Preparation method of silicon nitride powder

Abstract

The invention relates to the field of inorganic nonmetal powder materials, and discloses a preparation method of silicon nitride powder, which comprises the following steps: (1) mixing a surfactant, liquid ammonia and an organic solvent to form a to-be-reacted solution, and mixing the to-be-reacted solution with a silicon tetrachloride solution for substitution reaction to obtain a solution containing silicon imine suspended matters; the surfactant is selected from one or more of sodium dodecyl sulfate, polyvinylpyrrolidone, sodium stearate, sodium dodecyl benzene sulfonate, fatty acid methyl ester ethoxylate sulfonate, fatty acid sorbitan, polysorbate-20 and polysorbate-80; the surfactant is selected from one or more of sodium dodecyl sulfate, polyvinylpyrrolidone, sodium stearate, sodium dodecyl benzene sulfonate, fatty acid methyl ester ethoxylate sulfonate, fatty acid sorbitan, polysorbate-20 and polysorbate-80; (2) carrying out suspended matter extraction and impurity removal on the solution containing the silicon imine suspended matter to obtain silicon imine powder; (3) performing thermal decomposition on the silicon imine powder to obtain amorphous silicon nitride powder; and (4) crystallizing the amorphous silicon nitride powder to obtain the alpha-phase silicon nitride powder. The method can reduce the impurity content and agglomeration degree of the precursor so as to improve the quality of the silicon nitride powder.

Description

Technical Field

[0001] The present invention relates to the field of inorganic non-metallic powder materials, and particularly to a method for preparing silicon nitride powder. Background Art

[0002] Si3N4 ceramics have many excellent properties such as high hardness, high strength, low thermal expansion coefficient, low high-temperature creep, good oxidation resistance, strong thermal corrosion resistance, and low friction coefficient, and are one of the structural ceramic materials with the best comprehensive performance. The ammonolysis method synthesizes a silicon imide precursor through a liquid-phase method, obtains amorphous silicon nitride powder through preliminary calcination, and then crystallizes to obtain high-α-phase silicon nitride powder through calcination at a higher temperature. The prepared product has a higher purity and good sintering performance of the powder. However, in the synthesis process of the silicon imide precursor powder, the raw material liquid ammonia reacts violently with silicon tetrachloride, the reaction rate is too fast, the obtained precursor agglomerates severely, and the generated by-product ammonium chloride is coated by the precursor, resulting in a relatively large particle size of the silicon nitride powder and difficult to reduce the impurity chloride ion content to less than 100 ppm during the calcination process.

[0003] Currently, the method of reducing the reaction temperature is commonly used to slow down the reaction rate and reduce the precursor agglomeration to improve the quality of silicon nitride powder. However, in this method, the refrigeration energy consumption is relatively high, and after the temperature is lowered to below -40 °C, further reducing the temperature has little effect on the reaction rate, and it is difficult to achieve an ideal optimization effect. Summary of the Invention

[0004] In order to solve the technical problem of poor quality of silicon nitride powder prepared by the above ammonolysis method, the present invention provides a method for preparing silicon nitride powder, which uses a surfactant to reduce the impurity content and agglomeration degree of the precursor silicon imide to improve the quality of silicon nitride powder.

[0005] The specific technical solution of the present invention is as follows: A method for preparing silicon nitride powder, comprising the following steps: (1) Mix a surfactant, liquid ammonia and an organic solvent to form a reaction solution to be reacted, and mix the reaction solution to be reacted with a silicon tetrachloride solution for a substitution reaction to obtain a solution containing silicon imide suspension; the surfactant is selected from one or more of sodium dodecyl sulfonate, polyvinylpyrrolidone, sodium stearate, sodium dodecylbenzenesulfonate, fatty acid methyl ester ethoxylate sulfonate, sorbitan fatty acid ester, polysorbate-20, polysorbate-80; (2) Perform suspension extraction and impurity removal on the solution containing silicon imide suspension to obtain silicon imide powder; (3) Thermally decompose the silicon imide powder to obtain amorphous silicon nitride powder; (4) Perform crystallization treatment on the amorphous silicon nitride powder to obtain α-phase silicon nitride powder.

[0006] The present invention reduces the impurity content and agglomeration degree of silicon imide by adding a specific surfactant during the preparation of silicon imide. Specifically, on the one hand, the reaction between liquid ammonia and silicon tetrachloride is very violent. After adding the above-mentioned surfactant, the surfactant forms micelles to wrap the reactant droplets, forming an incompletely coated protective film, which hinders the contact between reactants, thereby reducing the reaction rate, enabling the by-product ammonium chloride to be fully dissolved in liquid ammonia and not participating in the agglomeration of silicon imide, thus greatly reducing the chloride ion content in silicon imide. On the other hand, for the silicon imide solid particles generated from silicon tetrachloride and liquid ammonia, the above-mentioned surfactant can coat the surface of the generated particles, modify the generated silicon imide particles, hinder the mutual agglomeration between particles, reduce the size of silicon imide particles, and improve the uniformity of particle size distribution. During the synthesis of silicon imide, by adding a specific surfactant, not only the chloride ion content in silicon imide is reduced, but also the agglomeration of silicon imide is reduced. The obtained silicon imide has high purity, small size and uniform distribution. After the above-mentioned silicon imide is subjected to thermal decomposition and crystallization treatment, α-phase silicon nitride powder with high purity, small size and uniform distribution can be obtained.

[0007] Preferably, in step (1), the dosage of the surfactant is 1-20% of the mass of liquid ammonia, specifically 1-8%.

[0008] Preferably, in step (1), the mass ratio of liquid ammonia to the organic solvent is about 6:1-6, and the organic solvent is selected from one or more of n-heptane, n-hexane, cyclohexane, n-pentane, benzene, toluene and xylene.

[0009] In the above technical solution, when an organic solvent that forms a liquid-liquid interface with liquid ammonia is selected from toluene, the added surfactant can adsorb at the interface to form a dense molecular layer, which will hinder silicon tetrachloride from crossing the interface to contact liquid ammonia, thereby reducing the reaction rate. When organic solvents such as n-hexane and benzene are selected, there is no obvious stratification with liquid ammonia, and liquid ammonia is diluted by the organic solvent, which can reduce the reaction rate and is beneficial to improving the dispersibility of the generated particles. After adding the surfactant, the reaction rate between silicon tetrachloride and liquid ammonia can be further reduced, and small-sized silicon imide particles are generated.

[0010] Preferably, in step (1), the substitution reaction is carried out under stirring conditions, the reaction temperature is -45°C to -10°C, and the reaction pressure is 0.06-4 MPa.

[0011] Preferably, in step (1), the volume fraction of silicon tetrachloride in the silicon tetrachloride solution is 1-20%, and the solvent is selected from one or more of n-heptane, n-hexane, cyclohexane, n-pentane, benzene, toluene and xylene.

[0012] In the above technical solution, since the reaction between liquid ammonia and silicon tetrachloride is very violent, diluting silicon tetrachloride with an organic solvent can reduce the reaction rate.

[0013] Preferably, in step (2), the extraction of the suspended matter is specifically as follows: the solution containing the silicon imide suspended matter is filtered to separate the silicon imide suspended matter, and then the silicon imide suspended matter is washed with liquid ammonia 1 to 15 times and filtered. The total amount of liquid ammonia used for washing is 4 to 20 times the volume of the silicon tetrachloride solution in step (1).

[0014] In the above technical solution, the silicon imide suspended matter is washed with liquid ammonia to further remove the residual ammonium chloride in the silicon imide suspended matter and reduce the impurity content in the silicon imide suspended matter.

[0015] Preferably, in step (2), the impurity removal includes: placing the extracted suspended matter in an impurity removal atmosphere and keeping it at a constant temperature of 200 to 400 °C for 0.5 to 2 hours to remove the residual chlorine impurities and organic solvents in the suspended matter. The impurity removal atmosphere is selected from one or more of nitrogen, helium, and argon.

[0016] In the above technical solution, by utilizing the characteristics of high-temperature decomposition of ammonium chloride and high-temperature volatilization of organic solvents, the chlorine impurities and organic solvents remaining in the silicon imide are further removed by high temperature.

[0017] Preferably, in step (2), the impurity removal further includes: placing the suspended matter from which the chlorine impurities and organic solvents have been removed in an impurity removal atmosphere and keeping it at a constant temperature of 500 to 900 °C for 0.5 to 2 hours to remove the residual carbon impurities in the suspended matter. The impurity removal atmosphere is selected from one or more of oxygen, nitrogen, helium, hydrogen, argon, ammonia, and carbon monoxide.

[0018] In the above technical solution, by utilizing the characteristics of high-temperature decomposition of organic solvents, the carbon impurities remaining in the silicon imide are removed by high temperature.

[0019] Preferably, in step (3), the thermal decomposition is specifically as follows: the silicon imide powder is placed in a nitrogen atmosphere with an air flow rate of 100 to 200 mL / min and thermally decomposed at 900 °C to 1300 °C for 0.5 to 4 hours.

[0020] In the above technical solution, between 900 °C and 1300 °C, the chemical bonds in the silicon imide molecule begin to break and recombine, gradually transforming into an amorphous silicon nitride structure. If the temperature is lower than 900 °C, the silicon imide decomposes insufficiently, and some silicon imide remains, which will lead to a decrease in the purity of the final product, and the incompletely decomposed substances will interfere with the formation of the subsequent crystal structure; if the temperature is higher than 1300 °C, the atomic migration is too intense, affecting the particle size distribution of the powder.

[0021] Preferably, in step (4), the crystallization treatment is carried out in a crystallization protective atmosphere at a temperature of 1200-1700 °C for 2-5 hours, and the crystallization protective atmosphere is selected from one or more of oxygen, nitrogen, helium, hydrogen, argon, ammonia, and carbon monoxide.

[0022] In the above technical solution, if the temperature is lower than 1200 °C, the molecular kinetic energy of amorphous silicon nitride is not enough to overcome the activation energy, the phase change process is slow or even difficult to occur, and α-phase silicon nitride cannot be effectively obtained; if the temperature is higher than 1700 °C, it will cause excessive crystal growth and coarse grains, affecting the performance of the powder.

[0023] Compared with the prior art, the present invention has the following advantages: (1) During the preparation of silazane, a specific surfactant is added, which reduces the agglomeration degree of silazane powder, makes the powder size small and evenly distributed, and at the same time reduces the chloride ion content in silazane, solving the problem that the internal ammonium chloride cannot be washed and dissolved due to the agglomeration of silazane. After the above silazane is thermally decomposed and crystallized, α-phase silicon nitride powder with high purity, small size and evenly distributed can be obtained; (2) Some of the selected organic solvents can be miscible with liquid ammonia. By reducing the liquid ammonia concentration and combining with the surfactant to reduce the reaction rate, the generated silazane particles are small and not easy to agglomerate; (3) In the preparation of silazane in the present invention, in addition to liquid ammonia washing, high-temperature impurity removal is also added. Utilizing the characteristics of easy decomposition of ammonium chloride, the residual chlorine impurities are removed at high temperature; hydrogen and carbon monoxide reducing atmospheres are used to remove oxygen from the powder; the carbon impurities formed after the volatilization and decomposition of the organic solvent are decomposed at high temperature or removed by reacting hydrogen and carbon monoxide. The chlorine impurities and carbon impurities remaining in the silazane are further removed. Detailed implementation manners

[0024] The present invention will be described below through specific examples. It should be understood that these examples are only used to illustrate the present invention and not to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, the changes and advantages that those skilled in the art can think of are included in the present invention, and the scope of protection of the present invention is defined by the appended claims and any equivalents thereof.

[0025] Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The raw materials and equipment used in this invention are conventional raw materials and equipment in the art and can be obtained from conventional commercial channels without special instructions; the methods used in this invention are conventional methods in the art without special instructions. The reaction equation of silicon tetrachloride and liquid ammonia involved in this invention is: SiCl4(l) + 6NH3(1) → Si(NH)2(s) + 4NH4Cl(s). In the following examples, the prepared α-phase silicon nitride powder was detected according to the GB / T 37258-2018 standard for silicon nitride ceramic powder.

[0026] Example 1: The present invention provides a method for preparing silicon nitride powder, comprising the following steps: (1) At -40 °C, 10 g of sodium dodecyl sulfate is added to a closed container, and then 500 g of liquid ammonia and 100 g of n-hexane are introduced into the closed container, stirred and mixed at a stirring rate of 80 rpm to form a reaction solution to be reacted; (2) 20 ml of silicon tetrachloride and 400 ml of n-hexane are mixed to prepare a silicon tetrachloride solution, and the silicon tetrachloride solution is added dropwise to the reaction solution to be reacted at a dropping rate controlled at 20 mL / min. Silicon tetrachloride reacts with liquid ammonia to generate silazane, and the by-product ammonium chloride formed is dissolved in liquid ammonia. After the reaction, a solution containing silazane suspension is obtained; (3) The solution containing silazane suspension is transferred to a filtration device, and the silazane suspension is filtered out. The silazane suspension is separated, washed 3 times with liquid ammonia and filtered, and then transferred into a thermal decomposition furnace. The total amount of liquid ammonia used for washing is 4 times the volume of the silicon tetrachloride solution; (4) The thermal decomposition furnace is heated to 300 °C, nitrogen is slowly introduced into the furnace, and the extracted suspension is placed in a nitrogen atmosphere and kept at a constant temperature for 1 hour to remove residual chlorine impurities and organic solvents in the suspension. Then nitrogen is switched to argon, and after complete replacement, the temperature in the furnace is raised to 550 °C and kept at a constant temperature for 1 hour to remove residual carbon impurities in the suspension, obtaining silazane powder; (5) In the thermal decomposition furnace, the argon in the furnace is replaced with nitrogen, the silazane powder is placed in a flowing nitrogen atmosphere with a nitrogen flow rate of 100 mL / min, and thermally decomposed at 950 °C for 2 hours to obtain amorphous silicon nitride powder; (6) The amorphous silicon nitride powder is transferred to a crystallization furnace for crystallization treatment. The temperature in the furnace is raised to 1450 °C, and the amorphous silicon nitride powder is in a flowing nitrogen atmosphere with a nitrogen flow rate of 100 mL / min and kept at a constant temperature for 2 hours to obtain α-phase silicon nitride powder.

[0027] The α-phase silicon nitride powder obtained by the above steps was detected. The α-phase content of the silicon nitride powder was 93.2%, the average particle size was 0.2 μm, the chloride ion content was 23 ppm, the oxygen content was 0.8%, and the carbon content was 0.08%.

[0028] In this example, the water content of sodium dodecyl sulfate used was <100 ppm, the closed container was a reaction kettle, the filtration equipment was a two-in-one filter, and the thermal decomposition furnace and the crystallization furnace were the same rotary furnace. After obtaining the amorphous silicon nitride powder, crystallization treatment was continued in this rotary furnace. It can be understood that the organic solvent can also be n-heptane, cyclohexane, n-pentane, benzene, toluene or xylene; the filtration equipment can also be a three-in-one filter or a ceramic membrane filter, etc.; the thermal decomposition furnace can also be a pusher kiln, a roller hearth kiln or a tunnel kiln, etc.; the crystallization furnace can also be a pusher kiln, a gas pressure furnace, a roller hearth kiln or a tunnel kiln, etc., and the crystallization furnace can be different equipment from the thermal decomposition furnace. In step (1) of this example, the self-vaporization of liquid ammonia made the pressure in the closed container during the reaction 0.07 Mpa; the surfactant can also be selected from polyvinylpyrrolidone, sodium stearate, sodium dodecylbenzenesulfonate, fatty acid methyl ester ethoxylate sulfonate, sorbitan fatty acid ester, polysorbate-20, polysorbate-80.

[0029] Example 2: The present invention provides a method for preparing silicon nitride powder, comprising the following steps: (1) At -30°C, 16 g of polyvinylpyrrolidone was added to a closed container, and then 600 g of liquid ammonia and 100 g of n-heptane were introduced into the closed container. Nitrogen was used to pressurize to 4 MPa, and stirring and mixing were carried out at a stirring rate of 100 rpm to form a liquid to be reacted; (2) 25 ml of silicon tetrachloride was mixed with 400 ml of n-heptane to prepare a silicon tetrachloride solution. The silicon tetrachloride solution was added dropwise to the liquid to be reacted, and the dropping rate was controlled at 50 mL / min. Silicon tetrachloride reacted with liquid ammonia to form silazane, and the by-product ammonium chloride was dissolved in liquid ammonia. After the reaction, a solution containing silazane suspension was obtained; (3) The solution containing silazane suspension was transferred to a filtration device, and the silazane suspension was filtered out. The silazane suspension was separated, washed 5 times with liquid ammonia and filtered, and then transferred into a thermal decomposition furnace. The total amount of liquid ammonia used during washing was 5 times the volume of the silicon tetrachloride solution; (4) The thermal decomposition furnace was heated to 250°C, helium was slowly introduced into the furnace, and the extracted suspension was placed in a helium atmosphere and kept at a constant temperature for 1 hour to remove the residual chlorine impurities and organic solvents in the suspension. Then the helium was switched to argon. After complete replacement, the temperature in the furnace was raised to 600°C and kept at a constant temperature for 0.5 hour to remove the residual carbon impurities in the suspension to obtain silazane powder; (5) In the thermal decomposition furnace, the argon gas in the furnace is replaced with nitrogen gas. The silicon imide powder is placed in a flowing nitrogen atmosphere with a nitrogen gas flow rate of 130 mL / min and thermally decomposed at 900 °C for 1.5 hours to obtain amorphous silicon nitride powder; (6) Transfer the amorphous silicon nitride powder to a crystallization furnace for crystallization treatment. Raise the temperature in the furnace to 1500 °C. The amorphous silicon nitride powder is in a flowing nitrogen atmosphere with a nitrogen gas flow rate of 130 mL / min and kept at a constant temperature for 2 hours to obtain α-phase silicon nitride powder.

[0030] For the α-phase silicon nitride powder prepared in the above steps, the α-phase content of the silicon nitride powder is 92.7%, the average particle size is 0.7 μm, the chloride ion content is 45 ppm, the oxygen content is 0.6%, and the carbon content is 0.05%.

[0031] In this example, the polyvinylpyrrolidone used has a water content of <100 ppm. The closed container is a reaction kettle, the filtration equipment is a three-in-one filter, and the thermal decomposition furnace and the crystallization furnace are the same rotary furnace. After obtaining the amorphous silicon nitride powder, the crystallization treatment is continued in this rotary furnace.

[0032] Example 3: The present invention provides a method for preparing silicon nitride powder, comprising the following steps: (1) At -10 °C, add 2.5 g of sodium stearate to a closed container, and then introduce 250 g of liquid ammonia and 100 g of toluene into the closed container. Pressurize with nitrogen gas to 4 MPa and stir and mix at a stirring rate of 150 rpm to form a reaction solution to be reacted; (2) Mix 5 ml of silicon tetrachloride with 95 ml of toluene to prepare a silicon tetrachloride solution. Dropwise add the silicon tetrachloride solution to the reaction solution to be reacted, and control the dropping rate at 20 mL / min. The silicon tetrachloride reacts with the liquid ammonia to generate silicon imide, and the generated by-product ammonium chloride dissolves in the liquid ammonia. After the reaction, a solution containing silicon imide suspension is obtained; (3) Transfer the solution containing the silicon imide suspension to a filtration device, filter to obtain the silicon imide suspension, separate the silicon imide suspension, wash the silicon imide suspension 15 times with liquid ammonia and filter, and then transfer it into a thermal decomposition furnace. The total amount of liquid ammonia used for washing is 20 times the volume of the silicon tetrachloride solution; (4) Raise the temperature of the thermal decomposition furnace to 200 °C, slowly introduce argon gas into the furnace, place the extracted suspension in an argon atmosphere and keep it at a constant temperature for 2 hours to remove the residual chlorine impurities and organic solvents in the suspension, and then switch the argon gas to nitrogen gas. After complete replacement, raise the temperature in the furnace to 500 °C and keep it at a constant temperature for 2 hours to remove the residual carbon impurities in the suspension to obtain silicon imide powder; (5) In the thermal decomposition furnace, the silicon imide powder is placed in a flowing nitrogen atmosphere with a nitrogen gas flow rate of 200 mL / min, and thermally decomposed at 1000 °C for 4 hours to obtain amorphous silicon nitride powder; (6) Transfer the amorphous silicon nitride powder to a crystallization furnace for crystallization treatment. Raise the furnace temperature to 1200 °C. The amorphous silicon nitride powder is in a flowing nitrogen atmosphere with a nitrogen gas flow rate of 200 mL / min, and keep the temperature constant for 5 hours to obtain α-phase silicon nitride powder.

[0033] Detect the α-phase silicon nitride powder prepared in the above steps. The α-phase content of the silicon nitride powder is 94.0%, the average particle size is 0.7 μm, the chloride ion content is 33 ppm, the oxygen content is 0.6%, and the carbon content is 0.12%.

[0034] In this example, the water content of sodium stearate used is <100 ppm, the closed container is a reaction kettle, the filtration equipment is a two-in-one filter, and the thermal decomposition furnace and the crystallization furnace are the same rotary furnace. After obtaining the amorphous silicon nitride powder, continue the crystallization treatment in this rotary furnace.

[0035] Example 4: The present invention provides a method for preparing silicon nitride powder, comprising the following steps: (1) At -45 °C, add 8 g of sodium dodecyl sulfonate to a closed container, and then introduce 100 g of liquid ammonia and 100 g of toluene into the closed container, stir and mix at a stirring rate of 80 rpm to form a reaction solution to be reacted; (2) Mix 5 ml of silicon tetrachloride with 200 ml of toluene to prepare a silicon tetrachloride solution. Dropwise add the silicon tetrachloride solution to the reaction solution to be reacted, and control the dropping rate at 20 mL / min. Silicon tetrachloride reacts with liquid ammonia to generate silicon imide, and the generated by-product ammonium chloride dissolves in liquid ammonia. After the reaction, a solution containing silicon imide suspension is obtained; (3) Transfer the solution containing silicon imide suspension to a filtration equipment, filter to obtain the silicon imide suspension, separate the silicon imide suspension, wash the silicon imide suspension 10 times with liquid ammonia and filter, and then transfer it into a thermal decomposition furnace. The total amount of liquid ammonia used for washing is 10 times the volume of the silicon tetrachloride solution; (4) Raise the temperature of the thermal decomposition furnace to 400 °C, slowly introduce nitrogen gas into the furnace, place the extracted suspension in a nitrogen atmosphere and keep the temperature constant for 0.5 hour to remove the residual chlorine impurities and organic solvents in the suspension, and then raise the furnace temperature to 700 °C and keep the temperature constant for 0.5 hour to remove the residual carbon impurities in the suspension to obtain silicon imide powder; (5) In the thermal decomposition furnace, the silicon imide powder is placed in a flowing nitrogen atmosphere with a nitrogen gas flow rate of 150 mL / min, and thermally decomposed at 1300 °C for 3 hours to obtain amorphous silicon nitride powder; (6) Transfer the amorphous silicon nitride powder to a crystallization furnace for crystallization treatment. Raise the temperature in the furnace to 1700 °C. The amorphous silicon nitride powder is in a flowing nitrogen atmosphere with a nitrogen flow rate of 150 mL / min and kept at a constant temperature for 2 hours to obtain α-phase silicon nitride powder.

[0036] Detect the α-phase silicon nitride powder obtained in the above steps. The α-phase content of the silicon nitride powder is 92.4%, the average particle size is 0.5 μm, the chloride ion content is 31 ppm, the oxygen content is 0.5%, and the carbon content is 0.05%.

[0037] In this example, the water content of sodium dodecyl sulfonate used is <100 ppm, the closed container is a reaction kettle, the filtration equipment is a two-in-one filter, and the crystallization furnace and the thermal decomposition furnace are the same rotary furnace. After obtaining the amorphous silicon nitride powder, continue the crystallization treatment in this rotary furnace. In step (1) of this example, the self-vaporization of liquid ammonia makes the pressure in the closed container during the reaction 0.06 Mpa.

[0038] Example 5: The present invention provides a method for preparing silicon nitride powder, comprising the following steps: (1) At -40 °C, add 10 g of dodecylsulfonic acid to a closed container, and then introduce 500 g of liquid ammonia and 100 g of n-hexane into the closed container, stir and mix at a stirring rate of 80 rpm to form a liquid to be reacted; (2) Mix 20 ml of silicon tetrachloride with 400 ml of n-hexane to prepare a silicon tetrachloride solution, and dropwise add the silicon tetrachloride solution to the liquid to be reacted at a dropping rate controlled at 20 mL / min. Silicon tetrachloride reacts with liquid ammonia to form silazane, and the by-product ammonium chloride is dissolved in liquid ammonia. After the reaction, a solution containing silazane suspension is obtained; (3) Transfer the solution containing silazane suspension to a filtration device, filter to obtain the silazane suspension, separate the silazane suspension, wash the silazane suspension 3 times with liquid ammonia and filter, and then transfer it to a thermal decomposition furnace. The total amount of liquid ammonia used for washing is 4 times the volume of the silicon tetrachloride solution; (4) Raise the temperature of the thermal decomposition furnace to 300 °C, slowly introduce nitrogen into the furnace, place the extracted suspension in a nitrogen atmosphere and keep it at a constant temperature for 1 hour to remove the residual chlorine impurities and organic solvents in the suspension, and then introduce a mixture of 4% hydrogen and 96% nitrogen. After complete replacement, raise the temperature in the furnace to 800 °C and keep it at a constant temperature for 1 hour to remove the residual carbon impurities in the suspension to obtain silazane powder; (5) In the thermal decomposition furnace, replace the argon in the furnace with nitrogen, place the silazane powder in a flowing nitrogen atmosphere with a nitrogen flow rate of 100 mL / min, and thermally decompose it at 950 °C for 2 hours to obtain amorphous silicon nitride powder; (6) Transfer the amorphous silicon nitride powder to a crystallization furnace for crystallization treatment. Raise the temperature in the furnace to 1450 °C. The amorphous silicon nitride powder is in a flowing nitrogen atmosphere with a nitrogen flow rate of 100 mL / min and held at a constant temperature for 2 hours to obtain α-phase silicon nitride powder.

[0039] Detect the α-phase silicon nitride powder obtained in the above steps. The α-phase content of the silicon nitride powder is 92.8%, the average particle size is 0.6 μm, the chloride ion content is 27 ppm, the oxygen content is 0.4%, and the carbon impurity content is 0.02%.

[0040] Comparative Example 1: This comparative example provides a method for preparing silicon nitride powder, including the following steps: (1) At -40 °C, introduce 500 g of liquid ammonia and 100 g of n-hexane into a closed container, stir and mix at a stirring rate of 80 rpm to form a reaction solution to be reacted; (2) Mix 20 ml of silicon tetrachloride with 400 ml of n-hexane to prepare a silicon tetrachloride solution. Dropwise add the silicon tetrachloride solution to the reaction solution to be reacted, and control the dropping rate at 20 mL / min. Silicon tetrachloride reacts with liquid ammonia to form silazane, and the by-product ammonium chloride formed is dissolved in the liquid ammonia. After the reaction, a solution containing silazane suspension is obtained; (3) Transfer the solution containing silazane suspension to a filtration device, filter to obtain the silazane suspension, separate the silazane suspension, wash the silazane suspension 3 times with liquid ammonia and filter, and then transfer it to a thermal decomposition furnace. The total amount of liquid ammonia used for washing is 4 times the volume of the silicon tetrachloride solution; (4) Raise the temperature of the thermal decomposition furnace to 300 °C, slowly introduce nitrogen into the furnace, place the extracted suspension in a nitrogen atmosphere and hold at a constant temperature for 1 hour to remove the residual chlorine impurities and organic solvents in the suspension. Then switch the nitrogen to argon. After complete replacement, raise the temperature in the furnace to 550 °C and hold at a constant temperature for 1 hour to remove the residual carbon impurities in the suspension to obtain silazane powder; (5) In the thermal decomposition furnace, replace the argon in the furnace with nitrogen. Place the silazane powder in a flowing nitrogen atmosphere with a nitrogen flow rate of 100 mL / min and thermally decompose it at 950 °C for 2 hours to obtain amorphous silicon nitride powder; (6) Transfer the amorphous silicon nitride powder to a crystallization furnace for crystallization treatment. Raise the temperature in the furnace to 1450 °C. The amorphous silicon nitride powder is in a flowing nitrogen atmosphere with a nitrogen flow rate of 100 mL / min and held at a constant temperature for 2 hours to obtain α-phase silicon nitride powder.

[0041] Detect the α-phase silicon nitride powder obtained in the above steps. The α-phase content of the silicon nitride powder is 91.5%, the average particle size is 1.1 μm, the chloride ion content is 600 ppm, the oxygen content is 0.7%, and the carbon content is 0.07%.

[0042] In this comparative example, the closed container used is a reaction kettle, the filtration equipment is a two-in-one filter, and the crystallization furnace and the thermal decomposition furnace are the same rotary furnace. After obtaining the amorphous silicon nitride powder, the crystallization treatment is continued in this rotary furnace.

[0043] Comparative Example 2 The present invention provides a method for preparing silicon nitride powder, comprising the following steps: (1) At -40°C, 10 g of dodecylsulfonic acid is added to a closed container, and then 500 g of liquid ammonia and 100 g of n-hexane are introduced into the closed container, and stirred and mixed at a stirring rate of 80 rpm to form a reaction solution to be reacted; (2) 20 ml of silicon tetrachloride and 400 ml of n-hexane are mixed to prepare a silicon tetrachloride solution, and the silicon tetrachloride solution is added dropwise to the reaction solution to be reacted at a dropping rate controlled at 20 mL / min. The silicon tetrachloride reacts with the liquid ammonia to form silazane, and the by-product ammonium chloride formed is dissolved in the liquid ammonia. After the reaction, a solution containing silazane suspension is obtained; (3) The solution containing the silazane suspension is transferred to a filtration device, and the silazane suspension is filtered out. The silazane suspension is separated, washed 3 times with liquid ammonia and filtered, and then transferred into a thermal decomposition furnace. The total amount of liquid ammonia used for washing is 4 times the volume of the silicon tetrachloride solution; (4) The thermal decomposition furnace is heated to 300°C, nitrogen is slowly introduced into the furnace, and the extracted suspension is placed in a nitrogen atmosphere and kept at a constant temperature for 1 hour to remove the residual chlorine impurities and organic solvents in the suspension to obtain silazane powder; (5) In the thermal decomposition furnace, the silazane powder is placed in a flowing nitrogen atmosphere with a nitrogen flow rate of 100 mL / min, and thermally decomposed at 950°C for 2 hours to obtain amorphous silicon nitride powder; (6) The amorphous silicon nitride powder is transferred to a crystallization furnace for crystallization treatment. The temperature in the furnace is raised to 1450°C. The amorphous silicon nitride powder is in a flowing nitrogen atmosphere with a nitrogen flow rate of 100 mL / min and kept at a constant temperature for 2 hours to obtain α-phase silicon nitride powder.

[0044] The α-phase silicon nitride powder prepared by the above steps is detected. The α-phase content of the silicon nitride powder is 93.7%, the average particle size is 0.6 μm, the chloride ion content is 25 ppm, the oxygen content is 0.5%, and the carbon content is 0.21%.

[0045] In this comparative example, the closed container used is a reaction kettle, the filtration equipment is a two-in-one filter, and the crystallization furnace and the thermal decomposition furnace are the same rotary furnace. After obtaining the amorphous silicon nitride powder, the crystallization treatment is continued in this rotary furnace.

[0046] Comparing the data of Example 1 and Comparative Example 1, it can be seen that the average particle size and chloride ion impurity content of the α-phase silicon nitride powder in Example 1 are significantly smaller than those of the α-phase silicon nitride powder in Comparative Example 1, which proves that adding a specific surfactant in the process of preparing silicon imide can reduce the size of the α-phase silicon nitride powder and improve its purity. Comparing the data of Example 1, 5 and Comparative Example 2, it can be seen that the carbon content in the α-phase silicon nitride powder of Example 1 and 5 is significantly smaller than that of the α-phase silicon nitride powder in Comparative Example 2, which proves that high-temperature decomposition can effectively remove carbon impurities in the powder. The data of Examples 1 to 5 show that the α-phase content of the silicon nitride powder prepared by the method of the present invention is higher than 92%, the average particle size is not more than 0.7 μm, the impurity chloride ion content is reduced to below 50 ppm, and the impurity carbon content is reduced to below 0.13 ppm. According to the regulations in the GB / T 37258-2018 standard for silicon nitride ceramic powder, the quality of the α-phase silicon nitride powder prepared by the method of the present invention is close to the highest grade of Class A, which indicates that the method of the present invention can prepare high-quality α-phase silicon nitride powder.

[0047] In the present invention, the raw materials and equipment used are all common raw materials and equipment in the art without special instructions; the methods used in the present invention are all conventional methods in the art without special instructions.

[0048] The above are only the preferred embodiments of the present invention, and do not limit the present invention in any way. Any simple modifications, changes and equivalent transformations made to the above embodiments according to the technical essence of the present invention still belong to the protection scope of the technical solution of the present invention.

Claims

1. A method for preparing silicon nitride powder, characterized in that: The following steps are involved: (1) mixing a surfactant, liquid ammonia and an organic solvent to form a reaction solution, and mixing the reaction solution with a silicon tetrachloride solution to carry out a substitution reaction to obtain a solution containing a silyl imine suspension; wherein the surfactant is selected from one or more of sodium dodecyl sulfonate, polyvinyl pyrrolidone, sodium stearate, sodium dodecylbenzene sulfonate, fatty acid methyl ester ethoxylate sulfonate, fatty acid sorbitan, polysorbate-20 and polysorbate-80; (2) extracting and removing impurities from the solution containing the silicimide suspension to obtain silicimide powder; (3) thermally decomposing the silicon imide powder to obtain amorphous silicon nitride powder; (4) The amorphous silicon nitride powder is crystallized to obtain α-phase silicon nitride powder.

2. The method for preparing silicon nitride powder according to claim 1, characterized in that: In step (1), the amount of surfactant used is 1 to 20% of the mass of liquid ammonia.

3. The method for preparing silicon nitride powder according to claim 1, characterized in that: In step (1), the mass ratio of liquid ammonia to organic solvent is about 6:1-6, and the organic solvent is selected from one or more of n-heptane, n-hexane, cyclohexane, n-pentane, benzene, toluene and xylene.

4. The method for preparing silicon nitride powder according to claim 1, characterized in that: In step (1), the substitution reaction is carried out under stirring conditions, the reaction temperature is -45°C to -10°C, and the reaction pressure is 0.06 to 4MPa.

5. The method for preparing silicon nitride powder according to claim 1, characterized in that: In step (1), the silicon tetrachloride content in the silicon tetrachloride solution is 1 to 20% by volume, and the solvent is selected from one or more of n-heptane, n-hexane, cyclohexane, n-pentane, benzene, toluene and xylene.

6. The method for preparing silicon nitride powder according to claim 1, characterized in that: In step (2), the suspension extraction is specifically as follows: filtering the solution containing the silicimide suspension to separate the silicimide suspension, then washing the silicimide suspension with liquid ammonia for 1 to 15 times and filtering, wherein the total amount of liquid ammonia used during washing is 4 to 20 times the volume of the silicon tetrachloride solution in step (1).

7. The method for preparing silicon nitride powder according to claim 1, characterized in that: In step (2), the impurity removal includes: placing the extracted suspension in an impurity removal atmosphere and maintaining the suspension at a constant temperature of 200 to 400° C. for 0.5 to 2 hours to remove residual chlorine impurities and organic solvents in the suspension, wherein the impurity removal atmosphere is selected from one or more of nitrogen, helium, and argon.

8. The method for preparing silicon nitride powder according to claim 7, characterized in that: In step (2), the impurity removal further includes: placing the suspension from which chlorine impurities and organic solvents have been removed in an impurity removal atmosphere and maintaining the suspension at a constant temperature of 500 to 900° C. for 0.5 to 2 hours to remove carbon impurities remaining in the suspension, wherein the impurity removal atmosphere is selected from one or more of oxygen, nitrogen, helium, hydrogen, argon, ammonia, and carbon monoxide.

9. A method for preparing silicon nitride powder according to any one of claims 1 to 8, characterized in that: In step (3), the thermal decomposition is specifically as follows: the silicene powder is placed in a nitrogen atmosphere with a gas flow rate of 100 to 200 mL / min, and thermally decomposed at 900° C. to 1300° C. for 0.5 to 4 hours.

10. A method for preparing silicon nitride powder according to any one of claims 1 to 8, characterized in that: In step (4), the crystallization treatment is carried out under a crystallization protective atmosphere at a temperature of 1200 to 1700° C. for 2 to 5 hours, and the crystallization protective atmosphere is selected from one or more of oxygen, nitrogen, helium, hydrogen, argon, ammonia, and carbon monoxide.