JPS62158133A - Production of silica glass - Google Patents

Abstract

PURPOSE:To prevent the cracking and crazing of a gel during the drying stage and to obtain a monolithic and large-sized dry gel by immersing a gel before drying in an aq. soln. of formamide, taking out the gel, and drying the gel. CONSTITUTION:Silicon alkoxide shown by the formula (where R is alkyl) and/or its polycondensation product are hydrolyzed to obtain a silica sol. The silica sol is allowed to stand and gelated, and the gel is dried and sintered to produce the silica glass. The gel before drying is immersed in an aq. soln. of formamide, taken out, and then dried. The aq. soln. of formamide contg. the water-soluble salt of an alkali metal and/or an alkaline earth metal is used as the aq. soln. of formamide.

Description

[Detailed description of the invention] (Industrial application field) The present invention is applicable to optics, semiconductor industry, and electronic industry.

This article relates to a method for producing silica glass used for physical and chemical purposes.

(Prior Art) Silica glass has recently been recognized as useful, particularly in the semiconductor industry, due to its heat resistance, chemical resistance, and excellent optical properties. The sol-gel method has recently been attracting attention as a new method for producing silica glass.

An example of a method for producing silica glass using the sol-gel method is as follows.

Silicon alkoxide represented by the general formula Si(OR)4 (where R is an alkyl group) and/or its polycondensate 2
For example, (RO)3Si ・(O8i(0几)2)n・
O8i (OR)! + (n = 0 to 8. R is a furkyl group) and water (pH may be adjusted with an alkali or acid)
is added and hydrolyzed to produce silica hydrosil. At this time, an appropriate alcohol may be added as a solvent so that the silicon alkoxide and water form a homogeneous system.

Furthermore, ultrafine particles of silica or the like may be added.

This silica hydrosil is allowed to stand still, heated, or added with a gelling agent to form a gel. Further, 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.

This sol-gel method has the following features.

Glass soot is deposited using an oxyhydrogen flame using fil S i Cla as a raw material (compared to the conventional silica glass manufacturing method), which can be manufactured at low temperatures, resulting in energy savings and low cost.

(2) Since the raw material is liquid, it is easy to purify.

High purity products can be obtained.

(3) Because they can be mixed in a liquid phase at room temperature.

AI! zOs+ Zr0z, Ti0zt BzCht
Silica glass uniformly doped with PzOs, Nb2O3, etc. can be obtained.

Due to these very useful features, much research has been conducted to date.

(Problems to be solved by the invention) However, in the sol-gel method, cracks and cracks tend to occur in the gel during the gel drying process, making it difficult to produce monolithic large-sized dried gels with a good yield. This problem exists, and methods have been used to control the evaporation rate of water and alcohol by making pinholes in the lid of the container containing the gel, but this method is insufficient.

An object of the present invention is to provide a method for manufacturing silica glass that solves the above-mentioned problems.

(To solve the problem, the inventors discovered that because the evaporation rate of the alcohol produced by hydrolysis of silicon alkoxide or the alcohol added as a solvent is high, it cannot keep up with the drying shrinkage of the gel, resulting in cracks and cracks.) Considering that this may be a contributing factor, the inventors have found that the evaporation rate can be controlled by substituting the alcohol solution W by immersing it in an aqueous formamide solution, and the above object can be achieved, leading to the completion of the present invention.

In the present invention, a silicon alkoxide represented by the general formula Si'(O8)4 (where R is an alkyl group) and/or a polycondensate thereof is hydrolyzed to obtain a silica sol.

The present invention relates to a method for producing silica glass in which the gel is then left to gel, dried, and sintered, and the gel before drying is immersed in an aqueous formamide solution, taken out, and dried.

The alkyl group of the silicon alkoxide and/or its polycondensate used in the present invention is preferably a methyl group or an ethyl group because it is economical and easy to replace with an aqueous formamide solution. 9 as a polycondensed metal of silicon alkoxide
For example, (CHsO)a Si・(O8i (OCH3)
2) n ・O8i (OCHa)s can be raised. It is preferable that the pH of water added for hydrolysis is adjusted in advance with an acid or alkali. The silicon alkoxide and/or its polycondensate and water are thoroughly mixed using a stirrer or the like in order to make the produced silica sol as uniform as possible. The generated silica sol is quickly transferred to a container and allowed to gel. During gelation, it is preferable to seal the container to prevent evaporation of alcohol from the gel formed, and the temperature during gelation is room temperature to 80℃.
°C is preferred. Next, this gel is taken out from the container and immersed in a formamide aqueous solution, and the alcohol solution in the gel (i-liquid of alcohol and water) is replaced with a formamide aqueous solution. In order to prevent this, it is preferable to conduct the reaction in an aqueous formamide solution in an aquarium.The formamide aqueous solution preferably contains formamide in an amount of xovo 1% or more.

If it is less than 10 vol 1, smooth progress of substitution is insufficient. When using a formamide aqueous solution containing water-soluble salts, the upper limit of the formamide content is preferably 50% by volume.

After the substitution, the gel is taken out of the formamide aqueous solution, placed on a plate with a smooth surface, dried in air, and solidified by shrinkage to form a dry gel. The appropriate temperature for drying is room temperature to 60°C.

The present invention consists of a single silica glass and trace amounts of alkali metals and/or alkaline earth metals (usually present as oxides).
applied to obtain silica glass containing. In order to obtain the latter silica glass, when the gel before drying is immersed in an aqueous formamide solution, it is preferable that the aqueous formamide solution contains at least one salt of the aqueous solution of the metal. Examples of salts in aqueous solutions include Li, Na+
KpRb, Cs, Mg, Ca, Sr,
Nitrate and hydrochloride of Ba.

Examples include acetate and sulfate, and their composition is selected so that they will eventually vitrify.

(Function) When a hardened gel of silica hydrosil is immersed in an aqueous formamide solution, alcohols such as methanol contained in the gel have a high evaporation rate.

Since the alcohol is replaced with a formamide aqueous solution that has a low evaporation rate, cracks and cracks can be prevented during the process of forming a dry gel. In addition, when drying gel, the evaporation rate is low, so there is no need to put it in a container and cover it with a lid, as in the conventional case, and there is no problem even if it is dried in an exposed state.

(Example) The present invention will be explained by examples.

Example 1 Dilute hydrochloric acid (0.1 mol/l) was added to silicon tetramethoxide Si (OCH3)4 at a volume ratio of 2=1 and stirred with a magnetic stirrer to form immiscible 2.
A liquid, homogeneous silica hydrosil was obtained. The diameter of the sol is 9
The mixture was placed in a glass Petri dish coated with Teflon (0[[ll[+]), sealed, and kept at 60°C to gel.

Once the gel was separated from the container, it was opened in a water bath and the gel was taken out. The removed gel was immersed in a 50 vol. 1% formamide aqueous solution overnight. Remove the gel and place it on a Teflon-coated plate with a smooth surface in air at 40°.
It was dried at C to obtain a dry gel. The dried gel obtained had no cracks or cracks.

Example 2 A dry gel was obtained by carrying out the same operation as in Example 1, except that dilute nitric acid (0.2 mol/l) was added to silicon tetramethoxide Si (OCH3)4 at a volume ratio of 2:1 and stirring. Obtained. The resulting dried gel has cracks.

There were no cracks at all.

Example 3 The same operation as in Example 1 was performed except that dilute hydrochloric acid (0.1 mol/l) was mixed with silicon tetraethoxide S i (OC2H5)4 at a volume ratio of 2:1 and stirred. Got the gel. The dried gel obtained did not have any splits or cracks.

Example 4 Silicon tetramethoxide S i (OCH3)4
and dilute hydrochloric acid (0.1 mol/l) in a volume ratio of 2:1.
When mixed and stirred at a ratio of 100 to 100%, a homogeneous silica hydrosil was obtained from an immiscible solution. Place the sol in a Teflon-coated glass petri dish with a diameter of 90 mm and seal it.
It was kept at 60°C to form a gel. Once the gel separates from the container,
It was opened and taken out in an aquarium. The removed gel was washed with 10 vo
1% formamide aqueous solution overnight. The gel was removed and dried on a Teflon-coated smooth surface dish in air at room temperature to obtain a dry gel.

The dried gel obtained had no cracks or cracks.

Example 5 Polycondensate of silicon tetramethoxide (CHsO)3
Si ・(O8i (OCH3)2 ), @08i
(OCH3)! , (centered at n=3) was used as the raw material, but a dry gel was produced in the same manner as in Example 1. The dried gel obtained did not have any splits or cracks.

Example 6 When dilute hydrochloric acid (0.1 mol/!) was added to silicon tetramethoxide (Si (OCHri4) at a volume ratio of 2:1 and stirred, heat was generated, and a homogeneous silica sol was formed from the two immiscible liquids. The sol was placed in a Teflon-coated glass Petri dish with a diameter of 90ff IuI and sealed.
It was kept at 50°C to form a gel. Once the gel was separated from the container, it was opened in a water tank and the gel was taken out.

The removed gel was heated to 50% by weight containing 5 wt% sodium nitrate.
It was immersed in a vol% formamide aqueous solution overnight.

The gel was removed and dried on a Teflon-coated smooth surface dish in air at 50°C to obtain a dry gel. There were no breaks or cracks in the dried gel.

Example 7 Silicon tetramethoxide polycondensate (CH30)3S
r・(O8i(OCH3)z ), -osi(OCHa
)s, (with n=3 as the center) was used as the raw material, but was produced in the same manner as in Example 1. There were no breaks or cracks in the dried gel obtained.

(Effects of the Invention) According to the present invention, cracks and cracks can be prevented from occurring during the gel drying process, and a monolithic large-sized dried gel can be obtained. Furthermore, since the gel can be dried while being exposed, the work is easier.

Agent Patent Attorney Kunihiko Wakabayashi; Procedural Amendment (Voluntary) February 2513, 1985

Claims (1)

[Claims] 1. A silicon alkoxide represented by the general formula Si(OR)_4 (where R is an alkyl group) and/or a polycondensate thereof is hydrolyzed to form a silica sol, then left to gel, dried and In the method for producing sintered silica glass, the gel before drying is immersed in an aqueous formamide solution and then taken out;
A method for producing silica glass characterized by drying. 2. The method for producing silica glass according to claim 1, wherein the formamide aqueous solution contains a water-soluble salt of an alkali metal and/or an alkaline earth metal.