KR20030022953A - A composition for transparent silica glass and a method of preparing silica glass using the same - Google Patents

Abstract

PURPOSE: A transparent silica glass composition comprising silica sol, and main and auxiliary gelling agents is provided. The resultant silica glass with good transparency and near theoretical density(2.202g/cm3) is manufactured by sintering silica compacts under vacuum, not conventional air or He atmosphere. CONSTITUTION: The method for manufacturing transparent silica glass comprises the steps of: (i) preparing a silica sol by mixing 40-60pts.wt. of silica powder, fumed silica(40-100nm), 1-5pts.wt. of dispersant such as tetramethyl ammonium hydroxide(TMAH) or tetraethyl ammonium hydroxide(TEAH), and 40-60pts.wt. of deionized water; (ii) adding 0.1-12.5pts,wt, of main gelling agent such as polyethylene amide and small amounts of auxiliary gelling agents, aliphatic ester compounds such as methyl lactate and ethyl formate, and optionally 0.1-3pts.wt. of plasticizers such as glycerin and ethylene glycol; (iii) pouring the silica sol into a mold and gelling; (iv) demolding to get a bulky gel; (v) drying at 30-50deg.C for 48hrs. or more; (vi) thermal treating at 5-50deg.C/hr to 300-600deg.C for removal of organics; (vii) thermal treating at 100deg.C/hr to 500-1000deg.C in Cl2 atmosphere for removal of hydroxide ions and metallic impurities; (viii) sintering at 1200-1700deg.C for 0.1-100hrs. at a vacuum between 10-1 and 10-6torr. The step for removing foams in silica sol is optionally added to the next of step (i).

Description

A composition for manufacturing transparent silica glass and a method for producing silica glass using the same {A COMPOSITION FOR TRANSPARENT SILICA GLASS AND A METHOD OF PREPARING SILICA GLASS USING THE SAME}

Field of invention

The present invention relates to a composition for preparing transparent silica glass and a method for producing silica glass using the same, and more particularly, to a method for manufacturing transparent silica glass by vacuum sintering a composition for preparing transparent silica glass and a dried molded article prepared therefrom. .

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 in economy.

In order to solve this problem, a sol-gel process using ultrafine particles 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 still 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 composition for producing transparent silica glass close to the theoretical density.

Another object of the present invention is to provide a method for producing transparent silica glass close to the theoretical density.

In order to achieve the above object, the present invention provides a silica powder, dispersant, deionized water, polyethyleneimide as a main gelling agent and a silica glass manufacturing composition comprising a water-soluble aliphatic ester compound of the acid as an auxiliary gelling agent.

The invention also comprises the steps of preparing a silica sol by mixing silica powder, dispersant and deionized water; Adding a polyethyleneimide as a main gelling agent and a water-soluble aliphatic ester compound of an acid as a cogelling agent to the silica sol; 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 composition for producing silica glass of the present invention comprises silica powder, dispersant, deionized water, polyethyleneimide as main gelling agent and water-soluble aliphatic ester compound of acid as cogelling agent.

The silica particles preferably have a specific surface area of 2 to 50 m ² / g and ultrafine particles having an average particle size of 40 to 100 nm. Fumed silica, which is double commercially available, may be preferably used.

Dispersants in the above components can be used for all compounds used in the production of conventional silica glass. Examples of such 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 in the composition, but also serve to electrostatically stabilize the sol in which the silica is dispersed.

In the present invention, a mixture of polyethyleneimide which is a main gelling agent and a water-soluble aliphatic ester compound of an acid which is an auxiliary gelling agent is used as a gelling agent for silica sol. It is preferable that the polyethyleneimide which is a main gelling agent has a molecular weight of 1,000,000 or less. If the molecular weight exceeds 1,000,000, the viscosity increase of the silica sol increases in the addition step of the gelling agent and subsequent steps, which is not preferable. Polyethyleneimide is preferably added in the form of an aqueous solution.

Examples of the water-soluble aliphatic ester compound of the acid used as the cogelling agent include ethyl lactate, methyl lactate, ethyl formate, methyl formate, ethyl glycol, methyl glycolate and the like. This gelling agent reacts with the dispersant to form an ionic compound and serves to reduce the pH of the silica sol.

The content of each component of the composition for preparing silica glass of the present invention can be easily determined according to the desired density of the silica glass. According to a preferred embodiment of the present invention, 40 to 60 parts by weight of silica powder, 0.5 to 5 parts by weight of dispersant, 40 to 60 parts by weight of deionized water, 0.1 to 20 parts by weight of polyethyleneimide as a main gelling agent and acid as an auxiliary gelling agent Provided is a composition for preparing silica glass comprising 0.5 to 5 parts by weight of a water-soluble aliphatic ester compound. The content of the gelling agent is adjusted based on the dispersant. It is preferable that it is 0.1-12.5 weight part with respect to 100 weight part of dispersing agents, and, as for polyethyleneimide, it is more preferable that it is 1.25-6.25 weight part. The water-soluble aliphatic ester compound of the acid, which is a cogelling agent, is used in a much smaller amount than the amount used in the conventional method.

The composition for preparing silica glass of the present invention may further include a plasticizer. As the plasticizer, polyhydric alcohol may be preferably used. Specific examples thereof include glycerin, ethylene glycol, 2-methylpropane-1,2,3-triol, and the like. The plasticizer is preferably used in an amount of 0.1 to 3 parts by weight.

Silica glass manufacturing method of the present invention comprises the steps of preparing a silica sol by mixing the silica powder, dispersant and deionized water; Adding a polyethyleneimide as a main gelling agent and a water-soluble aliphatic ester compound of an acid as a cogelling agent to the silica sol; 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 sintering the secondary heat-treated result in a vacuum atmosphere.

In the present invention, the silica powder, the dispersant, and the deionized water in the components of the silica glass manufacturing composition may be mixed, and the bubbles present in the silica sol may be removed using a vacuum pump before or after adding the main gelling agent and the auxiliary gelling agent.

The silica sol prepared as described above is preferably injected into the mold while maintaining fluidity before the gelation reaction occurs. The wet gel taken out of the mold is preferably dried 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 for 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 a silica glass 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

Blending apparatus of 1000 g of silica powder (Aerosil OX-50 (Degussa, Germany)), 1000 g of deionized water and 200 g of tetramethylammonium hydroxide (TMAH) having a specific surface area of 50 m ² / g or less and an average particle size of 40 nm or less The mixture was sufficiently mixed with 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 polyethyleneimide aqueous solution and 20 g of ethyl lactate were added to the sol as a gelling agent, and then the mixture obtained by uniformly mixing 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 to 500 ° C. at a temperature increase rate of 10 ° C./hr, and the first heat treatment was performed at this temperature for 5 hours to remove organic matter in the dried gel. 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 ⁻³ 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 100 g of aqueous polyethyleneimide solution and 20 g of ethyl lactate were used as a gelling agent and sintered at 1400 ° C. under vacuum atmosphere of 10 ⁻² Torr during vacuum sintering.

Example 3

A silica glass tube was prepared in the same manner as in Example 1, except that 100 g of aqueous polyethyleneimide solution and 10 g of ethyl formate were used as a gelling agent, and sintered at 1380 ° C. under a vacuum atmosphere of 10 ⁻⁴ 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.

In Comparative Example 1, crystallization occurred during sintering, which made silica glass impossible. Comparative Example 2 was able to produce a transparent silica glass, but contained at least 50 vol% more residual microbubbles than the silica glass obtained in Examples 1 to 3. Residual bubbles in the transparent silica glass were measured using a laser scattering method.

In the case of vacuum sintering a dry molding agent prepared using the silica glass composition of the present invention, it is possible to reduce microbubbles, improve transparency, and close to theoretical density, compared to sintering in a conventional air atmosphere or helium atmosphere. Transparent silica glass can be produced.

Claims (5)

A silica glass, a dispersing agent, deionized water, a composition for producing silica glass comprising polyethyleneimide as a main gelling agent and a water-soluble aliphatic ester compound of an acid as a cogelling agent. The composition of claim 1, wherein the polyethyleneimide has a molecular weight of 1,000,000 or less. According to claim 1, 40 to 60 parts by weight of silica powder, 0.5 to 5 parts by weight of dispersant, 40 to 60 parts by weight of deionized water, 0.1 to 20 parts by weight of polyethyleneimide and 0.5 to 5 parts by weight of water-soluble aliphatic ester compound of the acid Silica glass manufacturing composition. a) mixing silica powder, dispersant and deionized water to produce a silica sol; b) adding polyethyleneimide as main gelling agent and water-soluble aliphatic ester compound of acid as cogelling agent to said 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 4, 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.