A method for producing a silicon nitride powder

Abstract

This invention relates to the field of ceramic materials and provides a method for preparing silicon nitride powder, solving the problem of high residual chlorine / oxygen impurities due to the difficulty in removing byproducts during the preparation of silicon nitride in existing technologies. The method includes the following steps: S1, constructing an electrochemical cell: the anode material is a high-purity single-crystal silicon wafer; the cathode material is a graphite plate; the electrolyte is a mixed solvent of liquid ammonia and ethylenediamine with added LiCl electrolyte; S2, electrochemical ammonolysis reaction: under the action of an electric field, the silicon anode is oxidized to generate high-valence silicon-ammonia complex ions; the NH2 released from the liquid ammonia... ‑ The precursor migrates and combines towards the anode to generate a silane precursor containing Si-NH2 bonds; S3, the precursor is purified by washing with liquid ammonia, replacing with organic solvent and drying at low temperature; S4, the precursor is subjected to low-temperature pre-decomposition and high-temperature crystallization treatment under nitrogen protection to obtain high-purity α-Si3N4.

Description

Technical Field

[0001] This invention relates to the field of ceramic materials, and more particularly to a method for preparing silicon nitride powder. Background Technology

[0002] Silicon nitride (Si3N4) ceramics are widely used in high-end manufacturing fields such as aerospace, high-speed cutting tools, precision bearings, and electronic packaging due to their high strength, high hardness, high temperature resistance, corrosion resistance, and excellent self-lubricating properties. The prerequisite for preparing high-performance Si3N4 ceramics is obtaining high-purity, ultrafine Si3N4 powder with high α-phase content and narrow particle size distribution.

[0003] Currently, the mainstream methods for preparing Si3N4 powder mainly include direct nitriding, carbothermal reduction, and gas-phase / liquid-phase ammonolysis. Chinese Patent Publication No. CN116813353 A discloses a silicon nitride-based composite powder and its preparation and sintering methods. The silicon nitride-based composite powder consists of silicon nitride powder and a sintering aid coated on its surface. The preparation method involves preparing the sintering aid as an amino compound and dissolving it in liquid ammonia solution, then mixing silicon nitride and its precursor powder with it. Composite formation with the sintering aid is achieved through the adsorption of silicon nitride and its precursor powder. Finally, the silicon nitride and its precursor powder loaded with the sintering aid are pyrolyzed at 500–1500℃ to obtain the silicon nitride-based composite powder. The sintering method involves using silicon nitride-based composite powder coated with a low content of sintering aids as raw material, and sintering this silicon nitride-based composite powder at an temperature of 1500–1700°C. This method can obtain a dense silicon nitride sintered body with excellent mechanical properties under conditions of low sintering aid content. However, this method uses elemental metals (La, Y, Mg, etc.) as sintering aids, inevitably resulting in high levels of residual chlorine / oxygen impurities.

[0004] Chinese Patent Publication No. CN 109264677A discloses a method for preparing silicon nitride with a rich fibrous morphology: 5 wt% Fe powder and 3 wt% NH4Cl powder are weighed and added to Si powder. The powders are ball-milled and mixed evenly, then placed in a high-temperature tube furnace. Vacuum is applied, nitrogen is introduced, and the furnace is heated to 1300°C and held for 4 hours. After the reaction is complete, the furnace temperature is lowered to room temperature, and the silicon nitride product is collected. This invention utilizes the reaction of the added Fe powder and NH4Cl powder to form a reducing atmosphere containing hydrogen, which is beneficial for the vaporization of Si powder and catalyzes the formation of fibrous silicon nitride. The resulting silicon nitride powder is rich in fibers and has a large aspect ratio. However, this method has drawbacks, including high energy consumption and the potential introduction of metal / Cl. - It has defects such as halogen impurities and complex product phase composition. Summary of the Invention

[0005] Therefore, in view of the above problems, the present invention provides a method for preparing silicon nitride powder, which solves the problem that the by-products are difficult to remove and result in high residual chlorine / oxygen impurities when preparing silicon nitride in the prior art.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing silicon nitride powder includes the following steps: S1. Constructing an electrochemical cell: including an anode material, a cathode material, and an electrolyte system; the anode material is a high-purity single-crystal silicon wafer; the cathode material is a graphite plate; the electrolyte system includes a mixed solvent and an electrolyte, wherein the mixed solvent is a mixture prepared by mixing liquid ammonia and ethylenediamine in a volume ratio of (5-9):(1-5); the electrolyte is LiCl; S2. Electrochemical ammonolysis reaction: A DC voltage of 2.0-5.0V is applied between the electrodes of the electrochemical cell, and the current density is controlled at 10-50mA / cm². 2 The electrolysis time is 1-4 hours. Under the influence of the electric field: the silicon anode surface undergoes oxidation and dissolution, generating soluble high-valence silicon species, which dissolve in the electrolyte system in the form of silicon-ammonia complex ions; NH3 molecules in liquid ammonia are polarized and partially dissociated under the influence of the electric field, generating NH2. - Ions migrate towards the anode under the drive of an electric field; on the anode surface, the soluble high-valence silicon species react with the migrating NH2. - Coordination and nucleophilic reactions occur to generate silane precursors containing Si-NH2 bonds, wherein the silane precursors include Si(NH2)2, (NH2)2Si-NH-Si(NH2)2, and Si-O-NH2; S3. Collection of silane precursor: After the reaction is completed, the anode material is removed and rinsed repeatedly with liquid ammonia at 0-10℃ three to five times to peel the silane precursor off the electrode surface; the firmly attached thin film precursor is scraped off with a scraper and ultrasonically dispersed in liquid ammonia, and the suspension is collected. S4. Purification of silane precursor: The suspension collected in step S3 is filtered, the filtrate is discarded, the solid is collected, and then washed with liquid ammonia, replaced with organic solvent, separated by low temperature centrifugation and dried to obtain silane precursor powder. S5. High-temperature thermal decomposition crystallization: The dried silane precursor powder is loaded into a crucible and placed in a tube furnace. High-purity nitrogen is introduced and the temperature is raised to 400-600℃ at 8℃ / min, and held for 20-40 min for pre-decomposition to remove volatile components such as NH3 and H2. Then, the temperature is raised to 1400-1550℃ at 8℃ / min and held for 1-3 h to completely break and rearrange the Si-NH2 bonds to form α-Si3N4 crystals. The furnace is cooled to room temperature, the crucible is removed and purged with high-purity nitrogen to obtain silicon nitride powder.

[0007] The anodic half-reaction is as follows: In the system of liquid ammonia and ethylenediamine, the silicon anode surface loses electrons to generate high-valence silicon species, and the reaction process is shown in equation (1): (1) It then coordinates with NH3 to form a soluble silicon-ammonia complex ion, and the reaction process is shown in equation (2): (2) Electrolyte-side reaction: Liquid ammonia molecules are polarized and partially dissociated under the influence of an electric field, generating NH2. - The reaction process, which involves migration towards the anode, is shown in equation (3): (3) Next, soluble silicon-ammonia complex ions on the anode surface react with NH2. - A nucleophilic substitution / condensation reaction occurs, forming Si(NH2)2, the main component of the silane precursor. The reaction process is shown in equation (4): (4) And a small amount of (NH2)2Si-NH-Si(NH2)2 is formed, the reaction process is shown in equation (5): (5) Trace amounts of Si-O-NH2 are formed and can be removed in subsequent purification steps.

[0008] Furthermore, the purity of the high-purity single-crystal silicon wafer is ≥99.999%.

[0009] Furthermore, the purity of the graphite plate is ≥99.99%.

[0010] Furthermore, the temperature of the electrolyte system is controlled at -20°C using a low-temperature circulating bath.

[0011] Furthermore, in step S4, the liquid ammonia cleaning involves placing the solid in liquid ammonia and stirring it, then rinsing it with liquid ammonia 2-3 times to dissolve and remove soluble metal ions and polar impurities.

[0012] Furthermore, in step S4, the organic solvent replacement cleaning is performed by transferring the solid after cleaning with liquid ammonia to ethylenediamine, stirring for 30-60 minutes to dissolve and remove the trace ions and silicon-oxygen bond impurities adsorbed on the surface.

[0013] Furthermore, in step S4, the low-temperature centrifugation separation is performed by centrifuging at 5°C and 3000 rpm for 10-20 minutes.

[0014] Furthermore, in step S4, the drying process involves drying in a vacuum oven at 70°C and a vacuum level ≤10Pa for 16-24 hours.

[0015] By adopting the aforementioned technical solution, the beneficial effects of the present invention are as follows: This technical solution constructs an electrolysis system using liquid ammonia / ethylenediamine as the medium, and utilizes an electric field to drive the anodic oxidation and dissolution of silicon to generate high-valence silicon-ammonia complex ions, which then react with NH2. - Directed bonding. This process does not require the introduction of chlorine, thus avoiding the formation of Cl. - This eliminates the risk of residues and solves the problems of difficulty in removing byproduct NH4Cl and easy introduction of oxygen impurities in traditional methods. Attached Figure Description

[0016] Figure 1 This is the FT-IR image of the electrolyte after electrolysis in Example 1 of the present invention; Figure 2 This is the XPS spectrum of the anode product in Example 1 of the present invention. Detailed Implementation Example 1

[0017] A method for preparing silicon nitride powder includes the following steps: S1. Constructing an electrochemical cell: including an anode material, a cathode material, and an electrolyte system; the anode material is a high-purity single-crystal silicon wafer; the cathode material is a graphite plate; the electrolyte system includes a mixed solvent and an electrolyte, wherein the mixed solvent is a mixture prepared by mixing liquid ammonia and ethylenediamine in a volume ratio of 7:3; the electrolyte is LiCl; the purity of the high-purity single-crystal silicon wafer is ≥99.999%, and the purity of the graphite plate is ≥99.99%; the temperature of the electrolyte system is controlled at -20℃ using a low-temperature circulating bath. S2, Electrochemical Ammonolysis Reaction: A DC voltage of 3.5V is applied between the electrodes of the electrochemical cell, and the current density is controlled at 30mA / cm². 2 The electrolysis time was 2 hours. Under the influence of the electric field: the silicon anode surface underwent oxidation and dissolution, generating soluble high-valence silicon species, which dissolved in the electrolyte system in the form of silicon-ammonia complex ions; the NH3 molecules in the liquid ammonia were polarized and partially dissociated under the influence of the electric field, generating NH2. - Ions migrate towards the anode under the drive of an electric field; on the anode surface, the soluble high-valence silicon species react with the migrating NH2. - Coordination and nucleophilic reactions occur to generate silane precursors containing Si-NH2 bonds, wherein the silane precursors include Si(NH2)2, (NH2)2Si-NH-Si(NH2)2, and Si-O-NH2; S3. Collection of silane precursor: After the reaction is completed, the anode material is removed and rinsed repeatedly with liquid ammonia at 5°C three to five times to peel the silane precursor off the electrode surface; the firmly attached thin film precursor is scraped off with a scraper and ultrasonically dispersed in liquid ammonia, and the suspension is collected. S4. Purification of the silane precursor: The suspension collected in step S3 is filtered, the filtrate is discarded, and the solid is collected. The solid is then subjected to liquid ammonia washing, organic solvent displacement washing, low-temperature centrifugation, and drying to obtain silane precursor powder. The liquid ammonia washing involves placing the solid in liquid ammonia, stirring, and then rinsing with liquid ammonia three times to dissolve and remove soluble metal ions and polar impurities. The organic solvent displacement washing involves transferring the solid after liquid ammonia washing to ethylenediamine and stirring for 60 minutes to dissolve and remove surface-adsorbed trace ions and silicon-oxygen bond impurities. The low-temperature centrifugation involves centrifuging at 5°C and 3000 rpm for 10 minutes. The drying process involves drying in a vacuum oven at 70°C and 10 Pa for 24 hours. S5. High-temperature thermal decomposition crystallization: The dried silane precursor powder is loaded into a crucible and placed in a tube furnace. High-purity nitrogen is introduced and the temperature is raised to 500℃ at 8℃ / min and held for 30 min for pre-decomposition to remove volatile components such as NH3 and H2. Then, the temperature is raised to 1500℃ at 8℃ / min and held for 2 h to completely break and rearrange the Si-NH2 bond to generate α-Si3N4 crystal. The furnace is cooled to room temperature, the crucible is removed and purged with high-purity nitrogen to obtain silicon nitride powder.

[0018] During the high-temperature thermal decomposition process, the thermal decomposition process of Si(NH2)2 is shown in Equation (6). The main product is α-Si3N4, the by-product is gas, and there are no solid impurities. Because it is a monomer structure, the nucleation is uniform during thermal decomposition, and equiaxed, fine-grained Si3N4 is obtained. (6) The thermal decomposition process of (NH2)2Si-NH-Si(NH2)2 is shown in equation (7). The main product is α-Si3N4, which has a high nucleation density and produces particles with slightly larger crystals and more complete crystal forms. (7) The decomposition products of Si(NH2)2 and (NH2)2Si-NH-Si(NH2)2 silane precursors are both target silicon nitride, and there are no other solid residues. Si3N4 powder has the characteristics of good molding performance and excellent mechanical properties.

[0019] refer to Figure 1 After electrolysis, the electrolyte was analyzed using a Fourier transform infrared spectrometer (FT-IR spectrometer) at 3300 cm⁻¹. -1An absorption peak of the NH stretching vibration was observed nearby, at 1550 cm⁻¹. -1 An absorption peak of NH bending vibration was observed nearby, and at 820 cm⁻¹ -1 The presence of Si-N stretching vibration peaks nearby indicates the formation of a silane precursor containing Si-NH2 bonds in the electrolyte.

[0020] refer to Figure 2 X-ray photoelectron spectroscopy (XPS) analysis of the products on the anode surface revealed a characteristic peak with a binding energy of 102.5 eV in the Si 2p spectrum. This peak falls between elemental silicon (~99.5 eV) and silicon dioxide (~103.5 eV), which is consistent with the characteristic binding energy range of Si-N bonds. This indicates that silicon atoms and nitrogen atoms on the anode surface have successfully formed chemical bonds, namely, the formation of Si-NH2 bonds. Example 2

[0021] The difference from Example 1 is that the mixed solvent is a mixture prepared by mixing liquid ammonia and ethylenediamine in a volume ratio of 5:1; a DC voltage of 2.0V is applied between the two electrodes of the electrochemical cell, and the current density is controlled at 20mA / cm². 2 The electrolysis time was 2 hours. The dried silane precursor powder was placed in a crucible and then in a tube furnace. High-purity nitrogen gas was introduced, and the temperature was raised to 550°C at 8°C / min and held for 30 minutes for pre-decomposition to remove volatile components such as NH3 and H2. The temperature was then raised to 1550°C at 8°C / min and held for 1 hour to completely break and rearrange the Si-NH2 bonds to form α-Si3N4 crystals. The furnace was cooled to room temperature, the crucible was removed, and purged with high-purity nitrogen gas to obtain silicon nitride powder. Other technical solutions were the same as in Example 1. Example 3

[0022] The difference from Example 1 is that the mixed solvent is a mixture prepared by mixing liquid ammonia and ethylenediamine in a volume ratio of 9:5; a DC voltage of 5.0V is applied between the two electrodes of the electrochemical cell, and the current density is controlled at 10mA / cm². 2 The electrolysis time was 4 hours. The dried silane precursor powder was placed in a crucible and then in a tube furnace. High-purity nitrogen was introduced, and the temperature was raised to 600°C at 8°C / min and held for 30 minutes for pre-decomposition to remove volatile components such as NH3 and H2. The temperature was then raised to 1550°C at 8°C / min and held for 2 hours to completely break and rearrange the Si-NH2 bonds to form α-Si3N4 crystals. The furnace was cooled to room temperature, the crucible was removed, and purged with high-purity nitrogen to obtain silicon nitride powder. Other technical solutions were the same as in Example 1.

[0023] Although the invention has been specifically shown and described in conjunction with preferred embodiments, those skilled in the art should understand that various changes in form and detail may be made to the invention without departing from the spirit and scope of the invention as defined in the appended claims, all of which shall be within the scope of protection of the invention.

Claims

1. A method for preparing silicon nitride powder, characterized in that, Includes the following steps: S1. Constructing an electrochemical cell: including an anode material, a cathode material, and an electrolyte system; the anode material is a high-purity single-crystal silicon wafer; the cathode material is a graphite plate; the electrolyte system includes a mixed solvent and an electrolyte, wherein the mixed solvent is a mixture prepared by mixing liquid ammonia and ethylenediamine in a volume ratio of (5-9):(1-5); the electrolyte is LiCl; S2. Electrochemical ammonolysis reaction: A DC voltage of 2.0-5.0V is applied between the electrodes of the electrochemical cell, and the current density is controlled at 10-50mA / cm². 2 The electrolysis time is 1-4 hours. Under the influence of the electric field: the silicon anode surface undergoes oxidation and dissolution, generating soluble high-valence silicon species, which dissolve in the electrolyte system in the form of silicon-ammonia complex ions; NH3 molecules in liquid ammonia are polarized and partially dissociated under the influence of the electric field, generating NH2. - Ions migrate towards the anode under the drive of the electric field; On the anode surface, the soluble high-valence silicon species react with the migrating NH2. - Coordination and nucleophilic reactions occur to generate silane precursors containing Si-NH2 bonds, wherein the silane precursors include Si(NH2)2, (NH2)2Si-NH-Si(NH2)2, and Si-O-NH2; S3. Collection of silamine precursor: After the reaction is completed, the anode material is removed and rinsed repeatedly with liquid ammonia at 0-10℃ three to five times to peel the silamine precursor off the electrode surface; the silamine precursor is scraped off with a scraper and ultrasonically dispersed in liquid ammonia, and the suspension is collected. S4. Purification of silane precursor: The suspension collected in step S3 is filtered, the filtrate is discarded, the solid is collected, and then washed with liquid ammonia, replaced with organic solvent, separated by low temperature centrifugation and dried to obtain silane precursor powder. S5. High-temperature thermal decomposition crystallization: The dried silane precursor powder is loaded into a crucible and placed in a tube furnace. High-purity nitrogen is introduced and the temperature is raised to 400-600℃ at 8℃ / min, and held for 20-40 min for pre-decomposition to remove volatile components such as NH3 and H2. Then, the temperature is raised to 1400-1550℃ at 8℃ / min and held for 1-3 h to completely break and rearrange the Si-NH2 bonds to form α-Si3N4 crystals. The furnace is cooled to room temperature, the crucible is removed and purged with high-purity nitrogen to obtain silicon nitride powder.

2. The method for preparing silicon nitride powder according to claim 1, characterized in that, The purity of the high-purity single-crystal silicon wafer is ≥99.999%.

3. The method for preparing silicon nitride powder according to claim 1, characterized in that, The purity of the graphite plate is ≥99.99%.

4. The method for preparing silicon nitride powder according to claim 1, characterized in that, The temperature of the electrolyte system is controlled at -20℃ using a low-temperature circulating bath.

5. The method for preparing silicon nitride powder according to claim 1, characterized in that, In step S4, the liquid ammonia cleaning involves placing the solid in liquid ammonia, stirring it, and then rinsing it with liquid ammonia, repeating this process 2-3 times.

6. The method for preparing silicon nitride powder according to claim 1, characterized in that, In step S4, the organic solvent replacement cleaning is performed by transferring the solid after cleaning with liquid ammonia to ethylenediamine and stirring for 30-60 minutes.

7. The method for preparing silicon nitride powder according to claim 1, characterized in that, In step S4, the low-temperature centrifugation separation is performed by centrifuging at 5°C and 3000 rpm for 10-20 minutes.

8. The method for preparing silicon nitride powder according to claim 1, characterized in that, In step S4, the drying process involves drying in a vacuum oven at 70°C and a vacuum level ≤10Pa for 16-24 hours.

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