SU1680623A1 - Method for manufacturing silicon oxide aerogel blocks - Google Patents

Abstract

This invention relates to methods for producing azrogel silica blocks and allows for an increase in the yield of undeformed and intact blocks up to 97%. The method involves hydrolysis of orthosilicic acid esters dissolved in alcohol in the presence of a catalyst, gelation of the resulting mass in forms to form blocks of alcohol, aging of alcohol in alcohol, subsequent removal of alcohol, and the conversion of alcohol to airgel in an autoclave first with increasing temperature and pressure to supercritical for alcohol values, then with isothermal pressure reduction to atmospheric and further temperature reduction to room temperature, firing the airgel. The removal of alcohol and the conversion of alcohol are carried out in a sectional autoclave, and the molds with alcohol are closed with lids before being inserted into the autoclave.

Description

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The invention relates to the preparation of aerogels, in particular transparent blocks of silica airgel, used in Cerenkov detectors for analyzing charged high-energy particles, as well as heat and sound insulators.

Aerogels consisting of silica gel particles are highly porous structures with a large volume of macropores (with R 300 A) and low values of apparent density (0.08-0.3 g / cm).

The aim of the invention is to increase the yield of undeformed and intact silica airgel blocks when it is produced.

In the proposed solution, the synthesis of transparent blocks of silica airgel is carried out in several stages: alcohol production, aging, solvent removal, airgel calcination.

Alcohol synthesis is carried out from orthosilicic acid tetramethyl ester in the presence of an alkaline catalyst or orthosilicic acid tetraethyl ester in the presence of an acid catalyst.

Ammonium hydroxide is used as alkaline catalyst, hydrofluoric acid is used as acid.

To obtain transparent silica airgel blocks in the alcohol stage, at least 4–8 mol of water per 1 mol of tetra alkoxysilane is used to hydrolyze tetraalkoxysilane.

With a lower water content, the gel is milky in color; with a larger one, the time it takes for the alcohol to wash out residual water increases.

Aqueous organic solutions are used to create a homogeneous mixture.

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The catalyst is introduced into the aqueous-organic mixture immediately prior to the addition of the heteroalkoxysilane.

The solutions are poured together, mixed until the viscosity begins to change, and then poured into stainless steel molds, where the formation of alcohol occurs. To prevent the alcohol from sticking to the walls of the molds, the latter are internally coated with Fluorone.

Immediately after gel formation, according to NMR295, the initial tetraalkoxysilane is present in alcohol. Such an alcoholic gel is poured with an aqueous-alcoholic mixture containing the same amount of water as the gel, and the gel starts within 24 hours. After one day, the water-alcohol mixture was replaced by an old-fashioned alcohol gel for another 7-10 days.

Removal of the solvent from the alcohol is carried out in an autoclave under supercritical conditions for the solvent used. For methanol, the critical temperature, pressure and specific volume of methanol introduced into the autoclave are respectively

240 ° C, 79.9 bar, 3.67 CMJ / r.

The autoclave process is carried out in several stages: raising the temperature and pressure to supercritical, etching the alcohol by reducing the pressure at a constant temperature to atmospheric, reducing the temperature to room temperature.

Alkogel has low strength. To increase the stability of the gels by capillary pressures, a low solvent removal rate is necessary. In addition, to reduce the likelihood of compression and destruction of the sample, solvent removal must be carried out quickly. In the inventive method, the optimum temperature rise time is in the order of 24 hours.

Such a regime, however, does not exclude the formation of cracks in the gel or its deformations. In order for the gels to not become deformed, it is necessary to divide the autoclave into sections, and to close the forms with gels from above.

The separation of the autoclave into sections is carried out using a mold cassette. The cassette is manufactured in such a size that the distance between the cassette shelves and the walls of the autoclave is insignificant. In addition, the forms with gels on top are covered with glasses, whose dimensions are slightly larger than the sizes of the forms.

The process of the solvent removal step can even be intensified without compromising the integrity of the gel blocks. Thus, in the prototype, pressure reduction in the autoclave is carried out within 24 hours, in the proposed method - about 6 hours.

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After removal of the solvent, the airgel surface is hydrophobic due to the presence of methoxy groups on its surface. Condensed methanol is present in the pores. To remove the remaining methanol, the sample is heated in air at 400-500 ° C. In this case, methanol is oxidized to formaldehyde and formic acid, which are easily removed.

Unlike the prototype, in the proposed method, at the stage of solvent removal, the section of the autoclave is used, and the alcohol is isolated from the environment of the autoclave by closing the molds from above.

EXAMPLE 1. For the synthesis of alcohol, 172 ml of tetramethoxylan are taken, dissolved in 57.5 ml of methanol, the mixture is cooled to 0- (+4) ° C, B the prepared water-methanol mixture containing 86 ml water and 184 ml of methanol; while stirring, 0.2 ml of a 25% NH40H solution and 229.5 ml of a solution of tetramethoxysilane in methanol are successively introduced. Stirring is continued until the viscosity of the solution begins to change, then the solution is poured into square shapes of stainless steel with dimensions of 60x60x45 mm, where the formation of alcohol occurs.

After hardening, the alcohols are poured with a water-methanol mixture containing 8.6% by volume of water and left to age for a day. After 1 day, the water-methanol mixture is replaced with methanol, and the alcohols are aged under methanol for 9 days.

Before removing the solvent, the alcohols are washed with a fresh portion of methanol to remove residual water.

At the stage of removing the solvent, the forms with alcohol are covered with glass lids and placed in a cassette, which is a cylinder with shelves. The cassette is placed in an autoclave, and methanol is poured there in an amount that ensures the achievement of critical parameters. The autoclave is closed, a pressure of 90 kg / cm2 is created in it and, over a period of 20 hours, the temperature is raised from room temperature to 250-260 ° C. The pressure is maintained at a level of 90 kg / cm.

After reaching the maximum temperature, the valve is opened at the outlet of the autoclave and, keeping the temperature constant, methanol is removed from the system for 6 hours. The pressure is uniformly reduced to atmospheric. Methanol residues are pumped out of the system. After that, the autoclave is closed and slowly (due to the natural cooling of the autoclave) the temperature is lowered to room temperature.

The aerogels from the autoclave are transferred to a muffle furnace and burned in air at 500 ° C. Five blocks of silica airgel are obtained, intact and uncracked, having a density of about 0.175 g / cm and a refractive index of 1.036,

Seven syntheses were carried out, as a result of which 35 blocks of silica airgel were synthesized. All blocks were smooth, undeformed, 34 of them without cracks. The yield of whole blocks of silica airgel is 97.1%.

Similar syntheses were carried out according to the method described in the prototype. At the same time, the yield of undeformed blocks was only 31.4%.

PRI mme R 2. Synthesis of silica airgel is carried out as in Example 1, but 78.8 ml of tetraethoxysilane, 48.7 ml of water, 372.5 ml of a 96% ethanol solution and 4.0 ml Nrkonts.

At the aging stage, ethanol residues are replaced with methanol; at the solvent removal stage, methanol is already removed from the alcohol.

The proposed method can be obtained transparent blocks of airgel oxide

silicon with a density of 0.08-0.4 g / cm, having a refractive index of 1.017-1.04.

Claims (1)

Invention Formula The method of producing silica airgel blocks, including hydrolysis of orthosilicic acid esters dissolved in alcohol in the presence of a catalyst, gelation of the resulting mass in forms with the formation of blocks of alcohol in alcohol, the subsequent removal of alcohol and the conversion of alcohol to airgel in an autoclave, first with increasing the pressure and temperature to alcohol supercritical values, and then with an isothermal reduction of pressure to atmospheric and a further decrease in temperature to room temperature, roasting an airgel that differs That, in order to increase the yield of undeformed and intact airgel blocks, the removal of alcohol and the conversion of alcohol to airgel are carried out in a sectional autoclave, while the forms with alcohol are closed with lids before being installed in the autoclave.