Method and device for preparing silicon nitride ceramic balls by slope rolling titration method

Abstract

The invention discloses a method and a device for preparing silicon nitride ceramic balls by a slope rolling titration method. According to the slope rolling titration balling device, a slope rolling method is creatively adopted, the influence of gravity on the sphericity degree of liquid drops is reduced, and silicon nitride green balls with ultrahigh roundness can be obtained; and the problems that a compact core part cannot be obtained due to low-density rolling forming of the silicon nitride powder, the ball is not round due to traditional direct titration, and the small balls cannot be produced in quantity and are provided with indentations due to compression forming are fundamentally solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of silicon nitride ceramics, and particularly relates to a method and device for preparing silicon nitride ceramic balls by means of a slope rolling titration method. BACKGROUND

[0002] With the rapid development of the chip industry in China in recent years, with the rapid development of new energy vehicles and the chip industry, the application of silicon nitride substrates is becoming more and more widespread, and more stringent requirements for the purity of silicon nitride are put forward. At present, silicon nitride microbeads have gradually replaced zirconia microbeads and become the preferred grinding medium for fine grinding of silicon nitride powder in the semiconductor industry. Silicon nitride bearings are widely used due to their low density, small thermal expansion coefficient, high temperature resistance, corrosion resistance, long service life and other advantages. With the rapid development of precision machining and manufacturing, the demand for silicon nitride microbeads bearings will continue to increase, and the requirements for bearing quality and precision will also become higher and higher. Silicon nitride has high mechanical properties, heat resistance and chemical stability, and is a very excellent material in structural ceramics, so its application is also very widespread, and the demand for silicon nitride powder preparation is also increasing, and the use of silicon nitride microbeads has also increased significantly.

[0003] At present, the most commonly used process for preparing silicon nitride microbeads is press forming. Although this process is quite mature, it is mainly used for preparing silicon nitride balls with a size greater than 4 mm. However, press forming often leads to surface peeling, and a clear protruding waistband is formed in the middle of the ball. In order to solve this problem, diamond grinding machines are usually used for long-term grinding, which takes at least 7-10 days, undoubtedly increasing the complexity of processing. In addition, accurate control of concentricity is also a problem, making the later processing relatively difficult.

[0004] Due to the low density of silicon nitride and the hydrophobicity of silicon nitride powder, the use of rolling ball forming method is generally water-soluble glue, which makes it difficult to eat powder, granulation powder particles, and the infiltration and crushing of the particles become difficult during the ball forming process. The ball grows slowly, several granulation powder particles are connected to form a ball, the sphericity is poor, the density of silicon nitride powder is low, the rolling is too light, the ball and powder collision force is small during the ball forming process, which further increases the difficulty of rolling silicon nitride powder into balls, the roundness of the rolled ceramic green balls is poor, and the heart is not good. Zirconia is hydrophilic and has high density, so the rolling forming effect is good, but this rolling method cannot be used to roll silicon nitride balls.

[0005] Another method of silicon nitride microbeads is titration forming, the heart of the titration method is generally better, the traditional titration forming is to drop the slurry into the curing liquid to prepare the silicon nitride microbead green body, the sphericity of the microbead is affected by the titration height, the gravity of the earth, the solid content of the slurry and the viscosity of the slurry, causing the instability of the production process, and the size of the prepared microbead cannot be too large, the larger the sphericity is worse.

[0006] The method for preparing silicon nitride ceramic beads by the inclined rolling titration method of the application is a method for preparing silicon nitride microspheres with a wide size range, good surface, high sphericity and no defects in the heart by using a special titration curing process, which avoids the problems of the heart being not good, the difficulty in forming a ball, the unsuitability of the press forming for small balls and the sensitivity of the sphericity to the solid content of the slurry, the gravity of the ball dropping and the viscosity of the slurry in the traditional titration forming method. SUMMARY

[0007] The application aims to provide a method and device for preparing silicon nitride ceramic beads by the inclined rolling titration method, the silicon nitride microbeads prepared by the method have better sphericity, better polished surface, no hole in the cross section, fast forming and avoid the influence of the hydration of silicon nitride.

[0008] The application adopts the following technical scheme: A method for preparing silicon nitride ceramic beads by the inclined rolling titration method, comprising the following steps: S1, component weighing: 100-110 parts by mass of silicon nitride powder, 50-55 parts by mass of pure water, 1.5-2.0 parts by mass of curing agent and 0.5-1.0 parts by mass of thickening agent are weighed; S2, ball milling and mixing: the weighed components are put into a polyurethane ball mill tank, and silicon nitride grinding beads are added for ball milling and mixing, the silicon nitride grinding beads are 3-4 times the mass of the silicon nitride powder, and the mixing time is 1-2 h; S3, ball and material separation: the mixed and uniform slurry is poured into a vacuum funnel together with the silicon nitride grinding beads, and the slurry is sucked out to separate from the silicon nitride grinding beads; S4, slurry filtration: the separated slurry is filtered by a vacuum filter bag to remove the particulate impurities therein, so as to prevent the slurry from blocking the pipeline and the needle when dropping; S5, slurry viscosity adjustment and vacuum bubble removal: a proper amount of pure water is added to the filtered slurry to adjust the viscosity to a proper value, and then the slurry is put into a stirring vacuum bubble removal machine to remove the bubbles therein, so as to obtain the bubble-free silicon nitride slurry; S6, preparation of curing liquid: the coagulant and the pure water are prepared according to the ratio of 1-2 parts by mass of coagulant and 8-10 parts by mass of pure water, and stirred until completely dissolved into a transparent solution state to form the curing liquid; S7. Preparation of non-polar buffer layer liquid: Prepare 8-10 parts by weight of non-polar liquid and 1-3 parts by weight of interfacial tension eliminator, and stir thoroughly to form a non-polar buffer layer liquid. S8. Mixing of non-polar buffer layer liquid and curing liquid: Add the curing liquid prepared in S6 to the storage tank. After the curing liquid surface is calm, slowly pour the non-polar buffer layer liquid prepared in S7 onto the curing liquid to form a buffer layer. The curing liquid layer and the buffer layer form two immiscible liquid layers. S9. Dropping with the dropper needle: Add the bubble-free silicon nitride slurry obtained in S5 to the silicon nitride slurry storage tank, and use a multi-channel peristaltic pump to draw the silicon nitride slurry into the dropper needle to drop out the silicon nitride slurry droplets. S10. Forming of silicon nitride ceramic green spheres: A ramp is set at the junction of the curing liquid layer and the buffer layer. The ramp connects the buffer layer to the curing liquid layer. The position of the dripping needle is adjusted so that the dripping needle is aimed at the liquid above the buffer layer at the ramp. The silicon nitride slurry droplets enter the liquid of the buffer layer and roll down through the ramp. They first roll in the ramp section of the liquid of the buffer layer, and then quickly pass through the interface between the curing liquid layer and the buffer layer and enter the curing liquid to solidify. They continue to roll until they are completely solidified, forming silicon nitride ceramic green spheres. Due to the action of the interfacial tension eliminator, the liquid of the buffer layer loses its surface tension and the interface with the curing liquid layer is a horizontal plane, ensuring that there is no obvious interfacial resistance when the silicon nitride droplets pass through. The inclination of the ramp is adjusted appropriately to make the sphericity of the silicon nitride ceramic green spheres optimal. S11. Cleaning and drying: The silicon nitride ceramic green balls are cleaned and dried to obtain the final silicon nitride ceramic green balls. S12. Sintering: Silicon nitride ceramic green balls are sintered under nitrogen atmosphere and at atmospheric pressure according to a certain sintering regime to form silicon nitride ceramic balls. S13. Polishing: Polish the sintered silicon nitride ceramic balls with a vertical sand mill, add a certain proportion of polishing material, polish for a certain time, and then wash the polished silicon nitride ceramic balls with pure water and let them dry. S14. Sorting: The dried and polished silicon nitride ceramic balls are sorted by a sorting machine according to their particle size and sphericity. After sorting, silicon nitride ceramic balls with the final qualified particle size are obtained.

[0009] Preferably, in step S1, the curing agent is one or a mixture of sodium alginate, sodium citrate and low-fat pectin; the thickener is one or a mixture of xanthan gum, guar gum and carrageenan.

[0010] Preferably, in step S3, the diameter of the hole in the vacuum funnel is smaller than the diameter of the silicon nitride grinding beads used in mixing; in step S4, the pore size of the vacuum filter bag is 10-20 micrometers.

[0011] Preferably, the viscosity of the slurry in step S5 is such that a needle can drip relatively round silicon nitride slurry droplets.

[0012] Preferably, in step S6, the coagulant is one or a mixture of calcium sulfite, calcium sulfate, and calcium nitrate.

[0013] Preferably, in step S7, the non-polar liquid is one or a mixture of several of liquid paraffin, microcrystalline wax, and wax emulsion, and the surface tension eliminator is one or a mixture of several of tributyl phosphate, triethyl phosphate, and triisopropyl phosphate.

[0014] Preferably, in step S8, the liquid density of the buffer layer is less than the liquid density of the curing liquid layer. This allows the buffer layer to float above the curing liquid layer.

[0015] Preferably, in step S11, the cleaning process involves circulating pure water combined with ultrasonic waves and an ion exchange device; the drying temperature is 90-110℃.

[0016] Preferably, in step S12, the sintering regime is as follows: 0-600℃, sintering rate is 5-7℃ / min, holding time at 600℃ is 0.5-1.5h; 600-1100℃, sintering rate is 4-6℃ / min; 1100-1400℃, sintering rate is 3-5℃ / min; 1400-1450℃, sintering rate is 1.5-3℃ / min, holding time at 1450℃ is 20-40min; 1450℃-1500℃, sintering rate is 2-3℃ / min, holding time at 1500℃ is 20-40min. At 1500-1600℃, the sintering rate is 0.5-1.5℃ / min; hold at 1600℃ for 20-40 min; at 1600-1650℃, the sintering rate is 0.5-1℃ / min; hold at 1650℃ for 20-40 min; at 1650-1700℃, the sintering rate is 0.3-0.7℃ / min; hold at 1700℃ for 60 min; at 1700-1750℃, the sintering rate is 0.5℃ / min; hold at 1750-1750℃ for 90 min; then allow to cool naturally.

[0017] Preferably, in step S13, the polishing material is one or a mixture of two of corundum alumina and silicon carbide powder, with a particle size range of 20-30 micrometers, and the added mass of the polishing material is 10-15% of the silicon nitride ceramic balls.

[0018] An apparatus for preparing silicon nitride ceramic balls by a ramp rolling titration method includes a silicon nitride slurry storage tank, a storage tank, a cleaning device, a drying device, and a silicon nitride ceramic green ball collection tank. The storage tank contains a non-polar buffer layer forming a buffer layer and a curing liquid forming a curing liquid layer. A ramp plate is positioned between the buffer layer and the curing liquid layer, connecting the buffer layer to the curing liquid layer. The silicon nitride slurry storage tank is located above the ramp plate outside the storage tank, storing silicon nitride green ball precursor slurry. The silicon nitride slurry storage tank is connected to a dripping needle via a multi-channel peristaltic pump, with the dripping needle positioned below the ramp plate. A flexible conveyor belt is connected to the lower end of the ramp plate inside the storage tank. The flexible conveyor belt transports the product through the cleaning device and the drying device, ultimately reaching the silicon nitride ceramic green ball collection tank.

[0019] Preferably, the ramp plate is a multi-channel semi-open arc-shaped ramp plate with adjustable slope, and the surface of the ramp plate has a layer of weak polar film, which is made of silicone oil, perfluoropolymer or silane coupling agent.

[0020] Preferably, the flexible conveyor belt has cylindrical grooves and holes throughout its center to prevent the silicon nitride balls from moving during the conveyor belt's movement. The flexible conveyor belt is made of porous nylon, and there are fixing grooves on both sides of the flexible conveyor belt to engage the conveyor belt fixing wheels at the designated bending positions of the conveyor belt.

[0021] Preferably, the flexible conveyor belt is driven by a drive motor to drive the drive wheel, and the conveyor belt is guided and transported by the conveyor belt support guide wheel and the conveyor belt fixed wheel.

[0022] Preferably, the cleaning device contains pure water and includes an ion exchange device and an ultrasonic device. The ion exchange device filters out impurities washed out of the pure water; the ultrasonic device assists in accelerating the cleaning of impurities from the green pellets.

[0023] Preferably, the drying device is provided with a conveying channel for passing through a flexible conveyor belt. Heating resistance wires are installed at the upper and lower ends of the conveying channel. A blower is connected to the lower end of the drying device, and an air outlet is opened at the upper end.

[0024] Preferably, a silicon nitride ceramic green ball guide plate is provided between the tail end of the flexible conveyor belt and the silicon nitride ceramic green ball collecting tank. This guide plate helps the silicon nitride ceramic green balls to flow smoothly into the collecting tank.

[0025] The technical principle of this invention is as follows: This method aims to solve the problems of poor sphericity control in traditional titration of silicon nitride droplets during the rolling process, where the core of the droplet is not well-formed. The idea is to allow the silicon nitride slurry droplets to roll during titration, forming spherical shapes before curing. Specifically, since silicon nitride slurry is aqueous, it cannot be directly dripped into an aqueous curing solution, as the droplets would not have enough time to achieve optimal sphericity. Therefore, a non-polar solvent is first used to drop the silicon nitride slurry droplets into a non-polar buffer layer solvent. Because the slurry droplets are encapsulated by the non-polar solvent, their lowest energy state is close to a sphere. Then, they enter the curing solution for curing. When the slurry droplets pass through the interface between the curing solution and the non-polar buffer layer, they encounter significant resistance, which can even affect the sphericity of the spheres. Therefore, it is necessary to reduce this resistance. The main source of resistance is the viscous force of the non-polar solvent on the surface of the slurry droplets. To address this, the present invention introduces a surface tension eliminator. When the surface tension of the non-polar solvent is reduced to a very low level, the slurry droplets pass quickly through the interface between the non-polar solvent and the curing liquid. This allows the sphericity of the slurry droplets to be maintained. However, if the silicon nitride slurry droplets fluctuate, it will affect the sphericity and curing process of other droplets. Moreover, the presence of gravity will make the sphericity of the slurry droplets worse. This traditional droplet method can only titrate silicon nitride spheres with very small particle sizes. For particles larger than 0.5 mm, there may be a significant decrease in sphericity, or even droplet-shaped spheres. To address this, the present invention introduces a ramp plate and creates a separate channel for each slurry droplet, ensuring they do not interfere with each other. It was found that the small slurry droplets roll on the ramp, first rolling in the non-polar solvent buffer layer ramp section, then quickly passing through the interface into the curing liquid, where they solidify and continue rolling until fully cured. The resulting green body has excellent sphericity. It was also found that if the operation is not done properly, the small water-based slurry droplets can stick to the ramp plate. Therefore, a weakly polar film was introduced, and a smooth weakly polar film was coated on the surface of the ramp plate. The presence of this repulsive force ensures that the small droplets will not stick to the ramp plate due to the low rolling speed. The rolling channel of the slurry droplets on the ramp plate was changed to a rounded bottom surface, which allows the small droplets to maintain a more spherical shape. The sphericity of the cured silicon nitride green body is even better. Using the method of the present invention, even silicon nitride green bodies with a diameter of 1.0-4.0 mm can maintain a high degree of sphericity. The solidified green pellets themselves are not particularly strong. If a large number of them are piled up, the surface of the pellets will become flat and deteriorate. The present invention introduces a flexible conveyor belt to transport the solidified pellets out in sequence and clean and dry them in turn. The dried pellets are particularly strong and the surface will not be flattened even if a large number of them are piled up.

[0026] The beneficial effects of the present invention are: (1) The method of the present invention introduces a non-polar buffer layer to reduce surface tension for preliminary shaping of slurry droplets, and creatively introduces an independent rolling channel ramp plate and coats the surface with a weakly polar organic film. This not only allows each silicon nitride droplet to not interfere with each other and to spherize and solidify into a ball, but also the rolling effect of the ramp improves the sphericity of the solidified silicon nitride green ball. It can maintain a high sphericity even when rolling and titrating silicon nitride green balls of 1.0-4.0 mm. (2) The device of the present invention introduces a conveyor belt device and makes a special design for the conveyor belt, so that the solidified green balls are washed, dried and collected in sequence in a production line manner, achieving automated production, improving product consistency, and the production line operation avoids the serious hydration of silicon nitride green balls caused by long-term immersion in water, which affects subsequent sintering. (3) The slurry system is simple and stable with low glue content. It only needs simple cleaning to be dried and sintered directly. The surface sphericity is good and bright, which reduces the difficulty of polishing after sintering. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of one structure of the device of the present invention; Figure 2 yes Figure 1 A schematic diagram of a multi-channel, semi-open, arc-shaped bottom sloping plate; Figure 3 yes Figure 1 A schematic diagram of a flexible conveyor belt structure; Figure 4 This is a cross-sectional photograph of the 0.8 mm silicon nitride microspheres titrated in Example 1; Figure 5 This is a cross-sectional photograph of 0.8 mm silicon nitride microspheres prepared by the rolling method in Comparative Example 1; Figure 6 This is a cross-sectional photograph of a 0.8 mm microsphere prepared by the traditional droplet process in Comparative Example 2; In the diagram: 1. Silicon nitride slurry storage tank; 2. Silicon nitride sphere green body precursor slurry; 3. Multi-channel peristaltic pump; 4. Dropping needle; 5. Silicon nitride slurry droplets; 6. Silicon nitride ceramic green body spheres; 7. Buffer layer; 8. Multi-channel semi-open arc-shaped bottom ramp plate; 8-1. Semi-open arc-shaped bottom channel; 9. Flexible conveyor belt; 9-1. Fixing groove; 9-2. Cylindrical grooved hole; 10. Storage tank; 11. Conveyor belt support guide wheel; 12. Conveyor belt fixing wheel; 13. Curing liquid layer; 14. Cleaning device; 15. Drying device; 16. Heating resistance wire; 17. Blower; 18. Vent; 19. Silicon nitride ceramic green body sphere collection tank; 20. Silicon nitride ceramic green body sphere guide plate; 21. Pure water; 22. Ion exchange device; 23. Ultrasonic device; 24. Drive motor. Detailed Implementation

[0028] The technical solution of the present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings: Example: Figure 1 As shown, an apparatus for preparing silicon nitride ceramic spheres by inclined rolling titration includes a silicon nitride slurry storage tank 1, a silicon nitride sphere green precursor slurry 2, a multi-channel peristaltic pump 3, a dropping needle 4, silicon nitride slurry droplets 5, silicon nitride ceramic green spheres 6, a buffer layer 7, a multi-channel semi-open arc-shaped bottom inclined plate 8, a flexible conveyor belt 9, a storage tank 10, a conveyor belt support guide wheel 11, a conveyor belt fixing wheel 12, a curing liquid layer 13, a cleaning device 14, a drying device 15, a heating resistance wire 16, a blower 17, a vent 18, a silicon nitride ceramic green sphere collection tank 19, a silicon nitride ceramic green sphere guide plate 20, pure water 21, an ion exchange device 22, an ultrasonic device 23, and a drive motor 24.

[0029] The storage tank contains a non-polar buffer layer formed by a liquid buffer and a curing liquid formed by a liquid curing agent. A ramp is installed between the buffer layer and the curing liquid layer, connecting the buffer layer to the curing liquid layer. Above the ramp is a silicon nitride slurry storage tank located outside the storage tank. The silicon nitride slurry storage tank stores silicon nitride slurry precursor material for green body pellets. The silicon nitride slurry storage tank is connected to a dripping needle via a multi-channel peristaltic pump. The dripping needle is positioned below the ramp. A flexible conveyor belt is installed at the lower end of the ramp inside the storage tank. The flexible conveyor belt transports the product through a cleaning device and a drying device, until it reaches the silicon nitride ceramic green body pellet collection tank.

[0030] like Figure 2 As shown. The ramp plate is a multi-channel, semi-open, arc-shaped ramp plate with adjustable slope. The surface of the ramp plate has a layer of weak polar film, which is made of perfluoropolymer.

[0031] like Figure 3 As shown, the center of the flexible conveyor belt is covered with cylindrical grooves and holes 9-2. The conveyor belt is made of porous nylon. The two sides of the flexible conveyor belt are provided with fixing grooves 9-1, which are used to lock the conveyor belt fixing wheels into the bending positions of the conveyor belt.

[0032] The flexible conveyor belt is driven by a drive motor that drives the drive wheel, and the conveyor belt guides and transports the material through the conveyor belt support guide wheel and the fixed wheel of the conveyor belt.

[0033] The cleaning device contains pure water and is equipped with an ion exchange device and an ultrasonic device. The ion exchange device is used to filter out impurities washed out of the pure water; the ultrasonic device helps to accelerate the cleaning of impurities from the green pellets.

[0034] The drying device is equipped with a conveying channel for passing through a flexible conveyor belt. Heating resistance wires are installed at the upper and lower ends of the conveying channel. A blower is connected to the lower end of the drying device, and an air outlet is opened at the upper end.

[0035] A silicon nitride ceramic green ball guide plate is installed between the end of the flexible conveyor belt and the silicon nitride ceramic green ball collection tank. This guide plate helps the silicon nitride ceramic green balls to flow smoothly into the collection tank.

[0036] A method for preparing silicon nitride ceramic spheres by ramp rolling titration includes the following steps: Weigh 1000g of silicon nitride powder, 500g of pure water, 15g of sodium alginate, and 5g of xanthan gum. Place the above raw materials into a 5L drum ball mill jar, and add 3500g of silicon nitride beads with a particle size distribution of 3mm. Ball mill and mix for 2 hours. Separate the slurry using a vacuum device. Vacuum filter the separated slurry using a 20-micron filter bag. Adjust the viscosity of the filtered slurry by adding pure water according to its viscosity to a suitable viscosity for titration. Place the slurry with the adjusted viscosity into a vacuum defoamer to remove air bubbles, thereby obtaining the final silicon nitride drop slurry. Transfer the slurry to a silicon nitride slurry storage tank 1 in a slope rolling titration ball forming device of the present invention as the silicon nitride green ball precursor slurry 2 in the device of the present invention.

[0037] Preparation of curing liquid: Prepare the curing liquid according to the mass ratio of calcium nitrate: pure water = 3:100 and stir until it is colorless and transparent; then transfer the curing liquid to the storage tank 10 in the device of the present invention, and distribute it as the curing liquid layer 13 in the device of the present invention and keep the liquid surface calm.

[0038] Next is the preparation of the buffer layer liquid: Take an appropriate amount of liquid paraffin, add 10% of its mass of tributyl phosphate, and stir evenly to form a transparent solution. Place this solution into the storage tank 10 in the device of this invention. Its distribution is as shown in the buffer layer formed by the oily buffer solution in the device of this invention. Since the density of liquid paraffin is less than that of the curing liquid, it will be suspended above the surface of the curing liquid. Due to the effect of tributyl phosphate, the surface tension of liquid paraffin is reduced to a very low level, and the interface with the curing liquid is a horizontal line. This is to ensure that the silicon nitride slurry droplets can easily and quickly pass through the interface without much interfacial resistance. Due to the isolation of the ramp plate 8, a U-shaped connection is formed. Figure 1 In the storage tank 10, the liquid level in the buffer layer on the left side of the ramp plate is slightly higher than the liquid level in the solidified liquid on the right side. A six-channel peristaltic pump is used, with six drip needles 4 having equivalent pipe resistance. At least six corresponding semi-open arc-shaped bottom channels 8-1 are distributed on the ramp plate 8. The characteristics of these semi-open arc-shaped bottom channels are as follows... Figure 2 As shown, its surface is coated with a perfluoropolymer to obtain a smooth hydrophobic surface.

[0039] Then, the flexible conveyor belt 9 in the inclined rolling titration ball-forming device of the present invention is started by the conveyor belt drive motor 24. Then, the six-channel peristaltic pump is turned on and the size of the silicon nitride slurry droplet 5 is adjusted to 1.2mm by selecting a 0.6mm inner diameter droplet needle. By adjusting the inclination of the inclined plate 8, the silicon nitride slurry droplet first passes through the buffer layer and becomes a near-spherical droplet by its own surface tension. Then, it rolls down through the independent channel of the inclined plate to further shape the shape of the droplet and smoothly passes through the interface between the buffer layer and the curing liquid to continue rolling. Because the oily liquid in the buffer layer has a low density and low surface tension, the droplet gradually peels off the surface of the droplet as the ball rolls and floats to the buffer layer. The sulfur in the curing liquid layer is also removed. Calcium acid rapidly diffuses into the small, round droplets, gradually solidifying them into silicon nitride ceramic green balls 6. As the silicon nitride ceramic green balls roll further, solidification is complete, and they roll into the cylindrical groove holes 9-2 on the flexible conveyor belt 9 at the bottom of the ramp plate 8. As the flexible conveyor belt moves, the solidified silicon nitride ceramic green balls are first sent to the cleaning device 14 to wash away excess solidification liquid ions, and then sent to the drying device 15 to dry. The wet silicon nitride ceramic green balls shrink to obtain 1.0 mm silicon nitride ceramic green balls. When the flexible conveyor belt 9 moves to the bottom, the silicon nitride ceramic green balls are collected in the silicon nitride ceramic green ball collection tank 19 by the silicon nitride ceramic green ball guide plate 20 in the ramp rolling titration ball forming device of the present invention. The obtained silicon nitride green pellets were calcined under a nitrogen atmosphere at atmospheric pressure: the calcination regime was as follows: 0-600℃, sintering rate 5-7℃ / min, holding at 600℃ for 0.5-1.5h; 600-1100℃, sintering rate 4-6℃ / min; 1100-1400℃, sintering rate 3-5℃ / min; 1400-1450℃, sintering rate 1.5-3℃ / min, holding at 1450℃ for 20-40min; 1450℃-1500℃, sintering rate 2-3℃ / min, holding at 1500℃ for 20h. -40 min; at 1500-1600℃, the sintering rate is 0.5-1.5℃ / min; hold at 1600℃ for 20-40 min; at 1600-1650℃, the sintering rate is 0.5-1℃ / min; hold at 1650℃ for 20-40 min; at 1650-1700℃, the sintering rate is 0.3-0.7℃ / min; hold at 1700℃ for 60 min; at 1700-1750℃, the sintering rate is 0.5℃ / min; hold at 1750-1750℃ for 90 min; then allow to cool naturally.Then, polishing is performed using a vertical sand mill. The polishing ratio is silicon carbide powder to silicon nitride ceramic balls = 1:20 by mass, with a filling rate of 80%. Rough polishing is performed for 2 hours. Then, the circulating liquid in the sand mill is replaced with pure water, and the silicon nitride ceramic balls are allowed to self-polish for another 4 hours to obtain 0.8mm silicon nitride ceramic balls with a bright surface. Sorting: The balls are sieved through a screen and sorted by an inclined plane to remove unqualified balls. The final 0.8mm silicon nitride ceramic balls are the final product. The SEM image of its inlaid cross section is shown below. Figure 4 As shown, the core is uniform and intact, with good sphericity.

[0040] Comparative Example 1: 0.8 mm silicon nitride microspheres were prepared using a rolling molding method. The adhesive was prepared in a mass ratio of PVA:glycerol:triethanolamine:polyacrylic acid:pure water = 2:1:1:1:100. The mixture was stirred thoroughly to obtain a transparent adhesive solution. The rolling equipment was a sugar coating machine. 50 kg of silicon nitride granulation powder was added to the sugar coating machine, and the particles were gradually increased to 0.2 mm according to a suitable powder addition rate and adhesive addition time. Then, the 0.2 mm microspheres were sieved out, and the upper and lower limits were removed. 50 kg of the sieved 0.2 mm microspheres were taken out and added to the sugar coating machine. The powder addition rate and adhesive addition rate were gradually increased to 0.4 mm, and then the 0.4 mm microspheres were sieved out again, removing the upper and lower limits. This method of sieving every 0.2 mm increase was used until silicon nitride microspheres of approximately 1.0 mm were obtained. The subsequent sintering, polishing, and sorting of the silicon nitride microspheres were carried out in the same manner as in Example 1. The SEM image of the inlaid cross-section of the final 0.8 mm silicon nitride ceramic spheres is shown below. Figure 5 As shown, due to roll forming, the sphericity is relatively good, but the core is generally not very good. This is because silicon nitride itself has a low density, and the granulated powder particles are too light. When rolled in the coating machine, they cannot be crushed and made dense. Since the glue is water-soluble and silicon nitride itself is hydrophobic, the wettability of the silicon nitride granulated powder is poor. As a result, the fake particles of silicon nitride granulated powder cannot be quickly wetted and broken. The granulated powder particles themselves are not particularly dense. As the core of the ball, it is natural that there are more defects after sintering.

[0041] Comparative Example 2 uses a traditional droplet process to prepare silicon nitride microspheres, which differs from Example 1 in that... Traditional droplet-forming technology lacks a multi-channel, semi-open, arc-shaped, sloping bottom plate and a buffer layer, eliminating the need for a buffer liquid. After adjusting the slurry droplet height to a suitable level, the silicon nitride slurry droplet is directly dropped into the curing liquid, resulting in green silicon nitride spheres after curing. Other processes are the same as in Example 1, ultimately yielding 0.8 mm silicon nitride ceramic spheres. The SEM image of their inlaid cross-section is shown below. Figure 6As shown, ellipsoidal spheres are more common, while silicon nitride ceramic beads have poorer sphericity. Since the droplet needs to be supported by surface tension during free fall to form a sphere, theoretically, the smaller the droplet, the less the influence of gravity, the greater the surface tension, and the better the sphericity. In preparing 0.8mm silicon nitride ceramic spheres, the droplet particles before sphere formation are relatively large, making it difficult to achieve a perfectly round shape using only surface tension and brief free fall. Furthermore, when the droplet impacts the surface of the curing liquid at high speed, the reaction force of the curing liquid's surface tension inevitably causes slight deformation of the uncured droplet. Therefore, the resulting green silicon nitride spheres after curing will also exhibit elliptical characteristics.

[0042] It is evident that the silicon nitride microspheres prepared by the method and equipment of this invention have better sphericity, better polished surface, no pores in the cross-section, and faster molding, avoiding the influence of silicon nitride hydration.

[0043] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Other variations and modifications are possible without departing from the technical solutions described in the claims.

Claims

1. A method for preparing silicon nitride ceramic spheres by ramp rolling titration, characterized in that, It includes the following steps: S1. Component weighing: 100-110 parts by weight of silicon nitride powder, 50-55 parts by weight of pure water, 1.5-2.0 parts by weight of curing agent, and 0.5-1.0 parts by weight of thickener; S2. Ball milling and mixing: Place the weighed components into a polyurethane ball mill jar, and add silicon nitride grinding beads at the same time to ball mill and mix. S3. Particle separation: Pour the well-mixed slurry together with the silicon nitride grinding beads into a vacuum funnel, and suck out the slurry to separate it from the silicon nitride grinding beads. S4. Slurry filtration: The separated slurry is filtered using a vacuum filter bag to remove particulate impurities. S5. Slurry viscosity adjustment and vacuum degassing: Add a little pure water to the filtered slurry to adjust the viscosity to a suitable value, and then put the slurry into a stirring vacuum degassing machine to remove the air bubbles and obtain a bubble-free silicon nitride slurry. S6. Preparation of curing solution: Mix the coagulant and pure water in a certain proportion and stir until completely dissolved into a transparent solution to form the curing solution; S7. Preparation of non-polar buffer layer liquid: Prepare 8-10 parts by weight of non-polar liquid and 1-3 parts by weight of interfacial tension eliminator, and stir thoroughly to form a non-polar buffer layer liquid. S8. Mixing of non-polar buffer layer liquid and curing liquid: Add the curing liquid prepared in S6 to the storage tank. After the curing liquid surface is calm, slowly pour the non-polar buffer layer liquid prepared in S7 onto the curing liquid to form a buffer layer. The curing liquid layer and the buffer layer form two immiscible liquid layers. S9, Dropping with the dropper needle: The bubble-free silicon nitride slurry obtained in S5 is drawn into the dropper needle and dropped as a drop of silicon nitride slurry. S10. Forming of silicon nitride ceramic green spheres: A ramp is set at the junction of the curing liquid layer and the buffer layer. The ramp connects the buffer layer to the curing liquid layer. The position of the dripping needle is adjusted so that the dripping needle is aimed at the liquid above the buffer layer at the ramp. The silicon nitride slurry droplets enter the liquid of the buffer layer and roll down through the ramp. They first roll in the ramp section of the liquid of the buffer layer, and then quickly pass through the junction between the curing liquid layer and the buffer layer and enter the curing liquid to solidify. They continue to roll until they are completely solidified, forming silicon nitride ceramic green spheres. S11. Cleaning and drying: The silicon nitride ceramic green balls are cleaned and dried to obtain the final silicon nitride ceramic green balls. S12. Sintering: Silicon nitride ceramic green balls are sintered under nitrogen atmosphere and at atmospheric pressure according to a certain sintering regime to form silicon nitride ceramic balls. S13. Polishing: Polish the sintered silicon nitride ceramic balls with a vertical sand mill, add a certain proportion of polishing material, polish for a certain time, and then wash the polished silicon nitride ceramic balls with pure water and let them dry. S14. Sorting: The dried and polished silicon nitride ceramic balls are sorted by a sorting machine according to their particle size and sphericity. After sorting, silicon nitride ceramic balls with the final qualified particle size are obtained.

2. The method for preparing silicon nitride ceramic spheres by ramp rolling titration according to claim 1, characterized in that, In step S1, the curing agent is one or a mixture of sodium alginate, sodium citrate and low-fat pectin, and the thickener is one or a mixture of xanthan gum, guar gum and carrageenan.

3. The method for preparing silicon nitride ceramic spheres by ramp rolling titration according to claim 1, characterized in that, In step S6, the coagulant and pure water are mixed in a ratio of 1-2 parts by weight of coagulant and 8-10 parts by weight of pure water and stirred until completely dissolved into a transparent solution to form a solidified liquid.

4. The method for preparing silicon nitride ceramic spheres by ramp rolling titration according to claim 1, characterized in that, In step S6, the coagulant is one or a mixture of calcium sulfite, calcium sulfate, and calcium nitrate.

5. The method for preparing silicon nitride ceramic spheres by ramp rolling titration according to claim 1, characterized in that, In step S7, the non-polar liquid is one or a mixture of several of liquid paraffin, microcrystalline wax, and wax emulsion, and the surface tension eliminator is one or a mixture of several of tributyl phosphate, triethyl phosphate, and triisopropyl phosphate.

6. The method for preparing silicon nitride ceramic spheres by ramp rolling titration according to claim 1, characterized in that, In step S8, the liquid density of the buffer layer is less than the liquid density of the curing liquid layer.

7. The method for preparing silicon nitride ceramic spheres by ramp rolling titration according to claim 1, characterized in that, In step S12, the sintering conditions are as follows: 0-600℃, sintering rate is 5-7℃ / min, holding at 600℃ for 0.5-1.5h; 600-1100℃, sintering rate is 4-6℃ / min; 1100-1400℃, sintering rate is 3-5℃ / min; 1400-1450℃, sintering rate is 1.5-3℃ / min, holding at 1450℃ for 20-40min; 1450℃-1500℃, sintering rate is 2-3℃ / min, holding at 1500℃ for 20-40min. n; At 1500-1600℃, the sintering rate is 0.5-1.5℃ / min; hold at 1600℃ for 20-40 min; at 1600-1650℃, the sintering rate is 0.5-1℃ / min; hold at 1650℃ for 20-40 min; at 1650-1700℃, the sintering rate is 0.3-0.7℃ / min; hold at 1700℃ for 60 min; at 1700-1750℃, the sintering rate is 0.5℃ / min; hold at 1750-1750℃ for 90 min; then allow to cool naturally.

8. An apparatus for preparing silicon nitride ceramic spheres by a ramp rolling titration method, characterized in that, It includes a silicon nitride slurry storage tank, a storage tank, a cleaning device, a drying device, and a silicon nitride ceramic green ball collection tank. The storage tank is equipped with a non-polar buffer layer formed by liquid and a curing liquid formed by liquid. A ramp is provided between the buffer layer and the curing liquid layer, and the ramp connects the buffer layer to the curing liquid layer. The silicon nitride slurry storage tank is located above the ramp outside the storage tank. The silicon nitride slurry storage tank is connected to a dripping needle through a multi-channel peristaltic pump. The dripping needle is positioned below the ramp. A flexible conveyor belt is connected to the lower end of the ramp inside the storage tank. The flexible conveyor belt transports the product through the cleaning device and the drying device, until it reaches the silicon nitride ceramic green ball collection tank.

9. The apparatus for preparing silicon nitride ceramic spheres by ramp rolling titration according to claim 8, characterized in that, The ramp plate is a multi-channel semi-open arc-shaped ramp plate with adjustable slope. The surface of the ramp plate has a layer of weak polar film, which is made of silicone oil, perfluoropolymer or silane coupling agent.

10. The apparatus for preparing silicon nitride ceramic spheres by ramp rolling titration according to claim 8, characterized in that, The flexible conveyor belt has cylindrical grooves and holes throughout its center, and the flexible conveyor belt is made of porous nylon.

Images