KR20040056544A - A method of preparing transparent silica glass - Google Patents

Abstract

PURPOSE: Provided is a process for producing transparent silica glass having compactness close to the theoretical density by vacuum-sintering a dried article produced from a silica sol. CONSTITUTION: The process contains the steps of: preparing a hydrolysate by injecting a Si-based alkoxide to water; preparing a water slurry by dispersing a fine silica powder in water; preparing silica granules by injecting the Si-based alkoxide hydrolysate into the water slurry and drying, mechanically grinding, and calcining; obtaining the silica sol by dispersing the silica granules in water and milling; dispersing by injecting a high alkali dispersant into the silica sol and reducing a pH by adding a gelating agent; injecting the silica sol into a mold and gelating in the mold; and obtaining a humid gel by detaching the gelated article from the mold, drying the humid gel, heat-treating and sintering under the vacuum. The weight ratio of the silica powder and the solid content of the Si-based alkoxide hydrolysate is 99:1-1:99.

Description

A manufacturing method of transparent silica glass {A METHOD OF PREPARING TRANSPARENT SILICA GLASS}

Field of invention

The present invention relates to a method for producing transparent silica glass, and more particularly, to a method for producing transparent silica glass having a density close to the theoretical density by vacuum sintering a dry molded product prepared from a silica sol.

Prior art

As a method of manufacturing silica glass, the sol-gel process is a liquid phase process, which has high productivity and can freely control the composition of the product, and has an advantage of high economical efficiency since the manufacturing process is generally performed at a low temperature as compared to the existing method. Numerous methods have been attempted to produce bulk glass, in particular high silica glass, via a sol-gel process. As one of these methods, a sol-gel process using an alkoxide has been proposed. However, this method is limited in size and has a disadvantage of low economic efficiency.

In order to solve this problem, a sol-gel process using an ultrafine powder small enough to have colloidal properties has been proposed. According to this method, it is possible to manufacture a large size silica glass of a certain size or more, and to economically manufacture an over-cladding tube for an optical fiber, or a high purity silica glass for semiconductor manufacturing and various other applications. Brief description of the technique for preparing silica glass through the sol-gel process using such ultrafine particles is as follows.

In the sol-gel process, a process of (1) preparation of silica sol, (2) gelation of sol, (3) drying of gel, and (4) vitrification through heat treatment of dry gel are performed. Various processes are provided for the processes of (1) to (3), and the vitrification process through heat treatment of the dry gel (4), which is the final densification process, generally removes organic matter from the gel by first heat treatment of the dried gel. Subsequently, the gel from which the organic substance has been removed is sintered in a hydroxyl group and an impurity removing step and in an inert gas atmosphere such as air or helium to complete silica glass of high purity.

According to this method, large silica substrates can be prepared relatively easily through an adopted sol-gel process. However, in the case of sintering in air, it is very difficult to manufacture transparent silica glass that is close to the theoretical density due to excessive tendency of crystallization and excessive presence of residual pores in the sintered body as it rises to a high temperature during sintering.

On the other hand, when sintering under a gas atmosphere such as helium, which is the most widely known sintering method, the sintering performance is superior to that in air, and the side effects of crystallization of glass can be eliminated as much as possible. In the case of sintering under helium atmosphere, incomplete sintering still leaves an excessive amount of fine bubbles in the sintered body, which has the disadvantage of degrading the final glass. In order to remove such fine bubbles, a method of raising the sintering temperature to a higher temperature than that used in the sintering under helium atmosphere is used. Removal of microbubbles is not satisfactory.

In order to solve the above problems, the present invention is to provide a method for producing a transparent silica glass having a density close to the theoretical density.

In order to achieve the above object, the present invention comprises the steps of adding a Si-based alkoxide to water to make a hydrolyzate;

Dispersing the fine silica powder in water to prepare an aqueous slurry;

Adding a Si-based alkoxide hydrolyzate to the aqueous slurry and then drying, mechanically grinding and calcining to produce granule silica;

Redispersing the granular silica in water and milling to obtain a silica sol;

Adding a high alkaline dispersant to the silica sol to disperse and then adding a gelling agent to reduce the pH;

Injecting the silica sol into a mold to gel it in the mold; And

It provides a method for producing a silica glass comprising the step of sintering in a vacuum atmosphere after drying, heat treatment the wet gel obtained by removing the gelled molded body from the mold.

Hereinafter, the present invention will be described in more detail.

The first step of the silica glass production method of the present invention is to prepare a Si-based alkoxide hydrolyzate. Si-based alkoxide hydrolyzate is prepared by adding Si-based alkoxide to water and stirring. Preferred examples of Si-based alkoxides include tetraethylorthosilicate (TEOS; Si (OC ₂ H ₅ ) ₄ ), tetramethylorthosilicate and the like. Si-based alkoxide hydrolyzate has better sinterability than fumed silica and is much finer, thereby filling the gap between silica particles, densifying the molded body and improving sinterability.

Then fine silica powder is dispersed in water to prepare an aqueous slurry. The silica powder preferably has a specific surface area of 20 to 50 m ² / g and ultrafine particles having an average particle size of 30 to 100 nm. Fumed silica, which is commonly used, can be preferably used.

Si-based alkoxide hydrolyzate is added to the prepared aqueous slurry to have some fluidity. In order to increase the degree of mixing and dispersion during mixing, ball milling using a quartz ruler and a ball yields a slurry having a very low viscosity and excellent dispersibility. The ratio of the solid phase content of the silica powder and the Si-based alkoxide hydrolyzate is blended in a ratio of 99: 1 to 1:99 by mass ratio. The blending ratio of distilled water to total silica in the slurry is in the range of 5 to 80 wt%.

The silica aqueous slurry in the sol state thus obtained is completely dried to form a silica mass by granulation by a dry mechanical grinding process, and then calcined to obtain solid silica granules. In this case, the specific surface area of the silica granules is almost similar to that of the dry formed body.

Silica sol is prepared by redispersing the granular silica in water and then milling. At this time, a small amount of dispersant may be added. When the granular silica is put in water and ball milled using quartz jars and balls for grinding and dispersion, a silica sol having a very low viscosity is obtained. The optimum dispersion is determined by measuring the average particle size change and particle size distribution of the mill with ball milling time. Generally, the average particle size is 1 μm or less, preferably 0.5 μm or less, and the particle size distribution is a radian type. The optimum solids content in the silica sol is 35 to 55% by weight.

After dispersing by adding a highly alkaline dispersant to the silica sol, a gelling agent is added to reduce the pH. As the highly alkaline dispersant and the gelling agent, all compounds used in the production of conventional silica glass may be used, and are not limited to the compounds described below.

The highly alkaline dispersant raises the pH of the silica sol to 11 or more. Such highly alkaline dispersants include ammonium salt compounds, specific examples of which are quaternary ammonium hydroxide tetramethylammonium hydroxide (TMAH), or tetraethylammonium hydroxide (TEAH). Dispersants not only help the silica to be uniformly dispersed, but also serve to electrostatically stabilize the sol in which the silica is dispersed.

As the gelling agent, water-soluble aliphatic esters of acids may be used, and preferred examples thereof include ethyl lactate, methyl lactate, ethyl formate, methyl formate, ethyl glycolate and methyl glycolate. This gelling agent reacts with the high alkaline dispersant to form an ionic compound, and serves to reduce the pH of the silica sol, which is raised by the addition of the high alkaline dispersant. The gelling agent is added in an amount of 1.2 to 1.4 equivalents based on 1 equivalent of the high alkaline dispersant.

The silica sol prepared as described above is preferably injected into the mold while maintaining fluidity before the gelation reaction occurs. After the injection into the mold, it is preferable to seal in order to prevent evaporation of moisture. The pH of the silica sol injected into the mold begins to drop rapidly with time and solidifies rapidly when the pH is below 9.5. The solidification reaction time is influenced by the amount of solids, the amount of gelling agent and the reaction temperature, but it is generally preferred to be within 5 to 10 minutes.

Once solidification is complete, the gel will have sufficient strength and after a period of time the gel is removed from the mold. Drying of the wet gel taken out of the mold is preferably carried out at a temperature of 30 to 80 ° C. and a relative humidity of 60 to 80%.

The primary heat treatment process of the dried gel is carried out at a temperature increase rate of 5 to 50 ℃ / hr at a temperature of 300 to 600 ℃. This primary heat treatment removes organic matter present in the gel. The first heat treated gel is second heat treated at a temperature rising rate of 100 ° C./hr at a temperature of 500 to 1000 ° C. During this secondary heat treatment process, a high purity process is performed using a chlorine gas atmosphere, in which residual hydroxyl groups and other metallic impurities are removed. At the same time, oxygen or a fluorine compound gas is flowed to remove the chlorine component remaining in the silica molded body. At this time, the heat treatment is preferably carried out at a heating rate of 100 ℃ / hr at 900 to 1100 ℃.

In the present invention, unlike the conventional method of sintering the secondary heat-treated molded body in the air or helium atmosphere as described above, it is sintered in a vacuum atmosphere to provide silica glass which is excellent in transparency and close to the theoretical density (2.202 g / cm ³ ). do. The vacuum sintering process is carried out in a vacuum atmosphere of 10 ^-1 to 10 ^-6 Torr (Torr) to a temperature rising rate of 100 ℃ / hr up to 1200 to 1700 ℃ heat treatment at that temperature for 0.1 to 20 hours.

The following presents a preferred embodiment to aid the understanding of the present invention. However, the following examples are provided only to more easily understand the present invention, and the present invention is not limited to the following examples.

Examples and Comparative Examples

Example 1

First, 200 g of tetraethylorthosilicate (TEOS; Si (OC ₂ H ₅ ) ₄ ) was mixed with 800 g of distilled water with Si-based alkoxide and stirred vigorously at room temperature for 24 hours to complete hydrolysis. The temperature was again raised to 80 ° C. and maintained for 4 hours or more. At this time, the vessel in which the reaction is progressing was sealed so that there was no loss of moisture. After 4 hours at 80 ° C., the gelation reaction was completely completed. The obtained silica gel has a structure in which several hundreds to several tens of silica particles are connected.

An aqueous slurry was prepared by mixing 200 g of silica powder (Aerosil OX-50 (Degussa, Germany)) with a specific surface area of 50 m ² / g or less and 40 nm in average with distilled water. 1200 g of this aqueous slurry was mixed with 1000 g of TEOS hydrolyzate and placed in a high speed blender forcibly mixed for 10 minutes to maintain fluidity to some extent. In order to improve the mixing and dispersion of the aqueous slurry, the ball was milled for 8 hours using a quartz jar and a ball to obtain a slurry having a very low viscosity and a good dispersion. The prepared slurry is present in a sol state by uniformly mixing the ultrafine particles of Å unit obtained through the hydrolysis of TEOS and aero-sil with an average particle size of 40 nm.

The sol slurry was heated to 110 ° C. and maintained for at least 48 hours to obtain a completely dried silica mass. The silica mass was dried into granules of about 0.5 mm by dry grinding. The granules were calcined at 800 ° C. for 10 hours in air to obtain solid silica granules.

1000 g of silica granule powder was mixed with 1000 g of distilled water, and 0.3% of DARVAN-C was added thereto as a dispersant to aid dispersion. The mixture was first placed in a high speed blender and mixed for 10 minutes to prepare a slurry having considerable fluidity. The slurry was ball milled for 8 hours using a quartz jar and a ball to grind and disperse to obtain a silica sol having a very low viscosity.

Strong alkali TMAH (Tetramethyl ammonium hydroxide) was added to the silica sol in a mass ratio of 2.8 wt% to silica and stirred for at least 1 hour to increase the pH of the sol to 11 or more. Again, the sol was aged for 24 hours at room temperature, and then degassed in a vacuum vessel to remove bubbles. To this silica sol was added 1.2 to 1.4 equivalents of methyl formate (Methyl Formate) in an equivalent ratio to TMAH with slow stirring. Fully stirred silica sol was injected into the cone mold. After mold injection, the mold was sealed to prevent evaporation of moisture. As time passed, the pH of the silica sol injected into the mold began to drop sharply and rapidly solidified below pH 9.5. Once solidification is complete, hold for at least 24 hours to ensure that the gel has sufficient strength. After a period of time, the gel was removed from the mold and placed on a smooth surface. The obtained gel molded product is different depending on the thickness of the gel, but is maintained at room temperature for 14 days or more to remove 98% of moisture. The obtained dried gel was further heated to 110 ° C. for 24 hours and maintained at that temperature for 24 hours to completely remove the adsorbed water. The dried gel was heated to 500 ° C. at a temperature increase rate of 5 ° C./h, and heat-treated at this temperature for 5 hours to remove organic matter in the dried gel. The organic-free gel was heated to 900 ° C. at 100 ° C./hr and maintained at this temperature for 5 hours. At this time, the heat treatment process was carried out in a chlorine gas atmosphere to remove the hydroxyl group and metal impurities. Further, after performing chlorine gas heat treatment to 900 ° C, the chlorine component remaining in the silica molded body was removed by maintaining the oxygen atmosphere while raising the temperature to 1100 ° C at a rate of 100 ° C / hr.

Finally, silica glass was prepared by heating up to 1250 ° C. at a heating rate of 100 ° C./hr under a vacuum atmosphere of 10 ⁻⁴ Torr and sintering at this temperature for 1 hour.

Example 2

Silica glass was prepared in the same manner as in Example 1 except that the sintered at 1400 ° C. under vacuum of 10 ⁻³ Torr during vacuum sintering.

Comparative Example 1

Silica glass was prepared in the same manner as in Example 1 except that the mixture was sintered in a helium atmosphere.

Comparative Example 2

Silica glass was prepared in the same manner as in Example 1 except that the sintered in a helium atmosphere instead of a vacuum atmosphere.

The amount of residual microbubbles in the silica glass tubes prepared according to the above Examples and Comparative Examples was analyzed using a laser scattering method. Compared to the silica glass obtained in Comparative Example 1, it contained about 40 vol% less residual microbubbles than the silica glass obtained in Examples 1 and 2.

According to the method of manufacturing the silica glass of the present invention, compared to the case of vacuum sintering a dry molded body of a silica sol and sintering in a conventional air atmosphere or helium atmosphere, fine bubbles are reduced, transparency is high, and dense transparent silica is close to theoretical density. Glass can be prepared.

Claims (3)

Adding a Si-based alkoxide to water to make a hydrolyzate; Dispersing the fine silica powder in water to prepare an aqueous slurry; Adding a Si-based alkoxide hydrolyzate to the aqueous slurry and then drying, mechanically grinding and calcining to produce granule silica; Redispersing the granular silica in water and milling to obtain a silica sol; Adding a high alkaline dispersant to the silica sol to disperse and then adding a gelling agent to reduce the pH; Injecting the silica sol into a mold to gel it in the mold; And A method of producing silica glass, comprising the step of drying the wet gel obtained by removing the gelled molded body from the mold, followed by sintering under vacuum atmosphere after drying and heat treatment. The method of claim 1, wherein the ratio of the solid phase content of the silica powder and the Si-based alkoxide hydrolyzate is 99: 1 to 1:99 by mass ratio. The method of claim 1, wherein the sintering in a vacuum atmosphere is carried out at a temperature of 1200 to 1700 ℃ at a vacuum pressure of 10 ^-1 to 10 ^-6 Torr.