KR20040056547A - A method of preparing transparent silica glass - Google Patents

Abstract

PURPOSE: Provided is a process for producing transparent silica glass reducing fine pores and having excellent transparency and compactness close to the theoretical density by vacuum-sintering a dried article of a silica sol. CONSTITUTION: The process comprises the steps of: preparing the silica sol by injecting a silica powder and a high alkali dispersant into deionized water in which a polyethylene imine(PEI) having a molecular weight of 1,000-500,000 is dissolved; adding a gelating agent to the silica sol to reduce a pH; injecting the resultant sol into a mold and gelating; detaching the gelated article from the mold to obtain a humid gel; drying the humid gel; firstly heat-treating the dried gel at 300-600deg.C; secondly heat-treating the resultant at 500-1000deg.C; and sintering the resultant under a vacuum atmosphere.

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, the incomplete sintering still leaves an excessive amount of fine bubbles in the sintered body, which has the disadvantage of degrading the final glass quality. 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 preparing a silica sol by adding silica powder and a highly alkaline dispersant to deionized water in which polyethyleneimine is dissolved; Adding a gelling agent to the silica sol to reduce the pH; Injecting the sol into a mold to gel it in the mold; Removing the gelled formed body from the mold to obtain a wet gel; Drying the wet gel; Primary heat treating the dried gel at 300 to 600 ° C .; Second heat treatment of the heat-treated resultant at 500 to 1000 ° C; And it provides a method for producing a silica glass comprising the step of sintering the secondary heat-treated product in a vacuum atmosphere.

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

The first step of the silica glass production process of the present invention is to prepare a silica sol. The silica sol is prepared by mixing silica powder, a highly alkaline dispersant and polyethyleneimine-containing deionized water and then adding a gelling agent. Bubbles may be removed using a vacuum pump before or after the gelling agent is added.

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 double commercially available, may be preferably used.

Polyethyleneimine serves to increase the strength of the gel by being located between the silica particles when the gelling reaction of the silica sol proceeds as a binder to promote the adhesion between the particles. The molecular weight of polyethyleneimine is preferably between 1,000 and 500,000.

The high alkaline dispersing agent and the gelling agent in the above components can be used for all compounds used in the production of conventional silica glass, it is not limited to the compounds described below.

The highly alkaline dispersant raises the pH of the silica sol to 10 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 silica sol prepared as described above is preferably injected into the mold while maintaining fluidity before the gelation reaction occurs. 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 performed 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 by heating to a temperature of 1200 to 1700 ℃ in a vacuum atmosphere of 10 ^-1 to 10 ^-6 Torr (Torr) for 0.1 to 20 hours at that temperature.

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

Silica powder (Aerosil OX-50 (Degussa, Germany)) 1000 g of specific surface area of 50 m ² / g or less and average particle size of 40 nm or less, 1000 g of deionized water pre-dissolved 0.2 g of polyethyleneimine and tetramethylammonium hydroxide 200 g of (TMAH) was sufficiently mixed using a blending apparatus to obtain a sol in which silica powder was uniformly dispersed. Then, the sol was defoamed sufficiently for at least 10 minutes by using a vacuum pump, and the sol was stabilized by standing in air at least 24 hours. 50 g of ethyl lactate was added to the sol, followed by uniform mixing, and the mixture was injected into a tube forming mold while maintaining fluidity before gelation occurred. When gelation was completed, the wet gel was separated from the mold and dried for 72 hours in a thermo-hygrostat adjusted to 25 ° C and 80% relative humidity. Then, the dried gel was heated up to 500 ° C. at a temperature increase rate of 10 ° C./hr, and the organic material in the dried gel was removed by first heat treatment at this temperature for 5 hours. The organic material-free gel was heated to 900 ° C. at 100 ° C./hr and subjected to a second heat treatment, and maintained at this temperature for 5 hours. At this time, the secondary heat treatment process was carried out in a chlorine gas atmosphere to remove hydroxyl groups and metal impurities. Then, the chlorine component remaining in the silica molded body was removed by maintaining the oxygen atmosphere while raising the temperature at a rate of 100 ° C./hr to 1100 ° C. Finally, a silica glass tube was prepared by heating up to 1350 ° 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 tubes were 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 tubes were prepared in the same manner as in Example 1 except that the mixture was sintered in air instead of in a vacuum atmosphere.

Comparative Example 2

A silica glass tube 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. The silica glass obtained in Comparative Examples 1 and 2 contained more than 50 vol% of the remaining fine bubbles 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 molding agent of silica sol and sintering in a conventional air atmosphere or helium atmosphere, fine bubbles are reduced, transparency is high, and the density is close to theoretical density. Silica glass can be prepared.

Claims (4)

a) preparing a silica sol by adding silica powder and a highly alkaline dispersant to deionized water in which polyethyleneimine (PEI) is dissolved; b) reducing the pH by adding a gelling agent to the silica sol; c) gelling in the mold by injecting the sol prepared in step b) into the mold; d) removing the gelled formed body from the mold to obtain a wet gel; e) drying the wet gel; f) first heat treating the dried gel at 300 to 600 ° C; g) second heat treatment of the heat-treated resultant at 500 to 1000 ° C; And h) sintering the secondary heat-treated product under vacuum atmosphere Silica glass manufacturing method comprising a. The method of claim 1, wherein the polyethyleneimine has a molecular weight of 1,000 to 500,000. The method of claim 1, further comprising removing bubbles present in the silica sol by using a vacuum pump before or after the gelling agent of step b). 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.