Preparation method of silicon nitride ceramic with high thermal conductivity and high strength

Abstract

The invention discloses a preparation method of silicon nitride ceramic with high thermal conductivity and high strength, and belongs to the technical field of ceramic products. According to the invention, different ceramic sintering aids with the particle sizes of 0.05-0.5 [mu] m and 0.8-1.5 [mu] m are selected, and liquid phases are generated at different temperatures and regions through the matching combination of different particle sizes, so that the differentiated growth of silicon nitride ceramic grains in the sintering process is realized, and a double-grain structure is generated. The prepared silicon nitride substrate not only retains a short beta-Si3N4 crystal grain phase capable of improving the mechanical property, but also can generate a certain proportion of high-thermal-conductivity long beta-Si3N4 crystal grains. According to the innovative method, the thermal conductivity of the silicon nitride substrate is improved, and the mechanical property of the ceramic substrate is considered.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of ceramic products, in particular to a high-thermal-conductivity silicon nitride ceramic preparation method with high strength. BACKGROUND

[0002] With the increasing demand of people for the energy saving, high efficiency, high power and intelligentization of equipment, the application scenarios of high-power modules are expanding. Because high-power devices have advantages such as high efficiency, high reliability, high power density and intelligentization, they fully meet the needs of human development for equipment. Therefore, high-power modules are increasingly demanded in the fields of communication industry, photoelectric industry, consumer electronics and industrial applications. For example, in recent years, IBGT for new energy vehicles and multi-mode multi-frequency PA modules for consumer electronics. With the increase of transmission efficiency and energy, the requirement of high-power devices for heat dissipation is higher and higher. According to statistics, thermal failure is the first cause of device failure. High-power devices belong to the field of semiconductor circuits, and among the three kinds of substrates of alumina (Al2O3) substrate, aluminum nitride (AlN) substrate and silicon nitride (Si3N4) substrate, the silicon nitride substrate has the best comprehensive performance, and can best meet the requirements of high-power devices for high heat dissipation and high reliability. SUMMARY

[0003] The application realizes the differential growth of silicon nitride ceramic grains in the sintering process by selecting different ceramic sintering aids with particle sizes of 0.05-0.5 um and 0.8-1.5 um, and generating liquid phases at different temperatures and regions, so as to produce a bicrystal structure. The silicon nitride substrate prepared in this way not only retains the short beta-Si3N4 grain phase with improved mechanical properties, but also produces a certain proportion of long beta-Si3N4 grains with high thermal conductivity. Through this innovative method, the mechanical properties of the silicon nitride substrate are considered in the case of improving the thermal conductivity.

[0004] A high-thermal-conductivity silicon nitride ceramic preparation method with high strength, characterized in that the steps are as follows: S1, prepare Si3N4 powder with alpha phase content higher than 85%, the oxygen content of the Si3N4 powder is ≤2%, the amount is 80-95 parts; prepare rare earth element compound A, the amount is 3-15 parts; prepare rare earth element compound B, the amount is 3-15 parts.

[0005] S2, ball mill the Si3N4 powder to a particle size of 0.3-2 μm, ball mill the rare earth element compound A to a particle size of 0.05-0.5 μm, and ball mill the rare earth element compound B to a particle size of 0.8-1.5 μm, and mix all the powders after ball milling.

[0006] S3, add solvent and dispersant for molding, mix until uniform, vacuum degassing, complete the preparation of water-based flow casting slurry.

[0007] S4, after the water-based flow casting slurry is flow casted and dried, it is placed in an environment of 400-800℃ for debinding to obtain a silicon nitride green body.

[0008] S5, the silicon nitride green body is placed in a sintering furnace for sintering, the sintering temperature is 1600-1900℃, the holding time is 2-15h, and the heating rate is 1-15℃ / min.

[0009] Further, in step S1, the rare earth element compound A can be one or more components of oxide or halogen compound or nitride or carbide; the rare earth element compound B can be one or more components of oxide or halogen compound or nitride or carbide.

[0010] Further, in step S4, the debinding atmosphere can be air or nitrogen or argon or vacuum.

[0011] The principle of the application is explained as follows: The three-point bending strength of the mainstream silicon nitride substrate is ≥650MPa, and the fracture toughness is ≥6MPa·m (1 / 2), thermal conductivity ≥ 70 W / m-k. In order to further improve the three-point bending strength and fracture toughness of the silicon nitride ceramic substrate, when preparing the silicon nitride substrate, it is desirable that the silicon nitride substrate has finer grains, such as an average grain size ≤ 5 um, or even less than 3 um; in order to further improve the thermal conductivity of the silicon nitride substrate, it is desirable that the silicon nitride substrate has larger grains, such as an average grain size ≥ 5 um, or more preferably an average grain size greater than 10 um. Because there is a clear contradiction between the requirements of high mechanical properties and high thermal conductivity for the microstructure of the ceramic substrate, it is difficult to simultaneously achieve high thermal conductivity and high strength during the preparation of the silicon nitride substrate. For example, the thermal conductivity of the silicon nitride substrate can be easily increased to 85 W / m-k or even 90 W / m-k or more by increasing the maximum sintering temperature, but the three-point bending strength will decrease to below 600 MPa, or even below 550 MPa. By reducing the sintering temperature, the three-point bending strength of the ceramic substrate can be increased to more than 700 MPa, but the thermal conductivity is difficult to guarantee to be more than 80 W / m-k. At the same time, when adjusting the average grain size of the silicon nitride substrate by the high and low of the maximum holding temperature to match the different needs of customers, the repeatability of the sintering process adjustment in production brings great inconvenience.

[0012] Silicon nitride has a strong covalent bond crystal structure, and low self-diffusion, making it difficult to achieve sintering densification by solid phase diffusion, so it needs to rely on sintering aids. Common sintering aids include rare earth compounds, for example, the rare earth element is preferably at least one selected from Y (yttrium), La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Pm (promethium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium), Yb (ytterbium), Lu (lutetium), Ca, Mg, Al, Zr, Hf, Ti, W, Mo, etc. The sintering aid reacts with the silicon oxide compound on the surface of the silicon nitride powder to produce a liquid phase at high temperature, which promotes the sintering densification of silicon nitride. Among them, the type and particle size of the sintering aid have a great impact on the sintering performance of the ceramic. For example, when the particle size of the sintering aid is reduced from 1-1.5 um to 0.1-0.5 um, the temperature at which the liquid phase is generated will be reduced by more than 100°C. By having both small particles of 0.05-0.5 um and large particles of 0.8-1.5 um in the sintering aid, and by appropriate proportioning, differential growth of silicon nitride grains can be achieved, allowing the silicon nitride substrate to retain short β-Si3N4 grain phases that improve mechanical properties, and also produce a certain proportion of long β-Si3N4 grains that have high thermal conductivity. At the same time, through the interlocking structure of long and short grains, the mechanical properties of the ceramic substrate are considered in the case of improving the thermal conductivity. BRIEF DESCRIPTION OF DRAWINGS

[0013] In order to more clearly illustrate the technical solutions in the specific embodiments or the prior art of the present application, the drawings required to be used in the specific embodiments or the prior art description will be briefly introduced as follows.

[0014] Figure 1 Microstructure under the process of Example 1; Figure 2 Microstructure under the process of Example 2; Figure 3 Microstructure under the process of Comparative Example 1; Figure 4 Microstructure under the process of Comparative Example 2. DETAILED DESCRIPTION

[0015] 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, and any person under the inspiration of the present application or the combination of the present application with other prior art features will fall within the protection scope of the present application.

[0016] Example 1, the ceramic formula design is 92% Si3N4-5% Y2O3-3% MgO, the mass ratio of Y2O3 sintering aid with 0.2um grain is 30%, the mass ratio of Y2O3 with 1um grain is 70%, the mass ratio of MgO sintering aid with 0.2um grain is 40%, and the mass ratio of Y2O3 with 1um grain is 60%. The molding pattern is kept at a lower sintering temperature of 1650℃ for 3h, so as to promote the fine-grained Y2O3 and MgO to generate liquid phase at low temperature with the silicon oxide compound on the surface of the silicon nitride powder, promote the growth of part of the silicon nitride grains, and because the temperature is low and the liquid phase constantly plays a role, the growth of this part of the grains is limited to generate short β-Si3N4 grains with high mechanical properties; when the sintering temperature is increased to 1750℃ and kept for 5h, at this time, the large-grained Y2O3 and MgO generate liquid phase at high temperature with the silicon oxide compound on the surface of the silicon nitride powder, promote the growth of another part of the silicon nitride grains, and because the temperature is high at this time, another part of the silicon nitride grains grow faster under liquid phase sintering to generate long β-Si3N4 grains with high thermal conductivity. The ceramic grains are observed by SEM to have obvious bimodal grain structure, short grains D50=2um, and long grains D50=6um, and the microstructure diagram is as shown in Figure 1 .

[0017] Example 2: The ceramic formulation is designed as 92% Si3N4-5% Y2O3-3% MgO. In the Y2O3 sintering aid, the mass percentage of Y2O3 with a grain size of 0.2 μm is 70%, and the mass percentage of Y2O3 with a grain size of 1 μm is 30%. In the MgO sintering aid, the mass percentage of MgO with a grain size of 0.2 μm is 60%, and the mass percentage of Y2O3 with a grain size of 1 μm is 40%. The molded sample was sintered at a relatively low temperature of 1650℃ for 3 hours. This sintered at a low temperature, causing the fine-grained Y₂O₃ and MgO to react with the silicon oxides on the surface of the silicon nitride powder to form a liquid phase, promoting the growth of some silicon nitride grains. Due to the low temperature and the continuous evaporation of the liquid phase, the growth of these grains was limited, resulting in short β-Si₃N₄ grains with high mechanical properties. The sintering temperature was then increased to 1750℃ and held for 5 hours. At this temperature, the large and small-grained Y₂O₃ and MgO reacted with the silicon oxides on the surface of the silicon nitride powder to form a liquid phase, promoting the growth of other silicon nitride grains. Because of the higher temperature, the other silicon nitride grains grew faster under liquid phase sintering, resulting in long β-Si₃N₄ grains with high thermal conductivity. SEM observation showed that the ceramic grains exhibited a distinct bicrystalline structure, with short grains having a D₅₀ of 2 μm and long grains having a D₅₀ of 6 μm. The microstructure diagram is shown below. Figure 2 As shown.

[0018] Comparative Example 1: The ceramic formulation is designed as 92% Si3N4-5% Y2O3-3% MgO. In the Y2O3 sintering aid, the mass percentage of Y2O3 with a grain size of 0.2 μm is 95%, and the mass percentage of Y2O3 with a grain size of 1 μm is 5%. In the MgO sintering aid, the mass percentage of MgO with a grain size of 0.2 μm is 95%, and the mass percentage of Y2O3 with a grain size of 1 μm is 5%. The molded sample was sintered at a relatively low temperature of 1650℃ for 3 hours. This allowed the fine-grained Y₂O₃ and MgO to react with the silicon oxides on the surface of the silicon nitride powder to form a liquid phase at low temperature, promoting the growth of some silicon nitride grains. However, due to the low temperature and the continuous release of the liquid phase, the growth of these grains was limited, resulting in short β-Si₃N₄ grains with high mechanical properties. The sintering temperature was then increased to 1750℃ and held for 5 hours. At this temperature, the large and fine-grained Y₂O₃ and MgO reacted with the silicon oxides on the surface of the silicon nitride powder to form a liquid phase, promoting the growth of other silicon nitride grains. However, the liquid phase generated by the large grains was significantly insufficient, resulting in a significantly smaller number of long β-Si₃N₄ grains with high thermal conductivity. The microstructure diagram is shown below. Figure 3 As shown.

[0019] Comparative Example 2: The ceramic formulation is designed as 92% Si3N4-5% Y2O3-3% MgO. In the Y2O3 sintering aid, the mass percentage of Y2O3 with a grain size of 0.2 μm is 5%, and the mass percentage of Y2O3 with a grain size of 1 μm is 95%. In the MgO sintering aid, the mass percentage of MgO with a grain size of 0.2 μm is 5%, and the mass percentage of Y2O3 with a grain size of 1 μm is 95%. The molded sample was held at a relatively low sintering temperature of 1650℃ for 3 hours. This promoted the growth of some silicon nitride grains by generating a liquid phase at low temperature between the fine-grained Y₂O₃ and MgO and the silicon oxide compounds on the surface of the silicon nitride powder. Due to the low temperature and the continuous evaporation of the liquid phase, the growth of these grains was limited, resulting in the formation of short β-Si₃N₄ grains with high mechanical properties. However, the amount of liquid phase generated by the fine grains was significantly insufficient, leading to a significantly smaller number of short β-Si₃N₄ grains with high mechanical properties. When the sintering temperature was increased to 1750℃ and held for 5 hours, the large and fine-grained Y₂O₃ and MgO generated a liquid phase at high temperature with the silicon oxide compounds on the surface of the silicon nitride powder, promoting the growth of other silicon nitride grains. However, the amount of liquid phase generated by the large grains was significantly excessive, leading to a significantly larger number of long β-Si₃N₄ grains with high thermal conductivity. The microstructure diagram is shown below. Figure 4 As shown.

[0020] The physical properties measured for the above four process technologies are as follows: Item [Y2O3 ratio] MgO ratio Thermal conductivity Bending strength Fracture toughness Grain morphology Example 1 Fine:Coarse = 30:70 Fine:Coarse = 40:60 95 700 6.5 Short average D50 = 3 um; long grain D50 = 7 um Example 2 Fine:Coarse = 70:30 Fine:Coarse = 60:40 90 750 7 Short grain D50 = 2 um; long grain D50 = 6 um Comparative Example 1 Fine:Coarse = 95:5 Fine:Coarse = 95:5 70 850 7.5 Long grain deficient, average grain = 3; Comparative Example 2 Fine:Coarse = 5:95 Fine:Coarse = 5:95 100 600 6 Short grain deficient, average grain = 8;

Claims

1. A method for preparing high-thermal-conductivity silicon nitride ceramic with high strength, The method is characterized in that the steps are as follows: S1, preparing Si3N4 powder with an alpha phase content higher than 85%, wherein the oxygen content of the Si3N4 powder is less than or equal to 2%, and the amount of the Si3N4 powder is 80-95 parts; preparing a rare earth element compound A, and the amount of the rare earth element compound A is 3-15 parts; S2, ball-milling the Si3N4 powder to a particle size of 0.3-2 μm, ball-milling the rare earth element compound A to a particle size of 0.05-0.5 μm, and ball-milling the rare earth element compound B to a particle size of 0.8-1.5 μm; and mixing all the powders after the ball-milling is completed; S3, adding a solvent and a dispersing agent for molding, mixing until uniform, vacuumizing and degassing, and completing the preparation of the water-based flow-casting slurry; S4, drying the water-based flow-casting slurry after flow-casting, and placing the water-based flow-casting slurry in an environment with a temperature of 400-800 °C for degreasing to obtain a silicon nitride green body; S5, placing the silicon nitride green body in a sintering furnace for sintering, wherein the sintering temperature is 1600-1900 °C, the holding time is 2-15 h, and the heating rate is 1-15 °C / min. In step S1, the rare earth element compound A can be one or two or three or four components of an oxide or a halogen compound or a nitride or a carbide; and the rare earth element compound B can be one or two or three or four components of an oxide or a halogen compound or a nitride or a carbide.

2. The method according to claim 1, wherein the method comprises the following steps: In step S4, the degreasing atmosphere can be air or nitrogen or argon or vacuum. ​ 3. The method according to claim 1, wherein the method comprises the following steps: ​ ​

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