A composite aerogel and its preparation method

Abstract

The present invention belongs to the technical field of composite ceramic aerogel materials, and the present invention provides a composite aerogel and a preparation method thereof. The preparation method includes the following steps: mixing aluminum chloride hexahydrate, ethanol and water and carrying out a hydrothermal reaction to obtain an alumina sol; sequentially impregnating boron nitride aerogel in the alumina sol and 1,2-epoxypropane solution, and carrying out gel promotion after impregnation to obtain a BN@Al2O3 composite aerogel. The present invention uses an impregnation method to compound and obtain a composite aerogel. The composite aerogel retains the three-dimensional network fiber structure of the boron nitride aerogel, and at the same time enables alumina to be fully coated on the boron nitride aerogel fibers, making the composite aerogel have the advantages of low density, low dielectric constant, low thermal conductivity and high specific surface area, and has good application prospects in the fields of wave transmission, heat preservation, catalytic carriers, adsorption, etc.

Description

Technical Field

[0001] The present invention relates to the technical field of composite ceramic aerogel materials, and in particular to a composite aerogel and a preparation method thereof. Background Art

[0002] Microwave sintering is a process that uses microwave energy to efficiently heat materials to achieve the purpose of sintering. The difference between microwave sintering and conventional sintering processes lies in that it relies on the thermal conversion of microwave energy inside the sample for heating, rather than direct contact heating or radiation heating. Therefore, heat can be more evenly diffused inside the material, and at the same time, it has higher energy utilization efficiency. During microwave sintering, the dipole molecules in the material will rotate following the change of the electromagnetic field, and the internal friction generated by this rotational motion will be converted into heat energy. Compared with traditional heating methods, microwave sintering shows significant advantages in heating speed, energy consumption, sintering temperature, and heating uniformity. In addition, microwave sintering can also precisely control the microstructure of the sample by adjusting the microwave power, heating time, etc., and is suitable for the sintering of high-performance ceramics, cermet composites, nano-powders, etc. At the same time, because the heating rate of microwave sintering is extremely fast, it can greatly shorten the production cycle, improve energy utilization efficiency, and reduce production costs.

[0003] However, microwave sintering also has its own limitations and challenges. Due to the special principle and ultra-high heating rate of microwave sintering, its thermal insulation structure not only needs to have the basic physical and chemical properties of the traditional sintering thermal insulation structure, but also needs to take into account wave transmission, thermal shock resistance, etc., which puts higher requirements on the performance of the thermal insulation material. Since inorganic non-metallic materials have the characteristics of high temperature resistance and ablation resistance, they have currently become a research hotspot for microwave sintering thermal insulation structure materials. Generally, materials with low dielectric constant and low dielectric loss also have better wave transmission performance. Therefore, the existing wave-transmitting thermal insulation material systems can be mainly divided into alumina systems, silica systems, boron nitride systems, etc. However, there are few reports on the research of preparing boron nitride and alumina into composite aerogels as a potential microwave sintering thermal insulation material.

[0004] At present, the difficulties restricting the further application of alumina aerogel lie in that the preparation of bulk alumina aerogel requires processes such as solvent replacement, aging, and supercritical drying. There are problems such as long preparation cycle, high toxicity in the chemical process, high operation risk, and high technical requirements. Moreover, the alumina aerogel prepared by atmospheric pressure drying usually has problems such as large shrinkage rate and easy cracking due to the large surface tension during the drying process, and the obtained samples are usually in powder form. Boron nitride aerogel is a three-dimensional network structure composed of fiber winding. Growing alumina on boron nitride fibers can form a coating structure, with boron nitride fibers serving as the supporting framework for alumina, which can effectively improve problems such as easy cracking of alumina aerogel under atmospheric pressure drying. In addition, alumina also has a strengthening effect on boron nitride fibers, which can effectively improve the mechanical properties of the product. Therefore, how to provide a preparation method with simple operation and low cost to prepare a BN@Al 2 O 3 composite aerogel material has become an urgent problem to be solved by those skilled in the art. Summary of the Invention

[0005] In view of this, the present invention provides a composite aerogel and a preparation method thereof. The first object is to solve the technical problems such as poor mechanical properties and easy cracking existing in the existing aerogel materials; the second object is to solve the technical problems such as long cycle, high toxicity in the chemical process, and high operation risk existing in the preparation methods of the existing aerogel materials.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] The present invention provides a preparation method of a composite aerogel, comprising the following steps:

[0008] Mix aluminum chloride hexahydrate, ethanol and water and carry out a hydrothermal reaction to obtain an alumina sol; impregnate the boron nitride aerogel successively in the alumina sol and 1,2-epoxypropane solution, and carry out gel promotion after impregnation to obtain BN@Al 2 O 3 composite aerogel.

[0009] Further, the temperature for gel promotion is 20 - 40 °C, and the time for gel promotion is 5 - 120 min.

[0010] Further, the 1,2-epoxypropane solution comprises 1,2-epoxypropane and ethanol, and the molar ratio of 1,2-epoxypropane to ethanol is 1:1 - 10.

[0011] Further, the molar ratio of aluminum chloride hexahydrate, ethanol and water is 0.5 - 2:10 - 20:1 - 5.

[0012] Further, the temperature of the hydrothermal reaction is 30 to 60 °C, and the time of the hydrothermal reaction is 30 to 120 min.

[0013] Further, the impregnation time of the boron nitride aerogel in the alumina sol and the 1,2-epoxypropane solution is independently 5 to 20 min.

[0014] Further, after the promotion of gelation, aging treatment, solvent replacement, and drying treatment are sequentially carried out.

[0015] Further, the time of the aging treatment is 24 to 72 h, and the solvent used in the aging treatment is ethanol;

[0016] The solvent used for the solvent replacement is n-hexane, the time of the solvent replacement is 24 to 48 h, and the solvent is replaced every 10 to 12 h.

[0017] Further, the drying treatment includes atmospheric drying or freeze drying; the temperature of the atmospheric drying is 20 to 60 °C, and the time of the atmospheric drying is 2 to 6 h;

[0018] The temperature of the freeze drying is -20 to -60 °C, and the time of the freeze drying is 40 to 50 h.

[0019] The present invention also provides a composite aerogel prepared by the above preparation method.

[0020] It can be seen from the above technical solutions that, compared with the prior art, the beneficial effects of the present invention are as follows:

[0021] The composite aerogel prepared by the present invention retains the three-dimensional network fiber structure of the boron nitride aerogel, making the composite aerogel have the advantages of low density, low dielectric constant, low thermal conductivity, and high specific surface area. At the same time, alumina is fully coated on the boron nitride aerogel fibers. The introduction of alumina can enhance the bundling phenomenon of boron nitride fibers. As a reinforcing body of boron nitride fibers, the composite aerogel sample has good mechanical properties, making the composite aerogel have good application prospects in the fields of wave transmission, heat preservation, catalytic carriers, adsorption, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 SEM morphology diagram of the BN@Al 2 O 3 composite aerogel prepared in Example 1;

[0023] Figure 2 For the BN@Al prepared in Example 1 2 O 3 EDS energy spectrum diagram of the composite aerogel;

[0024] Figure 3 For the BN@Al prepared in Example 12 O 3 XRD image of the composite aerogel. Specific embodiments

[0025] The present invention provides a method for preparing a composite aerogel, comprising the following steps:

[0026] Mix aluminum chloride hexahydrate, ethanol and water and carry out a hydrothermal reaction to obtain an alumina sol; impregnate the boron nitride aerogel successively in the alumina sol and 1,2-epoxypropane solution, and carry out gel promotion after impregnation to obtain BN@Al 2 O 3 composite aerogel.

[0027] In the present invention, the preparation steps of the boron nitride aerogel are as follows: mix boric acid, melamine and a solvent to obtain a mixture, and the mixture is successively subjected to ultrasonic dispersion, drying and sintering to obtain the boron nitride aerogel.

[0028] In the present invention, the molar ratio of melamine to boric acid is 1:2 - 8, preferably 1:3 - 6, and more preferably 1:4 - 5; the total concentration of melamine and boric acid in the solvent is 1 - 10 g / mL, preferably 2 - 8 g / mL, and more preferably 4 - 6 g / mL; the solvent includes water and / or tert-butanol, preferably water.

[0029] In the present invention, the temperature of the mixing is 60 - 100 °C, preferably 70 - 90 °C, and more preferably 80 - 85 °C; the mixing time is 30 - 80 min, preferably 40 - 70 min, and more preferably 50 - 60 min;

[0030] The ultrasonic dispersion time is 10 - 20 min, preferably 12 - 18 min, and more preferably 14 - 16 min; the drying includes atmospheric drying or freeze-drying, preferably atmospheric drying; the temperature of the atmospheric drying is 20 - 60 °C, preferably 25 - 55 °C, and more preferably 30 - 50 °C; the atmospheric drying time is 2 - 6 h, preferably 2.5 - 5.5 h, and more preferably 3 - 5 h; the temperature of the freeze-drying is -20 - -60 °C, preferably -25 - -55 °C, and more preferably -30 - -50 °C; the freeze-drying time is 40 - 50 h, preferably 42 - 48 h, and more preferably 44 - 46 h; the sintering atmosphere includes nitrogen or argon, preferably nitrogen; the sintering temperature is 1000 - 1300 °C, preferably 1050 - 1250 °C, and more preferably 1100 - 1200 °C; the sintering time is 1 - 5 h, preferably 2 - 4 h, and more preferably 3 h.

[0031] In the present invention, the temperature of the promoting gel is 20 to 40 °C, preferably 25 to 35 °C, and more preferably 30 °C; the time for promoting the gel is 5 to 120 min, preferably 10 to 100 min, and more preferably 30 to 60 min.

[0032] In the present invention, the 1,2 - propylene oxide solution comprises 1,2 - propylene oxide and ethanol, and the molar ratio of 1,2 - propylene oxide to ethanol is 1:1 to 10, preferably 1:2 to 8, and more preferably 1:4 to 6.

[0033] In the present invention, the molar ratio of aluminum trichloride hexahydrate, ethanol and water is 0.5 to 2:10 to 20:1 to 5, preferably 0.8 to 1.6:12 to 18:2 to 4, and more preferably 1.0 to 1.5:14 to 16:3.

[0034] In the present invention, the temperature of the hydrothermal reaction is 30 to 60 °C, preferably 35 to 55 °C, and more preferably 40 to 50 °C; the time for the hydrothermal reaction is 30 to 120 min, preferably 40 to 100 min, and more preferably 50 to 80 min.

[0035] In the present invention, the impregnation time of boron nitride aerogel in alumina sol and 1,2 - propylene oxide solution is independently 5 to 20 min, preferably 8 to 16 min, and more preferably 10 to 15 min.

[0036] In the present invention, impregnating boron nitride aerogel in 1,2 - propylene oxide solution will produce a catalytic gel, and the temperature of the catalytic gel is 20 to 40 °C, preferably 25 to 35 °C, and more preferably 30 °C.

[0037] In the present invention, after the promotion of the gel, aging treatment, solvent replacement, and drying treatment are sequentially carried out.

[0038] In the present invention, the time for the aging treatment is 24 to 72 h, preferably 30 to 60 h, and more preferably 40 to 50 h; the solvent used for the aging treatment is ethanol; the temperature for the aging treatment is 20 to 40 °C, preferably 25 to 35 °C, and more preferably 30 °C;

[0039] The solvent used for the solvent replacement is n - hexane, the time for the solvent replacement is 24 to 48 h, preferably 30 to 40 h, and more preferably 33 to 36 h; the solvent is replaced every 10 to 12 h, preferably 11 h.

[0040] In the present invention, the drying treatment includes atmospheric drying or freeze-drying, preferably atmospheric drying; the temperature of the atmospheric drying is 20-60°C, preferably 25-55°C, more preferably 30-50°C; the time of the atmospheric drying is 2-6 h, preferably 2.5-5.5 h, more preferably 3-5 h;

[0041] the temperature of the freeze-drying is -20 to -60°C, preferably -25 to -55°C, more preferably -30 to -50°C; the time of the freeze-drying is 40-50 h, preferably 42-48 h, more preferably 44-46 h.

[0042] The present invention also provides the composite aerogel prepared by the above preparation method.

[0043] The technical solutions provided by the present invention will be described in detail below with reference to the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0044] Example 1

[0045] 2.52 g of melamine and 9.72 g of boric acid were added to 200 mL of deionized water, and the mixture was stirred and mixed in a water bath at 70°C for 40 min to obtain a transparent boron nitride precursor solution. Then, the transparent solution was placed in an ultrasonic cell disruptor for ultrasonic treatment for 15 min until the transparent solution turned into a milky white suspension. Then, it was transferred to a vacuum freeze dryer, and after sufficient pre-freezing, it was dried at -60°C for 48 h. After drying, the sample was placed in a tubular furnace and maintained at 1100°C for 3 h in an N 2 atmosphere to obtain boron nitride aerogel.

[0046] 7.26 g of aluminum chloride hexahydrate, 22.08 g of absolute ethanol and 2.16 g of deionized water were reacted at 30°C for 120 min to obtain an Al 2 O 3 sol.

[0047] The obtained boron nitride aerogel was immersed in the Al 2 O 3 sol for 10 min, and then transferred to a 1,2-epoxypropane solution (the molar ratio of 1,2-epoxypropane to ethanol is 1:2) and immersed at 25°C for 10 min for catalytic gelation. After that, the composite aerogel sample after two-step impregnation was allowed to stand at 25°C for 2 h to promote the gelation reaction. Then, the sample was transferred to an ethanol solution and aged at 25°C for 48 h. To avoid the structural collapse caused by surface tension during the drying process, the sample was transferred to n-hexane for solvent replacement and placed at room temperature for 48 h, and the solution was changed every 12 h during this period; finally, it was transferred to a freeze dryer, and after sufficient pre-freezing, it was freeze-dried at -60°C for 48 h to obtain BN@Al 2O 3 Composite aerogel.

[0048] Figure 1 The BN@Al prepared in this example 2 O 3 SEM morphology diagram of the composite aerogel, which was observed by the JSM-IT800 field emission scanning electron microscope of JEOL Ltd. It can be seen from Figure 1 that the generated BN@Al 2 O 3 maintains the original three-dimensional network porous structure of the BN aerogel, and the granular Al 2 O 3 is relatively evenly distributed on the BN fibers and in the voids of the BN aerogel.

[0049] Figure 2 The BN@Al prepared in this example 2 O 3 EDS energy spectrum diagram of the composite aerogel. It can be seen from Figure 2 that the Al and O elements are relatively evenly distributed on the BN fibers, forming a relatively complete coating structure, highlighting the success of the process.

[0050] Figure 3 The BN@Al prepared in this example 2 O 3 XRD image of the composite aerogel, which was detected by the SmartLab X-ray diffractometer of Rigaku Corporation. It can be seen from Figure 3 that the mechanical superposition of the BN and γ-Al 2 O 3 crystal planes confirms the successful synthesis of the sample, and there are no impurity peaks, indicating that the sample has a high purity.

[0051] Example 2

[0052] Prepare a 150 mL solvent by mixing deionized water and tert-butanol at a volume ratio of 1:1. Add 2.52 g of melamine and 6.48 g of boric acid to the solvent, stir and mix in a 60 °C water bath for 60 min to obtain a transparent boron nitride precursor solution. Then place the transparent solution in an ultrasonic cell disruptor for ultrasonic treatment for 15 min until the transparent solution turns into a milky white suspension. Then transfer it to a vacuum freeze dryer, fully pre-freeze it, and dry it at -60 °C for 72 h. After drying, place the sample in a tube furnace and keep it at 1200 °C for 3 h in an N 2 atmosphere to obtain boron nitride aerogel.

[0053] React 14.52 g of aluminum chloride hexahydrate, 22.08 g of absolute ethanol and 2.16 g of deionized water at 30 °C for 100 min to obtain Al 2 O3 Sol

[0054] The obtained boron nitride aerogel was immersed in Al 2 O 3 sol for 10 min, then transferred to a 1,2 - epoxypropane solution (molar ratio of 1,2 - epoxypropane to ethanol is 1:4) and immersed at 40 °C for 5 min for catalytic gelation. After that, the composite aerogel sample impregnated in two steps was left standing at 25 °C for 3 h to further promote the gelation reaction; then the sample was transferred to an ethanol solution and aged at 20 °C for 48 h. To avoid structural collapse caused by surface tension during drying, the sample was transferred to n - hexane for solvent replacement and placed at room temperature for 48 h, with the solution replaced every 12 h; finally, it was transferred to a thermostatic drum dryer and dried at 40 °C for 6 h to obtain the BN@Al 2 O 3 composite aerogel.

[0055] Example 3

[0056] Deionized water and tert - butanol were prepared into a 150 mL solvent at a volume ratio of 1:1. 2.52 g of melamine and 7.42 g of boric acid were added to the solvent, and the mixture was stirred and mixed in a 70 °C water bath for 60 min to obtain a transparent boron nitride precursor solution. Then the transparent solution was placed in an ultrasonic cell disruptor for ultrasonic treatment for 15 min until the transparent solution turned into a milky white suspension, and then transferred to a vacuum freeze - dryer. After sufficient pre - freezing, it was dried at - 60 °C for 48 h. After drying, the sample was placed in a tubular furnace and kept at 1300 °C for 3 h under N 2 atmosphere to obtain boron nitride aerogel.

[0057] 3.63 g of aluminum chloride hexahydrate, 9.214 g of absolute ethanol and 0.9 g of deionized water were reacted at 30 °C for 80 min to obtain Al 2 O 3 sol.

[0058] The obtained boron nitride aerogel was immersed in Al 2 O 3After 10 min in the sol, it was then transferred to a 1,2 - epoxypropane solution (molar ratio of 1,2 - epoxypropane to ethanol is 1:8) and impregnated at 25 °C for 10 min for catalytic gelation. After that, the composite aerogel sample that had undergone two - step impregnation was left standing at 25 °C for 3 h to promote the gel reaction; then the sample was transferred to an ethanol solution and aged at 30 °C for 48 h. To avoid structural collapse caused by surface tension during drying, the sample was transferred to a mixed solution of anhydrous ethanol and n - hexane with a volume ratio of 1:1 for solvent replacement, placed at room temperature for 24 h, then transferred to n - hexane for solvent replacement and placed at room temperature for 36 h, and the solution was changed every 12 h during this period; finally, it was transferred to an electro - thermal blast dryer and dried at 20 °C, 30 °C, and 40 °C for 2 h respectively to obtain BN@Al 2 O 3 composite aerogel.

[0059] Example 4

[0060] Deionized water and tert - butanol were prepared into a 150 - mL solvent with a volume ratio of 2:1. 3.78 g of melamine and 7.29 g of boric acid were added to the solvent, and stirred and mixed in a water bath at 80 °C for 60 min to obtain a transparent boron nitride precursor solution. Then the transparent solution was placed in an ultrasonic cell disruptor for ultrasonic treatment for 15 min until the transparent solution turned into a milky white suspension, and then transferred to a vacuum freeze - dryer. After sufficient pre - freezing, it was dried at - 40 °C for 72 h. After drying, the sample was placed in a tubular furnace and kept at 1300 °C for 2 h under N 2 atmosphere to obtain boron nitride aerogel.

[0061] 7.26 g of aluminum chloride hexahydrate, 27.64 g of absolute ethanol, and 2.16 g of deionized water were reacted at 30 °C for 90 min to obtain Al 2 O 3 sol.

[0062] The obtained boron nitride aerogel was impregnated in Al 2 O 3 sol for 10 min, then transferred to a 1,2 - epoxypropane solution (molar ratio of 1,2 - epoxypropane to ethanol is 1:6) and impregnated at 25 °C for 10 min for catalytic gelation. After that, the composite aerogel sample that had undergone two - step impregnation was left standing at 40 °C for 2 h to promote the gel reaction. Then the sample was transferred to an ethanol solution and aged at 40 °C for 36 h. To avoid structural collapse caused by surface tension during drying, the sample was transferred to n - hexane for solvent replacement, placed at room temperature for 36 h, and the solution was changed every 12 h during this period; finally, it was transferred to a freeze - dryer. After sufficient pre - freezing, it was freeze - dried at - 60 °C for 48 h to obtain BN@Al 2 O 3Composite aerogel.

[0063] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a composite aerogel, characterized in that: The following steps are involved: Aluminum chloride hexahydrate, ethanol and water are mixed and subjected to hydrothermal reaction to obtain alumina sol; boron nitride aerogel is impregnated in alumina sol and 1,2-propylene oxide solution in sequence, and after the impregnation is completed, gelation is promoted to obtain BN@Al2O3 composite aerogel.

2. The preparation method according to claim 1, characterized in that: The temperature for accelerating gelation is 20-40° C., and the time for accelerating gelation is 5-120 minutes.

3. The preparation method according to claim 1 or 2, characterized in that: The 1,2-propylene oxide solution comprises 1,2-propylene oxide and ethanol, and the molar ratio of the 1,2-propylene oxide to the ethanol is 1:1-10.

4. The preparation method according to claim 3, characterized in that: The molar ratio of the aluminum chloride hexahydrate, ethanol and water is 0.5-2:10-20:1-5.

5. The preparation method according to claim 1, 2 or 4, characterized in that: The temperature of the hydrothermal reaction is 30-60° C., and the time of the hydrothermal reaction is 30-120 min.

6. The preparation method according to claim 5, characterized in that: The immersion time of the boron nitride aerogel in the alumina sol and the 1,2-propylene oxide solution is independently 5 to 20 minutes.

7. The preparation method according to claim 1, 2, 4 or 6, characterized in that: After the gel promotion was completed, aging treatment, solvent replacement and drying treatment were carried out in sequence.

8. The preparation method according to claim 7, characterized in that: The aging treatment time is 24 to 72 hours, and the solvent used in the aging treatment is ethanol; The solvent used in the solvent replacement is n-hexane, the time of the solvent replacement is 24 to 48 hours, and the solvent is replaced every 10 to 12 hours.

9. The preparation method according to claim 8, characterized in that: The drying process includes normal pressure drying or freeze drying; the normal pressure drying temperature is 20 to 60° C., and the normal pressure drying time is 2 to 6 hours; The freeze drying temperature is -20 to -60°C, and the freeze drying time is 40 to 50 hours.

10. The composite aerogel prepared by the preparation method according to any one of claims 1 to 9.

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