A silicon nitride powder ball milling medium and its usage method

Abstract

The present invention discloses a silicon nitride powder ball milling medium and its usage method, belonging to the technical field of inorganic non-metallic materials. The ball milling medium of the present invention is composed of alkali metal hydroxide, alkali metal fluoride and deionized water, wherein the mass fraction of the alkali metal hydroxide is 2-20%, the mass fraction of the alkali metal fluoride is 8-20%, and the mass fraction of the deionized water is 60-90%. The method of the present invention consists of steps such as mechanical ball milling of silicon nitride powder in the ball milling medium, screening and filtering, washing and impurity removal, and vacuum drying. The silicon nitride powder prepared by the ball milling medium and its usage method of the present invention has remarkable advantages such as low oxygen content and small particle size, and moreover, the operation process is simple, safe and environmentally friendly, and has broad application prospects in the field of preparation of high-purity silicon nitride materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of inorganic non-metallic materials. More specifically, it relates to a silicon nitride powder ball milling medium and its usage method. Background Art

[0002] Silicon nitride ceramics have excellent mechanical properties, good high-temperature stability, and outstanding insulation and thermal conductivity. With the rapid development of application fields such as new energy vehicles, 5G communication, wind power generation, precision machine tools, and aerospace, stricter requirements have been put forward for the particle size and oxygen impurity content of silicon nitride powder used to prepare silicon nitride ceramics. Generally, the median particle size is required to be <1 μm and the oxygen impurity content is <1%. Currently, the median particle size of silicon nitride powder synthesized by traditional methods such as direct nitridation method and combustion synthesis method is generally 2 - 20 μm, and the oxygen impurity content is generally 0.3 - 3%. Usually, ball milling refinement treatment is required. However, during the ball milling refinement process of silicon nitride powder, surface oxidation occurs and oxygen impurities are introduced, resulting in an oxygen impurity content of 3 - 5% in the finished silicon nitride powder, which has an adverse impact on key properties such as the strength, toughness, dielectric, and thermal conductivity of the material, severely restricting the application of silicon nitride ceramics in high-end fields.

[0003] Currently, deionized water or anhydrous ethanol is generally used as the ball milling medium during the ball milling refinement process of silicon nitride powder, which will promote the surface oxidation of silicon nitride powder and cause a sharp increase in the oxygen impurity content of silicon nitride powder. In order to remove oxygen impurities in silicon nitride powder, hydrofluoric acid pickling is usually used, but this process will not only severely corrode the equipment but also cause great harm to the operating environment. Therefore, for the ball milling refinement process of silicon nitride powder, it is particularly important to develop a new type of ball milling medium and its process method.

[0004] After retrieval, relevant patents have been published on the method of using silicon nitride for ball milling. For example, in the Chinese patent "A Ball Milling Method for Improving the Particle Size and Distribution of Silicon Nitride", the application publication number is CN109650904A, and the application publication date is October 10, 2017. The ball milling method proposed in this invention uses deionized water as the ball milling medium. Another example is the Chinese patent "High-Purity Silicon Nitride Ultrafine Grinding Technology", the application publication number is CN1868965A, and the application publication date is May 23, 2005. The ball milling method proposed in this invention uses anhydrous ethanol as the ball milling medium. Summary of the Invention

[0005] 1. Technical Problems to be Solved by the Invention

[0006] The object of the present invention is to solve the problem that during the ball milling and refinement process of silicon nitride powder using existing conventional ball milling media, the surface of the silicon nitride powder will be oxidized, and the oxygen impurity content will increase sharply, which cannot meet the requirements of the high-tech and high-end equipment fields for the low oxygen content of silicon nitride powder. It is intended to provide a ball milling medium for silicon nitride powder and its usage method. By adopting the technical solution of the present invention, both the ball milling and refinement of silicon nitride powder can be achieved, and silicon nitride powder with a low oxygen content can be obtained, while taking into account safety and environmental protection, so it is suitable for industrial promotion and application.

[0007] 2. Technical solution

[0008] To achieve the above object, the technical solution provided by the present invention is as follows:

[0009] A ball milling medium for silicon nitride powder of the present invention comprises alkali metal hydroxide, alkali metal fluoride and deionized water. Among them, the mass fraction of alkali metal hydroxide in the ball milling medium is 2% - 20%, and the mass fraction of alkali metal fluoride in the ball milling medium is 8% - 20%, and the rest is deionized water. Specifically, the alkali metal hydroxide includes but is not limited to any one or more of lithium hydroxide, sodium hydroxide and potassium hydroxide, and the alkali metal fluoride includes but is not limited to any one or more of lithium fluoride, sodium fluoride, magnesium fluoride, calcium fluoride, strontium fluoride and barium fluoride.

[0010] The present invention promotes the mechanical force chemical reaction between the alkali metal hydroxide and the surface oxide layer of the silicon nitride powder during the ball milling process to form a soluble water glass phase, thereby playing a role in removing the oxygen impurities on the surface of the silicon nitride powder. The excessive fluoride in the ball milling medium exists in the form of microcrystals, which plays the role of "tiny grinding balls" during the ball milling process. It can not only accelerate the refinement of the silicon nitride powder, but also play a role in friction and peeling on the surface oxide layer of the silicon nitride powder and the adsorbed water glass phase, which is beneficial to the further removal of oxygen impurities in the silicon nitride powder.

[0011] A usage method of a ball milling medium for silicon nitride powder of the present invention comprises the following technological steps:

[0012] Step 1: Mechanical ball milling: Put silicon nitride grinding balls, the ball milling medium and silicon nitride powder into a ball milling tank, and carry out mechanical ball milling using a planetary ball mill. The ball milling time is 0.5 - 8 h; among them, the silicon nitride grinding balls are silicon nitride ceramic balls, and the particle size of the silicon nitride ceramic balls is 0.2 - 5 mm; the median particle size of the silicon nitride powder is 2 - 20 μm, and the oxygen content is 0.3 - 3%; the mass ratio of the silicon nitride powder, the ball milling medium and the silicon nitride grinding balls is 1:(5 - 50):(10 - 90).

[0013] The principle of this step is mainly to generate high mechanical energy through ball milling to promote the chemical reaction between alkali metal hydroxides and oxygen impurities on the surface of silicon nitride powder, so as to achieve the purpose of removing the oxygen impurities generated by the secondary oxidation of silicon nitride powder during ball milling. Alkali metal fluorides have very low solubility in the above-mentioned ball milling medium and mainly exist in the form of microcrystals, which will have a certain frictional effect on the surface of silicon nitride powder particles, facilitating the further stripping of the oxidized phase on the surface of silicon nitride powder particles.

[0014] Step 2: Screening and filtration: Use a sieve to screen the silicon nitride slurry and silicon nitride grinding balls, and then filter the silicon nitride slurry in a filter; the purpose of screening is to separate the silicon nitride slurry from the silicon nitride grinding balls, and the purpose of slurry filtration is to perform liquid-phase separation of the silicon nitride powder from the ball milling medium enriched with soluble water glass phase.

[0015] Step 3: Acid washing and impurity removal: Place the filtered silicon nitride slurry in a reaction kettle, add a washing solution and heat and stir, and then filter out the washing solution to obtain a silicon nitride filter cake; the washing solution is any one or a combination of deionized water, hydrochloric acid or nitric acid, and wash until the pH of the filtrate is 6-8. The purpose of acid washing is to remove the residual ball milling medium on the surface of silicon nitride powder particles and at the same time can remove a certain amount of metal impurities, which is beneficial to further improve the purity of silicon nitride powder.

[0016] Step 4: Vacuum drying: Dry the silicon nitride slurry after acid washing and filtration in a vacuum drying oven, with a vacuum degree of 0.1-1 Pa. The drying process is divided into two stages, namely low-temperature drying and high-temperature drying. First, dry at 40-60 °C for 6-12 h, and then heat up to 100-120 °C and dry for 2-4 h. Low-temperature drying first prevents the powder from hydrolysis and oxidation, while high-temperature drying can remove the bound water on the surface of silicon nitride powder particles.

[0017] By using the ball milling medium of the present invention for ball milling, not only can the ball milling and refinement efficiency of silicon nitride powder be improved, and silicon nitride powder with a low oxygen impurity content can be obtained, but also the operation process is simple, safe and environmentally friendly, and has a very broad application prospect in the industrial production field of high-quality silicon nitride powder.

[0018] 3. Beneficial effects

[0019] Adopting the technical solution provided by the present invention, compared with the prior art, it has the following beneficial effects:

[0020] (1) The ball milling medium of silicon nitride powder of the present invention, compared with the ball milling media such as water and ethanol used in the conventional ball milling process, can effectively remove the oxygen impurities generated by the secondary oxidation of silicon nitride powder during ball milling through mechanochemical reaction, and the oxygen impurity content of the finished silicon nitride powder is low.

[0021] (2) The method for using the silicon nitride powder ball milling medium of the present invention can promote the refinement of silicon nitride powder through mechanical force chemical reaction and the erosion effect of the ball milling medium compared with the conventional ball milling process, so as to obtain silicon nitride powder with finer particle size. Detailed implementation manners

[0022] The present invention will be further described below in conjunction with embodiments.

[0023] Embodiment 1

[0024] The method for using the silicon nitride powder ball milling medium in this embodiment is as follows:

[0025] Step 1: Prepare the ball milling medium: Prepare the ball milling medium with sodium hydroxide, sodium fluoride and deionized water, where the mass fraction of sodium hydroxide is 20%, the mass fraction of sodium fluoride is 20%, and the rest is deionized water;

[0026] Step 2: Mechanical ball milling: Place 10 g of silicon nitride powder with an oxygen impurity content of 3% and a median particle size of 20 μm in 500 g of the ball milling medium, then add 900 g of silicon nitride ceramic balls with a particle size of 5 mm, and ball mill for 8 h with a planetary ball mill;

[0027] Step 3: Screening and filtration: Screen the ball milled silicon nitride slurry with a 100-mesh sieve, and then filter the silicon nitride slurry with an acid and alkali resistant filter membrane;

[0028] Step 4: Washing and impurity removal: Wash the filtered silicon nitride filter cake with deionized water, and then wash the filtrate with deionized water until the pH = 8;

[0029] Step 5: Vacuum drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying, evacuate to 1 Pa, dry at 60 °C for 12 h, and then raise the temperature to 120 °C for drying for 4 h.

[0030] The particle size and oxygen content of the ball milled silicon nitride powder were detected with an oxygen content tester and a laser particle size analyzer respectively, and the results were an oxygen impurity content of 0.9% and a median particle size of 0.8 μm.

[0031] Embodiment 2

[0032] The method for using the silicon nitride powder ball milling medium in this embodiment is as follows:

[0033] Step 1: Prepare the ball milling medium: Prepare the ball milling medium with lithium hydroxide, lithium fluoride and deionized water, where the mass fraction of lithium hydroxide is 2%, the mass fraction of lithium fluoride is 8%, and the rest is deionized water;

[0034] Step 2. Mechanical ball milling: Place 3 g of silicon nitride powder with an oxygen impurity content of 0.3% and a median particle size of 2 μm in 15 g of ball milling medium, then add 30 g of silicon nitride ceramic balls with a particle size of 0.2 mm, and ball mill for 0.5 h using a drum-type ball mill;

[0035] Step 3. Screening and filtration: Screen the ball-milled silicon nitride slurry using a 150-mesh sieve, and then filter the silicon nitride slurry using an acid- and alkali-resistant filter membrane;

[0036] Step 4. Washing and impurity removal: Wash the filtered silicon nitride filter cake with hydrochloric acid with a mass fraction of 0.5%, and then wash the filtrate with deionized water until the pH = 6;

[0037] Step 5. Vacuum drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying, evacuate to 0.1 Pa, dry at 40 °C for 6 h, and then raise the temperature to 100 °C and dry for 2 h.

[0038] Use an oxygen content tester and a laser particle size analyzer to detect the particle size and oxygen content of the ball-milled silicon nitride powder respectively. The results are an oxygen impurity content of 0.2% and a median particle size of 0.5 μm.

[0039] Example 3

[0040] The method for using the ball milling medium of the silicon nitride powder in this example is as follows:

[0041] Step 1. Prepare the ball milling medium: Prepare the ball milling medium using sodium hydroxide, magnesium fluoride, and deionized water, where the mass fraction of sodium hydroxide is 8% and the mass fraction of magnesium fluoride is 15%, and the rest is deionized water;

[0042] Step 2. Mechanical ball milling: Place 20 g of silicon nitride powder with an oxygen impurity content of 1.5% and a median particle size of 4 μm in 300 g of ball milling medium, then add 800 g of silicon nitride ceramic balls with a particle size of 3 mm, and ball mill for 5 h using a vertical sand mill;

[0043] Step 3. Screening and filtration: Screen the ball-milled silicon nitride slurry using a 100-mesh sieve, and then filter the silicon nitride slurry using an acid- and alkali-resistant filter membrane;

[0044] Step 4. Washing and impurity removal: Wash the filtered silicon nitride filter cake with a mixed acid of hydrochloric acid with a mass fraction of 0.7% and nitric acid with a mass fraction of 0.3%, and then wash the filtrate with deionized water until the pH = 6.8;

[0045] Step 5. Vacuum drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying, evacuate to 0.5 Pa, dry at 50 °C for 6 h, and then raise the temperature to 120 °C and dry for 3 h.

[0046] The particle size and oxygen content of the ball-milled silicon nitride powder were measured using an oxygen content tester and a laser particle size analyzer, respectively. The results showed an oxygen impurity content of 0.7% and a median particle size of 0.8 μm.

[0047] Example 4

[0048] The method for using the ball-milling medium of silicon nitride powder in this example is as follows:

[0049] Step 1: Prepare the ball-milling medium: Prepare the ball-milling medium using sodium hydroxide, potassium hydroxide, strontium fluoride, barium fluoride, and deionized water. Among them, the mass fraction of sodium hydroxide is 5%, the mass fraction of potassium hydroxide is 5%, the mass fraction of strontium fluoride is 8%, the mass fraction of barium fluoride is 8%, and the rest is deionized water;

[0050] Step 2: Mechanical ball milling: Place 20 g of silicon nitride powder with an oxygen impurity content of 2.5% and a median particle size of 5 μm in 300 g of the ball-milling medium, and then add 800 g of silicon nitride ceramic balls with a particle size of 3 mm. Ball mill for 5 h using a planetary ball mill;

[0051] Step 3: Screening and filtration: Screen the ball-milled silicon nitride slurry using a 100-mesh sieve, and then filter the silicon nitride slurry using an acid- and alkali-resistant filter membrane;

[0052] Step 4: Washing and impurity removal: Wash the filtered silicon nitride filter cake with a mixed acid of hydrochloric acid with a mass fraction of 0.7% and nitric acid with a mass fraction of 0.3%, and then wash the filtrate with deionized water until the pH = 6.8;

[0053] Step 5: Vacuum drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying. Evacuate to 0.5 Pa, dry at 50 °C for 6 h, and then raise the temperature to 120 °C and dry for 3 h.

[0054] The particle size and oxygen content of the ball-milled silicon nitride powder were measured using an oxygen content tester and a laser particle size analyzer, respectively. The results showed an oxygen impurity content of 0.7% and a median particle size of 0.8 μm.

[0055] Example 5

[0056] The method for using the ball-milling medium of silicon nitride powder in this example is as follows:

[0057] Step 1: Prepare the ball-milling medium: Prepare the ball-milling medium using potassium hydroxide, magnesium fluoride, calcium fluoride, and deionized water. Among them, the mass fraction of potassium hydroxide is 10%, the mass fraction of magnesium fluoride is 6%, the mass fraction of calcium fluoride is 8%, and the rest is deionized water;

[0058] Step 2. Mechanical ball milling: Place 20 g of silicon nitride powder with an oxygen impurity content of 2% and a median particle size of 3 μm in 200 g of ball milling medium, then add 600 g of silicon nitride ceramic balls with a particle size of 2 mm, and ball mill for 7 h using a planetary ball mill;

[0059] Step 3. Screening and filtration: Screen the ball-milled silicon nitride slurry using a 100-mesh sieve, and then filter the silicon nitride slurry using an acid- and alkali-resistant filter membrane;

[0060] Step 4. Washing and impurity removal: Wash the filtered silicon nitride filter cake with nitric acid with a mass fraction of 0.5%, and then wash the filtrate with deionized water until the pH = 6;

[0061] Step 5. Vacuum drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying, evacuate to 0.5 Pa, dry at 50 °C for 8 h, and then raise the temperature to 110 °C and dry for 3 h.

[0062] Use an oxygen content tester and a laser particle size analyzer to detect the particle size and oxygen content of the ball-milled silicon nitride powder respectively. The results are an oxygen impurity content of 0.6% and a median particle size of 0.9 μm.

[0063] Example 6

[0064] The usage method of the ball milling medium for the silicon nitride powder in this example is as follows:

[0065] Step 1. Prepare the ball milling medium: Prepare the ball milling medium using lithium hydroxide, sodium hydroxide, lithium fluoride, magnesium fluoride, and deionized water, where the mass fraction of lithium hydroxide is 3%, the mass fraction of sodium hydroxide is 12%, the mass fraction of lithium fluoride is 5%, the mass fraction of magnesium fluoride is 10%, and the rest is deionized water;

[0066] Step 2. Mechanical ball milling: Place 20 g of silicon nitride powder with an oxygen impurity content of 1.3% and a median particle size of 4 μm in 300 g of ball milling medium, then add 800 g of silicon nitride ceramic balls with a particle size of 3 mm, and ball mill for 6 h using a planetary ball mill;

[0067] Step 3. Screening and filtration: Screen the ball-milled silicon nitride slurry using a 100-mesh sieve, and then filter the silicon nitride slurry using an acid- and alkali-resistant filter membrane;

[0068] Step 4. Washing and impurity removal: Wash the filtered silicon nitride filter cake with a mixed acid of hydrochloric acid with a mass fraction of 0.8% and nitric acid with a mass fraction of 0.2%, and then wash the filtrate with deionized water until the pH = 6.8;

[0069] Step 5. Vacuum drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying, evacuate to 0.5 Pa, dry at 50 °C for 6 h, and then raise the temperature to 120 °C and dry for 3 h.

[0070] The particle size and oxygen content of the ball-milled silicon nitride powder were measured using an oxygen content tester and a laser particle size analyzer respectively. The results showed that the oxygen impurity content was 0.5% and the median particle size was 0.7 μm.

[0071] Example 7

[0072] The method for using the ball-milling medium of the silicon nitride powder in this example is as follows:

[0073] Step 1: Prepare the ball-milling medium: Prepare the ball-milling medium using lithium hydroxide, sodium hydroxide, lithium fluoride, magnesium fluoride, calcium fluoride and deionized water. Among them, the mass fraction of lithium hydroxide is 3%, the mass fraction of sodium hydroxide is 12%, the mass fraction of lithium fluoride is 3%, the mass fraction of magnesium fluoride is 7%, the mass fraction of magnesium fluoride is 5%, and the rest is deionized water;

[0074] Step 2: Mechanical ball milling: Place 20 g of silicon nitride powder with an oxygen impurity content of 1.25% and a median particle size of 4 μm in 300 g of the ball-milling medium, then add 800 g of silicon nitride ceramic balls with a particle size of 3 mm, and ball mill for 6 h using a planetary ball mill;

[0075] Step 3: Screening and filtration: Screen the ball-milled silicon nitride slurry using a 100-mesh sieve, and then filter the silicon nitride slurry using an acid and alkali resistant filter membrane;

[0076] Step 4: Washing and impurity removal: Wash the filtered silicon nitride filter cake with a mixed acid with a hydrochloric acid mass fraction of 0.8% and a nitric acid mass fraction of 0.2%, and then wash the filtrate with deionized water until the pH = 6.8;

[0077] Step 5: Vacuum drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying, evacuate to 0.5 Pa, dry at 50 °C for 6 h, and then raise the temperature to 120 °C and dry for 3 h.

[0078] The particle size and oxygen content of the ball-milled silicon nitride powder were measured using an oxygen content tester and a laser particle size analyzer respectively. The results showed that the oxygen impurity content was 0.4% and the median particle size was 0.7 μm.

[0079] Comparative Example 1

[0080] The ball-milling method of this comparative example is as follows:

[0081] Step 1: Prepare the ball-milling medium: Prepare the ball-milling medium using deionized water;

[0082] Step 2: Mechanical ball milling: Place 20 g of silicon nitride powder with an oxygen impurity content of 1.25% and a median particle size of 4 μm in 300 g of the ball-milling medium, then add 800 g of silicon nitride ceramic balls with a particle size of 3 mm, and ball mill for 6 h using a planetary ball mill;

[0083] Step 3. Screening and Filtration: Screen the ball-milled silicon nitride slurry with a 100-mesh sieve, and then filter the silicon nitride slurry with an acid- and alkali-resistant filter membrane;

[0084] Step 4. Washing and Impurity Removal: Wash the filtered silicon nitride filter cake with a mixed acid of 0.8% hydrochloric acid and 0.2% nitric acid, and then wash the filtrate with deionized water until the pH = 6.8;

[0085] Step 5. Vacuum Drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying, evacuate to 0.5 Pa, dry at 50 °C for 6 h, and then raise the temperature to 120 °C and dry for 3 h.

[0086] Use an oxygen content tester and a laser particle size analyzer to detect the particle size and oxygen content of the ball-milled silicon nitride powder. The results are an oxygen impurity content of 3.2% and a median particle size of 1.5 μm.

[0087] Comparative Example 2

[0088] The ball-milling method of this comparative example is as follows:

[0089] Step 1. Prepare the ball-milling medium: Prepare the ball-milling medium with absolute ethanol;

[0090] Step 2. Mechanical ball milling: Place 20 g of silicon nitride powder with an oxygen impurity content of 1.25% and a median particle size of 4 μm into 300 g of the ball-milling medium, and then add 800 g of silicon nitride ceramic balls with a particle size of 3 mm. Ball mill with a planetary ball mill for 6 h;

[0091] Step 3. Screening and Filtration: Screen the ball-milled silicon nitride slurry with a 100-mesh sieve, and then filter the silicon nitride slurry with an acid- and alkali-resistant filter membrane;

[0092] Step 4. Washing and Impurity Removal: Wash the filtered silicon nitride filter cake with a mixed acid of 0.8% hydrochloric acid and 0.2% nitric acid, and then wash the filtrate with deionized water until the pH = 6.8;

[0093] Step 5. Vacuum Drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying, evacuate to 0.5 Pa, dry at 50 °C for 6 h, and then raise the temperature to 120 °C and dry for 3 h.

[0094] Use an oxygen content tester and a laser particle size analyzer to detect the particle size and oxygen content of the ball-milled silicon nitride powder. The results are an oxygen impurity content of 2.7% and a median particle size of 1.8 μm.

[0095] Comparative Example 3

[0096] The ball-milling method of this comparative example is as follows:

[0097] Step 1. Configure the ball milling medium: Use deionized water and absolute ethanol to configure the ball milling medium, where the mass fraction of deionized water is 50% and the mass fraction of absolute ethanol is 50%.

[0098] Step 2. Mechanical ball milling: Place 20 g of silicon nitride powder with an oxygen impurity content of 1.25% and a median particle size of 4 μm into 300 g of the ball milling medium, and then add 800 g of silicon nitride ceramic balls with a particle size of 3 mm. Ball mill for 6 h using a planetary ball mill.

[0099] Step 3. Screening and filtration: Screen the ball milled silicon nitride slurry using a 100-mesh sieve, and then filter the silicon nitride slurry using an acid and alkali resistant filter membrane.

[0100] Step 4. Washing and impurity removal: Wash the filtered silicon nitride filter cake with a mixed acid with a hydrochloric acid mass fraction of 0.8% and a nitric acid mass fraction of 0.2%, and then wash with deionized water until the pH of the filtrate is 6.8.

[0101] Step 5. Vacuum drying: Place the silicon nitride powder after washing and impurity removal into a vacuum drying oven for drying. Evacuate to 0.5 Pa, dry at 50 °C for 6 h, and then raise the temperature to 120 °C and dry for 3 h.

[0102] Use an oxygen content tester and a laser particle size analyzer to detect the particle size and oxygen content of the ball milled silicon nitride powder. The results are an oxygen impurity content of 3.1% and a median particle size of 1.7 μm.

[0103] The above schematically describes the present invention and its embodiments. This description is not restrictive, but only one of the embodiments of the present invention. In fact, it is not limited thereto. Therefore, if those of ordinary skill in the art are inspired by it and design a structural method and embodiment similar to this technical solution without creative work without departing from the purpose of the present invention, they shall fall within the protection scope of the present invention.

Claims

1. A silicon nitride powder ball milling medium, characterized in that: The ball milling medium is composed of alkali metal hydroxides, alkali metal fluorides and deionized water. The mass fraction of the alkali metal hydroxide in the ball milling medium is 2% - 20%, and the mass fraction of the alkali metal fluoride in the ball milling medium is 8% - 20%, with the rest being deionized water. Among them, the alkali metal hydroxide is used to react with the surface oxide layer of the silicon nitride powder to form a soluble water glass phase; the excessive alkali metal fluoride exists in the form of microcrystals and is used to frictionally peel off the oxide layer on the surface of the silicon nitride powder and the adsorbed water glass phase.

2. The silicon nitride powder ball milling medium according to claim 1, characterized in that: The alkali metal hydroxide includes but is not limited to any one or more of lithium hydroxide, sodium hydroxide and potassium hydroxide.

3. The silicon nitride powder ball milling medium according to claim 1 or 2, characterized in that: The alkali metal fluoride includes but is not limited to any one or more of lithium fluoride, sodium fluoride, magnesium fluoride, calcium fluoride, strontium fluoride and barium fluoride.

4. The usage method of the silicon nitride powder ball milling medium according to claim 1, characterized in that: It includes the following steps: Step 1: Mechanical ball milling; Mix the silicon nitride powder, the ball milling medium and the silicon nitride grinding balls and then carry out mechanical ball milling. Step 2: Screening and filtration; Use a sieve to screen the ball milled silicon nitride slurry and the silicon nitride grinding balls, and then place the silicon nitride slurry into a filter for filtration. Step 3: Washing and impurity removal; Place the filtered silicon nitride slurry into a reaction kettle, add the washing liquid and heat and stir, and then filter out the washing liquid to obtain a silicon nitride filter cake. Step 4: Vacuum drying; Place the silicon nitride filter cake after washing and impurity removal into a vacuum drying oven for drying.

5. The usage method of the silicon nitride powder ball milling medium according to claim 4, characterized in that: The silicon nitride grinding balls are silicon nitride ceramic balls, and the particle size of the silicon nitride ceramic balls is 0.2 - 5 mm.

6. The usage method of the silicon nitride powder ball milling medium according to claim 4, characterized in that: The median particle size of the silicon nitride powder is 2 - 20 μm, and the oxygen content is 0.3 - 3%.

7. The usage method of the silicon nitride powder ball milling medium according to claim 4, characterized in that: In Step 1, the mass ratio of the silicon nitride powder, the ball milling medium and the silicon nitride grinding balls is 1:(5 - 50):(10 - 90).

8. The usage method of the silicon nitride powder ball milling medium according to claim 4, characterized in that: The ball milling time in Step 1 is 0.5 - 8 h.

9. The usage method of the silicon nitride powder ball milling medium according to claim 4, characterized in that: The washing liquid in Step 3 is any one or a combination of deionized water, hydrochloric acid or nitric acid, and the washing is carried out until the pH of the filtrate is 6 - 8.

10. The usage method of the silicon nitride powder ball milling medium according to any one of claims 4 - 9, characterized in that: In Step 4, the vacuum degree is 0.1 - 1 Pa, and the drying process is divided into two stages. First, dry at 40 - 60 °C for 6 - 12 h, and then heat up to 100 - 120 °C and dry for 2 - 4 h.