058] The composite rare earth nitrate was dissolved in deionized water at a mass ratio of 1:10 and stirred at 60℃ and 500 rpm for 30 min. High-purity silicon powder (silicon powder to rare earth nitrate solution mass ratio 3:1) was added, and the mixture was stirred at 1000 rpm for 60 min, simultaneously ultrasonically dispersed (500W, 20kHz) for 30 min. The solid content of the slurry was adjusted to 50%, and then filtered through a 100-mesh sieve. Spray drying parameters: inlet air temperature 220℃, outlet air temperature 100℃, feed rate 15 kg / h, atomization pressure 0.5 MPa, atomizer speed 25000 rpm. This yielded a coated modified silicon powder with a coating thickness of approximately 45 nm.

[0059] Modified silica powder and pore-forming agent were mixed at a mass ratio of 65:35. The pore-forming agent was pre-dispersed (dried at 60℃ for 1 h, then ultrasonically dispersed for 20 min at 40 kHz and 200 W with the addition of deionized water and SDBS). The ball-forming machine was rotated at 140 r / min, and the total amount of binder added was 20%. After gradient drying, the composite microspheres were passed through 100-mesh and 500-mesh sieves to obtain green bodies with a particle size of 120~520 μm.

[0060] The sealed bag was filled with argon gas at a pressure of 0.1 MPa, the cold isostatic pressing pressure was 130 MPa, the pressurization rate was 4 MPa / min, the holding time was 35 min, and the depressurization rate was 3 MPa / min. The density of the pre-densified green compact was approximately 2.0 g / cm³. 3 The shell density is ≥88%.

[0061] (1) Gradient heating and core removal stage: Evacuate to ≤5×10 -4 Pa, high-purity nitrogen gas is introduced at a pressure of 0.1 MPa. Heating program: from room temperature to 200℃ at 2℃ / min, hold for 2h; to 400℃ at 3℃ / min, hold for 3h; to 600℃ at 4℃ / min, hold for 3h.

[0062] (2) Nitriding reaction stage: Maintain a nitrogen atmosphere at a pressure of 0.2 MPa. Increase the temperature from 600℃ to 1180℃ at a rate of 5℃ / min and hold for 7 hours.

[0063] (3) Densification sintering stage: switch to high-purity argon gas and pressurize to 130MPa. Increase to 1800℃ at 4℃ / min and hold for 5h.

[0064] (4) Gradual cooling: Maintain pressure at 130 MPa up to 800 °C. Cooling rate: 1.5 °C / min for 1700~1200 °C, 2 °C / min for 1200~800 °C, and 3 °C / min for below 800 °C. Gradually depressurize after 800 °C.

[0065] Same as in Example 1, ultrasonic cleaning followed by drying and grading.

[0066] Testing revealed that the hollow microspheres had a sphericity ≥0.93, a shell thickness / inner diameter ratio of 0.15~0.22, and an apparent density of 0.85~1.05 g / cm³. 3 The shell has a density ≥99.8%, compressive strength ≥650MPa, hardness ≥90HRA, and fracture toughness ≥7.0MPa·m. 1 / 2 The silicon nitride purity is ≥99.5%, β phase content is ≥94%, and residual silicon is ≤0.2%. The product has a concentrated particle size distribution, regular spherical shape, no cracks or collapses, and a batch-to-batch performance variation coefficient ≤3%. This embodiment verifies that the method of the present invention can prepare hollow silicon nitride microspheres with better performance under higher process parameters.

[0067] 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 high-purity silicon hollow microspheres, characterized in that, Includes the following steps: S1: High-purity silicon powder is modified by coating with rare earth nitrates to obtain modified silicon powder with a rare earth element coating layer on the surface. S2: The modified silicon powder obtained in step S1 is mixed with a pore-forming agent, and the modified silicon powder is uniformly coated on the surface of the pore-forming agent through a sphere-forming process to form a composite microsphere green body with a core-shell structure; the pore-forming agent is an organic polymer microsphere that can be completely removed during sintering. S3: The composite microsphere green body obtained in step S2 is subjected to cold isostatic pressing to obtain a pre-densified green body; S4: The pre-densified green blank obtained in step S3 is placed in a hot isostatic pressing sintering furnace and sintered in an integrated manner under a nitrogen-containing atmosphere and a high-pressure inert atmosphere. The sintering process includes a gradient heating and core removal stage, a nitriding reaction stage and a densification sintering stage, to obtain high-purity silicon hollow microspheres.

2. The method for preparing high-purity silicon hollow microspheres according to claim 1, characterized in that, The high-purity silicon powder mentioned in step S1 has a purity of ≥99.99% and a median particle size D50 of 0.3~0.5μm; the rare earth nitrate is selected from one or a mixture of two of Y(NO3)3·6H2O or La(NO3)3·6H2O, and the amount added is 5~8wt% of the mass of the high-purity silicon powder.

3. The method for preparing high-purity silicon hollow microspheres according to claim 1, characterized in that, The coating modification process described in step S1 specifically includes: dissolving rare earth nitrates in deionized water to form a rare earth nitrate solution, adding high-purity silicon powder and a dispersant, forming a uniform slurry after high-speed stirring and ultrasonic dispersion, and then spray drying to obtain coated modified silicon powder.

4. The method for preparing high-purity silicon hollow microspheres according to claim 1, characterized in that, The pore-forming agent mentioned in step S2 is one or more of polymethyl methacrylate microspheres, polystyrene microspheres or starch microspheres, with a particle size D50 of 50~200μm and a sphericity ≥0.95; the amount of pore-forming agent added is 20~40wt% of the modified silica powder mass.

5. The method for preparing high-purity silicon hollow microspheres according to claim 1, characterized in that, The ball forming process in step S2 uses a rolling ball forming machine with a turntable speed of 140~180 r / min and a liquid spraying volume of 4~7 kg / h. A binder is added during the forming process. The binder is an ethanol solution of phenolic resin with a mass concentration of 50~70%.

6. The method for preparing high-purity silicon hollow microspheres according to claim 1, characterized in that, The cold isostatic pressing process in step S3 has a pressure of 100~130MPa, a holding time of 30~35min, a pressurization rate of 3~4MPa / min, and a depressurization rate of 2~3MPa / min.

7. The method for preparing high-purity silicon hollow microspheres according to claim 1, characterized in that, The specific process of the gradient heating and core removal stage in step S4 is as follows: the temperature is raised from room temperature to 200℃ at a heating rate of 1~3℃ / min and held for 1~2h, then raised to 400℃ at a heating rate of 2~4℃ / min and held for 2~3h, and then raised to 600℃ at a heating rate of 3~5℃ / min and held for 2~3h; the pressure inside the furnace during the core removal stage is controlled at 0.05~0.1MPa, and inert gas is continuously introduced.

8. The method for preparing high-purity silicon hollow microspheres according to claim 1, characterized in that, The process conditions for the nitriding reaction stage in step S4 are as follows: heating from 600℃ to 1100-1200℃ at a heating rate of 3-5℃ / min, nitrogen pressure in the furnace is 0.1-0.2MPa, and holding for 5-7 hours; the process conditions for the densification sintering stage are as follows: heating from 1100-1200℃ to 1700-1800℃ at a heating rate of 2-4℃ / min, argon pressure in the furnace is 90-130MPa, and holding for 4-5 hours.

9. The method for preparing high-purity silicon hollow microspheres according to claim 8, characterized in that, The cooling process after sintering adopts a gradient cooling: the cooling rate is 1.5~2℃ / min in the 1700~1200℃ range, the cooling rate is 2~3℃ / min in the 1200~800℃ range, and the cooling rate from 800℃ to room temperature is 3~5℃ / min; during the cooling process, the furnace pressure is maintained at 90~130MPa until the temperature drops to 800℃, and then the pressure is gradually released.

10. A high-purity silicon hollow microsphere prepared by the method according to any one of claims 1 to 9, characterized in that, The hollow microspheres have a single closed hollow cavity, with an outer shell density ≥99.5% and an apparent density of 0.8~1.2 g / cm³. 3 Compressive strength ≥ 600 MPa, hardness ≥ 88 HRA, fracture toughness ≥ 6.5 MPa·m 1 / 2 The sphericity is ≥0.9, and the ratio of shell thickness to inner diameter is 0.15~0.3.