Method for synthesizing low-oxygen high-alpha-phase silicon nitride powder through direct nitridation of silicon sawdust

Abstract

The invention provides a method for synthesizing low-oxygen high-alpha-phase silicon nitride powder through direct nitridation of silicon sawdust, and belongs to the technical field of silicon nitride preparation. The method comprises the following steps: carrying out acid pickling, impurity removal and purification treatment on silicon sawdust to obtain a silicon raw material of which the main metal impurities are less than 10ppm and the oxygen content is less than 2%; the silicon raw material and a regulator are mixed according to a preset proportion and subjected to ball milling treatment, a mixed material is obtained, and the regulator comprises sodium chloride and carbon powder; introducing a gaseous nitrogen source, and carrying out vacuum gradient heating nitridation treatment on the mixed material to obtain a silicon nitride crude product; and sequentially carrying out acid pickling, solid-liquid separation, cleaning and drying post-treatment on the silicon nitride crude product to obtain the low-oxygen high-alpha-phase silicon nitride powder. According to the invention, the silicon sawdust is subjected to pickling purification, and the obtained silicon sawdust is directly nitrided at low temperature under the assistance of sodium chloride and carbon powder to synthesize alpha-phase silicon nitride; and then the high-dispersion and high-purity alpha-phase silicon nitride powder can be obtained through an acid pickling-water washing-drying process. The silicon nitride product prepared by the method can be used as a raw material of silicon nitride ceramic.

Description

Technical Field

[0001] The present invention belongs to the technical field of silicon nitride preparation, and particularly relates to a method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder. Background Art

[0002] Silicon nitride ceramics have the advantages of high strength, high hardness, good chemical stability, heat resistance, wear resistance, good insulation, and high theoretical thermal conductivity. Therefore, silicon nitride ceramics have great application potential in industries such as semiconductors, metallurgy, aerospace, machinery, photovoltaic, medical, and fine grinding. Silicon nitride powder has three crystal forms: α-Si3N4, β-Si3N4, and γ-Si3N4; among them, the α-phase and β-phase are the two most common silicon nitride crystals, both belonging to the hexagonal crystal system. Different application fields have different requirements for the phase state and purity of silicon nitride. Silicon nitride ceramics are usually sintered from α-phase silicon nitride powder as raw materials under the action of various additives. With the continuous expansion of the application of silicon nitride ceramics, the demand for α-phase silicon nitride powder will also increase.

[0003] In the photovoltaic industry, during the slicing process of crystalline silicon ingots, about 40% of high-purity silicon materials become silicon sawdust. The annual production reaches 100,000 tons, which is a huge volume. If not properly disposed of, it will bring huge waste and environmental burden. At present, the recycling and reuse of silicon sawdust only stay in the application with relatively low added value such as being used as a metallurgical additive. The high-value recycling and reuse of silicon sawdust has great practical significance.

[0004] Currently, the common methods for preparing α-Si3N4 powder mainly include direct nitriding method, carbothermal reduction method, silicon imide decomposition method, chemical vapor deposition method, and polysilazane pyrolysis method, etc. The carbothermal reduction method has the problem of carbon pollution; the chemical vapor deposition method has a complex process and high cost; the silicon imide decomposition method and polysilazane pyrolysis method are based on complex chemical processes and have high costs; the direct nitriding method of silicon powder has a simple process and low cost, and is adopted by most manufacturers. However, the direct nitriding method of silicon powder also has some disadvantages, mainly including a high reaction temperature (generally higher than 1350 °C) and a long synthesis period (more than 40 h). Due to the plastic and brittle mixed cutting characteristics of photovoltaic silicon sawdust, the surface of the silicon sawdust becomes amorphous and has high reaction activity, which is very suitable as a raw material for silicon nitride preparation.

[0005] In the existing process of synthesizing high-α-phase silicon nitride from silicon sawdust by nitridation, promoters such as calcium fluoride CaF₂ are added during the nitridation reaction. Although it increases the content of α-phase silicon nitride to a certain extent, the product still contains a relatively high proportion of oxygen. Oxygen is the most abundant negative impurity element in silicon nitride powder, and its excessive content will have a negative impact on the crystal structure, heat resistance, electrical properties, chemical stability, and mechanical properties of silicon nitride. When preparing silicon nitride materials, it is necessary to strictly control its oxygen content to ensure its excellent properties. Therefore, it is of great significance to directly nitride silicon sawdust to synthesize low-oxygen, high-α-phase silicon nitride powder. Summary of the Invention

[0006] In view of this, the purpose of the present invention is to provide a method for directly nitriding silicon sawdust to synthesize low-oxygen, high-α-phase silicon nitride powder, aiming to solve at least one technical problem in the background technology.

[0007] The present invention is implemented as follows:

[0008] A method for directly nitriding silicon sawdust to synthesize low-oxygen, high-α-phase silicon nitride powder, which includes the following steps:

[0009] Pickling, impurity removal, and purification treatment of silicon sawdust to obtain a silicon raw material with less than 10 ppm of main metal impurities and an oxygen content of less than 2%;

[0010] Mix the silicon raw material with a regulator in a preset ratio and perform ball milling treatment to obtain a mixed material;

[0011] Introduce a gaseous nitrogen source and perform vacuum gradient temperature rise nitridation treatment on the mixed material to obtain a crude silicon nitride product;

[0012] The crude silicon nitride product is sequentially subjected to pickling, solid-liquid separation, cleaning, and drying post-treatment to obtain the low-oxygen, high-α-phase silicon nitride powder;

[0013] Among them, the regulator includes sodium chloride and carbon powder.

[0014] Further, in the regulator, the dosage of sodium chloride is 3 wt% - 7 wt% of the silicon raw material, and the dosage of carbon powder is 1 wt% - 3 wt% of the silicon raw material.

[0015] Further, the step of introducing a gaseous nitrogen source and performing vacuum high-temperature nitridation treatment on the mixed material specifically includes:

[0016] Loosely load the mixed material into a ceramic crucible, place the ceramic crucible in the isothermal zone of the atmosphere furnace, and use high-purity nitrogen to displace the air in the furnace tube of the atmosphere furnace;

[0017] Exhaust the furnace tube of the atmosphere furnace, close the air outlet after completion, and fill the furnace tube of the atmosphere furnace with high-purity ammonia gas to 0.1 Mpa - 0.2 Mpa;

[0018] Heat up to 900-1000℃ at a rate of 5℃ / min-10℃ / min, keep warm for 1h-3h. After the heat preservation is completed, open the air outlet valve and release the pressure to normal pressure;

[0019] Then, high-purity ammonia is continuously injected at a flow rate of 80 ml / min to 120 ml / min, and the temperature is raised to 1100°C to 1200°C at a rate of 5°C / min to 10°C / min, and kept at this temperature for 3h to 5h, and cooled to room temperature with the furnace to obtain crude silicon nitride.

[0020] Furthermore, the purity of the high-purity nitrogen and high-purity ammonia is ≥99.999%.

[0021] Furthermore, the step of pickling, removing impurities and purifying the silicon sawdust specifically includes: pickling the silicon sawdust in a mixed acid solution of 1%-5% hydrofluoric acid and 5%-10% hydrochloric acid for 2h to 4h, then repeatedly washing with deionized water, and drying to obtain a silicon raw material with a main metal impurity of less than 10ppm and an oxygen content of less than 2%.

[0022] Furthermore, the solid-liquid ratio of the silicon sawdust to the mixed acid solution is 1:(10-20).

[0023] Furthermore, the crude silicon nitride product is sequentially subjected to acid washing, solid-liquid separation, washing, and drying to obtain the low-oxygen high-α-phase silicon nitride powder, which specifically includes:

[0024] The crude silicon nitride is placed in a mixed solution of 1%-5% hydrofluoric acid and 1%-5% nitric acid by mass concentration, and stirred, immersed and pickled for 30min-60min;

[0025] Collect the powder precipitate by suction filtration and rinse it repeatedly with deionized water at least 3 times;

[0026] After drying, silicon nitride powder with an α-phase content greater than 95% and an oxygen content less than 1% is obtained.

[0027] Furthermore, the ball milling time is 5h to 7h.

[0028] Furthermore, the silicon sawdust includes silicon sawdust generated by wire saw cutting during the whole process of squaring and slicing single crystal ingots and polycrystalline ingots.

[0029] The present invention uses acid washing to purify silicon sawdust, and the obtained silicon sawdust is directly nitrided at low temperature with the assistance of sodium chloride and carbon powder to synthesize α-phase silicon nitride; and then through the acid washing-water washing-drying process, highly dispersed and high-purity α-phase silicon nitride powder can be obtained. The silicon nitride product prepared by the present invention can be used as a raw material for silicon nitride ceramics.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) Through the synergistic effect of sodium chloride and carbon powder, the present invention can promptly and effectively reduce the dense oxide layer on the surface of silicon sawdust into gaseous silicon monoxide, promoting the rupture of the dense oxide layer and exposing the silicon to a nitrogen-containing atmosphere, enabling the rapid synthesis of silicon sawdust under low-temperature conditions.

[0032] (2) Using silicon sawdust generated during the slicing process of silicon wafers in the photovoltaic industry as raw materials to synthesize α-phase silicon nitride, the present invention realizes the high-value recycling of silicon waste and reduces the environmental protection pressure.

[0033] (3) By using sodium chloride and carbon powder to assist in the direct nitridation for synthesizing α-phase silicon nitride, the present invention reduces the reaction temperature, shortens the nitridation synthesis time, and reduces costs.

[0034] (4) Through a simple pickling - water washing - drying process for the products of the present invention, impurities in the products can be effectively removed and the dispersibility can be improved, thereby obtaining high-purity and highly dispersed α-phase silicon nitride powder.

[0035] (5) The process of the present invention is simple, has low requirements for equipment, low costs, a short production cycle, and is easy to achieve large-scale production. Description of the Drawings

[0036] Figure 1 XRD pattern of the pretreated silicon sawdust in Example 1 of the present invention;

[0037] Figure 2 SEM image of the pretreated silicon sawdust in Example 1 of the present invention;

[0038] Figure 3 XRD pattern of the silicon nitride product prepared in Example 1 of the present invention;

[0039] Figure 4 SEM image of the silicon nitride product prepared in Example 1 of the present invention. Detailed Embodiments

[0040] In order to make the objectives, technical solutions, and advantages of the present invention clearer, the following further elaborates on the present invention in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0041] A method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder, which includes steps S1 - S4.

[0042] S1. Pickling, impurity removal, and purification treatment of silicon sawdust;

[0043] The silicon sawdust includes silicon sawdust generated by wire saw cutting during the entire process of squaring and slicing single-crystal ingots and polycrystalline ingots.

[0044] This step is specifically as follows: The silicon sawdust is pickled in a mixed acid solution of 1%-5% hydrofluoric acid and 5%-10% hydrochloric acid for 2h to 4h, and then repeatedly washed with deionized water. After drying, a silicon raw material with main metal impurities less than 10ppm and oxygen content less than 2% is obtained.

[0045] In specific implementation, the solid-liquid ratio of the silicon sawdust to the mixed acid solution is 1:10 to 1:20. For example, it can be 1:10, 1:15, 1:20; the pickling time can be 2h, 3h, 4h; the mixed acid solution can be 3% hydrofluoric acid + 10% hydrochloric acid. The following examples are illustrated with a solid-liquid ratio = 1:20, pickling time = 3h, mixed acid solution = 3% hydrofluoric acid + 10% hydrochloric acid, 1% hydrofluoric acid + 5% hydrochloric acid, 5% hydrofluoric acid + 8% hydrochloric acid, but are not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0046] S2. Mix the silicon raw material and the regulator in a preset ratio and perform ball milling treatment to obtain a mixed material.

[0047] The regulator includes sodium chloride and carbon powder. Among them, the dosage of sodium chloride is 3wt%-7wt% of the silicon raw material, and the dosage of carbon powder is 1wt%-3wt% of the silicon raw material; the ball milling time is 5h to 7h.

[0048] In specific implementation, the dosage of sodium chloride can be 3wt%, 5wt%, 7wt% of the silicon raw material; the dosage of carbon powder is 1wt%, 3wt% of the silicon raw material; the ball milling time can be 5h, 6h, 7h, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0049] S3. Introduce a gaseous nitrogen source and perform vacuum gradient temperature-raising nitriding treatment on the mixed material to obtain a crude silicon nitride product.

[0050] Specifically, it includes:

[0051] 3-1. Loosely load the mixed material into a ceramic crucible, place the ceramic crucible in the isothermal zone of the atmosphere furnace, use a vacuum pump to evacuate to about -0.1Mpa, and then fill the furnace tube with high-purity nitrogen (purity ≥ 99.999%) to normal pressure to replace the air in the furnace tube of the atmosphere furnace.

[0052] 3-2. Exhaust the furnace tube of the atmosphere furnace. After completion, close the air outlet and fill the furnace tube of the atmosphere furnace with high-purity ammonia gas (purity ≥ 99.999%) to 0.1Mpa - 0.2Mpa.

[0053] 3-3. Heat it from the current temperature to 900°C - 1000°C at a rate of 5°C / min - 10°C / min, and keep it at this temperature for 1h - 3h. After the heat preservation is completed, open the air outlet valve to release the pressure until it reaches atmospheric pressure. In specific implementations, the heating rate can be 5°C / min or 10°C / min; the temperature can be 900°C, 950°C, or 1000°C; the heat preservation time can be 1h, 2h, or 3h. However, it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0054] 3-4. Continuously introduce high-purity ammonia gas (purity ≥ 99.999%) at a flow rate of 80ml / min - 120ml / min. At the same time, heat it from the current temperature to 1100°C - 1200°C at a rate of 5°C / min - 10°C / min, keep it at this temperature for 3h - 5h, and then cool it to room temperature in the furnace to obtain the crude silicon nitride product. In specific implementations, the flow rate can be 80ml / min, 100l / min, or 120ml / min; the heating rate can be 5°C / min or 10°C / min; the temperature can be 1100°C, 1150°C, or 1200°C; the heat preservation time can be 3h, 4h, or 5h. However, it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0055] S4. The crude silicon nitride product is successively subjected to pickling, solid-liquid separation, cleaning, and drying to obtain the low-oxygen high-α-phase silicon nitride powder.

[0056] 4-1. After being treated in step S3, the crude silicon nitride product is in a loose state. Immerse it in a mixed solution of hydrofluoric acid with a mass concentration of 1% - 5% and nitric acid with a mass concentration of 1% - 5% and stir for pickling for 30min - 60min.

[0057] 4-2. Filter by suction to collect the powder precipitate, and rinse it with deionized water at least 3 times repeatedly.

[0058] 4-3. After drying, a silicon nitride powder with an α-phase content greater than 95% and an oxygen content less than 1% is obtained.

[0059] Through the synergistic effect of sodium chloride and carbon powder, the present invention effectively and timely reduces the dense oxide layer on the surface of silicon sawdust into silicon monoxide gas, promotes the rupture of the dense oxide layer, exposes the silicon to the nitrogen-containing atmosphere, and enables the silicon sawdust to be rapidly synthesized under low-temperature conditions.

[0060] Example 1

[0061] A method for directly nitriding silicon sawdust to synthesize low-oxygen high-α-phase silicon nitride powder specifically includes:

[0062] S1. Weigh 100 g of recycled silicon sawdust, add 3% hydrofluoric acid and 10% hydrochloric acid according to a solid-liquid ratio of 1:20, and stir. Stir once every 30 minutes during the reaction process. The reaction proceeds for 3 hours. After the reaction is completed, take the precipitate and wash it 7 times with deionized water. Dry the obtained precipitate at 50 °C to obtain pretreated silicon sawdust, and obtain a silicon raw material with a main metal impurity of less than 10 ppm and an oxygen content of less than 2%; as Figure 1 shown is the X-ray diffraction pattern of the pretreated silicon sawdust obtained in this example. It can be seen that it is a single silicon phase, and the half-height width of the diffraction peak is broadened compared with that of crystalline silicon, which is related to the transformation of part of the surface crystalline silicon into amorphous state during the cutting process. As Figure 2 shown is the SEM scanning electron micrograph of the silicon sawdust powder after preliminary pickling pretreatment in this example. The microscopic morphology of the silicon sawdust powder is irregular flakes; these physical and chemical properties make the silicon sawdust very suitable as a raw material for silicon nitride synthesis.

[0063] S2. Weigh a certain amount of the silicon raw material pretreated in step S1, and then weigh 5% sodium chloride and 1% carbon powder relative to the mass of the silicon raw material. Place the three powders in a mixer and mix for 6 hours to obtain a mixed material.

[0064] S3. Loosely load the mixed material prepared in step S2 into a ceramic crucible, and then place the ceramic crucible in the isothermal zone of an atmosphere furnace; use a vacuum pump to evacuate to -0.1 Mpa, then fill the furnace tube with high-purity nitrogen to normal pressure, then exhaust at a flow rate of 100 ml / min for about 30 minutes, close the outlet, continue to fill with high-purity ammonia to 0.1 Mpa, then heat up to 1000 °C at a rate of 5 °C / min, hold for 1 h. After the holding is completed, open the outlet valve, relieve the pressure to normal pressure, and continue to fill with high-purity ammonia at a flow rate of 100 ml / min, while heating up to 1100 °C at a rate of 5 °C / min, hold for 3 h, and cool to room temperature with the furnace to obtain crude silicon nitride.

[0065] S4. Place the crude silicon nitride obtained in step S3 in a mixed solution of 3% hydrofluoric acid and 2% nitric acid by mass concentration, stir and soak for pickling for 60 min, filter and collect the powder precipitate, then rinse it repeatedly with deionized water for 7 times, and dry to obtain silicon nitride powder with an α-phase content of 97.6% and an oxygen content of 0.76%.

[0066] The X-ray diffraction pattern of the silicon nitride powder obtained in this example is as Figure 3 shown, and the SEM microscopic morphology diagram of the silicon nitride powder obtained in this example is as Figure 4 shown. It can be seen that the obtained product is mainly α-phase silicon nitride, and the α-phase content is relatively high.

[0067] Example 2

[0068] A method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder, specifically including:

[0069] S1. The same as in Example 1.

[0070] S2. Weigh a certain amount of silicon raw material obtained by pretreatment in step S1, and then weigh 7% sodium chloride and 3% carbon powder relative to the mass of the silicon raw material. Place the three powders in a mixer and mix for 6 hours to obtain a mixed material.

[0071] S3. Loosely load the mixed material prepared in step S2 into a ceramic crucible, and then place the ceramic crucible in the isothermal zone of an atmosphere furnace; use a vacuum pump to evacuate to -0.1 Mpa, then fill the furnace tube with high-purity nitrogen to normal pressure, then exhaust at a flow rate of 100 ml / min for about 30 minutes, close the air outlet, continue to fill with high-purity ammonia gas to 0.2 Mpa, then heat up to 900 °C at a rate of 5 °C / min, hold for 1 h. After the holding is completed, open the air outlet valve, release the pressure to normal pressure, then continuously fill with high-purity ammonia gas at a flow rate of 100 ml / min, and at the same time heat up to 1100 °C at a rate of 5 °C / min, hold for 3 h, and cool to room temperature with the furnace to obtain a crude silicon nitride product.

[0072] S4. The same as in Example 1, to obtain silicon nitride powder with an α-phase content of 96.8% and an oxygen content of 0.83%.

[0073] Example 3

[0074] A method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder, specifically including:

[0075] S1. The same as in Example 1.

[0076] S2. The same as in Example 1.

[0077] S3. Loosely load the mixed material prepared in step S2 into a ceramic crucible, and then place the ceramic crucible in the isothermal zone of an atmosphere furnace; use a vacuum pump to evacuate to -0.1 Mpa, then fill the furnace tube with high-purity nitrogen to normal pressure, then exhaust at a flow rate of 100 ml / min for about 30 minutes, close the air outlet, continue to fill with high-purity ammonia gas to 0.1 Mpa, then heat up to 1000 °C at a rate of 10 °C / min, hold for 3 h. After the holding is completed, open the air outlet valve, release the pressure to normal pressure, then continuously fill with high-purity ammonia gas at a flow rate of 100 ml / min, and at the same time heat up to 1100 °C at a rate of 5 °C / min, hold for 3 h, and cool to room temperature with the furnace to obtain a crude silicon nitride product.

[0078] S4. The same as in Example 1, to obtain silicon nitride powder with an α-phase content of 97.5% and an oxygen content of 0.92%.

[0079] Example 4

[0080] Method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder, specifically including:

[0081] S1. Same as Example 1.

[0082] S2. Weigh a certain amount of silicon raw material pretreated in step S1, and then weigh 5% sodium chloride and 3% carbon powder relative to the mass of the silicon raw material. Place the three powders in a mixer and mix for 6 hours to obtain a mixed material.

[0083] S3. Loosely load the mixed material prepared in step S2 into a ceramic crucible, and then place the ceramic crucible in the isothermal zone of an atmosphere furnace; use a vacuum pump to evacuate to -0.1 Mpa, then fill the furnace tube with high-purity nitrogen to normal pressure, then exhaust at a flow rate of 100 ml / min for about 30 minutes, close the air outlet, continue to fill with high-purity ammonia gas to 0.1 Mpa, then heat up to 1000 °C at a rate of 10 °C / min, hold for 3 h. After the heat preservation is completed, open the air outlet valve, relieve the pressure to normal pressure, then continuously fill with high-purity ammonia gas at a flow rate of 100 ml / min, and at the same time heat up to 1200 °C at a rate of 10 °C / min, hold for 3 h, and cool down to room temperature with the furnace to obtain crude silicon nitride.

[0084] S4. Same as Example 1 to obtain silicon nitride powder with an α-phase content of 95.6% and an oxygen content of 0.86%.

[0085] Example 5

[0086] Method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder, specifically including:

[0087] S1. Same as Example 1.

[0088] S2. Same as Example 1.

[0089] S3. Loosely load the mixed material prepared in step S2 into a ceramic crucible, and then place the ceramic crucible in the isothermal zone of an atmosphere furnace; use a vacuum pump to evacuate to -0.1 Mpa, then fill the furnace tube with high-purity nitrogen to normal pressure, then exhaust at a flow rate of 100 ml / min for about 30 minutes, close the air outlet, continue to fill with high-purity ammonia gas to 0.1 Mpa, then heat up to 1000 °C at a rate of 10 °C / min, hold for 3 h. After the heat preservation is completed, open the air outlet valve, relieve the pressure to normal pressure, then continuously fill with high-purity ammonia gas at a flow rate of 100 ml / min, and at the same time heat up to 1200 °C at a rate of 5 °C / min, hold for 5 h, and cool down to room temperature with the furnace to obtain crude silicon nitride.

[0090] S4. Same as Example 1 to obtain silicon nitride powder with an α-phase content of 98.6% and an oxygen content of 0.98%.

[0091] Comparative Example 1

[0092] Based on Example 1, in Comparative Example 1, the regulator was removed, and other steps and conditions were the same as those in Example 1.

[0093] Comparative Example 2

[0094] Based on Example 1, in Comparative Example 2, sodium chloride in the regulator was removed, and other steps and conditions were the same as those in Example 1.

[0095] Comparative Example 3

[0096] Based on Example 1, in Comparative Example 3, carbon powder in the regulator was removed, and other steps and conditions were the same as those in Example 1.

[0097] Comparative Example 4

[0098] Based on Example 1, in Comparative Example 4, the dosage of sodium chloride in the regulator was adjusted to 8% of the mass of the silicon raw material, and other steps and conditions were the same as those in Example 1.

[0099] Comparative Example 5

[0100] Based on Example 1, in Comparative Example 5, the dosage of carbon powder in the regulator was adjusted to 4% of the mass of the silicon raw material, and other steps and conditions were the same as those in Example 1.

[0101] Comparative Example 6

[0102] Based on Example 1, in Comparative Example 6, the gradient temperature rise nitriding step in S3 was adjusted to single temperature nitriding, and other steps and conditions were the same as those in Example 1.

[0103] Specifically, S3 in this comparative example was as follows: The mixture obtained in step S2 was loosely loaded into a ceramic crucible, and then the ceramic crucible was placed in the isothermal zone of the atmosphere furnace; vacuum pumping was carried out using a vacuum pump until -0.1 Mpa, then high-purity nitrogen was filled into the furnace tube until normal pressure, and then the gas was exhausted at a flow rate of 100 ml / min for about 30 minutes. The gas outlet was closed, and high-purity ammonia was continuously filled until 0.1 Mpa. Subsequently, the temperature was raised to 1100 °C at a rate of 5 °C / min, held for 4 h, and cooled to room temperature with the furnace to obtain the crude silicon nitride.

[0104] Comparative Example 7

[0105] Based on Example 1, in Comparative Example 7, the gradient temperature rise nitriding step in S3 was adjusted to gradient temperature drop nitriding, and other steps and conditions were the same as those in Example 1.

[0106] S3 of this comparative example is specifically as follows: The mixed material prepared in step S2 is loosely loaded into a ceramic crucible, and then the ceramic crucible is placed in the isothermal zone of an atmosphere furnace; a vacuum pump is used to evacuate to -0.1 Mpa, then high-purity nitrogen is filled into the furnace tube until normal pressure, and then the gas is exhausted at a flow rate of 100 ml / min for about 30 minutes. The gas outlet is closed, and high-purity ammonia is continuously filled until 0.1 Mpa. Subsequently, the temperature is raised to 1100 °C at a rate of 5 °C / min and held for 3 h. After the heat preservation is completed, the gas outlet valve is opened to release pressure to normal pressure, and high-purity ammonia is continuously introduced at a flow rate of 100 ml / min. At the same time, the temperature is adjusted to 1000 °C at a rate of 5 °C / min and held for 1 h, and then cooled to room temperature with the furnace to obtain the crude silicon nitride product.

[0107] Table 1

[0108] Object to be detected <![CDATA[α-Si3N4 content]]> Oxygen content Example 1 97.6％ 0.76％ Comparative Example 1 4.36％ 5.59％ Comparative Example 2 7.78％ 2.79％ Comparative Example 3 6.28％ 4.36％ Comparative Example 4 86.5％ 3.25％ Comparative Example 5 89.3％ 2.26％ Comparative Example 6 85.8％ 2.83％ Comparative Example 7 86.2％ 2.90％

[0109] It can be seen from the comparison of Comparative Examples 1 to 3 with Example 1 in the data of Table 1 that deleting the regulator or any component in the regulator, the α-phase content of the target product silicon nitride powder decreases sharply, and the oxygen content increases compared with that of silicon sawdust. The reason is that the rupture of the surface dense oxide layer requires the synergistic effect of the two regulators.

[0110] It can be seen from the comparison of Comparative Example 4 and Comparative Example 5 with Example 1 that when the content of sodium chloride or carbon powder in the regulator is excessive, the α-phase content of the target product silicon nitride powder decreases, and the oxygen content increases. The reason is that when sodium chloride or carbon powder is excessive, a dense oxide layer will be formed on the surface of silicon sawdust particles, hindering the nitridation of silicon sawdust and increasing the oxygen content at the same time.

[0111] It can be seen from the comparison of Comparative Example 6 and Comparative Example 7 with Example 1 that the α-phase content of the silicon nitride powder prepared by single-temperature nitridation or gradient cooling nitridation from high temperature to low temperature decreases. The reason is that the temperature range of 900-1000 °C is the stage of synergistic deoxidation of the regulator. Directly heating to the nitridation temperature makes the deoxidation incomplete and hinders the synthesis of silicon nitride.

[0112] The above-described embodiments only represent several implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation to the scope of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several deformations and improvements can be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the appended claims.

Claims

1. A method for directly synthesizing low-oxygen and high-α-phase silicon nitride powder by nitriding silicon sawdust, characterized in that The method includes the following steps: Performing pickling, impurity removal, and purification treatment on silicon sawdust to obtain silicon raw materials with main metal impurities less than 10 ppm and oxygen content less than 2%; Mixing the silicon raw materials and a regulator in a preset ratio and performing ball milling treatment to obtain a mixed material; Introducing a gaseous nitrogen source and performing vacuum gradient temperature rise nitriding treatment on the mixed material to obtain crude silicon nitride; The crude silicon nitride is successively subjected to pickling, solid-liquid separation, cleaning, and drying post-treatment to obtain the low-oxygen and high-α-phase silicon nitride powder; Among them, the regulator includes sodium chloride and carbon powder.

2. The method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder according to claim 1, characterized in that, In the regulator, the dosage of sodium chloride is 3 wt% - 7 wt% of the silicon raw materials, and the dosage of carbon powder is 1 wt% - 3 wt% of the silicon raw materials.

3. The method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder according to claim 1, characterized in that, The step of introducing a gaseous nitrogen source and performing vacuum high-temperature nitriding treatment on the mixed material specifically includes: Loosely loading the mixed material into a ceramic crucible, placing the ceramic crucible in the isothermal zone of a muffle furnace, and using high-purity nitrogen to replace the air in the furnace tube of the muffle furnace; Exhausting the furnace tube of the muffle furnace, closing the air outlet after completion, and filling the furnace tube of the muffle furnace with high-purity ammonia gas to 0.1 Mpa - 0.2 Mpa; Heating at a rate of 5 °C / min - 10 °C / min to 900 °C - 1000 °C, holding for 1 h - 3 h, after the holding is completed, opening the air outlet valve and releasing pressure to atmospheric pressure; Then continuously flushing in high-purity ammonia gas at a flow rate of 80 ml / min - 120 ml / min, while heating at a rate of 5 °C / min - 10 °C / min to 1100 °C - 1200 °C, holding for 3 h - 5 h, and cooling with the furnace to room temperature to obtain crude silicon nitride.

4. The method for directly synthesizing low-oxygen and high-α-phase silicon nitride powder by nitriding silicon sawdust according to claim 3, characterized in that, The purity of the high-purity nitrogen and high-purity ammonia gas ≥ 99.999%.

5. The method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder according to claim 1, wherein The step of performing pickling, impurity removal, and purification treatment on the silicon sawdust specifically includes: pickling the silicon sawdust in a mixed acid solution of 1% - 5% hydrofluoric acid and 5% - 10% hydrochloric acid for 2 h - 4 h, then repeatedly washing with deionized water, and drying to obtain silicon raw materials with main metal impurities less than 10 ppm and oxygen content less than 2%.

6. The method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder according to claim 5, characterized in that, The solid-liquid ratio of the silicon sawdust to the mixed acid solution is 1:(10 - 20).

7. The method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder according to claim 1, characterized in that, The step of the crude silicon nitride being successively subjected to pickling, solid-liquid separation, cleaning, and drying post-treatment to obtain the low-oxygen and high-α-phase silicon nitride powder specifically includes: Placing the crude silicon nitride in a mixed solution of 1% - 5% hydrofluoric acid and 1% - 5% nitric acid with a mass concentration and stirring and soaking for pickling for 30 min - 60 min; Performing suction filtration to collect the powder precipitate and repeatedly rinsing with deionized water at least 3 times; Drying to obtain silicon nitride powder with an α-phase content greater than 95% and an oxygen content less than 1%.

8. The method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder according to claim 1, wherein The ball milling time is 5 h - 7 h.

9. The method for directly nitriding silicon sawdust to synthesize low-oxygen and high-α-phase silicon nitride powder according to claim 1, wherein The silicon sawdust includes silicon sawdust generated by wire saw cutting during the entire process of slicing single-crystal ingots and polycrystalline ingots.

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