CN114603938B - Preparation method of layered polyimide/alumina composite aerogel - Google Patents

Abstract

A preparation method of layered polyimide/alumina composite airgel relates to a preparation method of alumina composite airgel. The present invention aims to solve the technical problem that the current polyimide is difficult to apply at high temperature. The invention prepares the upper and lower layered polyimide/alumina composite airgel, which is mainly used for heat protection in the fields of aerospace, space construction, automobile industry and the like in high temperature environment, and is used as heat insulation protection. Since polyimide will thermally decompose at high temperature, the aluminum oxide airgel is directly exposed to the high temperature environment during use, and the polyimide airgel is on the lower side of the temperature. By adjusting the relative temperature of the two Thickness, so that the temperature reaches its stable temperature range when it reaches the polyimide airgel, so as to realize the effect of high temperature insulation of alumina airgel and low temperature insulation effect of polyimide airgel, and play the role of mutual insulation Thermal advantage.

Description

A kind of preparation method of layered polyimide/alumina composite airgel

technical field

The invention relates to a preparation method of alumina composite airgel.

Background technique

In recent years, with the rapid development of the aerospace industry, more and more people pay more and more attention to advanced high temperature resistant and heat insulating materials. Al ₂ O ₃ airgel has the advantages of high specific surface area, low density, high porosity and extremely low thermal conductivity, etc., and has excellent high temperature resistance and heat insulation performance. It is an ideal thermal protection material for aerospace, but it is poor Mechanical properties severely limit their potential applications. As a representative of organic aerogels, polyimide (PI) airgel not only has the characteristics of good mechanical properties of polymer materials, but also has excellent high temperature stability and low thermal conductivity, which is unique to other organic aerogels. Incomparable, especially in the medium and low temperature heat insulation, it is expected to become a candidate material for heat insulation applications.

Contents of the invention

The present invention aims to solve the technical problem that the current polyimide is difficult to apply at high temperature, and provides a preparation method of layered polyimide/alumina composite airgel.

The preparation method of layered polyimide/alumina composite airgel of the present invention is carried out according to the following steps:

1. Prepare the water-soluble polyimide precursor solution: add the solvent N,N-dimethylacetamide (DMAc) into the there-necked flask, then add 4,4'-diaminodiphenyl ether (ODA), at room temperature Stir under low temperature for 20 minutes; under the condition of ice-water bath, add pyromellitic dianhydride in batches, and mechanically stir for 5h-6h. After the stirring is completed, wash the obtained solution with deionized water and soak for 24h-25h to obtain polyimide Precipitate the precursor, then dry it in an oven at 50°C to 55°C to obtain a polyimide precursor; mix the polyimide precursor, triethylamine and deionized water evenly, and stir at room temperature for 5h to 6h , the water-soluble polyimide precursor solution can be obtained;

The volume ratio of the mass of 4,4'-diaminodiphenyl ether to N,N-dimethylacetamide is 1g:(25mL～26mL);

The molar ratio of the pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is (1-1.05):1;

The specific operation of adding pyromellitic dianhydride in batches is as follows: a total of 6 times, half of the total amount is added for the first time, the remaining half is added for the second to fifth times, and the remaining half is added for the sixth time. Add all of them, the interval between the first four times is 15min~18min, the interval between the last two times is 20min~25min, and stir during the interval;

The mass ratio of described polyimide precursor, triethylamine and deionized water is 2:1:(17～18);

2. Preparation of Al ₂ O ₃ slurry: Mix aluminum sec-butoxide and water evenly, and stir in a water bath at 90°C to 95°C for 1h to 1.5h; add nitric acid to promote the hydrolysis and condensation reaction of the organic alcohol aluminum salt until the solution turns from turbid until the milky white gradually becomes clear, and then continue to stir at 90°C to 95°C for 9h to 10h to remove the residual sec-butanol and nitrate in the solution to obtain the Al ₂ O ₃ aqueous sol; the prepared Al ₂ O ₃ water The sol is left at room temperature for 72h~73h to age and increase the viscosity, after that, add Al ₂ O ₃ nanoparticles to make the solid phase content of Al ₂ O ₃ reach 15wt.%~17wt.%; add hydroxyethyl cellulose binder, and stirred in a water bath at 50°C to 55°C for 2h to 3h to obtain Al ₂ O ₃ slurry;

The mol ratio of described aluminum sec-butoxide and water is 1:(60～65);

The mass of the hydroxyethyl cellulose is 0.5% to 0.6% of the mass of _{the Al2O3} hydrosol;

3. Preparation of layered composite airgel: Add the Al ₂ O ₃ slurry prepared in step 2 into the polytetrafluoroethylene mold, let it stand to make the surface smooth, and then add the water-soluble polyimide precursor prepared in step 1 Because the two solutions are immiscible, and there are differences in viscosity and density, a natural layering phenomenon is formed. The polyimide is on the upper layer, and the Al ₂ O ₃ is on the lower layer. Liquid nitrogen is used as a cold source for directional freezing to the slurry. The material is completely frozen; use a freeze dryer with a pressure of 0.03mbar to freeze-dry the sample at -50°C to -60°C for 48h to 50h to remove moisture. After drying, heat the sample to 100°C to 105°C and keep it warm for 1h to 1.5h, then Heating to 200°C-205°C and keeping it warm for 1h-1.5h, then heating to 300°C-305°C and keeping it warm for 1h-1.5h, cooling naturally to room temperature to get layered PI/Al ₂ O ₃ composite airgel; all The heating rate is 3°C/min～5°C/min;

The polytetrafluoroethylene mold is a cuboid structure;

The directional freezing of the sides is to only freeze the two adjacent sides of the polytetrafluoroethylene mold.

Most of the traditional PI composite airgel preparation methods are to uniformly mix polyimide and other materials together to obtain composite materials with various performance enhancements. In the layered _PI /Al ₂ O ₃ composite airgel of the present invention, one layer is polyimide, and one layer is aluminum oxide _. The integrated preparation of glue not only exerts the good low-temperature heat insulation effect of PI airgel, but also makes up for the characteristics of PI airgel’s low temperature resistance and Al ₂ O ₃ airgel’s poor mechanical properties. The obvious superior heat insulation effect is expected to develop into a new type of heat insulation material.

The present invention adopts sol-gel method to prepare respectively Al ₂ O ₃ slurry and water-soluble polyimide precursor polyamic acid (PAA), by controlling the viscosity and density of the liquid, the two are naturally layered, and then through Directional freezing, freeze-drying, and heat treatment can obtain PI/Al ₂ O ₃ composite airgel with upper and lower layered structures, which can achieve the effect of Al ₂ O ₃ high temperature insulation and PI medium and low temperature insulation. The integrated design of PI/Al ₂ O ₃ airgel, on the one hand, can achieve good thermal insulation effect, on the other hand, it can also improve the overall mechanical properties of the composite material, providing a new idea for the research and development of thermal management materials.

Method of use and principle: The layered PI/Al ₂ O ₃ composite airgel prepared by the present invention is mainly used for thermal protection in aerospace, space construction, automobile industry and other fields in high-temperature environments, and is used as heat insulation protection. Imine will thermally decompose at high temperature. Therefore, during use, the alumina airgel is directly exposed to the high temperature environment, and the polyimide airgel is on the side with a lower temperature. By adjusting the relative thickness of the two, the When the temperature reaches the polyimide airgel, it reaches its stable operating temperature range, so as to realize the effect of high temperature insulation of alumina airgel and medium and low temperature insulation of polyimide airgel, and take advantage of each other's heat insulation advantages.

Description of drawings

Fig. 1 is the device schematic diagram that carries out lateral directional freezing in the step 3 of test one;

Fig. 2 is the physical photo of the sample after step 3 freeze-drying (before heat treatment) of test one;

Fig. 3 is the layered PI/ _Al of test one preparation O _Composite airgel (after heat treatment) physical photo;

Fig. 4 is the FT-IR figure of Al ₂ O ₃ part in the sample (before heat treatment) after step 3 freeze-drying of test one;

Fig. 5 is the FT-IR figure of PI airgel before and after heat treatment;

Fig. 6 _is the low magnification SEM image of the layered PI/ _Al2O3 composite airgel (after heat treatment) prepared in test one;

Figure 7 is a high-magnification SEM image of the layered PI/Al ₂ O ₃ composite airgel (after heat treatment) prepared in Experiment 1;

Fig. 8 is the TG curve _of the PI part in the layered PI/ _Al2O3 composite airgel (after heat treatment) that test one prepares;

Fig. 9 is the TG-DSC curve of the _Al2O3 part in _the sample after step 3 freeze-drying (before heat treatment) of test one;

Fig. 10 is a test diagram of the thermal insulation ability of the layered PI/Al ₂ O ₃ composite airgel prepared in Experiment 1.

Detailed ways

Embodiment 1: This embodiment is a preparation method of layered polyimide/alumina composite airgel, which is carried out in the following steps:

1. Preparation of water-soluble polyimide precursor solution: Since the PAA prepared by the conventional method quickly forms a precipitate when it encounters water, during the preparation of the layered PI/Al ₂ O ₃ airgel, the PAA is poured into the Al ₂ O 3 ₃ slurry, a layer of white precipitate will be quickly formed along the surface of PAA to wrap it, making it difficult to form a layered material with an upper and lower layered structure. In addition, because the freezing point of the organic solvent N,N-dimethylacetamide (DMAc) is very low, the sample is easy to melt before drying and difficult to store. In order to solve the above problems, this embodiment adopts the method of preparing water-soluble PAA so as to avoid the above problems, the specific process is as follows:

Add the solvent N,N-dimethylacetamide (DMAc) into the three-necked flask, then add 4,4'-diaminodiphenyl ether (ODA), stir at room temperature for 20min; Pyromellitic dianhydride, mechanically stirred for 5h-6h, after the stirring, the obtained solution was washed with deionized water, soaked for 24h-25h, and the polyimide precursor was precipitated, and then put into an oven at 50℃～ Dry at 55°C to obtain a polyimide precursor; mix the polyimide precursor, triethylamine and deionized water evenly, and stir at room temperature for 5h to 6h to obtain a water-soluble polyimide precursor solution ;

The volume ratio of the mass of 4,4'-diaminodiphenyl ether to N,N-dimethylacetamide is 1g:(25mL～26mL);

The molar ratio of the pyromellitic dianhydride to 4,4'-diaminodiphenyl ether is (1-1.05):1;

The specific operation of adding pyromellitic dianhydride in batches is as follows: a total of 6 times, half of the total amount is added for the first time, the remaining half is added for the second to fifth times, and the remaining half is added for the sixth time. Add all of them, the interval between the first four times is 15min~18min, the interval between the last two times is 20min~25min, and stir during the interval;

The mass ratio of described polyimide precursor, triethylamine and deionized water is 2:1:(17～18);

2. Preparation of Al ₂ O ₃ slurry: Mix aluminum sec-butoxide and water evenly, and stir in a water bath at 90°C to 95°C for 1h to 1.5h; add nitric acid to promote the hydrolysis and condensation reaction of the organic alcohol aluminum salt until the solution turns from turbid until the milky white gradually becomes clear, and then continue to stir at 90°C to 95°C for 9h to 10h to remove the residual sec-butanol and nitrate in the solution to obtain the Al ₂ O ₃ aqueous sol; the prepared Al ₂ O ₃ water The sol is left at room temperature for 72h~73h to age and increase the viscosity, after that, add Al ₂ O ₃ nanoparticles to make the solid phase content of Al ₂ O ₃ reach 15wt.%~17wt.%; add hydroxyethyl cellulose binder, and stirred in a water bath at 50°C to 55°C for 2h to 3h to obtain Al ₂ O ₃ slurry;

The mol ratio of described aluminum sec-butoxide and water is 1:(60～65);

The mass of the hydroxyethyl cellulose is 0.5% to 0.6% of the mass of _{the Al2O3} hydrosol;

3. Preparation of layered composite airgel: Add the Al ₂ O ₃ slurry prepared in step 2 into the polytetrafluoroethylene mold, let it stand to make the surface smooth, and then add the water-soluble polyimide precursor prepared in step 1 Because the two solutions are immiscible, and there are differences in viscosity and density, a natural layering phenomenon is formed. The polyimide is on the upper layer, and the Al ₂ O ₃ is on the lower layer. Liquid nitrogen is used as a cold source for directional freezing to the slurry. The material is completely frozen; use a freeze dryer with a pressure of 0.03mbar to freeze-dry the sample at -50°C to -60°C for 48h to 50h to remove moisture. After drying, heat the sample to 100°C to 105°C and keep it warm for 1h to 1.5h, then Heating to 200°C-205°C and keeping it warm for 1h-1.5h, then heating to 300°C-305°C and keeping it warm for 1h-1.5h, cooling naturally to room temperature to get layered PI/Al ₂ O ₃ composite airgel; all The heating rate is 3°C/min～5°C/min;

The polytetrafluoroethylene mold is a cuboid structure;

The directional freezing of the sides is to only freeze the two adjacent sides of the polytetrafluoroethylene mold.

Specific embodiment 2: The difference between this embodiment and specific embodiment 1 is that the volume ratio of the mass of 4,4'-diaminodiphenyl ether described in step 1 to N,N-dimethylacetamide is 1g : 25mL. Others are the same as the first embodiment.

Specific embodiment 3: The difference between this embodiment and specific embodiment 1 or 2 is that the molar ratio of pyromellitic dianhydride to 4,4'-diaminodiphenyl ether in step 1 is 1.01:1. Others are the same as those in Embodiment 1 or 2.

Embodiment 4: The difference between this embodiment and one of Embodiments 1 to 3 is that the mass ratio of the polyimide precursor, triethylamine and deionized water described in step 1 is 2:1:17. Others are the same as one of the specific embodiments 1 to 3.

Embodiment 5: This embodiment is different from Embodiment 4 in that: the molar ratio of aluminum sec-butoxide and water described in step 2 is 1:60. Others are the same as in Embodiment 4.

Embodiment 6: This embodiment is different from Embodiment 4 in that: the mass of hydroxyethyl cellulose described in step 2 is 0.5% of the mass of Al ₂ O ₃ hydrosol. Others are the same as in Embodiment 4.

Embodiment 7: This embodiment differs from Embodiment 4 in that: in step 2, Al ₂ O ₃ nanoparticles are added to make the solid phase content of Al ₂ O ₃ reach 15 wt.%. Others are the same as in Embodiment 4.

Embodiment 8: This embodiment is different from Embodiment 4 in that: in step 3, a freeze dryer with a pressure of 0.03 mbar is used to freeze-dry the sample at -50° C. for 50 hours to remove moisture. Others are the same as in Embodiment 4.

Embodiment 9: This embodiment is different from Embodiment 4 in that: after drying in step 3, heat the sample in argon to 100°C and keep it for 1h, then heat it to 200°C and keep it for 1h, and then heat it to 300°C ℃ and keep it warm for 1h, then naturally cool to room temperature to obtain layered PI/Al ₂ O ₃ composite airgel. Others are the same as in Embodiment 4.

Embodiment 10: This embodiment is different from Embodiment 4 in that all the heating rates in Step 3 are 5° C./min. Others are the same as in Embodiment 4.

The present invention is verified with following test:

Test 1: This test is a preparation method of layered polyimide/alumina composite airgel, which is carried out according to the following steps:

1. Preparation of water-soluble polyimide precursor solution: Add 100mL of solvent N,N-dimethylacetamide into a three-necked flask, then add 3.97g of 4,4'-diaminodiphenyl ether, Stir for 20min; under the condition of ice-water bath, add 4.38g of pyromellitic dianhydride in batches, mechanically stir for 5h, after stirring, wash the obtained solution with deionized water, soak for 24h, and obtain the polyimide precursor Precipitate, then put into an oven and dry at 50°C to obtain a polyimide precursor; 10g of polyimide precursor, 5g of triethylamine and 85g of deionized water are evenly mixed, stirred at room temperature for 5h, A water-soluble polyimide precursor solution can be obtained;

The specific operation of adding pyromellitic dianhydride in batches is as follows: a total of 6 times, half of the total amount is added for the first time, the remaining half is added for the second to fifth times, and the remaining half is added for the sixth time. Add all of them, the interval between the first four times is 15min, the interval between the last two times is 20min, and stir during the interval;

2. Preparation of Al ₂ O ₃ slurry: Mix aluminum sec-butoxide and water evenly, and stir in a water bath at 90°C for 1 hour; add nitric acid to promote the hydrolysis and condensation reaction of the organic alcohol aluminum salt until the solution gradually changes from turbid milky white to clear Then continue to stir at 90°C for 9h to remove the residual sec-butanol and nitrate in the solution to obtain Al ₂ O ₃ hydrosol; place the configured Al ₂ O ₃ hydrosol at room temperature for 72h to age And increase the viscosity, after that, add Al ₂ O ₃ nanoparticles to make the solid phase content of Al ₂ O ₃ reach 15wt.%; add hydroxyethyl cellulose binder, and stir in a water bath at 50°C for 2h to obtain Al ₂ O ₃ slurry;

The mol ratio of described aluminum sec-butoxide and water is 1:60;

The mass of the hydroxyethyl cellulose is 0.5% of the _Al2O3 hydrosol mass _;

3. Preparation of layered composite airgel: Add 20mL of the Al ₂ O ₃ slurry prepared in step 2 into a polytetrafluoroethylene mold, let it stand to make the surface smooth, and then add 20mL of the water-soluble polyimide prepared in step 1 Amine precursor solution, because the two solutions are immiscible, and the viscosity and density are different, forming a natural layering phenomenon, polyimide is in the upper layer, Al ₂ O ₃ is in the lower layer, and the side is directional frozen by using liquid nitrogen as a cold source Until the slurry is completely frozen; use a freeze dryer with a pressure of 0.03mbar to freeze-dry the sample 50 at -50°C to remove moisture. After drying, heat the sample to 100°C in argon and keep it warm for 1h, then heat it to 200°C and keep it warm 1h, then heated to 300°C and held for 1h, then naturally cooled to room temperature to obtain layered PI/Al ₂ O ₃ composite airgel; all heating rates were 5°C/min;

The polytetrafluoroethylene mold is a cuboid structure;

The specific method of directional freezing is: as shown in Figure 1, polytetrafluoroethylene mold 1 is placed in a corner of a rectangular foam box 2, two adjacent sides of polytetrafluoroethylene mold 1 and foam box 2 The two adjacent inner walls are closely attached, and then liquid nitrogen 3 is added to the foam box 2 to perform directional freezing on the side of the polytetrafluoroethylene mold 1 , and the liquid nitrogen cannot enter the inner cavity of the polytetrafluoroethylene mold 1 .

Fig. 2 is the physical photograph of the sample after step 3 freeze-drying (before heat treatment) of test one, _and Fig. 3 is the layered PI/ _Al of test one preparation O Composite airgel (after heat treatment) physical photograph, all is The lower layer is Al ₂ O ₃ airgel, which is white before heat treatment, with almost no volume shrinkage; the upper layer is PI airgel, which is light yellow before heat treatment, and turns golden yellow after thermal imidization, with a certain volume shrinkage.

Cut the sample (before heat treatment) after freeze-drying in step 3 of Test 1 from the layered part, and take the Al ₂ O ₃ part for testing. Figure 4 is its FT-IR diagram. By observing the infrared absorption peak, it can be seen that at 3090cm The peaks ^{at -1} , 1160cm ^-1 and 1065cm ^-1 respectively correspond to the absorption peak of symmetric ^stretching vibration, asymmetric bending vibration and closing peak of symmetric bending vibration ^of Al-OH. The Boehmite peaks correspond to the torsional vibration, stretching vibration and bending vibration of Al-O respectively, and the water absorption peaks appear near 3430cm ^-1 and 163cm ^-1 , corresponding to the stretching vibration and bending vibration of HOH. , there is an obvious absorption peak at 1380cm ^-1 , which represents the symmetrical absorption vibration of CH, indicating that there are certain organic groups in the sample.

Fig. 5 is the FT-IR figure of PI airgel before _and after heat treatment, and curve 1 is the PI part in the layered PI/ _Al2O3 composite airgel (after heat treatment) that test one prepares, and curve 2 is the step of test one 3. The PAA part in the sample after freeze-drying (before heat treatment), it can be seen that after the high-temperature thermal imidization process, the PAA airgel removes small molecule water and triethylamine to generate PI, which can be seen from the FT-IR figure It can be seen that both PAA and PI aerogels have absorption peaks around 1496cm ^-1 , indicating that benzene rings can exist stably at 300°C during high temperature treatment. The stretching vibration of the carbon-oxygen double bond of the amide bond (-CO-NH-) in PAA airgel appeared near the position 1660cm ^-1 , and the carbonyl group of the amide bond disappeared after imidization. The asymmetric and symmetric stretching vibration peaks of the carbon group on the imine ring appeared near the 1774cm ^-1 and 1715cm ^-1 positions of the PI airgel, and the stretching and stretching of the imine bond (CNC) in the imine ring appeared at the 1365cm ^-1 position Vibration peak, indicating that the dehydration ring closure of imidization is completed.

Fig. 6 is the low magnification SEM image of the layered PI/Al ₂ O ₃ composite airgel (after heat treatment) prepared in test one, and Fig. 7 is the layered PI/Al ₂ O ₃ composite airgel prepared in test one (heat treatment After) the high-magnification SEM image, it can be seen that in the process of directional freezing with the cold source on the side, the ice crystals will grow along the direction perpendicular to the cold source on the side, with a parallel layered structure, and the pore size is in the range of several microns to tens of microns. There is a structure like a "stent" between layers. This layered structure makes the properties of the material have obvious anisotropy, showing good compression resilience in the vertical direction, and this "layer-air-layer" similar sandwich structure makes the material's thermal insulation ability greatly Improvement helps the material to exert a good thermal insulation effect, which is very consistent with the ideal expectation before the experiment.

Fig. 8 is the TG curve of the PI part in the layered PI/ _{Al2O3 composite} airgel (after heat treatment) that test one prepares, and curve 1 corresponds to the ordinate on the left side, and curve 2 corresponds to the ordinate on the right side, from the figure It can be seen that the thermal decomposition initiation temperature of PI airgel is around 460 °C, and the thermal weight loss rate is the fastest at 573 °C, and its residual mass is still 90% of the initial mass at 515 °C, showing good thermal stability. sex.

Fig. 9 is the TG-DSC curve of the Al ₂ O ₃ part in the sample after step 3 freeze-drying (before heat treatment) of test one, and curve 1 corresponds to the ordinate on the left, and curve 2 corresponds to the ordinate on the right, from the TG curve ( According to curve 1), the heat treatment process of Al ₂ O ₃ airgel mainly has four steps of weight loss. The first weight loss step is between room temperature and 140°C, and the mass loss rate is 14.51%. In the DSC curve (curve 2), an endothermic peak appears around 95°C, which corresponds to the evaporation of water in the sample. The second weight loss step is between 140°C and 180°C, and the mass loss rate is 3.11%. There is an exothermic peak at 151°C, which may be caused by the pyrolysis and combustion of residual organic matter in the reaction. The third weight loss step is between 180°C and 255°C, and the mass loss rate is 5.14%. There is no obvious endothermic or exothermic peak in this process, but there is a small and broad exothermic peak, which represents the slow burning process of hydroxyethyl cellulose in the sample. The fourth weight loss step is between 255°C and 1300°C, and the mass loss rate is 11.1%. An obvious exothermic peak appeared at 338°C, which corresponded to the combustion process of butanol in the hydrolysis reaction product. A series of complex phase transitions will occur in alumina between 300°C and 1300°C, and finally transform from boehmite to α-Al ₂ O ₃ .

In order to characterize the thermal insulation ability of the layered PI/Al ₂ O ₃ composite airgel prepared in Test 1, the prepared composite airgel sample was placed on the surface of a heating table at 600 °C, with PI on it, and the sample size was 10×10 ×10(mm), heating for 600s to record the temperature change of the cold surface (upper surface) and the hot surface (lower surface) of the sample, as shown in Figure 10, curve 1 is the hot surface, curve 2 is the cold surface, it can be seen that the cold surface The surface temperature increased from the initial 30.7°C to 69.8°C at the end, and the temperature only increased by 39.1°C after 600s of high-temperature heat treatment at 600°C, which proved the excellent thermal insulation ability of PI/Al ₂ O ₃ composite airgel.

Claims (10)

1. A preparation method of layered polyimide/alumina composite aerogel is characterized in that the preparation method of the layered polyimide/alumina composite aerogel is carried out according to the following steps:

1. preparing a water-soluble polyimide precursor solution: adding a solvent N, N-dimethylacetamide into a three-necked bottle, then adding 4,4' -diaminodiphenyl ether, and stirring at room temperature for 20-25 min; under the condition of ice-water bath, adding pyromellitic dianhydride in batches, mechanically stirring for 5-6 h, washing the obtained solution with deionized water after stirring is finished, soaking for 24-25 h to obtain polyimide precursor precipitate, and then drying in an oven at 50-55 ℃ to obtain a polyimide precursor; uniformly mixing a polyimide precursor, triethylamine and deionized water, and stirring at room temperature for 5-6 h to obtain a water-soluble polyimide precursor solution;

the volume ratio of the mass of the 4,4' -diaminodiphenyl ether to the volume of the N, N-dimethylacetamide is 1g (25 mL-26 mL);

the molar ratio of the pyromellitic dianhydride to the 4,4' -diaminodiphenyl ether is (1-1.05) to 1;

the specific operation of adding the pyromellitic dianhydride in batches is as follows: the first time is divided into 6 times, half of the total amount is added for the first time, the rest half is added for the second time to the fifth time, the rest is added for the sixth time, the first four times are separated by 15 min-18 min, the last two times are separated by 20 min-25 min, and stirring is carried out at intervals;

the mass ratio of the polyimide precursor to the triethylamine to the deionized water is (2) - (17-18);

2. preparing Al ₂ O ₃ Slurry preparation: evenly mixing aluminum sec-butoxide with water, and stirring for 1-1.5 h in a water bath at 90-95 ℃; adding nitric acid to promote the hydrolysis and condensation reaction of the organic aluminum alkoxide until the solution is gradually changed from turbid milky color to clear, and then continuously stirring for 9-10 h at 90-95 ℃ to remove residual sec-butyl alcohol and nitrate radical in the solution to obtain Al ₂ O ₃ Hydrosol; prepared Al ₂ O ₃ The hydrosol is kept standing for 72-73 h at room temperature to be aged and the viscosity is increased, and then Al is added ₂ O ₃ Nanoparticles of Al ₂ O ₃ The solid content of (A) reaches 15wt.% to 17wt.%; adding hydroxyethyl cellulose adhesive, and stirring for 2-3 h in water bath at 50-55 ℃ to obtain Al ₂ O ₃ Sizing agent;

the molar ratio of the aluminum sec-butoxide to the water is 1 (60-65);

the mass of the hydroxyethyl cellulose is Al ₂ O ₃ 0.5 to 0.6 percent of hydrosol by mass;

3. preparing a layered composite aerogel: al prepared in the second step ₂ O ₃ Adding the slurry into a polytetrafluoroethylene mould, standing to flatten the surface, adding the water-soluble polyimide precursor solution prepared in the first step, wherein natural layering is formed due to the fact that the two solutions are immiscible and the viscosity and the density are different, and the polyimide is arranged on the upper layer, al is arranged on the upper layer ₂ O ₃ In the lower layer, liquid nitrogen is used as a cold source to perform directional freezing on the side surface until the slurry is completely frozen; freeze-drying the sample at-50 to-60 ℃ for 48 to 50 hours by using a freeze dryer with the pressure of 0.03mbar to remove water, heating the sample to 100 to 105 ℃ after the drying is finished, preserving the heat for 1 to 1.5 hours, then heating to 200 to 205 ℃ and preserving the heat for 1 to 1.5 hours, heating to 300 to 305 ℃ and preserving the heat for 1 to 1.5 hours, naturally cooling to the room temperature to obtain the layered PI/Al ₂ O ₃ Compounding aerogel; all ofThe heating rate is 3-5 ℃/min;

the polytetrafluoroethylene mould is of a cuboid structure;

the directional freezing of the side surface is to freeze only two adjacent side surfaces of the polytetrafluoroethylene mold.

2. The method for preparing the laminated polyimide/alumina composite aerogel according to claim 1, wherein the volume ratio of the 4,4' -diaminodiphenyl ether to the N, N-dimethylacetamide in the step one is 1g.

3. The method for preparing laminated polyimide/alumina composite aerogel according to claim 1, wherein the molar ratio of pyromellitic dianhydride to 4,4' -oxydianiline in step one is 1.01.

4. The method for preparing the laminated polyimide/alumina composite aerogel according to claim 1, wherein the mass ratio of the polyimide precursor, triethylamine and deionized water in the step one is 2.

5. The method for preparing a layered polyimide/alumina composite aerogel according to claim 1, wherein the molar ratio of aluminum sec-butoxide to water in step two is 1.

6. The method for preparing laminated polyimide/alumina composite aerogel according to claim 1, wherein the mass of hydroxyethyl cellulose in step two is Al ₂ O ₃ 0.5 percent of the mass of the hydrosol.

7. The method for preparing the layered polyimide/alumina composite aerogel according to claim 1, wherein Al is added in the second step ₂ O ₃ Nanoparticles of Al ₂ O ₃ Up to a solid content of 15wt.%.

8. The method for preparing a layered polyimide/alumina composite aerogel according to claim 1, wherein the sample is freeze-dried at-50 ℃ for 50h by using a freeze-dryer with a pressure of 0.03mbar in the third step to remove water.

9. The method for preparing the layered polyimide/alumina composite aerogel according to claim 1, wherein the sample is heated to 100 ℃ and insulated for 1h after the drying in the third step, then heated to 200 ℃ and insulated for 1h, heated to 300 ℃ and insulated for 1h, and naturally cooled to room temperature to obtain the layered PI/Al ₂ O ₃ And (3) compounding the aerogel.

10. The method for preparing the laminated polyimide/alumina composite aerogel according to claim 1, wherein all the heating rates in the third step are 5 ℃/min.

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