KR100722379B1 - A method of preparing transparent silica glass - Google Patents

Abstract

The present invention comprises the steps of first dispersing the fine silica powder followed by drying, mechanical grinding and calcining to produce a granule of silica; Secondary dispersing the sintered silica granules to obtain a silica sol in a slurry state; Mixing a highly alkaline dispersant with the silica sol and then adding a coagulant; Injecting the obtained 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.

Silica Glass, Vacuum Sintered, Sol-Gel

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 that is 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 most widely known as an improved sintering method, it shows an excellent sintering performance as compared to sintering in air and has the advantage of eliminating side effects due to crystallization of glass to the maximum. However, when sintering in a helium atmosphere, there is a disadvantage in that excessive fine bubbles remain in the sintered body due to incomplete sintering, thereby 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 granule of silica by first dispersing fine silica powder, followed by drying, mechanical grinding and calcining; Secondary dispersing the sintered silica granules to obtain a silica sol in a slurry state; Mixing a highly alkaline dispersant with the silica sol and then adding a coagulant; Injecting the obtained 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. In the present invention, fumed silica having an average particle size of 30 to 100 nm is used. It is not a problem when the amount of the silica powder is small when the ultrafine silica powder is directly dispersed in water, but when the amount of the silica powder is more than 40% by weight of the silica sol, the powder is fine and the viscosity increases rapidly and dispersion is There is a problem that becomes extremely difficult.

Therefore, in the present invention, in order to improve the dispersibility of the fine high purity silica powder, a silica dispersion is prepared through a process of primary dispersion, drying, mechanical grinding, calcining, and secondary dispersion, and then a silica sol is prepared therefrom.

The manufacturing process of the silica dispersion is described in detail as follows. Fine silica powder is added to water to obtain a homogeneously dispersed primary silica dispersion. The silica dispersion was heated to 110 ° C. and maintained for at least 48 hours to form a completely dried silica mass. This agglomerated silica is used to dry granulate the granular silica powder having an average particle size of about 0.5 mm. The granulated silica powder is calcined at 900 to 1100 ° C. for 1 to 5 hours to produce solid silica granules. At this time, the surface area of the silica powder of the granules is hardly reduced compared to the surface area of the silica powder used in preparing the primary dispersion. The silica powder of the granules has a microstructure of granules in which primary particles having an average particle size of 40 nm are combined to form secondary particles. The microstructure of these granules prevents breakage during final drying and facilitates dispersion of the silica particles. Silica sol in a slurry state is prepared by secondary dispersion by adding silica powder of sintered granules through calcination to water. In the second dispersion, 0.1 to 3 wt% of a dispersant may be added, and as the dispersant, a conventional dispersant used in preparing a conventional silica sol may be used. The silica sol is prepared by preparing a slurry having considerable fluidity using a high speed blender and the like and then milling to have a low viscosity. The optimum dispersion by the milling process is determined by measuring the average particle size distribution of the pulverized product according to the milling time. In general, the average particle size is preferably 1 μm or less, more preferably 0.5 μm or less, and the particle size distribution has a radian type. It is preferable.

Finally, a highly alkaline dispersant is added to the prepared silica sol to increase the pH of the sol to 11 or more. Ammonium hydroxide is used as the high alkaline dispersant, and specific examples thereof include tetramethylammonium hydroxide (TMAH) or tetraethylammonium hydroxide (TEAH). It is preferable that the usage-amount of a highly alkaline dispersing agent is 1 to 10 weight%. Degassing can then be carried out in a vacuum vessel to remove bubbles present in the silica sol.

To the silica sol, 1.2 to 1.4 equivalents of coagulant (gelling agent) is added to 1 equivalent of the high alkaline dispersant. Examples of coagulants include ethyl lactate, methyl lactate, ethyl formate, methyl formate, and the like.

The silica sol prepared as described above is preferably injected into the molding mold while maintaining fluidity before the gelation reaction occurs. As time passes, the pH of the silica sol injected into the mold begins to drop rapidly, and when the pH falls between pH 7.5 and 9.5, it solidifies rapidly. The time to completion of the coagulation reaction is influenced by the amount of solids and the amount of coagulant, but it is generally good within 5 to 10 minutes. The coagulation reaction can be shortened further by raising the temperature. 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 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 100 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 having a specific surface area of 50 m ² / g or less and an average particle size of 40 nm or less (Aerosil OX-50 (Degussa, Germany)) was dispersed in distilled water at a weight ratio of 30/100 to obtain a homogeneously dispersed silica sol. The sol 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 in air at a temperature between 900 and 1100 ° C. for about 1 to 5 hours to obtain solid silica granules. The powder sintered by the calcination process was mixed with 1000 g of distilled water, and 0.3 wt% of Darvan-C was added thereto as a dispersant to aid dispersion. This mixture was first placed in a high speed blender and mixed for 10 minutes to prepare a silica dispersion in the form of a slurry having considerable fluidity. This was again ball milled for 8 hours using a quartz jar and a ball to obtain a silica sol having a very low viscosity.

2.8% by weight of tetramethylammonium hydroxide (TMAH) was added to the silica sol, followed by stirring for 1 hour to 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. 1.2-1.4 equivalents of methyl formate was added slowly to the silica sol with respect to 1 equivalent of TMAH. The sufficiently stirred sol was injected into the tube forming mold. After injection of the silica sol the mold was sealed to prevent evaporation of moisture. Once gelling (coagulation) of the silica sol is complete, hold for at least 24 hours to ensure that the gel has sufficient strength. After some time the gel was removed from the mold and placed on a smooth surface. The obtained gel molded body was kept at room temperature for 14 days or more to remove 98% of moisture. The obtained dried gel was further heated up to 110 ° C for 24 hours and kept at that temperature for 24 hours to completely remove the adsorbed water.

Then, 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 hydroxyl groups and metal impurities. Furthermore, after performing chlorine gas heat treatment to 900 degreeC, the chlorine component which remained in a silica molded object was removed by maintaining an oxygen atmosphere, heating up at the rate of 100 degreeC / hr to 1100 degreeC. Finally, the silica glass tube was manufactured by heating up to 1400 ° C. at a temperature rising rate of 100 ° C./hr under a vacuum atmosphere of 10 ⁻² Torr and sintering at this temperature for 1 hour.

Example 2

A silica glass tube was prepared in the same manner as in Example 1 except that the product was sintered under a vacuum of 10 ^-4 Torr during vacuum sintering.

Comparative Example 1

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.

As a result of measuring the residual pore amount by using the laser scattering method, the silica glass prepared according to Comparative Example 1 contained more than 30 vol% of the remaining fine bubbles compared to the silica glass prepared according to 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 (6)

a) first dispersing the fine silica powder, followed by drying, mechanical grinding and calcining to produce granule silica; b) secondarily dispersing the sintered silica granules to obtain a silica sol in a slurry state; c) mixing a high alkaline dispersant into the silica sol and then adding a coagulant; d) injecting the obtained sol into the mold to gel it in the mold; e) removing the gelled molded body from the mold to obtain a wet gel; f) drying the wet gel; g) first heat treating the dried gel at 300 to 600 ° C .; h) secondary heat treatment of the heat treated product at 500 to 1000 ° C; And i) sintering the secondary heat-treated product under vacuum atmosphere Silica glass manufacturing method comprising a. The method of claim 1, wherein the silica powder of step a) is fumed silica having an average particle size of 30 to 100 nm. The method of claim 1, wherein the silica sol after the second dispersion of step b) has an average particle size of 1 µm or less and a particle size distribution having a radian type. The method of claim 1, wherein the coagulant of step c) is used in an amount of 1.2 to 1.4 equivalents based on 1 equivalent of the highly alkaline dispersant. The method of claim 1, further comprising removing bubbles present in the silica sol by using a vacuum pump after the step c). 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.