CN108585920B - Method for preparing continuous gradient porous ceramic by hydrogel layer-by-layer crosslinking - Google Patents

Method for preparing continuous gradient porous ceramic by hydrogel layer-by-layer crosslinking Download PDF

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CN108585920B
CN108585920B CN201810612014.7A CN201810612014A CN108585920B CN 108585920 B CN108585920 B CN 108585920B CN 201810612014 A CN201810612014 A CN 201810612014A CN 108585920 B CN108585920 B CN 108585920B
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CN108585920A (en
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叶枫
张标
叶健
刘强
高晔
叶凯
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Harbin Institute of Technology
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Abstract

The invention provides a method for preparing continuous gradient porous ceramics by utilizing hydrogel layer-by-layer crosslinking, which comprises the steps of preparing hydrogel precursor solution, adding ceramic powder, a dispersing agent and a defoaming agent in different proportions, and uniformly mixing to obtain a series of slurries with different solid contents; carrying out vacuum defoaming on the slurry with the lowest or highest solid phase content, spraying a cross-linking agent solution to the slurry, and then standing the slurry to enable the slurry to be cross-linked; sequentially repeating the steps of the slurry with other components according to the ascending order or the descending order of solid content until the thickness of the slurry meets the requirement, and soaking the slurry in deionized water to obtain a wet blank; compared with the prior art, the method can accurately realize the accurate gradient control of the material components, the porosity, the pore structure and the performance, and can be widely applied to the preparation of the continuous gradient porous ceramics.

Description

Method for preparing continuous gradient porous ceramic by hydrogel layer-by-layer crosslinking
Technical Field
The invention relates to a method for preparing a porous ceramic material, in particular to a method for preparing continuous gradient porous ceramic by crosslinking hydrogel layer by layer.
Background
The continuous gradient porous ceramic is a porous ceramic material with continuously changed components, porosity, pore structure and other microstructures, has wide application market in the fields of beverage, dairy product manufacture, medicine, bioengineering, catalysis, filtration, high-temperature flue gas separation and the like due to good high temperature resistance, corrosion resistance and chemical stability, and becomes one of the hot spots for porous ceramic research. However, the prior art has difficulty in realizing precise control of the porosity and microstructure between the continuous gradient porous ceramic layers.
In view of the above-mentioned drawbacks, the present inventors have made the present invention through long-term research and practice.
Disclosure of Invention
The invention provides a method for preparing continuous gradient porous ceramic by crosslinking hydrogel layer by layer, aiming at solving the problem of precise gradient control of porosity, microstructure and performance of porous ceramic, and the technical scheme is that the method comprises the following steps:
firstly, preparing a hydrogel precursor solution, adding ceramic powder in different proportions, sequentially adding a dispersing agent and a defoaming agent, and uniformly mixing to obtain a series of ceramic slurries with different solid contents;
secondly, placing the ceramic slurry with the lowest or highest solid content obtained in the first step in a vacuum environment for defoaming, pouring the ceramic slurry into a mold, spraying a cross-linking agent solution above the ceramic slurry, soaking the whole mold in the cross-linking agent solution, and standing;
thirdly, taking the ceramic slurry after the cross-linking in the second step as a substrate, and sequentially repeating the process in the second step for other slurries in the first step according to the ascending order or the descending order of the solid content until the ceramic slurry is completely cross-linked;
the fourth step: soaking the mold in deionized water to obtain a wet blank;
and fifthly, freeze-drying, removing glue and sintering the wet blank to finally obtain the porous ceramic with continuous gradient.
Preferably, the hydrogel precursor solution in the first step is a mixed solution of a hydrogel precursor and deionized water, and the concentration of the hydrogel precursor is 3-5 wt%,
preferably, the hydrogel precursor is one of polyvinyl alcohol, chitosan or polyacrylic acid.
Preferably, the ceramic powder in the first step is one or more of oxide, nitride, carbide or boride.
Preferably, the volume fraction of the ceramic powder in the ceramic slurry is 5-50 vol%, the mass of the dispersing agent is 0.1-1.5 wt% of the mass of the ceramic powder, and the mass of the defoaming agent is 0.2-1 wt% of the mass of the ceramic powder.
Preferably, the standing time in the second step is 1-5 hours.
Preferably, in the second step, the cross-linking agent is one of aldehydes, boric acid, N' -methylene-bisacrylamide, epichlorohydrin or sodium hydroxide, and the concentration of the cross-linking agent is 0.5-5 wt%.
Preferably, the freeze drying process in the fifth step is that the mixture is frozen at the temperature of-15 to-30 ℃ for 10 to 30 minutes and then is placed in a freeze dryer at the temperature of-40 to 0 ℃ for drying for 24 to 48 hours.
Preferably, the conditions of the gel discharging process in the fifth step are that the temperature is raised to 500-650 ℃ at a speed of 0.2-1 ℃/min and is kept for 1-3 hours.
Preferably, the sintering temperature in the fourth step is 600-2500 ℃.
Compared with the prior art, the method for preparing the continuous gradient porous ceramic by using the hydrogel layer-by-layer crosslinking has the beneficial effects that the method for preparing the continuous gradient porous ceramic by using the hydrogel layer-by-layer crosslinking can accurately realize the accurate gradient control of the material components, the porosity, the pore structure and the performance, and is widely applied to the preparation of the continuous gradient porous ceramic.
Drawings
Fig. 1 is a process flow chart of a method for preparing a continuous gradient porous ceramic by using hydrogel layer-by-layer crosslinking in example 1 of the present invention.
FIG. 2 is an electron micrograph of a continuous gradient porous ceramic prepared by hydrogel layer-by-layer crosslinking in example 1 of the present invention.
Detailed Description
Example 1
Referring to figures 1 and 2 of the drawings,
FIG. 1 is a process flow chart of a method for preparing a continuous gradient porous ceramic by crosslinking hydrogel layer by layer according to the present embodiment;
FIG. 2 is an electron micrograph of a continuous gradient porous ceramic prepared by crosslinking hydrogel layer by layer in this example, and the porous ceramic is filled with resin for easy observation.
The embodiment provides a method for preparing continuous gradient porous ceramic by utilizing hydrogel layer-by-layer crosslinking, which comprises the following steps:
firstly, weighing a hydrogel precursor, adding deionized water to prepare a hydrogel precursor solution with the concentration of 3 wt% to 5 wt%, adding ceramic powder in different proportions into the hydrogel precursor solution, then sequentially adding a dispersing agent and a defoaming agent, and uniformly mixing to obtain a series of ceramic slurries with different solid contents, wherein the hydrogel precursor is one of polyvinyl alcohol, chitosan or polyacrylic acid, the ceramic powder is one or more of oxide, nitride or carbide, and the dispersing agent is polyacrylic acid. Wherein the volume fraction of the mixed ceramic powder and hydrogel precursor solution is 5-50 vol%, the mass of the dispersing agent is 0.1-1.5 wt% of the mass of the ceramic powder, and the mass of the defoaming agent is 0.2-1 wt% of the mass of the ceramic powder;
secondly, defoaming the ceramic slurry with the lowest or highest solid content obtained in the first step in a vacuum environment for 20min, pouring the ceramic slurry into a mold, enabling the slurry to flow under the action of gravity and be flatly laid at the bottom of the mold, preparing a cross-linking agent solution with the concentration of 0.5-5 wt%, taking out a part of the cross-linking agent solution and pouring the part of the cross-linking agent solution into a spraying pot, slightly spraying the cross-linking agent solution above the slurry in the first step to enable the surface of the slurry to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 1-5 hours, wherein the cross-linking agent is one of aldehydes, boric acid, N' -methylene bisacrylamide, epichlorohydrin or sodium hydroxide;
thirdly, taking the ceramic slurry after the cross-linking in the second step as a substrate, and sequentially repeating the process in the second step on other slurries in the first step according to the ascending or descending order of solid content until the ceramic slurry is completely cross-linked, namely the thickness of the multilayer cross-linked film reaches the design requirement;
fourthly, soaking the mold in deionized water for 2 days, and replacing the deionized water every 12 hours to remove redundant cross-linking agent ions to obtain a wet blank;
fifthly, freeze-drying, removing glue and sintering the wet blank body obtained in the fourth step to finally obtain the porous ceramic with continuous gradient, wherein the freeze-drying process conditions are that the wet blank body is frozen for 10-30 minutes at the temperature of-15-30 ℃, and then the wet blank body is placed in a freeze dryer at the temperature of-40-0 ℃ for drying, and the drying time is 24-48 hours; the glue discharging process condition is that the temperature is raised to 500-650 ℃ at the speed of 0.2-1 ℃/min and is continued for 1-3 hours; the sintering temperature is 600-2500 ℃, the sintering can be carried out in an air furnace, a vacuum furnace, an atmosphere protection furnace or an air pressure furnace, different ceramics are sintered, the selected sintering furnace is different, the sintering system is different, and the sintering can be actually selected according to the requirements, for example, the air furnace can be selected when oxide ceramics are sintered, carbide ceramics can be sintered in the vacuum furnace or the atmosphere protection furnace, and nitride ceramics are sintered in the air pressure furnace preferably. In addition, different ceramic sintering temperatures, times and the like are different, each ceramic has different sintering characteristics, and process parameters can be reasonably changed according to the properties of different ceramics.
Therefore, the method for preparing the continuous gradient porous ceramic by utilizing the hydrogel layer-by-layer crosslinking realizes the accurate regulation and control of the microstructure of the gradient porous ceramic by the layer-by-layer casting and crosslinking of ceramic slurry with different solid phase contents. Therefore, the method can accurately realize the precise gradient control of the porosity, the pore structure and the performance of the material, and is widely applied to the preparation of continuous gradient porous ceramics.
Example 2
The embodiment provides a method for preparing continuous gradient porous ceramic by utilizing hydrogel layer-by-layer crosslinking, which comprises the following steps:
firstly, weighing a hydrogel precursor, adding deionized water to prepare a hydrogel precursor solution with the concentration of 3 wt%, and adding waterAdding ceramic powder in different proportions into a gel precursor solution, then sequentially adding a dispersing agent and a defoaming agent, and uniformly mixing to obtain a series of ceramic slurries with different solid-phase contents, wherein the hydrogel precursor is polyvinyl alcohol, and the ceramic powder is CaTiO3And the dispersant is polyacrylic acid. Wherein, the solid phase content of the ceramic powder in the ceramic slurry is respectively 5 vol%, 15 vol%, 25 vol%, 35 vol% and 45 vol%, and is respectively marked as ceramic slurry A, B, C, D and E. The mass of the dispersing agent is 0.1 wt% of the mass of the ceramic powder, and the mass of the defoaming agent is 0.2 wt% of the mass of the ceramic powder;
secondly, placing the ceramic slurry A in the first step in a vacuum environment for deaeration for 20min, pouring the ceramic slurry A into a mold, enabling the slurry to flow under the action of gravity and be flatly laid at the bottom of the mold, preparing a cross-linking agent solution with the concentration of 0.5 wt%, taking out a part of the slurry and pouring the part of the slurry into a spray pot, slightly spraying the cross-linking agent solution above the ceramic slurry A to enable the surface of the slurry to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 1 hour, wherein the cross-linking agent is aldehyde;
step three, placing the ceramic slurry B in the step one in a vacuum environment for defoaming for 20min, taking the ceramic slurry A after the cross-linking in the step two as a substrate, pouring the ceramic slurry B into a mold, enabling the ceramic slurry B to flow under the action of gravity and be flatly laid above the slurry A, slightly spraying the cross-linking agent solution above the ceramic slurry B to enable the surface of the ceramic slurry to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 1 hour; repeating the above process, and respectively crosslinking the ceramic slurry C, D and E to prepare a green body with 5 layers;
fourthly, soaking the mold in deionized water for 2 days, and replacing the deionized water every 12 hours to remove redundant cross-linking agent ions to obtain a wet blank;
fifthly, freeze-drying, removing glue and sintering the wet blank body obtained in the fourth step to finally obtain the porous ceramic with continuous gradient, wherein the freeze-drying process conditions are that the porous ceramic is frozen for 10 minutes at the temperature of minus 15 ℃, and then the porous ceramic is placed in a freeze dryer at the temperature of minus 40 ℃ for drying, and the drying time is 24 hours; the technological conditions of the rubber discharging are that the temperature is raised to 500 ℃ at the speed of 0.2 ℃/min and the temperature lasts for 1 hour; the sintering temperature is 1400 ℃, the sintering time is 2h, and the sintering is carried out in an air furnace.
Therefore, the method for preparing the continuous gradient porous ceramic by utilizing the hydrogel layer-by-layer crosslinking realizes the accurate regulation and control of the microstructure of the gradient porous ceramic by the layer-by-layer casting and crosslinking of ceramic slurry with different solid phase contents. Therefore, the method can accurately realize the precise gradient control of the porosity, the pore structure and the performance of the material, and is widely applied to the preparation of continuous gradient porous ceramics.
Example 3
This example is different from example 2 in that BaTiO for ceramic powder is used in the first step3Alternatively, the others are the same as in embodiment 2.
Example 4
This example is different from example 2 in that BaTi for the ceramic powder is used in the first step4O9Instead, the sintering temperature in the fifth step was 1000 ℃, and the rest was the same as in example 2.
Example 5
This example differs from example 2 in that in the first step, TiO is used for the ceramic powder2Instead, the sintering temperature in the fifth step was 1200 ℃, and the rest was the same as in example 2.
Example 6
The embodiment provides a method for preparing continuous gradient porous ceramic by utilizing hydrogel layer-by-layer crosslinking, which comprises the following steps:
firstly, weighing a hydrogel precursor, adding deionized water to prepare a hydrogel precursor solution with the concentration of 4 wt%, adding ceramic powder in different proportions into the hydrogel precursor solution, then sequentially adding a dispersing agent and a defoaming agent, and uniformly mixing to obtain a series of ceramic slurries with different solid contents, wherein the hydrogel precursor is chitosan, and the ceramic powder is Si3N4The dispersant is polyAcrylic acid. Wherein, the solid phase content of the ceramic powder in the ceramic slurry is respectively 10 vol%, 20 vol%, 30 vol%, 40 vol% and 50 vol%, and is respectively marked as ceramic slurry A, B, C, D and E. The mass of the dispersing agent is 1 wt% of the mass of the ceramic powder, and the mass of the defoaming agent is 0.6 wt% of the mass of the ceramic powder;
secondly, placing the ceramic slurry A in the first step in a vacuum environment for deaeration for 20min, pouring the ceramic slurry A into a mold, enabling the slurry to flow under the action of gravity and be flatly laid at the bottom of the mold, preparing a cross-linking agent solution with the concentration of 2 wt%, taking out a part of the slurry and pouring the part of the slurry into a spray pot, slightly spraying the cross-linking agent solution above the slurry A to enable the surface of the slurry to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 2.5 hours, wherein the cross-linking agent is sodium hydroxide;
step three, placing the ceramic slurry B in the step one in a vacuum environment for defoaming for 20min, taking the ceramic slurry A after the cross-linking in the step two as a substrate, pouring the ceramic slurry B into a mold, enabling the ceramic slurry B to flow under the action of gravity and be flatly laid above the slurry A, slightly spraying the cross-linking agent solution above the ceramic slurry B to enable the surface of the slurry to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 2.5 hours; repeating the above process, and respectively crosslinking the ceramic slurry C, D and E to prepare a green body with 5 layers;
fourthly, soaking the mold in deionized water for 2 days, and replacing the deionized water every 12 hours to remove redundant cross-linking agent ions to obtain a wet blank;
fifthly, freeze-drying, removing glue and sintering the wet blank body obtained in the fourth step to finally obtain the porous ceramic with continuous gradient, wherein the freeze-drying process conditions are that the porous ceramic is frozen for 20 minutes at the temperature of minus 20 ℃, and then the porous ceramic is placed in a freeze dryer at the temperature of minus 20 ℃ for drying, and the drying time is 36 hours; the technological conditions of the rubber discharge are that the temperature is increased to 600 ℃ at the speed of 0.6 ℃/min and the temperature is maintained for 1.5 hours; the sintering temperature is 1800 ℃, the sintering time is 1h, and the sintering is carried out in an air pressure furnace.
Therefore, the method for preparing the continuous gradient porous ceramic by utilizing the hydrogel layer-by-layer crosslinking realizes the accurate regulation and control of the microstructure of the gradient porous ceramic by the layer-by-layer casting and crosslinking of ceramic slurry with different solid phase contents. Therefore, the method can accurately realize the precise gradient control of the porosity, the pore structure and the performance of the material, and is widely applied to the preparation of continuous gradient porous ceramics.
Example 7
This example is different from example 6 in that AlN is substituted for the ceramic powder in the first step, and the rest is the same as example 6.
Example 8
This example is different from example 6 in that the ceramic powder in the first step is replaced with BN, and the rest is the same as example 6.
Example 9
The embodiment provides a method for preparing continuous gradient porous ceramic by utilizing hydrogel layer-by-layer crosslinking, which comprises the following steps:
firstly, weighing a hydrogel precursor, adding deionized water to prepare a hydrogel precursor solution with the concentration of 4 wt%, adding ceramic powder in different proportions into the hydrogel precursor solution, then sequentially adding a dispersing agent and a defoaming agent, and uniformly mixing to obtain a series of ceramic slurries with different solid phase contents, wherein the hydrogel precursor is polyvinyl alcohol, the ceramic powder is SiC, and the dispersing agent is tetramethylammonium hydroxide. Wherein, the solid phase content of the ceramic powder in the ceramic slurry is respectively 5 vol%, 15 vol%, 25 vol%, 35 vol% and 45 vol%, and is respectively marked as ceramic slurry A, B, C, D and E. The mass of the dispersing agent is 1.5 wt% of the mass of the ceramic powder, and the mass of the defoaming agent is 1 wt% of the mass of the ceramic powder;
secondly, defoaming the ceramic slurry A in the first step in a vacuum environment for 20min, pouring the ceramic slurry into a mold, enabling the ceramic slurry A to flow under the action of gravity and be flatly laid at the bottom of the mold, preparing a cross-linking agent solution with the concentration of 0.5 wt%, taking out a part of the ceramic slurry A and pouring the part of the ceramic slurry A into a spray can, slightly spraying the cross-linking agent solution above the ceramic slurry A in the first step to enable the surface of the ceramic slurry A to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 2 hours, wherein the cross-linking agent is boric acid;
step three, placing the ceramic slurry B in the step one in a vacuum environment for deaeration for 20min, taking the ceramic slurry A after the cross-linking in the step two as a substrate, pouring the ceramic slurry B into a mold, enabling the ceramic slurry B to flow and be flatly laid above the slurry A under the action of gravity, slightly spraying the cross-linking agent solution above the ceramic slurry B to enable the surface of the slurry to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 2 hours; repeating the above process, and respectively crosslinking the ceramic slurry C, D and E to prepare a green body with 5 layers;
fourthly, soaking the mold in deionized water for 2 days, and replacing the deionized water every 12 hours to remove redundant cross-linking agent ions to obtain a wet blank;
fifthly, freeze-drying, removing glue and sintering the wet blank body obtained in the fourth step to finally obtain the porous ceramic with continuous gradient, wherein the freeze-drying process conditions are that the porous ceramic is frozen for 30 minutes at the temperature of minus 10 ℃, and then the porous ceramic is placed in a freeze dryer at the temperature of minus 10 ℃ for drying, and the drying time is 48 hours; the technological conditions of the rubber discharging are that the temperature is raised to 650 ℃ at the speed of 1 ℃/min and the temperature is maintained for 3 hours; the sintering temperature is 1800 ℃, the sintering time is 2 hours, and the sintering is carried out in a vacuum furnace or an atmosphere protection furnace.
Therefore, the method for preparing the continuous gradient porous ceramic by utilizing the hydrogel layer-by-layer crosslinking realizes the accurate regulation and control of the microstructure of the gradient porous ceramic by the layer-by-layer casting and crosslinking of ceramic slurry with different solid phase contents. Therefore, the method can accurately realize the precise gradient control of the porosity, the pore structure and the performance of the material, and is widely applied to the preparation of continuous gradient porous ceramics.
Example 10
This example differs from example 9 in that the ceramic of the first stepPowder B4C was replaced, and the others were the same as in example 9.
Example 11
This example differs from example 9 in that in the second step the crosslinking agent is replaced by epichlorohydrin, otherwise the same as example 9.
Example 12
The embodiment provides a method for preparing continuous gradient porous ceramic by utilizing hydrogel layer-by-layer crosslinking, which comprises the following steps:
firstly, weighing a hydrogel precursor, adding deionized water to prepare a hydrogel precursor solution with the concentration of 5 wt%, adding ceramic powder in different proportions into the hydrogel precursor solution, then sequentially adding a dispersing agent and a defoaming agent, and uniformly mixing to obtain a series of ceramic slurries with different solid-phase contents, wherein the hydrogel precursor is polyacrylic acid, and the ceramic powder is TiB2The dispersant is triammonium citrate. Wherein, the solid phase content of the ceramic powder in the ceramic slurry is respectively 5 vol%, 15 vol%, 25 vol%, 35 vol% and 45 vol%, and is respectively marked as ceramic slurry A, B, C, D and E. The mass of the dispersing agent is 1.5 wt% of the mass of the ceramic powder, and the mass of the defoaming agent is 1 wt% of the mass of the ceramic powder;
secondly, defoaming the ceramic slurry A in the first step in a vacuum environment for 20min, pouring the ceramic slurry into a mold, enabling the ceramic slurry A to flow under the action of gravity and be paved at the bottom of the mold, preparing a cross-linking agent solution with the concentration of 1 wt%, taking out a part of the ceramic slurry A and pouring the part of the ceramic slurry A into a spray can, slightly spraying the cross-linking agent solution above the ceramic slurry A in the first step to enable the surface of the slurry to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 5 hours, wherein the cross-linking agent is N, N' -methylene bisacrylamide;
step three, placing the ceramic slurry B in the step one in a vacuum environment for defoaming for 20min, taking the ceramic slurry A after the cross-linking in the step two as a substrate, pouring the ceramic slurry B into a mold, enabling the ceramic slurry B to flow under the action of gravity and be flatly laid above the slurry A, slightly spraying the cross-linking agent solution above the slurry B to enable the surface of the ceramic slurry to be rapidly cross-linked, then soaking the whole mold in the cross-linking agent solution, and standing for 5 hours; repeating the process, and respectively crosslinking the ceramic slurry C, D and the E to prepare a green body with 5 layers;
fourthly, soaking the mold in deionized water for 2 days, and replacing the deionized water every 12 hours to remove redundant cross-linking agent ions to obtain a wet blank;
fifthly, freeze-drying, removing glue and sintering the wet blank body obtained in the fourth step to finally obtain the porous ceramic with continuous gradient, wherein the freeze-drying process conditions are that the wet blank body is frozen for 25 minutes at the temperature of minus 30 ℃, and then the wet blank body is placed in a freeze dryer at the temperature of minus 20 ℃ for drying, and the drying time is 48 hours; the technological conditions of the rubber discharging are that the temperature is raised to 650 ℃ at the speed of 0.5 ℃/min and the temperature is maintained for 3 hours; the sintering temperature is 1700 ℃, the sintering time is 2.5h, and the sintering is carried out in a vacuum furnace.
Therefore, the method for preparing the continuous gradient porous ceramic by utilizing the hydrogel layer-by-layer crosslinking realizes the accurate regulation and control of the microstructure of the gradient porous ceramic by the layer-by-layer casting and crosslinking of ceramic slurry with different solid phase contents. Therefore, the method can accurately realize the precise gradient control of the porosity, the pore structure and the performance of the material, and is widely applied to the preparation of continuous gradient porous ceramics.
Although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that various changes in the embodiments and/or modifications of the invention can be made, and equivalents and modifications of some features of the invention can be made without departing from the spirit and scope of the invention.

Claims (10)

1. A method for preparing continuous gradient porous ceramics by hydrogel layer-by-layer crosslinking is characterized by comprising the following steps:
firstly, preparing a hydrogel precursor solution, adding ceramic powder in different proportions, sequentially adding a dispersing agent and a defoaming agent, and uniformly mixing to obtain a series of ceramic slurries with different solid contents;
secondly, placing the ceramic slurry with the lowest or highest solid content obtained in the first step in a vacuum environment for defoaming, pouring the ceramic slurry into a mold, spraying a cross-linking agent solution above the ceramic slurry, soaking the whole mold in the cross-linking agent solution, and standing;
thirdly, taking the ceramic slurry after the cross-linking in the second step as a substrate, and sequentially repeating the process in the second step for other slurries in the first step according to the ascending order or the descending order of the solid content until the ceramic slurry is completely cross-linked;
the fourth step: soaking the mold in deionized water to obtain a wet blank;
and fifthly, freeze-drying, removing glue and sintering the wet blank to finally obtain the porous ceramic with continuous gradient.
2. The method for preparing a continuous gradient porous ceramic by using hydrogel layer-by-layer crosslinking as claimed in claim 1, wherein the hydrogel precursor solution in the first step is a mixed solution of a hydrogel precursor and deionized water, and the concentration of the hydrogel precursor is 3-5 wt%.
3. The method for preparing a continuous gradient porous ceramic by hydrogel layer-by-layer crosslinking according to claim 2, wherein the hydrogel precursor is one of polyvinyl alcohol, chitosan or polyacrylic acid.
4. The method for preparing the continuous gradient porous ceramic by utilizing the hydrogel layer-by-layer crosslinking as claimed in claim 2 or 3, wherein the ceramic powder in the first step is one or more of oxide, nitride, carbide or boride.
5. The method for preparing a continuous gradient porous ceramic by using hydrogel layer-by-layer crosslinking as claimed in claim 4, wherein the volume fraction of the ceramic powder in the ceramic slurry is 5-50 vol%, the mass of the dispersant is 0.1-1.5 wt% of the mass of the ceramic powder, and the mass of the defoamer is 0.2-1 wt% of the mass of the ceramic powder.
6. The method for preparing a continuous gradient porous ceramic using hydrogel layer-by-layer crosslinking as claimed in claim 1, wherein the standing time of the second step is 1 to 5 hours.
7. The method for preparing a continuous gradient porous ceramic by hydrogel layer-by-layer crosslinking according to claim 1, wherein the crosslinking agent in the second step is one of aldehydes, boric acid, N' -methylenebisacrylamide, epichlorohydrin or sodium hydroxide, and the concentration of the crosslinking agent is 0.5-5 wt%.
8. The method for preparing a continuous gradient porous ceramic by using hydrogel layer-by-layer crosslinking as claimed in claim 1, wherein the freeze-drying process in the fifth step is performed under the conditions of freezing at a temperature of-15 ℃ to-30 ℃ for 10 to 30 minutes, and then drying in a freeze-dryer at-40 ℃ to 0 ℃ for 24 to 48 hours.
9. The method for preparing a continuous gradient porous ceramic by hydrogel layer-by-layer crosslinking as claimed in claim 8, wherein the gel discharge process in the fifth step is carried out under the conditions of raising the temperature to 650 ℃ at a speed of 0.2-1 ℃/min and continuing for 1-3 hours.
10. The method for preparing a continuous gradient porous ceramic by hydrogel layer-by-layer crosslinking as claimed in claim 9, wherein the sintering temperature in the fourth step is 600-.
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