A method for preparing high-purity, highly dispersed nano-silicon nitride powder

Abstract

This invention discloses a method for preparing high-purity, highly dispersed nano-silicon nitride powder. The method involves ball milling raw materials including silicon dioxide powder, fatty acids, and a wet grinding agent, followed by a hydrothermal reaction with a carbon source to obtain a precursor powder. The precursor powder is then subjected to reduction nitridation and decarbonization to obtain the final product. The mass ratio of silicon dioxide powder, fatty acids, and the wet grinding agent is 100:0.1–1:10–50. The carbon source comprises 50–80% of the silicon dioxide powder mass by weight of carbon. This method, through fatty acid modification of silicon dioxide, not only effectively avoids particle agglomeration but also alters the interfacial tension of silicon dioxide. During the hydrothermal process, the fatty acid layer coating the silicon dioxide surface acts as a carbonization active site, uniformly carbonizing with the carbon source on the silicon dioxide surface. This effectively solves the problem of uneven mixing of silicon dioxide and carbon, improves the purity of the silicon nitride product, and significantly reduces the content of impurities such as oxygen and carbon in the product.

Description

Technical Field

[0001] This invention relates to a method for preparing silicon nitride powder, specifically a method for preparing high-purity, highly dispersed nano-silicon nitride powder, belonging to the field of inorganic non-metallic powder materials technology. Background Technology

[0002] Silicon nitride (Si3N4) is a high-performance ceramic material with excellent mechanical properties, heat resistance, corrosion resistance, and insulation properties. It is a compound composed of silicon and nitrogen elements, possessing various crystal structures, including α-Si3N4, β-Si3N4, and γ-Si3N4. Silicon nitride is a very hard and wear-resistant material with high hardness, high strength, and high toughness, making it suitable for manufacturing cutting tools, bearings, and abrasives. Furthermore, silicon nitride exhibits good corrosion resistance, allowing it to be used in the manufacture of chemical reactors, flue gas desulfurization equipment, and catalysts. It also possesses good insulation and thermal stability, enabling its use in the manufacture of high-temperature electronic components, high-temperature ceramics, and high-temperature seals. Additionally, silicon nitride exhibits excellent optical properties, making it suitable for the manufacture of optical devices and optical coatings.

[0003] Currently, methods for preparing silicon nitride powder include direct silicon powder nitridation, carbothermic reduction of silicon dioxide, self-propagating combustion, thermal decomposition, and chemical vapor synthesis. Direct silicon powder nitridation is an earlier method, where metallic silicon powder is heated in a nitrogen or ammonia atmosphere, causing the silicon powder to react directly with a nitrogen source to generate silicon nitride powder. Carbothermic reduction of silicon dioxide involves mixing carbon powder and silicon dioxide powder, then heating in a nitrogen or ammonia atmosphere, resulting in the reduction and nitridation of silicon dioxide to produce silicon nitride powder. Self-propagating combustion involves igniting silicon powder under high-pressure nitrogen, resulting in the self-propagating combustion of silicon nitride powder. Thermal decomposition involves first reacting SiCl4 and NH3 at low temperature to generate a silane precursor, then decomposing the silane precursor at high temperature to produce silicon nitride powder. Chemical vapor phase synthesis involves reacting a gaseous silicon source, such as SiCl4, SiH4, SiHCl3, or SiBr4, with NH3, N2, or H2 at high temperatures or under laser or plasma excitation to directly generate silicon nitride powder.

[0004] Compared to other methods, the direct nitridation of silicon powder and the carbothermic reduction of silica method have been industrialized on a large scale due to their lower cost. However, the direct nitridation of silicon powder is difficult to completely nitride silicon, making it difficult to control the crystal phase. The synthesized silicon nitride agglomerates require crushing, which can easily lead to contamination, and it is difficult to crush them to the nanoscale. Therefore, the direct nitridation of silicon powder is difficult to prepare high-purity, highly dispersed, high-α-phase nano-silicon nitride powder without free silicon. The silicon nitride powder prepared by the traditional carbothermic reduction of silica method has a high α-phase content and small particle size, but due to insufficient contact between carbon and silica, the Si-O bonds in the solid-phase reaction are difficult to be completely replaced by Si-N bonds, or side reactions may occur, generating silicon carbide, resulting in a high oxygen and carbon content in the product. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention aims to provide a method for preparing high-purity, highly dispersed nano-silicon nitride powder. This method modifies silicon dioxide with fatty acids, which not only effectively avoids particle agglomeration but also changes the interfacial tension of silicon dioxide. During the hydrothermal process, the fatty acid layer coated on the surface of silicon dioxide serves as a carbonization active site, and carbonizes uniformly with the carbon source on the surface of silicon dioxide. This effectively solves the problem of uneven mixing of silicon dioxide and carbon, improves the purity of the silicon nitride product, and significantly reduces the content of impurities such as oxygen and carbon in the product.

[0006] To achieve the above-mentioned technical objectives, the present invention provides a method for preparing high-purity, highly dispersed nano-silicon nitride powder. The raw materials, including silicon dioxide powder, fatty acid and wet grinding agent, are ball-milled and then mixed with a carbon source for hydrothermal reaction to obtain precursor powder. The precursor powder is then subjected to reduction nitriding and decarbonization in sequence to obtain the final product.

[0007] The mass ratio of the silica powder, fatty acid, and wet grinding agent is 100:0.1-1:10-50;

[0008] The carbon source is 50-80% of the mass of the silica powder, based on the mass of carbon.

[0009] In the preparation method provided by this invention, a fatty acid layer is coated on the surface of silica by wet ball milling to obtain highly dispersed silica powder. During the hydrothermal process, the carbon layer coated on the surface of silica serves as a carbonization active site and is uniformly carbonized with the carbon source on the surface of silica to form a nanoscale carbon shell, which is easy to remove quickly in the subsequent decarbonization process and avoids the increase in oxygen content in the product due to prolonged decarbonization.

[0010] In the preparation method provided by this invention, each reagent component must be strictly executed according to the above conditions. If the fatty acid content is too low, it will not be able to encapsulate the silica powder. If the fatty acid content is too high, it will lead to the introduction of too much carbon source in the subsequent process, prolonging the time required for decarbonization. If the amount of carbon source added is too low, it will lead to insufficient contact between silica and carbon, reducing the yield of silicon carbide. If the amount of carbon source added is too high, it will lead to an excessively thick carbon layer, prolonging the decarbonization time and causing excessive carbon and oxygen content in the product. In severe cases, it may even cause side reactions.

[0011] As a preferred embodiment, the silica powder has a purity of ≥99.999% and a particle size of less than 200 nm.

[0012] As a preferred embodiment, the fatty acid is at least one selected from oleic acid, stearic acid, lauric acid, linolenic acid, and isoleic acid. More preferably, the fatty acid is oleic acid and / or stearic acid.

[0013] As a preferred embodiment, the wet abrasive is at least one selected from water, ethanol, isopropanol, and acetone. As a preferred embodiment, the wet abrasive is ethanol.

[0014] As a preferred embodiment, the material obtained after ball milling is sequentially separated, filtered, dried, and sieved before being mixed with a carbon source for a hydrothermal reaction.

[0015] As a preferred embodiment, the ball milling conditions are: a rotation speed of 100–300 r / min and a time of 1–10 h.

[0016] As a preferred embodiment, the carbon source is a carbohydrate organic carbon source solution. More preferably, the organic carbon source solution is one of sucrose, glucose, fructose, maltose, and lactose solutions.

[0017] As a preferred embodiment, the content of all metal impurities in the organic carbon source solution is ≤0.1ppm. To ensure that the impurity content in the organic carbon source does not exceed the required standard, the present invention further purifies the organic carbon source with resin, thereby ensuring the purity of the carbon source.

[0018] As a preferred embodiment, the hydrothermal reaction conditions are: a reaction temperature of 120–200°C and a reaction time of 4–8 hours.

[0019] As a preferred embodiment, the reduction nitriding process of the precursor powder is carried out under a nitrogen atmosphere, and the reaction conditions are: reduction nitriding temperature of 1200-1400℃ and time of 4-12h.

[0020] As a preferred embodiment, the carbon removal process is oxidative carbon removal, and the oxygen source for oxidative carbon removal is oxygen and / or air.

[0021] As a preferred embodiment, the carbon removal conditions are: a temperature of 600–650°C and a time of 1–4 hours.

[0022] As a preferred embodiment, the silicon nitride powder contains less than 1 ppm of each metal impurity, less than 0.1% carbon, and less than 0.1% oxygen.

[0023] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0024] 1) In the preparation method provided by the present invention, a fatty acid layer is coated on the surface of silica by wet ball milling to obtain highly dispersed silica powder. In the hydrothermal process, the fatty acid layer coated on the surface of silica serves as a carbonization active site and is uniformly carbonized with the carbon source on the surface of silica to form a nanoscale carbon shell, which is easy to remove quickly in the subsequent decarbonization process and avoids the increase of oxygen content in the product due to long-term decarbonization.

[0025] 2) In the technical solution provided by the present invention, the surface treatment of silica with fatty acids improves the dispersion of silica after ball milling and refines the particle size. On the other hand, it changes the surface tension of silica powder, making its binding with the carbon source more compact and the carbon layer obtained by hydrothermal treatment more uniform and dense. Thus, the purity and dispersibility of the product are improved while ensuring that the product has a nanoparticle size. Detailed Implementation

[0026] To facilitate understanding of the present invention, a more comprehensive description will be given below with reference to specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the present invention.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0028] Example 1

[0029] A. 99.999% pure nano-spherical silica powder, oleic acid, and ethanol are added to a ball mill, wherein the oleic acid accounts for 0.1% of the silica and the ethanol accounts for 10% of the silica. The mixture is ball-milled, separated, filtered, dried, and sieved to obtain highly dispersed silica powder.

[0030] B. Highly dispersed silica powder and high-purity glucose solution were added to a hydrothermal reactor. The high-purity glucose solution was prepared by dissolving glucose in pure water and then purifying it with resin. The total carbon content of the solution was 50% of that of silica. A hydrothermal reaction was carried out at 200℃ for 4 hours. After filtration and drying, nanospheres with silica as the core and carbon as the shell were obtained.

[0031] C. Place the powder obtained in step B into an atmosphere furnace, introduce nitrogen gas, heat the atmosphere furnace to 1250℃ and hold for 10 hours, then cool down and remove the product.

[0032] D. The product obtained in step C is kept at 600°C for 4 hours in air atmosphere and then cooled to obtain silicon nitride powder.

[0033] E. Detection: The silicon nitride powder obtained in step D was tested according to GB / T16555-2017, determining the content of silicon nitride, carbon, and oxygen. The results showed that the silicon nitride content in the silicon nitride powder obtained in step D was 99.6%, of which the α-silicon nitride content was 94%, the carbon content was 0.05%, and the oxygen content was 0.06%. Trace metal elements in the silicon nitride powder were determined by inductively coupled plasma mass spectrometry, and the results are shown in Table 1.

[0034] Table 1

[0035]

[0036]

[0037] The particle size of silicon nitride powder was detected by laser diffraction. The results showed that the smallest particle size of silicon nitride powder obtained in step D was 30 nm, with 28% being below 100 nm and a D50 of 117 nm.

[0038] Example 2

[0039] A. 99.999% pure nano-spherical silica powder, stearic acid, and ethanol are added to a ball mill, wherein the stearic acid constitutes 0.2% of the silica and the ethanol constitutes 50% of the silica. The mixture is ball-milled, separated, filtered, dried, and sieved to obtain highly dispersed silica powder.

[0040] B. Highly dispersed silica powder and high-purity sucrose solution were added to a hydrothermal reactor. The high-purity sucrose solution was prepared by dissolving sucrose in pure water and then purifying it with resin. The total carbon content of the solution was 60% of that of silica. A hydrothermal reaction was carried out at 190℃ for 6 hours. After filtration and drying, nanospheres with silica as the core and carbon as the shell were obtained.

[0041] C. Place the powder obtained in step B into an atmosphere furnace, introduce nitrogen gas, heat the atmosphere furnace to 1350℃ and hold for 8 hours, then cool down and take out the product.

[0042] D. The product obtained in step C is kept at 650°C for 2 hours in air atmosphere and then cooled to obtain silicon nitride powder.

[0043] E. Detection: The silicon nitride powder obtained in step D was tested according to GB / T16555-2017, determining the silicon nitride, carbon, and oxygen content. The results showed that the silicon nitride content in the silicon nitride powder obtained in step D was 99.7%, of which the α-silicon nitride content was 93%, the carbon content was 0.04%, and the oxygen content was 0.04%. Trace metal elements in the silicon nitride powder were determined by inductively coupled plasma mass spectrometry, and the results are shown in Table 2.

[0044] Table 2

[0045]

[0046] The particle size of silicon nitride powder was detected by laser diffraction. The results showed that the silicon nitride powder obtained in step D had a minimum particle size of 30 nm, with 25% of the particles being less than 100 nm, and a D50 of 125 nm.

[0047] Comparative Example 1

[0048] A. 99.999% pure nano-spherical silica powder and high-purity glucose solution were added to a hydrothermal reactor. The high-purity glucose solution was prepared by dissolving glucose in pure water and then purifying it with resin; the total carbon content of the solution was 50% of the silica. A hydrothermal reaction was carried out at 200℃ for 4 hours. After filtration and drying, nano-spherical powder with silica as the core and carbon as the shell was obtained.

[0049] B. Place the powder obtained in step A into an atmosphere furnace, introduce nitrogen gas, heat the atmosphere furnace to 1250℃ and hold for 10 hours, then cool it down and take out the product.

[0050] C. The product obtained in step B is kept at 600°C for 4 hours in air atmosphere and then cooled to obtain silicon nitride powder.

[0051] D. Detection: The silicon nitride powder obtained in step C was tested according to GB / T16555-2017, determining the silicon nitride, carbon, and oxygen content. The results showed that the silicon nitride content in the silicon nitride powder obtained in step D was 94.3%, of which the α-silicon nitride content was 90%, the carbon content was 1.8%, and the oxygen content was 2.5%. Trace metal elements in the silicon nitride powder were determined using inductively coupled plasma mass spectrometry (ICP-MS), and the results are as follows:

[0052] Table 3

[0053]

[0054] The particle size of silicon nitride powder was detected by laser diffraction. The results showed that the particle size D50 of the silicon nitride powder obtained in step C was 1.26 micrometers.

[0055] Comparative Example 2

[0056] A. 99.999% pure nano-spherical silica powder, stearic acid, and ethanol are added to a ball mill, wherein the stearic acid constitutes 0.2% of the silica and the ethanol constitutes 50% of the silica. The mixture is ball-milled, separated, filtered, dried, and sieved to obtain highly dispersed silica powder.

[0057] B. Highly dispersed silica powder and sucrose solution were added to a hydrothermal reactor. The sucrose solution was prepared by dissolving sucrose in pure water, and the total carbon content of the solution was 60% of that of silica. A hydrothermal reaction was carried out at 190℃ for 6 hours. After filtration and drying, nanospheres with silica as the core and carbon as the shell were obtained.

[0058] C. Place the powder obtained in step B into an atmosphere furnace, introduce nitrogen gas, heat the atmosphere furnace to 1350℃ and hold for 8 hours, then cool down and take out the product.

[0059] D. The product obtained in step C is kept at 650°C for 2 hours in air atmosphere and then cooled to obtain silicon nitride powder.

[0060] E. Detection: The silicon nitride powder obtained in step D was tested according to GB / T16555-2017, determining the content of silicon nitride, carbon, and oxygen. The results showed that the silicon nitride content in the silicon nitride powder obtained in step D was 99.6%, of which the α-silicon nitride content was 93%, the carbon content was 0.05%, and the oxygen content was 0.04%. Trace metal elements in the silicon nitride powder were determined by inductively coupled plasma mass spectrometry, and the results are shown in Table 4.

[0061] Table 4

[0062]

[0063] The particle size of silicon nitride powder was detected by laser diffraction. The results showed that the particle size D50 of the silicon nitride powder obtained in step D was 130 nm.

[0064] For the reader's convenience, the above description focuses on representative examples of all possible embodiments, which illustrate the principles of the invention and demonstrate the best mode for carrying out the invention. This description does not attempt to exhaustively list all possible variations. Other variations or modifications not described may also be possible.

Claims

1. A method for preparing high-purity, highly dispersed nano-silicon nitride powder, characterized in that: The raw materials including silica powder, fatty acid and wet grinding agent are mixed with a carbon source after ball milling to perform a hydrothermal reaction to obtain a precursor powder; the precursor powder is subjected to reduction nitriding and carbon removal in sequence to obtain the silicon nitride powder; The mass ratio of the silica powder, the fatty acid and the wet grinding agent is 100:0.1-1:10-50; The carbon source is 50-80% of the mass of the silica powder; The fatty acid is at least one of oleic acid, stearic acid, lauric acid, linolenic acid and isoleic acid; and the wet grinding agent is at least one of water, ethanol, isopropyl alcohol and acetone; The hydrothermal reaction is performed at a temperature of 120-200℃ for 4-8h. The silicon nitride powder contains less than 1ppm of each metal impurity, less than 0.1% of carbon and less than 0.1% of oxygen.

2. The preparation method of high-purity and high-dispersion nano-silicon nitride powder according to claim 1, characterized in that: The purity of the silica powder is greater than or equal to 99.999%, and the particle size is less than 200nm.

3. The method according to claim 1, wherein the method is characterized by: The material obtained after the ball milling is subjected to separation, filtration, drying and sieving in sequence before being mixed with the carbon source to perform the hydrothermal reaction; the ball milling is performed at a speed of 100-300r / min for 1-10h.

4. The method according to claim 1, wherein the method is characterized by: The carbon source is a sugar organic carbon source solution; the content of each metal impurity in the organic carbon source solution is less than or equal to 0.1ppm.

5. The method according to claim 1, wherein the method is characterized by: The reduction nitriding of the precursor powder is performed in a nitrogen atmosphere at a temperature of 1200-1400℃ for 4-12h.

6. The method according to claim 1, wherein the method is characterized by: The carbon removal is oxidation carbon removal, and the oxygen source for the oxidation carbon removal is oxygen and / or air.

7. The method according to claim 1, wherein the method is characterized by: The carbon removal is performed at a temperature of 600-650℃ for 1-4h.

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