CN111908478A

CN111908478A - Preparation method of flexible silica aerogel

Info

Publication number: CN111908478A
Application number: CN202010794789.8A
Authority: CN
Inventors: 张瑜; 聂朝胤; 谢红梅
Original assignee: Southwest University
Current assignee: Southwest University
Priority date: 2020-08-10
Filing date: 2020-08-10
Publication date: 2020-11-10
Anticipated expiration: 2040-08-10
Also published as: CN111908478B

Abstract

The invention discloses a preparation method of flexible silica aerogel, which comprises the following steps: s1, dissolving CTAB in a mixed solvent to obtain a mixed solution, wherein the mixed solvent comprises ethanol, water and hydrochloric acid; s2, sequentially adding TEOS, MTES and DEDMS into the mixed solution under the stirring condition, and mixing and stirring to obtain silicon dioxide sol; s3, dropwise adding ammonia water with the concentration of 2-3 mol/L into the silica sol under the stirring condition, and mixing and stirring to obtain silica gel; the molar ratio of the ammonia to the DEDMS added in the S2 is 0.008-0.012: 0.015 to 0.025; and S4, carrying out aging treatment, cleaning and drying on the silica gel to obtain the flexible silica aerogel. It can improve the joint strength of aerogel skeleton, shortens the preparation time, improves preparation efficiency.

Description

Preparation method of flexible silica aerogel

Technical Field

The invention relates to preparation of an aerogel, in particular to a preparation method of a flexible silicon dioxide aerogel.

Background

Aerogels are considered the lightest solid materials in the world. Because of its ultra-high porosity and unique pore structure, aerogels have a number of important applications in thermal insulation, adsorptive separation, catalysts and catalyst carriers, optical devices, electrical devices, and the like. Aerogels mainly include oxide aerogels such as silica, carbon aerogels, cellulose aerogels and the like, and these aerogels are usually prepared by preparing a wet gel by a sol-gel process and then replacing the solvent in the wet gel with air to obtain the aerogel. The replacement drying process usually requires supercritical carbon dioxide drying or freeze drying, otherwise, the pores in the material collapse due to the action of capillary force, and the skeleton structure is damaged. The supercritical carbon dioxide drying and freeze-drying technology has the problems of high price, insecurity, environmental pollution and the like, so that the production cost of the aerogel is high, and the large-scale production preparation and industrial application of the aerogel are not facilitated.

The preparation of the silicon dioxide aerogel generally takes organosilicon precursors (tetraethoxysilane and methyl orthosilicate) as raw materials, and is the aerogel which is the most mature in preparation technology and application at present. The preparation process generally comprises the steps of forming a wet gel with a dendritic and three-dimensional network structure by using a sol-gel method, and removing a solution in the gel by combining a proper drying process to obtain the aerogel, but the method has the problems of long time (usually more than 2 days), low connection strength of a product skeleton structure and the like.

Disclosure of Invention

The invention aims to provide a preparation method of a flexible silica aerogel, which can improve the connection strength of an aerogel framework, shorten the preparation time and improve the preparation efficiency.

The preparation method of the flexible silica aerogel comprises the following steps:

s1, dissolving CTAB in a mixed solvent at the temperature of 30-50 ℃ to obtain a mixed solution, wherein the mixed solvent comprises ethanol, water and hydrochloric acid, and the molar ratio of CTAB, ethanol, water and hydrochloric acid is 0.00024-0.0003: 1.6-1.7: 3.6-3.7: 0.0001 to 0.00015;

s2, sequentially adding TEOS, MTES and DEDMS into the mixed solution under the stirring condition, and mixing and stirring to obtain the silica sol, wherein the molar ratio of TEOS, MTES, DEDMS to hydrochloric acid in the mixed solution is 0.04-0.05: 0.235 to 0.245: 0.015 to 0.025: 0.0001 to 0.00015;

s3, dropwise adding ammonia water with the concentration of 2-3 mol/L into the silica sol under the stirring condition, and mixing and stirring to obtain silica gel;

the molar ratio of the ammonia to the DEDMS added in the S2 is 0.008-0.012: 0.015 to 0.025;

and S4, carrying out aging treatment, cleaning and drying on the silica gel to obtain the flexible silica aerogel.

Furthermore, the concentration of the hydrochloric acid in the S1 is 0.08-0.15 mol/L.

Further, the aging temperature of the aging treatment in the S4 is 30-50 ℃, and the aging time is 3-6 h.

Further, the drying in S4 specifically includes: firstly, drying for 1-2 hours at the temperature of 40-50 ℃, and then drying for 1-3 hours at the temperature of 60-80 ℃.

Further, the ammonia water concentration in the S3 was 2.5 mol/L.

Further, the molar ratio of ammonia in the S3 and the DEDMS added in the S2 was 0.01: 0.021.

CTAB is the abbreviation for cetyl trimethyl ammonium bromide, TEOS is the abbreviation for tetraethoxysilane, MTES is the abbreviation for methyltriethoxysilane, DEDMS is the abbreviation for dimethyldiethoxysilane.

Compared with the prior art, the invention has the following beneficial effects.

1. According to the invention, the crosslinking degree of the aerogel framework is controlled by controlling the addition amount of DEDMS, the particle size of the aerogel framework is controlled by utilizing the concentration and the addition amount of ammonia water of an alkali catalyst, and a coarsened aerogel framework is obtained by combining the limitation of other components. And the time of the whole process flow can be shortened to 8-12h, the preparation time of the flexible silicon dioxide aerogel is greatly shortened, and the preparation efficiency is improved.

2. The invention controls and optimizes the adding proportion of each component, namely the molar ratio of TEOS, MTES, DEDMS, ethanol, water, CTAB, hydrochloric acid and ammonia water is 0.04-0.05: 0.235 to 0.245: 0.015 to 0.025: 1.6-1.7: 3.6-3.7: 0.00024 to 0.0003: 0.0001 to 0.00015: 0.008 to 0.012. Particularly, DEDMS and ammonia water are controlled, wherein if the addition amount of DEDMS is too small, effective connection cannot be formed among silica clusters, the aerogel framework is formed by stacking a large number of fragmented silica clusters, the connection among the silica clusters is loose, a large number of gaps exist, and the connection strength is weak. If the addition amount of DEDMS is too large, the silicon dioxide clusters further react to form smooth secondary spherical particles, and the interfaces among the spherical particles are sunken, so that the connection effect of the aerogel framework is weakened. If the addition amount of the ammonia water is too small, the polycondensation reaction of the system is insufficient, so that a complete three-dimensional network structure cannot be formed, namely a firm aerogel skeleton structure cannot be formed, and the flexibility is poor. If aqueous ammonia addition is too much, promoted condensation polymerization reaction rate for aerogel skeleton has not yet arrived to form complete structure and has just become the gel state, so that has more gaps between the secondary particle, and then connects comparatively loosely.

3. The invention adopts sectional drying, firstly drying for a period of time at a lower temperature, evaporating part of the solvent on the surface of the sample, simultaneously leading the temperature of the surface and the temperature of the interior of the sample to be consistent, and avoiding the cracking caused by the overlarge temperature difference between the surface and the interior of the sample. And then the temperature is raised for drying, and the residual solvent in the aerogel is quickly removed, so that the sample preparation time is shortened.

Drawings

FIG. 1 is an SEM image of a flexible silica aerogel prepared according to a first embodiment of the present invention;

FIG. 2 is an SEM image of a silica aerogel of comparative example one;

FIG. 3 is an SEM image of a silica aerogel of comparative example II;

FIG. 4 is an SEM image of a silica aerogel of comparative example III;

FIG. 5 is an SEM image of a silica aerogel of comparative example No. four;

FIG. 6 is an SEM image of a silica aerogel of comparative example five;

FIG. 7 is an SEM image of a silica aerogel of comparative example six;

FIG. 8 is an SEM image of a silica aerogel of comparative example seven;

FIG. 9 is a stress-strain plot of silica aerogels obtained with varying amounts of DEDMS added;

FIG. 10 is a graph showing stress-strain curves of silica aerogels obtained by adding ammonia water of different concentrations.

Detailed Description

The invention is described in detail below with reference to the figures and the specific embodiments.

In one embodiment, a method for preparing a flexible silica aerogel comprises the following steps: weighing raw materials, wherein the molar ratio of TEOS, MTES, DEDMS, ethanol, water, CTAB, hydrochloric acid and ammonia water is 0.045: 0.242: 0.021: 1.65: 3.67: 0.00027: 0.00012: 0.01, ethanol is absolute ethanol, water is deionized water, the concentration of hydrochloric acid is 0.12mol/L, and the concentration of ammonia water is 2.5 mol/L.

S1, dissolving CTAB in a mixed solvent at the temperature of 40 ℃ to obtain a mixed solution, wherein the mixed solvent comprises ethanol, water and hydrochloric acid.

And S2, sequentially adding TEOS, MTES and DEDMS into the mixed solution under the stirring condition, carrying out hydrolysis reaction, and mixing and stirring to obtain the silica sol.

And S3, dropwise adding ammonia water into the silica sol under the stirring condition, carrying out polycondensation reaction, and mixing and stirring to obtain the silica gel.

S4, aging the silica gel at 40 ℃ for 4h, washing the silica gel with absolute ethyl alcohol for three times, and then drying the silica gel in sections, wherein the drying is carried out for 1h at 50 ℃ and for 2h at 70 ℃ to obtain the flexible silica aerogel.

The obtained flexible silica aerogel is observed by a scanning electron microscope, and as a result, referring to fig. 1, the obtained aerogel framework is a coarsened structure, the connection of silica clusters is very tight, and gaps can hardly be seen.

The second embodiment is a preparation method of the flexible silica aerogel, the raw material component ratio, the hydrolysis reaction and the polycondensation reaction are the same as those in the first embodiment, except that the aging treatment and the drying treatment are carried out, the aging temperature of the aging treatment is 30 ℃, the aging time is 6 hours, and the drying treatment specifically comprises the following steps: drying is carried out firstly at a temperature of 40 ℃ for 2h and then at a temperature of 60 ℃ for 3 h. And observing the obtained flexible silica aerogel by adopting a scanning electron microscope, wherein the aerogel framework is of a coarsening structure.

Embodiment three, a preparation method of a flexible silica aerogel, the raw material component ratio, hydrolysis reaction, and polycondensation reaction operations are the same as those of embodiment one, except for aging treatment and drying treatment, the aging temperature of the aging treatment is 50 ℃, the aging time is 3 hours, and the drying treatment specifically comprises: drying is carried out firstly at a temperature of 45 ℃ for 1.5h and then at a temperature of 80 ℃ for 1 h. And observing the obtained flexible silica aerogel by adopting a scanning electron microscope, wherein the aerogel framework is of a coarsening structure.

Example four, a method for preparing a flexible silica aerogel, hydrolysis, polycondensation, aging, and drying are the same as in example one, except for the raw material ratios, the molar ratio of TEOS, MTES, DEDMS, ethanol, water, CTAB, hydrochloric acid, and ammonia is 0.04: 0.245: 0.025: 1.6: 3.6: 0.0003: 0.0001: 0.008, and the concentration of ammonia water is 3 mol/L. And observing the obtained flexible silica aerogel by adopting a scanning electron microscope, wherein the aerogel framework is of a coarsening structure.

Example five, a preparation method of a flexible silica aerogel, hydrolysis reaction, polycondensation reaction, aging treatment and drying treatment are the same as in example one, except that the raw material ratio is that the molar ratio of TEOS, MTES, dedims, ethanol, water, CTAB, hydrochloric acid and ammonia water is 0.05: 0.238: 0.016: 1.63: 3.67: 0.00028: 0.00013: 0.012 and the ammonia concentration was 2 mol/L. And observing the obtained flexible silica aerogel by adopting a scanning electron microscope, wherein the aerogel framework is of a coarsening structure.

The sixth embodiment is a method for preparing a flexible silica aerogel, in which the hydrolysis reaction, the polycondensation reaction, the aging treatment, and the drying treatment are the same as those of the first embodiment, except that the raw material ratio is that the molar ratio of TEOS, MTES, DEDMS, ethanol, water, CTAB, hydrochloric acid, and ammonia water is 0.042: 0.245: 0.024: 1.65: 3.68: 0.00026: 0.00012: 0.011 and the concentration of ammonia water is 2.1 mol/L. And observing the obtained flexible silica aerogel by adopting a scanning electron microscope, wherein the aerogel framework is of a coarsening structure.

Example seven, the effect of the amount of DEDMS added on the flexible silica aerogel structure was analyzed.

DEDMS with different contents is added according to the mixture ratio in the table 1, and the rest conditions are the same as the first embodiment.

TABLE 1 silica aerogels with varying DEDMS contents

	TEOS	MTES	DEDMS	Ethanol	Water (W)	CTAB	Hydrochloric acid	Aqueous ammonia
									Example one	0.045	0.242	0.021	1.65	3.67	0.00027	0.00012	0.01
Comparative example 1	0.045	0.251	0.012	1.65	3.67	0.00027	0.00012	0.01
									Comparative example No. two	0.045	0.233	0.03	1.65	3.67	0.00027	0.00012	0.01
Comparative example No. three	0.045	0.224	0.039	1.65	3.67	0.00027	0.00012	0.01

The silica aerogel structures of comparative example one, comparative example two, and comparative example three were observed with a scanning electron microscope, respectively, and the results are shown in fig. 2 to 4. As can be seen from fig. 2, when the amount of the DEDMS is too small, effective connection between the silica clusters cannot be formed, the aerogel framework is formed by stacking a large amount of fragmented silica clusters, the connection between the silica clusters is loose, and a large number of gaps exist, so that a coarsened aerogel framework structure cannot be formed. As shown in fig. 3, when the amount of the DEDMS is too large, the silica clusters further react to form smooth secondary spherical particles, and the interface between the spherical particles is recessed, which weakens the connection function of the aerogel framework. As the addition amount of DEDMS is further increased, the depressions between the secondary spherical particles are further deepened in fig. 4, compared to fig. 3, indicating that the connection of the aerogel skeleton is further weakened.

Mechanical property tests were performed on the first, second and third examples, respectively, to obtain corresponding stress-strain curves, and the results are shown in fig. 9. As can be seen from fig. 9, as the addition ratio of the dedims increases, the compressibility of the silica aerogel increases and then decreases, and when the molar ratio of the addition amount of the dedims is 0.021, the compressibility is as high as 78.2%, which is far higher than the values corresponding to other ratios. In addition, the young's moduli of the four groups of samples were 45kPa, 14kPa, 17kPa, and 20kPa, respectively, as the addition ratio was increased, and when the molar ratio of the addition amount of dedims was 0.021, the young's modulus was the smallest, indicating that the flexibility of the silica aerogel reached a higher level at this time.

Example eight, the effect of ammonia addition on the flexible silica aerogel structure was analyzed.

Ammonia water with different contents and different concentrations is added according to the mixture ratio in the table 2, and the rest conditions are the same as the first embodiment.

TABLE 2 silica aerogels with different contents and concentrations of ammonia

	MTES	Aqueous ammonia	Concentration of ammonia
				Example one	0.242	0.01	2.5mol/L
Comparative example No. four	0.242	0.002	0.5mol/L
				Comparative example five	0.242	0.018	4.5mol/L
Comparative example six	0.242	0.026	6.5mol/L
				Comparative example seven	0.242	0.034	8.5mol/L

The other component ratios not listed in table 2 were the same as in example one.

The silica aerogel structures of comparative examples four to seven were observed with a scanning electron microscope, respectively, and the results are shown in fig. 5 to 8. As can be seen from fig. 5, when the amount of ammonia added is too small and the concentration is low, the polycondensation reaction of the system is insufficient due to the too small amount of ammonia, so that a complete three-dimensional network structure cannot be formed, i.e., a firm aerogel skeleton structure cannot be formed. As can be seen from fig. 6, when the amount of ammonia added is too large and the concentration is high, the aerogel skeleton becomes significantly thinner and the secondary particles become significantly smaller compared to fig. 5. The higher concentration ammonia water promotes the condensation polymerization reaction rate, so that the aerogel framework has not yet formed a complete structure and becomes a gel state, and more gaps exist among secondary particles. As can be seen from fig. 7, with further increase of ammonia, the polycondensation reaction of the system is further accelerated, so that the secondary particles are not sufficiently grown to form a gel, and the aerogel framework is formed by aggregation of fine secondary particles. As can be seen from fig. 8, the silica skeleton was formed by aggregating finer particles as the amount of ammonia water was further increased.

And respectively carrying out mechanical property tests on the comparative example four, the comparative example five and the comparative example six of ammonia water with different concentrations to obtain corresponding stress-strain curves, and obtaining the results shown in the figure 10. As can be seen from FIG. 10, the compressible amount of the sample was the largest, i.e., 78.2%, when the ammonia water concentration was 2.5 mol/L. As the concentration of the ammonia water increases, the Young's moduli are respectively 38kPa, 14kPa, 32kPa and 79kPa, which shows that the flexibility of the silica aerogel reaches a higher level at an ammonia water concentration of 2.5 mol/L.

The above description is only for the preferred embodiment of the present invention and is not intended to limit the present invention, but all equivalent variations and modifications made according to the present invention should be included in the protection scope of the present invention.

Claims

1. The preparation method of the flexible silica aerogel is characterized by comprising the following steps:

s3, dropwise adding ammonia water with the concentration of 2-3 mol/L into the silica sol under the stirring condition to obtain silica gel;

2. The method for preparing a flexible silica aerogel according to claim 1 or 2, characterized in that: the concentration of the hydrochloric acid in the S1 is 0.08-0.15 mol/L.

3. The method for preparing a flexible silica aerogel according to claim 1 or 2, characterized in that: the aging temperature of the aging treatment in the S4 is 30-50 ℃, and the aging time is 3-6 h.

4. The method for preparing a flexible silica aerogel according to claim 1 or 2, wherein the drying in S4 is specifically: firstly, drying for 1-2 hours at the temperature of 40-50 ℃, and then drying for 1-3 hours at the temperature of 60-80 ℃.

5. The method for preparing a flexible silica aerogel according to claim 1 or 2, characterized in that: the ammonia water concentration in the S3 is 2.5 mol/L.

6. The method for preparing a flexible silica aerogel according to claim 1 or 2, characterized in that: the molar ratio of ammonia in the S3 and DEDMS added in the S2 is 0.01: 0.021.