JPH0259443A - Production of silica glass - Google Patents

Abstract

PURPOSE:To easily obtain large-sized silica glass free from cracks and crazes by using a specified solvent and adding polyvinyl acetate in a sol preparing stage when silica glass is produced by a sol-gel process. CONSTITUTION:Silicon alkoxide is hydrolyzed to prepare silica sol, this sol is converted into gel and dried and the resulting dry gel is sintered to produce silica glass. In the hydrolyzing stage, a solvent contg. tetrahydrofurfuryl alcohol is used as a solvent for the silicon alkoxide and polyvinyl acetate is added. The solvent used in combination with the tetrahydrofurfuryl alcohol may be alcohols, ethers, ketones or esters.

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. 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 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, silicon alkoxide including its polycondensate, for example (RO)ssi・(O31
(OR)z)-・O3i (OR)s, (n-o~8
, R: alkyl group)) is added with water (the pH may be adjusted with an alkaline earth metal) and hydrolyzed to form silica hydrosyl (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. Silica glass is obtained 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 (and it is difficult to produce a monolithic large-sized dried gel without cracks or cracks with a good yield).

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 provides a method for producing silica glass using a sol-gel method, in which a solvent containing tetrahydrofurfuryl alcohol is used as a solvent for silicon alkoxide during sol preparation. It is characterized by the addition of vinyl acetate.

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

The catalyst to be added together 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 tetrahydrofurfuryl alcohol 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 tetrahydrofurfuryl alcohol include methyl alcohol, ethyl alcohol, -propyl alcohol, 2-propyl alcohol,
At least one of alcohols such as butyl alcohol, ethers such as dimethyl ether, ketones such as acetone and ethyl methyl ketone, and esters such as ethyl acetate can be used.

The silicon alkoxide, polyvinyl acetate, solvent, and water are thoroughly mixed using a starter or the like to make the resulting sol as uniform as possible.

Alternatively, ultrasonic waves may be irradiated. Fine silica particles may be added when preparing the sol.

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.

In the drying process, instead of using a lid with holes, the gel is dried and shrunk and solidified in an appropriate atmosphere to form a dry gel.

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

The gelling process, drying process, and sintering process are carried out under commonly used conditions.
Leave at 100℃ for a few minutes to several 10 days, leave at room temperature to 200℃ for several hours to several 10 days, 1000 to 140 in an appropriate atmosphere
50~b at 0°C (Effect) The effects of tetrahydrofurfuryl alcohol and polyvinyl acetate are unknown, but they can control the production and growth of silica fine particles in the sol, and relieve 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, making it possible to increase the size of the gel.

Example 1 Adjust the molar ratio of silicone tetramethoxide:tetrahydrofurfuryl alcohol to 1=1, add methanol in the same amount as tetrahydrofurfuryl alcohol and stir, and then add polyvinyl acetate to silicone tetramethoxide 100 10 parts by weight were added to make a homogeneous solution. This solution has a concentration of 0. OI Tl0L/1 choline aqueous solution to 1 mole of silicon tetramethoxide
A silica sol was obtained by adding moles and mixing thoroughly. The obtained sol was placed in a Teflon-coated glass Petri dish with a diameter of 200 mm to a depth of 10 cm, sealed, and gelatinized at room temperature. Instead of gelatinized gelatin, it was dried at 60°C for 14 days, and then heated 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 without cracks or foaming. This silica glass has no devitrification or bubbles and is of high quality.

Further, as a result of analysis, the properties of this silica glass matched those of commercially available silica glass.

Example 2 Silicone tetramethoxide:tetrahydrofurfuryl alcohol; Adjust the molar ratio to 1:1, add half the amount of methanol to the tetrahydrofurfuryl alcohol and stir, and then add polyvinyl acetate to 100% by weight of silicone tetramethoxide. 10 parts by weight were added to make a homogeneous solution. To this solution, 4 mol of choline aqueous solution having a concentration of 0.01 mol/1 per 1 mol of silicon tetramethoxide was added 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 has no devitrification or bubbles and is 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 carried out except that tetrahydrofurfuryl alcohol was not used as the solvent and only methanol was used. As a result, the gel was likely to crack or crack during drying.

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

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 tetrahydrofurfuryl alcohol is used as a solvent for silicon alkoxide, and polyvinyl acetate is added.