Method for preparing SiC through silicon nitride assisted sintering

Abstract

The invention relates to a method for sintering high-purity silicon carbide with the assistance of silicon nitride, and belongs to the technical field of high-purity silicon carbide ceramic preparation. The preparation method comprises the following steps: (1) preparing silicon carbide powder and oxygen-containing silicon nitride according to a mass ratio of silicon carbide to oxygen-containing silicon nitride of 100: (5-15), and uniformly mixing to obtain mixed powder; the purity of the silicon carbide powder is larger than or equal to 99.99%, the silicon carbide powder is composed of alpha-SiC powder and beta-SiC powder according to the mass ratio A: (100-A), and the value range of A is 40-30; the mass percentage of oxygen in the oxygen-containing silicon nitride is 1-5%; (2) carrying out compression molding on the obtained mixed powder to obtain a molded body; and (3) the formed body is placed in a sintering furnace and sintered under the protective atmosphere or vacuum condition, the sintering temperature ranges from 2000 DEG C to 2200 DEG C, and the sintering time is larger than or equal to 2 h. The process is simple and controllable, the obtained product is excellent in performance, and industrial application is facilitated.

Description

TECHNICAL FIELD

[0001] The application relates to a method for sintering high-purity silicon carbide with the assistance of silicon nitride, and belongs to the technical field of high-purity silicon carbide ceramic preparation. BACKGROUND

[0002] Silicon carbide (SiC) is an important ceramic material, which has excellent high-temperature performance, hardness, chemical stability and thermal conductivity, and is widely used in industrial fields with strict requirements such as high temperature, wear resistance and oxidation resistance. The sintering of high-purity silicon carbide usually requires high temperature and high pressure, and there are problems such as relatively low sintering density and relatively poor mechanical properties in the sintering process. Therefore, how to improve the sintering density and mechanical properties of silicon carbide materials while maintaining high purity is a major problem in current silicon carbide research and industrial production.

[0003] At present, the existing preparation methods of high-purity silicon carbide ceramics mainly include recrystallization sintering, hot-pressing sintering and spark plasma sintering. Recrystallized silicon carbide (RSiC) is sintered at a high temperature of 2100-2500 DEG C, has excellent purity (>99.9%), but due to its high porosity, the density is low (about 2.5 g / cm 3 ), and the bending strength is only 55-160 MPa. Although hot-pressing sintering and spark plasma sintering can prepare high-density (about 3.15 g / cm 3 ) silicon carbide ceramics, they need to be carried out in a high-temperature and high-pressure environment, which has a very high requirement on the equipment and is not suitable for industrial production.

[0004] Other silicon carbide preparation methods, such as pressureless sintering, have good mechanical properties and temperature resistance, but they all add a proper amount of sintering aids (commonly used B4C, etc.), so that the finished silicon carbide ceramic device usually contains about 0.3%-0.5% B, and the purity cannot meet the use requirements in the semiconductor field. The finished product prepared by the reaction sintering process usually contains 10-15% free Si, so that it can generally only withstand a temperature below 1400 DEG C, and the free Si is easily corroded, such as patent application CN120157490A. SUMMARY

[0005] The present application attempts to use a proper amount of pre-oxidized silicon nitride to assist sintering to prepare high-purity and high-strength silicon carbide for the first time. The present application obtains a proper amount of active oxides (such as SiO2) on the surface of silicon nitride by pre-oxidizing a proper amount of silicon nitride, uses these active oxides to promote the sintering reaction of silicon carbide in the sintering process, significantly improves the sintering density, mechanical properties and thermal stability of silicon carbide, and for the first time obtains high-silicon carbide content and high-temperature-resistant products with a temperature greater than or equal to 1600 DEG C.

[0006] The application discloses a method for preparing SiC by using silicon nitride as an auxiliary sintering agent.

[0007] Step one

[0008] The mass ratio of silicon carbide to oxygen-containing silicon nitride is 100:5-15, silicon carbide powder and oxygen-containing silicon nitride are prepared and mixed uniformly to obtain a mixed powder; the purity of the silicon carbide powder is greater than or equal to 99.99%, and the silicon carbide powder is composed of alpha-SiC powder and beta-SiC powder at a mass ratio of A:(100-A), wherein the value range of A is 40-30; the mass percentage of oxygen in the oxygen-containing silicon nitride is 1-5%.

[0009] Step two

[0010] The mixed powder obtained in step one is subjected to compression molding to obtain a molded body.

[0011] Step three

[0012] The molded body is placed in a sintering furnace and sintered under a protective atmosphere or vacuum condition, the sintering temperature is 2000-2200 DEG C, and the sintering time is greater than or equal to 2h.

[0013] As preferred, the D50 particle size of the alpha-SiC powder is 150-200 mu m.

[0014] As preferred, the beta-SiC powder has two kinds, and the purity of each kind is greater than or equal to 99.999%, wherein the D50 particle size of fine-grained beta-SiC powder is 25-50 mu m, and the D50 of coarse-grained beta-SiC powder is 60-100 mu m. As further preferred, the mass ratio of the fine-grained beta-SiC powder to the coarse-grained beta-SiC powder is 20-15:40-55. In the application, the D50 particle size of the alpha-SiC powder is controlled to be 150-200 mu m, the D50 particle size of the fine-grained beta-SiC powder is controlled to be 25-50 mu m, the D50 of the coarse-grained beta-SiC powder is controlled to be 60-100 mu m, and the mass ratio of the fine-grained beta-SiC powder to the coarse-grained beta-SiC powder is controlled to be 20-15:40-55, which can optimize the particle distribution and uniformity in the sintering process, reduce the inter-particle voids, improve the sintering density, and achieve higher sintering density and mechanical properties, and meanwhile, the toughness and crack propagation resistance of the ceramic material are improved. The reason is that the fine-grained beta-SiC powder can promote the refinement of the crystal grains and improve the initial sintering rate, and the coarse-grained beta-SiC powder can promote the uniform growth of the crystal grains, so that a more uniform microstructure is obtained, the generation of cracks is avoided, and the mechanical properties and thermal stability of the material are enhanced.

[0015] As a further preferred solution, the SiC powder is composed of α-SiC, fine-grained β-SiC powder and coarse-grained β-SiC powder in a mass ratio of 40-30:20-15:40-55.

[0016] In the present application, the particle size of the silicon oxynitride powder is 15-35 microns. The present application controls the mass percentage of oxygen in the silicon oxynitride to be 1-5% because an appropriate amount of oxide can effectively promote the sintering process, and by forming an oxide (SiO x ) liquid phase or low-melting point phase at high temperatures, filling the gaps between silicon carbide particles, thereby improving the sintering density and mechanical properties of the material. If the oxygen content is too high, it will result in an excessively thick oxide layer, generating too much liquid phase or low-melting point phase, which may in turn lead to a decrease in the strength of the ceramic, and even the formation of a brittle phase, reducing the toughness and high-temperature resistance of the material. If the oxygen content is too low, it will result in an insufficient sintering process, failing to form sufficient liquid phase to promote the contact and growth of the grains, leading to low sintering density and unsatisfactory material density and mechanical properties.

[0017] In industrial applications, the silicon oxynitride powder is prepared by the following process:

[0018] The silicon nitride (Si3N4) powder with a purity of greater than or equal to 99.9% is used as the treatment object, and a certain thickness of an oxide layer is formed on its surface through pre-oxidation treatment. The pre-oxidation treatment can be performed by heating the above-mentioned silicon nitride powder to a temperature range of 800-1200°C in an air or oxygen atmosphere for 5-30 min, so as to oxidize the surface of the silicon nitride and generate an active oxide layer. The thickness and distribution of the oxide layer can be adjusted according to actual requirements to ensure that it can effectively promote the sintering process.

[0019] As a preferred solution, in step one, the silicon carbide: silicon oxynitride is 100:5-10 by mass ratio, and the silicon carbide powder and the silicon oxynitride are weighed and mixed uniformly to obtain a mixed powder. In the present application, the silicon carbide: silicon oxynitride is controlled to be 100:5-15 by mass ratio, preferably 100:5-10, because the oxide (such as SiO x ) in the silicon oxynitride can generate a liquid phase during high-temperature sintering, promoting the contact and grain growth of the silicon carbide particles, thereby improving the sintering density and mechanical properties, and controlling the proportion of the silicon oxynitride can avoid the negative effects of excessive oxide. If the amount of the silicon oxynitride is too high, it will result in excessive oxide, which in turn will lead to excessive liquid phase formation, reducing the strength and toughness of the material, and excessive oxygen elements will affect the purity of the final silicon carbide ceramic. If the amount is too low, it will result in insufficient oxide, failing to effectively promote the generation of liquid phase during the sintering process, and the density and mechanical properties of the material will be affected.

[0020] In the present application, each raw material can be mixed sufficiently by methods such as ball milling or planetary milling to ensure uniform distribution of each raw material. The amount of silicon nitride added can be adjusted within a controlled range according to specific needs to ensure that it plays an auxiliary role.

[0021] In the present application, the uniformly mixed powder is subjected to compression molding, and a conventional cold pressing method is used to obtain a molded body having a certain shape and size. When cold pressing is used, the pressing pressure is 30-50 MPa, and the pressure holding time is 5-15 minutes.

[0022] In the present application, the sintering time can be 4-10 hours, and the atmosphere can be argon, nitrogen, or vacuum. During sintering, silicon nitride reacts with silicon carbide to form a composite phase of silicon nitride and silicon carbide, promoting the growth of silicon carbide grains and thus increasing the sintering density.

[0023] In actual applications, after sintering, appropriate cooling and post-processing (such as grinding, polishing, etc.) are performed to obtain the final high-purity silicon carbide material.

[0024] The present application obtains high-quality products with a heat resistance temperature greater than or equal to 1600℃.

[0025] Principles and advantages

[0026] Principles of the present application:

[0027] The principle of the present application is to pre-oxidize the silicon nitride (Si3N4) powder to form a certain thickness of silicon oxide (SiOx) layer on its surface to promote the sintering process of high-purity silicon carbide (SiC). During sintering, the oxygen-rich layer on the surface of silicon nitride reacts with silicon carbide to form a liquid phase or a low-melting-point phase, reducing the sintering temperature and promoting the contact between silicon carbide particles and the growth of grains, thereby increasing the sintering density. In addition, the oxide (such as SiOx) can fill the pores between silicon carbide particles at high temperatures, reducing the porosity and improving the density and mechanical properties of the material. The surface oxide of silicon nitride can also effectively improve the thermal stability and oxidation resistance of high-purity silicon carbide material, ultimately making the sintered high-purity silicon carbide material exhibit better mechanical properties, thermal stability, and higher sintering density in a high-temperature environment.

[0028] Advantages:

[0029] 1. Increase sintering density: The oxide (such as SiOx) rich on the surface of silicon nitride can form a liquid phase during sintering, which fills the pores between silicon carbide particles and significantly increases the sintering density.

[0030] 2. Improve mechanical properties: The appropriate amount of silicon oxide can promote grain growth and reduce the voids at the grain boundaries, and the sintered high-purity silicon carbide material exhibits more excellent mechanical properties in terms of bending strength, hardness, etc.

[0031] 3. Improved thermal stability: The addition of an appropriate amount of silicon nitride improves the thermal stability and oxidation resistance of high-purity silicon carbide in high-temperature environments, extending its service life. The resulting product can be used in environments with temperatures up to 1600°C and above.

[0032] 4. Reduced sintering temperature: The sintering temperature is reduced by the fluxing effect of the silicon nitride surface oxide, making the sintering process more efficient and allowing high-density sintering to be achieved in a shorter time.

[0033] 5. Environmental friendliness: This method does not rely on traditional metal oxide additives, avoiding the use of substances that may be harmful to the environment and human health, and has good environmental friendliness. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 Figure 1 is a photograph of the SiC product in Example 1.

[0035] Figure 2 Figure 2 is a micrograph of the SiC product in Example 1.

[0036] Figure 3 Figure 6 is a micrograph of the SiC product in Example 6. DETAILED DESCRIPTION

[0037] The purity of the products in the examples and comparative examples of the present application is the percentage of the total mass of SiC and Si3N4 to the total mass of the product.

[0038] Example 1

[0039] Silicon nitride (Si3N4) powder (D50 particle size of 25 μm) was heated to 1200°C and oxidized for 30 minutes. Then, 100 g of high-purity silicon carbide powder (α-SiC, β-SiC-1, and β-SiC-2 powders in a mass ratio of 40:20:40, with D50 particle sizes of 200 μm, 25 μm, and 100 μm, respectively) was mixed with 5 g of the above pre-oxidized silicon nitride powder (D50 particle size of about 25 μm) by ball milling for 2 hours. The D50 particle size of the product after ball milling was 84 μm. The mixed powder was pressed using cold pressing technology (pressing pressure of 30 MPa, with a holding time of 10 minutes), and then sintered at 2200°C in an argon atmosphere for 10 hours. The sintered density of the resulting material was 94% TD (3.02 g / cm 3 ), the bending strength was 300 MPa, the purity of the product was 99.995%, and the silicon carbide content was greater than 95 wt%.

[0040] Example 2

[0041] The other conditions are identical to Example 1, except that the silicon nitride (Si3N4) powder (D50 particle size of 25 μm) is heated to 1200°C for 10 min.

[0042] The sintered density of the resulting material is 92% TD (2.95 g / cm 3 ), and the bending strength is 280 MPa. The purity of the product is 99.990%.

[0043] Example 3

[0044] The other conditions are identical to Example 1, except that the silicon nitride (Si3N4) powder (D50 particle size of 25 μm) is heated to 800°C for 30 min.

[0045] The sintered density of the resulting material is 90% TD (2.89 g / cm 3 ), and the bending strength is 276 MPa. The purity of the product is 99.989%.

[0046] Example 4

[0047] The other conditions are identical to Example 1, except that the silicon nitride (Si3N4) powder (D50 particle size of 25 μm) is heated to 1200°C for 30 min. Then, 100 g of high-purity silicon carbide powder (α-SiC, β-SiC-1 two powders are composed according to the mass ratio (40:60), and the D50 particle size is 200 μm 25 μm, respectively) is obtained. The sintered density of the resulting material is 88% TD (2.82 g / cm 3 ), and the bending strength is 269 MPa. The purity of the product is 99.991%.

[0048] Example 5

[0049] The other conditions are identical to Example 1, except that the silicon nitride (Si3N4) powder (D50 particle size of 25 μm) is heated to 1200°C for 30 min. Then, 100 g of high-purity silicon carbide powder (α-SiC, β-SiC-2 two powders are composed according to the mass ratio (40:60), and the D50 particle size is 200 μm 100 μm, respectively) is obtained. The sintered density of the resulting material is 84% TD (2.70 g / cm 3 ), and the bending strength is 257 MPa. The purity of the product is 99.991%.

[0050] Example 6

[0051] Silicon nitride (Si3N4) powder (D50 particle size of 25 μm) was heated to 1200℃ and oxidized for 30 min. Then, 100 g of high-purity silicon carbide powder (composed of α-SiC, β-SiC-1, and β-SiC-2 powders in a mass ratio of 40:20:40, with D50 particle sizes of 200 μm, 25 μm, and 100 μm respectively) was mixed with 10 g of the pre-oxidized silicon nitride powder and ball-milled uniformly for 2 hours. The particle size of the product after ball milling was 73 μm. The mixed powder was pressed into shape using cold pressing technology and sintered at 2200℃ for 4 hours under an argon atmosphere. The resulting product had a sintered density of 95%TD (3.05 g / cm³). 3 This is a high-purity silicon carbide material. The flexural strength of this material is 320 MPa, and the purity of the product is 99.995%.

[0052] Example 7

[0053] Silicon nitride (Si3N4) powder (D50 particle size of 25 μm) was heated to 1200℃ and oxidized for 30 min. Then, 100 g of high-purity silicon carbide powder (composed of α-SiC, β-SiC-1, and β-SiC-2 powders in a mass ratio of 40:20:40, with D50 particle sizes of 200 μm, 25 μm, and 100 μm respectively) was mixed with 15 g of the pre-oxidized silicon nitride powder and ball-milled for 2 hours. The resulting product had a particle size of 92 μm. The mixed powder was then pressed into shape using cold pressing technology and sintered at 2200℃ for 4 hours under an argon atmosphere. The resulting product had a sintered density of 93%TD (2.99 g / cm³). 3 This is a high-purity silicon carbide material. The flexural strength of this material is 298 MPa, and the purity of the product is 99.995%.

[0054] Comparative Example 1

[0055] Other conditions were the same as in Example 1, except that the silicon nitride was not pre-oxidized, and the sintering density of the resulting material was 88%TD (2.80 g / cm³). 3 The bending strength is 260MPa.

[0056] Comparative Example 2

[0057] All other conditions are the same as in Example 1, except that silicon nitride is replaced with an equal mass of silicon dioxide.

[0058] The sintered density of the obtained material is 81%TD (2.60 g / cm³). 3 The bending strength is 220MPa.

[0059] Comparative Example 3

[0060] Other conditions were identical to Example 1 except that silicon nitride (Si3N4) powder (D50 particle size of 25 μm) was heated to 1200 °C for 45 min. The sintered density of the resulting material was 91% TD (2.92 g / cm 3 ) and the flexural strength was 280 MPa. The purity of the product was 99.984%.

Claims

1. A method of silicon nitride assisted sintering for the production of SiC, characterized in that, It comprises the following steps: Step one According to the mass ratio, silicon carbide: oxygen-containing silicon nitride = 100:5-15, the silicon carbide powder and the oxygen-containing silicon nitride are prepared and mixed uniformly to obtain a mixed powder; the purity of the silicon carbide powder is ≥99.99%, and the silicon carbide powder is composed of α-SiC powder and β-SiC powder in a mass ratio of A:(100-A), wherein the value of A is 40-30; the mass percentage of oxygen in the oxygen-containing silicon nitride is 1-5%; Step two The mixed powder obtained in step one is pressed to form a shaped body; Step three The shaped body is placed in a sintering furnace and sintered under a protective atmosphere or vacuum conditions, with a sintering temperature of 2000-2200°C and a sintering time of greater than or equal to 2h.

2. The method of claim 1, wherein the method further comprises: The D50 particle size of the α-SiC powder is 150-200μm.

3. The method of claim 1, wherein the method further comprises: The β-SiC powder has two types, both with a purity of ≥99.999%, wherein the D50 particle size of the fine particle size β-SiC powder is 25-50μm, and the D50 of the coarse particle size β-SiC powder is 60-100μm.

4. The method of claim 3, wherein the silicon nitride is used as a sintering aid for the preparation of SiC. The mass ratio of the fine particle size β-SiC powder to the coarse particle size β-SiC powder is 20-15:40-55.

5. The method of claim 3, wherein the silicon nitride is used as a sintering aid for the preparation of SiC. The SiC powder is composed of α-SiC, fine particle size β-SiC powder and coarse particle size β-SiC powder in a mass ratio of 40-30:20-15:40-55.

6. The method of claim 1, wherein the method further comprises: The particle size of the oxygen-containing silicon nitride powder is 15-35μm.

7. The method of claim 1, wherein the method further comprises: The oxygen-containing silicon nitride powder is prepared by the following process: The silicon nitride powder with a purity of ≥99.9% is treated by pre-oxidation to form an oxidation layer on its surface; the pre-oxidation treatment is performed by heating the silicon nitride powder to a temperature range of 800-1200°C in an air or oxygen atmosphere for 5-30min to oxidize the surface of the silicon nitride.

8. The method of claim 1, wherein the method further comprises: In step one, the silicon carbide powder and the oxygen-containing silicon nitride are prepared and mixed uniformly according to the mass ratio of silicon carbide: oxygen-containing silicon nitride = 100:5-10 to obtain a mixed powder.

9. The method of claim 1, wherein the method further comprises: The uniformly mixed powder is pressed to form a shaped body with a certain shape and size by cold pressing; when cold pressing is used, the pressing pressure is 30-50MPa, and the holding time is 5-15min.

10. The method of claim 1, wherein the method further comprises: The sintering time is 4-10h, and the atmosphere is one of argon atmosphere, nitrogen atmosphere and vacuum atmosphere.

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