AU619301B2 - A method of preparing doped silica glass - Google Patents

Description

AUSTRALIA

PATENTS ACT 1952 Form COMPLETE SPECIFICATION

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: I Application Number: Lodged: S Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: SEIKO EPSON KABUSHIKI KAIFPA Address of Applicant: 4-1, NISHISHINJUKU, 2-CHOME, SHINJUKU,

TOKYO

JAPAN

Actual Inventor: Address for Service: GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Si Complete Specification for the invention entitled: A METHOD OF PREPARING DOPED SILICA GLASS The following statement is a full description of this invention including the best method of performing it known to me:- 1 I :1 I I O 0 2-- S* the glass,

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A METHOD FOR PREPARING DOPED SILICA GLASS This invention relates to the sol-gel method for preparing doped silica glass applicable for various uses such as optical fibres.

The sol-gel method for preparing doped silica glass is known to have many advantages including: relatively small energy requirements to form the glass, it is possible to form glass having better homogeneity than glass formed by conventional methods, high purity glass can be formed, it is possible to uniformly disperse dopants r -3into glass.

There are several known commercial sol-gel methods.

For example, Kamiya and his co-inventors developed a method of forming a TiO 2 -SiO 2 glass having a lower thermal expansion coefficient than a silica glass for use in the construction of astronomical telescopes.

(Japan Chemics Conference Bulletin, No. 10, 1571, 1981).

Furthermore, Sato and his colleagues developed a method of preparing a silica gel having additives such eas Ge, Ti, Zr, Ta, Nb and Sb for use as a preform for o: optical glass and various other uses. (Japanese Laid-Open Publication No. 57/191221).

However, the conventional methods of preparing silica glass have various disadvantages. In the method developed by Kamiya et al. substantial periods of time I: (in some instances in excess of 4 months) are necessary to complete the glass forming process, and in any event It has not been possible to form a TiO -SiO glass of any acceptably large size. The maximum size of dry gel formed in accordance with the method developed by Sato is about 4mm in diamete. and 50mm in length. Such dimensions will not enable the formation of a doped silica glass piece of an acceptable size (for example, a substrate of 6 square inches or rod of 20mm in diameter and 500mm in length).

Accordingly, it is an object of this invention to provide an improved method of preparing a silica glass which is large enough for industrial usage.

According to the present invention there is provided a method of preparing doped silica glass comprising the steps of: 3a preparing a sol solution by mixing together: a hydrolyzed solution formed from an alkyl silicate (represented by the general formula Si(OR) 4 wherein R stands for an alkyl group) and a suitable metal alkoxide (M(OR) where M is a metal) to adjust the refractive index (hereinafter referred to as "the dopant"), and

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4

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.4 Al MV~7 4 (ii) ultra fine particle silica (herein i referred to as "ammosil") formed by hydrolyzing an alkyl silicate with water or ammonia gas and water, adjusting the pH of said sol solution to a predetermined value, casting said sol solution into a container, gelling said sol solution to form a wet gel, drying said wet gel to form a dry gel, and '10 sintering said dry gel to form a transparent I a o glass.

i In accordance with a preferred embodiment of the con azpct of th invention the method of preparing doped silica glass comprises the following steps.

i :t5 First, a sol solution is preparing by mixing a i "hydrolyzed solution of alkyl silicate and a metal alkoxide ji with ultra fine particle silica obtained by hydrolyzing alkyl Ii silicate with ammonia water or ammonia gas and water.

The pH of the sol solution is adjusted to a predetermined value with ammonia, ammonia gas, an aqueous solution of ammonia or organic base.

Then the sol solution is cast into cylindrical S* container formed from hydrophobic material and is gelled.

The gel is dried to a dry gel and finally is sintered to form a transparent glass.

In order to enhance the yield of doped silica glass by the sol-gel method, the ultra fine particle silica is added, thereby reducing the occurrence of fractures in the dry gel during drying of the wet gel. Moreover, by adding ultra fine particle silica, the structure of the dry gel becomes non-porous and is less susceptible to cracks during sintering.

In order to facilitate the control of the particle diameter or dispersion of the silica particles, the ultra fine particle silica obtained by hydrolyzing alkyl silicate with ammonia water or ammonia gas and water (hereinafter referred to as "ammosil") is used in the present invention.

It has been found that there is a close relation between the strength of the dry gel, and the pH of the sol solution and the temperature at which the sol solution is Sgelled. For example, in case of using pure silica, it is favorable to adjust the temperature and the pH to between 0 and 50 C and between 3 and 6, respectively. Suitable bases to adjust the pH comprise ammonia water, ammonia gas, an aqueous solution of ammonia and organic base especially such as S'O triethylamine, an aqueous solution of triethylamine, pyridine, 3i 10 and aqueous solution of pyridine, aniline and an aqueous se" solution of aniline. On the other hand, bases including a S' i alkali metal ion such as sodium hydroxide and potassium hydroxide are not suitable since positive ions remaining in the glass crystallize.

o 2*5as In preparing relatively large dry gels, it is preferred that the container for the gel be formed from p, •hydrophobic material. Suitable materials comprise organic polymers such as polypropylene, polyethylene fluoride, polyvinyl chloride and polyethylene polystyrene and inorganic materials such as glass coated with one or more of the above organic polymers.

IIn preparing the cylindrical silica glass used as an optical fiber, germanium alkoxide, which increases the refractive index of the glass, is desirable as the metal alkoxide to be doped in the hydrolyzed solution.

As mentioned above, the yield of the transparent glass is enhanced by adding ammosil. However, since ammonia is used to synthesize ammosil, if the synthesized ammosil is directly added to the hydrolyzed solution, the sol solution abruptly gels which is not desirable. Accordingly, the pH of the ammosil preferably should be adjusted to be less than before adding to the hydrolyzed solution.

If the pure silica in ammosil is less than 20% of the silica included in the hydrolyzed solution of alkyl silicate, bubbles are likely to be retained in the glass during sintering, which causes fractures of the glass.

6 In cases where metal alkoxide is doped when alkyl silicate is hydrolyzed, the hydrolysis is carried out by adding water at a molar ratio in excess of 1 with respect to the alkyl silicate. However, if the amount of water exceeds 3 in a molar ratio, the excessive water directly reacts with the metal alkoxide, thereby preventing the dry gel being transformed into a transparent glass during sintering, as well as resulting in a non-uniform distribution of refractive indices. If the hydrolysis is carried out without alcoholic solvent, since the gelation occurs during operations and the yield is lowered, the hydrolyzed reaction solution should be kept at a temperature lower than 20 0

C.

The mean particle diameter of the ammosil is adjusted by controlling the amount of ammonia or alcohol. If 15 the mean particle diameter of ammosil is too small, bubbles *o remain in the gel during the sintering step and if the mean 0@ particle diameter thereof is too large the gels are likely to fracture during the drying step. The experiments by the S present inventors demonstrated that the suitable mean particle 20 diameter of ammosil for obtaining doped silica glass with a 0* good yield is between 0.01 and 0.lum. As the mean particle Sdiameter of ammosil increases, the dispersion of silica particles deteriorates. So, according to necessicy, it is Spreferred to apply ultrasonic vibration to the solution or to use centrifugal separation to uniformly disperse the silica S particles.

In gelling the sol solution to a gel, the strength of the resulting dry gel can be changed by adjusting the pH and the temperature of the sol solution. Moreover, it is possible to approximate the two parameters into one by controlling the gelling time. Through the experiments, it was found that between 3 and 100 minutes is a preferred gelling period.

If the volume ratio of the wet gel and the resulting transparent glass obtained by drying and sintering the wet gel is too high, a long time is required for drying, and fracture is likely to occur when shrinking, and if the volume ratio is i 7 too small the gel easily fractures when dried to a dry gel.

Our experiments demonstrated that the preferred volume of the resulting glass is between 5 and The step of drying the wet gel to a dry gel has the greatest influence to the yield of the glass and is a very i important step. It has been found that the most suitable conditions to dry a wet gel to a dry gel with a good yield in a short manufacturing period comprise placing the wet gel in a container with a lid having openings amounting to less than 15% of the surface area of the lid and heated to a temperature between 0 and 120 C at a heating rate of less than 120 0 C per 0 0 hour. It is acceptable to exchange the lid of the container in which the wet gel has been gelled with a lid having S openings amounting to less than 15% of the surface area of the 15 lid. However, it is more desirable to place the gel into i another container of larger capacity and provide spacers like saddles to reduce the contact area between the wet gel and the S container.

i The yield from sinte.ing and suitable conditions to S 20 enhance the yield are explained below.

s e In accordance with this invention, the dry gels are S• sintered in the following 5 steps.

S1) removing absorbed water 2) removing carbon 3) removing hydroxide group 4) removing chloride or fluoride and making the S 00 gel non-porous, and making the gel to be a transparent glass.

The step 1) of removing absorbed water has the greatest influence on yield. An amount of physical absorbed water in the dry gel can be removed by the heat treatment at a temperature of about 400 0 C. At this time, if the gel is abrup tly heated, the susceptibility for fractures increases, thereby lowering the yield. However, if a heating rate is too low, an excessive time is require and the manufacturing cost increases, Our experiments demonstrated that the upper limit of the acceptable heating rate which does not lower the yield

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8 is about 400 0 C per hour. In order to remove absorbed water, it is suitable to perform at least one step of maintaining the gel at a temperature selected from the range between room temperature and 400 C for at least 1 hour.

In step carbon is removed by heat treatment at a temperature between 400 and 1200 0 C. The step 2) is very efficient because, in this step, hydrochloride in the base is decomposed and the dehydrating condensation is accelerated, thereby reducing the remaining hydroxide group before treatment with chloride. Though not so much as in the above step the heating rate in step 2) also has an influence on yield. According to our experiments, the preferred heating 0 rate is between 30 and 400 0 C per hour and at least one step of 0 maintaining the gel at the temperature between 400 and 1200 C 15 for at least 1 hour should be followed.

to:::In step hydroxide group is removed. Because hydroxide group in doped silica glass absorbs light having a oo0o wavelength of about 1.39um, thereby causing a deterioration of 0 the optical fiber properties According to our experiments, 0 20 the preferred condition for removing hydroxide group is to 0 perform at least one step of heating the gel in the presence a 0° of a carrier gas and a hydroxide group removing agent at a flow rate of between 1 and 40% with respect to the carrier gas at a temperature between 700 and 1200 C for at least minutes. Suitable carrier gases comprise He, Ne, Ar and N 2 o Suitable hydroxide group removing agents comprise C1 2 SOC1, too SF 6

CF

4

C

2

F

6 and C 3

F

8 One purpose of step namely removing chloride or fluoride, is to prevent the hydroxide removing agent used in the preceding step from remaining in the glass, which causes bubbling. Chloride or fluoride should be removed by at least one step of heating the gel in the presence of the carrier gas as above and 02 at a flow rate between 1 and 100% with respect to the carrier gas at a temperature between 1000 and 1500 C.

Moreover, it is desirable to make the dry gel non-porous by at least one step of flowing He gas only into the oven.

9 In step the dry gel is transformed into a transparent glass by heat treatment. The temperature of the heat treatment depends on the mixing ratio of ammosil but a preferred temperature range is 1000 to 1600 C.

It is preferred that the heating rate in steps 3), 4) (including making the gel non-porous) and 5) is between and 10000C/hour.

This invention is explained in detail with reference to embodiments.

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.1 S 1.o• 10 Embodiment 1 of 0.02 normal hydrochloric acid was added to 62 4 g of the refined commercially available silicon ethoxide to carry out the partial hydrolysis with keeping the temperature of the solution at 5 C in order to prevent the abrupt gelation of the hydrolyzed solution.

I Still keeping the temperature at 5 C, 57g of tetra ethoxy germanium was added to the above solution and after the violent stirring, 152ml of water was added to complete the hydrolysis.

Separately from the above, the mixture solution of 180ml of 28% I0 ammonia water, 1.8Q of ethanol and 32 5 ml of water was added to the 1 mixture solution of 1. of silicon ethoxide and 1.7Q of ethanol and the solution was stirred at a room temperature and maintained still over one night. Then the resulting solution was condensed until the 0 S volume thereof became to 720ml and thus the ammosil solution including the ultra fine particle silica whose mean particle diameter was S At this tie, the pH of the solution was adjusted to 5.0 by 0.17 u m. At this time, the pH of the solution was adjusted to 5.0 by K Hi

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I I 11 the addition of 2 normal hydrochloric acid in order to avoid the abrupt gelation when the solution was added to the hydrolyzed solution prepared above.

The hydrolyzed solution and the ammosil solution were mixed to yield the sol in which the ratio of the effective glass component of he hydrolyzed solution and that of the ammosil solution was 4 to 6.

The pH of the resulting sol was adjusted to 3.5 by adding 0.2 normal ammonia water and the volume thereof was adjusted to 1700ml by adding water.

10 The sol adjusted as above was casted into a cylindrical container of polyethylene (6.0cm in inner diameter and 75.0cm in length) to about 80% of the capacity of the container 60cm) and the container was sealed. In 25 minutes at the room temprature of 30" C, the sol was gelled. After maintaining the resulting gel in the 15 container over one night, the cover of the container was exchanged with the cover having openings amounting to 1.0% of the surf-ace area.

Then the gel was heated from 30 to 60 0 C at a heating rate of 20 0 C per hour and dried. Thus a dry gel (4cm in outer diameter and 41cm in length) which did not fracture even at a room temperature was obtained.

out of 20 dry gels formed in the same manner fractured and 16 perfect dry gels were obtained, thus the yield being Then 16 dry gels were placed in the sintering oven and heated to 200'C at a heating rate of 30 0 C per hour and maintained at 200"C for 1 hour to remove absorbed water. The gels were further heated to 450 3

C

at a heating rate of 30°C per hour, maintained at 450 C for 5 hours, 12 heated to 950 0 C at a rate of 30'C per hour and maintained at 950"C for 1 hour to remove carbon.

The gels were next cooled down to 770C ata rate of 5 C per minute at which temperature the mixture gas of He and Cjz at a flowing amount ratio of 5 to 1 was flown into the oven for 2 hours and then heated to 900 C at a rate of 5 0 C per minutes and maintained at 900 C for 1 hour to remove hydroxide group. Then the temperature was raised to 1000'C at a heating rate of 1°C per minutes and the mixture gas of He and Oz at a flowing amount ratio of 3 to 1 was flown into the oven 10 for 1 hour to remove chloride. The gels were heated to 1200 3 C at a rate of 1 0 C per minutes with flowing He gas to make the gels nonporous and maintained at 1200 C for 1.5 hours to make the gels transparent to yield cylindric doped silica glasses of the high purity a o (3cm in diameter and 30cm in length). The degree of shrinkage from 15 the wet gel to the resulting glass was 10.5% and the germanium content of the glass was 3mol% with respect to silica.

In the above sintering process, no dry gel frnctured.

Accordingly, the yield in all steps in this embodiment was 80% and 16 S complete cylindrical doped silica glasses of the high purity were obtained.

Embodiment 2 4 ml of 0.01 normal hydrochloric acid and 105.2g of ethanol were added to 6 2 4 g of the refined commercially available silicon ethoxideand the soltion was violently stirred to carry out the partial hydrolysis. 93g of tetra isopropoxy germanium was added to the

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13 solution and violently stirred and then 18 4 ml of water was added to complete the hydrolysis.

Separately from the above, the mixture solution of 120ml of ammonia water of 28%, 1.51 of ethanol and 2 16ml of water was addd to the mixture solution of 4 4 4 ml of silicon methoxide and 1i of ethanol with stirring. The mixture solution was maintained for 1 day and was condensed under the reduced pressure to a predetermined concentration to yield the ammosil solution. The pH of the ammosil solution was adjusted to *0 The hydrolyzed solution and the ammosil solution were mixed and g o* stirred and the impurities were removed by the centrifugal separation.

The pH of the resulting solution was adjusted to 3.2 by bubbling ammonia gas to obtain the sol solution. The sol solution was casted into a cylindrical container of polyvinyl chloride (4.0cm in inner diameter and 140.0cm in length) to the height of 130cm. At the room temperature of 25 C, the sol ws gelled in 20 minutes. The gel was i- S maintained for 2 days and then was placed onto 100 cylindrical spacers (1cm in outer diameter and 60cm in length) of polyethylene fluoride S provided in a drying container of polypropylene having a cover 20 comprising openings amounting to 0.5% of the surface area. The gels in a drying container was heated from 25 0 C to 600C at a heating rate of 5"C per hour and maintained for 8 days, thus dried to a dry gel.

out of 20 dry gels formed in the same way as above fractured and 15 complete dry gels were obtained with the yield of The above 15 dry gels were placed in the sintering oven and heated to 3000C at a heating rate of 60 0 C per hour and maintained at I -14- 300 0 C for 5 hours to remove absorbed water. Then the gels were heated to 9500C at a heating rate of 1800C per hour and maintained at 950 C for 18 hours to remove carbon. Then the gels were cooled down to 770CC and the mixture gas of He and SOC12 at a flowing amount ratio of 5 to 2 was flown into the oven for 2 hours to remove hydroxide group.

The gels were then heated to 1220 C at a heating rate of 1 C per minutes with flowing He gas and maintained at 1220'C for 1.5 hours to yield the transparent glasses (2cm in outer diameter and 60cm in length).

In the above sintering process, 3 out of 15 dry gels fractured i and 12 complete cylindrical doped silica glasses were obtained with o, the yield of 80%. Accordingly, in all steps in this embodiment the cylindrical doped silica glasses of the high purity including S: germanium of 5mol% with.respect to silica with the yield of 15 Embodiment 3 S 32ml of 0.01 normal hydrochloric acid was added to 1248 g of the refined commercially available silicon ethoxide and the solution was violently stirred to carry out the hydrolysis.

The mixture solution of 5 4 0ml of 28% ammonia water, 5.252 of ethoanol and 11 of water was added to the mixture solution of 3 of silicon ethoxide and 5.251 of ethanol and the solution was stirred S: and maintained over one night. The resulting solution was condensed under the reduced pressure to a predetermined concentration and the pH of the solution was adjusted to 6.0 to yield the ammosil solution.

The hydrolyzed solution and the ammosil solution were mixed and 15 the ultrasonic vibration was applied to the solution to uniformly disperse the silica particle. The pH of the resulting solution was adjusted to 6.0 by the addition of 0.1 normal ammoia to yield the sol solution.

At 4 0' C, the sol was casted into a cylindrical container of polyethylene (4.0cm in inner diameter and 180cm in length) to the height of 160cm and the container was sealed. The sol was gelled in minutes. After maintained over one night, the cover of the j container was exchanged with the cover having openings amounting to 10 0.5% of the surface area of the cover and the gel was heated to 80 C I at a heating rate of 100 C per hour. The gel was maintained at S for 7 days and the dry gel (2.80cm in outer diameter and llcm in I length) which did not fracture even at the room temperature was obtained.

i 1 out of 12 dry gels formed under the same condition fractured s and 11 complete dry gels were obtained with the yield of 92%.

The above 11 dry gels were heated to 150 C at a heating rate of S U per hour, maintained at 1500C for 1 hour, heated to 400 0 C at a heating rate of 70 C per hour and maintained at 4000C for 1 hour to remove absorbed water.

Then the gels were heated to 9500C at a heating rate of 900C per hour and maintained at 950 0 C for 5 hour to remove carbon. Then by flowing the mixture gas of Ne and SFb at a flowing amount ratio of with rspect to Ne into the oven for 1.5 hours to remove hydroxide group. The gels were further heated to 1000 0 C at a heating rate of per minutes and flowing the mixture gas of Ne and Oz at a flowing _t 16 i amount ratio of 100% with respect to Ne gas for 2 hours to remove fluoride. Then by flowing only Ne gas into the oven for 2 hours, the gels were made non-porous.

The gels were heated to 1200 0 C at a heating rate of 60 C per minute for 3.5 hours to yield the transparent glasses.

I In the above sintering process, there was no fractures in 11 dry I gels and thus the cylindrical pure silica glasses (2cm in outer diamter and 83cm in length) were obtained with the yield of 100%.

i 6 i 0 Embodiment 4 i I I S 108ml of 0.01 normal hydrochloric acid and 2 10 4 g of ethanol were I added to 1248g of the refined commercially available silicon ethoxide and the solution was violently stirred to carry out the partial hydrolysis. Then il14g of tetra ethoxy germanium was added and the solution was violently added and then 368ml of water was added to complete the hydrolysis.

6 The mixture solution of 460ml of 28% ammonia water, 3.5Q of ethanol and 6 50ml of water was added to the mixture solution of 2 of I silicon ethoxide and 3.51 of ethonol and the solution was stirred at '20 the room temperature and maintained over on night. The solution was condensed to a predetermined concentration and the pH was adjted to Then the resulting solution and the hydrolyzed solution were mixed and the pH was adjusted to yield a sol solution. The sol solution was casted into a cylindrical container of polyvinyl chloride of 4 cm in inner diameter and 220cm in length. Thus a wet gel of 4cm in outer diameter and 200cm in length was obtained.

i 17 The obtained wet gel was placed in a container having openings i amounting to 0.2% of the surface area and dried. Then the gel was i jheated to 60 C at a heating rate of 100'C per hour and maintained at °C for 7 days. Thus a dry gel which did not fracture even at the I 5 room temperature was obtained.

S9 out of 20 dry gels formed under the same condition as above I fractured and 11 complete dry gels were obtained with the yield of i The dry gels were heated to 150 C at a heating rate of 600C per *150* i 10 hour,maintained at 1503C for 1 hour, heated to 400 0 C at a heating rate of 60 C per hour and maintained at 400 0 C for 1 hour to remove absorbed ,i water and OH group.

The gels were further heated to 900 0 C at a rate of 90 0 C per hour and maintained at 900 0 C for 5 hours to remove carbon. Then the mixture gas of He and CQ 2 at a flowing amount ratio of 40% with respect to He gas was flown into the oven for 1.5 hours to remove hydroxide group. Then the gels were heated to 1000'C at a heating H rate of 5 0 C per minutes and maintained at 10000C for 3 hours with flowing the mixture gas of He and 02 at a flowing amount ratio of with respect to He gas to remove chloride and to make the gels nonporous. Thereafter, the gels were heated to 1150 C at a heating rate of 10 C per minute and maintained at 1150'C for 5 hours to make the gels transparent and thus the doped silica glasses of 2cm in outer diameter and Im in length was obtained.

In the above sintering process, 2 dry gels fractured and in whole steps of this embodiment, 9 complete doped silica glasses were 18 obtained with the yield of Embodiment 432ml of 0.02 normal hydrochloride was added to 1248g of the refined commercially available silicon ethoxide and the solution was violently stirred at the room temperature.

The mixture solution of 360ml of 28% ammonia water, 3.5 of ethanol and 6 50 ml of water was addd to the mixture solution of 2 9 of silicon ethoxide and 3.5 of ethanol the the solution was stirred at i I the room temperature. After maintained over one night, the solution i o• S'J was condensed to a predetermined concentration and the pH thereof was adjusted to SThe hydrolyzed solution and the sol solution were mixed to yield I a sol solution. The sol solution was casted into a container of 0 Si polyvinyl chloride dof 5.0cm in inner diameter and 200cm in length and i gelled to a wet gel of 5cm in outer diameter and 2 7 0cm in length. The wet gel was re-placed into a drying container of polypropylene having openings amounting to 0.45% and then the same drying and sintering steps as in Embodiment 9 were followed. Thus a pure silica glass of 3cm in outer diameter and 9 4 0cm in length was obtained.

i 20 3 out of 20 dry gels formed under the same conditions fractured before sintering and 17 complete pure siica glass of the high purity I were obtained with the yield of Embodiment 6 162ml of 0.03 normal hydrochloric acid was added to 913g of the .4.

:1 i i 14 S 0D 0 SO S 55 11 S S 0 00

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_-7 19 refined commercially available silicon methoxide and the solution was violently stirred at 0 0 C. Then 4 9g of tetra methoxi germanium was added with violent stirring and then 288ml of water was added to complete the hydrolysis.

The mixture solution of 110ml of 28% ammonia water, 1.2 of ethanol and 288ml of water was added to the mixture solution of 59 2 ml of silicon nathoxide and 1.2 of methanol and the mixture was condensed to a predetermined concentration. The pH was adjusted to to yield the ammosil solution.

In the same way as in Embodiment 9 using the above solutions and a container of polyethylene, a wet gel of 4cm in outer diameter and 180cm in length was obtained. The cover of the container was replaced by a cover with openings amounting to 0.9% of the surface area of the cover and the wet gel was dried. Following the same steps as in Embodiment 9, a silica glass was obtained.

2 out of 20 glasses fractured and 18 complete doped silica glasses including germanium of 3mol% with respect to the effective glass component were obtained with the yield of Embodiment 7 32 4 ml of 0.01 normal hydrochloric acid was added to 12 48 g of the refined commercially available silicon ethoxide and the solution was sufficiently stirred. Then 11 4 g of tetra ethoxi germanium was added with stirring and finally 108ml of water was added to complete the hydrolysis.

The mixture solution of 180ml of 28% ammonia water, 1.8 of ethanol and 325m1 of water was added to the mixture solution of 19 of tetra ethoxi gemanium and 1.7? of ethonol with stirring. The resulting solution was condensed to a predetermined concentration and the pH was adjusted. To the resulting solution was added the hydrolyzed solution to adjust the pH. The sol was gelled in the container of polypropylene of 6cm in inner diameter and 100cm in length to a wet gel of 6 cm in outer diameter and 75cm in length. The wet gel was sintered under the same conditions as in Embodiment 9 and a transparent glass was obtained.

.10 2 out of 20 glasses formed in the same way as above fractured and 18 complete doped silica glasses were obtained with the yield of *s* However, the distribution of germanium in the core portion of the obtained doped silica glass was non-uniform and the quality of the I resulting glass was not very good.

S Reference Example 108ml of 0.01 normal hydrochloric acid was added to 1 248 g of the i Srefined commercially available silicon ethoxide and the solution was i stirred at the room temperature without adjusting the solution i.

S temperature. Then 5 7 g of tetra ethoxi germanium was added and the i 20 solution was sufficiently stirred. Thereafter 3 2 4 ml of water was added to the solution with stirring and the solution was gelled during the stirring.

7mbodiment 8 of 0.02 normal hydrochloric acid was added to 624g of the i

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21 refined silicon ethoxide and the solution was vilently stirred with keeping the solution temperature at 5 C to carry out the partial hydrolysis. Then 5 7 g of tetra ethoxy germanium was added with vilently stirring and then 152ml of water was added to complete the hydrolysis.

The mixture solution of 1 2 0ml of 28% ammonia water and 1.8 of ethanol and 325ml of water was added to the mixture solution of 1i of silicon ethoxide and 1.7) of ethanol and the solution was stirred at the room temperature. After maintained over one night, the solution was condensed to a predetermined concentration. Under the same conditions as in Embodiment 9, a transparent glass is formed. 10 out of 20 glasses fractured and 10 complete doped silica glasses were obtained with the yield of

S.

S

S

S*

10 *r S

S

S

Embodiment 9 15 80ml of 0.02 normal hydrochloric acid was added to 624g of the refined commercially available silicon ethoxide and the solution was violently stirred with keeping the solution temperature at 5 OC to carry out the partial hydrolysis. Then 5 7 g of germanium ethoxide was added to complete the substitutional reaction. Finally, 152ml of water was added to complete the hydrolysis.

The mixture solution of 180ml of 28% ammonia water, 1.8 of ethanol and 325ml of water was added to the mixture solution of 1 of silicon ethoxide and 1.7S of ethanol and the solution was stirred at the room temperature. After maintained over one night, the solution was condensed under the reduced pressure to a predetermined 22concentration and the pH was adjusted to 5.0 to yield the ammosil solution.

The hydrolyzed solution and the ammosil solution were mixed and the masses were get rid of the solution by the centrifugal separation.

Then by following the same drying and sintering processes, a doped silica glass of high quality was obtained.

2 out of 20 doped silica glasses formed under the same conditions fractured and 18 complete clindrical silica glasses were obtained with the yield of 0 i' As explained with reference to foregoing embodiments, in accordance with this invention, a pure cylindrical doped silica glass S* S of high quality applicable to a fiber core of an optical fiber of such as 2cm in outer diameter and im in length is provided with the good yield, which was impossible in the prior art.

i 0 9 i 6

Claims (23)

1. A method of preparing doped silica glass comprising the steps of: preparing a sol solution by mixing together: a hydrolyzed solution formed from an alkyl silicate (represented by the general formula SI(OR) 4 wherein R stands for an alkyl group) and a suitable metal alkoxide (M(OR) x where M is a metal) to adjust the refractive index (hereinafter referred to as "the dopant"), and (ii) ultra fine particles silica (herein referred to as "ammosil") formed by hydrolyzing an alkyl silicate with water or ammonia gas and water, adjusting the pH of said sol solution to a predetermined value, casting said sol solution into a 'container, gelling said sol solution to form a wet gel, drying said wet gel to form a dry gel, and sintering said dry gel to form a transparent glass.

2. The method as claimed in Claim 1, wherein the pH of said sol solution is adjusted by the addition of ammonia, ammonia gas, or aqueous solution of ammonia or an organic base.

3. The method as claimed in Claim 2, wherein the organic base comprises triethylamine, or aqueous solution of pyridine, aniline or an aqueous solution of aniline.

4. The method as claimed in any one of the preceding claims, wherein said sol solution is cast into t i 1 i i:: i r :-d os r i-i I r ;-B i .I i 24 a cylindrical container formed from a hydrophobic material.

The method as claimed in any one of the preceding claims, wherein said metal alkoxide is tetra alkoxy germanium represented by the general formula Ge(OR) 4

6. The method as claimed in any one of the preceding claims, wherein said sol solution is prepared by adding a solution containing said ammosil and having a pH previously adjusted to a predetermined value to the hydrolyzed solution, at a selected ratio.

7. The method as claimed in any one of the preceding claims, wherein said hydrolyzed solution is prepared by the steps of: partially hydrolyzing said alkyl silicate with water at a molar ratio between 1 and 3 with respect to said alkyl silicate at a temperature below adding said metal alkoxide in a suitable amount, and hydrolyzing any alkoxide group remaining in the solution.

8. The method as claimed in any one of the preceding claims, wherein said hydrolyzed solution is prepared by the steps of: adding water to a mixture solution of said alkyl silicate and alcohol at a volume ratio greater than with respect to said alkyl silicate, said water being between 1 and 3 at a molar ratio with respect to said alkyl silicate to carry out the partial hydrolysis, adding said metal alkoxide to the resulting solution, and hydrolyzing any alkoxide group remaining in the solution.

9. The method as claimed in any one of the preceding claims, wherein the mean particle diameter of said ammosil is adjusted to be between 0.01 and l.Opm.

I i 25 The method as claimed in any one of the .i preceding claims wherein said ammosil is uniformly dispersed in said sol solution by ultrasonic vibration and/or centrifugal separation.

11. The method as claimed in any one of the preceding claims, wherein the temperature and the pH of said sol solution are adjusted so that the gelation of the sol solution is completed in a period between 3 and 100 minutes.

12. The method as claimed in any one of the preceding claims, wherein the composition of said sol solution is adjusted so that the volume of said transparent glass obtained by drying and sintering said wet gel is between 5 and 15% of the volume of said wet gel.

13. The method as claimed in any one of the preceding claims, wherein, in said step of drying the wet gel, an open end of said container is covered with a lid having openings amounting to less than 15% of the surface area of said lid.

14. The method as claimed in any one of the preceding claims, wherein, in said step of drying the wet gel, said wet gel is removed from said container and placed into another container and an open end of the said other container is closed with a lid having Sopenings amounting to less than 15% of the surface area of the lid. _L

15. The method as claimed in any one of the preceding claims, wherein said wet gel is dried at a temperature between 0 and 120 0 C and heated to the temperature at a heating rate less than 1200°C per hour.

16. The method as claimed in any one of the preceding claims whereln said sintering step comprises the steps of: t .4 26 removing absorbed water removing carbon removing hydroxide group removing hydrochloride or fluoride and making the gel non-porous, and transforming the gel into a transparent glass.

17. The method as claimed in Claim 16, wherein absorbed water is removed by at least one step of heating the gel to a first selected temperature between and 400 C at a heating rate less than 400 C per hour and maintaining the gel at said first selected temperature for at least 1 hour.

18. The method as claimed in Claim 16 or 17, wherein carbon is removed by at least one step of heating the gel to a second selected temperature between 400 and 1200 0 C at a heating rate between 30 and 400 0 C per hour and maintaining at said second selected temperature for •at least one hour.

19. The method as claimed in any one of Claims 16 to 18, wherein hydroxide group is removed by at least one step of heating the gel to a third selected temperature between 700 and 1200 0 C in the presence of a carrier gas selected from the group comprising He, Ne, Ar and N 2 and an hydroxide group removing agent such as C 2 SOCk, SF 6 CF 4 C 2 F 6 and C 3 F 8 at a flow rate ratio between 1 and 40% with respect to said carrier gas.

The method as claimed in any one Claims 16 to 19, wherein after removing the hydroxide group, chloride or fluoride is removed by heating the gel to a fourth selected temperature between 1000 and 1500 0 C in the presence of the carrier gas selected from the group consisting of He, Ne, Ar and N 2 together with oxygen at a flow rate between 1 and 100% with respect to said carrier gas and making the gel non-porous by heating the 27 gel to the fourth selected temperature in the presence of pure He gas only.

21. The method as claimed in any one of Claims 16 to wherein the gel is made into a transparent glass by heating the gel to a fifth selected temperature between 1000 and 1600 0 C and maintaining said gel at said fifth selected temperature for a predetermined period of time.

22. The method as claimed in any one of Claims 16 to 21, wherein the heating rate in said steps of removing hydroxide group, of removing hydrochloride or fluoride and making the gel non-porous and of making the gel to be a transparent glass is between 0.5 and 1000 0 C per i hour.

"23. A method of preparing doped silica glass *substantially as hereinbefore described with reference to the examples. S" Dated this 16th day of June 1991 :SEIKO EPSON KABUSHIKI KAISHA By Its Patent Attorneys GRIFFITH HACK CO. Fellows Institute of Patent S Attorneys of Australia. H1