A method for preparing magnesium oxide aerogel

Abstract

The invention discloses a preparation method of magnesium oxide aerogel, which belongs to the field of aerogel technology. The preparation method includes: preparing a first reinforcing agent, preparing a second reinforcing agent, preparing a magnesium sol, preparing a silica sol, and mixing; the mixing, after mixing the magnesium sol, the silica sol, the first reinforcing agent, and the second reinforcing agent, ultrasonic stirring is performed, and the mixture is allowed to stand for aging under 40-45°C closed conditions, completely immersed in anhydrous ethanol, allowed to stand at 30-35°C, and supercritically dried in a carbon dioxide environment to obtain magnesium oxide aerogel. The magnesium oxide aerogel prepared by the present invention has strong hydrophobicity, high specific surface area, low thermal conductivity, and high strength.

Description

Technical Field

[0001] The present invention relates to the technical field of aerogels, and in particular to a method for preparing magnesium oxide aerogels. Background Art

[0002] Aerogel refers to a nano-scale porous solid material formed by the sol-gel method, in which a certain drying method is used to replace the liquid phase in the gel with gas. It has high porosity, high specific surface area, and low density, and therefore has excellent thermal insulation and adsorption properties. Furthermore, aerogel exhibits excellent performance in thermal, electrical, optical, acoustic, adsorption, catalytic and other aspects, and is widely used in aerospace, building energy conservation, chemical industry, electronics and electrical, biomedicine and other fields.

[0003] According to the classification of skeleton components, aerogels include silica aerogel, alumina aerogel, zirconia aerogel, zinc oxide aerogel, carbon aerogel, polyimide aerogel, polyurethane aerogel, etc. The physical and chemical properties of aerogels are different due to different skeleton components.

[0004] Magnesium oxide has the advantages of being non-toxic and antibacterial. The specific surface area of ​​magnesium oxide aerogel is nearly ten times that of powdered magnesium oxide. Magnesium oxide aerogel not only has obvious advantages in organic catalysis, but can also adsorb toxic gases. In the preparation of magnesium oxide aerogel, magnesium alkoxide or inorganic magnesium salt is commonly used as a magnesium source. When the magnesium source is magnesium alkoxide, the magnesium alkoxide is mainly hydrolyzed to generate Mg-OH, and Mg-OH undergoes dehydration condensation to generate Mg-O-Mg. However, the hydrolysis rate of magnesium alkoxide is fast, and a large amount of Mg-OH is generated in a short time. Furthermore, a gel with poor structural performance is formed in a short time, resulting in poor mechanical properties of the prepared magnesium oxide aerogel. When the magnesium source is inorganic magnesium salt, after the inorganic magnesium salt and the epoxide with proton absorption capacity are added to water, the inorganic magnesium salt exists in the form of magnesium ions in the water. Although the degree of hydrolysis of the magnesium ions is very small, some magnesium ions will still be hydrolyzed, making the aqueous solution acidic. The epoxide with proton absorption capacity can consume the hydrogen ions in the solution, promote the hydrolysis of magnesium ions, increase the Mg-OH in the aqueous solution, and a small part of the Mg-OH undergoes dehydration condensation to generate Mg-O-Mg. However, most of the Mg-OH will form magnesium hydroxide precipitate, affecting the formation of magnesium oxide aerogel.

[0005] There are two common solutions to the above problems: the first solution is to address the problem of magnesium alkoxide by adding glycerol to prolong the gel time; the second solution is to address the problem of inorganic magnesium salt by adding polyvinyl alcohol-type water-soluble polymers as a network structure directing agent.

[0006] However, in the above methods, the glycerol used in the first solution will cause the hydrophilicity of the prepared magnesium oxide aerogel to increase and the hydrophobicity to decrease, and further, the moisture resistance of the prepared magnesium oxide aerogel will be poor. In addition, it will also affect the application effect of the magnesium oxide aerogel in organic catalysis; the polyvinyl alcohol-type water-soluble polymer used in the second solution will not only cause the hydrophilicity of the prepared magnesium oxide aerogel to increase and the hydrophobicity to decrease, but also the amount of polyvinyl alcohol-type water-soluble polymer used will also affect the specific surface area, mechanical strength and thermal insulation performance of the magnesium oxide aerogel. A large amount of polyvinyl alcohol-type water-soluble polymer can effectively avoid the formation of magnesium hydroxide precipitation and increase the cross-linking density, thereby increasing the specific surface area and porosity of the magnesium oxide aerogel, and further improving the thermal insulation performance of the magnesium oxide aerogel. However, the mechanical strength of the hydrogel formed by the polyvinyl alcohol-type water-soluble polymer is low. As the amount of polyvinyl alcohol increases, the mechanical properties of the aerogel formed by the polyvinyl alcohol-type water-soluble polymer will decrease.

[0007] To address the above problems, a common solution is to add methyltrimethoxysilane during the preparation of magnesium oxide aerogel to improve the hydrophobicity of magnesium oxide aerogel and improve the mechanical properties of magnesium oxide aerogel. However, methyltrimethoxysilane can cause phase separation of the prepared magnesium oxide aerogel, affecting the specific surface area and porosity of the prepared magnesium oxide aerogel, and further resulting in a decrease in the thermal insulation performance of the prepared magnesium oxide aerogel.

[0008] Chinese patent CN108017073B discloses a method for preparing magnesium oxide aerogel using bischofite as raw material, the preparation method comprising the following steps: mixing bischofite, distilled water, an alcohol solvent, and a water-soluble polymer, stirring for 5-120 minutes to obtain a transparent solution, then adding an epoxy compound to the solution and stirring for 5-60 minutes to obtain a sol; sealing the obtained sol for gelation at a gelation temperature of 20-95°C and a gelation time of 2-20 hours to obtain magnesium oxide wet gel; and immersing the obtained magnesium oxide wet gel in distilled water. The method comprises the following steps: soaking and washing the wet gel for no less than three times, using an amount of distilled water no less than the volume of the wet gel, and then drying the wet gel to prepare the magnesium oxide aerogel; the method solves the problem that the magnesium oxide wet gel is prepared by using magnesium alkoxide, which is very easy to hydrolyze and polycondense, and the process is difficult to control; however, as can be seen from the embodiments of the patent, when the amount of the water-soluble polymer increases, the specific surface area increases, the thermal conductivity decreases, and the strength decreases, making it difficult to simultaneously achieve high specific surface area, low thermal conductivity, and high strength; and the magnesium oxide aerogel prepared by the method has poor hydrophobicity. Summary of the Invention

[0009] In view of the shortcomings of the existing technology, the present invention provides a method for preparing magnesium oxide aerogel. The prepared magnesium oxide aerogel has strong hydrophobicity, high specific surface area, low thermal conductivity and high strength.

[0010] In order to solve the above technical problems, the technical solutions adopted by the present invention are as follows:

[0011] A method for preparing magnesium oxide aerogel comprises: preparing a first reinforcing agent, preparing a second reinforcing agent, preparing magnesium sol, preparing silica sol, and mixing;

[0012] The first enhancer is prepared by mixing magnesium chloride hexahydrate and the first portion of deionized water, stirring at a stirring speed of 50-100 rpm for 10-30 minutes at room temperature, adding the first portion of polyvinyl alcohol 1788, continuing to stir for 10-30 minutes, adding ammonia water dropwise to adjust the pH to 11-11.5, controlling the dropping speed of ammonia water to 4-6 mL / min, continuing to stir for 30-35 minutes, centrifuging at a centrifugal speed of 7000-8000 rpm for 10-12 minutes, and discarding the upper The supernatant was vacuum dried at 70-80°C to obtain magnesium hydroxide powder; the second portion of polyvinyl alcohol 1788 and the second portion of deionized water were mixed, stirred at room temperature at a stirring speed of 50-100 rpm for 10-30 minutes, all the above magnesium hydroxide powders were added, and stirring was continued for 1.5-2 hours. The mixture was centrifuged at a centrifugal speed of 9000-10000 rpm for 10-12 minutes, and the precipitate was subjected to 3-4 freeze-thaw cycles. The mixture was vacuum dried at 70-80°C to obtain the first enhancer;

[0013] In the preparation of the first enhancer, the usage ratio of magnesium chloride hexahydrate, the first portion of deionized water, the first portion of polyvinyl alcohol 1788, the second portion of polyvinyl alcohol 1788, and the second portion of deionized water is 95-100 g: 500-550 mL: 1.4-1.5 g: 9-10 g: 500-550 mL;

[0014] The concentration of the ammonia solution is 2 mol / L;

[0015] The freeze-thaw cycle was performed by placing the pellet at -25°C to -20°C for 8-9 hours and at 24-27°C for 4-5 hours;

[0016] The second enhancer is prepared by mixing magnesium chloride hexahydrate, anhydrous ethanol, and deionized water, stirring the mixture at a stirring speed of 50-100 rpm at room temperature for 10-15 minutes, adding propylene oxide, continuing to stir for 10-15 minutes, standing for 45-50 hours, centrifuging the mixture at a centrifugal speed of 9000-10000 rpm for 10-12 minutes, discarding the supernatant, washing the precipitate with deionized water 3-4 times, vacuum drying the mixture at 70-80° C., mixing the mixture with hexadecyltrimethylammonium bromide and dimethyl sulfoxide, and stirring the mixture at room temperature for 30-40 minutes to obtain the second enhancer;

[0017] In the preparation of the second enhancer, the usage ratio of magnesium chloride hexahydrate, anhydrous ethanol, deionized water, propylene oxide, hexadecyltrimethylammonium bromide, and dimethyl sulfoxide is 90-100 g: 550-600 mL: 500-550 mL: 300-320 mL: 1.8-2 g: 100-110 mL;

[0018] The magnesium sol is prepared by mixing magnesium chloride hexahydrate, anhydrous ethanol, deionized water, and polyvinyl alcohol 2488, stirring at room temperature for 20-25 minutes, adding propylene oxide, and continuing to stir for 10-15 minutes to obtain magnesium sol;

[0019] In the preparation of the magnesium sol, the dosage ratio of magnesium chloride hexahydrate, anhydrous ethanol, deionized water, polyvinyl alcohol 2488, and propylene oxide is 100-110 g: 1800-2200 mL: 1000-1200 mL: 5-6 g: 460-500 mL;

[0020] The silica sol is prepared by mixing methyltrimethoxysilane, anhydrous ethanol, and deionized water, stirring the mixture at 40-45° C. and a stirring speed of 100-300 rpm for 10-20 minutes, adding a hydrochloric acid aqueous solution to adjust the pH to 3-3.5, continuing to stir for 20-25 hours, adding ammonia water to adjust the pH to 7.5-8, and continuing to stir for 10-15 minutes to obtain the silica sol;

[0021] In the preparation of the silica sol, the volume ratio of methyltrimethoxysilane, anhydrous ethanol and deionized water is 50-52:200-250:20-23;

[0022] The concentration of the hydrochloric acid aqueous solution is 1 mol / L;

[0023] The concentration of the ammonia solution is 2 mol / L;

[0024] The magnesium sol, silica sol, the first reinforcing agent and the second reinforcing agent are mixed, ultrasonically stirred, and allowed to stand for aging for 30-35 hours under a closed condition at 40-45° C., and then completely immersed in anhydrous ethanol, and allowed to stand for 48-60 hours at 30-35° C., with the anhydrous ethanol replaced once every 12 hours, and supercritically dried in a carbon dioxide environment to obtain magnesium oxide aerogel;

[0025] In the mixing, the dosage ratio of magnesium sol, silica sol, first reinforcing agent and second reinforcing agent is 2800-3000 mL:240-270 mL:14-16 g:100-110 mL;

[0026] The frequency of the ultrasonic stirring is 20-25kHz, the stirring speed is 150-200rpm, and the time is 30-40min;

[0027] The supercritical drying temperature is 50° C., the pressure is 15 MPa, and the time is 20-25 hours.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The preparation method of magnesium oxide aerogel of the present invention uses a first reinforcing agent and a second reinforcing agent in the preparation. When preparing the first reinforcing agent, magnesium hydroxide is first prepared in a polyvinyl alcohol aqueous solution, and then the magnesium hydroxide and the polyvinyl alcohol aqueous solution are mixed. Polyvinyl alcohol is adsorbed on the surface of the magnesium hydroxide, and then a freeze-thaw cycle is performed to obtain magnesium hydroxide with a surface coated with polyvinyl alcohol aerogel. After the first reinforcing agent is mixed with magnesium sol and silica sol, the polyvinyl alcohol aerogel on the surface of the first reinforcing agent can serve as a network structure guide to promote the formation of magnesium oxide gel. At the same time, the magnesium hydroxide in the first reinforcing agent can improve the strength of the magnesium oxide gel. When preparing the second reinforcing agent, magnesium chloride hexahydrate is first used as a magnesium source to obtain magnesium hydroxide with a high specific surface area, the surface of which contains Mg-OH. The magnesium hydroxide is then mixed with cetyltrimethylammonium bromide and dimethyl sulfoxide to obtain a magnesium hydroxide dispersion, namely the second reinforcing agent. The surface of the magnesium hydroxide in the second reinforcing agent is coated with cetyltrimethylammonium bromide, and the hydrophilic end of the cetyltrimethylammonium bromide is connected to the magnesium hydroxide. After the second reinforcing agent is mixed with magnesium sol and silica sol, the cetyltrimethylammonium bromide on the surface of the second reinforcing agent can promote the dispersion of the magnesium hydroxide. At the same time, the hydrophobic end can also combine with the silica sol to prevent particle growth, inhibit phase separation, and promote the connection between the silica sol and the second reinforcing agent.

[0030] (2) The magnesium oxide aerogel prepared by the present invention has strong hydrophobicity and a water contact angle of 127-131°;

[0031] (3) The magnesium oxide aerogel prepared by the present invention has high specific surface area, low thermal conductivity, high strength, and a density of 0.07-0.08 g / cm 3 , BET specific surface area is 725-758m 2 / g, compressive strength is 0.14-0.15MPa, and thermal conductivity is 0.020-0.023W / (m•K). DETAILED DESCRIPTION

[0032] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific embodiments of the present invention are now described.

[0033] The room temperature in the Examples and Comparative Examples was 23°C.

[0034] Example 1

[0035] A method for preparing magnesium oxide aerogel comprises the following steps:

[0036] 1. Preparation of a first enhancer: After mixing 95 g of magnesium chloride hexahydrate and 500 mL of deionized water, stir at room temperature at a stirring speed of 50 rpm for 10 min, add 1.4 g of polyvinyl alcohol 1788, continue stirring for 10 min, add ammonia water dropwise to adjust the pH to 11, control the ammonia water addition rate to 4 mL / min, continue stirring for 30 min, centrifuge at a centrifugal speed of 7000 rpm for 10 min, discard the supernatant, and vacuum dry at 70 ° C to obtain magnesium hydroxide powder; After mixing 9 g of polyvinyl alcohol 1788 and 500 mL of deionized water, stir at room temperature at a stirring speed of 50 rpm for 10 min, add all the above magnesium hydroxide powder, continue stirring for 1.5 h, centrifuge at a centrifugal speed of 9000 rpm for 10 min, discard the supernatant, and place the precipitate at -25 ° C for 8 h and at 24 ° C for 4 h, which is regarded as one freeze-thaw cycle. A total of three freeze-thaw cycles were performed, and vacuum dried at 70 ° C to obtain a first enhancer;

[0037] The concentration of the ammonia solution is 2 mol / L;

[0038] 2. Preparation of a second enhancer: 90 g of magnesium chloride hexahydrate, 550 mL of anhydrous ethanol, and 500 mL of deionized water were mixed, stirred at 50 rpm at room temperature for 10 min, 300 mL of propylene oxide was added, stirring was continued for 10 min, and the mixture was allowed to stand for 45 h. The mixture was centrifuged at 9000 rpm for 10 min, the supernatant was discarded, and the precipitate was washed three times with deionized water and dried in a vacuum oven at 70°C. The mixture was then mixed with 1.8 g of hexadecyltrimethylammonium bromide and 100 mL of dimethyl sulfoxide, and stirred at room temperature for 30 min to obtain a second enhancer.

[0039] 3. Preparation of magnesium sol: 100 g of magnesium chloride hexahydrate, 1800 mL of anhydrous ethanol, 1000 mL of deionized water, and 5 g of polyvinyl alcohol 2488 were mixed, stirred at room temperature for 20 min, 460 mL of propylene oxide was added, and stirring was continued for 10 min to obtain magnesium sol;

[0040] 4. Preparation of silica sol: 50 mL of methyltrimethoxysilane, 200 mL of anhydrous ethanol, and 20 mL of deionized water were mixed, stirred at 100 rpm at 40°C for 10 min, and aqueous hydrochloric acid was added to adjust the pH to 3. The stirring was continued for 20 h. Ammonia water was added to adjust the pH to 7.5, and the stirring was continued for 10 min to obtain silica sol;

[0041] The concentration of the hydrochloric acid aqueous solution is 1 mol / L;

[0042] The concentration of the ammonia solution is 2 mol / L;

[0043] 5. Mixing: After mixing 2800 mL of magnesium sol, 240 mL of silica sol, 14 g of the first reinforcing agent, and 100 mL of the second reinforcing agent, ultrasonically stir for 30 minutes, control the frequency of ultrasonic stirring to 20 kHz, and the stirring speed to 150 rpm. Under closed conditions at 40°C, stand for aging for 30 hours, completely immerse in anhydrous ethanol, stand for 48 hours at 30°C, replace anhydrous ethanol once every 12 hours, and perform supercritical drying in a carbon dioxide environment. Control the temperature during supercritical drying to be 50°C, the pressure to be 15 MPa, and the time to be 20 hours to obtain magnesium oxide aerogel.

[0044] The density of the magnesium oxide aerogel obtained in this example is 0.07 g / cm 3 , BET specific surface area is 725m 2 / g, compressive strength is 0.14MPa, thermal conductivity is 0.022W / (m•K), and water contact angle is 127°.

[0045] Example 2

[0046] A method for preparing magnesium oxide aerogel comprises the following steps:

[0047] 1. Preparation of the first enhancer: 98 g of magnesium chloride hexahydrate and 520 mL of deionized water were mixed, stirred at 80 rpm for 20 min at room temperature, 1.4 g of polyvinyl alcohol 1788 was added, and stirring was continued for 20 min. Ammonia water was added dropwise to adjust the pH to 11.2, and the ammonia water was added at a rate of 5 mL / min. Stirring was continued for 32 min, and the mixture was centrifuged at 7500 rpm for 11 min. The supernatant was discarded and the mixture was dried in vacuum at 75 ° C to obtain magnesium hydroxide powder. 9.5 g of polyvinyl alcohol 1788 and 520 mL of deionized water were mixed, and the mixture was stirred at 80 rpm for 20 min at room temperature. All the above-mentioned magnesium hydroxide powder was added, and the mixture was stirred for 1.5 h. The mixture was centrifuged at 9500 rpm for 11 min, and the supernatant was discarded. The precipitate was placed at -25°C for 8.5 h and at 25°C for 4.5 h, which served as one freeze-thaw cycle. A total of four freeze-thaw cycles were performed, and the mixture was vacuum-dried at 75°C to obtain a first enhancer.

[0048] The concentration of the ammonia solution is 2 mol / L;

[0049] 2. Preparation of a second enhancer: 95 g of magnesium chloride hexahydrate, 580 mL of anhydrous ethanol, and 520 mL of deionized water were mixed and stirred at 80 rpm at room temperature for 12 min. 310 mL of propylene oxide was added and stirring continued for 12 min. The mixture was allowed to stand for 48 h and centrifuged at 9500 rpm for 11 min. The supernatant was discarded and the precipitate was washed three times with deionized water and dried under vacuum at 75°C. The mixture was mixed with 1.9 g of hexadecyltrimethylammonium bromide and 105 mL of dimethyl sulfoxide and stirred at room temperature for 35 min to obtain a second enhancer.

[0050] 3. Preparation of magnesium sol: 105 g of magnesium chloride hexahydrate, 2000 mL of anhydrous ethanol, 1100 mL of deionized water, and 5.5 g of polyvinyl alcohol 2488 were mixed, stirred at room temperature for 23 min, 480 mL of propylene oxide was added, and stirring was continued for 12 min to obtain magnesium sol;

[0051] 4. Preparation of silica sol: 51 mL of methyltrimethoxysilane, 230 mL of anhydrous ethanol, and 22 mL of deionized water were mixed and stirred at 200 rpm at 42°C for 15 min. Aqueous hydrochloric acid was added to adjust the pH to 3.2 and stirring was continued for 22 h. Aqueous ammonia was added to adjust the pH to 7.7 and stirring was continued for 12 min to obtain silica sol;

[0052] The concentration of the hydrochloric acid aqueous solution is 1 mol / L;

[0053] The concentration of the ammonia solution is 2 mol / L;

[0054] 5. Mixing: After mixing 2900 mL of magnesium sol, 250 mL of silica sol, 15 g of the first enhancer, and 105 mL of the second enhancer, ultrasonically stir for 35 minutes, control the frequency of ultrasonic stirring to 22 kHz, and the stirring speed to 180 rpm. Under closed conditions at 42°C, stand for aging for 32 hours, completely immerse in anhydrous ethanol, stand for 60 hours at 32°C, replace anhydrous ethanol once every 12 hours, and perform supercritical drying in a carbon dioxide environment. Control the temperature during supercritical drying to be 50°C, the pressure to be 15 MPa, and the time to be 22 hours to obtain magnesium oxide aerogel.

[0055] The density of the magnesium oxide aerogel obtained in this example is 0.07 g / cm 3 , BET specific surface area is 758m 2 / g, compressive strength is 0.14MPa, thermal conductivity is 0.020W / (m•K), and water contact angle is 130°.

[0056] Example 3

[0057] A method for preparing magnesium oxide aerogel comprises the following steps:

[0058] 1. Preparation of a first enhancer: 100 g of magnesium chloride hexahydrate and 550 mL of deionized water were mixed, stirred at room temperature at a stirring speed of 100 rpm for 30 min, 1.5 g of polyvinyl alcohol 1788 was added, stirring was continued for 30 min, ammonia was added dropwise to adjust the pH to 11.5, the ammonia addition rate was controlled to 6 mL / min, stirring was continued for 35 min, centrifuged at a centrifugal speed of 8000 rpm for 12 min, the supernatant was discarded, and vacuum dried at 80° C. to obtain magnesium hydroxide powder; 10 g of polyvinyl alcohol 1788 and 550 mL of deionized water were mixed, stirred at room temperature at a stirring speed of 100 rpm for 30 min, all of the above magnesium hydroxide powder was added, stirring was continued for 2 h, centrifuged at a centrifugal speed of 10000 rpm for 12 min, the supernatant was discarded, the precipitate was placed at -20° C. for 9 h and at 27° C. for 5 h, which served as one freeze-thaw cycle, and a total of four freeze-thaw cycles were performed, and vacuum dried at 80° C. to obtain a first enhancer;

[0059] The concentration of the ammonia solution is 2 mol / L;

[0060] 2. Preparation of a second enhancer: 100 g of magnesium chloride hexahydrate, 600 mL of anhydrous ethanol, and 550 mL of deionized water were mixed, stirred at 100 rpm at room temperature for 15 minutes, 320 mL of propylene oxide was added, stirring was continued for 15 minutes, and the mixture was allowed to stand for 50 hours. The mixture was centrifuged at 10,000 rpm for 12 minutes, the supernatant was discarded, and the precipitate was washed four times with deionized water and dried in a vacuum at 80°C. The mixture was mixed with 2 g of hexadecyltrimethylammonium bromide and 110 mL of dimethyl sulfoxide, and stirred at room temperature for 40 minutes to obtain a second enhancer;

[0061] 3. Preparation of magnesium sol: 110 g of magnesium chloride hexahydrate, 2200 mL of anhydrous ethanol, 1200 mL of deionized water, and 6 g of polyvinyl alcohol 2488 were mixed, stirred at room temperature for 25 min, 500 mL of propylene oxide was added, and stirring was continued for 15 min to obtain magnesium sol;

[0062] 4. Preparation of silica sol: 52 mL of methyltrimethoxysilane, 250 mL of anhydrous ethanol, and 23 mL of deionized water were mixed and stirred at 300 rpm at 45°C for 20 min. Aqueous hydrochloric acid was added to adjust the pH to 3.5 and stirring was continued for 25 h. Ammonia water was added to adjust the pH to 8 and stirring was continued for 15 min to obtain silica sol;

[0063] The concentration of the hydrochloric acid aqueous solution is 1 mol / L;

[0064] The concentration of the ammonia solution is 2 mol / L;

[0065] 5. Mixing: After mixing 3000 mL of magnesium sol, 270 mL of silica sol, 16 g of the first reinforcing agent, and 110 mL of the second reinforcing agent, ultrasonic stirring was performed for 40 minutes, the frequency of ultrasonic stirring was controlled to be 25 kHz, the stirring speed was controlled to be 200 rpm, and the mixture was allowed to stand for 35 hours under closed conditions at 45°C. The mixture was completely immersed in anhydrous ethanol and allowed to stand for 60 hours at 35°C, with the anhydrous ethanol replaced once every 12 hours. The mixture was supercritically dried in a carbon dioxide environment, and the temperature during supercritical drying was controlled to be 50°C, the pressure to be 15 MPa, and the time to be 25 hours to obtain magnesium oxide aerogel.

[0066] The density of the magnesium oxide aerogel obtained in this example is 0.08 g / cm 3 , BET specific surface area is 740m 2 / g, compressive strength is 0.15MPa, thermal conductivity is 0.023W / (m•K), and water contact angle is 131°.

[0067] Comparative Example 1

[0068] Based on the preparation method of magnesium oxide aerogel in Example 2, the first step of preparing the first reinforcing agent was omitted, and in the fifth step of mixing, 3.8 g of polyvinyl alcohol 1788 and 11.2 g of magnesium hydroxide with an average particle size of 1 μm were used instead of adding 15 g of the first reinforcing agent;

[0069] The remaining operations are the same as those in Example 2.

[0070] The density of the magnesium oxide aerogel obtained in this comparative example is 0.11 g / cm 3 , BET specific surface area is 517m 2 / g, compressive strength is 0.08MPa, thermal conductivity is 0.029W / (m•K), and water contact angle is 115°.

[0071] Comparative Example 2

[0072] Based on the preparation method of magnesium oxide aerogel in Example 2, the second step of preparing the second reinforcing agent was omitted, and in the fifth step of mixing, a mixture of 1.8 g hexadecyltrimethoxysilane and 105 mL dimethyl sulfoxide was used instead of adding 105 mL of the second reinforcing agent;

[0073] The remaining operations are the same as those in Example 2.

[0074] The density of the magnesium oxide aerogel obtained in this comparative example is 0.16 g / cm 3 , BET specific surface area is 481m 2 / g, compressive strength is 0.13MPa, thermal conductivity is 0.034W / (m•K), and water contact angle is 133°.

[0075] Comparison of the density, BET specific surface area, compressive strength, thermal conductivity, and water contact angle of the magnesium oxide aerogels of Examples 1-3 and Comparative Examples 1-2 revealed that the density, BET specific surface area, compressive strength, thermal conductivity, and water contact angle of Comparative Example 1 were inferior to those of Example 2, while the density, BET specific surface area, and thermal conductivity of Comparative Example 2 were inferior to those of Example 2. This further illustrates that when polyvinyl alcohol is added alone, it can cause a decrease in the mechanical properties of the aerogel. Furthermore, it illustrates that when magnesium hydroxide is added alone, the density, BET specific surface area, thermal conductivity, and water contact angle of the aerogel are also affected due to the poor dispersibility of magnesium hydroxide and its poor binding force with the sol. It further illustrates that when hexadecyltrimethylammonium bromide is added alone, hexadecyltrimethoxysilane can cause phase separation in the aerogel, resulting in a decrease in the BET specific surface area and thermal conductivity and an increase in density.

[0076] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art will be able to modify the technical solutions described in the aforementioned embodiments or substitute equivalents for some of the technical features. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for preparing magnesium oxide aerogel, characterized in that: include: preparing a first reinforcing agent, preparing a second reinforcing agent, preparing a magnesium sol, preparing a silica sol, and mixing them; The first enhancer is prepared by mixing magnesium chloride hexahydrate and a first portion of deionized water, stirring at room temperature, adding a first portion of polyvinyl alcohol 1788, stirring, adjusting the pH to 11-11.5, stirring, centrifuging, discarding the supernatant, and vacuum drying to obtain magnesium hydroxide powder; mixing a second portion of polyvinyl alcohol 1788 and a second portion of deionized water, stirring at room temperature, adding magnesium hydroxide powder, stirring, centrifuging, subjecting the precipitate to 3-4 freeze-thaw cycles, and vacuum drying to obtain the first enhancer; The second enhancer is prepared by mixing magnesium chloride hexahydrate, anhydrous ethanol, and deionized water, stirring at room temperature, adding propylene oxide, stirring, standing, centrifuging, washing the precipitate, vacuum drying, mixing with hexadecyltrimethylammonium bromide and dimethyl sulfoxide, and stirring at room temperature to obtain the second enhancer; The magnesium sol, silica sol, the first reinforcing agent and the second reinforcing agent are mixed, ultrasonically stirred, allowed to stand and age under a closed condition at 40-45° C., completely immersed in anhydrous ethanol, allowed to stand at 30-35° C., and supercritically dried in a carbon dioxide environment to obtain magnesium oxide aerogel.

2. The method for preparing magnesium oxide aerogel according to claim 1, wherein In the preparation of the first enhancer, the usage ratio of magnesium chloride hexahydrate, the first portion of deionized water, the first portion of polyvinyl alcohol 1788, the second portion of polyvinyl alcohol 1788, and the second portion of deionized water is 95-100 g: 500-550 mL: 1.4-1.5 g: 9-10 g: 500-550 mL.

3. The method for preparing magnesium oxide aerogel according to claim 1, wherein In the preparation of the first enhancer, the freeze-thaw cycle is performed by placing the precipitate at -25°C to -20°C for 8-9 hours and at 24-27°C for 4-5 hours.

4. The method for preparing magnesium oxide aerogel according to claim 1, wherein In the preparation of the second enhancer, the usage ratio of magnesium chloride hexahydrate, anhydrous ethanol, deionized water, propylene oxide, hexadecyltrimethylammonium bromide, and dimethyl sulfoxide is 90-100 g: 550-600 mL: 500-550 mL: 300-320 mL: 1.8-2 g: 100-110 mL.

5. The method for preparing magnesium oxide aerogel according to claim 1, wherein The magnesium sol is prepared by mixing magnesium chloride hexahydrate, anhydrous ethanol, deionized water, and polyvinyl alcohol 2488, stirring at room temperature, adding propylene oxide, and stirring to obtain the magnesium sol.

6. The method for preparing magnesium oxide aerogel according to claim 5, wherein: In the preparation of the magnesium sol, the usage ratio of magnesium chloride hexahydrate, anhydrous ethanol, deionized water, polyvinyl alcohol 2488, and propylene oxide is 100-110 g: 1800-2200 mL: 1000-1200 mL: 5-6 g: 460-500 mL.

7. The method for preparing magnesium oxide aerogel according to claim 1, wherein The silica sol is prepared by mixing methyltrimethoxysilane, anhydrous ethanol and deionized water, stirring at 40-45° C., adjusting the pH to 3-3.5, stirring, adjusting the pH to 7.5-8, stirring to obtain the silica sol.

8. The method for preparing magnesium oxide aerogel according to claim 7, wherein: In the preparation of the silica sol, the volume ratio of methyltrimethoxysilane, anhydrous ethanol and deionized water is 50-52:200-250:20-23.

9. The method for preparing magnesium oxide aerogel according to claim 1, wherein In the mixing, the usage ratio of magnesium sol, silica sol, first reinforcing agent and second reinforcing agent is 2800-3000 mL: 240-270 mL: 14-16 g: 100-110 mL.

10. The method for preparing magnesium oxide aerogel according to claim 1, wherein: During the mixing, the frequency of the ultrasonic stirring is 20-25 kHz, the stirring speed is 150-200 rpm, and the time is 30-40 min; The supercritical drying temperature is 50° C., the pressure is 15 MPa, and the time is 20-25 hours.