CN110894166A - Preparation method of mullite porous graded fibrous material - Google Patents
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Abstract
A preparation method of mullite porous graded fibrous material relates to a preparation method of mullite graded material. The invention aims to solve the technical problem that the quality of whiskers and fibers cannot be accurately regulated and controlled in a large range in the conventional mullite porous graded fibrous material. Firstly, the invention uses the low-cost polycrystalline mullite fiber to be dedicated to the research of low-cost high-performance samples. The mullite graded fibrous material prepared by combining the control of fiber sedimentation and the injection method can regulate and control the proportion of whiskers and fibers in a graded structure on the premise of controlling low density.
Description
Technical Field
The invention relates to a preparation method of a mullite grading material.
Background
The mullite porous fibrous material is an aluminosilicate material and has the advantages of a three-dimensional network structure similar to a bird nest, good high-temperature mechanical property, good chemical stability, low thermal conductivity, good creep property, good thermal shock resistance and the like. The preparation process can be adjusted to meet the performance requirements of different application occasions, such as gas and wastewater filters, separation membranes, catalyst carriers, heat-insulating materials and other fields with special requirements on pore diameter and structure. At present, a plurality of synthesis methods are used for obtaining a mullite whisker/fiber hierarchical structure with a three-dimensional structure, and mainly comprise a fluorine catalytic gas phase reaction method, a molten salt method and the like. The preparation of mullite whiskers where different aluminum sources (e.g., bauxite, aluminum nitrate), different silicon sources (e.g., silica, tetraethylorthosilicate), different catalysts (e.g., aluminum fluoride, ammonium fluoride, silicon fluoride) have a significant impact on whisker morphology and synthesis conditions has also been attempted by many researchers.
At present, research on the grading material mostly focuses on exploration of a whisker growth mechanism and influence of the proportion of effective components (an aluminum source, a silicon source and a catalyst) in raw materials on the organizational structure and the performance of the grading material, the research on the influence of the whisker to fiber mass ratio in the grading structure on the organizational structure and the performance of the grading material is less, researchers mostly adopt a suction filtration method to change the mass ratio of the whisker to the fiber in the grading material, and the method cannot accurately regulate and control the quality ratio of the whisker to the fiber in a large range. On the basis, the development of a method for preparing the grading material with the adjustable whisker-to-fiber mass ratio is of great significance.
Disclosure of Invention
The invention provides a preparation method of a mullite porous graded fibrous material, aiming at solving the technical problem that the quality of whiskers and fibers cannot be accurately regulated and controlled in a large range in the existing mullite porous graded fibrous material.
The preparation method of the mullite porous graded fibrous material is carried out according to the following steps:
firstly, preparing a fiber blank:
①, cutting the polycrystalline mullite fiber into granular fiber by using scissors, sieving the granular fiber by a 40-mesh sieve, and repeatedly sieving the granular fiber until no slag balls are sieved out, wherein the diameter of the granular fiber is 0.5-3 mm;
②, putting the screened granular fibers obtained in the step ① into a soybean milk machine, adding water, and chopping for 20-22 min, wherein the rotation speed of the soybean milk machine is 10000-11000 r/min, and the mass ratio of the screened granular fibers obtained in the step ① to the water is 1 (100-110);
③, after chopping, pouring the whole mixture in the soybean milk machine in the step ② into a container, standing for 20-25 s, then pouring out and retaining the upper layer of fiber, and removing the slag balls at the bottom;
④, repeating the operation of step ③ four to five times to remove the slag balls, and filtering the upper layer fiber with gauze to remove water to obtain chopped fiber;
⑤, dispersing the fibers chopped in the step ④ into an acetic acid aqueous solution containing a binder, and then stirring for 30-35 min to obtain a slurry, wherein the mass of the chopped fibers is 1.5-2% of that of the acetic acid aqueous solution containing the binder, the concentration of acetic acid in the acetic acid aqueous solution containing the binder is 0.5-0.6 mol/L, the binder in the acetic acid aqueous solution containing the binder is chitosan, and the mass of the binder is 0.5-2% of that of the acetic acid aqueous solution containing the binder;
⑥, pouring the slurry prepared in the step ⑤ into a mould, putting the mould into a refrigerator, freezing at the temperature of-18 to-20 ℃ until the slurry is completely frozen, putting the mould into a freeze dryer, freeze-drying at the temperature of-60 to-65 ℃ and the vacuum degree of 150 to 200Pa for 72 to 74 hours, and finally demoulding to obtain a fiber blank;
secondly, preparing active slurry:
①, preparation of aluminum sol:
mixing aluminum sec-butoxide with water, refluxing and stirring for 1-1.5 h under the condition of 70-100 ℃ water bath, then adding dilute nitric acid aqueous solution to promote further hydrolysis, clarifying the solution from milky color, then continuously refluxing and stirring for 9-10 h under the condition of 70-100 ℃ water bath to obtain mixed sol, and putting the mixed sol into a constant-temperature stirring heater at 100-110 ℃ until the content of Al element in the mixed sol is 1-1.1 mol/L to obtain aluminum sol; the molar ratio of the aluminum sec-butoxide to the water is 0.12 (9.5-10); the mass ratio of the dilute nitric acid aqueous solution to the secondary aluminum butoxide is 1 (6-7);
②, preparation of silica sol:
pouring tetraethoxysilane into a beaker, adding absolute ethyl alcohol, adding deionized water to form a mixed solution, putting the mixed solution into a water bath at the temperature of 40-50 ℃ for heat preservation for 10-15 min, dripping hydrochloric acid aqueous solution, and preserving the heat for 1-1.5 h in the water bath at the temperature of 40-50 ℃ to obtain silica sol; the molar ratio of the ethyl orthosilicate to the deionized water is 1 (4-5); the molar ratio of the ethyl orthosilicate to the absolute ethyl alcohol is 1 (4-5); the molar ratio of the ethyl orthosilicate to the hydrogen chloride in the hydrochloric acid aqueous solution is 1 (7.5 multiplied by 10)-4～10-3) (ii) a The concentration of the hydrochloric acid aqueous solution is 0.2-0.3 mol/L;
③, mixing the aluminum sol obtained in the step II ① with the silica sol obtained in the step II ②, adding a catalyst, and stirring for 12-15 hours to obtain active slurry, wherein the catalyst is aluminum fluoride powder, the molar ratio of aluminum elements in the catalyst to aluminum elements in the aluminum sol is 3 (6-8), and the molar ratio of aluminum elements to silicon elements in the active slurry is 3 (1.1-1.3);
thirdly, injection:
injecting the active slurry in the step two into the fiber blank in the step one by using an injection device, wherein the injection device is provided with an injection head, the side wall of the injection head is provided with a plurality of holes, and the active slurry flows into the fiber blank through the holes in the side wall and the holes at the bottom of the injection head;
fourthly, freezing and freeze drying:
and putting the injected fiber blank into a refrigerator, freezing at the temperature of minus 18-minus 20 ℃ until the fiber blank is completely frozen, putting the sample into a freeze drier, freeze-drying for 72-74 h at the temperature of minus 50-minus 60 ℃ and the vacuum degree of 150-200 Pa, and taking out to obtain a graded sample blank.
Fifthly, heat treatment:
and (3) placing the graded sample blank prepared in the fourth step into a crucible, covering the crucible with a cover, then placing the crucible into a muffle furnace, heating the crucible to 800-1200 ℃ from room temperature at a heating rate of 3-5 ℃/min in the air atmosphere, preserving the heat at 800-1200 ℃ for 2-3 h, and then cooling along with the furnace to obtain the mullite porous graded fibrous material.
The invention is characterized in that:
compared with the porous fibrous material prepared by the traditional process methods such as the gravity precipitation method, the pressurized liquid discharge method, the vacuum filtration method and the like, the invention adopts the freezing injection molding method to change the viscosity of the slurry by adding the chitosan in the step one to control the fiber sedimentation so as to realize more accurate control on the density of the fiber blank; compared with the traditional method for preparing the graded fibrous material by blending and sintering the fiber and the active powder, the method firstly prepares a fiber blank with lower density, and then realizes the regulation and control of the proportion of the whiskers and the fibers in the graded fibrous material by controlling the dosage of the active slurry injected into the network structure of the fiber, thereby realizing the regulation and control of the performance of the graded fibrous material and further realizing the preparation of the optimal graded structure.
The invention has the beneficial effects that:
firstly, the invention uses the low-cost polycrystalline mullite fiber to be dedicated to the research of low-cost high-performance samples. The invention can control the low density (0.03 g/cm) of the mullite graded fibrous material prepared by combining the control of fiber sedimentation and the injection method3～0.08g/cm3) The ratio of whiskers to fibers in the hierarchical structure is regulated and controlled on the premise of (1).
Drawings
Fig. 1 is an SEM image of mullite porous graded fibrous material prepared in run one.
Detailed Description
The first embodiment is as follows: the embodiment is a preparation method of a mullite porous graded fibrous material, which is specifically carried out according to the following steps:
firstly, preparing a fiber blank:
①, cutting the polycrystalline mullite fiber into granular fiber by using scissors, sieving the granular fiber by a 40-mesh sieve, and repeatedly sieving the granular fiber until no slag balls are sieved out, wherein the diameter of the granular fiber is 0.5-3 mm;
②, putting the screened granular fibers obtained in the step ① into a soybean milk machine, adding water, and chopping for 20-22 min, wherein the rotation speed of the soybean milk machine is 10000-11000 r/min, and the mass ratio of the screened granular fibers obtained in the step ① to the water is 1 (100-110);
③, after chopping, pouring the whole mixture in the soybean milk machine in the step ② into a container, standing for 20-25 s, then pouring out and retaining the upper layer of fiber, and removing the slag balls at the bottom;
④, repeating the operation of step ③ four to five times to remove the slag balls, and filtering the upper layer fiber with gauze to remove water to obtain chopped fiber;
⑤, dispersing the fibers chopped in the step ④ into an acetic acid aqueous solution containing a binder, and then stirring for 30-35 min to obtain a slurry, wherein the mass of the chopped fibers is 1.5-2% of that of the acetic acid aqueous solution containing the binder, the concentration of acetic acid in the acetic acid aqueous solution containing the binder is 0.5-0.6 mol/L, the binder in the acetic acid aqueous solution containing the binder is chitosan, and the mass of the binder is 0.5-2% of that of the acetic acid aqueous solution containing the binder;
⑥, pouring the slurry prepared in the step ⑤ into a mould, putting the mould into a refrigerator, freezing at the temperature of-18 to-20 ℃ until the slurry is completely frozen, putting the mould into a freeze dryer, freeze-drying at the temperature of-60 to-65 ℃ and the vacuum degree of 150 to 200Pa for 72 to 74 hours, and finally demoulding to obtain a fiber blank;
secondly, preparing active slurry:
①, preparation of aluminum sol:
mixing aluminum sec-butoxide with water, refluxing and stirring for 1-1.5 h under the condition of 70-100 ℃ water bath, then adding dilute nitric acid aqueous solution to promote further hydrolysis, clarifying the solution from milky color, then continuously refluxing and stirring for 9-10 h under the condition of 70-100 ℃ water bath to obtain mixed sol, and putting the mixed sol into a constant-temperature stirring heater at 100-110 ℃ until the content of Al element in the mixed sol is 1-1.1 mol/L to obtain aluminum sol; the molar ratio of the aluminum sec-butoxide to the water is 0.12 (9.5-10); the mass ratio of the dilute nitric acid aqueous solution to the secondary aluminum butoxide is 1 (6-7);
②, preparation of silica sol:
pouring tetraethoxysilane into a beaker, adding absolute ethyl alcohol, adding deionized water to form a mixed solution, putting the mixed solution into a water bath at the temperature of 40-50 ℃ for heat preservation for 10-15 min, dripping hydrochloric acid aqueous solution, and preserving the heat for 1-1.5 h in the water bath at the temperature of 40-50 ℃ to obtain silica sol; the molar ratio of the ethyl orthosilicate to the deionized water is 1 (4-5); the molar ratio of the ethyl orthosilicate to the absolute ethyl alcohol is 1 (4-5); the molar ratio of the ethyl orthosilicate to the hydrogen chloride in the hydrochloric acid aqueous solution is 1 (7.5 multiplied by 10)-4～10-3) (ii) a The concentration of the hydrochloric acid aqueous solution is 0.2-0.3 mol/L;
③, mixing the aluminum sol obtained in the step II ① with the silica sol obtained in the step II ②, adding a catalyst, and stirring for 12-15 hours to obtain active slurry, wherein the catalyst is aluminum fluoride powder, the molar ratio of aluminum elements in the catalyst to aluminum elements in the aluminum sol is 3 (6-8), and the molar ratio of aluminum elements to silicon elements in the active slurry is 3 (1.1-1.3);
thirdly, injection:
injecting the active slurry in the step two into the fiber blank in the step one by using an injection device, wherein the injection device is provided with an injection head, the side wall of the injection head is provided with a plurality of holes, and the active slurry flows into the fiber blank through the holes in the side wall and the holes at the bottom of the injection head;
fourthly, freezing and freeze drying:
putting the injected fiber blank into a refrigerator, freezing at the temperature of minus 18-minus 20 ℃ until the fiber blank is completely frozen, putting the sample into a freeze dryer, freeze-drying for 72-74 h at the temperature of minus 50-minus 60 ℃ and the vacuum degree of 150-200 Pa, and taking out to obtain a graded sample blank;
fifthly, heat treatment:
and (3) placing the graded sample blank prepared in the fourth step into a crucible, covering the crucible with a cover, then placing the crucible into a muffle furnace, heating the crucible to 800-1200 ℃ from room temperature at a heating rate of 3-5 ℃/min in the air atmosphere, preserving the heat at 800-1200 ℃ for 2-3 h, and then cooling along with the furnace to obtain the mullite porous graded fibrous material.
The second specific embodiment is different from the first specific embodiment in that in the first step ②, the screened granular fibers obtained in the first step ① are put into a soymilk machine, water is added for chopping for 20min, the rotation speed of the soymilk machine is 10000r/min, the mass ratio of the screened granular fibers obtained in the first step ① to the water is 1:100, and the rest is the same as the first specific embodiment.
Third embodiment the third embodiment is different from the first or second embodiment in that the first ⑤ is that the fiber cut in the ④ step is dispersed in an acetic acid aqueous solution containing a binder, and then stirred for 30min to obtain a slurry, wherein the mass of the fiber cut is 1.5% of the mass of the acetic acid aqueous solution containing the binder, the concentration of acetic acid in the acetic acid aqueous solution containing the binder is 0.5mol/L, the binder in the acetic acid aqueous solution containing the binder is chitosan, and the mass of the binder is 0.5% of the mass of the acetic acid aqueous solution containing the binder.
Fourth embodiment this embodiment differs from the first to third embodiments in that in the first ⑥ the slurry prepared in the second step ⑤ is poured into a mold, the mold is placed in a refrigerator and frozen at a temperature of-18 deg.c until the slurry is completely frozen, then the mold is placed in a freeze dryer and freeze dried at-60 deg.c and a vacuum of 150Pa for 72h, and finally the mold is removed to obtain a fiber body.
Fifth embodiment the fifth embodiment is different from the fourth embodiment in that the concentration of the dilute aqueous nitric acid solution in the second step ① is 1 mol/L.
Sixth specific embodiment, the difference between the fourth specific embodiment and the sixth specific embodiment is that in the second step ③, the aluminum sol in the second step ① and the silica sol in the second step ② are mixed, then a catalyst is added, and stirring is carried out for 12 hours, so as to obtain an active slurry, wherein the catalyst is aluminum fluoride powder, the molar ratio of aluminum element in the catalyst to aluminum element in the aluminum sol is 3:7, the molar ratio of aluminum element to silicon element in the active slurry is 3:1.2, and the difference is the same as that in the fourth specific embodiment.
The seventh embodiment: the fourth difference between this embodiment and the specific embodiment is that: and step four, putting the injected fiber blank into a refrigerator, freezing the fiber blank to be completely frozen at the temperature of 18 ℃ below zero, putting the sample into a freeze dryer, freeze-drying the sample for 72 hours at the temperature of 50 ℃ below zero and the vacuum degree of 150Pa, and taking out the sample to obtain a graded sample blank. The rest is the same as the fourth embodiment.
The specific implementation mode is eight: the fourth difference between this embodiment and the specific embodiment is that: and fifthly, placing the graded sample blank prepared in the fourth step into a crucible, covering the crucible with a cover, then placing the crucible into a muffle furnace, heating the crucible to 1000 ℃ from room temperature at a heating rate of 3 ℃/min in the air atmosphere, preserving the heat for 2h at 1000 ℃, and then cooling the crucible along with the furnace to obtain the mullite porous graded fibrous material. The rest is the same as the fourth embodiment.
The invention was verified with the following tests:
test one: the test is a preparation method of mullite porous graded fibrous material, which is specifically carried out according to the following steps:
firstly, preparing a fiber blank:
①, cutting the polycrystalline mullite fiber into granular fiber by using scissors, sieving the granular fiber by a 40-mesh sieve, and repeatedly sieving the granular fiber until no slag balls are sieved out, wherein the diameter of the granular fiber is 2 mm;
②, putting the screened granular fibers obtained in the step ① into a soybean milk machine, adding water, and chopping for 20min, wherein the rotation speed of the soybean milk machine is 10000r/min, and the mass ratio of the screened granular fibers obtained in the step ① to the water is 1: 100;
③, after chopping, pouring the whole mixture in the soybean milk machine in the step ② into a container, standing for 20s, then pouring out and retaining the fibers on the upper layer, and removing the slag balls at the bottom;
④, repeating the operation of step ③ for four times to remove the slag balls, and filtering the upper layer fiber with gauze to remove water to obtain chopped fiber;
⑤, dispersing the fibers chopped in the step ④ into an acetic acid aqueous solution containing a binder, and then stirring for 30min to obtain a slurry, wherein the mass of the chopped fibers is 1.5% of that of the acetic acid aqueous solution containing the binder, the concentration of acetic acid in the acetic acid aqueous solution containing the binder is 0.5mol/, the binder in the acetic acid aqueous solution containing the binder is chitosan, and the mass of the binder is 0.5% of that of the acetic acid aqueous solution containing the binder;
⑥, pouring the slurry prepared in the step ⑤ into a mould, putting the mould into a refrigerator, freezing the mould at the temperature of 18 ℃ below zero until the slurry is completely frozen, putting the mould into a freeze dryer, freeze-drying the mould for 72 hours at the temperature of 60 ℃ below zero and the vacuum degree of 150Pa, and finally demoulding to obtain a fiber blank;
the test shows that the density of the fiber blank is 0.022g/cm3；
The fiber blank is a cylinder, the diameter of the bottom surface is 3.8cm, and the height is 2.8 cm;
secondly, preparing active slurry:
①, preparation of aluminum sol:
mixing aluminum sec-butoxide with water, refluxing and stirring for 1h under the condition of 70 ℃ water bath, then adding dilute nitric acid aqueous solution to promote further hydrolysis, changing the solution from milky color to clear, continuously refluxing and stirring for 9h under the condition of 70 ℃ water bath to obtain mixed sol, and putting the mixed sol into a constant-temperature stirring heater at 100 ℃ until the content of Al element in the mixed sol is 1mol/L to obtain aluminum sol; the molar ratio of the aluminum sec-butoxide to the water is 0.12: 10; the mass ratio of the dilute nitric acid aqueous solution to the secondary aluminum butoxide is 1: 7; the concentration of the dilute nitric acid aqueous solution is 1 mol/L;
②, preparation of silica sol:
pouring tetraethoxysilane into a beaker, adding absolute ethyl alcohol, adding deionized water to form a mixed solution, putting the mixed solution into a water bath at 40 ℃ for heat preservation for 10min, dripping hydrochloric acid aqueous solution, and preserving the heat in the water bath at 40 ℃ for 1h to obtain silica sol; the molar ratio of the ethyl orthosilicate to the deionized water is 1: 4; the molar ratio of the ethyl orthosilicate to the absolute ethyl alcohol is 1: 4; the molar ratio of the ethyl orthosilicate to the hydrogen chloride in the hydrochloric acid aqueous solution is 1:7.5 multiplied by 10-4(ii) a The concentration of the hydrochloric acid aqueous solution is 0.2 mol/L;
③, mixing the aluminum sol obtained in the step II ① with the silica sol obtained in the step II ②, adding a catalyst, and stirring for 12-15 hours to obtain active slurry, wherein the catalyst is aluminum fluoride powder, the molar ratio of aluminum elements in the catalyst to aluminum elements in the aluminum sol is 3:7, and the molar ratio of aluminum elements to silicon elements in the active slurry is 3: 1.2;
thirdly, injection:
injecting the active slurry obtained in the step two into the fiber blank obtained in the step one by using an injection device, wherein the injection device is provided with an injection head, the side wall of the injection head is provided with a plurality of holes, and 3mL of active slurry flows into the fiber blank through the holes in the side wall and the holes at the bottom of the injection head; injection depth: repeatedly moving the injection head up and down within the range of 0.9-2.8 cm away from the bottom layer of the fiber blank body for injection; injection speed: 0.05 mL/s; the injection head is a needle head;
fourthly, freezing and freeze drying:
putting the injected fiber blank into a refrigerator, freezing the fiber blank at the temperature of minus 18 ℃ until the fiber blank is completely frozen, putting the sample into a freeze drier, freeze-drying the sample for 72 hours at the temperature of minus 50 ℃ and the vacuum degree of 150Pa, and taking out the sample to obtain a graded sample blank;
fifthly, heat treatment:
and (3) placing the graded sample blank prepared in the fourth step into a crucible, covering the crucible with a cover, then placing the crucible into a muffle furnace, heating the crucible to 1000 ℃ from room temperature at a heating rate of 3 ℃/min in the air atmosphere, preserving the heat at 1000 ℃ for 2h, and then cooling along with the furnace to obtain the mullite porous graded fibrous material.
Fig. 1 is an SEM image of the mullite porous graded fibrous material prepared in the first experiment, from which it can be seen that the mullite fiber surface is a well-developed whisker secondary structure, which is needle-like and densely grown.
The mullite graded fibrous material prepared by combining the control of fiber sedimentation and the injection method in the test can control the low density (0.06 g/cm)3) And the ratio of the whiskers to the fibers in the hierarchical structure is regulated and controlled on the premise of low thermal conductivity (0.04667W/mK).
Claims (8)
1. A preparation method of a mullite porous graded fibrous material is characterized by comprising the following steps:
firstly, preparing a fiber blank:
①, cutting the polycrystalline mullite fiber into granular fiber by using scissors, sieving the granular fiber by a 40-mesh sieve, and repeatedly sieving the granular fiber until no slag balls are sieved out, wherein the diameter of the granular fiber is 0.5-3 mm;
②, putting the screened granular fibers obtained in the step ① into a soybean milk machine, adding water, and chopping for 20-22 min, wherein the rotation speed of the soybean milk machine is 10000-11000 r/min, and the mass ratio of the screened granular fibers obtained in the step ① to the water is 1 (100-110);
③, after chopping, pouring the whole mixture in the soybean milk machine in the step ② into a container, standing for 20-25 s, then pouring out and retaining the upper layer of fiber, and removing the slag balls at the bottom;
④, repeating the operation of step ③ four to five times to remove the slag balls, and filtering the upper layer fiber with gauze to remove water to obtain chopped fiber;
⑤, dispersing the fibers chopped in the step ④ into an acetic acid aqueous solution containing a binder, and then stirring for 30-35 min to obtain a slurry, wherein the mass of the chopped fibers is 1.5-2% of that of the acetic acid aqueous solution containing the binder, the concentration of acetic acid in the acetic acid aqueous solution containing the binder is 0.5-0.6 mol/L, the binder in the acetic acid aqueous solution containing the binder is chitosan, and the mass of the binder is 0.5-2% of that of the acetic acid aqueous solution containing the binder;
⑥, pouring the slurry prepared in the step ⑤ into a mould, putting the mould into a refrigerator, freezing at the temperature of-18 to-20 ℃ until the slurry is completely frozen, putting the mould into a freeze dryer, freeze-drying at the temperature of-60 to-65 ℃ and the vacuum degree of 150 to 200Pa for 72 to 74 hours, and finally demoulding to obtain a fiber blank;
secondly, preparing active slurry:
①, preparation of aluminum sol:
mixing aluminum sec-butoxide with water, refluxing and stirring for 1-1.5 h under the condition of 70-100 ℃ water bath, then adding dilute nitric acid aqueous solution to promote further hydrolysis, clarifying the solution from milky color, then continuously refluxing and stirring for 9-10 h under the condition of 70-100 ℃ water bath to obtain mixed sol, and putting the mixed sol into a constant-temperature stirring heater at 100-110 ℃ until the content of Al element in the mixed sol is 1-1.1 mol/L to obtain aluminum sol; the molar ratio of the aluminum sec-butoxide to the water is 0.12 (9.5-10); the mass ratio of the dilute nitric acid aqueous solution to the secondary aluminum butoxide is 1 (6-7);
②, preparation of silica sol:
pouring tetraethoxysilane into a beaker, adding absolute ethyl alcohol, adding deionized water to form a mixed solution, putting the mixed solution into a water bath at the temperature of 40-50 ℃ for heat preservation for 10-15 min, dripping hydrochloric acid aqueous solution, and preserving the heat for 1-1.5 h in the water bath at the temperature of 40-50 ℃ to obtain silica sol; the molar ratio of the ethyl orthosilicate to the deionized water is 1 (4-5); the molar ratio of the ethyl orthosilicate to the absolute ethyl alcohol is 1 (4-5); the molar ratio of the ethyl orthosilicate to the hydrogen chloride in the hydrochloric acid aqueous solution is 1 (7.5 multiplied by 10)-4～10-3) (ii) a The concentration of the hydrochloric acid aqueous solution is 0.2-0.3 mol/L;
③, mixing the aluminum sol obtained in the step II ① with the silica sol obtained in the step II ②, adding a catalyst, and stirring for 12-15 hours to obtain active slurry, wherein the catalyst is aluminum fluoride powder, the molar ratio of aluminum elements in the catalyst to aluminum elements in the aluminum sol is 3 (6-8), and the molar ratio of aluminum elements to silicon elements in the active slurry is 3 (1.1-1.3);
thirdly, injection:
injecting the active slurry in the step two into the fiber blank in the step one by using an injection device, wherein the injection device is provided with an injection head, the side wall of the injection head is provided with a plurality of holes, and the active slurry flows into the fiber blank through the holes in the side wall and the holes at the bottom of the injection head;
fourthly, freezing and freeze drying:
putting the injected fiber blank into a refrigerator, freezing at the temperature of minus 18-minus 20 ℃ until the fiber blank is completely frozen, putting the sample into a freeze dryer, freeze-drying for 72-74 h at the temperature of minus 50-minus 60 ℃ and the vacuum degree of 150-200 Pa, and taking out to obtain a graded sample blank;
fifthly, heat treatment:
and (3) placing the graded sample blank prepared in the fourth step into a crucible, covering the crucible with a cover, then placing the crucible into a muffle furnace, heating the crucible to 800-1200 ℃ from room temperature at a heating rate of 3-5 ℃/min in the air atmosphere, preserving the heat at 800-1200 ℃ for 2-3 h, and then cooling along with the furnace to obtain the mullite porous graded fibrous material.
2. The method for preparing the mullite porous graded fibrous material according to claim 1, wherein in the step one ②, the screened granular fibers obtained in the step ① are put into a soybean milk machine, water is added into the soybean milk machine for chopping for 20min, the rotation speed of the soybean milk machine is 10000r/min, and the mass ratio of the screened granular fibers obtained in the step ① to the water is 1: 100.
3. The method for preparing the mullite porous graded fibrous material as claimed in claim 1, wherein the step ⑤ comprises dispersing the fibers chopped in the step ④ into an aqueous acetic acid solution containing a binder, and stirring for 30min to obtain a slurry, wherein the mass of the chopped fibers is 1.5% of that of the aqueous acetic acid solution containing the binder, the concentration of acetic acid in the aqueous acetic acid solution containing the binder is 0.5mol/L, the binder in the aqueous acetic acid solution containing the binder is chitosan, and the mass of the binder is 0.5% of that of the aqueous acetic acid solution containing the binder.
4. The method of claim 1, wherein in step one ⑥, the slurry prepared in step ⑤ is poured into a mold, the mold is placed in a refrigerator and frozen at a temperature of-18 ℃ to completely freeze the slurry, then the mold is placed in a freeze dryer and freeze-dried at-60 ℃ and a vacuum degree of 150Pa for 72h, and finally the fiber blank is obtained after demolding.
5. The method according to claim 1, wherein the concentration of the dilute aqueous nitric acid solution in step two ① is 1 mol/L.
6. The method for preparing the mullite porous graded fibrous material as claimed in claim 1, wherein the aluminum sol obtained in the step two ① and the silica sol obtained in the step two ② are mixed in the step two ③, then the mixture is added with a catalyst, and the mixture is stirred for 12 hours to obtain an active slurry, wherein the catalyst is aluminum fluoride powder, the molar ratio of aluminum elements in the catalyst to aluminum elements in the aluminum sol is 3:7, and the molar ratio of the aluminum elements to the silicon elements in the active slurry is 3: 1.2.
7. The method for preparing the mullite porous graded fibrous material according to claim 1, wherein the step four is to put the injected fiber blank into a refrigerator, freeze the fiber blank at a temperature of 18 ℃ below zero until the fiber blank is completely frozen, put the sample into a freeze dryer, freeze-dry the sample for 72 hours at 50 ℃ below zero and under a vacuum degree of 150Pa, and take out the sample to obtain a graded sample blank.
8. The method for preparing the mullite porous graded fibrous material according to claim 1, wherein in the fifth step, the graded sample blank prepared in the fourth step is placed in a crucible, the crucible is covered by a cover, then the crucible is placed in a muffle furnace, the temperature is raised to 1000 ℃ from room temperature at the heating rate of 3 ℃/min under the air atmosphere, the temperature is kept for 2h at 1000 ℃, and then the mullite porous graded fibrous material is obtained after furnace cooling.
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