JP2666471B2 - Method for producing silica glass - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing silica glass used for optics, semiconductor industry, electronics industry, science and engineering, and the like.

[Prior art] Silica glass is an important material for semiconductor production due to its excellent heat resistance, corrosion resistance and optical properties. In addition, photomask substrates for optical fiber and IC production, and TFT substrates It is used for such purposes, and its use is being further expanded.

Conventional silica glass manufacturing methods 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 melted by oxidizing it at a high temperature in a plasma flame in an oxyhydrogen flame or the like. The method also requires a high temperature of 2000 ° C or more in the manufacturing process, which consumes a large amount of energy, requires materials that can withstand such a high temperature during manufacturing, and is economically difficult to obtain high-purity products. There are some problems in terms of quality and quality.

On the other hand, in recent years, a method of synthesizing silica glass at a low temperature called a sol-gel method has attracted attention. The outline is briefly described.

Si prepared by hydrolysis and polymerization of silicon alkoxide or by gas phase hydrolysis of silicon tetrachloride
The silica sol of water, an organic solvent, or a mixed solution of water and an organic solvent in which O ₂ fine particles are dispersed is gelled by standing, heating, and adding a gelling agent. Thereafter, the gel is evaporated and dried to obtain a silica dry gel. The dried gel is sintered in an appropriate atmosphere to obtain silica glass.

The method for producing silica glass by the sol-gel method has the following problems. That is, it is extremely difficult to uniformly disperse the SiO ₂ fine particles in water, an organic solvent, or the like, and large aggregates of the SiO ₂ fine particles tend to remain in the silica sol. Such agglomerates cause defects (voids) in the glass produced by sintering the gel, and cause deterioration in quality. Therefore, a method of irradiating the silica sol with ultrasonic waves to improve the dispersion of the SiO ₂ fine particles, and a method of further sintering the sintered glass under pressure to eliminate defects (voids) in the glass caused by the aggregates. However, any of these causes an increase in the number of steps and an increase in opportunities for impurity contamination.

To eliminate the step of dispersing fine SiO ₂ particles into the liquid phase, by hydrolyzing a silicon alkoxide in the presence of a base catalyst SiO ₂
A method has been proposed in which a sol containing fine particles is produced, and this is directly gelled.

[Problems to be Solved by the Invention] However, in this method, cracks and cracks are easily generated in the gel during the drying process. In order to prevent such gel destruction, it is necessary to increase the diameter of the SiO ₂ fine particles contained in the sol.Therefore, the water used in the hydrolysis of the alkoxide has a large excess and a large concentration with respect to the alkoxide. It must contain a base. Therefore, before gelation of the sol, a concentration step for removing a large excess of water was required, causing an increase in the number of steps. In addition, it is necessary to add an acid in order to bring the sol to a pH suitable for gelation, but this causes a large amount of salt formed in the sol to precipitate in the gel during the drying process and generate porosity when vitrified. Caused a problem.

In order to solve these problems, the present inventors have previously proposed a method of producing silica glass that does not require a step of dispersing SiO ₂ fine particles and a step of concentrating a sol before gelation, that is, a method in which silicon alkoxide is used as a basic catalyst. obtain a silica sol containing SiO ₂ fine particles is hydrolyzed with, SiO ₂ together with the sol is dried and hydrolyzed in the presence of polyethylene glycol and / or a derivative thereof a silicon alkoxide in the production method of silica glass sintered A method for producing silica glass characterized by adding an acid to a silica sol containing fine particles has been proposed (Japanese Patent Application No. 63-303356). However, even with this method, the yield of the dried gel was sometimes reduced during many batches. The present inventors have conducted intensive studies on the cause of such a decrease in the yield of the dried gel,
The present inventors have found that the pH of the silica sol when adding an acid to the silica sol containing SiO ₂ fine particles has an important relationship with the gel yield, and have completed the present invention.

[Means for Solving the Problems] That is, when an acid is added to a silica sol containing SiO ₂ fine particles produced by hydrolyzing a silicon alkoxide using a basic catalyst in the presence of polyethylene glycol and / or a derivative thereof, It has been found that when the pH of the gel is in the range of 3 to 5, cracking of the gel during the drying process is remarkably suppressed. Outside the pH of silica sol during the addition of acid from the range of 3-5, significantly agglomerated SiO ₂ particles in the sol, the sol and gel is not uniform, cracks in the drying process is likely to occur. In addition, defects such as pores increase in the glass obtained by sintering.

Hydrochloric acid, nitric acid, formic acid, acetic acid, propionic acid and the like can be used as the acid added to the silica sol containing the SiO ₂ fine particles.
These acids are preferably diluted with water or alcohol or a mixed solution thereof, rather than added to the silica sol as a concentrated liquid.

The silica glass is prepared by transferring the silica sol prepared as described above to a container such as a petri dish, gelling the mixture at room temperature to 70 ° C., and then drying at a temperature equal to or higher than room temperature to form a dry gel.
Further, it can be obtained by sintering by raising the temperature to 1000 to 1400 ° C. in the air, for example, in a known manner.

[Action] When acid is added to silica sol containing SiO ₂ fine particles produced by hydrolyzing silicon alkoxide using a basic catalyst in the presence of polyethylene glycol and / or a derivative thereof, the pH of the obtained silica sol is 3 to 5 Within this range, the yield of the dried gel is significantly improved. Although the details of this reason are unknown, the silica sol obtained by adding an acid
As shown in FIG. 1, the relationship between the pH and the gelation time of the sol is as follows:
Since the gel time reaches a maximum in the range of pH 3 to 5, especially in the vicinity of pH 4, the interaction (aggregation and the like) between the SiO ₂ fine particles in the silica sol is weakened in this pH range, and as a result, a uniform sol is obtained. It is presumed that the generation of cracks during the drying process was suppressed because the gel became uniform. In addition, the present invention
You are not bound by the above speculation.

[Example] (Example 1) 284 g of 2-propyl alcohol and 160 M aqueous solution of 0.01M choline
g was mixed, and 16.8 g of polyethylene glycol (molecular weight: 20,000) was added and dissolved. The resulting solution was slowly added to 340 g of tetramethoxysilane (Si (OCH ₃ ) ₄ ) to obtain a sufficiently mixed silica sol. This was allowed to stand at room temperature overnight to ripen. Then, a 0.1 M aqueous hydrochloric acid solution was added little by little to the silica sol with vigorous stirring to adjust the pH of the silica sol to 4. This was put into a 300 mm diameter stainless steel petri dish coated with Teflon, covered with aluminum foil, and gelled at 30 ° C. A hole was formed in the lid, and the lid was dried in a thermostat at 60 ° C. for 2 weeks, then heated to 120 ° C. and dried at that temperature for 1 day to obtain a dried gel. No cracks or cracks were observed in the obtained dried gel. The obtained dried gel was heated and sintered at 1300 ° C. in the air to obtain silica glass. The obtained silica glass had no defects such as voids.

Example 2 A 0.1 M aqueous hydrochloric acid solution was added little by little to a silica sol produced in the same manner as in Example 1 while stirring vigorously to adjust the pH of the silica sol to 3. This was gelled in the same manner as in Example 1,
Dried and sintered vitrified. No cracks or cracks were observed in the dried gel, and no defects such as voids were found in the obtained silica glass.

(Example 3) A 0.1 M aqueous hydrochloric acid solution was added little by little to a silica sol produced in the same manner as in Example 1 while stirring vigorously to adjust the pH of the silica sol to 5. This was gelled in the same manner as in Example 1,
Dry and sintered vitrified. The dried gel had no cracks or cracks, and the obtained silica glass had no defects such as voids.

[Effects of the Invention] According to the present invention, large-sized silica glass free from cracks and cracks and having few defects such as voids can be easily produced by a gel-sol method. The size is basically not limited, and it can be manufactured in any shape such as a plate shape, a rod shape, and a tubular shape.

Further, according to the present invention, silica glass can be manufactured at a lower cost than in the past, so that its application can be expanded not only in the field of photomask base materials for IC manufacturing and the like, but also in liquid crystal display base materials and the like, which have been conventionally used. .

[Brief description of the drawings]

FIG. 1 is a diagram showing the effect of the pH of silica sol on the gelation time of silica sol containing SiO ₂ fine particles. The gelation time represents the time required until gelation, with the time when a 1 M hydrochloric acid solution was added to the silica sol taken as zero.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Toshikatsu Shimazaki 48, Wadai, Tsukuba-shi, Ibaraki Hitachi Chemical Co., Ltd. Chikuzuki R & D Laboratory (56) References Showa 59-131539 (JP, A)

Claims (1)

(57) [Claims] 1. A silica sol containing fine particles of SiO ₂ is obtained by hydrolyzing a silicon alkoxide using a basic catalyst in the presence of polyethylene glycol and / or a derivative thereof, adding an acid to the silica sol, gelling and drying. A method for producing silica glass to be sintered, wherein the pH of the silica sol is 3 to 5 when an acid is added to the silica sol, and the gelation time is set to a maximum value. .