JPH0340927A - Production of silica glass - Google Patents

Abstract

PURPOSE:To prevent the crack of gel by adding silicon alkoxide after the addition of acid and then carrying out gelation and drying in the sol-gel process requiring neither the dispersion stage of fine SiO2 grains nor the concentration stage of sol prior to gelation. CONSTITUTION:Silicon alkoxide is hydrolyzed by using a basic catalyst in the presence of polyethylene glycol and/or its derivative, by which silica sol containing fine SiO2 grains is prepared. Subsequently, acid together with water are added to the above silica sol and further silicon alkoxide is added, which is allowed to gel and dried. By this method, large-sized silica glass free from crazing and cracking and minimal in defects, such as pores, can be obtained in high yield.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing silica glass used in optical applications, semiconductor industry, electronic industry, science and engineering, and the like.

[Prior Art] Silica glass has excellent heat resistance, corrosion resistance, and optical properties, so it is an important material indispensable for semiconductor manufacturing, and is also used as a photomask substrate for optical fiber and IC manufacturing, and TPT substrate. It is used for various purposes, and its applications are being expanded more and more.

Conventional methods for producing silica glass include a method in which natural quartz is melted in an electric furnace or an oxyhydrogen flame, or a method in which silicon tetrachloride is oxidized and melted at high temperature in an oxyhydrogen flame or a plasma flame. Since the manufacturing process requires high temperatures of 2000℃ or higher, it consumes a large amount of energy, and materials that can withstand such high temperatures are required during manufacturing, and it is difficult to obtain high purity materials, making it economical. , there are some quality problems.

On the other hand, in recent years, a method called the sol-gel method in which silica glass is grown at low temperatures has been attracting attention.

The outline will be briefly described below.

Si prepared by hydrolysis or polymerization of silicon alkoxide or gas phase hydrolysis of silicon tetrachloride
A silica sol of water, an organic solvent, or a water-organic solvent mixed solution in which O2 fine particles are dispersed is gelled by standing still, raising the temperature, adding a gelling agent, or the like. Thereafter, the gel is evaporated and dried to obtain a dry silica gel. Silica glass is obtained by sintering this dried gel in a suitable atmosphere.

The above sol-gel method for producing silica glass has the following problems. That is, it is very difficult to uniformly disperse SiO2 fine particles in water, organic solvents, etc., and large aggregates of 5iOz fine particles tend to remain in the silica sol. Such aggregates cause defects (pores) in the glass produced by sintering the gel, resulting in a decrease in quality. Therefore, in order to improve the dispersion of 5iOz fine particles,
A method of irradiating silica sol with ultrasonic waves and a method of further sintering sintered glass under pressure to eliminate defects (vacancies) in the glass caused by aggregates have been proposed, but both methods increase the number of steps. , causing increased opportunities for adulteration.

In order to eliminate the step of dispersing SiO2 fine particles into the liquid phase, silicon alkoxide is hydrolyzed in the presence of a base catalyst to form S
A method has been proposed in which a sol containing iO2 fine particles is prepared and the sol is directly gelled.

[Problems to be Solved by the Invention] However, with this method, cracks and cracks are likely to occur in the gel during the drying process. In order to prevent such destruction of the gel, it is necessary to increase the diameter of the 5iOz fine particles contained in the sol, and for this reason, the water used in the hydrolysis of the alkoxide must be in large excess with respect to the alkoxide and contain a concentrated base. must be included. Therefore, before gelling the sol,
A concentration step was required to remove a large excess of water, resulting in an increase in the number of steps. In addition, it is necessary to add acid to bring the sol to the appropriate pH for gelation, but this causes the large amount of salt that is generated in the sol to precipitate into the gel during the drying process, resulting in the creation of pores during vitrification. A problem arose that caused

In order to solve these problems, the inventors first developed a method for producing silica glass that does not require a step of dispersing SiO2 fine particles and a step of concentrating the sol before gelation. Decomposes into SiO2
A method for producing silica glass in which a silica sol containing fine particles is obtained, the sol is dried and sintered, and a silicon alkoxide is hydrolyzed in the presence of polyethylene glycol and/or its derivatives, and an acid is added to a silica sol containing 5iOz fine particles. We proposed a method for producing silica glass with special characteristics (Japanese Patent Application No. 303356/1983). However, even with this method, the yield of dried gel sometimes decreased in multiple batches. The present inventors have intensively investigated the cause of such a decrease in the yield of dried gel, and as a result, have completed the present invention.

[Means for solving the problem] That is, an acid is added to a silica sol containing SiO2 fine particles prepared by hydrolyzing silicon alkoxide using a basic catalyst in the presence of polyethylene glycol and/or its derivative, and then silicon Add alkoxide,
It has been found that by gelling and drying, cracking of the gel during the drying process is significantly suppressed.

The acid added to the silica sol containing 5iOz fine particles is as follows:
Hydrochloric acid, nitric acid, formic acid, acetic acid, propionic acid, etc. can be used. The amount of acid added is such that the pH of the silica sol containing SiO2 fine particles is 3 to 5.

Good results are obtained when the amount of water added together with the acid is 0.1 to 4 times the theoretical amount required for hydrolysis of the silicon alkoxide subsequently added.

After adding water together with the acid, as the silicon alkoxide to be added, use tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, a partial condensation polymerization thereof, or a mixture of these silicon alkoxides. be able to.

After adding water together with acid, the amount of silicon alkoxide to be added is 0.02 to 0.6 of the amount of silicon alkoxide used to prepare silica gel containing SiO2 fine particles.
molar ratio).

However, since the preferred range varies depending on the type of silicon alkoxide added, it is not limited to the above range.

Silica glass is produced by transferring the silica sol prepared as described above to a container such as a petri dish, gelling it by keeping it at room temperature to 70°C, and then drying it at a temperature higher than room temperature to form a dry gel. 1000-140 in air
It is obtained by raising the temperature to 0°C and sintering.

[Action: Water is added together with acid to a silica sol containing 5i02 fine particles prepared by hydrolyzing silicon alkoxide using a basic catalyst in the presence of copolyethylene glycol and/or its derivatives, and then silicon alkoxide is added to form a gel.] , the yield of dry gel is significantly improved. The details of the reason for this are unknown, but the silicon alkoxide added to the silica sol undergoes hydrolysis, and the resulting product acts as a binder to strengthen the bonds between the 5iOz fine particles in the gel, resulting in an increase in the strength of the gel. This is presumably because the occurrence of cracking during the drying process was suppressed. Note that the present invention is not limited to the above speculation.

[Example 1 (Example 1) 284 g of 2-propyl alcohol and 0. 160 g of OIM choline aqueous solution was mixed, and 16.8 g of polyethylene glycol (molecular weight 20,000) was added and dissolved therein.

The obtained solution was mixed with tetramethoxysilane (S f (
It was slowly added to 340 g of OCH3)4) and thoroughly mixed to obtain a silica sol.

This was allowed to stand overnight at room temperature to age. Next, 48.3 g of water and 0.8 ml of a 1M aqueous hydrochloric acid solution were added to the silica sol while stirring vigorously, and then 68 g of tetramethoxysilane was added. This was placed in a stainless steel Petri dish coated with Teflon and having a diameter of 300 mm, covered with aluminum foil, and allowed to gel at 30°C. Make a hole in the lid, 60
The gel was dried for 2 weeks in a constant temperature bath at 120° C., and then heated to 120° C. and dried at that temperature for 1 day to obtain a dry gel.

No cracks or breaks were observed in the dried gel obtained. The obtained dry gel was sintered by heating to 1800° C. in air to obtain silica glass. The obtained silica glass had no defects such as pores.

(Example 2) 24.2 mL of water was added to the silica sol prepared in the same manner as in Example 1 with 0.8 ml of 1M hydrochloric acid aqueous solution while stirring vigorously.
g, then 46.5 g of tetraethoxysilane
was added. This was gelled, dried, and sintered into vitrification in the same manner as in Example 1. No cracks or breaks were observed in the dried gel, and the obtained silica glass had no defects such as pores.

(Example 3) To the silica sol prepared in the same manner as in Example 1, 12.1 ml of 1M hydrochloric acid aqueous solution was added while stirring vigorously.
g and then 30.g of tetrapropoxysilane. Og was added. This was gelled, dried, and sintered into vitrification in the same manner as in Example 1. The dried gel had no cracks or breaks, and the obtained silica glass had no defects such as pores.

[Effects of the Invention] According to the present invention, large-sized silica glass without cracks or breaks and with few defects such as pores can be easily produced by a gel-sol method. There are basically no restrictions on its size, and it can be manufactured in any shape such as a plate, rod, or tube.

Claims (1)

[Claims] 1. In the presence of polyethylene glycol and/or its derivatives, silicon alkoxide is hydrolyzed using a basic catalyst to obtain a silica sol containing SiO_2 fine particles, water is added together with an acid to this silica sol, and then silicon alkoxide is added, A method for producing silica glass characterized by gelling, drying, and sintering.