Preparation method of hydrophobic modified silicon dioxide aerogel with low thermal conductivity

Abstract

The invention discloses a preparation method of hydrophobic modified silicon dioxide aerogel with low thermal conductivity. The preparation method comprises the following steps: by taking water glass-tetraethoxysilane as a composite silicon source and formamide-ethylene glycol as a retarder, hydrolyzing and mixing, and then adjusting the pH value to trigger sol-gel reaction to form wet gel; after tert-butyl alcohol aging and solvent replacement, hexamethyldisiloxane and hexamethyldisilazane dual modifiers are used for synchronous hydrophobic modification and sodium removal, and the hydrophobic modified low-thermal-conductivity silicon dioxide aerogel with a porous nanostructure is prepared through freeze drying. The method uses the composite silicon source to cooperate with the gel, so that the raw material cost is reduced; a double-modifier synchronous sodium removal process is green and efficient; the obtained aerogel has low thermal conductivity and excellent hydrophobicity, is suitable for the fields of building thermal insulation, industrial thermal insulation, aerospace and the like, and has the advantages of economy and environmental protection.

Description

Technical Field

[0001] The present invention belongs to the technical field of aerogel preparation, and particularly relates to a method for preparing hydrophobic modified low thermal conductivity silica aerogel by using low-cost sodium silicate (water glass) and tetraethyl orthosilicate as composite silicon sources, combining sol-gel method, surface modification, synergistic sodium removal and freeze-drying process. Background Art

[0002] Silica aerogel is a novel lightweight material with a nanoscale porous structure, and is known as "solid smoke" due to its extremely low density and unique three-dimensional network structure. This material exhibits excellent physical properties, including extremely low thermal conductivity, high porosity and high specific surface area, which make it have broad application prospects in the fields of high-efficiency heat insulation, acoustic noise reduction, optical devices, etc.

[0003] However, general methods for preparing silica aerogel have many disadvantages, which limit its large-scale application and industrial production. Firstly, traditional preparation methods usually rely on organosilicon sources such as methyl orthosilicate or tetraethyl orthosilicate, and these raw materials are expensive, resulting in high production costs and being difficult to meet the requirements of large-scale industrial production. Secondly, general drying processes usually adopt supercritical drying technology. This technology not only requires high-pressure equipment (usually operating pressure > 25 MPa, temperature > 40 °C), there are certain safety risks, but also has high energy consumption and low yield. The complexity and high cost of supercritical drying equipment further increase the production difficulty and economic burden.

[0004] At the same time, silica aerogel prepared solely with water glass as the silicon source has key performance shortcomings. The influence of sodium ions during the water glass gelation process easily leads to difficult control of the gelation reaction, resulting in uneven pore distribution and relatively large pore size of the formed wet gel, thereby causing an increase in the thermal conductivity of the aerogel. In addition, generally, unmodified silica aerogel is rich in silicon hydroxyl groups (Si-OH) on the surface and has strong hydrophilicity. In a humid environment, water molecules easily enter the nanopores of the aerogel through capillary action, leading to the following problems: (1) the gas in the pores is replaced by water molecules, increasing the heat conduction path and reducing the heat insulation performance; (2) a thermal bridge effect is formed, further weakening the heat insulation effect of the material; (3) the adsorption of water may cause deformation of the aerogel structure, affecting its long-term stability and durability. Although the hydrophobicity of the aerogel can be improved through surface modification, general modification methods usually require additional steps and expensive modifiers, increasing the complexity and cost of preparation.

[0005] Based on the above analysis, it is an urgent problem for those skilled in the art to develop a preparation method of hydrophobic modified low thermal conductivity silica aerogel, which is based on a cheap silicon source (such as sodium silicate water glass), supplemented with a small amount of tetraethyl orthosilicate as a structure regulator, combined with surface hydrophobic modification and a safe and controllable drying process, in order to achieve continuous and large-scale production, reduce the preparation cost, simplify the process flow, and improve the material properties. Summary of the Invention

[0006] Aiming at the problems that existing silica aerogels are prone to water absorption in humid environments, resulting in increased thermal conductivity and shortened service life, the present invention provides a preparation method of hydrophobic modified low thermal conductivity silica aerogel. This method uses low-cost sodium silicate as the silicon source, supplemented with a small amount of tetraethyl orthosilicate as a structure regulator, and combines the sol-gel method, surface modification technology and freeze-drying process to prepare silica aerogel with excellent hydrophobicity and low thermal conductivity. The present invention is assisted by tetraethyl orthosilicate, introduces hexamethyldisiloxane and hexamethyldisilazane as surface modifiers, and utilizes the chemical inertness between the sodium salt and the hydrophobic modifier to efficiently remove sodium ions during the hydrophobization process, while improving the hydrophobic performance of the aerogel, so that it can still maintain low thermal conductivity in humid environments. The freeze-drying process is used to replace the traditional supercritical drying technology, which not only reduces the production cost, but also simplifies the operation process and improves the production efficiency. The silica aerogel prepared by the present invention has many excellent properties such as high specific surface area, high porosity, stable mesoporous structure, excellent hydrophobic performance, good thermal stability and low thermal conductivity, and has broad application prospects in the fields of building insulation, industrial heat insulation, aerospace, etc.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for preparing hydrophobic modified low thermal conductivity silica aerogel using sodium silicate - tetraethyl orthosilicate as a composite silicon source: Add a formamide and ethylene glycol mixed retarder to sodium silicate, dropwise add a tetraethyl orthosilicate solution while stirring, and adjust the pH to neutral with an oxalic acid solution to form a hydrogel; after aging with tert-butanol and solvent replacement, use hexamethyldisiloxane and hexamethyldisilazane as double modifiers for synchronous hydrophobic modification and sodium removal, and obtain a hydrophobic modified low thermal conductivity silica aerogel with a porous nanostructure through freeze-drying. The specific steps are as follows:

[0009] Step S1: Prepare sodium silicate: Dissolve sodium silicate in deionized water, and obtain a sodium silicate solution through ultrasonic dispersion;

[0010] Step S2: Prepare a tetraethyl orthosilicate solution: Mix tetraethyl orthosilicate and deionized water with a volume ratio of 1:1, and gradually add a 10wt% oxalic acid solution dropwise during stirring to adjust the pH to 6.8, and stir for 4 minutes to completely hydrolyze tetraethyl orthosilicate to obtain a tetraethyl orthosilicate solution;

[0011] Step S3: Acid addition and gelation treatment: Add a mixed retarder composed of formamide and ethylene glycol to the sodium silicate solution prepared in Step S1, slowly add the tetraethyl orthosilicate solution prepared in Step S2 during stirring, and then gradually add 10 wt% oxalic acid solution to adjust the pH to 7.0 - 7.4 to obtain a hydrogel.

[0012] Step S4: Aging and solvent replacement: Immerse the hydrogel prepared in Step S3 in tert-butanol, seal and let it stand at room temperature for 24 hours of aging; replace the tert-butanol every 8 hours during aging to obtain the aged hydrogel.

[0013] Step S5: Surface modification: Immerse the aged hydrogel prepared in Step S4 in a tert-butanol solution of hexamethyldisiloxane and hexamethyldisilazane, and perform hydrophobic modification at room temperature for 18 hours to obtain a hydrophobically modified hydrogel.

[0014] Step S6: Freeze-drying: Place the hydrophobically modified hydrogel prepared in Step S5 at -70°C for 6 hours of freezing, and then perform vacuum drying at -70°C for 24 hours to obtain the hydrophobically modified low thermal conductivity silica aerogel.

[0015] Further, in Step S1, the modulus of sodium silicate is 2.0 - 3.0; the mass fraction of SiO2 in the sodium silicate solution is 2% - 6%.

[0016] Further, in Step S3, the volume ratio of the tetraethyl orthosilicate solution to the sodium silicate solution is 2:10.

[0017] Further, in Step S3, the molar ratio of silicon dioxide, formamide, and ethylene glycol in the sodium silicate solution is 1:2:1.

[0018] Further, in Step S4, the volume ratio of the tert-butanol replaced each time to the hydrogel is 1.2:1.

[0019] Further, in Step S5, the total volume ratio of hexamethyldisiloxane and hexamethyldisilazane to tert-butanol is 1:8, and the molar ratio of sodium silicate, hexamethyldisiloxane, and hexamethyldisilazane is 2:1:1.

[0020] The beneficial effects of the present invention are as follows:

[0021] (1) The present invention uses inexpensive and easily available water glass as the main silicon source, combines a unique retarder formula and an optimized pH strategy, avoids the problem of high cost of traditional orthosilicate silicon sources, fundamentally reduces the economic threshold of material production, and provides a practical solution for industrial scale production.

[0022] (2) By simultaneously introducing a small amount of tetraethyl orthosilicate as an auxiliary silicon source, the present invention effectively inhibits pore coarsening through the synergistic effect generated by the silicon source compounding, optimizes the three-dimensional pore structure of the gel network, forms a mesoporous structure with a concentrated pore size distribution, and endows the material with both high porosity and a fine nanostructure without substantially increasing the raw material cost.

[0023] (3) By selecting tert-butanol as the aging and solvent replacement medium, the present invention fully utilizes the unique physical and chemical properties of tert-butanol: tert-butanol has a low surface tension. Compared with traditional solvents such as water, it can penetrate and replace the water in the wet gel more effectively during the solvent replacement process, reducing the risk of collapse of the gel pore structure, thus ensuring the integrity of the porous structure of the aerogel. At the same time, the vapor pressure of tert-butanol is moderate, especially during the freeze-drying process, which is conducive to the rapid sublimation of the solvent, avoiding structural damage caused by solvent residue or freezing in the pores. This property effectively improves the drying efficiency and maintains the low thermal conductivity and high specific surface area of the aerogel.

[0024] (4) The present invention uses hexamethyldisiloxane and hexamethyldisilazane as double modifiers to form a hydrophobic layer on the pore walls of the aerogel through synergistic covalent bonding, effectively blocking the entry of moisture and maintaining low thermal conductivity and long-term stability. Utilizing the chemical inertness of the modifiers and sodium salts, sodium ions are synchronously removed during the phase separation process without the need for traditional anion exchange resin sodium removal and regenerant treatment, avoiding strong acidic pollution, eliminating the water washing and ion exchange steps before and after gelation, shortening the synthesis time and reducing solvent consumption, significantly enhancing the environmental protection and economy of the process, and providing a more efficient solution for large-scale production.

[0025] (5) The silica aerogel prepared by the present invention has many excellent properties such as high specific surface area, high porosity, a stable mesoporous structure, excellent hydrophobic properties, good thermal stability, and low thermal conductivity.

[0026] (6) The present invention uses the freeze-drying process to replace the supercritical drying technology, avoiding the complex operation and high energy consumption of high-pressure equipment, simplifying the preparation process, improving the production efficiency, and being suitable for continuous production.

[0027] (7) The hydrophobic modified low thermal conductivity silica aerogel prepared by the present invention has broad application prospects in the fields of building insulation, industrial heat insulation, aerospace, energy storage, etc., and can significantly improve the material properties and application efficiency in related fields. Description of the Drawings

[0028] Figure 1 It is the scanning electron microscope images of the water glass-tetraethyl orthosilicate hydrophobic modified low thermal conductivity silica aerogel (1) of Examples 1, 2, 3, 4 of the present invention and the hydrophobic modified silica aerogel (2) of Comparative Examples 1, 2, 3, 4. Figure 1(1), the scale bar is 50 nm; in Figure 1 (2), the scale bar is 200 nm.

[0029] Figure 2 These are the contact angle test pictures of the sodium silicate - tetraethyl orthosilicate hydrophobically modified low - thermal - conductivity silica aerogel in Examples 1, 2, 3, 4 of the present invention and the hydrophobically modified silica aerogel in Comparative Examples 1, 2, 3, 4.

[0030] Figure 3 These are the infrared spectra of the sodium silicate - tetraethyl orthosilicate hydrophobically modified low - thermal - conductivity silica aerogel (1) in Examples 1, 2, 3, 4 of the present invention and the hydrophobically modified silica aerogel (2) in Comparative Examples 1, 2, 3, 4; the abscissa is the wave number (cm -1 ), and the ordinate is the transmittance (%).

[0031] Figure 4 These are the N2 adsorption - desorption isotherm curves of the sodium silicate - tetraethyl orthosilicate hydrophobically modified low - thermal - conductivity silica aerogel (1) in Examples 1, 2, 3, 4 of the present invention and the hydrophobically modified silica aerogel (2) in Comparative Examples 1, 2, 3, 4; the abscissa is the relative pressure (P / P0), and the ordinate is the adsorption amount (cm 3 / g STP).

[0032] Figure 5 These are the BJH pore size distribution curves of the sodium silicate - tetraethyl orthosilicate hydrophobically modified low - thermal - conductivity silica aerogel (1) in Examples 1, 2, 3, 4 of the present invention and the hydrophobically modified silica aerogel (2) in Comparative Examples 1, 2, 3, 4; the abscissa is the pore diameter (nm), and the ordinate is the pore size distribution (cm 3 / g). Detailed Embodiments

[0033] In order to make the content of the present invention easier to understand, the technical solutions of the present invention are further described below in conjunction with specific embodiments, but the present invention is not limited thereto.

[0034] Example 1 (using sodium silicate powder (modulus 2.0 - 2.2) as the silicon source, tert - butanol as the aging agent, wt SiO2 = 2%)

[0035] (1) Preparation of sodium silicate aqueous solution:

[0036] Take 5 g of sodium silicate powder (modulus 2.0 - 2.2) and add it to 160 g of deionized water. Use an ultrasonic cleaner to disperse it evenly to make a sodium silicate aqueous solution with a mass fraction of SiO2 of 2%.

[0037] (2) Preparation of tetraethyl orthosilicate solution:

[0038] 16 mL of tetraethyl orthosilicate was taken respectively and the same volume of deionized water was added. During stirring, 10% oxalic acid solution was added dropwise to adjust the pH to 6.8, and stirring was carried out fully for 40 minutes to completely hydrolyze tetraethyl orthosilicate.

[0039] (3) Acid addition for gelation:

[0040] 1.08 mL of formamide and 3.06 mL of ethylene glycol were added to the aqueous sodium silicate solution. Then, during stirring, the prepared tetraethyl orthosilicate solution was slowly added to the sodium silicate solution. Finally, 10% oxalic acid solution was slowly added to the solution and the pH was adjusted to 7.0 - 7.4 to form a hydrogel.

[0041] (4) Aging and solvent replacement:

[0042] The wet gel was soaked in 192 mL of tert - butanol, sealed and left standing at room temperature for 24 hours of aging. During aging, tert - butanol was replaced every 8 hours until all the water in the wet gel was replaced, and the amount of tert - butanol replaced each time was 1.2 times the volume of the wet gel.

[0043] (5) Surface modification:

[0044] The aged wet gel was immersed in a tert - butanol solution containing 2.5 g of hexamethyldisiloxane and 2.5 g of hexamethyldisilazane for hydrophobic modification for 18 hours at room temperature.

[0045] (6) Freeze - drying:

[0046] The modified wet gel was frozen at - 70 °C for 6 hours and then freeze - dried for 24 hours to obtain a hydrophobic - modified low - thermal - conductivity silica aerogel.

[0047] Example 2 (using sodium silicate powder (modulus 2.3 - 2.5) as the silicon source and tert - butanol as the aging agent, wt SiO2 = 2%)

[0048] (1) Preparation of aqueous sodium silicate solution:

[0049] 5 g of sodium silicate powder (modulus 2.3 - 2.5) was taken respectively and added to 175 g of deionized water, and ultrasonic cleaner was used for uniform dispersion to prepare an aqueous sodium silicate solution with a SiO₂ mass fraction of 2%.

[0050] (2) Preparation of tetraethyl orthosilicate solution:

[0051] 17.5 mL of tetraethyl orthosilicate was taken respectively and the same volume of deionized water was added. During stirring, 10% oxalic acid solution was added dropwise to adjust the pH to 6.8, and stirring was carried out fully for 40 minutes to completely hydrolyze tetraethyl orthosilicate.

[0052] (3) Acid addition for gelation:

[0053] Add 0.93 mL of formamide and 2.63 mL of ethylene glycol to the sodium silicate aqueous solution respectively. Then, during stirring, slowly add the prepared tetraethyl orthosilicate solution to the sodium silicate solution. Finally, slowly add 10% oxalic acid solution to the solution and adjust the pH to 7.0 - 7.4 to prepare a hydrogel.

[0054] (4) Aging and solvent replacement:

[0055] Soak the wet gel in 210 mL of tert-butanol respectively, seal and let it stand at room temperature for aging for 24 hours. During aging, replace the tert-butanol every 8 hours until all the water in the wet gel is replaced. The amount of tert-butanol replaced each time is 1.2 times the volume of the wet gel.

[0056] (5) Surface modification and (6) freeze-drying are the same as in Example 1.

[0057] Finally, a hydrophobic modified low thermal conductivity silica aerogel is obtained.

[0058] Example 3 (using sodium silicate powder (modulus 2.6 - 2.7) as the silicon source and tert-butanol as the aging agent, wt SiO2 = 2%)

[0059] (1) Preparation of sodium silicate aqueous solution:

[0060] Take five portions of 5 g of sodium silicate powder (modulus 2.6 - 2.7) and add them to 175 g of deionized water respectively. Use an ultrasonic cleaner to disperse them evenly to prepare a sodium silicate aqueous solution with a SiO2 mass fraction of 2%.

[0061] (2) Preparation of tetraethyl orthosilicate solution:

[0062] Take 17.5 mL of tetraethyl orthosilicate respectively and add the same volume of deionized water. During stirring, slowly add 10% oxalic acid solution dropwise to adjust the pH to 6.8, and stir well for 40 minutes to completely hydrolyze the tetraethyl orthosilicate.

[0063] (3) Acid addition for gelation:

[0064] Add 0.89 mL of formamide and 2.52 mL of ethylene glycol to the sodium silicate aqueous solution respectively. Then, during stirring, slowly add the prepared tetraethyl orthosilicate solution to the sodium silicate solution. Finally, slowly add 10% oxalic acid solution to the solution and adjust the pH to 7.0 - 7.4 to prepare a hydrogel.

[0065] (4) Aging and solvent replacement:

[0066] Soak the wet gel in 210 mL of tert-butanol respectively, seal and leave it standing at room temperature for 24 hours of aging. Replace the tert-butanol every 8 hours during the aging process until all the water in the wet gel is replaced. The volume of tert-butanol replaced each time is 1.2 times the volume of the wet gel.

[0067] (5) Surface modification and (6) freeze-drying are the same as in Example 1.

[0068] Finally, a hydrophobic modified low thermal conductivity silica aerogel is obtained.

[0069] Example 4 (sodium silicate powder (modulus 2.8 - 3.0) as the silicon source, tert-butanol as the aging agent, wt SiO2 = 2%)

[0070] (1) Preparation of sodium silicate aqueous solution:

[0071] Take five portions of 5 g of sodium silicate powder (modulus 2.8 - 3.0) and add them to 180 g of deionized water respectively. Use an ultrasonic cleaner to disperse them evenly to make a sodium silicate aqueous solution with a SiO2 mass fraction of 2%.

[0072] (2) Preparation of tetraethyl orthosilicate solution:

[0073] Take 18 mL of tetraethyl orthosilicate respectively and add the same volume of deionized water. During stirring, slowly add 10% oxalic acid solution dropwise to adjust the pH to 6.8, and stir well for 40 minutes to completely hydrolyze the tetraethyl orthosilicate.

[0074] (3) Acid addition for gelation:

[0075] Add 0.82 mL of formamide and 2.30 mL of ethylene glycol to the sodium silicate aqueous solution respectively, and then slowly add the prepared tetraethyl orthosilicate solution to the sodium silicate solution during stirring. Finally, slowly add 10% oxalic acid solution to the solution and adjust the pH to 7.0 - 7.4 to make a hydrogel body.

[0076] (4) Aging and solvent replacement:

[0077] Soak the wet gel in 216 mL of tert-butanol respectively, seal and leave it standing at room temperature for 24 hours of aging. Replace the tert-butanol every 8 hours during the aging process until all the water in the wet gel is replaced. The volume of tert-butanol replaced each time is 1.2 times the volume of the wet gel.

[0078] (5) Surface modification and (5) freeze-drying are the same as in Example 1.

[0079] Finally, a hydrophobic modified low thermal conductivity silica aerogel is obtained.

[0080] Comparative Example 1 (sodium silicate powder (modulus 2.0 - 2.2) as the silicon source, tert-butanol as the aging agent, wt SiO2= 2%)

[0081] (1) Preparation of sodium silicate aqueous solution:

[0082] Take 5 g of sodium silicate powder (modulus 2.0 - 2.2) and add it to 160 g of deionized water. Use an ultrasonic cleaner to disperse it evenly to make a sodium silicate aqueous solution with a mass fraction of SiO2 of 2%.

[0083] (2) Acid addition for gelation:

[0084] Add 1.08 mL of formamide and 3.06 mL of ethylene glycol to the sodium silicate aqueous solution. Then, slowly add 10% oxalic acid solution to the solution during stirring and adjust the pH to 7.0 - 7.4 to make a hydrogel.

[0085] (3) Aging and solvent replacement:

[0086] Soak the wet gel in 192 mL of tert-butanol, seal and let it stand at room temperature for aging for 24 hours. Replace the tert-butanol every 8 hours during aging until all the water in the wet gel is replaced. The amount of tert-butanol replaced each time is 1.2 times the volume of the wet gel.

[0087] (4) Surface modification:

[0088] Immerse the aged wet gel in a tert-butanol solution containing 2.5 g of hexamethyldisiloxane and 2.5 g of hexamethyldisilazane for hydrophobic modification at room temperature for 18 hours.

[0089] (5) Freeze-drying:

[0090] Freeze the modified wet gel at -70 °C for 6 hours, and then perform freeze-drying for 24 hours to obtain a hydrophobic modified low thermal conductivity silica aerogel.

[0091] Comparative Example 2 (using sodium silicate powder (modulus 2.3 - 2.5) as the silicon source and tert-butanol as the aging agent, wt SiO2 = 2%)

[0092] (1) Preparation of sodium silicate aqueous solution:

[0093] Take five portions of 5 g of sodium silicate powder (modulus 2.3 - 2.5) and add them to 175 g of deionized water respectively. Use an ultrasonic cleaner to disperse them evenly to make a sodium silicate aqueous solution with a mass fraction of SiO2 of 2%.

[0094] (2) Acid addition for gelation:

[0095] Add 0.93 mL of formamide and 2.63 mL of ethylene glycol to the sodium silicate aqueous solution respectively. Then, slowly add 10% oxalic acid solution to the solution during stirring and adjust the pH to 7.0 - 7.4 to make a hydrogel.

[0096] (3) Aging and solvent replacement:

[0097] Soak the wet gel in 210 mL of tert-butanol respectively, seal and leave it still at room temperature, age for 24 hours, replace the tert-butanol every 8 hours during the aging process until all the water in the wet gel is replaced, and the amount of tert-butanol replaced each time is 1.2 times the volume of the wet gel.

[0098] (4) Surface modification and (5) freeze-drying are the same as in Comparative Example 1.

[0099] Finally, a hydrophobic modified low thermal conductivity silica aerogel is obtained.

[0100] Comparative Example 3 (sodium silicate powder (modulus 2.6 - 2.7) as the silicon source, tert-butanol as the aging agent, wt SiO2 = 2%)

[0101] (1) Preparation of sodium silicate aqueous solution:

[0102] Take five portions of 5 g of sodium silicate powder (modulus 2.6 - 2.7) and add them to 175 g of deionized water respectively, and use an ultrasonic cleaner to disperse them evenly to make a sodium silicate aqueous solution with a SiO2 mass fraction of 2%.

[0103] (2) Acid addition and gelation:

[0104] Add 0.89 mL of formamide and 2.52 mL of ethylene glycol to the sodium silicate aqueous solution respectively, and then slowly add 10% oxalic acid solution to the solution during stirring and adjust the pH to 7.0 - 7.4 to make a hydrogel body.

[0105] (3) Aging and solvent replacement:

[0106] Soak the wet gel in 210 mL of tert-butanol respectively, seal and leave it still at room temperature, age for 24 hours, replace the tert-butanol every 8 hours during the aging process until all the water in the wet gel is replaced, and the amount of tert-butanol replaced each time is 1.2 times the volume of the wet gel.

[0107] (4) Surface modification and (5) freeze-drying are the same as in Comparative Example 1.

[0108] Finally, a hydrophobic modified low thermal conductivity silica aerogel is obtained.

[0109] Comparative Example 4 (sodium silicate powder (modulus 2.8 - 3.0) as the silicon source, tert-butanol as the aging agent, wt SiO2 = 2%)

[0110] (1) Preparation of sodium silicate aqueous solution:

[0111] Take 5 g of sodium silicate powder (modulus 2.8 - 3.0) and add it to 180 g of deionized water respectively. Use an ultrasonic cleaner to disperse it evenly to make a sodium silicate aqueous solution with a mass fraction of SiO₂ of 2%.

[0112] (2) Acid - induced gelation:

[0113] Add 0.82 mL of formamide and 2.30 mL of ethylene glycol to the sodium silicate aqueous solution respectively. Then, during stirring, slowly add 10% oxalic acid solution to the solution and adjust the pH to 7.0 - 7.4 to make a hydrogel.

[0114] (3) Aging and solvent replacement:

[0115] Soak the wet gel in 216 mL of tert - butanol respectively, seal and let it stand at room temperature for 24 hours of aging. During aging, replace the tert - butanol every 8 hours until all the water in the wet gel is replaced. The amount of tert - butanol replaced each time is 1.2 times the volume of the wet gel.

[0116] (4) Surface modification and (5) freeze - drying are the same as in Comparative Example 1.

[0117] Finally, a hydrophobic - modified low - thermal - conductivity silica aerogel is obtained.

[0118] Performance testing

[0119] Table 1 Performance testing of Examples 1, 2, 3, 4 and Comparative Examples 1, 2, 3, 4

[0120]

[0121] Table 2 EDS element content analysis in Example 1

[0122] Element <![CDATA[W t %]]> <![CDATA[W t % sigma]]> <![CDATA[A t %]]> C 25.39 0.37 36.03 O 40.27 0.27 42.91 Na 1.60 0.05 1.19 Si 32.74 0.21 19.87 Total amount 100.00 100.00

[0123] Table 3 EDS element content analysis in Comparative Example 1

[0124] Element <![CDATA[W t %]]> <![CDATA[W t % sigma]]> <![CDATA[A t %]]> C 35.37 0.21 46.06 O 40.79 0.18 39.88 Na 6.44 0.06 4.38 Si 17.40 0.09 9.69 Total amount 100.00 100.00

[0125] As shown in Table 1, the specific surface area, pore volume, average pore diameter, thermal conductivity, and contact angle performance test results of Examples 1, 2, 3, 4 and Comparative Examples 1, 2, 3, 4 are shown respectively. The specific surface area of the examples gradually decreases from 1304.3176 m 2 / g (Example 1) to 971.50270 m 2 / g (Example 4); the specific surface area of the comparative examples gradually decreases from 533.5544 m 2 / g (Comparative Example 1) to 437.2544 m 2 / g (Comparative Example 4); as the modulus of sodium silicate increases, the specific surface area gradually decreases, which may be related to the densification of the internal structure of the material or the change in porosity. The specific surface area of the examples is generally higher than that of the comparative examples, especially at low moduli (1304.3176 m 2 / g for Example 1 and 533.5544 m 2 / g for Comparative Example 1). The pore volume of the examples gradually decreases from 1.7799 cm 3 / g (Example 1) to 1.2362 cm 3 / g (Example 4); the pore volume of the comparative examples gradually decreases from 1.6485 cm 3 / g (Comparative Example 1) to 0.9529 cm 3 / g (Comparative Example 4); as the modulus of sodium silicate increases, the pore volume decreases significantly. This indicates that as the modulus increases, the pore volume of the material gradually decreases, which may be related to the densification of the internal structure of the material or the change in porosity; the pore volume of the examples is generally higher than that of the comparative examples, especially at high moduli (0.9529 cm 3 / g for Comparative Example 1 and 1.2362 cm 3 / g for Example 1). The average pore diameter of the examples decreases from 4.6820 nm (Example 1) to 3.6865 nm (Example 2); for the comparative examples: the average pore diameter gradually decreases from 11.9358 nm (Comparative Example 1) to 6.8563 nm (Comparative Example 4); the change in the average pore diameter is highly correlated with the increase in the modulus of sodium silicate. The average pore diameter of the comparative examples is generally larger than that of the examples, especially at higher moduli (11.9358 nm for Comparative Example 3 and 3.6865 nm for Example 1), indicating that the comparative examples may have a larger pore structure at higher moduli. For the examples: the thermal conductivity gradually increases from 0.0267 W / m·K (Example 1) to 0.0310 W / m·K (Example 4); for the comparative examples, the thermal conductivity gradually increases from 0.0408 W / m·K (Comparative Example 1) to 0.0446 W / m·K (Comparative Example 4); as the modulus of sodium silicate increases, the thermal conductivity gradually increases. This indicates that as the modulus of sodium silicate increases, the thermal conductivity of the material gradually increases, which may be related to the densification of the internal structure of the material or the change in porosity; the thermal conductivity of the examples is generally lower than that of the comparative examples, especially at low moduli (0.0267 W / m·K for Example 1 and 0.0446 W / m·K for Comparative Example 1), indicating that the examples may have better thermal insulation performance at the modulus.

[0126] As shown in Table 2, the EDS elemental content analysis of Example 1 is presented. The oxygen (O) content is the highest, indicating the presence of a large amount of oxides in the sample, which is consistent with the chemical composition of silica aerogel; the carbon (C) content is the second highest, possibly originating from organic additives or surface modifiers used in the preparation process; the silicon (Si) content is 32.74%, which is one of the main components of silica aerogel; the sodium (Na) content is extremely low, originating from the sodium silicate raw material used in the preparation process, indicating that the introduction of tetraethyl orthosilicate can reduce the residual sodium ions. The standard deviation of each element is small, indicating that the elemental content is relatively evenly distributed in the sample, and the analysis results have high reliability. The high contents of oxygen and silicon elements confirm that the sample is mainly composed of silica, which is in line with the expected composition of silica aerogel; the relatively high carbon content may indicate that organic components were introduced during the preparation process, and carbon-containing compounds were used during the surface modification process; the low sodium content is the residual component in the raw material and has little impact on the overall performance. The EDS analysis results confirm that Example 1 is mainly composed of silica, and at the same time contains a certain amount of carbon and sodium. The presence of these elements is closely related to the preparation process and surface modification steps, further verifying the chemical composition and structural characteristics of the sample.

[0127] As shown in Table 3, the EDS elemental content analysis of Comparative Example 1 is presented. The oxygen (O) content is the highest, indicating the presence of a large amount of oxides in the sample, which is consistent with the chemical composition of silica aerogel; the carbon (C) content is the second highest, possibly originating from organic additives or surface modifiers used in the preparation process; the silicon (Si) content is 17.40%, which is one of the main components of silica aerogel; the sodium (Na) content is relatively high, originating from the residual sodium ions in the preparation process. The standard deviation of each element is small, indicating that the elemental content is relatively evenly distributed in the sample, and the analysis results have high reliability. The high contents of oxygen and silicon confirm that the sample is mainly composed of silica, which is in line with the expected composition of silica aerogel; the relatively high carbon content may indicate that organic components were introduced during the preparation process, and carbon-containing compounds were used during the surface modification process. The EDS analysis results confirm that Comparative Example 1 is mainly composed of silica, and at the same time contains a certain amount of carbon and sodium. The presence of these elements is closely related to the preparation process and surface modification steps, further verifying the chemical composition and structural characteristics of the sample.

[0128] The oxygen content of Example 1 is slightly higher than that of Comparative Example 1, which may indicate that more organic components or oxides were introduced during the preparation of Example 1; the sodium content of Comparative Example 1 is significantly higher than that of Example 1, which may indicate that more sodium ions remained during the preparation of Comparative Example 1; the silicon content of Example 1 is significantly higher than that of Comparative Example 1, indicating that the silica content in Example 1 is slightly higher.

[0129] Figure 1(1) Scanning electron microscope images of the water glass - tetraethyl orthosilicate hydrophobically modified low - thermal - conductivity silica aerogels in Examples 1, 2, 3, and 4 of the present invention; Figure 1 (2) Scanning electron microscope images of the hydrophobically modified silica aerogels in Comparative Examples 1, 2, 3, and 4 of the present invention. The SEM images of all samples show a relatively uniform surface morphology, indicating that the structure of the materials is relatively consistent during the preparation process. It can be observed that there are certain pore structures on the surface of the samples, which is consistent with the porous characteristics of silica aerogels; no obvious particle aggregation phenomenon is shown in the images, indicating that the particle distribution in the samples is relatively uniform. The SEM images show their similarity in surface morphology, indicating the stability of the preparation process. These samples have undergone hydrophobic modification treatment, and the SEM images may show different surface characteristics from the unmodified samples, such as a smoother surface or different pore structures.

[0130] Figure 2 Pictures of the contact angle tests of the water glass - tetraethyl orthosilicate hydrophobically modified low - thermal - conductivity silica aerogels in Examples 1, 2, 3, and 4 of the present invention and the hydrophobically modified silica aerogels in Comparative Examples 1, 2, 3, and 4. Example 1 shows relatively high contact angles, and the contact angles of all samples are greater than 115°, indicating that the hydrophobic modification in the examples may be more effective.

[0131] Figure 3 (1) Infrared spectra of the water glass - tetraethyl orthosilicate hydrophobically modified low - thermal - conductivity silica aerogels in Examples 1, 2, 3, and 4 of the present invention; Figure 3 (2) Infrared spectra of the hydrophobically modified silica aerogels in Comparative Examples 1, 2, 3, and 4 of the present invention. The infrared spectra of the examples and comparative examples show the characteristic peaks of silica aerogels (Si - O - Si bonds), indicating that silica aerogels have been successfully prepared in both cases. The infrared spectra of the examples show the characteristic peaks of C - H bonds and Si - CH3 bonds, indicating that hydrophobic modification treatment has been carried out. The absorption peak of the O - H bond in the hydrophobically modified samples should be weaker, indicating a reduction in surface hydroxyl groups and an increase in hydrophobicity.

[0132] Figure 4 (1) N2 adsorption - desorption isotherm curves of the water glass - tetraethyl orthosilicate hydrophobically modified low - thermal - conductivity silica in Examples 1, 2, 3, and 4 of the present invention; Figure 4(2) is the N2 adsorption-desorption isotherm curve diagram of the hydrophobic modified silica aerogel in Comparative Examples 1, 2, 3, and 4 of the present invention. The adsorption capacity of the examples is higher than that of the comparative examples under the same relative pressure, which may indicate that the examples have a larger specific surface area or more pores; the isotherm curves of the examples show more obvious Type IV characteristics, which may indicate that they have more mesoporous structures; the hysteresis loops of the examples are larger or have different shapes, which may indicate that the pore shapes and size distributions are different from those of the comparative examples. By comparing the N2 adsorption-desorption isotherm curve diagrams, the differences in the pore structures between the examples and the comparative examples can be analyzed. The examples may have a larger specific surface area or more mesoporous structures; the differences in the adsorption capacity and hysteresis loops can reflect the differences in the adsorption performance between the two, and the examples may have better adsorption performance; the examples have advantages in terms of pore structure and adsorption performance.

[0133] Figure 5 (1) is the BJH pore size distribution diagram of the sodium silicate-tetraethyl orthosilicate hydrophobic modified low thermal conductivity silica in Examples 1, 2, 3, and 4 of the present invention; Figure 5 (2) is the BJH pore size distribution diagram of the hydrophobic modified silica aerogel in Comparative Examples 1, 2, 3, and 4 of the present invention. The pore size distribution of Example 1 is relatively concentrated, mainly distributed in a relatively small pore size range (such as 5 - 10 nm), indicating that its pore structure is relatively uniform; with the increase of the sodium silicate modulus in Examples 1 - 4, the pore size distribution gradually shifts towards a smaller pore size direction, but does not maintain a relatively concentrated distribution; for example, the pore size distribution of Example 1 may be concentrated in the range of 10 - 15 nm; the pore size distribution of the examples is relatively concentrated, indicating that its pore structure is relatively uniform. With the increase of the SiO2 mass fraction, the pore size distribution gradually shifts towards a larger pore size direction, indicating that the pore structure of the material may change with the increase of the SiO2 concentration; the pore size distribution range of the examples is relatively narrow, indicating that its pore structure may be relatively single, suitable for applications requiring high thermal insulation and adsorption performance. The pore size distribution of Comparative Example 1 is relatively dispersed, mainly distributed in a relatively large pore size range (such as 10 - 15 nm), indicating that its pore structure is relatively open; with the increase of the sodium silicate modulus in Comparative Examples 1 - 4, the pore size distribution gradually shifts towards a smaller pore size direction, but still maintains a relatively dispersed distribution. For example, the pore size distribution of Comparative Example 3 may be concentrated in the range of 10 - 20 nm; the pore size distribution of the comparative examples is relatively dispersed, indicating that its pore structure is relatively open. With the increase of the sodium silicate modulus, the pore size distribution gradually shifts towards a smaller pore size direction, indicating that the pore structure of the material may change with the increase of the modulus; the pore size distribution range of the comparative examples is relatively wide, indicating that its pore structure may be relatively diverse, suitable for applications requiring a larger pore volume.

[0134] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope covered by the present invention.

Claims

1. A method for preparing hydrophobic modified low thermal conductivity silica aerogel using sodium silicate - tetraethyl orthosilicate as a composite silicon source, characterized in that: Add formamide and ethylene glycol mixed retarder to sodium silicate solution, dropwise add tetraethyl orthosilicate solution while stirring, and adjust the pH to neutral with oxalic acid solution to form hydrogel; after aging with tert-butanol and solvent replacement, use hexamethyldisiloxane and hexamethyldisilazane double modifiers for synchronous hydrophobic modification and sodium removal, and obtain hydrophobic modified low thermal conductivity silica aerogel with porous nanostructure by freeze-drying.

2. The method according to claim 1, wherein: The specific steps are as follows: Step S1: Prepare sodium silicate solution: Dissolve sodium silicate in deionized water and obtain sodium silicate solution by ultrasonic dispersion. Step S2: Prepare tetraethyl orthosilicate solution: Mix tetraethyl orthosilicate and deionized water with a volume ratio of 1:1, and gradually add 10wt% oxalic acid solution dropwise during stirring to adjust the pH to 6.8, and stir for 40 minutes to completely hydrolyze tetraethyl orthosilicate to obtain tetraethyl orthosilicate solution. Step S3: Acid gelation treatment: Add the mixed retarder composed of formamide and ethylene glycol to the sodium silicate solution prepared in Step S1, slowly add the tetraethyl orthosilicate solution prepared in Step S2 during stirring, and then gradually add 10wt% oxalic acid solution to adjust the pH to 7.0 - 7.4 to obtain hydrogel. Step S4: Aging and solvent replacement: Immerse the hydrogel prepared in Step S3 in tert-butanol, seal and stand at room temperature for aging for 24 hours; replace tert-butanol every 8 hours during aging to obtain the aged hydrogel. Step S5: Surface modification: Immerse the aged hydrogel prepared in Step S4 in the tert-butanol solution of hexamethyldisiloxane and hexamethyldisilazane, and carry out hydrophobic modification at room temperature for 18 hours to obtain the hydrophobic modified hydrogel. Step S6: Freeze-drying: Place the hydrophobic modified hydrogel prepared in Step S5 at -70°C for freezing for 6 hours, and then vacuum dry at -70°C for 24 hours to obtain the hydrophobic modified low thermal conductivity silica aerogel.

3. The method according to claim 2, wherein: In Step S1, the modulus of sodium silicate is 2.0 - 3.0; the mass fraction of SiO2 in the sodium silicate solution is 2% - 6%.

4. The method according to claim 2, wherein: In Step S3, the volume ratio of tetraethyl orthosilicate solution to sodium silicate solution is 1:

10.

5. The method according to claim 2, wherein: In Step S3, the molar ratio of silicon dioxide, formamide and ethylene glycol in the sodium silicate solution is 1:2:

1.

6. The method according to claim 2, wherein: In Step S4, the volume ratio of the tert-butanol replaced each time to the hydrogel is 1.2:

1.

7. The method according to claim 2, wherein: In Step S5, the total volume ratio of hexamethyldisiloxane and hexamethyldisilazane to the volume of tert-butanol is 1:8, and the molar ratio of sodium silicate, hexamethyldisiloxane and hexamethyldisilazane is 2:1:

1.

8. A hydrophobic modified low thermal conductivity silica aerogel prepared by the method according to any one of claims 1 - 7.

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