High-purity silicon nitride powder and method for preparing the same

Abstract

The application relates to the field of chemical material preparation, and particularly discloses a high-purity silicon nitride powder and a preparation method thereof. The preparation method of the high-purity silicon nitride powder is as follows: 30-40 parts of silicon dioxide nano particles are mixed with 12-40 parts of carbon black to obtain a blend by adding deionized water, the blend is stirred, and the blend is dried at 60-65 DEG C to obtain a silicon-carbon precursor; nitrogen is introduced into the carbon-silicon precursor, the temperature is increased to 1350-1500 DEG C, and the temperature is kept constant to obtain a crude product; ammonia is continuously introduced into the crude product, the temperature is kept constant at 1300-1400 DEG C to obtain a first purification product; the first purification product is cooled to 600-650 DEG C, oxygen is introduced, and the temperature is kept constant to obtain a second purification product; and the second purification product is cooled to obtain the product. The preparation method of the high-purity silicon nitride powder can prepare high-purity silicon nitride powder with a purity of 99.65% or above.

Description

Technical Field

[0001] This application relates to the field of chemical material preparation, and more specifically, it relates to a high-purity silicon nitride powder and a method for preparing the same. Background Technology

[0002] Silicon nitride (Si3N4) ceramics are known as "all-around ceramics" due to their high strength, high density, high hardness, and high elastic modulus. They also exhibit excellent thermal shock resistance and corrosion resistance, making them widely used in aerospace, electronics, biomaterials, and photovoltaic industries. High-quality silicon nitride powder requires a particle size of approximately 0.1–0.5 μm, an α-phase content >95%, an O content <0.9%, and a C content <0.2%. The purity of silicon nitride powder is the decisive factor for producing high-quality powder.

[0003] Regarding the aforementioned technologies, the inventors discovered that current methods for preparing silicon nitride powder mainly include carbothermal nitridation of SiO2, direct nitridation of Si powder, and chemical vapor deposition. Carbothermal reduction of silicon nitride powder is widely used in large-scale industrial production due to its inexpensive raw materials and simple equipment. However, carbothermal reduction typically uses ordinary silicon dioxide as the silicon source, resulting in coarse powder particles that are difficult to mix evenly with carbon black. This leads to inconsistent particle size and purity of the synthesized silicon nitride powder. Although repeated crushing and nitriding can reduce oxygen impurities in the powder to some extent, the high hardness of silicon nitride introduces other impurities during crushing, and it is difficult to reduce the particle size to below 1.0 μm. The resulting powder often contains a large amount of unreacted silicon monoxide, silicon dioxide, and free silicon impurities. Summary of the Invention

[0004] In order to reduce the content of impurities such as free silicon and oxygen in the product and improve the purity of silicon nitride powder, this application provides a high-purity silicon nitride powder and its preparation method.

[0005] In a first aspect, this application provides a method for preparing high-purity silicon nitride powder, employing the following technical solution: A method for preparing high-purity silicon nitride powder, by weight, includes the following steps: Mix 30-40 parts of silica nanoparticles with 12-40 parts of carbon black, add deionized water to obtain a blend, stir the blend for 2-3 hours, and dry it at 60-65℃ for 6-8 hours to obtain a silicon-carbon precursor. Nitrogen gas is introduced into the silicon carbide precursor and heated to 1350-1500℃. The temperature is maintained for 8-10 hours to obtain a crude product. Ammonia gas is then introduced into the crude product and the temperature is maintained at 1300-1400℃ for 3-4 hours to obtain a first-purified product. The product from the first purification is cooled to 600-650℃, oxygen is introduced, and the temperature is maintained for 5-6 hours to obtain the product from the second purification. The product from the second purification is then cooled to obtain the final product.

[0006] By adopting the above technical solution, using silica nanoparticles as the silicon source for the nitride reduction reaction, the particles are small and easier to mix with carbon black. By mixing silica nanoparticles and carbon black in the liquid phase to obtain a blend, it is possible to promote uniform mixing of silica nanoparticles and carbon black, so that they can be fully reduced by carbon black, which helps to form high-purity ultrafine silicon nitride powder.

[0007] After reacting silicon-carbon precursors with nitrogen at high temperatures, ammonia is introduced to facilitate the rapid conversion of unreacted silicon monoxide and a small amount of residual silicon dioxide into silicon nitride powder, resulting in a purer product phase. This is because ammonia has higher reactivity than nitrogen. The chemical bond formed by nitrogen and hydrogen atoms in ammonia molecules is more reactive than the nitrogen-nitrogen triple bond, making it easier to react and convert the silicon monoxide intermediate formed during the conversion of silicon dioxide into silicon nitride. Furthermore, the transformation from α-phase to β-phase silicon nitride does not occur in an ammonia atmosphere at 1300-1400℃, which is beneficial for improving the purity of the product phase.

[0008] After cooling the purified product, oxygen is introduced to remove remaining impurities such as carbon black. At a temperature of 600-650℃, oxygen and carbon black react to produce gas that leaves without affecting the silicon nitride product. This effectively performs secondary purification of the product, removes C impurities from silicon nitride, and improves the purity of silicon nitride.

[0009] The presence of multiple gases can effectively promote the reaction, remove byproducts and impurities generated during the preparation process, and efficiently purify silicon nitride products. The different flow rates of the multiple gases entering the reaction system can subject the particles to different forces, which help to break up agglomerates, make the particles better dispersed, make the reaction more complete, optimize the interaction between particles, improve the dispersibility of the product, and promote particle refinement.

[0010] Optionally, a method for preparing silica nanoparticles includes the following steps: A precursor solution was obtained by mixing and stirring a deionized water, ethanol and ammonium chloride solution in a volume ratio of 1:(7-8):(0.3-0.5). Add the composite silicon source dropwise to the precursor solution, stir, let stand for 2.5-3 hours, centrifuge, wash with anhydrous ethanol, and dry at 80-90℃ for 12-14 hours to obtain silicon dioxide nanoparticles. The volume ratio of the composite silicon source to the ammonium chloride solution is (1.5-1.8):1. The composite silicon source includes propyltriethoxysilane and tetraethyl orthosilicate in a volume ratio of 1:(1-2).

[0011] By adopting the above technical solution and using a composite silicon source, silica nanoparticles are prepared using the sol-gel method. This helps to obtain silica nanoparticles with more uniform particle size and better dispersibility. The uniform structure and good dispersibility help the silica nanoparticles mix better with carbon black in the liquid phase, avoiding agglomeration and accumulation between particles. This helps to promote the silicon nitride synthesis reaction, improve the purity of the reaction product, and promote the refinement of the silicon nitride particles. Propyltriethoxysilane and tetraethyl orthosilicate, as composite silicon sources, have high purity and can avoid the mixing of other impurities. Moreover, the synergistic effect of different components in the composite silicon source can promote a more stable structure of silica nanoparticles, thereby improving the purity and stability of silica nanoparticles.

[0012] Optionally, when cooling the secondary purified product, the secondary purified product is passed through liquid ammonia for 4-6 hours and then the liquid ammonia is evaporated to remove it.

[0013] By adopting the above technical solution, liquid ammonia has excellent cooling capacity, which can rapidly cool the secondary purification product in a relatively mild process, avoiding product structure changes or impurity formation that may be caused by temperature changes. Liquid ammonia also provides a relatively closed and pure environment, preventing secondary purification products from coming into contact with impurities in the external environment and causing secondary pollution. After cooling, liquid ammonia can be effectively removed by evaporation, ensuring that the product does not contain any impurities from liquid ammonia itself or its decomposition.

[0014] Optionally, the blend may be pretreated as follows before stirring: add ethanol to the blend, with a volume ratio of ethanol to deionized water of (0.5-0.8):100.

[0015] By adopting the above technical solution, the addition of ethanol can act as a dispersant to reduce the surface tension of silica nanoparticles, thereby reducing the surface energy and making it easier for silica nanoparticles to disperse in the liquid. This can prevent the aggregation of silica nanoparticles and promote a more uniform mixing of silica nanoparticles and carbon black in the liquid phase.

[0016] Optionally, hydrogen gas is introduced into the crude product simultaneously with the ammonia gas, and the flow rate ratio of the ammonia gas to the hydrogen gas is (2-3):1.

[0017] By adopting the above technical solution, in the first purification process, ammonia serves as the nitrogen source, providing the nitrogen element required for the formation of silicon nitride in the reaction, while hydrogen plays a reducing role. The gas molecules in the mixed gas flow have a higher diffusion rate and collision frequency during the reaction, which increases the opportunity for oxides such as silicon dioxide and silicon monoxide to come into contact with ammonia and hydrogen molecules, thereby improving the reaction rate and efficiency. In addition, the mixed gas generates a certain stirring effect on the reaction system, making the reactants react more uniformly in the reaction area, reducing the impact of local concentration differences on the reaction, effectively improving the conversion rate of the reaction in reducing the remaining impurities such as silicon dioxide and silicon monoxide to silicon nitride, and improving the purification efficiency.

[0018] Optionally, the flow rate of the nitrogen gas is 2.5-7.5 L / min.

[0019] By adopting the above technical solution, the nitrogen flow rate of 2.5-7.5 L / min is moderate, which can ensure that the nitrogen and silicon dioxide in the reaction system are in full contact. At the same time, it can promptly remove gases such as carbon monoxide generated in the reaction, and prevent the reaction from being too violent due to excessive flow rate, which would lead to an increase in product impurities. This can result in silicon nitride products with high purity and good cleanliness.

[0020] Optionally, the flow rate of the ammonia gas is 1.5-6.5 L / min.

[0021] By adopting the above technical solution, the gas flow rate of ammonia is moderate at 1.5-6.5 L / min, which can ensure that nitrogen is supplied to the reaction system in sufficient quantity. This allows unreacted silicon monoxide and unreacted silicon dioxide to come into full contact with ammonia, generating silicon nitride with high purity and good crystallinity. Although excessive flow rate can promote the reaction, it will result in excessive ammonia not participating in the reaction and causing a lot of waste.

[0022] Optionally, the oxygen flow rate is 1-5 L / min.

[0023] By adopting the above technical solution, oxygen can react with carbon black to generate carbon dioxide gas, thereby effectively removing C impurities from silicon nitride products. This is crucial for improving the purity of silicon nitride. A flow rate of 1-5 L / min allows the oxidation reaction to proceed relatively gently, avoiding excessive oxidation or damage to the silicon nitride products. These gentle oxidation conditions help maintain the structural integrity and performance stability of silicon nitride.

[0024] Secondly, this application provides a high-purity silicon nitride powder and its preparation method, employing the following technical solution: A high-purity silicon nitride powder, characterized in that it is prepared by a method for preparing high-purity silicon nitride powder.

[0025] By adopting the above technical solution, high-purity silicon nitride powder with a purity of over 99.65% can be obtained.

[0026] In summary, this application has the following beneficial effects: 1. This application uses the sol-gel method to prepare silica nanoparticles with more uniform particle size and better dispersibility using a composite silicon source. The silica nanoparticles are mixed with carbon black in the liquid phase, which effectively promotes the mixing of silica nanoparticles and carbon black. The nitriding reduction reaction and subsequent impurity removal are carried out in the atmosphere of mixed gas flow, which reduces the particle size of the product and significantly improves the purity of the silicon nitride powder.

[0027] 2. In this application, a mixed gas flow of ammonia and hydrogen is preferably used to purify the crude product after the nitridation reduction reaction. This can effectively remove the unreacted silicon monoxide intermediate and residual silicon dioxide, thereby further improving the purity of the silicon nitride powder product.

[0028] 3. In this application, liquid ammonia is preferably used for cooling to rapidly cool the purified silicon nitride product. The excellent cooling capacity of liquid ammonia and the relatively closed environment provided by liquid ammonia effectively prevent and reduce secondary pollution caused by contact between the product and impurities in the external environment during the cooling process, thus ensuring the purity of the silicon nitride powder. Detailed Implementation

[0029] The following embodiments provide a further detailed description of this application.

[0030] Preparation Example Preparation Examples 1-4 are examples of preparing silica nanoparticles. The ethanol used in the raw materials is anhydrous ethanol; propyltriethoxysilane is selected from Shanghai Yuanye Biotechnology Co., Ltd., T22627; tetraethyl orthosilicate is selected from Shanghai Yuanye Biotechnology Co., Ltd., T21985; and the mass concentration of ammonium chloride solution is 40%.

[0031] Preparation Example 1: Preparation of silica nanoparticles, including the following steps: S1. Add a solution of deionized water, ethanol and ammonium chloride in a volume ratio of 1:7:0.3 to a four-piece magnetic stirrer, set the temperature to 25℃ and stir until homogeneous to obtain a precursor solution. S2. Add the composite silicon source dropwise to the precursor solution in a four-piece magnetically heated stirrer. After the composite silicon source is completely added, continue stirring for 30 minutes, let stand for 2.5 hours, and then centrifuge. After centrifugation, wash with ethanol and dry in a vacuum drying oven at 80°C for 12 hours to obtain particles with an average diameter of 12 nm and a specific surface area of ​​692.38 m². 2 / g of silica nanoparticles. The composite silicon source is a mixture of propyltriethoxysilane and tetraethyl orthosilicate in a volume ratio of 1:1.5, and the volume ratio of the composite silicon source to the ammonium chloride solution is 1.5:1.

[0032] Preparation Example 2: Preparation of silica nanoparticles, including the following steps: S1. Add a solution of deionized water, ethanol and ammonium chloride in a volume ratio of 1:7.5:0.4 to a four-piece magnetic stirrer, set the temperature to 25℃ and stir until homogeneous to obtain a precursor solution. S2. Add the composite silicon source dropwise to the precursor solution in a four-piece magnetically heated stirrer. After the composite silicon source has been completely added, continue stirring for 30 minutes, let stand for 3 hours, and then centrifuge. After centrifugation, wash with ethanol and dry in a vacuum drying oven at 90℃ for 14 hours to obtain particles with an average diameter of 13 nm and a specific surface area of ​​686.26 m². 2 / g of silica nanoparticles. The composite silicon source is a mixture of propyltriethoxysilane and tetraethyl orthosilicate in a volume ratio of 1:2, and the volume ratio of the composite silicon source to the ammonium chloride solution is 1.6:1.

[0033] Preparation Example 3: Preparation of silica nanoparticles, including the following steps: S1. Add a solution of deionized water, ethanol and ammonium chloride in a volume ratio of 1:8:0.5 to a four-piece magnetic stirrer, set the temperature to 25°C and start stirring to obtain a precursor solution. S2. Add the composite silicon source dropwise to the precursor solution in a four-piece magnetically heated stirrer. After the composite silicon source is completely added, continue stirring for 30 minutes. After standing for 2.5 hours, uniform silica nanoparticles are obtained. After centrifugation, wash with ethanol and dry in a vacuum drying oven at 80℃ for 13 hours to obtain an average particle size of 11 nm and a specific surface area of ​​698.98 m². 2 / g of silica nanoparticles. The composite silicon source is a mixture of propyltriethoxysilane and tetraethyl orthosilicate in a volume ratio of 1:1, and the volume ratio of the composite silicon source to the ammonium chloride solution is 1.8:1.

[0034] Preparation Example 4: Preparation of silica nanoparticles, including the following steps: S1. Add a solution of deionized water, ethanol and ammonium chloride in a volume ratio of 1:7:0.3 to a four-piece magnetic stirrer, set the temperature to 25°C and start stirring to obtain a precursor solution. S2. Tetraethyl orthosilicate was added dropwise to the precursor solution in a four-piece magnetic stirrer. After the tetraethyl orthosilicate was completely added, stirring was continued for 30 minutes. After standing for 2.5 hours, uniform silica nanoparticles were obtained. After centrifugation, the nanoparticles were washed with ethanol and dried in a vacuum drying oven at 80°C for 12 hours, yielding an average particle size of 12 nm and a specific surface area of ​​695.38 m².2 / g of silica nanoparticles. The volume ratio of tetraethyl orthosilicate to ammonium chloride solution is 1.5:1. Example

[0035] Example 1: A high-purity silicon nitride powder, wherein the silicon dioxide nanoparticles used in the raw materials were prepared in Preparation Example 1; carbon black with a particle size of 21 nm; nitrogen with a purity of 99.999%; ammonia with a purity of 99.999%; hydrogen with a purity of 99.999%; oxygen with a purity of 99.999%; liquid ammonia with a purity of 99.9%; and anhydrous ethanol.

[0036] The above-mentioned high-purity silicon nitride powder is prepared by the following method: S1. Mix 350g of silica nanoparticles with 210g of carbon black, add 800mL of deionized water and stir until a thin paste is formed to obtain a blend. S2. Add 6 mL of anhydrous ethanol to the blend, place the blend on a four-piece magnetic stirrer and set the temperature to 25°C, stir for 3 h, and then dry in a vacuum drying oven at 60°C for 8 h to obtain the silicon-carbon precursor. S3. Place the silicon carbide precursor in a corundum sintering boat and heat it to 1400℃ in nitrogen gas with a flow rate of 4L / min at a heating rate of 10℃ / min. Hold the temperature for 10h to obtain the crude product. S4. Continue to simultaneously introduce ammonia gas at a flow rate of 2 L / min and hydrogen gas at a flow rate of 1 L / min into the crude product, and lower the temperature to 1340℃ at a rate of 5℃ / min. Hold the temperature for 3.8 h to obtain the first purified product. S5. Cool the primary purified product to 630℃, introduce oxygen at a flow rate of 2L / min, and keep it at this temperature for 5.4h to obtain the secondary purified product. S6. Pass the secondary purified product into liquid ammonia and cool it rapidly for 5 hours. Then evaporate the liquid ammonia at a temperature above -5°C.

[0037] Example 2: A high-purity silicon nitride powder, which differs from Example 1 in that the silicon dioxide nanoparticles are prepared by Example 2; The above-mentioned high-purity silicon nitride powder is prepared by the following method: S1. Mix 300g of silica nanoparticles with 120g of carbon black, add 600mL of deionized water and stir until a thin paste is formed to obtain a blend. S2. Add 3 mL of anhydrous ethanol to the blend, place the blend on a four-piece magnetic stirrer and set the temperature to 25 °C, stir for 2 h, and then dry in a vacuum drying oven at 65 °C for 6 h to obtain the silicon-carbon precursor. S3. Place the silicon carbide precursor in a corundum sintering boat and heat it to 1350°C in nitrogen gas at a flow rate of 2.5 L / min at a heating rate of 10°C / min. Hold the temperature for 10 h to obtain the crude product. S4. Continue to simultaneously introduce ammonia gas at a flow rate of 1.5 L / min and hydrogen gas at a flow rate of 0.5 L / min into the crude product, and lower the temperature to 1300℃ at a rate of 5℃ / min. Hold the temperature for 4 hours to obtain the first purified product. S5. Cool the primary purified product to 650℃, introduce oxygen at a flow rate of 1L / min, and keep it at this temperature for 5.7h to obtain the secondary purified product. S6. Pass the secondary purified product into liquid ammonia and cool it rapidly for 4 hours. Then evaporate the liquid ammonia at a temperature above -5°C.

[0038] Example 3: A high-purity silicon nitride powder, which differs from Example 1 in that the silicon dioxide nanoparticles are prepared by Example 3; The above-mentioned high-purity silicon nitride powder is prepared by the following method: S1. Mix 300g of silica nanoparticles with 240g of carbon black, add 850mL of deionized water and stir until a thin paste is formed to obtain a blend. S2. Add 6.8 mL of anhydrous ethanol to the blend, place the blend on a four-piece magnetic stirrer and set the temperature to 25 °C, stir for 3 h, and then dry in a vacuum drying oven at 60 °C for 8 h to obtain the silicon-carbon precursor. S3. Place the silicon carbide precursor in a corundum sintering boat and heat it to 1450°C in nitrogen gas at a flow rate of 7.5 L / min at a heating rate of 10°C / min. Hold the temperature for 9 hours to obtain the crude product. S4. Continue to simultaneously introduce ammonia gas at a flow rate of 6.5 L / min and hydrogen gas at a flow rate of 3 L / min into the crude product, and lower the temperature to 1360℃ at a rate of 5℃ / min. Hold the temperature for 3.5 h to obtain the first purified product. S5. Cool the primary purified product to 620℃, introduce oxygen at a flow rate of 4L / min, and keep it at this temperature for 5 hours to obtain the secondary purified product. S6. Pass the secondary purified product into liquid ammonia and cool it rapidly for 5 hours. Then evaporate the liquid ammonia at a temperature above -5°C.

[0039] Example 4: A high-purity silicon nitride powder, differing from Example 1 in that it is prepared by the following method: S1. Mix 400g of silica nanoparticles prepared in Preparation Example 1 with 400g of carbon black, add 950mL of deionized water and stir until a thin paste is formed to obtain a blend. S2. Add 5.7 mL of anhydrous ethanol to the blend, place the blend on a four-piece magnetic stirrer and set the temperature to 25 °C, stir for 3 h, and then dry in a vacuum drying oven at 65 °C for 8 h to obtain the silicon-carbon precursor. S3. Place the silicon carbide precursor in a corundum sintering boat and heat it to 1500℃ in nitrogen gas with a flow rate of 6L / min at a heating rate of 10℃ / min. Hold the temperature for 9h to obtain the crude product. S4. Continue to simultaneously introduce ammonia gas at a flow rate of 4 L / min and hydrogen gas at a flow rate of 2 L / min into the crude product, and lower the temperature to 1400℃ at a rate of 5℃ / min. Hold the temperature for 3 hours to obtain the first purified product. S5. Cool the primary purified product to 600℃, introduce oxygen at a flow rate of 5L / min, and keep it at this temperature for 6 hours to obtain the secondary purified product. S6. Pass the secondary purified product into liquid ammonia and cool it rapidly for 5 hours. Then evaporate the liquid ammonia at a temperature above -5°C.

[0040] Example 5: A high-purity silicon nitride powder, which differs from Example 1 in that the silicon dioxide nanoparticles are prepared by Preparation Example 4, while the other steps are the same as in Example 1.

[0041] Example 6: A high-purity silicon nitride powder, which differs from Example 1 in that anhydrous ethanol was not added to the blend in step S2, while the other steps are the same as in Example 1.

[0042] Example 7: A high-purity silicon nitride powder, which differs from Example 1 in that step S4 is as follows: continue to introduce ammonia gas at a flow rate of 2L / min into the crude product, lower the temperature to 1340℃ at a rate of 5℃ / min, and keep it at that temperature for 3.8h to obtain a first-purified product. All other steps are the same as in Example 1.

[0043] Example 8: A high-purity silicon nitride powder, which differs from Example 1 in that step S6 is: the secondary purified product is naturally cooled to room temperature, and the other steps are the same as in Example 1.

[0044] Comparative Example Comparative Example 1: A high-purity silicon nitride powder, differing from Example 1 in that it was prepared by the following method: S1. Mix 350g of silica nanoparticles prepared in Preparation Example 1 with 210g of carbon black and place them in a corundum calcining boat. Heat the mixture to 1400℃ in nitrogen gas with a flow rate of 4L / min at a heating rate of 10℃ / min and hold for 10h to obtain the crude product. S2. Continue to simultaneously introduce ammonia gas at a flow rate of 2 L / min and hydrogen gas at a flow rate of 1 L / min into the crude product, and lower the temperature to 1340℃ at a rate of 5℃ / min. Hold the temperature for 3.8 h to obtain the first purified product. S3. Cool the primary purified product to 630℃, introduce oxygen at a flow rate of 2L / min, and keep it at this temperature for 5.4h to obtain the secondary purified product. S4. Pass the secondary purified product into liquid ammonia and cool it rapidly for 5 hours. Then evaporate the liquid ammonia at a temperature above -5°C.

[0045] Comparative Example 2: A high-purity silicon nitride powder, which differs from Example 1 in that it is prepared by the following method: ammonia and hydrogen are not introduced into the crude product, and all other steps are the same as in Example 1.

[0046] S1. Mix 350g of silica nanoparticles with 210g of carbon black, add 800mL of deionized water and stir until a thin paste is formed to obtain a blend. S2. Add 6 mL of ethanol to the blend, place the blend on a four-piece magnetic stirrer and set the temperature to 25 °C, stir for 3 h, and then dry in a vacuum drying oven at 60 °C for 8 h to obtain the silicon-carbon precursor. S3. Place the silicon carbide precursor in a corundum sintering boat and heat it to 1400℃ in nitrogen gas with a flow rate of 4L / min at a heating rate of 10℃ / min. Hold the temperature for 10h to obtain the crude product. S4. Cool the crude product to 630℃, introduce oxygen at a flow rate of 2L / min, and keep it at this temperature for 5.4h to obtain the purified product. S5. Pass the purified product into liquid ammonia and cool it rapidly for 5 hours. Then evaporate the liquid ammonia at a temperature above -5°C.

[0047] Comparative Example 3: A high-purity silicon nitride powder, which differs from Example 1 in that step S5 was not performed. Step S6 specifically involves: passing the purified product into liquid ammonia and rapidly cooling it for 5 hours, then evaporating and removing the liquid ammonia at a temperature above -5°C. All other steps are the same as in Example 1.

[0048] Comparative Example 4: A method for preparing high-purity silicon nitride powder, comprising the following steps: S1. Load silica particles into a ball mill, add grinding media, and use the ball mill to grind and crush them to form 25-micron silica. S2. Nitrogen gas is introduced into the high-temperature container to remove the air inside the container, and then the power is supplied to raise the temperature to 300°C and hold it for 5 minutes. S3. Take out the crushed silica and dry it to obtain silica powder. At the same time, heat the high temperature container to 1500℃. Then, under the protective gas, spray the silica powder and carbon black into the high temperature container at a mass ratio of 1:2. Continue to introduce nitrogen into the high temperature container. Under the condition that the nitrogen flow rate in the furnace is stable, the silica powder and nitrogen react fully for 10 hours. S4. The silica dry powder is cooled to room temperature in the furnace, and then the nitrogen gas is stopped. The silicon nitride product is collected, crushed, and sieved to obtain high-purity silicon nitride powder.

[0049] Performance testing The high-purity silicon nitride powders obtained in Examples 1-8 and Comparative Examples 1-4 were tested using the following methods: the carbon and oxygen content in the silicon nitride powders was tested according to GB / T16555-2017, and the test data were recorded in Table 1; the free silicon content was tested using the colorimetric method, and the test data were recorded in Table 1; the impurity content of the synthesized silicon nitride powders was tested using ICP, and the purity of the silicon nitride powders was calculated, and the calculated data were recorded in Table 1.

[0050] The particle size of silicon nitride powder was determined by laser diffraction, and the test data are recorded in Table 1.

[0051] Analysis of test results: According to the performance test data of Examples 1-8 and Comparative Examples 1-4, the silicon nitride content of the high-purity silicon nitride powder of this application is all above 99.65%, the oxygen content is all <0.23%, the carbon content is all <0.7%, the free silicon content is all <0.1%, and the particle size D50 is all 0.42-0.57μm. The silicon nitride powder prepared by the preparation method of the high-purity silicon nitride powder of this application has high purity, low impurity content, and small particle size, which meets the requirements of high-quality powder.

[0052] Based on the detection data from Examples 1-4 and Comparative Example 1, it can be seen that the silica nanoparticles prepared by the sol-gel method, used as the silicon source for the nitride reduction reaction, have smaller particles, better dispersibility, and are easier to mix with carbon black. In Examples 1-4, silica nanoparticles and carbon black were stirred in a mixed liquid phase of deionized water and anhydrous ethanol, ultimately forming a sol-gel mixture with good dispersibility of both silica nanoparticles and carbon black. The silicon-carbon precursor obtained after drying this sol-gel mixture showed more thorough and uniform mixing between silica nanoparticles and carbon black compared to the mixture obtained by directly mixing silica nanoparticles and carbon black in the solid phase in Comparative Example 1. This facilitates the subsequent full reaction to generate high-purity silicon nitride and reduces the particle size of the product.

[0053] The detection data from Examples 1-5 show that the dispersibility of silica nanoparticles in the liquid phase affects the mixing of silica nanoparticles and carbon black, thus affecting the subsequent preparation of silicon nitride and the particle size of the final product. Using a composite silicon source, compared to using a single silicon source for the preparation of silica nanoparticles, yields silica nanoparticles with more uniform particle size and better dispersibility. The uniform structure and good dispersibility result in more uniform mixing of the silica nanoparticles with carbon black, which helps promote the synthesis of silicon nitride and improve product purity.

[0054] The detection data from Examples 1-4 and Example 6 show that the addition of ethanol helps the silica nanoparticles disperse in the liquid phase, and promotes the formation of a sol-gel mixture between the silica nanoparticles and carbon black in the liquid phase. This makes the mixing between the silica nanoparticles and carbon black more thorough and uniform, and promotes the subsequent formation of silicon nitride with higher purity.

[0055] According to the detection data of Examples 1-4, Example 7 and Comparative Example 2, ammonia can effectively remove oxygen impurities at a certain temperature, converting unreacted intermediate products such as silicon monoxide and residual silicon dioxide into silicon nitride, reducing the content of free silicon and oxygen. In the mixed gas flow of ammonia and hydrogen, the reaction is more complete and the effect is more significant.

[0056] The detection data from Examples 1-4 and Comparative Example 3 show that oxygen can effectively remove residual carbon impurities, and at appropriate temperatures, it does not affect the reaction products or increase other impurities.

[0057] According to the detection data of Examples 1-4 and Example 8, the purity of the product is affected by the natural cooling process. During the natural cooling process, the external influences cause the impurities such as free silicon and oxygen in the product to increase.

[0058] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A method for preparing high-purity silicon nitride powder, characterized in that, By weight, the following steps are included: Mix 30-40 parts of silica nanoparticles with 12-40 parts of carbon black, add deionized water to obtain a blend, stir the blend for 2-3 hours, and dry it at 60-65℃ for 6-8 hours to obtain a silicon-carbon precursor. Nitrogen gas is introduced into the silicon carbide precursor and heated to 1350-1500℃. The temperature is maintained for 8-10 hours to obtain a crude product. Ammonia and hydrogen gas are then introduced into the crude product and the temperature is maintained at 1300-1400℃ for 3-4 hours to obtain a first-purified product. The flow rate ratio of ammonia gas to hydrogen gas is (2-3):

1. The product from the first purification is cooled to 600-650℃, oxygen is introduced, and the temperature is maintained for 5-6 hours to obtain the product from the second purification. The product from the second purification is then cooled by passing it through liquid ammonia for 4-6 hours, and the liquid ammonia is evaporated to remove it, thus obtaining the final product.

2. The method for preparing high-purity silicon nitride powder according to claim 1, characterized in that, The method for preparing the silica nanoparticles includes the following steps: A precursor solution was obtained by mixing and stirring a deionized water, ethanol and ammonium chloride solution in a volume ratio of 1:(7-8):(0.3-0.5). Add the composite silicon source dropwise to the precursor solution, stir, let stand for 2.5-3 hours, centrifuge, wash with anhydrous ethanol, and dry at 80-90℃ for 12-14 hours to obtain silicon dioxide nanoparticles. The volume ratio of the composite silicon source to the ammonium chloride solution is (1.5-1.8):

1. The composite silicon source includes propyltriethoxysilane and tetraethyl orthosilicate in a volume ratio of 1:(1-2).

3. The method for preparing high-purity silicon nitride powder according to claim 1, characterized in that, The following pretreatment should be performed on the blend before mixing: Add ethanol to the blend, with the volume ratio of ethanol to deionized water being (0.5-0.8):

100.

4. The method for preparing high-purity silicon nitride powder according to claim 1, characterized in that: The flow rate of the nitrogen gas is 2.5-7.5 L / min.

5. The method for preparing high-purity silicon nitride powder according to claim 1, characterized in that: The flow rate of the ammonia gas is 1.5-6.5 L / min.

6. The method for preparing high-purity silicon nitride powder according to claim 1, characterized in that: The oxygen flow rate is 1-5 L / min.

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