Preparation method of stable spray granulation silicon nitride slurry and granulation powder

Abstract

The invention discloses a preparation method of stable spray granulation silicon nitride slurry and granulation powder, and belongs to the technical field of ceramic materials. According to the invention, absolute ethyl alcohol is adopted as a solvent, so that the problem that silicon nitride slurry is easy to hydrolyze is solved, and the temperature during spray granulation is reduced. On the basis that a dispersing agent is added into slurry, tetramethylammonium hydroxide is added into the slurry, the Zeta potential value of the slurry is increased, the slurry is made to be in an alkaline environment, dissociation of Si-OH groups is promoted, more negative charges are carried on the surface of silicon nitride, meanwhile, zwitterions released by the tetramethylammonium hydroxide form a double-electron layer around silicon nitride particles, and therefore the conductivity of the silicon nitride particles is improved. The dispersity of the particles in the slurry is improved. The silicon nitride powder is more uniformly distributed in a medium such as a binder due to the improvement of the dispersibility, and the silicon nitride slurry with low viscosity, good dispersibility, good stability and good fluidity can be prepared. The slurry can be more symmetrically gathered and formed in the spray granulation process, and the sphericity degree and apparent density of granulation powder are improved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of ceramic materials, and particularly relates to a preparation method of stable spray granulation silicon nitride slurry and granulation powder. BACKGROUND

[0002] Silicon nitride ceramics have excellent comprehensive performances such as high strength, high modulus, high hardness, high temperature resistance, corrosion resistance, oxidation resistance, thermal shock resistance and wear resistance, and structural stability, and have a prominent position in nitride ceramics, so that the silicon nitride ceramics are widely applied to high-speed, high-precision and long-life bearings in the fields of machine tool spindles, electric spindles, automobile engines, aerospace engines and wind power generation, and the prepared silicon nitride ceramic balls have the characteristics of light weight, long service life, low friction loss, self-lubrication and strong operation ability under dry friction conditions.

[0003] In the preparation process of the silicon nitride ceramic ball, the uniformity of the powder mixture has a great influence on the strength of the green body. The traditional mixing method includes mixing-drying-granulation-screening. Because the drying time is too long and the density difference between the components is large, the components in the original uniform dispersion state re-agglomerate and precipitate, and the manual granulation introduces pollutants, finally resulting in irregular particle shape, poor powder flowability, uneven particle size distribution, low bulk density of the powder, and reduced performance of the product.

[0004] The method for solving the above problems adopts a spray granulation process, uniformly ball-mills the components into a slurry, sprays the slurry into a granulation tower by using a high-pressure sprayer for atomization to form a granulation powder. The flowability and bulk density are improved by controlling the stability and viscosity of the slurry, the sphericity and particle size of the granulation powder, so that the powder can fill the mold under the same pressure, the density after sintering is effectively increased, and the porosity is reduced. At present, the water-based solvent is mostly used for spray granulation of the silicon nitride slurry. For example, a patent CN 113999019 A discloses a preparation method of silicon nitride ceramic granulation powder. Water is used as the solvent, the inlet temperature is 300-340 DEG C, sodium hexametaphosphate is used as the dispersant, and a binary water-based binder polyvinyl alcohol and polyethylene glycol are used to prepare the granulation powder. There are larger holes in the prepared granulation powder. Using water as the solvent, the silicon nitride is hydrolyzed to increase the oxygen content of the powder, and it is difficult to prepare a slurry with low viscosity by using water as the dispersion medium. When spray granulation is performed, a higher temperature is required, the drying time is long, the organic matter in the slurry may volatilize and cannot be well retained, the sphericity is poor at high temperature, and the subsequent forming and sintering mechanical properties and thermal conductivity are not good. SUMMARY

[0005] The application adopts anhydrous ethanol as a solvent to solve the problem of easy hydrolysis of silicon nitride slurry, and the temperature can be reduced during spray granulation by using anhydrous ethanol as a solvent. By adding a dispersant in the slurry and adding tetramethylammonium hydroxide in the slurry, the Zeta potential value of the slurry is improved, the slurry is in an alkaline environment, the dissociation of Si-OH groups is promoted, the surface of silicon nitride has more negative charges, and the anions and cations released by tetramethylammonium hydroxide form a double electron layer around the silicon nitride particles, improving the dispersibility of the particles in the slurry. The improvement of dispersibility makes the silicon nitride powder more uniformly distributed in the medium such as the binder, and a silicon nitride slurry with good low viscosity, dispersibility, stability and fluidity can be prepared. The slurry can be more symmetrically aggregated and formed during spray granulation, and the sphericity and bulk density of the granulated powder are improved.

[0006] The application discloses a preparation method of stable spray granulated silicon nitride slurry and granulated powder, and the steps are as follows: S1. A certain proportion of silicon nitride powder, sintering aid, dispersant and anhydrous ethanol are weighed and placed in a polyurethane ball mill jar, and medium balls are used for ball milling for 12-24 hours to form a first slurry, and the solid content of the slurry is 45%-55%; S2. Tetramethylammonium hydroxide is weighed and added into the first slurry while stirring until the absolute value of the Zeta potential of the slurry is stabilized at more than 40 mV, and then the ball milling is continued for 6-12 hours to form a second slurry; S3. A binder is weighed and added into the second slurry, and the ball milling is continued for 12-24 hours to form an anhydrous ethanol-based silicon nitride slurry; S4. The mixed anhydrous ethanol-based silicon nitride slurry is subjected to spray drying by using a centrifugal spray granulation tower, and the granulated powder is formed after sieving.

[0007] Further, the medium balls in step S1 are zirconium oxide balls, aluminum oxide balls, silicon nitride balls or silicon carbide balls.

[0008] Further, the purity of the silicon nitride powder in step S1 is more than 99%, and the particle size is 0.2-3 μm.

[0009] Further, the sintering aid in step S1 comprises two or more than two of Al2O3, MgO, Y2O3, MgSiN2, YH3 and YF3, and the purity of each component is more than 99%, and the particle size is 0.2-3 μm. Further, the dispersant in step S1 is PVP.

[0010] Further, the binder in step S3 is one of PEG, PVB and phenolic resin, the concentration of the sol is 5%-30%, and the viscosity of the slurry is 200-600 mPa·s.

[0011] Further, the hot air inlet temperature of the spray dryer in step S4 is 140-180℃, the outlet temperature is 60-90℃, the rotation speed of the atomizer is 5000-9000r / min, the feeding speed is 0.5-2.5L / min, and the pressure of the spray granulation pump is 750-950MPa. BRIEF DESCRIPTION OF DRAWINGS

[0012] Figure 1 The process flow chart of the application. DETAILED DESCRIPTION

[0013] The following examples are provided to better further understand the present application, and are not limited to the best mode, and do not constitute a limitation on the content and protection scope of the present application. Any person under the enlightenment of the present application or the combination of the present application with other prior art features can obtain any product same or similar to the present application, which falls within the protection scope of the present application.

[0014] Take 90g of silicon nitride powder, 3g of aluminum oxide, 7g of yttrium oxide, 1.2g of dispersant PVP, and 90g of anhydrous ethanol, and place them in a polyurethane ball mill tank. The medium ball is a silicon nitride ceramic ball, and the mass ratio of the mixed powder to the silicon nitride ceramic ball is 1:3. After ball milling for 12 hours (the rotation speed of the ball mill is 100-250r / min), a first slurry is prepared, and the solid content of the slurry is 53%. Take 0.2g of tetramethylammonium hydroxide, and add it to the first slurry in batches while stirring. It is measured that the absolute value of the Zeta potential of the slurry is stabilized at 43mV. Continue to ball mill for 6 hours (the rotation speed of the ball mill is 100-250r / min) to form a second slurry. Take 10g of a certain concentration of phenolic resin sol, and drop it into the second slurry. Continue to ball mill for 12 hours (the rotation speed of the ball mill is 100-250r / min) to prepare a silicon nitride slurry. It is measured by a viscometer that the viscosity of the slurry is 360mPa·s. After a period of standing, a 2mm thick layering phenomenon appears on the upper layer of the slurry. A centrifugal spray granulation tower is used to spray dry the mixed slurry. The hot air inlet temperature of the spray dryer is 140℃, the outlet temperature is 80℃, the rotation speed of the atomizer is 5000r / min, the feeding speed is 1.5L / min, and the pressure of the spray granulation pump is 8000MPa. After drying, the granulation powder is sieved through a screen. EXAMPLE

[0015] Take 90 g of silicon nitride powder, 3 g of aluminum oxide, 7 g of yttrium oxide, 1.2 g of dispersant PVP, 90 g of anhydrous ethanol, and put them into a polyurethane ball mill tank. The medium ball is a silicon nitride ceramic ball, and the mass ratio of the mixed powder to the silicon nitride ceramic ball is 1:3. After ball milling for 12 hours (the speed of the ball mill is 100-250 r / min), the first slurry is prepared. The solid content of the slurry is 53%. Take 0.8 g of tetramethylammonium hydroxide, and add it to the first slurry in batches while stirring. The absolute value of the Zeta potential of the slurry is measured to be stable at 52 mV. Continue to ball mill for 6 hours (the speed of the ball mill is 100-250 r / min) to form the second slurry. Take 10 g of a certain concentration of phenolic resin sol, and drop it into the second slurry. Continue to ball mill for 12 hours (the speed of the ball mill is 100-250 r / min) to prepare a silicon nitride slurry. The viscosity of the slurry is measured to be 270 mPa·s using a viscometer. The upper layer of the slurry does not appear to be stratified after a period of standing. A centrifugal spray granulation tower is used to spray dry the mixed slurry. The hot air inlet temperature of the spray dryer is 140°C, the outlet temperature is 80°C, the speed of the atomizer is 5000 r / min, the feeding speed is 1.5 L / min, and the pressure of the spray granulation pump is 8000 MPa. After drying, the granulation powder is sieved through a screen to form the granulation powder.

[0016] Comparative Example 1 Take 90 g of silicon nitride powder, 3 g of aluminum oxide, 7 g of yttrium oxide, 1.2 g of dispersant PVP, 90 g of anhydrous ethanol, and put them into a polyurethane ball mill tank. The medium ball is a silicon nitride ceramic ball, and the mass ratio of the mixed powder to the silicon nitride ceramic ball is 1:3. After ball milling for 12 hours (the speed of the ball mill is 100-250 r / min), the first slurry is prepared. The solid content of the slurry is 53%. Take 1.2 g of tetramethylammonium hydroxide, and add it to the first slurry in batches while stirring. The absolute value of the Zeta potential of the slurry is measured to be stable at 45 mV. Continue to ball mill for 6 hours (the speed of the ball mill is 100-250 r / min) to form the second slurry. Take 10 g of a certain concentration of phenolic resin sol, and drop it into the second slurry. Continue to ball mill for 12 hours (the speed of the ball mill is 100-250 r / min) to prepare a silicon nitride slurry. The viscosity of the slurry is measured to be 290 mPa·s using a viscometer. The upper layer of the slurry again appears to be stratified by about 1.6 mm after a period of standing. A centrifugal spray granulation tower is used to spray dry the mixed slurry. The hot air inlet temperature of the spray dryer is 140°C, the outlet temperature is 80°C, the speed of the atomizer is 5000 r / min, the feeding speed is 1.5 L / min, and the pressure of the spray granulation pump is 8000 MPa. After drying, the granulation powder is sieved through a screen to form the granulation powder.

[0017] Comparative Example 2 Take 90 g of silicon nitride powder, 3 g of aluminum oxide, 7 g of yttrium oxide, 1.2 g of dispersant PVP, 90 g of anhydrous ethanol, and place them in a polyurethane ball mill tank. The medium ball is a silicon nitride ceramic ball, and the mass ratio of the mixed powder to the silicon nitride ceramic ball is 1:3. After ball milling for 12 hours (the speed of the ball mill is 100-250 r / min), a first slurry is prepared, and the solid content of the slurry is 53%. Continue ball milling for 6 hours (the speed of the ball mill is 100-250 r / min) to form a second slurry, and measure the absolute value of the Zeta potential of the slurry to be 12 mV. Take 10 g of a certain concentration of phenolic resin sol and add it dropwise to the second slurry. Continue ball milling for 12 hours (the speed of the ball mill is 100-250 r / min) to prepare a silicon nitride slurry. Use a viscometer to measure the viscosity of the slurry to be 570 mPa·s. After a period of standing, a 3.4 mm layering phenomenon appears on the upper layer of the slurry. Use a centrifugal spray granulation tower to spray dry the mixed slurry. The hot air inlet temperature of the spray dryer is 140°C, the outlet temperature is 80°C, the speed of the atomizer is 5000 r / min, the feeding speed is 1.5 L / min, and the pressure of the spray granulation pump is 8000 MPa. After drying, sieve the granulation powder through a screen.

[0018] Table 1 Performance parameters of silicon nitride slurry, morphology of granulation powder, and bulk density in examples and comparative examples Examples Zeta potential of slurry absolute value mV Slurry viscosity mPa-s Granulated powder morphology Loose bulk density g / cm 3 ]]> Example 1 43 360 Predominantly spheroidal 0.932 Example 2 52 274 Spheroidal 0.954 Comparative Example 1 45 312 Predominantly apple-shaped 0.915 Comparative Example 2 12 575 Apple-shaped 0.832 As can be seen from Table 1, as the amount of tetramethylammonium hydroxide added to the slurry increases, the absolute value of the Zeta potential of the slurry increases, which promotes the uniformity and stability of the slurry. At the same time, the viscosity of the slurry decreases, the granulation powder formed after spray granulation has a high degree of sphericity, and the bulk density of the powder is large. However, excessive tetramethylammonium hydroxide can reduce the absolute value of the Zeta potential of the slurry and increase the viscosity of the slurry. The reason is that the excess tetramethylammonium hydroxide organic ions act as free ions, increasing the ion concentration in the solution, compressing the double electron layer, and weakening the electrostatic repulsion between silicon nitride particles. In addition, the excess tetramethylammonium hydroxide molecules form a network structure that greatly restricts the movement of silicon nitride particles, causing the silicon nitride slurry to flocculate and resulting in an increase in viscosity. In Comparative Example 2, the influence of the PVP dispersant on the absolute value of the Zeta potential of the slurry is small, and it is not possible to form a low-viscosity, high-stability slurry. The granulation powder formed is apple-shaped with large holes in the middle.

Claims

1. A method for preparing a stable spray-granulated silicon nitride slurry and granulated powder, characterized in that: the steps include: S1. Weigh out silicon nitride powder, sintering aid, dispersant and anhydrous ethanol in a certain proportion and place them in a polyurethane ball mill jar. Use media ball milling for 12-24 hours to form the first slurry. The solid content of the slurry is 45%-55%. S2. Weigh out tetramethylammonium hydroxide and add it to the first slurry while stirring until the absolute value of the zeta potential of the slurry stabilizes above 40mV. Continue ball milling for 6-12 hours to form the second slurry. S3. Weigh the binder, add it to the second slurry, and continue ball milling for 12-24 hours to form anhydrous ethanol-based silicon nitride slurry. S4. The mixed anhydrous ethanol-based silicon nitride slurry is spray-dried using a centrifugal spray granulation tower and then granulated after being screened and dried.

2. The method for preparing a stable spray-granulated silicon nitride slurry and granulated powder according to claim 1, characterized in that: The dielectric spheres mentioned in step S1 are zirconium oxide spheres, aluminum oxide spheres, silicon nitride spheres, or silicon carbide spheres.

3. The method for preparing a stable spray-granulated silicon nitride slurry and granulated powder according to claim 1, characterized in that: The silicon nitride powder mentioned in step S1 has a purity of over 99% and a particle size of 0.2-3 μm.

4. The method for preparing a stable spray-granulated silicon nitride slurry and granulated powder according to claim 1, characterized in that: The sintering aid mentioned in step S1 comprises two or more of Al2O3, MgO, Y2O3, MgSiN2, YH3, and YF3; the purity of each component is above 99%, and the particle size is 0.2-3μm.

5. The method for preparing a stable spray-granulated silicon nitride slurry and granulated powder according to claim 1, characterized in that: The dispersant in step S1 is PVP.

6. The method for preparing a stable spray-granulated silicon nitride slurry and granulated powder according to claim 1, characterized in that: The binder mentioned in step S3 is one of PEG, PVB, or phenolic resin; the concentration of the sol is 5%-30%; and the viscosity of the slurry is 200-600 mPa·s.

7. The method for preparing a stable spray-granulated silicon nitride slurry and granulated powder according to claim 1, characterized in that: In step S4, the hot air inlet temperature of the spray dryer is 140-180℃, the outlet temperature is 60-90℃, the atomizer speed is 5000-9000 r / min, the feeding rate is 0.5-2.5 L / min, and the spray granulation pump pressure is 750-950 MPa.

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