Method for preparing high-thermal-conductivity silicon nitride ceramic substrate based on silicon powder direct nitriding method and silicon nitride ceramic substrate

Abstract

The invention discloses a method for preparing a high-thermal-conductivity silicon nitride ceramic substrate based on a silicon powder direct nitridation method and a silicon nitride ceramic substrate, and belongs to the field of advanced ceramic preparation.The method comprises the following steps that S1, silicon powder and a composite sintering aid are subjected to ball milling and mixed in a medium to obtain mixed slurry, the composite sintering aid comprises Y2O3, MgO, Lu2O3 and CaF2, on the basis of the silicon powder, the composite sintering aid is prepared from Y2O3, MgO, Lu2O3 and CaF2; the weight of the composite sintering aid is 5-10% of the weight of the silicon powder; s2, drying the mixed slurry to obtain mixed powder; s3, pressing the mixed powder into a green body; s4, sintering: firstly nitriding and then sintering; s5, cooling; wherein Y2O3 accounts for 3%-6% of the weight of the silicon powder, MgO accounts for 1%-3% of the weight of the silicon powder, Lu2O3 accounts for 1%-2% of the weight of the silicon powder, and The method solves the problems of incomplete nitriding, high oxygen content and poor microstructure development in a silicon powder direct nitriding method.

Description

Technical Field

[0001] This invention relates to the field of advanced ceramics preparation, and in particular to a method for preparing high thermal conductivity silicon nitride ceramic substrates based on direct silicon powder nitridation and the silicon nitride ceramic substrates thereof. Background Technology

[0002] Silicon nitride ceramics are considered ideal materials for packaging and heat dissipation substrates of next-generation high-power electronic devices (such as IGBTs, LEDs, and power modules) due to their excellent mechanical strength, good electrical insulation, thermal expansion coefficient matching that of silicon, and potential high thermal conductivity.

[0003] Theoretically, the thermal conductivity of pure, defect-free silicon nitride crystals along the c-axis can reach 200-320 W / m·K. However, the thermal conductivity of actually prepared silicon nitride ceramics is much lower than this theoretical value. The main reasons are: 1) Oxygen impurities in the crystal lattice will dissolve into the Si3N4 crystal lattice, forming strong phonon scattering centers, which will seriously reduce the thermal conductivity; 2) Defects such as pores, grain boundary phases and amorphous phases in the microstructure will also hinder phonon transmission. In particular, the amorphous phase at the grain boundary is one of the main obstacles to phonon transmission.

[0004] Currently, the mainstream methods for preparing high thermal conductivity silicon nitride ceramics mostly use expensive silicon nitride powder as raw material. Although high thermal conductivity (such as 90-110 W / m·K) can be obtained, there are problems such as high equipment cost, complex process and high energy consumption, which limit its large-scale application. In contrast, the process route of using inexpensive silicon powder as raw material to generate silicon nitride through direct nitriding reaction and then densifying by sintering has a significant cost advantage.

[0005] In the prior art, there are also methods for preparing silicon nitride substrates using silicon powder. For example, patent CN105884376A discloses a "method for preparing silicon nitride ceramic substrates by silicon powder casting"; this method prepares a green blank through casting, followed by debinding and sintering. However, this method has the following limitations: (1) Low thermal conductivity: The thermal conductivity of its final products is only "greater than 80 W / m·K", usually concentrated in the range of 50-70 W / m·K, which is difficult to meet the stringent requirements of current high-end high-power devices for the thermal conductivity of heat dissipation substrate (>100 W / m·K).

[0006] (2) The process chain is long and prone to defects: The casting process requires the addition of a large amount of organic carriers (binders, plasticizers, etc.), and the subsequent debinding process is prone to cracking of the green body and the generation of pores. Furthermore, the carbon residue left by incomplete debinding will seriously affect the purity and performance of the ceramic.

[0007] (3) Insufficient control over microstructure: The sintering process of this method is relatively simple, and the control over the purification of lattice oxygen, abnormal growth of β-Si3N4 grains and crystallization of grain boundary phase is insufficient, resulting in severe phonon scattering and inability to break through the thermal conductivity barrier.

[0008] Therefore, developing a method to stably prepare silicon nitride substrates with thermal conductivity exceeding 110 W / m·K and excellent overall performance based on low-cost silicon powder raw materials, through innovative raw material systems and precise process control, has become a technical problem that urgently needs to be solved in this field and has important industrial application value.

[0009] The above background information is provided to facilitate understanding of the present invention and is not intended to be publicly known technology disclosed to the general public prior to the application of this invention. Summary of the Invention

[0010] To address the aforementioned shortcomings, this invention provides a method for preparing high thermal conductivity silicon nitride ceramic substrates based on direct silicon powder nitridation and the silicon nitride ceramic substrate itself, solving the problems of incomplete nitridation, high oxygen content, and poor microstructure development in the direct silicon powder nitridation method.

[0011] The technical solution is: a method for preparing high thermal conductivity silicon nitride ceramic substrates based on direct silicon powder nitridation, comprising the following steps: S1, silicon powder and composite sintering aid are ball-milled and mixed under medium conditions to obtain a mixed slurry, wherein the composite sintering aid includes Y2O3, MgO, Lu2O3 and CaF2, and the composite sintering aid accounts for 5% to 10% of the weight of silicon powder based on silicon powder; S2, drying the mixed slurry to obtain mixed powder; S3, the mixed powder is pressed into a blank; S4, sintering, sintering is performed by first nitriding and then sintering; S5, cooling; Among them, Y2O3 is 3%~6% of the weight of silicon powder, MgO is 1%~3%, Lu2O3 is 1%~2% and CaF2 is 0.5%~1.5%.

[0012] Furthermore, in S1, the medium is anhydrous ethanol, the ball milling time is 4-8 hours, and the ball-to-material ratio is 3:1 to 5:1.

[0013] Furthermore, the purity of the silicon powder is ≥99.99 wt%, and the average particle size D 50 The range is 0.5 μm to 2.0 μm.

[0014] Furthermore, in S4, the process of nitriding followed by sintering includes the following steps: S41, the billet is placed in a sintering furnace, a vacuum is drawn, and nitrogen is introduced; S42, Nitriding: Under a nitrogen atmosphere, a multi-stage programmed temperature nitriding process is performed, wherein the multi-stage programmed temperature nitriding process includes at least a first low-temperature pre-nitriding stage, a second medium-temperature main nitriding stage, and a third high-temperature phase transformation stage. S43, Gas Pressure Sintering: Gas pressure sintering is carried out under nitrogen pressure of 0.5MPa~2.0 MPa, and the sintering temperature is 1750℃~1850℃.

[0015] The first low-temperature pre-nitriding stage aims to slowly nitrid the surface of silicon powder to form a complete and dense silicon nitride surface layer, which encapsulates the unreacted silicon inside and prevents it from being lost or forming macroscopic pores due to silicon melting (silicon melting point is 1414℃) in subsequent stages caused by temperature increases.

[0016] The second intermediate-temperature main nitriding stage is the key stage in which silicon powder and nitrogen undergo a violent exothermic reaction to generate a large amount of α-Si3N4. By precisely controlling the heating rate and holding time, the nitriding reaction is ensured to be complete, and unreacted silicon is avoided.

[0017] The third high-temperature phase transformation stage aims to promote the initial transformation of Si3N4 into thermodynamically more stable β-S Si3N4 and grain growth, forming "seeds" for the abnormal growth of β grains during subsequent sintering, thus laying the foundation for finally obtaining a high aspect ratio grain structure.

[0018] The high-temperature and high-pressure process effectively promotes the liquid-phase sintering formed by sintering aids, drives rapid material migration, and fully densifies the material (density ≥ 99%). At the same time, the high-pressure nitrogen atmosphere inhibits the high-temperature decomposition of Si3N4 and drives the abnormal growth and development of β-SSi3N4 grains through a dissolution-precipitation mechanism, forming "rod-shaped" grains with high aspect ratio. These grains intertwine and interlock, forming a microstructure that is conducive to phonon transport.

[0019] Furthermore, the first low-temperature pre-nitriding stage involves heating to 1250℃~1300℃ at a rate of 2℃ / min~5℃ / min and holding at that temperature for 1 hour~3 hours; the second medium-temperature main nitriding stage involves heating to 1350℃~1420℃ at a rate of 1℃ / min~3℃ / min and holding at that temperature for 3 hours~8 hours; and the third high-temperature phase transformation stage involves heating to 1450℃~1500℃ and holding at that temperature for 1 hour~2 hours.

[0020] Furthermore, in S43, the heating rate of gas pressure sintering is 5℃ / min ~ 10℃ / min, and the holding time is 1 hour ~ 4 hours.

[0021] Furthermore, in S5, the cooling is controlled at 2℃ / min ~ 5℃ / min to below 1000℃, and then naturally cooled to room temperature with the furnace.

[0022] Furthermore, in S3, the mixed powder is pressed into a green body as follows: the mixed powder is first dry-pressed into green body one under a pressure of 100MPa~200 MPa, and green body one is then subjected to cold isostatic pressing treatment at 200MPa~300 MPa to become green body two.

[0023] The present invention also provides a silicon nitride ceramic substrate with high thermal conductivity.

[0024] A silicon nitride ceramic substrate with high thermal conductivity is prepared by the method described above.

[0025] Furthermore, this high thermal conductivity silicon nitride ceramic substrate has a thermal conductivity ≥110 W / mK and a bulk density ≥3.25 g / cm³. 3 The microstructure is mainly composed of interwoven long columnar β-S Si3N4 grains with an aspect ratio of not less than 5, and the grain boundary phase is a crystalline phase.

[0026] Invention principle: The core of this invention lies in the synergistic effect of raw material pretreatment, multi-stage precisely controlled nitriding process, and optimized composite sintering aid system to achieve simultaneous optimization of material purity, density, and microstructure.

[0027] First, high-purity, fine-particle-size spherical silicon powder is used to reduce the introduction of impurities at the source and provide high reactivity.

[0028] Second, a composite sintering aid system composed of Y2O3-MgO-Lu2O3-CaF2 was designed. Among them, Y2O3 and Lu2O3 (especially Lu2O3) can react with SiO2 on the surface of Si3N4 to generate high-melting-point, easily crystallized rare earth silicate grain boundary phases (such as Y2Si2O7, Lu2Si2O7), thereby "capturing" oxygen atoms, significantly reducing the lattice oxygen content, and reducing the amorphous phase at the grain boundaries; MgO can lower the liquid phase formation temperature and promote sintering densification; the introduction of a small amount of CaF2 is a key innovation, which can generate a trace amount of fluoride atmosphere at high temperature, effectively etching and purifying the surface of Si3N4 grains, further removing oxygen impurities, and significantly promoting the anisotropic growth of β-Si3N4 grains through the formation of a gas-liquid-solid transport mechanism, forming long columnar crystals with a high aspect ratio. Furthermore, a multi-stage controlled nitriding process is employed. First, a dense nitriding layer is formed at a low temperature to prevent silicon melt loss. Then, full nitriding is completed at a medium temperature. Finally, the α→β phase transformation is "seeded" and initially grown at a high temperature, providing an ideal precursor with high β phase content and development potential for subsequent sintering. Finally, gas pressure sintering under high-pressure nitrogen further promotes densification and drives the β-Si3N4 grains to fully develop and preferentially orient through a dissolution-precipitation mechanism, forming a highly interlocked microstructure conducive to phonon transport.

[0029] Invention effects: This invention achieves a major breakthrough in thermal conductivity: through the organic combination of composite sintering aids (especially the synergistic introduction of Lu2O3 and CaF2) and a multi-stage nitriding process, the Si3N4 lattice is effectively purified, significantly reducing the lattice oxygen content and promoting the crystallization of grain boundary phases at high temperatures. Combined with the well-developed, highly oriented β-Si3N4 long columnar interlocking structure formed by gas pressure sintering, the thermal conductivity of the silicon nitride substrate prepared by this invention is stable between 110 W / mK and 130 W / mK at room temperature, reaching a maximum of over 130 W / mK. This significantly surpasses the level of the existing silicon powder casting method (CN105884376A), which is typically below 80 W / mK, achieving a high-end product level using expensive silicon nitride powder as raw material.

[0030] The present invention has significant cost advantages: it uses inexpensive silicon powder (priced at about 1 / 3 to 1 / 5 of silicon nitride powder) instead of expensive silicon nitride powder as the starting material, which greatly reduces the cost of raw materials.

[0031] This invention features high process integration and stable product performance: the nitriding and sintering processes are completed continuously in the same equipment, reducing intermediate steps and contamination and performance damage to powders during transport, improving production efficiency, and ensuring consistent product performance. Compared to the casting method, it eliminates lengthy ball milling and debinding steps, resulting in a simpler and more reliable process flow.

[0032] The product of this invention exhibits excellent microstructure and mechanical properties: the final product has high density (≥99% of theoretical density), uniform grain size distribution, and forms an effective three-dimensional heat conduction pathway; due to the formed interlocking microstructure, the resulting substrate simultaneously possesses high three-point bending strength (≥600 MPa) and fracture toughness (≥6.5 MPa). 1 / 2 ). Attached Figure Description

[0033] Figure 1 This is a scanning electron microscope image of the cross-section of the silicon nitride substrate in Embodiment 1 of the present invention; Figure 2 This is a scanning electron microscope image of the cross-section of the silicon nitride substrate of Comparative Example 5 of the present invention. Detailed Implementation

[0034] The technical solution of the present invention will be described in detail below with specific embodiments. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention and not for limiting the claims of the present invention.

[0035] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "longitudinal," "lateral," "horizontal," "inner," "outer," "front," "rear," "top," and "bottom," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0036] In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, the terms "set up," "open," "installed," "connected," and "communicate" should be interpreted broadly. For example, they can refer to fixed connection, detachable connection, or integral connection; they can refer to direct connection or indirect connection through an intermediate medium; and they can refer to the connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0037] Unless otherwise specified, all inventions are existing technologies.

[0038] Unless otherwise specified, all materials involved in this invention are purchased from the market.

[0039] Example 1 A method for preparing a silicon nitride ceramic substrate includes the following steps: S1, take 1000 grams of spherical silicon powder (purity 99.99%, D 50 =1.0 μm), and composite sintering aid (Y2O3 50g, MgO 20g, Lu2O3 10g, CaF2 8g) were ball-milled in anhydrous ethanol in a planetary ball mill (with silicon nitride balls as the grinding medium, ball-to-material ratio 4:1, rotation speed 300 rpm) for 6 hours to obtain a mixed slurry.

[0040] S2, the mixed slurry is vacuum dried at 80℃ for 12 hours and then passed through a 150-mesh sieve to obtain mixed powder.

[0041] S3, the mixed powder is dry-pressed at 150 MPa to form a green body of 50mm×50mm×0.8mm. Green body one is then subjected to cold isostatic pressing at 250 MPa to form green body two.

[0042] S4, second sintering of green body, includes the following steps: S41, the second green billet is placed in an atmosphere pressure sintering furnace, and after being evacuated to 10⁻² Pa, nitrogen is introduced to bring it to atmospheric pressure.

[0043] S42, Nitriding: Heat to 1280℃ at 3℃ / min and hold for 2 hours; then heat to 1380℃ at 2℃ / min and hold for 5 hours; finally heat to 1480℃ at 1℃ / min and hold for 1.5 hours; the total nitrogen flow rate is 1 L / min.

[0044] S43, Gas Pressure Sintering: After nitriding, nitrogen is immediately introduced into the furnace to a pressure of 1.0 MPa, and then the temperature is raised to 1800℃ at a rate of 8℃ / min and held for 2 hours for gas pressure sintering.

[0045] S5, Cooling to form a silicon nitride ceramic substrate: After sintering, the temperature is controlled to be cooled to 1000℃ at 3℃ / min, and then naturally cooled to room temperature with the furnace.

[0046] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0047] SEM, such as Figure 1 SEM revealed its typical interlocking structure formed by the interweaving of long columnar β-Si3N4 grains.

[0048] Example 2 Compared with Example 1, the difference lies in the content of the composite sintering aid. In this example, the composite sintering aid contains 40g of Y2O3, 25g of MgO, 15g of Lu2O3, and 5g of CaF2.

[0049] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0050] Example 3 Compared with Example 2, the difference is that the nitriding process of S42 is as follows: the temperature is increased to 1280°C at 3°C / min and held for 2 hours; the temperature is increased to 1400°C at 1°C / min and held for 6 hours; finally, the temperature is increased to 1480°C at 1°C / min and held for 1.5 hours; the nitrogen flow rate throughout the process is 1 L / min.

[0051] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0052] Example 4 Compared with Example 3, the difference is that the gas pressure sintering of S43 is as follows: after nitriding, nitrogen is immediately introduced into the furnace to a pressure of 1.2 MPa, and then the temperature is raised to 1820°C at a rate of 8°C / min and held for 2 hours for gas pressure sintering.

[0053] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0054] Example 5 Compared with Example 1, the difference lies in the content of the composite sintering aid. In this example, the composite sintering aid contains 60g of Y2O3, 10g of MgO, 20g of Lu2O3, and 10g of CaF2.

[0055] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0056] Example 6 Compared with Example 5, the difference is that the nitriding process of S42 is as follows: the temperature is increased to 1300°C at 5°C / min and held for 1 hour; the temperature is increased to 1420°C at 1°C / min and held for 3 hours; the temperature is increased to 1500°C at 1°C / min and held for 1 hour.

[0057] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0058] Example 7 Compared with Example 6, the difference is that the gas pressure sintering of S43 is as follows: after nitriding, nitrogen is immediately introduced into the furnace to a pressure of 2.0 MPa, and then the temperature is raised to 1850°C at a rate of 8°C / min and held for 1 hour for gas pressure sintering.

[0059] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0060] Comparative Example 1 Compared with Example 1, the difference is that the composite sintering aid contains 13.5g of MgO, 54.2g of Lu2O3, and 20.3g of ZrO2.

[0061] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0062] Comparative Example 2 In this comparative example, the mixed powder was obtained according to method S1 of Example 1, and the green body was obtained according to methods S2 and S3 of Example 1, the difference being: The sintering and cooling of the green blanks in S4 and S5 adopted the sintering and cooling method of Example 7 in CN105884376A (i.e., the temperature was raised to 1400°C at a heating rate of 20°C / min and held for 2 hours, then the temperature was raised to 1850°C at a heating rate of 10°C / min and held for 2 hours, then the temperature was lowered to 1200°C at a cooling rate of 5°C / min, and then cooled in the furnace. The sintering atmosphere was 9 atm of nitrogen).

[0063] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0064] Comparative Example 3 This comparative example uses the same spherical silicon powder as Example 1, and the amount of sintering aid is the same as in Example 1 (i.e., 1000 grams, and the amount of sintering aid is 88 grams). The difference is: This comparative example uses the sintering aid of Example 7 of CN105884376A and prepares silicon nitride ceramic substrates according to the method for preparing silicon nitride ceramic substrates in Example 7 of CN105884376A (i.e., steps (1) to (3) of Example 7).

[0065] Comparative Example 4 The difference compared to Example 1 is that the composite sintering aid does not contain CaF2.

[0066] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0067] Comparative Example 5 Compared with Example 1, the difference is that the nitriding of S42 is as follows: the temperature is increased to 1280°C at 3°C / min and held for 2 hours; then the temperature is increased to 1380°C at 2°C / min and held for 5 hours; the nitrogen flow rate is 1 L / min throughout the process.

[0068] SEM, such as Figure 2 The results show that the β-Si3N4 grains have a small aspect ratio, indicating that the lack of a high-temperature phase transformation stage is not conducive to the formation of an ideal precursor structure, which affects the development of the microstructure of the final sintered body.

[0069] The performance of the fabricated silicon nitride ceramic substrate was tested, and the results are shown in Table 1 below.

[0070] Table 1 Performance testing of silicon nitride ceramic substrates In the performance test items in Table 1, the room temperature thermal conductivity was measured by the laser flare method (LFA), and the bulk density was measured by the Archimedes displacement method.

[0071] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a high thermal conductivity silicon nitride ceramic substrate based on a direct nitridation of silicon powder, characterized in that, Includes the following steps: S1, silicon powder and composite sintering aid are ball-milled and mixed under medium conditions to obtain a mixed slurry, wherein the composite sintering aid includes Y2O3, MgO, Lu2O3 and CaF2, and the composite sintering aid accounts for 5% to 10% of the weight of silicon powder based on silicon powder; S2, drying the mixed slurry to obtain mixed powder; S3, the mixed powder is pressed into a blank; S4, sintering, sintering is performed by first nitriding and then sintering; S5, cooling; Among them, Y2O3 is 3%~6% of the weight of silicon powder, MgO is 1%~3%, Lu2O3 is 1%~2% and CaF2 is 0.5%~1.5%.

2. The method of claim 1, wherein the silicon powder-based direct nitridation method for preparing a high-thermal-conductivity silicon nitride ceramic substrate is characterized by, In S1, the medium is anhydrous ethanol, the ball milling time is 4-8 hours, and the ball-to-material ratio is 3:1 to 5:

1.

3. The method of claim 1, wherein the silicon powder-based direct nitridation method for preparing a high-thermal-conductivity silicon nitride ceramic substrate is characterized by, The silicon powder has a purity ≥ 99.99 wt% and an average particle size D. 50 The range is 0.5 μm to 2.0 μm.

4. The method for preparing high thermal conductivity silicon nitride ceramic substrate by silicon powder based direct nitridation according to claim 1, characterized in that, In S4, the process of nitriding followed by sintering includes the following steps: S41, the billet is placed in a sintering furnace, a vacuum is drawn, and nitrogen is introduced; S42, Nitriding: Under a nitrogen atmosphere, a multi-stage programmed temperature nitriding process is performed, wherein the multi-stage programmed temperature nitriding process includes at least a first low-temperature pre-nitriding stage, a second medium-temperature main nitriding stage, and a third high-temperature phase transformation stage. S43, Gas Pressure Sintering: Gas pressure sintering is carried out under nitrogen pressure of 0.5MPa~2.0 MPa, and the sintering temperature is 1750℃~1850℃.

5. The method of claim 4, wherein the silicon powder-based direct nitridation method for preparing a high-thermal-conductivity silicon nitride ceramic substrate is characterized by, The first low-temperature pre-nitriding stage involves heating to 1250℃~1300℃ at a rate of 2℃ / min~5℃ / min and holding at that temperature for 1 hour~3 hours; the second medium-temperature main nitriding stage involves heating to 1350℃~1420℃ at a rate of 1℃ / min~3℃ / min and holding at that temperature for 3 hours~8 hours; the third high-temperature phase transformation stage involves heating to 1450℃~1500℃ and holding at that temperature for 1 hour~2 hours.

6. The method of claim 4, wherein the silicon powder-based direct nitridation method for preparing a high-thermal-conductivity silicon nitride ceramic substrate is characterized by, In S43, the heating rate of gas pressure sintering is 5℃ / min ~ 10℃ / min, and the holding time is 1 hour ~ 4 hours.

7. The method of claim 1, wherein the silicon powder-based direct nitridation method for preparing a high-thermal-conductivity silicon nitride ceramic substrate is characterized by, In S5, the cooling is controlled at 2℃ / min ~ 5℃ / min to below 1000℃, and then naturally cooled to room temperature with the furnace.

8. The method for preparing high thermal conductivity silicon nitride ceramic substrates based on direct silicon powder nitridation according to claim 1, characterized in that, In S3, the mixed powder is pressed into a green body as follows: the mixed powder is first dry-pressed into green body one under a pressure of 100MPa~200 MPa, and then green body one is subjected to cold isostatic pressing treatment at 200MPa~300 MPa to become green body two.

9. A silicon nitride ceramic substrate with high thermal conductivity, characterized in that, The high thermal conductivity silicon nitride ceramic substrate is prepared by the method described in any one of claims 1-8.

10. The high thermal conductivity silicon nitride ceramic substrate of claim 9, wherein, The high thermal conductivity silicon nitride ceramic substrate has a thermal conductivity ≥110 W / mK and a bulk density ≥3.25 g / cm³. 3 The microstructure is mainly composed of interwoven long columnar β-Si3N4 grains with an aspect ratio of not less than 5, and the grain boundary phase is a crystalline phase.

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