A method for directional freezing of ceramic slurry using a blast device

Abstract

The application relates to a method for directional freezing of ceramic slurry by using a blowing device, and relates to a method for directional freezing of ceramic slurry.The application aims to solve the technical problem that when liquid nitrogen is used as a cold source in the current directional freezing process based on ceramic slurry, the surface of aerogel is uneven, and the demand for ceramic materials with uniform directional pore structure obtained by directional growth of ice crystals along the z axis cannot be met.The application adopts the method of directional freezing by using liquid nitrogen as a cold source and combining with blowing to prepare ceramic aerogel with uniform internal pore diameter and smooth surface without protruding bulges.In the process, the growth of ice crystals is controlled by the liquid nitrogen cold source and regulated by the blowing device, so that the growth rate of ice crystals in the z direction is consistent, and thus a structure with uniform pore diameter distribution is obtained, thereby avoiding the problems of uneven internal pore diameter, inability to completely orient, surface protruding bulges and the like caused by traditional directional freezing.The process of the application is simple, and the prepared aerogel has a smooth surface and uniform internal pore size.

Description

Technical Field

[0001] This invention relates to a method for directional freezing of ceramic slurry. Background Technology

[0002] Ceramic aerogels, due to their low density, high specific surface area, and low thermal conductivity, have extremely broad application prospects in extreme environment insulation materials. There are many methods for preparing ceramic aerogels, and directional freezing is one of the most promising. Directional freezing involves placing one end of a ceramic slurry close to a cold source, creating a single temperature gradient within the slurry along the direction of the cold source. This causes ice crystals to grow in a designated direction. After drying, the internal ice crystals completely sublimate, resulting in a directionally ordered porous ceramic aerogel. To obtain a porous structure with small and uniform pore size, liquid nitrogen is generally used as the cold source. However, in this case, the cold source temperature is too low, and the mold cannot be completely insulated. This induces a temperature gradient in the xy plane, causing ice crystals to grow from the periphery towards the center. This pushes the slurry, causing the central surface to bulge, resulting in an uneven surface of the dried aerogel. This fails to meet the requirement of obtaining a ceramic material with a uniformly oriented porous structure through the directional growth of ice crystals along the z-axis. Summary of the Invention

[0003] The present invention aims to solve the technical problem that the aerogel surface is uneven when liquid nitrogen is used as a cold source in the current directional freezing process of ceramic slurry, which cannot meet the requirement of obtaining ceramic materials with uniform directional pore structure by directional growth of ice crystals along the z-axis. The invention provides a method for directional freezing of ceramic slurry using a blower.

[0004] The method of directional freezing of ceramic slurry using a blower device according to the present invention is carried out according to the following steps:

[0005] I. Assembly Equipment: The equipment consists of a liquid nitrogen container 1, a copper rod 2, a copper plate 3, an insulation layer 4, a slurry mold 5, and a blower 6;

[0006] A vertical copper rod 2 is fixed at the center of the bottom surface inside the liquid nitrogen container 1, and the center of the lower surface of the horizontally arranged copper plate 3 is placed on top of the copper rod 2; the slurry mold 5 is set on the upper surface of the copper plate 3, the central axis of the copper plate 3 coincides with the central axis of the slurry mold 5, and the side wall of the copper plate 3 and the inner wall of the slurry mold 5 are on the same vertical line; a heat insulation layer 4 is set between the outer wall of the slurry mold 5 and the inner wall of the liquid nitrogen container 1, and a liquid nitrogen inlet hole 4-1 is set on the heat insulation layer 4; a blower 6 is set directly above the slurry mold 5, and the air outlet of the blower 6 is vertically downward.

[0007] II. Targeted Freezing:

[0008] ① Add the ceramic aerogel slurry into the slurry mold 5, insert the three probes of the temperature recorder into the slurry to a depth of 1mm for real-time temperature monitoring, with two probes located at the edge of the slurry mold 5 and one probe located at the center of the slurry mold 5; then add liquid nitrogen into the liquid nitrogen container 1 through the liquid nitrogen inlet hole 4-1, plug the liquid nitrogen inlet hole 4-1, and start freezing;

[0009] ② When the temperature of one probe located at the edge of the slurry mold 5 reaches 0℃ or the temperature difference between one probe located at the edge of the slurry mold 5 and the probe located at the center exceeds 5℃ (the temperature at the center is higher), turn on the blower device 6 to blow air into the slurry mold 5 to suppress the growth of ice crystals in its xy plane; the test temperature of the three probes will produce three temperature differences. When the absolute value of the three temperature differences is less than 3℃ and the detection temperature of the three probes is greater than or equal to 0℃, turn off the blower device 6.

[0010] ③ Repeat the operation in ② above. When the blower 6 is in the off state, the absolute value of the three temperature differences is less than 3℃ and the detection temperature of the three probes is between -3℃ and 3℃. Maintain this state for 3 minutes to complete the directional freezing.

[0011] 3. Drying of slurry: Remove the three temperature probes, then take the slurry mold 5 out of the equipment and put it into the freeze dryer. Freeze it at -95℃ to -98℃ until it is completely frozen. Then freeze dry it under vacuum and -58℃ to -60℃ for 72h to 73h to ensure that all the ice crystals in the green body sublimate.

[0012] IV. Heat Treatment: The dried product from step three is placed in a muffle furnace for heat treatment. The heat treatment process is as follows: heating is performed sequentially at 100℃~110℃, 180℃~190℃, 400℃~410℃, 600℃~610℃ and 1000℃~1100℃ for 1h~1.2h each to obtain ceramic aerogel.

[0013] This invention employs a method combining liquid nitrogen as a cold source with directional freezing and forced air circulation to prepare ceramic aerogels with uniform internal pore size and a smooth, bulging-free surface. During this process, the growth of ice crystals is controlled by the liquid nitrogen cold source and regulated by the forced air circulation device, ensuring a consistent growth rate in the z-direction. This results in a structure with uniform pore size distribution, avoiding the problems of uneven internal pore size, inability to achieve complete orientation, and surface bulging caused by traditional directional freezing methods.

[0014] Previous researchers have often used liquid nitrogen as a cold source to improve the rate of directional freezing and obtain porous structures with small and uniform pore sizes. However, this method has two drawbacks: first, the excessively low temperature of the cold source causes a sudden drop in temperature around the mold, which cannot meet the directional growth requirements of ice crystals, resulting in disordered ice crystal growth and uneven pore structure; second, the excessively rapid freezing rate, combined with ice crystal growth from the cooler edges towards the center, causes surface bulges. This invention, by adding a blower to the existing directional freezing method, effectively solves the problem of surface bulges and effectively controls the temperature at the end furthest from the cold source (the upper part of the slurry), thus ensuring the freezing rate, reducing freezing time, and obtaining a porous structure with small and uniform pore sizes.

[0015] The process of this invention is simple, and the prepared aerogel has a smooth surface and uniform internal pore size. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the equipment used in step one of Experiment 1;

[0017] Figure 2 A photograph of the ceramic aerogel prepared for Experiment 1;

[0018] Figure 3 A photograph of the ceramic aerogel prepared for comparative experiments;

[0019] Figure 4 SEM image of the lower surface of the ceramic aerogel prepared in Experiment 1;

[0020] Figure 5 SEM image of the upper surface of the ceramic aerogel prepared for Experiment 1. Detailed Implementation

[0021] Specific Implementation Method 1: This implementation method is a method for directional freezing of ceramic slurry using a blower device, specifically carried out according to the following steps:

[0022] I. Assembly Equipment: The equipment consists of a liquid nitrogen container 1, a copper rod 2, a copper plate 3, an insulation layer 4, a slurry mold 5, and a blower 6;

[0023] A vertical copper rod 2 is fixed at the center of the bottom surface inside the liquid nitrogen container 1, and the center of the lower surface of the horizontally arranged copper plate 3 is placed on top of the copper rod 2; the slurry mold 5 is set on the upper surface of the copper plate 3, the central axis of the copper plate 3 coincides with the central axis of the slurry mold 5, and the side wall of the copper plate 3 and the inner wall of the slurry mold 5 are on the same vertical line; a heat insulation layer 4 is set between the outer wall of the slurry mold 5 and the inner wall of the liquid nitrogen container 1, and a liquid nitrogen inlet hole 4-1 is set on the heat insulation layer 4; a blower 6 is set directly above the slurry mold 5, and the air outlet of the blower 6 is vertically downward.

[0024] II. Targeted Freezing:

[0025] ① Add the ceramic aerogel slurry into the slurry mold 5, insert the three probes of the temperature recorder into the slurry to a depth of 1mm for real-time temperature monitoring, with two probes located at the edge of the slurry mold 5 and one probe located at the center of the slurry mold 5; then add liquid nitrogen into the liquid nitrogen container 1 through the liquid nitrogen inlet hole 4-1, then plug the liquid nitrogen inlet hole 4-1 and start freezing;

[0026] ② When the temperature of one probe located at the edge of the slurry mold 5 reaches 0℃ or the temperature difference between one probe located at the edge of the slurry mold 5 and the probe located at the center exceeds 5℃, turn on the blower 6 to blow air into the slurry mold 5; the test temperature of the three probes will generate three temperature differences. When the absolute value of the three temperature differences is less than 3℃ and the detection temperature of the three probes is greater than or equal to 0℃, turn off the blower 6.

[0027] ③ Repeat the operation in ② above. When the blower 6 is in the off state, the absolute value of the three temperature differences is less than 3℃ and the detection temperature of the three probes is between -3℃ and 3℃. Maintain this state for 3 minutes to complete the directional freezing.

[0028] 3. Drying of slurry: Remove the three temperature probes, then take the slurry mold 5 out of the equipment and put it into the freeze dryer. Freeze it at -95℃ to -98℃ until it is completely frozen. Then freeze dry it under vacuum and -58℃ to -60℃ for 72h to 73h to ensure that all the ice crystals in the green body sublimate.

[0029] IV. Heat Treatment: The dried product from step three is placed in a muffle furnace for heat treatment. The heat treatment process is as follows: heating is performed sequentially at 100℃~110℃, 180℃~190℃, 400℃~410℃, 600℃~610℃ and 1000℃~1100℃ for 1h~1.2h each to obtain ceramic aerogel.

[0030] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the blower device 6 mentioned in step one is a fan, and the temperature of the blower is room temperature. Everything else is the same as in Specific Implementation Method One.

[0031] Specific Implementation Method Three: This implementation method differs from Specific Implementation Method One or Two in that the insulation layer 4 mentioned in step one is polyethylene foam. Everything else is the same as in Specific Implementation Method One or Two.

[0032] Specific Implementation Method Four: This implementation method differs from one of Specific Implementation Methods One to Three in that the copper plate 3 and the slurry mold 5 mentioned in step one are both square. Everything else is the same as in one of Specific Implementation Methods One to Three.

[0033] Specific Implementation Method Five: This implementation method differs from Specific Implementation Method Four in that the copper plate 3 and the slurry mold 5 described in step one are fixed together with double-sided adhesive. Everything else is the same as in Specific Implementation Method Four.

[0034] Specific Implementation Method Six: This implementation method differs from Specific Implementation Method Five in that the ceramic aerogel slurry mentioned in step two is an Al2O3 aerogel slurry. Everything else is the same as in Specific Implementation Method Five.

[0035] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Method Six in that the Al2O3 aerogel slurry is prepared as follows: Aluminum sec-butoxide and deionized water are mixed at a molar ratio of 1:60 (aluminum sec-butoxide to H2O). The mixture is stirred in a 90°C water bath for 1 hour, resulting in a cloudy, milky-white solution. Nitric acid solution is slowly added to adjust the pH until the solution changes from milky white to clear and transparent. The addition of nitric acid solution is then stopped, and stirring continues for 9 hours to obtain Al2O3 hydrosol. The solution is then allowed to stand at room temperature for 2-3 days to age and increase viscosity. The aluminum concentration is then evaporated in an oil bath at 105°C until it reaches 1 mol / L. 10 wt.% alumina fiber and 2 wt.% hydroxyethyl cellulose are added and mixed. The mixture is stirred in a 50°C constant-temperature water bath for 2 hours and then cooled to room temperature to obtain a milky-white, viscous Al2O3 aerogel slurry. All other aspects are the same as in Specific Implementation Method Six.

[0036] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Method Seven in that step three involves freezing at -96°C until completely solid. Everything else is the same as in Specific Implementation Method Seven.

[0037] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Method Eight in that the vacuum degree in step three is 150 Pa to 200 Pa. Everything else is the same as in Specific Implementation Method Eight.

[0038] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Method Nine in that: in step four, the dried product from step three is placed in a muffle furnace for heat treatment. The heat treatment process is as follows: the heating rate is 1℃ / min, and the temperature is successively maintained at 100℃, 180℃, 400℃, 600℃, and 1000℃ for 1h to 1.2h each to obtain ceramic aerogel. Everything else is the same as in Specific Implementation Method Nine.

[0039] The invention was verified using the following experiments:

[0040] Experiment 1: This experiment demonstrates a method for directional freezing of ceramic slurry using a blower device, specifically carried out according to the following steps:

[0041] I. Assembly equipment: such as Figure 1As shown, the equipment consists of a liquid nitrogen container 1, a copper rod 2, a copper plate 3, an insulation layer 4, a slurry mold 5, and a blower 6; the blower 6 is a fan, and the temperature of the blower is room temperature; the insulation layer 4 mentioned in step one is polyethylene foam; the copper plate 3 and the slurry mold 5 are both square.

[0042] A vertical copper rod 2 is fixed at the center of the bottom surface inside the liquid nitrogen container 1, and the center of the lower surface of a horizontally arranged copper plate 3 is placed on top of the copper rod 2. A slurry mold 5 is set on the upper surface of the copper plate 3, with the central axis of the copper plate 3 coinciding with the central axis of the slurry mold 5, and the side wall of the copper plate 3 and the inner wall of the slurry mold 5 on the same vertical line. A heat insulation layer 4 is set between the outer wall of the slurry mold 5 and the inner wall of the liquid nitrogen container 1, and a liquid nitrogen inlet hole 4-1 is set on the heat insulation layer 4. A blower 6 is set directly above the slurry mold 5, with the air outlet of the blower 6 pointing vertically downward. The copper plate 3 and the slurry mold 5 are fixed together with double-sided adhesive.

[0043] II. Targeted Freezing:

[0044] ① Add the ceramic aerogel slurry into the slurry mold 5, insert the three probes of the temperature recorder into the slurry to a depth of 1mm for real-time temperature monitoring, with two probes located at the edge of the slurry mold 5 and one probe located at the center of the slurry mold 5; then add liquid nitrogen into the liquid nitrogen container 1 through the liquid nitrogen inlet hole 4-1, then plug the liquid nitrogen inlet hole 4-1 and start freezing;

[0045] The ceramic aerogel slurry is an Al2O3 aerogel slurry. The preparation method of the Al2O3 aerogel slurry is as follows: aluminum sec-butoxide and deionized water are mixed according to the molar ratio of aluminum sec-butoxide to H2O of 1:60. The mixture is stirred in a 90°C water bath for 1 hour. At this time, the mixed solution is turbid and milky white. Nitric acid solution is slowly added to adjust the pH until the mixed solution changes from milky white to clear and transparent. The addition of nitric acid solution is stopped, and stirring is continued for 9 hours to obtain Al2O3 hydrosol. Then, it is allowed to stand at room temperature for 2 to 3 days to age and increase the viscosity. Then, it is evaporated in an oil bath at 105°C until the concentration of aluminum element is 1 mol / L. Then, 10 wt.% of alumina fiber and 2 wt.% of hydroxyethyl cellulose are added and mixed. The mixture is stirred in a constant temperature water bath at 50°C for 2 hours and cooled to room temperature to obtain a milky white viscous Al2O3 aerogel slurry.

[0046] ② When the temperature of one probe located at the edge of the slurry mold 5 reaches 0℃ or the temperature difference between one probe located at the edge of the slurry mold 5 and the probe located at the center exceeds 5℃, turn on the blower 6 to blow air into the slurry mold 5; the test temperature of the three probes will generate three temperature differences. When the absolute value of the three temperature differences is less than 3℃ and the detection temperature of the three probes is greater than or equal to 0℃, turn off the blower 6.

[0047] ③ Repeat the operation in ② above. When the blower 6 is in the off state, the absolute value of the three temperature differences is less than 3℃ and the detection temperature of the three probes is between -3℃ and 3℃. Maintain this state for 3 minutes to complete the directional freezing.

[0048] 3. Drying of slurry: Remove the three temperature probes, then take the slurry mold 5 out of the equipment and put it into the freeze dryer. Freeze it at -96℃ until it is completely frozen. Then freeze-dry it under vacuum and 60℃ for 72 hours to ensure that all the ice crystals in the green body sublimate. The vacuum degree is 150Pa.

[0049] IV. Heat Treatment: The dried product from step 3 is placed in a muffle furnace for heat treatment. The heat treatment process is as follows: the temperature is increased at a rate of 1℃ / min, and the temperature is held at 100℃, 180℃, 400℃, 600℃ and 1000℃ for 1 hour each to obtain ceramic aerogel.

[0050] Comparative Experiment: This experiment uses the traditional directional freezing method. The difference from Experiment 1 is that the blower is not activated during step two. Everything else is the same as Experiment 1.

[0051] Figure 2 The image shows the physical specimen of the ceramic aerogel prepared in Experiment 1. The left image is its lower surface, i.e., the cold source end; the middle image is its upper surface, i.e., the end away from the cold source; and the right image is its three-dimensional view.

[0052] Figure 3 The images shown are of the ceramic aerogels prepared for comparative experiments. The left image shows the lower surface, i.e., the cold source end; the middle image shows the upper surface, i.e., the end furthest from the cold source; and the right image is a three-dimensional view.

[0053] from Figure 3 As can be seen, the upper surface of the sample prepared in the comparative experiment (the middle picture) has bulges, and compared with the cold source end (left picture), the surface of the non-cold source end shows visible traces of large ice crystal growth.

[0054] from Figure 2 As can be seen from the results, the sample prepared using the blower in Experiment 1 has a smooth upper surface without any bumps or bulges, and the sample has a good shape. Furthermore, the sample surfaces at both the cold source end and the non-cold source end are dense and have uniform pores.

[0055] Figure 4 This is a SEM image of the lower surface (cold source end) of the ceramic aerogel prepared in Experiment 1. Figure 5 The SEM image of the upper surface (non-cold source end) of the ceramic aerogel prepared for Experiment 1 clearly shows that the pore size in different regions is basically the same with no obvious difference, indicating that the growth of ice crystals was well controlled.

Claims

1. A method for directional freezing of ceramic slurry using a blower, characterized in that... The method of directional freezing of ceramic slurry using a blower is carried out according to the following steps: I. Assembly equipment: The equipment consists of a liquid nitrogen container (1), a copper rod (2), a copper plate (3), an insulation layer (4), a slurry mold (5), and a blower (6); A vertical copper rod (2) is fixed at the center of the bottom surface of the liquid nitrogen container (1), and the center of the lower surface of the horizontally arranged copper plate (3) is placed on the top of the copper rod (2); the slurry mold (5) is set on the upper surface of the copper plate (3), the central axis of the copper plate (3) coincides with the central axis of the slurry mold (5), and the side wall of the copper plate (3) and the inner wall of the slurry mold (5) are on the same vertical line; a heat insulation layer (4) is set between the outer wall of the slurry mold (5) and the inner wall of the liquid nitrogen container (1), and a liquid nitrogen inlet hole (4-1) is set on the heat insulation layer (4); a blower (6) is set directly above the slurry mold (5), and the air outlet of the blower (6) is vertically downward; II. Targeted Freezing: ① Add the ceramic aerogel slurry into the slurry mold (5), insert the three probes of the temperature recorder into the slurry to a depth of 1 mm for real-time temperature monitoring, with two probes located at the edge of the slurry mold (5) and one probe located at the center of the slurry mold (5); then add liquid nitrogen into the liquid nitrogen container (1) through the liquid nitrogen inlet hole (4-1), then plug the liquid nitrogen inlet hole (4-1) and start freezing; ② When the temperature of one probe located at the edge of the slurry mold (5) reaches 0℃ or the temperature difference between one probe located at the edge of the slurry mold (5) and the probe located at the center exceeds 5℃, turn on the blower (6) to blow air into the slurry mold (5); the test temperature of the three probes will generate three temperature differences. When the absolute value of the three temperature differences is less than 3℃ and the detection temperature of the three probes is greater than or equal to 0℃, turn off the blower (6); ③ Repeat the operation in ② above. When the blower (6) is in the off state, the absolute value of the three temperature differences is less than 3℃ and the detection temperature of the three probes is -3℃ to 3℃. Maintain this state for 3 minutes to complete the directional freezing.

3. Drying of slurry: Remove the three temperature probes, then take the slurry mold (5) out of the equipment and put it into the freeze dryer. Freeze it until it is completely solid at -95℃ to -98℃. Then freeze dry it for 72h to 73h under vacuum and -58℃ to 60℃ to ensure that all the ice crystals in the green body sublimate. IV. Heat Treatment: The dried product from step three is placed in a muffle furnace for heat treatment. The heat treatment process is as follows: heating is performed sequentially at 100℃~110℃, 180℃~190℃, 400℃~410℃, 600℃~610℃ and 1000℃~1100℃ for 1h~1.2h each to obtain ceramic aerogel.

2. The method for directional freezing of ceramic slurry using a blower according to claim 1, characterized in that... The blower device (6) mentioned in step one is a fan, and the temperature of the blower is room temperature.

3. The method for directional freezing of ceramic slurry using a blower according to claim 1, characterized in that... The insulation layer (4) mentioned in step one is polyethylene foam.

4. The method for directional freezing of ceramic slurry using a blower according to claim 1, characterized in that... The copper plate (3) and the slurry mold (5) mentioned in step one are both square.

5. A method for directional freezing of ceramic slurry using a blower according to claim 1, characterized in that... The copper plate (3) and the slurry mold (5) described in step one are fixed together with double-sided adhesive tape.

6. A method for directional freezing of ceramic slurry using a blower according to claim 1, characterized in that... The ceramic aerogel slurry mentioned in step two is an Al2O3 aerogel slurry.

7. A method for directional freezing of ceramic slurry using a blower according to claim 6, characterized in that... The preparation method of the Al2O3 aerogel slurry is as follows: Al2O3 aerogel and deionized water are mixed at a molar ratio of 1:60 (aluminum sec-butoxide to H2O). The mixture is stirred in a 90°C water bath for 1 hour. At this time, the mixed solution is turbid and milky white. Nitric acid solution is slowly added to adjust the pH until the mixture changes from milky white to clear and transparent. The addition of nitric acid solution is stopped, and stirring is continued for 9 hours to obtain Al2O3 hydrosol. Then, the mixture is allowed to stand at room temperature for 2 to 3 days to age and increase its viscosity. Then, the aluminum concentration is evaporated in an oil bath at 105°C until it reaches 1 mol / L. Then, 10 wt.% alumina fiber and 2 wt.% hydroxyethyl cellulose are added and mixed. The mixture is stirred in a 50°C constant temperature water bath for 2 hours and cooled to room temperature to obtain a milky white viscous Al2O3 aerogel slurry.

8. A method for directional freezing of ceramic slurry using a blower according to claim 1, characterized in that... In step three, freeze at -96°C until completely solid.

9. A method for directional freezing of ceramic slurry using a blower according to claim 1, characterized in that... The vacuum level in step three is 150 Pa to 200 Pa.

10. A method for directional freezing of ceramic slurry using a blower according to claim 1, characterized in that... In step four, the dried product from step three is placed in a muffle furnace for heat treatment. The heat treatment process is as follows: the heating rate is 1℃ / min, and the temperature is held at 100℃, 180℃, 400℃, 600℃ and 1000℃ for 1h to 1.2h respectively to obtain ceramic aerogel.

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