JPH02248342A - Production of silica glass - Google Patents

Abstract

PURPOSE:To obtain large-sized silica glass in a high yield without causing cracking or crazing by using polyvinyl acetate and a mixed solvent contg. acetylacetone at the time of preparing a sol when silica glass is produced by a sol-gel process. CONSTITUTION:Silicon alkoxide is hydrolyzed to prepare a silica sol and this sol is converted into gel. This gel is dried and the resulting dry gel is sintered to obtain silica glass. In this sol-gel process, a mixed solvent contg. acetylacetone such as a methanol-acetylacetone mixed solvent is used as a solvent for the silicon alkoxide and polyvinyl acetate is added in the stage for preparing the silica sol. By using the polyvinyl acetate and acetylacetone in combination, large-sized dry gel can easily be obtd. without causing cracking in the drying stage.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing silica glass used for optical applications, semiconductor industry, electronic industry, physical and chemical applications, 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 for optical fibers, photomask substrates for IC manufacturing, TPT substrates, etc. It has been used for many years, and its applications are expanding more and more.

Conventional methods for manufacturing silica glass include melting natural quartz in an electric furnace or oxyhydrogen flame, or oxidizing and melting silicon tetrachloride at high temperature in an oxyhydrogen flame or plasma flame. The manufacturing process requires high temperatures of 2000℃ or higher, consuming 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 issues.

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.

Silicon alkoxide represented by the general formula S i (OR) a (R: alkyl group) (in the present invention,
0 containing its polycondensate, for example (RO)ssi・(O31
(OR)tl ,,・O3i (OR)! , (n==
0 to 8, R: alkyl group)) is added with water (pH may be adjusted with an alkali or acid) and hydrolyzed to produce silica hydrosil (referred to as silica sol in the present invention).
At this time, an appropriate alcohol is generally added as a solvent so that the silicon alkoxide and water form a homogeneous system. This silica sol is gelled by standing still, increasing the temperature, adding a gelling agent, etc. Thereafter, the gel is evaporated to dryness to obtain a dry silica gel. - Obtain silica glass by sintering this dried gel in a suitable atmosphere.

(Problems to be Solved by the Invention) However, there are still unresolved problems in the production of silica glass by the sol-gel method. In particular, during the process of drying the gel, cracks and cracks are likely to occur in the gel, making it difficult to produce a monolithic large-sized dried gel without any cracks or cracks with a good yield.

In order to solve these problems, the inventors have already discovered a method to control the growth of silica particles in the sol and prevent the gel from cracking during drying by adding polyvinyl acetate during sol preparation. (Special application No. 62-294459)
However, when attempting to produce a larger dried gel using this method, cracks often occur during drying.

The present invention provides a method for manufacturing silica glass that does not cause cracks or cracks.

(Means for Solving the Problems) The present invention uses a solvent containing acetylacetone as a solvent for silicon alkoxide and also adds polyvinyl acetate during sol preparation in a method for producing silica glass by a sol-gel method. It is characterized by:

In the present invention, there are no particular restrictions on the alkyl group of the silicon alkoxide, but silicon alkoxides having a methyl group, ethyl group, propyl group, or butyl group are used from the viewpoint of hydrolysis resistance and gelation time. is preferable, and partially polymerized silicon alkoxides can also be used.

The catalyst added with water is not particularly limited, such as bases and acids, but
From the viewpoint of gelation time and ease of sintering of the resulting dry gel, a base gives more preferable results.

The molecular weight of polyvinyl acetate is not particularly limited in the present invention, but it is selected depending on the solubility in water and solvents and the degree of growth of silica fine particles in the sol.

The amount of acetylacetone used as a solvent for silicon alkoxide is not particularly limited in the present invention, but is selected depending on the degree of growth of silica fine particles in the sol.

Solvent components used with acetylacetone include:
Alcohols such as methyl alcohol, ethyl alcohol, 1-propyl alcohol, 2-propyl alcohol, butyl alcohol, ethers such as dimethyl ether,
At least one of ketones such as acetone and ethyl methyl ketone, and esters such as ethyl acetate can be used.

Silicon alkoxide, polyvinyl acetate, acetylacetone, solvent, and water are thoroughly mixed using a stirrer or the like to make the resulting sol as uniform as possible. Alternatively, ultrasonic waves may be irradiated.

Fine silica particles may be added during sol preparation.

The generated sol solution is quickly transferred to another container and allowed to gel. During gelation, it is preferable to seal the container to prevent the solvent from escaping from the generated gel, and the temperature during gelation is preferably 0° C. or higher.

During the drying process, replace the sealing lid with a lid with holes.
It is dried and shrunk and solidified in an appropriate atmosphere to form a dry gel.

Thereafter, silica glass is manufactured by sintering using a sol-gel method.

The gelling process, drying process, and sintering process are carried out under commonly used conditions.
Leave at 100°C for several minutes to several lθ days, leave at room temperature to 200°C for several hours to several lO days, and leave at 1000 to 140 in an appropriate atmosphere.
50~b at 0°C (Function) The details of the effects of acetylacetone and polyvinyl acetate are unknown, but they can be used to control the production and growth of silica fine particles in the sol, and to alleviate the stress generated in the gel during the drying process. It is thought that this contributed to the increase in the size of the gel.

Example 1 Silicon tetramer oxide: acetylacetone-1:
The molar ratio was measured to be 1:1, methanol was added in the same volume as acetylacetone, and the mixture was stirred.
Parts by weight were added to form a homogeneous solution. This solution has a concentration of 0.0
A 1 s+ol/1 choline aqueous solution was added in an amount of 4 moles of water to 1 mole of silicon tetramethoxide, and thoroughly mixed to obtain a silica sol. The obtained sol was placed in a Teflon-coated glass Petri dish with a diameter of 200 mm to a depth of 10 m, sealed, and gelatinized at room temperature. After gelatinization, the sealing lid was replaced with a lid with holes and dried at 60°C for 14 days, and then the temperature was raised to 170°C at a rate of 30°C/day to obtain a dry gel without any cracks or cracks. This dried gel was heated in air to 1300° C. at a rate of 60° C./hour to obtain silica glass free from cracks and foaming. This silica glass had no devitrification or bubbles and was of high quality. Further, as a result of analysis, the properties of this silica glass matched those of commercially available silica glass.

Example 2 Silicon tetramethoxide:acetylacetone-X:t
, S were weighed so as to have a molar ratio of 1/3 of the volume of acetylacetone, and the mixture was stirred. Furthermore, 10 parts by weight of polyvinyl acetate was added to 100 parts by weight of silicon tetramethoxide to form a homogeneous solution. A choline aqueous solution having a concentration of 0.01 mol/1 was added to this solution in an amount of 4 mol of water per 1 mol of silicon tetramethoxide, and thoroughly mixed to obtain a silica sol.

Thereafter, the same operation as in Example 1 was performed to obtain silica glass. This silica glass had no cracks or bubbles and was of high quality. Further, as a result of analysis, the properties of this silica glass matched those of commercially available silica glass.

Comparative Example The same operation as in Example 1 was performed except that acetylacetone was not used as the solvent and only methanol was used. As a result, the gel was prone to cracks and cracks during drying.

Example 3 Polycondensate of silicon tetramethoxide ((CH30)z St ・ (O・st (OCH
s)t)n・O3i (OCHs)3. Niacetylacetone (centered on n=2) was weighed out so that the molar ratio was 1:0.5, methanol was added 2.5 times the volume of acetylacetone, stirred, and polyvinyl acetate was added to silicone tetramethoxide. 10 parts by weight were added to 100 parts by weight of the polycondensate to form a homogeneous solution. A choline aqueous solution having a concentration of 0.01 mol/l was added to this solution in an amount of 3 mol of water per 1 mol of the polycondensate of silicon tetramethoxide, and the mixture was thoroughly mixed to obtain a silica sol. Thereafter, the same operation as in Example 1 was performed to obtain silica glass. The obtained silica glass had no cracks, breaks, or foaming, and was of high quality.

Example 4 Polycondensate of silicone tetramethoxide - Niacetylacetone - Weighed so that the molar ratio was 1:l, methanol was added 0.8 times the volume of acetylacetone and stirred, and then polyvinyl acetate was added to silicone tetramethoxide. 10 parts by weight were added to 100 parts by weight of the polycondensate to form a homogeneous solution. A choline aqueous solution having a concentration of 0.01 mol/1 was added to this solution in an amount of 3 mol of water per 1 mol of the polycondensate of silicone tetramethoxide and thoroughly mixed to obtain a silica sol. Thereafter, the same operation as in Example 1 was performed to obtain silica glass. The obtained silica glass had no cracks, cracks, or foaming, and was of high quality.

(Effects of the Invention) According to the present invention, it is possible to manufacture large silica glass into containers by the sol-gel method without generating cracks or breaks. There are basically no restrictions on its size, and its shape is not limited to plate-like ones, but rod-like and tubular ones can also be manufactured, making it possible to manufacture it at a lower cost than in the past.

In addition, the present invention allows silica glass to be manufactured at a lower cost than before, so it can be used not only in fields such as photomask substrates for IC manufacturing, which have traditionally been used, but also in fields where it has not been used due to its high price. □-7

Claims (1)

[Claims] 1. In a method for producing silica glass in which silicon alkoxide is hydrolyzed to form silica sol, this is gelled, dried to form a dry gel, and then sintered, at the step of hydrolyzing silicon alkoxide to form silica sol, A method for producing silica glass, characterized in that a solvent containing acetylacetone is used as a solvent for silicon alkoxide, and polyvinyl acetate is added.