JPH0512290B2 - - Google Patents

Description

Detailed Description of the Invention The present invention is a low-temperature synthesis method of silica glass using a sol-gel method using alkyl silicate and fine powder silica as raw materials and adjusting the pH to 3 to 6. Concerning how to make it in good condition.

Quartz glass is used in crucibles and boards during the IC manufacturing process.
It began to be used in diffusion furnaces, etc., and its usefulness was recognized, and as products with fewer hydroxyl groups and better optical uniformity were developed, it began to be used for various optical purposes. In particular, silica glass fibers for optical communications have been attracting attention recently.

As described above, quartz glass is used in various fields, and the scope of its use is expanding. However, the manufacturing cost of quartz glass is high, and its high price has become a problem. The sol-gel method has been attempted as a method for producing inexpensive, high-quality quartz glass.

As a method for obtaining large-sized quartz glass with good yield using the sol-gel method, ultrafine powdered silica is added to a sol prepared by hydrolyzing alkyl silicate, the pH is further adjusted to 3 to 6, and then dried at 50 to 90°C. death,
There is a method of sintering. This solution simultaneously solved the problem of cracks during dry gel production and the problems of cracks and cracks during sintering, and it became possible to manufacture fairly large quartz glass (4 inctφ or more) at low cost.

However, when manufacturing large quartz glass, warping occurs during the drying and shrinking process of the gel, and the warping cannot be completely eliminated. Furthermore, even if the drying conditions are precisely controlled and a flat dry gel is produced over a long period of time, there is a strong tendency for warping to occur during sintering.

Even if large quartz glass can be produced, if it cannot be obtained in the desired shape (for example, a flat plate), its application will be limited to a very small area.
When using it, a large amount of loss occurs and more steps are required. Considering its use in quartz substrates and boards for ICs, it is extremely important to obtain quartz glass with good flatness.

The object of the present invention is to solve the problem of warping that the conventional sol-gel method faces, and to produce quartz glass with good flatness, and more specifically, to produce quartz glass that maintains its shape during gelation.

Next, an outline of the present invention will be described.

The basic operation of the present invention is to gel the gel in a short period of time by adjusting the pH to 4 to 5, hold it in a sealed state near room temperature for 2 to 4 days, and then dry and sinter it.

The cause of warping is uneven drying. In a plate-shaped gel, the rate of solvent evaporation differs between the top surface and the bottom surface that is in contact with the container, resulting in partially different shrinkage rates. Furthermore, since a concentration gradient occurs, mass transfer also occurs in tetrahydroxysilane in addition to the solvent, resulting in partial differences in silica density. Sintering the dry gel produced in this manner does not yield flat quartz glass, and not only warps but also cracks may occur during the drying and sintering process.

A sol whose pH is adjusted to 3 to 6 gels in a short period of time, and then rapidly shrinks while extruding the solvent. Therefore, the gel is kept immersed in the solvent, and there are large differences in the drying speed of the gel.

After gelation, the process of shrinking and becoming a dry gel is as follows:
From a molecular perspective, a dehydration condensation reaction of tetrahydroxysilane occurs. A silica network structure is formed and becomes denser. Once the polycondensation reaction has progressed to a certain extent and the molecular arrangement has been determined, no mass transfer will occur and shrinkage due to drying and sintering will proceed evenly.

The reaction rate of the dehydration condensation reaction of tetrahydroxysilane is determined by the tetrahydroxysilane concentration, water concentration, hydrogen ion concentration, and temperature. If these factors can be uniformly controlled, the reaction will proceed uniformly. In the sol state, chemical uniformity can be controlled.

Therefore, we propose to keep the sol in closed conditions for a period of time until the network structure is formed, and then fill it with supersaturated steam to prevent the evaporation of the solvent, allowing the polycondensation reaction to proceed uniformly, and then drying and sintering. .

The dry gel produced using this method had extremely high flatness and maintained its flatness even after sintering. Furthermore, although the dry gel warped when dried very rapidly, it became flat when sintered.

Hereinafter, the present invention will be explained in detail based on Examples.

Example 1 440ml of ethyl silicate and 0.1N hydrochloric acid aqueous solution
360 ml was vigorously stirred to obtain a colorless and transparent homogeneous solution. Fine silica powder (Aerosil oX-50)
After gradually adding 150 g and thoroughly stirring, dispersion was performed using ultrasonic waves. Furthermore, 400g of sol was adjusted to pH4.5 by dropping 0.1N ammonia aqueous solution.
was transferred to a container with an inner diameter of 20 cm and a flat bottom, and the container was sealed tightly.

When the temperature was maintained at 20°C, it gelled after about 1 hour, and then gradually shrunk while maintaining its flatness. After 2 days, the gel had shrunk by about 4%, and the lid was replaced with a lid with 20 holes of 5 mm in diameter, and the temperature was gradually raised to 60°C and maintained. The subsequent shrinkage proceeded smoothly, and a dry gel with a diameter of 14 cm with very good flatness was obtained.

This dry gel was heated to 1250℃ at a heating rate of 180℃/hr.
When heated to ℃, colorless and transparent quartz glass with a diameter of 10 cm was produced. The flatness was extremely good.

Example 2 A sol prepared in the same manner as in Example 1 was transferred to a sealed container and kept at 20°C for 6 days. The gel had shrunk by about 6%. Thereafter, drying and sintering were carried out under the same conditions as in Example 1, and a dry gel and quartz glass with very good flatness were obtained.

Example 3 A sol prepared in the same manner as in Example 1 was transferred to a sealed container, kept at 20° C. for 2 days, and then replaced with a lid with 20 holes of 5 mm in diameter. When the temperature was gradually raised to 80°C and maintained, shrinkage did not proceed uniformly, and the resulting dry gel was warped.

However, when this dry gel was vitrified by heating to 1250°C at a heating rate of 180°C/hr, the warpage was eliminated and silica glass with very good flatness was obtained.

Example 4 A sol prepared in the same manner as in Example 1 was prepared using a sol with an inner diameter of 5 cm.
It was transferred to a cylindrical container with a height of 30 cm and left in a sealed container for 2 days. By making a pinhole in the lid and drying it at 60°C, an extremely straight rod-shaped dry gel was obtained.
By heating to 1250°C, rod-shaped quartz glass with a diameter of 2.5 cm and a length of 10 cm was produced. It had a similar shape to the inside of the container, and there was no warpage in the axial direction.

As described above, according to the method of the present invention, plate-shaped quartz glass can be easily manufactured with good flatness.
It can be used as a quartz base or board. Additionally, since it maintains the shape of the container during gelation, it is very effective in manufacturing rods and tubes.

Claims (1)

[Claims] 1. In a method for producing quartz glass by the sol-gel method using at least alkyl silicate and silica fine particles as raw materials, when creating a dry gel, the sol whose pH is adjusted to 3 to 6 is sealed in a container, kept for at least one day, and then opened half-open. A method for producing quartz glass characterized by heating and drying it.