Title of Invention

SILICA GLASS MONOLITH FABRICATING METHOD USING SOL-GEL PROCESS

Abstract

A silica glass monolith fabricating method using a sol-gel process. In the method, a first sol is formed by mixing 100 parts by weight of high-density silica containing powder with 100-300 parts by weight of deionized water, and rapidly dried, while the pH of the first sol is being controlled in the range between 9 and 11. The dried first sol is thermally treated at or above 600°C, and a second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water. The second sol is gelled in a mold, dried, thermally treated, and sintered. Thus, a high-purity silica glass monolith is obtained.

Full Text

SILICA GLASS MONOLITH FABRICATING METHOD USING SOL-GEL PROCESS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sol-gel process, and in particular, to a method for fabricating a silica glass monolith containing high-purity and high-density silica, for use in fabricating an optical fiber forming glass. 2. Description of the Related Art In general, many methods have been suggested to fabricate a glass monolith by a sol-gel process. However, silica glass obtained by using fumed silica powder only is vulnerable to cracking during a drying step, failing to find its general use. In a sol-gel process using silicon alkoxide, a glass body is homogeneous and transparent but has a very high shrinkage rate (i.e, higher than 60%), which makes it difficult to use the glass body for a large secondary glass tube for fabrication of an optical fiber. In the conventional method using fumed silica particles as shown in FIG. 1, a first sol is formed by dispersing the fumed silica particles in deionized vater to prevent cracking, gelled, dried, and powdered, and a second sol is formed by thermally treating the silica powder and re-dispersing the thermally treated silica powder in deionized water, gelled, dried, and sintered, so that the thus-obtained powder particles are 1A larger than original powder particles, the size of pores among the particles is increased, and thus a crack-free silica glass monolith is obtained. However, this method is not effective in achieving the purpose and requires the first sol gelation step. When fabricating a large glass monolith in the above conventional method, even the two sol dispersion steps cannot prevent cracking during the drying steps, and a long time is required to gel the first sol. SUMMARY OF THE INVENTION An object of the present invention is to provide a silica glass monolith fabricating method using a sol-gel process, which can remarkably reduce a time required to reach a second sol dispersion step by rapidly drying a first sol without a gelation step through control of the pH of the first sol. Another object of the present invention is to provide a silica glass monolith fabricating method using a sol-gel process, which can prevent cracking caused by capillary pressure during a drying step by increasing the size of powder particles and thus the size of pores among the particles. Still another object of the present invention is to provide a silica glass monolith fabricating method using a sol-gel process, which can prevent cracking during a second sol drying step by adding an appropriate amount of aqueous organic compound during the second sol drying step. - 2 - To achieve the above objects, there is provided a silica glass monolith fabricating method using a sol-gel process. In the method, a first sol is formed by mixing 100 parts by weight of high-density silica containing powder with 100-300 parts by weight of deionized water, and rapidly dried, while the pH of the first sol is being controlled in the range between 9 and 11. The died first sol is thermally treated at or above 600°C, and a second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water. The second sol is gelled in a mold, dried, thermally treated, and sintered. Thus, a high-purity silica glass monolith is obtained. 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 silica glass fabricating method using a sol-gel process; and FIG. 2 is a flowchart of a silica glass monolith fabricating method using the sol-gel process according to a preferred embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 2, fine particles containing high-density silica, preferably fumed silica powder (having a desirable size of 7-40mn) obtained by reacting silicon tetrachloride with oxygen is mixed will deionized water, preferably, deionized water, at a weight ratio of 1:1-1:3 in a high shear mixer, and then a homogeneously mixed - 3 - first sol is formed by a ball mill, according to a silica glass monolith fabricating method of the present invention. The pH of the first sol is adjusted in the range between 9 and 11 by adding an appropriate amount of ammonia liquid to the first sol, without gelation of the first sol. Then, the first sol is dried at or above 100°C in an electric oven or rapidly dried in a microwave oven. Particles are grown by coagulation by thermally treating the dried first sol at a temperature of 600-1200° C. The grown particles are re-dispersed in the same manner as for forming the first sol, thereby forming a second sol. During the formation of the second sol, an aqueous organic binder such as polyvinylalcohol can be added when necessary to prevent possible cracking during a drying step. Subsequently, the second sol becomes a homogeneous mixture by the ball mill and is gelled in a mold preferably easily detachable from a gel. It is more preferable to remove bubbles from the mixture before pouring the second sol in the mold by reducing the ambient pressure of the mixture below an atmospheric pressure. After the gelation, the moist gel is removed from the mold and dried. At or below 1100°C in a low-temperature furnace, an OH-group is removed from the dried gel free of cracks using chlorine, and remaining Chlorine the added binder are simultaneously removed therefrom using He gas. Then, the gel is sintered and glassified between 1100 ° C and a glass fusion point in a high-temperature furnace. The silica glass monolith fabricating method according to the present invention will be described with reference to an embodiment in a best mode, hereinafter. (Embodiment 1) - 4 - A first sol containing 25wt% of silica is formed by mixing 2200g of fumed silica powder having a specific surface area of 50m2/g and containing high-density silica with 6600g of deionized water and 50ml of 28% ammonia liquid. To obtain a homogeneous first sol, they are mixed with 18kg of silica balls having a diameter of 10mm at 90rpm for 24 hours in a ball mill. Then, the first sol is dried in a drier at 120°C for 24 hours, ground, classified in a 20 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 mixed with deionized water at a weight ratio of 1:1.2, blended for 15 minutes, and treated for 24 hours with additional 20g of polyvinylalcohol in the ball mill under the same conditions as for the first ball mill treatment. Here, 4.4g of ammonium fluoride is added to the resulting mixture 20 minutes before the second ball mill treatment is completed. Then, the formed sol is gelled in a mold for 48 hours. The mold is formed of teflon and divided into an upper portion, a lower portion, an outer tube-shaped portion, and a central rod. The dimensions of an object moldable by the mold are an inner diameter of 35mm, an outer diameter of 71mm, and a length of 1.3m. After the gelation, the rod is removed from the mold, the gel is dried in the mold for 2-3 days, and the mold is detached from the gel. The tube-shaped gel is dried at a room temperature and 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. Remaining moisture and organic material are removed from the dried gel at 900 ° C for 5 hours in a heat treatment furnace having a temperature rising speed of 100°C/hr. The thermally-treated gel is glassified in a furnace at an atmosphere of He and Chlorine gases. Here, dehydroxylation and glassification are performed at 600-1000 °C for 5 hours and at 1400 °C for one hour, respectively. The glassified and sintered tube has an inner diameter of 21mm, an outer diameter of 41mm, and a length of lm, and shows a shrinkage rate of about - 5 - 25%. As describe above, the silica glass monolith fabricating method using a sol-gel process according to the present invention reduces a processing time by rapidly drying a first sol without a gelation step and suppresses cracking during a drying step by adding an aqueous organic binder, as compared to the conventional method (see FIG. 1) including two sol dispersion steps and two sol gelation steps. In addition, the first sol is very rapidly dried without the first gelation step and thus a time required to reach the second sol dispersion step is reduced because the first sol is formed while its pH is being controlled to be alkaline with an additional alkaline material, the first sol is formed from original powder, dried, and thermally treated so that the size of powder particles and thus the size of pores among the particles are increased. As a result, capillary pressure-induced cracking is prevented during the drying step. Furthermore, the pore size and the adhesion force between particles are controlled by adding an appropriate amount of aqueous organic binder during the second gel drying step, thereby preventing possible cracking during the second gel drying step. While the invention has been shown and described withreference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that 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. - 6 - WE CLAIM : 1. A silica glass monolith fabricating method using a sol-gel process, comprising the steps of : forming a first sol pay mixing 100 parts by weight of high density silica containing powder with 100-300 parts by weight of deionized water; rapidly drying the first sol, while controlling the pH of the first sol in the range between 9 and 11 by adding aqueous ammonia to the first sol; thermally treating the dried first sol at or above 600°C; forming a second sol by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water; gelling the second sol in a mold; and drying, thermally treating, and sintering the second gel to thereby form a silica glass monolith. 2. The silica glass monolith fabricating method using a sol-gel process as claimed in claim 1, wherein the high density silica containing powder is fumed silica obtained by reacting silicon tetrachloride with oxygen, and the water is deionized water. 3. The silica glass monolith fabricating method using a sol- gel process as claimed in one of claims 1 and 2, wherein the obtained glass monolith is tube shaped. -7- A silica glass monolith fabricating method using a sol-gel process. In the method, a first sol is formed by mixing 100 parts by weight of high-density silica containing powder with 100-300 parts by weight of deionized water, and rapidly dried, while the pH of the first sol is being controlled in the range between 9 and 11. The dried first sol is thermally treated at or above 600°C, and a second sol is formed by mixing the thermally-treated first sol with 100-200 parts by weight of deionized water. The second sol is gelled in a mold, dried, thermally treated, and sintered. Thus, a high-purity silica glass monolith is obtained.

Documents:

00389-cal-1998-abstract.pdf

00389-cal-1998-claims.pdf

00389-cal-1998-correspondence.pdf

00389-cal-1998-description(complete).pdf

00389-cal-1998-drawings.pdf

00389-cal-1998-form-1.pdf

00389-cal-1998-form-2.pdf

00389-cal-1998-form-3.pdf

00389-cal-1998-form-5.pdf

00389-cal-1998-g.p.a.pdf

00389-cal-1998-priority document others.pdf

00389-cal-1998-priority document.pdf

389-cal-1998-granted-abstract.pdf

389-cal-1998-granted-claims.pdf

389-cal-1998-granted-correspondence.pdf

389-cal-1998-granted-description (complete).pdf

389-cal-1998-granted-drawings.pdf

389-cal-1998-granted-examination report.pdf

389-cal-1998-granted-form 1.pdf

389-cal-1998-granted-form 2.pdf

389-cal-1998-granted-form 3.pdf

389-cal-1998-granted-form 6.pdf

389-cal-1998-granted-gpa.pdf

389-cal-1998-granted-letter patent.pdf

389-cal-1998-granted-priority document.pdf

389-cal-1998-granted-reply to examination report.pdf

389-cal-1998-granted-specification.pdf

389-cal-1998-granted-translated copy of priority document.pdf