CN111848205A

CN111848205A - Method for preparing high-temperature-resistant aerogel heat-insulating material by normal-pressure drying

Info

Publication number: CN111848205A
Application number: CN202010669916.1A
Authority: CN
Inventors: 张恩爽; 李文静; 张昊; 杨洁颖; 张凡
Original assignee: Aerospace Research Institute of Materials and Processing Technology
Current assignee: Aerospace Research Institute of Materials and Processing Technology
Priority date: 2020-07-13
Filing date: 2020-07-13
Publication date: 2020-10-30
Anticipated expiration: 2040-07-13
Also published as: CN111848205B

Abstract

The invention relates to a method for preparing a high-temperature resistant aerogel heat insulation material by normal-pressure drying, which comprises the following steps: (1) preparing a nanowire solution, (2) dispersing the nanowire solution, (3) hydrolyzing a silicon source, (4) gelling, (5) performing hydrophobic modification, (6) drying at normal pressure, and (7) performing post-treatment. The method adopts the alumina nanowires with high length-diameter ratio to carry out three-dimensional lap joint, and adopts silica sol as a sintering aid to realize the generation of high-temperature stable phases and the strengthening of a nanometer framework. And (3) modifying the wet gel by adopting a hydrophobic reagent to realize a normal-pressure drying process. The invention realizes the low-cost and short-period preparation of the high-temperature (above 1400 ℃) resistant heat insulation material.

Description

Method for preparing high-temperature-resistant aerogel heat-insulating material by normal-pressure drying

Technical Field

The invention relates to the technical field of aerogel preparation, in particular to a method for preparing a high-temperature-resistant aerogel heat-insulating material by normal-pressure drying.

Background

Aerogel is a kind of nano porous material with three-dimensional network structure. As a solid with high porosity, the aerogel is filled with a large amount of gas. The porosity of the porous ceramic is as high as 80-99.8%, the typical size of the pores is 1-100 nm, and the specific surface area is 200-1000 m ²A density of as low as 3kg/m³The room temperature thermal conductivity can be as low as 0.012W/m.k. Due to the characteristics, the aerogel material has wide application potential in the aspects of thermal, acoustic, optical, microelectronic and particle detection. The preparation methods of the aerogel are numerous, however, the drying process is difficult to break through in the preparation, and due to the fine skeleton structure of the aerogel, in the normal pressure drying process, the material is shrunk due to the tension effect of the solvent, and the three-dimensional network structure is damaged. Due to the above limitations, the current methods for drying aerogels are still general methods based on supercritical drying, and a few kinds of aerogels can be freeze-dried or atmospheric-pressure dried. However, either supercritical drying or atmospheric drying results in high cost and long cycle time, which severely limits the widespread use of aerogel materials. Therefore, the development of an effective atmospheric drying method is an important issue for aerogel preparation.

In the existing research of the normal-pressure drying method, the framework is modified mainly by introducing a long-chain organic precursor, the tension in the drying process is resisted by utilizing a strong framework, or the surface of a wet gel framework is modified with an organic group, and the contraction problem in the drying process is avoided by utilizing the surface hydrophobicity. However, the existing atmospheric pressure drying method mainly uses silica aerogel, carbon aerogel and other oxide aerogel with low temperature resistance level as main materials. No atmospheric drying methods have been reported for aerogel materials resistant to higher temperatures (1200 ℃ C.).

In recent years, the nano ceramic fiber aerogel has attracted extensive attention of researchers due to good temperature resistance, elasticity and light weight. At present, most methods for preparing the nano ceramic fiber aerogel adopt electrostatic spinning to prepare nano fibers, and the nano fibers are dispersed and then are frozen and dried to prepare the nano ceramic fiber aerogel. The nanofiber aerogel prepared by the method has excellent comprehensive performance. However, electrospinning to produce aerogel materials has the limitation of high cost and not being easily scalable for production.

Chinese patent document CN101254449A discloses a preparation method of an oxide nanowire reinforced transparent aerogel block material, wherein sol and oxide nanowires are mixed to form composite gel, the composite gel is aged and dried to obtain the oxide nanowire reinforced transparent aerogel block material, the mass ratio of the oxide nanowires to the sol is limited to be 1: 0.5-1000, the diameter of the nanowires is 1-100nm, and the length-diameter ratio of the nanowires is 10-1000. But the sol is preformed rather than being formed in situ by hydrolysis as the silicon source material is mixed with the alumina nanowires. The nanowires play a reinforcing role and are low in content. The material system is used for preparing transparent aerogel, the temperature resistance of the material system is insufficient, the deformation degree in the drying process is large, and the heat insulation performance is poor.

With the development of science and technology, various fields put higher requirements on the temperature resistance and high-temperature heat insulation performance of heat insulation materials, so that the development of a low-cost and short-period method for preparing aerogel materials with high temperature resistance and high-efficiency heat insulation performance at high temperature is very needed. The patent provides a preparation method of a nanowire aerogel, and the aerogel has good temperature resistance. And the normal pressure drying is realized through the surface modification and the physical self-supporting effect to prepare the high temperature resistant and high performance aerogel heat insulation material.

Disclosure of Invention

In order to solve the technical problems in the prior art, the invention provides a method for preparing a high-temperature-resistant aerogel heat-insulating material by replacing complex processes such as supercritical drying, freeze drying and the like with normal-pressure drying.

The invention provides a method for preparing a high-temperature-resistant aerogel heat-insulating material by normal-pressure drying, which is characterized by comprising the following steps of:

(1) preparing a nanowire solution: preparing an aluminum oxide nanowire dispersion by a hydrothermal method;

(2) and (3) nanowire solution dispersion: adding the nanowire dispersion prepared in the step (1) into a solvent, and stirring and performing ultrasonic treatment to obtain a uniformly mixed solution, wherein the solid content in the solution is controlled to be 7-20 wt%;

(3) The hydrolysis process of the silicon source: adding a mixture of methyl orthosilicate and ethyl orthosilicate into the dispersion liquid in the step (2), and then stirring to enable silicon ester to generate hydrolysis reaction; after the hydrolysis of the silicon ester is completed, vacuumizing the solution to remove bubbles;

(4) and (3) gel process: after the bubbles are removed in the step (3), adding a catalyst, stirring uniformly, sealing and standing for gelation reaction to obtain gel, wherein the standing condition is 5h to 48h at 25 ℃, and then 1h to 144h at 80 ℃;

(5) hydrophobic modification process: and (3) placing the gel in n-hexane for solvent replacement, wherein the volume ratio of the gel to the solvent is 1: 10, replacing for 1-5 times, wherein each time of replacement is 1-5 days, the hydrophobic reagent is a mixture of trimethylchlorosilane and an organic solvent, and finally washing for 1-5 times in a pure solvent, wherein each time of 2-24 hours, and the pure solvent is the pure solvent of the used organic solvent;

(6) and (3) drying under normal pressure: carrying out normal pressure drying process on the wet gel after solvent replacement to obtain the aerogel, wherein the normal pressure drying process is respectively drying at room temperature for 12h to 72h, drying at 30 ℃ to 60 ℃ for 0.5h to 24h, and drying at 100 ℃ to 200 ℃ for 0.5h to 24 h;

(7) and (3) post-treatment process: and carrying out staged heat treatment on the prepared aerogel, wherein the staged heat treatment is respectively carried out for 0.1 to 20 hours at 500 to 700 ℃, for 0.1 to 20 hours at 900 to 1100 ℃, for 0.1 to 20 hours at 1100 to 1300 ℃ and for 1min to 200min at 1300 to 1500 ℃.

In the method for preparing the high-temperature-resistant aerogel heat-insulating material by normal-pressure drying, the hydrothermal method is carried out at 100-300 ℃ for 1-10 h.

In the method for preparing the high-temperature-resistant aerogel heat-insulating material by normal-pressure drying, the length-diameter ratio of the alumina nanowires is 10-1000.

In the method for preparing the high-temperature-resistant aerogel heat-insulating material by normal-pressure drying, the molar ratio of the methyl orthosilicate to the ethyl orthosilicate in the mixture of the methyl orthosilicate and the ethyl orthosilicate is 1:1-1: 10.

In the method for preparing the high-temperature resistant aerogel heat insulation material by normal-pressure drying, in the step (5), the organic solvent in the hydrophobic reagent is selected from the group consisting of n-hexane, ethanol and acetone, and the molar ratio of the trimethylchlorosilane to the organic solvent is 1:1-1: 10.

in the method for preparing the high-temperature resistant aerogel heat insulation material by normal-pressure drying, the catalyst in the step (4) is 1M ammonium fluoride.

In the method for preparing the high-temperature resistant aerogel heat insulation material by normal-pressure drying, the solvent in the step (2) is water and/or ethanol.

In the method for preparing the high-temperature resistant aerogel heat insulation material by normal-pressure drying, the ultrasonic treatment condition in the step (2) is 30 to 80kHZ for 250 min.

In the method for preparing the high-temperature resistant aerogel heat-insulating material by normal-pressure drying, the silicon ester is subjected to hydrolysis reaction in the step (3), and finally the final silicon: the weight ratio of aluminum is 3: 7.

in the method for preparing the high-temperature resistant aerogel heat insulation material by normal-pressure drying, the staged heat treatment in the step (7) is as follows: respectively treating at 500-700 deg.C for 0.2-6 h, at 900-1100 deg.C for 0.2-6 h, at 1100-1300 deg.C for 0.2-6 h, and at 1300-1500 deg.C for 1-40 min.

In a second aspect, the invention provides a high temperature resistant shaped nanocrystalline aerogel material prepared by the preparation method of the first aspect of the invention.

Compared with the prior art, the invention at least has the following beneficial effects:

(1) according to the invention, the nanowires with longer lengths are used as main body units for an assembling process, so that the low heat conductivity coefficient is ensured, and the integral temperature resistance of the material is improved due to the self-supporting effect of the three-dimensional network structure.

(2) The nanowire with the length-diameter ratio of 10-1000, preferably 20-200 can realize a physical cross wet gel network structure in a dispersion liquid, can effectively realize a sub-support effect in a normal pressure drying process, and avoids the problem of serious size shrinkage.

(3) The preparation method adopts the hydrophobic modification process in the gel stage, can reduce the surface tension removed by the solvent, adopts normal pressure drying to replace the supercritical drying process, greatly reduces the preparation cost and shortens the preparation period.

(4) The post-treatment process adopts a graded heat treatment process, so that the hydroxyl on the surface of the hydroxy aluminum oxide and the silicon-aluminum component react to generate a high-temperature stable phase and strengthen the framework, and the temperature resistance and the mechanical property of the material are effectively improved.

(5) The nanowire aerogel of preparing in this patent has high three-dimensional network overlap joint structure, realizes that the pore volume accounts for more than 99% of total volume, realizes the preparation of ultralow density aerogel.

(6) The silicon phase component is added in the composite material, and the silicon dioxide component and the alumina component generate a high-temperature resistant mullite phase at high temperature, so that the high-temperature stability of the material is realized.

Drawings

FIG. 1 is a flow chart of the preparation of the present invention.

Fig. 2 is an SEM image of the alumina nanowires prepared in embodiment 1.

Fig. 3 is a macroscopic optical photograph of the alumina nanowire aerogel prepared in embodiment 1.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. It should be apparent that the described embodiments are examples of some of the present invention, and should not limit the scope of the present invention. All other embodiments, which can be obtained by a person skilled in the art without any inventive step based on the embodiments of the present invention, are within the scope of the present invention.

The invention provides a preparation method of a high-temperature-resistant special-shaped nanocrystalline aerogel material in a first aspect, which comprises the following steps:

(3) the hydrolysis process of the silicon source: adding a mixture of methyl orthosilicate and ethyl orthosilicate into the dispersion liquid in the step (2), and then stirring, for example, stirring at a high speed to enable the silicon ester to have a hydrolysis reaction; after the hydrolysis of the silicon ester is completed, vacuumizing the solution to remove bubbles;

(4) and (3) gel process: after the bubbles are removed in the step (3), adding a catalyst, stirring uniformly, sealing and standing for gelation reaction to obtain gel, wherein the standing condition is 5-48h at 25 ℃ and 1-144h at 80 ℃;

(5) hydrophobic modification process: and (3) placing the gel in n-hexane for solvent replacement, wherein the volume ratio of the gel to the solvent is 1: 10 replacement is carried out for 1 to 5 times, each replacement is carried out for 1 to 5 days, the hydrophobic reagent is a mixture of trimethylchlorosilane and an organic solvent, and finally the mixture is washed in a pure solvent for 1 to 5 times, each time lasts for 2 to 24 hours, and the pure solvent is the pure solvent of the used organic solvent;

(6) And (3) drying under normal pressure: carrying out normal pressure drying process on the wet gel after the solvent replacement to obtain the aerogel, wherein the normal pressure drying process comprises the steps of drying at room temperature for 12-72h, drying at 30-60 ℃ for 0.5-24h and drying at 100-200 ℃ for 0.5-24h respectively;

(7) and (3) post-treatment process: the prepared aerogel is subjected to staged heat treatment, and is respectively treated at 500-700 ℃ for 0.1-20h, at 900-1100 ℃ for 0.1-20h, at 1100-1300 ℃ for 0.1-20h and at 1300-1500 ℃ for 1-200 min.

In the method for preparing the alumina nanowire aerogel thermal insulation material, the nanowire solution of the step (1) is prepared by the following steps: dissolving 1-30g of alumina nano powder (with the grain diameter of 5-50nm) in 10-300mL of water, adding 0.001-1mol/L of sulfuric acid as an adsorbent, and reacting at the temperature of 100 ℃ and 300 ℃ for 1-10h to obtain the alumina nano wire with the diameter of 10-300 nm and the length of 1-5 mu m.

Examples of the dispersion of the nanowire solution of the step (2) are: adding a certain amount of water and ethanol (volume ratio is 1: 1-10: 1) into the nanowire dispersion in the step (1), and uniformly mixing the solution by stirring and ultrasonic treatment, wherein the solid content of the solution is controlled to be 7-20 wt%.

An example of the silicon source hydrolysis process of step (3) is: taking 50g of the dispersion liquid obtained in the step (2), and adding the dispersion liquid into a reaction kettle with the volume of 1: 3-20g of a mixture of 1 n-methyl silicate and ethyl orthosilicate, and stirring at a high speed to hydrolyze the silicon ester, so that the solid content ratio of the final silicon to aluminum is 1: 9-5: 5; and vacuumizing the obtained mixed solution for 0.1-2 h under the conditions that the temperature is 10-50 ℃ and the vacuum degree is 0.1-0.3 MPa, and standing the obtained aluminum oxide nanowire/silica sol/boric acid mixed solution for 6-72h for defoaming.

An example of the step (4) gelling process is: step (3) after the methyl orthosilicate and the ethyl orthosilicate are completely hydrolyzed, adding 0.5-5g of catalyst (1M ammonium fluoride), uniformly stirring, sealing and standing for gelation reaction (5-48 h at 25 ℃ and 1-144h at 80 ℃);

examples of the hydrophobic modification process of step (5) are: and (3) placing the gel in n-hexane for solvent replacement, wherein the volume ratio of the gel to the solvent is 1: 10 substitutions 1-5 times, each for 1-5 days. Soaking a mixture (molar ratio of 1: 1-1: 10) of a hydrophobic reagent (trimethylchlorosilane) and an organic solvent (normal hexane, ethanol and/or acetone and the like) for 1-5 days for modification, and finally washing in a pure solvent (1-5 times, 2-24h each time).

An example of the step (6) normal pressure drying process is: drying the modified wet gel at normal pressure for 12-72h at room temperature, 0.5-24h at 30-60 deg.C, and 0.5-24h at 100-200 deg.C.

An example of the post-treatment process of step (7) is: the prepared aerogel is subjected to staged heat treatment, and is respectively treated at 500-700 ℃ for 0.2-6h, at 900-1100 ℃ for 0.2-6h, at 1100-1300 ℃ for 0.2-6h and at 1300-1500 ℃ for 1-40 min. Finally, the preparation of the high-temperature resistant aerogel heat insulation material is realized.

One or more of the above examples of steps (1) to (7) may be combined to form an example of the method of the present invention.

The alumina nanowire is prepared by a hydrothermal method and is carried out for 1-10h at the temperature of 100-300 ℃.

The aspect ratio of the alumina nanowire of the present invention is 10 to 1000, preferably 20 to 800, 30 to 700, 50 to 500 or 100 to 200. If the length-diameter ratio is less than 10, the nano rod is difficult to realize normal pressure drying through self-lapping action; if the length-diameter ratio is more than 1000, the strength of the nano-rod is crossed, and serious cross-linking and agglomeration phenomena occur. For example, comparative example 5 alumina nanowire aerogel material prepared with alumina nanowires having aspect ratios of 5 had too high a linear shrinkage.

The organic solvent in the hydrophobic reagent is n-hexane, ethanol and acetone, and the molar ratio of the trimethylchlorosilane to the organic solvent is 1: 1-1: 10. dilution of the nanowires with a suitable organic solvent allows the nanowires to freely stretch and to exist in a linear form in the final system to increase the strength of the alumina nanowire aerogel material, see, e.g., comparative example 2.

In the hydrolysis process of the silicon source in the step (3), the added volume is 1: 3-20g of a mixture of methyl orthosilicate and ethyl orthosilicate. If too little silicone ester is added, the system will not gel and hydrophobic modification will not be possible, see, for example, comparative example 3.

And (4) carrying out vacuum pumping treatment on the mixed liquid obtained after mixing and hydrolyzing the dispersion liquid and silicate ester in the step (3) so as to avoid the defect of air holes formed in the material.

The catalyst in step (4) of the present invention is 1M ammonium fluoride.

In the step (2) of the present invention, the solvent is water and ethanol.

The ultrasonic treatment condition in the step (2) is 30-80 kHZ and 20-50 min.

Silicon in step (3) of the present invention: the weight ratio of aluminum is 1: 9-5: 5, preferably 3: 7. if the silicon content is too low, the lapping among the nanowires is not fixed by enough silicon oxide, and the strength is weak; if the silicon content is too high, the silicon oxide content will reduce the temperature resistance of the material.

According to the invention, a silicon source substance is mixed with the alumina nanowire, and then the silicon source substance is subjected to hydrolysis in situ, wherein the meaning of in situ hydrolysis is that the silicon ester is hydrolyzed while being subjected to adsorption and weak interaction with the nanowire in the hydrolysis process.

In the mixture of methyl orthosilicate and ethyl orthosilicate, the molar ratio of methyl orthosilicate to ethyl orthosilicate is 1:1-1:20, preferably 1:1-1: 10.

In the silicon source hydrolysis process in the step (3), the generated silica sol particles can be adsorbed on the surface of the nanowire through physical adsorption to form a dispersion phase, so that the interaction between the nanoparticles is reduced, and the overall viscosity of a material system is further reduced.

The step (6) of the invention is that the modified wet gel is dried under normal pressure, and the process is that the wet gel is dried for 12 to 72 hours at room temperature, 0.5 to 24 hours at 30 to 60 ℃ and 0.5 to 24 hours at 100 and 200 ℃. The significance of the step drying is that the speed of water evaporation can be adapted to the water content in the wet gel, and the skeleton collapse caused by rapid and large amount of evaporation of water in the wet gel is avoided. For example, the alumina nanowire aerogel material of comparative example 4 prepared by one-step drying has an excessively small specific surface area and an excessively high thermal shrinkage rate.

The step (7) of the invention is to carry out staged heat treatment on the prepared aerogel, and the aerogel is respectively treated at 500-700 ℃ for 0.2-6h, at 900-1100 ℃ for 0.2-6h, at 1100-1300 ℃ for 0.2-6h and at 1300-1500 ℃ for 1-40 min. Finally, the preparation of the high-temperature resistant aerogel heat insulation material is realized. The significance of staged heat treatment is that different temperature ranges lead to different dehydration or crystal transformation reactions, which will induce a certain volume shrinkage of the material. The significance of the staged heat treatment is that the crystal form transformation process of each temperature range is completely reacted, the volume shrinkage rate is slowed down, and the collapse of the material structure is avoided. Meanwhile, sufficient time is provided for constructing a high-temperature stable phase in the staged heat treatment, and the high-temperature stable phase can also inhibit the shrinkage of the material. By the staged heat treatment of the present invention, optimal balance between dehydration and/or crystal form transformation and material volume shrinkage can be achieved, thereby obtaining a high temperature stable phase. The hydroxyl on the surface of the hydroxy aluminum oxide and the silicon-aluminum component react to generate a high-temperature stable phase and strengthen the framework, so that the temperature resistance and the mechanical property of the material are effectively improved. As can be seen from comparative example 6, without the staged heat treatment of the present invention, the material had significant dusting, weak strength, and insufficient temperature resistance.

The invention will be further illustrated by way of example, but the scope of protection is not limited to these examples.

Example 1

(1) 12g of alumina nano powder (with the particle size of 15nm) is dissolved in 100mL of water, and the solid content of the prepared solution is 10.7%. Adding 0.025mol/L sulfuric acid as an adsorbent to react for 8h at 230 ℃ to obtain the alumina nanowire with the diameter of two-dimensional 200nm and the length of 2-4 mu m.

(2) And (3) nanowire solution dispersion: adding a certain amount of water and ethanol (volume ratio is 2: 1) into the nanowire dispersoid in the step (1), stirring and ultrasonically treating to uniformly mix the solution, wherein the solid content of the solution is controlled at 10%.

(3) The hydrolysis process of the silicon source: taking 50g of the dispersion liquid obtained in the step (2), and adding the dispersion liquid into a reaction kettle with the volume of 1: 1, 7g of a mixture of methyl orthosilicate and ethyl orthosilicate, and stirring at a high speed to hydrolyze silicon ester, so that the solid content ratio of the final silicon-aluminum is 3: 7; vacuumizing the obtained mixed solution for 0.5h at the temperature of 25 ℃ and the vacuum degree of 0.1-0.3 MPa to obtain an alumina nanowire/silica sol/boric acid mixed solution, and standing for 12h for defoaming;

(4) and (3) gel process: step (3) after the methyl orthosilicate and the ethyl orthosilicate are completely hydrolyzed, adding 4g of catalyst (1M ammonium fluoride), uniformly stirring, sealing and standing for gelation reaction (24 hours at 25 ℃ and 48 hours at 80 ℃);

(5) Hydrophobic modification process: and (3) placing the gel in n-hexane for solvent replacement, wherein the volume ratio of the gel to the solvent is 1: 10 substitutions were made 2 times for 3 days each. The hydrophobic reagent (trimethylchlorosilane) and the mixture (the molar ratio is 1:4) of organic solvents (normal hexane, ethanol, acetone and the like) are soaked for 4 days for modification, and finally washed in pure solvents (2 times, 24 hours each time).

(6) And (3) drying under normal pressure: the modified wet gel was subjected to a drying process at normal pressure, which was carried out at room temperature for 24 hours, at 45 ℃ for 5 hours, and at 120 ℃ for 5 hours, respectively.

(7) And (3) post-treatment process: the prepared aerogel is subjected to staged heat treatment, and is treated at 600 ℃ for 1h, 1000 ℃ for 1h, 1200 ℃ for 1h and 1400 ℃ for 10min respectively. Finally, the preparation of the high-temperature resistant aerogel heat insulation material is realized.

The alumina aerogel prepared under these conditions had a specific surface area of 88m²G, density 0.18g/cm³The linear shrinkage after heat treatment at 1400 ℃ for 1 hour was 7.5%.

Example 2

Example 2 is essentially the same as example 1, except that: the alumina nanocrystal content in step 1 was 20g, and the solids content of the prepared solution was 18%.

The thermal insulation performance test of the alumina nanowire aerogel material in example 2 shows that the surface of the aerogel material has no color change and no shedding when lightly touching, and other performance indexes are shown in table 1.

Comparative example 1

Comparative example 1 is substantially the same as example 1 except that: the hydrophobic modification process of step 5 was not performed.

When the thermal insulation performance of the alumina nanowire aerogel material in comparative example 1 is tested, the surface of the aerogel material is free from color change and shedding after being lightly touched, and other performance indexes are shown in table 1.

Comparative example 2

Comparative example 2 is substantially the same as example 1 except that: the ethanol and water dilution process of step 2 was not performed.

In contrast to comparative example 2, the nanowires are sticky and knotted without dilution, and finally an aerogel material with a smooth surface cannot be formed.

Comparative example 3

Comparative example 3 is substantially the same as example 1 except that: 2g of silicon ester was added during the preparation.

The alumina nano wire in the comparative example 3 has less added silicone ester content, so that the system can not be gelled and hydrophobic modification can not be carried out.

Comparative example 4

Comparative example 4 is essentially the same as example 1 except that the step 6 drying process is at 150 ℃ for 2 h.

And (3) performing a heat insulation performance test on the alumina nanowire aerogel material in the comparative example 4, and finding that the surface of the nanowire aerogel material is not discolored and is not fallen off when the surface of the nanowire aerogel material lightly touches, wherein other performance indexes are shown in table 1.

Comparative example 5

Comparative example 5 is substantially the same as example 1 except that: in the step 1, the hydrothermal reaction time of the nanowire is 2 hours, and the low-length-diameter ratio nanowire with the diameter of 40nm and the length of 200nm is obtained.

And (3) performing a heat insulation performance test on the alumina nanowire aerogel material in the comparative example 5, and finding that the surface of the nanowire aerogel material is not discolored and is not fallen off when the surface of the nanowire aerogel material lightly touches, wherein other performance indexes are shown in table 1.

Comparative example 6

Comparative example 6.1 is essentially the same as example 1, except that: no post-treatment process was performed.

The heat insulation performance test of the alumina nanowire aerogel material in the comparative example 6.1 shows that the material has obvious powder falling phenomenon, weak strength and insufficient temperature resistance.

Comparative example 6.2 is essentially the same as example 1, except that: the work-up was carried out in 1 stage: 1000 ℃ for 4 h.

Comparative example 6.3 is essentially the same as example 1, except that: post-processing was performed in 2 steps: 600 ℃ for 2h, then 1000 ℃ for 2 h.

When the alumina nanowire aerogel material in the comparative example 6.2 is subjected to a heat insulation performance test, the silica component of the material is not completely sintered, so that the material is weak in strength and insufficient in temperature resistance.

The alumina nanowire aerogel material in the comparative example 6.3 is subjected to a heat insulation performance test, and the results show that the sintering process of the material is not subjected to step heating, the shrinkage is large, and the silicon oxide component is not completely sintered, so that the material is weak in strength and insufficient in temperature resistance.

Comparative example 7

Comparative example 7 is substantially the same as example 1 except that: spherical nanocrystals with a diameter of 13nm are used instead of nanowire material in step 1, and the subsequent steps are the same.

The results show that the obtained material has large drying shrinkage under normal pressure and unsatisfactory temperature resistance.

Comparative example 8

Comparative example 8 is substantially the same as example 1 except that: in step 3, the vacuum pumping process is not carried out;

SEM test shows that a large number of air holes exist in the prepared material, which causes defects.

Comparative example 9

Preparation of sol

160g of methyl orthosilicate and 160g of acetonitrile were weighed into a 500mL beaker, sealed with a preservative film and magnetically stirred for 1 min. After mixing evenly, 60g of hydrochloric acid with the concentration of 0.003mol/L is added as a catalyst, the process needs to be slowly added, and the mixture is stirred for 5min by magnetic force; adding the mixed solution into a 1000mL three-necked bottle, heating at 70 ℃, magnetically stirring, and refluxing for 30min to obtain a first solution of a silica sol precursor; 160g of methyl orthosilicate is added into the obtained first solution of the silica sol precursor, and the mixture is heated and magnetically stirred under the condition of 70 ℃ to react for 16h, so that silica sol (silicon dioxide sol) is obtained. Diluting the silica sol, evaporating out 300g of solvent contained in the silica sol, adding 600g of acetonitrile, uniformly mixing to obtain diluted silica sol, and refrigerating the diluted silica sol for later use.

② spherical nano crystal assembling process

Dissolving 3.7g of alumina spherical nanocrystalline powder in 34g of acetonitrile, uniformly stirring to obtain a first mixed solution, adding 8g of diluted silica sol serving as an adhesive into the first mixed solution, performing ultrasonic dispersion for 20min to obtain a second mixed solution, adding 2g of ammonia water with the concentration of 0.43mol/L into the second mixed solution, and continuing performing ultrasonic treatment for 20min to obtain the aerogel wet gel taking the oxide nanocrystals as the framework.

③ gelatinization and aging

And (3) placing the prepared aerogel wet gel in a mold, standing for 24h, and then placing in an oven at 60 ℃ for 48h to finish the gelling and aging processes.

Replacement of solvent

And taking out the gel after the completion of the gelation and the aging, putting the gel into ethanol with the volume being 10 times of that of the gel for solvent replacement, wherein the solvent replacement time is 3d, and the solvent replacement process is repeated for 3 times.

And fifthly, performing supercritical drying to prepare the aerogel material.

Process of heat treatment

And (3) heating the aerogel material to 1200 ℃ along with the furnace (the heat treatment temperature), wherein the heating rate is 10 ℃/min, keeping the temperature for 1h (the heat treatment time), and then cooling along with the furnace to room temperature to obtain the high-temperature-resistant aerogel material.

Comparative example 9 employs an assembly process of spherical nanocrystals instead of the heterogeneous nanocrystals of nanowires or nanorods, and the solvent system used is acetonitrile, and the drying process is a supercritical drying process.

The performance indexes of the high-temperature resistant special-shaped nanocrystalline aerogel materials in the examples 1-2 and the high-temperature resistant aerogel materials in the comparative examples 1-9 are shown in Table 1.

Comparative example 10

Comparative example 10 was prepared according to example 1 (i.e., high pressure supercritical drying) disclosed in CN 110282958A.

Specifically, comparative example 10 was conducted as follows.

S1, preparing the special-shaped nanocrystalline dispersion liquid: using aluminum oxide nano powder as a raw material, and dispersing 20g of the aluminum oxide nano powder in 500mL of aqueous solution, wherein the particle size of single particles of the nano powder is within the range of 10-200 nm; and (3) adding 15mL of 2mol/L hydrochloric acid serving as a catalyst (adsorbent) into the mixed solution of the alumina nanoparticles, placing the mixed solution into a reaction kettle with polytetrafluoroethylene serving as an inner container, sealing, and reacting at 240 ℃ for 3 hours to obtain the special-shaped nanocrystal dispersion.

S2, self-assembly process of the special-shaped nanocrystalline: and (3) fully mixing 30g of the prepared special-shaped nanocrystal dispersion liquid with 20g of silicic acid with the concentration of 4 wt%, fully stirring magnetons for 5 hours, and then carrying out ultrasonic treatment for 30min to obtain a mixed phase first solution for self-assembly of the special-shaped nanocrystals.

S3, gelation reaction process: 2g of NH with a concentration of 1 mol/L/was added to the mixed phase first solution ₄Fully stirring the solution F and magnetons for 0.5h, and then carrying out ultrasonic treatment for 30min to obtain a mixed phase second solution; and then, placing the mixed phase second solution at 25 ℃, vacuumizing for 0.1h under the vacuum degree of 0.5MPa, taking out the solution, and standing to obtain a gelation reaction solution.

S4, aging process: and (3) sealing the gelation reaction liquid, aging at 25 ℃ for 12h to fully lap the network, and aging at 60 ℃ for 72h in a water bath environment, wherein the humidity in the beaker is required to be more than 80%.

S5, drying: and (2) aging the gelation reaction liquid, performing a solvent replacement process by using ethanol, performing replacement for 3 times in 3 days each time to obtain silicon-aluminum wet gel, and performing supercritical drying by using absolute ethyl alcohol as a drying medium: and (2) loading the silicon-aluminum composite wet gel into supercritical drying equipment, placing the supercritical drying equipment into an autoclave, adding absolute ethyl alcohol into the autoclave, sealing the autoclave until the pressure in the autoclave is 25MPa and the temperature is 30 ℃, keeping the pressure and the temperature for 24 hours, and then discharging the absolute ethyl alcohol and fluid generated in the drying process to obtain the special-shaped nanocrystalline aerogel material.

S6, heat treatment process (post-treatment process): treating the special-shaped nanocrystalline aerogel material prepared in the step S5 at a low temperature of 300 ℃ for 5 hours in the first stage to enable the silicon-aluminum composite aerogel to generate a dehydroxylation process, so that the first-step framework of the silicon-aluminum composite aerogel is strong; after the steps are carried out, cooling the sample to room temperature, carrying out a second stage, and carrying out heat treatment for 3h at 600 ℃ to make the crystal form of the composite silicon-aluminum sol undergo initial transformation; when the sample in the previous step is cooled to room temperature, carrying out a third stage, carrying out heat treatment for 1h at 1200 ℃, and finally cooling to room temperature along with a furnace to obtain a high-temperature-resistant special-shaped nanocrystalline aerogel material with a strong structural skeleton; the heating rates of the heat treatment processes of the three stages are all 3 ℃/min.

Finally, it should be noted that: the above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims

1. The method for preparing the high-temperature-resistant aerogel heat-insulating material by normal-pressure drying is characterized by comprising the following steps of:

2. The process of claim 1, wherein the hydrothermal process is carried out at 100 ℃ to 300 ℃ for 1h to 10 h.

3. The method of claim 1, wherein the alumina nanowires have an aspect ratio of 10 to 1000.

4. The method of claim 1, wherein the mixture of methyl orthosilicate and ethyl orthosilicate has a molar ratio of methyl orthosilicate to ethyl orthosilicate in the range of 1:1 to 1: 20.

5. The method of claim 1, wherein in step (5) the organic solvent of the hydrophobic reagent is selected from the group consisting of n-hexane, ethanol and acetone, and the molar ratio of trimethylchlorosilane to the organic solvent is 1:1 to 1: 10.

6. the process of claim 1, wherein the catalyst in step (4) is 1M ammonium fluoride.

7. The method of claim 1, wherein the solvent in step (2) is water and/or ethanol.

8. The method of claim 1, wherein the sonication conditions in step (2) are between 30 and 80kHZ, between 2 and 50 min.

9. The method of claim 1, wherein the step (3) comprises hydrolyzing the silicon ester to form a silicon: the weight ratio of aluminum is 3: 7.

10. the method of claim 1, the staged heat treatment in step (7) being: respectively treating at 500-700 deg.C for 0.2-6 h, at 900-1100 deg.C for 0.2-6 h, at 1100-1300 deg.C for 0.2-6 h, and at 1300-1500 deg.C for 1-40 min.