Title of Invention	AN IMPROVED METHOD FOR FABRICATING SILICA GLASS USING SOL-GEL PROCESS.
Abstract	There is provided a high-purity silica glass fabricating method using a sol-gel process. In the method, a first sol its formed by mixing 100 parts by weight of fumed silica powder with 100-300 parts by weight of deionized water. The first sol is gelled, dried, powdered, and thermally treated. A second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water and 20-50 parts by weight of non-thermally treated original fumed silica powder. The second sol is gelled, dried, and sintered. Thus, a high-purity silica glass is formed.

Title of Invention

AN IMPROVED METHOD FOR FABRICATING SILICA GLASS USING SOL-GEL PROCESS.

Abstract

There is provided a high-purity silica glass fabricating method using a sol-gel process. In the method, a first sol its formed by mixing 100 parts by weight of fumed silica powder with 100-300 parts by weight of deionized water. The first sol is gelled, dried, powdered, and thermally treated. A second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water and 20-50 parts by weight of non-thermally treated original fumed silica powder. The second sol is gelled, dried, and sintered. Thus, a high-purity silica glass is formed.

Full Text	BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to an improved method for fabricating silica glass using a sol-gel process, and in particular, to a method for fabricating a silica glass tube containing high-purity, high density silica, using a sol-gel process. 2, Description of the Related Art In general, many methods have been suggested to manufacture a secondary cladding tube for use in fabricating an optical fiber, by a sol-gel process, Silica glass obtained by using only fumed silica powder is vulnerable to cracking during a drying because of very fine pores among particles process and cannot be generally used, A sol-gel method using silicon alkoxide allows a homogeneous and transparent glass body to be obtained, but the glass body has a very high shrinkage rate (i.e., higher than 60%)f which makes it difficult to use the glass body as a long secondary cladding tube (i.e., longer than 90cm} for fabrication of an optical fiber. As shown in Fig. 1, in the conventional method of fabricating a silica glass tube out of fumed silica particles, a first sol gel is formed by dispersing the Fine fumed silica particles in water to prevent cracking. Then, the first sol is gelled and dried. Silica powder Is obtained by grinding and classifying the dried first sol. Then, a 1 second sol is formed by thermally treating the silica powder and re-dispersing the thermally treated silica powder in water. The secondary sol is gelled, dried, and sintered. However, the above method remarkably decreases a packing rate in the powder and is limited in reducing the shrinkage rate during the drying process, because a silica glass tube is fabricated by dispersing, gelling, drying, powdering, thermally treating, re-dispersing, re-gelling, drying, and sintering the fumed silica. SUMMARY OF THE INVENTION An object of the present invention is to provide a method for fabricating a high-density, high-purity silica glass by adding original fumed silica powder during a secondary sol formation process and then performing a second gelation process. Another object of the present invention is to provide a high-purity silica glass fabricating method which prevents cracking of a dried silica gel tube and increases a packing rate by increasing the size of pores among silica particles. To achieve the above objects, there is provided a high-purity silica glass fabricating method using a sol-gel process. In the method, a first sol is formed by mixing 100 parts by weight of fumed silica powder with 100-300 parts by weight of deionized water. The first sol is gelled, dried, powdered, and thermally treated. A second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water and 20-50 parts by weight of non-thermally treated original fumed silica powder. The second sol is gelled, dried, and sintered. Thus, - 2 - a high-purity silica glass is formed. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS The above objects and advantages of the present invention will become more apparent by describing in detail a preferred embodiment thereof with reference to the attached drawings in which : Fig. 1 is a flowchart of a conventional method for fabricating a silica glass using a sol-gel method; Fig. 2 is a flowchart of a method for fabricating a highpuirity silica glass using the sol-gel method according to an embodiment of the present invention; and Fig. 3 is a graph showing the FT-IR spectra of the high-purity silica glass according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In a high-purity silica glass fabricating method according to an embodiment of the present invention as shown in Fig. 2, preferably 7-40nm fumed silica powder containing high-density silica is mixed with distilled deionized water at a weight ratio of 1:1-1:3 in a high shear mixer, and then a first homogeneous mixed sol is formed by a ball mill. The first sol is gelled and dried for a predetermined time. The dried first sol becomes powder through grinding and classification. Powder particles are grown, while being coagulated, by thermally treating the powder for 0.5 to 4 hours at about 600°C or higher. A second sol is formed by re-dispersing the grown particles in the same manner for the first sol. For formation of the second sol, 20-50 wt% original fumed silica powder based on the weight of the thermally 3 treated silica powder is added to and mixed with the grown powder to fill pores among the grown particles. At this time, an aqueous organic compound such as polyviny(alcohol is preferably added at an appropriate amount to the mixture in order to prevent cracking. Then, the second sol is poured into a mold of an intended shape (e.g., Kibe), gelled, and dried for a predetermined time. Moisture (and a hydroxyl-group) is removed from the dried gel at about 600-1000°C, using Chlorine gas, and remaining Chlorine is removed therefrom, using Helium gas. Then the dried second sol is sintered between 1350°C and a glass fusion point. Thus, a high-purity silica glass is fabricated. The above high-purity silica glass fabricating method of the present invention will be described referring to an embodiment in the best mode, as follows. (Embodiment 1) A first sol containing 25 wt% silica is formed by mixing 200g of fumed silica powder having a specific surface area 50m /g with 6000g of deionized water. To obtain a homogeneous first sol, the mixing process is performed in a ball mill at 90rpm for 24 hours by adding 16kg of silica balls having a diameter of 10mm .Then, the first sol is gelled, and moisture is vaporized from the gel at 120°C for 24 hours in a drier. The dried silica is ground, classified by a mesh sieve, and thermally treated at 1100°C for one hour in a heat treatment furnace having a temperature rising speed of 300°C/hr. The thermally treated powder is blended with water at a weight ratio of 1:1.2 for 15 minutes, and mixed with additional 20g of polyvinylalcohol for 24 hours in the ball mill under the same condition for forming the first sol. Then, the thus-formed second sol is mixed with 400g of non- 4 thermally treated fumed silica powder, 400g of deionized water, and 4.8g of ammonium fluoride for 6 hours In the ball mill. The sol is poured into a mold and gelled for 48 hours. Here, the mold is formed of teflon and divided into an upper portion, a lower portion, a tube-shaped outer portion, and a central rod. The dimensions of an object moldable by the mold are 3Snm in inner diameter, 71nrn in outer diameter, and 1.3m in length. Then, the central rod is removed and the sot in the mold is dried for 2-3 days at room temperature and a relative humidity of 80%. Then the mold is removed and the tube-shaped gel is dried for 10 days at a relative humidity of 80%. The tube-shaped gel is dried for 24 hours at 40°C, for 24 hours at 60°C, and then for 24 hours at 80°C. Then, remaining moisture and an organic material are removed from the dried gel by heating the gel for 5 hours at 900°C in a heat treatment furnace having a temperature rising speed of 100°C/hr. Finally, a high-purity silica glass tube is formed by glassifying the thermally treated gel at an atmosphere of Helium and Chlorine gases in a furnace. Here, dehydroxylation and glassification at performed at 600-1000°C for 5 hours and at 1400°C for one hour, respectively. Therefore, the IR transmittance of the high-purity glass fabricated in the above method at a wave number of 3400cm or above is higher than that of a conventional silica glass, as shown in Fig. 3. From the result, it is noted that the silica glass of the present invention is more transparent and has a lower OH content than the conventional one. For comparison, a silica glass tube was fabricated in the same shape as a comparative example in the same manner for embodiment 1, except that original silica powder is not added during formation of the second sol. 5 The shrinkage rates of comparative example 1 and embodiment 1 are measured as follows. The glass tube of the present invention is used as a secondary cladding tube for fabrication of an optical fiber, and other glass products, for example, optical lenses, can be fabricated in the same method by using different molds. In the first embodiment, the thermally-treated powder is mixed with water at a weight ratio of 1:1.2, blended for 15 minutes, and mixed with additional 20g of polyvinyl alcohol in the ball mill for formation of the second sol as shown in FIG. 2. Meanwhile, for formation of the second sol as shown in FIG. 2 in a second embodiment, the thermally-treated powder is mixed with water at a weight ratio of 1:1.2, blended for 15 minutes, mixed with additional 16kg of silica balls having a diameter of 10mm for 24 hours at 90rpm in the ball mill, and then mixed with 6 - additional 20g of polyvinyl alcohol in the ball mill. The subsequent steps are performed in the same manner as those in the first embodiment. Thus, a more excellent high-purity silica glass can be fabricated. For formation of the second sol as shown in FIG. 2 in a third embodiment, the thermally-treated powder is mixed with water at a weight ratio of 1: 1.2, blended for 15 minutes, and mixed with 20g of polyvinyl alcohol, 400g of fumd silica powder, 400g of deionized water, and 4.8g of ammonium fluoride. Then, the resultant mixture is mixed with additional 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in the ball mill, thereby obviating the need for the third ball mill step. Various values (e.g., weight ratio, weight, temperature, and length) described in the embodiments of the present invention are exemplary, and thus can be varied. As described above, the high-purity silica glass fabricating method using the sol-gel method according to the present invention are advantageous in that (1) the silica glass tube of the present invention is fabricated at a lower cost to have a lower OH content, a higher purity, and a comparable or more excellent quality than a conventional synthetic glass tube; (2) when the silica glass tube is used for fabrication of an optical fiber, a very cheap high-purity optical fiber can be obtained; and (3) cracking is prevented and a packing rate is increased by mixing a first thermally heated powder with original powder at an appropriate ratio, heating the mixture, and thus increasing the size of pores after particle growing, thereby removing additional cracking causes. While the invention has been shown and described with reference to a certain preferred embodimenuthereoa it will be understood by those skilled in the art that _ 7 -. - 8 - various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. WE CLAIM : 1. An improved method for fabricating silica glass using a sol-gel process, comprising the steps of: forming a first sol by mixing 100 parts by weight of fumed silica powder with 100-300 parts by weight of deionized water; gelling, drying, powdering, and thermally treating the first sol; forming a second sol by mixing the thermally-treated first sol with 100*200 parts by weight of deionized water and 20-50 parts by weight of non-therm ally treated original fumed silica powder; and gelling, drying, and sintering the second sol to thereby form a high-purity silica glass. 2. The method as claimed in claim 1, wherein the thermal treatment of the first sol is performed at 600°C for 0.5-4 hours, the second sol is gelled and dried in a mold, and the dried second sol is sintered at or below a glass fusion point. 3. The method as claimed in claims 1 and 2, wherein the obtained high-purity silica glass is tube-shaped. 4. An improved method for fabricating silica glass using a sol-gel process, comprising the steps of: forming a first homogeneous mixed sot by mixing fumed silica powder with distilled deionized water at a predetermined weight ratio in a ball mill; 9 forming a first gel by gelling the first sol; drying the first gel at a predetermined temperature for a predetermined time in a drier; forming powder out of the dried silica by grinding and classifying the dried silica; thermally treating the powder at a predetermined temperature for a predetermined time; mixing the thermally-treated powder with water at a predetemined weight ratio, blending the mixture for a predetermined time, and mixing the mixture with a predetermined amount of poly vinyl alcohol in the ball mill; forming a second sol by mixing the resultant mixture with fumed silica powder,deionized water, and ammonium fluoride in the ball mill; forming a tube-shaped gel by pouring the second sol in a mold, drying the sol for a predetermined time, and removing the mold from the sol; drying the tube-shaped gel at room temperature; and removing remaining moisture and Chlorine from foe dried tube-shaped gel at a predetermined temperature using Chlorine and Helium gases, and sintering the gel at a high temperature being a glass fusion point or below, 5. An improved method for fabricating silica glass using a sol-gel process, comprising the steps of: forming a first homogeneous mixed sol by mixing fumed silica powder with distilled deionized water at a weight ratio of 1:1-1:3, and mixing the mixture with 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in a ball mill; forming a first gel by gelling the first sol; drying the first gel at 120°C for 24 hours in a drier; 10 forming powder out of the dried silica by grinding the dried silica and classifying the ground silica by a 20 mesh sieve; thermally treating the powder in the range between 600 and 1100°C for one hour in a heat treatment furnace having a temperature rising speed of 300°C/hr; mixing the thermally-treated powder with water at a weight ratio of 1:1.2, blending the mixture for 15 minutes, and mixing the mixture with 20g of polyvinyl alcohol and 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in the ball mill; forming a second sol by mixing the resultant mixture with 400g of fumed silica powder, 400g of deionized water, and 4.8g of ammonium fluoride for 6 hours in the ball mill; forming a tube-shaped gel by pouring the second sol in a mold, gelling the sol for 48 hours, drying the gel at a relative humidity of 80% for 2-3 days, and removing the mold from the sol; drying the tube-shaped gel at a relative humidity of 80% for 10 days, at 40°C for 24 hours, at 60°C for 24 hours, and then at 80°C for 24 hours, and removing remaining moisture and organic material from the dried gel at 900°C for 5 hours in a heat treatment furnace having a temperature rising speed of 100°C/hr; and removing remaining moisture from the dried gel between 600 and 1100°C, using Chlorine gas, removing remaining Chlorine from the gef, using Helium gas, and sintering the gel between 1300°C and a glass fusion point. 6. An improved method for fabricating silica glass using a sol gel process, comprising the steps of: forming a first homogenous mixed sol by mixing fumed silica powder with distilled deionized water at a predetermined weight ratio in a ball mill; 11 forming a first gel by getting the first sol; drying the first gel at a predetermined temperature for a predetermined time in a drier; forming powder out of the dried silica by grinding and classifying the dried silica; thermally treating the powder at a predetermined temperature for a predetermined time; mixing the thermally-treated powder with water at a predetermined weight ratio, blending the mixture for a predetermined time, mixing the mixture in the ball mill, and adding a predetermined amount of pofyvinyf alcohol to the mixture; forming a second sol by mixing the resultant mixture containing the polyvinyl alcohol with fumed silica powder deionized water, and ammonium fluoride in the ball mill; forming a tube-shaped ge! by pouring the second sol in a moid,drying the sol for a predetermined time, and removing the mold from the sol; drying the tube-shaped gel at room temperature; and removing remaining moisture and Chlorine from the dried tube-shaped gel at a predetermined temperature using Chlorine and Helium gases, and sintering the gel at a high temperature being a glass fusion point or below. 7. An improved method for fabricating silica glass using a sol gel process, comprising the steps of: forming a first homogeneous mixed sol by mixing fumed silica powder with distilled deionized water at a weight ratio of 1:1-1:3, and mixing the mixture with 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in a ball mill; forming a first gel by gelling the first sol; 12 drying the first gel at 120°C for 24 hours in a drier; forming powder out of the dried silica by grinding the dried silica and classifying the ground silica by a 20 mesh sieve; thermally treating the powder in the range between 600 and 1100°C for one hour in a heat treatment furnace having a temperature rising speed of 3G0°C/hr; mixing the thermally-treated powder with water at a weight ratio of 1:1.2, blending the mixture for 15 minutes, mixing the mixture with 16kg of silica baits having a diameter of 10mm at 90rpm for 24 hours in the ball mill, and adding 20g of polyvinyl alcohol to the mixture; forming a second sol by mixing the resultant mixture containing the polyvinyl alcohol with 400g of fumed silica powder, 400g of deionized water, and 4.8g of ammonium fluoride for 6 hours in the ball mill; forming a tube-shaped gel by pouring the second sol in a mold, gelling the sol for 48 hours, drying the gel at'a relative humidity of 80% for 2-3 days, and removing the mold from the sol; drying the tube-shaped gel at a relative humidity of 80% for 10 days, at 40°c for 24 hours, at 60°C for24 hours, and then at 80°C tor 24 hours, and removing remaining moisture and organic material from the dried tube-shaped gel at 900°C for 5 hours in a heat treatment furnace having a temperature rising speed of 100°C/hr; and removing remaining moisture from the dried gel between 600 and 1100°C, using Chlorine gas, removing remaining Chlorine from the gel, using Helium gas, and sintering the gel between 1300°C and a glass fusion point. 8. An improved method for fabricating silica glass using a sol gel process, comprising the steps of: 13 forming a first homogenous mixed sot by mixing fumed silica powder with distilled deionized water at a predetermined weight ratio in the ball mill; forming a first gel by gelling the first sol; drying the first gel at a predetermined temperature for a predetermined time in a drier; forming powder out of the dried silica by grinding and classifying the dried silica; thermally treating the powder at a predetermined temperature for a predetermined time; mixing the thermally-treated powder with water at a predetermined weight ratio, and blending the mixture for a predetermined time; forming a second sol by mixing the mixture with a predetermined amount of poly vinyl alcohol, fumed silica powder, deionized water, and ammonium fluoride in the ball mill; forming a tube-shaped gel by pouring the second sol in a moid, drying the sol for a predetermined time, and removing the mold from the sol; drying the tube-shaped gel at room temperature; and removing remaining moisture and Chlorine from the dried tube-shaped get at a predetermined temperature using Chlorine and Helium gases, and sintering the gel at a high temperature being a glass fusion point or below. 9. An improved method for fabricating silica glass using a sol gel process, comprising the steps of : forming a first homogeneous mixed sol by mixing fumed silica powder with distilled deionized water at a weight ratio of 1:1-1:3, and mixing the mixture with 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in a ball mill; 14 forming a first gel by gelling the first sol; drying the first gel at 120° C for 24 hours in a drier; forming powder out of the dried silica by grinding the dried silica and classifying the ground silica by a 20 mesh sieve; thermally treating the powder in the range between 600 and 1100° C for one hour in a heat treatment furnace having a temperature rising speed of 300°C/hr; mixing the thermally-treated powder with water at a weight ratio of 1:1.2, and blending the mixture for 15 minutes; forming a second sol by mixing the mixture with 20g of polyvinyl alcohol, 400g of fumed silica powder, 400g of deionized water, and 4.8g of ammonium fluoride, and mixing the resultant mixture with 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in the ball mill; forming a tube-shaped gel by pouring the second sol in a mold, gelling the sol for 48 hours, drying the gel at a relative humidity of 80% for 2-3 days, and removing the mold from the sol; drying the tube-shaped gel at a relative humidity of 80% for 10 days, at 40 ° C for 24 hours, at 60 °C for 24 hours, and then at 80 °C for 24 hours, and removing remaining moisture and organic material from the dried tube-shaped gel at 900 °C for 5 hours in a heat treatment furnace having a temperature rising speed of 100°C/hr;and removing remaining moisture from the dried gel between 600 and 1100°C, using Cl gas, removing remaining Cl from the gel, using He gas, and sintering the gel between 1300°C and a glass fusion point. - 15 - There is provided a high-purity silica glass fabricating method using a sol-gel process. In the method, a first sol its formed by mixing 100 parts by weight of fumed silica powder with 100-300 parts by weight of deionized water. The first sol is gelled, dried, powdered, and thermally treated. A second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water and 20-50 parts by weight of non-thermally treated original fumed silica powder. The second sol is gelled, dried, and sintered. Thus, a high-purity silica glass is formed.

Full Text

BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to an improved method for fabricating silica glass using a sol-gel process, and in particular, to a method for fabricating a silica glass tube containing high-purity, high density silica, using a sol-gel process.
2, Description of the Related Art
In general, many methods have been suggested to manufacture a secondary cladding tube for use in fabricating an optical fiber, by a sol-gel process, Silica glass obtained by using only fumed silica powder is vulnerable to cracking during a drying because of very fine pores among particles process and cannot be generally used, A sol-gel method using silicon alkoxide allows a homogeneous and transparent glass body to be obtained, but the glass body has a very high shrinkage rate (i.e., higher than 60%)f which makes it difficult to use the glass body as a long secondary cladding tube (i.e., longer than 90cm} for fabrication of an optical fiber.
As shown in Fig. 1, in the conventional method of fabricating a silica glass tube out of fumed silica particles, a first sol gel is formed by dispersing the Fine fumed silica particles in water to prevent cracking. Then, the first sol is gelled and dried. Silica powder Is obtained by grinding and classifying the dried first sol. Then, a
1

second sol is formed by thermally treating the silica powder and re-dispersing the thermally treated silica powder in water. The secondary sol is gelled, dried, and sintered.
However, the above method remarkably decreases a packing rate in the powder and is limited in reducing the shrinkage rate during the drying process, because a silica glass tube is fabricated by dispersing, gelling, drying, powdering, thermally treating, re-dispersing, re-gelling, drying, and sintering the fumed silica.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for fabricating a high-density, high-purity silica glass by adding original fumed silica powder during a secondary sol formation process and then performing a second gelation process.
Another object of the present invention is to provide a high-purity silica glass fabricating method which prevents cracking of a dried silica gel tube and increases a packing rate by increasing the size of pores among silica particles.
To achieve the above objects, there is provided a high-purity silica glass fabricating method using a sol-gel process. In the method, a first sol is formed by mixing 100 parts by weight of fumed silica powder with 100-300 parts by weight of deionized water. The first sol is gelled, dried, powdered, and thermally treated. A second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water and 20-50 parts by weight of non-thermally treated original fumed silica powder. The second sol is gelled, dried, and sintered. Thus,
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a high-purity silica glass is formed.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The above objects and advantages of the present invention will become more
apparent by describing in detail a preferred embodiment thereof with reference
to the attached drawings in which :
Fig. 1 is a flowchart of a conventional method for fabricating a silica glass using a
sol-gel method;
Fig. 2 is a flowchart of a method for fabricating a highpuirity silica glass using
the sol-gel method according to an embodiment of the present invention; and
Fig. 3 is a graph showing the FT-IR spectra of the high-purity silica glass
according to the embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In a high-purity silica glass fabricating method according to an embodiment of
the present invention as shown in Fig. 2, preferably 7-40nm fumed silica powder
containing high-density silica is mixed with distilled deionized water at a weight
ratio of 1:1-1:3 in a high shear mixer, and then a first homogeneous mixed sol is
formed by a ball mill. The first sol is gelled and dried for a predetermined time.
The dried first sol becomes powder through grinding and classification. Powder
particles are grown, while being coagulated, by thermally treating the powder for
0.5 to 4 hours at about 600°C or higher. A second sol is formed by re-dispersing
the grown particles in the same manner for the first sol. For formation of the
second sol, 20-50 wt% original fumed silica powder based on the weight of the
thermally
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treated silica powder is added to and mixed with the grown powder to fill pores among the grown particles. At this time, an aqueous organic compound such as polyviny(alcohol is preferably added at an appropriate amount to the mixture in order to prevent cracking. Then, the second sol is poured into a mold of an intended shape (e.g., Kibe), gelled, and dried for a predetermined time. Moisture (and a hydroxyl-group) is removed from the dried gel at about 600-1000°C, using Chlorine gas, and remaining Chlorine is removed therefrom, using Helium gas. Then the dried second sol is sintered between 1350°C and a glass fusion point. Thus, a high-purity silica glass is fabricated.
The above high-purity silica glass fabricating method of the present invention will be described referring to an embodiment in the best mode, as follows.
(Embodiment 1)
A first sol containing 25 wt% silica is formed by mixing 200g of fumed silica powder having a specific surface area 50m /g with 6000g of deionized water. To obtain a homogeneous first sol, the mixing process is performed in a ball mill at 90rpm for 24 hours by adding 16kg of silica balls having a diameter of 10mm .Then, the first sol is gelled, and moisture is vaporized from the gel at 120°C for 24 hours in a drier. The dried silica is ground, classified by a mesh sieve, and thermally treated at 1100°C for one hour in a heat treatment furnace having a temperature rising speed of 300°C/hr. The thermally treated powder is
blended with water at a weight ratio of 1:1.2 for 15 minutes, and mixed with
additional 20g of polyvinylalcohol for 24 hours in the ball mill under the same
condition for forming the first sol. Then, the thus-formed second sol is mixed
with 400g of non-
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thermally treated fumed silica powder, 400g of deionized water, and 4.8g of ammonium fluoride for 6 hours In the ball mill. The sol is poured into a mold and gelled for 48 hours. Here, the mold is formed of teflon and divided into an upper portion, a lower portion, a tube-shaped outer portion, and a central rod. The dimensions of an object moldable by the mold are 3Snm in inner diameter, 71nrn in outer diameter, and 1.3m in length. Then, the central rod is removed and the sot in the mold is dried for 2-3 days at room temperature and a relative humidity of 80%. Then the mold is removed and the tube-shaped gel is dried for 10 days at a relative humidity of 80%. The tube-shaped gel is dried for 24 hours at 40°C, for 24 hours at 60°C, and then for 24 hours at 80°C. Then, remaining moisture and an organic material are removed from the dried gel by heating the gel for 5 hours at 900°C in a heat treatment furnace having a temperature rising speed of 100°C/hr. Finally, a high-purity silica glass tube is formed by glassifying the
thermally treated gel at an atmosphere of Helium and Chlorine gases in a furnace. Here, dehydroxylation and glassification at performed at 600-1000°C for 5 hours and at 1400°C for one hour, respectively.
Therefore, the IR transmittance of the high-purity glass fabricated in the above method at a wave number of 3400cm or above is higher than that of a conventional silica glass, as shown in Fig. 3. From the result, it is noted that the silica glass of the present invention is more transparent and has a lower OH content than the conventional one.
For comparison, a silica glass tube was fabricated in the same shape as a comparative example in the same manner for embodiment 1, except that original silica powder is not added during formation of the second sol.
5

The shrinkage rates of comparative example 1 and embodiment 1 are measured as follows.

The glass tube of the present invention is used as a secondary cladding tube for fabrication of an optical fiber, and other glass products, for example, optical lenses, can be fabricated in the same method by using different molds.
In the first embodiment, the thermally-treated powder is mixed with water at a weight ratio of 1:1.2, blended for 15 minutes, and mixed with additional 20g of polyvinyl alcohol in the ball mill for formation of the second sol as shown in FIG. 2. Meanwhile, for formation of the second sol as shown in FIG. 2 in a second embodiment, the thermally-treated powder is mixed with water at a weight ratio of 1:1.2, blended for 15 minutes, mixed with additional 16kg of silica balls having a diameter of 10mm for 24 hours at 90rpm in the ball mill, and then mixed with
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additional 20g of polyvinyl alcohol in the ball mill. The subsequent steps are performed in the same manner as those in the first embodiment. Thus, a more excellent high-purity silica glass can be fabricated. For formation of the second sol as shown in FIG. 2 in a third embodiment, the thermally-treated powder is mixed with water at a weight ratio of 1: 1.2, blended for 15 minutes, and mixed with 20g of polyvinyl alcohol, 400g of fumd silica powder, 400g of deionized water, and 4.8g of ammonium fluoride. Then, the resultant mixture is mixed with additional 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in the ball mill, thereby obviating the need for the third ball mill step.
Various values (e.g., weight ratio, weight, temperature, and length) described in the embodiments of the present invention are exemplary, and thus can be varied.
As described above, the high-purity silica glass fabricating method using the sol-gel method according to the present invention are advantageous in that (1) the silica glass tube of the present invention is fabricated at a lower cost to have a lower OH content, a higher purity, and a comparable or more excellent quality than a conventional synthetic glass tube; (2) when the silica glass tube is used for fabrication of an optical fiber, a very cheap high-purity optical fiber can be obtained; and (3) cracking is prevented and a packing rate is increased by mixing a first thermally heated powder with original powder at an appropriate ratio, heating the mixture, and thus increasing the size of pores after particle growing, thereby removing additional cracking causes.
While the invention has been shown and described with reference to a certain preferred embodimenuthereoa it will be understood by those skilled in the art that
_ 7 -.

- 8 -
various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

WE CLAIM :
1. An improved method for fabricating silica glass using a sol-gel process,
comprising the steps of:
forming a first sol by mixing 100 parts by weight of fumed silica powder with 100-300 parts by weight of deionized water; gelling, drying, powdering, and thermally treating the first sol; forming a second sol by mixing the thermally-treated first sol with 100*200 parts by weight of deionized water and 20-50 parts by weight of non-therm ally treated original fumed silica powder; and
gelling, drying, and sintering the second sol to thereby form a high-purity silica glass.
2. The method as claimed in claim 1, wherein the thermal treatment of the first
sol is performed at 600°C for 0.5-4 hours, the second sol is gelled and dried
in a mold, and the dried second sol is sintered at or below a glass fusion
point.
3. The method as claimed in claims 1 and 2, wherein the obtained high-purity
silica glass is tube-shaped.
4. An improved method for fabricating silica glass using a sol-gel process,
comprising the steps of:
forming a first homogeneous mixed sot by mixing fumed silica powder with distilled deionized water at a predetermined weight ratio in a ball mill;
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forming a first gel by gelling the first sol;
drying the first gel at a predetermined temperature for a predetermined time in a drier;
forming powder out of the dried silica by grinding and classifying the dried silica;
thermally treating the powder at a predetermined temperature for a predetermined time;
mixing the thermally-treated powder with water at a predetemined weight ratio, blending the mixture for a predetermined time, and mixing the mixture with a predetermined amount of poly vinyl alcohol in the ball mill;
forming a second sol by mixing the resultant mixture with fumed silica powder,deionized water, and ammonium fluoride in the ball mill;
forming a tube-shaped gel by pouring the second sol in a mold, drying the sol for a predetermined time, and removing the mold from the sol;
drying the tube-shaped gel at room temperature; and
removing remaining moisture and Chlorine from foe dried tube-shaped gel at a predetermined temperature using Chlorine and Helium gases, and sintering the gel at a high temperature being a glass fusion point or below,
5. An improved method for fabricating silica glass using a sol-gel process, comprising the steps of:
forming a first homogeneous mixed sol by mixing fumed silica powder with distilled deionized water at a weight ratio of 1:1-1:3, and mixing the mixture with 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours
in a ball mill;
forming a first gel by gelling the first sol;
drying the first gel at 120°C for 24 hours in a drier;
10

forming powder out of the dried silica by grinding the dried silica and classifying the ground silica by a 20 mesh sieve;
thermally treating the powder in the range between 600 and 1100°C for one hour in a heat treatment furnace having a temperature rising speed of 300°C/hr;
mixing the thermally-treated powder with water at a weight ratio of 1:1.2, blending the mixture for 15 minutes, and mixing the mixture with 20g of polyvinyl alcohol and 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in the ball mill;
forming a second sol by mixing the resultant mixture with 400g of fumed silica powder, 400g of deionized water, and 4.8g of ammonium fluoride for 6 hours in the ball mill;
forming a tube-shaped gel by pouring the second sol in a mold, gelling the sol for 48 hours, drying the gel at a relative humidity of 80% for 2-3 days, and removing the mold from the sol;
drying the tube-shaped gel at a relative humidity of 80% for 10 days, at 40°C for 24 hours, at 60°C for 24 hours, and then at 80°C for 24 hours, and removing remaining moisture and organic material from the dried gel at 900°C for 5 hours in a heat treatment furnace having a temperature rising speed of 100°C/hr; and
removing remaining moisture from the dried gel between 600 and 1100°C, using Chlorine gas, removing remaining Chlorine from the gef, using Helium gas, and sintering the gel between 1300°C and a glass fusion point.
6. An improved method for fabricating silica glass using a sol gel process, comprising the steps of:
forming a first homogenous mixed sol by mixing fumed silica powder with distilled deionized water at a predetermined weight ratio in a ball mill;
11

forming a first gel by getting the first sol;
drying the first gel at a predetermined temperature for a predetermined time in a drier;
forming powder out of the dried silica by grinding and classifying the dried silica;
thermally treating the powder at a predetermined temperature for a predetermined time;
mixing the thermally-treated powder with water at a predetermined weight ratio, blending the mixture for a predetermined time, mixing the mixture in the ball mill, and adding a predetermined amount of pofyvinyf alcohol to the mixture;
forming a second sol by mixing the resultant mixture containing the polyvinyl alcohol with fumed silica powder deionized water, and ammonium fluoride in the ball mill;
forming a tube-shaped ge! by pouring the second sol in a moid,drying the sol for a predetermined time, and removing the mold from the sol;
drying the tube-shaped gel at room temperature; and
removing remaining moisture and Chlorine from the dried tube-shaped gel at a predetermined temperature using Chlorine and Helium gases, and sintering the gel at a high temperature being a glass fusion point or below.
7. An improved method for fabricating silica glass using a sol gel process, comprising the steps of:
forming a first homogeneous mixed sol by mixing fumed silica powder with distilled deionized water at a weight ratio of 1:1-1:3, and mixing the mixture with 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in a ball mill;
forming a first gel by gelling the first sol;
12

drying the first gel at 120°C for 24 hours in a drier;
forming powder out of the dried silica by grinding the dried silica and classifying the ground silica by a 20 mesh sieve;
thermally treating the powder in the range between 600 and 1100°C for one hour in a heat treatment furnace having a temperature rising speed of 3G0°C/hr;
mixing the thermally-treated powder with water at a weight ratio of 1:1.2, blending the mixture for 15 minutes, mixing the mixture with 16kg of silica baits having a diameter of 10mm at 90rpm for 24 hours in the ball mill, and adding 20g of polyvinyl alcohol to the mixture;
forming a second sol by mixing the resultant mixture containing the polyvinyl alcohol with 400g of fumed silica powder, 400g of deionized water, and 4.8g of ammonium fluoride for 6 hours in the ball mill;
forming a tube-shaped gel by pouring the second sol in a mold, gelling the sol for 48 hours, drying the gel at'a relative humidity of 80% for 2-3 days, and removing the mold from the sol;
drying the tube-shaped gel at a relative humidity of 80% for 10 days, at 40°c for 24 hours, at 60°C for24 hours, and then at 80°C tor 24 hours, and
removing remaining moisture and organic material from the dried tube-shaped gel at 900°C for 5 hours in a heat treatment furnace having a temperature rising speed of 100°C/hr; and
removing remaining moisture from the dried gel between 600 and 1100°C, using Chlorine gas, removing remaining Chlorine from the gel, using Helium gas, and sintering the gel between 1300°C and a glass fusion point.
8. An improved method for fabricating silica glass using a sol gel process, comprising the steps of:
13

forming a first homogenous mixed sot by mixing fumed silica powder with distilled deionized water at a predetermined weight ratio in the ball mill;
forming a first gel by gelling the first sol;
drying the first gel at a predetermined temperature for a predetermined time in a drier;
forming powder out of the dried silica by grinding and classifying the dried silica;
thermally treating the powder at a predetermined temperature for a predetermined time;
mixing the thermally-treated powder with water at a predetermined weight ratio, and blending the mixture for a predetermined time;
forming a second sol by mixing the mixture with a predetermined amount of poly vinyl alcohol, fumed silica powder, deionized water, and ammonium fluoride in the ball mill;
forming a tube-shaped gel by pouring the second sol in a moid, drying the sol for a predetermined time, and removing the mold from the sol;
drying the tube-shaped gel at room temperature; and
removing remaining moisture and Chlorine from the dried tube-shaped get at a predetermined temperature using Chlorine and Helium gases, and sintering the gel at a high temperature being a glass fusion point or below.
9. An improved method for fabricating silica glass using a sol gel process, comprising the steps of :
forming a first homogeneous mixed sol by mixing fumed silica powder with distilled deionized water at a weight ratio of 1:1-1:3, and mixing the mixture with 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in a ball mill;
14

forming a first gel by gelling the first sol;
drying the first gel at 120° C for 24 hours in a drier;
forming powder out of the dried silica by grinding the dried silica and classifying the ground silica by a 20 mesh sieve;
thermally treating the powder in the range between 600 and 1100° C for one hour in a heat treatment furnace having a temperature rising speed of 300°C/hr;
mixing the thermally-treated powder with water at a weight ratio of 1:1.2, and blending the mixture for 15 minutes;
forming a second sol by mixing the mixture with 20g of polyvinyl alcohol, 400g of fumed silica powder, 400g of deionized water, and 4.8g of ammonium fluoride, and mixing the resultant mixture with 16kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in the ball mill;
forming a tube-shaped gel by pouring the second sol in a mold, gelling the sol for 48 hours, drying the gel at a relative humidity of 80% for 2-3 days, and removing the mold from the sol;
drying the tube-shaped gel at a relative humidity of 80% for 10 days, at 40 ° C for 24 hours, at 60 °C for 24 hours, and then at 80 °C for 24 hours, and removing remaining moisture and organic material from the dried tube-shaped gel at 900 °C for 5 hours in a heat treatment furnace having a temperature rising speed of 100°C/hr;and
removing remaining moisture from the dried gel between 600 and 1100°C, using Cl gas, removing remaining Cl from the gel, using He gas, and sintering the gel between 1300°C and a glass fusion point.

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There is provided a high-purity silica glass fabricating method using a sol-gel process. In the method, a first sol its formed by mixing 100 parts by weight of fumed silica powder with 100-300 parts by weight of deionized water. The first sol is gelled, dried, powdered, and thermally treated. A second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water and 20-50 parts by weight of non-thermally treated original fumed silica powder. The second sol is gelled, dried, and sintered. Thus, a high-purity silica glass is formed.

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Patent Number

208945

Indian Patent Application Number

00388/CAL/1998

PG Journal Number

33/2007

Publication Date

17-Aug-2007

Grant Date

16-Aug-2007

Date of Filing

10-Mar-1998

Name of Patentee

SAMSUNG ELECTRONICS CO.,LTD.,

Applicant Address

416, MAETAN DONG,PALDAL-GU, SUWON CITY,KYUNGKI-DO.KOREA.

Inventors:

#	Inventor's Name	Inventor's Address
1	YOUNG-MIN BAIK,	HANSHIN APT.,NO.106-511,SONGJONG-DONG,KUMI-SHI,KYONGSANGBUK-DO,
2	YOUNG-SIK YOON,	NO.539,OKGYE-DONG,KUMI-SHI,KYONGSANGBUK-DO,KOREA.
3	SUN-UK KIM,	NO.756,JAGOK-DONG.NAM-GU,POHANG-SHI,KYONGSANGBUK-DO,KOREA.
4	MYUNG-CHUL JUN,	SEUNGRI APT.NO.5-903,JIGOK-DONG,NAM-GU.PHONG-SHI,KYONGSANGBUK-DO,KOREA.

PCT International Classification Number

C03 B 8/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	7974/1997	1997-03-10	Republic of Korea