Method for preparing superfine alpha-phase silicon nitride powder

Abstract

The invention provides a method for preparing superfine alpha-phase silicon nitride powder. The method comprises the following steps: by taking metal salt, an additive and amorphous silicon nitride powder as raw materials and an organic solvent as a grinding solvent, carrying out ball milling under the protection of inert gas to obtain uniformly mixed slurry; drying the obtained slurry under the protection of inert gas, grinding and sieving; performing high-temperature crystallization treatment on the sieved powder in a protective atmosphere; and taking out the sample, cleaning with an acid solution and water in sequence, and drying to obtain the alpha-phase silicon nitride powder. The additive is salt and / or graphite powder which can be decomposed at high temperature to release gas. On the basis of taking metal salt (fused salt) as a medium, an additive is used. On one hand, carbonic acid and / or sulfate in the additive are / is decomposed at a high temperature, gas is released, the liquid phase fluidity and dispersity are improved, and mass transfer is enhanced; on the other hand, the graphite powder in the additive provides a nucleation position for amorphous silicon nitride crystallization, and crystallization is promoted.

Description

Technical Field

[0001] The present invention belongs to the technical field of silicon nitride ceramic powder, and particularly relates to a method for preparing ultrafine α-phase silicon nitride powder. Background Art

[0002] Silicon nitride ceramic materials have excellent properties such as high strength, high temperature resistance, corrosion resistance, radiation resistance, and thermal shock resistance. In high-tech fields such as electric vehicles, high-speed railways, aerospace, mechatronics, chemical metallurgy, and national defense, the application of silicon nitride materials is becoming more and more extensive, and in some large-scale national projects and cutting-edge technologies, it has become an indispensable key material. High-quality powder raw materials are a prerequisite for preparing ceramic materials with excellent properties. Common silicon nitride mainly has two crystal forms, α and β phases. In addition to the crystalline phase, silicon nitride also has an amorphous form.

[0003] By performing high-temperature crystallization treatment on amorphous silicon nitride powder, silicon nitride powder with high α-phase and high sintering activity can be obtained, and this powder is widely used in the preparation of fine silicon nitride ceramics. The existing crystallization treatment of amorphous silicon nitride is mainly carried out in a gas atmosphere (Paper 1: P.F. Liu, W. Xi, Yuan Li, et al. Synthesis of equiaxedSi 3 N 4 from Si(NH) 2 by adding seeds formed by in situ nitridation of silicon. Ceramics International, 2024, 50(16): 28411–28418; Paper 2: T. Itoh. Preparation of pure α-silicon nitride from silicon powder. Journal of materials science letters, 1990, 9: 19-20.).

[0004] The molten salt synthesis method utilizes the characteristics that substances are more uniformly mixed and diffuse faster in the liquid state, which can significantly reduce the synthesis temperature of high-temperature ceramic powder and shorten the synthesis time. The molten salt synthesis method is used to prepare powder materials of various component systems, such as oxides, multi-component oxides, metal or alloy powders, etc. (Patent 1: Si prepared by the molten salt method 3 N 4 / MgSiN 2Method for preparing high - thermal - conductivity silicon nitride ceramics from composite powder, ZL 202311505512.9; Patent 2: Preparation of fine - grained and low - oxygen molybdenum - silicon - boron alloy based on molten salt method, ZL202310061195.X; Patent 3: A method for low - temperature synthesis of rhombohedral boron nitride powder based on molten salt method and its preparation method, ZL201810338386.5; Patent 4: A method for growing gallium nitride single crystal by using a novel flux molten salt method, ZL200510115278.4; Paper 1: Zhao Yuehong, Gao Yun, Sun Yi, etc. Process research on low - temperature preparation of flaky alumina by molten salt method. Chemical Engineer, 2023, 10: 06 - 10; Paper 2: Cheng Dengfeng, Ke Changming, Zhang Jinhua, etc. Research progress on molten salt synthesis of carbon / nitride ceramic powders. China Ceramics, 2023, 59(2): 1 - 12.).

[0005] In the existing methods for preparing silicon nitride powder using salt as an additive, the main raw materials are silicon powder and amorphous silicon nitride powder, which are different from the raw materials in this invention. Moreover, the method of using silicon powder and salt requires reaction with nitrogen to generate silicon nitride (Paper 1: Q.Gu, J.Zhang, H.X.Li, et al. Synthesis ofα - Si 3 N 4 powder by high energy ballmilling assisting molten salt nitridation method at low temperature. Ceramics International, 2019, 45(15): 18445 - 18451; Paper 2: Z.N.Chai, J.Ding, C.J.Deng, etal. Ni - catalyzed synthesis of hexagonal plate - like alpha silicon nitride from nitridation of Si powder in molten salt media. Advanced Powder Technology, 2016, 27(4): 1637 - 1644; Patent: Method for preparing high - purityα - silicon nitride powder and high - purityα - silicon nitride powder, CN202010282836.0).

[0006] Traditional methods for crystallizing amorphous silicon nitride usually require long - time heat treatment under inert gas protection and high temperature, which have problems such as high crystallization temperature and uneven crystallization. Under inert gas protection, the solid - phase amorphous silicon nitride particles transform intoα - phase silicon nitride particles. The growth of particles is highly affected by the environment, the grain morphology is difficult to control, and the particles are prone to agglomeration and have poor dispersibility. Summary of the Invention

[0007] The present invention aims to solve the problems that during the transformation of amorphous silicon nitride into the α-phase at high temperature, it has uneven distribution in the liquid phase, difficult-to-control morphology, poor dispersibility, and too high crystallization temperature, and provides a method for preparing ultrafine α-phase silicon nitride powder.

[0008] The method for preparing ultrafine α-phase silicon nitride powder provided by the present invention comprises the following steps:

[0009] (1) Using metal salts, additives, and amorphous silicon nitride powder as raw materials, and an organic solvent as the grinding solvent, ball milling is carried out under the protection of an inert gas to obtain a uniformly mixed slurry;

[0010] (2) Drying the obtained slurry under the protection of an inert gas, grinding, and sieving;

[0011] (3) Performing high-temperature crystallization treatment on the sieved powder in a protective atmosphere;

[0012] (4) Taking out the sample in step (3), washing it successively with an acid solution and water, and drying to obtain α-phase silicon nitride powder.

[0013] In step (1) of the above method, the metal salt can remain in a liquid state in the range of 700 - 1600 °C and can dissolve in water at room temperature;

[0014] The metal salt can be selected from one or more of: sodium chloride, potassium chloride, calcium chloride, magnesium chloride, barium chloride, sodium fluoride, potassium fluoride, lithium fluoride, sodium bromide, potassium bromide, sodium silicate, sodium tungstate, potassium titanate, sodium sulfate, potassium sulfate;

[0015] In the raw materials, the mass ratio of the metal salt to the amorphous silicon nitride powder can be: 1 - 100:1;

[0016] The additive is 0.1% - 10% of the mass of the amorphous silicon nitride powder;

[0017] The additive is a salt that can decompose and release gas at high temperature and / or graphite powder;

[0018] The salt that can decompose and release gas at high temperature can be selected from one or more of: calcium carbonate, strontium carbonate, barium carbonate, lithium carbonate, sodium carbonate, potassium carbonate, calcium sulfate, barium sulfate, strontium sulfate;

[0019] The organic solvent can be selected from one or more of: absolute ethanol, acetone, isopropanol, ethyl acetate, n-hexane, dichloromethane, ether, methanol, tert-butanol, and cyclohexane;

[0020] The mass ratio of the amorphous silicon nitride powder to the organic solvent can be 1:1 - 100.

[0021] The ball milling uses silicon nitride balls as the grinding medium;

[0022] The rotation speed of the ball milling is 50 - 300 rpm; the ball milling time is 0.5 - 24 h;

[0023] In the above method step (2), the drying method is any one of vacuum drying, rotary evaporation drying, and spray drying;

[0024] The grinding and sieving are carried out under the protection of inert gas, using a mortar for grinding, and the mesh number of the sieve is 60 - 1000 meshes.

[0025] In the above method step (3), the heating rate of the crystallization is 5 - 100 °C / min, the crystallization temperature is 1200 - 1600 °C, the heat preservation time is 0.5 - 8 h, and the cooling rate is 10 - 200 °C / min;

[0026] The protective atmosphere is one or more of nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, and ammonia, and the gas pressure is 0.01 - 10 MPa.

[0027] In the above method step (4), first wash with hydrochloric acid solution 3 - 7 times, and then wash with deionized water 3 - 15 times;

[0028] The drying method is any one of vacuum drying, freeze drying, and rotary evaporation drying, and the drying is carried out under the protection of nitrogen.

[0029] The particle size of the obtained α-phase silicon nitride powder is 10 - 1000 nm.

[0030] The α-phase silicon nitride powder prepared by the above method also belongs to the protection scope of the present invention.

[0031] The technical principle of the present invention is:

[0032] Amorphous Si 3 N 4 (s) → α - Si 3 N 4

[0033] In the present invention, a metal salt (molten salt) is used as a medium. The molten salt can provide a stable thermodynamic environment, reduce the energy fluctuation during the crystallization process of the solute, and thus lower the crystallization temperature. In Patent CN 109608205 B (a method for preparing equiaxed α-phase silicon nitride powder), the selected molten salts are metal chlorides, fluorides, oxides, and sulfates (potassium sulfate, sodium sulfate, and magnesium sulfate). At the crystallization temperature, the densities of the liquid phase formed by the molten salt and the amorphous silicon nitride powder are different, and it is easy to have the problem of uneven distribution of solid-phase particles. On the basis of this patent, the present invention supplements the types of molten salts and additionally adds additives. The carbonate and / or sulfate in the selected additives decompose at the crystallization temperature, release gases, and during the process of the formed microbubbles escaping from the inside of the molten salt liquid to the liquid surface, they promote the flow of the molten salt and enhance the mass transfer process in the crucible. The graphite powder in the additives provides more nucleation sites and further promotes crystallization. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 SEM diagram of the silicon nitride powder prepared in Example 1 of the present invention.

[0035] Figure 2 XRD diagram of the silicon nitride powder prepared in Example 1 of the present invention.

[0036] Figure 3 SEM diagram of the silicon nitride powder prepared in Example 2 of the present invention.

[0037] Figure 4 XRD diagram of the silicon nitride powder prepared in Example 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The present invention will be further described in detail below in conjunction with the specific embodiments. The provided embodiments are only for clarifying the present invention, rather than limiting the scope of the present invention. The following provided embodiments can be used as a guide for those of ordinary skill in the art to make further improvements, and do not limit the present invention in any way.

[0039] The experimental methods in the following embodiments are all conventional methods unless otherwise specified, and are carried out according to the techniques or conditions described in the literature in this field or according to the product instructions. The materials, reagents, etc. used in the following embodiments can be obtained from commercial sources unless otherwise specified.

[0040] The sources of the reagents in the following embodiments are shown in the following table:

[0041]

[0042]

[0043] Example 1

[0044] (1) Mix 90 g of sodium fluoride, 0.3 g of barium carbonate, 0.2 g of graphite powder, 10 g of amorphous silicon nitride powder, and 500 g of ethanol. Using silicon nitride balls as the grinding medium, carry out ball milling under nitrogen protection (rotation speed is 300 rpm, ball milling time is 2 h) to obtain a uniformly mixed slurry.

[0045] (2) Dry the mixed slurry obtained in the above step under nitrogen protection (60 °C, rotary evaporation drying for 2 h), then grind it in a mortar and pass through a 200-mesh sieve to make the particles uniform.

[0046] (3) Transfer the powder after sieving in the above step to a crucible, and carry out high-temperature crystallization treatment under a flowing nitrogen atmosphere. The crystallization heating rate is 10 °C / min, the crystallization temperature is 1500 °C, the holding time is 2 h, and it is cooled to room temperature at a cooling rate of 10 °C / min.

[0047] (4) Take out the sample from step (3), wash it 5 times with 1 mol / L hydrochloric acid, and then wash it 5 times with deionized water.

[0048] (5) Freeze-dry for 72 h under the conditions of a temperature of -40 °C and a pressure of 1 Pa to obtain silicon nitride powder.

[0049] Detect the silicon nitride powder prepared in the above steps. The α-phase content is 95.4%, nearly equiaxed particles, and the average particle size is 0.5 μm.

[0050] Figure 1 It is the SEM image of the prepared silicon nitride powder.

[0051] Figure 2 It is the XRD pattern of the prepared silicon nitride powder.

[0052] Comparative Example 1

[0053] (1) Mix 90 g of sodium fluoride, 10 g of amorphous silicon nitride powder, and 500 g of ethanol. Using silicon nitride balls as the grinding medium, carry out ball milling under nitrogen protection (rotation speed is 300 rpm, ball milling time is 2 h) to obtain a uniformly mixed slurry.

[0054] (2) Dry the mixed slurry obtained in the above step under nitrogen protection (60 °C, rotary evaporation drying for 2 h), then grind it in a mortar and pass through a 200-mesh sieve to make the particles uniform.

[0055] (3) Transfer the powder after sieving in the above step to a crucible, and carry out high-temperature crystallization treatment under a flowing nitrogen atmosphere. The crystallization heating rate is 10 °C / min, the crystallization temperature is 1500 °C, the holding time is 2 h, and it is cooled to room temperature at a cooling rate of 10 °C / min.

[0056] (4) Take out the sample from step (3), wash it 5 times with 1 mol / L hydrochloric acid, and then wash it 5 times with deionized water.

[0057] (5) Freeze-dry for 72 h under the conditions of a temperature of -40 °C and a pressure of 1 Pa to obtain silicon nitride powder.

[0058] For the silicon nitride powder prepared by the above steps, the α-phase content is 91.2%, the particles are nearly equiaxed, and the average particle size is 1.2 μm.

[0059] Comparative Example 2

[0060] (1) Mix 10 g of amorphous silicon nitride powder and 500 g of ethanol, use silicon nitride balls as the grinding medium, and carry out ball milling under nitrogen protection (rotation speed is 300 rpm, ball milling time is 2 h) to obtain a uniformly mixed slurry.

[0061] (2) Dry the mixed slurry obtained in the above step under nitrogen protection (60 °C, rotary evaporation drying for 2 h), then grind it in a mortar and pass through a 200-mesh sieve to make the particles uniform.

[0062] (3) Transfer the powder after sieving in the above step to a crucible, and carry out high-temperature crystallization treatment under a flowing nitrogen atmosphere. The crystallization heating rate is 10 °C / min, the crystallization temperature is 1500 °C, the holding time is 4 h, and the temperature is reduced to room temperature at a cooling rate of 10 °C / min.

[0063] (4) Take out the sample from step (3), wash it 5 times with 1 mol / L hydrochloric acid, and then wash it 5 times with deionized water.

[0064] (5) Freeze-dry for 72 h under the conditions of a temperature of -40 °C and a pressure of 1 Pa to obtain silicon nitride powder.

[0065] For the silicon nitride powder prepared by the above steps, the α-phase content is 94.2%, the morphology is whiskers and crystalline grains, and the crystalline grains are short columnar.

[0066] Comparative Example 3

[0067] (1) Mix 10 g of amorphous silicon nitride powder and 500 g of ethanol, use silicon nitride balls as the grinding medium, and carry out ball milling under nitrogen protection (rotation speed is 300 rpm, ball milling time is 2 h) to obtain a uniformly mixed slurry.

[0068] (2) Dry the mixed slurry obtained in the above step under nitrogen protection (60 °C, rotary evaporation drying for 2 h), then grind it in a mortar and pass through a 200-mesh sieve to make the particles uniform.

[0069] (3) Transfer the powder after sieving in the above steps to a crucible, and under a flowing nitrogen atmosphere, conduct high-temperature crystallization treatment. The crystallization heating rate is 10 °C / min, the crystallization temperature is 1500 °C, the heat preservation time is 2 h, and it is cooled to room temperature at a cooling rate of 10 °C / min.

[0070] (4) Take out the sample in step (3), wash it 5 times with 1 mol / L hydrochloric acid, and then wash it 5 times with deionized water.

[0071] (5) Freeze-dry for 72 h under the conditions of a temperature of -40 °C and a pressure of 1 Pa to obtain silicon nitride powder.

[0072] For the silicon nitride powder prepared in the above steps, the α-phase content is 68.8%, the morphology is whiskers and crystalline grains, and the crystalline grains are short columnar.

[0073] Example 2

[0074] (1) Mix 90 g of sodium chloride, 1 g of calcium sulfate, 10 g of amorphous silicon nitride powder, and 500 g of ethanol, use silicon nitride balls as the grinding medium, and conduct ball milling under nitrogen protection (rotation speed is 300 rpm, ball milling time is 2 h) to obtain a uniformly mixed slurry.

[0075] (2) Dry the mixed slurry obtained in the above step under nitrogen protection (60 °C, rotary evaporation drying for 2 h), then grind it in a mortar and pass through a 200-mesh sieve to make the particles uniform.

[0076] (3) Transfer the powder after sieving in the above steps to a crucible, and under a flowing nitrogen atmosphere, conduct high-temperature crystallization treatment. The crystallization heating rate is 10 °C / min, the crystallization temperature is 1400 °C, the heat preservation time is 2 h, and it is cooled to room temperature at a cooling rate of 10 °C / min.

[0077] (4) Take out the sample in step (3), wash it 5 times with 1 mol / L hydrochloric acid, and then wash it 5 times with deionized water.

[0078] (5) Freeze-dry for 72 h under the conditions of a temperature of -40 °C and a pressure of 1 Pa to obtain silicon nitride powder.

[0079] For the silicon nitride powder prepared in the above steps, the α-phase content is 92.0%, and the particles are stacked flakes.

[0080] Figure 3 It is the SEM image of the prepared silicon nitride powder.

[0081] Figure 4 It is the XRD pattern of the prepared silicon nitride powder.

[0082] Example 3

[0083] (1) Mix 70 g of sodium fluoride, 0.3 g of lithium carbonate, 0.2 g of graphite powder, 10 g of amorphous silicon nitride powder, and 500 g of ethanol. Using silicon nitride balls as the grinding medium, carry out ball milling under nitrogen protection (rotation speed is 300 rpm, ball milling time is 2 h) to obtain a uniformly mixed slurry.

[0084] (2) Dry the mixed slurry obtained in the above step under nitrogen protection (60 °C, rotary evaporation drying for 2 h), then grind it and pass it through a 200-mesh sieve to make the particles uniform.

[0085] (3) Transfer the powder after sieving in the above step to a crucible, and carry out high-temperature crystallization treatment under a flowing nitrogen atmosphere. The crystallization heating rate is 10 °C / min, the crystallization temperature is 1250 °C, the holding time is 1.5 h, and it is cooled to room temperature at a cooling rate of 10 °C / min.

[0086] (4) Take out the sample from step (3), wash it 5 times with 1 mol / L hydrochloric acid, and then wash it 5 times with deionized water.

[0087] (5) Freeze-dry for 72 h under the conditions of a temperature of -40 °C and a pressure of 1 Pa to obtain silicon nitride powder.

[0088] Detect the silicon nitride powder prepared in the above steps. The α-phase content of the silicon nitride powder is 92.0%, the particles are near equiaxed particles, and the average particle size is 0.1 μm.

[0089] Example 4

[0090] (1) Mix 95 g of potassium fluoride, 0.5 g of lithium carbonate, 5 g of amorphous silicon nitride powder, and 500 g of ethanol. Using silicon nitride balls as the grinding medium, carry out ball milling under nitrogen protection (rotation speed is 300 rpm, ball milling time is 2 h) to obtain a uniformly mixed slurry.

[0091] (2) Dry the mixed slurry obtained in the above step under nitrogen protection (60 °C, rotary evaporation drying for 2 h), then grind it with a mortar and pass it through a 200-mesh sieve to make the particles uniform.

[0092] (3) Transfer the powder after sieving in the above step to a crucible, and carry out high-temperature crystallization treatment under the condition of a nitrogen gas pressure of 2.0 MPa. The crystallization heating rate is 10 °C / min, the crystallization temperature is 1300 °C, the holding time is 0.5 h, and it is cooled to room temperature at a cooling rate of 10 °C / min.

[0093] (4) Take out the sample from step (3), wash it 5 times with 1 mol / L hydrochloric acid, and then wash it 5 times with deionized water.

[0094] (5) Freeze-dry for 72 h under the conditions of a temperature of -40 °C and a pressure of 1 Pa to obtain silicon nitride powder.

[0095] Detect the silicon nitride powder obtained in the above steps, and obtain that the α-phase content of the silicon nitride powder is 95.8%, near equiaxed particles, and the average particle size is 0.4 μm.

[0096] Comparing the data of Example 1 and Comparative Example 1, it can be seen that in Example 1, sodium fluoride is used as the molten salt, and barium carbonate and graphite powder are used as additives. The silicon nitride powder obtained has fine particles and a near equiaxed morphology. In Comparative Example 1, no additives are used, and the particle size is larger. In Comparative Examples 2 and 3, no molten salt and additives are used, the crystallization rate is slow, and the crystal particle size is large. The data of Examples 1 to 4 show that the morphology and size of the α-phase silicon nitride powder prepared by the method of the present invention can be achieved by using different metal salts and additives, indicating that the method of the present invention can improve the quality of the α-phase silicon nitride powder.

[0097] The beneficial effects of the present invention are as follows:

[0098] (1) At high temperatures, the metal salt melts into a liquid phase. Compared with the technology of phase change crystallization in the gas phase, the liquid phase can provide a more uniform and stable temperature field and chemical field. The amorphous silicon nitride powder particles are more uniformly dispersed in the liquid phase, so the crystalline α-phase silicon nitride particles are also more uniform and smaller in size.

[0099] (2) The cations and anions in the metal salt can affect the nucleation and growth of silicon nitride, resulting in the inhibition of the growth of certain crystal planes of α-phase silicon nitride. Therefore, the morphology of the grains can be regulated to obtain particles with a single morphology.

[0100] (3) The carbonates and sulfates in the additives decompose at high temperatures to generate gases. Trace bubbles can slightly disturb the liquid phase, making the amorphous silicon nitride more uniformly dispersed and increasing the mass transfer in the liquid phase. The liquid phase formed by the additive and the metal salt has a low melting point, which accelerates crystallization. At the same time, the graphite powder in the additive provides more nucleation sites, reduces the nucleation energy, and increases the crystallization rate.

[0101] (4) The ions in the metal salt and the additive interact with the amorphous silicon nitride particles, making it easier to undergo phase change crystallization at a lower temperature. Under the combined action of the metal salt and the additive, the efficient crystallization temperature of amorphous silicon nitride can be reduced to 1250 °C, improving production efficiency and reducing energy consumption.

[0102] (5) Pickling the crystallized powder can effectively remove the residual additives and metal impurities in the raw materials. For the washed powder, vacuum drying, freeze-drying, and rotary evaporation drying methods are used to reduce the agglomeration of the powder during sedimentation and drying in water. Especially freeze-drying can effectively prevent the aggregation of primary particles of the powder to obtain ultrafine powder with controllable particle size.

[0103] The present invention has been described in detail above. For those skilled in the art, without departing from the spirit and scope of the present invention and without unnecessary experiments, the present invention can be implemented within a relatively wide range under equivalent parameters, concentrations and conditions. Although specific embodiments of the present invention are given, it should be understood that the present invention can be further improved. In short, according to the principle of the present invention, this application intends to cover any modifications, uses or improvements of the present invention, including those that depart from the scope disclosed in this application and are made by conventional techniques known in the art.

Claims

1. A method for preparing ultrafine α-phase silicon nitride powder, comprising the following steps: (1) using metal salt, additives, and amorphous silicon nitride powder as raw materials and an organic solvent as a grinding solvent, ball milling is performed under the protection of an inert gas to obtain a uniformly mixed slurry; (2) drying the obtained slurry under the protection of inert gas, grinding and sieving; (3) subjecting the sieved powder to high temperature crystallization treatment in a protective atmosphere; (4) taking out the sample from step (3), washing it with an acid solution and water in turn, and drying it to obtain α-phase silicon nitride powder; The additive is salt and / or graphite powder which can decompose and release gas at high temperature.

2. The method according to claim 1, characterized in that The metal salt can remain liquid in the range of 700-1600°C and can be dissolved in water at room temperature; The metal salt is selected from: one or more of sodium chloride, potassium chloride, calcium chloride, magnesium chloride, barium chloride, sodium fluoride, potassium fluoride, lithium fluoride, sodium bromide, potassium bromide, sodium silicate, sodium tungstate, potassium titanate, sodium sulfate, and potassium sulfate; In the raw materials, the mass ratio of the metal salt to the amorphous silicon nitride powder is 1-100:

1.

3. The method according to claim 1, characterized in that The additive is 0.1%-10% of the mass of the amorphous silicon nitride powder; The salt that can decompose and release gas at high temperature is selected from one or more of calcium carbonate, strontium carbonate, barium carbonate, lithium carbonate, sodium carbonate, potassium carbonate, calcium sulfate, barium sulfate, and strontium sulfate.

4. The method according to claim 1, characterized in that: The organic solvent is selected from one or more of anhydrous ethanol, acetone, isopropanol, ethyl acetate, n-hexane, dichloromethane, ether, methanol, tert-butanol and cyclohexane; The mass ratio of the amorphous silicon nitride powder to the organic solvent is 1:1-100.

5. The method according to claim 1, characterized in that The ball mill uses silicon nitride balls as grinding media; The ball milling speed is 50-300 rpm; the ball milling time is 0.5-24 h.

6. The method according to claim 1, characterized in that In step (2), the drying method is any one of vacuum drying, rotary evaporation drying and spray drying; The grinding and screening are carried out under the protection of inert gas, with mortar grinding and the mesh number of the screen being 60-1000 meshes.

7. The method according to claim 1, characterized in that In step (3), the crystallization heating rate is 5-100°C / min, the crystallization temperature is 1200-1600°C, the holding time is 0.5-8h, and the cooling rate is 10-200°C / min; The protective atmosphere is one or more of nitrogen, argon, hydrogen, carbon monoxide, carbon dioxide, and ammonia, and the gas pressure is 0.01-10MPa.

8. The method according to claim 1, characterized in that In step (4), the substrate is first washed with hydrochloric acid solution for 3-7 times, and then washed with deionized water for 3-15 times; The drying method is any one of vacuum drying, freeze drying and rotary evaporation drying, and the drying is performed under nitrogen protection.

9. Ultrafine α-phase silicon nitride powder obtained by the method according to any one of claims 1 to 8.

10. The ultrafine α-phase silicon nitride powder according to claim 9, characterized in that: The particle size of the α-phase silicon nitride powder is 10nm-1000nm.

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