JPH0350125A - Production of silica glass - Google Patents

Abstract

PURPOSE:To inhibit the occurrence of pores by hydrolyzing Si alkoxide in the presence of polyethylene glycol and/or a deriv. thereof, adding water, an acid and a mixture of specified Si alkoxides to the resulting silica sol, forming gel and carrying out sintering. CONSTITUTION:Si alkoxide represented by the formula (where R is alkyl), e.g. tetramethoxysilane is hydrolyzed with a basic catalyst in the presence of polyethylene glycol and/or a deriv. thereof to obtain silica sol contg. fine SiO2 particles. An acid such as hydrochloric acid is added to the silica sol to adjust the pH to 3-5, water is added and a mixture of two or more kinds of Si alkoxides having different alkyl groups, e.g. tetramethoxysilane and tetraethoxysilane is further added. The amt. of the water added is 0.1-4 times the theoretical amt. of water required to hydrolyze the added Si alkoxides. They are mixed and held at room temp. to 70 deg.C to form gel. This gel is dried at room temp. or above and the resulting dry gel is sintered by heating to 1,000-1,400 deg.C in the air to obtain silica glass.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing silica glass used in optics, semiconductor industry, electronic industry, science and engineering, etc.

[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 of synthesizing silica glass at low temperature called the sol-gel method 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, an organic solvent, etc., and large aggregates of SiO2 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 SiO2 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 was hydrolyzed in the presence of a base catalyst.
A method has been proposed in which a sol containing ty2 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 size of the SiO2 fine particles contained in the sol, and for this reason, the water used for hydrolyzing the alkoxide must be in large excess with respect to the alkoxide, and must 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 the step of dispersing SiO2 fine particles and the step of concentrating the sol before gelation, that is, in the presence of polyethylene glycol and/or its derivatives. Hydrolyze silicon alkoxide using a basic catalyst to form SiO
We proposed a method for producing silica glass characterized by obtaining a silica sol containing 2 fine particles, adding water together with an acid to this silica sol, then adding silicon alkoxide, gelling, drying, and sintering. However, even with this method, cracks or cracks sometimes occur in the gel during the drying process in multiple batches, resulting in a decrease in the yield of the dried gel. An object of the present invention is to provide a method for producing large-sized silica glass that prevents gel cracks and breaks during the drying process and has fewer defects such as pores.

[Means for Solving the Problems] To achieve the above object, the present inventors investigated various causes of cracks and cracks in gel during the drying process and methods for preventing the same, and found that cracks in gel are caused by shrinkage of gel. Although this kind of gel cracking that occurs during the process can be relatively suppressed by increasing the amount of silicon alkoxide poison added to the silica sol containing SiO2 fine particles, the gel becomes denser and gel cracking occurs at the end of the drying process. Furthermore, it was found that the densification of the gel that occurs when the amount of silicon alkoxide is increased can be suppressed by using a mixture of two or more types of silicon alkoxides (S i (OR) 4 ) having different alkyl groups (R). The present invention has been completed.

That is, in the present invention, silicon alkoxide is hydrolyzed using a basic catalyst in the presence of polyethylene glycol and/or its derivative to obtain a silica sol containing SiO2 fine particles, water is added together with an acid to this silica sol, and then silicon alkoxide is In the method for producing silica glass in which silicon alkoxide (S i (OR) 4) to be added is a mixture of two or more different types of silicon alkoxide (S i (OR) 4) and alkyl groups (R) (7), A method for producing silica glass is provided.

The acid added to the silica sol containing SiO2 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 the 5i02 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 with the acid, the mixture of silicon alkoxides to be added is two or more types selected from tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, or partial condensation polymerization of these. A mixture of silicon alkoxides can be used. For suitable combinations and compositions of mixtures of two or more silicon alkoxides, see
The composition of silica sol containing O2 fine particles, especially alcohol (
It is not particularly limited as it varies depending on the composition (derived from the hydrolysis product of silicon alkoxide and the solvent).

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 it.

[Function] Water is added together with an acid 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 derivatives, and then two different alkali groups (R) are added. More than one type of silicon alkoxide (S i (OR) 4
) is added to form a gel, the gel does not become densified even if a large amount of silicon alkoxide is added, and as a result, the yield of dry gel is significantly improved. Although the details of this reason are unknown, if water is added together with an acid to a silica sol containing SiO2 fine particles, and then a large amount of silicon alkoxide (for example, silicon butoxide) containing a large R is added, the Si02 fine particles will coagulate. For example, when two types of silicon alkoxides with different R (R1>R2) are added to silica sol, the larger silicon alkoxide itself containing R1 or the alcohol of the hydrolysis product will be added to the SiO2 fine particles and their secondary particles in the silica sol. It is presumed that it promotes growth and, as a result, suppresses densification of the gel. Note that the present invention is not limited to the above speculation.

[Example] (Example 1) 284 g of 2-propyl alcohol and 160 g of O0OIM choline aqueous solution were mixed, and 16.8 g of polyethylene glycol (molecular weight 20,000) was added and dissolved therein.

The resulting solution was diluted with tetramethoxysilane (Si (oC
It was slowly added to 340 g of H3)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 with vigorous stirring, and then a mixture of 153 g of tetramethoxysilane and 23.3 g of tetraethoxysilane was added. This is coated with Teflon and has a diameter of 3
The mixture was placed in a 00 mm stainless steel petri dish, covered with aluminum foil, and allowed to gel at 30°C. A hole was made in the lid and the mixture was dried for two weeks in a constant temperature bath at 60'C, and then the temperature was raised to 120C and dried at that temperature for one 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 1300° C. in air to obtain silica glass. The obtained silica glass had no defects such as pores.

(Example 2) 0.8 ml of 1M hydrochloric acid aqueous solution and 48.3 ml of water were added to the silica sol prepared in the same manner as in Example 1 with vigorous stirring.
g, and then a mixture of 136 g of tetramethoxysilane and 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 resulting silica glass was free of defects such as pores.

(Example 3) To the silica sol prepared in the same manner as in Example 1, 48.3 ml of water was added with 0.8 ml of 1M hydrochloric acid aqueous solution while gently stirring.
Then, a mixture of 102 g of tetramethoxysilane and 93.1 g of tetraethoxysilane 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.

Furthermore, according to the present invention, silica glass can be manufactured at a lower cost than before, so its application can be expanded not only to fields such as photomask substrates for IC manufacturing, which have been conventionally used, but also to liquid crystal display substrates, etc. .

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 by gelling, drying, and sintering, characterized in that the silicon alkoxide (Si(OR)_4) added is a mixture of two or more types of silicon alkoxides having different alkyl groups (R). Glass manufacturing method.