JPS62158123A - Preparation of silica glass - Google Patents

Abstract

PURPOSE:To prevent generation of crack or cleavage during drying of silica gel in the sol-gel process by carrying out hydrolysis of silicon alkoxide in aq. soln. contg. H3BO3, and dipping the gel prior to drying in aq. soln. of formamide. CONSTITUTION:Silicon alkoxide expressed by the general formula Si(OR)4 (where R is alkyl group) and/or a polycondensation product of the silicon alkoxide is hydrolyzed in aq. soln. contg. ca. 0.05-1mol/l H3BO3. Generated silica sol is transferred quickly to a vessel having smooth inside surface coated with Teflon(R), etc., and sealed tightly and gelled at ca. room temp. -80 deg.C. Then, the gel is discharged from the vessel and dipped in aq. soln. contg. >=ca. 10vol% formamide to replace alcohol in the gel with the aq. formamide soln. After finishing the replacement, the gel is discharged from the vessel and contracted and solidified in the air at ca. 50-60 deg.C to obtain dry gel. By this method, dry gel contg. no cracks nor cleavage is obtd. and monolithic large sized dry gel is prepd.

Description

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

This invention relates to a method for producing silica glass containing a trace amount of B2O3 used for physical and chemical purposes.

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

This is a sol-gel method.

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

Clicone alkoxide and/or polycondensate represented by the general formula Si (OR)4 (where R is an alkyl group) are polycondensates 2 thereof, such as (RO)s Si・(O8i (OR)2
). - Add water (pH may be adjusted with an alkali or acid) to 08i(OR)s (n = 0 to 8. R is an alkyl group) and hydrolyze it to obtain silica hydrosyl. At this time, a suitable alcohol may be added as a solvent so that the cricon alkoxide and water form a homogeneous system. Ultrafine silica particles or the like may be added. This silica hydrosil is placed in a container of a desired shape and gelled by standing, heating, or adding a gelling agent. Further, the gel is evaporated to dryness to obtain a dry gel.

Silica glass is obtained by sintering this dried gel in an appropriate atmosphere.

This sol-gel method has the following features.

(Glass soot is deposited using an oxyhydrogen flame using 115iCl!4 as a raw material.) It can be manufactured at a lower temperature than the conventional manufacturing method of silica glass, so it is energy saving and low cost.

(2) Since the raw material is liquid, purification is easy and a highly pure product can be obtained.

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

Silica glass uniformly doped with various dopants is obtained.

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

(Problems to be Solved by the Invention) However, the sol-gel method has the problem that cracks and fractures 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. A method has been used to control the evaporation rate of water and alcohol by making a pinhole 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.

(Means for Solving the Problem) The inventors discovered that because the evaporation rate of alcohol produced by hydrolysis of silicon alkoxide or alcohol added as a solvent is high, it is difficult to 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 with an aqueous formamide solution, thereby achieving the above object, and have completed the present invention.

In the present invention, a silica sol is obtained by hydrolyzing a silicon alkoxide and/or a polycondensate thereof represented by the general formula S r (OR) 4 (where R is an alkyl group).

A method for producing silica glass in which the hydrolysis is then carried out in an aqueous solution containing 1-1BO3, and the gel before drying is immersed in an aqueous formamide solution and then 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 can be easily replaced with an aqueous formamide solution. As a polycondensed metal of silicon alkoxide, 2
For example, (C) (a O)3S i・(O8i (OCH
3)2).・O8i (OCHs)a can be raised. H3BO3 in aqueous solution added for hydrolysis
The amount of is preferably 0.05 to 1 mol/l.

In addition to H2PO4, other acids may be added to adjust the pH. The silicon alkoxide and/or its polycondensate and the aqueous solution are mixed using a stirrer or the like in order to make the resulting silica sol uniform.

The generated silica sol is quickly transferred to a container with a smooth inner surface coated with Teflon or the like and allowed to gel. During gelation, it is preferable to seal the container in order to prevent evaporation of alcohol from the generated gel, and the temperature during gelation is preferably room temperature to 80°C.

Next, this gel is taken out from the container and immersed in an aqueous formamide solution to replace the alcohol in the gel with the aqueous formamide solution. This formamide aqueous solution may contain H3BOa. Furthermore, by incorporating water-soluble salts of alkali metals and/or alkaline earth metals, silica glass containing these metals can be obtained. The operation of removing the gel from the container is preferably carried out in a water bath or in an aqueous formamide solution in order to prevent evaporation of alcohol from the gel. Formamide aqueous solution contains formamide at 10 Vol.
Those containing 4 or more are preferred. 10 Vol! If it is less than %, smooth progress of substitution is insufficient. After the substitution, the gel is taken out from the formamide aqueous solution, placed on a plate with a smooth surface, dried in air, and solidified by shrinkage to form a dry gel.

Drying is preferably carried out quickly at 50 to 60°C, since this can prevent the precipitation of HsBOs.

(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 it is replaced with an aqueous formamide solution that has a lower evaporation rate than alcohol, it is possible to prevent cracks and cracks 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 Silicon tetramethoxide (Si(0CH3)4)
Dilute hydrochloric acid containing 0.5 mol/l of H3B03 (
0.1 mol/l) aqueous solution was added at a volume ratio of 2:1 and stirred with a magnetic stirrer to obtain a homogeneous silica sol from the two immiscible liquids. The diameter of the sol is 9001111
Place in a Teflon-coated glass petri dish and seal.

It was kept at 50°C to form a gel. When the gel separated from the container, it was opened in a water tank and the gel was taken out. The removed gel was 0.1
50voJ% containing mol/l H3B Os
It was immersed in an aqueous formamide solution overnight. The gel was removed and dried in air at 50° C. on a smooth Teflon-coated dish to obtain a dry gel. There were no breaks or cracks in the dried gel.

Example 2 Silicon tetramethoxide polycondensate (CH30)3S
r (O8+ (OC) (3) 2) nosi (QC)
The gel was produced in the same manner as in Example 1 except that 31 (centered at '=3) was used as the raw material.The dried gel obtained had no cracks at all.

(Effects of the Invention) According to the present invention, it is possible to prevent the occurrence of cracks and cracks during the gel drying process, and a large monolink dried gel can be obtained. Another advantage is that the gel can be dried with it exposed, making the work easier.

Since the glass contains 8203, it can be sintered at a lower temperature than glass containing silica alone.

Agent Patent Attorney Kunihiko Wakabayashi −゛

Claims (1)

[Claims] 1. Silicon alkoxide and/or its polycondensate represented by the general formula Si(OR)_4 (where R is an alkyl group) is hydrolyzed to form a silica sol, then left to gel, dried, and then sintered to produce silica glass. A method for producing silica glass, characterized in that the hydrolysis is carried out in an aqueous solution containing H_3BO_3, and the gel before drying is immersed in an aqueous formamide solution and then taken out and dried.