A method for preparing a high thermal conductivity silicon nitride ceramic

Abstract

The application relates to a preparation method of high-thermal-conductivity silicon nitride ceramics, which comprises the following steps: ① selecting alpha-Si3N4, beta-Si3N4 powder and sintering aid powder as raw materials; ② mixing the mixed powder raw materials selected in the above ① according to a certain proportion, vacuum ball milling for 24-48 hours for sufficient wet mixing, and drying in a vacuum drying box to obtain mixed raw materials; ③ pre-pressing the mixed raw materials obtained in the step ②, loading into a graphite mold, high-temperature treatment of the mixed powder in a low-pressure environment through a hot-pressing sintering furnace, and adding the mixed raw materials again for secondary vacuum ball milling; ④ pre-pressing the secondary ball-mixed raw materials obtained in the step ③, assembling a leaf talc composite block, and sintering under high temperature and high pressure by using a six-surface hydraulic press, so that the preparation of the high-thermal-conductivity silicon nitride ceramics is completed. The application is characterized in that large-particle silicon nitride seeds are prepared under vacuum conditions, the silicon nitride mixed powder is continuously crystallized under high-pressure driving, the thermal conductivity of the silicon nitride ceramics is improved, and the operability is high.

Description

Technical Field

[0001] This invention belongs to the field of manufacturing materials such as cutting tools and heating elements, specifically a method for preparing silicon nitride ceramic materials with high thermal conductivity. Background Technology

[0002] In recent years, with the strengthening of my country's military industry and the rapid development of aerospace, railway transportation, power electronics, and machinery manufacturing, the superior properties of silicon nitride ceramic materials, such as high hardness, high strength, high temperature resistance, and wear resistance, have led to their increasingly widespread commercial applications, mainly in cutting tools and bearing balls, thus increasing their commercial value. Since 2020, domestic sales of high-purity silicon nitride have been relatively good, and the high-value-added products corresponding to high-purity raw materials have enormous economic potential. Compared with traditional ceramics such as AlN and Al2O3, silicon nitride's excellent theoretical thermal conductivity has gradually made it a major heat dissipation material for electronic devices.

[0003] However, the actual thermal conductivity of current silicon nitride ceramic materials is still far lower than their theoretical thermal conductivity. Oxygen atoms react in the silicon nitride lattice to create silicon vacancies, causing lattice distortion and phonon scattering, thus reducing the thermal conductivity of silicon nitride ceramics. Simultaneously, solid-state sintering of the material is prone to particle agglomeration and bridging, resulting in poor local uniformity and severely affecting the transformation of silicon nitride crystal forms and grain growth, further reducing the thermal conductivity of silicon nitride ceramics. How to improve the thermal conductivity of silicon nitride ceramics and achieve large-scale production of silicon nitride ceramic thermally conductive substrates or cutting tools has become an urgent problem to be solved. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing silicon nitride ceramics with high thermal conductivity. This method is highly operable and produces silicon nitride ceramics with high thermal conductivity.

[0005] The technical solution of the present invention:

[0006] A method for preparing silicon nitride ceramics with high thermal conductivity includes the following steps:

[0007] ① Select α-Si3N4, β-Si3N4 powder and sintering aid powder as raw materials;

[0008] ② The mixed powder raw materials selected in ① above are mixed in a certain proportion, vacuum ball milled for 24-48 hours to fully wet mix, and then dried in a vacuum drying oven to obtain the mixed raw materials;

[0009] ③ The mixed raw materials obtained in ② are pre-pressed and placed into a graphite mold. The mixed powder is then subjected to high-temperature treatment in a low-pressure environment through a hot-press sintering furnace. After that, the mixed raw materials are added again for secondary vacuum ball milling.

[0010] ④ The secondary ductile iron mixed raw material obtained in ③ is pre-pressed and molded, pyrophyllite composite blocks are assembled, and sintered under high temperature and high pressure using a six-sided hydraulic press to complete the preparation of high thermal conductivity silicon nitride ceramics.

[0011] The beneficial effects of this invention are:

[0012] 1. This invention uses non-oxides as sintering aids. The entire material preparation process is carried out under vacuum conditions, which avoids the introduction of oxygen atoms and effectively prevents lattice distortion caused by the generation of silicon vacancies in the silicon nitride lattice by oxygen atoms, thereby improving the thermal conductivity of silicon nitride ceramics.

[0013] 2. This invention employs low-pressure high-temperature sintering technology, using a graphite cavity as a reaction vessel, allowing sintering aids and silicon nitride particles to melt and infiltrate each other at high temperatures. This effectively promotes uniform matrix composition, reduces defects such as "agglomeration" and "bridging" introduced by the powder mixing process, and prepares large-particle silicon nitride seed crystals under vacuum conditions. This allows the silicon nitride mixed powder to continue crystallizing under high pressure, thereby improving the thermal conductivity of silicon nitride ceramics.

[0014] 3. The present invention can effectively crush high-temperature sintered samples and fully mix them with raw materials through a secondary vacuum ball milling step.

[0015] 4. This invention uses a pyrophyllite composite cavity as a high-pressure reaction vessel to prepare high-purity silicon nitride ceramic sintered bodies under high temperature and pressure. This method is highly operable, eliminates contamination introduced by the powder mixing process, and produces silicon nitride with high thermal conductivity and stability.

[0016] 5. The silicon nitride ceramic material prepared by this invention has a microhardness ≥20GPa, a heat resistance temperature ≥1200℃, a bending strength ≥1000MPa, a yield rate of over 95%, and a thermal conductivity ≥65W / (m·K). Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0018] Figure 1 This is a cross-sectional view of the hot-pressed sintered graphite cavity assembly of the present invention.

[0019] Figure 2 This is a cross-sectional view of the pyrophyllite composite block cavity assembly in the process of this invention.

[0020] Figures 3-9 These are photos of the experimental process.

[0021] Figure label:

[0022] Graphite gasket 1; initial raw material block 2; graphite sleeve 3; pressure head 4; secondary raw material block 5; small graphite sheet 6; plug 7; graphite tube 8; large graphite sheet 9; copper sheet 10; conductive steel cap 11; pyrophyllite composite block 12. Detailed Implementation

[0023] To address the problems in the background art, this invention presents a method for preparing high thermal conductivity silicon nitride ceramics. This invention employs low-pressure, high-temperature sintering technology, using a graphite cavity as the reaction vessel. Large-particle silicon nitride seed crystals are prepared under vacuum conditions, followed by high-temperature, high-pressure sintering technology to allow silicon nitride to continue crystallizing within the pyrophyllite cavity under high pressure, thereby preparing high thermal conductivity silicon nitride ceramic materials. This method enables high-efficiency, mass production and industrial application.

[0024] The specific steps of this invention are as follows: using α-Si3N4 and β-Si3N4 powders (99.9%, particle size in the nanometer and micrometer ranges) as raw materials, and using fluorides (MgF2 or YF3, etc.) or Si or a mixture thereof (99.9%, particle size in the nanometer and micrometer ranges) as sintering aids, the materials are mixed in a certain proportion, vacuum ball milled for 24-48 hours to fully wet mix, and then dried in a vacuum drying oven to obtain the mixed raw materials. After the mixed powder is subjected to high temperature treatment (1500-2000℃) for 20 min-1 h in a low pressure (0-30MPa) environment in a hot press sintering furnace, the mixed raw materials are added again and vacuum ball milled for 10 min-1 h. The mixture is then sintered under high temperature and high pressure conditions (HPHT, 4.5-5.6GPa, 1350-1600℃) using a domestic six-sided hydraulic press to prepare silicon nitride ceramic materials with high thermal conductivity (microhardness ≥20GPa, heat resistance temperature ≥1200℃, flexural strength ≥1000MPa, yield rate ≥95%, thermal conductivity ≥65W / (m·K)). The method for preparing silicon nitride ceramics described in this invention involves high-temperature heat treatment, where sintering aids and silicon nitride particles interpenetrate, promoting a more uniform matrix composition and generating large silicon nitride grains as seed crystals. A secondary vacuum ball milling step effectively breaks down the high-temperature sintered sample, ensuring thorough mixing with the raw materials. Under high temperature and pressure, a high-purity silicon nitride ceramic sintered body is prepared. This method is highly operable, eliminates contamination introduced by the powder mixing process, and produces silicon nitride with high thermal conductivity and stability.

[0025] The present application will now be described in detail with reference to the accompanying drawings:

[0026] Figures 3-9 These are photos of the experimental process. Figure 3 The experimental photograph shows the first step of the process: pre-pressing of powder materials. Figure 4The experimental photograph shows the assembly of the hot-pressed sintered graphite cavity in step two. Figure 5 Step 3 of the experiment: Installation of the grinding jar for the high-energy ball mill. Figure 6 The experimental photograph shows the assembly of the cavity of the four-layer foliated phyllite composite block. Figure 7 Step five of the experimental photograph involves placing the pyrophyllite composite block and the six-sided top cylinder for air intake. Figure 8 Set the high-temperature and high-pressure synthesis parameters for step six of the experimental photographs. Figure 9 The experimental photographs are from step seven. After the experiment, bulk silicon nitride ceramic material was obtained.

[0027] Example 1

[0028] As attached Figure 1 , 2 As shown: 15g of α-Si3N4, β-Si3N4, and sintering aid (a mixture of MgF2, YF3, and Si) powder with a particle size of 3-5μm were selected as the initial raw material (the ratio of α-Si3N4 to β-Si3N4 was 3:1, and the ratio of the α-Si3N4 and β-Si3N4 mixture to the sintering aid (a mixture of MgF2, YF3, and Si) was 1:0.08). After thorough wet mixing by vacuum ball milling for 24 hours, the mixture was dried in a vacuum drying oven to obtain the final raw material. After being pre-pressed at 40MPa, the graphite gasket 1, the initial raw material block 2, and the graphite gasket 1 are placed in the reaction chamber consisting of the graphite sleeve 3 with an inner diameter of 35mm and the pressure head 4 (upper and lower pressure heads) in the order from bottom to top. After being heat-treated at 1700℃ and 20MPa for 30 minutes under vacuum, the secondary raw material is obtained by high-energy ball milling for 30 minutes. After being pre-pressed at 40MPa again, the secondary raw material block 5, the small graphite sheet 6, and the plug 7 are loaded into the graphite tube 8. The graphite tube 8 has an inner diameter of 17mm and a height of 16mm. The graphite cavity and components 9-11 are then placed into the pyrophyllite composite block 12, which has dimensions of 32.5mm×32.5mm×32.5mm and a cavity diameter of 18mm. The silicon nitride is sintered at high temperature and high pressure on a six-sided hydraulic press under the following conditions: 5GPa, 1450℃, sintering holding time of 30min, followed by slow cooling to room temperature. After depressurization, a silicon nitride ceramic block material with a diameter of 14mm×5mm is obtained.

[0029] Example 2

[0030] As attached Figure 1 , 2As shown: 15g of a mixture of α-Si3N4, β-Si3N4, and sintering aid YF3 with a particle size of 0.5-1μm was selected as the initial raw material (the ratio of α-Si3N4 to β-Si3N4 was 2:1, and the ratio of α-Si3N4 to β-Si3N4 and sintering aid YF3 was 1:0.02). After thorough wet mixing by vacuum ball milling for 24 hours, the mixture was dried in a vacuum drying oven to obtain the mixed raw material. After being pre-pressed at 40MPa, the graphite gasket 1, the initial raw material block 2, and the graphite gasket 1 are placed in the reaction chamber consisting of the graphite sleeve 3 with an inner diameter of 35mm and the pressure head 4 (upper and lower pressure heads) in the order from bottom to top. After being heat-treated at 1900℃ and 30MPa for 1 hour under vacuum, the secondary raw material is obtained by high-energy ball milling for 40 minutes. After being pre-pressed at 40MPa again, the secondary raw material block 5, the small graphite sheet 6, and the plug 7 are loaded into the graphite tube 8. The graphite tube 8 has an inner diameter of 17mm and a height of 16mm. The graphite cavity and components 9-11 are then placed into the pyrophyllite composite block 12. The dimensions of the pyrophyllite composite block 12 are 32.5mm×32.5mm×32.5mm, and the diameter of the composite cavity is 18mm. The silicon nitride is sintered at high temperature and high pressure on a six-sided hydraulic press. The sintering conditions are 5GPa, 1550℃, and sintering holding time of 1 hour. After sintering, the temperature is slowly reduced to room temperature. After depressurization, a silicon nitride ceramic block material with a diameter of 14mm×5mm is obtained.

[0031] Example 3

[0032] As attached Figure 1 , 2 As shown: 15g of β-Si3N4 powder with a particle size of 0.1-0.5μm and a mass of MgF2 sintering aid were selected as the initial raw material (the ratio of β-Si3N4 to MgF2 was 1:0.05). After thorough wet mixing by vacuum ball milling for 36 hours, the mixed raw material was dried in a vacuum drying oven. After pre-pressing at 40MPa, the graphite gasket 1, initial raw material block 2, and graphite gasket 1 were placed in the reaction chamber consisting of a 35mm inner diameter graphite sleeve 3 and pressure head 4 (upper and lower pressure heads) in the order of bottom to top. After vacuum heat treatment at 1500℃ and 10MPa for 1 hour, the secondary raw material was obtained by high-energy ball milling for 50 minutes. After pre-pressing again at 40MPa, the secondary raw material block 5, small graphite sheet 6, and plug 7 were loaded into the graphite tube 8, which has an inner diameter of 17mm and a height of 16mm. The graphite cavity and components 9-11 are then placed into the pyrophyllite composite block 12. The dimensions of the pyrophyllite composite block 12 are 32.5mm×32.5mm×32.5mm, and the diameter of the composite cavity is 18mm. The silicon nitride is sintered at high temperature and high pressure on a six-sided hydraulic press. The sintering conditions are 4.5GPa, 1600℃, and sintering holding time of 1 hour. After sintering, the temperature is slowly reduced to room temperature. After depressurization, a silicon nitride ceramic block material with a diameter of 14mm×5mm is obtained.

[0033] Example 4

[0034] As attached Figure 1 , 2 As shown: 15g of α-Si3N4, β-Si3N4, and sintering aid (a mixture of MgF2 and YF3) powder with a particle size of 10-15μm were selected as the initial raw material (the ratio of α-Si3N4 to β-Si3N4 was 4:1, and the ratio of the α-Si3N4 and β-Si3N4 mixture to the sintering aid (a mixture of MgF2 and YF3) was 1:0.1). After thorough wet mixing by vacuum ball milling for 48 hours, the mixture was dried in a vacuum drying oven to obtain the mixed raw material. After being pre-pressed at 40MPa, the graphite gasket 1, the initial raw material block 2, and the graphite gasket 1 are placed in the reaction chamber consisting of the graphite sleeve 3 with an inner diameter of 35mm and the pressure head 4 (upper and lower pressure heads) in the order from bottom to top. After being heat-treated at 2000℃ and 5MPa for 1 hour under vacuum, the secondary raw material is obtained by high-energy ball milling for 60 minutes. After being pre-pressed at 40MPa again, the secondary raw material block 5, small graphite sheets 6, and plugs 7 are loaded into the graphite tube 8. The graphite tube 8 has an inner diameter of 17mm and a height of 16mm. The graphite cavity and components 9-11 are then placed into the pyrophyllite composite block 12. The dimensions of the pyrophyllite composite block 12 are 32.5mm×32.5mm×32.5mm, and the diameter of the composite cavity is 18mm. The pyrophyllite composite block cavity is assembled using a "side-heating" heating method. The silicon nitride is sintered at high temperature and high pressure on a six-sided hydraulic press. The sintering conditions are 5.6GPa, 1350℃, and the sintering holding time is 1 hour. After sintering, the temperature is slowly reduced to room temperature. After depressurization, a silicon nitride ceramic block material with a diameter of 14mm×5mm is obtained.

[0035] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention; the actual structure is not limited thereto. Therefore, if those skilled in the art, inspired by this description, design similar structures and implementations to the above embodiments without departing from the technical essence of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A method for preparing silicon nitride ceramics with high thermal conductivity, characterized in that: The method includes the following steps: ① Select α-Si3N4, β-Si3N4 powder and sintering aid powder as raw materials; ② The mixed powder raw materials selected in ① above are mixed in a certain proportion, vacuum ball milled for 24-48 hours to fully wet mix, and then dried in a vacuum drying oven to obtain the mixed raw materials; ③ The mixed raw materials obtained in ② are pre-pressed and placed into a graphite mold. The mixed powder is then subjected to high-temperature treatment in a low-pressure environment through a hot-press sintering furnace. After that, the mixed raw materials are added again for secondary vacuum ball milling. ④ The secondary ball milling mixture obtained in ③ is pre-pressed into shape, pyrophyllite composite blocks are assembled, and sintered under high temperature and high pressure using a six-sided hydraulic press to complete the preparation of high thermal conductivity silicon nitride ceramics. In step ①, the sintering aid is MgF2 or YF3 or Si or a mixture thereof. In step ③, the low-pressure environment of the hot-pressing sintering furnace is 0-30MPa, the high-temperature treatment temperature is 1500-2000℃, and the treatment time is 20min-1h.

2. The method for preparing a high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: In step ①, the raw materials and sintering aids are all nano- or micron-sized powder materials, with a single particle size or a mixture of multiple particle sizes. The mass ratio of α-Si3N4 to β-Si3N4 is 0-4.0:1, and the mass ratio of the mixture of α-Si3N4 and β-Si3N4 to the sintering aid is 1:0.01-0.

1.

3. The method for preparing a high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: In step ③, the secondary ball milling is a vacuum high-energy ball milling, and the milling time is 10 min to 1 h.

4. The method for preparing a high thermal conductivity silicon nitride ceramic according to claim 1, characterized in that: In step ④, the secondary ball milled mixed raw materials are pre-pressed at 40MPa, the secondary raw material blocks are loaded into graphite tubes, the graphite tubes are placed into the cavity of the pyrophyllite composite block, and high-temperature and high-pressure sintering is carried out on a six-sided top press. The sintering conditions are 4.5-5.6GPa, 1350-1600℃, and the sintering holding time is 10min-2h.

5. The silicon nitride ceramic prepared by any one of the high thermal conductivity silicon nitride ceramic preparation methods according to claims 1-4 is used to manufacture cutting tools.

6. The silicon nitride ceramic prepared by any one of the high thermal conductivity silicon nitride ceramic preparation methods according to claims 1-4 is used to manufacture a thermally conductive substrate.

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