Silicon nitride with low oxygen content and high alpha phase as well as preparation method and application thereof

Abstract

The invention relates to the technical field of non-oxide ceramic powder, in particular to low-oxygen-content and high-alpha-phase silicon nitride as well as a preparation method and application thereof. The method comprises the following steps: mixing silicon powder and absolute ethyl alcohol, performing mechanical activation pretreatment to reduce the particle size of the silicon powder, increase the specific surface area and reduce the thickness of an oxide layer on the surface of the silicon powder, and performing vacuum drying to obtain activated silicon powder; and nitriding the activated silicon powder, carrying out two-stage cooling treatment, and then carrying out crushing and sieving treatment to obtain the low-oxygen-content and high-alpha-phase silicon nitride. Through the assistant-free negative-pressure silicon thermal nitriding and mechanical activation synergistic process, the problems that in the prior art, the oxygen content is high, the alpha-phase content is low and difficult to control, powder agglomeration is serious, the morphology is uneven, presintering is not facilitated, the purity is reduced and the cost is increased due to the fact that an assistant needs to be added, and industrialization is difficult due to the fact that the reaction is violent are solved.

Description

Technical Field

[0001] This invention relates to the field of non-oxide ceramic powder technology, specifically to a silicon nitride with low oxygen content and high α phase, its preparation method, and its application. Background Technology

[0002] The global energy structure is shifting from fossil fuels to electricity, and the rapid development of third-generation power semiconductors has placed new demands on ceramic substrates. In particular, as materials that support electronic devices and circuits, these substrates not only need to have excellent thermal conductivity but also good mechanical properties.

[0003] In recent years, silicon nitride (Si3N4) ceramic substrates have gradually become a research hotspot due to their combination of thermal conductivity and mechanical properties. The primary condition for preparing high-quality Si3N4 ceramics is the use of high-quality Si3N4 powder, which needs to possess characteristics such as high purity, high α-phase content, low oxygen content, and uniform morphology. Among these, oxygen (O) is the most significant factor affecting the thermal conductivity of silicon nitride; therefore, the oxygen content in Si3N4 powder is one of the most critical indicators for powders used as high thermal conductivity silicon nitride ceramic substrates.

[0004] Currently, Si3N4 powder prepared using direct nitriding or self-propagating methods has a high oxygen content. At the same time, the intense exothermic reaction leads to uneven powder morphology, low α-phase content, and difficulty in control. It is necessary to add additives such as α-Si3N4 powder as diluents and increase the synthesis atmosphere pressure, which increases costs and reduces powder purity, thus limiting industrial production. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a low-oxygen-content, high-α-phase silicon nitride, its preparation method, and its applications. Using silicon powder as raw material, the process involves first wet milling with anhydrous ethanol and vacuum low-temperature drying to obtain activated silicon powder. Then, under a negative pressure of 10 kPa to 30 kPa and a mixed atmosphere of nitrogen and hydrogen, pre-nitriding at 900°C to 1100°C and high-temperature nitriding at 1300°C to 1500°C are performed sequentially. Following this, two stages of controlled-rate cooling, crushing, and sieving are performed to obtain high-purity, low-oxygen-content, high-α-phase silicon nitride with adjustable α-phase. This invention overcomes the problems of high oxygen content, low and difficult-to-control α-phase content, the need for additives leading to decreased purity and increased cost, and the problem of violent reactions hindering industrialization in existing technologies through a synergistic process of additive-free negative pressure silicon thermal nitriding and mechanical activation.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first objective of this invention is to provide a method for preparing silicon nitride with low oxygen content and high α phase, comprising the following steps: S1. Silicon powder and anhydrous ethanol are mixed and then mechanically activated to reduce the particle size of silicon powder, increase the specific surface area, and reduce the thickness of the oxide layer on the surface of silicon powder. After vacuum drying, activated silicon powder is obtained. The role of anhydrous ethanol is to prevent silicon powder oxidation and control the oxygen content from the raw materials.

[0007] S2. In a mixed atmosphere of hydrogen and nitrogen, activated silicon powder is subjected to nitriding treatment. During the nitriding treatment, pre-nitriding is performed first. At this time, hydrogen reduces the silicon dioxide on the surface of the activated silicon powder to generate silicon monoxide. Silicon monoxide reacts with nitrogen in a gas-gas reaction to generate α-phase silicon nitride. Subsequently, nitrogen is adsorbed and dissociated on the silicon surface and diffuses through the surface nitriding layer into the interior of the silicon particles. Then, the temperature is continued to melt the free silicon and form liquid silicon. Liquid silicon reacts with nitrogen in a gas-liquid reaction until the silicon powder is completely nitrided. After the nitriding treatment is completed, silicon nitride with low oxygen content and high α-phase is obtained through two-stage cooling.

[0008] Preferably, the activated silicon powder has a particle size of 6μm~12μm and a specific surface area of ​​1m². 2 / g~3m 2 / g; Smaller particle size has a larger specific surface area, which promotes more complete contact with nitrogen and improves nitriding efficiency; if the specific surface area is too large, the nitriding exothermic reaction is intense and the heat is out of control, which will lead to a decrease in the α phase content and make it difficult to control.

[0009] Preferably, the mass ratio of anhydrous ethanol to silicon powder is 2~3:5, and the mass ratio of silicon nitride grinding balls to silicon powder is 2~4:1; wherein, the mass ratio of anhydrous ethanol to silicon powder is the solid content, and this mass ratio is the solid content of 50%~60%, and the mass ratio of silicon nitride grinding balls to silicon powder is the ball-to-material ratio, and a ball-to-material ratio of 4:1 has higher grinding efficiency.

[0010] Preferably, the nitriding conditions are as follows: in an environment with a negative pressure of 10 kPa to 30 kPa, a nitrogen flow rate of 20 L / min to 50 L / min, and a hydrogen flow rate of 1 L / min to 3 L / min, the temperature is first raised to 900℃ to 1100℃ at a heating rate of 3℃ / min to 5℃ / min for pre-nitriding for 1h to 2h, and then the temperature is raised to 1430℃ to 1500℃ at a heating rate of 1℃ / min to 3℃ / min for gas-liquid reaction of liquid-phase silicon for 1h to 2h.

[0011] Preferably, the two-stage cooling process is as follows: First, cool the temperature to 800℃~1000℃ at a rate of 50℃ / min, then cool it to room temperature at a rate of 15℃ / min.

[0012] Preferably, the mechanical activation pretreatment conditions are: ball milling at a speed of 30 r / min to 45 r / min for 3 h to 9 h.

[0013] Preferably, the vacuum drying conditions are: drying at 55℃~65℃ until completely dry.

[0014] Preferably, the purity of the silicon powder is greater than 99%. Low purity will affect the nitriding rate, and more importantly, it will reduce the purity of the final product and affect the product performance.

[0015] A second objective of this invention is to provide a silicon nitride with low oxygen content and high α-phase obtained by the above-described preparation method, wherein the α-phase content of the silicon nitride with low oxygen content and high α-phase is 80%~95%, and the oxygen content is 0.1wt.%~0.3wt.%. The α-phase content is determined according to JC / T2342-2015 "Methods for Phase Content Analysis of Silicon Nitride Materials", where α-phase content = α-phase mass / (α-phase mass + β-phase mass) × 100%.

[0016] A third objective of this invention is to provide the application of the aforementioned low-oxygen-content, high-α-phase silicon nitride in the preparation of ceramic substrates.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention provides a method for preparing silicon nitride with low oxygen content and high α phase. Silicon powder and anhydrous ethanol are mixed and then mechanically activated to reduce the particle size, increase the specific surface area, and reduce the thickness of the silicon powder oxide layer. The mixture is then vacuum dried to obtain activated silicon powder. In a mixed atmosphere of hydrogen and nitrogen, the activated silicon powder is nitrided. During nitriding, pre-nitriding is performed first, where hydrogen reduces the silicon dioxide on the surface of the activated silicon powder to generate silicon monoxide. Silicon monoxide reacts with nitrogen in a gas-gas reaction to generate α-phase silicon nitride. Subsequently, nitrogen is adsorbed and dissociated on the silicon surface and diffuses through the surface nitriding layer into the silicon particles. The temperature is then increased to melt the free silicon and form liquid silicon. Liquid silicon reacts with nitrogen in a gas-liquid reaction. After nitriding, a two-stage cooling process is performed to obtain silicon nitride with low oxygen content and high α phase. This invention provides a method for preparing silicon nitride with low oxygen content and high α phase, without additives, under negative pressure, with low oxygen content and adjustable α phase content. This method overcomes the problems of high oxygen content, low and difficult-to-control α phase content, severe powder agglomeration and uneven morphology, the need to add additives leading to decreased purity and increased cost, and violent reaction that makes industrialization difficult.

[0018] This invention employs an additive-free negative pressure nitriding process, which improves the purity of silicon nitride with low oxygen content and high α phase while reducing costs. Mechanical activation increases the specific surface area of ​​silicon powder, allowing for more thorough contact between silicon powder and nitrogen gas, reducing the thickness of the oxide layer on the silicon powder surface, shortening the reaction incubation period, and preferentially generating more α phase at low temperatures. By utilizing a low heating rate and segmented cooling, the α phase content of the silicon nitride with low oxygen content and high α phase is significantly increased. The negative pressure environment enhances the controllability of the reaction process, slows down the reaction intensity, and avoids localized overheating caused by concentrated silicon powder nitriding, thereby inhibiting the transformation of the α phase to the β phase. Furthermore, the negative pressure environment accelerates nitrogen diffusion, inhibiting the formation and growth of β-Si3N4 whiskers; simultaneously, negative pressure promotes gas-phase reactions, reduces atmospheric impurity concentration, and optimizes particle growth behavior by regulating airflow, which is beneficial for obtaining equiaxed, uniformly sized, highly sintered, and high-purity silicon nitride with low oxygen content and high α phase.

[0019] Using a mixture of hydrogen and nitrogen as the nitrogen source, hydrogen facilitates the reduction of the SiO2 layer on the surface of Si3N4, shortening the nitriding incubation period, lowering the reaction temperature, and enabling the silicon powder to nitride at low temperatures to generate more α-phase. Simultaneously, hydrogen reduces the oxides on the silicon powder surface, effectively reducing the increase of oxygen during the reaction and preventing oxygen from entering the product, thus effectively controlling the oxygen content of the product. Strict control of gas flow rate and purity prevents oxygen interference, further reducing the oxygen content.

[0020] In the ball milling stage, anhydrous ethanol is used as the solvent, and vacuum low-temperature drying is performed to avoid silicon powder oxidation, thus controlling the oxygen content of the product from the raw material stage. In summary, this invention achieves high and controllable α-phase content through mechanical activation and process synergy without the need for additives.

[0021] 2. The preparation method of the present invention is simple and low in cost, and the resulting silicon nitride with low oxygen content and high α phase has the characteristics of low oxygen content, high α phase, and adjustable properties. Specifically, the α phase content of the low oxygen content, high α phase silicon nitride is 80%~95%, and the oxygen content is 0.1wt.%~0.3wt.%.

[0022] 3. The synthesis process of this invention avoids the introduction of diluents such as silicon nitride and metal element catalysts such as iron, magnesium and copper in traditional high pressure, pressureless nitriding and silicothermic nitriding processes, thereby reducing costs, improving purity, and obtaining silicon nitride with low oxygen content and adjustable α phase content. Attached Figure Description

[0023] Figure 1 The X-ray diffraction patterns are of silicon nitride with low oxygen content and high α phase in Examples 1 to 4.

[0024] Figure 2The images are SEM images of silicon nitride with low oxygen content and high α phase from Examples 1 to 4, where (a) is Example 1, (b) is Example 2, (c) is Example 3, and (d) is Example 4. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to the data in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] It should be noted that the technical terms used in this invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased on the market or prepared by existing methods.

[0027] In existing technologies, silicon nitride powders prepared by direct nitriding or self-propagating methods typically have high oxygen content, and the intense exothermic reaction during the reaction results in a low α-phase content that is difficult to precisely control. To solve this problem, it is often necessary to add α-Si3N4 powder or other diluents or increase the synthesis atmosphere pressure. This not only increases production costs but also introduces impurities, reduces powder purity, and easily leads to agglomeration, affecting powder morphology and hindering pre-sintering, thus limiting its industrial application in the field of high-end ceramic substrates.

[0028] To address the problems existing in the prior art, this invention provides a method for preparing silicon nitride with low oxygen content and high α phase, comprising the following steps: mixing silicon powder and anhydrous ethanol and then performing mechanical activation pretreatment to reduce the silicon powder particle size, increase the specific surface area, and reduce the thickness of the silicon powder oxide layer, followed by vacuum drying to obtain activated silicon powder; nitriding the activated silicon powder in a mixed atmosphere of hydrogen and nitrogen, wherein the nitriding process first involves pre-nitriding, during which hydrogen reduces the silicon dioxide on the surface of the activated silicon powder to generate silicon monoxide, and silicon monoxide reacts with nitrogen in a gas-gas reaction to generate α phase silicon nitride, subsequently nitrogen is adsorbed and dissociated on the silicon surface and diffuses through the surface nitriding layer into the interior of the silicon particles; then the temperature is further increased to melt the free silicon and form liquid silicon, which reacts with nitrogen in a gas-liquid reaction; after the nitriding process, a two-stage cooling process is performed to obtain silicon nitride with low oxygen content and high α phase.

[0029] This invention systematically constructs a composite method for efficiently preparing high-quality silicon nitride powder by introducing a negative pressure environment, pretreating silicon powder through mechanical activation, and combining in-situ reduction under a hydrogen atmosphere, an additive-free process, and a two-stage cooling mechanism. Mechanical activation pretreatment enhances the reactivity of the silicon powder, lowers the reaction temperature, and increases the α-phase content. The negative pressure environment not only enhances the controllability of the reaction process and reduces the intensity of the silicon powder nitridation process, effectively avoiding the α→β phase transition caused by local overheating, but also promotes nitrogen diffusion and inhibits the formation and growth of β-Si3N4 whiskers. Simultaneously, the negative pressure condition strengthens the gas-phase reaction, helps reduce impurity concentration, and improves particle growth behavior by optimizing airflow, thereby obtaining powder with equiaxed shape, uniform particle size, and high sintering activity. This process successfully overcomes the problems of high oxygen content, difficulty in controlling the α phase, and dependence on high pressure or additives in traditional methods. Under conditions below atmospheric pressure and without additives, silicon nitride powder with low oxygen content (0.1wt.%~0.3wt.%) and high α phase content (80%~95%) is prepared. Its phase is tunable and its morphology is uniform, which can be directly used for the large-scale production of high thermal conductivity silicon nitride ceramic substrates.

[0030] To enable those skilled in the art to more clearly understand the technical solution of the present invention, the following will provide a detailed description in conjunction with specific embodiments: Example 1 A method for preparing silicon nitride with low oxygen content and high α phase includes the following steps: S1. Mix 240g of high-purity coarse silicon powder with 960g of silicon nitride grinding balls at a ball-to-powder ratio of 4:1. Add 160g of anhydrous ethanol to the mixture, based on a solid content of 60% for the high-purity coarse silicon powder. Mill the mixture in a rolling ball mill at 45r / min for 3 hours. Then, dry the mixture in a vacuum drying oven at 60℃ to obtain activated silicon powder with a particle size of 12μm and a specific surface area of ​​1.38m². 2 / g.

[0031] S2. Under a negative pressure of 25KPa~30KPa and a continuous flow of a mixed atmosphere of nitrogen and hydrogen, the activated silicon powder is first heated to 1100℃ at a rate of 5℃ / min for pre-nitriding for 1h; then the temperature is increased to 1450℃ at a rate of 3℃ / min and held at 1450℃ for 1.5h; after the holding period, the temperature is reduced to 800℃ at a rate of 50℃ / min, and then further reduced to 25℃ at a rate of 15℃ / min to obtain coarse powder; wherein the nitrogen flow rate is 38L / min and the hydrogen flow rate is 2L / min.

[0032] The coarse powder is crushed in a crusher and then passed through an 80-mesh sieve to obtain silicon nitride with low oxygen content and high α phase, with the molecular formula Si3N4.

[0033] The oxygen content of the low-oxygen-content, high-α-phase silicon nitride powder prepared in this embodiment is 0.195 wt.%, and the α-phase content is 80.1%.

[0034] Example 2 A method for preparing silicon nitride with low oxygen content and high α phase includes the following steps: S1. Mix 240g of high-purity coarse silicon powder with 960g of silicon nitride grinding balls at a ball-to-powder ratio of 4:1. Add 160g of anhydrous ethanol to the mixture, based on a solid content of 60% for the high-purity coarse silicon powder. Mill the mixture in a rolling ball mill at 30r / min for 9 hours. Then, dry the mixture in a vacuum drying oven at 60℃ to obtain activated silicon powder with a particle size of 9μm and a specific surface area of ​​1.7m². 2 / g.

[0035] S2. Under a negative pressure of 20 kPa to 25 kPa and a mixed atmosphere of nitrogen and hydrogen continuously introduced at a rate of 100 mL / min, the activated silicon powder is first heated to 1100 °C at a rate of 5 °C / min for pre-nitriding for 1 h; then the temperature is increased to 1430 °C at a rate of 3 °C / min and held at 1430 °C for 1.5 h; after the holding period, the temperature is reduced to 800 °C at a rate of 50 °C / min, and then further reduced to 25 °C at a rate of 15 °C / min to obtain coarse powder; wherein the nitrogen flow rate is 38 L / min and the hydrogen flow rate is 2 L / min.

[0036] The coarse powder is crushed in a crusher and then passed through an 80-mesh sieve to obtain silicon nitride with low oxygen content and high α phase, with the molecular formula Si3N4.

[0037] The oxygen content of the low-oxygen, high-α-phase silicon nitride powder prepared in this embodiment is 0.213 wt.%, and the α-phase content is 86.6%.

[0038] Example 3 A method for preparing silicon nitride with low oxygen content and high α phase includes the following steps: S1. Mix 240g of high-purity coarse silicon powder with 960g of silicon nitride grinding balls at a ball-to-powder ratio of 4:1. Add 160g of anhydrous ethanol to the mixture, based on a solid content of 60% for the high-purity coarse silicon powder. Mill the mixture on a rolling ball mill at 45r / min for 9h. Then, dry the mixture in a vacuum drying oven at 60℃ to obtain activated silicon powder with a particle size of 6μm and a specific surface area of ​​2.9m². 2 / g.

[0039] S2. Under a negative pressure of 15KPa~20KPa and a mixed atmosphere of nitrogen and hydrogen continuously introduced at 100mL / min, the activated silicon powder is first heated to 1100℃ at a rate of 5℃ / min for pre-nitriding for 1h; then the temperature is increased to 1450℃ at a rate of 3℃ / min and held at 1450℃ for 1h; after the holding period, the temperature is reduced to 800℃ at a rate of 50℃ / min, and then further reduced to 25℃ at a rate of 15℃ / min to obtain coarse powder; wherein the nitrogen flow rate is 20L / min and the hydrogen flow rate is 2L / min.

[0040] The coarse powder is crushed in a crusher and then passed through an 80-mesh sieve to obtain silicon nitride with low oxygen content and high α phase, with the molecular formula Si3N4.

[0041] The oxygen content of the low-oxygen-content, high-α-phase silicon nitride powder prepared in this embodiment is 0.235 wt.%, and the α-phase content is 91.5%.

[0042] Example 4 A method for preparing silicon nitride with low oxygen content and high α phase includes the following steps: S1. Mix 240g of high-purity coarse silicon powder with 960g of silicon nitride grinding balls at a ball-to-powder ratio of 4:1. Add 160g of anhydrous ethanol to the mixture, based on a solid content of 60% for the high-purity coarse silicon powder. Mill the mixture on a rolling ball mill at 45r / min for 9h. Then, dry the mixture in a vacuum drying oven at 60℃ to obtain activated silicon powder with a particle size of 6μm and a specific surface area of ​​2.9m². 2 / g.

[0043] S2. Under a negative pressure of 10 kPa to 15 kPa and a continuous flow of a mixed atmosphere of nitrogen and hydrogen, the activated silicon powder is first heated to 1100°C at a rate of 5°C / min for pre-nitriding for 1 hour; then the temperature is increased to 1450°C at a rate of 3°C / min and held at 1450°C for 1.5 hours; after the holding period, the temperature is reduced to 800°C at a rate of 50°C / min, and then further reduced to 25°C at a rate of 15°C / min to obtain coarse powder; wherein the nitrogen flow rate is 38 L / min and the hydrogen flow rate is 2 L / min.

[0044] The coarse powder is crushed in a crusher and then passed through an 80-mesh sieve to obtain silicon nitride with low oxygen content and high α phase, with the molecular formula Si3N4.

[0045] The oxygen content of the low-oxygen-content, high-α-phase silicon nitride powder prepared in this embodiment is 0.211 wt.%, and the α-phase content is 95.6%.

[0046] Example 5 A method for preparing silicon nitride with low oxygen content and high α phase includes the following steps: S1. Mix 240g of high-purity coarse silicon powder with 960g of silicon nitride grinding balls at a ball-to-powder ratio of 4:1. Add 160g of anhydrous ethanol to the mixture, based on a solid content of 60% for the high-purity coarse silicon powder. Mill the mixture on a rolling ball mill at 45r / min for 9h. Then, dry the mixture in a vacuum drying oven at 60℃ to obtain activated silicon powder with a particle size of 6μm and a specific surface area of ​​2.9m². 2 / g.

[0047] S2. Under a negative pressure of 15KPa~30KPa and a continuous flow of a mixed atmosphere of nitrogen and hydrogen, the activated silicon powder is first heated to 900℃ at a rate of 3℃ / min for pre-nitriding for 2h; then the temperature is increased to 1500℃ at a rate of 1℃ / min and held at 1500℃ for 2h; after the holding period, the temperature is reduced to 800℃ at a rate of 50℃ / min, and then further reduced to 25℃ at a rate of 15℃ / min to obtain coarse powder; wherein the nitrogen flow rate is 50L / min and the hydrogen flow rate is 3L / min.

[0048] The coarse powder is crushed in a crusher and then passed through an 80-mesh sieve to obtain silicon nitride with low oxygen content and high α phase, with the molecular formula Si3N4.

[0049] The α-phase content of the low-oxygen, high-α-phase silicon nitride prepared in Examples 1-4 is as follows: Figure 1 As shown, the percentages are 80.1%, 86.6%, 91.1%, and 95.6%, respectively.

[0050] The surface microstructures of the low-oxygen-content, high-α-phase silicon nitride prepared in Examples 1-4 are as follows: Figure 2 As shown, in Examples 1 and 2, the silicon nitride with low oxygen content and high α phase has a small amount of rod-shaped β-Si3N4 and some short rod-shaped β-Si3N4 that have not grown. In Examples 3 and 4, the surface of silicon nitride with low oxygen content and high α phase has significantly reduced rod-shaped particles and short rod-shaped β-Si3N4. The α phase is equiaxed particles, and the β phase is long rod-shaped. The reduction of rod-shaped structures indicates a decrease in the β phase content and an increase in the α phase content (which is consistent with the different α phase contents mentioned in the examples), which is consistent with the XRD pattern. In addition, the powder morphology is uniform, without large agglomeration, and the sintering activity is better.

[0051] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range, as well as any value between the two endpoints, can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

Claims

1. A method for preparing silicon nitride with low oxygen content and high α phase, characterized in that, Includes the following steps: Silicon powder and anhydrous ethanol are mixed and then mechanically activated and pretreated, followed by vacuum drying to obtain activated silicon powder. In a mixed atmosphere of hydrogen and nitrogen, activated silicon powder is subjected to nitriding treatment. During the nitriding process, pre-nitriding is first performed, during which hydrogen reduces the silicon dioxide on the surface of the activated silicon powder to generate silicon monoxide. Silicon monoxide reacts with nitrogen in a gas-gas reaction to generate α-phase silicon nitride. Subsequently, nitrogen is adsorbed and dissociated on the silicon surface and diffuses through the surface nitriding layer into the interior of the silicon particles. Then, the temperature is raised to melt the free silicon and form liquid silicon. Liquid silicon reacts with nitrogen in a gas-liquid reaction. After the nitriding treatment is completed, a two-stage cooling process is performed to obtain silicon nitride with low oxygen content and high α-phase.

2. The method for preparing silicon nitride with low oxygen content and high α phase according to claim 1, characterized in that, The activated silicon powder has a particle size of 6μm~12μm and a specific surface area of ​​1m². 2 / g~3m 2 / g.

3. The method for preparing silicon nitride with low oxygen content and high α phase according to claim 1, characterized in that, The nitriding conditions are as follows: in an environment with a negative pressure of 10 kPa to 30 kPa, a nitrogen flow rate of 20 L / min to 50 L / min, and a hydrogen flow rate of 1 L / min to 3 L / min, pre-nitriding is carried out at 900℃ to 1100℃ for 1 h to 2 h, followed by a gas-liquid reaction of liquid-phase silicon at 1430℃ to 1500℃ for 1 h to 2 h.

4. The method for preparing silicon nitride with low oxygen content and high α phase according to claim 1, characterized in that, The two-stage cooling process is as follows: first, the temperature is lowered to 800℃~1000℃ at a rate of 50℃ / min, and then lowered to room temperature at a rate of 15℃ / min.

5. The method for preparing silicon nitride with low oxygen content and high α phase according to claim 1, characterized in that, The conditions for mechanical activation pretreatment are: ball milling at a speed of 30 r / min to 45 r / min for 3 h to 9 h.

6. The method for preparing silicon nitride with low oxygen content and high α phase according to claim 1, characterized in that, The conditions for vacuum drying are: drying at 55℃~65℃ until completely dry.

7. A silicon nitride with low oxygen content and high α phase prepared by the preparation method according to any one of claims 1 to 6.

8. The low-oxygen-content, high-α-phase silicon nitride according to claim 7, characterized in that, The silicon nitride with low oxygen content and high α phase has an α phase content of 80% to 95% and an oxygen content of 0.1 wt.% to 0.3 wt.%.

9. The application of the low oxygen content, high α phase silicon nitride as described in claim 7 in the preparation of ceramic substrates.

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