KR100549423B1 - Manufacturing method of silica glass for sol-gel process - Google Patents

Abstract

The present invention provides a method for producing silica glass for a sol-gel process. The production method is a gel powder and silica particles obtained by hydrolysis of silicon alkoxide (Si (OR) ₄ ), wherein R is an alkyl group having 1 to 8 carbon atoms as a starting material. According to the present invention, it becomes easier to control the porosity of the wet gel. As such, when the porosity of the wet gel is adjusted within a predetermined range, not only the crack generation after drying can be effectively suppressed, but also the post-shrinkage shrinkage rate is improved as compared with the usual case. In addition, the sintering temperature of the gel is lowered by more than 50 ℃ compared to the case using only the silica particles.

Description

Manufacturing method of silica glass for sol-gel process

The present invention relates to a method for preparing silica glass for sol-gel process, and more particularly, to a method for preparing silica glass that can easily control the porosity of the wet gel to reduce cracking after drying and shrinkage after sintering. It is about.

Silica glass is generally transparent, chemically inert, has excellent thermal stability, strength, and the like, and has a low coefficient of thermal expansion. Due to these excellent properties, silica glass is usefully used as an optical fiber base material.

The optical fiber basically consists of an inner core and cladding with different refractive indices to allow total reflection of light in the core. In order to manufacture such an optical fiber, first, an optical fiber preform composed of a core rod and an overcladding tube surrounding the core rod is manufactured. The precursor is then heat treated and then stretched to produce the optical fiber.

Silica glass can be prepared using silicon alkoxide or fume silica as starting material.

First, a process of preparing silica glass using silicon alkoxide is performed by adding a solvent such as alcohol or water to the silicon alkoxide to perform a hydrolysis reaction. Subsequently, the result of the hydrolysis reaction of the silicon alkoxide is injected into a molding mold to perform molding to form a gel. At this time, the structure of the gel depends on the relative content ratio of silicon alkoxide, water, solvent, etc. in the hydrolysis reaction of silicon alkoxide or pH of the silicon alkoxide hydrolysis composition.

Thereafter, the obtained gel is dried for a predetermined time, and then heat treated at a temperature of about 700 ° C. or higher to obtain a silica glass tube.

However, the gel formed from the silicon alkoxide according to the above-described method has a problem that the shrinkage rate after drying is very large because it is subjected to very large stress due to small pores during drying. Therefore, in order to control the degree of drying in the drying yarn, drying is performed by using relaxed drying conditions that maintain relatively low temperature and high humidity, and controlling the drying degree by using a small hole in the mold lid. Prevents cracking during the process and improves yield.

However, this method not only takes considerable time to complete drying, but also has a limitation in producing a large-sized due to high shrinkage.

Meanwhile, chemical vapor deposition or a sol-gel process is widely used as a method for producing silica glass using fume silica. Among them, the chemical vapor deposition method has a problem in that the production cost increases because the production of solid silica glass by the gas phase reaction is low, and the manufacturing temperature is not only high temperature process up to about 1800 ° C., but also expensive equipment is used. .

On the other hand, the sol-gel (sol-gel) process is a liquid phase process is not only highly productive and freely control the composition of the product, but also the process is very economical because the process is made at a low temperature as a whole. In addition, by using a high purity material from the starting material, it is a very useful method for manufacturing a semiconductor photomask, high purity silica glass, and the like.

Looking at the method for producing a silica glass over cladding tube using a sol-gel process as follows.

First, fume silica and additives such as a dispersant, a plasticizer, and the like are dispersed in deionized water to form a sol. The sol formed is left to mature for a predetermined time.

The gelling agent is then added to the aged sol, which is then poured into a mold to gel. 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, a hydroxyl group removal reaction and a sintering reaction are performed with respect to the gel from which the organic material was removed, and silica glass is completed.

The gel formed from the fume silica according to the above-described method has the advantage that the shrinkage of the gel after drying is smaller than that of the silicon alkoxide, while the sintering temperature is higher and cracks still occur in the gel after drying.

In order to solve this problem, a method of preparing silica glass using fume silica and silicon alkoxide as a starting material has been proposed. This method is a method of producing silica glass by first hydrolyzing and gelling silicon alkoxide, then pulverizing using a grinder such as a blender, mixing fume silica thereto, and gelling, drying and heat-treating it.

However, this method has the advantage that the strength of the wet gel is improved to facilitate handling in subsequent processes, but the porosity of the wet gel is not easy to control. The porosity of the wet gel is generally closely related to cracking after drying and shrinkage after sintering. Therefore, it is desirable to control the porosity of the wet gel within a predetermined range. If the porosity of the wet gel is too large, the shrinkage ratio after the sintering process is excessively undesirable, and if the porosity of the wet gel is too small, a passage for water to escape during the drying process is not properly secured and the gel is subjected to excessive stress. This increases the possibility of cracking.

The technical problem to be solved by the present invention is to solve the problems described above to adjust the porosity of the wet gel within a predetermined range to suppress the occurrence of cracks after drying and at the same time to reduce the shrinkage after sintering manufacturing of silica glass for sol-gel process To provide a way.

In order to achieve the above technical problem, in the present invention, (a) forming a gel by hydrolyzing a silicon alkoxide (Si (OR) ₄ ), wherein R is an alkyl group having 1 to 8 carbon atoms; (b) mixing the gel with silica particles, a dispersant and deionized water to form a sol; (c) adding a gelling agent to the sol, injecting it into a molding mold to gel, and then demoulding; (d) drying the demolded product; And (e) heat-treating the dried gel; provides a method for producing a silica glass for sol-gel process comprising a.

The content of the hydrolyzate of the silicon alkoxide in the gel state formed from the step (a) is preferably less than 50% by weight based on the weight of the silica particles. When the content of the hydrolyzate of the silicon alkoxide is outside the above range, the effect of improving the porosity characteristics of the wet gel or the effect of lowering the sintering temperature of the gel is not preferable.

In the step (e), the vitrification of the gel is preferably performed at 1350 ° C. or lower, especially 1250 to 1350 ° C., under a helium gas atmosphere. The vitrification of the gel at such a temperature reduces the possibility of crystallization of the gel, thereby improving the physical properties of the silica glass.

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

First, water is added to the silicon alkoxide (Si (OR) ₄ ), where R is an alkyl group having 1 to 8 carbon atoms, and then hydrolyzed under an acid catalyst. This hydrolysis results in the formation of a gel with a network structure. Here, it is preferable to use tetraethylortho silicate or tetramethylorthosilicate as the silicon alkoxide, and the acid is not particularly limited, but a hydrochloric acid or sulfuric acid solution is used.

The gel formed is washed several times with distilled water, and it is preferable to repeat the washing process with water until the washing solution is neutral. The gel washed with water is mixed with fume silica, deionized water and a dispersant, and the mixture is ground to form a sol. Although the dispersant is not particularly limited, it is preferable to use tetramethylammonium hydroxide, the content of which is the level commonly used in the production of silica glass.

The gelling agent is added to the sol and then injected into a molding mold and gelled. As the gelling agent, ethyl lactate, methyl lactate, and the like are used, and the content thereof is the same as that used in the manufacture of silica glass as the dispersant.

When gelation is complete, the wet gel is separated from the molding mold and dried by standing in a constant temperature and humidity chamber at 20 to 40 ° C. and a relative humidity of 70 to 90% for one week.

Next, the dried gel is subjected to low temperature heat treatment. Looking at this low temperature heat treatment process in detail, the dried gel is maintained in a helium gas atmosphere, room temperature ~ 120 ℃ to remove the moisture remaining in the gel. Then, heat treatment is performed at 100 to 600 ° C. in an air atmosphere to remove organic matter remaining in the dried gel. The gel from which the organic matter is removed is heat-treated in a chlorine gas atmosphere at 600 to 900 ° C., and then cooled to room temperature to remove trace metal impurities present in the gel and hydroxyl groups chemically bonded to the surface of the gel.

Finally, a sintering step of vitrifying the low temperature heat-treated gel is performed. This sintering step is preferably carried out in a reactor different from the reactor in which the low temperature heat treatment step is performed. In this sintering step, heat treatment is performed in a helium gas atmosphere at 1100 to 1350 ° C. to produce a silica glass overcladding tube with no cracks and high purity.

In the present invention, in the vitrification process of the gel under helium gas atmosphere, the vitrification of the gel is more smoothly because the sintering temperature is lower than that of manufacturing silica glass by the sol-gel process using only fume silica (about 1500 ° C). Will be done. It is common that the higher the sintering temperature for vitrification of the gel, the higher the possibility of crystallization in addition to vitrification of the gel, thereby lowering the purity of the finally obtained silica glass.

As described above, in the manufacturing process of the present invention, the sintering temperature for vitrification is lowered by 50 ° C. or more as compared to the case of manufacturing silica glass by the sol-gel process using only fume silica. .

In addition, according to the method as described above, it is easy to adjust the porosity of the wet gel. The porosity of the wet gel varies depending on the drying conditions. If the porosity of the wet gel is too small, the shrinkage ratio after drying is preferable, but the stress is excessively stressed in the drying process, and cracking occurs after the drying process. Therefore, it is necessary to control the porosity of the wet gel in a desirable range so as to minimize shrinkage after drying and cracking after drying. In the present invention, many attempts have been made to overcome cracks caused by small pores during drying by using the primary particles by heat treatment at a high temperature (about 900 ° C.). However, this method requires a complicated and long process of dispersing, drying or gelling the primary particles and generating secondary particles through drying, heat treatment and grinding. In the present invention, this problem is replaced by a simple process and has the same effect as well as additional effects.

The silica glass manufacturing method of the present invention 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 about 20 mm, an outer diameter of about 60 mm, and a length of about 300 mm.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited only to the following Examples.

Example 1

0.1 M-HCl solution was added to 200 g of tetraethylorthosilicate and stirred for 60 minutes to form a gel.

The gel was washed several times with distilled water. This washing process was repeated until the wash solution was neutral.

To 900 g of water-washed gel, 820 g of silica powder (Aerosil OX-50, Degussa), 1500 g of deionized water, and 120 g of ammonium hydroxide were added and mixed, which was then ground to form a sol.

Subsequently, 50 g of ethyl lactate was added to the mixture, which was then injected into a molding mold, followed by molding to gelate.

When gelation was completed, the wet gel was separated from the molding mold and dried for 48 hours in a thermo-hygrostat controlled at 25 ° C. and a relative humidity of 80%.

Then, the dried gel was heated to 500 ° C. at a temperature increase rate of 50 ° C./h, and heat-treated at this temperature for 5 hours to remove organic matter in the dried gel. The gel from which the organic matter was removed was heated to 900 ° C. at 100 ° C./hr, and maintained at this temperature for 5 hours for vitrification. At this time, the vitrification process was carried out in a chlorine gas atmosphere to remove the hydroxyl group.

Finally, the silica glass tube was manufactured by heating up to 1350 degreeC by the temperature increase rate of 100 degreeC / hr in helium gas atmosphere, and sintering at this temperature for 1 hour. At this time, the inner diameter of the silica glass tube was 20 mm, the outer diameter was 60 mm, and the length 350 mm.

Example 2

Silica glass tubes were prepared in the same manner as in Example 1, except that tetramethyl orthosilicate was used instead of teraethyl ortho silicate.

Comparative Example 1

A mixture of 7500 g of silica powder, 7063 g of deionized water and 795 g of 25% TMAH aqueous solution was mixed for 5 minutes, and then 23 g of polyethyloxazoline was added thereto.

Then, 113 g of glycerin was added to the mixture, followed by uniform mixing for 30 minutes in a high shear mixer (20,000 rpm). The mixture was then aged at 20 ° C. for 20 hours.

337 g of ethyl lactate was mixed into the sol where the ripening was completed and mixed for 3 minutes. The sol was then injected into a molding mold and gelled.

After gelation was complete, the gel was aged for 2 hours. Then, the wet gel was removed from the molding mold, and then dried in a thermo-hygrostat adjusted to a temperature of 25 ° C. and a relative humidity of 80% for 20 days.

Then, the gel dried according to the above procedure was dried for 2 to 3 days at 130 ℃.

Subsequently, it heated up to 500 degreeC at the temperature increase rate of 50 degreeC / hr, and heat-processed at this temperature for 5 hours. The gel from which the organic material was removed was heated to 1000 ° C. at 100 ° C./hr, and maintained at this temperature for 5 hours for vitrification. At this time, the vitrification process was carried out in a chlorine gas atmosphere to remove the hydroxyl group.

Finally, in the same reactor as the reactor in which the heat treatment reaction was carried out, a silica glass tube was manufactured by heating up to 1500 ° C. at a heating rate of 100 ° C./hr under a helium gas atmosphere and sintering at this temperature for 1 hour. At this time, the inner diameter of the silica glass tube was 20mm, the outer diameter is 60mm, length 400mm, the shrinkage was about 27% compared to the dry gel.

Comparative Example 2

Using a blending apparatus, 1500 g of silica powder (Aerosil OX-50, Degussa) and 1500 g of deionized water were sufficiently mixed to form a sol.

Separately, 0.1 g of 0.1 M-HCl and 18 g of water were added to 57 g of tetraethylorthosilicate, followed by stirring. It was added to the sol and then mixed uniformly.

The mixture was then defoamed for 10 minutes using a vacuum pump.

After the defoaming operation was finished, the resultant was injected into a molding mold and molding was performed.

When gelation was completed, the wet gel was separated from the molding mold and dried for 48 hours in a thermo-hygrostat controlled at 25 ° C. and a relative humidity of 80%.

Then, the dried gel was heated to 500 ° C. at a temperature increase rate of 50 ° C./h, and heat-treated at this temperature for 5 hours to remove organic matter in the dried gel. The gel from which the organic matter was removed was heated to 900 ° C. at 100 ° C./hr, and maintained at this temperature for 5 hours for vitrification. At this time, the vitrification process was carried out in a chlorine gas atmosphere to remove the hydroxyl group.

Finally, the silica glass tube was manufactured by heating up to 1500 degreeC by the temperature increase rate of 100 degreeC / hr in helium gas atmosphere, and sintering at this temperature for 1 hour. At this time, the inner diameter of the silica glass tube was 20 mm, the outer diameter was 60 mm, and the length 400 mm.

In Examples 1-2 and Comparative Examples 1-2, the degree of crack initiation of the gel after drying and the shrinkage after sintering were investigated.

As a result, in Example 1-2, the degree of cracking after drying was reduced and shrinkage after sintering was also reduced as compared with Comparative Example 1-2.

According to the present invention, it becomes easier to control the porosity of the wet gel. As such, when the porosity of the wet gel is adjusted within a predetermined range, not only the cracking after drying can be effectively suppressed, but also the shrinkage after shrinkage is improved as compared with the usual case. In addition, the sintering temperature of the gel is lowered by about 50 ℃ compared to the case using only the silica particles.

Claims (3)

(a) hydrolyzing the silicon alkoxide (Si (OR) ₄ ), wherein R is an alkyl group having 1 to 8 carbon atoms to form a gel; (b) mixing the gel with silica particles, a dispersant and deionized water to form a sol; (c) adding a gelling agent to the sol, injecting it into a molding mold to gel, and then demoulding; (d) drying the demolded product; And (e) heat treating the dried gel; Method for producing a silica glass for sol-gel process, characterized in that the content of the hydrolyzate of the silicon alkoxide in the gel state formed from the step (a) is less than 50% by weight based on the weight of the silica particles. delete The method of claim 1, wherein in the step (e), the vitrification of the gel is carried out in a helium gas atmosphere, 1350 ℃ or less method for producing a silica glass for sol-gel process.