Preparation method of pressureless sintered silicon carbide porous ceramic

Abstract

The invention discloses a preparation method of pressureless sintered silicon carbide porous ceramics, which comprises the following steps: mixing silicon dioxide powder with the particle size of 0.5-2 mu m and silica powder with the particle size of 0.1-1 mu m to prepare pore-forming agent ceramic powder, adding an organic additive, ammonia water, ammonium polyacrylate, n-caprylic alcohol and sodium tripolyphosphate to prepare pore-forming agent granulation powder, pressing and crushing to obtain pore-forming agent particles, and sintering the pore-forming agent particles to obtain the pressureless sintered silicon carbide porous ceramics. And pressing and drying with silicon carbide granulation powder, and carrying out pressureless sintering to obtain the silicon carbide porous ceramic. The preparation process is simple, the obtained silicon carbide porous ceramic has excellent mechanical properties and high porosity, the pore size and porosity of the ceramic can be accurately controlled according to the particle size and the addition amount of the added pore forming agent particles, and the application requirements of different scenes are met; secondly, low-pressure pressing and pressureless sintering forming are adopted, damage to the surface of the product can be effectively reduced, energy loss is small, the production and manufacturing cost is remarkably reduced, and the application range is wide.

Description

Technical Field

[0001] The invention belongs to the field of porous ceramic materials, and in particular relates to a method for preparing pressureless sintered silicon carbide porous ceramics. Background Art

[0002] As a new type of inorganic membrane material matrix, silicon carbide porous ceramics have a huge specific surface area, high temperature resistance, wear resistance, excellent chemical stability, low density and good impact resistance. It is a ceramic material with both structural and functional properties, and is widely used in aerospace, chemical industry, bioenergy and other fields. However, due to its high brittleness and high hardness, silicon carbide porous ceramics are difficult to form and process. The forming process is the key technology for the preparation of silicon carbide ceramics and is the basic factor affecting the manufacturing cost and product yield of silicon carbide ceramics. Improving the forming process is particularly critical. At present, the production and preparation of silicon carbide porous ceramics still have problems such as high cost, complex process, and long preparation cycle. In particular, the preparation method of the pore-forming agent also has the disadvantage of complex preparation process. The mixed addition of multiple substances makes the prepared silicon carbide ceramics low in purity and relatively weak in corrosion resistance, which will undoubtedly hinder the development of silicon carbide ceramic technology to a certain extent.

[0003] Silicon carbide porous ceramics prepared by the traditional slurry method have a large number of surface defects and hollow pores formed in the porous ceramics due to the high-temperature decomposition of the template skeleton, which seriously affects the mechanical properties of the silicon carbide porous ceramics. In order to improve this defect, the existing technology improves the traditional method, such as adding silicon dioxide and polycarbosilane to silicon carbide powder, and making high-strength silicon carbide porous ceramics after dry pressing and sintering. However, this requires sintering under high pressure and high temperature, and the process is relatively complicated and not conducive to large-scale industrial production; or silicon powder, alumina powder and aluminum sol can be added to the silicon carbide slurry, and the organic foam impregnation method can be used to prepare the silicon carbide porous carrier. However, this method is difficult to control the coarseness and porosity of the silicon carbide skeleton, which is easy to cause pore blockage and reduce pore connectivity, and the purity of the silicon carbide ceramics is greatly affected.

[0004] CN102503521A discloses a method for preparing high-strength silicon carbide porous ceramics, by adding aluminum disilicide and polycarbosilane to silicon carbide powder, dry pressing and two-step sintering, to obtain high-strength silicon carbide porous ceramics. The added aluminum disilicide can promote the growth of silicon carbide grains, and polycarbosilane can properly form pores and control the porosity of the material. However, the powder of this technology needs to be dry pressed at 50MPa, which not only consumes a lot of energy, but also has a relatively complicated process, which greatly increases the preparation cost of silicon carbide porous ceramics.

[0005] CN118930312A discloses a method for preparing a silicon carbide porous ceramic material and its application. The method comprises the following steps: mixing silicon carbide and a porogen, ultrasonically dispersing the mixture in deionized water, and then filtering and drying the mixture to obtain a mixed powder; pressing the mixed powder under 300-400 MPa to obtain a ceramic matrix; pre-sintering the ceramic matrix at 800-900°C and then cooling the ceramic matrix to obtain a porous matrix; boiling the porous matrix under pressure in deionized water for 1-2 hours and then cooling the porous matrix to obtain a porous intermediate; sintering the porous intermediate at 1300-1400°C in an inert atmosphere to obtain a porous intermediate; After forming, cooling is performed to obtain a ceramic intermediate; boron oxide powder is spread on the upper surface of the ceramic intermediate, the temperature is raised to 450-500°C, and the temperature is kept for 2-3 hours, and then centrifuged to obtain a ceramic composite; the ceramic composite is nitrided at 1400-1600°C in a nitrogen atmosphere, and then cooled to obtain a silicon carbide porous ceramic material. The obtained silicon carbide porous ceramic material has an apparent porosity of up to 70% and a mechanical property of about 40%. However, the preparation process of the silicon carbide porous ceramic material and the porogen of the invention is complicated, which reduces the production efficiency and significantly increases the preparation cost of the silicon carbide porous ceramic.

[0006] Based on the above problems existing in the preparation of silicon carbide porous ceramics, a silicon carbide porous ceramic with simple preparation process and excellent performance is developed to meet different application requirements, which plays an important role in the transformation and upgrading of the silicon carbide porous ceramic field. Summary of the invention

[0007] The present invention provides a method for preparing pressureless sintered silicon carbide porous ceramics, wherein silicon dioxide powder with a particle size of 0.5-2 μm and silicon micropowder with a particle size of 0.1-1 μm are mixed to form pore-forming agent ceramic powder, and organic additives, ammonia water, ammonium polyacrylate, n-octanol, and sodium tripolyphosphate are added to form pore-forming agent granulation powder, which is pressed and crushed to obtain pore-forming agent particles, and then pressed, dried, and pressureless sintered with silicon carbide granulation powder to obtain silicon carbide porous ceramics. The preparation process of the present invention is simple, and the obtained silicon carbide porous ceramics have excellent mechanical properties and high porosity, and the pore diameter and porosity of the ceramics can be accurately controlled according to the particle size and amount of the added pore-forming agent particles, so as to meet the application requirements of different scenarios, and the pressureless sintering meets the production requirements of silicon carbide porous ceramics with high efficiency and high energy saving.

[0008] To achieve the above object, the present invention provides the following technical solutions:

[0009] A method for preparing pressureless sintered silicon carbide porous ceramics, the method comprising the following steps:

[0010] (1) mixing silicon dioxide powder and silicon micropowder in a certain weight ratio to form pore-forming ceramic powder;

[0011] (2) mixing the organic additive and distilled water and grinding them thoroughly, adding the pore-forming agent ceramic powder, ammonia water, ammonium polyacrylate and n-octanol prepared in step (1) and continuing grinding, spray granulating the obtained slurry to obtain a pore-forming agent granulated powder, and then spraying sodium tripolyphosphate into the pore-forming agent granulated powder while stirring;

[0012] (3) pressing the pore-forming agent granulated powder prepared in step (2) into a green body, and then crushing and sieving to obtain pore-forming agent particles;

[0013] (4) The pore-forming agent particles prepared in step (3) are mixed with silicon carbide granulated powder, pressed into a silicon carbide porous ceramic body, dried, and fired to obtain a pressureless sintered silicon carbide porous ceramic.

[0014] Furthermore, in step (1), the silicon dioxide powder and silicon micropowder are 60 parts by weight and 28 parts by weight respectively, the particle size of the silicon dioxide powder is 0.5-2 μm; the particle size of the silicon micropowder is 0.1-1 μm.

[0015] Furthermore, in step (2), the organic additive, distilled water, pore-forming ceramic powder, ammonia water, ammonium polyacrylate, and n-octanol are added in amounts of 2-3.5 parts, 100-120 parts, 90-110 parts, 0.5-2 parts, 1-5 parts, and 0.3-1 parts by weight, respectively; the sufficient grinding time is 1-3 hours; and the continued grinding time is 30-50 hours.

[0016] Furthermore, in step (2), the sodium tripolyphosphate is uniformly attached to the surface of the pore-forming agent granulation powder.

[0017] Furthermore, the pressing pressure in step (3) is 1-10 MPa; and the particle size of the pore-forming agent particles is 0.05-2 mm.

[0018] Furthermore, in step (4), the pore-forming agent particles and silicon carbide granulation powder are 30-80 parts and 90-110 parts by weight respectively; the pressing pressure is 2-15 MPa; the drying temperature is 80-150° C., and the drying time is 5-10 hours.

[0019] Furthermore, the firing temperature in step (4) is 2100-2200° C. and the firing time is 3-5 hours.

[0020] Furthermore, the preparation method of the organic additive in step (2) is: 40 parts by weight of dextrin, 14 parts by weight of polyvinyl alcohol, 14 parts by weight of polyethylene glycol, and 22.5 parts by weight of glycerol are uniformly mixed to prepare the organic additive.

[0021] Furthermore, the preparation method of the silicon carbide granulated powder in step (4) is:

[0022] ① 100 parts by weight of silicon carbide powder, 1.5 parts by weight of carbon black, 7.5 parts by weight of boron carbide and 2 parts by weight of graphite powder are uniformly mixed to prepare silicon carbide ceramic powder;

[0023] ② Add 2.75 parts of organic additives and 110 parts of distilled water by weight into a ball mill and grind for 1 hour to fully dissolve the organic additives and mix them evenly, then add 100 parts of silicon carbide ceramic powder prepared in step ①, 1.25 parts of ammonia water, 3 parts of polyacrylic acid, and 0.65 parts of n-octanol and continue grinding for 30 hours to obtain a slurry; transfer the slurry to a stirring barrel, add 0.3% and 0.1% of ammonium polyacrylate and defoaming agent n-octanol in the stirring barrel, stir for 4 hours, use a spray granulator to granulate, then put the granulated powder into a mixer and spray the release agent sodium tripolyphosphate while stirring, so that the release agent is evenly attached to the surface of the granulated powder to make silicon carbide granulated powder.

[0024] The preparation methods of the organic additives and silicon carbide granulated powder can be found in CN119100799A, an ultra-thick silicon carbide ceramic plate and its preparation method.

[0025] The invention also provides pressureless sintered silicon carbide porous ceramics prepared by the preparation method.

[0026] Beneficial effects:

[0027] (1) The pore-forming agent ceramic powder of the present invention is made by uniformly mixing micro-sized silica powder and silicon micropowder. During the firing process at 1200℃-1400℃, the silica powder and silicon micropowder generate silicon monoxide gas and volatilize, that is, SiO2+Si→2SiO↑, leaving voids in the product, thereby forming a porous structure. The use of micro-sized silica powder and silicon micropowder can promote the full progress of the reaction, ensure a high porosity, and have a simple preparation process. Based on the above reaction process, the pore size and porosity of silicon carbide porous ceramics can be precisely controlled by changing the particle size and addition amount of the pore-forming agent particles, and silicon carbide porous ceramics with different levels of mechanical properties and porosity can be obtained to meet the needs of different scenarios and expand the scope of application.

[0028] (2) Silicon monoxide (SiO) deposited at room temperature during the firing process of the present preparation method is a chemical raw material with high added value. When deposited at room temperature, it can be used as a negative electrode material to achieve efficient utilization of reaction waste.

[0029] (3) Compared with the existing preparation method of silicon carbide porous ceramics, the preparation process of the present invention is simple, and it can be pressed and formed within a pressure range of 1-15MPa, with low container loss and energy loss, which significantly reduces the production cost; secondly, pressureless sintering can effectively reduce product surface damage and improve the quality of ceramic products, which meets the high-efficiency and high-energy-saving production requirements of silicon carbide porous ceramics. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 This is a cross-sectional morphology diagram of pressureless sintered silicon carbide porous ceramics. DETAILED DESCRIPTION

[0031] The following will combine the contents in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] Preparation of experimental materials:

[0033] (1) Preparation of organic additives: 40 parts by weight of dextrin, 14 parts by weight of polyvinyl alcohol, 14 parts by weight of polyethylene glycol, and 22.5 parts by weight of glycerol were uniformly mixed to prepare an organic additive.

[0034] (2) Preparation of silicon carbide granulated powder:

[0035] ① 100 parts by weight of silicon carbide powder, 1.5 parts by weight of carbon black, 7.5 parts by weight of boron carbide and 2 parts by weight of graphite powder are uniformly mixed to prepare silicon carbide ceramic powder;

[0036] ② Add 2.75 parts of the organic additive prepared above and 110 parts of distilled water by weight into a ball mill and grind for 1 hour to make the organic additive fully dissolved and mixed evenly, then add 100 parts of silicon carbide ceramic powder, 1.25 parts of ammonia water, 3 parts of polyacrylic acid, and 0.65 parts of n-octanol and continue grinding for 30 hours to obtain a slurry; transfer the slurry to a stirring barrel, add 0.3% and 0.1% of ammonium polyacrylate and defoaming agent n-octanol in the stirring barrel respectively, and stir for 4 hours, use a spray granulator to granulate, and then put the granulated powder into a mixer and spray the release agent sodium tripolyphosphate while stirring, so that the release agent is evenly attached to the surface of the granulated powder to make silicon carbide granulated powder.

[0037] Example 1

[0038] 1. A method for preparing pressureless sintered silicon carbide porous ceramics, comprising the following steps:

[0039] (1) 60 parts by weight of silicon dioxide powder with a particle size of 0.8 μm and 28 parts by weight of silicon micropowder with a particle size of 0.4 μm are uniformly mixed to prepare a pore-forming ceramic powder;

[0040] (2) 3 parts of organic additives and 115 parts of distilled water are added to a ball mill by weight and ground for 1.5 hours to fully dissolve the organic additives and mix them evenly. Then, 100 parts of the pore-forming agent ceramic powder prepared in step (1), 1.5 parts of ammonia water, 4 parts of ammonium polyacrylate, and 0.8 parts of defoaming agent n-octanol are added and grinding is continued for 40 hours. The obtained slurry is subjected to spray granulation to obtain a pore-forming agent granulated powder. The pore-forming agent granulated powder is then placed in a mixer and sprayed with a release agent, sodium tripolyphosphate, while stirring, so that the release agent is evenly attached to the surface of the pore-forming agent granulated powder to obtain the pore-forming agent granulated powder;

[0041] (3) pressing the pore-forming agent granulated powder prepared in step (2) into a green body at 5.5 MPa, and then crushing and sieving to obtain pore-forming agent particles with a particle size of 0.1 mm;

[0042] (4) 30 parts by weight of the pore-forming agent particles obtained in step (3) and 100 parts by weight of silicon carbide granulated powder are prepared into a silicon carbide porous ceramic body under a pressure of 2 MPa, and then dried at 80°C for 5 hours, and then fired at 2100°C for 4 hours in a vacuum atmosphere to obtain a pressureless sintered silicon carbide porous ceramic.

[0043] 2. Performance Analysis

[0044] The performance of the silicon carbide porous ceramics obtained above was tested and analyzed, and the specific testing methods are as follows: Compressive strength test: The silicon carbide porous ceramic material prepared above was made into a test sample of 30mm×30mm×55mm, and a universal testing machine was used to apply a compressive load at a rate of 0.5m / min until the test sample was destroyed. The maximum load was recorded, which is the compressive strength; the porosity was tested by the water absorption method; and the pore size was tested by the bubble method.

[0045] The compressive strength of the silicon carbide porous ceramic prepared in this embodiment is tested to be 98Mpa; the porosity is 29.6%; the average pore size is 0.04mm; the cross-sectional morphology of the pressureless sintered silicon carbide porous ceramic is as shown in the figure Figure 1 As shown, it can be seen that the pores are evenly distributed and the pore sizes are uniform.

[0046] Example 2

[0047] 1. A method for preparing pressureless sintered silicon carbide porous ceramics, comprising the following steps:

[0048] (1) 60 parts by weight of silicon dioxide powder with a particle size of 0.8 μm and 28 parts by weight of silicon micropowder with a particle size of 0.4 μm are uniformly mixed to prepare a pore-forming ceramic powder;

[0049] (2) 3 parts of organic additives and 115 parts of distilled water are added to a ball mill by weight and ground for 1.5 hours to fully dissolve the organic additives and mix them evenly. Then, 100 parts of the pore-forming agent ceramic powder prepared in step (1), 1.5 parts of ammonia water, 4 parts of ammonium polyacrylate, and 0.8 parts of defoaming agent n-octanol are added and grinding is continued for 40 hours. The obtained slurry is subjected to spray granulation to obtain a pore-forming agent granulated powder. The pore-forming agent granulated powder is then placed in a mixer and sprayed with a release agent, sodium tripolyphosphate, while stirring, so that the release agent is evenly attached to the surface of the pore-forming agent granulated powder to obtain the pore-forming agent granulated powder;

[0050] (3) pressing the pore-forming agent granulated powder prepared in step (2) into a green body at 5.5 MPa, and then crushing and sieving to obtain pore-forming agent particles with a particle size of 1 mm;

[0051] (4) 55 parts by weight of the pore-forming agent particles obtained in step (3) and 100 parts by weight of silicon carbide granulated powder are prepared into a silicon carbide porous ceramic body under a pressure of 8 MPa, and dried at 100°C for 7.5 hours, and fired at 2150°C for 4 hours in a vacuum atmosphere to obtain a pressureless sintered silicon carbide porous ceramic.

[0052] 2. Results Analysis

[0053] The performance testing method is the same as that of Example 1. The compressive strength of the silicon carbide porous ceramic obtained in this example is tested to be 72 MPa; the porosity is 48.9%; the average pore size is 0.68 mm, and the pore size is uniform.

[0054] Example 3

[0055] 1. A method for preparing pressureless sintered silicon carbide porous ceramics, comprising the following steps:

[0056] (1) 60 parts by weight of silicon dioxide powder with a particle size of 0.8 μm and 28 parts by weight of silicon micropowder with a particle size of 0.4 μm are uniformly mixed to prepare a pore-forming ceramic powder;

[0057] (2) 3 parts of organic additives and 115 parts of distilled water are added to a ball mill by weight and ground for 1.5 hours to fully dissolve the organic additives and mix them evenly. Then, 100 parts of the pore-forming agent ceramic powder prepared in step (1), 1.5 parts of ammonia water, 4 parts of ammonium polyacrylate, and 0.8 parts of defoaming agent n-octanol are added and grinding is continued for 40 hours. The obtained slurry is subjected to spray granulation to obtain a pore-forming agent granulated powder. The pore-forming agent granulated powder is then placed in a mixer and sprayed with a release agent, sodium tripolyphosphate, while stirring, so that the release agent is evenly attached to the surface of the pore-forming agent granulated powder to obtain the pore-forming agent granulated powder;

[0058] (3) pressing the pore-forming agent granulated powder prepared in step (2) into a green body at 5.5 MPa, and then crushing and sieving to obtain pore-forming agent particles with a particle size of 2 mm;

[0059] (4) 80 parts by weight of the pore-forming agent particles obtained in step (3) and 100 parts by weight of silicon carbide granulated powder are prepared into a silicon carbide porous ceramic body under a pressure of 15 MPa, and dried at 120°C for 10 h, and fired at 2200°C for 5 h in a vacuum atmosphere to obtain a pressureless sintered silicon carbide porous ceramic.

[0060] 2. Results Analysis

[0061] The detection method is the same as that of Example 1. After detection, the compressive strength of the silicon carbide porous ceramic obtained in this example is 47 MPa; the porosity is 67.3%; the average pore size is 0.95 mm, and the pore size is uniform.

[0062] From the above examples, it can be seen that increasing the particle size and amount of the pore-forming agent particles can increase the porosity of silicon carbide porous ceramics and increase the pore size; however, as the porosity increases, the compressive strength decreases. The preparation method of the present invention can change the porosity and compressive strength of silicon carbide porous ceramics by adjusting the particle size and amount of the pore-forming agent particles, and can prepare silicon carbide porous ceramics according to actual use requirements to meet the use requirements of different scenarios.

[0063] Comparative Example 1

[0064] The difference between this comparative example and Example 1 is that the particle sizes of silicon dioxide powder and silicon micropowder are increased.

[0065] A method for preparing pressureless sintered silicon carbide porous ceramics comprises the following steps:

[0066] (1) 60 parts by weight of silicon dioxide powder with a particle size of 1.5 μm and 28 parts by weight of silicon micropowder with a particle size of 0.8 μm are uniformly mixed to prepare a pore-forming ceramic powder;

[0067] (2)-(4) are the same as steps (2)-(4) in Example 1.

[0068] The compressive strength of the silicon carbide porous ceramic prepared in this comparative example was tested to be 115 MPa, the porosity was 18.1%, and the average pore diameter was 0.045 mm.

[0069] Comparative Example 2

[0070] The difference between this comparative example and Example 1 is that the particle sizes of silicon dioxide powder and silicon micropowder are increased.

[0071] A method for preparing pressureless sintered silicon carbide porous ceramics comprises the following steps:

[0072] (1) 60 parts by weight of silicon dioxide powder with a particle size of 2.5 μm and 28 parts by weight of silicon micropowder with a particle size of 1.5 μm are uniformly mixed to prepare a pore-forming ceramic powder;

[0073] (2)-(4) are the same as steps (2)-(4) in Example 1.

[0074] The compressive strength of the silicon carbide porous ceramic prepared in this comparative example was tested to be 145 MPa, the porosity was 13.2%, and the average pore diameter was 0.05 mm.

[0075] It can be seen from Comparative Examples 1-2 and Example 1 that increasing the particle size of the raw silica powder and silicon micropowder reduces the porosity of the silicon carbide porous ceramic. This is because the excessively high particle size leads to insufficient contact between the silica powder and silicon micropowder, which reduces the degree of reaction, reduces the amount of volatilization of silicon monoxide gas, and thus reduces the porosity.

Claims

1. A method for preparing pressureless sintered silicon carbide porous ceramics, characterized in that: The method comprises the following steps: (1) mixing silicon dioxide powder and silicon micropowder in a certain weight ratio to form a pore-forming agent ceramic powder; (2) mixing the organic additive and distilled water and grinding them thoroughly, adding the pore-forming agent ceramic powder, ammonia water, ammonium polyacrylate and n-octanol prepared in step (1) and continuing grinding, spray granulating the obtained slurry to obtain a pore-forming agent granulated powder, and then spraying sodium tripolyphosphate into the pore-forming agent granulated powder while stirring; (3) pressing the pore-forming agent granulated powder prepared in step (2) into a green body, and then crushing and sieving to obtain pore-forming agent particles; (4) The pore-forming agent particles prepared in step (3) are mixed with silicon carbide granulated powder, pressed into a silicon carbide porous ceramic body, dried, and fired to obtain a pressureless sintered silicon carbide porous ceramic.

2. The preparation method according to claim 1, characterized in that: In step (1), the silicon dioxide powder and silicon micropowder are 60 parts and 28 parts by weight respectively. The particle size of the silicon dioxide powder is 0.5-2 μm; the particle size of the silicon micropowder is 0.1-1 μm.

3. The preparation method according to claim 1, characterized in that: In step (2), the organic additive, distilled water, pore-forming ceramic powder, ammonia water, ammonium polyacrylate and n-octanol are added in amounts of 2-3.5 parts, 100-120 parts, 90-110 parts, 0.5-2 parts, 1-5 parts and 0.3-1 parts by weight, respectively; the sufficient grinding time is 1-3 hours; the continued grinding time is 30-50 hours; the organic additive is prepared by uniformly mixing dextrin, polyvinyl alcohol, polyethylene glycol and glycerol.

4. The preparation method according to claim 1, characterized in that: In step (2), the sodium tripolyphosphate is uniformly attached to the surface of the pore-forming agent granulation powder.

5. The preparation method according to claim 1, characterized in that: The pressing pressure in step (3) is 1-10 MPa; the particle size of the pore-forming agent particles is 0.05-2 mm.

6. The preparation method according to claim 1, characterized in that: In step (4), the pore-forming agent particles and silicon carbide granulation powder are respectively 30-80 parts and 90-110 parts by weight.

7. The preparation method according to claim 1, characterized in that: The pressing pressure in step (4) is 2-15 MPa; the drying temperature is 80-150° C., and the drying time is 5-10 h.

8. The preparation method according to claim 1, characterized in that: The firing temperature in step (4) is 2100-2200° C. and the firing time is 3-5 hours.

9. Pressureless sintered silicon carbide porous ceramics prepared according to the preparation method according to any one of claims 1 to 8.

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