Preparation method of beta-SiC ceramic with low open porosity

Abstract

The invention discloses a preparation method of beta-SiC ceramic with low open porosity, and belongs to a preparation method of ceramic. The invention aims to solve the problem that the existing SiC sintering temperature is relatively high and cannot meet the preparation conditions of a nuclear fuel coated matrix material. The method comprises the following steps: 1, ball milling; 2, drying; 3, sintering; and 4, post-processing. The method is used for preparing the low-open-porosity beta-SiC ceramic.

Description

Technical Field

[0001] The invention relates to a method for preparing ceramics. Background Art

[0002] SiC ceramics are fine ceramics with good thermal stability, excellent high-temperature mechanical properties and high-temperature creep resistance. Among them, β-SiC ceramics have unique advantages such as radiation resistance, high thermal conductivity and high hardness. Therefore, they are widely used in the nuclear industry, and as a coating material to prevent nuclear fuel leakage is one of the important application scenarios. Low open porosity can ensure that the coated nuclear fuel will not leak out along the open pores in the ceramic matrix while ensuring the mechanical properties of the material, which can effectively improve the safety of nuclear fuel and greatly reduce the risk of nuclear leakage. However, the free mobility of carbon atoms and silicon atoms in SiC is extremely low. During sintering, the sintering temperature usually needs to be increased to increase the sintering driving force to achieve densification. However, on the other hand, β-SiC belongs to the low-temperature phase of silicon carbide. When the temperature is increased for sintering, it is often accompanied by a phase transition from β phase to α phase. At the same time, the fuel particles coated inside it cannot withstand the corresponding sintering temperature. Summary of the invention

[0003] The present invention aims to solve the problem that the existing SiC sintering temperature is high and cannot meet the preparation conditions of nuclear fuel coating matrix materials, and further provides a preparation method of low open porosity β-SiC ceramics.

[0004] A method for preparing low-open-porosity β-SiC ceramics is carried out according to the following steps:

[0005] 1. Ball milling:

[0006] Ball milling β-SiC nanopowder, sintering aid and deionized water to obtain slurry;

[0007] The β-SiC nanopowder is one or a combination of β-SiC nanopowder with a median particle size of 60 nm and β-SiC nanopowder with a median particle size of 3 μm;

[0008] 2. Drying:

[0009] The slurry is stirred and evaporated to dryness, and then dried and ground to obtain a dry powder;

[0010] 3. Sintering:

[0011] The dried powder is placed in a graphite mold, heated to the sintering temperature under mechanical pressure, then sintered, and finally cooled to room temperature and demolded to obtain a sintered sample;

[0012] 4. Post-processing:

[0013] The sintered sample is ground and polished to complete the preparation method of low open porosity β-SiC ceramics.

[0014] The beneficial effects of the present invention are:

[0015] 1. The present invention develops a method for preparing high thermal conductivity β-SiC ceramics with simple operation and rapidity, using high-purity β-SiC with different particle sizes as raw materials, and preparing β-SiC ceramics with low open porosity through mixing, sintering and post-treatment;

[0016] Second, the present invention adjusts the particle size and mixing ratio of the β-SiC powder in the raw material, the sintering temperature and time and other factors to adjust the grain size, density and density of the internal defects of the grains of the obtained β-SiC ceramic, and quickly obtains high thermal conductivity β-SiC ceramics with adjustable thermal conductivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 TEM results of the low open porosity β-SiC ceramic prepared in Example 1;

[0018] Figure 2 The XRD results of the low open porosity β-SiC ceramic prepared in Example 1;

[0019] Figure 3 The cross-sectional SEM result of the low open porosity β-SiC ceramic prepared in Example 2;

[0020] Figure 4 The thermal conductivity results of the low open porosity β-SiC ceramics prepared in Example 2;

[0021] Figure 5 TEM results of the low open porosity β-SiC ceramic prepared in Example 3;

[0022] Figure 6 The XRD results of the low open porosity β-SiC ceramic prepared in Example 3;

[0023] Figure 7 The cross-sectional SEM result of the low open porosity β-SiC ceramic prepared in Example 3;

[0024] Figure 8 The thermal conductivity results of the low open porosity β-SiC ceramics prepared in Example 3;

[0025] Fig. 9 The low open porosity β-SiC ceramic prepared in Example 3 was subjected to 1×10 15 cm -2 High-resolution transmission (HR-TEM) images after fluence irradiation;

[0026] Fig.10Density and open porosity data of the low open porosity β-SiC ceramics prepared in Examples 1 to 3. DETAILED DESCRIPTION

[0027] Specific implementation method 1: This implementation method is a method for preparing low-open-porosity β-SiC ceramics, which is carried out according to the following steps:

[0028] 1. Ball milling:

[0029] Ball milling β-SiC nanopowder, sintering aid and deionized water to obtain slurry;

[0030] The β-SiC nanopowder is one or a combination of β-SiC nanopowder with a median particle size of 60 nm and β-SiC nanopowder with a median particle size of 3 μm;

[0031] 2. Drying:

[0032] The slurry is stirred and evaporated to dryness, and then dried and ground to obtain a dry powder;

[0033] 3. Sintering:

[0034] The dried powder is placed in a graphite mold, heated to the sintering temperature under mechanical pressure, then sintered, and finally cooled to room temperature and demolded to obtain a sintered sample;

[0035] 4. Post-processing:

[0036] The sintered sample is ground and polished to complete the preparation method of low open porosity β-SiC ceramics.

[0037] This specific embodiment proposes a method for preparing β-SiC with low open porosity at a relatively low temperature to ensure nuclear safety under actual working conditions. The raw materials used are one or more high-purity β-SiC powders with different particle sizes. During the sintering process, the particle size and ratio of the raw materials used can be adjusted to adjust the grain size of the obtained β-SiC ceramic and the density of defects inside the grains, so that high thermal conductivity β-SiC ceramics with adjustable thermal conductivity can be quickly obtained. In addition, the densification sintering of β-SiC is achieved, and the open porosity of the sintered body is maintained at a relatively low level.

[0038] The beneficial effects of this embodiment are:

[0039] 1. This embodiment develops a method for preparing high thermal conductivity β-SiC ceramics with simple operation and rapidity, using high-purity β-SiC with different particle sizes as raw materials, and preparing low open porosity β-SiC ceramics through mixing, sintering and post-treatment;

[0040] 2. This embodiment adjusts the particle size and mixing ratio of the β-SiC powder in the raw material, the sintering temperature and time, etc., to adjust the grain size, density and density of the internal defects of the grains of the obtained β-SiC ceramic, and quickly obtain a high thermal conductivity β-SiC ceramic with adjustable thermal conductivity.

[0041] Specific embodiment 2: This embodiment is different from the specific embodiment 1 in that: the mass ratio of the β-SiC nanopowder with a median particle size of 60nm and the β-SiC nanopowder with a median particle size of 3μm described in step 1 is (0-9):1, and the purity of the β-SiC nanopowder is ≥99.9%; the sintering aid described in step 1 is B 4 C and C are mixed, and B 4 The mass ratio of C to C is 1:(1-4). Others are the same as the first embodiment.

[0042] Specific implementation method 3: This implementation method is different from specific implementation method 1 or 2 in that the mass of the sintering aid described in step 1 is 0.5% to 5% of the mass of the β-SiC nanopowder. The rest is the same as specific implementation method 1 or 2.

[0043] Specific embodiment 4: This embodiment differs from specific embodiments 1 to 3 in that the mass of the deionized water in step 1 is 1 to 9 times the mass of the β-SiC nanopowder. The rest is the same as specific embodiment 3.

[0044] Specific embodiment 5: This embodiment differs from specific embodiments 1 to 4 in that: in step 1, β-SiC nanopowder, sintering aid and deionized water are added to a polytetrafluoroethylene ball mill. 2 O 3 The small balls are used as grinding media. The ball milling is carried out for 3h to 20h under the conditions of a ball-to-material mass ratio of (9 to 15):1 and a ball milling speed of 150r / min to 400r / min. The grinding media is composed of Al2O3 with diameters of 5.5mm to 6.5mm, 9.5mm to 10.5mm and 14.5mm to 15.5mm. 2 O 3 The small balls are mixed in a mass ratio of 3:(2-5):(4-8). The rest is the same as the first to fourth embodiments.

[0045] Specific embodiment 6: This embodiment is different from specific embodiments 1 to 5 in that: in step 2, the slurry is stirred and evaporated to dryness at a temperature of 80°C to 100°C and a stirring speed of 200r / min to 400r / min, and then dried at a temperature of 60°C to 100°C for 8h to 24h, and finally ground and passed through a 360-mesh sieve to obtain a dried powder. The rest is the same as specific embodiments 1 to 5.

[0046] Specific embodiment 7: This embodiment differs from Specific embodiments 1 to 6 in that: the graphite mold described in step 3 is padded with 1 to 3 layers of graphite paper with a thickness of 0.1 mm to 0.5 mm. The rest is the same as Specific embodiments 1 to 6.

[0047] Specific embodiment 8: This embodiment differs from specific embodiments 1 to 7 in that in step 3, the temperature is raised to the sintering temperature in a vacuum or Ar environment, then sintered, and finally cooled to room temperature. The rest is the same as specific embodiments 1 to 7.

[0048] Specific embodiment 9: This embodiment differs from specific embodiments 1 to 8 in that: in step 3, under the condition of a mechanical pressure of 10MPa to 70MPa, the temperature is increased to a sintering temperature of 1700°C to 1900°C at a heating rate of 2°C / min to 10°C / min, and the sintering temperature is 1700°C to 1900°C under the condition of a mechanical pressure of 10MPa to 70MPa and a sintering temperature of 1700°C to 1900°C for 20min to 120min, and finally, under the condition of a mechanical pressure of 10MPa to 70MPa, the temperature is reduced to 300°C at a cooling rate of 2°C / min to 10°C / min, and then, under the condition of a mechanical pressure of 10MPa to 70MPa, the temperature is cooled to room temperature with the furnace. Others are the same as specific embodiments 1 to 8.

[0049] Specific embodiment 10: This embodiment differs from specific embodiments 1 to 9 in that: in step 4, abrasives are used for grinding until the external graphite paper is removed, and then a diamond grinding tool with 80-3000 mesh is used to remove surface scratches from coarse to fine, and finally a diamond polishing liquid is used for polishing; the abrasive is Al 2 O 3 , SiC, diamond or B 4 C. The rest is the same as Specific Embodiments 1 to 9.

[0050] The following examples are used to verify the beneficial effects of the present invention:

[0051] Embodiment 1:

[0052] A method for preparing low-open-porosity β-SiC ceramics is carried out according to the following steps:

[0053] 1. Ball milling:

[0054] β-SiC nanopowder, sintering aid and deionized water were added into a polytetrafluoroethylene ball mill. 2 O 3 The small balls were used as grinding media. Under the conditions of ball-to-material mass ratio of 10:1 and ball milling speed of 300 r / min, the slurry was obtained by ball milling for 10 h.

[0055] The β-SiC nanopowder is a combination of β-SiC nanopowder with a median particle size of 60 nm and β-SiC nanopowder with a median particle size of 3 μm;

[0056] The mass ratio of the β-SiC nanopowder with a median particle size of 60 nm to the β-SiC nanopowder with a median particle size of 3 μm is 3:1, and the purity of the β-SiC nanopowder is ≥ 99.9%; the sintering aid is B 4 C and C are mixed, and B 4 The mass ratio of C to C is 1:2;

[0057] The mass of the sintering aid is 1.5% of the mass of the β-SiC nanopowder;

[0058] The mass of the deionized water is 4 times the mass of the β-SiC nanopowder;

[0059] The grinding media are made of Al2O3 with diameters of 6 mm, 10 mm and 15 mm. 2 O 3 The pellets are mixed in a mass ratio of 3:4:6;

[0060] 2. Drying:

[0061] The slurry was stirred and evaporated to dryness at a temperature of 100°C and a stirring speed of 250 r / min, and then dried at a temperature of 60°C for 24 hours, and finally ground and passed through a 360-mesh sieve to obtain a dried powder;

[0062] 3. Sintering:

[0063] The dried powder was placed in a graphite mold, and the temperature was increased to 1900°C at a heating rate of 10°C / min under the conditions of Ar environment and mechanical pressure of 50MPa, and sintered for 30min under the conditions of Ar environment, mechanical pressure of 50MPa and sintering temperature of 1900°C, and finally cooled to 300°C at a cooling rate of 10°C / min under the conditions of Ar environment and mechanical pressure of 50MPa, and then cooled to room temperature and demolded under the conditions of Ar environment and mechanical pressure of 50MPa, to obtain a sintered sample;

[0064] 4. Post-processing:

[0065] The sintered sample was ground and polished to obtain low open porosity β-SiC ceramics.

[0066] The inner diameter of the graphite mold described in step 3 is 20 mm, and the inside of the graphite mold is padded with 3 layers of graphite paper with a thickness of 0.1 mm.

[0067] In step 4, abrasives are used for grinding until the external graphite paper is removed, and then an 80-mesh diamond grinding disc is used to grind in two vertical directions for 10 minutes by cross-grinding method, and then 250 mesh, 500 mesh, 800 mesh, 1500 mesh and 3000 mesh diamond grinding discs are replaced in sequence to repeat the grinding to remove surface scratches, and finally diamond polishing liquid is used to polish to a roughness of 100 nm; the abrasive is B 4 C.

[0068] Embodiment 2: This embodiment differs from Embodiment 1 in that the β-SiC nanopowder described in step 1 is a β-SiC nanopowder with a median particle size of 3 μm. The rest is the same as Embodiment 1.

[0069] Embodiment 3: The difference between this embodiment and embodiment 1 is that in step 3, the temperature is raised to the sintering temperature under vacuum environment, then sintered, and finally cooled to room temperature. The rest is the same as embodiment 1.

[0070] Figure 1 TEM results of the low open porosity β-SiC ceramic prepared in Example 1; there are a large number of micron-sized and submicron-sized grains in the ceramic. 50 =60nm nanopowder appears, resulting in a considerable number of nanocrystals in the grains. In addition, the figure shows that there are a large number of dislocations (black lines) inside the grains, which may be caused by the sintering and growth of different nanocrystals during the sintering process.

[0071] Figure 2 The XRD results of the low open porosity β-SiC ceramic prepared in Example 1; the main crystalline phase in the sintered ceramic is β-SiC, indicating that no phase transition from β phase to α phase occurs under the sintering condition of 1900°C, and the detected Al 2 O 3 It mainly comes from the Al 2 O 3 Debris.

[0072] Figure 3 This is the cross-sectional SEM result of the low open porosity β-SiC ceramic prepared in Example 2; the fracture surface shows obvious intergranular fracture morphology.

[0073] Figure 4 The thermal conductivity of the low-open-porosity β-SiC ceramic prepared in Example 2; the thermal diffusion coefficient reaches 42.680±0.151 mm 2 / s, the heat capacity reaches 0.646±0.004J / (g·K), and the thermal conductivity reaches 87.724±0.310W / (m·K).

[0074] Figure 5TEM results of low open porosity β-SiC ceramics prepared in Example 3; the grain size is similar to that in Example 1 ( Figure 1 ) are basically similar, and there are also a large number of dislocations inside the grains.

[0075] Figure 6 This is the XRD result of the low open porosity β-SiC ceramic prepared in Example 3; after sintering, SiC does not undergo a phase transition from β phase to α phase.

[0076] Figure 7 This is the cross-sectional SEM result of the low open porosity β-SiC ceramic prepared in Example 3; the cross section shows an intergranular fracture morphology.

[0077] Figure 8 The thermal conductivity of the low-open-porosity β-SiC ceramic prepared in Example 3; the thermal diffusion coefficient reaches 31.394±0.195 mm 2 / s, the heat capacity reaches 0.681±0.003J / (g·K), and the thermal conductivity reaches 67.645±0.421W / (m·K).

[0078] Fig. 9 The low open porosity β-SiC ceramic prepared in Example 3 was subjected to 1×10 15 cm -2 High-resolution transmission (HR-TEM) image after electron irradiation; after electron irradiation, point defects (bright white spots in HR-TEM) were observed in the SiC lattice.

[0079] Fig.10 The density and open porosity data of the low open porosity β-SiC ceramics prepared in Examples 1 to 3 are measured according to BS EN 1389-2003 (Advanced technical ceramics-Ceramic composites-Physical properties-Determination of density and apparent porosity). The results show that the preparation method of the examples can maintain the open porosity of the final ceramics at a relatively low level, and the open porosity of all examples is less than 0.3%.

[0080] Table 1 shows the low open porosity β-SiC ceramics prepared in Example 3 after 1×10 15 cm -2 Table of changes in mechanical properties before and after injection irradiation; the results show that after irradiation, the mechanical properties of ceramic samples decreased to varying degrees.

[0081] Table 1

[0082]

Claims

1. A method for preparing low-open-porosity β-SiC ceramics, characterized in that It is carried out in the following steps:

1. Ball milling: Ball milling β-SiC nanopowder, sintering aid and deionized water to obtain slurry; The β-SiC nanopowder is one or a combination of β-SiC nanopowder with a median particle size of 60 nm and β-SiC nanopowder with a median particle size of 3 μm; 2. Drying: The slurry is stirred and evaporated to dryness, and then dried and ground to obtain a dry powder; 3. Sintering: The dried powder is placed in a graphite mold, heated to the sintering temperature under mechanical pressure, then sintered, and finally cooled to room temperature and demolded to obtain a sintered sample; 4. Post-processing: The sintered sample is ground and polished to complete the preparation method of low open porosity β-SiC ceramics.

2. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that The mass ratio of the β-SiC nanopowder with a median particle size of 60 nm to the β-SiC nanopowder with a median particle size of 3 μm described in step 1 is (0-9):1, and the purity of the β-SiC nanopowder is ≥99.9%; the sintering aid described in step 1 is a mixture of B4C and C, and the mass ratio of B4C to C is 1:(1-4).

3. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that The mass of the sintering aid described in step 1 is 0.5% to 5% of the mass of the β-SiC nanopowder.

4. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that The mass of the deionized water described in step 1 is 1 to 9 times the mass of the β-SiC nanopowder.

5. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that In step 1, β-SiC nanopowder, sintering aid and deionized water are added to a polytetrafluoroethylene ball mill, and Al2O3 balls are used as grinding media. The grinding process is performed for 3 hours to 20 hours at a ball-to-material mass ratio of (9 to 15):1 and a ball mill speed of 150 r / min to 400 r / min. The grinding media is formed by mixing Al2O3 balls with diameters of 5.5 mm to 6.5 mm, 9.5 mm to 10.5 mm and 14.5 mm to 15.5 mm in a mass ratio of 3:(2 to 5):(4 to 8).

6. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that In step 2, the slurry is stirred and evaporated to dryness at a temperature of 80°C to 100°C and a stirring speed of 200r / min to 400r / min, and then dried at a temperature of 60°C to 100°C for 8h to 24h, and finally ground and passed through a 360-mesh sieve to obtain a dried powder.

7. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that The graphite mold described in step 3 is padded with 1 to 3 layers of graphite paper with a thickness of 0.1 mm to 0.5 mm.

8. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that In step three, the temperature is raised to the sintering temperature in a vacuum or Ar environment, then sintered, and finally cooled to room temperature.

9. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that In step three, under the condition of a mechanical pressure of 10MPa to 70MPa, the temperature is increased to a sintering temperature of 1700°C to 1900°C at a heating rate of 2°C / min to 10°C / min, and the sintering temperature is 1700°C to 1900°C at a mechanical pressure of 10MPa to 70MPa and sintering temperature of 1700°C to 1900°C for 20min to 120min. Finally, under the condition of a mechanical pressure of 10MPa to 70MPa, the temperature is reduced to 300°C at a cooling rate of 2°C / min to 10°C / min, and then cooled to room temperature with the furnace under the condition of a mechanical pressure of 10MPa to 70MPa.

10. The method for preparing a low open porosity β-SiC ceramic according to claim 1, characterized in that In step 4, abrasives are used for grinding until the external graphite paper is removed, and then a diamond grinding tool with a mesh size of 80 to 3000 is used to remove surface scratches from coarse to fine, and finally a diamond polishing liquid is used for polishing; the abrasives are Al2O3, SiC, diamond or B4C.

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