JPS63288921A - Production of silica glass - Google Patents

Abstract

PURPOSE:To prevent cracking and crazing when silica glass is produced by a sol-gel process, by using an org. solvent contg. formamide during hydrolysis to increase the strength of gel, substituting other org. solvent for the formamide, drying and sintering the gel. CONSTITUTION:When silicon alkoxide such as silicon tetramethoxide is hydrolyzed to produce silica hydrosol, an org. solvent contg. formamide is used as a solvent and produced sol is turned into gel to produce high strength gel. This gel is soaked in a liq. mixture of an org. solvent contg. no formamide, e.g., ethanol with water to substitute the liq. mixture for the formamide in the gel. The gel is then dried and sintered. The occurrence of large stress due to formamide is avoided and silica glass is produced.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing silica glass used for optics, semiconductor industry, electronic industry, physics and chemistry, etc.

(Prior Art) In recent years, the sol-gel method has been attracting attention as a new method for producing silica glass.

The method for producing silica glass using the sol-gel method is as follows.

Silicon alkoxide and/or its polycondensate represented by the general formula St (OR) a (where R represents an alkyl group), such as (RO)ssi(O3i(OR))n
Water (an acid or alkali may be added as a catalyst in advance) is added to O5i(OR)s (n=o~8) and hydrolyzed to obtain a silica hydrosol. At this time, an appropriate alcohol may be added as a solvent so that the silicon alkoxide and water form a uniform system.The silica sol solution is allowed to stand still, heated, or gelled by adding a gelling agent. Thereafter, the gel is dried to obtain silica gel. Silica glass is obtained by sintering this dried gel in a suitable atmosphere.

This sol-gel method has the following features.

(1) Since it can be produced at a lower temperature than the conventional vapor phase chemical vapor deposition method in which the soot produced by flame hydrolysis or the like using 5iC14 as a raw material is sintered and vitrified, it can save energy and reduce costs.

(2) Since it can be used as a raw material in a liquid state, it can be purified into a container to achieve high purity.

(3) Since it can be mixed in a liquid phase in a greenhouse, homogeneous glass can be produced even when mixed with other components.

(Problems to be Solved by the Invention) There are still unresolved problems in the production of silica glass by the sol-gel method, which has various advantages as described above.

In particular, there is a problem in that the gel tends to crack or crack during the process of drying the gel, making it difficult to produce a monolithic large-sized dried gel with a good yield. One of the causes of cracks and cracks is that stress is generated in the gel as water, alcohol, etc. evaporate during gel drying, and if this stress is greater than the strength of the gel, it is thought that the gel will crack or crack. There is.

As a countermeasure to this problem, methods have been taken to lower the porosity of the container and control the evaporation rate of water, alcohol, etc., but these methods are insufficient. Cracks and splits during the gelation and drying shrinkage processes are largely due to the internal structure of the gel and the drying conditions.

On the other hand, it has been reported that when producing silicate glass by the sol-gel method, the strength of the gel can be increased by using formamide as a solvent during sol preparation (M
material Re5earcb 5ocityS
ys+posia Proceedings Vol.
32p71-77 (1984)).

However, it was confirmed that when formamide is used as a solvent in a silica glass system, a large stress is generated due to the formamide remaining during drying, and the gel breaks.

The present invention provides a method for producing silicate glass that does not cause cracking or cracking.

(Means for Solving the Problems) The present invention considers that stress generated due to evaporation of water, alcohol, etc. during gel drying is a cause of cracks and cracks, and the present invention is designed to prevent the occurrence of such large stress. In order to avoid this, we created a sol using formamide as a solvent and increased the strength of the gel by gelling it, and then replaced the formamide in the gel with a mixed solution of other organic solvents and water to avoid large stress. This was done based on the discovery that the occurrence of this can be avoided.

As the silicon alkoxide used in the present invention, not only silicon alkoxide monomers but also polycondensates may be used. For example, (CHsO) ssI (O3i (OCRs)
t) nO5i(OCTo)i (n −0 to 8 ).

One or more of these can be used. As the alkyl group, a methyl group, an ethyl group, a propyl group, etc. are preferable.

To the water added for hydrolysis, mineral acids such as hydrochloric acid and nitric acid, organic acids such as formic acid and acetic acid as catalysts, and not only ammonia but also organic bases such as ethylenediamine as bases may be added in advance.

The organic solvent in the mixture of formamide and organic solvent added as a solvent is preferably one that is compatible with silicon alkoxide and water; for example, alcohols, ketones, etc. can be mentioned. One or more of these can be used. The silicon alkoxide, solvent, and water are thoroughly mixed using a starrer or the like in order to make the resulting sol as uniform as possible. Ultrasonic waves may also 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 in order to prevent solvents such as alcohol from escaping from the generated gel, and the temperature during gelation is preferably 0° C. or higher.

Next, this gel is immersed in a mixture of a formamide-free organic solvent and water to replace the formamide in the gel with this mixture. This mixture of organic solvent and water is preferably compatible with water.

Examples include alcohol/water and ketones/water. These organic solvents can be used alone or in combination.

After the formamide has been sufficiently replaced, the replacement solvent is removed, the lid is replaced with a hole, and the lid is dried under an appropriate atmosphere to shrink and solidify to form a dry gel. The temperature during drying is preferably at least a greenhouse temperature. Thereafter, it is sintered under conditions commonly used in the sol-gel method.

Example 1 Silicon tetramethoxide C5i (OCH3) 4)
19. Mix 2-1, 22.7 ml of ethanol, and 22.7 ml of formamide with thorough stirring. 0.0 in this solution
9.1 ml of 1 mol/It aqueous ammonia was added and thoroughly mixed to obtain a silica sol. The resulting sol has a diameter of 9
The mixture was placed in a Teflon-coated glass petri dish to a depth of 8, sealed, and allowed to gel at room temperature. After gelatinization, the gel was immersed for one day in a solution of ethanol and water in a volume ratio of 1:1 to perform solvent replacement with formamide. After this, after removing the displacement solvent, replace it with a lid with a hole at 60°C.
After drying for 10 days at 120° C., a dry gel without cracks or cracks was obtained. This dry gel is placed in a baking oven.
It was heated at 150°C to obtain silica glass with cracks and foam.

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 When a dry gel was prepared by performing the same operation as in Example 1 except that the solvent was not replaced with formamide in the gel, cracks and cracks occurred during drying, and a monolithic dry gel was obtained. I couldn't do it.

Example 2 Polycondensate of silicon alkoxide (CI(so)ssi(O5i(OCHs)z)nO5
i(QC)Is)3 (with n −3 at the center) 22
．． 7ml, ethanol 24.3m+1 and formamide 2
4.3 ml was added and stirred and mixed well.

This solution costs 0.01/j! A choline aqueous solution was added and thoroughly mixed to obtain a silica sol. This sol has a diameter of 90
The mixture was placed in a glass Petri dish coated with fi Teflon to a depth of 8 mm, sealed, and allowed to gel in a greenhouse. After gelation, add 1 part of the gel to a solution of ethanol and water in a volume ratio of 1=1.
The sample was soaked for 1 day, and the solvent was replaced with formamide. Thereafter, after removing the replacement solvent, the gel was dried at 60° C. for 10 days and then at 120° C. for 1 day in place of a lid with holes to obtain a dry gel without any cracks or cracks. This dry gel was placed in a baking oven for 11 minutes.
Silica glass without cracks or foaming was obtained by heating at 50°C.

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.

In addition, as a substitution solvent, ethanol/for example l-
These effects were also found in the propatool/water system.

(Effects of the Invention) According to the present invention, a monolithic dry gel without cranks or cracks can be produced by replacing formamide, which causes cracks in the gel during drying.

Furthermore, the amount of water in the glass obtained can be significantly reduced compared to the conventional sol-gel method.

Claims (1)

[Claims] 1. In the manufacturing method of silica glass, in which silicon alkoxide is hydrolyzed to form silica hydrosol, left to gel, and sintered after drying, an organic solvent containing formamide is used as a solvent during hydrolysis, and before drying. A method for producing silica glass characterized by immersing a gel in a mixture of a formamide-free organic solvent and water and then drying the gel.