KR20010001879A - Manufacturing method of silica glass for sol-gel process - Google Patents

Abstract

PURPOSE: A manufacturing method of silica glass by sol-gel process is provided to prevent formation of cracks after drying and reduce macro pores, resulting from a low-viscosity sol with high Si content by ultrasonic dispersion. The resultant silica glass having improved physical properties is applicable to optical fibers or semiconductors demanding high purity. CONSTITUTION: The method comprises the steps of: (i)mixing silica, dispersant such as tetramethyl ammonium hydroxide and tetraethyl ammonium hydroxide, binder such as polyethyloxazoline and polyvinyl acetate, gelation agent such as methyl formate, methyl lactate and ethyl lactate and deionized water; (ii)dispersing it to get 10-100cps viscosity of sol containing more than 40% of Si by 20-50kHz strength of ultrasonic waves; (iii)molding and gelling; (iv)demolding gel, then drying at 30-80°C and 65-80% of relative humidity; (iv)thermal-treating as the following steps: heating at 10-50°C/hr to 300-600°C for the elimination of remaining organics of gel, at 100°C/hr to 500-100°Cfor an effective time to separate gel and eliminate remained OH- in a Cl2 gas atmosphere, at 100°C/hr to 1100-1500°C for about 5hrs. in a He gas atmosphere.

Description

Manufacturing method of silica glass for sol-gel process

The present invention relates to a method for producing silica glass for sol-gel process, and more particularly, to a method for producing silica glass for sol-gel process, which can suppress cracking after drying and reduce the number of macro pores. .

The sol-gel process is a liquid phase process, which has high productivity and can freely control the composition of the product, and because the process is generally performed at a low temperature, it is very economical and is very useful when manufacturing high purity silica glass for optical fibers or semiconductors.

Hereinafter, a brief description of a method of manufacturing a silica glass overcladding tube using a sol-gel process is as follows.

First, silica particles are dispersed in water to form a sol. The sol formed is left to mature for a predetermined time. The aged sol is then poured into a mold and gelated. Once gelling is complete, the gel is separated from the mold and then dried.

Thereafter, the dried gel is first heat treated to remove organic matter in the gel. Subsequently, the hydroxyl group removal reaction and the sintering reaction are performed on the gel from which the organic material is removed to complete the silica glass overcladding tube.

Since the reactivity of the sol-gel process as described above depends on factors such as gelation reaction temperature, composition, pressure, pH, solvent, etc., these factors should be appropriately controlled to control the reactivity to the desired range.

On the other hand, when the molded gel is dried, there are many cracks and shrinkage and cracking occur in the sintering step. In order to solve this problem, a method using a dry control chemical additive (DCCA), a redispersion method, a supercritical drying method, a method using a polymer binder, and the like have been proposed.

The method of using a dry control chemical additive can minimize the difference in the local stresses of the specimen during drying by minimizing the local difference in the rate of evaporation of the solvent in the gel and by equalizing the thickness of the solid skeleton that can withstand the stress during drying. have. The result is a hardening of the gel which reduces the formation of cracks.

Briefly looking at the redispersion method, fine dry silica powder, ie, fumed silica, is dispersed in water to form a sol and then gelled. In the process of gelling, the silica particles form aggregates by hydrogen bonding. The formed aggregates are dried, then heat treated and ground to redisperse them in water. The redispersed product is gelled, then molded and sintered.

However, the above-described methods are not only very effective in suppressing cracks generated in the step of drying the gel or suppressing shrinkage after the sintering step, but also in the process itself.

Technical problem to be solved by the present invention is to solve the above problems by controlling the manufacturing process to produce a low viscosity sol with a high silica content, sol-gel process silica glass that can suppress the cracking after drying It is to provide a manufacturing method.

In the present invention to achieve the above technical problem. (a) mixing silica, dispersant, binder, gelling agent and deionized water, and then dispersing with ultrasound to form a sol;

(b) molding and gelling the sol;

(c) demolding the obtained gel to dry it;

(d) providing a method for producing silica glass for sol-gel process, comprising the step of heat-treating the dried gel.

It is more preferable in terms of dispersibility of the sol to perform mechanical dispersion before the ultrasonic dispersion in step (a). In the ultrasonic dispersion process of (a), the ultrasonic intensity is 20 to 50 kHz, and the viscosity of the sol formed from the step (a) is preferably 10 to 100 cps.

Hereinafter, a method of manufacturing silica glass according to the present invention will be described.

First, silica, additives and deionized water are mixed and then dispersed using ultrasonic waves to form a sol. Ultrasonic intensity, dispersion time, temperature during sonication, medium for delivering ultrasonic waves, pretreatment conditions before ultrasonic dispersion and the like are very important variables. Herein, the intensity of the ultrasonic wave is preferably 20 to 50 KHz, in particular 20 to 28 KHz, and the dispersion time is preferably the point of time when the viscosity decrease of the composition no longer occurs, that is, the time of saturation.

The sol thus formed is uniformly dispersed in constituents thereof, and can increase the silica content to 40% by weight or more. And the viscosity is very low as 10 to 100cps. Thus low viscosity has the advantage of easy handling in subsequent processes. In addition, if the ultrasonic dispersion is carried out as described above, some bubbles are also removed, thereby reducing the degassing time which is a subsequent process.

In the related art, only a method of mechanically dispersing by using a mixer such as a ball mill when mixing silica, an additive, and deionized water was mainly used. However, according to such a mechanical dispersion method, there is a limit in preparing a low viscosity sol.

On the other hand, by using ultrasonic dispersion or ultrasonic dispersion after mechanical dispersion as in the present invention it is possible to solve all these problems. In particular, using mechanical dispersion followed by ultrasonic dispersion, a sol in which the components are very uniformly dispersed can be obtained.

Although the said additive is not specifically limited, A dispersing agent, a binder, a plasticizer, a gelling agent, etc. are used. Herein, the dispersant, binder, plasticizer, and gelling agent are not particularly limited as long as they are materials commonly used in the production of silica glass. And the content of each substance is also normal.

The gelling agent is a water-soluble aliphatic ester of an acid selected from the group consisting of formic acid, lactic acid and glycolic acid, and specific examples thereof include methyl formate, methyl lactate, ethyl lactate, and the like. As the dispersant, tetramethylammonium hydroxide and tetraethylammonium hydroxide which are quaternary ammonium hydroxides are used. These materials 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 addition, a polyhydric alcohol (polyhydric alcohol) is used as a plasticizer. Specific examples include glycerin, ethylene glycol, 2-methylpropane-1,2,3-triol, and the like. As the binder, polyethyloxazoline, polyvinylacetate, or the like is used.

The sol formed according to the above procedure is molded and then gelled. Subsequently, when the gelation is completed after a predetermined period of time, the gelled resultant is de-molded.

The demolded gel is then dried at a temperature of 30-80 ° C. and a relative humidity of 65-80%.

The dried gel is then heat treated at 300 to 600 ° C. (raising rate: 10 to 50 ° C./hr) to remove organic matter remaining in the gel. Thereafter, the gel is heated to 500 to 1000 ° C. (heating rate: 100 ° C./hr) for heat treatment for a predetermined time in order to vitrify the gel. This heat treatment is carried out in a chlorine gas atmosphere to remove residual hydroxyl groups. Subsequently, a silica glass tube is completed by heating up at 1100-1500 degreeC (heating rate: 100 degreeC / hr) in a helium gas atmosphere, and heat-processing for about 5 hours.

The silica glass manufacturing method described above is a method that can be usefully applied regardless of the specifications of the silica glass overcladding tube. In particular, it is a very useful method for producing a large silica glass tube having an inner diameter of 10 to 50 mm, an outer diameter of 50 to 100 mm, and a length of 500 to 1000 mm.

Example 1

500 g of fume silica (Aerosil OX-50, manufactured by Degussa), 52 ml of a 25% by weight solution of tetraammonium hydroxide (TMAH), 500 g deionized water, 50 g ethyl lactate, and 2.1 g polyethyloxazoline This was then blended for 10 minutes using a high shear mixer. Subsequently, ultrasonic waves (strength: 20 kHz) were added to the reaction mixture for 2 hours to disperse to prepare a sol. The viscosity of the sol formed according to this procedure was 25 cps.

Subsequently, bubbles in the sol were removed using a vacuum pump, and then aged for 15 hours. This bubble-free sol was poured into a mold and gelated. At this time, an acrylic tube having an inner diameter of 100 mm and a stainless rod having an outer diameter of 33 mm were used as the mold.

Once gelling was complete, the wet gel was removed from the mold and dried for 4 days at 30 ° C. and 80 RH% in a thermo-hygrostat. Subsequently, the dried gel was heated to about 500 ° C. (raising rate: 50 ° C./hr) and heat-treated at this temperature for 5 hours to remove organic matter contained in the gel.

Thereafter, the resultant was heat treated at about 1000 ° C. (raising rate: 100 ° C./hr) for 5 hours in a chlorine (Cl ₂ ) gas atmosphere to remove residual hydroxyl groups in the gel. Subsequently, residual chlorine gas was removed using oxygen (O ₂ ) and helium (He) gas.

The resultant was sintered at about 1400 ° C. (heating rate: 100 ° C./hr) for 4 hours under a helium atmosphere to complete a silica glass tube. At this time, the inner diameter of the silica glass tube was 22 mm, the outer diameter was 67 mm, and the length was 1000 mm.

Example 2

A silica glass tube was prepared in the same manner as in Example 1 except that the ultrasonic intensity was changed to 26 kHz instead of 20 kHz.

Comparative example

A silica glass tube was prepared in the same manner as in Example 1 except that ultrasonic dispersion was not performed.

The degree of cracking after drying and the degree of shrinkage after heat treatment of the silica glass tubes prepared according to Examples 1-3 and Comparative Examples were investigated.

As a result, the silica glass tube prepared according to Example 1-3 was found to be less cracking after drying and the number of macro pores in the silica glass tube than in the case of Comparative Example 1-2. This is because when the ultrasonic dispersion is performed as in Example 1-3, the components are dispersed very uniformly and stably, thereby increasing the silica content in the sol and thus obtaining a low viscosity sol.

Ultrasonic dispersion according to the present invention can produce a low viscosity sol having a high silica content, reducing the degassing time of the sol, which is a subsequent process. As a result, while the occurrence of cracks after drying is suppressed, the number of macropores is reduced, thereby making it possible to manufacture silica glass with improved physical properties.

Claims (4)

(a) mixing silica, a dispersant, a binder, a gelling agent and deionized water and then dispersing with ultrasound to form a sol; (b) molding and gelling the sol; (c) demolding the obtained gel to dry it; (D) a method for producing a silica glass for sol-gel process comprising the step of heat-treating the dried gel. The method of claim 1, wherein the mechanical dispersion is performed before the ultrasonic dispersion treatment of step (a). The method of claim 1, wherein the intensity of the ultrasonic wave is 20 to 50KHz. The method of claim 1, wherein the viscosity of the sol formed from the step (a) is 10 to 100cps method for producing a silica glass for sol-gel process.