Silicon nitride green body and method for producing the same

Abstract

A silicon nitride green body and its preparation method are disclosed, belonging to the field of silicon nitride green body technology. This invention overcomes the problems of poor sphericity and low density of silicon nitride green bodies after compression molding in existing spray granulation processes. The preparation method of the silicon nitride green body of this invention includes the following steps: mixing silicon nitride micron powder, sintering aid, dispersant, binder, and deionized water, followed by ball milling to obtain a slurry; the mass ratio of the sintering aid, dispersant, binder, and silicon nitride powder is (1×10⁻⁶). ‑4 ~0.2): (1×10 ‑4 ~0.25): (1×10 ‑4 The slurry has a solid content of 40-55 wt.%, and is spray-granulated to obtain granules. The granules are then pressed into shape to obtain green bodies. This invention, by employing spray granulation technology and combining the proportions of silicon nitride micron powder, sintering aid, dispersant, binder, and the solid content of the slurry, can produce green bodies with high density and good uniformity.

Description

Technical Field

[0001] This invention belongs to the field of silicon nitride green blank technology, specifically relating to a silicon nitride green blank and its preparation method. Background Technology

[0002] Silicon nitride ceramics possess high strength, high toughness, high thermal conductivity, and excellent resistance to oxidation, creep, and high resistivity, making them one of the most widely used structural ceramics. With the continuous development of powder metallurgy technology, silicon nitride, as a structural ceramic material with superior comprehensive performance, has received increasing attention.

[0003] Silicon nitride ceramics can be prepared using classic ceramic manufacturing processes, mainly including three steps: powder synthesis, green body forming, and high-temperature sintering. The flowability of the powder directly affects the density and uniformity of the pressed green body, thus influencing the ceramic properties. Micron-sized ceramic powders have small particle sizes and large specific surface areas, resulting in high frictional resistance between powder particles and poor flowability. During pressing, they cannot uniformly fill the mold, easily creating voids and leading to low green body density. Summary of the Invention

[0004] Therefore, the technical problem to be solved by the present invention is to overcome the problems of poor sphericity of granules prepared by the existing spray granulation process and low density of silicon nitride green blanks after pressing and molding, thereby providing a silicon nitride green blank and its preparation method.

[0005] To this end, the present invention provides the following technical solution.

[0006] In a first aspect, the present invention provides a method for preparing a silicon nitride green blank, comprising the following steps:

[0007] Silicon nitride micron powder, sintering aid, dispersant, binder, and deionized water are mixed and ball-milled to obtain a slurry; the mass ratio of the sintering aid, dispersant, binder, and silicon nitride powder is (1×10⁻⁶). -4 ~0.2): (1×10 -4 ~0.25): (1×10 -4 ~0.1):1, the solid content of the slurry is 40~55wt.%.

[0008] The slurry is spray-granulated to obtain granules;

[0009] The granules are pressed into shapes to obtain green bodies.

[0010] Furthermore, the silicon nitride micron powder includes coarse powder and fine powder.

[0011] This spray granulation process is highly automated and can be used for the continuous production of silicon nitride spherical powder. It has high production efficiency, and powders with poor formability can be recycled and regranulated.

[0012] Furthermore, it also includes machining the formed green blank to meet the requirements for dimensional accuracy.

[0013] Furthermore, the silicon nitride micron powder satisfies at least one of the following conditions:

[0014] (1) The coarse powder has a particle size of 1.5 to 2.0 μm, and the fine powder has a particle size of ≤1.0 μm;

[0015] (2) The coarse powder accounts for 30% to 65% of the total mass of silicon nitride micron powder;

[0016] (3) The silicon nitride micron powder is α-Si3N4.

[0017] Furthermore, the sintering aid includes one or more of yttrium oxide, lanthanum oxide, calcium oxide, silicon dioxide, or magnesium oxide;

[0018] Furthermore, the dispersant includes one or more of sodium carboxymethyl cellulose, polyacrylamide, Darvan CN, and sodium hexametaphosphate;

[0019] Furthermore, the adhesive includes one or more of polyvinyl alcohol, polyacrylic acid, or polyethylene glycol.

[0020] Furthermore, the process parameters for spray granulation are as follows: feed rate 1.6–4.5 L / h, hot air inlet temperature 260–350 °C, outlet temperature 110–160 °C, atomizer speed 10000–15000 rpm, and spray pressure 0.4–1.0 MPa.

[0021] Furthermore, the particle size of the granules is ≤60 mesh.

[0022] Furthermore, the step of spray granulation of the slurry to obtain granules includes: spray granulation of the slurry, collecting the granulated particles, and drying and classifying them to obtain granules.

[0023] The grading process involves passing the powder through a 60-mesh sieve. The powder remaining on the sieve is then mixed with deionized water and regranulated.

[0024] Furthermore, the drying conditions are as follows: drying in an oven at 110–150°C for 2–20 hours.

[0025] Furthermore, the pressing process includes dry pressing and / or cold isostatic pressing.

[0026] Secondly, the present invention provides a silicon nitride green blank prepared according to the method.

[0027] Preferably, silicon nitride micron powder and deionized water are first mixed and ball-milled; then sintering aid, dispersant and binder are added and ball-milled a second time.

[0028] Silicon nitride micron powder has a small particle size and a large specific surface area, which leads to agglomeration. Firstly, mixing the silicon nitride micron powder with deionized water and then performing a first ball milling can break up the agglomeration of the silicon nitride micron powder, which is beneficial for the uniformity of subsequent mixing.

[0029] The technical solution of this invention has the following advantages:

[0030] 1. The method for preparing silicon nitride green blanks of the present invention includes the following steps: mixing silicon nitride micron powder, sintering aid, dispersant, binder and deionized water and then ball milling to obtain a slurry; wherein the mass ratio of the sintering aid, dispersant, binder to silicon nitride powder is (1×10⁻⁶). -4 ~0.2): (1×10 -4 ~0.25): (1×10 -4 ~0.1):1, the solid content of the slurry is 40~55wt.%; the slurry is spray-granulated to obtain granules; the granules are pressed into shape to obtain green bodies.

[0031] This invention employs spray granulation technology, combined with precise proportions of silicon nitride micron powder, sintering aids, dispersants, binders, and slurry solids content. This avoids sedimentation, separation, and re-agglomeration of components in the slurry, maintaining the original uniformity of the slurry while obtaining spherical powder with uniform particle size distribution. This ensures the uniformity of the green body and improves the tap density of the granules. Furthermore, the resulting granules have high sphericity, are free of donut-shaped or apple-shaped particles, and exhibit strong flowability. This effectively solves the problems of difficult pressing of silicon nitride ceramic micron powder and uneven green bodies after molding. The granules obtained using this method can completely fill the mold, resulting in high-density green bodies after pressing, free of internal defects, and with high strength. High-density, highly uniform green bodies can be produced. This method can be used for green body molding of large-size, complex-morphology silicon nitride ceramics, producing crack-free, high-density green bodies.

[0032] The method of this invention is simple, low-cost, more suitable for practical application, and has industrial value. Furthermore, water-based spray granulation technology is environmentally friendly and highly safe.

[0033] 2. The silicon nitride micron powder of this invention includes coarse powder and fine powder. By increasing the particle size distribution of the silicon nitride powder, the contact area between powder particles is increased, reducing voids after pressing, further improving the green body density and uniformity. Using some coarse powder can also reduce costs. Attached Figure Description

[0034] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0035] Figure 1 Here is an SEM image of the granules obtained in Example 1;

[0036] Figure 2 This is an SEM image of the fracture surface of the silicon nitride green blank prepared in Example 1. Detailed Implementation

[0037] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0038] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0039] The polyacrylamide used in the examples was purchased from Shanghai Aladdin, with an average Mn of 150,000; polyvinyl alcohol was purchased from Shanghai Aladdin, with a degree of hydrolysis of 85.0-90.0 mol% and a viscosity of 20.0-30.0 mPa; and polyacrylic acid was purchased from Shanghai Aladdin, with an average Mv of ~450,000.

[0040] Example 1

[0041] This embodiment provides a method for preparing a silicon nitride green blank, including the following steps:

[0042] Take 8 kg of coarse α-Si3N4 powder with a particle size of 1.55 μm and 2 kg of fine α-Si3N4 powder with a particle size of 0.8 μm, add deionized water, and ball mill at 160 rpm for 1 hour for the first mixing step. Then add 5.5 wt.% yttrium oxide, 1.6 wt.% magnesium oxide, 0.5 wt.% polyacrylamide, and 5.3 wt.% polyvinyl alcohol (in this example, the mass content of yttrium oxide, magnesium oxide, polyacrylamide, and polyvinyl alcohol are based on the total mass of silicon nitride micron powder), and ball mill for 10 hours. At this time, the solid content of the slurry is 45 wt.%.

[0043] The above slurry was spray-granulated. The spray granulator had a feed rate of 3.5 L / h, a hot air inlet temperature of 320℃, an outlet temperature of 130℃, an atomizer speed of 12000 rpm, and a spray pressure of 0.7 MPa. The collected powder after granulation was dried in an oven at 120℃ for 10 hours, and then passed through a 60-mesh sieve. The powder remaining on the sieve was added to a solvent and re-granulated.

[0044] The powder that passes through the sieve is the obtained granules, such as... Figure 1 As shown, the granules were poured into a circular mold with a diameter of 200 mm and dry-pressed under a pressure of 100 MPa, followed by cold isostatic pressing under a pressure of 200 MPa. The resulting green body showed no cracking and had a density as high as 1.83 g / cm³. 3 SEM images of the fracture surface of the green billet are shown below. Figure 2 As shown.

[0045] Example 2

[0046] This embodiment provides a method for preparing a silicon nitride green blank, including the following steps:

[0047] Take 7.5 kg of coarse α-Si3N4 powder with a particle size of 1.8 μm and 2.5 kg of fine α-Si3N4 powder with a particle size of 1.0 μm, add deionized water, and ball mill at 160 rpm for 1 hour for the first mixing step. Then add 5.5 wt.% yttrium oxide, 3.2 wt.% alumina, 1 wt.% sodium hexametaphosphate, and 5.0 wt.% polyacrylic acid (in this example, the mass content of yttrium oxide, alumina, sodium hexametaphosphate, and polyacrylic acid are based on the total mass of silicon nitride micron powder), and ball mill for 10 hours. At this time, the solid content of the slurry is 48 wt.%.

[0048] The above slurry was spray-granulated. The spray granulator had a feed rate of 3.5 L / h, a hot air inlet temperature of 330℃, an outlet temperature of 130℃, an atomizer speed of 14000 rpm, and a spray pressure of 0.6 MPa. The collected powder after granulation was dried in an oven at 120℃ for 10 hours, and then passed through a 60-mesh sieve. The powder remaining on the sieve was added to a solvent and re-granulated.

[0049] The powder that passes through the sieve is the obtained granules. These granules are poured into a 200mm diameter circular mold and dry-pressed at 80MPa. Subsequently, they undergo cold isostatic pressing at 200MPa. The resulting green body shows no cracking and has a density as high as 1.85g / cm³. 3 .

[0050] Example 3

[0051] This embodiment provides a method for preparing a high-performance silicon nitride green compact, including the following steps:

[0052] Take 7.0 kg of coarse α-Si3N4 powder with a particle size of 1.5 μm and 3.0 kg of fine α-Si3N4 powder with a particle size of 1.0 μm, add deionized water, and ball mill at 160 rpm for 1 hour for the first mixing step. Then add 5.5 wt.% yttrium oxide, 1.5 wt.% calcium oxide, 0.8 wt.% silica, 0.4 wt.% Darvan C-N, and 5.5 wt.% polyacrylic acid (in this example, the mass content of yttrium oxide, calcium oxide, silica, and Darvan C-N are based on the total mass of silicon nitride micron powder), and ball mill for 10 hours. At this time, the solid content of the slurry is 53 wt.%.

[0053] The above slurry was spray-granulated. The spray granulator had a feed rate of 3.2 L / h, a hot air inlet temperature of 360℃, an outlet temperature of 1430℃, an atomizer speed of 14000 rpm, and a spray pressure of 0.8 MPa. The collected powder after granulation was dried in an oven at 120℃ for 10 hours, and then passed through a 60-mesh sieve. The powder remaining on the sieve was added to a solvent and re-granulated.

[0054] The powder that passes through the sieve is the obtained granules. These granules are poured into a 200mm diameter circular mold and dry-pressed at 80MPa. Subsequently, they undergo cold isostatic pressing at 200MPa. The resulting green body shows no cracking and has a density as high as 1.96g / cm³. 3 .

[0055] Example 4

[0056] This embodiment is basically the same as Embodiment 1, except that only 10 kg of α-Si3N4 coarse powder with a particle size of 1.55 μm is used. Under the same granulation process, spherical granules can still be obtained, and green bodies without internal defects can be obtained after molding, but the density is reduced to 1.75 g / cm³. 3 .

[0057] Comparative Example 1

[0058] This comparative example is basically the same as Example 1, except that the binder is 15 wt.% polyvinyl alcohol. Under the same granulation process, the resulting granules exhibit a large number of donut-shaped morphologies, and the green density after molding is reduced to 1.54 g / cm³. 3 .

[0059] Comparative Example 2

[0060] This comparative example is basically the same as Example 1, except that the slurry solid content is 58 wt.%. Under the same granulation process, the granulated powder obtained has a large difference in particle size, with more hollow particles appearing. At this time, the fluidity of the granulated powder is reduced, the filling ability of the mold during the pressing process decreases, the green body after molding is uneven, and local cracks appear. At this time, the density of the green body is reduced to 1.48 g / cm³.3 .

[0061] Test case

[0062] The pore size distribution of the green bodies prepared in the examples and comparative examples was tested according to GB / T 21650.1-2008 Part 1: Mercury intrusion porosimetry.

[0063] The test results are shown in Table 1.

[0064] Table 1 Green body parameters

[0065]

[0066]

[0067] As can be seen from the comparison between Examples 1-4 and Comparative Examples 1-2, the density of the green body prepared by the present invention is significantly improved. The green body of the present invention has a lower nanoscale micropore size and no micrometer-scale micropores, resulting in significantly improved uniformity.

[0068] A comparison of Examples 1 and 4 shows that when silicon nitride micron powder includes both coarse and fine powder, the density of the green body can be further increased.

[0069] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a silicon nitride green compact, characterized in that, Includes the following steps: Silicon nitride micron powder, sintering aid, dispersant, binder, and deionized water are mixed and ball-milled to obtain a slurry; the mass ratio of the sintering aid, dispersant, binder, and silicon nitride powder is (1×10⁻⁶). -4 ~0.2): (1×10 -4 ~0.25): (1×10 -4 ~0.1): 1, the solid content of the slurry is 40~55 wt.%; The silicon nitride micron powder includes coarse powder and fine powder; The coarse powder has a particle size of 1.5~2.0 μm, and the fine powder has a particle size ≤1.0 μm; The coarse powder accounts for 30% to 65% of the total mass of the silicon nitride micron powder; The adhesive is one of polyvinyl alcohol, polyacrylic acid, or polyethylene glycol; The slurry is spray-granulated to obtain granules; The granules are pressed into shapes to obtain green bodies.

2. The method for preparing silicon nitride green blank according to claim 1, characterized in that, The silicon nitride micron powder is α-Si3N4.

3. The method for preparing silicon nitride green blank according to claim 1 or 2, characterized in that, At least one of the following conditions must be met: (1) The sintering aids include one or more of yttrium oxide, lanthanum oxide, calcium oxide, silicon dioxide, or magnesium oxide; (2) The dispersant includes one or more of sodium hydroxymethyl cellulose, polyacrylamide, Darvan CN, and sodium hexametaphosphate.

4. The method for preparing a silicon nitride green blank according to any one of claims 1 or 2, characterized in that, The process parameters for spray granulation are as follows: feed rate 1.6~4.5 L / h, hot air inlet temperature 260~350℃, outlet temperature 110~160℃, atomizer speed 10000~15000 rpm, and spray pressure 0.4~1.0 MPa.

5. The method for preparing a silicon nitride green blank according to any one of claims 1 or 2, characterized in that, The particle size of the granules is ≤60 mesh.

6. The method for preparing a silicon nitride green blank according to any one of claims 1 or 2, characterized in that, The step of spray granulation of the slurry to obtain granules includes: spray granulation of the slurry, collecting the granulated particles, drying and classifying them to obtain granules.

7. The method for preparing a silicon nitride green blank according to claim 6, characterized in that, The drying conditions are to dry in an oven at 110~150℃ for 2~20 hours.

8. The method for preparing a silicon nitride green blank according to any one of claims 1 or 2, characterized in that, The compression molding includes dry pressing and / or cold isostatic pressing.

9. A silicon nitride green blank prepared by the method according to any one of claims 1-8.

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